Hydrothermal alteration of the peridotites cored at the ocean/continent boundary of the Iberian margin: petrologic and stable isotope evidence

Abstract

A ridge of strongly serpentinized, plagioclase-bearing peridotite crops out at the boundary between the Atlantic oceanic crust and the Galicia continental margin (western Spain). These peridotites have been mylonitized at high-temperature, low-pressure conditions and under large deviatoric stress during their uplift. After this main ductile deformation event, the peridotite underwent a polyphase metamorphic static episode in the presence of water, with the crystallization of Ti- and Cr-rich paragasites at high-temperature (800°-900°C) interaction with a metasomatic fluid or alkaline magma. Introduction of water produced destablization of the pyroxenes and the subsequent development of hornblendes and tremolite at temperatures decreasing from 750° to 350°C. The main serpetinization of the peridotite occurred at a temperature below 300°C, and possibly around 50°C, as a consequence of the introduction of a large amount of seawater. Finally, calcite derived from seawater precipitated in late-formed fractures. -from Authors