Sub-recent volcanism in Northern Patagonia: A tectonomagmatic approach

Abstract

The Crater Basalt Volcanic Field (CBVF) in northern Patagonia (∼42°S, Chubut province) is located 400 km eastward of the current Peru–Chile Trench and to the southwest of the Meseta de Somuncura, in a back-arc position in the extra-Andean Patagonia. The CBVF lava flows cover glaciation related terraces and present stream valleys. The occurrence of CBVF is related with the Gastre Fault System (GFS), a very significant tectonic feature about 40 km wide. The CBVF volcanics cover a surface of ∼700 km2 , occurring mainly as lava flows and scoria cones. CBVF volcanics have relatively high contents of MgO (6–9 wt.%), Cr (136–289 ppm) and Ni (25–198 ppm), and classify as alkali basalts, basanites and trachybasalts. Geothermometry indicates a crystallization temperature of ∼1140 °C and a ƒO2 of −1.0 1 to 0.0 (log FMQ units). The petrographic and geochemical characteristics of CBVF products suggest that the magma originated from a garnet-bearing lherzolite mantle source with asthenospheric characteristics, as a result of decompression, partial melting and ascent. These melts would have risen through a system of deep faults like the Gastre Fault System, following the last Quaternary glacial event.