Metamorphic evolution of a Tonian eclogite associated with an island arc of the southern Brasiliano Orogen

Abstract The early evolution of the southern Brasiliano Orogen is recorded in the Sao Gabriel Terrane, which consists of two Neoproterozoic (Tonian and Cryogenian) juvenile arcs, ophiolite complexes and supracrustal rocks. This study aims at providing new insights into the geodynamics processes and subduction process of this orogen by investigating the first Tonian eclogite (named Tres Vendas eclogite) discovered in the southern Brasiliano Orogen. To determine the tectono-metamorphic evolution, we integrated field work with petrography, scanning electron microscopy, electron microprobe analyses of minerals and in situ LA-ICP-MS U-Pb geochronology with Lu-Hf and rare earth elements in zircon. The preserved high-grade metamorphic assemblage in these rocks has garnet, rutile, phengite, and omphacite, indicating that these rocks are (retro) eclogites. The presence of omphacite with up to 50% jadeite indicates minimum burial pressures of ≥15 kbar that are consistent with forward modeling of the garnet compositions. Omphacite was altered to fine-grained symplectites of hornblende, oligoclase, and diopside or augite with minor quartz during rapid exhumation. The second stage of retrogression involved reaction of garnet and diopside to plagioclase and hornblende symplectites (kelyphites) at amphibolite facies conditions. Kelyphitic hornblende-garnet pairs indicate peak temperatures of 710–760 °C. Whole rock composition and Zr–Y characteristics are consistent with tholeiitic island arc basalts, while their garnet compositions point to subduction zone eclogites. U-Pb ages of zircon cores yielded 909.5 ± 5.3 Ma and metamorphic rims 891.9 ± 8.2 Ma. Zircon age-corrected eHf isotope compositions range between +4 and +10, reflecting the moderately juvenile nature of these rocks. Zircon cores are distinguished by higher rare earth element (REE) contents and steeper middle REE to heavy REE (MREE-HREE) patterns, higher Th/U ratios and higher Sr and Ca contents than the metamorphic rims, and the rims are likely coeval with the kelyphites. The combined data indicate that the Tres Vendas eclogites belong to a Tonian arc and were buried to >45 km, most likely in a subduction channel. The data from the first eclogite in southern Brazil have significant consequences for the interpretation of the Tonian evolution of the Brasiliano Orogen and, by extension, the Rodinia framework.