东昆仑黑刺沟金矿区晚奥陶世花岗斑岩的成因:锆石U-Pb年龄、岩石地球化学和Sr-Nd-Pb-Hf同位素制约

Genesis of Late Ordovician granite porphyry at Heicigou gold deposit,East Kunlun:constraints from zircon U-Pb age,geochemistry and Sr-Nd-Pb-Hf isotopic systematics

黑刺沟金矿区位于东昆仑造山带西段,区内花岗斑岩呈脉状沿北西向构造与金矿化带近平行产出,是近年勘查工作新圈定的与金矿化有成因关系的酸性岩脉。对研究区花岗斑岩进行LA-ICP-MS锆石U-Pb测年,全岩地球化学及Sr-Nd-Pb-Hf同位素研究,探讨其形成时代、岩石成因、构造环境、岩浆源区等。研究结果表明,花岗斑岩锆石U-Pb年龄为445.8±2.4 Ma(MSWD=0.36),形成时间为晚奥陶世。岩石SiO_(2)含量在75.17%~78.94%之间,K_(2)O+Na_(2)O含量为5.49%~7.07%,K_(2)O/Na_(2)O=0.01~0.03,相对贫钾富钠,贫MnO(0.05%~0.07%)、MgO(0.08%~0.25%)、P_(2)O_(5)(0.07%~0.08%)和TiO_(2)(0.15%~0.18%),属低钾钙碱性岩石系列。A/CNK值为0.92~0.99,A/NK值为1.1~1.14,为准铝质岩石。稀土元素总量介于132.4×10^(-6)~183.95×10^(-6)之间,LREE/HREE=8.04~8.78,微量元素显示Ba、Th、Pb相对富集,K、P、Ti亏损,微量元素组成特征具有I型花岗岩特征。Sr-Nd-Pb-Hf同位素显示,样品具有高的I_(Sr)值(0.7133~0.7158),低ε_(Nd)(t)值(-4.45~-5.15),t_(DM)年龄为1.54~1.6 Ga,初始铅同位素比值(^(206)Pb/^(204)Pb)_(i)=18.218~18.375,(^(207)Pb/^(204)Pb)_(i)=15.63~15.738,(^(208)Pb/^(204)Pb)_(i)=38.22~38.544,锆石ε_(Hf)(t)值为-11.12~+0.13,t_(DM2)=1.42~1.65 Ga。综合认为,花岗斑岩形成于晚奥陶世原特提斯洋向北俯冲消减环境下的陆缘弧环境,岩浆来源于下地壳中—古元古代物质的部分熔融。

The Heicigou gold deposit is located in the southern segment of the East Kunlun Orogen.The graniteporphyry in the area which is parallel to the gold metallogenic belt along the NW trending structure,is a newly delineated felsic dike related to the gold mineralization in recent years.Based on LA-ICP-MS zircon U-Pb dating,Sr-Nd-Pb-Hf isotopic systematics,and whole-rock geochemistry,the formation age,petrogenesis,tectonic setting,and magma source of the granite porphyry are discussed in detail.The results show that the zircon U-Pb age of the granite porphyry is 445.8±2.4 Ma(MSWD=0.36),indicating an age of the Late Ordovician.The granite porphyry contains 75.17%~78.94%SiO_(2),10.22%~12.8%Al_(2)O_(3),5.49%~7.07%K_(2)O+Na_(2)O,K_(2)O/Na_(2)O ranging from 0.01 to 0.03,0.05%~0.07%MnO,0.08%~0.25%MgO,0.07%~0.08%P_(2)O_(5),and 0.15%~0.18%TiO_(2),suggesting that the rock belongs to the low-K calc-alkaline rock series.The A/CNK and A/NK values are 0.92~0.99 and 1.1~1.14,respectively,indicating that the rock is metaluminuous.TheΣREE concentration ranges from 132.4×10^(-6)to 183.95×10^(-6),with the LREE/HREE ratios varying between 8.04 and 8.78.The primitive mantle-normalized trace element spidergrams are characterized by enrichment in Ba,Th,and Pb and depletion in Nb,Ta,and Ti,similar to I-type granites in volcanic arc settings.The granite porphyry exhibits high I_(Sr)(0.7133~0.7158),(^(207)Pb/^(204)Pb)_(i)but low ε_(Nd)(t)(-4.45~-5.15)and ε_(Hf)(t)(-11.12~+0.13),similar to magmas derived from the lower crust.It is believed that the granite porphyry was formed in a continental margin arc setting due to the northward subduction of the Proto-Tethys Ocean in the Late Ordovician,and that the magma was originated from the partial melting of the lower crust.