U.S. Department of the Interior

U.S. Geological Survey

Professional Paper 1752

Geology of the Northern Part of the Harcuvar Complex, 

West-Central Arizona
Geology of the Northern Part of the 

Harcuvar Complex, West-Central Arizona

By Bruce Bryant and J.L. Wooden

Professional Paper 1752

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

DIRK KEMPTHORNE, Secretary

U.S. Geological Survey

Mark D. Myers, Director

U.S. Geological Survey, Reston, Virginia: 2008

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Suggested citation:

Bryant, Bruce, and Wooden, J.L., 2008, Geology of the Northern Part of the Harcuvar Complex, West-Central Arizona: 

U.S. Geological Survey Professional Paper 1752, 52 p.
iii

Contents

Abstract...........................................................................................................................................................1

Introduction.....................................................................................................................................................1

Previous Work .......................................................................................................................................3

Present Work.........................................................................................................................................5

Geology............................................................................................................................................................5

Rock Units ..............................................................................................................................................6

Proterozoic and Cretaceous Layered Gneiss .........................................................................6

Proterozoic Granodiorite to Granite Gneiss ............................................................................7

Proterozoic Porphyritic Granodiorite to Granite Gneiss .......................................................8

Paleozoic and Mesozoic Metasedimentary Rocks................................................................8

Jurassic Metagabbro, Metadiorite, and Amphibolite ........................................................10

Late Cretaceous Granite of Tank Pass ...................................................................................11

Oligocene(?) and Miocene Swansea Plutonic Suite ...........................................................12

Rocks Probably Correlative with the Swansea Plutonic Suite ..........................................15

Geochronology.............................................................................................................................................15

Proterozoic Granitic Rocks ...............................................................................................................15

Jurassic Plutonic Rocks ....................................................................................................................15

Rocks Migmatized in the Cretaceous..............................................................................................16

Swansea Plutonic Suite.....................................................................................................................19

Whole-Rock and Feldspar Pb-Isotopic Compositions..................................................................19

Geochemistry................................................................................................................................................19

Analytical Methods.............................................................................................................................19

Proterozoic Granitic Rocks ...............................................................................................................21

Jurassic Plutonic Rocks ....................................................................................................................23

Cretaceous Plutonic Rocks...............................................................................................................23

Swansea Plutonic Suite.....................................................................................................................23

Comparison with Poachie Crust and Possible Origin ..........................................................31

Relation Between Magmatism, Extension, and Core Complex Formation.......................32

Structure........................................................................................................................................................32

Faults. ..................................................................................................................................................32

Major Folds ..........................................................................................................................................34

Dikes. ..................................................................................................................................................34

Lineation...............................................................................................................................................37

Foliation ................................................................................................................................................37

Minor Folds ..........................................................................................................................................41

Older Foliation......................................................................................................................................42

Summary of Structural and Metamorphic Events..................................................................................43

Acknowledgments.......................................................................................................................................44

References Cited..........................................................................................................................................44

Appendix. Location and Description of Analyzed Samples..................................................................49
iv

Figures

1. Map showing location of northern Harcuvar complex and core complexes of 

 west-central Arizona and adjacent part of California.........................................................2

2. Geologic map of the northern Harcuvar complex showing locations of 

 analyzed and(or) dated rocks ..................................................................................................4

3–12. Photographs of:

 3. Migmatitic layered gneiss and granite in the eastern Harcuvar Mountains ..............7

 4. Layered mylonite from layered gneiss in the eastern Buckskin Mountains ...............7

 5. Mylonitic granodiorite gneiss from the eastern Buckskin Mountains.........................8

 6. Mylonitic coarse-grained granite gneiss from the central Buckskin Mountains ......9

 7. Thin layers of marble in migmatitic layered gneiss in the eastern 

 Buckskin Mountains .......................................................................................................10

 8. Mylonitic biotite hornblende gneiss derived from gabbro containing 

 trondhjemite sills that have been deformed into “fish” .............................................11

 9. Mylonitic biotite granite in the eastern Buckskin Mountains correlated 

 with the Late Cretaceous granite of Tank Pass in the Harcuvar Mountains..........12

 10. Mylonitic biotite granodiorite from the felsic phase of the Swansea 

 Plutonic Suite ....................................................................................................................13

 11. Mylonitic porphyritic granite or monzogranite from inclusion in felsic 

 phase of the Swansea Plutonic Suite ...........................................................................13

 12. Small intrusion of granodiorite of the felsic phase of the Swansea Plutonic 

 Suite into layered migmatitic gneiss ............................................................................14

13. U-Pb zircon concordia diagram for mylonitic granitic gneiss and mylonitic 

 porphyritic granite and porphyritic granodiorite inclusion in felsic rocks 

 of the Swansea Plutonic Suite ..............................................................................................18

14. U-Pb zircon concordia diagram for sample B–566 of mylonitic horblende diorite 

 from the eastern Buckskin Mountains. MSWD, mean standard weighted 

 deviation....................................................................................................................................18

15. U-Pb zircon concordia diagram for sample HA–118 from a migmatitic-looking 

 granite layer in layered gneiss in the eastern Harcuvar Mountains ..............................18

16. U-Pb concordia diagrams for gabbro and granite of the Swansea Plutonic 

 Suite and combined with inclusions in the suite................................................................20

17. Whole-rock and feldspar lead (Pb) isotopic compositions of some rocks from 

 the northern Harcuvar complex and Mesozoic and Tertiary plutonic rocks in the 

 Whipple Mountains .................................................................................................................22

18. Chemical diagrams for rocks of the northern Harcuvar complex......................................24

19. More chemical diagrams for rocks of the northern Harcuvar complex ...........................26

20. Chrondite-normalized rare-earth element contents of igneous rocks in the 

 northern Harcuvar complex...................................................................................................27

21. Northeast-southwest crustal section through the Buckskin Mountains based on 

 geophysical and geologic data .............................................................................................33
v

22. Areas of structural analyses in the northern Harcuvar complex.......................................33

23. Structures in the Buckskin Mountains formed before, during, and after 

 mylonitization in the Miocene................................................................................................35

24. Photographs showing low-angle brittle faults in the eastern Buckskin Mountains.......36

25. Photographs showing features in mylonitized rock..............................................................38

26. Structures in the Buckskin Mountains synchronous with mylonitization.........................39

27. Structures in the Little Buckskin Mountains formed during and after 

mylonitization during the Miocene ..........................................................................................40

28. Structures in the eastern Harcuvar Mountains formed before and during 

 mylonitization............................................................................................................................40

29. Photograph showing hinge of northeast-trending fold formed by mylonitic 

 granite gneiss............................................................................................................................42

30. Photograph showing minor tight folds in mylonitic biotite-hornblende gneiss ..............42

Tables

1. U-Pb ages of rocks in the northern Harcuvar complex........................................................16

2. Analytical data and ages of zircons from the northern part of the Harcuvar 

 complex in the lower plate of the Rawhide detachment fault in the Buckskin 

 and eastern Harcuvar Mountains.........................................................................................17

3. Whole-rock and feldspar (F) Pb-isotopic compositions of rocks from the 

 northern Harcuvar complex and from Cretaceous and Tertiary plutonic 

 rocks from the Whipple Mountains ......................................................................................21

4. Major-oxide and trace-element concentrations in some Proterozoic 

 granitic rocks and inclusions in the Swansea Plutonic Suite..........................................25

5. Major- and trace-element concentrations in some Jurassic and Cretaceous 

 plutonic rocks from the northern part of the Harcuvar complex....................................28

6. Major-oxide and trace-element concentrations in rocks of the 

 Swansea Plutonic Suite..........................................................................................................30
Abstract

In west-central Arizona near the northeast margin of the 

Basin and Range Province, the Rawhide detachment fault 

separates Tertiary and older rocks lacking significant effects 

of Tertiary metamorphism from Precambrian, Paleozoic, and 

Mesozoic rocks in the Harcuvar metamorphic core complex 

below. Much of the northern part of the Harcuvar complex 

in the Buckskin and eastern Harcuvar Mountains is layered 

granitic gneiss, biotite gneiss, amphibolite, and minor pelitic 

schist that was probably deformed and metamorphosed in 

Early Proterozoic time. In the eastern Buckskin Mountains, 

Early and Middle Proterozoic plutons having U-Pb zircon ages 

of 1,683±6.4 mega-annum (Ma) and 1,388±2.3 Ma, respectively, intruded the layered gneiss. Small plutons of alkaline 

gabbro and diorite intruded in Late Jurassic time. A sample 

of mylonitized diorite from this unit has a U-Pb zircon age 

of 149±2.8 Ma. In the Early Cretaceous, amphibolite facies 

regional metamorphism was accompanied by partial melting and formation of migmatite. Zircon from a granitic layer 

in migmatitic gneiss in the eastern Harcuvar Mountains has 

a U-Pb age of 110±3.7 Ma. In the Late Cretaceous, sills and 

plutons of the granite of Tank Pass were emplaced in both the 

Buckskin and eastern Harcuvar Mountains. In the Buckskin 

Mountains those intrusions are locally numerous enough to 

form an injection migmatite. A pluton of this granite crops out 

over almost half the area of the eastern Harcuvar Mountains.

Paleozoic and Mesozoic sedimentary rocks were caught 

as slices along south-vergent Cretaceous thrusts related to the 

Maria fold and thrust belt and were metamorphosed beneath a 

thick sheet of Proterozoic crustal rocks. 

Inception of volcanism and basin formation in upperplate rocks indicates that regional extension started at about 

26 Ma, in late Oligocene. The Swansea Plutonic Suite, 

composed of rocks ranging from gabbro to granite, intruded 

the lower-plate rocks in the Miocene and Oligocene(?). 

Granite and a gabbro from the suite have a U-Pb zircon age of 

21.86±0.60 Ma. Previously published 40Ar/39Ar ages of hornblende suggest that some of the Swansea Suite is Oligocene. 

The felsic rocks contain numerous inclusions ranging from 

porphyritic granite to porphyritic granodiorite. A sample from 

one inclusion has a U-Pb zircon age of 1,409±6.3 Ma. A discordia line for the U-Pb zircon data from the Swansea Plutonic 

Suite has an upper intercept at 1,408±3.4 Ma. The Swansea 

Plutonic Suite probably formed by interaction between mantle 

material and plutonic rocks at least as old as Middle Proterozoic. An irregular layer in the middle crust, which is thickest 

under and adjacent to the Buckskin Mountains, may be the 

level where that interaction took place.

During extensional deformation these rocks and all the 

older rocks were displaced southwest from beneath the rocks 

of the Colorado Plateau transition zone below an area extending 50–80 kilometers northeast of the Buckskin Mountains 

as far as Bagdad, Arizona, or beyond. At that time the rocks 

were variably mylonitized, and a northeast-trending lineation 

formed. Much of the evidence for the complex sequence of 

structural events preserved in these rocks in the western Harcuvar Mountains has been obliterated in the northern Harcuvar 

complex by Miocene deformation.

Introduction

A belt of metamorphic core complexes crops out in 

southeastern California and west-central Arizona in the Basin 

and Range Province southwest of the margin of the 

Colorado Plateau (fig. 1). The complexes consist of crystalline 

rock (commonly mylonitized) below a low-angle detachment 

fault. Overlying the detachment fault are unmetamorphosed 

Tertiary sedimentary and volcanic rocks, metamorphosed 

Mesozoic and Paleozoic sedimentary and volcanic rocks, and 

Proterozoic and younger plutonic and metamorphic rocks 

lacking pervasive mylonitic structures. Crystalline rocks below 

detachment faults in the Rawhide, Buckskin, Little Buckskin, Harcuvar, Harquahala, Little Harquahala, and Granite 

Wash Mountains have been informally named the Harcuvar 

metamorphic core complex (Rehrig and Reynolds, 1980), or 

simply Harcuvar complex, because in some areas it contains 

unmetamorphosed igneous rocks. The area described in this 

report, the northern part of the Harcuvar complex, includes the 

lower-plate rocks in the Buckskin, Little Buckskin, and eastern 

Harcuvar Mountains. This area is about 180 km northwest of 

Phoenix, Ariz., and 60 km southwest of Bagdad, Ariz. (fig. 1). 

Because only the valley of the Bill Williams River separates 

the Rawhide from the Buckskin Mountains and the Rawhide 

Geology of the Northern Part of the Harcuvar Complex, 

West-Central Arizona

By Bruce Bryant and J.L. Wooden
2 Geology of the Northern Part of the Harcuvar Complex, West-Central Arizona

Figure 1. Location of northern Harcuvar complex and core complexes of west-central Arizona 

and adjacent part of California. Area of figure 2 outlined.

100 KILOMETERS

Las Vegas

Phoenix

Bagdad

NV AZ

CA

Colorado

Plateau Basin and Range 

Transition 

Zone

33°

35°

116° 114° 112°

Core complexes

Colorado River

x

x

x

x

McCoy basin

North border of Maria

 fold and thrust belt

Eldorado

Mtns

EXPLANATION

x

Dead Mtns

Sacramento Mtns

Chemehuevi Mtns

Whipple Mtns

Harcuvar Mtns

Little Buckskin Mtns

Buckskin and 

Rawhide Mtns

Mesquite Mtns

Moon Mtns

Granite Wash Mtns

Little Harquahala Mtns

Harquahala Mtns

White Tank 

Mtns

South Mtn

Bouse

Hills

Poachie Range
Introduction 3

Mountains occupy only about 20 percent of the core complex 

in those two ranges, we here include the Rawhide Mountains 

with the Buckskin Mountains for the sake of brevity. The 

rocks below the surrounding detachment fault are part of the 

northern Harcuvar complex. In the Buckskin Mountains, that 

complex is about 40 km long in a northeast-southwest direction, and 20 km wide, and is exposed over about 800 km2

. 

Upper-plate rocks within the Buckskin Mountains occur in 

two synforms bounded by the Rawhide detachment fault 

that divide the complex into three antiforms, each cored by 

lower-plate metamorphic and igneous rocks. From northwest 

to southeast, they are called the Planet Peak, Clara Peak, and 

Ives Peak antiforms (fig. 2) (Rehrig and Reynolds, 1980; 

Spencer and Reynolds, 1989a). The folds and the detachment 

fault are cut by northwest-trending normal and reverse faults 

(Spencer and Reynolds, 1989a). Also included in this study 

are the rocks of the Harcuvar complex forming the eastern part 

of the Harcuvar Mountains. Southwest movement of the lower 

plate composed of the complex relative to the upper plate has 

been estimated as more than 50 km (Reynolds and Spencer, 

1985), 75±20 km (Spencer and Reynolds, 1989b), and 66±8 

km (Spencer and Reynolds, 1991). That distance and direction 

of separation places the rocks now exposed in the Buckskin 

Mountains beneath the Colorado Plateau transition zone in an 

area extending to 20 km northeast of Bagdad, Ariz. Seismic 

reflection data indicate that the detachment fault in the subsurface extends from its outcrop at the northeast margin of the 

Buckskin Mountains to the Colorado Plateau transition zone 

where it soles into a midcrustal reflection horizon at a depth 

of 18 km about 35 km northeast of the Buckskin Mountains 

(Clayton and Okaya, 1991), which is approximately below 

U.S. Highway 93. If this interpretation is correct, the rocks 

now exposed beneath the detachment fault in the eastern part 

of the Buckskin Mountains were at those depths when extension started.

The northern Harcuvar complex lies north of the Late 

Cretaceous Maria fold and thrust belt, an east-west-trending 

belt in which Proterozoic basement rocks moved southward 

over Paleozoic and Mesozoic rocks, including the Upper 

Jurassic to Upper Cretaceous (Barth and others, 2004) McCoy 

Mountains Formation (Reynolds and others, 1986; Spencer 

and Reynolds, 1990a). The Phanerozoic rocks were buried by 

the thrust sheet(s) deeply enough so that a greenschist facies 

regional metamorphic mineral assemblage formed in them 

during or after the thrusting.

Mylonitic lower-plate rocks similar to those in the 

Buckskin Mountains are exposed in the eastern part of the 

Harcuvar Mountains. These rocks are included in the northern 

Harcuvar complex. Rocks of the northern Harcuvar complex 

are also exposed in the Little Buckskin Mountain, a small 

antiform between the eastern parts of the Buckskin and 

Harcuvar Mountains.

Detachment faults bounding the lower-plate rocks in the 

Rawhide, Buckskin, and Harcuvar Mountains have been called 

the Rawhide, Buckskin, and Bullard faults, respectively. The 

Whipple Mountains detachment fault, which separates 

lower-plate rocks from upper-plate rocks in the Whipple 

Mountains 25 km west-northwest of the west end of the 

Buckskin Mountains, has been extended in cross sections 

to 8 km north-northwest of the detachment fault bounding 

the lower plate rocks in the Buckskin Mountains (Howard 

and others, 2000). Because the faults bounding these core 

complexes are parts of a single detachment, following the 

interpretations of Spencer and Reynolds (1991) and Reynolds 

and Spencer (1985), we apply the name Rawhide to this fault 

because it is better exposed in the Rawhide Mountains than in 

the other ranges in Arizona.

Previous Work

Lower-plate rocks in the northern Harcuvar complex 

have received little study. Brief descriptions were published 

of rocks from some mining districts (Bancroft, 1911). Reconnaissance geologic maps of Yuma and Mohave Counties show 

units of older Precambrian granite gneiss and schist and Mesozoic sedimentary rocks in the area of the northern Harcuvar 

complex. An area of Mesozoic sedimentary rock was mapped 

in the southwestern corner of the Buckskin Mountains. Overlying Paleozoic, Mesozoic, and Tertiary sedimentary rocks of 

the upper plate were mapped as partly in thrust fault contact 

and partly in depositional contact with the underlying rocks of 

the lower plate (Wilson, 1960; Wilson and Moore, 1959).

In a pioneering study of detachment faults, Shackelford 

(1976; 1989) mapped the lower-plate rocks in the Rawhide 

Mountains. He mapped most of the rocks as mylonitic gneiss, 

which he characterized as very homogeneous on a large 

scale and lacking any continuous marker horizons. He also 

mapped small areas of metasedimentary rock in the mylonitic 

gneiss. He favored a plutonic igneous protolith for the mylonitic gneiss and suggested that the enclosed small bodies of 

metasedimentary rock were pendants in the granitic terrane. 

At the southeast edge of the Rawhide Mountains a pluton of 

partly foliated quartz monzonite, intruded by small plugs of 

nonfoliated hornblende diorite along its south edge, intrudes 

the mylonitic gneiss. Shackelford inferred a Mesozoic or 

Tertiary age for these plutonic rocks. He recognized the parallelism between well-developed, pervasive mineral lineation in 

the mylonites below the detachment fault with slickenlines on 

and near the detachment fault. 

In a reconnaissance study, Rehrig and Reynolds (1980) 

interpreted the layered gneisses in the Harcuvar complex to be, 

at least in part, of metasedimentary origin. In the Planet–Mineral Hill area in the western Buckskins, Spencer, Reynolds, 

and Lehman (1989a, b) mapped a sill-like body of intermediate to mafic dioritic(?) intrusive rock just below the detachment fault. In that publication and others on mining districts in 

the upper-plate rocks of the Buckskin Mountains, Spencer and 

Reynolds lumped the lower-plate rocks into a unit of mylonitic 

crystalline rocks of Proterozoic to Cenozoic age. Also, in the 

Reid Valley area they mapped a unit of metasedimentary rock 

composed of marble, siliceous marble, quartzite, calcareous 

quartzite, phyllite, and metasandstone. Spencer and Welty 
13 75 564 EXPLANATION Proterozoic to Miocene upper-plate rocks and Miocene to Holocene post- detachment faulting rocks Brecciated lower-plate rocks Miocene Swansea Plutonic Suite Late Cretaceous granite of Tank Pass Jurassic metagabbro, metadiorite, and amphibolite Mesozoic and Paleozoic metasedim

Proterozoic porphyritic granodiorite 

Proterozoic granodiorite to granite 

Cretaceous and Proterozoic layere

Detachment fault; dotted wher hachure on upper plate. Arrow  direction and amount of dip Low-angle fault in lower-plate 

concealed. Hachures on hang

High-angle fault; dotted where 

and number show direction am

Antiform; arrow shows directiSynform; arrow shows directio

Location and number of analy

Inferred mylonite front. Hachu

nonmylonitic rocks

Bill Williams River

34°15’

34°

114° 113°45’ 113°30’ 1

MYLONITE FRONT ?

AlamoLake

Antiform

Harcuvar Mountains

Little Buckskin Mountains

Clara

Peak

Antiform

Lincoln Ranch Synform

Rawhide 

Fault

Swansea

Antiform

65

70 55

80

16

Planet

Peak

Swansea

Mineral Swansea Fault

Planet

Peak

561

60

68

20

15

65

469 495 496

464

463

563

50

75

65

45

75

268

566

564

565

93-2

665

93-1

587

85

Ives

BP

118

Wash

Fault

Buckskin

Mountains

Synform 55

12

1111

562

Peak

Rawhide fault

Lincoln Ranch Fault 38 65 RV 30

IP

Rawhide fault

65

Bouse Hills

0 

Figure 2. Geologic map of the northern Harcuvar complex showing locations of analyzed and(or) dated rocks. Letter prefixes of sample numbers nofrom Bryant (1995). BP, Battleship Peak. IP, Ives Peak. RV, Reid Valley.
Geology 5

(1989) describe the mineral deposits in the Buckskin Mountains, some of which occur in the lower-plate rocks.

Osborne (1981) and Woodward (1981) conducted 

detailed mapping and structural studies of the upper-plate and 

lower-plate rocks near Swansea in the central and western 

Buckskin Mountains.

Marshak and Vander Meulen (1989) described the structure of metasedimentary and underlying basement rocks in the 

southwestern Buckskin Mountains and subdivided the metasedimentary rocks into map units based on detailed mapping 

of that area (Marshak and others, 1987).

Seismic studies through the Buckskin Mountains show 

a thickened middle crust and a thinned lower crust above a 

subhorizontal Moho at 26–28-km depth. No faults offsetting 

the seismically determined layers were detected (McCarthy 

and others, 1991; Clayton and Okaya, 1991).

K-Ar, 40Ar/39Ar, and fission-track dating studies (Richard and others, 1990; Bryant and others, 1991) show that the 

lower-plate rocks in the northern Harcuvar complex cooled 

below about 500°C in Late Cretaceous to early Tertiary time 

after a Cretaceous metamorphic and plutonic event and cooled 

below about 280°C in middle Miocene time after undergoing mylonitization under upper greenschist facies conditions. 

At this time new biotite formed and older biotite was completely degassed, but hornblende was only locally partially 

degassed, as indicated by 40Ar/39Ar spectra. Foster and others 

(1993) showed that fission-track dates of apatite decrease from 

southwest to northeast in the Harcuvar and Buckskin Mountains from 21 to 14 Ma and 16 to 13 Ma, respectively. They 

interpreted these data as showing a slip rate on the Rawhide 

detachment fault of 7–8 mm/yr. Brady (2002), in a 15-km 

transect confined to the central part of the Buckskin Mountains, concluded on the basis of (U-Th)/He dating of apatite 

that part of the lower plate had a slip rate of 4.2 mm/yr over 

more than 3 my from about 14 to 11 Ma. Carter and others 

(2004) concluded, based on (U-Th)/He apatite dating, that slip 

rate along the Rawhide fault in the Harcuvar Mountains was 

2.6 km/my (2.6 mm/yr) during the time interval 23–15 Ma but 

increased to 30 km/my (30 mm/yr) after 15 Ma.

Present Work

In 1986 and 1987, during mapping of the Alamo Lake 

1:100,000 quadrangle, lower-plate rocks of the Buckskin and 

Little Buckskin Mountains were mapped in reconnaissance 

fashion. In 1987 Bryant helped with the reconnaissance mapping of the Harcuvar Wilderness Study Area (Drewes and 

others, 1990) and in 1988 completed reconnaissance mapping 

in the eastern Harcuvar Mountains.

Wooden, as part of his regional studies of crystalline rocks in the Southwestern United States, obtained the 

geochronologic and Pb isotope data from 1987 to 1993. 

Preliminary reports (Bryant and Wooden, 1989; Bryant, 

1992b; Bryant, Wooden, and Nealey, 1993) on this work 

show the distribution of the rock types, briefly summarize 

their characteristics, and announce the discovery of a suite of 

Oligocene(?)–early Miocene plutonic rocks, which crop out 

over a significant part of the northern Harcuvar complex in the 

Buckskin Mountains. The results of mapping in the northern 

Harcuvar complex are shown on the Alamo Lake 1:100,000 

quadrangle (Bryant, 1995). In addition, Bryant and others 

(1996) gave a summary of the geochronology and metamorphic history of the northern Harcuvar complex. Here we 

present our field, structural, chemical, and isotopic data and 

discuss the history and origin of the rock units in the northern 

part of the Harcuvar complex in the Buckskin and eastern 

Harcuvar Mountains.

In this report we refer to features that can be located on 

the 1:100,000-scale Alamo Lake quadrangle and to many 

detailed features that must be located on the 1:24,000 topographic quadrangle maps that are available throughout the 

area. The small scale of figure 2 allows only an overview of 

the entire northern Harcuvar complex. 

