VOLCANOES OF THE MCCULLOUGH MOUN by pengxiang

VIEWS: 45 PAGES: 45

									VOLCANOES OF THE MCCULLOUGH
MOUNTAINS, S OUTHERN N EVADA

Eugene Smith
Denise Honn
Racheal Johnsen

Department of Geoscience
University of Nevada, Las Vegas
Las Vegas, NV 89154-4010

Abstract

       The McCullough Mountains preserves a unique record of Miocene volcanism in the

western Lake Mead area of Nevada. The first recorded volcanism with a source in the

McCullough Mountains produced dacite and basalt of the Cactus Hill, Eldorado Valley,

McCullough Wash, and Colony volcanoes in the central McCullough Mountains (18.5-15.2 Ma).

These units lie on Precambrian basement and locally on the regionally extensive Peach Springs

Tuff. The next event produced the dominant volcanic construct, the McCullough stratovolcano

that accumulated over 400 m of andesite lava, agglo merate, and breccia of the Eldorado Valley

volcanic section (15.55 Ma) in the central and northern McCullough Mountains. Eruptions

occurring after 15.2 Ma are lower in volume and are mainly on the flanks of the McCullough
April 16, 2007, Volcanoes of the McCullough Mountains, Southern Nevada—Smith, Honn and Johnsen



Mountains stratovolcano. These include the eruption of (1) the McCullough Pass caldera and

outflow tuff (14.1 Ma), (2) Hidden Valley andesite including 300 m of andesite lavas erupted

from local centers (mainly cinder cones), (3) four Hidden Valley volcanoes on the west flank of

the McCullough stratovolcano (Mt. Ian, Mt. Sutor, Center Mountain, and Mt. Hanna) (13.07

Ma), and (4) the Henderson dome complex on the northern flank of the McCullough

stratovolcano. The volcanic rocks in the McCullough Mountains are calc-alkaline and vary in

composition from rhyolite to basalt. Intermediate compositions (andesite and dacite) prevail,

while basalt and rhyolite are rare. The trace element chemical signature displayed by volcanic

rocks in the McCullough Mountains (low Nb, Ti, Zr, P compared to primitive mantle) is an

indication of either a magma source in the continental lithosphere or lithospheric contamination.

Rhyolite and dacite probably formed by partial melting of crust. Mafic magmas (basalt and

andesite) either originated by melting of lithospheric mantle or reflect asthenospheric magmas

contaminated in the lithosphere.



Introduction

          The McCullough Mountains provide a unique view of pre-extensional volcanism in the

northern part of the Colorado River Extensional Corridor. Nearby volcanic terr ains in the

Eldorado and River Mountains and Highland Range were tilted and dismembered during mid-

Miocene extension (Anderson, 1971; Smith et al., 1990, Darvall, 1991, Faulds and Olsen, 2002;

Olsen, 1996; Olsen et al., 1999), but the McCullough Mountains were not extensively faulted

and volcanoes remained relatively intact. The McCullough Mountains lie in the southern Basin

and Range Province near the western border of the Colorado River Extensional Corridor (Figure


                                                                 2
April 16, 2007, Volcanoes of the McCullough Mountains, Southern Nevada—Smith, Honn and Johnsen



1). The range trends north-south from the City of Henderson in the Las Vegas Valley to the New

York Mountains of California. The McCullough Mountains were not studied in detail until the

mid-1980s due to their lack of mineral wealth and perceived minor role in terms of answering

questions of regional tectonic significance. Early studies in the McCullough Mountains by

Hewett (1956), Longwell (1965), and Bingler and Bonham (1973) provided the first descriptions

of the volcanic section. Carnotite occurrences in caliche on the west side of the McCullough

Mountains prompted Kohl (1978) to study parts of the Hidden Valley area. His mapping

identified the source of the Carnotite as the Erie Tuff (now identified as the Tuff of Bridge

Spring) and located dikes, volcanic domes and lava flows. Studies by UN LV began in 1984 as

part of a joint study with Lawford Anderson of the University of Southern California to

characterize the geology of the southern McCullough Wilderness area (Anderson et al., 1985).

Since this initial study, work by UNLV has established the volcanic stratigraphy and identified

over 15 volcanoes in the central and northern parts of the McCullough Mountains. The recent

establishment by the US Bureau of Land Management (BLM) of the Sloan Canyon National

Conservation Area in the northern and central McCullough Mountains has prompted more

detailed geological and volcanological studies of many of the volcanoes (Honn and Smith,

2005). This paper is a progress report of our work on the volcanoes of the McCullough

Mountains and provides a geologic and stratigraphic framework for future studies.



Geologic Summary

          The McCullough Mountains form the west side of the Eldorado Valley, an alluvial basin

ringed by mountains consisting of mainly Tertiary volcanic, plutonic and sedimentary rocks

(Figure 1). The Eldorado Valley and McCullough Mountains lie within the northern Colorado
                                                                 3
April 16, 2007, Volcanoes of the McCullough Mountains, Southern Nevada—Smith, Honn and Johnsen



River extensional corridor, a 50–100-km-wide extensional belt that formed during Miocene

crustal extension and rifting (e.g., Howard and John, 1987; Faulds et al., 1990). Both magmatism

and crustal extension migrated to the north in the extensional corridor during Miocene time

(Smith and Faulds, 1994; Faulds et al., 1999). In most places in the corridor, magmatism

occurred 1 to 4 m.y. before crustal extension. Surprisingly, there was little magmatic activity

during the period of peak extension (e.g., Gans et al., 1998; Smith and Faulds, 1994; Faulds et

al., 1995; Gans and Bohrson, 1998). Crustal extension in ranges near the southern part of the

Eldorado Valley began about 16 Ma and ceased about 11 Ma. In mountains bounding the

northern part of the Eldorado Valley, extension began at about 14 Ma and terminated between 8

and 10 Ma (Anderson, 1971; Gans et al., 1994; Faulds, 1995; Faulds et al., 1994, 1995). While

magmatism spread to the north in the Colorado River Extensional corridor during Miocene time,

volcanic activity in individual fields lasted for considerable periods of time. In the McCullough

Mountains, for example, there is a two million year record of volcanism, but this record is longer

if regional units like the Peach Springs Tuff (18.5 Ma) with a source outside the McCullough

Mountains are included.


          During the period of active volcanism in the McCullough Mountains (16-12 Ma) coeval

volcanism occurred in neighboring ranges. Volcanic rocks in each range for the most part

erupted from local volcanoes and few volcanic units extend from range to range (Figure 2).

Exceptions are the Tuff of Bridge Spring (15.2 Ma) and the Peach Springs Tuff (18.5 Ma). Both

are regional units that are excellent stratigraphic marker beds (Figure 2).


          Precambrian basement rocks (1.7 Ga) composed mainly of granite paragneiss, granite and

monzongranite form a buttress on the west side of the McCullough Mountains and locally crop

                                                                 4
April 16, 2007, Volcanoes of the McCullough Mountains, Southern Nevada—Smith, Honn and Johnsen



out at low elevation on the east side of the range (Boland, 1996). The volcanic section lies

directly on Precambrian basement and the Paleozoic and Mesozoic strata so common in the

Spring Mountains to the west and Colorado Plateau to the east are missing in the McCullough

Mountains. The location of the McCullough Mountains on the west flank of the Kingman Arch,

a structural uplift that extends from central Arizona to the Las Vegas area (Lucchitta, 1966;

Young and Brennan, 1974; Bohannon, 1984; Herrington, 2000) may explain this relationship.

According to Herrington (2000), nearly 5.5 km of Mesozoic and Paleozoic strata were removed

from the Kingman Arch between the onset of Sevier thrust faulting (146 Ma) and the deposition

of the Peach Springs Tuff (18.5 Ma). Based on fission track thermochronology, much of the

uplift occurred at about 70 Ma coincident with the Laramide Orogeny. The Kingman Arch,

therefore, may represent the westernmost Laramide uplift in the western United States

(Herrington, 2000). Locally a thin (1-5 m) conglomerate containing well rounded clasts of

quartzite and carbonate (up to 5 m in size) crops out between Precambrian crystalline rocks and

the volcanic section. Herrington (2000) suggested that the conglomerate was shed from the rising

Kingman Arch during late Cretaceous-Early Tertiary time and represents the stripping of the

Paleozoic and Mesozoic cover during the formation of the arch. In two localities an ash-flow

tuff crops out above the basal breccia but below the Eldorado Valley section. The southern

exposure was identified as Peach Springs Tuff (Wells and Hillhouse, 1989) by its distinctive

paleomagnetic pole. At the McCullough Pass locality (Johnsen and Smith, 2007) ash- flow tuff

crops out above the basal conglomerate and below the Eldorado Valley volcanic section

suggesting that it is Peach Springs Tuff. Stratigraphic position makes this interpretation

compelling, further study, however is necessary to verify the presence Peach Springs Tuff at this

location.

