Volcano evolution and eruptive flux on the thick crust of the Andean Central Volcanic Zone: 40Ar/39Ar constraints from Volcan Parinacota, Chile

Abstract

The 163 k.y. history as well as the chemi- cal and 46 km 3 volumetric evolution of Vol- can Parinacota are described in detail by new mapping, stratigraphy, and 57 40 Ar/ 39 Ar ages determined from groundmass or sani- dine crystals in basaltic andesitic to rhyolitic lavas. A more precise chronology of eruptions and associated eruptive volumes of this cen- tral Andean volcano, which was built upon 70-km-thick crust, provides a more com- plete view of how quickly volcanic edifi ces are built in this setting and how their mag- matic systems evolve during their lifetime. Development of the complex involved initial eruption of andesitic lava fl ows (163-117 ka) followed by a rhyodacite dome plateau (47- 40 ka) synchronous with the onset of the building of a stratocone (52-20 ka), which was later destroyed by a debris avalanche ~3 times larger than that at Mount St. Helens in 1980. Dome plateau emplacement occurred faster and later than has previously been published, implying a compressed duration of cone building and introducing a preced- ing 65 k.y. hiatus. Debris avalanche timing is refi ned here to be older than 10 but younger than 20 ka. Rapid postcollapse rebuilding of the volcanic edifi ce is delineated by 16 groundmass and whole-rock 40 Ar/ 39 Ar ages, which include some of the youngest lava fl ows dated by this method. Increase in cone- building rate and a continued trend toward more mafi c compositions following collapse imply an inter-relationship between the pres- ence of the edifi ce and fl ux of magma from the feeding reservoir. Cone-building rates at Parinacota are similar to those at other well- dated volcanoes on thinner crust; however, the distributed basaltic volcanism prevalent in those other arcs is virtually absent both at Parinacota and elsewhere in the Central Volcanic Zone. This suggests that while the hydrous, calc-alkaline magmas that make up the central volcanoes are not signifi cantly retarded by thick crust, primitive, dry basalts might be.