The opening ceremony was held at “Alliance Franco-camerounaise” of Garoua, Cameroon and the opening address was given by the Governor of the Northern representing the Minister of Scientific and Technical Research (Cameroon).

After the opening ceremony, the rest of the day (January 5) was devoted to paper presentations on the geology of central Africa and the neighbouring regions. The session was chaired by Dr J.V. Hell Director of IRGM. Most of the papers presented were related to preliminary results of the research of various teams during the first year of the project. The 17 abstracts accepted for oral presentation were grouped in two sessions, the general theme being “the improvement of the geochronological database of central Africa”.

The business meeting that was planned at the end of the oral presentations was finally postponed, as some participants from abroad arrived only on the evening of January 5. Finally, the business meeting held during the field trip on 8 and 9 January 2004.The following points were agreed:

-         The web site of the project (www.igcp470.org) will be operational by the end of January 2004.

-         Dr Jennifer Tait was unanimously elected as the secretary of the IGCP-470;

-         All the current activities (sub-projects) will be on the web site as soon as possible so that all members are aware of what is done by each other.

-         After debating on the demand presented by Dr Valentin Kanda of the Demacratic Republic of Congo (DRC), the IGCP-470 third annual meeting will be held in Kinshasa (DRC) and will be hosted the University and Centre for Geological and Mining Research of Kinshasa. The field trip for this conference is planned for the “Bas Congo” region. The general theme of the meeting is “The late Mesoproterozoic-early Neoproterozoic events and low latitude glaciations connexions with the Pan-African orogenic system in central Africa “. The first circular of that meeting will be issued as soon as possible.

The business meeting also focussed of the evaluation of the activities of the project. The results of most of the current activities were presented during the oral presentation. The ongoing and new collaborative sub-projects are:

-         the paleomagnetic studies of Neoproterozoic to Paleozoic involving one PhD student,

-         the metamorphic studies (P-T path reconstruction) involving two PhD students,

-         the metallogenic studies on the rutile, Ni and Cr deposit North of the Congo craton,

-         the geochronology of the geological units of the Pan-African belt,

-         the Neoproterozoic plutonic complex in Chad involving one PhD student,

-         the carbonates of Central African Republic with a post-doctoral researcher,

-         the geochemical and petrological studies of Pan-African granitoids. A particular recommendation is that these studies must also focuss on granitoids that contain some economic minerals, especially Nb and Ta,

-         the development of the GIS database, as a tool for an update of the geological and tectonic map of central Africa,

-         the use of ASM as a tool to study granitoids of northern Gabon and southern Cameroon,

-         the Geology of the West Congolian belt in Gabon, Republic of Congo, Democratic Republic of Congo and Angola,

-         The geology of eastern Nigeria and correlation with western Cameroon.

The leader of each of these sub-projects will prepare a short presentation for the web site.

The five days field trip in northern Cameroon and south-western Chad was attended by 23 researchers. The geological context of both regions is made up of: (1) the Paleoproterozoic basement; (2) the Neoproterozoic unit comprising the Poli succession in Cameroon and the Lere succession in Chad; (3) the Paleozoic unit known as Mangbai series in Cameroon.

Paleoproterozoic basement

 The Paleoproterozoic basement in northern Cameroon comprises (1) a meta-sedimentary or volcano-sedimentary sequence including amphibole-biotite and biotite-garnet gneisses, meta-arkoses and metaquartzites often associated with iron formations, scattered bands of garnet-pyroxene amphibolites, garnetiferous and calc-silicate rocks; and (2) metaplutonic rocks of dominant dioritic and granodioritic composition. The Paleoproterozoic structures have been completely reworked during the Pan-African orogeny and only in some cases are relict granulitic assemblages preserved. This is consistent with the lower intercepts of zircons, titanite U-Pb ages, and Sm/Nd garnet-whole rock ages all around 600 Ma. The upper intercepts of zircons from metasedimentary and metaigneous rocks point for the climax of the tectonic, metamorphic and plutonic activities at ca 2.1 Ga. T_DM ages show that both recycling of an Archean crust and Paleoproterozoic accretion were recorded.

