Mantle Lithospheric Beneath the South American Plate: Heterogeneity Compositons and Process

CONCEIÇÃO, ROMMULO VIEIRA; KOESTER, EDINEI; MALLMANN, GUILHERME; SCHILLING, MANUEL; CHEMALE JR, FARID; BERTOTTO GUSTAVO WALTER; KAWASHITA, KOSHI; CINGOLANI, CARLOS ALBERTO; HERVÉ, FRANCISCO; WEBER, M B I

San Pablo

Simposio; Simposio 40 anos de Geocronologia no Brasil; 2004

Instituto de Geociencias- Universidade de São Paulo

In order to discuss the heterogeneity composition and process associated to the mantle lithospheric beneath the South American Plate, studies have been carried on in the Patagonia region (Argentina) and Andes region (Colombia). Both regions present centimetric ultramafic mantle xenoliths brought to the surface by alkaline volcanic activities, lavas and pyroclastic rocks, related to a back-arc setting. The  presence of an active subduction zone, with distinct oceanic plates (Cocos, Nazca and Antarctic) underlying the South American Plate is a special geotectonic environment, in which the accretion of distinct terranes modifies and produces diversity in the mantle. New data have been added to this discussion and are hereby described. In the Mercaderes region (Colombia), mantle xenoliths are represented by spinel websterites, (16 kbar, 1065oC) with MORB signature, 87Sr/86Sr = 0.7030 and 143Nd/144Nd = 0.5129; garnet-peridotite/pyroxenite xenoliths (29-35 kbar, 1250-1295oC) with an OIB signature, 87Sr/86Sr = 0.7043 and 143Nd/144Nd = 0.5129 and garnet peridotite (> 38 kbar, 1140-1175oC), which could represent OIB source area, enriched in Nd and low Sr radiogenic (87Sr/86Sr = 0.7041 and 143Nd/144Nd = 0.5135). The Cerro Redondo (Argentina) present spinel lherzolites and harzburgites mantle xenoliths with OIB signature, representing different stages of interaction with the host basalt during their ascension to the earth surface. The Sr-Nd isotopic composition for whole-rock Cerro Redondo xenoliths (87Sr/86Sr = 0.7050-0.7052, 143Nd/144Nd = 0.5129) and the host lava (87Sr/86Sr = 0.7045, 143Nd/144Nd = 0.5127) suggest a mixing line, in response to the basalt-xenolith interaction. The Cerro del Mojon ultramafic xenolith suite (Argentina) comprise spinel dunite and harzburgites (16-19 kbar, 1000oC) with high 87Sr/86Sr (0.7126) and spinel lherzolites (14-15 kbar, 940oC) with low 87Sr/86Sr (0.7031-0.7045). The Estancia Alvarez ultramafic xenoliths (Argentina) is represented by spinel harzburgites xenoliths (11-18 kbar, 900oC) presenting high 87Sr/86Sr (0.7298). In western South American Plate the main tectonic regime since the Paleozoic has been interpreted as an active convergent margin, which was responsible for the continental growth due to accretion of different terranes. Also, this accretion of terranes has caused addition/metasomatism of material into the lithospheric mantle below the continental crust. Since there are distinct isotopic signatures for these mantle xenoliths, heterogeneity compositions and process should be present. In Mercaderes region, MORB-related mantle xenoliths represent melts of upper mantle, while OIB-related mantle xenoliths are related to a mantle plume (hot spot) modified or not by subducted oceanic lithosphere and/or delaminated continental lithosphere. Regarding the mantle evolution in the Cerro del Mojon and Estancia Alvarez, these high isotopic Sr values could be related to the presence of metasomatic processes, in response to the slab related sediment (EM-II), while low values are inferred for deeper mantle source. The most important process in the Cerro Redondo was the interaction between OIB mantle and basaltic melt, producing an enrichment in radiogenic Sr with the increasing of basaltic interaction. Chromatographic effects also have been observed in some xenoliths from these areas, in response to the percolation of a metasomatic agent through the mantle.