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A review of the current knowledge of lithostratigraphy, geochronology, mineralogy, alteration, fluid chemistry and structural data is provided in order to discuss the main controls on mineralization in the Crixás gold deposit and the existing structural framework and evolution. Gold mineralization at Crixás represents orogenic upper mesothermal to lower hypothermal types developed within the overturned Paleoproterozoic (Rhyacian) stratigraphic sequence. The structural data indicates that upright folding of the stratigraphy and formation of a distinct S1 foliation characterizes D1 deformation. This controls the formation of laminated quartz veins, precipitation of Au-rich sulfides, and the development of NW trending orebodies. Localized F2 folding of D1 structures and strong L2 stretching lineations characterizes the D2 event. This localized F2 folding created oreshoots trending W-WNW within distinct orebodies. The D2 deformation is associated with the precipitation of native Au. The ‘bulk plunge’ of these orebodies is a combination of both plunges defined by D1 and D2 deformation events. A critical assessment of lithostratigraphic and structural data reveal two possible tectonic scenarios operating during the Paleoproterozoic. The first possible tectonic scenario considers sagduction and the development of a regional-scale synclinorium, followed by tilting and localized folding of D1 structures. The second scenario, which is similar to the current model, considers accretion producing a synclinorium, followed by thrusting. Based on the current knowledge, both models are considered to be permissible and a series of focused research studies are proposed to test both the current and the new tectonic models.

Early Neoproterozoic metaigneous rocks occur in the central part of the Kaoko–Dom Feliciano–Gariep orogenic system along the coasts of the southern Atlantic Ocean. In the Coastal Terrane (Kaoko Belt, Namibia), the bimodal character of the ca. 820–785 Ma magmatic suite and associated sedimentation sourced in the neighbouring pre-Neoproterozoic crust are taken as evidence that the Coastal Terrane formed as the shallow part of a developing back arc/rift. The arc-like chemistry of the bimodal magmas is interpreted as inherited from crustal and/or lithospheric mantle sources that have retained geochemical signature acquired during an older (Mesoproterozoic) subduction-related episode. In contrast, the mantle contribution was small in ca. 800–770 Ma plutonic suites in the Punta del Este Terrane (Dom Feliciano Belt, Uruguay) and in southern Brazil; still, the arc-like geochemistry of the prevalent felsic rocks seems inherited from their crustal sources. The within-plate geochemistry of a subsequent, ca. 740–710 Ma syn-sedimentary volcanism reflects the ongoing crustal stretching and sedimentation on top of the Congo and Kalahari cratons. The Punta del Este–Coastal Terrane is interpreted as an axial part of a Neoproterozoic “Adamastor Rift”. Its opening started in a back-arc position of a long-lasting subduction system at the edge of a continent that fragmented into the Nico Perez–Luis Alves Terrane and the Congo and Kalahari cratons. The continent had to be facing an open ocean and consequently could not be located in the interior of the Rodinia. Nevertheless, the early opening of the Adamastor Rift coincided with the lifetime of the circum-Rodinia subduction system.

