Comparative Study On The Volcanic Rocks Of Dahalajunshan Formation And Yishijilike Formation, Awulale-Gongnaisi Area, Yining Block, Xinjiang

Carboniferous volcanic rocks are widespread in the Western Tianshan Mountain, Xinjiang. The volcanic rocks of Dahalajunshan Formation and Yishijilike Formation are 90% of Carboniferous volcanic rocks in Awulale-Gongnaisi area, Yining Block. They are closely related with mineralization, correctly recognize and identify their tectonic setting is the key to reveal the closure of Paleozoic ocean basin and transformation process of tectonic regime. This paper takes the volcanic rocks of Dahalajunshan Formation and Yishijilike Formation as the study objects, which exposed in the research area. On the basis of field investigation and variously previous geological data, through anew drawing of the reginal geological map and reasonable division of volcanic stratigraphic squence, we mastered their temporal and spatial distribution, rock association, regional variation, divided the eruption cycles, established the geochronological framework, summarized the geochemical characteristics, comparative study on the differences between two sets of volcanic rocks in the aspects of geological and geochemical characteristics, explored their different tetonic attributes, analysed source characteristics and genetic types. The above aspects provided new evidences for exploration of Late Paleozoic tectonic evolution in the western Tianshan Mountain. By comparative study on volcanic rocks of Dahalajunshan Formation and Yishijilike Formation, made the following understanding:1. The volcanic rocks of Dahalajunshan Formation are composed of basalt, andesite, dacite, rhyplite and homogeneous pyroclastic rocks, overall is a set of continuous outcrop of basic, intermediate and acid volcanic rock association. The thickness ratio of the lava and the pyroclastic rock is 3.4:6.6, the basic, intermediate and acid volcanic lava is 1:8:1. The outcrop thickness gradually increases from west to east. The volcanic rocks of Yishijilike Formation are mainly composed of meso basaltic(basalt, basalt-andesite) and acid(rhyplite) components, lack or absence of andesite. The rock association has obviously discontinuous compositon, and shows the characteristics of bimodal volcanic rocks. The thickness ratio of the lava and the pyroclastic rock is 4.3:5.7, the basic, intermediate and acid volcanic lava is 4:3:3. The outcrop thickness changes with large varied magnitude, gradually increases firstly and then decreases from west to east.2. The volcanic rocks of Dahalajunshan Formation are dominated by explosive facies, and the overflow facies are secondary. The outbreak index generally is relatively large, they are divided into 5~6 eruption cycles. However, the volcanic rocks of Yishijilike Formation are dominated by overflow facies, and the explosive facies are secondary. The outbreak index generally is relatively small, they are divided into 3~5 eruption cycles.3. The age of volcanic rocks of Dahalajunshan Formation is 343 Ma to 317 Ma, mainly belongs to Early Carboniferous. The age of volcanic rocks of Yishijilike Formation is 319 Ma to 294 Ma, mainly belongs to Late Carboniferous. Regional volcanic activity has the characteristics of long duration and large scale. The volcanism gradually became younger from west to east in research area.4. The volcanic rocks of Dahalajunshan Formation are mainly classified into calc-alkaline series. They are generally enriched in LILE and LREE, and relatively depleted in HFSE and HREE, have obvious Nb, Ta and Ti negative anomalies. Therefore, they show the geochemical characteristics of island arc or active continental arc volcanic rock. The volcanic rocks of Yishijilike Formation are classified into calc-alkaline and shoshonite series. They are strongly enriched in LILE and LREE, and relatively depleted in HFSE and HREE. They are similar to the characteristics of rift volcanic rock.5. Basaltic magma of Dahalajunshan Formation was derived from partly melting mantle wedge, which was metasomatized with fluid(melt) precipitated from subtractive sheet space. Andesitic magma was derived from the root of arc crust and affected by mantle wedge. Acidic magma was derived from the stable lower crust source, mixed with substances from mantle magma. Lithosphere delamination of orogen belt roots caused mantle material upwelling of asthenosphere in Late Carboniferous. Basaltic magma of Yishijilike Formation was derived from partly melting upper mantle substances. Felsic magma was formed by remelting of Precambrian basement, which caused by underplating of basaltic magma to the lower crust. Andesitic magma was the result of the exchange of basic and acidic magma occurred in the contact area.6. The volcanic rocks of Dahalajunshan Formation were formed in active continental margin environment, as the result of South Tianshan Ocean and North Tianshan Ocean opposingly subducted beneath the Yining Block. After the end of subduction, the research area entered into post-collsion extension stage. Therefore, bimodal volcanic rocks of Yishijilike Formation were the product of the initial period of extensional process. South Tianshan Ocean closed in Late Silurian to Early Carboniferous, North Tianshan Ocean finally closed in the end of Early Carboniferous. The research area formed early rift and then gradually matured with the extensional regime in the end of Late Carboniferous to Early Permian, actually entered into intracontinental evolution stage in Late Permian.