Abstract
The pre-Upper Cretaceous basement rocks distributed in the Notsuharu area, eastern Kyushu, are classified structurally from bottom to top into the Chokai Unit, the Ultramafic Unit and the Hikata Unit. The Chokai Unit known as the Asaji metamorphic rocks consists mainly of pelitic, psammitic and mafic rocks, and chert. Metamorphic zonation and the chemical compositions of metamorphic garnet and biotite indicate that the metamorphism of the Chokai Unit is due to the thermal effects of the syntectonic intrusion of the Nioki granite. Metamorphic conditions are estimated to be approximately 610°C at 2 kbar. SHRIMP Zircon U-Pb dating of the Nioki granite shows a concordia age of 134.7 ± 2.8 Ma (2σ). The uppermost Hikata Unit consists of non-metamorphic sedimentary rocks, and forms a southeastern vergenced nappe structure (Hikata nappe). The Hikata nappe truncates the thermal structure within the Chokai Unit, and is unconformably overlain by the Upper Cretaceous Onogawa Group. Therefore, the movement of the Hikata nappe can be limited to the period between the intrusion of the Nioki granite (ca. 135 Ma) and initial sedimentation of the Onogawa Group (ca. 100 Ma).
