Geoquímica, evolução magmática e condições de cristalização do pluton São João do Sabugi, RN

Abstract

The São João do Sabugi pluton, nearby the homonymous city in the Rio Grande do Norte state, is one of the most expressive individual occurrences of shoshonitic rocks within the Rio Piranhas-Seridó Domain of the Borborema Province. Therefore, a detailed study of these rocks allows a better understanding of the genetic and evolutive processes of the shoshonitic suite in the northern Borborema Province. The pluton was object of petrographic and litochemical studies, including geochemical modelling, in order to characterize their magmatic differentiation processes and intensive crystallization parameters. The identified lithotypes were grouped into three petrographic facies: (1) gabbro-dioritic facies (more mafic rocks), of restrict occurrence, made of dioritic and gabbroic rocks with andesine-labradorite (An45-63%) and the mafic paragenesis clinopyroxene ± orthopyroxene + amphibole ± biotite; (2) monzodioritic, composed of mafic to intermediate rocks, from diorites and monzodiorites to quartz-monzonites, with the mafic paragenesis amphibole + biotite (as well as pyroxenes in the more mafic varieties) and oligoclase-andesine (An22-45%); and (3) granodioritic, with the most felsic lithotypes within the pluton, varying from quartz monzonites to granodiorites, with biotite as the main mafic, as well as higher contents of orthoclase. Mafic and intermediate enclaves and xenoliths of the Jucurutu Formation (calc-silicate rocks and paragneisses) are common in the monzodioritic facies. The studied rocks are metaluminous, alcali-calcic and magnesian, and have shoshonitic affinity. They show relatively high contents of Ba, Rb, Th, K, and light rareearth elements, and pronounced negative Nb anomaly in primitive mantle-normalized diagrams. The clinopyroxenes and amphiboles in the monzodiorites are chemically classified as augite and ferro- and magnesium-hornblende, respectively, whereas the biotite is a relatively Mg-enriched (0.44 < Mg/(Mg+Fe) < 0.65) variety. Available geochemical data suggest the studied rocks are cogenetic. Major and trace element modeling reveal that the least evolved compositions of the gabbro-dioritic facies can be generated through ~12% of partial melting of a metasomatized mantle source. From these precursor magmas, the monzodioritic and granodioritic facies can be derived after ~60 to 80% of fractional crystallization associated to the assimilation of ~15% of country-rock materials from the Jucurutu Formation. The São João do Sabugi pluton crystallized at temperatures between 788 816oC, and pressures of 5 6 kbar, under relatively oxidizing conditions (1.2<NNO<1.4).