Evolution of the Platoro Caldera Complex 

and Related Volcanic Rocks, Southeastern 

SanJuan Mountains, Colorado 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 852 
Evolution of the Platoro Caldera Complex 

and Related Volcanic Rocks, Southeastern 

San Juan Mountains, Colorado 

By PETER W. LIPMAN 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 852 

A study of relations between ash-flow eruptions, 

lava-flow activity, and caldera structure at a 

mineralized volcanic center 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON 1975 
UNITED STATES DEPARTMENT OF THE INTERIOR 

ROGERS C. B. MORTON, Secretary 

GEOLOGICAL SURVEY 

V. E. McKelvey, Director 

Library of Congress Cataloging in Publication Data 

Lipman, Peter W. 

Evolution of the Platoro caldera complex and related volcanic rocks, southeastern San Juan Mountains, Colorado. 

(Geological Survey Professional Paper 852) 

Includes bibliographical references and index. 

Supt. of Docs. No.: I 19.16:852 

1. Volcanism-San Juan Mountains. 2. Volcanic ash, tuff, etc.-San Juan Mountains. 

I. Title. II. Series: United States Geological Survey Professional Paper 852. 

QE461.L626 552'.2'0978838 74-20902 

For sale by the Superintendent of Documents, U.S. Government Printing Office 

Washington, D.C. 20402- Price $3.25 (paper cover) 

Stock Number 2401-02586 
CONTENTS 

/Abstract ................................................ . Page Page 

1 Lavas and related rocks of the Platoro caldera 

.IJntroduction ............................................ . 2 complex-Continued 

Present investigation .................................... . 2 Intrusive rocks-Continued 

/Geologic setting ......................................... . 6 Granitic intrusions-Continued 

,..Prevolcanic rocks ........................................ . 8 Jasper stock................................. 84 

"'Early intermediate-composition rocks (Conejos Formation) .. . 8 Bear Creek stock cluster . . . . . . . . . . . . . . . . . . . . . . 84 

Vent facies .......................................... . 12 Other stocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86 

Volcaniclastic facies ................................. . 13 Porphyro-aphanitic intrusions . . . . . . . . . . . . . . . . . . . . . 86 

Upper lava unit ..................................... . 13 Andesitic rock-s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86 

Ash-flow sheets ......................................... . 14 Rhyodacitic rocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87 

Tuff of Rock Creek .................................. . 14 Quartz latitic and rhyolitic rocks . . . . . . . . . . . . . . 88 

Treasure Mountain Tuff ............................. . 16 Late basalt and rhyolite ·(Hinsdale Formation) . . . . . . . . . . . . . . 90 

Lower tuff ...................................... . 19 Basaltic lavas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90 

La Jara Canyon Member ........................ . 25 Rhyolitic lavas and tuffs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93 

Middle tuff ..................................... . 30 Mixed lavas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97 

Ojito Creek Member ............................. . 33 Surficial deposits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101 

Ra Jadero Member .............................. . 36 Structural evolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101 

Upper tuff ...................................... . 40 Pre-Tertiary structures. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101 

Younger ash-flow sheets and interlayered lavas ........ . 41 Tertiary volcanic structures . . . . . . . . . . . . . . . . . . . . . . . . . . . 101 

Masonic Park Tuff .............................. . 41 San Juan caldera cluster . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101 

Fish Canyon Tuff ............................... . 47 Platoro caldera . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103 

Carpenter Ridge Tuff ............................ . 49 Premonitory volcanism . . . . . . . . . . . . . . . . . . . . . . . . . . . 103 

Wason Park Tuff ................................ . 53 Ash-flow eruptions and caldera collapse . . . . . . . . . . . . 104 

Snowshoe Mountain T~ff ........................ . 54 Resurgent uplift.................................. 107 

Interlayered lava flows ........................... . 54 Postresurgence volcanism . . . . . . . . . . . . . . . . . . . . . . . . . . 107 

Sheep Mountain Andesite .................... . 55 Summitville caldera . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108 

Huerto Andesite ............................. . 55 Arcuate faulting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108 

Volcanics of Table Mountain ................. . 55 Topographic wall................................. 109 

Lavas and related rocks of the Platoro caldera complex .... . 55 Postcollapse volcanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109 

Rhyodacite of Fisher Gulch .......................... . 56 Caldera-related regional faulting . . . . . . . . . . . . . . . . . . . . . . . 110 

Summitville Andesite ................................ . 58 Rio Grande fault zone . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110 

Andesite of Summit Peak ............................ . 67 Pass Creek-Elwood Creek-Platoro fault system . . . . . . 110 

Volcanics of Green Ridge ............................ . 67 Late Cenozoic extensional faulting . . . . . . . . . . . . . . . . . . . . . 112 

Rhyolite unit ................................... . 67 Alteration and mineralization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113 

Andesite-rhyodacite unit ......................... . 68 Summitville district . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113 

Porphyritic rhyodacite unit ....................... . 70 Stunner and Gilmore districts . . . . . . . . . . . . . . . . . . . . . . . . . 114 

Rhyodacite of Park Creek ............................ . 70 Platoro district . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114 

Quartz latite of South Mountain ..................... . 73 Jasper area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114 

Rhyolite of Cropsy Mountain ......................... . 74 Crater Creek area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114 

Volcaniclastic rocks (Los Pinos Formation) ............ . 76 Cat Creek area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114 

Intrusive rocks ...................................... . 78 Economic significance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115 

· Granitic intrusions .............................. . 78 Petrologic evolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115 

Alamosa River stock ......................... . 78 References cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120 

Cat Creek stock ............................. . 82 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125 

ILLUSTRATIONS 

Page 

FIGURE 1. Index map showing location of San Juan volcanic field . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 

2. Index map of the southeasterp San Juan Mountains, showing areas covered by published topographic maps . . . . . . . 4 

3. Diagram showing approximate relations between Si02 content and phenocryst constit~ents of volcanic rocks . . . . . . . 5 

4. Histograms of Si02 contents for major divisions of Tertiary volcanic rocks in the San Juan Mountains . . . . . . . . . . . . 7 

5. Map showing outcrop ~reas of the vent facies and volcaniclastic facies of the Conejos Formation . . . . . . . . . . . . . . . . . . 10 

6. Photograph and photomicrograph showing porphyritic andesite of the Conejos Formation . . . . . . . . . . . . . . . . . . . . . . . . . 13 

III 
IV CONTENTS 

Page 

FIGURE 7. Photographs of cone and explosion breccia from the Adams Fork volcano . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 

8. Photograph showing a thick section of the clastic facies of the Conejos Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 

9. Map showing outcrop area, approximate thickness of selected sections, and inferred original depositional extent of the tuff 

of Rock Creek and the lower tuff of the Treasure Mountain Tuff . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 

10. Photograph showing densely welded tuff of Rock Creek . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 

11. Photograph showing weakly welded tuff of Rock Creek . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 

12. Graphs showing petrologic features of the major ash-flow members of the Treasure Mountain Tuff . . . . . . . . . . . . . . . . 22 

13. Geologic map of the Banded Peak area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 

14. Map showing outcrop area, approximate thickness of selected sections, and inf~rred original depositional extent of the La 

Jara Canyon Member, Treasure Mountain Tuff........................................................... 26 

15. Photograph of ash-flow boundary within densely welded devitrified tuff of the La Jara Canyon Member . . . . . . . . . . . 28 

16. Photomicrographs of ash-flow boundary within densely welded devitrified tuff of the La Jara Canyon Member . . . . . 29 

17. Photographs showing field appearance of intracaldera La Jara Canyon Member.................................. 30 

18. Photomicrographs of intracaldera La Jara Canyon Member . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 

19. Photograph showing moderately welded La Jara Canyon Member . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 

20. Diagram showing geometric model for volume calculations of the three major ash-flow sheets of the Treasure Mountain 

Tuff................................................................................................... 33 

21. Generalized geologic map of the Black Mountain area . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 

22. Photograph showing well-bedded ash-fall tuffs of the middle tuff of the Treasure Mountain Tuff . . . . . . . . . . . . .. . . . . .. 36 

23. Map showing outcrop area, approximate thickness of selected sections, and inferred original depositional extent of the 

Ojito Creek Member, Treasure Mountain Tuff............................................................ 37 

24. Photomicrographs of the Ojito Creek and Ra Jadero Members.................................................. 38 

25. Map showing outcrop area, approximate thickness of selected sections, and inferred original depositional extent of the Ra 

Jadero Member, Treasure Mountain Tuff . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 

26. Map showing outcrop area and inferred original depositional extent of the Masonic Park Tuff ......... ; . . . . . . . . . . 44 

27. Photograph showing ash-flow sheets along the Rio Grande valley near Masonic Park . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 

28. Graphs showing phenocryst variations in a section of the Masonic Park Tuff . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46 

29. Photograph showing crudely bedded welded tuff near the base of the upper member of the Masonic Park Tuff..... 46 

30. Map showing outcrop area and inferred original depositional extent of the Fish Canyon Tuff. . . . . . . . . . . . . . . . . . . . . . 48 

31. Map showing outcrop area and inferred original depositional extent of the Carpenter Ridge Tuff . . . . . . . . . . . . . . . . . . 50 

32. Graphs showing petrographic variations in a section of the Carpenter Ridge Tuff . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 

33. Photomicrographs of Carpenter Ridge Tuff.................................................................... 52 

34. Graph showing inverse correlation between Si02 and total phenocryst contents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53 

35. Photograph showing Snowshoe Mountain Tuff interlayered with late volcanic products from the Platoro caldera complex 54 

36. Map showing distribution of postcollapse lavas and related monzonitic intrusives of the Platorc caldera complex . . . 57 

37. Photograph and photomicrograph of rhyodacite of Fisher Gulch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59 

38. Photomicrographs of the lower member of the Summitville Andesite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 

39. Photographs showing features of the lower member of the Summitville Andesite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64 

40. Photographs showing intracaldera volcaniclastic sedimentary rocks of the Summitville Andesite . . . . . . . . . . . . . . . . . . . 65 

41. Photomicrographs of characteristic rock types of volcanics of Green Ridge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68 

42. Photograph showing volcaniclastic sedimentary rocks of the Los Pinos Formation overlain and fused by thick lava flow 70 

43. Photomicrographs of typical lavas of the late viscous domes around the margins of the Summitville caldera . . . . . . . 72 

44. Photograph showing rhyolite of Cropsy Mountain.............................................................. 74 

45. Photomicrographs of mixed-lava complex at North Mountain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76 

46. Diagrammatic cross section through the Cat Creek volcano. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77 

47. Map showing distribution of major groups of intrusive rocks in the southeastern San Juan Mountains . . . . . . . . . . . . . 79 

48. Photomicrographs of rocks of the Alamosa River stock . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83 

49. Photomicrographs of rocks of the Bear Creek stock cluster. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85 

50. Photograph· and photomicrographs showing phenocryst variations at the margin of a rhyodacite porphyry sill . . . . . . 88 

51. Photograph showing typical exposure of quartz latite porphyry dike . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89 

52. Photograph showing low-angle flow grooves in quartz latite porphyry dike ...................................... ·. 89 

53. Map showing distribution of basaltic and rhyolitic lavas of the Hinsdale Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91 

54. Photomicrographs of common basalt and rhyolite types of the Hinsdale Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93 

55. Graph showing total alkalis and Si02 for basalts of the southeastern San Juan Mountains and the northern end of the Rio 

Grande depression . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93 

56-58. Photographs showing the Beaver Creek vent-dome complex: 

56. Feeder dike . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96 

57. Depositional units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97 

58. Upper part of the vitrophyric zone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98 

59. Photomicrographs showing petrographic variations within the mixed lava on the southwest side of Handkerchief Mesa 99 

60. Photographs and photomicrographs of the Handkerchief Mesa mixed-lava complex............................... 100 

61. Map showing distribution of faults, stocks, and areas of hydrothermal alteration in the southeastern SanJuan Mountains 102 

62-65. Photographs: 

62. Creede caldera from the north . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103 

63. Depositional contact against south caldera wall below Conejos Peak . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105 
CONTENTS v 

FIGURES 62-65. Photographs-Continued Page 

64. Depositional relations at west wall of Platoro caldera, south slope of Prospect Mountain . . . . . . . . . . . . . . . . . . . 106 

65. Displacement in faulted ridge east of Crater Creek . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111 

66. Si02 variation diagram for major igneous units of the Platoro caldera complex . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118 

67. Outlines of Platoro caldera complex in Colorado and Aso caldera in Japan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120 

TABLES 

Page 

TABLE 1. Generalized Tertiary volcanic stratigraphy of the San Juan Mountains . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 

2. Analyses of lava flows of the Conejos Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 

3. Summary of petrologic features of ash-flow tuffs of the southeastern San Juan Mountains . . . . . . . . . . . . . . . . . . . . . . . . 15 

4. Analyses of the Treasure Mountain Tuff and the tuff of Rock Creek . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 

5. Estimated volumes of ash-flow sheets related to the Platoro caldera complex . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 

6. Analyses of the younger ash-flow sheets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 

7. Analyses of lava flows interlayered with the younger ash-flow sheets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56 

8. Summary of general features of lavas and related rocks of the Platoro caldera complex . . . . . . . . . . . . . . . . . . . . . . . . . . . 58 

9. Analyses of lava flows and related rocks of the Platoro caldera complex . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60 

10. Analyses of intrusive rocks of the Platoro caldera area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80 

11. Analyses of basaltic and rhyolitic lavas of the Hinsdale Formation............................................... 94 

12. Measurements of slopes of Platoro caldera walls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106 
EVOLUTION OF THE PLATORO CALDERA COMPLEX AND RELATED 

VOLCANIC ROCKS, SOUTHEASTERN SAN JUAN MOUNTAINS, COLORADO 

By PETER w. LIPMAN 

ABSTRACT 

Volcanic rocks in the San Juan Mountains constitute the largest 

erosional remnant of a once nearly continuous volcanic field that extended over much of the southern Rocky Mountains and adjacent areas 

in Oligocene and later time. Recent regional studies have shown that 

the gross petrologic evolution of the San Juan remnant of this field was 

relatively simple, beginning with intermediate-composition lavas and 

breccias, followed closely in time by more silicic ash-flow tuff, and 

ending with a bimodal association of basalt and alkali rhyolite. 

In the southeastern San Juan Mountains, voluminous early lavas 

and breccias of the Conejos Formation-mainly alkali andesite, 

rhyodacite, and mafic quartz latite-were erupted from numerous 

scattered central volcanoes onto an eroded tectonically stable terrane. 

These rocks were erupted mostly during the interval 35-30 m.y. 

(million years) ago, and they volumetrically represent about two-thirds 

of the volcanism. 

About 30 m.y. ago, major volcanic activity changed to explosive ashflow eruptions of quartz latite and low-silica rhyolite that persisted until about 26 m.y. ago. Source areas for the ash flows are marked by 

large calderas in the central San Juan Mountains. Among the earliest 

and most southeastern of these is the Platoro caldera complex, a composite collapse structure about 20 kilometers in diameter that formed 

as a result of ash-flow eruption of the 29- to 30-m.y.-old Treasure 

Mountain Tuff. Three major ash-flow sheets of the Treasure Mountain 

Tuff-the La Jara Canyon, Ojito Creek, and Ra Jadero Members in 

ascending order-are approximately coextensive and cover about 5,000 

square kilometers in the southeastern San Juan Mountains. Major 

collapse of the Platoro caldera occurred during eruption of phenocrystrich quartz latitic ash flows that form the La Jara Canyon Member, 

and late ash flows of this member accumulated within the collapsing 

caldera to a thickness of more than 800 meters. The core of the 

collapsed block was resurgently uplifted to form an unbroken 

homoclinally tilted block, in contrast to the fractured domical uplifts 

that characterize other known resurgent calderas; and the marginal 

moat was filled by as much as 1 kilometer of lavas and interbedded 

volcaniclastic sedimentary rocks of the lower member of the Summitville Andesite (name reinstated in this report). 

Renewed ash-flow eruption of the quartz latitic Ojito Creek and Ra 

Jadero Members resulted in further collapse to form the Summitville 

caldera within the northwest part of the Platoro caldera. No resurgence 

is associated with this late caldera, but it was also filled to overflowing 

by a thick accumulation of upper member lavas of the Summitville 

Andesite. 

Porphyritic rhyodacitic to rhyolitic lavas, and genetically related 

dikes and granitic stocks, were emplaced repeatedly around margins of 

the Platoro caldera complex during the interval29-20 m.y., with dated 

events at 29.1, 26.7-27.8, 25.8, 22.8, and 20.2 m.y. Volcaniclastic 

sedimentary rocks, interbedded with these late lavas, record erosion of 

the primary volcanic topography, and young ash-flow sheets erupted 

from other calderas northwest of Platoro show variations in distribution and thickness near the Platoro caldera complex that correlate with 

the growing constructional topography of the caldera-margin 

volcanoes. One of these sheets is the Masonic Park Tuff, a new name in 

this report. 

In the early Miocene the character of volcanism changed notably. 

Whereas the Oligocene volcanic rocks are predominantly intermediate 

lavas and genetically related more silicic ash-flow tuffs, the younger 

rocks are a bimodal association of basalt and highly silicic alkali 

rhyolite of the Hinsdale Formation. Locally interbedded intermediatecomposition rocks are, at least in places, mixed-lava complexes with 

basaltic and rhyolitic end members. Basalt and minor rhyolite were 

erupted intermittently through the Miocene and Pliocene, and at one 

time they formed a widespread veneer over most of the older volcanic 

terrane. 

The general petrologic progression of the postcollapse igneous activity at the Platoro caldera complex is toward more silicic rocks as age 

decreases, from voluminous dark andesites (56-59 percent Si02) to 

minor scattered lava domes of porphyritic rhyolite (73-76 percent Si02) 

that are contemporaneous with nearby widespread flows of alkali 

olivine basalt. This postcollapse igneous activity at the Platoro caldera 

therefore spans the regional transition from predominantly andesitic 

volcanism to the bimodal basalt-rhyolite suites. 

The major Tertiary structural features of the southeastern San Juan 

Mountains, in addition to the calderas, are two major northwesttrending fault systems that run between several of the major calderas: 

(1) the Rio Grande fault zone that extends from the northeast side of 

the Creede caldera toward the north rim of the Summitville caldera 

and (2) the Pass Creek-Elwood Creek-Platoro fault system that connects the Mount Hope caldera with the Platoro caldera complex. These 

major zones had lengthy histories of recurrent movement, but their initiation and much of their total displacement occurred during the 

period of ash-flow eruption and major caldera collapse. In the 

southeastern area, however, these faults were reactivated during later 

Cenozoic antithetic block faulting that is related to regional eastward 

tilting of the volcanic rocks toward the San Luis Valley in response to 

development of the Rio Grande rift depression to the east. 

Hydrothermal alteration and ore deposition of late Oligocene and 

early Miocene age is locally associated with the postcollapse volcanics 

and intrusives around the margins of the Platoro caldera complex. 

Economic deposits occur where the northwest-trending Pass CreekElwood Creek-Platoro fault system intersects the ring fractures of the 

caldera complex at the Summitville, Stunner, and Platoro mining districts. Other mineralized areas, at Crater Creek, Jasper, and Cat 

Creek, are also localized by caldera-margin faulting and igneous activity. The mineralization of these areas, which had previously 

appeared anomalous, is therefore comparable to other calderaassociated mineralization in the central and western San Juan Mountains. 

The Oligocene andesite to quartz latite associations of the Platoro 

1 
2 PLATORO CALDERA COMPLEX, SAN JUAN MOUNTAINS, COLORADO 

caldera comple~, as well as similar ash-flow assemblages elsewhere in 

the San Juan field, contrast with other continental-interior ash-flow 

associations which are predominantly rhyolitic in composition. Close 

spatial and temporal association of the quartz latitic ash-flow tuffs and 

the voluminous andesitic lava flows in the Platoro area, both within 

and outside the calderas, indicates that the ash-flow tuffs represent 

differentiated upper parts of a magma body of predominantly intermediate composition that was similar to the composition of the 

andesitic lavas and monzonitic stocks. The Oligocene San Juan 

volcanic associations in general resemble andesitic associations that 

are found along most of the Pacific margin, where continental plates 

override oceanic crust along active trench-Benioff zone systems. A 

related mechanism probably operated in the Western United States 

until late Cenozoic time. 

The contrast between the Oligocene intermediate to low-silica 

rhyolitic magmas and the later basaltic and rhyolitic bimodal magmas 

implies either different conditions of magma generation or different 

processes of fractionation for the two magmatic assemblages. This 

petrologic change coincides approximately in time with nearby 

development of the Rio Grande depression, a major rift that is the local 

expression of widespread late Cenozoic crustal extension. The change 

from predominantly intermediate to bimodal basalt-rhyolite 

volcanism, approximately concurrent with initiation of late Tertiary 

crustal extension, seems to be characteristic of Cenozoic volcanism for 

much of the Western United States. These changes also coincided in 

time with changes in the geometry of sea-floor spreading recorded in 

magnetic anomaly patterns in the northeast Pacific, apparently reflecting initial intersection of North America with the East Pacific Rise 

and destruction of the trench-Benioff zone system. 

INTRODUCTION 

This report describes the evolution of the Platoro 

caldera complex and related middle Tertiary volcanic 

rocks that cover about 5,000 km2 (square kilometers) in 

the southeastern San Juan Mountains, Colo. (fig. 1). 

The caldera complex is a composite collapse structure 

about 20 km (kilometers) in diameter that formed as a 

result of ash-flow eruptions of the upper Oligocene 

·Treasure Mountain Tuff. Close spatial and temporal 

association of the quartz latitic ash-flow tuffs with 

voluminous andesitic lava flows, both within and outside 

the caldera, .indicates that the ash-flow tuffs represent 

differentiated upper parts of a magma body of 

predominantly intermediate composition that was 

similar to the composition of the andesitic lavas. Significant hydrothermal alteration and copper-molybdenumgold mineralization are associated with prolonged postcollapse igneous activity that was localized around 

margins of the caldera complex. 

The Platoro caldera complex was first recognized during reconnaissance mapping of the southeastern San 

·Juan Mountains during 1965-67, as part of a project 

directed by T. A. Steven to prepare a geologic map of the 

Durango 2-degree sheet on a scale of 1:250,000 (Steven, 

Lipman, Hail, Barker, and Luedke, 1974). Features of the 

Platoro caldera complex ·have been described in 

preliminary form (Lipman and Steven, 1970), but 

because the reconnaissance mapping for the Durango 

project was deemed inadequate to establish firmly the 

complex geology of the caldera area, two 15-minute 

quadrangles that cover this area (fig. 2) were mapped in 

greater detail during 1970-71. This report is mainly concerned with the detailed geology of the Platoro caldera 

area (Lipman, 1974), but also included is considerable 

discussion-especially in the stratigraphic descriptions-of adjacent parts of the southeastern San Juan 

Mountains that are illustrated only by the 2-degree 

Durango map. Frequent reference will be made to 

topographic and geologic features illustrated by these 

two maps, hereafter referred to as the Durango map and 

the Platoro map, and the serious reader will find it convenient to have both maps at hand. 

The only significant previous regional study of the volcanic rocks of the southeastern San Juan Mountains was 

summarized by Larsen and ci'O:)S (1956) in their description 

of the geology and petrology of the entire San Juan 

region. Study of the southeastern part of this large region 

was mainly detailed reconnaissance; fieldwork was 

mostly by Larsen, assisted by J. F. Hunter from 1911 to 

1917 and by C. S. Ross in 1920-21 (Larsen and Cross, 

1956, preface). This work, although done without benefit 

of modern concepts of ash-flow tuffs and their relation to 

caldera structures, succeeded in delimiting the regional 

stratigraphy fairly well, except within the Platoro 

caldera area. Previously, Patton (1917) had presented an 

excellent early description of the geology and ore 

deposits of the Platoro-Summitville area. The rock units 

distinguished by Patton differed significantly from those 

used by subsequent workers in the area, but Patton's 

stratigraphic interpretations generally seem to have 

been valid. Steven and Ratte (1960) made a detailed 

study of the geology and ore deposits of the Summitville 

mining district; although their study was hampered by a 

lack of information on regional stratigraphy or on 

relations of the Summitville rocks to the thenunrecognized Platoro caldera structure, the local 

geologic sequence they determined for the Summitville 

district remains valid. Fragmentary 19th and early 20th 

century studies in the Summitville-Platoro area, concerned primarily with the mineralization, were summarized by Patton (1917) and Steven and Ratte (1960). 

PRESENT INVESTIGATION 

The present report is based on a total of about 15 

months' field work during the summers of 1965-67 and 

1970-71. Reconnaissance mapping of the entire 

southeastern San Juan Mountains in 1965-67 for the 

1:250,000 Durango map (Steven, Lipman, Hail, Barker, 

and Luedke, 1974) was done on 1:40,000 black-and-white 

aerial photographs, with an intermediate stage of compilation on 1:24,000 and 1:62,500 topographic 

quadrangle maps wherever available. The area of the 

Platoro caldera complex was remapped in greater detail 

in 1970-71 by use of 1:16,000 color aerial photographs for 

most of the area; this work was compiled at 1:24,000 and I 
PRESENT INVESTIGATION 3 

106" 

. •• I 

. I ~. • 

I I 0 Durango // 

gl CQ!,Q.RAOO ______ · ______ LUA'-- ~1 NEW MEXICO '1tl1 I 

~~ ~.,:~,~:.~: 00 SAN ~D~uAN 0 25 50 MILES I I I I I I I I I I I I I 0 25 50 KILOMETERS 

EXPLANATION 

Alluvial deposits Calderas of the San Juan volcanic field 

~ ''/·' B, Bachelor LC, Lake City 

BZ, Bonanza P, Platoro 

Volcanic rocks Intrusive rocks C, Creede S, Silverton 

Shaded area underlain by CP, Cochetopa Park SJ, San Juan 

Treasure Afowuain Tuff G, La Garita SL, San Luis 

H,Mount Hope U, Ute Creek 

L, Lost Lake UN, Uncompahgre 

FIGURE 1.-lndex map showing location of the San Juan volcanic field, calderas of the San Juan Mountains. and ash-flow tuffs erupted from the 

Platoro caldera complex. In part modified from Cohee (1961). 

reduced for publication as a single 1:48,000-scale map 

(Lipman, 1974). Assistance in fieldwork was ably given 

by Russel F. Burmester in 1966, Harry R. Covington in 

1970, and David A. Johnston in 1971. This report was 

written during the winter of 1971-72. 

For this report a series of 12 summary maps, showing 

distribution, thickness, and other features of major 

volcanic units in the southeastern San Juan Mountains, 

was prepared from the Durango map, the Platoro map, 

and data from unpublished compilations. On the summary maps the outcrop distribution is slightly 

generalized, and small patches of overlying units are in 

places omitted for clarity. The thickness values were 

determined from 1:24,000 quadrangles with 40-foot contours, except for the southwestern part of the area, where 

1:62,500 quadrangles with 80-foot contours (fig. 2) were 

used. Most thicknesses were estimated to the nearest 30 

m (30 meters, or 100 feet), but thicknesses for very thin 

parts of units were estimated to the nearest 15 m. 

Terminology of ash-flow tuffs generally is that of R. L. 

Smith (Smith, 1960a, b; Ross and Smith, 1961). 

Petrographic interpretations are based on study of approximately 800 thin sections from the southeastern San 

Juan Mountains, supplemented by several hundred 

more from adjacent parts of the volcanic field being 

studied by T. A. Steven. Modal compositions were determined for about 200 specimens; most were determined 

by point-counting methods on a single 25x40 mm thin 

section. Spacing of point-count traverses was adjusted to 

yield 400-800 phenocryst points over the area of the entire section, wherever possible. For a few phenocryst-poor 

rocks this number of points could not be attained, even 
4 PLATORO CALDERA COMPLEX, SAN JUAN MOUNTAINS, COLORADO 

with the closest possible (0.3 mm) traverses. Modal compositions are tabulated in this report only for analyzed 

specimens, but discussion of compositions of the 

0 5 10 15 

various units is also based on other unpublished modal 

data. In addition to the author's determinations, some 

modes were counted by James Kirkpatrick, Harry 

20 KILOMETERS 

EXPLANATION 

[H:!:] Mapped by P. W. Lipman, 1965-67, 1970-71 

l,;{i :;:·\:.:;:j Mapped by T. A. Steven, 1965-66 

'""""""""""' Limit of exposed volcanic rocks 

Caldera wall (topographic)- Dashed where 

covered by younger rocks 

FIGUKE 2.-Index map of the southeastern San Juan Mountains, showing areas covered by published 15- and ? 1/2-minute topographic and geologic 

maps and indicating sources of unpublished geologic data utilized in following figures. Names of topographic maps are underscored. 
PRESENT INVESTIGATION 5 

Covington, Thomas Steven, Leonard Schmidt, and 

James Ratte. Modes listed only to the nearest percent 

concentration were estimated rather than counted. 

Most stated compositions of phenocryst minerals are 

based on routine thin-section examination, but feldspar 

compositions of several samples from each major unit 

were determined more precisely. Discussions of compositions and structural states of alkali feldspars are 

based on X-ray measurements of phenocryst separates, 

by use of the "three-peak" method of Wright (1968) or 

the unit-cell refinement technique· of Wright and 

Stewart (1968). Plagioclase compositions were determined routinely by maximum extinction angles normal 

to 010, using the high-temperature curves of Deer, 

Howie, and Zussman (1963); more precise determinations for some plagioclases were made by universalstage methods. Partial chemical analyses of some 

feldspars, as well as a few biotites and clinopyroxenes, 

were determined by microprobe techniques. 

All available seemingly satisfactory chemical analyses 

of volcanic rocks from the southeastern San Juan field 

are given in tables 2, 4, 6, 7, and 9-11. Of the 129 

analyses, 94 are new; the others are from Patton (1917), 

Larsen and Cross (1956), and Steven and Ratte (1960). A 

few analyses of rocks that apparently have been altered 

significantly have been excluded from the tables. 

The major-oxide analyses have been recalculated to 

100 percent free of volatiles in order to facilitate comparisons among specimens having different amounts of 

secondary water and carbonates. Secondary hydration is 

especially evident in the glassy rocks. Silicic glasses contain at most a few tenths of a percent water upon initial 

cooling (Ross and Smith, 1955), whereas some analyzed 

glasses reported here contain as much as 2.5 percent 

H20+. In general, glassy rocks have not been newly 

analyzed for this study because of the possibility of 

alkali exchange with ground water (Lipman, 1965; 

Truesdell, 1966), but some previously published 

analyses (Larsen and Cross, .1956; Steven and Ratte, 

1960) are of glassy samples. All C02 in the analyses has 

been calculated out as CaC03 if the carbonate mainly 

fills cavities such as vesicles but as C02 if the carbonate 

mainly replaces phenocrysts or groundmass. The 

analytically determined contents of H20 and C02 are 

listed in the tables following the recalculated analyses. 

Most of the new major-oxide analyses were made by 

the rapid methods of Shapiro and Brannock (1962) and 

Shapiro (1967); a few samples were analyzed by the standard wet-chemical methods of Peck (1964). Most of the 

minor-element analyses were made by the quantitative 

spectrographic methods of Bastron, Barnett, and 

Murata (1960); results are reported to two significant 

figures and have an overall accuracy of ± 15 percent; 

they are less accurate near limits of detection, where 

only one digit is significant. Elements that are reported 

as zero were not detected. A few minor-element analyses 

were made by the semiquantitative spectrographic 

methods of Myers, Havens, and Dunton (1961) and are 

reported as midpoint values of logarithmic third 

divisions; the precision of a reported value is approximately plus or minus one division at 68-percent 

confidence or two divisions at 95-percent confidence. All 

analyses were made in the laboratories of the U.S. 

Geological Survey between 1966 and 1972. 

Names chosen for the volcanic rocks require discussion, because no single classification system has yet been 

accepted widely enough to be used without comment. 

Because most of the rocks in the area studied are at least 

partly aphanitic or glassy, names based upon chemical 

composition are necessary for consistency. Because the 

middle Tertiary volcanic rocks of the San Juan field tend 

to form a single variation series ranging from mafic to 

silicic (Lipman and others, 1969, fig. 3), it has seemed 

expedient to divide this series somewhat arbitrarily by r 

percent Si02: I <~.f 

\f~ . c Y\1.-JfV 

Rock name .·fercent Si02 ~\c.f\c.tsftJ 1-' Basalt . . . . . . . . . . . . . . . . . <52 (Jo'- 0 

Andesite . . . . . . . . . . . . . . . 52-60 \ 

Rhyodacite . . . . . . . . . . . . 60-65 ( k\ c. · 

Quartz latite . . . . . . . . . . . 65-70.....---'--

Rhyolite . . . . . . . . . . . . . . . >70 

These simple divisions have the advantage for the San 

Juan rocks of coinciding approximately with phenocryst 

variations recognizable in the field (fig. 3). Where 

chemical analyses are not available, rocks are classified 

by petrographic similarity to analyzed samples; in uncertain cases, terms such as "basaltic," "intermediatecomposition," and "rhyolitic" are used in a loose sense. 

Such usage should be apparent. in context. Mineralname modifiers are used only for rocks containing the 

minerals as phenocrysts, but the term "quartz latite" is 

used for a specific compositional type whether or not 

phenocrystic quartz is present. 

Basalt 

Quartz 

Sanidine 

Plagioclase ? 

Biotite 

Hornblende 

Augite 

Rhyolite 

Si0 2 , IN PERCENT 

FIGURE 3.-Approximate relations between Si02 content and 

phenocryst constituents of volcanic rocks of the southeastern San 

Juan Mountains, as related to volcanic rock names used in this 

report. 
6 PLATORO CALDERA COMPLEX, SAN JUAN MOUNTAINS, COLORADO 

Natural remanent magnetic polarities of many of the 

volcanic units, including all the major ash-flow sheets, 

were determined in the field during 1966-67 by use of a 

portable transistorized fluxgate magnetometer (Doell 

and Cox, 1962). At each locality, determinations were 

made on three to five oriented samples, collected at 10-

to 20-m intervals along the outcrop; determinations were 

made at as many as 30 localities for a single volcanic unit. 

The measurements, though neither exceptionally sensitive nor precise, were clearly consistent-especially for 

the ash-flow tuffs-except for a few outcrops that 

probably were lightning struck. Numerous polarity 

changes were found within the ash-flow sequence (table 

3), and the polarity determinations have provided a 

valuable confirmatory correlation technique. The field 

polarity determinations of many of the units have been 

confirmed by more precise laboratory determinations 

(M. E. Beck, Jr., and James Diehl, written commun., 

1971). 

All isotopic ages reported here for rocks of the 

southeastern San Juan Mountains were determined by 

H. H. Mehnert of the U.S. Geological Survey, unless indicated otherwise; most of the radiometric results have 

already been reported elsewhere (Steven and others, 

1967; Lipman and others, 1970; Mehnert and others, 

1973). In relating the radiometric ages to Cenozoic time 

units, the Geological Society of London Phanerozoic 

time scale (Harland and others, 1964, p. 26) has been 

used. 

I especially thank my colleague Thomas A. Steven, with 

whom many of the regional interpretations were jointly 

developed; without his recurrent encouragement and 

friendly criticism this study might never have been undertaken and certainly would have been less adequately 

completed. 

GEOLOGIC SETTING 

The San Juan volcanic field, which covers about 

25,000 km2 in southwestern Colorado and adjacent parts of 

New Mexico (fig. 1), is the largest erosional remnant of a 

once nearly continuous volcanic field that extended over 

much of the southern Rocky Mountains in Oligocene and 

later time (Steven and Epis, 1968). Throughout the San 

Juan remnant of this volcanic field, the general volcanic 

sequence was relatively simple (table 1): initial 

intermediate-composition lavas and breccias, followed 

closely in time by more silicic ash-flow tuffs, and ending 

with a compositionally bimodal association of basalt and 

rhyolite (Lipman and others, 1970). The general 

petrology of the main volcanic groupings of the San Juan 

field can be seen from a histogram of SiO~ contents (fig. 

4). 

In the western part of the San Juan region, a Late 

Cretaceous and Paleocene episode of volcanic activity 

(Burbank and Lovering, 1933, p. 297-298; Shoemaker, 

1956, p. 162; Dickinson and others, 1968), which occurred contemporaneously with Laramide uplift, 

\ 

TABLE I.-Generalized Tertiary volcanic stratigraphy of the San Juan 

Mountains 

[Modified from Lipman, Steven, and Mehnert (1970)] 

LATE BASALTS AND RHYOLITES: 

Basalt of the Servilleta Formation (3.6-4.5 

m.y.) 

Hinsdale Formation 

Basalt (4.7-23.4 m.y.) 

Rhyolite (4.8-22.4 m.y.) 

Sunshine Peak Tuff (ash-flow tuff of Lake 

City caldera, 22.5 m.y.) 

MAIN ASH-FLOW TUFFS: 

Snowshoe Mountain Tuff (>26.4 m.y.) 

Nelson Mountain Tuff 

Rat Creek Tuff 

Wason Park Tuff 

Mammoth Mountain Tuff (26.7 m.y.) 

Carpenter Ridge Tuff 

Fish Canyon Tuff (27.8 m.y.) 

Masonic Park Tuff (28.2 m.y.) 

Sapinero Mesa Tuff 

Dillon Mesa Tuff 

Blue Mesa Tuff 

Tuff of Ute Ridge (28.4 m.y.) 

Treasure Mountain Tuff 

Ra Jadero Member 

Ojito Creek Member 

La Jara Canyon Member (29.8 m.y.) 

Tuff of Rock Creek 

LAVAS AND RELATED ROCKS 

ERUPTED CONCURRENTLY WITH 

THE ASH-FLOW TUFFS: 

Local andesitic to quartz latitic flows and 

breccias that intertongue with the ashflow sequence in and near their source 

calderas; Fisher Quartz Latite (26.4 

m.y.) overlies the entire ash-flow sequence 

EARLY INTERMEDIATE LAVAS AND 

BRECCIAS: 

Andesitic to quartz latitic rocks of the 

Conejos Formation and related units 

(31.1-34.7 m.y.) 

preceded the middle Tertiary volcanism. Because both 

igneous suites are dominated by intermediate compositional types, they are not easily separated in some 

places. 

Following Laramide upwarping of the Uncompahgre 

arch, Needle Mountain uplift, San Luis highlands, and 

related structures in early Tertiary time, the sedimentary cover was eroded from the upwarped areas; and 

aprons of fluviatile gravel, sand, and clay of the 

Telluride Conglomerate and Blanco Basin Formation 

were deposited around the margins. The age and interrelations of these formations have never been determined with certainty, but in recent years both the 

Telluride· and the Blanco Basin have been correlated, on 

lithology and stratigraphic position, with the San Jose 

Formation of early Eocene age in the central part of the 

San Juan Basin (Kelley, 1952; VanHouten, 1957; Dunn, 

1964). 

After deposition of the Telluride and Blanco Basin 

sediments, volcanism broke out over an area larger than 

the now-preserved San Juan volcanic field. Initially, 

intermediate-composition lavas and breccias were 

erupted from numerous local volcanoes. Most centers 

erupted monotonously uniform sequences of alkali 

andesite, rhyodacite, or mafic quartz latite, but a few 

produced rocks ranging from basaltic andesite to rhyolite 

(Lipman, 1968). Basalt is absent or extremely rare. 

These early intermediate-composition rocks _constitute 

about two-thirds of the volume of the San Juan field, 

were deposited over more than 25,000 km~, and probably 

had an original volume of at least 40,(XX) km:3 (cubic 
GEOLOGIC SETIING 7 

kilometers). They were formed mostly during the interval 35-30 m.y. (million years) ago (Lipman and others, 

1970). In the southeastern San Juan Mountains such 

rocks have been collectively designated the Conejos Formation (Cross and Larsen, 1935, p. 69). 

About 30 m.y. ago, major volcanic activity changed to 

explosive ash-flow eruptions of quartz latite and lowsilica rhyolite that persisted until about 26 m.y. ago. 

More than 16 separate ash-flow sheets have been 

recognized (table 1); the source areas for these sheets 

are marked by at least 14 large calderas (fig. 1) in the 

western and central San Juan Mountains (Steven and 

Ratte, 1965; Luedke and Burbank, 1968; Steven and 

Lipman, 1968). The original extent of the ash-flow tuffs 

(about 25,000 km2) was similar to that of the ~arly intermediate lavas, but their volume (about 20,000 km3) 

was only about half as great. Some individual ash-flow 

10 

5 

0 

20 

15 

10 

Cl) 5 w 

Cl) 

>-

~ 0 '------'--== z 

~ 20 u. 

0 

cr: 

~ 15 

~ 

::::> 

z 

Late basalts and rhyolites 

Lavas and related rocks 

erupted concurrently 

with ash·flow tuffs 

Ash-flow tuffs 

Si01 , IN PERCENT 

FIGURE 4.-Histograms of Si02 contents for major divisions of Tertiary 

volcanic rocks in the San Juan Mountains. Histogram interval is 2 

percent Si02. Analyses are from Larsen and Cross (1956), Varnes 

(1963), Bromfield (1967), Ratte and Steven (1967), Olson, Hedlund, 

and Hansen (1968), Doe, Lipman, Hedge, and Kurasawa (1969), and 

Lipman (1968, 1969, this report). 

sheets were very large, spreading over as much as 15,000 

km2 and having a volume as great as 3,000 kma. 

Intermediate to silicic lava flows and breccias are interlayered with and overlie the ash-flow sequence (table 

1). Although difficult to estimate because of irregular 

distribution and extensive erosion, the initial volume of 

these lavas probably aggregated between 5,000 and 

10,000 km3• Some of the lavas adjacent to caldera-collapse 

structures are petrographically and chemically similar to 

the ash-flow tuffs and are clearly related genetically to 

the pyroclastic rocks. Other more mafic lavas that are indistinguishable lithologically from the early 

intermediate-composition lavas and breccias are also interlayered with the ash-flow sequence, and they represent a continuation and waning of the early type of eruptions. Some of these are spatially unrelated to the 

caldera sources of the ash-flow eruptions, but several of 

the calderas-Platoro, Uncompahgre, San Juan, and 

probably Bonanza-were filled to overflowing by such 

rocks after collapse. 

A few small epizonal stocks and laccoliths cut the 

volcanic pile, especially near the margins of calderas in 

the northern, western, and southeastern parts of the 

volcanic field; numerous dikes and less regular satellitic 

intrusions radiate outward from some of these bodies. 

Although Larsen and Cross (1956, pl. 1) thought most of 

these intrusives to be comparable in age with the early 

intermediate lavas and breccias, some of the stocks in~ 

trude the ash-flow sequence in both the western (Bromfield, 1967, p. 57) and the southeastern San Juan Mountains and are demonstrably younger. The intrusive rocks 

are mainly monzonites and granodiorites; they are 

chemically similar to the early intermediate flows and 

breccias, and distinctly less silicic than most rocks of the 

ash-flow sequence. 

Locally associated with these caldera-margin intrusives and similarly situated postcollapse lavas are 

striking areas of hydrothermal alteration and important 

Pb-Zn-Ag-Au-Cu-Mo mineralization, especially in the 

Silverton-Ouray area, at Creede, and near Summitville. 

Mineralization in the San Juan region appears to correlate with the margins of calderas that are 

characterized by complex postcollapse structural 

features and igneous activity (Steven, Luedke, and Lipman, 1974). 

Near the end of the period of major ash-flow eruptions, 

both tectonic environment and nature of volcanic activity changed notably. The eastern part of the volcanic 

field was broadly warped, regionally faulted, and tilted 

to the east in response to initial development of the Rio 

Grande rift depression (Lipman and Mehnert, 1969). 

Near the Platoro caldera complex this deformation also 

produced a broad anticline, which plunges gently to the 

northeast from near Summitville to the vicinity of Del 

Norte. Southeast of the anticlinal axis the volcanic se-
8 PLATORO CALDERA COMPLEX, SAN JUAN MOUNTAINS, COLORADO 

quence dips 5°-10° to the east, toward the San Luis 

Valley; northwest of the axis the sequence dips about 10° 

to the north, toward the valley of the Rio Grande. This 

broad deformation has important effects on the levels of 

exposure within the Platoro caldera complex: in general, 

higher structural levels are exposed on the east and north 

sides, lower levels ori the west and sou:th sides. 

Concurrently with this deformation, widespread 

basalt and local silicic alkali rhyolite flows were 

emplaced (fig. 4). Most of these rocks constitute the 

Hinsdale Formation, which is mainly alkali olivine 

basalt and basaltic andesite, with associated small 

scattered rhyolite plug domes, ranging in age from about 

24 to 5 m.y. The youngest major ash-flow sheet in the 

San Juan field, erupted from the Lake City caldera 

about 22.5 m.y. ago (H. H. Mehnert, written commun., 

1971; see also Lipman and others, 1970, p. 2344), is 

petrographically similar to rhyolites of the Hinsdale Formation and can be considered as part of the same 

volcanotectonic assemblage. Basaltic lavas of the 

Hinsdale Formation range from scattered highly eroded 

remnants in central parts of the field to widespread 

sheets surrounded by relatively less eroded cinder cones 

near the Rio Grande along the southeast margin of the 

field. These upper Cenozoic basalt flows, now much 

eroded, probably originally covered 15,000-20,000 km:!, 

but total volume may have been no more than 1,000 km: 1

• 

The rhyolites of the Hinsdale Formation, except for the 

ash-flow sheet from Lake City caldera, were relatively 

minor, with an initial area of a few hundred square 

kilometers and volume of a few tens of cubic kilometers. 

A younger sequence of largely tholeiitic basalts, the 

Servilleta Formation, accumulated within the Rio 

Grande depression adjacent to the southeastern part of 

the San Juan Mountains from about 4.5 to 3.5 m.y. ago 

(Ozima and others, 1967; Aoki, 1967; Lipman, 1968). 

The marked change between the Oligocene intermediate to low-silicia rhyolitic magmas and the later 

bimodal basaltic and rhyolitic magmatic association in 

the San Juan field and adjacent parts of the southern 

Rocky Mountains implies different conditions of magma 

generation for the two suites. This petrologic change 

coincides approximately with initial nearby development of the Rio Grande depression, a major rift that is 

the local expression of widespread late Cenozoic crustal 

extension. Progression from predominantly intermediate 

eruptions to bimodal basalt-rhyolite volcanism, approximately concurrent with initiation of late Tertiary 

crustal extension, is characteristic of much of western 

Cordilleran North America. This intracontinental 

volcanotectonic transition seems to correlate with times 

at which tectonic boundaries between North American 

and Pacific plates underwent drastic changes 

(Christiansen and Lipman, 1972). 

PREVOLCANIC ROCKS 

Two small outcrops of Precambrian granite gneiss, exposed along the lower Conejos River valley (Durango 

map), are the tops of hills buried by.the middle Tertiary 

volcanic rocks; a more extensive terrane of Precambrian 

crystalline rocks is exposed beneath the volcanics 30 km 

to the south, in the Tusas Mountains of New Mexico 

(Barker, 1958; Bingler, 1968; Butler, 1971). 

Cretaceous sedimentary rocks, including the Dakota 

Sandstone, Mancos Shale, Mesaverde Formation, Lewis 

Shale, Pictured Cliffs Sandstone, Fruitland Formation, 

and Kirtland Shale (Dunn, 1964; Durango map), are 

widely distributed around the southwest margin of the 

San Juan Mountains, where the mountains merge with 

the San Juan Basin. The Cretaceous and Tertiary 

Animas Formation, consisting of interbedded shale, 

arkosic sandstone, and conglomerate, locally contains 

abundant volcanic detritus of andesitic to rhyolitic composition that records a Late Cretaceous episode of 

volcanism (Larsen and Cross, 1956, p. 56). 

After Laramide deformation in Late Cretaceous and 

early Tertiary time that produced dips of the Animas 

Formation locally as high as 50° (Platoro map), 

the uplifted sedimentary cover was eroded, and nonvolcanic sediments of the Blanco Basin Formation were 

deposited around the margins of the highlands. The 

Blanco Basin Formation apparently formed as discontinuous valley-fill deposits, with the Tertiary volcanic 

rocks in places resting directly on older deformed 

sedimentary rocks. 

EARLY INTERMEDIATE-COMPOSITION ROCKS 

(CONEJOS FORMATION) 

All the early intermediate-composition lavas and breccias in the southeastern San Juan Mountains were 

assigned by Cross and Larsen (1935, p. 69) to the Conejos 

Formation, named for th~ great cliff exposures in the 

Conejos River Canyon. As thus defined, the Conejos Formation is similar in age and petrology to other named 

formations elsewhere in the San Juan volcanic 

field-including the San Juan and the Lake Fork Formations in the western part of the volcanic field and the 

West Elk Breccia at the north edge of the field (Lipman 

and others, 1970). Equivalent early intermediate rocks in 

the northeastern part of the field include the Beidell and 

Tracy Creek Quartz Latites (Larsen and Cross, 1956, p. 

83-86) and the Rawley Andesite and other unnamed 

rocks of the Bonanza area (Burbank, 1932). 

The early intermediate-composition rocks of the 

Conejos Formation were erupted from . numerous 

volcanoes in the southeastern San Juan Mountains. Individual centers mostly erupted monotonously uniform 

sequences of alkali andesite, rhyodacite, or mafic quartz 

latite, although at a few volcanoes the rocks range from 
EARLY INTERMEDIATE-COMPOSITION ROCKS 9 

basaltic andesite to rhyolite (Lipman, 1968). Flanking have covered a middle Tertiary hill of Lewis Shale and 

the central volcanoes are volcaniclastic sedimentary- Animas Formation. Others have interpreted this contact 

rocks of the Conejos Formation that reflect penecontem- as a fault (Dunn, 1964; Sinclair, 1963); but excellent exporaneous erosion of the constructional volcanic posures along the stream channel indicate that the contopography; these clastic rocks represent approximately tact is extremely irregular, having variations in strike of 

half the volume of the Conejos Formation. Accordingly, as ·much as 60°. Along the lower Conejos River, rocks of 

the Conejos Formation has been divided into a vent the Conejos Formation were deposited around and over hills 

facies, consisting principally of lava f1ows, f1ow bre~.;;, of Precambrian granite gneiss having more than 150m of 

and explosion breccias that represent the primary exposed relief; and 20 km to the south, along the canyon 

products of the early-intermediate volcanic activity, and of Rio de los Pinos in New Mexico, the Conejos Formaa volcaniclastic facies, consisting of the mudflow brec- tion buried a rugged Precambrian terrane which had at 

cias, conglomerates, and other volcanic sedimentary least 300m of relief (Butler, 1971, p. 294). The lower condeposits that resulted from erosion of the central tact of the Conejos Formation along the southwest front 

volcanoes (fig. 5). Two additional local units of of the San Juan Mountains is commonly obscured by 

stratigraphic and structural significance, which are in landslides. 

essence subdivisions of the vent facies, are separated on Andesite and rhyodacite lava flows !Q:§Q_m thick are 

the Platoro map: a thick local accumulation of cone the predominant compositional types of the Conejos Forbreccias that mark a major vent on the southwest side of mation, which has less voluminous amounts of more 

the Platoro caldera and an upper lava unit that inter- silicic rocks. Most of the lava flows are entirely 

fingers with or overlies the Tuff of Rock Creek and the devitrified, and the predominant colors of rocks are 

lower tuff of the Treasure Mountain Tuff. various shades of dark gray, brown, and red. Basal 

Rocks of the volcaniclastic facies tend to fill in the vitrophyres occur only in silicic rhyodacite and more 

basins and other topographically low regions between leucocratic rocks. Basal flow breccias are common, and 

the central volcanoes, but the two facies of the Conejos in places much of the flow appears pervasively autobrecFormation are penecontemporaneous and tend to inter- ciated. These lava types also make up the fragmental 

finger complexly in map pattern. Such interfingering con_stituent§ 9f the volcaniclastic facies. 

was only locally worked out in detail for the Platoro and Ash-flow tuff is very rare in the Conejos Formation, 

Durango maps. In general, areas that contain more than constituting much less than 1 percent of the unit by 

about 50 percent primary volcanic products have been volume. The few tuffs recognized are typically weakly 

mapped as vent facies, because minor sedimentary in- welded, less than 10 m thick, and traceable laterally for 

terbeds between thick lava flows are typically poorly ex- only a few hundred meters. They are phenocryst poor 

posed. In contrast, areas mapped as volcaniclastic facies and probably are rhyodacitic in composition. One such 

consist overwhelmingly of reworked sedimentary tuff is well exposed in a roadcut along the Conejos River 

material, as local lavas and flow breccias tend to be con-· road immediately south of the southern Platoro 

spicuous in such sequences and therefore are easily quadrangle boundary. 

mapped separately. Although the division of the Conejos The eight analyzed rocks of the Conejos Formation 

Formation into vent and volcaniclastic facies is relative- range from 53.8 to 67.0 percent SiO:! (table 2). A more 

ly crude in comparison with the delineation of other volumetrically representative sampling would probably 

mapped units, this distinction serves well to identify the show that the bulk of the early intermediate rocks are 

approximate locations of Conejos volcanoes and provides within a more restricted compositional rang~, between 

insight into the volcanic topography before eruption of about 57 and 62 percent SiOz, with a mean near 60 perthe ash-flow sheets. cent Si02 (fig. 4). 

The Conejos Formation was deposited on a surface of The Conejos Formation and equivalent formations are 

substantial local relief, and the underlying rocks range in largely or entirely Oligocene in age, as indicated by K-Ar 

age from Tertiary to Precambrian. In places the Conejos results for nine samples from seven different centers in 

Formation rests on the Eocene Blanco Basin Formation, the San Juan field, which range in age from 31.1 to 35.0 

the Tertiary and Cretaceous Animas Formation, the m.y. (Lipman and others, 1970, table 2). Three of these 

Cretaceous Lewis Shale, or Precambrian. crystalline ages are from the Conejos Formation within- the area of 

rocks. The Animas and the Lewis were deformed-they this report (fig. 5): 32.4 m.y. for biotite and hornblende 

dip as much as 50° (Platoro map)-and were eroded from a rhyodacite flow breccia at Navajo Peak, 32.4 m.y. 

before deposition of the subhorizontal Blanco Basin and for biotite from a rhyolite dike at Summer Coon volcano, 

Conejos Formations. Along the East Fork of the San and 31.1 m.y. for plagioclase from an andesite f1ow at 

Juan River, just above the junction with Quartz Creek, · ·Snowball Park. Although none of these samples 

mudflow deposits of basal Conejos Formation appear to necessarily represents the actual beginning of volcanic 
10 PLATORO CALDERA COMPLEX, SAN JUAN MOUNTAINS, COLORADO 

0 5 

0 5 10 

D Younger volcanic rocks 

Conejos Formation 

Vent facies and upper lava unit 

Volcaniclastic facies 

10 15 20 MILES 

15 20 KILOMETERS 

EXPLANATION 

* 

Sample locality (table 2) 

Approximate location of volcanic vent 

Limit of exposed volcanic rocks 

Caldera wall (topographic)- Dashed where 

covered by younger rocks 

FIGURE 5.-0utcrop areas of the vent facies and volcaniclastic facies of the Conejos Formation, southeastern San Juan Mountains. Based on mapping by P. W. Lipman, 1965-67, 1970-71. 
EARLY INTERMEDIATE-COMPOSITION ROCKS 

TABLE 2.-Analyses of lava flows of the Conejos Formation 

[Sample localities shown in fig. 5. Analyses 3-5 are from Larsen and Cross (1956, table 21). Other major-oxide analyses are by rapid methods by 

P. L. D. Elmore, Samuel Botts, Gillison Chloe, Lowell Artis, H. Smith, John Glenn, and James Kelsey. Minor-element analyses 2 and 8 are 

by quantitative methods by Norma Rait; remaining analyses are by J.D. Fletcher. <,less than; leaders(_ _ .), looked for but not found] 

Vent facies Upper lava unit 

Sample---------

Field No.------

1 

71J-6 

Wl76172 

2 

70L-32 

Wl74434 

3 

SVU3 Con262 Con591 

6 

71J-7 

Wl76173 

7 

71J-8 

Wl76174 

8 

66L-16 

Lab. No.------- Wl68530 

Major oxides (weight percent), recalculated without H20 and C0 2 (original values listed separately below) 

s i02-----------

Al20 3----------

Fe203----------

Feo------------

Mgo------------

cao------------

Na2o-----------

K2o------------

Ti02-----------

P20s-----------

Hno------------

TOTAL-----

H20+ ----------

H2o- ---------- C02------------

53.8 

16.7 

4.1 

5. 8 

4. 4 

8. 3 

3.1 

2.0 

1.2 

• 43 

. 20 

100.03 

. 60 

1.3 

<.05 

55.7 

17.0 

4.9 

4. 2 

3.1 

6. 2 

3.4 

3.1 

1.4 

.81 

. 21 

100.02 

1.8 

. 65 

1.1 

58.95 

16.57 

3. 95 

3. 61 

2.82 

6. 58 

3. 38 

2. 64 

1.02 

. 36 

.ll 

99.99 

. 72 

.80 

.00 

60.62 

16.26 

4.82 

2.25 

2. 22 

6. 39 

3. 70 

2. 65 

1.00 

.10 

100.01 

.12 

.44 

.00 

61.23 

17.71 

5.48 

. 56 

2.01 

5. 73 

3. 23 

2. 97 

. 64 

.32 

.ll 

99.99 

1. 64 

. 46 

63.3 

17.3 

3.4 

1.4 

1.7 

3. 7 

4. 3 

4.0 

. 69 

.14 

.13 

100.06 

.so 

1.1 

.12 

67.0 

16.7 

2.9 

. 61 

.86 

1.7 

4. 3 

5. 2 

. 56 

.10 

.08 

·100. 01 

. so 1.6 

<,05 

59.8 

18.3 

5. 6 

1.1 

1.4 

5.8 

3.4 

3.1 

1.1 

. 27 

.06 

99.93 

. 70 

.80 

<.OS 

Norms (weight percent) 

Quartz---------

Orthoclase-----

Albite---------

Anorthi te------

Wollas toni te---

Enstatite------

Ferrosili te----

Magnet i te------

Hema t i te-------

Ilmeoi te-------

Apati te--------

s. 37 

u. 55 

26.ll 

25.85 

s. 29 

11.01 

s. so s. 97 

2. 34 

1.02 

8. 20 

18.39 

28.97 

21.87 

1. 56 

7. 75 

1. 75 

7.07 

2. 56 

1. 92 

12.79 

15.62 

28.57 

22.26 

3. 36 

7.01 

1.89 

s. 72 

1. 94 

. 85 

14.51 

15.65 

31.31 

19.92 

4. 92 

5. 59 

4. 66 . 

1.60 

1.91 

17.00 

17.53 

27.34 

25.06 

. 54 

4. 99 

. 31 

5. 27 

1. 22 

. 75 

13.43 

23.46 

36.17 

16.30 

.40 

4. 31 

3.02 

1. 28 

1. 31 

. 34 

17.33 

30.82 

36.35 

7.96 

2.14 

• 61 

2.44 

1.07 

. 24 

14.80 

18.14 

28.58 

25.76 

.60 

3. 57 

. 56 

s. 24 

2.14 

.63 

Minor elements (parts per million) 

11--------------

Ba-------------

Be-------------

Ce------------- Co-------------

Cr-------------

Cu-------------

Ga-------------

La-------------

Mo-------------

Nb-------------

Ni-------------

Pb-------------

Sc-------------

Sn-------------

Sr-------------

v-------------- Y--------------

Yb------------- Zr-------------

560 

24 

58 

66 

20 

100 

30 

16 

20 

40 

2000 

300 

40 

2 

100 

2400 

<2 

16 

59 

490 

20 

17 

22 

480 

180 

<40 

4 

370 

1500 

3 

21 

20 

80 

20 

10 

14 

680 

80 

30 

3 

280 

uoo 

2 

5 

20 

100 

20 

10 

10 

390 

20 

30 

3 

200 

1400 

<4 

2 

17 

30 

<4 

34 

5 

40 

360 

60 

30 

320 

Phenocrysts (volume percent) 

Plagioclase----

Biotite--------

Hornblende-----

Augite---------

Hypers thene----

Opaques--------

Groundmass----- 97 

17 

1 

80 

18 

1.5 

3 

2 

1.5 

74 

33 

2. 5 

3. 5 

2 

59 

25 

10 

• 5 

• 5 

4 

60 

13.1 

2.9 

• 2 

1.2 

.9 

81.7 

11.6 

3. 2 

1.1 

84.1 

9.8 

1.6 

.9 

87.7 

7 

SAMPLE DESCRIPTIONS 

1. Dense dark-gray basaltic andesite, containing sparse small (1-2 

mm) tabular phenocrysts of plagioclase (An,&-&r>), set in a coarsely microcrystalline intergranular groundmass of plagioclase, augite, opaques, and iddingsite pseudomorphs after olivine. Saddle Creek road, at 11,400 ft. 2. Gray platy plagioclase andesir.e, containing about 20 percent 

phenocrysts. (See fig. 68.) Tabular euhedral plagioclase 

phenocrysts (An,r.-M) are as much as I em long and are charged with small inclusions of glass and other phenocrysts. Some car- bonate is present, replacing both phenocrysts and ground mass, as is typical for this rock type. Cat Creek rc.ad, north side of Cat Creek, at about 8,800 ft. 3. Gray pyroxene andesite, in which all phenocrysts are less than I mm across. "Near the head of t.he Alamosa River, and about I mile southeast of Summit Peak" (Larsen and Cross, 1956. table 21, No. II). Vent-facies assignment of this sample is un- certain. 4. Dark-red-brown pyroxene-hornblende rhyodacite, containing "abundant white plagioclase in stout tablets about I mm long, 

and some pyroxene • • • along sheep trail near crest of Nigged wlackl Mountain, at altitude of 10,000 feet" (Larsen an ross, 1956, table 21, No. 12). 5. Dark-red-brown hornblende rhyodacite. "The north base of Mcintyre Point [Peak]" (Larsen and Cross, 1956, table 21, No. 15). 6. Gray porphyritic rhyodacite, containing phenocrysts of plagioclare (An,o-:;o) as much as 4 mm in diameter, set in a microcrys~alline intergranular ground mass. Biotite is oxidized, and hornblende is brown and resorbed. Saddle Creek road, at just above 11,000 ft. 7. Red-brown porphyritic quartz latite, containing phenocrysts of plagioclase (An,&-M) as much as 3 mm in diameter, set in a fine-grained pilotaxitic groundmass. Chemically anomalous: high KtO and low CaO. This flow immediately underlies flow of No.6. Saddle Creek road, at just below 11,000 ft. 8. Dark-brownish-gray andesite, containing small phenocrysts less than 1 mm in diameter, set in a fine:grained pilotaxitic 

ground mass. 1.3 km south of Jacobs Hill on 8,912-ft knob 

11 

. .,/ 
12 PLATORO CALDERA COMPLEX, SAN JUAN MOUNTAINS, COLORADO 

activity in middle Tertiary time, the samples do suggest 

that most of the early intermediate-composition activity 

was during a very restricted span of time. A limit on the 

termination of Conejo~ volcanism is provided by a date 

of 29.8 m.y. for the overlying La Jara Canyon Member of 

the Treasure Mountain Tuff (Lipman and others, 1970, 

table 3, No. 1). 

VENT FACIES 

As mapped, the vent facies of the Conejos Formation 

consists mainly of intermediate-composition lava f1ows, 

flow breccias, and explosion breccias from widely 

scattered central volcanoes. Interbedded with these 

rocks are volcaniclastic sedimentary rocks that are too 

thin or poorly exposed to map separately. These 

Oligocene central volcanoes were extensively eroded and 

largely mantled in their own debris shortly after formation. 

Perhaps the most conspicuous volcanoes of the Conejos Formation are the Summer Coon and Baughman 

Creek centers at the north edge of the area of this report 

(fig. 5; Durango map; Lipman, 1968), where erosion has 

exposed spectacular dikes radiating outward from central stocks, which are flanked by outward dipping lavas. 

These two Conejos volcanoes are also distinctive in that 

they contain a variety of lava types ranging from basaltic 

andesite to rhyolite (52-73 percent Si02: Lipman, 1968; 

Mertzman, 1971). At each volcano, however, most of-the 

exposed rock is monotonous olivine andesite (56-58 percent Si02), forming a crudely stratified cone of explosion 

breccia that intertongues with minor lavas on the lower 

flanks. After a period of quiescence marked by erosion of 

the andesitic cone, volumetrically minor rhyolite was 

erupted first, then varied quartz latitic and rhyodacitic 

lava types (Lipman, 1968). This late sequence is thought 

to reflect progressive eruption of the fractionated upper 

part of a magma chamber of predominantly andesitic 

composition. 

The Platoro caldera appears to have formed within a 

cluster of five or more Conejos volcanoes, parts of which 

have been caved away by caldera collapse. These Conejos volcanoes tend to be petrologically uniform, having 

few clearly comagmatic intrusives at present levels of exposure. In general,·intrusives of Conejos age are difficult 

to distinguish from· the numerous younger intrusives 

that are clearly related to the postcaldera activity. These 

Conejos volcanoes have not been studied in detail, and 

only general aspects of their compositional variations 

and primary constructional geometry are known. 

In the thick pile of Conejos lavas east of the caldera, 

from the Alamosa River to the head of Rock Creek (fig. 

5), the predominant lava type is a distinctive dark-gray 

andesite that contains large tabular phenocrysts of 

labradorite (f~g. 6B). Interlayered with the~e lavas are 

sparsely porphyritic gray andesites· and, high in· the sequence, red-brown lavas that cont3:.in small phenocrysts 

of plagioclase and augite, that are of more silicic 

appearance, and that may be rhyodacites. The vent for 

this near-source accumulation has not been determined 

with confidence, but it probably lay within the collapsed 

eastern side of Platoro caldera (fig. 5). 

