O Complexo Juiz de Fora no Sul do Rio de Janeiro: geocronologia, petrogênese, evolução tectônica e sua relação com o Orógeno Minas-Bahia

Abstract

The Juiz de Fora Complex (JFC) is one of the basement units of the Occidental Terrane in the Central Ribeira belt, regarded as part of the reworked border of the São Francisco paleocontinent. The JFC comprises metamorphosed rocks in granulite facies with varied geochemical compositions, deformational fabrics and ages that refer its polycyclic character. This work presents new zircon U-Pb (SHRIMP) ages and Lu-Hf (LA-ICPMS) data, besides whole-rock Sr and Sm-Nd (TIMS) isotopic and elementary data from JFC orthogranulites, located on Southwest Rio de Janeiro state. The main goal of this study is to constrain the Paleoproterozoic Evolution of this unit, as well as, magma sources of its protholiths. Six samples, chosen by their element compositions, were selected for U-Pb and Lu-Hf analysis. The tholeiitic enderbitic granulite present age of ca. 2.44 Ga. Three charno-enderbitic protoliths with TTG and sanukitoid signatures yielded crystallization ages from ca. 2.2 to 2.18 Ga. A mafic granulite has age of ca. 2.13 Ga. One sample of the high-K leucocratic charnockitic group yielded a crystallization age of ca. 1.78 Ga. Metamorphic rims around the magmatic cores present ages from 600 to 580 Ma confirming the Neoproterozoic overprint, related to Ribeira Belt. Inherited zircon crystals from ca. 2.53 to 2.38 Ga were identified in two samples. The isotopic data show that arc related Siderian magmatism (2.45-2.38 Ga?) is juvenile to moderately juvenile εNd(t) from -0.1 to +3.2; εHf(t) from -8 to +3). The arc-related Rhyacian magmatism is heterogeneous (εNd(t) from -0.6 to -4,8; εHf(t) from -11.7 to +0.2). Both magmatic episodes present juvenile siderian and crustal archean sources (TDM Nd: 2.62 to 2.31 Ga; TDM Hf 3.25 to 2.49 Ga) with rhyacian magmas showing major crustal contribution on its genesis. There is a predominance of Paleoproterozoic juvenile sources (Nd TDM) for Juiz de Fora Complex. The minor Archean crustal contribuition occurs in minor scale. The envisaged geological evolution from JFC starts at Siderian (ca. 2.45-2.38 Ga?) with crystallization of the arc-related tholleitic diorite marking the initial stage of arc development. The granodioritic protholiths represent a heterogeneous stage of Juiz de Fora arc between ca. 2.20 and 2.18 Ga. The third arc-related magmatic episode is constrained by a tholeiitic gabbro (IAT) with moderately juvenile signature and crustal contamination by two previous magmatic events. The collision between Juiz de Fora and Mantiqueira Complexes is attributed by metamorphic rims and grains dated at ca. 2035±30 Ma on this mafic granulite. After the cratonization, a Statherian (1.78 Ga) episode of intra-plate magmatism was detected and correlates to the Borrachudos Suite. During the Brasiliano orogeny, 600-580 Ma granulite facies metamorphism, coupled with deformation, is recorded in the studied samples. This study is important for paleogeographic reconstructions on the southern São Francisco Craton because of the discovery of more Siderian rocks in the JFC and by Paleoproterozoic metamorphism. The Minas Accretionary Orogen seems to be diachronic and the magmatic episodes from Juiz de Fora Complex and Mineiro Belt very similar, with occurrence of TTG and sanukitoid plutons on both units