JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 93, NO. B12, PAGES 14,835-14,855, DECEMBER 10, 1988

Geology and Petrology of the Woods Mountains Volcanic Center, Southeastern California: Implications for the Genesis of

Peralkaline Rhyolite Ash Flow Tuffs

MICHAEL MCCURRY

Department of Earth Sciences, New Mexico State University, Las Cruces

The Woods Mountains Volcanic Center is a middle Miocene silicic caldera complex located at the transition from the notthem to the southern Basin and Range provinces of the western United States. It consists of a trachyte- trachydacite-rhyolite-peralkaline rhyolite association of lava flows, domes, plugs, pyroclastic rocks, and epiclastic breccia. Volcanism began at about 16.4 Ma, near the end of a local resurgence of felsic to intermediate magrnatism and associated crustal extension. Numerous metaluminous high-K trachyte, trachydacite, and rhyolite lava flows, domes, and pyroclastic deposits accumulated from vents scattered over an area of 200 km 2 forming a broad volcanic field with an initial volume of about 10 km 3. At 15.8 Ma, about 80 km 3 of metaluminous to mildly peralkaline high-K rhyolite ash flows were erupted from vents in the western part of •he field in three closely spaced pulses, resulting in the formation of a trap door caldera 10 km in diameter. The •,sh flows formed the Wild Horse Mesa Tuff, a compositionally zoned ash flow sheet that originally covered an area bf about 600 km 2 to a maximum thickness of at least 320 m. High-K trachyte pumice lapilli, some of which are intimately banded with rhyolite, were produced late in the two later eruptions. Intracaldera voIcanism from widely distributed vents rapidly filled the caldera with about 10 km 3 of high-K, mildly peralkaline, high-silica rhyolite lava flows and pyroclastic deposits. These are interlayered with breccia derived from the caldera scarp. They are intruded by numerous compositionally similar plugs, some of which stmcturally uplifted and fractured the center of the caldera. The center evolved above a high-K trachyte magma chamber about 10 km in diameter that had developed and differentiated within the upper crest at about 15.8 Ma. Petrologica1, geochemical, and geophysical data are consistent with the idea that a cap of peralkaline rhyolite magma formed within the trachyte chamber as a result of fractional crystallization within chemical boundary layers.

INTRODUCrION

Many recent studies, primarily of ash flow tuff deposits, have demonstrated that compositional, thermal, and mineralogical zoning characterize many silicic magma chambers in the middle to upper continental crust. However, the mechanisms of zoning, in particular, the relative importance of diffusion, convection and fractional crystallization in producing zoning, and the relationships between the zoning mechanisms, overall chamber evolution (including thermal and compositional boundary conditions), and eruptive history are subjects of ongoing debate [e.g., Hildreth, 1979, 1981, !983; Michael, 1983a, b; McBirney et al., 1985; Carrigan and Cygan, 1986; Spera eta!., 1982; Huppert and Sparks, 1984].

This paper summarizes the geology and petrology of the Woods Mountains Volcanic Center, located in southeastern California. The center is well suited for an analysis of magma chamber evolution

favorable conditions, evolve at the top of intermediate composition magma chambers in the upper crust as a result of the interaction of fractional crystallization and boundary layer processes.

GEOLOGIC Sin'riNG

The Woods Mountains Volcanic Center is situated in the eastern

Mojave Desert in a region where the southern Basin and Range province thins to the north around the western margin of the Colorado Plateau and is transitional into the northern Basin and Range province [Eaton, 1980]. It is located 40 km to the east of the Mojave Block, as defined by Dokka [1983], although this feature probably assumed its distinct tectonic identity 5-10 m.y. after the formation of the center. The Woods Mountains Volcanic Center

evolved toward the end of a widespread pulse of late Oligocene [cf. Lambert et al., 1987] to late Miocene [e.g., Anderson, 1977; Glazner and Supplee, 1982], high-K calc-alkaline volcanism, after

based on the study of Ossociated volcanic rocks because of excellent a long period of local early to mid-Tertiary magmatic and tectonic exposures of unaltered parts of nearly the entire rock sequence, the quiescence [Hewett, 1956]. It is spatially and temporally associated occurrence of isotopically distinctive country rocks, geophysical with regional, roughly east-west directed ex•nsion and crustal anomalies, caldera structures, and the occurrence of large, thinning [Davis, 1980; Dokka, 1986; Wust, 1986; Spencer, 1985] y. compositionally zoned ash flow tuff deposits. Previous The volcanic rocks were deposited on a rugged topography reconnaissance studies of the Woods Mountains area and details of consisting dominantly of lower Proterozoic granitic and mapping, analytical, and field geophysical procedures are metamorphic rocks [Goldfarb et al., 1988] and Jurassic to summarized by McCurry [1985]. Rock classification follows the Cretaceous granitic rocks [Beckerman et aL, 1982]. recommendations of the International Union of Geological Sciences Studies of volcanic rocks in nearby areas suggest that volcanism [Le Bas et al., 1986]; mineral abbreviations follow Kretz [1983]. was, for the most part, restricted to scattered stratovolcano The term "high-K" follows the usage of Ewart [1979]. My complexes. The volcanic rocks within these complexes reached 3- interpretation of the geology and petrology of the center supports 4 km thick in some areas [Anderson, 1971, 1977; Smith, 1982; the idea first described by Shaw [1974], and elaborated on by Schmidt and Smith, 1987; Glazner, 1981; Lukk, 1982]. others [cf. McBirney et al., 1985], that silicic caps will, under

Copyright 1988 by the American Geophysical Union.

Paper number 7B7033. 0148-0227/881007B-7033505.00

Metaluminous, intermediate compositions dominate in man•t of these areas [e.g., Anderson, 1971; Smith, 1982; Lukk, 1982]. Bimodal, basalt-rhyolite, assemblages also occur but are not time transgressive after the intermediate rocks [e.g., Glazner, 1981; cf. Christiansen and Lipman, 1972]. Some of these volcanic centerrs developed calderas [e.g., Lambert et al., 1987; Schmidt and Smith,

14,835

14,836 MCCURRY: MIOCENE PIeRALKALINE VO•, MOJAVE DESIST, CALII•,NIA

Pinto Mountain

115o15 '

Lanfair Valley

Alluvium

Undivided basalt and andesite flows (-•10 Ma)

Hackberry Mountain

ß ß ß ß ß ß ß

ß

ß Tortoise Shell Mountain Rhyolite (--15.8 Ma)

Wild Horse Mese Tuff (15.8 Ma)

Map

o o d s•;)M o u n t a i n s

lind Hills•

Hackberry Spring Volcanics (16.4 - 15.8 Ma)

0 10 km i

ß .

Undivided Miocene rocks (17- 20 Ma?)

Pre-Tertiary rocks

Fig. 1. Generalized geologic map of the Woods Mountains area. Unpatterened areas are covered by late Tertiary and Quatemary alluvium; faults, heavy line (ball on downthrown side); stars indicate the locations of nine measured stratigraphic sections (iV[. McCurry, unpublished data, 1987); A-A' refers to the location of the cross section illustrated in Figure 2.

1987; McCurry, 1982] and have been identified as the sources of some ash flow tuffs. A less voluminous later Miocene phase of dominantly mafic volcanism has also been recog•d in some areas [e.g., Otton, 1982].

Exposures of pre-Woods Mountains Volcanic Center volcanic rocks in the immediate vicinity of the Woods Mountains are illustrated in Figure 1; stratigraphic relationships are illustrated in Figure 2. Characterization of the original stratigraphy, chronology, distribution, and thicknesses of these rocks is hindered by a widespread cover of younger volcanic rocks, locally intense

hydrothermal alteration, and deposition on a deeply incised terrain. Preliminary work suggests that they were originally thin, a maximum of 100-200 m thick, and spatially discontinuous, consisting mosfiy of rhyolite lava flows and ash flow tuffs. One of the ash flow tuffs yielded a sanidine K-Ar date of 18.0 + 0.2 Ma (E. H. McKee, personal communication, 1980) and is probably correlative with the Peach Springs Tuff [of. Glazner et al., 1986]. Minor epiclastic sandstone, breccia, and lacustrine sediments are intercalated in the sequence; those at the base appear to have been locally derived and are barren of volcanic rock fragments. The

McCuv•¾: 1VI•• Pm•axa• VOLCANISM• MoJAv• D•mT, CnL•A 14,837

Generalized Cross Section and Stratigraphic Sections of the Woods Mountains Area

Ts Tr Tb

Taf •" Tss

pT

Wild Horse Mesa

Tba

. ß . .

pT

pT

W •'• Fig. 2. Diagrammatic east-west cross section of the Woods Mountains area (see Figure 1 for location) and representative features of the stratigraphy. An inferred pluton is illustrated beneath the eastem Woods Mountains. HSV, Hackberry Spring Volcanics; WHMT, Wild Horse Mesa Tuff; TSMR, Tortoise Shell Mountain Rhyolite; pT, pre-Tertiary rocks; Muv, undivided early to middle Miocene volcanic rocks; Tss, conglomerate and feldspathic sandstone; Tar, Peach Springs Tuff; Tb, basaltic lava flow; Tr, rhyolite dome and crumble breccia; Ts, lacustrine rocks; Tba, basalt and andesitc lava flows; Tsl, lacustrine rocks.

Tsl

Muv

Woods Mountains Hackberry Mountain . --

volcanic rocks may be contemporaneous with and genetically related to 17-19 Ma, intermediate to felsic dikes exposed 15 km to the east [Spencer, 1985]. However, they are probably not genetically related to the Woods Mountains Volcanic Center.

The cause of volcanism in the eastern Mojave Desert and its relationship with coeval tectonic events are poorly understood. Some of the hypotheses proposed include are volcanism [Cross and Pilger, 1978], transition from arc to back transform activity [Eaton, 1984], migration of the Mendocino fracture zone [Glazner and Supplee, 1982], passage of a gap in the subducted plate [Dickinson and Snyder, 1979], and changes in the thermal and stress fields of the lithosphere resulting from formation of the Mendocino triple junction [Atwater, 1970; Christiansen and Lipman, 1972]. None of these models are well constrained in this area.

EVOLUTION OF THE VOLCakNIC •

Volcanic rocks of the Woods Mountains Volcanic Center may be divided into three lithostratigraphic units: (1) Hackberry Spring Volcanics, (2) Wild Horse Mesa Tuff, and (3) Tortoise Shell Mountain Rhyolite. The Wild Horse Mesa Tuff was previously referred to informally by McCurry [1982, 1985] as the Hole-in-the- Wall Tuff. The spatial distribution of the units and their stratigraphic relationships are illustrated in Figures 1 and 2, respectively. The volcanic rocks vary from 61 to 76% in silica content, had an original volume of about 100 km 3, and accumulated over a period of about 0.6 m.y. Volcanism culminated at 15.8 Ma with the eruption of 80 km 3 of ash flow deposits and the formation of a 10-km-diameter caldera. Features of the rocks are summarized

below, and a model for the evolution of the center is presented.

Hackberry Spring Volcanics

Volcanism at the Woods Mountains Volcanic Center began with the incremental buildup of a heterogeneous volcanic field consisting of high-K trachyte, trachydacite, and rhyolite. The present and inferred original distributions of these rocks are illustrated in Figures 1, 2, and 3. The resultant volcanic rock sequence, the Hackberry Spring Volcanics, consists of interlayered, gently dipping to horizontal lava flows, domes, ash flow and fallout tephra, dikes, and plugs and contains little or no interbedded epiclastic rocks. Exposures occur discontinuously, mostly because of extensive coverage by later volcanic rocks, over an area of about 150 km 2 to a maximum thickness of about 200 m. No strong pattern was recog•d in the spatial distribution or thickness of the deposits or associated hypabyssal intrusions; rather the volcanism seems to have occurred from numerous, widely scattered vents that are located in a crudely east-west trending zone, forming a broad volcanic field (Figure 3). Simple interpolations between exposures suggest that the field had an original extent of about 200 km 2 and a volume of about 10 km 3.

The Hackberry Spring Volcanics were emplaced between about 16.4 and 15.8 Ma, based on K/At dates of 16.4 + 0.6 and 15.5 + 0.8 Ma on hornblende and sanidine phenocryst separates, respectively, from the basal vitrophyre zones of lava flows at the base and at the top of the sequence (J. Nakata, personal communication, 1984). The age of the youngest unit is also constrained by a date of 15.8 + 0.4 Ma on the conformably overlying Wild Horse Mesa Tuff.

Best exposures occur at Hackberry Mountain (Figure 1). Here the horizontal to gently dipping Hackberry Spring Volcanics

14,838 M•Y: MIOCENE PImAIXALINE VOI,CANISM, MOJAVE DESE•. CALIFORNIA

Evolution of the Woods Mountains Volcanic Center

(A)

(D)

N

16,4 - 15,8 m,y,b,p,

0 10km

15.8 m.y.b.p.

0 10km

15.8 m.y.b.p. (q.•) 15.8 m.y.b.p.

