Yaounde Group Research Articles

Relative active tectonic in the metamorphic rocks of the Yaounde group: insights from geomorphic indices and topographic analysis

Relative active tectonic in the metamorphic rocks of the Yaounde group: insights from geomorphic indices and topographic analysis

Geochemical characterization of the metasedimentary rocks of the Yaounde Group within the southernmost North Equatorial tectonic belt: insights into geodynamic evolution

Geochemical characterization of the metasedimentary rocks of the Yaounde Group within the southernmost North Equatorial tectonic belt: insights into geodynamic evolution

Petrography and geochemistry of weathered pyroxenite boulders from Mamb, Ngong and Nkolmbong (Pan-African Yaoundé group, Cameroon): tracking the Ni, Co, Cr and Cu mineralization processes during the alteration of ultramafic rocks

In this study, mineralogical and geochemical characteristics of weathered pyroxenites boulders of Mamb, Ngong and Nkolmbong areas (Pan-African Yaounde group) are determined in order to understand the mineralisation processes of Ni, Co, Cr and Cu during the early stage of weathering of ultramafic rocks. These pyroxenites are intruded in micaschist at Mamb and Ngong or in migmatitic gneiss at Nkolmbong. Fresh samples are made up of clinopyroxene, orthopyroxene, amphibole and accessory biotite, plagioclase, chlorite and sulphides. Their peculiar chemical features include high Ni (58–194 ppm), Co (46–137 ppm), Cr (875–3017 ppm), Zn (37–185 ppm) and Cu (27–159 ppm) contents. REE contents vary in a large range (ΣREE 59–478 ppm) with LREE-enrichment (LREE/HREE 3–6) and negative Ce and Eu anomalies. The thin (≤ 10 cm) weathered rims comprise alternating black streaks and red beds made up of kaolinite, gibbsite, goethite, hematite, halloysite and maghemite. The alteration intensity is marked by a decrease of the rock density and an increase of its porosity and Chemical Index of Alteration (CIA) values. The supergene alteration of fresh pyroxenites boulders yields Ni-, Co-, Cu-poor, but Cr-slightly enriched weathered rims. Conversely, the lateritization of pyroxenite boulders having previously undergone a hydrothermal alteration into chlorite-rich rocks leads to the formation of clay-, Ni-, Co-, and Cu-rich cortices. These cortices are peeled off during weathering and got mixed with the soils formed under and around the boulders. So, the current weathered rims are probably related to the recent climatic variations and the soils are the host of potential concentration in Ni, Co, Cu and Cr.

Geochemical, mineralogical and sedimentological analyses of reworked sediments (new) in the syn- to post-rift Middle Cretaceous-Quaternary detrital deposits from western Atlantic margin of Cameroon: evidence from sedimentation-erosion alternation in the context of passive margin evolution

Reworked detrital sediments were discovered in the syn-rift to post-rift Cretaceous to Quaternary (Pleistocene) detrital deposits of the Douala coastal sub-basin in the eastern margin of the South Atlantic Ocean at West Cameroon. This contribution presents the geochemical, mineralogical, and sedimentological data to constrain the origin of these reworked sediments. The investigated samples are characterized by the high values of Plagioclase Index of Alteration (PIA, 97.83–99.90), Chemical Index of Alteration (CIA, 71.9–99.08), and Chemical Index of Weathering (CIW, 98.67–99.90). Data gleaned from these indices suggest that the reworked sediments and their potential source rocks experienced severe weathering. The A-CN-K plot along with the mineralogical indicates a probably post-depositional K-enrichment. The high light rare earth elements (LREE)/heavy rare earth elements (HREE) ratios (15.82–79.13) suggest that the reworked were mainly derived from felsic igneous rocks. This interpretation is further confirmed by the Zr versus TiO2, TiO2/Al2O3, Th/Co versus La/Sc, and La/Th versus Th/Yb plots, which indicate felsic igneous and silicic source rocks. Meanwhile, the high variability of LREE/HREE ratios (15.82–79.13) and the positive Eu anomalies (1.10–1.32) of studied samples could indicate a little contribution of mafic source rocks. Analysis of the distribution of the heavy minerals (e.g. tourmaline, aluminum silicate, and rutile) revealed that these felsic and silicic source rocks are mainly granite and gneiss, probably belonging to the Nyong Group and the Western part of the Neoproterozoic Yaounde Group. The morphoscopic analyses of the host rocks (very angular to angular shapes) have revealed that the reworked sediments have proximal to subproximal onshore origins, probably from former sedimentary deposits, as is indicated by variation of their color. The tectonic discriminant diagram indicates the Oceanic Island Arc and Active Continental Margin which are inconsistent with the geological history of the Douala coastal sub-basin and could be explained by the recycling effect experienced by studied sediments which have been deposited in the coastal plain to the fluvial environment in the Low System Track context.

