The discovery and limplications for the India-Eurasia Plate collision of the Nianbo Formation adakitic rocks from Yangyi basin in Middle Gangdise Belt
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摘要:
冈底斯中段羊易盆地发现始新世年波组埃达克质火山岩,岩石组合为英安岩、流纹岩、火山熔岩等。为确定其形成时代、成因及其地质构造意义,对其进行了岩相学、LA-ICP-MS锆石U-Pb测年及地球化学研究。结果表明,用于定年的锆石为岩浆成因,其206Pb/238U年龄加权平均值为55±0.8 Ma,喷出时代为始新世。地球化学特征显示,该套火山岩属弱过铝质钙碱性岩石系列,具富Si(SiO2=67.69%~71.93%)、高Al(Al2O3=13.13%~16.16%)、低Mg(MgO=0.52%~0.76%)、高Sr(Sr=345×10-6~875×10-6)、低Y(Y=4.40×10-6~11.30×10-6)及Yb(Yb=0.81×10-6~1.49×10-6)特征,稀土元素总量在116.86×10-6~352×10-6之间,轻、重稀土元素分馏较明显,(La/Yb)N=23.31~43.66,Eu异常不明显(δEu=0.8~1.15)。在原始地幔标准化蛛网图中,富集大离子亲石元素Rb、U、Th,亏损高场强元素Nb、P、Ti,为C型埃达克质火山岩地球化学特征。综合区域资料,羊易盆地年波组埃达克岩是加厚下地壳部分熔融形成的,说明在55±0.8 Ma左右拉萨地体的南缘部分地区已增厚到50 km左右。
Abstract:The Eocene Nianbo Formation Adakitic volcanic rocks were discovered for the first time from the Yangyi basin in the Middle Gangdise Belt.This set of adakitic volcanic rocks consists of dacite, rhyolite, and volcanic lava.In order to determine their formation age, origin and geological tectonic significance, the authors conducted LA-ICP-MS zircon U-Pb geochronologic, petrographic and geochemical studies.The results show that the zircons used for dating are of magmatic origin, and the zircon 206Pb/238U dating yielded 55±0.8 Ma, which suggests that the volcano erupted in Eocene.Geochemical characteristics show that the volcanic rocks belong to a series of weakly peraluminous calc-alkaline rocks, and have rich Si(SiO2 of 67.69%~71.93%), high Al(Al2O3 of 13.13% ~ 16.16%), low Mg(MgO of 0.52%~ 0.76%), higher Sr(Sr of 345×10-6~875×10-6), and lower Y(Y of 4.40×10-6 ~11.30×10-6)and Yb(Yb of 0.81×10-6 ~1.49×10-6).The total rare earth element values vary in the range of 116.86×10-6~352×10-6, exhibiting obvious fractionation between light and heavy REE((La/Yb)N=23.31~43.66), with no obvious δEu anomaly(0.8~1.15).Enrichment of LILE(Rb, U and Th)and depletion of HFSE(Nb, P and Ti)are displayed in the primitive mantle normalized multi-element variation diagram, indicating geochemical characteristics of C-type adakitic volcanic rocks.Based on regional information, the authors hold that adakitic rocks of Nianbo Formation in Yangyi basin were formed by partial melting of thickened lower crust, indicating that some areas of the southern margin of Lhasa were thickened to about 50 km around 55±0.8 Ma.
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Key words:
- Nianbo Formation /
- C-type adakitic rocks /
- zircon U-Pb age /
- Middle Gangdise Belt /
- petrology
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表 1 年波组火山岩LA-ICP-MS锆石U-Th-Pb测年结果
Table 1. LA-ICP-MS zircon U-Th-Pb isotopic compositions of Nianbo Formation volcanic rocks
点号 含量/10-6 Th/U 同位素比值 年龄/Ma Pb Th U 207Pb/206Pb±1σ 207Pb/235U±1σ 206Pb/238U±1σ 206Pb/238U±1σ 01 5 273 470 0.6 0.0514±0.0030 0.0602±0.0035 0.0085±0.0002 54.7±1.1 02 1 83 93 0.9 0.0568±0.0297 0.0543±0.0278 0.0082±0.0007 52.7±4.703 4 328 391 0.8 0.0530±0.0034 0.0594±0.0040 0.0082±0.0002 52.5±1.2 04 4 323 320 1.0 0.0484±0.0044 0.0585±0.0055 0.0088±0.0002 56.6±1.4 05 3 211 219 1.0 0.0467±0.0060 0.0526±0.0067 0.0083±0.0003 53.3±1.7 06 2 138 154 0.9 0.0437±0.0074 0.0497±0.0078 0.0085±0.0003 54.3±2.0 07 4 278 420 0.7 0.0395±0.0022 0.0456±0.0029 0.0085±0.0002 54.2±1.6 08 2 155 165 0.9 0.0739±0.0066 0.0829±0.0069 0.0085±0.0003 54.6±1.909 7 619 582 1.1 0.0523±0.0019 0.0623±0.0028 0.0087±0.0003 