GEOCHEMISTRY OF PROTEROZOIC MAFIC ROCKS FROM THE PASSOS NAPPE (MINAS GERAIS, BRAZIL): TECTONIC IMPLICATIONS TO THE EVOLUTION OF THE SOUTHERN BRASILIA BELT

Abstract

Major and trace element compositions, including REE, are used to infer the tectonic setting of the mafic magmatism associated with the metasediments of the Araxa Group in the Passos nappe. The pre-metamorphic protoliths are sub-alkaline tholeiitic basalts, separated in three compositional groups: (i) high-TiOi (HTi) group (TiO2 > 2%) enriched in P2O5, Fe2O3 tot, incompatible elements, and light-REE (LaN/SmN between 2,0 and 2,9); (ii) low-TiOa (LTi), relatively depleted in these elements (LaN/SmN between 1,4 and 2,6); and (ii) a subordinated group represented by basalts of E-MORB composition, with LaN/SmN between 0.80 and 0.97. The first two groups bear close similarities with the High and Low-TiOa magmas from Continental Flood Basalt provinces related to Gondwanaland break-up (Parana-Etendeka, Karroo). The E-MORB like group represents magmatic contributions from depleted mantle sources. The HTi group samples are concentrated in the lower section, but occur along the whole stratigraphic column, whereas the LTi samples also occur in the lower portion of the nappe, and the MORB-like basalts were found only in the upper portion of the stratigraphic pile. The observed distribution of magma types favors the hypothesis of sin-depositional magmatism and indicates that the initial sedimentation in the Passos nappe metasedimentary pile (Lower Unit) was related to continental extension in association with CFB-type magmatism (HTi and LTi types), which continued along the rest of the preserved sedimentary record. During the sedimentation of the Upper Unit of the Passos nappe, in which relatively deeper marine conditions probably prevailed, MORB-like magmas were formed, probably related to more severe lithospheric extension and thinning. However, the persistence of continental magmatism in this stage is an indication that oceanic floor was not formed during lithospheric thinning in the southern segment of the Brasilia belt.