LOW-P-HIGH-T METAMORPHISM OF THE AGGENEYS TERRANE, NAMAQUA METAMORPHIC COMPLEX, SOUTH AFRICA.

The Aggeneys terrane is a central part of the western Namaqua Metamorphic Complex (NMC) that is most renowned for hosting world-class massive sulphide mineralisation. Peak metamorphism reached upper amphibolite facies conditions, in contrast to the granulite facies conditions experienced by many of the surrounding terranes in the western NMC. Pseudosection modelling of metapelitic and metamafic lithologies constrains peak P-T conditions at 650 ± 20°C and 5 ± 1 kbar, corresponding to an apparent geotherm of 130°C/kbar. Prograde metamorphism involved heating and concomitant burial from conditions of andalusite stability, and initial retrograde metamorphism appears to have involved cooling, rather than decompression, resulting in an anticlockwise path. Whereas the absolute P-T conditions for the Aggeneys terrane are lower than that of the granulite terranes, metamorphism of the entire area occurred under similar, highly elevated apparent thermal gradients and was dominated by heating and cooling along similar P-T paths. However, peak metamorphism in the Aggeneys terrane is inferred to have occurred during the c. 1.17 to 1.13 Ga O'okiepian Orogeny whereas in other terranes it occurred during the younger, c. 1.07 to 1.03 Ga Namaquan Orogeny. The style of metamorphism experienced by the western NMC requires higher than average lithospheric heat flow generally attributed to mafic underplating, or elevated crustal heat production that can be caused by the slow burial of highly radiogenic crust. Both processes occur over protracted periods of time and affect large areas of the crust, such that the apparent diachronicity of peak metamorphism in the NMC, and the mechanisms by which it might have occurred require further investigation. [ABSTRACT FROM AUTHOR]