Crustal melting in the alborán domain: constraints from xenoliths of the Neogene Volcanic Province

Metapelitic xenoliths enclosed in the Crd-Grt-bearing lavas of the Neogene Volcanic Province of SE Spain retain evidence of partial melting and relevant information on the mechanisms and P-T conditions of crustal anatexis, preserved by rapid exhumation and cooling during eruption. Both at El Joyazo and Mazarrón, microstructures show that anatexis was accompanied by foliation development, implying that the xenoliths represent portions of a deforming crystalline basement, partially molten before being enclosed in the dacite.

At El Joyazo, the xenoliths have a marked restitic composition, are made of Bt-Pl-Sil-Grt-graphite (±Ilm, Crd, Her, Qtz), and contain abundant leucogranitic glass. Primary glass inclusions in all minerals indicate that the whole restite assemblage crystallised in the presence of melt, which is only possible by a disequilibrium melting mechanism due to very rapid heating rates. Variable degrees of graphite crystallinity point to syn-anatectic crystallisation of graphite, implying that the main stage of anatexis took place under fluid-present conditions. Further melting of biotite to hercynitic spinel was probably fluid-absent. Mass balance calculations among glasses, xenoliths and probable metapelitic protoliths from the basement of the Betic Cordillera indicate degrees of melting in the range of 35–60 wt. %.

Crustal anatexis took place at 5–7 kbar, 850 ± 50 °C, and was followed by a further melting stage at T>900°C, probably when the xenoliths were already incorporated into the dacite. Calculated pressures approximate the actual Moho depth in the region (ca. 21 km), and suggest that partial melting of the xenoliths occurred close to the crust-mantle boundary. The very high temperatures, the absence of HP relicts, and the syn-anatectic pseudomorphs of sillimanite after andalusite observed in many xenoliths at Mazarrón, are difficult to reconcile with a model of decompression melting, and rather suggest regional scale (isobaric) heating related to emplacement at shallow depth of asthenospheric mantle and/or mantle derived magmas.