Precambrian Melasyenite 

of Ute Creek, X ' J San : -. , Juan

Mountains, Colorado- 11

Chemistry, Petrology, and 

Strontium Isotopes

i

I*

GEOLOGICAL SURVEY .BULLETIN 1311-C

Precambrian Melasyenite 

of Ute Creek, San Juan 

Mountains, ColoradoChemistry, Petrology, and 

Strontium Isotopes

By FRED BARKER, Z. E. PETERMAN, and R. F. MARVIN

CONTRIBUTIONS TO GENERAL GEOLOGY 

GEOLOGICAL SURVEY BULLETIN 1311-C

Hornblende-biotite melasyenite, exposed 

in an erosional window through Tertiary 

and Quaternary rocks, is similar in age 

and composition to rocks near Powderhorn, Colorado

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1970
UNITED STATES DEPARTMENT OF THE INTERIOR 

WALTER J. HIGKEL, Secretary

GEOLOGICAL SURVEY 

William T. Pecora, Director

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

Washington, D.C. 20402 - Price 20 cents (paper cover)
CONTENTS

Page

Abstract_______________________________________________________ Cl

Introduction._____________________________________________________ 2

Areal geology-____________________________________________________ 2

Petrography and analytical data___________-_-_--_________--___--__ 3

Petrology__ _____________________________________________________ 9

References cited.._________________________________________________ 14

ILLUSTRATIONS

Figure 1. Map showing outline of Precambrian melasyenite body of Ute

Creek_____________________._________.___._ C3

2. Alk-F-M diagram__________________________________-.____ 7

3. Ternary plot of normative minerals-________________________ 8

4. Plot of age versus Sr87/Sr 86 ratio____________-____-_--_--_--. 13

TABLES

Table 1. Chemical analyses, C.I.P.W. rock norms, and modal analyses of 

Precambrian melasyenite of Ute Creek, and analyses of augite 

syenite and shonkinite from the Powderhorn area ___________ C5

2. Analytical data and K-Ar ages of biotite from melasyenite of

Ute Creek________________________________. 10

3. Analyses of syenites, shonkinites, and lamprophyres from

Colorado, New Mexico, and Montana____________________ 11

4. Rb-Sr analytical data and calculated values of the ratio Sr 87/Sr8s,

1,400 m.y. ago, for melasyenite of Ute Greek.______________ 13

CONTRIBUTIONS TO GENERAL GEOLOGY

PRECAMBRIAN MELASYENITE OF UTE CREEK, SAN JUAN

MOUNTAINS, COLORADO CHEMISTRY, PETROLOGY,

AND STRONTIUM ISOTOPES

By FRED BARKER, Z. E. PETERMAN, and K. F. MARVIN

ABSTRACT

A stock of hornblende-biotite melasyenite is exposed in an erosional window 

through Tertiary rocks and Quaternary deposits at Ute Creek in the northcentral San Juan Mountains, Colo. This rock contains about 22-49 percent hornblende, 17-25 percent biotite, 17-38 percent inicrocline-naicroperthite, 3-17 percent 

plagioclase, and 3-9 percent quartz. Chemical analyses show approximately 54 

percent Si02, 12 percent ALO3, 7.5 percent FeO+Fe2O3, 7-10 percent MgO, 5-6.5 

percent CaO, 2.3 percent Na2O, and 5 percent K2O. Petrographically, this intrusive 

is an oversaturated melasyenite, but it shows marked chemical kinship with 

potassic shonkinites and lamprophyres. The presence of high trace amounts of 

barium, strontium, and phosphorus supports this affinity. Crystallization of 

quartz from such a hydrous magma is due to early and median crystallization of 

hornblende and biotite which, compared with augite, are deficient in SiO2 and 

cause the remaining liquid to become residually enriched in SiOa. Water content, 

by stabilizing hornblende and biotite, thus is an important controlling factor; if 

this magma had contained less H»O, it would have precipitated augite and would 

have produced a shonkinite.

The Precambrian age of the melasyenite is given by K-Ar apparent ages of 

1,380±50 m.y. (million years) and 1,410±50 m.y. on biotite of two samples. The 

Ute Creek intrusive is similar in age and composition to the potassic augite 

syenite and shonkinite of the Powderhorn area, which lies about 45 miles 

north-northeast.

The ratio Sr^/Sr86 of the melasyenite, corrected for an age of 1,400 m.y., is 

0.7031. This value compares well with a predicted figure of 0.7022 in the mantle 

and also is lower than the ratios (corrected to 1,400 m.y.) found in nearby preexisting crustal rocks. Thus, origin of the melasyenite magma from the mantle, 

with little or no assimilation of crustal material, is not unlikely.

