Petrology, mineralogy and geochemistry of granulites of the Ford River and North River belts of northern Labrador, Canadian Shield

The Proterozoic granulites of the Ford River belt lie along the western flanks of the Archean granulites of the North River region of Nain Province in northern Labrador. Studies of mineral assemblages and the sequence of specific reactions in the hi9h-grnde rocks rewal the transitional facies character between the granoblastites and granulites in the study area. Using ACF plots of analyzed rocks of the Ford River and North River regions, hornblende-granulite facies rocks of bio + opx + pl + gar and bio + hb + opx + pl ± gar assemblages of transitional character have been identified. The biotites are dark brown with high Ti content and plot in the middle range of the eastonite-siderophyllite series. Orthopyroxenes range in composition /'ram En3 s to EnG o. Their low Ca, Al, Cr and Ni contents indicate that they are metamorphic orthopyroxenes. Garnets are predominantly almandine with Mg-component varying from 10-28°0 and Ca-component from 5 to 19°0. Hornblendes are aluminous tschermakite-pargasites. Distribution of Fe-A4g in coexisting f'emic minerals indicates that they were /'armed under equilibrium conditions. Phase relation studies suggest that pressures dicl not exceed 8 kb and temperatures were not above 850° C during regional metamorphism. The major-element geochemistry of Ford River and North River high-grade rocks compares with that of' the amphibolite-grade Uivak gneisses of Nain Province and shows predominantly a calc-alkaline trend. There is duality in their lithophile element distribution pattern. LTL elements are moderately or strongly depleted and SIL elements are enriched compared to the Uivak gneisses. The present data provide further evidence that certain geochemical features appear to be common to all granulite facies rocks, irrespective of their age. The clepletecl nature of granulites can only be attributed to the fractionating effects of high-grade regional metamorphism.