CONICET | Buscador de Institutos y Recursos Humanos

ABSTRACT: In this work very low-grade metamorphic assemblages found in the Northern Patagonian Andes (Argentina) are summarized. On the basis of previous studies, several localities with high concentration of secondary minerals have been delimited. According to their mineralogy, textures and structures three different stages which evidence a progressive decrease in temperature have been established. The first one (stage I) was the consequence of a regional metamorphism that reached green-schist grade (350°C and 2kbars). During this stage, primary phenocrysts broke down to produce actinolite, grossular, andradite, chlorite, albite, prehnite, titanite, clinozoicite, pistacite and calcite. In the subsequent stages, hydrothermal precipitation increased meanwhile temperature decreased. Thus, zeolites, chalcosilicates, calcite and silica varieties started to be generated filling cavities. During stage II, temperature decreased below 220°C and wairakite, yugawaralite, laumontite, pectolite, dachiardite, celadonite, thomsonite, pumpellyite and c/s interestratiphied started to grow. Prehnite, adularia, titanite and albite were also produced but only as minor species. These minerals alter phenocrysts or fill the amygdules of basalts and andesites. Stage III is characterized by a decrease in temperature (below 180°C) and the crystallization of secondary minerals due to hydrothermal precipitation inside open spaces. During this last event, the major number of alkaline zeolites was generated filling joints of metabasites and granitoids. Although Ca-stilbite is the most frequent species, analcime, natrolite, barrerite, offretite, chabazite, stellerite, heulandite, mordenite, scolecite, mesolite, quartz, calcite, cristobalite and smectites were also produced. Keywords: zeolites, low-grade metamorphism, Northern Patagonian Andes.