55.7±1.6 10 8 279 182 1.5 0.4249±0.0359 1.4128±0.1824 0.0203±0.0017 129.5±10.611 4 210 323 0.7 0.0492±0.0037 0.0688±0.0058 0.0101±0.0003 65.0±1.812 3 164 268 0.6 0.0436±0.0046 0.0502±0.0056 0.0085±0.0004 54.6±2.5 13 5 423 376 1.1 0.0533±0.0031 0.0622±0.0041 0.0085±0.0002 54.2±1.3 14 3 171 320 0.5 0.0475±0.0042 0.0536±0.0043 0.0084±0.0002 53.8±1.4 15 2 195 203 1.0 0.0669±0.0054 0.0777±0.0062 0.0084±0.0003 54.2±1.616 6 250 521 0.5 0.0476±0.0024 0.0565±0.0030 0.0087±0.0002 55.5±1.2 17 1 87 129 0.7 0.0019±0.0063 0.0016±0.0073 0.0085±0.0003 54.6±1.718 2 120 208 0.6 0.0458±0.0065 0.0550±0.0077 0.0087±0.0003 55.5±1.7 19 4 122 217 0.6 0.1636±0.0130 0.2367±0.0206 0.0104±0.0002 66.9±1.520 3 134 221 0.6 0.0518±0.0048 0.0599±0.0059 0.0085±0.0003 54.4±1.6 注:表中双删除线为谐和度低于90%且不参与年龄计算的数据 表 2 年波组火山岩主量、微量和稀土元素分析结果
Table 2. Major, trace elements and REE compositions of the Nianbo Formation volcanic rocks
样品编号 YQ1 YQ2 YQ3 YQ4 YQ5 YQ6 YQ7 SiO2 68.69 71.93 70.44 69.04 70.15 67.78 67.69 TiO2 0.43 0.28 0.38 0.45 0.43 0.29 0.32 Al2O3 15.56 13.13 14.29 14.8 14.65 14.67 16.16 Fe2O3 1.83 1.67 1.54 2.49 1.95 0.66 1.31 FeO 0.76 0.38 0.64 0.35 0.57 1.24 0.79 MnO 0.037 0.017 0.041 0.054 0.056 0.038 0.029 MgO 0.76 0.52 0.65 0.68 0.53 0.57 0.54 CaO 1.56 1.06 1.28 1.48 1.53 2.3 2.41 Na2O 2.97 2.65 3.04 3.87 3.76 3.06 3.14 K2O 4.34 5.05 5.25 5.12 4.99 3.49 4.07 P2O5 0.21 0.18 0.25 0.29 0.29 0.13 0.11 CO2 0.54 0.11 0.14 0.53 0.63 0.35 0.72 H2O- 1.46 2.18 1.58 0.76 0.57 2.39 1.38 H2O+ 1.59 1.59 1.34 0.71 0.53 2.76 1.89 烧失量 3.22 3.62 2.68 1.6 1.42 6.28 3.81 A/NK 1.62 1.34 1.34 1.24 1.26 1.67 1.69 A/CNK 1.25 1.12 1.1 1.01 1.02 1.13 1.16 DI 83.8 89.5 88.2 87.7 88.1 81.8 81.4 SI 7.18 5.12 5.83 5.47 4.55 6.37 5.52 AR 2.06 2.19 2.28 2.81 2.74 2.13 2.02 σ 2.04 2.02 2.48 3.08 2.8 1.67 2.06 La 56.8 57.3 70.2 84.6 89.2 27.3 29.9 Ce 105 104 125 161 154 52.9 57.7 Pr 11 11.1 13.3 16.7 17.8 5.91 6.38 Nd 38.4 38.4 46.3 59.6 65.4 21.2 23.5 Sm 5.34 5.33 6.46 8.33 9.2 3.19 3.57 Eu 1.46 1.24 1.69 2.15 2.24 1.05 0.94 Gd 3.79 3.87 4.53 5.86 6.51 2.18 2.55 Tb 0.53 0.53 0.62 0.79 0.88 0.31 0.37 Dy 1.73 1.7 1.96 2.47 2.82 1.02 1.38 Ho 0.32 0.32 0.36 0.43 0.49 0.21 0.28 Er 0.91 0.95 1.1 1.33 1.51 0.52 0.73 Tm 0.15 0.16 0.17 0.19 0.2 0.11 0.14 Yb 1.09 1.16 1.3 1.39 1.49 0.81 0.92 Lu 0.2 0.21 0.24 0.26 0.27 0.16 0.15 Y 6.72 7.4 8.02 9.36 11.3 4.4 6.07 ΣREE 226.72 226.27 273.23 345.09 352 116.86 128.51 LREE 218 217.37 262.95 332.38 337.84 111.55 121.99 HREE 8.72 8.9 10.28 12.71 14.16 5.31 6.52 LREE/HREE 24.99 24.43 25.58 26.14 23.85 21.01 18.71 (La/Yb)N 37.38 35.43 38.73 43.66 42.94 24.24 23.31 δEu 0.94 0.8 0.91 0.89 0.84 1.15 0.9 δCe 0.97 0.95 0.94 0.99 0.89 0.97 0.98 Cr 13.9 9.89 10.6 15.3 15.2 17.5 19.6 Hf 7.91 5.29 8.3 10.6 9.17 5.39 3.93 Sr 467 345 517 629 624 875 701 V 51.2 38.9 48.2 51.9 48.2 42.5 49.6 Zr 182 160 236 266 268 161 162 Cu 93.4 20.6 12.2 22.1 18.7 13.5 15.4 Zn 97.5 47.2 53.4 66.4 58.3 41.5 53.4 Rb 235 401 364 314 297 130 182 Pb 68.2 90.7 98.3 100 99.2 62 61.4 Th 46.9 67 73.6 74.1 75 27 26.1 U 9.29 10.5 12.5 12.4 11.6 3.77 2.66 Li 35.9 32.6 26.1 17.5 32.6 4.48 25.5 Nb 13.5 19.5 19.3 19.2 18.5 7.07 7.08 Ta 1.44 1.4 1.44 1.45 1.46 0.69 0.68 K 35998.76 41901.35 43597.9 42544.07 28977.83 33814.92 35998.76 P 927.84 765.24 1078.99 1273.87 564.68 498.3 927.84 Ti 2597.89 1664.82 2306.04 2681.76 1712.06 1938.64 2597.89 注:主量元素含量单位为%,微量和稀土元素含量单位为10-6 -
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