U-PB GEOCHRONOLOGY OF LABRADORIAN AND LATER EVENTS IN THE GRENVILLE PROVINCE, EASTERN LABRADOR

U-Pb dating of selected rock units in the Hawke River and Groswater Ba y terranes of the Grenville Province in eastern Labrador has refined t he timing of metamorphism and magmatism during the Labradorian Orogeny (1710-1620 Ma). In the northern part of the Hawke River terrane, a ma jor migmatization event is confirmed between 1671(-3)(+4) Ma for a ton alitic gneiss and a newly determined age of 1662 +/- 3 Ma for a granit e dyke that cross-cuts the migmatized fabric of its host. This event c an be correlated with similar coeval activities in the Lake Melville t errane to the south and Labradorian thrusting at the Grenville front. In the Paradise metasedimentary gneiss belt, high-grade metamorphism h as been dated in two localities: in pillowed mafic volcanic flows asso ciated with the metasedimentary gneisses which produced new zircon gro wth at 1645 Ma in a garnet-amphibolite assemblage, and in pegmatitic m elt pods hosted in high-grade pelitic gneiss at 1638(-3)(+11) Ma. The time of this metamorphic event places a minimum depositional age on th e supracrustal assemblage in the Paradise metasedimentary gneiss belt that conflicts with a previously reported age for sedimentation at les s than or equal to 1627 Ma. The 1645 Ma metamorphic event was coeval w ith emplacement of a layered mafic intrusion (1644 +/- 2 Ma) in the Gr oswater Bay terrane, a large granite body between the Groswater Bay te rrane and the Hawke River terrane (1645(-5)(+7) Ma), which is a minimu m age for juxtaposition of the two terranes (but inferred from other d ata to be 1665 Ma), and leucogabbro from the White Bear Arm complex (g reater than or equal to 1640 Ma), which is adjacent to the Paradise me tasedimentary gneiss belt. The faulted contact between the White Bear Arm complex and the Paradise metasedimentary gneiss belt is intruded b y the similar to 100-km-long Paradise Arm pluton at 1639 +/- 2 Ma, ind icating contact between the two domains at this time. Pegmatitic phase s in the White Bear Arm complex indicate later magmatism at 1629(-2)(3) Ma. Current geological models argue for accretion of terranes (neve r widely separated) at 1665 Ma followed by crustal thickening and upli ft between 1645 and 1639 Ma. Deformation in the Hawke River terrane du ring the Pinwarian event (ca. 1500-1470 Ma) is suggested by complex da ta for monazite and baddeleyite from the White Bear Arm complex and fr om titanite data for rocks elsewhere in the Hawke River terrane. Grenv illian metamorphic effects are present in the region but do not appear to have had a major impact.