Rb–Sr Isotopic Studies of the Lac Croche Complex Grenville Province, Quebec

Abstract

The Lac Croche complex is a deformed basin-shaped body of igneous origin, situated northeast of the Morin anorthosite pluton. Rb–Sr isotopic studies of the rock-types comprising the complex yield the following results: (a) The mangeritic augen gneiss, leuconoritic gneiss, monzonite, and monzonorite crystallized approximately 1124 m.y. ago with an initial 87Sr/86Sr ratio of 0.7042, compatible with these rocks being derivatives from a common source; (b) The K-feldspar – quartz gneiss crystallized approximately 1143 m.y. ago with an 87Sr/86Sr ratio of 0.7025; and (c) the granite crystallized approximately 961 m.y. ago with an 87Sr/86Sr ratio of 0.7084. Because the mangeritic augen gneiss and leuconoritic gneiss were deformed with the complex and the monzonite and monzonorite crystallized after this deformation, and because these rock types define an isochron, we conclude that the deformation of the Lac Croche complex took place approximately 1124 m.y. ago. If this deformation was a result of the emplacement of the Morin pluton, then this latter body also had to be emplaced at this time. These events constitute the main penetrative deformation in the area and represent the Grenville orogenic event.