reverse age relations of talc and tremolite deduced from reaction textures in metamorphosed siliceous dolomites of the southern damara orogen namibia the regional metamorphosed siliceous dolomites of the southern damara orogen namibia show a distinct talc bearing zone on the lower metamorphic grade side and a distinct tremolite bearing zone on the higher metamorphic grade side separated by the occurrence of the fivemineral assemblage i tremolite talc calcite dolomite quartz

forty years of ttg research abstract ttgs tonalite trondhjemite granodiorite are one of the archetypical lithologies of archaean cratons since their original description in the 1970s they have been the subject of many studies and discussions relating to archaean geology in this paper we review the ideas concepts and arguments brought forward in these 40 years and try to address some open questions both old and new the late 1960s and the 1970s mark the appearance of grey gneisses ttg in the scientific literature during this period most work was focused on the identification and description of this suite and the recognition that it is a typical archaean lithology ttgs were already recognised as generated by melting of mafic rocks this was corroborated during the next decade when detailed geochemical ttg studies allowed us to constrain their petrogenesis melting of garnet bearing metamafic rocks and to conclude that they must have been generated by archaean geodynamic processes distinct from their modern counterparts however the geodynamic debate raged for the following 30 years as many distinct tectonic scenarios can be imagined all resulting in the melting of mafic rocks in the garnet stability field the 1990s were dominated by experimental petrology work a wealth of independent studies demonstrated that melting of amphibolites as well as of mafic eclogites can give rise to ttg liquids whether amphibolitic or eclogitic conditions are more likely is still an ongoing debate from 1990s onwards one of the key questions became the comparison with modern adakites as originally defined these arc lavas are reasonably close equivalents to archaean ttgs pending issues largely revolve around definitions as the name ttg has now been applied to most archaean plutonic rocks whether sodic or potassic irrespective of their hree contents this leads to a large range of petrogenetic and tectonic scenarios a fair number of which may well have operated concurrently but are applicable only to some of the rocks lumped together in the ever broadening ttg bin

horizontal structures in granulite terrains a record of mountain building or mountain collapse in many high temperature 800 c granulite terrains the development of characteristic horizontal structures occurred during the metamorphic culmination and was followed by isobaric cooling the absolute magnitude of isobaric cooling commonly 300 c implies cooling intervals of the order of the thermal time constant of the continental lithosphere 100 m y such prolonged isobaric cooling implies that no significant erosional denudation followed the development of the horizontal structures and thus precludes the prograde deformation being responsible for significant crustal thickening rather the prograde deformation more probably records bulk crustal thinning during extensional collapse of a previously thickened crust possibly triggered by detachment of a thickened thermal boundary layer at the base of the lithosphere

recognition of 3850 ma water lain sediments in west greenland and their significance for the early archaean earth a layered body of amphibolite banded iron formation bif and ultramafic rocks from the island of akilia southern west greenland is cut by a quartz dioritic sheet from which shrimp zircon 206pb 207pb weighted mean ages of 3865 11 ma and 3840 8 ma 2 sigma can be calculated by different approaches three other methods of assessing the zircon data yield ages of 3830 ma the bifs are interpreted as water lain sediments which with a minimum age of approximately 3850 ma are the oldest sediments yet documented these rocks provide proof that by approximately 3850 ma 1 there was a hydrosphere supporting the chemical sedimentation of bif and that not all water was stored in hydrous minerals and 2 that conditions satisfying the stability of liquid water imply surface temperatures were similar to present carbon isotope data of graphitic microdomains in apatite from the akilia island bif are consistent with a bio organic origin mojzsis et al 1996 extending the record of life on earth to 3850 ma life and surface water by approximately 3850 ma provide constraints on either the energetics or termination of the late meteoritic bombardment event suggested from the lunar cratering record on earth

the continental and oceanic crust and lithosphere in the archaean isostatic thermal and tectonic models simple isostatic models of the development of selected archaean basins and greenstone belts can be used to constrain models of the archaean lithosphere geophysical geochemical and thermal data imply that in southern africa the late archaean crust was probably comparable in thickness to the modern continental crust and the whole lithosphere was perhaps over 80 100 km thick two archaean belts studied may have developed by stretching of the order of 1 3 1 6 1 isostatic and thermal data and petrological assumptions allow the construction of simple models of the archaean oceanic lithosphere oceans were probably deep mid ocean ridges were probably subaqueous and fed by komatiitic liquids calc alkaline volcanism in the archaean may have been produced as a consequence of the subduction of hydrated komatiitic oceanic crust into a hot low viscosity asthensophere

