Geochemical and Sr-Nd-Os Isotopic Characteristics of Middle Silurian Volcanic Rocks in Northwest of the Xiemisitai Mountains, West Junggar and Its Tectonic Implications
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摘要: 西准噶尔谢米斯台山火山岩近期报道形成于志留纪而非之前认为的中泥盆世，但有关中志留世火山岩特征及构造意义研究较弱。本文对西准噶尔谢米斯台山西北段乌兰浩特中志留世玄武岩、玄武安山岩样品进行了系统的地球化学主量、微量元素与Sr-Nd-Os同位素特征分析。火山岩样品表现出高MgO（5.15%~7.81%）、镁值（56.0~60.7）和低CaO（4.56%~5.09%）、FeOT/MgO（1.15~1.40）的特征，属于高镁火山岩类。玄武岩的εNd（t）值为+2.65，87Sr/86Sr和187Os/188Os初始比值分别为0.70439和0.4901。略显富集的同位素比值与高Th/Yb值（2.2~2.7）、低Ba/La值（17.4~33.4）表明其可能是受洋底沉积物熔体交代地幔部分熔融的产物，并遭受了一定程度地壳混染。火山岩样品为钙碱性，具有轻稀土和大离子亲石元素富集、Nb和Ta亏损等弧岩浆特征，推测乌兰浩特中志留世火山岩形成于与准噶尔洋板片由南向北俯冲有关的弧后环境。
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关键词:
- 中志留世 /
- 火山岩 /
- Sr-Nd-Os同位素 /
- 弧后 /
- 西准噶尔
Abstract: Volcanic rocks in the Xiemisitai Mountains, in Northwest Junggar were recently considered to form in the Silurian rather than the Devonian period. However, the characteristic and tectonic setting of Middle Silurian volcanic rocks in this area are poorly constrained. In this study, major and trace elements and Sr-Nd-Os isotopic composition analyses were synthetically conducted on the Middle Silurian basalt and basaltic andesite in northwest of the Xiemisitai Mountains, West Junggar. These volcanic samples are characterized by high MgO (5.15%-7.81%) with Mg of 56.0-60.7, low CaO (4.56%-5.09%) and FeOT/MgO (1.15-1.40), similar to those of high-Mg volcanic rocks. Basalts have εNd(t) of 2.65, (87Sr/86Sr)i of 0.70439, and (187Os/188Os)i of 0.4901. The slightly enriched isotopic signatures, high Th/Yb (2.2-2.7) and low Ba/La (17.4-33.4) indicate that they originated from depleted mantle metasomatized by melts from subducted sediments and suffered from crustal contamination. The volcanic rocks are calc-alkaline and exhibit arc-like geochemistry with the enrichment of light REE and large ion lithophile elements. It is suggested that these rocks were probably formed in a back-arc setting due to northward subduction of the Junggar Oceanic slab.-
Key words:
- Middle Silurian /
- volcanic rocks /
- Sr-Nd-Os isotopes /
- back-arc /
- West Junggar
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表 1 火山岩的主要氧化物测试结果(单位为%)
Table 1. Major oxides compositions (%) of volcanic rocks
元素 玄武岩
(08711B-02)玄武安山岩
(08711B-03)SiO2 50.83 56.23 TiO2 0.704 0.634 Al2O3 16.49 15.43 Fe2O3T 10.00 8.00 MnO 0.135 0.148 MgO 7.81 5.15 CaO 4.56 5.09 Na2O 3.75 3.78 K2O 0.94 2.55 P2O5 0.255 0.203 LOI 4.41 2.65 总计 99.88 99.86 
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表 2 火山岩的微量元素和稀土元素含量(单位为μg/g)测试结果
Table 2. Compositions of trace elements and rare earth elements (μg/g) of volcanic rocks
元素 玄武岩
08711B-02)玄武安山岩
(08711B-03)Li 28.9 12.9 Be 1.1 1.33 Sc 24.1 22 V 218 168 Cr 8.93 36.1 Co 26 20.3 Ni 9.48 18.6 Cu 64.9 75.1 Zn 79.7 81.5 Ga 18 16.3 Ge 1.25 1.18 Rb 31.2 67 Sr 455 411 Y 22.3 20.4 Zr 89.4 173 Nb 5.09 6.02 Cs 2.24 1.27 Ba 261 578 La 15 17.3 Ce 30.7 33.2 Pr 3.92 4.01 Nd 16.7 16 Sm 4.06 3.59 Eu 1.22 1.18 Gd 3.77 3.42 Tb 0.6 0.53 Dy 3.74 3.29 Ho 0.82 0.74 Er 2.23 2.06 Tm 0.35 0.32 Yb 2.18 2.14 Lu 0.34 0.34 Hf 2.25 3.72 Ta 0.31 0.39 Pb 12.3 26.7 Th 4.78 5.72 U 1.28 1.56
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表 3 火山岩样品08711B-02的同位素比值测试结果
Table 3. The isotopic data of the volcanic rock sample No.08711B-02
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