Research On Petrology And Geochemistry Of The Carboniferous Volcanic Rocks And Their Lu Anomaly In Northeastern Junggar Basin

The northeastern of Junggar basin in Xinjiang is located in key parts of central asia orogenic belt(CAOB), undergoing an evolution and change including many different stages. Research on Volcanic petrology and Geochemistry of this area has great significance to the Earth’s deep information extraction, defining tectonic evolution background and inversion magmatic evolution. The northeastern of Junggar basin,including Wulungu Depression and Luliang Uplift,is chosen as the key research area. The research objects include Carboniferous volcanic lava and pyroclastic rocks that drilled by Ying-1well, Wucan-1 Well, Dibei-1 Well and Dibei-2 Well.Core and debris of borehole and their geochemistry, logging and seismic data are analyzed Systematically.A lot of work on rock slice identification,geochemical testing of main and trace elements are carried out.The Petrology characteristics of Carboniferous volcanic rocks and their volcanic sources information and tectonic background are discussed through petrology and geochemical study of those rocks.Significant Lu anomaly was discovered in Carboniferous volcanic rocks of this area. Trace elements and Rare Earth Elements simulation that restricted by Lu anomaly are carryed out.Also,the geological significance of Lu anomaly is discussed.The main achievement and understanding of this work is listed as below:By the research of petrology, Carboniferous volcanic rocks in northeast of Junggar basin are be divided into three Rock types, which are volcanic lava rocks, pyroclastic rocks, volcanic-sedimentary rocks,and four lithofacies, explosive phase,effusive facies, volcanic conduit facies and volcanic sedimentary rocks Phase.Organic Geochemistry data indicates that Wulungu depression was under a semi-restricted environment at early Carboniferous. Sedimentary water become Salt significantly and the proportion of terrestrial organic matter reduced at the later stage of early Carboniferous. Pumiceous peperite is discovered through petrology reseach in Wulungu depression, which indicates that under-sea Magma intrusion in Wulungu depression is widespread in the later stage of early Carboniferous. Through the research of sedimentary background, the submarine volcanic-sedimentary facies in this area was divided into four subfacies,which are crushed lava rock deposits subfacies, fan delta sedimentary subfacies, storm sedimentary subfacies and underwater fan sedimentary subfacies. The research of Volcanic-sedimentary facies is an important enlightenment to geological exploration.The ratio of active elements in fluid, such as Rb、U、Ba、Sr and Pb, and HFSE or REE(always treated as the inactive elements in fluid) is a significant index for reflectting the enrichment of active elements in subducted plates.The values of Sr/Nd and U/Th are low in volcanic lava, which indicate that the effect of water and fluid from subducted plates on mantle source is little. At the same time,Y content in samples is high and Sr/Y is low.This is significantly different from adak melts thate generated from oceanic crust, which shows that the contribution of subducted plates melt to geochemical composition of volcanic lava is low. The samples of Carboniferous volcanic rocks has a low Ba/Th, relative high content of Th and Pb, and low value of Ce/Th and Ce/Pb, which is similar to addition of subducted sediment. The content of magnesium is high, the distribute pattern of trace elements and rare earth elements is similar to that of andesite from setouchi and central ryukyu. As a result, the melt induced by sediment of subducted plates has a significant effect on the source zone of Carboniferous volcanic lava of this area. Element geochemical characteristics and tectonic setting discrimination diagrams indicate that carboniferous volcanic rocks in the northeastern of Junggar basin was formed in tectonic setting of continental island arc.The enrichment anomaly of heavy rare earth element Lu is significant in the carboniferous volcano rock samples in northeast area of Junggar basin, which is very rare both at home and abroad. With the similar and stable geochemical properties in magma evolution, the enrichment anomaly of Lu provides an important information for us to research the evolution of magma.By analysis the characteristics of rare earth element of server geochemistry components that Involved in the formation process of arc magma,the Involvement of subduct sediment melt is considered as the main reason for cause the anomaly of Lu in Carboniferous volcanic rock.According to the research of source information of Carboniferous volcanic rocks in this area,the trace elements and rare earth element of Carboniferous volcanic rocks in well Ying 2 has been simulated by using trace elements and earth element geochemical simulation model.The simulation results are in good agreement with the geochemical characteristics of the samples, which confirms that the melt of the subducted plate sediments is involved in the mantle source region, and has an important contribution to the formation of the carboniferous volcanic rocks. Therefore, Lu enrichment anomaly can be used as an important identify indicators of involvement of sediment melt in arc magma source at Kalamaili struture zone or around its extension cord.