METAMORPHIC CONDITIONS OF A GARNET-BEARING GNEISS FROM THE HIGH-PRESSURE BELT, WESTERN GRENVILLE PROVINCE

A garnet-bearing felsic gneiss from the high-pressure belt in the Grenville Province is examined to determine its metamorphic conditions. The felsic gneiss was collected adjacent to the retrogressed eclogite locality ~16 km northwest of Mattawa, Ontario. The studied sample contains a mineral assemblage of garnet, quartz, amphibole, K-feldspar, plagioclase, biotite, rutile, zircon, apatite, titanite, and ilmenite. Garnet porphyroblasts, ~2–4mm and subhedral to anhedral in shape, contain inclusions of quartz, amphibole, K-feldspar, and plagioclase. Quartz is up to ~800 µm, subhedral, and present as inclusions within garnet as well as in the matrix. Amphibole is up to ~2.1mm and subhedral, commonly occurring in the matrix but also in inclusions. K-feldspar is ~800–1000 µm and occurs as subhedral to anhedral with plagioclase as inclusions and in the matrix. Plagioclase is subhedral to anhedral, commonly occurring throughout the matrix and in inclusions. Euhedral biotite, up to ~630 µm, occurs adjacent to amphibole in the matrix. Accessory zircon, apatite, rutile, titanite, iron oxides, and ilmenite occur in the matrix. Garnet displays a homogenous core in grossular (Gr) and almandine (Alm) contents with a slightly zoned rim by varying pyrope (Py). The composition of garnet is Gr₂₇Alm₆₂Py₇ at the core and Gr₂₇Alm₆₂Py₅ at the rim. The Mg# [=Mg/(Fe+Mg)] in garnet decreases from 0.10 at the core to 0.07 at the rim. Petrological modeling was conducted using Perple_X (ver. 6.9.0) and the thermodynamic data of Holland & Powell (2011) along with suitable solution models to retrieve metamorphic pressure-temperature (P-T) conditions. Isopleths of X_Ca and Mg# indicate P-T conditions of 15.4 kbar, 715 ℃ of the core and 15.3 kbar, 690 ℃ of the rim, with the phase assemblage of K-feldspar, plagioclase, garnet, melt, white mica, omphacite, quartz, rutile with or without biotite. At these conditions, modal melt increases from 0.08 % to 1.64%. The retrieved P-T conditions are lower than previous estimates on retrogressed eclogite, due to the significant diffusion modification of garnet composition. Further study is needed to better constrain metamorphic conditions of the host rock and modal melt to determine its tectonic significance.