Results of a comprehensive geochemical study (major and trace elements, and isotopes of Sr, Nd, Pb, Hf) of Cretaceous volcanic rocks from the Chukotka area in northeastern Russia are presented. Synthesis of available geological and geochronological data suggests diachronous onset of activity of the Okhotsk-Chukotka volcanic belt (OCVB), the largest magmatic province in the region. The OCVB consists of ca. 106 km³ of volcanic rocks. At 106-105 Ma, subduction-related magmatism initiated in the southern and central segments of the OCVB. In the Central and Northern Chukotka areas, where the northern OCVB is exposed, onset of arc magmatism occurred ca. 10 m.y. after extension-related magmatism of the Chaun igneous province at 109-104 Ma. Mafic rocks from the OCVB yield (⁸⁷Sr/⁸⁶Sr)^{80 Ma} of 0.7033 to 0.7047, εNd^{80 Ma} of 0.0 to 7.10, εHf^{80 Ma} of 4.12 to 12.88, (²⁰⁶Pb/²⁰⁴Pb)^{80 Ma} of 18.11 to 18.42, and (²⁰⁸Pb/²⁰⁴Pb)^{80 Ma} of 37.96 to 38.21. Volcanic rocks from the Chaun province, as well as OCVB rocks from Northern Chukotka, originate from a relatively enriched source and have (⁸⁷Sr/⁸⁶Sr)^{80 Ma} of 0.7088 to 0.7100, εNd^{80 Ma} of - 5.81 to - 3.42, εHf^{80 Ma} of - 3.40 to - 0.25, (²⁰⁶Pb/²⁰⁴Pb)^{80 Ma} of 18.69 to 18.90, and (²⁰⁸Pb/²⁰⁴Pb)^{80 Ma} of 38.65 to 38.86. No definitive across-arc elemental or isotopic zonation of the OCVB has been revealed, probably because of wide-scale crustal melting and subsequent contamination of mantle-derived melts. However, there is a clear along-arc isotopic zonation. In our interpretation, this results from heterogeneity of the subcontinental lithospheric mantle, which likely was a major contributor to the magma source. The similar isotopic signatures of silicic (dominantly crust-derived) and mafic (mantle-derived) volcanic rocks in each OCVB segment imply that remelting of juvenile mafic underplated material was the main process responsible for the crust-derived magma generation. These data from the major Cretaceous magmatic provinces of northeast Asia are synthesized in context of existing plate tectonic reconstructions.

1. The initiation of the OCVB activity was diachronous, probably resulting from variations in the arc-normal component of relative plate motion along the strike of the continental margin. At ca. 106–105 Ma, the formation of the larger, NE-trending (in present-day coordinates) part of the volcanic belt began with the eruptions of dominantly mafic and intermediate calc-alkaline magmas. Typically, mantle protolith of these magmas was characterized by depleted (DM-like) isotopic composition, although there is also evidence for contribution from relatively enriched sources. During the same time period, the extensional (or perhaps transtensional) tectonic regime caused generation and emplacement of strongly enriched shoshonitic magmas within the former Arctic Alaska–Chukotka block that formed shallow-level plutons and related minor volcanic units of the Chaun igneous province.
2. After the re-arrangement of plate kinematics in the North Pacific at ca. 95 Ma, the formation of the northern, SE-trending part of the OCVB began. The activity of the entire OCVB was accompanied by extremely intense thermal reworking and melting of the continental crust, with the production of significant volumes of silicic magmas. The strong crustal contamination of mantle-derived melts disguised the geochemical cross-arc zonation typical of many other subduction-related volcanic belts.
3. The Okhotsk–Chukotka belt has a clear along-arc isotopic zonation, which likely results from different compositions of the subcontinental lithospheric mantle under several continental blocks that were assembled together shortly before the onset of OCVB activity. Commonly, the isotopic ratios of silicic (crust-derived) and mafic (mantle-derived) volcanic rocks are similar within a single OCVB segment, suggesting that remelting of juvenile mafic underplated material was the major process responsible for the crustal magma generation.
4. The ‘upper basalts’ of the OCVB formed during decreasing heat flow, after the crystallization of large crustal magma chambers. The youngest mafic rocks of the OCVB are slightly enriched relative to other mafic rocks of the volcanic belt, possibly because of the addition of melts from relatively deep sources. However, the available data suggest that the ‘upper basalts’ represent the natural final stage of the OCVB activity, rather than the product of a postulated superposed rifting event.