flow of anisotropic ice from the epica core a new test apparatus our objective is to measure the mechanical properties of anisotropic ice of the epica ice core from dome concordia antarctica through the full depth of the borehole in deformation experiments under simulated ice sheet conditions of temperature and pressure we propose to undertake experiments on the epica core using both a conventional triaxial test apparatus and a new true triaxial test apparatus incorporating automated tomographic imaging of the ice fabric evolution during deformation we present the design of this new apparatus together with our testing methodology for epica ice the new apparatus is capable of deforming an ice specimen up to 200 mm x 100 mm x 40 mm under servo controlled biaxial loading with a superimposed confining pressure of 50 mpa and at temperatures down to 40 degreesc highly unusual problems arising from the true triaxial nature of the apparatus and tomographic imaging are discussed

carbonado revisited insights from neutron diffraction high resolution orientation mapping and numerical simulations abstract one of the most controversial diamond types is carbonado as its origin and geological history are still under debate here we investigate selected carbonado samples using neutron diffraction and high resolution orientation mapping in combination with numerical simulations neutron diffraction analyses show that fine grained carbonado samples exhibit a distinct lack of crystallographic preferred orientation quantitative crystallographic orientation analyses performed on transmission electron microscope tem sections reveal that the 2 10 m grains exhibit locally significant internal deformation such features are consistent with crystal plastic deformation of a grain aggregate that initially formed by rapid nucleation characterized by a high number of nucleation sites and no crystallographic preferred orientation crystal plastic deformation resulted in high stress heterogeneities close to grain boundaries even at low bulk strains inducing a high degree of lattice distortion without significant grain size reduction and the development of a crystallographic preferred orientation observed differences in the character of the grain boundary network and internal deformation structures can be explained by significant post deformation annealing occurring to variable degrees in the carbonado samples differences in intensity of crystal bending and subgrain boundary sharpness can be explained by dislocation annihilation and rearrangement respectively during annealing grain energy is reduced resulting in distinct changes to the grain boundary geometry grain scale numerical modelling shows that anisotropic grain growth where grain boundary energy is determined by the orientation of a boundary segment relative to the crystallographic orientation of adjacent grains results in straight boundary segments with abrupt changes in orientation even if the boundary is occurring between two triple junctions forming a zigzag pattern in addition in diamond anisotropic grain growth results in triple junctions that rarely show 120 angles our results support the interpretation that carbonados may have undergone at least 2 or 3 stages of development with rapid nucleation crystal plastic deformation to low strains and variable degrees of post deformation annealing such a history is commonly observed in earth s crustal or mantle rocks hence for carbonados it is not necessary to invoke an extraordinary and or extraterrestrial origin and history the combination of methods utilized here promises to help advance our understanding of diamond and diamond aggregates in the future

evolution of pore microstructures during healing of grain boundaries in synthetic calcite rocks the morphologies of calcite grain boundaries were analyzed to provide insight into the evolution of pore networks in unfractured rock two synthetic calcite rocks were fabricated by hot isostatically pressing hip ing dried analytical grade powders of pure caco3 and caco3 plus 5 al2o3 at 600 c and 200 mpa confining pressure for 3 hours hip 1 some samples were hiped a second time at different temperatures and pressures to investigate the stability of the structures hip 2a c sem and tem were used to image both grain faces and grain boundary cross sections structures on grain faces vary from open shallow basins with peripheral rims to labyrinths of irregular ridges and channels to isolated circular depressions all of these structures are mirrored across the plane between grain faces the grain size in both the single and two phase samples increased markedly during hip 1 migrating boundaries either dragged pores along or broke away leaving grain interiors dotted with small voids the structures present after hip 1 were not stable but evolved considerably in a way dependent on the conditions of the hip 2 confining pressure had the most pronounced effect with low confining pressure the grain boundary porosity evolved into isolated circular depressions but the total pore volume did not noticeably decrease with high confining pressure the pore volume virtually disappeared the structures present after hip 1 are strikingly similar to those that develop in intragranular cracks during healing we infer that grain boundaries and intragranular cracks heal by similar processes decomposition localized melting impurities and anisotropic surface energies played no evident role in forming the grain boundary structures the timing of the formation of the porosity and of the subsequent healing processes is more difficult to ascertain some structures appear to have evolved gradually throughout the constant high temperature stage of hiping the most obvious structures however appear to have evolved on grain boundary cracks that opened during cooling

composition and source of fluids in high temperature graphite bearing granitoids associated with granulites examples from the southern marginal zone limpopo complex south africa abstract p t conditions fluid regime and carbon isotope composition of graphite and fluid inclusions from garnet sillimanite bearing leucocratic tonalites trondhjemites and granites associated with orthopyroxene bearing granulite metapelites in the southern marginal zone smz of the limpopo complex south africa are presented in the paper re integrated compositions of perthitic alkali feldspars and antiperthitic plagioclase as well as p t and t xco2 phase equilibria modeling using perple x software indicate that the granitoids began to crystallize at temperatures of 900 940 c and pressures of 7 9 kbar and were equilibrated with a fluid phase with xco2 0 5 0 6 as is recorded in dense fluid inclusions in quartz a small fraction of a saline fluid accumulated during cooling only average 13cpdb values for graphite 6 52 to 8 65 and fluid inclusions 2 50 to 5 58 from the granitoids differ substantially from the values which have been obtained in previous studies for graphite from the surrounding smz metapelites isotope data thus indicate that fluids associated with the granitoid magmas of the smz originated from a source unrelated to the host metapelites in terms of the heavy isotope signatures of carbon the granitoids might carry fluids which have been produced during devolatilization of mafic rocks amphibolites interlayered with hydrothermal carbonate veins in the adjacent granite greenstone successions of the kaapvaal craton that have been buried underneath the smz granulites during their exhumation the cratonic rock could serve as a source for the trondhjemite magmas intruding the smz granulites in this scenario the studied granitoids crystallized from these magmas variously contaminated by metapelitic material graphite precipitated via reduction of co2 rich fluid as the result of dissolution of pyrrhotite rich metapelites in the magmas

isotopic and geochemical evidence for crust mantle interaction during late archaean crustal growth abstract the charnockites and enderbites of the north marginal zone of the limpopo belt in southern zimbabwe represent a suite of dry magmas of tonalite trondhjemite granodiorite ttg affinity which crystallised directly in the lower crust they were emplaced between 2 6 and 2 7 ga and were derived from a source which showed remarkable lead isotope homogeneity unlike many other lower crustal rocks they have low k rb and low k u ratios and have not experienced depletion of their heat producing elements th u k and rb and thus represent an unusual suite of granulites in this study we have combined the constraints of trace element and isotope geochemistry to characterise the nature of the protolith to the nmz enderbites and charnockites we use a combination of high values calculated lead and strontium isotope initial ratios and old nd model ages to show that the enderbites and chamockites were derived from a protolith which was in part formed more than 500 ma before magma genesis we argue from the very high values that this protolith constituted two separate components an old high low sm nd crustal component and a younger low high sm nd mantle derived component both of which were homogenised by extensive mixing prior to 2 7 ga the presence of an older crustal component contrasts with other archaean ttg suites worldwide which show little evidence for a crustal prehistory the process of mantle crust interaction during late archaean crustal growth in southern zimbabwe invites comparison with modern convergent margins such as the andes

pro and retrograde p t evolution of granulites of the beit bridge complex limpopo belt south africa constraints from quantitative phase diagrams and geotectonic implications interpretations based on quantitative phase diagrams in the system cao na2o k2o tio2 mno feo mgo al2o3 sio2 h2o indicate that mineral assemblages zonations and microstructures observed in migmatitic rocks from the beit bridge complex messina area limpopo belt formed along a clockwise p t path that path displays a prograde p t increase from 600 c 7 0 kbar to 780 c 9 10 kbar pressure peak and 820 c 8 kbar thermal peak followed by a p t decrease to 600 c 4 kbar the data used to construct the p t path were derived from three samples of migmatitic gneiss from a restricted area each of which has a distinct bulk composition 1 a k al rich garnet biotite cordierite sillimanite k feldspar plagioclase quartz graphite gneiss 2 a k poor al rich garnet biotite stauro lite cordierite kyanite sillimanite plagioclase quartz rutile gneiss and 3 a k al poor fe rich garnet orthopyroxene biotite chlorite plagioclase quartz rutile ilmenite gneiss preservation of con tinuous prograde garnet growth zonation demonstrates that the pro and retrograde p t evolution of the gneisses must have been rapid occurring during a single orogenic cycle these petrological findings in combination with existing geochronological and structural data show that granulite facies metamorphism of the beit bridge metasedimentary rocks resulted from an orogenic event during the palaeoproterozoic c 2 0 ga caused by oblique collision between the kaapvaal and zimbabwe cratons abbreviations follow kretz 1983

the limpopo mobile belt a result of continental collision the 600 km long limpopo mobile belt is discussed within the frame of a proterozoic supercontinent model piper 1976 evidence is presented that the rhodesian and kaapvaal cratons may have been separated by distances of more than 1000 km of oceanic crust from about 3350 ma ago the kaapvaal craton appears to have been driven intermittently n nw and then wnw against the rhodesian craton forming the ne sw trending collision zone the limpopo mobile belt and all the major fold and fracture patterns found this movement would be similar to the oblique movement of the pacific plate into the aleutian trench when collision ceased around 2500 ma ago it is likely that the great dyke and other complexes intruded along release fractures formed at right angles to the compression

the effect of different second phase particle regimes on grain growth in two phase aggregates insights from in situ rock analogue experiments the aim of the study was to investigate the effect of rigid second phases on grain growth of a matrix phase for this purpose variable mixtures of norcamphor as the matrix phase with glass beads 0 08 0 51 volume fraction as second phase were used to perform see through rock analogue experiments under static conditions at constant temperatures 50 c irrespective of the second phase content grain size evolution of all mixtures can be subdivided into a stage of continuous grain growth a transient stage and a stage of a finally stabilized grain size on the grain scale the second phases affect the migrating grain boundaries either by pinning by single particles by multiple particles or even by particle clusters summed up over the entire aggregate these pinning regimes affect the average bulk grain size of the matrix grains such that the changes in matrix grain size directly correlate with the amount of second phases their dispersion and their degree of clustering in this way the matrix grain size decreases with increasing second phase content which can be expressed as a zener relationship originating from the modification of an ordinary grain growth law a new mathematical expression is defined which allows the calculation of changes in the matrix grain size as a function of different second phase volume fractions and particle sizes such models will be helpful in the future to predict microstructural changes in polymineralic rocks at depth