Geology

Cretaceous–Early Proterozoic layered gneiss crops out 

over much of the Planet Peak and Ives Peak antiforms in the 

Buckskin Mountains (fig. 2). Early and Middle Proterozoic 

and Cretaceous granite, Jurassic diorite and gabbro, and 

Tertiary gabbro and granite intrude the gneiss (fig. 2). Paleozoic and Mesozoic metasedimentary rocks (not all between 

distinguished in fig. 2) occur as sheets below the detachment 

fault and as lenses in the layered gneiss. All of these rocks are 

mylonitized in various degrees. The mylonitic rocks generally 

have a well-developed, northeast-trending mineral lineation. 

About a quarter of the lower plate in the Buckskin Mountains 

is underlain by distinctive heterogeneous, variably mylonitized, plutonic rock we assign to the Swansea Plutonic Suite 

(Bryant and Wooden, 1989). This rock unit crops out over 

much of the Clara Peak antiform in the middle of the Buckskin 

Mountains; it extends on to the southern flank of the Planet 

Peak antiform and forms a sill just below the detachment 

fault on the northwestern flank of the Planet Peak antiform. 

In the southwestern part of the Buckskin Mountains, it locally 

extends south of the Lincoln Ranch synform.

In the Little Buckskin and eastern Harcuvar Mountains, 

rocks of the Swansea Plutonic Suite are absent except for 

very few microdiorite dikes. Cretaceous granite is much more 

extensively exposed in the eastern Harcuvar Mountains than 

in the Buckskin or Little Buckskin Mountains. Layered Early 

Proterozoic to Cretaceous gneiss is widespread in the northern 

Harcuvar complex.
6 Geology of the Northern Part of the Harcuvar Complex, West-Central Arizona

Rock Units

Proterozoic and Cretaceous Layered Gneiss

Layered gneiss forms much of the northern Harcuvar 

complex. Some of the freshest and most extensive exposures 

are along the canyon of the Bill Williams River between 

Alamo Dam and Reid Valley. Many washes in the Buckskin 

and Rawhide Mountains have good exposures, and Ives Peak, 

the highest peak in the Buckskins (not named on the USGS 

topographic maps, but indicated by the 1,197-m spot elevation on the Alamo Lake 1:100,000 quadrangle), has cliffs of 

layered gneiss containing sills of Cretaceous granite.

The layered gneiss is a heterogeneous unit, but its variations are similar throughout the northern Harcuvar complex. 

It consists of layers of biotite-quartz-feldspar gneiss, granite 

gneiss, biotite-hornblende gneiss, hornblende-biotite gneiss, 

amphibolite, and minor amounts of biotite-muscovite schist 

(see figs. 3, 4, 24, 25, 29, 30). In many places the granite layers grade into the nongranitic layers, and the rock is a migmatite. Layers range from a centimeter to more than 10 meters 

thick. Foliation is everywhere well developed, and many of 

the rocks are mylonitized to various degrees. The mylonitic 

texture is best developed and most pervasive in the eastern 

Buckskin and Harcuvar Mountains and in the Little Buckskin Mountains. In the western Buckskins, in many outcrops, 

mylonitic texture is only locally developed, and then only in 

the granitic layers. Many of the felsic layers contain porphyroclasts of feldspar 0.5 to 2 cm in diameter. Much of the 

amphibolite is not mylonitized or is only weakly mylonitized. 

Layering ranges from very well defined to faint in the more 

granitic parts of the unit. Concordant and discordant pegmatite lenses, layers, and dikes are widespread. In some areas 

the lenses are the remains of stringers and dikes disrupted by 

ductile deformation during Tertiary mylonitization or some 

older episode of shearing. Where the characteristic effects of 

Tertiary mylonitization are lacking, the rocks are well foliated. 

The layered gneiss locally contains numerous sills of leucocratic garnet-bearing biotite granite, especially near mapped 

bodies of that granite, which we correlate with the Late 

Cretaceous granite of Tank Pass of the Harcuvar and Granite 

Wash Mountains. In a few places the leucocratic granite cuts 

older granite layers in the layered gneiss. Here and there in the 

layered gneiss unit are pods and lenses of gabbro, metagabbro, 

and hornblendite as much as 4 m thick and 20 m long. Locally, 

the gabbro contains segregations of hornblende-plagioclase 

pegmatite as much as 10 cm in diameter and having a grain 

size of 2 cm. The mafic rocks range from unmetamorphosed to 

locally converted to amphibolite. 

 The layered gneiss unit contains rocks with a variety 

of textures and compositions. Some amphibolites, even in 

the eastern Buckskin Mountains, do not show the effects of 

Tertiary mylonitization. In hornblende and biotite-hornblende 

gneiss, the hornblende grains commonly form porphyroclasts. The effects of Tertiary mylonitization are much more 

pervasive in rocks containing quartz than in rocks that lack 

it. The diverse rocks of the layered gneiss unit have a variety 

of textures depending on their mineralogy and the severity 

of mylonitization they underwent. Although the effects of 

mylonitization are much more pervasive in the eastern than the 

central and western Buckskin Mountains, weakly mylonitized 

rocks can be found in the eastern Buckskin Mountains and 

strongly mylonitized rocks in the western Buckskin Mountains. No well-defined contact separates areas of more and less 

pervasive mylonitization.

In strongly mylonitized quartz-feldspar rocks, the feldspars commonly form porphyroclasts in a finer grained matrix 

of recrystallized, cataclastically broken down material. Quartz 

may form porphyroclasts, which tend to be ductilely elongated, but in many rocks it forms elongate lenses of recrystallized quartz. Biotite may form porphyroclasts that commonly 

show the effects of deformation. They are bent or, in some 

cases, folded. In many rocks the biotite was reconstituted during mylonitization and forms small synkinematic grains in the 

matrix.

In the least mylonitized quartz-feldspar rocks, those 

minerals are commonly about 0.5 mm in diameter, but some 

feldspar grains are several millimeters long. Quartz has a 

mosaic texture, either filling in around the feldspar grains or 

as aggregates as much as several millimeters long. Biotite is 

synkinematic to postkinematic. In parts of the rocks, finer 

grained feldspar (probably recrystallized mortar) rims the 

larger feldspar grains. 

Mafic rocks range from metagabbro to amphibolite and 

hornblendite. Amphibolite occurs as layers or lenses (see 

fig. 25C) in the gneiss and as zones and margins on metagabbro bodies. Few amphibolites are mylonitized, but those 

that are, generally have actinolite formed at the expense of 

hornblende.

Muscovite-biotite schist commonly has large muscovite 

porphyroclasts that are deformed into lenticular “fish” that 

show sense of shearing. Biotite forms sparse smaller porphyroclasts and is commonly seen as grains inferred to be 

synkinematic with the mylonitization. One sample from NW 

corner sec. 19, T. 10 N., R. 12 W. contains porphyroclasts of 

sillimanite as much as 1 mm long in addition to muscovite as 

much as 2 mm long in a sericitic matrix.

The layered gneiss unit has not been directly dated and 

apparently has a long and complex history. It is intruded by 

plutons of granitic rock. In one small area, some samples of 

the plutonic rocks have been dated as Early Proterozoic and 

some as Middle Proterozoic (see “Proterozoic Granitic Rocks” 

in Geochronology section). The gneiss contains granitic layers 

that appear to be of two generations. One type of granitic layer 

has a range in texture and composition and in many places 

has gradational contacts with gneiss of various compositions. 

These granitic layers appear to have formed during an event 

involving migmatization and partial melting. The other type of 

granitic layer is composed of relatively homogeneous granite 

and leucogranite that we interpret to be intrusive and correlative with the granite of Tank Pass, which has a U-Pb zircon 
Geology 7

age of 78–80 Ma in the western Harcuvar Mountains (DeWitt 

and Reynolds, 1990).

The protolith of the layered gneiss unit is difficult to 

determine. Most of the rocks have igneous compositions ranging from granite to gabbro but locally are layers of biotitemuscovite schist that represent aluminous rocks that could 

have been shale in a section rich in tuffaceous sedimentary 

rock or a horizon of weathering on volcanic rock. Some pods 

of mafic and ultramafic rock retain igneous texture in whole or 

in part. Some of these bodies are of Jurassic age (see “Jurassic 

Plutonic Rocks” in Geochronology section), but perhaps some 

are of Proterozoic age.

Proterozoic Granodiorite to Granite Gneiss

Coarse- to fine-grained mylonitic and nonmylonitic 

biotite and hornblende-biotite granodiorite gneiss form several plutons in the Buckskin Mountains. In the northeastern 

Buckskin Mountains, this unit is generally coarse grained 

and contains porphyroclasts of feldspar as much as 7 cm long 

(fig. 5A). The rock is pervasively mylonitized and displays 

a well-developed northeast-trending lineation. Stringers and 

lenses of aplite and pegmatite as much as 7 m thick occur in 

the granite. The granite commonly contains amphibolite xenoliths. Locally, it has concordant streaks, inclusions, and layers 

of more biotitic biotite-quartz-feldspar and feldspar porphyroclast biotite-quartz-feldspar gneiss. 

The contact with the layered gneiss unit is difficult to 

locate precisely. In places, the rock mapped in this unit is a 

medium-grained biotite granite (fig. 5B); in one place, that 

rock type cuts the coarse-grained granite. At the scale of the 

mapping, differentiating the two rock types was not feasible.

In the southern part of the central Buckskin Mountains, 

rocks of the unit are generally medium grained but contain 

zones of coarse-grained, porphyritic granite (fig. 6) like that 

of the porphyritic granite unit. Foliation is well developed, but 

the northeast-trending lineation is only locally present except 

in mylonite zones. The best developed mylonite is directly 

beneath a unit of metasedimentary rock that dips southwest off 

the southwestern end of the Ives Peak antiform (fig. 2). Stringers, lenses, and dikes of pegmatite as much as 2 m thick cut 

the granite. The granite contains xenoliths of amphibolite and 

layered gneiss. 

In both the northeastern and south-central areas, sills 

of leucogranite and aplite, locally containing garnet, cut the 

plutonic rocks. These are interpreted to be of Cretaceous age 

(see “Late Cretaceous Granite of Tank Pass”). In the western 

Buckskin Mountains intrusions of fine- to medium-grained, 

mylonitized granitic rock in this unit are interpreted to be of 

Miocene age. Other aplitic and pegmatitic stringers and lenses 

are probably of Proterozoic age (fig. 5A)

The textures of the granitic gneisses in the two outcrop 

areas differ because of differences in degree of cataclasis and 

ductile deformation during Miocene time. Megascopically, 

the contrast is most obvious in the development of the 

northeast-trending mineral lineation. In the northeastern 

Buckskin Mountains that lineation is generally well developed 

throughout the rock; to the southwest it tends to be restricted 

to local zones. 

In the northeastern Buckskin Mountains, quartz forms 

aggregates of 0.05–0.2-mm grains as much as 3 mm long and 

a few ductilely deformed porphyroclasts as much as 1.5 mm 

long. Plagioclase (oligoclase) occurs in porphyroclasts as 

much as 4 mm in diameter. Potassic feldspar forms anhedral 

porphyroclasts as much as 1 mm in diameter. Biotite as much 

Figure 3. Migmatitic layered gneiss and granite in the 

eastern Harcuvar Mountains. Granitic layer is similar 

to analyzed sample HA–118, which was collected 

within about 10 meters from this exposure. Smith Peak 

71⁄2-minute quadrangle, in bottom of canyon 500 meters 

from south line and 30 meters from east line of sec. 7, T. 

8 N., R. 10 W. Hammer is 22 centimeters long.

Figure 4. Layered mylonite from layered gneiss in the 

eastern Buckskin Mountains. Derived from calc-silicate 

gneiss and biotite-quartz-feldspar gneiss containing 

lenses of pegmatite. Alamo Dam 71⁄2-minute quadrangle, 

sec. 16, T. 10 N., R. 13 W. in wash 460 meters from south 

line and 200 meters from west line.
8 Geology of the Northern Part of the Harcuvar Complex, West-Central Arizona

as 1 mm in diameter is synkinematic to locally postkinematic. 

The two feldspars, quartz, and biotite form a matrix to the 

larger grains.

In the south-central Buckskin Mountains, anhedral 

quartz is 0.3–1 mm in diameter and forms mosaic-textured 

aggregates. Locally. the quartz is strained and broken down 

into subindividuals. Plagioclase (oligoclase) forms anhedral 

porphyroclasts as much as 2 mm in diameter, which are locally 

bent. Potassic feldspar forms porphyroclasts as much as 5 mm 

in diameter. Synkinematic biotite is generally less than 1 mm 

in diameter but locally as much as 3 mm in diameter. The 

largest grains are locally bent. Quartz and feldspar with a grain 

size of 0.1–0.5 mm and a granoblastic texture form the matrix 

of the feldspar porphyroclasts. Mylonitic zones in the granite 

gneiss of the south-central Buckskin Mountains have textures 

similar to the mylonitic granitic rocks of the northeastern 

Buckskin Mountains.

Accessories in rocks of both areas are allanite, epidote, 

sphene, apatite, opaque mineral, zircon, and myrmekite. 

Secondary minerals are chlorite from biotite, epidote, sericite, 

and carbonate from plagioclase.

Proterozoic Porphyritic Granodiorite to Granite 

Gneiss

Well-foliated porphyritic granodiorite to granite gneiss 

underlies a small area east of Butler Pass in the southwestern 

part of the Buckskin Mountains, which is exposed beneath 

lower-plate metasedimentary rock in two places. In many 

places, feldspar porphyroclasts 2–3 cm long in a finer grained, 

well-foliated matrix form a flaser structure. The gneiss 

contains concordant to slightly discordant sills of pegmatite 

and aplite 0.1–0.3 m thick. A 2-m-diameter body of coarsegrained hornblendite is included in the porphyritic granite 

gneiss in one place. The porphyritic rock has a gradational 

contact with equigranular granite gneiss; zones of porphyritic 

rock occur in the equigranular rock near the contact.

The porphyritic granodiorite is apparently closely 

related to the adjacent equigranular granite. We have no 

geochronological data for that body, but we believe that it 

is Proterozoic. No intrusions resembling it were found in 

the slice of Paleozoic and Mesozoic metasedimentary rock 

overlying it along a ductile fault, but many intrusions in that 

slice appear to be related to the Swansea Plutonic Suite.

Paleozoic and Mesozoic Metasedimentary 

Rocks

At the southwesern margin of the Buckskin Mountains, 

Phanerozoic metasedimentary rocks form a gently southwestdipping fault slice between mylonitic Proterozoic plutonic 

rocks below and nonmylonitic Early Proterozoic rocks and 

early Miocene granitic rocks above. The nonmylonitic rocks 

are exposed in the Bouse Hills (fig. 1), which are south of 

the western Buckskin Mountains. The slice of metasedimentary rocks crops out over about 16 km2

 on and southwest of 

Battleship Peak (fig. 2; Marshak and Vander Meulen, 1989). 

Elsewhere in the Buckskin Mountains, similar rocks form 

numerous smaller slices below the detachment fault above and 

within the mylonitized plutonic rocks. These rocks have not 

been found in the eastern Harcuvar Mountains.

The slice at Battleship Peak is approximately concordant 

with the foliation in the underlying metamorphosed plutonic 

rocks. However, as Marshak and Vander Meulen (1989) 

pointed out, it is discordant in detail. Near the base of the 

metasedimentary rocks, the augen gneiss, much of which does 

not have pervasive northeast-trending mylonitic lineation, is 

intensely mylonitized. In one place, the margin of a sill of 

biotite-quartz-feldspar porphyry is foliated. The porphyry is 

probably a part of the Miocene Swansea Plutonic Suite. On 

the east side of Battleship Peak, foliation in nonlineated augen 

gneiss below the metasedimentary rock dips gently north. The 

Figure 5. Mylonitic granodiorite gneiss from the eastern 

Buckskin Mountains. Alamo Dam 71⁄2-minute quadrangle. In 

wash, SE1⁄4SE1⁄4 sec. 36, T. 10 N., R. 13 W. A, Fold in mylonitic 

foliation. Granitic rock contains aplite and pegmatite 

stringers. Hammer is 22 centimeters long. B, Elongate feldspar 

porphyroclasts. Lens cap is about 4 centimeters in diameter.

A

B
Geology 9

base of the metasedimentary rock dips very gently southwest. 

At the contact between metasedimentary and basement rock, 

the augen gneiss is intensely sheared along planes dipping 

parallel with the base of the metasedimentary rock and has a 

northeast-trending lineation. In one locality northeast-trending 

lineation occurs in the basement rocks only within 2 m of the 

contact with the metasedimentary rocks. In other places the 

lineated rock below the contact is as much as 5 m thick. At 

another locality the foliation in porphyritic granodiorite is 

concordant with the base of the metasedimentary rock only 

within about 4 m of the contact. A 1- to 1.5-m-thick sill of 

Oligocene(?)-Miocene Swansea Plutonic Suite rock is parallel 

with the mylonitic foliation in the porphyritic granite 2–3 m 

below the contact.

The metasedimentary rocks consist of marble, quartzite, 

sandy marble, calcareous quartzite, and calc-silicate schist 

and gneiss. The marble is white, light gray, pale yellowish 

gray, pale greenish gray, pale brownish gray, yellowish gray, 

or yellowish brown and stands out in the darker gray 

and brownish-gray-weathering layered gneiss. The 

yellowish brown and yellowish-gray marbles are generally 

dolomitic. Quartzite is white, pale gray, greenish gray, and, 

where calcareous, weathers brown. Calc-silicate gneiss and 

schist are pale green, greenish gray, or dark greenish gray and 

difficult to distinguish in outcrop from fine-grained biotitequartz-feldspar mylonite or mylonitic fine-grained diorite or 

granodiorite of the Swansea Plutonic Suite.

Slices of metasedimentary rock are locally abundant. 

An almost continuous slice crops out along the northwestern flank of the Ives Peak antiform as far northeast as Reid 

Valley. About 4 km north of Battleship Peak, that slice is in 

fault contact with mylonitized Miocene plutonic rocks of 

the Swansea Plutonic Suite. A zone of moderately to very 

gently north-dipping slices of metasedimentary rock in the 

layered gneiss northeast of Reid Valley extends to the northeastern margin of the Buckskin Mountains near Alamo Dam 

(fig. 7). The slices in that zone are as much as 2 km long 

and 20 m thick, and three or more slices are 50–150 m thick, 

measured perpendicular to foliation and layering, but are too 

thin to show at 1:100,000 scale.

North of that zone, slices of similar rock occur scattered in the layered gneiss. Another concentration of 

slices as much as 500 m long is 3 km north of Swansea 

and extends about 3.5 km to the northeast. The slices are 

at the contact with and in the layered gneiss within 200 m 

of the contact of the Swansea Plutonic Suite. In that area 

the Swansea Plutonic Suite intrudes some of the slices of 

metasedimentary rock.

Slices of marble are as small as 0.5 m thick and 10 

m long. They tend to pinch and swell due to the ductility 

contrast between the marble and the adjacent quartzo-feldspathic layered gneiss.

Sills and lenses of pegmatite and leucocratic granite, 

probably of Cretaceous age, are widespread throughout 

the slices of metasedimentary rock. They have been folded 

along with the metasedimentary rock and locally are very 

weakly foliated. Some of the layers thought to be part of 

the metasedimentary strata in the field are sills proved to 

be mylonitized fine-grained biotite or biotite-hornblende 

diorite, quartz diorite, or granodiorite when examined in thin 

section. In some, igneous texture is preserved. These are 

probably part of the Swansea Plutonic Suite. Others consist 

of mylonitized, coarser grained granitic rock and resemble 

many of the layers in the layered gneiss unit. They may 

represent tectonic intercalations.

In the Battleship Peak area the metasedimentary rocks 

can be divided into six map units that change thickness and 

character along strike, according to Marshak and others 

(1987) and Marshak and Vander Meulen (1989, who also 

suggest that metasedimentary rocks were derived from both 

Paleozoic and Mesozoic protoliths, but did not correlate 

them with specific formations. They felt that the top unit 

probably had a Paleozoic protolith, and the unit underlying 

it had a Mesozoic protolith, although facing criteria were 

not found in those metamorphic rocks. They interpret the 

sequence as overturned by folding or repeated along a thrust 

fault.

Thin sections show that the metasedimentary rocks are 

completely recrystallized and that no sedimentary textures 

and structures are preserved other than layering that probably represents transposed bedding. Calcite and dolomite 

are fine to medium grained. Quartz occurs in elongate, ductilely deformed grains and aggregates of grains that are as 

Figure 6. Mylonitic coarse-grained granite gneiss from the 

central Buckskin Mountains. Flattened feldspar mostly 1 to 

2 centimeters long and 0.5 centimeter thick, but some are as 

much as 4 centimeters long. Butler Pass 71⁄2-minute quadrangle. 

On ridge on west side of easternmost major wash in the eastcentral part of the quadrangle, 130 meters S. 24oE. from spot 

elevation 2,374, and 3,120 feet S. 80o E. from spot elevation 2,579 

on Battleship Peak. Pencil is 12 centimeters long.
10 Geology of the Northern Part of the Harcuvar Complex, West-Central Arizona

much as 1 mm long. Common calc-silicate minerals in impure 

quartzites, marbles, and calc-silicate schist and gneiss are epidote and tremolite-actinolite. Amphibole is in grains as much 

as 1 mm long aligned with the northeast-trending lineation. 

Some of the amphibole is green to olive green and is probably 

actinolitic hornblende. In some samples the composition of the 

amphibole differs between layers of different compositions. 

Some of the larger amphibole grains have a lenticular outline 

and are probably porphyroclasts. In a thin section perpendicular to the axis of one tight fold, the amphibole forms polygonal 

arcs around the nose of the fold, which is cut by shears parallel 

with its axial plane. In an adjacent layer, quartz grains are 

ductilely elongated in a northeast trend on the axial plane of 

the fold. In this case deformation and recrystallization of the 

quartz continued along the axial plane of the fold after folding 

and amphibole crystallization.

About 20 percent of thin sections of this unit that we 

examined contain diopside. In some rocks it is in bent or 

broken grains as much as 3 mm in diameter, which are partly 

altered to calcite, actinolite, or a very fine “dust” within the 

grains. In other rocks the diopside appears to be in equilibrium 

with amphibole and(or) epidote. Many diopside grains in these 

rocks display well-developed polysynthetic twinning.

A few of the metasedimentary rocks contain euhedral to 

anhedral, pale yellowish brown garnet as much as 0.6 mm in 

diameter that is partly altered to calcite, epidote, and(or) chlorite. In one sample the garnet is apparently contemporaneous 

with epidote and amphibole in the main mineral assemblage.

Synkinematic to postkinematic biotite is as much 

as 0.3 mm long, and lenticular and bent porphyroclasts of 

biotite are as much as 1 mm long. 

The textures and mineralogy of the metasedimentary 

rocks suggest that they may have been metamorphosed twice, 

once probably in the Cretaceous and again 

during the Miocene extensional deformation. The earlier 

mineral assemblage consists of epidote, amphibole, diopside, 

garnet, and biotite and formed under amphibolite facies 

conditions. During Miocene shearing, new biotite, quartz, 

epidote, carbonate, actinolite, and chlorite formed under 

greenschist facies conditions.

Jurassic Metagabbro, Metadiorite, and 

Amphibolite 

Variously metamorphosed diorite and gabbro at the 

southern edge of the eastern Buckskin Mountains in the southcentral part of T. 10 N., R. 12 W. were previously assigned a 

Proterozoic age (Bryant and Wooden, 1989). However, U-Pb 

dating of zircon from a metadiorite in that area indicates that 

the rocks are about 148 Ma or Late Jurassic, as described 

herein.

The metadiorite and metagabbro crop out in an area 

about 5 km long and 1 km wide and contain much layered 

gneiss. The area underlain by these rocks is mapped as one 

pluton but appears to consist of numerous intrusions of 

various dimensions in the layered gneiss or to contain many 

inclusions of gneiss. The mafic rocks range from metadiorite 

and metagabbro having some relict igneous texture to biotitehornblende-plagioclase gneiss, hornblende-biotite schist, and 

amphibolite. The layered gneiss between the intrusions is 

commonly mylonitized. The mafic rocks are cut by sills of 

felsic rock and pegmatite.

One particularly good exposure of well-foliated, ductilely 

deformed gabbro is in the Alamo Dam 71⁄2-minute quadrangle 

and in a wash 280 m N. 80 W. from hill 2461 in the southwest 

corner of sec. 31, T. 10 N., R. 12 W. There, the metagabbro is 

cut by two types of sills, both of which have been drawn out 

into distinctively shaped fragments (“fish”) indicating that the 

lower rock moved southwest relative to the upper rock (fig. 8). 

Both sills are trondhjemite. One is gray and contains biotite 

and the other is white and contains muscovite. They differ 

from sills of leucocratic rock in the layered gneiss, which are 

granite where we have studied them.

