Geochemistry and Tectonic Setting of Granites from Shire Region, Ethiopia
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摘要: 埃塞俄比亚北部施瑞地区具有造山前、造山期和造山后3种花岗岩类型, 全岩Sm-Nd等时线测年结果表明, 造山前和造山后花岗岩成岩年龄分别为824.4±15.5 Ma和517.9±5.8 Ma.3类花岗岩主量元素和稀土微量元素成分存在明显差异, 其中造山前花岗岩属于低钾过铝质花岗岩, 稀土分配模式属轻稀土弱富集型, 富集大离子亲石元素, 亏损P和Ti高场强元素;造山期花岗岩为准铝质高钾钙碱性花岗岩, 稀土分配模式属轻稀土富集型, 富集大离子亲石元素和高场强元素;造山后花岗岩为弱过铝质高钾钙碱性花岗岩, 稀土分配模式具强烈铕亏损的海鸥型, 富集大离子亲石元素, 明显亏损P和Ti高场强元素.综合研究表明: 造山前和造山期花岗岩均为I型幔源花岗岩, 构造环境处于被动大陆边缘-火山岛弧环境;造山后花岗岩为A2型壳源主花岗岩, 是在洋盆关闭和阿拉伯-努比亚地盾成熟后, 由减薄的地壳部分熔融产生.Abstract: There are pre-, syn-, and post-orogenic granites in Shire region of northern Ethiopia. Whole rock Sm-Nd isochron dating results show that diagenetic age of pre-, and post-orogenic ages are 824.4±15.5 Ma and 517.9±5.8 Ma respectively. Major, rare-earth and trace elements of three kinds of granite are obviously different. The pre-orogenic granites are relatively low in MgO and high in SiO2, belonging to the peraluminous series granite, with low K content and slight light REE enrichment, high large-ion lithophile element abundance and relative depletion of HFSE. The syn-orogenic granite belongs to the quasi-aluminous rock and high-potassium calc-alkaline granite, with the light REE enriched pattern, high large-ion lithophile element and HFSE abundances. The post-orogenic granite belongs to the weakly peraluminous high-potassium calc-alkaline granite, which has the chondrite-normalized REE patterns in the type of "gull" with severe depletion in Eu. The large-ion lithophile elements are concentrated, while P and Ti are severely depleted here. Comprehensive study shows both the pre- and syn-orogenic granites are I-type mantle-source granites in the passive continental margin-volcanic island arc tectonic setting; the post-orogenic granites are A2-type crust-source major granites, which were formed from the melting of thinner crust after the closure of oceanic basins and the growth of Arabian-Nubian shield.
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Key words:
- granite /
- geochemistry /
- tectonics /
- Sm-Nd isotope /
- Ethiopia /
- Shire
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图 3 花岗岩K2O-SiO2图解(a)和A/NK-A/CNK图解(b)(据Maniar and Piccoli, 1989)
Fig. 3. K2O-SiO2 diagram (a) and A/NK-A/CNK classification diagram(b) for granitoids
图 6 各期花岗岩(Na2O+K2O)-104×Ga/Al(据Whalen et al., 1987)、Y-SiO2(据Collins et al., 1982)、Nb-Y-3Ga(据Eby, 1992)和Rb/Ba-Rb/Sr(据Sylvester, 1998)判别图
Fig. 6. (Na2O+K2O)-104×Ga/Al、Y-SiO2、Nb-Y-3Ga and Rb/Ba-Rb/Sr discrimination diagrams of granitoids
图 7 花岗岩构造环境判别图解(据Pearce et al., 1984; Pearce, 1996)
syn-COLG.同碰撞花岗岩;VAG.火山弧花岗岩;WPG.板内花岗岩;ORG.洋脊花岗岩
Fig. 7. Tectonic environment discrimination of granitoids
图 8 花岗岩构造环境R1-R2判别(据Batchelor and Bowden, 1985)
