Early Silurian medium-grade shear zone in the La Huerta range, Western Pampean Ranges, Argentina: implications for Gondwana margin evolution

CASTRO DE MACHUCA, BRÍGIDA; ARANCIBIA, GLORIA; MORATA, DIEGO; BELMAR, MAURICIO; PONTORIERO, SANDRA

Mendoza

Simposio; Gondwana 12: Geological and Biological Heritage of Gondwana; 2005

Academia Nacional de Ciencias

After regional metamorphism, local mylonitization along discrete NW-SE to NNW-SSE striking ductile shear zones occurred in the Sierra de La Huerta, Western Sierras Pampeanas area . This event would have been related to uplift and decompression during the later stages of the orogenesis (452 - 459 Ma), probably associated with the accretion of the Precordillera terrane to the southwestern Gondwana margin. A younger 40Ar/39Ar plateau age (90% of 39Ar released) of 432±4 Ma (integrated age of 428±4 Ma and isochron age of 432±2 Ma, MSWD=0.56, 40Ar/36Ar intercept 320±50), was obtained in this study for rather homogeneous hornblende porphyroclasts (MgO= 12.07 - 15.02 wt%, Al2O3= 10.45 - 12.47 wt%, CaO= 11.11 - 12.25 wt%), from a NNW-SSE trending metagabbro mylonite collected in a reverse shear zone in Quebrada Sanjuanina, Sierra de La Huerta. Estimation of P conditions based on the Al content of metamorphic amphibole gives values ranging from 5.9 to 7.3 kbar. These age and P values are interpreted as conditions during the latest medium-grade ductile deformation in the area, and provide evidence that orogenesis was active at least until the Early Silurian.