THE ORIGIN OF BIMODAL VOLCANISM, WEST-CENTRAL ARIZONA

Tertiary volcanic rocks in the Castaneda Hills and northwestern part of the Artillery Peak 15' quadrangles, west-central Arizona, are interbedded in predominantly continental clastic sedimentary rocks and minor limestone. Gravity glide blocks composed of Paleozoic(?) limestone and quartzite and Precambrian(?) granitic rocks and a monolithologic megabreccia composed dominantly of fragments of Mesozoic(?) metavolcanic rocks are also interbedded in the sedimentary sequence. The sediments accumulated in northwest-trending normal fault-controlled basins. The normal faults are mostly high-angle to the northeast but are low-angle listric faults to the southwest. The inverse stratigraphy represented by the megabreccia and glide block rock types indicates progressive tectonic denudation of the plastically attenuated and mylonitized gneiss of the Harcuvar metamorphic core complex, first by coalescence of listric faults to form the Rawhide Mountains detachment fault, then by "free-glide" gravity gliding.; The volcanic suite in the Castaneda Hills quadrangle is strongly bimodal; rocks with 55 to 71 weight percent SiO(,2) are absent. Five volcanic units are present: the older basalts (18.7 and 16.5 m.y.), quartz-bearing basalts (13.7 and 12.4 m.y.), rhyolite lavas and tuffs (15.1 to 10.3 m.y.), mesa-forming basalts (13.1 to 9.2 m.y.), and megacryst-bearing basalts (8.6 to 6.8 m.y.). The basalts contain groundmass olivine and titanaugite phenocrysts and are alkali-olivine basalts. Most of the rhyolites contain over 75 weight percent SiO(,2).; Sr isotopic evidence suggests that some of the basalts (('87)Sr/('86)Sr(,i) = .7036) are partial melts of upper mantle material. The chemical composition of some of the megacrysts supports a high-pressure origin (high Ca-Tsch, high Mg/Mg+Fe* in clinopyroxenes, high Al(,2)O(,3) in titanomagnetites, presence of ferrian pleonaste). Low Mg/Mg+Fe* values and presence of phenocrysts in these basalts indicate they underwent crystal fractionation prior to eruption. Other basalts with ('87)Sr/('86)Sr(,i) = .7077 and .7062 were probably derived from lower crustal granulites. The rhyolites have initial Sr ratios significantly higher than the basalts (('87)Sr/('86)Sr(,i) = .7093, .7141) indicating that they were not differentiated from the basalts. Partial melting of a 1.3 b.y.-old lower crustal granulite satisfactorily explains the isotopic ratios of the rhyolites. Shortly after emplacement of some of the rhyolites, vapor phase transfer during devitrification caused K to replace Na.; The inception of fundamentally basaltic volcanism and basin-range extension were nearly simultaneous and occurred at about 20 m.y. ago throughout the northern and southern Basin and Range provinces. Calc-alkalic volcanism locally persisted through the initial stages of basin-range faulting, but is insignificant on a province-wide scale. In some areas, basin-range deformation was manifested as low-angle listric faults within a thin suprastructure. Elsewhere, more "typical" high-angle basin-range faults penetrated to greater depths. Mid-Tertiary normal faulting occurred throughout the Basin and Range, but the extent of its development is unknown. The importance of early- or mid-Tertiary strike-slip faulting is also unknown. The only plate tectonic model which satisfactorily explains what is known about the tectonic history of the Basin and Range is an inter- or back-arc spreading model. Extension in the Southern Basin and Range resulted from a relaxation of Farallon-North American plate subduction-caused compression as the San Andreas transform system lengthened. Nearly simultaneous extension in the northern Basin and Range resulted from a slowdown in absolute velocity of the North American plate and consequent backarc spreading. The detailed history is undoubtedly more complex due to crustal inhomogeneities inherited from Paleozoic, Mesozoic, Laramide, and early- and mid-Tertiary deformational and magmatic events.