新潟県北部地域の白亜紀後期〜新第三紀中期の火成岩類のフィッション-トラック年代

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Various kinds of igneous rocks ranging from late Cretaceous to middle Neogene in age, are widely distributed in northern part of Niigata Prefecture. Among them late Cretaceous igneous rocks mentioned here are mainly composed of the Kogawa type granite and the so-called Asashi rhyolites. The Neogene pyroclastic deposits in this area are divided into the Shimoseki Formation, Formation, Atsumi Group, Oizumi Formation, Kitaoguni (Budo) Formation, and Sumikawa Formation in descending order (TAKAHAMA, 1976). The results of fission-track age determinations of accessory zircons derived from above mentioned Cretaceous igneous rocks and volcanics of the Tertiary deposits are baiefly described in this note. According to the method described by TAMANYU (1975), the fission-track ages were obtained by logarithm population of track densities on zircons. Separated zircons were divided into two groups. Spontaneous fission-track densities (ρs) were counted in a group of zircons by logarithm population of track densities registered on crystals. Induced fission-track densities (ρi) were obtained from another group, after irradiation of thermal neutrons. The obtained ages and some notes from them are following: (λ_f=6.85×10^<17>y^<-1>) Shimoseki Formosation (Rhyolitic tuff) 22.3 m.y. Asahi Formation (Rhyolite lava) 20.7 m.y. Kitaoguni Formation (Rhyolitic welded tuff) 32.3 m.y. Budo Formation (Rhyolitic welded tuff) 46.4 m.y. Sumikawa Formation ((Quartz porphyry) 50.6 m.y. Kogawa type granite (Biotite adamerite) 76.3 m.y. Asahi rhyolites (Quarts pophyry)141.6 m.y. 1) The proposed ages in the present work fundamentally correspond to those from stratigraphic successions of this area. 2) On the basis of the present fission-track ages and previous reported K-Ar ages, the igneous activities ranging from late Cretaceous to middle Neogene in this area may be divided into at least five stages as shown in table 4. 3) Sumikawa Formation, which occupies the lowest stratigraphic horizon in the Green Tuff member of this area, has been generally believed to be correlatable with the Nyudozaki igneous rocks of Monzen group, typically developed in the Oga Peninsula (TAKAHAMA, 1976). The fission-track age of Sumikawa Formation is however 50.3 m.y., and those of Nyudozaki igneous rocks are 71.3, 78.2 m.y. (TAMANYD 1976), 54.4 m.y. (SDZUKI, 1978) and K-Ar ages are 30, 37 m.y. (OGUCHI et at., 1976). Accordingly, volcanic activities revealed by the Sumikawa Formation are concluded to have occurred during the Palaeogene.
1979-01-25

新潟県北部地域の白亜紀後期〜新第三紀中期の火成岩類のフィッション-トラック年代

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