AN EAST-WEST TRANSECT THROUGH THE ANDES AT 36-38 DEGREES S

The volcanoes Llaima, Tolhuaca, Callaqui, Copahue, Caviahue and back arc lavas provide a crossection through the quaternary Andes at 36-38 ^oS. The arc rocks are mainly andesites to dacites, with back arc basalts southeast of Caviahue (Loncopue graben near Laguna Blanca) and Cochiquito volcano further north near Barrancas. The latter overlap with the lava fields of Payun Matru which have an intracratonic character. The arc rocks show a regional trend from west to east with increasing evidence for arc enrichment in the volcanoes nearest to the trench. The La/Yb values increase from Llaima (3-4) to Caviahue (5-8) to Copahue (9-13). The Callaqui samples have an intermediate position in most trace element ratio diagrams between Copahue and Caviahue, whereas the Tolhuaca samples plot within the Llaima array. A Ba/Nb versus Nb/Zr graph shows high Ba/Nb in Llaima and Tolhuaca samples (80-170), intermediate at Copahue-Caviahue (Ba/Nb = 35-75) and still lower in the back arc basalts of the Loncopue graben and Barrancas area. The Nb/Zr values increase with decreasing Ba/Nb, whereas MORB and OIB rocks cover the full range of Nb/Zr (0.01-0.15) at low Ba/Nb (1-10). We interpret these relations as the result of decreasing contributions of the subducted slab going east, with decreasing amounts of water as well (as indicated by glass inclusion data and phenocryst phase equilibria). The variations in Nb/Zr and La/Yb may be a result of the variable degrees of partial melting, with low La/Yb and Nb/Zr in the wet near-trench arc rocks and high Nb/Zr and La/Yb in the less water-rich back arc basalts. The radiogenic isotope systematics of these rocks show overlapping 87Sr/86Sr values, whereas Pb isotope data are currently too sparse to detect any trends. The back arc basalts show a wide range of isotope ratios, with some of the most mafic and least arc-like back arc basalts trending towards the low 206Pb/204Pb field of the slab-window magmas in the back arc in central and southern Patagonia. The occurrence of arc like magmas in the far back arc near Barrancas and the chemical similarity between Caviahue and Callaqui rocks suggests that the mantle enrichment by arc processes may not strictly be related to the current subduction regime. Earlier, less steep subduction (pre 5Ma) may have fertilized the mantle wedge with a different spatial pattern.