Petrological Characteristics And Its Tectonic Implication Of Xia Dawu Formation Of Lower Triassic In The North Of Hua Shixia Area Of Eastern Kunlun

In this thesis,1/1000scale is used systematically for measuring sections in the field work and indoor work to study the age of Xiadawu Formation, petrological characteristics and tectonic significance in northern part of Huashixia area, eastern Kunlun. The conclusions are listed as follows:(1) Ammonite and bivalve fossils are firstly reported in Xiadawu Formation which are suggested to form in early Triassic medium-term by the analysis of their biostratigraphy. This work enhances the accuracy of the stratigraphy of Xiadawu Formation.(2) Combined with field observations and analysis of rock slice,8types of lithology of Xiadawu Formation are divided, including of ignimbrite, tuff, tuffite, tuffaceous mudstone, tuffaceous siltstone, tuffaceous limestone, arkosic sandstone and mudstone. On base of these analysis,13volcano-sedimentary cycles of volcaniclastic rocks of Xiadawu Formation are divided and each cycle is divided into several sub-cycles, which shows the frequent volcanic activity.(3) According to the synthetic study of volcaniclastic rocks, ammonite and bivalve fossils, the conclusion is that these volcaniclastic rocks maybe formed in the calc-alkaline island arc volcanic in the extrusion background. In the early and middle Triassic, this area was deep-sea sedimentary environment. There is a volcanic sedimentary record that the oceanic crust of Bayan Har remnant ocean in Paleo-Tethys was the northward occurrence of B-type subduction and it also provided a new evidence for the existence of the the Bayan Har remnant ocean.