Coronitic metagabbro and eclogite from the Grenville Province of western Quebec: interpretation of U–Pb geochronology and metamorphism

Abstract

We present new U–Pb and metamorphic data on high-pressure coronitic metagabbros from three distinct structural settings in the Parautochthonous Belt of the Grenville Province in western Quebec. Intrusive ages are (i) [Formula: see text], for metagabbro close to the Grenville Front, correlative with the Sudbury dykes, defined in Ontario; (ii) [Formula: see text] for an eclogitized lens at the base of the highest structural level (SL4), a new age for mafic magmatism in the western Grenville; and (iii) [Formula: see text] for metagabbro from SL4, interpreted as correlative with metagabbros from the Algonquin and Shawanaga domains in Ontario. Metamorphism in all cases is Grenvillian, with the best constrained age of 1069 ± 3 Ma for the metagabbro of SL4. Metamorphic grade increases from the Grenville Front to the south. The mafic rocks preserve relict igneous textures overprinted by garnet + clinopyroxene that developed as coronas and (or) pseudomorphs after igneous phases. The highest grade metagabbros contain omphacite and some lack primary plagioclase, therefore being eclogites. However, interpretation of textures and mineral chemistry indicates that they were equilibrated during decompression (at 1350 MPa and 720 °C, sample 51; and at 1200 MPa and 740 °C, sample 29), so maximum depths of burial remain unconstrained. Their evolution is interpreted as follows: (i) high-pressure metamorphism by burial of the Laurentian margin under accreted terranes thrust toward the northwest between 1080 and 1060 Ma; (ii) residence at intermediate crustal levels, for a few tens of millions of years; and (iii) rapid exhumation by renewed thrusting that led to the emplacement of the high-pressure units over the northerly adjacent structural units of the Parautochthonous Belt.