Geochemical Characteristics of Rare Earth Elements in Deep Geological Profile of Qitianling Intrusion, Hunan Province
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摘要: 湖南骑田岭白石渡—飞仙镇剖面是我国深部探测专项工作实施的重点剖面之一，前人对该剖面穿过的骑田岭岩体稀土元素特征开展了大量研究工作，但并未系统地就岩体、风化壳及其邻近的地层等各地质单元稀土元素地球化学特征作出分析。本文运用电感耦合等离子体质谱技术，对该剖面穿切的5种地质单元(灰岩岩屑、砂岩岩屑、第四纪残破积黏土、花岗岩及花岗岩风化壳)的稀土元素含量进行分析。结果表明：①剖面中各地质体的稀土配分曲线均为轻稀土富集型，但稀土总量存在显著的差异，骑田岭花岗岩风化壳的稀土含量最高(均值605×10-6)，灰岩的稀土含量最低(均值154.3×10-6)；对于相同岩石类型的岩性，其稀土总量也存在着较大差异，往往是越靠近岩体的地层其稀土元素含量越高；②结合REEs-Y/ΣREEs、ΣREEs-δEu图解及前人对花岗岩中锇同位素示踪结果，推测骑田岭花岗岩混入了较多的幔源物质；③骑田岭花岗岩风化壳的稀土含量(RE2O3平均含量814.8×10-6)超过了离子吸附型稀土矿床的边界品位(重稀土RE2O3 0.05%，轻稀土0.07%)。研究指出，骑田岭矿集区的稀土是否以离子吸附状态存在以及能否构成工业矿床有待进一步评价。Abstract: Baishidu-Feixianzhen profile in Qitianling intrusion of Hunan Province is one of the most important profiles carried out in Sinoprobe. Despite the fact that a large amount of previous research had been carried out using various methods on rare earth elements (REEs) characteristics of Qitianling granite complex, no systematical study has been undertaken on geochemical characteristics from different geological units including limestone, sandstone, quaternary eluvial clays, the plution and the weathering crust. Analysis of the REEs in the profile through Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) has been carried out and is reported in this paper. The results indicate that all geology units have LREE enrichment pattern, while a great difference exists between ΣREEs. For the total content of REEs in Qitianling Granite, the weathering crust has the highest level of ΣREEs with an average value of 605×10-6, and the limestone has the least ΣREEs with an average value of 154.3×10-6. In addition, the closer to the intrusions, the higher ΣREEs contents have in strata, even for the same lithology rock. In combination with REE-Y/ΣREE, ΣREE-δEu diagrams and Os Isotope tracer results, Qitianling granitoids might have mixed with some mantle materials. The content of REEs in the Qitianling Granite weathering crust (average value of RE2O3 is 814.8×10-6) are more than the cutoff grade of ion-absorbed-type REEs deposit (HREE RE2O3 0.05% and LREE 0.07%) in the main, which deserves further exploration.
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表 1 各地质体稀土元素的平均含量
Table 1. REE average content of Qitianling intrusion and the layers
岩性 样品数量 w/10-6 --- --- δEu δCe La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Y ΣREEs LREEs HREEs 花岗岩风化壳 14 148.19 236.21 32.38 105.83 20.11 1.98 16.68 2.81 16.14 3.17 9.61 1.37 8.69 1.38 85.60 604.55 544.70 59.84 9.35 12.54 0.32 0.85 花岗岩岩屑 31 140.55 241.08 29.84 97.46 18.02 1.55 14.38 2.45 14.40 2.86 8.53 1.26 7.96 1.29 80.71 581.64 528.52 53.12 10.38 13.29 0.28 0.91 灰岩 31 27.26 49.17 7.20 28.62 5.65 0.87 4.64 0.82 4.51 0.89 2.71 0.37 2.28 0.35 22.61 135.32 118.78 16.54 7.73 9.02 0.59 0.91 砂岩 28 50.42 85.37 11.29 44.54 7.97 1.48 6.26 1.10 6.02 1.20 3.75 0.52 3.24 0.49 31.22 223.64 201.08 22.56 9.14 12.13 0.62 0.89 黏土 30 27.66 54.81 6.25 23.61 4.42 0.86 3.64 0.64 3.62 0.74 2.30 0.32 2.08 0.33 19.60 131.27 117.61 13.65 8.33 9.20 0.63 0.97 
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表 2 地层中不同岩性的稀土含量分配情况
Table 2. REEs distribution of different lithologies in stratum
岩性 w(ΣREEs)>100×10-6 w(ΣREEs)<100×10-6 稀土平均含量w/10-6 样品数量 含量均值 样品数量 含量均值 灰岩 11 310.1 20 39.2 135.3 砂岩 24 253.5 4 44.7 253.5 黏土 17 174.1 13 75.2 131.3
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