Textures in the metadiorite and metagabbro range from 

granoblastic to cataclastic. One granoblastic-textured rock has 

anhedral plagioclase (An34-46) showing normal and weak oscillatory zoning and forming a mosaic texture with a grain size of 

Figure 7. Thin layers of marble in migmatitic layered gneiss 

in the eastern Buckskin Mountains. Three light-colored, locally 

discontinuous marble layers as much as 3 meters thick pinch 

and swell. Alamo Dam 71⁄2-minute quadrangle. Canyon wall on 

tributary to Ives Wash in SE1⁄4SE1⁄4 sec. 15, T. 10 N., R. 13 W.
Geology 11

0.5 mm. Anhedral to subhedral blue-green hornblende as much as 

2 mm long is in elongate aggregates as much as 

6 mm long. This texture probably is the result of Cretaceous 

metamorphism without any overprint by later events. A rock 

showing severe cataclastic deformation has anhedral, rounded, 

locally bent, porphyroclasts of plagioclase (An28) as much as 

1 mm in diameter and anhedral dark green porphyroclasts of 

hornblende as much as 1 mm long in a matrix of quartz, plagioclase, and hornblende with a grain size of 0.05–0.3 mm. Zones 

of microbreccia or ultramylonite containing rounded grains of 

plagioclase and hornblende in a cryptocrystalline to glassy matrix 

cut the rock.

Late Cretaceous Granite of Tank Pass

Light-colored granite forms numerous sills and 

small plutons in the layered gneiss in the Buckskin and 

Little Buckskin Mountains and large sill-like bodies in 

the eastern Harcuvar Mountains. There the main body 

is at least 200 m thick. Its top is subconcordant with 

layering in the country rock, and no floor is exposed. 

Many sills of granite intrude the country rock overlying the main body. We correlate this granite with the 

granite of Tank Pass of the western part of the Harcuvar 

Mountains, which has a U-Pb zircon date of 78–80 

Ma (DeWitt and Reynolds, 1990). A sample of a 5- to 

30-cm-thick dike of leucocratic granite from the Harcuvar Mountains, about 2.5 km south of the area of this 

study, has a zircon age of 70±2 Ma, based on singlecrystal determinations. Only one crystal gave a concordant analysis. Other grains analyzed are discordant 

due to inheritance from rocks of both Early and Middle 

Proterozoic age. Uncertainties in the lower intercepts 

of regressions based on the discordant grains overlap 

a 78–80-Ma zircon date on the granite of Tank Pass of 

the western Harcuvar Mountains (Isachsen and others, 

1999). In the western Harcuvar Mountains, the granite 

of Tank Pass is cut by the granodiorite of Granite Wash, 

hornblende from which has a plateau age of 78.6±0.4 

Ma (Richard and others, unpublished data cited in 

Isachsen and others, 1999). Since the granite of Tank 

Pass is foliated even where not mylonitized and the dike 

dated is not, the dike may be somewhat younger than 

the granite of Tank Pass.

In the Buckskin Mountains, sills of the granite of 

Tank Pass are especially numerous adjacent to mapped 

plutons, and the plutons have complex, discordant 

contacts where they pass into sills. In the Planet Peak 

area in the western part of the range, many intrusions of 

leucocratic granite were combined to form a map area 

of granite. In the Little Buckskin Mountains, where no 

plutons were mapped, sills probably related to the granite of Tank Pass are numerous, and other workers might 

map a body of granite on the crest of that range. 

Concordant pegmatite lenses, stringers, and knots 

as much as 1 m thick cut the granite in places. These 

pegmatites are widespread in the layered gneiss country 

rock, and they cut the slices of metasedimentary rock. 

The granite locally contains inclusions of layered gneiss 

from the wallrock. 

The granite of Tank Pass (fig. 9) is a mylonitic, 

leucocratic, medium- to coarse-grained biotite and 

muscovite-biotite granite containing sparse accessory garnet and having a color index of about 5. It has 

a well-developed mylonitic foliation parallel to the 

foliation in the layered gneiss country rock. Northeasttrending, stretched mineral lineation ranges from very 

well developed to absent.

Figure 8. Mylonitic biotite hornblende gneiss derived from 

gabbro containing trondhjemite sills that have been deformed 

into “fish.” Analyzed samples Appendix nos. 11, 13, and 14 are 

from this outcrop. Plane of photograph approximately parallel 

with northeast-trending lineation. Northeast is to the left. South 

side of wash, Alamo Dam 71⁄2-minute quadrangle. Sec. 31, T. 10 N., 

R. 12 W., 140 meters from south line, 420 meters from east line. 

Lens cap is about 4 centimeters in diameter. 

Some rocks of the unit contain subhedral plagioclase phenocrysts as much as 4 mm long. They contain secondary epidote, 

suggesting that during metamorphism the plagioclase was decalcified to its present andesine composition from labradorite, the 

composition typical of gabbros.

Biotite is a major constituent of many rocks of the unit. 

It occurs in synkinematic to postkinematic crystals as much as 

1.5 mm long. In a few rocks the grains have been bent during 

younger deformation.

Accessory minerals include epidote that appears to be contemporaneous with metamorphic biotite and plagioclase, apatite, 

sphene, opaque minerals, zircon, and allanite. Chlorite and some 

of the epidote formed late in the history of the rock.
12 Geology of the Northern Part of the Harcuvar Complex, West-Central Arizona

Quartz is in aggregates of ductilely deformed grains 

as much as 3 mm long. Plagioclase, generally An12-22, is in 

anhedral to subhedral porphyroclasts as much as 5 mm long. 

Potassic feldspar forms porphyroclasts as much as 4 mm long. 

Synkinematic and postkinematic biotite flakes are generally 

0.5 mm, but locally as much as 1.5 mm, in diameter. In a few 

rocks, muscovite is intergrown with biotite. All these minerals also form a matrix of recrystallized cataclastic material. 

Myrmekite is widespread. Accessory pale red anhedral to 

subhedral garnet is as much as 2 mm in diameter. Other accessory minerals are apatite, sphene, zircon, opaque mineral, and 

allanite. Secondary minerals are chlorite from biotite, carbonate in late veinlets, and sericite.

Oligocene(?) and Miocene Swansea Plutonic 

Suite

In the Buckskin Mountains a distinctive group of plutonic 

rocks forms the Clara Peak antiform and extends across the 

Swansea synform to the southeastern flank of the Planet Peak 

antiform and locally to the northwestern flank of the Ives Peak 

antiform (fig. 2). These plutonic rocks underlie about 370 km2

of the lower plate in the Buckskin and Rawhide Mountains but 

are partly covered by upper-plate and younger, postdetachment 

rocks. These rocks are mapped as the Swansea Plutonic Suite 

(Bryant and Wooden, 1989) and consist of a variety of rocks 

Figure 9. Mylonitic biotite granite in the eastern Buckskin 

Mountains correlated with the Late Cretaceous granite of 

Tank Pass in the Harcuvar Mountains. Southwest part of 

Alamo Dam 71⁄2-minute quadrangle about 200 meters from 

analyzed sample (Appendix no. 16; table 5). In wash S. 56o W. 

from spot elevation 3,110. Knife is 7 centimeters long.

that are well mixed on a scale that precludes distinguishing 

the separate rock types at a scale of 1:100,000. 

The rocks of the Swansea Plutonic Suite range from 

gabbro to granite. Gabbro and diorite are medium to coarse 

grained, locally porphyritic only locally mylonitized, and 

intruded by more felsic and intermediate rocks of the 

suite. The felsic and intermediate rocks are fine to medium 

grained and variously porphyritic and have a well-developed mylonitic texture and a northeast-trending mineral 

lineation formed by ductilely deformed quartz grains and 

aggregates (fig. 10). Aplite is a minor phase, which intrudes 

the other rocks sparsely and is mylonitized.

Another major component of the complex is porphyritic granite, which is found in the felsic phase as bodies a 

meter to tens of meters thick. The porphyritic granite is not 

found to be included in or cutting the mafic rocks. Fine- to 

medium-grained granites cut the porphyritic granite. Contacts between the porphyritic granite and the felsic phase 

are generally sharp but locally gradational. The porphyritic 

granite has well-developed foliation and lineation and contains K-feldspar phenocrysts (now porphyroclasts) as much 

as 3 cm in diameter (fig. 11). The porphyritic granite does 

not resemble the wallrock exposed in the Buckskin Mountains, nor are bodies of porphyritic granite concentrated 

in any particular area in the felsic phase of the Swansea 

Plutonic Suite in the Buckskin Mountains. 

Approximate modes of Swansea Plutonic Suite rocks 

show that they range from gabbro to leucomonzogranite 

according to the IUGS classification (Streckeisen, 1973). 

Fifteen to 20 percent of the samples studied are either 

diorite or granodiorite. Color index ranges from 2 to 62. 

About 15 percent of the rocks examined have a microcrystalline matrix composing 25 percent or more of the rock 

and consisting of feldspar and probably quartz and, locally, 

some very fine grained biotite. Another 7 percent of the 

rocks have a similar matrix that forms less than 25 percent 

of the rock.

 The mafic rocks contain euhedral to subhedral plagioclase 0.5–5 mm long, which is generally calcic andesine 

and displays normal and, less commonly, oscillatory 

zoning. Locally, the crystals are strained and bent. Anhedral 

to subhedral hornblende as much as 3 mm long fills in 

around the plagioclase grains. Some mafic rocks contain a 

minor amount of biotite in primary grains as much as 1 mm 

in diameter and as secondary grains 0.1 mm in diameter. 

Accessory minerals are sphene, apatite, and opaque 

minerals. Secondary minerals are chlorite, actinolite, and 

epidote. Because these rocks contain andesine and have a 

color index of less than 50, they are diorites according to 

the IUGS classification (Streckeisen, 1973), but a few have 

a color index of greater than 50, indicative of gabbro in that 

classification. Geochemical composition of two samples of 

mafic rock indicates that they are gabbros when compared 

with a compilation of chemical compositions of rocks 

called gabbro from the literature by Le Maitre (1976). (See 

section “Geochemistry” herein.)
Geology 13

Figure 10. Mylonitic biotite granodiorite from the felsic phase 

of the Swansea Plutonic Suite. Texture and composition similar 

to analyzed and dated sample (Appendix no. 25, tables 2 and 

5). Swansea 71⁄2-minute quadrangle. Sec. 19, T. 10 N., R. 15 W., 

430 meters from east line, 30 meters from south line. Face of 

outcrop parallel northeast-trending mineral lineation. Knife is 

7 centimeters long.

Figure 11. Mylonitic porphyritic granite or monzogranite from 

inclusion in felsic phase of the Swansea Plutonic Suite. Reid 

Valley 71⁄2-minute quadrangle. Section 8 or 9, T. 10 N., R. 14 W. 

U-Pb zircon age indicates that these inclusions are derived from 

Middle Proterozoic basement rock (tables 1 and 2; fig. 16). Pencil 

is 7 centimeters long.

Most of the rocks that contain any quartz have a mylonitic foliation because the quartz deformed ductilely under the 

conditions of metamorphism during extensional deformation. 

The quartz is interstitial to feldspars, and it forms aggregates 

as long as 3 mm composed of 0.05- to 0.2-mm-diameter 

grains. The individual grains in the aggregates have strain 

shadows in most rocks, but in a few they are in mosaic-textured aggregates, suggesting that in those rocks recrystallization of the quartz lasted longer than deformation of the rock. 

In a few rocks the quartz crystals as much as 1 mm in diameter 

have not been broken into smaller grains. Plagioclase ranges 

from euhedral phenocrysts to anhedral porphyroclasts 1–4 mm 

long, is calcic oligoclase to sodic andesine, and displays normal and oscillatory zoning. Locally, the plagioclase grains are 

bent and broken. Potassic feldspar is in anhedral to subhedral 

porphyroclasts 1–7 mm long, some of which are strained, 

bent, and broken. Some potassic feldspar is weakly perthitic, 

and some has poorly developed microcline twinning. Brown 

biotite forms bent porphyroclasts 0.5–2.5 mm in diameter. 

Brownish-green, new biotite 0.05–0.2 mm in diameter occurs 

in the matrix of recrystallized cataclastic material. Green to 

dark green, anhedral to subhedral hornblende forms grains 

as much as 1.3 mm long, some of which have rounded edges 

and are probably porphyroclasts. Accessory epidote, allanite, 

sphene, apatite, zircon, and opaque minerals complete the 

main mineral assemblage. Allanite is commonly rimmed by 

epidote. Secondary minerals are epidote, chlorite, actinolite, 

and carbonate and locally are a major component where all 

the biotite and hornblende have been altered. Many rocks have 

such a fine-grained matrix that it is difficult to tell whether it 

represents results of cataclastic breakdown of larger grains or a 

matrix of a porphyritic rock.

Because the inclusions of porphyritic granodiorite to 

granite are a component of the Swansea Plutonic Suite but 

are in areas too small to map at 1:100,000, we describe them 

here. The rocks contain aggregates of quartz with a grain size 

of 0.05–0.3 mm as much as 2 mm long. The quartz grains are 

usually strained but locally recrystallized into mosaic-textured 

areas. Anhedral porphyroclasts of plagioclase are locally bent 

and broken. Potassic feldspar forms porphyroclasts as much 

as 1.2 cm long, which are fractured, strained, and broken. The 

potassic feldspar is locally perthitic and shows microcline 

twinning. Biotite occurs as locally bent porphyroclasts as 

much as 0.5 mm in diameter and as new, very small grains 

0.1 mm and less in diameter. Green to dark green hornblende 

is in porphyroclasts as much as 1 mm long. Accessory 

minerals are allanite commonly rimmed by epidote, sphene, 

apatite, epidote, opaque mineral, and zircon. Epidote and 

chlorite are secondary minerals from alteration of plagioclase 

and biotite.

The various rocks have sharp and gradational contacts 

with each other and with their country rocks. Diffuse contacts 

with country rock are formed by numerous small intrusions of 
14 Geology of the Northern Part of the Harcuvar Complex, West-Central Arizona

Swansea rocks near the contact. Inclusions of country rock are 

numerous in some areas and range from a meter to hundreds 

of meters in length. Pegmatites are rare in the suite except 

locally near contacts with country rock. Most of the area 

mapped as the Swansea Plutonic Suite is underlain by mixed 

felsic, intermediate, and mafic rocks with felsic and intermediate rocks dominant. An area about 2 km in diameter and 

two smaller areas north of Reid Valley in the central Buckskin 

Mountains are underlain predominantly by mafic rocks. An 

area more than 2 km long and offset by a northwest-trending 

fault with apparent left-lateral displacement was mapped 

south of Swansea in the western Buckskin Mountains 

(Bryant, 1995). Except for a few dikes, no rocks of this suite 

were identified in the eastern Harcuvar Mountains.

The map relations of the Swansea Plutonic Suite indicate that it lies above layered gneiss in the eastern Buckskin 

Mountains. At least locally, such as 2–4 km downstream from 

the Alamo Dam, the contact is a brittle fault. However, in 

some places of relatively good exposures, such as the west part 

of sec. 29, T. 11 N., R. 14 W., the Swansea rocks are above 

the layered gneiss along a contact dipping gently in about the 

same direction as the overlying detachment fault, and no significant faulting is apparent along that contact. One exposure 

in that area shows that the Swansea plutonic rock cuts the 

layering of the layered gneiss. The rocks are mylonitized, but 

the low-dipping contact apparently does not represent a major 

ductile fault zone. Another exposure in the same area shows 

Swansea rocks interlayered in the layered gneiss unit. In secs. 

19 and 20, T. 10 N., R. 15 W., on the southeastern flank of the 

Planet Peak antiform, the low-angle contact of the Swansea 

with overlying layered gneiss above cuts lenses of metasedimentary rock in the layered gneiss unit. The complex intrusive 

relations between the Swansea rocks and the layered gneiss 

wallrock are exposed locally along this part of the contact. 

On the northwestern flank of the Ives Peak antiform in secs. 

21 and 22, T. 9 N., R. 15 W., the contact of Swansea rocks is 

along a gently dipping brittle fault subparallel with the overlying detachment fault. It is above a slice of metasedimentary 

rock that is in ductile fault contact with layered gneiss below. 

The Swansea contact may be steep for 2 km southwest of 

Alamo Dam, but the dip of the contact is not well constrained 

due in part to poor exposures and in part to lack of detailed 

mapping. The same applies to the contact 5 km west of Alamo 

Dam. Probably the overall shape of the Swansea Plutonic 

Suite mass is that of a northeast-southwest-trending lens parallel to the regional Tertiary extension and mineral lineation. 

Although the contacts are brecciated in many localities, in 

other localities they are not. Apparently the contact is not primarily a fault but was a favorable horizon for late brittle fault 

movements associated with the overlying detachment fault.

In the eastern Buckskin Mountains an area of layered 

gneiss 2 km long probably represents the floor of the Swansea 

rocks, and the contact may be a low-angle brittle fault like that 

exposed on the north side of the canyon of the Bill Williams 

River 2–4 km downvalley from Alamo Dam. The contact of 

the body of layered gneiss surrounded by Swansea rocks in 

the Rawhide Mountains is poorly exposed. Breccia is found 

in many areas near the contact but seems to be lacking in others. Consequently, that contact is not shown as a fault on the 

Alamo Lake 1:100,000 geologic map (Bryant, 1995).

Sparse small intrusions of Swansea plutonic rocks 

(fig. 12) are found in the layered gneiss away from the contact. 

The slices of metamorphosed sedimentary rock contain sills 

of variously mylonitized igneous rocks that are probably part 

of the Swansea Plutonic Suite. Sparse northwest-trending 

dikes of unmetamorphosed diabase and hornblende diorite 

and variably foliated medium-grained granodiorite cut the 

layered gneisses and are probably related to the Swansea 

Plutonic Suite. The granodiorite dikes contain phenocrysts of 

Figure 12. Small intrusion of granodiorite of the felsic phase 

of the Swansea Plutonic Suite into layered migmatitic gneiss. 

Both rocks were mylonitized during the Miocene. Alamo Dam 

71⁄2-minute quadrangle at north end of spillway of Alamo dam. 

quartz, biotite, plagioclase, and hornblende as much as 5 mm 

in diameter. The dikes are most numerous in the southeastern 

part of T. 9 N., R. 15 W. and the northeastern part of T. 8 N., 

R. 15 W. These dikes resemble those in the Harcuvar and 

Harquahala Mountains (Drewes and others, 1990; DeWitt and 

others, 1988; Richard and others, 1990). One such dike from 

the Harquahala Mountains has a 22.3-Ma 40Ar/39Ar total gas 

age on hornblende (Richard and others, 1990). K-Ar ages of 

hornblende and biotite from another northwest-trending dike 

of diorite in the Harquahala Mountains are 28.6±1.9 Ma and 

22.1±1.3 Ma, respectively (Shafiqullah and others, 1980). 

Hornblende from an undeformed diorite sill in the Harcuvar 

Mountains has a K-Ar date of 25.7 Ma (Rehrig, 1982). In the 

Harquahala Mountains, dikes of this age are predominantly 

gabbro but include some rhyolite and dacite (DeWitt and 

others, 1988).
Geochronology 15

Rocks Probably Correlative with the Swansea 

Plutonic Suite

In the eastern Bouse Hills, south of the southwestern 

part of the Buckskin Mountains, a pluton of granodiorite to 

granite lacks mylonitic textures and has a biotite K-Ar date 

of 20.5±1.1 Ma (Spencer and Reynolds, 1990b). The granite 

intrudes Proterozoic crystalline rocks, which are overlain by 

24.0- to 19.5-Ma volcanic rocks of various compositions. An 

upper felsic volcanic unit unconformably overlies tilted older 

volcanic rocks and has a biotite K-Ar date of 20.5 Ma, the 

same age as the pluton (Spencer and Reynolds, 1990b). No 

chemical data are available for that pluton. The older volcanic 

sequence contains extrusive rocks that are the same age as the 

Swansea Plutonic Suite and may be extrusive equivalents. The 

Bouse Hills are interpreted to be a continuation of the lowerplate rocks of the Buckskin Mountains (Spencer and Reynolds, 

1991). If so, they are above the mylonitized metasedimentary 

rocks of the Battleship Mountain slice at the southwestern part 

of the Buckskin Mountains. A southwest-dipping mylonite 

front must separate the granite of the eastern Bouse Hills from 

metasedimentary rocks in the Buckskin Mountains (sec. C–C', 

Bryant, 1995). Although mapped separately from the Swansea 

Plutonic Suite (Bryant, 1995), the granitic rock in the Bouse 

Hills is probably correlative with the Swansea Plutonic Suite.

In the northeastern part of the Bouse hills, mediumgrained hornblende-biotite granodiorite and hornblende diorite 

intergrade. Clots of mafic minerals define a foliation, and the 

rocks contain a few cognate inclusions of diorite.

The granitic rocks in the northeastern Bouse Hills consist 

of euhedral to subhedral plagioclase An35-22 with normal and 

oscillatory zoning in grains as much as 3 mm long, subhedral 

to anhedral hornblende as much as 2 mm long, interstitial 

anhedral quartz as much as 3 mm in diameter, and anhedral 

to subhedral biotite as much as 2 mm in diameter. Accessory 

minerals are sphene, zircon, apatite, and opaque mineral. 

Secondary minerals are chlorite from biotite and sericite from 

plagioclase.

Geochronology

U-Pb zircon dating of rocks in the northern Harcuvar 

complex (tables 1 and 2) in the Buckskin Mountains shows 

that Early Proterozoic and Middle Proterozoic plutonic rocks 

cut the layered gneisses, which must be Early Proterozoic or 

older. A pluton of diorite and gabbro in the eastern Buckskin 

Mountains is of Late Jurassic age. A granitic layer in the migmatitic layered gneisses in the eastern Harcuvar Mountains is 

of Early Cretaceous age, showing that at least part of the migmatization is of that age. The Swansea Plutonic Suite, which 

is widespread in the Buckskin Mountains, is of Oligocene(?) 

and early Miocene age, similar to that of widespread volcanic 

rocks above the Rawhide detachment fault and of plutonic 

rocks in other core complexes of the region.

Proterozoic Granitic Rocks

Sample B–565A (Appendix sample 6) is- fine to mediumgrained mylonitic granite, and sample B–565 (Appendix 7) is 

mylonitic porphyritic granite. These samples were taken about 

25 m apart at a locality in the eastern Buckskin Mountains in 

sec. 31, T. 10 N., R. 12 W. Sample B–562 (Appendix sample 

9) is mylonitic porphyritic granite from an inclusion 0.8 m 

thick concordant with the mylonitic foliation in the felsic 

phase of the Swansea Plutonic Suite west of Swansea. Four 

zircon fractions from B–565 define a chord with an upper 

intercept at 1,638.2±6.4 Ma. Four zircons each from B–565 

and B–562 define chords with upper intercepts of 1,388.5±2.3 

and 1,408±1.5 Ma, respectively (fig. 13). Note that the lower 

intercepts of all three discordia lines are at about 22 Ma. Since 

the granitic rocks that intrude the layered gneiss in the lowerplate rocks are of both Early and Middle Proterozoic ages, we 

designate all the mapped metaplutonic rocks as either Early or 

Middle Proterozoic, pending more detailed information that 

would allow us to differentiate them in the field. Elsewhere in 

the Harcuvar complex, U-Pb geochronologic studies of Proterozoic plutonic rocks are at present (2006) unavailable.

Jurassic Plutonic Rocks

Sample B–566 (Appendix sample 12) represents a 

mylonitized hornblende diorite in a gabbro and diorite body 

forming a pluton on the south side of the east end of the Buckskin Mountains. Five zircon fractions define a discordia line 

with a lower intercept at 149±2.8 Ma and an upper intercept 

at 1,653±150 Ma (fig. 14). The value of the upper intercept 

suggests that Early Proterozoic crust contributed to the origin 

of the rock. A sixth fraction of zircon is substantially off that 

chord and was not used in calculating the regression.

This Late Jurassic pluton is north and northeast of any 

known igneous rocks of that age in the region. The closest 

known Jurassic igneous rocks of an age similar to the pluton 

in the eastern Buckskin Mountains are in the Granite Wash 

Mountains about 45 km to the south-southwest. There, sills 

and probable flows of basalt to trachyandesite (Gleason and 

others, 1999) are in sedimentary rocks of the McCoy Mountains Formation (Laubach and others, 1987). Ages of detrital 

zircon from that formation in western Arizona are all Jurassic 

or older (Spencer and others, 2005). In the New Water Mountains about 30 km southwest of the Granite Wash Mountains, 

an andesitic lava flow in the McCoy Mountains Formation 

has a U-Pb zircon age of 154±2.1 Ma (Spencer and others, 

2005). Initial ΣNd isotopic values of about +5 from two of the 

most mafic igneous rocks in the McCoy Mountains Formation 

in the Granite Wash Mountains suggest that those rocks were 

derived from depleted mantle sources. Rocks having a variety 

of smaller to negative ΣNd values may be due to variable 

interaction with those magmas with crustal rocks or mantlesource heterogeneity (Gleason and others, 1999). The basin 

in which the McCoy Mountains Formation was deposited 

has been interpreted to be an extension of the Bisbee basin of 
16 Geology of the Northern Part of the Harcuvar Complex, West-Central Arizona

Table 1. U-Pb ages of rocks in the northern Harcuvar complex.

