Quartzites and granulites containing the sapphirine–quartz association are found in the py-roxene–granulite-facies Archean Napier complex in the Tula Mountains and Amundsen Bay areas of Enderby Land, Antarctica (50°30′ to 53°E; 67°S). Quartzites from the Tula Mountains consist of SiO2, Al2O3, MgO, and FeO and contain virtually no alkalis or CaO. At the peak of metamorphism, the mineral assemblage in these quartzites is interpreted to be quartz–sapphirine–orthopyroxene–rutile±plagioclase. Cordierite is present, mostly as overgrowths on sapphirine, and appears to have formed from the reaction between sapphirine and quartz after the peak of metamorphism. Sapphirines are somewhat more siliceous than the 7MgO · 9Al2O3 · 3SiO2 composition and the proportion of the iron end member ranges from 6.8 to 23.5 mole %. Fe3+ contents do not exceed 18% of the total Fe in sapphirine. The proportion of Al to total cations in orthopyroxene ranges from 0.072 to 0.117, increases linearly with FeSiO3 content, and is extrapolated to be 0.048 in a hypothetical iron-free orthopyroxene. On the basis of the compositions of coexisting pyroxenes, the Al content of hypothetical iron-free orthopyroxene associated with sapphirine, and the compositions of coexisting plagioclase and garnet, metamorphic temperatures and pressures are estimated to be 900°±30°C and 7±1 kbar. Colorless sapphirine (2.40 wt% FeO) with overgrowths of sillimanite is associated with orthopyroxene, quartz, biotite, and plagioclase in a magnesian quartzite (MgO = 4.24 wt%; Fe2O3 = 0.51 wt%) at Reference Peak, an exposure south of the Tula Mountains–Amundsen Bay areas. This association may represent the univariant reaction sapphirine + quartz = sillimanite + orthopyroxene in the iron-free system and may have crystallized at lithostatic pressures greater than 7 kbar and temperatures near 900°C.

This content is PDF only. Please click on the PDF icon to access.

First Page Preview

First page PDF preview
You do not have access to this content, please speak to your institutional administrator if you feel you should have access.