藏北北羌塘盆地晚三叠世古风化壳地质地球化学特征及其意义

对最近在藏北羌塘盆地菊花山、肖茶卡、石水河等地区新发现的肖茶卡组顶部的粘土质角砾岩的地质、地球化学特征研究表明:其岩石矿物成分、结构构造、角砾层产状及剖面演化序列等与古风化壳的形成及其特征较为一致；相对于基岩，古风化壳的主量元素表现为CaO、CO2的淋失和SiO2、Al2O3、Fe2O3、K2O、Na2O、MnO、P2O5等不同程度的相对富集；古风化壳中的粘土质风化残积层的稀土元素具有明显的富集，表现为LREE含量远大于HREE的特征，存在明显的Ce的正异常以及Eu的负异常；微量元素除Mo、Sn和U外，粘土质风化残积层的其余元素含量均有明显的增加。这些特征从另一角度证实了北羌塘盆地晚三叠世存在明显的沉积间断。晚三叠世肖茶卡组顶部古风化壳的发现，为了解区域地壳发展史、古气候、古地理演化、层序地层的划分与对比、羌塘含油气盆地分析等提供了重要的依据。

The Late Triassic Paleo-weathering Crust in the Qiangtang Basin, Northern Tibet: geology, geochemistry and significance

Recent developments in the research on the geology and geochemistry of the newly-discovered clayey breccias at the top of the Xiaochaka Formation in the Juhuashan,Xiaochaka and Shishuihe areas,Qiangtang Basin,northern Tibet have disclosed that mineral compositions,textures and structures,occurrences and sectional architectures of the clayey breccias agree well with those of the paleo-weathering crust in these areas.Compared with the bedrocks,the paleo-weathering crust is characterized by the leaching out of CaO and CO2,and relative enrichment of SiO2,Al2O3,Fe2O3,K2O,Na2O,MnO and P2O5.The clayey weathered residues in the paleo-weathering crust are characterized by marked enrichment of REEs,much higher contents of LREEs than those of HREEs,markedly positive Ce anomalies and negative Eu anomalies,and a noticeable increase in other element contents besides Mo,Sn and U from the clayey weathered residues.The characteristics cited above have documented that there was a sharp depositional hiatus in the Qiangtang Basin during the Late Triassic.The discovery of the paleo-weathering crust at the top of the Upper Triassic Xiaochaka Formation has provided important evidence for the understanding of regional history of crustal,paleoclimatic and paleogeographic evolution,sequence stratigraphic division and correlation and Qiantang petroliferous basin analysis.