Riacian Augen gneisses in Piranhas River-Seridó Domain - Borborema Province, Brazil Northeast

Abstract

Augen gneisses occurring at the Fazenda Genezaré in the Serra da Formiga east of Caicó, at the border of the São Vicente-Florânia (RN) nucleus, and in exposures SW and NW of São José do Seridó (RN) were studied by geologic mapping, petrography, lithogeochemistry and U-Pb dating. These rocks consist of biotite augen gneisses, and granitic/granodioritic orthogneisses, with porphyro-granoblastic textures and light gray to pink colors. Their mineralogy consists of perthitic microcline, oligoclase, quartz, besides biotite and amphibole, with epidote, titanite, chlorite, opaque minerals, apatite, zircon and allanite as accessory minerals. Petrography indicates that these rocks were affected by amphibolite facies metamorphism, and a later greenschist facies retrometamorphism. They have a prominent low-angle NW-SE foliation containing a high rake stretching lineation. Lithogeochemistry indicates high-K, metaluminous to peraluminous and calc-alkaline geochemical signatures related to a collisional environment. U-Pb dating of zircon (LA-ICP-MS) provided an upper intercept age of 2252 ± 17 Ma and 2171 ± 20 Ma, for the rocks found north of Florânia (RN), and east of Genezaré, respectively. In the latter values of 2.84 Ga TDM and ƐNd(0) of -20.76 were also obtained. The data allow attribution of the augen gneisses to the Rhyacian period, and are consequently associated with to the magmatism which formed the Caicó Complex.