Cl
C2 CONTRIBUTIONS TO GENERAL GEOLOGY

INTRODUCTION

The existence of a body of Precambrian melasyenite at Ute Creek, 

in the north-central San Juan Mountains of Colorado, was first 

pointed out by Cross and Larsen (1935, p. 21, pi. 1). Later, Larsen and 

Cross (1956, p. 29) briefly described this rock. This syenite was revisited by members of the U.S. Geological Survey in 1965 and 1966 

during geologic mapping for the Durango 1:250,000 sheet, being 

prepared under the general direction of T. A. Steven. The senior 

author of the present paper has restudied the Precambrian rocks of 

this area (Barker, 1969). The present paper presents petrographic 

data, major- and minor-element contents, and petrologic interpretations by Barker, Sr isotopic analyses by Peterman, and two K-Ar age 

determinations by Marvin. The radiometric age measurements were 

made to determine whether the melasyenite is of Precambrian age or 

whether it is of Cambrian age and is related to the complex at Iron 

Hill of the Powderhorn area (Larsen, 1942), which lies 41-46 miles 

north-northeast of Ute Creek and which recently was restudied by 

Hedlund and Olson (1968).

AREAL GEOLOGY

There are two exposures of melasyenite in the valley of Ute Creek. 

Both are erosional windows bounded on all sides by Tertiary and 

Quaternary deposits. The larger window is 31^-Si^ miles southwest 

of the mouth of Ute Creek at Rio Grande reservoir (fig. 1). The 

smaller exposure is 3 miles west-southwest and is not discussed in this 

report. Whether or not these two masses are parts of the same stock is 

not known.

The large mass of melasyenite is well exposed and underlies about 

11/2 square miles. The melasyenite is fresh, massive, homogeneous in 

outcrop, and dark pinkish green to pinkish gray. Dikes of fine- to 

medium-grained pink massive microcline alaskite cut the melasyenite. 

The dikes range in thickness from a few inches to about 25 feet. They 

lie at various attitudes and are found throughout the melasyenite. 

They are similar in composition to a stock of alaskite in Rincon La 

Osa, 5 miles south, and to alaskite dikes that cut schist of the Precambrian Uncompahgre Formation, 6 miles south. The latter two 

intrusives probably are related to the nearby batholith of Precambrian 

Eolus Granite (Barker, 1969).
MELASYENITE, SAN JUAN MOUNTAINS, COLORADO C3

1 MILE

FIGURE 1. Precainbrian melasyenite body (patterned area) of TJte Creek described in this report. Tertiary volcanic and Quaternary glacial deposits are 

unpatterned. Sample localities BSJ-136, -138, -139, and -140 are shown. 

Elevations shown are in feet above mean sea level.

PETROGRAPHY AND ANALYTICAL DATA

The melasyenite of Ute Creek consists principally of hornblende, 

biotite, microcline, and oligoclase. The stock shows minor variation 

from place to place in its proportions of minerals, but its texture and 

mineralogy are virtually uniform. The rock is seriate inequigranular, 

and most of the minerals are %-3 mm (millimeters) in maximum 

dimension. Grains of microcline, however, commonly are 5-10 mm in 

size. Hornblende, plagioclase, and biotite occur as anhedra and subhedra, but microcline and quartz are anhedral only. Plagioclase is 

the only mineral to show more than very mild alteration to deuteric 

minerals.

Hornblende occurs typically as single prismatic grains, but aggregates of several 1- to 2-mm grains and of many y10- to %-mm grains 

are scattered throughout the melasyenite. The hornblende is pleochroic
C4 CONTRIBUTIONS TO GENERAL GEOLOGY

with X=pale brown, Y=olive green, and Z=green. Local bleaching 

to very pale green and straw colors has occurred in a few grains per 

thin section. The angle between z and c is about 28°.

Biotite is present as chunky tablets and irregularly shaped grains, 

partly showing ragged edges. Its pleochroism is X=straw, Y=Z= 

olive brown.

Microcline has formed both as %- to 2-mm anhedra, which are 

interstitial to hornblende, biotite, and plagioclase, and as 2- to 10-mm 

blocky anhedra. Grid twinning is well developed, and most of the 

larger grains are carlsbad twinned. Most grains contain 10-30 percent 

by volume of veinlets and patches of albite, and so this mineral actually 

is microperthite. A mild, widespread alteration to sericite and a very 

fine grained brown clay (?) has affected most of the microcline.

Plagioclase occurs as stubby laths and anhedra. The bulk of most 

grains is sodic oligoclase, but rims of albite are common. Grains in 

contact with microcline typically are myrmekitic. Sharply defined to 

faded albite twinning is present in all grains, and carlsbad twinning 

is abundant. The cores of most grains are partly to wholly sericitized, 

and veinlets of sericite extend outward toward the margins of many 

of these grains.

Quartz typically is present as interstitial single and compound 

grains, %-l mm in size, that show mild undulatory extinctions. None 

of the quartz shows any evidence of being xenocrystic.

Equant to blocky elongate grains of graphic granite are a minor 

but important part of the melasyenite. The units of quartz and alkali 

feldspar are typically 0.02-0.03 mm thick.

Colorless clinopyroxene occurs only as residual cores in a few 

of the hornblende grains and probably was one of the first minerals 

to crystallize. This mineral is dusted with very fine grained opaque 

inclusions, and so in aggregate it is pale brown. Several grains show 

(100) twinning, which in one slide extends outward as twin planes 

in the rim of hornblende.

