Petrogenesis of Early Silurian Intrusive Rocks in Yeniujishan Area of South Qilian: Constraints from Geochemistry and LA-ICP-MS Zircon U-Pb Geochronology
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摘要: 对南祁连野牛脊山地区侵入岩进行了岩石地球化学和锆石LA-ICP-MS U-Pb年代学研究，进而约束其岩石成因和形成的构造背景。该地区侵入岩主要岩性为花岗闪长岩、似斑状花岗闪长岩和似斑状二长花岗岩。地球化学特征显示：岩石具有高硅、富铝、富钾、富碱和贫钛、铁、镁、钙的特征，其w（SiO2）=64.69%~78.03%，w（Al2O3）=7.44%~14.16%，w（K2O）=2.98%~5.23%，w（Na2O+K2O）=5.13%~7.92%，（TiO2）=0.55%~0.94%，w（FeO）=1.52%~5.00%，w（CaO）=1.11%~3.43%，w（MgO）=0.78%~2.49%，岩石里特曼指数δ=0.91~2.89，A/CNK为0.84~1.23，为准铝质-弱过铝质的钙碱性系列岩石。岩石稀土元素总含量量较低（ΣREE为157.89×10-6~299.15×10-6），相对富集轻稀土元素、大离子亲石元素（Rb、K）和Th，亏损重稀土元素，Ba，Ce、Hf、Zr、Sm、Y、Yb等明显低于洋脊花岗岩，具有明显的Eu负异常（δEu=0.47~0.66）。综合地质地球化学资料，显示该侵入岩属于S型花岗岩且来源于地壳物质的重熔。由w（Y）-w（Nb）图解和R1-R2构造环境判别图解可得知研究区内花岗岩与南祁连造山带板块碰撞有关。在似斑状花岗闪长岩中获得LA-ICP-MS锆石U-Pb年龄为（443.9±2.1） Ma，侵位时代为早志留世。结合区域构造演化分析，认为野牛脊山地区侵入岩形成于板块俯冲碰撞的地球动力学背景下。Abstract: In order to to constrain its petrogenetic and tectonic processes, the geochemistry and LA-ICP-MS zircon U-Pb dating of intrusive rocks from Yeniujishan area in South Qilian has been conducted in this paper. The intrusive rocks in this area are mainly composed of granodiorite, porphyritic granodiorite and monzonite granite. Geochemical data show these intrusive rocks are characterized by rich SiO2 (64.69%~78.03%),Al2O3 (7.44%~14.16%), K(2.98%~5.23) and alkali(5.13%~7.92%), but low TiO2 (0.55%~0.94%), FeO (1.52%~5.00%), CaO (1.11%~3.43%) and MgO(0.78%~2.49%). Their Rittmann indexes (δ) range from 0.91 to 2.89, while their aluminum saturation index (A/CNK) vary from 0.84 to 1.23, belonging to the calc-alkaline series, with the characteristics of metaluminous-weakly peraluminous rocks. These rocks have low ΣREE values (157.89×10-6~299.15×10-6), which are relatively enriched in LREE, large ion lithophile elements (Rb, K) and Th element. But, they are relatively depleted in HREE, while their Ce,Hf,Zr,Sm,Y and Yb values are obviously lower than the ones of mid-ocean ridge granitoids, with strong negative Eu anomaly (δEu=0.47~0.66). The geological and geochemical data suggest that these intrusive rocks are S-type granite, which was formed by the re-melting of crustal material. The discrimination diagrams for tectonic settings (w(Y)-w(Nb) and R1-R2) show that these granites are related to the plate collision of South Qilian orogenic belt. The U-Pb dating of porphyritic granodiorite yields an age of (443.9±2.1)Ma, indicting that these rocks were emplaced in Early Silurian. In combination with the comprehensive analysis of tectonic evolution, it is believed that these intrusive rocks in Yeniujishan area formed under the geodynamic background of subduction collision.
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Keywords:
- Early Silurian /
- granite /
- geochemistry /
- zircon U-Pb dating /
- Yeniujishan /
- South Qilian
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