                                                                 5
April 16, 2007, Volcanoes of the McCullough Mountains, Southern Nevada—Smith, Honn and Johnsen



          The basal volcanic rocks in the McCullough Mountains are correlated with the Patsy

Mine section (Anderson, 1971)(18.5 to 15.2 Ma) and were named the Eldorado Valley volcanic

section by Schmidt (1987) (Figures 3 and 4). This unit contains rhyolite and dacite domes, debris

aprons and block and ash deposits as well as compound basalt to andesite cinder cones and broad

shield volcanoes (Johnsen and Smith, 2007). Boland (1996) identified two additional units that

may be age equivalent to the Eldorado Valley volcanic section. These are the dacite and rhyolite

domes of the Colony volcano and andesite flows of the McCullough stratovolcano (Figures 3 and

4).


          Separating the Eldorado Valley Volcanic section from younger units is the Tuff of Bridge

Spring, a regional unit dated at 15.2 Ma (Bridwell, 1991). The Tuff of Bridge Spring is a

moderately welded dacite ash-flow tuff originally described by Anderson (1971) and studied in

detail by Smith et al. (1993) and Morikawa (1994). The tuff is densely welded and vitric at the

base and grades upward to a moderately welded tuff. The base of the tuff is commonly oxidized

and has a distinctive orange color. The source of the Tuff of Bridge Spring may be in the

Eldorado Mountains (Figure 1) (Gans et al., 1994; Druschke et al., 2004). Faulds et al. (2001)

identified the Tuff of Mt. Davis in the Highland Range based on a 15.0 Ma age, a distinctive

paleomagnetic pole, and characteristic mineralogy. In the Highland Range it is separated from

the Tuff of Bridge Spring by a sedimentary unit, but in areas where this unit is missing, the Tuff

of Mt. Davis lies directly on the Tuff of Bridge Spring (Figure 2). In this case it is difficult to

separate the units. The Tuff of Bridge Spring in the McCullough Mountains appears to be a

single unit and it is not clear whether the Tuff of Mt. Davis crops out in the range. Bridwell

(1991) dated the Tuff of Bridge Spring at 15.2 Ma in the McCullough Mountains, however a
40
     Ar/39 Ar date of 15.02 ± 0.08 Ma (Spell et al., 2001) suggested a correlation to the Tuff of Mt.
                                                      6
April 16, 2007, Volcanoes of the McCullough Mountains, Southern Nevada—Smith, Honn and Johnsen



Davis. Because of this uncertainty and the historical use of the term, Tuff of Bridge Spring is

used in this paper as the name for this unit.


          Above the Tuff of Bridge Spring are the basalt and andesite flows and scoria of the

Pumice Mine basalt. Flows erupted from at least one cinder cone in the central McCullough

Mountains and according to Sanford (2000) are coeval with the formation of the McCullough

Pass caldera. This interpretation is disputed, and discussed in more detail later in this paper.

Schmidt (1987) discovered and described the McCullough Pass caldera and related outflow unit;

the McCullough Pass tuff. Studied in more detail by Sanford (2000), caldera formation was

precisely dated using the 40 Ar/39 Ar technique at 14.1 Ma (Sanford, 2000; Spell et al., 2001).


          The Hidden Valley volcanic section lies on the McCullough Pass tuff and consists of

dacite flows and domes and an overlying section of andesite flows. The northern McCullough

Mountains lack exposures of the Tuff of Bridge Spring; therefore in this area the Hidden Valley

lies directly on the Eldorado Valley volcanic section (Figures 3 and 4). Work by Boland (1996)

determined basalt stratigraphy and provided a geochemical database.


          The Sloan volcanic section (13.07 ± 0.02 Ma; Gans unpublished 40 Ar/39 Ar biotite date) in

the Hidden Valley area (Figures 1, 3 and 4) consists of four volcanoes arranged about Hidden

Valley; Mount Hanna, Mt. Ian, Center Dome, and Mount Sutor. Eruptions from the Mount Sutor

volcano produced a thick section of aphyric dacite by a process of hot, dry lava fountaining

(Bridwell, 1991). Few faults cut this volcanic section, so it is likely that it formed after the major

phase of extension.




                                                                 7
April 16, 2007, Volcanoes of the McCullough Mountains, Southern Nevada—Smith, Honn and Johnsen



          The youngest volcanic feature is the Henderson dome complex at the northern tip of the

McCullough Mountains (Figure 3 and 4). Originally interpreted as a caldera (Smith et al., 1993),

new mapping (Honn et al., 2007) indicates that it is a complex of domes aligned along an arc

concave to the north. Domes are associated with flows, pyroclastic flow and mesobreccia. The

10 km diameter dome complex cuts the tilted and faulted Eldorado Valley and Hidden Valley

sections and clearly formed post-extension.



Eldorado Valley Volcanoes


          The volcanoes of the Eldorado Valley section represent the oldest volcanism (18.5 to

15.2 Ma) yet recognized in the McCullough Mountains. The section crops out below the Tuff of

Bridge Spring (15.2 Ma) and above Peach Springs Tuff (18.5 Ma) in the central part of the range.

An 40 Ar/39 Ar date for an andesite flow toward the top of the section is 15.55 ± 0.02 Ma (Gans

unpublished 40 Ar/39 Ar whole rock date). Based on the work of Johnsen and Smith (2007), the

Eldorado Valley section contains at least four volcanic centers. The largest is the Cactus Hill

volcano, a broad (2 km diameter) basalt-andesite cone comprised of a 200 m thick section of

basalt and andesite flows and agglomerates. Basalt (46-52% SiO 2 ) with olivine and

clinopyroxene phenocrysts in a vitric matrix is interbedded with andesite (55-57% SiO 2 )

containing hornblende, olivine and clinopyroxene. Three basalt dikes (each 1-3 m wide) cut the

Cactus Hill volcano. In addition, eight dacite (61-64% SiO 2 ) domes crop out on the west flank of

the volcano. Dacite contains small plagioclase and clinopyroxene phenocrysts and xenocrysts,

and glomerocrysts of clinopyroxene and plagioclase. Debris aprons associated with dacite domes




                                                                 8
April 16, 2007, Volcanoes of the McCullough Mountains, Southern Nevada—Smith, Honn and Johnsen



are interbedded with basalt and andesite suggesting contemporaneous mafic and felsic

volcanism.


          Located 2 km to the southwest of the Cactus Hill volcano is the McCullough Wash dome

complex composed of dacite (63-65% SiO2 ) with large (2-4 mm) plagioclase phenocrysts,

hornblende, and glomerocrysts of plagioclase and clinopyroxene. Breccia representing dome

growth and collapse crops out about the McCullough Wash center. The complex consists of at

least 6 coalescing dacite domes. Domes typically have a light gray glassy carapace surrounding

a phenocryst-rich (~50%) flow banded core.


          The Eldorado Valley Volcano, a series of dacite domes and flows, may in part be the

source of a 250 m thick breccia unit (Eldorado Valley breccia) that crops out between and is

interbedded with rocks of the McCullough Wash and Cactus Hill volcanoes. The Eldorado

Valley domes commonly have a core composed of flow banded vitric dacite surrounded by a rim

of vitrophyre. Flows consist of a basal vitrophyre overlain by flow banded, platy, and vitric

dacite with a carapace of vesicular dacite breccia. Eldorado Valley breccia is a block and ash

deposit containing bombs and clasts. Bombs are 10 cm to 6 m in size and are recognized by

radial fractures and breadcrust surfaces (Figure 5). Clasts range in size from <1 cm to 3 m.

Bombs and clasts are usually vitric aphanitic dacite (68% SiO 2 ), but may include biotite and

hornblende phenocrysts; all contain small clinopyroxene/plagioclase glomerocrysts. The

Eldorado Valley breccia represents both debris aprons from dome growth and deposits from

partial collapse of domes.




                                                                 9
April 16, 2007, Volcanoes of the McCullough Mountains, Southern Nevada—Smith, Honn and Johnsen



         Rhyolite to dacite flows and domes and high-silica andesite flows comprise the Colony

volcano (Boland, 1996). Dacite and andesite contain phenocrysts of plagioclase, biotite, and

clinopyroxene. Phenocrysts of sanidine (up to 6 mm in size), plagioclase (to 5 mm), Fe-Ti minerals,

biotite, and clinopyroxene are typical of rhyolite flows and dome. The presence of large and

abundant biotite phenocrysts easily distinguishes the Colony rocks from other volcanic units.

