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

[摘要] ENGLISH ABSTRACT: The Cretaceous Ambohiby Complex is an alkaline ring complex located in the central part ofMadagascar and covers a mountainous area of approximately 225km2. The complexintrudes into Precambrian basement gneisses and consists of the following rock types in achronological order; gabbros, monzonite, alkali-syenite, micro-granite and granites. Bothmafic and felsic rocks are dominated by sodic mineralogies. Pyroxenes are generallyaegirine, aegirine-augite, and hedenbergite and commonly occur in granites, micro-granites,syenites and monzonite. In gabbros and mafic dykes, augite is the more commoncomposition. Amphiboles are represented by bluish to brownish-green varieties witharfvedsonite to eckermannite compositions in granites, and magnesia-arfvedsonitecompositions in micro-granites. Ferro-edenite is present in some alkali-syenites andmonzonite. Feldspars are usually single phase and are therefore hypersolvus. In granites,micro-granites and alkali-syenites, path and string perthite is very common. Graphicintergrowth of quartz and alkali feldspars is also common in granites and some alkalisyenites.Major elements variation diagrams plotted against SiO2 indicate that the mafic and felsicrocks of the Ambohiby Complex were formed by processes similar to those of Fractionalcrystallization. Chondrite normalised mafic rocks have slightly positive Eu anomalies whilethe felsic rocks have negative Eu anomalies, indicating fractionation of plagioclase feldspars.The Chondrite normalised gabbroic rocks shared similar trends of heavy rare earth withChondrite normalised Marion Hotspot data. This suggests that the basaltic parent magma forthe Ambohiby Complex, possibly related to the Marion hotspot plume. The Fractionalcrystallization model with an inclusion of olivine in the mineral assemblage seems to fit verywell with the actual Ambohiby felsic end member rocks (i.e. granites). It is therefore clearthat differentiation mainly occurred by fractional crystallization but variable initial Sr and Ndvalues indicate the magmas assimilated crustal material during emplacement. The Rb-Srgeochronology gave an age of 90±2.4 Ma for the intrusion of the Ambohiby Complex, whichconfirms that the Ambohiby Complex is associated with the Gondwana break-up. In additionthe Marion Hotspot plume is believed to have been located in the southern tip of the island ataround 90 Ma ago.

[发布机构] Stellenbosch University