Metasomatism and melting in carbonated peridotite xenoliths from the mantle wedge: The Gobernador Gregores case (Southern Patagonia)

Spinel-facies mantle xenoliths occur in a diatreme cutting through the Neog ene Southern Patagonia Plateau at Gobernador Gregores (Santa Cruz Province, Argentina). This plateau is in a back-arc position with respect to the Chi le trench. Xenoliths differ in their whole-rock composition from other Sout h America occurrences, having higher CaO/Al2O3 ratios and in some samples, TiO2 enrichment, whereas the Na2O/Al2O3 variation range is similar. Three a ssemblages can be distinguished. Assemblage 1, in anhydrous protogranular l herzolites and harzburgites, contains clinopyroxene with a depleted major a nd trace element composition, indicating premetasomatic depletion processes . This assemblage fully recrystallized to Assemblage 2 (amphibole +/- phlog opite +/- Cl-apatite-bearing) during a metasomatic episode. This causes cli nopyroxene to acquire geochemical characteristics often attributed to carbo nate-melt metasomatism. Noticeably, amphibole is markedly enriched in Nb (u p to 298 ppm), especially when depleted in Ti. A further event, related to decompression during xenolith uplift to the surface, induces closed-system (perhaps with the exception of CO2 addition) disequilibrium melting of Asse mblage 2, dominantly of amphibole. It is found in pockets (where amphibole is a residual phase) consisting of Na Si-rich glass and carbonate (Mg-rich calcite) drops, and in veins originating from the pockets (Assemblage 3). E uhedral olivine, clinopyroxene and spinel crystallize only in the silicate glass. So do new, euhedral apatite crystals when glass is in contact with p revious Assemblage 2 apatite. Textural evidence and comparison with experim ental work suggest that silicate glass and carbonates are the result of unm ixing of a former homogeneous melt. Because of the different flow rates of carbonate and silicate melt, the xenoliths become enriched in carbonate, wh ich is found in the veins, during their migration. Thus, the high CaO/Al2O3 ratio of whole rocks provides inconclusive evidence of carbonatite metasom atism. This factor, and other minor deviations from the expected results of carbonatite metasomatism, lead us to hypothesize an aqueous, Cl-rich fluid , possibly slab derived, as an alternative agent. Amphibole, resulting from reactive porous flow of this agent in the mantle, could fully explain the observed geochemical features, as indicated by estimates of its partition c oefficients.