Wedge-shaped euhedra and subhedra of sphene lie in and marginal 

to biotite grains. The sphene is faintly pleochrbic from pale straw 

to pale straw yellow. The grains range in size from about %0 to 

V/2 mm5 but most are in the range ^o-1/^ nun.

Magnetite, in octahedral to anhedral grains %Q-y% mm in size, 

occurs mostly as concentrations in hornblende.

Apatite, in hexagonal prisms %o~l% mm l°ng> is scattered through 

the rock. Zircon is the least abundant accessory mineral.

Four specimens were collected across the exposure of melasyenite 

(fig. 1). Modal and chemical analyses of these are given in table 1.
MELASYENITE, SAN JUAN MOUNTAINS, COLORADO C5

TABLE 1. Chemical analyses, C.I.P.W. rock norms, and modal analyses of Precambrian melasyenite of Ute Creek, and analyses of augite syenite and shonkinite 

from the Powderhorn area

[Localities: BSJ-136, -138, -139, and -140 from northwest of Ute Creek, as shown In flg. 1; augite syenite 

and shonkinite from Wildcat Gulch, 7.6 miles north-northwest of Powderhorn (Hunter, 1925, pi. 1). 

Tr., trace; n.a., not analyzed]

...... BSJ-136 BSJ-138 BSJ-139 BSJ-140 Augite 

syenite

Shonkinite

Chemical analyses 1

Si02 __. _____-___._.

A1203 - -- -----

Fe203 ----- - -----

FeO---___ 

MgO___--. _ -

CaO____-_-_-_ .-

Na2O--___- _ -----

K,O__-_ __-_- 

H20+__-__------

H2O-__-_------_-_-

TiOj- ___-_ -

PaO« ----- 

MnO__ - 

C02 _. ______________

Cl._____. ___.____-

F

Subtotal-. _ .

Total __ -_ .

.__._ 54.25

..___ 12.99

_____ 2.23

.___. 5.96

.__._ 7. 12

..___ 5.56

._ - 2. 34

.___. 5.06

1 22

._.-_ .20

.____ .98

.____ .79

..___ .14

.24

.____ .06

.____ 99.49

.__-_ . 16

. _-_ 99. 33

53. 13 

11.49 

2. 14 

5. 74 

10. 05 

6.55 

2. 06 

4. 83 

1. 24 

. 04 

. 77 

. 66 

. 15 

. 18 

. 08 

. 30

99. 41 

. 15

99. 26

54. 23 

12. 02 

2. 04 

5. 31 

9. 38 

6. 06 

2. 25 

4.90 

1. 28 

. 16 

. 69 

. 60 

. 14 

. 01 

. 07 

. 28

99. 42 

. 14

99.28

54. 93 

13. 16 

2. 03

5.47 

7. 35 

5. 13 

2. 43 

5. 55 

. 94 

. 18 

. 87 

. 69 

. 14 

. 02 

. 05 

. 35

99. 29 

. 16 .

99. 13

54. 99 

12. 98 

3. 13 

3. 92 

5. 50 

5. 67 

2. 83 

7. 08 

. 58 

.41 

. 99 

1. 00 

. 13

n.a. 

n.a.

99. 99

50.86 

11. 14

2.93 

5.21 

11. 26 

6. 97 

1. 73 

5. 85 

.95 

. 64 

. 84 

. 79 

. 13

n.a. 

n.a.

100. 02

C.I.P.W. rock norms

Albite _ ___-_____-.

Halite _ ____ _ _ _.

Magnetite.... _ _ _.

Apatite _ _ _ _ _.

Fluorite __ _ _ _ -

1. 21

. __ 29.90

.-_._ 19.36

.____ 10.23

.-_._ . 10

.____ 3.49

..___ 17.73

..___ 7.74

._-__ 3.23

._ __ 1. 86

. . - 1. 87

. ___ . 65

.__.- .55

60. 80

____ 37. 13

28.54 

16. 84 

8. 15 

. 13 

7.06 

17. 93 

5. 57 

4. 97 

1. 70 

3. 10 

1. 46 

1. 56 

. 56 

.41 

53. 67 

44. 34

28. 95 

18. 52 

8. 50 

. 12 

6.57 

19. 09 

5. 87 

2. 99 

1. 01 

2.96 

1. 31 

1.42 

. 52 

. 02 

56. 09 

41. 77

32. 80 

20. 19 

8. 81 

. 08 

4. 04 

17. 50 

6. 86 

. 56 

. 24 

2. 94 

1. 65 

1. 63 

. 66 

. 04 

61. 88 

36. 15

See footnotes at end of table.

365-570 7C
C6 CONTRIBUTIONS TO GENERAL GEOLOGY

TABLE 1. Chemical analyses, C.I.P.W. rock norms Continued

Sample........................... BSJ-136 BSJ-138 BSJ-139 BSJ-140 Augite Shonkinite

syenite

Modal analyses 2

Quartz. _____

Microcline. _ _ _ _ _ _.

Plagioclase _ _ ____ .

Graphic granite _ _ .

Biotite _ ___ _ _ .

Hornblende. _ _ _ _ .

Pyroxene.

