Ocean Island Rock Assembly and Its Tectonic Significance in Tangga⁃Sumdo Area, Tibet
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摘要: 西藏松多(超)高压变质带对认识古特提斯洋的演化具有十分重要的作用,然而目前关于该带代表的洋盆早期演化记录发现较少,制约了对松多古特提斯洋盆演化的理解. 对唐加地区的洋岛型岩石组合进行了野外地质特征、岩浆岩全岩地球化学和锆石U⁃Pb定年研究.唐加地区洋岛型岩石组合的野外地质特征具有典型的“二元结构”,下层为玄武岩基底,上层为灰岩(大理岩)盖层和塌积砾岩,在上部盖层中还可见变质玄武岩夹层和大理岩与绿片岩互层的现象,与“佛得角型”洋岛类似. 两件玄武岩和一件辉绿玢岩脉的锆石U⁃P定年结果分别为330 Ma、310 Ma和307 Ma,为早石炭世晚期-晚石炭世. 玄武岩和辉绿玢岩均具有较高的TiO2、P2O5和(Na2O+K2O)含量,稀土元素和微量元素组成与OIB类似,显示明显的Nb、Ta富集,在判别图解中落在OIB和板内玄武岩区域,其岩浆源区可能为尖晶石-石榴子石二辉橄榄岩地幔,熔融深度较小.结合前人研究,初步认为松多古特提斯洋早石炭世晚期可能已经发育了初始洋盆,在早石炭世晚期到晚石炭世为慢/超慢速扩张,形成了具有“佛得角型”洋岛特征的唐加地区洋岛型岩石组合.Abstract: The Sumdo (Ultra)high Pressure Metamorphic Zone plays an important role in understanding the evolution of the Paleo Tethys Ocean in Tibet. However, there are few records about the early evolution of the ocean basin on behalf of this zone, which restricts the understanding of the evolution of the Paleo Tethys Ocean. In this paper, we have studied the field geological characteristics, magmatic whole rock geochemistry and zircon U⁃Pb chronology of the ocean island rock assemblages in Tangga area. The field geological characteristics of the ocean island rock assemblages in Tangga area show the typical "double layer structure", with basalt basement in the lower layer, limestone (marble) cover layer and colluvial conglomerate in the upper cover layer, and metamorphic basalt interbedded with marble and tuff in the upper cover layer, which is similar to the "Cape Verde" ocean island. Zircon U⁃Pb geochronology of two basalts and a diabase porphyrite dike is 330 Ma, 310 Ma and 307 Ma, respectively, indicating the least Early Carboniferous to Late Carboniferous. Both basalt and diabase porphyrite have high content of TiO2, P2O5 and (Na2O+K2O). The rare earth elements (REE) and trace element compositions are similar to those of OIB, showing significant enrichment of Nb and Ta, which fall in OIB and intraplate basalt regions in the discriminant diagram. The REE partition curve and the trace element spider diagram are similar to those of OIB, showing obvious Nb and Ta enrichment. The magma source area of basalt and diabase porphyrite may be the spinel⁃garnet peridotite mantle, with a small melting depth, indicating that the overlying lithosphere may be a new⁃born ocean crust with a small thickness. Combined with previous studies, we preliminarily consider that the Sumdo Paleo Tethys Ocean developed an initial ocean basin in the least Early Carboniferous, and it expanded slowly/ultra⁃slowly from the least Early Carboniferous to the Late Carboniferous, forming the ocean island rock assemblages with the characteristics of "Cape Verde" in Tangga area.
