Deepest Exposed Crust Of Brazil-Geochemistry Of Paleoproterozoic Depleted Santa Maria Chico Granulites

The deepest crustal processes of Brazil are investigated in the Santa Maria Chico Granulitic Complex of Rio Grande do Sul State. Depth of ca. 30 km is inferred from garnet-bearing mafic granulite geobarometry for the deformation of the bimodal basic/acid sequence of Late Archean age. This low-K tholeiitic magmatism occurred in an island-arc environment, which accreted to the Atlantica supercontinent in a collisional episode at ca. 2.1 Ga during the Transamazonian Cycle. Although.this magmatism is strongly depleted in lithophile elements, its geochemistry is still magmatic and shows that the metamorphism did not disturb the original distribution of the elements in the host rocks significantly. The associated trondhjemites could have been the differentiated portions of the basic magmas, but more likely were the result of the partial melting of the garnet-bearing mafic residue. This suite of high-grade rocks is one of several protoliths for the generation of the abundant K-granites of the 600 Ma Brasiliano Cycle from southern Brazil. This high-grade rock association is rather comparable to the Lewisian Complex of Scotland.