藏南吉隆淡色花岗岩地球化学特征、成因机制及其构造动力学意义

Geochemical Characteristics of the Leucogranites from Gyirong,South Tibet:Formation Mechanism and Tectonic Implications

藏南吉隆淡色花岗岩体位于大喜马拉雅淡色花岗岩带的中部,是吉隆地区藏南拆离系剪切带上部的重要组成部分。地球化学特征显示,岩石具有高SiO_2(72.09%~74.02%)、Al_2O_3(14.54%~15.59%)和K_2O(4.55%~5.59%)含量,高K_2O/Na_2O比值(1.12~1.55)和A/CNK值(1.14~1.18),属于高钾钙碱性过铝质S型花岗岩。富集大离子亲石元素Rb和放射性生热元素U,亏损Ba、Nb、Sr和Zr等元素,具有明显的轻重稀土元素分异和Eu负异常(δEu=0.37~0.54)。具有高的Rb/Sr比值(3.6~9.7)和低的CaO/Na_2O比值(0.15~0.25),指示源区为泥质岩区;(^(87)Sr/^(86)Sr)_i和ε_(Nd)(t)变化范围分别为0.7548~0.7586和-14.0^-13.1,与大喜马拉雅变泥质岩的Sr-Nd同位素组成一致;锆石边部的ε_(Hf)(t)介于-16.0^-8.5之间,位于大喜马拉雅变泥质岩中碎屑锆石的演化线上,表明淡色花岗岩的源岩为大喜马拉雅变泥质岩。岩石(^(87)Sr/^(86)Sr)_i较高而Sr浓度较低,且随着Ba浓度的增加,Rb/Sr比值降低,表明淡色花岗岩是无水条件下白云母脱水熔融形成的,部分熔融可能与藏南拆离系(STDS)伸展拆离导致的深部构造减压密切相关。吉隆淡色花岗岩的形成反映了地壳伸展减薄背景下,构造减压导致的深部地壳物质中含水矿物(白云母)脱水熔融并沿向北伸展的STDS侵位的构造动力学过程。

Two sub-parallel massive granite belts, the Greater Himalayan leucogranite belt to the south and the North Himalayan Gneiss Dome granite belt to the north, are situated in the youngest and most magnificent Himalayan orogen on earth. These granites are important records of the tectonic-magmatic evolution and orogenesis mechanism. The Gyirong Miocene leucogranite pluton is located in the center of the Greater Himalayan leucogranite belt, and constitutes the upper part of the South Tibet Detachment Systems （STDS） ductile shear zone in the Gyirong area. The leucogranites are characterized by high SiO2 （72.09%-74.02%）, Al2O3 （14.54%-15.59%）, K2O （4.55%-5.59%）, and high K2O/Na/O （1.12-1.55） and A/CNK （1.14-1.18）, and enrichment of Rb and U, depletion of Ba, Nb, Sr, and Zr, and moderate fractionation between LREE and HREE, and strong negative Eu anomalies （δEu=0.37-0.54）. These features demonstrate that the Gyirong leucogranites are of high potassium calc-alkaline and peraluminous. The leucogranites show high Rb/Sr ratios （3.6-9.7） and low CaO/Na2O ratios （0.15-0.25）, typical of the pelite source. The （87Sr/S6Sr）i and εNd（t） values range from 0.7548 to 0.7586 and from -14.0 to -13.1, respectively, and can be compared with the metapelites in the Greater Himalayan Crystalline Complex （GHC）. Similarly, the εHf（t） values of-16.0 to -8.5 can be interpreted to be results of reworking of the GHC metapelite. Thus, we suggest the Gyirong leucogranites were generated from partial melting of the GHC metapelite. The low Sr content, relatively high （875r/86Sr）i values, and decrease Rb/Sr ratios with increasing Ba concentrations of the leucogranites are consistent with the trend of muscovite dehydration melting which was possibly triggered by the structural decompression responding to STDS activity. In conclusion, we propose the Gyirong leucogranites reflect the dynamic processes of structural decompression, dehydration melting and emplacement of the melt along the STDS under a tectonic regime of crustal extension and thinning.