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

The Mont Rigaud Intrusion is a syeno-granitic stock located in the St. Lawr ence River-Ottawa River Graben and surrounded by sedimentary rocks of the S t. Lawrence Lowlands. The intrusion is circular and divided into three unit s: an outer ring of hornblende syenite, an inner ring of feldspar-hornblend e 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 un its are comagmatic and were intruded within a short period of time. Rock sa mples from the intrusion have a geochemical Frofile typical of within-plate A-type granites. The pluton is enriched in high field strength elements an d rare earth elements and is characterized by negative Eu anomalies in the more fractionated units. The epsilon Nd 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 u nderlying 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 s tudies 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 S t. 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 characteriz ed by alkaline, silica-undersaturated intrusions with minor oversaturated s yenitic intrusions, such as the Mont Rigaud Stock. Rift related magmatic ac tivity ended with the extrusion of the Tibbit Hill volcanics in the Mont Su tton area of southwestern Quebec.