| The Central Asian Orogenic Belt?CAOB?,which includes Altaides,Junggar orogenic belt,Junggar terrane and Tianshan orogenic belt,has experienced complex geodynamic evolution,including the subduction of Paleo-Asian Oceanic plates,collision and orogeny,and post-orogenic extension with a series of magmatism.As an important part of the CAOB,the Northern Xinjiang area has experienced complicated geodynamic evolution involving the oceanic plates subduction,continental collision,crustal thickening and post-orogenic extension.The Junggar terrane is critical to reveal the Paleozoic ocean-continent transformation and tectonic evolution of the Northern Xinjiang.Previous studies have been conducted on the early Paleozoic tectonic setting and magmatism in Junggar terrane,but these are still limited and many are even controversial,systematic geological research and accurate petrology,geochemistry,chronology of early Paleozoic volcanic rocks is lacking.Here we report detailed geological survey,new insitu U–Pb ages of zircon,geochemistry,and whole-rock Sr-Nb-Pb isotopic data of two Early Paleozoic volcanic rocks from the Hulugou and Daliugou Formation,situated in Harlik-MoQinulla,Eastern Junggar terrane.Combined with regional geology,the petrogenesis,magma evolution,and eruption age are proposed,along with the formation environment and tectonic evolution of Eastern Junggar terrane.The main conclusions are as follows:?1?Early Silurian volcanic rocks are continental in nature,with layered and porous basalt.Hulugou Formation?S1h?volcanic rocks are oriented along Harlik mountain with NWW-trending,which overlie Middle-Late Ordovician Qiaganbulake Formation(O2-3q)in the form of angular unconformity.The volcanic rocks consist of trachytic basalt,basaltic trachytic andesite,basaltic andesite,andesite and breccia lava,and breccia tuff with corresponding compositions.Effusive facies lava and pyroclastic rocks are displayed in the lower and upper part of S1h volcanic rocks respectively.Daliugou Formation?S1d?volcanic rocks expose in MoQinulla mountain in the north of the study area,which overlies Ordovician Miaoergou Formation(O2-3m)in its south with faulted contact or angular unconformity,overlied by Jurassic Badaowan Formation?J2b?in its north with angular unconformity,and intruded by the late Carboniferous granite in its east.The volcanic rocks consist of tephrite,basaltic trachytic andesite,trachytic andesite,trachytic andesite brecciated lava and pyroclastic rock with relative compositions.?2?The zircon U-Pb age of Hulugou Formation basaltic andesite,lithic breccia tuff and basaltic trachytic andesite are 441.0±2.2 Ma,440.1±3.2 Ma and 443.1±6.9Ma respectively,indicating that S1h volcanic rocks formed in the early stage of early Silurian.While zircon U-Pb dating of Daliugou Formation trachytic andesite brecciated lava show the age of 434.4±2.2 Ma,which suggests a late stage of early Silurian.These two groups of volcanic rocks all formed in early Silurian,with Hulugou Formation being older than Daliugou Formation.?3?Hulugou Formation volcanic rocks are alkaline or tholeiite in nature,with SiO2ranging from 46.64%to 57.5%,Na2O+K2O from 4.10%to 7.61%,TiO2 from 0.77%to2.37%,and FeOt from 7.08%to 11.19%.These volcanics are rich in Na?K2O/Na2O=0.030.65?.However,Daliugou Formation volcanic rocks are classified into strong alkaline-alkaline series with SiO2 ranging from 45.6%to 56.16%,TiO2 from0.85%to 2.23%and FeOt from 5.88%to 12.8%.They also display relatively high alkali?Na2O+K2O=4.94%9.10%?and Rittmann index??2.949.68?.These two groups of volcanic rocks have the low MgO abundance?Mg#ranging from 39.0 to 53.0,and 25.1to 58.7 respectively?,indicating that they are evolved.Early Silurian volcanic rocks are similar in the rare earth and trace elements.?REE of Hulugou formation alkaline volcanic rocks range from 68.9×10-66 to 187.3×10-6,and they also display slight LREE enrichment??La/Yb?N=1.814.95?and weak negative Eu anomalies??Eu=0.871.04?.While?REE of tholeiite volcanic rocks range from66.7×10-6 to 103.5×10-6,with their?La/Yb?N ratios from 1.81 to 4.95 and?Eu from 0.78to 1.However,Daliugou Formation volcanic rocks have higher?REE,from 121.3×10-6to 280.7×10-6,with strong LREE enrichment??La/Yb?N=4.038.27?and weak negative Eu anomalies??Eu=0.761.05?.These rocks are all enriched in large ion lithophile elements?LILE:Ba,U?and Th,Pb,but are relatively depleted in high field strength elements?HFSE:Nb,Ta,Ti?with positive Pb and slight negative Zr,Hf anomalies.In conclusion,The Early Silurian volcanic rocks have higher alkalinity,and are richer in LILE and LREE?higher LREE/HREE?.Hulugou Formation volcanic rocks display limited initial 87Sr/86Sr?0.704390.70480?and small ranges in?Nd?t??+3.04+3.56?and Nd model ages?0.820.86 Ma?.Similarly,Daliugou Formation volcanic rocks show variable(87Sr/86Sr)i?0.703290.70464?,?Nd?t??+2.13+4.16?and Neoproterozoic Nd model ages?0.770.91 Ma?,with(208Pb204Pb)t ranging from 17.9556 to 18.3169,(207Pb204Pb)t from 15.5426 to15.5729,and(208Pb204Pb)t from 37.6433 to 38.3656.?4?The initial 87Sr/86Sr?0.703290.70480?,positive?Nd?t??+2.13+4.16?,and Nd model ages?0.770.91Ma?of these volcanic rocks,along with the low Pb isotopic content,show the juvenile depletion mantle source characteristics.However,the high alkali,LILE enrichment and low(143Nd/144Nd)i values?0.51223?indicate that mantle source was metasomatized by early slab-derived fluids.Early Silurian volcanic rocks originated from partial melting of metasomatic mantle peridotite triggered by upwelling asthenosphere.All rocks have low Mg#?25.356.2?and low Cr and Ni contents,indicating fractionation of olivine and clinopyroxene.Weak negative Eu anomalies suggest a small amount of plagioclase fractionation.Variable(87Sr/86Sr)i,Th/Ce?0.020.05?and Th/La?0.12?indicate that crustal contamination played an important role in the generation of the magmas.?5?Hulugou Formation volcanic rocks are characteristic within-plate basalt and island arc basalt.We speculate that their formation can be linked to post-collision magmatism in extensional setting after the collision between Junggar terrane and Altai-Mongolia terrane in Paleo-Asian Ocean.Daliugou Formation alkaline volcanic rocks formed in the intraplate rift setting,which may represent the stage of Karamaili ocean intracontinental initial extension.Combining regional geology and Zr-Zr/Y,Ti/1000-V diagram,our working hypothesis is that there was a collision orogeny in the early Paleozoic and the tectonic setting of Early Silurian was not the island arc setting.The volcanic rocks in our study show that Early Paleozoic is a key stage of tectonic transition from collisional extrusion to extension in Northeastern Junggar terrane,providing new evidence and implications to the early Paleozoic tectonic evolution of the Northern Xinjiang. |
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