A NEW PRACTICAL INDEX OF CHEMICAL WEATHERING FOR GRANITIC ROCKS

Quantitative determination of chemical weathering rate of rocks is a fundamental work for environmental and engineering geology. The chemical weathering of rocks proceeds by water-rock interaction. During the weathering, some alkaline and alkali-earth elements are easily leached from rocks. On the other hand, the residual elements are re-distributed to secondary minerals. This is a fundamental system on the chemical weathering of rocks. Therefore, this system is widely used for the chemical weathering indices using whole rock chemistry (e.g. Vogel, 1975; Nesbitt and Young, 1982; Harnois, 1988). The indices are based on the chemical dissolution rate (CDR) of weathered rock. The CDR means the degree of depletion of mobile components relative to immobile components during weathering (e.g. Harnois, 1988). We found that the CDR of fresh granitic rocks does not take a constant value. The CDR changed with the increase in SiO2 content. It is meaning that the base line of weathering index does not fix. In general, chemical compositions of granitic rocks primary varied with their magmatic process like differentiation, magma mixing, assimilation, and etc. (e.g. Petersen 1980; LeBel et al., 1985; Larsen and Smith, 1990; Leterrier, 1994). The change in CDR of the fresh rocks is owing to their magmatic compositional variation. In order to overcome the problem, we propose a new practical chemical weathering index. The index is evaluated the degree of chemical weathering of granitic rocks on the basis of the magmatic compositional variation. The index based on the petrogenesis of rocks would be taken into consideration to the case for other igneous, sedimentary, and metamorphic rocks because they also have wide chemical variation with genesis.