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Key words:
- Tibet /
- Tangga /
- paleo tethys ocean /
- oceanic island-type rock assemblage /
- OIB /
- tectonics
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图 1 (a) 青藏高原及其邻区大地构造单元划分简图(据Metcalfe, 2013修改); (b)唐加-松多地区地质简图
据西藏唐加1:5万区域地质调查报告和西藏松多1:5万区域地质调查报告修改,图中的年龄数据引自Yang et al.(2009);Li et al.(2009);Cheng et al.(2012, 2015);Weller et al.(2016);Wang et al.(2018);Zhang et al.(2018);段梦龙等(2019);王斌(2019)
Fig. 1. (a)Tectonic framework of the Tibetan Plateau and adjacent area (modified from Metcalfe, 2013); (b)Geological sketch map of the Tangga⁃Sumdo area
图 4 洋岛岩浆岩锆石稀土元素球粒陨石标准化配分曲线和U⁃Pb年龄谐和图(球粒陨石数据引自Sun and McDonough, 1989)
Fig. 4. Zircon Chondrite⁃normalized REE patterns diagram and U⁃Pb zircon Concordia of representative zircon grains from magmatites of ocean island(Chondrite data from Sun and McDonough, 1989)
图 5 西藏唐加地区洋岛岩浆岩Nb/Y⁃Zr/TiO2×0.000 1图解(据Winchester and Floyd, 1976)
Fig. 5. Nb/Y vs. Zr/TiO2×0.000 1 plot (after Winchester and Floyd, 1976) for magmatites of ocean island in Tangga area, Tibet
图 6 西藏唐加地区洋岛岩浆岩球粒陨石标准化稀土元素配分图和原始地幔标准化微量元素蛛网图
球粒陨石、原始地幔和OIB/E⁃MORB/N⁃MORB数据引自Sun and McDonough(1989)
Fig. 6. Chondrite⁃normalized REE patterns, and primitive⁃mantle⁃normalized spider diagrams for magmatites of ocean island in Tanggaarea, Tibet
图 7 西藏唐加地区洋岛岩浆岩Nb×2⁃Zr/4⁃Y图解(Meschede, 1986)和Ti/100⁃Zr⁃Y×3图解(Pearce and Norry, 1979)
a. Nb×2⁃Zr/4⁃Y判别图;板内碱性玄武岩落在AⅠ和AⅡ区,板内拉斑玄武岩落在AⅡ和C区,P⁃MORB落在B区;N⁃MORB落在D区,火山弧玄武岩落在C和D区;b. Ti/100⁃Zr⁃Y×3判别图,板内玄武岩落在D区,洋中脊玄武岩落在B区,低钾拉斑玄武岩落在A和B区,钙碱性玄武岩落在C和B区
Fig. 7. Nb×2 vs. Zr/4⁃Y plot (Meschede, 1986) and Ti/100 vs. Zr⁃Y×3 plot (Pearce and Norry, 1979) for magmatites of ocean island in Tangga area, Tibet
图 8 (a) (La/Nb)PM⁃(Th/Nb)PM图解; (b) La/Sm⁃Sm/Yb图解(Aldanmaz et al., 2000); (c)Th/Yb⁃Nb/Yb图解(Pearce, 2008); (d)TiO2/Yb⁃Nb/Yb图解(Pearce, 2008)
温木朗洋岛岩浆岩数据来自Wang et al.(2019)
Fig. 8. (a) (La/Nb)PM vs. (Th/Nb)PM plot; (b) La/Sm vs. Sm/Yb plot(Aldanmaz et al., 2000); (c) Th/Yb vs. Nb/Yb plot(Pearce, 2008); (d) TiO2/Yb vs. Nb/Yb plot(Pearce, 2008)
表 1 唐加地区洋岛岩浆岩锆石U-Pb同位素测试结果
Table 1. Zircon U⁃Pb isotope test results of ocean island magmatites in Tangga area
点号 Th
(10-6)U
