Reactions destroying detrital monazite in greenschist-facies sandstones from the Witwatersrand basin, South Africa

Abstract

Detrital monazite in sandstone and conglomerate from the Witwatersrand Supergroup, South Africa, displays reaction textures indicating that monazite has undergone replacement after sediment deposition, and most probably during regional greenschist-facies metamorphism. In sandstones with moderate- to high-Ca bulk compositions, detrital allanite is preserved in heavy mineral bands, whereas co-existing detrital monazite is replaced by secondary allanite, apatite and ThSiO4. In sandstones with low-Ca bulk compositions, detrital monazite is replaced by florencite, apatite and ThSiO4 or pseudomorphed by metamorphic monazite, apatite and Th-silicate. The monazite alteration phases are commonly intergrown with metamorphic quartz, chlorite and sericite. Large (~0.5 mm) grains of metamorphic monazite are present in the same samples as detrital monazite pseudomorphs. This suggests that REE are mobile on a scale of millimetres to centimetres in these rocks, and that detrital grains are a likely source of components for metamorphic monazite. In other low-Ca sandstones, monazite forms composite grains that comprise a core surrounded by a pore-filling, inclusion-rich rim. The monazite cores have pronounced negative Eu anomalies and higher concentrations of U, Th, Ca and Y than the rims. Previous SHRIMP U-Pb dating of the cores gives dates of ~3.0 Ga and ~2.85 Ga, indicating a detrital origin. The monazite rims lack Eu anomalies and generally have lower Th contents than the cores.They result from dissolution and reprecipitation of monazite, and the ages of the rims suggest that replacement occurred at 2.045 Ga in the West Rand goldfield and at 2.12 Ga in the Welkom goldfield. Our results indicate that detrital igneous monazite is unstable under mid-greenschist facies conditions (~350 C) and may be replaced by low-Th metamorphic monazite, allanite, apatite, florencite and ThSiO4, as well as matrix minerals such as quartz, chlorite and sericite.