K–Ar geochronology of the southwestern Brooks Range, Alaska

The southern Brooks Range schist belt is characterized by metasediments with subordinate metabasites and metafelsites. Blueschists occur within the schist belt from the Chandalar Quadrangle westward to the Baird Mountains Quadrangle.We report 76 new 40K–40Ar mineral ages from 47 metamorphic and igneous rocks in the southwestern Brooks Range. The pattern of radiometric ages reflects the complex geologic history of this area. Local and regional radiometric ages provide evidence that the schist belt has, at least in part, undergone a Late Precambrian blueschist facies metamorphism. Silurian to Devonian carbonates and Middle Devonian felsic volcanics were later deposited on this Precambrian basement. Subsequent deformations produced the present pattern of isoclinal, recumbent folds, resulting in the interlayering of rocks of markedly different ages and lithologies.The entire schist terrane was metamorphosed to greenschist facies in Mesozoic time, probably as a result of Jurassic–Cretaceous northward overthrusting of oceanic crust and upper mantle, resulting in the resetting of most K–Ar mica ages to the mid-Cretaceous. A series of apparent 40K–40Ar ages intermediate between Late Precambrian and mid-Cretaceous are interpreted as indicating varying amounts of partial argon loss from older rocks during the Mesozoic metamorphism. Some glaucophanes contain excess radiogenic 40Ar.