The thick accumulation of lava f1ows north of the 

caldera, in the Pinos Creek and San Francisco Creek 

areas, also appears to represent the flanks of a large 

stratovolcano centered farther south, within the caldera 

area (fig. 5). These lavas are petrographically different 

from those to the east and consist of light-brown to gray 

hornblende-bearing rhyodacitic lavas associated with 

brown or red-brown augite-plagioclase rhyodacites or 

andesites. 

West of the caldera the vent-facies lavas and breccias 

near the top of the formation consist largely of platyplagioclase andesite (fig. 6A) like that east of the 

caldera. These lavas represent the thin margin of a late 

volcano that must have had its center farther to the 

northeast. 

On the southwest side of Platoro caldera, in the 

Adams Fork area; a cone of andesitic explosion breccia 

about 5 km in diameter is largely mantled by mudf1ow 

breccias of the volcaniclastic facies (Platoro map; fig. 5). 

The dominant rock type of the Adams Fork cone is gray 

to brown andesite that either is nonporphyritic or contains sparse small phenocrysts of plagioclase and augite. 

The cone breccias of explosive origin are monolithologic, 

angular to subrounded, crudely bedded and sorted, and 

largely lacking in fine-grained matrix. They dip outward 

10°-35° from a central area (fig. 7A, 7B). In places the 

explosion breccias, especially the vesicular varieties, are 

indurated and agglutinated-apparently as a result of 

deposition while still hot (fig. 7C). Interlayered with the 

explosion breccias of the cone complex are local bedded 

fine-grained tuff and, in places, mudflow breccias thatdiffer from the explosion breccia in having poorer sorting 

and more abundant matrix; in contrast with the regional 

mudflow deposits of the volcaniclastic facies, these have 

steep primary dips down the slope of the volcanic cone ... 

Just south of Platoro caldera on the upper slopes of 

'Conejos Peak, several thick flows of distinctive 

hornblende.!plagioclase rhyodacite dip gently to the 

south and may represent the south flank of a relatively 

young Conejos volcano centered to the north within the 

caldera (fig. 5). In contrast, older, sparsely porphyritic 

andesitic or rhyodacitic lava flows, which interfinger 

complexly with mudflow deposits of the volcaniclastic 

facies on "the lower west slope of Conejos Peak, seem to 

be the northwest flank. of a thick pile of sparsely 

porphyritic andesite or rhyodacite that accumulated to 

.maximum thickness in the South Fork of the Conejos 

(fig. 5), where individual viscous flows are as much as 

150m thick. 

The thick accumulation of vent-facies rocks, near 
EARLY INTERMEDIATE-COMPOSITION ROCKS 13 

FIGURE G.-Porphyritic andesite of the Conejos Formation. Ji, 

Porphyritic andesite lava, containing large platy plagioclase 

phenocrysts. Elwood Creek road, at about 10,000 feet. . 

Photomicrograph of similar platy-plagioclase andesite; large 

euhedral plagioclase phenocrysts (An45-ss) are charge~mall igclusions of glass and microphenocrysts; rounded phenocryst at lower 

right is augite. Cat Creek road, north side of Cat Creek at about 

8,800 feet. Analyzed sample 2, table 2. 

Willow Mountain on the southeast side of the Platoro 

caldera (fig. 5), apparently represents the southeast 

flank of another stratovolcano, the center of which lay 

just within the area of subsequent caldera collapse. 

These lavas are mainly gray or brown andesite or 

rhyodacite containing fairly abundant small phenocrysts 

of augite and plagioclase; hornblende-bearing rhyodacite 

is present locally. 

Other major accumulations of vent facies of the Conejos Formation (fig. 5) include a pile of platy-plagioclase 

andesite at least 500 m thick centered near the mouth of 

the East Fork of the San Juan River, a pile of dark 

andesitic lavas and breccias at least 25 km across in the 

Chalk Mountains-Navajo Peak-Chama Peak area that 

was examined only cursorily, and an accumulation of 

distinctive olivine-augite andesite exposed along the 

lower Conejos River in Colorado and also near the mouth 

of Rio de los Pinos just across the State line in New Mexico (Butler, 1971). 

VOLCANICLASTIC FACIES 

The volcaniclastic facies of the Conejos Formation 

(fig. 8) consists of mudflow breccias and conglomerates, 

accompanied by minor amounts of better sorted tuffaceous sandstone; all were formed by erosion of the 

vent-facies rocks just described. 

The mudflow deposits are volumetrically by far the 

most important. They typically are crudely bedded and 

poorly sorted, having abundant sandy or tuffaceous 

matrix between heterogeneous angular to rounded 

blocks. The mudflow breccias differ from explosion breccia of the vent facies in having more abundant matrix, 

poorer sorting, lithologically heterogeneous fragmental 

material, and subhorizontal dips. Lithologic variations 

in the fragmental material from place to place clearly are 

related to petrology of source rocks, and sufficiently 

detailed mapping would enable the volcaniclastic facies 

to be subdivided by provenance. 

The mudflow breccias and other volcaniclastic rocks 

filled topographic depressions between the central 

volcanoes of the Conejos Formation. In places, 

monotonous sequences of such sedimentary deposits accumulated to thicknesses in excess of 700 m (fig. 8): 

Elsewhere, interfingering between vent and 

volcaniclastic facies is very complex, such as on the west 

slope of Conejos Peak and in Rio Blanco (Platoro map). 

UPPER LAVA UNIT 

Lava flows that intertongue with or overlie the tuff of 

Rock Creek or the lower tuff of the Treasure Mountain 
14 PLATORO CALDERA COMPLEX, SAN JUAN MOUNTAINS, COLORADO 

A 

B 

Tuff locally have been mapped as the upper lava unit of 

the Conejos Formation to determine important structural and stratigraphic relations near the Platoro caldera 

complex (Platoro map). Flows of the upper lava unit 

overlie all rocks of the volcaniclastic facies; in essence, 

the upper lava unit is part of the vent facies, and its 

separation into a distinct unit is arbitrary in the sense 

that it cannot be distinguished petrographically from 

older rocks of the vent facies and can only be separated 

from them where ash-flow tuff intervenes. As thus 

defined, the Conejos Formation includes all 

intermediate-composition volcanic rocks in the 

southeastern San Juan Mountains that predate the major collapse of Platoro caldera during eruption of the La 

Jara Canyon Member of the Treasure Mountain Tuff. 

The upper lava unit of the Conejos Formation has been 

recognized at Chama Peak, in the Elk Creek area, north 

c 

FIGURE 7.-Adams Fork volcano. A, Eroded cone breccia of Conejos 

Formation, showing steep primary dips. Along Continental Divide, at 

head of North Fork of Conejos River. B, Typical explosion breccia 

of Adams Fork volcano , Conejos Formation, showing 

monolithologic composition, angularity, and crude sorting. 

Fragments are dense dark-gray andesite. Southeast side of Haskell 

Rincon, at 11,300 feet. C, Agglutinated explosion breccia, consisting of monolithologic blocks of rounded vesicular red-brown 

andesite or rhyodacite in fine-grained matrix of same material. At 

11,800 feet at end of ridge northeast of triangulation point 

"Velvet." 

of Elwood Creek, south of Pass Creek, west of Bear 

Creek, south of Del Norte, in the Rock Creek area, north 

of the Terrace Reservoir, and along Ojito Creek (fig. 5). 

Typically it consists of only one or two flows of dark-gray 

or brown sparsely porphyritic andesite or rhyodacite. 

ASH-FLOW SHEETS 

In contrast to the few small ash-flow tuffs locally interlayered with the early intermediate lavas and breccias 

of the Conejos Formation the ash-flow tuffs overlying the 

early intermediate rocks in the southern San Juan 

Mountains are widespread and numerous. These quartz 

latitic and rhyolitic ash-flow sheets were derived from 

the Platoro caldera complex and from younger calderas 

in the central part of the San Juan volcanic field. Nine of 

these ash-flow sheets are of sufficient size and importance to be mapped and described separately. Some 

general petrologic features of these sheets are summarized in table 3. 

TUFF OF ROCK CREEK 

The tuff of Rock Creek, the oldest sizable ash-flow 

sheet in the southeastern San Juan Mountains, apparently represents the first significant pyroclastic eruptions from the Platoro area. It is here described as an informal unit, distinguished from the Treasure Mountain 
ASH-FLOW SHEETS 15 

Summit Peak 

~-- ~ La Jara Canyon Member 

' ~ .. ( 

FIGURE 8.-Thick section of the volcaniclastic facies of the Conejos Formation, on the great cliffs east of Summit Creek. Thin rim at top of cliffs is 

La Jara Canyon Member of Treasure Mountain Tuff; below it is exposed approximately 700 m of crudely sorted and bedded subhorizontal 

volcaniclastic rocks, mostly mudflow breccias of andesitic and rhyodacitic composition. Photograph taken from Rio Blanco trail at 10,700 

feet (Platoro quadrangle). 

TABLE 3.-Summary of petrologic features of ash-flow sheets of the southeastern San Juan Mountains 

[Magnetic polarity: N, normal; R, reversed] 

Unit 

Young ash-flow sheets: 

Snowshoe Mountain Tuff 1 ••••••••••••• 

Wason Park Tuff 1 ................. . . . 

Carpenter Ridge Tuff ... . 

Fish Canyon Tuff . . . .. . . . . 

Masonic Park Tuff .. . 

Treasure Mountain Tuff: 

Upper tuff ' ........ . 

Ra Jadero Member. 

Ojito Creek Member . . . 

Middle tuff ' . . . . La Jara Canyon Member 

Lower tuff ' .. 

Tuff of Rock Creek . . . 

Number 

of 

analyses 

3 

6 

6 

11 

1 

4 

9 

Sio, Phenocryst 

content content 

(percent) (percent) 

63-67 34-58 

68-72 25-38 

61-74 5-26 

66-69 34-52 

68 38-52 

3 -70 5-15 

64-66 10-17 

68-71 6-12 

' -70 5-10 

65-70 20-45 

69 3-11 

64-65 2· 3 

1 Data, other than magnetic polarities and K-Ar ages, are from Ratte and Steven (1967). 

2 Assemblage of several small ash-flow sheets. 

3 Estimated. 

4 No sanidine. 

Sanidine- Magnetic 

plagioclase polarity 

ratio 

0.01-0.1 N 

.3 . 5 R 

.2 -5.0 R 

.1 .3 N 

0- .01 R 

.1 .3 R 

.2 .6 R 

0- .02 N 

0- .02 N,R 

0- .03 R 

0- .03 R 

(') N 

Source caldera 

Creede ............ .. ... ........ ... .... (Near Creede) . ............. . . . . . . . . . . . . 

Bachelor. . ... .. .. . ... 

La Garita ... · ··· · ······· ·· ·· ·· ·· ·· ·· ···· Mount Hope ........ .. .. . . . . . . . . . . 

Platoro caldera complex 

Summitville caldera . .... ....... ... .. . do ... ........... ···· ············ Platoro caldera complex . ..... .... .... .... Platoro caldera ..... ·········· ······ Platoro caldera complex .. .... ..... ... ... do . . . . . . . . . .... . ........ ... ... . 

K-Ar 

age 

(m.y.) 

>26.4 

<26.7 

>26.7 

27.8 

28.2 

29.8 
16 PLATORO CALDERA COMPLEX, SAN JUAN MOUNTAINS, COLORADO 

Tuff by absence of biotite phenocrysts and by its 

relatively mafic composition. The tuff of Rock Creek 

rests on lavas and breccias of the main body of the 

Conejos Formation; it is overlain by andesitic or 

rhyodacitic lava flows that have also been assigned to 

the Conejos Formation (upper lava unit) because a contact cannot readily be located between the two groups of 

intermediate-composition rocks where the tuff of Rock 

Creek is absent. 

The tuff of Rock Creek is confined to the northeast sector of the area (fig. 9) and is everywhere less than 100m 

thick. Its best exposures and maximum thickness are in 

the Rock Creek drainage (fig. 10), where it filled broad 

valleys on the northeast flank of the large stratovolcano 

of Conejos age centered somewhere near Bennett Peak. 

The tuff of Rock Creek thins upslope and is overlapped 

by the upper lava unit of the Conejos Formation (Platoro 

map). The tuff of Rock Creek appears to have flowed 

down the flank of the stratovolcano from a source 

somewhere in the volcanic highlands to the southwest, 

within the area of the subsequently formed Platoro 

caldera. 

The tuff of Rock Creek forms a single, apparently simple cooling unit. Whether weakly or densely welded, it 

commonly contains large black scoria blocks as much as 

75 em long in a red-brown or orange-brown fragmental 

matrix (fig. 11). The scoria blocks tend to be elongate 

and to define a crude foliation even in weakly welded 

rock, in<;licating primary shape orientation during 

flowage and emplacement as well as any later welding 

compaction. At the south margin of the tuff, in the Cat 

Creek area, the scoria blocks decrease in size as the tuff 

sheet thins, and the resulting brown lithic-rich 

pyroclastic brecCia becomes poorly exposed and difficult 

to separate from other volcanic breccias of the Conejos 

Formation. In contrast, at the most northern outcrops 

near Del Norte, the tuff of Rock Creek is a partly welded 

glass, in which the black scoriallenses are surrounded by 

gray glassy fragmental matrix. . 

Phenocrysts in the tuff of Rock Creek are sparse (3-8 

percent) and are plagioclase and minor amounts of 

augite and Fe-Ti oxides; biotite is absent. One sample 

contains hypersthene, but it may be xenocrystic. Small 

andesitic lithic fragments are very common. 

The· tuff of Rock Creek is relatively mafic-a 

rhyodacite or mafic quartz latite, as might be predicted 

from its phenocryst mineralogy. A bulk sample and an 

individual scoria block both contain 64-65 percent Si02 

(table 4); this is the least silicic ash-flow tuff in the 

southeastern San Juan Mountains. Because the tuff of 

Rock .Creek contains only sparse phenocrysts, its bulk 

composition and ._.groundmass composition differ only 

slightly. The-mafic. composition .of the groundmass contrasts with that of the other ash-flow sheets whose 

relatively mafic bulk-rock compositions (table 3) are due 

to abundant phenocrysts in a significantly more silicic 

groundmass. 

The volume of the tuff of Rock Creek is estimated as 

about 10 km3 (table 5), but this figure is very approximate because exposures are fragmentary (fig. 9). No 

caldera collapse feature has been preserved that can be 

related to eruption of the tuff of Rock Creek, but subsidence need not necessarily have accompanied this 

small-volume eruption. (See Smith, 1960a, fig. 3.) 

Field measurements, not checked in the laboratory, 

suggest that the tuff of Rock Creek (like the overlying 

upper lava unit of the Conejos Formation) has normal 

magnetic polarity. 

The tuff of Rock Creek has not been dated directly, 

but it is bracketed by K-Ar determinations of other units 

(Lipman and others, 1970). At Del Norte the tuff of Rock 

Creek is underlain by hornblende rhyodacite lavas 

related to the Summer Coon volcano of Conejos age, 

which has yielded radiometric dates as young as 32.4 

m.y. The tuff of Rock Creek is overlain by the La Jara 

Canyon Member of the Treasure Mountain Tuff, dated 

as 29.8 m.y., and its "Source from within the Platoro 

caldera area suggests that it may only slightly predate 

the beginning of Treasure Mountain eruptions. 

TREASURE MOUNTAIN TUFF 

The Treasure Mountain Tuff has been redefined (Lipman and Steven, 1970) from the Treasure Mountain 

Rhyolite of Larsen and Cross (1956) to include the units 

present at the type locality at Treasure Mountain, about 

20 km west of Summitville, Colo, (fig. 9), plus additional 

intertonguing units farther south. 

As redefined, the Treasure Mountain Tuff is a coextensive assemblage of ash-flow tuffs, largely or entirely 

related to the Platoro caldera complex. Three large ashflow sheets in the Treasure Mountain Tuff that have 

been mapped over nearly 5,000 km2 have been formally 

designated as members: the La Jara Canyon, Ojito 

Creek, and Ra Jadero Members, in ascending order (Lipman and Steven, 1970). Less widespread ash-fall and 

ash-flow tuffs below the La Jara Canyon Member, 

between the La Jara Canyon and Ojito Creek Members, 

and above the Ra Jadero Member have been informally 

designated the lower, middle, and upper tuffs, respectively. 

Excluded from the Treasure Mountain Tuff as here 

defined are thick sections of welded ash-flow tuffs, 

erupted from other volcanic. centers, that were mapped 

as Treasure Mountain Rhyolite by Larsen and Cross 

(1956, pl. 1) in the western, central, and northeastern 

San Juan Mountains. The largest of these, a sequence of 

four major ash-flow sheets in the western Sa~ Juan 

Mountains, was designated the Gilpin Peak Tuff by 

Luedke and Burbank (1963), but the Gilpin Peak Tuff is 
ASH-FLOW SHEETS 

106°15' 

0 5 10 15 20 MILES 

0 5 10 15 20 KILOMETERS 

EXPLANATION 

Outcrops shown by darker pattern, inferred original extent by lighter pattern 

Younger volcanic rocks Inferred original depositional margins 

Lower tuff, Treasure Mountain Tuff .. 

~j Tuff of Rock Creek 

.Jo Approximate thickness - In meters 

Sample locality (table 4) 

of the tuff of Rock Creek and (or) 

the lower tuff of the Treasure 

Mountain Tuff - Dashed where approximate; queried where uncertain 

"'"""'""""""' Limit of exposed volcanic rocks 

Caldera wall (topographic)- Dashed 

where covered by younger rocks 

17 

1o6·oo· 

FIGURE 9.-0utcrop area, approx_imate thickness of selected sections, and inferred original depositional extent of the tuff of Rock Creek and the 

lower tuff of the Treasure Mountain Tuff. Based on mapping by P.W. Lipman, 1965-67, 1970-71. 
18 PLATORO CALDERA COMPLEX, SAN JUAN MOUNTAINS, COLORADO 

now known to be equivalent to older parts ofthe ash-flow 

sequence (Sapinero Mesa Tuff, Dillon Mesa Tuff, Blue 

Mesa Tuff, and underlying tuffs) described by Olson, 

Hedlund, and Hansen (1968) in the Gunnison River area 

(Lipman and others, 1970, p. 2340-2341; Lipman and 

others, 1973). These tuffs were all erupted from centers 

in the western San Juan Mountains, including San 

Juan, Uncompahgre, Lost Lake, and Ute Creek calderas 

(fig. 1). A second body of ash-flow tuff, mapped as 

Treasure Mountain Rhyolite by Larsen and Cross (1956, 

pl. 1) in the northeastern San Juan Mountains, has been 

reassigned to Patton's (1916) Bonanza Tuff by Mayhew 

(1969) (Knepper and Marrs, 1971) and is inferred to have 

been erupted from a caldera near Bonanza (fig. 1). One 

major ash-flow sheet in the southeastern San Juan 

Mountains, generally mapped by Larsen and Cross 

(1956) in their Treasure Mountain Rhyolite, is thought 

to have been erupted from a caldera centered near 

Mount Hope (fig. 1) and is described as the Masonic 

Park Tuff, a new name in this report. (See also Lipman 

and others, 1970; Steven and Lipman, 1973.) 

Units of the Treasure Mountain Tuff are 

petrographically fairly uniform quartz latites and less 

abundant low-silica rhyolites. All contain phenocrysts of 

plagioclase and biotite, and most contain augite; 

sanidine is characteristically sparse (table 3), and quartz 

is absent. Phenocryst contents and welding 

characteristics vary widely, but phenocrysts are typically more abundant and welding more dense in the three 

major ash-flow sheets that constitute the formal members 

than in the more local informal tuff members. Similar 

chemistry, phenocryst mineralogy, and areal distribution of the units of the Treasure Mountain Tuff indicate 

that the units are closely interrelated. Despite the 

similarities, the units can be distinguished in part by 

differences in proportion of phenocrysts (fig. 12) and by 

FIGURE 10.- Densely welded tuff of Rock Creek forms gently dipping palisades in middle distance (arrow). The tuff of Rock Creek is underlain here 

by mudflow breccias of the Conejos Formation and is overlain by andesitic and rhyodaciti c lavas of t he upper lava unit, Conejos Formation. 

Pintada Mountain, in left distance, consists of Conejos lava on the northeast wall of Platoro caldera. Photographed from Rock Creek road at 

divide above Lost Cabin Ranch. 
ASH-FLOW SHEETS 19 

FIGURE 11.-Weakly welded tuff of Rock Creek, showing characteristic large black scoria blocks in red-brown fragmental matrix. West side of 

Horseshoe Mountain, at about 10,300 feet (Horseshoe Mountain quadrangle). 

differences in their natural remanent magnetic polarities 

(table 3). The Ojito Creek Member and part of the middle tuff have normal polarities; all the other units of the 

Treasure Mountain have reverse polarities. Of the three 

widespread formally named ash-flow sheets, the La Jara 

Canyon Member is significantly higher in total 

phenocrysts than the other two, and the Ra Jadero has 

significantly more sanidine (fig. 12). 

Si02 contents in 22 analyses of Treasure Mountain 

Tuff (table 4) range from 64.2 to 71.2 percent. Most of 

the bulk-rock compositions thus are quartz latite; a few 

phenocryst-poor tuffs and pumice blocks are low-silica 

rhyolite. In general, Si02 increases as total phenocrysts 

decrease (fig. 12B), a relation especially evident if 

variations within individual ash-flow sheets are examined. Such examination indicates that much of the 

total compositional variation of the Treasure Mountain 

is due to differing amounts of phenocrysts. The groundmass composition for all the tuffs appears to be lowsilica rhyolite. Although all Tertiary volcanic rocks of 

the San Juan field of any given Si02 content tend to be 

similar in other oxide contents within the scatter of the 

chemical data, the Treasure Mountain Tuff is slightly 

higher in both alkalis and is lower in CaO than most of 

the younger volcanic rocks of the area, including both 

the younger ash-flow sheets and the lavas and intrusives 

of the caldera complex (fig. 66). 

LOWER TUFF 

The lower tuff of the Treasure Mountain consists of a 

varied assemblage of biotite-bearing ash-flow tuffs and a 

minor amount of interlayered ash-fall tuff; these ashflow and ash-fall tuffs are thought to mark the beginning 

of silicic volcanic activity at the area that was subsequently to develop into the Platoro caldera complex. The 

lower tuff rests on lavas, breccias, or mudflow conglomerates of the Conejos Formation everywhere except 

for a small area south of Rock Creek, where it directly 

overlies the tuff of Rock Creek. The lower tuff is overlain 

by the La Jara Canyon Member, except in a few places 
20 PLATORO CALDERA COMPLEX, SAN JUAN MOUNTAINS, COLORADO 

TABLE 4.-Analyses of Treasure 

[Sample localities shown in figs. 9, 14, 23, and 25. Analyses 3-5, 8, 9, and 11 are from .Larsen and Cross (1956, table 21). Other major-oxide analyses are by rapid methods 

methods by J. C. Hamilton; remaining analyses are by quantitative methods by 

La Jara Canyon Kember Oj ito Creek Kember 

Tuff of Rock Creek Lower tuff Intracaldera tuff Outflow sheet Intracaldera Lower unit 

Sample--------- 1 2 3 4 6 7 10 11 12 13 14 

Field No------- 65L-17D-m 65L-17D-s. Con595 Con596 Con563 66L-208 66L-13-A Con597 SV20 66L-22 Con532 66L-19-B 70L-40-G 70L-4D-H 

Lab. No-------- w-168454 w-168453 Wl68068 Wl68461 ------ ----- Wl68462 Wl68463 Wl76179 Wl76180 

Major oxides (weight percent) calculated without H20 and C02 (original values listed separately below) 

5102----------- 64.3 64.6 65.15 69.09 65.03 65.1 66.1 66.88 68.11 69.9 65.18 67.7 69.6 '70.4 

Al20 3---------- 17.0 17.3 16.27 15.05 16.82 17.6 17.3 16.17 15.96 17 .o 17.40 ; 16.0 16.3 15.4 

Fe2o 3---------- 5.17 3. 7 4.48 2. 77 3. 69 2. 7 3.9 2.99 2. 78 2. 5 3.89 2.1 2.2 1.2 

FeD------------ . 32 • 9 • 54 • 36 • 30 .81 .14 .49 • 70 .12 • 53 .06 .24 .85 

KgD------------ • 55 . 7 2.06 .12 1.06 1.4 .69 .16 .61 . 66 • 56 .65 • 36 • 33 

cao------------ 3. 24 2.8 3. 69 2. 23 3.47 3.6 2. 7 2.86 2.18 2.1 1.88 3.0 1.4 1.2 

Na2o----------- 3.85 4.1 2.81 4. 28 4. 38 3.9 4. 3 4. 64 4.00 3. 5 4.00 4.6 4.1 4.6 

K2o------------ 4.05 4.6 4.05 5. 26 4.44 3. 7 4.0 4.87 4.80 3.4 5.46 5.1 5.2 5.4 

Ti02----------- .94 1.0 . 66 .69 .60 .87 . 63 .63 .65 .41 .91 .47 .42 • 39 

P 20 s----------- .41 • 37 .28 .14 . 20 .21 .18 .17 .20 .14 .20 .08 .oo .00 

Kno------------ .12 .09 ----- ----- ----- .07 .16 .14 ----- .12 ----- .14 .08 .08 

TOTAL------- 99.95 100.16 99.99 99.99 99.99 99.96 100.10 100.00 99.99 99.85 100.00 99.90 . 99.90 99.87 

H20+- ---------- . 52 .69 2.90 .47 1.06 .60 .84 .10 . 75 .50 1.05 . 51 .43 .49 

H2o- ---------- .48 .71 2.08 .43 . 20 .90 . 56 .26 .47 .40 1.03 .10 . 57 . 37 

C02 ----------- <.OS <,05 ----- ----- ----- . 20 < .05 ----- ----- .21 ----- <.OS <.OS <.OS 

Norms (weight percent) 

~rtz--------- 19.81 17.58 23.17 20.33 14.74 19.47 19.67 16.27 21.47 30.88 16.81 15.3 21.83 19.63 

Orthoclase----- '23.95 27.06 23.93 31.09 26.23 21.66 23.43 28.78 28.33 20.37 32.23 30.32 31.04 31.19 

Albite--------- 32.57 34.44 23.73 36.22 37.03 32.73 36.12 39.26 33.86 30.03 33.79 39.16 35.04 39.26 

Anorthite------ 13.43 11.24 16.45 6. 31 13.14 16.28 12.42 8. 91 9.51 9. 63 7.98 7. 72 7.02 5. 20 

Corundum------- 1. 38 1. 53 1. 24 1. 31 1.41 . 70 3.93 . 2.00 1.19 

Wollastonite--- 1.19 .77 1. 74 2.45 . 34 

Enstatite------ l. 36 1. 65 5.12 • 30 2. 63 3. 55 1.72 .40 1. 52 1.64 1. 39 1. 63 . 91 .83 

Ferros ill te---- .06 

Magnet! te------ • 24 • 35 . 21 . 37 1. 75 

Hematite------- 5.17 3. 5 4.48 2. 77 3.69 2. 51 3.86 2.84 2.52 2.53 3.89 2.11 2. 22 

Ilmenite------- .94 1.93 1.14 .77 .64 1.64 .45 1. 20 1. 23 .52 1.11 .43 .68 • 75 

Titanite------- .69 .66 • 61 

Rutile--------- .44 .07 .29 .13 • 33 .06 

Apatite-------- .96 .87 • 67 . 33 .48 . 51 .43 .40 .48 . 34 .48 .19 

Minor elements (parts per million) 

11-------------- 20 20 <20 <20 <20 

Ba------------- 140 140 1000 1000 1000 1100 1300 1200 

Ce------------- 100 

Co------------- 5 <4 10 5 <4 <4 

Cr------------- 7 3 3 2 2 

Cu------------- 58 100 10 14 36 12 7 8 

Ga------------- 10 20 150 20 20 20 20 20 

La------------- 70 90 80 

Ni------------- 45 240 7 9 6 4 

Pb------------- 30 50 200 40 30 40 20 20 

Sc------------:- 16 16 10 12 8 7 6 7 

Sn------------- 30 

Sr------------- 250 250 1000 240 240 160 240 200 

v-------------- 70 20 20 

Y-------------- 50 50 30 40 40 40 30 30 

Yb------------- 3 3 3 

Zr------------- 390 450 150 250 250 310 3PO 320 

Phenocrysts (volume percent)· 

Plagioclase---- 2. 7 28 34.2 24.6 25 21 13.9 6.0 5. 2 5.9 

Sanidine------- 5 .8 .4 1.1 Tr. 

Quartz---------

Biotite-------- Tr. Tr. 4. 3 3.8 2.4 .8 .8 .9 

Augite--------- .1 Tr. 1 1.8 . 3 .4 .1 

Hornblende----- 1 1.5 Tr. 

Opaques-------- .4 1 2 1 2.4 1.4 2 2 .9 1 .4 .2 .2 

Groundmaas----- 96.8 95 88 56 56.5 69.5 69 72 81.3 92 92.7 93.8 93.0 

SAMPLE DESCRIPTIONS 

1. Densely welded devitrified purple-brown matrix of ash-flow sheet from near base, where 

sheet is about 100 m thick. North side of North Fork Rock Creek near mouth, at about 

8,850 ft. (See fig. 9.) 

2. Dark-gray devitrified scoriaceous pumice block. Same locality as No. 1. 

3. Pale-purple porous tuff, containing hydrated glass. Anomalously low in Na20. "West 

slope of Mcintyre Point [Peak], at an altitude of 9,650 feet" (Larsen and Cross, 1956, 

table 21, No. 55). (See fig. 9.) 

4. Dark-red-brown devitrified tuff, containing porous lighter colored streaky pumice lenses. 

"West slope of Mcintyre Point [Peak], at an altitude of 9,R40 feet" (Larsen and Cross, 

1956, table 21, No. 50). (See fig. 9.) 

5. Dense purple-gray devitrified welded tuff, probably propylitically altered. "East slope 

of Cornwall Mountain, at altitude of 10,500 feet" (Larsen and Cross, 1956, table 21, 

No. 22). (See fig. 14.) 

6. Relatively fresh densely welded devitrified tuff, showing well- preserved shard and pumice 

foliation (fig. 17B). Platoro Reservoir road, 1.2 km southwest ofRito Gato. (See fig. 14.) 

7. Pale-red-brown densely welded devitrified tuff from near top of unit. Ojito Creek, at 

8,516-ft bench mark. (See fig. 14.) 

8. Dark-red-brown· devitrified welded tuff. "West slope of Mcintyre Mountain [Peak] at 

altitude of 10,000 feet" (Larsen and Cross, 1956, table 21, No. 33). (See fig. 14.) 

9. Densely welded devitrified tuff, "West slope of Treasure Mountain at altitude of 11,200 

feet" (Larsen and Cross, 1956, table 21, No. 35). (See fig. 14.) 

10. Brick-red densely welded finely devitrified tuff from near top of ash-flow sheet. 

. Phenocrysts are small and fairly sparse probably because of winnowing. Along jeep trail, 

at 8,750 ft, 1.4 km southwest of junction between Hot Creek and Piedrosa Creek. (See 

fig. 14.) 

11. Densely welded devitrified reddish-brown tuff. "Along trail from Alamosa River to the 

head of Cat Creek and 2 miles north of Alamosa ranger station" (Larsen and Cross, 

1956, table 21, No. 24). (See fig. 23.) 

12. Densely welded devitrified light-brown tuff. Seems anomalously low in Si02 and high in 

CaO. Cliffs east of Ojito Creek, 2 km north of junction with Hot Creek. (See fig. 23.) 
ASH-FLOW SHEETS 21 

Mountain Tuff and tuff of Rock Creek 

by P. L. D. Elmore, Samuel Botts, Gillison Chloe, Lowell Artis, H. Smith, John Glenn, and James Kelsey. Minor-element analysis 6 is by semiquantitative 

J. D. Fletcher. <. less than. All samples except 1 and 2 are Treasure Mountain Tuff) 

Ojito Creek Member--Continued Ra Jadero Kember 

Upper unit Pumice blocks, upper unit Bulk rock Pumice block 

15 16 17 18 19 20 21 22 23 24 Sample No. 

70L-40-F 66L-13-C 66L-13-B 70L-40-Fp 66L-13-Bp 70L-4D-B 70L-4D-C 70L-40-A 66L-118 66L-4-A Field No, 

Wl16181 Wl68464 W168458 W1761812 W168457 W176177 W176178 W176176 W168460 W168459 Lab. No. 

Major oxides (weisht percent) calculated without H20 and co2 (original values listed separately below)--Continued 

68.0 68.6 68.8 70.4 71.2 64.2 64.8 65.3 6S.6 70.4 Si02 

16.4 16.4 16.5 15.7 15.8 17.3 17.3 17 .o 16.6 15.9 Al203 

2. 3 2.4 2.8 1.0 1.3 3.4 3. 5 3. 5 3.6 1.8 Fe203 

• 61 .12 • 27 .89 .63 1.1 .77 .57 • 32 .16 FeO 

• 46 .60 .57 • 33 • 29 .91 .81 • 79 .96 .38 KgO 

2.0 1.8 1.9 1.2 1.1 3.0 2.4 2. 3 2. 7 1.1 CaO 

4.6 4. 7 4. 3 3.9 4.2 4.2 4. 2 4. 2 4. 2 4.0 Na20 

s.o 4.6 4.0 6.0 5.0 4.8 s. 2 s. 3 4.9 5.4 K20 

. so .47 .60 • 39 • 39 .77 • 73 • 72 • 73 • 57 Ti02 

.02 .10 .15 .oo .07 .12 .09 .11 .18 .11 P20s 

.09 .15 .12 .11 .09 .10 .u .18 .06 .06 KnO 

99.98 99.94 100.01 99.92 100.07 99.90 99.91 99.97 99.85 99.88 TOTAL 

.46 • 28 .64 .45 .49 .45 .41 .51 .53 • 56 H20+ 

.64 • 22 • 56 • 22 .19 .so • 79 .49 .16 .64 H2o-

<.OS <.OS .12 <,05 <.OS <.05 < .05 <.OS <.05 <.OS C02 

Norms (weight percent)--Continued 

17.57 

29.35 

38.60 

9. 66 

.ll 

1.14 

.82 

1.77 

• 94 

.OS 

18.81 

27.36 

40.03 

8. 33 

• 56 

1. 50 

2. 42 

• 58 

.17 

• 24 

24.13 

23. so 36.24 

8.61 

2.01 

1.42 

2. 75 

• 82 

.17 

• 36 

21.51 

35.18 

33.30 

6.01 

.62 

.83 

• 35 

1.46 

• 75 

25.35 

29.35 

35.17 

5:07 

1. 74 

• 73 

1. 21 

.48 

• 73 

.17 

13.51 

28.66 

35.90 

13.77 

• 20 

2. 26 

1.69 

2.27 

1.46 

.2!1 

14.14 

30.51 

35.98 

11.46 

.53 

2.02 

• 73 

3.04 

1. 38 

.22 

14.56 

31.09 

35.96 

10.82 

• 35 

1.96 

• 34 

3. 31 

1. 36 

.26 

15.37 

29.21 

35.85 

u.so 

2. 39 

3.63 

.81 

• 37 

.15 

.43 

23,84 

32.00 

33.72 

4.84 

1. 75 

.94 

1.84 

.48 

.32 

• 27 

Quartz 

Orthoclase 

Albite 

Anorthite 

Corundum 

Wollastonite 

Enstatite 

FerrosiUte 

Magnetite 

Hematite 

Ilmenite 

·Titanite 

Rutile 

Apatite 

Minor elements (parts per million)--Continued 

1500 

10 

20 

100 

20 

8 

340 

30 

30 

3 

280 

<20 

1400 

<4 

2 

14 

20 

6 

30 

7 

240 

40 

180 

<20 

2000 

<4 

<2 

16 

20 

18 

40 

8 

240 

40 

250 

1300 

7 

20 

70 

20 

6 

240 

20 

30 

280 

<20 

2100 

<4 

<2 

15 

20 

12 

40 

8 

150 

40 

300 

20 

1400 

6 

7 

20 

20 

90 

3 

20 

13 

440 

60 

30 

3 

300 

20 

1800 

7 

5 

24 

20 

100 

5 

20 

14 

490 

so 

30 

3 

320 

20 

2000 

6 

4 

20 

20 

100 

6" 

20 

13 

440 

40 

30 

4 

290 

<20 

880 

5 

7 

32 

20 

10 

40 

17 

180 

so 

400 

<20 

1600 

<4 

<2 

14 

20 

30 

30 

9 

180 

40 

380 

B 

Ba 

Ce 

Co 

Cr 

Cu 

Ga 

La 

Ni 

Pb 

Sc 

Sn 

Sr 

v 

y 

"Yb 

Zr 

Phenocrysts (volume percent)--Continued 

12.8 

.1 

1.3 

Tr. 

.4 

85.3 

10.3 

1.7 

• 3 

.s 87.2 

11.9 

1.3 

Tr. 

.8 

86.0 

7.1 

1.3 

• 2 

93.4 

2. 7 

.9 

.2 

• 3 

95.9 

13.5 

1.3 

2.0 

.6 

.9 

81.7 

11.3 

1.7 

1.8 

.4 

.6 

84.3 

11.7 

.9 

1.4 

• 5 

1.0 

84.5 

u.s 

2.1 

2.1 

.s 

.6 

83.2 

7. 3 

• 7 

1.3 

.6 

90.1 

Plagioclue 

Sanidine 

Quartz 

Biotite 

Augite· 

Hornblende 

Opaques 

Groundmass 

SAMPLE DESCRIPTIONS-Continued 

13. Densely welded devitrified light-brown tuff from near base of ash-flow sheet (fig. 24A). At 

about 8,450 ft, along southwest base of Chiquita Peak, 2.4 km southeast of Terrace 

Reservoir spillway. (See fig. 23.) 

14. Densely welded dark-gray nearly vitrophyric weakly devitrified tuff from just above basal 

vitrophyre. About 2 m below No. 13. (See fig. 23.) 

15. Densely welded devitrified gray tuff, containing large feathery black pumice lenses (fig. 

248), from middle of upper unit. At about 8,500 ft, same locality as No. 13. (See fig. 23.) 

16. Very densely welded devitrified red-brown tuff from near base o(upper unit, near margin 

of ash-flow sheet, where lower unit is absent. Large pumice lenses are oxidized and 

lineate. Cliffs north or Ojito Creek road near 9,169-ft bench mark. (See fig. 23.) 

17. Densely welded devitrified tuff from near top of upper unit, where pumice lenses are 

black. Same locality as No. 16. (See fig. 23.) 

18. Black pumice lens from upper part of upper unit. Same locality as No. 15. (See fig. 23.) 

19. Black pumice lens from upper part of upper unit. Same locality as No. 17. (See fig. 23.) 

20. Dark-gray weakly devitrified tuff from just below upper vitrophyre (fig. 24C). At about 

8. 600 ft, same locality as No. 13. (See fig. 25.) · · 

21. Red-brown densely welded devitrified tuff from lower part of sheet, just above basal 

vitrophyre. At about 8,500 ft, same locality as No. 13. (See fig. 25.) 

22. Red-brown densely welded devitrified tuff from upper part of ash-flow sheet, just below 

upper vitrophyre. At about 8,600 ft, same locality as No. 13. (See fig. 25.) 

23. Red-brown densely welded devitrified tuff from interior of ash-flow sheet. Northeast of 

Terrace Reservoir, 0.2 km north of 8,685-ft bench mark along Cat Creek road (See fig. 

25.) 

24. Large red-brown devitrified scoriaceous puinice lens from upper part of ash-flow sheet. 

Same locality as No. 23 (fig. 25). 
22 PLATORO CALDERA COMPLEX, SAN JUAN MOUNTAINS, COLORADO 

1-

z 

w 

u 

a: 

w 

a.. 

w 

:E 

::J 

...J 

0 

> 

~ 

~-

(J) 

>-

a: 

u 

0 

z 

w 

J: 

a.. 

...J 

<( 

to 

1-

50 

0 

0 0 

0 

0 0 

0 

0 

+ 

+ + 

+ 

+ + 

EXPLANATION 

* Upper tuff 

6. Ra Jadero Member 

c Ojito Creek Member 

X Middle tuff 

o La Jara Canyon Member 

+ Lower tuff 

0 6. * 6. t::.. 

A6. t::.. * t::.. 

6. 

* 

o~----~~--~~~~~----~~--~~~~ 

~ 0.01 0.1 1.0 

1-

z 

w 

u 

a: 

w 

a.. 

w 

:E 

::J 

...J 

0 

> 

~ 

~-

(J) 

>-

50 

a: 20 u 

0 

z 

w 

J: 

a.. 

...J 

<( 

1-

~ 10 

A SANIDINE: PLAGIOCLASE RATIO 

~ 0 

EXPLANATION X + Bulk rock Pumice block 

Ra Jadero Member 

-~ ® !E Bulk rock, Bulk rock, Pumice block, 

lower unit upper unit upper unit 

· Ojito Creek Member 

0 • Intracaldera tuff Outflow sheet 

La Jara Canyon Member 

·X 

----.x ' x x·---. '® 

-- ' ® 

---~ ',---+ ' ' ' ~ ''l:... . 181" ~ Anomalous/ 

'~ 

' 

B Si02 , IN ~EIGHT PERCENT 

FIGURE. 12.-Petrolokic features of. the ~ajor ash-flo~ members of the 

Treas.ure Mountain Tuff. A, Relations bet~een feldspar proportions 

and total phenocryst contents. B, Relations between Si02 and total 

phenocryst contents. · 

where andesitic lavas intervene. Intermediatecomposition lava flows that occur locally between the 

lower tuff and the La Jara Canyon or that are interlayered within the lower tuff are included in the 

Conejos Formation (upper lava unit), because these 

lavas could not be separated readily from other Conejos 

lavas in places where the lower tuff has wedged out. 

Such andesitic flows occur within the lower tuff near 

Elwood Creek (fig. 9) and in the Banded Peak area (figs. 

9, 13). Andesitic lavas occur between the lower tuff and 

the La Jara Canyon Member at Chama Peak, in an area 

of at least 50 km2 from the head of Elk Creek to Banded 

Peak, north of Elwood Creek, south of Pass Creek, west 

of Bear Creek, nortl; of the Terrace Reservoir, and along 

Ojito Creek. 

DISTRIBUTION AND THICKNESS 

Cover by younger volcanic units and extensive erosion 

have left only a fragmentary record of the distribution of 

the lower tuff of the Treasure Mountain, but existing 

outcrops clearly indicate that the lower tuff covered 

sizable areas but did not form a continuous sheet (fig. 9). 

Most plausibly, several irregularly elongate lobes extended in a crude radial pattern outward from the area of 

the subsequent Platoro caldera. These lobes extended as 

much as 35 km to the west and south, farther than later 

members of the Treasure Mountain Tuff. 

Maximum thicknesses-as much as 200 m-are 

generally at intermediate distances from the caldera area 

(fig. 9), but some local thickness variations are substantial. Deposition of the lower tuff in local valleys that 

were cut into the Conejos Formation can be seen on outcrop scale at many places in the southeastern San Juan 

Mountains; easily accessible examples occur on the canyon walls above the Conejos River north of Elk Creek. 

This distribution suggests deposition controlled by 

several broad valleys that extended irregularly outward 

from a central highland of eroded stratovolcanoes of 

Conejos age in the Platoro area. Such a paleogeographic 

reconstruction is consistent with the distribution of vent 

and volcaniclastic facies of the Conejos Formation, as 

discussed previously, and also with features of the distribution and thickness of later ash-flow sheets of the 

Treasure Mountain. 

The volume of the lower tuff of the Treasure Mountain 

is estimated to be about 30 km3, but this figure is very 

uncertain. (See table 5.) No single ash-flow sheet within 

the lower tuff is likely to have had an original volume exceeding 5 km 3• 

LITHOLOGIC DESCRIPTION 

The lower tuff of the Treasure Mountain is an 

assemblage of numerous seemingly local, small ash-flow 

sheets and minor beds of ash-fall tuff. Except where 

densely welded units predominate, the lower tuff is 

relatively poorly exposed. Ash-flow units of the lower tuff 
ASH-FLOW SHEETS 23 

are relatively phenocryst poor (fig. 12), commonly are 

only slightly welded, and are distinguished from the tuff 

of Rock Creek and the minor ash-flow tuffs within the 

underlying Conejos Formation by the presence of 

phenocrystic biotite. Total phenocryst content is 

generally in the range 5-10 percent, and phenocrysts consist of plagioclase, sparse biotite and opaques, and rare 

augite. Much of the lower tuff, including all fairly thick 

accumulations, is outside the.Platoro map area and accordingly has not been studied in detail. 

Where the lobe of lower tuff extending west from the 

Platoro area is thickest and best exposed-in the area 

from Treasure Mountain to Wolf Creek Pass-it consists of at least three separate cooling units of 

phenocryst-poor (5-10 percent) ash-flow tuff, each 20-30 

m thick, that grade from gray or tan nonwelded margins 

into light-purplish-brown densely welded devitrified interiors. Eastward from Treasure Mountain, these units 

thin, and welding decreases. At the east end of this lobe, 

however, the lower tuff, which is well exposed on the 

southwest slope of Prospect Mountain and along the 

Stunner-Summitville road, is again brown phenocrystpoor (5-10 percent) densely welded devitrified tuff and 

has faint bedding on a scale of 10-50 em (centimeters) 

defined by subtle variations in number and size of 

pumice fragments and phenocrysts. This welded bedded 

tuff is believed to represent agglutinated ash fall that 

was deposited so close to its source that it was still sufficiently hot to weld (Lipman and others, 1966, p. F18). 

A few meters of similar bedded welded tuff, mapped as 

lower tuff of the Treasure Mountain because of the 

presence of biotite phenocrysts, is also present east of the 

Platoro caldera area along the Terrace Reservoir road. 

These tuff beds are separated from the overlying La Jara 

Canyon Member by two dark-gray nonporphyritic 

andesitic lava flows, assigned to the upper lava unit of 

the Conejos Formation. 

In the Rock Creek area the lower tuff consists of at 

least two thin ash-flow sheets. The upper sheet contains 

more phenocrysts than normal (about 10 percent) and, 

where thin, is densely welded to a black vitrophyre. 

Where the sheet is thickest, as much as 50 m, its center 

has been devitrified to dark-red-brown tuff that contains 

dark-gray scoriaceous pumice lenses as much as 30 em 

long and abundant andesitic lithic fragments a few centimeters across. The lower sheet is as much as 30m thick 

and consists of phenocryst-poor (5 percent) lighter 

colored tuff of more silicic appearance; this tuff is also 

densely welded and devitrified in thick sections. 

In the southern area, from the Conejos River to 

Banded Peak, two main ash-flow sheets are present in 

the lower tuff. The lower sheet is as much as 40 m thick, 

but it varies abruptly in thickness owing to irregular 

preeru ption topography. It consists of white to pink nonwelded to partly welded tuff containing only about 5 percent phenocrysts. Small light-colored pumice lapilli and 

dark volcanic rock fragments are abundant. Overlying 

this unit is a sheet consisting mostly of densely welded 

dark-brown devitrified tuff, characterized by lithophysal 

gas cavities a few centimeters in diameter and by 

collapsed pumice lenses as much as 60 em long; this 

sheet grades upward into less welded pink tuff at its top. 

It contains 6-8 percent phenocrysts and sparse lithic 

fragments. In the Banded Peak area (fig. 13) a thick 

dark-gray andesite or rhyodacite lava flow (Conejos Formation, upper lava unit), containing small plagioclase 

and augite phenocrysts, is locally present between these 

two ash-flow sheets. In other places the upper sheet is 

absent and the La Jara Canyon Member of the Treasure 

Mountain directly overlies the lava flow. On the ridge 

crest east of Rio Chama, two or three separate nonwelded to partly welded phenocryst-poor tuff units that 

resemble the lower ash-flow sheet described above are 

overlain directly by the La Jara Canyon Member. 

AGE AND MAGNETIC POLARITY 

No samples of the lower tuff have been dated 

radiometrically, but two dated lava flows from the 

Conejos Formation provide a maximum limit on its age 

(Lipman and others, 1970, table 2). At Snowball Park an 

andesitic lava, more than 700 m below the lower tuff, 

yielded a K-Ar age of 31.1 m.y., and at Navajo Peak a 

rhyodacite, deep in a volcanic pile overlain around its 

margins by lower tuff," was dated at 32.4 m.y. The lower 

tuff is overlain by the La Jara Canyon Member, well 

dated at 29.8 m.y. (Lipman and others, 1970, table 3, no. 

1), and petrologic affinities with the La Jara Canyon 

Member suggest that the lower tuff of the Treasure 

Mountain is only slightly older. 

Densely welded units of the lower tuff have yielded 

seemingly reliable field determinations indicating 

reverse magnetic polarity at these sites: the Wolf Creek 

Pass road, the south slope of Treasure Mountain, the 

Rock Creek area, the lower Conejos River area, and 

Banded Peak. Laboratory demagnetization studies have 

not yet been made. 

SOURCE 

The nature of the lower tuff-a diverse assemblage of 

local ash-flow units that have not been successfully correlated over sizable areas-and the fragmentary outcrop 

pattern make any discussion of source somewhat hazardous. Nevertheless, it seems fairly certain that at least 

most of the ash-flow units. included in the lower tuff 

represent premonitory eruptions of the volcanic events 

that led to formation of the Platoro caldera complex. 

The major features of the lower tuff in support of such an 

interpretation are (1) the radial distribution of lobes, 

converging toward the caldera area (fig. 9), (2) the 

presence, near the caldera margins, of agglutinated ash 
24 PLATORO CALDERA COMPLEX, SAN JUAN MOUNTAINS, COLORADO 

0 2 3 MILES 

0 2 3 KILOMETERS 

FIGUI!E 13 (above and right).-Geologic map of the Banded Peak area, showing complex intertonguing relations between andesitic lava flows and 

units of the Treasure Mountain Tuff. Loc·ation of area shown in figure 9. 
EXPLANATION 

frm\i9:t%/l !: ::.: :-?~:.:· . .-:~1 Landsiide deposits Glacial deposits 

G 

Intrusive quartz latite porphyry 

Basalt, Hinsdale 

-

Formation 

Los Pinos Formation 

Andesite of 

Summit Peak 

G 

Upper lava unit 

Tee ra:~<H Volcani- Vent 

clastic facies 

facies 

c 

1::::::::1m:~::::::::1 Masonic Park Tuff 

~ 

~ 

Ojito 

Creek 

Member 

Ra J adero and 

Ojito Creek 

Members 

Middle 

~ tuff 

R 

La Jara ~anyon Member 

:§ 

J: § Lower unit - of lower tuff 

"' 0 

'Ci)' 

c 

0 

u 

Contact 

Fault 

Dashed where approximately located; 

dotted where concealed. Bar and 

ball on downthrown side 

ASH-FLOW SHEETS 25 

falls that are necessarily near-source deposits, and (3) 

the general petrologic affinity with subsequent units of 

the Treasure Mountain Tuff. 

LA JARA CANYON MEMBER 

The La Jara Canyon Member is a multiple-How compound cooling unit of phenocryst-rich quartz latite that 

makes up the first widespread ash-f1ow sheet in the 

eastern and central San Juan Mountains; eruption of 

these ash f1ows resulted in collapse of the main Platoro 

caldera. The La Jara Canyon Member was named (Lipman and Steven, 1970) for cliff exposures along the rims 

of La Jara Canyon, about 27 km southeast of Platoro 

(fig. 14). It rests conformably on the lower tuff of the 

Treasure Mountain over much of its extent, but where 

the lower tuff was not deposited, the La Jara rests directly on eroded Conejos lavas and breccias. The La Jara Canyon Member is in general overlain by the middle tuff 

of the Treasure Mountain, although in the northeastern 

part of the mapped area where the middle tuff is thin or 

absent, the La Jara Canyon is directly overlain by the Ra 

Jadero Member of the Treasure Mountain. 

DISTRIBUTION AND THICKNESS 

The distribution, thickness, and other features of the 

La Jara Canyon Member are most readily described in 

terms of two assemblages: (1) an outf1ow sheet that 

spread widely over an erosional surface of low relief cut 

in the Conejos Formation and previously partly mantled 

by the lower tuff of the Treasure Mountain, and (2) a 

thick local accumulation of tuff that ponded within the 

caldera concurrently with subsidence. 

Preserved remnants of the outflow sheet (fig. 14) indicate that it extended outward 20-30 km in nearly ever 

direction from Platoro caldera. The record is fragmentary, however, especially on the southwest side. In the 

three areas where the original depositional margin of the 

La Jara Canyon Member can be established with confidence-near the West Fork of the San Juan River, 

along the Rio Grande between South Fork and Del 

Norte, and in the lower Conejos River drainage-the 

ash-f1ow tuffs wedge out against the subdued erosional 

remnants of volcanoes of Conejos age. Elsewhere, the 

location of the depositional edge of the sheet is conjectural. If the original margins of the body were better 

known, they would probably appear irregularly lobate, 

owing to topographic irregularities in the preeruption 

surface, rather than smoothly subcircular as 

schematically shown in figure 14. 

Variations in thickness and welding features of the La 

Jara Canyon also indicate significant topographic 

irregularity near the rims of the Platoro caldera. In 
26 

.eo 

PLATORO CALDERA COMPLEX, SAN JUAN MOUNTAINS, COLORADO 

0 5 10 15 20 MILES 

0 5 10 15 20 KILOMETERS 

EXPLANATION 

Outcrops shown by darker pattern, inferred original extent by lighter pattern 

Younger volcanic rocks Inferred original depositional 

margins of La J ara Canyon Member -

La Jara Canyon Member Dashed where approximate; 

queried where uncertain 

Approximate thickness - In meters 

.s Sample locality (table 4) 

""'""'"'""'" Limit of exposed volcanic 

rocks 

-60- Isopach - Dashed where approximate. 

Interval 60 meters 

Caldera wall (topographic)- Dashed 

where covered by younger rocks 

FIGURE 14.-0utcrop area, approximate thickness of selected ·sections, and inferred original depositional extent of the La Jara Canyon Member, 

Treasure Mountain Tuff. Based on mapping by P. W. Lipman, 1965-67, 1970-71. 
ASH-FLOW SHEETS 27 

general, the caldera area apparently was topographically 

high before the eruption. This is especially clear on the 

south side of the caldera, south of Willow Mountain, 

where the La Jara Canyon Member fills south-trending 

valleys between ridges of Conejos lava. Although the less 

welded upper parts of the La Jara Canyon have been 

erosionally removed from this area, the weldingcrystallization zonations indicate that the ash-flow 

deposits were thin and discontinuous. Farther south, the 

ash-flow deppsits are several times thicker and form a 

continuous sheet. Depositional wedging of the La Jara 

Canyon against erosional topography is also well displayed north of Platoro caldera, especially at Horseshoe 

Mountain, near Del Norte Peak, and a few kilometers 

northwest of North Mountain. In each of these areas, 

younger units of the Treasure Mountain Tuff lap higher 

onto Conejos lavas than does the La Jara Canyon 

Member. In the one area where the La Jara Canyon is 

preserved on the caldera rim, at Prospect Mountain on 

the west side of the caldera, it thins toward the caldera 

from a much thicker accumulation farther west. These 

relations clearly indicate that the cluster of volcanoes of 

Conejos age in the Platoro caldera area remained, during 

eruption of the Treasure Mountain, as topographic highs 

having at least several hundred meters of relief above the 

surrounding aprons of volcaniclastic sedimentary rocks 

of the Conejos Formation. 

The preserved thickness of the outflow sheet .of the La 

Jara Canyon Member varies greatly but is commonly 

about 50-100 m. Its maximum preserved thickness, 

about 250m, is north of Summit Peak, but the thickness 

decreases within a few kilometers to the southeast to less 

than 20 m (fig. 14). This ash-f1ow sheet is about 100 m 

thick at the La Jara Canyon area and about 60 m thick 

at Treasure Mountain. Crude isopachs have been 

plotted for the La Jara Canyon Member wherever the 

data are adequate (fig. 14). Near the volcanoes of Conejos age clustered in the Platoro caldera area, the isopachs 

define the radial valleys described above. Farther from 

the caldera area thickness variations in the La Jara Canyon Member are more subdued, and the average 

thickness is greater. The zone of maximum accumulation of the La Jara Canyon is concentric to the caldera, 

especially on its south side, and roughly coincides with 

alluviated basins between volcanoes of Conejos age in 

the caldera area and volcanoes along the north and south 

margins of the mapped area. 

Despite the wedging out of the La Jara Canyon outflow sheet toward the Platoro caldera, the ash-flow tuff 

accumulated to great thickness within the caldera, a 

relation which indicates beginning of caldera collapse 

during the eruption and which characterizes many San 

Juan ash-flow eruptions (Steven and Lipman, 1968). Exposures of the intracaldera tuff are mainly on the 

Cornwall Mountain resurgent block. Near Jasper, where 

its top is eroded and its base is not exposed, the intracaldera tuff is continuously exposed at elevations 

from 9,100 feet to over 12,000 feet on the northeast side of 

Cornwall Mountain (Platoro map). Even after correction 

for the gentle southwest dip of the resurgent block, the 

exposed thickness is greater than 800 m. Thinner sections of the La Jara Canyon within the caldera are exposed where the tuff wedges against the topographic 

walls of the caldera, especially on the southwest side 

(figs. 14, 63). 

LITHOLOGIC DESCRIPTION 

Densely welded devitrified tuff, which constitutes 

most of the La Jara Canyon Member, is typically light 

red brown and forms rounded blocky outcrops that 

weather to a sandy grus. A black basal vitrophyre is commonly present where the ash-flow sheet is 50-100 m 

thick; where the sheet is very thick or very thin, the zone 

of devitrification generally extends down into the zone of 

partial welding. (See Smith, 1960b.) Nonwelded to partly welded buff or gray glassy tuff at the base of the sheet 

is typically only a few meters thick and is rarely exposed. 

Where not removed by erosion, several tens of meters of 

gray partly welded tuff showing evidence of vapor-phase 

crystallization overlies the densely welded central part of 

the sheet. This upper vapor-phase zone is characterized 

by closely spaced, somewhat irregular subhorizontal 

joints 10-30 em apart that are parallel to the compaction 

foliation. 

The ash-flow sheet only locally displays clear evidence 

of compound cooling, as at the Terrace Reservoir dam, 

where a partly welded zone of gray tuff about 10m thick 

separates two zones of red-brown more densely welded 

tuff. Contacts between individual ash-flow units are also 

generally obscure, as is typical of thick densely welded 

ash-flow sheets (Smith, 1960a, p. 811; Ratte and Steven, 

1967, p. H29-H33); but in a few places, 5- to 10-cm-thick 

winnowed horizons containing fewer and smaller 

phenocrysts than typical appear to mark the tops of flow 

units (figs. 15, 16). 

Most of the La Jara Canyon Member inside Platoro 

caldera appears very different from the outflow sheet, so 

different, in fact, that during initial stages of field study 

the equivalence was not recognized. 1 In the interior of 

the resurgent block, near Platoro, the intracaldera La 

Jara Canyon is dark-gray very dense propylitized rock 

with closely spaced nearly vertical joints 10-40 em apart 

(fig. 17A). With rare exceptions (figs. 17 B, 18B), the 

compaction foliation is obscure, both in the field and under the microscope (figs. 17A, 18A). Fine-grained secondary epidote, calcite, and chlorite have partly replaced 

'Patton (1917, p. 33-35) described the intracaldera tuff precisely enough that to a modern 

reader it is clearly a welded tuff. With impressive insight, he designated these rocks the 

"biotite latite of the Treasure Mountain Rhyolite." He did, however, separate an underying 

unit near Platoro, "the Palisade andesite," which he considered to be part of the Conejos Formation; actually, these rocks constitute the more propylitically altered lower part of the intracaldera La Jar Canyon Member. 
28 PLATORO CALDERA COMPLEX, SAN JUAN MOUNTAINS, COLORADO 

the devitrified groundmass and plagioclase and augite 

phenocrysts. Biotite phenocrysts are extremely oxidized 

and contain abundant exsolved fine-grained Fe-Ti oxides. 

The size and abundance of phenocrysts in the La Jara 

Canyon Member vary appreciably but were not studied 

in detail. In general, phenocrysts are more abundant (40-

of the unit elsewhere; this variation may also possibly be 

due to sorting during emplacement of the ash flows. 

Andesitic lithic fragments are common in the La Jara 

Canyon Member, especially near and within the caldera 

(fig. 19). The most common fragments are dark gray, 

aphanitic, and angular to slightly rounded; they are 2-10 

em in diameter and rarely constitute more than a few 

FIGURE 15.-Ash·flow boundary within densely welded devitrified tuff of the La Jara Canyon Member. North side of Elwood Pass road, along 

tributary of Elwood Creek, at 10,800 feet. Boundary is defined by thin resistant ledge (arrow), about 10 em thick, that consists of shard-rich 

tuff at the top of the lower unit, from which phenocrysts have been winnowed. Note hammer (circled) for scale. 

50 percent of the rock) and smaller (0.5-1.0 mm 

(millimeters) in diameter) in the intracaldera tuff than 

in the outflow sheet (20-35 percent phenocrysts; 1-2 mm 

in diameter). The abundance of phenocrysts within the 

caldera is probably due to near-source density concentration of phenocrysts relative to ash shards during 

emplacement (Fisher, 1966; Lipman, 1967), and the 

smaller average size may reflect a larger percentage of 

broken grains. Around the northern margin of the outflow sheet, from Del Norte to Treasure Mountain, the 

plagioclase phenocrysts are conspicuously more tabular 

in outline than are the large blocky feldspar phenocrysts 

percent of the tuff. In one atypical small area within the 

caldera (southeast of the Lily Pond, on the Cornwall 

Mountain resurgent block), andesitic blocks as much as 

25 em across constitute about a third of the tuff. 

However, large tongues of landslide breccia, such as 

those interlayered with the intracaldera tuff of Creede, 

San Juan, Uncompahgre, and Lake City calderas 

(Steven and Ratte, 1965, p. 42-43; Lipman and others, 

1973, p. 639-640), have not been recognized in the La 

Jara Canyon Member within Platoro caldera. 

All rocks of the La Jara Canyon Member are quartz 

latite, but Si02 content ranges from 65 to almost 70 per-
ASH-FLOW SHEETS 29 

A 

FIGURE 16.-Photomicrographs of rock from ash-flow boundary within 

densely welded devitrified tuff of the La Jara Canyon Member. 

North side of Elwood Pass road, along tributary of Elwood Creek, at 

10,800 feet. A, Shard-rich tuff containing sparse small broken 

phenocrysts from base of resistant ledge shown in figure 15; compare 

cent (table 4). The most mafic rocks are the intra caldera 

tuffs, a fact suggesting that the later tuff, which erupted 

concurrently with caldera subsidence, represents the 

lower more mafic part of a differentiated magma 

chamber. Similar well-documented examples of this 

process occur elsewhere' in the San Juan volcanic field 

(Steven and Ratte, 1964; Steven and Lipman, 1968) and 

in the Western United States (Lipman and others, 1966). 

However, the most mafic tuffs are also the most 

phenocryst rich (fig. 12B), and there seems to be a fairly 

good inverse correlation between phenocryst content and 

Si02 content. Calculated groundmass compositions of all 

analyzed samples are in the range 71-73 percent Si02, 

and some of the differences in rock composition seem to 

be due to variable sorting of phenocrysts during eruption 

and emplacement rather than to primary magmatic 

differences. 

AG E AND MAGNETIC POLARITY 

The preferred K-Ar radiometric age of the La Jara Canyon Member at Treasure Mountain is 29.8 m.y. (mean 

B 

with B. B, Normally textured tuff containing more abundant and 

larger phenocrysts than A. Plagioclase phenocrysts are largely 

replaced by carbonate, which obscures the outlines of individual 

phenocrysts. Sample from just above the resistant ledge shown in 

figure 15. 

of biotite and plagioclase from the basal vitrophyre; Lipman and others, 1970, table 3, No. 1). Phenocrysts from 

two samples of intracaldera tuff (Lipman and others, 

1970, table 3, Nos. 2, 3) gave somewhat discordant K-Ar 

ages that range from 27.0 to 28.5 m.y. These slightly 

younger ages probably reflect argon loss due to heating 

by later caldera-related igneous activity at such volcanic 

centers as the Summitville-Green Ridge zone (29-20 

m.y.). 

The La Jara Canyon Member is everywhere 

characterized by reversed magnetic polarity, as indicated by about 25 widely spaced field determinations 

by the author and by laboratory demagnetization studies 

of 12 oriented cores from four localities by M. E. Beck, 

Jr., and James Diehl (written commun., 1971). 

VOLUME 

Inferences about the original extent of the three main 

members of the Treasure Mountain Tuff (figs. 14, 23, 25) 

suggest a simple geometric model for estimation of 

original volume of these tuffs (fig. 20). This model 
30 PLATORO CALDERA COMPLEX, SAN JUAN MOUNTAI S, COLORADO 

A 

FIGURE 17 .-Field appearance of intra caldera La Jara Canyon Member. 

A, Typical exposure of densely welded propylitized intracaldera La 

Jara Canyon Member. Closely spaced vertical jointing is most conspicuous feature; welding and compaction textures are 

assumes that the outflow deposits have the crosssectional geometry of a double wedge that thins both 

toward the distal edge of the sheet and toward a 

topographically high caldera area as discussed previously. The intracaldera La Jara Canyon Member is inferred 

to be thick in the center of the caldera and thin toward 

the walls. (See cross sections, Platoro map.) This model 

is obviously much too simple, but the fragmentary 

record of the original extent of the sheets indicates that 

more detailed analysis would be unprofitable. The 

volume estimates are probably conservative, and the upper limit of the inferred possible range may well be more 

valid for some of the units. The original volume of the 

La Jara Canyon Member is estimated as 585 km3, but it 

could have been as great as 1000 km3 (table 5). 

B 

characteristically obscure in this rock type. Above Platoro on 

Stunner Pass road, at 10,100 feet. B, Densely welded intracaldera La 

Jara Canyon Member, showing exceptionally conspicuous foliation. 

