Partial melting of metapelites in the Gagnon terrane below the high-pressure belt in the Manicouagan area (Grenville Province): pressure-temperature (P-T) and U-Pb age constraints and implications

摘要

Metapelites of the parautochthonous Gagnon terrane at the footwall of the high-pressure (high-P) belt in the Manicouagan area (central Grenville Province) preserve an impressive textural record of partial melting reactions, mainly in polymineralic inclusions within garnet. The dominant textures were developed within the pressure-temperature (P-T) field of the continuous dehydration melting reaction biotite + kyanite (or sillimanite) + plagioclase + quartz = garnet + K-feldspar + melt, with sillimanite instead of kyanite in the southern part of the footwall. Inferred P-T paths have a hair-pin form in the range of 750-850 deg C and 1000-1500 MPa for the kyanite-bearing rocks. Monazite crystallization ages are consistently late Grenvillian, either 995 or 985 Ma, and one sample contains monazite of both ages. Two of these samples also contain inherited monazite with ages of 1738 + - 5 and 1719 + - 30 Ma, indicative of an earlier metamorphic event. Tonalite and diorite from the same area yield Archean zircon crystallization ages and titanite ages of 961 + - 3 and 956 9 + - 4 Ma, the youngest in the Manicouagan region. The late Grenvillian metamorphism was of comparable intensity but approx 50 Ma younger than in the overlying high-P belt in this area and therefore seems to be unrelated to the emplacement of the latter over the Parautochthonous Belt, as previously suggested. Rather, this younger metamorphism suggests a reactivation of the footwall by underthrusting of the Gagnon terrane during the waning stages of convergence, and a link with a major coeval post-tectonic thermal event farther south in the hinterland.