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International audience; The Dialé-Daléma metasedimentary series is exposed in the Kédougou-Kéniéba Inlier that corresponds to the northwestern branch of the Eburnean orogenic belt in the southern West African Craton. Here we conducted a U–Pb geochronological study on metapelites, metagraywackes and metavolcanic breccia of the Dialé-Daléma metasedimentary series in order to identify the sedimentary sources and establish the lithostratigraphic sequence of the Kedougou-Kenieba Inlier. This new U–Pb geochronological dataset from five samples representing different stratigraphic levels yield a dominant population of Paleoproterozoic detrital zircon grains with ages ranging from c. 2200 to 2100 Ma. The youngest weighted mean ages at c. 2120 Ma are identical for all five samples within error and provide a maximum deposition age for the sediments of the Dialé-Daléma series. The dominant ages are similar to those obtained on metamorphosed plutonic and volcanic rocks of the Mako belt and thus suggest a dominant proximal source for the clastic sediments of the Dialé-Daléma series and a distal or geographically isolated position relative to the Archean Leo-Man or Réguibat craton nuclei. Deposition ages are only a few Myr older than available ages for metamorphism and intrusion of plutons forming the Saraya batholith. This is consistent with deposition along an active convergence zone marked by the succession, within a few tens of Myr, of (1) magmatic accretion of the Mako plutonic and volcanic rocks in the context of volcanic arc, associated with local uplift, exhumation and erosion at c. 2200-2160 Ma, (2) deposition of clastic sediments forming the Dialé-Daléma series at c. 2120-2110 Ma followed by (3) their burial and exhumation at c. 2090–2060 partially contemporaneous with (4) the intrusion of the Saraya batholith at c. 2080–2070 Ma.

International audience; This paper presents new petrological, geochemical, isotopic (Nd) and geochronological data on magmatic rocks from the poorly known southern Ouaddaï massif, located at the southern edge of the socalled Saharan metacraton. This area is made of greenschist to amphibolite facies metasediments intruded by large pre-to syn-tectonic batholiths of leucogranites and an association of monzonite, granodiorite and biotite granite forming a late tectonic high-K calc-alkaline suite. U-Pb zircon dating yields ages of 635 ± 3 Ma and 613 ± 8 Ma on a peraluminous biotite-leucogranite (containing numerous inherited Archean and Paleoproterozoic zircon cores) and a muscovite-leucogranite, respectively. Geochemical fingerprints are very similar to some evolved Himalayan leucogranites suggesting their parental magmas were formed after muscovite and biotite dehydration melting of metasedimentary rocks. A biotite-granite sample belonging to the late tectonic high-K to shoshonitic suite contains zircon rims that yield an age of 540 ± 5 Ma with concordant inherited cores crystallized around 1050 Ma. Given the high-Mg# (59) andesitic composition of the intermediate pyroxene-monzonite, the very similar trace-element signature between the different rock types and the unradiogenic isotopic signature for Nd, the late-kinematic high-K to shoshonitic rocks formed after melting of the enriched mantle and further differentiation in the crust. These data indicate that the southern Ouaddaï was part of the Pan-African belt. It is proposed that it represents a continental back-arc basin characterized by a high-geothermal gradient during Early Ediacaran leading to anatexis of middle to lower crustal levels. After tectonic inversion during the main Pan-African phase, late kinematic high-K to shoshonitic plutons emplaced during the final post-collisional stage.

Greenstone belts preserved in the West African Craton (2300-2070 Ma) provide key information for understanding the petrogenetic processes and geodynamic setting for juvenile crust formation during a nascent Paleoproterozoic plate tectonic regime. New geochemical and isotopic data are presented for the magmatic suites exposed in Sefwi Greenstone Belt, SW Ghana. We define five distinct plutonic suites, with extrusive equivalents, including: 1) two generations of high SiO2, Na2O-rich tonalite-trondhjemite-granodiorite (TTG) granitoids; 2) calc-alkaline, LILE-enriched diorites; 3) tholeiitic pyroxenite, gabbro and layered mafic intrusives; 4) high-K quartz monzonites; and, 5) incompatible element-rich, two-mica granites and muscovite leucogranites. Trace element patterns of most magmatic suites display negative Nb-Ta, P and Ti anomalies and weakly positive Pb anomalies, suggesting magma genesis in a subduction setting with minor crustal contamination/assimilation. Complementary zircon U-Pb and Lu-Hf data indicate contemporaneous emplacement of diorites (epsilon Hf-(t) = + 2.5 to + 5.4) and low-HREE TTGs (epsilon Hf-(t) = + 4.3 to + 7.6) between ca. 2159 and 2153 Ma, derived from contrasting sources, namely the partial melting of the metasomatised mantle wedge and high-pressure partial melting of low-K mafic sources, respectively. High-HREE TTGS are attributed to the shallow, partial melting of mafic material. The emplacement of high-K quartz monzonites (epsilon Hf-(t) = +2.5 to +5.2) at ca. 2135 Ma reveals mixing of mantle-derived magmas and remelting of existing TTGs in the crustal pile. Consistently positive epsilon Hf-(t) values indicate predominantly radiogenic, though heterogeneous, magma sources during crustal growth, derived from precursory material extracted from the depleted mantle and an older crustal component with a minimum age of 2650 Ma. Magmatogenesis of two-mica granites and leucogranites (ca. 2092 and 2081 Ma) is interpreted to be the product of crustal anatexis during an oceanic arc-arc collisional event, with a proposed suture parallel to the NW margin of the Sefwi Belt. We hypothesise that the formation and accretion of the West African Craton reflects the rapid amalgamation of individual oceanic arc terranes, providing potential insight into the transitional plate tectonic regime that may have characterised the Earth between 3000 and 2000 Ma.

