The Late Jurassic magmatic protoliths of the Mikabu greenstones in SW Japan: A fragment of an oceanic plateau in the Paleo-Pacific Ocean

Abstract

The Mikabu greenstones form accreted fragments of an ancient oceanic plateau, which are currently incorporated within the Cretaceous high P/T-type Sanbagawa metamorphic belt in SW Japan. Previous geological and petrological studies indicate that protoliths of the Mikabu greenstones were mafic/ultramafic volcano-plutonic rocks of oceanic plateau and/or islands. Despite their rock assemblage and metamorphic nature, their protolith age has been poorly constrained. By measuring U-Pb age of igneous zircon grains using LA-ICPMS, this study identified for the first time a Late Jurassic age for the protoliths of the Mikabu greenstones in the eastern Kii Peninsula (the Shurei complex). With an overlap within 2-sigma error, measured U-Pb ages of zircon grains have a weighted mean age of 154.6 ± 1.6 Ma (the Kimmeridgian, Late Jurassic), which is slightly younger than the radiolarian ages (Middle Jurassic) of deep-sea pelagic cherts associated with the Mikabu greenstones. This age difference indicates that mafic magmas intruded pelagic deep-sea sediments to form a volcanic edifice onto the pre-existing oceanic lithosphere. New measurements of trace element composition of the dated zircon grains further confirmed that the dated zircon grains are more similar to those in oceanic crusts rather than in continental crusts. In addition, the trace element composition suggests that the host mafic magma of the zircon grains was derived from depleted mantle similar to the mid-oceanic ridge-type source, rather than fertile mantle source like Hawaii and Iceland. On the basis of the common occurrence of similar mafic rocks from the Shatsky Rise in the modern NW Pacific Ocean, we conclude that the protoliths of the Mikabu greenstones had formed a part of an oceanic plateau/island, which was generated by the Late Jurassic intra-plate mafic magmatism in ancient mid-Pacific, probably together with the Shatzky Rise.