Petrogenesis of A-Type Granite and Geological Significance of Bure Area, Western Ethiopia
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摘要: 为进一步约束埃塞俄比亚西部地区区域地质构造演化，选取埃塞俄比亚西部布雷地区岩浆岩开展岩石地球化学、年代学等分析.结果显示：布雷岩体具有相对较高的SiO2(66.4%~68.5%)、K2O(4.56%~4.87%)及铁值[TFeO/(TFeO+MgO]为0.75~0.79，相对较低的MgO(0.80%~1.3%)、CaO(2.01%~2.36%)和A/CNK(0.89~0.96)，富集Zr、Hf、Y、Ga等元素，亏损Sr、P、Ti和Eu元素，显示了A型花岗岩的特征.锆石LA-ICP-MS U-Pb加权平均年龄为631±4 Ma，代表了东非造山运动后伸展阶段.锆石εHf(t)值为3.2~9.1，具有一定的不均一特征.布雷A型花岗岩形成于碰撞造山的后伸展环境.岩石地球化学及Hf同位素特征表明布雷岩体主体可能来源于减薄的岩石圈地幔玄武质岩浆，其后经历了广泛的结晶分异作用而形成.
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关键词:
- 锆石U-Pb年代学 /
- 地球化学 /
- A型花岗岩 /
- 埃塞俄比亚西部布雷地区
Abstract: In order to furtherly constrain the evolution of regional geological structure in the western Ethiopia, we selected the magmatic rocks from the Bure area of western Ethiopia for petrogeochemical and chronologic analysis.The Burequartz monzonite in western Ethiopia has relatively high SiO2(66.4% to 68.5%), K2O (4.56% to 4.87%), and iron value (TFeO/(TFeO+MgO) is 0.75 to 0.79), and relatively low MgO (0.8% to 1.3%), CaO (2.01% to 2.36%), and A/CNK (0.89~0.96), withthe characteristic that areemriched in elements such as Zr, Hf, Y, and Ga, and depleted Sr, P, Ti, and Eu elements, which reveal the feature of A-type granite. The zircon LA-ICP-MS U-Pb age of the rock is 631±4 Ma (MSWD=2.9), representing the extension stage of the East African orogenic movement. The εHf(t) values of Bure A-type granite range from 3.2 to 9.1 (average=7.3), showing a relatively inhomegeneous feature. The Bure A-type granite formed in the post-stretching environment of the East African orogenic movement. Thewhole rock geochemistryand Hf isotopic characteristicssuggest that the Bure A-type granite may originated from basaltic magma in the thinned lithospheric mantle, and then formed through extensive crystallization differentiation.-
Key words:
- zircon U-Pb geochronology /
- geochemistry /
- A-type granite /
- Bure area, western Ethiopia
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图 1 阿拉伯-努比亚地盾地质简图(据Johnson et al., 2011修改)
Fig. 1. Geological map of Arabian⁃Nubian Shield (after Johnson et al., 2011)
图 2 西埃塞俄比亚地体区域地质简图(据Allen and Tadesse, 2003修改)
Fig. 2. Sketch of regional geology of western Ethiopian terrain (after Allen and Tadesse, 2003)
图 3 布雷岩体岩石手标本(a)及镜下特征(b)
Fig. 3. Hand specimen photograph (a) and photomicrograph (b) of Burequartz monzonite
图 4 埃塞俄比亚西部布雷地区石英二长岩锆石U-Pb同位素年龄谐和图(a)和锆石稀土元素球粒陨石模式图(b)
Fig. 4. U-Pb concordia diagram (a) and chondrite normalized REE distribution pattern (b) for Bure quartz monzonite zircon
图 5 石英二长岩TAS图解(a)；SiO2-K2O图解(b)；Na2O-K2O图解(c)；A/NK-A/CNK图解(d)
a. 据Middlemos(1985)；d. 据Maniar and Piccoli(1989)；蓝色数据来自本文研究样品；紫色数据来自Kebede and Koeberl(2003)
Fig. 5. TAS diagram (a), SiO2-K2O diagram (b), Na2O-K2O diagram (c), and A/NK-A/CNK diagram(d)
图 6 埃塞俄比亚西部布雷地区石英二长岩微量元素原始地幔标准化蛛网图(a)和稀土元素球粒陨石标准化分布图(b)
b. 据Sun and McDonough(1989)；蓝色数据来自本文研究样品；红色数据来自Kebede and Koeberl(2003)
Fig. 6. Primitive mantle normalized trace element spider diagram (a) and chondirite normalized REE distribution diagram (b)
图 7 (a) εHf(t)-Age图解；(b) TDM2年龄直方图
Fig. 7. (a) εHf(t)-Age diagram; (b) Histogram of TDM2 ages
图 8 A型花岗岩判别图
a. TFeO/MgO-Zr+Nb+Ce+Y图解(Whalen et al., 1987)；b. K2O+Na2O/CaO-Zr+Nb+Ce+Y图解(Whalen et al., 1987)；c. TFeO/(TFeO+MgO)-SiO2图解(Frost et al., 2001；Frost and Frost, 2011)；d. K2O+Na2O-CaO-SiO2图解(Frost et al., 2001；Frost and Frost, 2011)；蓝色数据来自本文研究样品；紫色数据来自Kebede and Koeberl(2003)
Fig. 8. Discriminant diagrams of A-type granites
图 9 (a) La/Sm-Sm/Yb图解(Kay and Mpodozis, 2001)；(b) La/Sm-La图解(Allegre and Minster, 1978)；(c) Rb/Sr-Sr图解(Rollinson, 1993)；(d) (La/Yb)N-La图解(Rollinson, 1993)
Fig. 9. (a) La/Sm vs. Sm/Yb diagram; (b) La/Sm vs. La diagram(Allegre and Minster, 1978); (c) Rb/Sr vs. Sr diagram (Rollinson, 1993); (d) (La/Yb)N vs. La diagram (Rollinson, 1993)
图 10 构造判别图
a. Rb-Y+Nb图解(Pearce et al., 1984)；b. Y-Nb-3Ga (Eby，1992)
Fig. 10. Tectonic discriminant diagrams
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