strength and anisotropy of foliated rocks with varied mica contents abstract we have shortened 15 schists and gneisses with varying compositions 15 75 mica by volume at angles of 45 and 90 to foliation s to investigate the influence of micas on the strength and anisotropy of foliated rocks at the conditions tested t 25 c p c 200 mpa and e 10 5 s 1 compressive strengths vary by a factor 4 in the 45 and 90 orientations and individual rock types exhibit directional responses ranging from isotropic to strongly anisotropic trends of decreasing strength decreasing anisotropy and increasing ductility are observed with increasing mica content strains in all samples were localized within inclined shear zones accommodated by dislocation slip and grain scale microkinking in micas and microcracking in all other silicates stress strain response and grain scale deformation microstructures both indicate that mechanical behavior is strongly influenced by the concentration and spatial arrangement of micas shear zone formation is associated either with discrete brittle fracture transitional strain softening or steady strength ductile shear in both brittle and transitional samples extensive microcracking of strong quartzo feldspathic bridges occurs and stress drop magnitudes decrease with decreasing spacing and increasing overlap of adjacent critically oriented micas steady strength ductile shear zones develop in samples containing domains of interconnected micas and isotropic mechanical response is observed in those in which micas are contiguous in nearly all directions

reworking of sialic crust as represented in late archean age gneisses northern labrador regionally reworked sialic crust has been recognized in the form of 2 800 m y old nebulitic rocks kiyuktok gneisses with high initial 87sr 86sr ratios 0 7081 and pb isotope ratios that suggest a history of the source back to at least 3 500 m y b p the field geochemical and isotopic data are consistent with these rocks having formed by in situ mobilization of granitic components in the 3 600 m y old uivak gneisses by the introduction of aqueous fluids some 2 800 m y ago contemporary but subordinate tonalitic granodioritic orthogneisses derived from discrete intrusive bodies have contrasting sr isotope compositions some display low initial 87sr 86sr ratios 0 7030 that suggest derivation from a source with mantlelike sr isotope composition however most of the late archean age intrusive rocks display higher initial 87sr 86sr ratios 0 7056 0 7059 recognition of these contrasting isotopic patterns suggests that processes of late archean crust formation and stabilization in the north atlantic craton were more complex than previously realized crustal reworking due to interaction of old crust with aqueous fluids must have been accompanied by the introduction of granitic melts derived by melting of ancient deep crustal rocks or contamination of juvenile granitic melts by ancient deep crustal material

deformation of chlorite mica aggregates in cleaved psammitic and pelitic rocks from islesboro maine u s a abstract chlorite white mica aggregates are large porphyroblasts found in weakly metamorphosed and deformed pelitic and psammitic rocks throughout the world they are usually considered to originate either as detrital white mica layers surrounded by crystallized chlorite or as synkinematic porphyroblasts in chlorite muscovite aggregates observed at islesboro the chlorite layers were found to represent prekinematic porphyroblasts that have been strongly deformed and rotated during cleavage development the muscovite layers in the aggregates formed by crystallization along split and dislocated 001 surfaces in the chlorite or by alteration within intensely deformed sections of chlorite grains such as kink bands

tem and xrd determination of crystallite size and lattice strain as a function of illite crystallinity in pelitic rocks average crystallite size and mean square strain of illite in rock specimens and clay separates were measured independently in tem images and by single line fourier voigt method profile analysis of the c 1 nm peak of xrd patterns for a prograde sequence of pelitic rocks illite crystallinity indices 0 17 0 58 d2h from the gaspepeninsula quebec the tem determined crystallite sizes in clay separates approxi mate those determined by fourier profile analyses and those calculated from illite crystallinity indices by the scherrer equation with the exception of the diagenetic sample the crystallite sizes and mean square strains of illite in rock samples exhibit a trend similar to that determined by profile analyses but the average crystallite sizes are up to five times larger than those measured for clay separates tem images show that all rock samples have a wide range of crystallite sizes and the proportions of larger crystallites increase with metamorphic grade the diagenetic illite is defect rich fine grained mean thickness by volume c 70 nm 1m d material anchizonal illite tends to occur as separate aggregates of small 1m d and larger 2m 1 crystals c 200 nm comprising arrays of subparallel coalescing packets the epizone sample has thick c 400 nm defect free crystals of muscovite occurring in stacks of parallel layers or subhedral crystals intergrown with large angle boundaries cleaved crystals that are free of intracrystalline layer terminations are dominant in clay separates of all samples having ranges of smaller sizes with volume average thicknesses of c 43 43 and 81 nm c 14 28 67 nm by the voigt method respectively for the three zones the results suggest that illite crystallinity indices do not provide a direct measure of a single microstruc tural state of illite in rocks although they yield consistent limits for average crystallite sizes for the anchizone 23 48 nm here therefore they serve as a general parameter of the degree of recrystallization on a relative basis in part because the contributions of all peak broadening variables mixed layering size and strain decrease regularly with prograde regional metamorphism of pelites the microstructural changes caused by rock disaggregation are probably a function of those variables as well the data collectively demonstrate a trend from metastable defect rich small crystals towards a stable assemblage of larger defect free crystals through dissolution of strained crystals and neocrystallization consistent with the ostwald step rule

