The discovery of the Paleoproterozoic metamorphic rocks in Ulanhot, Inner Mongolia, and its geological significance
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摘要:
在乌兰浩特东白音乌苏一带新发现一套斜长角闪片岩、透辉透闪岩、斜长角闪片麻岩的岩石组合，岩石普遍经高绿片岩相-低角闪岩相变质，经受了后期多期构造改造。斜长角闪片岩经原岩恢复为基性火山岩，锆石晶形、阴极发光、背散射图像及高Th/U值等特征显示为岩浆锆石。采用LA-ICP-MS锆石U-Pb测年技术，在变质岩系中获得了1864.1±7.3Ma的同位素年龄，时代归属为古元古代，该年龄为岩浆形成年龄。此年龄限定了该套变质岩系的形成时代，证明乌兰浩特地区可能存在前寒武纪结晶基底。与区域上松嫩地块南部新发现的古元古元代岩浆锆石年龄进行对比发现，松嫩地块西缘可延伸至乌兰浩特一带。
Abstract:A set of amphibolite schist, diopside tremolite schist and amphibolite gneiss supracrustal rock assemblage was newly discovered in Dongbaiyinwusu area. The rocks have generally experienced high greenschist facies and low amphibolite facies metamorphism and the multiple tectonic transformation of plagioclase in later stage. The hornblende schist protolith comprised basic volcanic rocks, whose zircon crystal, cathodoluminescence, back scattering image and high Th/U ratio characteristics suggest magmatic zircon. By using zircon U-Pb dating LA-ICP-MS technology, the authors obtained the isotopic age of 1864.1±7.3Ma, suggesting Paleoproterozoic and representing magma formation age, which constrains the formation age of supracrustal rocks and proves the existence of Precambrian crystalline basement in Ulanhot area. A comparison with the Paleoproterozoic magmatic zircon ages of southern Songnen massif shows that the west margin of Songnen massif can extend to the Ulanhot area.
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Keywords:
- Ulanhot /
- Palaeoproterozoic /
- metamorphic series
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图 2 内蒙古乌兰浩特市东古元古代斜长角闪片岩岩组人工露头实测剖面
Figure 2. Measured section of eastern Proterozoic epidermal rocks in Ulanhot, Inner Mongolia
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图 3 斜长角闪片岩和斜长角闪片麻岩宏观照片
Figure 3. Macrophotograph of plagioclase amphibole schist and plagioclase amphibole gneiss
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图 4 变质岩系中脉体的褶皱变形
Figure 4. Fold deformation of the middle vein of metamorphic rock series
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图 7 （al-alk）-c图解
Ⅰ—钙质泥岩；Ⅱ—白云质泥灰岩；Ⅲ—粘土岩；Ⅳ—中酸性凝灰岩；Ⅴ—角斑岩；Ⅵ—细碧岩-玄武岩；Ⅶ—二长安山质凝灰岩；Ⅷ—英安质凝灰岩
Figure 7. Diagram of(al-alk)-c
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图 8 样品野外和镜下照片
a—古元古代变质岩系岩貌特征；b—样品PM028B101斜长角闪片岩镜下照片；c—样品PM028B102斜长角闪片麻岩镜下照片；dsk—透光透闪矽卡岩；sch—黑云斜长片岩；hos—斜长角闪片岩；gnt—斜长角闪片麻岩；δμ—闪长玢岩；λ—流纹岩；γπ—花岗斑岩
Figure 8. Field features and microphotograph
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图 10 PM028B101斜长角闪片岩年龄谐和图
Figure 10. PM028B101 age concordent diagram of zircon in plagioclase amphibole schist
表 1 古元古代变质岩系主量元素分析结果
Table 1 Major element of the Paleoproterozoic metamorphic rocks
