Phlogopite-bearing fossil plume and EM II component evidences in Patagonia, registered in alkaline basalts from Southern Argentine (36ºS-44ºS)

JALOWITZKI, T.L.R.; CONCEIÇÃO R.V.; ORIHASHI, Y.; BERTOTTO G.W.; GERVASONI F.

Davos (Suiza)

Conferencia; Goldschmidt Conference; 2009

The Patagonian Volcanic Field composed of late Cretaceous to Quaternary volcanism is widely distributed in a continental extra back-arc geotectonic environment. Eleven monogenetic volcanoes accompanied with ultramafic xenoliths are situated from 36‹13ŒS to 44‹52ŒS, and were selected to be studied. These volcanoes are dominantly composed of basanite to alkaline basalt, which are divided into two groups, based on mineralogy, geochemical and isotope compositions. The Group I shows low SiO2 concentration than Group II and both Groups present similar variation in REE patterns (LREE > HREE), but Group I has LREE > HREE compared to Group II. Group II shows HFSE depletion with Ba, Pb and Sr enrichment in the spidergram, while Group I shows Rb and K depletion with HFSE enrichment. The Rb, K and HREEs depletion suggests that the Group I source might be derived from phlogopite-bearing garnet peridotite. Group I alkaline basalts have lower 87Sr/86Sr (Group I = ~0.703; Group II = ~0.704) and similar ƒÃNd (Group I = 3.57 to 6.74; Group II = 0.99 to 5.62) and 143Nd/144Nd (Group I = 0.51282 to 0.51298; Group II = 0.51268 to 0.51292) values, compared to Group II ones. Geochemical trends; incompatible element behavior (e.g. Ba/Nb, Pb/Ce; Group I < Group II) and REE suggest that Groups I and II originated from the similar sub-continental mantle sources, but were undergone to different metasomatism processes. Groups I and II were generated by nearly <3% melting of an OIB-like garnet peridotite, but the Group II has enriched mantle (EMII) characteristics possibly inherited from on-going subduction-related metasomatism, while Group I demonstrates the OIBlike signature, which might result from phlogopite-bearing fossil plume in the subcontinental lithosphere.