Microstructural analysis and P–T conditions of the Azul megashear zone, Tandilia, Buenos Aires province, Argentina

Abstract

A microstructural analysis was carried out on mylonitic rocks of the Azul megashear zone (AMSZ), Tandilia, which were formed in a range of metamorphic conditions from lower greenschist to amphibolite facies. Tailed porphyroclasts are common and mostly symmetric. Scarce asymmetric rotated porphyroclasts show both sinistral and dextral senses of shear. In sections parallel to the mylonitic foliation, porphyroclasts are round. The AMSZ is probably related to the late Transamazonian orogenic cycle and may be due to NNE–SSW-directed convergence. In weakly deformed protolith and protomylonites, quartz deforms by dynamic recrystallization, mainly subgrain rotation in dislocation creep Regime 2. K-feldspar porphyroclasts and plagioclase show scarce fracturation and deform by dynamic recrystallization along grain boundaries. Quartz microstructures in mylonites indicate predominantly Regime 3 grain boundary migration recrystallization. Feldspar structures indicate recrystallization through the nucleation and growth of new grains at grain boundaries. The temperatures of deformation from mineral assemblages in the CNKFMASH system in four bulk compositions are in the range of 400–450 °C, and the pressures are more than 6 kb.