The Kedougou-Kenieba inlier is the westernmost exposure of Birimian crust in the West African Craton. The inlier is host to several multi-million ounce gold deposits (e.g., Sadiola, Yatela, Loulo, Massawa, and Gounkoto) and a large iron ore resource (i.e. Faleme). Although the inlier has received increasing academic attention over the last decade, only limited data exists for its north-eastern part, which contains the world-class Sadiola-Yatela gold camp. New field data, U-Pb magmatic zircon ages, and whole rock geochemistry data from the north-eastern Kedougou-Kenieba inlier are presented and integrated with published data in order to generate a revised tectono-magmatic framework to gold mineralisation for the westernmost Birimian terranes. The granitoids from the Sadiola-Yatela region display a temporal evolution from ca. 2140 to 2080 Ma calc-alkaline metaluminous plutons (e.g., diorite, hornblende-biotite-granodiorite) to ca. 2080–2060 Ma high-K calc-alkaline, metaluminous to slightly peraluminous granites (e.g., biotite-monzogranite). The least-evolved rocks are associated with flat REE patterns and correspond to plutons of the ca. 2140 Ma Finman complex and the ca. 2115 Ma Sadiola quartz-feldspar porphyry. The vast majority of the Eburnean (ca. 2115–2060 Ma) granitoids in the Sadiola-Yatela region are characterised by negative Nb-Ta anomalies, marked Pb positive anomalies, and high LILE/HFSE ratios. These Eburnean granitoids share a close affinity with relatively hydrous, oxidised magmas and source regions. The polycyclic deformation recorded in sedimentary and magmatic rocks of the region includes a period of early convergence (D 1s ), followed by a period of fold-and-thrust tectonics (D 2s ), and later transcurrent tectonics (D 3s -D 4s ). The bulk of the gold mineralisation in the region occurred during D 3s , with sinistral displacement and hydrothermal fluid circulation along the Senegal-Mali Shear Zone and steep NNE-trending shear zones connected to structural traps in the Kofi basin. The results of this study, when integrated with published data, indicate an evolution from primitive island arc magmas to increasingly evolved high-K calc-alkaline granitic melts as a result of crustal thickening during the Eburnean orogeny combined with fractionated crystallisation and melting-assimilation-storage-homogenisation processes. The late Eburnean tectono-thermal event between ca. 2080 and 2060 Ma is proposed as the geodynamic engine that empowered gold mineralisation in the Kedougou-Kenieba inlier.

International audience; The ~8-Moz Sadiola Hill gold deposit is located in the Kédougou-Kénieba inlier, a window of deformed ca. 2200 to 2050 Ma rocks that crop out in eastern Senegal and western Mali. The geology of the inlier differs from other Paleoproterozoic granite-greenstone belts and sedimentary basins by the abundance of carbonate rocks. The Sadiola Hill gold deposit occurs within 3 km of the Senegal-Mali shear zone, and country rocks in the region have undergone polycyclic deformation. The lithostratigraphy of the open pit consists of impure limestones overlain by younger sequences of detrital sedimentary rocks, which include arenite, wacke, and siltstone. The sedimentary rocks have been subjected to regional greenschist-facies metamorphism and have been intruded by multiple generations of synkinematic, calc-alkaline stocks and dikes. The bulk of the ore is hosted within a N-S- to NNW-trending, 10- to 50-m-wide, brittle-ductile dilational shear zone defined as the Sadiola shear zone. Gold lodes also occur along an array of steep NNE-trending shear zones. Geometric and kinematic analyses indicate that the ore-hosting structures were undergoing sinistral displacement at the time of mineralization, locally defined as the D3s NNW-SSE-shortening event. Hydrothermal alteration is polyphase and includes an early high-temperature calc-silicate phase (i.e., porphyroblastic growth of actinolite-tremolite) followed by a potassic phase (i.e., biotite-calcite-quartz ± K-feldspar-tourmaline-actinolite) that was synchronous with ore mineral deposition. Paragenetic studies reveal a multistage ore development that includes an As-rich sulfide stage, followed by an Au-Sb stage. The ore is associated with a metal enrichment suite of Au-As-Sb ± Cu-Fe-W-Mo-Ag-Bi-Zn-Pb-Te. The Sadiola Hill deposit shares a similar relative timing and structural setting to that of other world-class orogenic gold systems in the West African craton (e.g., Ashanti, Loulo), with gold deposition occurring during a period of transcurrent tectonics, after the cessation of region-wide compressional deformation. The high-temperature paragenesis at Sadiola Hill is atypical of gold mineralization in the craton and indicates that the late Eburnean tectono-magmatic activity between ca. 2080 and 2060 Ma played an important role in the dynamics of hydrothermal fluid circulation along the Senegal-Mali shear zone.

