Petrogenesis And The Tectonic Settingof The Lower Paleozoic Volcanic Rocks In The Northern Belt Of West Kunlun

Large-scale of the late Paleozoic volcanic rocks exposed in the north part of the West Kunlun belt. The volcanic rocks in this period are not only the magma production in the shift stage of tectonic regime, but also are closely related to the regional mineralization. Therefore, we carry out the research in the Carboniferous-Permian volcanic rocks in the north part of the West Kunlun orogenic belt seriously, which is important to understand the Paleo-Tethys evolution represented as Ma Zha- Kang Xiwa suture zone,and the research also provides geological background important basic geological data to the Carboniferous Kunlun type volcanic massive sulfide deposit. On the basic of previous work, this paper takes the volcanic rocks exposed in the north part of the West Kunlun belt as an object of study. by systematically studying of the aspects of petrography, geochemistry and geochronology, we discussed the petrogenesis of the volcanic rocks and its tectonic setting. Finally we made the following understanding:1. The volcanic rocks of Wuluate formation are overall composed of basalt and rhyolite, and the Yishake formation is mainly andsite, basaltic andsite and basalt. The basalts and dacites are the mainly component of the Aqiang formation.2. The basalts in the Wuluate formation are classified into tholeiitic magma series and the rhyolites are ca-alkaline series. Accoding to the trace element composition, the basalts show the geochemical characteristics of E-BABB, while the rhyolites are similar to the LCC. In the Yishake formation, the volcanic rocks are high-Na tholeiitic magma series. With the remarkable difference in REE and the trace element normalized patterns, they are divided into three groups. YSK-01 extremely is silmilar to the N-MORB, who are in low REE compositon. YSK-02 have a higer tatal value of REE and its normalized pattern is same as the E-MORB. YSK-03 rocks are completely different from YSK-01 or YSK-02.They are generally enriched in LREE ang LILE, depleted in HREE and HFSE, and show obvious Nb, Ta negative anomalies. Therefore, they are similar to the IAB. The rocks in the Aqiang formation are classified into tholeiitic- ca-alkaline magma series. The Sukedaiya region volcanic rocks are mainly basalts, which show a combination of characteristics of E-MORB. The Yeyike region is composed of basalts and dacite. The former are similar to the N-MORB, and the later are same as the LCC.3. The basic volcanic rocks in the Wuluate formation come from the depleted mantle. The basalts magma is the production of the partial melting(5%~10%) in the spinel lherzolite. The heat carried by the mantle material led to the partial melting of the lower crust, and then the rhyolites are produced. The volcanic rocks of Yishake formation are also derived from the different degrees partial melting in the spinel lherzolite, YSK-01(10%), YSK-02(20%~30%) and YSK-03(5%~7%). On the whole, the basalts of aqiang formation went through a higher degree of the partial melting in spinel lherzolite to a lower degree of the partial melting in garnet lherzolite. The Yeyike region(10%~20%) is different from the Sukedaiya region(20%~40%).4. All the volcanic rocks in the north part of the West Kunlun belt may be formed in the back-arc basins, created during the subduction of the Paleo- Tethys. In the early, the Yishake formation was in the arc environment, and then the tectonic setting translated into back-arc basins. The Aqiang formation was formed in the back-arc basin,and the ocean basin was deeper, which showed “Future ocean basin” sedimentary characteristics.5. In the carboniferous period, the tectonic setting of the West Kunlun orogenic belt occurred a dramatic transformation. With the further cracking, the basic volcanic rocks began to appear in the back-arc basin sediments. The back-arc basins formed in the late ordovician continued to expand, the Paleo- Tethys ocean appeared with the real oceanic crust and lithosphere, In the later period of the early Carboniferous, although the Paleo Tethys ocean was still in the expansion, the edge of the ocean lithosphere invasion and extinction had already begun, similar to the Pacific Ocean, the ocean began to subduct to the both sides. In the early Permian, the Paleo Tethys ocean subducted rapidly to the both sides, at the same time, the constant extension lead to the formation of the Aqiang-Kuerliang-Qialong-Aoyitage back-arc basins. Then, at the peak accumulation of the back-arc extension, the carboniferous- Permian volcanic rocks formed in the north part of the West Kunlun belt.