MID-CRETACEOUS MAGMATISM OF THE BERING STRAIT REGION (SEWARD PENINSULA, ST. LAWRENCE ISLAND, AND CHUKOTKA): A REVIEW WITH NEW ZIRCON U-PB AGES

Cretaceous magmatism covers a vast area on both sides of the Bering Strait. Chukotka volcanism includes the Okhotsk-Chukotka volcanic belt (OCVB) from 106–77 Ma (Akinin and Miller, 2011). Volcanics from Provideniya are poorly dated but similar in age based on cross-cutting plutons (Calvert, 1999) and flora. St. Lawrence Island (STL), between Alaska and Chukotka, has trachytes and rhyolites with 106–101 Ma zircon ages, coeval with the oldest OCVB rocks. In Chukotka there are Albian (116-100 Ma) extension-related granitoids and Turonian (92-88 Ma) calc-alkaline tin-bearing granites (Akinin et al., 2012). The potassic intrusion at Cape Dezhnev is 110 Ma. Little Diomide granite is 109 Ma. On STL the western plutonic rocks are 113–99 Ma. STL has an alkaline (potassic) suite including 112 Ma syenite and 102-100 Ma “syenodiorite”. Seward Peninsula also has older, small-volume potassic rocks in the Kigluaik Mts. at 110 Ma. This was followed by 91 Ma diorite intrusions. The composite Kigluaik pluton has an 85±1 Ma felsic cap and 84±2 Ma mafic enclaves (neither previously dated) overlapping with an 84 Ma diabase dike swarm. In the Bendeleben Mts., sillimanite-bearing leucogranites are ~104-98 Ma followed by 91-84 Ma dioritic to monzogranitic plutons and dikes. The Darby pluton is 98 Ma. The Grand Singatuk pluton with miarolitic cavities is 78 ± 2 Ma, likely coeval with tin granites on Seward Peninsula and Chukotka, and represents the end of the Cretaceous magmatic episode in the Bering Strait region. Ar-Ar cooling ages are generally 2-4 m.y. younger than zircon ages except in the Bendelebens where 104-98 Ma granites yield 84–82 Ma ages.

Magmatism in the region can be broadly characterized as Andean-type, represented by the OCVB and volcanic rocks in Provideniya and STL. North of this belt, plutonism associated with extensional gneiss domes may be related to back-arc extension from a southward-migrating slab. The transition from highly potassic to high-K calc-alkaline to S-type magmatism likely reflects early small-degree partial melting of the mantle and increasing incorporation of crustal components. The presence of mafic plutonic rocks may reflect higher magnitude extension and/or exhumation to expose deeper crustal levels.