WEATHERING OF A TRIASSIC BASALT IN THE EL HAJEB 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 basal t is composed mainly of plagioclase (34 vol% labradorite), clinopyroxe ne (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 depo sition of smectite, talc, quartz and calcite in vesicles and open frac tures; 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 pseudomorp hed by Al-montmorillonite and Mg-Fe-saponite, respectively, in coheren t saprolite. In the friable saprolite, the Al-beidellite is the only s econdary product, except for pyroxene, which is replaced by iron oxide at the top of the profile. Differences in weathering intensity and co nditionin each part of the profile are responsible for chemical and mi neralogical differences in the secondary products. (C) 1997 Published by Elsevier Science Limited.