PARTIAL LITHOSPHERIC FOUNDERING BENEATH THE PUNA PLATEAU, NORTHWEST ARGENTINA EVIDENCED BY TRACE ELEMENT AND ISOTOPE GEOCHEMISTRY OF LATE CENOZOIC, SMALL VOLUME BASALTS

Lithospheric foundering has drawn increasing attention as an important contributor to continental plateau formation, especially as a driver for increased elevation, extension and mafic magmatism. This contribution focuses on the young (ca. 7 Ma to present) mostly mafic magmatism that led to the creation of monogenetic and simple polygenetic volcanoes throughout the Puna Plateau of NW Argentina. Lavas from these volcanoes provide a means to evaluate the recent petro-tectonic development of the plateau and, in combination with basement intrusive rocks, determine the isotopic composition and long term evolution of the lithosphere beneath the central Andean back-arc domain. Mafic samples have trace element concentrations and isotopic values typical of an enriched magma source region. We propose the mafic magmas are sourced from an aged, metasomatized sub-continental lithospheric mantle (SCLM). The lavas have isotopic values nearly identical to those of Early Ordovician Famatinian gabbro and granodiorite. We suggest the most primitive Puna lavas and Famatinian magmas originated from the same SCLM. This implies that a portion of the SCLM has remained intact beneath NW Argentina for at least the last ~485 million years. Comparison to coastal Jurassic igneous rocks and mantle xenoliths from the nearby Salta rift system suggests the SCLM has been removed from convection, being chemically decoupled from the depleted mantle. This has been the case for hundreds of millions of years despite long term tectono-magmatic activity along the proto-Andean and Andean margin and within the continental interior. Our data almost certainly rule out large delaminating bodies, meaning the SCLM has been thinned, in accord with geophysical and mass balance arguments, but not fully removed. Partial or piecemeal removal of the lithosphere beneath the Puna Plateau is thus put forth as the foundering mechanism during the Late Cenozoic.