The Mount Distin assemblage: Neoproterozoic metasedimentary rocks in the Nome Complex of Seward Peninsula, Alaska, USA

Abstract

Proterozoic to Paleozoic rocks involved in Jurassic and earliest Cretaceous (Brookian) orogenesis and subsequently extended, metamorphosed at high temperature and intruded by Cretaceous plutons, are widespread on the Seward Peninsula, Alaska, USA. These blueschist and greenschist facies metasedimentary and metaigneous rocks make up the highly deformed Nome Complex. We describe and bracket the age of a distinct Precambrian succession in the Nome Complex, the Mount Distin assemblage (informal). The Mount Distin assemblage consists of marble, impure marble, calc-schist, quartz mica schist, and lesser quartzite that was originally a shelfal succession. Metamafic bodies, originally dikes and/or sills, are minor but common components. Three new U-Pb detrital zircon (DZ) samples are characterized by numerous peaks between 1.5 Ga and 1.0 Ga, with minor ages of 1000−900 Ma and 2000−1500 Ma and only 1−2 grains 3.0−2.5 Ga, indicating maximum depositional ages of 1000−900 Ma. A new U-Pb zircon age for an orthogneiss lens in marble is 661 ± 3 Ma like others previously dated in the Nome Complex. Orthogneiss ages provide an upper age limit for the Mount Distin assemblage of ca. 687 Ma, the oldest reported date. Zircons in three other orthogneiss bodies are interpreted as inherited as they have a range of U-Pb ages like those in their country rocks. The Mount Distin assemblage DZ ages are like those from schist-bearing carbonate sequences in the Central and Schist belts of the Brooks Range and inherited zircon populations in Neoproterozoic plutons on Wrangel Island, Russia. The discovery and documentation of the Mount Distin assemblage explains the previously reported enigmatic lack of country rocks to Neoproterozoic orthogneiss bodies, establishes the continuity of older rocks across the entire extent of the Arctic Alaska−Chukotka microplate, and further confirms this microplate’s Baltica affinities.