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OACSTPCD

LA-ICP-MS zircon U-Pb age of the Middle Jurassic granodiorite is 172.7±1.1 Ma in Gaxiangou ar-ea,northern Great Xing'an Range.The whole-rock SiO2 content ranges from 62.81%to 68.78%with higher whole-alkali(ALK)and Al2O3 and lower FeOT,MgO and TiO2 tontents.Rittman index(σ)is from 2.27 to 5.33,Na2O/K2O=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.The ∑REE=(117.70-146.32)×10-6,and(La/Yb)N =11.70-18.10,show a right-leaning distribu-tion 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 ele-ments Nb,Ta and Ti indicate that the source rocks are crust-derived.P2O5 decreases with increasing SiO2 con-tent and TZr 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.