Petrogenesis of Peraluminous Granites, Monashee Mountains, Southeastern Canadian Cordillera

Abstract

Two-mica granites that locally contain garnet and sillimanite occur as dikes, sills, and sheets up to 50 m thick within the sillimanite zonc of the Monashee Mountains in the southeastern Canadian Cordillera of British Columbia. Syn-kinematic and post-kinematic granites are recognized. U-Pb dating of zircon demonstrates that the syn-kinematic granites are 100.4±0.3 Ma old, based on duplicate concordant single abraded zircon analyses. Other zircons have slightly older Pb/Pb dates, indicating small amounts of inherited zircons. Monazites are 99±10 Ma old. Post-kinematic granites have 62.5±0.2 Ma zircon ages and 634+0.1 Ma monazite ages. High initial 87 ratios (0.71492–0.74181) and evidence of Precambrian Pb inheritance indicates that both syn- and post-kinematic granites were derived from a crustal source. Geobarometric estimates suggest that both generations of granites equilibrated at 6–8 kb (22–30 km). Zircon and monazite saturation temperatures range from 660–824°C and indicate that these minerals were liquidus phases early in the crystallization history of the granites. Because monazite saturation temperatures generally exceed those of zircon, it is possible that some monazites may be inherited. Apatite saturation temperatures in excess of 900°C suggest that both generations of granites contain source inherited apatite. Syn- and post-kinematic granites have essentially identical major and trace element chemistries. Syn-kinematic granites have steep light rare earth element (LREE) enriched patterns with pronounced negative Eu anomalies. The REE patterns of post-kinematic granites range from steep LREE enriched patterns with negative Nd and Eu anomalies to flat patterns with low LREE contents, negative Nd anomalies, and both positive and negative Eu anomalies. Modelling of REE, Rb, Sr, and Ba contents demonstrates that syn-kinematic gramtes could have been generated by a low degree of partial melting (with 10–25% feldspar fractionation of the melt) of Late Proterozoic Horsethief Creek Group metapelites leaving a monazite-bearing upper amphibolite facies residue. Post-kinematic granites were produced by partial melting of a geochemically and isotopically similar metapelitic source. The suite of post-kinematic granites can be related by a small amount (up to 0.1%) of monazite crystal fractionation.