CORONITIC METAGABBRO AND ECLOGITE FROM THE GRENVILLE PROVINCE OF WESTERN QUEBEC - INTERPRETATION OF U-PB GEOCHRONOLOGY AND METAMORPHISM

We present new U-Pb and metamorphic data on high-pressure coronitic me tagabbros from three distinct structural settings in the Parautochthon ous Belt of the Grenville Province in western Quebec. Intrusive ages a re (i) 1217(-10)(+15) for metagabbro close to the Grenville Front, cor relative with the Sudbury dykes, defined in Ontario; (ii) 1403(-11)(+1 4) Ma for an eclogitized lens at the base of the highest structural le vel (SL4), a new age for mafic magmatism in the western Grenville; and (iii) 1218(-34)(+53) Ma for metagabbro from SL4, interpreted as corre lative with metabbros from the Algonquin and Shawanaga domains in Onta rio. Metamorphism in all cases is Grenvillian, with the best constrain ed age of 1069 +/- 3 Ma for the metagabbro of SLA. Metamorphic grade i ncreases from the Grenville Front to the south. The mafic rocks preser ve relict igneous textures overprinted by garnet + clinopyroxene that developed as coronas and (or) pseudomorphs after igneous phases. The h ighest grade metagabbros contain omphacite and some lack primary plagi oclase, therefore being eclogites. However, interpretation of textures and mineral chemistry indicates that they were equilibrated during de compression (at 1350 MPa and 720 degrees C, sample 51; and at 1200 MPa and 740 degrees C, sample 29), so maximum depths of burial remain unc onstrained. Their evolution is interpreted as follows: (i) high-pressu re metamorphism by burial of the Laurentian margin under accreted terr anes thrust toward the northwest between 1080 and 1060 Ma; (ii) reside nce at intermediate crustal levels, for a few tens of millions of year s; and (iii) rapid exhumation by renewed thrusting that led to the emp lacement of the high-pressure units over the northerly adjacent struct ural units of the Parautochthonous Belt.