A metasomatized lithospheric mantle contribution to the genesis of Li-rich magmas in slab-window settings

The geological factors controlling Li abundances in the mantle and mantle-derived magmas rising through the continental lithosphere, as well as their implications for Li ore genesis in the shallow crust, are still under debate. Here, we look from the mantle source perspective at those mechanisms that may boost the Li inventory in continental arc magmas, by characterizing a set of sub-arc mantle xenoliths from the southern Andes (Coyhaique volcanic field, western Patagonia). The mineral trace element signatures and oxygen fugacity estimates (FMQ > +3) in these peridotite xenoliths are consistent with those expected for a mantle wedge fluxed by oxidizing, subduction-related silicate melts rich in slab-derived volatile components. However, our data support that subduction-related metasomatism did not enhance significantly the Li inventory of the sub-arc lithospheric mantle, at odds with the generally claimed major role of slab-derived fluids in enriching Li in the supra-subduction mantle wedge. Major and trace element compositions of minerals also record transient thermal and chemical anomalies fingerprinting the interaction with OIB-type alkaline melts, which percolated through the shallow (7.2-16.8 kbar) and hot (952-1054 °C) mantle wedge in response to the opening of an asthenosphere slab window and ridge collision. This alkaline metasomatism produced exceptionally high Li abundances (6-15 ppm) in metasomatic clinopyroxene, promoting the generation of a Li-rich and fertile lithospheric mantle wedge. A numerical model suggests that low degrees (< 10%) of partial melting of this alkaline-metasomatized sub-arc lithospheric mantle generates primitive melts having Li abundances (⁓13 ppm) much higher than average subduction-zone basalts. Extreme differentiation by fractional crystallization of these Li-rich magmas may provide an effective pathway for enhancing the fold enrichment process required for a magmatic rock to effectively source ore deposition in the shallow arc crust.