Potassium-Rich Alkaline Intrusive Rocks of Western Alaska

Abstract

Alkaline intrusive rocks occur in a 300-km-long belt of small plutons and intrusive complexes that extends from west-central Alaska to the Bering Sea. The occurrence of a massif of similar rocks on the easternmost tip of the Chukotsk Peninsula suggests the belt may extend into Siberia. These are highly potassic and subsilicic rocks that were emplaced at about mid-Cretaceous time (Albian and Cenomanian). The belt of alkaline potassic rocks cuts across two different geologic provinces. The eastern half of the belt is in the Yukon-Koyukuk volcanogenic province of Mesozoic age; the western half is in the Seward Peninsula province, which is comprised mainly of metamorphic and sedimentary rocks ranging in age from Precambrian to Paleozoic. The Yukon-Koyukuk province is an unusual setting for alkaline rocks, commonly thought to be typical of stable continental platform or shield areas. The possibility that the volcanic rocks of the Yukon-Koyukuk province rest directly on oceanic crust together with the regional extent of the potassium-rich alkaline rocks suggests that the potassic magma may have originated in the mantle. Confinement of the alkaline rocks to the general boundary area between the two geologic provinces suggests this highly faulted area was a zone of structural weakness along which deep-seated alkaline magma was emplaced. This magma may have influenced the composition of other plutonic rocks that are closely associated in time and space with the alkaline potassic rocks.