Metamorphic Evolution of the Precambrian Basement of Alberta

The metamorphic facies of the subsurface Precambrian of Alberta are charted for the first time as a contribution to the new tectonometamorphic map of the Canadian Shield. Inferences about facies are based primarily on petrographically determined mineral assemblages. A few P–T determinations from results of electron-microprobe analyses are available. The chronology of metamorphic events is based primarily on extensive and previously reported U–Pb, Rb–Sr, K–Ar, 40Ar–39Ar and Sm–Nd geochronology. New U–Pb, 40Ar–39Ar and Sm–Nd dates are reported for critical localities. All of Alberta was affected by a protracted series of early Proterozoic events. Late Archean (2700–2800 Ma) sialic crust in the southern half of the province was thermally overprinted at about 1800 Ma. In northern Alberta, older crust was dynamically, thermally and metasomatically reworked by a series of high-temperature events between 1750 and 1950 Ma.