Conditions of metamorphism and fluid characteristics of the high-pressure complex of the shackleton range, antarctica

The Early Paleozoic metamorphic complex of the Shackleton mobile belt followed a clockwise P-T loop from the staurolite-chlorite-muscovite subfacies of the epidote amphibolite facies (T = 490–540°, P = 4.5–5.0 kbar) to a high-pressure garnet-biotiteorthoclase subfacies of the almandine amphibolite facies T = 700–740°, P = 8.0–11.0 kbar) on the prograde branch, and then a sharp drop in pressure in the retrograde branch stage under conditions close to isothermal (T > 650°). P = 6.8 kbar at T = 750° is the maximum pressure obtained in fluid-inclusion thermobarometry. Cryometric measurements of the CH4/CO.2 ratios in fluid inclusions occupy a region within the log fo2 field where a carbon dioxide-methane fluid existed, which is consistent with the calculations of log fo2 based on mass-spectrometric and gaschromatographic analyses.