KOOLEN METAMORPHIC COMPLEX, NE RUSSIA - IMPLICATIONS FOR THE TECTONICEVOLUTION OF THE BERING STRAIT REGION

Structural culminations of midcrustal metamorphic rocks are found on b oth 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 ba sic 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 gneisse s and high-grade metamorphic rocks is exposed. The succession consists of a lower sequence of granitic gneisses and an upper sequence of bio tite-rich gneisses, quartzofeldspathic gneisses, lesser amphibolite an d marble, and gneisses and schists with an increasing abundance of int ercalated marble and calc-silicate units toward the top. All rocks are strongly foliated and exhibit north-south trending stretching lineati ons, Deformation occurred during sillimanite-grade metamorphism concur rent with partial melting of the crust. Metamorphic conditions varied from 7 to 3 kbar and from 700 degrees-500 degrees C. Three fractions o f monazite from a deformed pegmatite yielded ages of 104 Ma. Igneous m onazite from undeformed biotite granite yielded a U-Pb age of 94 Ma, i ndicating peak metamorphism and deformation is Cretaceous. Relations i n 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 similar to 105 and 90 M a. Our findings, together with regional relations, suggest that wholes ale crustal extension or extensional collapse of the crust affected th is region, perhaps during Pacific-ward migration of subduction. The re sults do not support large amounts of east-west shortening between Nor th America and Russia predicted by poles of rotation related to openin g of the North Atlantic in the Late Cretaceous and Tertiary.