The Serra da Graciosa A-type granites and syenites, southern Brazil. Part 1: Regional setting and geological characterization

Abstract

The Serra da Graciosa region includes important occurrences of granites and. syenites of the Graciosa A-type Province (formerly Serra do Mar Suite), southern Brazil. Using fieldwork, petrography, and remote sensing imagery, we characterize the geology of the plutons in the region. Five individual plutons were recognized. Two correspond to the previously defined. Marumbi and Anhangava Plutons. We divide the former "Graciosa Pluton" into three new plutons: Capivari, Órgãos, and Farinha Seca. The plutons are elliptical with northeast-southwest orientation. Two petrographic associations can be recognized: an alkaline association that includes peralkaline and metaluminous hypersolvus alkalifeldspar granites and syenites (Anhangava, Farinha Seca, Órgãos), and an aluminous association composed of metaluminous and weakly peraluminous subsolvus granites (Capivari, Órgãos, Anhangava, Marumbi). Occurrences of each association are limited to one individual pluton or to portions of a pluton, and the age relationships are not well established. Monzodioritic rocks are found marginal to the Órgãos and Farinha Seca Plutons, and interaction with silicic magmas locally produced hybrid, quartz syenites (Farinha Seca Pluton). Geothermobarometry indicates emplacement at shallow crustal levels (P = 2 ±0.6 kbar), and crystallization temperatures within the interval 900-700°C for the granitic and syenitic rocks, and. 1000-750°C for the monzodioritic rocks.