A geological synthesis of the Precambrian shield in Madagascar
Résumé
vailable U-Pb geochronology of the Precambrian shield of Madagascar is summarized and integrated into a synthesis of the region's geological history. The shield is described in terms of six geodynamic domains, from northeast to southwest, the Bemarivo, Antongil-Masora, Antananarivo, Ikalamavony, Androyan-Anosyan, and Vohibory domains. Each domain is defined by distinctive suites of metaigneous rocks and metasedimentary groups, and a unique history of Archean (similar to 2.5 Ga) and Proterozoic (similar to 1.0 Ga, similar to 0.80 Ga, and similar to 0.55 Ga) reworking. Superimposed within and across these domains are scores of Neoproterozoic granitic stocks and batholiths as well as kilometer long zones of steeply dipping, highly strained rocks that record the effects of Gondwana's amalgamation and shortening in latest Neoproterozoic time (0.560-0.520 Ga).
The present-day shield of Madagascar is best viewed as part of the Greater Dharwar Craton, of Archean age, to which three exotic terranes were added in Proterozoic time. The domains in Madagascar representing the Greater Dharwar Craton include the Antongil-Masora domain, a fragment of the Western Dharwar of India, and the Neoarchean Antananarivo domain (with its Tsaratanana Complex) which is broadly analogous to the Eastern Dharwar of India. In its reconstructed position, the Greater Dharwar Craton consists of a central nucleus of Paleo-Mesoarchean age (>3.1 Ga), the combined Western Dharwar and Antongil-Masora domain, flanked by mostly juvenile "granite-greenstone belts" of Neoarchean age (2.70-2.56 Ga). The age of the accretionary event that formed this craton is approximately 2.5-2.45 Ga. The three domains in Madagascar exotic to the Greater Dharwar Craton are the Androyan-Anosyan, Vohibory, and Bemarivo. The basement to the Androyan-Anosyan domain is a continental terrane of Paleoproterozoic age (2.0-1.78 Ga) that was accreted to the southern margin (present-day direction) of the Greater Dharwar Craton in pre-Stratherian time (>1.6 Ga), and rejuvenated at 1.03-0.93 Ga with the creation of the Ikalamavony domain. The Vohibory domain, an oceanic terrane of Neoproterozoic age was accreted to the Androyan-Anosyan domain in Cryogenian time (similar to 0.63-0.60 Ga). The Bemarivo domain of north Madagascar is a terrane of Cryogenian igneous rocks, with a cryptic Paleoproterozoic basement, that was accreted to the Greater Dharwar Craton in latest Ediacaran to earliest Cambrian time (0.53-0.51 Ga).