Very low grade metamorphism associated with a geothermal system in basic volcanic rocks of the Middle Jurassic, Lonco Trapial Formation, Paso Berwin, Extra-Andean Chubut

Abstract

The basic volcanic rocks of the Lonco Trapial Formation, have a primary mineralogy mainly composed of olivine, calcic plagioclase and pyroxene, and present a large quantity of amygdales. Their metamorphic mineralogy is characterized by zeolites, clay and micaceous minerals. The main mineral association includes celadonite, montmorillonite, heulandite, analcime, gonnardite and subordinate levyne and minerals of the wairakite-analcime series. This association belongs to the zeolite facies and its origin was associated with a geothermal system. The analogy with known zeolite zones in equivalent geothermal systems and the use of the petrogenetic grid for zeolite parageneses, in the system An-Ab-SiO₂-H₂O, permits to establish a range of temperature between 80°-180°C. A single metamorphic hidrothermal event would have taken place in presence of a CO₂ very poor fluid phase, whose chemical composition possibly changed from a neutral character with low a(SiO₂) and moderate a(K₂O) (allowing precipitation of smectites and celadonite) to a neutral-alkaline character with high a(SiO₂) that allows heulandite to be stable.