zircon ages defining deposition of the palaeoproterozoic soutpansberg group and further evidence for eoarchaean crust in south africa abstract the precise age of the volcano sedimentary soutpansberg group which was deposited upon the palala shear belt separating the kaapvaal craton from the central zone of the limpopo belt has long been debated the soutpansberg group is subdivided into a lower and upper succession which are separated from each other by a prominent regional unconformity zircon grains from silicic pyroclastic rocks of both successions were investigated in order to constrain the timing of deposition of the soutpansberg group rocks the zircon grains of the investigated samples from both successions yield a wide range of ages spanning from 1831 to 3937 ma most of the zircon grains have rounded shapes however it is not clear whether they are mainly xenocrystic or detrital or have been rounded by resorption in a silicic magma chamber the youngest zircon grain ages obtained come from the lower succession and are 1832 9 and 1831 15 ma we interpret these youngest zircon grains as magmatic grains that have been rounded by resorption this view is corroborated by the fact that no magmatic rocks of this particular age have been observed in the kaapvaal craton or the central zone of the limpopo belt and that no apparent sedimentary admixtures are present in the well exposed pyroclastic rocks we therefore conclude that deposition of the soutpansberg volcano sedimentary succession commenced around 1830 ma the soutpansberg rocks were deposited apparently over a lengthy period of time ca 230 ma as provided by the published age of 1604 ma for pyroclastic rocks of the upper succession in botswana zircon grain age spectra of our soutpansberg samples show prominent peaks at 2 0 2 6 and 3 2 ga indicating the central zone of the limpopo belt as the source area but excludes the adjacent northern part of the kaapvaal craton the oldest zircon grain identified in the soutpansberg samples has an age of 3937 4 ma one of the oldest zircon grain ages yet reported from the african continent

the normal grain growth behaviour of nominally pure calcitic aggregates the normal grain growth behaviour of four different but all nominally pure calcite powders 99 analytic grade calcite 99 7 chalk 99 97 crushed iceland spar 99 95 chelometric grade calcite has been investigated as a function of temperature 550 600 650 700 c and confining pressure 100 190 mpa under both dry and hydrostatic pfluid ptotal conditions the initial particle size of both the analytic grade and chelometric grade calcite was about 5 m and that of the chalk was about 3 m while the experiments on the iceland spar were conducted on powders of three different initial particle sizes 3 4 7 5 38 5 m on each material at each pressure temperature condition 6 to 15 experiments equally spaced in log time from 15 minutes to 50 days were conducted under dry conditions all four materials recrystallized to aggregates which contained less than 2 porosity and which had a grain size of between 4 and 20 m depending on the initial particle size subsequently the aggregates coarsened by normal grain growth with the kinetics of the growth process being controlled by the rate at which the grain boundaries could drag the residual pores with them as they migrated under nominally identical conditions both the mechanism and rates of pore drag differed greatly for the different materials implying that this process is highly sensitive to trace solute impurity concentrations this sensitivity renders the task of providing a systematic account of dry calcite grain growth kinetics highly problematic under hydrostatic conditions all the powders followed the same normal grain growth kinetics in which the growth process was rate controlled by diffusion through the pore fluid on the grain boundaries an activation enthalpy of 162 6 kj mol 1 and an activation volume of 34 35 cm3 mol 1 was obtained for this process

can grain size sensitive flow lubricate faults during the initial stages of earthquake propagation recent friction experiments carried out under upper crustal p t conditions have shown that microstructures typical of high temperature creep develop in the slip zone of experimental faults these mechanisms are more commonly thought to control aseismic viscous flow and shear zone strength in the lower crust upper mantle in this study displacement controlled experiments have been performed on carbonate gouges at seismic slip rates 1 m s 1 to investigate whether they may also control the frictional strength of seismic faults at the higher strain rates attained in the brittle crust at relatively low displacements 900 c grain boundary sliding gbs deformation mechanisms the observed micro textures are strikingly similar to those predicted by theoretical studies and those observed during experiments on metals and fine grained carbonates where superplastic behaviour has been inferred to a first approximation the measured drop in strength is in agreement with our flow stress calculations suggesting that strain could be accommodated more efficiently by these mechanisms within the weaker bulk slip zone rather than by frictional sliding along the main slip surfaces in the slip zone frictionally induced grainsize sensitive gbs deformation mechanisms can thus account for the self lubrication and dynamic weakening of carbonate faults during earthquake propagation in nature

microstructural evolution during strain localization in dolomite aggregates abstract dolomite aggregates deformed by dislocation creep over a wide range of conditions t 700 1000 c effective pressure of 900 mpa strain rates of 10 7 10 4 s strain weaken by up to 75 of the peak differential stress microstructural study of samples shortened to different finite strains beyond the peak differential stress shows that strain becomes highly localized within shear zones by high temperature creep processes with no contribution of brittle cracking at low strains 8 dolomite deforms homogeneously by recrystallization accommodated dislocation creep at progressively higher sample strains deformation is localized into narrow shear zones made up of very fine 3 m recrystallized grains and relict porphyroclasts 20 100 m finely recrystallized dolomite grains in the shear zones are largely dislocation free and localized shear is facilitated by diffusion creep in contrast original dolomite grains and porphyroclasts in shear zones have high dislocation densities and do not deform after shear zone formation calculated strain rates in the shear zones are two to three orders of magnitude faster than the imposed bulk strain rate of the samples and these strain rates are consistent with predictions of the diffusion creep flow law for fine grained dolomite

magnetic susceptibility as a tool to study deformed calcite with variable impurity content 1 a set of calcite mylonites were systematically sampled along the basal shear zone of the morcles nappe located in the southwestern swiss alps to investigate the relationship among crystallographic preferred orientation cpo chemistry and anisotropy of magnetic susceptibility ams second phase minerals are 10 vol of the rock the substitution of trace impurities for ca in the calcite crystal lattice in particular fe mn mg and sr varies among the samples with total values ranging from 1450 190 ppm in the most pure sample to 2450 320 ppm for the most impure sample fe shows the largest variation among the sample group and varies by a factor of nearly four from the purest 230 30 ppm to the most impure sample 840 110 ppm all samples have negative bulk susceptibility and the concentration of ferromagnetic in a broad sense phases is very low a correlation is identified between trace element chemistry cpo of calcite and the magnetic susceptibility the bulk susceptibility and the ams vary systematically with fe and mn elemental concentration in the matrix calcite and this variation is apparent for the entire length of the shear zone a simple model that takes into account the cpo and trace element chemistry is used to explain the resulting bulk susceptibility and ams calcite mylonites and marbles from other locations fit the model indicative of a general relationship between cpo chemistry and ams in calcites where second phase minerals are sparse for lithologies that are dominated magnetically by calcite with a deficiency of secondary phases magnetic susceptibility can be used to rapidly screen rock samples for the variation of paramagnetic trace elements fe and mn and cpo which can then be used for detailed microfabric studies

mechanical and microstructural development of carrara marble with pre existing strain variation abstract a series of two stage torsion experiments on carrara marble were conducted to constrain the influence of known prior deformation on rheological response and microstructural evolution comparison with previous experimental data on carrara marble flow and fabric evolution during single stage torsion deformation experiments provided direct insights into the significance of initial deformation at various conditions our experiments were conducted at 727 c temperature and 300 mpa confining pressure while maintaining a constant strain rate of 3 10 4 s 1 on the periphery of the cylindrical samples under these conditions the marble is known to deform in power law n 6 10 ductile flow all torsion experiments were performed with a paterson type gas medium testing machine equipped with a torsion actuator module prior d1 and subsequent d2 deformation are accomplished by two torsion experiments in sequence on same sample segments the effect of d1 strain history is investigated during d2 by applying counter clockwise torsion to a sandwich sample consisting of three segments with different d1 rotation sense d2 samples experienced continued first and reversed shearing deformation in top centre and bottom segments respectively d2 bulk strain was chosen equal to d1 strain in top and bottom segments d1 experiments followed the typical single stage deformation behaviour of carrara marble under the applied experimental conditions yielding was followed by strain hardening until a peak stress was reached at a shear strain around 1 after which work softening occurred weakening gradually evolved into a constant stress regime during hardening a shear microstructure and crystallographic preferred orientation cpo developed afterwards the volume fraction of smaller dynamic recrystallised grains increased continuously resulting in a recrystallisation microstructure and cpo at shear strains of 5 and higher the new d2 experiments displayed a strain variation between homogeneously deformed sample segments that increased with increasing d1 strain the stress strain behaviour of the d2 bulk sandwich samples showed less pronounced work hardening and softening when compared with single stage deformation experiments furthermore constant flow stress was attained at increasingly lower strain with increasing d1 strain in most d2 segments fabric development is equivalent to single stage experiments at corresponding absolute strain the fabrics differed markedly from those in single stage experiments in the case of reversed two stage deformation at moderate strain d1 shear strain 1 and 2 6 experiments show that grain shape by shearing of relict grains is defined by finite strain and thus affected by strain reversal recrystallisation is controlled by absolute strain and not influenced by strain reversal

the effect of dissolved magnesium on diffusion creep in calcite we experimentally tested a series of synthetic calcite marbles with varying amounts of dissolved magnesium in a standard triaxial deformation machine at 300 mpa confining pressure temperatures between 700 and 850 c stresses between 2 and 100 mpa and strain rates between 10 7 and 10 3 s 1 the samples were fabricated by hot isostatic pressing of a mixture of calcite and dolomite at 850 c and 300 mpa the fabrication protocol resulted in a homogeneous fine grained high magnesian calcite aggregate with minimal porosity and with magnesium contents between 0 07 and 0 17 mol mgco3 at stresses below 40 mpa the samples deformed with linear viscosity that depended inversely on grain size to the 3 26 0 51 power suggesting that the mechanisms of deformation were some combination of grain boundary diffusion and grain boundary sliding because small grain sizes tended to occur in the high magnesium calcite the strength also appeared to vary inversely with magnesium content however the strength at constant grain size does not depend on the amount of dissolved magnesium and thus the impurity effect seems to be indirect at stresses higher than 40 mpa the aggregates become non linearly viscous a regime we interpret to be dislocation creep the transition between the two regimes depends on grain size as expected the activation energy for diffusion creep is 200 30 kj mol and is quite similar to previous measurements in natural and synthetic marbles deformed at similar conditions with no added magnesium

exhumation of limpopo central zone granulites and dextral continent scale transcurrent movement at 2 0 ga along the palala shear zone northern province south africa abstract the palala shear zone is part of a major tectonic lineament in southern africa which extends over 1000 km from central botswana to the soutpansberg in south africa the lineament is situated between the central zone of the limpopo belt and the kaapvaal craton at its main exposure in the koedoesrand window 50 km e of ellisras northern province the palala shear zone comprises a c 15 km wide mylonite zone which can be subdivided into a northern central and southern domain this study addresses the tectono metamorphic history of the palala shear zone in the koedoesrand window and its role in the tectonics of the limpopo belt in order to do this structural and petrographical work was combined with geochronology age data from relic mineral parageneses show that metapelites from the central domain underwent granulite facies metamorphism around 2 6 ga however because of an intense proterozoic overprint field relationships of the archean event are largely disrupted during a second high grade event at c 2 0 ga granulite facies metamorphism was reached in the northern domain and the adjacent limpopo central zone charnockites were emplaced in the northern domain after 2 1 ga and were affected by retrograde amphibolite and greenschist facies metamorphism around 1 97 ga post orogenic transcurrent faulting under sub greenschist facies conditions characterizes the central and southern domains field observations and microstructural investigations indicate a dextral shear sense for the retrograde mylonitisation these findings document that the palala shear zone played an important role during the exhumation of the proterozoic granulites in the limpopo central zone associated with a transpressive orogeny between 2 05 and 1 95 ga low grade post orogenic transcurrent faulting in the palala shear zone at c 1 9 ga may be associated with the transtensional opening of the soutpansberg and palapye grabens on the basis of the tectono metamorphic history in the koedoesrand window and regional geophysical geochemical and sequence stratigraphic data the palala shear zone is interpreted as a proterozoic suture zone along which the archean kaapvaal and zimbabwe provinces were juxtaposed

