PRE-GRENVILLIAN EVOLUTION AND GRENVILLIAN OVERPRINTING OF THE PARAUTOCHTHONOUS BELT IN KEY-HARBOR, ONTARIO - U-PB AND FIELD CONSTRAINTS

The Parautochthonous Belt in the region of Key Harbour, Ontario, is co mposed of Early Proterozoic migmatitic para- and orthogneiss and Mid-P roterozoic granitoids, which were reworked during the Grenville orogen y. Grenvillian deformation is localized into anastomosing arrays of hi gh-strain shear zones enclosing elongate bands and lozenges of rock su bjected to lower and near-coaxial strain. Crosscutting relationships p reserved in the low-strain domains document two pre-Grenvillian pluton ic and tectonometamorphic events, which are bracketed in age by U - Pb zircon geochronology. A 1694 Ma leucogranite intrudes, and provides a minimum age for, high metamorphic grade gneisses formed during an ear lier tectonometamorphic event (D1-M1). The leucogranite was intruded b y mafic dykes, deformed, and metamorphosed at uppermost amphibolite fa cies during D2-M2, before the emplacement of Mid-Proterozoic granitoid s at ca. 1450 Ma. Following the emplacement of gabbro dykes and pods a t ca. 1238 Ma, the area was overprinted by granulite to uppermost amph ibolite facies metamorphism (Grenvillian), for which monazites provide a minimum age of ca. 1035 Ma. Titanite U-Pb ages of 1003 - 1004 Ma re cord cooling through 600-degrees-C. A regionally important swarm of ea st - west-trending posttectonic pegmatite dykes dated by U-Pb zircon a t 990 Ma provides a minimum age for Grenvillian ductile deformation. T he present data support the contention that the Parautochthonous Belt in the Key Harbour area consists in part of reworked midcontinental cr ust of Early to Mid-Proterozoic age.