From the plutonic root to the volcanic roof of a continental magmatic arc : a review of the Neoproterozoic Araçuaí orogen, southeastern Brazil.

Abstract

The Araçuaí-West Congo orogen (AWCO) is one of the various components of the Brasiliano/Pan- African orogenic network generated during the amalgamation of West Gondwana. In the reconstructions of Gondwana, the AWCO, encompassing the Araçuaí orogen of South America and the West Congo belt of Southwestern Africa, appears as a tongue-shaped orogenic zone embraced by the São Francisco-Congo craton. Differing from the vast majority of the known orogens owing to its singular confined setting, the AWCO contains a large amount of orogenic igneous rocks emplaced in all stages of its tectonic evolution. We present new and revised information about the oldest Ediacaran granitic assemblage, the G1 Supersuite, which together with the Rio Doce Group defines the Rio Doce magmatic arc, and then we propose a new tectonic setting for the arc. Field relationships and mineralogical compositions of the G1 Supersuite allow us to characterize three lithofacies associations, Opx-bearing rocks, enclaverich Tonalite–Granodiorite and enclave-poor Granite– Tonalite, suggesting different crustal levels are exposed in the central part of the Araçuaí orogen. The region is interpreted to represent a tilted crustal section, with deep arc roots now exposed along its western border. Chemically, these plutonic associations consist mostly of magnesian, metaluminous to slightly peraluminous, calcalkaline to alkali-calcic and medium- to high-K acidic rocks. The dacitic and rhyolitic rocks of the Rio Doce Group are mainly magnesian, peraluminous, calcic to calc-alkaline, and medium- to high-K acidic rocks. Zircon U–Pb data constrain the crystallization of the granitoids between ca. 625 and 574 Ma, while the age of the metamorphosed volcanic rocks is around ca. 585 Ma. Thus, within errors, these rock associations likely belong to the same magmatic event and might represent the subduction- related, pre-collisional, evolution of the Araçuaí orogen. In addition, whole-rock Sm–Nd isotopic compositions show variable negative εNd(t) values between −6.7 and −13.8, and TDM model ages varying from 1.39 to 2.26 Ga, while εHf(t) vary between −5.2 and −11.7, with TDM ages from 1.5 to 2.0 Ga. Thus, predominantly constructed upon Paleoproterozoic (Rhyacian) basement, the Rio Doce arc shows crustal sources largely prevailing over mantle sources, providing a well-studied example to be compared with similar orogenic settings around the world.