Timing of the Late Cretaceous ignimbrite flare-up at the eastern margin of the Eurasian Plate: New zircon U-Pb ages from the Aioi-Arima-Koto region of SW Japan

The Kinki district in the Inner Zone of southwest Japan is characterized by Late Cretaceous volcanic rocks of the Aioi and Arima groups and the Koto Rhyolites (from west to east). These rocks are dominated by pyroclastic flow deposits related to caldera-forming events. Here, we present new laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) zircon U-Pb ages for the Aioi Group (four isolated caldera-filling deposits); the lower, middle, and upper parts of the Arima Group and one intrusive rock (the Kashihara Quartz Gabbro); and the lower and upper parts of the Koto Rhyolites, including the oldest volcanic unit within these rhyolites. These analyses yielded ages of ca. 86-82 Ma for the Aioi Group, ca. 83-81 Ma for the Arima Group, 78.6 Ma for the Kashihara Quartz Gabbro, and ca. 74-73 Ma for the Koto Rhyolites. The Aioi Group represents a cluster of calderas and the ages obtained for individual units in this group differ from the ages of adjacent units. The Arima Group and the Koto Rhyolites both consist of pyroclastic flow deposits associated with caldera-forming events, and the ages of these rocks are all the same within error. This suggests that the Aioi Group represents a series of individual caldera-forming eruptions that are distinct from the Arima Group and the Koto Rhyolites, which formed during a single stage of caldera formation. The U-Pb ages presented here indicate that the Late Cretaceous caldera-forming eruptions in the study area occurred at intervals of > 1 Myr and represent individual events that lasted for < 1 Myr. The oldest volcanic unit within the Kinki district is similar in age to the oldest volcanic unit within the Chubu district, suggesting that caldera-forming eruptions in southwest Japan commenced at ca. 90 Ma.