WEATHERING OF A QUATERNARY GLASS-RICH BASALT IN BAKRIT, MIDDLE ATLAS MOUNTAINS, MOROCCO - COMPARISON WITH A GLASS-POOR BASALT

We compared the disintegration processes and mineralogic and chemical evolution pathways of two Quaternary basalts at Bakrit and Ifrane, wea thered in the same physiographic and hydrologic conditions, but differ ing in texture according to the quantity of glass present. At Bakrit, quite abundant glass favoured the formation of a microfissure network throughout the rock and its disintegration without any distinct weathe ring front. As a result, basaltic sand with polymineral grains and a c layey-silty matrix were produced. At Ifrane, weathering of a glass-poo r basalt produced only a clayey-silty saprolite. In glass-rich basalts , secondary minerals formed in microfissures and were 2/1 clay-mineral rich. In glass-poor basalts, secondary minerals formed mainly within primary minerals and were 1/1 clay-mineral rich. Because glass could b e easily dissolved, it protected the minerals of close chemical compos ition, especially the plagioclases. The order of basalt-mineral weathe ring (olivine, labrador, augite, Fe-Ti oxides) was modified when glass was abundant (glass, olivine, augite, labrador, Fe-Ti oxides). (C) El sevier, Paris.