The distribution of igneous rocks beneath Mont Mégantic (the easternmost Monteregian) as revealed by gravity

Abstract

At the surface, Mont Mégantic, the easternmost and most imposing of the Monteregian hills, is composed of 77.5% leucogranite and syenite and 22.5% gabbro. Nonetheless, the intrusion is characterized by a major positive Bouguer anomaly of +20 mGal. A computed three-dimensional gravity adumbration is consonant with the intrusion having the form of a vertical prismatic column with a flat base at a depth of 10 km. The base appears to coincide with the unconformity atop Precambrian basement rocks. Gabbro composes 96% of the adumbration. Leucogranite and syenite make up the remainder and are confined to an uppermost layer no more than about 1 km thick. The subvolcanic texture of the granite and presence of miarolitic cavities at Mégantic show the shallow level of the intrusion, the present surface perhaps having been no deeper than 1 or 2 km. This is the same as the depth of emplacement elucidated at Mont Royal, at the opposite end of the Monteregian chain, which in turn points to uniform uplift and erosion since Early Cretaceous time across the entire area, from the shield and well into the Appalachians. The mechanism of intrusion at Mégantic appears to be the same as that we proposed earlier for the eight classical Monteregians.