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Department of Earth Sciences, University of Lowell, Lowell, MA 01854, U.S.A.
The Monteregian Hills and White Mountain provinces consist of stocks, plugs, ring-dyke complexes and several large granite bodies emplaced into Precambrian gneisses, flat-lying Cambro-Ordovician sediments and the deformed Lower Palaeozoic section of the Appalachian fold belt. Felsic rocks dominate in the Appalachian fold belt, while elsewhere mafic and ultramafic rocks are significant components of the plutons. Igneous activity extended from 240 to 90 Ma ago with two major periods of magmatism, correlated with events in the opening of the N Atlantic Ocean, occurring between 200–165 Ma and 140–110 Ma ago.
Five major rock series have been identified: (1) undersaturated CO2-rich rocks, carbonatite and alnöite; (2) moderately to strongly undersaturated diorites-nepheline syenites; (3) slightly undersaturated to slightly oversaturated pyroxenites-gabbros-diorites-syenites; (4) alkali syenite-quartz syenite-granite; (5) metaluminous biotite granite. Series (1), (2) and (3) magmas were drawn from an isotopically depleted mantle which was enriched in incompatible elements shortly before or synchronous with melting. These magmas were produced by variable degrees of melting of garnet or spinel lherzolite. Series (4) and (5) magmas represent partial melts of a heterogeneous crustal section consisting of both meta-sedimentary and meta-igneous rocks of either Grenville (Precambrian) or Lower Palaeozoic age.
