西藏朗县地区增生楔杂岩带90Ma岛弧型深成岩浆活动和意义

The 90 Ma island-arc type plutonism in the subduction- accretionary complex in Langxian County area, Tibet

碰撞前增生楔带的岩浆活动在世界许多缝合带均较强烈。为证实雅鲁藏布江带是否存在碰撞前增生楔岩浆活动,本文报道了朗县英云闪长岩岩株的地质特征,以及岩石学、锆石U-Pb定年、全岩地球化学和Sr-Nd同位素数据。结果表明,朗县岩体变形较弱,侵位于强烈变形的蛇绿混杂岩中（蛇绿岩年龄（145.7±2.5）Ma）,时代为（92.58±0.72）Ma（MSWD=1.8）,岩石属于钙碱性系列,高镁、富钠,相容元素（Cr=71.48×10^-6-86.74×10^-6,Co=22.32×10^-6-23.52×10^-6,Ni=33.51×10^-6-36.31×10^-6）含量高,富含大离子亲石元素Rb、Ba和放射性生热元素Th、Pb、U、K、Sr等,明显亏损Nb、Ta、Zr、Hf（Ti）等非活动性元素,轻稀土明显富集,重稀土亏损（LaN/YbN=6.95-7.44）,具有不明显的铕异常,δEu值为0.82-0.91,εNd（t）值变化于2.68-3.23。在以上基础上,结合前人研究成果,认为朗县英云闪长岩体形成于增生楔岛弧环境,与雅鲁藏布江洋俯冲-消减带向南迁移密切相关,是90 Ma的洋脊俯冲地球动力背景下亏损地幔和增生楔熔体混合的产物。

The pre-collision magmatism took place in the accretionary wedges of many orogens. This paper presents the geological features of Langxian tonalite stock, zircon U-Pb data,and geochemical and Sr-Nd isotopic data for the tonalite to discuss whether island-arc type magmatism took place in the accretionary wedges of Yarlung Zangbo belt. The results indicate that the deformation degree of stock is distinctively lower than that of its wall rocks which comprises ophiolite mélange（145.7±2.5Ma）, and the age ofthe stock is（92.58 ± 0.72）Ma（MSWD=1.8）. The diorite is characterized by calc- alkaline series, and high Mg, Na,abundances of compatible elements（Cr=71.48×10^-6-86.74×10^-6,Co=22.32×10^-6-23.52×10^-6,Ni=33.51×10^-6-36.31×10^-6）. The rock is enriched in Rb, Ba, Th, Pb, U, K, and depleted in Nb,Ta,Zr,Hf（Ti）. REE patterns show strong enrichment of LREE and depletion of HREE（LaN/YbN=6.95-7.44）, with indistinct Eu anomaly（δEu=0.82-0.91）. Meanwhile, the values of εNd（t） vary from 2.68 to 3.23. Based on these results in combination with previous data, the authors suggest that Langxian diotite intrusion was formed in an accretionary wedge island arc setting, related to southward migration of the Yarlung Zangbo subduction, and derived from the magma mixture of depleted mantle and subduction accretionary complex in the ocean ridge subducting at 90 Ma or so.