Geochemistry, geochronology and Hf isotopic compositions of metagabbro dykes on the northwestern margin of Oulongbuluke micro-block on the northern margin of Qaidam Basin
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摘要:
侵位于柴北缘构造带欧龙布鲁克地块西北缘达肯大坂岩群中的辉长岩脉岩石Al2O3、CaO含量较高，P2O5、TiO2含量较低，属于钙碱性系列。稀土元素总量低（43.17×10-6~57.53×10-6），LREE/HREE值为1.82~2.77，稀土元素配分曲线整体表现为右倾型。岩石Ti亏损，具高ThN/NbN值（3.75~8.03）和低Nb/La值（0.44~0.84）特征。辉长岩锆石的176Hf/177Hf值介于0.282813~0.282979之间，εHf（t）值为9.25~15.11，Hf模式年龄（TDM）分布在378~610Ma之间，平均494Ma。综合研究认为，辉长岩岩浆源区为软流圈地幔且后期可能受到壳源富集组分的混染。LA-ICP-MS锆石U-Pb定年结果表明，辉长岩脉形成于357±4Ma，即早石炭世。结合区域地质背景，认为早石炭世柴北缘仍处于后造山伸展的构造演化阶段，而辉长岩脉是后造山伸展阶段柴北缘造山带去根、软流圈地幔上涌作用形成的。
Abstract:Metagabbro dykes were emplaced in the Dakendaban Rock Group on the northwestern margin of the Oulongbuluke Block, northern margin of Qaidam. They are characterized by higher Al2O3, CaO but lower P2O5, TiO2, belonging to subalkalic calcalkaline series. Meanwhile, the rocks have lower Σ REE and LREE/HREE ratios between 1.82~2.77, with the chondritenormalized REE showing right-inclined patterns. The rocks are characterized by low Ti and Nb/La(0.44~0.84), but high ThN/NbN (3.75~8.03). The εHf(t)values vary in the range of 9.25~15.11, 176Hf/177Hf ratios vary from 0.282813 to 0.282979, and Hf model ages vary from 378Ma to 610Ma. Comprehensive researches show that the magma of metagabbro dykes was derived from asthenosphere mantle, and was probably contaminated by the crust. LA-ICP-MS zircon U-Pb dating of metagabbro dykes in the Dakendaban Rock Group yielded the formation age of 357±4Ma. Combined with regional geological background, the authors hold that the northwestern margin of the Qaidam Basin was still in the extensional tectonic stage in Early Carboniferous, and the rocks were formed under the condition of unrooting of the north of Qaidam orogenic belt and the upwelling of asthenospheric mantle.
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Key words:
- metagabbro dykes /
- geochemistry /
- geochronology /
- Hf isotopic /
- northern margin of the Qaidam
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表 2 辉长岩LA-ICP-MS锆石U-Th-Pb同位素测年结果
Table 2. LA-ICP-MS zircon U-Th-Pb isotopic analyses of metagabbro
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① 辜平阳,董增产,陈锐明,等.青海阿尔金1:5万打柴沟(J46E005011、J46E005012、J46E005013、J46E006011、J46E006012、J46E006013)等6幅区调.2016.
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