Petrogenesis of the Laguna del Maule volcanic complex, Chile (36� S)

Abstract

From 33°–42° S in central-south Chile, there are numerous volcanoes which form part of the Andean magmatic arc caused by subduction of the Nazca plate beneath western South America. The <0.3 m.y. old Laguna del Maule volcanic complex at 36° S is in a transition region between volcanoes at 33°–34° S formed dominantly of hornblende-bearing andesite and volcanoes south of 37° S dominantly composed of basalt and basaltic andesite. The Laguna del Maule complex ranges in composition from basalt (∼0.3 m.y.) to rhyolite (post-glacial). Although there is abundant evidence for magma mixing, basalt and rhyolite have similar Sr and Nd isotopic ratios, thereby requiring that the mixing members had the same isotopic ratios (87Sr/ 86Sr ∼0.70419 and 143Nd/144Nd ∼0.51274). In contrast, dacitic dikes and a volcanic neck which also have evidence for magma mixing are isotopically distinct. Major and trace element abundances are consistent with a genetic relationship between the basalt and rhyolite, either by low-pressure, plagioclase-dominated, fractional crystallization or by partial melting of a plagioclase-rich assemblage. There is no evidence that the rhyolites contain more of a crustal component than the associated basic volcanics.