Orthopyroxenes From Granulite Rocks of the Wilson Terrane (victoria Land, Antarctica) - Crystal-chemistry and Cooling History

The compositions of 17 orthopyroxenes from granulites facies rocks of the Wilson Terrane (Ross Orogen), determined by microprobe analysis, are consistent with a metamorphic origin. Three groups may be distinguished : a) orthopyroxenes with high Al2O3 content (2.85-5.33 weight%) and X(Mg) = 0.46-0.60, from garnet-cordierite granulites ; b) orthopyroxenes with low Al2O3 (less than 0.9 wt%) and X(Mg) = 0.49-0.62, from enderbites ; c) orthopyroxenes with very low Al2O3 (less than 0.4 wt%) and X(Mg) = 0.70, from two-pyroxene mafic granulites. These differences do not reflect different metamorphic conditions, but are due to local mineral assemblages. Zoning occurs in few orthopyroxenes from garnet-cordierite and mafic granulites, in both cases due to a retro-grade evolution. The crystal chemistry and site populations of two Al-rich and one Al-poor specimen were determined by X-ray single crystal diffraction. The most relevant structural differences are the decrease in M1 and M2 polyhedral dimensions, and an increase in TB, both induced by Al. The high degree of Fe2+-Mg order (k(D) between 0.017 and 0.023) is coherent with slow cooling. Closure temperatures of the ordering process are between 214-degrees and 243-degrees-C, with cooling rates in the range expected for a regional metamorphic terrane.