THE LINJIA FORMATION IN BENXI, EAST LIAONING PROVINCE ASSIGNED TO MIDDLE TRIASSIC: Evidences from Zircon, Fossil and Stratigraphy
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摘要:
林家组出露于辽宁东部本溪林家崴子一带.自小林贞一1942年记载以来，其时代一直有很大的争议.争议跨度从第三系到上二叠统.20世纪80年代初，张武等依据动植物化石和地层对比，提出了林家组属于中三叠世的观点.由于林家组产出的植物化石十分复杂，既有古生代，又有中生代分子，且没有锆石年龄证据，故林家组时代仍然存疑.近年来，在林家组底部找到了变质凝灰质粉砂岩，其中有很多锆石.根据30颗保存较好锆石测年结果，发现4颗最年轻锆石的206Pb/238U平均年龄是239.5±4.1 Ma.另外，最近前甸子地区林家组发现了少林肋木（Pleuromeia shaolinii）.因此尽管林家植物群存在二叠纪和三叠纪均有分布的重要植物，但结合锆石定年、地层对比、岩性特征、平行不整合接触关系、三叠纪代表植物及中三叠世到早白垩世均有分布的昆虫索德蠊属来综合分析，可以确认林家组确实属于中三叠世.
Abstract:The Linjia Formation is outcropped in Linjiawaizi area, Benxi, eastern Liaoning Province. Since recorded by Kobayashi in 1942, its age has long been a great controversy. In the early 1980s, Zhang et al. suggested that the Linjia Formation should be assigned to Middle Triassic on the basis of the plant fossils fish scales, insect wings, spores and pollens, as well as stratigraphic correlation. As the components of Linjia flora are very complicated with both Paleozoic and Mesozoic elements but without evidences of zircon ages, the age of Linjia Formation has not yet been determined. Recently, the metamorphic tuffaceous siltstones containing lots of zircons were found at the bottom of Linjia Formation. According to the dating results of 30 grains of zircon crystal, the average 206Pb/238U age of 4 newest zircons was 239.5±4.1 Ma. In addition, Pleuromeia shaolinii was lately discovered from Linjia Formation in Qiandianzi area. Although some significant fossil plants of Linjia flora are distributed striding cross Permian and Triassic, combined with the zircon dating, stratigraphic correlation, lithologic characteristics, parallel unconformity relation, typical Triassic fossil plants and the Middle Triassic-Early Cretaceous insect fossil Sogdoblatta, it is confirmed that the Linjia Formation belongs to Middle Triassic.
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Key words:
- zircon U-Pb age /
- Pleuromeia /
- Sogdoblatta /
- Linjia Formation /
- Middle Triassic /
- Benxi of Liaoning Province
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表 1 林家组变质凝灰质粉砂岩锆石U-Pb年龄
Table 1. Zircon U-Pb ages of the metamorphic tuffaceous siltstones in Linjia Formation
样品号 含量/10-6 年龄/Ma Pb Th U 207Pb/206Pb 1σ 206Pb/238U 1σ SYNU-LJ-L1-01-01 73 65 186 1902 35 1630 20 SYNU-LJ-L1-01-02 152 104 338 1922 31 1885 35 SYNU-LJ-L1-01-03 20 412 299 287 69 255 4 SYNU-LJ-L1-01-04 5 147 97 187 168 213 4 SYNU-LJ-L1-01-05 159 201 332 1873 35 1861 29 SYNU-LJ-L1-01-06 125 22 210 2539 46 2383 65 SYNU-LJ-L1-01-07 202 170 285 2555 31 2524 27 SYNU-LJ-L1-01-08 153 151 236 2566 40 2459 47 SYNU-LJ-L1-01-09 285 275 401 2532 29 2511 26 SYNU-LJ-L1-01-10 289 147 439 2564 36 2491 41 SYNU-LJ-L1-01-11 240 65 652 1906 32 1674 23 SYNU-LJ-L1-01-12 150 108 345 1910 29 1833 19 SYNU-LJ-L1-01-13 283 185 530 2174 32 2136 24 SYNU-LJ-L1-01-14 100 61 164 2529 40 2346 30 SYNU-LJ-L1-01-15 46 17 67 2679 33 2671 40 SYNU-LJ-L1-01-16 107 45 261 1885 35 1865 32 SYNU-LJ-L1-01-17 212 94 568 1847 30 1660 18 SYNU-LJ-L1-01-18 80 68 177 1898 47 1854 22 SYNU-LJ-L1-01-19 614 404 1394 1950 33 1856 31 SYNU-LJ-L1-01-20 43 357 913 280 61 239 5 SYNU-LJ-L1-01-21 199 57 531 1813 30 1684 22 SYNU-LJ-L1-01-22 231 87 626 1831 25 1684 21 SYNU-LJ-L1-01-23 148 78 263 2487 31 2221 25 SYNU-LJ-L1-01-24 141 193 204 2509 37 2338 30 SYNU-LJ-L1-01-25 282 254 506 2457 28 2126 33 SYNU-LJ-L1-01-26 168 87 465 1803 30 1597 18 SYNU-LJ-L1-01-27 235 159 864 1795 32 1326 36 SYNU-LJ-L1-01-28 185 153 598 1754 35 1367 20 SYNU-LJ-L1-01-29 19 306 312 265 78 251 4 SYNU-LJ-L1-01-30 108 110 298 1798 39 1542 22 
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表 2 林家组属于中三叠世的锆石、古生物和地层证据
Table 2. Evidences of zircons, fossils and stratigraphy for Linjia Formation assigned to Middle Triassic
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