Petrology and Geodynamic Context of Metabasic Rocks of Nemba Complex in the West Congo Fold Belt (Republic of Congo)

The West Congo belt contains Paleoproterozoic and Neoproterozoic Units which are deformed during Panafrican event. The Neoproterozoic Unit contains rift and post rift deposits. The rift formation is made of metabasite, volcanic and volcano-sedimentary rocks. The metabasite constitutes the Nemba Complex which is considered put into place in a continental or oceanic context. Samples from Nemba Complex collected along the “Congo ocean railway realignment” are analyzed. Major elements indicate a low potassic subalkaline affinity of the samples. Classification diagrams based of major elements shows that the rocks are a tholeiitic serie constituted of andesitic basalts, sub-alkaline andesitic basalts and a sub-alkaline to alkaline basalts. Some samples with a calc-alkaline signature indicate a crustal contamination. The traces elements plots indicate that the rocks are the MORB. The spectrum of multi-elements is characteristic of continental rocks with a lithospheric origin. This is an agreement with many works on the Nemba Complex and the alkali plutons and acid volcanic rocks associated which attribute there a rift context.


Introduction
The west Congo belt ( Figure 1) outcrops along the Atlantic eastern margin from South Gabon to the North Angola around 1400 km in length and 150 to 300 km passive margin [5]. The rift formations are represented by Sounda Group containing from bottom to top, the Nemba Complex, the Kakamoeka and Mvouti Subgroups [5]. The Nemba Complex consists of metagabbros, metabasalts, amphibolites, and green schists. The Kakamoeka Subgroup is made of volcanic and volcano-sedimentary rocks (conglomerates, quartzites, graphitic schists, ignimbrites, rhyolites, tuffs, metabasic rocks) whose equivalent is dated in DRC between 920 ± 8 Ma and 917 ± 14 Ma [7]. TheMvouti Subgroup is constituted by black schists, chlorito-schists, quartzites, meta-arkoses and meta-sandstones.The post rift is represented by the Mayombe Group which contains the Moussouva Subgroup, the lower Tillite Formation and the Louila Subgroup. The Moussouva Subgroup is made of quartzites and schists. The lower Tillite is a glaciogenic formation with a black calcareous shale matrix and a doleritic sills intercalations in DRC. The Louila Subgroup contains meta-gres locally conglomeratic, quartzites, quartzo-schists and schists with intercalation of limestones and calcshists [8] [19] [22] [23]. The whole formation is intruded in the western domain by rifting contemporaneous granites (Mfoubou and Mont Kanda granites in Congo and Noqui granite in Democratic Republic of Congo).
The paleoproterozoic formations are deformed by eburnean phase generating a fold oriented NE-SW [8] [13] [20] which are reactive during the Panafrican orogenic events (West-Congolian) building a D1 and D2 folds oriented NW-SE associated respectively by cleavage (S1) verging to the NE and crenulation cleavage (S2) often straighten and without clear verging [20] [21] [24]. A third tardy-orogenic phase characterized by a second crenulation cleavage (S3) oriented NE-SW folding a D1 and D2 structures is described in the Loémé Formation (or Guéna Subgroup) [8] [21]. In the Mayombe, deformation is also accompanied by thrust pulling the formation of internal domain stacked upon the formation of the external domain. The intensity of deformation decreases from SW (internal domain) to the NE of the belt (external domain).
The metamorphism is an amphibolite facies grade in the internal domain. It decreases to the NE where it becomes greenschist facies grade in the external domain and anchizone facies grade near the adjoining Niari basin.

