Evidence of Quartz , Feldspar and Amphibole Crystal Plastic Deformations in the Paleoproterozoic Nyong Complex Shear Zones Under Amphibolite to Granulite Conditions ( West Central African Fold Belt , SW Cameroon )

The Paleoproterozoic is characterised by a global-scale period of crust formation, extensive mafic to granitic magmatism, crustal reworking and intracrustal partial melting. Since a dramatic strength drop is associated with the presence of melt in crystallising or melting rocks, Paleoproterozoic continental deformation is thought to have been largely accommodated by shearing of high-grade gneisses and syntectonic granitoids. The Paleoproterozoic Nyong complex, which is made up of volumetrically dominant granitoids, metapelites and granitic gneisses, quartzites and banded iron formations; has been affected by the Eburnean/Transamazonian polyphase ductile and brittle deformation responsible of S1/2 foliation and S2 schistosity, L2 stretching and mineral lineations, F3 mesoand large-scale folds, S2/C3 structures and faults. Its shear zone strain anisotropies represent a natural laboratory to study the effect of crystal plastic deformation of felsic minerals and amphibole as predicted by numerical experiments and tectonic models. This paper describes the geometry of the deformation and focus on microstructural evidence of dynamic recrystallization features that were active under granulite conditions (>800 °C, >8.5 Kbar). These thermal conditions can be extended in similar Proterozoic worldwide shear zones.


Introduction
Geological and geophysical boundaries show that deformation in the Earth's crust is heterogeneous, with large displacements confined into faults and shear zones.Since last decades, significant progress has been made in understanding the microstructural evolution of crystallizing or melting rocks in natural and numerical experiments and tectonic models (e.g.While, 1973While, , 1977;;Urai et al., 1986;Vernon, 2000;Hirth et al., 2001;Stipp et al., 2002;Passchier & Trouw, 2005;Zibra et al., 2010Zibra et al., , 2012;;Zibra, 2012).A close spatial and temporal relationship between pluton emplacement and shear zones has been documented in various geological settings (see Rosenberg, 2004 for a review; Weinberg et al., 2004).Shear zones display microstructurally very different strain anisotropy to protoliths.They are associated to mineral dynamic recrystallizations where magmatic and metamorphic textures are overprinted by later deformations or annealing processes and the physical conditions (pressure, temperature, strain rate, differential stress, and water content).They offer to geologists, best examples of mineral recrystallization from microscopic to mesoscopic and megascopic scales.These deformations may differ from one portion to another through different fabrics, reflecting subsequent stages of syndeformational crystallization.Therefore, interpretation of microstructure and orientation and intensity of fabrics is critical in order to constrain space/time/temperature/deformation relationships during rock crystallization (Stipp et al., greenstone rock types that have experienced the Eburnean/transamazonian orogeny (e.g.Maurizot et al., 1986;Nédélec et al., 1993;Feybesse et al., 1998;Penaye et al., 2004).Herein, we present its new structural data at the western border of the Congo Craton.In particular, we focus on dynamic recrystallizations and apply its mechanisms with reference to natural deformation microstructures in quartz, feldspar and amphibole and with respect of the inferred deformation temperature from the Nyong complex shear zones in comparison to undeformed protoliths (Figure 1).For these objectives, we used the light microscopy and standard methods of microfabric analysis (Passchier & Trouw, 2005) and models of crystal plastic deformation of quartz (Stipp et al., 2002) and feldspar (Kurse et al., 2001).

Material Studied
Oriented samples (Figure 2) from the NyC foliations and shear zones were sampled in Mbigame (Ow294), Angonfeme (Ow309) and Akongo (Ow310) clinopyroxene syenites for the Akongo group.The Kama group was sampled on Abang Betsenga (Ow374) and Akok (Ow326) metagranodiorites, Enguingli gneisses (Ow354) and Abang-Betsenga banded iron formation (BIF).Above samples were prepared for XZ high-quality slides for petrographical and microtectonic studies, especially quartz, feldspar and amphibole plastic deformations.Investigations were carried out on a Leica LMP microscope of the Institute of Geology TU-Freiberg in Germany.Mineral abbreviations are from Kretz (1983).These petrographical and microtectonic studies focused on metagranodiorites and synkinematic syenites, the main rock types for the Kama and Akongo groups, respectively.

