Geology, petrology and tectonic setting of the Late Jurassic ophiolite in Hokkaido, Japan

摘要

The Gokurakudaira Formation, which has a N–S zonal distribution within a latest Jurassic greenstone belt in Hokkaido Island, Japan, constitutes the uppermost ultramafic–mafic unit of the Horokanai Ophiolite. The following three hypotheses for the origin of the ophiolite have been proposed: (1) a mid-oceanic ridge; (2) an oceanic plateau; and (3) an island arc. The Gokurakudaira Formation can be subdivided into four zones extending NNW to SSE, from east (Zone I) to west (Zone IV), based on lithofacies and areal distribution. Zones I and III consist of aphyric tholeiite resembling back-arc basin basalt (BABB), while Zone II is characterized by the coexistence of BABB-like tholeiite along with high-Mg andesite. Zone IV has a different lithology from the other zones, and is composed mainly of picrite and thick sedimentary sequences of island arc tholeiite (IAT) type andesitic subaqueous pyroclastic deposits and terrigenous sediments. These stratigraphic and petrological characteristics of the Gokurakudaira Formation cannot be explained by the oceanic plateau or mid-oceanic ridge models, but they could correspond to the marginal sea model, as in the Lau Basin. Therefore, we conclude that the Horokanai Ophiolite was formed in the Late Jurassic in a marginal basin above a supra-subduction zone on the margin of the Asian continent.