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

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Abstract
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 arc‐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 arc that was being fragmented during extension, probably as the result of rollback of the north dipping subducting slab during Late Cretaceous time.