Appendix

sample 

no.

Unit

Field 

sample

no.

Age (Ma) Comment

7 Mylonitic fine- to medium-grained granodiorite to granite gneiss B–565 1,388.4±2.3 Early Miocene Pb loss

6 Mylonitic porphyritic granite gneiss B–565A 1,638.2±6.4 Early Miocene Pb loss

9 Mylonitic porphyritic granite inclusion in Swansea Plutonic Suite B–562 1,408±1.5 Early Miocene Pb loss

12 Mylonitic hornblende diorite B–566 149±2.8 Early Proterozoic 

 inheritance

15 Mylonitic granite layer in migmatitic gneiss HA–118 110±3.7 Early Proterozoic 

 inheritance

18

25

Hornblende gabbro

Mylonitic granite

B–563

B–561 21.86±0.60 Middle Proterozoic  inheritance

southeastern Arizona and adjacent parts of Mexico and New 

Mexico; the basin was formed by passive continental rifting 

when Jurassic arc magmatism retreated due to foundering of 

the subducted slab (Lawton and McMillan, 1999).

Late Jurassic dikes of the Independence dike swarm and 

related small intrusions are found in eastern California from 

the central Sierra Nevada on the north to the southern Mojave 

Desert on the south. The rocks are predominantly mafic but 

range to silicic and alkalic. Numerous interpretations of their 

tectonic significance have been made, and none has been 

established as the one preferred to date (Carl and Glanzer, 

2002). 

In the Granite Wash Mountains, the McCoy Mountains 

Formation overlies Jurassic volcanic rocks correlated with 

Planet Volcanics of Reynolds and Spencer (1989). This unit 

consists of felsic metavolcanic rocks and is mapped in the 

hanging wall of the Rawhide detachment fault at the west end 

of the Buckskin Mountains. The Planet Volcanics are about 

160–155 Ma based on isotopic analysis of zircon (Reynolds 

and others, 1987) and are the most northerly dated Jurassic 

volcanic rocks of a regional blanket of felsic metavolcanic 

rock in southwestern Arizona. Probably correlative metavolcanic rock is found in the hanging wall of the Rawhide detachment on the north side of the eastern Buckskin Mountains 

(Shackelford, 1976; Lucchitta and Suneson, 1994b).

The presence of a Jurassic plutonic rock in the eastern 

Buckskin Mountains certainly indicates that either magmatic 

effects of the extension that formed the McCoy basin continued a considerable distance north of where its deposits are 

preserved or the processes responsible for the Independence 

dike swarm extended much farther east than presently known. 

If we move the Harcuvar complex 60 km northeast relative to 

the upper plate, that rock should have originated close to lat. 

34°30' and long. 113°, the northeast corner of the Alamo Lake 

30'× 60' quadrangle and at a depth of 10 km or more beneath 

the present land surface. No Jurassic plutonic rocks are known 

at the surface in that area. 

Rocks Migmatized in the Cretaceous

Sample HA–118 (Appendix sample 15) from the eastern Harcuvar Mountains is from a layer of granite gneiss like 

that shown in figure 3 in a well-layered sequence of biotitequartz-feldspar gneiss, biotite plagioclase porphyroclast 

gneiss, granite gneiss, and pegmatite. These rocks, which 

are variously mylonitic, are typical of much of the northern 

Harcuvar complex and are shown as an Early Proterozoic and 

Cretaceous layered migmatitic gneiss unit on the Alamo Lake 

1:100,000-scale geologic map (Bryant, 1995). Three fractions 

of zircon from the granite gneiss layer define a discordia line 

with a lower intercept at 110±3.7 Ma and an upper intercept 

at 1,669±21 Ma (fig. 15). This indicates that at least some, 

and perhaps all, of the migmatization in the Early Proterozoic 

layered gneisses occurred in Cretaceous time. Without more 

data we hesitate to conclude that no migmatization occurred in 

Proterozoic time. The only comparable data from similar rocks 

in the region are from Mesquite Mountain about 45 km westsouthwest of the southwest margin of the Harcuvar complex 

in the Buckskin Mountains. There, a sill of biotite granite in a 

migmatitic complex has a U-Pb zircon date of 67.2±1.4 Ma, 

which is interpreted to be the time of injection and migmatization in response to thickening of the crust in the Maria fold 

and thrust belt (Knapp and Walker, 1989). This date is younger 

than the 78–80 Ma date on the granite of Tank Pass (DeWitt 

and Reynolds, 1990), to which we relate the numerous sills 

and small intrusions in the Buckskin Mountains and the larger 

plutons in the eastern Harcuvar Mountains. It is similar to the 

70±2 Ma age of zircon based on only one concordant analysis (Isachsen and others, 1999) from a dike in the Harcuvar 

Mountains 2.5 km south of the area of this study. Garnetbearing granite pegmatite, syn- to late kinematic in relation 

to latest movement of a thrust fault in the Moon Mountains 

50 km southwest of the Buckskin Mountains, has a similar 

U-Pb zircon age of 71.1±6.7 Ma (Knapp and Walker, 1989).
Table 2. Analytical data and ages of zircons from the northern part of the Harcuvar complex in the lower plate of the Rawhide detachment fault eastern Harcuvar Mountains.[App. no., Appendix number; Field sam. no., field sample number; wt., weight; ppm, parts per million; ±%, plus or minus percent; Ma, mega-annum; mg, milligrams]

Measured ratios Corrected ratios 

App.no. 

Field 

sam.

no.

Fraction

Sam.

wt.

(mg)

U

(ppm)

Pb

(ppm)

206Pb/

204Pb

208Pb/

206Pb

206Pb/

238U ±%

207Pb/

235U ±%

207Pb/

208Pb ±%

206P

238

Proterozoic rocks

6 B-565A Mylonitic porphyritic granite (granodiorite to granite gneiss)

NM102-163 12.5 895 203 336 0.082 0.22091 0 3.056 0.2 0.10034 0.1 1,NM-63 12 104 230 11,100 0.082 0.21685 0 2.993 0.3 0.10013 0.2 1,M63-102 4.6 84.1 18 764 0.080908 0.2046 0.07 2.920 0.12 0.10065 0.09 1,M102-163 6.2 629 137.8 421 0.081419 0.21396 0.18 2.965 0.19 0.10051 0.04 1,

7 B-565 Mylonitic granite (granodiorite to granite gneiss)

NM+102 19.2 198 323 491 0.126 0.15479 0 1.880 0.4 0.08809 0.1 9NM-63 15.4 208 375 17,190 0.13 0.17118 0 2.085 0.4 0.08836 0.1 1,NM63-102 223 375 478 0.132777 0.158834 0.06 1.926 0.06 0.08795 0.02 9M-63 181 321 604 0.137069 0.16658 0.08 2.022 0.09 0.08805 0.03 9M63-102 6.8 189 316 340 0.139948 0.15683 0.14 1.900 0.15 0.08788 0.03 9

9 B-562 Mylonitic porphyritic granite (inclusion in Swansea Plutonic Suite)

NM+163 11.9 425 107.3 31.250 0.168 0.23294 0 2.864 0.23 0.0089 0.1 1,NM-100 10.5 423 101.4 23,810 0.173 0.22059 0.11 2.709 0.13 0.08909 0.07 1,M+163 9.2 533 132 25,000 0.156 0.2298 0.07 2.825 0.16 0.08919 0.13 1,M-100 11.2 414 97.2 24,100 0.170 0.2164 0.12 2.660 0.19 0.08917 0.14 1,

Jurassic rocks

12 B-566 Mylonitized hornblende diorite (metagabbro and amphibolite)

NM+163 7.4 518 9.6 266 0.090 0.01882 0.08 0.12671 0.56 0.04884 0.54 1NM100-130 8.6 445 10.4 270 0.084 0.02469 0.07 0.17496 0.36 0.05139 0.34 15NM-100 13.6 422 11.1 487 0.085 0.02664 0.09 0.20501 0.6 0.05582 0.56 16M+163 6.5 506 11.7 285 0.066 0.02387 0.08 0.16555 0.8 0.05029 0.78 15M100-130 8.5 431 11.2 110 0.106 0.02539 0.05 0.18796 0.17 0.05368 0.16 16M-100 8.7 42.8 11.9 408 0.085 0.02828 0 0.22932 0.67 0.05882 0.62 17

Cretaceous rocks

15 Ha-1118 Granite mylonite layer in layered migmatitic gneiss.

NM+130 7.1 573 21 543 0.08 0.03671 0.44 0.39703 0.58 0.07844 0.34 23NM-100 6.4 679 26 526 0.084 0.03748 0.35 0.40969 0.36 0.07928 0.05 23M-All 721 36 684 0.084 0.04947 0.23 0.584 0.27 0.08563 0.14 3

Tertiary rocks

25 B-561 Granite mylonite (Swansea Plutonic Suite)

NM-63 5.6 177 9.4 133 0.156 0.005 0.16 0.04234 0.6 0.06141 0.55 32NM102-163 11.3 144 11 116 0.161 0.00705 0.16 0.06723 0.28 0.06919 0.22 45M-63 9.8 230 11.5 109 0.183 0.00455 0.16 0.03617 0.54 0.05759 0.49 29M102-163 155 17.1 105 0.154 0.01014 0.15 0.10604 0.28 0.07584 0.22 65 

18 B-563 Hornblende gabbro (Swansea Plutonic Suite)

NM+130 11.4 261 1.64 145 0.526 0.00385 0.32 0.027 2.7 0.05176 2.5 24M-100 10.8 306 2.5 76.5 0.770 0.00365 0.41 0.02454 5.7 0.04881 5.4 23
18 Geology of the Northern Part of the Harcuvar Complex, West-Central Arizona

Figure 13. U-Pb zircon concordia diagram for mylonitic granitic gneiss and mylonitic 

porphyritic granite and porphyritic granodiorite inclusion in felsic rocks of the Swansea 

Plutonic Suite. Sample B–565, mylonitic granitic gneiss and sample B–565A mylonitic 

porphyritic granite from the eastern Buckskin Mountains; sample B–562, inclusion near 

Swansea in the central Buckskin Mountains. MSWD, mean standard weighted deviation.

Figure 14. U-Pb zircon concordia diagram for sample B–566 

of mylonitic horblende diorite from the eastern Buckskin 

Mountains. MSWD, mean standard weighted deviation.

Figure 15. U-Pb zircon concordia diagram for sample 

HA–118 from a migmatitic-looking granite layer in layered 

gneiss in the eastern Harcuvar Mountains. MSWD, mean 

standard weighted deviation.

1,000

1,200

1,400

1,600

0.30

0.22

0.18

0.14

0.10

1 2 3 4

B–565

B–565A

B–562

207Pb 235U

206Pb

238

intercepts at

22±5 and 1,408±1.5 Ma

MSWD=0.40

intercepts at

22±5 and 1,388.5±2.3 Ma

MSWD+1.8

intercepts at 

22±5 and 1,638.2±6.4 Ma

MSWD=7.1

U 0.26

0.36

0.28

0.32

0.16

0.20

0.24

0.10 0.12 0.14 0.16 0.18 0.20 0.22 0.24

120

140

160

180

200

206Pb

238U

207Pb/ 235U

intercepts at

(MSWD=33)

B-566

149.4±2.8 and 1,653±150 Ma

500

400

300

200

100

0

0

0.06

0.06

0.10

0.04

0.02

0.2 0.4 0.6 0.8

206 Pb

238 U

207Pb U 235

intercepts at

(MSWD=0.96)

HA–118

110±3.7 and

1,669±20 Ma
Geochemistry 19

Swansea Plutonic Suite

Sample B–561 (Appendix sample 25) of mylonitic 

granite from the felsic phase of the Swansea Plutonic Suite 

north-northeast of Swansea and sample B–563 (Appendix 

sample 18) of hornblende gabbro from the pluton mapped in 

the Alamo Lake 1:100,000 quadrangle and crossed by the Bill 

Williams River. U-Pb isotopic analyses of four zircon fractions from the felsic phase and two fractions from a sample 

of the mafic phase fall on a chord with a lower intercept 

of 21.86±0.60 Ma and an upper intercept of 1,427±26 Ma 

(fig.16A). The value of the upper intercept suggests that the 

felsic rocks were derived in part from crust containing Middle 

Proterozoic rock. Analyses of four zircon splits from sample 

B–562 of porphyritic granite, which forms numerous inclusions in the felsic phase of the Swansea Plutonic Suite, fall on 

a chord with an upper intercept at 1,409±6.3 Ma and a lower 

intercept at 31±110 Ma (fig.16B). Thus the porphyritic granite 

has an age similar to the upper intercept for the felsic phase of 

the Swansea Plutonic Suite. When all the zircon data, except 

the two points from the gabbro, are plotted together, they form 

a chord with an upper intercept of 1,408.5±3.4 Ma and a lower 

intercept of 21.49±0.34 Ma (fig.16C). 40Ar/39Ar plateau ages by Joan Fryxell (Richard and others, 1990) of hornblende from the mafic rocks of the Swansea 

Plutonic Suite range from 29.9 to 26.2 Ma. We assume that the 

hornblende samples came from gabbro and diorite occurring 

as small bodies not mappable at 1:100,000. The youngest of 

the hornblende ages is based on a better plateau than the older 

ages. This suggests that the discrepancies with the U-Pb zircon 

isotopic age are due to excess 40Ar or contamination with 

xenocrysts, as Nelson and others (1992) have demonstrated 

for laccoliths in the Henry and La Sal Mountains of Utah. 

The published age spectra of hornblendes from the Swansea 

Plutonic Suite, however, lack the obvious features of those 

of the hornblendes that gave mixed older ages for the Utah 

plutons. Unfortunately, the zircons we analyzed and the hornblende Fryxell analyzed are not from the same samples. The 

field relations, in our opinion, favor a close time relationship 

between the felsic and mafic members of the suite. 

Whole-Rock and Feldspar Pb-Isotopic 

Compositions

Whole-rock and feldspar Pb-isotopic analyses (table 3) 

of rocks from the northern Harcuvar complex fit the regional 

relations between Proterozoic Pb-isotopic crustal provinces as 

described by Wooden and Miller (1990), Wooden and Dewitt 

(1991), and Bryant and others (2001). There are relatively 

few analyses from Proterozoic rocks in the northern Harcuvar 

complex compared to the nearby Colorado Plateau transition, 

and most of those in the northern Harcuvar complex are from 

Mesozoic and Tertiary igneous rocks.

On a greatly enlarged part of a Pb207/Pb204 vs. Pb206/Pb204

diagram (fig.17A), lead isotope ratios cluster in tight groups 

between reference lines for the Mojave and central Arizona 

provinces from Wooden and DeWitt (1991). Only a few of 

the Pb isotope ratios in rocks of the Poachie region could be 

shown here because of their wide range. We show them here 

because the Poachie region, according to our favored tectonic 

interpretation, was about 15 km above the rocks of the northern Harcuvar complex before regional extension. The range 

of Pb-isotope ratios in the rocks analyzed in the northern Harcuvar complex resembles that of rocks in the Poachie region 

(Bryant and others, 2001). Most of the northern Harcuvar 

samples are from Phanerozoic igneous rocks rather than from 

Proterozoic rocks, which were used to determine the reference 

lines. The Poachie region is near the eastern margin of the 

Mojave Pb-isotope province and has a larger range of Pbisotopic ratios than the Mojave province to the west. The Pb 

isotopic ratios in the Poachie region differ significantly from 

those in the central Arizona province to the east (Bryant and 

others, 2001). Tight clustering of the lead-isotope contents of 

the various rock types in the Swansea Plutonic Suite indicates 

that the lead-isotope content of the magma was very homogeneous despite the heterogeneous aspect of the rocks. Feldspar 

from the inclusion of Proterozoic granitic rock in the Swansea 

Plutonic Suite is less radiogenic than feldspars from other 

Proterozoic rocks analyzed in the northern Harcuvar complex. 

The inclusion is interpreted as residuum from partial melting 

of the Proterozoic crust leading to reduction of the U/Pb ratio 

in the incompletely melted rock, which was incorporated in 

the magma.

None of the suites of Cretaceous or Tertiary plutons in the 

Whipple Mountains have as uniform Pb isotopic composition 

as the Swansea Plutonic Suite, although most of them form 

fairly well defined groups on the diagrams. These groups fall 

between the Mojave and central Arizona reference lines on the 

207Pb/204Pb vs. 206Pb/204Pb diagram (fig. 17A). The members of 

the 89-Ma suite have a greater spread in their Pb isotopic composition than the other Tertiary igneous rock suites analyzed 

(fig. 17A,B). This may be due to the effects of Late Cretaceous 

regional metamorphism or to difficulty in accurately identifying rocks belonging to that suite.

Geochemistry

Analytical Methods

Ten major elements were determined by wavelength 

dispersive X-ray fluorescence spectrometry (Taggart and others, 1990). In most samples, Rb, Sr, Y, Zr, and Nb and in some 

samples, Cr, Ni, Cu, Zn, Ba, La, and Ce were determined 

by energy-dispersive XRF by using a Kevex 7000 EDXRF 

analyzer (King and Lindsay, 1990). Rare-earth elements were 

determined by inductively coupled plasma-mass spectrometry by using an internal standard for drift correction, matrix 

effects, and oxide formation calculation (Lichte and others, 
20 Geology of the Northern Part of the Harcuvar Complex, West-Central Arizona

Figure 16. U-Pb concordia diagrams for gabbro and granite of the Swansea Plutonic Suite and combined with inclusions in the 

suite. A, Sample B–561 (open squares) and sample B–563 (open triangles) represent a mylonitic granite from north-northeast of 

Swansea and a hornblende gabbro from a pluton cut by the Bill Williams River northwest of Reid Valley, respectively (Appendix 

samples 25 and 18). B, Sample B–562 from an inclusion of mylonitic porphyritic granodiorite in mylonitic granite west of Swansea 

(Appendix sample 9). C, Sample B–561 (open triangles) and sample B–562 (open squares) plotted together showing isotopic 

relations between the Swansea Plutonic Suite and its inclusions. MSWD, mean standard weighted deviation. 

1,240

1,280

1,360

1,400

1,410 0.25

0.23

0.24

0.22

0.21

0.20

2.2 2.4 2.6 2.8 3.0 3.2

206 Pb

238 U

MSWD=.0.30

207Pb 235 U

1,320

80

60

40

20

207Pb 235 U

0.04 0.06 0.08 0.10

intercepts at

0 0.02

0

0.004

0.008

0.012

206 Pb

238

U

0.016

400

600

800

1,000

1,200

1,400

0 1 2 3

0.24

0.16

0.20

0.12

0.08

0.04

0

206 Pb

238 U

207Pb 235U

intercepts at

1,408.5 ±3.4 and

 21.49±0.34 Ma

(MSWD=6.7)

A B

C

B–562

intercepts at

31±110 and

1,409.0±6.3 [±8.3] Ma

B–561 and B–563

21.86±0.60 and

1,427±26 Ma

MSWD=8.4

B–561 and B–562
Geochemistry 21

1987). Uranium and thorium were determined by delayed 

neutron counting following neutron irradiation of the sample 

(McKown and Knight, 1990). Fluorine and FeO were determined by ion-selective electrode potentiometry (Pribble, 1990) 

and potentiometric titration (Papp and others, 1990), respectively. In some samples some rare earths and other minor 

elements were determined by instrumental neutron activation 

analysis (Baedecker and McKown, 1987). Rare-earth elements 

shown in diagrams are normalized to chondrite values from 

Hanson (1980).

Proterozoic Granitic Rocks

Isotopic dating shows that Early and Middle Proterozoic 

granitic rocks form part of the northern Harcuvar metamorphic 

complex. Overprints by both Mesozoic and Tertiary structural 

and metamorphic events make it difficult to separate rocks 

of the two ages in the field without more detailed study than 

presented here. The distinctively coarse-grained, porphyritic, 

granitic inclusions in felsic rocks of the Swansea Plutonic 

Suite are probably all of Middle Proterozoic age, and one of 

the other six samples of Proterozoic granitic rock analyzed has 

been dated as Middle Proterozoic. Unfortunately we have no 

analysis of the one granitic rock dated as Early Proterozoic. 

The Proterozoic granitic rocks are generally alkali-calcic 

(fig. 18A; table 4) and average in alkali content (fig. 18B). The 

inclusions of Middle Proterozoic granitic rocks in the Swansea 

Plutonic Suite are more potassic than the other analyzed 

Proterozoic granitic rocks. The inclusions are Fe-rich whereas 

the other granitic rocks are Mg-rich to average (fig. 18C). 

Table 3. Whole-rock and feldspar (F) Pb-isotopic compositions of rocks from the northern Harcuvar complex and from 

Cretaceous and Tertiary plutonic rocks from the Whipple Mountains. Ages in the Whipple Mountains based on Wright 

and others (1986). 

[no., number]

Measured Calculated ∆

Appendix 

sample 

no.

Age

(Ma)

Field 

sample 

no.

206Pb/

204Pb

207Pb/

204Pb

208Pb/

204Pb

238U/ 204Pb235U/ 204Pb232Th/

204Pb

Th/

U

207Pb/

204Pb

 6 1,640 B-565AF 19.163 15.668 38.971 10.1 8.7 36.8 3.7 3.9

 7 1,400 B56AF 19.409 15.676 41.782 10.9 9.0 58.7 6.3 2.1

 9 1,400 B-562 17.155 15.494 37.192 3.5 3.3 16.7 4.8 7.5

12 149 B-566F 17.871 15.526 37.663 5.8 4.3 22.0 3.8 3.2

15 110 HA-118F 18.965 15.573 38.919 9.4 5.8 36.2 3.8 -3.6

16 70 B-564 19.201 15.665 39.512 10.2 8.6 43.0 4.2 3.1

17 22 B-469 18.831 15.631 39.070 9.0 7.6 37.9 4.2 3.7

18 22 B-563 18.856 15.636 39.156 9.1 7.7 38.9 4.3 3.9

22 22 B-495C 18.865 15.635 39.172 9.1 7.7 39.1 4.3 3.7

25 22 B-561 18.879 15.638 39.188 9.1 7.8 39.3 4.3 3.9

25 22 B-561F 18.862 15.636 39.127 9.1 7.7 38.5 4.2 3.8

Whipple Mountains

 89 NP-3 18.258 15.582 38.574 7.1 6.5 32.3 4.6 6.3

 89 89WP-12 18.508 15.617 38.900 7.9 7.1 36.0 4.5 5.6

 89 89WP-32 18.509 15.618 39.096 7.9 7.2 38.2 4.8 5.7

 89 89WP-6 18.139 15.585 38.329 6.7 6.1 29.6 4.4 6.3

 89 89WP-5 18.123 15.582 38.431 6.7 6.0 30.7 4.6 6.1

 89 89WP-23 17.912 15.560 38.152 6.0 5.3 27.5 4.6 6.1

 89 89WP-9 17.876 15.536 37.872 5.8 4.6 24.4 4.2 4.1

 89 89WP-3 19.071 15.673 39.106 9.8 8.9 38.4 3.9 5.3

 89 MR-194 17.871 15.553 38.206 5.8 5.1 28.2 4.8 5.9

 89 MR-162 18.149 15.542 38.330 6.7 4.8 29.6 4.4 1.2

 73 MR-322C 18.309 15.605 39.016 7.3 6.8 37.3 5.1 6.5

 73 MR-292 18.947 15.676 39.074 9.4 9.0 38.0 4.1 6.9

 73 Mr-322 18.280 15.579 38.761 7.2 5.9 34.4 4.8 4.2

 73 Mr-302 18.252 15.572 38.826 7.1 5.7 35.2 5.0 3.8

 26 WT-1 19.120 15.622 39.080 9.9 7.3 38.1 3.8 -0.3

 26 89WP-4 19.101 15.542 38.949 9.9 7.9 36.6 3.7 1.9

 26 89WP-13 19.024 15.648 38.954 9.6 8.1 36.6 3.8 3.3

 26 89WP-31 19.037 15.563 39.006 9.7 8.3 37.2 3.8 3.7

 26 89WP-16 19.098 15.647 38.896 9.9 8.1 36.0 3.6 2.5

 19 WE-38 18.556 15.587 39.053 8.1 6.2 37.8 4.7 2.1

 19 WE-7 18.294 15.592 39.077 7.2 6.4 38.0 5.3 5.4

 19 WE-93A 18.504 15.612 39.104 7.9 7.0 38.3 4.8 5.2

 19 WSW-166 18.216 15.577 38.976 7.0 5.9 36.9 5.3 4.7
22 Geology of the Northern Part of the Harcuvar Complex, West-Central Arizona

Figure 17. Whole-rock and feldspar lead 

(Pb) isotopic compositions of some rocks 

from the northern Harcuvar complex and 

Mesozoic and Tertiary plutonic rocks in the 

Whipple Mountains (Wright and others, 

1986). F indicates determination on feldspar. 

A, 207Pb/204Pb vs. 206Pb/204Pb. Dashed lines are 

1.7–giga-annum (Ga) reference isochrons from 

Wooden and DeWitt (1991). Proterozoic rocks 

from the Poachie region from table 8, Bryant 

and others (2001). B, 208Pb/204Pb vs.206Pb/204Pb. 

