The nepheline syenite and syenite gneisses of Monmouth and Glamorgan Townships represent the most southwesterly extent of the Ontario nepheline syenite-syenite gneiss belt. They lie between the Glamorgan granite gneiss batholith to the north and the Anstruther and Cheddar batholiths to the southeast. A prominent feature of the area is the wedge of the Glamorgan gabbro in the southwest. The nepheline syenite and syenites form sinuous wedges and bands which become more continuous to the northeast, and have a southerly dip. They are intimately associated with Grenville marbles, amphibolites and other metasedimentary rocks, with which they are intercalated. Cross-cutting relationships are only rarely observed. The nepheline-bearing rocks are very variable ranging from gneissic varieties to massive pegmatites, with igneous textures and cross-cutting relationships preserved in the area near the Glamorgan-Monmouth Townships boundary. The rock types at 16 different localities are described in some detail by Armstrong (in Hewitt, 1961, pp. 35-55) as are a number of named pegmatites, some of which have been quarried. The nepheline- bearing rocks are variable in mineralogy but generally are albite- nepheline-biotite gneisses, sometimes with hastingsitic amphibole, sodic hedenbergite, garnet, microcline, scapolite, perthite, zircon or magnetite. Nepheline ranges from an accessory to more than 50% of the rock. In the central part of the area rocks described as theralites (Tilley and Gittins, 1961) and consisting of andesine, nepheline, clinopyroxene, amphibole and accessory apatite, magnetite and olivine and with primary igneous textures are found. Pegmatites are abundant throughout the area, usually comprising plagioclase and nepheline, but with variable biotite, zircon, sodalite, hornblende, calcite and garnet. Apart from the absence of nepheline the syenites are generally similar petrographically to the nepheline syenites. Analyses of rocks and coexisting nepheline and feldspar will be found in Tilley and Gittins (1961), and a few modes in Hewitt (1961, p. 50).
HEWITT, D.F. 1957. Haliburton-Bancroft area. Ontario Department of Mines, Map No. 1957b.
HEWITT, D.F. 1961. Nepheline syenite deposits of southern Ontario. Annual Report, Ontario Department of Mines, 69(8): 1-194.
MACINTYRE, R.M., YORK, D. and MOORHOUSE, W.W. 1967. Potassium-argon age determinations in the Madoc-Bancroft area in the Grenville Province of the Canadian Shield. Canadian Journal of Earth Sciences, 4: 815-28.
TILLEY, C.E. and GITTINS, J. 1961. Igneous nepheline-bearing rocks of the Haliburton-Bancroft province of Ontario. Journal of Petrology, 2: 38-48.