PRESSURE-TEMPERATURE PATH OF THE HIGH-PRESSURE BELT IN EASTERN SECTION OF THE WESTERN GREENVILLE PROVINCE

Felsic gneiss and retrogressed eclogite from Mattawa area in eastern section of the high-pressure (HP) belt in western Grenville Province are examined to retrieve metamorphic pressure-temperature (P-T) paths. The felsic gneiss is composed of garnet, amphibole, biotite, K-feldspar, plagioclase, and quartz, along with accessory apatite and zircon. Garnet in the felsic gneiss displays a relatively homogenous core with slightly zoned rims. From core to rim, the garnet shows compositional variation of X_Mg (=Mg/(Fe+Mg)) from 0.08 to 0.06 and grossular content decreasing from 0.28 to 0.26. Pseudosection modeling of the sample along with garnet zoning yielded an equilibrium P-T condition at 1.5 GPa, 730 °C. Two studied retrogressed eclogites contained the assemblage of garnet, omphacite, amphibole, phengite, biotite, and rutile at peak pressure, but is overprinted by lower-pressure metamorphism. Rutile is only found in the core to mantle section of garnet, whereas ilmenite-bearing assemblages are predominately present in the garnet rim and the matrix. Polymineralic inclusions of plagioclase + K-feldspar ± ilmenite ± biotite ± amphibole ± apatite ± sulfides are found in garnet, which are interpreted as crystallized melt that captured during garnet growth. The core of garnet is modified by diffusion, but the garnet displays slight zoning from mantle to inner rim, X_Mg (=Mg/(Fe+Mg)) ranging from 0.23 to 0.28 and grossular from 0.17 to 0.19 for garnet in one sample, while X_Mg varying from 0.21 to 0.28 and grossular from 0.20 to 0.18 in the other sample. Pseudosection modeling of the retrogressed eclogites considering chemically zoned garnet yielded an exhumation P-T path from 1.85 ± 0.2 GPa, 680 ± 30°C in a rutile-bearing assemblage to 1.2 ± 0.1 GPa, 880 ± 40 °C in an ilmenite-bearing assemblage. This study demonstrates that the HP terrane was exhumed along with heating.