Fe-Ti oxides

Apatite __ _ __.

Sphene __ _________

Zircon __ _ _ ___.

.___ 9

.___ 17

.--- 17

.___ 3.5

.___ 25

.__- 27.5

.__. Tr.

.___ Tr.

.___ Tr.

.___ Tr.

Tr.

3. 5

30.

6. 5

Tr.

21. 5

35. 5

2

1

Tr.

Tr.

Tr.

3

27. 5

3.

1

16. 5

49

Tr.

Tr.

Tr.

Tr.

Tr.

5

38

15

2

17

22. 5

Tr.

Tr.

Tr.

Tr.

Tr.

1 Analysts: BSJ-136, -138, -139, and -140 by V. C. Smith; augite syenite and shonkinite by George Steiger 

(Hunter, 1925, p. 67, 73).

2 All values rounded to the nearest J^ percent.

The modes in table 1 show (1) less than 10 percent quartz in all 

specimens, (2) a dominance of microcline over oligoclase in three of 

the four specimens, and in BSJ-136, these minerals are proportioned 

roughly 1:1, (3) 40 percent dark minerals in BSJ-140 and 53-66 

percent in the other three specimens, and (4) more hornblende than 

biotite in all specimens. Thus, we class specimens BSJ-138, -139, and 

-140 as hornblende-biotite melasyenite, and BSJ-136 as hornblendebiotite melamonzonite. On the basis of proportions of minerals only, 

however, the three specimens that contain more than 50 percent dark 

minerals, if hypabyssal, would be classed as lamprophyre of the 

variety biotite vogesite (Johannsen, 1937, p. 32-39). The usual definition of lamprophyre, however, specifies a fine-grained or fine-grained 

porphyritic texture (Eosenbusch, 1907, p. 653) and "notable idiomorphism" of the ferromagnesian minerals in the groundmass 

(Knopf, 1936, p. 1749); so the term "lamprophyre" is not applicable 

to the Ute Creek rocks.

Notable features of the modal analyses are the sympathetic variations of the amounts of hornblende versus plagioclase and of biotite 

versus microcline. The rock with the most hornblende, BSJ-139, has 

the least biotite. Also, biotite-rich BSJ-136 contains less hornblende 

than BSJ-138 and BSJ-139. Compositional differences, especially of 

MgO, are responsible for these variations in mineral content.

In the four chemical analyses (table 1), the melasyenite shows only 

slight variations in percentages of SiO2 , Fe2O3 , FeO, Na2O, and K2O. 

The silica content of about 54 percent and potash content of about 5 

percent are characteristic. Alumina shows a moderate range, from 

11.49 to 13.16 percent. Magnesia and lime, however, show variations of
MELASYENITE, SAN JUAN MOUNTAINS, COLORADO C7

30-40 percent of the amounts present the former from 7.12 to 10.05 

percent and the latter from 5.13 to 6.55 percent. The central part of 

the body, as now exposed, contains more CaO and MgO than do the 

margins. Specimens BSJ-138 and -139 contain more MgO and CaO, 

as well as less A12O3 and SiO2 , than do BSJ-136 and -140. These 

differences also result in higher amounts of hornblende in the modes 

of BSJ-138 and -139. On an Alk-F-M diagram (fig. 2), the four

ALK 

(Na2 0 + K2 0)

FIGURE 2. Alk-F-M diagram of four analyzed melasyenites of Ute 

Creek, and of augite syenite (AS) and shonkinite (SH) of the Powderhorn area (Hunter, 1925, p. 67, 73), in molecular proportions. Weight 

percentages of CaO in each sample are given in parentheses after the 

sample numbers.

analyses very nearly lie on a line radial to the MgO corner. This 

arrangement, in conjunction with the fact that the most magnesian 

melasyenite also is the most calcareous, means that the four rock compositions are roughly related by a simple gain or loss of MgO and 

CaO. We thus suggest that these variations in CaO and MgO are due 

to differentiation, in the very early stages of crystallization, by loss or 

gain of diopsidic pyroxene and perhaps of olivine also.

Rock norms of the melasyenite are given in table 1. These are especially interesting in that three show normative olivine and one
C8 CONTRIBUTIONS TO GENERAL GEOLOGY

(BSJ-136) is quartz normative. The back-to-back ternary plot (fig. 3) 

of normative feldspars, pyroxene, olivine, and quartz shows that the 

four rocks straddle the line between normative olivine and quartz.

FELDSPAR /__________136* lja________________________\ PYROXENE 

(Or + Ab + An) V »140 7 (Wo + En + Fs)

\ / 

QUARTZ QUARTZ

FIGURE 3. Ternary plot of normative minerals of the melasyenite of TJte Creek 

and of augite syenite (AS) and shonkinite (SH) of the Powderhorn area 

(Hunter, 1925, p. 67, 73). Feldspar and pyroxene are plotted against olivine 

in the upper part of the diagram and against quartz in the lower part of the 

diagram. Abbreviations: Ab, albite; An, anorthite; En, enstatite; Fa, fayalite; 

Fo, forsterite; Fs, ferrosilite; Or, orthoclase; and Wo, wollastonite.

