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.