PETROGRAPHIC AND MINERALOGIC CHARACTERIZATION OF THE BETSIAKA GROUP, NORTHERN MADAGASCAR

Geology

The Betsiaka Group is one of the four groups forming the Bemarivo Domain in northern Madagascar. This domain represents a juvenile Neoproterozoic volcanic arc with some older, pre-Neoproterozoic components. Along with the Sahantaha Group, the Betsiaka Group represents the oldest formations in the Bemarivo Domain. Sahantaha Group is in the southern part of the domain, whereas the Betsiaka Group is found in the northernmost region. Betsiaka group is composed of metamorphic felsic rocks, amphibolites, and impure marbles. It is separated from the Permo-Triassic sedimentary cover to the north by a normal fault and is adjacent to the juvenile Neoproterozoic volcanic rocks of the Milanoa Group to the south. This study aims to redefine and recharacterize the Betsiaka Group and its formations through field observations, petrographic, and mineralogical analyses. Petrographic and mineralogical studies together offer a thorough understanding of rocks and minerals, enabling accurate rock classification and tracking of geological events. Samples from the Betsiaka Group have been analyzed under transmitted light of petrography microscope in combination with EDS analyses. These studies, combined with field observations, have allowed us to categorize the Betsiaka Group into two distinct formations: the Mantalimanera and Andafialava formations. The Mantalimanera Formation consists of metasedimentary sequences, including felsic rocks, impure marble, and calcsilicate rocks, which are only slightly affected by fluid alteration. In contrast, the Andafialava Formation features mafic orthoamphibolite sequences interlayered with biotite micaschist and some paragneiss. These mafic sequences are strongly affected by fluid alteration. Textural and mineralogical analyses reveal retrograde metamorphism within the Betsiaka Group, a characteristic also observed in other groups of the Bemarivo Domain.

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