向阳组三叶虫动物群的分布——兼论奥陶纪印支地体在滇西的地质界线

PALAEONTOLOGICAL CONSTRAINS ON THE EXTENT OF THE ORDOVICIAN INDO-CHINA TERRANE IN WESTERN YUNNAN

奥陶纪Darriwilian早、中期的向阳组三叶虫动物群仅分布于滇西东部红河断裂和南定河 澜沧江断裂之间 ,这一狭长地带代表奥陶纪印支地体的北延部分。

Geologically, eastern West Yunnan has recently been considered as an independent tectonic unit, the Simao Terrane (Wu et al., 1995) or the Lanping-Simao Block (Jin, 1996), or as a part of the Qiangbei-Changdu-Simao Microcontinent (Fan and Wang, 1994). However, as pointed out by Metcalfe (1988, 1992, 1996), the area may have formed the north extension of the Indo-China Terrane. It is bounded to the west by the Sibumasu Terrane and to the east by the Yangtze Block in Yunnan, but the complicated faulting, the superimposed tectonics and the metamorphism of rocks in this area make it difficult to define its exact extent. So far, there are several different opinions about its location (Text-fig.1):1) between the Honghe-Jinshajiang Fault and Lancangjiang Fault, 2) between the Ailaoshan-Xiaojinhe Fault and Lancangjiang Fault, 3) between the Ailaoshan-Jinshajiang Fault and Lancangjiang Fault, 4) between the Ailaoshan-Jinshajiang suture zone and Changning-Menglianon suture zone-Lancangjiang Fault. The evidence to support the views is largely from regional geological data of either Proterozoic or Late Palaeozoic. Ordovician clastic rocks are sporadically distributed in eastern West Yunnan, but only the Xiang- yang Formation (early to middle Darriwilian) is fossiliferous. The trilobite fauna, consisting mainly of Ogyginus, Hanchungolithus, Liomegalaspides and Neseuretus, was previously recorded from Xiang- yang, Dali (Text-fig.2, Loc.4) by Zhou et al. (1998). Since then further field investigation has been made and similar trilobites have been collected from the Xiangyang Formation at Manghei, Cangyuan (Loc.1); Shizhongshan, Jianchuan (Loc.2); Azhengleng and Dangziluo, Lijiang (Loc.3); and Baimazhai (Loc.5) and Mengxie (Loc.6), Jinping (Text-fig.2; Pl.Ⅰ and Ⅱ). Judging from the distribution of the Xiangyang Formation and its trilobite fauna, we believe that the Ordovician Indo-China Terrane should cover an area between the Nandinghe-Lanchanjiang Fault and Honghe Fault in Yunnan.