Probing the deep mantle wedge in an active subduction zone: Xenoliths from the Mercaderes Volcanic District, Southern Colombia

In subduction zones, the mantle wedge is the domain where liquid phases released by the oceanic subducting plates cause mantle hydration, metasomatism, and partial melting, producting the arc crust. However, direct petrologic information on mantle wedges overlying active subduction zones remains scarce. Here we constrain key aspects of the petrological evolution of the supra-subduction mantle underneath the active volcanic district of Southern Colombia (Mercaderes) by studying a unique suite of peridotite and pyroxenite xenoliths hosted by andesites and lamprophyres. These garnet-bearing peridotite, websterite, clino- and orthopyroxenite xenoliths equilibrated between 2.5 and 4 GPa and 1100-1200 degrees C. The studied rocks display considerable heterogeneity in terms of lithotypes, textures, mineral abundance and extent of metasomatic overprint. Most samples display finegrained mylonitic textures overprinting coarse-grained peridotite and garnet pyroxenite precursors; the EBSD study of undeformed garnet websterite samples suggests crystallization from melts and/or melt-rock reaction processes. Modal enrichment in clinopyroxene as well as in the alkalis and LREE contents in some garnet peridotite implies interaction with alkaline basaltic melts, whereas enrichment in silica, Pb and Sr in the garnetbearing orthopyroxenites records interaction with metasomatic, slab-derived liquids. Some garnet clinopyroxenite xenoliths display marked positive Sr anomalies and minor Eu anomalies, indicating derivation of these rocks from Mg-rich, plagioclase-bearing (and garnet-absent) low-pressure cumulates. Their equilibration in the garnet field implies that after crystallization these clinopyroxenites were transported to deeper levels in the mantle, as the result of foundering into the mantle of the dense arc roots, as already documented in Mercaderes and elsewhere in the Andes. In conclusion, the Mercaderes xenoliths probe a heterogeneous and chemically active domain of the Colombian mantle wedge, where deep sinking lower crustal rocks, melt metasomatism by distinct venues of chemically different melts, and deformation predate fast exhumation by volatile-rich melts.