Linking P-T path with development of discontinuous phosphorus zoning in garnet during high-temperature metamorphism - an example from Lutzow-Holm Complex, East Antarctica

Abstract

Garnet porphyroblasts contained in the garnet-sillimanite gneiss from the Lützow-Holm Complex at Skallevikshalsen, East Antarctica, have a phosphorus-poor core and phosphorus-rich rim. The core/rim boundary of the phosphorus zoning is discontinuous. The irregular shape of this core/rim boundary together with the pressure difference between the core and the rim inferred from the difference in aluminosilicate inclusions (kyanite in the core and sillimanite in the rim) suggests that this is the resorption/reprecipitation boundary. The difference between phosphorus concentrations in the garnet core and rim is accompanied by a change in phosphate inclusions in the garnet. Apatite and monazite are included in the phosphorus-poor garnet core, whereas monazite alone is included in the phosphorus-rich garnet rim. Utilizing the core/rim boundary as a contemporaneous surface when comparing different garnet grains, the timing of the discontinuous phosphorus-zoning formation (and thus, the garnet resorption) and change in the phosphate assemblage can be correlated to the pressure-temperature path of the garnet-sillimanite gneiss. The phosphorus-poor core of the garnet mainly formed during the prograde stage in the kyanite to sillimanite stability fields under which apatite probably buffered the phosphorus-content of garnet, and the phosphorus-rich garnet rim possibly crystallized from the melt at the retrograde stage near the vapor saturated solidus under which monazite alone (without apatite) probably did not buffer the phosphorus-content of garnet. Garnet resorption occurred during the decompression stage between these two garnet growth stages.