The errors of sample analyses were propagated by quadratic addition of the external uncertainty observed for the standards to the reproducibility and within run precision off each unknown analysis. The instrumental set-up and further details of the analytical method applied are given by Buhn et al. The masses , and are measured with ion counters, and U was analyzed on a Faraday cup. The signal of Hg was monitored on an ion counter for correction of the isobaric interference between Hg and Pb. The signals during ablation were taken in 40 cycles of 1 sec each.
For data evaluation, only coherent intervals of signal response were considered. Data reduction was performed with an Excel spreadsheet developed by one of the authors that considers blank values, zircon standards composition and errors propagation. Consequently, the Isoplot calculated errors were modified in order to incorporate this uncertainty level and, hence, represent a more realistic age in terms of the analytical limitations of the method.
Two main types of leucosomes have been distinguished on the basis of petrographic and structural criteria:. They are observed in both metapelitic gneisses and metabasites where they are sometimes associated with ultramafic dykes. Injected leucosomes are either:.
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Field occurrences of injected leucosomes a and in situ leucosomes c , and detail of microstructure of injected leucosomes b and in situ leucosomes d. The average mineral composition of garnet is presented in table 1. Garnet forms solid solution Figure 4 of almandine X Fe: Accessories are monazite, zircon, apatite and often sillimanite, calcite, tourmaline and muscovite.
They are observed in both metapelite and metabasite Figure 3 c , structured or not, medium to coarsed-grained, mainly composed of quartz, plagioclase, K-feldspar and garnet. The perthitic composition ranges from An 18 Ab 82 to An 20 Ab Garnets Figure 3 d. The mineral composition of garnet is given in table 1. They form a solid solution of almandine X Fe: Kyanite Figure 3 d.
Muscovite, monazite, zircon, rutile, apatite and oxides represent the accessory phase.
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In the X Fe vs Al VI biotite diagram Figure 5 , there is a significant difference between in situ leucosomes and injected leucosomes. In situ leucosomes are enriched in both aluminium and iron, whereas injected leucosomes are depleted with respect to the same elements. In situ leucosomes plot in granite while injected leucosomes plot in granite and quartz monzonite fields Figure 6. The rocks are silica rich with a SiO 2 content ranging from In the K 2 O vs SiO 2 diagram, the in situ leucosomes plot in the calc-alkaline domain while the injected leucosomes show high K-calcalkaline.
Classification diagram of plutonic rocks  showing the localization of the studied leucosomes in the granite field.
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In major elements variation diagrams vs SiO 2 Figure 8 , the two rock types form well defined clusters, and taken together define more or less well-defined trends. Trace elements concentrations of the different leucosomes are listed in table 3.
Selected elements are plotted against SiO 2 in figure Chondrite-normalized REE the normalizing values are from Jahn et al. Symbols as in figure 6. The REE patterns lead to the distinction of two groups of injected leucosomes: These geochemistry behaviours are in accordance with subdivisions done in petrography. These samples were chosen because they are more enriched in monazite crystals than other samples.
The analyzed monazite crystals are of two main types: No systematic core-to-rim optical age zonation of were observed. This observation implies that monazite crystallize from melting liquid. This allows the calculation of ages composed of several single analyses by regression through zero  and of weighted means . Two groups of ages, corresponding probably to the two events of monazite crystallisation, are recognised in the diagrams of mineral chemistry of monazite element vs Th-U-Pb chemical ages Figures 13 and The first event younger occurs at around Ma and the second older at around Ma.
On this diagram, the two generations of monazite previously described are clearly individualized. The average age of. Harker diagrams of selected major elements. These two groups of ages seem to belong to the same thermal event because the petrography study reveals that all the monazite crystals are included in biotite flakes.
Statistically, all these data can be considered as belonging to the same crystal population. The analyzed zircons crystals did not show any sign of a complex history. The obtained concordant ages correspond to the melting age during the Pan-African orogeny in Cameroon.
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For the leucosomes occurring in metabasite, sixteen grains of zircons were analyzed by the same method. Two types of zircon crystals zoned and no zoned were identified in metasediment. Zoned zircon crystals are observed in the leucosomes from the metasediment whereas no zoned zircon crystals were selected from paleosome. The analytical results of no zoned zircons are listed on table 8. The ages obtained define a Discordia. Monazite ages from single analyses are given with minimal error see text.
Zoned zircon crystals were also analyzed and their radiometric results are given in table 9.
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