Formation of zeolitic assemblages during very low grade metamorphism of Paleogene volcanic rocks, south of Confluencia, Neuquén, Argentina.

Abstract

A metamorphic assemblage of very low grade metamorphism in Paleogene volcanic rocks from the Andesitic Formation, Confluencia, Neuquén Province, is studied. The protolith is basaltic and andesitic. The secondary minerals are calcic, calcosodic and sodic zeolites. Wairakite is the first zeolite to crystallize, together with other minerals like albite, adularia, pumpellyite, titanite, pectolite, pyrite, native copper, hematite, smectite and quartz in the matrix and in phenocrysts. They were followed by laumontite, yugawaralite, and sodic wairakite in the matrix and in amygdales. Later on, only in amygdales, sodic wairakite was replaced by scolecite and mesolite, whereas laumontite and yugawaralite were replaced by heulandite and calcic clinoptilolite. Then barrerite, tetranatrolite and paranatrolite replaced albite and adularia was replaced by illite. These assemblages are in accordance with a pervasive geothermal field type, very low grade metamorphism in the zeolite facies. The equilibrium assemblage wairakite-yugawaralite-albite indicates P-T conditions of <0,5kb and <220°C. At temperatures lower than 200°C in the zeolite facies heulandite group minerals, scolecite and mesolite formed replacing the first association. In a second event, at temperature lower than 150°C, barrerite, paranatrolite and tetranatrolite precipitated in joints together with stilbite and stellerite. Other process, formed analcime by replacement of barrerite. Then, an increase in X_CO2 favoured the precipitation of calcite in joints. The thermal waters were neutral to slightly alkaline, the ƒO₂ conditions were in the hematite - pyrite stability field, the P_H2O / P_total ratio was high and then, decrases in the joints, at the same time that X_C2O was increasing; it was low during the main process. The geothermal system was heated by Early Miocene magmatic intrusions. Then, in the Late Miocene? a tensional fracturing permitted the action of new fluids.