INSIGHTS INTO TRANSCRUSTAL PROCESSES BENEATH CONTINENTAL MONOGENETIC VOLCANOES FROM MINOR CENTERS ON THE BOLIVIAN ALTIPLANO

Plio-Pleistocene lavas erupted from monogenetic volcanic centers on the Eastern Bolivian Altiplano display geochemical characteristics typical of Central Andean lavas (13-27^oS, 74-66^oW) with respect to major and trace element abundances and chemical signatures diagnostic of continental arc magmatism: LILE and LREE enrichment, Nb-Ta depletion. Their associated Sr-isotopes however record some of the most isotopically enriched signatures observed in volcanic rocks (<5 Ma) from the Central Andes. ⁸⁷Sr/⁸⁶Sr ratios range from 0.7080 to 0.7169 and correlate with indices of differentiation, a clear indication of crustal contamination. In addition, this range in ⁸⁷Sr/⁸⁶Sr ratios is greater than observed at any nearby Central Andean arc volcano and implies a complex, multi-stage petrogenetic pathway through the crust. Of particular note is crustal xenolith-bearing nature of these lavas, several of which are partially-melted.

The presence of quenched anatectic melts within the xenolith suite permits the direct characterization of potential contaminants. These glasses are isotopically heterogeneous on the sub-mm scale and may provide more realistic constraints on the composition of crustal components to small volume Central Andean magmas. Bulk mixing, AFC, and EC-AFC modelling provided numerous non-unique solutions, supporting a scenario in which crustal assimilation likely occurred during turbulent ascent.

Monogenetic volcanic centers of comparable age extend westward toward the active arc and are used to evaluate across-arc changes in magma petrogenesis. From Zr/Nb systematics, evidence for shallow level storage (low Sr/Y), increasing crustal thickness from west to east (from FeO_T-MgO systematics), previously published Helium isotope data, and seismic survey results, it is inferred that the crustal contribution to mantle-derived magmas at monogenetic centers immediately behind the Central Andean arc-front and on the Central Altiplano is lacking. Here, ascending melts traverse relatively thin lithosphere, potentially as a result of earlier, partial, lithosphere removal. In contrast, lavas erupted on the Eastern Altiplano experienced extensive upper crustal contamination, likely due to the influence of a thick lithosphere resulting from westward underthrusting of the Brazilian Craton.