The eight classical Monteregian hills at depth and the mechanism of their intrusion

Abstract

The eight classical Monteregian hills are monadnocks on the St. Lawrence Lowlands and adjacent Appalachian foothills in a swath that sweeps 80 km eastward from Montréal. Gravity anomalies suggest the presence of about 200 km3 of mafic and ultramafic Monteregian rocks at depth. Mounts Royal, Saint-Bruno, and Rougemont are interpreted to be pluglike intrusions atop large laccoliths that were fed by magma that spread laterally along the buried Precambrian–Paleozoic unconformity. Mounts Saint-Hilaire, Saint-Grégoire, and Yamaska lie at higher stratigraphic levels in flat-lying sedimentary host rocks. These six intrusions filled the lower parts of breccia pipes formed by the explosive upward escape of volatiles. Late-stage settling of the cooling intrusions dragged downward an encircling collar of baked host rocks. The two easternmost hills (Brome and Shefford) are interpreted to be thin intrusive sheets emplaced along Appalachian thrusts. Stepwise emplacement of magma in the thick cover rocks in the east promoted contamination and may account for the presence of quartz-bearing felsic rocks. Igneous rocks along the deeply buried unconformity in the east and felsic rocks, all undetectable by gravity, could add substantially to the total volume of the Monteregians. The absence of Monteregian intrusives west of Montréal (apart from Oka) is explained by the removal through erosion of Paleozoic cover rocks. The Monteregian intrusives developed only in cover rocks; feeders in the Precambrian basement are possibly small and may be covered by Quaternary deposits. Monteregian magmatism was a major event, out of all proportion to the small intrusions presently exposed, and may have emplaced as much as 1000 km3 of igneous rock.