THE 8 CLASSICAL MONTEREGIAN HILLS AT DEPTH AND THE MECHANISM OF THEIRINTRUSION

The eight classical Monteregian hills are monadnocks on the St. Lawren ce Lowlands and adjacent Appalachian foothills in a swath that sweeps 80 km eastward from Montreal. Gravity anomalies suggest the presence o f about 200 km(3) of mafic and ultramafic Monteregian rocks at depth. Mounts Royal, Saint-Bruno, and Rougemont are interpreted to be pluglik e intrusions atop large laccoliths that were fed by magma that spread laterally along the buried Precambrian-Paleozoic unconformity. Mounts Saint-Hilaire, Saint-Gregoire, and Yamaska lie at higher stratigraphic levels in flat-lying sedimentary host rocks. These six intrusions fil led the lower parts of breccia pipes formed by the explosive upward es cape of volatiles. Late-stage settling of the cooling intrusions dragg ed downward an encircling collar of baked host rocks. The two easternm ost hills (Brome and Shefford) are interpreted to be thin intrusive sh eets emplaced along Appalachian thrusts. Stepwise emplacement of magma in the thick cover rocks in the east promoted contamination and may a ccount 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 Mon treal (apart from Oka) is explained by the removal through erosion of Paleozoic cover rocks. The Monteregian intrusives developed only in co ver rocks; feeders in the Precambrian basement are possibly small and may be covered by Quaternary deposits. Monteregian magmatism was a maj or event, out of all proportion to the small intrusions presently expo sed, and may have emplaced as much as 1000 km(3) of igneous rock.