GEOCHRONOLOGY OF THE CANADIAN SHIELD IN NORTHEASTERN ALBERTA: I. ANDREW LAKE AREA

Abstract

Field work and petrographic investigation in the Precambrian of northeastern Alberta near Andrew Lake indicate a complex geologic history involving at least three major diastrophic periods. U–Pb, Rb–Sr, K–Ar, and several model lead dates determined on amenable phases of representative samples of the major rock units may be placed in three groups, roughly corresponding to the major diastrophic periods. All mica K–Ar dates are close to ~1 800 m.y., while the zircon, uraninite, and allanite U–Pb dates fall into two groups at ~1 900 m.y. and ~2 250 m.y, A whole-rock Rb–Sr isochron plot of selected basement gneiss samples also yields two groups of dates at ~1 900 m.y. and ~2 250 m.y. When the geologic history of seven selected representative rock samples is judged by petrographic examination of the dated minerals, the petrographically estimated response of the minerals to sequential geologic events is reflected with good correlation by the various dates determined on the minerals. Some discrepancies in this latter correlation are noted and include such things as a relatively 'old' zircon in a young intrusive plug, the apparent absence of primary intrusive rocks at the time of the oldest (zircon dates) and the youngest (K–Ar mica dates) diastrophic periods.