EMPLACEMENT MECHANISM OF A MESOZOIC CAMPTONITE DIKE, WINOOSKI, VERMONT

Dikes represent magma conduits that develop either along preexisting structural weaknesses in rocks or along an echelon fracture array that is produced during magma emplacement by a hydrofracture-type mechanism. Dikes emplaced along an echelon fracture array will develop features such as dike-parallel joints, horns, and bridges. During the Mesozoic Era dikes were emplaced in structurally complex Lower Paleozoic rocks of the Champlain Valley. The orientation of these dikes has been used to infer paleostress directions even though the emplacement mechanism of these dikes is not well known.

A 0.75 m camptonite dike (N19W, 81SW) is exposed along the Winooski River just west of Colchester Avenue. The dike was emplaced in the Cambrian Winooski Dolostone which consists of thin beds (15 cm average) that strike N18E and dip 9SE. A regional joint set, roughly N65E, 88SE, is present in the dolostone. The dike occurs as three offset segments with horns and bridges present at the contact offsets. Dike-parallel joints are well developed in the dolostone within 40 cm of the dike. The contact offsets are also the location of 1.5 to 3 m wide joint zones (N32E, 87NW and N22E, 79NW) which contain 15 to 20 joints per zone. The joint zones cut the camptonite dike and therefore appear to be a later structural feature. The presence of horns, bridges, and dike-parallel joints suggests that this dike was emplaced within an echelon fracture array that was created during emplacement by a hydrofracture-type mechanism.