% 样品号 PM028B92—1 PM028B101 PM028B103 PM028B102—1 PM028B94—1 PM028B98—2 PM028B116—1 镜下定名 斜长角闪片岩 斜长角闪片岩 斜长角闪片岩 黑云阳起片岩 角闪斜长片岩 黑云斜长片岩 黑云斜长片岩 SiO2 49.26 49.40 54.50 48.92 61.80 71.42 66.08 TiO2 2.46 2.22 2.65 2.52 0.88 0.42 0.59 Al2O3 12.00 11.96 15.49 12.36 14.81 13.70 15.69 Fe2O3 4.52 1.59 2.67 4.48 3.36 2.86 3.25 FeO 7.81 9.64 4.21 7.94 3.15 0.50 0.62 MnO 0.20 0.18 0.14 0.19 0.10 0.055 0.059 MgO 7.31 7.65 3.73 7.37 2.63 1.12 1.85 CaO 9.76 10.29 7.99 10.01 5.42 2.61 4.00 Na2O 2.52 2.49 4.38 2.63 3.66 4.59 4.05 K2O 0.17 1.09 2.06 1.06 1.86 1.56 1.45 P2O5 0.38 0.35 1.13 0.37 0.27 0.11 0.19 烧失量 2.34 1.74 1.18 1.50 1.30 1.56 2.36 总计 98.7 98.6 100.1 99.4 99.2 100.5 100.2 注:测试单位为黑龙江地质矿产测试应用研究所 
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表 2 PM028B101斜长角闪片岩锆石U-Th-Pb同位素分析结果
Table 2 PM028B101 zircon U-Th-Pb isotopic sample analytical results of plagioclase amphibole schist
样品号 含量/10-6 207Pb/206Pb 207Pb/235U 206Pb/238U 232Th/238U 207Pb/206Pb 207Pb/235U 206Pb/238U Pb U 比值 1σ 比值 1σ 比值 1σ 年龄/Ma 1σ 年龄/Ma 1σ 年龄/Ma 1σ 1 174 370 0.12 0.001 5.28 0.077 0.33 0.003 0.6 1883 22 1866 12 1848 15 2 225 487 0.11 0.002 5.13 0.079 0.33 0.004 0.7 1869 18 1842 13 1816 20 3 635 2102 0.11 0.001 3.97 0.057 0.25 0.002 0.1 1843 14 1628 12 1463 11 4 250 540 0.12 0.001 5.26 0.057 0.33 0.002 0.6 1881 17 1863 9 1844 12 5 11 301 0.05 0.002 0.13 0.057 0.02 0.000 1.4 209 67 128 4 124 1 6 147 309 0.11 0.001 5.19 0.057 0.33 0.003 0.6 1862 18 1851 12 1838 17 7 167 384 0.11 0.001 5.09 0.057 0.33 0.002 0.7 1854 17 1834 9 1815 11 8 6 148 0.05 0.002 0.20 0.057 0.03 0.000 0.5 272 93 185 6 179 2 8-1 138 292 0.11 0.001 5.25 0.057 0.33 0.003 0.5 1876 19 1861 10 1845 13 9 230 483 0.11 0.001 5.13 0.057 0.33 0.003 0.6 1856 17 1841 9 1835 13 10 207 414 0.11 0.001 5.28 0.057 0.33 0.005 0.6 1866 17 1866 16 1860 24 11 703 2403 0.11 0.001 4.12 0.057 0.26 0.003 0.4 1852 24 1658 13 1507 17 12 239 503 0.11 0.001 5.20 0.057 0.33 0.003 0.6 1856 19 1853 9 1857 14 13 140 319 0.11 0.001 5.26 0.057 0.33 0.003 0.4 1872 17 1862 10 1852 14 14 142 352 0.11 0.001 5.06 0.057 0.33 0.003 0.3 1856 16 1830 10 1814 13 15 7 285 0.05 0.002 0.13 0.057 0.02 0.000 0.3 154 87 128 4 127 1 16 854 1574 0.12 0.001 5.37 0.057 0.34 0.003 0.9 1880 19 1880 11 1877 16 17 440 975 0.11 0.001 5.24 0.057 0.33 0.002 0.5 1862 12 1859 9 1853 12 18 199 449 0.11 0.001 5.15 0.057 0.33 0.004 0.6 1839 18 1845 12 1848 20 19 225 433 0.11 0.001 5.27 0.057 0.33 0.004 0.9 1863 12 1864 12 1862 20 20 319 700 0.11 0.001 5.21 0.057 0.33 0.004 0.6 1856 20 1854 10 1859 18 21 184 379 0.11 0.001 5.16 0.057 0.33 0.004 0.6 1850 17 1846 10 1842 17 22 57 123 0.11 0.001 5.11 0.057 0.33 0.004 0.5 1847 22 1838 12 1832 21 23 69 153 0.11 0.001 5.15 0.057 0.33 0.003 0.5 1843 22 1844 11 1843 15 24 75 153 0.11 0.001 5.29 0.057 0.33 0.003 0.6 1876 22 1868 12 1859 16 25 199 450 0.12 0.001 5.38 0.057 0.34 0.003 0.3 1884 19 1882 11 1877 15 26 135 269 0.11 0.001 5.29 0.057 0.34 0.004 0.6 1869 26 1867 14 1864 21 27 5 193 0.05 0.003 0.17 0.057 0.02 0.000 0.4 391 110 160 7 145 2 28 110 226 0.12 0.002 5.42 0.06 0.34 0.008 0.6 1898 23 1888 22 1878 37
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