Origin of bimodal , southern Basin and Range province, west-central Arizona

NEIL H. SUNESON* Department of Geological Sciences, University of California, Santa Barbara, California 93106 IVO LUCCHITTA U.S. Geological Survey, 2255 North Gemini Drive, Flagstaff, Arizona 86001

ABSTRACT production of basaltic and rhyolitic from the Earth's crust and mantle requires extremely heterogenous source regions. As- Miocene volcanic rocks in the Castaneda Hills area, west- thenospheric upwelling associated with basin-range extensional tec- central Arizona, are interbedded with continental clastic sedimen- tonism probably produced the heating event that caused partial tary rocks, minor limestone, gravity glide blocks of Precambrian(?) melting and basaltic magma generation at different levels in the and Paleozoic(?) rocks, and monolithologic megabreccia. The sed- mantle. Partial melting in the lower crust to produce rhyolitic imentary and volcanic units dip to the southwest and are offset by probably was caused by the intrusion of magma. northwest-trending listric and high-angle normal faults. The listric The basaltic and rhyolitic magmas formed in separate source faults coalesce at the Rawhide detachment fault, which overlies regions, rose independently, and erupted at the same time and mylonitic gneiss. place. The volcanic suite is strongly bimodal; rocks with 55 to 71 wt % SiC>2 are rare. On the basis of age, geomorphic position, and petro- INTRODUCTION graphy, five volcanic units can be distinguished: older (18.7 and 16.5 m.y. old), quartz-bearing basalts (13.7 and 12.4 m.y. old), In his classic study of the geology of Ascension Island, Daly and tuffs (15.1 to 10.3 m.y. old), mesa-forming basalts (1925) recognized that intermediate-composition lavas of the ocea- (13.1 to 9.2 m.y. old), and megacryst-bearing basalts (8.6 to 6.8 m.y. nic basalt- association are much less abundant than the old). Most of the basalts contain groundmass olivine and titanau- and silicic members. Volcanic suites with this "Daly gap" are gite phenocrysts and are alkali-olivine basalts. Many con- usually expressed in plots of weight percentage of Si02 versus tain more than 75 wt % Si02- volume. The term "bimodal" as now used refers to volcanic fields in The initial whole-rock Sr isotopic composition of the basalts which and are scarce and basalt and rhyolite were indicates that they are partial melts of an isotopically vertically erupted at about the same time and place. heterogenous mantle. The chemical composition of some of the Bimodal suites are not restricted to the oceanic basalt-trachyte megacrysts in megacryst-bearing basalts with 87Sr/86Sn equal to association. Bimodal volcanic suites in other petrologic and tec- .7035 and .7038 supports a high-pressure mantle origin. The low tonic settings include the ferrobasalt-peralkaline silicic suite asso- (.7034) Sr ratio and lack of evidence for mixing with young rocks ciated with continental spreading (Baker and others, 1977; Weaver, indicate that the quartz-bearing basalts were also derived from the 1977); basalt-high-silica rhyolite suite in crustal areas undergoing mantle. Other basalts with 87Sr/86Sn >0.705 probably were extension (Christiansen and Lipman, 1972; Noble, 1972); the basalt- derived from old, lithospheric mantle with a high Rb/Sr ratio and high-alkali rhyolite suite over mantle hot spots (Johnson and oth- do not appear to be contaminated with old, upper-crustal material. ers, 1978); and bimodal calc-alkaline suites erupted on continental The rhyolites have initial Sr isotopic ratios of 0.7093 and terranes over subduction zones (McBirney, 1969; McDowell and 0.7141. These ratios indicate that the rhyolites were not differen- Clabaugh, 1979). tiated from the basalts. Partial melting of 1.3-b.y.-old lower-crustal The common association of bimodal volcanism and exten- material with Rb/Sr = 0.10 to 0.19 satisfactorily explains the iso- sional tectonism is well known, but the relation of the silicic mag- topic ratios of the rhyolites. Granulite, which may constitute the mas to the mafic ones is controversial. Are the rhyolites and basalts lower crust in this part of Arizona, has Rb/Sr ratios similar to those derived from the same source by a partial melting or differentiation required to produce the rhyolites. K substituted for Na during cool- mechanism, for example, crystal fractionation (Meyer and Sigurds- ing and devitrification in some of the rhyolites. son, 1978), vapor transfer (Arana and others, 1973), liquid immisci- Partial melting of upper-mantle peridotite and old lower- bility (Hamilton, 1965), or are they derived from different sources, crustal granulite from 19 to 7 m.y. ago in the Castaneda Hills area for example, basalt from the mantle and rhyolite from the lower produced the bimodal volcanic suite. The nearly contemporaneous crust (Lipman and others, 1978)? Another possibility is that bimod- al volcanic rocks result from normal differentiation processes, * Present address: Geothermal Exploration Division, Chevron Re- accompanied by the preferential eruption of mafic magmas, which sources Company, P.O. Box 7147, San Francisco, California 94120-7147. are very fluid, and of silicic magmas, which are less dense and less

Geological Society of America Bulletin, v. 94, p. 1005-1019, 8 figs., 3 tables, August 1983.

1005

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viscous (due to high H2O and halogen contents) than magmas of intermediate composition (Baker and others, 1977). A fourth possi- bility was proposed by Weaver (1977), who suggested that "the middle stages of crystallization [at the Emuruangogolak, Kenya ], representing the transition from basaltic to trachytic liquids, [were] rapidly traversed and that the volume of interme- diate magma existing at any time [was] relatively small" (p. 228). Miocene bimodal volcanic rocks characterize much of the Basin and Range province of the western United States. Eruption of these rocks accompanied normal faulting that began between about 20 and 15 m.y. ago (McK.ee, 1971; Christiansen and Lipman, 1972; Noble, 1972; Snyder and others, 1976). We have studied a basalt-rhyolite suite in west-central Arizona that may be typical of similar suites elsewhere in the western United States. Field map- ping, chemical analyses, and isotopic dating of this suite contribute to our understanding of the origin of these rocks. The study area is in west-central Arizona near the eastern mar- gin of the southern Basin and Range province about 40 km south- west of the edge of the Colorado Plateau (Fig. 1). Mapped quadrangles include the Castaneda Hills 15' quadrangle and the northwest corner of the Artillery Peak 15' quadrangle (Fig. 2).

STRATIGRAPHY AND STRUCTURE

The Castaneda Hills area contains Precambrian crystalline and metamorphic rocks, Paleozoic(?) and Mesozoic(?) metamorphic rocks, and Tertiary and Quaternary metamorphic, sedimentary, and volcanic rocks (Fig. 3). The Tertiary section consists mostly of interbedded nonmarine coarse-grained clastic sedimentary rocks, limestone, and gypsum, and silicic and mafic volcanic rocks. A widespread, unstratified, monolithologic megabreccia is also inter- bedded in the sedimentary-volcanic sequence. Minor unconformi- ties within and between units suggest that most of the sedimentary units were deposited in subsiding structural and topographic basins that formed during an early period of listric faulting (sequence I Figure 1. Location map showing Castaneda Hills area and rocks) and a later period of high-angle normal faulting (sequence II major physiographic provinces in the southwestern United States. rocks). Boundary between the Great Basin and southern Basin and Range provinces from Eaton and others (1978, p. 75-78). In the Artillery Mountains area immediately east of the study area, Lasky and Webber (1949) divided the Tertiary rocks into seven major units separated by angular or erosional unconformi- ties: the Eocene(?) Artillery Formation, Miocene(?) volcanic rocks, lower Pliocene(?) Chapin Wash Formation and Cobwebb Basalt, ment fault. As noted by Shackelford (1980), the detachment fault upper Pliocene(?) Sandtrap Conglomerate, lower Pleistocene formed by the coalescing of listric normal faults along a zone of basalt, upper Pleistocene alluvium, and Recent alluvium. Our map- weakness near the top c f the lower-plate mylonitic gneiss. Faults ping has shown that most of the unconformities recognized by that offset the Rawhide detachment fault are rare but, where pres- Lasky and Webber (1949) are local. This fact and the K-Ar isotopic ent, are for the most part high-angle normal faults. dates on the volcanic rocks (Suneson and Lucchitta, 1979, and The age of inception of listric faulting is unknown, but the Table 1) indicate that the Tertiary sedimentary-volcanic sequence is absence of thick sedimentary deposits that might have accumulated Miocene to early Pliocene. in listric fault-bounded basins prior to eruption of the older basalts In the Castaneda Hills area, the Rawhide detachment fault suggests that most faulting occurred after eruption of the older (Shackelford, 1980) separates an upper plate of brittlely deformed, basalts. The age of latest movement on the detachment fault is mainly fluviolacustrine and volcanic rocks from a lower plate of constrained by the ages of the volcanic rocks in the upper plate. The ductilely deformed mylonitic gneiss (Fig. 2). Gravity glide blocks older basalts (18.7 and 16.5 m.y. old) dip steeply and are rotated by composed of Precambrian(?) plutonic rocks, Paleozoic(?) quartzite listric normal faults. The mesa-forming and quartz-bearing basalts and limestone, and Mesozoic(?) metasedimentary and metavolcanic (13.7 to 9.2 m.y. old) are flat-lying or slightly tilted and offset by rocks are interleaved in the Tertiary section. The Tertiary sedimen- high-angle normal faults. Some of the rhyolites (15.1 to 10.3 m.y. tary and volcanic rocks strike northwest, dip southwest, and are old) are offset by closely spaced listric faults. This evidence suggests generally displaced along northwest-striking faults along which the that most listric faulting and displacement on the Rawhide detach- northeast side has generally moved relatively down. The southwest ment fault ceased by about 14 m.y. ago and was followed by high- rotation of beds suggests a listric-fault geometry. Listric faults, angle normal faulting. These later faults locally offset the which rotate the overlying beds, merge with the Rawhide detach- detachment fault. The upper age limit of the high-angle faults is

