Provenace study of detrita minerals in the sandstones from the Japanese Islands

古今趋势、岩屑种类分类、重矿物的模态和化学分析为砂岩物源分析和解释提供了强有力的依据。锆石、独居石等碎屑矿物的年龄分布模式为单粒年龄分析的发展提供了一种强有力的物源研究方法。在本研究中，我们分析了日本列岛不同地块砂岩中的碎屑独居石。它们形成于大陆架，起源于东亚大陆地区。此外，我们收集了来自亚洲大陆的现代砂岩，并将其年龄数据与日本群岛砂岩中的年龄数据进行了比较。结果表明：1 .日本列岛砂岩中的碎屑矿物大部分来自朝鲜半岛或包括上海半岛在内的相当地区。2 .北海道白垩系至第三系砂岩和舞鹤带二叠系至三叠系砂岩的年龄分布与阿穆尔河砂的年龄分布相似。在四国至关东地区的岛东带第三系砂岩中也观察到类似的模式。3 .古长江和黄河的贡献有限。类似的年龄模式只在九州和冲绳岛曼东带第三系砂岩中观察到。通过这项年龄研究，解决了许多问题。最显著的是重矿物的溶解和形成。石榴石是砂岩中抗蚀性最强的矿物之一。然而，富钙石榴石容易分解。因此，砂岩中石榴石的化学组成高度局限于无ca侧。这表明，如果我们仅仅根据石榴石的化学成分得出结论，就会对其出处产生误导。独居石是低变质条件下的分解矿物之一。它通常分解成褐榴石。但在极低的温度条件下，碎屑独居石分解成无钍独居石。在物源研究中应注意砂岩中重矿物的溶蚀、分解和形成。少

Paleo-current trends, the classification of species of rock fragments and, modal and chemical analyses of heavy minerals provide strong basis for analysis and interpretation of sandstone provenence. The age distribution patterns of detrital minerals such as zircon and monazite give us a powerful approach to the provenance study since the development of single grain age analyses. In this study, we analyzed detrital monazites in the sandstones from various terranes in the Japanese Islands. They were formed at the continental shelf and have been derived from regions in the East Asian continent. In addition, we collected present sands from the Asian continent to compare the age data wit h those in the sandstones in the Japanese Islands.The comparison revealed the following conlustions :1:Detrital minerals in most of the sandstones in the Japanese Islands were derived from the Korean Peninsual or its equivalent areas including the Shangdon Peninsual. These sandstones have bimodal age ditrib … More ution with peaks at 250 Ma and 1900 Ma.2:Cretaceous to Tertiary sandstones in Hokkaido and Permian to Triassic sandstones in the Maizuru belt have age distribution similar to the sands in Amur River. Similar pattern was observed in the Tertairy sandstones in the Shimanto belt from the Shikoku to Kanto regions.3:Contribution from the Paleo-Yangtze and Yellow rivers is restricted. The similar age pattern is observed only in the Tertiary sandstones in the Shimanto belt in Kyushu and Okinawa.Through this age study, many problems were cleared. The most remarkable ones are dissolution and formation of heavy minerals. Garnet is one of the most resistant minerals in the sandstones. However, Ca-rich garnets are easily decomposed. As a result, the chemical compositions of garnet in the sandstones are highly restricted into Ca-free side. This shows the misleading of the provenance if we conclude only based on the chemical compositions of garnet. Monazite is one of the decomposition minerals at the low-grade metamorphic conditions. It usually decomposed into allanite. But at very low temperature conditions, detrital monazite decomposed into Th-free monazite. We should becareful about the dissolution, decomposition and formation of heavy minerals in the sandstones at the provenance study. Less