A Research on the Genesis of the Conglomerate of Dachang Layer of Middle Permian in Qinglong,Guizhou

There is a conglomerate formation lies in the middle and upper of Dachang Layer in Qinglong antimony deposit,which occurs as saccate and lenticular. By studying the depositional sequences,structures,tectonics and mineral assemblages of this conglomerate of Dachang Layer,it shows the conglomerate presents an abnormal depositional sequence and is consisted of basalt,which indicates this conglomerate was transformed directly by Emei basalt and without having characteristics of channel facies. Furthermore,the special "argillization edge"tectonic of gravels is a direct evidence that the basalt had a massive hydrolysis in the seawater. Moreover,the propylitic mineral assemblages,located in interstitial material,demonstrate the conglomerate had suffered( volcanic) epithermal transformation. The interstitial materials show constructive central island with the ratio of Zr/Hf( 30. 7 ~ 43. 4,mean 38. 0) close to the ratio of volcanic tuff,indicating that the pyroclast( volcanic ash)participated in diagenesis during basalt was transforming into gravels. Being compared with basalt,the gravels andinterstitial materails are relatively lack of Na( Na2O) and enriched in K( K2O),but they have the same REE pattern models and have a regular variation of total REE: the highest is basalt,then is gravel and the lowest is interstitial material. The analysis of depositional setting shows that the crust had been differently uplifted,which was caused by the Emei Mantle Plumes. The uplifted one was exposed and denuded,then formed disconformity plane. In addition,relatively descended one was continuing to deposit,in this case Dachang Layer is the product of continuous deposition above the Maokou limestone,formed in the restricted margin of ELIP zone of nappe. The forming mechanism of Dachang Layer is that incandescent basalt was chilled in the sea-water,meanwhile the pyroclast participated in deposition,forming ELIP foreset bed( shattered lava breccia) and tuff then deposited above the Maokou limestone. Among evolved process of later stage,incandescent basalt flows cooled and shrank,forming columnar joints,and Dachang Layer conglomerate was formed by abrasion after being interacted with tide flows and coastal current.