A NEW GEOLOGICAL MAP OF THE PRECAMBRIAN SHIELD OF MADAGASCAR

The new 1:1,000,000 scale Geological Map of Madagascar, published under the auspices of the Project de Gouvernance des Resources Minerales of Madagascar, portrays the Precambrian shield in terms of six geodynamic building-blocks, from southwest to northeast, the Vohibory, Androyen-Anosyen, Ikalamavony, Antananarivo, Antongil-Masora, and Bemarivo domains. The Archean nucleus consists of the Antongil-Masora and Antananarivo domains. The eastern Antongil-Masora domain consists of Paleo- to Mesoarchean TTG gneisses (3.3-3.2 Ga) and abundant metawacke, BIF, and metamafic gneiss dated between 3.2 and 2.5 Ga. The western Antananarivo domain consists mostly of juvenile Neoarchean orthogneiss, but it also includes distinctive belts of metamafic gneiss and schist best interpreted as recrystallized Archean greenstone belts (Tsaratanana Units). The Antongil-Masora and Antananarivo domains represent a piece of an even larger craton – the Greater Dharwar Craton – which was amalgamated at 2.5-2.45 Ga. Three domains exotic to this Archean nucleus are the Vohibory, Androyen-Anosyen, and Bemarivo domains. The basement to the southern Androyen-Anosyen Domain is a continental terrane of Paleoproterozoic age (2.0-1.8 Ga) that was accreted to the Antananarivo domain in pre-Stratherian time (~1.8-75 Ma). The Ikalamavony domain, situated between the Antananarivo and Androyen-Anosyen domains, is a Mesoproterozoic terrane of volcano-sedimentary and intrusive igneous rocks (magmatic arc) built on continental crust of the Androyen-Anosyen and Antananarivo domains (1.04-0.95 Ga). The Vohibory Domain, an oceanic terrane of early Neoproterozoic age, was accreted to the Androyen-Anosyen domain in Cryogenian time (~0.64-0.62 Ga). The Bemarivo domain of north Madagascar, is an exotic fragment of Cryogenian igneous rocks with a cryptic Paleoproterozoic basement, that was accreted to Antananarivo and Antongil-Masora domains in Ediacaran- Cambrian time. Superimposed within and across these domains are scores of granitic stocks and batholiths, as well as kilometer long zones of steeply dipping, highly-strained rocks, which record the effects of Gondwana’s convergence and amalgamation in latest Neoproterozoic time.