The Late-Paleozoic Volcanic Rocks In The Eastern Bogda Of Eastern Tianshan: Petrology, Geochemistry And Structure Property

Tianshan orogenic belt was an important part of the Central Asian orogenic belt and also treated as one of the key research areas of its evolution. As one of the branches of the Tianshan orogenic belt, Bogda orogenic belt was the key research area of the Tianshan orogenic belt. So far, there were many different opinions about the late Paleozoic tectonic attributes of the Bogda orogen, such as the rift valley, island arcs and back-arc basin. The cause of the above controversy is the different understanding of the Bogda late Paleozoic volcanic-sedimentary rocks and the shortage of detailed study on volcanic rock geochemistry. Aiming at the problems existed in the research, this article summarized the research achievements of predecessors, investigated the key sectional of the eastern Bogda late Paleozoic volcanic-sedimentary rocks, ascertained its rock assemblage, rock geochemistry and chronology characteristics, analysised the late Paleozoic volcanic-sedimentary system and sedimentary environment, explored its the volcanic sedimentary sequence, sedimentary structure pattern and regional chronology framework features, demonstrated the formation of volcanic rocks and its tectonic attributes, restored the late Paleozoic tectonic evolution process of the eastern Bogda Belt.From its volcano-sedimentary sequence, we can conclude that the overall sedimentary environment of this area is marine(the early Devonian to the middle Devonian) to continental sea(the late Devonian), marine(the early Carboniferous to the late Carboniferous) to continental sea(the end of the late Carboniferous to the early Permain), continental(the middle to late Permain) transformation trend, during the late Paleozoic. Geochronology study showing that, the age of the late Paleozoic volcanic rocks of the eastern Bogda belt is 400 Ma to 290 Ma. There were obvious phases in the late Paleozoic volcanism of the eastern Bogda belt, which can be divided into the Devonian(400 Ma to 360 Ma) and the Carboniferous-Permian(350 Ma to 270 Ma) two times, and the last can also be divided into the early-middle Carboniferous(345 Ma to 330 Ma), the end of the early Carboniferous to the middle of the late Carboniferous(320 Ma to 305 Ma) and the end of the late Carboniferous to the begin of the early Permian(300 Ma to 290 Ma) three main activity.The late Paleozoic volcanics petrology and geochemistry analysis of the eastern Bogda belt showing that, first, the volcanic rocks of the early Devonian were mainly classified into ca-alkline to high kalium calc alkalic magma series. They showed the geochemistical characteristics of active continental arc rock. The late Devonian volcanic rocks had the different tectonic attributes with the early Devonian volcanic rocks. They were belonging to continental basalts, which were strongly contaminated by the crustal materials, and were mainly classified into tholeiitic series, formed in extensional environment. Second, the Carboniferous-Early Permian volcanic rocks in the eastern Bogda belt were mainly classified into the basic and the acidic series, which constituted the bimodal volcanic rocks. The basic rocks were low Ti/Y magmatic type, mainly belonged to tholeiitic series, had almost flat REE distribution patterns, generally enriched in LREE, slightly enriched HREE, possessed of a weak differentiation between LREE an HREE elements, had obvious Nb, Ta and Ti negative anomalies. These characteristics of the basic volcanic rocks indicated that they had geochemical features indicative of intraplate basalts formed within continent and originated from the partial melting(Less than twenty percent) of the spinel phase mantle peridotite to garnet phase mantle peridotite at the certain depth about 60 to 80 kilometers, which was similar with the volcanic source area from the OJP, BSE and N-MORB under the AFC influence, and suffered a certain degree of crustal or lithosphere contamination during the process. The acidic series had the similar geochemical characteristics as the basic rocks, which revealed it may be evolved from the basic magma’s crystallization differentiation.Combined with the type and age of the ophiolite from northern Xinjiang, the regional unconformity surface under the late Devonian, late Devonian to early Carboniferous epoch continuity sedimentary sequence, we can conclude that there was a broad and strong tectonic movements during the late Devonian, which might be related to the closing of the palaeozoic ocean basin of the Tianshan Bogda area before the late Devonian. Since late Devonian, the eastern Bogda belt had entered a new stage of tectonic evolution: post collision intraplate extension stage. And also we concluded that the evolution progress of the eastern Bogda belt can be divided into 3 stages during late Paleozoic, the ocean-land evolutionary stages before the late Devonian, the sea-land evolutionary stages(post-collisional extension) during the late Devonian to the early Permian and the middle to late Permian intracontinental evolution stage.