Abstract

Calbuco volcano is a Late Pleistocene-Holocene composite stratovolcano located at 42 degrees 20' S, in the southern region of the Southern Volcanic Zone of the Andes (SSVZ; 37 degrees-46 degrees S). In contrast to basalt and basaltic andesite, which are the dominant lava types on the volcanic front from 37 degrees to 42 degrees S, Calbuco lavas are porphyritic andesites which contain a wide variety of crustal xenoliths. They have SiO2 contents in the 55-60% range, and have comparatively low K2O, Rb, Ba, Th and LREE abundances relative to other SSVZ centers. Incompatible element abundance ratios are similar to those of most SSVZ volcanics, but Sr-87/Sr-86 and Nd-143/Nd-144 are respectively higher and lower than those of adjacent volcanic centers. Basalts from nearby Osorno stratovolcano, 25 km to the northeast, are similar to other basaltic SSVZ volcanoes. However, basalts from several minor eruptive centers (MEG), located east of Calbuco and Osorno volcano along the Liquine-Ofqui fault zone (LOFZ), are enriched in Ba, Nb, Th and LREE, and have higher La/Yb and lower Ba/La, K/La and Rb/La. Sr-87/Sr-86 and Nd-143/Nd-144 in MEC basalts are respectively lower and higher than those of Osorno and Calbuco lavas. We suggest that MEC basalts were produced by lower extents of mantle melting than basalts from Osorno and other SSVZ stratovolcanoes, probably as a result of lower water content in the source of MEC basalts. Calbuco andesites formed from basaltic parents similar to Osorno basalts, by moderate pressure crystallization of a hornblende-bearing assemblage accompanied by crustal assimilation. Hornblende stability in the Calbuco andesites was promoted by the assimilation of hydrous metasedimentary crustal rocks, which are also an appropriate endmember for isotopic trends, together with magma storage at mid-crustal depths. The unique characteristics of Calbuco volcano, i.e. the stability of hornblende at andesitic SiO2 contents, low Nd-143/Nd-144 and high Sr-87/Sr-86, and abundant crustal xenoliths, provide evidence for crustal assimilation that is not apparent at more northerly volcanoes in the SSVZ.