Rb–Sr Isotopic Ages and Petrologic Studies of the Rocks in the Lac St. Jean Area, Quebec

The granulite facies meta-igneous and metasedimentary rocks that surround the Lac St. Jean anorthosite yield an Rb–Sr age of 1480 m.y. Intrusive monzonite, possibly genetically associated with the anorthosite, was emplaced slightly prior to 1513 m.y. ago. pre-anorthosite rocks were variously affected by the intrusion of the anorthosite and related rocks and affected again by the Grenvillian orogeny. Intrusive monzonite, granite, and syenite associated with the Grenvillian orogeny yield Rb–Sr ages between 1300 and 1000 m.y. Amphibolite facies rocks to the east of the anorthosite–monzonite granulite terrain were not as intensely affected during the Grenvillian orogeny. These granitic paragneisses record a metamorphic age of 1380 m.y. which suggests, in this area, continuous metamorphism between 1550 and 1000 m.y. ago. Thermal peaks at 1500 and 1100 m.y. are considered probable.