(10-6)Th/U 同位素比值 同位素年龄(Ma) 207Pb/235U 1σ 206Pb/238U 1σ 207Pb/235U 1σ 206Pb/238U 1σ S19T42-01 27 68 0.40 0.324 0 0.020 4 0.048 8 0.000 9 285 16 307 5 S19T42-02 1 284 675 1.90 0.353 8 0.007 1 0.050 9 0.000 7 308 5 320 4 S19T42-03 94 256 0.37 4.833 7 0.079 1 0.333 4 0.003 8 1 791 14 1 855 18 S19T42-04 148 226 0.65 0.352 9 0.013 6 0.049 6 0.000 7 307 10 312 4 S19T42-05 51 771 0.07 5.015 4 0.068 3 0.336 8 0.003 2 1 822 12 1 871 15 S19T42-06 239 255 0.93 1.635 3 0.029 0 0.165 5 0.001 2 984 11 988 6 S19T42-07 99 393 0.25 5.825 5 0.096 7 0.340 7 0.003 6 1 950 14 1 890 17 S19T42-08 769 933 0.82 0.353 1 0.007 3 0.049 3 0.000 6 307 6 310 4 S19T42-09 116 276 0.42 10.845 2 0.162 9 0.518 7 0.005 0 2 510 14 2 694 21 S19T42-10 299 532 0.56 4.642 1 0.078 8 0.307 7 0.003 8 1 757 14 1 729 19 S19T42-11 238 359 0.66 4.478 3 0.079 3 0.325 4 0.004 2 1 727 15 1 816 20 S19T42-12 122 179 0.68 0.360 6 0.015 7 0.047 7 0.000 7 313 12 300 4 S19T42-13 78 397 0.20 1.725 0 0.027 0 0.176 5 0.001 7 1 018 10 1 048 9 S19T42-14 116 167 0.70 0.344 4 0.014 6 0.049 4 0.000 6 300 11 311 4 S19T42-15 152 1 705 0.09 5.002 6 0.063 9 0.339 3 0.002 7 1 820 11 1 883 13 S19T43-01 875 650 1.35 0.363 5 0.007 8 0.049 7 0.000 5 315 6 313 3 S19T43-02 134 78 1.72 2.180 2 0.046 8 0.203 3 0.002 0 1 175 15 1 193 11 S19T43-03 95 229 0.42 3.631 4 0.053 5 0.274 7 0.002 5 1 556 12 1 564 13 S19T43-04 66 42 1.57 2.032 9 0.060 4 0.193 1 0.002 3 1 127 20 1 138 13 S19T43-05 1 492 720 2.07 0.353 3 0.006 9 0.047 8 0.000 3 307 5 301 2 S19T43-06 449 357 1.26 0.367 6 0.009 4 0.048 3 0.000 5 318 7 304 3 S19T43-07 689 522 1.32 0.365 4 0.006 8 0.048 6 0.000 4 316 5 306 2 S19T43-08 100 100 1.00 0.349 1 0.017 8 0.048 9 0.000 7 304 13 308 4 S19T43-09 155 276 0.56 3.663 5 0.054 6 0.276 0 0.002 2 1 563 12 1 571 11 S19T43-10 342 303 1.13 0.343 9 0.012 2 0.048 2 0.000 4 300 9 304 3 S19T43-11 41 1 275 0.03 7.306 0 0.108 2 0.370 5 0.004 3 2 150 13 2 032 20 S19T43-12 84 237 0.36 5.185 5 0.074 5 0.334 2 0.002 5 1 850 12 1 859 12 S19T43-13 757 502 1.51 0.353 7 0.008 6 0.049 9 0.000 5 307 6 314 3 S19T43-14 412 280 1.47 0.358 4 0.010 9 0.049 2 0.000 5 311 8 310 3 S19T43-15 358 307 1.17 0.353 9 0.011 3 0.049 9 0.000 5 308 9 314 3 S20T12-01 66 200 0.33 0.930 8 0.032 6 0.118 6 0.003 2 668 17 723 19 S20T12-02 151 247 0.61 4.703 5 0.117 8 0.347 8 0.009 2 1 768 21 1 924 44 S20T12-03 