ARCHEAN METASEDIMENTARY ROCKS FROM THE NORTHWESTERN PONTIAC SUBPROVINCE OF THE CANADIAN SHIELD - CHEMICAL CHARACTERIZATION, WEATHERING AND MODELING OF THE SOURCE AREAS

The northwestern Pontiac Subprovince is dominated by a monotonous greywacke sequence, metamorphosed and deformed during the Late Archaean Kenoran orogeny. Limited chemical weathering of the source rocks is suggested by: (1) low values of the chemical index of alteration, (2) no preferential leaching of Ca, Sr and Ba, (3) well defined correlations among MgO, K2O and Al2O3, and (4) small variations of Cr/Ni. The source was dominantly felsic as indicated by high Th/Sc (average=0.39), La/Co (1.06), Zr/Cr (0.7), Zr/Y (11.2) and SiO2/MgO (19.4). The composition of these metasedimentary rocks can be modelled by mixing of three end-member components: (1) tonalites/trondhjemites (approximately 65%) in order to explain the SiO2, Al2O3, Th, U, Zr, Sr, and Ba abundances, Na/K ratios, strong depletion in HREE and HFSE relative to LREE and LILE, low Th, Y and K20, and fractionated La/Yb (19.4), (2) komatiites (approximately 5%) in order to explain the high Cr (220 ppm) and Ni (66 ppm) contents, and (3) tholeiitic basalts (approximately 30%) accounting for the observed TiO2 (0.62 wt%), V ( 1 44 ppm) and Sc (17 ppm) concentrations. The source region may be considered as a deeply eroded greenstone belt intruded by abundant tonalite-trondhjemite plutons; possibly now located to the south (and east) in the Grenville Province or possibly to the north, underlying the dominantly volcanic Abitibi Subprovince.