Colony dacite and rhyolite flows are thick (up to 100 m), platy, and locally flow banded and

autobrecciated. Dacite flows are interbedded with breccia interpreted as debris flow based on

poor sorting, lack of internal layering, and wide range of clast sizes (clay to boulders)

supported in an open framework of a finer- grained matrix. Colony dacite domes are platy,

and slightly coarser grained than Colony flows. Colony dacite is locally intruded by dikes of

Eldorado Valley andesite.


         Below the Colony dacite is the Ivan Canyon andesite (Boland, 1996) and Precambrian

basement. The Ivan Canyon andesite consists of 5- 10 massive plagioclase-clinopyroxene

andesite flows that sit on Precambrian basement. Basement rock in this area is coarse- grained

red granite containing large (up to 3 cm) potassium feldspar crystals. Granite intrudes biotite

gneiss and is similar in texture to coarse- grained "Rapakivi" granites that crop out in the Lucy

Gray Range and the central McCullough Mountains (Duebendorfer and Christensen, 1995).



McCullough Pass Caldera

                                                        Introduction


          The McCullough Pass caldera is the best studied of the volcanic centers in the

McCullough Mountains and is the source of the McCullough Pass tuff. Kohl (1978) noted
                                                                10
April 16, 2007, Volcanoes of the McCullough Mountains, Southern Nevada—Smith, Honn and Johnsen



rhyolite dikes and domes in the caldera area but did not recognize the caldera. Schmidt (1987),

and Smith et al. (1988) provided the first description of the McCullough Pass caldera. This early

work was followed by a more detailed study of the caldera by Sanford (2000) and Spell et al.

(2001).


          The McCullough Pass caldera (Figure 4) was created by the eruption of the McCullough

Pass tuff at 14.10 Ma, but the duration of volcanic activity associated with the caldera was short.

The time between caldera collapse and the eruption of the McCullough Pass tuff and the

youngest intracaldera rhyolite (Capstone Rhyolite) is about 100,000 years (Spell et al., 2001).

The McCullough Pass caldera is small (2.4 km by 1.5 km in diameter) with an elliptical shape

elongate east-west with embayments to the northwest and southeast. Intracaldera units include

rhyolite and basalt flows, high-silica rhyolite ash-flow tuff, surge deposits, rhyolite domes,

rhyolite and basalt dikes and volcaniclastic units.


          Rhyolite domes lie in a circular pattern in the eastern half of the caldera with five domes

forming a prominent ridge just inside the eastern caldera wall. Domes also cluster in the

northwestern embayment. Dikes generally trend north-south and occur only in the central and

western portions of the caldera. The caldera wall is composed dominantly of the Tuff of

Bridge Spring and Eldorado Valley basalt. Megabreccia blocks (4 to 6 m in size) of the Tuff of

Bridge Spring occur in intracaldera units just inside the southwestern wall.


          The McCullough Pass caldera is a topographic low surrounded by ridges capped by 65-150

m of Hidden Valley volcanic section that are 50-100 m higher than the highest points within the

caldera. The fact that Hidden Valley basalt does not crop out within the McCullough Pass

caldera suggests that intracaldera units may have been a topographic high and barrier to the
                                                                11
April 16, 2007, Volcanoes of the McCullough Mountains, Southern Nevada—Smith, Honn and Johnsen



flow of Hidden Valley basalt and andesite. Less resistant intracaldera rock eroded more

easily than the more resistant Hidden Valley units forming an erosional basin (Sanford,

2000). The present caldera depression, therefore, is a topographic basin produced by

differential erosion rather than caldera collapse. Although caldera rocks are faulted it is more

common for regional faults to terminate at the caldera wall suggesting that caldera formation

at 14. 1 Ma occurred after the major pulse of extension in the McCullough Mountains.




                                               Intracaldera Stratigraphy


          Intracaldera volcanic rocks of the McCullough Pass caldera were named the Jean Lake

rhyolite, Pumice Mine basalt (now the McCullough Pass basalt), Ramhead rhyolite and the

Capstone rhyolite by Sanford (2000) (Figure 4) and are discussed in stratigraphic order below.


          The Jean Lake rhyolite forms the base of the intracaldera section and consists of rhyolite

flows and domes, breccia and ash-flow deposits mainly in the eastern part of the caldera. The most

prominent features are five rhyolite domes and associated block and ash and pyroclastic surge

deposits that form an arc just inside the caldera. Rhyolite contains phenocrysts of sanidine,

plagioclase, quartz and biotite in a glassy matrix.


          Sanford (2000) identified intracaldera basalt as Pumice Mine basalt. We dispute this

stratigraphic assignment and argue that intracaldera basalt is younger than the Pumice Mine

basalt as defined by Schmidt (1987). Evidence for this assertion includes the observation that

Pumice Mine basalt in the caldera lies on Jean Lake rhyolite and beneath Ramhead rhyolite and

clearly formed after caldera collapse. Outside of the caldera, the Pumice Mine basalt lies
                                                                12
April 16, 2007, Volcanoes of the McCullough Mountains, Southern Nevada—Smith, Honn and Johnsen



below the McCullough Pass tuff; the unit responsible for caldera collapse. Clearly there are

two basalts; one erupted before caldera collapse and the other during post-caldera activity. We

retain the name Pumice Mine basalt for older mafic flows above the Tuff of Bridge Spring and

below the McCullough Pass tuff and propose the name McCullough Pass basalt for the

intracaldera unit.


          Outcrops of McCullough Pass basalt in the western part of the caldera consist of three to

five flows that are each less than 5 m thick. In the eastern part of the caldera, McCullough

Pass basalt crops out as a 150 m section of homogenous rock lacking flow boundaries or

agglomerate. The western contact between the homogenous basalt and intracaldera rhyolite is

nearly vertical but is not a fault contact. When inspected in detail, the contact is irregular with

dikes of basalt cutting into neighboring rhyolite. Toward the top of the section, basalt becomes

oxidized and vesicular and is associated with small volcanic bombs. We interpret the thick

section of homogenous basalt as a subjacent pluton with vertical intrusive contacts. The

pluton grades upward into a volcanic section.


          The Ramhead rhyolite is composed of flow, ash- flow tuff and surge deposits interbedded

with volcaniclastic deposits and McCullough Pass basalt. Ramhead rhyo lite cons ists of

several meters of thinly bedded volcaniclastic units overlain by a 1-2 m thick pyroclastic

surge, and four 2-3 m thick pyroclastic flows. The ash- flow tuffs contain phenocrysts of

sanidine, plagioclase and biotite, sparse pumice clasts and andesite xenoliths in a fine-grained

matrix.


          The Capstone rhyolite is the youngest intracaldera unit (13.98 ± 0.04 Ma; Spell et al.,

2001). It consists of rhyolite domes, flows and dikes. Capstone rhyolite forms a dome complex
                                                                13
April 16, 2007, Volcanoes of the McCullough Mountains, Southern Nevada—Smith, Honn and Johnsen



in the northwestern caldera embayment and a cluster in the south-central part of the caldera. A

Capstone rhyolite flow, up to 50 m thick, caps the highest peaks within the caldera. Capstone flows

are massive and contain large blocks of devitrified pumice, up to 0.5 m in size, in a glassy

matrix. Both the matrix and pumice blocks contain sanidine and plagioclase phenocrysts.



                                        The McCullough Pass outflow units


          McCullough Pass outflow is a composite unit that includes ash flow tuff, volcaniclastic

breccia and conglomerate and extends about 12 km to the northeast, but only 2 km to the south of

the caldera (Figure 4). The ash- flow tuff probably responsible for caldera collapse crops out near

the bottom of the outflow sequence. The tuff is generally poorly welded and contains pumice (up

to 6 cm in size) and rhyolite clasts in a matrix of glass shards and small pumice fragments.

Common phenocrysts are sanidine, plagioclase, biotite and quartz. Sphene and hornblende are

also present but in minor amounts. Abrupt changes in thickness reflect pre-flow topography on a

surface composed of Tuff of Bridge Spring and Pumice Mine basalt.


          Volcaniclastic units are composed of conglomerate, sandstone and debris flows

containing clasts of McCullough Pass tuff, rhyolite domes, and Precambrian gneiss. These

deposits have an irregular distribution and are thickest (135 m) within 2 km of the caldera wall

(Schmidt, 1987).




