大兴安岭北段嘎仙沟地区中侏罗世花岗闪长岩锆石U-Pb年龄、地球化学特征及其地质意义

Zircon U-Pb age,geochemical characteristics and geological significance of Middle Jurassic granodiorite in Gaxiangou area,northern Great Xingan Range

大兴安岭北段嘎仙沟地区中侏罗世花岗闪长岩的LA-ICP-MS锆石U-Pb年龄为172.7±1.1 Ma。全岩SiO_(2)含量为62.81%~68.78%,全碱(ALK)和Al_(2)O_(3)含量较高,FeO^(T)、MgO和TiO_(2)含量较低;里特曼指数σ=2.27~5.33,Na_(2)O/K_(2)O=1.19~3.57,Mg^(#)=38.0~49.0,属于中高钾钙碱性系列;铝饱和指数A/CNK=0.91~0.97,为准铝质岩石。稀土总量为(117.70~146.32)×10^(-6),(La/Yb)_(N)值为11.70~18.10,呈轻稀土富集、重稀土亏损的右倾型配分模式;具轻微的铕负异常(δEu=0.86~0.98),表明源区没有明显的斜长石残留或分离结晶;较高的Zr/Hf、Nb/Ta和Sr/Y值,低Y和Yb含量,富集大离子亲石元素Rb、Sr和Ba,亏损高场强元素Nb、Ta和Ti,指示源岩具有壳源特征。P_(2)O_(5)随SiO_(2)含量的增加而降低,T_(Zr)值为700.50~781.20℃,表明花岗闪长岩的成因类型为I型花岗岩,并具有C型埃达克岩特征。结合区域上的资料表明,中侏罗世花岗闪长岩形成于由碰撞向后碰撞过渡环境,蒙古-鄂霍茨克洋在中侏罗世的构造体系表现为由挤压向伸展、由地壳增厚向减薄的构造体制转换,与中侏罗世岩浆活动明显减弱相对应。

LA-ICP-MS zircon U-Pb age of the Middle Jurassic granodiorite is 172.7±1.1 Ma in Gaxiangou area,northern Great Xingan Range.The whole-rock SiO_(2)content ranges from 62.81%to 68.78%with higher whole-alkali(ALK)and Al_(2)O_(3)and lower FeO^(T),MgO and TiO_(2)tontents.Rittman index(σ)is from 2.27 to 5.33,Na_(2)O/K_(2)O=1.19-3.57,Mg^(#)values from 38.0 to 49.0,aluminum saturation index(A/CNK)=0.91-0.97,illustrating the granodiorite belongs to medium-high potassium calc-alkaline series and metaluminous rocks.TheΣREE=(117.70-146.32)×10^(-6),and(La/Yb)_(N)=11.70-18.10,show a right-leaning distribution pattern with enrichment in light rare earth elements and relative depletion in heavy rare earth elements.Slightly Eu negative anomaly(δEu=0.86-0.98)suggests that there is no significant plagioclase remains or fractional crystallization in the source region.The high Zr/Hf,Nb/Ta and Sr/Y ratios,low Y and Yb contents,enrichment of the large ionic lithophilic elements Rb,Sr and Ba,and depletion of the high-field-strength elements Nb,Ta and Ti indicate that the source rocks are crust-derived.P_(2)O_(5) decreases with increasing SiO_(2)content and T_(Zr) values range from 700.50 to 781.20℃.These indicate that the diagenetic type of this granodiorite is an I-type granite with C-type edakite characteristics.Combined with the regional data,the Middle Jurassic granodiorites were formed in the transition from collision to post-collision,and the Mongolian-Okhotsk Ocean tectonic system in the Middle Jurassic was characterized by a tectonic regime shift from extrusion to extension,and from crustal thickening to thinning,which is corresponding to the significant decrease of magmatic activity in the Middle Jurassic.