Petrology of the late Pan-African Guider syenite pluton (Cameroon): Implications for magma genesis and emplacement
Résumé
The Guider pluton, located in the eastern border of the North-West Cameroon domain close to the Mayo-Kebbi domain, is one of the rare high potassic magmatic bodies in the northern portion of Central African Orogenic Belt. Previously studied for its structural and geochronological aspects, its petrogenesis remained uncertain. Petrography, mineralogy, and whole rock geochemistry data are presented. The pluton is made of pink and grey quartz syenites and contains numerous enclaves of quartz diorites. These enclaves are highly concentrated at the center of pluton, previously identify as the pluton's feeder zone. The transition between syenitic types is gradational while the contact between grey quartz syenite and quartz diorite is sharp. Geochemically, they display a high-K signature. Our results including field observations, petrography, mineral chemistry and geochemistry suggest that these rocks derive from a single parental magma differentiated by fractional crystallization and sedimentation in a magmatic chamber. These results also suggest that the parental magma was generated by partial melting of metasomatized lithospheric mantle. Combining previous results with former structural data, we envisage that the Guider pluton derive from two successive steps: (i) a static step of fractional crystallization and sedimentation in a deep magmatic chamber; and (ii) a dynamic step during which the stratified magma ascent toward the upper crust, the evolved syenitic magma being tapped first and the diorite later. The magmatic structures are consistent with this magma emplacement scenario.