It is generally considered that the lower boundary of the Sinian system in central Yunnan is defined by a sharp unconformity left by the Jinning orogeny, but there are two different views for the age of its lower limit, that is 900 or 800 m.y.A volcanic rock series was found by the author in the lower part of the Chengjiang Formation during the geological research in Luoci area of the central Yunnan. The Chengjiang Formation in this area may be correlated with the standard Chengjiaug Formation of the central Yunnan, it is also a red molasse formation of the continental fades, and laid over the Hcishantou Formation of the Kunyang Group with a sharp unconformity, and is over laid by the Doushantuo Formation with a weak unconformity. The volcanic rock series is occurred in the lower part of the Chengjiang Formation, and closely laid on the basal conglomerate. The rocks are mainly of basaltic lava and tuff, breccia and agglomerate, and are characterized by rich in alkaline. It represented a continental type of eruption.The Chengjiang Formation may be divided into two parts in Luoci area; the lower part is the continental basalts which may be correlated with the Suxiong Formaion of the western Sichuan, and the upper part is the red detrital deposit of river origin which may be correlated with the Chengjiang Formation of the central Yunnan. Therefore, it seems to he the transitional type between the Suxiong Formation and the Cheug-jian Formation, which are the contemporaneous deposits, but of different facies.The isotopic dating samples of the volcanic rock were collected by Institute of Geology, Academia Sinica from the base of the Chengjiang formation, and its age was determined as 885 m.y., this is an only age of the Chengjiang Formation itself in Yunnan. The age of the paisanite intruded into the volcanic rock was determined as 705 m.y. by Institute of Geology, Ministry of Metallurgical Industry. In addition, the age of the dacite which lie in the middle part of Suxiong Formation in western Sichuan is 822 m.y., and the age of carbonaceous slate of the Liubatang Formation in central Yunnan is 1002 m.y.. According to these isotopic dating data (the isochronie age of Rb-Sr), an age sequence may be obtained as follows:Doushantuo Formation——Chengjiang movement—— 7.05 m.y.The middle part of Soxiong Formation 822 m.y.The basal part of Chengjiang Formation 885 m.y.——Jinning movement—— 900 m.y.Liubatang Formation 1002 m.y.Thus, the age of the Jinning movement should be put at about 900 m.y., so that, to put the age of the lower limit of the Sinian system at about 900 m.y. is suitable in Yun. nan.
Sun Jiacong. DISCOVERY OF THE VOLCANIC ROCK SERIES IN THE CHENGJIANG FORMATION AT LUOCI AREA AND DISCUSSION ON THE AGE OF THE BASAL LIMIT OF THE SINIAN SYSTEM, YUNNAN[J]. Chinese Journal of Geology, 1985, (4): 354-363.