158 m bp (E)

Fig. 3. An interpretation of the evolution of the Woods Mountains Volcanic Center. Patterned areas represent regions covered by volcanic deposits at the time indicated at the top of the respective diagrams. (a) Formation of the Hackberry Spring Volcanics field. (b) Emplacement of the three members of the Wild Horse Mesa Tuff and trapdoor caldera collapse resulting in formation of the Woods Mountains caldera. (c) Filling of the caldera by early deposits of the Tortoise Shell Mountain Rhyolite. (d) Caldera collapse and emplacement of later lava flows, pyroclastic deposits, and breccia deposits of the Tortoise Shell Mountain Rhyolite. (e) Emplacement of numerous plugs and doming of the center of the Woods Mountains caldera, as the last phase of formation of the Tortoise Shell Mountain Rhyolite. (t) Intedayered basaltic and andesitic lava flows extruded from north trending dikes near the westem margin of the Woods Mountains caldera.

unconformably overlie generally gently dipping lower to middle Miocene(?) volcanic and epiclastic rocks and Mesozoic granitic rocks. Domes, lava flows, and densely welded ash flow tuffs are the dominant rock types. Most domes and flows overlie comagmatic layers of fallout vitric lapilli-tuff, lapillistone, and tuff breccia from 1 to several meters thick. Based on their limited exposures, the largest units had an original volume of close to 1 km 3, although most were much smaller. The volcanic rocks are intruded by compositionally similar plugs and dikes (e.g., Table 2, analyses HSV4 and HSVI). Two well-exposed dikes that

apparently fed two of the largest units at Hackberry Mountain (HSV1 and HSV4, Table 1) trend from west to west-northwest. The trends of these dikes are consistent with the trend of the volcanic field as a whole and with the trend of numerous 17-19 Ma, intermediate to felsic composition dikes 15 km to the east at Homer Mountain [Spencer, 1985]. The localized north-south extensional effects of crustal flexure described by Spencer for Homer Mountain may thus have extended to Hackberry MountaM.

Hackberry Springs Volcanics are characterized by the association high-K trachyte-trachydacite-rhyolite (Tables 1 and 2). Moderately

MCCU'RRY: MI(X:F/qE •ALIN-E VOLCANISM, MOJAVE DESl•RT, CALIFORNIA 14,839

TABLE 1. Representative Phenocryst Modes of the Hackberry Spring Volcanics •nit

HSV1 .HSV2a HSV2b HSV2c HSV3 HSV4 HSV[ [tSV5 [tSV6 HSV7a HSV7!• •SV7•: Vol. 8.7 36.2 12.4 9.4 37.6 21.0 42.1 23.0 30.7 13.4 12.9 9.2 Max. 2.5 8 3 1 10.3 3.1 1 7 5 3 2 1 PI 6.7 11.1 5.1 1.5 - 0.1 - 10.2 22.6 8.3 1.2 0.2

An- 35-45 3046 28-38 28-30 - 20-33 - 37-53 29-45 26-30 nd nd Sa* - 18.8 5.6 6.1 36.5 19.4 40.3 8.1 1.5 4.3 10.4 8.3 Bt 0.6 0.3 1.0 0.8 0.6 0.7 0.2 2.6 3.9 0.6 1.0 0.4

Cpx - 2.2 0.2 0.4 0.1 0.4 1.1 0.7 0.8 mnr 0.2 0.1 Opx - 1.2 ...... mnr - - - Hbl 1.3 ...........

Spn ........... 0.1 Opaques 0.1 2.6 0.5 0.6 0.4 0.4 0.5 1.2 1.5 0.2 0.1 0.1 Zm tr - tr tr tr? tr tr 0.1 tr tr tr tr

Ap mnr mnr mnr mnr •r mnr mnr mnr 0.4 tr mnr tr pc-'r 1000 1000 1840 1000 2070 !000 1956 1000 !000 1000 1000 1635

HSV1 - HSV7c are arranged in stratigraphic order (1, lowest; 7c, highest); HSVI is from a 1-kin-diameter trachyte porphyry plug that is intruded into a comagmafic unit; HSV4, a moderately to densely welded ash flow tuff. Other units are HSV1, a 2-km-diameter rhyolite dome; HSV2a to 2c, three overlapping, comagmatic trachyte lava flows; HSV3, rhyolite lava flow; HSV5, rhyolite lava flow; HSV6, a 3.5-km-diameter trachydacite dome; HSV7a to 7c, three, comagmafic rhyolite lava flows. Vol., volume percent; Max., maximum size of phenocryst (pl in HSV1, 2b, 2c, 6, 7a; sa in HSV 2a, 3, 4, 5, 7b, 7c); An, range of anorthite content within normally to oscillatory zoned plagioclase based upon the A-normal petrographic technique [cf. Deer et al., 1966]; Sa*, sanidine and anorthoclase are not differentiated; nd, not determined; PCT, point counts on standard thin sections; tr, trace; mnr, minor.

porphyritic high-K trachyte is the volumetrically dominant rock type, followed by rhyolite and then trachydacite. All rocks are porphyritic, and contain the phenocryst assemblage alkali feldspar + plagioclase > augire + biotite + opaques > apatite. Subhedral to euhedral sanidine is the dominant alkali feldspar, although anorthoclase (identified petrographically by its characteristic tartan twinning [cf. Deer et al., 1966] occurs in some units. Plagioclase varies from andesine to oligoclase in composition and most commonly occurs as subhedral to euhedral, moderately normal or oscillatory zoned laths. Augire occurs as pale green, euhedral to subhedral prisms; in many units it has been psuedomorphed by fine-grained amphibole. Biotite occurs as euhedral, strongly pleochroic (brown to light yellowish brown) plates. Total phenocryst contents vary from 9 to 35% (one plug contains 38% phenocrysts). The more mafic rocks contain hypersthene and andesine, whereas the more silicic rocks contain oligoclase

(commonly mantied by sanidine), accessory zircon, and in some cases, sphene. Quartz is absent, and pleochroic, brown to brownish green amphibole (probably edenite or hornblende) occurs in only one unit, a well-exposed 2-km-diarneter dome at the base of the sequence. Total phenocryst percentage and maximum size generally decrease from approximately 35 to 9% and from 3-10 to 1-3 mm across, respectively, while the ratio of sanidine/plagioclase generally increases with silica content.

Most units of the Hackberry Spring Volcanics exhibit phenocryst textures that are consistent with fractional crystallization, such as normally zoned plagioclase and alkali felspar overgrowths on plagioclase in silicic units [McCurry, 1985]. However, the two most mafic units HSV2a and HSV6 (Tables 1 and 2) exhibit textures that are suggestive of magma mixing, such as strongly embayed hypersthene phenocrysts mantled by clinopyroxene, strongly embayed plagioclase phenocrysts with reversely zoned

TABLE 2. Representative Whole Rock Chemical Analyses of the Hackberry Spring Volcanics Unit

HSV1 HSV2a HSV3 HSV4 HSVI HSV5 HSV6 HSWa HSV7b HSV7c

SiO 2 67.3 61.2 69.5 ' 66.0 65.6 ' 69.6 ' 65.7 73.0 71.7 74.'1 TiO 2 0.45 1.22 0.88 0.92 0.93 0.42 0.89 0.37 0.34 0.27 AI20 • 15.0 15.7 15.8 17.0 17.2 15.1 15.4 14.3 14.1 13.9 FeO* 2.45 8.67 1.86 3.38 3.02 2.31 5.38 1.44 1.40 1.22 MnO 0.03 0.07 0.02 0.03 0.06 0.07 0.04 0.04 0.05 0.06

MgO 0.45 0.59 0.29 0.27 0.40 0.38 0.46 0.25 0.16 0.14 CaO 2.78 2.97 1.47 1.58 1.67 1.82 2.68 0.82 0.82 0.91

Na20 4.2 4.4 4.3 4.5 4.7 3.7 4.3 3.9 4.2 4.4 K20 3.89 4.16 5.21 5.70 5.98 4.23 4.18 5.29 5.34 5.25 LOI 1.37 0.86 1.38 1.67 1.99 1.19 0.70 1.31 1.03 0.58 Total 97.92 99.84 100.71 101.05 101.55 98.82 99.73 100.72 99.14 100.83

Rb, ppm 74 76 51 66 69 85 70 120 120 130 Sr 650 400 290 193 114 320 430 100 100 34 Zn 40 61 16 47 47 26 53 31 28 30 Zt 310 330 510 505 535 270 330 350 340 280

SiO2* 69.7 61.3 70.0 66.4 66.1 71.3 66.3 73.4 73.1 73.9 P.I. 0.74 0.75 0.80 0.80 0.81 0.71 0.75 0.85 0.90 0.93 Cm - - 0.41 0.56 - 1.13 - 0.67 - -

Major elements were determined by XRF on fused disks and Rb, St, Zn, and Zr on pressed disks. Total Fe is given as FeO*; LOI = sample weight loss on ignition for 30 min at 1000øC in a Pt crucible; SiO2* is total SiO 2 where the analyses have been normalized to 100% on an anhydrous bases; P.I. (peralkaline index), molar ratio of (Na20 + K20)/A1203; Cm, CIPW normatire corundum, where FeO* is recalculated to FeO and Fe20 3 by the method of Irvine and Baragar [1971]. The whole rock analytical data for the major elements listed in this and in subsequent tables are significant at the lc• confidence level, based upon replicate analyses; relative standard deviations for replicate analyses of trace clements are less than 10%. These data are available on MS- DOS and Macintosh fonnaued disks in ASCII file format; requests will also be accepted over BITNET at ES009 @ NMSUVM1.

14,840 McCXIRRY: MIOCENE Pl•.ALKALIl• VOtCAlqlSM, MOJAVE DESi•, CAL•

1.0

0.5

0.0

10.0 -

7.5

5.0

2.5

0.0

18-

16

14-

12"

[]

x

x

[]

Whole Rock Chemical Variation Diagrams

4.0

• • o [] L) 2.0

[] &•x ' , ' ' ß • 0.0

6.0

!:• Z 4.0

• ß [ 2.0

ß 6.0

[]

[] + • 4.0

x

[]

Q HSV

+ WHMT-LM

x WHMT-MM

ß WHMT-UM

ß TSMR

[]

i I -- -i --

' , , , ' ] 2.0

X [] [] [] X [] []

' I ' I

[]

' I ' I

1.5

1.0

0.5

0.0

600 []

400 • []

200 [] [] [] []

, , 0 ' ' ß a-Am'_ --. '-' ß , . [

70 80 60 70 SiO2 SiO2

Fig. 4. Whole rock chemical variation diagrams expressed as weight percent oxides (data from Tables 2, 3, and 5). Open squares, Hackberry Spring Volcanic; pluses, crosses, and triangles are for the lower, middle, and upper members respectively, of the Wild Horse Mesa Tuff; solid squares are for the Tortoise Shell Mountain Rhyolite.

I

rims, and strongly embayed and rounded alkali feldspar rock types at the volcanic center and the absence of phenocrysts that phenocrysts. These features are consistent with the idea of Glazner occur in some peraluminous rocks, such as topaz, garnet, or [1985] and Glazner and Turner [1984] that intermediate sillimanite. Incipient clay alteration is apparent in many of the composition rocks in the Mojave Desert were formed by mixing of samples and as a result may have significantly increased their basaltic and rhyolitic parental magmas [cf. Novak and Mahood, 1986].

The rocks vary in normalized anhydrous silica content from 61 to 74%. Representative whole rock chemical analyses of the Hackberry Spring Volcanics are listed in Table 2. They vary from metaluminous to mildly peraluminous. The most distinctive chemical features are very low ratios of CaO/SiO 2 and MgO/SiO 2 and high (Na20+K20)/SiO 2 and K20/Na20 ratios in comparison to Miocene volcanic rocks of similar silica content occurring elsewhere in the Mojave Desert. These characteristics are retained throughout the entire Woods Mountains Volcanic Center rock sequence. The rocks have many features in common with the widespread Tertiary "biotite rhyolite" association of the western United States described by Ewart [ 1979].

Although most are metaluminous, some rock analyses listed in Table 2 are distinguished by small mounts of normafive corundum. This is unusual, given the dominance of metaluminous

original A1203/(Na20 + K20 ) ratios. On this basis it is tentatively concluded that corundum nonnative values are a result of weak

alteration and that the rocks were originally metaluminous. Considerable scatter is apparent in silica-oxide covariation

diagrams of various units of the Hackberry Spring Volcanics (Figure 4). Correlation coefficients for least squares fits of the data vary from 0.88 to 0.71 for TiO2, A1203, FeO*, MgO, CaO, and Sr and from 0.55 to 0.27 for Na20, K20 , Rb, St, and Zr. Covariation trends for Na20, K20, Rb, Zn, and Zr are not significant at the 95% confidence level. The scatter indicated by these values exceeds reasonable estimates of analytical and sampling errors. In addition, scatter in the data cannot be easily explained by the effects of alteration because a similarly low level of correlation is apparent between intensity of clay alteration of the rocks, loss on ignition (LOI) values, and A1203/(CaO + Na20 + K20 ) ratios. The wide scatter in the da_ta apparently reflects original magmatic variations.

An overall lack of time dependence in whole rock chemical

MCCURRY: MIOCENE PERALK•I• VOLCANISM, MOJAVE DESERT. CALIFORNIA 14,841

TABLE 3. Whole Rock Chemical Analyses of the Wild Horse Mesa Tuff

' ' ' w e be Middle Member ' •pper Member ' R1 R49 R89 R112 T130 A158 A159 A160 R161 R164

SiO 2 69.7 74.1 74.8 76.3 60.7 72.6 71.8 67.9 70.9 73.8 TiO 2 0.16 0.14 0.12 0.14 1.28 0.23 0.24 0.24 0.26 0.24 A1203 13.4 13.6 14.4 13.5 16.5 13.4 13.1 13.4 13.6 13.3 FeO* 1.71 1.62 1.47 1.56 7.23 1.40 1.44 1.47 1.47 1.40 MnO 0.11 0.09 0.11 0.10 0.09 0.12 0.11 0.12 0.12 0.09

MgO 0.32 0.17 0.09 0.15 0.86 0.38 0.26 0.45 0.39 0.! 8 CaO 0.72 0.25 0.04 0.09 3.67 0.42 0.35 0.55 0.61 0.64 NaeO 2.1 4.0 4.3 4.2 4.0 2.6 2.6 2.6 2.5 4.9 K20 5.4 5.2 4.8 4.8 4.4 5.5 5.9 4.8 5.3 5.2 LOI 5.85 1.66 0.32 0.55 2.36 .....

