西藏羌塘盆地古近纪康托组沉积物源及构造背景分析

Provenance and tectonic setting analysis of the Paleogene Kangtuo Formation in the Qiangtang Basin,Xizang(Tibet)

沉积岩中的主微量元素和稀土元素特征可以指示其物源、构造背景和沉积环境等。康托组地层沉积时代为古近纪,是青藏高原早期隆升后首先沉积的陆相碎屑岩,对该地层的物源信息及构造背景分析对于研究羌塘盆地新生代演化和高原隆升过程具有积极意义。本文对羌塘盆地半岛湖地区康托组地层进行了地层学、岩石学、元素地球化学和黏土含量分析,研究了其物源特征及构造背景。研究区康托组主要为岩屑砂岩、岩屑石英砂岩,分选性和磨圆度较差,发育平行层理。对17件样品进行元素地球化学分析测试,结果显示中等的化学蚀变指数CIA值(69.57~80.81),较高的化学风化指数CIW值(81.07~95.02)和斜长石风化指数PIA值(77.32~93.96),表明物源区整体经历了较强的化学风化作用。Th/U、Sc/Th、Rb/Sr等微量元素比值接近大陆上地壳值,结合稀土元素的配分模式,表明近源快速堆积的特点。根据TiO_(2)/Zr、Co/Th、La/Sc、Th/Sc、Cr/Zr值和δEu、δCe的负异常,结合相关判别图解说明康托组物源主要来自于长英质岩石。La—Th—Sc、Th—Sc—Zr/10、TiO_(2)—Fe_(2)O_(3)^(T)+MgO、Al_(2)O_(3)/SiO_(2)—Fe_(2)O_(3)^(T)+MgO和主量元素双因素构造判别图解表明物源区主要为被动大陆边缘。结合构造演化分析,指示了半岛湖地区康托组形成于陆内沉积环境。

Objectives:The Kangtuo Formation is the first deposited continental-face clastic rock after the early uplift of the Qinghai—Xizang(Tibet))Plateau,the analysis of the provenance information and tectonic background of this strata is of positive significance for studying the evolution process of the Qiangtang Basin in the Cenozoic and the plateau uplift.In this study,we tested and analyzed the stratigraphy,lithological,geochemical and clay content features of the Kangtuo Formation in the Bandaohu area of the Qiangtang Basin,and studied its provenance characteristics and tectonic background.Methods:The X-ray fluorescence(XRF)and high-resolution inductively coupled plasma mass(HR-ICPMS)are used to test the major and trace elements of the Paleogene Kangtuo Formation sandstones in the Bandaohu area;the X-ray diffraction(XRD)is used to test the clay mineral composition of the same samples.All the analysis are measured in the Analytical Laboratory,Beijing Research Institute of Uranium Geology,China.Results:For the Kangtuo Formation sandstones,SiO_(2)(58.22%~71.91%)and CaO(12.72%~21.47%)are the most abundant oxides;Al_(2)O_(3)(1.08%~2.45%)and Fe_(2)O_(3)^(T)(0.98~1.94)are the second most abundant oxides;while other oxides like Na_(2)O,K_(2)O,MnO,TiO_(2),P_(2)O_(5) are fewer than 1%.Elements Si and Ca are obviously enriched and other elements are apparently depleted compared to UCC.The total rare earth element(ΣREE)contents of the Kangtuo Formation sandstones are(34.40~67.10)×10^(-6)(avg.47.90×10^(-6)),which are fewer than that of the UCC and have a significant negative Eu anomaly.Conclusion:The CIA(69.57~80.81),CIW(81.07~95.02)and PIA(77.32~93.96)values indicate that intensities of weathering in the source area were strong.The A—CN—K diagram and the low K_(2)O contents of the samples suggest that no K-metasomatism occurred in these sandstones.The ratios of trace elements such as Th/U,Sc/Th,Rb/Sr are similar to those of the continental crust.considering the REE distribution mode of the Kangtuo Formation,we conclude that this formation may be formed during a rapid accumulating process and was close to its provenance.The ratios of TiO_(2)/Zr,Co/Th,La/Sc,Th/Sc,Cr/Zr and the negative anomalies ofδEu,δCe,combined with the discriminant diagrams,show that the provenance of the Kangtuo Formation mainly comes from felsic rocks.The La—Th—Sc,Th—Sc—Zr/10 and major element two-factor structural discriminant diagrams indicate that the source area is mainly a passive continental margin.Combined with basin evolution characteristics,the Kangtuo Formation of Bandaohu area may be formed in intracontinental sedimentary environment.