Oligocene complex and calc-alkaline tuffs and lavas of the Indian Peak , Nevada and Utah

MYRON G. BEST US. Geological Survey and Department of Geology, Brigham Young University, Provo, Utah 84602 ERIC H. CHRISTIANSEN Department of Geology, Brigham Young University, Provo, Utah 84602 RICHARD H. BLANK, JR. US. Geological Survey, Denver, Colorado 80225

ABSTRACT eruptive sequence consists of several cooling subject of recent controversy (Whitney and units of trachydacite tuff containing small to Stormer, 1985; Johnson and Rutherford, 1989; The Indian Peak volcanic field is represen- modest amounts of plagioclase and two Grunder and Boden, 1987). tative of the more than 50,000 km3 of ash- pyroxenes. Compared to contemporaneous volcanic flow tuff and tens of in the Great These dominantly high-K calc-alkaline fields around the Colorado Plateaus to the east Basin that formed during the Oligocene-early rocks are a record of the biih, maturation, (Fig. l), the -10,000 km3 of ash-flow deposits Miocene "ignimbrite flareup" in southwest- and death of a large, open, continental in the Indian Peak volcanic field is an order of ern North America. The field formed about magma system that was probably initiated magnitude greater than in the Marysvale field 32 to 27 Ma in the southeastern and sustained by influx of mafic magma de- (Steven and others, 1984) but similar to that of and consists of the centrally positioned Indi rived from a southward-migrating locus of the San Juan (Steven and Lipman, 1976) and Peak caldera complex and a surrounding magma production in the mantle. The small Mogollon-Datil fields (RattC and others, 1984). blanket of related ash-flow sheets distributed volumes of chemically diverse andesitic rocks In these three fields, however, individual ash- over an area of about 55,000 km2. The field were derived from separately evolving flow eruptions were on the average smaller, and has a volume on the order of 10,000 km3. A magma bodies but are modified representa- more calderas formed than in the Indian Peak. cluster of two obscure source areas and four tives of the mantle power supply. Recurrent The Indian Peak field also differs from the other calderas comprise the -80 x 120 km caldera production of very large batches (some three in its overwhelming dominance of silicic complex. Only minor volumes of and greater than 3,000 km3) of quite uniform da- ash-flows over intermediate lava flows, a charac- two pyroxene andesite lavas were extruded cite magmas appears to have required combi- teristic aspect of Oligocene-early Miocene rocks episodically throughout the lifetime of the nation of andesite magma and crustal silicic of the Great Basin (Best and others, 1989a). magma system that formed the field, chiefly material in vigorously convecting chambers. This paper summarizes the eruptive history of during its youth and old age. Compositional data indicate that are the Indian Peak volcanic field and the basic Six ash-flow sequences alternate between polygenetic. As the main locus of mantle compositional characteristics of the rocks and rhyolite and dacite in a volume ratio of about magma production shied southward, tra- concludes with a provisional history of the 1:8, and a culminating seventh is trachytic. chydacite magma could have been produced magma system that provides a basis for continu- The first, fourth, and sixth tuff units are of by fractionation of andesitic magma withiin ing more detailed analytical studies of its ther- rhyolite that contains sparse to modest the crust. mochemical evolution. amounts of phenocrysts, chiefly plagioclase and biotite, and abundant lithic and pumice INTRODUCTION REGIONAL GEOLOGIC SETTING lapilli; these deposits are confined within the caldera complex and form multiple and com- We document a large-volume, cyclic eruptive Tertiary volcanic rocks are virtually the only pound cooling units that are normally zoned sequence of rhyolite, dacite, and trachydacite magmatic rocks of Phanerozoic age in the with respect to bulk chemical composition ash-flow tuffs and coeval andesite and rhyolite southeastern Great Basin. They were deposited and crystal type, content, and size. The sec- lava comprising the Indian Peak volcanic field on an erosional surface of modest relief carved ond, third, and fifth tuff sequences are of astride the southern Utah-Nevada state line. into a thick, upper Proterozoic through lower crystal-rich dacite that forms extensive simple Tuffs were derived from a centrally positioned Mesozoic miogeoclinal sedimentary sequence. cooling-unit outflow sheets and partial cal- magma locus marked by the Indian Peak cal- During the Cretaceous Sevier orogeny, this se- dera fillings of compound cooling units. Each dera complex, a cluster of four known calderas quence was folded and thrust-faulted and subse- dacite unit contains similar amounts of plagi- and two inferred source areas. The dacite tuffs quently locally overlain by lower Tertiary oclase, biotite, hornblende, quartz, two py- are representative of the Monotony composi- sedimentary deposits (Stewart, 1980). A grow- roxenes, and Fe-Ti oxides; trace amounts of tional type of tuff which dominates the late ing body of data documents episodes of locally sanidiine and titanite also occur in the young- Oligocene of the Great Basin (Best and others, extreme crustal extension (as much as 300%) in est. Cognate inclusions in the dacites show 1989a; compare the "monotonous interme- the middle Cenozoic era (for example, Wust, only slight intra- and inter-unit differences diates" of Hildreth, 1981). The origin of similar 1986) before, during, and after volcanism and in bulk chemical composition. The seventh crystal-rich dacite tuff in the San Juan field is the generally before the classic high-angle Basin and

Geological Society of America Bulletin, v. 101, p. 1076- 1090, 8 figs., 3 tables, August 1989. INDIAN PEAK VOLCANIC FIELD, NEVADA AND UTAH

\ I ANDESlTlC LAVA PLATEAUS \ a AND DEBRIS FLOWS \ 1 ,FELSIC INTRUSIVE ROCKS

L CALDERA MILES 400 I 0 KILOMETERS

Figure 1. Generalized distribution of Oligocene and lower Miocene (mostly 34 to 17 m.y. old) magmatic rocks and known calderas in the Great Basin of Nevada and western Utah and in the .... Marysvale, San Juan, and Mogollon-Datil volcanic fields. The Mogollon-Datil field includes considerable intermediate composition lava flows which are not separately distinguished. Data from Lipman (1984), Ratti! and others (1984), Sargent and Roggensack (1984), Steven and others (1984), and Stewart and Carlson (1976). Dotted line is approximate margin of Colorado Plateaus. ---

