Petrology, geochemistry, Ar-Ar isotopes of an arc related calk-alkaline pluton from Mamb (Pan-African Yaounde group, Cameroon): A testimony to the subduction of a hot oceanic crust

In Central Africa, the Pan-African Yaounde group is made up of migmatites and gneisses exhumed and thrusted on micaschists. In a window in the micaschists, the Mamb metagabbro massif occurs as km-size and ca 20m-high hill crosscut by a 15m-thick dyke of meta-hornblendite. Minor facies are biotitites, garnetites, and granitic veins. Metagabbro comprises pargasite, biotite, labrador-andesine, pyrope- andradite- grossular-rich almandine, epidote, Fe-Ti oxide, quartz, scapolite, rutile, zircon, and late metamorphic muscovite, whereas meta-hornblendite is made up of abundant edenite-pargasite, and minor amounts of plagioclase, biotite and epidote. Textural features such as epidote needles in plagioclase and rutile exsolutions in amphibole suggests that metamorphism (P-T conditions: 610 degrees C and 9.1 Kb) post-dated magma emplacement and crystallisation. Based on 40Ar/39Ar thermochronology of amphibole and micas, the proceedings of metamorphic processes extended from 601 to 586 Ma. Chemically, metagabbros and meta-hornblendites display hybrid adakitic and calk-alkaline characteristics. Mantle-normalised element patterns of metagabbros are marked by negative Nb, Ta, Zr, Hf, Ti, Y and positive Sr and Pb anomalies parallel to metadiorites of Yaounde. Meta-hornblendite patterns showdepletion in Th, K, and Pb similarly to alkaline rocks but high values of Y/Nb (> 3) and Zr/Nb (11-52) ratios point to a subalkalic affinity. Characteristics of these metagabbros and meta-hornblendites point to heterogeneous origins by partial melting of a dehydrated subducted hot oceanic crust and the fusion of a metasomatised mantle wedge. Metadiorites from Yaounde are arc related rocks. Thus it appears that the island arc setting described at Boumnyebel extends eastwards up to Yaounde, and probably up to Lomie in the south-eastern Cameroon. The northwards extension of this setting is yet to be established taking into account the presence of Archaean-Paleoproterozoic formations north of the continental scale Sanaga shear zone. At the regional scale, the best model to explain the presence of adakites and classic arc magmas at Mamb, Bape, Bangangte and Fomopea is a south-north subduction of the Pan-African Yaounde hot oceanic crust under the Archaean Adamawa-Yade block. (C) 2021 Elsevier B.V. All rights reserved.

Automated summary

Geologists in Central Africa have conducted in-depth research on the rocks in the region, offering new insights into magma intrusion and metamorphic processes, revealing the formation mechanisms of these rocks.