Zircon U – Pb Dating of the Akashima Formation , Oga Peninsula , Akita Prefecture , Japan

Comprehensive zircon U–Pb ages have been obtained using laser-ablation ICP-MS from igneous rocks in the Akashima Formation exposed in the northwestern part of the Oga Peninsula, Akita Prefecture, Japan. The Akashima Formation, mainly composed of dacitic welded lappilli-tuff, was previously considered to represent part of the type locality for the Miocene “Green Tuff” in the Northeast Japan Arc. However, recent SHRIMP zircon U–Pb dating showed that at least part of the Akashima Formation was formed in the Late Cretaceous at 72 Ma (Kano et al., 2012), raising questions about the age and distribution of the formation. We have newly analyzed seven tuff samples that broadly cover the exposures that have been mapped as the Akashima Formation. All of the obtained igneous zircon U–Pb ages cluster at ~72 Ma, concordant with the recent SHRIMP zircon ages, confirming that the Akashima Formation was generated during Late Cretaceous episodic volcanism. However, three samples analyzed from the abundant granitic xenoliths entrained in the volcanic breccia of the Akashima Formation yielded distinctively older ages of ~92 Ma, indicating that an unexposed Late Cretaceous granitic basement underlies the Oga Peninsula region. Furthermore, NE-trending porphyritic rhyolite dikes that intrude the Akashima Formation showed much younger ages of ~32 Ma. These Oligocene ages are contemporaneous with the previously reported radiometric ages of the rhyolitic lava and volcaniclastic rocks of the overlying Monzen Formation, suggesting that these dikes represent the magma feeding system of the Monzen Formation volcanism. These new ages provide important temporal constraints for the tectonic reconstruction of pre-Cenozoic Northeast Japan Arc before the formation of the Sea of Japan.