Regional UHT metamorphism with widespread, primary CO2-rich cordierite in the Bakhuis Granulite Belt, Surinam: A feldspar thermometry study

Abstract

A metapelite area in the Paleoproterozoic Bakhuis Granulite Belt (BGB) in western Surinam shows mineral assemblages diagnostic of ultrahigh-temperature metamorphism (UHTM): high-Al orthopyroxene + sillimanite + quartz and sapphirine + quartz. Metapelites elsewhere in the belt contain cordierite + sillimanite. Locally associated high-Al orthopyroxene might indicate that the cordierite + sillimanite assemblage was formed by UHTM. The Mg-rich cordierite has an elevated birefringence, up to blue second order, which is evidence for a considerable amount of CO2 in its lattice. Given the absence of a reliable model for CO2-bearing cordierite, phase equilibrium modelling cannot be applied to determine the metamorphic conditions of these rocks. Instead, ternary feldspar thermometry, using the Fuhrmann and Lindsley model, was applied to mesoperthite and antiperthite in 25 samples of mainly metapelite throughout the BGB. The temperatures determined range between 900 and 1050 °C. The temperatures are minimum estimates for the metamorphic temperature because the feldspar crystallized upon cooling from melts generated during peak metamorphism. These results imply that most of the BGB witnessed UHT metamorphism and that the widespread assemblage cordierite + sillimanite ± high-Al orthopyroxene indeed formed under UHTM conditions. The widespread formation of primary cordierite is unusual for UHTM terranes and is probably the result of its considerable CO2 content.