Zircon Pb-Pb and U-Pb systematics of TTG rocks in the Congo Craton: Constraints on crust formation, magmatism, and Pan-African lead loss


Authors: Shang CK, Siebel W, Satir M, Chen F, Mvondo JO

Published in: Bulletin of Geosciences, volume 79, issue 4; pages: 205 - 219; Received 5 February 2004; Accepted in revised form 30 July 2004;

Keywords: Ntem complex, Congo craton, TTG suite, Pb-Pb evaporation, U-Pb zircon dating, Pan-African, Archaean,

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Tonalite-Trondhjemite-Granodiorite (TTG) intrusions in the Sangmelima region of the Ntem complex represent a major Archean accretion event in the Congo craton. These intrusions are characterized by calc-alkaline rocks of a charnockitic suite, a tonalitic suite, and granodiorites. Zircons from these rock types have high Th/U ratios. Their internal structure is characterized by oscillatory zoning of magmatic origin, with recrystallization overprints and relict cores. Zircon 207Pb/206Pb evaporation data are interpreted to indicate crystallization ages, inheritance, and later disturbance. 207Pb/206Pb zircon data point to a Mesoarchaean emplacement between 2800 and 2900 Ma, with the intrusion of rocks of the charnockitic suite being followed by granodiorites and the tonalitic suite. While older age data (> 2900 Ma) suggest the existence of an older crust prior to TTG emplacement, younger data (< 2800 Ma) are strong indications for Pb loss due to post-emplacement reworking of the TTG suite. However, high proportions of a common Pb component suggest Pb gain. The Pb-Pb systematics of zircon therefore suggests a complex post-magmatic history for the Sangmelima TTG. Conventional U-Pb zircon dating confirms this observation, suggesting that the Pan-African tectonothermal event was the principal cause of the disturbance of zircon Pb-Pb systematics in the Sangmelima region.