A new geological framework for south-central Madagascar, and its relevance to the "out-of-Africa" hypothesis
Résumé
The Precambrian shield of south-central Madagascar, excluding the Vohibory region, consists of three geologic domains, from north to south: Antananarivo, Ikalamavony-Itremo, and Anosyen-Androyen. The northern Antananarivo domain represents the Neoarchean sector of the Greater Dharwar Craton amalgamated at 2.52-2.48 Ga. The Greater Dharwar Craton is overlain by several groups of Mesoto Neoproterozoic supracrustal rocks (Ambatolampy, Manampotsy, Ampasary, Sahantaha, and Maha Groups) each with a common and diagnostic signature of Paleoproterozoic detrital zircons (2.2-1.8 Ga). The central domain (Ikalamavony-Itremo) consists of two distinct parts. The Itremo Sub-domain, in the east, is a structurally intercalated sequence of Neoarchean gneiss and shallow marine metasedimentary rocks of Paleo-Mesoproterozoic age (Itremo Group), the latter with Paleoproterozoic detrital zircons ranging in age between 2.2 and 1.8 Ga. The Ikalamavony Sub-domain, to the west, contains abundant volcano-clastic metasediments and lesser quartzite (Ikalamavony Group), formed between 1.03 Ga and 0.98 Ga, and intruded by igneous rocks (Dabolava Suite) of Stenian-Tonian age. Structurally intercalated with these are sheets of Neoarchean gneiss (∼2.5 Ga) and Neoproterozoic metaclastic rocks (Molo Group). Like the Itremo Group, quartzite of the Ikalamavony Group has detrital zircons of Paleoproterozoic age (2.1-1.8 Ga). The southern domain of Anosyen-Androyen consists of a newly recognized suite of Paleoproterozoic igneous rocks (2.0-1.8 Ga), and stratified supracrustal rocks also having Paleoproterozoic detrital zircons (2.3-1.8 Ga). The contact between the Anosyen-Androyen and Ikalamavony-Itremo domains, formerly known as the Ranotsara-Bongolava shear zone, is a tightly folded and highly flattened boundary that was ductilely deformed in Ediacaran time. It is roughly equivalent to the Palghat-Cauvery shear zone in south India, and it defines approximately the boundary between the Archean Greater Dharwar Craton (to the north) and the Paleoproterozoic terrane of Anosyen-Androyen (to the south).