Geological and petrological study of volcano-plutonic complexes, Southwest Jpan

日本西南部内带广泛分布着白垩纪至古近纪的火山岩和花岗质岩基，它们形成了火山-深成杂岩。本项目包括对日本西南部白垩纪至古近纪杂岩的构造地质学、岩石学、地球化学研究。从这些研究中，我们发现了新的大锅，即，分别在冈山县的新见地区和山口市的新见caidron和山口大锅。白垩纪Fukuga组主要由流纹岩-流纹英安岩熔岩和火山碎屑岩组成，其间侵入了一个花岗质岩株，表明火山岩和花岗质岩组成了一个火山-深成岩杂岩。10个流纹岩样品的福贺组和8个花岗岩样品产生类似的，定义明确的Rb-Sr全岩等时线。福贺岩套的等时线为86.8 * 2。 ...更多信息 花岗岩的等时线年龄为85.0 × 3.1Ma（2siguma），初始Sr同位素比值为0.70526 × 0.00023。所有的流纹岩和花岗岩给出了一个单一的组合等时线为87.0 - 3.1马（2 siguma）的初始比率为0.70526 * 0.000025。斜长石、钾长石、黑云母花岗岩全岩样品的等时线年龄为79.7 * 5.3 Ma（2 siguma），初始比值为0.70521 * 0.00017。岩石学、岩石化学和Sr、Nd同位素资料表明，福河组流纹岩和花岗质岩石具有共同的物源，与中国地区白垩纪火山岩相比，它们的物源是稍富集的（可能是下地壳）。在日本西南部山阴地区西部的须佐地区，海相须佐群、山岛火山岩和小山辉长岩的产出关系密切。山岛火山岩主要由海底块状熔岩、玻屑岩、玄武安山岩的假流水脉和补给脉及其改造沉积物组成。本文根据近年来在须佐地区获得的年代学和地球化学资料，结合野外资料，认为山岛-小山火成岩是一个与中中新世日本海开孔有关的海底火山-深成岩体。少

Cretaceous to Paleogene volcanic rocks and granitic batholiths are extensively distributed in the Inner zone of Southwest Japan and they formed volcano-plutonic complexes. This project includes structural geology, petrology, geochemistry on the Cretaceous to Paleogene complexes in Southwest Japan.From these studies, we found new cauldrons, i.e ., Niimi caidron and Yamaguchi cauldron in the Niimi area, Okayama Prefecture and Yamaguchi city ; respectively. These cauldrons show volcano-plutonic complexes.The Cretaceous Fukuga Formation, which is mainly composed of rhyolite to rhyodacite lavas and pyroclastic rocks, is intruded by a granitic stock, suggesting that the volcanic and granitic rocks comprise a volcano-plutonic complex.The rhyolites and granites have similar major and trace element chemistry apart from Ba. Ten rhyolite samples of the Fukuga Formation and eight granite samples yield similar, well-defined Rb-Sr whole rock isochrons. The Fukuga suite gives an isochron of 86.8 * 2. … More 8 Ma (2 siguma) with an initial Sr isotope ratio of 0.70544 * 0.00024, and the granites give an isochron of 85.0 * 3.1 Ma (2 siguma) with an initial ratio of 0.70526 * 0.00023. All rhyolites and granites give a single combined isochron of 87.0 3.1 Ma (2 siguma)with the initial ratio of 0.70526 * 0.000025. While plagioclase, K-feldspar, whole rock samples from a biotite granite give a younger isochron age of 79.7 * 5.3 Ma (2 siguma)with initial ratio of 0.70521 * 0.00017. The petrographical, petrochemical and Sr and Nd isotope data indicate that the rhyolite of the Fukuga Formation and the granitic rocks were derived from a common source, which were slightly enriched source (probably lower crust), contrasting with other Cretaceous volcanic rocks in the Chugoku district.In the Susa area western San-in district, Southwest Japan, marine Susa Group, Yamashima volcanic rocks and Koyama gabbros occur in close relation. The Yamashima volcanic rocks, presumed to lie on the Susa Group, consist mainly of submarine massive lavas, hyaloclastites, pseudopillow and feeder dykes of basaltic andesites and their reworked deposits. On the basis of newly obtained and reported geochronological and geochemical data and field evidence of igneous rocks in the Susa area, it is argued that the Yamashima-Koyama igneous rocks are a submarine volcano-plutonic complex produced in relation to the opening of Japan Sea in the Middle Miocene. Less