| Research Abstract |
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.
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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
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