Range normal faulting and extension in the late broad zone of voluminous late Oligocene-early graphic expression in the present terrain which is Cenozoic era. Many recent interpretations of the Miocene volcanic rocks extends across Nevada dominated by northerly trending basins and total amount of this more or less east-west ex- and Utah into southwestern Colorado (Fig. 1). ranges produced by subsequent block faulting; tension suggest that it varied greatly from place Within this zone, chiefly calc-alkaline, highly nearly half of the caldera complex now lies bur- to place through time (for example, Coney and potassic intermediate to silicic volcanism was ied beneath alluvium-filled basins. Most impor- Harms, 1984; Wells and Heller, 1988; Taylor, in gradually supplanted by bimodal -rhyolite tantly, gravity gradients mark the southwestern press), precluding a reliable single-value estimate volcanism in the late Cenozoic era. Within the and southern margin of the complex which is of the stretching of Great Basin tuff sheets. We eastern Indian Peak volcanic field, early, middle, apparently filled with thick, low-density tuff or use a figure of 50% east-west crustal extension and late Miocene episodes of generally high- underlain by low-density intrusive rocks. With- since deposition in restoring the distribution of silica rhyolitic activity were accompanied by ex- out the gravity data, the southern margin can be the different sheets and in calculating their origi- trusion of mafic lavas, but not until the last only indirectly located, even in the mountain nal volumes. episode had their content of K20 and Si02 de- ranges, because of an extensive cover of younger The Cenozoic volcanic history of the central creased to the level of true basalt (Best and oth- volcanic rocks. western United States is dominated by a broad ers, 1980, 1987~). Aeromagnetic data (Zietz and others, 1976, southward sweep of essentially calc-alkaline ig- 1978) also disclose the general location of the neous activity (Cross and Pilger, 1978) while GEOPHYSICAL EXPRESSION OF Indian Peak caldera complex. oceanic lithosphere was subducting beneath the THE CALDERA COMPLEX Geologic mapping corroborates the com- western margin of North America. Between pound nature of the geophysically expressed about 30 and 25 Ma, this transgressive activity Although bearing the overprint of basin and caldera complex. Topographic margins of four decelerated or even stagnated in the southern range faulting, gravity data delineate the -80 x nested calderas are indicated by striking discon- Great Basin (Best and others, 1989a) where the 120 km Indian Peak caldera complex (Fig. 2). tinuities in thickness of tuff deposits and by Indian Peak volcanic field developed. This None of the Oligocene calderas has any topo- coarse breccia of older rocks shed off caldera 1078 BEST AND OTHERS walls. Short segments of caldera-bounding faults generally high K20 concentrations that qualify 35-34-m.y.-old rhyolitic tuffs and lavas were are locally exposed or indirectly expressed by them as a high-K calc-alkaline suite (Ewart, emplaced in and near the volcanic complex small dacite lava domes. The sources of two 1979). prior to its development (Best and others, additional ash-flow sheets are approximately Voluminous eruption of lavas did not precede 1987%; Loucks and others, 1989). located by the distribution of the sheets and by nor accompany ash-flow eruptions from the In- Volumes of ash-flow tuff deposits in Figure 3 clast size. dian Peak complex, in contrast to other nearly were calculated using restored areas based on contemporaneous volcanic fields in the south- 50% east-west crustal extension and the method ERUPTIVE HISTORY western United States (Marysvale, Utah- of Ekren and others (1984). The lack of a tight Steven and others, 1984; Mogollon-Datil, New control on the amount of extension is a possible The Indian Peak volcanic field was domi- Mexico-RattC and others, 1984; and San Juan, major source of error in the listed areas and nated by eruption of voluminous ash flows Colorado-Lipman, 1975; Fig. I). Had large volumes. For example, the estimated volume of (Figs. 3,4, and 5); lavas are much less extensive, piles of lava existed, it is unlikely that all of the the outflow member of the Wah Wah Springs and deposits of pyroclastic surge and fall are edifices as well as peripheral alluvial deposits Formation is about 1,500 km3 if an east-west rare. Rock analyses (Tables 1,2, and 3) indicate would have been completely engulfed. Exten- extension of loo%, which we consider excessive, that the volcanic field is typical of the Great sive lavas do not occur in resurgent parts of the is used. Several additional factors also contribute Basin as a whole (Best and others, 1989a); the calderas, which locally expose Paleozoic rock on to the uncertainty of areas and volumes, as fol- rocks have relatively low Fe/Mg ratios and the caldera floor. Only small volumes of lows. (1) Edge of outflow sheet may have been eroded before deposition of younger sheet. (2) Areas of older calderas engulfed by younger are uncertain. (3) The thickness of intracaldera tuff now obscured by resurgence, basin-and- range faulting, and younger deposits in known calderas and concealed sources is uncertain. Vol- ume estimates of intracaldera dacite tuff in Figure 3 are probably conservative; volumes of Cottonwood Wash and Isom tuffs do not in- clude possible thick accumulations within their sources. (4) The existence of predeposition to- pography created variably thick deposits, espe- cially of older tuff sheets. (5) A final factor is the porosity of minor weakly and rarely nonwelded parts of some outflow sheets that has not been compensated to a dense-rock equivalent.

Escalante Desert Formation and the Pie Valley Caldera

Initial eruptions about 32 Ma from the Indian Peak caldera complex produced several hundred cubic kilometers of a bimodal assemblage of rhyolite tuff and minor rhyolite and two- pyroxene andesite lava of the Escalante Desert Formation (Fig. 3; Best and Grant, 1987). The crystal-poor Marsden Tuff Member is almost everywhere propylitically altered and is generally characterized by conspicuous xenoliths 0 20 miles 40 60 I of sedimentary rock and rhyolite; variations in - -4 0- 20 km 40 60 their size and concentration indirectly indicate the location of an obscure source (the Pine Val- Figure 2. Bouguer gravity in and around the Indian Peak caldera complex. Southwestern ley caldera) largely engulfed in younger calderas comer of the map area and area north of about 38" show horst-graben structure of the Basin (Best and Grant, 1987). Phenocrysts consist of and Range province (compare with topographic features in Fig. 4). Thick accumulation of plagioclase and minor quartz, sanidine, biotite, relatively low density tuff filing two and locally three superposed calderas produces gravity Fe-Ti oxides, apatite, and zircon (Fig. 6); these lows along the major west-northwest axis of the elliptical caldera complex. Identified calderas increase in abundance upward. based on unpublished and published geologic mapping of M. G. Best and associates (see The younger Lamerdorf Tuff Member con- References Cited) are, from oldest to youngest: Pme Valley, PV; Indian Peak, IP; White Rock, trasts with the Marsden in its greater areal extent WR; Mount Wilson, MW. Areas from which the Cottonwood Wash (CW) and Isom (IS) tuffs (Best and Grant, 1987), generally thinner, more were probably derived are denoted by diagonal ruling (compare Figs. 5A and 5D). Assumed densely welded cooling units that have local crustal density is 2.67 g/cm3; contour interval is 10 mgal, with progressively more negative basal vitrophyres, more phenocrysts of larger areas below -200 mgal more strongly stippled. size, and conspicuous xenoliths of rhyolite. The INDIAN PEAK VOLCANIC FIELD, NEVADA AND UTAH

Figure 3. Stratigraphic rela- tions and ages of rocks in the Indian Peak volcanic field (Best and Grant, 1987, average K-Ar age of 26 m.y. of the Isom For- mation cited in this reference appears to be at least 1 m.y. too young based upon 40~r/39~r determinations of Alan Deino, 1988, written commun.). AU units except Isom belong to Needles Range Group. Vertical scale is not linear and shows only stratigraphic position. Areas in km2 and volumes in km3. Lower of two values for tuff member of Lund Formation is volume outside White Rock caldera; upper value is for tuff inside caldera based on a con- servative estimate for thickness of 1 km; locally it may be as much as 3 km.

Lamerdorf contains minor amounts of clinopy- cooling unit of crystal-rich dacite ash-flow tuff. cooling unit of dacite tuff similar to the outflow roxene, hornblende, Fe-Ti oxides, apatite, and Thickest sections (as much as 300 m, Fig. 5A) tuff member but containing conspicuous lithic zircon, in addition to more abundant plagioclase are commonly marked by gray, near-basal vit- fragments and variable but commonly slightly and biotite. rophyres. The distribution and thickness of the more abundant phenocrysts of quartz (Best and Rhyolite and andesite lava flows are more tuff, its vitrophyre, and local lithic clasts suggest Grant, 1987; Best and others, 1987a). Together, widespread in the Escalante Desert Formation a source between the northern Needle and Forti- the volume of the intra- and extra-caldera tuff is than in the younger formations within the cal- fication Ranges and south of the Snake Range. on the order of 3,000 km3. dera complex and occur chiefly in and near its These ranges themselves disclose no direct evi- Less quartz, more hornblende, and smaller north and east sectors. dence of a nearby fault-bounded depression. phenocryst size (especially biotite) distinguish Andesite lavas are generally nonvesicular, Nonetheless, gravity data (Fig. 2) permit a bur- the Wah Wah Springs from the otherwise sim- two-pyroxene plagioclase rocks, but phenocrysts ied source for the - 1,500 km3 of tuff. ilar Cottonwood Wash Tuff (Fig. 6). There are of biotite and hornblende also occur in lavas in The Cottonwood Wash Tuff has a phenocryst only minor differences in the chemical composi- the southern Fortification Range. Andesite lavas assemblage dominated by plagioclase and un- tions of bulk samples of these two high-K calc- include both high (greater than about 5%) and commonly large books of biotite (up to 8 mm alkaline dacites (Table 3; Figs. 7 and 8). low MgO varieties (Figs. 7 and 8); the magne- across); embayed bipyramidal quartz; horn- Little andesite lava was erupted during the sian andesites also have higher K20 and lower blende (small grains inconspicuous in hand middle history of the volcanic field when volum- Al2O3 concentrations. sample), two pyroxenes, Fe-Ti oxides, apatite, inous eruptions of dacite occurred. Only small Chemical analyses of the rhyolites of the Es- and zircon make up the rest (Fig. 6). Like other outcrops of andesite of possible Wah Wah calante Desert Formation show that the Lamer- dacites in the Indian Peak field, the Cottonwood Springs age have been found within the caldera dorf tuff and most of the lavas are low-Si02 Wash has relatively low Zr concentrations for its complex (Table 1). Sixty kilometers north of the rhyolite and even dacite (67% to 72% SiOz; Si02 content (Fig. 8). caldera complex, an olivine-bearing, high-MgO Table 2 and Fig. 7). The Lamerdorf tuff displays basaltic andesite lies between the Cottonwood slight vertical chemical zonation with Si, K, and Wah Wah Springs Formation and Wash and Wah Wah Springs tuffs. This lava Rb depleted and Al, Fe, Ca, Sr, Zr, and Ba Indian Peak Caldera shows compositional similarity to those in the enriched in the upper part of the tuff. Most sam- Escalante Desert Formation (Table I). ples of the rhyolitic lavas and Lamerdorf tuff are Emplaced at about 29.5 Ma, the tuff of the enriched in K, Ba, Ti, and Zr and depleted in Ca Wah Wah Springs Formation comprises a Ryan Spring Formation compared to younger rhyolites (Fig. 8). widespread simple cooling unit of crystal-rich dacite that ranges up to 520 m thick outside of The Ryan Spring Formation consists mostly Cottonwood Wash Tuff its resurgent Indian Peak caldera source (Best of several hundred cubic kilometers of variably and Grant, 1987; Figs. 2, 4, and 5B). The intra- welded rhyolite tuff that is virtually identical in The next eruptive event at 30.6 Ma formed caldera member of the formation consists of hand sample to the rhyolite tuffs of the Escalante the Cottonwood Wash Tuff, generally a simple breccia, intrusive rocks, and a thick compound Desert Formation. The basal or Greens Canyon 1080 BEST AND OTHERS

