Abstract
Abstract The Andringitra Range (Madagascar) is made of potassic to ultrapotassic syenites (K2O=8–11 weight%; K2O/Na2O=2–8). It is a new example of this rare type of magmatism, here related to the Late Precambrian Panafrican orogeny. In the Pic Boby area, the main rock type contains pyroxene (diopside) and amphibole (edenite), and is nearly silica-saturated. A subordinate type is an undersaturated diopside- and phlogopite-bearing syenite. Very high K, Ba, Rb, Sr and LREE contents, together with moderate to high Mg# (33–54), suggest a parental magma derived from the enriched (metasomatized) lithospheric mantle. Fractional crystallisation and crustal assimilation may be responsible for the most evolved compositions.
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