sr and nd isotope systematics of francistown plutonic rocks botswana implications for neoarchaean crustal evolution of the zimbabwe craton the francistown plutonic rocks at the south western margin of the zimbabwe craton consist of three igneous suites sanukitoid tonalite trondhjemite granite ttg suites and high k granites the ttg suite is subdivided into high aluminum ttg ha ttg and low aluminum ttg la ttg sub suites their rb sr isotope systems were partially homogenized by post crystallization thermo tectonic events in which hydrothermal solutions and migmatization played an important role therefore the rb sr isochron age of 2427 54 ma can only be regarded as a lower limit to the francistown plutonic rock age the large errors in the sm nd isochron dates of francistown granitoids indicate that these dates are not really constrained in this study we compared the rock types of francistown and adjacent areas adopting the precise u th pb single zircon shrimp ages from the vumba area as references for ttg and sanukitoid suites the age we adopted is ca 2 7 ga which is close to their depleted mantle sm nd model ages tdm for high k granites the age adopted is ca 2 65 ga which is also close to their sm nd isochron age the highest endt values of sanukitoids and ttg are 2 1 and 2 3 respectively the positive endt values and trace element geochemistry support partial melting of a depleted mantle and young oceanic crust for the genesis of sanukitoid and the ttg suites respectively the lowest endt values of sanukitoids and ttgs are 1 0 and 1 1 respectively indicating contamination by continental crust up to 10 and 14 respectively the endt values of ttg decrease with decreasing al2o3 and sr contents and increasing eu negative anomalies eu eu suggesting that the ttg magmas underwent a coupled fractionation crystallization and crustal contamination and that the la ttg was the product of the fractionation and contamination of the ha ttg sub suite in contrast negative endt values for the high k granites from 0 4 to 3 5 indicate the involvement of la ttg and some materials from an old continental crust in their genesis the products of partial melting of both oceanic and continental crusts at the south western margin of the zimbabwe craton occurred within a short time interval from 2 7 to 2 65 ga ago suggesting that the francistown plutonic rocks were formed in a active continental margin environment where a young ocean plate limpopo oceanic plate subducted underneath an old continental plate zimbabwe craton

the effect of dissolved magnesium on creep of calcite ii transition from diffusion creep to dislocation creep we extended a previous study on the influence of mg solute impurity on diffusion creep in calcite to include deformation under a broader range of stress conditions and over a wider range of mg contents synthetic marbles were produced by hot isostatic pressing hip mixtures of calcite and dolomite powders for different intervals 2 30 h at 850 c and 300 mpa confining pressure the hip treatment resulted in high magnesian calcite aggregates with mg content ranging from 0 5 to 17 mol both back scattered electron images and chemical analysis suggested that the dolomite phase was completely dissolved and that mg distribution was homogeneous throughout the samples at the scale of about two micrometers the grain size after hip varied from 8 to 31 m increased with time at temperature and decreased with increasing mg content 3 0 mol grain size and time were consistent with a normal grain growth equation with exponents from 2 4 to 4 7 for samples containing 0 5 17 0 mol mg respectively we deformed samples after hip at the same confining pressure with differential stresses between 20 and 200 mpa using either constant strain rate or stepping intervals of loading at constant stresses in a paterson gas medium deformation apparatus the deformation tests took place at between 700 and 800 c and at strain rates between 10 6 and 10 3 s 1 after deformation to strains of about 25 a bimodal distribution of large protoblasts and small recrystallized neoblasts coexisted in some samples loaded at higher stresses the deformation data indicated a transition in mechanism from diffusion creep to dislocation creep at stresses below 40 mpa the strength was directly proportional to grain size and decreased with increasing mg content due to the reductions in grain size at about 40 mpa the sensitivity of log strain rate to log stress n became greater than 1 and eventually exceeded 3 for stresses above 80 mpa at a given strain rate and temperature the stress at which that transition occurred was larger for samples with higher mg content and smaller grain size at given strain rates constant temperature and fixed grain size the strength of calcite in the dislocation creep regime increased with solute content while the strength in the diffusion creep regime was independent of mg content the results suggest that chemical composition will be an important element to consider when solid substitution can occur during natural deformation

the motloutse complex and the zimbabwe craton limpopo belt transition in botswana abstract the archaean rocks of northeastern botswana are divided into named lithostratigraphic complexes each a different assemblage of stratiform lithologies granitoids and gneisses and each with differences in metamorphic grade structural style and metallic mineralisation the motloutse complex lies between four other complexes it includes granitoids and orthogneisses typical of the zimbabwe craton to the east detached segments of the quartzite carbonate pelite association of the anomalous matsitama schist belt to the west and migmatites similar to those of the limpopo belt to the south but is itself a distinctive lithological assemblage it shows a higher grade of metamorphism than the adjacent cratonic terranes but lacks the granulites of the limpopo belt it is delineated by relatively abrupt changes in structural style and fabric orientation of which most if not all coincide with shear zones or faults it is partly bounded against the limpopo belt by the magogaphate shear zone but also flanks the western boundary of the limpopo belt central zone west of 27 35 e the motloutse complex was formed in the late archaean between 2 7 ga and 2 5 ga ago in a zone of plutonism which separated a crustal fragment bearing the depositional basin of the matsitama shelf type sequences from the developing greenstone belts to the east during regional deformation elements of the matsitama belt and associated paragneisses were thrust from the southwest over the active magmatic zone and were then infolded with and underplated by migmatites orthogneisses and granitoids of the developing motloutse complex the terranes which were thus accreted to the southwestern margin of the zimbabwe craton during this episode of deformation and crustal thickening d1 d3 were then cut across by dextral movement on the magogaphate shear zone during d4 so juxtaposing the limpopo belt with the motloutse complex the area of the motloutse complex was previously regarded as a transitional boundary between the craton and the mobile belt but is here seen instead as an intracratonic accretionary assemblage on which the ene wsw limpopo trend was partly superimposed during d4 shearing delineation of lithostratigraphic complexes elsewhere in the zimbabwe craton and the limpopo belt may be possible and can be expected to clarify the pattern to which these provinces were assembled during the archaean prior to d4 deformation

two orogenic events separated by 2 6ga mafic dykes in the central zone limpopo belt southern africa abstract the limpopo belt of southern africa is a typical early precambrian orogen that experienced two high grade metamorphic events which are a key for understanding its tectonic evolution there has been a long standing debate on whether the neoarchean c 2 65 ga or the palaeoproterozoic c 2 0 ga tectono thermal event records continental collision the clear field relationship between deformed mafic dykes and the surrounding rocks is a powerful tool to help reconstructing the deformation history mafic dykes intruded the 3 3 3 1 ga sand river gneiss suite in the central zone of the limpopo belt near musina south africa and were classified in the past into older causeway dykes and younger stockford dykes according to assumed field relationships deformational patterns and doubtful rb sr dating our detailed field observations suggest that both dyke types belong to the same generation and cut the foliation and anatectic melt veins in the sand river gneiss suite they also experienced strong post emplacement deformation and high grade metamorphism so they separate two high grade metamorphic events shrimp zircon dating yielded a likely emplacement age of 2 6 ga for two causeway dyke samples as well as metamorphic ages of 2002 4 ma and 2002 8 ma the zircons of two stockford dyke samples yielded igneous emplacement ages of 2607 5 and 2604 6 ma respectively and metamorphic ages of 2014 13 ma and 2005 11 ma combining these data with similarities in the dyke geochemistry we propose that the causeway and stockford dykes formed at the same time but had different orientations our preferred tectonic model for the evolution of the central zone of the limpopo belt involves continental collision between the zimbabwe and kaapvaal cratons at 2 65 ga followed by intracontinental transpressive orogeny and high grade metamorphism at 2 0 ga we therefore propose that the widespread 2 6 ga mafic dykes in the central zone formed in an extensional environment during post collisional orogenic collapse both the neoarchaean and palaeoproterozoic orogenic events resulted in high grade metamorphism and are separated in time by emplacement of the 2 6 ga mafic dykes

fast diffusion along mobile grain boundaries in calcite experimental measurements of grain boundary diffusion are usually conducted on static boundaries despite the fact that grain boundaries deep in the earth are frequently mobile in order to explore the possible effect of boundary mobility on grain boundary diffusion rates we have measured the uptake of 44ca from a layer of 44ca enriched calcite powder during the static recrystallization of a single crystal of calcite at 900 c a region about 500 m wide adjacent to the powder layer is heterogeneously enriched in 44ca and complex zoning patterns including sharp steps in composition and continuous increases and decreases in 44ca content are developed in metamorphic rocks these would normally be interpreted in terms of changes in pressure or temperature rayleigh fractionation or episodic fluid infiltration these explanations cannot apply to our experiments and instead the zoning patterns are interpreted as being due to variations in grain boundary migration rate we have applied an analytical model which allows the product of grain boundary diffusion coefficient and grain boundary width dgb to be calculated from the grain boundary migration rate and the compositional gradient away from the powder layer the value of dgb in the mobile grain boundaries is at least five orders of magnitude greater than the published value for static boundaries under the same conditions in order to allow the scale of chemical equilibrium and hence textural evolution to be predicted under both experimental and geological conditions we present quantitative diffusion regime maps for static and mobile boundaries in calcite using both published values and our new values for grain boundary diffusion in mobile boundaries enhanced diffusion in mobile boundaries has wide implications for the high temperature rheology of earth materials for geochronology and for interpretations of the length and time scales of chemical mass transport moreover zones of anomalously high electrical conductivity in the crust and mantle could be regions undergoing recrystallization such as active shear zones rather than regions of anomalous mineralogy water or melt content as is generally suggested

unraveling the record of successive high grade events in the central zone of the limpopo belt using pb single phase dating of metamorphic minerals abstract dating of relic metamorphic assemblages can provide important information about the timing and character metamorphic grade and or pt evolution of early high grade episodes in polymetamorphic provinces using pb stepwise leaching of metamorphic silicates we have dated multiple granulite facies metamorphic episodes in the central zone cz of the limpopo belt ages of 2 52 ga were obtained from sillimanite and cogenetic garnet and ages of about 2 01 ga from titanite garnet and clinopyroxene together with new and published conventional age data from accessory phases and in the context of combined petrological and structural data these results lead us to a reinterpretation of the tectono metamorphic history of the cz three distinct high grade events at about 3 2 3 1 ga 2 65 2 52 ga and 2 0 0 05 ga are recognized each of these is suggested to correspond to a tectonic episode of distinct character a for the 3 2 ga event magmatic activity can mainly be identified best represented for example by the sand river gneisses or the messina layered intrusion the field relationships concerning the tectonometamorphic history of this early archean event are largely erased by at least two high grade metamorphic overprints b late archean 2 6 2 52 ga low pressure granulite facies metamorphism was associated with voluminous granitic and charnockitic plutonism the anticlockwise p t evolution of these granulites probably reflects deep crustal processes associated with magmatic underplating or in plating contemporaneous with vertical crustal growth of the zimbabwe craton around 2 6 ga c during the proterozoic event 2 05 1 95 ga tectonic thickening was caused by the collision of the kaapvaal and zimbabwe cratons the cz was squeezed between these two cratons and as a consequence underwent high pressure granulite facies metamorphism with a clockwise p t evolution the structural metamorphic and geochronological data can be best explained with a tectonic model that describes this final event as a dextral transpressive orogeny

