The discovery of sedimentary intercalations in Emeishan basalt in the Zhaotong area, Northeastern Yunnan: the new evidence of episodic eruption
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摘要:
首次在滇东北昭通地区峨眉山玄武岩中识别出3套沉积夹层,显示峨眉山玄武岩主喷发期内至少存在3次较长的喷发间歇期,以沉积夹层为亚旋回划分标志,将研究区峨眉山玄武岩划分为4个喷发亚旋回,沉积夹层的存在为峨眉山玄武岩幕式喷发提供了新的关键性证据。研究区峨眉山玄武岩第三亚旋回和第四亚旋回之间的沉积夹层沉积厚度大、沉积特征明显,本次工作选取其中的玄武质细砂岩进行了LA-ICP-MS测年,获得锆石U-Pb年龄为261.6 ± 0.6 Ma,代表了昭通地区峨眉山玄武岩主喷发期的时间,同时限定了沉积夹层的沉积时间下限。峨眉山玄武岩的喷发中心和火山机构展布可能受到区域上深大断裂的影响和控制,喷发形式为多期次多点位喷发,而沉积夹层发育于玄武岩多点喷发形成的小型山间凹陷汇水盆地。
Abstract:Three sets of sedimentary intercalations were systematically identified in Emeishan basalt in the Zhaotong area, Northeastern Yunnan for the first time, indicating that there were at least three long eruption intermissions during the main eruption period of Emeishan basalt.Taking the sedimentary intercalations as the subcycle division mark, the Emeishan basalt in the study area could be divided into four eruption subcycles.The existence of sedimentary intercalation provides a new key evidence for the episodic eruption of Emeishan basalt.The sedimentary intercalations between the third and fourth subcycles of Emeishan basalt in the study area have large sedimentary thickness and obvious sedimentary characteristics.In this work, the basaltic fine sandstone was selected for LA-ICP-MS dating, and the zircon U-Pb age was 261.6 ± 0.6 Ma.This age represents the time of the main eruption period of Emeishan basalt in the Zhaotong area, and defines the lower limit of the sedimentation time of the sedimentary intercalation.The eruption center and volcanic mechanism distribution of Emeishan basalt may be influenced and controlled by the deep faults in the region.The eruption form is multi-stage and multi-point eruption, while the sedimentary intercalations are developed in a small intermountain depression catchment basin formed by multi-point eruption of basalt.
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Key words:
- U-Pb dating /
- sedimentary intercalation /
- Emeishan basalt /
- Zhaotong /
- geological survey engineering /
- Yunnan Province
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表 1 PM027-2样品LA-ICP-MS锆石U-Th-Pb同位素分析结果
Table 1. The LA-ICP-MS zircon U-Th-Pb isotopic analysis results of PM027-2 sample
编号 元素含量/10-6 Th/U 同位素比值 年龄/Ma 谐和度
/%Pb Th U 207Pb/
206Pb1σ 207Pb/
235U1σ 206Pb/
238U1σ 207Pb/
206Pb1σ 207Pb/
235U1σ 206Pb/
238U1σ 208Pb/
232Th1σ PM027-2-1 5.6 126.1 97.7 1.2907 0.0509 0.0044 0.294 0.028 0.04132 0.00077 236 180 262 22 261 5 256 14 100 PM027-2-2 10.5 196.0 221.0 0.8869 0.0549 0.0037 0.312 0.024 0.04098 0.00078 408 139 276 19 259 5 275 16 107 PM027-2-3 12.6 217.0 231.8 0.9362 0.0514 0.0026 0.300 0.018 0.04238 0.00067 259 109 266 14 268 4 262 12 99 PM027-2-4 5.9 59.4 120.7 0.4921 0.0515 0.0035 0.295 0.022 0.04161 0.00077 1008 122 825 42 760 14 756 