Déformation et hydratation du manteau lithosphérique sous le craton du Kaapvaal

Déformation et hydratation du manteau lithosphérique sous le craton du Kaapvaal

Le craton du Kaapvaal est un domaine continental épais et froid resté stable depuis sa formation durant l’Archéen (Griffin et al., 2004; Pearson et al., 1995; Shirey et al., 2002). Les péridotites du Kaapvaal montrent des compositions appauvries en CaO et Al2O3 et des Mg# dans l’olivine élevés, suggérant qu’elles sont le résultat de forts taux de fusion partielle (Boyd and Mertzman, 1987). Pourtant, ces compositions appauvries et le géotherme froid caractéristiques des cratons ne suffisent pas à générer des contrastes de viscosité suffisamment élevés pour expliquer la stabilité et la longévité des cratons (Doin et al., 1997; Lenardic and Moresi, 1999). Partant de l’idée que l’épisode de fusion partielle ayant formé les cratons avait dû entrainer une déshydratation de la racine, plusieurs études ont ainsi proposé qu’un contraste de densité suffisant pouvait être lié à de très faibles teneurs en hydrogène dans l’olivine (Doin et al., 1997; Pollack, 1986). Pourtant, une étude récente de Peslier et al. (2010) révèle que des teneurs en hydrogène dans l’olivine de xénolites mantelliques du Kaapvaal sont souvent élevées (jusqu’à 100 wt. ppm H2O), à l’exception des échantillons les plus profonds (<10 wt. ppm H2O). De plus, de multiples épisodes métasomatiques ont été identifiés dans les péridotites du craton du Kaapvaal (e.g. Boyd and Mertzman, 1987; Griffin et al., 2003b; Kelemen et al., 1998; Nixon et al., 1981; Simon et al., 2007). Le travail présenté dans ce chapitre se base sur l’étude de 50 xénolites péridotitiques échantillonnés dans 9 diatrèmes kimberlitiques différents au sein du craton du Kaapvaal. Ces péridotites présentent donc une large distribution latérale, mais aussi verticale. L’analyse des microstructures et des OPRs révèle que les échantillons granulaires à gros grains qui prédominent parmi les péridotites du Kaapvaal enregistrent une déformation par fluage dislocation, suivie par une longue période de recuit. Ces observations supportent un épisode de déformation ancien, probablement associé à la formation de la racine, suivie d’un refroidissement lent et d’une longue période de quiétude. Les textures et compositions révèlent également de multiples évènements métasomatiques à la répartition géographique hétérogène. L’analyse combinée des microstructures et des OPRs montre que les épisodes  Déformation et hydratation du manteau lithosphérique sous le craton du Kaapvaal métasomatiques à l’origine des fortes compositions modales en orthopyroxene sont pré- à post-cinématiques. Le métasomatisme causé par des fluides riches en K est quand à lui postcinématique. L’analyse des textures et des OPRs des mylonites cratoniques suggèrent une déformation plus tardive et locale. Les teneurs en hydrogène sont variables, mais ont tendance à augmenter jusqu’à une profondeur d’environ 150 km (~150 wt. ppm H2O, calibration de Bell et al. (2003)). Les échantillons les plus profonds sont quant à eux presque secs. L’absence de corrélation entre les teneurs en hydrogène et le Mg# de l’olivine suggèrent une réhydratation postérieure à la formation de la racine cratonique appauvrie, par métasomatisme lié à la percolation des fluides ou magmas riches en eau ou par contamination durant le transport des xénolites à la surface par les kimberlites. Or, une étude magnétotellurique récente montre que les données de conductivité (Evans et al., 2011) sont mieux expliquées par un modèle montrant une variation de la teneur en hydrogène dans l’olivine similaire à celle mesurée (Fullea et al., 2011), suggérant que ces données pourrait être représentatives de l’ensemble de la racine cratonique. Que cette réhydratation soit extensive ou plus hétérogène, elle n’a toutefois pas à ce jour entraîné une remobilisation de la racine d’après les textures très recuites des péridotites cratoniques.

