Dating the geological history of the northwestern North American Plate: Re-Os isotopic analyses of sulfides from western Yukon ultramafic complexes

The Western Yukon contains an assemblage of schists, basalts and ultramafic rocks. The basalts were recently dated (U-Pb zircon) as Lower Permian (282 Ma, Escayola et al, in prep). The ultramafic massifs are composed of dunite, harzburgite and cpx harzburgite, with replacive dunite layers and orthopyroxenite veins. Dunites host irregularveins of chromitite and disseminated Cr-spinel. The peridotites are refractory in terms of whole rock Al2 O3 content (ca 1%), olivine Mgup to 0.91) and Spinel Cr0.5), suggesting initial high degrees of partial melting. Microstructures show that they have reacted with intergranular melts prior to cooling in the lithosphere, leading to crystallization of olivine, cpx and spinel at the expense of opx. Coarse and intergranular cpx show re-fer tilization by reaction with melts representing 8-9% melting in the garnet stability field. A two-stage partial melting/melt-rock reaction history is suggested. In situ Re-Os isotopic analyses on primary sulfides show peaks of TRD model ages at 2.5-2.0 Ga, 1.2-1.0 Ga and 650-700 Ma. We propose that these ages record the main events of the western North American plate's history from an Archean origin, supercontinent breakup, accretion of Rodinia (1.2-1.0 Ga) and the break-up of Rodinia to form the Panthalasa Ocean (Neoproterozoic). The main peak at 700 Ma may represent the evolution from sub-continental mantle to a passive margin-ocean transition environment.