格尔木河流域富钾花岗岩中成盐元素风化迁移的时空演变

The spatial-temporal variation of weathering and migration of salt-forming elements in potassium-rich granites of the Golmud River catchment

位于东昆仑山中段的格尔木河流域分布着大面积的花岗岩,其中局部出露富钾花岗岩,研究这些富钾花岗岩的分布特征,K、B、Li等元素的含量变化以及它们风化迁移的时空演变过程,对揭示下游察尔汗盐湖的成盐成矿规律具有重要的意义。本研究采集格尔木河流域昆仑河、西大滩和小干沟不同河段不同风化程度的岩石/碎屑样品,采用常微量元素分析、岩石薄片显微照片、风化指数、光释光定年(OSL)等方法,系统研究了该流域内的花岗岩分布、类型及其成盐元素迁移的时空变化规律。结果显示:①昆仑河、西大滩区段内主要出露灰白色中细粒二长花岗岩,属于中等钾含量花岗岩;小干沟区段主要出露灰白、肉红色高钾斑状二长花岗岩组合,钾含量较高,为主要的富钾基岩分布区;②风化程度由低到高的花岗岩样品中,K元素含量逐步降低,B、Li元素含量逐渐升高;在空间分布上,K、B元素含量在小干沟区段最高,Li元素在昆仑河区段最高;③流域内风化程度不同的花岗岩样品的地球化学数据能够很好地响应化学风化势指数(WPI)和烧失量(LOI)的变化,说明这两种化学风化指标在区域上对花岗岩的风化具有高度的敏感性;④小干沟坡积-冲洪积沉积剖面形成于95.9±10.5~17.7±1.0 ka之间,该剖面中WPI和K、B、Li元素含量的变化说明花岗岩中元素的风化迁移与流域内冰期-间冰期旋回气候导致的化学风化强度变化密切相关,同时也为察尔汗盐湖提供了丰富的成盐离子。

Located in the middle part of the east Kunlun Mountains,the Golmud River catchment is covered with a large area of granite,especially the potassium-rich granite in local areas.In order to understand the evolution of distal Qarhan Salt Lake,it is important to study the distribution of these granites,the variation of the K,B,and Li elements,and their temporal and spatial migration.In this study,the authors collected samples from granites and their weathered detrital materials along Kunlun River section,Xidatan River section,and Xiaogangou section of Golumd River catchment.Based on major and trace element analysis,rock slice identification,chemical weathering index,and optical stimulated luminescence dating(OSL),the authors investigated the salt-forming elements in the granites and their weathered products.The results are as follows:①In the Xidatan and Kunlun River catchment,gray-white,medium-fine grained late Variscan granulites are predominant,with intermediate level potassium content;In the Xiaogangou River section,there are mainly gray-white and flesh red late Yanshanian porphyritic adamellites,which are potassium-rich granites,and thus these areas are advantageous metallogenic targets of potassium;②K content gradually decreases and B,Li element content gradually increases with the increasing weathering degree in the granite.The Xiaogangou River section has high K,B content,but the Kunlun River section shows high Li content;③The chemical weathering indexes,such as WPI,LOI,are well coupled with the element content variations of different weathering degree samples,indicating that these two chemical weathering indexes are sensitive to the variation of weathering degree;④The alluvial sediments section of Xiaogangou River catchment has OSL ages from 95.9±10.5 ka in the bottom layer to 17.7±1.0 ka in the upper layer,and the WPI index,K,B and Li content variation curves in the sections indicate that the salt-forming elements migrations in the granites in the Golmud River catchment seem to have been related to the chemical weathered degree change associated with the glacial-interglacial cycles since late Pleistocene,and these elements have provided abundant saline minerals for the Qarhan Salt Lake.