Geochemistry and tectonic setting of the Double Mountain volcanic complex, northeastern Brooks Range, Alaska and regional correlation with Devonian magmatism in the northern Cordillera
Creators: name: Lockett, Alec
Publisher: name: University of Iowa
Year: 2025
Subjects: subject: arc
subject: Arctic Alaska
subject: Brooks Range
subject: Devonian
subject: tectonic
subject: public abstract
subject: Geochemistry
subject: FOS: Earth and related environmental sciences

description: Establishing the Paleozoic tectonic setting of the northeastern Brooks Range is critical to understanding the timing and transition from an active to passive margin setting in Arctic Alaska. Previous mapping in the Arctic 1:250,000 Quadrangle (Brosgé et al., 2001) revealed unnamed Paleozoic volcanic rocks and clastic units that are key to constraining this tectonic evolution. New mapping results and geochemical analysis improve characterization of the stratigraphic and structural relationships in this sequence exposed in the Arctic D-1 SW and NW Quadrangles. We identify informal units: the Upper Devonian Double Mountain complex, which includes a felsic, mafic and shale member, the Lower Mississippian Chandalar unit; and an Unnamed dikes unit. The Chandalar unit overlies the Double Mountain complex on a prominent angular unconformity. Both units are structurally overlain by the Kayak Shale and Lisburne Group. The Brookian thrust fault separating these units defines a shallowly plunging antiform in the Arctic Quadrangle with the volcanic and clastic units exposed in a structural window.
description: All Double Mountain complex and Unnamed dikes rocks record elevated LREE with respect to HREE, negative Nb and Ti anomalies, and shallow HREE profiles that define primitive mantle-normalized trace element signatures characteristic of magma in continental arc settings. Initial 87Sr/86Sr and εNd values were calculated using a 373 Ma eruptive age: 87Sr/86Sr(t) values range from 0.70593 to 0.71750, with most samples around 0.708 to 0.709, and εNd(t) values range from -7.4 to -4.8; the Unnamed dikes range from 0.70326 to 0.71468 and -8.6 to -1.7, respectively. Radiogenic initial 87Sr/86Sr values and slightly evolved εNd signatures suggest the Double Mountain igneous rocks were sourced from melting of continental material. These isotopic signatures are less evolved than the Old Crow suite in northern Yukon (North Slope subterrane), but they are similar to Upper Devonian Beaver Creek (Coldfoot subterrane) and Arrigetch Peak (Hammond subterrane) igneous rocks in the central and western Brooks Range. The Upper Devonian Double Mountain complex has a penetrative foliation and quartz microstructures resulting from grain boundary migration recrystallization, a microstructural fabric that is lacking from the overlying Chandalar unit. Additionally, the Ulungarat Formation exhibits a contractional cleavage formed by pressure solution transfer. These observations suggest contractional deformation must have occurred at least through the Late Devonian, which conflicts with the Eifelian (early Middle Devonian) timing constraint for the end of the Romanzof orogeny. Although we cannot preclude the possibility that there is a distinct episode of pre-Ulungarat deformation, evidence of widespread Late Devonian deformation in Arctic Alaska suggests Romanzof and Ellesmerian activity may reflect a long period of contractional deformation.