New constraints on the architecture, deformation sequence and tectonic evolution of the sparsely exposed Palaeoproterozoic Sefwi Greenstone Belt (ca. 2200-2070 Ma) in SW Ghana are derived from the integration of field observations, structural mapping and interpretation of regional airborne geophysical data. Our findings provide new insights into the tectonic evolution of the West African Craton during the Palaeoproterozoic Eburnean Orogeny (ca. 2150-2070 Ma). Domains of contrasting metamorphic grade comprising high-grade paragneisses and meta-volcanic rocks are tectonically juxtaposed against low-grade volcano-sedimentary packages. The earliest deformation event is characterised by a bedding-parallel high-grade metamorphic foliation (S-1) that is axial planar to isoclinal folds formed during NNW-SSE shortening and late-D-1 partial melting. This crustal thickening event generated biotite +/- muscovite granites emplaced parallel to the NW margin of the Sefwi Belt between ca. 2092 and 2081 Ma, providing a minimum age for D-1. D-2 deformation is characterised by a moderate to steeply dipping, ENE-WSW to NE-SW striking, penetrative S-2 mineral foliation. Within the Sefwi Belt, coaxial F-2 fold axes and L-2 stretching lineations pitch shallowly ENE-WSW to NE-SW, oblique to the major shear zones, indicative of ENE-WSW transtension and associated constrictional deformation. Differential exhumation of middle and lower crustal segments along NNE-striking D-2 extensional detachments was coincident with sinistral oblique reactivation of NE-SW regional-scale shear zones. The timing of exhumation is constrained by metamorphic monazite U-Pb ages at ca. 2073 Ma. Subsequent E-W shortening is associated with dextral reactivation of regional NE-SW striking shear zones and refolding of earlier structures away from the major shear zones. We propose that the NW margin of the Sefwi Belt represents the collision (D-1) of southern Ghana and central and NW Ghana/Ivory Coast which record different tectonic histories. Furthermore, we suggest the amalgamation of the West African Craton is the product of episodic collisional orogenesis, providing new insights into the evolution of orogenic processes during the Palaeoproterozoic.

Batholith emplacements within a continental margin may bear witness of a magmatic input lasting for several million years. Consequently, the geochemical signatures of such sections are complex, and their understanding in terms of petrological processes, is crucial. The Arequipa section of the Coastal Batholith of Southern Peru was discontinuously constructed during several periods of magmatic activity, from the Jurassic to the Paleocene (200–175 Ma, and 90–60 Ma). Thermobarometric data on amphiboles indicates two main levels of emplacement at the batholith scale, the deepest between 5 and 7 km in depth and the second around 3.5 km. The present day outcropping of these different units at the same elevation argue for a large vertical movement along the Lluclla Fault System between 76 and 68 Ma. Both major/trace element contents and Nd-Sr isotopes show a large variability that is not random. The data dispersion is consistent with a two-staged evolutionary model of the magmatic arc, inspired by the MASH model: (i) an early stage dominated by hybridization and fractional crystallization processes, (ii) a late stage in which magmas were homogenized and mainly evolved by fractional crystallization. The change from one stage to another is controlled by the thermal state of the crustal arc section, especially the Deep Crustal Hot Zone.

The breakup of supercontinents is accompanied by the emplacement of continental flood basalts and dike swarms, the origin of which is often attributed to mantle plumes. However, convection modeling has showed that the formation of supercontinents result in the warming of the sub-continental asthenospheric mantle (SCAM), which could also explain syn-breakup volcanism. Temperature variations during the formation then breakup of supercontinents are therefore fundamental to understand volcanism related to supercontinent cycles. Magmatic minerals record the thermal state of their magmatic sources. Here we present a data mining analysis on the first global compilation of chemical information on magmatic rocks and minerals formed over the past 600 million years: a time period spanning the aggregation and breakup of Pangea, the last supercontinent. We show that following a period of increasingly hotter Mgrich magmatism with dominant tholeiitic affinity during the aggregation of Pangea, lower-temperature minerals crystallized within Mg-poorer magma with a dominant calc-alkaline affinity during Pangea disassembly. These trends reflect temporal changes in global mantle climate and global plate tectonics in response to continental masses assembly and dispersal. We also show that the final amalgamation of Pangea at similar to 300 Myr led to a long period of lithospheric collapse and cooling until the major step of Pangea disassembly started at similar to 125 Myr. The geological control on the geosphere magma budget has implications on the oxidation state and temperature of the Earth's outer envelopes in the Phanerozoic and may have exerted indirect influence on the evolution of climate and life on Earth.

The Cordillera Blanca batholith (12–5 Myr) forms the highest Peruvian summits and builds the footwall of the Cordillera Blanca normal fault (CBNF). Even if several models have been proposed, the processes driving both the exhumation of the Cordillera Blanca and extensional deformation along the CBNF are still debated. Here we quantify the emplacement depth and exhumation of the batholith of the northern Peru arc from the late Miocene to present. Based on a compilation of crystallization ages and new thermobarometry data in the Cordillera Blanca batholith, we propose that the batholith was emplaced at a depth of ~3 km in successive sills from 14 to 5 Ma. By contrast, the younger rocks exposed at the surface were emplaced the deepest (i.e., ~6 km) and are located close to the CBNF, suggesting post 5 Ma tilting. Furthermore, a formal inversion of the thermochronologic data indicates an increase of the exhumation rates in the Cordillera Blanca during the Quaternary. The higher predicted exhumation rates correlate with areas of high relief, both in the northern and central part of the Cordillera Blanca, suggesting that Quaternary valley carving by glaciations have a significant impact on the latest stage of the Cordillera Blanca exhumation (2–0 Ma).