preferred crystallographic orientation development during the plastic and superplastic flow of calcite rocks abstract grain size sensitive flow of fine grained rocks such as are often found in mylonitic shear zones is commonly inferred to be accomplished by grain boundary sliding so that no preferred crystallographic orientation is expected to develop pre existing fabrics are supposed to weaken and grain shapes to remain equant however many fine grained mylonites commonly exhibit marked preferred crystallographic orientation whilst displaying a stable but weak grain shape fabric we have examined preferred crystallographic orientation and shape fabric development during the flow of synthetic ultrafine grained 700 c the strong crystal plastic fabric develops more slowly but is not eliminated through the broad transition into true superplastic flow this is interpreted to reflect an important role of intracrystalline plasticity in the accommodation of grain boundary sliding from experiments designed to investigate the extent of survival or modification of an initial twinning fabric through recrystallization and subsequent hot working it was shown that although the microstructure could be totally annealed the early formed fabric survived even subsequent high temperature crystal plastic or superplastic flow this may explain the frequent occurrence of strong low temperature fabrics in calcite ultramylonites

paleowattmeters a scaling relation for dynamically recrystallized grain size during dislocation creep mineral grains often evolve to a stable size dictated by the deformation conditions we suggest that grain size evolution during deformation is determined by the rate of mechanical work provided that other elements of microstructure have achieved steady state and that the dissipation rate is roughly constant then changes in internal energy will be proportional to changes in grain boundary area if normal grain growth and dynamic grain size reduction occur simultaneously then the steady state grain size is determined by the balance of those rates a scaling model using these assumptions and published grain growth and mechanical relations matches stress grain size relations for quartz and olivine rocks with no fitting for marbles the model also explains scatter not rationalized by assuming that recrystallized grain size is a function of stress alone when extrapolated to conditions typical for natural mylonites the model is consistent with field constraints on stresses and strain rates

post deformational annealing of calcite rocks abstract the evolution of microstructure and crystallographic preferred orientation cpo during post deformational annealing was studied on three calcite rock types differing in purity and grain size carrara marble 98 calcite mean grain size of 115 m solnhofen limestone 96 5 m and synthetic calcite aggregates 99 7 m samples were first deformed in torsion at 727 c at a shear strain rate of 3 10 4 s 1 to a shear strain of 5 and subsequently heat treated at 727 c for various durations between 0 and 24 h microstructures and cpos were analysed by optical microscopy image analysis and electron backscatter diffraction ebsd all rock types deformed in the dislocation creep field at the same applied conditions but their microstructures and cpos after deformation and after annealing differed depending on starting grain size and material composition in carrara marble and in the synthetic calcite aggregate a strong cpo developed during deformation accompanied by dynamic recrystallisation with significant changes in grain size during annealing widespread grain growth and subtle changes of cpo occurred and equilibrated foam microstructures were approached after long annealing times the cpo is the only feature in annealed samples indicating an earlier deformation phase although it is not always identical to the cpo formed during deformation in the more impure solnhofen limestone secondary phases on grain boundaries suppressed grain boundary mobility and prevented both the formation of a recrystallisation cpo during deformation and grain size modification during deformation and annealing

the behaviour of nd and pb isotopes during 2 0 ga migmatization in paragneisses of the central zone of the limpopo belt south africa and botswana we have carried out major and trace element analyses and nd pb isotope measurements on different components of two migmatite outcrops in the limpopo belt lose quarry botswana and palala south africa in order to assess the effect of migmatization during a high grade metamorphic event on sm nd systematics and nd model ages the outcrops studied are located in the mahalapye tshipise straightening zone at the south western part of the limpopo central zone close to its contact with the kaapvaal craton leucosomes of peraluminous granitic composition were formed by incongruent biotite melting in an fe rich metagreywacke lose quarry and in a metapelite palala respectively monazite u pb and garnet sm nd ages yield close to 2 0 ga for the high grade event confirming other recent work in the central zone the chemical compositions of the main mineral phases feldspar biotite and garnet indicate minimum melt formation at t 750 c and low pressure isocon plots and pb isotope data indicate that the partial melts are locally derived however lil trace element distribution between the leucosomes and paleosomes indicate the incomplete equilibration of the nascent melts with the residual feldspars and biotite in the paleosomes sm nd results on leucosomes and paleosomes of the metagreywacke migmatite show considerable scatter when calculated back to 2 0 ga the paleosomes fall in a narrow range of nd 4 and give tdm model ages between 2 45 and 2 66 ga the nd values of leucosomes are on average four units lower causing them to have apparent tdm ages of 2 7 2 92 ga on average 250 ma older than those of the paleosomes the ree th p2o5 and zr budgets of paleosomes and leucosomes indicate dominance of the lree by monazite and the hree by apatite incorporation of proportionally more monazite than apatite into the melt is inferred as the cause for the nd isotope discrepancy and hence the uncertainty of the average protolith mantle derivation age in contrast pb pb results define an apparent age of 2025 29 ma reflecting nearly full pb isotope exchange in the case of the palala metapelitic migmatite the nd values at 2 0 ga of all but two migmatitic constituents cluster between 7 and 9 5 pointing out extensive nd isotope equilibration during migmatization whereas the pb isotopes yield an errorchron date of 2311 65 ma indicating incomplete pb isotope exchange here garnet dominates the hree distribution whereas the lree are controlled by feldspar and monazite considering that nd isotope equilibration was nearly reached during migmatization nd model ages on the whole rocks represent a good approximation of those of the original protolith 3 0 3 2 ga in the regional context the pb isotope data from the lose quarry locality combined with the nd model age uncertain but within the range of 2 5 2 9 are reminiscent of the northern marginal zone of the limpopo belt and the southern zimbabwe craton whereas the nd and pb isotope data from the palala locality are typical for the limpopo southern marginal zone and the northern kaapvaal craton

greenstone belts at the northernmost edge of the kaapvaal craton timing of tectonic events and a possible crustal fluid source abstract the hout river shear zone a complex thrust system marks the boundary between the low to medium grade metamorphic granitoid greenstone northern kaapvaal craton province and the high grade southern marginal zone of the limpopo complex to the north of it the giyani greenstone belt located on the northern kaapvaal craton immediately adjacent to the hout river shear zone is in a key position for studying the tectonics of this shear zone we have mapped two central transects through the greenstone belt dated metamorphic minerals by 207 pb 206 pb stepwise leaching and 40 ar 36 ar and compared our results with data from the pietersburg polokwane and rhenosterkoppies greenstone belts which are close to the hout river shear zone further west the se domain of the ne sw striking giyani greenstone belt appears to constitute a nw vergent nappe movement along the hout river shear zone is consistently sw vergent varying in character between frontal and lateral ramps and nappe the nw domain of the giyani greenstone belt is dominated by ultramafic schists with mafic and banded iron formation bif intercalations it is considered a separate tectonic unit from the se domain in which mafic intrusive and pillow volcanic rocks are prevalent with intercalated clastic sediments correlations between this se domain and the lower mafic unit of the pietersburg greenstone belt and with the mafic bif association of the rhenosterkoppies greenstone belt are possible ages for peak metamorphism in the immediate footwall of the hout river shear zone from this and previous work yield a weighted mean age of 2717 28 ma 95 confidence documenting a hot iron metamorphism a garnet age of 2833 38 ma in the se domain of the giyani greenstone belt overlaps with ages from 2854 to 2870 ma for youngest zircons in syntectonic intrusions within the giyani and the pietersburg greenstone belts as well as youngest detrital zircons in the probably syntectonic clastic sedimentary uitkyk formation of the latter these ages constrain a tectonometamorphic event in both belts termed here the pgb lwaji orogeny amphibole 40 ar 39 ar dates from the hout river shear zone from this and previous work and shear zones within the giyani greenstone belt yield disturbed age spectra with step ages mainly in the 2550 2670 ma range the amphiboles are chemically heterogeneous and the ages can be shown to reflect mixtures of older and younger amphibole generations due to reactivation of the shear zones and fluid activity after the main event in the hout river shear zone thermal modelling shows that in spite of the very low content of radioactive heat producing elements in the southern marginal zone its lower crust after moderate crustal thickening could be heated to granulite facies conditions in 30 60 ma a much shorter time than the 120 ma age difference between the pgb lwaji orogeny and the hout river shear event a causal link between the two tectonic events is thus precluded on the other hand previously low grade supracrustal rocks from the kaapvaal craton buried at lower crustal levels during hout river shear event could provide a source for fluids up to about 80 ma after the event

modeling episodic fluid release events in the ductile carbonates of the glarus thrust the exposed glarus thrust displays midcrustal deformation with tens of kilometers of displacement on an ultrathin layer the principal slip zone psz geological observations indicate that this structure resulted from repeated stick slip events in the presence of highly overpressured fluids here we show that the major characteristics of the glarus thrust movement localization periodicity and evidence of pressurized fluids can be reconciled by the coupling of two processes namely shear heating and fluid release by carbonate decomposition during this coupling slow ductile creep deformation raises the temperature through shear heating and ultimately activates the chemical decomposition of carbonates the subsequent release of highly overpressurized fluids forms and lubricates the psz allowing a ductile fault to move tens of kilometers on millimeter thick bands in episodic stick slip events this model identifies carbonate decomposition as a key process for motion on the glarus thrust and explains the source of overpressured fluids accessing the psz

a linear hf isotope age array despite different granitoid sources and complex archean geodynamics example from the pietersburg block south africa abstract combined u pb and lu hf isotope data from zircon populations are widely used to constrain hadean archean crustal evolution linear hf isotope age arrays are interpreted to reflect the protracted internal reworking of crust derived from the depleted mantle during a short lived magmatic event and related 176lu 177hf ratios are used to constrain the composition of the reworked crustal reservoir results of this study however indicate that hf isotope age arrays can also result from complex geodynamic processes and crust mantle interactions as shown by u pb and lu hf isotope analyses of zircons from well characterized granitoids of the pietersburg block pb northern kaapvaal craton south africa apart from scarce remnants of paleoarchean crust most granitoids of the pb with ages between 2 94 and 2 05 ga n 32 define a straight hf isotope age array with low 176lu 177hf of 0 0022 although they show a wide compositional range were derived from various sources and emplaced successively in different geodynamic settings the crustal evolution occurred in five stages i predominately mafic crust formation in an intra oceanic environment 3 4 3 0 ga ii voluminous ttg crust formation in an early accretionary orogen 3 0 2 92 ga iii internal ttg crust reworking and subduction of ttg derived sediments in an andean type setting 2 89 2 75 ga iv post collisional high k magmatism from both mantle and crustal sources 2 71 2 67 ga and v alkaline magmatism in an intra cratonic environment 2 05 2 03 ga the inferred array results from voluminous ttg crust formation during stage ii and involvement of this crust during all subsequent stages by two different processes i internal crust reworking through both partial melting and assimilation at 2 89 2 75 ga leading to the formation of biotite granites coeval with minor ttgs and ii subduction of ttg derived sediments underneath the pb causing enrichment of the mantle that subsequently became source for high k granitoids and mafic rocks at 2 68 and 2 05 ga some scatter along the array might have resulted either from significant assimilation of ancient crust intracrustal lu hf fractionation or melting of heterogeneous mantle sources those results show that without any information about the nature and composition of zircon hosting granitoids hf isotope age data are of limited use to constrain hadean archean magmatogenesis and geodynamics

