Igneous history of the Koyukuk terrane, western Alaska: Constraints on the origin, evolution, and ultimate collision of an accreted island arc terrane

Abstract

The Koyukuk terrane of western Alaska consists of volcanic, volcaniclastic, and plutonic rocks which range from Late Paleozoic to Early Cretaceous in age. The terrane crops out in a U-shaped belt which is roughly paralleled by outer belts of ultramafic rocks, oceanic plate basalts and cherts, and retrograded blueschist facies rocks of continental protolith. These rocks have been interpreted as components of a volcanic arc terrane that collided with the North American continental margin in Early Cretaceous time. The Koyukuk terrane consists of four time-stratigraphic units: (1) pre-Middle Jurassic basalts, (2) Middle and Late Jurassic granitic rocks, (3) lower Lower Cretaceous volcanic rocks, and (4) upper Lower Cretaceous volcanic rocks. Limited chemical data from the basalts of unit 1 indicate that they were erupted in a nonarc tectonic environment, possibly in an oceanic island or back arc setting. Units 2, 3, and 4 have the characteristics of subduction-related volcanic rocks (i.e., depleted Nb and Ta and enriched alkaline elements, relative to the light rare earth elements). Unit 3 contains tholeiitic, calc-alkaline, and alkaline rocks with chondrite-normalized rare earth element patterns that range from flat (La N /Yb N = 1) to highly light rare earth element enriched (La N /Yb N > 15). The highly alkaline or shoshonitic lavas were erupted toward the end of unit 3 time (Valanginian) during the final stages of arc-continent collision. These alkaline lavas could have been derived by very small degrees of partial melting of a similar source to that of the earlier arc lavas. Unit 4 lavas are also alkaline or shoshonitic, but their incompatible element composition indicates that they were derived from a different source than that of the earlier arc lavas. These late alkaline lavas are chemically similar to crosscutting mid-Cretaceous plutons whose isotopic compositions (Arth et al., this issue ( a )) suggest derivation by partial melting of distinctly older subcontinental lithosphere. We speculate that the parental magmas of unit 4 lavas may also have been derived by partial melting of this subcontinental mantle which was underthrust beneath the Koyukuk arc terrane during the final stage of arc-continent collision.