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

Abstract

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 CO 2 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. Keywords Mafic megacrysts • Clinopyroxenite • Glimmerite • Melilitite melt • Calatrava volcanic field • Alkaline circum-Mediterranean province Resumen Megacristales de clinopiroxeno, anfíbol y flogopita aparecen en los depósitos piroclásticos ricos en cristales, enclaves y xenolitos, de los volcanes de El Aprisco y Cerro Pelado (campo volcánico de Calatrava). Estos megacristales muestran una composición química similar a los cristales que forman los enclaves clinopiroxeníticos asociados, incluyendo los poco comunes enclaves ricos en flogopita (glimmeritas). El magma volcánico es de textura porfídica, mostrando una compleja población de fenocristales y macrocristales máficos, con núcleos residuales, que sugieren formen una suite cogenética con aquellos. Las estimaciones geobarométricas indican que los megacristales, así como los núcleos de fenocristales y los enclaves clinopiroxeníticos representan acumulados de alta presión, formados entre 12-16 kbar, en el manto litosférico superior (de 35 a 55 km). La variabilidad composicional de estos minerales máficos apunta a un proceso de diferenciación controlado por la cristalización de olivino, clinopiroxeno, anfíbol y flogopita. La cristalización de minerales máficos hidra...

Deep Magma Storage Revealed by Multi-Method Elemental Mapping of Clinopyroxene Megacrysts at Stromboli Volcano

“…One of the most widely used tests for equilibrium is the Fe-Mg exchange coefficient K D (Fe-Mg) cpx−melt , with an equilibrium range refined at 0.28 ± 0.08 by Putirka (2008). Whilst our data return equilibrium Fe-Mg conditions (Supplementary Figure 5), the K D (Fe-Mg) cpx−melt coefficient is known to be dependent on temperature and melt composition (Putirka, 2008;Villaseca et al, 2019), and does not respond to potential kinetic effects during crystal growth (Mollo et al, 2013;Mollo and Hammer, 2017). In fact, the value of 0.28 ± 0.08 represents the weighted arithmetic mean of a much broader equilibrium range of 0.04-0.68 obtained by regressing clinopyroxene-melt compositions from phase equilibrium experiments (Putirka, 2008).…”

Anatomy of Intraplate Monogenetic Alkaline Basaltic Magmatism

“…However, in different systems storage and crystallization may occur over a variety of different levels. Clinopyroxene thermobarometry revealed a bimodal distribution in the Calatrava Volcanic Field (Spain) (Villaseca et al, 2019). These authors interpreted the formation of cumulates within the lithospheric mantle as well as crystallization within the crust.…”

Depleted lherzolite xenoliths from the leucititic Morrón de Villamayor volcano (Calatrava volcanic field, Spain)

“…The xenolith reaction with high-T host melts (1250-1300°C are experimental liquidus temperatures for silica-undersaturated alkaline magmas; Perinelli et al, 2008) may explain the high temperature values obtained in these MVM secondary clinopyroxenes. Accordingly, estimated P-T crystallization conditions of clinopyroxene-2 transits from the MVM subcontinental lithospheric mantle (MVM primary paragenesis) toward the P-T path of the CVF alkaline magmatism (Villaseca et al, 2019b) (Fig. 6), indicating their variable neoformation conditions during volcanic transport.…”

“…South Tajo basin geotherm (red line) fromJiménez-Díaz et al (2012). P-T path of the Calatrava volcanism fromVillaseca et al, 2019b.…”