elastic strength of the slave craton at 1 9 gyr and implications for the thermal evolution of the continents the secular cooling of the earth since its formation together with the law of radioactive decay imply that the earth s internal heat generation and heat loss must have been two to three times higher in archaean and proterozoic times than today but this inference is difficult to test directly here we present a flexural analysis of the proterozoic kilohigok basin in the archaean slave craton of canada which shows that 1 9 gyr ago the craton had an effective elastic thickness of only 12 4 km because the present effective elastic thickness of the region is 100 25 km these results imply a two to fourfold decrease in the thermal gradient beneath the slave craton since 1 9 gyr and indicate that at 1 9 gyr 700 myr after the accretion of the continental lithosphere forming the craton the thick continental root beneath this part of the canadian shield had not yet formed

diapirism bulk homogeneous shortening and transcurrent shearing in the archaean dharwar craton the holenarsipur area southern india abstract qualitative strain data are spatially integrated in order to define archaean foliation trajectories and variations in strain intensity and ellipsoid type in the holenarsipur area dharwar craton southern india the study provides constraints on the bulk geometry the progressive deformation and the kinematic history of this region of archaean continental crust two major structural events have shaped the study area i a regional diapiric event contemporaneous with a magmatic event and ii a later sinistral transcurrent ductile shear event which has locally reoriented both foliations and stretching lineations the entire holenarsipur supracrustal belt underwent a common tectono metamorphic history that included two metamorphic events each event corresponds temporally to a distinct strain field the first a regional amphibolite facies event was contemporaneous with the main tectonic event and probably with the intrusion of halekote trondhjemites the second occurred under greenschist facies conditions and was restricted to a late transcurrent shear zone at the eastern boundary of the greenstone belt the strain pattern prior to transcurrent shearing cannot be explained by a thrusting event unless all trace of this event had been erased by the diapirism we interpret all structures older than the transcurrent shearing to be the direct result of vertical movements linked to body forces augmented by minor horizontal crustal shortening

tectonic implications of metamorphism and gravity field in the penokean orogen of northern michigan published geologic data together with new metamorphic pressure and temperature data and gravity data provide strong evidence for northward directed thrusting of lower proterozoic rocks in northern michigan and wisconsin metamorphic studies indicate pressures of 7 kbar at watersmeet node and 4 kbar at the peavy node the republic metamorphic node located 30 km to the north of peavy attained pressures of 2 3 kbar all three metamorphic highs reached temperatures in the range of 600 650 c and are correlated with areas of negative gravity anomalies relative to surrounding regions in the peavy high the center of the gravity anomaly lies on the sillimanite isograd and we suggest that the source of this negative anomaly may be remobilized underlying archean gneiss the highest grade rocks of the republic node lie well within a residual gravity anomaly which is interpreted to have a very shallow source and the watersmeet high lies above gneiss domes which show negative gravity anomalies a north south geologic cross section constructed from a two dimensional gravity model which extends northward from the dunbar dome in the magmatic arc terrane of wisconsin across a terrane boundary interpreted as a suture and marked by the niagara fault and across the peavy metamorphic node suggests that the dunbar dome was transported northward onto the continental foreland during the penokean orogenic event these north verging fold thrust sheets or nappes produced prominent tectonic thickening at the peavy and watersmeet as indicated by the relatively high metamorphic pressures there the thrust sheets evidently thinned to the north as suggested by the relatively low pressures at republic we suggest that the archean continental edge defined by the steep gradient in the regional northeast trending gravity high lies beneath the magmatic rocks of wisconsin and thus extends south of the niagara fault suture

models for the archean thermal regime a variety of different regionalized thermal evolution models are presented all models include an oceanic style of heat loss in which mantle convection penetrates to the surface in the form of spreading and subducting plates in continental regions the lithosphere is assumed to act as a conductive lid on mantle convection that can be either layered or mantle wide the continental lithosphere is defined in different ways it can be of fixed thickness insensitive to temperature or extend to some fixed cutoff temperature a thermally controlled rheological lithosphere when a fixed temperature is used widespread melting occurs in the archean and one has the further option of allowing these melts to erupt onto the surface thus providing a new heat transfer mechanism and also a general subsidence of the continental lithosphere due to the continuous volcanic loading the models are first used to determine the concentration of heat producing elements required to have a present day mantle potential temperature of 1350 c starting from a high temperature 2000 c 4 5 billion years ago the results depend primarily on whether the mantle is layered if layered 15 30 ppb uranium normalized to the silicate portion of the earth and withk u 104 th u 3 8 are needed while mantle wide cases need uranium concentrations in excess of 35 ppb the models are also used to calculate the thermal structure of continental lithosphere through time only lithospheres of fixed thickness 175 km can explain the recent discovery of diamonds with model ages of 3 2 3 4 billion years for a thick lithosphere to have existed in the archean it would seem most likely that its thickness be the result of some chemical distinction rather than simply its rheological properties such chemically distinct lithosphere would have greater likelihood of surviving to the present which raises the issue of biased preservation