                                                                14
April 16, 2007, Volcanoes of the McCullough Mountains, Southern Nevada—Smith, Honn and Johnsen




McCullough Stratovolcano
                                                        Introduction


          Weber and Smith (1987) and Boland (1996) identified a stratovolcano in the

northern McCullough Mountains comprised of basalt and andesite flows interlayered

with volcaniclastic units (Figure 4 and 6). Boland (1996) divided stratovolcano

stratigraphy into two units and informally named them the Fog Ridge and Farmer

Canyon andesites. The relationship between the older Farmer Canyon and younger

Fog Ridge andesites with the underlying Colony dacite and Ivan Canyon andesite

is important for correlation of these units to others in the McCullough Mountains.

The Farmer Canyon andesite ends abruptly against Colony dacite and Ivan Canyon

andesite indicating that Colony dacite stood as a topographic high and prevented

lava flows of Farmer Canyon andesite from traveling farther to the south (Figure

6). The Fog Ridge section, however, sits unconformably on all underlying units .

This relationship suggests that the Colony-Ivan Canyon section was beveled by

erosion prior to the eruption of the Fog Ridge andesite (Boland, 1996) . Lacking

radiometric dates for these units stratigraphic correlation is difficult. We argue,

however, that the Farmer Canyon andesite is correlative with Eldorado Valley

volcanic section and the Fog Ridge andesite with the Hidden Valley section .

                                                                15
April 16, 2007, Volcanoes of the McCullough Mountains, Southern Nevada—Smith, Honn and Johnsen



Accordingly, in this paper we use the terms Eldorado Valley and Hidden Valley

instead of Farmer Canyon and Fog Ridge . Furthermore, we suggest that the

McCullough stratovolcano is a composite feature formed over a period of about 2 m.y.

Its history began with the formation of a stratovolcano of Eldorado Valley andesite

and ended after a period of erosion with eruption of Hidden Valley andesite

representing a number of centers on the flanks of the stratovolcano.



                                                  Volcanic stratigraphy


         The Eldorado Valley section (450 m thick) (Figure 3) forms the core of the

McCullough stratovolcano and is characterized by altered andesite flows, agglo merate s

and numerous dikes. The lower part of the Eldorado Valley section contains massive,

porphyritic andesite flows with abundant phenocrysts (24%) of plagioclase, Fe-Ti minerals, and

clinopyroxene, and small (<1.5 mm) phenocrysts of biotite and oxyhornblende. Flows form

short lobes and are not traceable laterally for more than 1 km. This part of the section is

highly altered and outcrops are commonly dark pink to red in color. Unlike the rest of the

Eldorado Valley volcanic section, the andesite flows of the lower part of the Eldorado Valley

lack agglomerate units. Numerous andesite dikes that are finer grained and more resistant to

erosion than the host rock intrude the section.


         The flows of the upper part of the Eldorado Valley section of the McCullough

stratovolcano are porphyritic and fine grained with phenocrysts (15-45%) of plagioclase,

clinopyroxene, olivine, and Fe-Ti minerals. Most outcrops vary in color from gray to red and


                                                                16
April 16, 2007, Volcanoes of the McCullough Mountains, Southern Nevada—Smith, Honn and Johnsen



are highly altered. In contrast to the lower part of the section agglomerate is the dominate

rock type and andesite flows rare. Numerous dikes varying in compos ition fro m dacite to

andesite intr ude the Eldorado Valley section. Dikes are prevalent throughout the upper

Eldorado Valley andesite, but none intrude the overlying Hidden Valley andesite. This

relationship is additional evidence for a period of erosion between the deposition of the upper

part of the Eldorado and the Hidden Valley sections.


         The Hidden Valley andesite (Figure 3) of the McCullough stratovolcano (290 m

thick) consists of thin basaltic- andesite and andesite flows interbedded with agglomerate and

breccia. Flows are porphyritic and contain 20-40% phenocrysts of plagioclase, olivine, and

clinopyroxene. Alteration is rare in the Hidden Valley section and outcrops are usually black

to gray in color. I n contras t to the Eldorado Valley sectio n, Hidde n Va lle y andesite

breccia is generally thinner and flows thicker (3 m) and more numerous.



                                             Evidence for a Stratovolcano



         Geologists commonly perceive volcanoes as thick stacks of lava flows; however,

continental stratovolcanoes usually form as broad features composed mainly of fragmental

material (Cas and Wright, 1988). Lava flows tend to be short, and they commonly

interfinger with pyroclastic flows and volcaniclastic deposits. On some stratovolcanoes,

pyroclastic and volcaniclastic deposits make up as much as 70-80 % of the volume of the

volcano.




                                                                17
April 16, 2007, Volcanoes of the McCullough Mountains, Southern Nevada—Smith, Honn and Johnsen



          Although the volcanic stratigraphy of stratovolcanoes is quite variable with vertical

and lateral lithologic changes over short distances, consistent lateral facies changes are

commonly observed from proximal to distal position relative to the cone (Cas and Wright,

1988). Near the vent area, dikes, altered flows and volcaniclastic breccias, and domes form the

main edifice of the volcano. About the cone, debris is deposited into valleys eroded into the

slopes of the stratovolcano. Distal to the cone are thick sequences of alluvia l, mudflow and

debris flow deposits (Cas and Wright, 1988).


          The deposits of the Eldorado Valley sequence in the northern McCullough Mountains

compare favorably with this general model of a stratovolcano. Short interfingering altered lava

flows, breccias, and dike swarms are consistent with the near vent area of a stratovolcano.

However, volcaniclastic sandstones and epiclastic units that are common to more distal

stratovolcano facies are missing from the section. The vent area for the stratovolcano has not

been identified, but it is possible that the Railroad Pass pluton (14.99 ± 0.08; Gans unpublished
40
     Ar/39 Ar date, 1997) may occupy the conduit (Figure 1 and 6). This quartz monzonite pluton

intrudes a highly altered Eldorado Valley sequence cut by hundreds of dikes. Dike abundance

and the degree of alteration decreases to the west away from the pluton. Accordingly, a transect

from the pluton to the eastern side of the McCullough Mountains may represent a section across

the volcano from conduit to flank flows (Figure 6).

          The Hidden Valley lava flows may have erupted from several sources each identified by

red welded scoria, volcanic bombs and brecciated basalt and agglomerate. We suggest that the

Hidden Valley flows represent eruptions from local vents (cinder cones) on the flank of the

McCullough stratovolcano.


                                                                18
April 16, 2007, Volcanoes of the McCullough Mountains, Southern Nevada—Smith, Honn and Johnsen




Hidden Valley Volcanoes
          Deposits of the four Hidden Valley volcanoes comprise the Sloan volcanic section that

sits stratigraphically above Hidden Valley andesite (Figure 4). The volcanoes are located about

Hidden Valley, a broad interior basin, whose origin may be either volcanotectonic or structural

(Bridwell, 1991) (Hirsch et al., 2006). The volcanoes include Mt. Sutor, Mt. Ian, Center

Mountain, and Mt. Hanna and vary in composition from andesite to dacite.


          Mt. Hanna is the oldest of the volcanoes and erupted from a single vent. A section of

andesite up to 307 m thick with a volume of about 9 km3 surrounds a vent occupied by a

volcanic neck composed of silicified and brecciated andesite. Andesite is fine-grained and

commonly trachytic. Phenocrysts are rare (<1%) and consist of highly embayed and pitted

plagioclase and biotite. Bridwell (1991) suggested that the Mt. Hanna andesite erupted at high

temperature (1000 degrees C) with water content of less than 2% by a lava- fountaining

mechanism. The eruption resulted in lava flows composed of agglutinated spatter that are

analogous to a hot, dry ash-flow tuff.


          Center Mountain is a dome complex occupying a large tuff ring formed by a pyroclastic

unit with a matrix of blocky nonvesicular glass shards. Armored mud balls and discontinuous

stringers of dacite suggest hydromagmatic activity and lava fountaining. The Center Mountain

dacite erupted within the tuff ring and produced thick, viscous dacite with a volume of about

0.44 km3 . Dacite contains plagioclase and biotite phenocrysts in a matrix of aligned plagioclase

microlites.




                                                                19
April 16, 2007, Volcanoes of the McCullough Mountains, Southern Nevada—Smith, Honn and Johnsen



          The Mt. Ian volcano is formed by numerous domes which occupy tuff rings of black ash

and lapilli. Domes are characterized by andesite with foliation defined by platy slabs commonly

dipping inward toward the conduit but becoming horizontal over the vent. Single domes are

usually less than 0.5 km in diameter. The fine-grained andesite contains phenocrysts (5-35%) of

plagioclase, oxidized biotite, orthopyroxene, and iddingsitized olivine. The groundmass

consists of microlites of plagioclase.


