Precambrian mafic dykes from the Superior Province of the Canadian Shield in Ontario and Quebec range in age from 2600 to 1100 m.y. and represent at least five different swarms. Included in this study are dykes of the Archaean Matachewan swarm, early Proterozoic and Archaean swarms, and the Middle Proterozoic Sudbury and Abitibi swarms. Archaean and early Proterozoic dykes are quartz tholeiites while Middle Proterozoic dykes are chiefly olivine tholeiites. The Middle Proterozoic dykes are enriched in incompatible and partially compatible elements relative to the Archaean and early Proterozoic dykes. All dykes have variably enriched light REE patterns and relatively low Ti/Zr (65-80) and Zr/Nb (5-15) ratios. The Archaean and early Proterozoic dykes have some trace-element distributions in common with calc-alkaline basalt, while the Middle Proterozoic dykes are more like oceanic-island and continental-rift basalts. Relatively low Mg numbers (34-56) and low Ni contents (<100 ppm) indicate significant olivine fractionation in all dykes. Significant variations in incompatible and partially compatible element abundances both within and between dyke swarms demand heterogeneous mantle sources. The increased importance of enriched mantle sources beneath the Superior Province in the Middle Proterozoic may reflect increased recycling of sediments into the mantle after a period of rapid continental growth in the late Archaean. (Authors' abstract)-A.W.H.
|Number of pages||14|
|Journal||Geological Association of Canada, Special Paper|
|Publication status||Published - 1987|