the variability and predictability of axisymmetric hurricanes in statistical equilibrium abstractthe variability and predictability of axisymmetric hurricanes are determined from a 500 day numerical simulation of a tropical cyclone in statistical equilibrium by design the solution is independent of the initial conditions and environmental variability which isolates the intrinsic axisymmetric hurricane variability variability near the radius of maximum wind is dominated by two patterns one associated primarily with radial shifts of the maximum wind and one primarily with intensity change at the time mean radius of maximum wind these patterns are linked to convective bands that originate more than 100 km from the storm center and propagate inward bands approaching the storm produce eyewall replacement cycles with an increase in storm intensity as the secondary eyewall contracts radially inward a dominant time period of 4 8 days is found for the convective bands which is hypothesized to be determined by the time scale over which subsidence from previous bands suppresses convection a

late archaean early proterozoic co2 streaming through the lower crust and geochemical segregation considerable evidence exists for large scale migration of co2 rich fluids through the lower continental crust in the late archaean and early proterozoic with transfer of heat producing elements and h2o to higher levels the evidence manifest in the large charnockitic terrains of this age includes deformation related charnockitic alteration of gneisses nearly ubiquitous co2 rich fluid inclusions in minerals depletion of large ion lithophile elements lile particularly rb relative to typical upper crustal rocks similar depletion of 18o in paragneisses and petrographic and geochemical evidence of open system replacement of amphibole gneiss by charnockite a conveyor belt upper mantle source appears to have been necessary for prolonged co2 supply in high grade crustal metamorphism most plausibly by deep subduction of marine carbonate under continental interiors this plate tectonic mode seems consonant with crustal thickening and orogenic patterns inferred for most charnockitic terrains and with co2 transport mechanisms constrained by experimental petrology

underplating of oceanic lithosphere in the archean a possible mechanism for the formation of archean komatiites abstract the thermal evolution of a subducting archean oceanic lithosphere suggests that amphibole and phlogopite can transport water to depths of about 100 km water released from the oceanic crust hydrates the overlying depleted peridotitic lithosphere lowering its melting temperature and causing a high degree of partial melting temperatures at the top of the archean asthenosphere only about 150 c higher than the present value are sufficient to produce over 50 melting in the bottom of the overlying lithosphere under wet conditions and generate peridotitic komatiites

tectonic evolution in the archaean and proterozoic kre high heat flow probably favoured global hotspot activity with crustal growth of iceland type through vertical magmatic accretion the tectonic environment for the generation of about 3 9 2 5 ga granite gneiss greenstone terrains with volurnlnous production of tonalite trondhjemite granodlorite magmas is still uncertain and both piate ma and intrapiate scenarios arc possible deformanon atylea in greenstones and adjacent high grade orthognersses with intercalated shallow water supracrustal assemblages suggest extensive horizontal shortening and crustal interstacking this probably resulted from collisional and or rotational motion and produced significant intracrustal melting as early as 3 9 ga by the end of the archaean and locally much earlier crustal thicknesses reached 35 50 km or more and the cxtl aordinary hhlding to erogenic deformation may have been due to delamination and crustal subduction towards the end of the proterozoic evidence for modern style plate tectonics becomes abundant through the recognition of laland a16 accretion ophiolite obducdon foreland thrust and fold belts and exotic terranes one of the best documented example for this tectonic environment is the arabian nubian shield 4lthough it is doubtful whether modern plate tectonic settings can serve as analogues for all precambrian tectonic styles it i clear that global crustal evolution since the early archaean was governed by the motion of plates which repeatedly came togethel tv fotm supercontinents and then dlspersetl again we shall not understand the detailed mechanism of precambrian crustal itlton cs before we learn horn lo irltejpret recent large scale crustal deformation and its causes

near surface thermal chemical boundary layer convection at infinite prandtl number two dimensional numerical experiments chemical differentiation and convective removal of internal heat make the earth s lithosphere a thermal and a chemical boundary layer thin layers of chemically light material form near the earth s surface and become embedded within the cold thermal boundary layer associated with interior heat removal the likelihood of near surface thermal and chemical boundary layer interactions influencing the earth s thermotectonic evolution prompts the models presented herein a simplified system consisting of a chemically light layer within the upper thermal boundary layer of a denser thermally convecting layer is explored through a suite of numerical experiments to see how its dynamic behaviour differs from similar well studied thermal boundary layer systems a major cause of differences between the two systems resides in the ability of the deformable near surface chemical layer to alter the effective upper thermal boundary condition imposed on the convectively unstable layer below in thermal equilibrium regions of chemical boundary layer accumulation locally enforce an effectively near constant heat flux condition on the thermally convecting layer due to the finite thermal conductivity of chemical boundary layer material for cases in which chemical accumulations translate laterally above the unstable layer the thermal coupling condition between chemical boundary layer material and the unstable layer below is one of non equilibrium type i e the thermal condition at the top of the convectively unstable layer is time as well as space variable a second major cause of differences is that for the thermal chemical system chemically induced rheologic variations can offset or compete with those due to temperature more specifically the presence of chemically weak material can lubricate convective downwellings allowing for enhanced overturn of an on average strong upper thermal boundary layer both of these factors have low order effects on internal flow structure and heat loss and lead to dynamic behaviour in which chemical boundary layer deformation is not only driven by flow in the thermally convecting interior layer but also feeds back and alters this flow some implications of this in regard to elucidating how near surface chemical boundary layer deformation e g continental tectonics might interact with and influence mantle convection are discussed