Platoro Reservoir road. Analyzed sample 6, table 4. 

MIDDLE TUFF 

The middle tuff, the most widespread informally 

named unit of the Treasure Mountain, consists of several 

thin weakly welded ash-flow deposits and locally thick 

interbedded ash-fall tuff. The middle tuff rests on the La 

Jara Canyon Member, except where lava flows of Summitville Andesite occur along the contact, as at the 

mouth of the West Fork of Park Creek (Platoro map) and 

near Black Mountain in the Spectacle Lake quadrangle 

(figs. 2, 21). The middle tuff is overlain by the Ojito 

Creek Member, or by the Ra Jadero Member where the 

Ojito Creek is absent. Flows of Summitville Andesite 

also locally intertongue with the middle tuff, as at Mcintyre Peak in the Osier quadrangle (fig. 2). 
ASH-FLOW SHEETS 31 

The distribution and thickness variations of the middle tuff are more like those of the lower tuff (fig. 9) than 

any of the major Treasure Mountain ash-flow sheets; a 

separate distribution map has not been prepared. The 

middle tuff is present on all sides of the caldera, except 

the southwest. Like other units of the Treasure Mountain, the middle tuff wedges out toward the caldera rim. 

It is especially thick and widely distributed west of the 

caldera, in the Treasure Mountain-Wolf Creek Pass 

. area, and ·along the east front of the San Juan Mountains 

from the Alamosa River to the south. Maximum 

thicknesses, about 100 m, are at intermediate distances 

from the caldera rim, as at Treasure Mountain and along 

the lower Conejos River. No middle tuff is present 

between the La Jara Canyon and Ojito Creek Members 

of the Treasure Mountain in the Summit PeakBlackhead Peak area or around Banded Peak, suggesting 

that little of this unit spread to the southwest. The middle tuff is also relatively thin, typically no more than 20 

A 

FIGURE · lB.-Microscopic appearance of intracaldera La Jara Canyon 

Member. A, Photomicrograph of rock shown in figure 17A . 

Phenocrysts are mostly plagioclase and biotite, with plagioclase 

partly altered to carbonate and epidote and biotite to chlorite, a 

m, north of the caldera in the Del Norte Peak-Horseshoe 

Mountain area. 

The middle tuff consists of local relatively thin and 

weakly welded ash-flow tuffs, interlayered with ash-fall 

tuffs. In general, both tuff types are only weakly indurated, and accordingly, exposures tend to be poor. 

The middle tuff is the only unit of the Treasure Mountain that contains appreciable thicknesses of ash-fall 

tuff. Where best exposed, as along the lower Alamosa 

River (fig. 22), as much as 10 m of white to buff bedded 

tuff is exposed within single outcrops, and adjacent 

slopes undoubtedly c,9nceal more. Individual beds, 

typically 2-50 em thick, differ mainly in size and abundance of phenocrysts, pumice fragments, and andesitic 

lithic debris. Local crossbedding and channeling indicate minor reworking of the ash-fall deposits concurrently with deposition. Although dips are everywhere 

gentle, broad undulations appear to reflect local 

irregularities in the depositional topography which are 

B 

typical propylitic alteration assemblage. Original shard texture of 

groundmass is completely obscured by devitrification and alteration . 

B, Photomicrograph of rock shown in figure 17B. Densely welded 

shard texture still discernible; compare with A. 
32 PLATORO CALDERA COMPLEX, SAN JUAN MOUNTAINS, COLORADO 

FIGURE 19.-Moderately welded La Jara Canyon Member, containing abundant small andesitic lithic fragments. East slope of Montezuma Peak 

at about 12,000 feet, just southwest of Cataract Creek stock. 

presumably due to slight erosion of the weakly welded 

top of the La Jara Canyon Member. Near the roadcut 

shown in figure 22, a broad valley is well exposed in cross 

section, showing a "synclinal" draping of ash-fall tuff 

overlain by a lens of local glassy partly welded ash-flow 

tuff that w~s deposited in the channel and that contains 

a concentration of lithic rubble near its base along the 

channel axis. 

The ash-flow deposits of the middle tuff are thin local 

nonwelded to partly welded sheets, mostly too thin to 

have devitrified during cooling. At Treasure Mountain, 

where the middle tuff is about 70 m thick, it contains 

four separate 10 to 20-m thick ash-flow sheets of nonwelded to partly welded glassy tuffs. All are relatively 

phenocryst poor, containing 5-10 percent total 

phenocrysts-mainly plagioclase, accompanied by 

biotite and sparse augite. Petrologically similar ash-flow 

deposits, although not necessarily exactly correlative, 

also characterize the middle tuff to the east and south of 

Platoro caldera. However, at the one place where middle 

tuff underlies the Ojito Creek within the eastern moat of 

Platoro caldera (Platoro map), this unit consists of a 

rather different densely welded devitrified ash-flow 

deposit that is exposed over a thickness of at least 60 m 

where it wedges against the caldera wall; it contains 

abundant andesitic fragments and about 20 percent 

phenocrysts of plagioclase , biotite, and augite 

(completely altered to carbonate). 

Ash-flow deposits of the middle tuff appear to be 

largely or entirely characterized by reverse remanent 

magnetic polarities west of the caldera, as indicated by 

TABLE 5.-Estimated uolumes of ash-flow sheets related to the Platoro 

caldera complex 

Best Range of 

Unit estimate alternative 

(km3) estimates 

(fig. 20) (km3) 

Tuff of !Wck Creek . . . . . .. . . . . .......... . 10 5·20 

Treasure Mountain Tuff: 

Lower tuff (many separate ash-flow sheets) .. . 30 20-40 

La Jara Canyon Member . . . . . ...... ...... . 580 500·1,000 

Middle tuff (many separate ash-flow sheets) .. 30 20-40 

Ojito Creek Member . .. .... . .............. . 50 40-70 

Ra Jadero Member. . . . . . . . . . . . . . . . . . ... . 120 100·150 

10 5-20 

830 690·1,340 

Upper tuff (several separate ash-flow sheets) . ·---:::-:-----:::::-:--::-:::--

Total ..... .. .... . ......... . . .. ....... . 
ASH-FLOW SHEETS 33 

field determinations near Wolf Creek Pass, but at least 

one of the ash-flow sheets southeast of the caldera (the 

one exposed in the channel along the lower Alamosa 

River canyon) has normal polarity, as measured both in 

the field and in the laboratory. 

OJITO CREEK MEMBER 

The Ojito Creek Member of the Treasure Mountain 

Tuff is a relatively widespread thin single cooling unit of 

quartz latitic ash-flow tuff which is inferred to have been 

erupted from the Summitville caldera, nestled within 

the northern part of the main Platoro caldera. The Ojito 

Creek Member was named for mesa rim exposures at the 

head of Ojito Creek (fig. 23), about 22 km east of Platoro 

(Lipman and Steven, 1970). Even better exposures along 

the lower Alamosa River canyon on the southwest slope 

of Chiquita Peak in sees. 24 and 25, T. 36 N ., R. 6 E., are 

designated here as the principal reference section. 

The Ojito Creek Member commonly rests on the middle tuff. Where the middle tuff is absent, as inside the 

Platoro caldera and along the continental divide from 

AB = lOkm 

AC = 40km. 

Ra Jadero Member 

Ojito Creek Member 

X y 

0 0.05 

0 .05 

La Jura Canyon Member 1.5 .15 

V~lume. of Ojito Creek Member calculated on basis of deposition over 120° arc (see 

fig. 23); volumes of other members, on basis of 360° arc 

FIGURE 20.-Geometric model for volume calculations of the three major ash-flow sheets of the Treasure Mountain Tuff. See text for 

details. X andy are approximate thickness~s, in kilometers. 

Summit Peak south, the Ojito Creek Member rests 

directly on the La Jara Canyon. The Ojito Creek is 

generally overlain by the Ra Jadero M·ember, but the 

two densely welded cliff-forming zones are so close 

together that in most exposures it is not clear that two 

separate ash-flow sheets are present. In the southwestern 

part of the area, where the Ojito Creek Member was 

·deposited slightly beyond the Ra Jadero, the Ojito Creek 

is overlain by various younger units-mainly Masonic 

Park Tuff, Los Pinos Formation, or basalt of the 

Hinsdale Formation. 

DISTRIBUTION AND THICKNESS 

The Ojito Creek Member. is most extensive south and 

southeast of the Platoro caldera (fig. 23), where it forms 

an apparently originally continuous sheet that is uniformly 15-30 m thick. This main sheet of the Ojito Creek 

extends south beyond the Conejos River, at least as far 

as the La Jara Canyon Member. Individual units of the 

Treasure Mountain have not been recognized farther 

south in New Mexico (Butler, 1971); if the Ojito Creek is 

present there, it is only weakly welded. Wedging out of the 

Ojito Creek Member on the southeast flank of the 

Platoro caldera highland is well exposed in the Alamosa 

River canyon and at the head of Ojito Creek (fig. 23); a 

similar wedging out of the La Jara Canyon Member (fig. 

14) occurs somewhat higher on the outer caldera slope. 

The western extent of the Ojito Creek Member is uncertain, but most likely only a few small thin lobes extended west from the Platoro caldera. Wedging out 

against older units is shown in small exposures at the 

mouth of the West Fork of Park Creek, at Summit Peak, 

and probably on Conejos Peak. The magnetic polarity of 

the outcrops on Conejos Peak could not be measured, 

owing to the effects of lightning strikes; thus, this identification of the Ojito Creek is less certain here than in 

the other exposures. 

The Ojito Creek also was deposited within the main 

Platoro caldera, in the eastern part of the moat between 

the resurgent block and the topographic wall. It is exceptionally densely welded there and is as much as 180 m 

thick-four times as thick as anywhere outside the 

caldera. The Platoro caldera moat appears to have been 

lowest in its eastern part during deposition of the Ojito 

Creek, because this tuff is not exposed elsewhere within 

the caldera. 

LITHOLOGIC DESCRIPTION 

Exposures of the Ojito Creek Member are mostly 

densely welded devitrified red-brown tuff, in places having a distinctive waxy luster imparted by the devitrified 

groundmass. A black vitrophyre commonly present near 

the base of the sheet is rarely exposed. The upper 5-10 m 

of the sheet generally consists of partly welded pinkishbrown or gray tuff characterized by vapor-phase 

crystallization. This zone was variably eroded shortly 
34 PLATORO CALDERA COMPLEX, SAN JUAN MOUNTAINS, COLORADO 

30" 

1 MILE 

0 .5 1 KILOMETER 

FIGu~E 21 (above and right).-Generalized geologic map of the Black Mountain area, showing local flows of Summitville Andesite along contact 

between La Jara Canyon Member and middle tuff. In places, both the middle tuff and the Ojito Creek Member wedge out against the 

andesite flows. Two feeder dikes for the andesite are exposed in the cliffs above the Conejos River. 

after deposition, and in places the Ra Jadero Member 

rests directly on the densely welded interior of the Ojito 

Creek; this relation is strikingly exposed along the lower 

Alamosa River canyon on the lower southwest slope of 

Chiquita Peak. 

In most outcrops the Ojito Creek Member appears as a 

simple cooling unit, although probably it is the result of 

numerous separate ash flows erupted rapidly and welded 

together. However, on the southeast side of the caldera, 

from Chiquita Peak to Ojito Creek, the ash-flow sheet 

clearly displays compound cooling, with a welldeveloped partial welding break a few meters thick 
ASH-FLOW SHEETS 35 

EXPLANATION 

Landslide deposits 

Basalt, Hinsdale Formation 

..... 

..... 

::s 

Ec: 

·;;; 

~ ::s 

0 

Lower member of the ::E 

Summitville Andesite ~ 

Conejos Formation, 

vent facies 

::s 

~ 

~ 

E-

~:_:_:~-~~·:·:::] 

