Abstract
The Sakhalin-Hokkaido orogenic belt along the western margin of the Okhotsk Sea is an important early Cenozoic accretionary orogen related to subduction of the Paleo-Pacific oceanic plate. In order to study the Eocene evolution of this accretionary orogen and related magmatism, we carried out zircon UPb dating, whole-rock elemental and SrNd isotopic analyses on the Langeri and Val'za granitic plutons that intruded the East Sakhalin accretionary complex in the central Sakhalin Island. Zircon UPb dating results revealed that the Langeri and Val'za plutons were emplaced in the late Eocene at 38–36 Ma. Whole-rock elemental data indicate that granitoids are mainly peraluminous S-type with subordinate I-type granites, and all granitic samples show arc-like geochemical affinity. Whole-rock SrNd isotopic results show consistent low initial 87Sr/86Sr ratios (0.7042 to 0.7049), positive εNd(t) values (+1.7 to +3.1) and young Nd two-stage model ages (611 to 717 Ma). Elemental and isotopic data reveal that granitic rocks of the Langeri and Val'za plutons were generated from remelting of metasediments and amphibolites in the accretionary complex. From comprehensive analyses of regional tectonics, we concluded that the late Eocene granitic rocks were generated in a syn-collisional tectonic setting, marking the timing for formation of the Sakhalin-Hokkaido orogenic belt. The middle-late Eocene welding of the Okhotsk Sea Plate and the Eurasia Plate is the most plausible mechanism that caused crustal thickening beneath the Sakhalin Island, and anatexis of metasediments and oceanic crustal materials in the lower crust might be responsible for the formation of late Eocene granitic rocks. Considering the Eocene tectonic events in NE Asia, late Paleocene to early Eocene subduction of the Kula-Pacific ridge beneath the Hokkaido Island and Eocene initial rift of pull-apart basins along strike-slip fault system in the Hokkaido and Sakhalin Islands may also contribute to the emplacement of Eocene magmatism in the Sakhalin-Hokkaido orogenic belt.
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