Monteregian Hills II. Petrography, Major and Trace Element Geochemistry, and Strontium Isotopic Chemistry of the Eastern Intrusions: Mounts Shefford, Brome, and Megantic

The Monteregian Hills consist of a series of alkaline intrusions and associated dikes and sills emplaced along a linear west-east trend extending from 35 km west of Montreal, Quebec, to 190 km east of Montreal. The igneous activity occurred between 117 Ma and 141 Ma and the age data show a distinct bimodality, with strongly undersaturated magmas emplaced at c. 118 Ma and slightly undersaturated magmas emplaced at c. 136 Ma. The eastern intrusions largely consist of gabbros, diorites, and a variety of felsic rocks as compared to the more mafic character of the western intrusions which largely consist of pyroxenites, gabbros, and diorites. The eastern intrusions were emplaced through a thick sequence of folded and faulted geosynclinal sediments which seems to have played a role in determining their felsic character. Mounts Brome and Shefford are located approximately 80 km east of Montreal. The dominant rock type at Mount Shefford is a diorite which has been intruded by arcuate bodies of pulaskite and nordmarkite. Mount Brome consists of an outer crescent-shaped gabbro body, composed of several cyclical units, and a large syenite body divided into a slightly quartz-undersaturated unit (pulaskite) and a slightly oversaturated unit (nordmarkite). The two units are chemically similar, but the nordmarkite has elevated initial Sr ratios suggesting that crustal contamination is responsible for its oversaturation. Both complexes have been intruded by nepheline-bearing diorites, and at Mount Brome, foyaites, tinguaites, and laurdalites. Mount Megantic is located approximately 190 km east of Montreal and consists of an outer normarkite annulus, separated by a gabbro-dioritic body from an inner granite plug. Five different magma types have been identified in the eastern Monteregian Hills. Type 1, precursor to the gabbros and syenites at Mounts Shefford and Brome, is an alkali picrite generated by limited partial melting of a garnet lherzolite, with subsequent evolution controlled by the removal of olivine, pyroxene, and plagioclase. Type 2, precursor to the gabbros and nordmarkites at Mount Megantic, is apparently produced by moderate partial melting of a spinel lherzolite, and this magma subsequently evolved to a quartz-saturated residuum. Type 3, which produced the rocks of the strongly undersaturated series, is equivalent to the basanite magmas generated by small degrees of melting of a spinel lherzolite, which formed similar rocks in the western Monteregian Hills. The high concentration of incompatible elements and high-charge-density cations in these magmas indicates that the mantle source regions were enriched in these elements. Type 4, Mount Shefford nordmarkite, and Type 5, Mount Megantic granite, magmas apparently originated in the crust by partial melting of, respectively, amphibolite or granulite facies metadiorite and metagraywacke sources.