Alluvial and glacial deposits 

I::::Itr:w:~:::~:::l ~asonic Park Tuff 

~~~ Ojito Creek Member 

Middle tuff 

Lii Jara Canyon Member 

Lower tuff 

Contact 

Dashed where approximately located 

Fault 

Dotted where concealed. Bar and ball on downthrown side 

within the devitrified zone. The two cooling units in this 

area also have contrasting fragmental constituents. The 

lower unit is densely welded, red brown, and relatively 

phenocryst poor (6-9 percent phenocrysts); it contains 

sparse lithic fragments and obscure small pumices (fig. 

24A). This is the characteristic lithology of the Ojito Creek 

Member elsewhere. The upper unit, in contrast, contains 

larger and more abundant phenocrysts. (12-17 percent) 

and conspicuous large dark wispy pumice fragments as 

much as 30 em long (fig. 24B); rock fragments are fairly 

abundant, and the devitrified shard matrix is light gray 

to pale brown, distinctly lighter colored than in the lower 

unit. The upper unit appears to be quite local and has 

only been found southeast of the caldera; it appears to be 

absent from the relatively thick Ojito Creek within the 

caldera. 

The Ojito Creek Member within the southeastern part 

of Platoro caldera is petrographically similar to the 

phenocryst-poor lower unit just described, except that 

the basal vitrophyre is lacking, and pumices near the 

base of the ash-flow sheet are larger and are exceptionally compacted to lineate lenses about 1 em thick and as 

much as 40 em long in the direction of lineation. The 

unit within the caldera has a chalky appearance, owing to 

intense devitrification; and in the upper third or half of 

the unit, where degree of welding decreases, vapor-phase 

crystallization predominates. 

The Ojito Creek contains phenocrysts of plagioclase, 

biotite, Fe-Ti oxides and sparse augite. The sparse 

hornblende in a few samples may be xenocrystic. In 

general, the Ojito Creek Member contains fewer 

phenocrysts than the other two major ash-flow sheets of 

the Treasure Mountain; in addition, it can generally be 

distinguished from the La Jara Canyon Member by its 

lower augite content and from the Ra Jadero Member by 

its lower sanidine content (fig. 12). 

The Ojito Creek tuffs are quartz latitic; Si02 content 

of six bulk-rock samples of the outflow sheet ranges only 

from 67.7 to 70.4 percent (table 4). Within this limited 

range, the phenocryst-rich tuffs of the upper unit appear 

to be distinctly more mafic (68.0-68.8 percent Si02) than 

two of the three samples of the phenocryst-poor lower 

unit (69.6-70.4 percent SiOz). The one sample of the 

lower unit that has only 67.7 percent Si02 (table 4, No. 

12) is thought to be abnormal, because it is also 

anomalous in other oxides, such as CaO and total iron. 

Pumice lenses from the upper unit contain fewer 

phenocrysts than the enclosing tuff, and two analyses 

(table 4, Nos. 18, 19) are compositionally very similar to 

bulk-rock analyses of the lower unit. This similarity 

suggests that petrologic differences between the two 

units are largely due to sorting and winnowing of fme 

shards and ash from the fragmental matrix of the upper 

unit during eruption and emplacement; such sorting has 

also been described in other ash-flow deposits (Fisher, 

1966; Lipman, 1967). One old analysis of the Ojito Creek 

Member from within the Platoro caldera (table 4, No. 

11) indicates that the intracaldera tuff is more mafic 

than the outflow sheet; but because the intracaldera tuff 

petrographically resembles silicic parts of the outflow 

sheet, probably this sample is anomalous, or the analysis 

is erroneous. 

AGE AND MAGNETIC POLARITY 

The Ojito Creek Member has not been radiometrically 

dated, but its age is bracketed by dates (Lipman and 

others, 1970) for the La Jara Canyon Member (29.8 m.y.) 

and for the overlying Masonic Park Tuff (28.2 m.y.). In 

addition, ring faults of the Summitville caldera, from 

which the Ojito Creek is thought to have erupted, 

localized the emplacement of the Alamosa River stock, 

dated at 29.1 m.y. This suggests eruption of the Ojito 

Creek and Ra Jadero Members between 29.8 and 29.1 

m.y. This age bracket is sufficiently tight that little 

would be gained by radiometric determinations on the 
36 PLATORO CALDERA COMPLEX, SAN JUAN MOUNTAINS, COLORADO 

Ojito Creek, because the precision of individual determinations by current techniques is approximately ± 3 

percent-an uncertainty as great as the limits imposed 

by the dated rocks. 

The Ojito Creek everywhere has normal magnetic 

polarity, as indicated by at-least 30 field determinations, 

supplemented by laboratory study of oriented cores from 

one locality by M. E . Beck, Jr. , and James Diehl 

(written commun., 1971). The normal polarity of the 

Ojito Creek, in contrast to the reverse polarities of the Ra 

Jadero and La Jara Canyon Members, was of particular 

assistance in initial unravelling of stratigraphic complexities in the field, especially in correlation of units within 

the southeastern part of Platoro caldera with the outflow 

Treasure Mountain sequence. 

RA JADERO MEMBER 

The Ra Jadero Member, a widespread thin densely 

welded quartz latitic tuff, is the last major ash-flow sheet 

of the Treasure Mountain Tuff. Like the Ojito Creek, it 

is thought to have been erupted from the Summitville 

caldera. The Ra Jadero Member was named (Lipman 

and Steven, 1970) for exposures in Ra Jadero Canyon, 

# ~ ... 

.. -".·r -... . --

about 26 km southeast of Platoro (fig. 25). The principal 

reference section is the same as that selected for the 

Ojito Creek-the area of excellent cliff exposures along 

the lower Alamosa River canyon, southwest of Chiquita 

Peak, in sees. 24 and 25, T. 36 N., R. 6 E. 

TheRa Jadero Member rests on lower members of the 

Treasure Mountain Tuff over most of its extent: commonly on the Ojito Creek Member or on the middle tuff; 

directly on the La Jara Canyon Member in 

the northeastern area where the Ojito Creek was not 

deposited. In the Park Creek area the Ra Jadero is 

separated from the Ojito Creek by lava flows of Summitville Andesite (Platoro map), and in a few places the Ra 

Jadero laps up onto rocks of the Conejos Formation that 

stood high during the ash-flow eruptions. TheRa Jadero 

Member is generally overlain by the Masonic Park Tuff; 

in areas where the Masonic Park was eroded or was never 

deposited, the Ra Jadero is overlain by Los Pinos Formation or by basalt of the Hinsdale Formation. 

DISTRIBUTION AND THICKNESS 

TheRa Jadero Member is a widespread thin unit (fig. 

25); among the units of Treasure Mountain Tuff, its ex-

-~ --

FIGURE 22.-Well-bedded ash-fall tuffs of the middle tuff of the Treasure Mountain Tuff, overlain by glacial outwash gravel. Alamosa River canyon, west of Chiquita Peak. 
'107"00' 

.,, 

ASH-FLOW SHEETS 

0 5 10 15 20 MILES 

0 5 10 15 20 Kl LOM ETERS 

EXPLANATION 

Outcrops shown by darker pattern, inferred original extent by lighter pattern 

Younger volcanic rocks 

Ojito Creek Member 

Approximate thickness - In meters 

Sample locality (table 4) 

Inferred original depositional 

margins of Ojito Creek MemberDashed where approximate; 

queried wherl! uncertain 

""""""'"""" Limit of exposed volcanic rocks 

Caldera wall (topographic) - Dashed 

where covered by younger rocks 

37 

106"00' 

F IGURE 23.-0utcrop area, approximate thickness of selected sections, and inferred original depositional extent of the Ojito Creek Member, 

Treasure Mountain Tuff. Based on mapping by P. W. Lipman, 1965-67, 1970-71. 
38 PLATORO CALDERA COMPLEX, SAN JUAN MOUNTAINS, COLORADO 

FIGURE 24.-Photomicrographs of the Ojito Creek and Ra Jadero 

Members of the Treasure Mountain Tuff. Cliffs on southwest slope 

of Chiquita Peak, lower Alamosa River valley. A, Phenocryst-poor 

lower unit (about 6 percent phenocrysts, mainly of plagioclase and 

biotite), Ojito Creek Member. Lithic fragments are sparse, and 

welded shard texture is evident in light-brown devitrified matrix. 

Sample from just above basal vitrophyre. Analyzed sample 13, table 

4). B, Phenocryst-rich upper unit (about 14 percent phenocrysts of 

plagioclase, biotite, and sparse augite), Ojito Creek Member. Lithic 

fragments more abundant than in lower unit; angular corner of 

andesite fragment shows at lower left. Sample from about 5 m below 

top of cliff that marks welded interior of ash-flow sheet. Analyzed 

sample 15, table 4. C, Densely welded devitrified interior of Ra 

Jadero Member, containing about 18 percent large blocky 

phenocrysts of plagioclase, sanidine, and biotite in dark-red-brown 

devitrified matrix. Note andesite lithic fragments at left edge and 

upper center. Analyzed sample 20, table 4. 

tent is second only to that of the La Jara Canyon 

Member (fig. 14). Preserved outcrops of the Ra Jadero 

(fig. 25) suggest that it spread uniformly across the subhorizontal constructional surface on top of older ash-flow 

sheets of the Treasure Mountain Tuff for 15-20 km from 

the Platoro caldera complex in all directions, except 

perhaps to the southwest. Distal edges of Ra J adero are 

preserved west of the caldera near Sheep Mountain, to 

the north near Del Norte, and to the south near Osier 

Mountain; the original extent of the unit to the east is 
ASH-FLOW SHEETS 

0 5 10 15 20 MILES 

0 5 10 15 20 KILOMETERS 

EXPLANATION 

Outcrops shown by darker pattern, inferred original extent by lighter pattern 

Younger volcanic rocks 

Ra Jadero Member 

Approximate thickness - In meters 

Sample locality (table 4) 

Inferred original depositional margins 

of Ra Jadero Member - Dashed 

where approximate ; queried where 

uncertain 

Limit of exposed volcanic rocks 

.L.....L.....L Caldera wall (topographic) - Dashed 

where covered by younger rocks 

39 

FIGURE 25.- 0utcrop area, approximate thickness of selected sections, and inferred original depositional extent of the Ra Jadero Member, 

Treasure Mountain Tuff. Based on mapping by P. W. Lipman, 1965-67, 1970-71 . 
40 PLATORO CALDERA COMPLEX, SAN JUAN MOUNTAINS, COLORADO 

unknown. Like the older units of the Treasure Mountain, 

the Ra Jadero Member also clearly thins and wedges out 

toward the Platoro caldera complex (fig. 25). The Ra 

Jadero ash-flow sheet is relatively thin-typically 10-40 

m-everywhere on the outflow plateau that surrounds 

Platoro caldera. Its maximum thickness, about 60 m, occurs in the area east of Treasure Mountain (fig. 25). 

Like the Ojito Creek Member, the Ra Jadero accumulated to its greatest thickness within the 

northeastern part of the main Platoro caldera. On the 

north side of Bennett Peak, the Ra Jadero Member is as 

much as 125 m thick-twice as thick as anywhere outside the caldera. 

By use of the simple geometric model already 

described (fig. 20), the original volume of the Ra Jadero 

Member is estimated at 100-150 km3, with a preferred 

best estimate of 120 km3 (table 5). This volume is only 

about 20 percent of the volume estimated for the La Jara 

Canyon, but it is double the volume estimated for the 

Ojito Creek. 

LITHOLOGIC DESCRIPTION 

In most exposures the Ra Jadero consists of dark-redbrown densely welded tuff underlain by a conspicuous 

black basal vitrophyre zone about 5 m thick. Where the 

total thickness of the Ra Jadero is less than about 15-20 

m, an upper thinner vitrophyre zone is commonly present as well. The base of the Ra Jadero is generally 

covered by debris and has only been observed at one 

place-on an inaccessible cliff exposure along the lower 

Alamosa River canyon on the southwest slope of Chiquita Peak. There, the Ra Jadero was deposited on an 

irregular surface of low relief eroded in the partly welded 

upper vapor-phase zone of the Ojito Creek Member. The 

two ash-flow sheets are separated by only about 0.5 m of 

bedded tuff, and the nonwelded basal tuff of the Ra 

Jadero grades upward within about 1m into the densely 

welded basal vitrophyre zone. 

Just above the basal vitrophyre the matrix of the Ra 

Jadero becomes devitrified, but the large collapsed 

pumice lenses remain glassy, resulting in a colorful rock 

with black lenses in a red-brown matrix. Higher in the 

unit, where the rock is most intensely devitrified, the 

pumices are a chalky purple brown with a fine porous 

texture, resulting from fine secondary vesiculation 

within the pumices during devitrification. In places the 

size of the pumice blocks increases upward, .. and 

northeast of the Terrace Reservoir (Platoro map), single 

lenses are as long as 0.5 m. 

TheRa Jadero is fairly phenocryst rich, typically containing 12-18 percent phenocrysts of plagioclase, 

sanidine, biotite, and augite. In contrast to other major 

units of the Treasure Mountain, the Ra Jadero contains 

more abundant sanidine (fig. 12A), and the plagioclase 

phenocrysts are exceptionally large and blocky in outline 

(fig. 24C). 

The Ra Jadero is also characterized by abundant 

volcanic rock fragments of varied intermediate compositional types. These fragments tend to be 1-5 em in 

diameter and locally amount to as much as 5-10 percent 

of the rock. In hand specimen the Ra Jadero appears 

heterogeneous, as a result of the large pumice blocks, the 

abundant rock fragments, and the large size of 

phenocrysts. 

Despite their heterogeneous appearance, four 

analyzed bulk-rock samples of Ra Jadero tuffs are uniform in chemical composition (table 4) and are 

characterized by Si02 contents (64.2-65.6 percent) lower 

than those of any other outflow units of the Treasure 

Mountain Tuff. These relatively mafic compositions are 

surprising, in light of the greater abundance of 

phenocrystic sanidine in the Ra J adero tuffs than in 

other major Treasure Mountain ash-flow sheets. A 

typical large pumice lens from the upper part of the Ra 

Jadero Member is phenocryst poor and rather silicic 

(70.4 percent; table 4, No. 24), however, and is similar in 

composition to pumice blocks of the Ojito Creek 

Member. This suggests that the :relatively mafic· bulkrock compositions of the Ra Jadero Member may be due 

to concentration of phenocrysts in the tuffaceous matrix, 

during eruption and emplacement, by sorting and winnowing mechanisms similar to those already discussed 

for the Ojito Creek and for the intracaldera La Jara Canyon tuffs. The andesitic fragments in the bulk-rock 

samples probably also contribute to the relatively mafic 

bulk compositions of the Ra Jadero Member, although 

abundant xenoliths were avoided in selecting material 

for analysis. In any case, the relations between bulk-rock 

and pumice compositions of the Ojito Creek and Ra 

Jadero Members show that bulk samples of these units 

do not closely approximate the preeruption magmatic 

compositions. 

AGE AND MAGNETIC POLARITY 

The Ra Jadero Member has not been directly dated, 

but its age is bracketed by radiometric dates of 29.8 and 

29.1 m.y., by use of the same reasoning as already 

presented in discussing the age of the Ojito Creek 

Member. TheRa Jadero Member has reverse remanent 

magnetic polarity. 

UPPER TUFF 

The upper tuff is the most restricted and least studied 

unit of the Treasure Mountain. It is present mainly 

north of the Platoro caldera, where it includes a distinc-. 

tive densely welded phenocryst-rich ash-flow sheet and 

minor less welded local tuffs. Relatively thin partly· 

welded tuffs at the top of the Treasure Mountain Tuff 

near Wolf Creek Pass and south of Del Norte are also 

probably parts of the upper tuff, although poor exposures permit interpretation of these rocks as weakly 

welded upper parts of the Ra Jadero Member. The upper· 
ASH-FLOW .SHEETS 41 

tuff everywhere rests on the Ra Jadero Member and is 

overlain by Masonic Park Tuff. Correlatives of the upper 

tuff have not been recognized south of the caldera complex. 

The distinctive phenocryst-rich ash-flow sheet of the 

upper tuff is 20-40 m thick where best exposed-at 

Bennett Peak just within the Platoro caldera, and on the 

slopes of Del Norte Peak outside the caldera to the north. 

At these localities, it is mainly densely welded red-brown 

devitrified tuff, with a conspicuous black vitrophyre 

zone near its base. It contains about 25 percent 

phenocrysts of plagioclase, sanidine, biotite, and augite. 

In places, it resembles the La Jara Canyon Member in 

hand specimen, but the higher ratio of sanidine to 

plagioclase-as great as 1:2 (fig. 12A)-is diagnostic. 

A densely welded tuff sheet that is about 20m thick on 

the ridge between Burro and Pinos Creeks (Platoro 

map), apparently sandwiched between lavas of Summitville Andesite just within the Platoro caldera, is distinguished by abundant tabular plagioclase phenocrysts, 

accompanied by biotite and augite, but is entirely lacking in sanidine. Numerous separate ash flows are present 

within the unit, as indicated by conspicuous partings 

well defined by concentrations of pumices and andesitic 

lithic fragments. This unit has not been identified 

elsewhere, and its inclusion in the upper tuff is tentative. 

YOUNGER ASH-FLOW SHEETS AND INTERLA YERED LA VAS 

Overlying the Treasure Mountain Tuff over much of 

its extent is a sequence of younger ash-flow sheets that 

were erupted in late Oligocene time from calderas in the 

central part of the San Juan field. Although not directly 

related to the evolution of the Platoro caldera complex, 

these ash-flow sheets are described briefly because (1) 

variations in their distribution and thickness provide 

much information on the changing topography resulting 

from intermittent late volcanic activity in the Platoro 

caldera area, and (2) petrologic features of the younger 

sheets permit illuminating contrasts and comparisons 

with the Treasure Mountain Tuff. General petrologic 

features of these ash-flow sheets-the Masonic Park, 

Fish Canyon, Carpenter Ridge, Wason Park, . and 

Snowshoe Mountain Tuffs-are summarized in table 3, 

and available chemical analyses are listed in table 6. 

Lava flows interlayered with these ash-flow sheets are 

also discussed briefly. 

MASONIC PARK TUFF 

The Masonic Park Tuff is named here for the great 

palisade exposures at the type area along the Rio Grande 

valley near Masonic Park, 5 km northwest of South Fork 

(fig. 26). It consists· of two large ash-flow sheets of 

phenocryst-rich quartz latite that originally spread over 

much of the southeastern San Jua:r;t Mountains. The 

Masonic Park Tuff, generally . mapped by Larsen and 

Cross (1956) at the top of their Treasure Mountain 

Rhyolite; is separated from the Treasure Mountain Tuff 

and renamed here, because it is believed to have been 

erupted from a different source, the Mount Hope 

caldera (fig. 26). Recently, this unit has been informally 

called the tuff of :tylasonic Park (Lipman and others, 

1970; Steven and Lipman, 1973). 

The two ash-flow sheets of the Masonic Park Tuff, 

designated as the lower and upper mem hers, seem 

petrographically indistinguishable. Near the Mount Hope 

caldera they interfinger complexly with penecontemporaneous lavas and breccias of the Sheep Mountain 

Andesite and other lava units (Steven and 

Lipman, 1973). Away from this caldera, the Masonic 

Park Tuff generally overlies upper units of the Treasure 

Mountain Tuff and is overlain by the Fish Canyon Tuff. 

South of an east-west zone running approximately from 

Summitville to Green Ridge where the Fish Canyon 

seems never to have been deposited, the Masonic Park is 

overlain by volcaniclastic sedimentary rocks of the Los 

Pinos Formation or by basalt of the Hinsdale Formation. 

DISTRIBUTION AND THICKNESS 

The lower member of the Masonic Park Tuff has been 

identified only west and southwest of the Mount Hope 

caldera; it crops out in the West Fork of the San Juan 

River and in Goose Creek (fig. 26) and has also been 

traced 25 km west of the mapped area, as far as Huerto 

Creek (Durango map). In these places the lower member 

is typically 30-50 m thick; its maximum thickness, in 

Goose Creek, is about 150 m. 

The upper member of the Masonic Park overlies the 

lower member in the West Fork of the San Juan River, 

where it is generally about 50-75 m thick, but in places it 

wedges out against the Sheep Mo~ntain Andesite. 

Where the upper member occurs above the lower 

member in Goose Creek, it is as much as 300m thick, 

and at Masonic Park the upper member reaches its 

maximum known thickness-400 m, with no base exposed (fig. 26). East and southeast of the Mount Hope 

caldera only the upper member is present; it extends 

east as far as the San Luis Valley and south about 25 km 

into New Mexico (Butler, 1971). Although these exposures are discontinuous (fig. 26), only one ash-flow 

sheet of Masonic Park lithology is present, and there 

seems little doubt of its identity as the upper member. 

Over this vast area the upper member forms a relatively 

uniform thin sheet of partly welded tuff 15-50 m thick. 

The Masonic Park Tuff is not present within the 

Platoro caldera or high on its outer slopes. Although the 

distribution of the Masonic Park shown in figure 26, is in 

part conJectural, depositional wedging of the Masonic 

·Pa:rk on the flanks of the Platoro caldera complex is 

clearly evident from mapped relations northwest of 

Summitvi~le, ·in the Del Norte Peak area, near Summit 
42 PLATORO CALDERA COMPLEX, SAN JUAN MOUNTAINS, COLORADO 

TABLE G.-Analyses of the 

[Sample localities shown in figs. 26, 30, and 31 and (or) described below. Analyses 8, 11, and 20 are from Larsen and Cross (1956, table 21); analyses 

from unpublished data provided by T. A. Steven and J. C. Ratte. Major-oxide analyses 1, 5, 9, 10, 12, 13, 15-17, and 19 are by rapid methods 

methods are No. 2 by F. H. Neuerburg, No. 3 by V. C. Smith, and Nos. 4, 6, and 7 by C. L. Parker. Minor-element analyses 4, 6, 

by Norma Rait.>, more than; <, less than] 

Masonic Fish Canyon Tuff Park 

Tuff Intra-caldera (La Garita '"!ember) Outflow 

Sample---------

Field No-------

Lab. No--------

1 

65L-127 

2" 

C-987 

C-1165 

3 

s- 352-A 

0100281 

4 

S-292-C 

0100011 

5 

C-95 

151485 

6 

C-162-B 

0100009 

7 

S-292-B 

0100010 

8 9 10 

L+C n-37 OS-34-A 65L-80 

W-168465 W-168466 

Major oxides (weight percent), recalculated without H20 and C02 

Si02----------- 68.2 66.00 66.38 66.56 66.6 67.04 67.42 65.86 66.6 66.8 

Al20 3---------- 16.4 16.20 .15.87 15.93 16.1 15.77 15.41 15.81 16.3 17 .o Fe2o3---------- 2. 6 4.00 3.07 3. 92 4.0 2. 39 3. 75 3.51 3.4 3.9 FeO------------ 0 70 .28 1. 28 • 29 0 20 1. 61 . 27 .95 .61 .08 MgO------------ 0 76 1.14 1.48 1. 27 .94 1.10 1.20 1.43 1.2 • 55 

CaO--_: _________ 2. 8 3.10 3. 39 3. 35 3.4 3.05 3.41 3. 73 3.4 3.4 Na 2o----------- 3.4 3.27 3. 75 3.89 3.9 3. 52 3. 77 3.65 3.6 3.6 K20------------ 4. 7 5.17 3.92 3. 97 4.0 4. 66 3.97 4.11 3.8 3. 7 nor---------- .49 0 54 .56 0 52 0 53 .56 . 51 .71 .57 .53 

P 20 s----------- .00 .21 . 21 .20 0 21 . 20 .20 .23 .18 .21 

MnO------------ .10 .09 .08 .10 .06 .09 .09 ----- 0 22 .10 

Total----- 100.14 100.00 99.99 100.00 ----- 99.99 100.00 99.99 99.88 99.86 H2CH----------- 0 69 .88 . 34 . 30 0 48 . 33 .22 .57 .48 .18 

H20- ---------- .71 • 98 .84 • 36 • 23 .15 • 31 .81 .41 0 31 COr----------- <.05 1.10 .04 .02 .10 .01 . 23 ----- .15 <.05 

Norms (weight percent) 

Quartz---------

Orthoclase-----

Albite---------

Anor thi te------

Corundum-------

Wollastonite---

Enstatite------

Ferrosilite----

Magnet i te------

Hema ti te-------

Ilmenite-------

Ti tani te-------

Rutile---------

Apatite--------

23.45 

27.8 

28.6 

13.7 

0 7 

1.9 

1.15 

1.8 

0 93 

19.4 

30.6 

27.7 

14.05 

.07 

2.8 

3. 99 

.8 

.12 

.49 

20.8 

23.16 

31.7 

14.89 

• 22 

3. 7 

2.8 

1.15 

1.06 

0 50 

20.51 

23.43 

32.86 

14.3 

• 29 

3.16 

3. 9 

.8 

0 21 

0 5 

26.65 

23.67 

33.03 

14.6 

.01 

2. 34 

4.0 

0 55 

. 59 

.49 

21.13 

27 0 52 

29.76 

13.48 

.15 

2. 74 

. 22 

3.46 

1.07 

.48 

22.25 

23.47 

31.89 

13.39 

.77 

2. 99 

3. 75 

.77 

0 24 

.48 

19.96 

24.27 

30.88 

14.63 

.98 

3. 56 

1.01 

2.81 

1. 35 

0 55 

22.0 

22.71 

30.81 

15.87 

.32 

3.02 

1.04 

2.72 

1.08 

.43 

23.78 

22.05 

30.72 

15.62 

1. 30 

1. 38 

3.93 

. 39 

0 33 

. 50 

Minor elements (parts per million) 

11--------------

Ba-------------

Co-------------

Cr-------------

Cu-------------

Ga-------------

Ni-------------

Pb-------------

Sc-------------

Sn-------------

Sr-------------

V--------------

Y--------------

Yb-------------

Zr-------------

1400 

<4 

2 

17 

30 

<4 

340 

5 

40 

360 

60 

30 

320 

1000 

8 

3 

4 

10 

7 

30 

10 

0 400 

80 

40 

3 

200 

2000 

7 

5 

20 

30 

5 

20 

10 

<10 

150 

100 

30 

3 

150 

930 

5 

5 

12 

20 

97 

10 

660 

7Q 

28 

3 

220 

1500 

7 

7 

15 

30 

<3 

20 

10 

<10 

150 

100 

30 

3 

150 

1500 

7 

5 

15 

30 

<3 

20 

10 

<10 

150 

100 

30 

3 

100 

<20 

1200 

7 

5 

24 

20 

7 

40 

12 

320 

40 

140 

<20 

1300 

6 

5 

18 

20 

8 

30 

10 

280 

40 

220 

Phenocrysts (volume percent) 

Plagioclase----

Sanidine-------

Quartz---------

Biotite--------

Augite---------

Hornblende-----

Sphene---------

Opaques--------

Groundmass-----

32.6 

4. 3 

1.7 

1.0 

60.4 

30.0 

6. 5 

3.9 

4.0 

3.4 

.1 

2. 4 

49.7 

33.8 

4. 7 

4. 2 

4.1 

2. 5 

• 4 

0 9 

48.8 

28.0 

4. 2 

3.1 

6. 5 

2.4 

• 2 

2. 9 

52.7 

29.0 

5. 5 

4. 7 

3. 9 

2.9 

.1 

1.9 

52.0 

30 

4 

2 

4 

Tr . 

2 

55 

27 .o 

6.0 

3.0 

5.0 

3.0 

2.0 

54.0 

20 

5 

2 

4 

2 

0 5 

2 

64.5 

SAMPLE DESCRIPTIONS 

1. Devitrified brown densely welded tuff. At about 10,600 ft, on slope east of Lake Creek (tributary of Wolf Creek). (See fig. 26.) 2. Densely welded devitrified La Garita Member. Dump of Outlet Tunnel, East Willow Creek, Creede district (beyond area of fig. 30). 3. Densely welded devitrified La Garita Member. North side of Cochetopa Creek, at about 10,800 ft, 0.7 km northeast of junction with Canyon Diablo (beyond area of fig. 30). 4. Bulk sample of densely welded La Garita Member. (For analysis of pumice fragment, see No. 7.) At about 12,900 ft, on cirque 

headwall, 0.5 km southeast of 13, 718-ft bench mark "La Garita" along Continental Divide (beyond area of fig. 30). 5. Densely welded devitrified La Garita Member. At about 10,100 ft, 

along East Willow Creek (beyond area of fig. 30). 6. Densely welded devitrified Phoenix Park Member. At about 10,000 

ft, on ridge east of Phoenix Park (beyond area of fig. :JU). 7. Devitrified pumice fragment from La Garita Member. Same sample and locality as No. 4. 8. Pale-red-brown devitrified Fish Canyon Tuff. "Alder Creek" (Larsen and Cross, 1956, table 21, No. 37). (See fig. 30.) 9. Light-brown densely welded devitrified Fish Canyon Tuff from near base of unit. On 8,023-ft hill, 2.6 km southeast of East Butte. (See 

fig. 30.) 10. Light-gray densely welded devitrified Fish Canyon Tuff, from thick accumulation within the Mount Hope caldera. At about 10,850 ft, in gully north of Archuleta Creek, 3.3 km southeast of Mount Hope. 

(see fig. 30.) 11. Gray densely welded Fish Canyon Tuff. "San Cristobal quadrangle; mouth of canyon of Texas Creek, at altitude of 9,050 feet" (Larsen 
ASH-FLOW SHEETS 

younger ash-flow sheets 

14 and 18 ore from Olson, Hedlund, and Hansen (1968, table 1); analysis 13 is from unpublished data provided by W. R. Hansen: analyses 2-7 are 

by P. L. D. Elmore, Samuel Botts, Gillison Chloe, Lowell Artis, H. Smith, John Glenn, and James Kelsey. Major-oxide analyses by standard 

and 7 ore by semiquantitative methods by J. C. Hamilton; analyses by quantitative methods are 9, 10, and 12 by J. D. Fletcher and 15-17 

Fish Canyon Tuff--Continued 

Carpenter Ridge Tuff 

Out f1ow--Cont inued 

11 

L+C 21-38 

12 

66L-96-8 

W-168467 

13 86-62-39 

14 

G-2384 

152868 

15 

67L-133-G 

W-174427 

16 

65L-43-B 

W-168540 

17 

65L-33-B 

W-168541 

18 

G-3474 

152866 

19 

65L-4 7 

w-168539 

20 

L+C 21-47 

Sample 

Field No. 

Lab. No. 

Major oxides (weight percent), recalculated without H21l and co2--Continued 

66.93 68.7 70.2 70.6 60.6 66.2 71.6 71.9 .73.8 74.80 Si02 

15.58 16.3 14.2 16.15 20.0 17.0 15.2 15.1 14.3 13.51 Al203 2. 52 2. 3 3. 6 1.7 ). 2 3. 3 2. 2 1.4 1.0 .98 Fe2o3 1.14 1.2 . 36 .16 1.2 • 33 .16 . 23 .45 • 33 FeO 

. 86 . 69 1.4 • 26 1.3 . 90 .44 . 26 .44 ----- MgO 

3. 74 3.0 2. 7 1.4 5. 2 3.5 1.9 1.2 1.5 .77 CaO 

3. 68 3. 4 2. 9 4.1 3.8 3. 7 3. 2 4.0 2.8 3. 76 Na 20 

4. 63 3. 4 4.1 4. 9 ). 5 4.4 4.9 5.4 5.4 5.65 K20 

.50 .44 .40 . 3 .60 .44 . 30 . 23 • 25 .20 Ti02 

• 36 . 20 .19 .08 • 41 .15 .06 .08 .oo ----- P205 

. 06 .07 .09 .08 .os .14 .00 .08 .07 ----- MnO 

100.00 100.00 100.13 99.93 99.89 100.06 99.96 99.88 100.01 100.00 Total 

• 56 • 57 1.3 . 90 . 69 >. 45 . 74 .44 H20+ 

. 66 • 33 >1. 6 >. 78 1.7 1.2 . 91 . 96 .18 H20-

----- <.05 . 05 .06 <.05 <.05 <,05 .16 <.05 ----- C02 

Norms (weight percent)--Continued 

20.04 

27.34 

31.12 

12.32 

1.61 

2.13 

2. 40 

.86 

• 96 

.86 

27.53 

19.84 

30.99 

13.81 

1. 56 

1. 72 

2. 90 

. 34 

. 83 

.48 

30.40 

24.15 

24.46 

12.11 

• 54 

3.47 

2. 94 

3. 38 

• 76 

. 45 

24.9 

29.0 

34.8 

6.4 

1.7 

• 65 

1.7 

. 51 

.03 

.19 

13.05 

20.74 

32.32 

22.91 

1. 55 

). 34 

2. 53 

1.46 

1.14 

• 98 

19.66 

25.98 

31.14 

16.24 

• 20 

2.24 

• 25 

3.10 

.83 

• 36 

29.83 

28.92 

26.74 

9. 21 

1. 32 

1.09 

2. 24 

.45 

.06 

.14 

24.94 

32.13 

34.07 

5.4 7 

• 59 

.65 

• 34 

1.18 

.44 

.19 

33.19 

31.93 

23. 29' 

7. 59 

1.12 

1.09 

.94 

. 37 

.48 

29.79 

33.41 

31.77 

3. 31 

. 21 

.49 

.64 

• 38 

Quartz 

Orthoclase 

Albite 

Anorthite 

Corundum 

Wollastonite 

Enstatite 

Ferrosilite 

Magnetite 

Hematite 

Ilmenite 

Titanite 

Rutile 

Apatite 

Minor elements (parts per million)--Continued 

<20 

1200 

6 

4 

24 

20 

7 

40 

8 

260 

30 

190 

13,000 

<4 

4 

10 

20 

8 

10 

1000 

40 

<40 

2 

960 

4400 

<4 

4 

13 

20 

<4 

50 

8 

<20 

490 

60 

<20 

400 

2400 

<4 

2 

8 

20 

<4 

90 

6 

<20 

140 

<20 

30 

330 

680 

<4 

2 

8 

20 

<4 

60 

5 

<20 

56 

<20 

30 

220 

8 

Ba 

Co 

Cr 

Cu 

Ga 

Ni 

Pb 

Sc 

Sn 

Sr 

v 

y 

:tb 

Zr 

Phenocrysts (volume percent)--Continued 

18.0 

6.0 

3.0 

3.0 

3.0 

• 5 

1.0 

65.0 

20 

5 

1.5 

4 

2 

• 5 

2 

65 

19.9 

4. 8 

3. 5 

2. 7 

2. 3 

.1 

1.1 

65.5 

28.0 

2. 4 

.o 

1.4 

.1 

1.3 

66.0 

27.9 

2.4 

3.9 

1.4 

1.5 

62.9 

11.6 

4.4 

1.4 

.6 

.4 

81.6 

4.9 

4.8 

.6 

. 3 

89.4 

5. 5 

2. 5 

• 3 

• 3 

• 2 

91.0 

o. 2 

4. 7 

• 2 

• 2 

94.1 

4.0 

2. 5 

• 5 

.5 

94.0 

Plagioclase 

Sanidine 

Quartz 

Biotite 

Augite 

Hornblende 

Sphene 

Opaques 

Groundmass 

SAMPLE DESCRIPTIONS-Continued 

and Cross, 1956, table 21, No. 39; beyond area of fig. 30). 16. 12. Dark-gray cognate scorioceous pumice block. Roodcut along U.S. Dark-brown densely welded devitrified mafic top of ash-flow sheet (fig. 33B). Some locality as No. 15. Highway 160 along Pass Creek, at about 9,000 ft. (see fig. 30.) 17. 13. Densely welded devitrified light-gray Fish Canyon Tuff. Anomalous- ly high in Fe20a and MgO and low in both alkalis. Sopinero 

9uodrangle: NEV.SWV. sec. 28, T. 48 N., R. 4 W. (beyond area of 

Light-red-brown densely welded devitrified tuff from lower part of unit, just above basal vitrophyre zone. Some locality as No. 15, at about 9,250 ft. 

fig. 30). 14. Massive light-brown devitrificd Fish Canyon Tuff. Gateview quadrangle SEV.SEV. sec. 10, T. 47 N., R. 3 W. (Olson and others, 

1968, tnble I, No. II· beyond oren of fig. 30). 15. Dnrk-grny scorin block (fig. 33C) from upper port of ash-flow sheet. (For 1111nlysis of bulk rock, sec No. 16.) Mensured section on west. slope of Benvcr Mountain, nbout 0.6 km northeast of Benver Creek Reservoir spillway, at nbout 9,500 ft. (See fig. 31.) 

18. Cinnamon-brown devitrified welded tuff. Goteview quadrangle, NEV. SE V. sec. 2, T. 46 N ., R. 3 W. (Olson and others, 1968, table 1, No. 12; beyond area of fig. 31). 

19, Pale-purple densely welded devitrified tuff from lower part of unit. North side of upper Pork Creek, on 10,500-ft knob, 0.9 km northeast of junction with West Fork. (See fig. 31.) 20. Li~ht-brown devitrified welded tuff. "San Cristobal quadrangle, 

ndge west of Bennett Creek at altitude of 11,180 feet" (Larsen and Cross, 1956, table 21, No. 47; beyond area of fig. 31). 

43 
44 PLATORO CALDERA COMPLEX, SAN JUAN MOUNTAINS, COLORADO 

I06•J5' 

Monte 

Vistao ,.. 

: 

::;~""''"''~ ·t" 

;"'~<'It" Blackhead 

.f,,,""''~,~ Peak)( ~,,.., .. ~ 

.,, 

~~ .... /';_~ ~./ ... ":.... . 

~w,t<),, ... f .) 

,,,<c 

\ 

' ~0 , ...... '"""''"'•'' 

~"? ( 

">)-...... "\, .. , Navajo~ 

Peak ~ x ::}'1: ..... ,,,.,,.ot 

0 5 10 15 20 MILES 

0 5 10 15 20 KILOMETERS 

EXPLANATION 

Outcrops shown by darker pattern, inferred original extent by lighter pattern. 

Younger volcanic rocks 

Masonic Park Tuff 

Sample locality (table 6) 

Inferred original depositional 

margins - Dashed where appro.ximate; queried where uncertain 

Limit of exposed volcanic rocks 

Caldera wall (topographic) - Dashed 

where covered by younger rocks 

c::. 

106·oo· 

FIGURE 26. -Outcrop area and inferred original depositional extent of the Masonic Park Tuff. Based on mapping by P. W. Lipman, 1965-67, 1970-

71, and by T. A. Steven, 1965-66. 
ASH-FLOW SHEETS 45 

Peak, and at the head of Rio Chama. Although the 

Masonic Park Tuff in the southeastern San Juan Mountains is therefore rather similar in distribution to the 

Treasure Mountain Tuff, it extends much farther to the 

northwest, where it also accumulated to maximum 

thickness. The evidence for a source of the Masonic Park 

in the Mount Hope caldera seems compelling despite the 

somewhat eccentric distribution, as discussed in a later 

section. The Masonic Park therefore appears to have 

spread widely to the south and east on the flat plains underlain by Treasure Mountain units, but it lacked the 

momentum to ride up the outer slopes of the Platoro 

caldera area, which appears to have remained as a 

topographic high throughout the Oligocene ash-flow activity. In addition, volcanoes of Summitville Andesite 

may have presented barriers to the Masonic Park ash 

flows northwest of Platoro caldera, as indicated by the 

wedging out of these tuffs against andesite near the head 

of Park Creek (Platoro map). 

LITHOLOGIC DESCRIPTION 

The Masonic Park is petrologically uniform 

phenocryst-rich quartz latite, varying mainly in degree 

of welding. Where it is thickest, in the Masonic Park 

area, its cooling zonation is conspicuously compound-three major cliff-forming zones of red-brown 

densely welded tuff are separated by benches of less 

welded yellowish-brown tuff (fig. 27). In detail the zonation is more intricate, and many separate ash flows are 

clearly present. In contrast, the thinner sections of 

Masonic Park Tuff, along the edge of the San Luis Valley 

and elsewhere in the southern part of the volcanic field, 

generally approximate a simple cooling zonation, with 

the tuff only moderately densely welded in the interior of 

the sheet. In these areas the slight degree of welding of 

the Masonic Park results in great cavernous-weathering 

outcrops. Compound cooling is evident locally in this 

region, however, as along the Cat Creek road at the east 

edge of the Platoro map. Where basal parts of the sheet 

are exposed, welding has been insufficent to form a 

vitrophyre zone, and vapor-phase crystallization is 

typically well developed throughout the unit. 

Over most of its extent the Masonic Park Tuff contains 40-50 percent phenocrysts (table 3), mostly 

plagioclase but accompanied by conspicuous biotite and 

augite. Sanidine is absent or extremely sparse; quartz 

and hornblende are absent. In distal portions of the 

sheet, especially near the Colorado-New Mexico line, 

total phenocryst content is lower-20-30 percent in the 

interior of the sheet-and at the top of the unit a winnowed shard-rich tuff contains only about 10 percent 

small fragmental phenocrysts (fig. 28). Small fragments 

of gray or red-brown andesite a few centimeters across 

are abundant and make up as much as 5 percent of the 

rock. These are especially conspicuous in the less welded 

zones. 

FIGURE 27.-Ash-flow sheets along the Rio Grande valley near Masonic Park. In foreground are Masonic Park Tuff (Tmp) overlain by Fish Canyon Tuff (Tfc) near the east wall of La Garita caldera. Up the valley, within the caldera, are Carpenter Ridge Tuff (Tcr), Mammoth Mountain Tuff (Tm) , and Wason Park Tuff (Twp) t hat form the caldera fill. The compound cooling zonation of the Masonic Park Tuff is reflected 

by the alternation of benches and cliffs within this unit. 
46 PLATORO CALDERA COMPLEX, SAN JUAN MOUNTAINS, COLORADO 

' 30 I . I . "" "- I I 

·'\. I· I • (f) \. a: ·I • I \. w 

~ rw \ r :;: 20 

. \ I ~ I • I w· I I (f) 

I I I <{ 

ttl 

> 

w • I ~ I· 0 I I I ttl 

<{ 

r- 10 I • I· • I J: I I I l9 

w 

J: .I •I . I . I I . I 

I I I. 

0 

10 20 30 0.00 O.D1 0 .02 0 10 

TOTAL PHENOCRYSTS., 

IN VOLUME PERCENT 

SANIDINE:PLAGIOCLASE RATIO BIOTITE, IN PERCENT 

OF TOTAL PHENOCRYSTS 

FIGURE 28.- Phenocryst variations in a section of the Masonic Park 

Tuff along Rio de los Pinos in New Mexico. Trends are shown by 

dashed lines. Near the top of the sheet, total phenocrysts (and 

average diameter of crystals) decrease, whereas proportions of 

phenocrysts remain fairly constant. This relation probably results 

from winnowing effects in far-traveled ash flows (here about 70 km 

from the caldera rim), with larger phenocrysts settling preferentially 

near the source, and fine ash and broken small phenocrysts concentrating in the moving ash flow (Lipman, 1967). Similar vertical 

variations, probably due to winnowing, also characterize the Wason 

Park Tuff (Ratte and Steven, 1967, fig. 17). 

The single chemical analysis of the Masonic Park Tuff 

(table 6, No. 1) shows about 68 percent SiOz. This sample contains fewer phenocrysts than average; a more 

typical phenocryst-rich sample might be slightly less 

silicic. However, the general petrologic uniformity of the 

Masonic Park suggests that any chemical variation is 

likely to be small and to be due more to sorting 

phenomena during eruption and emplacement (Fisher, 

1966; Lipman, 1967, p. 321-322) than to compositional 

variations in the source magma. 

The Masonic Park Tuff is generally similar in 

petrography to the La Jara Canyon Member of the 

Treasure Mountain but typically is more phenocryst rich 

and contains more augite. The augite tends to be 

replaced by carbonate and clay minerals, even where the 

rest of the rock seems unaltered. 

In a few places south and west of the Mount Hope 

caldera-most notably northwest of Wolf Creek Pass and 

in upper Turkey Creek-crude bedding, defined by 

variations in size and abundance of pumices and 

phenocrysts, occurs within the welded lower few meters 

of the upper member (fig. 29). This bedded welded tuff is 

believed to represent agglutinated near-source ash fall, a 

depositional process previously described for similar 

rocks of the lower tuff of the Treasure Mountain near the 

Platoro caldera rim. 

The occurrence of bedded welded tuff of the Masonic 

Park near the rim of Mount Hope caldera is one of the 

criteria that indicate eruption of this unit from the 

Mount Hope caldera. Other evidence leading to this interpretation are : (1) the central position of the caldera , 

adjacent to the thickest accumulation of Masonic Park 

Tuff (fig. 26), (2) the intertonguing of Masonic Park Tuff 

with Sheep Mountain Andesite and other locally derived 

lavas near the rim of Mount Hope caldera , and (3) the 

stratigraphic constraints on the age of the caldera- in 

particular, the Mount Hope caldera must have formed 

just before eruption of the Fish Canyon Tuff (from the 

La Garita caldera), because it is filled by this tuff 

FIGURE 29.- Crudely bedded welded tuff near the base of the upper 

member of the Masonic Park Tuff, believed to represent near-source 

agglutinated ash fall. Bedding is defined by various sizes and abundance of pumice fragments and phenocrysts. Base of ash-flow sheet 

is indicated by arrows; note person (circled) for scale. 
ASH-FLOW SHEETS 47 

AGE AND MAGNETIC POLARITY 

A single K-Ar age determined on biotite from the 

Masonic Park Tuff is 28.2 m.y. (Armstrong, 1969; Lipman and others, 1970), in excellent agreement with 

radiometric ages of underlying and overlying units. The 

remanent magnetic polarity determined in the field and 

in the laboratory seems to be everywhere reverse; low 

magnetic intensities, however, did complicate field interpretation of results from some weakly welded outcrops. 

FISH CANYON TUFF 

The Fish Canyon Tuff, a phenocryst-rich quartz latitic 

ash-flow sheet named by Olson, Hedlund, and Hansen 

(1968), is the most widespread (at least 15,000 km2) and 

voluminous (more than 3,000 km3) single ash-flow sheet 

in the San Juan volcanic field, and it ranks among the 

largest in the world (Smith, 1960a, fig. 3). The Fish Canyon was erupted about 27.8 m.y. ago (Steven and 

others, 1967; Lipman and others, 1970) from the great La 

Garita caldera (fig. 1), which is 35-40 km in diameter. 

Collapse of the La Garita caldera during the ash-flow 

eruptions permitted thick intracaldera accumulation of 

these tuffs, resulting in a unit analogous to the thick 

intracaldera-thin outflow association of the La Jara Canyon Member of the Treasure Mountain. The thick ashflow sheet within the La Garita caldera, called the La 

Garita Quartz Latite (Steven and Ratte, 1964), is now 

known to be the intracaldera equivalent of the Fish Canyon Tuff and has been redefined as the La Garita 

Member of the Fish Canyon Tuff (Steven, Lipman, and 

Olson, 1974). 

The Fish Canyon Tuff extends from north of the Gunnison River (Olson and others, 1968) southwest as far as 

the flanks of the Needle Mountains (Durango map) and 

southeast to Green Ridge (fig. 30). Within the La Garita 

caldera the La Garita Member is exposed to a thickness 

of nearly 1 km, with the top of the section eroded and the 

base exposed only around the margins of the caldera 

(Steven and Ratte, 1965). 

Only the southeastern part of the vast area covered by 

the Fish Canybn is within the present mapped area (fig. 

30). In the southeastern San Juan Mountains the Fish Canyon, with only local exceptions near its distal edges, 

rests on Masonic Park Tuff and is overlain by Carpenter 

Ridge Tuff. In places Huerto Andesite intervenes 

between the Fish Canyon and the Carpenter Ridge, and 

the Fish Canyon near its distal edges locally rests on 

units older than the Masonic Park, especially near 

topographically high parts of the Platoro caldera complex. · 

In the southeastern San Juan Mountains the Fish Canyon is much less extensive than the underlying Masonic 

Park Tuff, apparently because it was blocked by growing 

volcanoes around the north margin of the Platoro caldera 

complex. The Fish Canyon Tuff wedges out slightly higher 

on the outer north slopes of the Platoro caldera complex 

than does the Masonic Park (fig. 26, 30). Unlike the 

Masonic Park, some Fish Canyon Tuff accumulated 

within the caldera complex, in the low northeastern part 

of the Platoro caldera moat that had previously been 

partly filled with upper units of the Treasure Mountain 

Tuff (figs. 23, 25). Only a single small patch of Fish Canyon Tuff is preserved south or east of the caldera complex, in a local erosional valley cut in Masonic Park Tuff 

(fig. 30), and it seems unlikely that any sizable volume of 

the Fish Canyon was ever deposited in this region. The 

main area of Fish Canyon along the San Luis Valley 

thins and wedges out north of Green Ridge, probably 

reflecting the growing accumulation of the volcanics of 

Green Ridge around the volcano centered at Cat Creek 

(described later). Similarly, northwest of the Platoro 

caldera, the Fish Canyon wedges out against a calderamargin accumulation of the upper member· of the Summitville Andesite, against which the Masonic Park and 

the Ra Jadero Member of the Treasure Mountain had 

earlier wedged out even farther from the caldera rim. 

The maximum thickness of the Fish Canyon Tuff in 

the southeastern San Juan Mountains is in the 

topographic depression of the previously formed Mount 

Hope caldera (fig. 30), in the headwaters of the South 

Fork of the Rio Grande, where the tuff forms a single 

thick cooling unit continuously exposed from the valley 

bottom, at an elevation near 9,000 feet, to the top of 

Mount Hope, at 12,834 feet. Even with due allowance for 

a gentle northeastern dip, the exposed Fish Canyon here 

is nearly 1 km thick, even though the top of the ash-flow 

sheet has been eroded and the base is nowhere exposed 

within the caldera. The Fish Canyon Tuff thins abruptly 

across the Mount Hope caldera walls, and outside this 

caldera it is typically 20-200 m thick. Exceptional 

thicknesses of the Fish Canyon, as much as 500 m, also 

accumulated in downfaulted blocks at the head of the 

West Fork of the San Juan River (Steven and others, 

1969). 

The Fish Canyon Tuff is a phenocryst-rich quartz 

latite (table 6) that petrographically is perhaps the most 

distinctive ash-flow unit in the San Juan field. It contains 35-50 percent phenocrysts, including plagioclase, 

sanidine, quartz, biotite, hornblende, and accessory 

sphene. No other major ash-flow sheet in the field contains hornblende (and lacks augite) as the second most 

abundant mafic after biotite. The quartz phenocrysts, 

which are very rare in or absent from other quartz latitic 

tuffs of the field, are also distinctive: they occur as 

rounded pale amethystene pellets as much as 5 mm in 

diameter that are riddled with wormy glass inclusions 

whose texture is caused by resorption. The sanidine 
48 PLATORO CALDERA COMPLEX, SAN JUAN MOUNTAINS, COLORADO 

0 5 10 15 20 MILES 

0 5 10 15 20 KILOMETERS 

EXPLANATION 

Outcrops shown by darker pattern, inferred original extent by lighter pattern 

GIJ Younger volcanic rocks Inferred original depositional marginsDashed where approximate; queried 

10 

Fish Canyon Tuff where uncertain 

Sample locality (table 6) 

""'""'""""'" Limit of exposed volcanic rocks 

Caldera wall (topographic) - Dashed 

where covered by younger rocks 

106'00' 

FIGURE 30.- 0utcrop area and inferred original depositional extent of the Fish Canyon Tuff. Based on mapping by P. W. Lipman, 1965-67, 1970-

71, and by T . A. Steven, 1965-66. 
ASH-FLOW SHEETS 49 

phenocrysts, which are exceptionally common in comparison with other quartz latitic tuffs of the San Juan 

field, are relatively potassic-X-ray and microprobe 

data for eight samples indicate compositions between 

Or67 and Or76· Finally, the presence of abundant 

accessory sphene is unique. 

Analyses of all the bulk-rock samples of the Fish Canyon from the southeastern and central San Juan Mountains, including those from the intracaldera tuffs, cluster 

between 66 and 68 percent Si02; one sample of a cognate 

block is slightly higher in Si02 (table 6, No. 12). Two 

samples from the Gunnison River area, near the north 

edge of the ash-flow sheet, contain more Si02-as much 

as 70.6 percent (table 6, Nos. 13, 14)-possibly owing to 

winnowing of mafic phenocrysts during emplacement. 

The Fish Canyon has normal remanent magnetic 

polarity (table 3). 

CARPENTER RIDGE TUFF 

The Carpenter Ridge Tuff, a phenocryst-poor rhyolitic 

ash-f1ow sheet named by Olson and others (1968), is 

probably the second largest ash-flow sheet in the San 

Juan field, surpassed only by the Fish Canyon Tuff. The 

Carpenter Ridge Tuff was erupted from the Bachelor 

caldera (fig. 1); its thick intracaldera equivalent, formerly called the Bachelor Mountain Rhyolite in the Creede 

district (Steven and Ratte, 1965), has been redefined as 

the Bachelor Mountain Member of the Carpenter Ridge 

Tuff (Steven, Lipman, and Olson, 1974). In the 

southeastern San Juan Mountains (fig. 31) the 

Carpenter Ridge Tuff generally rests on Fish Canyon 

Tuff or Huerto Andesite; locally near its distal margins it 

lies on lavas or volcaniclastic sedimentary rocks of the 

Los Pinos Formation, derived from the postcollapse 

volcanoes around the north margin of the Platoro caldera 

complex. Generally, the Carpenter Ridge is overlain by 

the Wason Park Tuff; beyond the area of deposition of 

the Wason Park, the Carpenter Ridge is overlain by the 

Los Pinos Formation or basalt f1ows of the Hinsdale Formation. 

DISTRIBUTION AND THICKNESS 

The Carpenter Ridge Tuff has a distribution very 

similar to that of the Fish Canyon, extending from north 

of the Gunnison River (Olson and others, 1968), 

southwest to the Needle Mountains area (Durango 

map), and southeast at least as far as the north slopes of 

Green Ridge (fig. 31). A small area of thin partly welded 

phenocryst-poor tuff, shown as Carpenter Ridge south of 

the Alamosa River (fig. 31), is of somewhat uncertain 

correlation; although this tuff has an appropriate 

phenocryst assemblage, it lacks the dense welding that 

characterizes the Carpenter Ridge Tuff elsewhere, even 

in similarly thin accumulations. 

The outf1ow sheet of Carpenter Ridge Tuff is thin and 

uniform in thickness, having accumulated to more than 

200-m thickness only in a few areas of rough fault-block 

topography such as at the head of the West Fork of the 

San Juan River (fig. 31). Within the Bachelor caldera, 

however, the equivalent Bachelor Mountain Member is 

more than 1 km thick (Steven and Ratte, 1965, p. 22), 

showing that collapse accompanied at least the later 

stages of eruption of the Carpenter Ridge Tuff. 

The Carpenter Ridge Tuff wedges out around the 

north outer slopes of the Platoro caldera complex, much 

like the preceding Fish Canyon, but slightly farther from 

the caldera rim (figs. 30, 31). Unlike the Fish Canyon 

Tuff, however, the Carpenter Ridge has not been found 

within this caldera. Although no caldera-margin lavas or 

derivative volcaniclastic sedimentary rocks have been 

found between the Fish Canyon and Masonic Park 

Tuffs, by the time of eruption of the Carpenter Ridge the 

caldera-margin volcanoes had grown sufficiently large 

that lavas and detrital material spread far enough to be 

preserved between the Fish Canyon and Carpenter Ridge 

Tuffs, especially north of North Mountain (Platoro map) 

and north of Green Ridge (Durango map). Thus, the contrast in distributions between the Fish Canyon and 

Carpenter Ridge Tuffs (figs. 30, 31) on the one hand, and 

the Masonic Park Tuff (fig. 26) on the other, is clearly 

tied by stratigraphic relations to marginal volcanic activity of the Platoro caldera complex. 

Further local change in the topography is shown by the 

wedging out of the Carpenter Ridge against the Fish Canyon Tuff within the Mount Hope caldera (fig. 31; 

Durango map). This relation demonstrates that the Fish 

Canyon 'ruff within Mount Hope caldera was uplifted 

and tilted between the times of deposition of these two 

units, apparently mainly by movement along a major 

fault up Beaver Creek (fig. 61); the Carpenter Ridge 

appears to wedge out depositionally against this fault 

scarp. In essence this movement represents a weak 

delayed resurgence of the Mount Hope caldera. 

LITHOLOGIC DESCRIPTION 

The Carpenter Ridge Tuff is typically a simple cooling 

unit, consisting mainly of densely welded red-brown tuff, 

almost everywhere having a black basal vitrophyre a few 

meters thick. Sections thicker than about 50 m commonly contain a well-developed central lithophysal zone, in 

which round or elliptical gas cavities average about 5 em 

in diameter; in areas of lithophysal development, the 

welded devitrified tuff is commonly bleached to more 

pastel red, brown, or salmon colors. Where welding 

decreases near the top of the unit, the tuff is light gray 

owing to vapor-phase crystallization. 

The Carpenter Ridge Tuff is readily distinguished 
50 PLATORO CALDERA COMPLEX, SAN JUAN MOUNTAINS, COLORADO 

Osier~ain 

\Y 

0 5 10 15 20 MILES 

0 5 10 15 20 KILOMETERS 

EXPLANATION 

Outcrops shown by darker pattern, inferred original extent by lighter pattern 

Younger volcanic rocks 

Carpenter Ridge Tuff 

Sample locality (table 6) 

Inferred original depositional 

margins - Dashed where approximate ; queried where uncertain 

Limit of exposed volcanic rocks 

Caldera wall (topographic) - Dashed 

where covered by younger rocks 

106"00' 

FIGURE 31.-0utcrop area and inferred original depositional extent of the Carpenter Ridge Tuff. Based on mapping by P. W. Lipman, 1965-67, 

1970-71, and by T. A. Steven, 1965-66. 
ASH-FLOW SHEETS 51 

from all other units in the southeastern part of the San 

Juan field by its abundant sanidine and by its low total 

phenocryst content. Phenocrysts in general constitute 

about 5 percent of the tuff and consist of subequal 

amounts of sanidine (OrGs) and plagioclase (An2s-4o), accompanied by a minor amount of biotite and very sparse 

augite. Pumice fragments are abundant but smal~ the 

compacted lenses rarely being more than 5 em long. 

Small intermediate-composition volcanic fragments are 

present but rare. The typical pheno·cryst-poor Carpenter 

Ridge Tuff contains about 73 percent Si02 (table 6) and 

is the most rhyolitic ash-flow sheet in the southeastern 

San Juan Mountains. 

In the vicinity of Beaver Mountain, the Carpenter 

Ridge Tuff is about 120 m thick and displays atypical 

but extremely interesting variations in petrography and 

chemistry (figs. 32, 33). The lower part of the ash-flow 

sheet is the typical phenocryst-poor rhyolite described 

above (fig. 33A); however, from about 60 m above the 

u. 

u. 

::::> 

1-

w 

(!) 

c 

a: 

a: 

w 

1-

z 

w 

0... 

a: 

<( 

u 

WASON PARK 

TUFF 

Partly welded 

devitrified (vaporphase) tuff 

Densely welded 

devitrified 

phenocryst-poor 

tuff 

0 

0 

0 

·----0 

0---------------· 0 

0· -----· 

0 

0----· 

0 

NonweldedtQPartly 

welded~s~~-----L----~-----L----~ 

FISH CANYON 0 40 0 

0 

0 

0 

base to the top of the sheet, total phenocryst content 

gradually increases from about 5 percent to more than 25 

percent (fig. 33B), the proportion of sanidine decreases, 

augite becomes abundant, and bulk-rock Si02 content 

decreases from about 73 to 66.5 percent. Large dark 

scoriaceous pumices become conspicuous (fig. 33C); 

these contain sparse sanidine, and the single analyzed 

sample has only 60.6 percent Si02 (table 6, No. 15). The 

phenocryst-poor rhyolitic pumices that characterize the 

lower part of the unit persist to the top of this. section, 

however, and occur in the same samples as the dark, 

mafic pumices. The quartz latitic upper part of the 

Carpenter Ridge Tuff in this area is clearly a mixture of 

shards and pumice fragments ranging in composition 

from rhyolite to low-silica rhyodacite. Similar 

mechanical mixing during eruption and emplacement 

characterizes other such compositionally zoned ash-flow 

sheets (Lipman, 1967, p. 320-321). 

Such compositiOnal zonations in large ash-flow 

120 

0 

100 

en 

a: 

w 80 1-

w 0 0 

~-

0 ~ 

w· 

en 

<( 

60 0-------------··· CD --------· 

w 

> .0 ·---------0-------------. CD 

<( 

1-

J: 

40 (!) 

w 0 :;c 

O·• 

20 

0 

TUFF TOTAL PHENOCRYSTS, TOTAL MAFIC PHENOCRYSTS, 

1.5 

SANIDINE:PLAGIOCLASE 

RATIO 

BULK-ROC.K 

IN VOLUME PERCENT IN VOLUME PERCENT 

EXPLANATION 

·------------0 In pumice fragment In groundmass 

Si02 CONTENTS, 

IN WEIGHT PERCENT 

FIGURE 32.-Petrographic variations in a section of the Carpenter Ridge Tuff, southwest slope of Beaver Mountain. Total phenocrysts and mafics 

increase upward, whereas Si02 content and sanidine:plagioclase ratios decrease. Phenocryst contents of uppermost sample are anomalously 

low, owing to lesser welding and possibly also to winnowing effects. Pumices vary widely in phenocryst content and in places are clearly 

different in composition from enclosing groundmass. 
52 PLATORO CALDERA COMPLEX, SAN JUAN MOUNTAINS, COLORADO 

FIGURE 33.-Photomicrographs of Carpenter Ridge Tuff, illustrating 

effects of the compositional zonation in the Beaver Mountain area. 

(See fig. 32.) A, Basal vitrophyre, containing sparse phenocrysts 

(about 5 percent) of sanidine (S), plagioclase (P), and biotite (not 

visible). Note welded shard texture; dark round spot on left side is a 

small devitrification spherulite having radial extinction. Sample is 

from about 5 m above base of ash-flow sheet. Immediately adjacent 

devitrified sample (table 6, No. 19) contains 73.8 percent SiO,. B, 

Densely welded devitrified quartz latitic upper part of ash-flow 

sheet, containing about 25 percent phenocrysts. Note andesitic lithic 

fragment (L) at bottom center. Sample is from about 75 m above 

base of sheet and contains 66.2 percent SiO,. Analyzed sample 16, 

table 6. C, Large dark-gray scoria block, from same part of ash-flow 

sheet as B, containing about 37 percent phenocrysts of plagioclase 

(P), biotite (B), and augite (A). Sample contains 60.6 percent Si02• 

Analyzed sample 15, table 6. 

sheets-becoming more mafic and phenocryst-rich upward (figs. 32, 34)-are fairly common elsewhere in the 

San Juan field (Ratte and Steven, 1964; P . W. Lipman 

and T. A. Steven, unpub. data), as well as in other parts of 

the world. (See summary in Lipman and others, 1966, p. 

F39-F41.) Each of these occurrences seems best interpreted as the result of progressive eruption from a single 

source chamber containing a compositionally zoned 

magma. The sequence of compositional variations in the 

ash-flow sheet represents, in inverted order, the original 

compositional zonation that existed in the magma 
ASH-FLOW SHEETS 53 

chamber at the time of eruption; for the Carpenter Ridge 

Tuff the original magmatic zonation was downward from 

phenocryst-poor rhyolite to phenocryst-rich rhyodacite. 

Although only a local feature of the Carpenter Ridge, the 

striking compositional zonation in the Beaver Mountain area demonstrates that even this most rhyolitic ashflow sheet of the southeastern San Juan Mountains was 

closely associated with more mafic material in its 

magma chamber before eruption. 

Although the outflow Carpenter Ridge Tuff and the intracaldera Bachelor Mountain Member are similar in 

phenocryst content and in remanent magnetic polarity, 

these two facies of a single ash-flow sheet so differ in 

general appearance that their equivalence is not readily 

apparent. The extreme dense welding and fluidal 

appearance (Ratte and Steven, 1967, figs. 4-7) of the 

dark-gray lower part (Willow Creek unit) of the intracaldera Bachelor Mountain Member is unknown outside the caldera in the red-brown Carpenter Ridge Tuff. 

Upper parts of the intracaldera Bachelor Mountain 

Member (Campbell Mountain and Windy Gulch units) 

contain abundant· volcanic lithic fragments, in contrast 

to the paucity of such material in the outflow Carpenter 

Ridge. The intracaldera tuff has undergone pervasive extreme potassium metasomatism (Ratte and Steven, 

1967, table 2), and therefore the two facies of the ashflow sheet do not resemble each other chemically. 

Nevertheless, the equivalence of the two facies seems 

clearly established by the general distribution of the 

Carpenter Ridge Tuff around the Bachelor caldera, by 

similarities in phenocryst petrology and in 

paleomagnetism, and by similarity in stratigraphic posi-

~ 

z 

w 

75 

u 70 a: 

w 

Q. 

1-

J: 

(.!) 

w 

~ 

z 

-. 65 

0 

u; 

x. 

'• ~ "): X EXPLANATION 

' "'("-. x Mammoth Mountain Tuff 

"-. • Carpenter Ridge Tuff 

......... .......__ 

" " ......... , 

.................. 

......... ......... 

......... ,X 

......... 

X 

10 20 "30 40 

TOTAL ~HENOCRYSTS, IN VOLUME PERCENT 

FIGURE 34.-lnverse correlation between Si02 and total phenocryst contents for two compositionally zoned ash-flow sheets of the San Juan 

volcanic field. Data from table 6, Ratte and Steven (1967), and P. W. 

Lipman (unpub. data). Approximate correlation trends drawn by inspection. 

tion both inside and outside the caldera with respect to 

the underlying Fish Canyon Tuff and the overlying 

Mammoth Mountain (table 1) and Wason Park Tuffs 

(Steven, Lipman, and Olson, 1974). 

AGE AND MAGNETIC POLARITY 

The Carpenter Ridge Tuff has not been dated 

radiometrically, but its age is bracketed by K-Ar determinations (Lipman and others, 1970) on the underlying 

Fish Canyon Tuff (27.8 m.y.) and on Mammoth Mountain Tuff (26.7 m.y.) that overlies it in the Creede area 

(table 1). The Carpenter Ridge Tuff has reverse remanent magnetic polarity. 

WASON PARK TUFF 

The Wason Park Tuff, named by Steven and Ratte 

(1964), is a generally densely welded ash-f1ow sheet of 

moderately phenocryst rich low-silica rhyolite, believed 

to have been erupted from a source now subsided 

beneath the Creede caldera (Ratte and Steven, 1967, p. 

H34). Because the Wason Park is a relatively minor unit 

in the southeastern San Juan Mountains, a separate distribution map has not been prepared. In this area, the 

Wason Park rests on Carpenter Ridge Tuff and is 

overlain by Snowshoe Mountain Tuff or by basalt flows 

of the Hinsdale Formation (Durango map). Where the 

Wason Park was partly confined by the wall of the La 

Garita caldera in the northwestern part of the mapped 

area (fig. 31), it is as much as 300 m thick, but scattered 

remnants that are preserved to the southeast as far as 

Wolf Creek Pass, Fox Mountain, and Beaver Mountain 

are generally no more than 50 m thick. Like the 

Carpenter Ridge Tuff (fig. 31), the Wason Park wedges 

out against the uplifted block of Fish Canyon Tuff 

within the Mount Hope caldera. 

In the southeastern San Juan region the Wason Park 

Tuff appears to be a simple cooling unit. Where the unit 

is densely welded, a black vitrophyre 3-5 m thick near 

the base is overlain by brick red-brown tuff containing 

streaky light-gray pumice lenses as much as 50 em long 

(Ratte and Steven, 1967, fig. 16). Less welded devitrified 

tuff near the top of the unit is gray, owing to vapor-phase 

crystallization; and where the Wason Park is thin near 

its depositional margins in the southeastern area, it is 

partly welded and is gray throughout. 

The Wason Park contains 20-30 percent phenocrysts, 

mainly plagioclase (An2s-4o), sanidine (about Orno), 

biotite, and sparse augite. The plagioclase phenocrysts 

tend to be distinctively tabular, fairly unbroken, and 

parallel to the compaction foliation of the tuff. Foreign 

volcanic fragments are sparse, especially as compared to 

other ash-f1ow units already described. Seven chemical 

analyses of the Wason Park, listed by Ratte and Steven 

(1967, table 14), indicate 68-72 percent Si02, on a 

volatile-free basis. In comparison with other ash-flow 
54 PLATORO CALDERA COMPLEX, SAN JUAN MOUNTAINS, COLORADO 

tuffs from the Creede area (Carpenter Ridge, Mammoth 

Mountain, and Snowshoe Mountain Tuffs), the Wason 

Park seems to be slightly higher in alkalis and lower in 

CaO, MgO, and total iron . . 

The age of the Wason Park is bracketed in the Creede 

district by ages of 26.7 m .y. on the underlying Mammoth 

Mountain Tuff and 26.4 m.y. on overlying lavas of 

the Fisher Quartz Latite (Lipman and others, 1970). The 

Wason Park Tuff has reverse remanent magnetic polarity. 

SNOWSHOE MOUNTAIN TUFF 

The Snowshoe Mountain Tuff (Steven and Ratte, 

1964, 1965), the youngest ash-flow sheet in the central 

and eastern San Juan field, was erupted from Creede 

caldera a little more than 26.4 m.y. ago (Steven and 

others, 1967; Lipman and others, 1970). This ash-flow 

sheet accumulated to a thickness of at least 1,000 m 

(base not exposed) within Creede caldera as a result of 

subsidence concurrent with eruption (Steven ana Ratte, 

1965, p. 40, 59). By contrast, the outflow sheet is thin 

and weakly welded, and only a few patches have escaped 

erosion. The largest of these remnants is .in the 

northwestern part of the area described in this 

report-at Handkerchief Mesa and on Trout, Beaver, 

and Fox Mountains (Durango map). In most of these 

places the Snowshoe Mountain Tuff has been protected 

from erosion by the capping basalt flows of the Hinsdale 

Formation . The Snowshoe Mountain Tuff rests on the 

Wason Park Tuff, except at the south end of 

Handkerchief Mesa (Platoro map), where it wedges 

between lavas and volcaniclastic sedimentary rocks of 

the Los Pinos Formation (fig. 35) that were eroded from 

lava domes around the north margin of Platoro caldera. 

The outflow Snowshoe Mountain Tuff is a simple cooling unit of light-gray tuff 10-100 m thick that is rarely 

well exposed because it is only weakly welded. Where the 

tuff is best exposed, erosion along large crudely columnar joints has produced crumbling conical ribs 

("tepees"). The outflow Snowshoe Mountain contrasts 

strikingly with the much thicker intracadera part of this 

tuff sheet, which shows dense welding and compound 

cooling. These contrasts are analogous to those already 

described for the La Jara Canyon Member of the 

Treasure Mountain Tuff, the Fish Canyon Tuff, and the 

Carpenter Ridge Tuff within and outside their source 

calderas. 

The Snowshoe Mountain Tuff is a phenocryst-rich 

quartz latite (table 3) containing common small angular 

red-brown or gray fragments of intermediatecomposition volcanic rocks. In the outflow sheet, 

phenocrysts total about 40 percent; of this, about threefourths are plagioclase (An3o-so) and the remainder is 

divided subequally among biotite, augite, Fe-Ti oxides, 

sanidine, and quartz. The presence of sanidine (0r7J-?s) 

and quartz in a low-silica quartz latite (64-67 percent 

Si02; Ratte and Steven, 1967, table 18) is notable and is 

in contrast with the phenocryst mineralogy of chemically 

similar quartz latites of the Treasure Mountain and 

Masonic Park Tuffs. 

The Snowshoe Mountain Tuff can be only slightly 

older than the caldera-related ring lavas of Fisher Quartz 

Latite, well dated at 26.4 m.y. (Steven and others, 1967; 

Lipman and others, 1970). It has normal remanent 

magnetic polarity. 

FIGURE 35.-Snowshoe Mountain Tuff, erupted from Creede caldera, 

interlayered with late volcanic products from the Platoro caldera 

complex. Light-gray slightly welded Snowshoe Mountain Tuff (Tsm) 

rests on volcaniclastic sediments of the Los Pinos Formation (Tip) 

and is overlain by dark-gray lavas of the rhyodacite of Park Creek 

(Tpd). South end of Handkerchief Mesa. (See fig. 31.) 

INTERLAYERED LAVA FLOWS 

Andesitic to quartz latitic lava flows are interlayered 

with the younger ash-flow sheets at numerous horizons 

in the central San Juan Mountains (Durango map); only 

a few of these-those that are significant in the 

southeastern part of the field-are discussed briefly 

here. 
LA VAS AND RELATED ROCKS 55 

SHEEP MOUNTAIN ANDESITE 

The Sheep Mountain Andesite is a sequence of dark 

andesitic lavas, named by Larsen and Cross (1956). At 

the type locality northwest of Wolf Creek Pass (fig. 26), 

Sheep Mountain Andesite is interlayered with the 

members of the Masonic Park Tuff and apparently 

represents a local group of andesitic stratovolcanoes 

around the margins of the Mount Hope caldera. 

.Although the Sheep Mountain as mapped by Larsen and 

Cross (1956) included andesitic rocks from volcanoes of 

various ages scattered widely over the San Juan field, 

the name "Sheep Mountain," to have any current utility, should be restricted to andesites in and near the type 

locality that are penecontemporaneous with the Masonic 

Park Tuff and that are related to evolution of the Mount 

Hope caldera. The name is therefore changed from 

Sheep Mountain Formation to Sheep Mountain 

Andesite. In this area Sheep Mountain flows and breccias occur below the lower mem her of the Masonic Park, 

between the two members, and above the upper 

member. The maximum thickness of the Sheep Mountain, about 250m., is on the west slopes of Saddle Mountain (Wolf Creek Pass quadrangle). 

The Sheep Mountain Andesite typically consists of 

lavas and flow breccias of dark andesite and perhaps 

some rhyodacite; interlayered with these are minor 

amounts of sedimentary rocks. Most of the lavas are 

nonporphyri tic to sparsely porphyritic; small 

phenocrysts of plagioclase (An4o-Go) and augite are the 

common types. The two analyses of Sheep Mountain 

lavas contain 56.1 and 57.3 percent Si02 (table 7). 

Petrologically similar dark andesites that overlie the 

Masonic Park Tuff along the Continental Divide west of 

the Platoro caldera complex .(Platoro map) are informally designated the andesite of Summit Peak. Although 

these flows lie at approximately the same stratigraphic 

position as the Sheep Mountain Andesite, they are most 

plausibly interpreted as part of the postcollapse volcanic 

activity related to the Platoro center. 

HUERTO ANDESITE 

The Huerto Andesite, an assemblage of andesitic lava 

flows and associated breccias named by Larsen and 

Cross (1956, p. 143) and also described by Steven and 

Ratte (1965, p. 32-34) as the Huerto Formation, is locally 

present between the Fish Canyon and Carpenter Ridge 

Tuffs, mainly in the northwestern part of the mapped 

area (fig. 31). At the head of the West Fork of the San 

Juan River, these andesites buried an irregular faultblock topography and are locally as much as 500 m thick 

(Steven and others, 1969). In the South Fork of the Rio 

Grande the Huerto occuts discontinuously and is 

nowhere more than 50 m thick. 

Typioally, the Huerto consists of a succession of thin 

flows of distinctive porphyritic andesite that contains 

tabular phenocrysts of calcic plagioclase (Ans5·Gs) as 

much as 2 em in diameter and that is virtually identical 

in petrography and chemistry to the platy-plagioclase 

andesite already described in the Conejos Formation 

(fig. 6). Huerto Andesite is dark gray, except for 

vesiculated upper and lower parts of flows which are redbrown owing to oxidation. Analyzed andesites of the 

Huerto contain 54-59 percent Si02; like the other platyplagioclase andesites, they are characteristically higher 

in Alz03 and lower in MgO than less porphyritic dark 

andesites of similar Si02 content in other units in the 

southeastern San Juan Mountains (table 7). 

VOLCANICS OF TABLE MOUNTAIN 

The volcanics of Table Mountain (Steven and Lipman, 1973) are a complex assemblage of lavas and breccias, ranging from dark fine-grained rhyodaci tes to 

lighter coarsely porphyritic quartz latites. They form a 

thick local accumulation between the Carpenter Ridge 

and Wason Park Tuffs northwest of the South Fork of 

the Rio Grande. These rocks were erupted from local 

vents near Table Mountain and farther to the southwest. 

Their maximum preserved thickness of about 500 m is on 

the west slopes of Table Mountain and near the head of 

Goose Creek. 

LAVAS AND RELATED ROCKS 

OF THE PLATORO CALDERA COMPLEX 

The lavas and related rocks described in this section 

provide the main record of an intricate sequence of 

lengthy postcollapse igneous activity within and around 

the margins of the Platoro caldera complex (fig. 36). 

These rocks were emplaced over the interval 29-20 m.y. 

ago and are generally divisible into two groups: (1) older 

nonporphyritic to sparsely porphyritic dark andesite and 

rhyodacite, including the rhyodacite of Fisher Gulch, the 

members of the Summitville Andesite, the andesite of Summit Peak, and the major andesitic unit of the volcanics 

of Green Ridge; and (2) younger, more silicic lavas that 

characteristically contain large P.henocrysts of feldspar 

as well as biotite and augite or hornblende-the upper 

rhyodacite unit of the volcanics of Green Ridge, the 

rhyodacite of Park Creek, the quartz latite of South 

Mountain, and the rhyolite of Cropsy Mountain. Concurrent intrusive rocks, including granitic stocks and 

outward-radiating andesitic to rhyolitic dikes, seem to 

mark cores of some of the same volcanoes whose dissected cone remnants are represente~ by the lavas. 

Interbedded volcaniclastic sedimentary rocks, including 

part of the Los Pinos Formation, record erosion of the 

primary volcanic constructional topography 

penecontemporaneously with its formation. Some 

general features of the postcollapse lavas and related 

rocks are summarized in table 8. 
.56 PLATORO CALDERA COMPLEX, SAN JUAN MOUNTAINS, COLORADO 

TABLE 7 .-Analyses of lava flows interlayered with the younger ash-flow sheets 

[Analysis 1 is by standard methods by C. L. Parker; other analyses are from Larsen and Cross (1956, tables 

· 21, 23). Tr., trace) 

Sample---------

Field No-------

Lab. No--------

Si02----------- Al203----------

Fe20]----------

Feo------------

MgQ------------

cao------------

Na2o-----------

K20------------

Ti02-----------

P20s-----------

Mno------------

TOTAL-------

H20.. ----------

H20- ---,-------

C02------------

Quartz---------

Orthoclase-----

Albite---------

Anorthite------

Wollas toni te---

Enstatite------

Ferrosili te----

Hagnetite------

Hema ti te-------

llmenite-------

Apa t 1 te--------

Plagioclase----

Biotite--------

Hornblende-----

Aug! te---------

Hypersthene----

011 vine--------

Opaques--------

Groundmilss-----

Sheep Mountain Andesite Huerto Andesite Volcanics of Table Mountain 

1 

Sl83A 

Dl00028 

2 

SV9 

3 4 

SC2391 SC2291 

5 

LaG1540 

Major oxides (weight percent), recalculated without H20 and C02 

56.11 

18.16 

2.93 

4.94 

2.93 

7. 34 

3.40 

2.25 

1.15 

.49 

. 30 

100.00 

. 64 :27 } .72 

7.80 

13.27 

28.80 

27.65 

2. 32 

7. 29 

5. 32 

4. 24 

2.18 

1.15 

25 

3 

1 

2 

60 

57.33 54.89 58.19 

16.05 19.36 17.83 

5. 72 4. 76 4.13 

3.15 4.47 4.02 

3.93 1.17 2.11 

6.15 6.88 6. 71 

3.18 3.25 3.60 

2.84 3.03 2.03 

1.28 1.40 .82 

.28 .63 . 39 

.08 .15 .16 

100.00 100.00 100.00 

• 57 .16 

1.4 1.19 .94 

Norms (weight percent) 

11.36 8.85 12.67 

16.79 17.88 12.00 

26.88 27.48 30.41 

21.15 29.32 26.51 

3.16 • 30 1. 78 

9. 79 2.91 5.26 

2.24 2.91 

6.69 6.91 6.00 

1.11 

2.44 2. 66 1. 56 

• 65 1.49 .91 

Phenocrysts (volume percent) 

25 20 

Tr. Tr. 

6 2 Tr. 

1 2 Tr. 

4 1 1 

55 74 90 

SAMPLE DESCRIPTIONS 

60.17 

17.53 

3.69 

3. 35 

2.04 

6.10 

3.84 

2.03 

.92 

• 32 

100.00 

.71 

.17 

14.94 

12.00 

32.46 

24.63 

1.47 

5.08 

1. 58 

5. 34 

1. 76 

.77 

19 

2 

68 

6 

LaG1534 

60.76 

17.15 

4. 76 

2.45 

1.66 

5.91 

3.16 

2.82 

.92 

• 31 

.08 

100.00 

.88 

.60 

.00 

18.00 

16.65 

26.75 

24.28 

1.24 

4.14 

5. so 

.97 

1. 75 

• 74 

26 

4 

63 

1. Gray andesite, containing abundant small (1-2 mm) phenocrysts in a fine-grained pilotaxitic ground- mass. Pyroxenes are fresh, but olivine is altered to dark-brown iddingsite. Ridge between Goose and Leopard Creeks, at 9,950 ft, 2.5 km northeast of Lake Humphreys (Spar City quadrangle). Previously 

