INVESTIGADORES
BERTOTTO Gustavo Walter
congresos y reuniones científicas
Título:
Evidence for strong depletion, followed by multiple refertilisation, in the mantle column of the extra-Andean backarc (Paso de Indios, Argentina)
Autor/es:
MAZZUCCHELLI MAURIZIO; PONCE ALEXIS; BERTOTTO GUSTAVO W.; ZANETTI ALBERTO; BRUNELLI DANIELE; GIOVANARDI TOMMASO; ARAGÓN, EUGENIO; BERNARDI, MAURO
Reunión:
Congreso; 87° Congresso della Società Geologica Italiana e 90° Congresso della Società Italiana di Mineralogia e Petrologia; 2014
Resumen:
In the central part of the Chubut province, 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. Here, the petrochemical processes experienced by spinel-facies mantle xenoliths, hosted in Eocene basalts of the Matilde lava flow remnants, the León volcano and the Chenque dike, are presented. 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. 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% (Chenque and León) and 14 to 18% (Matilde). 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 100 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 DM. V-to-U-shaped REE patterns are also shown by Chenque lherzolites and harzburgites. Their MHREE 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. This represents a unique example for the Patagonian region, where the mantle is usually completely overprinted by multiple stages of melt/fluid migration.