17.0 17.4 17.8 18.2 18.6 19 19.4

37

37.5

38

38.5

39

39.5

40

40.5

41

41.5

15.48

15.5

15.52

15.54

15.56

15.58

15.6

15.62

15.64

15.66

15.68

15.7

42

566F

562

HA–118F

564

565F

B565F

B565AF

564

565AF

Swansea Plutonic Suite

Precambrian, Jurassic, and Cretaceous rocks

Whipple Mountains

19 Ma plutons

26 Ma plutons

73 Ma plutons

89 Ma plutons

Plutons Northern Harcuvar Complex

B

x

x x

x

x

562

566F

HA–118F

17.0 17.4 17.8 18.2 18.6 19.0 19.4

Northern Harcuvar Complex Whipple Mountains

Plutons

Poachie Region

F

F

F

F

A

Central Arizona

Mojave

Swansea Plutonic Suite

Proterozic rocks

Proterozoic, Jurassic, and

Cretaceous rocks

19 Ma

26 Ma

73 Ma

89 Ma

207 204 Pb Pb

206 204 Pb Pb

206 204 Pb Pb

208 204 Pb Pb

x x x x

x
Geochemistry 23

In the Poachie region to the northeast, which once may 

have overlain some of the rocks that now form the northern 

Harcuvar complex, Early Proterozoic plutonic rocks are 

both Mg- and Fe-rich. In that region the Middle Proterozoic 

Signal Granite is Fe-rich, whereas other Middle Proterozoic 

rocks studied are Mg-rich (Bryant and others, 2001). The 

Proterozoic granitic rocks of the northern Harcuvar complex 

in the Buckskin Mountains are metaluminous to weakly 

peraluminous (fig. 18D), although the four samples we know 

to be Middle Proterozoic are peraluminous, as is the Signal 

Granite of the Poachie region.

 Uranium/Thorium (U/Th) ratios and U and Th contents 

are highly variable except that the U and Th contents of the 

inclusions in the felsic rocks of the Swansea Plutonic Suite 

are lower than that of the other Proterozoic granitic rocks 

(fig. 19A). Zirconium contents are 58–259 ppm except in the 

inclusions where they are 344–550 ppm (fig. 19B). Cerium 

(Ce) contents range from 51 to 198 ppm, and the Ce contents 

of the inclusions are in the higher part of that range (fig. 19D). 

Strontium (Sr) contents of the Proterozoic granitic rocks range 

from 207 to 959 ppm whereas contents of the inclusions in the 

felsic phase of the Swansea Plutonic Suite are relatively uniform at 350–408 ppm (fig. 19C). Rubidium/Strontium (Rb/Sr) 

ratios range widely compared to the younger igneous rocks in 

the northern Harcuvar complex and are similar to those in the 

Proterozoic rocks of the Poachie region (Bryant and others, 

2001).

Total rare-earth elements in rocks we analyzed range 

from 350 to 437 ppm. The range is similar to that of the 

Proterozoic rocks of the Poachie region, except for the Signal 

Granite, which has a higher rare-earth content (Bryant and 

others, 2001). Chondrite normalized (CN) light-rare-earth-element vs. heavy-rare-earth-element (LREE/HREE) enrichment 

[(La/Yb)CN] ranges from 8.5 to 11. Chondrite normalized 

rare-earth element diagrams (fig. 20A) show higher total REE 

content compared to the younger rocks, a moderate slope in 

the LREEs and a gentle slope in the HREEs. The diagrams for 

both the dated Middle Proterozoic rocks and an undated rock 

for which we have REE analytical data have small europium 

anomalies (fig. 20A; sample B–93–1, fig. 20E). In the Poachie 

region, REEs in most of the Middle Proterozoic plutonic rocks 

have Eu anomalies, whereas those anomalies are much less 

common among the Early Proterzoic rocks (Bryant and others, 

2001). 

Jurassic Plutonic Rocks

The Jurassic rocks (table 5), which were identified in 

a single pluton in the southern part of the eastern end of the 

Buckskin Mountains (fig. 2), have a distinctive geochemical 

signature in relation to other units for which we have data in 

the northern Harcuvar complex. We have not dated any of the 

more felsic varieties, which probably were emplaced as sills 

or dikes of related magma in the diorite and gabbro during the 

Jurassic igneous event. The felsic material could be of Tertiary 

or Cretaceous age, but the geochemistry of the felsic rocks in 

the mafic rock seems to fit that of the enclosing rock rather 

than the Tertiary or Cretaceous igneous rocks (figs. 18,19). 

Also, textures of the felsic rocks do not resemble those of the 

Tertiary Swansea Plutonic Suite.

The most mafic rock analyzed is alkalic, and the other 

rocks from the pluton analyzed form trends from alkalic to 

calcic, very potassic to very sodic, metaluminous to peraluminous, and very Fe-rich to Mg-rich to average with increasing 

SiO2 content (table 5; figs. 18A,B,C,D). The most silicic rock 

in the pluton has a high U and Th content (fig. 19A), whereas 

the other rock types have low U and Th contents. Zirconium 

contents are 60–146 ppm (fig. 19B). Strontium contents are 

high (820–1,300 ppm) except for the most silicic rock, which 

has 440 ppm (fig. 19C). Cerium contents are 41–54 ppm 

except in the gabbro, which contains 101 ppm (fig. 19D). In 

contrast to the Proterozoic igneous rocks, the Jurassic suite, 

except for its most siliceous member, has lower Th, U, Zr, and 

Ce contents (fig. 19A,B,D). 

Total REE content decreases from 222 in the most mafic 

rock to 106 ppm in the most felsic rock. Light-rare-earth vs. 

heavy rare-earth element (LREE/HREE) enrichment ranges 

widely [(La/Yb)CN=6.9–35.9], and the enrichment values 

are not systematic with SiO2 content. The most silicic rock 

(sample B–93–2b, fig. 20B) has the lowest enrichment value, a 

flat REE pattern, and a prominent negative europium anomaly. 

No similar rocks are known nearby. 

Cretaceous Plutonic Rocks

The two Cretaceous rocks analyzed (table 5) apparently 

each have different origins. One (HA–118) is a granite layer 

in migmatitic gneiss and has a U-Pb zircon age of 110 Ma. 

This rock may represent a locally derived partial melt formed 

during a Cretaceous metamorphic event. The other (B–564) is 

a granite intrusion correlated with younger Cretaceous granitic 

rocks in the Harcuvar Mountains.

Both rocks are alkali-calcic, sodic, and weakly peraluminous (fig. 18A,B,C,D), but B–564 is Mg-rich whereas HA–118 

is average. HA–118 shows a very strong LREE enrichment 

[(La/Yb)CN=47.7], whereas B–564 is only weakly enriched in 

LREE [La/Yb)CN=4.2] (fig. 20C). Total REE content of the 

granitic layer is 131 ppm compared to 52 ppm for the intrusion. 

Swansea Plutonic Suite

Rocks of the Swansea Plutonic Suite (table 6) range 

from 60 to 72 percent SiO2 for the felsic types, and typical 

mafic rocks have about 50 percent SiO2. Petrographic studies 

indicate that some rocks of the suite probably would fill the 

50 to 60 percent SiO2 gap in the rocks analyzed. Rocks of the 

suite are alkali-calcic, Mg-rich, average to sodic, and mildly 

metaluminous to mildly peraluminous, except for the gabbro, 

which is strongly metaluminous (fig. 18).

Thorium contents (fig. 19A) are relatively uniform except 

that the most felsic rocks (>65% SiO2) have higher 
24 Geology of the Northern Part of the Harcuvar Complex, West-Central Arizona

A B

C

D

Swansea Plutonic Suite

Cretaceous rocks

Jurassic rocks

Inclusions in felsic Swanseas Plutonic Suite rocks

Proterozoic rocks

SiO2, IN WEIGHT PERCENT

A/CNK

SiO2, IN WEIGHT PERCENT

(FeTO3.8998)/FeTO3.8998+MgO)

SiO2 IN WEIGHT PERCENT

K2O/Na2O+K2O

very potassic

potassic

average

sodic

very sodic

hidden

open squares

calc-alkalic

alkali-calcic

alkalic

calcic

Very Mg-rich Mg-rich Average Fe-rich

10

9

8

7

6

4

5

3

80

70

60

50

40

40 50 60 70 80

0 0.2 0.4 0.6 0.8

40 45 50 55 60 65 70 75 0.6

0.7

0.8

0.9

0

1.1

1.2

40

50

60

80

70

0.5 0.6 0.7 0.8 0.9

Na2O+K2O, IN WEIGHT PERCENT

EXPLANATION

Very Fe-rich

SiO2, IN WEIGHT PERCENT

Figure 18. Chemical diagrams for rocks of the northern Harcuvar complex. A, SiO2 vs. Na2O+K2O. Fields of alkalinity 

from Anderson (1983) and DeWitt (1989). B, SiO2 vs. K2O/Na2O+K2O. Fields from Fridrich and others (1998). C, SiO2 vs. 

Fe/Fe+Mg. Fields from Fridrich and others (1998). D, SiO2 vs. A/CNK.
Geochemistry 25

Table 4. Major-oxide and trace-element concentrations in some Proterozoic granitic rocks and inclusions in the Swansea Plutonic 

Suite (Tsp), northern Harcuvar metamorphic complex. 

[nd, not determined; LOI, loss on ignition; FeTO3, total Fe as Fe2O3; <, less than]

Proterozoic granitic rocks Inclusions in Tsp

Appendix no. 1 2 3 4 5 6 8 9 10

Field no. B-268A B-93-1 B-587 B-665 B-653 B-565A B-495-92 B-562 B-1111

Lab no. D-363785 D-363786 D-363787 D-363787 D-363788 D-288864 D-5254487 D-288864 D-525489

Job no. UD76 VK74 UD76 UD76 UD76 SL11 VH78 SL11 VH78

Major-oxide concentration, in weight percent1

SiO2 67.1 68.4 68.5 68.6 70.6 71.7 60.9 65.2 65.5

Al2O3 16.9 14.0 15.3 16.3 14.8 13.1 14.9 15.5 14.7

FeTO3 2.58 4.10 2.72 1.48 1.94 3.82 7.84 5.02 4.84

MgO 1.01 0.82 1.05 0.50 0.57 0.68 1.66 1.01 1.09

CaO 3.91 2.47 2.01 2.37 1.51 1.87 4.38 3.49 2.35

Na2O 4.46 2.98 3.99 4..24 3.18 3.07 3.00 2.84 2.77

K2O 1.95 4.59 4.54 4..47 5.82 4.64 3.76 5.06 5.71

TiO2 0.27 0.70 0.39 0.36 0.26 0.54 1.35 0.85 0.83

P2O5 0.21 0.26 0.13 0.13 0.08 0.23 0.52 0.29 0.31

MnO 0.06 0.09 0.03 <0.02 0.04 0.07 0.11 0.05 0.06

LOI 1.14 0.57 0.77 0.54 0.47 0.41 0.71 0.61 1.00

Total 98.45 98.41 98.66 98.67 98.8 99.72 99.13 99.92 99.16 2

FeO 1.65 1.99 2

F 0.03 0.02

Trace-element concentration, in parts per million3

Nb <10 21 8 8 15 20 24 13 15

Rb 71 180 183 141 243 157 98 95 100

Sr 856 235 503 959 260 206 380 359 405

Zr 58 250 140 162 139 259 560 344 380

Y 11 64 10 5 21 51 59 45 45

Ba 1,190 1,150 1,140 1,670 1,070 750 1,700 1,800 2,150

Ce 122 168 69 58 51 nd 198 nd 146

La 54 82 26 10 16 nd 70 nd 63

Cu 6 <10 <5 6 <5 5 18 29 25

Ni <5 <10 24 5 7 <4 10 7 10

Zn 56 70 52 45 63 72 89 59 30

Trace-element concentration, in parts per million4,5

Cs nd 3.3 nd nd nd nd nd nd nd

Hf nd 8.37 nd nd nd nd nd nd nd

Ta nd 2.8 nd nd nd nd nd nd nd

La nd 88.5 nd nd nd 76.4 nd 81.6 nd

Sc nd 11.3 nd nd nd nd nd nd nd

Ce nd 177 nd nd nd 151 nd 177 nd

Pr nd nd nd nd nd 17.6 nd 23.3 nd

Nd nd 76 nd nd nd 60.0 nd 89.3 nd

Sm nd 15.7 nd nd nd 11.4 nd 18.0 nd

Eu nd 2.3 nd nd nd 2.17 nd 3.69 nd

Gd nd nd nd nd nd 9.22 nd 14.1 nd

Tb nd 2 nd nd nd 1.46 nd 2.24 nd

Dy nd nd nd nd nd 9.06 nd 12.7 nd

Ho nd nd nd nd nd 1.79 nd 2.42 nd

Er nd nd nd nd nd 4.90 nd 6.43 nd

Tm nd nd nd nd nd 0.75 nd 0.91 nd

Yb nd 6.97 nd nd nd 4.54 nd 5.10 nd

Lu nd 0.89 nd nd nd 0.65 nd 0.66 nd 6

U 1.41 7.4 5.5 3.77 3.89 3.86 1.32 0.981 0.899 6

Th 8.02 44.3 11.8 5.6 4.8 22.8 9.09 2.2 5.6 1

Major oxides determined by wavelength dispersive X-ray fluorescence: J.R. Evans, J. Taggart, A. Bartel, D.F. Siems, and J.S. Mee, analysts.

2

FeO determined by potientiometric titration and F by ion-selective electrode potentiometry: J. Sharkey and E. Brandt, analysts.

3

Trace elements determined by energy-dispersive X-ray fluorescence: J. Kent, D.F. Siems, J.E. Taggart, and J.S. Mee, analysts.

4

Rare earth and trace elements determined by neutron activation analysis in No. 2: G.A. Wandless, analyst.

5

Rare earths determined by inductively coupled plasma-mass spectrometry in Nos. 6, 8: R. Moore analyst.

6

Th and U determined by delayed neutron activation analysis except for No. 2: E.J. Knight, R.B. Vaughn, D. McKown, and R.E. McGregor, analysts.
26 Geology of the Northern Part of the Harcuvar Complex, West-Central Arizona

Swansea Plutonic Suite

Cretaceous rocks

Jurassic rocks

Inclusions in felsic Swansea Plutonic Suite rocks

Proterozoic rocks

A B

C D

SiO2, IN WEIGHT PERCENT

0 10 20 30 40 50

0

2

4

6

8

10

Th, IN PARTS PER MILLION

U, IN PARTS PER MILLION

Th/U=2

Th/U=4

Th/U=6

Includes 3 points

40 50 60 70 80

100

0

200

300

400

500

600

SiO2, IN WEIGHT PERCENT

Zr, IN PARTS PER MILLION

2,000

1,500

1,000

500

0

40 50 60 70 80

Sr, IN PARTS PER MILLION

Si02, IN WEIGHT PERCENT

40 50 60 70 80

150

200

100

50

0

Ce, IN PARTS PER MILLION

EXPLANATION

Figure 19. More chemical diagrams for rocks of the northern Harcuvar complex. A, U vs. Th; B, SiO2 vs. Zr; C, SiO2 vs. Sr; D, SiO2 

vs. Ce.
Geochemistry 27

La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

1

10

100 100

10

1

La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

100

10

1

La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

100

10

1

La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

100

1

10

La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

B–562

B–565

B–93-2C

B–566

B–93-2A

B–93-2B

B–564

HA–118 B–495C

B–563

B–463

B–469

B–93–1

B–495C

B–564

B–93–2A

A B

C

D

E

Figure 20. Chrondite-normalized rare-earth element contents of igneous rocks in the northern Harcuvar complex. A, 

Middle Proterozoic granite (B–565) and inclusion of porphyritic granite in felsic rock of the Swansea Plutonic Suite (B–562). 

B, Jurassic plutonic rocks. Mylonitic biotite-hornblende gneiss (B–93–2C) and mylonitized diorite (B–566) from main body of 

pluton and well-foliated biotite trondhjemite and mylonitic muscovite trondhjemite from disrupted late-phase dikes in pluton. 

C, Cretaceous rocks. Mylonitic biotite granite from pluton (B–564) and mylonitic granitic layer in layered migmatitic gneiss 

(HA–118). D, Swansea Plutonic Suite. Gabbros (B–563 and B–469), aplite sill in felsic rocks (B–495C), and mylonitic granite, 

the most siliceous of the samples analyzed from the suite (B–463). The latter two samples define the range of REE contents 

of the eight samples analyzed of the felsic unit in the suite. E, Samples of igneous rocks of four ages having SiO2 contents of 

about 68 percent. Proterozoic (B–93–1), Jurassic (B–93–2A), Cretaceous (B–564), and Miocene (B–495C). 
28 Geology of the Northern Part of the Harcuvar Complex, West-Central Arizona

Table 5. Major- and trace-element concentrations in some Jurassic and 

Cretaceous plutonic rocks from the northern part of the Harcuvar complex. 

[nd, not determined; <, less than]

Jurassic rocks Cretaceous rocks

Appendix no. 11 12 13 14 15 16

Field no. B-93-2C B-566 B-93-2A B-93-2B HA-118 B-564

Lab no. D-529418 D-288868 D-529416 D-529417 D-363380 D-288866

Job number VK74 SL11 VK74 VK74 UD58 SL11

Major oxide concentration, in weight percent1

SiO2 42.9 57.3 66.7 75.9 70.6 71.1

Al2O3 18.3 17.5 17.1 13.9 14.9 15.9

FeTO3 14.0 7.74 3.26 0.86 2.16 1.6

MgO 5.61 3.53 1.19 0.15 0.44 0.48

CaO 8.57 7.04 4.18 1.50 1.93 2.04

Na2O 2.56 3.52 4.44 4.44 3.98 4.73

K2O 2.95 1.37 1.22 1.87 4.19 3.61

TiO2 1.84 0.65 0.34 0.1 0.24 0.25

P2O5 1.18 0.30 0.13 0.05 0.13 0.08

MnO 0.17 0.09 0.03 0.01 0.03 <0.02

LOI 0.91 1.29 0.41 0.45 0.22 0.44

Total 98.08 99.06 98.59 98.78 98.6 99.81 2

FeO nd 4.29 nd nd 0.99 0.73 2

F nd 0.02 nd nd 0.01 0.01

Trace-element concentration in parts per million3

Nb <10 <10 <10 76 <10 12

Rb 87 47 36 28 83 113

Sr 1,150 820 1,300 440 445 605

Zr 142 83 146 60 90 100

Y 30 21 10 86 4 14

Trace-element concentration in parts per million3,4

Ba 2,350 800 1.250 1,150 1,630 960

Cu 38 67 37 11 7 5

Ni 34 49 <10 <10 <2 <4

Zn 140 52 32 <10 58 38

Co 35.8 29 7.8 0.96 4 5

Cr 46 550 <20 <20 <1 <2
Geochemistry 29

Table 5. Major- and trace-element concentrations in some Jurassic and 

Cretaceous plutonic rocks from the northern part of the Harcuvar complex.—

Continued

[nd, not determined; <; less than]

Jurassic rocks Cretaceous rocks

Trace-element concentration in parts per million5,6

Cs 1.6 nd 0.49 0.15 nd nd

Hf 2.7 nd 3 3.9 nd nd

Ta 0.40 nd 0.14 8 nd nd

Sc 25.2 nd 4.65 6.93 nd nd

La 45.1 23.7 32.5 19 18 9.7

Ce 101 46.4 54.1 41.4 35 20.2

Pr nd 6.3 nd nd 3.9 2.5

Nd 56 26.2 18 23 16 10.1

Sm 13.3 6.16 3.05 8.49 2.8 2.37

Eu 3.27 1.64 0.76 0.59 0.68 0.59

Gd nd 5.14 nd nd 1.6 1.48

Tb 1.3 0.87 0.24 2.02 0.14 0.22

Dy nd 5.01 nd nd 0.74 1.48

Ho nd 0.98 nd nd 0.11 0.29

Er nd 2.42 nd nd 0.26 1.08

Tm nd 0.38 nd nd 0.03 0.23

Yb 2.3 2.31 0.59 11.7 0.21 1.44

Lu 0.3 0.25 0.096 1.64 nd 0.24 7

Th 1.1 <1.5 3.34 29.8 1.3 2.4 7

U 1.81 <0.4 0.73 9 0.29 1.56

1

Major oxides determined by wavelength dispersive X-ray fluorescence: D.F. Siems, J.S. 

 Mee, J.E. Taggart, A.J. Bartel, and K. Stewart, analysts.

 2FeO determined by potentiometric titration and F by ion-selective electrode potentiometry: 

 C.S. Papp, E.Brandt, and J. Sharkey, analysts.

 3Trace elements determined by energy-dispersive X-ray fluorescence: J.R. Evans, J. Kent, 

 and R.R. Larson analysts.

 4Trace elements determined by X-ray spectroscopy: P.H. Briggs, and D.C. Fey, analysts. 

 Trace elements in Nos. 10, 12, 13 determined by energy-dispersive X-ray fluorescence 

 except for Co determined by inductively coupled plasma-mass spectrometry.

 5Trace elements and rare earths in Nos. 10, 12, 13 determined by instrumental neutron 

 activation analysis: G.A. Wandless, analyst.

 6Rare earths determined by inductively coupled plasma-mass spectrometry: R. Moore, 

 and G.O. Riddle, analysts.

 7Th and U determined by delayed neutron counting: R.B. Vaughn, D.M. McKown, 

 and R.E. McGregor, analysts. Th and U determined by instrumental neutron activation 

 analysis in Nos. 10, 12, 13.
30 Geology of the Northern Part of the Harcuvar Complex, West-Central Arizona

Table 6. Major-oxide and trace-element concentrations in rocks of the Swansea Plutonic Suite.

[nd, not determined; <, less than; >, greater than]

Appendix

No. 17 18 19 20 21 22 23 24 25 26

Field no. B-469 B-563 B-458A B-495B B-562-92 B-495C B-464 B-496 B-561 B-463

Lab no. D-288861 D-28288863 D-363374 D-363371 D-525488 D-288860 D-288865 D-363373 D-288862 D-363372

Job no. SL11 SL11 UD58 UD58 VH78 SL11 SL11 UD58 SL11 UD58

Major-oxide concentration, in weight percent1

SiO2 50 50.4 60.6 61.6 64.4 67.8 68.7 70.9 71.2 72.2

Al2O3 16.5 17.9 18.0 18.0 17.3 16.3 16.3 14.7 15.1 14.0

FeTO3 9.26 8.54 4.57 4.06 3.61 2.70 2.52 1.93 2.03 1.45

MgO 7.80 7.45 1.98 1.66 1.31 1.13 1.03 0.73 0.74 0.47

CaO 10.2 9.95 4.47 4.32 3.67 2.76 3.14 2.22 1.94 1.49

Na2O 2.82 3.09 4.32 4.49 4.49 4.33 4.34 3.88 4.12 3.65

K2O 0.73 0.64 3.22 2.89 2.93 3.63 2.86 3.74 4.06 4.69

TiO2 1.0 0.84 0.55 0.55 0.53 0.38 0.39 0.25 0.25 0.18

P2O5 0.26 0.09 0.21 0.21 0.20 0.13 0.11 0.1 0.08 0.06

MnO 0.15 0.15 0.06 0.05 0.03 0.03 0.03 0.03 0.02 <0.02

LOI 1.55 1.45 0.69 0.61 0.56 0.39 0.57 0.37 0.48 0.36

Total 100.28 100.05 97.98 97.83 99.03 99.19 99.99 100.02 99.57 99.03 2

FeO 5.99 5.76 2.62 2.27 nd 1.40 1.40 0.89 0.92 0.60

F 0.01 >0.01 0.04 0.03 nd 0.02 0.01 0.02 <0.01 0.01

Trace-element concentration in parts per million3

Nb <10 <10 <10 <10 <10 <10 <10 11 13 15

Rb 27 18 84 87 82 92 75 103 107 115

Sr 726 524 939 951 960 786 836 524 552 334

Zr 116 94 202 187 200 194 168 110 110 86

Y 18 16 7 7 12 7 10 9 14 11

Trace-element concentration, in parts per million4

Ba 360 300 2,080 1,580 1,850 1,500 1,600 1,090 1,300 976

Co 35 41 13 12 41 9 7 5 6 4

Cr 370 240 22 24 nd 18 7 7 8 5

Cu <2 100 32 178 <10 32 4 34 4 3

Ni 130 110 21 19 <10 15 5 3 9 5

Zn 63 43 52 48 20 45 45 38 20 23

Trace-element concentration, in parts per million5

La 23.6 10.6 23 22 nd 43.6 27.3 22 35.2 19

Ce 47.2 22.0 45 42 nd 75.6 50.1 40 63.2 35

Pr 6.3 3.0 5.3 4.8 nd 8.4 6.0 4.3 7.1 3.8

Nd 24.9 12.4 22 20 nd 27.9 21.9 16 23.3 14

Sm 5.47 3.20 3.5 3.3 nd 4.96 4.56 2.5 4.16 2.6

Eu 1.63 1.01 0.94 0.90 nd 1.15 1.14 0.61 0.86 0.60

Gd 4.36 2.91 3.2 2.7 nd 3.28 3.26 1.6 3.23 2.1

Tb 0.78 0.58 0.41 0.38 nd 0.40 0.49 0.23 0.44 0.26

Dy 4.50 4.08 2.7 2.3 nd 1.81 2.66 1.6 2.58 2.2

Ho 0.88 0.77 0.52 0.43 nd 0.28 0.46 0.30 0.48 0.44

Er 2.41 2.10 1.3 1.1 nd 0.72 1.10 0.82 1.47 1.3

Tm 0.34 0.33 0.18 0.15 nd 0.089 0.17 0.10 0.22 0.17

Yb 2.24 2.14 1.1 0.93 nd 0.55 0.89 0.87 1.33 1.3

Lu 0.32 0.29 nd nd nd 0.10 0.12 nd 0.20 nd 6

Th 2.4 1.6 2.4 2.3 3.60 7.22 3.5 4.1 7.91 9.07 6

U 0.479 0.16 0.546 0.485 0.816 0.936 1.07 1.37 1.88 2.44

1

Major oxides determined by wavelength-dispersive X-ray fluorescence: J.E. Taggart, A. Bartel, D.F. Siems, and J.S. Mee, analysts.

