Characteristics and geological significance of Early Proterozoic turbidite in Yuanjiang of central Yunnan Province
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摘要:
对滇中元江地区调查研究发现一套以钙质浊积岩-碎屑浊积岩组合为特征的岩石序列。测制的剖面由2个钙质浊积岩-碎屑浊积岩旋回组成，发育大量不完整的鲍马序列组合及粒序层理、斜层理、包卷层理、槽模构造、透镜状构造等典型浊积沉积构造，且岩石成熟度及分选度都较低。上部还发育一套凝灰质球粒石英岩，粒序层理发育，在其中获得了LA-ICP-MS锆石U-Pb年龄1102±76Ma和1713±48Ma（YJgzn-03），具有明显铅丢失现象；对侵入地层中的石英正长斑岩进行年代学研究，获得了LA-ICP-MS锆石U-Pb年龄349±13Ma和1732±13Ma（YJgzn-02），上部未发生明显铅丢失的平均年龄为1718±30Ma，应为成岩期年龄。两者的锆石都具环带结构，Th/U值都大于0.4，都明显发生了铅丢失，且形成时代相近，应为同期岩浆作用的结果。推测该套地层沉积或受到后期热蚀变年龄为1713±48~1718±30Ma，其沉积年龄应早于古元古代晚期。对比区域昆阳群及前人划分的古元古代地层，认为这套地层下伏于昆阳群之下，应属于早于古元古代晚期（1.7Ga）的碳酸盐前缘-陆棚斜坡浊积岩体系，建议依据地名定义为古元古代"曼林组"（Pt1m）。
Abstract:In this study, the authors identified a sedimentary sequence of calcareous-clastic turbidites through geological survey in Yuanjiang County of central Yunnan Province. The sedimentary sequence section consists of two calcareous-clastic turbidite forms, and contains a series of sedimentary tectonics, such as Boma sequence, graded bedding, oblique bedding, slump structures and flute marks. These sedimentary rocks show low maturity and sorting under the microscope. At the upper section, there exists about a tuffed globular quartzite bed 4m in thickness, whose LA-ICP-MS zircon U-Pb ages are 1102±76Ma and 1713±48Ma(YJgzn-03), and the zircon exhibits the loss of common Pb; another sample of quartz-orthophyre that intruded into the turbidite bed has the LAICP-MS zircon U-Pb ages 349±13Ma and1732±13Ma(YJgzn-02), and the sample without loss of common Pb has the mean age of 1718±30Ma, which could be regarded as the intrusive age of quartz-orthophyre. All of the zircons in the two samples that show the same ages are marked by girdle structure in CL images and Th/U>0.4, characterized by loss of common Pb. According to the zircon ages of tuff and intrusive rocks (quartz-orthophyre), the turbidite bed might have been deposited earlier than 1713±48~1718±30Ma. Based on correlation with the Proterozoic sedimentary beds and Kunyang Group, the authors hold that the sedimentary sequence of calcareous-clastic turbidite the authors studied in this paper should be formed earlier than Kunyang Group, being products of Proterozoic form or even earlier. The authors suggest that it should be named Proterozoic"Manlin Formation"because it is near Manlin Village in Yuanjiang County.
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表 1 凝灰岩样品YJgzn-03锆石U-Th-Pb同位素分析结果
Table 1. Zircon U-Th-Pb isotopic compositions of the tuff sample YJgzn-03
测点号 Pb Th U Th/U 207Pb/206Pb 207Pb/235U 206Pb/238U 207Pb/206Pb 207Pb/235U 206Pb/238U 10-6 比值 1σ 比值 1σ 比值 1σ 年龄/Ma 1σ 年龄/Ma 1σ 年龄/Ma 1σ 1 40.7 78.1 136 0.57 0.1005 0.0025 3.7304 0.0974 0.2673 0.0038 1635 46.8 1578 20.9 1527 19.5 2 89.4 194 276 0.70 0.1010 0.0022 3.9139 0.0838 0.2792 0.0035 1643 33.5 1617 17.4 1587 17.8 3 169.1 415 498 0.83 0.1250 0.0071 4.7422 0.3685 0.2584 0.0042 2029 101 1775 65.2 1482 21.7 4 99.9 321 314 1.02 0.0921 0.0020 3.2584 0.0737 0.2546 0.0034 1470 40.1 1471 17.6 1462 17.6 5 212 1222 849 1.44 0.0784 0.0015 1.9492 0.0369 0.1792 0.0021 1167 37.0 1098 12.7 1062 11.5 6 57.8 138 186 0.74 0.0965 0.0020 3.4144 0.0727 0.2551 0.0028 1567 39.7 1508 16.8 1465 14.5 7 44.7 103 142 0.72 0.0941 0.0020 3.3147 0.0734 0.2539 0.0031 1511 40.4 1485 17.3 1458 15.9 8 126.5 318 359 0.89 0.0991 0.0017 3.8290 0.0687 0.2783 0.0030 1609 31.6 1599 14.5 1583 15.3 9 180.3 320 535 0.60 0.0990 0.0017 3.8270 0.0696 0.2786 0.0033 1606 32.9 1598 14.7 1585 16.7 10 70.2 278 287 0.97 0.0759 0.0014 1.9837 0.0404 0.1888 0.0025 1094 37.3 1110 13.8 1115 13.7 11 49.3 127 135 0.94 0.1021 0.0022 3.8716 0.0836 0.2742 0.0034 1663 34.3 1608 17.5 1562 17.1 12 87.5 192 248 0.77 0.0996 0.0016 3.8264 0.0620 0.2779 0.0030 1617 29.8 1598 13.1 1581 15.1 13 143.5 387 372 1.04 0.1028 0.0015 4.0180 0.0639 0.2824 0.0030 1676 25.9 1638 13.0 1603 15.2 14 60.1 147 170 0.86 0.0985 0.0020 3.6514 0.0746 0.2688 0.0030 1595 33.2 1561 16.3 1535 15.5 15 129.9 421 493 0.85 0.0813 0.0012 2.2899 0.0362 0.2036 0.0019 1229 29.6 1209 11.2 1195 10.2 16 65.7 182 243 0.75 0.0818 0.0014 2.4052 0.0399 0.2135 0.0024 1240 34.1 1244 11.9 1247 12.7 17 165.5 382 723 0.53 0.0785 0.0011 2.0806 0.0338 0.1914 0.0021 1161 27.3 1142 11.2 1129 11.2 18 52.4 106 144 0.74 0.1002 0.0018 3.9534 0.0749 0.2859 0.0034 1628 33.3 1625 15.4 1621 16.9 19 141.0 474 358 1.32 0.0990 0.0014 3.7054 0.0573 0.2708 0.0028 1606 22.1 1573 12.4 1545 14.2 20 63.5 169 172 0.99 0.0995 0.0016 3.7504 0.0707 0.2719 0.0030 1617 34.3 1582 15.2 1550 15.0 21 87.0 200 230 0.87 0.1028 0.0017 4.0389 0.0712 0.2839 0.0027 1676 25.5 1642 14.4 1611 13.5 22 128.7 274 344 0.80 0.1007 0.0015 3.9718 0.0578 0.2858 0.0031 1637 27.8 1628 11.8 1621 15.5 
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