Insights into the Assembly of Arctic Alaska: Provenance and Geochemical Data from the Doonerak Fenster and Endicott Mountains Allochthon

Abstract

The Doonerak fenster of the central Brooks Range contains the Apoon assemblage, a suite of Early Paleozoic mafic-to-intermediate volcanic and siliciclastic rocks. The fenster exposes deep structural levels of the Brooks Range between regions of Laurentian and non-Laurentian origin in Arctic Alaska. During the Mesozoic to Cenozoic formation of the Brooks Range, Upper Devonian to Triassic siliciclastic and carbonate rocks of the Endicott Mountains allochthon were thrusted northward and juxtaposed over the top of this volcanic suite. New geochemical data indicates that the Apoon assemblage formed as part of an island arc complex. The assemblage contains basaltic-to-andesitic rocks with diverse textures, including pillow basalts and cross-cutting dikes and sills. Enrichment in large ion lithophile elements (e.g. Cs, Rb, Ba, Th), depletion in high field strength elements (e.g. Nb, P, Zr, Ti), and chondritenormalized rare-earth element (REE) trends within the Apoon volcanics are consistent with formation in an island arc setting and corroborate previous work. Detrital zircon spectra from volcaniclastic portions of the Apoon assemblage show a prominent unimodal 440-530 Ma population centered on 504 Ma with only minor populations potentially derived from Laurentia. The siliciclastic units unconformably overlying the Apoon assemblage (Ellesmerian sequence) and those of the Endicott Mountains allochthon mostly lack detritus from this 504 Ma population and are instead dominated by 400-440 Ma populations centered on ~420 Ma. While this is dissimilar from the Apoon assemblage, detritus of this age is common in rocks of the Canadian Arctic, Svalbard, and eastern Greenland and is associated with the Caledonian orogeny. The coexistence of Caledonian detritus alongside 800-550 Ma populations within samples from the northern margin of the fenster also indicates a connection to the Timanian orogen of northern Baltica. The youngest portions of the Apoon assemblage also show significant Laurentian and Caledonian age detritus. This shift in provenance could be explained by the closure of an oceanic basin, providing newly integrated sediment pathways. Collectively, evidence presented here indicates that the Apoon assemblage formed as part of an island arc, which initially formed in a relatively isolated geographic position and received detritus from mostly proximal sources (single unimodal 504 Ma population). This arc progressively approached and eventually docked onto the margin of northwest Laurentian during closure of the northernmost Iapetus Ocean, eventually amalgamating regions of Laurentian and non-Laurentian origin. Thus, the Doonerak fenster preserves an island arc complex and marks the location of a Paleozoic suture between Laurentian and non-Laurentian portions of Arctic Alaska.

Structural Origins of the Doonerak Anticline, Central Brooks Range, Alaska

“…Subsequent mapping was completed at the more accessible eastern end of the anticline adjacent to the Dalton Highway at the 1:63,360 scale as part of the Trans-Alaska Crustal Transect (TACT) project (Moore et al, 1997) and the 1:36360 scale by Phelps (1987) and Julian (1989). More recent work in the study area has included geochemical and geochronological sampling, but no major revisions to previous maps (Frier, 2017;Strauss et al, 2017).…”

“…The 2015 field season targeted the northern margin of the structure at the contact between the Apoon assemblage, Ellesmerian sequence, and Endicott Mountains allochthon. This first season was principally for the thesis work of W. P. Frier (2017). I served as field assistant, but the 2015 data and observations were integrated with those from the 2016 season for this work.…”

“…The Hunt Fork Shale has a pair of Caledonian-aged peaks (~360 and ~420 Ma) in samples from the Doonerak anticline area and elsewhere. Samples of Sillyasheen Mountain graywacke(Frier, 2017) may have a single peak within this same Caledonian age range. These peaks are younger than the peaks seen in Apoon assemblage samples Dillon et al (1986).…”

“…anticline and on correlative units to the west. Representative detrital zircon geochronology probability density plots are shown inFigure 33from published work byFrier (2017) andStrauss et al (2017). Their work was conducted primarily along the northern portion of the Doonerak anticline.…”

“…Stacked detrital zircon geochronology probability density plots from samples ofFrier (2017) and accumulated plots from multiple samples fromStrauss et al (2017). Modified fromFrier (2017). Color bars represent potential Laurentian source domains.…”