High-strain zones bound and separate the high-grade tectono-metamorphic terranes from low-grade greenstone belts in the North-Western parts of Ghana. These belts are bounded by granitoid domains characterized by two main episodic pulses of magmatic intrusive events, which occurred between 2213 Ma and 2086 Ma. High-strain zones are thought to play a significant role in creating fluid pathways, particularly for partially molten material from lower crustal sources to the upper crust. In this study, a three-dimensional thermo-mechanical model has been used to explore the evolution of high-strain zones and relief under compressional and simple shear boundary conditions. Different orientations of a system of branched strike-slip faults were tested. The effects of the frictional angle and density contrast on the evolution of relief were also compared in this study. The resulting model indicates domains of tensile vs. compressional strain as well as shear zones. This shows that the internal fault zones as well as the host rock in between the faults behave as relatively weaker domains than the external regions. Under both applied compressive and simple shear boundary conditions, these weakened domains constitute preferential zones of tensile and shear strain accommodations in the upper crust, which may favour infilling by deeper partially molten rocks. This processes is suggested by the authors as the most likely processes to have occurred in pre-existing branched shear zones systems in North-Western Ghana during the Eburnean orogeny (around 2.20-2.10 Ga). The orientations of faults in these models play an important role in controlling the evolution of relief and localized deformation. In particular, greatest the largest relief is obtained when faults dip parallel to each other and when they are inclined at depth, as they thus facilitate strain rotation and material transfer from depth. The host rock density does not play a primary role in producing relief compared to variations in friction angle at crustal scale of our model. Relief increases by 200-300 m when the host rock density is increased by 200 kg/m(3), whereas relief reduces by about 1200 m when decreasing the host rock friction from phi =20 degrees to 10 degrees. This study suggests a model for interpreting the evolution and locus of exhumation of partially molten rocks in North-Western Ghana.

International audience; Constraining the way in which continental deformation is accommodated in time and space is essential to reconcile past plate movements with geological observations. Kinematic reconstructions of the Iberia-Europe plate boundary are still debated. Here we focus on an inverted Mesozoic rift basin, the Cameros basin, which is part of the Iberian chain. We use a combination of detrital low-temperature thermochronological techniques to define the time-temperature evolution of the basin from Mesozoic rifting to Cenozoic collision. Zircon fission-track analyses of Oligocene-Miocene sedimentary rocks yield two main age populations at 17010 and 10010Ma, reflecting (i) an Early Jurassic thermal event related to the Atlantic-Alpine Tethys opening and (ii) an Albo-Cenomanian thermal event related to the Bay of Biscay opening. Thermal modeling of combined zircon fission-track, apatite fission-track, and apatite (U-Th-Sm)/He data reveals that collision-related cooling of the Cameros basin started during the Paleocene (60Ma). A second cooling/exhumation phase of the basin is recorded from 35 to 25Ma. Initial cooling occurred after a protracted postrift period characterized by persistence of high geothermal gradients, a feature also recognized in the Pyrenees. Our results show that the Iberian chain shared the same Early to Late Jurassic tectonothermal evolution with the Atlantic-Alpine Tethyan rifted margins. From the Albian onward, the thermal evolution of the Cameros basin was very similar to that of the Pyrenees. This study shows that the preservation in mountain ranges of a succession of rifting events provide important clues for plate reconstructions.

High-grade (amphibolite-granulite facies) tectono-metamorphic domains in the Sefwi terrane of Ghana are separated from adjacent lower-grade (greenschist facies) greenstone belts by two main shear zones. The high-grade rocks presumably exhumed along the sinistral shear zones during the D2 ENE-WSW transtension (similar to 2073 Ma). To better understand the role boundary conditions and the spatial relationship of faults play in the exhumation of partially molten lower crust in the Sefwi terrane, ten 3D thermomechanical models have been constructed. The results show that the normal component of velocity boundary conditions mainly controls the exhumation (8-10 km) of the lower crust along pre-existing faults, while the exhumation in the relay zones between faults is controlled by the obliquity between the applied extensional velocity vector and the vertical wall on which it is applied. The strike of the exhumation belt made of partially molten lower crust rocks in the relay zone is sub-orthogonal to the horizontal maximum stretching axis. The isostatic compensation from low-density upper mantle to overlying crust (thinning) is higher under transtension than under extension. The lower crust exhumation influenced by inherited shear zones (ductile) can be used to better understand the loci of the high-grade rocks in the Sefwi terrane. We suggest that the Kukuom-Juaboso domain composed of amphibolite-migmatite facies rocks probably resulted from the concentration of partially molten rocks in the relay zone between the Ketesso and Kenyase shear zones during the D2 ENE-WSW transtension. The two shear zones probably underwent two main stages for growth and maturation from the D1 to D2 phases. The regional exhumation of the high-grade rocks in the Sefwi terrane probably occurred within < 5 Ma.

High-grade tectono-metamorphic domains in the Sefwi terrane of Ghana are separated from adjacent low-grade greenstone belts by two main shear zones. The high-grade rocks are thought to have been exhumed along sinistral shear zones during the D2 ENE-WSW transtension. To better understand the role boundary conditions and the spatial relationship of faults play in the re-distribution of partially molten lower crust, ten 3D thermo-mechanical models have been constructed. The results show that the normal component of velocity boundary conditions mainly controls the exhumation of the lower crust which occurred along pre-existing faults, while the exhumation in the relay zones between faults is controlled by the ratio of extension rate to shear rate applied at the boundaries. The strike of the exhumation belt made of partially molten lower crust rocks in the relay zone is sub-orthogonal to the transtension direction. The isostatic compensation from low-density upper mantle to overlying crust (thinning) is higher under transtension than under extension. The lower crust exhumation influenced by inherited shear zones can be used to better understand the loci of the high-grade rocks in the Sefwi terrane. We suggest that the Kukuom-Juaboso domain composed of up to amphibolite-migmatite facies probably resulted from the concentration of partially molten rocks in the relay zone between the Ketesso and Kenyase shear zones during the D2 ENE-WSW transtension. The two shear zones probably underwent two main stages for growth and maturation from the D1 to D2 deformation phases. The regional exhumation of the high-grade rocks in the Sefwi terrane probably occurred within a duration of less than 5Ma.