2

FeO determined by potentiometric titration and F by ion-selective electrode potentiometry: E. Brandt and C.S. Papp, analysts.

3

Trace elements determined by energy-dispersive X-ray fluorescence: J.R. Evans, and J. Kent, analysts.

4

Trace elements determined by X-ray spectroscopy: D.L. Fey and P.H. Briggs, analysts. No. 26 determined by energy-dispersive X-ray fluorescence: 

 J. Kent, analyst.

5

Rare earths determined by inductively coupled plasma-mass spectrometry: G.O. Riddle and R. Moore, analysts.

6

Th and U determined by delayed neutron activation analysis: R.E. McGregor, R.B. Waughn, and D. M. McKown, analysts.
Geochemistry 31

Th contents (as much as 9 ppm). Th/U ranges from 2.6 to 6.7 

in the felsic members of the suite and averages 4.1. Th/U for 

the two mafic members analyzed is 5.0 and 10.0. On a Th/U 

plot, a best fit line is Th/U~5. 

Barium (Ba) contents are highest (1,500–2,000 ppm) in 

intermediate rocks (60–65 percent SiO2) and fall to 1,000 ppm 

in the most felsic (70 percent or greater SiO2). The mafic rocks 

(50 percent SiO2) have Ba contents of 300–360 ppm. Cerium 

contents are relatively uniform at about 50 ppm but range 

from 30 to 70 ppm (fig. 19D). Zirconium ranges from about 

100 ppm in the mafic rocks, is as much as 200 ppm in the 

intermediate rocks, and drops to 100 ppm in the most felsic 

rocks (fig. 19B). Strontium contents are 500–700 ppm in the 

mafic rocks, 900 ppm in the intermediate rocks, and as low 

as 300 ppm in the most felsic members of the suit (fig. 19C). 

Rubidium contents are 18–37 ppm in the mafic rocks, about 

90 ppm in the intermediate rocks, and as much as 115 ppm in 

the most felsic rocks.

Rare earth abundances in the Swansea Plutonic Suite 

range from 83 to 169 ppm, light to heavy REE enrichment 

chrondite normalized [CN] La/Yb=3–53, and heavy REE 

enrichment [CN] Yb=2.6–10.8. Felsic rocks in the suite are 

depleted in HREE compared to the mafic rocks (fig. 20D), 

indicating possible removal of the HREE by pyroxene with or 

without amphibole or garnet during evolution of the magma. 

A fine-grained aplite sill (sample B–495C) intruded along 

the mylonitic foliation in the felsic phase of the suite has the 

greatest light to heavy rare earth enrichment and appears to be 

the end product of the process.

Comparison with Poachie Crust and Possible 

Origin

If we assume that the northern Harcuvar complex rocks 

were drawn out from beneath the Colorado Plateau from the 

vicinity of the Poachie region and areas to the northeast, it 

might be instructive to compare data on the two sets of rocks. 

Seismic data indicate that the northern Harcuvar complex 

came from depths of about 18 km (Clayton and Okaya, 1991). 

In the Poachie region, 25 kilometers from the northeastern 

edge of the exposures of the northern Harcuvar complex, felsic 

volcanic rocks of the same age as the Swansea Plutonic Suite 

form exogenous domes indicating that little denudation of 

the crust has occurred there since the early Miocene (Bryant, 

1995; Bryant and others, 1991). The rocks of the Swansea 

Plutonic Suite differ in chemistry from the granitic rocks at 

the surface in the Poachie region and from their Proterozoic 

wallrock in the Buckskin Mountains. The upper intercept of 

the line in the U-Pb concordia diagram of samples from the 

Swansea Plutonic Suite and the Middle Proterozoic U-Pb 

age of zircons from inclusions of granitic rock indicate that 

Middle Proterozoic crust was involved in the generation of the 

magmas that formed the suite (fig. 16C). The uniform Pb isotopic composition of the Swansea rocks suggests that the melt 

was well mixed. The contrast of the Pb isotopic composition 

between the rocks of the Swansea Plutonic Suite and one such 

determination on an inclusion of Middle Proterozoic granitic 

rock suggest that the melt was not primarily derived from 

material like that in the inclusion but that the Middle Proterozoic granitic rock and any other crustal rocks that contributed 

to the magma were completely homogenized, except for some 

parts of the zircons contributed by those rocks, with melts 

from the lower crust and(or) upper mantle.

Much more thorough studies of the 16.4-Ma Searchlight 

(Bachl and others, 2001) and 15.7-Ma Aztec Wash (Falkner 

and others, 1995) plutons in the Colorado River extensional 

corridor in the Eldorado Mountains about 200 km north-northwest of the Buckskin Mountains influence our interpretations 

of the Swansea Plutonic Suite. The Aztec Wash pluton most 

resembles the Swansea Plutonic Suite, although it lacks the 

well-developed mylonitic foliation of the felsic and intermediate rocks of the Swansea Plutonic Suite because of its position 

in an accommodation zone in the extended terrane (Faulds and 

others, 1988). The Aztec Wash pluton is composed of granite 

and diorite and minor amounts of intermediate, more mafic, 

and more silicic rocks. The mafic and felsic rocks mingled 

extensively. The pluton was emplaced at levels of 5 km or 

less, and it is inferred on the basis of Pb, Sr, and Nd isotopic 

compositions to have formed from old enriched lithospheric 

mantle that underwent limited contamination from the crustal 

rocks it passed through on its way to the upper crust. A 

concordia plot of U-Pb zircon analyses has an upper intercept 

indicating contribution by Early Proterozoic crust or Cretaceous rocks derived from that crust.

The chemistry of the Aztec Wash pluton contrasts to 

some degree with that of the Swansea Plutonic Suite. Mafic 

rocks of that pluton have high incompatible element concentrations: K2

O is about 3 weight percent; Ba, 1,600 ppm; and 

LREE, 350 times chondrite, in contrast to the mafic rocks in 

the Swansea Plutonic Suite, for which K2O is about 0.7 weight 

percent, Ba, about 330 ppm, and LREE is 75 times chondrite. 

Total REE abundances in the Aztec Wash pluton range from 

94 to 570 ppm, much higher than the 65–169 ppm range in 

rocks analyzed in the Swansea Plutonic Suite. However, if we 

do not count the rare earth contents of the rocks in the Aztec 

Wash pluton that have SiO2 contents of 52–57 percent, a range 

of silica content lacking in the analyzed samples from the 

Swansea Plutonic Suite, the range for the Aztec Wash pluton is 

reduced to 94–334 ppm compared to 83–169 ppm for rocks in 

the 60–75 percent SiO2 range for the Swansea Plutonic Suite. 

The Aztec Wash pluton is richer in Th and U than the Swansea 

Plutonic Suite. Thorium ranges from 1.7 to 52 ppm, compared 

to 1.6–9.1 ppm for the Swansea rocks, and uranium from 0.6 

to 11.7 ppm in the Aztec Wash pluton compared to 0.48 to 

2.44 ppm in the Swansea Plutonic Suite. Th/U ratios of the 

Aztec Wash pluton and the Swansea Plutonic Suite are similar. 

A possible interpretation of the origin of the Swansea 

Plutonic Suite is that it was derived from lithospheric mantle 

less enriched than that giving rise to the Aztec Wash pluton 

or from asthenospheric mantle. The magma homogenized its 

crustal component either by being hotter or by spending more 
32 Geology of the Northern Part of the Harcuvar Complex, West-Central Arizona

time in the lower crust than the material that formed the Aztec 

Wash pluton.

The Swansea Plutonic Suite crops out over a much larger 

area than the Aztec Wash pluton. Is its present configuration 

due to large-scale ductile deformation during and(or) after 

emplacement? Is it all of the same age, or was it emplaced 

as the lower plate was drawn southwestward? Interpretation 

of seismic data shows a bulbous layer having middle crustal 

velocities and as much as 15 km thick beneath the Buckskin 

Mountains (fig. 21). The 6.35–6.5 km/sec velocities in this 

layer could represent diorite or a mixture of mafic and silicic 

rocks (McCarthy and others, 1991). The layer pinches out 

near Bagdad in the transition zone northeast of the Buckskin 

Mountains and thins to the southwest in the Basin and Range 

away from the Buckskin Mountains. This layer might be a 

source for the Swansea Plutonic Suite now forming part of the 

upper crust exposed in the Buckskin Mountains. Limited data 

show that it is thicker along the belt of Miocene intrusions 

in the lower Colorado River extensional corridor than to the 

southeast or northwest. The layer thickens to 10 km beneath 

the Whipple Mountains, in which Miocene intrusive rocks 

also are exposed (McCarthy and others, 1991). McCarthy 

and Parsons (1994) speculate that the bulge in the midcrustal 

layer beneath the Buckskin Mountains is caused by addition of 

mantle-derived mafic magma and crustal flow of silicic rocks 

into a zone of large extension.

Relation Between Magmatism, Extension, and 

Core Complex Formation.

The Buckskin Mountains comprise the only large core 

complex in the region where a significant area of Miocene 

plutonic rocks is exposed. The only Tertiary igneous rocks 

exposed in the Harcuvar and Harquahala Mountains are early 

Miocene microdiorite dikes. Lister and Baldwin (1993) relate 

pulses of ductile deformation in core complexes to intrusion of 

igneous material and attribute differential uplift of the bounding detachment fault to those intrusions. On the other hand, 

Spencer and others (1996) favor the hypothesis that the large 

magnitude extension apparent in west-central Arizona adjacent 

to the Colorado Plateau transition zone was caused by a crust 

thickened during Cretaceous structural and igneous events that 

produced an elevated land mass and crustal root in that region. 

Variations in the few K-Ar and 40Ar/39Ar dates available are 

related to position along the northeast-southwest direction 

of extension and denudation of the core complex rather than 

to proximity to the outcrop area of the Swansea Plutonic 

Suite. Biotite was completely reequilibrated in Miocene time 

throughout the Buckskin Mountains. Biotite from an inclusion 

in Miocene plutonic rock has the same cooling age as biotite 

from many kilometers away. In the Harcuvar Mountains, the 

effects of Tertiary ductile deformation diminish in the middle 

of the range and in the Harquahala Mountains are confined to 

the northeastern end of the range (Rehrig and Reynolds, 1980). 

Miocene igneous rocks in these ranges are only in dikes, and 

those ranges apparently are composed of rock that came from 

a shallower depth than that in the lower plate exposed in the 

Buckskin Mountains (Richard and others, 1990).

Structure

Structures in the lower-plate rocks in the Buckskin 

Mountains and the eastern Harcuvar Mountains are high-angle 

reverse and normal faults, low-angle normal faults, mylonitic 

foliation and lineation, nonmylonitic foliation and lineation, 

and folds. We will discuss them from youngest to oldest in the 

order from relatively well dated structures to less well dated 

ones.

For the purposes of structural analysis the Buckskin 

Mountains have been divided into three parts: the eastern 

Buckskin Mountains east of the Lincoln Ranch fault zone, 

the central Buckskin Mountains between the Lincoln Ranch 

fault zone and the Swansea fault and a line projected along 

strike to the southeast, and the western Buckskin Mountains 

west of the Swansea fault and that line (fig. 22). Structures in 

the Little Buckskin Mountains are treated separately. In order 

to clarify the ages of the structures, the foliation and lineation in the Oligocene(?) and Miocene Swansea Plutonic Suite 

are compiled separately from structures in the older rocks. 

In the eastern Harcuvar Mountains, structures in the Alamo 

Dam SE and ECP 71⁄2-minute quadrangles west of longitude 

113o22'30"are compiled separately from those in the Smith 

Peak and Date Creek SW 71⁄2-minute quadrangles east of longitude 113o22'30". Within each of these areas, structures in the 

Cretaceous granite of Tank Pass are compiled separately from 

those in the older rocks in order to determine whether any 

pre-Late Cretaceous structures could be detected in the older 

rocks. Except for the rocks containing northwest-trending mineral lineation, we used only the data we collected rather than 

all the data shown on the Harcuvar wilderness study area map 

(Drewes and others, 1990) so that any bias in the observations 

would be as uniform as possible.

Faults

The youngest structures are northwest-trending, highangle reverse and normal faults, which offset the Rawhide 

detachment fault. They may be as young or younger than the 

youngest movement on the Sandtrap Wash fault, a northwesttrending fault just northeast of the Rawhide Mountains, which 

displaces 9.2-Ma basalt flows (Lucchitta and Suneson, 1994a; 

Bryant, 1995).

The Lincoln Ranch fault zone, first mapped by Wilson 

(1960), is an en echelon series of faults that extends from 

about the crest of the Planet Peak antiform to the southern 

edge of the Buckskin Mountains. Strands of the fault zone 

generally dip moderately to steeply northeast but locally dip 

steeply southwest. The Rawhide detachment fault and 
Structure 33

Exposures of mylonitic northern

Harcuvar complex in the 

Buckskin Mountains

10

20

30

0

10

20

30

Upper crust

Middle crust

Lower crust

50 KILOMETERS

?

Moho

Bagdad

8.0 km/sec

6.55 km/sec

6.35 km/sec

6.0-6.25 km/sec

Basin and Range Transition zone

Southwest Northeast

?

Rawhide fault

Mylonite front?

DEPTH, IN KILOMETERS 

PER METER

0

mean

sea

level

Figure 21. Northeast-southwest crustal section through the Buckskin Mountains based on geophysical and geologic 

data. Modified from McCarthy and others (1991) with added data based on Clayton and Okaya (1991) and McCarthy and 

Parsons (1994). Details in seismic velocity variations at near the top of the upper crust not shown. Dotted lines indicate 

inferred velocity discontinuities. Irregular discontinuous lines indicate reflective horizons. These lines are shown where the 

northern Harcuvar complex is exposed in the Buckskin Mountains because the rock is mylonitized. The northeastern-dipping 

reflective horizon projects to the Rawhide fault on the northeastern side of the Buckskin Mountains. At the southern end of the 

Buckskin Mountains a mylonitic front is interpreted to be above the thick, southwest-dipping slice of Paleozoic and Mesozoic 

metasedimentary rocks so that nonmylonitic Miocene granitic rock in the Bouse Hills is in the lower plate of the Rawhide fault 

as interpreted by Spencer and Reynolds (1991). km/sec, kilometers per second; Moho, Mohorovicic discontinuity. 

10 KILOMETERS

34° 

114° 113°45 113°30 113°15'

CB EB

WB

ECP

SP

LB

Figure 22. Areas of structural analyses in the northern Harcuvar 

complex. EB, eastern Buckskin Mountains; CB, central Buckskin 

Mountains; WB, western Buckskin Mountains. In the Harcuvar 

Mountains: SP, Smith Peak 71⁄2-minute quadrangle and part of the 

Date Creek Ranch SW 71⁄2-minute quadrangle; ECP, ECP 71⁄2-minute 

quadrangle and part of the Alamo Dam SE 71⁄2-minute quadrangle. LB, 

Little Buckskin Mountains.

ˇ '
34 Geology of the Northern Part of the Harcuvar Complex, West-Central Arizona

overlying upper-plate rocks are displaced down to the 

southwest so that upper-plate rocks of the Swansea and 

Lincoln Ranch synforms are bounded on the northeast by the 

Lincoln Ranch fault. The net displacement on much of the 

fault is reverse. Slickenlines in a few exposures of the fault 

zone are subhorizontal (Spencer and Reynolds, 1989b; one 

additional locality found in this study) indicating that the 

most recent movement was dominantly strike slip. The map 

pattern suggests displacement dominantly in a vertical sense 

and a minimum displacement of 400 m in the Lincoln Ranch 

synform.

Southeast of the stepover in major displacement along 

the Lincoln Ranch fault zone are numerous shear zones having 

northwest trends and generally steep to moderate southwest 

dips. These zones consist of 1–2 m of brecciated or highly 

fractured rock and locally contain quartz and quartz-carbonate 

veins, fluorite, and secondary copper minerals. Specular hematite and secondary copper minerals also occur along fractures 

in rock of the shear zones. These shear zones have been prospected and locally mined for copper, lead, silver, and gold in 

the Bluebird subdistrict of the Alamo mining district (Spencer 

and Welty, 1989).

The Swansea fault cuts the Swansea synform, extends 

northwest beyond the Buckskin Mountains beneath younger 

rocks, and apparently dies out about 2 km southeast of the 

Swansea synform. Northwest of Swansea, rocks on the southwest side of the fault are displaced about 200 m up in relation 

to those on the northeast (Spencer and Reynolds, 1989b). 

East of Battleship Peak, northwest-trending shear zones 

generally dip steeply to moderately northeast and locally 

contain quartz veins. Both the veins and the adjacent fractured 

rock locally contain specular hematite and chrysocolla in the 

Green Streak subdistrict of the Midway district (Spencer and 

Welty, 1989).

The Mineral Wash fault in the western Buckskin Mountains cuts through the entire width of the exposed lower-plate 

rocks and extends to the northwest through upper-plate rocks. 

At Planet Peak the southwest side is displaced upward at least 

400 m. Dips of the Mineral Wash fault range from 50° southwest to 80° northeast along the northern part to 50° northeast 

near the crest of the Clara Peak antiform (Spencer, Reynolds, 

and Lehman, 1989b; Spencer and Reynolds, 1989b).

The northwest-trending faults are perpendicular to the 

trend of the antiforms and synforms in the Buckskin Mountains and a regional stretched mineral lineation in the mylonitic rocks of the lower plate.

Statistical analyses of observed brittle faults show mean 

trends of N. 42° W. and N. 45° W. and 60° northeast dips 

in the central and eastern Buckskin Mountains and N. 57°

W. and 76° northeast dip in the western Buckskin Mountains (fig. 23D, H, K). In the eastern and western Buckskin 

Mountains some of the northwest-trending faults dip steeply 

southwest, and some steep faults have northerly or northeasterly trends. 

Scattered, gently dipping faults are probably related to 

the Rawhide detachment fault. These faults are mappable 

below the Rawhide detachment fault in the southeastern part 

of the eastern end of the Buckskin Mountains, on the southern 

side of the eastern end of the Little Buckskin Mountains, and 

on the northern side of the Harcuvar Mountains southeast of 

the Little Buckskin Mountains. The latter two might be parts 

of one slice that extends beneath the valley separating the 

Harcuvar Mountains from the Little Buckskin Mountains. 

These faults have zones of brecciated mylonite along them as 

much as 100 m thick. One extensive such fault in the Lincoln 

Ranch synform in the eastern Buckskin Mountains is mapped 

in the valley of the Bill Williams River southwest of Alamo 

Lake (fig. 24A). This fault could not be followed north of 

the Clara Peak antiform and probably dies out in that area. 

Breccia along it has a sharp lower contact mapped as the fault 

and a gradational upper contact with nonbrecciated mylonite. 

Displacements of these faults seem to be minor, but lack of 

piercing points makes precise determinations impossible. 

Other low-angle faults probably associated with late 

movements on the Rawhide detachment are best exposed in 

canyons and are best identified where the mylonitic foliation is 

discontinuous across them (fig. 24B).

Major Folds

Northwest-trending folds locally deform the detachment 

fault bounding the lower-plate rocks in the Centennial Wash 

quadrangle on the northern side of the Rawhide Mountains 

(Lucchitta and Suneson, 1994b) and on the northern side of 

the eastern Buckskin Mountains south of Alamo Lake (Bryant, 

1995). 

The next youngest structural features are the major 

northeast-trending antiforms and synforms in the detachment 

fault. The antiforms bring up the detachment fault and the 

underlying lower-plate rocks, which form the Buckskin, Little 

Buckskin, and Harcuvar Mountains (fig. 2). Measured dips of 

the detachment fault adjacent to the mountains range from less 

than 5° to 70° (Lucchitta and Suneson, 1994b; Spencer and 

Reynolds, 1989b; Reynolds and Spencer, 1984). The structural 

relief of these northeast-trending folds in the Buckskin Mountains is a few hundred meters (Spencer and Reynolds, 1989b). 

The structural relief of the synform between the Buckskin and 

Harcuvar Mountains is probably as much as 1,000 m (Spencer 

and Reynolds, 1989b; Bryant, 1995).

Dikes

The next youngest structural feature is a set of northwesttrending dikes of diabase, gabbro, and diorite. Some are 

undeformed, and others are at least locally foliated. Although 

these dikes have the same trend as the faults that offset the 

Rawhide detachment fault, they must be significantly older, 

for similar dikes are not found in nearby upper-plate rocks. 

None of the dikes in the Buckskin Mountains have been dated, 

but one from a well-developed swarm of similar dikes in 

the Harquahala Mountains has an 40Ar/39Ar total gas age on 

hornblende of 22.3 Ma (Richard and others, 1990). Similar 
Structure 35

Foliation in

metasediments Foliation planes lacking NE lineation Older lineation Faults

Eastern Buckskin

Mountains

Central Buckskin

Mountains

Western Buckskin

Mountains

A B C D

E

F G H

I J K

N = 66

N

c= 1, 5, 10%

c= 1, 3, 6, 9%

c= 1, 3, 5% c= 2, 5, 8%

c= 2, 5,12%

c= 2, 5, 8%

c= 1, 3, 7, 10%

N N

N = 93

N

N = 46

N

N = 74

N N

N = 32

N

N = 92

N N

N = 31

N

Figure 23. Structures in the Buckskin Mountains formed before, during, and after mylonitization in the Miocene. This and 

other structural diagrams are equal-area projections in the lower hemisphere with planes of projection horizontal and 

north at the top. Contours show percentage (%) of points falling within 1 percent of the area of the diagram. N, number of 

points; C, contours.
36 Geology of the Northern Part of the Harcuvar Complex, West-Central Arizona

Figure 24. Low-angle brittle faults in the eastern Buckskin Mountains. A, Zone 

of breccia in layered gneiss along the canyon of the Bill Williams River about 4 

kilometers northeast of Reid Valley. Breccia separates layered mylonitic gneiss 

below with regular structure from brittlely deformed mylonite with less regular 

structure above. View towards northeast from 1,200-foot knoll west of the Bill 

Williams River 170 meters east of the center of the east line of sec. 7, T. 10 N., R. 

13 W., Reid Valley 71⁄2-minute quadrangle. B, Low-angle brittle fault puts mylonitic 

layered gneiss with west dips above mylonitic layered gneiss with north dips. Alamo 

Dam 71⁄2-minute quadrangle. Section 11, T. 10 N., R. 13 W., 300 meters from south 

line and 510 meters from east line. About 1,500 meters horizontally from detachment 

fault and probably less than 500 meters and possibly as little as 200 meters vertically 

below it.

A

B
Structure 37

dikes in the Harcuvar Mountains have mainly northwest 

trends, but a few have northeast trends (Drewes and others, 

1990), and in the western Harcuvar Mountains the dikes trend 

northwest (Reynolds and Spencer, 1993).

Lineation

The next youngest structures are mylonitic foliation and 

lineation found in all the rock types in the northern Harcuvar complex. Most, and perhaps all, of the mylonitization is 

younger than the 22-Ma rocks of the Swansea Plutonic Suite. 

Mylonitic textures are lacking in the rocks of the Bouse Hills, 

which are interpreted to be in the lower plate (Spencer and 

Reynolds, 1991) but above a southwest-dipping mylonitic 

front that lies concealed beneath younger Tertiary and Quaternary rocks between the Buckskin Mountains and the Bouse 

Hills (Bryant, 1995, sec. C–C'). Most of the foliation is mylonitic and is formed by aligned biotite and, locally, by flattened 

feldspar and(or) quartz grains. The rocks have mesoscopic 

and microscopic features indicative of cataclastic breakdown 

of larger minerals, recrystallization, and ductile deformation 

of some minerals at conditions where some biotite was not 

recrystallized, although it completely lost argon in the samples 

dated (Rehrig, 1982; Spencer, Shafiqullah, and others, 1989). 

