[SGC49-03] Geological and petrological study of Miocene felsic volcanic rocks in the Kamo area, Niigata Prefecture
キーワード：火山岩、珪長質火山岩、日本海拡大、背弧拡大、岩石学
The igneous activity in the Northeastern Japan arc at the Middle Miocene, which is related to the buck-arc spreading of the Sea of Japan (20 to 15Ma), is characterized by a bimodal igneous activity with large amounts of mafic and felsic volcanic rocks (e.g. Shuto et al., 2006). Many previous studies have been discussed magma generation and evolution process of their basaltic to andesitic rocks. However, there are few published discussions for petrological study of rhyolitic rocks. This study was performed to clarify the petrological characteristics of the Middle Miocene felsic volcanic rocks in the Kamo area, north-central Niigata Prefecture.
The Miocene series in this study area are classified into Tonoiri Formation, Otani Formation, Nanatani Formation and Minamiimogawa Formation in ascending order, and unconformably cover the basement of the Lower Jurassic accretionary complex (Kudo et al., 2011). Tonoiri Formation mainly consists of conglomerate and dacite lava. Otani Formation is composed of mudstone, pyroclastic rock and rhyolite intrusive rock. Nanatani Formation is composed of massive mudstone, pyroclastic rock, rhyolite intrusive rock and dolerite dike. Minamiimogawa Formation is composed of massive mudstone, pyroclastic rock and rhyolite intrusive rock. Previous study reported fission-track ages of 14Ma (Nanatani Formation) and 8Ma (Shigekurayama volcanic rock unit in Minamiimogawa Formation) from some rhyolite intrusive bodies in this area (Kudo et al., 2011).
Tonoiri dacite lava display aphyric texture and contains microphenocryst of pseudomorphic amphibole and plagioclase. Otani and Nanatani rhyolite intrusive rocks are divided into two types by phenocrystic mineral assemblages, Type-1 (hornblende + plagioclase) and Type-2 (plagioclase ± quartz). Nanatani dolerite display subophitic texures and composed of clinopyroxene, pseudomorphic olivine and plagioclase. Shigekurayama volcanic rock unit rhyolites are divided into three types by petrographical features, Type-3 (hornblende + orthopyroxene ± quartz), Type-4 (hornblende + biotite ± quartz) and Type-5 (hornblende + biotite ± quartz, pumiceous rhyolite). Type-3 and Type-4 rhyolites are observed in intrusive bodies. Type-5 rhyolite is observed in pyroclastic deposits.
Miocene volcanic rocks in this study area are plotted on subalkali rock field in SiO2 vs Na2O + K2O diagram. In SiO2 vs K2O diagram, Nanatani dolerite and felsic rocks are plotted on Low-K and High-K series, respectively. Rhyolites in this study display two differentiation trends, Group-1 (Type-1 and Type-2) and Group-2 (Type-3, Type-4 and Type-5), in some variation diagrams. In alumina saturation index diagram, Group-1 is plotted on peraluminous rock field, and Group-2 is metaluminous rock field. Zr/Nb ratios of Group-1 and Group-2 are also different such as Zr/Nb ratios are 22.9 and 17.8, respectively.
From the petrological features of rhyolites, differentiation trends of both rhyolite groups are difficult to explained by simple crystallization differentiation from mafic magma (e.g. Nanatani low-K dolerite magma). These results suggest the possibility that rhyolite magmas were formed by the different magma generation and evolution process.
Kudo et al, 2011, Geology of the Kamo district.1:50000 Geol. Map, GSJ, 162 p.
Shuto et al, 2006, Lithos, 86, 1-33 p.
The Miocene series in this study area are classified into Tonoiri Formation, Otani Formation, Nanatani Formation and Minamiimogawa Formation in ascending order, and unconformably cover the basement of the Lower Jurassic accretionary complex (Kudo et al., 2011). Tonoiri Formation mainly consists of conglomerate and dacite lava. Otani Formation is composed of mudstone, pyroclastic rock and rhyolite intrusive rock. Nanatani Formation is composed of massive mudstone, pyroclastic rock, rhyolite intrusive rock and dolerite dike. Minamiimogawa Formation is composed of massive mudstone, pyroclastic rock and rhyolite intrusive rock. Previous study reported fission-track ages of 14Ma (Nanatani Formation) and 8Ma (Shigekurayama volcanic rock unit in Minamiimogawa Formation) from some rhyolite intrusive bodies in this area (Kudo et al., 2011).
Tonoiri dacite lava display aphyric texture and contains microphenocryst of pseudomorphic amphibole and plagioclase. Otani and Nanatani rhyolite intrusive rocks are divided into two types by phenocrystic mineral assemblages, Type-1 (hornblende + plagioclase) and Type-2 (plagioclase ± quartz). Nanatani dolerite display subophitic texures and composed of clinopyroxene, pseudomorphic olivine and plagioclase. Shigekurayama volcanic rock unit rhyolites are divided into three types by petrographical features, Type-3 (hornblende + orthopyroxene ± quartz), Type-4 (hornblende + biotite ± quartz) and Type-5 (hornblende + biotite ± quartz, pumiceous rhyolite). Type-3 and Type-4 rhyolites are observed in intrusive bodies. Type-5 rhyolite is observed in pyroclastic deposits.
Miocene volcanic rocks in this study area are plotted on subalkali rock field in SiO2 vs Na2O + K2O diagram. In SiO2 vs K2O diagram, Nanatani dolerite and felsic rocks are plotted on Low-K and High-K series, respectively. Rhyolites in this study display two differentiation trends, Group-1 (Type-1 and Type-2) and Group-2 (Type-3, Type-4 and Type-5), in some variation diagrams. In alumina saturation index diagram, Group-1 is plotted on peraluminous rock field, and Group-2 is metaluminous rock field. Zr/Nb ratios of Group-1 and Group-2 are also different such as Zr/Nb ratios are 22.9 and 17.8, respectively.
From the petrological features of rhyolites, differentiation trends of both rhyolite groups are difficult to explained by simple crystallization differentiation from mafic magma (e.g. Nanatani low-K dolerite magma). These results suggest the possibility that rhyolite magmas were formed by the different magma generation and evolution process.
Kudo et al, 2011, Geology of the Kamo district.1:50000 Geol. Map, GSJ, 162 p.
Shuto et al, 2006, Lithos, 86, 1-33 p.