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    敦煌地块中酸性火山岩锆石U–Pb年代学、岩石成因及对古亚洲洋俯冲的指示

    Zircon U–Pb geochronology, petrogenesis of intermediate–acid volcanic rocks in Dunhuang block and its indiction of subduction of Paleo–Asian Ocean

      摘要
    • 摘要:
      研究目的 敦煌地块地处塔里木板块、中亚造山带和祁连造山带的结合部位，其形成时代、构造属性等一直存在争议，本文以出露于敦煌地块东北缘的中酸性火山岩为研究对象，尝试对解决以上问题提供证据。
      研究方法 通过对3件英安岩样品进行LA–ICP–MS锆石U–Pb测年、对中酸性火山岩进行岩石地球化学分析，探讨该套火山岩的形成时代、岩石成因及构造属性等。
      研究结果 锆石U–Pb年代学表明3件英安岩样品的成岩年龄为（533.2±6.1）Ma、（527.7±6.1）Ma、（514.9±3.4）Ma，时代为早寒武世。研究区出露的安山岩具有典型高镁安山岩特征，如SiO2含量51.4%~55.98%，具有高MgO（5.81%~12.31%）和Mg#（59~72）、高Cr（166×10−6~1020×10−6）和Ni（41.4×10−6~169×10−6）以及低FeOT/MgO（0.79~1.56）比值的特征，此外安山岩也具有高的Ti/Zr、Ti/Y比值和低的Rb/Sr比值，具有高的La/Nb、Ba/Nb和Ba/La比值。中酸性火山岩富集大离子亲石元素（Rb、Ba、U、K）及LREE，亏损高场强元素（Nb、Ta、Zr、Ti）及HREE，具有近平坦的右倾稀土配分模式，以及较低的Nb/La（0.36~0.46）和Hf/Th（1.02~1.34）比值和较高的Hf/Ta（5.71~8.60）、La/Ta（21.72~29.50）比值，显示了岛弧火山岩的地球化学属性。
      结论 该套中酸性火山岩为敦煌群之上早寒武世火山岩盖层，形成于岛弧环境，可能由俯冲的早寒武世古亚洲洋洋壳熔体（流体）交代地幔楔部分熔融作用形成，且在上升过程中受到地壳物质的混染，根据火山岩形成时代及岩石地球化学特征认为古亚洲洋的俯冲作用最早可追溯至早寒武世。

       

      Abstract:
      This paper is the result of geological survey engineering.
      Objective Dunhuang block is located at the junction of Tarim Plate, Central Asian Orogenic Belt and Qilianshan Orogenic Belt. Its formation age and tectonic attributes have always been controversial. This paper takes the intermediate–acid volcanic rocks exposed in the northeastern margin of Dunhuang block as the research object, and attempts to provide evidence for solving the above problems.
      Methods In this paper, LA–ICP–MS zircon U–Pb dating of three dacite samples and geochemical analysis of intermediate–acid volcanic rocks were carried out to explore the formation age, petrogenesis and tectonic attributes of the volcanic rocks.
      Results LA–ICP–MS zircon U–Pb geochronology indicates that three dacite samples form at (533.2±6.1) Ma, (527.7±6.1) Ma and (514.9±3.4) Ma, belonging to Early Cambrian. The andesite exposed in the area has typical characteristics of high–Mg andesite, e.g., SiO2 contents (51.4%−55.98%), high MgO contents (5.81%−12.31%) and Mg# values (59−72), high Cr (166×10−6−1020×10−6) and Ni (41.4×10−6−169×10−6) contents, and low FeOT/MgO (0.79−1.56) ratios. Otherwise, the andesites have high Ti/Zr, Ti/Y ratios and low Rb/Sr ratios, high La/Nb and Ba/Nb and Ba/La ratios. The intermediate–acid volcanic rocks are enriched in large ionic lithophile elements (Rb, Ba, U, K) and light rare earth elements, depleted in high field strength elements (Nb, Ta, Zr, Ti) and heavy rare earth elements, with characteristics of near–flat right–leaning rare earth distribution pattern, low Nb/La (0.36−0.46) and Hf/Th (1.02−1.34) ratios, high Hf/Ta (5.71−8.60) and La/Ta (21.72−29.50) ratios, indicative of the geochemical properties of island arc volcanic rocks.
      Conclusions It is considered that the volcanic rocks should be the Early Cambrian volcanic caprock above Dunhuang Group. The volcanic rocks in the area should be formed in the island arc environment, which may be formed by partial melting of metasomatic mantle wedge in the subduction of the Early Cambrian Paleo–Asian Ocean crust melt (fluid), and contaminated by crustal materials during the ascending process. According to the formation age of volcanic rocks and the characteristics of petrogeochemistry, the subduction of the Paleo–Asian Ocean can be traced back to the Early Cambrian.

       

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