Chemistry of Post 12 Ma Los Frailes Volcanic Complex Ignimbrites in Bolivia and the Role of Magmatism in the Uplift of the Central Andean Altiplano Plateau

KAY, S.M., KELLER, CB., COIRA, B., JIMÉNEZ, N., CAFFE, P.J.

San Francisco

Congreso; AGU Fall Meeting; 2010

The giant (~2000 km3) backarc Los Frailes volcanic complex in the BolivianAltiplano of the Central Andes records the late Oligocene to Pleistocene magmatichistory of the Altiplano plateau whose time and processes of uplift are controversial.Most previous studies of the Los Frailes complex have concentrated on pre-12 Mavolcanic rocks that host Sn-Ag deposits including the world’s largest silver depositat Cerro Rico. Here, we combine sparse pre-existing chemical data from post 12 Mavolcanic rocks with new major and trace element analyses to show that thevoluminous post-12 Ma ignimbrites are predominantly peraluminous, K-richbiotite-bearing andesites and dacites (58-69% SiO2) that can contain calcicfeldspar, orthopyroxene, cordierite, and ilmenite. Trace element data show that theignimbrites have very steep REE patterns (La/Yb = 30-90) marked by HREEdepletion (Sm/Yb = 5-9) requiring garnet as a deep-crustal restitic phase in a verythick crust. The presence of cordierite crystals in some ignimbrites requires finalequilibration of the erupted magmas at depths of less than ~14 km (< 450 MPa).Other prominent chemical features include LIL enrichment (to 90 ppm La), non-arclike Ba/La ratios (<20) and HFSE/LREE ratios (La/Ta=35-50) as high as those infrontal arc magmas. Sparse 87Sr/86Sr ratios near 0.712 show the magmas containsignificant crustal contributions. The least silicic ignimbrites with the lowest La/Ybratios (25-35) are the voluminous ~7 Ma Livicucho/Condor Nasa andesites/dacitesin the northern part of the complex. These ignimbrites erupted during an ~ 8.5 to~6 Ma peak in backarc ignimbrite activity in the northern Puna to the south and theAltiplano to the north. These eruptions are approximately coincident with the 10-7Ma period of rapid plateau uplift proposed by Molnar and Garzione (2007). Otherwidespread ignimbrites from across the main part of the Los Frailes plateau havechemical features in common with an ignimbrite flow dated at 2.2 Ma by Barke et al.(2007). Together, these ignimbrites are consistent with a voluminous eruption orseries of eruptions at < 3 Ma (a possible erupted volume of ~800 km3) in the regionabove the modern mantle and crustal seismic low velocity anomalies attributed todetached lithosphere and crustal partial melt zones (Beck and Zandt 2002). Overall,the Frailes ignimbrites are best interpreted as hybrid mixtures of decompressionmantle-derived magmas and deep crustal melts that pooled at shallow levels beforeeruption.