Total 99.47 100.83 100.45 101.39 101.09 96.65 95.80 91.53 95.15 99.75

Rb, plan 130 190 - - 120 ..... Sr 12 46 - - 390 37 22 43 42 55 Ba ..... 18 19 21 35 25 La ..... 50 49 53 49 48 Y ..... 68 71 75 66 53 Zr 430 450 390 408 447 447 466 416 SiO2* 74.5 74.7 7•7 7•.7 61.5 75.1 75.0 74.2 74.5 74.0 P.I. 0.90 0.85 0.90 0.69 1.03 Crn 2.79 0.94 2.06 1.23 - 2.40 1.87 2.92 2.67

Upper Member R167 R169 R170 R179 R190 R201 R212 R214 T205 T208

SiO 2 73.1 73.7 75.5 74.4 71.8 75.0 74.7 71.9 67.2 67.4 TiOe 0.25 0.24 0.25 0.24 0.23 0.25 0.26 0.24 0.54 0.63 A1203 13.1 13.0 13.2 13.1 12.5 13.0 13.3 12.9 16.6 17.1 FeO* 1.39 1.40 1.35 1.40 1.31 1.44 1.46 1.44 2.03 2.32 MnO 0.08 0.09 0.10 0.10 0.11 0.11 0.10 0.11 0.09 0.10

MgO 0.16 0.09 0.09 0.11 0.09 0.09 0.10 0.32 0.22 0.48 CaO 0.48 0.07 0.10 0.18 0.35 0.11 0.10 0.70 0.84 0.53 NaeO 4.7 5.0 5.0 5.0 4.8 4.9 5.1 2.1 5.8 5.5 KeO 5.3 5.0 5.1 5.1 4.9 5.0 5.2 6.6 6.0 6.1 LOI

Total 98.56 98.59 100.69 99.63 96.09 99.90 100.32 96.31 99.32 100.16 Rb, plan .......... Sr 17 3 7 7 10 8 5 24 22 17 Ba 25 18 20 30 18 53 55 37 102 107 La 42 48 40 41 46 52 50 49 191 215 Y 48 62 56 62 68 68 58 66 37 49 Zr 436 409 418 434 412 428 441 449 804 906 SiO2* 74.2 74.8 75.0 74.7 74.7 75.1 74.5 74.7 67.7 67.3 P.I. 1.03 1.05 1.04 1.05 1.06 1.04 1.05 0.97 0.93 Cm ....... 1.02 - 0.49

Sample number prefixes are as follows: A, rhyolitic air fall tephra, R, rhyolitic ash How tuff, T, trachyte pumice. The number to the right of the sample prefix indicates the stratigraphically vertical distance above the base of a measured section of the Wild Horse Mesa Tuff (Figure 5). All samples yielding anhydrous totals of less than 96% consist of hydrated volcanic glass; the other samples are devitrified. Peralkaline indices are only listed for devitrified samples. Major element analyses of the lower member and middle members determined by XRF on pressed powders of fused rock + lithium metaborate and Rb, St, and Zr on pressed rock powders. Major and trace element analyses of the upper member were determined by Inductively Coupled Plasma- Atomic Emission Spectrometry (ICP-AES). Analyses of the same sample by both XRF and ICP-AES are indistinguishable at the level of precision indicated here.

composition among the different units is indicated by the absence of significant correlations between composition and stratigraphic position. The weak compositional covariance of the Hackberry Spring Volcanics and lack of a time-composition correlation between major units indicate that they evolved more or less independently of each other, within small isolated magma chambers.

In contrast to the lack of systematic compositional variations between the different units of the Hackberry Spring Volcanics, strong compositional zoning occurs within two of the units. Both units, HSV2 and HSV7, consist of three overlapping lava flows

culminate in rocks that are more similar in whole rock chemical

composition and phenocryst assemblage to the overlying Wild Horse Mesa Tuff than any other unit in the Hackberry Spring Volcanics. Whole rock chemical analyses are listed in Table 2 as samples numbered HMV7a, b, and c. Most prominent among changes in chemical composition are stratigraphically upward decreases in the MgO, TiO 2, and Sr contents from 0.25 to 0.14 wt %, 0.37 to 0.27%, and 100 to 35 ppm, respectively, and increases in Rb from 120 to 130 ppm and peralkaline indices from 0.85 to 0.93. (cf. analyses of the Wild Horse Mesa Tuff, Table 3). In addition, there is a prominent upward increase in the

and interbedded tephra deposits. Vertical variations in phenocryst sanidine/plagioclase ratio from 2 to 42 and an upward decrease in mode and whole rock chemical composition are documented in Tables 1 and 2 (only for HSV7). In both cases there is a strong upward increase in sanidine/plagioclase ratio, and there are upward decreases in phenocryst concentration and maximum size. Vertical variations within HSV7 are particularly important because the trends among the three constituent, overlapping lava flows

the maximum size and concentration of phenocrysts from 3 to 1 mm and 13.4 to 9.2%, respectively. K/At dates on the uppermost lava flow and on the Wild Horse Mesa Tuff are indistinguishable. Subsequent rhyolite units of the Woods Mountains Volcanic Center have much less chemical variation than the Hackberry Spring Volcanics, suggesting that heterogeneous magmas were trapped

14,842 McCXamY: 1VII• PERALKALINE VOLCANISM, MOJAVB DESERT. CALIFORNIA

Rhyolite lava flow

Ve.a-y light gray block and ash-flow tuff; highly variable thickness

Rhyolite lava flow

Interbedded very light gray air-fall tephra and nonwelded ash flow ruff

Trachyte pumices .... Numerous lithophysal cavities; aecreasing in aounaance upwams .. Medium to dark reddish gray rock; platey. jointed; .sparsely porphynUq at the base increasing to 5% phenocrysts at m.e top wire sa >> cpx, opx, t•t, Fe-Ti oxides, and accessory zm, apafite, ana pe-rrierite

• Columnar •o!nting 2 m of Plinian hit fall tuff and !apilli tuff; very sparsely pophyritic (sa);

Lithic fragment segregations; numerous fragm.en• of Mesozoic grin.. itic rocks and Precambrian metamorphic rocks; lith•c fragment segregauons;

numerous large pumices Salmon colored; contains 10% phenoc.rysts with sa >>. qtz, cpx, bt, opx, pl, Fe-Ti oxides, and accessory zrn, apafite, and perrieritc

Light to medium gray rock

Cavernous weathering

Very sparsely porphyfi'tic (dominantly sa); numerous flow units, this unit is salmon colored hnd is very weakly columnar jointed

Medium to dark gray; numerous ,li. thic fr4.g.ment se ,g,r.e. ga.fions; the,.dominant lithic is a sparsely porphyritic, medium reddish gray litho•dal rhyonte

Very light grey beds

Fig. 5. A simplifted stratigraphic section of the northwestern Woods Mountains, illustrating the major stratigraphic features of the Wild Horse

"standard ignimbrite unit" [cf. Sparks et al., 1973]. Contacts between units are primarily distinguished by moderately sorted tuff at the base a few centimeters thick (layer 2a) that grades upward into a nonsorted zone from a few centimeters to a few meters thick that is enriched by about a factor of 10 or more in lithic fragn, ents (L zones). Ash cloud deposits (layer 3) occur at the tops, and ground surge deposits (layer 1) occur at the base of some of the flow units. However, layers of fallout tephra are absent except between the members. Each member is interpreted to have been emplaced in what was essentially a single major eruption.

Age. A sanidine separate from a single, glassy, nonwelded pumice fragment from the top of the middle member has been K/Ar dated at 15.8 + 0.4 Ma (K. Howard, personal communication, 1985). Time interv• between the emplacement of the members is poorly constrained. However, they were sufficient to have produced small but significant changes in the chemistry of the erupted magmas and enough to have produced cooling breaks. Conversely, the time intervals between eruptions were short enough so that the member contacts were not significantly eroded. Contacts between the members are planar in numerous excellent exposures, despite the highly friable nature of the rocks at the contacts.

Original spatial distribution and volume. All three members of the Wild Horse Mesa Tuff occur in areas where upper units have been protected from erosion (e.g., at Pinto Mountain and in the western Woods Mountains, Figure 1) or where erosion has not removed the upper, erosionally resistant unit (e.g., Wild Horse Mesa, Figure 1), suggesting that they originally had a similar spatial distribution. In addition, the paucity of post-Woods Mountains Volcanic Center faulting and the high erosional resistance of the bulk of the upper two members suggest that

Mesa Tuff. h is based on a 1:48 measured stratigraphic section [McCurry, present exposures give a reasonable indication of the original 1985; also unpublished data, 1987]. Bar to the left of the column indicates distribution; an interpretation based on simple interpolations intense devitrificafion, solid; partial devitrificafion, diagonal fine; glassy, no pattern; pattern on the column indicate dense welding, dash; moderate welding, ellipses; no welding, circles; horizontal contacts are shown for the major flow units.

before reaching the surface. The upper unit of the Hackberry Spring Volcanics may indicate that a fundamental change had taken place in the evolution of the magmatic system. It is the first signal that a large volume of magma had accumulated and differentiated beneath the Woods Mountains at 15.8 Ma, just prior to the extrusion of the Wild Horse Mesa Tuff. It may also presage increasingly extreme differentiation within the chamber with time.

Wild Horse Mesa Tuff

The Wild Horse Mesa Tuff is a comagmatic sequence of dominantly rhyolite ash flow tuffs that form a large group of mesas over an area of 600 km 2 (Figure 1). Major stratigraphic features of the tuff are illustrated in Figure 5. Exposures of the horizontal to gently southeast dipping deposits vary from 20 to 320 m thick, and they have a distinctive layer-cake appearance that results from welding and devitrificafion zonation.

The tuff is divided into lower, middle, and upper members on the basis of stratigraphically coincident discontinuities in devitrification and welding zonations, phenocryst assemblage and abundance, and whole rock chemical composition (Figures 5 and 6 and Tables 3 and 4). Each member is a cooling unit that consists of multiple flow units [cf. Fisher and Schmincke, 1984], many of which are well characterized by depositional bedforms of the

between current exposures is shown in Figure 3. The lobate pattern illustrated in Figure 3 suggests strong

topographic control on the original distribution of the tuffs. Three of the four major lobes can be understood in terms of exhumed palcotopographic features. The southern lobe was channeled between north trending paleo-Colten Hills and Fenner Hills (Figure 1). The western and northern lobes ponded in topographic depressions against the northern paleo-Providence Mountains and New York Mountains, respectively. That these areas were basins is indicated by the occurrence of the thickest exposed sections of pre-Wild Horse Mesa Tuff lacustrine sediments at these localities [McCu•ry, 1985]. The maximum exposed thickness of Wild Horse Mesa Tuff, 320 m, occurs within the western lobe about 12 km

west-southwest of the inferred source vents (Figure 7). A prominent absence of ash flow tuff exposures northeast of the volcanic center in Lardair Valley suggests that that area stood at a topographically high level during the middle Miocene.

Estimates of the original volumes of the members can be made given the inferred spatial distribution of Figure 3 and simple interpolations between nine measured stratigraphic sections [McCurry, 1985, also unpublished data, 1987]. The locations of the sections are shown in Figure 1. Based upon these criteria, the volumes of the members are estimated at 40 km 3 (lower member), 20 km 3 (middle member), and 20 km 3 (upper member). These values correspond to original magma volumes of 23, 14, and 20 km 3, when density differences are taken into account [McCurry, 1985, also unpublished data, 1987].

Lower member. The lower member of the Wild Horse Mesa

Tuff is a very sparsely porphyritic, generally nonwelded, and glassy to weakly welded and devilrifled compound cooling unit. It

M•Y: MI• •.INI! VOI. EANISM, MOIAVE DES•T, CALIFORNIA 14,843

Stratigraphically Vertical Variations in Phenocryst Assemblages

Phenocryst sa PI Qtz Bt Cpx Opx Fay Zrn Per Ap Meg Hem Py content

0 5 10 15%

I i

i i

i

(I,.

I I

, ;--i"- I

f I

i t I

I i , I I J

Fig. 6. Stratigraphically vertical variations in the assemblage of phenocrysts within the rhyolite component of the Wild Horse Mesa Tuff and an overlying lava flow of the Tortoise Shell Mountain Rhyolite. The diagram is based upon petrographic analysis and point counting of 30 samples collected from a measured stratigraphic section in the western Woods Mountains [McCurry, 1985; also unpublished data, 1987]. A simplified column is shown for reference along the left margin (el, Figure 5). Phase assemblages for trachyte pumices at the tops of the lower and upper members are not shown (see the text for a description of the petrography of these pumices). Solid lines are for volumetrically dominant phases; dashed lines are for phases that occur at lower concentrations.

consists of at least 11 flow units that are light to dark gray or salmon colored where nonwelded and glassy and light to medium reddish gray where welded and devitrified. Flow units vary from several meters to 70 m thick and have a maximum cumulative

thickness of at least 130 m. A maximum of 24 m of fallout tuff, lapilli tuff, and lapillistone occur at the base of the member at Hackberry Mountain, thinning to about 30 cm in the western Woods Mountains; they are absent elsewhere. Lithic fragments are dominantly accessory rhyolite but also include fragments similar to granitic and metamorphic rocks exposed in nearby areas and generally make up from 1 to 10% of the tuffs. Devitrification and welding zones cross flow unit contacts and are highly variable in intensity, although there is a general tendency for an upward increase in both welding and devitriflcation.

Flow units at and near the base contain approximately 0.1% (by volume) total phenocrysts, with sanidine (Or37; Figure 8) approximately equal to magnetite >> plagioclase (Ab75An18Or 7) +

augite + fayalite (Figure 6 and Table 4). Prominent changes upward through the member include increases in the size and abundance of sanidine (to 1 mm and 1%, respectively), the appearance of trace to minor amounts of biotite, zircon, and ilmenite, and the disappearance of fayalite.

