PRESSURE-TEMPERATURE PATH OF MIGMATITE FROM LAC DUMOINE TERRANE, WESTERN GREENVILLE PROVINCE

A migmatite sample from the Lac Dumoine terrane, western Grenville Province was studied to retrieve the cooling pressure-temperature (P-T) path. The migmatite was collected in an amphibolite adjacent to a granitoid intrusion in the high-pressure (HP) belt in western Quebec. The diatexite contains an assemblage of garnet, quartz, biotite, amphibole, clinopyroxene, plagioclase and K-feldspar along with minor titanite and accessory apatite, zircon, and rutile. An amphibolitic enclave of consists of amphibole, biotite, plagioclase and garnet is present. Matrix quartz, plagioclase and K-feldspar display equigranular texture. Two generations of garnet, large (900–3000 µm) and small (~200–900 µm) are observed. Monomineralic inclusions in both small and large garnets contain biotite, quartz and feldspars. Polymineralic inclusions within large anhedral garnets consist of biotite, quartz, feldspar, apatite, zircon, and garnet. Polymineralic inclusion and crack in garnet filled by clinopyroxenes and feldspars, which is interpreted as decrepitated melt, is also present. Garnet composition varies from Alm₅₈Grs₃₀Py₁₁Sp₁ to Alm₆₄Grs₂₇Py₅Sp₄ from core to rim. Plagioclase (Ab_70-96An_10-26Kfs_1-2) and K-feldspar (Ab_6-12An₂Kfs_86-92) are present in the matrix and in garnet. Amphibole in the matrix is ferro-pargasite and potassic-ferro-pargasite with 2.80–3.12 p.f.u. Fe²⁺ and 0.34–0.38 p.f.u. K. Amphibole-plagioclase thermometry yielded a temperature range at 700–800 °C. Garnet and biotite in the matrix equilibrated at 660–760 °C. Matrix titanite yielded a temperature range of 800–900 °C using Zr-in-titanite thermometry, assuming P = 1.5 GPa. The same thermometry yielded a temperature range of 850–900 °C for titanite in garnet at 1.5 GPa. Petrological modeling considering garnet zoning yielded a near-isobaric cooling path at ~1.5 GPa from 900 °C to 700 °C. This study demonstrated that the anatectic rock crystallized at lower crustal conditions through near-isobaric cooling.