滇东南老城坡片麻状花岗岩地球化学特征、锆石U-Pb年龄及其意义

Geochemistry and Ziron U-Pb Chronology of the Laochengpo Gneissic Granite in the Southeast Yunnan Area and their Implications

为探讨中越边境Song Chay穹窿中片麻状花岗岩的形成及演化,对穹窿北端的老城坡片麻状花岗岩进行了年代学和地球化学研究。结果表明,老城坡片麻状花岗岩的LA-ICP-MS锆石U-Pb年龄为415.4±5.2Ma,属于过铝质高钾钙碱性S型花岗岩,Si O2含量为73.48%～77.16%,全碱含量较高（K2O＋Na2O=7.40%～7.86%）,K2O/N2O=0.99～1.75（均值为1.30〉1）,铝饱和指数（A/CNK）为1.01～1.14。大离子亲石元素K和Rb相对富集,Ba、Sr亏损明显;高场强元素Th、U、Hf相对富集,Ti、Nb亏损,其中Ti强烈亏损。结合区域地质背景初步推断,岩浆源岩可能来自早期地壳物质部分熔融的产物;广西运动时期云开地体与桂滇—北越地体发生碰撞,导致老城坡岩体侵位于华南褶皱带。

To further explore the formation and evolution of gneissic granite in the Song Chay dome of the Sino-Vietnamese border,the paper studied the chronology and geochemistry of the Laochengpo gneissic granite in the north of the dome. The results showed that the Laochengpo gneissic granite is a peraluminous,high K and calc-alkaline S-type granitoid,with a LAICP-MS zircon U-Pb dating age of 415. 4 ± 5. 2 Ma. The granite contains 73. 48% —77. 16% of Si O2,high total alkalis（ K2O ＋ Na2O = 7. 40% —7. 86%） and K2O/N2O = 0. 99- 1. 75（ with a mean of 1. 30）,and high A/CNK of 1. 01—1. 14.The LILE elements K and Rb are relatively enriched,Ba and Sr are relatively depleted,the HFSE elements Th,U,and Hf are relatively enriched,and Ti and Nb are relatively depleted with Ti is depleted the most in the Laochengpo gneissic granite. On the basis of the regional geological background,this study suggested that the magma source may be a product of partially melted early crustal materials; the Yunkai terrane collided with the Yunnan-North Vietnam terrane during the Guangxi movement,which resulted in the emplacement of the Laochengpo plutons into the South China fold belt.