Petrology And Geochemistry Of The Ynezhao—Pipasi Metamorphic Volcanic Rocks In The West-Qinling And Their Tectonic Significance

The working area is located mostly in Wudu area of Gansu province,, betwee n Bailongjiang orogenic belt and Bikou block. The Devonian-Carboniferous clastic sediment and metamorphic volcanic rocks are well constrained by the TazangLueya ng fault in the North and Wenxian-Mianxian fault in the South. Geological elemen ts are bordered each other by south-directed thrust-nappe faults. In terms of te ctonic styles and deformation, the zone can be classified into front thrust syst em, middle thrust-nappe system and hind thrust system, which represent the gradu al deformation features of different tectonic location along the collision oroge ny. Both the basic composition and imbricate structural framework are comparable with MianLue suture zone to the East. Yuezhao-Pipasi metamorphic volcanic rocks are exposed between Devonian str ata and the Bikou block in the south of study area. Based on geochemical classif ication two serial volcanic rocks can be assigned, one is bimodal and another is alkaline. The bimodal volcanic rocks consist of dacitic rhyolite and tholeiitic basalts characterized by low K and high Na concentration. The REE distribution patterns of tholeiitic basalts show the flat style; and their trace elements geo chemistry is similar to MORB. But the low Nd and high Th concentration imply con tinental contaminating in magma process. It suggests that the tholeiitic basalts were formed in a continental rift to a mature basin setting. Geochemistry and g eology features of Yuezhao-Pipasi metamorphic volcanic rocks are comparable with the Heigouxia metamorphic volcanic rocks, but apparently differ from the Bikou volcanic rocks. All above evidences imply that the Yuezhao-Pipasi metamorphic vo lcanic rocks are formed during the opening stage of the west branch of Mian-Lue paleo-ocean. In the light of the comprehensive study of sedimentary settings, structure , tectonics, magmatism together with the analysis of regional feature of Qinling orogenic belt, the evolution processes of the study area can be recognized thre e stages, initiation and spreading of the limited oceanic basin (D-C), subductio n and collision of the limited basin (C-T), and post orogenic intra-continental stage (T-Q).