Evolução da porção Nordeste do Domínio Bacajá a partir de dados U-Pb e Lu-Hf em zircão e Sm-Nd em rocha total

Abstract

The north-northeast border of the Amazonian Craton is defined as an extensive paleoproterozoic belt whose evolution is related to the Transamazonic Cycle (2.2 - 1.95 Ga). In the southern segment of this belt, in the Central Brazil Shield, is located the Bacajá Domain that consists of portions formed in the Archean and Rhyacian orogens. This Domain is composed of intrusive granitoids and charnockitoides, orthognaisses, granulites, migmatites and supracrustal rocks. In this study, by petrographic analysis, geochronological data U-Pb and Lu-Hf in zircon and Sm-Nd in total rock, it was possible to elaborate an evolution of the continental crust of this region from the Neoarchean to the Rhyacian, highlighting the relationship of the Archean terranes with the granitoids/charnockitoides correlated to the Transamazonic Cycle. Orthognaisses located to the north of Tucuruí Sheet (Aruanã Complex) present strong features of retrometamorphism identified in typical high metamorphic textures (granoblastic/interlobed textures) by association with a lower grade hydrated mineral amphibole assembly (Anf) and biotite (Bt). Coronitic textures and orthopyroxene (Opx) pseudomorphs for Anf and Bt and the presence of antipertites are also evidences that corroborate this phenomenon. These orthognaisses are 2630 ± 15 Ma, the oldest record obtained in this study. They represent fragments of a reworked crust (ɛHf(t) have subchondritic values of -0.3 to -1.7 and ɛNd(t) = - 3.08 to -2.98) whose model ages Hf-TDM C and Nd-TDM indicate that the material giving rise to this orthognaisses was extracted from the mantle in the Mesoarchean (3.0-3.2 Ga). The Rhyacian intrusive suites are characterized by an association of massive granitoids and charnockitoids with well-defined hypidiomorphic granular texture and rocks deformed by the effects of shear zones. In this area is also described a xenolith of orthogranulite characterized by polygonal granoblastic texture. These rocks were placed in the period of 2.12-2.09 Ga and 2.08-2.06 Ga and crystals of the orthogranulite identify a high-grade metamorphic event ca. 2.09 Ga. The identified zircons of the 2.12 Ga orthogranulite are interpreted as related to a magmatic arc (pre-collisional stage) on the edge of a Neoarchean continent. In addition, the isotopes of Pb and Hf show that the 2.12 Ga rock formed from an accreted neoarcan crust (ca. 2.6 Ga), which suffered a high grade 2.09 Ga metamorphic event (in a decompression stage). Finally, the igneous rocks from 2.08 to 2.06 NE of the Bacajá domain formed from the continental crust accreted during the Neoarchean (2.8-2.5 Ga) and that the post-collisional magmatic event of DB extended to 2.06 Ga.