Emplacement and Evolution of The Nlonako Ring Complex in The Southern Domain of The Cameroon Line

Abstract

The Cameroon Line (CL) appears as a SW-NE straight line characterized by an intense volcanic activity of basaltic composition with alkaline plutonic complexes, including the Nlonako ring complex (NRC) emplaced in the southern part of the CL. Many hypotheses have been proposed to explain the origin and petrogenetic evolution of these complexes but very few of these works focus or attempt to propose their structural emplacement model, though structural data recorded by them can permit such studies. Using petrographic, structural data and space images, we propose an emplacement model of the NRC and other ring complexes along the CL. The NRC is a ring complex of 10 km diameter slightly elongated in the NNE-SSW direction mainly composed of plutonites among which syenites, gabbros, diorites and biotite-amphibole granites, and a few volcanites made up of rhyolites and basalts occurring as veins or boulders in syenites. This complex was emplaced as sill intrusive body within Pan-African k-feldspars megacrysts granites and gneiss host rocks under fractures control. These fractures developed as result of stress release consecutive to the readjustment of the crust during and after the late Cretaceous general extension, therefore facilitating the upwelling of the mantle plume and the generation of magma that vertically uplift. The progressive magmatic pressure decreases after the NRC emplacement in addition to conjugated fractures networks developed at superficial crust level finally lead to the cauldron subsidence of the NRC summit. This subsidence was facilitated by the downward sliding of rocks along the WNW-ESE fault, finally leading to the formation of a caldera at the summit of the NRC. The NRC and other anorogenic complexes aligned along the CL are located in a tension gash form by the Cretaceous sinistral activation of the N70E Adamawa fault. This left lateral wrench movements developed a tension gash, overprinted on pre-existing transcurrent mega-faults with as result, the development of "en-echelon" fractures. Stress release during the late Cretaceous general extensional phase probably accelerated the uplift of magmas and emplacement of ring complexes along the "en-echelon" regional fractures in an extensional setting during the Tertiary (60 - 30 Ma). This therefore explained the alignment of anorogenic complexes along the N30E CL corridor that are highly correlated to lineament networks and Pan-African megafaults

Structural Interpretation of Lineament from Nlonako Area and Surroundings: Contribution to Pan-African Tectonic Reconstruction

“…Structures clearly identified in the filter directions were considered as major while those observed not in all the directions were considered as minor structures. Major structures were named faults and minor structures named fractures after comparing with geological maps from [8] and [11]. A tentative validation of the results is provided based on SRTM image and our previous [8] conducted in the same area.…”

“…In k-feldspar megacrysts granite, Bt-Cpx granite displays N20°E48°SE and N35E44NW conjugated planes whose directions are parallel to k-feldspar megacrysts. In Singalngal, N-S (N10E) and E-W (N95E) k-feldspar megacrysts granite veins with 15 to 45 cm thickness are observed in gneiss [11].…”

“…According to [11] this complex was emplaced as sill intrusive body during the Tertiary [12]- [14]) in a Pan-African k-feldspars megacrysts granites and gneiss host rocks under fractures control consecutive to the late Cretaceous general extension. After its emplacement, the NAC experienced the cauldron subsidence of its summit, facilitated by the development of conjugated faults and the downward sliding of rocks along the NW-SE fault, leading to the formation of caldera at its summit [11]. The NAC is made up of syenites, biotite -pyroxene granite gabbro, rhyolite and basalt intruded in granite-gneissic basement [11].…”

“…After its emplacement, the NAC experienced the cauldron subsidence of its summit, facilitated by the development of conjugated faults and the downward sliding of rocks along the NW-SE fault, leading to the formation of caldera at its summit [11]. The NAC is made up of syenites, biotite -pyroxene granite gabbro, rhyolite and basalt intruded in granite-gneissic basement [11]. Syenite, which represents the dominant rock type (about 90% of the rock types) of the NAC, displays porphyritic heterogranular texture made up of quartz, k-feldspar, plagioclase, pyroxene, zircon and opaque minerals.…”

“…Rhyolite and basalt outcrop as dyke in syenite and gneiss and displays porphyric microlitic texture. More detailed petrographic data of the NAC are given by [9], [14] and [11]. According to [15], the megacryst granite basement rock of the NAC, is synkinematic granite whose emplacement was totally controlled by a N30° sinistral shear zone.…”