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Origin of Bimodal Volcanism, Southern Basin and Range Province, West-Central Arizona

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Origin of Bimodal Volcanism, Southern Basin and Range Province, West-Central Arizona Origin of bimodal volcanism, 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 magma 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 magmas probably was caused by the intrusion of basalt 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 basalts (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 rhyolite lavas 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-trachyte association are much less abundant than the old). Most of the basalts contain groundmass olivine and titanau- mafic and silicic members. Volcanic suites with this "Daly gap" are gite phenocrysts and are alkali-olivine basalts. Many rhyolites 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 andesites and dacites 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 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/94/8/1005/3419196/i0016-7606-94-8-1005.pdf by guest on 29 September 2021 1006 SUNESON AND LUCCHITA 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 volcano Emuruangogolak, Kenya rift], 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.
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