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Petrology and geochemistry of lower crustal granulites from the Geronimo Volcanic Field, southeastern Arizona

1990; Elsevier BV; Volume: 54; Issue: 12 Linguagem: Inglês

10.1016/0016-7037(90)90294-u

1872-9533

Pamela D. Kempton, Russell S. Harmon, Chris Hawkesworth, S. Moorbath,

Geochemistry and Geologic Mapping

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. Oxygenisotope values of whole-rock samples range from +5.9 to +8.4%. SMOW and are consistent with igneous protoliths for the GVF granulites. The predominance of δ18O values greater than +6.0%. 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 143Nd144Nd values and Proterozoic Nd-depleted-mantle model ages (1.2–1.4 Ga) for these rocks require that Precambrian material 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 208Pb204Pb vs. 206Pb204Pb 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.