IGCP 304 Granulite facies Conference, Adirondacks, USA, 16-22 Mai, 1992.
P-T-X GRIDS FOR MAFIC GRANULITES : APPLICATIONS TO THE ANORTHOSITIC COMPLEX FROM THE SOUTH OF MADAGASCAR
NICOLLET C., URA 10, Dpt de Géologie, 5, Rue Kessler, Clermont Fd, France
In the South of Madagascar, massifs of anorthosites (from 25 to 100 km2 large) are associated with abundant mafic granulites and coronitic 0l metagabbros. Thirty parageneses of the HP and IP Hb-granulite facies may contain Opx, Cpx, Pl, Gt, brown Hb, Q, Biot, Ilm in the granulites and, in addition, 0l and Sp in the metagabbros. Many parageneses are observable at the scale of several hectares, due to the wide compositional range of the rocks. Such a diversity can be graphically explained only if the usual solid solutions (Fe-Mg, NaSi-CaAl) are taken in account in the conventional end member petrogenetic grids (SAFCH). The incorporation of a solid solution (Fe-Mg) gives an additional variance to the mineralogical assemblages : invariant points become univariant lines; univariant lines become divariant assemblages; etc. In this P-T-XMg, grid, projected onto the PT plane, T (or P)- XMg sections allow us to appreciate the diversity of the parageneses and the complexity of the coronites in the 0l metagabbros. However, these diagrams are pseudo-binary and the incorporation of an other solid solution (Na-Ca) remove the reactions by enlarging the stabilité field of the Pl-Hb-bearing assemblages. In this way, the mineralogical variations around one crystal of 0l (or Opx) may be explained by variations of the Pl composition.
Changes in aH20 may also account for some of the diversity of the encountered parageneses ; moreover, at high aH20, the SACFH grid is unstable relative to melting reactions (as indicated by the occurence of basic migmatites in the granulitic complex from the South of Madagascar). A P-T-XH20 grid may be drawn, provided that a liquid phase (L) is included in the system. Two examples of infiltration of fluids in fractures will be considered. Infiltration of fluids in fractures is presumed to be an important process in the lower crust, because some of the involved reactions have negative volume changes of the solid phases : rocks lose volume, opening channelways for fluids. However, the use of T-X, aH20 diagrams needs great care, in this case, because infiltration processes imply mass transfert so that the system is no longer isochemical.