Temperatures of formation in Triassic cataclasites of Cordillera Domeyko, Antofagasta, Chile.

Abstract

The cataclasites here studied represent exhumed fault zones, which form long and narrow interlaced fringes striking northwest and cross-cutting Paleozoic rocks in the Domeyko Cordillera. The contact relationships as well as whole rock K-Ar determinations, restrain the formation of the cataclasites to the Triassic. They belong to a tectonic environment related with continental 'rifts' which dominated the southwestern border of Gondwana during this period. In these rocks the minerals show pervasive brittle deformation, especially feldspars and quartz which are intensively fractured. Microstructural features allow to estimate the temperature of formation of these cataclasites in the range of 250 to 300°C. A neoformed mineral association recristalized in the matrix of these rocks consists of chlorite+calcite+albite±epidote. The use of chlorite geothermometers allowed to compel that the cataclasites formed at temperatures of 250-280°C, which are consistent with temperatures estimated by observations of their microstructure and neoformed mineralogical association. On assuming a temperature gradient of 40-70°C/km, as in most continental 'rifts', a depth on formation for the cataclasites between 3,5 and 7 km is inferred.