Tectonic control on sub-recent volcanism in northern Patagonia.

MJ HALLER

Guangzhou, China

Congreso; IAVCEI - International Conference on Continental Volcanism; 2006

The Crater Basalt Volcanic Field (CBVF) in northern Patagonia (~ 42° S) is located 300 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 pours out in present stream valleys. The occurrence of CBVF is related with the Gastre Fault System (GFS), a very significant tectonic feature of about 40 km wide. The CBVF volcanics cover a surface of ~ 700 km2, occurring mainly as lava flows and scoria cones. The lava flows are mainly pahoehoe type with pressure ridges, tumuli and microflow structures on their surface. 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 traquybasalts.  Geothermometry indicates a crystallization temperature of ~ 1140º C and a ƒO2 of  -1.0 1 to 0,0 (log FMQ units). The petrographic and geochemical characteristics of CBVF products are suggestive of a garnet-bearing lherzolite magma source with asthenospheric characteristics, wich went under decompression melting and ascent. These melts would have risen through a system of deep faults like the Gastre Fault System, after the last Quaternary glacial event.