Methods
Samples were prepared for XZ high-quality slides for petrographical and microtectonic studies, especially quartz, feldspar and amphibole plastic deformations.Investigations were carried out on a Leica LMP microscope of the Institute of Geology TU-Freiberg in Germany.Mineral abbreviations are from Kretz (1983).These petrographical and microtectonic studies focused on metagranodiorites and synkinematic syenites, the main rock types for the Kama and Akongo groups, respectively.
Bedding (S 0 ), foliation (S n ), lineation (L n ), axial planes of folds (F n+1 ), fold axes (B n+1 ), S-C foliation-shear fabrics and faults were identified and measured in outcrops.The foliations were classified by the metamorphic minerals that define them.Stretching lineations as ripple marks and flow lines were measured for kinematical and geometrical criteria.Optical microscopy was applied in standard thin sections oriented parallel to XZ sections of the finite strain ellipsoid (normal to the foliation and parallel to the stretching lineation) to select microstructures indicating ductile deformation and identify critical metamorphic mineral assemblages as well as to guide quartz, feldspar and amphibole dynamic recrystallization features and mechanisms associated to temperature conditions (Kurse et al., 2001;Stipp et al., 2002;Passchier & Trouw, 2005).
Dynamic or plastic recrystallizations were defined as deformation-induced reworking of grain sizes, shapes or orientations with little or no chemical change (Guillopé and Poirier, 1979).These happened when the primary or protolith mineral recrystallization temperature intervals is reached later during tectonothermal events.Plastic recrystallized minerals display several microstructures such as bulging (BLG), sub grain rotation recrystallization (SGR), grain boundary migration (GBM) and sub grain area reduction (SGAR) from low-, medium-, high-to highest-temperatures, respectively (Figure 3).The BLG that occurs at lower temperature consists of micrometric local slow boundary migration or apophyses into or out of porphyorclasts.It undergoes progressive SGR with separation from old grain by bridging sub grain boundary or new and small grain apparitions surrounding old cores under medium temperature.The GBM that occurs at high-temperature, are characterized by undulated mineral boundaries.The SGAR defines the final plastic deformation stage of the old mineral under highest temperature conditions.New and small grains with undulated boundaries disappeared for idiomorphic blasts with triple points replacing the old core minerals.Details of dynamic recrystallization stages and mechanisms can be consulted in Kurse et al. (2001), Stipp et al. (2002), Passchier and Trouw (2005), Zibra (2010Zibra ( , 2012)), Zibra et al. (2012).If these assertions are accepted, plastic recrystallizations define a reworked deformation affecting primary minerals such as porphyroclasts and porphyroblasts.Kurse et al., 2001;Stipp et al., 2002;Passchier & Trouw, 2005) 5. Results

Magnetite-Bearing Quartzites
Magnetite-bearing quartzites correspond to BIF.They show red colour, foliation and composed mainly of quartz and opaques in hand sample.Under the microscope (Figure 4a), BIF are mesocratic with a granoblastic texture, composed essentially of quartz and magnetites with accessories, muscovites and detrital zircons.Quartz (1-3 mm) is represented by hypidio-to idiomorphic coarse to stretched blasts.It is polycrystalline defining the SGAR fabric with undulose and patchy extinctions.They form quartzite layers corresponding to the composite S 1/2 foliation that derived from the transposition of S 1 in S 2 during D 2 .Quartz also forms the L 2 mineral and stretching lineations and contains zircon inclusions.Magnetites (0.5-2 mm) outline isolated or mineral aggregate blasts.They form discontinuous mafic layers equivalent to S 1/2 foliation too.Magnetites are stretched parallel to quartz band ribbons and form the L 2 stretching and mineral lineations whereas muscovite (1-2 mm) participates in the felsic layers.The magnetite-bearing quartzites define two mineral associations, (1) sedimentary paragenesis symbolized by the association quartz1 + magnetite1 + zircon and equivalent to the s 0/1 foliation and, (2) metamorphic paragenesis characterised by the association quartz2 + magnetite2 ± muscovite corresponding to the S 2 foliation.

