Two‐stage decompression in garnet‐bearing mafic granulites from Sostrene Island, Prydz Bay, East Antarctica

Mafic garnet‐bearing granulites from Sostrene Island, 150 km southwest of Davis Station on the coast of Prydz Bay, East Antarctica, exhibit two‐stage symplectic coronas on garnet, formed after peak metamorphic conditions (M1). An outer corona of Opx (Mg 66 ) + Pl (An 94–97 ) + minor Hbl mantles a fi...

Full description

Bibliographic Details
Published in:Journal of Metamorphic Geology
Main Authors: THOST, D. E., HENSEN, B. J., MOTOYOSHI, Y.
Format: Article in Journal/Newspaper
Language:English
Published: Wiley 1991
Subjects:
Online Access:http://dx.doi.org/10.1111/j.1525-1314.1991.tb00520.x
https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1111%2Fj.1525-1314.1991.tb00520.x
https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1525-1314.1991.tb00520.x
Description
Summary:Mafic garnet‐bearing granulites from Sostrene Island, 150 km southwest of Davis Station on the coast of Prydz Bay, East Antarctica, exhibit two‐stage symplectic coronas on garnet, formed after peak metamorphic conditions (M1). An outer corona of Opx (Mg 66 ) + Pl (An 94–97 ) + minor Hbl mantles a finer‐grained inner corona of Opx (Mg 67 ) + Pl (An 95–96 ) + Spl (Mg 36 ). Both symplectites contain minor ilmenite–magnetite intergrowths. The finer‐grained symplectite also occurs along a fracture cleavage in the garnet. The outer corona originated during a second metamorphic event (M2) via the reaction Grt + Cpx (Hbl) + SiO 2 = Opx + Pl (1), whereas the inner corona formed later in response to decompression and minor deformation, resulting in the fracture cleavage in the garnet, according to the reaction Grt = Opx + Pl + Spl (2). The grossular content of the garent ( X Grs = 0.168) is almost exactly that which is required for the stoichiometric breakdown by reaction (2) (calculated X Grs = 0.167). The mafic rocks are silica undersaturated, and the SiO 2 for reaction (1) was most probably derived externally from the surrounding felsic gneisses. Preferred P–T estimates for M1 based on garnet core (Prp 40 Alm 42 Grs 17 Sps 1 )–matrix Opx–Cpx–Hbl pairs are c. 10 kbar at 980°C. The fine‐grained symplectite formed post‐peak M2 at c. 7 kbar and 850°C. The enclosing felsic gneisses yield pressure estimates of between 5 and 7 kbar, which compare with conditions of c. 6 kbar and 775°C in the nearby Bolingen Islands. These lower P–T estimates are considered to be representative of the widespread 1100‐Ma metamorphic event recognized in outcrops along the Prydz Bay coast. The high‐ P , high‐ T estimates derived from the garnet relics provide evidence for an earlier, possibly Archaean, high‐grade metamorphic event.