Potassic mafic magmatism in the Kigluaik gneiss dome, northern Alaska: A geochemical study of arc magmatism in an extensional tectonic setting

The 90 Ma Kigluaik pluton forms the core of a granulite facies gneiss dome and is part of an extensive belt of Cretaceous plutons in northern Alaska and Russia. The dome consists of Precambrian to Paleozoic gneisses metamorphosed during a pre-120 Ma high-pressure event and a Late Cretaceous high-temperature event. The pluton is essentially bimodal in composition. A ''mafic root'' consists of granodiorite, quartz monzodiorite, tonalite, and diorite. This unit appears to be cut by gabbroic dikes or lenses and is overlain by a 1-km-thick ''felsic cap'' of biotite granite. Separating these units is a zone of mafic enclaves, with pillow shapes and crenulate margins, within a granodiorite matrix. The mafic root is depleted in Nb and enriched in K and other large-ion lithophile elements, including the light rare earth elements; the felsic cap is geochemically similar but with higher concentrations of incompatible elements. Sr and Nd isotopic compositions near bulk earth suggest that a mantle-derived parental magma fractionated and assimilated crustal rocks during its history. The mafic root has trace element characteristics similar to Early Cretaceous are-related volcanic rocks and Late Cretaceous potassic plutons in the Yukon-Koyukuk Basin of Alaska. It is also similar to other high-K mafic plutons formed above continental subduction zones. The Cretaceous tectonic setting of the Seward peninsula was likely characterized by a continental are that was being fragmented during extension, probably as the result of rollback of the north dipping subducting slab during Late Cretaceous time.