Foliations
Two foliation types, S 0/1/2 in BIF and paragneisses and, S 2 in pre-to syntectonic granitoids are encountered within the NyC.The S 0 bedding is completely transposed to the S 0/1 foliation during first deformation phase D 1 .S 0/1 relict foliation is preserved in hinges of F 2 intrafolial rootless folds.It is a typical millimetric to centrimetic gneissic foliation, defined by the shape preferred orientation of biotite, muscovite, ribbon quartz and feldspar; ribbon quartz, magnetite and muscovite in BIF microscopically.Parageneses quite similar, attest to transposition of S 0/1 foliation is during the D 2 deformation phase to the composite S 0/1/2 foliation.The S 2 foliation is axial planar to folded veins concentrated at edges of mesoscopic boudins.It forms melanocratic and leucocratic layers in metagranodiorites, syntectonic clinopyroxene syenites.Microscopically, S 2 foliation is represented by quartz + plagioclase + amphibole + biotite ± microcline ± opaques paragenesis in metagranodiorites and, quartz + plagioclase + microcline + omphacite + actinolite + biotite ± opaques ± apatite ± titanite + ilmenite + magnetite + zircon in synkienatic syenites.Both S 0/1/2 and S 2 foliations are mainly oriented WSW-ENE to N-S, sub-parallel to magmatic and metamorphic layering limited by blastomylonitic S/C shear zones.Their average values correspond to northern and southern sides are oriented to 037 29 and 166 71 respectively.These sides defined F 3 large-scale fold (Figure 2a).

Lineations
L 2 includes stretching and mineral lineations.The L 2 stretching lineation developed during D 2 , is defined by elongate ribbon quartz and magnetites, quartzo-feldspathic aggregates, stretched and boudinaged feldspars.The L 2 mineral lineation is outlined by prismatic amphibole in synkinematic syenites.Both lineations are parallel, oriented WSW-ENE to WNW-ESE with an average value of 070 30.B 2 fold axes display similar plunges (Figure 2b).

Shear Zones
Shear zones are characterised by their fine grain sizes and anisotropy on the contrary of protoltihs and their S n foliation (Figures 2a, b).They constitute one of the main structural imprints of the late-tectonic D 3 phase and the cooling stage.They are grey to light grey show by alignment of dominant felsic and mafic minerals and secant or parallel to the foliation, oriented mainly ~E-W, NE-SW, NNE-SSW.They dissect protoliths as well as the S n foliation, F 3 meso-and large-scale folds, L 2 stretching and mineral lineations, B 2 boudins, feldspathic and quartzo-feldspathic lenses that characterised the D 2 .They define S-C structures and S 3 schistosities.
In magnetite-bearing quartzites, metagranodiorites and clinopyroxene syenites, shear zones show from outer to inner zones, drop of magmatic or metamorphic grain sizes, equivalent to the raise of crystal plastic deformation stages.In BIF and metagranodiorites, metamorphic quartz and feldspar represented by secondary fine grains size blasts, recrystallized dynamically, progressively until the primary or protolith minerals complete disappearance.
In clinopyroxene syenites, from outer to inner zone, quartz, feldspar and actinolite display increase of crystal plastic deformation stages.They define the recrystallized anisotropy of shear zone fabrics in which, old protolith magmatic minerals are progressively replaced by secondary fine grain size ones, until their complete disappearance.

