IGCP 348/368, Intern. Symposium on Geol. and mine. Resources of Madagascar - 1997.

C. NICOLLET, JM MONTEL, S. FORET, JE MARTELAT, R. RAKOTONDRAZAFY* and JM LARDEAUX **

Dpt de Géologie, Clermont Ferrand, ** Lyon (France) et *Antananarivo

Madagascar displays a huge portion of the lower crust and its study is fundamental for the understanding of the evolution of this part of the crust. Unfortunately, our knowledge of this domain is limited. At this time, numerous works (structural, geochemical, petrological) are in progress in Madagascar, but there are only a few geochronological data. In this context, the fast, non-expensive, reliable electron microprobe monazite dating method, using the Th-U-Pb system, is particularly efficient to obtain many ages (see Montel et al., 1996, Chem. geol., 37-53). We applied this method in Madagascar.

DATING OF THE UPLIFT IN THE SOUTH OF MADAGASCAR

In the granulitic basement Southern Madagascar, three large areas can be distinguished, from East to West : the Androyan, the Bekily and the Vohibory blocks, separated by transcurrent shear zones : the Bongolava-Ranotsara, the Beraketa and the Ampanihy shear zones. This geometry is interpreted as the result of crustal extrusion towards the S-SE of the South of the island during the Mozambican collision. (Martelat et al., 1995 ; 1997 ; Lardeaux et al., this congress)

Granulitic rocks were dated inside and outside the shear zones :

Results are consistent with available datings carried out by conventional methods (Andriamarofahatra et al., 1990 ; Paquette et al., 1994 ; Kröner et al., 1996).

Concerning the late Panafrican event, the ages are younger in the shear zones : 450 - 500 Ma in the Beraketa zone and 375 - 481 Ma in the Bongolava-Ranotsara lineament, than inside the blocks : 523 - 558 Ma. We suggest that, in the southern part of the island, the peak of the HT metamorphism at 560-550 Ma is followed by a general cooling until 500 Ma. Then, deformation and resetting due to hydrothermal processes during uplift, are restricted, in space and time, along the shear zones. The Bongolava-Ranotsara lineament, especially, plays a role until the opening of the Canal of the Mozambique (from Jurassic to Cretaceous).

DATING OF THE POLYMETAMORPHIC GRANULITES OF VERY HIGH TEMPERATURES OF THE ANDRIAMENA FORMATION, NORTH OF MADAGASCAR

In the Northern, very high temperatures (> 900°C) aluminous granulites, associated with mafic - ultramafic belts, show complex mineralogical associations with Opx, Saph, Sill, Gt, Cord, Orthoamph, Stl, Biot, Q, Corundum, Sp, Rut. The diversity of these associations reflects the complex retrograde evolution of these rocks, from a primary association: aluminous Opx (or Sill) - Saph - Q - Gt - Rut to a retrograde paragenesis : Orthoamph - Cord - Gt or Q - Sill + / - Biot. However, many rocks are only partially reequilibrated and show varied coronitic assemblages. Single zircon evaporation ages (Guérrot and al., 1991) of the granulites of the region yielded 780 My and 2.3 to 3.2 Ga.

The monazites of thin sections of 4 rocks representing various stages of retrogression have been dated by the electron microprobe. Populations of ages are in the range 450 My and 3.2 Ga, with an intermediate population around 1.7 Ga. The distribution of these populations depends on the petrological positions of the dated crystals. The retrogressed sample A 6-3 (Orthoamph, Cord, Q, Sill, + / - Biot) displays a single population (731 + / -24 My) while sample An 4c, with preserved primary association yields many early Proterozoic ages. In the reactional assemblages, ages spread between the two extremes, but depend on the petrographical position : old ages (until 3.2 Ga) in primary relic minerals (Gt) and more recent ages in coronitic minerals.

IN CONCLUSION, the granulites of Madagascar demonstrate that the in-situ dating method of monazite by e-probe, applied on accurately chosen samples and appropriate petrographical locations, is an efficient tool for geochronological exploration.