Publication: Metamorfismo de alta-P en las rocas volcánicas submarinas de Espasante (II):
Condiciones P-T y evolución reaccional
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Publication Date
2016
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Sociedad Geológica de España.
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Abstract
El bloque tectónico de metavulcanitas de la Mélange de Somozas se caracteriza por registrar un evento de alta-P ocurrido durante su subducción. Este evento se refleja únicamente en la matriz hialoclastítica, que desarrolló una mineralogía compleja favorecida por la alteración hidrotermal de fondo oceánico previa que modificó su composición primaria y favoreció el desarrollo de fases minerales de alta-P durante la subducción. La modelización termodinámica de las metahialoclastitas ha permitido determinar las condiciones P-T de la asociación mineral del pico metamórfico, constituida por moscovita+paragonita+granate+clorita+distena+hematites+epidota+ cuarzo+rutilo. Las pseudosecciones, calculadas en el sistema KNCFMnMASHTO, indican unas condiciones para el pico metamórfico de aproximadamente 17.5-18 kbar de presión y 550ºC de temperatura. La margarita aparece como mineral retrógrado en las muestras, y se forma por debajo de los 11 kbar de presión. La trayectoria P-T propuesta para estas rocas indica una drástica descompresión y un rápido ascenso del bloque de metavulcanitas a través del canal de baja viscosidad en el cual se desarrolló la mélange en sí.
The studied tectonic block of submarine metavolcanic rocks in the Somozas Mélange records a high-P metamorphic event developed during subduction. This metamorphic event is only clearly recorded in the hyaloclastitic matrix of the pillow-breccias. It is suggested that the first hydrothermal metamorphism affecting the hyaloclastic rocks, that strongly modified its chemical composition, favored the development of high-P mineral phases during subduction. Pseudosection modeling of the metahyaloclastites has allowed establishing the P-T conditions of the metamorphic peak mineral assemblage (muscovite+paragonite+garnet+chlorite+kyanite+hematite+epidote+quartz+rutile). Several pseudosections calculated in the KNCFMnMASHTO system indicate P-T peak conditions of 17.5-18 kbar and ca. 550ºC. Margarite appears as a retrograde mineral formed at pressure conditions below 11 kbar. The P-T path evolution shows a drastic exhumation of the metavolcanic block developed after a subduction event. The exhumation was driven by return flow in the low-viscosity subduction channel in which the mélange developed.
The studied tectonic block of submarine metavolcanic rocks in the Somozas Mélange records a high-P metamorphic event developed during subduction. This metamorphic event is only clearly recorded in the hyaloclastitic matrix of the pillow-breccias. It is suggested that the first hydrothermal metamorphism affecting the hyaloclastic rocks, that strongly modified its chemical composition, favored the development of high-P mineral phases during subduction. Pseudosection modeling of the metahyaloclastites has allowed establishing the P-T conditions of the metamorphic peak mineral assemblage (muscovite+paragonite+garnet+chlorite+kyanite+hematite+epidote+quartz+rutile). Several pseudosections calculated in the KNCFMnMASHTO system indicate P-T peak conditions of 17.5-18 kbar and ca. 550ºC. Margarite appears as a retrograde mineral formed at pressure conditions below 11 kbar. The P-T path evolution shows a drastic exhumation of the metavolcanic block developed after a subduction event. The exhumation was driven by return flow in the low-viscosity subduction channel in which the mélange developed.