| As an important part of the Central orogenic belt of China, the study area adjoins the Pamirs to the west, the Eastern Kunlun to the east, the Tarim Basin to the north and the Tibet Plateau to the south. It has an important implication on the research of the evolution of the Proto-Tethys and the Paleo-Tethys during Early Paleozoic to Early Mesozoic. Igneous rocks are widely distributed due to the intensive magmatic activities. Because of the severe natural conditions of the area, geology survey is difficult to conduct. Till now, the research on the magmatic arc of the Western Kunlun is still in the starting stage. The Datong pluton in the Early Paleozoic magmatic arc intruded into Mesoproterozoic metamorphic basement during the mid-Caledonian and itself was intruded by Early Mesozoic plutons. Located between the Oytag-Kudi suture zone and the Mazha-Kangxiwa suture zone, it is considered as the product of Tethys in different stages of evolution. In this study, we focus on the petrography, major and trace elements, zircon U-Pb chronology, zircon Hf isotopic data and whole-rock Sr-Nd isotopic data of the shoshonitic and adakitic rocks as well as their magmatic source, formation mechanism and evolution processes. Furthermore, we also discussed the genetic relationship between the shoshonitic and adakitic rocks, and the evolution history of this area.The shoshonitic rocks of the Datong pluton are syenite. They contain high Si O2(58.16%~69.29%), K2O(3.91%~6.58%), moderate Al2O3(15.34%~17.11%) and K2O+Na2O(8.07%~ 10.17%), low Ti O2(0.19%~0.77%) and Mg O(0.39%-2.06%). For trace elements, they are enriched in LREE, LILE, depleted in HREE and HFSE, with variably negative Eu anomalies. The contents of εHf(t) range from-4.52 to 0.59, with an average of-1.58. The Isr values vary from 0.709214 to 0.710023, 143Nd/144 Nd values from 0.512179 to 0.512224 and εNd(t) from-3.23 to-3.92. The zircon U-Pb dating for the shoshonitic rocks yielded weighted mean ages of(434.7±0.9~443.6±0.9)Ma. The geochemistry of the shoshonitic rocks suggests that their melt derived from the mantle metasomatism with high-silicon melt during the southward subduction of Proto-Tethys.The adakitic rocks consist of biotite monzonitic granite and quartz monzonitic granite. They occur as small isolated pluton and dykes penetrated into shoshonitic Datong pluton. They contain high Si O2(≥60.34%), Al2O3(≥14.73%), total alkali(K2O+Na2O≥6.40%), Sr(≥504 μg/g) and Sr/Y(with an average of 70.88), low Mg O(≤2.35%) and Y(≤17.20μg/g). They are obviously enriched in LREE, LILE, depleted in HREE and HFSE, with variably negative Eu anomalies. The zircon U-Pb dating for these rocks from both isolated and dykes within the Datong pluton yields weighted mean ages of(443.6±1.4 Ma~462.0±1.0) Ma. The zircon εHf(t) values for the adakitic rocks range from-7.28 to 4.56, with an average of-0.84. Their Isr values vary from 0.709807 to 0.713363, 143Nd/144 Nd values ranging from 0.512193 to 0.512225 and εNd(t)values varing from-2.97 to-3.50. Combined with the regional tectonic evolutionary history, we consider that the adakitic rocks were reaction products of mantle wedge and the melt derived from the melting of the oceanic crust and sediment on it during the southward subduction of the Proto-Tethys.Based on this study, we suggeat that the Proto-Tethys was subducting southward during the Late Ordovician. Because of this subduction, intensive magmatic activities occurred in supra-subduction zone and formed a series of Early Paleozoic magmatic arc. Because of the complex tectonic environment and the long-term subduction of the Proto-Tethys, the shoshonitic and adakitic rocks formed throughout the formation the Datong Pluton and without obvious chronological order. |
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