RAPID METAMORPHISM RECORDED BY GRENVILLIAN RETROGRADE ECLOGITE

The Grenville province is the largest and structurally complex orogenic belts in the Mesoproterozoic, representing the amalgamation of the supercontinent Rodinia. The Mattawa Terrane, part of the Algonquin Domain in Northern Ontario, hosts some of the most deeply-buried metamorphic rocks in the orogen. Assessing the pressure-temperature-time history of metamorphic rocks is key to deciphering the complex evolution of the orogenic belt. The Mattawa Terrane consists of large areas of orthogneiss of Paleoproterozoic to Neoarchean model ages. Mafic lenses resembling deformed dikes crop out to the north of Mattawa, ON. The mafic gneisses contain relict mineral assemblage and decompression textures indicative of HP eclogite precursors, and undergo retrograde resetting to various degrees. In the best-preserved samples, garnet porphyroblasts surrounded by plagioclase coronae are hosted in fine-grained symplectic intergrowths of diopside + plagioclase which compositionally reintegrates into a omphacite composition (Na/(Na+Ca) = 0.5). Elevated manganese concentration at garnet rims is evident of retrogressive resorption. According to phase equilibria analysis, the garnet and integrated omphacite compositions indicate that the peak metamorphic condition to be ~20 kbar at 860 ⁰C in eclogite facies, in a supersolidus plagioclase assemblage. The condition is consistent with the zircon-in-rutile thermometer. Despite the high temperature, chemical zoning in garnet is partially preserved. Forward modeling of diffusion profiles in garnet constraints the duration of peak metamorphic stage to be ~0.2 Myr, suggesting a short-duration regional metamorphism within the overall protracted Grenville orogenesis. Very few examples of Precambrian short-duration regional metamorphism have been reported, and this result could potentially be evidence of operative tectonics similar to modern plate tectonics in Mesoproterozoic.