unpublished analysis provided by T. A. Steven. 2. Platy-plagioclase andesite. Wolf Creek Pass· quadrangle "in Beaver Creek, at an altitude of 9,000 feet" (Larsen and Cross, 1956, table 21, No. 7). 3. Platy-plagioclase andesite. "San Cristobal quadrangle, south of Huerto Peak, at altitude of 12,000 feet" (Larsen and Cross, 1956, table 21, No. 10). 4. Dense dark-gray andesite, containing "scattered white plagioclase tablets about 1 mm long * * * San 

Cristobal quadrangle, from the ridge west of the east fork of Woodfem Creek" (Larsen and Cross, 

1956, table 21, No. 14). 

5. Dense gray hornblende rhyodacite. Spar City quadrangle, "shoulder west of Mount Hope at 

altittiae of 12,500 feet" (Larsen and Cross, 1956, table 23, No. 2). 6. Gray hornblende-pyroxene rhyodacite. Spar City quadrangle, "ridge east of Table Mountain, at altitude of 12,350 feet" (Larsen and Cross, 1956, table 23, No. 3). 

The general petrologic progression of the postcollapse 

activity is toward more silicic types as age decreases 

(table 9): from voluminous dark andesites (56-59 percent 

Si02) to small scattered lava domes of porphyritic 

rhyolite (75-77 percent Si02) that are contemporaneous 

with nearby widespread flows of alkali olivine basalt. 

This postcollapse igneous activity at Platoro caldera 

therefore spans the regional ttansition from 

predominantly andesitic volcanism in the Oligocene to 

bimodal basalt-rhyolite suites in Miocene and Pliocene 

time (Lipman and others, 1970). 

RHYODACITE OF FISHER GULCH 

The rhyodacite of Fisher Gulch is a single thick 

biotite-plagioclase rhyodacite lava flow that represents 

one of the first major lava eruptions within the Platoro 

caldera after its collapse and resurgence. Although this 

rhyodacite flow is exposed only locally, over about 12 

km2 in the southeastern moat of Platoro caldera (fig. 36), 

it has been distinguished from the overlying Summitville 

Andesite because its distinctive phenocryst mineralogy 

and more silicic composition are petrologically transitional toward the Treasure Mountain Tuff. Where its 
''· • 

P 1 a t o r o .C a 1 d e r a 

--petrographic reference 

alteration, propylitic 

andesite 

Lithics 

porphyritic 

apatite 

aplite 

augite 

carbonate pseudomorph 

reaction rim 

basalt 

biotite 

oxidized 

to chlorite 

compositional zonation 

devitrification 

devitrification s pherul i te 

d is e qui 1 i b r i urn 

flow layering 

hornblende 

oxidized margins 

hypersthene 

monzonite 

olivine 

iddingsite 

orthoclase 

oxides 

in basalt 

Page 

31 

63 

38, 52 

13 

76 

82 

13, 52, 63 73 

59 

69, 85 

93 

52, 59(?), 69, 

82, 85, 99 

73, 76 

31 

52 

93 

52, 98 

73, 76 

93 

63 

63, 93 

63, 85 

82, 85 

93 

99 

82 

63, 6 9, 99 

93 
"' ... 

plagioclase 52, 69, 99 

AN 35-45 73 

AN 40-45 82 

An 45-55 13 

glomeroporphyritic 76 

euhedral 59 

replaced with carbonate 29' 31, 82 

zoned 63 

em.bayed 76 

propylitic 31 

quartz 85, 99 

ernbayed 73, 76, 93 

rhyodacite 59, 63' 88 

rhyolite 93 

sanidine 52, 69, 73' 93' 99 

resoebed 76 

euhedral 93 

sphene 76 

s pherul i te 52 

textures 

pilotaxitic 63' 69' 73' 76 

hyalopil itic 63 

intergranular 99 

perlitic 73 

granophyric 82 

phanero crystalline 82 

skeletal 93 

tuff 

shard rich 29' 31 

vescile 101 

vitrophyre 

welded tuff 52 

bas a 1 52 

xenocryst, feldspar 99 
107'00' 

Trout Mountainx 

Mount 

xHope 

xMetroz 

Mountain 

Rhyolite of 

~ Cropsy Mountain 

Quartz latite ~ of South Mountain 

Rhyodacite ~ of 

Park Creek 

Volcanics of 

Green Ridge 

LAVAS AND RELATED ROCKS 

106'15' 

Osier M~untain 

0 5 10 15 20 MILES 

0 5 10 15 20 KILOMETERS 

EXPLANATION 

c=J 

Monzonite 

.£ ~ .~ { II L__j )i Up ber 

Lower member 

[[] 

Rhyodacite of Fisher Gulch 

.1 s Sample locality (table 9) 

"""""""""" Limit of exposed volcanic 

rocks 

* 

Caldera wall (topographic) -

Dashed where covered by 

younger rocks 

Volcanic vent 

57 

106'00' 

FIGURE 36.-Distribution of postcollapse lavas and related monzonitic intrusives of the Platoro caldera complex . Based on mapping by P. W. Lipman, 1965-67, 1970-71. 
58 PLATORO CALDERA COMPLEX, SAN JUAN MOUNTAINS, COLORADO 

TABLE B.-Summary of general features of lavas and related rocks of the Platero caldera complex 

[Rhyolitic lavas of the Hinsdale Formation are included for comparison] 

Rock unit 

Approximate 

Si02 content 

(percent) 

Distinguishing phenocrysts 

Total 

phenocrysts 

Age 

(m.y.) 

Rhyolite of the Hinsdale Formation .............. . 75-77 

71-73 

68-70 

60-64 

Quartz, anorthoclase (Or.a-.s) ......................... . 1-15 

10-25 

20-35 

20-30 

4.8-21.9 

20.2 

22.8 

Rhyolite of Cropsy Mountain ..................... . Quartz, sanidine (Orss), hornblende .................... . 

Quartz latite of South Mountain ................ . Quartz, sanidine (0r7s), hornblende .................... . 

Rhyodacite of Park Creek ........................ . Biotite, augite ........................................ · >25.8, <27.8 

Volcanics of Green Ridge: 

Porphyritic rhyodacite unit .................... . 60-66 

58-62 

2

72 

... do ............................................... . 20-30 

(I) 

27.4-27.8 

>27.4, <28.2 

<28.2(?) 

Andesite-rhyodacite unit ...................... . 

Rhyolite unit ................................. . 5 

Summitville Andesite: 

Upper member ............................... . 

Lower member ............................... . 

Rhyodacite of Fisher Gulch ...................... . 

58-62 

56-60 

62-64 Biotite, augite ....................................... . 

>29.1 

>29.1, <29.8 

<29.8 

I Small, sparse. 

2 Estimated. 

base is exposed along the caldera wall, this flow rests on 

Conejos Formation; within the caldera, it lies on the La 

Jara Canyon Member or on thin tuffaceous sedimentary 

rocks that overlie the La Jara Canyon (Platoro map). 

The maximum exposed thickness of the rhyodacite of 

Fisher Gulch, about 350 m, is along the Lake Fork of the 

Conejos River (Platoro map), but the flow thins abruptly 

against the caldera wall and against the resurgent block. 

A small area of similar biotite-plagioclase rhyodacitic 

lava is exposed low within the caldera-fill sequence for a 

few hundred meters along the Alamosa River near the 

mouth of California Gulch (Platoro map), but 

equivalance of this lava flow to the main rhyodacite of 

Fisher Gulch is uncertain, owing to the gap of about 5 

km between the two areas. 

Most rhyodacite of Fisher Gulch is light to medium 

gray, but where oxidized, the rhyodacite is light red 

brown. Flow foliation, as defined by alternating light 

and dark layers and by mineral alinement, is well 

developed in much of this unit. In general, the flow foliation dips uniformly 10°-25°, but it steepens to nearly 

. vertical where the rhyodacite was deposited against the 

south wall of the caldera (fig. 37). The foliation is also 

consistently steep (60° -90°) over an elliptical area of 

about 1 km2 along the Conejos River just south of Fisher 

Gulch (Platoro map) that is thought to reflect the vent 

area. In detail, the foliation seems abruptly to fan outward to more gentle attitudes, as is characteristic of 

vent-dome complexes elsewhere (Williams, 1932). 

The rhyodacite of Fisher Gulch contains about 20 percent phenocrysts, most of which are oscillatory-zoned 

plagioclase (An3o-5o); biotite and sparse Fe-Ti oxides are 

the only other phenocrysts seen in most thin sections. 

Clinopyroxene was originally present, as indicated by 

carbonate pseudomorphs (fig. 37B); sparse augite was 

found in one sample from the vent area. Dark-gray 

andesitic inclusions, a few centimeters to a meter across, 

are locally abundant. Most of these inclusions are 

angular to subrounded, and in one area they are distinctively rimmed by replacement carbonate (fig. 37A). At 

Big Lake at the head of the Lake Fork (Platoro map), 

andesitic inclusions and the rhyodacitic matrix are intricately intermixed in a marble-cake style that suggests 

that both components were mobile magmas at the time 

of mixing. 

The rhyodacite of Fisher Gulch resembles the La Jara 

Canyon Member of the Treasure Mountain rather closely, both in general petrographic appearance and, especially, in the abundance of andesitic inclusions, which 

are rare in the other lava flows of the region. The large 

unbroken euhedral phenocrysts in the rhyodacite lava 

(fig. 37 B) are diagnostic, however, in contrast to the 

smaller, broken phenocrysts of the ash-flow tuffs. 

In chemical composition this unit is a typical 

rhyodacite: two fresh samples from the vent and the lava 

flow contain 62.5 and 63.8 percent Si02, respectively 

(table 9, Nos. 1, 2). Another analysis of'the flow, from a 

-.zone of intense vapor-phase crystallization, seems high 

in Si02 content (66.4 percent) and is anomalous in 

several other oxides (high Fe203 and low CaO and Na20; 

table 9, No. 3). 

SUMMITVILLE ANDESITE 

The term Summitville Andesite is here applied to the 

thick local assemblage of dark sparsely porphyritic lavas 

and related rocks that accumulated within the Platoro 

caldera complex after collapse. The Summitville 

Andesite is divided into a lower member, which filled the 

Platoro caldera after eruption of the La Jara Canyon 

Member of the Treasure Mountain Tuff, and an upper 

member, which filled the Summitville caldera after 

eruption of the Ojito Creek and Ra Jadero Members. 

The name Summitville Andesite was first proposed for 

almost the same sequence of rocks by Patton (1917, p. 

35-38), who correctly noted its approximate stratigraphic 

equivalence to the Sheep Mountain Andesite elsewhere 

in the San Juan field. The name "Summitville" was subsequently abandoned by Larsen and Cross ( 1956, p. 124), 
LAVAS AND RELATED ROCKS 59 

A 

FIGURE 37 .-Rhyodacite of Fisher Gulch. A, Steeply dipping flow layering where lava flow banked against the south caldera wall. Dark 

andesitic inclusions are irregularly rimmed by white replacement 

carbonate. Road cut at 10,800 feet, along logging road above the Lake 

Fork, northeast ridge of Conejos Peak. B, Photomicrograph showing 

euhedral unbroken phenocrysts of plagioclase (P) and biotite (B). 

(Note textural contrast with compositionally similar welded tuff of 

the intracaldera La Jara Canyon Member, fig. 18.) Arrows indicate 

carbonate pseudomorphic after augite. Analyzed sample 2, table 9. 

who, lacking knowledge of the significance of the Platoro 

and Summitville caldera structures, concluded that 

"***the great flows*** near Summitville to which the 

name Summitville andesite was applied, as well as the 

rhyolitic rocks underlying them and originally believed 

to belong to the Treasure Mountain***have been found 

to lie below the Treasure Mountain and to belong to the 

Conejos (formation)." Because the present study has 

shown that Patton was basically right and Larsen and 

Cross wrong, the term "Summitville Andesite" is a 

valid, useful name and is therefore reinstated. 

The lower member of the Summitville Andesite is 

defined here to include the andesitic lavas and related 

p 

B 

-· 

... . . 

~ .. • ~l 

. '

rocks within and near the Platoro caldera that are 

younger than the La Jara Canyon Member of the 

Treasure Mountain Tuff and older than the Ojito Creek 

Member; thus, in places the lower member is interlayered with the middle tuff of the Treasure Mountain. The upper member includes lavas within and near 

the Summitville caldera that are younger than the Ojito 

Creek Member of the Treasure Mountain Tuff and older 

than the Masonic Park Tuff. 

DISTRIBUTION AND THICKNESS 

As reinstated, the Summitville Andesite is a local sequence of andesitic lavas, breccias, and interlayered 

volcaniclastic sedimentary rocks that are confined to an 

area of about 500 km2 in and near the Platoro caldera 

complex (fig. 36). Each member is exposed over a vertical distance of nearly 1 km within the Platoro and 

Summitville calderas, and original thickness must have 

been appreciably greater for each member, because the 

top is eroded and the base not exposed in the sections of 

maximum thickness. The original volume of the Summitville Andesite is difficult to estimate because of these 

exposure problems, but it must have been at least 

several hundred cubic kilometers, perhaps nearly 500 

km3. 
60 

Sample---------

Field No------

Lab. No--------

Si02-----------

Al203----------

Fe20 3---------- Feo------------

Mgo------------

cao------------

Na 2o-----------

K2o------------

Ti02-----------

P20s-----------

Hno------------

Total-----

H20+ ----------

H2o- ----------

C02 -----------

Quartz---------

Corundum-------

Orthoclase-----

Albite---------

Anorthite------

Wollastonite---

Ens tat 1 te------

Ferros ili te----

Magnetite------

Hematite-------

Ilmenite------- Rutile-:. _______ 

Apatite--------

&--------------

Ba-------------

Be--------'-----

Ce-------------

Co-------------

Cr-------------

Cu-------------

Ga-------------

La-------------

Ho-------------

Nb-------------

Ni-------------

Pb-------------

Sc-------------

Sn-------------

Sr-------------

v-------------- Y-------------- Yb-------------

Zr-------------

Quartz---------

Sanidine-------

Plagioclase----

Bio t 1 te--------

Hornblende-----

Augite---------

Hypersthene----

Sphene---------

Opaques--------

Groundmass-----

PLATORO CALDERA COMPLEX, SAN JUAN MOUNTAINS, COLORADO 

Rhyodacite 

of 

Fisher Gulch 

1 2 3 4 

71L-38 71L-17 71L-18 71J-l 

Wl76167 Wl76165 Wl76166 Wl76170 

TABLE 9.-Analyses of lava flows and 

[Sample localities shown in fig. 36. Analyses 8, 11, and 14-16 are from Larsen and Cross 

3); analyses 10 and 22 are from Patton (1917). All other major-oxide analyses are by 

John Glenn, and James Kelsey. Minor-element analyses by standard methods are 

Norma Rait; analyses 24-29 are by semiquantitative methods by J. L. Harris. >, 

Summitville Andesite Volcanics of Green Ridge 

Andesite 

of 

Lower member Upper member Summit Peak Andesite-rhyodacite unit 

5 6 7 8 9 10 11 12 13 14 71J-2 67L-126 67L-125 SV 136 SA-8 p 11 sv 115 66L-48-A 66L-7 Con 320 

Wl76171 Dl33704W Dl33703W ------ A-159 ----- ------ W-168532 W-168531 ------- Major oxides (weight percent), recalculated without H20 and C02 (original values listed separately below) 

62.5 63.8 66.4 56.4 56.6 57.3 60.6 58.46 58.83 60.94 60.95 58.8 60.7 65.98 18.3 17.2 16.8 17.9 17.4 17.0 17.4 17.77 18.30 17.50 15.53 17.2 17 .o 16.76 2.0 2. 9 5. 2 2. 7 4.0 5.0 3. 2 4.57 3. 30 3.94 3.27 3.5 2.6 2.95 2. 2 2. 3 . 70 5. 5 4. 5 3.4 2.8 3. 51 3.04 1. 98 3.14 3. 9 3. 7 1. 28 1.5 1.4 . 73 3. 2 3.1 3.1 2. 3 2. 82 2.41 2.11 2.60 3.0 3.0 1. 20 

4. 3 3.8 2.0 6.4 6. 3 5. 6 5.0 4. 61 5.91 5. 58 5.01 6,4 5. 5 3. 71 4.0 3.8 3.0 3.4 3.4 ). 6 3. 9 3. 52 3. 70 3.48 4.11 2.8 3. 3 3. 39 4.2 3.8 4. 3 2.8 3.0 3.1 3.4 2.97 3.13 3.23 3.58 2.9 2.7 3.86 . 73 • 72 • 74 1.1 1.1 1.2 • 92 1. 40 .84 1.00 1.06 .97 .88 • 59 .17 .16 .16 .41 .41 .43 • 39 . 37 .42 .11 • 75 • 30 .48 • 21 

.11 .09 .07 .15 .16 .11 .14 .00 .11 .13 ----- .10 .09 .06 

100.01 99.97 100.10 99.96 99.97 99.84 100.05 100.00 99.99 100.00 100.00 99.87 99.45 99.99 . 50 • 45 1.1 • 64 1.2 • 53 .40 . 86 1. 30 . 97 .83 • 69 .92 2. 69 

1.1 • 55 1.6 • 76 1.1 • 57 .91 • 63 • 32 . 60 .43 .41 .18 .44 . • 20 1.6 <.05 <,05 • 75 .18 <.05 1.45 .94 1. 35 ----- <.05 <.05 ----- Norms (weight percent) 

11.86 17.05 27.60 6. 64 8.06 9. 37 12.06 13.44 10.07 17.68 11.69 13.39 14.85 2~. 33 • 37 4.03 1. 27 1. 61 • 75 

24.70 22.44 25.58 16.79 17.64 18.55 19.81 17.56 18.50 19.42 21.17 17.32 16.24 22.83 33.64 32.13 25.29 28.34 28.74 30.85 32.67 29.77 31.25 29.93 . 34.76 23.94 27.57 .28. 68 19.60 17.76 8. 64 25.31 23.40 20.79 20.17 20.43 24.11 18.57 13.'35 25.52 23.65 17.06 

• 21 1. 57 2.11 1. 70 .83 1.03 2·. 76 1. 70 • 21 3.81 3. 58 1. 82 8.09 7.69 7. 82 5.82 7.03 6.01 5. 33 6.48 7.55 7.61 2. 99 1.43 . 76 6. 24 3.44 .42 1. 28 • 35 1. 68 1. 31 2.90 3. 27 2. 96 4.17 • 34 3. 97 5. 82 7. 20 4. 71 6.63 4. 78 3. 96 4. 74 5.13 3.84 2.64 

4. 92 1. 28 1.13 1. 39 1. 36 1.41 2.15 2.15 2. 31 1. 76 2. 66 1.60 1.95 2.02 1.84 1.66 1.11 

.41 • 39 . 39 .96 . 98 1.01 . 91 .87 .99 . 27 1. 77 • 72 1.13 .49 

Minor elements (parts per million) 

1000 920 BOO 720 520 920 1095 1200 1400 

<2 <2 

10 18 15 23 12 16 13 

5 5 5 4 8 12 3 5 50 28 28 34 26 56 64 36 46 35 20 20 20 20 20 21 20 20 20 80 90 80 100 90 74 62 

4 6 <5 <5 

20 20 20 20 20 24 24 

4 5 5 12 19 <5 8 18 20 80 <10 20 20 <50 <50 40 60 12 14 9 30 24 23 24 14 12 

<20 <20 <20 <20 

840 680 410 1200 960 520 440 1200 1000 

60 90 80 180 180 200 120 180 120 

20 20 20 40 30 45 52 30 20 

2 2 2 3 2 3 4 

160 160 180 130 130 220 206 140 100 

Phenocrysts (volume percent) 

9 

22.3 23.6 23 23.9 30.1 7.6 72 27 27.1 20.3 13 

4. 9 5. 2 5 

2.1 7. 2 5. 7 2.9 5 5. 2 4. 3 

3.9 1.5 1.0 3. 5 1.8 2.1 

2. 4 2.1 2 2.8 1.9 1.4 2 2.1 • 7 2 

68.3 69.1 70 95 63.2 60.9 87.1 63 63.8 71.6 79 

See page 62 for sample descriptions. 
LA VAS AND RELATED ROCKS 

related rocks of the Platoro caldera complex 

(1956, tables 21, 23); analyses 9, 18-21, and 23 ore from Steven and Ratte (1960, tables 2, 

rapid methods by P. L. D. Elmore, Samuel Botts, Gillison Chloe, Lowell Artis, H. Smith, 

Nos. 1-5 by J. D. Fletcher, Nos. 6 and 7 by Harriet Neiman, and Nos. 12, 13, and 17 by 

more than; <, less than; Tr., trace] 

Volcanics of 

Green Ridge--Continued 

Porphyritic Rhyodacite Quartz Lot1te 

rhyodacite of of 

Rhyolite of Cropsy Mountain 

unit Pork Creek South Mountain RhyoU te flows Mix-lava complex at North Mountain 

15 16 17 lg 19 20 21 22 23 24 25 26 27 ' 2g Con 2 Con 606 66L-4g-c SM 244 SA-2 SH 291 SA-l p 39 SH 23Al 70L-l39-C 70L-l59 70L-l58-B 70L-l5g-A 70L-l57 

----- ------- lol-l6g533 139604 A 153 139603 A 152 ----- 139602 Wl74656 Wl74660 Wl74659 Wl7465g Wl74657 

Major oxides (weight percent), recalculated without H20 and C02 (original values listed separately below)--Continued 

60. g4 62.29 66.9 60.5 6g. 2 68.4 70.5 70.73 72. 3 64.0 64.9 64.7 65.g 6g, 6 16.41 16.32 17.4 17.3 15.0 15.1 15.4 14.64 14.6 l7 .1 16,g 15.8 16.2 15.4 s. 82 3. 40 3.1 4.0 2. 5 2.4 2.4 .65 1.3 4.0 4.06 3. 7 3.9 3. 5 ,g6 2. 35 .77 2. 3 1.0 1.2 • 54 1.13 .52 . 69 • 69 .89 .65 • 36 2. 96 2.06 • 72 1.9 1.2 • 70 .44 .85 . 62 . 54 . 51 1.6 .69 • 39 

5.17 4. 79 ).8 5. 7 ), 2 2.8 1.4 2. 36 1.63 2.8 2. 7 3. 5 2. 5 2.03 3.11 3. 77 ), 7 3. 7 ), 7 4.0 4.1 4. 29 3.8 5.0 4.6 4. 3 4. 7 4.4 3.84 ), 75 2. 6 3.0 4. 2 4. 3 4.4 4. 66 4. 7 4. 3 4. 3 4.1 4. 2 4. 3 • 73 1.01 . 46 .89 • 55 . 53 .49 . 51 • 29 .92 .92 .81 • 78 . 70 .18 . 27 • 36 . 52 • 39 • 41 . 32 .18 • 27 . 51 .48 . 39 • 36 • 26 

.08 .00 .07 . 22 .09 .12 .02 .oo .04 .08 .06 .08 .08 .07 

100.00 100.01 99.88 100.03 100.03 99.96 100.01 100.00 100.07 99.94 100.00 97 ,g7 99.86 100.01 

>, 90 1.84 . 36 >1. 3 • 64 >1. 2 • 55 2.45 >2. 5 .63 . 56 • 41 . 57 .48 .12 • 44 . 99 • 59 .06 • 4 7 .94 .18 • 73 .82 

----- ----- <.05 1.4 1. 99 1.7 o.oo ----- .05 <.05 <.05 <.OS <.05 <.OS 

Norms (wei9ht percent)--Continued 

14.19 14. 6g 26.82 14.70 23.28 22.93 27.09 22.65 27.74 13.1 16.7 15.6 17.0 22.8 2. 28 2.17 • 97 . 29 • 78 . 21 • 57 22.67 22.13 15.53 -17.67 24.98 25.50 25.78 27.50 27.70 25.7 25.2 24.4 25.1 25.1 26.29 31.85 31.64 31.40 31.28 33.89 34.84 36.25 31.90 41.98 38.7 36.7 39.4 36.9 19.50 16.57 16.67 21.78 11.80 10.49 4.82 6.97 6. 36 10.7 10.5 11.5 10.2 8. 3 

2.07 2. 25 1. 24 .71 • 24 1.48 o.oo o.oo 1.4 0.00 7. 36 5.12 1. 79 4. 62 2.92 1. 74 1.09 2.11 1. 55 1.3 1.3 4.01 1.7 .96 

.68 

.92 4.64 1. 36 5.45 1.82 2.83 • 39 .95 . 98 • 78 .09 

5.18 • 20 2.19 • 26 1. 24 .41 2.15 .65 4.05 4.06 3. 2 3.9 3. 5 1. 38 1. 92 .88 1. 68 1.05 1.01 • 92 • 97 • 54 1.6 1.6 1.5 1.5 .92 

.06 .09 • 21 .43 .65 .86 1. 22 • 93 .97 • 77 .44 .63 1.2 1.1 .93 .86 .62 

Minor elements (parts per million)--Continued 

2300 2000 2000 2000 2000 1500 

1.5 1.5 1.5 2 2 

500 300 500 300 300 

10 10 15 10 

5 100 70 70 70 30 14 7 10 7 10 15 20 15 15 15 15 15 

150 150 100 100 100 

3 3 

15 15 15 15 15 8 0 30 30 30 70 15 7 10 10 15 5 10 10 10 10 7 <20 

860 1500 1500 1500 1500 1000 50 70 70 70 70 50 30 20 20 20 20 20 

2 2 2 2 2 150 300 300 300 300 200 

Phenocrysts (volume percent)--Continued 

2 3 2 2. 2 0. 2 0.1 0.3 1 2.5 2 3 2 ).1 • 3 • 2 .8 1.5 3 . 25 23 24.9 22 10 13 10 9. 3 1 1 3. 7 6. 5 15 5 10 6. 2 5 4 4 4 2.1 Tr.? .2 .3 1 3 1 2 3.0 .8 .1 .2 .3 • 5 • 5 

1.5 • 3 .5 

Tr. Tr. 3 2 2.1 1.5 2 2 2 • 7 • 3 • 3 • 3 1 1 60 61 63.8 70 80 75 80 81.7 98 98 94 8g 75 

61 

29 Sample 

70L-l6l Field No. 

Wl74661 Lab. No. 

72.4 Si02 

14.2 Al203 

1.8 Fe203 .49 FeO 

.49 MgO 

1.5 CaO 

3.9 Na 2o 4.6 K20 

.43 Ti02 .11 P20s 

.06 MnO 

99.9g Total 

.82 H20+ 

• 78 H2o- <,05 C02 

28.6 Quartz 

.42 Corundum 

27.1 Orthoclase 

32.7 Albite 

6.8 Anorthite 

Wollastonite 

1.2 Enstatite 

Ferrosil1te 

• 53 Magnetite 

1.5 Hematite 

.81 Ilmenite 

Rutile 

• 26 Apatite 

B 

1000 Ba 

5 Be 

100 Ce 

Co 

15 Cr 

1 Cu 

15 Ca 

70 La 

3 Ho 

15 Nb 

Ni 

15 Pb 

3 Sc 

Sn 

500 Sr 

20 v 20 y 

2 Yb 

150 Zr 

1.5 Quartz 

2. 5 Sanidine 

13.5 Plagioclaae 

1.5 Biotite 

Hornblende 

Augite 

Hyperathene 

Tr. Sphene 

1 Opaques 

80 Croundmaaa 
62 PLATORO CALDERA COMPLEX, SAN JUAN MOUNTAINS, COLORADO 

TABLE 9.-Analyses of lava flows and related rocks of the Platoro caldera complex-Continued 

SAMPLE DESCRIPTIONS 

,· · 1. Dark-gray dense devitrified rock having steep obscure flow layering at vent area. Phenocrysts of plagioclase as much as 5 mm across are intricately zoned and resorbed, biotite is oxidized, and augite is partly replaced by carbonate. Platoro Road, along · Conejos River, about 0.9 km north of junction with Beaver Lake Creek. 2. Dark dense lava, similar to No. 1, having steep well-developed flow layering (fig. 37A) where· flow banked against caldera wall. Contains large inclusions of sparsely 

porphyritic andesite, with irregular rims of replacement carb9nate along contacts. Groundmass is more finely devitrified than No. 1,-and augite is completely replaced by carbonate (fig. 37 B). At 10,800 ft, along logging road on northeast ridge of Conejos Peak, 1.6 km southeast of Big Lake. 3. Pale-purplish-gray devitrified lava that is chalky because of intense vapor-phase 

crystallization. Composition appears anomalous: high in SiO" and Fe"O" and low CaO and Na10. About 100 m west of locality of No. 2 along the logging road. - 4. Dark-gray dense aphanitic andesite, containing small microphenocrysts-of plagioclase less than 1 mm long and even smaller sparse augite microphenocrysts in a finely 

microcrystalline intergranular groundmass. At 11,400 ft, along Saddle Creek Road on northeast ridge of Conejos Peak, 1.4 km north of upper switchback. 5. Dense·gray andesite, containing abundant plagioclase phenocrysts 1-2 mm in diameter and smaller pyroxenes in a very finely crystalline intergranular ground mass. About 200 m southeast of locality of No. 4, near crest of ridge along road. 

6. Coarsely crystalline red-brown andesite .. See fig. 388 for description and location. 7. Sparsely porphyritic dark andesite. See fig. 38A for description and location. · 8. Dark-gray fine-grained seriate andesite flow. "Gold Creek at an altitude of 11,400·feet" 

(Larsen and Cross, 1956, table 21, No. 3). 1 9. Dark-gray fine-grained dense andesite. Biotite occurs in finely granular aggregates and may be secondary. Other ferromagnesian minerals are completely altered, but clots of chlorite and carbonate probably are replacement products of augite. "Road cut just east of the bridge across Wightman Fork and 500 feet west of the mouth of Cropsy Creek" (Steven and Ratte, 1960, table 2, No. 3). 10. Augite andesite, containing "plagioclase and augite phenocrysts in an altered ground- mass that probably consisted of plagioclase, augite, and magnetite. From creek bed of _Cropsy Gulch" (Patton, 1917, p. 31). · 11. Dark-gray dense. plagioclase-pyroxene rhyodacite. "Higher in silica and potash than usual. The phenocrysts are variable in size and most of them are· less than 1 mm long. West slope of Summit Peak, at an altitude of 12,80Q feet" (Larsen and Cross, 1956, table 21, No.8). . 12. Dark-gray dense andesite. See fig. 41C for description and location. · - 13. Dark-red-brown finely porphyritic rhyodacite. Texture much like No. 12, but phenocrysts are slightly larger-as much as 1 mm in diameter. At 9,400 ft, on northwest ridge of Chiquita Peak, about 0.3 km from summit. 14. Light-gray finely porphyritic rhyodacite to quartz latite. Described as devitrified, but analysis is problematic because of high H10+. "The rock is seriate porphyritic, the larger phenocrysts are about 1 mm across, and they grade into submicroscopic cryst(!ls in the groundmass." "Saddle, 1/2 mile north of Silver Mountain" ·tarsen and Cro'ss, 

1956, table 21, No. 16). 

The two members of the Summitville Andesite. are 

similar in general appearance and in composition; this 

similarity causes major mapping problems, because the 

members can only be reliably distinguished by 

stratigraphic relations with units of the Treasure Mountain Tuff. The Summitville Andesite also is similar in 

appearance and composition to the andesite of Summit 

Peak and, to a considerable extent, to intermediatecomposition lavas of the Conejos Formation. Near the 

caldera complex, however, the Conejos Formation tends 

to be more propylitically altered than otherwise sitnilar 

rocks of the Summitville Andesite. 

Discrimination between the two members of Summitville Andesite is most confidently made in the eastern 

part of Platoro caldera, where upper units of the 

Treasure Mountain Tuff are present, and within the 

Summitville caldera, where the lower member of the 

Summitville Andesite is unlikely to be exposed. In the 

western parts of the caldera complex, especially south of 

the Alamosa River, division into lower and upper 

members is tenuous and is based mainly on the presence 

of some local intertonguing weakly welded ash-flow tuffs 

along Gold Creek that are uncertainly identified as the 

upper tuff and, possibly, the Ra Jadero Member of the 

Treasure Mountain (Platoro map). 

The high stratigraphic position of some (probably 

rhyodacitic) lava flows and interbedded sedimentary 

-rocks near the top of Red Mountain along the southeast 

rim of the Platoro caldera (Platoro map) suggests that 

15. Red-brown porphyritic rhyodacite, containing plagioclase phenocrysts as long as 2 em. A clast in the Los Pinos Formation, almost certainly derived from the volcanics of Green Ridge. "North slope of Green Ridge, above Goat Ranch" (Larsen and Cross. 19.56. table 23, No. 8). 16. Gray glassy porphyritic rhyodacite, much like No. 15. Also a clast in the Los Pinos For- mation. "Northeast slope of Green Ridge, ridge east of house, at altitude of 8..'i00 feet" (Larsen and Cross, 1956, table 21, No. 9). 17. Red-brown porphyritic quartz latite. Contains phenocrysts of plagioclase as long as I em in a dusty submicroscopically devitrified groundmass. Chemically anomalous: K10 is low; CaO, P20r., and perhaps Si02 are high. South slope of Jacobs Hill. at about 9.:300 ft. 

18. Dark-gray porphyritic rhyodacite. In addition tq the large phenocrysts :J-10 mm across, the finely devitrified groundmass contains small plagioclase laths about 0.1-0.2 mm long that define a flow foliation. "Collected along the eastern branch of Park Creek due west of the summit of South Mountain" (Steven and Ratte, 1960, table :3. No.1). 19. Gray porphyritic quartz latite, containing feldspar and quartz phenocrysts varying from one to several centimeters in diameter. Carbonate replaces feldspar phenocrysts. as well as groundmass. Modal estimate very approximate, because of coarse phenocryst size. "About 2,400 feet east of the summit of South Mountain" (Steven and Ratte. 1960. table 3, No. 3). 20. Gray porphyritic quartz latite, generally similar to No. 19. "Collected from within 200 feet" of No. 19 (Steven and Ratte, 1960, table 3, No. 2). 21. Gray porphyritic quartz latite to rhyolite, in general similar to Nos. 19 and 20, hut relatively free of carbonate. "The core of the South Mountain volcanic dome. collected · from the nose of the ridge extending east from the Iowa mine workings at an altitude of about 11,700 feet" (Steven and Ratte, 1960, table 3, No.4). 22.. Black vitrophyric rhyolite. "Contains sanidine, plagioclase, biotite, and hornblende in a · fresh black glass. From Lookout Mountain" (Patton, 1917, p. 32). 23. Black vitrophyric rhyolite. "Collected about two-thirds of the way up the south face of North Mountain" (Steven and Ratte, 1960, table 3, No. 6). 24. Dark-gray phenocryst-poor rhyodacite. See fig. 4.5C for description. Near base of lava flow, at about 12,00 ft, above small pond 0.6 km northwest of west peak ( 12,.568 ft) of North Mountain. 25. Dark-gray phenocryst-poor rhyodacite, much like No. 24. Near base of flow, at about 

12,375 ft, at saddle 0.4 km southwest of west peak (12,568 ft). 

26. Intermediate-comJ)9Sition phase, showing extreme disequilibrium. Well-developed flow layering, with abrupt local variations in phenocryst content. At 12,400 ft. along ridge 0.:1 km southwest of west peak (12,568 ft). 27. Intermediat~-composition phase, showing phenocryst disequilibrium. See fig. 4.5H for sample description. Very similar to No. 26 but has higher phenocryst content: only J.S m from No. 26. _ 28. · Typical light-gray phenocryst-rich rhyolite. See fig. 45A for sample description. At 12, 12f, 

ft," southeast of main ridge, about 9.2 km from 12,410-ft spot elevation. 29. Dark-gray nearly, vitrophyric margin of vent intrusive. At southwest contact on ridge, at about 11,925 ft. 

these rocks may be correlative with the andesiterhyodacite unit of the volcanics of Green Ridge; but 

without firm evidence, such as control from the ash-flow 

stratigraphy, the rocks on Red Mountain have been tentatively included in the Summitville Andesite. 

LITHOLOGIC DESCRIPTION 

~.Lavas of the Summitville Andesite consist of sparsely 

porphyritic dark andesite and minor rhyodacite (fig. 38). 

Phe~ocrysts are generally less than 10 percent of the 

rock, are rarely more than 20 percent, and are typically 

sma_ll-2 mm or less. Tabular or blocky plagioclase 

(An35-55), less abundant equant augite, and opaque oxides are the dominant phenocrysts; a few samples contain sparse hypersthene. Biotite and hornblende are 

rare. Seven samples of these lavas range in Si02 content 

fro~ 56.4·to 60.9 percent (table 9, Nos. 4-10). 

These petrologic features characterize especially the 

great sequence of lower member lavas exposed over a 

thickness of at least 500 m in the eastern caldera moat, 

along the Alamosa River canyon east of Jasper (Platoro 

map)·. At least 12 monotonously uniform ·sparsely 

porphyritic dark andesitic flows, each 10-40 m thick and 

tr~ceable laterally for 100-1,000 m, constitute the bulk of 

this seCtion and are separated by only minor andesitic 

sandstone interbeds. 

Th~ lowe~ member is somewhat more variable in its 

other major accumulation, in the southwestern part of 

the Platoro caldera moat (Platoro map). The lowest flow 
LAVAS AND RELATED ROCKS 63 

FIGURE 38.-Photomicrographs of the lower member of the Summitville 

Andesite, northeast of Conejos Peak . .cl., Typical sparsely porphyritic 

dark andesite. Finely devitrilied groundmass with a pilotaxitic texture 

surrounds small phenocrysts of plagioclase (P) (An,"·ool, augite (A), 

hypersthene (hy), Fe-Ti oxides (ft), and sparse small brown 

hornblende (hb) with opaque oxidized margins. Saddle Creek road 

at 10,100 feet. Analyzed sample 7, table 9. B, Coarsely crystalline 

andesite that forms widespread thick flow i;:;-southwestern part of 

Platoro caldera. Contains phenocrysts of abundant small strongly 

zoned plagioclase (An,o-55), a few obscure augite and hypersthene, 

and serpentine pseudomorphs after olivine. Plagioclase, pyroxenes, 

and Fe-Ti oxides in intergranular ground mass are relatively coarse, 

averaging about 0.1 mm in diameter. Saddle Creek road at 10,700 

feet. Analyzed sample 6, table 9. £. Dark sparsely porphyritic 

andesite with a partly glassy groundmass. Phenocrysts of plagioclase 

(P) (An,5), small augite, and sparse Fe-Ti oxides in a hyalopilitic 

ground mass. Sample from 11,600 feet, just below lip of cirque basin 

on north side of Conejos Peak. 

in this area, directly overlying the rhyodacite of Fisher 

Gulch, is a distinctive red-brown-weathering andesite 

that is chemically similar to other Summitville lavas 

(table 9, No. 6) but that is distinguished by a fairly 

coarse devitrification texture (fig. 38B). Abundant 

plagioclase laths 0.5-2 mm long define a diabasic texture 

readily visible in hand specimen. This flow is as much as 

150m thick where it forms a conspicuous continuous cliff 

between Fisher Gulch and Beaver Lake Creek (Platoro 

map); there, it was interpreted as a sill by Larsen and 
64 PLATORO CALDERA COMPLEX, SAN JUAN MOUNTAINS, COLORADO 

Cross (1956, pl. 1, p. 106), presumably because of its 

thickness and coarse texture. However, the same flow or 

a texturally similar single flow is traceable nearly continuously to the west, up the Lake Fork to the north side 

of Conejos Peak (fig. 63), where it directly overlies the in-

. tracaldera La Jara Canyon Member and has a flowbrecciated basal contact. 

Overlying this distinctive flow is a sequence of at least 

five flows in the Forest King area. There, the common 

sparsely porphyritic andesite flows (fig. 38A) are interlayered with darker andesitic flows characterized by 

a partly glassy pilotaxitic groundmass texture (fig. 38C). 

These partly glassy flows tend to form relatively thick 

local accumulations rather than laterally extensive 

sheets, and they are locally characterized by intricate 

closely spaced joints that define irregularly curved 

columns 0.5-1 m in diameter and 4-10 min length. The 

bulbous shape, glassy textures, and irregular columnar 

jointing suggest that these lavas may have been 

deposited on wet sediments or in shallow water. 

Unusual agglutinated andesitic breccias having steep 

foliations are preserved at the top of the section of Summitville Andesite along the southern caldera wall. These 

breccias seem to fringe the Summitville Andesite around 

the north side of Conejos Peak but are well exposed only 

on the lip of the major cirque basin on the north side of 

the peak (fig. 39A). There, the breccia forms the top and 

margin of an otherwise normal andesite flow, about 60 m 

thick, which was deposited against the Conejos FormaA 

FIGURE 39.- Features of lower member of Summitville Andesite along 

south wall of Platoro caldera. A, View of Conejos Peak from near the 

head of t he Lake Fork. Upper part of Conejos Peak consists of 

hornblende rhyodacite lavas of the Conejos Formation that define 

the south wall of Platoro caldera. Lava flows in foreground and flooring the cirque basin on Conejos Peak are intracaldera lavas of the 

tion on the caldera wall. The breccia varies laterally and 

vertically from a weakly indurated mass containing 

monolithologic blocks of scoriaceous and dense lava, to 

an indurated compact breccia, to an extremely indurated breccia that has undergone flowage to such an 

extent that it appears flow layered (fig. 39B). All 

gradations occur both vertically and laterally within a 

few meters in continuous excellent exposures. The flow 

layering is very steep, varying from 50° to nearly 90°, 

and appears most plausibly to reflect draping and compaction against the steep caldera wall. This relation 

would only seem possible if these lavas were originally 

erupted outside the caldera, enabling brecciation, 

agglutination, and secondary flowage to take place as the 

lava cascaded over the wall of the caldera to the floor. 

Where not intensely hydrothermally altered, most 

lavas of the upper member of the Summitville Andesite 

are little different in petrography from the abundant 

sparsely porphyritic lavas of the lower member. The best 

exposed sections, on Cornwalls Nose and along the 

Wightman Fork (Platoro map), are both more than 800 

m thick. In these sections the flows differ from lavas of 

the lower member to the east in being thicker, less 

tabular or laterally traceable, and in places pervasively 

internally brecciated. (See detailed description in 

Steven and Ratte, 1960, p. 11-12.) These features suggest 

near-source deposition. 

Interlayered with the typical sparsely porphyritic 

andesites in the upper member are more coarsely 

B 

lower member. Arrow indicates location of agglutinate breccia in B. 

B, Intensely flowed and compacted agglutinate breccia of lower 

member where it was deposited against steep slope of south caldera 

wall. This depositional foliation dips 70°-90°. On lip of cirque basin 

at 11,800 feet on north side of Conejos Peak. 
LAVAS AND RELATED ROCKS 65 

porphyritic dark lavas that contain phenocrysts of 

plagioclase (An3o-so ) as much as 1 em in diameter. Total 

phenocryst content of such flows varies from about 5 to 

25 percent. In some flows the phenocrysts are 

heterogeneously distributed, with more phenocrysts concentrated in the interiors. These coarsely porphyritic 

lavas, although not studied chemically, are probably 

more s1licic than typical Summitville Andesite. The 

coarsely porphyritic flows become more abundant upward in the lava sequence of the upper member between 

Wightman Fork and Marble Mountain (Platoro map), and 

they appear to constitute a transitional petrologic type 

between Summitville Andesite and the overlying coarsely porphyritic biotite-bearing rhyodacite of Park Creek. 

In the Bonito Peak-Marble Mountain area, some of the 

highest flows included in the upper member because 

they are interlayered with the more characteristic 

sparsely porphyritic andesite contain sparse 

phenocrystic biotite in addition to the ubiquitous 

plagioclase and augite. These are clearly transitional to 

the Park Creek unit and probably are rhyodacitic in 

composition. 

Upper member lavas that are exposed over a sizable 

area near the head of Park Creek (fig. 36) are different in 

lithology and thickness from those within the caldera. In 

the Park Creek area the upper member consists of only a 

few relatively thin flows that tend to be laterally persistent. In addition to the typical sparsely porphyritic 

andesite, a few flows of coarsely porphyritic platyplagioclase andesite are similar to the common type 

described in the Conejos Formation (fig. 6) and also to 

the flows interlayered higher in the ash-flow sequence 

(Huerto Andesite). These outflow andesites of the upper 

member are well bracketed stratigraphically by the Ojito 

Creek and Ra Jadero Members of the Treasure Mountain Tuff (Platoro map), in contrast to the intracaldera 

lavas of the upper member, which lack well-defined 

stratigraphic relations with the upper members of the 

Treasure Mountain. 

VOLCANICLASTIC ROCKS 

About a fourth or a· third of the Summitville Andesite 

consists of volcaniclastic sedimentary rocks, mainly gray 

sandstone and buff shale, that were eroded from the 

primary andesitic accumulations (fig. 40). These 

sedimentary rocks interfinger intricately with the lavas 

and breccias, and they could only easily be mapped 

separately on the Platoro map locally where they constitute most of the formation. Most of the sediments apparently were deposited in a deltaic environment, but 

some may represent shallow-water lake deposits. In 

places where clay-rich volcaniclastic rocks are interlayered with lavas of the Summitville Andesite, as, 

for example, high on the north side of Red Mountain 

(Platoro map), great landslides characteristically have 

developed. 

B 

FIGURE 40.-lntracaldera volcaniclastic sedimentary rocks of the Summitville Andesite. A, Volcaniclastic rocks at east caldera walL Lightgray bedded tuffaceous sediments are deltaic or shallow-water lake 

deposits within caldera. Distant timbered slopes are andesitic lavas 

and mudflow breccias of Conejos Formation on caldera walL Outcrops high on caldera wall (arrow) here consist of agglutinated 

bedded ash fall material that appears to have been welded as it was 

deposited, presumably because of proximity to its source. Alamosa 

River road, just west of Ranger Creek. B, Intracaldera sandstone and 

shale containing plant remains. Rito Gato, along Platoro Reservoir 

road. 

The southeast caldera moat, north of Red Mountain, 

apparently contained the thickest accumulation of intracaldera sediments; about 300 m is preserved there. 

These sedimentary rocks interfinger to both the 

northeast and the southwest with intermediatecomposition lavas. To the northwest the sedimentary 

rocks originally lapped against the Cornwall Mountain 

resurgent block of the intracaldera La Jara Canyon 

Member of the Treasure Mountain Tuff; this contact 

was subsequently modified by faulting. Toward the 

resurgent block, the predominantly sandstone deposits 

coarsen and grade into monolithologic fanglomerate 

breccia containing blocks of the La Jara Canyon 
66 PLATORO CALDERA COMPLEX, SAN JUAN MOUNTAINS, COLORADO 

Member that are as much as 1 m in diameter and that 

represent the gray partly welded top of the ash-flow 

sheet. This relation demonstrates that the resurgent 

block was already a structural and topographic high 

when the lower member of the Summitville Andesite 

filled the caldera moat. 

Poorly preserved, fragmented carbonized plant 

remains have been found in the intracaldera sedimentary rocks in several places: most abundantly along 

roadcuts and streams near the mouth of Rito Gato (fig. 

40B) and in a few small slumped outcrops along Beaver 

Lake Creek at an elevation of about 10,050 feet (Platoro 

map). Preliminary study by Estella Leopold of the U.S. 

Geological Survey (written commun., 1972) indicated 

that rocks from the Beaver Lake Creek locality 

(U.S.G.S. paleobot. loc. D4772, samples A-E; NE 1/4 sec. 

1, T. 35 N., R. 4 E., elev. 10,000 ft) contain an excellently 

preserved pollen assemblage. The main forms recognized 

are: 

Pinus 

Picea, large form 

Selaginella cf. S. densa 

Typha cf. T. angustifolia 

Ephedra cf. E. nevedensis, cf. E. torreyana, and an Ephedra 

species typical of White River Group 

Chenopodiaceae undet. and Sarcobatus 

Acer of A. negundo 

cf. Juniperus 

cf. Salix 

Ulmaceae, p4 and p5 

Abies cf. A. venusta 

Cruciferae? 

cf. Parthenocissus 

Carya? 

Acrostichium type 

Dodonaea? 

Juglans or Pterocarya 

Juglans 

Compositae? (contaminant?) 

Leopold reported that "this flora is intermediate in 

aspect between flora of the Florissant Lake Beds (lower 

Oligocene) and flora of the Creede Formation (uppermost Oligocene). It is more diverse than the Creede flora 

and yet resembles the flora at Creede more than it does 

that of the Florissant Lake Beds. It is certainly Oligocene 

and probably late Oligocene in age." 

Exceptionally tuffaceous intracaldera sandstones and 

conglomerates that have been extremely silicified to 

white, buff, and light-purple dense slabby rocks occur 

along the divide just within the east wall of the Summitville caldera, where they have been the cause of the misleading name of "Marble Mountain" on topographic 

maps. These rocks, which apparently originated as tuffaceous deltaic or lake deposits, differ conspicuously 

from sedimentary rocks of the lower member to the east, 

outside the caldera, that are of andesitic provenance. 

This contrast in sediment types, in conjunction with 

significant local differences in lava types, marks the 

topographic wall of the Summitville caldera. Marble 

Mountain is the only place where this topographic boundary is preserved within the Platoro caldera complex. 

Elsewhere, the margin of the Summitville caldera is 

marked by the Cornwall fault, which represents the 

structural boundary of the caldera rather than its 

topographic wall, or is covered by postcaldera lavas. 

SOURCE VENTS 

Most of the source vents for the lavas of the Summitville Andesite are poorly understood. Two main lava accumulations of the lower member, in the eastern Platoro 

caldera moat along the lower Alamosa River and in the 

southwestern moat northwest ofConejos Peak (fig. 36), 

are separated by the thick accumulation of volcanclastic 

sedimentary rocks northeast of Red Mountain. The 

eastern accumulation clearly interfingers with sediments 

to the east toward the caldera wall, and the thickness of 

individual flows appears to increase to the west, right up 

to truncation by the younger Summitville caldera. These 

lavas probably represent the east flank of a large intracaldera andesitic stratovolcano, whose core is farther 

to the west, within the Summitville caldera area. Like 

the eastern accumulation, the southwestern accumulation of lavas of the lower member has no exposed source 

within the Platoro caldera. The source could have been 

to the north, within the area that later collapsed to form 

the Summitville caldera, or it could have been outside 

the caldera, as is perhaps suggested by the relations on 

the north side of Conejos P~ak (fig. 39). Possibly, the Canyon Diablo stock (fig. 36) marks the dissected core of a 

volcano from which these lavas were erupted. 

The eruptive source of the lavas of the upper member 

is equally obscure. Thickness, local lateral irregularity, 

and chaotic brecciation of upper member lavas in the 

Corn walls Nose-Wightman Fork area suggest that these 

lavas are not far from their source and that they perhaps 

constitute part of a cone complex. Measurements of flow 

attitudes are too few and inconsistent to make this conclusion firm, however, and the extreme alteration to the 

northwest along the Alamosa River further impedes interpretation. Tentatively, the 29.1-m.y. Alamosa River 

stock is interpreted as the central intrusive of an eroded 

volcano source for the main mass of the intracaldera upper member lavas. This interpretation is based mainly on 

appropriateness of compositions (andesitic = monzonitic), radiometric age (older than Masonic Park 

Tuff), and location (intrusive into the thick chaotic nearsource portion of the upper member lavas). 

AGE AND MAGNETIC POLARITY 

The lower member of the Summitville Andesite is 

younger than 29.8 m.y., the well-established age of the 

underlying La Jara Canyon Member of the Treasure 
LAVAS AND RELATED ROCKS 67 

Mountain Tuff. Much of the thick section of the upper 

member of the Summitville Andesite is intruded by and 

therefore older than the 29.1.-m.y. Alamosa River stock 

(Lipman and others, 1970). A few topographically and 

stratigraphically high patches of the upper member 

might be slightly younger than 29.1 m.y. but cannot be 

younger than 28.2 m.y., the date on the overlying 

Masonic Park Tuff. 

Both lower and upper members of the Summitville 

Andesite seem to be characterized by normal magnetic 

polarities, as indicated by preliminary determinations 

on 12 separate flows (M. C. Beck, Jr., and James Diehl, 

written commun., 1971). 

ANDESITE OF SUMMIT PEAK 

The andesite of Summit Peak is named for a small 

group of scattered erosional remnants of intermediatecomposition lavas and breccias that overlie the Masonic 

P1;1rk Tuff along the continental divide west of the 

Platoro caldera complex. Because these lavas cap the 

volcanic sequence wherever they are exposed, their exact 

age is uncertain, as is their correlation with 

intermediate-composition lavas interlayered with the 

ash-f1ow sequence elsewhere. The andesite of Summit 

Peak is similar in lithology and is approximately 

equivalent in age to the andesite-rhyodacite unit of the 

volcanics of Green Ridge on the opposite side of the 

caldera, and it appears to represent approximately similar 

postcollapse caldera-margin igneous activity. 

The Summit Peak rocks may also be equivalent to 

f1ows of Sheep Mountain Andesite that interfinger with. 

and overlie the Masonic Park Tuff to the west near Wolf 

Creek Pass. However, because the andesite of Summit 

Peak is restricted to the west margins of the Platoro 

caldera complex and has a close spatial association with 

the upper member of the Summitville Andesite, it is 

most validly considered as a continuation of the Summitville Andesite activity. The Summit Peak f1ows and 

the Sheep Mountain Andesite are difficult to distinguish lithologically, although the andesite of Summit 

Peak may be slightly more silicic on the average and may 

include more rhyodacite. North of Summit Peak, where 

the Masonic Park Tuff wedges out, the formational 

assignment of several small isolated hills of andesitic 

rocks is tenuous (Platoro map). 

By far the thickest accumulation of this unit is at 

Summit Peak, where at least 10 f1ows form an aggregate 

thickness of about 200 m. Most of these f1ows are 

andesite that contains 10-20 percent small phenocrysts 

of plagioclase and augite, but some are probably 

rhyodacite. The only chemical analysis of the andesite of 

Summit Peak (table 9, No. 11) is clearly andesitic. One 

f1ow of porphyritic platy-plagioclase andesite is similar 

to the distinctive rock type that is so abundant in the 

Conejos Formation (fig. 6); this platy-plagioclase flow is 

one of the rare recurrences of that type within the postcollapse lava sequence of the Platoro caldera complex, 

although it is the dominant lithology of the Huerto 

Andesite within the ash-flow sequence farther to the 

northeast. 

VOLCANICS OF GREEN RIDGE 

The volcanics· of Green Ridge are a heterogeneous accumulation of lava flows and breccias, ranging in composition from andesite to rhyolite. These rocks represent 

the erosional remnants of a stratovolcano, as much as 25 

km in basal diameter, that developed near the east rim 

of the Platoro caldera complex (fig. 36). This postulated 

former stratovolcano is called the Cat Creek ·volcano, 

because it appears to be centered at the Cat Creek stock, 

which is interpreted as the intrusive core of the volcano. 

The volcanics of Green Ridge appear to have originally 

dipped outward 5°-15° from this central area; at present, 

dips are steeper on the east and more nearly horizontal 

on the west, as a result of 5°-10° regional eastward tilting 

of the southeastern San Juan Mountains toward the San 

Luis Valley. Three informal subdivisions recognized 

within the volcanics of Green Ridge are, in ascending 

sequence: the rhyolite. unit, the andesite-rhyodacite unit, 

and the porphyritic rhyodacite unit. These three units 

are separately mapped on the Platoro map but not on 

the maps accompanying this report. 

RHYOLITE UNIT 

The rhyolite unit of the volcanics of Green Ridge consists of buff to white phenocryst-poor rhyolitic lava, f1ow 

breccia, and minor pumiceous tuff. It seems to represent 

the initial and areally most restricted activity of the Cat 

Creek volcano. It is preserved only over a few square 

kilometers near the head of Cat Creek, where it forms 

parts of the walls and roof of the Cat Creek stock at the 

core of the volcano (Platoro map). 

Within about 100 m of the Cat Creek stock, the 

rhyolite has been recrystallized to dense blocky hornfels 

(fig. 41A, B) that crops out as erosion-resistant cliffs and 

jagged ribs; elsewhere, the rhyolite breaks down readily 

to small f1akes and scree and is therefore poorly exposed 

on subdued slopes. Primary structural features of the 

rhyolite are obscure in the well-exposed hornfels because 

of recrystallization, but elsewhere the rhyolite is 

characterized by fine f1ow layers a few millimeters to a 

few centimeters thick. The flow layering is broadly subhorizontal but is highly irregular locally and has not 

been studied in detail. Many outcrops are of 

monolithologic breccia, consisting of blocks a few centimeters to at least 1 m in diameter that probably 

resulted from flow brecciation of the lava. Rarely, near 

the base of the rhyolite where the rhyolite rests on the 

Conejos Formation, thin beds of lapilli tuff of similar 

rhyolitic composition seem to have resulted from explosive ash and pumice eruptions. 
68 PLATORO CALDERA COMPLEX, SAN JUAN MOUNTAINS, COLORADO 
LAVAS AND RELATED ROCKS 69 

The basal contact of the rhyolite is irregular, showing 

about 200 m of exposed relief; this indicates either that 

the rhyolite covered very rugged erosional topography or, 

more likely, that the rhyolite is intrusive in its lower 

parts and constitutes the remnants of an intrusive· 

extrusive vent· dome complex. The entire area of rhyolite 

seems plausibly to have been a single lava dome, 

although the fragmentary exposures, hornfelsic 

recrystallization and brief nature of this study make this 

conclusion only tentative. 

The rhyolite contains only about 1 percent 

phenocrysts (fig. 41A). These are typically small, no 

more than 1 mm in diameter, and are plagioclase (An2o·3o) 

accompanied by sparse flakes of biotite. The rhyolite 

unit has not been analyzed chemically, but petrographic 

similarity to analyzed phenocryst·poor rhyolitic lavas in 

the San Juan field, such as the rhyolite of Summer Coon 

volcano (Lipman, 1968, table 1, Nos. 4-6), suggests that 

the rhyolite unit of the volcanics of Green Ridge contains 

70-72 percent Si02. 

.The rhyolite is younger than the Conejos Formation 

and older than the Cat Creek stock. Unlike upper units 

of the volcanics of Green Ridge, the rhyolite cannot be 

related directly in.age to the sequence of ash-flow sheets, 

because it is nowhere in contact with them. Clasts of 

petrographically similar rhyolite in the Los Pinos Formation, where the Los Pinos overlies the Masonic Park 

Tuff and is covered by the andesite-rhyodacite unit in 

the Green Ridge area, suggest that the rhyolite formed a 

topographic high that was being eroded at this time. In 

addition, the distribution of the rhyolite unit, restricted 

to the core of the volcano, indicates that emplacement of 

the rhyolite unit was the initial activity of this volcano 

and probably occurred after deposition of the 28.2-m.y. 

Masonic Park Tuff. 

FIGURE 41 (left).-Photomicrographs of characteristic rock types of 

volcanics of Green Ridge. A, Rhyolite unit, about 300m west of contact with monzonite stock, South Cat Creek. Single small 

phenocryst of plagioclase (Anzs) is surrounded by cryptocrystalline groundmass that displays a mottled texture probably due to 

weak contact metamorphism. B, Rhyolite unit, about 1m from contact with monzonite stock, South Cat Creek. Phenocrysts are not 

recognizable; rock has been recrystallized to a fine microcrystalline 

mosaic of plagioclase, sanidine, and quartz. C, Andesite-rhyodacite 

unit, south slope of Jacobs Hill at about 9,350 feet. Finely devitrified 

dark groundmass having a pilotaxitic texture surrounds abundant 

microphenocrysts of plagioclase (An40·4s), augite, Fe-Ti oxides, and 

serpentine pseudomorphs after olivine that are surrounded by reaction rims of finely granular augite. Analyzed sample 12, table 9. D, 

Porphyritic rhyodacite unit. Finely devitrified interior of thick flow 

contains about 30 percent phenocrysts of plagioclase (P), biotite (B), 

augite (not visible), olivine (not visible), and Fe-Ti oxides (ft). 

Northeast side of Green Ridge at about 9,200 feet. Radiometrically 

dated sample (Lipman and others, 1970, table 4, No. 5). 

ANDESITE-RHYODACITE UNIT 

The andesite-rhyodacite unit of the volcanics of Green 

Ridge consists of dark-gray to red-bro'Wn lavas and breccias of finely porphyritic andesite and rhyodacite. This 

unit constitutes much of the exposed volcanics of Green 

Ridge near Chiquita Peak and in the Green Ridge area, 

as well as all the probably correlative rocks in the Silver 

Mountain-Bennett Peak area to the west (fig. 36). 

Although correlation of the rocks of this western area 

with the Cat Creek volcano is uncertain and some of 

these lavas may have been derived from a local center 

near Silver Mountain, the rocks are included in the 

andesite-rhyodacite unit on the basis of generally similar 

lithology and age. 

The andesite·rhyodacite unit overlies the Masonic 

Park Tuff in the Chiquita Peak-Green Ridge area, 

whereas the approximately equivalent rocks to the west, 

in the Silver Mountain-Bennett Peak area, are above the 

Fish Canyon Tuff within the Platoro caldera. In both 

areas, volcaniclastic sedimentary rocks of the Los Pinos 

Formation locally lie between ash-flow tuffs and overly: 

ing lavas (fig. 42). North of Green Ridge, the conglomerate of the Los Pinos Formation that occurs 

between the Fish Canyon and Carpenter Ridge Tuffs 

(Durango map) contains clasts that apparently were 

derived from the andesite-rhyodacite unit, as well as 

from the overlying porphyritic rhyodacite unit. In the 

Chiquita Peak and Green Ridge areas, the andesiterhyodacite unit is capped by the porphyritic rhyodacite 

unit and in places also by basalt flows of the Hinsdale 

Formation. 

Most of the andesite-rhyodacite unit was erupted 

before the porphyritic rhyodacite, but these two 

lithologies locally interfinger. At Jacobs Hill just 

southwest of Chiquita Peak (fig. 36) a thin flow of 

porphyritic rhyodacite (not shown on the Platoro m·ap) 

occurs beneath the andesite-rhyodacite unit. A similar interfingering may occur near the crest of Chiquita Peak, 

east of the detailed Platoro map area. 

Included in the andesite-rhyodacite unit are many 

somewhat petrographically varying lava flows and breccias. Individual lava flows are typically 10-20 m thick, 

although a single thick flow on the southeast side of 

Silver Mountain is at least 125 m thick. Interbedded 

volcaniclastic sedimentary rocks seem to be minor. 

Some of the flows are nonporphyritic, but most contain 

5-15 percent small phenocrysts (mostly about 1 mm in 

diameter), mainly of plagioclase (An3o·so) and augite. An 

exceptional flow on the west slope of Bennett Peak contains about 25 percent phenocrysts of plagioclase (An4o- 55 ) 

augite, hypersthene, and Fe-Ti oxides. Some lava~ 

that appear megascopically nonporphyritic actually contain abundant microphenocrysts of plagioclase and 

pyroxene no more than 0.5 mm in diameter (fig. 41C). 

Three chemical analyses of this unit vary from 58.8 to 
70 PLATORO CALDERA COMPLEX, SAN JUAN MOUNTAINS, COLORADO 

almost 66 percent SiOz, calculated volatile free; 

however, the highest SiOz value is an old analysis 

(Larsen and Cross, 1956, table 21, No. 16) that shows 

more than 3 percent HzO (table 9, No. 14), and it may be 

anomalous. Petrographic comparisons with other 

·analyzed intermediate-composition lava flows from the 

southeastern San Juan area suggest that most samples of 

this unit would contain between about 57 and 64 percent 

SiOz. 

At Green Ridge the age of the andesite-rhyodacite unit 

is bracketed by the underlying 28.2-m.y. Masonic Park 

Tuff and by the overlying porphyritic rhyodacite unit, 

dated at about 27.4 m.y. (Lipman and others, 1970). At 

the time of eruption of the 27.8-m.y. Fish Canyon Tuff, 

the Cat Creek volcano appears to have been sufficiently 

large to interfere with spreading of this ash-flow sheet to 

the south, as already discussed, yet intermediatecomposition lavas of the andesite-rhyodacite unit continued to erupt after emplacement of the Fish Canyon, 

as indicated by the relations at Silver Mountain. 

PORPHYRITIC RHYODACITE UNIT 

The porphyritic rhyodacite unit of the volcanics of 

Green Ridge consists of a few thick light-gray lava flows 

of coarsely porphyritic rhyodacite and minor quartz 

latite. The porphyritic rhyodacite is exposed mainly 

along the crest and on the northeast slopes of Green 

Ridge; smaller patches are preserved on the northwest 

ridge of Chiquita Peak, on Jacobs Hill, and on the high 

double knob about 5 km farther west (fig. 36). In various 

places, the porphyritic rhyodacite overlies volcaniclastic 

sedimentary rocks of the Los Pinos Formation derived 

from early activity of the Cat Creek volcano, overlies and 

interfingers with the andesite-rhyodacite unit of the 

volcanics of Green Ridge, and is overlain by basalt flows 

of the Hinsdale Formation. 

The porphyritic rhyodacite is characterized by 25-40 percent large phenocrysts, including plagioclase (An3o-4s) 

grains as long as 1 em, biotite, and hornblende or 

augite (fig. 41D). The augite-bearing lavas contain a second type of mafic phenocryst, identified as olivine in 

one nearly glassy sample, . but these phenocrysts are 

generally altered completely to fine-grained serpentinetype minerals. Three analyses of this unit range from 

60.8 to 66.9 percent SiOz (table 9, Nos. 15-17). The two 

samples that are relatively low in SiOz are actually clasts 

from the Los Pinos Formation (Larsen and Cross, 1956, 

p. 191); these contain augite and are petrographically 

similar to the flow that caps Green Ridge. In contrast, 

the single analyzed augite-free hornblende-bearing sample is distinctly more silicic and is a quartz latite. The 

change, at about 65 percent SiOz, from augite to 

hornblende as the second most abundant mafic after 

biotite in the coarsely porphyritic lavas also occurs in 

many of the lavas around Summitville. 

FJGURE 42.-Volcaniclastic sedimentary rocks of the Los Pinos Formation overlain and fused by a thick nonporphyritic andesite or 

rhyodacite lava flow of the volcanics of Green Ridge (arrows indicate 

base of flow). South side of Silver Mountain, at 11,700 feet. Note person for scale. 

RHYODACITE OF PARK CREEK 

The rhyodacite of Park Creek is a new name for the 

predominantly porphyritic biotite-bearing assemblage of 

thick lavas, flow breccias, and minor pyroclastic rocks 

that were erupted as lava-dome complexes around the 

northwest margin of the Summitville caldera after its 

final collapse. This rhyodacite is similar in petrology and 

age to the porphyritic rhyodacite unit of the volcanics of 

Green Ridge, just described, and the two units are distinguished mainly by their separate areas of accumulation, which reflect separate sources. The rhyodacite of 

Park Creek essentially consists of the rocks described by 

Steven and Ratte (1960, p. 14-18) as the rhyodacite lavas 

of the lower member of the Fisher Quartz Latite, but use 

of the name Fisher Quartz Latite was subsequently 

restricted to lavas and breccias that are localized around 

the Creede caldera and that postdate the ash-flow se-
LAVAS AND RELATED ROCKS 71 

quence there, leaving the Summitville assemblage 

without a formal designation (Steven and Ratte, 1965, p. 

43-44). 

The lavas of the rhyodacite of Park Creek are concentrated along the northern and western topographic walls 

of the Summitville caldera, and they are especially thick 

and well exposed in upper Park Creek (fig. 36). These 

lavas rest partly on the upper member of the Summitville Andesite within the caldera and partly on older 

rocks of the caldera wall that range from the Conejos 

Formation to the younger ash-flow sheets. North and 

northeast of the caldera rim the rhyodacite of Park Creek 

interfingers with and overlies volcaniclastic sedimentary 

rocks of the Los Pinos Formation that formed by nearly 

contemporaneous erosion and reworking of the 

rhyodacitic lavas. Locally, the rhyodacite of Park Creek 

is overlain by distinctly younger rocks, such as the 

quartz latite of South Mountain, the rhyolite of Cropsy 

Mountain, and the basaltic lavas of the Hinsdale Formation. 

Individual flows of the rhyodacite of Park Creek are as 

much as 150 m thick, especially where well exposed· 

along the continental divide west of Summitville. Each 

flow typically contains a basal flow breccia a few meters 

thick that is overlain by a thick dark-gray vitrophyre 

zone as much as 10m thick (fig. 65) and then by generally structureless lighter gray devitrified rock in the interior 

and upper parts of the flow. Flow layering is well 

developed only near the basal vitrophyre and near the 

top of the flow, where it tends to be steeply inclined. (See 

Christiansen and Lipman, 1966, fig. 3.) The geometry of 

many individual flows is complex, owing to emplacement on irregular topography adjacent to the caldera 

wall. In general, individual flows were not separated in 

the field, because exposures and time were limited. 

Three vent structures were recognized for the 

rhyodacite of Park Creek; additional vents probably are 

present but are covered by younger volcanic rocks or are 

simply obscure owing to poor exposure. The largest vent 

feature is a mass of intrusive rhyodacite porphyry, about 

3 km long and almost 1 km wide, that is well exposed 

along the divide between the East and West Forks of 

Park Creek abouf 5 km northwest of the caldera rim (fig. 

36). This body, which has the form of a thick stubby sill 

dipping about 40°-50° to the northeast, is localized by 

the Summitville fault, a major fault on the northeast 

side of the Elwood Creek fault zone (fig. 61). The intrusive porphyry is continuous to the southwest into a 

thick extrusive flow, characterized by gently dipping 

f1ow layering (Platoro map), which differs only slightly 

in appearnce from intrusive parts of the vent complex; 

compared with the flow, the intrusive is slightly lighter 

in color owing to coarser groundmass crystallization and 

is slightly more massive, less clearly f1ow layered, and 

more regularly jointed. This intrusive and the fault that 

localized it have tilted the welded tuffs of its roof to near 

vertical dips along Park Creek (Platoro map, cross section D-D · ). 

A similar but smaller vent was the source of the 

isolated patch of lava between Summitville and Jasper 

(fig. 36). The western end of this body is clearly intrusive, cutting steeply through lavas of the upper 

member of the Summitville Andesite, but it merges with 

gently dipping flow-layered lavas on the crest of the 

ridge (Platoro map). Because this body contains sparse 

large sanidine phenocrysts in association with both 

hornblende and augite, especially ,in its intrusive part, 

and is petrographically transitional toward typical 

quartz latite of South Mountain, its correlation with the 

rhyodacite of Park Creek is uncertain. 

A third rather different vent structure, which forms a 

steep hill near the northwest caldera rim (elev. 11,874 

feet; Platoro map), apparently is the eroded remnant of 

a pyroclastic cone. Here, layers of irregularly outward 

dipping bedded agglutinated tuff alternate with variably 

indurated monolithologic breccia that consists of blocks 

of rhyodacite. 

The rhyodacite of Park Creek typically contains about 

20-30 percent·phenocrysts (fig. 43A). Blocky plagioclase 

(An3o-5o) crystals as much as 10 mm in diameter constitute 75-90 percent of total phenocrysts ·and are accompanied by lesser amounts of biotite, augite, and Fe-Ti 

oxides. Hornblende is rarely present instead of augite. 

Quartz and sanidine are absent or very sparse. The 

petrography of this unit is described in more detail by 

Steven and Ratte (1960, p. 17). 

The rhyodacite of Park Creek can generally be distinguished from Summitville Andesite by the presence of 

biotite and larger more abundant plagioclase 

phenocrysts; it differs from the quartz latite of South 

Mountain in paucity of quartz and sanidine and in the 

general presence of augite rather than hornblende as the 

second mafic. However, in a few atypical lava flows included in this unit for convenience, the phenocrysts are 

few and small, and biotite is sparse (fig. 43B); such rocks 

occur high in the unit, above the Snowshoe Mountain 

Tuff at the south end of Handkerchief Mesa (fig. 35), 

and also low in the unit, along the upthrown side of the 

fault zone west of Elwood Pass (Platoro map). These 

rocks seem to be more mafic than the typical coarsely 

porphyritic rhyodacite and are petrographically transitional toward Summitville Andesite. Within the 

caldera, especially, there may be no significant 

depositional hiatus between Summitville and Park 

Creek volcanic activity: as previously noted, porphyritic 

sparsely biotitic local lavas of transitional composition 

and texture are also interlayered with the upper member 

of the Summitville Andesite near the northern rim of the 

caldera complex. 

The rhyodacite of Park Creek has not been dated 
72 PLATORO CALDERA COMPLEX, SAN JUAN MOUNTAINS, COLORADO 
LAVAS AND RELATED ROCKS 73 

radiometrically, but its age can be approximated by 

reference to other units. The lowest flows outside the 

caldera complex are above the Fish Canyon Tuff (27 .8 

m.y., ta·ble 3) and below the Carpenter Ridge Tuff 

(>26.7 m.y., table 3). Lower lavas of the Park Creek unit 

that are petrographically transitional toward Summitville Andesite may represent yet earlier activity than any 

of the flows that can be stratigraphically related to the 

ash-flow sequence outside the caldera. Upper flows of 

this unit overlie the Snowshoe Mountain Tuff (;>26.4 

m.y.) at the south end of Handkerchief Mesa (fig. 35), 

and other late flows are cut by a sanidine porphyry dike 

in Schinzel Meadows that has been dated at 25.8 m.y. 

(Lipman and others, 1970, table 4, No. 6). The 

rhyodacite of Park Creek is also overlain by the quartz 

latite of South Mountain, dated at 22.8 m.y., as is discussed later. 

Thus, most of the rhyodacite of Park Creek seems to 

have been erupted between about 28 and 26 m.y. ago, 

but significantly earlier or later activity is possible. 

QUARTZ LATITE OF SOUTH MOUNTAIN 

The name "quartz latite of South Mountain" is given 

to a coarsely porphyritic lava dome and related rocks at 

least 300 m thick, near the west rim of the Summitville 

caldera (fig. 36). Although this body covers an area of 

only about 5 km2, it is of special interest as the host rock 

of the Summitville ore body. The quartz latite of South 

Mountain was discussed by Steven and Ratte (1960) but 

was referred to as the quartz latitic lavas of the lower 

member of the Fisher Quartz Latite, a usage subsequently abandoned by these authors. The quartz latite of 

South Mountain rests partly on dark lavas of the upper 

member of the Summitville Andesite and partly on the 

porphyritic rhyodacite of Park Creek. Because these 

FIGURE 43. (left).-Photomicrographs of typical lavas of the late 

viscous domes around the margins of the Summitviile caldera. A, 

Typical rhyodacite of Park Greek. Large phenocrysts of plagiocl:;e 

(P) (An35-45), augite (A), and biotite (B) with oxidized margins are 

set in a matrix of microphenocrysts of the same minerals and 

groundmass. Prominent nose at head of Park Creek, east of 

Summitville-Stunner road at about 11,400 feet. B, Rhyodacite of 

Park Creek, mafic type. Biotite phenocrysts are -:'bsent; about 25 

percent small phenocrysts of plagioclase (P), augite (A), and Fe-Ti 

oxides (ft) are surrounded by fine-grained groundmass with pilotax-

.ill£. texture. South end of Handkerchief Mesa, overlying the 

Snowshoe Mountain Tuff. (See fig. 35.) C, Quartz latite of South 

Mountain. Large phenocrysts, some showi~g disequilibrium relations, 

are resorbed embayed guartz (Q), sanidine (S), rounded plagioclase 

(P) with dusty marginal inclusions, and abundant smaller dark oxidized biotite and hornblende. Northeast shoulder of Cropsy~­

tain at about 12,200 feet. J1. Rhyolite of Cropsy Mountain. Large 

rounded and embayed phenocrysts of quartz (Q), sanidine (S) 

around plagioclase (P), green hornblende (H), and brow~e (not 

visible) in perlitic glassy groundmass containing abundant 

microlites that define swirly flow foliation. Basal vitrophyre, from 

saddle east of North Mountain, at about 12,300 feet. 

rocks-the quartz latite of South Mountain-were 

described in detail by Steven and Ratte (1960, p. 18-22, 

33-34) only the general features of this unit and a few 

new interpretations are summarized here. 

Where least altered, the quartz latite of South Mountain is a light-gray thoroughly devitrified massive lava in 

which crude flow layering is discernible locally; primary 

volcanic features of much of the main body at South 

Mountain have been obscured by intense acid-sulfate 

alteration. This quartz latite typically contains 20-30 percent phenocrysts, consisting of sanidine (Or7s-77), 

plagioclase (An2o-4o), quartz, biotite, and hornblende 

(fig. 43C). Although broadly similar in appearance to the 

underlying rhyodacite of Park Creek, this unit is distinguishable by the presence of sanidine, quartz, and, 

generally, hornblende rather than augite. Rounded and 

resorbed phenocrysts locally indicate disequilibrium 

relations (fig. 43C). The quartz latite of South Mountain 

is especially coarsely porphyritic over an area about 1 km 

in diameter, just south of Summitville ·(fig. 36; Steven 

and Ratte, 1960, pl. 2), where the sanidine 

phenocrysts-commonly showing Carlsbad twinning-are as long as 5 em. Some of these sanidines are 

rounded by resorption and are rimmed by sodic 

plagioclase, resulting in a rapakivi texture. This area 

was interpreted by Steven and Ratte (1960, p. 33-34) as 

the core of the lava dome, overlying the vent. Their interpretation has been confirmed by recent exploratory 

drilling that demonstrates that this coarse porphyry extends downward below any plausible projection, based 

on the exposed marginal contacts of the body, were it 

interpreted as a simple lava flow. 

An outlying area of sanidine-quartz-hornblende 

quartz latite, along the east side of upper Park Creek 

(Platoro map; Steven and Ratte, 1960, p. 24), has been 

included in the quartz latite of South Mountain because 

of general petrologic similarity and because it is 

hydrothermally altered and intruded by unaltered 

porphyry that is interpreted as a vent of the rhyolite of 

Cropsy Mountain. It is not clear whether these Park 

Creek outcrops are a marginal remnant of the dome 

centered at South Mountain or whether they represent 

nearly contemporaneous accumulation from a separate 

source. 

Also assigned to this unit are lavas and flow breccias 

at the south end of Cropsy Mountain (Steven and Ratte, 

1960, p. 18-19) that contain a sanidine-plagioclasebiotite-augite phenocryst assemblage and that seem 

petrographically transitional toward the rhyodacite of 

Park Creek. These rocks apparently are local accumulations around a separate vent. 

The quartz latite of South Mountain seems to contain 

about 68-70 percent Si02 (table 9, Nos. 19-21), but two of 

the three analyses contain abundant C02, making precise 

interpretation difficult. 
74 PLATORO CALDERA COMPLEX, SAN JUAN MOUNTAINS, COLORADO 

. The age of the main lava dome at South Mountain is 

about 22.8 m.y., as indicated by K-Ar determinations on 

phenocrystic biotite (22.8 m.y.} and sanidine (22.9 m.y.) 

from a seemingly unaltered sample near the east margin 

of the dome (Mehnert and others, 1973). The 

petrographically transitional augite-bearing quartz 

latite at the south end of Cropsy Mountain may be appreciably older, however, because it may correlate with a 

dike of generally similar sanidine-quartz porphyry about 

3 km to the west which contains carbonate 

pseudomorphs after augite and which has yielded a 

· biotite K~Ar age of 25.8 m.y. (Lipman and others, 1970, 

table 4, No. 6). 

RHYOLITE OF CROPSY MOUNTAIN 

The youngest coarsely porphyritic lava flows in the 

Summitville area, named the rhyolite of Cropsy Mountain, are petrographically similar to but slightly more 

silicic than the quartz latite of South Mountain (table 

8). These rocks were described as the upper member of 

the Fisher Quartz Latite by Steven and Ratte (1960, p. 

22-25, 34-36) but later were excluded from the Fisher 