          Mt. Sutor dacite covers a large area to the north and west of Hidden Valley and is

composed of massive flows and domes of biotite dacite. Eruptive centers are domes intruding

tuff rings. In the north, Mt. Sutor dacite forms a broad dome intruded by a hypabyssal dacite

stock. Dacite contains phenocrysts and glomerocrysts of plagioclase, biotite, and

clinopyroxene. Dacite of the stock is coarser grained but has the same mineralogy as Mt. Sutor

dacite flows.




Henderson dome complex

          The Henderson Dome Complex is a series of dacite domes and flows located just south of

Henderson Nevada at the northern tip of the McCullough Mountains (Figure 4). The geometry

of the complex is simple with an arc of dacite domes to the south; the source of dacite coulees

that flowed 1-2 km to the north. Biotite-dacite domes intruded the McCullough stratovolcano

and are associated with flows that have surface and basal breccias containing small (1-10 cm in

diameter) clasts of dacite of the Henderson dome complex and andesite of the McCullough


                                                                20
April 16, 2007, Volcanoes of the McCullough Mountains, Southern Nevada—Smith, Honn and Johnsen



stratovolcano. A dacite flow in the northeastern part of the complex filled a paleovalley on the

flank of the McCullough stratovolcano. This flow forms the summit of Black Mountain

(containing TV and other communication equipment), a prominent peak that forms the backdrop

to the City of Henderson. The earliest eruptions produced a dacite tuff which lies unconformably

on Hidden Valley andesite flows of the McCullough stratovolcano. The tuff is approximately 20

meters thick and grades from a pumice poor lithic rich base to a pumice rich lithic poor top.

Lithic fragments are mainly andesite of the McCullough Mountains stratovolcano. Several

dacite and andesite dikes cut dacite domes. Dikes average 2 meters in width and are 20 to 500 m

in length and contain partially resorbed phenocrysts, multiple populations of plagioclase

(resorbed anhedral, and euhedral), polycrystalline pyroxene, and abundant xenocrysts.



Geochemistry

          The volcanic rocks in the McCullough Mountains are calc-alkaline and vary in

composition from rhyolite to basalt (Table 1). Intermediate compositions (andesite and dacite)

prevail, while basalt and rhyolite are rare (Figures 7). Mafic rocks associated with the

McCullough stratovolcano and the Cactus Hill volcano are andesites (52-60 wt. % SiO 2 ).

Intermediate rocks of the Eldorado Valley and Sloan volcanic sections are andesite and dacite.

While average values portrait important information, individual volcanoes sometimes display

considerable variation. An example is the McCullough stratovolcano. Boland (1996) collected

52 samples from major flow units from the base of the Eldorado Valley to the top of the Hidden

Valley section (nearly 800 m of section) and demonstrated variation in stratovolcano andesite

from 50 to 60 wt. % SiO 2 without any consistent trend either up or down section. MgO, TiO 2 ,


                                                                21
April 16, 2007, Volcanoes of the McCullough Mountains, Southern Nevada—Smith, Honn and Johnsen



Fe2 O3 , CaO and P2 O5 decrease with increasing SiO2 while Na2 O and K 2 O increase. Trace

elements scatter and show no clear relationship when plotted versus SiO 2 . Boland (1996)

suggested that these relationships indicate a mixing and/commingling between at least two

batches of andesite magma in the middle to upper crust. Fractional crystallization may have

occurred but magma mixing/commingling appeared to be the dominant differentiation process.


          A Common trace element pattern prevalent throughout the stratigraphic section is low

Nb, P, Ti and Zr when normalized to primitive mantle (Figures 8). McCullough Pass rhyolite,

however, displays greater depletions of these elements when compared to Eldorado Valley and

Hidden Valley andesite and dacite. An especially diagnostic element is Zr. Sloan andesite and

dacite are characterized by high (>500 ppm) Zr while Hidden Valley and Eldorado Valley have

between 300 and 400 ppm Zr. McCullough Pass rhyolite, andesite and basalt typically have low

Zr (200-300 ppm). The Pumice Mine basalt is high in Zr (617 ppm) while the McCullough Pass

basalt has much lower values (221 ppm Zr).


          The trace element chemical signature displayed by volcanic rocks of the McCullough

Mountains is an indication of either a magma source in the continental lithosphere or lithospheric

contamination. Rhyolite and dacite almost certainly formed by partial melting of crust. Mafic

magmas (basalt and andesite) either originated by melting of lithospheric mantle or reflect

asthenospheric magmas contaminated in the lithosphere (Wilson, 1989).


          Chemistry in Table 1 is summarized from several studies completed over a period of over

20 years. Furthermore, analyses were done by a variety of techniques in several analytical

laboratories. As a result, differences in chemistry between rock types in some cases may reflect



                                                                22
April 16, 2007, Volcanoes of the McCullough Mountains, Southern Nevada—Smith, Honn and Johnsen



analytical quality and technique; therefore comparisons between datasets must be viewed

cautiously.



Discussion

          We propose the following geologic sequence for volcanism in the McCullough

Mountains (Figures 4 and 6).


     1. Volcanism occurred on the west flank of the Kingman Arch uplifted in late Cretaceous

          time. Because Paleozoic and Mesozoic sections were stripped during the early Tertiary,

          the volcanic section was mainly erupted on an irregular surface composed of Precambrian

          basement rock. Peach Springs Tuff (18.5 Ma) covered parts of the southern and central

          part of the range from a source outside of the McCullough Mountains and forms the base

          of the volcanic section.


     2. Eruption of dacite domes and basalt from the Cactus Hill, Eldorado Valley, McCullough

          Wash, and Colony volcanoes in the central McCullough Mountains (18.5-15.2 Ma).

          Dome formation and collapse events produced the voluminous block and ash deposits of

          the Eldorado Valley breccia. These events probably record the most explosive activity in

          the McCullough Mountains.


     3. The McCullough stratovolcano accumulated over 400 m of andesite lava, agglomerate,

          and breccia of the Eldorado Valley volcanic section (18.5-15.2 Ma) in the central and

          northern McCullough Mountains. Although the Colony volcano was partially covered by




                                                                23
April 16, 2007, Volcanoes of the McCullough Mountains, Southern Nevada—Smith, Honn and Johnsen



          products of the McCullough stratovolcano, it served as a topographic barrier preventing

          lavas from flowing farther to the south.


     4. The Tuff of Bridge Spring with a source outside of the McCullough Mountains swept

          through the southern and central parts of the range (15.2 Ma). Tuff of Bridge Spring lies

          on the Eldorado Valley volcanic section in the McCullough Pass area but does not crop

          out to the north. Presumably the Colony and McCullough volcanoes were topographic

          highs during the eruption of the Tuff of Bridge Spring causing the tuff to flow to the

          south around the southern flanks of these volcanoes.


     5. Cinder cones of the Pumice Mine basalt erupted basalt flows in the McCullough Pass

          area.


     6. The McCullough Pass caldera erupted explosively and produced the McCullough Pass

          tuff (14.1 Ma). Intracaldera activity resulted in numerous rhyolite domes, dikes and

          related pyroclastic activity. A basalt plug and flows also contribute to intracaldera fill.

          The Colony and McCullough volcanoes were barriers to the northward flow of the

          McCullough Pass tuff because it appears to pinch out to the north against exposures of

          Colony dacite.


     7. After a period of quiescence, activity continued at the McCullough stratovolcano

          producing over 300 m of andesite lavas erupted fro m local centers (mainly cinder cones)

          distributed on the flanks of the older Eldorado Valley cone. This activity occurred after

          the eruption of the Tuff of Bridge Spring (15.2 Ma) but the upper age is presently not

          well constrained.


                                                                24
April 16, 2007, Volcanoes of the McCullough Mountains, Southern Nevada—Smith, Honn and Johnsen



     8. Eruptions from the Hidden Valley volcanoes produced four volcanoes on the west flank

          of the McCullough stratovolcano (Mt. Ian, Mt. Sutor, Center Mountain, and Mt. Hanna)

          (13.07 Ma). Eruptions of dacite and andesite domes and flows are arranged about the

          Hidden Valley. E ruptions from Mt. Hanna resulted in lava flows composed of

          agglutinated spatter that are analogous to a hot, dry ash- flow tuff.


     9. The Henderson dome complex formed on the northern flank of the McCullough

          stratovolcano and produced dacite domes, flows and a thin pyroclastic flow deposit. The

          age of this event is not known precisely, but the dome complex cuts tilted and eroded

          lavas of the McCullough stratovolcano.


          We regard the Eldorado Valley volcanic section (McCullough stratovolcano, Colony and

volcanoes in the central McCullough Mountains) as the dominant volcanic construct. Other

younger events represent eruptions on the flanks of the stratovolcano.


