CRUSTAL XENOLITHS, THEIR ZIRCONS AND APATITES: INVESTIGATING THE CONTINENTAL BASEMENT BENEATH THE CENTRAL ANDES

The chemical similarity between Earth’s continental crust and continental arc andesites implies a crucial petrogenetic link between continental arc magmatism and crustal growth. The Pleistocene-aged monogenetic centers of Pampa Aullagas and Quillacas on the Bolivian Eastern Altiplano erupted through c. 80 km of continental crust and are hosts to a petrologically diverse suite of crustal xenoliths. These xenoliths, collectively, represent a long history of crustal terrane accretion and subduction zone magmatism along the western margin of South America. The sampled suite comprises igneous and metamorphic lithologies including microdiorites, granodiorites, gneisses, garnet-mica schists, and granulites. This study reports new whole-rock geochemical data, in-situ analysis of accessory apatite, and U-Pb geochronology of zircons in the sampled xenoliths with the aim of providing constraints on the petrogenesis of the suite and consequently, on the timescales of crustal differentiation, accretion, and growth beneath the modern-day Bolivian Altiplano.

Whole-rock geochemical data confirms compositional heterogeneity between the sampled xenoliths, as expected based on observed lithological and mineralogical variations. New U-Pb zircon ages from igneous xenoliths record plutonism associated with Neogene (and younger) arc magmatism while previously dated zircons in metamorphic xenoliths range from the Devonian to the Paleoproterozoic (McLeod et al., 2013, GSA Bull. v. 125). Preliminary apatite REE data from two igneous xenoliths display variable but distinct geochemical characteristics (n=73): e.g. La_N/Ce_N (average 0.74 vs. 0.99), Eu anomalies range from 0.23-0.39 vs. 0.57-0.77, LREE signatures: La_N/Sm_N at 0.36-2.6 vs. 1.77-2.71, and variable HREE depletion: Gd_N/Lu_N 3.95-16.18 vs. 2.78-9.57, respectively. It is anticipated that this new dataset will allow crustal domain boundaries beneath Pampa Aullagas and Quillacas to be evaluated, potentially leading to a renewed understanding of the tectonic block arrangement of the region. Ultimately, the bulk geochemical data combined with in-situ analysis of accessory phases will yield vital insights into the processes and events that have established the continental basement on which the modern-day Andean Cordillera is constructed.