The supergiant Obuasi gold deposit is the largest deposit in the Paleoproterozoic Birimian terranes of West Africa with 62 Moz of gold (past production + resources). The deposit is hosted in the Paleoproterozoic Kumasi Group sedimentary rocks composed of carbonaceous phyllites, slates, psammites, and volcaniclastic rocks intruded by different generations of felsic dikes and granites. A three-stage deformation history is defined for the district. The D1Ob stage is weakly recorded in the sedimentary rocks as a layer-parallel fabric and indicates that bedding parallel shearing occurred during the early stage of deformation at Obuasi. The D2Ob is the main deformation stage affecting the Obuasi district and corresponds to a NW-SE shortening. Tight to isoclinal folding, as well as intense subhorizontal stretching, occurred during D2Ob, parallel with the plane of a pervasive NE-striking subvertical foliation (S2Ob). Finally, a N-S shortening event (D3Ob) refolded previously formed structures and formed a distinct ENE-striking, variably dipping S3Ob cleavage that is domainal in nature throughout the deposit. Two economic styles of mineralization occur at Obuasi and contribute equally to the gold budget. These are (1) gold-bearing sulfides, dominantly arsenopyrite, mainly disseminated in metasedimentary rocks and (2) native gold hosted in quartz veins that are as much as 25 m wide. Microstructural evidence, such as strain shadows surrounding gold-bearing arsenopyrite parallel with S2Ob, but folded by S3Ob, indicates that the sulfides were formed during D2Ob. Concentrations of as much as 700 ppm Au are present in the epitaxial growth zones of the arsenopyrite grains. Although the large mineralized quartz veins are boudinaged and refolded (indicating their formation during D2Ob), field and microanalytical observations demonstrate that the gold in the veins is hosted in microcracks controlled by D3Ob, where the S3Ob cleavage crosscuts the quartz veins in the main ore zones. Thus, these observations constitute the first evidence for multiple stages of gold deposition at the Obuasi deposit. Futhermore, three-dimensional modeling of stratigraphy, structure, and gold orebodies highlights three major controls on oreshoot location, which are (1) contacts between volcaniclastic units and pre-D1 felsic dikes, (2) fault intersections, and (3) F3Ob fold hinges. The maximum age for the older disseminated gold event is given by the age of the granites at 2105 ± 2 Ma, which is within error of hydrothermal rutile in the granites of 2098 ± 7 Ma; the absolute age of the younger gold event is not known.

The Alamoutala gold deposit is located in the Kedougou-Kenieba inlier, a window of Paleoproterozoic rocks that crop out in eastern Senegal and western Mali. The deposit is part of the ~3-Moz Au Yatela district and produced 308,400 oz Au between 2002 and 2012. Country rocks in the Alamoutala open pit consist of carbonate rocks, arenites, wackes, and siltstones. The sedimentary rocks have been subjected to polycyclic deformation and regional greenschist-facies metamorphism. Synkinematic, calc-alkaline, intermediate quartz-feldspar porphyry stocks intruded the metasedimentary rocks at 2083 ± 7 Ma (U-Pb zircon). Amphibolite-facies contact metamorphism of carbonate rocks resulted in the formation of a high-temperature-low-pressure magnetite-bearing skarn. Field relationships and microtextural data, however, indicate that economic gold mineralization is shear hosted and occurred during retrograde contact metamorphism. Geometric and kinematic analyses indicate that the ore-hosting structures were undergoing sinistral-reverse displacement at the time of mineralization. Ore minerals deposited synchronous with a potassic hydrothermal alteration assemblage defined as biotite + calcite + pyrite ± K-feldspar, actinolite-tremolite, quartz, muscovite, and tourmaline. Petrographic studies reveal that ore-related sulfides largely consist of pyrite with lesser pyrrhotite and chalcopyrite. The ore also contains accessory Ag-Sb-As-Hg-Bi-Te-Mo-W-Zn-Pb-Ni-Co-U-bearing mineral species. The Alamoutala deposit represents two distinct hydrothermal systems in which a locally developed magnetite-bearing skarn was overprinted by a region-wide orogenic gold event in the late Eburnean.