Whole rock chemical analyses of samples from the bottom, middle, and top of the member indicate that the rocks are mildly peraluminous high-K rhyolites. No strong and systematic trends occur among the analyses except those that correlate with intensity of alteration, the presence of fine-grained carbonate (common in some highly porous samples), intensity of vapor phase mineralization, and contamination by xeno]iths that were too small to be removed during sample preparation. All analyses contain normafive corundum, unusual given other features that are more typical of metaluminous rocks such as high concentrations of Zr, and the presence of phenocrysts of clinopyroxene and fayalite. Therefore, as in the case of the Hackberry Spring Volcanics, it is

14,844 M•Y: MIOCE• •AL• VOLCANISM, MO;IAVE I•s•, CALIFORNIA

c• "• o

McCkJRRY: MIool• P•RALKALINB VOIf..ANISM, MO•AV• DF_,SBRT, CALIPORNIA 14,845

lO

.Ol

Upper Member

' ' I ' I ' ' I' ' I ' I '• I

0 2 4 6 8 10 12

10

1 it;tile Mem•r .1

.Ol ß1 i [ i i i [ •i ' .001 , . [ . 0 2 4 6 8 10 12

semiquantitative microprobe analyses [McCurry, 1985]), are coincident with a cooling break in the Wild Horse Mesa Tuff about half way up from the bottom of the tuff (Figure 6). The break is coincident with the contact between two flow units and is the basis

for dividing the lower from the middle member of the Wild Horse Mesa Tuff.

The middle member is a moderately porphyritic, moderately to densely welded, and glassy to devitrified simple cooling unit. It consists of at least six flow units, similar in color to those described previously but also having a distinctive dark reddish gray color where densely welded. Flow units vary from a few meters to 75 m thick and have a cumulative maximum cumulative thickness of 150

m. The member rests directly upon a layer 3 fine ash deposit several centimeters thick at the top of the lower member. There are no other intervening fallout deposits. As in the case of the lower member lithics constitute 1-10% of the tuffs. The dominant lithic

type is accessory rhyolite in the lower two flow units; however, granitic and metamorphic lithics, resembling Mesozoic and Precambrian rocks exposed in nearby areas, are common in the upper four flow units.

Unusual lithic fragment segregations that are similar to structures formed experimentally in gas-fluidized beds [cf. Rowe and Nienow, 1976; Wilson, 1984] occur in proximal facies of both the lower and middle members and are particularly conunon near the top of the middle member in the western Woods Mountains. They consist of an assemblage of multilithologic, randomly oriented, unsorted lithic fragments in a matrix of shards and pumice. The matrix merges imperceptibly into the surrounding parts of the deposit. The lithics are similar in size and lithology to those in the surrounding parts of the deposit, but they are concentrated by a factor of from 5 to 20 times their average abundance in the deposit. The morphology of the structures is highly variable. Many occur as ellipsoidal swarms from l0 cm to I m across, in some cases with vertical tails. Some have amoeboid or vertically elongated shapes. The tops of some of the swarms are,sharply bent into the horizontal, as if sheared by late stage movement within the flow.

10 Horizontal, lensoid concentrations of lithies occur in correlative flow units throughout the extent of the Wild Horse Mesa Tuff,

1 suggesting that segregation structures similar to those occurring in •. the Woods Mountains originally were widespread but that the • .1 horizontal motion of the more proximal deposits stagnated while the

.01 Or

.001 0r55/ 0 2 4 6 8 10 12

Distance (kin) so

Fig. 7. Maximum lithic fragment sizes for flow units within the Wild Horse Mesa Tuff. The most common lithic is lithoidal rhyolite, followed by quartz monzonite, and gneiss. Distance is measured radially away from the probable vent zone; symbols represent the averages of the five largest lithics 4o on an outcrop for individual ash flow units that are correlated at the starred localities; open triangles are for beds near the base of the respective members; solid triangles are for beds near the tops of the middle and upper members and the middle of the lower member, cross is for a bed located near the top of the lower member.

tentatively concluded that the corundum normative values are a Ab result of minor alteration or contamination and that the original samples were metaluminous.

Middle member. A distinctive break in total phenoeryst content and maximum size from 1 to 11% and 1 to 3 ram, respectively, and the appearance of two new phenocryst phases, quartz and perrierite (differentiated from similar appearing allanire on the basis of

Fig. 8. Alkali feldspar phenocryst compositions in weight percent, on Ab- An-Or diagrams, for the lower, middle, and upper members of the Wild Horse Mesa Tuff, and two lava flows in the Tortoise Shell Mountain Rhyolite. A, L, M, and U refer to stratigraphic intervals for the respective members (Table 4); T refers to a trachyte sample from the top of the upper member. Bars circumscribe the range of microprobe analyses obtained from different samples.

14,846 McCtm•¾: MI(X:F_2qE PERALKALINE VOLCANISM, MOJAVE D•ERT. CALaaORNIA

Wo

Hd TSMR

// D• Hd

/ // 701 # / IT] u.. M L /

En

E n 9o 80 70 60 50 Fs Fig. 9. •roxene phm•st c•sifions • weight •ent, • -•-Wo-Fs diagr•s, for •e middle and u•er mem•m of •e W•d Horse •esa T•f •d for a sample from one lava flow • •e Tortoise She• Mountain •yolite. L, M, and U •fer to stratigmp•c •te•als for •e middle member •able 4); a•ow• •es •cate Fe-enfic•ent trend for cl•opymxenes in • midge mem• T mfem to tmchyte s•ples from •e midge and up•r mem•rs; fie l•es co•ect com•sifions of coe•s•g cl•opymxcnes and o•opyroxenes.

deposits were still vigorously degassing. These features support the suggestion of numerous authors that fluidization is an important process during the transport of some ash flows [cf. Sparks, 1978; Wilson, 1984].

The rhyolite component of the middle member is slightly more siliceous than the lower member (75.7% as compared to 74.7%, on a normalized, anhydrous basis) but is otherwise similar to it in whole rock major element composition (Table 3).

Strong vertically systematic changes occur in the content and chemical composition of phenocrysts in the middle member. Some of these are illustrated in Figures 6, 8 and 9 and Table 4. Basal flow units contain approximately 11% total phenocrysts, with sanidine >> quartz > ferroaugite (commonly partially altered around the margins to gruneritc) > Fe-Ti oxides (mosfiy magnetite) + biotite + plagioclase (An30 based on the A-normal petrographic technique; commonly manfled by sanidine) + accessory zircon, perrieritc, and apatite. Total phenocryst content increases to 14% at the top of the member, primarily because of an increase in the abundance of sanidine. Vertical variations in the assemblages of phenocrysts are illustrated in Figure 6. Orthopyroxene (XFs = 0.25) occurs in tuffs slightly above the base of the member; plagioclase decreases in abundance upward through the member and is absent in the top flow unit; and a trace of pyrite or pyrrhotite (identified by a qualitative microprobe analysis) occurs in the uppermost part of the member. In addition, roughly linear increases occur vertically through the tuff in the molar Mg/Mg+Fe ratios of clinopyroxene (Figure 9) and biotite phenocrysts from 0.31 to 0.73 and 0.36 to 0.43, respectively, and in Or/Or+Ab ratios of sanidine, from 0.36 to 0.43 (Figure 8).

Upper member. The upper member of the Wild Horse Mesa Tuff is a densely welded simple cooling unit that is easily recognized by a distinctive, 6- to 10-m-thick, yellowish brown, weakly to moderately welded, columnar jointed zone at the base that grades upward into erosionally resistant, densely welded, platy jointed, dark reddish gray rock (Figure 5). It caps many of the mesas in the Woods Mountains area.

The member consists of at least three flow units that are from 3 to

30 m thick and has a maximum cumulative thickness of 55 m.

Lithics are similar to those in lower members but are less abundant

(a maximum of about 5%) and generally smaller. The ash flow tuff overlies a maximum of 2.4 m of interbedded fallout tuff and lapilli

tuff. The fallout tephra is thickest and coarsest in the western Woods Mountains and pinches out systematically 10 km to the north and west. In the western Woods Mountains the uppermost bed of fallout tuff grades imperceptibly upward over a few centimeters into the basal ash flow unit.

Phenocryst assemblages are illustrated in Figure 6. The dominant assemblage consists of euhedral to subhedral sodic sanidine (Figure 8) >> Mg-rich augite (Figure 9) = magnetite > biotite, orthopyroxene (Figure 9), and ilmenite, accessory zircon, and a trace of apafite. Plinian fallout tephra and the basal part of the ash flow tuff are very sparsely porphyritic. Small amounts of fayalite occur only in the Plinian tephra. Phenocryst concentration increases upward through the fin'st ash flow unit, to approximately 5%; fayalite disappears, and minor amounts of subhedral to euhedral, bipyrarnidal quartz appear near the top of the unit. Coarse pumices occur at the top of the farst unit, and they are overlain by several centimeters of f'me-grained, well-sorted tuff. The overlying flow unit is similar to the top of the lower flow unit, except for the absence of quartz. The lack of quartz in the upper flows units is curious. There is no evidence of a cooling break at the contact. Rocks above and below, including the fine-grained tuff, are all densely welded. In addition, the compositions of phenocrysts from the basal flow unit and upper flow units are almost identical. Perhaps the vent location shifted somewhat, draining a slightly different part of the magma chamber.

Stratigraphically vertical variations in phenocryst composition are much less pronounced than those of the middle member (Figures 8 and 9 and Table 4). Upward through the deposit, sanidine phenocrysts become slightly more potassic (Or0.356 to Or0.355). However, variations among clinopyroxene, orthopyroxene, and Fe- Ti oxides are near the resolution of microprobe, or within the range of variation that occ• within individual mineral grains.

The upper member of the Wild Horse Mesa Tuff is remarkably uniform in whole rock major and trace element composition and Sr and Nd isotopic composition. Representative chemical analyses are listed in Table 3 and illustrated in Figure 4. They are high-K, mildly peralkaline (peralkaline indices = 1.03 to 1.06 for the nonhydrated samples, Table 2) and contain about 74.5% SiO 2 on a normalized anhydrous basis. The peralkaline character of these rocks is also demonstrated by the presence of prisms of sodic amphibole in the matfix and in cavities of vapor-phase-altered rocks (S. W. Novak, personal communication, 1988). Variations in the alkali contents of other samples listed in Table 3 correlate positively with ignition losses, indicating that the variations are a result of hydration [cf. Noble, 1967]. Variations in CaO and Sr are a result of the presence of variable amounts of fine-grained carbonate in porous s:maples. However, an increase in Ba near the top of the upper member is probably reflective of a true magmatic variation. Subtle but significant variations occur between the upper and middle members, most prominenfiy upward increases in TiO 2 from 0.14 to 0.25%, in total alkalies, K20/Na20, and (Na20+K20)/A1203 ratios, and a slight decrease in silica content from approximately 76 to 75%.

No significant vertical variation occurs in whole rock rare earth elements (REE) and Nd and Sr isotopic data of the rhyolite component of the member (D. Musselwhite, unpublished data, 1986). Initial 87Sr/86Sr isotopic and •[d values for the rhyolite are about 0.711 and -7, respectively. These values are about the same as those for the lower and middle members. Trachyte pumices at the top of the upper member have values of about 0.709 and -7. Both the trachyte and rhyolite are strongly light REE (LREE)- enriched (Figure 10). Trachyte putnice lapilli at the top of the upper member are more LREE-enriched (chondrite normalized La = 400;

M•Y: MI• P!•RALK•!I• VO•ANISM, MOJAVE D•l•r. CAL• 14,847

1,000

• 100 .;-

_e 10 o

i i i i i i i i LaCeNdSmEu Tb Yb Lu

Fig. 10. Chondrite normalized rare-earth element concentrations based upon instrumental neutron activation analysis (I•AA) for coexisting rhyolite and trachyte pumices from the top of the upper member of the Wild Horse Mesa Tuff (based upon unpublished data by D. Musselwhite, University of California, Los Angeles, 1982). The stippled field encloses the ranges of values for rhyolite samples collected at several intervals from the base to the top of the member.

La/Yb = 40) than rhyolite pumices from the same stratigraphic level (chondrite normalized La = 200; La/Yb = 10). These rhyolite pumices also have a strong negative Eu anomaly, whereas the trachyte has none, or possibly a slight positive Eu anomaly.

Recenfiy obtained oxygen isotopic data also reflect the isotopic homogeneity of the Wild Horse Mesa Tuff, as well as its geochemical continuity with the overlying Tortoise Shell Mountain Rhyolite (A. F. Glazner, personal communication, 1987). Sanidine separates from rhyolite pumices from the middle and upper members of the Wild Horse Mesa Tuff yield •)lSO values of 7.4, and 7.4, respectively, and from a rhyolite pumice from the Tortoise Shell Mountain Rhyolite the sanidine separate yields a •)lSO value of 7.5.

Trachyte pumices. High-K trachyte pumice and pumice consisting of intermingled rhyolite and trachyte components occur in the upper parts of both middle and upper members. They are rare and only locally exposed in the middle member [McCurry, 1985]. However, they constitute up to 5-10% of the total pumice population at the top of the upper member and have been identified throughout the lateral extent of the Wild Horse Mesa Tuff where the deposits have been protected from erosion.

