Composition、 Age And Origin Of The Ophiolite Melange In Yushigou Of The North Qilian

The North Qilian Orogenic Belt lies between Qilian Plate and the southwest margin of North China Plate(Alashan Block), heading from north to west and from south to east. Its east part is adjacent to the Tongxin- Guyuan Fault which is dextral strike-slip fault. Its west part is cut off by the Altyn Tagh Fault which is a sinistral strike-slip fault. From the south to the north, the North Qilian Orogenic Belt can be divided into different tectonic units including subtractive fragments of oceanic crust on the north hillside of Tuolai Mountain, subduction of high-pressure metamorphic rocks, island arc volcanic rocks on Hexi Corridor Nanshan, back-arc basin in the Hexi Corridor, and etc.According to the data collected, Yushigou Ophiolite, towards east, connects with Chuancigou Ophiolite, Zhamashen Ophiolite, Xiaobabao Ophiolite, which constitutes the whole oceanic spreading ridge type Ophiolite Belt.Through the various detailed research on wildland geochemical feature, the rock microscopic observation, geochemical characteristics of rocks and cumulate gabbro dating of Yushigou Melange Ophiolite, and combined with the results of previous studies, the following preliminary understandings are obtained:The Melange Ophiolite of the studied area consists of the Cumulate Gabbro, the Early Ordovician Yingou Volcanic Rock Group, the Metamorphic Peridotite, Gabbro Wall and Diabase Wall, the Deep-sea Sedimentary Rock in Early Ordovician, and etc;The north side of the Melange Ophiolite thrusts nappe upon Hong Quan Group(P3h) of the Permian Yaogou Group. For the south side, from north to south, it thrusts nappe upon Yanghugou Group(C2y) of the Carboniferous and the Dahuanggou formation of Permian(P1—2d). For the west side, it forms the fault contact with the schist of Proterozoic(Pt1Tb). The east side is out of the working area. In the inner Melange Ophiolite, the contact relationship among different rock slices is fault contact;The age of Cumulate Gabbro is 513.4 ±6.4Ma, which shows that formation time of the Ophiolite was about from Early Cambrian to Early Ordovician;Study on the geochemistry shows that the basalt- diabase unit on the top of the Yushigou Melange Ophiolite owns the typical N-MORB features, suggesting that the Yushigou Melange Ophiolite is the product of the expansion of the North Qilian Ocean.