Orebodies in the Siguiri district, a world-class Paleoproterozoic orogenic gold camp located in the Birimian of northeastern Guinea, are typically represented by cryptic subvertical damage zones that host a high density of mineralized veins. Although no large regional fault system was recognized, observations from five representative deposits of the Siguiri district (Sanu Tinti, Bidini, Kami, Kosise, and Sintroko PB1) show that these orebodies are locally controlled by incipient structures and spread across three distinct structural and lithostratigraphic domains. Two shale-dominated peripheral domains adjoin a central domain whose lithostratigraphy is dominated by medium- to coarse-grained graywacke and this domain hosts the bulk of the gold endowment of the district. The three domains exhibit similar structural elements that can be described within a four-stage deformation scheme. The first deformation event (D 1S ) is poorly constrained and interpreted to have been an N-S compressional event. It included development of minor folds with W- to WNW-gently plunging fold axes without a clear axial planar cleavage. The main and second deformation event (D 2S ) is interpreted to have been associated with E-W to ENE-WSW- directed compression. The D 2S event was responsible for forming the dominant N-trending structural grain of the district and creating interference patterns between F 1S and F 2S folds. The third event (D 3S ) developed progressively from D 2S compression into an early-D 3S E-W- to ENEWSW-directed transpression and a late-D 3S NNW-SSE-directed-transtension responsible for most of the gold mineralization in the Siguiri district. The fourth and last event (D 4S ) was an NW-SE-oriented compressional event responsible for the localized overprinting of veining by a steep to shallowly dipping NNE-SSW ductile cleavage. Late-D 3S gold-bearing mineral occurrences formed along subvertical N-S reverse faults, NE-trending dextral shear zones, WNW-trending sinistral faults, and E-trending normal relay faults developed or reactivated early-D 3S . Mineralization is expressed as mineralized shear zones or subvertical damage zones, characterized by a 10- to 15-m-wide zone of dense quartz-carbonate-sulfide veining, or disseminated gold-bearing sulfides. The mineralized veins consistently strike ENE-WSW, are steeply dipping, and commonly have a conjugate geometry at the mesoscale. Finite strain analysis of deformation, including analysis of folds, faults, and conjugate mineralized vein sets, is consistent with a stress switch from a compressional (D 2S ) to transpressional deformation (termed early-D 3S ). Results of paleostress analysis on conjugate mineralized vein sets that formed late during D 3S indicate that the stress field ranged from extensional to strike-slip, sometimes within the same vein locality. The late-D 3S deformation is interpreted to have been a transtensional event. The first change in the orientation of the principal stress axes is related to a switch from a far field-dominated to a body force-dominated stress field reducing the deviatoric component on the stress tensor. The second change in the orientation of the principal stress axes, from early-D 3S transpression to late-D 3S transtension, suggests that σ 1 and σ 2 were similar in magnitude, which facilitated localized stress switches. In the Siguiri district, the early-D 3S and late-D 3S stress switches, which occurred at both a local and regional scale, enhanced the fracture permeability and were critical for the establishment of active fluid pathways leading to the formation of a world-class gold system.

