MAFIC VOLCANISM ON THE PUNA PLATEAU, NORTHWEST ARGENTINA: IMPLICATIONS FOR LITHOSPHERIC FOUNDERING

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 mafic magmatism that led to the creation of monogenetic 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 samples originated from the same SCLM. This implies at least a thin portion of the SCLM has remained intact beneath NW Argentina for the last ~485 million years. Comparison to coastal Jurassic igneous rocks and mantle xenoliths from the nearby Salta rift system suggests the SCLM is chemically decoupled from the depleted mantle to the west and east. 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, suggesting instead partial or piecemeal removal of the lithosphere beneath the Puna plateau.