          The volcanic section in the McCullough Mountains is coeval with faulting related to

regional extension. Although, displacement and stratal rotation are minor compared to that in

adjacent ranges (e.g., Anderson, 1971), numerous faults cut the Eldorado Valley section and the

Tuff of Bridge Spring and die out upward in the McCullough Pass section. Hidden Valley

andesite is locally faulted but younger volcanic sections (Sloan, Henderson dome complex) are

largely unaffected by faulting. Boland (1996) noticed that the central and northern McCullough

Mountains is a broad synform with axis passing through McCullough Pass and Hidden Valley.

Many of the volcanoes in the McCullough Mountains are located on or near the axis of this dip

reversal.


                                                                25
April 16, 2007, Volcanoes of the McCullough Mountains, Southern Nevada—Smith, Honn and Johnsen




Acknowledgements

          We especially thank R. Ernest Anderson for his support and encouragement during early

phases of our study of volcanic and plutonic rocks in the Lake Mead area. His pioneering efforts

set the stage for all of the work that followed. Funding from the Bureau of Land Management to

geologically characterize the Sloan Canyon National Conservation Area is greatly appreciated.

We especially thank Lola Henio, Charles Carroll, Robert Taylor and Robbie McAbboy (BLM)

for their support and encouragement. UNLV graduate students Casey Schmidt, Hayden

Bridwell, Kelly Boland, Aaron Sanford, and Juliana Herrington and undergraduate students

Tracy Tuma Switzer and Mike Weber made major contributions to the study of McCullough

Mountains. Much of the work reported in this paper is based on their work. We also thank Paul

Umhoefer (NAU) for inviting us to participate in this volume. Colleagues that helped shape our

thinking about magmatism in the McCullough Mountains and the northern Colorado Extensional

Corridor include Jim Faulds (NBMG), Rod Metcalf (UNLV), Terry Spell (UNLV), Ernie

Duebendorfer (NAU) and Calvin Miller (Vanderb ilt University).



References Cited

Anderson, R.E., 1971, Thin skin distension in Tertiary rocks of southeastern Nevada: Geological

          Society of America Bulletin, v. 82, p. 43-58.


Bingler, E.C. and Bonham, H.F., 1973, Reconnaissance geologic map of the McCullough Range

          and adjacent areas, Clark County, Nevada: Nevada Bureau of Mines and Geology Map

          45



                                                                26
April 16, 2007, Volcanoes of the McCullough Mountains, Southern Nevada—Smith, Honn and Johnsen



Bohannon, R.G., 1984, Nonmarine sedimentary rocks of Tertiary age in the Lake Mead region,
          southeastern Nevada and northwestern Arizona: U.S. Geological Survey Professional
          Paper 1259, 72 p.

Boland, K.A., 1996, The petrogenesis of andesites produced during regional extension: examples

          from the northern McCullough Range, Nevada and Xitle volcano, Mexico [M.S. thesis],

          Las Vegas, University of Nevada, 127 p.


Bridwell, Hayden L., 1991, The Sloan Sag: A mid-Miocene volcanotectonic depression, north-

          central McCullough Mountains, southern Nevada [MS thesis]: Las Vegas, University Of

          Nevada, 147 p.


Cas, R.A.F., Wright, J.V., 1988, Volcanic Successions, Chapman and Hall, London, 528 pp.


Darvall, P., 1991, Miocene extension and volcanism in the Eldorado Mountains, southeast

          Nevada, U.S.A. [Master of Science Thesis]: Monash University, Clayton, Australia, 129

          p.


Druschke, Peter; Honn, Denise; McKelvey, Matt; Nastanski, Nico le; Rager, Audrey; Smith, E.I.,
          and Belliveau, Robert, 2004, Volcanology of the northern Eldorado Mountains, Nevada:
          new evidence for the source of the tuff of Bridge Spring: Geological Society of America
          Abstracts with Programs, Vol. 36, No. 5, p. 431.

Faulds, J.E., 1995, Geologic map of the Mount Davis Quadrangle, Nevada and Arizona: Nevada
          Bureau of Mines and Geology Map 105.

Faulds James E., Feuerbach Daniel L., Miller Calvin F., and Smith Eugene I., 2001, Cenozoic

          evolution of the Northern Colorado River Extensional Corridor, southern Nevada and

          northwestern Arizona: : in Erskine, M.C., Faulds, J.E., Bartley, J.M., and Rowley, P.D.,


                                                                27
April 16, 2007, Volcanoes of the McCullough Mountains, Southern Nevada—Smith, Honn and Johnsen



          The Geologic Transition, High Plateaus to Great Basin-A Symposium and Field Guide,

          The J.H. Mackin Volume, Utah Geological Association Publication 30 and Pacific

          Section American Association of Petroleum Geologists Publication GB 78, p. 239-271.


Faulds, J.E., Bell, J.W., and Olson, E.L., 2002, Geologic map of the Nelson SW quadrangle,

          Clark County, Nevada: Nevada Bureau of Mines and Geology Map 134.


Faulds, J.E., Gans, P.B., and Smith, E.I., 1994, Spatial and temporal patterns of extension in the

          northern Colorado River extensional corridor, northwestern Arizona and southern

          Nevada: Geological Society of America Abstracts with Programs, v. 26, no. 2, p. 51


Faulds, J.E., Geissman, J.W., and Mawer, C.K., 1990, Structural development of a major

          extensional accommodation zone in the Basin and Range province, northwestern Arizona

          and southern Nevada: implications for kinematic models of continental extension:

          Geological Society of America Memoir 176, p. 37-76.


Faulds, J.E., Smith, E.I., and Gans, Phil, 1999, Spatial and temporal patterns of magmatism and

          extension in the Northern Colorado River Extensional Corridor, Nevada and Arizona: A

          preliminary report: in Faulds, J.E., Cenozoic geology of the Northern Colorado River

          Extensional Corridor, southern Nevada and northwestern Arizona: Economic

          implications of regional segmentation structures, Nevada Petroleum Society 1999 field

          trip guidebook, Reno, Nevada, p. 171-183.


 Gans, P.B., and Bohrson, W.A., 1998, Suppression of volcanism during rapid extension in the

          Basin and Range Province, United States: Science, v. 279, p. 66-68.



                                                                28
April 16, 2007, Volcanoes of the McCullough Mountains, Southern Nevada—Smith, Honn and Johnsen



Gans, P.B., Landau, B. and Darvall, P., 1994, Ashes, ashes, all fall down: Caldera- forming

          eruptions and extensional collapse of the Eldorado Mountains, southern Nevada:

          Geological Society of America Abstracts with Programs, v. 26, no. 2, p. 53.


Herrington, J.M., 2000, Evolution of the Kingman Arch, southern Nevada [M.S. thesis]: Las

          Vegas, University of Nevada, 83 p.


Hewett, D.F., 1956, Geology and mineral resources of the Ivanpah Quadrangle, California and

          Nevada: U.S. Geological Survey Professional Paper 275, 172 p.


Hirsch, A.C., Snelson, C.M., Smith E.I., 2006, An Integrated Geophysical Study of Hidden

          Valley, Central McCullough Range, NV: Characterization of a Volcanotectonic Terrain:

          EOS Transactions, AGU, v. 87, no. 52, Fall Meeting Supplement, Abstract NS41A-1114.


Honn, D.K. and Smith, E.I., 2005, Volcanoes of the McCullough Range, southern Nevada: A

          window into the pre-extensional history of the Colorado River extensional corridor:

          Geological Society of America Abstracts with Programs, Vol. 376, No. 7, p. 229-230.

Honn, D.K., Johnsen, R., Smith, E.I., 2007, Volcanic Centers of the Northern McCullough
          Range, Southern Nevada USA: a View of Pre- Extensional Volcanism in the Colorado
          River Extensional Corridor: EOS Transactions, AGU, v. 88, no. 23, Joint Assembly
          Supplement, Abstract V23A-04.

Howard, K.A., and John, B.E., 1987, Crustal extension along a rooted system of imbricate low-
          angle faults: Colorado River extensional corridor, California and Arizona: Geological
          Society of London, Geological Society Special Publications, v. 28, p. 299-311.


Irvine, T.N. and Baragar, W.R.A., 1971, A guide to the chemical classification of the common
          volcanic rocks. Canadian Journal of Earth Sciences, v. 8, p. 523-548.


                                                                29
April 16, 2007, Volcanoes of the McCullough Mountains, Southern Nevada—Smith, Honn and Johnsen



Johnsen, Racheal and Smith, E.I., 2007, Evidence for dome collapse and coeval mafic- felsic

          volcanism in the central McCullough Range, Nevada: Geological Society of America

          Abstracts with Programs, Rocky Mountain section abstract 15-1.