64 242 0.26 1.160 8 0.035 3 0.140 9 0.003 8 782 17 850 21 S20T12-04 162 181 0.90 0.376 1 0.033 7 0.051 7 0.001 9 324 25 325 12 S20T12-05 261 265 0.98 4.216 5 0.107 3 0.313 0 0.008 2 1 677 21 1 756 40 S20T12-06 167 726 0.23 1.138 8 0.031 2 0.132 5 0.003 5 772 15 802 20 S20T12-07 290 650 0.45 8.341 1 0.203 9 0.395 4 0.010 2 2 269 22 2 148 47 S20T12-08 78 249 0.31 0.380 1 0.031 6 0.052 3 0.001 8 327 23 328 11 S20T12-09 82 344 0.24 0.377 0 0.027 9 0.052 0 0.001 7 325 21 327 10 S20T12-10 37 1 184 0.03 4.501 7 0.112 2 0.289 8 0.007 5 1 731 21 1 640 37 S20T12-11 444 717 0.62 0.380 6 0.033 3 0.052 9 0.001 9 327 24 332 11 S20T12-12 161 195 0.83 0.374 1 0.020 9 0.052 1 0.001 5 323 15 327 9 S20T12-13 137 275 0.50 0.668 3 0.026 7 0.083 0 0.002 3 520 16 514 13 S20T12-14 116 228 0.51 4.374 6 0.125 0 0.311 0 0.008 1 1 708 24 1 745 40 S20T12-15 60 286 0.21 10.529 8 0.282 5 0.435 5 0.011 3 2 483 25 2 331 51 S20T12-16 228 280 0.82 0.372 7 0.064 9 0.053 7 0.003 0 322 48 337 18 S20T12-17 226 518 0.44 1.406 2 0.040 8 0.142 0 0.003 7 892 17 856 21 S20T12-18 65 297 0.22 3.839 2 0.109 0 0.266 9 0.006 9 1 601 23 1 525 35 S20T12-19 172 457 0.38 0.375 8 0.068 4 0.055 5 0.002 8 324 50 348 17 S20T12-20 33 97 0.34 1.460 4 0.058 8 0.142 7 0.003 9 914 24 860 22 表 2 唐加地区洋岛岩浆岩全岩地球化学测试结果
Table 2. Whole⁃rock geochemical test results of ocean island magmatites in Tangga area
样品 S20T12H1 S20T12H2 S20T12H3 S20T12H4 S19T43H1 S19T43H2 S19T43H3 S19T43H4 S19T43H5 S19T42H1 S19T42H2 S19T42H3 S19T42H4 SiO2 47.18 57.86 55.30 55.41 48.80 48.48 51.74 46.86 46.61 55.36 50.65 55.55 50.96 Al2O3 15.98 15.44 15.49 15.53 14.23 14.59 14.00 14.29 13.97 15.44 15.76 15.58 15.91 FeOT 12.38 9.01 9.29 9.31 20.31 20.00 18.21 22.19 21.63 11.18 11.73 10.28 10.72 CaO 6.39 3.03 3.78 3.80 4.93 3.73 3.48 4.31 3.58 3.79 5.10 3.79 5.13 MgO 8.91 2.47 3.96 3.97 0.96 1.14 1.14 0.92 1.73 5.04 6.31 5.15 6.50 K2O 0.87 3.97 3.26 3.28 1.05 1.29 0.99 1.51 1.03 0.64 0.23 0.64 0.24 Na2O 2.43 2.58 2.50 2.51 2.18 2.80 3.27 1.93 2.90 2.40 2.66 2.36 2.62 TiO2 1.96 1.85 1.86 1.88 4.63 4.36 4.37 4.49 4.73 2.45 2.18 2.36 2.12 P2O5 0.25 0.36 0.33 0.33 0.42 0.40 0.41 0.41 0.40 0.29 0.30 0.29 0.30 MnO 0.17 0.13 0.14 0.14 0.14 0.12 0.13 0.12 0.16 0.19 0.19 0.19 0.18 LOI 3.33 3.16 3.69 3.69 2.33 2.67 2.10 