Geochronology and lithostratigraphic characterisation of the Siguiri district in the Palaeoproterozoic Birimian terrane of West Africa has provided a reappraisal of the Siguiri Basin stratigraphy. This new insight into the evolution of the central part of the basin further highlights first order controls on the location of the world class Siguiri orogenic gold district. Three metasedimentary formations occur in the Siguiri district: the Balato, Fatoya and Kintinian formations. These formations consist of fine-grained organic-rich shales, mudstone and siltstone/greywacke/limestone interbeds, decimetre- to metre-thick graded greywacke beds and debris flow deposits. The Balato Formation displays low-energy, hemipelagic marine or lake sediments, whereas the Fatoya Formation consists of distal turbidite deposits (thin-bedded turbidites and channel-fill deposits). Altogether they form a basal regressive sequence. The overlying Kintinian Formation is shale-rich, and contains a stack of polymict conglomeratic interbeds (up to 100 m in thickness) around the base of the Formation in the Siguiri district. This conglomerate unit transects the whole Siguiri Basin and is interpreted to be the product of repeated subaqueous debris flow deposits and represent olistostromes (or melanges of sedimentary origin) with various autochthonous and allochthonous clasts. The youngest detrital zircon age population for each of the Balato, Fatoya and Kintinian formations yields ages of 2113 ± 10 Ma, 2113 ± 5 Ma and 2124 ± 7 Ma respectively, thus constraining the maximum age of deposition for each of these formations. The minimum age of deposition of these sedimentary rocks is constrained by a crosscutting volcanic breccia dated at 2092 ± 5 Ma, and by the Malea monzogranite intrusion dated at 2089 ± 12 Ma. Comparison of the sedimentary facies and geochronology of the Siguiri sedimentary rocks with other sedimentary basins in the West African Craton, suggests that the Kintinian Formation is part of the Lower Tarkwa Group, best described as “late orogenic basin” sediments. The Kintinian olistostromes are inferred to highlight the onset of renewed tectonic activity associated with the first phases of compression during the Eburnean Orogeny in the Siguiri Basin at ca. 2115 Ma. The peculiar morphology of these olistostromes compared to the bulk of the Siguiri Basin sedimentary rocks, illustrates a major change in lithofacies. This change is interpreted to have developed along fundamental basement structures, controlling the early architecture of the Siguiri Basin as well as the location of world-class gold mineralisation in the Siguiri district. These fundamental structures are thought to have acted as first order fluid pathways during the E-W progressive compressional event responsible for orogenic gold mineralisation in the Siguiri district.

The relative strengths of constituent minerals in a deforming poly-mineralic rock depend on the grain size distributions of the various phases, the operating deformation mechanisms in those phases, and the topology of the microstructure. It is observed that more than one deformation mechanism operates, and the resulting constitutive relations (especially for calcite) are extremely variable. To explore such relations, deformation mechanism maps for each mineral are required together with mixing relations for multiple deformation mechanisms. We construct reference deformation mechanism maps for calcite and alkali-feldspars that are based on theoretical expressions for constitutive relations, together with experimental or molecular dynamic estimates of constitutive parameters. A thermodynamically based mixing relation is employed that enables bulk constitutive relations to be established for mixtures of mineral phases and deformation mechanisms with a range of grain sizes. This provides a basis for comparison and understanding of experimental results. We map out fields on deformation mechanism maps where switches in the relative strengths of calcite and quartz and of feldspar and quartz occur. We introduce the concept of geometrically necessary deformation mechanisms that minimise the energy of the system.

The Yatela gold deposit is located in the Kedougou-Kenieba inlier (KKI), a window of ca. 2200-2050 Ma rocks that are exposed in eastern Senegal and western Mali. The geology of the KKI differs from other Paleoproterozoic granite-greenstone belts and sedimentary basins by the abundance of carbonate rocks. The Yatela deposit occurs within 8 km of the regional-scale Senegal–Mali Shear Zone. Country rocks in the Yatela region have been subjected to polycyclic deformation and regional greenschist-facies metamorphism. A syn-kinematic diorite stock has intruded the metasedimentary sequences in the open pit and is associated with a hornblende-hornfels contact aureole. Field relationships and micro-textural data indicate that the primary gold mineralisation is shear-hosted. The similar relative timing and structural setting between the Yatela primary gold mineralisation and other world-class deposits in the region (e.g., Loulo, Lawrence et al., 2013a; Massawa, Treloar et al., 2014; Sadiola Hill, Masurel et al., in press) suggest that regional orogenic gold mineralisation occurred during a period of transcurrent tectonics, after the cessation of regional compressional deformation. The primary gold mineralisation at Yatela, however, is low-grade and sub-economic. It is hosted by marbles and, to a lesser extent, diorite. The primary ore is pyrite-rich, with abundant chalcopyrite, minor arsenopyrite and accessory Zn–Pb–Sb–Fe–Ag–Co–Ni-bearing mineral species. Post-Birimian surficial dissolution of hydrothermally altered and mineralised host marbles resulted in the creation of troughs, which were draped and infilled with a ferruginous dissolution residue enriched in gold. This auriferous residuum formed the economic resource mined at Yatela until decommissioning in 2013. The Yatela gold deposit is unique with respect to mineralisation types encountered in West Africa because an auriferous residuum of economic interest (>1 Moz) derives from an underlying sub-economic Birimian occurrence. This study provides a new appreciation for the economic interest of Yatela-style orebodies and their prospectivity in other gold provinces of the West African Craton where carbonates are present, such as in the Siguiri basin in Guinea and the Ity region in western Cote d’Ivoire.