Kohl, M.S., 1978, Tertiary volcanic rocks of the Jean-Sloan area, Clark County, Nevada and

          their possible relationship to Carnotite occurrences in caliches [M.S. thesis]: Los

          Angeles, University of California, 116 p.


Longwell, C.R., Pampeyan, E.H., Bowyer, B., and Roberts, R.J., 1965, Geology and mineral
          deposits of Clark County, Nevada: Nevada Bureau of Mines Bulletin 62, 218 p.

Lucchitta, I., 1966, Cenozoic geology of the upper Lake Mead area adjacent to the Grand Wash
          Cliffs, Arizona [Ph.D. dissertation]: University Park, Pennsylvania State University.

Morikawa, S.A., 1994, The geology of the tuff of Bridge Spring: southern Nevada and

          northeastern Arizona [Master of Science Thesis]: University of Nevada, Las Vegas, 165

          p.


Olsen, E., 1996, Geometry and kinematics of an extensional anticline: Highland Spring Range,

          southern Nevada [M.S. Thesis]: Iowa City, University of Iowa, 77 p.


Olsen, E., Faulds, J.E., and Harlan, S.S., 1999, Miocene extension and extension-related folding

          in the Highland Range, southern Nevada: implications for hydrocarbon exploration:

          Nevada Petroleum Society Guidebook 14, p. 97-114.


Sanford, Aaron L., 2000, Geologic history of the McCullough Pass caldera [MS thesis]: Las

          Vegas, University of Nevada, 111 p.




                                                                30
April 16, 2007, Volcanoes of the McCullough Mountains, Southern Nevada—Smith, Honn and Johnsen



Schmidt, Casey S., 1987, A mid-Miocene caldera in the central McCullough Mountains, Clark

          County, Nevada [MS thesis]: Las Vegas, University of Nevada, 78 p


Smith, E.I., and Faulds, J.E., 1994, Patterns of Miocene magmatism in the northern Colorado

          River extensional corridor (NCREC), Nevada, Arizona and California: Geological

          Society of America, Abstracts with Programs, v. 26, no. 5, p. 93.


Smith, E.I., Feuerbach, D.L, Naumann, T.R. and Mills, J.E., 1990, Geochemistry and evolution

          of mid-Tertiary igneous rocks in the Lake Mead area of Nevada and Arizona: in

          Anderson, J.L., Cordilleran Magmatism: Geological Society of America Memoir 176, p.

          169-194.


Smith, E.I., Morikawa, S.A., Martin, M.W., Gonzales, D.A. and Walker, J.D., 1993, Tuff of

          Bridge Spring: a mid-Miocene ash- flow tuff, northern Colorado River extensional

          corridor, Nevada and Arizona: Geological Society of America, Abstracts with Programs,

          v. 25, no. 5, p. A148.


Smith, E.I., Schmidt, C.S., and Mills, J.G., 1988, Mid-Tertiary volcanoes of the Lake Mead area

          of southern Nevada and Northwestern Arizona: in Weide, D.L., and Faber, M.L., This

          Extended Land, Geological Journeys in the southern Basin and Range, Geological

          Society of America, Cordilleran Section Field Trip Guidebook; UNLV Department of

          Geoscience, Special Publication No. 2, p. 107-122.


Spell, T.L., Smith, E.I., Sanford, Aaron, Zanetti, K.A., 2001, Systematics of xenocrystic
          contamination: preservation of discrete feldspar populations at McCullough Pass Caldera
          revealed by 40 Ar/39 Ar dating: Earth and Planetary Science Letters, v. 190, p. 153-165.


                                                                31
April 16, 2007, Volcanoes of the McCullough Mountains, Southern Nevada—Smith, Honn and Johnsen



Weber, M.E., and Smith, E.I. 1987, Structural and geochemical constraints on the reassembly
          mid-Tertiary volcanoes in the Lake Mead area of southern Nevada: Geology, v. 15, p.
          553-556.

Wells, R.E., Hillhouse, J.W., 1989, Paleomagnetism and tectonic rotation of the lower Miocene

          Peach Springs Tuff: Colorado Plateau, Arizona, to Barstow, California: Geological

          Society of America Bulletin, v. 101, p. 846–863.

Wilson, M., 1989, Igneous Petrogenesis. Boston, MA: Unwin Hyman, 450 pp.

Young, E. D., J. L. Anderson, H. S. Clarke, and W. M. Thomas (1989). Petrology of biotite-

          cordierite- garnet gneiss of the McCullough Range, Nevada I: Evidence for Proterozoic

          low pressure fluid-absent granulite grade metamorphism in the southern Cordillera:

          Journal of Petrology, v. 30, p. 39-60.


Young, R.A. , and Brennan, W.J., 1974, The Peach Springs Tuff: Its bearing on the structural

          evolution of the Colorado Plateau and the development of Cenozoic drainage in Mohave

          County, Arizona : Geological Society of America Bull., v. 85, p. 83-90.




                                                                32
April 16, 2007, Volcanoes of the McCullough Mountains, Southern Nevada—Smith, Honn and Johnsen



Figures:


1. Index map showing the location of the McCullough Mountains in the Lake Mead area.


2. Regional correlation of stratigraphic units to the south of Lake Mead. Note that the Tuff of

          Bridge Spring and the Peach Springs Tuff are the only units that extend from range to

          range.


3. Stratigraphic summary of the McCullough Mountains.


4. Geologic compilation map of the McCullough Mountains.


5. Photo of a large bomb in the Eldorado Valley breccia. The bomb is 5 m wide and 3 m high.

          Note the radial fractures and columnar jointing at the margin of the bomb.


6. Diagrammatic cross sections from west to east and north to south across the northern

          McCullough Mountains.


7. Chemical classification of volcanic rocks in the McCullough Range using the scheme of Irvine

          and Baragar (1971). Volcanic rocks are subalkaline (based on a normative nepheline,

          olivine, quartz plot); calc-alkaline (AMF diagram), and vary in composition from rhyolite

          to basalt (CI=color index; An=normative anorthite content).


8. Chemical variation for major and trace elements for volcanic rocks of the McCullough

          Mountains.




                                                                33
April 16, 2007, Volcanoes of the McCullough Mountains, Southern Nevada—Smith, Honn and Johnsen




Table 1. Summary of Geochemistry for Volcanic Rocks of the McCullough Mountains

    Sample        Tch-a1      Tch-d2      Tch-b3      Tmw-d4         Tmw-b5      Tev-d6          Tev-b7
       SiO2        54.43       61.92       63.16        64.88         62.81       61.46           65.56
     Al2O3         15.67       15.60       16.00        15.61         15.96       15.72           15.49
      TiO2          1.14        0.81        0.67         0.60          0.68        0.85            0.57
     Fe2O3          7.16        4.73        4.42         3.91          4.55        4.81            3.80
      MgO           5.32        2.45        1.82         1.56          2.11        2.48            1.89
      Na2O          3.44        3.80        4.02         3.72          3.77        3.81            3.70
       K2O          3.24        3.82        3.87         4.26          3.94        4.00            4.02
      MnO           0.11        0.07        0.07         0.06          0.08        0.06            0.06
       CaO          7.08        4.37        3.49         3.48          4.01        4.38            3.33
      P2O5          0.61        0.38        0.31         0.27          0.34        0.40            0.24
        LOI         2.34        1.92        1.74         1.71          1.70        1.92            2.28
      Total       100.54       99.87       99.57       100.06         99.95       99.89          100.94
         Sc           19          10                        8             8          11               8
          V          184         104                       66            79         109              63
         Ni          104          34                       19            19          33              26
         Cu           48          28                       24            25          29              22
         Ga           18          20                       19            21          20              19
         Rb           66          92                      108           115          81             112
         Sr         1273         990                      841          1001        1003             765
          Y           24          21                       21            21          25              19
         Zr          362         335                      296           300         348             279
         Nb           16          13                       16            16          13              14
         Ba         1607        1595                     1454          1453        1579            1467
         La           79          66                       61            61          58              65
         Hf           10          10                        9            14           9               9
         Pb           13          16                       24            22          15              24
         Th           20          18                       15            13          19              14
________________
1
  Average value of Cactus Hill basalt and andesite (7).
2
  Average value of Cactus Hill domes (5).
3
  Value of Cactus Hill bo mb, found below a dome in b reccia.
4
  Average value of McCullough Wash domes (3).
5
  Average value of McCullough Wash bombs, found below and about domes (2).
6
  Average value of Eldorado Valley do mes (3).
7
  Average value of Eldorado Valley bo mbs in the Eldorado Valley breccia (13).
         Analyses 1-7 by X-ray fluorescence spectrometry (XRF)




