青藏高原东北缘多福屯陆内基性火山岩的40Ar/39Ar同位素定年和地球化学特征及其构造启示

目的探讨青藏高原东北缘多福屯地区钠质基性火山岩与青藏高原隆升及南北构造带活动之间的关系。方法运用地球化学和同位素Sr-Nd-Pb特征分析及全岩40Ar/39Ar同位素进行研究。结果全岩40Ar/39Ar年龄结果显示,多福屯钠质基性火山岩的形成时代为96.21 Ma±2.10 Ma,为晚白垩世(K2)早期。岩石元素-同位素地球化学特征表明,该套火山岩岩浆形成于部分熔融。火山岩表现为特征的DUPAL(高放射成因铅)异常,属似OIB性质的板内碱性—偏碱性玄武岩,源区具有DM与EMⅡ混合特点。结论多福屯火山岩相关的岩浆活动源于板块边界动力学引起的远程大陆陆内构造效应,即印度板块与欧亚大陆的早期碰撞(100～85 Ma),由于藏北板块的刚性特点,应力实现远程传递到高原东北缘,导致南北复合构造带在西秦岭西段发生构造扰动,诱发了软流圈地幔的部分熔融,形成多福屯火山岩原生岩浆。

40Ar/39Ar isotopic dating,geochemistry and their tectonic implications of Duofutun Na-rich mafic volcanic rocks,the Northeastern margin of the Qinghai-Tibet Plateau

Aim To reveal the relationship between the Duofuton Na-rich mafic magmatism and the tectonic activity of the S-N trending tectonics,induced by the early uplifting of the Qinghai-Tibet Plateau.Methods Petrology,major and trace element,Sr-Nd-Pb isotopic research,and 40Ar/39Ar isotopic dating.Results The whole-rock 40Ar/39Ar age data suggest that the Duofuton Na-rich volcanics were formed at the interval of 96.21 Ma±2.10 Ma(early in K2).The formation of the Duofuton Na-rich magma was mainly controlled by partial melting,and generally not affected by fractional crystallization.The characteristics of elemental and Sr-Nd-Pb isotopic geochemistry indicate that the Duofuton Na-rich volcanics are OIB-like alkaline-subalkaline WPB and the source region exhibits a mixture of DM and EMⅡ.Conclusion It can be interpreted that the Duofuton Na-rich magma was generated by partial melting an asthenospheric mantle,induced by the tectonic activity of the S-N trending tectonics in the western part of WQ in the Late Cretaceous(K2,100~85 Ma),possibly due to the initial collision between the Eurasian Plate and a microcontinent in Northwest Indian Plate.