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Figure 2. Geologic map of Castañeda Hills area. I, II, and III refer to sedimentary units described in text and in Figure 3. Symbols for volcanic units are described in text. Rawhide detachment fault is shown by hachured line, normal faults by line with bar and ball (dotted where concealed), and vent areas of volcanic rocks by asterisk. Bill Williams Mountains (B W M) are in southwest corner of map, Castañeda Hills (C H) in northwest corner, Aubrey Peak (A P) in east-central part.

about 7 m.y., the approximate age of the largely unfaulted shows that the vents of many of the mafic and silicic flows are near megacryst-bearing basalts. each other (Fig. 2); for example, the thickest accumulation of older basalts (and probable vents) was intruded by rhyolite plugs, and VOLCANIC UNITS both were overlain by megacryst-bearing basalts in the Castaneda Hills. In the Bill Williams Mountains, megacryst-bearing basalt Field, pétrographie, chemical, and isotopic evidence indicates dikes intrude rhyolite dikes. Major-element analyses confirm the that the volcanic rocks in the Castaneda Hills area form a bimodal suspicion based on field and petrographic work, that intermediate- suite. Detailed mapping (Lucchitta and Suneson, unpub. data) composition rocks (here, those with about 58 tb 71 wt % SÍO2) are

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Older Basalts (Tbo) SEQUENCE HI The older basalts form piles of flows, flow breccias, and prob- able source vents as much as 500 m thick near the Castaneda Hills. ALLUVIAL DEPOSITS. South of Aubrey Peak, these basalts are relatively thin flows inter- MEGACRYST 5.4(?)-8.6MY POST- TECTONIC 6454LT bedded in sequence I sedimentary rocks (Fig. 2). The older basalts 6 AGES are highly faulted and generally poorly exposed. Consequently, Major unconformity individual flows cannot be mapped where the basalts are thick. The flows are typically weathered, fractured, amygdaloidal, and cut by calcite veinlets. Most flows contain phenocrysts of plagioclase (avg SEQUENCE H An^o) (0% to 4%), iddingsitized olivine (1 % to 3%), and rare titanau- gite in an intersertal to intergranular groundmass of plagioclase, INTERIOR-BASIN DEPOSITS, 9.2-I3.7MY MESA-FORMING clinopyroxene, olivine, opaque minerals, and altered glass. A few B4S4LT BASIN-RANGE FAULTING S AGES flows are extremely porphyritic and contain 20% 15-mm plagioclase phenocrysts. Glomeroporphyritic clots of phenocrysts are common. Major unconformity

io conformity Quartz-Bearing Basalts (Tbq) I0.3-I5J MY SILICIC 9 AGES VOLC. RX. Three quartz-bearing basalt flows and an associated feeder dike are exposed as high-standing to subdued ridges and mesas in SEQUENCE I the Castaneda Hills and near Aubrey Peak. The rocks range from 'OLOE R* fresh and dense to weathered and amygdaloidal and contain 3% to FLU VI O LACUSTRINE BA 54 LT 5% sieve-textured plagioclase (avg An55), rare, partially iddingsit- 2 AGES ROCKS. ized olivine, rare titanaugite, and 1% to 5% rounded, embayed LISTRIC FAULTING quartz phenocrysts. The groundmass is intergranular to intersertal and is composed of plagioclase, clinopyroxene, olivine (rare to NEO&ENE 10%), opaque minerals, and variable amounts of altered glass. The MICROFOSSILS © quartz phenocrysts are surrounded by reaction rims of radiating clinopyroxene crystals. Except for the presence of quartz pheno- MoLjor unconformity crysts, these basalts are similar to the mesa-forming basalts de- BASEMENT scribed below.

Figure 3. Stratigraphy of the Castaneda Hills area. Numbers Silicic Volcanic Rocks (Tvs) on left show age ranges of different volcanic units. Quartz-bearing basalts are included with the mesa-forming basalts. Interbedded rhyolitic volcaniclastic rocks, flows, and intrusives as much as 150 m thick occur throughout the area absent. K-Ar isotopic work verifies field evidence that the mafic and mapped. The flows form short, stubby masses near the rhyolite silicic rocks erupted at about the same time, from about 19 to 7 m.y. domes. The thickest accumulations of volcaniclastic rocks, which ago. Hence, the volcanic rocks in the Castaneda Hills area form a are chiefly reworked and airfall tuff, are near the domes. This bimodal suite because mafic and silicic rocks erupted at about the suggests the volcaniclastic rocks are associated with the domes and same time and in the same place. were probably not derived from great distances.

TABLE 1. K-Ar AGE DATA FOR ROCKS FROM THE CASTANEDA HILLS AREA

40 40 Sample no. Unit Location % K2O Arr (x 10"" mol/g) % Arr Age ± 1 (m.y.)

CH-49 (WR) Older basalts 34°28'57"N, 2.320 6.280 61.5 18.7 10.3 113°58'00"W CHNW-154 (San) Rhyolite 34°27'45"N, 11.29 22.29 58.1 13.7 ±0.2 113°57'49"W CHSE-I35A (WR) Mesa-forming 34°21'08"N, 1.181 1.933 24.4 11.3 ± 0.7 basalt 113°48'26"W AP-19 (WR) Mesa-forming 34°24'54"N, 2.330 3.079 38.8 9.2 ± 0.3 basalt 113°40'22"W MSMF(WR) Megacryst-bearing 34°25'16"N, 1.166 1.439 35.1 8.6 ± 0.3 basalt 114°01'27"W CH-79 (Pig) M egacryst-bearing 34°23'08"N, 0.563 0.436 14.9 5.4 ± 0.6 basalt 113°58'32"W

Note: constants used are those recommended by Steiger and Jäger (1977).