Tuff Member of the Ryan Spring Formation Tuff of the Ryan Spring Formation is almost Mackleprang (Table 2) and, like many normally commonly contains clasts of Wah Wah Springs wholly confined within the Indian Peak caldera zoned ash-flow deposits, it contains fewer and tuff instead of sedimentary xenoliths as is char- (Best and Grant, 1987; their Figs. 4 and 5) smaller phenocrysts (Fig. 6). The Mackleprang acteristic of the otherwise similar but older which probably also wntains its source. In the Tuff lacks the sparse quartz and sanidine pheno- Marsden tuff. In the moat of the resurgent In- Atlanta, Nevada, open-pit gold mine, thick, al- crysts found in the lower unit. Compared to the dian Peak caldera in the northern White Rock tered tuff is banked against silicified Paleozoic similarly zoned Lamerdorf Tuff and the rhyolite Mountains (Best and others, 1989b), the Greens carbonate rocks (Willis and others, 1987). Lo- lavas of the Escalante Desert Formation, the Canyon Tuff is a wmpound cooling unit as cally derived slabs of brecciated Paleozoic rocks Ryan Spring tuffs have slightly lower concentra- much as 1.4 km thick that consists of five wol- appear to lie on the tuff, suggesting recurrent tions of Ti, Fe, Mg, Y, Zr, and Ba (Fig. 8). Like ing units with local vitrophyres near their bases. collapse of the Indian Peak caldera as tuff of the the Lamerdorf Tuff, however, the Ryan Spring Directly overlying the Greens Canyon is the Ryan Spring Formation was erupted. tuffs are low-SOz, high-K, calc-alkaline rhyo- upper, or Mackleprang Tuff Member of the The similarities in age, distribution, and min- lites (Fig. 7). Ryan Spring Formation that is more strongly eralogy suggest that the Greens Canyon and Minor volumes of andesite that occur beneath porphyritic and in most places cannot be distin- Mackleprang tuffs are, respectively, the lower the Mackleprang tuff in the southeastern seg- guished in outcrop from the Lamerdorf Tuff and upper parts of a single eruption sequence ment of the Indian Peak caldera have been as- Member of the Escalante Desert Formation; in that is wmpositionally normally zoned. The signed to the Ryan Springs Formation. These the field, these "twins" can be distinguished only Greens Canyon is slightly more evolved (lower two-pyroxene plagioclase andesites (Table 1) by reference to stratigraphic position. Ti, Al, Fe, Mg, and Zr and higher Si) than the locally contain hornblende.

Figure 4. Present maximum extent of major tuff sheets of the Indian Peak volcanic field around the caldera complex (heavy line, dashed where less certain around obscure source areas). Topographic contours at 5,000,7,500, and 10,000 fi. Compare Figures 2 and 5. Formation Escalante Deren Wah Wah Springs Ryan Spring Lund lsom Estimated -- accuracy Sample 1 2 3 4 5 6 7 8 9 10 I1 12 (i2 sigma)

Field LAM-9 MIL-l TET-9 TET-9 BRNJ BUCK-8 MIN-8 STM-8 STM-8 STM-8 STM-8 GLE-6 number -71-2 - 13-2 -7748 -140-2 -23-1 -169-3 -1694 -128-1 -130-1 -961

Si02 Ti02 A'2°3 Fe203 MnO MgO CaO Na20 K20 '2'5 TOTAL

I. Glassy matrix; lat. 38'18'48*, long. I 13'30'00-. 2. Holwslalline: uwer flow in andsite vile immediatelv belnw Cottonwood Wash TuE lat. 38'17'58-. lone. 113"13'3'. 3. ~li~htl;alteral,nbw ahnr Waodcn T;R Member, la; 38°1).16.. long 1 13°34'16- 4. Hol~rrall~nc,flow ah,ur Warden Tuff Mrmbcr: la1 38°10'58'. long 1 13°30'31'. 5. Basaltif andesile lava flow comprised of phenccrysts of olivine in a mat& ofplagiocl~,awte, glass, and magnetite. betwecn oufflow tuff member of Wah Wah Springs Formation and Cottonwood Wash Tuff;lar 38°50'20". long. 113'28.25'. 6. Altered matrix. fr& phencctysu; lat. 38'16'20". long. 113'51'42". 7. Holocmralline flow wntainine.. oaniallv . chloritmd .ovroxene. .ohenccmts . on townraohic. - . rim of Indian Peak caldera: lar. 38O22'4". lone. 113"56'6". S Glassv matnk. abmdant hornhlrnde and mmor blot~tcphenwr).cn nllh plagicclw and two pyrorcmes, lal. 38'2'41-, long. 1 13O48'52'. 9. Holccrystallinc,sltghtl, . . altered hyponthenr @hmccr)~s:immd~ately ahove luff;a hsx of anhue pdc. IJI 3812'?4', long. 113'49'19- 10. Glassy matrix, at top of andesite pile; lat. 3Rb1'58", long. 113~50'00'. I I. Strong porphyritic, glmy matrix; lat. 38'239-, long. 1 13°50'10-. 12. Holoc~ralline,sparse hornblende phenccrysu; below tulis; lat. 38"lCSO'. long. 114"6'2-. Oxide values have been recalculated to 100%on a wlatile-free basis. Analytical total for 10 oxides in parentheses.

