Late Triassic bivalves associated with a hydrothermal vent system in the Yidun Island Arc （SW China） of the eastern Tethys

• Published in: 中国科学：地球科学英文版 2011, Vol.54 (12), p.1864-1870
• Main Author: LIN Li ZHU LiDong PANG YanChun SHA JinGeng Franz T. FURSICH FU XiuGen WANG XinLi
• Format: Article
• Language: English
• Subjects: 东特提斯; 中国西南地区; 义敦岛弧; 双壳类动物群; 晚三叠世; 构造背景; 火山成因块状硫化物矿床; 热液系统
• ISSN: 1674-7313; 1869-1897

The Yidun Island Arc in the Three Rivers （Jinsha River, Lancang River, Nujiang River） region of southwestern China is one of the most important Kuroko-type volcanogenic massive sulfide deposits （VMS） in China. Intra-arc rifting of Yidun Island occurred during the Late Carnian-Norian when VMS deposits such as the Gacun Pb-Zn-Cu deposit were formed. A bivalve fauna was found in fine-grained tuffaceous slate and in mineralized tuffaceous siltstone containing very high contents of Pb （45.01-103.37 ppm） and Zn （135.78-300.03 ppm） of the upper Tumugou Formation in the Changtai-Gacun volcanicsedimentary rift basin. Stratigraphically, the bivalve-bearing beds are equivalents of the Gacun Pb-Zn-Cu deposits. The diversity of this bivalve fauna is very low. It consists mainly of the thin-shelled, epibyssate suspension-feeding bivalves Pergamidia eumenea and Parapergamidia changtaiensis, the burrowing large, elongated, suspension-feeding Trigonodus keuperinus and Unionites？ sp., and occasional specimens of the endobyssate suspension-feeding Trigonodus？ sp. and the deep burrowing suspension-feeding Pleuromya markiamensis. Individuals of the first four taxa are so abundant that the specimens are sometimes concentrated in shell beds, probably indicating a gregarious habit. This bivalve fauna is associated with internal moulds of cylindrical, slightly conical tubes most likely produced by a worm-shaped organism. Composition, morphology, diversity, and high abundance of this fauna, chemical features of the surrounding sediment, and the tectonic setting all suggest that this bivalve fauna lived in a deep-water environment in or around a hydrothermal vent system.