In the middle member, trachyte pumices and interbanded trachyte-rhyolite pumices are up to 15 cm across. The pumices (61.5% SiO 2, on a normalized, anhydrous basis, Table 3) are very dark gray and contain 14% phenocrysts of sanidine • labradorire > augitc •. biotite •. opaques > orthopyroxene > olivine + accessory

phases are presented in Table 4. The unusual assemblage of phases in these pumices, the occurrence intense resorption of plagioclase and sanidine phenocrysts, and the similarity of the compositions of orthopyroxene in rhyolite and trachyte pumices (Table 4) indicate that the trachyte pumices are hybrids. Orthopyroxene phenocrysts in the trachyte are similar in major and minor element oxide composition to those occurring in pumices from the rhyolite part of middle member (Figure 8) and suggest that the rhyolite was one of the parental magmas. The more mafic parent is unidentified. However, the trachyte is isotopically more manfielike than the rhyolite (D. Musselwhite, personal communication, 1986). It is tentatively concluded that the trachyte was produced by partial mixing of a manfie-derived alkali basalt or alkaline basaltic andesire magma and the rhyolite just prior to the extrusion of the middle member [cf. Novak and Mahood, 1986].

Trachyte pumices at the top of the upper member are easily recognized in the field by their dark color, strongly porphyritic character, and large size, up to 2 m across in proximal areas, compared to a maximum of 30 cm for rhyolite pumices. These pumices (approximately 67.3% SiO 2, on a normalized, anhydrous basis, Table 3) contain about 25% total phenocrysts that are a maximum of about 8 mm across. The assemblage is sanidine (Or42) >> plagioclase (An30, based upon the A-normal petrographic technique) + augite + biotite + opaques >> zircon + apatite, similar to the coexisting rhyolite. All except sanidine and plagioclase are euhedral to subhedral, are compositionally distinctive from the same phases in the rhyolite, and are compositionally uniform throughout, indicating that they were originally in equilibrium with the trachyte magma. Plagioclase phenocrysts are strongly embayed, and many have thick rims of sanidine. Sanidine phenocrysts are large (up to 8 mm across) and have a sieve texture suggesting rapid crystallization. They are slightly more potassic than sanidine phenocrysts in neighboring rhyolite pumices (Figure 8).

Source. Although the vents are not directly exposed, the locations of at least some, and probably most, of the source vents for the Wild Horse Mesa Tuff ash flows are well constrained to a

zone less than a few kilomaters long and a few hundred meters wide in the western Woods Mountains, on the east side of the

easternmost exposures of Wild Horse Mesa Tuff (Figure 7). The vent zone is primarily constrained by the occurrence of coarse coignirnbrite lagfall deposits [cf. Wright and Walker, 1977] and by a spatially systematic exponential increase in the size and abundance of lithie fragments within individual beds toward the zone (Figure 7). Rounded and angular inclusions of quartz monzonite, similar to basement rocks exposed in situ in nearby areas, are up to 14 m in maximum dimension west of the inferred vent zone. These fragments occur within the Wild Horse Mesa Tuff on the outside of the caldera and were therefore ejected from the same vents as the enclosing tuff and are not fault scarp rubble. The exponential coarsening patterns that characterize many of the beds constrain the locations of vents to within several hundred

meters to the east of the easternmost exposures of some flow units (Figure 7) because further extrapolation of the curves yields unrealistically large fragment sizes.

Fallout tephra are much thicker (24 m) at the base of the lower member at Hackberry Mountain, 12 km east of the inferred vent zone, than they are I km west of the zone (0.3 m). The fallout tephra has not been studied in detail but appears to be comagmatic with the overlying ash flow units of the lower member of the Wild Horse Mesa Tuff. If both fallout and ash flow units were erupted during a continuous eruption, then the vent location may have shifted rapidly from east to west. However, a simpler explanation

zircon, apatite, and amphibole. Chemical analyses of some of these is that the vent location remained fixed and that the variations in the

14,848 McCtmItY: MI• P!•O, LKALINE VOLCANISM, MillAVE DESI•RT, CALIFOICNIA

thickness of the tephra were produced by a strong westerly wind. The source of the lower part of the upper member is the least

well constrained by lithic dispersal patterns (Figure 7). This is primarily because this unit generally contains fewer, and smaller, lithics than most flow units of the earlier two members. However,

anomalous variations in welding and thickness of the upper member, and systematic pinching out and thinning of comagmatic Plinian fallout tephra beneath the member, away from the inferred vent zone [McCurry, 1985] are consistent with the idea that the source vents were located near those for the lower and middle

members. The upper member is consistent in its welding profile and thickness over most of its spatial extent, probably because terrain irregularities had been leveled by the previous members. However, the unit thins rapidly from 55 to less than 2 m thick in a north trending zone extending approximately 2 km west from the caldera margin. The thinning is accompanied by a rapid decrease in the intensity of welding and devitrification. Trachyte pumices and layer 3 deposits occur near and at the top of the unit, respectively, indicating that the thinning did not result from erosion. The basal contact of the unit is flat; therefore thinning is not a result of topography. The most likely thinning mechanism is probably a partial eastward directed drain back of the ash flow tuff over caldera-related scarps. It is also plausible that it resulted from a hydraulic jump [cf. Clark, 1984].

The inferred vent zone illustrated in Figure 7 is adjacent to north- northeast trending, caldera-related faults and fault scarps in the western Woods Mountains (Figures land 2) [McCurry, 1985]. Proximal deposits are draped over early caldera-related fault scarps and are cut by later collapse-related faults [McCurry, 1982, 1985]. The spatial and temporal association of vent zone and faults suggests that venting occurred nearby to the east of presently exposed faults, through other inferred caldera-related faults.

Initial Phase of Caldera Formsion

Eruption of the Wild Horse Mesa Tuff initiated at least two periods of caldera collapse that are centered on the eastern Woods

[McCurry, 1985]. I'4o similar faults were found to the east. However, deposits filling the depression wedge out in that direction, indicating that the collapse was hinged. The original caldera was therefore a trapdoor-shaped depression, deepest on the west side, and about 10 km across.

The inferred caldera margin parallels contours of a near-circular 34 mGal negative residual gravity anomaly (G. Henzel, personal communication 1982). Some of the features of the anomaly are summarized by McCurry [1985]. Although the anomaly is probably mostly the result of a shallow pluton (G. Henzel, personal communication, 1982), the steepest gradient occurs on the west side. This is consistent with the deduced shape of the caldera since the caldera fill would be expected to be less dense than the surrounding country rocks.

It is possible that the faulting reported here is tectonic in origin (R. L.Christiansen, personal communication, 1987). The faults roughly parallel numerous, extension-related Miocene faults in the region. However, no similar, contemporaneous faulting occurs elsewhere in the Woods Mountains area. I believe that the faulting overlap in time with eruption of the Wild Horse Mesa Tuff, proximity to the inferred source vents, geometry of the faults, and geophysical data indicate that the faults formed primarily as a result of caldera collapse. The north-northeasterly trends of most of the faults indicate a secondary regional tectonic influence.

Tortoise Shell Mountain Rhyolite Extrusion of the Wild Horse Mesa Tuff was immediately

followed by voluminous rhyolite intracaldera volcanism. Lava flows are volumetrically dominant and are well exposed in the central and eastern Woods Mountains. The flows are 10-70 m

thick, have glassy margins of flow breccia, and aphanitic interiors. They have a spatial distribution of several tenths of a cubic kilometer for the smallest, up to at least 10 km 2 for the largest. In some cases, flows can be traced to near-vertical feeder dikes [McCurry, 1985]. The dikes vary from approximately 5 to 15 m across, have no preferred azimuthal orientation, and occur

Mountains. The best exposures of caldera boundary faults occur in throughout the caldera, suggesting that the floor of the caldera was the western Woods Mountains (Figure 1). Here, the caldera is highly fractured during the caldera collapse. distinguished by a system of high angle faults and buttress Intracaldera lava flows are interbedded with compositionally unconformities (Figures 1 and 2). They are exposed in a similar nonwelded ash flow and pyroclastic surge deposits and curvilinear, north-northeast to northeast trending zone about 5 km vitric fallout tuff, lapilli tuff, and tuff breccia. The interstratified long and several hundred meters wide. The first phase of collapse deposits are generally a few centimeters to several meters thick. is indicated by fault scarps consisting of rocks of the lower member Some contain coarse lithics that are rarely up to 1 m across. The of the Wild Horse Mesa Tuff that are overlain and buffed by the lithics are dominantly of accessory rhyolite but also include sparse middle member. The faults are steplike, with the downdropped fragments of granitic and metamorphic country rocks and fragments side to the east. Approximately 60 m of cumulative offset can be resembling the Hackberry Spring Volcanics. documented with present exposures, although the total may be Plugs. At least 13 rhyolite plugs, ranging from 30 to 900 m much larger. The second phase of caldera collapse is abnormal across were intruded into the caldera fill late during the because the relative movement along the collapse-related faults was emplacement of the Tortoise Shell Mountain Rhyolite (Figure 1). eastside up. In these cases, downdrop to the east resulted from the Several occur in an arcuate zone near the western margin of the progressive eastward rotation of fault blocks. These structures are caldera, suggesting that they were extruded from ring fractures, but well exposed in the southeastern part of the Woods Mountains. most occur in a distinctive cluster in the eastern Woods Mountains The faults cut both the Wild Horse Mesa Tuff as well as the near the center of the caldera. The plugs are elliptical to near- overlying part of the Tortoise Shell Mountain Rhyolite. However, circular in plan view and have steep, inward dipping contacts, interbedded, nonwelded pumice flow deposits and fallout deposits resulting in a funnellike morphology. The contacts commonly have at the base of the Tortoise Shell Mountain Rhyolite wedge out a well-developed concentric zonation. An outer zone of glassy rapidly to the west over the fault blocks, indicating that the rotation pumice grades inward into moderately to densely welded of the fault blocks began after the extrusion of the upper member of pumiceous rhyolite, then into a dense, brecciated vitrophyre the Wild Horse Mesa but prior to the extrusion of the first units of consisting of angular blocks of dense rhyolite in a matrix of welded the Tortoise Shell Mountain Rhyolite. The time interval between pumice, further inward to strongly flow banded lithoidal rhyolite the eruption of the upper member and the initiation of block rotation (in which the flow banding parallels the contacts), and finally was brief because no erosion was observed at the contact between inward into massive to sparsely vesicular lithoidal rhyolite. The the Tortoise Shell Mountain Rhyolite and Wild Horse Mesa Tuff border zone varies from 1 to 5 m thick. In many cases the lava

MCOmRY: 1VI•• PSRAI, JO, L• VOLCAmSM, MOSAV• DESERT, CALIlt3RNIA 14,849

TABLE 5. Representative Whole Rock Chemical Analyses of the Tortoise Shell Mountain Rhyolite

Sample T_ _vpe Flows SD Plugs $D

SiO 2 75.8 0.7 74.9 0.2 TiO2 0.19 0.02 0.20 0.01 A120 • 13.0 0.2 12.9 0.1 FeO* 1.28 0.13 1.30 0.10 MnO 0.09 0.01 0.09 0.01

MgO 0.08 0.02 0.06 0.01 CaO 0.26 0.11 0.22 0.06

NazO 4.7 0.2 4.7 0.2 KzO 4.94 0.08 4.92 0.05 LOI 0.42 0.13 0.34 0.06 Total 100.76 - 99.63 -

Rb, ppm 166 6 166 5 Sr 5 8 6 6 Zn 70 11 73 14 Zr 361 65 381 68 SiO2* 75.7 0.7 75.4 0.2 P.I. 1.01 0.02 1.01 0.01

SD, 1 o standard deviation of 20 analyses of at least eight different flows, and seven different plugs.

flows and tephra deposits through which the plugs were intruded are bowed upward and moderately to strongly silicified near the contacts. The rhyolite plugs are identical in chemical composition (Table 5) and overlap in the assemblages of phenocrysts with the older volcanic rocks, indicating that the volcanic and shallow intrusive rocks are consanguineous.

Volume. The original volume of the Tortoise Shell Mountain Rhyolite is estimated to be l0 km 3. This estimate assumes that the rocks filled a caldera with an area of 70 km 2, to a maximum thickness of 300 m, and that the caldera has a simple trapdoor shape. It is poorly constrained because the maximurn thickness of the deposits is unknown. The maximum exposed thickness of 300 m occurs in the western Woods Mountains, and the unit pinches out at Hackberry Mountain. The original volume might be as high as 20 km 3 because an interpretation of a reversed 2-km seismic refraction profile 1 km west of Hackberry Mountain indicates that the deposits are at least 300 m thick there and therefore that they do

were never observed together in the same sample. Sanidine compositions overlap with those of the Wild Horse Mesa Tuff (Figure 8). Quartz and plagioclase were only observed in one unit (a plug near the western margin of the caldera). Phenocryst textures are subhedral to euhedral, and no evidence of strong disequilibrium was observed. Various combinations occur among the ferromagnesian and accessory phases; however, some apparently are mutually exclusive; perrieritc was never observed with sphene, and biotite was never found in the same samples with edenitc. Lava flows, tuffs, and plugs overlap in whole rock and phenocryst composition and mineral assemblage. In general, no correlation was observed between age, mechanism of eraplacement, and phenocryst assemblage among the different units in point counts (1000-2500 points on standard thin sections) of 32 samples from eight flows and three plugs examined in detail [McCurry, 1985, also unpublished data, 1988].

