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
Abstract
The giant (~2000 km3) backarc Los Frailes volcanic complex in the Bolivian Altiplano of the Central Andes records the late Oligocene to Pleistocene magmatic history 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 Ma volcanic rocks that host Sn-Ag deposits including the world’s largest silver deposit at Cerro Rico. Here, we combine sparse pre-existing chemical data from post 12 Ma volcanic rocks with new major and trace element analyses to show that the voluminous post-12 Ma ignimbrites are predominantly peraluminous, K-rich biotite-bearing andesites and dacites (58-69% SiO2) that can contain calcic feldspar, orthopyroxene, cordierite, and ilmenite. Trace element data show that the ignimbrites have very steep REE patterns (La/Yb = 30-90) marked by HREE depletion (Sm/Yb = 5-9) requiring garnet as a deep-crustal restitic phase in a very thick crust. The presence of cordierite crystals in some ignimbrites requires final equilibration 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-arc like Ba/La ratios (<20) and HFSE/LREE ratios (La/Ta=35-50) as high as those in frontal arc magmas. Sparse 87Sr/86Sr ratios near 0.712 show the magmas contain significant crustal contributions. The least silicic ignimbrites with the lowest La/Yb ratios (25-35) are the voluminous ~7 Ma Livicucho/Condor Nasa andesites/dacites in 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 the Altiplano to the north. These eruptions are approximately coincident with the 10-7 Ma period of rapid plateau uplift proposed by Molnar and Garzione (2007). Other widespread ignimbrites from across the main part of the Los Frailes plateau have chemical 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 or series of eruptions at < 3 Ma (a possible erupted volume of ~800 km3) in the region above the modern mantle and crustal seismic low velocity anomalies attributed to detached lithosphere and crustal partial melt zones (Beck and Zandt 2002). Overall, the Frailes ignimbrites are best interpreted as hybrid mixtures of decompression mantle-derived magmas and deep crustal melts that pooled at shallow levels before eruption.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2010
- Bibcode:
- 2010AGUFM.T13D..08K
- Keywords:
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- 1031 GEOCHEMISTRY / Subduction zone processes;
- 1065 GEOCHEMISTRY / Major and trace element geochemistry;
- 8104 TECTONOPHYSICS / Continental margins: convergent;
- 8178 TECTONOPHYSICS / Tectonics and magmatism