Mantle plume fixity has long been a cornerstone assumption to reconstruct past tectonic plate motions. However, precise geochronological and paleomagnetic data along Pacific continuous hotspot tracks have revealed substantial drift of the Hawaiian plume. The question remains for evidence of drift for other mantle plumes. Here, we use plume-derived basalts from the Mid-Atlantic ridge to confirm that the upper-mantle thermal anomaly associated with the Azores plume is asymmetric, spreading over ~2,000 km southwards and ~600 km northwards. Using for the first time a 3D-spherical mantle convection where plumes, ridges and plates interact in a fully dynamic way, we suggest that the extent, shape and asymmetry of this anomaly is a consequence of the Azores plume moving northwards by 1–2 cm/yr during the past 85 Ma, independently from other Atlantic plumes. Our findings suggest redefining the Azores hotspot track and open the way for identifying how plumes drift within the mantle., Tectonic plate motions are often reconstructed based on the assumption that mantle plumes are fixed within the mantle. Here, the authors provide geochemical and geodynamic evidence to suggest that the asymmetry of the Azores thermal anomaly can be explained by northward motion of the Azores plume.

Greenstone belts in the West African Craton (WAC) are separated by several generations of granitoids intruded at ca. 2.18-1.98 Ga. Simultaneous folding and exhumation play an important role in the formation of greenstone-granitoid belts. However, the overall tectonic regime and origin of granitoids remain controversial. In this study, we present the estimates of the mantle potential temperature (Tp) for the WAC, which yields values of about 1500-1600 degrees C, pressure estimates of initial and final melting yield values of about 3.7-5.2 GPa and 1-1.3 GPa, respectively. Subsequently, 2D thermo-mechanical models have been constructed to explore the width of volcano-sedimentary basin on spatial-temporal evolution of diapirs that emplaced in the lower-middle crust during compression. The models show that the width of the volcano-sediment layer plays an important role in the formation mechanisms of greenstone-granitoid belts. The lower crust beneath sedimentary sequences is deformed into a buckle fold during the first compressional stage, through which relief uplifts slowly. Subsequently, the buckle fold is further deformed into several individual folds. Diapirs made of lower crust rocks ascend and emplace in the middle-upper crust resulting from instability. Benefitting from the mantle temperature, the pressure estimates and the numerical modelling results, a new geodynamic model was constructed. This model indicates that a series of sheet-like granitoids possibly derived from either subducted melanges, lower crust and/or mantle melting that are accumulated at depths of the subcontinental mantle would channel along diapirs before feeding the upper crust. When the granitoids arrive at the solidified lids of the diapirs, they would favour migrating horizontally and intrude into the upper crust through weakening zones between the diapirs. Our geodynamic model also suggests an asymmetry of structures between the upper and middle-lower crust, with the dome-like granitoids overlying high-grade sedimentary synforms and high-grade diapirs underlying low-grade greenstone belts.

The Marbat basement (Sultanate of Oman) belongs to the Neoproterozoic accretion domain of the Arabian-Nubian shield. We present new geochronological, petrological and geochemical data as an extension of our previous study (Denele et al., 2017) re-interpreting this basement as a metamorphic core complex (MCC). We showed that this MCC consists of a metamorphic unit (Juffa complex) separated by an extensional detachment from a plutonic unit (Sadh complex and Tonalite plutons). Geochemical data show that the Juffa metasediments correspond to volcanogenic graywackes, suggesting deposition in front of a juvenile magmatic arc. New in situ U-Pb zircon data show that the protolith of the Juffa paragneisses is Tonian in age (960-830 Ma). The Juffa complex recrystallized under amphibolite facies conditions (950 MPa, 630 degrees C) corresponding to thermal gradient of 17-18 degrees C/km, i.e. close to that observed in fore-arc environment subjected to tectonic underplating. It was then retrogressed (< 400 MPa, < 400 degrees C) during exhumation of the MCC. Phase assemblages of the Sadh complex and Tonalite plutons record magmatic temperatures estimated at 700-840 degrees C and 610-840 degrees C from hornblende and zircon-saturation thermometry, respectively; pressures are grossly estimated at ca. 250 MPa. These data and structural evidence led us to consider that the Sadh complex is a plutonic unit emplaced during exhumation of the MCC. Hence, we re-interpret the previously published zircon core U-Pb ages of the Banded gneisses (ca. 860-830 Ma) as inherited, and zircon rim ages (ca. 815 Ma) as the age of their intrusion. This suggests that the parent magma of all these granitoids could have interacted with volcanic material equivalent to the Juffa complex. Overall, the Marbat area developed during the Tonian and involved (i) deposition of volcanosediments (Juffa complex) in an island-arc environment (960-830 Ma); (ii) metamorphism under middle pressure conditions during tectonic underplating at ca. 820-815 Ma, followed by emplacement of plutonic rocks in the upper crust during initiation of the MCC at ca. 815-810 Ma; and (iii) late-tectonic intrusions associated with MCC amplification at ca. 800-790 Ma (Mahall intrusives and Tonalite plutons).