TABLE 2. CHEMICAL COMPOSITION OF RHYOLITE LAVA FLOWS AND TUFFS

Stratigraphic Escalante Desert Formation Ryan Spring Ripgut unit Formation Formation lsom Formation Rhyoltte lava flow member Lamerdof Tuff Member Lower Upper Lower Upper Greens Mackleprang Canyon Lower Upper Lower Upper Sample I 2 3 4 5 6 7 8 9 10 I1 12 13 14 15 16

Field MIN-8 MSP-9 TET-9 TET-9 HFW- HFW- MLLR-6 MIN-8 ATL-l MTWlLS STM-8 MLLR-6 MLLR-6 MLLR-9 number 40-10 -37-1 43-3 -75-1 IA 2A I -61-2 -68-1 50 -169-5 -17-1 -17-2 -164-3

Si02 T,02 A1203 Fe203 MnO ME0 cao Na20 K20 '2'5 TOTAL

I. Felsitis lat. 38°17'42", long. 1 1395'00'. 2. Fclsitiq lat. 38'3'18". long. 113'31'40". 3. Felsitif; lat. 38°13'12", long. 1 13°34'17". 4. FeUtif; lat. 38°13'18", long. 11391'49". 5. Average of 3 analyses from Campbell (1978). 6. Basal vitmphyre; lat. 3890'32", long. 113'50'57". 7. Average of 4 analyses from Campbell (1978). 8. Basal vitmphyre: lat. 38'30'32.. long. 113'50'57". 9. Basal vitmphyre; la1 38'17'43.. long. 1 14O9'45". 10. Devitrilied base of unit; lat. 38'20'5". long. 113"55'30'. I I. Basal vitrophyre; lat. 38'257'. long. 114'2'2". 12. Felisitc; 3X014'6", long. 114'22'40". 13. Felsitic; tat. 38'1'43". long. 113"48'57". 14. Felsitic; poikilitic biotite in matrix: oldest of 3 cooling units; lat. 38'16'24". long. 114'1 1'25' 15. Felritic; youngest of 3 cool~ngunits; lat 38°16'19', long. 114°1 1'14'. 16. Basal viaophyre; lal. 38'15'18", long. 114°12'55". BEST AND OTHERS

Figure 5. Present extent and thickness (100-m contours) of major tuff sheets around source calderas. Zero (0) indi- cates unit is absent between older and younger units. Di- agonally ruled areas in 5A and 5D are probable source areas. Dotted lie is outer perimeter of Indian Peak and White Rock calderas (compare with Fig. 2). Heavy line is topographic margin of caldera related to particular tuff. In 5B, inner ring fault of the Indian Peak caldera is orna- mented by ball and bar. In 5C, stars denote flow-layered dacite bodies of Lund mineralogy. Thickness of Isom tuff south of its source partly from unpublished mapping of R. E. Anderson.

0 KILOMETERS 60

- I /? -- I / ?30\, B. WAH WAH SPRINGS \

112O + 39O

KILOMETERS 60

Lund Formation and the White Rock Caldera Lund tuff appears to be similar to that of the 1989b) where massive Lund and younger Rip Wah Wah Springs Formation (Fig. 3). gut and Isom tuffs that fill the caldera thicken Eruptions at 27.9 Ma produced another The perimeter of the White Rock caldera is southward from a few tens of meters to as much voluminous crystal-rich dacite tuff that consti- indirectly marked by gravity gradients on the as 2 km. In the northern Wilson Creek Range tutes most of the Lund Formation and created south and west (Fig. 2) and by geologic relations (Willis and others, 1987), the topographic mar- the resurgent White Rock caldera that engulfed elsewhere (Best and Grant, 1987). The topo- gin is marked by the southward thickening tuff the southwestern part of the older Indian Peak graphic margin is well exposed in the northern member of the Ripgut Formation. Beneath the caldera (Figs. 2 and 5C). The volume of the White Rock Mountains (Best and others, Ripgut, breccia of Wah Wah Springs and Ryan INDIAN PEAK VOLCANIC FIELD, NEVADA AND UTAH 1083

1+ so 115' LUND +

.O

I 0 MILES 50 .o .o 0 KILOMETERS 60 .o .o

'0 115' 113O 112 +

Figure 5. (Continued). I I '5 -0

.o '0 / 100 \ ?8 745 5 .150+ .420

?18

a 0 MILES 5 0 100 0 KILOMETERS 60

.o + + + 37O

Spring tuffs, Paleozoic carbonate rocks, and, lo- tuff and breccia in the Fairview Range (20 km placed along the caldera-bounding fault marking cally, granitic rock occur as wedges intercalated due west of Mount Wilson; Fig. 4). the structural margin of the caldera. within intracaldera Lund tuff. If unfaulted, the Five small masses of porphyritic and locally Southward from Atlanta, Nevada, the young- intracaldera Lund tuff and breccia could be as flow-layered dacite similar to the Lund tuff er Ripgut tuff thickens and then thins and much as 3 km thick in the northern Wilson occur inside the northern topographic margin of pinches out north of Mount Wilson (Willis and Creek Range. Reconnaissance work has re- the caldera (Fig. 5C). These are interpreted to be others, 1987). This lens-shaped mass of tuff de- vealed a similar intracaldera sequence of Lund shallow intrusions and lava-flow remnants em- fines a moat between the topographic wall of the BEST AND OTHERS

TABLE 3. CHEMICAL COMPOS~ONOF DAClTE TUFFS

Stratigraphic Cottonwood Wash Tuff Wah Wah Springs Formation Lund unit Formation OuUlow tuff member lntracaldera member

Sample I 2 3 4 5 6 7 8 9 LO LI 12 13 14

Field HFW-8 HFW-8- ERN-2 ERN-2 HFW-8 HFW-8- BRN-I ERN-I HAM-9- MLLR-6- MLLR6 GLE-6- MINS-8- MLLR-9- number* 153-IBV 153-ICD M P 153-3BV 153-3CD M P 129-IP 63-IP 64-2X 98-IX 61-4BD 164-1X

Si02 Ti02 A1203 Fez03 MnO MgO CaO Na20 K20 '2'5 TOTAL

*V, vitrophyre at base of ash-flow sheet; D, devivified densely welded just above basal vitmphyre; P, pumice lapilli or block in tuff;X nonvesicular vitrophyre block in tuff; M, tuff matrix enclosing pumice lumps. I. Basal vitrophyre; lat. 3S033'25-, long. 113°51'20" 2. Devitrified densely welded tuff immediately above basal vitmphyn;. . laL 3E033'25', long. 113°5L'2W 3. Matnx of IuRaround pumace blocks near baw of weakly welded unit la!. 38°4YlW, long. 113°28'30' 4. Pum~ceblwk from ueaklr welded base of una la!. 38'49'10'. . lone.- 11398'W 5. Basal vitmphyre; laL 38"34'34', long. 1 13°51'26- 6. ~evitrifieddknsel~welded tuff immediately above basal vitmphyre; lat. 3S034.34", long. 113°51'20" 7. Matrix of tuff around ~umicelaoilli 4-6 m above base of weallv welded unit lat. 3S048'W.. lone.- L13"27'30" 8. Pumice lapilli 4-6 m above bsse of weakly welded unit lat. 38'48'~, long. I1397'W 9. Pumice block from weakly welded top of unit lat. 38°24'12', long. I l4W35- 10. Pumice block from weakly welded top of uniS lat. 38°18'51", long. 114"8'48' I I. Nonvesicular vitrophyre block within a few meten of top of unit lat. 38"18'37', long. 114097" 12 Nonva~cularvtlrophyrc block wttbtn a feu meten of top of un4 lat 38°17'57'. long 114°6'45' 13 Dcvtlnfied dcnul) welded tufftmmed~atelv above basal vllru~hvre.. . kL 38'20'5'. lone 1 13°55'W 14. Nonvericular viwophyre block in tuff in probable vent mass; lal. 38'15'16', long. 114'12'24"

