Orthopyroxenes from granulite rocks of the Wilson Terrane (Victoria Land, Antarctica): crystal chemistry and cooling history

Abstract

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 Al 2 O 3 content (2.85-5.33 weight%) and X Mg =0.46-0.60, from garnetcordierite granulites; b) orthopyroxenes with low Al 2 O 3 (less than 0.9 wt%) and X Mg =0.49-0.62, from enderbites; c) orthopyroxenes with very low Al 2 O 3 (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 doe to a retrograde 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 Fe 2+ -Mg order (k D between 0.017 and 0.023) is coherent with slow cooling. Closure temperatures of the ordering process are between 214 ° and 243 ° C, with cooling rates in the range expected for a regional metamorphic terrane