Strain patterns within mantle rocks and surrounding coarse-grained felsic granulites from the Kutna Hora Crystalline Complex in the Variscan Bohemian Massif have been studied in order to assess their strain coupling. The studied rock association occurs within low-strain domains surrounded by fine-grained granulite and migmatite. The Doubrava peridotite contains closely spaced and steeply dipping layers of garnet clinopyroxenite, which are parallel to the NE–SW-striking, high-temperature foliation in nearby granulites, while the Uhrov peridotite lacks such layering. The Spacice eclogite is not associated with peridotite and shows upright folds of alternating coarse- and fine-grained varieties bearing NE–SW-striking axial planes. Electron back-scattered diffraction measurements revealed full strain coupling between clinopyroxenites and coarse-grained granulites in the S1 fabric that is superposed on the S0 fabric preserved in peridotites. The B-type olivine lattice preferred orientation (LPO) characterizes the S0 fabric in peridotites and its reworking is strongly controlled by the presence of macroscopic clinopyroxenite layering. The S1 in clinopyroxenites and coarse-grained granulites is associated with the LS-type clinopyroxene LPO and prism slip in quartz respectively. While the S1 fabric in these rock types is accompanied invariably by a sub-vertical stretching lineation, the S1 fabric developed in reworked Uhrov peridotite is associated with strongly planar axial (010) type of olivine LPO. The peridotites with the S0 fabric are interpreted to be relicts of a fore-arc mantle wedge hydrated to a various extent above the Saxothuringian subduction zone. The prograde metamorphism recorded in peridotites and eclogites occurred presumably during mantle wedge flow and was reaching UHP conditions. Strain coupling in the S1 fabric between clinopyroxenites and granulites at Doubrava and upright folding of eclogites at Spacice document a link between tectonic and magmatic processes during orogenic thickening, coeval with intrusions of the arc-related calcalkaline suites of the Central Bohemian Plutonic Complex (c. 360–345 Ma). Juxtaposition of peridotites and granulites could be explained by a rheological heterogeneity connected to the development of clinopyroxenite layering in the upper mantle and a previously published model of a lithospheric-scale transpressional arc system. It invokes vertical shearing along NE–SW trending, sub-vertical foliations in the upper mantle that could have led to an emplacement of mantle bodies into the granulitized, orogenic root in the sub-arc region. Clearly, such a transpressional arc system could represent an important pathway for an emplacement of deep-seated rocks in the orogenic lower crust.