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TABLE 2. MAJOR-ELEMENT CHEMICAL ANALYSES OF ROCKS FROM THE CASTAÑEDA HILLS AREA

Quartz-bearing Rhyolites Older basalts basalts Aubrey Hills Sample CH-61 CH-65 ILCH- CH-16 CHSE- CHNW- CHNE-E CHNE- CH-07A CH-42 AP-04 oxide 77-1 26 58B I53D 125

Si02 48.43 49.83 48.71 53.73 54.99 53.54 74.23 76.95 75.89 74.35 75.38 AI2O3 15.29 18.89 16.96 15.13 14.99 15.76 12.00 11.19 11.94 12.09 11.78 Fe203 6.73 8.38 6.83 5.63 5.24 2.20 0.56 0.78 0.87 0.65 1.77 FeO 2.27 0.86 1.67 4.14 3.75 7.11 0.42 n.d. 0.15 0.09 0.05 MgO 6.37 2.42 5.60 4.06 3.71 4.82 n.d. 0.05 0.22 0.10 0.38 CaO 10.21 8.03 9.95 7.51 7.30 8.16 0.65 1.12 1.32 0.70 0.26

Na20 3.18 3.46 3.38 3.62 3.35 3.35 3.11 1.28 2.74 3.21 0.83 K2O 2.31 4.12 0.74 2.12 2.31 1.92 5.14 7.76 6.12 4.99 8.80 + H2O 1.26 0.65 1.51 0.69 0.82 0.24 3.73 0.47 0.14 3.37 0.69 H20" 0.67 0.61 2.74 0.33 1.27 0.31 0.36 0.44 0.13 0.21 0.33 Ti02 1.47 1.75 1.37 1.99 1.95 1.85 0.13 0.11 0.08 0.11 0.07

p2o5 0.88 0.73 0.24 0.31 0.35 0.33 0.03 0.02 0.03 0.02 0.02 MnO 0.13 0.06 0.09 0.11 0.10 0.14 0.03 0.02 0.04 0.03 0.05

Co2 0.06 0.96 0.19 0.10 0.06 0.03 0.10 0.71 0.92 0.03 0.10 Total 99.26 100.75 99.98 99.47 100.19 99.76 100.49 100.90 100.59 99.95 100.51 q 1.63 7.02 9.76 2.63 35.19 39.15 35.76 35.96 35.90 c 0.44 0.50 0.61 0.25 0.69 or 13.75 24.47 4.57 12.59 13.63 11.37 30.23 45.42 35.95 29.41 51.74 ab 26.44 29.43 29.88 30.80 28.29 28.42 26.19 10.73 23.05 27.09 6.99 an 20.78 23.97 30.21 18.87 19.01 22.35 2.39 0.48 0.53 3.15 0.52 ne 0.36 wo 10.05 1.10 7.71 6.64 6.05 6.63 en 8.69 2.42 14.57 10.17 9.22 12.03 0.12 0.55 0.25 0.94 fs 8.46 0.15 0.05 fo 5.11 2.55 fa mt 3.51 1.78 7.98 6.75 3.20 0.81 0.38 0.94 0.12 hm 4.36 8.42 5.91 0.16 0.58 0.77 0.60 1.68 il 2.81 1.96 2.72 3.80 3.70 3.52 0.25 0.04 0.15 0.21 0.13 ru 0.09 ap 2.10 1.74 0.59 0.74 0.83 0.78 0.07 0.21 0.07 0.05 0.05 cc 0.14 2.19 0.45 0.23 0.14 0.07 . 0.23 1.60 2.08 0.07 0.23 1.79*

Fe203 2.96 9.74 4.09 1.36 1.40 0.31

FeO m

FeO a 10.58 7.21 6.96 10.50 9.69 8.14 Fe203 a 2.11 1.44 1.39 2.10 1.94 1.63 Mg values 51.76 37.43 58.92 40.80 40.56 46.31

Note: all oxides in weight percent. Subscripts "m" = measured; "a" = adjusted, as described in text. *Sphene

Three major silicic centers are located in the Aubrey Peak area, dized to opaque minerals. The rhyolites in the Castañeda Hills are Castañeda Hills, and Bill Williams Mountains (Fig. 2). In these similar to those near Aubrey Peak but more crystal-rich (5% to 15% areas, the lavas tend to form topographic highs, and the tuffs tend phenocrysts). Rhyolites of the Bill Williams Mountains, by con- to form intervening valleys. The volcaniclastic rocks consist of trast, are sparsely porphyritic (< 5% phenocrysts) and lack ferro- light-colored, well-stratified to massive pumiceous pyroclastic rocks magnesian minerals. Groundmass textures of the rhyolites range and reworked tuff represented by lapilli- and lithic-rich volcanic from perlitic to granophyric. Micropoikilitic and spherulitic to axi- sandstone and conglomerate. The lava flows and plugs are glassy or olitic textures are locally common. devitrified, flow-banded, and locally brecciated. The intrusives typ-

ically have glassy selvages. Miarolitic cavities are uncommon. The Mesa-Forming Basalts (Tbmf) pyroclastic and reworked tuffs are highly altered and zeolitized. The rhyolite flows and intrusives near Aubrey Peak contain 0% The mesa-forming basalts cap high mesas in the Castañeda to 12% sanidine, quartz, and oligoclase phenocrysts and trace Hills area. The mesas are generally underlain by 1 to 15 massive amounts of biotite, hornblende, zircon, and opaque minerals. The flows separated by thin layers of flow rubble or sediments. The vent feldspars are generally euhedral, and the quartz is euhedral to area of the flows in the southeastern part of the study area is pre- rounded and highly embayed. Biotite is partly to completely oxi- served; the vents for the other mesa-forming basalts are probably

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TABLE 2. (Continued)

Rhyolites Castañeda Hills Bill Wms. Mesa-forming Mtns. basalts Sample CHNW- CHNW- CHNW- CHNW- CHSW- CH-47 CHS W- CHSE- oxide 154D 155D 156D 159D 158D III 135

Si02 76.52 73.91 71.62 76.12 75.80 50.82 52.66 51.69 AI2O3 11.97 11.95 13.95 11.07 12.32 17.05 16.32 17.25

Fe203 0.74 0.86 1.97 0.85 1.27 4.24 4.35 2.47 FeO 0.10 0.35 0.40 n.d. 0.03 5.09 4.12 6.31 MgO n.d. 0.0 6 0.21 n.d. n.d. 5.30 5.16 5.67 CaO 0.27 0.90 1.29 1.34 1.12 9.25 8.60 9.28

Na20 1.84 2.74 2.89 1.09 3.84 3.52 3.23 3.29 K20 7.91 5.12 6.06 8.56 4.71 1.36 1.52 1.19 + H2O 0.21 2.88 0.45 0.30 0.23 0.34 0.80 0.40

H2O- 0.15 0.69 0.15 0.30 0.25 0.60 0.87 0.29

Ti02 0.15 0.21 0,41 0.11 0.11 1.54 1.26 1.36 P2Os 0.03 0.05 0.10 0.04 0.03 0.50 0.43 0.38 MnO 0.02 0.04 0.06 0.02 0.03 0.14 0.14 0.14 co2 0.16 0.06 0.15 1.04 0.10 0.09 0.06 0.34 Total 100.07 99.82 99.71 100.84 99.84 99.84 99.52 100.06 q 35.47 36.73 29.32 37.00 33.94 0.77 5.12 0.95 c 0.33 0.52 0.88 or 46.71 30.31 35.91 50.55 27.88 8.05 9.02 7.03 ab 15.56 23.23 24.53 8.90 32.55 29.83 27.87 27.82 an 0.13 3.77 4.81 0.28 2.47 26.75 25.43 28.77 ne wo 0.88 6.42 5.94 5.27 en 0.15 0.53 13.22 12.91 14.11 fs 3.57 2.16 7.56 fo fa mt 0.65 0.30 6.16 6.33 3.58 hm 0.74 0.41 1.77 0.80 1.27 il 0.25 0.40 0.78 0.13 2.93 2.40 2.58 ru 0.02 0.08 ap 0.07 0.12 0.24 0.07 1.19 1.02 0.90 cc 0.36 0.14 0.34 2.30 0.23 0.21 0.14 0.77 0.11** 0.11*

Fe203 0.83 1.06 0.39

FeOra

FeO a 9.35 8.79 7.91

Fe203 a 1.87 1.76 1.58 Mg values 50.26 51.13 56.10

**Na-carbonate; *Sphene

buried by flows. The rocks are typically black, locally vesicular or by different cooling rates of individual flows; this appears to be the amygdaloidal, and they weather to large boulders coated with case for some of the Manganese Mesa basalts. Other differences, desert varnish. Where the interiors of the flows are exposed, the however, are probably a reflection of the facts that each mesa basalts weather spheroidally. Crude columnar jointing is locally represents a distinct eruptive center and each center consists of present. many individual flows, each of which was fed by an essentially The four mesa-forming basalts in the Castañeda Hills area can homogenous small magma chamber. be distinguished by the presence or absence of key minerals. All

contain 5% to 15% phenocrysts of sieve-textured and euhedral to Megacryst-Beairing Basalts (Tbmx) rounded plagioclase (avg Anso) and partially to completely id- dingsitized olivine in an intergranular, intersertal, or diktytaxitic More than 20 megacryst-bearing basalt flows occur through- groundmass composed of plagioclase, clinopyroxene, and opaque out the Castañeda Hills area. Some older flows form low mesas minerals. One flow lacks groundmass olivine, another lacks clino- and flowed oblique to the present drainage systems. Younger flows pyroxene phenocrysts, and a third contains abundant glass. Some are topographically low and follow present topography. The vents of the differences in texture and mineralogy probably were caused for many of these flows are small, eroded scoria cones or steeply

