Abstract

The Doonerak fenster of the central Brooks Range, Alaska, exposes a unique package of Cambrian–Devonian(?) volcanic and sedimentary basement rocks (Apoon assemblage) within the Mesozoic–Cenozoic Brookian fold-and-thrust belt. Recognition of a major pre-Mississippian unconformity within the fenster led to previous correlations between the Apoon assemblage and age-equivalent strata in the North Slope subterrane of the composite Arctic Alaska–Chukotka microplate. Previous age constraints on the Apoon assemblage are limited to a handful of Cambrian–Devonian paleontological collections and hornblende K-Ar and 40Ar/39Ar ages from mafic dikes ranging from ca. 520 to 380 Ma. We conducted U-Pb and Hf isotopic analyses on igneous and sedimentary units in the Apoon assemblage to test links with the North Slope subterrane and assess the tectonic and paleogeographic setting of this early Paleozoic arc complex. Igneous zircon from a leucogabbro in the Apoon assemblage analyzed using a sensitive high-resolution ion microprobe with reverse geometry (SHRIMP-RG) provided a 207Pb-corrected 206Pb/238U age of 462 ± 8 Ma (2σ). Detrital zircon analyzed using laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) from volcaniclastic and tuffaceous strata of the Apoon assemblage yielded a spectrum of unimodal and polymodal age populations, including prominent age groups at ca. 490–420, 540–520 Ma, 1250–960, 1500–1380, 1945–1750, and 2830–2650 Ma. Lu-Hf isotopic data from the ca. 490–420 Ma age population, including the Middle Ordovician leucogabbro, are highly juvenile (εHf ∼+7–10), implying a distinct lack of crustal assimilation during Ordovician–Silurian arc magmatism. In context, these data suggest that a juvenile arc complex, herein referred to as the Doonerak arc, marks a prominent tectonic boundary between pre-Mississippian crustal fragments of Laurentian and non-Laurentian affinity in the Arctic Alaska–Chukotka microplate. The U-Pb geochronologic and Hf isotopic data also provide a connection to Taconic–Caledonian arc magmatism along the edge of the Iapetus Ocean and linkages between the Apoon assemblage and the Descon Formation and plutonic equivalents of the Alexander terrane (Prince of Wales Island, Alaska).

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