himalayan tibetan analogies for the evolution of the zimbabwe craton and limpopo belt abstract the limpopo orogeny which lasted from 2700 to 2600 ma was characterised by nnw sse directed shortening this resulted in both crustal thickening accommodated by folding and nnw directed thrusting and associated crustal extrusion whereby a number of crustal blocks including the central zone of the limpopo belt was displaced to the west southwest along wsw ene trending shear zones this was part of a regionally distributed phase of orogenic deformation which affected the adjoining kaapvaal and zimbabwe cratons from the witwatersrand basin in the south to the harare greenstone belt in the north this deformation which migrated northwards with time postdated an earlier phase of crustal accretion during which the archaean crustal unit now marked by the kaapvaal and zimbabwe cratons and the limpopo belt grew towards the north northwest shortening of this unit during the limpopo orogeny may be regarded as the last stage of the crustal accretion process this tectonic process of continental growth by accretion followed by crustal shortening with associated crustal extrusion has many features in common with tibetan tectonics from the mesozoic to the present

grain growth control of isotope exchange between rocks and fluids isotope exchange between fluid and rocks has been traditionally considered to be rate limited by two elementary processes lattice diffusion in the matrix minerals and dissolution into the fluid followed by precipitation from it in this study we show the results of high pressure experiments on 18 o water infiltration into quartzite that point to a third highly efficient process grain growth accompanied by migration of the grain boundaries gbs that are isotopically enriched by gb diffusion and surface exchange we predict on the basis of a mass transfer mechanism discrimination diagram that this gb sweeping is the primary control on isotopic equilibration under hydrothermal conditions in various fine grained rocks with low fluid fraction grain growth should be considered when interpreting and simulating isotope and chemical composition of rocks and fluids

an isotopic and geochemical study of the northern kaapvaal craton and the southern marginal zone of the limpopo belt are they juxtaposed terranes abstract to test existing lateral accretion models for the archean kaapvaal craton in south africa metapelites and leucocratic granitoids from two different provinces the granulite facies southern marginal zone of the limpopo belt and the northern part of the adjacent greenschist to amphibolite facies kaapvaal craton have been studied both provinces resemble each other in terms of isotope and trace element characteristics they show a general depletion of incompatible elements independent from the grade of metamorphism this observation conflicts with theories involving mobilisation during granulite facies metamorphism a further common fingerprint is the unusually high cr and ni concentrations in the metapelites in addition they define a very narrow range of nd model ages 2 9 3 05 ga constraining the average crustal age of their cratonic source region the granitoid gneisses show somewhat larger variations of nd model ages indicating some age heterogenity within the crust however the data from both provinces completely overlap the southern marginal zone and the kaapvaal craton also show uniform low 207 pb 204 pb relative to their 206 pb 204 pb ratios that differ clearly from other southern african provinces these results support the view that the southern marginal zone represents a part of the kaapvaal craton exposed at a deeper crustal level it rules out a hypothesis that both provinces are separate amalgamated terranes furthermore there is very little difference between granitoids and metapelites which indicates that the latter largely derived from the former the exposure of the granulite facies metapelites of the southern marginal zone at the same level as the lower grade metamorphic rocks of the kaapvaal craton indicates uplift along a sharp tectonic boundary

grain growth in synthetic marbles with added mica and water evolution of grain size in synthetic marbles was traced from compaction of unconsolidated powder through primary recrystallization and normal grain growth to a size stabilized by second phases to form the marbles reagent grade caco3 was mixed with 0 1 and 5 volume mica and heat treated under pressure with added water densification with negligible recrystallization occurred within one hour at 500 c and 500 mpa confining pressure primary recrystallization occurred at 500 550 c causing increases of grain size of factors of 2 5 resulting samples had uniform grain size gently curved grain boundaries and near equilibrium triple junctions they were used subsequently for normal grain growth studies normal grain growth occurred above 550 c at 800 c grain size d increased from 7 m d0 to 65 m in 24 hours growth rates fit the equation d n d 0 n kt where k is a constant and n 2 6 minor amounts of pores or mica particles inhibit normal grain growth and lead to a stabilized grain size dmax which depends on the size of the second phases and the inverse of their volume fraction raised to a power between 0 3 and 1 once dmax is reached normal growth continues only if second phases are mobile or coarsen or if new driving forces are introduced that cause unpinning of boundaries normal grain growth in solnhofen limestone was significantly slower than in pure synthetic marble suggesting that migration is also inhibited by second phases in the limestone

differences in grain growth of calcite a field based modeling approach normal grain growth of calcite was investigated by combining grain size analysis of calcite across the contact aureole of the adamello pluton and grain growth modeling based on a thermal model of the surroundings of the pluton in an unbiased model system i e location dependent variations in temperature time path 2 3 and 1 3 of grain growth occurs during pro and retrograde metamorphism at all locations respectively in contrast to this idealized situation in the field example three groups can be distinguished which are characterized by variations in their grain size versus temperature relationships group i occurs at low temperatures and the grain size remains constant because nano scale second phase particles of organic origin inhibit grain growth in the calcite aggregates under these conditions in the presence of an aqueous fluid these second phases decay at a temperature of about 350 c enabling the onset of grain growth in calcite in the following growth period fluid enhanced group ii and slower group iii growth occurs for group ii a continuous and intense grain size increase with t is typical while the grain growth decreases with t for group iii none of the observed trends correlate with experimentally based grain growth kinetics probably due to differences between nature and experiment which have not yet been investigated e g porosity second phases therefore grain growth modeling was used to iteratively improve the correlation between measured and modeled grain sizes by optimizing activation energy q pre exponential factor k0 and grain size exponent n for n 2 q of 350 kj mol k0 of 1 7 1021 mns 1 and q of 35 kj mol k0 of 2 5 10 5 mns 1 were obtained for group ii and iii respectively with respect to future work field data based grain growth modeling might be a promising tool for investigating the influences of secondary effects like porosity and second phases on grain growth in nature and to unravel differences between nature and experiment

impact of grain size evolution on necking in calcite layers deforming by combined diffusion and dislocation creep abstract natural pinch and swell structure in centimetre thick calcite layers shows a reduction of grain size from swell towards pinch however the impact of grain size evolution on necking and pinch and swell formation is incompletely understood we perform zero dimensional 0d and 2d thermo mechanical numerical simulations of calcite layer extension to quantify the impact of grain size evolution on necking for bulk extension rates between 10 12 s 1 and 10 14 s 1 and temperatures around 350 c for a combination of diffusion and dislocation creep we calculate grain size evolution according to the paleowattmeter grain size is proportional to mechanical work rate or the paleopiezometer grain size is proportional to stress numerical results fit three observations i significant thickness variations along the layer after extension ii grain size reduction from swells towards pinches and ii dislocation creep dominated deformation in swells and significant contribution of diffusion creep in pinches modelled grain size in pinches 10 60 m and swells 70 800 m is close to observed grain size in pinches 21 6 m and in swells 250 1500 m in the models grain size evolution has a minor impact on necking and viscous shear heating and grain size evolution have a negligible thermal impact

crystallization processes in migmatites an important prerequisite for interpreting microstructures in plutonic and metamorphic rocks is understanding crystallization processes this study uses crystal size distribution csd and grain shape parameters to investigate mineral reaction controlled crystal growth and melt solidification in migmatites of the bayerische wald variscan orogeny germany partially molten rocks were studied because they allow the simultaneous investigation of crystallization of a melt and the growth of solid products of a metamorphic reaction csd and shape factors were obtained for three different minerals 1 cordierite in a diatexite where the melt remained in the system 2 k feldspar in a segregated k feldspar quartz leucosome and 3 plagioclase from a segregated plagioclase quartz leucosome cordierite crystals exhibit a nearly linear csd pattern and subeuhedral crystal shapes k feldspar grains display an approximately linear csd except for a marked increase at large crystal size classes k feldspar grains are characterized by strongly lobate grain boundaries the csd of plagioclase crystals is nearly linear with a slight decrease at small crystal size classes plagioclase shape factors also indicate lobate grain boundaries however with lower values than k feldspar cordierite csd and shape data are consistent with interface controlled crystallization the csd indicates that nucleation and growth is the rate limiting step during the cordierite producing reaction in contrast to porphyroblast growth in solid rocks where mass transfer is typically rate limiting csd combined with information on the duration of crystallization yields growth rates of 10 17 to 10 18 m s the plagioclase data are consistent with magmatic crystallization and are well described by the communicating neighbor theory the k feldspar data also indicate magmatic crystallization the overproduction of large grains of k feldspar reflects the interplay between nucleation and growth rates at the initial stage of crystallization because of different environments between solid and anatectic rocks the csd may help to decipher metamorphic from anatectic migmatites

garnet bearing low sr and high sr singelele leucogranite a record of neoarchean episodic melting in collisional setting and paleoproterozoic overprint in the beit bridge complex southern africa abstract the study presents field petrographic mineral chemical bulk rock geochemical u pb zircon and 40ar 39ar amphibole biotite geochronologic characteristics of the singelele leucogranite which constitutes a prominent neoarchean magmatic event in the beit bridge complex terrane of the limpopo complex southern africa the singelele leucogranite is magnesian to ferroan calcic to alkalic and peraluminous two groups of granites are distinguished on the basis of bulk rock sr content and garnet chemistry low sr with low xmg garnet and high sr with high xmg garnet major and trace element characteristics show that these two groups were probably generated by a fluid absent melting of crustal rocks like metagreywacke at different depths in collision tectonic setting spatial distribution and available geochronologic data indicate that these two groups represent two episodes of singelele leucogranite magmatism low sr at 2 68 2 65 ga and high sr at 2 63 2 62 ga the singelele leucogranite occurring close to major fold structures and faults shear zones are locally affected by anatectic and metasomatic paleoproterozoic overprint at 2 0 ga 2037 21 ma and 2015 15 ma u pb for zircon overgrowths zoning of coarse garnet grains with a decrease of xca in the melting overprinted rocks reflects decompression of about 3 4 kbar the metasomatic overprint produces cl f rich biotite 2 and amphibole phase equilibria modeling shows a lack of equilibrium between these minerals and earlier garnet and indicates modification of the garnet composition by metasomatism 40ar 39ar ages for amphibole 2043 18 ma 1994 12 ma and biotite 2 1985 11 ma 1967 11 ma are consistent with the zircon overgrowth ages

