Late Paleozoic Volcanic Rocks Tectonic Evolution And Its North-south Comparison In The Eastern Tianshan

The Eastern Tianshan is located at the eastern intersection of the Junggar-Kazakhstan plate,the Tarim plate,and the Siberian plate,which is a key part of the accretion and orogeny in Central Asia.The Late Paleozoic was a crucial period for the transformation of the ocean-land pattern in the Tianshan orogenic belt.Under the complex tectonic background caused by large-scale ocean-land subduction and land-land collision,volcanic rocks of this period were broadly distributed in the Eastern Tianshan section.By using the study of petrology,rock geochemistry,and zircon U-Pb chronology of the igneous rocks in the Eastern Tianshan section,the timing of the magmatism in the late Paleozoic stage of the Tianshan was precisely determined,and the source regions of the magmatic rocks in various structural units were inferred as well using the nature and diagenesis process.The Devonian-Carboniferous igneous rocks in the southern area of the Tuha Basin in the eastern Tianshan show that they are rich in K,Ru,Ba,and other large ion lithophilic elements(LILE),and poor in Nb,Ta,P,Th,and HREE(heavy rare earth elements),Ce,Ti,and other high field strength elements(HFSE).They are also slightly rich in LREE(light rare earth elements)such that light and heavy rare earth elements can be differentiated easily.However,the Eu is not noticeable.From the Early Carboniferous to the Late Carboniferous,the degree of fractionation between light and heavy rare earth elements gradually decreased,and Eu changed from a slightly negative anomaly to no anomaly.The main trace and rare earth elements are similar to island-arc volcanic rocks.The standardized distribution model of rare earth element chondrites of acidic lava in the Early Permian transitioned to the seagull pattern with obviously Eu negative anomalies,similar to typical continental rift rhyolite.The standardized distribution patterns of rare earth element chondrites in the Early Permian basic lava are close to the flat type,and the rare earth curve is similar to the continental porphyritic basalt and continental rift basalt.The tectonic locations of the southern Tuha Basin in the eastern Tianshan in the early Paleozoic are the eastern section of the northern Tianshan Ocean and the southern margin of the Tuha block.The area experienced two tectonic evolution stages in the Late Paleozoic:Early Devonian-Late Carboniferous oceanic crust northward subduction of oceanic and continental evolution stage;Permian ocean basin closure and post-collision orogenic stage,post-collision in-plate extension stage.The rare earths of Early Devonian igneous rocks in the northern Tuha Basin of the Eastern Tianshan are right-inclined distribution models,rich in the large ion lithophile elements such as K,Rb,and Ba,and very poor in the high field strength elements such as Nb and Ta,and slightly poor in Ti.The geochemical characteristics of volcanic rocks in the subduction zone are similar.The late Devonian volcanic rare earth elements have a near flat rare earth element patterns of enrichment of Ba and U,large ion lithophile element,obviously negative anomalies of Nb,Ta,and Sr,and weakly negative anomalies of Ti,similar to island arc volcanics.Carboniferous-early Permian rare earth element chondrites with acidic lava are all rich in LREE,and the negative anomalies are clear.The content of rare earth elements is significantly larger.The enrichment of Ba and U and the relative depletion of high field strength elements such as Nb,Ta,Ti,P,and Sr are similar to typical continental rift rhyolites,and they are completely different from island arc acidic lavas.The standardized distribution patterns of Carboniferous-early Permian rare earth element chondrite meteorites with basic lava are close to flat type;LREE is slightly enriched,the differentiation between light and heavy rare earth elements is weak,the europium anomaly is not noticeable,and trace elements suggest great enrichment of the large ion lithophile element(LILE)and slight enrichment of the high field strength elements(HFSE).The rare earth curve is similar to continental porphyritic basalt and continental rift-type basalt.The tectonic location of the northern part of the Tuha Basin in the eastern Tianshan in the early Paleozoic is the southeast side of the Junggar branch of the oceanic basin(i.e.Kalamali oceanic basin).It is believed that this area predominantly underwent three stages of tectonic evolution in the Late Paleozoic:the evolutionary stage of oceanic land subduction which gradually subsided before the Late Devonian,Late Devonian-Early Permian plate extension which caused the Bogda Rift Collision,and Inland evolution stage of Middle-Late Permian.