An erupted migmatite from Cerro del Hoyazo,SE Spain

The almandine-bearing biotite-cordierite-labradorite dacite of the Cerro del Hoyazo is part of the Neogene volcanic range in SE Spain, extending roughly from Cabo de Gata to Cartagena. About 1 vol. % of the lava consists of rock inclusions, measuring over 1 cm, made up of almandine-biotite-sillimanite gneiss, quartz-cordierite gneiss and spinel-cordierite rock. On the ground of their abundance, chemical composition, mineral content and structure, the first and the second type are interpreted as restite inclusions and the third type as recrystallized restite. These restites and the dacitic magma were derived syngenetically from a (semi-) pelitic rock sequence by means of anatexis: the (semi-)pelitic rocks separated into a granitoid melt and Al-rich restites. Euhedral almandine crystals found in the glass base of the dacite have a pre-magmatic origin, and may be compared directly to those in the restites. Another type of inclusion is represented by basic igneous rocks of varying grain size, comprising mainly basaltoid rocks and quartz-rich gabbros. These inclusions commonly bear some restite fragments of the kind mentioned above, and therefore are interpreted as representing basic magma of deeper origin that has absorbed some anatectic material. In part, the composite basic melt thus formed crystallized under plutonic conditions and fragments of the resulting quartz-rich gabbro were incorporated in a later stage in the dacitic melt. Another portion of the composite basic magma was incorporated in the dacitic melt (probably shortly before the eruption of the dacitic magma) as magma blebs (Ø ?1–20 cm) which subsequently crystallized in part, some of them showing a comparatively fine-grained border zone. Two possible hypotheses are suggested for the time relation between the anatexis of the (semi-) pelitic complex and the appearance of the basic magma: (a) the anatexis was of a regional nature, and was in progress when the basic magma entered the stage (and assumedly triggered the eruption of the granitoid magma); (b) the regional anatexis took place considerably earlier and the basic magma intruded an essentially solid migmatite complex, which was then melted down (contact anatexis) and subsequently erupted.