太原西山上古生界含煤地层最大沉积年龄的碎屑锆石U-Pb定年约束及地层意义

Constraints on U-Pb Dating of Detrital Zircon of the Maximum Depositional Age for Upper Paleozoic Coal-Bearing Strata in Xishan,Taiyuan and Its Stratigraphic Significance

太原西山是华北地区上古生界含煤地层标准层型剖面所在地，自1882年以来，已广泛进行了古生物学、地层学、沉积学等方面的工作，取得了大量的研究成果。但是，由于缺乏绝对年代学数据，年代地层界线多次变更，从而影响了地层划分与对比。为了确定太原西山晚古生代含煤地层的沉积时间，应用LA-MC-ICPMS对该剖面太原组、山西组及下石盒子组作为标志层的8个砂岩样品进行了碎屑锆石U-Pb定年，确定了它们的最大沉积年龄，为地层形成时限的确定提供了依据。太原组底部晋祠砂岩及上部的七里沟砂岩的YC1δ（24-）年龄分别为296±4Ma和277土2Ma，且在七里沟砂岩-A中，62颗锆石构成了271～301Ma年龄谱，其单颗粒最年轻年龄（YSG）271±7Ma代表了其最大沉积年龄。山西组底部北岔沟砂岩及其上覆下石盒子组底部的骆驼脖子砂岩所获得的U-Pb年龄均大于七里沟砂岩，不能用来限定山西组的最大沉积年龄。与太原组牙形石生物地层的比较表明，太原组沉积于早二叠世是确定无疑的，但是，碎屑锆石U_Pb定年结果表明太原组包含Asselian、Sakmarian、Artinskian和Kungurian4个时期的沉积，甚至进入了中二叠世，而非牙形石确定的只为Asselian期沉积，石炭-二叠系界线可能位于晋祠砂岩底界或其以下。结合前人对太原西山地区及华北晚古生代盆地的沉积学、岩石学、事件地层学等的结果，认为不能排除在晚石炭世-早二叠世存在盆内岩浆与火山活动的可能性。

The standard stratotype section of Late Paleozoic coal-bearing strata of North China located in Xishan, Taiyuan. Since 1882, this section has been widely investigated on paleontology, stratigraphy, and sedimentology and fruitful achievement has been obtained. However, due to the lack of absolute chronology data, the chronostratigraphic boundaries have been changed several times, which affects the division and correlation of strata. To determine the timing for the formation of Late Paleozoic coal-bearing strata in Xishan, Taiyuan, a laser ablation multi-collector inductively coupled plasma mass spectrometry （LA-MC-ICPMS） was applied to date the U-Pb age of detrital zircon. Eight sandstone samples from the marker layers of strata division and correlation from Taiyuan, Shanxi and Xiashihezi Formations were analyzed and their maximum depositional ages were determined. Jinci sandstone in the lower part and and Qiligou sandstone in the upper of Taiyuan Formation showed YCIδ（2＋） ages are 296±4 Ma and 277±2 Ma, respectively. 62 zircons from Qiligou sandstone-A, constitute an age spectrum of 271-301 Ma, and the minimum age of the single zircons （YSG） is 271±7 Ma, representing the maximum depositional age. The U-Pb ages derived from both the Beichagou sandstone at the bottom of the Shanxi Formation and the Luotuobozi sandstone at the bottom of the Lower Shihezi Formation are greater than that of the Qiligou sandstone, so they cannot be used to constrain the maximum depositional age of the Shanxi Formation. Comparison with the conodont biostratum of the Taiyuan Formation indicates that the Taiyuan Formation was undoubtedly formed during the Early Permian. However, the U-Pb dating results of the detrital zircon reveal that the Taiyuan Formation deposited for four stages, including the Asselian, Sakmarian,