RETROGRADE EVOLUTION OF THE VERY-HIGH-TEMPERATURE GRANULITES FROM
C. NICOLLET, R. RAMBELOSON and D. VIELZEUF (Département de Géologie, URA 10, 5 rue Kessler, Clermont-Ferrand 63038, France et E.E.S.S., Antananarivo, Madagascar)
In Madagascar, extensive granulite terrains formed in response to Very - High - T (>900°C) metamorphism. In the North, aluminous granulites, associated with belts of basic-ultrabasic magmatic rocks, indicate high grade conditions (T=1000°C-P=6Kb). The mineral associations are complex with 3 generations of Opx, at least 2 of sapphirine, Sill and garnet. Cordierite, Q, rutile, ilmenite, corundum, green spinel, ±feldspars are also present. The high - T paragenesis was : Al rich Opx (or Sill) - Sapph - Q - Ga - Rut.
The diversity of the mineral associations reflects the complex rétrograde evolution of these rocks, recorded by the following reaction textures : Al - rich Opx contains Ga lamellae (Al - rich Opx = Opx + Ga). Sapph is armoured by Sill and Opx at the contact with Q (Sapph + Q = Opx + Sill). Around Ga, coronas of Opx + Sapph or Opx + Sill or Opx + Cord are interpreted in terms of the following reactions : Ga = Opx + Sapph (± Sill) or Ga + Q = Opx + Sill or Ga + Q = Opx + Cord. Cord replaces Ga through the equilibrium Ga + Sill + Q = Cord. Sill - Opx contacts are rimmed by Sapph + Cord (Opx + Sill = Cord + Sapph) or Cord (Sill + Opx + Q = Cord). Cord is subsequently pseudomorphosed by a fine- agregate of Opx + Sill ± Q, implying the same reaction, but in the opposite direction.
Hydration of these rocks produces orthoarmphibole - Cord - Ga or Q ± staurolite, biotite gneisses, according to the reactions : Opx + Q + V = Anthophyllite, Ga + Q + V = Gedrite + Cord, Gd + Sill + V = Stl + Cord.
The complexity of these reaction textures make these difficult to interpret in terms of a simple PT trajectory. However, most of them seem to be consistent with an overall near isobaric cooling from VHT. It is suggested that the VHT metamorphism and the associated magmatic intraplating could be the consequences of an extreme thermal perturbation during continental lithospheric thinning in an extensional regime.