STITCHING TOGETHER THE UNGAVA OROGEN, NORTHERN QUEBEC - GEOCHRONOLOGICAL (TIMS AND ICP-MS) AND GEOCHEMICAL CONSTRAINTS ON LATE MAGMATIC EVENTS

Late magmatic activity in the Ungava Orogen of northern Quebec is mani fest as granitic dykes and small, rare plutons that crosscut all tecto no-stratigraphic elements of the orogen. Conventional U-Pb geochronolo gy (thermal ionization mass spectrometry (TIMS)) on one particularly i mportant pluton that cuts all these domains (the Lac Duquet monzograni te) indicates its age of emplacement at 1742.2 +/- 1.3 Ma. This undefo rmed and nonmetamorphosed pluton postdates the youngest structures in the orogen (D-4 folds), thereby constraining the timing of the latest deformation to > 1742 Ma. Laser ablation inductively coupled plasma - mass spectrometry (ICP-MS) on zircons from the same sample identified a large range in Pb-207/Pb-206 ages of inherited grains from 1.7 to 3. 2 Ga, corresponding to the ages of the host rocks for the pluton. This high-K peraluminous monzogranite pluton contains moderate to high con centrations of large ion lithophile elements and fractionated and enri ched light rare earth elements, similar in composition to the surround ing continental crust and to other crustally derived granites. Initial Sr-87/Sr-86 values of 0.7040-0.7051 and epsilon(Nd) ranging from -4.4 to -9.7 indicate incorporation of a significant amount of older mater ial in the petrogenesis of the pluton. It is proposed that anatexis of the surrounding continental crust due to structural thickening during the waning stages of the Ungava orogeny resulted in the generation of the Lac Duquet pluton and was the source for its inherited zircons.