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TABLE 2. (Continued)

Mesa-forming Megacryst-bearing basalts basalts Sample CHSE- AP-02 CHSE- CH-28 ILCH- ILCH- CH-78 CH-92 CH-96 CH-109 ILCH- oxide 164 166 77-8 77-22 77-41

Si02 51.38 58.10 45.91 47.54 47.52 48.96 47.53 48.78 47.07 47.13 45.69

AI2O3 16.87 15.75 17.18 16.54 16.86 17.28 17.43 17.54 17.18 17.02 14.85 Fe203 3.85 2.02 5.30 8.45 4.50 4.73 6.21 3.86 7.90 6.43 6.85 FeO 5.34 4.76 6.12 3.85 5.78 6.18 4.46 6.80 3.56 5.70 5.13 MGO 6.57 4.49 6.73 5.44 6.90 6.22 6.21 5.80 5.63 5.96 11.49 CaO 8.85 6.77 10.90 9.61 9.11 9.19 7.96 9.99 7.95 9.81 8.86 Na20 3.18 3.27 3.06 3.18 3.82 3.22 3.73 3.26 3.72 3.22 2.58 K2O 1.18 2.56 0.64 1.05 1.24 1.21 1.93 1.07 1.83 0.78 0.34 + H2O 0.25 0.74 0.69 0.72 0.95 0.69 0.97 0.27 0.95 0.97 1.30 H2O- 0.31 0.38 0.60 0.93 0.33 0.55 0.62 0.27 0.52 0.48 0.89 Ti02 1.34 0.82 2.16 2.29 2.06 1.94 2.17 2.05 2.19 2.44 1.42 p2o5 0.34 0.15 0.41 0.38 0.53 0.34 0.54 0.40 0.48 0.49 0.19 MNO 0.15 0.11 0.16 0.18 0.17 0.16 0.17 0.16 0.16 0.15 0.14 co2 0.19 0.32 0.49 0.19 0.06 0.08 0.26 0.17 0.21 0.04 0.18 Total 99.80 100.24 100.35 100.35 99.83 100.75 100.19 100.42 99.35 100.62 99.91 q 1.59 8.79 1.42 c or 6.99 15.09 3.77 6.18 7.34 7.10 11.38 6.30 11.05 4.65 2.06 ab 26.96 27.60 25.80 26.81 28.94 27.04 31.48 27.47 32.16 27.48 22.34 an 28.33 20.69 31.14 27.66 25.24 28.91 25.07 29.94 25.31 29.93 28.59 ne 1.86 0.01 wo 5.11 4.10 7.09 6.76 6.76 5.70 3.84 6.57 4.35 6.54 5.83 en 16.40 11.16 7.58 13.50 5.00 11.89 3.31 8.99 4.54 14.08 17.76 fs 4.70 5.91 1.62 1.11 3.48 3.87 1.33 1.04 fo 6.39 8.56 2.44 8.50 3.78 6.86 0.62 8.08 fa 1.51 2.09 0.79 1.79 0.07 0.52 mt 5.59 2.92 7.66 6.34 6.54 6.81 8.62 5.57 5.77 9.40 10.16 hm 4.05 0.25 4.09 il 2.55 1.55 4.09 4.33 3.92 3.66 4.11 3.88 4.25 4.67 2.76 ru ap 0.81 0.35 0.97 0.90 1.26 0.80 1.28 0.94 1.16 1.17 0.46 cc 0.43 0.73 1.11 0.43 0.14 0.18 0.59 0.39 0.49 0.09 0.42

Fe203 0.72 0.42 0.87 2.19 0.78 0.77 1.39 0.57 2.22 1.13 1.34

FeO m FeO a 9.02 6.17 11.51 14.01 10.21 10.80 11.50 10.11 9.30 9.86 9.84 Fe203 a 1.80 1.23 2.30 2.80 2.04 2.16 2.30 2.02 1.86 1.97 1.97 Mg values 56.49 56.47 51.03 40.90 54.64 50.66 49.04 50.56 51.90 51.86 67.54

dipping dikes that strike N10° to 15°E. The rocks are characterized iddingsite and euhedral to rounded and embayed. The rare spinel by large (1 to 8 cm long), rounded megacrysts of plagioclase and and opaque megacrysts are also rounded and embayed. The pyroxene with smaller olivine, spinel (pleonaste), and magnetite in a groundmass texture of most of the megacryst-bearing flows is inter- black, fine-grained matrix. The silicate megacrysts have a vitreous granular with small clinopyroxene and opaque grains poikilitically luster, conchoidal fracture, and poor cleavage. The megacrysts enclosed in larger plagioclase laths. Altered glass is locally present range downward in size to that of phenocrysts or large, ground- in trace amounts. A dark mica (biotite or phlogopite) is a minor mass-sized crystals; hence, the distinction between megacryst and interstitial phase in some of the basalts. Calcite and/or clay miner- phenocryst or microphenocryst is arbitrary. The amount of meg- als typically fill vesicles and vugs. acrysts and the proportions of different megacrysts vary widely between flows; however, the total volume of megacrysts in the flows Volume of Volcanic Rocks probably does not exceed 1% or 2%. In thin section, plagioclase megacrysts (An3o to Anyo) are typi- On the basis of detailed geologic mapping (Lucchitta and cally rounded, with a slightly zoned interior to strongly zoned mar- Suneson, unpub. data), the volumes of the five volcanic units were gin. Normal and reverse zoning are common. The cores of most estimated. The estimates are probably minimum values because individual megacrysts are uncorroded and surrounded by a sieve- (1) in many areas, the base of the unit is not exposed, (2) rocks textured zone, which, in turn, is surrounded by a thin, uncorroded outside the study area were not included, and (3) the present out- rim. Clinopyroxene megacrysts are highly rounded, and some are crop area of the units was used in the calculations rather than the partially seive-textured. Olivine megacrysts are generally altered to original extent. However, these minimum values are tempered by

