Zircon U-Pb ages, petrogenesis and tectonic setting of basic rocks in Gudui area, southern Tibet
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摘要:
藏南古堆地区处于喜马拉雅带北东段, 雅鲁藏布江缝合带南部, 措美大火成岩省东南部, 区内基性岩分布较广泛, 主要呈脉状侵位于侏罗纪—三叠纪地层中。为探讨古堆地区基性岩的成因及其构造动力学背景, 对区内基性岩进行了锆石U-Pb年龄、岩石地球化学等研究。结果表明, SHRIMP锆石206Pb/238U同位素测年得出辉绿岩年龄加权平均值为137.7±0.9 Ma;辉长岩年龄加权平均值为133.8±0.8 Ma, 其形成时代为早白垩世, 与措美大火成岩省OIB型基性岩形成时代相近(130~136 Ma)。岩石地球化学证据表明, 基性岩具有高TiO2和P2O5特点, 不具有Nb-Ta槽特征, 微量元素比值(Ce/Zr、Zr/Nb、Zr/Y、Th/Yb)及稀土元素配分模式与板内洋岛玄武岩(OIB)相似。综合研究表明, 古堆地区基性岩源于富集地幔, 并且基性岩浆在上侵过程中未受到地壳混染, 其形成于板内构造环境, 可能是Kerguelen地幔柱早期活动的产物。对古堆地区基性岩的研究, 为近一步认识藏南地区早白垩世基性岩的成因及其大地构造背景提供了地质年代学和岩石地球化学方面的证据。
Abstract:The Gudui area in southern Tibet is located in the northeast section of the Himalaya belt, the south of the Yarlung Zangbo River suture zone, and the southeast of the cuomei large igneous province.The basic rocks in the area are widely distributed, mainly located in the Jurassic-Triassic strata.In order to explore the origin and structural dynamic background of basic rocks in Gudui area, zircon U-Pb chronology and rock geochemistry of basic rocks in the area are studied in this paper.The results show that the SHRIMP zircon 206Pb/238U isotopic dating shows that the weighted average age of diabase is 137.7±0.9 Ma, and the weighted average age of gabbro is 133.8±0.8 Ma, its formation age is early Cretaceous, which is similar to that of OIB basic rocks in Cuomei igneous Province(130~136 Ma).Rock geochemical evidence shows that the basic rocks have high TiO2 and P2O5 characteristics, not Nb-Ta groove characteristics, and the trace element ratios(Ce/Zr, Zr/Nb, Zr/Y, Th/Yb)and rare earth distribution patterns are similar to those of intraplate ocean island basalt(OIB).The comprehensive study shows that the basic rocks in gudui area originated from the enriched mantle, and the basic magma was not contaminated by the crust during the upwelling process.It was formed in the intraplate tectonic environment and may be the product of the early activities of the Kerguelen mantle plume.Through this study of basic rocks in Gudui area, geological chronology and rock geochemistry evidence are provided for further understanding the genesis and tectonic background of Early Cretaceous basic rocks in the southern Tibet.
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表 1 古堆地区辉绿岩和辉长岩SHRIMP锆石U-Th-Pb分析结果
Table 1. SHRIMP zircon U-Th-Pb analysis results of diabase and gabbro in Gudui area
测试点号 206Pbc/% 含量/10-6 232Th/238U 207Pb*/206Pb* ±% 207Pb*/235U ±% 206Pb*/238U ±% 206Pb/238U
年龄/MaU Th 206Pb* 辉绿岩 1 1.21 549 1156 10.5 2.17 0.0473 13.6 0.14 13.6 0.0220 1.3 140.3 1.8 2 0.68 933 1914 17.3 2.12 0.0533 9.7 0.16 9.7 0.0214 1.0 136.5 1.4 3 1.71 585 1118 11.1 1.97 0.0447 16.9 0.13 17.0 0.0216 1.3 137.9 1.8 4 0.49 589 1061 11.1 1.86 0.0501 4.7 0.15 4.8 0.0219 0.9 139.7 1.2 5 0.83 835 2244 15.4 2.78 0.0457 6.8 0.13 6.8 0.0213 0.8 136.1 1.1 6 1.01 483 903 9.1 1.93 0.0479 6.3 0.14 6.4 0.0218 1.0 138.8 1.3 7 0.54 397 684 7.3 1.78 0.0488 8.6 0.14 8.8 0.0214 1.4 136.4 1.9 8 0.35 774 1972 14.2 2.63 0.0482 3.7 0.14 3.8 0.0213 0.8 136.0 1.0 9 0.64 427 799 8.1 1.93 0.0518 9.0 0.16 9.1 0.0220 1.1 140.3 1.6 10 0.30 1282 3840 23.9 3.09 0.0487 2.8 0.15 3.2 0.0216 1.5 137.9 2.1 辉长岩 1 2.28 210 92 3.8 0.44 0.0449 18.1 0.13 18.2 0.0205 1.7 130.8 2.2 2 0.00 1302 1962 23.4 1.51 0.0497 1.8 0.14 1.9 0.0209 0.5 133.6 0.7 3 1.74 1043 2010 19.1 1.93 0.0534 8.2 0.15 8.2 0.0209 0.8 133.6 1.1 4 2.04 991 2028 18.3 2.11 0.0485 9.5 0.14 9.6 0.0211 0.9 134.5 1.2 5 1.52 408 810 7.5 2.05 0.0439 17.0 0.13 17.1 0.0210 1.4 134.3 1.8 6 1.13 1020 2629 18.7 2.66 0.0427 10.2 0.12 10.2 0.0211 1.2 134.4 1.7 7 0.68 628 1343 11.5 2.21 0.0471 7.5 0.14 7.6 0.0212 1.2 135.5 1.6 8 1.31 818 2446 15.1 3.09 0.0490 8.2 0.14 8.3 0.0212 0.9 135.2 1.2 9 0.52 718 1567 12.8 2.26 0.0519 3.9 0.15 4.1 0.0207 1.0 131.8 1.3 
