Characteristic mineralogy of arc-related cumulate gabbros: Implications for the tectonic setting of gabbroic plutons and for andesite genesis

Abstract

Several mineralogically distinct suites of gabbroic cumulates occur as xenoliths in basalt and andesite from active arc volcanos and in plutons exposed in deeply eroded arcs. Calcic (An85–100) plagioclase and moderately Fe-rich (Fo60–80) olivine commonly occur together in arc cumulate gabbros but not in cumulate gabbros from mid-ocean ridges, ocean islands, or tholeiitic layered intrusions. Compositions of coexisting olivine and plagioclase in gabbro cumulates can establish the tectonic setting of ancient volcano-plutonic complexes. In deeply eroded arcs, gabbroic cumulates typically occur in zoned plutons that also contain gabbros, diorites, and quartz diorites of basaltic to andesitic composition. In many cases, it is clear that the cumulate gabbros are chemically and physically related to the gabbro-diorite (= basalt-andesite) fractionation trends seen in the zoned plutons. Zoned plutons in deeply eroded arcs provide physical evidence that crystal fractionation plays an important role in andesite petrogenesis.