Weathering Inrensity and Investigation of Weathring Indices in som Soils Developed on Igneous Rocks in West-north of Iran

This study was conducted to assess different chemical weathering indices and to evaluate the weathering rates of soils developed on volcanic (hornbelende andesite، pyroxene andesite and dacite) and plutonic (alkali granite، granodiorite، monzodiorite، syenite and pyroxene diorite) igneous rocks in the northwestern Iran. Representative soil profiles were described and soil samples were collected and analyzed for selected chemical and physical properties. Total concentrations of major elements and trace element (Zr، V، Ti and Y) were determined with ICP-OES and ICP-MS، respectively. Significant correlation coefficients were obtained between soil properties (clay percent، pedogenic iron and bulk density) and Ba، B/A، B/R، CIA، CIW، PIA، PWI and the WR chemical weathering indices. These indices are based on the ratio of a group of mobile oxides to one or more immobile oxides and are suitable for explaining the weathering rate of the soils developed on igneous rocks in this semiarid region. A-CN-K and MFW ternary plots showed that the soils developed on volcanic rocks (hornbelende andesite> pyroxene andesite> dacite) were more weathered than those on the plutonic parent rocks (alkali granite، granodiorite، monzodiorite، syenite، pyroxene diorite). Ba and CIA weathering indices predicted weathering trend such as MFW ternary plot، and it seems these two weathering indices are the most suitable weathering indices after W index (or MFW ternary plot) in this semiarid region. Ca، Na and K elements are presented in chemical weathering formulas of these two indices. These elements are in the feldspar minerals structures which are the most mineral in the earth crust.