Discovery of Paleoproterozoic Crustally Derived Carbonatite in the Northern Altyn Tagh
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摘要: 报道了阿尔金东段北缘的古元古代晚期壳源火成碳酸岩全岩地球化学和锆石SHRIMP定年结果.它们明显切割围岩, 以岩株状、岩脉状产出, 包裹不同类型围岩包体.岩石主要由方解石、透辉石组成, 含石英、长石等长英质矿物.岩石高CaO(20.56%~39.80%), SiO2含量变化较大(23.33%~54.06%), 稀土总量低(TREE=28.36×10-6~63.01×10-6)、弱负铕异常(Eu/Eu*=0.69~0.83)、轻重稀土分离不强((La/Yb)N=9.20~16.85), 相对富集大离子亲石元素Sr、Rb、Ba、Th, 亏损高场强元素Nb、Ti、P.锆石具核-边结构, 边部锆石普遍存在不明显的环带, 具变质深熔锆石的结构特征, 其207Pb/206Pb加权平均年龄为1 931±18 Ma.它们是高级变质作用条件下米兰岩群不纯大理岩深熔作用的产物.Abstract: This paper reports whole-rock geochemical compositions and zircon dating results of late Paleoproterozoic crustally-derived carbonatite in the northern Altyn Tagh. The carbonatite cuts contry rocks and occurs as stocks or dikes and contains different types of contry rocks. It is mainly composed of calcite, diopside, and a few silicate minerals (such as, feldspar, quartz, etc.). Samples of the carbonatite are characterized with high CaO contents (20.56%-39.80%), a large range of SiO2 contents (23.33%-54.06%), low TREE (28.36×10-6-63.01×10-6), weakly negative Eu anomalies (Eu/Eu*=0.69-0.83), moderately fractionated REE patterns ((La/Yb)n=9.20-16.85). Compared with Sr, Rb, Ba, Th (LILE), the high field strength elements (HFSE), especially Nb, Ti and P, are strongly depleted. Zircons of the carbonatite show core-rim textures, and commonly show weakly zoning. SHRIMP U-Pb zircon dating result of the carbonatites is 1 931±18 Ma. All these threw lights on that the carbonatite are formed by the anatexis of impure marbles from Milan group.
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Key words:
- derived carbonatite /
- geochronology /
- geochemical geochemistry /
- Petrology /
- northern Altyn Tagh
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图 1 阿尔金北缘地质简图示火成碳酸岩的分布(据拉配泉地区基岩区侵入岩分布图改编;刘永顺等,2010)
Fig. 1. Sketched geological map of the northern Altyn Tagh showing carbonatite distribution
图 2 阿尔金北缘火成碳酸岩野外照片
a.火成碳酸岩侵入英云闪长质片麻岩; b.火成碳酸岩与围岩片麻状闪长岩的接触带呈强变形特征; c.火成碳酸岩与围岩斜长角闪岩的接触带呈强变形特征;d.火成碳酸岩呈分支脉状切割米兰岩群斜长角闪岩; e.火成碳酸岩切割石榴黑云斜长片麻岩和斜长角闪岩,接触界线波状不平; f.火成碳酸岩中长条状英云闪长质片麻岩包体,有碳酸岩岩脉侵入切割包体; g.火成碳酸岩中椭圆状英云闪长质片麻岩包体,相互之间存在反应; h.火成碳酸岩中不同形态的片麻状闪长岩包体,片麻理方向不同
Fig. 2. Field features of carbonatites in the northern Altyn Tagh
图 3 阿尔金北缘碳酸岩地球化学图谱
a.稀土配分曲线;b.微量元素蛛网图; 标准化数据分别取自Boynton(1984)和Pearce(1982)
Fig. 3. REE pattern (a) and trace element spider diagram (b) of carbonatite in the northern Altyn Tagh
图 5 阿尔金北缘火成碳酸岩(TW1119-1)的锆石阴极发光图像
图中圆圈为锆石SHRIMP U-Pb测年位置,其编号与表 2中的一致
Fig. 5. Cathodoluminescence images of zircons from carbonatite in the northern Altyn Tagh
表 1 阿尔金北缘古元古代火成碳酸岩元素组成
Table 1. Chemical compositions of carbonatite in the northern Altyn Tagh
