大兴安岭中段五岔沟地区早白垩世花岗质岩地球化学特征及其成因

Geochemical characteristics and petrogenesis of Early Cretaceous granitoids in Wuchagou region of central Great Xing'an Range

对大兴安岭中段五岔沟地区晚中生代花岗质岩石进行了锆石U-Pb年代学、Lu-Hf同位素和全岩地球化学研究。锆石U-Pb测年结果显示,该区晚中生代花岗质岩石形成于134~127 Ma,为早白垩世岩浆活动的产物。在地球化学特征上,这些花岗质岩石普遍具有高硅(SiO_2=65.21%~78.43%)、富碱(K_2O+Na_2O=6.77%~9.66%)、贫CaO(0.10%~2.55%)和MgO(0.07%~1.30%)的特征,富集Rb、Th、U等大离子亲石元素,亏损Nb、Ta、Ti、P等高场强元素,属于高钾钙碱性I型花岗质岩石。锆石ε_(Hf)(t)=+4.9^+8.4,t_(DM2)=654~874Ma,表明其源于新元古代期间新增生的地壳物质的部分熔融,结合区域研究资料,认为兴安岭中段五岔沟地区早白垩世花岗质岩的形成与古太平洋板块的俯冲作用关系密切。

The authors studied the zircon U-Pb geochronogy,Lu-Hf isotopic compositions and whole-rock major and trace element data for Late Mesozoic granitiods in the Wuchagou area of the central Great Xing＇an Range,NE China. The zircon U-Pb dating shows that the granitiods were formed in Early Cretaceous,ranging from 134 to127 Ma in age. Geochemically,the granitiods are characterized by high silica（ SiO_2= 65. 21% - 78. 43%） and total alkali concentrations（ K_2O ＋ Na_2O = 6. 77% - 9. 66%）,but low concentrations of CaO（ 0. 10% - 2. 55%）and MgO（0. 07% - 1. 30%）. They are also enriched in large-ion lithophile elements（e. g. Rb,Th,U） and depleted in high field elements（ e. g. Nb,Ta,Ti,and P）,indicating that they can genetically be ascribed to high--K calc-alkaline I-type granites. The positive ε_（Hf）（t） values（ ＋ 4. 9 to ＋ 8. 4） of the Early Cretaceous granites and their corresponding Hf two-stage model ages of 654 Ma to 874 Ma indicate that the parental magma was derived from partial melting of Neoproterozioc juvenile crustal material. Combining with the regional research data,the authorsconsidered that the formation of Early Cretaceous granites in the Wuchagou area was related to the subduction of Paleo-Pacific plate.