大兴安岭中部乌兰浩特地区林西组碎屑岩锆石U-Pb年龄、地球化学特征及其构造意义

Zircon U-Pb age, geochemical characteristics and tectonic implications of detrital zircon from Linxi Formation in the Wulanhaote area, central Da Hinggan Mountains

内蒙古乌兰浩特地区出露一套缺少化石、遭受接触变质作用的厚层碎屑岩。在该套地层采集碎屑锆石样品,进行碎屑锆石U-Pb测年研究。80个锆石分析点的年龄可分为3个主要年龄区间:242~294Ma,年龄加权平均值为263.6±3.3Ma(n=50,MSWD=4.9),峰值年龄为261Ma;301~381Ma,峰值年龄为348Ma;454~530Ma,峰值年龄为487Ma。另有7个锆石分析点的年龄分别为824Ma、836Ma、859Ma、867Ma、1279Ma、1556Ma、2447Ma。261Ma的峰值年龄限定了该地层的沉积下限,即属晚二叠世林西组。主量元素SiO2含量为67.81%,Al2O3为18.22%,MgO为1.44%,CaO为0.41%,Na2O为1.54%,K2O为3.90%,K2O/Na2O为1.15~10.21;Al2O3/(CaO+Na2O)为4.87~26.38;稀土元素总量介于162.39×10^-6~223.46×10^-6之间,平均值为200.88×10^-6,负Eu异常,Ce为1.00~1.05,轻稀土元素富集,重稀土元素亏损,微量元素以亏损Nb、Ta、Sr,富集Rb、Ba、La、Ce、Pb、Nd、Sm为特征。以上分析表明,林西组形成于活动大陆边缘构造背景。依据碎屑锆石年龄谱值信息,林西组沉积物源具多样性和复杂性,除东北各地块,同时存在华北板块和西伯利亚板块的物源信息,说明林西组沉积时期华北板块与西伯利亚板块可能已经开始俯冲碰撞过程。

There are a set of thick layers of rocks in Wulanhaote of Inner Mongolia which are lack of fossil and suffered from contact metamorphism. LA-ICP-MS U-Pb isotope test was performed on the zircons from the clastic rock samples. The concordant-near concordant ages of 80 zircons were obtained. The 80 detrital zircons can be divided into three groups according to age and frequency distribution characteristics: 242~294Ma with peak age at 261 Ma;301~381Ma with peak age at 348, and 454~530Ma with peak age at 487Ma. In addition, other 7 zircons have ages of 824Ma, 836Ma, 859Ma, 867Ma, 1279Ma, 1556Ma and 2447Ma. The weighted average age for the youngest age group is 263.6±3.3Ma (MSWD=4.9, n=50), consistent with the minimum peak age of 261 Ma. It is inferred that the maximum deposition age of the Linxi Formation is 261 Ma. The major chemical composition of these rocks is SiO2 67.81%, Al2O3 18.22%,MgO 1.44%, CaO 0.41%,Na2O 1.54% and K2O 3.90%, with K2O/Na2O between 1.15 and 10.21, A12O3/(CaO + Na2O) between 4.87 and 26.38. The value of rare earth element ΣREE is in the range of 162.39 × 10^- 6~223.46 × 10^- 6, averagely 200.88 × 10^- 6.δEu is in the range of 0.58~0.71,averagely 0.64.δCe is between 1.00 and 1.05. The clastic rocks are characterized by LREE enrichment and HREE depletion. Trace elements are characterized by depletion of Nb, Ta, Sr and enrichment of Rb, Ba, La, Ce, Pb, Nd, Sm.These data show that the Linxi Formation was formed in an active continental marginal structural background. Based on detrital zircon age profile information, the authors hold that sediment sources in Linxi Formation were diverse and complex, in addition to blocks in the northeast, there existed provenance information for the North China plate and the Siberia plate at the same time, suggesting that the North China plate and the Siberia plate probably began the subduction collision process during the deposition period of the Linxi Formation.