Abstract
Abstract In Japan and Korea, some Lower Cretaceous terrigenous clastic rocks yield detrital chromian spinels. These chromian spinels are divided into two groups: low‐Ti and high‐Ti. The Sanchu Group and the Yuno Formation in Japan have both groups, whereas the Nagashiba Formation in Japan and the Jinju Formation in Korea have only the low‐Ti spinels. High‐Ti spinels are thought to have originated in intraplate‐type basalt. Low‐Ti spinels (higher than 0.6 Cr#) were probably derived from peridotites, which are highly correlated with an arc setting derivation and possibly with a forearc setting derivation. Low‐Ti spinels are seen in the Sanchu Group, the Nagashiba Formation and the Jinju Formation. Low‐Ti spinels from the Yuno Formation are characterized by low Cr# (less than 0.6) and these chromian spinels appear to have been derived from oceanic mantle‐type peridotite, including backarc. According to maps reconstructing the pre‐Sea of Japan configuration of the Japanese Islands and the Korean Peninsula, the Korean Cretaceous basin was comparatively close to the Southwest Japan depositional basins. It is possible that these Lower Cretaceous systems were sediments mainly in the forearc and partly in the backarc regions. The peridotite might have infiltrated along major tectonic zones such as the Kurosegawa Tectonic Zone (= serpentinite melange zone) in which left lateral movement prevailed during the Early Cretaceous.
Highlights
It is known that the Late Mesozoic proto-JapaneseIslands were located along the eastern margin of the Asian continent before the formation of theSea of Japan (Otofuji & Matsuda 1983; Taira et al.1989)
We describe the occurrence and chemistry of detrital chromian spinels obtained from non-marine and shallow marine sediments from four areas in Japan and Korea, and discuss the tectonic implication of the Early Cretaceous eastern border of the Asian continent
The zone consists of numerous lenticular bodies of Paleozoic sedimentary rocks, high P–T type metamorphic rocks, amphibolites and orthogneisses, Fig. 1 Distribution of pre-Cretacous terranes of Southwest Japan. [NTZ, Nagato Tectonic Zone; KTZ, Kurosegawa Tectonic Zone; MTL, Median Tectonic Line.]
Summary
Islands were located along the eastern margin of the Asian continent before the formation of the. We describe the occurrence and chemistry of detrital chromian spinels obtained from non-marine and shallow marine sediments from four areas in Japan and Korea, and discuss the tectonic implication of the Early Cretaceous eastern border of the Asian continent. The zone consists of numerous lenticular bodies of Paleozoic sedimentary rocks, high P–T type metamorphic rocks, amphibolites and orthogneisses, Fig. 1 Distribution of pre-Cretacous terranes of Southwest Japan (modified from Ichikawa 1990). We examine detrital chromian spinels collected from four areas: Sanchu, Konomori and Yuno in Southwest Japan and Kunwi in the Gyeongsang Basin, Korea (Fig. 2). Detrital chromian spinels (25 grains) from the Nagashiba Formation in the Konomori area were examined
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