Emplacement of three granitic plutons in Southeastern Ontario, Canada

Abstract

The manner and history of emplacement of three post-tectonic granitic plutons in Hastings and Peterborough counties, Ontario, were studied using lithological and structural mapping, petrographic studies (including modal analyses, chemical analyses, radio-metric and decrepitometric studies), and petrofabric studies. The country rocks consist of Grenville metasedimentary rocks which were folded in two stages along northeast-and east-southeast-trending axes respectively, before emplacement of the plutons. The elliptical (5 by 3 miles) Chandos Lake pluton is made up of a central body of foliated syenite-monzonite, surrounded by later foliated biotite granite grading to a granodiorite. Both components are steep-dipping, crosscutting, funnel-shaped bodies emplaced by forceful intrusion at the site of a pre-existing structural basin. The elongated body of the foliated Wollaston biotite-hornblende granite (6 by 1.5 miles) was forcefully intruded along the eastern flank of a regional anticline as several steep, east-dipping bodies of granite, separated by narrow screens of paragneissic country rocks, parts of which were granitized to a granite gneiss. The composite Deloro pluton (20 square miles) consists of an earlier crescent-shaped (on plan), forceful intrusion of perthite granite grading to syenite along the western border, followed closely by the intrusion of a smaller body of granophyric granite in the east. The biotite granite-granodiorite of Chandos Lake pluton and the Wollaston biotite-hornblende granite show unsystematic variations of the modal quartz (Q)-feldspars (F)-mafic minerals (M) and the quartz (Q)-potassium feldspar (Kf)-plagioclase (PI) proportions and of the α activity; assimilation of varied rock types by the granite magmas accounts for such variations. In the Deloro pluton assimilation of gabbro-diabase by the granite magma accounts for the syenitic western border, but magmatic differentiation by fractional crystallization caused a systematic trend of acidification toward the central and eastern parts of the pluton and the location of the Q-Pl-Kf plots of the perthite granites within the thermal valley of Bowen. The cooling of the plutonic rocks was accompanied by extensive recrystallization of quartz in all the plutons, formation of myrmekite and of albite rims on plagioclase in the Chandos Lake and Wollaston plutons, and segregation of albite from the perthites in the Deloro pluton.