the role of second phases for controlling microstructural evolution in polymineralic rocks a review abstract we present a state of the art review of the microstructural evolution in rocks under static and deformational conditions first the general concepts and processes are introduced using monomineralic aggregates then they are expanded into the more complex context of polymineralic rocks with a dominant matrix phase the first part of this contribution delivers information on sample strategies to quantify polymineralic microfabrics based on comparisons between microfabrics of monomineralic and polymineralic rocks we use the common knowledge collected over the past decades for monomineralic systems and discuss the differences to polymineralic ones in terms of microstructures modal compositions spatial distribution of phases crystallographic preferred orientations and associated processes the article puts particular emphasis on the effect of coupled grain growth mass transfer processes and deformation mechanisms we speculate on the effect of mineral reactions during the evolution of microstructures and rheology in polymineralic aggregates at different metamorphic conditions at the end of the article we demonstrate the great potential of grain size evolution maps as microstructural tool to unravel the geological history of polymineralic rocks that evolved under a variety of geodynamic situations

ultrahigh temperature granulites and magnesian charnockites evidence for neoarchean accretion along the northern margin of the kaapvaal craton abstract shrimp u pb analysis of zircon grains from an ultrahigh temperature mg al granulite from the northern margin of the kaapvaal craton southern marginal zone shows ages of 2 72 ga for the peak metamorphism geochemical characteristics of syn tectonic charnockites in the area are similar to magnesian charnockites typically formed in subduction settings in contrast to the prevailing models on peak metamorphism in the context of neoarchean continent collision between archean zimbabwe and kaapvaal cratons resulting in the formation of the limpopo complex we relate the neoarchean events to the accretion of this marginal block to the rest of the kaapvaal craton in this model the subduction event is marked by the formation of magnesian charnockites and the ultrahigh temperature metamorphism dates the collision event

proterozoic transpressive deformation in the northern marginal zone limpopo belt zimbabwe a major tectono metamorphic event affected the southern half of the northern marginal zone of the limpopo belt at 2 0 ga structural analysis along a 43 km long profile perpendicular to strike shows extension inclined toward the northeast extension parallel to axes of folds that uplift the southeast of the section i e the central zone of the limpopo belt and dextral shear these deformation patterns developed coevally the lineations form distinct tightly clustered groups that occur together in the profile there is no evidence for superimposed deformation or deformed lineations nor are lineations observed cross cutting each other this single episode of deformation can therefore be called transpression petrographic and thermobarometric evidence support this kinematic picture in that decreases of both pressure 2 5 kb and temperature 150 c are documented from southeast to northwest across strike 39 ar 40 ar step heating of metamorphic hornblende from amphibolites has yielded ages of

effect of water on the strength and microstructure of carrara marble axially compressed at high temperature in this study we aimed to i answer the question if water affects the strength of marble deforming at high temperature and ii establish criteria that allow assessment of the role of water in microstructure development in marble samples of carrara marble were axially compressed at temperatures ranging 600 1000 8c and strain rate w10 k5 s k1 with or without addition of 0 4 2 1 wt water water added samples were found to be slightly weaker than dry samples microstructurally all samples showed grain flattening and twinning at 600 8c and dynamic recrystallization dominated by grain boundary migration at higher temperature grain boundaries in wet samples showed isolated or semi continuous remnants of fluid pockets the average grain roundness r and fractal dimension d relating grain diameter d and grain perimeter p pfd d for wet recrystallized samples were systematically lower than for dry samples at 600 8c high pore fluid pressure probably enhanced dilatation of wet samples and helped grain boundary sliding gbs thereby lowering the strength at higher temperature fluid enhanced grain boundary migration served as a recovery process reducing the contribution of gbs to creep overall water has little effect on strength and microstructure of marble experimentally deformed at high temperature q 2005 elsevier ltd all rights reserved

grain growth in the forsterite diopside system abstract grain growth experiments in dunite clinopyroxenite and wehrlites with various forsterite diopside ratios were performed to investigate the effect of modal composition on grain growth kinetics in the earth s uppermost mantle the experiments were conducted using a piston cylinder apparatus at 1200 c and 1 2 gpa for 2 763 h under dry conditions normal grain growth ngg occurred in dunite clinopyroxenite and relatively forsterite poor wehrlites 70 vol of forsterite grain growth rates of forsterite and diopside in relatively forsterite poor wehrlites were much slower than those in dunite and clinopyroxenite in the forsterite rich wehrlites 80 vol of forsterite ngg of diopside and abnormal grain growth agg of forsterite occurred the growth rate of diopside was significantly slower than that in clinopyroxenite while the growth rate of forsterite by agg was found to be similar to that by ngg in dunite the presence of 20 vol diopside had the effect of inhibiting the forsterite grain growth during the initial stage resulting in agg of forsterite thus overtaking the growth rate in dunite our experimental results suggest that there would be a significant difference in grain growth rate and consequently in effective viscosity between olivine rich peridotites depleted mantle and relatively olivine poor peridotites fertile mantle in the case of grain size sensitive creep variation of mean grain sizes in the upper mantle would result not only from differences in temperature and phase assemblage but also from the variation of modal compositions

thermally induced grain growth of calcite marbles on naxos island greece the island of naxos is composed of an elliptically shaped structural and thermal dome of miocene age peak metamorphic temperatures within the central migmatite complex exceeded 700 c decreasing to about 300 c at the most distant exposures on the island equigranular calcite marbles which outcrop together with metapelites and metabasites over the whole island show a systematic pattern of increasing grain size towards the central migmatite complex with a significant discontinuity in the pattern corresponding approximately with the 500 c isotherm the microstructures and grain size distributions in the marbles are consistent with normal grain growth the variation of grain size with peak temperature attained can be explained equally well by the assumptions that a a maximum grainsize had developed particularly at higher temperatures or that b the grain size had been frozen in by a combination of cooling and coarsening both of which combine to reduce the rate of grain growth

discrimination of annealed and dynamic fabrics consequences for strain localization and deformation episodes of large scale shear zones abstract large scale shear zones are often characterized by long lasting deformation with complex strain localization behavior based on a drill core across polymineralic carbonate mylonites of the doldenhorn nappe helvetic alps switzerland the present study uses spatially resolved microstructural variations to discriminate unambiguously deformation microstructures from annealed ones to infer the deformation mechanisms and to reconstruct the strain localization history these goals require quantitative microstructural and textural analyses of sample suites in sections across the shear zone particularly the occurrence of two distinct zener trends i e the relationship between grain size and volume fraction of matrix and second phase grains allows for the identification of annealed from dynamic microstructures the data presented here indicate that continued shearing after peak metamorphic conditions and during exhumation induced cooling resulted in progressive strain localization at the base of the shear zone while the inactive parts of the formerly high temperature shear zone were progressively overprinted by annealing

shear zones of the limpopo belt and adjacent granitoid greenstone terranes implications for late archaean collision tectonics in southern africa abstract granulite metamorphism in the 2 65 ga limpopo belt of southern africa is related to crustal thickening due to late archaean collision tectonics the central zone of the belt is a major thrust sheet of distinctive lithological structural and isotopic character separated from adjacent terranes by crustal scale shear zones and the northern and southern marginal zones are up thrust deep level granulite facies terranes of the adjacent cratons geometrically the marginal zones are mirror images and the limpopo belt is essentially symmetrical in n s section conventionally regarded as a n s collisional orogeny the gross structural and lithological differences between the central zone and the marginal zones kinematic data from shear zones and geophysical data showing the three dimensional geometry of the central zone are here used to suggest that the direction of emplacement of the central zone was also the collision direction this was from northeast to southwest with the upthrust outwardly directed marginal zones developing in mechanical response to this collision

grain growth kinetics of quartz and calcite aggregates grain growth rates for novaculite flint and jasper have been determined between 1000 and 800 c and 15 to 2 kb water pressure a pre treatment established an equilibrium microstructure and a nearly uniform mean grain diameter bar d 0 at 1000 c and 15 kb the grain growth data are linear on a log bar d bar d 0 vs log time plot and there is no significant difference between the three samples although the jasper contained 5 10 vol iron oxide at lower pressures and temperatures the growth curves are approximately parallel to the 1000 c and 15 kb data but growth is slower and that for jasper is slower than for the other two the growth rate is strongly dependent on the solubility of quartz in the intergranular fluid and the activation energy is about 11 kcal mole the data are used to estimate the growth rate of quartz at metamorphic conditions and to help evaluate the conditions where growth of dynamically recrystallized grains may be significant the growth rate of calcite grains s

experimental sytectonic dehydration of serpentinite under conditions of controlled pore water pressure we have studied the mechanical behavior of serpentinite under conditions favouring dehydration to olivine talc water at temperatures ranging between 300 and 600 and at total pressures from 100 to 300 mpa constant stress stress relaxation and constant displacement rate testing techniques permitted a wide range of deformation rates and paths to be accessed effective confining pressure was held constant during the dehydration process by means of a controlled pore water pressure system stress supported by the samples was almost independent of deformation rate at temperatures below those required for the onset of the breakdown reaction at 500 and 600 c however the strength was linearly dependent on the inverse of the deformation rate and independent of effective confining pressure over the range investigated microstructural studies revealed that the weakening was associated with sliding in narrow shear zones containing ultrafine grained 0 1 m olivine produced in the breakdown reaction the enhanced deformability is interpreted in terms of flow of the olivine in these shear zones by diffusion accommodated grain boundary sliding these experimental results suggest that metamorphic transformations affect the deformability of rocks and demonstrate enhancement of deformability through the transient existence of fine grained reaction products the results may also be applicable to the mechanics of deformation processes in oceanic transform faults and give some indication of the degree of weakening that might be expected from grain size reduction by dynamic recrystallization in plastically deforming rocks

porphyroblast microstructures a review of current and future trends many recent papers show how porphyroblast microstructures play an important role in a wide range of structural and metamorphic studies this paper reviews ten current applications of these microstructures 1 porphyroblast growth timing criteria 2 tracking progressive foliation development relative to changing metamorphic conditions 3 timing of pluton emplacement relative to deformation and metamorphism 4 finite longitudinal strain determinations 5 kinematics and porphyroblast rotation 6 use of linear fabrics preserved in porphyroblasts 7 porphyroblasts and folding mechanisms 8 inclusion trail orientations and orogenic processes 9 inferring shear strain rates from porphyroblast growth rates and 10 in situ age determinations although there is still no concensus on the interpretation of some porphyroblast microstructures a bright future lies ahead as traditional and newly developed techniques of microstructural analysis are combined with modern chemical and microprobe techniques to provide an increased understanding of the relationships between deformation and metamorphism in a wide range of metamorphic settings

effects of solute segregation on the grain growth kinetics of orthopyroxene with implications for the deformation of the upper mantle abstract the kinetics of static grain growth in a pure synthetic orthopyroxene and orthopyroxene of natural composition were investigated to determine the effects of solute segregation on grain growth in all experiments the grain size of the synthetic orthopyroxene increases considerably in contrast the natural orthopyroxene shows no perceptible grain growth at the same conditions high resolution x ray compositional mapping confirms that the natural orthopyroxene has large concentrations of aluminum and calcium cations segregated on and near its grain boundaries this segregation is believed to be responsible for the slow grain growth kinetics in the natural orthopyroxene slow grain growth may permit long term deformation in the diffusion creep regime as has been observed in some deep mantle xenoliths as such it may provide a means of weakening the lithosphere

