Late Mesozoic silicic magmatism of the North Chukotka area (NE Russia): Age, magma sources, and geodynamic implications

The Cretaceous Okhotsk-Chukotka volcanic belt (OCVB) is one of the largest subduction-related volcanic provinces of the Earth. It is thought to be related with the subduction of paleo-Pacific plates under the collage of terranes of NE Asia accreted during Jurassic and Early Cretaceous time. The OCVB comprises a remarkably high portion of silicic rocks, up to 80-90% in some segments. Within the Central Chukotka segment of the belt, volcanic sequences reveal a rather uniform ⁴⁰Ar/³⁹Ar and U-Pb isotopic age of near 89-87 Ma. But the felsic volcanic unit with coeval subsurface intrusives named the 'Berlozhya magmatic assemblage' (BMA) yields much older zircon U-Pb ages (146.0 ± 2.4 for rhyolitic tuff and 145.5 ± 1.8 Ma for related granodiorite). The relationship of the BMA with the active boundary between the Chukotka-Arctic Alaska microcontinent and the Anyui-Angayucham oceanic basin is suggested. BMA rocks seem to be undeformed, and their age puts constraints on the timing of main compressional events in the North Chukotka area. We present analytical data for both BMA and OCVB silicic magmatic rocks, including the first isotopic compositions of Sr, Nd, Pb, and Hf for the 1500 km long northern part of the OCVB. We infer that felsic magmas of both BMA and OCVB were produced by the melting of the continental crust, without a significant direct contribution from mantle sources. However, the crustal protolith could contain some ancient island arc complexes, which affected the chemical and isotopic composition of magmatic derivatives. The BMA exhibits a relatively uniform isotopic composition (⁸⁷Sr/⁸⁶Sr_i = 0.7057 to 0.7070, ε_Ndⁱ = - 0.51 to - 0.12, ε_Hfⁱ = 4.41 to 4.92, ²⁰⁶Pb/²⁰⁴Pbⁱ = 18.50 to 18.57, and ²⁰⁸Pb/²⁰⁴Pbⁱ = 38.23 to 38.31), and its chemical diversity likely results from crystal fractionation. The protolith of OCVB rhyolites has more variable isotopic characteristics (⁸⁷Sr/⁸⁶Sr_i = 0.7032 to 0.7082, ε_Ndⁱ = - 4.06 to - 2.84, ε_Hfⁱ = - 1.56 to 3.77, ²⁰⁶Pb/²⁰⁴Pbⁱ = 18.56 to 18.81, and ²⁰⁸Pb/²⁰⁴Pbⁱ = 38.21 to 38.63), and reveals at least three end-members. Each of them is distinct from the source of the Berlozhya assemblage, thus suggestive of compositional layering of the crust. Nd and Hf model ages of all studied rocks correspond to Neoproterozoic. The estimated Nd model age of the OCVB protolith is slightly older than that for the BMA, but both fall into the Neoproterozoic interval (1000 to 800 Ma).