The Geological Characteristics, Age And Tectonic Environment Studies About Delishitan Ophiolites In Buqingshan, South Of East Kunlun Mountains

Delisitan ophiolite is located in the southern East Kunlun Mountains, west of A'nyemaqen-Buqingshan ophiolite tectonic belt. It separates East Kunlun orogenic belt in its north and Hoh Xil-Bayan Har orogenic belt in its south. Based on the detailed field trip survey, measurement and plotting of main section and indoor slide analysis, detailed studies about distribution, trend direction, scale, occurrence, rock association and petrography characteristics of Delisitan ophiolite had done in this thesis. Besides, petrogeochemistry tests and isotopic dating analysis were carried out on the samples to discuss the formation envoironment and time of Delisitan ophiolite from the aspect of geochemistry and isotopic geochronology. The preliminary understanding was made as follows:Delisitan ophiolite extends NWW direction, parallel to the strike direction of south East Kunlun faults. The lithological combination within the ophiolite zone is relatively integrate. From bottom to top, there are mainly dark green altered harzburgite(serpentinite) and iherzolite, which show tectonic contact relationship with the adjacent rocks. Diabase porphyry is interscalated in the serpentine in dyke form and the dike width ranges from 0.5m to 6m. To its north, there is dark gray massive gabbro, whose acumulation is not obvious. The relationship between gabbro and the south serpentinite is tectonic contact. The uppermost of the ophiolite is gray-black altered basalt with serious fragmentation, and pillow structure can be seen in the some region. It shows tectonic contact relationship with the gabbro. Gray-black deep-sea silty mudstone sedimentary rocks overlies the Delisitan ophiolite, and the relationship between silty mudstone and basalt is tectonic contact.There is a narrow SiO2 content range of serpentinite in Delisitan ophiolite combination. It has large amount of Mg, low in Ti and depleted in K and Na. The gabbro and diabase porphyry belong to the sub-alkaline series, and they both locate in the mafic accumulate region in the ACM diagrams. The basalt belongs to the sub-alkaline tholeiite series. Delisitan metaperidotite chondrite-normalized REE curve is extensive U pattern, and the cumulate peridotite chondrite-normalized REE curve is flat pattern with slight light rare loss. Gabbro, diabase porphyry and basalt'chondrite-normalized REE curve is remarkably similar, they all aer the light rare earth depleted pattern. On the primitive mantle normalized trace element curve, there is a significant large-ion lithophile elements fluctuation in every unit of Delisitan ophiolite and the high field strength elements are relatively flat without Nb, Ta and Ti deficit and there's no island arc or oceanic island characters, showing almost no continental crust contamination. In addition, Delisitan basalt's composition is very similar to normal mid-ocean ridge basalt. All these above demonstrate Delisitan ophiolite is probably formed in a normal mid-ocean ridge.Delisitan gabbro LA-ICP-MS zircon U-Pb weighted average age is (516.4±6.3) Ma, which mainmasts the Delisitan ophiolite forming age is the early Cambrian, which shows that both paleo-Tethys ophiolite and Early Paleozoic Proto-Tethys ophiolite exsit in A'nyemaqen-Buqingshan tectonic belt, south East Kunlun Mountain.Through comprehensive studies of detailed field geological features, petrology, petrogeochemistry and isotopic chronology, conclusions are reached that the Early Paleozoic Proto-Tethys A'nyemaqen ocean clamped by East Kunlun block and Hoh Xil-Bayan Har black was probably formed from the late Neoproterozoic to early Cambrian. It began subduct to the north in the late Cambrian and the oceanic crust stopped expansion in Middle-Late Ordovician. Following the continuous subduction, the A'nyemaqen ocean was coverted to residual sea basin in the mid-Ordovician. The collision between north East Kunlun block and the south Hoh Xil-Bayan Har block occurred in the early Silurian, which resulted the close of Proto-Tethys oceanic basin.