Published K-Ar ages of biotite in the Buckskin Mountains 

range from 17 to 13 Ma, suggesting that these micas became 

closed systems during and after the formation of mylonitic 

foliation and lineation. 

The lineation is formed by ductilely streaked-out 

grains of quartz, biotite, or amphibole (fig. 25A). Rehrig and 

Reynolds (1980) first suggested that the mylonitic foliation 

and lineation in the Buckskin and Harcuvar Mountains 

were related to northeasterly extension during late Tertiary 

time. Now that we know that a substantial area of 20-Ma 

rock in the Buckskin Mountains has mylonitic foliation and 

lineation similar to that in the older rocks, there is no doubt 

that the mylonitic foliation and lineation are of Miocene 

age. Megascopic textures, such as the shape and orientation 

of porphyroclasts of mica or feldspar and of components of 

differing ductility in the layered gneisses, indicate that rock 

lower in the crust flowed southwest in relation to overlying 

rock (figs. 8, 25B; Spencer and Reynolds, 1989b).

Differential ductility of layers in the mylonite is especially well displayed where some layers are composed of 

marble (fig. 7). Another marked ductility contrast is between 

layered felsic gneiss and mafic rock (fig. 25C).

The lineation trends northeast, parallel with the direction 

of regional extensional deformation, and is about parallel with 

the axes of the large folds in the detachment fault that caused 

the northeast-trending ranges composed of lower-plate rocks. 

The mylonitic foliation and lineation become less pervasive 

toward the southwest in the Buckskin and Harcuvar Mountains, yet the statistical pattern of the foliation changes little.

The change in the degree of development of the 

northeast-trending lineation is best illustrated by dividing 

the number of readings of foliation in the pre-Miocene rocks 

containing northeast-trending lineation by the number of 

readings on foliation in those rocks lacking that lineation. 

The results are the following: eastern Buckskins, 50; central 

Buckskins, 3.6; and western Buckskins, 2.4. In the Harcuvar 

Mountains no foliation lacking northeast-trending lineation 

was recorded in the Smith Peak and Date Creek Ranch SW 

quadrangles at the eastern end of the range, but in the ECP 

Peak and Alamo Dam SW quadrangles to the west, the ratio 

is 8.4.

The statistical means of mylonitic lineation in the Buckskin Mountains are similar throughout the range and in both 

Miocene and older rocks. The mean lineations among the 

three geographical areas and the Miocene and pre-Miocene 

rocks are N. 43° W. to N. 49° W. (fig. 26C, D, I, J, O, P). 

Mean plunge of lineation in the pre-Miocene rocks range 

from 2° southwest in the eastern Buckskin Mountains to 11°

southwest in the western Buckskin Mountains. In the Swansea 

Plutonic Suite of the eastern Buckskin Mountains (fig. 26D), 

mean plunge is notably steeper than in the older rocks and 

ranges from 13° to 33° because of a statistical difference in 

dip of the foliation in that area (fig. 26B). The Little Buckskin 

Mountains have a mean lineation of N. 42° E. plunging 3.5°

northeast, similar to that in the Buckskin Mountains but with 

a different direction of plunge (fig. 27B). These directions are 

more northerly than the N. 60° E. trend of the Buckskin and 

Little Buckskin antiforms.

In the Harcuvar Mountains, the mean trends of the mylonitic lineation are the same in the Late Cretaceous granite and 

the older rocks (fig. 28C, D, G, H). They differ, however, from 

those in the ranges to the north. In the ECP and Alamo Dam 

SW 71⁄2-minute quadrangles, the mean is about N. 55° E. (fig. 

28G, H), whereas to the northeast it is N. 63° E. (fig. 28C, D, 

L). The northeastern end of the Harcuvar antiform is bent to 

the south, and the more easterly trend of the mean lineation 

azimuth is probably related to that bend.

Foliation

Foliation generally has gentle dips, except for the foliation in the Swansea Plutonic Suite in the eastern Buckskin 

Mountains (fig. 26B). In some areas, poles to foliation form a 

well-developed girdle around a northeast-trending axis. This is 

best shown in the Little Buckskin Mountains where the axis of 

the girdle is N. 55° E., almost the same as the N. 60° E. trend 

of that range (fig. 27A).

In the Buckskin Mountains, poles to foliation in preMiocene rocks form poorly developed girdles around gently 

plunging N. 70o E. axes in the eastern and central areas (fig. 

26A,G) and N. 58° E. in the western area (fig. 26M). The N. 

58° E. axis for the western Buckskin Mountains is parallel 

with the trend of the range and the folds in the detachment 

fault within the range. In the central Buckskin Mountains, 

the trend of these folds is about N. 60° E., whereas in the 

eastern Buckskin Mountains the trend of axis of the Ives Peak 
38 Geology of the Northern Part of the Harcuvar Complex, West-Central Arizona

A

B

C

Figure 25. Features in mylonitized rock. 

A, Typical northeast-trending mineral 

lineation on mylonitic foliation plane in 

mylonitic biotite-quartz-feldspar gneiss. 

Alamo Dam 71⁄2-minute quadrangle. In 

wash, sec. 32, T. 10 N., R. 12 W., 380 

meters from north line and 770 meters 

from west line. Knife is 8 centimeters 

long. B, Imbricated pegmatite in 

mylonitized granitic rock, North is to the 

left. Alamo Dam 71⁄2-minute quadrangle. 

On south side of wash in sec. 6, T. 9 N., 

R. 12 W., 540 meters from north line and 

230 meters from west line. Hammer is 

32 centimeters long. C, End of lens of 

less ductile rock ranging in texture from 

amphibolite to metagabbro in more 

ductile, mylonitic, layered migmatitic 

gneiss. Mafic rock forms lens as much as 

4 meters thick and more than 20 meters 

long. Alamo Dam 71⁄2-minute quadrangle. 

In wash sec. 15, T. 10 N., R. 13 W., 330 

meters from south line and 330 meters 

from east line. Hammer is 32 centimeters 

long.
Mylonitic foliation in pre-Tertiary rocks Mylonitic foliation in Swansea Plutonic Suite Mineral lineation in pre-Tertiary rocks Mineral lineation in Swansea Plutonic Suite Ax

Axial planes of isoclinal and tight folds

Eastern Buckskin

Mountains

Central Buckskin

Mountains

Western Buckskin

Mountains

A B C DG H I J

M N O P

E 

K 

Q 

N = 603

N

c= 1, 3, 6, 9%

c= 1, 4, 7, 10%

c= 1, 4, 6, 8% c= 1, 4, 7, 11% c= 2, 5, 10, 15% c= 2, 5, 10%

c= 1, 3, 6, 10% c= 1, 5, 10% c= 1, 4, 8, 12%

c= 1, 5, 10% c= 1, 5, 10, 15% c= 2, 4, 8% c= 1, 5, 10, 15% 

N = 38

N

N = 498

N

N = 32

N

N = 57

N

N = 265

N

N = 55

N

N = 255

N

N = 48

N N 

N = 215

N

N = 71

N

N = 186

N

N = 65

N N 

Figure 26. Structures in the Buckskin Mountains synchronous with mylonitization during the Miocene.
40 Geology of the Northern Part of the Harcuvar Complex, West-Central Arizona

Mylonitic foliation Mylonitic lineation Faults

c= 1, 5, 10, 15% c= 1, 5, 10, 15%

N A N B N C

N = 56 N = 56

Figure 27. Structures in the Little Buckskin Mountains formed during and after mylonitization.

c= 1, 4, 8, 12%

c= 1, 5,10%

pole to axial plane

axis c= 2, 4, 7% c= 1, 10, 20%

c= 1, 8, 16, 24% c= 1, 5, 12, 20% c= 1, 5, 15, 25%

c= 1, 5, 15, 25% c= 1, 5, 15, 30% c= 1, 5, 15, 30%

N = 63

N

N = 47

N

N = 75

N

N = 45

N

A B C D

E F G H

I J K L

N = 25

N

N = 55

N

N = 35

N

N = 42

N

N

N = 49

N N

N = 24

N

Smith Peak

and Date Creek

Ranch SW

ECP Peak

and Alamo

Dam SE

Eastern

Harcuvar

Mountains

KXlg foliation

Minor folds Nonmylonitic foliation Nonmylonitic lineation Lineation E 4km

Kg foliation KXlg lineation Kg lineation

Figure 28. Structures in the eastern Harcuvar Mountains formed before and during mylonitization. Kg, granite of Tank Pass; 

KXlg, Proterozoic and Cretaceous layered gneiss.
Structure 41

antiform curves to a N. 80° E. trend. Thus, the N. 70° E. trend 

of the axes of the girdle formed by poles to foliation in the 

eastern Buckskin Mountains is approximately parallel with 

average trend of the larger structure.

The mylonitic foliation in the Swansea Plutonic Suite 

forms a different pattern. Poles to foliation form poorly developed girdles around northwest-trending axes ranging from N. 

47° W. in the eastern Buckskin Mountains (fig. 26B) to N. 8°

W. in the western Buckskin Mountains (fig. 26N). The axes to 

the girdle are horizontal in the western and central Buckskin 

Mountains and plunge 15° northwest in the eastern Buckskin 

Mountains. The Swansea Plutonic Suite underlies the Clara 

Peak antiform and the Swansea synform, two of the major 

northeast-trending folds in the Buckskin Mountains. One 

might assume that the pattern of foliation reflects those structures, as the pattern of foliation in the pre-Miocene rocks does. 

Northwest-trending folds in the Swansea area are documented 

by Osborne (1981) and Woodward (1981). Those folds do not 

deform the detachment fault nearby, but since they deform 

mylonitic foliation in Miocene rocks, they must be of Miocene 

age. Detailed structural studies in the Battleship Peak area 

show that poles to foliation in the Precambrian rocks form a 

well-developed girdle around a horizontal northwest-trending 

axis, and poles to foliation in metasedimentary cover rocks 

form a poorly developed girdle of similar orientation (Marshak 

and Vander Meulen, 1989). 

The difference in the statistical pattern of the foliation 

in the western Buckskin Mountains obtained by Woodward 

(1981), Osborne (1981), and Marshak and Vander Meulen 

(1989) compared to the one described here is probably due to 

the small size of the areas they studied compared to the much 

larger area here compiled. The diagram of foliation presented 

here may reflect the effects of both the earlier folding along 

northwest trends and the later folding along northeast trends.

In the Harcuvar Mountains, foliation similarly tends to be 

gently dipping, and poles to the foliation form girdles perpendicular to northeast-trending axes (fig. 28A, E, B, F). The 

foliation diagrams from both the Late Cretaceous granite and 

the older rocks have similar patterns. In the Smith Peak and 

the Date Creek Ranch SW 71⁄2-minute quadrangles (fig. 28A, 

B), the poles to foliation form a girdle whose axis trends N. 

74° E. and plunges gently southwest. The trend of the Harcuvar antiform is about N. 75° E. at its eastern end. In the ECP 

and Alamo Dam SW 71⁄2-minute quadrangles, the average axis 

of the girdle formed by the poles to foliation in the Cretaceous 

and older rocks is about N. 65 E. (fig. 28E, F), and the antiform trends N. 60° E.

Foliation and bedding in Phanerozoic metasedimentary 

rocks were compiled separately to see if the pattern differed 

from that in older or younger rocks. The metasedimentary 

rocks form slices in the older rocks. Data from the large slice 

south of Battleship Peak (Marshak and Vander Meulen, 1989) 

are not compiled on our diagrams, but we compare those 

data with our data from other slices. The pattern formed by 

poles to foliation in the metasedimentary rocks (fig. 23A, E) 

resembles that in the older layered gneiss. Detailed study in 

the Battleship Peak area where the metasedimentary slice is as 

much as 1,200 m thick (Marshak and others, 1987) produces a 

similar result; the poles to foliation in the basement rocks and 

the metasedimentary cover have the same distribution pattern 

(Marshak and Vander Meulen, 1989).

Few foliation planes in the eastern Buckskin Mountains 

lack northeast-trending stretched mineral lineation. They have 

various attitudes, although most dip gently to moderately like 

the mylonitic foliation (fig. 23B). In the central and western 

Buckskin Mountains, the poles to foliation lacking northeasttrending lineation form moderately developed girdles with 

northwest-trending axes and horizontal to gently northwest 

plunge (fig. 23F, I). We tried to detect the pattern of the older 

foliation by plotting the poles to all the foliation planes that 

contain the older northwest-trending lineation. We combined 

data from the Buckskin Mountains and the Harcuvar Mounains in order to have enough points to make a contour diagram. The pattern we obtained is not distinctly different from 

others; it resembles that from the metasedimentary rocks in the 

eastern Buckskin Mountains (fig. 23A).

In the Little Buckskin Mountains, the direction of the 

mylonitic lineation (fig. 27B) has two statistical highs about 

22° apart. This could be caused by an irregularity in the path 

of ductile flow of the rock during formation of the lineation or 

a fold younger than the lineation. The distribution of the foliation does not show similar highs, but any irregularity formed 

during formation of the lineation or late deformation of the 

lineation would have been around a steeply plunging axis and 

would not have shown well on the gently dipping mylonitic 

foliation.

Minor Folds

Tight and isoclinal minor folds (figs. 29, 30) are widespread in the mylonitic layered gneiss throughout the northern 

Harcuvar complex. Axial planes of these folds are parallel 

with mylonitic foliation (fig. 26E, K, Q). Axes may have 

diverse trends in folds seen in one large outcrop or nearby 

outcrops, but they tend to have a weak to moderate statistical 

northeast trend (fig. 26F, L, R). 

These folds are more numerous in the eastern Buckskin 

Mountains than in the central and western Buckskin 

Mountains. In the eastern Buckskin Mountains, poles to axial 

planes form a poorly developed girdle around an axis trending 

N. 37° E. and plunging 7 southwest (fig. 26E). The plunge of 

this girdle is similar to that of the mean value of the mylonitic 

lineation in the pre-Tertiary rocks of the eastern Buckskin 

Mountains. Axes of these folds are generally gently plunging 

and have varied strikes (fig. 26F). However, the statistical 

mean of their strike is N. 62° E. plunging 5° southwest. These 

are the B1 folds of Shackelford (1976, 1989) in the Rawhide 

Mountains, which form the northern part of the eastern 

Buckskin Mountains as used in this report. The distribution of 

the fold axes found by Shackelford in that part of the eastern 

Buckskin Mountains area differs from our compilation for the 

entire eastern Buckskin Mountains in that no concentration 
42 Geology of the Northern Part of the Harcuvar Complex, West-Central Arizona

of axial trends is apparent. The folds in the Rawhide 

Mountains tend to plunge southerly more steeply than in 

the entire area of the eastern Buckskin Mountains because 

the foliation in the mylonitic gneiss, to which the axial 

planes of the B1 folds are parallel, has southwesterly dips. 

No such folds were found in the mylonitic foliation in the 

Oligocene(?) and Miocene Swansea Plutonic Suite, but 

some were found in mylonite derived from Proterozoic 

granitic rock in the eastern Buckskin Mountains.

In the central Buckskin Mountains, axial planes have 

gentle dips parallel to that of the foliation (fig. 25K), and 

the axes are scattered but have some concentration in a 

northeasterly direction (fig. 25L). In the western Buckskin 

Mountains, these folds similarly have axial planes parallel with foliation (fig. 25Q), but the axes have a better 

developed northeast trend (fig. 25R) than in the central 

Buckskin Mountains. In the Battleship Peak area of the 

western Buckskin Mountains, folds of this type occur 

in the cover rocks and were designated class B folds by 

Marshak and Vander Meulen (1989). The basement rocks 

in the area of their study are plutonic and lack the layering that shows this type of fold elsewhere in the Buckskin 

Mountains. Class A folds are rootless isoclinal folds with 

thick hinges, and they grade to class B folds. A compilation of the axial planes and axes of the two types (called 

F1 and F2 in fig. 8 of Marshak and Vander Meulen, 1989) 

shows that their axial planes are parallel to the foliation, 

and their axes are distributed around the compass but 

have concentrations in east and northeast directions and 

gentle plunges to the west and southwest down the foliation planes. This pattern of folding is identical to that in 

the layered gneisses (fig. 26Q,R).

Tight and isoclinal folds in the eastern Harcuvar 

Mountains form a pattern similar to that of the foliation 

and lineation. Poles to the axial planes form a girdle with 

a horizontal, northeast-trending axis. The fold axes trend 

northeast and are subhorizontal, but a few have other 

strikes and are also horizontal (fig. 28I).

Older Foliation

Perhaps the oldest structures in the lower-plate 

rocks in the region are foliation planes that strike north 

to northeast and dip steeply. Only a few outcrops of 

layered gneiss in the area of this report have that attitude. In the Harcuvar Mountains (Reynolds and Spencer, 

1993; Drewes and others, 1990) and in the Harquahala 

Figure 29. Hinge of northeast-trending fold formed by mylonitic 

granite gneiss. Alamo Dam 71⁄2-minute quadrangle. Sec. 24, T. 10 N., 

R. 13 W., 550 meters from south line and 750 meters from north line. 

Hammer is 32 centimeters long.

Figure 30. Minor tight folds in mylonitic biotitehornblende gneiss. Reid Valley 71⁄2-minute quadrangle. 

In wash, sec. 29, T. 10 N., R. 13 W., 850 meters from south 

line and 210 meters east of quadrangle boundary. Hammer 

head is 18 centimeters long.
Summary of Structural and Metamorphic Events 43

Mountains (DeWitt and others, 1988), foliation of that trend is 

attributed to Early Proterozoic deformation and metamorphism 

because such trends are typical of the Proterozoic rocks in the 

Colorado Plateau transition zone to the northeast, although 

variations from this attitude are present (Anderson and others, 

1955; Bryant, 1992a; Karlstrom and Bowring, 1993).

In the western Harcuvar Mountains, four deformational 

events of Cretaceous age based on structural overprinting 

relations are found (Reynolds and Spencer, 1993). Two of 

them formed new foliation and northwest-trending lineation. 

Evidence for the older one is confined to the westernmost part 

of the Harcuvar Mountains. The younger one is younger than 

the granite of Tank Pass and is probably the one that formed 

the northwest-trending lineation in the Buckskin Mountains 

and eastern Harcuvar Mountains.

In the eastern Harcuvar Mountains, foliation planes 

lacking prominent northeast-trending mineral lineation have 

a distribution statistically similar to that of the mylonitic 

foliation. (Compare fig. 28J with 28A, B, E, F.) The lack of 

difference in the patterns of foliation containing the northwesttrending lineation and the foliations containing the Miocene 

northeast-trending mylonitic lineation suggests that the gently 

to moderately dipping foliation may have formed in part or 

entirely during one or more deformations during the Cretaceous. Some or all of such foliation may have formed during 

Cretaceous thrusting in which Paleozoic and Mesozoic sedimentary and volcanic rocks were overidden by thrust sheets 

of Proterozoic rock. Two of the deformations in the western 

Harcuvar Mountains were associated with south-to-southwest 

vergent thrusting, and they are correlated with the southwest 

movement on the Hercules thrust system in the Granite Wash 

Mountains and southward movement on the Harquahala thrust 

in the Harquahala Mountains (Reynolds and Spencer, 1993). 

In order to get the Phanerozoic rocks in the upper plate of 

the Rawhide detachment in the Buckskin Mountains down 

to depths sufficient for regional greenschist facies metamorphism, thrust sheets of Proterozoic basement rock must have 

overidden them during Cretaceous deformation in the Maria 

fold and thrust belt (fig. 1). Slices of metamorphosed Paleozoic and possibly Mesozoic rock exposed in the northern 

Harcuvar complex in the Buckskin Mountains may represent 

the remains of slices initially formed during thrusting. The 

Miocene extensional deformation could not have caused those 

rocks to be buried, but it exhumed the rocks buried during the 

Cretaceous orogeny.

Summary of Structural and 

Metamorphic Events

Layered gneisses of the northern Harcuvar complex are 

intruded by Early Proterozoic granitic rocks and consequently 

are of Early Proterozoic age or older. Because no Archean 

rocks have yet been found in the region, the gneisses are considered to be of Early Proterozoic age. They were deformed 

and perhaps migmatized in the Early Proterozoic. The protoliths of the nongranitic layers in the gneisses have igneous 

compositions ranging from basalt to rhyolite, but locally some 

layers were derived from shale. Probably the rocks were metamorphosed to at least amphibolite facies at that time because 

upper-plate Proterozoic rocks in the Poachie Range and east 

of the Harcuvar Mountains were metamorphosed at that grade. 

Granitic magmas intruded the lower-plate rocks during the 

Middle Proterozoic.

We do not know whether Middle Proterozoic diabase 

intruded the rocks of the Harcuvar complex. Sheets, dikes, and 

small intrusions of diabase are widespread in the Proterozoic 

rocks of the transition zone and in the upper-plate rocks of the 

Basin and Range province to the north. If diabase intruded 

during the Middle Proterozoic, it would have been converted 

to amphibolite during Cretaceous metamorphism and would 

be difficult to distinguish in the field from older mafic metaigneous rocks.

In Late Jurassic time, diorite, trondhjemite, and, possibly, 

granite intruded rocks in the eastern Buckskin Mountains. The 

only Jurassic plutons we recognize in the northern Harcuvar 

complex are in that area; elsewhere, there may be small, 

unrecognized bodies of this age.

In Early Cretaceous time about 110 m.y. ago, the rocks 

were migmatized and metamorphosed under amphibolite 

facies conditions. They were thrust south or southwest over 

Paleozoic and Mesozoic sedimentary and volcanic rocks. 

Probably much of the older structure in the Proterozoic rocks 

was obliterated by the formation of gently dipping foliation 

associated with thrusting and regional metamorphism and 

migmatization in Early and Late Cretaceous time. Some of the 

sedimentary rocks were caught up as lenses in the fault zones 

and were metamorphosed under amphibolite facies conditions. 

In Late Cretaceous time, large and small plutons and many 

sills of granite and pegmatite intruded the Proterozoic basement rocks and the slices of metasedimentary rock. Uplift and 

cooling in latest Cretaceous and early Tertiary time followed.

 In early Miocene and possibly late Oligocene, diorite, 

gabbro, granodiorite, and granite of the Swansea Plutonic 

Suite intruded the Proterozoic basement rocks before or during 

extension. The more felsic rocks of the suite carry numerous 

xenoliths of Middle Proterozoic granitic rock derived from 

deeper levels in the crust. Temperatures and pressures were 

suitable for ductile flow of the felsic rocks. The intrusive mass 

now forms a semiconcordant northeast-southwest-trending 

body parallel to the direction of regional extension. Some 

northwest-trending gabbro dikes were emplaced at about the 

same time. Extension by ductile flow formed foliation and 

lineation in many of the Miocene plutonic rocks. The ductile 

flow brought rocks out from 15–20-km depth beneath the 

transition zone and up into a brittle environment in which 

the detachment fault zone now bounding the Buckskin and 

Harcuvar core complexes formed. Cooling ages of the lowerplate rocks are younger than the stratigraphic ages of many of 

the rocks in the upper plate. Extension continued to at least 13 

m.y. ago, and movement on the detachment fault in the eastern 
44 Geology of the Northern Part of the Harcuvar Complex, West-Central Arizona

Buckskin Mountains continued until at least that time. Weak 

regional extension continued in the upper plate northeast of 

the Buckskin Mountains until at least 9 m.y. ago, as indicated 

by tilting of basalts of that age.

Acknowledgments

Steve Reynolds and Jon Spencer encouraged me to start 

mapping in the Buckskin Mountains and furnished much 

valuable advice and good times in the field. This research was 

supported by the Geologic Framework, National Geologic 

Mapping, Pacific-Arizona Crustal Experiment (PACE), and 

COGEOMAP programs of the U.S. Geological Survey. Early 

in this work, L. David Nealey helped me get started in studying the geochemistry of the plutonic rocks in the Buckskin 

Mountains. We appreciate technical reviews of this report by 

Jon Spencer and John C. Reed, Jr.

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Geological Survey Open-File Report 90–558, p. 166–172.

Wilson, E.D., 1960, Geologic map of Yuma County, Arizona: 

University of Arizona and Arizona Bureau of Mines, scale 

1:375,000.

Wilson, E.D., and Moore, R.T., 1959, Geologic map of 

Mohave County, Arizona: University of Arizona and Arizona Bureau of Mines, scale 1:375,000.

Wooden, J.L., and DeWitt, Ed, 1991, Pb isotopic evidence for 

the boundary between the Early Proterozoic Mojave and 

Central Arizona crustal provinces in western Arizona in 

Karlstrom, K.E., ed., Proterozoic geology and ore deposits of Arizona: Arizona Geological Society Digest 19, 

p. 27–50.

Wooden, J.L., and Miller, D.M., 1990, Chronologic and 

isotopic framework for Early Proterozoic crustal evolution 

in the eastern Mojave desert region: Journal of Geophysical 

Research, v. 95, no. B12, p. 20133–20146.

Woodward, R.J., 1981, The structural geology of the Swansea area, east-central Buckskin Mountains, Yuma County, 

Arizona: Los Angeles, University of Southern California, 

unpublished Master’s thesis, 106 p.

