Publication: Mineral chemistry of megacrysts and associated clinopyroxenite
enclaves in the Calatrava volcanic field: crystallization processes
in mantle magma chambers
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Publication Date
2019
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Universidad Complutense de Madrid
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Abstract
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 hidratados en el manto facilitaría la exsolución de CO2 y la subsecuente ebullición del fundido, posibilitando la fragmentación de los márgenes semicristalinos de la cámara magmática y la excavación de la roca mantélica encajante. Esta fragmentación profunda explicaría también la compleja variedad de cristales, enclaves y xenolitos atrapados por los magmas volcánicos. Se han encontrado dos tipos de clinopiroxenos (verde e incoloro) que aparecen como antecristales (núcleos de macrocristales/fenocristales) y también en los enclaves piroxeníticos. La coexistencia de ambos tipos de clinopiroxeno en los zonados cristalinos de las clinopyroxenitas sugiere que deben ser cogenéticos, representando precipitados de fundidos de distinto grado evolutivo, pero posiblemente de un mismo magma fraccionante. Este estudio propone un modelo de ascenso y origen de magmas ricos en cristales y xenolitos que puede ser útil para explicar otros tipos volcánicos que transportan complejos cargamentos de cristales profundos, como ocurre frecuentemente en la provincia volcánica circum-Mediterránea.
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.
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.
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article (https ://doi.org/10.1007/s4151 3-019-00101 -3) contains
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