Reaction textures in calc‐silicate granulites from the Bolingen Islands, Prydz Bay, East Antarctica: implications for the retrograde P–T path

• Published in: Journal of Metamorphic Geology
• Main Authors: MOTOYOSHI, Y., THOST, D. E., HENSEN, B. J.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 1991
• Subjects: Geochemistry and Petrology; Geology; East Antarctica; Prydz Bay; Bolingen Islands; Antarc*; Antarctica
• Online Access: http://dx.doi.org/10.1111/j.1525-1314.1991.tb00524.x; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1111%2Fj.1525-1314.1991.tb00524.x; https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1525-1314.1991.tb00524.x

Calc‐silicate granulites from the Bolingen Islands, Prydz Bay, East Antarctica, exhibit a sequence of reaction textures that have been used to elucidate their retrograde P–T path. The highest temperature recorded in the calc‐silicates is represented by the wollastonite‐ and scapolite‐bearing assemblages which yield at least 760°C at 6 kbar based on experimental results. The calc‐silicates have partially re‐equilibrated at lower temperatures (down to 450°C) as evidenced by the successive reactions: (1) wollastonite + scapolite + calcite = garnet + CO 2 , (2) wollastonite + CO 2 = calcite + quartz, (3) wollastonite + plagioclase = garnet + quartz, (4) scapolite = plagioclase + calcite + quartz, (5) garnet + CO 2 + H 2 O = epidote + calcite + quartz, and (6) clinopyroxene + CO 2 + H 2 O = tremolite + calcite + quartz. The reaction sequence observed indicates that a CO 2 was relatively low in the wollastonite‐bearing rocks during peak metamorphic conditions, and may have been further lowered by local infiltration of H 2 O from the surrounding migmatitic gneisses on cooling. Fluid activities in the Bolingen calc‐silicates were probably locally variable during the granulite facies metamorphism, and large‐scale CO 2 advection did not occur. A retrograde P–T path, from the sillimanite stability field ( c. 760°C at 6 kbar) into the andalusite stability field ( c. 450°C at <3 kbar), is suggested by the occurrence of secondary andalusite in an adjacent cordierite–sillimanite gneiss in which sillimanite occurs as inclusions in cordierite.