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The Quatro Ilhas Granitoids are syntectonic to a major NE-SW dextral strike-slip shear zone at the northeastern part of the Dom Feliciano Belt in Southern Brazil. They comprise high-K calc-alkaline, coarse-grained porphyritic granodiorite to monzogranite emplaced at 625 to 615 Ma. Their foliation is marked by the alignment of biotite and euhedral to stretched K-feldspar phenocrysts and dips gently to steeply to the SE. Mineral and stretching lineations plunge at a low angle to both NE and SW. Several fine-grained dioritic to granitic synplutonic sheets crosscut the porphyritic granites parallel or oblique to the foliation. This assemblage is affected by regional tectonics, resulting in centimetre-to metre-scale, NW-verging asymmetrical folds, whose axis is parallel to the observed lineations. Progressive verticalization of foliation due to folding leads to local steeply-dipping, dominantly dextral shear zones. The observed structures and coeval strike-slip and contractional structures point to oblique/inclined transpressive deformation of the monzogranite. The fine-grained granites are geochemically grouped into low- and high-K types. The first group is relatively depleted in K2O and enriched in Ba and Sr, whereas the second resembles A-type granitoids. The low-K intrusions are interpreted to be crustally-derived, and the high-K type results from differentiation of tholeiitic magmas with crustal contamination. The presence of both gently-dipping and steeply-dipping structures active during the emplacement of these granitoids is further evidence of the transpressive character of the regional tectonic event. It is interpreted as a marker of the transition between the main collision and overall oblique transpression (650–635 Ma) and strict strike-slip tectonics (≤611 Ma) during the post-collisional period of the Dom Feliciano Belt construction. |
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