White Rock caldera and its resurgent wre to the east of the Nevada-Utah state line in NW% sec. East of Mount Wilson, on the northeast side south. 36, T. 29 S., R. 20 W. in the northeastern wrner of the Mount Wilson caldera, a clastic member The most notable wmpositional difference of the White Rock Peak 7%minute quadrangle. overlies the tuff member of the Ripgut Forma- between the Lund and the older dacite ash-flow The type area for the Ripgut Formation is the tion. These accumulations of bedded tuff, silt- deposits in the Indian Peak volcanic field is the western slope of Mount Wilson in sections 25, stone, sandstone, and local conglomerate aggre- small amount of sanidine and titanite. Quartz is 26, 35, and 36, T. 5 N., R. 67 E. along the gating no more than a few tens of meters in more abundant and clinopyroxene and ortho- southern margin of the Schoolmarm Basin 7% thickness represent another post-rhyolite erup- pyroxene are sparse; they are found only as minute quadrangle, Nevada (Willis and others, tive hiatus. anhedral wres within hornblende phenocrysts. 1987). Here, along the northern wall of the Only the northern segment of the Mount Wil- The major-element composition of the Lund tuff Mount Wilson caldera that was the source of the son caldera is clearly defined around its name- lies within the range observed for the other da- tuff member of the formation, southward- sake in the northern Wilson Creek Range (Fig. cites from the magma system; however, there are thinning wedges of the breccia member are in- 2; Willis and others, 1987). Mount Wilson itself small differences in Rb, Sr, Zr, and Ba (Table 3; tercalated with the tuff member. These two is underlain by as much as 2 km of the tuff Fig. 8). members, which comprise the intracaldera se- member of the Ripgut Formation. Southward in quence, overlie the tuff member of the Lund the Wilson Creek Range, as much as several Ripgut Formation and the Mount Formation and in turn are overlain by the Blawn hundred meters of rhyolite tufk and lava flows Wilson Caldera Formation (Best and others, 1987~).The breccia of the Blawn Formation (Best and others, A third sequence of rhyolite ash flows erupted member is composed of clasts of Lund tuff shed 1987c) deposited about 23 to 18 Ma wnceal the from within the caldera wmplex after deposi- off the topographic wall of the Mount Wilson southern margin of the Mount Wilson caldera. tion of the Lund tuff and possibly well before caldera and has either a matrix of Ripgut tuff or As defined by Best and Grant (1987), the emplacement of the Isom Formation about 27 comminuted Lund tuff. An easily accessible, youngest unit in the Needles Range Group is the Ma. These eruptions formed several hundred well-exposed reference section for the tuff Lund Formation. Since that report was written, cubic kilometers of tuff and created the small member of the Ripgut Formation is in the however, the Ripgut Formation has been found Mount Wilson caldera. This tuff and associated Atlanta 7% minute quadrangle north of White to be an integral part of the Indian Peak volcanic landslide breccias and epiclastic deposits within Rock-Bailey Spring to the top of hill 8046 field and is therefore now included as the the source caldera comprise the Ripgut Forma- which is capped by Isom tuff. This section youngest formation in the Needles Range tion, a new formal unit defined here. The name is in an unsurveyed area at 38'25'10"N and Group. is taken from Ripgut Springs located about 1 km 114O22'5"W. The tuff member of the Ripgut Formation is INDIAN PEAK VOLCANIC FIELD, NEVADA AND UTAH 1085

rhyolite and petrographically similar to the older gioclase, biotite, hornblende, quartz, and trace calc-alkaline compositions, it appears that the Marsden and Greens Canyon tuffs (Fig. 6). As amounts of titanite and Fe-Ti oxides. It is a low- Isom magmas were derived from the Indian in other rhyolite tuffs in the complex, volcanic silica rhyolite like the Ryan Spring tuffs. Peak system. xenoliths are locally abundant; in the Ripgut, Cooling units are everywhere densely welded these clasts are of Wah Wah Springs and Lund Isom Formation and commonly have black basal vitrophyres a tuffs. As many as three ash flows comprise the meter or two thick. Vugs partly filled with compound cooling unit in which intensity of Trachytic ash flows erupted about 27 Ma vapor-phase minerals are typical; both vugs and welding and compaction range from dense, formed one to three tuff cooling units of the pumice lumps are commonly flattened and black or dark brown vitrophyre as much as 40 Isom Formation (Anderson and others, 1975) in elongated, presumably due to secondary flowage m thick upward into a nonwelded, glassy lapilli and near the caldera complex. The distribution after deposition. In some thick sections within and ash mixture. and thickness of the formation (Fig. 5D) and the Indian Peak caldera complex, flow units of The sequence of Ripgut cooling units is wm- size of pumice clasts point to a source north of Isom mineralogy that lack eutaxitic texture may positionally zoned (Table 2 and Figs. 6 and 8). Modena as suggested by Rowley and others be lava-like, ash-flow tufk formed by secondary The basal unit has only a few percent small (1979). Widespread Miocene volcanic rocks flowage after deposition on an uneven surface phenocrysts of plagioclase, biotite, quartz, sani- (Best, 1987) and alluvium, however, conceal the (compare Ekren and others, 1984). In some dine, and Fe-Ti oxides. This unit is a high-Si02 source area of the more than 1,300 km3 of py- places within the southern part of the White rhyolite with low concentrations of CaO (0.7 roclastic material. Based on this overlapping Rock caldera, Isom tuffs have intensely com- wt%), MgO (0. I%), Ti02, Fe203, P2O5, Sr, and source area, a time lapse on the order of only 1 pressed pumice blocks as much as 40 cm in Zr (Table 2 and Fig. 8). The upper unit on m.y. since eruption of the Lund tuff and accord- diameter that are ptygmatically folded. These Mount Wilson has about 5% phenocrysts of pla- ingly less for the Ripgut tuff, and their high-K features, together with small amounts of pheno-

AGE AVERAGE MODAL COMPOSITION OF TUFFS (volume %) LAVA FLOWS ve volume)

I p.>.:.'.'.'..:..:.:,..>,::.:',;: .;:,:'.~.;;:..;.'~<~:'..':.,'.Q:.._. . .: ...... ::: .. w.'.:B...... I:.'::HC:,.',[;~M. . .. 28 . __ _ _ . .

IIIIIM 1 P I B RYAN P lBU~ I +'+' WAH WAH SPRINGS I

w I zD < 0 X rn ?-4 u, rn --I rn

H M -{+ -{+ c ESCALANTE # +B+M +S DESERT IQl I I I 0 5 10 15 2 0 2 5 3 0 3 5 4 0 4 5 Figure 6. Modal proportions of phenocrysts in tuffs of the Indian Peak volcanic field. Phenocryst proportions are on a whole-rock basis free of lithic fragments, which occur in significant quantities in rhyolite tuffs. Dacite tuffs are stippled bars; rhyolite tuffs and the trachydacite Isom tuff are open bars. Thickness of bars does not represent duration of eruption of deposit. Data on units labeled # &om Dinkel (1969), Kreider (1970), Best and others (1973), Anderson and others (1975), and Grant (1978) are averages of many analyses; modes of other units are based on at least 1,500 points counted in a single thii section supplemented by mineral separations. This single mode is considered representative of the unit judging from examination of hundreds of outcrops. P, plagioclase; B, biotite; Q, quartz; H, hornblende; C, clinopyroxene; 0, orthopyrox- ene; X, orthopyroxene plus cliopyroxene; M, Fe-Ti oxides, chiefly magnetite; S, sanidine; T, titanite. Plus signs precede accessory phases. Most rocks contain trace amounts of apatite and zircon. The right side of the figure shows the relative volumes of rhyolite and andesite lava flows in and near the caldera complex. BEST AND OTHERS

crysts of plagioclase and two pyroxenes, suggest a relatively high temperature of emplacement. The Isom is the most distinctive unit in the Indian Peak volcanic field. It is the only tuff in which pyroxenes rather than biotite and hom- blende dominate as mafic phases; the Isom pos- Figure 7. IUGS classitication (Le sess the same mineral assemblage as most of the Bas and others, 1986) of volcanic andesitic lavas. Apatite commonly occurs as in- rocks from the Indian Peak vol- clusions in pyroxene, and zircon is sparse. canic field. AN analyses have been Andesitic lavas that erupted late in the evolu- recalculated to 100% volatile free. tion of the volcanic field underlie the Isom tuff Triangles represent andesites of all in several localities, mostly in the southern part ages. Cognate inclusions in tuffs are of the caldera complex. These two-pyroxene shown as filed symbols, other sam- (and locally hornblende-bearing) andesites have ples with letters. been included in the Isom Formation. In the A Andesites E Escalante southern Needle Range, lavas of this age have Desert compositional characteristics transitional to the C Cottonwood s Spring distinctive Isom tuff, including high Zr and K20 Wash concentrations (Fig. 8). W Wah Wah R Ripgut Springs COMPOSITIONAL RELATIONS L Lund I lsom