 Geological setting

The Kaapvaal craton (South Africa) is an assemblage of Archean terranes that extends over more than 12,000 km2 . It is mainly composed by granitoids and gneisses that enclose narrow greenstone belts, but Upper Archean and Lower Proterozoic basins cover most of it (Begg et al., 2009; de Wit et al., 1992). Crustal formation occurred essentially between ~3.7 and ~3.1 Ga, followed by terrane assembly and stabilization of the craton between 3.1 and 2.6 Ga (de Wit et al., 1992; Griffin et al., 2003). Formation of the Kaapvaal mantle root as early as 3.3–3.5 Ga is indicated by Re–Os whole-rock data on peridotite xenoliths (Pearson et al., 1995) and on sulfides in diamond inclusions (Shirey et al., 2002). These conclusions were confirmed by Re–Os analyses of sulfide phases in peridotite xenoliths, which indicate that the Kaapvaal mantle root formed prior to 3 GPa, that is, previously to or simultaneously to the formation of the crust, implying that each terrane carried its own keel during the craton assembly (Griffin et al., 2004). Recent electrical conductivity measurements suggest that the lithosphere beneath the central Kaapvaal craton is currently defined by a high resistivity layer 200–250 km thick (Evans et al., 2011). High seismic velocities imaged in body and Rayleigh wave tomography also indicate a 250 km thick root beneath most of the craton (Chevrot and Zhao, 2007; James et al., 2001). Body-wave data suggest however that the high-velocity cratonic root may locally attain depths of 300 km (James et al., 2001). A slightly thinner high-velocity layer, 175–250 km thick, was imaged recently using SH-waves (Begg et al., 2009). These lithospheric thicknesses estimated from geophysical data are slightly higher, but still consistent with the 185 to 215 km thick Kaapvaal lithosphere constrained by xenolith thermobarometry (Eaton et al., 2009). Following its stabilization, the Kaapvaal craton was affected by several magmatic events. The most important is the Bushveld complex, which intruded the Kaapvaal craton at 2.05 Ga (Scoates and Friedman, 2008). Another major magmatic event was the extrusion of the Karoo large igneous province at 182 Ma (Riley et al., 2005), which is associated with the Gondwana breakup. The craton was also affected by numerous kimberlitic eruptions. Kimberlitic pipes were mostly emplaced between the Late Jurassic and the Cretaceous (Kramers and Smith, 1983), but also erupted between 1650 and 1200 Ma (Kramers and Smith, 1983) and between 530 and 255 Ma (Allsopp et al., 1985; Kramers and Smith, 1983; Phillips et al., 1998). These kimberlitic pipes contain xenoliths of the cratonic mantle that have been extensively studied for their microstructures, petrology, and geochemistry. Kaapvaal mantle xenoliths may be classified in two groups: (1) coarse-grained peridotites, which have dominantly refractory compositions and (2) fine-grained sheared peridotites, which are equilibrated at high temperatures and pressures and have, on average, more fertile compositions (Boullier and Nicolas, 1975; Boyd and Mertzman, 1987; Boyd and Nixon, 1975; Nixon et al., 1981). Petrological and geochemical data on Kaapvaal coarse-grained nodules reveal that they represent a highly refractory lithospheric mantle residue, implying ~40% melt extraction, which was subsequently affected by several metasomatic episodes (e.g., Boyd and Mertzman, 1987; Griffin et al., 2003; Kelemen et al., 1998; Nixon et al., 1981; Simon et al., 2007). The high orthopyroxene/olivine ratio that characterizes many lowtemperature Kaapvaal xenoliths, for instance, is usually attributed to Si-enrichment due to interaction with subduction-related fluids or intraplate hydrous melts (Bell et al., 2005; Kelemen et al., 1998; Simon et al., 2007; Wasch et al., 2009). Lu–Hf and Sm–Nd model ages on garnet and orthopyroxene clots in peridotite xenoliths from Kimberley suggest multiple, rather than a single Si-enrichment episode, ranging from 1.3 to 1.1 Ga to Neoproterozoic (Wasch et al., 2009). Kaapvaal xenoliths and xenocrysts also display geochemical evidence for refertilization, which added basaltic components like Fe, Ca and Al to a depleted protolith (Griffin et al., 2003). Crystallization of diopside, lherzolitic garnet, and phlogopite has been proposed to result from interactions with kimberlitic–carbonatitic fluids (Grégoire et al., 2003; Simon et al., 2007). Recent geochemical data indicate that diamonds and subcalcic garnets also result from interaction with reduced asthenospherederived fluids, corroborating the hypothesis that the cratonic mantle was originally essentially composed by highly refractory harzburgites and dunites (Malkovets et al., 2007). Melt-related metasomatism has also been identified in sheared peridotites, which may have acquired their fertile composition by interactions with asthenosphere-derived melts shortly before the kimberlitic eruption (O’Reilly and Griffin, 2010). In the present study, we will refer to all such changes in modal composition due to melt or fluid–rock interactions as modal metasomatism. 

