Eclogite with unusual atoll garnet from the southern Armorican Massif, France: Pressure-temperature path and geodynamic implications

Abstract

Eclogites form in different geotectonic environments such as subduction zones or roots of overthickened continental crust. The specific environment can be identified by the construction of pressure-temperature (P-T) trajectories. We determined the P-T path of eclogite from the Les Essarts high-pressure unit, Southern Armorican zone in western France. The eclogite contains unusual atoll garnet characterized by a younger generation (Grt2) that replaced the core of an older generation (Grt1). Large-area (cm2) X-ray mapping with the electron microprobe was applied to determine the quantities of Grt1 and Grt2 in the studied eclogites. Detailed characterization of the mineral chemistry was undertaken followed by thermodynamic modelling. The assemblage of amphibole, quartz, rutile, and ilmenite enclosed in Grt1 formed in the amphibolite facies during a first metamorphic event. Grt1 grew at P-T conditions around 20 kbar and 540 °C probably after nearly isothermal burial during a second metamorphic cycle. An isobaric heating to ca. 650 °C followed at which Grt2 and, thus, atoll garnet formed. A time period of ca. 15 million years for exhumation in the temperature range 620–655 °C was estimated by diffusional modelling of the contact between Grt1 and Grt2. We correlate the Southern Armorican zone with the Malpica-Tuy zone (NW Spain) that contains eclogite and related rocks with a similar metamorphic evolution. We outline a geodynamic scenario in which the eclogite-facies rocks of both zones were tectonically eroded from the upper continental plate by a subducting plate to reach maximum depths of 70–90 km in the Late Devonian. Exhumation to depths of ca. 45–50 km occurred in a subduction channel just before continent-continent collision started.