During its 5-m.y. lifetime, the magma system wide. Although perhaps only a result of en- andesite) extruded well beyond the caldera beneath the developing Indian Peak caldera hanced differentiation, Isom tuffs show within- complex northeast of Crystal Peak, contains complex sporadically erupted rhyolite and an- plate tendencies because of high Rb and Nb phenocrysts of olivine instead of hypersthene. desite lavas and cyclically erupted much larger abundances. Rocks in the Indian Peak volcanic Like volcanic rocks found in other subduction volumes of rhyolite and still more voluminous field are similar in composition and mineralogy settings, all of the andesitic rocks have negative dacite ash flows. Modal and bulk chemical to those related with other middle Cenozoic cal- Nb anomalies (on chondrite normalized dia- compositions of these rocks provide some dera complexes in the Great Basin (Best and grams), with the deepest anomalies found in the general insights into the nature and evolution of others, 1989a). high MgO lavas. the magma system. Elemental concentrations were determined by Andesitic Lavas Rhyolitic Tuffs and Lava Flows X-ray fluorescence spectrometry at BYU using the Norrish and Hutton (1969) method of sam- Minor volumes of intermediatecomposition Rhyolitic tuffs and lavas of the caldera com- ple preparation and data reduction. Major ele- lava were extruded from numerous vents in and plex are a petrographically similar suite of rocks ments were determined on fused glass disks and near the Indian Peak caldera complex during the that contain less than about 15% phenocrysts minor and trace elements on undiluted pressed lifetime of the magma system but chiefly during (Fig. 6), chiefly plagioclase (generally andesine), rock powders. Samples were analyzed in dupli- deposition of the Escalante Desert Formation at and minor biotite and Fe-Ti oxides; some sam- cate and tied to calibration lines of several NIM -32 Ma. Most are petrographically the same in ples also have hornblende, quartz, clinopyrox- and USGS rock standards. Total iron is reported that they contain phenocrysts of calcic plagio- ene, or sanidine that together generally make up as Fe203. Estimates of precision are shown in clase, augite, hypersthene, and magnetite in a less than 1%. Apatite and zircon are trace Table 1. matrix of the same plus variable amounts of constituents. With few exceptions, the compositional va- brown glass. A few flows of all ages (such as The rhyolitic rocks are all slightly peralumi- riety of rocks in the Indian Peak volcanic field numbers 9 and 12 in Table 1) also contain nous and have high K20/Na20 ratios whether (subalkaline, metaluminous andesite to rhyolite) hornblende and biotite. Chemically, the inter- or not they are vitrophyres. Although most sam- is broadly similar to those in other continental mediatecomposition lava flows are variable and ples are low-Si02 rhyolites, Si02contents range magmatic arcs that developed over subducting range from basaltic andesite to dacite (Fig. 7). from 68%to 77% on an anhydrous basis (Table oceanic lithosphere (Ewart, 1979). In terms of Two flows that plot in the dacite field are not 2). Only two units can be considered as high- the lack of Fe enrichment, relatively low Ti02 petrographically different from those that lie in Si02rhyolites, a lava from the Escalante Desert concentrations (less than I%), and position on a the andesite field. Although most are high-K, Formation and the basal Ripgut tuff. These normative plagioclase versus A1203 diagram they range to low-K and no single value of K57.5 high8i02 rocks, which contain only sparse (Irvine and Baragar, 1971), they are calc- (Gill, 1981) exists. This variety suggests that quartz phenocrysts, differ from other rhyolitic alkaline; the rock series is calcic to calc-alkalic they cannot be comagmatic. The most dktinc- rocks in their lower concentrations of Zr (less as defined by Peacock (1931). Except for some tive variation occurs in MgO concentrations. than 150 ppm), Fe203, Ti02, and MgO; it is andesitic lavas, both holocrystalline and glassy High and low MgO andesites were extruded notable, however, that compared to the low- rocks of the Indian Peak volcanic field are de- throughout the lifetime of the system. Flows Si02 rhyolites, the high-Si02 rhyolites are not cidedly potassic and fall in the high-K field of with greater than 4.5 wt% MgO (numbers 3,4, more enriched in the incompatible elements Rb Ewart (1979). On the tectonic discrimination 5, and 9 in Table 1) have lower A1203 and or Nb nor depleted in sanidinecompatible ele- diagrams of Pearce and others (1984), which are P2O5 and higher K20, K20/Na20, and ments such as Ba. based on Rb, Nb, and Y abundances, the rhyo- Fe203/Ti02 than other flows at comparable All of the rhyolitic tuffs are normally zoned, lites and dacites also show their similarity to Si02 concentrations. The most primitive flow both petrographically and chemically. In addi- subduction-related volcanic-arc granites world- (no. 5) of the high-MgO type (a basaltic tion to its phenocryst zonation, the Lamerdorf INDIAN PEAK VOLCANIC FIELD, NEVADA AND UTAH 1087

