Petrology and geochemistry of the Canadian Shield of northeastern Alberta

The Alberta Shield is considered to represent an Archean gneissic terrain, which partially melted during a period of metamorphism at bout 1900 Ma producing four major peraluminous intrusive complexes. The remnant, central belt of granodioritic to quartz-monzonitic gneiss, of I-type granitoid composition, is flanked on its western argin by fairly homogeneous protons of biotite quartz monzonite to granite (Slave and Arch Lake Granitoids), and on its eastern margin by heterogeneous hornblende, biotite granodiorite to quartz monzonite (Wylie Lake and Colin Lake Granitoids). Field relationships, normative feldspar compositions, modeling of large ion lithophile elements, and comparisons with relevant experimental data, combine to indicate that the Slave and Arch Lake Granitoids could have been erived from about 40 percent partial melting of the central gneiss belt at approximately five kb(?). A restite assemblage of plagioclase, biotite, and hornblende was partially incorporated into his crystal-melt system, which was sufficiently ductile to be mplaced diapirically, while quartz-K-feldspar-plagioclase cotectic rystallization took place in the melt. R-mode factor analysis suggested that the Nb, Y and Zr compositions of the Slave and Arch Lake Granitoids were directly inherited from heterogeneities in the gneissic progenitor and were unrelated to crystal fractionation/partial melting processes. The Wylie Lake and Colin Lake Granitoids ate similar in composition to the central gneiss belt and were probably produced by a more limited partial melting of the same source.

The mean composition of the Alberta Shield is significantly higher in Si, K, and Ba than an estimate for the whole Churchill Province but is similar in composition to the exposed Saskatchewan Shield. The central gneiss belt is enriched in Ba and Si, compared to the mean composition of the Canadian Shield but has similar concentrations of alkalies, Sr, Zn, and the high field strength elements Ti, Zr, Y. and Nb.