A Polymetamorphic Complex in the Eastern Alaska Range

Abstract

A polymetamorphic basement complex is exposed in the Rainbow Mountain-Gulkana Glacier area, south of the Denali fault, in the Eastern Alaska Range. Reconstruction of the polycyclic history is based on a sequence of mineral assemblages, on structural relationships, and on associated units which have participated in less than the full sequence of metamorphic events. Preliminary work indicates four principal cycles of recrystallization, although little of the history of each cycle is yet known. These four events with their characteristic mineral facies and tentative ages are: (4) Mesozoic syn- or post-intrusive recrystallization in greenschist facies marginal to quartz diorite stocks, (3) Pre-Devonian synkinematic greenschist metamorphism, (2) Amphibolite facies metamorphism and migmatization of Paleozoic(?)-Precambrian(?) age, and (1) Precambrian granulite facies metamorphism. The presence of a granulite-bearing terrane in the Alaska Range, as well as the known occurrence of granulites at two other widespread localities, strongly points to the existence of a Precambrian basement underlying a large part of Alaska. This Alaska Range polymetamorphic unit is structurally and lithologically related to the Birch Creek Schist across the Denali fault. The two units most probably share a common history of deformation and recrystallization, and a continuing study of these relationships may lead to a better understanding of the entire block of metamorphic rocks in Interior Alaska.