Evidence for referti l isation of a strongly depleted mantle column in the extra-Andean backarc (Paso de Indios, Argentina)

ZANETTI ALBERTO; PONCE ALEXIS D.; BERTOTTO GUSTAVO W.; MAZZUCCHELLI MAURIZIO; BRUNELLI DANIELE; GIOVANARDI TOMMASO; ARAGÓN, EUGENIO; BERNARDI, MAURO

Marrakech

Congreso; 6th International Orogenic Lherzolite Conference; 2014

In the central part of the Chubut province (Patagonia, Argentina), close to the town of Paso de Indios, there are several outcrops of Cenozoic basalts carrying spinel-facies ultramafic xenoliths. In this area, located in the extra-Andean back-arc region, basaltic necks and dikes outcrop between 43º 36′ ? 43º 50′ S and 68º 53′ ? 69º 02′ W, along with remnants of lava flows, divided in two groups of Paleocene and Eocene age. This volcanism was generated by extensional tectonic related to a transform plate margin episode that affected the southern South America active margin from the Paleocene to the Oligocene, as the Aluk plate detached and a slab window opened beneath the study area. Some Paso de Indios mantle xenoliths were described in regional works focused on the Patagonian mantle (e.g. Rivalenti et al., 2004, J. S. Am. Earth Sci. 17, 121-152), evidencing peculiar records of large degrees of partial melting, along with the metasomatic overprint of strongly LILE-enriched, HFSE-depleted melts. The aim of this work is to characterize the petrochemical processes experienced by the mantle beneath the Paso de Indios region, investigating spinel-facies ultramafic xenoliths hosted in Eocene basalts of the three magmatic bodies, namely the Matilde lava flow remnants, the León volcano and the Chenque dike. The studied samples are mainly spinel-facies harzburgites and clinopyroxene(Cpx)-poor lherzolites, with some dunites. The Chenque xenoliths mainly present porphyroclastic to equigranular texture, whereas those from Matilde and León volcanoes have coarse-grained to porphyroclastic textures. Estimated equilibrium temperatures based on pyroxenes solvus range from 800 to 940°C, showing the lowest values so far exhibited by Patagonia mantle peridotites and indicating a provenance from shallow mantle sectors. The refractory character of the mineral assemblages is matched by the major element mineral compositions, which are mostly Al-poor and Mg-and-Cr rich. Spinel composition is consistent with melt extraction from 8 to 14% for Chenque and León samples, and from 14 to 18% for the Matilde samples. The estimated degree of melting rises up to 24% considering the literature spinel data. However, the occurrence of melt-related open-system processes is suggested by local trends of positive correlation between Na and Cr# in Cpx, being fully confirmed by the trace element compositions. Cpxs from a harzburgitic sample from León volcano show composition rich in U, Th, Sr and LREE. The Matilde harzburgites ubiquitously show Cpx with transient U-shaped REE patterns. The LREE fractionation is very strong, with LaN up to 150 and minimum at the M-HREE region between 0.1-1xCI. The HREE level content (LuN down to 1) is consistent with 20-23% fractional melting of spinel facies DM. V-to-U-shaped REE patterns are also shown by Cpx from Chenque lherzolites and harzburgites. Their M-HREE are more fractionated than that expected in residue after spinel facies basal removal, thus suggesting an onset of the partial melting at garnet facies conditions. Other Chenque lherzolites seem to result from a more extensive refertilisation processes led by LREE-enriched to LREE-depleted melts. The latter gave rise to transient LREE-depleted sinusoidal patterns through reaction with the depleted ambient peridotite. It is, thus, concluded that the shallow mantle column beneath Paso de Indios records an incomplete refertilisation of strongly depleted protoliths. As far as we are aware, this represents a unique example for the Patagonian region, where the mantle usually results completely overprinted by multiple stages of melt migration.