Is the petrographical PTt path consistent with the real thermal path ? The example of the polymetamorphic Ultra-High Temperature granulites of Andriamena (Madagascar).

GONCALVES Philippe, NICOLLET Christian, MONTEL Jean-Marc, LEFEVRE Brieuc, PAQUETTE Jean-Louis, LARDEAUX Jean-Marc and PIN Christian.

Due to their refractory behaviour, Mg-Al granulites preserve numerous coronitic textures providing plenty information to reconstruct continuous petrographical paths. In most cases, these granulites have suffered a polymetamorphic history. Without geochronological data, it is thus difficult to interpret them and infer tectonic interpretations. Our study area is located in the Andriamena mafic gneiss belt (North-Central Madagascar). It is characterized by a polymetamorphic and magmatic history since 2,5 Ga to 530 Ma and by the occurrence of sapphirine-bearing granulite relics and metapelitic migmatites. Petrographical investigations of both rock types clearly show two different PT evolutions :

- Mg-granulites preserve UHT assemblages (grt0-spr0-qz / opx1-sil1-qz) suggesting minimal PT conditions of 1050°C, 11kbar. Subsequently, a sequence of complex coronitic textures (spr2-crd2 / opx2-spr2-crd2 / opx2-crd2) suggest an isothermal decompression (ITD) from 11 to 7kbar at T>900°C. Finally, the crd2 are overprinted by an opx3-sil3-qz symplectite implying a return into the opx-sil-qz stability field probably through an isobaric cooling (IBC) at about 7kbar.

- Migmatites record a heating-cooling path at about 7kbar, without any significant change in pressure. Partial melting was achieved by biotite dehydration reactions at T<850°C, followed by back melting reactions with the residual silica-undersatured melt. PT conditions of the retrograde path are similar to the IBC of the Mg-granulites.

Multi-method geochronology (microprobe U-Th-Pb, U-Pb, Lu-Hf, Sm-Nd) are used to constrain these PT evolutions. In-situ electron microprobe dating has the advantage to combine textural features, chemical compositions and ages to distinguish several episodes of monazite growth.

The Mg-granulite monazites included in garnets yield the oldest age of about 2,5 Ga. We suggest that this age reflect the age of the UHT metamorphism. Matrix monazites in close association with the opx3-sil3-qz symplectites yield an age of 771±18 Ma. Chemical and textural features suggest a new episode of monazite crystallisation contemporeanously with the IBC. Migmatite monazites yield an U-Th-Pb age poorly constrained of about 770 Ma. U-Pb and Lu-Hf/Sm-Nd garnet geochronology better constrain them.

Geochronological data imply that the petrographical path recorded in the Mg-granulites cannot obviously be interpreted as a continuous PT path in a single metamorphic event. Two thermal events occurred : firstly, late Archean UHT metamorphism was followed by a cooling to the steady state geotherm in a single tectonothermal event. At 770 Ma, the second thermal event occured at lower pressure. Therefore, it suggests that the ITD is a fictive path joining these two events : the 2,5 Ga high pressure event and the 770 Ma low pressure one. The Neoproterozoic event could be the consequence of a thermal perturbation caused by the emplacement of mafic intrusions at 787±16 Ma (Guérrot et al., 1993) in a continental arc setting in relation with the subduction of the Mozambique ocean (Handke et al., 1999).