                                                                34
April 16, 2007, Volcanoes of the McCullough Mountains, Southern Nevada—Smith, Honn and Johnsen



Table 1 (continued)

    Sample Tfr-a8 Tfc-a9   Tic-a10 Tcd11   Thv-a12  Tpm-a13 Tmpb14  Thdc-t15 Thdc-f16
       SiO2 54.64  55.64     54.15 65.73     57.84    49.59  50.06    64.46       61
     Al2O3  16.25  16.40     15.06 14.99     16.84    16.01  16.34    16.26       16
      TiO2   1.23    1.03     1.28   0.55     0.74     1.28   1.24     0.61     0.98
     Fe2O3   7.17    6.18     6.26   3.12     6.60     9.53   8.86     3.68     4.79
      MgO    4.40    3.37     3.40   0.97     3.41     7.15   7.68     1.51     1.98
      Na2O   3.25    3.23     3.45   3.76     3.78     3.31   2.83     3.97     4.05
       K2O   3.33    3.65     4.12   5.20     3.91     1.36   1.39     3.94     4.21
      MnO    0.11    0.10     0.20   0.06     0.10     0.14   0.14    0.077     0.07
       CaO   6.80    5.65     7.58   3.20     5.34     9.48   9.53     4.26     4.80
      P2O5   0.65    0.55     0.57   0.21     0.43     0.55   0.45    0.231     0.40
        LOI                                   2.28     1.94   2.10              1.50
      Total 97.83  95.80     96.07 97.79    101.27   100.34  98.51    99.45    99.83
         Sc                              6                      32        9       10
          V                            66                                50      110
         Ni    58      21       80       4                     135        9       14
         Cu                              8                               11       47
         Ga                            16                                17       18
         Rb    78      87      107    158       84       28     26      111      117
         Sr   974     939      836    484     1113     1038    918      654      722
          Y    24      17       23     24                       33       32       38
         Zr   347     351      406    289      400      617    221      321      362
         Nb    17      17       23     21                         7      21       24
         Ba  1344   1507      1066    816     1578      610    824     1455     1368
         La    80      84       98     83       78       48     45       56       63
         Hf      8       8      10       8        7       6       4       9       10
         Pb                            24                       10       18       18
         Th    14      14       24     22       13        5       6      17       22
________________
8
 Average value of Fog Ridge andesite fro m Bo land (1996).
9
  Average value of Farmer Canyon andesite fro m Boland (1996).
10
   Ivan Canyon andesite fro m Boland (1996).
11
   Average value of Colony Dacite fro m Boland (1996) and this study (4).
         Analyses 8-11, major elements by XRF, trace elements by INAA and ICP-MS
12
   Average value of Hidden Valley andesite fro m Sch midt (1987).
13
   Average value of Pu mice Mine andesite and bas alt fro m Sch midt (1987).
         Analyses 12-13, major elements by ICP, trace elements by INAA.
14
   Average value of McCullough Pass basalt from Sanford (2000).
         Analysis 14, major elements by XRF, trace elements by ICP-MS.
15
   Henderson dome co mplex tuff.
16
   Average value for Henderson dome co mplex flows (2).
         Analyses 15-16, major and trace elements by XRF



                                                                35
April 16, 2007, Volcanoes of the McCullough Mountains, Southern Nevada—Smith, Honn and Johnsen




Table 1 (continued)

     Sample Tmpa17 Tmrh18                 Tmcs19      Tmjl 20     Ttbs21      Tmha22 Tcmd23            Tmia24   Tmsd25
        SiO2  59.67  73.64                   75.52      74.54         64.89    59.91        68.06       58.61    63.22
      Al2O3   17.11  13.04                   12.92      13.88         15.74    17.00        15.65       17.07    16.24
       TiO2    0.85   0.24                    0.17        0.22         0.52     0.60         0.14        0.64     0.44
      Fe2O3    6.10   1.29                    0.82        1.06         2.73     6.68         3.46        7.36     5.30
       MgO     3.06   0.37                    0.25        0.27         1.01     1.74         0.49        1.89     1.12
       Na2O    3.41   3.27                    3.45        3.55         4.16     3.70         3.71        3.72     3.59
        K2O    3.49   4.72                    4.80        4.96         5.43     4.35         5.56        4.25     5.41
       MnO     0.13   0.07                    0.06        0.04         0.07     0.16         0.16        0.16     0.17
        CaO    4.72   1.92                    0.82        0.85         2.31     3.98         1.73        4.25     2.70
       P2O5    0.56   0.04                    0.02        0.02         0.07     0.84         0.25        0.85     0.34
         LOI   1.71   4.22                    1.69        0.85         1.76     0.57         1.01        0.64     0.98
       Total  99.11  98.60                   98.83      99.39         97.51    99.54       100.21       99.43    99.49
          Sc     14      3                        3          3                     6            5           6        7
           V                                                                      78           16          85       26
          Ni    108                            135                      18
          Cu
          Ga
          Rb     75    159                      26        176          138        90             118      84      119
          Sr    989    185                     918        155          421      1834             707    1850     1474
           Y     43     20                      33          30          26                        12
          Zr    305    196                     221        178          448       579             488     626      710
          Nb     10     26                        7         18          31
          Ba  1169     570                     824        611         1042      1899         1755       1979     1869
          La     69     58                      45          56                    91          116         86      115
          Hf      7      5                        4          5                     8            9          7       11
          Pb     18     30                      10          31
          Th     14     22                  6        23        27       14                       21       13       19
17
   Average value of McCullough Pass andesites from Sanford (2000).
18
   Value of McCullough Pass, Ramhead rhyolite fro m Sanford (2000).
19
   Average value of McCullough Pass, Capstone rhyolite fro m Sanford (2000).
20
   Average value of McCullough Pass, Jean Lake rhyolite fro m Sanford (2000).
          Analyses 17-18, major elements by XRF, trace elements by ICP-MS
21
   Average value of Tuff of Bridge Spring fro m Morikawa (1994).
          Analysis 21, major and trace elements by XRF
22
   Average value of Mount Hanna andesite from Bridwell (1991).
23
   Average value of Center Mountain dacite fro m Bridwell (1991).
24
   Average value of Mount Ian andesite from Bridwell (1991).
25
   Average value of Mount Sutor dacite from Bridwell (1991).
          Analyses 22-25, major and trace elements by XRF.




                                                                 36
April 16, 2007, Volcanoes of the McCullough Mountains, Southern Nevada—Smith, Honn and Johnsen




Figure 1. Index map showing the location of the McCullough Mountains in the Lake Mead area.




                                                                37
April 16, 2007, Volcanoes of the McCullough Mountains, Southern Nevada—Smith, Honn and Johnsen




Figure 2. Regional correlation of stratigraphic units to the south of Lake Mead. Note that the Tuff of Bri dge
Spring and the Peach S prings Tuff are the only uni ts that extend from range to rang e.




                                                                38
April 16, 2007, Volcanoes of the McCullough Mountains, Southern Nevada—Smith, Honn and Johnsen




Figure 3. Stratigraphic summary of the McCullough Mountains.




                                                                39
April 16, 2007, Volcanoes of the McCullough Mountains, Southern Nevada—Smith, Honn and Johnsen




Figure 4. Geologic compilati on map of the McCullough Mountains.



                                                                40
April 16, 2007, Volcanoes of the McCullough Mountains, Southern Nevada—Smith, Honn and Johnsen




Figure 4 (continued). Map explanation and stratigraphic summary.


                                                                41
April 16, 2007, Volcanoes of the McCullough Mountains, Southern Nevada—Smith, Honn and Johnsen




Figure 5. Photo of a large bomb in the El dorado Valley breccia. The bomb is 5 m wi de and 3 m high. Note the
radial fractures and columnar jointing at the margin of the bomb.




                                                                42
April 16, 2007, Volcanoes of the McCullough Mountains, Southern Nevada—Smith, Honn and Johnsen




Figure 6. Diagrammatic cross sections from west to east and north to south across the northern McCullough
Mountains.




                                                                43
April 16, 2007, Volcanoes of the McCullough Mountains, Southern Nevada—Smith, Honn and Johnsen




Figure 7. Volcanic rocks are subalkaline (based on a normati ve nepheline, oli vine, quartz pl ot); calc -alkaline
(AMF di agram), and vary in composition from rhyolite to basalt (CI=color index; An=normati ve anorthi te
content). Classification based on Irvine and B arag ar (1971).




                                                                44
April 16, 2007, Volcanoes of the McCullough Mountains, Southern Nevada—Smith, Honn and Johnsen




Figure 8. Chemical vari ation for major and trace elements for volcanic rocks of the McCullough Mountains.




                                                                45

								
To top