The 1.5 Ga metamorphic gabbro at the Lixi area, S: chuan Province in the western margin of the Yangtze Block: Implications for the breakup of the Columbia supercontinent
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摘要:
扬子陆块西缘黎溪—菜子园—通安地区前寒武纪地质体分布广泛，是探讨扬子陆块前寒武纪地质演化的重要载体。对会理黎溪地区变辉长岩开展岩石学、全岩地球化学及锆石U-Pb定年研究。结果显示，黎溪变辉长岩形成于1508±21 Ma；在岩石地球化学组成上，它们属于亚碱性拉斑岩石系列，总体上略微富集轻稀土元素，高场强元素Nb、Ta、Zr、Hf等无明显亏损，大离子亲石元素Ba、Sr等不同程度富集。通过岩石成因分析认为，岩浆在上升过程中经历了一定程度的分离结晶作用，但并未受到明显的地壳混染，母岩浆可能来自软流圈地幔。地球化学和区域地质特征综合分析表明，黎溪变辉长岩与富集洋中脊型玄武岩的地球化学特征相似，可能形成于洋中脊及附近的海山环境，是Columbia超大陆裂解在扬子陆块的响应。
Abstract:Precambrian strata are widely distributed in the Lixi-Caiziyuan-Tong'an area, which is an important window to study Precambrian geological evolution of the western margin of the Yangtze Block.Petrology, whole-rock geochemistry and zircon U-Pb chronology of metamorphic gabbro in the Lixi area of Huili were studied.The results show that Lixi metamorphic gabbro was formed at 1508±21 Ma.In terms of petrogeochemical composition, it belongs to the subalkaline tholeiitic series, and is slightly enriched in light rare earth elements (LREE) on the whole, with no obvious depletion of high field strength elements such as Nb, Ta, Zr and Hf.Under the influence of later weathering, large ionic lithophile elements such as Ba, Sr incompatible elements are enriched in varying degrees.According to the petrogenesis analysis, the magma experienced a certain degree of separation and crystallization during the ascent, but was not obviously contaminated by the crust, and the mother magma might have come from the depleted asthenosphere mantle.The comprehensive analysis of geochemical and regional geological characteristics shows that the geochemical characteristics of Lixi metamorphic gabbro are similar to the E-MORB basalt, which might be formed in mid-ocean ridges or nearby seamount environment as a response to the breakup of Columbia supercontinent in the Yangtze Block.
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Key words:
- Yangtze Block /
- gabbro /
- Mesoproterozoic /
- geochemistry /
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/ - Columbia supercontinent
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表 1 黎溪变辉长岩主量、微量和稀土元素分析结果
Table 1. Analytical results of the major, trace and rare earth elements of the Lixi metamorphic gabbro
元素 19HQ01- YQ1 19HQ01- YQ2 19HQ01- YQ3 19HQ01- YQ4 19HQ01- YQ5 元素 19HQ01- YQ1 19HQ01- YQ2 19HQ01- YQ3 19HQ01- YQ4 19HQ01- YQ5 SiO2 46.01 46.79 46.93 47.48 46.37 Zr 81.0 65.9 69.2 71.8 74.0 TiO2 1.24 1.26 1.28 1.31 1.35 Nb 9.30 9.22 9.58 9.82 10.0 Al2O3 16.75 16.89 16.85 16.82 16.77 Sn 0.63 0.74 0.75 0.72 0.73 TFeO 17.64 16.67 17.09 16.53 17.23 Cs 0.55 0.65 0.45 0.47 0.51 MnO 0.20 0.18 0.18 0.18 0.18 Ba 153 125 107 111 111 MgO 6.72 6.27 6.53 6.29 6.65 La 11.4 11.1 11.5 11.9 12.1 CaO 7.85 8.11 8.46 8.33 8.30 Ce 23.1 22.9 24.2 24.5 24.1 Na2O 2.38 2.72 2.72 2.85 2.54 Pr 3.09 3.08 3.23 3.40 3.25 K2O 0.33 0.35 0.32 0.29 0.31 Nd 13.3 13.5 14.6 14.9 14.2 P2O5 0.15 0.14 0.15 0.16 0.16 Sm 2.88 2.94 3.19 3.02 3.09 烧失量 4.42 3.97 3.85 3.63 4.11 Eu 1.09 1.16 1.20 1.25 1.20 总计 100.21 99.85 100.43 100.15 100.24 Gd 3.01 3.11 2.99 3.27 3.14 FeO 4.90 4.80 5.25 5.00 5.08 Tb 0.52 0.51 0.52 0.55 0.55 Li 23.1 18.8 16.6 17.5 18.7 Dy 3.00 3.15 3.25 3.35 3.26 Be 0.58 0.58 0.47 0.52 0.52 Ho 0.58 0.64 0.64 