The backarc mantle lithosphere in Patagonia, South America

The backarc Patagonia mantle underwent pervasive regional re-crystallisation that left only remnants of the pre-existing mantle. Anhydrous xenoliths (dominantly lherzolites and harzburgites, few dunites) are predominant. Xenoliths containing hydrous phases (lherzolites and harzburgites, rare wehrlites) abundantly occur only at G. Gregores. Bulk-rock and clinopyroxene trace element patterns range from predominant LREE-depleted, LILE-enriched and variably Nb-depleted, to LREE-enriched patterns, with negative Zr, Hf and Ti spikes when amphibole is present. V-shaped trace element patterns are found only in rare non-recrystallised xenoliths. Trace element abundances in potential melts calculated from clinopyroxene are similar to arc magmas, but in southern Patagonia fade eastwards to E-MORB-like compositions. Trace element characteristics of bulk-rock and pyroxenes are interpreted as the consequence of two processes: (1) melting in the region of thermal inversion of the wedge, triggered by infiltration of hydrous components; (2) reactive porous flow of the melts into the overlaying mantle. The component triggering melting is inferred to be slab-derived in the western occurrences and a garnet-facies, asthenosphere-derived melt in the eastern occurrences as a consequence of wedge thickening. Differences between northern and southern Patagonia are interpreted to be due to variable contribution of slab components to the wedge. Compared to the southern region, slab-derived melts are tentatively attributed to the subduction of older and colder segments of the Nazca plate in the North.