Continuation of caldera collapse into period of lava extrusion. One of the more unusual aspects of the Woods Mountains Volcanic Center is that significant caldera collapse continued after the eruption of voluminous ash flows, into the following phase of lava extrusion [cf. Christiansen, 1979]. As previously stated, at least two periods of caldera collapse occurred in association with ash flow extrusion. Three lava flows and

interbedded tephra overlie proximal parts of the outflow sheets west of the caldera, indicating that the original caldera was fn'st filled in and then overflowed to the west with intracaldera-derived flows

and tephra. The direction of transport for the flows is deduced as outward from the caldera because, first, the flows pinch out away from the caldera margin from within 1-5 km, second, because no suitable vents are exposed outside of the caldera despite excellent exposures of rocks in the caldera margin area, and third, because the flows locally produced directional features in the underlying tephra deposits that are consistent with east-to-west transport [McCurry, 1985]. This sequence of three flows was subsequently downfaulted to the east along a north trending fault. Neither the flows nor the Wild Horse Mesa Tuff are exposed east of the fault, indicating that the flows were downfaulted at least 270 m. The downfaulting produced a large eastward facing scarp. Subsequent

not thin evenly across the caldera [McCurry, 1985]. intracaldera volcanism produced compositionally similar flows and Age. The average of two K/At dates on the deposits is 15.8 Ma tephra that are inset against this scarp and that are interbedded with

(J. Nakata, personal communication, 1985), identical to the age of fans of coarse breccia shed from the scarp. The breccia contains the Wild Horse Mesa Tuff.

Chemical composition and phenocryst assemblages. Flows, tephra deposits, and plugs of the Tortoise. Shell Mountain Rhyolite are nearly indistinguishable in whole rock chemical composition, phenocryst assemblage, and phenocryst composition from the upper member of the Wild Horse Mesa Tuff. Representative analyses are listed in Tables 5 and 6, and whole rock analyses are given on variation diagrams in Figure 4. The rocks are remarkably homogeneous, high-K, mildly peralkaline rhyolites. Near chemical homogeneity is illustrated in Table 5 by the small standard deviations among 21 analyses of at least eight lava flows and analyses of seven different plugs; many of the standard deviations are of the order of the analytical uncertainty.

The rhyolitic volcanic rocks and shallow intrusive rocks are aphyric to sparsely porphyritic. Phenocryst contents vary from less than 0.1-7%. The most common assemblage is sanidine (Or36 - Or43; Figure 8)>> opaques •. augRe (Figure 9)+ a trace of zircon. Trace amounts of sphene and perrieritc occur in many samples, and a trace of apatite occurs in a few. Minor amounts of euhedral biotite and edenitc phenocrysts occur in about a third of the 32 thin sections that were examined. Edenitc was identified on the basis of

numerous large fragments of the older rhyolite and pinches out 2-3 km east of the scarp [McCurry, 1985]. This later phase of volcanism produced flows that again filled in the caldera and overflowed the caldera margins, in this case to the north and south by from 1 to 3 km (Figure 1). The near chemical uniformity of the intracaldera rocks and absence of interbedded lacustrine suggest that the caldera was filled in rapidly after each phase of collapse.

TABLE 6. Representative Microprobe Analyses of Phenccrysts Within the Tortoise Shell Mountain Rhyolite

Samplg Sa(1 • Sa(2) Ct>x(1 • Ed(2)

SiO 2 65299 65161 •2.23 49.22 TiO2 - - 0.22 1.14 A120 • 18.80 18.53 0.54 4.46 FeO* 0.28 0.26 9.62 11.76 MnO - - 2.48 1.83

MgO - - 13.29 16.38 CaO 0.20 0.19 19.83 9.80

Na20 6.96 6.41 0.93 2.67 K20 6.65 7.34 - 0.78 Total 98.88 98.34 99.14 98.04

Samples (1) and (2) come from separate lava flows. Ed, edenitc, microprobe analyses [McCurry, 1985]. The two hydrous minerals following the classification of Hawthorne [1981].

14,850 MCOmRY: 1Vh• PERALKALnqE VOLCAmSM, MOIAVE DESERT. CALIFORNIA

TABLE 7. Whole Rock Analyses of Mafic Lava Flows

Sample 73514 7382 73723

SiO 2 57.0 46.0 47.9 TiO 2 1.17 2.44 1.25 A1203 16.7 15.1 17.5 FeO* 6.21 8.35 8.07

MnO 0.11 0.11 nd

MgO 4.37 9.87 5.53 CaO 7.70 12.42 10.25

Na20 3.5 2.5 3.2 K20 1.91 1.62 0.77 LOI 0.96 2.34 0.73 Total 99.63 100.75 95.20

Rb, ppm 45 18 5 Sr 460 1630 410 Y 31 45 24 Zn 62 93 57 Zr 290 470 170 Cr 56 240 31

SiO2* 7.8 46.7 50.7

Major elements determined by XRF on fused disks and trace elements on pressed disks; samples arranged in stratigraphic order, left-bottom, right- top.

Doming

Intrusion of a dense cluster of plugs domed and faulted the east central part of the caldera (Figure 1). Flows and tephra deposits are bowed upward from 6 ø to 30 ø near the cluster of plugs and are broken by outward radiating high-angle faults. The youngest volcanic rocks are faulted, but none of the faults appears to cut the plugs. Offsets along the faults vary from less than a few meters to at least 10 m and, based on vertical slickensides, are dip-slip. Some of the faults splay inward toward the plugs into prominent Y- shaped segments. The pattern of faulting and dips in the older rocks indicates that the central part of the caldera was domed upward from 100 to 200 m.

Angular unconformities of from 10øto 30 ø occur within sequences of fallout tephra deposits that are interbedded with lava flows near the cluster of plugs. The unconformities only occur in rocks near the top of the volcanic sequence and were not observed at all in other parts of the caldera. In addition, one lava flow appears to merge upward into the top of one of the plugs [McCurry, 1985]. They indicate that plug intrusion and doming overlapped in time with the latest stages of intracaldera volcanism.

Mafic Lava Flows

Several marie lava flows unconformably overlie the Tortoise Shell Mountain Rhyolite in the western Woods Mountains (Figures 1 and 2 and Table 7). The first, a sparsely porphyritic olivine bearing andesitc, yielded a K/At whole rock date of approximately 10 q- 0.6 Ma (D. Musselwhite, personal communication, 1985). This flow and subsequent moderately porphyritic alkali basalt and aphyric, olivine normative, subalkaline basalt flows were all extruded from well-exposed, generally north trending dikes located just outside as well as just within the western margin of the Woods Mountains caldera [McCurry, 1985]. Whole rock chemical analyses of these flows are listed in Table 7. A more detailed description is given by McCurry [1985]. The long hiatus between the Tortoise Shell Mountain Rhyolite and mafic flows probably rules out any close genetic connection.

DISCUSSION

The Woods Mountains Volcanic Center evolved through a buildup-climax-decay life cycle beginning at 16.4 Ma and lasting

about 0.6 m.y., during which time, approximately 100 km 3 of high-K trachyte to peralkaline high-silica rhyolite were extruded. The major phases of evolution are summarized in Figure 3. An evolutionary pattern is apparent in which there is a progressive increase in the degree of differentiation of the erupted products within the trachyte-trachydacite-rhyolite-peralkaline rhyolite association with time. The earliest volcanic rocks are metaluminous

trachyte, trachydacite, and rhyolite. Differentiation indices (abbreviated DI) vary from 73 to 94. These are followed sequentially by the extrusion of metaluminous rhyolite (lower and middle members of the Wild Horse Mesa Tuff; DI = 95), mildly peralkaline rhyolite (upper member of the Wild Horse Mesa Tuff; DI = 95), and finally mildly peralkaline, high-silica rhyolite (Tortoise Shell Mountain Rhyolite; DI = 97). Trachyte pumice lapilli at the top of the Wild Horse Mesa Tuff are similar to trachyte lava flows, ash flow tuffs, and plugs extruded early in the evolution of the center and are isotopically similar to the coexisting rhyolite pumices, suggesting that the metaluminous trachyte is parental to the peralkaline rhyolite.

The Woods Mountains Volcanic Center is one of the most

distinctive centers of Miocene magmatism in the Mojave Desert area. It is distinguished in two principal ways. First, whole rock chemical compositions are unusually alkaline, and the rocks are unusually depleted in MgO and CaO in comparison to rocks occurring in adjacent areas. It is apparently the only center in the region in which a large volume of peralkaline high-silica rhyolite magma was extruded. Second, it is distinguished by an unusually low intensity of middle to late Miocene faulting. This is despite its location about midway between two highly extended regions to the west and east. Is there a genetic connection between the two anomalous features of the center? One possible explanation for at least some of the features has been suggested by D. Miller (personal communication, 1984) in reference to the apparent paucity of calderas in the region. Intense detachment faulting could be expected to inhibit the formation of upper crustal magma chambers by highly fracturing the upper crust. If highly differentiated, peralkaline rhyolite magmas require significant residence times in the upper crust in which to form (e.g., by in situ fractional crystallization), then these types of magmas, as well as genetically related calderas and large-volume ash flow tuffs would be favored in areas of less extension. Mahood [1984] points out that peralkaline volcanic centers are conspicuously absent from areas of rapid mid-Tertiary extension in the western United States. The idea that the upper crust in the Woods Mountains area underwent less extension during the middle Miocene than some surrounding areas is consistent with regional Bouguer gravity data (K. Mickus, unpublished data, 1987) and seismic data [Fuis, 1980]. It is also consistent with the absence of exposed detachment faults in the area [cf. Spencer, 1985], the near-horizontal attitudes of the rocks, and the occurrence of sparse high-angle faults. The unique aspects of the center may therefore be understood in terms of the unique tectonic evolution of the area.

Although anique in the Mojave Desert, many features of the Woods Volcanic Center are similar to those of well studied mildly peralkaline, Cenozoic centers in the northern Basin and Range [e.g., Novak and Mahood, 1986; Noble and Christiansen, 1974], and Mexico [Mahood et al., 1985; Mahood, 1981]. These centers all evolved within a weak extensional tectonic environment and in a

common pattern in which the buildup of a compositionally heterogeneous volcanic field was followed by extrusion of dominantly peralkaline rhyolite ash flows and associated caldera collapse and finally by intracaldera volcanism that partially or completely filled in the original depression. Many underwent a

MCChJRRY: 1VI•tx:F.• P!•P. ALKALINI• VOLCANISM) MOJAV• D•E•T. CALIFORNIA 14,851

transition from metaluminous to peralkaline compositions during the two. One interpretation is that the metaluminous trachyte is the volumetrically dominant ash flow extrusion phase [Novak, parental to the peralkaline rhyolite. Novak and Mahood [1986], 1984; Noble et al., 1974; Mahood, 1981], as did the Wild Horse based primarily upon studies of the Kane Springs Wash caldera, Mesa Tuff. The evolution of the Woods Mountains Volcanic suggested that a combination of anorthoclase fracfionation of a

Center closely parallels the 14 Ma Kane Springs Wash volcanic trachyte parent magma, when combined with elevated fluorine and center of southcentral Nevada [Novak, 1984; Novak and Mahood, chlorine contents in the chamber, could produce the associated 1986] during the ash flow tuff exmasion and caldera formation peralkaline rhyolite. Is fractional crystallization ofhigh-K trachyte phase of activity. In both, the extrusions of metaluminous rhyolite magma capable of forming mildly peralkaline high-silica rhyolite ash flows were followed by exmasions of mildly peralkaline magmas at the Woods Mountains Volcanic Center? How are rhyolite ash flows. Both have similar whole rock, major element phenocrysts separated from residual liquid in viscous silicate compositions. However, the mildly peralkaline upper member of magmas? What constraints can be placed on the time over which the Wild Horse Mesa Tuff is distinguished from the corresponding the process takes place, and is the time span reasonable in the Kane Wash Tuff in that both trachyte and rhyolite pumices contain tectonic and thermal contexts of the original magma chambers? biotite, whereas the Kane Wash Tuff contains no hydrous phases. These questions are addressed in a first-order model of the

The occurrence of biotite in peralkaline tuffs is highly unusual magmatic evolution of the Woods Mountains Volcanic Center that (S. W. Novak, personal communication, 1987). However, biotite follows from theoretical and experimental work initiated by Shaw phenocrysts are a common minor phase in most of the Wild Horse [ 1974] and McBirney [ 1980]. Mesa Tuff, including the peralkaline upper member. They also occur within several peralkaline flows and one peralkaline plug of the Tortoise Shell Mountain Rhyolite [McCurry, 1985]. In the case of the upper member of the Wild Horse Mesa Tuff, biotite is rarely observed in standard thin sections but is observed in heavy mineral separates from all six stratigraphic intervals that were sampled

Magma Chamber Evolution

The previous existence of a large, shallow magma chamber is implied by the large original volume of the Wild Horse Mesa Tuff (equal to 55 km 3 of magma), a genetically related caldera, and near-

[McCurry, 1985, also unpublished data, 1987]. Most are from coincident, large, near-circular gravity and magnetic anomalies bulk samples of the tuff and may contain xenocrysts of biotite. However, biotite also occurs within heavy mineral separates from carefully cleaned rhyolite pumices from the top of the member; these are less likely to be xenocrysts. The grains generally occur as black (rare) and as brassy colored, euhedral, hexagonal plates. Microprobe analyses indicate that the more common brassy colored grains are partially weathered to montmorillonite [McCurry, 1985]. The probable occurrence of intratelluric phenocrysts of biotite in the Wild Horse Mesa Tuff suggests that the original magma contained

[McCurry, 1985]. If the erupted material represented less than 10% of the chamber volume, as is likely [Smith and Shaw, 1973, 1975], the original volume of the chamber was more than 550 km 3, corresponding to the volume of a sphere with a radius of 5 km. A granitic pluton of this size and shape would account for the geophysical anomalies and is consistent with the 10-km diameter of the Woods Mountains Cauldron. The chamber is referred to here

as the Woods Mountains Magma Chamber. As a first-order approximation, it is assumed that the Woods

an unusually high concentration of H2 ̧ relative to alkali rhyolites of Mountains Magma Chamber was drained from the top down during similar composition. the extrusion of each member of the Wild Horse Mesa Tuff and that

The evolution of the Woods Mountains Volcanic Center is the deposits in the western Woods Mountains are a continuous distinguished from many otherwise similar volcanic centers in the record of the eruption. A vertical profile of the chamber is then remarkable homogeneity of lavas extruded after caldera collapse and obtained by inverting the stratigraphy of the deposits. The by the continuation of caldera collapse into the following phase of intracaldera lava exu'usion. In addition, some of these other systems erupted trachyte lavas flows following the caldera collapse [e.g., Novak, 1984]. No such trend was observed at the Woods Mountains center. In addition, unlike some mildly peralkaline systems and many strongly peralkaline systems in which postcaldera volcanism occurs primarily from a central vent [e.g., Mahood, 1984; Novak, 1984], most intracaldera volcanic rocks were derived from widely scattered vents. The last phase of magmatic activity, characterized by the intrusion of numerous plugs, was concentrated at the center of the caldera. However, these rocks are also compositionally indistinguishable from the earlier intracaldera rocks.