Volcanic units and their crystals carry a memory of hidden magma reservoirs. Here, through a statistical assessment of a worldwide compilation of volcanic bulk-rock and associated mineral data, we can quantify the proportion of crystals that appear in disequilibrium with their bulk-rock composition, over a large range of temperature conditions and magma types. Minerals in these erupted units are frequently in disequilibrium with their bulk-rock chemistries, particularly in crystal-rich material of intermediate composition (similar to dacites, with up to 60%-80% disequilibrium). The disequilibrium crystals can be grouped in two categories: (1) dominantly more evolved than their hosts (in mafic magmas), or (2) dominantly less evolved than their hosts (in intermediate and silicic magmas). We suggest that (1) crystal accumulation, and (2) mineral entrainment from preexisting shallower mush zones, together explain the presence of more evolved crystals in mafic systems, while mingling/mixing between resident intermediate to silicic magmas and more mafic recharges in upper crustal mush zones accounts for dominantly less-evolved character of minerals in intermediate to silicic volcanic rocks. The transition between the two domains occurs in the mid-crust, at intermediate composition and temperature (similar to 1000-1100 degrees C), coincident with the SiO 2 increase and FeO tot/density decrease seen in evolving magmas. This statistical information supports other petrological and geophysical evidence indicating dominantly mafic lower crustal mush zones, producing intermediate magmas that then ascend to shallower levels (building a granodioritic upper crust) and sometimes differentiate further to rhyolite/granite/pegmatite.

We investigate the development of a Neoproterozoic metamorphic core complex (MCC) in the Marbat area (southern Arabian Peninsula). The MCC is a gneiss dome emplaced in a releasing bend along a dextral strike-slip fault system. Dome emplacement-related extension was at a high angle to the long axis of the dome and 65 degrees from the strike-slip fault system. Shortening is recorded in the upper plate of the MCC in a direction normal to the extension direction. Extensional emplacement of the gneiss dome accommodated lower crust exhumation and contributed to the formation of a ductile strike-slip releasing bend from an initial fault step. The Marbat example allows defining transversal stretching-dominated strike-slip MCCs, which would represent the early development stages of longitudinal stretching-dominated strike-slip MCCs. Conversely, longitudinal stretching-dominated strike-slip MCCs produced at low finite extension would result from strike-slip-dominated transtension. This study bears important implications for lower crust exhumation under pulla-part basin systems in obliquely convergent or divergent settings.

The Kiaka gold deposit is a major resource in West Africa, with measured and indicated resources of 124 Mt at 1.09 g/t Au (3.9 Moz) and inferred resources of 27 Mt at 0.83 g/t Au (0.8 Moz). Located within the Manga-Fada N'Gourma greenstone and plutonic belt in south of the Burkina Faso, the deposit is hosted by a metamorphosed volcano-sedimentary sequence of lithic-, quartz-biotite metagreywackes, aluminosilicate-bearing metapelites and garnet-orthopyroxene-bearing schists and volcanic units. Structural observations indicate four local deformation events: DK 1 , DK 2 and DK 3 and DK 4 . Respectively, these events are linked to regional D 1 E-W compression, D 2 NW-SE compression and lastly, D 3 - and D 4 -related reactivations along D 2 shear zones. The S 2 foliation and D 2 shear zones are developed during lower amphibolite facies metamorphism whereas retrogression occurs during D 3-4 reactivations along these shear zones at upper greenschist facies conditions. The emplacement of a dioritic intrusion, dated at 2140 ± 7 Ma (Concordia U-Pb age on magmatic zircon), is interpreted to be contemporaneous with sinistral displacement along mineralized, NE-trending D 2 shear zones. The intersection of these shears zones and the Markoye shear zone (dextral-reverse D 1 and sinistral-reverse D 2 reactivations) controlled the final geometry of the host rocks and the ore zones. Four subparallel elongated ore bodies are mainly hosted within D 2 -related shear zones and some are developed in an apparent axial plane of a F 2 isoclinal fold. Detailed petrographic studies have identified two main types of hydrothermal alteration associated with two stages of gold mineralization. The stage (1) corresponds to replacement zones with biotite and clinozoisite during the D 2 event associated with pyrrhotite ± pyrite, chalcopyrite (disseminated gold stage). The stage (2) occurs during reactivations of the D 2 -related auriferous shear zones (vein stage) and is characterized by diopside ± actinolite D 3 veins and veinlets and D 4 pervasive muscovite, ± chlorite, ± calcite in quartz-carbonate vein selvages and associated with pyrrhotite + arsenopyrite ± electrum, ± native gold and tellurobismuthite. The latter stage (2) could be divided into two sub-stages based on mineralogy and crosscutting relationship. A weighted average Re-Os pyrrhotite age at 2157 ± 24 Ma (Re-Os age based on 3 replicates) constraints the timing of the disseminated gold stage and represents the first absolute age for gold mineralization in the Manga Fada N'Gourma area. The timing of gold at Kiaka may be also coeval with one of the two lode gold event at ∼ ca. 2.16–2.15 Ga and occurred concomitant with tectono-thermal activity during Eo-Eburnean orogeny. The study of the Kiaka gold deposit emphasizes the importance of a multi-scale and multidisciplinary approach (field observations, petrography geothermobarometry and geochronology) to decipher the polyphase character of some Paleoproterozoic gold deposits.