plate tectonics on the early earth limitations imposed by strength and buoyancy of subducted lithosphere the tectonic style and viability of modern plate tectonics in the early earth is still debated field observations and theoretical arguments both in favor and against the uniformitarian view of plate tectonics back until the archean continue to accumulate here we present the first numerical modeling results that address for a hotter earth the viability of subduction one of the main requirements for plate tectonics a hotter mantle has mainly two effects 1 viscosity is lower and 2 more melt is produced which in a plate tectonic setting will lead to a thicker oceanic crust and harzburgite layer although compositional buoyancy resulting from these thick crust and harzburgite might be a serious limitation for subduction initiation our modeling results show that eclogitization significantly relaxes this limitation for a developed ongoing subduction process furthermore the lower viscosity leads to more frequent slab breakoff and sometimes to crustal separation from the mantle lithosphere unlike earlier propositions not compositional buoyancy considerations but this lithospheric weakness could be the principle limitation to the viability of plate tectonics in a hotter earth these results suggest a new explanation for the absence of ultrahigh pressure metamorphism uhpm and blueschists in most of the precambrian early slabs were not too buoyant but too weak to provide a mechanism for uhpm and exhumation

dynamics and structural development of metamorphic core complexes the development of metamorphic core complexes mccs in a thickened continental lithosphere is studied using fully coupled thermomechanical numerical code accounting for elastic brittle ductile properties of constituent rocks for mccs to develop the middle lower crust and the sub moho mantle are required to be weak enough to flow laterally so as to simultaneously feed the exhuming dome and enable the moho to keep a flat geometry the conditions are satisfied with initial moho temperatures of 800 c or higher with crustal thicknesses of 45 km or greater and with initial effective viscosities lower than 1020 pa s and 1022 pa s in the lower crust and the underlying mantle respectively a compositional mainly density anomaly with the properties of granite is placed centrally in the crust to localize strain at the onset of deformation during a first stage of upper crust necking the deformation pattern is relatively symmetrical and dominated by graben formation in the upper crust when the first ductile layers reach the surface a second stage of dome amplification and widening occurs dome amplification is accommodated by horizontal flow in the ductile crust giving an early symmetrical pattern of conjugate shear zones with no obvious detachment zone the system then rapidly becomes asymmetric with the localization of a detachment zone along one dome limb further accommodating dome widening thus the exhumation process of a metamorphic dome results in the progressive development of a detachment zone depending on initial moho temperature the detachment zone can migrate in space or die out and be replaced by a new one with an opposite dip

neoarchean lithospheric strengthening and the coupling of earth s geochemical reservoirs this paper explores the secular evolution of the height of an isostatically balanced collisional mountain belt in the context of ongoing convergence we show that until the neoarchean continents were unable to sustain topography 2500 m during the neoarchean the continental lithosphere evolved through a rheological threshold allowing for the development of signifi cant topography the consequence of the strengthening of the continental lithosphere is fundamental for the coupling of the earth s geochemical reservoirs the neoarchean was a period of global changes during which exogenic envelopes recorded major shifts in composition toward modern values we propose that during the neoarchean the exogenic envelopes which were until then coupled to the mantle through hydrothermal processes and volcanism also became coupled to the continental crust through reliefgenerating tectonics processes and erosion hence changing the balance between mantle versus crustal interaction with the exogenic earth

a case for late archaean continental emergence from thermal evolution models and hypsometry abstract the secular cooling of the earth s mantle and the growth of the continental crust together imply changes in the isostatic balance between continents and oceans in the oceanic bathymetry and in the area of emerged continental crust the evolution of these variables is of fundamental importance to the geochemical coupling of mantle continental crust atmosphere and ocean to explore this further we developed a model that evaluates the area of emerged continental crust as a function of mantle temperature continental area and hypsometry in this paper we investigate the continental freeboard predicted using different models for the cooling of the earth we show that constancy of the continental freeboard 200 m is possible throughout the history of the planet as long as the potential temperature of the upper mantle was never more than 110 210 c hotter than present such numbers imply either a very limited cooling of the planet or most likely a change in continental freeboard since the archaean during the archaean a greater radiogenic crustal heat production and a greater mantle heat flow would have reduced the strength of the continental lithosphere thus limiting crustal thickening due to mountain building processes and the maximum elevation in the earth s topography rey p f coltice n neoarchean strengthening of the lithosphere and the coupling of the earth s geochemical reservoirs geology 36 635 638 2008 taking this into account we show that the continents were mostly flooded until the end of the archaean and that only 2 3 of the earth s area consisted of emerged continental crust by around 2 5 ga these results are consistent with widespread archaean submarine continental flood basalts and with the appearance and strengthening of the geochemical fingerprint of felsic sources in the sedimentary record from 2 5 ga the progressive emergence of the continents as shown by our models from the late archaean onward had major implications for the earth s environment particularly by contributing to the rise of atmospheric oxygen and to the geochemical coupling between the earth s deep and surface reservoirs