- 500 Formation. The younger, dacite-associated rhyo- A I II 8-AA~ lites have lower concentrations of Zr, Ti02, and 33 400 - I E K20 and significantly higher CaO at similar 6- A E~ Si02 concentrations (Fig. 8). Moreover, silicic 5 A n 3 rocks of the Escalante Desert Formation extend - - 300- A A n to lower silica contents; some lavas included in '?+hW-k.n A S ON 4- Z - 168 200- s this unit are alkali-rich dacite or trachydacite '~5R (Fig. 7). Thus, all of the silicic rocks of the Esca- 2 - AAA~: A*.* 8 100- lank Desert Formation have compositional sim- E R ilarities to the Isom. Ol~~~I.~~~~~~~~~~~~~~~, oi~~~~r~~~~~~~n~~~~~~~r~As a group, Oligocene calc-alkaline rhyolitic rocks differ significantly from early and middle 1.2- 8-~ Miocene rhyolites erupted in the same area (Best -A and others, 1987~).The genetically distinct, A AA~ younger highSiOz magmas have abundant 6 - A phenocrysts of quartz and sanidine, were erupted A n 0.8-i - A, A I 5 -A Y I I in small volumes from numerous local vents to 3 3 A L* El - 4- - 0.6- form flows, domes, and tuffs, are strongly en- N ???.WE E 8 - 0 riched in F and other incompatible elements, I ' AA% AA 'Wsk 0.4- and have higher Fe/Mg ratios (Christiansen and 2 - v. S others, 1986). 0.2 - E 1pP R E IEEgV Dacite Tuffs 010s~.--~r~~b-8~8..~..~~0 , , , , , , , , , , , , , I , . , , . , , All three dacite tuff units contain about 40% phenocrysts, more than half of which are zoned calcic andesine (Fig. 6). Other phenocrysts in- clude biotite, hornblende, quartz and minor to trace amounts of clino- and ortho-pyroxene, Fe- Ti oxides (chiefly cubic), zircon, and apatite. The Lund is the only unit that contains titanite and sanidine. Sue of phenocrysts varies within and between units. Overall, the Cottonwood Wash Tuff has the largest phenocrysts, and this character prevails almost to the top of the de- posit. In contrast, the Wah Wah Springs outflow member is finest grained (about 2 mm or less) in A Andesites E Escalante its lower part and coarsest (5 mm or so) at the Desert top. Crystal size in the Lund is rather uniform throughout. C Cottonwood S Ryan Spring Wash Cognate inclusions are ubiquitous in the da- I W Wah Wah R Ripgut cite tuff units. Most of these were pumiceous Springs lapilli compacted with the host tuff, but rarely L Lund I Isom pumice blocks to as much as 30 cm in diameter have been found in weakly welded tuff. The suite of inclusion samples also includes three (nos. 11, 12, and 14) that are weakly vesicular to nonvesicular vitrophyres. Although the cog- nate relation of all of the inclusions to their host Figure 8. Si02variation diagrams for volcanic rocks from the Indii Peak volcanic field. See tuff is indicated by similarities of characteristic caption for Figure 7 for additional information. phenocryst assemblage and bulk composition of each unit, the origin and significance of the non- vesicular inclusions is problematic, and further Tuff Member of the Escalante Desert Formation dance, and type (Fig. 6), with the appearance of investigations are underway. Such inclusions shows slight chemical zonation from base to top, hornblende in the upper part as well as the have been noted in other ash-flow deposits (for with the top being enriched in Ti, Al, Fe, Ca, strongest chemical zonation of the rhyolitic tuffs example, Noble and others, 1974). Na, Sr, and Ba relative to the base. Ryan Spring (Figs. 7 and 8). Although dacite tuft3 are petrographically tuffs show the same sense and magnitude of ver- The younger rhyolitic tuft3 (Ryan Spring and strikingly different from andesitic lava flows and tical zonation in composition, as well as in phe- Ripgut) which appear to have been emplaced from rhyolitic tuft3 and lava flows, bulk chemi- nocryst proportions and size; in addition, quartz relatively soon after eruption of the voluminous cal compositions of dacites slightly overlap the and sanidine disappear upward. The Ripgut tuff dacite ash flows, are chemically distinct from the extreme variants of these units and bridge be- also shows phenocryst zonation-in size, abun- rhyolite lavas and tuffs of the Escalante Desert tween them to form an overall continuous com- BEST AND OTHERS positional spectrum that ranges from the low contain higher elemental concentrations of Ti, Thus, compared with dacite-associated rhy- MgO andesites to the low Zr rhyolites (Table 2 Al, Fe, Mg, Ca, P, and Sr, and lower of Si, Na, olitic rocks of the Ryan Spring and Ripgut For- and Fig. 8). Part of the chemical variation in the and K. Microprobe analyses (W. P. Nash, 1985, mations, Isom magmas were erupted at higher dacite tuffs may be a consequence of mechanical written commun.) of glass in cognate inclusions temperature (see also next section), contain py- fractionation of crystals and lighter glass during from the base of the outflow and from the top of roxenes instead of biotite as major mafic eruption and emplacement of the crystal-rich the intracaldera Wah Wah Springs tuff show phenocrysts, and have higher alkali contents. ash-flows (Sparks and Huang, 1980). The vol- that both glasses are rhyolitic with low Fe/Mg Nonetheless, compared to rhyolite lava of the ume of far-traveled ash can be a significant, even ratios and no analytically significant differences. Escalante Desert Formation whose extrusion in- equivalent, volume compared to the associated Thus, inclusion compositions suggest some itiated the eruptive lifetime of the Indian Peak ash-flow deposits. G. A. Izett (1987, oral com- small, but stratigraphically systematic, zonation magma system, the Isom has similar high Zr and mun.) has found beds of ash near the base of the in the Wah Wah Springs ash flows. Further low Fe203/Ti02 and CaO/A1203 ratios. Per- Arikaree Group in Nebraska that have age and evaluation of chemical variation in these large- haps the genetic conditions which led to these crystal composition appropriate for the Needles volume dacite deposits must include additional distinctive compositions were nearly reproduced Range dacite tuffs. Some sort of glass/pheno- comparisons of intra- and extracaldera tuffi and at the beginning and close of eruptive activity of cryst fractionation probably explains the differ- cognate inclusions, especially their phase com- the system. Following the suggestion of Meen ences between analyses 3/4 gnd 7/8 which positions (compare with Grunder and Boden, (1987), it is possible that high-pressure fraction- represent matrix/pumice pairs from the Cot- 1987). ation of pyroxene from a more mafic parent led tonwood Wash and Wah Wah Springs tuffs. Compositions of cognate inclusions of the to substantial enrichment of K and incompatible Elemental enrichments (Ti, Al, Fe, Mg, Ca, Sr, Cottonwood Wash and Lund tuffs are distinct elements without a commensurate increase in P, and Zr) and depletions (Si, K, and Rb) are for most elements from each other and from Si02. Clinopyroxene-dominant fractionation consistent with the accumulation of plagioclase, those in the Wah Wah Springs (Fig. 8; Table 3). could have produd the low CaO/A1203 ratios zircon, apatite, and mafic minerals. Linear varia- Moreover, based on bulk compositions of tuff in the differentiates. tion trends in the dacites (Fig. 8) suggest ande- and inclusions, the Lund is the least evolved of sitic and crustal silicic materials were both the dacites. Nonetheless, it has the most quartz Generalities Regarding P-T-Xin the important in the development of the magmas. of the three, contains sanidine, a smaller propor- Magma System Scott and others (1971) report initial 87~r/86~rtion of mafic phenocrysts, and no free pyroxene. ratios that range from 0.7088 to 0.7099 for pla- As total abundance of phenocrysts is about the Comparisons of the phenocryst assemblages gioclases from dacitic tuffs of the Needles Range same in all the dacites, slight but significant dif- found in rocks of the Indian Peak field with Group which are consistent with a substantial ferences in intensive parameters, rather than dif- experimentally determined phase relations for crustal component in these rocks. ferences in extent of crystallization or composi- similar compositions allow some inferences to In light of the contention of Whitney and tion, must have prevailed in the dacite magma be drawn about the intensive parameters of Stormer (1985) that large-volume dacite tufi bodies. erupted magmas. are compositionally uniform, it is important to The experiments of Johnson and Rutherford examine the evidence for zonation in the dacites Isom Trachydacite Tuff (1989) conducted on the Fish Canyon Tuff are of the Indian Peak volcanic field. Contrasts be- probably most applicable to the Indian Peak da- tween immobile-element concentrations in basal Compared to other tuffs in the volcanic field cites, particularly the Lund, which has the same vitrophyres and immediately overlying devitri- at the same Si02 content, the trachytic Isom phenocryst assemblage of quartz, plagioclase, fied tuffs of a single deposit (nos. 1 and 2, and 5 Formation has higher concentrations of Ti02, sanidine, biotite, hornblende, ilmenite, magne- and 6) could reflect a slight vertical zonation or Zr, Ba, Rb, and K20, and much lower CaO tite, and titanite. Experiments show that this simply mechanical fractionation of crystals and concentrations. In spite of the high alkali con- mineral assemblage is stable at relatively low glass during eruption and emplacement. Alter- tent, the Isom rocks are not peralkaline and by PH,o/P,d, less than 0.5, and at relatively low natively, differences between more mobile ele- strict IUGS usage (Le Bas and others, 1986), temperatures of about 760 "C. The assemblage, ments could have developed during hydration most samples are trachydacite because of their however, is not particularly pressure sensitive, and devitrification of the glass. high normative quartz content. Moderate that is, stable from 2 to 5 kb, but measured A better test for compositional uniformity is a Fe/Mg ratios place them in the calc-alkaline aluminum concentrations in hornblende in the comparison of cognate inclusions from within a field (Fig. 7). Thus, the Isom tufh are substan- Lund suggest a pressure of crystallization of single deposit. Bulk chemical compositions of tially different from trachytes