43 100 PM027-2-6 4.7 96.5 85.1 1.1340 0.0485 0.0049 0.277 0.028 0.04140 0.00095 124 183 248 22 262 6 249 15 95 PM027-2-7 3.4 46.8 65.9 0.7097 0.0534 0.0050 0.298 0.029 0.04134 0.00089 346 180 265 23 261 6 272 19 102 PM027-2-8 4.1 57.2 81.1 0.7053 0.0539 0.0046 0.295 0.025 0.04088 0.00088 367 153 262 20 258 5 259 16 102 PM027-2-9 8.8 94.4 190.7 0.4950 0.0552 0.0034 0.312 0.020 0.04150 0.00072 530 336 289 47 259 11 273 46 105 PM027-2-11 47.0 956.0 799.0 1.1965 0.0551 0.0018 0.324 0.014 0.04279 0.00051 416 75 285 11 270 3 280 11 106 PM027-2-12 12.5 276.1 214.3 1.2884 0.0534 0.0029 0.304 0.019 0.04146 0.00061 346 115 270 15 262 4 255 11 103 PM027-2-13 3.2 61.5 58.3 1.0549 0.0481 0.0063 0.264 0.035 0.04130 0.00120 104 234 238 28 261 7 242 18 91 PM027-2-14 7.1 146.0 119.5 1.2218 0.0498 0.0036 0.293 0.023 0.04264 0.00078 186 143 261 18 269 5 267 14 97 PM027-2-15 2.5 33.3 46.9 0.7100 0.0504 0.0068 0.279 0.038 0.04250 0.00120 213 251 250 30 268 7 283 26 93 PM027-2-17 3.8 65.2 69.3 0.9408 0.0506 0.0051 0.288 0.030 0.04188 0.00098 223 191 257 24 264 6 273 18 97 PM027-2-18 10.3 226.1 176.0 1.2847 0.0499 0.0031 0.284 0.019 0.04149 0.00071 190 121 254 15 262 4 260 12 97 PM027-2-19 7.2 179.2 114.1 1.5706 0.0520 0.0040 0.292 0.023 0.04126 0.00088 285 140 260 18 261 5 259 12 100 PM027-2-20 33.2 815.0 540.4 1.5081 0.0515 0.0020 0.297 0.014 0.04166 0.00053 515 162 289 22 263 5 252 17 100 PM027-2-23 4.5 99.6 78.1 1.2753 0.0510 0.0049 0.295 0.029 0.04196 0.00082 241 186 262 23 265 5 251 14 99 PM027-2-24 3.3 44.9 64.2 0.6994 0.0564 0.0057 0.320 0.033 0.04140 0.00110 468 183 282 25 262 7 250 20 108 PM027-2-25 7.8 111.6 145.5 0.7670 0.0540 0.0036 0.312 0.022 0.04192 0.00072 371 129 276 17 265 4 271 14 104 PM027-2-26 2.5 28.4 51.0 0.5569 0.0533 0.0064 0.291 0.034 0.04090 0.00100 342 218 259 27 258 6 265 24 100 PM027-2-27 8193.0 1396.0 1763.0 0.7918 0.1261 0.0022 5.988 0.210 0.34380 0.00420 2044 45 1974 31 1905 20 1873 66 107 PM027-2-28 17.0 234.3 353.7 0.6624 0.0540 0.0024 0.312 0.017 0.04170 0.00055 371 99 276 13 263 3 264 11 105 PM027-2-29 13.1 304.9 217.4 1.4025 0.0535 0.0028 0.309 0.018 0.04162 0.00069 350 102 273 14 263 4 264 12 104 PM027-2-30 19.4 111.7 236.1 0.4731 0.0566 0.0021 0.551 0.026 0.07000 0.00110 476 76 446 17 436 7 442 22 102 PM027-2-31 21.0 91.5 122.1 0.7494 0.0682 0.0025 1.301 0.060 0.13690 0.00190 875 72 846 26 827 11 820 35 102 PM027-2-32 2.9 39.8 58.7 0.6787 0.0486 0.0058 0.273 0.034 0.04124 0.00100 129 231 245 27 261 6 242 20 94 PM027-2-33 3.7 75.1 68.1 1.1028 0.0530 0.0052 0.302 0.031 0.04115 0.00093 329 190 268 24 260 6 253 15 103 PM027-2-34 8.1 127.9 153.7 0.8321 0.0512 0.0034 0.300 0.022 0.04146 0.00070 250 136 266 17 262 4 262 13 102 PM027-2-35 14.0 193.7 275.7 0.7026 0.0540 0.0025 0.307 0.016 0.04116 0.0006100 371 91 272 12 260 4 258 12 105 -
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