Abstract: S07C_O_16Spinel-facies mantle xenoliths occurring in pyroclastic deposits at Gobernador Gregores in the backarc southern Patagonia, ~400 km E of the actual Chile trench, display geochemical signatures ascribed to either carbonatite metasomatism (Gorring and Kay, 2000) or hydrous silicate melts (Laurora et al., 2001). Carbonatite metasomatism in this region is unexpected because subduction of young carbonated oceanic crust (Nazca Plate) occurred during mid-Miocene. Alternatively, introduction of carbonatitic melts/fluids could be related to either the heating of the lithospheric mantle by upwelling slab-window asthenosphere or a rising plume, as suggested for Pragauniyeu xenolithoccurrence (Ntaflos et al., 2001). The xenoliths show both anhydrous and hydrous mineral assemblages. The former (lherzolites and harzburgites) have geochemical composition consistent with melt–assisted partial melting. The latter (amphibole ± phlogopite ± apatite–bearing) display modal and chemical characteristics held as indicators of carbonatitic metasomatism. However, part of these xenoliths has high TiO2/Al2O3 and those with the most pronounced carbonatitic signatures modally trend towards harzburgite. Bulk-rock, clinopyroxene and amphibole show Zr, Hf and Ti negative anomalies, which increase at decreasing Na2O and HFSE concentrations. Steady variation trends between xenoliths with and without carbonatitic characteristics suggest a control by reactive porous flow of a unique agent, inferred to be initially a ne-normative hydrous basalt. Appearance of amphibole may explain the variation of Ti anomaly, whereas the presence of Ti-Zr-oxides is required to explain Zr and Hf anomalies. Modest HFSE anomalies in the infiltrating melt may be acquired during percolation of garnet-facies peridotite.
|Data di pubblicazione:||2004|
|Autore/i:||ZANETTI A; RIVALENTI G; MAZZUCCHELLI M; VANNUCCI R; CINGOLANI CA|
|Titolo:||Metasomatism by OIB-like alakaline melts in the back-arc Patagonian lithospheric mantle: the Gobernador Gregores case (Santa Cruz Province, Argentina).|
|Nome del convegno:||IAVCEI General assembly 2004. Volcanism and its Impact on Society|
|Luogo del convegno:||Pucon (Chile)|
|Data del convegno:||14-19/12/2004|
|Tipologia||Abstract in Atti di Convegno|
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