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 intrusionsand associated dikes and sills emplaced along a linear west-easttrend extending from 35 km west of Montreal, Quebec, to 190km east of Montreal. The igneous activity occurred between 117Ma and 141 Ma and the age data show a distinct bimodality, withstrongly undersaturated magmas emplaced at c. 118 Ma and slightlyundersaturated magmas emplaced at c. 136 Ma. The eastern intrusionslargely consist of gabbros, diorites, and a variety of felsicrocks as compared to the more mafic character of the westernintrusions which largely consist of pyroxenites, gabbros, anddiorites. The eastern intrusions were emplaced through a thicksequence of folded and faulted geosynclinal sediments whichseems to have played a role in determining their felsic character. Mounts Brome and Shefford are located approximately 80 km eastof Montreal. The dominant rock type at Mount Shefford is a dioritewhich 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 bodydivided into a slightly quartz-undersaturated unit (pulaskite)and a slightly oversaturated unit (nordmarkite). The two unitsare chemically similar, but the nordmarkite has elevated initialSr ratios suggesting that crustal contamination is responsiblefor its oversaturation. Both complexes have been intruded bynepheline-bearing diorites, and at Mount Brome, foyaites, tinguaites,and laurdalites. Mount Megantic is located approximately 190km 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 easternMonteregian Hills. Type 1, precursor to the gabbros and syenitesat Mounts Shefford and Brome, is an alkali picrite generatedby limited partial melting of a garnet lherzolite, with subsequentevolution controlled by the removal of olivine, pyroxene, andplagioclase. Type 2, precursor to the gabbros and nordmarkitesat Mount Megantic, is apparently produced by moderate partialmelting of a spinel lherzolite, and this magma subsequentlyevolved to a quartz-saturated residuum. Type 3, which producedthe rocks of the strongly undersaturated series, is equivalentto the basanite magmas generated by small degrees of meltingof a spinel lherzolite, which formed similar rocks in the westernMonteregian Hills. The high concentration of incompatible elementsand high-charge-density cations in these magmas indicates thatthe mantle source regions were enriched in these elements. Type4, Mount Shefford nordmarkite, and Type 5, Mount Megantic granite,magmas apparently originated in the crust by partial meltingof, respectively, amphibolite or granulite facies metadioriteand metagraywacke sources.