St Lawrence Granite

A more or less continuously exposed batholith of 30x10 km with a number of outliers, the St Lawrence granite cuts sharply across Proterozoic lavas, pyroclastics and volcanogenic sediments that are conformably overlain by Cambrian sediments. There is a well developed contact aureole containing andalusite, cordierite etc. The granite is composed essentially of quartz and K-feldspar with albite in some specimens together with small and variable amounts of alkali amphibole, aegirine, biotite, fluorite, magnetite and hematite. In the northern parts of the batholith there are riebeckite-bearing volcanics which are interpreted as being extrusive equivalents of the granite. Fluorite veins are widespread but do not occur in the western lobe, and rhyolite porphyry dykes of essentially the same mineralogy as the granite occur. The alkali amphiboles of the granite have been investigated by Strong and Taylor (1984). Colman et al. (1984), in an oxygen isotope study, investigated the nature and extent of fluid involvement in metasomatism at the contacts of the granite.