鄂尔多斯盆地南缘晚奥陶世钾质斑脱岩——SHRIMP测年及其成因环境

K-Bentonites from the Jinsushan Formation of Late Ordovician,Southern Ordos Basin: SHRIMP Dating and Tectonic Environment

鄂尔多斯盆地南缘上奥陶统金粟山组以深水碳酸盐岩沉积为特征,含多层桔黄色凝灰质粘土岩。矿物和化学成分分析表明,其主要由伊利石和伊蒙混层粘土矿物组成,含少量石英、长石和锆石等中酸性岩浆矿物,富K2O,属钾质斑脱岩;微量元素分析显示本区斑脱岩的源岩为中酸性岩浆成因,源于同碰撞火山弧构造环境。应用SHRIMP技术对其中的锆石进行了U-Pb测年,取得了(451.5±4.9)～(452.1±5.1)Ma、(457.5±5.1)Ma和(465.8±8.3)Ma 3组谐合年龄,前两者分别与欧美广布的Millbrig-Kinnekulle和Deicke斑脱岩同时,但15种化学组分的多元统计分析显示,金粟山组的斑脱岩与欧美同期著名的斑脱岩可能并不同源。形成本区斑脱岩的火山凝灰质可能源自沿商丹洋盆北缘展布的火山弧喷发,而鄂尔多斯盆地南缘晚奥陶世发生的强烈沉降和沉积转换可能与北秦岭弧后盆地的拉伸与扩张密切相关。

The Late Ordovician Jinsushan Formation in southern Ordos basin is characterized by carbonate of deep marine facies,and contains several marked tuffaceous clay beds of orange color.Mineral and geochemical analysis show that the clay beds mainly consist of illite and mixed illite-montmorillonite in composition,with some quartz,feldspar and zircon minerals that derived from intermediate to acidic magma,rich in K2O,and are of typical K-bentonites.Analysis on trace elements and geochemical composition of the bentonites suggests that their source rock is likely derived from syn-collision island-arc volcanism.SHRIMP U-Pb dating on zircons from the bentonites has yielded three sets of ages：（451.5±4.9） to（452.1±5.1） Ma,（457.5±5.1）Ma and（465.8±8.3）Ma,with the former two identical to those of the well-known Millbrig-Kinnekulle and Dicke bentonites in Laurentia and Baltica.However,stepwise discriminant analysis on chemical composition of the bentonites and comparison with those from the Midcontinent of United States indicate that they are not congruent in chemical fingerprints,therefore may not have resulted from the same episodes of the giant eruptions.The tuffaceous materials in the Jinsushan bentonites were most likely from volcanic eruptions of the island-arcs along north side of the Shang-Dan oceanic basin.The strong basement subsidence at southern margin of the Ordos basin and the significant change in the sedimentary framework there in early Late Ordovician may be best interpreted as depositional responses to the tectonic extension and subsiding initiated in the back-arc basin north to the Erlangping island-arc.