Geological Society of America, Annual Meeting, NOVEMBRE 2000, Reno, USA

In-situ electron microprobe monazite dating of the complex retrograde evolution of UHT granulites from Andriamena (Madagascar) : apparent petrographical path vs PTt path.

GONCALVES Philippe, NICOLLET Christian and LARDEAUX Jean-Marc, Univ. Blaise Pascal, Clermont-Ferrand, FRANCE

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     North-central Madagascar consists of late Archean crust reworked during Neoproterozoic. In this high grade metamorphic terrains displaying a polymetamorphic history, monazite can suffer several episodes of cristallisation or resorption, implying a difficult age interpretation. Using in-situ electron probe dating, we attempt to determine the different monazite growth generations.
     Our study area is located on the late Archean Andriamena mafic gneiss belt, characterized by the occurence of Al-Mg granulites preserving UHT assemblages (spr1-grt1-q/opx1-sil1-q) at about 1050°C, 10kbar. The retrograde evolution is recorded by a complex sequence of coronitic assemblages (spr2-cd2/opx2-spr2-cd2/opx2-cd2) suggesting an ITD from 10kbar to 6-7kbar. Finally, the cd2 is partially substituted in a symplectite of opx3-sil3-q implying an IBC after the ITD. At the same time, hydratation produces oamph-cd-bio-q assemblage. The aim of this work is to constrain the timing of the UHT metamorphism and the true PTt path.
     When monazites occurs as armoured inclusions in grt1 porphyroblast, they yield an age of 2,5Ga interpreted as peak UHT metamorphism age. All ages recorded in monazites located in the retrograde assemblage are lower than 1,1Ga : then, we can not constrain the late archean retrograde PTt path.
      Monazites associated with retrograde assemblages can yield until 3 age populations in a single cristal :
      1)Older cores can indicate a previously unrecognized event at 1,0-1,1Ga in Madagascar. The signification of this event in Gondawana is still debated.
      2)All samples have registered an early Neoproterozoic age (700-800Ma). The microtextural relationships between the monazites and the metamorphic assemblages show that this event is synchronous with the petrographical apparent ITD-IBC path. Furthermore, partial melting occured at the same time and PT conditions (900°C, 6-7kbar).
      3)A latest Neoproterozoic event, appears only in the hydrated granulites as little overgrowth probably associated with a process of dissolution-precipitation.

Using this in-situ dating method, we point of view that the deduced petrological path can not be interpreted directly in terms of a PTt trajectory. At least, two high grade metamorphisms clearly affect the north-central Madagascar at both 2,5Ga (UHT) and 750Ma (apparent ITD, IBC and partial melting). We interpret the apparent ITD as a fictive path joining the HP 2,5Ga event and the 750Ma event at a lower pressure.