SYMPLECTITES IN ECLOGITE AS RECORDERS OF DEEP CRUST EXHUMATION IN A MIGMATITE DOME

The occurrence of eclogites as bands or lenses within migmatite terranes has been recognized for almost a century. The significance of these high-pressure (HP) rocks hosted in felsic lithologies, which record seemingly discordant high-temperature, low-pressure (HT-LP) metamorphic conditions, has yet to be fully understood. In the Montagne Noire migmatite dome, French Massif Central, coeval crystallization of HP-metamorphic zircon in eclogite and monazite in the enclosing gneisses suggests that dome rocks may be sourced from the mid to deep crust, at depths much greater than the HT-LP conditions recorded by the gneisses and migmatites. The eclogites chemically and texturally conserve a better memory and a more detailed history of sourcing and exhumation than the gneisses. Specifically, the Montagne Noire eclogite rocks contain abundant symplectites, a texture observed in all eclogite localities and especially common in eclogites associates with migmatite/gneiss (e.g. Western Gneiss Region). We observe symplectites formed from the breakdown of omphacite to form plagioclase + Ca-pyroxene ± Ca-amphibole, as well as symplectites formed from the breakdown of garnet to form plagioclase + orthopyroxene ± orthoamphibole. In eclogites, symplectites are often interpreted to have formed as a result of rapid decompression. Variations in symplectite assemblages, degree of symplectization, and coarseness of symplectite lamellae in eclogites from the margin and core of the dome provide insights into their exhumation history. We generate high spatial resolution, fully quantified Electron Probe Micro Analysis maps of symplectites in eclogites from three eclogite localities in the Montagne Noire, and investigate spatial trends in the composition of individual phases and micro-chemical subdomains. The microchemical and microtextural information contained in these textures provides constraints on the P-T conditions, rates of reactions, and chemical environment (e.g. presence of fluids) these rocks experienced during exhumation. Better constraints on the metamorphic history contained in both the relict eclogite assemblage and symplectized domains provide insight into processes by which deep crust is exhumed during the late stages of orogenesis.