2.82 2.91 2.96 4.50 3.67 5.30 总合 99.86 99.88 99.61 99.85 99.97 99.59 99.84 99.84 99.66 99.75 99.62 99.86 99.96 Cr 236.7 765.3 70.1 250.6 2.51 2.03 3.85 2.94 3.17 153.1 178.1 212.4 271.0 Ni 81.4 219.6 32.4 83.5 42.4 34.6 37.5 29.0 43.0 78.5 86.5 115.6 133.1 Rb 33.36 23.34 52.55 34.82 5.73 5.78 23.07 13.00 7.72 19.22 20.18 9.11 10.17 Sr 186.4 230.0 170.6 194.6 291.2 258.8 320.1 324.4 283.4 181.8 194.5 272.9 298.2 Y 26.77 24.54 19.86 26.65 30.60 25.82 36.35 27.51 28.07 28.20 30.88 35.32 39.10 Zr 331.2 201.1 302.0 348.6 211.7 210.4 211.1 206.5 193.6 268.6 284.2 206.6 228.3 Nb 56.83 27.98 50.44 57.28 27.87 26.12 26.85 26.89 25.52 47.89 52.50 30.51 34.64 Ba 598.9 259.4 773.4 633.8 362.6 105.6 345.0 156.6 308.3 121.7 131.9 81.5 88.8 La 26.68 29.11 22.82 26.95 16.78 16.47 30.71 17.26 19.88 25.82 30.61 22.98 26.28 Ce 61.03 63.48 51.03 61.92 26.70 27.41 49.31 32.82 34.40 58.78 65.11 51.67 57.00 Pr 7.61 7.56 6.43 7.73 5.68 5.61 8.33 5.78 6.17 6.96 8.00 6.35 7.14 Nd 28.97 30.91 24.73 30.04 25.94 25.34 36.41 26.56 27.36 27.51 30.83 26.10 29.60 Sm 6.32 6.37 5.39 6.51 6.59 6.30 8.36 6.83 6.66 5.97 6.50 6.09 6.85 Eu 1.92 1.99 1.57 1.94 2.31 2.26 2.75 2.60 2.51 1.62 1.80 1.84 2.08 Gd 6.04 6.29 4.92 6.17 7.58 6.61 9.05 7.57 6.89 6.01 6.73 6.88 7.59 Tb 0.98 1.01 0.79 0.96 1.16 1.04 1.33 1.12 1.05 0.94 1.05 1.10 1.24 Dy 5.63 5.92 4.53 5.55 6.64 6.08 7.42 6.42 6.09 5.51 5.99 6.64 7.46 Ho 1.14 1.23 0.93 1.14 1.35 1.21 1.43 1.27 1.21 1.12 1.23 1.39 1.57 Er 3.22 3.38 2.62 3.23 3.64 3.34 3.87 3.42 3.34 3.17 3.45 3.95 4.37 Tm 0.47 0.48 0.39 0.48 0.50 0.45 0.52 0.47 0.47 0.45 0.48 0.57 0.64 Yb 2.96 2.96 2.49 2.99 3.04 2.84 3.09 2.83 2.75 2.78 3.01 3.56 3.90 Lu 0.47 0.45 0.38 0.46 0.44 0.41 0.45 0.41 0.41 0.41 0.45 0.53 0.59 Hf 7.45 5.49 8.19 7.67 6.25 6.33 6.24 6.18 5.99 7.58 6.48 5.82 5.23 Ta 3.74 2.03 3.51 3.71 1.71 1.61 1.66 1.65 1.57 2.87 3.48 1.90 2.38 Pb 4.49 2.81 5.19 4.82 19.59 19.55 17.61 21.07 18.78 4.89 5.15 3.66 3.99 Th 9.22 1.68 10.55 9.39 1.28 1.21 1.91 1.39 1.36 4.50 5.36 2.90 3.25 U 1.80 0.98 2.01 1.86 0.41 0.42 0.48 0.42 0.44 1.29 1.36 0.68 0.75 Ti 11 766 11 112 11 172 11 296 27 752 26 134 26 194 26 913 28 352 14 685 13 067 14 120 12 688 注:主量元素含量单位为%,微量和稀土元素为10-6. -
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