Ambohiby Massif, Ambatolampy Commune, Tsiroanomandidy District, Bongolava, Madagascari

Regional Level Types
Ambohiby Massif	Massif
Ambatolampy Commune	Commune
Tsiroanomandidy District	District
Bongolava	Region
Madagascar	Country

Abstract of Mukosi (2012):

The Cretaceous Ambohiby Complex is an alkaline ring complex located in the central part of Madagascar and covers a mountainous area of approximately 225 km². The complex intrudes into Precambrian basement gneisses and consists of the following rock types in a chronological order: gabbros, monzonite, alkali-syenite, micro-granite and granites. Both mafic and felsic rocks are dominated by sodic mineralogies. Pyroxenes are generally aegirine, aegirine-augite, and hedenbergite and commonly occur in granites, micro-granites, syenites and monzonite. In gabbros and mafic dykes, augite is the more common composition. Amphiboles are represented by bluish to brownish-green varieties with arfvedsonite to eckermannite compositions in granites, and magnesia-arfvedsonite compositions in micro-granites. Ferro-edenite is present in some alkali-syenites and monzonite. Feldspars are usually single phase and are therefore hypersolvus. In granites, micro-granites and alkali-syenites, path and string perthite is very common. Graphic intergrowth of quartz and alkali feldspars is also common in granites and some alkali-syenites. Major elements variation diagrams plotted against SiO2 indicate that the mafic and felsic rocks of the Ambohiby Complex were formed by processes similar to those of fractional crystallization. Chondrite normalised mafic rocks have slightly positive Eu anomalies, while the felsic rocks have negative Eu anomalies, indicating fractionation of plagioclase feldspars. The chondrite normalised gabbroic rocks shared similar trends of heavy rare earth with chondrite normalised Marion Hotspot data. This suggests that the basaltic parent magma for the Ambohiby Complex, possibly related to the Marion hotspot plume. The fractional crystallization model with an inclusion of olivine in the mineral assemblage seems to fit very well with the actual Ambohiby felsic end member rocks (i.e. granites). It is therefore clear that differentiation mainly occurred by fractional crystallization, but variable initial Sr and Nd values indicate the magmas assimilated crustal material during emplacement. The Rb-Sr geochronology gave an age of 90±2.4 Ma for the intrusion of the Ambohiby Complex, which confirms that the Ambohiby Complex is associated with the Gondwana break-up. In addition, the Marion Hotspot plume is believed to have been located in the southern tip of the island at around 90 Ma ago.

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References