Keywords: riverbed granite, chemical weathering, transferring and concentration

Up to now, the research on the origin of granite, the process of granite weathering and the features of regolith have been done and a large number of academic thesis and works have been published, but the research on the process and features of weathering of riverbed granite have hardly been done. In the early days of 1999, clearing up sediment and regolith on the foundation of dam in Three Gorges of Yangtze River, granite riverbed were exposed which offered a good opportunity for researchers to research riverbed granite weathering features. The chemical weathering features of riverbed granite are main researched in this paper.

1.Sample collection. The ways of sample collection were divided into:* between upper cofferdam and lower cofferdam, the 22 samples were equidistantly collected along riverbed two sides on a horizontal elevation of 28m; ?in lower 100m of dam location,the 50 samples were collected along NE45( direction cross section of river valley, dividing different 3 elevation(riverbed, flood plain, valley slope) and 6 typical columnars section.

2. Samples treatment and analysis. All chemical composition of granite samples were analyzed by X rays fluorescent slice. Further, weathering ratio, chemical element transferring ratio, the coefficient of average dripping and washing, the intensity of weathering, SiO2/Al2O3, Al2O3/Fe2O3, (CaO+MgO)/Al2O3, K2O+Na2O/Al2O3 and so on were calculated in greater detail.

3. Results analysis. The desilication rate of riverbed granite with 7.1% is > valley slope granite with 2.5% and < flood plain granite with 9.7%. Al2O3/Fe2O3, is in order of riverbed granite >flood plain granite > valley slope granite. ? (CaO+MgO)/Al2O3 of riverbed granite is the greatest of three landforms position. The dissolved and lossed rate of K2O and Na2O of riverbed granite with 0.71% is > valley slope granite with 0.67% and < flood plain granite with 1%. The increase rate of R2O3/(RO+R2O) of riverbed granite with 0.57% is the lowest of three landforms position. … absolute weathering intensity of riverbed granite is the greatest of three landforms position.The relative weathering intensity of riverbed granite with 7% is > valley slope granite with 3.75% and < flood plain with 7.78%.

4.Discussion. In flowing water environment for long term, chemical reaction (hydration, hydrolysis, solution, oxidation etc.) and the transferring rate of easy solution mineral of riverbed granite are faster than valley slope granite. But, because regolith of granite are eroded by flowing water and are not dissolved further, riverbed granite weathering is in the early stage condition of chemical weathering. In the period, K+ and Na+ are displace with H+ and are washed away as flowing water, and CaO and MgO are relatively concentrated. The other reasons is that CO2 is much littler and carbonic acidization is more slight in riverbed granite than in valley slope regolith.

In a word, the chemical weathering process of riverbed granite is different from that valley slope and flood plain granite in many aspects.