Sm‐Nd Geochemistry and U‐Pb Geochronology of the Mont Rigaud Stock, Quebec, Canada: A Late Magmatic Event Associated with the Formation of the Iapetus Rift

Abstract

The Mont Rigaud Intrusion is a syeno‐granitic stock located in the St. Lawrence River–Ottawa River Graben and surrounded by sedimentary rocks of the St. Lawrence Lowlands. The intrusion is circular and divided into three units: an outer ring of hornblende syenite, an inner ring of feldspar‐hornblende syenite, and an inner core of quartz syenite and granite. Geochemical, U‐Pb, and Nd isotopic data gathered from the stock indicate that all three units are comagmatic and were intruded within a short period of time. Rock samples from the intrusion have a geochemical profile typical of within‐plate A‐type granites. The pluton is enriched in high field strength elements and rare earth elements and is characterized by negative Eu anomalies in the more fractionated units. The εNd values from the two syenitic units range from +1.5 to +2.2, while the granite yields values of about +0.5. The Nd isotopic data lie below depleted mantle values but above those of the underlying Grenville crust (−2.5). Thus, the initial syenitic magma probably did not form by crustal melting but may be a mantle‐derived melt that has undergone an assimilation–fractional crystallization process. U‐Pb zircon studies of the granite unit yielded an age of 564 +10/−8 Ma, consistent with the ages of other rift‐related intrusions. Magmatism associated with the St. Lawrence River–Ottawa River Rift falls into two major periods. The first, from 615 to 588 Ma, is characterized by intrusion of mafic dike swarms of tholeiitic affinity. The second, from 577 to 554 Ma, is mostly characterized by alkaline, silica‐undersaturated intrusions with minor oversaturated syenitic intrusions, such as the Mont Rigaud Stock. Rift‐related magmatic activity ended with the extrusion of the Tibbit Hill volcanics in the Mont Sutton area of southwestern Quebec.