Geochemical and isotope evidence of Nazca Plate-derived components in the Backarc Mantle of Central Patagonia: The Cerro de los Chenques case

Abundant spinel-facies mantle xenoliths (dominantly lherzolites and subordinate harzburgites and dunites) occur in the alkaline lavas of the Quaternary Cerro de los Chenques volcano, in the Andean backarc ~200 km east from the Southernmost Southern Volcanic Zone. Major element variation trends in clinopyroxene show decreasing fertility with increasing Mg#. Clinopyroxene REE patterns are: 1) LREE-depleted and flat from Sm to Yb at 4 x PM (Primitive Mantle) concentration (in lherzolites) passing to LEE-enriched clinopyroxenes with spoon-shaped profile; 2) only slightly LREE-depleted or enriched and flat from Sm to Yb, but at concentrations ranging from ~2 to ~4 x PM (in cpx-poor lherzolites); 3) LREE-enriched, steadily fractionated (in cpx-poor lherzolites and harzburgites). All the clinopyroxenes exhibit Th, U and Sr enrichment (increasing in the LREE-enriched samples) and Nb, Ta, Pb and Ti depletion. These features are consistent with a variety of metasomatic processes ranging from complete re-equilibration with the metasomatic agent, to fluid-assisted melting, to percolation into a fertile mantle under decreasing melt mass and consequent chromatographic-like trace element fractionation. Isotopically, the clinopyroxenes form a tight array from a DM region representing the premetasomatic lithosphere, towards a high-Sr, low-Nd region, overlapping that of the Patagonia adakites and arc basalts. This array differs from that of the backarc basalts for having higher Sr at a given Nd isotope value and trends towards the composition of the Nazca sediments. Isotope enrichment positively correlates with the abundance of LILE and LREE and negatively correlates with the Mg# value and Ti, Y and HREE abundances of clinopyroxene, thus indicating that the metasomatic end-member triggering melting of the pristine spinel-facies lithospheric mantle was enriched in LILE and LREE. The better candidate as metasomatic agent, suggested both by isotopes and trace elements, is a slab-derived component containing a large contribution from sediments. However, the slab depth beneath C. Chenques is probably in excess of 180 Km and any component released from the slab had to cross a very thick mantle segment before reaching the spinel-facies level sampled by the xenoliths. Experimental work indicates that at the depth of interest the main water carrier is phengite both in basalts and sediments and that the slab component was probably released as a supercritical fluid having a K-rich, granitic composition and leaving a garnet-dominated residuum. Crystallisation of phlogopite during the upwards travel depletes the fluid in K and Rb and “freezes” its Sr isotope signature. The fluid reaching the spinel-facies has probably an adakitic composition and it is strikingly similar to the potential melts estimated from clinopyroxenes having the highest radiogenic isotopes.