藏南亚东地区中新世岩浆活动及构造意义

Abstract

<p>喜马拉雅淡色花岗岩是研究造山带构造-岩浆演化历史的岩石探针。亚东淡色花岗岩位于高喜马拉雅序列中部, 其成因机制和构造意义有待进一步的研究和讨论。本研究提供了新的亚东地区淡色花岗岩和花岗片麻岩的全岩地球化学和锆石年代学数据。亚东淡色花岗岩具有相对较高的SiO₂、K₂O含量, 和相对较低的CaO、FeO^T、MgO含量, 属于过铝-强过铝质、高钾钙碱性花岗岩。它们具有较低的稀土含量, 相对富集的LREE含量, 明显的Eu负异常。这些地球化学特征同典型的高喜马拉雅淡色花岗岩极为一致。本研究获得了顶噶二云母花岗岩约23Ma和21Ma的两期结晶年龄, 并且认为该地区高喜马拉雅结晶岩系的部分熔融可能早在36~30Ma就已经开始了。获得亚东淡色花岗岩的<em>ε</em>_Nd(<em>t</em>)值和(⁸⁷Sr/⁸⁶Sr)_i分别为-16.7~-13.8和0.754080~0.791009, 与亚东变沉积岩的Sr-Nd同位素组成极其一致, 而花岗片麻岩具有明显更高的<em>ε</em>_Nd(<em>t</em>)值和(⁸⁷Sr/⁸⁶Sr)_i, 分别为-9.0~-8.8和0.847193~0.866306。因此, 我们认为亚东淡色花岗岩源自高喜马拉雅结晶岩系中变沉积岩的部分熔融, 且不含或极少有花岗片麻岩的物质贡献。综合全岩地球化学、变质岩石学及年代学研究, 我们认为高喜马拉雅结晶岩系在埋藏加厚的过程中就已经发生了白云母为主的脱水部分熔融, 在随后的伸展构造阶段发生大规模减压熔融, 产生大量中新世淡色花岗岩。<br></p>

Himalayan leucogranites provide a natural probe to study tectonic-magmatic evolution history of continental collisional orogen. The Yadong leucogranites are located in the middle of the Higher Himalayan Sequence (HHS), and their petrogenesis and tectonic implications need to be further investigated and discussed. To address this issue, we carried out new whole-rock geochemistry and zircon geochronology study for leucogranites and granitic gneiss from Yadong area. The leucogranites are peraluminous and high-K calc-alkaline rocks, and characterized by high SiO2 and K2O concentrations (69.4%~75.5% SiO2 and 2.88%~6.70% K2O), but low CaO, FeOT, MgO concentrations (0.44%~1.74% CaO, 0.29%~2.53% FeOT and 0.04%~0.57% MgO), enriched LREE patterns and obviously negative Eu anomalies, which are consistent with typical High Himalayan leucogranites. LA-ICP-MS zircon U-Pb dating yields crystallization ages of ca. 21.0Ma and 23.0Ma, and partial melting in Yadong may have started as early as 36~30Ma. Yadong leucogranites have