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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。
The Banghu-Nujiang junction belt in Tibet has become an important porphyry copper-gold mineralization belt in China. The Dailong porphyry-epithermal copper- - Nujiang metallogenic belt, the most eye-catching prospecting discovery. In this paper, the whole rock geochemistry and zircon LA-MC-ICP-MS U-Pb zircon dating of the Namumen granite porphyry in the Bulung ore-bearing area are studied for the first time. The monzonitic granitic porphyries have the SiO 2 content of 66.31% -67.31%, Al 2 O 3 content 15.16% -15.33%, MgO 1.30% -1.47%, CaO 1.53% -1.65%, TFe 2 O 3 3.29% -3.74% and K 2 O 3.72% ~ 3.98%, Na_2O is 4.48% ~ 4.96%, K_2O / Na_2O is 0.80 ~ 0.89, the aluminum saturation index is 1.00 ~ 1.07, and the Reiteman index is 2.82 ~ 3.43, which belongs to the weak kaolinite-alkaline weak peraluminous rocks. It is obviously enriched in light rare earths and large ion lithophile elements (Th, U, K, Pb and Rb), depleted in heavy rare earths and high field strength elements (Ta, Nb, Ti and Zr), the total amount of rare earths is 131.81 × 10 ~ (-6) ~ 151.59 × 10 ~ (-6), the ratio of light rare earth to heavy rare earth is 10.67 ~ 10.97, (La / Yb) _N is 14.03 ~ 15.23, δEu is 0.75 ~ 0.78, The characteristics of the typical island arc magmatic rocks show that the monzonitic granite porphyry may be partly melted by the mantle wedge which is caused by the dewatering of the oceanic plate subducting slab due to the subduction zone background. The La / Nb value of the granitic porphyry is 3.36-3.50 and the value of La / Ta is 40.71-42.79, indicating that the magma might be contaminated by the crustal material during its rising process. The zircons have obvious magma zoning. The zircon U-Pb dating shows that the Th / U value is 0.56 ~ 1.10 and the rock diagenetic age is 112.6 ± 1.3Ma. According to the previous data, there are two obvious major diagenesis periods of 117 ~ 124 Ma and 107 ~ 112 Ma in the island arc magmatic rocks in the Duolong ore district, Magmatic late. According to a comprehensive analysis, the Tengchong-Nujiang Tethys Ocean has not been closed during the Early Cretaceous (112.6 ± 1.3Ma) and is still subducting northward beneath the Qiangtang block.