乌兰浩特地区中性火山岩LA-ICP-MS锆石U-Pb年龄、地球化学特征及成因探讨

LA-ICP-MS zircon U-Pb age, geochemistry and genesis of intermediate volcanic rocks in the Ulanhot area, Inner Mongolia

乌兰浩特地区出露一套以辉石安山岩为主的中性火山岩组合。LA-ICP-MS锆石U-Pb测年结果显示,该套火山岩锆石U-Pb年龄为130.0±1.5 Ma和129.9±1.4 Ma,表明其形成于早白垩世。辉石安山岩SiO 2含量为57.84%~60.86%,Al 2O 3含量为16.49%~17.76%,A/CNK>1.1,属过铝质岩石;ALK值为11.19%~11.87%,σ为1.08~1.98;∑REE为(128.69~145.06)×10^-6,(La/Sm)N为4.64~5.24,(La/Yb)N为6.58~8.09,轻稀土元素富集程度高且轻、重稀土元素分馏明显;Eu异常不明显,大离子亲石元素Rb、Ba、K、Sr富集,高场强元素Nb、Ti、P、Ta亏损,推断该中性火山岩的形成与蒙古—鄂霍次克洋闭合陆壳加厚之后的岩石圈伸展有关。

A set of intermediate volcanic rock assemblages dominated by pyroxene andesite is exposed in the Ulanhot area.The LA-ICP-MS zircon U-Pb dating results show that the volcanic rocks have two zircon U-Pb ages of 130.0±1.5 Ma and 129.9±1.4 Ma,suggesting that the volcanic rocks were formed in the Early Cretaceous.Pyroxene andesite has SiO 2 content of 57.84%~60.86%,Al 2O 3 of 16.49%~17.76%,A/CNK>1.1,suggesting that the rock belongs to peraluminous rock.ALK value is 11.19%~11.87%,σis 1.08~1.98.∑REE of(128.69~145.06)×10^-6,(La/Sm)N of 4.64~5.24 and(La/Yb)N of 6.58~8.09 show enrichment of light rare earth elements and distinct fractionation of light and heavy rare earth element.Weak Eu anomaly,enrichment of large ion lithophile elements(Rb,Ba,K and Sr)and depletion of high field strength elements(Nb,Ti,P and Ta)suggest that its formation is related to the lithospheric extension which followed continental crust thickening after closure of the Mongolian-Okhotsk Ocean.