玉龙铜(钼)矿带多霞松多岩体成因:锆石U-Pb年代学和岩石地球化学约束

Petrogenesis of Duoxiasongduo masses in Yulong Cu(Mo)ore belt:Constraints from zircon U-Pb geochronology and petrogeochemistry

玉龙铜(钼)矿带的成因机制和动力学背景一直存在争议。本文选择目前研究程度较低的多霞松多岩体,对该岩体含矿斑岩开展岩相学、年代学、岩石地球化学及锆石Hf同位素分析。研究表明,多霞松多岩体主要由二长花岗斑岩和碱长花岗斑岩组成,其中,二长花岗斑岩的主要造岩矿物为钾长石、斜长石和石英,碱长花岗斑岩的主要造岩矿物为钾长石和石英。两种岩石属于高钾钙碱性—钾玄质侵入岩,具有富碱高钾的特征;亏损Nb、Ti等高场强元素,富集Rb、Th、U等大离子亲石元素和Pb;稀土元素含量较高(ΣREE=145.04×10^(-6)~290.91×10^(-6)),富集轻稀土(LREE/HREE=6~37)。铕表现为从二长花岗斑岩的弱负异常(Eu/Eu^(*)=0.66~0.84)到碱长花岗斑岩的中等负异常(Eu/Eu^(*)=0.35~0.39)。锆石U-Pb定年显示,二长花岗斑岩的成岩年龄为38.3±0.6 Ma,碱长花岗斑岩的成岩年龄为37.8±1.0 Ma,两者形成时代基本一致,均属于喜马拉雅期,表明在多霞松多地区始新世存在一期重要的岩浆热液Cu-Mo成矿事件。多霞松多含矿斑岩的锆石ε_(Hf)(t)范围在+1.3~+2.6之间。结合元素和同位素地球化学特征,指示多霞松多含矿斑岩可能来源于亏损地幔分异的新生玄武质下地壳,受控于印度板块与欧亚板块碰撞诱发的金沙江走滑断裂系统。

The formation genetic mechanism and dynamic background of in the Yulong Cu(Mo)ore belt remain controversial.In this study,we present a systematic dataset of whole-rock major and trace elements,zircon U-Pb dating and in situ zircon Hf isotopes,to constrain the petrogenesis of masses in the Duoxiasongduo area.The Duoxiasongduo mass is composed of monzogranite porphyry and alkali feldspar granite porphyry.Monzogranite porphyry is mainly composed of potassium feldspar,plagioclase and quartz,and alkali feldspar granite porphyry is composed of potassium feldspar and quartz.Petrogeochemical analysis shows that both of the rocks belong to high-K calc-alkaline to shoshonitic intrusions with high alkali and potassium.Moreover,these rocks are depleted in high field strength elements(HFSE,e.g.,Nb and Ti),but enriched in the large-ion lithophile elements(LILE,e.g.,Rb,Th and U)and Pb.Rare earth elements(REE)content ranges from 145.04×10^(-6) to 290.91×10^(-6).The ratio of LREE/HREE is relatively high(6~37).Eu varies from weak negative anomalies(Eu/Eu^(*)=0.66~0.84)in the early stage to moderate negative anomalies(Eu/Eu^(*)=0.35~0.39)in the late stage.The zircon U-Pb dating results show that the monzogranite porphyry and alkali feldspar granite porphyry crystallized at 38.3±0.6 Ma and 37.8±1.0 Ma,respectively.Both of them were formed in the same age,belonging to the Himalayan age,indicating that there was an important magmatic hydrothermal Cu-Mo mineralization event in the Eocene in the Duoxiasongduo area.The ε_(Hf)(t)values of the zircons from the two types of rocks range from+1.3 to+2.6.Combined with the geochemical characteristics of elements and isotopes,the Duoxiasongduo ore bearing porphyry may be the product of juvenile basaltic lower crust which was derived from depleted mantle,which was possibly formed controlled by the Jinshajiang strike-slip fault system induced by the collision between the Indian Plate and the Eurasian Plate.