Wright, J.E., Anderson, J.L., and Davis, G.A., 1986, Timing of 

plutonism, mylonitization, and decompression in a metamorphic core complex, Whipple Mountains, CA: Geological Society of America Abstracts with Programs, v. 18, 

no. 2, p. 201.
Appendix. Location and Description of Analyzed Samples 49

Appendix. Location and Description of 

Analyzed Samples

1. Mylonitic medium- to coarse-grained granitic rock containing sills of garnet-bearing leucogranite and pegmatite 

in Proterozoic granodiorite to granite gneiss unit. 930 m 

N., 59°E. from spot elevation 3,110. SE11⁄4 sec. 1, T. 9 

N., R. 13 W., 34°08'52"N., 113°33'40"W., Alamo Dam 

71⁄2-minute quadrangle, La Paz County. Outcrop in wash 

80 m north of right-angle turn. Anhedral quartz to 1.5 mm 

in diameter. The largest grains are between pulled-apart 

fragments of plagioclase crystals. Plagioclase (An30) in 

porphyroclasts as much as 4 mm long, biotite as much as 

1 mm long, and potassic feldspar in a matrix with a grain 

size of 0.05–0.3 mm. Accessory sphene, allanite, apatite, 

and zircon. Secondary chlorite from biotite and carbonate. 

Major-oxide and trace-element concentrations, see table 4.

Field no. B–268A.

2. Well-foliated porphyritic biotite granite containing 

potassic feldspar crystals as much as 2 cm long from 

Proterozoic granodiorite to granite gneiss unit. 820 m S. 

26° W. from spot elevation 2,177, NW corner sec. 15, 

T. 8 N., R. 15 W., 34°02'38"N., 113°47'53"W., Butler 

Pass 71⁄2-minute quadrangle, La Paz County. Quartz as 

much as 2 mm in diameter in aggregates of anhedral and 

strained grains, subhedral plagioclase (An20) as much as 

2 mm long, anhedral potassic feldspar as much as 2 cm 

long containing inclusions of plagioclase, myrmekite, and 

biotite, synkinematic biotite as much as 3 mm long, accessory apatite, sphene, epidote, and zircon. A well-recrystallized porphyritic granite. Major-oxide and trace-element 

concentrations, see table 4. Field no. B–587.

3. Medium- to fine-grained biotite granite gneiss from Proterozoic granodiorite to granite gneiss unit. Outcrop 60 m 

west of 2,404-ft knob on ridge in north-central sec. 2, T. 

8 N., R. 15 W., 34°04'09"N., 113°47'14"W., Butler Pass 

71⁄2-minute quadrangle, La Paz County. Anhedral strained 

quartz as much as 0.5 mm in diameter, anhedral plagioclase (An20) as much as 1.2 mm long, anhedral potassic 

feldspar as much as 1.6 mm long, synkinematic biotite as 

much as 2 mm long, accessory sphene, opaque mineral, 

allanite, epidote, and apatite, and secondary chlorite from 

biotite and epidote. A completely recrystallized plutonic 

rock showing few of the cataclastic features typically 

associated with Tertiary mylonitization. Major-oxide and 

trace-element concentration, see table 4. Field no. B–587.

4. Well-foliated fine-grained phase of granite gneiss from 

Proterozoic granodiorite to granite gneiss unit. Outcrop 

on 2,383-ft knob on ridge in north part of sec. 5, T. 8 

N., R. 14 W., 34°04'10"N., 113°44'36"W., Butler Well 

71⁄2-minute quadrangle, La Paz County. Anhedral quartz, 

plagioclase (An20), potassic feldspar with a grain size of 

about 0.8 mm, synkinematic biotite as much as 1 mm 

long, accessory sphene, epidote, and apatite. A completely 

recrystallized plutonic rock. Major-oxide and traceelement concentrations, see table 4. Field no. B–665.

5. Biotite granite gneiss from Proterozoic granodiorite to 

granite gneiss unit. Has flattened feldspar grains to 5 mm 

long. Local northeast-trending lineation. Cut by pegmatite stringers 2–30 cm thick. From outcrop on ridge 

at 2,140-ft altitude, 34°04'42"N., 113°43'04"W., Butler 

Well 71⁄2-minute quadrangle, La Paz County. Anhedral 

quartz as much as 1 mm in diameter. strongly strained and 

broken into subindividual grains, anhedral plagioclase 

(An20) as much as 2 cm in diameter, anhedral microcline 

as much as 2 mm in diameter, synkinematic biotite as 

much as 1 mm long, accessory sphene, allanite, epidote, 

opaque mineral, and zircon and secondary chlorite from 

biotite. A completely recrystallized plutonic rock showing 

minor cataclastic effects. Major-oxide and trace-element 

concentrations, see table 4. Field no B–653.

6. Mylonitic porphyritic granite from Proterozoic granodiorite to granite gneiss unit in slab slumped from outcrop 

1,085 m N., 41°W. from spot elevation 2,461 25 m down 

wash from Field no. 565. Sec. 31 T. 10 N., R. 12 W., 

34°09'59"N., 113°32'50"W., Alamo Dam 71⁄2-minute 

quadrangle, La Paz County. Anhedral quartz as ductilely 

deformed grains as much as 4 mm long and as lenticular 

aggregates as much as 6 mm long, plagioclase (An26)) as 

much as 0.7 mm in diameter, anhedral potassic feldspar as 

much as 1.5 mm in diameter and a lenticular mass 1 cm 

long, possibly a metaphenocryst, and synkinematic to 

postkinematic biotite as much as 0.3 mm long, in a matrix 

of recrystallized quartz, plagioclase, and potassic feldspar 

having a grain size of 0.05–0.3 mm. Accessory sphene, 

opaque mineral, allanite, apatite, and zircon and secondary chlorite from biotite. U-Pb age determination, see 

figure 13. Major-oxide and trace-element concentrations, 

see table 4. Field no. B–565A.

7. Mylonitic granite from Proterozoic granodiorite to granite 

gneiss unit. Outcrop in wash 1,110 m N., 41°W. from spot 

elevation 2,461. Sec. 31, T. 10 N., R. 12 W., 34°09'59"N., 

113°32'50"W., Alamo Dam 71⁄2-minute quadrangle. 

Fine to medium grained and intruded by pegmatites and 

leucogranite. Anhedral quartz as ductilely deformed 

grains as much as 1.5 mm long and aggregates as much 

as 2 mm long. Anhedral, partly sausseritized plagioclase 

(An25) as much as 1 mm in diameter. Anhedral potassic feldspar as much as 1 mm in diameter. Chlorite after 

synkinematic biotite. Accessory biotite, allanite, sphene, 

apatite, zircon, and opaque mineral. Secondary epidote 

and concentrated in a veinlet perpendicular foliation. 

Carbonate in thin films parallel foliation. U-Pb zircon age 

determination, see figure 13. Field no. B–565.
50 Geology of the Northern Part of the Harcuvar Complex, West-Central Arizona

8. Mylonitic porphyritic granodiorite cut by aplite dikes 

of the youngest phases of the Swansea Plutonic Suite. 

Interpreted to be inclusion in Swansea Plutonic Suite. 

Content and size of potassic feldspar porphyroclasts 

vary. Spoil from cut along abandoned railroad to 

Swansea. 470 m N. 37° W. from Bench Mark 1608 on 

Lincoln Ranch road. North part sec. 16, T. 9 N., R. 15 

W., 34°07'26"N., 113°49'29"W., Butler Pass 71⁄2-minute 

quadrangle, La Paz County. Fractured and strained 

porphyroclasts of potassic feldspar as much as 1.2 cm 

long, subhedral plagioclase (An32) porphyroclasts as 

much as 2 mm long, anhedral ductilely drawn-out quartz 

grains as much as 0.5 mm long interstitial to the feldspar 

porphyroclasts, anhedral to subhedral porphyroclasts of 

dark green hornblende as much as 0.7 mm long, biotite 

as bent porphyroclasts as much as 1 mm long and 0.1 

mm and smaller recrystallized grains. Accessory sphene, 

allanite, apatite, epidote, and opaque mineral. Majoroxide and trace-element concentrations, see table 4. Field 

no. B–495–92.

9. Mylonitic porphyritic granodiorite or granite from 

0.8-m-thick inclusion concordant with mylonitic foliation 

in Swansea Plutonic Suite from roadcut in Swansea road 

550 meters N., 35° E. from township corner at 1,689-ft 

altitude. SW11⁄4 sec. 31, T. 10 N., R. 15 W., 34°09'46"N., 

113°51'58"W., Swansea 71⁄2-minute quadrangle, La Paz 

County. Quartz 0.05–0.2 mm in diameter forms aggregates of ductilely drawn-out grains and local mosaictextured aggregates. Potassic feldspar in broken porphyroclasts as much as 1 cm long having some microcline 

twinning, partly altered plagioclase in broken porphyroclasts as much as 3 mm in diameter, biotite as much as 

0.4 mm long and new biotite <0.2 mm, epidote contemporaneous with both the old and new biotite, and very fine 

grained quartz and feldspar mortar. Accessory sphene, 

allanite, epidote, and opaque mineral, and secondary chlorite from biotite. Neomineralization during mylonitization 

epidote, quartz, and biotite. U-Pb age determination, see 

figure 13. Major-oxide and trace-element concentrations, 

see table 4. Field no. B–562.

10. Mylonitic porphyritic granite cut by dikes of aplite, the 

youngest member of the Swansea Plutonic Suite. Strongly 

mylonitized and cut by fractures containing epidote. 

Outcrop on east side of wash 710 m N. 88° E. of top of 

hill 1452. At south margin of sec. 23, T. 10 N., R. 15 

W., 34°10'52"N., 113°47'12"W., Swansea 71⁄2-minute 

quadrangle, La Paz County. Ductilely elongated and 

strained mosaic-textured aggregates of quartz broken 

porphyroclasts of locally perthitic potassic feldspar as 

much as 2 cm long, having locally developed microcline 

twinning, porphyroclasts of sericitized plagioclase as 

much as 2 mm long, secondary chlorite from biotite, 

and epidote. Very fine grained quartz-feldspar mortar 

containing chlorite and epidote between porphyroclasts 

and in zones parallel with the mylonitic foliation. 

Accessory allanite, zircon, and opaque mineral. 

Major-oxide and trace-element concentrations, see table 4. 

Field no. B–1111.

11. Mylonitic biotite-hornblende gneiss from outcrop in 

wash. Cut by biotite trondhjemite and muscovite trondhjemite sills that are now disrupted into “fish” by ductile 

deformation. 280 m N. 79° W. from spot elevation 2,461 

in SE part of sec. 31, T. 10 N., R. 12 W., 34°09'34"N., 

113°32'31"W., Alamo Dam 71⁄2-minute quadrangle, La 

Paz County. Anhedral plagioclase (An39) as much as 

1 mm in diameter and locally bent and broken subhedral 

blue-green hornblende as much as 3 mm long. Accessory 

minerals are apatite, zircon, sphene, epidote, and opaque 

mineral. Major-element and trace-element concentrations, 

see table 5. Field no. B–93–2C.

12. Mylonitized hornblende diorite or gabbro having feldspar porphyroclasts as much as 2 cm in diameter and 

variable feldspar content. Contains some layers of more 

felsic mylonite. Scattered porphyroclasts as much as 2 

cm in diameter. Contains a 10-cm-thick pegmatite lens. 

Some fractures with epidote along them. Outcrop in wash 

140 m south of spot elevation 2,461, NE corner sec. 6, T. 

9 N., R. 12 W., 34°09'27"N., 113°32'22"W., Alamo Dam 

71⁄2-minute quadrangle, La Paz County. Anhedral rounded 

porphyroclasts of plagioclase (An28) as much as 1 mm in 

diameter and dark green hornblende as much as 1 mm 

long in a matrix containing quartz, plagioclase, and hornblende having a grain size of 0.05–0.3 mm. Accessory 

allanite, apatite, sphene, and opaque mineral and secondary chlorite, epidote, and carbonate. A zone of microbreccia or ultramylonite concordant with the foliation cuts 

the rock and has a glassy or cryptocrystalline matrix and 

rounded grains of hornblende and plagioclase. U-Pb age 

determination, see figure 14. Major-oxide and traceelement concentrations, see table 5. Field no. B–566.

13. Well-foliated biotite trondhjemite gneiss cuts metadiorite. Outcrop in wash 280 m N. 79° W. from spot elevation 2,461 in SE part of sec. 31., T. 10 N., R. 12 W., 

34°09'34"N., 113°32'31"W., Alamo Dam 71⁄2-minute 

quadrangle. Anhedral plagioclase (An40) as much as 

1 mm in diameter, anhedral quartz 0.1-0.4 mm in grain 

size ranging from ductilely deformed to mosaic-textured 

aggregates between feldspar grains. Synkinematic biotite 

as much as 3 mm long. Accessory apatite, opaque mineral, and epidote. Major-oxide and trace element concentrations, see table 5. Field no. B–93–2A.

14. Well foliated muscovite trondhjemite gneiss cuts 

metadiorite. Outcrop in wash N. 79° W. from spot 

elevation 2,461, SE part sec. 31, T. 10 N., R. 12 W., 

34°09'34"N., 113°32'31"W., Alamo Dam 71⁄2-minute 

quadrangle, La Paz County. Anhedral plagioclase (An20)) 

generally about 0.5 but as much as 1 mm in diameter, 

quartz ductilely strung out to 2 mm long but mostly 
Appendix. Location and Description of Analyzed Samples 51

0.2–0.5 mm in diameter. Synkinematic muscovite as 

much as 2 mm long, accessory potassic feldspar, biotite, 

allanite, epidote, and opaque see table 5. Field no. 

B–93–2B.

15. Streaky, well-foliated, granite layer in well-layered 

sequence of biotite-quartz-feldspar gneiss, biotite plagioclase porphyroclast gneiss, granite gneiss, and some pegmatite. Proterozoic and Cretaceous layered gneiss unit. 

Outcrop contains lenticular zones of less mylonitized rock 

having folds with NE-trending axes and variable axial 

plane. Mylonitic rock has NE-trending mineral lineation. 

About 60 percent of rock is mylonitic. Outcrop 50 m west 

of sharp bend in wash 520 m N., 45o W. from spot elevation 3,375, W. margin sec. 7, T. 8 N., R. 10 W., 34°02'57", 

N., 113°20'55" W., Smith Peak 71⁄2-minute quadrangle, La 

Paz County. Mosaic-textured quartz plagiocase (An~20), 

and potassic feldspar 0.2–0.5 mm in diameter and a few 

porphyroclasts of potassic feldspar and plagioclase as 

much as 3 mm in diameter. Synkinematic to postkinematic biotite 0.05–0.25 mm long. Accessory sphene, apatite, zircon, allanite, myrmekite, and opaque mineral and 

secondary chlorite from biotite. U-Pb zircon age determination, see figure 15. Major-oxide and trace-element 

concentrations, see table 5. Field no. HA–118.

16. Fine- to medium-grained mylonitic biotite granite outcrop 

at sharp bend in one of channels in wash 910 m S. 49o 

W. from spot elevation 3,110. 34°08'16" N., 113°34'38" 

W., Alamo Dam 71⁄2-minute quadrangle, La Paz County. 

Quartz in anhedral grains as much as 2 mm long, anhedral 

plagioclase (An20) porphyroclasts as much as 2 mm long, 

anhedral, slightly perthitic potassic feldspar in a matrix 

of those minerals having a grain size of 0.1–0.3 mm. 

Synkinematic biotite as much as 0.5 mm long tends to 

be in aggregates. Accessory myrmekite, allanite, opaque 

mineral, sphene, zircon, and apatite. Secondary chlorite 

from biotite. Major-oxide and trace-element concentrations, see table 5. Field no. B–564.

17. Medium-grained hornblende gabbro of the Swansea 

Plutonic Suite weakly and locally foliated. Outcrop on 

west-facing slope of hill 750 m N. 38° E. of spot elevation 

1,668. NE1/4 sec. 13, T. 9 N., R. 16 W., 34°07'22" N., 

113°52'37" W., Powerline Well 71⁄2-minute quadrangle, 

La Paz County. Euhedral to subhedral normally zoned 

plagioclase (An40) as much as 3 mm long, and anhedral 

to subhedral brown to olive green, green, and light green 

partly altered hornblende as much as 3 mm long. Some 

new grains of light green actinolite. Accessory apatite and 

sphene and secondary chlorite from hornblende. Epidote and chlorite in a late fracture <0.1 mm wide. A few 

discontinuous shear zones contain chlorite and actinolite. 

Major-oxide and trace-element concentrations, see table 

6. Field no. B–469.

18. Coarse-grained hornblende gabbro of the Swansea Plutonic Suite locally has some weakly developed foliation 

and some fractures containing epidote. Outcrop in wash 

400 m S. 71° W. from spot elevation 1,236, south part sec. 

10, T. 10 N., R. 14 W. 34°13'03" N., 113°42'19" W., Reid 

Valley 71⁄2-minute quadrangle, La Paz County. Euhedral 

to subhedral plagioclase (An40) as much as 5 mm long 

and brown, green, and light green, subhedral to anhedral, 

partly altered hornblende as much as 5 mm long interstitial to plagioclase grains. Accessory apatite and opaque 

mineral and secondary Mg-rich chlorite in fractures in 

feldspars. Cut by a shear zone with mortar along it. U-Pb 

zircon age determination, see figure 16. Major-oxide 

and trace-element concentrations, see table 6. Field no. 

B–563.

19. Quartz-bearing biotite-hornblende diorite of the Swansea 

Plutonic Suite from outcrop on road west of Swansea road 

north of Midway. 310 m S. 64° W. from spot elevation 

1,629, NW1⁄4 sec. 26, T. 9 N., R. 16 W., 34°05'47" N., 

113°54'07" W., Powerline Well 71⁄2-minute quadrangle, 

La Paz County. Euhedral to subhedral normally zoned 

plagioclase (An40-28) as much as 3 mm long locally bent, 

broken, and sericitized, biotite as much as 1.5 mm long, 

locally bent, anhedral to subhedral dark green hornblende 

as much as 1 mm long, interstitial quartz 0.1–0.2 mm in 

diameter, accessory sphene, allanite, apatite, and opaque 

mineral, and secondary sericite and chlorite. An igneous 

texture with a weak local cataclastic overprint. Majoroxide and trace-element concentrations, see table 6. Field 

no. B–458A.

20. Mylonitic hornblende-bearing biotite granodiorite gneiss 

of the Swansea Plutonic Suite from cut along abandoned 

railroad to Swansea. 470 m N. 37° W. from Bench Mark 

1608 on Lincoln Ranch road. N. central sec. 16, T. 9 

N., R. 15 W., 34°07'26" N., 113°49'29" W., Butler Pass 

71⁄2-minute quadrangle, La Paz County. Quartz as ductilely elongate grains and aggregates as much as 2 mm 

long, bent and broken euhedral to anhedral rounded grains 

of plagioclase as much as 3 mm long, porphyroclasts of 

potassic feldspar as much as 3 mm long, and two generations of biotite. Older brown bent biotite as much as 

0.8 mm long and very fine grained greenish-brown biotite 

that formed during mylonitization. Dark green hornblende 

in locally broken porphyroclasts as much as 1.3 mm long. 

Quartz-feldspar mortar between larger grains. Accessory 

sphene, allanite, apatite and opaque mineral and secondary epidote and chlorite. Major-oxide and trace-element 

concentrations, see table 6. Field no. B–495B.

21. Mylonitic porphyritic biotite granodiorite of the Swansea 

Plutonic Suite from roadcut on Swansea road 550 m N. 

35o E. from township corner at 1,689-ft altitude. SW1⁄4 

sec. 31, R. 10 N., R. 15 W., 34°09'46" N., 113°51'58" 

W., Swansea 71⁄2-minute quadrangle, La Paz County. 

Ductilely deformed quartz as strung out aggregates as 
52 Geology of the Northern Part of the Harcuvar Complex, West-Central Arizona

much as 2 mm long having a strained mosaic texture and 

average grain size of 0.05 mm, anhedral to subhedral 

plagioclase (An25), as much as 2 mm long, having local 

oscillatory zoning, anhedral to subhedral potassic feldspar in porphyroclasts as much as 0.4 mm long, locally 

showing weakly developed microcline twinning, and a 

matrix of quartz, feldspar, and new biotite with a grain 

size of <0.05 mm except for epidote, which is as long as 

0.15 mm. Accessory apatite, sphene, allanite, zircon, and 

opaque minerals. Major oxide and trace-element concentrations, see table 6. Field no. B–562–92.

22. Aplite sill 1 m thick concordant with mylonitic foliation 

in the Swansea Plutonic Suite. Latest phase of the suite. 

Cut in abandoned railroad grade to Swansea 470 m N. 

37° W. from Bench Mark 1608 on Lincoln Ranch road. 

North-central sec. 16, T. 9 N., R. 15. W., 34°07'26" N., 

113°49'29" W., Butler Pass 71⁄2-minute quadrangle, La 

Paz County. Euhedral phenocrysts of plagioclase (An30) 

as much as 3 mm long in a well-foliated, fine-grained 

matrix of 0.01–0.05 mm quartz, feldspar, and biotite and 

accessory sphene, allanite, epidote, and opaque mineral 

and secondary chlorite. Major oxide and trace-element 

concentrations, see table 6. Field no. B–495C.

23. Mylonitic potassic feldspar-bearing porphyritic granodiorite of the Swansea Plutonic Suite from dump of mine 

shaft at symbol 380 m N. 63° E. from spot elevation 

1,572, north edge sec. 28, T. 9 N., R. 16 W. 34°05'58" N., 

113°55'38" W., Powerline Well 71⁄2-minute quadrangle, 

La Paz County. Subhedral plagioclase (An25) as much as 

1 mm long, having some oscillatory and normal zoning 

potassic feldspar as much as 1 mm in diameter and quartz 

in aggregates of recrystallized grains as much as 2 mm 

long in a matrix of quartz, feldspar, and biotite with a 

grain size of 0.3 mm. Mortar of quartz, feldspar and new 

biotite <0.15 mm. Accessory sphene, hornblende, allanite, 

and opaque mineral and secondary chlorite and epidote. 

Major-oxide and trace-element concentrations, see table 

6. Field no. B–464.

24. Medium-grained, somewhat mylonitic monzogranite 

of the Swansea Plutonic Suite from top of 1,800-ft hill 

440 m N. 5° W. from Bench Mark 1608 on Lincoln Ranch 

road. NW1⁄4 sec. 16, T. 9 N., R. 15 W., 34°07'29" N., 

113°49'42" W., Butler Pass 71⁄2-minute quadrangle, La Paz 

County. Quartz in aggregates of ductilely deformed grains 

as much as 5 mm long, plagioclase (An29) and perthitic 

potassic feldspar in porphyroclasts as much as 2 mm long, 

and bent and sheared brown biotite as much as 1 mm 

long in a very fine grained matrix of quartz, feldspar, and 

greenish-brown biotite. Accessory sphene, zircon, apatite, 

and opaque mineral. Major-oxide and trace-element concentrations, see table 6. Field no. B–496.

25. Mylonitized granite of the Swansea Plutonic Suite 

containing feldspar porphyroclasts as much as 5 mm 

long. Outcrop in wash 840 m N. 81° E. from spot elevation 1,261 on top of hill. NE1⁄4 sec. 20, T. 10 N., R. 15 

W. 34°11'45" N., 113°50'13" W., Swansea 71⁄2-minute 

quadrangle, La Paz County. Quartz in matrix mostly 

0.1–0.2 mm but some elongate ductilely deformed grains 

as much as 2 mm long. Plagioclase (An~20)as much as 

3 mm long and potassic feldspar with poorly developed 

microcline twinning as porphyroclasts as much as 4 mm 

in diameter in a matrix of quartz, feldspar, and biotite with 

a grain size of as much as 0.5 mm. Biotite recrystallized 

during mylonitization has a grain size <0.15 mm. Accessory opaque mineral, apatite, zircon, sphene, and allanite. 

Secondary chlorite and epidote. U-Pb concordia diagram, 

see figure 16. Major-oxide and trace-element concentrations, see table 6. Field no. B–561.

26. Mylonitic medium-grained granite of the Swansea Plutonic Suite containing feldspar porphyroclasts as much 

as 5 mm long from crest of ridge 370 m S. 70° E. from 

spot elevation 2,008. West part sec. 22, T. 9 N., R. 16 W., 

34°06'30" N., 113°55'13" W., Powerline Well 71⁄2-minute 

quadrangle, La Paz County. Quartz in ductilely deformed 

grains as long as 0.3 mm, anhedral to subhedral, locally 

perthitic potassic feldspar in strained and broken porphyroclasts as much as 4 mm long, anhedral to subhedral 

plagioclase (An17) in porphyroclasts 3 mm long in matrix 

of quartz-feldspar mortar containing synkinematic biotite 

typically 0.05 mm but longer in the strain shadow of 

porphyroclasts. Some biotite as much as 0.6 mm long is 

included in porphyroclasts or is bent. Accessory sphene, 

zircon, and opaque mineral and secondary chlorite from 

biotite. Major-oxide and trace-element concentrations, see 

table 6. Field no. B–463.
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