Mantle Metasomatism at the Edge of a Retreating Subduction Zone: Late Neogene Lamprophyres from the Island of Kos, Greece

The island of Kos at the eastern end of the South Aegean Volcanic Arc hosts numerous late Neogene amphibole-and mica-bearing lamprophyres. The Sr and Nd isotope compositions of these mantle-derived magmatic rocks correlate negatively, extending from the undepleted end of the mantle array towards values typical for continental crust. The crust-like signature is linked to the addition of subducted Mediterranean sediments, which dominate the Pb isotopic composition of all lamprophyres. The mantle endmember of the Sr-Nd isotope mixing array is strongly enriched in incompatible trace elements and shows anorogenic affinities. Whole-rock geochemistry indicates that the lamprophyres originate from depleted lithospheric mantle above the Hellenic subduction zone that was modified by (1) K-rich silicic partial melts or supercritical fluids from subducted metasediments and (2) melts that originate from the asthenosphere below the subducting slab. The first metasomatic component formed phlogopite-orthopyroxene-rich veins, whereas the second component formed amphibole-bearing clinopyroxenites. Subsequent melting preferentially affected these enriched domains. Variable dilution by contributions from ambient peridotite and mixing between the two components caused the large chemical variation observed in the lamprophyres. The asthenosphere-derived component represents an incipient melt with a carbonatite-like trace element signature (e. g. superchondritic Nb/Ta and Zr/Hf; low Ti/Eu). This metasomatic agent probably was introduced into the mantle wedge along ruptures in the slab of the retreating Hellenic subduction zone. Lamprophyres dominated by the sediment signature have isotopic and chemical similarities to basaltic rocks erupted along the South Aegean Volcanic Arc. The much stronger enrichment in incompatible trace elements of the lamprophyres is related to only minor dilution by melts from the ambient mantle. Partial melting in the lithospheric mantle is attributed to extensional tectonics, probably during a stage of rapid slab rollback, with a limited availability of fluid as a fluxing agent. Lamprophyre emplacement occurred along sinistral en echelon structures in a crustal-scale shear zone, which separates Anatolia from the faster extending Aegean back-arc basin.