①地幔分异;②碰撞前;③碰后抬升;④晚造山;⑤非造山;⑥同碰撞;⑦后造山
Fig. 8. R1-R2 multi-cation diagram for tectonic environment discrimination of granitoids
表 1 花岗岩主量元素(10-2)、稀土微量元素(10-6) 分析结果
Table 1. Major (10-2), REE and other element (10-6) contents of granitoids
岩性样号 造山前花岗岩 造山期花岗岩 造山后花岗岩 QA042 QA044 QA045 QA046 QA047 QA051 QA037 QA038 QA039 QA040 QA041 BB318 BB319 BB320 BB321 BB322 SiO2 71.63 72.11 71.61 71.56 69.65 76.41 61.83 64.73 65.28 66.00 64.20 75.45 74.91 74.66 73.13 72.98 TiO2 0.23 0.23 0.22 0.22 0.24 0.14 0.51 0.49 0.49 0.46 0.49 0.04 0.02 0.08 0.09 0.10 Al2O3 13.05 13.46 13.16 13.71 13.75 12.12 15.07 14.88 14.78 14.75 14.91 13.63 14.20 12.72 14.12 13.96 Tfe 4.23 4.09 3.70 3.85 3.94 2.75 4.41 4.29 4.06 3.28 3.84 1.06 0.89 1.56 1.69 1.87 MnO 0.24 0.21 0.15 0.11 0.12 0.12 0.13 0.15 0.16 0.08 0.10 0.06 0.05 0.07 0.08 0.10 MgO 0.79 0.71 0.75 0.88 0.97 0.39 3.28 3.10 2.44 2.34 2.82 0.14 0.14 0.33 0.28 0.26 CaO 2.86 3.23 2.71 2.95 3.17 1.14 3.70 3.14 2.84 2.58 3.42 0.40 0.26 0.87 1.05 0.98 Na2O 3.92 3.88 3.17 3.44 3.64 4.24 3.97 3.86 3.77 3.78 3.46 4.73 4.94 3.81 4.50 3.93 K2O 0.64 0.83 1.31 1.09 1.26 1.09 3.56 3.82 4.12 4.17 3.15 3.93 3.95 3.78 3.59 4.67 P2O5 0.041 0.049 0.035 0.046 0.048 0.008 0.301 0.187 0.172 0.176 0.198 0.001 0.001 0.010 0.020 0.013 LOI 0.98 0.96 1.33 0.92 1.65 0.97 1.27 1.23 1.77 1.29 1.78 0.63 0.66 0.97 0.92 0.93 Total 98.69 99.84 98.29 98.91 98.58 99.46 98.30 100.15 100.15 99.13 98.62 100.10 100.05 98.88 99.49 99.83 分异指数DI 76.56 75.89 77.23 75.93 75.05 87.50 70.00 72.75 75.67 77.66 70.56 95.40 95.87 91.87 90.86 91.31 A/NK 1.82 1.85 1.99 2.00 1.87 1.49 1.45 1.42 1.38 1.37 1.64 1.13 1.14 1.23 1.25 1.21 A/CNK 1.06 1.02 1.14 1.12 1.05 1.18 0.88 0.92 0.93 0.96 0.97 1.07 1.10 1.06 1.07 1.05 Rb 13.90 15.90 24.40 24.10 24.90 24.50 103.00 117.50 124.50 129.50 91.90 316.00 325.00 235.00 256.00 298.00 Ba 584.0 681.0 1 105.0 1 100.0 1 175.0 670.0 1 355.0 1 340.0 1 340.0 1 345.0 1 445.0 24.8 14.9 186.5 174.0 307.0 Th 2.30 2.11 1.87 2.15 2.08 1.74 8.75 11.90 10.00 13.05 6.97 14.35 29.00 19.65 31.90 29.30 U 1.44 1.27 1.14 1.20 1.15 0.65 3.20 3.13 4.46 4.54 5.11 7.28 16.15 9.37 12.50 12.90 Ta 0.20 0.10 0.20 0.10 0.10 0.20 0.80 0.80 0.50 0.60 0.60 14.20 12.30 3.30 5.30 5.60 Nb 4.00 2.80 3.20 2.70 2.80 3.80 8.70 8.80 8.50 8.40 5.50 35.60 76.10 32.00 51.00 50.70 Pb 23.00 67.00 43.00 5.00 7.00 5.00 40.00 76.00 43.00 25.00 18.00 48.00 56.00 34.00 36.00 41.00 Sr 262.0 238.0 292.0 264.0 245.0 137.0 1 290.0 1 195.0 1 025.0 923.0 1 075.0 20.2 8.3 98.2 116.0 118.5 Zr 229 61 65 70 70 111 171 182 206 252 185 39 157 76 121 117 Hf 5.80 2.00 2.00 2.10 2.20 3.50 5.40 5.70 6.50 6.90 4.90 4.80 14.60 3.90 6.40 6.20 La 10.90 11.50 8.20 15.00 10.80 5.80 35.60 31.40 33.20 19.90 25.80 8.30 4.20 14.20 19.80 17.50 