Petrology, geochemistry, Ar[sbnd]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 Yaoundé 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 20 m-high hill crosscut by a 15 m-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, FeTi 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 °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 Yaoundé. Meta-hornblendite patterns show depletion 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 Yaoundé are arc related rocks. Thus it appears that the island arc setting described at Boumnyebel extends eastwards up to Yaoundé, and probably up to Lomié 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, Bangangté and Fomopea is a south-north subduction of the Pan-African Yaoundé hot oceanic crust under the Archaean Adamawa-Yade block.

Geophysical Data Processing for the Delineation of Tectonic Lineaments in South Cameroon

The processing of aeromagnetic and gravity data of the Northern part of Congo Craton (South Cameroon region), between latitudes 2°30’-3°30’ N and longitudes 12°-13° E, permitted the determination of the structural features ccurring within the Precambrian basement (Ntem Complex) southwards and the Pan-African belt (Yaounde Group) northwards. The maxima of the Horizontal Gradient within the study area, were obtained using the Blakely and Simpson method (1986). Those maxima were used to trace the magnetic lineaments of the study area. Furthermore, the Total Horizontal derivative of the Tilt derivative applied on the residual grid of Bouguer anomaly guaranteed the enhancement of linear structures which were automatically extracted using the CET Grid Analysis algorithm. The superimposition of both magnetic and gravity lineaments allowed us to display the structural framework of the area, whose major trending directions are E-W, ENE-WSW, and NE-SW. These major lineament directions are likely to be linked to one or more than a single tectonic event such as the ENE-WSW/NE-SW trends, considered as the subduction direction of the Congo craton beneath the Pan-African belt. These trends may be linked to the Eburnean orogeny and are also said to be connected to the Central African Shear Zone (CASZ). The geophysical lineaments identified in the study are defined as potential targets along which mineralization may have been formed, considering the economic potential of the area.

U/Pb LA-SF-ICP-MS dating of detrital zircons in Cretaceous-Tertiary kaolins in the Douala Sub-Basin, Cameroon: A Neoproterozoic provenance

Detrital zircon geochronology is a reliable provenance tool used to trace known zircon age populations from their metamorphic or igneous source to their present location in sedimentary basins. This paper presents U/Pb LA-SF-ICP-MS data of detrital zircons in Cretaceous-Tertiary kaolins in the Douala Sub-Basin in order to determine their provenance. The minimum ages of parent rocks which kaolinised were determined using U-Pb LA-SFICP-MS dating of zircons in the kaolin deposits. Four main zircon populations were identified from radiogenic dating: the 1st between 550 and 650 Ma, the 2nd between 950 and 1050 Ma, the 3rd around 1600 Ma and the 4th between 2800 and 3200 Ma. These four zircon populations belong to the Proterozoic (Neo-, Meso- and Paleoproterozoic) and the Archean. The minimum ages of parent rocks which contained the primary minerals that were kaolinised, reflected by the youngest weighted averages of zircon populations varied between 588 ± 2 Ma and 612 ± 2 Ma, all belonging to the Ediacaran Period (Neoproterozoic), respectively. Ages of zircons in Cretaceous-Tertiary kaolins suggested that the zircons formed during two main tectonic events: the Eburnean orogeny, during which older zircons crystallised and the Pan-African orogeny, during which younger zircons crystallised. The main identified sources of these zircons are the Archean Ntem Complex, the Paleoproterozoic Nyong Group and the Neoproterozoic Yaounde Group.

New igneous zircon Pb/Pb and metamorphic Rb/Sr ages in the Yaounde Group (Cameroon, Central Africa): implications for the Central African fold belt evolution close to the Congo Craton

Three meta-igneous bodies from the Yaounde Group have been analyzed for their petrography, geochemistry, and 207Pb/206Pb zircon ages. According to their geochemical patterns, they represent meta-diorites. The meta-plutonites yielded identical zircon ages with a mean of 624 ± 2 Ma interpreted as their intrusion age. This age is in agreement with previously published zircon ages of meta-diorites from the Yaounde Group. The meta-diorites derived mainly from crustal rocks with minor contribution from mantle material. The 87Rb/86Sr isochron ages of one meta-diorite sample and three meta-sedimentary host rocks are significantly younger than the obtained intrusion age. Therefore, they are not related to igneous processes. 87Rb/86Sr isochron ages differ from sample to sample (599 ± 3, 572 ± 4, 554 ± 5, 540 ± 5 Ma) yielding the oldest Neoproterozoic age (~600 Ma) for a paragneiss sample at a more northern location. The youngest Rb/Sr isochron age (~540 Ma) was obtained for a mica schist sample at a more southern location closer to the border of the Congo Craton. The 87Rb/86Sr whole rock-biotite ages are interpreted as cooling ages related to transpressional processes during exhumation. Therefore, several discrete metamorphic events related to the exhumation of the Yaounde Group were dated. It could be shown by Rb/Sr dating for the first time that these late tectonic processes occurred earlier at more distant northern locations of the Yaounde Group and lasted at least until early Cambrian (~540 Ma) more closely to the border of the Congo Craton.

