Abstract

A gravity survey has been made over a predominantly granitic terrain in the Wabigoon volcanic–plutonic subprovince of the Superior Province of the Canadian Shield. To supplement the preexisting regional coverage, 410 new gravity stations were established with an accuracy in the Bouguer gravity of about one milligal. The main feature of the area is a large negative anomaly that occurs over the Basket Lake granodiorite-to-quartz-monzonite intrusion. Suprisingly, no similar anomaly is found over a somewhat similar pluton (the Indian Lake intrusion) which is located about 20 km southeast of the Basket Lake body.Density studies show that both intrusions are lighter than the surrounding gneissic rocks. Gravity model studies indicate that the density contrast at the edge of the Basket Lake intrusion must extend to a depth of about 8 km, whereas the density contrast associated with the Indian Lake intrusion extends only to about 2 km depth. The difference between the intrusions can be correlated with their geological characteristics, the latter being more potassic and more uniform than the former, and sometimes feldspar porphyritic. The Indian Lake intrusion is thus interpreted as a relatively late, post-kinematic intrusion, exposed near its laterally expanded top. The Basket Lake intrusion is interpreted as a synkinematic, mezozonal to catazonal batholith of typical Archean type.

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