Polyphase Metamorphism in the Canadian Shield of Northeastern Alberta

The Precambrian polymetamorphic complex of granitoids and metasediments in northeastern Alberta reveals two distinct cycles of metamorphism. During the Archean metamorphic cycle, metasediments were metamorphosed under high pressure granulite conditions (M1). In a second cycle, probably related to remobilization during the Aphebian, the metasediments were subjected to conditions of granulite-amphibolite transitional facies retrogressing to greenschist facies metamorphism. Coexisting garnet-biotite and garnet-cordierite pairs in the metasediments allows estimation of P-T conditions during the granulite-amphibolite transitional facies, identifying a moderate-pressure granulite facies event (M2.1) and a low-pressure amphibolite facies overprint (M2.2). The widespread formation of granitoids by anatexis and ultrametamorphism during M2.1 is indicated by their Aphebian ages and high initial strontium ratios. Some granitoids show minerals characteristic of both cycles of metamorphism, but others contain minerals characteristic of the second cycle only.

All rocks were subjected to a last retrograde phase under greenschist facies conditions (M2.3), especially along mylonitic fault zones. Low-grade metasedimentary rocks underwent only greenschist facies metamorphism and are considered younger than the other metamorphic rocks. Unmetamorphosed Helikan sediments of the Athabasca Formation lie unconformably on the older crystalline rocks.