Early post-collisional granitic and coeval mafic magmatism of medium- to high-K tholeiitic affinity within the Neoproterozoic Southern Brazilian Shear Belt

The post-collisional stage of the Brasiliano/Pan-African Orogenic Cycle in the Garopaba region, southern Brazil, began at 630–620 Ma by the emplacement of large granitic massifs with associated mafic rocks. The Paulo Lopes Suite (PLS) is composed of metaluminous to slightly peraluminous biotite-hornblende Paulo Lopes and Garopaba granitoids, and by the Silveira Gabbro, containing augite, pigeonite and orthopyroxene. Mafic microgranular enclaves and mutual intrusive relations attest to the coeval character of mafic and felsic magmas. The PLS intruded during regional transcurrent shearing, and microstructures indicative of high-temperature solid-state deformation are described in the mafic and granitic rocks. Mafic rocks are medium- to high-K tholeiitic and show mineralogical and geochemical features, including isotopic ones that are compatible with magmas from sources modified by previous lithospheric subduction, such as OIB-EM1. The PLS granitic magmas are interpreted as evolved from a mafic magma that was compositionally equivalent to the Silveira Gabbro, contaminated by crustal melts or by more complex mechanisms; hence, they may be considered as granitic magmas of tholeiitic affinity. Transcurrent shearing seems to be essential for promoting the generation of mantle-derived magmas and their interaction with crustal melts, with consequent generation of abundant granitoids. Basic magmas have resulted from melting of heterogeneous mantle reservoirs, and their highly evolved character is indicated by their low contents of MgO, Ni and Cr. The compositional similarity of PLS granitoids, particularly the Garopaba Granitoids, with A-type granitoids discriminates them from the medium- to high-K granitoids of mature magmatic arcs, and this difference probably reflects the tholeiitic character of their primary magmas, typical of the early magmatic stages of post-collisional settings in southern Brazil. Trace element and isotopic geochemical data indicate subduction-modified (OIB) mantle sources for the mafic magmas, which is coherent with the post-collisional character of this magmatism.