大兴安岭扎兰屯地区格根敖包组碎屑锆石U-Pb年代学、地球化学特征及其地质意义

格根敖包组广泛分布在大兴安岭中段扎兰屯地区,为一套偏中性的火山岩及碎屑岩组合,主要为安山岩、安山质火山碎屑岩、英安岩及细-粉砂岩,夹薄层泥岩。笔者对格根敖包组中细碎屑岩进行了锆石LA-ICP-MS U-Pb年代学及古生物地层学研究,结果表明:格根敖包组细碎屑岩形成于晚石炭世至早二叠世,锆石U-Pb年龄集中于348.9,385.8,428.0和507.3 Ma 4个峰值;粉砂岩中存在Artisia sp., Eusigillaris sp.等化石。格根敖包组碎屑岩主要以细粒杂砂岩和细粒长石(岩屑)砂岩为主,主量元素平均质量分数w(SiO₂)=69.04%,w(Al₂O₃)=14.76%,w(MgO)=1.05%,w(CaO)=0.66%,w(Na₂O)=2.34%,w(K₂O)=2.65%;镁铝比值M为3.05~9.98,平均值7.18;稀土元素总量∑REE平均184.46×10^-6((124.06~261.96)×10^-6),δEu平均值0.71,δCe平均值0.99,LREE富集,HREE亏损。上述结果表明,格根敖包组地层形成于大陆岛弧-活动陆缘附近,古地理显示为温暖潮湿气候下的海陆交互相沉积环境。

U-Pb Geochronology of Detrital Zircon and the Constraint of Geochemistry from the Gegen'aobao Formation in Middle of Zalantun Area of Da Hinggan Mountains and Its Tectonic Significance

The Gegen’aobao Formation is widely exposed in the middle of Da Hinggan area.It is a combination of neutral partial volcanic and clastic rocks with main composition of andesite,andesitic pyroclastic rocks,dacite,fine-siltstone,and thin mudstone.We obtained the age of fine grained clastic rock in Gegen’aobao Formation by means of LA ICP MS zircon U Pb dating;and also discussed the paleontology stratigraphy of Gegen’aobao Formation.The results show that the fine clastic rock was formed in Late Carboniferous to Early Permian.Four main ages were observed,348.9,385.8,428.0 and 507.3 Ma.Several fossils like Artisia sp.,Eusigillaris sp.were found in siltstone.Fine-grained greywacke and feldspathic greywacke (debris)sandstones are the major clastic rocks in the Gegen’aobao Formation.The average bulk chemical composition of these rocks is SiO2 69.04%,Al2 O3 14.76%, MgO 1.05%,CaO 0.66% ,Na2 O 2.34%,and K2 O 2.65% in weight.Mg/Al ratio (M)is 3.05 to 9.98,with an average value of 7.18.The total weight of REE (∑REE)ranges from 124.06 × 10 -6 to 261.96× 10 -6 with the average value of 184.46 × 10 -6 .The value forδEu andδCe are 0.71 and 0.99 respectively.The clastic rocks are characterized by LREE enrichment and HREE depletion.These results indicate that Gegen’aobao Formation was formed between the continental island arc and the active continental margin,under a warm and humid paleoclimate condition.