erupted from about 2.5 kb using the calibration of Johnson pumice lumps from the base and top of the out- younger caldera complexes in the Great hasin and Rutherford (1989). The presence of the flow tuff member of the Wah Wah Springs which have higher Fe/Mg ratios and are com- oxygen-buffering assemblage titanite-magnetite- Formation (nos. 8 and 9 in Table 3 and Fig. 8) monly peralkaline (Novak and Mahood, 1986); quartz-clinopyroxene-hornblende in the Lund are sufficiently different in some elements (Ti, however, the Isom is similar to trachytic rocks tuff is consistent with its crystallization at rela- Fe, and P) to exceed analytical and geological from the middle Tertiary Trans-Pews volcanic tively high fugacity of oxygen, nearly 2 log units uncertainties. Greater zonation in the Wah Wah field (Henry and others, 1988). above the QFM buffer (Noyes and others, Springs tuff is evident in a comparison of inclu- The high Zr concentrations of the Isom de- 1983). Such high oxygen fugacity is consistent sions from the outflow and intracaldera deposits. mand a high eruption temperature (Watson and with the over-all calc-alkaline character of the Inclusions from the intracaldera member are Harrison, 1983). Magnetite-ilmenite and two- magmas erupted from the Indian Peak magma significantly and systematically more mafic than pyroxene geothermometry indicates an equili- system. The experiments of Johnson and Ruth- those from even the top of the outflow tuff and bration temperature of about 950 "C. erford (1989) show phase assemblages which INDIAN PEAK VOLCANIC FIELD, NEVADA AND UTAH 1089 lack sanidine (as in the Wah Wah Springs and fined sources covering a present area of about 80 ically similar, compositionally zoned, lithic-rich Cottonwood Wash tuffs) are formed at tempera- x 120 km. This caldera complex and related rhyolites and more voluminous, crystal-rich da- tures of 800 to 900 OC. Nusbaum (1988) reports volcanic rocks are the surface manifestation of cites and a final trachydacite ash flow. Rather Fe-Ti oxide temperatures for the Wah Wah an underlying, major magmatic system that similar conditions of magma generation and Springs of 800 to 880 OC and oxygen fugacity of vented on the order of 10,000 km3 of magma evolution, as well as extent of crystallization about 2 log unit above QFM. about 32 to 27 Ma. prior to eruption, were recurrently achieved in We noted earlier that the intracaldera tuff of 2. Eruptions from this open magma system, the magma system. the Wah Wah Springs Formation commonly which included discrete magma chambers 6. In detail, however, conditions of rhyolite has slightly more quartz than does the outflow whose size, shape, and constituents changed and dacite magma generation differed somewhat tuff. According to the experiments of Clemens with time, yielded a variety of andesites, dacites, through the lifetime of the system. For example, and Wall (1981) and Naney (1983) which show rhyolites, and trachydacite rocks that cannot Escalante Desert silicic rocks are chemically dis- positive dP/dT slopes for the quartz phase have been strictly comagmatic, that is, derived tinct from younger rhyolites, and modes of the boundary, a slightly greater pressure and/or from a single magma body. These rocks are calc- Ryan Spring tuffs show their affinity to the Wah lower water content could account for this dif- alkaline with low to moderate Fe/Mg ratios, Wah Springs dacite (phenocryst assemblages ference. Either of these differences is compatible low TiOz, inferred high oxygen fugacities, and, which lack sanidine), whereas the Ripgut shows with a somewhat deeper source for the intracal- in the andesitic rocks, sizable Nb depletions. similarities to the Lund (both have sanidine and dera Wah Wah Springs tuff. The less evolved With the exception of some andesites, all are titanite). The erupted Lund magma apparently chemical composition but greater quartz content potassic with K20>Na20. Although also show- partially crystallized under different conditions of the Lund tuff could be explained by the same ing many calc-alkaline characteristics, low-silica from the other dacites. relative shift in P and water content, or a lower rhyolites and a trachydacite ash-flow with dis- 7. Eruption of Ryan Spring and Ripgut rhyo- eruption temperature as indicated above. tinctive enrichments of incompatible elements lite magma appears to have followed closely in Rhyolitic rocks of the Indian Peak caldera were erupted, respectively, early and late in the time after the voluminous dacite eruptions, complex have near-liquidus phenocryst assem- evolution of the magma system. rather than the reverse, because epiclastic depos- blages dominated by plagioclase and biotite with 3. In contrast to the Marysvale and San Juan its overlie each of the rhyolite deposits but not little or no hornblende, sanidine, or quartz, even volcanic fields that are marginal to the Colorado the dacites. The oldest rhyolitic magmas from though concentrations of Si02 and K20 are Plateaus, no protracted extrusion of andesitic the Indian Peak system, forming the chemically high, as in the lower Ripgut tuff. The experi- lava flows preceded ash-flow eruptions from the distinctive Escalante Desert Formation, were as- ments of Naney (1983) and Clemens and Wall Indian Peak magma system in the Great Basin. sociated with no precursory dacite, possibly be- (1981) suggest that this assemblage requires Only trivial extrusions (an estimated few percent cause only enough mantlederived thermal water concentrations in the magma in excess of of the volume of silicic tuff) occurred during its power had been inserted into the crustal system 3 to 4 wt%. The expansion of the quartz stability 5-m.y. lifetime. Chemically variable two-pyrox- to generate a quasi-bimodal rhyolite-andesite field relative to plagioclase with increasing pres- ene andesitic magmas erupted throughout the association. sure documented by Tuttle and Bowen (1958), history of the system from vents both within and 8. It is improbable that rhyolitic magma ex- Clemens and Wall (1981), and Naney (1983), outside the caldera cluster. Although small in isted as a capping differentiate of a composition- also limits the crystallization of the quartz-poor volume, these extrusions are considered to mani- ally stratified magma body immediately before rhyolites of the Indian Peak magma system to fest a mantle input produced above a subduct- eruption of dacite ash flows. This conclusion relatively low pressures, less than 4 kb. ing slab of lithosphere. Their variable and follows from (a) the absence of zoned rhyolite to The two-pyroxene plagioclase assemblage of evolved compositions, however, must reflect dif- dacite cooling units; (b) inappropriate time rela- the Isom tuff was reproduced by Naney's (1983) ferentiation in an open system, including varia- tionships (Escalante Desert rhyolitic magmas experiments with a low-silica granite at tempera- ble mafic magma recharge, crustal contamina- were erupted about 2 m.y. before the succeeding tures which range from 900 to 980 OC (at 8 kb) tion, crystal fractionation, and mixing with large-volume dacite of the Cottonwood Wash and at 820 to 920 OC (at 2 kb) at water concen- silicic crustal melts. The greatest volumes of an- Tuff, and the occurrence of epiclastic deposits trations less than 3.5%in the magma. The exper- desite lava flows were extruded early and late in suggest a hiatus between eruption of rhyolite iments of Johnson and Rutherford (1989) the life of the system. A reduced mid-life rate of and following dacite); and (c) the difficulty of suggest that the Isom assemblage is stable at low andesite eruption could reflect the difficulty of withdrawing thousands of cubic kilometers of pressures (<2 kb) and temperatures of about 900 ascent through overlying less dense silicic dacite magma from a chamber without tapping OC. These suggestions of high temperature are in magma bodies as the magma system matured. any rhyolite magma which hypothetically accord with the geological evidence and with 4. The voluminous dacite tuffs and the capped a zoned chamber. Vigorous convection the relatively high temperatures from mineral trachydacite Isom tuff were derived from in dacite magma chambers could have pre- geothermometers. sources that migrated southward through time cluded the existence of gravitationally stable (Fig. 2). This transgression mimics in rate and caps of rhyolite. DISCUSSION AND CONCLUSIONS direction the pattern of volcanic activity in the 9. Distinctive enrichments in TiOz, alkalies, entire Great Basin (Cross and Pilger, 1978; Best and especially Zr occur in rhyolitic and trachytic Important conclusions that can be drawn and others, 1989a) and suggests that sites of rocks formed during waxing and waning stages from the foregoing information include the magma generation or eruption shifted in re- of the Indian Peak magma system compared to following. sponse to regional dictates. rhyolitic rocks of its mid-life. These differences 1. The Indian Peak caldera complex is a clus- 5. The history of the Indian Peak magma sys- reflect differences in magma source conditions ter of four nested calderas and two poorly de- tem is marked by cyclic eruptions of petrograph- or subsequent diversification processes. 1090 BEST AND OTHERS

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A,, 1984, Cordilleran metamorphic mre wm- strontium isotope evolution model for Cenmoic magma genesis, eastern plexes: Cenozoic extensional relics of Mmicmmpreaion: Geology, Great Basin, U.S.A.: Bulletin Volcanologique,v. 35, p. 1-26. ago through an established program of roving v. 12, p. 5s554. Sparks, R. S., and Huang, T. C., 1980, The volcanological signilicance of BYU summer field geology courses that has Crm, T. A., and Pilger, R. H., Jr., 1978, Coosvaiots on absolute motion and deep-sea ash layers associated with ignimhrites: Geological Magadne, plate interaction inferred from Cenozoic igneous activity in the westem v. 117, p. 425436. produced much of the data reported here; he has United States: American Journal of Science, v. 278, p. 865-902. Steven, T. A,, and Lipman, P. W., 1976, Calderas of the San Juan volcanic Dinkel, T. 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