FIELD RELATIONSHIPS, PETROLOGY, AND AGE OF NEOPROTEROZOIC GRANITIC AND SYENITIC PLUTONS IN THE ANTIGONISH HIGHLANDS, NOVA SCOTIA, CANADA

The Antigonish Highlands have characteristic features of Avalonia, including Neoproterozoic arc-related volcanic and plutonic rocks and Cambrian-Silurian cover sequences. Also present in the southern part of the highlands are scattered plutons previously inferred to be of Devonian-Carboniferous age. Although contacts are not exposed, these plutons were interpreted to have intruded Neoproterozoic and Silurian volcanic and sedimentary rocks. This project focuses on the field relationships, petrology, age, and tectonic implications of these plutons. One of the plutons, Hunter Lake, consists of tonalite and minor granodiorite, and is likely to be closely related to quartz diorite of the Neoproterozoic (based on previously determined K-Ar ages) Eden Lake Complex, an interpretation consistent with lack of evidence for contact metamorphism in the surrounding fossiliferous Silurian rocks. The other plutons consist of varying proportions of granitic and syenitic rocks. The granitic rocks include distinctive alkali feldspar granite and syenogranite with quartz eyes and interstitial granophyric texture indicative of shallow emplacement. The syenitic rocks contain aegirine and in some samples riebeckite, indicative of peralkaline composition. An additional pluton (Haggarts Lake), previously mapped as Neoproterozoic diorite, is also of syenitic composition. An alkali feldspar granite sample from one of the plutons (Gunn Lake) yielded a preliminary U-Pb (zircon) upper intercept age of about 605 Ma, consistent with the concordant U-Pb (zircon) age of 610 ± 3 Ma reported previously for the petrographically similar Cape Porcupine Granite at the Strait of Canso. These ages suggest that at least some (and probably all) of the plutons in the Antigonish Highlands previously inferred to be Devonian-Carboniferous are Neoproterozoic, and that such plutons are much more abundant than previously recognized. The widespread occurrence of Neoproterozoic peralkaline granite and syenite is especially significant, as such rocks also occur in the northern Antigonish Highlands but are not known in Avalonia elsewhere in Maritime Canada. Mineral analyses and whole-rock geochemical studies are in progress to better define the petrogenesis and tectonic implications of these plutons.