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 norther n Alaska and Russia. The dome consists of Precambrian to Paleozoic gne isses 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 monz odiorite, tonalite, and diorite. This unit appears to be cut by gabbro ic dikes or lenses and is overlain by a 1-km-thick ''felsic cap'' of b iotite granite. Separating these units is a zone of mafic enclaves, wi th 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 fel sic cap is geochemically similar but with higher concentrations of inc ompatible elements. Sr and Nd isotopic compositions near bulk earth su ggest that a mantle-derived parental magma fractionated and assimilate d crustal rocks during its history. The mafic root has trace element c haracteristics similar to Early Cretaceous are-related volcanic rocks and Late Cretaceous potassic plutons in the Yukon-Koyukuk Basin of Ala ska. It is also similar to other high-K mafic plutons formed above con tinental subduction zones. The Cretaceous tectonic setting of the Sewa rd peninsula was likely characterized by a continental are that was be ing fragmented during extension, probably as the result of rollback of the north dipping subducting slab during Late Cretaceous time.