Zircon U-Pb age of bertonite of Upper Ordovician Wufeng Formation in Southeast Yangtze platform and its geological significance
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摘要:
扬子板块东南缘思南地区上奥陶统顶部-下志留统底部地层中普遍发育一层斑脱岩。选取思南县罗湾沱上奥陶统五峰组剖面进行LA-ICP-MS锆石U-Pb测年,获得其年龄加权平均值为449±11Ma,限定了该区域奥陶系-志留系地层沉积年龄,为扬子板块东南缘思南地区奥陶系-志留系界线附近的火山喷发事件、地层年代学研究提供了较可靠的证据。449±11Ma的年龄与扬子板块北西大巴山一带的年龄数据高度吻合,但略晚于华北克拉通西南缘奥陶系顶部斑脱岩的年龄(449.0~465.8Ma)。其所代表的火山活动可能与华夏板块向扬子板块俯冲有关。晚奥陶世-早志留世晚期,多期次的火山喷发事件影响了当时的海洋化学条件、生态圈环境,造成中-晚奥陶世以来的生物灭绝事件及冰川启动。
Abstract:A layer of bentonite is usually developed in upper Ordovician and lower Silurian strata of southeast Yangtze platform in Sinan area. Sampling was done in Wufeng Formation of upper Ordovician and Longmaxi Formation of lower Silurian in Luowantuo of Sinan, high precision U-Pb dating was conducted and as a result, the stratigraphic sedimentary age of Wufeng Formation and Longmaxi Formation in this area were constrained. The data obtained provide reliable evidence for volcanic eruption and stratigraphic chronology near Ordovician and Silurian boundary of southeast Yangtze platform in Sinan area. The age data are correlated well with the data obtained in Dabashan of northwest China, which is a little later than the age of Ordovician bentonite in southeast of the North China Craton (449.0~465.8Ma). The volcanic activity was probably related to the subduction of Cathaysian plate to Yangtze plate. During Middle-Late Ordovician and late Early Silurian, many volcanic eruptions influenced the chemical condition and ecological environment of the ocean in that period and caused the extinction and glacier initiation since middle-late Ordovician.
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Key words:
- Southeast Yangtze platform /
- bentonite /
- zircon U-Pb age /
- volcanic eruption /
- extinction
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表 1 贵州思南罗湾沱斑脱岩LA-ICP-MS锆石U-Th-Pb同位素分析结果
Table 1. LA-ICP-MS zircon U-Th-Pb analytical data of the bentonites from the Luowantuo section in Sinan, Guizhou
编号 元素含量/10-6 Th/U 同位素比值 表面年龄 Pb 232Th 238U 207Pb/206Pb 207Pb/206Pb 207Pb/235U 207Pb/235U 206Pb/238U 206Pb/238U 207Pb/206Pb 207Pb/206Pb 207Pb/235U 207Pb/235U 206Pb/238U 206Pb/238U 208Pb/232Th 208Pb/232Th 谐和度 1σ 比值 1σ 比值 1σ 比值 年龄值/Ma 1σ 年龄值/Ma 1σ 年龄值/Ma 1σ 年龄值/Ma 1σ PM24-15-1 8 133 80 1.665 0.1066 0.009 0.9913 0.0829 0.0677 0.0017 1743 153 699 42 422 10 442 15 50% PM24-15-2 10 154 93 1.656 0.0561 0.002 0.5992 0.0208 0.0773 0.0019 457 69 473 13 486 11 406 11 97% PM24-15-3 11 180 102 1.765 0.0811 0.003 0.8461 0.0307 0.0754 0.0018 1233 62 623 17 469 11 409 11 71% PM24-15-4 12 185 102 1.810 0.0871 0.005 0.8950 0.0566 0.0738 0.0022 1363 106 469 30 459 13 