西藏错那洞淡色花岗岩地球化学特征、成岩时代及岩石成因

Geochemical characteristics,age and genesis of Cuonadong leucogranite,Tibet

错那洞淡色花岗岩是西藏北喜马拉雅淡色花岗岩带的重要组成部分。通过地球化学分析揭示其具有富硅(SiO_2含量为74.20%~74.52%)、贫铁(Fe_2O_3含量为0.04%~0.20%,FeO含量为0.40%~0.58%)、贫镁(MgO含量为0.06%~0.14%)、钙碱性(σ为2.15~2.32)、强过铝质(A/CNK为1.11~1.15)的地球化学特征。稀土元素总量较低(∑REE为47.24×10^(-6)~57.59×10^(-6)),轻稀土元素富集(LREE为39.85×10^(-6)~49.23×10^(-6)),重稀土元素亏损(HREE为6.91×10^(-6)~8.68×10^(-6)),有明显负Eu异常(0.49~0.80);富集Rb、Th、U、K等大离子亲石元素,亏损Nb、Ta、Zr、Ti等高场强元素。锆石U-Pb测年结果显示,错那洞淡色花岗岩形成中新世(21 Ma),属北喜马拉雅淡色花岗岩晚阶段峰值期(24~12 Ma)产物。锆石εHf(t)值为负值,且变化较大(-3.92^-17.64),说明其岩浆源区为壳源,以变泥质岩为主,可能存在多种物质组分的混合。初始岩浆结晶温度应不超过675~702℃,构造背景为后碰撞环境,是高喜马拉雅结晶岩系在板片快速折返过程中发生减压熔融而形成的产物。

The Cuonadong leucogranite is one of the most important parts of North Himalayan leucogranite belt. Geochemical data show that Cuonadong leucogranite is the calcium alkaline and strong peraluminous granite with high silica （SiO2 is 74.20%～74.52%）, poor iron （Fe2O3 is 0.04%～0.20%, FeO 0.04%～0.58%）, poor magnesium （MgO is 0.06%～0.14%）, and calcium alkaline （σ is 2.15～2.32） as well as strong peraluminous nature （A/CNK is 1.11～1.15）. Its total rare earth is relatively low （∑REE is 47.24×10-6～57.59×10-6）, with enrichment of LREE （LREE is 39.85×10-6～49.23×10-6）, depletion of HREE （HREE is 6.91×10-6～8.68×10-6） and obvious negative Eu anomaly （0.49～0.80）. It is characterized by an enrichment of some large ion lithophile elements （Rb, Th, U, K） and loss of high field strength elements （Nb, Ta, Zr Ti）. Zircon U-Pb dating results show that the Cuonadong leucogranite＇s formation time is Miocene （21 Ma）, which belongs to the peak of the late stage of north Himalayan leucogranite （24～12 Ma）. Zircon εHf（t） values are negative with a wide variation （-3.92～-17.64）, which shows that its magma source was the crust, mostly from the metapelite, with the probable mixing of many kinds of materials in its source. The petrogenesis of Cuonadong leucogranite is that the high Himalayan crystalline rock series experienced decompression melting during the rapid pull-back of the plate in the post-collision tectonic setting, with the initial magma crystallization temperature being 675～702℃.