Geochronology, Geochemistry Of Volcanic Rocks From The Yamansu Formation In The Eastern Tianshan, Xinjiang:Implication For The Closure Of The Paleo-TianShan Ocean

The Jueluotage belt is an important polymetallic metallogenic belt in the East Tianshan of Xinjiang. The volcanic rocks from Yamansu Formation display east-west zonation within the metallogenic belt, and were controlled by Aqikekuduke and Yamansu faults in its south and north respectively. The study of the Yamansu volcanic rocks can help to solve the problem of tectonic background in Jueluotage belt of the eastern Tianshan during Paleozoic and reconstruct the tectonic evolutional process of Paleo-TianShan Ocean in the Paleozoic, and it also can provide the geological evidence for the prediction of metallogenic regularities and establishment of prospecting model.Due to the limited geochronology and geochemistry study of Yamans volcanic rocks, we choose the volcanic rocks of Yamansu Formation as our research object. Based on the distribution of volcanic rocks and the regional data and research results, we have made the detailed research in petrology, geochemistry and geochronology, including lots of field work, the detailed observations of petrography, precised geochronology, whole-rock major and trace element composions, Sr-Nd isotopes. Through the above research, we make some conclusion below:1The assemblage and geochemical characteristics of volcanic rocks from Yamansu FormationThe assemblages of volcanic rocks from Yamansu Formation are made of basalt, andesite, dacite, rhyolite and intermediate-acid pyroclastic rock, and these volcanic rocks belong to the sub-alkaline rocks of calc-alkaline series, which are mainly the medium and high potassium calc-alkaline series and minor shoshonite series. Geochemical characteristics of major elements show a good correlation between SiO2and the other oxides; all samples show similar REE and spider patterns with enriched LILE and LREE, depleted Nb, Ta and HREE, suggesting they are the products of co-magmatic evolution. Meanwhile, the volcanic rocks have depleted magnesium, enriched calcium, alkali, potassium, high FeO/MgO ratios (1.03～5.67) and the K/Rb ratios (104.9～619.05), relatively high La/Nb、Pb/Ce、Ba/La、Ta/Yb、Zr/Nb and Th/Ta ratios, indicating the geochemical characteristics of continental marginal arc.2The age of Yamansu Formation volcanic rocksZircon U-Pb ages of volcanic rocks from Yamansu Formation in the eastern Tianshan Jueluotage belt are:348±1.7Ma (MSWD=1.15) in Shaquanzi to the east,335.9±2.4Ma (MSWD=1.03) of western Kumtag sandy ridges to the middle;334±2.5Ma(MSWD=1.02) of Heijianshan to the west. All the ages indicate the crystallization age of the volcanic rocks from Yamansu Formation is Early Carboniferous. The zircon U-Pb age of volcanic rocks has a degressive trend from the east to west, suggesting that Carboniferous magmatic activities of the Jueluotage zone may start from the east and extend to the west.3The magma evolution and source of volcanic rocks from Yamansu formationThese volcanic rocks show low Isr, positive and consistent εNd(t) and relatively consistent depleted mantle Nd model ages. the characteristics of trace elements and Sr-Nd isotopic shows that the magma source could be derived from equilibrium partial melting of the mantle wedge which metasomatized by subduction-related fluids.There are the fractional crystallizations of pyroxenes,plagioclases and other mineral in the evolution of magma; The value of ISr and εNd(t) show a good correlation with SiO2,suggesting that the contamination of continental crust material in the evolution of magma.From the diagram of and La/Nb-Ba/Nb,We can see that La/Nb and Ba/Nb value of volcanic rocks is highly than continental crust,It can’t be formed by contaminated mantle-derived magma(with low La/Nb and Ba/Nb valu) or the mixed crust-mantle material, so we infer that there are limited contribution of continental crust material in the process of magma evolution.The major mechanism of controlling the magma evolution are equilibrium partial melting,fractional crystallization and low degree of crustal contamination process (AFC).4Tectonic setting of volcanic rocks from Yamansu FormationThe characteristics of rock assemblages, geochemistry, magma evolution and forming age of volcanic rock from Yamansu Formation indicate that these volcanic rocks are the products of magmatism which is related to the subduction of Paleo-TianShan Ocean, and the tectonic settings should be the interarc basin upon the continental marginal arc.5Implication to the close of Paleo-TianShan OceanAccording to the ages of magmatic rocks in the middle Tianshan massif, the Jueluotage belt and Dananhu, and the age distributions of captured zircon in volcanic rocks from Yamansu Formation, as well as the possible ophiolite occurrence, we infer that the Jueluotage belt and Dananhu arc are connected with the middle-tianshan massif, sharing the common basement. The northern Paleo-TianShan Ocean subducted towards south to the plate that the basement represented, and resulted in cracking and expansion during carboniferous, then produced large amounts of calcium alkali series volcanic rocks, finally formed the Yamansu interarc basin.