LATE CENOZOIC MAGMATISM OF THE BOLIVIAN ALTIPLANO

Small basalt to dacite volcanic centers are distributed sparsely over the Bolivian Altiplano, behind the Andean volcanic front. Most are Pli ocene to Recent in age, and are characterized by textural and mineralo gical disequilibrium with abundant xenoliths and xenocrysts. True phen ocrysts are rare in the more mafic samples. Compared with Recent volca nic rocks from Andean stratovolcanoes, the Bolivian centers overlap in major element trends. Incompatible element contents tend to be higher , particularly in the eastern Altiplano. The ranges of isotopic compos itions reflect ubiquitous crustal contamination. Pb isotope compositio ns are dominated by Pb from isotopically heterogeneous basement, resul ting in a wide scatter of data lying between inferred crustal composit ions and showing little overlap with possible mantle sources in the re gion. Rocks sampled from the Bolivian Altiplano were probably derived from asthenospheric mantle and subjected to extensive open system diff erentiation during ascent through the 70 km thick crust of the region. Major element trends are largely controlled by the fractionating phas e assemblage (olivine, clinopyroxene and amphibole). Trace element and isotope systematics, however, defy realistic attempts at modeling due to the geographic scatter of samples, the uniformity of compositions at a given center, and the heterogeneity of the contaminant. Neverthel ess, there are first order systematic trace element variations that ap pear to relate to the geometry of the subduction zone. In particular, LILE/HFSE (exemplified by Ba/Nb), and Zr/Nb ratios decrease from the a re front eastward into the Altiplano. These variations are not easily reconciled with control by crustal contamination alone. A model is pro posed in which the asthenospheric source is fluxed by high Ba/Nb slab- derived fluid to induce melting. Beneath the are, high fluid flux incr eases the Ba/Nb ratio of the asthenosphere and leads to high degrees o f partial melting (high Zr/Nb). To the east, lower or no fluid flux le ads to low Ba/Nb and low degrees of partial melting (low Zr/Nb). Melti ng in the back are region of the Altiplano may be facilitated by litho spheric delamination that leads to decompression melting of counter-fl owing asthenosphere. There is no unequivocal evidence that requires a significant role for the lithospheric mantle.