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The Koto Rhyolites of late Cretaceous age consist of two units. Each unit contains volcanic and subvolcanic parts which are probably co-magmatic. The older unit is formed of the Kaiwara Welded Tuff and the Hatasho Quartz Porphyry, the younger of the Yatsuoyama Pyroclastic Rocks and Inugami Granite Porphyry. The first igneous activity was the eruption of the Kaiwara Welded Tuff which was deposited on the Paleozoic basement rocks of the area. This eruptive episode was followed by shallow intrusion of the Hatasho Quartz Porphyry. The area around Yatsuoyama subsequently collapsed to form a northwest-trending graben. A thin lake deposit, the Fukaya Formation, accumulated in the graben unconformably on the older volcanic and subvolcanic rocks which had been partly eroded. This area was then covered during the second episode of igneous activity by the Yatsuoyama Pyroclastic Rocks of the younger unit. Finally, the Inugami Granite Porphyry intruded mainly along the boundary faults of the graben. Development of faults across the graben synchronous with emplacement of the porphyry suggest forceful intrusion. Forty-three samples from the Koto Rhyolites were analysed for Rb, Sr, Y, and Zr, and also for Ca and K. The overall abundance of these elements and the general variations that resulted from magmatic differentiation are typical of those displayed by felsic igneous rocks elsewhere. The abundance of Rb, Sr, and Zr and the ratios of K/Rb and Rb/Sr are significantly different for rocks of each of the two volcanic and two subvolcanic units. The distribution patterns of the elements between the Kaiwara Welded Tuff and the Hatasho Quartz Porphyry of the older unit, however, are very similar to those between the Yatsuoyama Pyroclastic rocks and the Inugami Granite Porphyry of the younger unit. Volatiles, K, and Rb were apparently concentrated upward, and Ca, Sr, and Zr downward in the magma chambers present during the two periods. Magma in the upper parts of the chambers erupted several times to deposit the pyroclastic rocks, mainly welded tuff. Shortly after the eruptions, the remnant magma in the chambers intruded into shallower parts of the crust to form the quartz porphyry and granite porphyry.

IGNEOUS ACTIVITY OF THE KOTO RHYOLITES IN THE YATSUOYAMA DISTRICT, SOUTHEAST OF LAKE BIWA