METASOMATIC ENRICHMENT OF THE NORTHERN ALTIPLANO LITHOSPHERE CONSTRAINED BY ISOTOPIC AND TRACE ELEMENT CHARACTERISTICS OF BIOTITE MONZOGABBRO AND PYROXENITE XENOLITHS

A diverse suite of xenoliths was collected from Quaternary mafic to intermediate alkaline lavas of the northern Altiplano Plateau to constrain the composition of the lower lithosphere and potential sources of metasomatism. One group of xenoliths found at the Pleistocene Quimsachata volcano of Southern Peru (200 km inboard of the active arc) consists of biotite-rich monzogabbros and pyroxenites, likely representing cumulate bodies or intrusive mafic veins in the lower crust or upper mantle. These xenoliths are characterized by biotite (up to ~40 modal %) + augite (up to ~40 %) + plagioclase + fluorapatite ± forsterite ± alkali feldspar ± hornblende with minor Fe-Ti oxides. Hydrogen stable isotopes (δ²H) elicit potential volatile sources and range from -33 to -59 ‰ (vs VSMOW) for three of the four samples analyzed, consistent with hydrous minerals and melt inclusions in arc magmas and theoretical predictions for subduction derived fluids. However, low hydrogen yields may indicate dehydrogenation or oxidation of hydrous phases which may have driven isotopic fractionation, shifting δ²H to higher values. Lower δ²H values seen in one sample (-74 to -78 ‰) are consistent with the isotopic signature of mantle-sourced hydrogen (-80 ± 10 ‰). Trace element abundances point towards enrichment by slab-derived fluids, with characteristic enrichment in fluid mobile elements (e.g., Rb, Ba, Pb, La, Ce) relative to more conservative trace elements (e.g., Zr, Hf, Yb, Lu) and notable depletions in Nb and Ta concentrations. Isotopic and trace element data closely matches that of the shoshonitic volcanic host rock (δ²H of biotite phenocrysts = -46 ‰, nearly identical trace element patterns), suggesting a close relationship, perhaps as cumulates involved in magmatic fractionation towards intermediate alkaline compositions. Conversely, these lithologies may be intrusive corollaries to nearby occurrences of ultrapotassic minettes and kersantites. In either case, these xenoliths yield important information on magmatic processes and metasomatic alteration of the Altiplano lithosphere. An Oligocene episode of shallow subduction, which preceded the ongoing phase of bimodal backarc volcanism, is a likely suspect for enrichment of volatiles and incompatible elements in the continental lithosphere.