Petrogenesis of the Ambohiby Complex, Madagascar and the role of the Marion Hotspot Plume

The Cretaceous Ambohiby Complex is an alkaline ring complex located in the west central part of Madagascar and covers a mountainous area of approximate 225 km~2.The complex intrudes into Precambrian basement gneisses and is dominated by alkaline mafic to felsic rocks with sodic mineralogies. Pyroxenes are generally aegirine,aegirine-augite,and hedenbergite and commonly occur in granites,