New zircon ages and regional significance for the evolution of the Pan-African orogen in Madagascar

New (207)pb/(206)pb Single zircon evaporation ages for granulites, gneisses and granites in southern and central Madagascar record a widespread Pan-Af rican metamorphic and magmatic event in the period c. 650-556 Ma, but also earlier ages in the range 1890-1710 Ma, inherited from protolith material a nd reflecting heterogeneous crustal sources. South of the Ranotsara shear z one, metasedimentary gneisses and granulites contain an early population of detrital zircons with ages in the range 1890-1740 Ma; a detrital grain wit h an age of 899 +/- 2 Ma suggests that some sedimentary protoliths were dep osited later than c. 900 Ma. Metamorphic zircons have a mean age of 564.2 /- 0.9 Ma. North of the Ranotsara shear zone, our data provide information on the age of source material of metamorphic rocks: 788.6 +/- 0.7 Ma for the time of e mplacement of the granitic precursor of a granulite-facies charnockite and 650.9 +/- 0.9 Ma for the protolith age of an amphibolite-facies migmatitic gneiss. A structurally conformable alkali granite sheet with a crystallizat ion age of 568.7 +/- 1.6 Ma contains xenocrystic zircons, one of which has an age of 1229.6 +/- 1.0 Ma, inherited from the source of the anatectically derived material. The post-tectonic, alkalic Carion granite has an emplace ment age of 556.0 +/- 1.7 Ma and provides a minimum age for granulite- and amphibolite-facies metamorphism. Our field data indicate that extensional s hear zones are common in central Madagascar, locally controlling amphibolit e-facies retrogression of granulite-facies rocks and the emplacement of cru stal melt granites. These events record the widespread extensional collapse of the Pan-African orogen in Madagascar.