A new example of potassic to ultrapotassic magmatism: the Andringitra syenites (Madagascar).

The Andringitra Range (Madagascar) is made of potassic to ultrapotassic sye nites (K2O = 8-11 weight%; K2O/Na2O = 2-8). It is a new example of this rar e type of magmatism, here related to the Late Precambrian Panafrican orogen y. In the Pic Boby area, the main rock type contains pyroxene (diopside) an d amphibole (edenite), and is nearly silica-saturated. A subordinate type i s an undersaturated diopside- and phlogopite-bearing syenite. Very high K, Ba, Rb, Sr and LREE contents, together with moderate to high Mg# (33-54), s uggest a parental magma derived from the enriched (metasomatized) lithosphe ric mantle. Fractional crystallisation and crustal assimilation may be resp onsible for the most evolved compositions. (C) 2001 Academie des sciences / Editions scientifiques et medicales Elsevier SAS.