Crystallization conditions during the differentiation of the El Metate volcano lavas (Michoacán-Guanajuato Volcanic Field, México)

Abstract

El Metate is a shield volcano located in the southern sector of the Michoacan-Guanajuato Volcanic Field, one of two largest monogenetic volcanic fields of the Transmexican Volcanic Belt. It was active c. 4.700 ± 200 years B.P and emitted about fifteen calcalkaline lava flows showing variable differentiation degrees. Temperatures calculated from mineral-liquid geothermobarometers for olivine, plagioclase and pyroxene, suggest that olivine was the earliest fractionating phase (1232–1198 °C), followed by plagioclase (1162–1126 °C), orthopyroxene (1147–1027 °C) and clinopyroxene (1147–1018 °C). Pressure estimations indicate that crystallization started at ~7 kbar and progressed up to surface levels. Water contents in the melts during crystalliztion of plagioclase is estimated at ~1.6%. Temperatures calculated on the basis of Al content in amphibole, provide a crystallization range between 995 and 922 °C, at an average pressure of 3.5 kbar and water contents between 5.2% and 6.9%. Although these values could agree with a scenario where amphibole represents a late crystallization phase along the previous fractionating sequence, the systematic presence of disequilibrium textures, which are also observed occasionally in other phases, suggest that other possibilities such as open-system crystallization cannot be discarded.