Geochemical characteristics and zircon U-Pb geochronology of the Baijiafen augamite granite in Anshan of the North China platform and its significance
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摘要:
太古宙TTG岩石的成因及年代学研究是理解前板块构造和板块构造开始及其演化过程的关键,是陆壳增生的重要标志,为太古宙地质体研究的热点和难点之一。对鞍山地区的白家坟岩体进行了地球化学研究和锆石U-Pb测年。数据结果显示,该岩体具有正片麻岩特征,Si、Na含量高,TFeO、MgO、CaO含量低,轻、重稀土元素分异程度低,且具有一定变化。稀土元素总量偏低(12.6×10-6~119.34×10-6),LREE/HREE值为8.98~13.11。微量元素富集K、Rb、Ba,亏损Nb、Ce、Zr、Sm、Ti、Y、Yb;K/Rb平均值为229.37,Rb/Sr平均值为0.58,Ba/Sr平均值为3.28,Y/Nb平均值为3.36。表明该岩体为岩浆作用的产物,也有部分为壳内再循环的产物或受到陆壳物质影响。测得207Pb/206Pb年龄加权平均值为3816±14Ma,锆石具有清晰的振荡环带结构,所有核部锆石分析点的Th/U值在0.05~0.54之间,具有岩浆锆石的特点,代表了白家坟奥长花岗质岩石的结晶时代。3613~3651Ma和3331Ma两组年龄数据代表早太古代的一次岩浆事件及重要的陆壳增生时期的热事件。
Abstract:The genesis and chronology of Archaean TTG rocks is the key to understanding the problems concerning the beginning of the pre-plate tectonics and plate tectonics and their evolution process. It is an important sign of continental crust accretion and one of the hotspots and difficulties in the study of Archaean geologic bodies. Geochemical characteristics and zircon U-Pb dating of Baijiafen intrusion were studied in Anshan area. The data show that the rock mass has the characteristics of orthogneiss, with high Si, Na content, low TFeO, MgO, CaO content, low light and heavy REE content and some changes. The REE content of rocks is low, REE is 12.6×10-6~119.34×10-6 and LREE/HREE ratio is 8.98~13.11. Trace elements of rocks are characterized by enrichment of K, Rb and Ba and depletion of Nb, Ce, Zr, Sm, Ti, Y and Yb; K/Rb mean ratio is 229.37, Rb/Sr mean ratio is 0.58, Ba/Sr mean ratio is 3.28, and Y/Nb mean ratio is 3.36, suggesting that the rock mass is the product of magmatism, with a part being the product of intracrustal recycling or being affected by continental crustal materials. The weighted average age of 207Pb/206Pb is 3816±14Ma. The zircon has clear oscillating zonal structure. Th/U ratios of all core zircon analytical points range from 0.05 to 0.54. It has the characteristics of magmatic zircon and represents the crystallization age of Baijiafen trondhjemite. The age data of 3613~3651Ma and 3331Ma represent a magmatic event in the early Archaean and a thermal event in the important period of continental crust accretion.
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Key words:
- trondhjemite /
- TTG rock /
- zircon U-Pb dating /
- Baijiafen /
- Anshan
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表 1 白家坟奥长质花岗岩元素组成
Table 1. Element composition of Baijiafen trondhjemites
样号 9011[5] A9158[5] A9303[5] A9505-2[5] A9505-3[5] CH28[5] A9616C[5] PM030TY004① 3105 3106 SiO2 74.29 76.59 74.4 73.43 75.84 73.59 73.61 73.12 77.07 74.24 TiO2 0.41 0.47 0.51 0.34 0.80 0.59 0.90 0.99 1.49 1.20 Al2O3 15.62 13.79 14.66 14.05 13.77 14.59 14.69 13.38 11.80 13.00 Fe2O3 0.21 0.81 0.85 0.82 0.34 0.54 0.35 1.64 0.89 1.06 FeO 0.92 0.43 0.18 0.22 0.13 0.56 0.66 0.93 1.03 1.13 MnO 0.03 0.03 0.03 0.03 0.02 0.03 0.06 0.04 0.04 0.05 MgO 0.22 0.41 0.14 0.13 0.02 0.34 0.25 0.97 0.35 0.46 CaO 1.48 0.33 0.82 1.19 0.44 1.08 1.01 1.14 0.43 0.80 Na2O 4.72 4.25 4.28 5.20 3.79 4.20 5.22 4.04 3.65 3.95 K2O 1.88 2.22 4.71 1.79 5.68 3.52 2.50 2.69 3.13 3.6 P2O5 0.00 0.03 0.03 0.03 0.01 0.02 0.03 0.044 0.05 0.07 K2O/Na2O 0.40 0.52 1.10 0.34 1.50 0.84 0.48 0.67 0.86 0.91 DI 86.97 91.76 93.12 90.71 95.96 90.12 90.82 86.46 