Les plutons de Guider et de Bossoum-Pologozom (chaîne panafricaine au Nord-Cameroun): analyses pétrographique, structurale, magnétique, géochronologique et implications géodynamiques

The Guider pluton (GP), emplaced ~593 Ma in an orthogneissic basement, is mainly constituted by transalcaline diorite and syenite. These formations are thought to result from a large partial melting of a basaltic source with fractional crystallization, crustal assimilation and metasomatism. The Bossoum-Pologozom pluton (BPP) is made of ferropotassic syenite and syenogranite deriving from a granodioritic magma by fractional crystallization. The country-rocks of GP (orthogneiss), emplaced at ca. 632 Ma, have been affected by a dextral transpressive shear deformation N-S to NNE-SSW characterized by a dominant pure shear that began around 612 Ma. The microstructures and AMS suggest that the emplacement of GP took place before the end of this transpressive deformation of its country rocks. The BPP was emplaced around 566 Ma ago, within orthogneissic formations and syntectonic granites. The syntectonic granites are coeval with a shear episode that post-dates the previous transpressive episode. The sense shear of this second event remains unclear. The internal structures of the PBP suggest that this pluton is syn- to late-tectonic with respect to the second episode of transpression. This regional transpression results from the collision between the Mayo-Kébbi domain and the NW-Cameroun domain which ended after ca. 566 Ma. This study also suggests that the trans-alkaline magmatism of the BPP is similar and contemporaneous to those of some plutons of the East-Nigeria and Nordeste in Brazil, and that the second transpressive event of the area under study is contemporaneous to the Patos transpressive shear zone system.Le pluton de Guider (PG), mis en place vers 593 Ma dans des orthogneiss, est composé principalement de diorite et de syénites transalcalines. Le magma parent de ce pluton serait issu d'une large fusion partielle d'une source basaltique, et les différents faciès résulteraient de processus de cristallisation fractionnée, assimilation crustale et métasomatisme. Le pluton de Bossoum-Pologozom (PBP) est composé de syénite et de syénogranite ferro-potassiques issues d'un magma granodioritique essentiellement par cristallisation fractionnée. L'encaissant du PG qui s'est mis en place vers 632 Ma, a subi une déformation transpressive à cisaillement pur dominant dextre, orienté N-S à NNE-SSW, qui a débuté vers 612 Ma. Les microstructures et l'ASM suggèrent que sa mise en place a eu lieu avant la fin de la déformation transpressive de son encaissant. Le PBP s'est mis en place vers 566 Ma, au sein d'orthogneiss et granites syntectoniques. L'installation des granites syntectoniques est synchrone d'un second épisode cisaillant, postérieur à l'épisode transpressif dextre sus-cité. Le sens de cisaillement de ce second épisode reste incertain. Les structures internes du PBP suggèrent qu'il est syn- à tardi-tectonique du second épisode de la transpression. Cette transpression régionale est la conséquence de la collision entre les domaines Mayo-Kébbi et NW-Cameroun qui se serait achevée vers 566 Ma. Enfin le magmatisme transalcalin du PBP est semblable et contemporain du magmatisme de certains plutons de l'Est du Nigéria et du Nordeste brésilien, ce qui à suggère que le second épisode de transpression du secteur de Guider est contemporain des grands cisaillements de la zone de Patos

The Pan-African quartz-syenite of Guider (north-Cameroon): Magnetic fabric and U-Pb dating of a late-orogenic emplacement

International audienceThe small Guider pluton (70 km2) in-between the cities of Maroua and Garoua (north Cameroon) is one of the quartz-syenite bodies that intrude Pan-African orthogneisses, and that are aligned along the western side of the NNE-directed Poli-Léré volcano-sedimentary corridor. The Guider syenite is here studied for its internal structures, using field and anisotropy of magnetic susceptibility measurements, and for its emplacement age of 593 ± 4 Ma (U-Pb zircon), ∼20 Ma younger than the beginning of gneissification of the country-rocks, mainly made of deformed granodiorites and diorites. In these orthogneisses, constantly NNE-trending vertical foliations, variously plunging stretching lineations and fold axes, as well as kinematic markers attest to a transpressive and possibly dextral regime. The Guider syenite, rich in K-feldspar and magnetite, displays typical magmatic microstructures with incipent solid-state features such as chess-board subgrains in quartz. Its magnetic fabric displays a well-defined feeding-zone in its centre and western side, attesting to its unrooted nature. However, N- to NE-trending lineations at its northern and southern ends, parallel to the overall linear structure of the country-rocks, point to its late-orogenic emplacement. Our results reveal that the NE-trending and dextral transpressive shear zone that acted in the Guider area ended after the emplacement of the syenite body; they specify the age of collage of the NW-Cameroon domain with the Mayo-Kébbi domain. They also show that the tectonic evolution of Guider area is closer to the E-Nigerian domain, to the west, than to its eastern neighbouring Mayo-Kébbi and Adamawa-Yadé domains. The connection of our sector with E-Brazil is briefly discussed

Tracking Airborne Pollution with Environmental Magnetism in A Medium-Sized African City

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Petrology of the late Pan-African Guider syenite pluton (Cameroon): Implications for magma genesis and emplacement

International audienceThe 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

Microstructures and magnetic fabrics of the Ngaoundéré granite pluton (Cameroon): Implications to the late-Pan-African evolution of Central Cameroon Shear Zone

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Zircon dating and mineralogy of the Mokong Pan-African magmatic epidote-bearing granite (North Cameroon)

International audienceWe present the mineralogy and age of the magmatic epidote-bearing granite composing most of the Mokong pluton, in the Central Africa orogenic belt (North Cameroon). This pluton intrudes Neoproterozoic (~830 to 700 Ma) low- to high-grade schists and gneisses (Poli-Maroua group), and is crosscut or interleaved with bodies of biotite granite of various sizes. The pluton is weakly deformed in its interior, but solid-state deformation increases toward its margins marked by narrow mylonitic bands trending NNE-SSW. The magmatic epidote granitic rocks are classified as quartz monzodiorite, granodiorite, monzogranite, and syenogranite. They are medium- to coarse-grained and composed of K-feldspar + plagioclase + biotite + amphibole + epidote + magnetite + titanite + zircon + apatite. In these granites, the pistacite component [atomic Fe+3/(Fe3+ + Al)] in epidote ranges from 16 to 29 %. High oxygen fugacity (log ƒO2 - 14 to -11) and the preservation of epidote suggest that the magma was oxidized. Al-in hornblende barometry and hornblende-plagioclase thermometry indicate hornblende crystallization between 0.53 and 0.78 GPa at a temperature ranging from 633 to 779 °C. Zircon saturation thermometry gives temperature estimates ranging from 504 to 916 °C, the latter being obtained on samples containing inherited zircons. U/Pb geochronology by LA-ICP-MS on zircon grains characterized by magmatic zoning yields a concordia age of 668 ± 11 Ma (2 σ). The Mokong granite is the only known occurrence magmatic epidote in Cameroon, and is an important milestone for the comparison of the Central Africa orogenic belt with the Brasiliano Fold Belt, where such granites are much more abundant