Whole-rock and mineral chemistry characterization of contrasting granitoids, constraints on the source of the Vieirópolis NYF-type pegmatites, Northeastern Brazil

摘要

The Vieirópolis pegmatite field is characterized by amazonite pegmatites inserted in the Borborema Province. The Serra Branca pegmatite is hosted by the Serra Branca granites, which intruded partially the Serra Negra granites, and stands out by its amazonite mineralization. To comprehend the crystallization conditions and the source of this pegmatite and associated Serra Branca and Serra Negra granites, petrographic, mineral chemistry, geochronological and whole-rock geochemical data were used. The Serra Branca pegmatite is Cs-, Rb-, Pb-, Nb-, and Ta-rich. The Serra Branca granites (563 ± 2 Ma) are metaluminous to weakly peraluminous, crystallized under high fO2 conditions, pressure of 2.68-2.80 kbar and temperatures ranging from 667-670°C, with many zircon grains showing inherited cores of Paleoproterozoic ages. The Serra Negra granites (594 ± 4 Ma) comprises metaluminous granites crystallized under high fO2 conditions, temperatures within 711-751°C, pressure varying from 4.72 to 5.42 kbar. Both granites resulted from distinct partial melting degrees of a source similar to the Caicó Complex orthogneisses. The Serra Branca pegmatite geochemical data suggests that it was originated by extreme fractionation of the Serra Branca granites magma. The Serra Branca amazonite pegmatite is the first pegmatite mineralogically and geochemically characterized as Nb-Y-F (NYF)-type from the gadolinite subtype in the Borborema Province.