Weathering of a Triassic basalt in the Elhajeb region, Middle Atlas (Morocco)

The Elhajeb basalt is part of a tholeiitic volcanic series associated with the first stages of the opening of the North Atlantic. This basalt is composed mainly of plagioclase (34 vol% labradorite), clinopyroxene (28 vol%, augite and pigeonite), titanomagnetite (6 vol%) and glass. Two post-deposition processes have affected the Elhajeb basalt: (1) post-magmatic hydrothermal circulation inducing secondary mineral deposition of smectite, talc, quartz and calcite in vesicles and open fractures; and (2) weathering of basalt by meteoric water. The first step in the weathering of basalt induces the transformation of glass to Mg-Fe-saponite. In a second step, plagioclase and pyroxene are pseudomorphed by Al-montmorillonite and Mg-Fe-saponite, respectively, in coherent saprolite. In the friable saprolite, the Al-beidellite is the only secondary product, except for pyroxene, which is replaced by iron oxide at the top of the profile. Differences in weathering intensity and conditionin each part of the profile are responsible for chemical and mineralogical differences in the secondary products. (C) 1997 Published by Elsevier Science Limited.