Contents of some trace elements in the melasyenite were determined 

by the semiquantitative spectrographic method by G. W. Sears, Jr. 

These analyses show, in parts per million:

Barium...____________________.___________________ 3, 000-5, 000

Beryllium..________________________________ 1. 5-2

Cobalt. ____________________________________ 20-30

Chromium._________________________________ 500-1, 000

Copper..--.____-----__________-____________ 30-70

Lanthanum_________________________________________________ 70-150

Nickel-_______________- ___.____._____________________- 150-200

Lead.___________________________._________ 30

Vanadium. ___________________.________________ 150-300

Yttrium_____.____________________________________________ 10-50

Zirconium.___________________________________ _ ___________ 70-150
MELASYENITE, SAN JUAN MOUNTAINS, COLORADO C9

The following elements were not detected: Antimony, arsenic, bismuth, cadmium, gold, lead, molybdenum, niobium, palladium, platinum, silver, tantalum, tellurium, tungsten, and zinc. The values for 

chromium are noteworthy. The association of conspicuous amounts 

of cobalt, chromium, copper, and nickel with 70-150 ppm (parts per 

million) of lanthanum is unusual. The melasyenite contains 120-159 

ppm of rubidium and 1,017-1,088 ppm of strontium (table 4).

PETROLOGY

Three aspects of the melasyenite deserve further comment. (1) 

Quartz has formed in all of this rock in spite of the fact that three 

of the four specimens analyzed are olivine normative. (2) The melasyenite not only shows a close compositional affinity to the potassic 

augite syenite and shonkinite of the Powderhorn area but probably 

is also of the same age. (3) The melasyenite also shows compositional 

kinship to other alkalic-mafic igneous rocks, especially those of 

Tertiary age in the Navajo-Hopi country of Arizona and New Mexico, 

in Spanish Peaks, Colo., and in Little Belt Mountains, Mont.

The melasyenite contains about 3-10 percent quartz (table 1), including that in the graphic granite. However, three of the analyzed 

melasyenites have 0.80-6.67 percent olivine in their C.I.P.W. norms. 

Quartz apparently crystallized because the early and median crystallization of so much relatively silica-poor hornblende and biotite resulted in silica enrichment of the last one-third or one-fourth of the 

magma. We estimate that the hornblende in the melasyenite contains 

42-46 percent SiO2 (based on compilation of Deer and others, 1963a, 

p. 275-281), that the biotite probably contains about 37 percent SiO2 

(based on an analysis of biotite in minette by Williams, 1936, p. 166), 

and that the microcline perthite and oligoclase contain roughly 64 

percent of SiO2 (based on analyses reported by Deer and others, 

1963b, p. 38, 112). Augite in this type of magma apparently is more 

siliceous than biotite or hornblende, because Larsen and W. F. Jenks 

(Larsen, 1942, p. 46) found that the augite in a shonkinite near 

Powderhorn contains 49 percent SiO2 . Thus, in the melasyenite magma 

of Ute Creek, the 40-57 percent hornblende and biotite present contains appreciably less SiO2 than the remaining 60-43 percent of the 

rock. Simple calculations (with densities assumed for biotite and 

hornblende) indicate that the liquid remaining after crystallization 

of the mafic minerals contained more than 65 percent SiO2 , and this 

resulted in crystallization of quartz and graphic granite from the last 

fraction of magma. Had the original magma been less hydrous and 

had more than very minor amounts of the more siliceous mineral 

augite formed instead of hornblende, the late liquid would have con-
CIO CONTRIBUTIONS TO GENERAL GEOLOGY

tained less than 65 percent SiO2 , and a nepheline shonkinite would 

have formed. Thus, the water content, and hence the activity of H2O 

in the magma, can exert a strong influence on the course of crystallization in this case, causing the hydrous silica-poor minerals to crystallize first and so producing a silica-oversaturated residual magma from 

which feldspars and quartz crystallized.

The melasyenite of Ute Creek is compositionally similar to augite 

syenite and shonkinite of the Powderhorn area, as shown by the 

analyses in table 1. One petrographic difference, however, is that the 

Powderhorn rocks typically contain the anhydrous mafic minerals 

augite and olivine which almost certainly indicate crystallization 

under less hydrous conditions than the Ute Creek body. Subordinate 

amounts of dark quartz-bearing hornblende syenite and biotite syenite, 

however, do occur in the Powderhorn region (Hunter, 1925, p. 70-71), 

and their brief petrographic descriptions indicate that they are very 

similar to the melasyenite of Ute Creek. Gravitationally driven migration of H2O toward the upper regions of these intrusives (Kennedy, 

1955), while these magmas were largely liquid, may have caused 

these variations in H2O content. Larsen and Cross (1956, p. 37-38) 

pointed out that the alkalic-mafic rocks of the Powderhorn area are 

potassic, in distinction to the sodic, ultramafic, and mafic rocks of the 

nearby complex at Iron Hill (Larsen, 1942). The potassic syenites are 

cut by dikes of pegmatite and aplite, and Larsen and Cross considered 

them to be Precambrian. The Iron Hill rocks, in contrast, are not 

cut by such dikes.

