Crust and Lithospheric Melting Under Plateaus: A Petrologic and Geochemical Pilot Study of Pliocene Volcanic Rocks West of Lake Titicaca (Southern Peru)

Abstract

Volcanic rocks of the Pliocene Barroso formation in southern Peru were collected for a preliminary petrologic study. These volcanic rocks are typically found as aerially extensive 5-25 meter thick lava flows immediately west of Lake Titicaca and are interlayered with sedimentary continental and lacustrine deposits. Regionally, they represent the latest volcanic products in an area where various forms of subduction-related magmatism have generated distinct arc segments since the Cretaceous. Barroso flows are more mafic than previous volcanic products and formed at a time when the main arc was being formed significantly inboard; consequently, it is plausible that they are formed because of secondary processes operating on the plateau and not the main slab dehydration-related melting above the slab. On a TAS diagram the samples are high-K calc-alkaline series while some are shoshonitic and range from trachyte to basaltic trachyandesite. They are mafic to intermediate in silica concentration and have relatively high concentrations of MgO (<7%) and FeO (<9%). Trace elemental concentrations are indicative of a near-adakitic composition. Overall, these petrographic and geochemical characteristics are like other relatively small volume magmatic products found elsewhere on the plateau near local “bobber-type” basins (Ducea et al., 2013; Murray et. al., 2015) and where magmatism was interpreted to be triggered by small scale delamination events or heating overthickened crust. Thermodynamic forward modeling using major element composition suggest that these rocks were generated at pressure/temperature conditions corresponding to the lower part of the lithosphere, perhaps including the lowermost crust. The most likely source rocks are peridotites mixed with various pyroxenite-rich cumulates pre-existent in the crust (also found as rare xenoliths in some Barroso lavas). We speculate that these dense assemblages provide the negative buoyancy responsible for the topographic low represented by Lake Titicaca. The basin will presumably invert when the dense anomaly will detach and founder in the mantle.