吉南板石沟地区英云闪长质片麻岩LA-ICP-MS锆石U-Pb定年、地球化学特征及地质意义

以吉南板石沟地区的英云闪长质片麻岩为例,探讨其成因及演化机制。研究表明该片麻岩具有富硅(SiO_2=66.61%~72.35%)高铝(Al_2O_3=13.56%~16.18%)富钠(Na_2O=3.65%~4.69%)贫钾(K_2O=1.13%~2.25%)的特点,稀土元素配分曲线右倾,富集Ba、Rb等大离子元素,亏损Ni、Ta等高场强元素,属于高铝TTG片麻岩,其与高硅埃达克岩特征相似。对该片麻岩锆石进行了LA-ICP-MS U-Pb年龄测试,其形成年龄为(2 619±28)Ma,属于新太古代时期产物。锆石Lu-Hf同位素分析结果显示岩浆在形成演化过程中较少受到地幔物质混染,两阶段模式年龄T_(DM2)为2 976~2675 Ma。板石沟地区英云闪长质片麻岩可能是在俯冲环境下含玄武质洋壳部分熔融产生,认为华北克拉通东北部地区在新太古代时期可能存在一个由洋壳俯冲到弧陆碰撞造山带即吉—辽—冀弧陆碰撞增生造山带。

LA-ICP-MS zircon U-Pb dating,geochemical characteristics and geological significance of tonalitic gneisses from Banshigou area of southern Jilin

The authors discusse the origin and evolution mechanism of tonalitic gneisses from Banshigou area of southern Jilin. It is indicated that the TTG gneisses are characterized by high Si(SiO2=66.61%~72.35%), Al(Al2O3=13.56%~16.18%) and Na(Na2O=3.65%~4.69%), but low K(K2O=1.13%~2.25%). The chondrite normalized REE patterns are characterized by right inclining type. Trace element analysis shows the enrichment of large ion lithophile elements (e.g., Ba and Rb) and strong depletion in high field strength elements (e.g., Ni and Ta).The gneisses belong to the high aluminum TTG gneisses with geochemical characteristics similar to the high silicon adakite. LA-ICP-MS zircon U-Pb age data indicate that the rocks were crystallized at (2 619±28)Ma, so they are formed in Neoarchean. Zircon Lu-Hf isotopic analyses show that the magma is rarely mixed with the mantle material during its evolution, and the corresponding two-stage model age(TDM2) is mainly between 2 976 Ma and 2 675 Ma.Therefore, tonalitic gneisses of Banshigou area are probably produced by partial melting of basaltic oceanic crust in the subduction environment. The authors suggest that there probably existed an orogenic belt (Ji-Liao-Ji arc-continent collision orogenic belt) in Neoarchean in the northeast North China Craton.