Petrographic and geochemical constraints on the evolution of the Matarazzo Sequence, Arroio Grande Ophiolite, Brazil: Evidence from migmatites and marbles

Abstract

This study comprises the amphibolites, marbles, granitoids, metadiabases, and skarns of the Matarazzo Sequence (Arroio Grande Ophiolite, Dom Feliciano Belt, Brazil), which are here, for the first time, discussed from a migmatite perspective. The contribution presents novel petrographic and elemental geochemical data for the Sequence. Petrographic evidence demonstrates that high degrees of partial melting of the paleosomes (amphibolites) led to the development of millimeter-to meter-sized networks of mesocratic neosome veins (diorite to quartz-monzodiorite). The partial melting of the neosomes, in turn, led to the formation of micrometer-to meter-sized veins and pods of leucosomes (quartz-syenite to monzogranite). Garnet- and epidote-rich skarn layers were generated by the marble/melt interaction. The synkinematic nature of the partial melting event is evidenced by macrostructures such as rotated amphibolite schollen, neosome/leucosome veins concordant to the NE-SW-trending regional shear structures, and accumulation of melt in the hinge of folds. The minimum crystallization temperature of the neosomes and leucosomes are constrained, respectively, at 709 ± 31 and 732 ± 63 °C (Zr saturation thermometry). The geochemical data show that the trace element behavior of the paleosomes, neosomes and leucosomes are similar, strengthening the hypothesis of a genetic relationship and the interpretation that the metadiabases and amphibolites are the protoliths of the diatexites. The geochemical signature of the metadiabases and amphibolites suggest magmatic protoliths developed in an oceanic back-arc setting. The composition of the associated marbles is also consistent with this tectonic setting, reinforcing previous hypotheses that suggest that the studied ophiolite represents metamorphic remnants of an oceanic back-arc basin (South Adamastor paleo-ocean). The minimum metamorphic and obduction age of the Arroio Grande Ophiolite is, so far, marked by the U–Pb crystallization age of the studied leucosomes at 640 Ma. Since the same age is recorded in migmatitic orthogneisses from the southernmost sector of the Punta del Este Terrane in Uruguay, we suggest that the Punta del Este Terrane reached, synchronically, partial melting conditions at 640 Ma.