Neoproterozoic unit

The Neoproterozoic unit (Poli in Cameroon and Lere in Chad) is dominated by interleaved metavolcanic and metavolcanoclastic rocks. Metavolcanic rocks include alkaline and calc-alkaline metaryholites, and tholeiitic metabasalts. Volcaniclastic rocks are composed of either purely volcanogenic clastic rocks (mainly tuffs) or variably reworked clastic rocks of dominantly intermediate composition (metagreywackes). Occasionally, conglomeratic layers with granite pebbles occur within the volcaniclastic sequence. All these rocks underwent a poly-phased tectono-metamorphic evolution comprising a tangential deformation with gently dipping schistosity followed by a compressional folding with subvertical axial plane and late development of strike slip faults that controlled the emplacement of late-collisional granitoids. One of the most important is the Tcholliré-Banyo sinistral shear zone which separates the Neoproterozoic unit from the Paleoproterozoic basement. The metamorphism is of medium to high pressure type (garnet-kyanite-staurolite) and reaches the migmatization in lower units. The associated syn- and post-collisional plutonism evolves from calc-alkaline to alkaline compositions. Attempts to constrain the evolution of the Poli basin permitted to establish its early-Neoproterozoic age and its closure between 640 and 580 Ma corresponding to the age of the metamorphism and syncollisional granitoids. The key dates are: (1) in the Poli basin, the 830 +11/-10 Ma on Goldyna metarhyolite, the ca 700-800 Ma on volcaniclastic schists and gneisses, and the 630-620 Ma on syncollisional granitoids; T_DM ages are mainly Neoproterozoic, but some are older (mostly lower than 1.4 Ga) and are interpreted, at the light of zircon data, as representing mixing ages between young volcanic sources coeval with the development of the basins, and detrital sources from an old basement.

Paleozoic Unit

It is represented by numerous WNW-ENE to NW-SE oriented massifs, the most important being the Balche and Mangbai hills in the Poli and Figuil regions. They comprise alternating or superposed sedimentary and volcanic rocks. Sedimentary infilling consists of breccias and conglomerates with pebbles of associated volcanic rocks and nearby Neoproterozoic units, coarse to medium grained sandstones. Volcanic rocks comprise trachytes, rhyolites, ignimbrites and continental tholeiitic basalts. A composite Rb-Sr isochron from volcanic rocks collected from both Mangbai and Balché massifs gave an age around 580 Ma; but the deposit may be younger as pebbles from felsic dykes or even from Godé-type granites are found in conglomerates.

The objectives of the field trip

After the field trip of December 2002 in southern Cameroon which focused on the external zone of the Pan-African belt in central Africa, this field trip in the internal zone was organized to solve crucial problems for our understanding of the evolution of the belt. Below are some of them:

(1)   Regarding the tectonic and metamorphic evolution, the target region for the field trip offers better conditions for kinematic reconstructions during the polyphased tectonic events, on the contrary to the southern Cameroon where the late nappe tectonic has obliterated most of the previous structures. On the other hand, it is important to have a clear picture of this part of the belt dominated by late-collisional shear zones in contrast with southern Cameroon where nappe tectonic is the main structural feature. Finally, it is important to solve the question of granulitic assemblages in the Pan-African belt in central Africa: in northern Cameroon, they are attributed to the Paleoproterozoic (because they appear relictic) to that of the southern Cameroon which are definitely Pan-African. In other words, do we have in the belt north of the Congo craton an equivalent of the Paleoproterozoic Nyong series documented during the 2002 IGCP field trip?

