The study area covered a small part of the Boromo belt near the village of Pouni ( Figure 2 ). The outcropping rocks are mainly basalts and lamprophyres.

Basalts are massive and dark ( Figure 3(a) ). They outcrop in the form of small hills containing boulders or clusters of boulders. Macroscopic observation shows that basalt is aphyric. Microscopically ( Figure 3(b) ), the basalt consists almost entirely of amphibole, in the process of destabilisation into actinote or chlorite. Small amounts of biotite, plagioclase and quartz islands can be seen. The sample observed under the microscope is criss-crossed with quartz-plagioclase veinlets.

Lamprophyres are dark coloured rocks ( Figure 4(a) ). In the field they occur as small boulders or clusters of boulders. Microscopically, lamprophyres are composed mainly of plagioclase, pyroxene and amphibole. The amphiboles are altered to muscovite. Opaque colours are associated with ferromagnesian minerals.

The granodiorite is light in colour and medium to slightly coarse grained ( Figure 5(a) ). The visited outcrops have no measurable structures. Microscopically ( Figure 5(c) ), the white minerals are predominantly plagioclase, followed by quartz. Potassium feldspars, represented by microcline, are present in small amounts. The ferromagnesian minerals are biotite and amphibole. Biotite is slightly more common than amphibole. Auxiliary minerals include epidote, zircon and opaque.

It is a very poorly structured rock with biotite as the only ferromagnesian mineral. Biotite granite is leucocratic and the texture is essentially medium grained ( Figure 5(b) ). Microscopically ( Figure 5(d) ), biotite granite is composed of straight quartz, plagioclase and potassic feldspar. The other white minerals are plagioclase and potassium feldspar in more or less equal proportions. Accessory minerals are mainly sphene, zircon, epidote and a few opaques.

The results of the geochemical analyses indicate silica contents of between 50% and 52% ( Table 1 ). In the AFM ternary diagram of [11] ( Figure 6 ), sample TP07

has a highly ferriferous tholeitic affinity and is similar to the Nassara, Houndé and Boromo basalts [9] [10] [12] . The MgO-Al₂O₃-K₂O diagram indicates that sample TP06 is a lamprophyre similar to those from Nassara in Burkina Faso and Syama in Mali ( Figure 6(b) ).

The Zr/Y ratio of sample TP 07 is 2.86 ( Table 2 ). According to the Zr/Y ratio defined by [13] , the Pouni basalt (sample TP 07) has a tholeiitic affinity like the Nassara basalts [10] and the other Paleoproterozoic basalts of the Man/Leo shield [8] .

Table 3 shows the results of the geochemical analysis of samples TP 06 and TP 07.

Continued

Although we cannot classify sample TP 06, as has been done in other studies [10] , we note that its REE spectrum is more or less similar to those of Nassara and Syama in Mali ( Figure 7(a) ). The REE spectrum of sample TP 06 shows an enrichment in light REE compared to heavy REE and a slight positive anomaly less pronounced in Europium (Eu/Eu* = 1.27). This suggests that the spectrum is slightly fractionated with a (La/Yb)N ratio of 4.

The REE spectra in Figure 7(b) compare that of sample TP07 with that of the Nassara basalt in Burkina Faso [10] . The REE spectrum of sample TP07 is flat with an insignificant Europium anomaly (Eu/Eu* = 0.87). This is similar to that of the Nassara basalts and suggests that the plagioclases are little or not fractionated.

Condie (1999) [14] define mid-ocean basalts or MORBs (La/Nb > 1.4) and oceanic shelf basalts (La/Nb < 1.4) on the basis of the La/Nb ratio. According to this ratio, sample TP 07 (La/Nb = 1.38 ppm) could be defined as an oceanic shelf basalt.

The negative Ta, Nb and Ti anomalies could define a late orogenic to post- collisional environment for sample TP 06.

The geochemical data ( Table 4 ) show that the silica content of the granitoids ranges from 60% to 77%, clearly indicating that they are acid rocks. In the molar ratio diagram (A/CNK = [Al₂O₃]/[CaO] + [Na₂O] + [K₂O]), the granitoids are type I and are metaluminous to peraluminous ( Figure 8(a) ). In the diagram of [15] the granitoids are calc-alkaline to potassic calc-alkaline ( Figure 8(b) ).

Table 5 shows the average composition of major and minor elements and rare earths in the granitoids of the study area. The evolution of the major elements can be appreciated by correlating their percentages with respect to silica in the different diagrams of [16] of the major elements ( Figures 9(a)-(f) ). As silica increases, elements such as TiO₂, Al₂O₃, MgO and CaO decrease, while Na₂O and K₂O increase. Granitoids are moderately potassic, with the K/Na ratio varying between 0.16 and 0.43.

The REE spectra normalised to the C1 chondrite from [17] show relatively high LREE contents (0.17 < LREE < 38.5) compared to HREE ( Table 6 and Figure 10(a) ). This reflects a very good fractionation of the REE spectra ( Figure 10(a) ). The (La/Yb)N ratios range from 10 to 26 ( Table 6 ) and the Eu anomaly

(Eu/Eu* from 0.18 to 0.97) is weakly positive for samples TP 04, TP 02, TP and TP 15. Only sample TP 08 shows a strong negative anomaly. This indicates a good fractionation of the plagioclases. However, the granitoids in the study area are rich in Sr and Ba but poor in Ta and Nb ( Table 2 ). These results reflect a good distribution of feldspars and interactions between the different mineral phases.

The multi-element diagrams normalised to EMORB according to [17] are characterised by an enrichment in LILE (Cs, Ba, K, Sr, Nd) compared to HFSE ( Figure 10(b) ). Sample TP 08 has relatively low Ba and Sr contents compared to the other granitoid samples. The profiles are almost parallel and there is a very pronounced anomaly in Ti and moderate anomalies in Nb, Th and P. Overall, these different anomalies depend on the fractionation of potassic minerals, ferromagnesians and plagioclases.

In the geotectonic diagrams of [18] , the granitoids of the study area are located in a volcanic arc environment ( Figure 11(a) ) or in the range of syn-plate collision and volcanic arc environment granitoids ( Figure 11(b) ). In the diagram in [19] , all the granitoids are located in a volcanic arc environment ( Figure 11(c) ). This shows that they were emplaced in an active tectonic context.