PETROLOGY AND GEOCHEMISTRY OF LOWER CRUSTAL GRANULITES FROM THE GERONIMO VOLCANIC FIELD, SOUTHEASTERN ARIZONA

Mafic to intermediate composition granulite xenoliths (plagioclase + clinopyroxene +/- orthopyroxene +/- K-feldspar +/- amphibole) occur in Pliocene to Recent alkali basalts from the Geronimo Volcanic Field (GVF), southeastern Arizona, USA. The range of compositions and mineral assemblages observed suggests that the ultimate derivation of these rocks is from a variety of protoliths and that more than one mechanism has operated during the geologic evolution of the lower crust in this area. Oxygen-isotope values of whole-rock samples range from + 5.9 to +8.4 parts per thousand SMOW are are consistent with igneous protoliths for the GVF granulites. The predominance of delta-O-18 values greater than +6.0 parts per thousand suggests that a component of older crust or meta-sediment is involved in the petrogenesis of most granulites in the suite.

Two-pyroxene, two-feldspar granulites (meta-diorites) have major and trace element characteristics similar to estimates of post-Archaen lower crust (WEAVER and TARNEY, 1984). Low Nd-143/Nd-144 values and Proterozoic Nd-depleted-mantle model ages (1.2-1.4 Ga) for these rocks require that Precambrian materials exists in the lower crust of southeastern Arizona, either as the meta-diorites themselves or as older crust available for melting during production of the meta-diorite protoliths. K-feldspar-free granulites have more mafic compositions and their trace element characteristics are consistent with a cumulate origin (positive Eu anomaly, low Rb/Sr ratios, and high Sr concentrations). A negative correlation of Pb-208/Pb-204 vs. Pb-206/Pb-204 suggests that the meta-cumulate granulites represent mixing between Basin and Range age lavas with older meta-diorite crust and is, thus, evidence for Cenozoic underplating of the lower crust beneath the Basin and Range.

A summary of granulite xenolith compositions worldwide demonstrates that the lower crust is heterogeneous and more mafic than current estimates of its average composition.