One of the most important petrological features of some intermediate and silicic volcanic centers in North America and

Mexico is the occurrence of mixed magma pumices at the top of medium to large volume ash flow tuffs [e.g., Smith, 1979; Hildreth, 1981]. Bimodal peralkaline rhyolite and comagmatic metaluminous to peralkaline trachyte pumices have been documented within ash flow tuffs at several volcanic centers [e.g., Novak, 1984; Novale and Mahood, 1986; Mahood et al., 1985;

following interpretation is based specifically on features of the upper member of the Wild Horse Mesa Tuff.

Intensive variables are estimated on the basis of mineral equilibria constraints (Table 8). Temperatures and oxygen fugacity were calculated using the oxide recalculation procedure of Carmichael [ 1967] and Fe-Ti oxide solution model of Spencer and Lindsley [1981] and Anderson and Lindsley [1986]. They are probably accurate to about ñ30øC and 0.5 log units, respectively. Calculations of water concentration are based on sanidine + biotite

+ magnetite equilibria and follow procedures described by Hildreth [1977]. A total pressure of 200 MPa is assumed in these calculations. Although this pressure is consistent with indirect field data and with a geophysical model of the chamber based on gravity and magnetic data (G. Henzel, personal communication, 1982), it is not well constrained. Some preliminary calculations for the lower member suggest a much higher pressure [cf. Stormer and Whitney, 1985]. A higher pressure would lower the estimates for water concentration but would probably not change the interpretation sufficiently to alter fundamentally the conclusions of this study.

Calculations of magma density and viscosity follow Bottinga et Noble and Christiansen, 1974]. The common bimodal association al. [1982] and Shaw [1972], respectively (Table 8). Total Fe as of pumices (as well as interbanded trachyte-rhyolite pumices) and FeO is recalculated to FeO and Fe20 3 after Kilinc et al. [1983]. the stratigraphic location of trachyte pumices at the top of ash flow Corrections have been made to the trachyte because of the presence tuff sheets strongly suggest a common genetic connection between of 25% phenocrysts. The viscosity correction follows Roscoe

14,852 McCtmRY: MI• PERALKALINE VOLCANISM, Mo•nvz Dl•s•, CAI.U•lU•,

TABLE& Intensive Variables

l•O•ø½ ,, 933øC

Sample T, øC logfo a X•h o p(103 kg m '3) q(10• Pa s) p(103 kg m -3) •1(10• pa s) uM(I) 933 - 10.9 2.88 2.34 2.64" 2.33 5.30 UMCU) 799 -12.9 ..... UM(L) 805 -12.6 - 2.11' 1.78' 2.09* 0.12' UM(A) 807 -12.5 ..... MM(T) 945 -11.1 ..... MM(M) 782 -15.2 .....

Fe-Ti oxide temperatures and oxygen fugacity, and weight percent H20 (based on sa + bt + mt equilibria) calculated using data in Table 4 (see the text). UM(T), MM(T) - trachyte pumices from the tops of the upper and middle members respectively; UM(U), UM(L), UM(A) are rhyolite pumices from stratigraphic intervals R214, R161, A159 (Table 4) of the upper member, MM(M) is a pumice from the middle of the middle member, p, xl - density and viscosity of original trachyte and rhyolite magmas, calculated for the maximum probable range of temperature within the boundary layer (see Figure 11).

* Density and viscosity values for sample UM(L) assume a concentration of 6 wt % H20 in the liquid.

[1952]. These resulted in increases of 4% in density and a factor of 2 in viscosity over the phenocryst free liquid values.

An interpretation of the state of the Woods Mountains Magma Chamber just prior to eruption of the upper member of the Wild Horse Mesa Tuff is illustrated in Figure 11. A layer of compositionally nearly uniform but mineralogically zoned peralkaline rhyolite at 805øC, represented by the rhyolite component of the upper member of the Wild Horse Mesa Tuff, overlies a much larger volume of hotter, but more dense porphyritic trachyte, as represented by the trachyte pumice and trachyte component of mixed rhyolite-trachyte pumice at and near the top of the upper member of the Wild Horse Mesa Tuff. As a working hypothesis it is assumed that the rhyolite was formed by fractional crystallization of a trachyte magma containing 2.88% H20. A strong temperature gradient is maintained between the two by separated cellular convection. Roofward migration of H20 produced a density gradient that inhibited convection near the top of the rhyolite layer and also lowered the liquidus temperature of the rhyolite, inhibiting the formation of phenocrysts.

A first-order test of the model assumes fractional crystallization of phases present as phenocrysts in the trachyte to obtain the

rhyolite and uses the least squares mixing model of Bryan et al. [1969]. A statistically close fit is obtained for the major element data (Table 9). Application of the fractionation factors is also roughly consistent with most of the trace element data based on partition coefficients from Hanson [ 1978], given fractionation of about 0.2% zircon to account for variation in Zr. However, the crossover of REE trends implies an unusually high concentration of LREE in the crystal residue. Matrix/phenocryst partition coefficients have not yet been de•ed for the Wild Horse Mesa Tuff. Crystallization of a higher proportion of titanomagnetite, biotite, and possibly augite (a phenocryst in the Wild Horse Mesa Tuff trachyte pumices) could cause the crossover but reduces the quality of fit for the major elements. Fractionation of a small amount of petrierite might account for the crossover and would not significantly affect the major elements (S. W. Novak, personal communication, 1988). Therefore, with the possible exception of the LREE, the fractionation model is in concordance with all major and trace element data, as well as Nd and Sr isotopic data, given a small amount of assimilation of country rocks of the type that are exposed in the area and occurring as xenoliths in the volcanic rocks (D. J. DePaolo, personal communication, 1986). The dominance

(A) Phenocryst content 0 25%

(B) Stagnant zone

P - 2 Kbar

_ • •

/

Trachyt.

magma Fig. 11. Model of the Woods Mountains Magma Chamber as it existed just prior to the extrusion of the upper member of the Wild Horse Mesa Tuff. (a) An illustration of the cylindrically shaped magma chamber and corresponding vertical gradients in total phenocryst concentration (volume percent). Thickness of the rhyolite is purely diagrammatic, as are arrows that illustrate inferred regions of convective overturn. (b) Features of a rhyolite boundary layer formed by the crystallization of trachyte magma at the margin of the chamber. Assumed values of heat loss from the chamber and resultant mass flux (m) into the boundary layer are shown.

McCuaa¾: • la•aa•,utn[• Votc. nm•, Mosnv• Dinrex, C. at.nnmma 14,853

TABLE 9. Mixing Model

Trachv" Rhv $ft P1 'Bt Mag '']]m Rhy(m• SiO 2 67.70 74.90 65.05 61.09 38.74 0.12 0.02 74.80 TiO 2 0.54 0.25 - - 8.06 13.37 44.00 0.24 A1203 16.70 13.07 19.92 24.40 13.58 2.78 0.33 12.67 FeO* 2.05 1.41 0.30 - 12.38 78.88 49.66 1.41 MnO 0.09 0.10 - - 0.26 1.09 1.05 0.17 MgO 0.22 0.10 - - 16.81 3.61 4.86 0.16 CaO 0.85 0.11 1.08 5.87 0.03 0.14 0.08 0.14 Na20 5.80 4.97 6.21 7.76 0.93 - - 5.41 Kz, O 6.10 5.09 7.44 0.88 9.21 - - 5.16 Proportions of Phases (by weight) Trachyte 2.382 Percent Sa -1.233 89.4 PI -0.093 6.8 Bt -0.011 0.8

Mag -0.030 2.1 Itm -0.013 0.9

Residual = 0.26; fractionation factor = 0.58' Analyses are normalized to 100% on an anhydrous basis. Trachyte (Trachy) and rhyolite (Rhy) analyses are from Table 3, samples T205 and R201, respectively; mineral analyses are from Table 4 and correspond to phenocrysts occurring in a trachyte pumice from the top of the upper member of the Wild Horse Mesa Tuff. P1 is weakly nomaally zoned and has a composition of approximately An30 on the basis of the Carlsbad-Albite technique [cf. Deer et al., 1966]; a composition corresponding to An28Ab67Or5 is assumed in these calculations. Rhy(m) is the least squares fit of the mixing model; Residual is the sum of the squares of the differences between Rhy and Rhy(m); Fractionation factor is the mass fraction of trachyte fractionally crystallized, in the proportions of phenocrysts shown (negative numbers), in order to leave a residual having the composition of Rhy(m); percent is the relative weight proportions of phenocryts fractionated from the trachyte.

of sanidine in the model is consistent with its observed

predominance in both trachyte and rhyolite. Although the fractional crystallization model accounts for the

observed features of presumed parent and daughter magmas and is reasonable on petrological grounds, the mechanism is less clear. Crystal settling can probably be ruled out because of excessively low settling rates [cf. Marsh and Maxey, 1985]. One plausible mechanism was first introduced in the context of magma chamber formation by McBirney [1980] and Shaw [1974] and has more x'ecently been elaborated on by others [e.g., McBirney et al., 1985]. Buoyant chemical boundary layers form along magma chamber wails as a result of preferential fractional crystallization along '/he relatively cool surfaces (Figure 11). The inferred rhyolite chemical boundary layer within the Woods Mountains Magma Chamber would be about 10% less dense than the parental trachyte (Table 8), given fractionation in the proportions indicated in Table 9, at a pressure of 200 MPa. Assuming that this layer lacks strength, it will move buoyantly upward and collect incrementally at the top of the chamber. Mass and energy balance calculations suggest a maximum stratification rate of 0.05 m yr-1. This is within the range of stratification rates deduced for similar sized systems on the basis of different criteria [Spera and Crisp, 1981]. A 100-m-thick layer of peralkaline rhyolite magma, about equal to the volume of magma extruded to form the Wild Horse Mesa Tuff and intracaldera lavas

combined, could thus have formed in about 2000 years. This time span is well within the range for which the chamber would be expected to remain dominantly molten [cf. Spera, 1980]. Frequent eruptions would inhibit the peralkaline rhyolite magma formation process. Therefore, in a tectonically extended terrain such as the Mojave Desert in which the upper crust has been highly fractured, large volumes of peralkaline rhyolites either will not occur or will tend to occur within the regions of least upper crustal extension, as was apparently the case for the Woods Mountains Volcanic Center. In conclusion, fractional crystallization of a trachyte parental

magma produced a peralkaline rhyolite daughter magma at the Woods Mountains Volcanic Center [cf. Novak and Mahood, 1986]. This study supports the idea first implied by Shaw [1974] and later elaborated on by others [e.g., McBirney, 1980; McBirney et al., 1985] that boundary layer processes play an important role in the differentiation of some magma chambers.

Acknowledgments.. I thank W. R. Seager, C. R. Carrigan, G. A. Mahood, R. L. Christiansen, S. W. Novak, and R. I. Tilling for detailed and constructive reviews of the manuscript. I also thank A. L. Boettcher for providing financial and moral support during the course of this work. This study was supported in pan by NSF grant EAR-7816413 to A. L. Boettcher, and by GSA penrose grant 2494-79.

Anderson, D. J., and D. H. Lindsley, New (and final!) models for the Ti- magnetite/ilmenite geothermometer and oxygen barometer, Eos Trans. AGU, 66, 416, 1986.

Anderson, R. E., Thin skin distension in Tertiary rocks of southeastern Nevada, Geol. Soc. Am. Bull., 82, 43-58, 1971.

Anderson, R. E., Composite stratigraphic section of Tertiary volcanic rocks in the Eldorado Mountains, Nevada, U.S. Geol. Soc. Open File Rep., 77-483, 1977.

Atwater, T., Implications of plate tectonics for the Cenozoic tectonic evolution of western North America, Geol. Soc. Am. Bull., 81, 3513- 3535, 1970.

Beckemaan, G. M., J.P. Robinson and J. L. Anderson, The Teutonia batholith: A large intrusive complex of Jurassic and Cretaceous age in the eastern Mojave Desert, California, in Mesozoic-Cenozoic Tectonic Evolution of the Colorado River Region, California, Arizona, and Nevada, edited by E.G. Frost and D. L. Martin, pp. 205-220, Cordilleran Publishers, San Diego, Calif., 1982.

Bottinga, Y., D. F. Weill and P. Richer, Density calculations for silicate liquids, I, Revised method for aluminosilicate compositions, Geochim. Cosmochirn. Acta, 46, 909-919, 1982.

Bryan, W. B., L. W. Finger and F. Chayes, Estimating proportions in petrographic mixing equations by least-squares approximation, Science, 163, 926-927, 1969.

Burnham, C. W., Magmas and hydrothemaal fluids, in Geochemistry of Hydrothermal Ore Deposits, 2nd ed., edited by H. L. Barnes, pp. 71- 136, John Wiley, New York, 1979.

Carmichael, I. S. E., The iron-titanium oxides of salic volcanic rocks and their associated ferromagnesian silicates, Contrib. Mineral. Petrol., 14, 36-44, 1967.