Quartz Latite (Steven and Ratte, 1965, p. 43-44; Lipman 

and others, 1970, p. 2341-2342). The rhyolite of Cropsy 

Mountain is everywhere unaltered, in striking contrast 

to the widespread alteration of the underlying rocks, including the Summitville Andesite, rhyodacite of Park 

Creek, and quartz latite of South Mountain. 

The rhyolite of Cropsy Mountain is exposed as discontinuous erosional remnants which cap the ridge from 

South Mountain to Lookout Mountain and which cover 

larger areas around North Mountain (fig. 36). In most 

and perhaps all these places, the rhyolite of Cropsy 

Mountain is a single flow of uneven thickness. Variation 

in thickness is due to irregular preemption topography 

and to erosion of the flow top. On Cropsy Mountain the 

exposed thickness of the unit is typically 50-75 m, but 

north of North Mountain a single flow is at least 300m 

thick. Even though only one flow is present at each 

locality, several distinct flows are likely, as indicated by 

variations in petrology and by recognition of several 

areally separated vents. 

These flows are characterized by a basal dark-gray to 

black vitrophyre, 5-10 m thick, overlain by light-gray or 

pink devitrified rhyolite. Where the base is best exposed, 

as on the west face of Lookout Mountain (fig. 44), the 

vitrophyre is underlain by a few meters of flow breccia in 

which dark-gray dense blocks and lighter colored 

vesiculated blocks are enclosed in a white tuffaceous 

matrix. Flow layering, defined by variations in 

devitrification, is typically distinct in lower and upper 

parts of the flow. In lower parts of the flow the layering 

generally dips gently, but locally it is intricately flow 

folded; inclined ramp structures characterize upper 

parts of the flow in most outcrops. 

The rhyolite of Cropsy Mountain is fairly uniform in 

phenocryst petrology, except for the rocks related to the 

vent west of North Mountain described later. Typically, 

the rhyolite contains 20-30 percent phenocrysts of 

sanidine (Ors5), plagioclase (Anl5-ao ), quartz, biotite, and 

hornblende. The sanidine phenocrysts are large and 

blocky and show conspicuous Carlsbad twinning; quartz 

FIGURE 44.-Rhyolite of Cropsy Mountain (Tcr). Single thick flow, 20.2 

m.y. old, has a distinct dark basal vitrophyre ( vi ) and rests unconformably on hydrothermally altered lavas of the upper member of the 

Summitville Andesite (Tsu). Lookout Mountain as seen from the 

Stunner-Summitville road, just west of Iron Creek. 

occurs as rounded resorbed pellets. This assemblage is 

similar to that of the quartz latite of South Mountain, 

except that both feldspars are somewhat more sodic. 

Three vent structures related to eruption of the 

rhyolite of Cropsy Mountain have been recognized: (1) 

on the west shoulder of South Mountain, (2) at 

Sheepshead near Prospect Mountain, and (3) west of 

North Mountain. Shallow intrusives of similar 

petrography probably fed other vents, the surficial parts 

of which have been eroded. Such probable intrusive 

equivalents of the rhyolite of Cropsy Mountain are 

described in a separate section, however, because of the 

difficulty in distinguishing many of them from intrusives 

related to the quartz latite of South Mountain. 
LA VAS AND RELATED ROCKS 75 

The vent on the west shoulder of South Mountain is 

represented by an arcuate dike with steeply dipping flow 

foliation that either merges with or cuts extrusive 

rhyolite of Cropsy Mountain at the ridge crest; this 

feature is described in more detail by Steven and Ratte 

(1960, p. 35). 

The vent at Sheepshead is largely intrusive at the present erosional level; it consists of a stubby funnel-shaped 

dike that bulges outward on its southeastern side where 

it apparently approached the surface. In petrographic 

appearance it is identical to the flow on Cropsy Mountain. The Sheepshead vent intrusion is fresh rock, in contrast to the surrounding intensely altered lava flows of 

the Summitville Andesite. 

Perhaps the most interesting vent structure-and the 

only one examined in detail during this study-apparently formed at the crest of a pyroclastic cone and is 

exposed on the prominent shoulder west of North Mountain. Here, a steeply dipping elliptical intrusive mass, 

characterized by roughly concentric steeply dipping flow 

layering, is continuously traceable upward into a subhorizontal flow of unaltered lava more than 100m thick 

that lies on hydrothermally altered rhyodacite of Park 

Creek (Platoro map). Especially interesting are conspicuous compositional heterogeneities in both the intrusion and the lava flow. The most striking variation seen 

in the field is in total phenocryst content, which varies 

from only about 2 percent to at least 25 percent (fig. 45), 

both within the flow and in the vent. In the vent intrusion, concentric shells that consist of light-gray or pink 

devitrified rock and that differ by 5-10 percent in total 

phenocryst content are separated by black vitrophyre 

zones 1-2 m thick. In the surface flow, the phenocryst 

content varies both vertically and laterally. Lowest 

phenocryst contents are near the base of the flow, especially at distances of more than a few hundred meters 

from the vent. The phenocryst content generally increases upward in the flow, although adjacent subhorizontal flow layers in the interior locally vary discernibly in total phenocryst content. In upper parts of the 

flow, where layering generally defines steeply inclined 

ramp structures .and is intricately contorted in detail, 

phenocryst heterogeneities from layer to layer are especially conspicuous. 

Rocks of this complex that contain the fewest 

phenocrysts, only about 2 percent (fig. 45C), are dark 

gray and have a· conspicuously slabby jointing due to 

trachytic groundmass texture. The sparse scattered 

phenocrysts, consisting of quartz, plagioclase, sanidine, 

biotite, and hornblende, are small-mostly 0.5 mm or 

less-and irregular in outline owing to resorption. 

Among the phenocrysts are a few curious euhedral hexagonal inclusion-filled crystals as much as 0.8 mm in 

diameter that seem to be apatite. The feldspars are 

cloudy, owing to small glass inclusions, and some 

plagioclase crystals have clear rims of more calcic composition. These rocks are rhyodacitic in composition, 

containing 64-65 percent Si02 (table 9, Nos. 24, 25). 

The phenocryst-rich rocks (fig. 45A), which have 20-25 

percent phenocrysts (table 9, Nos. 28, 29), are distinctly 

more silicic, .containing as much as 72.4 percent 

Si02-about typical for the rhyolite of Cropsy Mountain. 

Quartz and feldspar phenocrysts in these rocks also show 

some rounding, resorption, and dusty inclusions, but 

these features are relatively minor. The phenocrysts are 

as much as 1 em in diameter, much larger than 

phenocrysts in the phenocryst-poor parts of the complex. 

These rocks are texturally within the normal range of the 

rhyolite of Cropsy Mountain. 

Intermediate compositional and textural types (fig. 

45B; table 9, Nos. 26, 27) are the most abundant. These 

types are typically flow layered, and phenocryst size 

changes . conspicuously from layer to layer. 

The heterogeneous textures and compositions within 

the vent and related flow at· North Mountain clearly 

demonstrate mixing of magmas of significantly different 

compositions before eruption. A rough calculation indicates that the groundmass compositions of the more 

silicic rocks of the complex (table 9, Nos. 28, 29) are even 

more silicic than the bulk compositions, as is typical for 

porphyritic volcanic rocks in the San Juan field (Larsen 

and Cross, 1956, figs. 53-56) and elsewhere. The compositional differences between phenocryst-poor 

rhyodacite and phenocryst-rich rhyolite cannot be due to 

the different phenocryst contents. It is also not 

clear whether the observed compositional extremes truly 

represent the end-member compositions of the magma 

before mixing, or whether the observed end members are 

themselves partial mixes of even more extreme initial 

compositional contrasts. A mixed-lava complex of silicic 

rhyolite and basalt, described in the section on the 

Hinsdale Formation, formed at the south end of 

Handkerchief Mesa at about the same time as the 

mixed-lava complex at North Mountain. 

The rhyolite of Cropsy Mountain has been well dated 

at the type area at 20.2 m.y., as indicated by concordant 

K-Ar determinations on sanidine, plagioclase, biotite, 

and hornblende phenocrysts from the same sample 

(Steven and others, 1967; Lipman and others, 1970), but 

the rocks assigned to this unit in the North Mountain 

area could possibly be somewhat different in age. 

In both petrology and age the rhyolite of Cropsy 

Mountain is transitional toward the Miocene and 

Pliocene silicic alkali rhyolites of the Hinsdale Formation (table 11), but its position marginal to the Summitville caldera and its petrographic affinities with older 

coarsely porphyritic lavas indicate that it is part of the 

postcollapse lava sequence of the Platoro-Summitville 

center. 
76 PLATORO CALDERA COMPLEX, SAN JUAN MOUNTAINS, COLORADO 

FIGURE 45.-Photomicrographs of mixed-lava complex at North Mountain, part of the rhyolite ofCropsy Mountain. A, Typical phenocrystrich rhyolite. Contains about 25 percent i arge phenocrysts of 

sanidine (upper left) , resorbed quartz (upper right), 

glomeroporphyritic plagioclase (right center), oxidized biotite (lower 

left), and (not shown) sparse hornblende, sphene, and apatite. 

Analyzed sample 28, table 9. f1 Intermediate-composition phase 

showing extreme disequilibrium among phenocrysts. Large rounded 

plagioclase phenocrysts have dusty outer margins and glass inclusions because of partial melting; partly resorbed sanidine 

phenocrysts (upper center) are rounded and embayed. Smaller 

phenocrysts include biotite, hornblende, augite, and apatite. 

Analyzed sample 27, table 9. ~ Phenocryst-poor rhyodacite from 

base of complex. Sparse small em bayed plagioclase irains and a few 

euhedral crystals of apatite (lower left) containing oriented dark 

acicular inclusions around their margins are the only phenocrysts in 

fine-grained devitrified pilotaxitic ground mass. Analyzed sample 24, 

table 9. 

VOLCANICLASTIC ROCKS (LOS PINOS FORMATION) 

The Los Pinos Formation as described by Larsen and 

Cross (1956, p. 185-192) consists of widespread 

volcaniclastic sedimentary rocks in the southeastern San 

Juan Mountains that overlie the ash-flow sequence 

(their Treasure Mountain Rhyolite) and underlie and interfinger with basalt flows of the Hinsdale Formation. 

Some of the volcaniclastic rocks in the Platoro caldera 

area, which largely represent erosional debris washed 

f'rom the postcollapse volcanoes, were mapped as Los 

Pinos Formation by Larsen and Cross; use of this term is 
LAVAS AND RELATED ROCKS 77 

herein continued, even though the Los Pinos Formation 

in the type locality farther south seems to consist mainly 

of younger material derived from other centers. Discussion of the postcaldera lavas and related rocks in terms 

of two groups-the near-source accumulations that 

formed volcanic cones or lava domes and the surrounding aprons of volcanic sedimentary rocks-is thus 

parallel to previous discussion of the early intermediatecomposition lavas and breccias of the Conejos Formation 

in terms of a vent facies and a volcaniclastic facies. 

In the Platoro map area, aprons of volcaniclastic 

sedimentary rocks of approximately similar age are 

marginal both to the Cat Creek volcano (volcanics of 

Green Ridge) east of the caldera and to the accumulation 

of the rhyodacite of Park Creek in the Summitville area. 

The sedimentary sequence derived from the Cat Creek 

volcano was mapped by Larsen and Cross (1956) as part 

of the Los Pinos Formation (as were most of the lavas 

here included in the volcanics of Green Ridge). In the 

Cat Creek-Green Ridge area the volcaniclastic sedimentary rocks generally rest on Masonic Park Tuff and are 

overlain by younger lavas of the Cat Creek volcano (fig. 

46). North of Green Ridge and farther from the volcanic 

center, however, some of these volcaniclastic rocks are 

interlayered with the ash-flow sequence, between the 

Fish Canyon and Carpenter Ridge Tuffs (Durango map). 

In contrast, in the Summitville area, marginal 

volcaniclastic sedimentary rocks derived from the 

rhyodacite of Park Creek were not previously mapped 

separately from the rhyodacite, although they are 

similar in age, lithology, and genesis to rocks of the Los 

Pina:; Formation to the east. These rocks are extensive in the 

Park Creek and Beaver Creek drainages (Platoro map), 

where they are exposed below the rhyodacite of Park 

Creek near the caldera rim and where they interfinger with the ash-flow sequence away from the volcanic 

centers. As in the Green Ridge area, the volcaniclastic 

rocks here occur between the Fish Canyon and Carpenter 

Ridge Tuffs, and higher tongues of similar volcaniclastic 

rocks wedge below and above the Snowshoe Mountain 

Tuff at the south end of Handkerchief Mesa (fig. 35). 

Farther south, the Los Pinos Formation occurs mainly 

as a widespread eastward-thickening alluvial wedge that 

overlies the ash-flow sequence and that i's interlayered 

with Miocene and Pliocene basalt flows of the Hinsdale 

Formation. These rocks appear to have accumulated as 

. . . . ~ . 

great coalescing alluvial fans along the east front of the 

San Juan Mountains; accumulation was a response to 

eastward tilting of the Oligocene volcanic sequence concurrent with structural development of the San Luis 

Valley graben system (Lipman and Mehnert, 1969). 

Although these Los Pinos rocks are therefore younger 

than the Oligocene volcaniclastic accumulations near 

the Platoro caldera complex, both areas of the Los Pinos 

Formation are generally similar in lithology and 

therefore are diffcult to separate except where control is 

provided by radiometrically dated interlayered basalt 

flows. In the southern San Juan region contrasting 

lithologies of clasts in the Los Pinos Formation reflect 

derivation from local centers south of the Platoro caldera 

complex, such as the one along the continental divide at 

the head of Elk Creek (fig. 13) and others farther south 

(Butler, 1971, p. 295-297). 

The Los Pinos Formation is weakly indurated and 

therefore rarely well exposed. Areas underlain by the Los 

Pinos Formation are typically characterized by subdued 

topography mantled by cobbles and boulders that were 

weathered out of conglomeratic units. Accordingly, conglomeratic rocks seem at first to be exceptionally abundant in the Los Pinos Formation, but the sparse good exposures show that most of the unit consists of poorly indurated volcaniclastic sandstones commonly containing 

abundant tuffaceous material. Two thin nonwelded 

white ash-flow sheets, each about 5 m thick, are exposed 

in an exceptioally fine cut along the lower Conejos River, 

just south of Los Mogotes (Durango map). 

The Los Pinos Formation includes upper Oligocene, 

Miocene, and Pliocene rocks. The oldest part of the formation is probably the volcaniclastic sequence that is interlayered with the ash-flow sequence between the Fish 

Canyon (27.8 m.y.) and Carpenter Ridge (>26.7 m.y.) 

Tuffs around the north and east sides of the Platoro 

center. In northern New Mexico, a local rhyolitic ash1 flow tuff interbedded with conglomerates of the Los 

Pinos Formation has been dated at 25.9 m.y. (Bingler, 

1968, p. 36). Southeast of the Platoro caldera complex, 

sedimentary rocks of the Los Pinos Formation are interlayered with basalt flows of the Hinsdale Formation, 

one of which has been dated at 17.7 m.y., and the upper 

part of the Los Pinos Formation intertongues with 

Hinsdale basalt of the Los Mogotes volcano, which 

seems to be about 5 m.y. old (Lipman and others, 1970) . 

. , .. 

. ; .· 

FtGutu; 46.-Diagrammatic cross section through the Cat Creek volcano, showing general relations between volcanics of Green Ridge (black) and 

surrounding apron of volcaniclastic rocks of the Los Rinos Formation (stippled). As the volcano grew, lavas spread over an enlarging area, so' 

in local sectiuns the lavas generally overlie the penecontemporaneous volcaniclastic sedimentary rocks. 
78 PLATORO CALDERA COMPLEX, SAN JUAN MOUNTAINS, COLORADO 

INTRUSIVE ROCKS 

Intrusive rocks in the southeastern San Juan region, 

consisting of numerous andesitic to rhyolitic dikes and 

scattered larger granitic stocks, tend to cluster around 

and radiate out from the Platoro caldera complex (fig. 

47). Although some of these intrusives can be linked to 

individual units of the caldera-related lavas described in 

the preceding sections, the intrusive rocks are described 

separately because the precise volcanic associations of 

many are ambiguous. 

(table 10, Nos. 2-5). In contrast, the northwestern part of 

the intrusion is texturally and compositionally complex 

and is also the locus of intense hydrothermal alteration 

(fig. 61). In this area ~ separately mapped porphyritic 

phase is distinctly more silicic (about 65 percent SiOz; 

table 10, Nos. 6, 7) than the equigranular monzonite and 

varies greatly in texture; the average size of phenocrysts 

ranges from only slightly larger than the equigranular 

grains of the monzonite to at least 1 em, and the groundmass texture ranges from aphanitic to phaneritic (fig. 

48B, C). The most aphanitic parts of the porphyritic 

GRANITIC INTRUSIONS phase generally are intensely hydrothermally altered, so 

About 10 granitic stocks, ranging in size from less than relations with the other porphyritic types are obscure. 

0.5 to at least 5 km across, intrude the volcanic sequence The porphyritic phase is at least locally intrusive into 

near the Platoro caldera complex, and several large lac- the equigranular monzonite, as first recognized by 

colithic granitic bodies are localized along the boundary Calkin (1971, p. 237), but in the southwestern part of the 

between the Conejos Formation and prevolcanic rocks stock the porphyritic phase grades into equigranular 

along the southwest margin of the San Juan Mountains monzonite. Although several porphyritic phases of 

(fig. 47). Although these rocks were considered to be different ages may be present, it seems preferable to 

related to the Conejos Formation by Larsen and Cross regard them all as late phases of the Alamosa River 

(1956, p. 104-105), most of the stocks around the Platoro stock, rather than as a separate intrusive, as suggested 

area are younger than the Treasure Mountain Tuff, as by Calkin (1971). 

discussed below. An even more silicic phase of the Alamosa River stock 

These bodies consist mainly of texturally variable (71 percent SiOz; table 10, No. 8) is represented by late 

monzonite, in places transitional into diorite or irregular aplite veins, rarely more than 10 em thick, that 

granodiorite (table 10). Locally, the granitic rocks are locally have chilled margins against the monzonite (fig. 

equigranular, but most are porphyritic, with plagioclase 48D) . 

. the conspicuous phenocryst. Two pyroxenes are also Many of the dikes in the western part of the mapped 

typically present as early-formed euhedral minerals; area radiate outward from the area of the Alamosa River 

these tend to be variably altered, the hypersthene to stock and seem to be closely related to it. These dikes are 

chlorite or serpentine and the augite to chlorite, car- described in the section on porphyro-aphanitic inbonate, or tremolite. Interstices between plagioclase and trusions. 

pyroxenes are characteristically occupied by various The Alamosa River stock intrudes the intracaldera La 

amounts of quartz and orthoclase-in places with a Jara Canyon Member; it also seems to be partly structendency toward granophyric intergrowths-and by tually controlled by the Cornwall fault that bounds the 

biotite and Fe-Ti oxides. Because good brief Summitville caldera on its south side (fig. 61), 

petrographic descriptions of many of these rocks are suggesting that the stock may be younger than the Ojito 

given by Larsen and Cross (1956, p. 104-110), the follow- 1 Creek and Ra Jadero Members of the Treasure Mouning short discussions of the individual bodies concen- tain as well. Biotite from a sample of the equigranular 

trate on the major compositional and textural variations. f!lOnzonite (fig. 48A; table 10, No. 2) yielded a K-Ar age 

These intrusives were studied only briefly; several that of 29.1 m.y., which is compatible with all available 

are well exposed and internally complex are deserving of geologic and ·geochronologic constraints (Lipman and 

more detailed investigation. others, 1970). 

ALAMOSA RIVER STOCK 

The Alamosa River stock (fig. 47, loc. A) is the largest 

of the granitic intrusives, about 3 by 7 km, and is 

elongate to the northeast, apparently owing to emplacement along the Cornwall faplt (fig. 61). Contacts are 

steep, except in the Alum Creek-Bitter _Creek area 

(Platoro map), where the north contact dips broadly outward and has many irregular apophyses. 

South of the Alamosa River, the stock consists of 

little-altered fine- to medium-grained equigranular 

monzonite (fig. 48A); it is fairly uniform in texture but 

varies in composition from about 57 to 62 percent SiOz 

Exposures of equigranular monzonite of the Alamosa 

River stock at elevations of over 12,000 feet along the 

divide between the Alamosa and Conejos Rivers present 

a significant paleotopographic problem. This position· 

must be close to the regional late Oligocene land surface, 

as marked by the level of ash-flow sheets of the Treasure 

Mountain Tuff on adjacent high ridges (Platoro map). 

Reasona~ly, even a shallow subvolcanic intrusive could 

have been emplaced and cooled at a such a high level 

only if it were covered by a thick local volcanic accumulation that stood significantly above the regional 

terrain. Thus, intrusion of the Alamosa River stock 
107"00' 

/ 

Mount 

xHope 

' 

xMetroz 

Mountain 

Wolf Creek" 

Pass 

v 

'-· I '? 

1-

/ 

0 

0 

Monzonite and associated rocks 

Quartz latite and rhyolite porphyry 

LAVAS AND RELATED ROCKS 

5 

106"30' 106"15' 

Green Ridge 

Osier M~untain 

5 10 15 20 MILES 

10 15 20 Kl LOM ETERS 

EXPLANATION 

Rhyodacite porphyry 

Andesite porphyry 

Sample locality (table I 0) 

""""'"""""" Limit of exposed volcanic 

rocks 

Caldera wall (topographic) -

Dashed where covered by 

younger rocks 

79 

106"00' 

FIGURE 47.-Distribution of major groups of intrusive rocks in the southeastern San Juan Mountains. Based on mapping by P. W. L:ipman 

1965-67, 1970-71. A, Alamosa River stock. B, Cat Creek stock. C, Terrace Reservoir laccolith. D, Jasper stock. E. Bear Creek stock cluster; Eb, 

Bear Creek stock; Ec, Crater Creek stock; Ee, Elwood Creek stock. F, Wightman Fork stock. G, Canyon Diablo stock. H, Cataract Creek stock. 

I, Jackson Mountain laccolith. J, V Mountain laccolith. K, Squaretop laccolith. L, Cornwall Mountain stock. M, Ojito Creek plug. N, Blanco 

Basin sill. 0, Trail Lake plug. 
80 

Sample---------

Field No-------

Lab. No--------

Si02-----------

Al20 3----------

Fe203----------

Feo------------

Mgo------------

Cao------------

Na2o-----------

K2Q------------

Ti02-----------

P20s-----------

Mno------------

TOTAL-------

H20+ ----------

H2Q- ----------

C02 -----------

Quartz---------

Corundum-------

Or thee lase-----

Albite---------

Anor th i te------

Wollastonite---

Ens tat i te------

Ferrosili te----

Magnet i te---~-­

Hema t i te-------

Ilmenite-------

Ti tani te-------

Rut i le---------

Apa t i te--------

Ag-------------

Ba-------------

Be-------------

Ce-------------

Co-------------

Cr-------------

Cu-------------

Ga-------------

La-------------

Mo-------------

Nb-------------

Ni-------------

Pb-------------

Sc-------------

Sr-------------

v--------------

v-------------- Yb-------------

Zr-------------

Quartz---------

Or thoc1ase-----

P1agioc1ase----

Bioti te--------

Hornb1ende-----

Aug1 te---------

Hypers thene----

Opaq ues--------

Groundmass-----

I 

PLATORO CALDERA COMPLEX, SAN JUAN MOUNTAINS, COLORADO 

1 

71L-34 

Wl76169 

56.5 

17 .o 

4.8 

4. 6 

3. 7 

5.4 

3. 7 

2. 8 

1.0 

0 30 

0 22 

100.02 

2.1 

.33 

0 29 

7.12 

16.53 

31.57 

21.36 

1. 42 

9. 29 

3.13 

6. 91 

1. 97 

1 

88 

.71 

2 

67L-113 

Wl74423 

3 

P57 

Alamosa River s toe k 

4 

66L-155 

Wl68451 

5 

ST-128 

Wl68450 

TABLE 10.-Analyses of intrusive 

[Sample localities shown in fig. 47: Analyses 3 and 19 are from Patton (1917). All other 

Chloe, Lowell Artis, H. Smith, John Glenn, and James Kelsey. Minor-element 

No. 22 by J. L. Harris; by quantitative methods are Nos. 2, 6, 7, 9-14, 

Harriet Neiman. <, less than] 

6 

ACL-2363 

Wl74424 

7 

ACL-2964 

Wl74425 

71L- 31 

Wl76168 

9 

70L-16 

Wl744 32 

Intrusive complex at Cat Creek 

10 

70L-10 

Wl74429 

11 

70L-14-A 

Wl74431 

12 

66L-51 

Wl74426 

13 

70L-ll 

Wl7'4430 

Major oxides (weight percentL ~ecalculated with H20 and C02 (original values listed separately below) 

56.6 

16.8 

4. 5 

3.8 

3. 6 

6.1 

3. 3 

2.8 

0 91 

0 48 

.13 

99.02 

1.0 

0 25 

< .05 

10.4 

16.8 

28.4 

22.3 

2.02 

8. 8 

1. 95 

6. 5 

.1. 73 

1.13 

2200 

7 

20 

110 

15 

30 

14 

24 

1000 

200 

<40 

6 

380 

4. 7 

15.1 

51.2 

5. 2 

14.3 

4. 7 

4. 8 

60.32 

14.67 

5.14 

3.03 

2. 92 

4. 52 

3. 91 

4. 21 

0 99 

.12 

.13 

99.94 

• 76 

.09 

.04 

10.3 

24.8 

33.1 

10.1 

4.8 

7 0 3 

7 0 4 

.03 

1.9 

. 29 

59.9 

16.5 

3.05 

3. 9 

3. 3 

5. 5 

3. 5 

2. 9 

0 90 

0 40 

.17 

100 0 02 

0 70 

0 28 

0 05 

12. 3 

17.4 

29 0 3 

20.7 

1.6 

8.1 

3.4 

4.4 

1.7 

0 94 

0. 5 

1000 

3 

100 

20 

so 

70 

15 

100 

7 

7 

30 

200 

15 

500 

150 

so 

3 

300 

3.8 

21.3 

so. 2 

2 0 7 

13.2 

6.1 

3. 3 

61.7 

16.2 

3. 5 

2.97 

2 0 5 

3. 99 

3. 6 

4.1 

0 97 

0 29 

0 29 

100.11 

1.5 

.44 

.18 

64.9 

15.5 

3.8 

2.0 

1.5 

4 0 3 

3. 6 

3.6 

0 54 

0 31 

.12 

100.17 

1.4 

0 27 

<.OS 

65.3 

16.0 

2.8 

2.4 

1.5 

3. 9 

3. 3 

3.6 

0 57 

0 27 

.11 

99 0 75 

.80 

. 30 

< .05 

Norms (weight percent) 

13.5 

24.2 

30.3 

15.9 

.82 

6.1 

1.5 

5.04 

1.8 

. 68 

20.7 

21.03 

30.1 

15.7 

1.5 

3.8 

0 00 

5.4 

.04 

1.02 

0 72 

21. 3 

21.5 

28 0 3 

17 0 8 

3. 8 

1.4 

4.1 

1.07 

0 65 

Minor elements (parts per million) 

0. 3 

1000 

3 

100 

15 

10 

30 

15 

100 

7 

5 

15 

30 

15 

500 

100 

30 

3 

300 

13 

11 

so 

10 

18 

14 

4300 

80 

<40 

3 

260 

1900 

<2 

22 

60 

20 

10 

11 

480 

90 

<40 

3 

300 

Phenocrysts (volume percent) 

5 

20 

55 

10 

5 

5 

10 

25 

45 

10 

10 

25 

45 

7 

10 

70.9 

14.3 

1.4 

0 93 

0 66 

.88 

2.8 

6. 9 

0 52 

.47 

.06 

99.82 

0 63 

• 33 

<.OS 

26.34 

1. 68 

41.15 

23.91 

1. 26 

1. 63 

1. 67 

0 26 

1.00 

1.12 

23.9 

52.4 

18.7 

2.1 

2. 9 

59.6 

18.1 

3. 7 

2.9 

2. 4 

5.07 

4 0 2 

3.1 

0 36 

0 38 

.11 

99.92 

.77 

.43 

<.OS 

9. 2 

18.3 

35.9 

21.2 

.61 

5.9 

1.8 

5.4 

.69 

.91 

2500 

<2 

28 

200 

23 

20 

19 

21 

480 

200 

<40 

<2 

170 

3.1 

13.9 

53.4 

3.1 

11.7 

8. 2 

4. 5 

61.4 

17 0 55 

3.4 

1.9 

2.1 

5.9 

3. 2 

3. 3 

0 72 

0 34 

.12 

99.93 

1.4 

0 23 

1.6 

16.1 

19.5 

27.1 

23.8 

1.3 

5.1 

4. 6 

2.1 

1.4 

.81 

2200 

<2 

10 

27 

24 

20 

14 

360 

100 

<40 

3 

120 

26.4 

5. 3 

3.1 

65.2 

63.2 

16.7 

2 0 7 

2.6 

z; 1 

4.05 

3. 7 

3. 5 

.81 

0 34 

.11 

99.81 

0 55 

.19 

< .05 

16.4 

20.9. 

31.7 

17.8 

5. 3 

1.4 

3. 96 

1.5 

.81 

2800 

<2 

10 

22 

34 

40 

15 

12 

320 

90 

<40 

<2 

160 

5.8 

24.5 

43.1 

3.0 

13.7 

4.8 

4. 2 

66.2 

17.1 

2. 2 

1.7 

1.3 

3.9 

3. 7 

2. 9 

0 53 

0 23 

.09 

99.85 

1.8 

.71 

1.0 

23.2 

17 0 2 

31.7 

18.1 

3.4 

0 55 

3. 2 

1.0 

0 54 

2800 

<2 

7 

22 

20 

8 

440 

40 

<40 

<2 

150 

19.7 

4.1 

2. 2 

2. 2 

71.8 

66.2 

17.1 

2. 5 

1.5 

1.1 

3.4 

3. 97 

3. 3 

0 54 

0 22 

.09 

99.92 

1.3 

0 26 

0 24 

22.4 

19.3 

33.6 

15.2 

2. 8 

3. 7 

.02 

1.03 

0 53 

2600 

<2 

20 

3 

2 

70 

8 

13 

20 

10 

10 

340 

30 

<40 

<2 

190 

See page 82 for sample descriptions. 
LAVAS AND RELATED ROCKS 

rocks of the Platora caldera area 

major-oxide analyses are by rapid methods by P. L. D. Elmore, Samuel Botts, Gillison 

analyses by semiquantitative methods are Nos. 4, 5, and 15 by H. W. Worthing and 

and 23 by Norma Rait, Nos. 1, 8, and 16-18 by J. D. Fletcher, and No. 20 by 

Boor Creek Cataract Creek Canyon Diablo Jasper stock Wightman Fork Porphyro-aphanitic intrusives 

stock stock stock stock 

14 15 16 17 18 19 20 21 22 

67L-84-8 66B-13A 71J-59 70L-95 70L-91 Pl2 68L-110 GL-8 66L-244-C 

W174428 ------- Wl7617 5 Wl76184 Wl76183 ------ 0133706 ------ Wl68070 

Major oxides (weight percent), recalculated without H20 and C02 (original values listed separately below)--Continued 

58.9 59.1 62.5 61.6 56.6 66.25 66.5 67.3 67.4 

17.1 17.3 17.4 15.9 17.4 15.52 17.1 15.7 16.2 

2. 7 3. 9 6.0 3. 7 3. 3 . 67 3.1 2.4 3.1 

5. 4 3. 2 • 24 3.1 5. 3 3.11 .80 1.8 . 97 

3.1 2. 9 1.1 2. 8 3.4 1. 99 1.1 1.5 1.2 

5.1 5.8 4. 9 4. 2 7.0 4.03 2. 7 2.4 3.2 

3. 2 3. 99 3.6 3.4 3.1 3.19 3.8 4.0 4. 3 

2. 9 2.6 3.0 4.0 2.4 4.01 4.1 4.0 2.4 

1.0 • 63 . 69 .87 1.1 .77 .60 . 55 . 73 

. 38 .43 • 27 .17 . 33 • 38 .12 . 24 • 29 

.16 • 25 .16 .12 .16 .08 .08 .08 .08 

99.94 100.10 99.86 99.86 100.09 100.00 100.00 99.97 99.87 

1.2 1.3 . 49 1.0 1.6 1.44 1.0 .63 .43 

• 56 . 57 • 23 • 27 ~ 33 .80 .82 • 23 .42 

<.05 . 39 <.05 < .05 .15 3. 75 . 62 < .05 < .05 

Norms (weight percent) --Continued 

11.9 10.5 18.42 13. 35 9.08 20.31 21.65 21.56 25.01 

1.89 .90 1.22 

17.5 15.1 17.86 23.94 14.50 23.67 24.25 23.74 14.32 

26.7 33.8 30.69 29.13 25.96 26.98 32.12 34.00 36.73 

22.8 21.7 22. 35 15.96 26.42 16.19 12.43 10.38 14.12 

1.8 1. 68 2. 48 . 54 

7. 6 7.1 2. 76 7.06 8.40 4. 96 2.81 3. 75 3.02 

6. 3 2.02 1. 54 5. 51 4.03 • 57 

4.0 5. 6 5. 29 4. 75 . 97 1.12 3. 50 1. 28 

6.04 2. 30 2.25 

1. 9 1.2 .86 1.65 2.14 1.47 1.13 1.05 1. 38 

• 26 

.13 

. 89 1.02 • 64 . 41 • 78 . 90 . 29 • 57 .69 

Minor elements (parts per m1llion)--Cont1nued 

0. 5 

2200 1000 970 640 560 880 700 

<2 3 <2 2 

100 300 

16 15 10 20" 22 15 

19 10 7 70 3 3 30 

57 15 18 68 88 9 20 

20 15 20 20 20 19 20 

100 100 90 100 50 tOO 

3 4 

3 20 30 20 23 10 

14 7 4 24 9 <5 30 

70 20 20 20 <50 7 

17 10 21 20 30 8 20 

570 700 850 600 1240 510 1000 

160 100 50 140 240 65 70 

<40 30 30 30 30 38 20 

4 3 3 2 3 2 2 

180 200 200 300 130 260 150 

Phenocrysts (volume percent)--Continued 

6. 5 5.8 3 

21.5 28.7 5 

48.1 30 52 42.6 65 25 25 

3.0 2 4 

15 2 

12.9 12.3 20 

4.0 4. 7 10 

4. 3 5 6 5.9 5 3 1 

50 35 65 60 

81 

23 Sample No. 

66L-15 Field No. 

Wl68529 Lab. No. 

67.5 Si02 

16.4 Al203 

2. 2 Fe203 

1.0 FeO 

1.1 MgO 

3. 6 CaO 

3. 9 Na 20 

3. 5 K20 

.40 Ti02 

• 22 P20s 

.04 MnO 

99.86 TOTAL 

• 26 H20+ 

. 23 H2o-

<.05 C02 

22.28 Quartz 

• 78 Corundum 

20.86 Orthoclase 

33.28 Albite 

16.56 Anorthite 

Wollastonite 

2. 76 Enstatite 

Ferrosilite 

2. 21 Magnetite 

.69 Hematite 

.77 Ilmenite 

Titanite 

Rutile 

• 53 Apatite 

Ag 

1500 8a 

Be 

Ce 

Co 

8 Cr 

16 Cu 

20 Ga 

La 

Mo 

Nb 

6 Ni 

<20 Pb 

7 Sc 

550 Sr 

40 v 30 y 

Yb 

240 Zr 

2.1 Quartz 

6. 3 Orthoclase 

28.5 Plagioclase 

4. 2 Biotite 

• 5 Hornblende 

1.6 Augite 

Hyper a thene 

• 7 Opaques 

57.7 Groundmass 
82 PLATORO CALDERA COMPLEX, SAN JUAN MOUNTAINS, COLORADO 

TABLE 10.-Analyses of intrusive rocks of the Platoro caldera area-Continued 

SAMPLE DESCRIPTIONS 

I. Greenish-gray porphyritic andesite. See fig. 48C for sample description and location. 

2. Gray medium-grained monzonite. See fig. 48A for sample description and location. 

3. Monzonite. ''Contains orthoclase, plagioclase, quartz, pyroxene, and biotite. Anomalously high in K,O and low in Al,O,. and P,O,,. Near sawmill above Stunner" (Patton, 1917, 

p. 31-32). 

4. Relatively coarse grained monzonite. Petrographically generally similar to No. 2. Augite 

is fairly fresh, but orthopyroxene is pseudomorphed to chlorite and serpentine. Biotite 

grains are small, irregular, and interstitial with quartz and orthoclase. Roadcut on 

Alamosa River road, just east of Bitter Creek. 

5. Gray fine-grained monzonite. Feldspars are cloudy, biotite is absent, and pyroxenes are 

thoroughly altered-augite to tremolite, and hypersthene to chlorite. Scattered grains 

of secondary epidote as much as 0.2 mm in diameter; also some secondary carbonate. 

Roadcut on north side of Stunner Pass, at about 10,200 ft. 

6. Porphyritic quartz monzonite from drill hole. See fig. 488 for sample description and 

location. 

7. Porphyritic quartz monzonite from 2,964-ft depth in drill hole in Alum Creek. Sample 

provided by W. S. Calkin. Petrographically similar to No. 6. 

8. Light-gray aplite vein in monzonite. See fig. 48D for sample description and location. 

9. Dark-gray fine-grained fresh monzonite. Slightly porphyritic; plagioclase tablets 1-2 mm 

long are set in matrix of plagioclase and mafics averaging about 0.5 mm in diameter. 

Quartz and orthoclase are interstitial. Augite is fresh, but hypersthene is altered to 

chlorite. Anomalously low in TiO,. Along North Fork Cat Creek, at 9,250 ft. 

10. Gray porphyritic rhyodacite dike. Plagioclase phenocrysts are as much as 4 mm long; 

augite is largely replaced by carbonate. Anomalously low in Fe and slightly high in CaO 

(note high CO,). Northwest side of Negro Gulch, at about 9,500 ft. 

11. Gray medium-grained equigranular monzonite. Petrographically much like No.4. Along 

South Fork Cat Creek, at 9,150 ft. 

12. Light-gray porphyritic quartz latite from Terrace Reservoir laccolith. Collected from 

talus above Alamosa River road, at northwest end of laccolith. 

13. Greenish-gray porphyritic quartz latite. Blocky plagioclase phenocrysts are as much as 

5 mm across and relatively fresh; mafic minerals that apparently originally included 

both biotite and augite are completely replaced by chlorite, epidote, and carbonate. 

North side of South Fork Cat Creek, at about 9,450 ft. 

14. Medium-grained equigranular monzonite. Much like Nos. 4 and 11, but hypersthene is 

seems best interpreted as a final stage in the evolution of 

an upward-moving body of magma, which, at a lower 

crustal level, had earlier fed volcanic eruptions to form a 

large stratovolcano. Perhaps, the lavas of the upper 

member of the Summitville Andesite, which are similar 

in composition to the monzonite, are preserved remnants 

of the lower northern flank of this volcano within the 

Summitville caldera. 

CAT CREEK STOCK 

The Cat Creek stock, the large intrusive complex 

centered about 3 km east of the Platoro caldera rim (fig. 

47, loc. B), is thought to represent the intrusive core of 

the volcano that erupted the volcanics of Green Ridge. 

This intrusive complex has a texturally uniform core, 

about 3 km in diameter, consisting of fine-grained 

equigranular monzonite (60-63 percent Si02; table 10, 

Nos. 9, 11); the core is enclosed by a finer grained 

porphyritic margin around the south and east sides of 

the stock and is flanked in other directions by scattered 

satellitic laccolithic bodies of similar porphyry (fig. 47). 

The marginal porphyry is too highly altered to determine 

its original composition, but two of the satellitic laccoliths are compositionally transitional between 

rhyodacite and quartz latite (table 10, Nos. 12, 

13)-distinctly more silicic than the equigranular monzonite of the core. 

, The Terrace Reservoir laccolith (fig. 47, loc. C), south 

of the major exposure of the Cat Creek stock, is a biotiteaugite-plagioclase porphyry (table 10, No. 12). It has 

been cut through by the Alamosa River (fig. 47) and is 

well exposed on both north and south walls of the valley. 

only slightly altered. East side of Bear Creek, at about 11,800 ft. 

15. Porphyro-aphanitic facies of monzonite, containing abundant elongate phenocrysts of 

plagioclase and hornblende as much as 1-3 mm long in a fine-grained granular groundmass. Northwest shore of the small lake within the area of the stock. 

16. Granodiorite porphyry, containing abundant plagioclase laths that range in length from 

less than 0.1 mm to 3 mm. Presence of fine-grained granular groundmass is only evident in thin section. Rim of cirque basin, at about 12,200 ft, 0.3 km northwest of unnamed lake at head of Canyon Diablo. 

17. Medium-grained equigranular monzonite. Petrographically much like No. 4, but both 

pyroxenes are altered, and orthoclase is extensively sericitized. Striking granophyric interstitial textures. Edge of beaver-pond dam, about 150m east of abandoned Miser mill 

site, south of the Alamosa River at Jasper. 

18. Fine-grained equigranular diorite. A few percent plagioclase phenocrysts as much as 3 

mm long are set among groundmass grains that have an average diameter of about 0.5 

mm. Augite is partly altered to tremolite, and hypersthene has been completely 

replaced by chlorite. Wightman Fork road, at 10,350 ft. 

19. Porphyritic quartz latite dike. "Contains phenocrysts of glassy sanidine, plagioclase, 

quartz, and biotite in a very fine grained groundmass. From dike in Cropsy Gulch" 

(Patton, 1917, p. 31). 

20. Porphyritic quartz latite from Cornwall Mountain stock. Plagioclase phenocrysts are 

blocky and as much as 6 mm in diameter; mafics are completely altered, with sericite 

replacing biotite and carbonate pseudomorphing augite. Fine granular groundmass 

(average diameter 0.05 mm) is about two-thirds sanidine and one-third quartz. West 

fork of Robinson Gulch, at about 10,600 ft. 

21. Porphyritic quartz latite sill. Northeast side of Blanco Basin, east of Squaretop Mountain. Previously unpublished analysis provided by David Gottfried, 

22. Light-gray porphyritic quartz latite plug. Plagioclase phenocrysts (An,.,., .. ,) are as much 

as 1 em in diameter; similar sanidine and somewhat smaller quartz are rounded and 

embayed. Hornblende is nearly opaque from exsolved Fe-Ti oxides. Modal estimate is 

only a rough approximation because of large phenocrysts. Anomalously low in K,O. 1 

km north of Trail Lake, at about 12,200 ft (fig. 13). 

23. Light-gray porphyritic quartz latite. Petrographically similar to No. 22, except for 

presence of augite and generally less severe oxidation of mafic phenocrysts. East side of 

Ojito Creek road, at 8,700 ft. 

Outcrops south of the river are near the edge of the laccolith, where it wedges out completely from a thickness 

of about 75 m, and provide the best exposures of the roof 

of the intrusion. North of the river, where the laccolith 

bulges to as much as 250m of exposed thickness, the roof 

has been largely eroded. In this area, the subhorizontal 

basal contact of the laccolith is well exposed; the resisFIGURE 48 (right).-Photomicrographs of rocks of the Alamosa River 

stock. A, Medium-grained equigranular monzonite from summit of 

Telluride Mountain. Euhedral plagioclase (P),(An4o--ls) and altered 

pyroxenes, with interstices filled by large biotite (B) and orthoclase 

(0) or by granophyric quartz-orthoclase intergrowths. Analyzed 

sample 2, table 10; biotite from this sample has been dated 

radiometrically at 29.1 m.y. (Lipman and others, 1970, table 4, No. 

9). B, Porphyritic quartz monzonite from 2,363-foot depth in drill 

hole in Alum Creek. Sample provided by W. S. Calkin. Large 

plagioclase phenocrysts (P) (An3~-4o) and small biotites (B) in a 

phanerocrystalline sugary-textured groundmass intergrowth of 

quartz and alkali feldspar. Analyzed sample 6, table 10. C, 

Porphyritic andesite from roadcut, Stunner-Summitville road at 

10,100 feet elevation, stream crossing south of Lookout Mountain. 

An exceptionally mafic part of the rhyodacite porphyry phase of the 

stock. Large plagioclase phenocrysts (An:~~-.m) are marginally altered 

to carbonate in fine-grained groundmass; pyroxene phenocrysts (not 

visible) are pseudomorphed by epidote and carbonate. This rock 

type is typically much more altered than this sample indicates. 

Analyzed sample 1, table 10. D, Chilled border of aplite vein, in contact with fine-grained equigranular monzonite, from bridge abutment across the Alamosa River at Stunner. The fine-grained monzonite (A) is sharply truncated by a selvage of microcrystalline 

porphyry (B) that contains small phenocrysts of plagioclase and 

orthoclase .. This porphyry, in turn, is sharply truncated by finegrained aplite that becomes coarser toward interior of vein (C). 

Coarse aplite consists of subequal amounts of quartz and orthoclase, 

accompanied by a few percent plagioclase. Analyzed sample 8, table 

10. 
LAVAS AND RELATED ROCKS 83 
84 PLATORO CALDERA COMPLEX, SAN JUAN MOUNTAINS, COLORADO 

tant rocks of the intrusion form the overhanging upper 

walls of caves along the contact, just a short distance 

above the main Alamosa River roadway. 

The wallrocks of the intrusive complex at Cat Creek 

are lavas and breccias of the Conejos Formation and, 

around much of the central stock, the rhyolite unit of the 

volcanics of Green Ridge, Although the marginal contacts of the central stock vary widely in attitude, 

generally moderate outward dips indicate that the present level of exposures is close to the original top of the 

rather irregularly shaped stock. Over a sizable area, especially along the ridge between Deer Creek and the 

North Fork of Cat Creek (Platoro map), the contact 

between the equigranular monzonite and the overlying 

rhyolite unit is nearly horizontal, and the intensely 

recrystallized rhyolite (fig. 41B) clearly forms the roof of 

the stock. 

The Cat Creek stock and related rocks have not been 

dated radiometrically, but their age can be inferred with 

some confidence if their correlation with the volcanics of 

Green Ridge is valid. Although the monzonitic rocks 

nowhere intrude the ash-flow sequence, the area of 

alteration that extends outward from the central stock 

(fig. 61) has-contrary to Larsen and Cross (1956, p. 

110)-intensely affected the units of the Treasure Mountain Tuff, including the Ra Jadero Member, especially in 

the tributary of Cat Creek known as "The Canyon" 

(Platoro map). Furthermore, dikes that radiate from the 

central stock in Cat Creek (fig. 47) intrude the Treasure 

Mountain and Masonic Park Tuffs, as well as the overlying volcanics of Green Ridge (Platoro map). These 

relations, in conjunction with the focal position of the 

monzonite stock with respect to the outward-dipping 

volcanics of Green Ridge, favor an interpretation of the 

Cat Creek stock as intrusive into its comagmatic 

volcanics, an interpretation analogous to that developed 

for the Alamosa River stock. A similar paleotopographic 

argument, requiring the presence of a thick volcanic 

cover over the Cat Creek stock during its emplacement, 

also applies. These relations all suggest emplacement of 

the Cat Creek stock toward· the end of eruption of the 

volcanics of Green Ridge, about 27 to 27.5 m.y. ago. 

JASPER STOCK 

The Jasper stock, a subequant body about 2 km 

across centered just north of the old mining district of 

Jasper (fig. 47, loc. D), is erosionally expa;ed vecy near the 

level of its roof. The walls of the stock dip 30°-50° outward along much of the east and west sides, and where 

the northern contact is well exposed at the head of ~urnt. 

Creek (Platoro ·map), it is broadly flat, although 

irregular in detail. The stock is lithologically 

heterogeneous including porphyritic monzonite, 

porphyritic biotite rhyodacite, and fine-grained sparsely 

porphyritic intrusive rocks of andesitic composition and 

texture. These diverse rock types are seemingly intergradational; but exposures are mostly poor and 

relations are widely obscured by intense hydrothermal 

alteration (fig. 61). The intrusive nature of the andesite, 

which is petrographically similar to many adjacent lavas 

of the Summitville Andesite, is evident from crosscutting contact relations, especially where the wallrocks 

have volcaniclastic textures. The margins of the stock 

consist mainly of the andesitic rocks; the porphyritic 

monzonite is exposed only locally in the interior, especially along the ridge between Jasper and Burnt 

Creeks (Platoro map). Monzonitic rocks would probably 

be more abundant deeper within the stock. Emplacement of the stock seems to be localized by the eastern 

ring-fault zone of the Summitville caldera (fig. 61), and 

the small dikelike body of monzonite exposed south of 

the alluvium around Jasper (fig. 47) may be a southern 

extension of the Jasper stock along this fault zone. 

The Jasper stock, like the 29.1-m.y. Alamosa Riv~r 

stock, is younger than the Summitville caldera, as indicated by its intrusion along the ring-fracture zone. 

Also, the stock cannot be younger than about 28 m.y., as 

indicated by relations east of Spring Creek; there, the 

lower member of the Summitville Andesite has been 

hydrothermally altered by the Jasper stock but is unconformably overlain by the unaltered andesite-rhyodacite 

unit of the volcanics of Green Ridge (Platoro map). The 

27.8-m.y. Fish Canyon Tuff occurs along this unconformity farther to the east, at the southeast end of Silver 

Mountain (Platoro map). Like the Alamosa River stock, 

the Jasper stock may be a subvolcanic intrusive related 

to an eroded vent for the upper member of the Summitville Andesite, which occurs as a chaotic near-source accumulation in adjacent parts of the Summitville 

caldera, as at Cornwalls Nose and in the Wightman Fork 

area. 

BEAR CREEK STOCK CLUSTER 

The Bear Creek stock is a steep-sided body of monzonite that is spectacularly exposed vertically for nearly 

1 km on the rugged castellated ridge between Bear and 

Crater Creeks (fig. 47, loc. Eb). Satellitic to the 

northwest and north are two smaller stocks that are less 

well exposed at the mouth of Elwood Creek and east of 

lower Crater Creek (fig. 47, locs. Ee and Ec). 

These three seemingly related stocks vary widely in 

texture: from exceptionally coarse equigranular monzonite (fig. 49A), to rather fine-grained equigranular 

monzonite (fig. 49B), to porphyritic monzonite (fig. 

49C). Each of the three stocks locally shows this diversity of textures; relations among the different textural 

types have not been determined adequately but appear 

to be gradational, with porphyritic phases concentrated 

near contacts. The Bear and Crater Creek stocks seem to 

consist mainly of equigranular monzonite of variable 
LAVAS AND RELATED ROCKS 85 

FIGURE 49.-Photomicrographs of rocks of the Bear Creek stock cluster. 

A, Exceptionally coarse equigranular monzonite. Euhedral laths of 

plagioclase (P) (about An,o) and euhedral equant grains of augite (A) 

and hypersthene (H), fringed by opaque alteration products, are surrounded by large spongy orthoclase (0), quartz (Q), and biotite (B) 

grains as much as 5 mm in diameter. Bear Creek stock at about 

12,000 feet, in cirque basin southwest of Crater Lake. B, Typical finegrained equigranular monzonite. Paragenetic relations generally 

similar to A, but interstitial quartz and sanidine are less abundant, 

and augite is extensively replaced by a secondary amphibole. Bear 

Creek stock, east side of Bear Creek, at about 11,200 feet. C, Contact 

between recrystallized breccia of the Conejos Formation (C) and 

fine-grained porphyritic monzonite (M). Phenocrysts in porphyritic 

monzonite are plagioclase (P) (An, 5) and augite (A). West contact of 

Elwood Creek stock, south of West Fork of San Juan River, at about 

9,200 feet. 

coarseness, whereas the Elwood Creek stock seems to be 

exposed near roof level and consists primarily of coarsely 

porphyritic monzonite. 

Despite the large area of intense hydrothermal alteration around the north margin of the stock cluster (fig. 

61), the monzonite is fairly fresh and in places shows less 

alteration, especially of pyroxenes, than is typical of 

other intrusions of the caldera area. Chemically, the 

monzonite seems typical of the area; the one analyzed 

sample-a medium-grained equigranular monzonite of 

the Bear Creek stock-contains about 59 percent Si02 

(table 10, No. 14). 
86 PLATORO CALDERA COMPLEX, SAN JUAN MOUNTAINS, COLORADO 

The age of the Bear Creek and related stocks cannot be 

determined closely from field relations, but these monzonites seem to be among the youngest of the caldera 

area. The wallrocks are mainly lavas and breccias of the 

Conejos Formation, but the south margin of the Bear 

Creek stock intrudes the La Jara Canyon Member of the 

Treasure Mountain Tuff (contrary to the observations of 

Larsen and Cross 1956, p. 107). The three stocks also 

appear to be intruded along anti are largely younger than 

the Elwood Creek fault system, which, to the east, 

offsets lavas of the rhyodacite of Park Creek (fig. 61, 65). 

No satisfactory younger limit can be placed on the age of 

these intrusives, however. 

OTHER STOCKS 

A small irregular intrusive exposed for less than 1 km2 

along the upper Wightman Fork (fig. 47, loc. F) consists 

of fairly uniform and unaltered fine-grained dark-gray 

phanerocrystalline rock that is slightly more mafic than 

other granitic stocks of the caldera area, containing only 

56.6 percent SiOz (table 10, No. 18). The walls of the 

Wightman Fork intrusive converge upward, as shown by 

good exposures on the rugged northeast side of the 

stream valley (Platoro map). The exposed level appears 

to represent the upper part of a body that may increase 

in size downward, possibly to connect with the northeast 

end of the Alamosa River stock, which is exposed in 

Bitter Creek just across the divide to the west. 

An elliptical stock of light-gray porphyro-aphanitic to 

equigranular monzonite, intrusive into lavas and breccias of the Conejos Formation, is well exposed in 

glaciated terrane over an area of about 2 km2 at the cirque head of Canyon Diablo southwest of Conejos Peak 

(fig. 47, loc. G). This body, which has been examined 

only briefly, seems to lack the hydrothermal alteration 

that is associated with most other monzonitic stocks of 

the caldera area. A petrologically diverse swarm of 

intermediate-composition porphyro-aphanitic dikes that 

are well exposed on the cirque rim (Platoro map) are 

confined to the interior of this stock and may represent a 

late phase of the same igneous activity. The Canyon 

Diablo stock is slightly more silicic than most other monzonites of the caldera area, containing about 62.5 percent 

SiOz (table 10, No. 16). An ag~ more precise than postConejos cannot be determined for the Canyon Diablo 

stock by field relations. Relations of the Summitville 

Andesite on the north side of Conejos Peak indicate an 

'eruptive source outside the caldera to the southwest (fig. 

39 and related text discussion) and suggest the possibility that this stock may be the subvolcanic core of another 

center for the intracaldera andesitic lavas. 

A small stock about half a kilometer across, at the 

head of Cataract Creek (fig. 47, loc. H), is well exposed 

in glaciated terrane at the level of its roof and is relatively little altered. This stock is texturally heterogeneous 

and ranges from plagioclase-augite porphyry having a 

greenish-gray aphanitic groundmass to equigranular 

fine-grained gray monzonite distinguished by the 

presence of hornblende in addition to augite and biotite. 

This is the only hornblende-bearing monzonite in the 

area. Chemically, this monzonite (table 10, No. 15) is little different from the nearby hornblende-free Bear Creek 

stock (table 10, No. 14). The Cataract Creek stock is intrusive into the Treasure Mountain Tuff, to a level at 

about the base of the La Jara Canyon Member. 

Detached roof remnants of recrystallized La Jara Canyon Member float within the stock at about the same 

level at which this tuff unit occurs in the walls, indicating that the stock was emplaced passively and did 

not significantly bulge its roof. Numerous related dikes 

cut the Cataract Creek stock and extend outward from 

it-many more than could be shown on the Platoro map. 

Three large laccolithic bodies of fine-grained dark 

granitic rock are each exposed over several square 

kilometers along the southwest margin of the San Juan 

Mountains: at Jackson Mountain, V Mountain, and 

Squaretop (fig. 57, locs. I, J, K). These bodies are 

localized near the contact between the Conejos Formation and prevolcanic rocks. They were examined cursorily during this study, and little can be added to the 

descriptions given by Larsen and Cross (1956, p. 104-

106). They may be somewhat more mafic than the 

granitic rocks near the Platoro caldera, as indicated by a 

single analysis from the V Mountain laccolith (Larsen and 

Cross, 1956, table 21, No. 2) that shows about 56 percent 

SiOz. None of these intrusions has been dated 

radiometrically, and no very significant constraints can 

be placed on their ages from the field relations, other 

than that they are younger than basal Conejos Formation. 

PORPHYRO-APHANITIC INTRUSIONS 

Radiating out to the west from several of the granitic 

intrusions, especially the Alamosa River stock (fig. 47), 

are numerous dikes of porphyro-aphanitic texture that 

range in composition from andesite to rhyolite. Similarly 

textured intrusive rocks also occur locally as plugs and a 

few small stocks. ANDESITIC ROCKS 

Dark-gray dikes, commonly containing small 

phenocrysts of plagioclase and augite or plagioclase and 

hornblende but lacking. biotite, are probably mostly 

andesitic in composition, although chemical analyses are 

lacking. A few of these may be sufficiently silicic to be 

rhyodacites, as judged from petrographic comparisons 

with analyzed lavas. Most of the andesitic dikes are 

narrow-rarely more than a few meters thick-and they 

are typically poorly exposed and are traceable along 

strike for only short distances. Many additional dikes of 

this group undoubtedly were overlooked in field map-
LAVAS AND RELATED ROCKS 87 

ping, in contrast to the more conspicuous silicic types, 

most of which are thought to have been mapped. In exceptionally good exposures, as where the country rocks 

are weakly consolidated mudflow breccia, a few individual andesitic dikes have been traced for at least 1 

km. 

The andesitic dikes form a well-developed radial 

system around the Cat Creek stock and are especially 

numerous around the west side of the Alamosa River 

stock (fig. 47). The andesitic dikes are the most 

numerous and widespread type in the Platoro area, and 

they extend outward farther than the more silicic types. 

The andesitic group of dikes seems to merge at the 

southwest margin of the volcanic field with the wellknown swarm of mafic dikes that extends along the east 

edge of the San Juan Basin for at least 75 km, as far 

south as northern New Mexico (fig. 1). 

The andesitic dikes are likely of several ages, which 

can only locally be satisfactorily distinguished. Many of 

them are undoubtedly part of the prolonged postcollapse 

igneous events around the margins of the Platoro caldera 

complex, especially the radial dikes around the Cat 

Creek and Alamosa River stocks. Some of these dikes 

have conspicuous petrographic similarities to nearby 

intermediate-composition lavas, such as the andesiterhyodacite unit of the volcanics of Green Ridge east of 

the caldera complex, or the andesite of Summit Peak to 

the west. Other andesitic dikes are almost certainly 

related to the Conejos Formation, however, and these are 

difficult to identify confidently because of the similar 

lithologies. Best candidates for dikes of Conejos age are 

some with the distinctive platy-plagioclase texttire (fig. 6) 

typical of many Conejos lavas, and also a few divergently 

trending east-west dikes in the western headwaters of 

the upper Conejos River that seem to be crudely radial to 

the Adams Fork volcano of Conejos age (fig. 7). 

Two small plugs on opposite sides of the Adams Fork 

(Platoro map) may also be of Conejos age, especially the 

southern one that is intrusive into the flank of the 

Adams Fork cone. This plug consists of dense dark nonporphyritic rock that would be taken for an andesitic 

flow were it not exposed in crosscutting relations over a 

vertical distance of at least 250 m. The other plug, a 

light'"gray porphyritic monolithologic intrusion breccia 

that is well exposed on the ridge crest between the 

Adams Fork and the head of the Alamosa River (Platoro 

map), is of less certain age. 

RHYODACITIC ROCKS 

Light gray or greenish-gray dikes and a few sills (fig. 

50), which contain phenocrysts of biotite in addition to 

plagioclase and augite or hornblende, are probably mostly rhyodacitic, although some may be low-silica quartz 

latite. The phenocrysts in these biotite-bearing dikes are 

typically larger than those in the andesitic dikes, and 

plagioclase 'crystals in some attain a diameter of at least 

1 em. The rhyodacitic dikes are thicker than andesitic 

ones-commonly 5-10 m or more-and they are better 

exposed and more continuous along strike. Several 

rhyodacitic dikes at the head of the Alamosa River (fig. 

47), are continuously exposed for about 3 km. 

The rhyodacitic dikes are probably entirely related to 

postcaldera volcanism, because this compositional type 

is rare in the Conejos Formation. Rhyodacitic dikes that 

radiate outward from the Cat Creek stock are similar in 

petrography to the porphyritic margins of this stock and 

to the satellitic laccoliths that have already been mentioned; these intrusive rocks may be comagmatic with 

the porphyritic rhyodacite unit of the volcanics of Green 

Ridge. Similarly, the dikes of this type in the Summitville area are probably related to eruption of the 

rhyodacite of Park Creek, and the great concentration of 

rhyodacitic dikes in the upper Alamosa River area (fig. 

47) suggests that a large volcanic accumulation of 

porphyritic rhyodacite may once have been erupted 

there. 

A subequant stock of biotite-bearing porphyry about 

2 km across, which is petrographically similar to the 

rhyodacitic dikes, intrudes. the intracaldera welded tuff 

of the La Jara Canyon Member on Cornwall Mountain 

(fig! 47, loc. L). This stock is compositionally transitional toward quartz latite (table 10, No. 20) and is 

similar in petrography and chemistry to the intracaldera 

La Jara Canyon (compare table 4, Nos. 5, 6), which 

represents the last volcanism from the caldera during 

collapse. This similarity, together with the central location of the stock in the resurgent block, suggests that the 

stock· is a cupola of the magma body that caused 

resurgence. This interpretation, discussed more fully in 

the section on the Platoro caldera, is illustrated in cross 

sections of the Platoro map (sections C-C' and D-D '). 

A similar stock of much-altered porphyry 

characterized by a rather variable phenocryst content is 

rather poorly exposed at the south foot of Prospect. 

Mountain and seems to be at the focus of the great concentration of rhyodacitic dikes along the upper Alamosa 

River (fig. 47; Platoro map). 

Many of the best exposed intrusives of biotite 

rhyodacite porphyry (and also more silicic porphyritic 

bodies) are characterized by dark phenocryst-poor 

margins (fig. 50). In some of the intrusives, such 

phenocryst-poor margins may be due to flowage differentiation (Bhattacharji and Smith, 1964). In others, the 

abundance of mafic groundmass minerals in the 

marginal zone, as compared with the interior (fig. 50B, 

C), suggests the possibility of significant compositional 

differences within the magmas that formed differe·nt 

parts of a single intrusive. 
88 PLATORO CALDERA COMPLEX, SAN JUAN MOUNTAINS, COLORADO 

FIGUU: 50.-Phenocryst variations at the margin of a rhyodacite 

porphyry sill. At 12,400 feet on the east slope of the 12,866-foot peak 

on the Continental Divide, at the head of Prospect Creek. A , Dark 

phenocryst-poor upper margin of the sill is about 50 em thick and 

grades abruptly (within 1 em) to lighter colored phenocryst-rich interior below. Approximate top of sill is shown at upper edge of 

photograph. B, Photomicrograph of the phenocryst-poor margin. C, 

Photomicrograph of the phenocryst-rich interior. The groundmass 

chemistry has not been studied, but the lower mafic mineral content 

of the groundmass of the interior, as compared with the margin, 

suggests that the interior may be a more silicic rock. 

Q UARTZ LATITIC AND RHYOLITIC ROC KS 

Coarsely porphyritic light-gray quartz latite and lowsilica rhyolite, characterized by quartz and sanidine 

phenocrysts, form the largest and most spectacular dikes 

and plugs in the region (fig. 51). The dikes are commonly 

more than 10 m thick and locally as much as 100 m 

thick, and many are continuously exposed for several 

kilometers (fig. 47; Platoro map). The dikes of sanidinequartz porphyry are the dominant type along the 

northeast side of the Elwood Creek fault zone (figs. 47, 

61), where most trend northwest. Although several large 

dikes of this type trend southwest from the area of the 

Alamosa River stock, the overall distribution is 

significantly different from that of the more mafic dikes. 

The sanidine-quartz porphyries appear to be intrusive 
LAVAS AND RELATED ROCKS 89 

FIGURE 51.-Typical exposure of large quartz Iatite porphyry dike, about 40 m thick (for scale, note the truck parked near the left contact of the 

dike). Poorly exposed wallrock is upper member of Summitville Andesite. Biotite from this dike has yielded a K-Ar age of25.8 m.y. (Lipman 

and others, 1970, table 4, No. 6). Summitville-Stunner road, at South Fork of Iron Creek in Schinzel Meadows. 

equivalents of both the quartz latite of South Mountain 

and the rhyolite of Cropsy Mountain, but these rocks are 

so similar that further subdivision is at present impractical. 

A few exceptionally well exposed dikes have spectacular marginal grooves along their contacts that 

plunge gently to moderately toward the area of the 

Alamosa River stock ·(fig. 52). Many of these dikes also 

have marginal vitrophyre zones-a feature rarely 

observed in the more mafic dikes. 

Ma:;t of the larger sanidine-quartz latitic and rhyolitic 

intrusives are some distance outside the caldera complex 

(fig. 47). An elliptical quartz latitic plug about 1 km 

across, containing sparse large sanidine and quartz, intrudes the outflow Treasure Mountain Tuff along lower 

Ojito Creek (fig. 47, loc. M). This plug is structurally interesting because it acted as a piston that punched up its 

roof (probably lower tuff of the Treasure Mountain 

overlain by upper lava unit of the Conejos Formation) 

and also locally deflected upward the welded tuff 

wallrocks adjacent to the intrusive contact (Platoro 

map). This plug consists of quartz latite that contains 

FIGURE 52.-Wall of large quartz Iatite porphyry dike displaying lowangle flow grooves. The grooves are 5-10 em deep and about 25-50 em 

apart; the plunge of the grooves and the strike of the dike are to the 

northeast, toward the area of the Alamosa River stock. At about 

12,600 feet, on the west slope of the 12,866-foot peak on the Continental Divide, at the head of Prospect Creek. 
90 PLATORO CALDERA COMPLEX, SAN JUAN MOUNTAINS, COLORADO 

about 67 percent Si02 (table 10, No. 23); it seems 

chemically and petrographically intermediate between 

the other porphyritic rhyodacites and quartz latites of 

the caldera complex. 

In upper Park Creek, along the northeastern side of 

the Elwood Creek fault zone, a small steep-sided plug on 

the north side of the valley and a stubby inclined sill on 

the south side both consist of exceptionally light colored 

porphyry that contains abundant rounded quartz and 

small sanidine ppenocrysts. Che:Jllical analyses are not 

available, but these rocks seem to be exceptionally 

silicic, perhaps transitional to the Hinsdale rhyolite 

type. 

At Sheepshead, just east of Propsect Mountain 

(Platoro map), a small plug of unaltered quartz-sanidine 

porphyry about 300 m across intrudes intensely 

hydrothermally altered Summitville Andesite. It seems 

petrographically identical to the rhyolitic lava flow on 

Lookout Mountain (fig. 44) and is almost certainly an 

example of an intrusive genetically related to the 

rhyolite of Cropsy Mountain. A much smaller plug of 

this type, only about 40 m across, crops out on the west 

slope of Lookout Mountain. 

To the southwest, on the east side of Blanco Basin (fig. 

47, loc. N), a large sill at least 100 m thick of typical 

quartz latite porphyry (table 10, No. 21) is poorly exposed beneath landslide deposits. Prior to erosion, it 

probably was continuous across the valley to the north 

with a petrographically similar irregular body which has 

the shape of a stubby dike that flattens downward into a 

sill-like orientation. 

Farther to the southeast, near Trail Lake northeast of 

Banded Peak (figs. 13; 47, loc. 0), a small plug of 

porphyritic quartz latite (67 percent Si02; table 10, No. 

22) that contains large sanidine and quartz phenocrysts 

intrudes the ash-flow sequence as well as basalt flows of 

the Hinsdale Formation. This is the only place where a 

sanidine-quartz intrusive can be shown by field relations 

to be younger than Hinsdale basalt, although late 

porphyritic lavas of the caldera margin and basalt flows 

have overlapping radiometric ages in the Summitville 

area. 

The porphyritic quartz latite and rhyolite intrusives 

are phenocryst rich (15-40 percent) except along borders, 

where some bodies are depleted in phenocrysts, as are 

the rhyodacites already discussed (fig. 50). Glassy 

margins of some of these intrusives have very intricate 

textures that indicate major phenocryst-groundmass 

disequilibrium. Sanidine phenocrysts are in places as 

much as 10 em long; some are ovoid in outline, riddled 

with small glassy inclusions, and mantled by rims of sodic 

plagioclase (An10-15). The quartz phenocrysts typically 

show remnants of bipyramidal outline but are rounded 

and embayed by resorption. In addition to sanidine and 

quartz, these rocks contain smaller phenocrysts of sodic 

plagioclase (Ani0-25), biotite, Fe-Ti oxides, and, commonly, hornblende. A few contain augite in place of, or 

in addition to, hornblende. Available analyses of the 

sanidine-quartz intrusives show a range of only 67 to 68 

percent Si02 (table 10, Nos. 21~23), all near the compositional range of the quartz latite of South Mountain. 

Phenocryst variations (abundant quartz and sanidine) 

indicate that at least some sanidine-quartz intrusives, 

such as the plugs in upper Park Creek and at Prospect 

Mountain (Sheepshead), are more silicic and are similar 

to the rhyolite of Cropsy Mountain, to which many of the 

intrusives seem likely related. 

In the rare places where cross-cutting contacts with 

other dikes can be observed, the sanidine porphyry dikes 

are invariably the youngest. Biotite from one dike has 

been dated at 25.8 m.y. (fig. 51; Lipman and others, 

1970, table 4, No.6). The close relations of other intrusives 

of this type with the 22.8-m.y. quartz latite of South 

Mountain and with the 20.2-m.y. rhyolite of Cropsy 

Mountain indicate that the sanidine porphyry dikes 

were emplaced over a time span of several million years. 

LATE BASALT AND RHYOLITE 

(HINSDALE FORMATION) 

Miocene and Pliocene volcanic activity in the 

southeastern San Juan region produced a widespread 

but thin veneer of basaltic lavas, with which are 

associated local rhyolite lavas and tuffs that are high in 

silica and alkalis. This basalt-rhyolite association contrasts markedly with the voluminous intermediate to 

silicic Oligocene lavas and ash-flow tuffs that make up 

most of the San Juan volcanic field. This contrast was 

implied by Larsen and Cross (1956), who placed all the 

late basalt and rhyolite of the San Juan region together 

in a single unit, the Hinsdale Formation. Usage of the 

term "Hinsdale Formation" is continued here, even 

though new radiometric and chemical data show that 

the postcollapse porphyritic lavas around the margins of 

the Platoro caldera complex bridge the transition from 

the intermediate-composition eruptions to the bimodal 

(basalt-rhyolite) associations. This upper Cenozoic 

bimodal volcanic association is believed to reflect fundamental aspects of the evolution of the San Juan 

Mountains and adjacent regions (Lipman and others, 

1970; Christiansen and Lipman, 1972). 

BASALTIC LAVAS 

Although dark-gray basaltic lavas of the Hinsdale Formation are present over most of the southeastern San 

Juans only as scattered erosional remnants that cap high 

mesas (fig. 53), they appear originally to have con-
1o7·oo· 

LATE BASALT AND RHYOUTE 

Mount 

,.Hope 

q,etroz 

Mountain 

Wolf Creek 

Pass>< 

'" \ 

0 5 

0, 5 10 

~ 

~Basalt 

Rhyolite 

Sample locality (table II) 

106°30' 106.15' 

Osier ~untain 

10 15 20 MILES 

15 20 Kl LOM ETERS 

EXPLANATION 

* Basaltic vent 

"""'"""'"""" Limit of exposed volcanic 

rocks 

Caldera wall (topographic)-

Dashed where covered by 

younger rocks 

91 

1o5•oo· 

FIGURE 53.-Distribution of basaltic and rhyolitic lavas of the Hinsdale Formation. Based on mapping by P.W. Lipman, 1965-67, 1970-71. 
92 PLATORO CALDERA COMPLEX, SAN JUAN MOUNTAINS, COLORADO 

stituted a widespread veneer over much of the region. At 

various places the basalt flows rest on most of the older 

units, and along the east front of the San Juan Mountains they interfinger with volcaniclastic rocks of the Los 

Pinos Formation. Individual flows are 5-50 m thick, but 

most are no more than 10 m thick. The thickest flows apparently represent ponded valley fillings. In places, as 

many as 15 thin flows with only minor interbedded 

sediments are exposed in cliff sections no more than 100 

m high, as at Del Norte Peak, east of Red Mountain, and 

along the south flank of Los Mogotes shield volcano 

where it is cut by the Conejos River (fig. 53). 

Patches of basaltic lava flows from near Red Mountain 

to south of the La Jara reservoir (fig. 53; Platoro map) 

were named the latite of Red Mountain and were mistakenly interpreted as Quaternary in age by Larsen and 

Cross (1956, p. 207). This age was assigned because these 

lavas were interpreted as having flowed down the 

modern drainage of La Jara Creek. More detailed mapping has demonstrated, however, that these basalts are 

preserved along the down thrown side of the La J ara 

Reservoir fault (fig. 61) and that a distinctive sequence 

of three flows east of the reservoir, that decreases upward 

in olivine phenocrysts, correlates across the fault with 

basalts mapped as Hinsdale Formation both by Larsen 

and Cross (1956, pl. 1) and in this study (Platoro map). 

Vent areas for the basaltic lavas are mostly obscure, 

but a notable exception is the relatively young and little 

eroded Los Mogotes shield at the southeast edge of the 

area (fig. 53). Vent areas that lack surviving constructional form are distinguishable mainly by thick accumulations of bedded red-brown cinders and scoria 

blocks. In places these beds show primary dips of as 

much as 25° (fig. 60A), and constitute eroded remnants 

of cinder cones. Such cinder accumulation marking vent 

areas have been recognized in the Handkerchief MesaBeaver Creek area, south of the mouth of the South Fork 

of the Conejos River, at Horse Park at the east end of 

Green Ridge, and along the ridge east of Red Mountain 

(fig .. 53). East of Red Mountain, at the head of the large 

Silver Lakes landslide scar (Platoro map), a central 

basaltic plug is surrounded by outward-dipping interbedded lava flows and ci;nders. (The reddish-brown 

beds interlayered with lavas on Red Mountain proper are 

not, as thought by Larsen and Cross (1956, p. 207), 

bedded cinders of a basaltic crater; rather, they are tuffaceous sandstones and conglomerates of the Platoro 

caldera fill that owe their conspicuous color to baking by 

an overlying intermediate-composition flow of Summitville Andesite.) 

The Hinsdale basalts in the southeastern San Juan 

region appear to have erupted intermittently over much 

of Miocene and Pliocene time (Lipman and others, 

1970). A basalt flow near Beaver Creek north of Summitville (fig. 53), believed to represent an old part of the 

Hinsdale Formation because it predates associated 

silicic volcanism, yielded a whole-rock K-Ar age of 23.4 

m.y. A basalt flow interlayered low in the Los Pinos Formation along La Jara Creek gave a whole-rock age of 17.7 

m.y. A high flow and a dike from the relatively little 

eroded Los Mogotes shield volcano yielded ages of 4. 7 

and 5.3 m.y., respectively. 

Although Pleistocene basalts do not appear to be present in the southeastern San Juan Mountains, young 

basalt flows in the Tusas Mountains of northern New 

Mexico that have been called the Brazos Basalt by 

Doney (1968) are only slightly eroded and are probably 

Pleistocene in age (Larsen and Cross, 1956; Bingler, 

1968); these basalts are similar in petrography and 

chemistry to older basalts of the Hinsdale Formation to 

the north (Lipman, 1969). 

Despite this great range in age, all late basaltic rocks 

of the southeastern San Juan region (and for that 

matter, the entire volcanic field) have generally similar 

petrography and chemistry. These ba~alts are typically 

dense and fine grained (fig. 54A); they contain small 

phenocrysts of olivine, commonly altered to iddingsite, 

and in places sparse phenocrysts of plagioclase or diopsidic augite. The groundmass of all the rocks consists of 

fine-grained plagioclase, olivine, and augite; the texture 

is typically intergranular but tends toward pilotaxitic in 

some of the most silicic lavas. 

Rounded xenocrysts of quartz jacketed by finely 

granular augite, and of oligoclase mantled by andesine or 

labradorite, occur sparsely in many of the flows and 

abundantly in a few (Larsen and others, 1938; Lipman, 

1969, pl. 2; Doe and others, 1969). The origin of these 

xenocrysts is in doubt. They may have been derived from 

Precambrian plutonic rocks (Doe and others, 1969); they 

may represent disequilibrium remnants from highpressure crystallization in the upper mantle (Nicholls 

and Carmichael, 1971); or they may have resulted from 

mixing with phenocryst-bearing rhyolitic magma as discussed under "Mixed Lavas." 

The basaltic rocks contain 50-59 percent Si02 (table 

11). Most of the more silicic rocks contain abundant 

xenpcrysts, suggesting that they represent contaminated 

lavas originally of more mafic composition. All the 

basalts are high in K20 and in total alkalis. Even the 

basalts lowest in silica typically contain more than 1 percent K20, and this oxide is as high as 3-4 percent in some· 

of the relatively silicic xenocrystic basaltic andesites. 

Total alkalis range from 4 to 8 percent (fig. 55). Such 

high sodium and potassium contents apparently are 