crustal heat production and style of metamorphism a comparison between two archean high grade provinces in the limpopo belt southern africa abstract different characteristics of late archean metamorphism in the northern and southern marginal zones of the limpopo belt in southern africa are examined in the light of differences between these provinces in contents of heat generating elements in the northern marginal zone intrusion of charnoenderbitic magmas lasted from 2 75 to 2 58 ga accompanied by repeated low to medium pressure granulite facies metamorphism with some anticlockwise p t paths in contrast the medium pressure granulites of the southern marginal zone experienced peak metamorphism at 2 69 ga without much evidence of plutonism at that time followed by near isobaric cooling at approximately 6 5 kbar as a basis for our approach we used steady state geotherms calculated for the deeply eroded marginal zones for late archean times on the basis of their average k th and u concentrations and assuming low basal heat flow values of 20 mw m 2 the southern marginal zone and the adjacent northern kaapvaal craton are both characterised by low average th and u concentrations th 1 1 and 1 5 ppm u 0 3 and 0 65 ppm which are not a consequence of high grade metamorphism the average th and u concentrations in the zimbabwe craton 8 8 and 2 2 ppm are much higher than those in the kaapvaal craton and those found in the northern marginal zone are even higher 13 8 and 2 6 ppm in the southern marginal zone peak metamorphic temperatures are much above reasonable steady state geotherms and peak conditions could only have been attained through transient very high basal heat flux which in the context of a subduction zone might have resulted from slab breakoff or slab windows the northern marginal zone in contrast has such a large internal heat generation that high grade metamorphic conditions plot close to steady state geotherms even with basal heat flux values of 20 mw m 2 and minor crustal thickening or basal heat flux increase would have sufficed to exceed fluid absent tonalite solidus temperatures in the lower crust the repeated high grade metamorphism and charnoenderbite intrusions over a period of 1 7 108 years in the province can be explained by this observation both marginal zones of the limpopo belt can be understood as archean active continental margins of the kaapvaal and zimbabwe craton respectively while the problems of explaining the metamorphism in the southern marginal zone i e the source of the required heat are similar to those faced in many present day margins or orogens the northern marginal zone is non uniformitarian in the sense that its extreme heat generation rate could not be a property of any geological province today

boudinage as a material instability of elasto visco plastic rocks abstract pinch and swell structures are commonly interpreted to evolve out of viscosity contrasts which are induced by geometric interactions and material imperfections from materials science an additional localization phenomenon is well established where localization emerges out of steady state for critical material parameters and or loading rates here we investigate the conditions under which this second type of instabilities prevails and whether geological materials necessarily require a trigger by imperfections in order to generate instability we focus on imperfections in terms of grain size variations embedded in a deformation environment controlled by thermo mechanical feedbacks we introduce a random distribution of grain sizes over two orders of magnitude in a central layer embedded in a matrix with a diffusion creep rheology the rheology of the layer evolves with dislocation and diffusion creep as end member deformation mechanisms applying pure shear extension the 3 layer model is subjected to natural deformation conditions the central layer quickly establishes a viscous steady state as a natural response of the system due to relaxation and energy optimization upon continued loading localization then intriguingly arises out of a homogeneous state we present an analysis which confirms that this type of instability is indeed physically admissible using vibration analysis we verify the robustness of the numerical solution by first identifying the natural mode shapes and frequencies of the simulated structure and material parameters including geometric imperfections in a second step the eigenmodes are perturbed and superposed to the initial conditions we conclude that this pattern of perturbations guides the onset of strain localization boudinage can therefore be seen both as a geometric problem and or a material bifurcation which evolves out of homogeneous state the latter class offers the great possibility of extracting fundamental material parameters out of localized structures directly from field observations

geologic history of the central zone of the limpopo complex the west alldays area abstract new field structural petrologic and age data show that tectonic styles and metamorphic histories in the west alldays area of the central zone of the limpopo complex southern africa can be linked to neo archean and paleoproterozoic high grade granulite facies tectonometamorphic events the styles comprise 1 a regionally developed high grade s2 gneissic fabric that evolved into a regional d2 system of sheath folds mapped as circular to oval shaped structures with lineations plunging steeply to the wsw and 2 a system of mainly n s trending and north verging d3 shear zones characterized by high grade tectonites straight gneisses with well developed s3 gneissic fabrics in the west alldays area the superimposition of d3 onto the regional d2 fold pattern produced the kilometer scale n s trending baklykraal shear zone which before this study was mapped as the baklykraal fold and interpreted as part of the regional d2 fold pattern in the central zone the paleoproterozoic high

three tier transition of neoarchean ttg sanukitoid magmatism in the beit bridge complex southern africa abstract neoarchean ttg sanukitoid associations of contrasting scales occur within the beit bridge complex terrane of the limpopo complex in southern africa these include the smaller 2 65 2 63 ga avoca granitoid and the voluminous 2 73 2 64 ga alldays granitoid this study characterizes the wide compositional spectrum preserved in these two granitoids the elliptical avoca pluton consists of a biotite amphibole orthopyroxene clinopyroxene bearing core that is dominantly trondhjemite with less dominant tonalite and granodiorite variants and a thin amphibole biotite bearing granite rim with local occurrence of two pyroxene bearing metabasite boudins while both the core and rim rocks exhibit a linear fabric the granite in addition preserves a penetrative foliation field relations of granite enclaves in the core rocks together with available ages indicate that the core rocks intruded the granite the foliated biotite amphibole bearing alldays granitoid contains inclusions of older supracrustals and rocks of the messina layered intrusion and is widely distributed compositionally it include tonalites and granodiorites and to a lesser extent trondhjemites both the avoca core and rim rocks are characterized by difference in mineral chemistry with the mafic minerals mg rich in the ttg core while they are fe rich in the granite and metabasite in comparison biotite is mg rich and amphibole is fe rich in the alldays granitoid two groups of alldays ttg can be delineated in terms of whole rock geochemical characteristics and are comparable to the low to medium pressure ttg groups delineated by moyen 2011 while the avoca ttg is similar to the high pressure ttg group the lowest silica samples from each group of granitoid have geochemical characteristics comparable to archean sanukitoids with those from the avoca granitoid similar to low ti sanukitoids and those from the alldays granitoid similar to low ti and high ti sanukitoids separate petrogenetic models are suggested for different phases of the avoca core with the trondhjemite tonalites considered as high pressure melts of metabasalt while the granodiorite with lower sio2 content higher k2o and mgo contents and higher incompatible element contents than the trondhjemite tonalites is a product of hybridization of earlier ttg melts and peridodite granite from the avoca rim are low pressure melts of pre existing crustal lithologies the two groups of alldays ttg with lower sr y ratios than the avoca ttg are considered as low to medium pressure melts of metabasalt whose progressive interaction with peridotitic mantle at shallower angles account for the unique composition of alldays low ti and high ti sanukitoids taken together with their spatial and temporal transition from southeastern 2 73 2 72 ga low pressure ttg low ti sanukitoid to central 2 65 2 64 ga medium pressure ttg high ti sanukitoid to northwestern 2 63 ga high pressure ttg low ti sanukitoid parts of the beit bridge complex the three tier transition of ttg sanukitoid magmatism argues for the southern margin of the beit bridge complex to represent an active arc in the neoarchean

complexity of characterizing granitoids in high grade terranes an example from the neoarchean verbaard granitoid limpopo complex southern africa abstract identifying the spatial distribution or extent of individual granitoids in archean high grade terranes can be complicated with the effect of overprint events resulting in localized varieties the case study presented here highlights this issue from the limpopo complex high grade terrane in southern africa the beit bridge complex segment preserves a variety of temporally and spatially associated neoarchean granitoids of the different granitoids the 2 65 2 62 ga biotite bearing verbaard granitoid is characterized by a conspicuous network of garnet bearing leucocratic veins unlike these leucocratic veins with more or less straight line outline garnet bearing leucosomes with irregular outline are associated with surrounding migmatitic metapelites and have biotite bearing melanocratic margins garnet from the verbaard granitoid and migmatitic metapelite have comparable mineral chemical characteristics the most common variety is characterized by low xcagrt 0 03 0 04 and the less common one has higher xcagrt 0 05 0 06 granitoid 0 08 migmatitic metapelite no rim core rim zoning is observed in garnets from the host verbaard granitoid and metapelite however garnets in the leucocratic veins and leucosomes exhibit distinct zoning with a decrease of xca from 0 08 to 0 03 phase equilibria modeling using perple x software reveals that such variation in grossular content of garnets reflects decompression from 9 5 to 6 5 kbar at 760 800 c for both rocks during decompression melts were segregated trapping garnet including the high xca variety both the verbaard granitoid and migmatitic metapelite were affected by the segregated melts difference in composition between the modeled melts and leucosome leucocratic vein is accounted in terms of different degree of a melt loss whole rock geochemical characteristics of garnet and leucocratic vein free domains of the verbaard granitoid indicate that they are similar to medium pressure ttgs derived from the melting of metabasalts and are comparable to those of the alldays granitoid thus the verbaard granitoid represents a local manifestation of the voluminous widespread alldays granitoid magmatism in the specific verbaard area

evolution of continental crust in southern africa abstract nd isotopic data from the zimbabwe and kaapvaal cratons and the limpopo kalahari namaqualand and damara mobile belts imply that over 50 of present day continental crust in this region had separated from the mantle by the end of the archaean and that

archean magmatic granulites diapirism and proterozoic reworking in the northern marginal zone of the limpopo belt the northern marginal zone nmz of the limpopo belt southern africa is a high grade gneiss belt dominated by magmatic granulites of the charnoenderbite suite which intruded minor mafic ultramafic and metasedimentary rocks between 2 74 and 2 57 ga the intrusive rocks have crustal and mantle components and occur as elliptical bodies interpreted as diapirs peak metamorphism p 800 mpa t 800 850 c occurred at ca 2 59 ga the highly radiogenic nature of the rocks in the nmz supplemented by heat from mantle melts led to heating and diapirism culminating in the intrusion of distinctive porphyritic charnockites and granites horizontal shortening and steep extrusion of the nmz during which crustal thickening was limited by high geothermal gradients contrast with overthickening and gravitational collapse observed particularly in more recent orogens the granulites were exhumed by the end of the archean the pervasive late archean shortening over the whole of the nmz contrasts with limited deformation on the zimbabwe craton possibly owing to the strengthening effect of early crust in the craton in the southeast of the nmz strike slip kinematic indicators occur within the transition zone and the triangle shear zone where dextral shearing reworked the archean crust at ca 1 97 ga