Pan-African metamorphic evolution in the southern Yaounde Group (Oubanguide Complex, Cameroon) as revealed by EMP-monazite dating and thermobarometry of garnet metapelites

Garnet-bearing micaschists and paragneisses of the Yaounde Group in the Pan-African Central African Orogenic Belt in Cameroon underwent a polyphase structural evolution with the deformation stages D 1–D 2, D 3 and D 4. The garnet-bearing assemblages crystallized in course of the deformation stage D 1–D 2 which led to the formation of the regional main foliation S 2. In X Ca– X Mg coordinates one can distinguish several zonation trends in the garnet porphyroblasts. Zonation trends with increasing X Mg and variably decreasing X Ca signalize a garnet growth during prograde metamorphism. Intermineral microstructures provided criteria for local equilibria and a structurally controlled application of geothermobarometers based on cation exchange and net transfer reactions. The syndeformational P– T path sections calculated from cores and rims of garnets in individual samples partly overlap and align along clockwise P– T trends. The P– T evolution started at ∼450 °C/7 kbar, passed high-pressure conditions at 11–12 kbar at variable temperatures (600–700 °C) and involved a marked decompression toward 6–7 kbar at high temperatures (700–750 °C). Th–U–Pb dating of metamorphic monazite by electron microprobe (EMP-CHIME method) in eight samples revealed a single period of crystallization between 613 ± 33 Ma and 586 ± 15 Ma. The EMP-monazite age populations between 613 ± 33 Ma enclosed in garnet and 605 ± 12 Ma in the matrix apparently bracket the high temperature–intermediate pressure stage at the end of the prograde P– T path. The younger monazites crystallized still at amphibolite-facies conditions during subsequent retrogression. The Pan-African overall clockwise P– T evolution in the Yaounde Group with its syndeformational high pressure stages and marked pressure variations is typical of the parts of orogens which underwent contractional crustal thickening by stacking of nappe units during continental collision and/or during subduction-related accretionary processes.

Pan-African tectonic evolution in central and southern Cameroon: transpression and transtension during sinistral shear movements

Kinematic analysis of the central Cameroon shear zone (CCSZ) and its Sanaga fault relay, indicate early sinistral shear movement (phase D 2) that was later followed by a dextral shear movement (phase D 3) during the Pan-African orogeny. The correlation of tectonic events among the CCSZs, thrusting of the Yaounde Group and the deformation in the Lom Group indicate a diachronous deposition history of these groups, where the Yaounde Group is pre-kinematic while the sedimentary and magmatic rocks of the Lom basin are syn-kinematic. Sinistral shear movements along the CCSZ and Sanaga faults are correlated with metamorphism and thrusting of the Yaounde granulites onto the Congo craton, on one hand, and to the opening of the Lom pull-apart basin, oblique to the shear zone, on the other. Kinematic interactions between shear and thrust movements characterize transpression, whereas interactions between shear and oblique normal fault movements characterize transtension. Resulting kinematic indicators show that the Lom basin represents a sinistral transtensional relay of the Sanaga fault. Greenschist-facies metamorphism in the Lom Group rocks dominantly affected by a monophase tectonic evolution were achieved during the late dextral shear movements along the Sanaga fault.

The ACP/EEC Conciliation and Arbitration Rules

in March 1990, the Council of Ministers of the African, Caribbean and Pacific Group of States and the European Economic Community (ACP/ EEC) approved a set of rules for the settlement of disputes arising out of contracts awarded under the European Development Fund (EDF) by conciliation or arbitration. Thus, a new set of conciliation and arbitration rules was added to those already in existence. Why was there a need to do so? And what new arrangements did it introduce which were not available to parties in dispute under the existing rules? The ACP/EEC relationship is based on a series of Conventions between the EEC and the ACP Group of States since 1975. This relationship itself developed from the earlier relationship between the francophone African countries, formerly colonies of France, and the EEC under the Yaounde Convention, so called because it was concluded in Yaounde in Cameroon in 1969. The United Kingdom was then not a member of the Community, and none of its former colonies with which it still continued to have strong aid and trade ties was involved. After the United Kingdom became a member in 1974, the Yaounde Group of States was enlarged to include the anglophone countries in Africa, the Caribbean and the Pacific, which had formerly been British colonies. A new Convention with broader application was entered into. It was done in Lome in the Republic of Togo, and became known as the ‘Lome Convention’. The 4th Convention in the series, all of which have been signed in Lome, was signed by the State parties involved in December 1989. Each of the previous Conventions had run for a period of five years. The basic provisions of Lome IV on the other hand will continue for ten years, although various matters, for example, the renewal and size …