EMPLACEMENT MECHANISM OF MESOZOIC DIKES IN THE CHAMPLAIN VALLEY: EXAMPLES FROM CHARLOTTE BEACH AND THE REDSTONE QUARRY

Compared to other regions of New England where Mesozoic dikes occur with a larger spread in orientation trends, the majority of dikes emplaced in Lower Paleozoic rocks in the Champlain Valley trend roughly east west. Dikes represent magma conduits that develop either under the influence of preexisting structural weaknesses in rocks or in response to the stress field present during emplacement of the magma. If influenced by the stress field the dike will be oriented perpendicular to s₃ and will intrude an echelon fracture array produced by a hydrofracture-type mechanism. Dikes emplaced along an echelon fracture array will develop features such as dike-parallel joints, horns, and bridges. Is the strong east-west alignment of the Champlain Valley dikes influenced by any preexisting structure in the Lower Paleozoic rocks?

Seven bostonite dikes emplaced in the Ordovician Stony Point Shale are exposed at Charlotte Beach. The dikes range in size from 0.25 to 1.78 m wide. All of these dikes strike N81E to N87E and dip 73NW. Bedding (N2W, 90) and a well-developed cleavage (N12W, 71NE) appear not to have influenced the emplacement direction of these dikes. Individual dikes occur as offset segments with well-developed horns and bridges. Dike-parallel joints are present and along the south contact of one dike extend 90 cm in front of a horn. Three camptonite dikes emplaced in the Cambrian Monkton Quartzite are exposed at the Redstone Quarry in Burlington. The quartzite strikes N22W and dips 3NE. A regional joint set is present in the quartzite with three main trends; roughly east west, N60E and N60W. The northern most camptonite dike has at least 20 dike-parallel joints in a zone that extends roughly 2 m from the dike contact. The presence of dike-parallel joints, offset dikes, horns and bridges suggest that these dikes were emplaced along an echelon fracture array that developed during dike emplacement by a hydrofracture-type mechanism.