first evidence of archean mafic dykes at 2 62 ga in the yilgarn craton western australia links to cratonisation and the zimbabwe craton abstract the archean yilgarn craton in western australia hosts at least five generations of proterozoic mafic dykes the oldest previously identified dykes belonging to the ca 2408 2401 ma widgiemooltha supersuite we report here the first known archean mafic dyke dated at 2615 6 ma by the id tims u pb method on baddeleyite and at 2610 25 ma using in situ shrimp u pb dating of baddeleyite aeromagnetic data suggest that the dyke is part of a series of ne trending intrusions that potentially extend hundreds of kilometres in the southwestern part of the craton here named the yandinilling dyke swarm mafic magmatism at 2615 ma was possibly related to delamination of the lower crust during the final stages of assembly and cratonisation and was coeval with the formation of late stage gold deposit at boddington paleogeographic reconstructions suggest that the yilgarn and zimbabwe cratons may have been neighbours from ca 2690 ma to 2401 ma and if the zimbabwe and kaapvaal cratons amalgamated at 2660 2610 ma the 2615 ma mafic magmatism in the southwestern yilgarn craton may be associated with the same tectonic event that produced the ca 2607 2604 ma stockford dykes in the central zone of the limpopo belt paleomagnetic evidence and a similar tectonothermal evolution including coeval low pressure high temperature metamorphism voluminous magmatism and emplacement of mafic dykes support a configuration where the northern part of the zimbabwe craton was adjacent to the western margin of the yilgarn craton during the neoarchean worldwide reliably dated mafic dykes of this age have so far been reported from the yilgarn craton the limpopo belt and the sao francisco craton

shear zones bounding the central zone of the limpopo mobile belt southern africa abstract contrary to previously suggested north directed thrust emplacement of the central zone of the limpopo mobile belt we present evidence indicating west directed emplacement the central zone differs from the marginal zones in rock types structural style and isotopic signature and is an allochthonous thrust sheet it is bounded in the north by the dextral tuli sabi shear zone and in the south by the sinistral palala shear zone which are crustal scale lateral ramps published gravity data suggest that the lateral ramps are linked at depth and they probably link at the surface in a convex westward frontal ramp in the vicinity of longitude 26 30 e in eastern botswana two phases of movement the first between 2 7 and 2 6 ga and the second between 2 0 and 1 8 ga occurred on both the tuli sabi and the palala shear zones

the high temperature static recovery and recrystallization behaviour of cold worked carrara marble the static recovery and recrystallization behaviour of carrara marble has been investigated as a function of pre strain 8 27 and annealing temperature 500 700 c quantitative estimates of the rate of recovery were obtained by comparing the stress strain curves generated at a temperature of 426 c a confining pressure of 170 mpa and a strain rate of 3 10 4s 1 before and after a period of isostatic annealing curves of fractional recovery r as a function of log time t show a characteristic sigmoidal shape in which the rate of recovery dr d logt is initially small increases rapidly and finally becomes small again the acceleration of recovery occurs at shorter times both with increasing pre strain and with increasing annealing temperature microstructural examination of the annealed material shows that the acceleration of recovery corresponds to the onset of recrystallization and that prior to the onset of recrystallization recovery is controlled by the development of a subgrain structure whereas after recrystallization has gone to completion it is controlled by the intensification and growth of this substructure the results have been interpreted within the perspective of hart s state variable description of inelastic deformation this analysis shows that the decay of the mechanical state of the marble during annealing can be treated as a two stage static recovery process substructure formation substructure coarsening with the recrystallization marking the transition between these two stages

u th pb and lu hf systematics of zircon from ttg s leucosomes meta anorthosites and quartzites of the limpopo belt south africa constraints for the formation recycling and metamorphism of palaeoarchaean crust abstract results of more than 400 in situ u th pb and lu hf zircon isotope analyses combined with cathodoluminescence images and field relationships reveal that the oldest magmatic rocks of the limpopo belt musina area formed during the palaeoarchaean between 3 40 and 3 27 ga experienced a first anatexis at 3 24 3 07 ga m1 and were overlain by sediments mainly between 3 24 and 3 0 ga subsequently the infra and supracrustal rocks together underwent two high grade metamorphic overprints a first during the neoarchaean at 2 65 ga m2 and a second at 2 025 ga m3 hafnium isotope data additionally reveal that the palaeoarchaean magmatic rocks result from substantial recycling of much older crust this is well reflected by nearly chondritic hf int obtained from primary magmatic zircon domains of four sand river ttg gneiss samples hf 3 27 3 28 ga 1 3 to 1 8 from leucosomes within the sand river gneisses hf 3 40 ga 2 0 hf 3 28 ga 1 5 and from two meta anorthosite samples of the messina layered intrusion hf 3 35 ga 0 1 1 4 crustal recycling is also supported by hafnium isotope data of detrital zircon grains from two quartzite samples these data reflect a general increase of the hf int between 3 65 and 3 2 ga from ca 4 0 to 0 0 indicating that an eo to palaeoarchaean crust was mixed with juvenile mantel material in the hinterland of the limpopo belt the present study also demonstrates that hf isotope data provide a powerful tool which makes it possible to distinguish zircon domains formed during distinct magmatic or metamorphic events from such affected by a pseudomorphic alteration even if the investigated zircon grain underwent multiple alteration processes

thrust exhumation of the neoarchean ultrahigh temperature southern marginal zone limpopo complex convergence of decompression cooling paths in the hanging wall and prograde p t paths in the footwall

grain size distribution and rheology of the upper mantle abstract the physical processes that govern the grain size of rocks in the upper mantle are examined the analysis is based on the experimental data on creep recrystallization and grain growth in dunites and on a theoretical model for the thermomechanical structure of the cooling moving lithosphere the grain size of rocks is shown to be determined by the in situ stress only at the deeper part where the temperature is high enough to allow significant strain rate above this depth the microstructures record the thermomechanical history of rocks rather than the in situ stress in the case of the oceanic lithosphere where the thermomechanical history is best known the following features of grain size distribution are found at the uppermost mantle where the amount of grain growth is limited the grain size is determined by the initial value and the growth rate and where the effect of grain growth dominates it increases with depth when the amount of grain growth becomes large and the grain size reaches the steady state size corresponding to the ambient stress while the rock is hot enough to deform the grain size is then determined by the applied stress this grain size is however frozen when the rock gets cool and the strain rate becomes too small to induce any further dynamic recrystallization thus at the intermediate depth region the grain size records the fossil frozen stress at which the microstructures of rock have been frozen since the frozen stress increases with age the grain size in this depth interval decreases with depth finally the grain size below this level reflects the in situ stress and increases with depth its extent being dependent on the nature of return flow in the deep mantle thus the grain size versus depth relation may show a sigmoid curve the qualitative features of this curve may be similar also in the case of the continental lithosphere if a similar thermal event i e the intrusion of hot material and subsequent cooling occurs the results are quite consistent with the observed depth variation of olivine grain size in peridotite nodules ave lallemant et al 1980 the present model suggests that the depth of minimum grain size 65 and 150 km at the continental rift zone and the shield region respectively corresponds to that where the mechanical properties of the upper mantle change from elastic to ductile at tectonic stress levels 1 mpa and in the geological time scale this result leads to a new definition of the thickness of lithosphere in terms of its rheological properties this thickness is about twice as large as that inferred from the flexure of lithosphere but approximately equal to seismic thickness the model suggests the importance of grain growth as well as dynamic recrystallization and plastic flow in determining the texture of upper mantle rocks and therefore seismic anisotropy

p t conditions mechanism and timing of the localized melting of metapelites from the petronella shear zone and relationships with granite intrusions in the southern marginal zone of the limpopo belt south africa

dolomite microstructures between 390 and 700 c indications for deformation mechanisms and grain size evolution abstract dolomitic marble on the island of naxos was deformed at variable temperatures ranging from 390 c to 700 c microstructural investigations indicate two end member of deformation mechanisms 1 diffusion creep processes associated with small grain sizes and weak or no cpo c rystallographic p referred o rientation whereas 2 dislocation creep processes are related with larger grain sizes and strong cpo the change between these mechanisms depends on grain size and temperature therefore sample with dislocation and diffusion creep microstructures and cpo occur at intermediate temperatures in relative pure dolomite samples the measured dolomite grain size ranges from 3 to 940 m grain sizes at tmax 450 c show an arrhenius type evolution reflecting the stabilized grain size in deformed and relative pure dolomite the stabilized grain size is five times smaller than that of calcite at the same temperature and shows the same arrhenius type evolution in addition the effect of second phase particle influences the grain size evolution comparable with calcite calcite dolomite mixtures are also characterized by the same difference in grain size but recrystallization mechanism including chemical recrystallization induced by deformation may contribute to apparent non temperature equilibrated mg content in calcite

behaviour of rigid objects during deformation and metamorphism a test using schists from the bolton syncline connecticut usa the behaviour of spherical versus highly ellipsoidal rigid objects in folded rocks relative to one another or the earth s surface is of particular significance for metamorphic and structural geologists two common porphyroblastic minerals garnet and staurolite approximate spherical and highly ellipsoidal shapes respectively the motion of both phases is analysed using the axes of inflexion or intersection of one or more foliations preserved as inclusion trails within them we call these axes fias for foliation inflexion intersection axes for staurolite this motion can also be compared with the distribution of the long axes of the crystals schists from the regionally shallowly plunging bolton syncline commonly contain garnet and staurolite porphyroblasts whose fias have been measured in the same sample garnet porphyroblasts pre date this fold as they have inclusion trails truncated by all matrix foliations that trend parallel to the strike of the axial plane however they have remarkably consistent fia trends from limb to limb the fias trend 175 and lie 25 nnw from the 020 strike of the axial trace of the bolton syncline the plunge of these fias was determined for six samples and all lie within 30 of the horizontal eleven of these samples also contain staurolite porphyroblasts which grew before during and after formation of the bolton syncline as they contain inclusion trails continuous with matrix foliations that strike parallel to the axial trace of this fold the staurolite fias have an average trend of 035 15 ne from the 020 strike of the axial plane of this fold the total amount of inclusion trail curvature in staurolite porphyroblasts about the axis of relative rotation between staurolite and the matrix i e the fia is greater than the angular spread of garnet fias although staurolite porphyroblasts have ellipsoidal shapes their long axes exhibit no tendency to be preferentially aligned with respect to the main matrix foliation or to the trend of their fia this indicates that the axis of relative rotation between porphyroblast and matrix the fia was not parallel to the long axis of the crystals it also suggests that the porphyroblasts were not preferentially rotated towards a single stretch direction during progressive deformation five overprinting crenulation cleavages are preserved in the matrix of rocks from the bolton syncline and many of these result from deformation events that post date development of this fold staurolite porphyroblast growth occurred during the development of all of these deformations most of which produced foliations staurolite has overgrown and preserved as helicitic inclusions crenulated and crenulation cleavages i e some inclusion trail curvature pre dates porphyroblast growth the deformations accompanying staurolite growth involved reversals in shear sense and changing kinematic reference frames these relationships cannot all be explained by current models of rotation of either or both the garnet and staurolite porphyroblasts in contrast we suggest that the relationships are consistent with models of deformation paths that involve non rotation of porphyroblasts relative to some external reference frame further we suggest there is no difference in the behaviour of spherical or ellipsoidal rigid objects during ductile deformation and that neither garnet nor staurolite have rotated in schists from the bolton syncline during the multiple deformation events that include and post date the development of this fold