Mineral compositions and pressure–temperature estimates

Chemical compositions of olivine, orthopyroxene, clinopyroxene, garnet and spinel were analyzed using a Cameca SX100 electron microprobe at Microsonde Sud facility, in Montpellier (France). Analysis conditions were a 20 kV accelerating voltage and a 10 nA probe current. Core and rim compositions were analyzed systematically. For each mineral phase, three to four grains were measured. The aim was to determine pressure and temperature equilibrium conditions of the selected peridotites to constrain their depth distribution. Equilibrium temperatures were calculated using the two pyroxenes Fe– Mg exchange geothermometer from Brey and Köhler (1990), which has an uncertainty of ±30 °C. In highly-depleted garnet harzburgites that contained no clinopyroxene, the geothermometer of O’Neill and Wood (1979), based on Fe–Mg partitioning between garnet and olivine, which has uncertainties around ±60 °C, was used. For garnetand clinopyroxene-free harzburgites, we used the geothermometer Li et al. (1995), based on Fe–Mg exchange between olivine and spinel, which yield temperatures within ±50 °C. Equilibrium pressures were calculated using the orthopyroxene–garnet barometer of Nickel and Green (1985), which has uncertainties of ±0.2 GPa. For the spinelbearing peridotites, pressures were estimated based on the equilibrium temperatures and the geotherm that best fits the equilibrium pressure and temperature data for the garnet-bearing samples. 3.2. Electron-backscattered diffraction (EBSD) Crystallographic preferred orientations (CPO) of olivine, pyroxenes, and garnet in all 50 samples were measured at the SEM-EBSD facility in Geosciences Montpellier by indexation of EBSD patterns produced by interaction between an electron beam with the crystals in thin sections tilted at 70° to the horizontal. Measurements were performed in a JEOL JSM 5600 scanning electron microscope using an acceleration voltage of 17 kV and a working distance of 23 mm. Maps covering almost entirely each thin section were performed using sampling steps of 100, 40 or 30 μm, depending on grain size. Indexation rates range from 40 to 75% depending on the extent of fracturing and serpentinization in the xenolith. Phlogopite is usually poorly indexed. Indexation is also poor in very fine-grained layers in samples displaying mylonitic or fluidal mosaic textures. Orthopyroxene was rarely misindexed as clinopyroxene. Errors in the measurements were reduced by careful post-acquisition data treatment, controlled by comparison between EBSD maps and microscopic observations. Modal composition, grain sizes, and shape-preferred orientations were also obtained from EBSD maps. Crystal-preferred orientation data is displayed in pole figures, presented as lower hemisphere stereographic projections. To avoid over-representation of large grains, data were plotted as one point per grain. When the foliation and lineation could be identified, the orientation of the main crystallographic directions: [100], [010] and [001] for olivine and pyroxenes, was plotted relatively to the principal axes of the deformation ellipsoid X, Y, and Z. However, in most coarse-grained samples, the identification of the foliation and lineation was not possible and thin sections were cut in random orientations. To allow easy comparison among different samples, we rotated the CPO of all samples into a common orientation, in which the maximum concentration of orthopyroxene [001] axes and of the olivine [010] axes are parallel to the E–W and the N–S directions of the pole figure, respectively. This choice was based on the observation that [001] is the only known glide direction in orthopyroxene; plastic deformation tends therefore to align this axis in the flow direction (cf. review in Frets et al., 2012). This choice allowed presenting the CPO without making an ad-hoc hypothesis on the dominant glide direction in olivine. When the orthopyroxene CPO was too dispersed, the maximum concentration of olivine [100] or [001] axes, depending on which had the strongest concentration, was placed in the E–W direction of the pole figure.