Slab-related metasomatism in the Patagonia backarc lithosphere of the Andean subduction: the Cerro de los Chenques case.

Recent research on the backarc mantle lithosphere in Patagonia has shown that it suffered a regional, pervasive, re-crystallisation and that the regions to the north and south of the latitude of the actual triple point (40°30’) between the Antartctica, Nazca and South America plate present important differences in the regional distribution of metasomatic components. Subduction of the Chile ridge is believed to have induced an interruption in the subducting plate (slab window, D’0razio et al. 2001, Gorring and Kay, 2001), upraise and melting of the sub-slab astenosphere, producing the abundant plateau and postplateu OIB like lavas, which record slab component only in the westernmost occurrences (Stern et al., 1990). The Cerro de los Chenques locality occurs immediately to the north of the latitude of the Triple point, it overlays the subducted Nazca plate and occurs at the edge of the putative slab window. It is, therefore, of especial importance for revealing the processes and constraining the metasomatic components of the mantle wedge in a complex geotectonic situation.Xenoliths occurring in alkaline lavas are predominately granular to porphyroblastic spinel-facies lherzolites, being harzburgites and dunites less abundant. They do not contain hydrous phases, but may contain trace amounts of glass derived from infiltration of the host basalt. They record equilibrium temperature in the range of 900-1100°C. Bulk-rock and mineral phases exhibit major element variation trends apparently consistent with melting and melt removal processes. This is also the case for moderately incompatible elements (e.g. HREE) in bulk rock and clinopyroxene, but not for highly incompatible elements.In lherzolites, clinopyroxene varies from moderately LREE depleted (Cen/Ybn 0.26 – 0.91) to moderately LREE-enriched (Cen/Ybn up to 2.6), being the REE pattern virtually flat in the range Eu – Lu at a 2 to 5 xPM concentration level. Harzburgitic clinopyroxene is LREE-enriched and the whole REE pattern varies from U-shaped to steadily fractionated (Cen/Ybn 14.4) In dunite, cpx is slightly LREE enriched. All the pyroxenes are enriched in Th, U and La with respect to Nb and Ta and depleted in Ti and Pb with respect to Gd and Ce, respectively. The 87Sr/86Sr value in clinopyroxene ranges 0.70279 – 0.70423 and 143Nd/144Nd 0.51279 – 0.51349. In the Sr-Nd isotope diagram, clinopyroxenes constitute a tight array (R2 = 0.79) between a highly depleted end-member and a component enriched in 87Sr/86Sr with respect to BE. The C. Chenques array runs at higher 87Sr/86Sr than the field of the Gobernador Gregores xenoliths and it is followed also by the xenoliths from P. Indios and E. Alvares (northern Patagonia) and Fraile and Cumbres (south of the volcanic gap). It is also on line at the enriched side with the isotope values of the C. Pampa, Lautaro and Aguilera adakites (those closest to the present occurrence).Isotope variations correlate with bulk-rock and cpx composition, so that the increase of 87Sr/86Sr correlates in cpx with an increase of the Mg# value and Sr, Zr, LREE, Pb, Th and U concentrations and with the decrease of Na, Fe, Ti, V, Y, Nb and HREE concentrations. Obviously the 143Nd/144Nd variation with respect to the geochemical parameters has the opposite behaviour of Sr. Assuming higher porosity in depleted peridotites, these data may indicate that a metasomatic agent affected a mantle that suffered previous variable depletion, being the amount of the metasomatic component proportional to the depletion. Alternatively, the variation trends may indicate melting triggered by the addition of a metasomatic agent and removal of the resulting hybrid melt. The increase of the highly incompatible element concentrations with increasing isotope fertility favours the first possibility. The trace element - isotope relationships reveal that the added component was enriched in LILE, LREE, Zr and depleted in Nb and Ti. These are characteristics expected in components derived from the subducting slab. The similarity of the potential melts estimated from clinopyroxene with the Patagonia arc magmas supports this conclusion. More detailed information on the composition of the subducted plate is provided by the correlations of isotopes with Th/La, Pb/U, Ce/Pb, Nb/U and Zr/Hf, showing that the metasomatic component was enriched in Pb with respect to U, in Ce with respect to Pb, in Th with respect to La, thus suggesting contribution of altered oceanic crust, pelagic sediments and, possibly, continental detritus components occurring in the trench. However, Klein and Karsten (1995) have documented the occurrence of Chile ridge basalts having arc-like geochemistry. Subduction of the Nazca plate related to one of these segments may control the composition of the metasomatic agent.