Geochemical Explorations of the Brooks Range
Plutonic rocks of the Brooks Range comprise much less than 1% of the surface area of the Range; this is fairly unusual for a major compressional orogeny where subduction persisted for millions of years. Unfortunately, the paucity of plutonic rocks limits the extent of geochemical work possible in the Range. Also, the amount of work done on the available rocks is minimal. As mentioned, plutonic rocks in the Brooks Range fall into two age categories; middle Paleozoic and Proterozoic, based on U-Pb dating.
A series of Paleozoic-age plutons extend in an E-W belt along the core of the Brooks Range. The total area underlain by this plutonic complex is about 4500 square kilometers. Detailed mapping of the plutons in unavailable and geochemical data is sparse. Most of the plutons are granitic in composition, highly evolved, peraluminous, and contain two micas. Many of the plutons are associated with Sn-W skarn formations. Dating (U-Pb and Rb-Sr) of the plutonic rocks indicate crystallization ages of 400-370 Ma.
Plutonic rocks at Mount Angayukaqsraq, in the western Brooks Range, underlie an area of four square kilometers. The rocks are gabbro and leucogabbro, later intruded by granodiorite and highly evolved (>75% SiO2) alkali granite. Zircon U-Pb dating from this intrusion yields an age of 750± 6 Ma, probably representing age of intrusion.
Volcanic rocks in the Brooks Range, excepting some tuffs found in sedimentary sequences, are entirely limited to the oceanic Angayucham terrane.
The Angayucham terrane, previously mentioned as an ophiolite and the structurally highest thrust sheet in the Brooks Range, is composed of two separate thrust sheets; one with pillow basalts and chert and a second with amphibolite, peridotite and gabbro. The second sheet actually overlies the first, making this an appear as an "inverted" ophiolite. Major-element analyses of the basalts classifies them as hypersthene-normative olivine tholeiites (MORB). None of the basalts have an ocean-island chemical signature. Ages based on radiolarian chert nodules range from Triassic in the Central Belt to Jurassic in the southern range.