Geothermobarometry Of Very Low-Grade Metamorphic Pelites Of The Vendian?Early Cambrian Puncoviscana Formation (Cordillera Oriental, NW Argentina)

DO CAMPO, MARGARITA; COLLO, GILDA; NIETO, FERNANDO

EUROPEAN JOURNAL OF MINERALOGY (PRINT)

E SCHWEIZERBARTSCHE VERLAGS

Lugar: Stuttgart; Año: 2013 vol. 25 p. 429 - 451

0935-1221

The Vendian-Early Cambrian Puncoviscana Formation is a pelite-greywacke turbidite sequence affected by polyphasedeformation cropping out extensively in the Cordillera Oriental of northwestern Argentina. Previous X-ray diffraction and analyticalhigh-resolution TEM studies on southern locations found anchizonal grade and proposed medium-high pressure metamorphismfollowed by higher thermal conditions. We have determined the chemical composition of mica and chlorite with EDX on smalluncontaminated areas selected using SEM in backscattered electron mode. The application of mica-chlorite geothermobarometry tomineral grains defining two different foliations has produced consistent pressure/temperature results based on the intersection of sixreactions: 9 kbar/250°C peak conditions were followed by an isothermal decompression to 3 kbar and a successive increase oftemperature to nearly 350°C at 1.5 kbar. The shales from the overlying Mesón and Santa Victoria Cambro?Ordovician groups do notcontain chlorite, thus chlorite-mica geothermobarometry could not be applied. According to the range of KIobtained (0.41-0.68) and their mineralogical assemblage (illite + kaolinite + corrensite + Qtz + Pl + Kfs), these rocks have notsurpassed diagenetic conditions, which implies a minimum temperature difference with the Puncoviscana Formation of 100°C,consistent with the sharp angular unconformity between the two units.