SYNTHESIS OF GEOCHRONOLOGIC, GEOCHEMICAL, METAMORPHIC, AND PALEONTOLOGICAL DATA ON ROCKS OF OCEANIC ORIGIN IN NORTHERN ALASKA

SIWIEC, Benjamin R. and TILL, Alison, U.S. Geol Survey, 4200 University Dr, Anchorage, AK 99508-4626, [email protected]

We present a synthesis of existing data on Paleozoic and Mesozoic oceanic rocks of northern Alaska. These rocks are known by a variety of terrane names, such as the Angayucham terrane in the southern Brooks Range, the Tozitna terrane farther south (along the Yukon river), and the Rampart and Christian volcanics to the east (near the Porcupine River). Angayucham terrane basalts were proposed to have originated as Devonian to Permian oceanic plateaus or seamounts in a paleo-ocean that has been called the Angayucham ocean. Other Angayucham terrane rocks were reported to be Triassic to Jurassic or Early Cretaceous mafic igneous rocks with arc chemical affinities. The Angayucham ocean is considered to have been in its constructional phase before the Triassic based on the lack of documented Paleozoic arc-volcanic rocks. Both Angayucham sequences were emplaced over the Arctic Alaska continental margin beginning in the Middle or Late Jurassic as this ocean closed. The specific tectonic elements and events involved in the ocean basin closure and the early stage of the subsequent collisional episode are not well understood. For example, mafic Angayucham and Tozitna terrane rocks have been metamorphosed at prehnite-pumpellyite-, greenschist-, and blueschist-facies conditions, yet the age(s) and setting of these metamorphic events is not known and they are not included in tectonic models. Although radiolarian and conodont ages from sections of Angayucham basalt range from Middle Devonian to Early Jurassic, isotopic (Ar-Ar, K-Ar, U-Pb) ages from gabbros, plagiogranites, diorites, amphibolites, pegmatites, and schists range from 344 Ma (Mississippian) to 138 Ma (Early Cretaceous). Improved tectonic models for the nucleation of the Brooks Range orogeny will depend on improved understanding of the oceanic rocks and their history. We present the specific locations and synthesized geochronologic, metamorphic, paleontologic, and geochemical fingerprinting data for the Angayucham terrane and related rocks in Alaska as a tool to address this problem.