TSK 11 Göttingen 2006 Kouankap
 Textures in mylonitised gran-
 ite from Banefo area in the
 central part of the Cameroon
 Central Shear Zone (central
 Africa) — Kinematics and
 gradian deformation indica-
 tors Poster
 Nono Gus D. Kouankap1
 The North Equatorial Panafrican belt
 is constituted by orthogneissified meta-
 morphic and magmatic sets linked to
 tectonometamorphic events of the neo-
 proterozoic orogeny. This belt whose
 formations are situated in the North of
 the congo craton is divided into three
 distinct geodynamic domains: a south-
 ern domain, a northern domain and a
 central domain. The deformation and
 metamorphism relationships are a func-
 tion of the different geodynamic do-
 mains of the belt.
 The central domains, to which belongs
 the Banefo region, is affected by large
 strike-slip faults among which is the cen-
 tral cameroonian shear zone. This do-
 main is marked out by numerous syn-
 tectonic granitoids. At Banefo (NE
 Bafoussam, West Cameroon), those
 massifs are mainly constituted by my-
 lotinised orthogneisses and granites of
 various degrees. The country rock is
 made up of gneiss and amphibolite. In
 the Banefo massifs, the orthogneiss is
 the least deformed petrographic unit.
 It shows a heterogranular granoblas-
 tic texture with a mineral assemblage
 formed by quartz, K-feldspars, biotite,
 etc., within the orthogneiss, remains of
 magmatic fluidality can still be observed
 marked by feldspar megacrystal.
 1 Université de Yaoundé I, Départe-
 ment des Sciences de la Terre, BP : 812
 Yaoundé-Cameroun
 Figure 1: C/S/C′ structure
 Field observations reveal a net progres-
 sion of the intensity of deformation of
 orthogeisses to ultramylotinised gran-
 ites having an identical chemical com-
 position as the orthogneisses. The my-
 lonitisation is marked by a reduction
 in grain size, in the percentage of the
 megacrystals of feldspars and by the
 flattened, elongated and stretched forms
 of minerals, with the main ones includ-
 ing:
 • orthoclase, which is present as elon-
 gated crystals with sigmoidal and
 elliptical shapes, with oblique fis-
 suration planes at the long axes of
 the mineral;
 • quartz which is in the form of
 rubans and xenomorphic stretched
 crystals;
 • biotite which is less abundant, with
 crystals disposed parallelly at the
 borders of orthoclase clasts, the
 latter showing shadow zones with
 quartz grains.
 The organization of those minerals in
 the rock forms the S/C′ structure whose
 identification in the field permitted to
 determine the sinistral trend of the
 shear movement.
 1
Kouankap TSK 11 Göttingen 2006
 Figure 2: S/C′ structure
 These S/C′ structures evolve with the
 intensity of the deformation; from the
 orthogneisses where they are observed,
 they pass to mylonites sensus stricto
 with S/C/C′ structures (Fig. 1) and fi-
 nally to the ultramylonites where one
 observes the S/C′ structure in which
 C is parallel and confused with S
 (Fig. 2). The gradian of the deforma-
 tion is marked by stretched crystals of
 quartz, by the reduction of the sizes
 and proportions of feldspar megacrys-
 tals. This has implications at the level
 of orthogeissification of structures, re-
 duction of the angle between S and C,
 the increase in the density of C and the
 appearance of the C′ plane, such that in
 the ultramylonites, S is almost parallel
 to C and the angle between C and C′
 attains its maximum.
 2