0.67 0.65 Sc 27.9 28.0 27.4 26.2 26.8 Er 1.59 1.64 1.68 1.67 1.71 V 190 182 186 184 184 Tm 0.24 0.22 0.22 0.24 0.23 Cr 108 97.6 93.0 91.1 94.0 Yb 1.47 1.49 1.50 1.61 1.57 Co 63.6 48.2 48.1 48.2 50.5 Lu 0.22 0.21 0.20 0.21 0.21 Ni 122 97.5 99.0 95.4 106 Hf 1.82 1.80 1.83 1.90 1.89 Cu 22.5 15.8 16.3 14.5 18.9 Ta 0.51 0.49 0.57 0.57 0.57 Zn 242 203 200 196 210 Tl 0.082 0.091 0.054 0.064 0.077 Ga 18.3 17.8 17.6 17.5 18.2 Pb 4.56 3.46 3.53 3.60 3.74 Rb 8.12 11.6 7.69 7.32 7.68 Th 0.96 0.89 0.90 0.94 0.91 Sr 374 407 394 405 399 U 0.32 0.27 0.28 0.31 0.38 Y 15.6 15.4 15.8 16.1 16.0 注：主量元素含量单位为%，微量和稀土元素含量单位为10-6 
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表 2 黎溪变辉长岩(19HQ01-TW3)锆石U-Th-Pb同位素分析结果
Table 2. Results of zircon U-Th-Pb dating of the metamorphic gabbro in the Lixi area
测点 含量/10-6 Th/U 同位素比值1σ 同位素年龄/Ma 谐和度 Pb 232Th 238U 207Pb/206Pb 207Pb/235U 206Pb/238U 207Pb/206Pb 207Pb/235U 206Pb/238U 19HQ01-TW3-1 836 1730.48 446.07 3.88 0.1112 0.0033 2.2491 0.0716 0.1464 0.0029 1820 54 1196 22.4 881 16.4 69% 19HQ01-TW3-2 1657 2761.53 564.59 4.89 0.0967 0.0021 2.4359 0.0534 0.1830 0.0026 1561 41.2 1253 15.8 1083 14.1 85% 19HQ01-TW3-3 322 310.82 182.17 1.71 0.0944 0.0023 3.6668 0.0947 0.2815 0.0042 1517 44.6 1564 20.6 1599 21.0 97% 19HQ01-TW3-4 798 1144.22 629.85 1.82 0.0960 0.0021 2.6126 0.0596 0.1970 0.0023 1547 41.2 1304 16.8 1159 12.4 88% 19HQ01-TW3-5 1448 3054.24 788.11 3.88 0.1137 0.0028 2.3153 0.0710 0.1465 0.0022 1859 44.4 1217 21.8 881 12.3 68% 19HQ01-TW3-6 479 421.88 335.51 1.26 0.0933 0.0021 3.5466 0.0817 0.2758 0.0037 1494 47.2 1538 18.3 1570 18.9 97% 19HQ01-TW3-7 1259 1308.32 557.37 2.35 0.0909 0.0019 3.5114 0.0772 0.2797 0.0036 1456 39.4 1530 17.4 1590 18.3 96% 19HQ01-TW3-8 1312 1903.14 878.39 2.17 0.0954 0.0021 2.8980 0.0689 0.2197 0.0030 1536 41.2 1381 18.0 1281 16.0 92% 19HQ01-TW3-9 804 949.23 285.53 3.32 0.0961 0.0032 3.2235 0.1188 0.2424 0.0047 1550 63.0 1463 28.6 1399 24.4 95% 19HQ01-TW3-10 1294 1351.32 536.14 2.52 0.0957 0.0024 3.5294 0.0898 0.2678 0.0039 1543 48.1 1534 20.2 1529 19.9 99% 19HQ01-TW3-11 1310 1326.42 556.29 2.38 0.0922 0.0023 3.3088 0.0784 0.2609 0.0046 1472 49.2 1483 18.5 1494 23.7 99% 19HQ01-TW3-12 1521 1673.09 651.29 2.57 0.0925 0.0025 3.3878 0.0908 0.2650 0.0043 1477 51.9 1502 21.0 1515 21.8 99% 19HQ01-TW3-13 687 1031.30 366.32 2.82 0.0920 0.0022 2.7955 0.0766 0.2191 0.0033 1466 44.4 1354 20.5 1277 17.6 94% 19HQ01-TW3-14 1679 2055.85 643.76 3.19 0.0955 0.0020 3.1003 0.0664 0.2351 0.0029 1539 39.7 1433 16.5 1361 15.1 94% 19HQ01-TW3-15 500 470.35 330.54 1.42 0.0914 0.0020 3.5399 0.0879 0.2791 0.0036 1457 40.7 1536 19.7 1587 18.1 96% 19HQ01-TW3-16 515 529.36 255.18 2.07 0.0941 0.0030 3.4657 0.1090 0.2677 0.0049 1510 60.8 1519 24.8 1529 24.9 99% 19HQ01-TW3-17 985 1067.38 540.71 1.97 0.0931 0.0020 3.4677 0.0825 0.2691 0.0039 1500 40.6 1520 18.8 1536 19.7 98% 19HQ01-TW3-18 979 1084.37 330.57 3.28 0.0954 0.0022 3.6112 0.0889 0.2743 0.0040 1536 48.6 1552 19.6 1562 20.2 99% 19HQ01-TW3-19 1065 1141.42 597.66 1.91 0.0916 0.0022 3.3787 0.0844 0.2665 0.0036 1459 44.8 1499 19.6 1523 18.3 98% 19HQ01-TW3-20 190 154.66 217.08 0.71 0.0953 0.0028 3.5223 0.1123 0.2675 0.0048 1544 55.6 1532 25.2 1528 24.4 99%
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② 四川省地质局攀西地质大队区调二队. 1∶5万关河幅区域地质测量报告. 1994.
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