FUNDAMENTAL PETROLOGY OF THE EAST HILL SUITE, MONT SAINT-HILAIRE, QUÉBEC, CANADA

The Mont Saint-Hilaire plutonic complex is the product of three magmatic episodes (Currie et al. 1986). Of the three resulting rock suites, the East Hill suite is the youngest and most geochemically evolved and hosts most of the more than 300 minerals that have been described from Mont Saint-Hilaire. Many earlier studies have been mineralogical in nature or have considered the petrology in a more cursory fashion, usually reporting only one or two general lithologies. We find that the East Hill suite consists of at least six distinct rock types. Melanocratic nepheline syenite serves as the host rock for a series of in situ differentiates: autoliths of two-feldspar nepheline syenite, eudialyte-nepheline-aegirine syenite, leucocratic perthite-nepheline syenite & biotite lamprophyre. Crosscutting the major lithologies are decimeter-scale pegmatite dikes and nearly bimineralic, centimeter-scale dikes composed of alkali feldspar and aegirine. Trace element plots indicate a continuous evolutionary progression through the major lithologies, suggesting a genetic relationship. The East Hill suite was subjected to sodium and chlorine metasomatism from interaction with late-stage brines. Sodalite pseudomorphs after nepheline are pervasive; much of the potassium feldspar is altered and albitized; and sodium-rich minerals, such as dawsonite, are present as reaction rims on alkali feldspar and zeolites. The pegmatite dikes are comparable chemically and mineralogically to the more evolved differentiates, notably to the eudialyte-nepheline-aegirine syenite, exhibiting similar chemical enrichments, albeit to a greater degree. Mont Saint-Hilaire is the result of anorogenic plutonism, therefore, NYF-type pegmatites are to be expected; however, the chemistry of East Hill suite pegmatites does not correspond to the classic NYF-type pegmatite: F is present in modest quantities, but Cl dominates, and Nb is subordinate to Zr and Y to Ce in terms of bulk chemistry and diagnostic minerals. Thus, the East Hill suite pegmatites might better characterize a new pegmatite type representing alkaline systems: the ZCC pegmatite.

Reference

Currie, K.L., Eby, G.N. & Gittins, J. (1986) The petrology of the Mont Saint-Hilaire complex, southern Quebec: An alkaline gabbro-peralkaline syenite association. Lithos 19, 65-81.