minimum continental velocities with respect to the pole since the archean abstract paleomagnetic apparent polar wander paths provide a method for estimating minimum plate velocities which can be extended much further into the geologic past than can be estimated based on sea floor magnetic anomalies minimum velocities can be determined from the rate of change of latitude as derived relative to a fixed pole such latitudinal velocities have been determined for the center of mass of the north american baltic siberian and african shields since the archean the results indicate that plate velocities in the past have at times easily exceeded those for present day continents and that they often were equivalent to present day oceanic velocities although there are peaks and troughs through time north american velocities are significantly greater prior to one billion years ago than those of siberia or baltica

growth and deformation mechanisms of talc along a natural fault a micro nanostructural investigation this paper documents the occurrence of large amounts of talc within a continental normal fault the talc in reaction is deformation enhanced and occurs by the interaction between dolostones and silica rich hydrothermal fluids in the high strain foliated fault core talc forms an interconnected network of oriented 001 lamellae 200 300 nm thick locally associated with minor tremolite fibres up to 300 nm in diameter the talc structure is affected by several strain induced defects among which 001 interlayer delamination that produces talc sublamellae down to 10 30 nm thick micro nanostructural observations definitely point to a predominant deformation mechanism of 001 frictional sliding further enhanced by pervasive delamination that gives rise to an almost infinite number of possible sliding surfaces these effects have fundamental implications in fault mechanics resulting in significant fault weakening

crustal shear zones and thrust belts their geometry and continuity in central africa the precambrian orogenic belts of africa are often defined by ductile shear zones which developed in response to large displacements and which mark orogenic fronts between mobile and stable parts of the crust they are thought to represent the major crustal reflectors seen by seismic reflection profiling in younger orogenic belts these orogenic fronts are connected by shear zones that transfer displacement or accommodate different displacements between orogenic segments smaller shears within an orogenic belt occur as a result of differential movements these shear zones are seen to pass from flat lying to steep structures and may have a thrust or strike slip sense they compare with the staircase trajectories characteristic of foreland thrust belts in common with thrust belts the geometry of the shear zones can be used to estimate displacement direction as can regional extensional fabrics developed in the associated high strain tectonites central africa has been previously described as a complex network of late proterozoic mobile belts the recognition of similar displacements and time equivalence in these belts allows their reinterpretation in terms of a linked thrust and strike slip shear zone system an example is the damaran lufilian zambezi and ukingan system these orogenic belts share a similar displacement picture and broad time equivalence and were apparently linked in a lower crustal shear zone of continental dimensions this shear zone system appears to have developed under a single tectonic framework

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

growth of early archaean crust in the ancient gneiss complex of swaziland as revealed by single zircon dating abstract we present new single grain zircon ages derived from ion microprobe and evaporation analysis for rocks of the ancient gneiss complex agc of swaziland southern africa that enable us to document a more extended history of early archaean crustal growth in the eastern kaapvaal craton than previously documented the oldest rocks appear to be strongly flattened tonalitic trondhjemitic orthogneisses dated as 3644 4 ma compston and kroner 1988 which are in faulted contact with rocks of the barberton greenstone belt bgb in an agc enclave in northwest swaziland and which also record subsequent thermal metamorphic plutonic events at 3580 3500 3430 3200 and 3000 ma these rocks are tectonically interbanded with 3200 ma old granodiorites and k rich leucogranites thereby providing evidence for a severe and important post 3 2 ga deformation event that is also inferred for 3500 to 3200 ma old tonalitic to trondhjemitic gneisses in the agc of northeast swaziland in central swaziland zircons from the oldest tonalitic gneiss of the agc have an age of 3560 ma whereas younger additions of granitoid which also intrude the dwalile greenstone enclave in the agc record ages of 3450 3460 ma intense ductile deformation has repeatedly obliterated original intrusive contacts so that preserved field relationships often cannot resolve the above chronology chemical and whole rock isotopic data are consistent with derivation of some agc gneisses from mantle sources and thus imply very short crustal residence times for their granitoid precursors but this is not generally true for the entire agc we suggest that at least some gneisses resulted from intracrustal melting perhaps during crustal thickening events as recorded in granulite parageneses within the agc documentation of 3 5 ga or older granitoid crust in all major regions of the agc now exposed suggests the presence of continental terrains of up to at least 100 km in diameter before the barberton greenstone succession was deposited the combined geochronologic and structural metamorphic data for the agc and bgb reveal a complex history of growth and deformation for the eastern kaapvaal craton that lasted for at least 600 ma from 3 64 to about 3 ga ago the magmatic and tectonic evolution of the agc was broadly similar to that of the bgb and suggests that early archaean greenstone and gneiss terrains were coeval but formed and evolved in different settings and at different crustal levels our age data and the structural metamorphic evolution of the eastern kaapvaal craton are compatible with models suggesting crustal growth through vertical magmatic underplating and early crustal thickening uplift and erosion as a result of tectonic interleaving due to repeated horizontal shortening