TABLE 2. Analytical data and K-Ar ages of biotite from melasyenite of Ute Creek

[Decay constants K*>: X0=4.72X10-1° per yr; X«=0.584X1<H0 per yr. Abundance: K«=1.22X1<H g/g K. 

Analysts: R. F. Marvin, H. H. Mehnert, and Violet Merritt]

Sample

BSJ-139 2____ 

BSJ-140 2 _ __

KsQi Radiogenic Percentage Apparent 

(weight Ar" (moles Radiogenic radiogenic age (m.y.) 

percent) per gram) Ar*0/K« AH0 ±2<r

9.64;9.60 2.979X10-8 0.122 

(9.62 avg) 

9.01; 8.81; 2.717X10-8 .119 

9.08; 9.13 

(9.01 avg)

99 1, 410±50 

99 1, 380±50

1 Potassium determinations made by flame photometer with lithium internal standard.

2 Melasyenite samples collected at approximately lat 37°42'30" N., long 107°24'40" W., in Hinsdale County;

We have determined the K-Ar ages of biotite from BSJ-139 and 

BSJ-140 as I,410dt50 m.y. and 1,380 ±50 m.y., respectively, using the 

general techniques of Evernden and Curtis (1965). Our analytical 

data are given in table 2. These two determinations fix a younger limit 

of about 1,400 m.y. on the age of the melasyenite. Furthermore, we
MELASYENITE, SAN JUAN MOUNTAINS, COLORADO Gil

suggest that the melasyenite is 1,400-1,460 m.y. old because of the 

position of the l,460-m.y.-old (Silver and Barker, 1968; Bickford 

and others, 1969) Eolus batholith less than 4y2 miles south (Larsen 

and Cross, 1956, pi. 1), and because biotite from a sample of the Eolus 

collected 12 miles south of the melasyenite gave an apparent K-Ar 

age of 1,390±50 m.y. (E. E. Zartman and E. F. Marvin, unpub. data). 

Kadiometric ages of the Powderhorn rocks, obtained by the K-Ar 

method on biotite, indicate that the augite syenite and shonkinite are 

about 1,400 m.y. old and also that the complex at Iron Hill is of 

Cambrian age (J. C. Olson and E. F. Marvin, unpub. data). Thus, 

there is a real possibility that these Precambrian potassic-mafic intrusives of the Powderhorn area were comagmatic with the melasyenite 

of Ute Creek.

TABLE 3. Analyses of syenites, shonkinites, and lamprophyres from Colorado, 

New Mexico, and Montana

Ute Creek, Colo. Shiprock, 

N. Mex.i

Spanish Peaks, Colo.2

Melasyenite Biotite 

(table 1) lamprophyre

Si02 ________

A12 03 _______

Fe2O3 _______

FeO________

MgO__ ____

CaO________

Na2O_

K20__ ______

_____ 53.13-54.93

11.49-13.16

_ __ 2. 03-2. 23

_____ 5.31-5.96

_____ 7. 12-10. 05

5. 13-6. 55

_ _ _ 2. 06-2. 43

_____ 4. 83-5. 55

51. 80 

11. 10 

3. 55 

3.42 

8. 15 

7.95 

2. 25 

5.97

Shonkinite Augite Biotite-augite 

lamprophyre lamprophyre

52. 90 

10. 20 

3. 61 

4.44 

9. 06 

6.91 

2. 20 

6. 36

47. 63 

12. 01 

4. 20 

4.99 

8. 31 

7. 28 

1. 98 

5.40

46.37 

11. 98 

5.05 

5. 16 

8. 38 

9.33 

2. 84 

4.34

Little Belt Mountains, Mont. 3

Shonkinite Biotite Melasyenite Augite-hornblende 

lamprophyre (yogoite) syenite

Si02 __________

A12O3 _________

Fe2 03 ______-_

FeO____.__ _

MgO_____ ___

CaO_________-

Na2O__ ______

K2 0______-._

48.49

12. 29

2.88

5. 77

9.91

9.65

2.22

4.96

52. 26 

13. 96 

2. 76 

4.45 

8. 21 

7. 06 

2. 80 

3. 87

54.42 

14.28 

3. 32 

4. 13 

6. 12 

7. 72 

3.44 

4. 22

61. 65 

15. 07 

2. 03 

2. 25 

3. 67 

4. 61 

4.35 

4.50

1 Williams (1936, p. 166); analysis by Prank Herdsman.

2 Knopf (1936, p. 1780-1781); analyses by George Steiger and R. B. Ellestad. 

s Weed and Pirsson (1895, p. 478); analyses by W. F. Hillebrand.

The compositional similarity of the Ute Creek intrusive to some 

of the Tertiary lamprophyres, shonkinites, and dark syenites of the
C12 CONTRIBUTIONS TO GENERAL GEOLOGY

Rocky Mountain area can be seen in table 3. This similarity is sustained by analyses of one shonkinite and three lamprophyres of vogesitic type from the Barker area of the Little Belt Mountains (I. J. Witkind, unpub. data). The melasyenite also contains relatively large 

amounts of Sr, Ba, and P2O5 , and thus shows another compositional 

similarity with lamprophyres (Knopf, 1936, p. 1781; Turner and 

Verhoogen, 1960, p. 255). These likenesses almost certainly indicate 

a similar mode of origin, despite the obvious differences of geologic 

environment and age.

