西藏南木林县拉隆地区花岗闪长岩体的时代、岩石地球化学特征及构造背景

Geochronology,Geochemistry and Tectonic Settings of Granodiorite in Lalong Area,Namling,Tibet

冈底斯印支期构造-岩浆-成矿的研究比较缺乏.以出露在冈底斯隆格尔-念青唐古拉复合古岛弧带的拉隆花岗闪长岩体为研究对象,通过野外地质调查、岩石学、LA-ICP-MS锆石U-Pb定年、岩石地球化学和锆石Lu-Hf同位素等方面的研究,探讨了其岩石成因和形成的构造背景.结果显示,拉隆花岗闪长岩体的LA-ICP-MS锆石U-Pb年龄为223.5±3.9Ma^225.4±4.9Ma,形成于晚三叠世(T3),属印支晚期构造-岩浆活动的产物;花岗闪长岩体表现出低Ti(w(TiO2)=0.46%~0.73%),富Si(w (SiO2)=63.24%~67.94%)、Al(w (Al2O3)=14.14%~15.85%)、Na(w (Na2O)=2.58%~3.04%)、K(w(K2O)=3.06%~4.89%),高钾钙碱性-钾玄岩系列和弱过铝质的主量元素地球化学特征;岩体稀土总量ΣREE为138.62×10-6~255.53×10-6,配分模式为轻稀土元素富集的右倾斜型,其(La/Yb)N值为8.35~17.90,具明显的负Eu异常(Eu/Eu*=0.54~0.69),无Ce异常(Ce/Ce*=0.92~1.02);微量元素表现出Ba、Nb、Ta、Sr、Ti、P元素亏损和Rb、K、Th、U、Zr、Hf元素富集的特征.岩体的εHf(t)为-6.4~2.5,其两阶段模式年龄tDM2为980~1 479Ma,相当于念青唐古拉群形成的地质时代.拉隆花岗闪长岩体属A型花岗岩,为古老下地壳物质部分熔融的岩浆产物,并有少量地幔物质的加入,形成于古特提斯俯冲碰撞形成的岛弧背景下局部拉张的构造环境.

The study on the tectono-magmatic mineralization of the Gangdese Indosinian is relatively deficient. This paper takes Lalong granodiorite masses located in the Gangdese Lunggar-Nyainqentanglha tectono-magmatic belt as the research object. The field geological surveying,petrology, LA-ICP-MS zircon U-Pb dating, geochemistry and Hf isotope data are systematically studied to discuss the petrogenesis and tectonic settings.It is found that the LA-ICP-MS zircon U-Pb ages show a weighted average age between 223.5±3.9 Ma and 225.4±4.9 Ma, indicating that they formed inLate Triassic epoch, and they are probably tectonic-magmatic products of late period of Indosinian. The rocks show low Ti(w(TiO2)=0.46%-0.73%), rich in Si(w(SiO2)=63.24%-67.94%), Al(w(Al2O3)=14.14%-15.85%), Na(w(Na2O)=2.58%-3.04%) and K(w(K2O)=3.06%- 4.89% ), belonging to the high-K calc-alkaline to shoshonite series and weakly peraluminous;enrichment in REE, and the ΣREE is 138.62×10^-6 to 255.53×10^-6. The rare earth element distribution pattern is the right tilt type of the light rare earth element enrichment, the (La/Yb)N=8.35-17.90, with negative Eu anomaly (Eu/Eu*=0.54-0.69) and without Ce anomaly(Ce/Ce*=0.92-1.02). The rocks show depletion in Ba,Nb,Ta,Sr,Ti and P;enrichment in Rb,K,Th,U,Zr and Hf in the trace elements. The εHf(t)=-6.4-2.5, coinciding with two-phase model age (tDM2) of the crust of 980-1 479 Ma, and a same geologic age with the Nyainqentanglha Group. Lalong granodiorite masses belong to A-type granite. They are partially melted magmatic products induced by ancient lower crust material, and accompanied by a small amount of mantle material. They are formed by the local extension of the tectonic setting in the island arc environment formed by the ancient Tethys subduction.