Sampling
21 samples are collected along the "Congo ocean railway realignment", between the Pk 12 and Pk 13 from Nkougni to Dolisie town. In this area outcrops largely the Nemba Complex formation with intercalation in small part of Loukoula Subgroup.
The Nemba Complex formation is essentially made of amphibolites and greenschists (Figures 2(a)-(c)). The amphibolites are in massive hard beds and the greenschists are strongly cleavage. Locally the two facies contains veins filled by quartz or calcite. The associated fold are long flank and short flank and a slight NE verging. The whole rocks shown a crenulation cleavage (S2) strongly straightens. The quartzo-schists (Figure 2(d)) are essentially made of quartzo-feldspathic levels alternating with wealthy muscovite levels. The shows also a International Journal of Geosciences

Analytical Procedures
Geochemical analyses of major, trace and rare earth elements were done to evaluate the petrogenesis of rocks. Major, trace and REE elements were determined after powdered samples in fusion with LiBO 2 and dilute by HNO3 and performed by inductively coupled plasma-mass spectrometry (ICP-MS) and inductively coupled plasma atomic emission spectrometry (ICP-AES) in the "Centre de Recherches Pétrographique et Géochimique" (CRPG) of Nancy, France.

Geochemistry and Petrographic Classification
Representative analyses are given in Table 1.    vs. MgO displays a dispersal data with a weak coefficient of correlation.
Nb/Yb diagram of [31] plots some samples in the EMORB domain while the major part is situated above this domain with a weak dispersion (Figure 7(b)). showing higher REE (ƩREE = 149 -183) concentration and contains Eu negative anomaly (Figure 8(a)). Spectrum of multi-elements normalized at primitive mantle of [33] shows also the bimodal comportment of samples with   (Figure 8(b)).

Discussion
The studied basic samples come from the Nemba Complex now considered synrift Neoproterozoic formation dated at 915 ± 8 Ma age SHRIMP gotten from meta-gabbro zircon, [5] and associated with volcanic (rhyolite) and volcano sedimentary rocks of Kakamoeka Subgroup whose equivalent called Mayumbian Group is dated in DRC between 920 ± 8 Ma and 917 ± 14 Ma [7].

Magmatic Signature
Major elements data (Table 1)

Crustal Contamination
The LOI data of the geochemistry analysis are low to medium and shows that is also described by [5] in the Nemba Complex. We can conclude that Nemba basalts are derived from primitive mantle (PM).
The spectrum of normalized multi-elements at primitive mantle of [33] displays (Figure 8 Tholeiite samples mark a difference with the calc-alkaline samples by the lower concentration in Th rendering the evolution from tholeiitic tendency to calc-alkaline tendency. The spectrum of multi-elements is characteristic of continental rocks with a lithospheric origin and is in agreement with a filiation of tholeiitic/calc-alkaline transition.

Geodynamic context
The Zr vs. Ti diagram of [30] displays that a majority of samples plots in the Arc Island lavas and MORB (Figure 7(a)). Tree samples are situated in the Within-Plate lavas. The meta-basalts with low-Ti are produced in intra-cratonic environment and the meta-basalts with high Ti are alimented by melting produced in the modified mantle in subduction across the oceanic crust [34]. [41] shows that active continental margin is characterized by basic rocks with high International Journal of Geosciences and 920 Ma is described like flood continental basalt put into place in rift context [6]. In Brazil, in Araçuài belt, the time between 920 Ma and 900 Ma will correspond to the development of continental rift in which were take place basalts of Macaùbas and Dom Silvério [46].
Data presented in this study shows that the majority of samples from Nemba situated between Pk12 and Pk13 have tholeiite signature and a few samples havecalc-alkaline signature but both have continental affinity. Isotopic data are necessary to complete chemical study in Nemba Complex to solve the geodynamic context of the west Congo fold belt.

Conclusion
The studied Nemba meta-basalts display a geochemistry composition in tholeiite majority. The evolution of the trace and rare earth elements shows a put into place in subduction context with a contamination by lithospheric crust of Insular Arc type. However, the association of the basic Nemba Complex with the plutonic rocks (Mfoubou and Mt Kanda granites) and effusive rocks (rhyolite and pyroclastic) of Kakamoeka Subgroup suggest the put into place in rift context.
This assumption is in agreement with data gotten in the whole Araçuài-West Congo (A-WCO) belt.