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From closure of the Mozambique Ocean to gondwana breakup: New evidence from geochronological data of the Vohibory terrane, Southwest Madagascar

Abstract:

A multi-thermochronometer study of basement rocks, using combined Pb-Pb zircon, U-Th-total Pb monazite, U-Pb SHRIMP titanite, 40Ar/ 39Ar biotite, and apatite fission track data, was performed to derive a detailed cooling history for the Vohibory terrane, southwest Madagascar. The main metamorphism at ca. 640-610 Ma is interpreted to represent the closure of the Mozambique Ocean and the subsequent accretion of the Vohibory arc terrane to the active continental margin of Azania (proto-Madagascar). Contemporaneous with the assumed main Gondwana-forming collision between Azania/India and the Congo craton at ca. 535 Ma, slow cooling (8.1°-3.6°C/m.yr.) indicates extrusion of the Vohibory block. This central part of the East African orogen reached thermal equilibrium between 500 and 350 Ma. During late Carboniferous/Triassic rifting between East Africa and Madagascar, the Vohibory terrane was the favored starting point for crustal extension, causing basement rock cooling (up to 5.3°C/m.yr.) and heating (up to 1.6°C/m.yr.). The Jurassic passive margin evolution was accompanied by rift locus migration to the west of the Vohibory block. The resulting rift geometry and associated sedimentation caused flexural isostatic response and inversion of the Vohibory part of the late Carboniferous/Triassic rift. © 2008 by The University of Chicago.