Quartz
Quartz displays different dynamic recrystallization features in protoliths and shear zones from outer to inner zones (Figures 5, 6).In outer zones of metagranodiorites and BIF protoliths, quartz fabrics consist of metamorphic quartz2 blasts.They derived from the recrystallization of quartz1 clasts, relict D 1 and S 1 foliation into quartz2 during the D 2 in BIF (Figures 5a, b).These quartz2 (≥ 2 mm) are lengthened and hypidiomorphic blasts with undulose and patchy extinctions and GBM types.They define felsic layers corresponding to the S 0/1/2 foliation in BIF and S 2 in metagranodiorites.In syn-D 2 clinopyroxene syenites, quartz fabrics consist of magmatic quartz2 clasts in outer zones.They are syn-D 2 quartz2 (≤500m) types, hypidiomorphic ~parallel to the L 2 actinolite lineation and contribute in S 2 foliation.Above quartz2 types display the quartz3 BLG crystal plastic deformation, suggesting the existence of late-tectonic phase that occurred after the cooled stage under low-thermal condition (< 400 °C).
In inner zones, quartz fabrics are well developed quartz3 dynamic recrystallized blast types (Figures 6).
Magmatic relict quartz1 in metagranodiorites, metamorphic quartz2 blasts in metagranodiorites and BIF as well as quartz2 in syn-D 2 clinopyroxene syenites are recrystallized into quartz3 (≤200m) that ranges from GBM to GBAR types, suggesting their occurrence at middle-to higher-thermal conditions estimated between 500-850 °C.These quartz3 form the shear zone felsic layers that define S-C structures and in favourable cases, S 3 schistosities.8).Feldspar fabrics and dynamic recrystallization features are better expressed in synkinematic syenites than in metagranodiorites.They include magmatic relict phenoclasts and recrystallized blasts.Feldspar1 phenoclasts (≥ 2 mm) show cracked minerals with kinked twins that still represent D 1 and S 1 .
They are recrystallized in feldspar2 blasts (≤50 0m) forming felsic layers, equivalent to S 2 foliation.Feldspar2 is dynamically recrystallized in feldspar3 during D 3 .This crystal plastic deformation started by kink bands, then grew outer and along cracks and continue towards the centre of feldspar2 old clasts and blasts in feldspar3, until their complete disappearance (Figures 8e, f).Feldspar3 (≤200 m) is represented by kinked clasts, type-1 to type-2 SGR and GBM blasts bordered by less pronounced SGR shape anisotropy to SGAR (Figures 8g, h).Feldspar3 define with quartz3, felsic layers equivalent to S 3 schistosity and define S-C structures.These crystal plastic deformations suggest their recrystallization at higher thermal condition estimated between 500-850 °C.Brittle feldspar characterized D 4 and occurs under low-temperatures (<450 °C) conditions.

Amphibole
Actinolites provide several fabrics that described their deformations and specially, dynamic recrystallization features in synkinematic clinopyroxene syenite protoliths and shear zones (Figures 9).Actinolites define the S 2 foliation as well as the L 2 mineral lineation.Its S i internal schistosities describe various angles with the S e external schistosities equivalent to S 0/1/2 and S 2 foliations, respectively.These schistosities provide kinematic criterion of its rotations from D 2 to D 3 .In shear zones, actinolite2 dynamically recrystallized define fine SGR amphibole3 blasts displaying S-C structures.These amphibole3 suggest at least their emplacement under amphibolite-granulite facies (500-850 °C) conditions.Brittle amphiboles as well as feldspars provide the brittle and post-Eburnean/Transamazonian crystal plastic deformation features (Figure 9a).These amphibole brittle fabrics occurred under low-temperature (<450 °C) conditions, lower than in shear zones.8).Feldspar fabrics and dynamic recrystallization features are better expressed in synkinematic syenites than in metagranodiorites.They include magmatic relict phenoclasts and recrystallized blasts.Feldspar1 phenoclasts (≥ 2 mm) show cracked minerals with kinked twins that still represent D 1 and S 1 .
They are recrystallized in feldspar2 blasts (≤500m) forming felsic layers, equivalent to S 2 foliation.Feldspar2 is dynamically recrystallized in feldspar3 during D 3 .This crystal plastic deformation started by kink bands, then grew outer and along cracks and continue towards the centre of feldspar2 old clasts and blasts in feldspar3, until their complete disappearance (Figures 8e, f).Feldspar3 (≤200m) is represented by kinked clasts, type-1 to type-2 SGR and GBM blasts bordered by less pronounced SGR shape anisotropy to SGAR (Figures 8g, h).Feldspar3 define with quartz3, felsic layers equivalent to S 3 schistosity and define S-C structures.These crystal plastic deformations suggest their recrystallization at higher thermal condition estimated between 500-850 °C.Brittle feldspar characterized D 4 and occurs under low-temperatures (<450 °C) conditions.

