STRUCTURE AND PETROLOGY OF THE CARIBOU LAKE INTRUSIVE BODY, ONTARIO, CANADA

Abstract

The Caribou Lake intrusive body, located in the Canadian shield, covers an area of 7 square miles. Its east-west dimension is 5½ miles; it attains a width of 2 miles but in the east tapers to a narrow constriction, 650 feet wide. Mapping of the foliation pattern, of the topographic highs and lows which reflect the rhythmic banding in the intrusive body, and of the gneissosity in the surrounding gneisses reveals the presence of two synclinal areas in the western part of the intrusive body. The northern syncline is 2½ by 1½ miles, the southern syncline covers 1 square mile. If the two synclinal areas represent channels through which magma ascended, then it should be possible to correlate structure with petrology. The petrological and geochemical evidence, however, points to a differentiation path that can be traced from the narrow constriction at the eastern tip of the intrusive body to the synclinal areas in the west. This evidence is summed up: (1) Ultrabasic rocks occur only near the eastern tip. (2) Hypersthene decreases and augite increases as the norite is traced westward across the intrusive body. (3) The Fe/Mg ratio in hypersthene and the Na-Si/Ca-Al ratio in plagioclase increase progressively westward. (4) MgO decreases to the west as FeO, Fe 2 O 3 , and alkalies increase. (5) Nickel, copper, and chromium decrease progressively to the west. The intrusive body and surrounding area was mapped by aeromagnetic methods. An anomalous magnetic high of strong gradient was defined at the narrow constriction in the east where petrological and geochemical evidence indicate the presence of the funnel. This combination of observations leads to the interpretation that the channel is located at the eastern tip of the intrusive body. The significance of the two synclinal areas is a matter of conjecture. It appears that structural data alone are inadequate to determine the shapes of basic intrusive bodies.