International audience; Including past production, current indicated and inferred resources, Wassa is a 5 Moz poly-deformed early-orogenic gold deposit located on the eastern flank of the Ashanti Belt, in southwest Ghana. It is hosted by metamorphosed volcanic, intrusive and sedimentary rocks of the Sefwi Group (ca. 2260–2160 Ma). Early mineralization has an Eoeburnean age (2164 ± 22 Ma, Re–Os on pyrite) and is characterized by quartz veins, by a carbonate alteration of the host rocks, and by deformed gold-bearing pyrite. Remobilization of this gold occurred during the late stages of the Eburnean Orogeny (~ 2.1 Ga) and is associated with quartz-carbonate veins with visible gold and euhedral pyrites.

New petrological and geochronological data are presented on high-grade ortho- and paragneisses from northwestern Ghana, forming part of the Paleoproterozoic (2.25-2.00Ga) West African Craton. The study area is located in the interference zone between N-S and NE-SW-trending craton-scale shear zones, formed during the Eburnean orogeny (2.15-2.00Ga). High-grade metamorphic domains are separated from low-grade greenstone belts by high-strain zones, including early thrusts, extensional detachments and late-stage strike-slip shear zones. Paragneisses sporadically preserve high-pressure, low-temperature (HP-LT) relicts, formed at the transition between the blueschist facies and the epidote-amphibolite sub-facies (10.0-14.0kbar, 520-600 degrees C), and represent a low (similar to 15 degrees Ckm(-1)) apparent geothermal gradient. Migmatites record metamorphic conditions at the amphibolite-granulite facies transition. They reveal a clockwise pressure-temperature-time (P-T-t) path characterized by melting at pressures over 10.0kbar, followed by decompression and heating to peak temperatures of 750 degrees C at 5.0-8.0kbar, which fit a 30 degrees Ckm(-1) apparent geotherm. A regional amphibolite facies metamorphic overprint is recorded by rocks that followed a clockwise P-T-t path, characterized by peak metamorphic conditions of 7.0-10.0kbar at 550-680 degrees C, which match a 20-25 degrees Ckm(-1) apparent geotherm. These P-T conditions were reached after prograde burial and heating for some rock units, and after decompression and heating for others. The timing of anatexis and of the amphibolite facies metamorphic overprint is constrained by in-situ U-Pb dating of monazite crystallization at 2138 +/- 7 and 2130 +/- 7Ma respectively. The new data set challenges the interpretation that metamorphic breaks in the West African Craton are due to diachronous Birimian basins' overlying a gneissic basement. It suggests that the lower crust was exhumed along reverse, normal and transcurrent shear zones and juxtaposed against shallow crustal slices during the Eburnean orogeny. The craton in NW Ghana is made of distinct fragments with contrasting tectono-metamorphic histories. The range of metamorphic conditions and the sharp lateral metamorphic gradients are inconsistent with hot orogeny' models proposed for many Precambrian provinces. These findings shed new light on the geodynamic setting of craton assembly and stabilization in the Paleoproterozoic. It is suggested that the metamorphic record of the West African Craton is characteristic of Paleoproterozoic plate tectonics and illustrates a transition between Archean and Phanerozoic orogens.

The aim of this paper is to explore the link between the metamorphic record of a Palaeoproterozoic orogeny and its thermo-mechanical evolution, in order to discuss the burial and exhumation processes of moderate- to high-pressure volcano-sedimentary rocks (6-8 to >10 kbar) in the Birimian Province (2.2-2.0 Ga) of the West African Craton. Metamorphic data collected in Burkina Faso, southwest Ghana and eastern Senegal suggests a Palaeoproterozoic Birimian crust dominated by moderate apparent geothermal gradients of 20-30 degrees C/km (M2a), which fit a purely conductive thermal crustal regime. This produced greenschist- to amphibolite-facies metamorphic assemblages that developed during the Eburnean orogeny (similar to 2.1 Ga), associated with regional shortening and granitoid intrusions. The M2a gradient is superimposed on an early thermal regime (M1: 6-8 kbar). At around 25 km depth, TTGs reached appropriate PT conditions to start melting. The buoyant and low-viscosity partially molten material, comprising a fraction of CaO-poor melt and residual material, then ascended through the overlying sediments. It entrained upwards lower crustal material as well as the surrounding sediments. This scenario explains the important breaks in metamorphic conditions observed in the Birimian province between the thin slivers ( 8 kbar) preserved in the thermal aureoles of granites and the adjacent synforms of greenstones, which consist of low-grade metasediments (P < 6 kbar; T < 450 degrees C). This scenario is in agreement with a significant recycling of the TTGs in the genesis of CaO-poor granitoid melts. We discuss the implications of our results for the tectono-metamorphic and mechanical evolution of Palaeoproterozoic orogenies.

The basement of Madagascar has been submitted to a strong reworking during a late Pan-African (D2) shearing event. The influence of the relevant fluid transfer on phase relationships is investigated on a calc-silicate gneiss belonging to the Ikalamavony domain, deformed and retrogressed, from low-temperature amphibolite to upper-greenschist metamorphic conditions. The rock is characterized by different generations of pyroxene growing with epidote as coronitic structures around relics of garnet and magnetite. Phase diagram sections are commonly used to study phase relationships as a function of pressure and temperature in closed-systems. Here, the thermodynamic system has been considered as only open for the fluid components (i.e. H 2 O, CO 2 and O 2 ) in order to discuss the interaction between fluid flow and mineral phase evolution occurring during D2 deformation. Prediction in terms of phase composition and modal proportion of P – T – M H2O–CO2–O2 phase diagrams (using Perple-X calculations) are in good agreement with natural observations. They suggest that a large input of aqueous-carbonic fluid throughout the “paragneiss” during the exhumation of the rocks led to the destabilisation of the early Fe 3+ -rich garnet (andradite) and magnetite. Thermodynamic modelling illustrates how the replacement of such mineral assemblage by Fe 2+ -rich pyroxene (diopside), sphene, epidote and plagioclase (anorthite-rich) then amphibole (actinolite) and K-feldspar has preferentially been controlled by the amount of H 2 O and CO 2 brought through minor shear zones during the D2 reworking, rather than by change of the effective bulk rock composition.

