New isotopic constraints on age and origin of Mesoarchean charnockite, trondhjemite and amphibolite in the Ntem Complex of NW Congo Craton, southern Cameroon

Abstract

Integrated in-situ analyses of zircon U–Pb ages and Hf–O isotopes are reported for charnockite, trondhjemite and associated amphibolite in the Archean Ntem Complex of the NW Congo Craton, southern Cameroon. The charnockite crystallized at ∼2.92Ga, coincident with the timing of extensive occurrence of high-temperature magmatism possibly related to mantle plume activity worldwide. They are characterized by zircon δ18O values of ∼5.9‰ and ɛHf(t) values of ∼0.0, with Hf model ages of ∼3.5–3.3Ga. On the basis of the new isotopic and literature data we suggest that the charnockites were generated by partial melting of a Paleoarchean mafic crustal source possibly triggered by 2.92Ga mantle plume activity. The trondhjemite and associated amphibolite protolith crystallized synchronously at ∼2.87–2.86Ga. The trondhjemite exhibits zircon δ18O values of ∼6.0‰ and negative ɛHf(t) values of ca. −3.7, with Hf model ages of ∼3.8–3.5Ga, whereas the amphibolite has positive zircon ɛHf(t) values of ∼2.3 and δ18O values of ∼4.6‰, lower than that (∼5.3‰) of zircons from juvenile mantle-derived magmas. The trondhjemite was most likely generated by high-pressure partial melting of an Eoarchean crustal protolith possibly triggered by the synchronous mafic magmatism. An ancient ∼3.8Ga-old proto-crust may have existed in the NW Congo Craton.