431 11 97% PM24-15-5 12 194 126 1.540 0.0583 0.002 0.5437 0.0175 0.0676 0.0016 543 65 441 12 422 6 365 9 95% PM24-15-6 13 219 122 1.796 0.0572 0.002 0.5853 0.0184 0.0742 0.0019 498 64 468 12 461 11 397 11 98% PM24-15-7 13 298 140 2.129 0.0680 0.002 0.5816 0.0215 0.0620 0.0015 870 74 469 14 468 9 334 9 99% PM24-15-8 15 249 147 1.701 0.0772 0.007 0.7651 0.0774 0.0708 0.0020 1128 185 577 45 441 12 409 17 73% PM24-15-9 16 292 146 2.005 0.0589 0.001 0.5990 0.0178 0.0738 0.0022 565 54 477 11 459 9 401 11 96% PM24-15-10 16 274 158 1.738 0.0649 0.003 0.6374 0.0254 0.0717 0.0022 772 91 491 16 446 10 401 11 91% PM24-15-11 17 359 162 2.209 0.0643 0.002 0.5902 0.0186 0.0666 0.0017 754 57 471 12 416 10 350 9 87% PM24-15-12 18 368 164 2.244 0.0716 0.002 0.7075 0.0248 0.0714 0.0019 976 54 543 15 445 11 387 10 80% PM24-15-13 19 372 171 2.178 0.0602 0.002 0.5744 0.0179 0.0692 0.0016 609 63 461 12 432 10 398 11 93% PM24-15-14 21 382 182 2.096 0.0921 0.005 0.9370 0.0730 0.0722 0.0026 1469 96 471 38 449 16 397 14 95% PM24-15-15 25 501 223 2.244 0.0570 0.001 0.5623 0.0172 0.0716 0.0019 500 54 453 11 446 11 409 11 98% PM24-15-16 25 198 293 0.675 0.0911 0.003 0.8808 0.0296 0.0700 0.0016 1450 58 461 16 446 10 470 16 96% PM24-15-17 30 407 289 1.406 0.0554 0.001 0.5900 0.0156 0.0774 0.0019 432 54 471 10 480 12 421 12 97% PM24-15-19 32 284 193 1.476 0.0679 0.002 1.1544 0.0292 0.1240 0.0036 865 49 433 14 421 20 687 16 96% PM24-15-20 43 368 426 0.863 0.0947 0.003 1.1306 0.0409 0.0867 0.0021 1522 66 768 19 536 13 357 10 64% PM24-15-21 44 383 246 1.557 0.0870 0.003 1.4879 0.0435 0.1246 0.0037 1361 57 925 18 757 21 768 21 79% PM24-15-24 48 336 293 1.146 0.1251 0.006 2.0987 0.1313 0.1209 0.0050 2031 82 1148 43 736 28 755 31 56% PM24-15-25 52 527 290 1.817 0.0958 0.003 1.5431 0.0420 0.1169 0.0032 1546 52 948 17 713 19 649 19 71% PM24-15-27 59 653 428 1.525 0.0674 0.002 0.9245 0.0337 0.0986 0.0029 852 50 420 18 409 7 537 15 91% PM24-15-28 68 277 385 0.720 0.0821 0.002 1.7185 0.0581 0.1506 0.0045 1250 36 465 22 432 6 808 24 92% PM24-15-29 69 471 502 0.938 0.1260 0.005 1.8561 0.0873 0.1067 0.0030 2043 76 1066 31 654 17 685 18 52% PM24-15-30 71 351 466 0.753 0.1131 0.005 1.8192 0.0716 0.1187 0.0035 1850 84 1052 26 723 20 854 22 62% PM24-15-31 73 655 635 1.031 0.1102 0.004 1.3490 0.0520 0.0891 0.0024 1802 67 867 22 550 14 494 21 55% PM24-15-32 76 506 559 0.906 0.1005 0.003 1.5457 0.0523 0.1113 0.0034 1635 62 949 21 680 20 590 18 67% PM24-15-33 81 384 551 0.697 0.0811 0.002 1.4118 0.0443 0.1259 0.0029 1233 53 454 19 435 16 588 17 95% PM24-15-34 117 720 742 0.970 0.1018 0.002 1.7841 0.0541 0.1266 0.0028 1657 44 457 20 427 16 657 14 93% -
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