A low‐grade metamorphic model for the Miocene volcanic sequences in the Andes of central Chile

Abstract

Calc‐alkaline basic volcanic rocks in a c. 600 m thick sequence of Miocene age, the Valle Nevado stratified sequence (VNSS), have been affected by very low grade metamorphism characterised by mineral assemblages of the zeolite facies. Metastable conditions prevailed, most of the igneous minerals being wholly or partially preserved. The main metamorphic phases are mafic phyllosilicates and zeolites of calco‐sodic and calcic composition. The intensity of the metamorphism was controlled by depth and by hydrothermal activity related to volcanic centres. From top to bottom of the sequence, the zeolites vary from heulandite‐clinoptilolite through mordenite to laumontite, whereas the phyllosilicates show a compositional transition from tri‐smectite to smectite/chlorite with up to 75% chlorite layers. Strong fluid/rock interaction took place only at flow levels with a high primary porosity and permeability as illustrated by: (a) the similarity of the REE patterns of the basaltic host flows and the secondary zeolites; and (b) the contrast in composition observed between phyllosilicates in vesicular tops as opposed to the massive (central) parts of the same flow. The thermal gradients acting during the metamorphic event were high and are estimated at 150–175°C/km. The metamorphic zonation is interpreted as the result of a rapid accumulation of considerable volumes of rock generated by volcanic centres connected with geothermal field activity and characterised in places by caldera collapse.