International audience; The timing of onset of modern-style plate tectonics is debated. The apparent lack of blueschist metamorphism1--a key indicator of modern plate subduction2--in rocks aged more than about 1 billion years calls into question the existence of plate tectonics during the Archaean and Palaeoproterozoic eras3, 4. Instead, plate tectonics and subduction could have either not occurred at that time5, or could have proceeded differently6 owing to warmer conditions in the early Earth mantle7. Here we use thermodynamic models8, 9, 10 to investigate the formation conditions of metamorphic minerals in the 2.2-2.0 Gyr old West African metamorphic province. We find a record of blueschist metamorphism in these rocks. We show that minerals such as chlorite and phengite formed at high pressures of 10-12 kbar, low temperatures of 400-450 °C and under a geothermal gradient of 10-12 °C km−1. These conditions are typical of modern subduction zones. We therefore suggest that modern-style plate tectonics existed during the Palaeoproterozoic era. We conclude that ancient blueschist metamorphism may exist in other parts of the world, but the identification of these rocks has so far been hampered by methodological problems associated with deciphering their pressure and temperature evolution.

International audience; Despite the consensus that TTGs, the main constituents of the Archaean continental crust worldwide, originated by partial melting of garnet-bearing amphibolites, natural evidence is scarce. A large variety of Archaean amphibole-rich rocks, including migmatitic amphibolites and hornblende-rich cumulates, was exhumed as a tectonic melange in the Inyoni shear zone (ISZ) near Barberton (South Africa), likely at the time of emplacement of the 3.2 Ga Nelshoogte-Badplaas plutons. This unique collection provides the opportunity to investigate partial melting of garnet-amphibolites, as well as the differentiation processes occurring in the TTG magmas en route to the surface. The ISZ migmatitic amphibolites are characterized by quartz-plagioclase leucosomes in equilibrium with garnet, amphibole, titanite ± epidote. Garnet compositions are characterized by high almandine and grossular contents. Actually, the leucosomes and neosomes likely lost part of their melt component and mesosomes may have been also modified towards restite-rich compositions due to melt segregation. Restite-bearing rocks are very iron-rich according to their high contents in garnet. Besides, the Mg-rich compositions of the hornblende cumulates and their REE distribution patterns resemble those of the nearby TT (tonalite-trondhjemite) plutons. Thermobarometric estimates from the ISZ migmatitic rocks cluster in the range 720-800 °C and 1.1-1.2 GPa for the melting reaction and the derived geothermal gradient is ca 17-22 °C/km. These conditions are consistent with either water-present melting or, more likely, water-absent epidote-dehydration melting. Such a geothermal gradient is therefore assigned to the genesis of large volumes of medium-pressure TTG magmas, that will likely form at greater depth. Unmelted garnet-bearing amphibolites correspond to slightly lower conditions. The magmatic cumulates crystallized at ca 0.6 GPa. The cumulate rocks evidence that the TTG parental magmas en route to the surface experienced fractional crystallization of a large amount of amphibole before crystallizing as tonalite-trondhjemite plutons at slightly shallower depths (ca 0.5 GPa).

International audience; The basement of central Madagascar displays two contrasted structural patterns. The first one (D1) is characterized by north-striking foliations that are gently dipping to the west and carry W- to WSW-plunging lineations, whereas the second one (D2) is characterized by steeper foliations that are striking to the NNE and lineations that are either subhorizontal or gently plunging to the SSW. The younger pattern is related to late-Panafrican tectonics along the major Angavo shear zone that is about 1000 km in length and 40 km in width with apparently little offset. Deformation in the Angavo zone induced interference folding on both sides. The D2 event is characterized by low pressures (ca 400 MPa) and high temperatures (up to 790 degrees C) responsible for prograde granulite facies conditions, that resulted from heat transfer due to magma and fluid advection in the Angavo shear zone. The D2 event is pinned at 550 +/- 11 Ma by a new monazite age from a reoriented Andringitrean granite near Ankaramena. A new suite of amphibole and biotite Ar-Ar geochronological data enables to retrace the thermochronogical evolution inside and outside the Angavo shear zone. Combined with new structural results from the western interference zone. these ages yield a better understanding of the late-Panafrican history of central Madagascar. No diachronism is observed along the strike of the Angavo shear zone. Conversely, amphibole and especially biotite ages decrease from West to East, i.e. towards the shear zone. These new ages range from 511 to 469 Ma. A 1-D conductive model constrains the thermal effect in relation with the Angavo shear zone to be restricted to a lateral distance of ca 60 km (in map view) for a maximum heating duration of 20 Myr. This is in agreement with the Ar-Ar data and with the width of the observed interference zone. Following this episode of deformation and heat transfer, the estimated cooling rates of the Angavo shear zone range from to 15 to 6 degrees C/Myr, respectively before and after 515 Ma. A post-collision intracontinental setting is suggested for the Angavo shear zone, which is regarded as a remote effect of the Kuunga Orogeny between India and Antarctica.

