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