metamorphism denudation and sea level in the archean and cooling of the earth abstract over long enough time scales an antagonistic balance acting between tectonics uplift and rifting and the sedimentary cycle erosion and deposition probably keeps continental surfaces close to sea level we argue that this balance constitutes the primary mechanism for keeping long term continental freeboard constant by regulating the continental thickness an important implication is that the long term uplift or down warp history of ancient continental segments provides direct information on secular changes in the isostasy of the adjacent ocean basins through time the amount of uplift can be estimated from the regional metamorphic grade the burial pressures at present exposure of ten undisturbed archean granite greenstone segments are estimated to be 1 5 0 5 kb from this we infer a maximum long term uplift and erosion of 5 2 km for stable continent since 3 0 ga implying a mean continental thickness of 46 km at the time of crustal stabilization for a constant mean ocean depth isostasy requires a palaeo oceanic crust 14 2 km thick in the middle archean alternatively if little continent existed then the oceans would have been shallower and the palaeo oceanic crust 21 2 km thick assuming plate spreading did occur in the archean potential temperatures in the shallow mantle at 3 ga were 90 30 c or 170 20 c for no continent hotter than those prevailing today these temperatures translate into time averaged cooling rates for the shallow mantle of 57 c ga 1 and are comparable to estimates of the present day cooling rate for the whole earth the low burial pressures are a strong argument against the existence of catastrophic mantle flush events in the geological past which have been predicted by some mantle convection models taken together these constraints suggest a plume related geological setting i e oceanic plateau like flood basalt for the initial stage of greenstone belt evolution

continental thermal and tectonic regimes during the archaean metamorphic pressures estimated from mineral assemblages in archaean high grade terrains may be used to constrain geothermal regimes in the archaean continental crust in two ways first they may be combined with temperature estimates to yield bounds on the heat flux during the thickening events that produced the metamorphism the pressure temperature data do not require any greater supply of heat to the base of the continental crust in the archaean than is supplied at the present day second the pressures alone may be used to infer the height of archaean mountains these mountains appear to have had elevations comparable with those of present day mountain belts it seems probable that the elevation of present day belts is limited by the creep strength of the lithosphere and in view of the strong temperature dependence of creep in earth materials this second observation also implies that archaean continental thermal regimes were similar to present day ones these observations together with the common

structural development of an archean orogen western point lake northwest territories the point lake orogen in the central archean slave province of northwestern canada preserves more than 10 km of structural relief through an eroded antiformal thrust stack and deeper anastomosing midcrustal mylonites fault restoration along a 25 km long transect requires a minimum of 69 km slip and 53 km horizontal shortening in the western part of the orogen the basal decollement places mafic plutonic volcanic rocks over an ancient tonalitic gneiss complex ten kilometers to the east in the keskarrah bay area slices of gneiss unroofed on brittle thrusts shed molasse into several submerged basins conglomerates and associated thinly bedded sedimentary rocks are interpreted as channel levee and overbank facies of this thrust related sedimentary fan system the synorogenic erosion surface at the base of the conglomerate truncates premetamorphic or early metamorphic thrust faults formed during foreland propagation while other thrusts related to hinterland progressing imbrication displace this unconformity tightening of synorogenic depositional troughs resulted in the conglomerates present localization in synclines to the west of associated thrust faults and steepening of structural dips eastern parts of the orogen consist of isoclinally folded graywackes composed largely of mutti and ricci lucchi turbidite facies b c and d interpreted as submarine fan deposits eroded from a distant volcanic arc thrust faults in the metasedimentary terrane include highly disrupted slate horizons with meter scale duplex structures and recrystallized calcmylonites exhibiting sheath folds and boudin trains with very large interboudin distances the sequence of fabric development and the overall geometry of this metasedimentary terrane strongly resembles younger forearc accretionary prisms conditions of deformation along the thrusts parallel the regional metamorphic zonation amphibolite facies in the basal decollement through greenschist facies shear zones to cataclastic crush zones in the region of emergent thrusts in keskarrah bay depth differences can account for only half of the metamorphic gradient thermal profiles which increased downwards in obducted greenstone belts and synthrusting plutonism explains other high metamorphic gradients a tectonic model involving the collision of an accretionary prism with a continental margin best explains the structural and sedimentological evolution of the orogen

