Original paper

Electron microprobe dating and thermobarometry of Neoproterozoic metamorphic events in the Kekem area, Central African Fold Belt of Cameroon

Tchato, Depesquidoux; Tchaptchet Schulz, Bernhard; Nzenti, Jean-Paul

Abstract

Th-U-Pb dating of metamorphic monazite by electron microprobe (EMP) and P-T path reconstruction from sillimanite-bearing garnet-biotite gneisses provide new insight into the Pan-African structural and metamorphic evolution in the Kekem area in the Central African Fold Belt of Cameroon. Two events can be distinguished: (1) crystallization of garnet-bearing assemblages and monazite during a Pan-African high temperature metamorphism and (2) subsequent deformation with pervasive mylonitisation. Geothermobarometry based on cation-exchange and net transfer reactions in garnet-bearing assemblages in low-strain domains (Ca- and Mg-rich garnet core compositions and enclosed plagioclase and biotite) yielded 720 °C/12 kbars. The Mg-rich and Ca-poor garnet rims crystallized at 750 °C/7 kbars. The mylonitic foliation was formed during subsequent cooling and decompression. Two groups of monazite ages have been recognized; an older group of monazite grains is preserved in low-strain domains and defines ages from 580 to 570 Ma. A slightly younger group of grains situated in high-strain mylonitic domains displays ages between 564 and 552 Ma. The data imply Pan-African high-temperature amphibolite-facies metamorphism along a clockwise P-T path. The recorded P-T evolution involves a marked variation in pressure, which is typical of collisional crustal thickening.