HYDROTHERMAL AND METEORIC WEATHERING IN T HE TRIASSIC BASALT OF THE MID-ATLAS MOUNTAINS OF MOROCCO

Triassic basalts in the Mid-Atlas Mountains of Morocco are represented by lava flow sequences. in a vertical transect, consisting of al leas t four lavas, the basal unit is covered by a paleosol relic. The basal t is tholeiitic with an ophitic texture, and has a mineralogical assem blage characterised by the presence of pumpellyite and chlorite, indic ating that the volcanic rocks have been subjected to hydrothermal weat hering at high temperatures. The upper part of the basal lavas shows s igns of meteoric weathering which is marked by a change in colour (gre en to reddish brown), presence of Fe oxy-hydroxides, transformation of chlorites to smectites, loss of Ca, Na, and Mg, and an accumulation o f Fe and Si. This data indicates that the profile was altered before b eing covered by the overlaying lava flows. The presence of kaolinite a nd smectite-type minerals suggest that weathering took place under a c onfined and a moderately-drained environment. The loss of kaolinite in the upper part of the palaeoprofile indicates exposure to thermal met amorphism by overlying basaltic lavas. Copyright (C) 1996 Elsevier Sci ence Ltd.