The assessment of the weathering intensity of Emeishan basalt based on rock blocks（Ⅰ）：Geochemistry of weathered basalt blocks

Abstract：Emeishan basalt is a typical medium with low permeability. The weathering of basalt rock mass results from basalt block weathering. The weathered basalt block has a shell-like structure formed by core stone which is surrounded by one or more layers of saprolite crusts. From the block surface inward, the nearer it approaches the geometrical center of the block, the lower the degree of weathering becomes. The weathering process of the basalt can be divided into two stages：the early period and the mid-late period. Bivalent iron turns into trivalent iron without significant loss and relative enrichment of components during the first stage. Bivalent iron is oxidized continuously with the leaching of mobile components (Si, Ca, Mg, Na, K) and the relative enrichment of immobile components (Al, Ti, ΣFe) during the second stage. As the intensity of weathering increases, the relative content of FeO decreases and that of Fe2O3 increases, but there is little change in the relative content of ΣFe during the early period of weathering. With the increase of weathering degree, the relative content of SiO2, CaO, MgO, Na2O, K2O and FeO decreases monotonically while that of Al2O3, TiO2, Fe2O3 and LOI increases monotonically during the mid-late stage of weathering. During the whole weathering process of the basalt, the relative content of FeO and Fe2O3 is most sensitive to the degree of weathering. Changes in the iron species caused by oxidation of iron-bearing minerals throughout the weathering process should serve as the key factor which can be used to evaluate the weathering degree of Emeishan basalt.