Carrigan, C. R., and R. T. Cygan, Implications of magma chamber dynamics for sorer-related fracfionafion, J. Geophys. Res., 91, 11,451- 11,461, 1986.

Christiansen, R. L., Cooling units and composite sheets in relation to caldera structure, Spec. Pap. Geol. Soc. Am., 180, 29-42, 1979.

Christiansen, R. L., and P. W. Lipman, Cenozoic volcanism and plate- tectonic evolution of the western United States, part 2, Late Cenozoic, Proc. R. Soc. London, 271,249-284, 1972.

Clark, S., The relevance of hydraulic jumps to pyroclastic flows, Eos Trans. AGU, 65, 1142, 1984.

Cross, T. A., and R. H. Pilger, Constraints on absolute plate motion and plate interaction inferred from Cenozoic igneous activity in the western United States, Am. J. Sci., 278, 865-902, 1978.

Davis, G. A., Problems of intraplate extensional tectonics, western United States, in Continental Tectonics, pp. 84-95, National Research Council, Washington, D.C., 1980.

Deer, W. A., R. A. Howie and J. Zussman, An Introduction to the Rock- Forming Minerals, John Wiley, New York, 1966.

Dickinson, W. R., and W. S. Snyder, Geometry of triple junctions and subducted slabs related to San Andreas transform, J. Geophys. Res., 84, 561-572, 1976.

Dokka, R. K., Displacements on late Cenozoic strike-slip faults of the central Mojave Desert, California, Geology, 11, 305-308, 1983.

Dokka, R. K., Patterns and modes of early Miocene crustal extension, central Mojave Desert, California, Spec. Pap. Geol. Soc. Am., 208, 75- 95, 1986.

Eaton, G. P., Geophysical and geological characteristics of the cmst of the Basin and Range province, in Continental Tectonics, pp. 96-113, National Research Council, Washington, D.C., 1980.

Eaton, G. P., The Miocene Great Basin of western North America as an extending back-arc region, Tectonophysics, 102,275-295, 1984.

14,854 l•u•Y: • Pm•• Vo!•.•, McoAv• D•T, CAL•A

Ewan, A., A review of the mineralogy and chemistry of Tertiary-Recent dacitic, latitic, rhyolitic, and related salic volcanic rocks, in Tro•uthjemites, Dacites, and Related Rocks, pp. 13-122, edited by F. Barker, Elsevier, New York, 1979.

Fisher, R. V., and H.-U. Schmincke, Pyroclastic Rocks, Springer-Verlag, New York, 1984.

Fuis, G. S., Crustal structure of the Mojave Desert, U.S. Geol. Sur. Open File Rep., 81-503, 36-38, 1980.

Glazner, A. F., Cenozoic evolution of the Mojave Block and adjacent areas, Ph.D. thesis, 175 pp., Univ. of Calif., Los Angeles, 1981.

Glazner, A. F., Petrology of Miocene volcanic rocks from the Mojave Desert: Interaction of mafic magmas with old continental crest (abstract), Geol. Soc. Am. Abstr. Programs, 17, 356, 1985.

Glazner, A. F., and J. A. Supplee, Migration of Tertiary volcanism in the southwestern United States and subruction of the Mendocino fracture zone, Earth Planet. Sci. Lea., 60, 429-436, 1982.

Glazner, A. F., and R. D. Turner, rtSrff6Sr variations in Tertiary volcanic rocks from the Mojave Desert and their bearing on crustal composition and structure (abstract), Geol. Soc. Am. Abstr. Programs, 16, 520, 1984.

Glazner, A. F., J. E. Nielson, K. A. Howard and D. M. Miller, Correlation of the Peach Springs Tuff, a large-volume Miocene ignimbrite sheet in Califbrnia and Arizona, Geology, 14, 840-843, 1986.

Goldfarb, R. J., D. M. Miller, R. W. Simpson and D. B. Hoover, Mineral resources of the Providence Mountains Wilderness Study Area, San Bernardino County, California, U.S. Geol. Surv. Bull. 1712-D, 1988.

Hanson, G. N., The application of trace elements to the petrogenesis of igneous rocks of granitic composition, in Trace Elements in Igneous Petrology, edited by C. J. A11egre and S. R. Hart, pp. 26-43, Elsevier, New York, 1978.

Hawthorne, F. C., Crystal chemistry of amphiboles, in Amphiboles and Other Hydrous Pyriboles - Mineralogy, edited by D. R. Veblen, pp. 1- 102, Reviews in Mineralogy, 9A, Mineralogica1 Society of America, Washington, D.C., 1981.

Hewett, D. F., Geology and mineral resources of the Ivanpah quadrangle California and Nevada, U.S. Geol. Surv. Prof. Pap., 275, 172 pp., 1956.

Hildreth, W., The magma chamber of the Bishop Tuff: Gradients in temperature, pressure, and composition, Ph.D. thesis, 328 pp., Univ. of Calif., Berkeley, 1977.

Hildreth, W., Evidence for the origin of compositional zonation in silicic magma chambers, Spec. Pap. Geol. Soc. Am., 180, 43-75, 1979.

I, Basic principles and experimental simulations, J. Volca•iol. Geotherm. Res., 24, 1-24, 1985.

McCurry, M., The geology of a late Miocene silicic volcanic center in the Woods and Hackberry mountains area of the eastern Mojave Desert, San Bernardino County, California, in Mesozoic-Cenozoic Tectonic Evolution of the Colorado River Region, California, Arizona, and Nevada, edited by E.G. Frost and D. L. Martin, pp. 433-439, Cordilleran Publishers, Santa Diego, Calif., 1982.

McCurry, M., The petrology of the Woods Mountains volcanic center, San Bernardino County, California, Ph.D. thesis, 403 pp., Univ. of Calif., Los Angeles, 1985.

Michael, P. J., Chemical differentiation of the Bishop Tuff and other high- silica magmas through crystallization processes, Geology, 11, 31-34, 1983a.

Michael, P. J., Comment and reply on 'chemical differentiation of the Bishop Tuff and other high-silica magmas through crystallization processes': Reply, Geology, 11,622-623, 1983b.

Noble, D.C., Sodium, potassium and ferrous iron contents of some secondarily hydrated natural silicic glasses, Am. Mineral., 52,280-285, 1967.

Noble, D.C., and R. L. Christiansen, Black Mountain volcanic center, in Guidebook to the Geology of Four Tertiary Volcanic Centers in Central Nevada, Rep. Nev. Bur. Mines Geol., 19, 27-34, 1974.

Novak, S. W., Eruptive history of the rhyolitic Kane Springs Wash volcanic center, Nevada, J. Geophys. Res., 89, 8603-8615, 1984.

Novak, S. W., and G. A. Mahood, Rise and fall of a basalt-trachyte- rhyolite magma system at the Kane Springs Wash caldera, Nevada, Contrjb. Mineral. Petrol., 94,352-373, 1986.

Orton, J. K., Tertiary extensional tectonics and associated volcanism in west-central Arizona, in Mesozoic-Cenozoic Tectonic Evolution of the Colorado River Region, California, Arizona, and Nevada, edited by E. G. Frost and D. L. Martin, pp. 143-157, Cordilleran Publishers, San Diego, Calif. 1982.

Roscoe, R., The viscosity of suspensions of rigid spheres, Br. J. Appl. Phys., 3, 267-269, 1952.

Rowe, P. N., and A. W. Nienow, Particle mixing and segregation in gas fluidized beds. A review, Powder Technol., 15, 141-147, 1976.

Schmidt, C. S., and E. I. Smith, The McCullough Pass caldera: A mid- Miocene caldera in the central McCullough Mountains, Clark County, Nevada, Geol. Soc. Am. Abstr. Programs, 19, 447, 1987.

Shaw, H. R., Viscosities of magmatic silicate liquids: An empirical method of prediction, Am. J. Sci., 272, 870-893, 1972.

Hildreth, W., Gradients in silicic magma chambers: Implication for Shaw, H. R., Diffusion of H20 in granitic liquids, part I, Experimental lithospheric magmatism, J. Geophys. Res., 86, 10,153-10,192, 1981.

Hildreth, W., Comment and reply on 'chemical differentiation of the Bishop Tuff and other high-silica magmas through crystallization processes': Comment, Geology, 11,622-623, 1983.

Huppert, H. E., and R. S. J. Sparks, Double-diffusive convection due to crystallization in magmas, Annu. Rev. Earth Planet. Sci., 12, 11-37, 1984.

Kilinc, A., I. S. E. Carmichael, M. L. Rivers and R. O. Sack, The ferric- ferrous ratio of natural silicate liquids equilibrated in air, Contrib. Mineral. Petrol., 83, 136-140, 1983.

Kretz, R., Symbols for rock-forming minerals, Am. Mineral., 68, 277-279, 1983.

Lambert, J. R., R. K. Dokka and A. K. Baksi, Lead Mountain caldera complex: Earliest Tertiary magmatism in the central Mojave Desert,

data; part 11, Mass transfer in magma chambers, in Geochemical Transport and Kinetics, edited by A. W. Hofmann et al., pp. 139-170, Carnegie Institute of Washington, Washington, D.C., 1974.

Smith, E. I., Geology and geochemistry of the volcanic rocks in the River Mountains, Clark County, Nevada and comparisons with volcanic rocks in nearby areas, in Mesozoic-Cenozoic Tectonic Evolution of the Colorado River Region, California, Arizona, and Nevada, edited by E. G. Frost and D. L. Martin, pp. 41-54, Cordilleran Publishers, San Diego, Calif., 1982.

Smith, R. L., Ash flow magmatism, Spec. Pap. Geol. Soc. Am., 180, 5- 27, 1979.

Smith, R. L., and H. R. Shaw, Volcanic rocks as geologic guides to geothermal exploration and evolution, Eos Trans. AGU, 54, 1213, 1973.

Smith, R. L., and H. R. Shaw, Igneous-related geothermal systems, U.S. California (abstract), Geol. Soc. Am. Abstr. Programs, 19, 738, 1987. Geol. Surv. Circ., 726, 58-83, 1975.

Le Bas, M. J., R. W. Le Maitre, A. Streckeisen and B. Zanettin, A chemical Sparks, R. S. J., Gas release rates from pyroclastic flows: an assessment of classification of volcanic rocks based on the total alkali-silica diagram, J. the role of fluidization in their eraplacement, Bull. Volcanol., 41, 2-9, Petrol., 27, 745-750, 1986.

Lukk, M. E., The geology and geochemistry of the Tertiary volcanic rocks of the northeastern half of the Clipper Mountains, eastern Mojave Desert, M.S. thesis, 120-pp., Univ. of Calif., Riverside, 1982.

Mahood, G. A., A summary of the geology and petrology of the Sierra La Primavera, Jalisco, Mexico, J. Geophys. Res., 86, 10137-10152, 1981.

Mahood, G. A., Pyroclastic rocks and calderas associated with strongly peralkaline magmatism, J. Geophys. Res., 89, 8540-8552, 1984.

Mahood, G. A., C. M. Gilbert and I. S. E. Carmichael, Peralkaline and metaluminous mixed-liquid ignimbrites of the Buadalajara region, Mexico, J. Volcanol. Geotherm. Res., 25, 259-271, 1985.

1978.

Sparks, R. S. J., S. Self and G. P. L. Walker, Products of ignimbrite eruption, Geology, 1, 115-118, 1973.

Spencer, J. E., Miocene low-angle normal faulting and dike eraplacement, Homer Mountain and surrounding areas, southeastem Califomia and southernmost Nevada, Geol. Soc. Am. Bull., 96, 1140-1155, 1985.

Spencer, K. J., and D. H. Lindsley, A solution model for coexisting iron- titanium oxides, Am. Mineral., 66, 1189-1201, 1981.

Spera, F. J., Thermal evolution of plutons: a parameterized approach, Science, 207, 299-301, 1980.

Marsh, B. D., and M. R. Maxey, On the distribution and separation of Spera, F. J., and J. A. Crisp, Eruption volume, periodicity, and caldera crystals in coitvecting magma, J. Volcanol. Geotherm. Res., 24, 95-150, area: Relationships and inferences on development of compositional 1985. zonation in silicic magma chambers, J. Volcanol. Geotherm. Res., 11,

McBimey, A. R., Mixing and unmixing of magmas, J. Volcanol. 169-187, 1981. Geotherm. Res., 7, 357-371, 1980. Spera, F. J., D. A. Yuen and S. J. Kirschvink, Thermal boundary layer

McBimey, A. R., B. H. Baker and R. H. Nilson, Liquid fractionation, part convection in silicic magma chambers: Effects of temperature-dependent

Mc'Ct•Y: 1VI•ooiNE PKO, AiJ•J.J• VOLCA.•SM• MOJAVE DESERTø CALIFORNIA 14,855

rheology and implications for thermogravitational chemical fractionation, J. Geophys. Res., 87, 8755-8767, 1982.

Stormer, J. C., Jr., and J. A. Whimey, Two feldspar and iron-titanium oxide equilibria in silicic magmas and the depth of origin of large volume ash flow tuffs, Am. Mineral., 70, 52-64, 1985.

Wilson, C. J. N., The role of fiuidization in the emplacement of pyroelastic flows, 2, Experimental results and their interpretation, J. Volcanol. Geotherm. Res., 20, 55-84, 1984.

Wright, I. V., and G. P. L. Walker, The ignimbrite source problem: Significance of a co-ignimbrite lag-fall deposit, Geology, 5, 729-732, 1977.

Wust, S. L., Regional correlation of extension directions in Cordilleran metamorphic core complexes, Geology, 14, 828-830, 1986.

M. McCurry, Department of Earth Sciences, New Mexico State University, Box 3AB, Las Cruces, NM 88003.

(Received February 9, 1987; revised February 15, 1988; accepted April 21, 1988.)