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(1) using maximum exposed thicknesses of units in the calculations rocks containing between 58.1 and 71.6 wt % silica are absent. and (2) assuming these thicknesses are true and not increased by Therefore, the volcanic rocks in the Castañeda Hills area represent repetition of section through faulting. Despite these limitations, the a bimodal suite similar to those described by Christiansen and Lip- following estimates (in km3) show that the volume of basalt man (1972) in the Basin and Range province. rhyolite lavas, and tuff are of the same order of magnitude: older One approach to determining the origin of basalts is based on basalts, 8; quartz-bearing basalts, 0.2; silicic volcanic rocks, 34; Mg-values. Irving and Green (1976) showed that basaltic liquids in mesa-forming basalts, 3; megacryst-bearing basalts, 0.4. Approxi- equilibrium with undepleted mantle peridotites have Mg-values +2 mately 12 km3 of basalt was erupted in the Castañeda Hills area at [= 100 Mg/(Mg+ Fe )] between 66 and 75. The mafic volcanic rocks about the same time (middle to late Miocene) as was 34 km3 of in the Castañeda Hills area have low Mg-values (40.8 to 56.5 using rhyolite. FeO values adjusted to Fe2C>3/FeO = 0.2) (Table 2). This suggests that these basalts are not primary liquids derived from undepleted PETROLOGY AND CHEMISTRY mantle lherzolite. Two possible explanations for the low Mg-values of the Castañeda Hills mafic volcanic suite can be eliminated using The mafic volcanic rocks in the Castañeda Hills area are petro- Sr isotopic and experimental evidence. (1) Sr isotopic evidence (dis- graphically similar to upper Cenozoic basalts found in the Basin cussed below) indicates that upper-crustal contamination of and Range province (Leeman and Rogers, 1970; Best and Brimhall, mantle-derived partial melts is unlikely. (2) A depleted mantle 1974). These rocks contain groundmass olivine, calcic clinopyrox- caused by a previous partial melting event would be enriched in Mg, ene (commonly Ti-rich), and calcic andesine to sodic labradorite. so that subsequent partial melts would have higher Mg-values. The Hence, most basalts in the Basin and Range province are alkali abundance of phenocrysts, especially olivine, in the basalts suggests basalts as defined petrographically by Yoder and Tilley (1962), de- that crystal fractionation probably occurred. The crystallization spite the absence of nepheline or olivine in the norm. and removal of olivine enriched in Mg relative to the host magma Analyses of the Tertiary volcanic units (Table 2) show that would result in lowered Mg-values. Much recent work on the origin of rhyolites relies on II trace-element data (Hildreth, 1981). These data are lacking for the 10 rhyolites in the Castañeda Hills area. Nevertheless, the major- 9 element compositions indicate that most of these rocks are proba- 8 bly similar to rhyolites that are part of bimodal suites elsewhere in the Basin and Range province. An important feature of the major- 7 element chemistry of some of the rhyolites in the study area is the O 6 V A OJ considerable scatter in the NajO and K.2O values, but less scatter in * 5 the Na2Ü + K2O values (Fig. 4). If one assumes that K2O approxi- 4 0 mately equaled NajO in the rhyolitic magma (K.20/Na20 = 0.5 to 3 2 I 0 A

4 <> % > O 3 X CM • < 2 >

O 4 > CM < 0

O eCM

-a—, , , 44 48 52 56 60 64 68 72 76 80 SLO.

Figure 4. K2O, Na20, and total alkalies versus SÍO2 of volcanic rocks from the Castañeda Hills area. Symbols: (> , older basalts; O , quartz-bearing basalts; A > rhyolites, Aubrey Hills; , Figure 5. Rhyolites from the Castañeda Hills area plotted on rhyolites. Castañeda Hills; £> rhyolites, Bill Williams Moun- normative quartz-albite-orthoclase triangle. Minimum melting tains; X , mesa-forming basalts; • , megacryst-bearing basalts. points from Tuttle and Bowen (1958). Symbols same as in Figure 4.

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2), Figure 4 suggests that K.2O replaced Na2Ü in a base exchange implication that the basalts containing the megacrysts were proba- reaction (Lipman, 1965, p. 19) during primary crystallization, bly generated below the crust is further supported by the chemical hydration, secondary devitrification, or leaching by ground water. similarity of the megacrysts to minerals in ultramafic nodules of This substitution of K for Na also suggests that the excess K2O is presumed mantle origin. not incorporated in vapor phase minerals found in vugs and miaro- litic cavities in the rhyolite, which were avoided when preparing the Plagioclase Megacrysts samples for analysis. Figure 5 is a plot of normative quartz and feldspar composi- Molecular percentages of Or, Ab, and An of the plagioclase tions for the rhyolites in the Castañeda Hills area in the Q-Ab-Or megacrysts from the megacryst-bearing basalts are plotted in Figure triangle. Also shown are the ternary eutectics at different PH20 6. Although most of the plagioclase megacrysts are calcic labrador- values as determined by Tuttle and Bowen (1958). Whereas rhyo- ite or sodic bytownite, their composition ranges from about An23 to lites from the Castañeda Hills area with a K.20/Na20 ratio less than An93. The megacrysts are both normally and reversely zoned. A 5%

2 plot near the 1,000 or 500 bar PH2O eutectic, rhyolites high in K.2O difference in An content between crystal rim and center is common, do not plot near the eutectics. In general, rhyolites with the highest but a few samples show as much as a 30% difference. The composi- K.20/Na20 ratios plot farthest away from the eutectics. Thus, most tion of plagioclase megacrysts does not vary systematically from of the high-K^O rhyolites plot far outside the contours drawn by flow to flow. Tuttle and Bowen (1958, p. 41) that include all of the analyzed rocks in Washington's Tables. This implies that, whereas the chemical compositions of the more "normal" rhyolites were probably con- trolled by crystal-liquid or liquid-state differentiation, the same origin cannot be used to explain the high-K20 rhyolites. This sup- ports the suggestion based on the K.2O, Na20, and total alkalis versus silica diagrams that the high-K^O rhyolites do not represent original magma compositions. The high-K^O rhyolites in the Castañeda Hills area have these characteristics: (1) all are from shallow intrusives; (2) several of these intrusives have hydrated glassy margins; (3) all of the analyzed high-K^O rhyolites are crystalline; and (4) petrographic evidence of K replacing Na (for example, potassic feldspar rims on plagioclase phenocrysts) is absent. These characteristics indicate that K replaced Na during primary crystallization (as defined by Lipman, Figure 6. Feldspar composition triangle with plagioclase meg- 1965) of rhyolite plugs and that most of the K is concentrated in the acryst analyses from the Castañeda Hills area. Field boundaries and groundmass crystals. The glassy margins of the intrusives would not names from Irving (1974). be preserved if secondary devitrification had occurred, and leaching or secondary hydration occurs primarily in glassy rocks. Agron and Irving (1974) has summarized analyses of feldspar megacrysts Bentor(1981) proposed an almost identical mechanism, which they from alkaline basalts. The megacrysts range in composition from termed "potassium metasomatism," to explain the high-K lavas, labradorite to sanidine; most plot in the andesine and anorthoclase tuffs, and hypabyssal porphyries of the Precambrian Biq'at Haya- fields on the ternary composition plot for feldspars. The type of reah volcanic massif in the Sinai-Negev Desert. feldspar megacryst appears to be related to the K2O content of the There is some experimental evidence to support this hypothe- basalt host rock. Binns and others (1970) reported anorthoclase- sis. At temperatures of 250 to 700 °C, K is preferentially partitioned bearing lavas with 1.52% to 3.41% K2O values, and Laughlin and into alkali feldspars (Orville, 1963). Fluids rich in both Na and K. others (1974) reported K2O values of 1.6 to 2.7 for anorthoclase- are likely to be present in a slowly crystallizing rhyolite intrusive. bearing lavas in New Mexico and Arizona. Anorthoclase meg- These migrating alkali-rich fluids would ensure constant levels of acrysts are contained in basaltic lavas with 1.90% to 3.37%'K2O in both alkalis in the glass, but only K would be concentrated in the southeastern Arizona (Evans and Nash, 1979). In contrast, the crystalline products (Kesler and Weiblen, 1968). Stewart (1979) basaltic lavas of the Pinacate have 0.90% to 1.71% proposed a similar mechanism to explain the production of high- K.2O and contain only plagioclase megacrysts (Gutmann, 1977), and K2O magmas. The substitution of K for Na in some of the Cas- plagioclase megacryst lavas from Nigeria contain 1.54% to 1.93% tañeda Hills area rhyolites is different from the slight Na-enrich- K20 (Frisch and Wright, 1971). In the Castañeda Hills area, the ment effect noted by Hildreth (1981) in the Bishop Tuff. This may basalts contain 0.64% to 1.93% K2O, which is consonant with a be due to liquid-state enrichment in the tuff and solid-state "altera- plagioclase composition of the megacrysts. tion" in the rhyolites described in this report. Pyroxene Megacrysts CHEMISTRY OF MEGACRYSTS IN BASALTS Analyses of pyroxene megacrysts are plotted in the pyroxene Electron microprobe analyses of megacrysts in the megacryst- quadrilateral in Figure 7. Most of the pyroxene megacrysts contain bearing basalts (Table IV in Suneson, 1980) are similar to those of considerably more AI2O3 and slightly more TÍO2 than typical igne-

megacrysts from other localities around the world that are comT ous pyroxenes (Deer and others, 1966, p. 120). monly regarded as high-pressure assemblages (Irving, 1974). The The megacrysts are slightly to strongly zoned with respect to

