Large-scale deformation of crystalline rocks coupled with metasomatic recrystallization during the Hudsonian orogeny (~1800 m.y. ago) determined the main features of a large portion of the western Canadian Precambrian basement (the Athabasca mobile zone) and its subsequent geological history. The following are the main changes resulting from deformation and metasomatic recrystallization in the Athabasca mobile zone:(a) Significant increase in K2O and Rb, significant decrease in CaO, MgO, Na2O, and Sr, and almost unchanged Al2O3 and SiO2. These changes reflect the relative mobility of major oxides. The very marked change in the Rb/Sr ratio constitutes an important disturbing factor for isotopic age dating.(b) Marked increase in potassium feldspar combined with very marked increase in secondary minerals (epidote, chlorite, muscovite), clear-cut decrease in plagioclase and primary dark minerals, and only a slight increase in quartz. The coexistence of newly-formed microcline and low-grade minerals cannot be explained by disequilibrium.(c) Decrease in the average density of deformed and K-metasomatized complexes, and the probable subsequent long-term isostatic adjustments in the western Canadian sedimentary basin.