Deformation, metamorphism and partial melting conditions of the Porto Belo Complex south of the Major Gercino Shear Zone, Bombinhas (SC-Brazil)

Transpressional settings, typical of oblique collisions, result in strain partitioning, initially characterized by thrust tectonics that progrades to and coexists with strike-slip tectonics. In the central-eastern portion of the Catarinense Shield, the Porto Belo Complex (PBC) comprises the basement of the dextral strike-slip, NE-trending Major Gercino Shear Zone (MGSZ). This shear zone has controlled the emplacement of successive magma pulses in the post-collisional stage of the Brasiliano Orogenic Cycle. The study area is located south of MGSZ, where the PBC records structures related to thrust tectonics. An integrated approach of structural geology and petrography was used in order to constrain the conditions of deformation and metamorphism in the rocks of the complex. In the study area, the PBC comprises orthogneisses, granodiorites, and amphibolites, with subordinate migmatites. The orthogneisses show mm-thick, NE-striking gneissic banding (Sb) which dips at shallow angles towards SE. The granodiorites are porphyritic and show NS-striking magmatic foliation that dips at moderate angles towards E. The amphibolites are finely-laminated and form boudinaged layers. These rocks are affected by isoclinal, recumbent and horizontal inclined folds, with top-to-the SW vergence. However, kinematic indicators as deformed xenoliths and asymmetric porphyroclasts show top-to-the-NW tectonic transport. Partial melting features are found in all rock types, but produce only small volumes of in situ and in source leucosome. Microstructures and mineral paragenesis suggest temperature conditions of mid- to upper amphibolite facies for the generation of the orthogneisses and crystallization of syntectonic granodiorites. (author)