U-Pb ages and Hf isotope composition of zircon from the Shimanto accretionary complex: Evidence for heterogeneous sources

要旨

U-Pb zircon age spectra determined for detrital zircon from the Shimanto Accretionary Complex suggest that the sediments forming the Shimanto belt are derived from essentially the same sources as the protoliths of the Sanbagawa Belt. The prominent Early Jurassic peaks seen in the age spectra of both belts suggest that SE China is not a major source of the detrital zircons. The age spectrum of our sample from the east coast of Shikoku indicates that the outcrop of the Tochidani Unit of the Shimanto Belt extends over a larger distance than previously thought, i.e. eastwards to the coast. The age barcode of the sample from the western coast of the Kii Peninsula agrees well with that of the overlying unit, whereas the age distribution of the sample from the south coast of the Kii Peninsular is unique. Hf isotopic compositions determined for a subset of the dated zircons fall into two groups, one with εHf(t) = –8 to +12 and the other with εHf(t) = –22 to –12. The former may be derived from sources located within the current Japanese archipelago and the latter from outside sources. These outside sources may be largely located in southern/south-eastern South Korea as indicated by the abundance of Early to Middle Jurassic zircon in the Shimanto Belt and the highly and negative εHf(t) values measured for zircon from Korean plutons.

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