AMS fabrics and emplacement model of Butiá Granite, an Ediacaran syntectonic peraluminous granite from southernmost Brazil

Abstract

The post-collisional stage of the Brasiliano/Pan-African Cycle in Southern Brazil is marked by metaluminous and peraluminous granites controlled by a transcurrent shear zone system. In southernmost Brazil, the sinistral, NE-trending Dorsal de Canguçu Transcurrent Shear Zone (DCTSZ) is the best known structure that conditioned these peraluminous granites. Despite its poorly-developed linear fabric, the emplacement of the NNW-elongate Butiá Granite (BG) northwest of the DCTSZ is interpreted to have been controlled by a dextral transcurrent shear zone. Thus, an anisotropy of magnetic susceptibility (AMS) study was performed in the BG aiming to constrain its emplacement mechanism and the relation of the granite with the regional shear zone system. Magnetic mineralogy was investigated through hysteresis loops, thermomagnetic and IRM acquisition curves and SEM analysis. These experiments show a dominant contribution of paramagnetic phases and a small content of low-coercivity (e.g., magnetite and titanomagnetite) and high-coercivity (e.g., hematite) remanence-carrying minerals. In spite of the presence of minor ferromagnetic grains, the BG magnetic anisotropy fabric is interpreted as dominantly controlled by paramagnetic biotite crystals. Magnetic susceptibility ranges between 0.1 and 8.0 × 10−5 SI. Shape parameter (T) ranges from 0.272 to 0.908, and anisotropy degree (P) ranges from 1.073 to 1.266, increasing from the inner portion of the pluton to its margins. Magnetic fabrics, microstructures and field relations suggest that magma ascent and emplacement were controlled by a NNW-trending dextral transcurrent shear zone. The presence of S-C magmatic fabric and high temperature (ca. 650 °C), solid-state deformation at the margins confirm that the pluton was deformed during its cooling process. Close to the host-rocks, magnetic foliation dips steeply towards W or E, and magnetic lineation plunges steep to moderately, indicating dominant flattening there. Shallow-plunging lineation parallel to the NW- to NNW-striking foliation is found away from the pluton margins. Foliation becomes less steep towards the BG northeastern portion and the presence of roof pendants in this area suggests the proximity to the roof zone. The combination of buoyancy forces and the partitioning of regional strain into simple and pure shear is in accordance with a transpressive regime. These results also suggest a time-space relationship between the NNW-dextral shear zone that controlled the emplacement of the Butiá Granite (ca. 629 Ma) and the sinistral, NE-trending Dorsal de Canguçu Transcurrent Shear Zone, responsible for the emplacement of peraluminous granites during the early post-collisional stage (ca. 634 - 610 Ma) of the Brasiliano/Pan-African Cycle in southernmost Brazil.