Geologic reconnaissance of Lautaro Volcano, Chilean Patagonia

Lautaro is the volcano closest to the Chile Triple Junction in the Andean Austral Volcanic Zone. The volcanic edifice of 3,607 m a.s.l. stands out on the north-western part of the plateau of the South Patagonian Ice Field. The volcano basement is composed of low-grade meta-pelites that are cut by tabular intrusive bodies of hornblende-biotite granite. The volcano is almost completely covered by ice and the study was performed on volcanic detritus present in terminal moraines of the Lautaro Glacier and the tephra on the surface of the O'Higgins Glacier. The terminal moraines of Lautaro Glacier contain fragments of light grey dacite rich in plagioclase and hornblende phenocrysts. Some dacite blocks show prismatic jointing, suggesting an origin of hot emplacement and subsequent rapid cooling, possibly resulted from collapse of a steep lava front. Some samples have glassy groundmass and rhyolite-like flow texture, with presence of mafic inclusions and bread crust bomb texture. The surface of the O'Higgins Glacier is covered by lapilli-size pumice fallout deposit. There are many dirt cones covered by volcanic ash composed of pumice fragments, volcanic glass, quartz, plagioclase, biotite, hornblende, and orthopyroxene. The ash and pumice are similar in chemical composition, both indicating an adakitic signature. At least three layers of fallout deposits have been recognised in parts of the surface of the glacier, which may correspond to the latest known eruptions.