PARTIAL MELTING OF ECLOGITE DUE TO PROGRADE BREAKDOWN OF AMPHIBOLE: AN EXAMPLE FROM THE ALGONQUIN TERRANE, WESTERN GRENVILLE PROVINCE, CANADA

Retrogressed eclogites from the Algonquin terrane in the western Grenville Province, Canada, were studied to better understand the partial melting of metabasite at high pressure. The studied samples contain the peak mineral assemblage garnet, omphacite, biotite, plagioclase, and rutile coexisting with melt, but amphibole and phengite were present on the prograde path due to crystallized mineral inclusions in garnet. Both microscale and mesoscale textures show that the eclogites were partially melted. Melt-related textures, such as polymineralic inclusions, cuspate textures and neoblasts, demonstrate that pockets of melt were present in the eclogites. Polymineralic inclusions of plagioclase + K-feldspar + ilmenite ± amphibole ± biotite ± apatite in garnet crystallized from melt, which was captured during garnet growth. Sharp cuspate boundaries from plagioclase to other phases (e.g. garnet and clinopyroxene) mimic the original melt-solid interface. Neoblasts of garnet with euhedral crystal faces were formed as a peritectic phase during melting of the eclogites. Leucocratic veins without external connections, present in the eclogite pod at outcrop scale, are interpreted as crystallized melt pockets which derived internally. Pseudosection modeling of one eclogite sample considering chemically zoned garnet yielded a prograde pressure-temperature (P-T) path from 1.5 GPa, 700 °C to 1.75 GPa, 780 °C, both under suprasolidus conditions. Along this prograde path, modal percentages of garnet and melt increased in response to the breakdown of amphibole along with minor involvement of phengite. The calculated melt composition is consistent with the crystallized trondhjemitic melt documented in the samples. The melt in the modeled rock crystallized during exhumation to P-T conditions of 0.6–0.8 GPa, 710–750 °C, which were estimated using empirical thermobarometry. This study shows that eclogites from Proterozoic collisional orogens can be partially melted on the prograde path, contributing to differentiation of lower continental crust and leading to crustal weakening.