Geochemistry of the gneissic basement complex near Valemount, British Columbia: further evidence for a varied origin

Abstract

Rare-earth elements and other trace elements have been determined by activation analysis and X-ray fluorescence for representative samples from each of the four blocks of basement gneisses near Valemount, eastern British Columbia. Patterns in mafic and tonalitic gneisses are generally as expected, but the granite–gneisses have very large negative europium anomalies, up to Eu*/Eu = 18, indicating multistage histories involving plagioclase fractionation. Modelling shows that plagioclase fractionation alone is insufficient to account for these anomalies without intervention of other phases: apatite control is suggested, among other possibilities. The granite–gneisses also contain exceptionally low magnesium (0.1–0.2%), phosphorus (<300 ppm), scandium (<0.07 ppm), and cesium (<0.5 ppm). After partial melting is considered as a possible mode of origin, it is concluded that the granite–gneisses are final, small-scale fractionates from enriched tholeiite magmas. These might be associated with upper Proterozoic rifting processes. Previously published conclusions regarding the protolith of each subset of the gneisses are confirmed; likewise, the earlier conclusion that the gneisses cannot be correlated across the Southern Rocky Mountain Trench is substantiated in detail.