Carbonatite and Alkaline Igneous Rocks in the Brent Crater, Ontario

Abstract

THE Brent crater (46° 04′ N., 78° 29′ W.) is cut into the Pre-Cambrian gneisses of central Ontario. On the surface the crater is marked by a poorly defined depression 18,000 ft. in diameter, with a sharply denned central crater 11,000 ft. in diameter and about 200 ft. deep. Drilling¹ has shown that the central crater is partly filled with flat lying Ordovician strata underlain by a thick lens of brecciated Pre-Cambrian material. A hole drilled approximately in the centre of the structure^1,2 passed through about 800 ft. of Ordovician strata, 1,900 ft. of breccia of various kinds, and beginning at a depth of 2,716 ft. penetrated breccia with a matrix of igneous textured trachyte for more than 100 ft. Below this “melt zone” brecciation died out very rapidly. Because of petrographic features found in the breccias, Brent has been widely accepted as a holotype of a “fossil” meteorite crater^1–4. The Brent crater occurs on or near the Ottawa–Bonnechere graben, however, a major structure with a length of some hundreds of miles⁵, along which circular alkaline and carbonatitic complexes were intruded in Cambro–Ordovician time^6,7. These facts suggest a possible endogenetic origin for the crater. In an attempt to resolve this problem we examined the petrographic and chemical composition of twenty-three samples of drill core from the central hole and nine samples collected on the surface. It was found petrographically that the samples fell into four groups. Underlying the Ordovician were about 100 ft. of breccias with substantial amounts of glass (often of several types), below which were 1,700 ft. of breccias with little or no glass. At a depth of 2,700 ft., 16 ft. of breccia rich in glass was again encountered grading into inclusion rich trachyte of igneous aspect. About 50 ft. of breccia rich in glass occurred below the trachyte, passing abruptly into relatively undeformed country rocks. A specimen from one of the dykelets cutting the breccia poor in glass was composed of extremely fine grained carbonates, minute needles of a pleochroic blue mineral and abundant opaque dust.