Ce 90.40 21.60 17.10 29.40 20.70 13.90 71.60 62.90 65.40 45.10 53.50 6.20 23.80 30.50 44.90 40.30 Pr 2.85 3.10 2.19 4.03 2.87 2.23 7.72 6.67 6.79 5.81 6.58 3.13 1.46 3.61 5.35 4.72 Nd 12.60 13.50 9.90 17.30 12.80 11.20 31.10 26.90 26.50 21.70 25.90 17.60 7.30 15.90 23.40 20.90 Sm 3.32 3.44 2.68 4.24 3.49 3.52 5.76 4.87 4.70 3.89 5.10 9.92 4.26 5.48 8.22 7.48 Eu 0.71 0.76 0.72 0.81 0.74 0.76 1.21 1.04 0.92 0.84 1.26 0.22 0.05 0.29 0.35 0.36 Gd 3.64 3.59 3.02 4.22 3.79 3.94 3.80 3.15 3.10 2.67 3.74 11.40 4.94 6.41 10.00 9.88 Tb 0.61 0.59 0.50 0.70 0.59 0.72 0.47 0.38 0.37 0.33 0.52 2.08 1.46 1.33 2.07 2.10 Dy 4.19 3.95 3.52 4.51 4.09 5.23 2.48 2.13 2.00 1.70 3.10 12.95 12.85 9.48 14.85 15.15 Ho 0.95 0.89 0.78 1.02 0.95 1.27 0.45 0.40 0.35 0.33 0.63 2.54 3.15 2.10 3.27 3.42 Er 3.11 2.78 2.49 3.17 2.97 4.36 1.21 1.12 0.99 0.86 1.86 8.75 13.15 7.09 11.10 11.75 Tm 0.48 0.45 0.40 0.49 0.47 0.71 0.17 0.14 0.14 0.13 0.31 1.59 2.73 1.17 1.83 1.90 Yb 3.39 3.05 2.72 3.41 3.29 5.04 1.11 1.04 0.90 0.91 2.17 13.00 23.40 8.46 13.10 13.60 Lu 0.59 0.50 0.45 0.57 0.56 0.84 0.16 0.15 0.14 0.15 0.36 2.02 3.75 1.26 2.01 2.08 Y 28.40 25.60 22.40 29.40 27.40 35.50 13.70 12.50 11.00 9.80 18.70 77.60 67.00 77.30 120.50 130.50 ∑REE 166.14 95.30 77.07 118.27 95.51 95.02 176.54 154.79 156.50 114.12 149.53 177.30 173.50 184.58 280.75 281.64 LR/HR 7.12 3.41 2.94 3.91 3.08 1.69 15.53 15.72 17.21 13.73 9.31 0.84 0.63 1.88 1.75 1.52 δEu 0.69 0.72 0.85 0.64 0.68 0.69 0.81 0.83 0.75 0.82 0.92 0.07 0.04 0.17 0.13 0.14 Sm/Nd 0.26 0.25 0.27 0.25 0.27 0.31 0.19 0.18 0.18 0.18 0.20 0.56 0.58 0.34 0.35 0.36 Nb/Ta 63.00 135.00 49.50 173.00 128.00 56.00 38.88 33.63 53.00 36.17 43.17 1.24 0.59 4.82 4.42 3.73 (La/Yb)N 1.91 2.24 1.79 2.61 1.95 0.68 19.04 17.93 21.90 12.98 7.06 0.38 0.11 1.00 0.90 0.76 注: A/NK=Al2O3/(Na2O+K2O), 分子比;A/CNK=Al2O3/(CaO+Na2O+K2O), 分子比;DI=Qz+Or+Ab+Ne+Kp;稀土元素球粒陨石标准化值据Herman(1971). 表 2 造山前和造山后花岗岩Sm-Nd同位素分析结果及特征比值
Table 2. Sm-Nd isotopic analysis and characteristic ratios of of pre-orogen and post-orogen granitoids
样号 Sm(10-6) Nd(10-6) 147Sm/144Nd 143Nd/144Nd±1σ εNd(0) fSm/Nd 造山前花岗岩 QA042 2.756 10.930 0.152 5 0.512 660±0.000 005 0.43 -0.22 QA044 2.882 11.950 0.146 0 0.512 632±0.000 003 -0.12 -0.26 QA045 2.460 9.463 0.157 3 0.512 691±0.000 006 1.03 -0.20 QA046 3.694 15.710 0.142 3 0.512 613±0.000 004 -0.49 -0.28 QA047 2.909 11.420 0.154 0 0.512 673±0.000 005 0.68 -0.22 QA051 2.965 9.815 0.182 7 0.512 830±0.000 006 3.75 -0.07 造山后花岗岩 BB319 3.825 7.102 0.325 9 0.513 301±0.000 008 12.93 0.66 BB320 4.465 15.930 0.169 6 0.512 766±0.000 005 2.50 -0.14 BB321 6.122 21.640 0.171 2 0.512 775±0.000 004 2.67 -0.13 BB322 5.992 21.720 0.166 9 0.512 767±0.000 006 2.52 -0.15 -
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