The basement domes of the central part of western Alps may result either from a multistage tectonic evolution with a dominant horizontal shortening component, an extensional behaviour, or both. The Ambin massif belongs to the “Briançonnais” domain and is located within the HP metamorphic zone. It was chosen for a reappraisal of the tectonic evolution of the Internal Alps in its western segment. Structural investigations have shown that Alpine HP rocks were exhumed in three successive stages. The D1 stage was roughly coeval with the observed peak metamorphic conditions and corresponds to a non-coaxial regime with dominant horizontal shortening and north movement direction. Petrological observations and P-T estimates show that the exhumation process was initiated during D1, the corresponding mechanism being still poorly understood. The D2 stage took place under low-blueschist facies conditions and culminated under greenschist facies conditions. It developed a retrogressive foliation and pervasive shear-zones at all scales that locally define major tectonic contacts. D2 shear zones show a top-to-east movement direction and correspond actually to large-scale detachment faults responsible for the juxtaposition of less metamorphic units above the Ambin basement and thus to a large part of the exhumation of HP rocks toward the surface. D2 shear zones were subsequently deformed by D3 open folds, large antiforms (e.g. the Ambin dome) and associated brittle-ductile D3 shear-bands. The D1 to D3 P-T conditions and P-T path of the blueschists occurring in the deepest part of the Ambin dome, was estimated by using the multi-equilibrium thermobarometric method of the Tweeq and Thermocalc softwares. Peak pressure conditions, estimated at about 14–16 Kb, 500oC, are followed by a nearly-isothermal decompression that occurred concurrently with the major D1–D2 change in the ductile deformation regime. Eastwards, the Schistes Lustrés units exhibit a similar geometry on top of the Gran Paradiso dome but exhibit opposite D2 movement direction. Lower-grade units are lying above higher-grade units, the shear zones occurring in between being similar to Ambin’s D2 detachments. Thus at regional scale, the D2 detachments seem to form together with the Ambin shear-zones, a network of conjugate detachments. Such a pattern suggests that the exhumation history is mostly controlled by a D2+D3 crustal-scale vertical shortening resulting in the thinning of the previous tectonic pile formed during D1. The slab-break off hypothesis may explain such an extensional behaviour within the western Pennine domain. It is suggested that the thermo-mechanical rebound of the residual European slab initiated between 35 and 32 Ma the fast exhumation of the previously thickened orogenic wedge (stack of D1 HP slices). It was immediately followed by a collapse of the wedge that may correspond to the E-W Oligocene extensional event responsible for the opening of rifts in the West European platform.

International audience; The Ambin and South Vanoise Briançonnais basement domes, located within the HP metamorphic zone, were chosen for a reappraisal of the early tectonic evolution of the Internal Alps in their western segment. Finite strain analyses have shown that HP rocks were exhumed in two main successive stages, more or less expressed according to their structural position in the massifs. A partitioning of deformation, from core to rim of the domes, is thus evidenced. The D1 stage was roughly coeval with the observed peak metamorphic conditions and corresponds in the core of the Ambin dome, to a non-coaxial regime with dominant horizontal shortening and N to NW movement direction. The D2 stage, well-expressed in the upper part of the domes, took place under low-blueschist facies conditions and culminated under greenschist facies conditions. It developed a retrogressive foliation and pervasive E-verging shear-zones at all scales that locally define major Φ2 tectonic contacts. The D2 shearing is responsible for the development of spectacular F1–F2 fold interference structures recognised in the western part of the South Vanoise dome. In this area, we interpret the steeply-dipping HP-fabric (S1) defining the axial planes of the first generation of now-upright folds (F1) as linked to an early flat-lying nappe edifice (Φ1) subsequently steepened by the D2 shearing. Geometry and kinematic characteristics for the early D1 nappe-forming event are strongly consistent with most evolutionary models assuming that oceanic to European and Apulian crust-derived nappes were initially exhumed during N to NW-verging overthrusting events, synthetic to the subduction. They tend also to suggest that the Penninic HP-units may have suffered an early exhumation by low-angle thrusting, rather than by vertical extrusion.

Petrological and structural observations from the Ambin pre-alpine basement dome and from its Brianconnais and Piedmont covers show an early D1 nappe-forming event overprinted by a major D2 (+ D3) ductile shearing deformation. The D1 event is characterised by garnet-blueschist facies metamorphic assemblages retrogressed to greenschist facies conditions during D2 then D3 stages near the top of the dome. North-verging D1 structures preserved in the core of the dome are consistent with alpine evolutionary models, in which exhumation of HP–LT metamorphic alpine rocks occurs initially in a north–south direction. To cite this article: J. Ganne et al., C. R. Geoscience 336 (2004).

Journal of physics / Conference Series 499, 012012 (2014). doi:10.1088/1742-6596/499/1/012012, Rocks are commonly polycrystalline systems presenting multi-scale chemical and structural heterogeneities inherited from crystallization processes or successive metamorphic events. This work illustrates how spatially resolved analytical techniques coupled with thermodynamic approaches allow rock compositional variations to be related to large-scale geodynamic processes. Emphasis is placed on the contribution of quantitative chemical imaging to the study of 2.2-2.0 Gy old metamorphic rocks from the West African Craton. A thorough analysis of elemental chemical maps acquired on rock thin sections enabled high pressure relic minerals to be located and re-analyzed later with precise point analyses. The pressure-temperature conditions of crystallization calculated from these analyses are typical of modern subduction zones. These results push back the onset of modern-style plate tectonics to 2.15 Gy, i.e. more than one billion years earlier than was consensually accepted. The second part of the paper describes the imaging capabilities offered by the new ultra-bright diffraction limited synchrotron sources. Experimental data acquired with the Maia detector at beamline P06 at Petra III as well as simulations of μ-XRF spectra that will be generated at the SRX beamline at NSLS-II are presented. These results demonstrate that cm2 large chemical maps can be acquired with submicron spatial resolution and a precision suitable for thermobarometric estimates, with dwell time smaller than 1 millisecond. The last part of the paper discusses the relevance of utilizing recent X-ray fluorescence nanoprobes for diagenetic to low grade metamorphism applications., Published by IOP Publ., Bristol