Thesis (MSc)--Stellenbosch University, 2012. / ENGLISH ABSTRACT: The Cretaceous Ambohiby Complex is an alkaline ring complex located in the central part of
Madagascar and covers a mountainous area of approximately 225km2. 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 alkalisyenites.
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.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/71665 |
| Date | 12 1900 |
| Creators | Mukosi, Ndivhuwo Cecilia |
| Contributors | Miller, Jodie, Macey, Paul, Stellenbosch University. Faculty of Science. Dept. of Earth Sciences. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | English |
| Type | Thesis |
| Format | 126 p. : ill., maps |
| Rights | Stellenbosch University |
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