metamorphic evolution of aluminous granulites from labwor hills uganda sapphirine cordierite quartz and spinel cordierite quartz form relic assemblages of probable archaean age in fe rich aluminous metapelites from labwor hills uganda and reflect an unusually high temperature metamorphism 1 000 c at pressures in the vicinity of 7 9 kbars and a o2 near the magnetite hematite buffer subsequent reaction textures include the replacement of spinel and cordierite by sillimanite and hypersthene and formation of sapphirine hypersthene k feldspar quartz symplectites which are interpreted as pseudomorphs after osumilite a petrogenetic grid appropriate to these assemblages suggests these reaction textures may be due to cooling at constant or increasing pressure and constant a o2 or decreasing a o2 at constant temperature and pressure the former interpretation is supported by the coexistence of ilmenohematite and magnetite during the development of the reaction textures and by the comparatively low al2o3 contents of secondary hypersthene this pressure temperature path implies that 1 metamorphism occurred at deep levels within normal thickness crust probably less than 40 45 km thick due to an extreme thermal perturbation induced either by emplacement of mantle derived magmas or by thinning of the subcontinental lithosphere in an extensional tectonic regime 2 the excavation and surface exposure of the granulites is due to a subsequent postgranulite facies metamorphism crustal thickening most probably involving their incorporation into an allochthonous upper crustal thrust sheet during the formation of the mozambique foldbelt

the narryer gneiss complex of the yilgarn block western australia a segment of archaean lower crust uplifted during proterozoic orogeny the narryer gneiss complex of the yilgarn block is a key segment of the western australian precambrian shield it is a regional granulite facies terrain comprised of predominantly quartzo feldspathic gneisses derived from granitic intrusions c 3 6 3 4 ga old granulite facies metamorphism occurred c 3 3 ga ago and conditions of 750 850 c and 7 10 kbar are estimated for the mukalo creek area mca near errabiddy in the north the p t path of the mca has been derived from metamorphic assemblages in younger rocks that intruded the gneisses during at least three subsequent events and this path is supported by reaction coronas in the older gneisses there is no evidence for uplift immediately following peak metamorphism of the mca and a period of isobaric cooling is inferred from the pressures recorded in younger rocks pressures and temperatures estimated from metadolerites which intruded the older gneisses during granite greenstone tectonism at about 2 6 ga and during early proterozoic thrusting show that the errabiddy area remained in the lower crust although it was probably reheated during the younger events isothermal uplift to upper crustal levels occurred at c 1 6 ga ago and was followed by further deformation and patchy retrogression of high grade assemblages the effects of younger deformation cooling and reheating can be discerned in the older gneisses but as there has been no pervasive deformation or rehydration the minerals and microstructures formed during early archaean granulite facies metamorphism for the most part are retained the mca remained in the lower crust for about 1700 ma following peak metamorphism and some event unrelated to the original metamorphism was required to exhume it uplift occurred during development of the capricorn orogen when some 30 35 km were added to the crust beneath the errabiddy area the recognition of early proterozoic thrusting plus crustal thickening suggests that the capricorn orogen is a belt of regional compression which resulted from convergence of the yilgarn and pilbara cratons

experimental test of theory for the stability of partially saturated vertical cut slopes abstractthis paper extends culmann s vertical cut analysis to unsaturated soils to test the extended theory unsaturated sand was compacted to a uniform porosity and moisture content in a laboratory apparatus a sliding door that extended the height of the free face of the slope was lowered until the vertical cut failed digital images of the slope cross section and upper surface were acquired concurrently a recently developed particle image velocimetry piv tool was used to quantify soil displacement the piv analysis showed strain localization at varying distances from the sliding door prior to failure the areas of localized strain were coincident with the location of the slope crest after failure shear strength and soil water characteristic parameters of the sand were independently tested for use in extended analyses of the vertical cut stability and of the failure plane angle experimental failure heights were within 22 3 of the heights predicted using the extended theory

on the metamorphic history of an archaean granitoid greenstone terrane east pilbara western australia using the40ar 39ar age spectrum technique abstract age spectrum analyses of blue green hornblendes from amphibolites from the western shaw belt east pilbara western australia indicate an age of at least 3200 ma for early regional metamorphism ages on hornblende and muscovite from the narrow contact zone with the adjacent yule batholith probably date updoming of the granitoid gneiss terranes at 2950 ma hornblendes from within the shaw batholith and from a contact zone of a post tectonic granitoid yield ages of 2840 2900 ma indicating either prolonged high temperatures within the granitoid gneiss terranes or a separate thermal pulse associated with the intrusion of post tectonic granitoids the preservation of very old hornblendes in a narrow greenstone belt surrounded by massive granitoid gneiss domes indicates that remarkable contrasts in metamorphic geotherms existed over short distances during the late archaean suggesting that updoming occurred during a period of rapid tectonism

