Sapphirine + quartz association from Archean rocks in Enderby Land, Antarctica

TLDR: In the Tula Mountains and Amundsen Bay areas of Antarctica, 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 as discussed by the authors.

Abstract: Quartzites and granulites containing the sapphirine-quartz association are found in the pyroxene-granulite-facies Archean Napier complex in the Tula Mountains and Amundsen Bay areas of Enderby Land, Antarctica (50'30' to 53'E; 67\"5). Quartzites from the Tula Mountains consist of SiOr, AlrOr, 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 TMgO ' 9AlrO3 . 3SiO, composition and the proportion of the iron end member ranges from 6.8 to 23.5 mole Vo. Fe3+ contents do not exceed lSVo 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'a30'C and 7+l kbar. Colorless sapphirine (2.40wtVo FeO) with overgrowths of sillimanite is associated with orthopyroxene, q\\aftt, biotite, and plagioclase in a magnesian quartzite (MgO : 4.24 wt%o; Fe2O, : O.5l wtVo) at Reference Peak, an exposuie 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 90ooc.