The geological inventory of the Variscan Bohemian Massif can be summarized as a result of Early Devonian subduction of the Saxothuringian ocean of unknown size underneath the eastern continental plate represented by the present-day Tepla-Barrandian and Moldanubian domains. During mid-Devonian, the Saxothuringian passive margin sequences and relics of Ordovician oceanic crust have been obducted over the Saxothuringian basement in conjunction with extrusion of the Tepla-Barrandian middle crust along the so-called Tepla suture zone. This event was connected with the development of the magmatic arc further east, together with a fore-arc basin on the Tepla-Barrandian crust. The back-arc region – the future Moldanubian zone – was affected by lithospheric thinning which marginally affected also the eastern Brunia continental crust. The subduction stage was followed by a collisional event caused by the arrival of the Saxothuringian continental crust that was associated with crustal thickening and the development of the orogenic root system in the magmatic arc and back-arc region of the orogen. The thickening was associated with depression of the Moho and the flux of the Saxothuringian felsic crust into the root area. Originally subhorizontal anisotropy in the root zone was subsequently folded by crustal-scale cusp folds in front of the Brunia backstop. During the Visean, the Brunia continent indented the thickened crustal root, resulting in the root's massive shortening causing vertical extrusion of the orogenic lower crust, which changed to a horizontal viscous channel flow of extruded lower crustal material in the mid- to supra-crustal levels. Hot orogenic lower crustal rocks were extruded: (1) in a narrow channel parallel to the former Tepla suture surface; (2) in the central part of the root zone in the form of large scale antiformal structure; and (3) in form of hot fold nappe over the Brunia promontory, where it produced Barrovian metamorphism and subsequent imbrications of its upper part. The extruded deeper parts of the orogenic root reached the surface, which soon thereafter resulted in the sedimentation of lower-crustal rocks pebbles in the thick foreland Culm basin on the stable part of the Brunia continent. Finally, during the Westfalian, the foreland Culm wedge was involved into imbricated nappe stack together with basement and orogenic channel flow nappes.

A detailed field study reveals a gradual transition from high-grade solid-state banded orthogneiss via stromatic migmatite and schlieren migmatite to irregular, foliation-parallel bodies of nebulitic migmatite within the eastern part of the Gfohl Unit (Moldanubian domain, Bohemian Massif). The orthogneiss to nebulitic migmatite sequence is characterized by progressive destruction of well-equilibrated banded microstructure by crystallization of new interstitial phases (Kfs, Pl and Qtz) along feldspar boundaries and by resorption of relict feldspar and biotite. The grain size of all felsic phases decreases continuously, whereas the population density of new phases increases. The new phases preferentially nucleate along high-energy like–like boundaries causing the development of a regular distribution of individual phases. This evolutionary trend is accompanied by a decrease in grain shape preferred orientation of all felsic phases. To explain these data, a new petrogenetic model is proposed for the origin of felsic migmatites by melt infiltration from an external source into banded orthogneiss during deformation. In this model, infiltrating melt passes pervasively along grain boundaries through the whole-rock volume and changes completely its macro- and microscopic appearance. It is suggested that the individual migmatite types represent different degrees of equilibration between the host rock and migrating melt during exhumation. The melt topology mimicked by feldspar in banded orthogneiss forms elongate pockets oriented at a high angle to the compositional banding, indicating that the melt distribution was controlled by the deformation of the solid framework. The microstructure exhibits features compatible with a combination of dislocation creep and grain boundary sliding deformation mechanisms. The migmatite microstructures developed by granular flow accompanied by melt-enhanced diffusion and/or melt flow. However, an AMS study and quartz microfabrics suggest that the amount of melt present did not exceed a critical threshold during the deformation to allow free movements of grains.

This work contributes to the experimental investigations of the origin and 3-D orientation of micropores in low porosity crystalline rocks. The origin and spatial orientation of microporosity in two eclogites with different microstructures were studied by 1) quantitative and qualitative microstructural analysis of grains and grain boundaries, 2) measurement of lattice preferred orientation using the SEM-EBSD method and 3) experimental measurement of velocity of elastic P-waves in spherical samples in 132 directions under confining pressures up to 400 MPa. Results show good correlation between the elastic properties and the orientation of grain boundaries and cleavage planes in clinopyroxene. The magnitude and anisotropy of velocity change with pressure shows that microporosity in the fine-grained sample is relatively large and strongly preferentially oriented, whereas it is significantly lower and less preferentially oriented in the coarse-grained sample. Seeing that the lattice preferred orientation of clinopyroxene is similar in both samples we can deduce from velocity changes that the grain size of the rock forming minerals controls the amount of microporosity. Also, the orientation of microporosity depends mostly on preferred orientation of grain boundaries and somewhat less on the orientation of cleavage planes. Grain boundaries are therefore the most important contributors to the bulk microporosity in the studied rocks.