complex 3d strain patterns in a synkinematic tonalite batholith from the aracuai neoproterozoic orogen eastern brazil evidence from combined magnetic and isotopic chronology studies abstract this work combines structural and geochronological data to improve our understanding of the mechanical behaviour of continental crust involving large amount of magma or partially melted material in an abnormally hot collisional belt we performed a magnetic and geochronological u pb study on a huge tonalitic batholith from the neoproterozoic aracuai belt of east brazil to determine the strain distribution through space and time anisotropy of magnetic susceptibility combined with rock magnetism investigations supports that the magnetic fabric is a good proxy of the structural fabric field measurements together with the magnetic fabrics highlight the presence in the batholith of four domains characterized by contrasted magmatic flow patterns the western part is characterized by a gently dipping orogen parallel ns magmatic foliation that bears down dip lineations in agreement with westward thrusting onto the sao francisco craton eastward the magmatic foliation progressively turns sub vertical with a lineation that flips from sub horizontal to sub vertical over short distances this latter domain involves an elongated corridor in which the magmatic foliation is sub horizontal and bears an orogen parallel lineation finally the fourth narrow domain displays sub horizontal lineations on a sub vertical magmatic foliation oblique n150 e to the trend of the belt u pb dating of zircons from the various domains revealed homogeneity in age for all samples this together with the lack of solid state deformation suggests that 1 the whole batholith emplaced during a magmatic event at 580 ma 2 the deformation occurred before complete solidification and 3 the various fabrics are roughly contemporaneous the complex structural pattern mapped in the studied tonalitic batholith suggests a 3d deformation of a slowly cooling large magmatic body and its country rock we suggest that the development of the observed 3d flow field was promoted by the low viscosity of the middle crust that turned gravitational force as an active tectonic force combining with the east west convergence between the sao francisco and congo cratons

implications of melting for stabilisation of the lithosphere and heat loss in the archaean abstract the increased depth and volume of melting induced in a higher temperature archaean mantle controls the stability of the lithosphere heat loss rates and the thickness of the oceanic crust the relationship between density distributions in oceanic lithosphere and the depth of melting at spreading centres is investigated by calculating the mineral proportions and densities of residual mantle depleted by extraction of melt fractions the density changes related to compositional gradients are comparable to those produced by thermal effects for lithosphere formed from a mantle which is 200 c or more hotter than modern upper mantle if archaean continental crust formed initially above oceanic lithosphere the compositional density gradients may be sufficient to preserve a thick archaean continental lithosphere within which the archaean age diamonds are preserved the amount of heat advected by melts at mid ocean ridges today is small but heat advected by melting becomes proportionally more important as higher mantle temperatures lead to a greater volume of melt and as the rate of production of oceanic plates increases archaean tectonics could have been dominated by spreading rates 2 3 times greater than now and with mantle temperatures between ca 1600 c and 1800 c at the depth of the solidus mid ocean ridge melting would produce a relatively thick but light refractory lithosphere on which continents could form protected from copious volcanism and high mantle temperatures

petrology and retrograde p t path in granulites of the kanskaya formation yenisey range eastern siberia the kanskaya formation in the yenisey range eastern siberia is a newly studied example of retrogression of granulite facies rocks the formation consists of two stratigraphical units the lower kuzeevskaya group and the upper atamanovskaya group rocks from both of these units show rare reaction textures such as replacement of cordierite by garnet sillimanite and quartz silimanite coronas around spinel and corundum and garnet rims around plagioclase in metabasites while plagioclase rims around garnet can be seen in associated metapelites the paragenesis quartz orthopyroxene sillimanite is a feature of the kuzeevskaya group in many samples chemical zoning of garnet and cordierite shows an increase in mg from core to rim as well as the reverse biotite garnet cordierite sillimanite quartz as well as spinel biotite garnet cordierite sillimanite quartz assemblages were studied using geothermometers and geobarometers based on both exchange and net transfer reactions perchuk lavrent eva 1983 aranovich podlesskii 1983 gerya perchuk 1989 detailed investigation of 10 samples including 1000 microprobe analyses revealed decompression first stage followed by the near isobaric cooling of the granulites from geological studies the 7 km total thickness of the sequence closely corresponds to the pressure difference 2 2kbar measured by geobarometers in the samples taken from different levels in the sequence individual samples yield p t paths ranging from 100 c kbar to 140 c kbar depending on their locations with respect to the large tarakskiy granite pluton in places the 100 c kbar path changed to the 140 c kbar due to the influence of the intrusion in a p t diagram these trajectories are subparallel lines whose p t maxima define the archaean geotherm between 3 1 and 2 7 ga determined isotopically a petrological model for p t evolution of the kanskaya formation is proposed

