Geothermobarometry of very low-grade metamorphic pelites of the Vendian?Early Cambrian Puncoviscana Formation (NW Argentina). European Journal of Mineralogy

DO CAMPO, M.; COLLO, G.,; F. NIETO

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 polyphase deformation cropping out extensively in the Cordillera Oriental of northwestern Argentina. Previous X-ray diffraction and analytical high-resolution TEM studies on southern locations found anchizonal grade and proposed medium?high pressure metamorphism followed by higher thermal conditions. We have determined the chemical composition of mica and chlorite with EDX on small uncontaminated areas selected using SEM in backscattered electron mode. Samples were prepared and analysed as usual for and using in the SEM similar conditions to electron microprobe analyses. The application of mica-chlorite geothermobarometry to mineral grains defining  two different foliations has produced coherent pressure/temperature results based on the intersection of six reactions. We have found 9 kbar/250 ºC peak conditions followed by an isothermal decompression to 3 kbar and a successive increase of temperature to nearly 350 ºC at 1.5 kbar. The shales from the overlying Mesón and Santa Victoria Cambro?Ordovician groups do not contain chlorite, thus chlorite-mica geothermobarometry could not be applied. According to the range of KICIS  obtained (0.41 - 0.68D°2q) and their mineralogical assemblage (illite ± kaolinite ± corrensite + Qtz + Pl ± Kfs) these rocks have not surpassed diagenetic conditions, which implies a minimum temperature difference with the Puncoviscana Formation of 100 ºC, coherent with the sharpangular unconformity between the two units.