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表 2 古堆地区基性岩主量、微量和稀土元素分析结果
Table 2. Analysis results of major, trace and rare earth elements of basic rocks in Gudui area
元素 辉长岩 辉绿岩 元素 辉长岩 辉绿岩 PM4QY002 11PM101 11PM102 11PM301 11PM302 11Q6 11Q8 PM4QY002 11PM101 11PM102 11PM301 11PM302 11Q6 11Q8 SiO2 52.14 49.85 48.76 49.18 48.14 51.70 50.61 Lu 0.69 0.48 0.31 0.49 0.54 0.74 0.72 TiO2 3.64 2.93 2.84 4.13 2.51 2.42 2.22 Y 48.98 34.71 35.19 36.53 42.41 55.17 49.69 Al2O3 13.96 14.68 13.86 14.36 13.88 14.36 13.34 ΣREE 325.49 128.53 265.92 184.2 235.31 220.31 191.63 Fe2O3 3.02 3.24 2.70 2.32 2.92 2.12 1.86 LREE 287.48 103.89 235.78 156.72 203.2 184.7 160.01 MnO 0.15 0.17 0.16 0.15 0.17 0.16 0.15 HREE 38.01 24.64 30.14 27.48 32.11 35.61 31.62 MgO 3.54 3.53 6.36 4.48 3.63 5.33 5.42 LREE/HREE 7.56 4.22 7.82 5.7 6.33 5.19 5.06 CaO 6.52 7.51 8.90 6.79 6.66 7.65 8.50 Na2O 3.84 3.82 3.07 4.39 2.83 2.74 2.46 (La/Yb)N 8.73 2.37 12.19 5.1 7.12 5.3 5.09 K2O 1.63 1.37 0.12 1.15 0.91 1.44 1.31 FeO 7.48 8.59 8.56 8.91 8.14 7.95 7.34 δEu 1.01 1.02 1.4 1.12 0.92 1.34 1.36 P2O5 0.66 0.53 0.34 0.47 0.51 0.29 0.26 Ba 433 501.5 155.7 205.8 201.8 355.4 279.4 Mg# 46.00 42.52 57.22 47.51 44.53 54.69 57.07 Co 40.8 24.2 45.5 52.2 36.5 43.83 41.65 烧失量 2.89 2.69 3.19 2.52 9.66 2.02 2.49 Cu 27 11.4 20.1 18.8 6 — — 总计 99.47 98.91 98.86 98.85 98.96 98.18 97.96 Ni 17.6 18.1 49.6 18.7 2 20.33 24.01 σ 3.27 3.93 1.77 4.97 2.72 2.01 1.87 Sc 19.6 17.6 26.8 23.6 16.7 26.3 25.4 Na2O+K2O 5.47 5.19 3.19 5.54 3.74 4.18 3.77 Sr 441.6 128.9 121.7 183.8 127.3 394.3 412.3 Zn 119.8 99.7 100.9 103 110.5 100.5 95.43 K2O/Na2O 0.42 0.36 0.04 0.26 0.32 0.53 0.53 Cr 12.2 37.5 132.6 18.8 11.8 131.7 115.8 Rb 22.2 125.3 5.0 20.4 26.1 50.3 49.92 A/CNK 0.70 0.68 0.65 0.69 0.72 0.72 0.73 Nb 61.7 27.15 18.19 41.99 29.24 26.44 28.56 La 57.72 10.69 38.74 24.02 37.03 37.34 32.84 Ta 3.54 1.69 0.97 2.54 1.45 0.72 0.63 Ce 119.2 44.51 94.78 65.64 85.91 78.77 68.17 W 0.68 2.64 0.91 0.62 0.41 2.11 1.59 Pr 15.94 6.37 13.7 9.15 11.56 10.23 8.78 Pb 13.8 26 5.7 13.7 20.9 21.65 24.77 Nd 75.72 32.31 69.13 45.03 54.38 43.64 37.5 Th 3.64 23.55 1.4 3 4.87 5.8 5.61 Sm 14.45 7.58 13.58 9.56 11.13 10.34 8.86 U 1.06 2.43 0.28 0.69 0.43 0.61 0.81 Eu 4.45 2.43 5.85 3.32 3.19 4.38 3.86 Sn 8.3 3.74 3.92 2.35 5 2.82 2.07 Gd 11.96 6.68 11.28 8.25 9.64 9.35 8.17 Zr 914.8 387.8 332.1 598.3 175.3 273.6 216.3 Tb 2.08 1.37 1.9 1.53 1.83 1.72 1.49 Li 17.2 39.8 35.7 31.5 74.1 36.92 25.66 Dy 10.39 7.13 8.67 7.69 9.14 10.39 9.08 Hf 12.8 10.2 5.4 6.8 7.3 7.21 10.82 Ho 2.04 1.47 1.59 1.57 1.83 2.05 1.83 V 237.8 139.7 271.4 312 256.4 273.2 257.3 Er 5.38 3.76 3.7 4.02 4.76 5.45 4.91 Au 0.1 0 0 0 0.07 0.82 0.88 Tm 0.73 0.52 0.41 0.55 0.64 0.86 0.79 Sb 1.12 0.72 0 0.57 0.54 0 0 Yb 4.74 3.23 2.28 3.38 3.73 5.05 4.63 注：主量元素含量单位为%，微量和稀土元素含量单位为10-6，其中Au为10-9
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