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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致谢: 野外工作得到武警黄金第四支队一中队全体官兵的帮助，测试过程中得到核工业北京地质研究院分析测试研究中心和北京科荟测试有限公司相关人员的帮助，审稿专家提出了建设性的修改意见，在此一并致以诚挚的感谢。
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图 1 研究区大地构造位置(a)及地质简图(b)
Figure 1. Tectonic location (a) and simplified geological map (b) of the study area
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图 2 梅勒图组火山岩显微照片
a—安山岩（+）；b—安山质晶屑熔结凝灰岩（+）；Pl—斜长石；Opx—单斜辉石；Mgt—磁铁矿；（+）—偏光显微镜下的正交偏光
Figure 2. Microphotographs of Meiletu Formation volcanics
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图 3 梅勒图组火山岩锆石阴极发光（CL）图像
a—安山质晶屑熔结凝灰岩；b—安山岩
Figure 3. CL images of zircons from Meiletu Formation volcanics
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图 4 研究区梅勒图组火山岩锆石U-Pb谐和图
Figure 4. U-Pb concordia diagrams of zircons from Meiletu Formation volcanic rocks in the study area
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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. SiO2-K2O/TiO2 diagram for Meiletu Formation volcanic rocks
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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 
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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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