characteristic of most upper Tertiary basaltic rocks from 

the Basin and Range and the Southern Rocky Mountains provinces (Moore, 1962; Leeman and Rodgers, 1970). 

A plot of total alkalis against silica for basalts of the 

Hinsdale Formation from the southeastern San Juan 

volcanic field shows that these basalts fall in the range of 
LATE BASALT AND RHYOLITE 93 

FIGURE 54.- Photomicrographs of common basalt and rhyolite types of 

the Hinsdale Formation. A. Dense basaltic lava, containing a few 

small olivine phenocrysts( arrows) in a fi ne-grained groundmass of 

plagioclase, olivine, augi!;e, and Fe-Ti oxides. Phenocryst at the bottom is skeletal. High lava flow on west rim of Handkerchief Mesa, 

just north of the mixed-lava complex. Analyzed sample 13, table 11. 

B, Silicic alkali rhyolite, containing small phenocrysts of sodic 

alkali basalts of Hawaii (fig. 55). In contrast, nearby upper Pliocene basalts of the southern San Luis Valley and 

the Rio Grande rift (Servilleta Formation) are much lower 

in alkalis and are tholeiitic in composition (fig. 55; Aoki, 

1967; Lipman, 1969). 

RHYOLITIC LA VAS A D T FFS 

Rhyolites of the Hinsdale Formation are petrologically 

distinctive white or light-gray rocks which are high in 

silica and alkalis and which are characterized by 

phenocrysts of quartz and sodic alkali feldspar. These 

contrast with the Oligocene rhyolites, which are lower in 

silica , contain both plagi oclase and s anidine 

phenocrysts, and generally lack phenocrystic quartz . 

Two major accumulations of rhyolitic rocks of t he 

Hinsdale Formation have been recognized: one around 

the Summitville caldera (fig. 53) and the other around 

the Lake City caldera in t he northwestern part of the 

volcanic field (fig. 1; Durango map). The rhyolites 

around the Summitville caldera form several small lava 

domes, each as thick as 200 m but nowhere covering 

e (Or,o), qua rtz. and sparse plagioclase, biotite, and Fe-Ti oxides. Single arrow indicates resorbed quartz phenocryst that shows 

remnant bipyramidal outline. Double arrow indicates euhedral 

sanidine enclosing rounded grain of sodic plagioclase (An, 5) in extinction position. Sample is devitrified and displays weak flow layering. East side of rhyolite plug dome at Grayback Mountain. 

Analyzed sample 12, table 11. 

fz 

w 

u 

a: 

w 

Q. 

fI 

(.!) 

8 

+ 2 0 

z "' 

~8 

0 

50 

0 .. 

52 54 

0 

0 + 

EXPLANATION 

+ Servilleta Formation 

Hinsda le Formation: 

Basalt of Los Mogo tes volcan,o 

~ Xenocrystic basaltic rocks 

o Other basalts 

56 58 60 

Si0 1 , IN WEIGHT PERCENT 

FIGL'RE 55.-Total alkalis and Si02 for basalts of the southeastern an 

Juan Mountains (Hinsdale Formation) and the northern end oft he 

Rio Grande depression (Servilleta Formation). Dashed line is boun -

dary between alkalic and tholei itic basalts of Hawaii (Macdonald 

and Katsura, 1964). Ana lyses are from table 11 and from Aoki 

(1967) and Lipman (1969 and unpub. data). 
94 PLATORO CALDERA COMPLEX, SAN JUAN MOU?'-JTAINS, COLORADO 

TABLE 11.-Analyses of basaltic and 

[Sample localities shown in fig. 53. Analyses 6 and 7 are from Larsen and Cross (1956, table 25). Other major-oxide analyses are by rapid methods by 

titative spectrographic analyses by J. D. Fletcher; other analyses 

Basalt Xenocrystic basalt 

Sample--------- 3 5 8 10 

Field No------- 67L-107-B 70L-31-B 66L-26 65L- 32 66L-98 Con21 66L-20 66L-244-A 66L-27 

Lab. No-------- Wl69529 Wl74433 Wl68066 Wl68063 W16842S W168065 Wl68069 Wl68423-B 

Major oxides (weight percent), recalculated without H20 and C02 (original values listed separately below) 

SiOr---------- so. 5 51.2 52.4 52.8 53.0 55.43 55.89 57.4 57.5 59.6 

Al203---------- 15. 3 15.6 14.6 17.7 15.5 15.85 14.98 14. 5 15.3 16.3 

Fe 2o3---------- 4. 5 4. 7 4. 6 s. 9 4.4 4. 58 4. 31 3.0 3. 5 2.0 

FeO------------ 6. 2 s. 4 7.0 4.1 5. 6 4. 60 4. 25 4. 9 4. 6 4. 6 

~gO------------ 7. 5 6. 6 6.4 4.1 5. 3 4. I 9 3. 92 s. 7 4. 8 2. 8 

CaO------------ 7. 9 7. 5 8.1 7. 6 7. 4 n. 70 6. 39 6. 7 6. 3 5.4 

Na 2o----------- 3. 6 4. 6 3. 1 3. 5 3. 5 3. 90 4. 38 3. 5 3.8 4. 3 

K20------------ 1.5 1.6 1.3 2. 2 2. 3 3. 32 3. 38 2.4 2. 2 3.1 

Ti02----------- 1.8 1.8 1.8 1.4 2.1) 1.46 I. 41 1.3 1.3 1.2 

P20s----------- . 87 . 75 . 42 . 52 . 66 . 99 . 39 . 38 . 55 

MnO------------ .18 .16 . 07 .14 .16 .06 . 12 .12 .11 .07 

TOTAL------- 99.85 99.91 99.79 99.96 99.82 100.09 100.02 99.91 99.79 99.92 

H20+ ---------- I. 80 1.0 . 54 . 72 . 56 .13 . 72 .43 . 44 . 75 

H20- ---------- . 65 .71 . 36 . 88 . 40 . 61 . 42 . 47 .46 . 65 

C02 ----------- <. 05 <. 05 <. 05 .09 <.OS . 21 < .05 <.OS 

CIPW norms (weight percent), from r<'ca1culated analyses 

Quartz--------- 3.84 4. 71 2. 69 2.18 2.85 6.87 7. 77 7.89 

Corundum-------

Or thee lase----- 9.16 9. 63 7.81 13.29 13. 71 19.63 19.97 14.34 13.12 18.61 

Albite--------- 30.59 38.78 26.66 29.40 29.88 32.94 37.01 29.94 32.45 36.11 

Anorthite------ 20.91 17.13 21.89 26.00 I 9. 69 15.77 11.26 16.40 18.08 15.90 

Wollastonite--- s. 38 6.40 6. 57 3. 39 s. 25 7. 28 5.84 s. 90 4. 36 3.02 

Enstatite------ 13.78 5. 60 15.95 10.18 13. 32 10.42 9. 77 14.10 12.06 7.09 

Ferrosili te---- 3. 71 1.13 • 6. 21 . 51 3. 68 2. 37 2.14 4. 65 3. 65 4.84 

Fors teri te----- 3. so 7. 63 

Fayoli te------- 1. 04 1. 69 

Magnetite------ 6. 59 6. 79 6. 63 8. 59 6. 44 6. 64 6. 24 4. 40 5.12 2. 95 

Hematite-------

Ilmenite------- 3. 33 3. 48 3. 4 7 2. 72 3. 83 2. 'i6 2. 67 2. 50 2. 49 2. 32 

Rutile---------

Apatite-------- 2. 05 1. 78 . 99 I. 23 1. 55 2. 33 . 93 . 91 1. 30 

Minor elements (parts oer million) 

Ba- ------------ 680 3200 500 700 1000 700 700 1500 

Be--..!~--------- <2 <1 2 <1 <1 1 

Ce------------- 100 100 100 300 

Co------------- 28 10 50 30 70 50 30 50 

Cr------------- 240 5 200 15 70 200 200 70 

Cu------------- 76 36 70 50 30 100 50 30 

Ga------------- 20 10 10 15 70 10 150 30 

La------------- 70 100 100 70 70 100 

Mo------------- 3 3 3 3 

Nb------------- 10 10 10 10 10 

Ni------------- 110 10 300 150 300 150 70 

Pb------------- 20 15 500 15 7 700 

Sc----------.:_ __ 24 11 30 30 150 20 20 70 

Sn------------- 70 150 

Sr------------- 1400 200 700 1500 3000 700 1000 2000 

v-------------- 190 90 200 200 150 200 200 150 

Y-------------- 60 <40 50 50 50 50 30 30 

Yb------------- <2 5 5 5 5 3 3 

Zr------------- 140 130 100 150 300 200 200 400 

Phenocrysts (volume percent) 

Quartz--------- Tr. Tr. 

Sanidine------- Tr. 

Plagioclase---- Tr. 24.4 3 Biotite--------

Hornblende-----

Augite--------- 7. 3 

01 i vine---·----- 2. 4 Tr. 

Opaques-------- 1.1 1 1 

Groundmass----- 95 100 95 64.8 95 97 97 90 90 95 

See page 96 for sample descriptions. 
LATE BASALT AND RHYOLITE 

rhyolitic lavas of the Hinsdale Formation 

P. L. D. g1more, Samuel Botts, Gillison Chloe, Lowell Artis, H. Smith, John Glenn, and James Kelsey. Minor-element analyses I, 2, and 12 are quanurc by scmiquunt.itative methods by J. C. Hamilton. Tr., trace] 

II 

651.-161-A 

W168067 

76.2 

13. I 

.32 

• 44 

• IS 

. 69 

4. 1 

4. 8 

.11 

.02 

'14 

100.07 

.04 

.16 

<.OS 

35.00 

. 70 

26. !!4 

32.46 

3. 27 

. 38 

. 63 

. 47 

. 21 

.05 

30 

7 

100 

3 

200 

70 

7 

so 

300 

30 

10 

20 

2 

100 

100 

Rhyolite 

12 

7ot-u s-s Wl76185 

13 

70L-146-8 

W1 74649 

14 

70L-147-C 

W174651 

Mixed-lava complex of Handkerchief Mesa 

15 

70L-148 

Wl74654 

16 

70L-147-D 

Wl74652 

17 

70L-147-E 

Wl74653 

18 

70L-14 7-A 

Wl74650 

19 

70L-151 

Wl746SS 

Sample 

Field No. 

Lab. No. 

Major oxides (weight percent), recalculated without H20 and C02 (original values listed separately be1ow)--Continued 

76.6 

13. 3 

. 65 

• 32 

.08 

. 23 

4.4 

4. 3 

. 06 

. 00 

.10 

100.04 

.31 

.14 

<, 05 

33.63 

. 89 

25. 53 

37.40 

l. IS 

. 20 

. 14 

. 94 

.!1 

28 

18 

4 

30 

70 

llO 

70 

30 

16 

20 

2 

I 30 

2. 9 

6. 3 

. 9 

1.2 

88.7 

51.6 

16.2 

4.1 

5. 7 

5. 6 

7.4 

3.8 

2. 7 

1.9 

. 64 

.18 

99.82 

1.2 

1.0 

<.OS 

56.0 

16.0 

4.4 

4.1 

4. 7 

6.4 

3. 98 

3. 2 

• 63 

. 37 

.19 

99.97 

. 68 

. 92 

<.OS 

63. 3 

15.9 

3.05 

2. 2 

2. 3 

4. 4 

4. 2 

3. 96 

.04 

• 4 7 

.12 

99.94 

.81 

. 29 

<.OS 

64.8 

16.1 

2. 5 

1.6 

1.5 

2. 9 

4. 3 

5.1 

. 78 

.19 

.11 

99.88 

1.0 

• 63 

< .05 

67.8 

16.1 

2. 4 

• 60 

. 55 

1.7 

4. 5 

5. 5 

. 54 

.13 

.'18 

99.90 

. 37 

. 29 

<.OS 

68.0 

15.8 

2. 2 

. 69 

. 57 

1.6 

4. 7 

5. 5 

. 5 

.14 

.10 

99.80 

. 66 

.14 

< .05 

ClPW non•~ (weight percent), from recalculated analvses--Continued 

o. 00 

. 00 

IS. 8 

32.2 

!9. 3 

s. 5 

7.7 

2. 3 

4. 4 

1.5 

s. 96 

.00 

3. 7 

.00 

!.5 

I 500 

l 

200 

so 

ISO 

20 

IS 

100 

3 

15 

100 

20 

2000 

ISO 

30 

3 

200 

4.1 

95.9 

2. 4 

.00 

18.7 

33. 7 

I 6. 5 

5. 4 

!1.7 

3. 2 

. au 

. 00 

h. 4 

.00 

1.2 

.00 

. 87 

I 2. 8 

.00 

23.4 

35. 2 

I 3. I 

2. 3 

5.8 

1.7 

.00 

.00 

4. 4 

. 00 

.08 

. 00 

1.1 

13.2 

.on 30. I 

3n. 2 

9. b 

1.5 

3. 8 

.00 

.00 

.00 

3. 3 

. 24 

1.5 

.00 

. 4o 

16.2 

. 00 

32.7 

38. 3 

7. 3 

. l 5 

1.4 

.oo 

.00 

.00 

. 63 

I. 98 

1.03 

.00 

. 31 

Minor elements (parts per million)--Continued 

1000 

I 

100 

30 

200 

20 

IS 

70 

3 

10 

100 

7 

l 5 

I SOD 

100 

30 

3 

200 

2000 

1.5 

500 

15 

70 

so 

15 

I 50 

5 

10 

30 

I 5 

10 

2000 

70 

30 

3 

200 

I 500 

3 

500 

10 

50 

10 

IS 

150 

5 

20 

30 

10 

10 

700 

so 

30 

3 

200 

I 500 

3 

500 

10 

5 

15 

ISO 

3 

20 

10 

5 

700 

30 

30 

3 

200 

Phenocrvsts (volume percent)-~Continued 

o. 2 

1.7 

. 9 

3. 6 

93.6 99 

0. 5 

1.4 

3. 2 

.8 

1.4 

. 6 

. 9 

91.2 

0.8 

3. 2" 

7. 5 

.7 

. 9 

Tr . 

l.l 

85.8 

15.6 

.00 

32.8 

40. I 

5. 5 

. 64 

1.4 

.00 

.00 

.00 

1. 08 

1.5 

. 95 

.00 

. 33 

1500 

3 

500 

15 

7 

15 

150 

5 

15 

15 

7 

700 

30 

30 

3 

300 

0. 6 

3. 7 

7. 4 

. 5 

1.1 

1.3 

85.4 

70.5 

15. 3 

2. 2 

. 2 

. 34 

1.2 

4.03 

5. 5 

. 42 

.06 

.10 

99.85 

. 55 

.15 

<.OS 

22.9 

. 63' 

32.7 

34. I 

5. 6 

.00 

.85 

.00 

.00 

.00 

.00 

2. 2 

. 64 

.09 

.14 

1000 

5 

500 

7 

10 

I SO 

5 

20 

I 5 

5 

200 

20 

20 

2 

200 

1.1 

3. 4 

3. 2 

.8 

• 2 

. 5 

90.8 

Si02 

Al203 

Fe2o3 FeO 

MgO 

MnO 

Ouartz 

Corundum 

Orthoclase 

Albite 

Anorthite 

Wollastonite 

Enstatite 

Ferrosilite 

Forster ite 

Favolite 

Magnet! te 

Hematite 

Ilmenite 

Rutile 

Apatite 

Ba 

Be 

Ce 

Co 

Cr 

Cu 

Ga 

La 

Mo 

Nb 

Ni 

Pb 

Sc 

Sn 

Sr 

Vb 

Zr 

Ouartz 

Sanidine 

Plagioclase 

Biotite 

Hornblende 

Augite 

Olivine 

Opaques 

Groundmass 

95 
96 PLATORO CALDERA COMPLEX, SAN JUAN MOUNTAINS, COLORADO 

TABLE 11.-Analyses of basaltic and rhyolitic lauas of the Hinsdale Formation-Continued 

SAMPLE DESCRIPTIONS 1. Dark.gray dense olivine basalt. Euhed.ral small olivine phenocrysts, mostly less than 1 

mm in diameter and partly altered to iddingsite, are set in a very fine grained intergranular groundm.ass. Cliffs east of Beaver Creek, north end of Upper Beaver Meadow, at about 10,500 ft. 2. Dark-gray dense olivine basalt. Consists of intergranular fine intergrowth of olivine, 

augite, and plagioclase. Only a few dusty resorbed plagioclase xenocrysts are as much as 

1 mm in diameter. Conspicuous light-gray mottling on weathered surfaces is due to concentration of slightly coa.rser plagioclase in the groundmass. Lip of landslide headwall, 2.0 km northeast of Red Mountain, at 11,000 ft. 3. Vesicular gray olivine basalt. Olivine phenocrysts are as much as 2 mm in diameter, 

euhedral with 1 tendency tow&Id skeletal outlines, and marginally altered to iddingsite. 

Groundmass texture is intergranular but tends toward diktytaxitic: equant grains of 

augite, olivine, and opaques are between plagioclase laths about 0.5 mm long; interstices Sie filled by dusty brownish tachylitic material. One xenocryst of quartz in thin 

secti9u is jacketed by rind of finely granular augite. Mouth of Ra Jadero Canyon, at 

a bout 8, 400 ft. 4. Exceptionally porphyritic gray basalt, containing abundant plagioclase laths 1-2 mm 

Ion&, equant augite phenocrysts as much as 4 mm in diameter, and smaller altered 

olivines. Talus along Beaver Creek road, 0.2 km west of Race Creek. 

5. Dark-gray dense basaltic andesite. Small euhedral or skeletal olivine phenocrysts, mostly 

1-2 mm in diameter and partly altered to iddingsite, are set in a relatively coarse 

pilotaxitic groundmass that contains plagioclase laths as much as 0.5 mm long. Southeast side of Green Ridge, along Cat Creek road, at about 8,100 ft. 6. VesiculSI black basalt. "The few phenocrysts are about 0.5 mm across. From northeast 

shoulder of ·Green Ridge, half way to top" (Larsen and Cross, 1956, table 25, No. 6). 

7. Similar to sample 6. No location given, other than Green Ridge (Larsen and Cross, 1956, table 25, No. 7). 8. Gray vesiculSI xenocrystic basaltic andesite. Xenocrysts of quartz as much as 5 mm 

across are embayed and rimmed by augite plagioclase forms anhedral grains 1-3 mm in 

diameter; cleSI cores (about An2o ) Sie surrounded by a wormy, partly melted zone and, 

in places, by a more calcic overgrowth that forms a clear rim on the wormy zone. (For 

more than 1 km2 in outcrop area. These erosional remnants tend to be preserved near their vents, and vent 

structures are exposed for three of the bodies-at 

Elephant and Grayback Mountains within the Platoro 

map area and at the head of Beaver Creek just to the 

north. 

The Beaver Creek body, which intrudes nearly contemporaneous lower Miocene basalt of the Hinsdale Formation (Lipman and others, 1970), is perhaps the most 

interesting. The lowest exposures of this rhyolite are 

beds of crudely sorted pumice and ash fall that have 

primary dips of 15°-25° away from the vent area. At the 

vent, a steeply dipping dike of flow-layered devitrified 

rhyolite extends upward (fig. 56), flattens, and merges at 

the original surface of the pyroclastic cone with subhorizontally flow layered lava (fig. 57) . At the base of the 

FIGURE 56.-Feeder dike for the Beaver Creek vent-dome complex is the 

ridge in foreground; it intrudes basalt flows of the Hinsdale Formation that form the mesa rims on both sides of the rhyolite. 

photomicrograph, see Lipman, 1968, pl. 2.) Ridge crest, about 0.7 km east of junction between Hot Creek and Ojito Creek. 9. Dense gray xenocrystic basaltic andesite, much like No.8. At small pond, 0.7 km north of Trail Lake. 10. Gray slab by andesite, containing elongate vesicles. Sparse resorbed xenocrysts of quartz. 

Augite phenocrysts, as much as 3 mm in diameter, are set in pilotaxitic groundmass. Ridge crest, north of Ra Jadero Canyon, at about 9,200 ft. 11. Obsidian cores ("apache tears"), collected from glassy perlitic flow breccia, rhyolite of Beaver Creek. Contains less than 1 percent microphenocrysts of plagioclase (avg diameter about 0.1 mm). Dated sample (Lipman and others, 1970, table 5, No.7). At about 11,400 ft, on north side of hill reaching altitude of 11,529 ft. 12. Light-gray rhyolite, containing resorbed phenocrysts of Quartz and sadie sanidine in fine- ly devitrified flow-layered groundmass (fig. 548). At 12,400 ft, on prominent rib, east 

side of Gray back Mountain. 13. Fine-grained dense black basalt. Underlies mixed-lava complex. Petrographically much like No.1. From main gully on southwest side of Handkerchief Mesa, at about 11,000 ft (fig. 54A). 14. Basaltic phase of mixed-lava complex. See fig. 59B for description. At about 11,300 ft, 0.3 km southeast of locality of No. 13. 15. Dark-gray nonporphyritic vesicular rhyodacite. Flow appears to overlie the mixed-lava 

complex along its northwest side. Contains about 1 percent microphenocrysts of augite 

that average about 0.5 mm in diameter. At about 11,400 ft, along north mesa rim , 0.9 km northeast of locality of No. 13. 16. Typical gray xenocrystic interior of main flow. Much like No.8. Same locality as No. 14. 

17. Light-gray phenocryst-rich lower part of main flow. Resorbed feldspars and quartz are as much as 55 mm in diameter. Downhill from locality of No. 14, at about 11,100 ft. 18. Scoriaceous red-brown top of upper flow. Phenocrysts petrographically very similar to 

No. 17. Uphill from locality of No. 14, at about 11,400 ft. 

19. Light-gray rhyolitic phase. See fig. 59A for description. Northeast rim of Handkerchief Mesa, at about 11,700 ft, 0.5 km southwest of Poage Lake. 

lava flow, overlying the pyroclastic cone, is several tens 

of meters of poorly sorted unstratified flow breccia. A 

conspicuous cooling zonation is evident within the flow 

breccia. The lower zone consists of loose glassy blocks 

that become indurated and compacted upward to form a 

vitrophyre zone of hydrated gray perlite in which outlines of fragmental blocks are still faintly visible. Within 

the perlite are preserved scattered remnant nodules of 

nonhydrated black obsidian ("apache tears") and redbrown spherulites that mark the inception of devitrification (fig. 58). The spherulites increase upward in size 

and abundance and eventually coalesce to form completely devitrified rhyolite near the base of the originally 

fluidal lava, which is marked by well-defined subhorizontal flow layering (fig. 57). 

Devitrified rhyolite of the Hinsdale Formation is a 

petrographically uniform light-gray, light-pink, or white 

chalky rock containing 10 percent or less small 

phenocrysts (1-2 mm in diameter) of quartz and alkali 

feldspar in subequal amounts, accompanied by minor 

biotite, sphene, and Fe-Ti oxides (fig. 54B). The quartz 

occurs as variably rounded and resorbed bipyramidal 

euhedra, and the alkali feldspar is sodic sanidine (0r4o-45). 

Two of the flow domes at the head of Beaver Creek 

contain sparse small phenocrysts of sodic plagioclase 

(about An15), commonly enclosed in alkali feldspar 

phenocrysts (fig. 54B). These rocks are the most silicic in 

the volcanic field; the few available analyses show 76-77 

percent Si02 (table 11, Nos. 11-12). 

The only dated rhyolite of the Hinsdale Formation 

near the Platoro caldera complex, from the Beaver Creek 

vent-dome complex (figs. 56-58), has yielded an age of 

21.9 m.y. by K-Ar analysis on obsidian and a date of 21.2 

m.y. by fission-track methods. This rhyolite is partly intrusive into 23.4-m.y. basalt of the Hinsdale Formation 
LATE BASALT AND RHYOLITE 97 

FIGURE 57 .- Depositional units of the Beaver Creek vent-dome complex. Lowest visible dipping layers are bedded pumice fall of the pyroclastic 

cone. These are overlain by unstratified massive flow breccia, the upper part of which has been fused to a dark vitrophyre by the overlying 

lava flow, itself devitrified and subhorizontally flow layered. On the back side of this outcrop, the flow-layered lava is continuous into the 

steeply dipping vent dike (fig. 56), which cuts down through the dipping pumice fall of t he pyroclastic cone. 

(Lipman and others, 1970). The petrologically related 

rhyolite near the Lake City caldera (Sunshine Peak 

Tuff) in the northwestern part of the volcanic field has 

yielded similar K-Ar ages ranging from 22.0 to 22.9 m.y. 

(H. H. Mehnert, written commun., 1971), but a 

petrologically similar rhyolitic lava flow near No Agua, 

N.Mex., at the southeastern edge of the San Juan field, 

has a fission-track age of only 4.8 m.y. (Lipman and 

others, 1970). 

MIXED LAVAS 

At two localities near the Platoro caldera area, the 

rhyolitic and basaltic lavas of the Hinsdale Formation 

are intimately intermingled, forming rhyolite-basalt 

mixed-lava complexes that are similar to the well-known 

complex on Gardiner River in Yellowstone National 

Park (Wilcox, 1944). The smaller of the Platoro occurrences, on the northwest slope of Cornwall Mountain, 

is largely covered by talus and landslide deposits and 

hence was not studied in detail. The other mixed-lava 

complex, however, is superbly exposed along the steep 

slopes of Handkerchief Mesa (fig. 53; Platoro map). 

The Handkerchief Mesa mixed-lava complex occupies 

a broad northeast-trending paleovalley about 1 km wide, 

near the south end of Handkerchief Mesa (Platoro map), 

and it is well exposed on cliffs about 150 m high on both 

southwest and northeast sides of the mesa. Exposures 

are generally poor on the flat mesa top. Before eruption 

of the mixed lavas, this area had been a center for 

basaltic eruptions of the Hinsdale Formation, as indicated by dissected basaltic cinder cones on the 

northeast side of the mesa, some of which show primary 

dips as great as 20° (fig. 60A). On the southwest side of 

the mesa, a sequence of four or five rather typical finegrained dense Hinsdale basalt flows underlies the mixed 

lavas (fig. 54A; table 11, No. 13). The rhyolitic Beaver 

Creek vent-dome complex (figs. 56-58) lies only about 1 

km north of the Handkerchief Mesa mixed-lava complex, but age relations between these two bodies are unclear. 
98 PLATORO CALDERA COMPLEX, SAN JUAN MOUNTAINS, COLORADO 

" .. · , - • 

... . ;-... ~~-

' .. 

·- '\ ~- ..::.·,---... ,.-.... 

. . .::::. ... 

- ... ~ _ ....... 

.. 

FIGURE 58.-Upper part of the vitrophyric zone of the Beaver Creek 

vent-dome complex. Initial stage of devitrification is development of 

scattered red-brown spherulites, 1-3 em in diameter, commonly 

nucleated around a feldspar phenocryst. Gray hydrated perlite sur· 

rounding the spherulites contains scattered black nonhydrated obsidian nodules ("apache tears"). 

Relations are simplest on the southwest side of the 

mesa, where the main body of mixed lava forms a mass 

100 m thick, interpreted as a flow, that is overlain by a 

flow 20 m thick having a scoriacous top and a wellexposed' basal flow breccia. Each flow, though 

heterogeneous in detail, tends to become more mafic and 

darker upward. The rhyolitic rock is light to medium 

gray and contains about 10 percent phenocrysts, including fairly large (up to 3mm in diameter) sanidine, 

plagioclase, and quartz phenocrysts, accompanied by 

small biotite and augite (fig. 59A). All phenocrysts are 

rounded, resorbed, and generally texturally complex. In 

the higher, more mafic parts of these flows the feldspar 

and quartz phenocrysts become smaller and sparser and 

even more intricately resorbed (fig. 59B). The most 

mafic parts contain olivine and lack biotite. On the 

southeast side of the mesa, these compositional 

variations are generally gradational, and sharply 

bounded internal contacts are absent. In places, 

however, swirly flow layering is defined by variations in 

color, phenocryst content, and, presumably, chemical 

composition. Locally, both flows contain angular to 

rounded well-defined masses of sparsely porphyritic 

dark basalt (table 11, No. 14); these inclusions vary in 

size from a few centimeters (fig. 59C) to at least 5 m. Except for the mafic inclusions and local flow layering, 

however, these rocks are texturally and compositionally 

within the spectrum of xenocrystic basaltic lavas of the 

Hinsdale Formation. 

In contrast, on the northeast side of Handkerchief 

Mesa, chaotically heterogeneous relations in the mixedlava complex are superbly exposed in the cliffs above 

Poage Lake (Platoro map), where the complex is intrusive into preexisting basaltic cinder cones (fig. 60A). 

Here, the relatively homogeneous intermediatecomposition rocks that constitute the bulk of the mixed 

lavas on the other side of the mesa are sparse, and more 

obviously rhyolitic or basaltic types predominate. 

Abrupt compositional discontinuities occur every few 

meters in most outcrops, and no masses of relatively 

homogeneous rock are sufficiently large to be mapped, 

even on a 1:24,000 map scale. 

Near the margins of the complex, light-gray rhyolitic 

dikes, sills, and irregular bodies a few centimeters to at 

least 10m thick intricately vein bedded basaltic cinders 

of the local precomplex cinder cones (fig. 60A, B). These 

rhyolitic intrusives are completely devitrified, and the 

larger bodies have caused fusion, induration, local 

flowage, and peculiar patchy recrystallization effects in 

the bedded cinder deposits. In contrast, some of the 

smaller rhyolitic intrusives intricately intruded the 

basaltic cinders but did not significantly modify the textural features (fig. 60C, D). 

The Handkerchief Mesa mixed-lava complex shows 

the same kinds of compositional and textural relations, 

as described by Wilcox (1944) for the complex on Gardiner River, that indicate that both the basalt and the 

rhyolite were simultaneously mobile and at least partly 

liquid at the time of eruption. Perhaps the most significant evidence for mixing is (1) the disequilibrium 

xenocrysts of quartz and alkali feldspar in the basalt 

that are identical to the phenocrysts of Hinsdale 

rhyolite and (2) the augite crystals of the rhyolite that 

resemble phenocrysts in basaltic phases of the complex. 

Although gradations are complete from basaltic to 

rhyolitic compositions, the end members are less extreme than typical rhyolitic and basaltic lavas of the 

Hinsdale Formation, suggesting that no completely uncontaminated magma has survived in the complex. 

Inclusions of basalt are preserved in rhyolitic matrix but 

never the reverse, as would be predicted from the lower 

solidus temperature of a rhyolitic melt. At the intrusive 

parts of the complex near Poage Lake, basalt inclusions 

that were derived from the country-rock cinder cones can 

be difficult to distinguish from basalt that was mobile 
LATE BASALT AND RHYOLITE 

FIGURE 59.-Photomicrographs showing petrographic variations within 

the mixed lava on the southwest side of Handkerchief Mesa. "-

Rhyolitic phase, containing large rounded phenocrysts of sodic 

sanidine j§), plagioclase (P), and qu~, accompanied by 

smaller phenocrysts of biotite (B), augite (A), and Fe-Ti oxides (ft). 

Note convolute flow lamination. Analyzed sample 19, table 11. B, 

Basaltic phase, containing sparse large resorbed xenocrysts of feldspar (F) and qu~ and small phenocrysts of a~A) and 

olivine (0) (altered to iddingsite) in a fine-grained mafic groundmass of intergranular texture. Analyzed sample 14, table 11. C, 

Small rounded basaltic inclusion in rhyolitic matrix. 

magma at the time of emplacement. The involvement of 

basaltic magma seems required by the large volume of 

mafic material that homogenized with the rhyolite to 

produce the largely intermediate-composition 

xenocrystic flows exposed on the southwest side of the 

mesa. Recognition of both intrusive and extrusive phases 

for the mixed-lava complex at Handkerchief Mesa (and 

also for the complex at North Mountain, involving the 

rhyolite of Cropsy Mountain, described earlier) indicates 

that the complex resulted from mixing of magmas prior 

to eruption rather than from surficial collision between a 

rhyolitic and basaltic lava flow, as postulated for the 

complex on Gardiner River by Wilcox (1944). A vent 

origin for the Gardiner River mixed lavas has also been 

suggested by Hawkes (1945), and this hypothesis is now 

favored also by Wilcox (oral commun., 1971). Finally, 

the large volume of relatively uniform xenocrystic mafic 

99 
100 PLATORO CALDERA COMPLEX, SAN JUAN MOUNTAINS, COLORADO 

FIGURE 60.-Handkerchief Mesa mixed-lava complex , showing 

relations between intrusive rhyolite and basaltic cinder cones. 

A Dissected basaltic cinder cone, just north of mixed-lava complex, 

northeast rim of Handkerchief Mesa. Primary dips of bedded cinders 

are as great as 20°. For scale, note person within circle. B, Irregular 

intrusive contact between dense devitrified rhyolite (above) and less 

resistant basaltic cinders (below). In detail, basaltic rocks are intricately veined by rhyolite. Arrow indicates location of sample 

shown in C. Cliff above Poage Lake, northeast side of Handkerchief 

Mesa. C, Photomicrograph of contact between intrusive rhyolite 

(above) and basaltic scoria (below) . Note intricate infiltration of 

rhyolite into scoria without destroying vesicular textures; lightcolored vesicles at bottom of photograph are empty. Rhyolite contains 

large phenocrysts of sanidine (S), plagioclase (P), and quartz (not 

visible); smaller crystals of biotite and augite not readily visible. 

Location given by arrow in B. D, Photomicrograph showing small 

fragments of scoriaceous basalt enclosed by rhyolite. Same locality 

as C; about 5 em from contact. 

lava emplaced within about 1 kilometer of the 

heterogeneous vent complex suggests that some other 

xenocrystic basaltic lavas of the Hinsdale Formation 

may be due to relatively shallow magmatic mixing of 

basalt and rhyolite, rather than to crustal contamination 

or metastable survival of high-pressure phenocryst 

phases. 
STRUCTURAL EVOLUTION 101 

SURFICIAL DEPOSITS 

Surficial deposits were mapped in the Platoro caldera 

area only where they are sufficiently thick and 

widespread to obscure the bedrock geology. Units distinguished on the Platoro map include glacial moraine, 

glacial outwash gravels, landslide deposits, rock glaciers, 

talus, colluvium, alluvial-fan deposits, and alluvium. 

STRUCTURAL EVOLUTION 

Pre-Tertiary structural features of the San Juan 

Mountains are little known because of widespread cover 

by younger volcanic rocks. The major Tertiary structures 

over most of the region are volcanic in origin and include 

numerous large calderas (fig. 1) that collapsed as a result 

of the voluminous ash-flow eruptions and the associated 

postsubsidence doming and faulting. In the southeastern 

area, however, these volcanic structures, largely of 

Oligocene age, are overprinted by later Cenozoic block 

faulting related to development of the Rio Grande rift 

depression in the San Luis Valley to the east (fig. 1, 61). 

PRE-TERTIARY STRUCTURES 

Pre-Tertiary structural features in the southeastern 

San Juan Mountains, although incompletely known, are 

significant to interpretation of some later Tertiary structures that seem to follow similar trends. Laramide deformation, which was superimposed on Precambrian and 

middle Paleozoic features (Muehlberger, 1960), began in 

the Late Cretaceous and probably continued until early 

Eocene time (Dunn, 1964). The major structures formed 

at this time are anticlinal and block-fault uplifts, similar 

to those elsewhere in the southern Rocky Mountains, including the Uncompahgre arch, Needle Mountains uplift, and Brazos arch (fig. 1; Kelley, 1957). These uplifts 

apparently partly or completely coalesced to produce a 

major northwest-trending highland almost 500 km long 

(King, 1969), although relations have been largely 

obscured along the middle sector by the Tertiary San 

Juan volcanic field. In the southeastern San Juan Mountains, Mesozoic sedimentary rocks that were folded during Laramide deformation underlie the Tertiary sequence along the southwest margin of the volcanic field. 

The axis of the main Laramide highland lay farther to 

the northeast under the volcanic cover, as indicated by 

the few small outcrops of Pre cam brian rocks along the 

lower Conejos River, the more widespread Precambrian 

exposures to the south in New Mexico, and the abundant 

Precambrian detritus in the prevolcanic Animas and 

Blanco Basin Formations. 

TERTIARY VOLCANIC STRUCTURES 

Mter erosion of the Mesozoic sedimentary cover from 

the Laramide uplifts and after concurrent fluvial deposition of the Blanco Basin Formation around margins of 

these highlands, large-scale volcanism broke out 

-probably in early Oligocene time-over much of the 

southern Rocky Mountain region (Steven and Epis, 

1968). 

In the San Juan field, which constitutes the largest 

single erosional remnant of this middle Tertiary 

volcanism, major activity began about 35 m.y. ago (Lipman and others, 1970) when many stratovolcanoes began 

to form by eruptions scattered over most of the area of 

the now-preserved volcanic field. These eruptions were 

overwhelmingly intermediate in composition, consisting 

mainly of andesitic and rhyodacitic lavas that now form 

the vent facies of the Conejos and related formations. 

Some volcanoes were very large, with basal diameters of 

as much as 20 km and original heights of at least 3 km. 

Basins and valleys between these volcanoes were filled 

by mudflow and other volcaniclastic deposits washed 

from the primary volcanic structural features. These early intermediate-composition rocks constitute about twothirds of the volume of the entire middle Tertiary activity in the San Juan field. Where now exposed, the vent 

and volcaniclastic facies together average about 1 km in 

thickness, and reconstructions of original distribution 

indicate that great additional volumes. of these rocks 

have been removed by erosion. 

During the early intermediate-composition activity, 

areas that were subsequently to be sites of the first 

calderas-such as the San Juan-Uncompaghre (Lipman 

and others, 1973), Bonanza (Burbank, 1932; Knepper 

and Marrs, 1971), and Platoro areas-were volcanic 

highlands marked by clusters of andesitic stratovolcanoes. Ash-flow tuffs are rare through most of 

the early intermediate sequence, but near the top of the 

sequence in the Platoro area, a change in the volcanic activity was premonished by eruption-from within the 

cluster of andesitic volcanoes there-of the rhyodacitic 

tuff of Rock Creek, the first sizable ash-flow sheet in the 

southeastern San Juan Mountains. 

SAN JUAN CALDERA CLUSTER 

The major middle Tertiary structural features of the 

San Juan field are the 14 or more calderas (fig. 1) and the 

related doming and faulting that resulted from eruption 

of the ash-flow sequence (table 1). The calderas range in 

diameter from less than 10 to almost 50 km (Durango 

map), and the largest caldera, La Garita, collapsed as a 

result of eruption of the most voluminous ash-flow sheet, 

the Fish Canyon Tuff. The volume of erupted material is 

roughly proportional to the size of related collapse, as 

seems to be generally true (Smith, 1960a, p. 834). 

A few of the San Juan calderas are simple collapse 

depressions that were subsequently filled by younger 

volcanic materials, but most are resurgent calderas 

(Smith and Bailey, 1968) in which renewed magmatic 
102 PLATORO CALDERA COMPLEX, SAN JUAN MOUNTAINS, COLORADO 

0 5 10 15 20 MILES 

0 5 10 15 20 KILOMETERS 

EXPLANATION 

- Hydrothermal alteration 

Monzonite stocks 

-.1-.. Fault - Dotted where concealed. 

Bar and ball on downthrown side 

"""""'"""'"'" Limit of exposed volcanic 

rocks 

Caldera wall (topographic) -

Dashed where concealed 

FIGURE 61.-Distribution of faults, stocks, and areas of hydrothermal alteration in the southeastern San Juan Mountains. Based on mapping by P. 

W. Lipman, 1965-67, 1970-71, and by T. A. Steven, 1965-66. 
STRUCTURAL EVOLUTION 103 

prei3sure after collapse caused uplift of the central part of 

the caldera. The principal elements of little-eroded 

calderas of this type are a topographic wall and rim, a 

central mountain representing the floor of the caldera 

typically uplifted to heights as great or greater than the 

rim, and a moatlike structural valley between. The 

topographic walls of the calderas are characteristically 

scalloped by arcuate reentrants that resulted from 

landsliding and slumping shortly after initial caldera 

collapse. The actual structural boundary of caldera 

collapse is rarely exposed but lies approximately 

beneath the sedimentary or volcanic fill in the moat. The 

best preserved example of a simple resurgent caldera of 

this type in the San Juan field is the Creede caldera (fig. 

62) (Steven and Ratte, 1965; Ratte and Steven, 1967). 

The older calderas of the San Juan field are variably 

complicated by later features, such as overlapping 

younger calderas, fill by younger volcanic units, faulting, 

and erosional modifications. All of these tend to cut 

across and obscure primary topographic and structural 

elements of the calderas. 

PLATORO CALDERA 

The Platoro caldera originated by collapse about 30 

m.y. ago as a result of eruption of ash flows to form the 

La J ara Canyon Member of the Treasure Mountain Tuff. 

The northwestern part of the caldera has been complicated by further collapse that formed the Summitville 

caldera during eruption of later ash-flow sheets of the 

Treasure Mountain Tuff. The Platoro caldera is crudely 

rectangular, about 18 by 23 km, and trends northeast (fig. 

61). The core of the caldera was resurgently uplifted, 

shortly after collapse, to form the Cornwall Mountain 

resurgent block that is still topographically high, but 

otherwise the primary elements of the caldera are no 

longer well expressed topographically. 

Eruption of the La Jara Canyon Member from Platoro 

caldera concurrent with subsidence is clearly indicated 

by several kinds of evidence. The La Jara Canyon tuffs 

were deposited more or less uniformly around the 

caldera, except for gaps related to preexisting 

topography. The La Jara Canyon was the first major 

ash-flow sheet in the southeastern San Juan region large 

enough to cause major caldera collapse, yet the Platoro 

caldera existed as a depression before completion of 

these eruptions, as shown by local plastering of the La 

Jara Canyon Member across the outer caldera walls 

(figs. 63, 64) and by the thick intracaldera accumulation 

of these tuffs. Within and near the caldera, the La Jara 

Canyon is more propylitically altered and more densely 

welded and contains more abundant and larger lithic 

fragments, all features that indicate near-source deposition. 

PREMONITORY VOLCANISM 

Stages in the development of Platoro caldera can be 

readily described in terms of the stages outlined by 

Smith and Bailey (1968) for resurgent calderas in 

general. Stage 1 in the resurgent-caldera cycle-regional 

tumescence-refers to magma insurgence, broad doming, and initial formation of a ring-fracture system. 

Direct structural evidence of tumescence is absent in 

the Platoro area, which was by this time a constructional topographic highland of clustered andesitic 

FIGURE 62.- View of Creede caldera from the north, showing central resurgent dome, outer topographic wall, and structural moat. Rio Grande has 

excavated the soft lacustrine caldera fill of Oligocene age that filled the moat. From Ratte and Steven (1967, color frontispiece). 
104 PLATORO CALDERA COMPLEX, SAN JUAN MOUNTAINS, COLORADO 

stratovolcanoes. Indeed, such evidence is rare at resurgent 

calderas (Smith and. Bailey, 1968, p. 636), with the 

notable exception of Timber Mountain caldera in 

Nevada (Christiansen and others, 1965). However, the initial events in development of Platoro caldera-the 

premonitory eruption of biotite-bearing quartz latitic and 

rhyolitic ash flows to form the lower tuff of the Treasure 

Mountain-seem to reflect the rise of silicic magma to a 

shallow crustal level for the first time in the area. Such 

rise of magma must be the basic condition for 

tumescence and doming. 

In addition, development of the intermediatecomposition volcano cluster of Conejos age and the erup- . 

tion of the rhyodacitic ash-flow tuff of Rock Creek may 

well reflect earli.er stages of tumescence. The inferred. 

locations of vents for the clustered early andesitic 

volcanoes in the Platoro area are all close to the margins 

of the subsequent caldera collapse (fig. 5). This distribution may have been an early manifestation of broad insurgence and the resultant·ring-fracture formation over a 

rising andesitic magma body that was subsequently to 

differentiate and feed the caldera-forming eruptions. A 

close relation between the andesitic lavas and more 

silicic ash-flow activity has already been inferred, and 

the development of several large early calderas of the 

San Juan field-Platoro, San Juan, Uncompahgre, and 

Bonanza-over clusters of andesitic stratovolcanoes 

seems significant in interpreting the origin and development of the ash-flow magmas. 

ASH-FLOW ERUPTIONS AND CALDERA COLLAPSE 

Ash-flow eruptions and caldera collapse represent 

both stage 2 and stage 3 of the resurgent-caldera cycle of 

Smith and Bailey(1968), but at Platoro caldera the 

stages overlapped too complexly to be discussed 

separately. 

At some optimum time, for reasons not well understood but perhaps related to progressive effects of the 

premonitory ash-flow eruptions of the lower tuff, the 

ring-fracture ·zone became sufficiently well established 

to trigger eruption of much more voluminous ash flows. 

Once these culminating eruptions began, they produced 

numerous ash flows so rapidly that no evidence of erosion between flows has anywhere been observed, and 

only locally was interruption sufficient to permit 

development of even partial cooling breaks. 

Because most of the early andesitic volcanoes had 

been extensively eroded and the intervening basins filled 

with the resultant detritus, these ash flows were able to 

spread 30-40 km in all directions from the ring-fracture 

zone, depositing approximately 350 km3 of tuff 

(estimated by use of model in fig. 20) as the outflow 

sheet of the La Jara Canyon Member (fig. 14). This great 

single cooling unit of phenocryst-rich quartz latitic ashflow tuff originally covered an area of nearly 5,000 km2• 

Before completion of these rapid and voluminous eruptions, drainage of much of the magma chamber caused 

the chamber roof to collapse along the ring-fracture zone 

and become a downdropped central cauldron block. 

As a resul~ of ash-flow eruptions continuing concurrently with caldera collapse, the later ash flows were 

largely ponded within the caldera; they wedge out part 

way up the outer walls (fig. 63). In a few low sectors of 

the caldera wall, the outflow sheet was deposited on the 

upper part of the wall and was draped over the rim (fig. 

64). Within the caldera these tuffs accumulated to a 

thickness in excess of 800 m, as indicated by exposures 

on the resurgent block near Jasper from the valley bottom almost to the crest of Cornwall Mountain. The total 

original volume of the intracaldera tuff of the La Jara 

Canyon is estimated at 235 km3 (fig. 20), although this 

figure is very approximate because the base of this accumulation is exposed only near its edges at the outer 

caldera walls. The intracaldera tuffs were somewhat more 

mafic and phenocryst rich than the initially erupted 

tuffs that formed the widespread thin outflow sheet. 

These differences probably reflect sorting during eruption, as well as limited preeruption magma differentiation that was similar in origin to the more extensive fractionation described for other ash-flow sheets in the San 

Juan field (fig. 34; Ratte and Steven, 1964). 

The concurrence of eruption an,d collapse at the 

Platoro caldera clearly differs from the timing of these 

events at other carefully studied areas, such as the 

Valles caldera in New Mexico, where the last erupted 

units are no thicker inside the caldera than outside, 

demonstrating that eruption was virtually complete 

before major subsidence occurred (Smith and Bailey, 

1968, p. 638). In the San Juan volcanic field, caldera 

collapse contemporaneous with eruption appears to have 

been a common process; other thick intracaldera accumulations of ash-flow tuff that demonstrate concurrent eruption and collapse are the Snowshoe Mountain Tuff at Creede caldera, the Carpenter Ridge Tuff at 

Bachelor caldera, and the Fish Canyon Tuff at La Garita 

caldera (table 3). Similar relations prevail at the San 

Luis and Lake City calderas (fig. 1; Durango map) and 

at several calderas in southern Nevada {Christiansen 

and others, 1965; Byers and others, 1969). Collapse 

. probably occurs concurrently only with ash-flow eruptions of very large volume; in some such eruptions initial 

collapse appears to have coincided approximately with 

abrupt compositional changes in the ash-flow sheets 

(Lipman and others, 1966, p. F24-F25; Byers and others, 

1969, p. 86). 

The overlapping timing between eruption of the La 

Jara Canyon ash flows and collapse of the caldera shows 

that the collapse resulted from the withdrawal of 

magma, with the emptied part of the magma chamber 

being filled by the subsided block. This timing also 
STRUCTURAL EVOLUTION 105 

suggests that the volume of collapse should approximate 

the volume of erupted magma, as has been shown for 

other calderas of this type (Smith, 1960a, p. 834). A 

minimum volume of collapse at Platoro caldera can be 

estimated if the resurgent block, as bounded by the arcuate California Gulch fault on the southeast and by the 

hinge line of the Platoro fault zone on the southwest (fig. 

61), is assumed to coincide approximately with the outline of original block subsidence before subsequent 

enlargement of the caldera by slumping and landsliding. 

This geometry suggests an original diameter of the subsided block of about 12 km and an original area of about 

110 km2, including the northwest sector, which subsequently collapsed into the Summitville caldera. The 

total vertical collapse of Platoro caldera, about 2.5 km, 

was estimated from the combined thickness of the 

postresurgence lavas within the caldera (about 1 km at 

Red Mountain) and the intracaldera La Jara Canyon 

Member (assumed to be 1.5 km; fig. 20). The resultant 

calculated volume of collapse, 275 km3, is less than the 

crude approximation of the initial volume of the La Jara 

Canyon, 500-1,000 km3, or, preferably, 580 km3 (table 5). 

Alternatively, the volume of collapse can be calculated 

by using a cone-shaped model and by assuming collapse 

across the entire 20-km rim-to-rim diameter of the 

caldera; for a 2.5-km maximum subsidence at the center 

of the cone, the volume of collapse would be about 400 

km3, still appreciably less than the estimated 580 km3 of 

the La Jara Canyon Member, although perhaps a not 

unreasonable agreement, considering the assumptions 

involved. An additional factor, not considered, is the 

probability that the precollapse andesitic volcano cluster 

within the caldera area had a mean elevation that was 

higher than the caldera rim; a mean excess height of 

these volcanoes of only 0.5 km could account for the entire 

discrepancy. 

The caldera wall must have been modified by 

landsliding during collapse of the main cauldron block. 

FIGURE 63.-View of Conejos Peak from Hillman Park road, showing densely welded cliff-forming ash-flow tuff of the La Jara Canyon Member 

(Ttj) within the caldera in depositional contact against the south caldera wall. All rocks at right side of photograph are lavas and breccias of 

Conejos Formation (Tc). Top rim, above La Jara Canyon Member within caldera, is intracaldera lava flow of the lower member of the Summitville Andesite (Tsl) . 
106 PLATORO CALDERA COMPLEX, SAN JUAN MOUNTAINS, COLORADO 

FIGURE 64.-South slope of Prospect Mountain, above Lake de Nolda, showing depositional relations at west wall of Platoro caldera. Subhorizontal units of Conejos Formation (Tc) and lower tuff (T t I) of Treasure Mountain Tuff are truncated by topographic wall of caldera, over which 

La Jara Canyon Member ( Ttj) is draped. Compaction foliations in the La Jara Canyon are as high as 60° on steepest slopes. Inside the 

caldera, subhorizontal dark lavas of the Summitville Andesite (Ts) lap out against the dipping welded tuffs of the La Jara Canyon Member. 

The main ring-fracture collapse would have initially 

formed a steep-walled depression, at least 1 kilometer 

deep, into which the steep walls subsequently slumped, 

as indicated by large lobate scars and by shallow dips of 

the 'outer walls. A particularly large slump scar, more 

than 5 km across, is evident in the Bennett Peak area 

(fig. 61; Platoro map), and smaller scallops are evident 

elsewhere around the rim; such features are also 

characteristic of other large calderas, such as Valles in 

New Mexico (Smith and others, 1970), Timber Mountain in Nevada (Christiansen and others, 1965), and 

Creede in Colorado (Steven and Ratte, 1965, pl. 1). Dips 

on the outer walls of Platoro caldera are gentle (9° -31 °) 

at all localities (table 12) where the caldera wall has been 

preserved below the Summitville Andesite and is now 

exposed by erosion. 

The paucity of landslide-derived megabreccia interlayered with or overlying the intracaldera La Jara Canyon is surprising, inasmuch as such deposits are substantial in otherwise analogous calderas such as Timber 

Mountain (Christiansen and others, 1965, p. B46), 

Creede (Steven and Ratte, 1965, p. 42), and Lake City 

(Lipman and others, 1973, p. 640). The necessary conclusion would seem that most slumping and landsliding at 

Platoro caldera occurred before deposition of the intracaldera La Jara Canyon Member at the levels now exposed, and such an interpretation is illustrated in the 

cross sections that accompany the map of the caldera 

TABLE 12.-Measurements of slopes of Platoro caldera walls (from 

Position 

of wall 

Platoro map) 

Locality 

North side . . . . Bennett Mountain . . . ... . . Northeast side . Rock Creek ............. . East side . . .. . Lower Alamosa River ..... . Southwest side Upper Conejos River (fig. 63) 

West side ... , . Upper Alamosa River ..... . 

Vertical 

exposure 

(m) 

200 

110 

500 

500 

150 

Dip 

(degrees) 

9 

10 

28-31 

19 

29 
STRUCTURAL EVOLUTION 107 

(Platoro map, sections C-C'; D-D' ). One small patch of 

monolithologic andesitic crackle breccia of landslide 

origin, derived from the Conejos Formation, is exposed 

as a lens (too thin to be shown on the geologic map) 

beneath the intracaldera La Jara Canyon along the 

southwest caldera wall just north of the Adams Fork of 

the Conejos River. These landslide deposits are the only 

recognized unit within the caldera that are likely to 

represent stage 4 of Smith and Bailey's resurgentcaldera cycle-preresurgence sedimentation and 

volcanism. No preresurgence volcanic activity has been 

identified within the caldera. 

RESURGENT UPLIFT 

The thick tuffs of the La Jara Canyon Member within 

the Platoro caldera are topographically and structurally 

high as a result of uplift of a central block shortly after 

caldera collapse; this uplift represents stage 5 of the 

resurgent-caldera cycle. Early resurgence is 

demonstrated by the presence of monolithologic talus· 

breccias which are derived from the weakly welded upper part of the intra caldera La J ara Canyon and which 

intertongue with lavas of Summitville Andesite adjacent 

to the resurgent block. Such talus breccias are especially 

well exposed at the head of California Gulch and in an 

isolated outcrop on the east slope of Mammoth Mountain, opposite the mouth of Fisher Gulch. 

The Platoro caldera differs in structure from other 

known resurgent calderas, where resurgence-has resulted 

in formation of a fairly complete structural dome (fig. 62; 

Smith and Bailey, 1968). At Platoro caldera, the 

resurgence uplifted a block that dips 10°-25° 

homoclinally to the southwest and that is bounded on its 

east side by the arcuate California Gulch fault (fig. 61). 

This normal fault has a large displacement that accommodated most of the resurgent uplift, as indicated by the 

adjacent early-formed talus breccias and fanglomerates. 

The minimum displacement along the California Gulch 

fault is at least 900 m, as indicated by the differences in 

level between the top of the intracaldera La Jara Canyon 

Member, where exposed in the caldera moat barely 

above the level of the Alamosa River east of Jasper, and 

the highest exposures of this tuff on the resurgent block 

on Cornwall Mountain (Platoro map). The California 

Gulch fault splays and seems to decrease in displacement to the southwest. Additional curved faults, concave to the northwest and north and related to resurgent 

uplift, are probably buried beneath the rhyodacite of 

Fisher Gulch and the Summitville Andesite on the south 

side of the caldera. Any such concealed faults could not 

have displacements as large as the California Gulch 

fault. These arcuate faults probably mark the ringfracture system along which subsidence occurred, 

because the La Jara Canyon Member on the main 

resurgent block is unfaulted and appears to have been 

deposited over a core area that retained coherence during caldera collapse. The northwestern part of the 

resurgent block is missing, however, having caved into 

the younger Summitville caldera by collapse along the 

Cornwall fault (fig. 61). 

On the southwest side of the resurgent block, the intracaldera La Jara Canyon tuff is almost continuously 

traceable across the filled caldera moat and has at most 

only small fault displacement. Dips of the tuff define a 

synclinal sag at the caldera moat; the tuff dips off the 

resurgent block and then rises again toward the 

depositional wedge-out on the caldera wall. This relation 

is well exposed along the Lake Fork of the Conejos River, 

where the moat-filling rhyodacite of Fisher Gulch covers 

the intracaldera tuff only for a distance of about half a 

kilometer east of Big Lake, and exposures of the intracaldera tuff are near the top of the unit, as indicated 

by the prevalence of weakly welded punky tuff. A similar 

relation is spectacularly exposed in the Platoro Reservoir 

area along the upper Conejos River, where weakly 

welded intracaldera tuff dips toward the moat from both 

the outer caldera wall (fig. 64) and the resurgent block, 

and the trough of the moat is filled by subhorizontal 

lavas and volcaniclastic rocks of Summitville Andesite. 

Here, the northwest-trending Platoro fault zone (fig. 61) 

approximately marks the hinged margin of the resurgent 

block, even though the faults of this zone have displacements limited to a few tens of meters, and at least 

some of this displacement occurred significantly later 

than uplift of the resurgent block. 

The cause of the asymmetrical resurgence at Platoro 

caldera, in contrast to the typical domical uplift at other 

resurgent calderas (fig.62; Smith and Bailey, 1968), is 

uncertain, but it may be related to asymmetry of the 

preceding subsidence. As already noted, the 

northeastern part of the Platoro caldera moat seems to 

have collapsed deeper than other areas, as indicated by 

the accumulation there of upper ash-flow units of the 

Treasure Mountain Tuff and the Fish Canyon Tuff, 

which are not found elsewhere within the caldera. This is 

not unique; mildly asymmetrical subsidence has been 

documented at Timber Mountain caldera (Carr and 

Quinlivan, 1968, p. 102), and a strongly asymmetrical 

trapdoor type of subsidence is evident for the Silverton 

and Cochetopa Park calderas in the San Juan field (fig. 

1; Lipman and others, 1973, p. 640; T. A. Steven, written 

commun., 1971). At Platoro caldera, the side that 

subsided the most deeply also achieved the greatest 

resurgent uplift. This relation would favor a hydrostatic 

rebound hypothesis (Smith and Bailey, 1968, p. 648) for 

the origin of resurgent uplift at Platoro. 

POSTRESURGENCE VOLCANISM 

After resurgence was virtually complete, the 

rhyodacite of Fisher Gulch and the lavas of the lower 
108 PLATORO CALDERA COMPLEX, SAN JUAN MOUNTAINS, COLORADO 

member of the Summitville Andesite accumulated 

within the caldera moat and lapped unconformably onto 

the uplifted central block. This volcanism marks the 

beginning of stage 6 of the resurgent-caldera cycle. The 

thick rhyodacite lava flow of Fisher Gulch was clearly 

erupted from within the caldera, along the southeastern 

ring-fracture zone; it represents a typical ring lava dome, 

similar to lava domes recognized at most other resurgent 

calderas. The sources of the andesitic lavas are less 

certain. Some may have flowed down the caldera w&lls 

into the moat from volcanoes beyond the rim, as 

indicated by relations already described in the Conejos 

Peak area (fig. 39). Elsewhere, as at the thick 

accumulation just east of Jasper, the source probably lay 

within the caldera, in this case to the west near the fault 

margin of the younger Summitville caldera. These lavas 

are 600-800 m thick in several parts of the Platoro 

caldera moat;. where best preserved near Red Mountain 

against the southeast wall, they are almost 1 km thick. 

These andesitic lavas represent a continuation of the 

same type of volcanic activity that characterized the 

Conejos Formation, with which they are readily confused 

in the field. Continued andesitic volcanic activity during 

ash-flow eruptions characterizes other parts of the San 

Juan volcanic field as well, and this relation is 

significant in reconstructing the petrologic evolution of 

the volcanism, as discussed later. 

These intracaldera lavas are interlayered with and 

overlie shallow-water lake and deltaic sedimentary rocks 

in several areas, most notably near Rito Gato and Beaver 

Lake Creeks in the southwestern caldera moat and near 

Ranger Creek in the eastern part of the caldera (fig. 40). 

Association of postresurgence volcanism with lake 

sediments and caldera fill is characteristic of resurgent 

calderas (Smith and Bailey, 1968, p. 641). 

The postresurgence evolution of the Platoro caldera 

was interrupted or ended by development of the 

Summitville caldera, although in a sense the postcollapse volcanism around the younger caldera forms a 

continuum with the postresurgence activity of Platoro 

caldera, especially in terms of compositional evolution of 

the lavas. Notably lacking, however, from the 

postresurgence activity around the Platoro caldera 

before development of the Summitville structure is any 

indication of hydrothermal alteration or solfataric 

activity that would be representative of stage 7, terminal 

activity of the resurgent-caldera cycle. 

SUMMITVILLE CALDERA 

Several features indicate the probable existence of a 

late collapse, here described as the Summitville caldera, 

related to upper units of the Treasure Mountain Tuff 

within the northwestern part of the Platoro caldera (fig. 

61). The Ojito Creek and Ra Jadero Members were 

erupted after the moat of the Platoro caldera was nearly 

filled by lavas of the lower member of the Summitville 

Andesite, because these tuffs are found within the 

caldera only in a few places above thick accumulations 

of andesitic lavas (Platoro map). The Ojito Creek and Ra 

Jadero tuffs are nearly coextensive with the La Jara 

Canyon Member and must have been erupted from the 

same general area. Although the volumes of these upper 

two members are much less than the volume of the La 

J ara Canyon, on the order of 50 and 120 km3, respectively· 

(table 5), they are sufficiently large to indicate the 

likelihood of associated caldera collapse (Smith, 1960a, 

fig. 3). In addition to the volume and general 

distribution of the ash-flow tuffs, the interpretation of 

the Summitville caldera is based mainly on (1) the 

Cornwall fault, which is thought to represent the main 

ring-fracture fault on the southeast side of the caldera, 

(2) fragmentary exposures of the topographic wall, 

especially on the northeast side, and (3) the 

concentration of late igneous activity and mineralization 

around the caldera margins. Many aspects of the 

development of this caldera are poorly understood, 

however, because of the thick intracaldera fill (upper 

member of Summitville Andesite), the absence of 

postcollapse resurgence that might have exposed 

internal structures, and the extensive late intrusion and 

hydrothermal alteration. 

The Summitville caldera is inferred to have collapsed 

twice, once after eruption of the Ojito Creek Member 

and again after the Ra Jadero. This interpretation is 

based on the hypothesis that both units are sufficiently 

large to have caused collapse, but recurrent collapse cannot be demonstrated from the structural features of the 

caldera. Near the northwest rim of the caldera, in upper 

Park Creek, the upper member of Summitville Andesite 

intervenes between the Ojito Creek and Ra Jadero 

Members and seemingly is continuous back into the 

western area of intracaldera andesite, indicating major 

caldera-fill volcanism at this time. In contrast, any lava 

erupted after Ra Jadero-related collapse seems to have 

been confined to the caldera area, because andesite has 

been recognized overlying this unit near the caldera rim. 

ARCUATE FAULTING 

The most conspicuous expression of the Summitville 

caldera is the arcuate northeast-trending Cornwall fault, 

first described and named by Patton ( 1917, p. 62). This 

fault truncates the central resurgent block of 

intracaldera La Jara Canyon tuff and juxtaposes lavas of 

the Summitville Andesite against the block for more 

than 800 m of elevation. Although the position, shape, 

and offset of this fault are thus anomalous with respect 

to the geometry of the Platoro caldera, features of the 

fault can be reasonably interpreted as an expression of 

the bounding ring-fracture zone on the southeast side of 

the Summitville caldera (fig. 61). 
STRUCTURAL EVOLUTION 109 

This fault cannot be a simple ring structure, against 

which the upper member of the Summitville Andesite 

was deposited unconformably after caldera collapse; 

appreciable renewed movement took place along the 

break after deposition of the adjacent andesite. The 

Summitville Andesite is locally altered, and about 25 em 

of gouge occurs along the contact in the single outcrop 

where the actual fault zone is exposed, a.t about 11,400 

feet just west of the prominent rib 1.5 km southeast of 

Cornwalls Nose. Significant downward displacement of 

the Summitville Andesite, along the Cornwall fault and 

also along a parallel fault to the north (fig. 61), seems 

required by the low position of the rhyodacite of Park 

Creek east of Wightman Fork (fig. 36), which seems to 

have accumulated in a structural depression. Paucity of 

detritus interlayered with the exposed Summitville 

Andesite-detritus that could have been derived from 

the adjacent intracaldera La Jara Canyon-also argues 

against interpretation of the contact as a simple 

depositional unconformity against a faultline scarp. 

Therefore, if the Cornwall fault originated as a ring 

fr~cture of the Summitville caldera, as seems likely, 

significant additional displacement occurred after the 

caldera was filled by Summitville Andesite. 

Movement on the Cornwall fault appears to have been 

largely or entirely complete by the time of emplacement 

of the 29.1-m.y. Alamosa River stock and the 

approximately contemporaneous Jasper stock, both of 

which appear to be localized by the fault zone. This 

timing, predating eruption of the next succeeding major 

ash-flow sheet, the 28.1-m.y. Masonic Park Tuff, 

brackets the development of the Cornwall fault in the 

same narrow time span as indicated for the eruption of 

the Ojito Creek and Ra Jadero Members. 

TOPOGRAPHIC WALL 

About a 180° sector of the topographic wall of the 

Summitville Andesite on the caldera wall to the east. In 

comparison with lower member andesites in the Platoro 

caldera, the upper member lavas within the Summitville 

caldera tend to be exceptionally thick, poorly stratified, 

and highly brecciated. Also, flows containing large 

phenocrysts of plagioclase are more abundant in the 

upper member than in the lower member. 

West of Marble Mountain, the caldera wall is mostly 

concealed by surficial deposits and by thick viscous flows 

of the rhyodacite of Park Creek that cover rough 

topography interpreted as the caldera margin. A small 

patch of outflow La Jara Canyon Member resting on 

Conejos Formation along the Summitville-Del Norte 

road just north of the Platoro map area (fig. 14; Durango 

map) provides a key northern constraint on the location 

of the caldera wall in this sector. 

In a small area just east of North Mountain, the 

Summitville caldera wall seems to be marked by the 

contact between dark sparsely porphyritic andesite, 

which is continuous over a large area to the south and is 

considered typical of the upper member of Summitville 

Andesite, and the underlying more altered red-brown 

lavas, which contain abundant small phenocrysts of 

plagioclase, augite, and, in places, hornblende and 

which are interpreted as part of the Conejos Formatior).. 

The location of the caldera wall southwest of 

Summitville is uncertain. Both caldera wall and caldera 

fill seem to be members of the Summitville Andesite, 

but the contact is obscured by intense hydrothermal 

aJteration. The contact between members of 

Summitville Andesite, shown southwest of the Alamosa 

River stock (fig. 36), is tenuous because it depends on 

uncertain identification of locally intervening weakly 

welded ash-flow tuffs within the Platoro caldera as the 

Ra Jadero Member and the upper tuff of the Treasure 

Mountain. 

Summitville caldera appears to have survived erosion, POSTCOLLAPSE VOLCANISM 

mainly on the north and northwest sides. Because the fill Perhaps the most interesting feature of the evolution 

of Summitville caldera lacks good stratigraphic markers, of the Summitville caldera is the prolonged sequence of 

such as the ash-flow units that so clearly define the postcollapse volcanism and related alteration and 

Platoro caldera walls (figs. 63, 64), the wall is mostly mineralization, beginning with the filling of the caldera 

defined by a boundary between lavas of the Conejos I to overflowing by the upper member of the Summitville 

Formation and lithologically similar intracaldera·l Andesite. Whereas only minor silicic postcollapse 

andesite. Much of the wall is difficult to locate precisely, volcanism and alteration occurred around margins of the 

and exposures are poor in several key areas. main Platoro caldera, a complex sequence of 

Relations are clearest on the northeast side, near postcollapse igneous activity continued for almost 10 

Marble Mountain, despite the fact that both caldera m.y. around margins of the Summitville caldera. 

wall and caldera fill seem to be members of the Structural features of the younger caldera and 

Summitville Andesite. In this area, mapped as upper associated igneous activity also seem to have localized 

member is a distinctive sequence of interlayered sparsely most of the major alteration and mineralization in the 

and coarsely porphyritic lavas overlain by the silicified southeastern San Juan Mountains, as discussed in the 

white bedded tuff of Marble Mountain. These rocks lap. section on economic significance. Postcollapse 

unconformably onto a markedly different sequence of intermediate to silicic lava flows and related intrusives, 

lavas and volcaniclastics of the lower member of the localized around margins of the Summitville caldera, 
110 PLATORO CALDERA COMPLEX, SAN JUAN MOUNTAINS, COLORADO 

were emplaced over the interval 29-20 m.y., with dated 

events at 29.1 m.y. (Alamosa River stock), between 27.8 

and 26.7 m.y. (lower flows of rhyodacite of Park Creek 

that wedge between Fish Canyon and Carpenter Ridge 

Tuffs), <26.5 (upper rhyodacite of Park Creek that 

overlies the Snowshoe Mountain Tuff), 25.8 m.y. (dated 

dike of quartz latite porphyry), 22.8 m.y. (quartz latite of. 

South Mountain), and 20.2 m.y. (rhyolite of Cropsy 

Mountain): More extensive dating of the voluminous 

postcaldera lavas and intrusives would likely disclose 

additional events. Isopach maps of the young ash-flow 

sheets that were erupted from other calderas northwest 

of Platoro (fig. 1) show variations in distribution and 

thickness in the Platoro ~rea (figs. 26, 30, 31) that 

correlate with the growing constructional topography of 

these caldera-margin volcanoes. 

CALDERA-RELATED REGIONAL FAULTING 

Within the region of the San Juan caldera cluster, 

three major fault-graben systems trend northwest, 

between or tangential to several of the major calderas 

(Durango map). These include the Clear Creek graben 

that tangentially connects the west sides of Creede and 

San Luis calderas (Steven, 1967), the Rio Grande fault 

zone that runs from the northeast side of Creede caldera 

across the La Garita caldera wall toward the north side 

of the Summitville caldera (fig. 61), and the Pass CreekElwood Creek-Platoro fault system that connects Mount 

Hope caldera with the Platoro caldera complex (fig. 61). 

Locally determinable ages of faulting show that these 

major zones had lengthy histories of recurre'nt 

movement, but their initiation and much of their total 

displacement seem to have occurred approximately 

contemporaneously with ash-flow activity and major 

caldera collapse. From a broad perspective, these fault 

zones appear to reflect attenuation related to a general 

resurgence of the roof of the composite batholith that is 

inferred to underlie the entire caldera cluster (Plouff and 

Pakiser, 1972), in contrast to the specific resurgences of 

individual calderas, each of which is inferred to overlie a 

shallow cupola of the batholith. 

RIO GRANDE FAULT ZONE 

The Rio Grande fault zone consists of several major 

northwest-trending faults, extending about 50 km from 

the southeast flank of the La Garita Mountains (Steven 

and Ratte, 1965) to south of Del Norte Peak (fig. 61). 

These faults, though clearly diminishing in 

displacement to the southeast, probably extend even 

farther, perhaps to the rim of the Platoro caldera 

complex, but they are concealed beneath surficial 

deposits and are lost in areas of poor exposure on the 

heavily timbered north slopes of the caldera. In the 

South Fork area and along the Rio Grande to the 

northwest, this fault zone forms a well-defined graben 

followed by the river, with offs~ts of as much as 350m on 

individual faults, but the graben is asymmetrical, and 

the general sense of displacement along the zone is a 

downdrop of the region to the southwest. 

The Rio Grande fault zone appears to have been 

recurrently active over a period of at least 5 m.y. in late 

Oligocene and early Miocene time. At the northwest end 

of the zone, the major faults along the flank of the La 

Garita Mountains coincide closely with the margin of the 

Bachelor caldera, where it caved away the southwest 

flank of the La Garita resurgent dome during eruption of 

the Carpenter Ridge Tuff (26.7-27.8 m.y.), and these 

faults probably originated then (Steven and Ratte, 1965, 

p. 61). A little farther to the southeast, these faults offset 

the Nelson Mountain Tuff that immediately preceded 

eruption of the Snowshoe Mountain Tuff and associated 

formation of the Creede caldera (table 1). Some faults of 

the Rio Grande zone predate and are covered by the 

great lava dome of Fisher Quartz Latite along the Creede 

caldera rim at Wagonwheel Gap,· which is dated at 26.5 

m.y. (Armstrong, 1969; Lipman and others, 1970). 

Southeast of South Fork in the Beaver Mountain area, 

the entire ash-flow sequence~including the· outflow 

Snowshoe Mountain Tuff-is faulted; but at h~ast some 

of the faults are ove~lain by basalt flows of the Hinsdale 

Formation that show no offset (Durango map). At the 

southeast end of the zone around Del Norte Peak, 

however,· Hinsdale basalts-including the one dated at 

23.4 m.y.-are offset as much as 50 m. 

PASS CREEK-ELWOOD CREEK-PLATORO FAULT SYSTEM 

The Pass Creek-Elwood Creek-Platoro fault system 

(fig. . 61) consists of a geometrically complex group of 

faults, characterized by northwest trends,· that extends 

about 35 km from the margin of Mount Hope caldera 

across the southwest side of the Platoro caldera complex. 

As just described for the Rio Grande fault zone, different 

ages of movement along different parts of the system can 

also be documented here. The Pass Creek-Elwood CreekPlatoro fault system is ·also of particular interest in the 

evolution of the Platoro caldera complex because it 

localized much of the postcollapse igneous activity and 

the associated alteration and mineralization (fig. 61). 

The Pass Creek zone, at the northwest end of the 

system, consists of only a few faults that define a small 

graben. Several dikes follow these faults (fig. 47) and 

widespread weak hydrothermal alteration in and close to 

the graben affects rocks as young as the Fish Canyon 

Tuff. The major displacement along these faults appears 

to be younger than the 27.8-m.y. Fish Canyon but older 

than the Huerto Andesite (>26.7 m.y.), which unconformably covers a faultline scarp at least 75 m high along 

the major northwestern fault of this zone. 

The Elwood Creek zone includes some of the most 

intricate and largest displacement faults in the San Juan 

volcanic field (fig. 65). About 15 faults of irregular trend 
STRUCTURAL EVOLUTION 111 

extend outward from the west side of the Summitville 

caldera and define a stubby complex graben that 

terminates near the Bear Creek stock, where most of 

these faults tend to converge (fig. 61). At this focal area 

the cumulative displacement of the graben faults is at a 

maximum; the La Jara Canyon Member, exposed along 

the bottom of lower Crater Creek (Platoro map), is 

downdropped as much as 550 m. This area is also a locus 

for intrusion of monzonite, hydrothermal alteration, and 

sulfide mineralization. In contrast to the complex fault 

pattern on the southwest side of the graben, the 

bounding faults on the northeast side tend to be a more 

regular series of antithetic normal faults, with 

displacements down to the southwest. These faults 

extend toward but apparently do not quite join faults of 

the Pass Creek zone. Some faults in this graben system 

may have been initiated during subsidence of the 

Summitville caldera, as suggested by major 

downdropping of an area just to the west of the caldera, 

especially the large triangular block west of Prospect 

Mountain (fig. 61). However, much of the displacement 

around Elwood and Crater Creeks is clearly younger 

than the 27.8-m.y. Fish Canyon Tuff, the undated 

rhyodacite of Park Creek (fig. 65), and the Carpenter 

Ridge Tuff (>26.7 m.y.). 

In a real sense, the Summitville fault is part of the 

boundary system on the northeast side of the Elwood 

Creek fault zone. It forms a well-defined continuous 

structure for 15 km from Park Creek to the Alamosa 

River and merges to the southeast with the Platoro fault 

zone for another 10 km (fig. 61). Interpretation based on 

cross sections suggests that at least one more fault of 

similar geometry is buried by younger rocks farther to 

the northeast (Platoro map, section D-D' ). The 

Summitville fault in the upper Park Creek area has a 

maximum displacement of 300-350 m, down to the 

southwest, but its location near the buried margin of the 

Summitville caldera suggests that it may have begun as 

FIGURE 65.-Faulted ridge east of Crater Creek, showing displacement in thick rhyodacite of Park Creek well defined by offset of the dark-colored 

basal vitrophyre. This area is near the focus of maximum displacement on the Elwood Creek fault zone. On near ridge, lavas of the upper 

member of the Summitville Andesite rest on weakly welded light-gray tuff at the top of the La Jara Canyon Member. Arrow indicates large 