样号 Bb1119-1 P15Bb29-1 P15Bb18-1 D1291-1Gs* P7Bb4-1 SiO2 54.06 40.19 29.44 28.36 23.33 TiO2 0.27 0.24 0.32 0.25 0.12 Al2O3 5.44 4.37 5.28 4.1 1.63 Fe2O3 0.83 0.16 0.1 - 0.11 FeO 2.02 1.78 2.71 1.39 0.84 MnO 0.05 0.03 0.03 0.02 0.06 MgO 2.56 3.32 2.78 1.94 1.37 CaO 20.56 29.78 36.30 36.56 39.80 Na2O 0.77 0.76 0.49 0.45 0.29 K2O 0.75 0.86 0.64 0.65 0.31 P2O5 0.02 0.06 0.09 0.07 0.07 LOI 11.69 17.76 21.94 25.36 31.46 La 8.05 12.6 12.5 8.49 6.08 Ce 15.8 24.7 23.6 13.7 11.1 Pr 1.74 2.67 2.44 1.59 1.13 Nd 6.31 9.65 8.51 5.64 4.19 Sm 1.21 1.66 1.46 0.98 0.78 Eu 0.26 0.35 0.32 0.27 0.16 Gd 0.91 1.39 1.12 0.98 0.53 Tb 0.15 0.24 0.18 0.13 0.09 Dy 0.97 1.27 1.05 0.83 0.52 Ho 0.18 0.23 0.19 0.17 0.1 Er 0.6 0.72 0.56 0.47 0.31 Tm 0.08 0.09 0.08 0.08 <0.05 Yb 0.59 0.61 0.5 0.44 0.31 Lu 0.08 0.09 0.09 0.08 <0.05 Y 4.82 6.74 5.43 4.61 2.98 TREE 41.75 63.01 58.03 38.46 28.36 σEu 0.73 0.69 0.74 0.83 0.72 (La/Yb)N 9.2 13.93 16.85 13.01 13.22 Ba 227 241 243 251 97.7 Rb 24.9 29.4 14.1 17.3 13.1 Sr 646 1089 1154 1648 909 Zr 50.5 51.7 34.3 60.7 34.2 Nb 2.71 3.03 3.5 2.47 1.27 Th 1.54 2.58 2.43 2.16 1.86 Pb 4.67 12.6 8.43 4.13 Zn 55.5 44.7 70.9 15.3 Cu 11.9 9.66 12.1 3.46 Ni 28.2 29.6 31.2 24 V 59.2 35.6 47.2 16.8 Cr 61.5 34.5 35.7 77.8 14.4 Hf 1.52 1.34 1.02 1.58 0.74 Sc 9.85 6.6 7.8 4.74 3.26 Ta 0.23 0.14 0.29 0.17 0.09 Co 9.6 9.33 13.4 5.84 Li 7.5 12 8.6 1.94 U 1.36 2.11 1.52 2.15 1.38 *引自拉配泉地区1∶5万区调报告; 常量元素单位为%,稀土和微量元素单位为10-6. 表 2 阿尔金北缘火成碳酸岩(Tw1119-1)锆石SHRIMP U-Pb年龄
Table 2. U-Pb SHRIMP data of zircons from carbonatite in the northern Altyn Tagh
点号 wB/(μg·g-1) Th/
U同位素原子比率 误差
相关表面年龄(Ma) 206Pb* U Th 207Pb*/206Pb* ±% 207Pb*/235U ±% 206Pb*/238U ±% 206Pb/238U 207Pb/206Pb 1.1ME 117 395 53 0.14 0.122 22 0.55 5.78 3.9 0.343 3.8 0.99 1 901±63 1 989± 9.9 2.1IN 98.9 308 51 0.17 0.144 06 0.56 7.42 3.5 0.374 3.4 0.987 2047±60 2 277± 10 3.1IN 240 790 281 0.37 0.144 93 0.37 7.04 3.4 0.353 3.4 0.994 1 947±57 2 287± 6.3 4.1IN 169 450 158 0.36 0.168 2 0.68 10.1 3.5 0.436 3.4 0.981 2 331±67 2 540±11 5.1IN 130 461 72 0.16 0.163 46 0.44 7.39 3.4 0.328 3.4 0.992 1 828±54 2 492± 7.3 5.2ME 196 612 23 0.04 0.108 1 3.3 5.56 4.7 0.373 3.4 0.72 2 044±60 1 768±60 6.1ME 243 984 53 0.06 0.120 3 0.44 4.77 3.4 0.2876 3.4 0.992 1 630±49 1 961±7.8 7.1ME 176 511 56 0.11 0.119 17 0.46 6.58 3.4 0.4 3.4 0.991 2 171±63 1 944± 8.2 8.1ME 110 421 45 0.11 0.119 02 0.56 4.99 3.9 0.304 3.9 0.99 1 712±59 1 942±10.0 9.1ME 73.1 241 22 0.09 0.108 4 2.5 5.27 4.2 0.352 3.4 0.813 1 946±58 1 773±45 10.1ME 122 413 41 0.10 0.114 8 0.6 5.45 3.5 0.344 3.4 0.985 1 907±57 1 877±11 10.2ME 212 685 70 0.11 0.112 3 1.3 5.58 3.6 0.36 3.4 0.932 1 983±58 1 837±24 11.1ME 147 513 44 0.09 0.116 5 0.5 5.37 3.4 0.334 3.4 0.989 1 858±55 1 903±9.0 12.1ME 134 443 44 0.10 0.117 57 0.52 5.73 3.4 0.353 3.4 0.988 1 951±57 1 920±9.4 13.1ME 102 341 42 0.13 0.123 3 0.84 5.93 3.5 0.349 3.4 0.971 1 929±57 2 005±15 14.1ME 227 714 44 0.06 0.118 01 0.5 6.01 3.4 0.369 3.4 0.989 2 026±59 1 926±8.9 15.1ME 174 542 43 0.08 0.117 44 0.43 6.06 3.4 0.375 3.4 0.992 2 051±60 1 918±7.7 16.1ME 135 499 53 0.11 0.115 74 0.49 5.01 3.5 0.314 3.4 0.99 1 760±53 1 892±8.9 17.1IN 199 580 93 0.17 0.143 85 0.59 7.92 3.7 0.399 3.6 0.987 2 165±66 2 274±10 注:误差为1σ;Pb*指示放射成因铅;所有同位素比率已对测得的204Pb进行了校正.样品数据由地科院北京离子探针中心测试. -
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