A study of geochronology and geochemistry of volcanic rocks from Meiletu Formation in Horqin Right Wing Front Banner, Inner Mongolia
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摘要:
通过对科右前旗一带梅勒图组火山岩LA-ICP-MS锆石U-Pb定年和元素地球化学分析,获得梅勒图组安山质晶屑熔结凝灰岩及安山岩的喷发年龄分别为125.6±1.6Ma和120.2±2.2Ma,指示梅勒图组火山岩的形成时代介于120~126Ma之间,形成于早白垩世中期。岩石具有高的SiO2(60.4%~65.3%)和全碱(7.0%~9.5%)含量,铝饱和指数A/CNK为0.89~0.98,属于钾玄岩及高钾钙碱性系列;稀土元素总量为124×10-6~169×10-6,(La/Yb)N值为5.83~13.4,负Eu异常微弱(δEu=0.84~1.05);在原始地幔标准化微量元素蛛网图上,样品富集Rb、Ba、Th、U、K、La、Nd,亏损Nb、Ta、P和Ti,Sr/Y值介于19.4~29.7之间。其岩石源区为富集地幔,岩浆上升过程中受到地壳物质的混染,岩石形成于板内伸展环境,其地球动力学背景可能与古太平洋板块的俯冲有关。
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关键词:
- LA-ICP-MS锆石U-Pb定年 /
- 地球化学 /
- 梅勒图组 /
- 火山岩 /
- 内蒙古科右前旗
Abstract:Through LA-ICP-MS zircon U-Pb dating and elemental geochemical analysis of Meiletu Formation volcanic rock in Horqin Right Wing Front Banner, the authors obtained the eruption ages of 125.6±1.6Ma and 120.2±2.2Ma for andesite crystallinoclasitc welded tuff and andesite in Meiletu Formation, indicating that the forming ages of Meiletu Formation volcanic rock are between 120 and 126Ma in the Early Cretaceous. The rock has high SiO2 (60.4%~65.3%) and total alkali content (7%~9.5%), with aluminum index A/CNK being 0.89~0.98, suggesting shoshonitic and high-K calc alkaline series; rare earths range from 124×10-6 to 169×10-6, and (La/Yb)N values are in the range of 5.83~13.4, with weak negative Eu anomalies (δEu=0.84~1.05); on the primitive mantle normalized trace element spider diagram, there exist enrichment of Rb, Ba, Th, U, K, La and Nd and depletion of Nb, Ta, P and Ti, with Sr/Y ratio between 19.4 and 29.7. The rock source area should be enriched mantle, and the magma was contaminated by crustal material during the process of magma rising. The rock was formed in an intraplate extensional environment, and its geodynamic background might have been related to subduction of paleo-Pacific plate.
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1. 研究目的(Objective)
研究区地处鄂尔多斯盆地北部伊盟隆起中部的塔然高勒地区,其东侧紧邻纳岭沟铀矿,西侧为大营铀矿,区内地层整体为一向西南缓倾的单斜构造,含铀目的层为中侏罗统直罗组下段。塔然高勒铀矿的发现有望使大营和纳岭沟相连,成为世界级的铀矿田。
2. 研究方法(Methods)
依靠煤田和石油勘查钻孔资料“二次”开发的创新性思路,中国地质调查局天津地质调查中心在鄂尔多斯盆地东北缘塔然高勒地区通过收集分析研究区煤田勘查、开发钻孔资料以及区域地质、矿产、物探、化探、遥感及水文地质等资料,开展铀矿战略选区;编制研究区目的层系列图件,分析铀矿成矿地质条件,圈定找矿靶区;优选放射性异常钻孔和成矿有利地段进行钻探施工,分析放射性异常和含矿目的层的空间位置以及矿体分布、矿石质量等特征,初步评价铀资源潜力。
3. 研究结果(Results)
排查研究区煤田测井资料,发现潜在铀矿孔164个,潜在铀矿化孔65个。自然伽玛强度值较高,放射性异常钻孔形态连片、成带特征明显,规模较大。通过对优选的放射性异常钻孔和成矿有利地段进行钻探施工,共发现铀矿工业孔14个,铀矿化孔26个,矿区达到大型铀矿产地规模;铀矿体平面上总体呈北东—南西向展布,矿体平均埋深526 m,由北东向南西方向埋深逐渐增大。垂向上,铀矿体主要赋存于中侏罗统直罗组下段下亚段的灰色砂体中,矿体形态为板状。矿石岩性为中粗粒砂岩,碎屑物胶结类型以接触式、孔隙式胶结为主,铀的存在形式以吸附态为主,含铀矿物主要为铀石。
研究区目前共圈定6个工业矿体,矿体厚度变化范围为1.30~7.50 m,平均值为4.33 m;矿体平均品位为0.0342%,矿体平米铀量变化范围为1.76~ 11.20 kg/m2,平均值为3.49 kg/ m2。
目前主要对研究区东部矿体进行钻探工程控制,而矿体向西南方向深部的延伸情况及其西部矿体的分布情况均未进行有效控制,矿区仍具有较大的铀矿找矿潜力。
4. 结论(Conclusions)
(1)鄂尔多斯盆地东北缘塔然高勒铀矿的发现拓展了该地区的铀矿找矿空间,扩大了鄂尔多斯盆地东北部铀资源基地规模。
(2)研究区与周边的大营、纳岭沟铀矿具有相似的优越铀矿成矿地质条件,铀源丰富、泥砂泥结构完整、辨状河道砂体发育,炭屑、有机质等找矿标志明显。通过加大该地区的铀矿调查力度,有望使该地区成为世界级铀矿田。
(3)该地区铀矿的发现是利用煤田资料“二次开发”技术思路快速寻找砂岩型铀矿的成功范例之一,该技术方法极大减少了铀矿调查周期,节省了大量勘查成本。
5. 致谢(Acknowledgements)
感谢李建国、李效广及项目组成员的支持。
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图 7 梅勒图组火山岩K2O/TiO2与SiO2相关性判别图[34]
Figure 7.
