内蒙古赵井沟新元古代花岗岩成因:年代学、地球化学及Sr-Nd-Pb-Hf同位素约束

Petrogenesis of the Neoproterozoic Granites in Zhaojinggou,Inner Mongolia:Constraints from Geochemistry,Geochronology and Sr-Nd-Pb-Hf Isotopic Composition

为研究内蒙古中部新元古代的地质作用过程与岩浆演化,在详细的野外调查和岩相学观察的基础上,对华北板块北缘大青山赵井沟地区发现的新元古代花岗岩开展了全岩主微量元素、锆石年代学及Sr-Nd-Pb-Hf同位素分析。分析结果显示,该区域新元古代花岗岩具有高硅(72.54%~75.67%)、富碱(8.70%~10.15%),贫Ca及低Fe、Mg的特点,Ga/Al(2.90~3.44)较高,具有明显的负Eu异常,其稀土配分曲线呈“海鸥型”展布,Rb、Th、U、Ta等元素较为富集,Ba、Sr、P和Ti等元素强烈亏损,显示出造山后A型花岗岩的地球化学特征;具有震荡环带结构的岩浆成因锆石的LA-ICP-MS谐和年龄为(765±5.0)Ma,表明该花岗岩为新元古代岩浆结晶演化的产物;样品εNd(t)变化于-7.3^-2.7,εHf(t)为-15.9^-6.3,与负的εHf(t)对应的TDM2(地壳二阶段模式年龄)介于2045~2662 Ma,暗示该花岗岩的初始物质源于较为古老的地壳物质,但存在幔源混入,这可能是伸展构造背景下地幔岩浆上涌所致。综合区域研究成果及地球化学特征,内蒙古赵井沟新元古代A型花岗岩的形成主要与华北板块北缘古元古代地壳物质的部分熔融有关,是对Rodinia超大陆裂解的岩浆响应。

To study the Neoproterozoic geological process and magmatic evolution in the central part of Inner Mongolia in the northern margin of North China plate,the Neoproterozoic granites distributed in Zhaojinggou area,Daqing Mountain,Inner Mongolia were canvassed in the field and petrographic observation in laboratory.Chemical components of whole-rock major and trace elements,zircon chronology,and Sr,Nd,Pb,and Hf isotopes were analyzed.Results show that the Neoproterozoic granites are characteristic of high silicon(content 2.54%~75.67%),rich alkali(8.70%~10.15%),poor in Ca,and low in Fe,Mg contents.The Ga/Al value(2.90~3.44)is high,with obvious negative Eu anomaly.The REE distribution curve is“seagull-wing”shaped.Rb,Th,U,Ta etc.are relatively rich,Ba,Sr,P,Ti etc.are strongly depleted,showing post-orogenic A-type fingerprint in geochemical characteristics.The LA-ICP-MS age of zircon with oscillatory ring structure is(765±5.0)Ma,indicating that the granite is the product of the Neoproterozoic magmatic activities.TheεNd(t)value of the sample varies from-7.3 to-2.7,theεHf(t)value from-15.9 to-6.3,and the T DM2(crustal two-stage model age)value that corresponds to the negativeεHf(t)value is between 2045 Ma and 2662 Ma,suggesting that the granite is originated from an old crustal material source that mixed with some mantle materials,which may be caused by the upwelling of mantle magma under the background of extensional tectonics.Based on the regional research results and geochemical characteristics,the formation of the Neoproterozoic A-type granites is mainly related to the partial melting of the Paleoproterozoic crustal materials in the northern margin of North China plate,which is a magmatic response to the breakup of the Rodinia supercontinent.