Geochemistry and tectonic setting of the Double Mountain volcanic complex, northeastern Brooks Range, Alaska and regional correlation with Devonian magmatism in the northern Cordillera

The Brooks Range in Arctic Alaska comprises many tectonic pieces called terranes or subterranes. Terranes are large packages of fault-bounded rocks that have some internal stratigraphic characteristic that allows them to be differentiated from other terranes; subterranes are further divisions of terranes based on similar characteristics. The link between how these Proterozoic—Paleozoic terranes were transported and early Paleozoic subduction was initiated along western North America is unclear. This is partly because the tectonic setting in the northeastern (NE) Brooks Range during the Paleozoic remains controversial. Most Paleozoic paleogeographic reconstructions place the NE Brooks Range next to the Canadian Arctic margin, which experienced contractional deformation during the Late Devonian-Early Mississippian in what is known as the Ellesmerian orogeny, followed by rifting in the Early –Middle Mississippian. In contrast, some researchers contend that the NE Brooks Range experienced contractional deformation well before the Ellesmerian orogeny, during the Early – Middle Devonian in an event called the Romanzof orogeny.

In the NE Brooks Range, between the North Slope subterrane and Endicott Mountain Allochthon (or subterrane), there are unnamed volcanic and sedimentary rocks that have been mapped but lack age control and geochemical data. The goal of this study is to characterize these unnamed volcanic rocks and determine whether they are related to a rift or collisional tectonic environment. Furthermore, I compare igneous geochemical data to other igneous rocks in Arctic Alaska, specifically, the Coldfoot (Schist belt), Hammond (Central belt), and North Slope subterranes.

Based on my new mapping, I have identified three informal units: the Upper Devonian Double Mountain complex, the Lower Mississippian Chandalar unit, and an Unnamed dikes unit. I designate the Double Mountain unit as a “complex” because it includes poorly understood bimodal volcanic rocks as well as several clastic units that are a significant part of the package. The Chandalar unit was deposited unconformably on the Double Mountain complex. Certain trace and rare earth elements were used in geochemical analysis for classification and tectonic discrimination due to their relative immobility in mineral fractionation. Trace element signatures of volcanic rocks of the Double Mountain complex are characteristic of arc lavas. Strontium and neodymium isotopic data provide information on the maturity of the magma and what kind of crust it was derived from, and they also suggest significant contribution from continental crustal material.

This study also found that the Upper Devonian Double Mountain complex has a penetrative deformational fabric that the younger unconformably overlying Chandalar unit lacks. Since the felsic igneous rocks from the complex have been dated to be about ca. 373 Ma, this implies that the Romanzof orogeny was likely not a separate deformational event from the Ellesmerian orogeny. Rather, the northeastern Brooks Range likely experienced a prolonged period of contractional deformation.