multiple zircon growth within early archaean tonalitic gneiss from the ancient gneiss complex swaziland abstract u pb age determinations by ion microprobe reveal multiple episodes of zircon growth and recrystallization within a single sample of tonalitic gneiss from the ancient gneiss complex the oldest episode at3644 4ma 2 produced the dominant type of zircon characteristically purplish brown and massive in texture this probably constitutes unaltered zircon precipitated from the original magma recrystallization accompanied and obscured by early pb loss took place within the oldest grains at3504 6ma and3433 8ma whole new grains grew at these times also we interpret the post 3644 ma growth as due to episodic deformational and metamorphic events that transformed the tonalite pluton into foliated banded gneiss in addition many grains are visibly overgrown by two layers of younger zircon of different colour and texture dated at2986 20ma and2867 30ma euhedral finely zoned whole grains having the 2986 ma age are present also evidently contributed by very thin felsic veins associated with the nearby lochiel granite the age of3644 4ma combined with precise zircon u pb dating of volcanics from the onverwacht group reported elsewhere demonstrates that at least part of the ancient gneiss complex is older than the barberton greenstone belt

origin of the earth s crust abstract the earth s earliest oceanic crust which was probably worldwide in extent appeared by 4 5 ga and likely formed at ocean ridges it may have been comprised chiefly of komatiite with basalt becoming more important with time the earliest continental crust appears to have formed by 4 ga in response to wet partial melting of mafic sources in descending slabs it was of tonalite granodiorite composition and of local extent continental crust has grown by magma additions thrust stacking accretion of arcs and welding of sedimentary prisms to continental margins periods of rapid continental growth are recorded at 2 7 3 0 ga and 1 7 1 9 ga the former of which is most significant and may reflect catastrophic eclogite driven subduction of basalt well documented secular changes that reflect one or a combination of 1 cooling of the earth 2 rapid continental growth in the late archean or 3 reworking of the continents include a decrease in komatiite or after 2 7 ga a long term change in 87 sr 86 sr in marine carbonates a decrease in ni cr in pelites after 2 5 ga changes in the composition of greenstone volcanic rocks and the increasing importance of alkaline igneous rocks and blueschists after 1 ga u pb zircon dates from igneous rocks do not support worldwide episodicity or synchronicity of orogeny all of the terrestrial planets may have had magma oceans and komatiite driven plate tectonics at 4 5 ga differences in planetary crustal histories largely reflect different cooling rates of the planets with only earth and possibly venus cooling slowly enough to sustain plate tectonics

did intense volcanism trigger the first late ordovician icehouse comment details of the climate evolution during the middle and late ordovician are increasingly debated with implications for understanding both greenhouse icehouse transitions and the late ordovician mass extinction we applaud buggisch et al 2010 1 for attempting to distinguish among different

origin of granite greenstone patterns yilgarn block western australia the yilgarn block of western australia consists mainly of archean granite greenstone terrain with oval areas of granitoid rocks fringed by arcuate greenstone belts in a typical part of this terrain the outcrop pattern is shown to result from large scale dome and basin fold interference structures rather than diapiric emplacement of steep sided batholiths the greenstones and underlying granitoid rocks were initially deformed together in a subhorizontal tectonic regime as a result the granitoid rocks recrystallized and a gneissosity developed subparallel to the stratigraphy of the greenstones thick subhorizontal monzogranite sheets were later emplaced into the lower part of the greenstone sequence subsequent deformation generated two successive sets of folds with vertical axial surfaces at a high angle to each other these folds formed dome and basin interference structures ranging from a few centimetres to more than 50 km in diameter