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certain oxides. In general, the margins of megacrysts are enriched in that an increase in Ca is due to crystallization under conditions of Ti and Ca and depleted in Mg and Na with respect to cores. AI, Cr, decreasing pressure. This relation, if applicable to Ca content in and Fe do not appear to vary systematically from core to margin. olivine megacrysts and phenocrysts in alkaline basalts, would imply The decrease in Mg from core to rim probably results from crystal- that the Ca-poor (Mg-rich) olivines crystallized at higher pressures lization under decreasing temperature. The increase in Ti and than did the Ca-rich olivines. decrease in Na at the rim may be the result of pyroxene crystalliza- Olivine is a common constituent of ultramafic inclusions in tion under conditions of decreasing pressure (Wass, (979). basalts but is not common as a megacryst. The only published Although a decrease in silica activity during clinopyroxene crystal- studies of olivine megacrysts in basaltic lavas (Evans and Nash, lization could also result in continuous zoning from core to rim, the 1979; Binns and others, 1970) indicate that the compositions of Ti-rich rims suggest that a decrease in asioj is less important in olivine megacrysts can vary widely and can be similar to those of controlling the chemical composition of the megacrysts than is a olivines in either the Cr-diopside or Al-augite series.

decrease in Ptotai- The pyroxene megacrysts in basalts from the Castañeda Hills Spinel Megacrysts area are similar in composition to clinopyroxene crystals in ultra- mafic inclusions of the Al-augite series of Wilshire and Shervais Spinel megacrysts from the megacryst-bearing basalts include (1975) or in the group II inclusions of Frey and Prinz (1978). Textur- ferrian pleonaste and titanomagnetite. The end-member composi- al relationships and melting experiments suggest that inclusions tions of the pleonastes are approximately 90% spinel and hercynite containing Cr-diopside are accidental xenoliths derived from the and 10% magnetite and ulvospinel. The pleonastes are essentially upper mantle (Frey and Green, ¡974), wheieas Al-augite represents unzoned, although Ti and Mg may decrease slightly from center to fragments of dikes that crystallized in the upper mantle (Wilshire rim. and Shervais, 1975) or are high-pressure cumulates (Wilkinson, The pleonaste megacrysts are similar in composition to meg- 1975). The Al-rich and Cr-poor clinopyroxene megacrysts from the acrystic pleonastes reported from Nigeria (Frisch and Wright, Castañeda Hills area are probably similar in origin to inclusions of 1971), Australia (Binns and others, 1970), southeastern Arizona the Al-augite series. (Evans and Nash, 1979), and Algeria (Conquere and Girod, 1968), They also resemble those found in the group II ultramafic inclu- Olivine Megacrysts sions described by Frey and Prinz (1978). The megacrysts contain considerably less C^Oj (0.10 to 4.7 versus 2.7 to 47.9 wt %) and Large, megacrystic olivine crystals are rare or absent in most of more Fe, AI, and Mg than do spinels in the group I inclusions. the basalt Hows in the Castañeda Hills area. Those that were ana- The titanomagnetite megacrysts contain 55% to 40% ulvo- lyzed vary in composition from Fogg to F045 (Fig. 7). This large spinel and 20% to 55% magnetite. The titanomagnetite megacrysts range in composition may reflect differences in the pressure of crys- are slightly zoned; Ti increases and AI decreases from core to rim. tallization. The chemical composition of some of the olivines indi- Fe, Mn, and Mg show no detectable systematic variation across the cates high-pressure crystallization. In fact, many "megacrysts" of crystals. There is no systematic difference in the chemical composi- high-pressure origin can be distinguished from "large phenocrysts" tion of titanomagnetite megacrysts between flows. of lower-pressure origin only on the basis of chemical composition. The titanomagnetite megacrysts in the Castañeda Hills area are Olivine megacrysts from some flows are more Mg- and Ni-rich and similar to opaque megacrysts in southeastern Arizona (Evans and contain less Ca than those from other flows. AH olivine megacrysts Nash, 1979), Nigeria (Frisch and Wright, 1971), and Australia show an increase in Ca from core to rim. Stormer (1973) suggested (Binns and others, 1970). All are Cr-poor, with 15% to 30% TÍO2, 1% to 10% AI2O3, and 1% to 5% MgO. The Castañeda Hills meg- acrysts are slightly more Fe-rich than the others, with FeO values ranging as high as about 78%. WO WO Opaque minerals are generally absent or unreported in ultra- mafic inclusions; the only reported analysis is that of an ilmenite (52.1 wt % TÍO2, 38.2 wt % FeO) in a group II inclusion from San Carlos, Arizona (Frey and Prinz, 1978). This ilmenite clearly con- tains more Ti and less Fe than do typical titanomagnetite megacrysts. In summary, the megacrysts from the megacryst-bearing basalt flows and plugs in the Castañeda Hills area are chemically similar to megacrysts found elsewhere. In addition, the clinopyroxene (titaniferous Al-augite) and spinel (ferrian pleonaste) megacrysts are chemically similar to those found in ultramafic inclusions of the Al-augite series of Wilshire and Shervais (1975). The plagioclase megacrysts in the Castañeda Hills area are chemically similar to megacrysts reported from other KzO-poor alkalic basalts. The cli- nopyroxene megacrysts show the effects of continued crystalliza- tion under conditions of decreasing temperature and pressure. Figure 7. Pyroxene composition triangle with pyroxene and Some of the olivine megacrysts appear to be of a high-pressur olivine megacrvst analyses from the Castañeda Hills area. Field origin; others appear to be large phenocrysts that crystallized nea boundaries and names from Deer and others (1966). the surface.

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TABLE 3. Rb-Sr ANALYTICAL DATA FOR BASALTS AND RHYOLITES, CASTAÑEDA HILLS AREA

87 86 87 86 Sample Unit K Cone. Rb Cone, Sr Cone, Sr/ Sr0* Sr/ Sr; ±2 0 no. (ppm) (ppm)

CH-61, WR Older basalts 2.403% 21.6 ± 0.6 928 ± 13 .7076 .7076 .0001 .7078 .7078 CHSE-164, WR Mesa-forming basalt 1.153% 17.0+ 1.1 546 ±4 .7062 .7062 .0001 CHNW-156D, San Rhyolite 12.10% 50.3 ± 6.8 529 ± 7 .7094 .7094 .0001 .7092 .7092 AP-04D, San Rhyolite 11.44% 138 ± 12 18.81 ±0.03 .7179 .7141** .0003 .7180 .7142** CH-588, WR Quartz-bearing 1.849% 24.2 ± 0.3 368 ± 1 .7034 .7034 .0002 basalt IL-CH-77-22, WR Megacryst-bearing n.a. 20.6 ± 0.7 466 ± 3 .7038 .7038 .0002 basalt IL-CH-77-8, WR Megacryst-bearing 1.135% 13.1 ± 0.4 653 ± 6 .7035 .7035 .0004 basalt •Corrected to NBS987 = .710145 (Moore and others, 1973). Duplicate analyses shown. "Corrected for .2128 g Sr contamination factor = -.0001.