In their summary discussion of potash-rich basic volcanic rocks 

and lamprophyres, Turner and Verhoogen (1960, p. 235-256) pointed 

out that lamprophyres commonly are associated with granitic rocks. 

The melasyenite of Ute Creek shows this association in that it probably was emplaced at the same time as or shortly after the Eolus batholith and in turn was intruded by alaskite dikes. However, the Ute Creek 

body does not exhibit the otherwise common occurrence of granite or 

quartz xenoliths in biotitic lamprophyres and potassic trachybasalts 

(Williams, 1936, p. 159-161; Turner and Verhoogen, 1960, p. 256).

The genesis of the melasyenite of Ute Creek thus is tied to the much 

larger problem of the origin of magmas that are potassic and mafic 

and generally have low to medium silica contents. Hypotheses of 

origin by direct injection of a partially melted mantle rock, by differentiation of mantle-derived magma, by assimilation of potassic crustal 

rock by mantle-derived magma, or by some other means cannot be 

properly tested by work on one rock type of such a small areal extent 

as that at Ute Creek. Nevertheless, a general knowledge of the Sr87/ 

Sr86 ratios both of the preexisting rocks of the Needle Mountains uplift 

immediately to the south and southwest (Bickford and others, 1969) 

and of the melasyenite appears to be of potential help in separating a 

strictly mantle origin from one involving crustal contamination of a 

mantle-derived magma.

Table 4 presents Sr and Kb data for the melasyenite. We calculated 

initial Sr87/Sr86 ratios from the analytical values of Sr87/Sr86 as corrected for the accumulation of radiogenic Sr87, produced by the radioactive decay of Kb87 in 1,400 m.y., and from using the measured abundances of Kb. A mean value of 0.7031 for the four samples may be 

compared with the predicted ratio of Sr87/Sr86 in the mantle 1,400 

m.y. ago and with the 1,400-m.y. Sr87/Sr86 ratios of nearby crustal 

rocks.
MELASYENITE, SAN JUAN MOUNTAINS, COLORADO C13

TABLE 4. Rb-Sr analytical data and calculated values of the ratio Sr87/Sr86, 1,400

m.y. ago, for melasyenite of Ute Creek 

[Rb and Sr values by X-ray fluorescence method. Sr standards: USGS Shelf, 0.7100; MIT Shelf, 0.7080]

Sample (BSJ- )

136__ __ .... ___

138. ____ ._

139... ____ .._...

Mean value .

Rb

(ppm)

_ 135

_ 120

_ 125

_ 159

Sr 

(ppm)

1,039 

1,085 

1, 088 

1,017

Rb/Sr

0. 130 

. Ill 

. 115 

. 156

Rb»/Sr"

0. 377 

. 322 

. 333 

. 542

(Present) (1,400 m.y . ago »)

0. 7109 0 

.7094 

. 7094 

.7118

7034 

7031 

7029 

7029 

7031

i Calculated using Rb9': X0 = 1.39X10-» yr-'.

In figure 4, the isotopic data are illustrated on a strontium evolution 

diagram. The variation of Sr87/Sr86 in the mantle is assumed to increase linearly from a value for achondritic meteorites of 4.55 b.y.

0.694

3000 2000 1000

AGE, IN MILLION YEARS

FIGURE 4. Plot of age versus Sr^/Sr88 ratio, showing model curve and approximate limits of Sr^/Sr86 in the mantle, and age (Sr87/Sr88 ) relations of samples 

BSJ-136, -138, -139, and -140.

(billion years) ago to a present mean Sr87/Sr86 value for oceanic 

basalts (Hedge, 1966). The upper and lower limits of this curve are 

based on the range of values for oceanic basalts.

