Rui Dias is kindly acknowledged for having encouraged this publication, Peter Watkins for technical assistance with the ICP-AES analysis at Imperial College, Oscar Carvalho for field assistance and José Francisco Santos for fruitful discussions. We are also very grateful to Colombo Tassinari, Liliane Petronilho and Kei Sato for technical help during sample preparation and Rb-Sr and Sm-Nd mass spectrometry at the University of São Paulo (Brasil). This investigation was carried out in the scope of two research projects (PRAXIS 2/2.1/CTA/94 and POCTI/CTA/35630/99), financially supported by the Portuguese Foundation of Science and Technology (FCT).