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 petrogr aphically determined mineral assemblages. A few P-T determinations from res ults of electron-microprobe analyses are available. The chronology of metam orphic events is based primarily on extensive and previously reported U-Pb, Rb-Sr, K-Ar, Ar-40-Ar-39 and Sm-Nd geochronology. New U-Pb, Ar-40-Ar-39 an d Sm-Nd dales are reported for critical localities. All of Alberta was affe cted by a protracted series of early Proterozoic events. Late Archean (2700 -2800 Ma) sialic crust in the southern half of the province was thermally o verprinted at about 1800 Ma. In northern Alberta, older crust was dynamical ly, thermally and metasomatically reworked by a series of high-temperature events between 1750 and 1950 Ma.