四川盆地红层古风化壳的地球化学特征

GEOCHEMICAL CHARACTERISTICS OF THE ANCIENT WEATHERED CRUST IN SICHUAN RED BASIN

源于四川盆地红层各层夷平面古风化壳,是重要的农业区域。 厚1-10余米,较完好的剖面有三个分解带。从深层第Ⅰ分解带至地表第Ⅲ分解带,物理和化学风化逐渐加深。粘粒SiO_2/Al_2O_3Ⅰ>Ⅱ>Ⅲ,风化系数Ⅲ>Ⅱ>Ⅰ。夷平面愈高,风化愈深。

The ancient weathered crust studied occurs on the razed surface of various physiographic periods at different altitudes in the Sichuan red basin. A typical section of the weathered crust can be divided into three weathered belts from the surface to the basement: the clastic clay belt (Belt Ⅰ), the clastic breccia-clastic clay belt (Belt Ⅱ), and the clastic breccia belt (Belt Ⅲ). These belts are different in clay mineral compostion, reflecting varying degree of chemical weathering.Wide variations are recognized in the abundance of chemical elements in the column of the ancient weathered crust. In comparison with the underlying purple parent rocks, Fe2O3> Al2O3>TiO2>SiO2 are relatively concentrated in the ancient weathered crust, while CaO> P2O5>MnO>Na2O>MgO>K2O are relatively depleted due to leaching in order of Belt Ⅰ>Ⅱ>Ⅲ.The extent of chemical weathering of the acient weathered crust in the Sichuan red basin is closely to the time of formation of the razed surface. For this reason, the contents of Al2O3 and Fe2O3 or their concentration values, and the leaching amounts of CaO, MgO, K2O, Na2O, and P2O5 are higher in the ancient weathered crust at high altitudes than those at low altitudes.