Early Miocene island arc tholeiite in the Mineoka Belt: Implications for genetic relationship with the Izu–Bonin–Mariana (IBM) arc

摘要

The Mineoka Belt in central Japan is a Paleogene accretionary complex with the various kinds of volcanic rocks and plutonic rocks formed in multiple ages. A basaltic lapilli tuff derived from the Hota Group was collected from the Mineoka Belt, Boso Peninsula to reveal the genetic relationship of the Mineoka Belt with the Izu–Bonin–Mariana (IBM) arc. The whole–rock composition of the basaltic lapilli tuff shows island arc tholeiite affinity, and the zircon U–Pb dating yields about 18 Ma. The geochemical signatures of the basaltic lapilli tuff are similar to those of the Eocene plutonic rocks in the Mineoka Belt, but the zircon U–Pb age is much younger than those of the plutonic rocks. The idea that the arc–related rocks in the Belt are fragments of the IBM arc can explain the two different ages of the Mineoka arc–related rocks. The island arc plutonic rocks are derived from IBM middle to lower crust formed during Eocene to Oligocene, whereas the Early Miocene basaltic rock is most likely to be a fragment of arc products formed in the IBM volcanism at the end of the Miocene back–arc spreadings.