Discrimination between mantle- and crustal-derived Andean calc-alkaline lavas

The Zr vs. Nb/Zr diagram enables a clear discrimination between the lavas e rupted along the normal segments of the Andean are and those of the intense ly thickened Altiplano-Pu (n) over tilde a. This is consistent with the con trasted origin of the calc-alkaline magmas, which are essentially mantle-de rived in the are and mainly crustal in the Altiplano-Pu (n) over tilde a. T he lavas from the thickened part of the are (central Andes) are divided alm ost equally between the two discriminating fields, which is consistent with a strong crustal influence in magma genesis. These results demonstrate the interest and limits of the Zr vs. Nb/Zr diagram for interpreting ancient m agmatic rocks. ((C) Academie des sciences/ Elsevier, Paris.).