(2)   Regarding the pre-collisional evolution of the belt, data are still lacking to constrain the events which predated the collision dated between 640-580 Ma. The presence of intense pre-kinematic plutonic rocks and the low grade meta-volcanic and meta-volcaniclastic rocks of northern Cameroon and south-western Chad are suitable geochronological material to understand the pre-collisional evolution of the belt (development of basins and subduction related plutonism).

(3)   Regarding the Chadian Precambrian basement, this trip was important not only because of correlations purpose, but also as a precious opportunity, for scientists to visit together and discuss the basement rocks of this country since many years. Field observations and sampling during the trip and the subsequent laboratory analyses are an important step for a broader view on the Pan-African belt in central Africa.

(4)   Regarding the paleomagnetic studies, since the IGCP-470 launching of activities a sub-project on “Snowball Earth, Cambrian True Polar Wander, and Neoproterozoic Geography: Paleomagnetic Research in Central Africa” sponsored by the VWF has been initiated. The northern Cameroon is important as it displays deposits (e.g. Balche and Mangbai type series) attributed to early Paleozoic which is a crucial geological time for this study.

Field trip Itinerary

The field itinerary was organized as follows:

(1)   Day one was devoted to a cross section from the Poli series to the Paleoproterozoic basement in the Buffle Noir region; this allowed to document the progressive D2 deformation evolving from coaxial to non coaxial regime and the resulting mylonitic structures related to Tcholliré Banyo shear zone which separates the Poli series and the Paleoproterozoic basement;

(2)   Day two was devoted to the western part of the Poli series with examination of typical synkinematic granitoids (ca. 630 Ma metadiorite at Tete bridge) and the Poli low-grade volcanic and volcaniclastic schist. This section showed that, in contrast with the buffle Noir area, the main structural features are folds that show locally a non-coaxial deformation regime. A discussion on the Paleozoic deposit at Hoy village led to the necessity for detailed studies to solve the question of whether or not the deposit represents a Pan-African molasse and on the necessity of accurate deposition age determination;

(3)   Day three focussed on a cross section accross migmatites and plutonic complexes between Garoua and Figuil (Cameroon) allowed a transition to south-western Chad;

(4)   Day four was devoted to the Neoproterozoic magmatic rocks (tonalites, gabbro-diorites, metadiorites) of the Lere region (Chad);

(5)   Day five was devoted to the detail study of the Neoproterozoic schist at Mayo Binder (Chad) as to compare it to that of Poli (Cameroon).

During the trip, samples were collected and teams constituted to solve some of the major problems among which: (1) the question of very long length of the time range (900-700 Ma) during which the Poli-Leré basin is assumed to have developed, (2) the timing of the deformation, (3) the mechanism of exhumation of Paleoproterozoic granulites on Pan-African times and, (4) the precision on the age deposit of the Mangbai-type series.

3 post-conference trips took place.
- The cross section from the north Cameroon to the Congo craton to collect samples for metamorphic studies for two PhD students,
- A visit of the Ni-Co deposit of south-eastern Cameroon to collect sample for metallogenic and isotopic studies,
- A cross section in the Yaoundé series to collect heavy minerals for studies to understand the origin of the rutile deposit in the Neoproterozoic units north of the Congo craton.

Difficulties

The main problem in the organization of this field conference started when the original plan to hold the meeting in Central African Republic was cancelled following the political situation in that country. The most important problem was the funds from UNESCO which are still not yet received at the time of writing this report. However, we did our best to bring most of the participants from the other countries of the region to the meeting.

The Local Committee of the organization of the meeting was as follows:
Dr J. Bassahak, Garoua-Cameroon
Dr J. Penaye, Garoua-Cameroon
J.C. Doumnang, N’djaména-Chad
J.V. Hell, Yaoundé-Cameroon
M. Sey Ndjekoundan, N’djaména, Chad
Dr S.F. Toteu, Garoua-Cameroon

By

Dr S. Felix Toteu

Leader of the IGCP-470

Centre for Geological and Mining research (CRGM)

B.P. 333, Garoua, Cameroon