Slab-derived fluid metasomatism in mantle lithosphere from Agua Poca volcano, Argentina N-Patagonia

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

Vancouver (Canada)

Conferencia; Goldschmidt Conference; 2008

The Quaternary Agua Poca volcano (37º01’S - 68º07’W) is located in the West of the La Pampa Province, Northernmost of Argentine Patagonia, and is part of the extensive basaltic volcanic event of the Payenia Geological Province, related to the Andean extra back arc setting. The Agua Poca is a monogenetic pyroclastic volcano, composed of an intercalation of spatter and cinder layers and host ultramafic mantle xenoliths. The xenoliths show protogranular and protogranular to porphyritic textures, and is composed of olivine (Fo89-91), clinopyroxene (En47-52, Fs2-5, Wo45-49), orthopyroxene (En89-91, Fs8-10, Wo1-2) and spinel (sp) [Mg#76-80; Cr7-12 (100Cr/(Cr+Al))]. The studied xenoliths are basically anhydrous sp-lherzolite and subordinate anhydrous sp-websterite and sp-plclinopyroxenite with whole rock Mg# from 88 to 91. Geothermobarometry data in clinopyroxene indicates equilibrium pressure and temperature trend between 10 to 20 kbar and 900 to 1200ºC. Agua Poca mantle xenoliths are characterized by flat Sun & McDonough (1989) primitive mantle normalized HREE and MREE patterns, and depletion of LREE compared to HREE (CeN/YbN = 0.15–0.50), with exception of the AP10 (1.46) sample. Multi-elemental diagram shows Th, Nb, La and Ce depletion, expressive Pb enrichment and, in lesser intensity, Ba, U, Ta and Ti enrichment. Batch melting modeling, using REE fractionation, indicates up to 10% melting from a primitive mantle composition. Pb/Ce, Ba/Nb, Sr/Ce, Ta/Hf, Nb/Hf and U/Th ratios, together with REE and isotopic (87Sr/86Sr = 0.700536–0.705021) data suggest that the selective enrichment in Pb and LILE (Ba and Sr), and moderate Sr isotopic ratios could be generated by interactions of a weakly depleted mantle lithosphere with water rich fluids released from a subduction slab in the compressive tectonic setting.