conductive incubation and the origin of dome and keel structure in archean granite greenstone terrains a model based on the eastern pilbara craton western australia 1 the archean east pilbara granite greenstone terrane epggt of the pilbara craton in western australia preserves a prolonged record of voluminous mafic dominated volcanism and felsic plutonism commencing at circa 3515 ma and ultimately resulting in the development of spectacular dome and keel structures early crustal growth was dominated by basaltic magmatism and the regional development of a 12 18 km thick greenstone sequence by circa 3335 ma overlying widespread 3470 3430 ma tonalite trondhjemite granodiorite ttg suites emplaced into the middle crust stratigraphic and structural relationships imply that voluminous granitoid plutonism and crustal scale dome and keel formation in the southeastern epggt was not initiated until circa 3325 ma but proceeded rapidly over the interval 3325 3308 ma this scenario is consistent with a conductive incubation period of 10 100 myr duration following the burial of radiogenic granitic crust beneath the accumulated greenstone pile for heat production parameters appropriate to the 3515 3308 ma epggt we show that the burial of 3470 3430 ma granitoid rich crust beneath the 3335 ma euro basalt and its precursors led to long term crustal temperature increases of 170 234 c at 35 km depth with a corresponding reduction in effective viscosity of 2 5 orders of magnitude in combination with the greenstone over granitoid density inversion the changes in effective viscosity of the mid to deep crust due to the burial of heat sources may have provided the crucial trigger for the initiation of dome and keel formation similarly cooling and strengthening of the crust during syndoming exhumation of the heat production potentially terminated large scale dome and keel amplification

fluids in granulite facies metamorphism a comparative oxygen isotope study on the south india and adirondack high grade terrains oxygen isotope compositions have been determined for samples from the south india archean to pro terozoic and northwest adirondack late proterozoic high grade terrains delta 18 o values of charnockites and related orthogneiss protolith from the prograde amphibolite facies to granulite facies transition zone in south india range from 5 9 to 8 4 charnockites khondalites sillimanite gneisses and retrogressed gneisses from the interior of the granulite facies terrain average 2 higher than orthogneisses of the transition zone individual minerals in incipient charnockitic net veins as at kabbaldurga karnataka and ponmudi kerala have delta 18 o identical to minerals of their non charnockitic host rocks quartz delta 18 o values of the northwestern adirondack granites and gneisses average 3 4 higher than those from the indian rocks the two terrains are thus isotopically distinct which may reflect differences in premetamorphic history or different degrees of communication with external

inverse age stratification in the archaean crust of the superior province evidence for infra and subcrustal accretion from high resolution u pb zircon and monazite ages abstract new u pb mineral ages from the agawa migmatite terrain in the southern superior province canada together with published ages from neighbouring high and low grade terrains reveal a pattern of progressive younging of plutonic and metamorphic processes with depth and provide insights into the mechanisms controlling late archaean cratonisation the following ages are reported 1 about 2670 ma for a first zircon generation about 2660 ma for a second zircon generation and 2649 2 ma for monazite all from a leucocratic layer in migmatites 2 2666 2 ma for a first zircon generation 2663 1 5 ma for a second zircon generation and 2642 3 ma for titanite all from an amphibolite 3 2654 4 ma for monazite from a granodiorite and 4 2637 2 ma for monazite from a granite together with petrographic observations and chemical and morphological features of the zircons the ages indicate the occurrence of a protracted period of magmatism and metamorphism commencing with migmatisation at 2670 2665 ma followed by injection of calc alkalic melts and metasomatism at 2663 2660 ma and concluded by the intrusion of granites and pegmatites and extensive hydrothermal activity between 2660 and 2635 ma the timing and the style of plutonism and metamorphism in the agawa migmatite terrain contrast with the situation observed in the adjacent but tectonically higher level batchawana greenstone belt where major metamorphism occurred about 2675 ma ago and plutonism was concluded at 2668 ma ago on the other hand it resembles the evolution of the high grade portion of the kapuskasing zone where the orogenic activity continued until after 2630 2620 ma ago these age relationships are interpreted as reflecting downward vertical accretion of the archaean crust the lower crust experienced magmatism and metamorphism long after conclusion of these processes at shallower crustal levels the late tectonic regime produced granulites at depth transferring granitic material rich in h 2 o and heat producing elements to intermediate crustal levels these processes were probably accompanied by crustal thickening through the addition of mantle derived material

an overview of charnockite abstract charnockite a quartzfeldspathic rock containing orthopyroxene is an important component of high grade deep crustal terrains the origin of charnockite by both igneous and metamorphic processes not yet clearly defined has major bearing on the evolution of continents