92.01 90.38 A/CNK 1.25 1.38 1.07 1.11 1.04 1.15 1.12 1.15 1.26 1.09 K 15600 18421 39082 14853 47131 29208 20744 22321 25243 19998 AR 2.26 2.69 3.77 2.69 5.00 2.94 2.93 2.73 2.49 2.42 σ 1.39 1.24 2.58 1.59 2.74 1.94 1.94 1.50 1.35 1.82 Sr 376 109 192 272 73 213 292 209 208 311 Rb 70 107 103 68 96 173 101 116 112 99 Ba 729 625 541 521 377 888 941 710 687 901 Th 5.00 10.00 2.40 5.00 7.00 5.00 7.00 12.20 6.00 6.50 Nb 2.00 3.00 5.60 2.00 2.00 2.00 2.00 6.85 2.00 2.00 Ce 9.59 19.46 4.32 3.53 7.50 22.83 24.05 39.50 19.86 23.1 Zr 103 101 35 41 31 42 25 112 38 29 Y 3.00 5.00 5.60 5.00 11.00 3.00 10.00 8.06 3.00 3.00 Yb 0.41 0.47 0.51 0.34 0.80 0.59 0.90 0.99 0.78 0.56 La 6.55 10.44 2.62 3.19 5.14 13.19 12.01 21.3 14.86 13.55 Pr 2.45 1.63 0.30 0.50 0.87 1.75 1.86 3.31 1.55 1.68 Nd 5.40 5.85 1.90 2.03 3.39 8.47 5.85 11.4 7.88 10.45 Sm 1.78 1.09 0.51 0.40 0.89 1.86 1.44 2.06 1.77 1.65 Eu 0.46 0.49 0.26 0.23 0.27 0.59 0.76 0.71 0.59 0.40 Gd 1.56 1.35 0.79 0.56 0.93 1.37 1.87 1.86 1.23 1.81 Tb 0.30 0.30 0.08 0.30 0.30 0.30 0.32 0.26 0.30 0.26 Dy 0.76 0.93 0.75 0.46 1.18 1.06 0.95 1.45 0.96 0.94 Ho 0.20 0.20 0.17 0.10 0.12 0.10 0.12 0.29 0.10 0.13 Er 0.49 0.37 0.33 0.33 0.81 0.71 0.69 0.82 0.86 0.41 Tm 0.10 0.10 0.09 0.10 0.12 0.10 0.12 0.15 0.15 0.10 Lu 0.10 0.10 0.09 0.10 0.10 0.30 0.15 0.15 0.10 0.12 ΣREE 30.15 42.78 12.72 12.17 22.42 53.22 51.09 84.25 42.32 75.64 LREE/HREE 6.69 10.20 3.53 4.31 4.14 10.75 8.98 13.11 8.45 11.63 LaN/YbN 11.46 15.93 3.68 6.73 4.61 16.04 9.57 15.43 14.21 12.56 δEu 0.83 1.23 1.25 1.49 0.90 1.08 1.42 1.09 1.02 1.41 LaN/SmN 2.38 6.18 3.32 5.15 3.73 4.58 5.38 6.68 3.86 5.11 GdN/YbN 3.15 2.38 1.28 1.36 0.96 1.92 1.72 1.55 1.86 1.42 注:主量元素含量单位为%,微量和稀土元素含量单位为10-6 表 2 白家坟奥长花岗岩锆石U-Th-Pb测年数据
Table 2. U-Th-Pb dating data of zircons from Baijiafen trondhjemites
样品及分析点号 Th U Th/U 同位素比值 同位素年龄/Ma 10-6 207Pb/206Pb 1σ 207Pb/235U 1σ 206Pb/238U 1σ 207Pb/206Pb 1σ 207Pb/235U 1σ 206Pb/238U 1σ 38亿年的古老锆石 TW1-19 45 161 0.277 0.3737 0.6220 40.0998 0.2494 0.7783 0.0118 3806 5 3773 15.5 3710 42.9 TW1-5 97 220 0.440 0.3795 0.6808 41.086 0.2797 0.7853 0.0121 3830 9.3 3797 16.6 3736 43.7 TW1-14 18 342 0.052 0.3788 0.6631 41.2076 0.2732 0.7889 0.0119 3827 8.5 3800 16.1 3749 43 TW1-6 72 245 0.292 0.3783 0.6741 41.5578 0.2801 0.7967 0.0125 3825 6.1 3809 16.2 3777 45.1 TW1-16 136 309 0.440 0.3739 0.6649 41.2792 0.2745 0.8008 0.0121 3807 8.5 3802 16.1 3792 43.4 TW1-1 168 312 0.537 0.3465 0.4730 29.2678 0.1384 0.6126 0.0093 3692 8.9 3463 16 3080 37 TW1-15 76 249 0.306 0.3683 0.6384 39.3841 0.2514 0.7756 0.0117 3784 8.9 3755 16.2 3701 42.7 TW1-12 47 162 0.289 0.3630 0.6158 36.9265 0.2274 0.7377 0.0115 3763 9 3692 16.6 3562 42.8 TW1-18 85 328 0.258 0.3606 0.5455 33.1948 0.1811 0.6676 0.0101 3752 9.8 3586 16.3 3297 39.1 TW1-11 38 392 0.097 0.3600 0.5255 33.5527 0.1763 0.676 0.0102 3750 6.5 3597 15.6 3329 39.3 36亿年的锆石 TW1-8 185 354 0.523 0.3292 0.6700 40.09981 0.2687 0.7783 0.0118 3613 10.2 3009 16 2190 28 TW1-4 67 457 0.146 0.3318 0.4700 41.08603 0.1931 0.7853 0.0121 3625 7.1 3144 15.4 2447 30.7 TW1-7 64 340 0.187 0.3372 0.6400 41.20755 0.2637 0.7889 0.0119 3650 9.7 3261 16 2666 32.9 TW1-17 51 250 0.204 0.3375 0.5900 41.55782 0.2452 0.7967 0.0125 3652 8.9 3665 16.1 3688 42.6 33亿年的锆石 TW1-3 109 215 0.509 0.2744 0.4000 26.02707 0.1041 0.6881 0.0104 3331 6.3 3348 15.4 3375 39.8 -
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