Clay mineral assemblages in weathered basalt profiles from central and southern Portugal: climatic significance

Abstract

Mineralogical and chemical changes during weathering of basalts from central and southern Portugal were studied. Both regions have a temperate Mediterranean type climate, the Algarve being more arid. The areas concerned are the Lisbon (72 Ma) and the Runa Volcanic Complexes (100–70 Ma), and Rio Maior (160–130 Ma) in central Portugal, and Budens (170–180 Ma) in the south. The basalts are generally rich in olivine phenocrysts. Clay minerals are in general well crystallised and comprise smectites, associated with 1:1 minerals (principally halloysite), illite and palygorskite. Kaolin minerals prevail in topographic highs, whilst smectites predominate in lower areas. Smectites were analysed from the <2 μm fraction and from weathered microsites on olivine crystals. Both approaches identified dioctaedral smectites. Smectites from the south (Budens) have higher Na contents and lower Mg concentrations than those from central Portugal. This, together with different climatic or microclimatic conditions, may explain the smectite–analcime association in the first case, and smectite–palygorskite in the second. Smectites can thus generate one or the other of these minerals. In weathered microsites on olivine crystals, iron oxides were noted as well as the possible replacement of smectites and Fe oxides by calcite. The general weathering features appear to be similar to those found on basalts in other temperate climates, i.e. the formation of secondary minerals with a predominance of smectites or 1:1 minerals according to geomorphic location. However, the degree of aridity may affect the stability of smectites, which seem to evolve either to palygorskite or to analcime.