B.S. Thesis. Advisor: Suzanne Mahlburg Kay
The Frailes volcanic complex of the Bolivian Altiplano plateau is the largest and most prominent ignimbrite of the Andean Central Volcanic Zone. With 2000 km3 of exposed volcanic deposits dating from 25 Ma to the present, the complex provides insight into the processes of large-volume silicic melt formation in a back-arc setting. However, the voluminous 0-10 Ma main body of the Frailes complex remains poorly studied, as the majority of the literature focuses on a small region of 12-14 Ma Sn-Ag mineralization at Cerro Rico – the world’s largest silver deposit. Here, we present geochemical analyses of 25 representative samples from our fieldwork on the Frailes ignimbrite. Whole-rock major element analysis by X-ray fluorescence spectroscopy showed the ~ 7 Ma samples to be highly peraluminous, potassic (>5% K2O) andesitic to rhyodacitic welded volcanic tuffs with an Al-rich, chemically reduced mineral assemblage including biotite, calcic feldspar, magmatic cordierite, and ilmenite. Trace element analysis by instrumental neutron activation revealed steep rare earth patterns with Sm/Yb > 5.0, requiring the presence of garnet in a deep-crustal, high-pressure restitic assemblage. In contrast, the presence of cordierite indicates crystallization at low pressure (< 450 MPa), at depths of less than 14 km. In addition, geochemical analyses combined with the geocronological work of Barke et al. (2007) support a recent (~2 Ma) age for the ignimbrites of the main Frailes Meseta. These ignimbrites are consequently best explained by mixing of a mantle- derived melt with a reduced, lower crustal component at great depths to produce hybrid magmas that ascended and evolved to form shallow crustal (<14 km) magma bodies before erupting in catastrophic, caldera-forming events at ~2 Ma.
Keller, C.B. (2010). Chemical Constraints on the Origin of the Frailes Volcanic Complex in the Central Andean Altiplano Plateau, Bolivia. B.S. Thesis, Cornell University. doi: 10.31237/osf.io/y6kv4