Pedogenesis STRONTIUM ISOTOPE GEOCHEMISTRY

Several lines of evidence suggest that the megacrysts have a Seven samples, representing each of the volcanic units in the high-pressure origin. (1) Megacrysts are commonly associated with Castañeda Hills area, were selected for Sr isotopic analysis (Table mantle-derived ultramafic nodules, which suggests that the meg- 3). For the rhyolites, sanidine phenocryst separates were analyzed acrysts and basaltic host rock are also mantle-derived (Irving, 1974; to minimize the effects of addition of Sr by ground water with an Binns and others, 1970). (2) Marginal corrosion of the megacrysts isotopic composition different from that of the rock. For the indicates that they were out of equilibrium with the host magma at basalts, whole-rock samples were analyzed. near-surface pressures. (3) The predominance of clinopyroxene Three basalt analyses average .7036 ± .0002 (one quartz- over olivine is significant in terms of pressure of formation. Exper- bearing and two megacryst-bearing basalts) and two others (an imental work by Green and Ringwood (1967) showed that, at 9 older and a mesa-forming basalt) are significantly higher (.7062 and kbar, olivine is the liquidus phase in an alkali-olivine basaltic liquid, .7077). The three basalts with low initial 87Sr/86Sr ratios are isoto- followed by clinopyroxene. At 13.5 kbar, orthopyroxene and clino- pically similar to other upper Tertiary basalts in the southern Great pyroxene are the first phases to crystallize. At 18 kbar, clinopyrox- Basin and Mojave Desert area and probably represent magmas ene is the liquidus phase. Therefore, the abundance of clino- derived from partial melting of suboceanic-like mantle and not con- pyroxene megacrysts, the scarcity of olivine megacrysts, and the taminated with crustal material (Leeman, 1982). These basalts have absence of orthopyroxene megacrysts suggest that some of the 87Sr/86Sr¡ values ranging from approximately .7025 to .7045 megacrysts crystallized at pressures greater than 18 kbar (70 km). (Hedge and Noble, 1971; Leeman, 1970). Such low isotopic ratios (4) The presence of spinel megacrysts indicates that the magma was also characterize basalt on the western edge of the Colorado Pla- generated at pressures greater than about 8 kbar, the lower stability teau north of the Grand Canyon (Leeman, 1974). The analyzed limit of spinel peridotites (Kushiro and Yoder, 1966). (5) The clino- mesa-forming basalt and older basalt have ratios of .7062 and pyroxenes are Al-rich, with most of the A1 contained in the Ca- .7077, respectively, which are similar to those of basalts from Tschermak molecule (4.4% to 15.8%). The enrichment of the southwestern Nevada and east-central California. Models for the Ca-Tsch component in clinopyroxenes is a result of crystallization origin of these radiogenic basalts include (1) contamination of at high pressures (Aoki and Kushiro, 1968). (6) The high AI2O3 mantle-derived basalt by crustal material; (2) derivation from the content of the titanomagnetite megacrysts also suggests a high- lower crust; and (3) partial melting of old, isotopically closed man- pressure origin (Binns and others, 1970, p. 158). However, the pres- tle peridotite. The origin of the radiogenic basalts in the Castañeda ence of plagioclase megacrysts implies that crystallization of at least Hills area in terms of these models is discussed below. some megacrysts occurred at depths less than 30 km (8 kbar), the 87Sr/86Sr¡ versus Sr and Rb/Sr were plotted (Fig. 8) to evalu- upper stability limit of plagioclase peridotites. Equilibration pres- ate the role of crustal contamination in generating CH-61 (older sures ranging from 8 to 18 kbar represent depths of 30 to 70 km. basalt) and CHSE-164 (mesa-forming basalt). If magma with an Crustal thickness in the Basin and Range province ranges from 87Sr/86Sr¡ ratio of .7036 were contaminated by crustal material with about 20 km in the Imperial Valley region in southern California to a high Rb content, high Rb/Sr ratio, and high 87Sr/86Sr ratio, about 35 km along its eastern and western margins (Thompson and plots of87Sr/86Sr¡ versus Sr would show an inverse correlation and Burke, 1974). The chemical compositions of the megacrysts there- 87Sr/86Sr¡ versus Rb/Sr a positive correlation. A systematic corre- fore indicate that the megacryst-bearing basalts were derived from lation between these values is not evident (Fig. 8), but if it exists at the upper mantle, a conclusion supported by the Sr isotopic evi- all, it is the reverse from what would be expected if a magma with dence given below. 87Sr/86Sr = .7036 were contaminated with Rb-enriched material. If

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basalt with an 87Sr/86Sr ratio of .7036 were contaminated by There are several problems with the hypothesis that some of 1.3-b.y.-old crust (approximate age of Precambrian in the the basalts in the Castaneda Hills area were derived from the lower Castaneda Hills area), the product should plot on the 1.3-b.y. line crust. Although the composition of the lower crust in this part of shown in Figure 8A or on a line parallel to it. This does not occur. the southern Basin and Range province is unknown, it is possible Hence, a simple bulk-contamination model does not appear to that it is composed of mafic granulite gneisses with or without explain the Sr-isotope geochemistry of the basalts in the Castaneda anorthosites and mafic-ultramafic bodies (Fountain and Salisbury, Hills area. 1981). However, these rocks would have to be almost completely melted to generate basaltic magmas. Leeman's (1970) model also requires an early partial melting event; however, there is little evi- .07 dence for such an event in the area. The removal of silicic and intermediate material from the lower crust may be related to the .06 development of the Harcuvar-Buckskin metamorphic core com- .05 plex. Mesozoic(?) plutonic rocks constitute part of the complex RB .04 (Shackelford, 1976; Rehrig and Reynolds, 1980), but the geochem- SR istry of the plutonic rocks and their relation to Tertiary volcanism .03 are unknown, and so the hypothesis cannot be verified. .02 Leeman (1977, 1982) later suggested that many of the radio- .01 genic basalts in the Basin and Range province were derived by partial melting of old (more than 1 b.y.) mantle material ("ancient 900 sub-continental lithospheric mantle") with a higher Rb/Sr value 800 than oceanic-type mantle. He noted that the Sierran subprovince in 700 southern Nevada and eastern California may be underlain by old SR mantle material that partially melted to form the high 87Sr/86Sr ( ppn> ) goO basalts that erupted there in the late Cenozoic. Following Leeman's 500 (1977, 1982) ideas, a possible explanation for the isotopically differ- ent basalts in the Castaneda Hills area is that the older and mesa- 400 forming basalts (high 87Sr/86Sr) are partial melts of old mantle 300 material, whereas the quartz- and megacryst-bearing basalts are .7032 .7040 .7048 .7056 .7064 .7070 .7078 .7086 derived from "young," oceanic-type mantle. This implies that both 87SR/É'86SR , types of mantle are present in the same area and that vertical het- erogeneity within the mantle may be as important as its lateral Figure 8. 87Sr/86Sr¡ versus Rb/Sr and Sr for basalts from the heterogeneity. Castaneda Hills area. Symbols same as in Figure 4. The depth of partial melting required to produce the isotopi- cally different basalts is unclear. Leeman (1977) suggested that the Any model used to explain the Sr isotopic composition of the radiogenic alkalic basalts typical of oceanic islands were generated basalts in the Castaneda Hills area must account for the eruption of at greater depths than were mid-ocean ridge basalts with low lavas with significantly different 87Sr/86Sr ratios at approximately 87Sr/86Sr values. If the source regions for the basalts in the Basin the same time and in the same place. The older basalts are more and Range province are analogous to those in the ocean basins, radiogenic than the youngest basalts, but the difference in their ages the old, subcontinental lithospheric mantle would underlie is only about 10 m.y. The quartz-bearing basalts and some of the "younger," oceanic-type mantle. It is difficult to visualize how mesa-forming basalts are nearly contemporaneous (13 m.y. old) but replenished and presumably convected mantle could overlie unde- have significantly different Sr isotopic ratios. (.7034 and .7062, pleted and "stagnant" mantle. A more likely explanation for the respectively). This indicates that the source region of the basalts nearly simultaneous eruption of high- and low- 87Sr/86Sr basalts beneath this part of the southern Basin and Range province is in the Castaneda Hills area is that the latter were derived from deep, strongly heterogeneous with respect to strontium isotopes. upwelling asthenospheric mantle with a low Rb/Sr ratio and the Leeman (1970) proposed a model in which radiogenic late former from old, subcontinental lithospheric mantle that was Cenozoic basalts were derived from a previously partially melted "welded" onto the lower crust more than 1 b.y. ago. mafic lower crust. Such a partial melting event would have removed The origin of the quartz-bearing basalts merits discussion. the silicic and intermediate fraction of the lower crust and lowered Quartz-bearing basalts and basaltic andesites have erupted in a its Rb/Sr ratio. He suggested that the remaining mafic fraction, variety of tectonic environments in the western United States in the with low Rb/Sr and high 87Sr/86Sr ratios, was partially melted and Cenozoic. The compositional and petrographic similarity of these erupted during the Miocene. Although the Sr isotopic composition rocks suggests a common origin (Best and Brimhall, 1974), but of pre-mid-Miocene volcanic rocks in this part of the Basin and there is no consensus on this problem. The low 87Sr/86Sr¡ isotopic Range province is unknown, 87Sr/86Sr¡ values from Oligocene and ratio of quartz-bearing basalts precludes an origin that involves early Miocene volcanic rocks from southern Arizona range from significant contamination by old crustal rocks. In the Castaneda .7069 to .7096 (Shafiqullah and others, 1978). These values are Hills area, the qaartz-bearing basalt (CH-58B) has an 87Sr/86Sr¡ similar to those of the radiogenic basalts in the Castaneda Hills ratio of .7034, identical to that of the mantle-derived megacryst- area. Shafiqullah and others (1978, p. 239) concluded that partial bearing basalts. Contamination by nonradiogenic younger (Meso- melting of lower crustal rocks could produce the isotopic ratios zoic and Paleozoic) rocks would be difficult to detect isotopically. observed. Contamination by these sources is unlikely, however, because