Three mantle eclogite xenoliths from the Robert Victor mine in South Africa were studied. The calculated pT conditions are pS4.5 GPa and T between 950 and 1145 8C. The clinopyroxene has a jadeite component between 0.19 and 0.52. In-situ, at these compositions and temperatures clinopyroxene (omphacite) has the high temperature C2/c structure, which has perfect cation disordering. The lattice preferred orientation (LPO) of clinopyroxene in the samples of different microstructure and composition has been measured by EBSD. The LPO for all three samples is characterized by a strong alignment of the [001] axes with the lineation, typical of the ‘L-type’ or ‘LS-type’ patterns for omphacite. These data disagree with a cation-ordering model for dislocation slip, which predicts a ‘S-type’ LPO pattern with the [001] axes forming a girdle in the foliation plane. Numerical simulation using the VPSC model has been undertaken to show the influence of the h110i{1 � 10} slip system on the LPO development in axially symmetric compression and extension, and pure and simple shear. The activity of the h110i{1 � 10} system should be very sensitive to changes in the cation ordering as the clinopyroxene goes towards the low temperature fully ordered P2/n structure. Increasing the critical resolved shear stress on this system from 1 to 10, resulted in a decrease of the slip activity of 80%, but produced no significant changes to the LPO. The most important slip systems for LPO development are [001](100), [100](010) and [001](010) in all simulations. The LS-index is introduced to quantify the symmetry of omphacite LPO. The index is based on the eigenvalues of the (010) and [001] pole figures, and has a value of one for the end-member L-type, zero for the end-member S-type and intermediate values for LS-types. The naturally deformed omphacite LPOs have LS-indexes of between 0.61 and 0.85 confirming they are LS-types, but closer to the L-type end-member. In the VPSC models the S-type end-member develops in axially symmetric compression (LS-indexz0), whereas the L-type end-member develops in axially symmetric extension (LS-indexz1). In simple and pure shear the LS-type fabric develops with LS-index of between 0.55 and 0.60. The VPSC simulations show that the LS-index is nearly constant with increasing strain for a constant macroscopic velocity gradient, which prescribes the constant strain path. We conclude that development of S- and L-type LPOs in omphacite is controlled by the strain path and that the LS-index is a useful quantitative indicator of fabric symmetry. q 2005 Elsevier Ltd. All rights reserved.

Microstructures from naturally deformed marbles were investigated from a nappe pile with inverted Barrovian metamorphic zones. Detailed microstructural and textural work combined with existing PT data allows us to correlate microstructural types with tectonic events. Type 1 microstructure related to D1 continental underthrusting shows highly asymmetrical grain boundaries, increasing grain size and increasing intensity of lattice-preferred orientation with increasing metamorphic grade. These characteristics suggest that dislocation creep with concommitant grain boundary migration operated during the simple shear dominated underthrusting regime. Paleopiezometric and strain rate estimates show a decrease of flow stress with increasing metamorphic temperature and a strain rate of around 10−14 s−1. Thrust related microstructure Type 2 is developed in Upper and Lower Nappes and is characterised by grain size reduction, increase of grain boundary symmetry and weaker textures. Microstructural and textural studies indicate dislocation creep with possible contribution of grain boundary sliding. Sub-grain rotation paleopiezometry and strain rate estimates show a decrease in flow stress with respect to the Type 1 microstructure and strain rate increase to around 10−12 to 10−13 s−1. Type 3 microstructure is developed in thrust related shear zones in the Parautochthon and syn-convergent extensional zones in the top of the nappe pile. This microstructure shows a high contribution of diffusion creep indicating low values of flow stress and significant weakening of marble layers. The general feature of studied area is the localisation of deformation into marbles during thrusting in low metamorphic grades while in higher grades marbles are not exploited as lubricating layers.