大兴安岭北段龙江地区早白垩世晶洞花岗岩的成因及构造意义

Petrogenesis of Early Cretaceous miarolitic monzogranite in Longjiang region of northern Great Xing’an Range and its tectonic implications

对大兴安岭北段龙江地区头道沟花岗岩进行岩相学、锆石U-Pb年代学、全岩地球化学和Lu-Hf同位素研究,限定花岗岩形成时代、成因类型及构造背景。研究区花岗岩为二长花岗岩,发育晶洞构造,锆石U-Pb年龄为129±2 Ma,暗示其为早白垩世岩浆作用的产物。全岩地球化学数据显示这些二长花岗岩样品具有高硅(SiO 2=74.23%~75.88%)、富钾(K 2O=4.45%~5.96%)、贫铁(TFe 2O 3=0.86%~1.30%)和贫镁(Mg^#=22.5~32.9)的特征,为准铝质-弱过铝质系列(A/CNK=1.03~1.11),结合其负铕异常(δEu=0.50~0.85)特征,认为研究区二长花岗岩为高分异I型花岗岩。样品富集轻稀土元素和Rb、K等大离子亲石元素,亏损重稀土元素和Nb、Ta、Ti、P等高场强元素,类似于典型的活动大陆边缘花岗岩。Lu-Hf同位素分析显示研究区二长花岗岩具有正εHf(t)值(+6.6^+12.3)和较年轻的二阶段模式年龄(T DM2=549~760 Ma),暗示其岩浆源于新元古代新增生的地壳物质的部分熔融。结合中生代大兴安岭北段构造-岩浆演化特征,认为龙江地区早白垩世头道沟岩体的形成与古太平洋板块西向俯冲引起的岩石圈拆沉作用有关。

Petrography,zircon U-Pb geochronology,whole-rock geochemistry and Lu-Hf isotopic compositions for Toudaogou miarolitic monzogranite in Longjiang area of northern Great Xing an Range have been studied to investigate their precise geochronological framework,petrogenesis and tectonic setting.The results show that Toudaogou pluton is composed mainly of monzogranite and displays miarolitic structures.The Toudaogou miarolitic monzogranite was emplaced in the Early Cretaceous with an age of 129±2 Ma.Geochemically,these monzogranites are characterized by high silica(SiO 2=74.23%~75.88%),potassium-rich(K 2O=4.45%~5.96%),poor iron(TFe 2O 3=0.86%~1.30%),low Mg^#values(22.5~32.9)and negative Eu anomalies(δEu=0.50~0.85),with the nature of metaluminous to weakly peraluminous,suggesting that they belong to highly fractionated I-type granite.They are also enriched in LREE and large-ion lithophile elements(LILEs;e.g.,Rb and K),and depleted in high field strength elements(HFSEs;e.g.,Nb,Ta,Ti,and P),and which bear resemblances to granite formed in an active continental margin setting.Primary zircons from the studied monzogranite haveεHf(t)values of+6.6 to+12.3,with juvenile Hf two-stage model ages(T DM2)of 549 Ma to 760 Ma,indicating that the parental magma was generated by partial melting of Neoproterozoic accreted crustal materials.Combined with the tectonic evolution characteristics of the northern Great Xing an Range during the Mesozoic,the authors advocated that the generation of the Early Cretaceous miarolitic monzogranite in Longjiang area was related to the lithospheric delamination caused by the westward subduction of the Paleo-Pacific plate.