Subduction- vs- Intraplate-Type Melt Migration in the Alboran Lithospheric Mantle: Insights From the Tallante Xenoliths (Betic Cordillera, SE Spain)

The Alboran Sea region has been affected since the late Oligocene by widespread eruption of tholeiitic to calc-alkaline magmas followed by Late Neogene alkaline basalts. These magmatic episodes are related to Neogene lithospheric extension beneath the Alboran domain, as a consequence of slab roll-back. According to recent models, subduction of oceanic lithosphere caused continental-edge delamination of subcontinental lithosphere, associated with upwelling of plume-type mantle sources. The Alboran lithospheric mantle thus constitutes a unique setting to investigate the effects of subduction- and intraplate-type metasomatism. Here we present a microstructural and geochemical study of mantle xenoliths from the Cabezo Tallante Late Neogene alkaline volcanic center (SE Spain). These xenoliths record multiple episodes of reactive porous melt percolation, and melt entrapment, tracking their progressive extension-related uplift from P > 20 Kb to 7-10 Kb. This is documented by i) crystallization of undeformed olivine replacing pyroxene porphyroclasts, and unstrained opx overgrowing undeformed olivine and pyroxene porphyroclasts, in porphyroclastic spinel peridotites, ii) development of annealed equigranular structure, likely enhanced by heating during melt percolation, iii) crystallization of interstitial (plag±ol±opx) aggregates between mantle minerals in porphyroclastic and equigranular xenoliths. Cpx in equigranular peridotites have smooth trace element spectra characterized by slight LREE depletion; computed equilibrium liquids have a tholeiitic-transitional affinity. Diffuse melt percolation was followed by intrusion of melts with distinct chemical affinity. The first event is documented by the intrusion of cm-sized gabbronoritic veins, showing a fine-grained opx reaction rim against the host peridotite. Similar gabbronoritic lithotypes were previously documented and ascribed to slab-derived melts. A quite remarkable textural feature in these veins is the occurrence of small but diffuse Cl-rich apatite crystals, mostly included in plagioclase. Cpx in the host peridotite, partly corroded by gabbronoritic apophyses, are enriched in Th,U, L-M-REE, relative to cpx in both sp-and plag-peridotites, whereas they preserve low Nb and Ta contents. Plag and opx in the vein are also LREE-enriched. Textural and chemical features indicate that parental melts to the gabbronoritic veins were Si-saturated, enriched in LILE and volatile (Cl) components and depleted in Nb,Ta, consistent with subduction-related metasomatic melts. The latest magmatic event recorded in the Tallante xenoliths is documented by the intrusion of cm- thick dikelets of Ti-rich amphibole pyroxenites, with clear alkaline affinity. Cpx and amph in the pyroxenites display similar convex-upward LREE-MREE spectra, and amphibole has high Nb,Ta concentrations. Pyroxenite intrusion provide the opportunity to observe the local effects of alkaline metasomatism. Cpx in the reaction zone close to pyroxenites are enriched in Ti and L-M-REE, approaching the composition of pyroxenite cpx. Extension-related uplift of the Tallante xenoliths was thus accompanied by interaction with melts of different sources tracking the transition from subduction- to intraplate-type magmatism in the Alboran domain.