METAMORPHIC P-T CONDITION OF A MAFIC META-INTRUSION IN HUNTSVILLE, ONTARIO

The Central Gneiss Belt of the Grenville Province in Ontario contains distinct domains categorised by lithologic units, metamorphic grades, and internal structures. The Huntsville subdomain of the Algonquin domain hosts massive high-grade felsic gneiss, in which mafic lenses crop out sporadically. This study focuses on a 10-m sized mafic body embedded in grey migmatite, resembling a mafic intrusion, in Huntsville, Ontario. The apparent contact features a dark margin consisting of biotite and hornblende in the mafic gneiss and concentrated leucosomes in the host rock with local injection spikes. The mafic gneiss consists of garnet + hornblende + plagioclase + biotite + quartz in the matrix. 5 mm-sized dark nodules of hornblende + biotite intergrowth are surrounded by coronae of garnet + plagioclase. EPMA analyses on garnets of two textural relationships show different compositional patterns. The largely homogeneous matrix garnet indicates a weak decrease in Mg# towards the rim. The corona garnet has a higher Ca content and Mg# than the matrix garnet, and shows a decreasing trend in Ca from the core to the rim; Mn content is more concentrated towards the rim proximal to the hornblende + biotite nodule. In an equilibrium phase diagram calculated in NCKFMASHTO model system, the garnet compositions trace a clockwise path of cooling and decompression with thermal relaxation. The peak temperature ~840 °C at 9-12 kbar, consistent with the high Ti contents in biotite (up to 5.3 wt.% TiO₂), indicates a geotherm of ~21 °C/km. The metamorphic peak condition and the overall P-T path suggests conductive heat transport in a thickened continental crust, analogous to the continental collision in the modern plate-tectonic regime.