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

The volcanoes Llaima, Tolhuaca, Callaqui, Copahue, Caviahue and back arc lavas form a cross-section through the quaternary Andes at 36-38^°S. The arc rocks are andesites to dacites, with back arc basalts southeast of Caviahue (Loncopue graben) and Cochiquito volcano further north near Barrancas. The arc rocks show a regional trend from west to east with 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), probably reflecting decreasing degrees of partial melting. The Callaqui samples have an intermediate position in most trace element ratio diagrams between Copahue and Caviahue, whereas the Talhuaca samples plot within the Llaima array. A Ba/Nb versus Nb/Zr graph shows high Ba/Nb in Llaima and Talhuaca samples (80-170), intermediate at Copahue-Caviahue (Ba/Nb = 35-75) and still lower values in the back arc basalts. The Nb/Zr values increase with decreasing Ba/Nb, whereas MORB and OIB rocks cover a wide range of Nb/Zr (0.01-0.15) at low Ba/Nb (1-10). We interpret these cross-arc relations as the result of decreasing contributions of the subducted slab going east, including decreasing amounts of water. The radiogenic isotope systematics of these rocks show overlapping ⁸⁷Sr/⁸⁶Sr, and some back arc basalts share the low ²⁰⁶/₂₀₄ Pb ratios with the slab window magmas further south in Patagonia. The phosphorous chemistry of these rocks has some unexplored aspects. LREE/P diagrams show arc rocks with elevated La concentrations at low P above the mantle array, whereas most Barrancas, Loncopue and slab window rocks trend to higher P values (0.4-0.8 wt.% P₂O₅) at lower La levels below the mantle array. These relations become more pronounced for the HREE, with a tail to high P values for the back arc and slab window magmas. We speculate that the host phase of P in the subducted complex remains refractory until great subduction depths: at high T-P, when the apatite (??) finally breaks down, the P is released to the overlying mantle wedge. We hypothesize that the subducted complex becomes fractionated during the ongoing subduction process, loosing its most mobile elements and water below the main arc, whereas the refractory elements are transported to greater depth where they contribute to back arc magma formation.