Age-Dating Studies on Precambrian Rocks at Jasper, Alberta
C. R. Evans, Department of Geology and Mineralogy, University Museum, Oxford, England. J. Steiner, Geology Department, Australian National University, Canberra, A.C.T. J. L. Weiner Geology Department, University of Alberta, Edmonton, Alta.
Whole-rock and muscovite potassium-argon dates from selected low-grade metamorphic rocks of Precambrian age in the Canadian Rocky Mountains near Jasper, Alberta, were obtained to ascertain the age of the source terrain and the age of the metamorphism. Ten argillites gave whole-rock dates ranging from 270 to 485 million years, dates which are apparently intermediate between the age of the source material and that of the metamorphism. "Up-dating" increases with the degree of orientation of muscovite in the cleavage plane, and with the ratio of the muscovite in the cleavage plane to that in the plane of the bedding. Detrital muscovite from four arenaceous rocks gave dates ranging from 1,050 to 1,780 million years. "Up-dating" increases with decreasing grain size, so that, within the range investigated, the cube of the median diameter of the muscovite is apparently proportional to the radiometrie date. To determine the age of the source terrain and the age of the metamorphism, muscovite with a diameter greater than 1 mm. and Jess than 0.04 mm. would be required. The original sediments were probably derived from the Churchill province of the Canadian Shield or its extension under the Interior Plains, and metamorphism took place in post-Carboniferous time.
Mots Clés:
Alberta, Development, Developments, Laramide orogeny, Metamorphism, muscovite, Precambrian Rocks, University of Alberta, Metamorphism, Mica, micas, Precambrian, Rock, Rocks
