GSA Annual Meeting in Phoenix, Arizona, USA - 2019

Paper No. 78-3
Presentation Time: 9:05 AM


MCLEOD, Claire1, DAVIDSON, Jon2, DE SILVA, Shanaka3 and VELAZQUEZ SANTANA, Liannie C.1, (1)Department of Geology and Environmental Earth Science, Miami University, Oxford, OH 45056, (2)Earth Sciences, Durham University, Durham, DH1 3LE, United Kingdom, (3)College of Earth, Ocean, and Atmospheric Sciences, Oregon State Univeristy, 104 CEOAS Administration Building, Corvallis, OR 97331

Plio-Pleistocene lavas erupted from monogenetic volcanic centers on the Eastern Bolivian Altiplano display geochemical characteristics typical of Central Andean lavas (13-27oS, 74-66oW) 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 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.