Data for the melasyenites project back to a mean Sr87/Sr86 value 

of 0.7031 at 1,400 m.y., which is within the predicted range (0.7010- 

0.7035) for mantle-derived rocks of this age. Thus, the value of 0.7031 

for the melasyenites is compatible with a mantle-derivation for these 

rocks.
C14 CONTRIBUTIONS TO GENERAL GEOLOGY

The immediate wallrocks of the melasyenite probably were quartzite 

and schist of the Uncompahgre Formation (Larsen and Cross, 1956, 

pi. 1). This formation, however, forms a synclinorium of no great 

depth (Fred Barker, unpub. data) and is underlain by a complex of 

amphibolite, quartzo-feldspathic gneiss, and granitic rocks that are 

about 1,700-1,800 m.y. old. The Eolus batholith intruded both the 

Uncompahgre Formation and the underlying complex 1,460 m.y. ago 

(Silver and Barker, 1968; Bickford and others, 1969) and is the 

youngest rock known to have been in this area when the melasyenite 

was intruded. Of these rocks, the one with the lowest Sr87/Sr86 ratio 

1,400 m.y. ago is the amphibolite found in the Twilight Gneiss (Fred 

Barker, Z. E. Peterman, and E. A. Hildreth, unpub. data), whose 

value was about 0.7037. The other rocks had values 1,400 m.y. ago, as 

calculated from data given by Bickford, Wetherill, Barker, and 

Lee-Hu (1969), ranging from 0.7060 for Eolus Granite to 0.7310 for 

the l,720-m.y.-old granite at Whitehead Gulch, which lies 4 miles 

south of Silverton and 13 miles west of the Ute Creek deposit. The 

high strontium content of the melasyenite would render the Sr87/Sr86 

values rather insensitive to small amounts of contamination by granitic 

rocks of the area. However, the lack of petrographic evidence indicating xenocrystic or xenolithic material in the syenites and the low 

Sr87/i3r86 initial values indicate that a crustal component need not be 

invoked as a significant feature in the petrogenetic history of these 

alkalic rocks. This interpretation is in accord with strontium isotopic 

data on carbonatites and on related alkalic rocks from other areas 

(Powell and others, 1966).

Barker, Fred, 1969, Precambrian geology of the Needle Mountains, southwestern

Colorado: U.S. Geol. Survey Prof. Paper 644-A, 35 p. 

Bickford, M. E., Wetherill, G. W., Barker, Fred, and Lee-Hu, Chin-nan, 1969,

Precambrian Rb-Sr chronology in the Needle Mountains, southwestern

Colorado: Jour. Geophys. Research, v. 74, p. 1660-1676. 

Cross, Whitman, and Larsen, E. S., Jr., 1935, A brief review of the geology

of the San Juan region of southwestern Colorado: U.S^ Geol. Survey Bull.

843,138 p. 

Deer, W. A., Howie, R. A., and Zussman, Jack, 1963a, Chain silicates, v. 2 of

Rock-forming minerals: New York, John Wiley & Sons, Inc., 379 p. 

 1963b, Framework silicates, v. 4 of Rock-forming minerals: New York,

John Wiley & Sons, Inc., 435 p. 

Evernden, J. F., and Curtis, G. H., 1965, The potassium-argon dating of Late

Cenozoic rocks in East Africa and Italy: Current Anthropology, v. 6, p.

343-385. 

Hedge, C. E., 1966, Variations in radiogenic strontium found in volcanic rocks:

Jour. Geophys. Research, v. 71, no. 24, p. 6119-6120.
MELASYENITE, SAN JUAN MOUNTAINS, COLORADO C15

Hedlund, D.C., and Olson, J. C., 1968, Geologic map of the complex of alkalic

rocks at Iron Hill, Gunnison County, Colorado: U.S. Geol. Survey open-file

map. 

Hunter, J. F., 1925, Pre-Cambrian rocks of Gunnison River, Colorado: U.S. Geol.

Survey Bull. 777,94 p. 

Johannsen, Albert, 1937, The intermediate rocks, v. 3 of A descriptive petrography

of the igneous rocks: Chicago, 111., Univ. Chicago Press, 360 p. 

Kennedy, G. C., 1955, Some aspects of the role of water in rock melts, in Poldervaart, Arie, ed., Crust of the earth a symposium: Geol. Soc. America Spec.

Paper 62, p. 489-503. 

Knopf, Adolph, 1936, Igneous geology of the Spanish Peaks region, Colorado:

Geol. Soc. America Bull., v. 47, no. 11, p. 1727-1784. 

Larsen, E. S., Jr., 1942, Alkalic rocks of Iron Hill, Gunnison County, Colorado:

U.S. Geol. Survey Prof. Paper 197-A, G4 p. 

Larsen, E. S., Jr., and Cross, Whitman, 1956, Geology and petrology of the San

Juan region, southwestern Colorado: U.S. Geol. Survey Prof. Paper 258,

303 p. 

Powell, J. L., Hurley, P. M., and Fairbairn, H. W., 1966, The strontium isotopic

composition and origin of carbonatites, in Tuttle, O. F., and Gittins, John,

eds., Carbonatites: New York, John Wiley & Sons, Inc., p. 365-378. 

Rosenbusch, Harry, 1907, Mikroskopische Physiographic der massigen Gesteine:

E. Schweizerbartsche Verlagshandlung, Stuttgart, v. 2, pt. 1, 716 p. 

Silver, L. T., and Barker, Fred, 1968, U-Pb zircon results, pt. 1 of Geochronology

of Precanibrian rocks of the Needle Mountains, southwestern Colorado, in

Abstracts for 1967: Geol. Soc. America Spec. Paper 115, p. 204-205. 

Turner, F. J., and Verhoogen, John, 1960, Igneous and metamorphic petrology

[2d ed.] : New York, McGraw-Hill Book Co., 694 p. 

Weed, W. H., and Pirsson, L. V., 1895, Igneous rocks of Yogo Peak, Montana : Am.

Jour. Sci., 3d ser., v. L, no. 300, p. 467-479. 

Williams, Howel, 1936, Pliocene volcanoes of the Navajo-Hopi country: Geol.

Soc. America Bull., v. 47, p. 111-172.

O