NEPHELINE GNEISSES OF THE WOLFE BELT, LYNDOCH TOWNSHIP, ONTARIO: I. STRUCTURE, STRATIGRAPHY, AND PETROGRAPHY

Abstract

The Wolfe Belt is a sequence of banded gneisses characterized by nepheline and other minerals with abnormally high sodium contents. Two main units can be recognized. On the north side of the belt and extending along its entire mapped length are leurocratic feldspathic gneisses, generally nepheiine-bearing. These gneisses form a unit up to 350 ft thick and over 4 miles long in which also occur bands of amphibolite, crystalline limestone, and quartzite. The southern half of the belt consists of calc-silicate gneiss up to 230 ft thick with intercalated horizons of metamorphic pyroxenite and 'iron-formation'.These lithotogical units are interlayered and contain within them well-developed banding, which is attributed to relict supracrustal layering. Nepheline may be present in all of the units except the quartzite, but its distribution is irregular, although influenced to a large degree by the stratigraphy. The species and compositions of the associated mafic minerals are related to the nepheline content of the rocks as well as to the bulk rock composition.The Wolfe Belt is interpreted as being of metasedimentary origin, the parent rocks having been metasomatized during the Grenville orogenic period. The only igneous phase present is leucocratic alkali–syenite, which intrudes the nepheline gneisses and modifies them.