dike of quartz latite porphyry. View is to the north from 12,866-foot peak on the Continental Divide at the head of Prospect Creek. 
112 PLATORO CALDERA COMPLEX, SAN JUAN MOUNTAINS, COLORADO 

part of the caldera ring-fault system and may have had 

an original displacement in the opposite sense. An even 

earlier inception of the Summitville fault is hinted by 

onlap relations of Treasure Mountain Tuff against lavas 

of the Conejos Formation on the upthrown side of the 

fault east of Lost Mine Creek in the upper Park Creek 

drainage (Platoro map). These relations are not 

conclusive because this buried topography of Conejos 

age does not necessarily have to be fault related. In 

upper Park Creek the major movement on the 

Summitville fault is clearly younger than the Carpenter 

Ridge Tuff (26.7-27.8 m.y.), which is tilted to nearvertical dips in the vicinity of the fault (Platoro map). 

The major movement probably preceded deposition of 

the overlying volcaniclastics of the Los Pinos Formation 

and the interlayered 26.4-m.y. Snowshoe Mountain Tuff 

(fig. 35). If so, it is slightly younger than the principal 

movement in the Pass Creek zone .. Substantially 

younger movement farther southeast along the 

Summitville fault is probably indicated by displacement 

of the 22.8-m.y. quartz latite of South Mountain, which 

was erupted from a vent localized near the fault (Steven 

and Ratte, 1960, pl. 2). Near Summitville the quartz 

latite of South Mountain is displaced only slightly; on 

the southeast side of South Mountain it is offset as much 

as 100 m, but this estimate is complicated by the. 

possibility that the lava dome was deposited against an 

existing faultline scarp in this area. Steven and Ratte 

(1960, p. 37) suggested movement of the Summitville 

fault following emplacement of the 20.2-m.y. rhyolite of 

Cropsy Mountain, but evidence for this age of movement 

seems inconclusive: one key area between South 

Mountain and Cropsy Mountain is exposed entirely as 

talus and solifluction debris, and the faulted lavas 

assigned to this unit west of North Mountain by Steven 

and Ratte (1960, pl. 2) seem equally plausibly 

interpreted as a correlative of the quartz latite of South 

Mountain or as a representative of some intermediate 

stage of activity. In this area, the mixed-lava complex at 

North Mountain, correlated with the rhyolite of Cropsy 

Mountain by Steven and Ratte and also in this report, 

was clearly erupted from a vent localized along the 

Summitville fault, but the complex itself is unfaulted. 

The main Summitville fault seems to end north of the 

Alamosa River, but near its terminus, en echelon faults 

of the Platoro fault zone appear. Faults in the Platoro 

zone, as in other parts of the system, trend 

northwestward and define an asymmetrical graben that 

is regionally downdropped to the southeast. These 

faults, though important loci for alteration and 

mineralization, seem to have relatively small offsets that 

decrease to the southeast. At the pass between Stunner 

and Platoro, the easternmost and largest fault of the 

zone displaces the contact between the La Jara Canyon 

Member and the Summitville Andesite at least 100m; in 

contrast, along the ridge south of Platoro, no fault of this 

zone has more than 50 m of offset (Platoro map). Faults 

of the Platoro zone are younger than the 29.1-m.y. 

Alamosa River stock, but no satisfactory minimum age 

limit is determinable from the field relations. 

LATE CENOZOIC EXTENSIONAL FAULTING 

In the southern and eastern parts of the San Juan 

Mountains, the northwest-trending zones of normal 

faulting that were active during the time of caldera formation merge with generally more northward-trending 

normal faults. Many of these faults offset basalt flows of 

the Hinsdale Formation and seem to be late Cenozoic 

structures related to regional eastward tilting of the 

volcanic field toward the San Luis Valley. 

Most of these late faults trend N. 20° E. to N. 30° W. 

and tend to be high-angle normal faults having small 

displacements down to the west. These faults define a 

well-developed antithetic pattern; the dominant sense of 

displacement is opposite to the eastward dip of the. 

volcanic strata, and similar stratigraphic levels are 

repeatedly exposed. Offsets as small as 10m have been 

mapped where possible (fig. 13; Durango map) to define 

the style of deformation. Few of the faults have displacements greater than 50 m, with the notable exception of the La Jara Reservoir-Cumbres Pass fault system 

that extends about 45 km from near the southeast rim of 

Platoro caldera to the New Mexico line (fig. 61) and 

beyond for an additional 8 km south (Bingler, 1968). 

This fault system has about 250m of displacement east 

of La Jara Reservoir and about 300m of displacement at 

Cumbres Pass, although between these places near the 

Conejos River the system diminishes to a large 

monoclinal flexure with no more than 10-20 m of actual 

fault displacement. 

Other areas in which faulting of this style has been 

recognized are south of Del Norte, near La J ara Creek, 

around Osier Mountain, and around Banded Peak (fig. 

61). The north-trending faults west of Summit Peak may 

also be part of this association; the age of movement, 

however, cannot be proved, because young basalts are 

absent there. Because these faults are mostly of small 

displacement, they have been recognized mainly where 

the stratigraphically well-controlled ash-flow sequence has 

been preserved. Similar faults were surely missed during 

mapping of the large areas underlain by the lithologically montonous lavas and breccias of the Conejos Formation. 

The age of this faulting has yet to be tightly established, but in all the areas mentioned above, except 

for Summit Peak, basalts of the Hinsdale Formation are 

faulted. Dated basalts as young as 4.7 m.y. and as old as 

17.7 m.y. (Lipman and others, 1970) are cut by these 

faults, and the dated basalts are underlain by older flows 

that. are too altered for reliable radiometric age deter-
ALTERATION AND MINERALIZATION 113 

minations to be made. The regional basaltic volcanism 

encompassed much and perhaps all of Miocene and 

Pliocene time (Lipman and others, 1970). Tilting and 

faulting also continued throughout much of the late 

Cenozoic, as indicated by progressively steeper dips of 

the older basalt flows along the southeast margin of the 

San Juan field and by eastward thickening of the intervening alluvial wedges of Los Pinos Formation. The 

merging of the late Cenozoic faults with the older 

caldera-related fault systems to the northwest also 

suggests that some caldera-related faults were reactivated at various later times to accommodate extensional deformation. This is suggested especially by the 

contrast between the northwest end of the Rio Grande 

fault zone, which seems to have been dormant after 26.5 

m.y., and the southeast end, where 23.4-m.y. basalt 

flows were involved in the faulting. 

The antithetic pattern of this late faulting is 

characteristic of extensional deformation, as is the block 

faulting of the Basin and Range province (Nolan, 1943; 

Thompson, 1960; Stewart, 1971). This, together with the 

regional eastward tilting of the southeastern San Juan 

region toward the San Luis Valley, suggests that the 

block faulting and eastward · tilting are marginal 

manifestations of downdropping and rifting of the San 

Luis Valley segment of the Rio Grande depression. The 

Rio Grande depression is a major rift structure (Chapin, 

1971) that is a local expression of widespread late 

Cenozoic crustal extension in western North America 

(Hamilton and Myers, 1966). The San Luis Valley segment of this structure is as much as 50 km across and 

may contain as much as 10 km of upper Cenozoic 

sedimentary fill, as indicated by gravity data (Gaca and 

Karig, 1966). 

ALTERATION AND MINERALIZATION 

In recent years it has become increasingly clear that 

most major ore deposits of the San Juan Mountains, 

such as those near Creede, Silverton-Telluride-Ouray, 

and Lake City, are closely related to the large calderas 

that collapsed as a result of the voluminous ash-flow 

eruptions in Oligocene time (Steven and Ratte, 1965: 

Luedke and Burbank, 1968; Steven and Lipman, 1968; 

Lipman and others, 1973). The Summitville-Platoro district seemed to be an exception (Steven and Ratte, 1960; 

Steven, 1968), but recognition of the Platoro caldera and 

of the sequence of igneous events related to its development has provided at least partial answ~rs to the major 

questions concerning the localization of the mineral 

deposits of the Summitville-Platoro district and nearby 

mineralized areas. 

Mineralization at Summitville and in certain nearby 

areas is now known to be closely related to intrusive and 

extrusive centers localized along marginal ring structures formed during the compound subsidence of the 

Platoro caldera complex. Outlying mineralization was 

controlled mainly by outward-extending fracture zones. 

The localization of mineralization in these zones is thus 

closely comparable to that of other highly mineralized 

areas in the central and western San Juan Mountains 

(Steven, 1968; Luedke and Burbank, 1968). Although no 

detailed· investigations of individual mining districts 

were undertaken in the present study, the structural 

relations and the timing of ore deposition at individual 

mineralized areas are reinterpreted in terms of the evolving caldera complex. 

SUMMITVILLE DISTRICT 

The alteration and mineralization at Summitville 

represent a late stage in the complex sequence of related 

igneous and hydrothermal episodes along the western 

margin of the Summitville caldera. As described by 

Steven and Ratte (1960), the gold-silver-copper ore in 

the Summitville district was deposited in a very shallow 

volcanic environment within a then recently erupted 

volcanic dome of coarsely porphyritic lava-the 2.2.8-

m.y. quartz latite of South Mountain. Both the ventdome complex and the associated alteration and 

mineralization are near the intersection of the Summitville fault with the buried ring-fracture zone of the Summitville caldera (fig. 61). The Summitville fault lies 

along the northeast margin of the Pass Creek-Elwood' 

Creek-Platoro fault system; the Summitville ore deposit, 

as well as the major mineralization in the Stunner and 

Platoro districts to the southeast, is situated approximately at intersections of this regional fault system 

with ring-fracture zones of the Platoro caldera complex. 

The ore at Summitville occurs in intensely altered 

pipes and irregular tabular masses of quartz-alunite rock 

that replaced the quartz latite along northwest-trending 

fracture zones. Metallic minerals, chiefly pyrite and 

enargite, fill irregular vugs that formed by local intense 

leaching of the quartz-alunite rock. The quartz-alunite 

masses are surrounded successively by soft argillically 

altered envelopes (illite-kaolinite zone) and by pervasively propylitized rocks (montmorillonite-chlorite zone). 

The alteration was interpreted by Steven and Ratte 

(1960) to have resulted from shallow solfataric activity 

similar to that associated with recent volcanism .. 

The close spatial association between hydrothermal 

alteration and ore deposition within the core of the. lava 

dome, directly overlying the vent, indicates that the 

mineralization was closely related to and followed shortly after emplacement of the quartz latite. This interpretation is supported by isotopic age data, which show 

that the emplacement of the quartz latite and the 

alteration and mineralization were coincident at about 

22.5 m.y. within the limits of analytical precision 

(Mehnert and others, 1973). 
114 PLATORO CALDERA COMPLEX, SAN JUAN MOUNTAINS, COLORADO 

STUNNER AND GILMORE DISTRICTS 

As described by Patton (1917, p. 98-101), quartzpyrite veins with ore shoots containing gold and silver 

tellurides were deposited locally in the Stunner and 

Gilmore districts within variably altered monzonite of 

the Alamosa River stock. The veins near Stunner strike 

generally northwest and dip steeply; they lie along small 

faults and fractures at the northern end of the Platoro 

fault zone. This vein mineralization is largely limited to 

the south side of the Alamosa River and is less well 

developed in the more intensely altered rock north of the 

river. In contrast, the mineralogically similar veins of the 

Gilmore district, also south of the river, strike N. 30°-40° 

E. and are in little-altered monzonite. These quartz 

veins have localized the breakaway zones of slump 

blocks on the steep north slopes of the Gilmore district 

(Patton, 1917, p. 57-60), and major slumping has impeded both geologic studies and mineral development. 

In the Stunner area, the north margin of the Alamosa 

River stock and much adjacent wallrock are intensely 

altered. Calkin (1967, p. 123; 1971) noted that the 

porphyritic phase of the Alamosa River stock (which he 

called the "Alum Creek Porphyry") is the focus of the 

most intense hydrothermal alteration. This porphyritic 

phase contains locally anomalous quantities of several 

metals (Calkin, 1967, p. 144-146) and in places is cut by 

numerous closely spaced quartz veinlets containing 

abundant pyrite and sparse molybdenite. 

Some dikes and plugs of coarse porphyry similar to 

that in the quartz latite dome at Summitville cut both 

the Alamosa River stock and the adjacent altered rock, 

and these in turn were variously altered (Calkin, 1967, p. 

74). Sharp and Gualtieri (1968) described zoned concentrations of lead, copper, molybdenum, and zinc near one of 

these porphyry dikes. 

PLATORO DISTRICT 

Most veins in the Platoro district are persistent northnorthwest-trending quartz-pyrite veins in the thick mass 

of intracaldera La Jara Canyon Member exposed on the 

resurgent block of Platoro caldera. Local ore shoots on 

these veins, containing gold tellurides and silver sulfosalts (Patton, 1917, p. 89-96), supplied most of the ore 

produced in early mining in the district. Although most 

of the La Jara Canyon tuff within the caldera is 

propylitically altered, hydrothermal alteration related to 

mineralization is restricted to local argillic zones flanking the quartz-pyrite veins (fig. 61). Mineralization in 

the Platoro district was recently studied by Bird (1972). 

The veins in the Platoro district are about on strike 

with some of the stronger veins in the Stunner district to 

the north, and both sets of veins follow the Platoro fault 

zone. This fault zone and the associated mineralization are younger than the Summitville caldera, and the 

Platoro veins are therefore only incidentally located within 

the resurgent block of the Platoro caldera. Ore deposits 

are uncommon within resurgent cores of other calderas 

in the San Juan Mountains; deposits in the Creede district-the closest structural analogy to the Platoro district-lie within the resurgent Bachelor caldera but are 

primarily related to radial graben faults of the Creede 

caldera (fig. 1; Steven and Ratte, 1965). 

JASPER AREA 

The Jasper district, along the east margin of the Summitville caldera about 12 km east of Summitville, is 

another area of intensely altered rock associated with a 

granitic stock (fig. 61). It is the only sizable area of intense alteration in the southeastern San Juan Mountains 

that does not lie along the Pass Creek-Elwood CreekPlatoro fault system. Much of the alteration is within the 

stock, which is exposed near the level of its roof. 

Numerous shallow prospects within the stock seem to 

have disclosed no vein deposits of sufficient size to be 

minable, although the possibility of disseminated 

mineralization may not have received much attention 

during early exploration. Quartz-pyrite veins with ore 

shoots containing gold. and silver are localized along the 

south margin of the highly altered rocks, according to 

Patton (1917, p. 105-108), but no modern studies of this 

area have been published. 

CRATER CREEK AREA 

Several major faults of the Elwood Creek fault zone 

that define a complex radial graben extend westward 

from the Summitville caldera (fig. 61) and converge 

toward the north end of. the Bear Creek stock cluster. 

Hydrothermally altered rock is apparent at places along 

these faults, and some rather extensive areas within the 

faulted zone and around the stocks are pervasively 

argillized. Numerous dikes of coarsely porphyritic 

rhyodacite and quartz latite fill subparallel fractures 

within and near the graben (fig. 47). 

Shallow prospect pits are scattered throughout the 

faulted and discontinuously altered area, and the Crater 

Creek drainage near the northeast end of the stock 

cluster has been intermittently explored in recent years 

(1967-70). This exploration has disclosed several veins, 

ranging in width from a few centimeters to several 

meters, that contain significant quantities of sphalerite 

and silver-bearing galena. The veins thus far discovered 

seem to be along minor fractures rather than along the 

major faults of the area, which are mostly poorly exposed 

and have not yet been explored. 

CAT CREEK AREA 

An extensive area of rather sporadic weak alteration 

and local pyritization extends southeastward from the 

Cat Creek stock, about 5 km east of the Platoro caldera 

wall (fig. 61). Numerous prospect pits and small adits 

were noted during the present study, but the altered rock 
PETROLOGIC EVOLUTION 115 

was not examined closely. No mineral production seems 

to have been recorded from this area, and the possible 

mineral potential is unknown. 

ECONOMIC SIGNIFICANCE 

Near Summitville and Platoro, the major foci of igneous intrusion, hydrothermal alteration, and 

mineralization seem to be localized along the ringfracture zone of the Summitville caldera, especially 

where it is intersected by the northwest-trending Pass 

Creek-Elwood Creek-Platoro fault system. The Platoro 

caldera apparently exerted little control either on exposed hypabyssal intrusions or on hydrothermal activity 

related to the intrusions. 

The areas of most widespread and intense alteration 

between Summitville and the Alamosa River show 

evidence of repeated intrusion and hydrothermal activity. The composite monzonite to quartz monzonite 

Alamosa River stock was intruded first, and a large block 

along its north margin was pervasively altered. Alteration was largely localized around the late porphyritic 

phase in the northern part of the stock, and anomalous 

concentrations of metals were introduced locally 

(Calkin, 1967; Sharp and Gualtieri, 1968). These altered 

rocks are overlain by unaltered rhyodacite lavas of Park 

Creek, erupted from the vicinity of Summitville, and are 

locally cut by coarsely porphyritic rhyodacitic and 

quartz latitic dikes and plugs. Similar coarsely 

porphyritic quartz latite of South Mountain, erupted 

from a vent along the northwest-trending Summitville 

fault, was altered and mineralized; ores from this center 

have provided most of the production from the Platoro 

caldera area. 

The whole area from Summitville south across the 

Alamosa River and east to Jasper thus is a zone of 

recurrent intrusion, extrusion, and hydrothermal alteration and mineralization controlled by the ring-fracture 

zone of the Summitville caldera. The environment is 

similar to that in the intensely altered and mineralized 

Red Mountain district along the northwest m·argin of the 

Silverton caldera in the western San Juan Mountains 

(fig. 1; Burbank, 1941; Luedke and Burbank, 1968). The 

ores are also similar; pyrite and enargite are common in 

both districts, although numerous other ore minerals, including abundant sulfosalts of copper and silver, are present at Red Mountain. The gangues consist of pipelike 

masses of strongly leached silicified rocks that formed by 

replacement of preexisting volcanic or shallow intrusive 

rocks in shallow solfataric environments. 

Deeper parts of this type of volcanic 

association-characterized by repeated hypabyssal intrusion of differentiated monzonitic to quartz monzonitic stocks, widespread and intense hydrothermal 

alteration, and local anomalous concentrations of gold, 

silver, copper, and molybdenum-are similar to those in 

which porphyry-type disseminated deposits of copper 

and molybdenum occur throughout western North 

America. Now-exposed levels at Summitville are shallow 

and within the volcanic pile (Steven and Ratte, 1960, p. 

50-51), whereas most porphyry-type deposits seem to 

have formed at lower volcanic levels or in the stocks 

beneath the volcanic pile. Favorable zones for exploration thus still exist at depth. 

The monzonitic and quartz monzonitic compositions 

of the known intrusive bodies in the SummitvilleAlamosa River area are more normally associated with 

predominantly copper-bearing porphyry deposits 

(Stringham, 1966) than with predominantly 

molybdenum-bearing deposits, where mineralization 

seems related to relatively silica-rich rhyolite and 

granite porphryry (Wallace and others, 1968; Carpenter, 

1968). The abundance of copper and the dearth of 

molybdenum in the known ores at Summitville further 

suggest this association. Molybdenum and copper, 

however, both are present in the mineralized area 

around Alum Creek, south of Summitville (Calkin, 1967, 

p. 144-147; Sharp and Gualtieri, 1968). 

Mineral deposits in the northwest-trending fracture 

zones range from quartz-pyrite veins with shoots of gold 

and silver tellurides and associated silver and copper sulfosalt minerals in the Stunner, Gilmore, and Platoro districts to silver-bearing galena-sphalerite veins near 

Crater Creek. A close analogy exists between the environment of the lead-zinc-silver veins along Crater 

Creek and that of the major producing base-metal veins 

in radial faults around the margins of the Creede caldera 

(Steven and Ratte, 1965) and the Lake City and Silverton calderas (Luedke and Burbank, 1968). The Crater 

Creek area is appreciably farther from the apparent 

focus of intrusive and hydrothermal activity between 

Summitville and the Alamosa River than are the 

Stunner, Gilmore, and Platoro districts. This general 

distribution of gold, silver, and copper· relatively near the 

source and of silver, lead, and zinc more distant is 

reminiscent of the metal zoning around many western 

mining districts, as previously suggested by Calkin 

(1967, p. 154). The northwest-trending fracture zones 

have been poorly explored and seemingly deserve more 

attention. 

PETROLOGIC EVOLUTION 

Petrologic studies on rocks of the Platoro caldera complex and related volcanic rocks of the San Juan field are 

still in progress (1972). Accordingly, petrologic evolution 

will not be interpreted in any great detail at this time, 

but a few features of petrogenetic significance are 

documented so well in the Platoro area that some discussion seems appropriate. 

Perhaps the most interesting feature of the Platoro 

caldera complex is the close spatial, temporal, and 
116 PLATORO CALDERA COMPLEX, SAN JUAN MOUNTAINS, COLORADO 

petrogenetic association of andesites with more silicic igneous rocks in a resurgent-caldera ash-flow setting. In 

general, calderas and related ash flows seem divisible 

into two broad groups (Smith and Bailey, 1968): the 

Krakatoan type (resurgence rare), which is associated 

with large andesitic volcanoes in a eugeosynclinal 

continental-margin or island-arc environment, and the 

GlenCoe type (resurgence common), which is associated 

with more silicic volcanism and intrusive activity in an 

epicontinental cratonic environment. Contrasts between 

continental-margin and continental-interior volcanism 

in the Western United States have also been emphasized 

recently by Gilluly (1971), who regards Cenozoic 

volcanic activity along the western continental margin 

as genetically related to plate-tectonic events but who 

considers volcanism as far from the continental margin 

as the San Juan field (about 1,500 km) as fundamentally 

different and unrelated to motions of tectonic plates. 

However, the rocks of the San Juan field, and of the 

Platoro caldera complex in particular, constitute a 

predominantly andesitic association in a continentalinterior setting, so the numerous resurgent calderas do 

not fit simply into either Krakatoan or GlenCoe types. 

Furthermore, the San Juan volcanic field seems to be the 

inner part of a broad middle Tertiary zone of 

predominantly andesitic volcanism in the Western 

United States that can be interpreted as related to a 

complex subduction boundary between American and 

Pacific tectonic plates (Lipman and others, 1971, 1972). 

The relations at the Platoro caldera complex strongly 

suggest that the quartz latitic and low-silica rhyolitic 

ash-flow tuffs are differentiates of andesitic magma. In 

the San Juan region in general, the caldera structures 

resulting from ash-flow eruptions are within the more extensive region of vents for the early intermediatecomposition rocks (Lipman and others, 1970, fig. 1). The 

Platoro caldera formed within a cluster of only slightly 

older andesitic volcanoes of the Conejos Formation (fig. 

5); shortly after eruption of the quartz latitic La Jara Canyon Member and caldera collapse, the caldera filled 

with andesitic lavas that are virtually indistinguishable 

from the precaldera lavas. Further ash-flow eruptions formed the upper two widespread sheets of Treasure 

Mountain Tuff and caused collapse of the Summitville 

caldera, which was subsequently filled by similar 

andesitic lavas. These andesitic lavas and more silicic 

tuffs of the Platoro caldera complex define a _£Q!!!-

positional continuum for all major oxides (fig. 66) . 

Similar compositional sequences are displayed by a few 

individual differentiated early intermediate centers, 

such as Summer Coon volcano .(Lipman, 1968; Mertzman, 1971). 

The continued availability of andesitic magma during 

later ash-flow eruptions of the San. Ju~m field is indicated by similarly interlayered andesites such as the 

Sheep Mountain Andesite, the Huerto Andesite, and the 

partly andesitic volcanics of Table Mountain (Steven 

and Lipman, 1973). Regionally, the nonsystematic alternating eruption of andesitic, rhyodacitic, and quartz 

latitic lavas indicates that this range of magma types 

was available during the time of caldera formation. 

Many of the ash-flow sheets, such as those of the 

Treasure Mountain Tuff, are monotonous quartz latites 

that are only slightly more silicic than the associated 

lavas; but other large ash-flow sheets are rhyolites. Compositional zonations from rhyolite upward into quartz 

latite within individual ash-flow sheets, such as the 

Carpenter Ridge Tuff (fig. 32), extend over the entire 

compositional range of all the Oligocene ash-flow sheets 

(figs. 4, 66) and are interpreted as representing in inverse 

order the vertical differentiation of the source magma 

chambers.' The most mafic parts of the sheets, compositionally as low as 61 percent Si02 (table 6, No. 15), 

bridge the compositional transition between the lav:as 

and the more silicic tuffs (figs. 4, 66). 

In addition, at several calderas, including the Platoro 

caldera complex, postcollapse intrusions along the ring 

fractures are granodiorite or monzonite, compositionally 

similar to the andesitic lavas that preceded and followed 

the ash-flow eruptions. The caldera structures thus are 

thought to have formed by collapse resulting from eruption of silicic magmas in cupolas at the tops of larger, 

more mafic batholiths. The andesitic and related lavas 

probably represent the composition of the greater parts 

of these batholiths (mainly diorite, granodiorite, and 

monzonite), and the ash-flow sheets represent their 

differentiated granitic tops. 

The overall area of the San Juan caldera cluster (fig. 

1) also coincides with a large negative gravity anomaly, 

the dimensions of which are compatible with the subsurface presence of a large shallow batholith,JPlouff and 

Pakiser, 1972). The evolution of the San Juan caldera 

~luster suggests progressive rise, differentiation, and 

crystallization of this batholith inward from its margins. 

The earliest calderas-Platoro, San Juan, Uncompahgre, and Bonanza-are at the edges of the gravity 

anomaly (fig. 1), and the ages of calderas generally converging toward the center of the cluster near Creede. 

(The only major exception, Lake City caldera, is about 4 

m.y. younger than any of the others, and its tuffs are 

different in petrology; probably it is best considered as a 

separate event.) The Platoro caldera complex is just out-

. side the gravity low (Plouff and Pakiser, 1972). This 

location, in conjunction with the rela.tively unfractionated petrologic character of the Treasure Mountain 

Tuff and its associated caldera-filling lavas, suggest that 

the magmas related to the Platoro caldera complex were 

derived from the batholith at an early stage ~n its evolu .. 

tion, when it was still fairly deep, relatively undifferen: 

tiated, and somewhat different in shape than in its final 
PETROLOGIC EVOLUTION 117 

form. The younger ash-flow sheets, which are related to the 

central San Juan caldera cluster within the core of the 

gravity anomaly, tend to be more fractionated, as indicated by compositional zonations of the sheets, and 

the postcaldera lavas are more silicic than at Platoro. 

Except for the resurgent uplift after its first collapse, 

the Platoro caldera complex is strikingly similar to some of 

the great Pleistocene calderas of the Japanese volcanic 

chain, such as Aso in Kyiishii (Matumoto, 1943; 

W.illiam~ 1941). Like the Platoro caldera complex, Aso 

caldera 1s a composite collapse structure from which 

three major ash-flow sheets were erupted (Ono, 1965). 

The caldera is about 17 by 25 km and has walls 

somewhat rectangular in plan and strikingly scalloped 

by slumping; the geometry is closely similar to that of 

the Platoro caldera complex (fig. 67). Before the ash-flow 

eruptions, the site of the Aso caldera was among a cluster 

of large andesitic stratovolcanoes within the central 

volcanic highland of Kyiishii Island. The related ashflow tuffs were deposited as a nearly continuous volcanic 

blanket for about 15 km out in every duectwn from the 

caldera r1m, and they foilowed maJor radial valleys outward from. the central highland for as much as 75 km 

(fig. 67). Although this extent is greater than that 

recognized for the Treasure Mountain Tuff, a great 

proportion of distal tuff deposits of the Aso eruptions is 

confined to the deep valleys. The Aso ash-flow tuffs are 

distinctly more silicic in bulk composition than the 

precaldera andesite, and at least two of these sheets are 

compositionally zoned, from basal phenocryst-poor 

rhyodacite upward into phenocryst-rich alkali andesite. 

The composition change, becoming more mafic upward, 

apparen~ly reflects in inverse order the compositional 

zonation in the magma chamber, in which more silicic 

differentiated magma overlay more mafic magma (Lipman, 1967). The composition of the last erupted 

andesitic tuff is generally similar to that of the 

precaldera andesitic lavas. As at Platoro caldera, the 

postcollapse volcanism at Aso (which is still in progress) 

is andesitic in compositio;;-;nd produced large cones of 

andesitic lava and scoria on the caldera floor. 

Circum-Pacific andesitic arcs, such as Japan, are 

clearly associated with subduction at the convergence of 

lithospheric plates, where chains of andesitic volcanoes 

are alined above dipping Benioff seismic zones (Dickinson and Hatherton, 1967; !sacks and others, 1968; 

Hamilton, 1969). Volcanoes of the active arcs, although 

mostly less alkalic than andesites of the San Juan region, 

are characterized by systematic variations in chemistry, 

especially in alkalis, that are transverse to the arc and 

that correlate with ~epth to the Benioff seismic zone 

(Kuno, 1959; Dickinson and Hatherton, 1967; Dickinson, 

1970; Jakes and White, 1972). The lower and middle 

Tertiary volcanic rocks of the Western United States, including those of the San Juan field, also display 

transverse variations in alkali content, with alkalis 

generally increasing toward the continental interior 

(Merriam and Anderson, 1942; Moore, 1962; Lipman 

and others, 1971). The compositions of these 

continental-interior andesites, although generally more 

alkalic than modern arc andesites, are within the spectrum of circum-Pacific andesites (Lipman and others, 

1972, fig. 7). For example, the Oligocene andesites of the 

San Juan field are compositionally similar to Quaternary andesites of the Chilean Andes (Lipman and 

others, 1972, fig. 6), where stratovolcanoes of alkali 

andesite are also associated with voluminous rhyolitic 

ash-flow tuffs (Zeil and Pichler, 1967; Pichler and Zeil, 

1969) in a plate-convergence setting (Hamilton, 1969; 

James, 1971). These similarities strongly suggest that 

the middle Cenozoic andesitic rocks of the Western. 

United States, including those of the San Juan field, 

were also genetically related to a subduction system. The 

great width of this volcanic· belt, extending as much as 

1,500 km from the continental margin, can be interpreted in terms of a complex imbricate subduction 

system (Lipman and others, 1971, 1972). 

At about the beginning of Miocene time, the volcanic 

associations in the San Juan region changed notably. 

Whereas the Oligocene volcanic rocks are predominantly 

intermediate-composition lavas and genetically related 

ash-flow tuffs, the younger rocks are largely a bimodal 

association of alkali olivine basal~ and high-silica alkali 

rhyolite. Basalt and minor rhyolite were erupted intermittently through the Miocene and Pliocene, and at one 

time they formed a widespread veneer over most of the 

older volcanic terrane. 

The general petrologic progression of the prolonged 

postcollapse igneous activity (29-20 m.y.) at the Platoro 

caldera complex is also toward rock types that are more 

silicic as age decreases, from voluminous dark andesites 

(56-59 percent Si02) to minor scattered lava domes of 

porphyritic rhyolite (73-76 percent Si02) that are con- \ 

temporaneous with nearby widespread flows of alkali 

olivine basalt. The · postcollapse igneous activity at 

Platoro caldera therefore spans the regional transition 

from predominantly andesitic volcanism to bimodal 

basalt-rhyolite association. 

The contrast between the Oligocene intermediate to 

low-silica rhyolite magmas and the later basaltic and 

rhyolitic bimodal magmas implies either different conditions of magma generation or different processes of fractionation for the two magmatic assemblages. This 

petrologic change coincides approximately in time with 

nearby development of the Rio Grande depression, a 

major rift that is the local expression of widespread late 

Cenozoic crustal extension (Hamilton and Myers, 1966; 

Scholz and others, 1971). The change from predominantly intermediate to bimodal basalt-rhyolite volcanism, 

approximately concurrent with initiation of late Tertiary 
118 

1-

z 

w 

u 

a: 

~ o"' 

~ ~ (!) 

iii 

3: 

z 

en' 

w 

Cl 

x 

0 

0 ... cu 

z 

2 

PLATORO CALDERA COMPLEX, SAN JUAN MOUNTAINS, COLORADO 

t:.x 0 

* X * .. OJ DO 0 *oQ& .A. .A. r! i' * ff x* * x&._ ~ .A 0 

* 

X 

X 

* 0 

~ 

EXPLANATION 

X 

* 

t> Late basalt and rhyolite (xenocrystic lavas excluded) 

* Lavas interlayered with younger ash-flow sheets 

Younger ash-flow sheets 

• Intrusive rocks of Platoro caldera complex 

o Lavas of Platoro caldera complex 

o Treasure Mountain Tuff 

x Early intermediate lavas 

0 0 

0 

0 

.. .. p f' 0 

0 

.. 

0 .. 

x~~oo~--0 

.. ~ ooo-x 

o 0" o. o f o• q,o 0 ........ 0 0 

.. . . 

o._ 

0 

CtJ 0 

0 

0 

0 

Cb 

~ D o o0 ° o 

0 

.. 

.. 

.. 

0 

0 

0 

* 

._•._*.rao ..0 x ooo &o• x~:, o ooo o o~Do ._ o >* 0 o _. L:ll'< 0 ~. 'lt & D 0 • M'ICO ;&. 0 • • 8 X *~ ~ A >< lfi xr A A ~ • • 0 X 0 0 0 

Si02 , IN WEIGHT PERCENT 

FIGURE 66.-Si02 variation diagram for major igneous units of the Platoro caldera complex. All analyses are from tables of this report. 
PETROLOGIC EVOLUTION 119 

6. 

6. )( 

6. 

b.t. 6. X 

X 6. ... * 6. Cb 0 X 

*• X 

0& x *rn ... 0 ... 0 ... oo lQ ... ... o"% u 0 ... ~ 0 

'\ ... ... ~ x o odO • .,.P"" 

g .~· • 10 0 • 0 oo oo&o •O 

0 0 0 ... 0 8 bbo 0 0 0 X 

0 ~ 0 

0 

0 ... . 

11 6. 

6. 

1-

z b.t. 6. 

w 

u ... a: * 6. X 

w 6.~ * 0.. ... 1- 0 6. 0& ... :I: Q) 6. * *o 

(.!1 u. X 

jjj Ill 

lQ 6. 0& All< X#' 6. 3: ""x ... Q) 0 !lD ... ... ~ u. 6. @OD< a ... vi ...J 6. 0 <( 0 0 ... w 1- c 0 X Do¢ 0. x 1- 6. 6. . ... X 

0 6. * 0 0 0 ~ ... ... ... 0 °o :!;1 ~ 

x*ct 0 0~ ... 0 • 0< ~· ... ~ ... ""o 00 0 

0 ... 0 ... ... 0 00 ... 0 * *o D<& 0 0 ... 0 .oP.oo ~ 0 X 

0 ~~ ~ \ 

0 X 

* ... 8 oo oo •• ~ ~ O Oo O 00 De 0c9~'b 0 0 

2.0 6. 6. 

6. 6. 

6. 

1.5 6.6. 6. * X 0 

o" X d' 

j: 1.0 *oo X 

cr x'EI ... CJ 0 

* 0 ~ ... oo 0~ 

~ "" o o o& 0 ... x o <». r:P t.o "" o X 00 ,. 0 0 0 

0.5 ....... )<Ito 0~ 00~ • 0 ... 0 • oo t>o 0 0 

0 . 0 

q.a 

Si0

2 , IN WEIGHT PERCENT 

FIGURE 66. - Continued. 
120 PLATORO CALDERA COMPLEX, SAN JUAN MOUNTAINS, COLORADO 

0 

I 

0 

ASO 

I 

10 

10 

PLATORO 

20 

I 

I 

30 MILES 

I I 

20 30 KILOMETERS 

FIGURE 67.-0utlines of Platoro caldera complex in Colorado and Aso caldera in Japan, showing extent of related ash-flow tuffs (shaded). 

Both maps at same scale. Platoro data are from figure 14 of this report; Aso data are from Matumoto (1943). 

crustal extention, seems to be characteristic of Cenozoic 

volcanism for much of the Western United States 

(Christiansen and Lipman, 1972). These changes also 

coincided in time with changes in the geometry of seafloor spreading recorded in magnetic anomaly patterns 

in the northeast Pacific (McKenzie and Morgan, 1969; 

Atwater, 1970), apparently reflecting initial intersection of 

North America with the East Pacific Rise and destruction of 

the trench-Benioff zone system. 

REFERENCES CITED 

Aoki, Ken-lchico, 1967, Petrography and petrochemistry of latest 

Pliocene olivine-tholeiites of Taos area, northern New Mexico, 

U.S.A.: Contr. Mineralogy and Petrology, v. 14, p. 190-203. 

Armstrong, R. L., 1969, K-Ar dating of laccolithic centers of the 

Colorado Plateau and vicinity: Geol. Soc. America Bull., v. 80, no. 

10, p. 2081-2086. 

Atwater, Tanya, 1970, Implications of plate tectonics for the Cenozoic 

tectonic evolution of western North America: Geol. Soc. America 

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INDEX 

[Italic page numbers indicate major references] 

A Page Analysts-Continued Page Calderas-Continued Page 

Adams l<'ork ...................................... 87 

Adams Fork volcano ........................... 12, 87 

Age, andesitic dikes ............................... 87 

Bear Creek stock cluster ....................... 86 

calderas of San Juan cluster .................. 116 

Carpenter Ridge Tuff .......................... 53 

Cat Creek stock ............................... 84 

Conejos Formation ............................. 9 

Cornwall fault ............................... 109 

granitic stocks ................................ 78 

Hinsdale Formation ....................... 92, 96 

Jasper stock .................................. 84 

La Jara Canyon Member of 

Treasure Mountain Tuff ............... 29 

Los Pinos Formation .......................... 77 

· lower tuff of Treasure 

Mountain Tuff ........................ 23 

Masonic Park Tuff ............................ 47 

Mount Hope caldera .......................... .46 

Ojito Creek Member of Treasure 

Mountain Tuff ........................ 35 

ore deposition at Summitville ................. 113 

quartz latite of South Mountain ........... 74, 113 

quartz loti tic and rhyolitic dikes ................ 90 

Ro Jadero Member of Treasure 

Chloe, Gillison ............... 11, 20, 42, 60, 80, 94 

Elmore, P. L. D .............. 11, 20, 42, 60, 80, 94 

Fletcher, J. D ................ 11, 20, 42, 60, 80, 94 

Glenn, John ................. 11, 20, 42, 60, 80, 94 

Hamilton, J. C ......................... 20, 42, 94 

Harris, J. L ............................... 60, 80 

Kelsey, James .............. 11, 20, 42, 60, 80, 94 

Neiman, Harriet .......................... 60, 80 

Neuerburg, F. H .............................. 42 

Parker, C. L .............................. 42, 56 

Roit, Norma ...... .' ................ 11, 42, 60, 80 

Smith, H .................... 11, 20, 42, 60, 80, 94 

Smith, V. C ................................. .42 

Worthing, H. W ............................... 80 

Andesite of Summit Peak ............... 55, 62, 67, 87 

Animas Formation ................................. 8 

Ash-fall tuff ...................................... 31 

agglutinated ........................... 23, 46, 71 

Ash-flow lobes ............................. 22, 23, 25 

Ash-flow sheets ................................ 7, 14 

accumulation concurrent with subsidence ....... 25 

compositional zonation ................... 51, 116 

first widespread ............................... 25 

geometric model .............................. 30 

magnetic polarities ......................... 6, 15 

most voluminous ............................. .47 

Creede ...................... 53, 54, 103, 104, 114 

Glen Coe type ............................... 116 

island arc environment ....................... 116 

Japanese .................................... 117 

Krakatoan type .............................. 116 

La Garita ................................ 47, 104 

Lake City ............................ 93, 97, 116 

Mount Hope. See Mount Hope caldera. 

northeastern San Juan Mountains .............. 18 

Platoro. See Platoro caldera. 

relation to earlier volcanoes ................... 101 

relation to mineralization .................. 7, 113 

relative ages ................................. 116 

resurgence .............................. 101, 107 

cause of assymmetry ..................... 107 

Mount Hope caldera ....................... 49 

Platoro caldera ........................... 107 

San Juan caldera cluster .................. 110 

stage 1 in cycle .......................... 103 

stage 2 in cycle .......................... 104 

stage 3 in cycle .......................... 104 

stage 4 in cycle .......................... 107 

stage 5 in cycle .......................... 107 

stage 6 in cycle .......................... 108 

stage 7 in cycle .......................... 108 

Silverton ............................... 107, 115 Mountain Tuff ....................... .40 

regional extensional faulting ................... 112 

rhyodacite of Park Creek ....................... 73 

overlying Treasure Mountain Tuff ............. .41 

petrologic features ............................. 15 

relation between outflow sheets 

Summitville. See Summitville caldera. 

Timber Mountain ............................ 107 

Valles ....................................... 104 rhyolite of Cropsy Mountain ................... 75 

Rio Grande fault zone ........................ 110 

San Juan volcanic field ....................... 101 

Snowshoe Mountain Tuff ...................... 54 

Summitville Andesite .......................... 66 

Summitville fault ............................ 112 

tuff of Rock Creek ............................. 16 

volcanics of Green Ridge ................... 69, 70 

Wason Park Tuff .............................. 54 

Alamosa River canyon ........ 31, 33, 34, 36, 40, 62, 82 

and intracaldera tuffs ........... 25, 27, 53 

source magma composition .................... 116 

B 

Bachelor Mountain Member of Carpenter 

Ridge Tuff ........................ 49, 53 

Bachelor Mountain Rhyolite ....................... 49 

Banded Peak area ............................. 23, 90 

western San Juan Mountains ................... 18 

California Gulch ................................. 107 

Canyon Diablo stock .......................... 66, 86 

Carpenter Ridge Tuff ......................... 49, 104 

Cat Creek .................................... 16, 67 

Cat Creek stock .................... 67, 82, 84, 87, 114 

Cat Creek volcano ...................... 47, 67, 77, S2 Cataract Creek stock .............................. 86 

Chemical composition, alkali content 

of San Juan andesites ................ 117 Alamosa River stock ....................... 35, 78, 109 

hydrothermal alteration ............... 78, 114, 115 

mineralization .......................... 114, 115 

radiating dikes ..................... 78, 86, 87, 89 

Basalt-rhyolite association .................... 90, 117 

Batholith ................................... 110, 116 

Bear Creek stock .............................. 84, 11 

Bear Creek stock cluster ................. . 84, 111, 114 

Carpenter Ridge Tuff .......................... 51 

Conejos Formation ............................. 9 

Fish Canyon Tuff ............................. 49 

Hinsdale basalts .............................. 92 

radiometric date .............................. 78 Beaver Creek .............................. 49, 77, 96 Huerto Andesite ............................... 55 relation to Summitville Andesite ............... 66 Beaver Lake Creek ........................ 63, 66, 108 La Jara Canyon Member of 

Alteration ...................................... . 113 Beaver Mountain ............................. : .... 51 Treasure Mountain Tuff ............... 28 

Cat Creek stock .......................... 84, 114 Seidell Quartz Latite ............................... 8 magma of San Juan volcanic field ............. 116 Treasure Mountain Tuff ....................... 84 Bennett Peak ...................... 16, 40, 41, 69, 106 Masonic Park Tuff ........................... .46 quartz latite of South Mountain ................ 73 

See also Hydrothermal alteration; Mineralization. Bibliography ..................................... 120 

Bimodal volcanic association .................. 90, 117 

Ojito Creek Member of Treasure 

Mountain Tuff ........................ 35 Alum Creek Porphyry ............................ 114 Blanco Basin ..................................... 90 quartz latite of South Mountain ................ 73 Analyses, ash-flow sheets overlying Treasure Blanco Basin Formation ......................... 6, 8 Ro Jadero Member of Treasure Mountain Tuff ....................... .42 Bonanza Tuff ..................................... 18 Mountain Tuff ....................... .40 Conejos Formation ............................ 11 Bonito Peak ...................................... 65 relation to phenocryst content ........ 5, 19, 29, 40 

Hinsdale Formation ........................... 94 Brazos Arch ..................................... 101 rhyodacite of Fisher Gulch ..................... 58 

intrusive rocks ................................ 80 Brazos Basalt ..................................... 92 rhyolite of Cropsy Mountain ................... 75 

lava flows overlying Treasure Burnt Creek ...................................... 84 San Juan volcanic rocks .................. · ...... 6 

Mountain Tuff ........................ 56 Treasure Mountain Tuff .................. 19, 116 

methods ....................................... 5 c tuff of Rock Creek ............................. 16 

postcollapse Platoro caldera volcanics of Green Ridge ................... 69, 70 

lava flows ............................ 60 Calderas ...................................... 7, 101 Wason Park Tuff .............................. 53 

Treasure Mountain Tuff ....................... 20 Aso ......................................... 117 See also Analyses. 

tuff of Rock Creek ............................. 20 Bachelor ........................ 49, 104, 110, 114 Chiquita Peak ................... 33, 34, 36, 40, 69, 70 

See also Chemical composition. central San Juan Mountains .................. .41 Cinder cones ............................... 92, 97, 98 

Analysts, Artis, Lowell ............ 11, 20, 42, 60, 80, 94 Cochetopa ................................... 107 Comagmatic intrusive-extrusive association ... 78, 82, 84 

Botts, Samuel ............... 11, 20, 42, 60, 80, 94 cratonic environment ......................... 116 Conejos Formation .......................... 7, 8, 101 

125 
126 INDEX 

Conejos Formation-Continued Page Faults-Continued Page Lava Flows-Continued Page 

age ............................................ 9 Rio Grande zone ............................. 110 postcollapse Platoro caldera ................ 55, 58 

analyses ................................... 9, 11 Summitville .................... 71, 111, 112, 113 Lithology, Alamosa River stock ..................... 78 

associated intrusives ................ 78, 84, 86, 87 Summitville caldera ring-fracture andesite of Summit Peak ...................... 67 

chemical composition ........................... 9 zone ................ 35, 84, 108, 113, 115 Carpenter Ridge Tuff ......................... .49 

cone breccias .............................. 9, 12 Fieldwork .......................................... 2 Conejos Formation ............ 9, 12, 13, 62, 65, 67 

definition ..................................... 14 Fish Canyon Tuff ............................ 47, 104 Fish Canyon Tuff ............................ .47 

lithology ...................... 9, 12, 13, 62, 65, 67 Fisher Gulch ...................................... 63 granitic stocks ................................ 78 

radiometric dates ............................... 9 Fisher Quartz Latite ........................ 70, 73, 74 Hinsdale Formatio.n .................... 92, 96, 98 

similar later volcanic activity ................. 108 Forest King area .................................. 64 Huerto Andesite ........................ 55, 65, 67 

upper lava unit ............................. 9, 13 Fossils ............................................ 66 La Jara Canyon Member of Treasure 

vent facies .............................. 9, 12, 77 Mountain Tuff ................. 27, 46, 58 

volcaniclastic facies ..................... q, 13, 77 Gilpin Peak Tuff .................................. 16 lower tuff of Treasure Mountain Tuff ........... 22 

volcanoes ..................... 8, 9, 12, 16, 27, 101 Goose Creek .................................. 41, 55 Masonic Park Tuff ........................... .45 

Conejos Peak ....................... 12, 33, 64, 66, 86 Grabens .................. , .......... 77, 110, 113, 114 middle tuff of Treasure Mountain Tuff .......... 32 

Conejos River .......................... 8, 12, 22, 107 Gravity anomaly ................................. 116 mixed-lava complexes ......................... 98 

Cornwall fault ....................... 66, 78, 108, 109 Grayback Mountain ............................... 96 most distinctive ash-flow unit ................. .47 

Cornwall Mountain ............................ 27, 97 Green Ridge ........................ 47, 49, 69, 70, 77 Ojito Creek Member of Treasure 

Cornwall Mountain resurgent block ............ 65, 103 Gunnison River ................................ 47,49 Mountain Tuff ........................ 33 

cause of resurgence ....................... 87, 107 quartz latite of South Mountain ............ 71, 73 

intracaldera tuff exposure ................. 27, 107 H, I, J Ra Jadero Member of Treasure 

volume of initial collapse ..................... 105 Mountain Tuff ....................... .40 

Cornwall Mountain stock .......................... 87 rhyodacite of Fisher Gulch ..................... 58 

Cornwalls Nose ........................... 64, 66, 109 Handkerchief Mesa ......................... 71, 97, 98 rhyodacite of Park Creek ....................... 71 

Correlation, andesite of Summit Peak ............... 67 Hinsdale Formation ............................ 8, 90 Sheep Mountain Andesite ...................... 55 

Conejos Formation ............................. 8 age ....................................... 92, 96 Snowshoe Mountain Tuff ...................... 54 

Hinsdale basalts .............................. 92 analyses ...................................... 94 Summitville Andesite ................... 62, 71, 84 

Sheep Mountain Andesite ...................... 67 basaltic lavas ............................. 90, 97 tuff of Rock Creek ............................. 16 

volcanics of Green Ridge ................... 62, 67 chemical composition .......................... 92 volcanics of Green Ridge ................ 67, 69, 70 

Crater Creek stock ........................... 84, 114 cinder cones .............................. 97, 98 Wason Park Tuff ....... : . ..................... 53 

Cretaceous sedimentary rocks ......... , ............. 8 faulting ..................................... 112 Lookout Mountain ............................ 74, 90 

Cropsy Mountain .............................. 73, 74 lithology ............................... 92, 96, 98 Los Mogotes shield volcano .................... 77, 92 

Crustal extension, coincidence mixed-lava complexes ........................ . 97 Los Pinos Formation ....................... 69, 70, 76 

with petrologic change ............. 8, 117 radiometric dates .......................... 92, 96 

Cumbres Pass .................................... 112 rhyolitic lavas and tuffs ....................... . 93 M vents ..................................... 92 

D,E xenocrysts ................................ 92, 98 

Horseshoe Mountain ............................... 27 Magmas, differentiation 

before eruption ........ 12, 29, 52, 104, 116 

Del Norte Peak ............................... 27, 41 Huerto Andesite ............................ 55, 65, 67 differentiation of rhyodacitic intrusives .......... 87 

Dikes ...................................... 7, 78, 86 Huerto Formation ................................. 55 mixing before eruption .............. 75, 92, 98, 99 

Adams Fork volcano ........................... 87 Hydrothermal alteration ............................ 7 11,1agnetic polarities ................................. 6 

Alamosa River stock ................ 78, 86, 87, 89 Alamosa River stock .................. 78, 114, 115 ash-flow sheets ............................. 6, 15 

andesitic ..................................... . 86 Bear Creek stock cluster .............. 85, 111, 114 Carpenter Ridge Tuff .......................... 53 

Cat Creek stock ........................... 84, 87 Jasper stock ............................. 84, 114 Fish Canyon Tuff ............................ .49 

Cataract Creek stock .......................... 86 Pass Creek fault zone ......................... 110 La Jara Canyon Member of Treasure 

cutting Alamosa River stock .................. 114 resurgent-caldera cycle ........................ 108 Mountain Tuff ........................ 29 

distribution ............................... 87, 88 

Elwood Creek fault zone ................. : ..... 88 

Bee also Alteration; Mineralization. 

lntracaldera tuffs ................................. 104 

lower tuff of Treasure Mountain Tuff ........... 23 

Masonic Park Tuff ........................... .47 

mafic ........................................ 87 Carpenter Ridge Tuff .......................... 53 middle tuff of Treasure Mountain Tuff .......... 32 

most spectacular .............................. 88 Fish Canyon Tuff ............................ .47 Ojito Creek Member of Treasure 

Pass Creek fault zone ......................... 110 geometric model . · ............................. 30 Mountain Tuff .................... 33, 36 

quartz latitic and rhyolitic .................... . 88 indicator of concurrent eruption and collapse ... 104 Ra Jadero Member of Treasure 

rhyodacitic ................................... . 87 La Jara Canyon Member of Treasure Mountain Tuff ....................... .40 

rhyolitic ...................................... 98 Mountain Tuff ........................ 27 Snowshoe Mountain Tuff ...................... 54 

upper Alamosa River .......................... 87 lithology ...................................... 27 Summitville Andesite .......................... 67 

Durango map ...................................... 2 relation to outflow sheets ............... 25, 27, 53 Treasure Mountain Tuff ....................... 19 

Intrusive rocks ................................. 7, 78 tuff of Rock Creek ............................. 16 

Elephant Mountain ............................... 96 Isopach maps ................................. 3, 110 Wason Park Tuff .............................. 54 

Elwood Creek stock ............................... 84 Isotopic dates. See Radiometric dates. Mammoth Mountain ............................. 107 

Elwood Pass ...................................... 71 Mapping .......................................... 2 

Eruption concurrent with caldera collapse .......... 103 Jacobs Hill ................................... 69, 70 Maps, summary .................................... 3 

Eruption from differentiated Jasper stock ............. : ............... 84, 109, 114 Marble Mountain ......................... 65, 66, 109 

magma chamber .......... 12, 2~, 52, 104 

L 

Masonic Park Tuff ............................ 18, 41 

Mineralization ................................. 7, 113 

F,G La Garita Member of Fish Canyon Tuff ............. 47 

Alamosa River stock ..................... 114, 115 

Bear Creek stock cluster ...................... 111 

Fault-graben systems ............................. 110 

Faults, along Beaver Creek ........................ .49 

antithetic ............................... 112, 113 

California Gulch ............................. 107 

La Garita Quartz Latite .......................... .47 

La Jara Canyon ................................... 25 

La Jara Canyon Member of Treasure 

Mountain Tuff ............ 25, 46, 58, 103 

Jasper stock ................................. 114 

level of exposure of deposits ................... 115 

metal zoning ................................. 115 

relation to caldera margins ................. 7, 113 

Cornwall ........................ 66, 78, 108, 109 

Creede caldera ............................... 114 

Laccoliths .................................. 7, 82, 86 

Lake Fork Formation ............................... 8 

Platoro fault zone ............................ 114 

Summitville caldera margin ................... 108 

Elwood Creek zone ................... 88, 110, 114 

influence on mineralization .............. 113, 115 

La Jara Reservoir ............................. 92 

Lake sediments .................................. 108 

Landslides ................................... 65, 106 

Laramide structural features ...................... 101 

Summitville ore body ..................... 73, 113 

See also Alteration; Hydrothermal alteration. 

Mining districts ................................. . 113 

La Jara Reservoir-Cumbres Pass system ........ 112 

Pass Creek-Elwood Creek-Platoro 

Latite of Red Mountain ............................ 92 

Lava domes ................................ 70, 73, 93 

Mixed-lava complexes ............................ . 97 

Cornwall Mountain ............................ 97 

system ..................... 110, 113, 115 ring ......................................... 108 evidence for mixing ............................ 98 

Pass Creek zone .............................. 110 Lava flows, analyses ............. , ...... 11, 56, 60, 94 Gardiner River in Yellowstone 

Platoro caldera ring-fracture zone ..... 104, 107, 113 early intermediate-composition ............ 6, 8, 13 National Park ..................... 97, 98 

Platoro zone ................... 107, 110, 112, 114 late basalts and rhyolites ..................... . 90 Handkerchief Mesa ........................... . 97 

regional caldera-related ....................... 110 overlying Treasure Mountain Tuff ............. . 54 North Mountain ......................... 75, 112 
INDEX 127 

Page Platoro caldera-Continued Page Page 

Modal compositions ................................ 3 relation to Conejos volcanoes .............. 12, 104 Sheepshead ................................ 74, 75, 9o 

Mount Hope ..................................... .47 resurgence ................................... 107 Sills .......................................... 87, 90 

Mount Hope caldera ........................ 41, 45, 47 ring-fracture zone ................... 104, 107, 113 Silver Mountain ............................... 69, 70 

age .......................................... .46 similarity to caldera in Japan ................. 117 Snowball Park ................................. 9, 23 

marginal volcanoes ............................ 55 topography before ash-flow Snowshoe Mountain Tuff ..................... 54, 104 

resurgence .................................... 49 eruption ........................... 9, 27 Sorting of phenocrysts during eruption 

Mudflow deposits, Conejos ......................... 13 topography during ash-flow and emplacement .......... 29, 35, 40, 51 

eruption ....................... 22, 27, 45 Sources, Carpenter Ridge Tuff .................... .49 

N,O tumescence .................................. 103 Los Pinos Formation .......................... 69 

volume of collapse ............................ 105 lower tuff of Treasure 

Navajo Peak ................................... 9, 23 wedging of tuffs at topographic walls ....... 27, 30 Mountain Tuff ........................ 23 

Needle Mountains ............................. 47, 49 Platoro fault zone .................. 107, 110, 112, 114 Masonic Park Tuff ..................... 41, 45, 46 

Needle Mountains uplift .......................... 101 Platoro map ....................................... 2 Ojito Creek Member of Treasure 

Nomenclature .. : ................................... 5 Platoro Reservoir area ............................ 107 Mountain Tuff ........................ 33 

North Mountain ...................... 27, 74, 75, 109 Plugs ........................................ 88, 114 Ra Jadero Member of Treasure 

Adams Fork .................................. 87 Mountain Tuff ........................ 36 

Ojito Creek ................................... 33, 89 Ojito Creek ................................... 89 Snowshoe Mountain Tuff ...................... 54 

Ojito Creek Member of Treasure Park Creek ................................... 90 Summitville Andesite ............... 66, 82, 84, 86 

Mountain Tuff ................... 33, 108 Trail Lake .................................... 90 volcanics of Green Ridge ................... 67, 82 

Ore deposits. See Mineralization. Poage Lake ....................................... 98 Wason Park Tuff .............................. 53 

Outcrop distribution ................................ 3 Pollen ............................................ 66 See alsoVents. 

Outflow sheets, relation to intracaldera tuffs .. 25, 27, 53 Porphyry ore deposits ............................. 115 South Mountain ........................... 73, 74, 75 

Precambrian rocks ................................. 8 Spring Creek ...................................... 84 

p Preliminary work ................................... 2 Stocks ......................................... 7, 78 

Previous studies .................................... 2 Alamosa River. See Alam99a River stock. 

Paleotopography, Alamosa River stock area .......... 78 Prospect Mountain ......................... 23, 27, 87 Bear Creek .............................. 84, 111 

Cornwall Mountain resurgent block ............. 66 Prospect Mountain stock ........................... 87 Bear Creek cluster .................. . 84, 111, 114 

Mount Hope caldera area ...................... 49 Pumice fragments, sorting during eruption Canyon Diablo ............................ 66, 86 

Plotoro caldera area ................ 22, 27, 45, 55 emplacement ......................... 51 Cat Creek ..................... 67, 82, 84, 87, 114 

See also Topography. Pyroclastic cones ........................... 71, 75, 96 Cataract Creek ................................ 86 

Park Creek ......................... 65, 71, 73, 77, 90 Cornwall Mountain ............................ 87 

Petrography ....................................... 3 Q,R Crater Creek ............................. 84, 114 

Petrologic change, coincidence Elwood Creek ................................. 84 

with crustal extension ............. 8, 117 Quartz latite of South Mountain .............. 73, 112 Jasper ............................... 84, 109, 114 

Petrologic evolution ........................... 8, 115 

postcollapse activity ........................... 56 

Petrology, ash-flow sheets ........................... 15 

transition to Hinsdale Formation ............... 75 

age ...................................... 74, 113 

associated intrusives ........................ 89, 90 

ore deposits ............................. 113, 115 

Prospect Mountain ............................ 87 

Stratigraphy, volcanic .............................. 6 

Stratovolcanoes. See Volcanoes. 

Structural evolution ............................. . 101 transition to quartz latite of Ra Jadero Member of Treasure Subduction ...................................... 117 South Mountain ...................... 71 Mountain Tuff ................... 36, 108 Summer Coon volcano .................. 9, 12, 69, 116 Phenocrysts, Carpenter Ridge Tuff .................. 51 Radiometric dates .................................. 6 Summit Peak ..................................... 67 determination of composition .................... 5 Alamosa River stock ........................... 78 Summitville Andesite ............................. . 58 

Fish Canyon Tuff ............................ .47 Conejos Formation .............................. 9 age ................................. · .......... 66 La Jara Canyon Member of Treasure 

Mountain Tuff ........................ 28 Hinsdale Formation ....................... 92, 96 

igneous rocks at Summitville caldera .......... 110 

associated intrusives ........................... 90 

lithology ........................... ; ... 62, 71, 84 lower tuff of Treasure 

Mountain Tuff ........................ 23 La Jara Canyon Member of Treasure 

Mountain Tuff ........................ 29 

Plat<iro caldera moat .................... 107, 108 

relation to Alamosa River stock ................ 66 Masonic Park Tuff ........................... .45 Masonic Park Tuff ............................ 47 stratovolcano remnant ......................... 82 Ojito Creek Member of Treasure mineralization at Summitville ................. 113 Summitville caldera .......................... 109 Mountain Tuff ........................ 35 

quartz latitic and rhyolitic intrusives ........... 90 

quartz latite of 

South Mountain ................. 74, 113 

stratigraphic relations at caldera wall .......... 109 

Summitville caldera .................. 33, 36, 103, 108 

Ra Jadero Member of Treasure 

Mountain Tuff ....................... .40 rhyolite of Cropsy Mountain ................... 75 

sanidine porphyry dike ........................ 90 

collapse ..................................... 108 

Hinsdale rhyolites ............................. 93 relation to bulk-rock Ranger Creek .................................... 108 influence on intrusion ......................... 78 composition ................. 5, 19, 29, 40 Rawley Andesite ................................... 8 lava-dome complexes .......................... 70 rhyodacite of Fisher Gulch ..................... 58 ReconnaiBBance mapping ............................ 2 margin .............................. 66, 108, 109 

rhyodacite of Park Creek ....................... 71 Red Mountain . . . . . . . . . . . . . . . . . . . . . . . . 62, 65, 92, 108 postcollapRe volcanism ................... 109, 117 rhyolite of Cropsy Mountain ................... 74 

rhyolite of Park Creek ......................... 75 

Snowshoe Mountain Tuff ...................... 54 

Regional faulting ............................ 110, 112 

Regional tilting ...................... 67, 77, 112, 113 

Rhyodacite of Fisher Gulch .............. . 56, 107, 108 

radiating fracture zones ....................... 113 

ring-fracture zone ............ 35, 84, 108, 113, 115 

topographic walls ................. 66, 71, 108, 109 

sorting during eruption 

and emplacement ...... 29, 35, 40, 51, 104 Rhyodacite of Park Creek .................. 70, 87, 109 

Rhyolite of Cropsy Mountain ................ 74, 89, 90 

Summitville fault ................... 71, 111, 112, 113 

Sunshine Peak Tuff ............... · ................ 97 Summitville Andesite .......................... 62 

Treasure Mountain Tuff ....................... 18 Rio Grande fault zone ............................ 110 

Rio Grande rift depreBBion ................. 7, 113, 117 

Surficial deposits ................................. 101 

tuff of Rock Creek ............................. 16 

volcanics of Green Ridge ................... 69, 70 Rito Gato .................................... 66, 108 

Rock Creek ................................... 16, 23 

T,U 

Wason Park Tuff .............................. 53 Table Mountain ................................... 55 Pinos Creek ....................................... 12 

Plate tectonics .............................. 116, 117 

Platoro caldera .................................. . 103 

ash-flow eruptions ............................ 104 

s 

Saddle Mountain .................................. 55 

San Francisco Creek ............................... 12 

Telluride Conglomerate ............................. 6 

Terminology .................................... 3, 5 

Terrace Reservoir ............................. 23, 27 

Terrace Reservoir laccolith ......................... 82 collapse ................................. 25, 104 

marginal volcanoes ............................ 49 

moat ........................................ 107 

east side ............................... 33, 66 

northeast side ....................... 47, 107 

southeast side ..................... 56, 65, 101:1 

southwest side .................... 62, 66, 107 

postcollapse igneous activity ................... 55 

postresurgence volcanism ................ 107, 117 

premonitory eruptions .................... 23, 104 

San Jose Formation ................................ 6 

San Juan Formation ................................ 8 

San Juan volcanic field ........................ 6, 1.01 

relation to plate tectonics ..................... 116 

San Luis Valley graben system ..................... 77 

Servilleta Formation ............................ 8, 93 

Shards, sorting during eruption 

and emplacement ..................... 51 

Sheep Mountain Andesite ...................... 55, 67 

Sheep Mountain Formation ........................ 55 

Topography, before ash-flow eruption ............ 9, 27 

Conejos ................................. 9, 22, 27 

during ash-flow eruption ................ 22, 27, 45 

Platoro caldera walls .................... 105, 106 

pre-Conejos .................................... 9 

resurgent calderas ............................ 103 

Summitville caldera walls ......... 66, 71, 108, 109 

See also Paleotopography. 

Tracy Creek Quartz Latite .......................... 8 

Treasure Mountain ... : ..................... 16, 23, 29 
128 INDEX 

Page Vents-Continued Page Volume-Continued Page 

Treasure Mountain Rhyolite ................... 16, 41 rhyodacite of Park Creek ....................... 71 Fish Canyon Tuff ............................ .47 

Treasure Mountain Tuff .......................... . 16 rhyolite of Cropsy Mountain ............ 73, 74, 75 Hinsdale basalts ............................... 8 

alteration ..................................... 84 See also Sources. La Jara Canyon Member of Treasure 

associated intrusives ................ 78, 86, 87, 89 Volcanic stratigraphy ............................... 6 Mountain Tuff ................... 30, 105 

chemical composition ..................... 19, 116 Volcaniclastic rocks, Conejos Formation ....... 9, 13, 77 lavas interlayered with ash-flow 

compositional zonation ....................... 116 Los Pinos Formation ......................... . 76 sheets ................................. 7 

La Jara Canyon Member .......... 25, 46, 58, 103 Summitville Andesite ......................... . 65 lower tuff of Treasure 

lower tuff .................................... . 19 Volcanics of Green Ridge ............... 62, 67, 84, 87 Mountain Tuff ........................ 22 

middle tuff .................................. . 30 Volcanics of Table Mountain ....................... 55 main ash-flow sheets of Treasure 

Ojito Creek Member ... 00 •••• 00 •• 00 ••• 00 •• 33, 108 Volcanoes, Adams Fork ........................ 12, 87 Mountain Tuff ........................ 29 

Ra Jadero Member ....................... 36, 108 Alamosa River stock area ...................... 82 Ojito Creek Member of Treasure 

radiometric date .............................. 29 Baughman Creek .............................. 12 Mountain Tuff ....................... 108 

transition to later flows ........................ 56 Cat Creek ......................... 47, 67, 77, 82 Platoro caldera collapse ....................... 105 

upper tuff ................................... .40 circum-Pacific ............................... 117 Ra Jadero Member of Treasure 

Tuff of Rock Creek ............................... . 14 Conejos. 00 00 •• 00 00 00 00 • 00 00 ••• 8, 9, 12, 16, 27, 101 Mountain Tuff ................... 40, 108 

Tuffs. See Ash-flow sheets; Intracaldera tuffs. Los Mogotes shield ........................ 77, 92 Summitville Andesite .......................... 59 

Turkey Creek .................................... .46 Mount Hope caldera margin ................... 55 tuff of Rock Creek. 00 •••• 00 00 ••••••••• 00 ••••••• 16 

Platoro caldera area ........................... 55 

Uncompahgre arch ............................... 101 Platoro caldera margin ....................... .49 

relation to caldera sites .................. 101, 104 w,x 

v Summer Coon ..................... 9, 12, 69, 116 

Summitville Andesite ......................... .45 

Summitville caldera area ...................... 66 Wason Park Tuff . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..... 53 

Vent-dome complexes ....................... 69, 93, 96 Summitville caldera margin ................... 110 West Elk Breccia ................................... 8 

Beaver Creek .............................. 96, 97 Volume, ash-flow sheets ........................ 7, 32 Wightman Fork ........................ 64, 65, 66, 86 

Vents, Conejos Formation ........................... 9 ash-flow sheets as related to Wightman Fork stock .............................. 86 

Hinsdale Formation ....................... 92, 96 caldera collapse ................. 101, 105 Willow Mountain .............................. 13, 27 

mixed-lava complex at North Mountain ........ 112 early intermediate-composition 

quartz latite of South Mountain ............... 112· rocks ............................. 6, 101 Xenocrysts, Hinsdale Formation ................ 92, 98 

1:r U.S. GOVERNMENT PRINTING OFFICE: 1974-677-306/22