Experimental petrology, geochemistry and petrography of mantle xenoliths from Prahuaniyeu Volcano, Northern Patagonia, Argentine

GERVASONI F.; CONCEIÇÃO R.V.; BERTOTTO G.W.; GALLAS M.R.; JALOWITZKI, T.L.R.

Davos (Suiza)

Conferencia; Goldschmidt Conference; 2009

The Prahuaniyeu volcano, Northern Patagonia, Argentine, belongs to the North Patagonian Massif and is characterized by OIB-like alkali basalts that host ultramafic xenoliths from the lithospheric mantle, which possibly were stable in the garnet-spinel transition zone. The xenoliths are sp-lherzolites, harzburgites and sp-websterites. Simplectitic texture between the spinel and orthopyroxene indicates the destabilization of garnet (gr) due to decreasing of pressure, suggesting that the reaction ol1+opx1+cpx1+gr1 = ol1+opx1+sp1+opx2+cpx2 took place. Geochemical patterns, normalized to primitive mantle (Sun & McDonough,1989), suggest that the xenoliths come from a depleted mantle, with heavy rare earth elements (REE) fractionation compared to light REE (CeN/YbN = 16,14 - 26,87). They are enriched in calchophile (W,Pb, Sn, Sb) and in LILE (Ba and Sr) elements. The HFSE (Th, Zr, Hf and Y) are depleted, though Nb and Ta are enriched. These evidences indicate melting processes, followed by mantle metasomatic processes associated to liquids or melts related to the subduction plate, and liquids with primitive characteristics. The xenoliths were submitted to pressures of ~2GPa and temperature of ~1300°C. These experiments resulted in the growth of minerals such as ol, opx, cpx, sp, gr and a liquid phase, enriched in CaO, SiO2, MgO, and FeOtotal, indicating that these rocks were in equilibrium in the mantle, in the garnet stable zone.