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

Miocene volcanic rocks in the Castaneda Hills area, west-central Arizona, are interbedded with continental clastic sedimentary rocks, minor limestone, gravity glide blocks of Precambrian(?) and Paleozoic(?) rocks, and monolithologic megabreccia. The sedimentary and volcanic units dip to the southwest and are offset by northwest-trending listric and high-angle normal faults. The listric faults coalesce at the Rawhide detachment fault, which overlies mylonitic gneiss. The volcanic suite is strongly bimodal; rocks with 55 to 71 wt % SiO 2 are rare. On the basis of age, geomorphic position, and petrography, 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), rhyolite lavas and tuffs (15.1 to 10.3 m.y. old), mesa-forming basalts (13.1 to 9.2 m.y. old), and megacryst-bearing basalts (8.6 to 6.8 m.y. old). Most of the basalts contain groundmass olivine and titanaugite phenocrysts and are alkali-olivine basalts. Many rhyolites contain more than 75 wt % SiO 2 . The initial whole-rock Sr isotopic composition of the basalts indicates that they are partial melts of an isotopically vertically heterogenous mantle. The chemical composition of some of the megacrysts in megacryst-bearing basalts with 87 Sr/ 86 Sr i equal to .7035 and .7038 supports a high-pressure mantle origin. The low (.7034) Sr ratio and lack of evidence for mixing with young rocks indicate that the quartz-bearing basalts were also derived from the mantle. Other basalts with 87 Sr/ 86 Sr i > 0.705 probably were derived from old, lithospheric mantle with a high Rb/Sr ratio and do not appear to be contaminated with old, upper-crustal material. The rhyolites have initial Sr isotopic ratios of 0.7093 and 0.7141. These ratios indicate that the rhyolites were not differentiated from the basalts. Partial melting of 1.3-b.y.-old lower-crustal material with Rb/Sr = 0.10 to 0.19 satisfactorily explains the isotopic ratios of the rhyolites. Granulite, which may constitute the lower crust in this part of Arizona, has Rb/Sr ratios similar to those required to produce the rhyolites. K substituted for Na during cooling and devitrification in some of the rhyolites. Partial melting of upper-mantle peridotite and old lower-crustal granulite from 19 to 7 m.y. ago in the Castaneda Hills area produced the bimodal volcanic suite. The nearly contemporaneous production of basaltic and rhyolitic magma from the Earth9s crust and mantle requires extremely heterogenous source regions. Asthenospheric upwelling associated with basin-range extensional tectonism probably produced the heating event that caused partial melting and basaltic magma generation at different levels in the mantle. Partial melting in the lower crust to produce rhyolitic magmas probably was caused by the intrusion of basalt magma. The basaltic and rhyolitic magmas formed in separate source regions, rose independently, and erupted at the same time and place.