西藏改则多龙矿集区地堡那木岗矿床钾玄质二长花岗斑岩锆石LA-MC-ICP-MSU-Pb年龄、地球化学特征及其地质意义

Zircons LA-MC-ICP-MS U-Pb Ages,Petrochemical,Petrological and Its Significance of the Potassic Monzonitic Granite Porphyry from the Duolong Ore-concentrated District,Gaize County,Xizang(Tibet)

西藏班公湖—怒江结合带已经成为我国重要的斑岩铜金矿成矿带,改则县多龙斑岩—浅成低温热液型铜(金)矿集区是近几年班公湖—怒江成矿带最为瞩目的找矿发现。本文首次对多龙矿集区地堡那木岗矿床二长花岗斑岩进行全岩地球化学分析及锆石LA-MC-ICP-MS U-Pb测年。二长花岗斑岩SiO_2为66.31%~67.31%,Al_2O_3为15.16%~15.33%,MgO为1.30%~1.47%,CaO为1.53%~1.65%,TFe_2O_3为3.29%~3.74%,K_2O为3.72%~3.98%,Na_2O为4.48%~4.96%,K_2O/Na_2O为0.80~0.89,铝饱和指数为1.00~1.07,里特曼指数为2.82~3.43,属于为高钾钙碱性弱过铝质岩石。其明显富集轻稀土及大离子亲石元素(Th、U、K、Pb及Rb),亏损重稀土及高场强元素(Ta、Nb、Ti及Zr),稀土总量为131.81×10^(-6)~151.59×10^(-6)之间,轻稀土与重稀土比值为10.67~10.97,(La/Yb)_N为14.03~15.23,δEu为0.75~0.78,具有中等负铕异常,显示出典型岛弧岩浆岩的特征,表明二长花岗斑岩可能是处于俯冲带背景下由洋壳俯冲板片脱水产生流体交代地幔楔发生部分熔融形成。二长花岗斑岩La/Nb值为3.36~3.50,La/Ta值为40.71~42.79,指示了岩浆在上升的过程中可能遭受到地壳物质的混染作用。其锆石具有明显的岩浆结晶环带,锆石U-Pb测年显示,Th/U值为0.56~1.10,岩石成岩年龄为112.6±1.3Ma。结合前人资料可知,多龙矿集区内岛弧岩浆岩存在117~124 Ma和107~112 Ma两个明显的主要成岩期,地堡那木岗矿床二长花岗斑岩形成于矿集区岩浆动活后期。综合分析认为,班公湖—怒江特提斯洋在早白垩世晚期(112.6±1.3Ma)尚未关闭,仍正在向北俯冲于羌塘地块之下。

Objective：The Duolong copper （gold） ore concentration district, located in the north of Bangong Co—Nujiang suture zone, is the most outstanding ore prospecting achievement in recent years. It is the first discovered super large epithermal porphyry Cu （Au） deposit in Xizang （Tibet）. Methods：This articale did the first study to the monzonitic granite porphyries via LA MC ICP MS ziron U Pb dating and whole rock geochemistry analysis collected from the Dibaonamugang deposit in the Duolong copper （gold） ore concentration district.Results：The monzonitic granite porphyries in the district belong to potassium calc alkaline and weakly peraluminous rocks. They have SiO2 values of 66.31% to 67.31%, Al2O3 values of 15.16% to 15.33% , MgO values of 1.30% to 1.47%, CaO values of 1.53% to1.65%, TFe2O3 values of 3.29% to 3.74%, K2O values of 3.72% to 3.98%, Na2O values of 4.48% to 4.96%. These rocks show K2O/Na2O ratios of 0.80 to 0.89, A/CNK ratios of 1.00 to 1.07, Terman index of 2.82 to 3.43. The monzonitic granite porphyries are systematically enriched in large ion lithophile elements（LILE： Th、U、K、Pb and Rb） and LREE, relatively deplted in high strength elements（HFSE：Ta、Nb、Ti and Zr） and HREE. Total Coneentartion of REE range from 131.81×10^-6 ~ 151.59×10^-6, with LREE/HREE 10.67~10.97, （La/Yb）N 14.03~15.23 and middle negative Eu anomalies（δEu=0.75~0.78）. The geochemical characteristics of these intrusions are consistent with those of arc type magmas worldwide. This indicates the arc granitic magma was generated by partial melting of mantle wedges replaced by the fluid which is formed by the dehydration of the oceanic crust subdution plate in the subdution zone. In addition, its high La/Nb ratios （3.36~3.50） and high La/Ta ratios （40.71~42.79） suggest that the magma might be contaminated by crustal mantle. Zicrons coming from the monzonitic granite porphyry have obvious oscillatory zone, yielding Th/U ratios of 0.56 to 1.10. Its have zircon LA MC ICP MS U Pb age of 112.6±1.3Ma. Conclusions：Combined with the published data, this ore concentration district contains two obvious main diagenesis stage （117~124Ma, 107~112Ma, respectively）, suggesting Bangong Co—Nujiang ocean basin was still subducting during ca. 112.6 Ma and the closure time of the ocean should be later than early Cretaceous （112.6±1.3Ma）.