REACTION TEXTURES IN GRANULITES FROM FOREFINGER POINT, ENDERBY LAND, EAST ANTARCTICA : AN ALTERNATIVE INTERPRETATION ON THE METAMORPHIC EVOLUTION OF THE RAYNER COMPLEX

Aluminous granulites from Forefinger Point in Enderby Land, East Antarctica, preserve a variety of reaction textures which demonstrate a part of the P-T evolution. On the basis of the petrographical evidence, the following reactions are inferred : orthopyroxene+sillimanite+quartz=cordierite, orthopyroxene+sillimanite=cordierite+sapphirine, garnet+quartz=orthopyroxene+cordierite, garnet=orthopyroxene+cordierite+sapphirine. These reactions are visualized on a simplified (Fe, Mg) O-Al_2O_3-SiO_2 diagram, and are interpreted on P-T and P-_ diagrams, respectively. The reactions imply nearly isothermal decompression after the peak metamorphic conditions. The prograde segment of the P-T path is inferred by relict spinel and corundum inclusions invariably rimmed by sapphirine in silica-undersaturated granulites. A possible reaction is spinel+corundum+cordierite=sapphirine. The petrogenetic grid for a model FMAS system indicates that the decompressional P-T path passed between two invariant points; [Spl] 11±1 kbar at 1040±15℃, and [Qtz] 8-9 kbar at 950℃. Assuming a constant temperature, a P-_ diagram is used to demonstrate that the variety of reactions depends on bulk chemical compositions. Several geothermobarometers, including garnet-orthopyroxene, garnet-orthopyroxene-plagioclase-quartz, garnet-plagioclase-sillimanite-quartz, GRAIL, GRIPS and sapphirine-spinel are applied. The garnet-orthopyroxene and sapphirine-spinel geothermometers yield around 700-850℃, and these values are considerably lower than those inferred from the petrogenetic grid mentioned above. On the basis of the petrographical interpretation and preliminary results of SHRIMP dating (SHIRAISHI, pers. commun.), it is concluded that the Forefinger Point granulites probably do not represent a reworked portion of the Archaean Napier Complex, but rather, they may be the result of Cambrian events in the East Antarctic Shield