Neoproterozoic metamorphic events in the kekem area (central domain of the Cameroon north equatorial fold belt): P-T data

Abstract

The Kekem area (southwestern part of the central domain of the Cameroon North Equatorial Fold Belt) is composed of high-grade migmatitic gneisses in which two lithological units are distinguished: (i) a metasedimentary unit (garnet- sillimanite-biotite-gneisses and garnet-biotite-gneisses) interpreted as a continental series; and (ii) meta-igneous rocks comprising mafic pyroxene gneisses, amphibolites, and orthogneisses. These units recrystallised under HT-MP conditions (T=700-800‹C, P . 0.5-0.8GPa) and were deformed in relation to a major tangential tectonic event with the NNE-SSW kinematic direction. The lithological association and its tectono-metamorphic evolution show striking similarities with the Banyo and Maham III gneisses, suggesting that the extensional depositional environment envisaged for this formation can be extended farther west. P-T calculations in this contribution provide new data on the Pan-African structural and metamorphic evolution of the metapelites and metabasites in the basement of the Kekem area. The results show two distinct events: (1) crystallization during a Pan-African high temperature metamorphic event and, (2) subsequent deformation and high temperature mylonitization. The data imply a high-temperature amphibolite-facies metamorphism along a clockwise P-T path. The recorded P-T involves a marked variation in pressure, which is typical of collisional crustal thickening. The contrasted metamorphic evolution between areas located to the south of the Central Cameroon Shear Zone
(CCSZ; high pressure: Yaounde, Ntui-Betamba), and those located to the north (low pressure: Banyo, Tibati), along with widespread remains of Paleoproterozoic crust, suggest important crustal thickening during Pan-African tangential tectonics in southern Cameroon. As a consequence, the CCSZ is not simply a late Pan-African transpressive shear zone but appears to have been formerly a major intracontinental thrust zone.

Key words: HT metamorphic event / Kekem/ Cameroon / Collisional crustal thickening / Neoproterozoic /
North Equatorial Fold Belt.

La region de Kekem est formee de gneiss anatectiques de haut-degre dans lesquels deux ensembles sont reconnus: (i) un ensemble metasedimentaire (gneiss a grenat, sillimanite et biotite, et gneiss a grenat et biotite) considere comme depose a lLinterieur dLun continent, (ii) des roches meta-ignees comprenant des gneiss a grenat et pyroxene,
des amphibolites de composition mafique, et des orthogneiss. Ces deux ensembles ont recristallise (T=700-800‹C, P. 0,5-0,8GPa) et ont ete deformes en relation avec un episode majeur de tectonique tangentielle de direction cinematique NNE-SSW. Cette association lithologique et son evolution tectono-metamorphique montrent une similitude remarquable avec les formations panafricaines de Banyo et Maham III du domaine central de la chaine, suggerant que le contexte extensif de depot envisage pour ces dernieres peut etre etendu plus a lLouest. Les donnees nouvelles sur lLevolution tectono-metamorphique des metasediments et des metabasites panafricains du socle de la region de Kekem mettent en evidence un metamorphisme panafricain de haute temperature suivi dLune mylonitisation a haute temperature dans le facies amphibolite de haut degre selon un chemin P-T horaire. Les conditions P-T enregistrees se caracterisent par une variation de pression typique dLun epaississement crustal. LLevolution metamorphique panafricaine contrastee entre le domaine au sud du cisaillement Centre Camerounais

(CCC; haute pression: Yaounde, Ntui-Betamba) et le domaine nord-CCC (basse a moyenne pression: Banyo, Tibati) ainsi que la presence ubiquiste de reliques dLune croute ancienne paleoproterozoique, suggerent un important epaississement crustal, lors dLun episode de tectonique tangentielle. Le CCC nLest donc pas un simple accident panafricain transpressif, mais apparait bien avoir ete prealablement un  chevauchement intracontinental majeur.

Mots-cles : Metamorphisme de Haut degre/Kekem/ Cameroun/ Epaississement crustal/ Neoproterozoique /
Chaine Nord Equatorial