Volcanism in the Kaiser Spring field of west-central Arizona postdates detachment faulting and crustal extension in the Basin and Range Province to the south. The volcanic products of this field include a diverse assemblage of rhyolitic and basaltic lavas that include topaz- and garnet-bearing high-silica rhyolites (HSR), hornblende-phyric low-silica rhyolites (LSR), tholeiitic basalts (TB) that vary from subalkaline to alkaline, alkali-olivine basalts (AOB), and a quartz-bearing mafic andesite (QMA). Extrusion of HSR lavas clearly preceded eruptions of LSR lava; however, both occurred concurrently with the effusion of basaltic lavas along the periphery of the silicic field. Compared with LSR lavas, the HSR are compositionally evolved, being strongly depleted in feldspar-compatible elements and having a variable, but high, content of incompatible elements. Rayleigh fractionation calculations and analyses of LSR matrix glass are consistent with derivation of HSR magmas by extensive crystallization of LSR parent melts. Both LSR and HSR lavas are characterized by ϵNd of -6 to -1.2, 87Sr/86Sri of 0.7056 to 0.7086, and unradiogenic 206Pb/204Pb of 16.96 to 17.32. Differentiated mafic lavas have Mg values of 54 to 58. The AOB is isotopically similar to basalts from the San Francisco volcanic field, having ϵNd of +12, 87Sr/86Sr of 0.7042, and 206Pb/204Pb of 18.18.

Two samples of TB differ markedly from the AOB, having ϵNd of -6.0 and -7.7, 87Sr/86Sr of 0.7073 and 0.7082, and 206Pb/204Pb of 17.87 and 18.39. With the exception of lower 206Pb/204Pb values, the TB lavas are isotopically similar to lower crustal mafic xenoliths from central Arizona. The QMA has ϵNd of -2.0 and 87Sr/86Sr of 0.7048, but a Pb isotopic composition that is similar to that of the rhyolites. At least three components are needed to model the combined Sr, Nd, and Pb isotopic compositions of the Kaiser Spring basalts and rhyolites. The isotopic composition of the AOB is consistent with its derivation from an OIB-like subcontinental mantle reservoir. The rhyolites were derived primarily from a comparatively unradiogenic crustal source, perhaps coupled with subordinate mantle input. The OMA formed by mixing of this same crustal component with mantle-derived basalt. The isotopic compositions of the TB lavas indicate that a second crustal component contributed to their geochemistry. This component may be mafic lower crust that is similar to amphibolite and eclogite xenoliths from central Arizona. The TB may represent contaminated mantle-derived melts or melts formed by lower crustal anatexis. The Pb isotopic compositions of the HSR, LSR, TB, and QMA compare favorably with the isotopic composition of local Precambrian granitoids, and indicate that the crustal components of the Kaiser Spring lavas belong to the Mojave crustal terrane. ¿ American Geophysical Union 1989