P-T-t reconstruction of a coesite-bearing retroeclogite reveals a new UHP occurrence in the Western Gondwana margin (NE-Brazil)

Abstract

The ultra-high pressure metamorphic rocks of the West Gondwana Orogen are the earliest evidence of deep subduction on Earth. The West Gondwana Orogen resulted from the closure of the Pharusian-Goianides Ocean in the Late Neoproterozoic, leading to the formation of the Santa Quitéria Continental Magmatic Arc (SQCMA) in NE Brazil and the Forquilha Eclogitic Zone, located on the western margin of the SQCMA, suggesting subduction from W to E. This research investigated and reports the occurrence of a coesite-bearing eclogite on the eastern margin of SQCMA, bringing new insights into the regional tectonic evolution of the West Gondwana Orogen. Although extensively retrogressed during exhumation, the coesite-bearing eclogite still records UHP conditions. The multiple stages of the pressure-temperature path were reconstructed by combining textural relationships, integrating symplectite domains in compositional maps, phase equilibrium modeling and Zr-in-rutile thermometry. The pressure peak is estimated at 35.8 ± 5.2 kbar, reached at a temperature from 700 ± 15 °C to 804 ± 15 °C. Retrogression formed symplectite and corona textures at ca. 18 kbar and 760–860 °C. Our results indicate a clockwise P-T path that is characterized by a nearly isothermal decompression associated with a slight temperature increase after the baric peak, followed by decompression and cooling. Geochronological data indicate that the UHP rocks were metamorphosed in the Brasiliano/Panafrican orogenic event during the Western Gondwana amalgamation in the Neoproterozoic. Remnants of intra-oceanic arcs were described at the SQCMA eastern margin. We propose two main tectonic models for the evolution of these rocks: i) double subduction with convergent polarities to E-SE and W-NW, which can be correlated to the scenario proposed to the Hoggar belt in NW Africa and is also supported by geophysical data, and ii) exhumation of the eclogite as a diapir in the mantle wedge.