甘肃龙首山青山堡花岗闪长岩锆石U-Pb年龄、地球化学特征及地质意义

Zircon U-Pb dating age,geochemical characteristics and geological significance of Qingshanbu granodiorite in Longshou Mountain area,Gansu

为探讨龙首山东段青山堡花岗闪长岩的形成时代、岩石类型及其构造环境,采用LA-ICP-MS锆石U-Pb同位素测年,获得其年龄为(451.5±3.2)Ma,属晚奥陶世岩浆作用的产物;岩石地球化学分析结果表明,青山堡花岗闪长岩中w(SiO_(2))为67.06%~68.23%,全碱含量为6.36%~6.93%,w(Na_(2)O)>w(K_(2)O),A/CNK值为0.99~1.02,属弱过铝质高钾钙碱性I型花岗岩,具有Al_(2)O_(3)含量中等、富钾、钠而贫镁、铁、钛、锰的特征,轻稀土富集、重稀土亏损,富集微量元素Rb、Th、U而亏损Nb、P、Ti,表明岩浆来源于下地壳物质的部分熔融。结合区域构造演化,认为青山堡花岗闪长岩形成于北祁连洋板块向阿拉善地块俯冲的环境下,说明至少在451 Ma之前,北祁连洋板块在龙首山东段仍处于俯冲阶段,但已消减殆尽,为北祁连洋的闭合和陆弧碰撞阶段的开始提供了年龄上限约束。

In order to study the formation age,rock type and tectonic environment of Qingshanbu granodiorite in the eastern part of Longshou Mountain area,LA-ICP-MS zircon U-Pb isotopic dating method is used,the age of zircon is(451.5±3.2)Ma,which is the product of Late Ordovician magmatism.The result of rock geochemical analysis shows that the content of SiO_(2) in Qingshanbu granodiorite is 67.06%-68.23%,the content of total alkali(Na_(2)O+K_(2)O) is 6.36%-6.93%,and Na_(2)O>K_(2)O,A/CNK value is 0.99-1.02.It belongs to weakly peraluminous and high potassium calc-alkaline I-type granite.Granodiorite has the char-acteristics of medium Al_(2)O_(3) content,rich in K and Na but poor in Mg,Fe,Ti and Mn.Light rare earth ele-ments are enriched and heavy rare earth elements are depleted,while trace elements Rb,Th and U are en-riched and Nb,P and Ti are depleted.The characteristics indicates that the magma comes from the partial melting of lower crustal materials,and might be contaminated by a small amount of mantle derived materi-als.Combined with the regional tectonic evolution,it is considered that the Qingshanbu granodiorite is formed in the process of the subduction of the North Qilian ocean plate to the Alashan block.At least before 451 Ma,the North Qilian ocean plate is still at the subduction stage in the eastern part of Longshou Moun-tain area,but it has all disappeared.The dating result provides an upper age limit constraint for the closure of the North Qilian ocean and the beginning of continent arc collision stage.