Anchimetamorphism in rocks from low Paleozoic in the NW of Argentina

Abstract

Anchimetamorphism in rocks from low Paleozoic in the NW of Argentina?. Measurements of the crystallinity of illite permit a quantitative evaluation of metamorphism in the Negro Peinado and Volcancito formations in the Sierra de Famatina, Falda Ciénega formation in the Puna, and Puncoviscana formation in the Cordillera Oriental.

The method Kibler proposed originally was employed, which consists of measuring the width of the peak (001) of the fraction <2 µm, in the half of its height. The relative crystallinity C.R. values of Weber was estabilished by using quartz as extern standard. "The reasons to use the fraction 2 mµ as indicator are clearly explained, as well as the choice of thick and thin preparates and their dispersal methods. The use of the Esquevín method (1969) as metamorphism indicator was succesful only with the rocks of the Falda Ciénega formation.

The crystailinity of the illite does not suffer the influence of the external backgroung in wich it is, within the anchimetamorphism medium; but in the diagenetic background the presence of the organic matter (coal or bitumen) delays its recrystallization.

For the classification of the rocks the squeme proposed by Teichmüller et al. (1979) was followed. According to this, the processed samples from Negro Peinado and Volfancito and Puncoviscana formations belong almost completely to the anchimetamorphism, while those from the Falda Ciénega formation are predominantly epimetamorphic.