羌塘盆地侏罗系元素地球化学特征与成盐层位分析

摘要

The gypsum-bearing strata and high-salinity spring have been found widely in the Jurassic strata of the Qiangtang basin in recent years,suggesting a potential for salt mineralization in the area.This study performed a comprehensive geochemical analysis in the Jurassic sediments in the Yanshiping area of the Qiangtang basin,combined with the sedimentary environments analysis,to reveal sea level fluctuations of Tethys Ocean,paleoclimate changes and potential of potassium deposit formation in the area.In order to obtain a clear indication of paleo-proxies and using the sediments with gypsum,carbonate and detrital material,we employed the water and acetic acid to distinguish the elements from water-soluble salts, carbonates and the silicate minerals in the bulk sediments.The results show that,① the Fe/Mn ratios and CaCO3 content in the bulk samples indicate that Buqu and Suowa formations display high sea level with low Fe/Mn and high carbonate content due to the transgression of the Tethys Ocean;however,the Quemocuo and Xiali formations were in the regression stage with high Fe/Mn and low carbonate content.② The chemical weathering proxies based on the oxides content in the silicate minerals,e.g.,the ratio of Na2 O/Al2 O3 ,and Na2 O/K2 O in the acid-insoluble fraction and chemical index of alternation (CIA),exhibit the much more arid paleoclimate with weak chemical weathering in the source areas periods of Buqu and Xiali formations,while the relatively less arid climate with strong chemical weathering in periods of Quemocuo and Suowa formations.③ The K+ and Na+ contents in water soluble salts,which are sensitive proxies of salt formation,show the much higher concentrations in the Xiali Formation and low concentrations in other formations.Summarized by provenance,tectonics,paleoclimate,and palegeographical conditions, the results indicate the Xiali Formation has the great potential of potassium deposits in the Qiangtang basin.Because Tethys Ocean retreating from the east to the west of the Qiangtang basin in Jurassic and Yanshiping locates in the eastern margin of the basin,there is no potassium deposits formation but some gypsum outcrop in this area.The further work of potassium deposits exploration should focus on the Xiali Formation in the central and western part of the Qiangtang basin.%近年来在羌塘盆地侏罗系发现的大面积石膏层位和高盐度的盐泉水，显示羌塘盆地侏罗纪可能具备成盐地质条件。本文通过对羌塘盆地雁石坪地区侏罗系元素地球化学分析，揭示了羌塘盆地侏罗纪特提斯洋海平面与古气候变化以及蒸发岩沉积条件。全岩 Fe／Mn 比值和 CaCO3含量揭示布曲组和索瓦组沉积时期，由于特提斯洋海水入侵，盆地沉积大量海相碳酸盐岩，盆地海平面处于高位时期；雀莫错组和夏里组沉积时期处于海退后的海平面低值期。酸不溶态元素比值 K2 O／Al2 O3、Na2 O／Al2 O3以及 Na2 O／K2 O 和化学蚀变指数（CIA）揭示雁石坪地区布曲组和夏里组沉积时期气候相对干旱，而雀莫错组和索瓦组沉积时期相对湿润。可溶态 K＋、Na＋含量仅在夏里组出现高值，指示了该沉积时期有利于成盐。综合物源、古构造、古气候和古地理条件，本文认为羌塘盆地侏罗系夏里组是最具成盐潜力的层位，今后成盐研究重点应该放在盆地中－西部地区侏罗系夏里组。