辽东三叠纪弟兄山岩体SHRIMP U-Pb年龄、地球化学特征及其地质意义

SHRIMP锆石U-Pb年龄测定表明，辽东半岛弟兄山岩体的侵位时代为三叠纪（205.2±2.1 Ma），是华北东部三叠纪花岗岩的一部分.全岩岩石化学分析结果显示，弟兄山花岗岩具有高SiO₂、Al₂O₃、K₂O，低TiO₂、Na₂、MnO和CaO的特征，K₂O+Na₂变化范围为7.88%~9.28%，K₂O/Na₂ ≥ 1.16~1.46；CaO/Na₂=0.08~0.23，铝指数A/CNK=0.95~1.10，并且在矿物组合中出现白云母，属准铝-过铝质花岗岩.在SiO₂-Zr图解中，所有样品点均落在S型花岗岩区域中.以上特征均显示该花岗岩为准铝-过铝质S型花岗岩.稀土曲线和稀土参数表现出强烈的轻、重稀土分异特征和明显的Eu负异常特征，反映源区岩浆形成后发生过斜长石或其他富Ca矿物的分离结晶作用，是典型准铝-过铝质花岗岩的稀土元素特征.在原始地幔标准化的微量元素蛛网图上，所有花岗岩均富集Rb、Th，明显亏损Nb、Ta、Sr和Ti.所有样品的Rb=133×10^-6~360×10^-6，绝大多数样品高于花岗岩的平均值（200×10^-6）；Sr（25×10^-6~135×10^-6）和Ba（48×10^-6~507×10^-6）明显低于花岗岩的平均值（Sr 300×10^-6，Ba 830×10^-6），Ba、Sr亏损反映岩浆经历了较为完全的分离结晶作用；大离子亲石元素Rb、Th富集，Nb和Ta亏损显示陆壳物质为岩浆的源岩.上述特征表明岩浆物质来源于陆源碎屑岩石.结合区域构造演化历史，认为弟兄山岩体是库拉-太平洋板块向欧亚大陆俯冲的产物，是印支晚期华北岩石圈处于弱伸展状态背景的响应.

ZIRCON SHRIMP U-Pb AGE AND GEOCHEMISTRY OF THE TRIASSIC DIXIONGSHAN ROCK MASS IN LIAODONG PENINSULA: Geological Significance

The zircon SHRIMP U-Pb dating result indicates that the Dixiongshan rock mass in Liaodong Peninsula, as a part of the Triassic granites found in the east of North China, was emplaced in the Triassic epoch (205.2±2.1 Ma). The whole rock chemical analysis shows that the Dixiongshan granites are characterized by high SiO 2, Al₂O₃ and K₂O and low TiO₂, Na₂, MnO and CaO, with K₂O+Na₂ ranging from 7.88% to 9.28%, K₂O/Na₂≥1.16-1.46, CaO/Na₂=0.08-0.23, A/CNK=0.95-1.10. Furthermore, muscovite appears in the mineral assemblage, which means the rock mass belongs to metaluminous-peraluminous granite. In the SiO₂-Zr diagram, all the sample points fall in the S-type granite area. All the characteristics above suggest that the Dixiongshan granites be of metaluminous-peraluminous S-type. The REE curves and parameters show intense fractionation between LREE and HREE and distinct negative anomaly of Eu, suggesting that a fractional crystallization happened to plagioclase or other Ca-rich minerals after the formation of magma in this area, which are typical REE characteristics for metaluminous-peraluminous granites. In the primitive mantle-normalized trace element spider diagram, all the granites are enriched in Rb and Th, but distinctly depleted in Nb, Ta, Sr and Ti. For all samples, Rb=133×10^-6-360×10^-6, most of which are higher than the average value of granites(200×10^-6). Sr(25×10^-6-135×10^-6) and Ba (48×10^-6-507×10^-6) are obviously lower than the average value of granites (Sr=300×10^-6, Ba=830×10^-6). The depletion of Ba and Sr reflects the magma experienced a relatively complete fractional crystallization. The enrichment of large ion lithophile elements (LILEs, such as Rb and Th) and the depletion of Nb and Ta indicate that the continental crust is the source rock of magma. The characteristics above show that the magma substance is from the terrigenous clastic rock. Combined with the history of regional geodynamical evolution, it is thought that the Dixiongshan rock mass is a product of the subduction of the Kula-Pacific plate towards Eurasia and response of weak extensional setting of North China lithosphere in the Late Indosinian epoch.