Mineral chemistry of megacrysts and associated clinopyroxenite enclaves in the Calatrava volcanic field: crystallization processes in mantle magma chambers

Clinopyroxene, amphibole and phlogopite megacrysts appear in the crystal- and xenolith-rich pyroclastic deposits of the Cerro Pelado and the El Aprisco volcanoes (Calatrava volcanic field). These megacrysts display a similar composition to crystals forming clinopyroxenite and rare phlogopite-rich (glimmerite) enclaves. The host magmas are highly porphyritic, showing a complex population of mafic macrocrysts and phenocryst cores. Most of these crystals are chemically similar, suggesting that they constitute a cogenetic suite of phenocrystic origin. Geobarometric estimations indicate that megacrysts and enclaves represent high-P cumulates, mostly formed at about 12–16 kbar within the upper lithospheric mantle (35 to 55 km). The compositional variability of the analyzed minerals indicates a differentiation process controlled by fractionation of olivine, clinopyroxene, amphibole and phlogopite. The crystallization of hydrous mafic minerals at mantle depths facilitated CO2 exsolution and subsequent boiling of the host magma, thus triggering the fragmentation of the semi-crystallized margin of the magma chamber and the excavation of mantle wall-rocks. This deep fragmentation could also explain the complex variety of crystals, enclaves and xenoliths dragged by the volcanic magmas. Two clinopyroxene types (green and colourless) have been found, both as antecrysts (macrocryst/phenocryst cores) and within enclaves. The coexistence of these clinopyroxenes within zoned crystals in clinopyroxenites suggests that they might be cognate, representing primitive and evolved products of a single fractionating magma. This study provides a model for the ascent of crystal- and xenolith-rich magmas that could be regarded in other alkaline volcanics carrying complex crystal cargos from the Cenozoic circum-Mediterranean area.