Origin and Evolution of the Piên Mafic-Ultramafic Complex, Southern Brazil

Abstract

A suite of mafic and ultramafic rocks exposed at Piên, SE Brazil, were affected by at least three recrystalli-zation episodes, the earliest one producing high-grade granulites about 2,000 m.y. ago (Transamazonic cycle), later partly converted into amphibolite-facies lithologies during the Brasiliano cycle (650 m.y.) and serpentin-ites and talc shists. The predominant ultramafic rocks are meta-olivine pyroxenites, meta-websterites, meta-norites, serpentinites (mainly former harzburgites) and magnesian schists (mainly former orthopyroxene-rich rocks). The mafic suite presents granoblastic meta-gabbros, amphibolites and amphibole gneisses, composi-tionally akin to high-alumina basalts and olivine tholeiites. Bulk chemical composition shows a trend of Fe enrichment and probable continuity in chemical tendencies among ultramafic and mafic members. Mineral analyses of oliviries, spinels and especially orthopyroxenes and clinopyroxenes in ultramafitites show a chemical trend compatible with their derivation from recrystallized differentiated (layered ?) complexes. Compositional variations of various coexisting mineral assemblages suggest that the granulitic high-grade re-equilibration occurred at temperatures from about 750 to 880°C and minimum pressures of about five to seven kb. Present petrography, outcrop patterns and chemical data suggest that the main primary minerals in ultramafitites were Mg-rich olivine, Al-orthopyroxene and Al-subcalcic clinopyroxenes. Experimental studies of high-pressure crystallization of basalts show that such minerals can fractionate away from an olivine tholeiite magma-at a pressure interval from about seven to about 12 kb.-yielding a series of ultramafic rocks and a residue which on many accounts matches the composition of the Piên metagabbros and associated rocks.