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Mesozoic and Paleozoic rocks occur only as thin tectonic slices in magma. In either case (partial melting of granulite or contamina- the Castaneda Hills area, so that rising magma would have little tion by deep ), the rhyolites were derived by lower-crustal contact with them. Furthermore, these slices are absent in many processes. areas where the quartz-bearing basalts crop out. Like other quartz-bearing mafic volcanic rocks in the western SUMMARY AND IMPLICATIONS United States, those in the Castaneda Hills area contain olivine. The coexistence of quartz and olivine may be explained in two The basalts and rhyolites in the Castaneda Hills area are petro- ways: (1) Nicholls and others (1971) showed that quartz can crystal- graphically, chemically, and isotopically similar to other bimodal lize from basaltic liquids at pressures between 15 and 25 kbar. This volcanic rocks in the southern Basin and Range province. The corresponds to 55 to 85 km, clearly within the mantle. As noted by megacryst-bearing and quartz-bearing basalts probably were de- Stormer (1972), olivine may crystallize at lesser depths, where rived from mantle sources and underwent crystal fractionation, quartz might remain as incompletely resorbed refractory pheno- most likely at several levels, within the mantle and lower crust. The crysts. (2) The upper mantle beneath the southern Basin and Range most probable source is a low-Rb/Sr asthenospheric mantle. The province may be partly composed of eclogite (Hausel and Nash, more radiogenic basalts, represented by one sample each of the 1977), Wyllie (1971) showed that at pressures greater than 20 kbar, older basalts and of the mesa-forming basalts, probably were garnet + clinopyroxene + quartz + liquid is the stable assemblage of derived from higher levels in the mantle, although interaction with an eclogitic composition in the presence of less than .5% water. If lower-crustal mafic granulite cannot be ruled out. This upper- quartz remains a refractory phase under decreasing pressure, the mantle source region has a higher Rb/Sr value and is older ("stag- liquid composition may become more mafic, allowing olivine to nant") than the underlying mantle. Leeman (1977, p. 157) termed crystallize. this, "ancient sub-continental lithospheric mantle." The rhyolites The initial Sr isotopic ratios of two rhyolites in the Castaneda probably represent the low-melting fraction of lower-crustal granu- Hills area are .7093 and .7141. These are slightly higher than those litic rocks. Some of the rhyolite plugs underwent near-surface determined by Rehrig and others (1980) for rhyolites from the Vul- replacement of Na by K, probably during initial cooling. ture Mountains (.7083 and .7089) and significantly higher than Late Tertiary melting of upper-mantle and lower-crustal rocks those of post-mid-Miocene rhyolites from the northern Basin and began in this part of the southern Basin and Range province about Range (.7023 to .7057) (Noble and others, 1973). They are also 18 m.y. ago. The production of basaltic and rhyolitic magmas was significantly higher than the isotopic ratios of nearly coeval basalts. the result of a heating event that is probably associated with basin- The different ratios of the basalts and rhyolites indicate that the range tectonism throughout the western United States. This heating rhyolitic magmas were not derived from basaltic magmas by differ- produced basaltic magmas by partial melting in a vertically hetero- entiation, provided that there was no contamination, and assuming geneous upper mantle and rhyolite magmas by partial melting of there are no high 87Sr/86Sr basalt intrusives at depth. Radiogenic undepleted lower crust. The mechanism of Tertiary magma genera- basalts that could have differentiated to produce the rhyolites may tion in the Castaneda Hills area is essentially identical to that de- have crystallized at depth, but there is no evidence for them. The scribed by Hildreth (1981, Fig. 15A), except that basalt was locally high ratios of the rhyolites suggest a crustal source region. Assum- erupted within and between the "rhyolitic dome clusters." On the ing a 1.3-b.y. age and 87Sr/86Srj = .704 for the basement rocks in basis of the Sr isotopic work reported here, the vertical scale on this area (Marvin and Cole, 1978), Rb/Sr ratios of .10 and .19 Hildreth's (1981) Figure 15A would be changed to show rhyolite would be required to generate the observed Sr isotopic ratios of magma being generated in the lower crust. However, the volcanism .7093 and .7141, respectively, if the rhyolites are partial melts of can be viewed as fundamentally basaltic. It is likely that the partial those basement rocks. The Rb/Sr ratios of granites similar to those melting in the lower crust that produced the rhyolitic magmas was exposed in the Castaneda Hills area (I. Lucchitta and N. Suneson, caused by intrusion of mantle-derived basalt dikes. The extensional unpub. data) range from 1.4 to 25.6 (Marvin and Cole, 1978) and stress regime that accompanied the suppressed mixing thus are too high to produce the isotopic ratios observed in the of the rhyolite and basalt (Hildreth, 1981, p. 10178). The basalts rhyolites. This implies that the rhyolites were not derived from and rhyolites that reached the surface thus form a bimodal suite upper-crustal sources. Lower-crustal granulitic rocks, however, are typical of middle and late Miocene volcanic suites in the western a possible source if they are present in this part of the Basin and United States. Range. The Rb/Sr ratio of granulites ranges from about .01 to .25 (Heier, 1964). Thus, 1.3-b.y.-old granulite with Rb/Sr ratios of ACKNOWLEDGMENTS about .10 to .19 could provide the source material for the rhyolites. The lowest melting fraction of such a granulite would probably be a This paper is a summary of the senior author's Ph.D. disserta- liquid of rhyolitic composition. tion at the University of California, Santa Barbara. Special thanks Alternatively, the rhyolites may result from contamination by are extended to J. M. Mattinson, W. S. Wise, R. V. Fisher, and G. old upper-crustal rocks of a rhyolitic differentiate of basalt. From K. Van Kooten (University of California, Santa Barbara) for many 16% to 30% contamination of a rhyolitic liquid (87Sr/86Sr = .7034) thought-provoking discussions. B. M. Myers, J. C. Von Esson, and with Dells granite (Marvin and Cole, 1978), which is probably sim- E. H. McKee(U.S. Geological Survey) are gratefully acknowledged ilar to that in the Castaneda Hills area, is required to produce the for their help with the K-Ar part of the project. Chemical analyses observed isotopic ratios in the rhyolites. However, the heat content were done by M. Taylor, P. Klock, H. N. Elsheimer, M. Villareal, of the rhyolite magma is probably insufficient to partially melt the and J. H. Tillman (U.S. Geological Survey). The paper benefited granite, remain liquid, and continue to rise. This suggests that con- greatly from reviews by M. A. Kuntz, E. W. Wolfe, B. M. Crowe, tamination could occur only at relatively great depths with the and S. H. Evans, Jr. granite near its melting temperature before intrusion of the rhyolite The work was conducted under the auspices of the U.S. Geo-

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logical Survey's Western Arizona Tectonics Project. The field work Science, v. 277, p. 833-861. on which this paper is based was carried out jointly by the authors Hamilton, W., 1965, Geology and petrogenesis of the Island Park of rhyolite and basalt, eastern Idaho: U.S. Geological Survey Professional over several years. Additional financial support included a Geologi- Paper 504-C, 37 p. cal Society of America Penrose Grant (no. 2398-78), a University of Hausel, W. D., and Nash, W. P., 1977, Petrology of Tertiary and Quater- California Patent Fund Grant, and a University of California nary volcanic rocks, Washington County, southwestern Utah: Geologi- Research Travel Grant for Graduate Students, all to Suneson. cal Society of America Bulletin, v. 88, p. 1831-1842. Hedge, C. E., and Noble, D. C., 1971, Upper Cenozoic basalts with high Sr87/Sr86 and Sr/Rb ratios, southern Great Basin, western United States: Geological Society of America Bulletin, v. 82, p. 3503-3510. 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