Amphibole
Actinolites provide several fabrics that described their deformations and specially, dynamic recrystallization features in synkinematic clinopyroxene syenite protoliths and shear zones (Figures 9).Actinolites define the S 2 foliation as well as the L 2 mineral lineation.Its S i internal schistosities describe various angles with the S e external schistosities equivalent to S 0/1/2 and S 2 foliations, respectively.These schistosities provide kinematic criterion of its rotations from D 2 to D 3 .In shear zones, actinolite2 dynamically recrystallized define fine SGR amphibole3 blasts displaying S-C structures.These amphibole3 suggest at least their emplacement under amphibolite-granulite facies (500-850 °C) conditions.Brittle amphiboles as well as feldspars provide the brittle and post-Eburnean/Transamazonian crystal plastic deformation features (Figure 9a).These amphibole brittle fabrics occurred under low-temperature (<450° C) conditions, lower than in shear zones.

Discussion
Magnetite-bearing quartzite, metagranodiorite and synkinematic clinopyroxene syenite petrographic studies from the Kama and Akongo group reveal in the NyC, the coexistence of the magmatic, metamorphic and retromorphic fabrics, respectively (Feybesse et al., 1998;Owona, 2008;Owona et al., 2011;Penaye et al., 2004).Granoblastic textures in BIF and metagranodiorites provide evidence of reactivation related to Eburnean/Transamazonian orogeny (Feybesse et al., 1998;Owona et al., 2011;Penaye et al., 2004).Protolith, metamorphic and retromorphic parageneses were identified in BIF and metagranodiorites, associated to their emplacement, deformation and uplift.Magmatic clasts and blasts such as quartz with undulose and patchy extinction, magnetites and detrital zircons in BIF; quartz with undulose and patchy extinction, perthitic plagioclase and antiperthitic microcline with kinked twins and biotite, opaques, apatite and zircon in metagranodiorites represent the D 1 deformation phase.D 1 can be interpreted as the sediment deposit as well as earliest granitoids emplacement phase that may range between ~2515 Ma in Gabon (U/Pb-Zr, Bassot et al., 1987) and 2100 Ma in Cameroon (U/Pb-Zr, Penaye et al., 1993;Sm/Nd, Toteu et al., 1994).These relict clasts form the S 1 foliation and the first mineral assemblages in both rock types.
D 3 is characterized by crystal plastic deformations varying in term of features, intensity and thermal conditions, in protoliths and from outer to inner shear zones.BIF, metagranodiorite and syn-D 2 clinopyroxene syenite protoliths show essentially BLG of quartz, brittle feldspar and amphibole.These dynamic recrystallization features attest to low and late-tectonic evidence under low-thermal conditions (<350 °C) occurring after the cool period.This low thermal character of late-tectonic phase, confined in shear zones justifies lack of an overall S 3 schistosity.

Figure 8 .Figure 9 .
Figure 8. Feldspar SGR, GBM and SGAR dynamic recrystallization features from outer to inner shear zones.(a-d) Relict feldspar2 surrounded by feldspar3 SGR-GBM, S-C to C-S structures in outer zone.(e, f) Relict feldspar2 surrounded by feldspar3 SGR-GBM and S-C structure in middle zone.(g, h) Late relict feldspar2 to complete recrystallization of feldspar3 GBM coexisting with quartz SGAR, S-C structure and S3 schistosity.Major recrystallization of SGR feldspar3 blasts along the X axis, indicates the σ3 orientations, the consequent anisotropies, and S2-C3 relationships.Note the recrystallization feature evolutions in agreement with increasing temperature from outer to inner shear zones BLG, SGR, GBM and SGAR thermal conditions as in Figure 8