A low-grade metamorphic model for the Miocene Volcanic sequences in the Andes of central Chile

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 ver y low grade metamorphism characterised by mineral assemblages of the zeolit e facies. Metastable conditions prevailed, most of the igneous minerals bei ng wholly or partially preserved. The main metamorphic phases are mafic phy llosilicates and zeolites of calco-sodic and calcic composition. The intens ity of the metamorphism was controlled by depth and by hydrothermal activit y related to volcanic centres. From top to bottom of the sequence, the zeolites vary from heulandite-clino ptilolite 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 fl ow 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 s econdary zeolites; and (b) the contrast in composition observed between phy llosilicates in vesicular tops as opposed to the massive (central) parts of the same flow. The thermal gradients acting during the metamorphic event w ere high and are estimated at 150-175 degrees C/km. The metamorphic zonatio n is interpreted as the result of a rapid accumulation of considerable volu mes of rock generated by volcanic centres connected with geothermal field a ctivity and characterised in places by caldera collapse.