SPINEL-PERIDOTITES FROM SIERRA BAGUALES (PATAGONIA) : INSIGHTS INTO PARTIAL MELTING AND REFERTILIZATION OF MANTLE WEDGE PERIDOTITES IN BACK-ARC SETTINGS

Mantle xenoliths are abundant in many different localities throughout Patagonia. They are found in plateau and post plateau alkaline volcanism (Miocene to Pleistocene) from the Patagonian back-arc region. The latter is due to the subduction of the Chile triple junction point, where the Nazca and the Antarctica plates subduct under the South American plate, leading to the opening of a slab window. Mantle xenoliths from these localities are important because they could provide information about the Andean mantle wedge. In the present study, mantle xenoliths, which include spinel bearing dunites, harzburgites and lherzolites, were collected in Sierra Baguales, Austral Patagonia, around 300 km east of the subduction trench in Chile. We use petrographic analyses, geothermometry as well as major and trace elements geochemistry in order to identify the different processes which have affected this lithospheric mantle. Harzburgites and lherzolites yield equilibration temperatures between 850°C and 950°C using the Wells (1977) and Brey and Köhler (1990) thermometers. Olivine, cpx and orthopyroxene Mg# range from 90 to 92.3, 89 to 94.7 and 90.5 to 92.6 respectively. Spinel is magnesiochromite and has Cr# between 25 to 50% from the most fertile to the most depleted peridotites. There are 3 distinct groups of REE patterns in cpx: hump-shaped (group 1), hump-shaped with LREE enrichment (group 2) and flat (group 3). We estimate, based on the best fit of MREE and HREE content in these cpx, that the initial melting process, which depleted these peridotites, starts in the garnet stability field (0-4%) followed by moderate melt extraction in the spinel stability field (6-9%). Secondary melt enrichment, associated to chromatic effects related to melt/fluid circulation, explain the LREE enrichment observed in cpx from peridotite samples. Consequently, it is possible to understand the contribution of slab-related fluids versus alkaline liquids to the geochemistry of the studied peridotites.