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We present new whole-rock geochemical data from the Brooks Range ophiolite (BRO) together with new mineral chemistry data from the BRO, South Sandwich forearc, Izu-Bonin forearc, and Hess Deep. The analyses reveal that the Brooks Range ophiolite (BRO) was most likely created in a forearc setting. We show that this tectonic classification requires the Brookian orogeny to begin at ~163-169 Ma. The middle-Jurassic BRO contains abundant gabbros and other intrusive rocks that are geochemically similar to lithologies found in other forearc settings. Based on major, minor, and trace element geochemistry, we conclude that the BRO has clear signals of a subduction-related origin. High-precision olivine data from the BRO have a forearc signature, with possible geochemical input from a nearby arc. The Koyukuk terrane lies to the south of the Brooks Range; previous studies have concluded that the BRO is the forearc remnant of this arc-related terrane. These studies also conclude that collision between the Koyukuk Arc and the Arctic Alaska continental margin marks the beginning of the Brookian orogeny. Since the BRO is a forearc ophiolite, the collision between the Koyukuk Arc and the continental margin must have coincided with obduction of the BRO. Previously determined 40Ar/39Ar ages from the BRO’s metamorphic sole yield an obduction age of 163-169 Ma. Since the same collisional event that obducts the BRO also is responsible for the Brookian orogeny, we conclude that the BRO’s obduction age of ~163-169 Ma marks the beginning of this orogenic event.

Tectonochemistry of the Brooks Range Ophiolite, Alaska