CONICET | Buscador de Institutos y Recursos Humanos

This study focuses on the analysis of pseudotachylytes from the Azul Megashear Zone (AMSZ), located in the Río de la Plata craton, central Argentina. Glassy and microcrystalline pseudotachylytes occur in Paleoproterozoic gneisses, granite and granulite mylonites. Several ductilely deformed pseudotachylyte veins were produced in a ductile regime or in the brittle-ductile transition, while others were formed during a brittle late event related with uplift or exhumation of the shear zone. Pseudotachylytes form linear to broadly sinuous veins or injected complex networks. Fault veins are parallel to the mylonitic foliation or at low angle (15º-20º) to the foliation. Vein thickness varies between 1mm to 20 cm. Injection veins are perpendicular or oblique to the mylonitic foliation and branching off fault veins. Taking into account matrix versus polycrystalline clast ratio, two main types of veins it can be distinguished: a) veins with more than 80% of matrix and rare polycrystalline clasts; and b) veins with a clast proportion that fluctuates between 70% and 80% with rare matrix. Partially melted quartz and feldspar clasts with embayed rims, flow structures and alignment of polycrystalline clasts (mainly quartz) provide evidence for a melt origin of the pseudotachylytes exposed at the Cerro Negro locality. The average bulk compositions of pseudotachylyte veins and the host rocks are similar; generally pseudotachylyte matrix are enriched in Fe2O3, MgO, CaO and TiO2 and depleted in SiO2, K2O and Na2O, indicating preferential melting of amphibole and biotite rather than quartz and feldspar.