Ti-Augite in Sediments of the Jianghan Plain as Tracing Mineral: Implication for the Evolution of the Yangtze River
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摘要: 江汉平原是长江穿过三峡地区后的第一个大型卸载盆地,其沉积物是采取“从源到汇”物质追踪思路进行三峡贯通和长江演化示踪研究的理想载体,含钛普通辉石是长江上游(攀西地区)广泛分布的特征矿物,可以作为长江三峡贯通研究的示踪矿物.通过对位于盆地沉积中心的周老孔岩心沉积物及长江和汉江现代沉积物中重矿物(粒级为0.063~0.125 mm)中的辉石进行提取、丰度统计和矿物化学成分分析,一方面,发现长江和汉江现代沉积物中辉石的物源完全不同,其中长江的辉石主要来自攀西地区;另一方面,周老孔岩心从深度104 m处开始出现含钛普通辉石,并且由此处向上辉石含量明显增加、辉石的类型组成明显变化,表明在岩心深度104 m(古地磁年龄1.1 Ma左右)位置处长江三峡已经贯通.而深度45 m处及其上部的辉石类型组成与现代长江相似,表明在古地磁年龄0.4 Ma左右长江已经基本发育成现代的规模.Abstract: The Jianghan plain is the first large-scale offloading basin of the Yangtze River after the formation of the Three Gorges. Its sediment deposits should reflect that the Yangtze River used to flow through. Ti-augite, a characteristic rock of Panxi area, where the formational conditions is unique in the Yangtze River basin, can be used as a tracing mineral to analyze the formation of the Three Gorges. The analysis of the percentage content and the electron microprobe analysis of the pyroxenes in Zhoulao core fine sediments between 0.063 mm and 0.125 mm show that the content of pyroxenes increases after 104 meters and Ti-augites appear. This line of change (104 m) dated of about 1.1 Ma attest a change of sediment provenance. So we conclude that the Three Gorges was formed around the time 1.1 Ma. From that time, materials from upper Three Georges has deposited in the Jianghan basin as a result of the forming of the Yangtze River. The characteristics of the pyroxenes composing the first 45 meters dated around (0.45 Ma) and those of the current Yangtze River are alike, meaning that during that period the Yangtze River already develops to its current pattern.
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Key words:
- Jianghan plain /
- Quaternary /
- sediments /
- Ti-augite /
- evolution of the Yangtze River /
- geophysics
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表 1 周老镇钻孔岩心(0.063~0.125 mm粒级)各样品重矿物中普通辉石的电子探针统计结果
Table 1. Electron microprobe analysis of augite of samples (in 0.063~0.125 mm size fraction) in Zhoulao core
岩心深度(m) 15 30 45 61 87 96 104 120 127 143 165 197 229 285 探针片数(个) 1 1 1 1 2 2 3 2 1 1 1 1 1 1 鉴定颗粒数(颗) 135 112 91 86 279 312 500 143 184 132 144 103 154 97 探针实验数(颗) 15 11 7 8 24 16 36 9 8 14 15 10 10 10 含钛普通辉石(颗) 3 1 0 0 1 0 2 0 0 0 0 0 0 0 表 2 周老镇钻孔岩心、长江现代沉积物(0.063~0.125 mm粒级)以及攀西地区部分岩体中含钛普通辉石的电子探针数据(%)
Table 2. Electron microprobe analysis of pyroxenes of amples (in 0.063~0.125 mm size fraction) in Zhoulao core, the sediments of Yangtze River and Ti-augite of rocks from the Panxi rift (%)
取样位置描述 探针点号 Na2O K2O Cr2O3 MgO CaO MnO Al2O3 TiO2 FeO* SiO2 Total 岩心深度15 m g1-3-1 0.337 - 0.341 15.355 20.041 0.194 2.562 1.302 8.400 51.110 99.642 岩心深度15 m g1-4-2 0.268 0.008 0.390 15.210 21.960 0.093 3.750 1.404 6.027 50.381 99.491 岩心深度15 m g1-6 0.321 - 0.232 14.887 19.297 0.259 2.257 1.050 9.830 51.811 99.944 岩心深度30 m g2-7 0.332 0.015 0.233 11.954 23.354 0.235 1.104 1.080 9.202 51.997 99.506 岩心深度87 m Z6-2 0.333 0.016 3.770 14.514 20.689 0.074 3.079 0.912 4.658 46.030 94.075 岩心深度104 m z8-a-1 0.456 0.010 0.018 14.818 20.466 0.288 3.244 1.247 8.475 51.244 100.266 岩心深度104 m 2002-8-6 0.385 0.031 1.703 14.855 20.932 0.082 3.572 1.079 4.889 47.165 94.693 罗龙(长江现代沉积物) LL-1 0.494 - - 13.860 20.846 0.267 3.315 1.560 8.525 51.354 100.221 罗龙(长江现代沉积物) LL-4 0.243 0.005 0.101 14.629 20.995 0.174 4.471 1.595 7.857 49.799 99.869 罗龙(长江现代沉积物) LL-6 0.483 0.005 0.131 14.315 21.274 0.237 3.221 1.148 7.338 50.247 98.399 罗龙(长江现代沉积物) LL-7 0.493 - 0.474 15.158 22.286 0.104 2.185 1.921 6.321 50.671 99.613 重庆(长江现代沉积物) chq-5 0.640 - 0.009 14.950 21.798 0.196 3.699 1.302 6.697 50.012 99.303 重庆(长江现代沉积物) chq-6 0.447 0.001 - 14.188 20.256 0.249 3.221 1.511 8.807 50.994 99.674 重庆(长江现代沉积物) chq-9 0.501 - 0.007 14.336 23.206 0.116 4.036 1.326 5.426 50.488 99.442 重庆(长江现代沉积物) chq-10 0.444 - 0.060 14.559 21.207 0.173 3.507 1.435 7.295 50.865 99.545 宜昌(长江现代沉积物) JT-2 0.606 0.017 0.057 14.519 21.180 0.159 3.525 1.629 7.430 50.587 99.709 宜昌(长江现代沉积物) JT-5 0.409 0.004 - 14.840 21.596 0.178 3.270 1.031 7.991 50.457 99.776 宜昌(长江现代沉积物) JT-8 0.638 - - 14.613 21.316 0.196 3.656 1.584 7.563 50.333 99.899 宜昌(长江现代沉积物) JT-9 0.473 0.024 - 14.992 21.064 0.265 2.389 1.145 8.007 51.765 100.124 峨眉山玄武岩① M4-2 0.330 - 0.080 10.590 19.940 0.300 2.630 1.550 16.200 48.200 99.830 峨眉山玄武岩① R12-4 0.440 - 0.170 15.780 20.030 0.150 4.560 1.180 7.220 51.310 100.840 峨眉山玄武岩① D12-2-2 0.310 - 0.660 14.180 20.630 0.130 2.900 0.970 5.900 50.670 99.690 峨眉山玄武岩① Z17-1-1 0.400 - 0.160 15.870 18.200 0.210 3.250 1.080 9.610 51.950 100.750 攀枝花岩体② LG020 0.570 - - 14.160 21.580 0.400 2.710 1.020 8.430 50.860 99.750 罗茨鸡街岩体③ g-43-1 0.750 0.020 0.050 14.270 24.040 0.060 3.620 1.290 5.290 50.710 100.110 注:*数据表示全铁;“-”数据表示微量(<0.010);①数据引用自黄开年(1988);②数据引用自万睿等(1986);③数据引用自吴静等(1994). -
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