表 1 梅勒图组火山岩LA-ICP-MS锆石U-Th-Pb分析结果
Table 1. LA-ICP-MS zircon U-Th-Pb data for the volcanic rocks of the Meletu Formation
测点号 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 207Pb/206Pb±1σ 207Pb/235U±1σ 206Pb/238U±1σ 01 9.44 171 349 0.49 0.0544±0.0035 0.1418±0.0098 0.01921±0.00044 387±146 135±9 123±3 02 15.8 360 526 0.69 0.0542±0.0021 0.1466±0.0057 0.01976±0.00034 389±81 139±5 126±2 03 11.79 191 357 0.54 0.0525±0.0042 0.1484±0.0130 0.02030±0.00062 306±177 141±12 130±4 04 10.59 177 392 0.45 0.0506±0.0025 0.1397±0.0070 0.01993±0.00039 220±113 133±6 127±2 05 7.04 106 293 0.36 0.0549±0.0036 0.1410±0.0086 0.01912±0.00042 409±148 134±8 122±3 06 9.13 209 290 0.72 0.0548±0.0035 0.1401±0.0077 0.01964±0.00050 406±144 133±7 125±3 07 22.7 517 741 0.70 0.0531±0.0021 0.1466±0.0061 0.01994±0.00039 332±91 139±5 127±2 08 7.65 124 288 0.43 0.0522±0.0033 0.1394±0.0087 0.01934±0.00046 300±146 133±8 123±3 09 5.29 69.3 211 0.33 0.0543±0.0040 0.1419±0.0102 0.01955±0.00053 383±167 135±9 125±3 10 14.21 295 481 0.61 0.0557±0.0039 0.1463±0.0089 0.02005±0.00050 443±156 139±8 128±3 11 6.66 117 267 0.44 0.0559±0.0036 0.1455±0.0192 0.01962±0.00048 456±146 138±8 125±3 01 8.06 119 333 0.36 0.0520±0.0020 0.1332±0.0047 0.01877±0.00038 283±89 127±4 120±2 02 20.6 191 463 0.41 0.0533±0.0016 0.2475±0.0083 0.03366±0.00077 346±67 225±7 213±5 03 6.14 161 240 0.67 0.0492±0.0027 0.1371±0.0167 0.01910±0.00099 167±123 130±15 122±6 04 29.9 77.8 765 0.10 0.0525±0.0009 0.2316±0.0050 0.03203±0.00062 309±6 212±4 203±4 05 15.2 22.3 22.5 0.99 0.1587±0.0038 9.964±0.632 0.45496±0.02845 2442±41 2431±59 2417±126 06 43.2 64.2 60.6 1.06 0.1643±0.0019 10.07±0.205 0.44372±0.00799 2502±19 2441±19 2367±36 07 557 780 797 0.98 0.1598±0.0020 9.970±0.244 0.45229±0.01110 2454±22 2432±23 2405±49 08 16.4 111 394 0.28 0.0544±0.0013 0.2460±0.0069 0.03279±0.00055 387±56 223±6 208±3 09 8.07 236 302 0.78 0.0485±0.0017 0.1250±0.0049 0.01866±0.00034 124±83 120±4 119±2 10 4.91 119 177 0.67 0.1124±0.0650 0.1257±0.0089 0.01951±0.00079 1839±1276 120±8 125±5 11 13.6 207 545 0.38 0.0508±0.0013 0.1358±0.0031 0.01954±0.00042 232±57 129±3 125±3 12 2.39 37.5 53.0 0.71 0.0549±0.0052 0.2413±0.0242 0.03157±0.00066 409±210 220±20 200±4 13 4.85 95.6 203 0.47 0.0511±0.0020 0.1290±0.0060 0.01832±0.00043 256±95 123±5 117±3 14 4.92 126 196 0.64 0.0520±0.0025 0.1333±0.0075 0.01850±0.00053 287±111 127±7 118±3 15 201 176 306 0.58 0.1595±0.0072 9.881±0.292 0.44929±0.00803 2450±76 2424±27 2392±36 16 182 205 268.6 0.76 0.1611±0.0018 10.12±0.275 0.45537±0.01127 2478±18 2446±25 2419±50 17 102.6 132.5 151.6 0.87 0.1577±0.0019 9.793±0.211 0.45130±0.00986 2431±20 2416±20 2401±44 表 2 梅勒图组火山岩主量、微量及稀土元素分析结果
Table 2. Analyses of major, trace elements and REE for Meiletu Formation volcanic rocks
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