Koolen metamorphic complex, NE Russia: Implications for the tectonic evolution of the Bering Strait region

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Abstract
Structural culminations of midcrustal metamorphic rocks are found on both sides of the Bering Strait in Alaska and Russia and occur within a magmatic belt of Cretaceous age. Geologic mapping in the Koolen Lake‐Lavrentia Bay region of the Chukchi Peninsula, Russia, outlines the basic relations between deformation, metamorphism, and magmatism in one of these structural culminations, the Koolen metamorphic complex. Here, a 10–15 km‐thick, southwest dipping structural succession of gneisses and high‐grade metamorphic rocks is exposed. The succession consists of a lower sequence of granitic gneisses and an upper sequence of biotite‐rich gneisses, quartzofeldspathic gneisses, lesser amphibolite and marble, and gneisses and schists with an increasing abundance of intercalated marble and calc‐silicate units toward the top. All rocks are strongly foliated and exhibit north‐south trending stretching lineations. Deformation occurred during sillimanite‐grade metamorphism concurrent with partial melting of the crust. Metamorphic conditions varied from 7 to 3 kbar and from 700°–500°C. Three fractions of monazite from a deformed pegmatite yielded ages of 104 Ma. Igneous monazite from undeformed biotite granite yielded a U‐Pb age of 94 Ma, indicating peak metamorphism and deformation is Cretaceous. Relations in the Koolen complex are similar to those in the Kigluaik gneiss dome, Seward Peninsula, Alaska, where upper amphibolite to granulite facies metamorphism and deformation occurred between ∼105 and 90 Ma. Our findings, together with regional relations, suggest that wholesale crustal extension or extensional collapse of the crust affected this region, perhaps during Pacific‐ward migration of subduction. The results do not support large amounts of east‐west shortening between North America and Russia predicted by poles of rotation related to opening of the North Atlantic in the Late Cretaceous and Tertiary.