甘肃北山红岩井西南似斑状二长花岗岩LA-ICP-MS锆石U-Pb定年及其地质意义

在1∶5万红泉等5幅区域地质调查中，对红岩井西南似斑状二长花岗岩体进行了岩石地球化学和锆石U-Pb测年研究，并在该岩体中获得了LA-ICP-MS锆石U-Pb年龄为（402.2±4.4）Ma，属早泥盆世。岩石化学和地球化学特征表明似斑状二长花岗岩体富Si（SiO2为68.12%~75.76%）、富碱（ALK=7.69%~9.91%），铝饱和指数A/CNK＝1.02~1.05，里特曼指数σ＝1.8~3.8，平均为2.38；属钙碱性准铝质S型花岗岩系列；ΣREE＝175.4~296.76，球粒陨石标准化分布模式图上显示具海鸥型特征，富集HFSE、Y，强烈亏损Ba、Sr、Eu、P、Ti。构造图解反映出该花岗岩体形成于同碰撞-碰撞后，结合测区的地质特征及同位素年代特征，说明北山主造山期应为加里东末期。

Zircon LA-ICP-MS U-Pb Dating of the Porphyritic Monzonitic Granite from Southwest Hongyanjing in Beishan Area,Gansu Province and Its Significance

In this paper, the chronologic and geochemical researches were carried out on the porphyritic monzonitic granite in Southwest Hongyanjing.The zircon LA-ICP-MS U-Pb dating results show that the zircon U-Pb age of the intrusions is (402.2±4.4)Ma, which indicates that this intrusion was formed in the Early Devonian.Petrochemical and Geochemical data indicate that the porphyritic monzonitic granite are characterized by high Si (68.12%~75.76%)and high alkali (7.69%~9.91%), with A/CNK value of 1.4~1.5.Such geochemical characteristics, such as the rittmann index in sigma(σ)=1.8~3.8 (average 2.38), indicate that this intrusions belongs to the calc-alkaline strong peraluminous series.It displays higher total REE content(ΣREE=175.4~296.76).Trace elements HFSE and Y are well concentrated, but Ba, Sr, Eu, P, and Ti are poorly concentrated.The comprehensive analysis also suggests that the granite is strongly peraluminous S-type granite of the calc-alkaline suite,which was formed in the post-orogenic tectonic environment.