archaean plate tectonics revisited 1 heat flow spreading rate and the age of subducting oceanic lithosphere and their effects on the origin and evolution of continents a simple model which relates the rate of seafloor creation and the age of the oceanic lithosphere at subduction to the rate of continental accretion can successfully explain the apparent differences between archaean and phanerozoic terrains in terms of plate tectonics the model has been derived using the following parameters 1 the spreading rate at mid ocean ridges 2 the age of the oceanic lithosphere at the time of subduction 3 the area age distribution of the seafloor 4 the continental surface area as a fraction of the total surface area of the earth and 5 the erosion rate of continents as a function of continental surface area and the total number of continental masses observations in phanerozoic terranes suggest that there are profound differences in the nature and volume of subduction zone igneous activity depending upon the age of the oceanic lithosphere being subducted and the nature of the overriding plate that is either continental or oceanic the subduction of young oceanic lithosphere less than 50 m y old which is thermally buoyant appears to result in a reduced volume of igneous activity most of the igneous activity caused by subduction of young oceanic lithosphere is either siliceous plutonism or bimodal tholeiitic rhyolitic volcanism when very young lithosphere is being subducted 50 m y old appears to result in greater volumes of igneous activity including the eruption of andesitic magmas thus andesites could only begin to be abundant in the rock record when older oceanic lithosphere began to be subducted our model predicts that as the earth aged and as heat flow from the interior of the earth diminished the proportion of old oceanic lithosphere being subducted increased fundamentally changing the nature of subduction zone igneous activity and the rate of continental accretion if the subduction of old oceanic lithosphere results in an 8 10 times greater volume of subduction zone magmatism our model predicts or explains all of the following observed features of earth history 1 archaean terranes appear to record two periods of rapid continental accretion between 3 8 and 3 5 b y ago and between 3 1 and 2 6 b y ago 2 there are very few differences and many marked similarities between rocks from archaean terranes and equivalent rocks from phanerozoic terranes 3 the total continental area appears to have remained essentially constant for the past 2 b y 4 archaean andesites are comparatively rare and the relative abundances of mafic and siliceous rocks appear to change during the archaean and the proterozoic with siliceous volcanics becoming proportionately more abundant in the geologic record with time 5 plutonic tonalites and trondhjemites appear to have been relatively much more abundant during the archaean plate tectonics is thus shown to have evolved over time due to a gradually decreasing rate of creation of oceanic lithosphere meaning that archaean tectonics and phanerozoic tectonics are but two points on an evolutionary continuum

evidence for syntectonic crystallization for the mudstone to slate transition at lehigh gap pennsylvania u s a data primarily for phyllosilicates have been obtained for the continuous transitional sequence from mudstone to slate with well developed slaty cleavage at lehigh gap pennsylvania and for slates from quarries in the same area samples were studied by optical microscopy powder x ray diffraction and electron microprobe analysis with emphasis on transmission and analytical electron microscopy mineral grains are virtually free of deformation induced strain concomitant with the gradual development of cleavage normal to bedding the following changes are observed or confirmed 1 the orientation of phyllosilicate grains changes discontinuously from being preferentially parallel to bedding to being parallel to cleavage 2 crystal imperfections as expressed in layer terminations low angle grain boundary like features and other defects decrease in density 3 complex mixed layering is replaced by homogeneous packets of layers of single phases and 4 illite transforms to muscovite with increase in k a1 and change from a lmd to 2m polytype slaty cleavage apparently develops due in part to pressure solution of phyllosilicates oriented parallel to bedding mass transport of components and crystallization to form new grains parallel to cleavage it reflects transitions from imperfect metastable phases toward ordered stable phases in a low temperature 225 c metamorphic environment

kinks in mica role of dislocations and 001 cleavage abstract kinking in natural and experimentally deformed biotite and muscovite has been investigated by transmission electron microscopy tem it is found that complex microstructures exist in the vicinity of kink band boundaries kbbs gentle bending of 001 planes plays an important role in the kinking process where 001 planes are bent several degrees edge dislocations are observed in complex walls between active slip planes together with broad bands of dislocations parallel to 001 microfracturing and dilation occur parallel to 001 planes and the kbb a model is presented to explain the origin of observed kbbs which may be applicable to other crystalline materials with only one active slip plane

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

40ar 39ar dating constraints on the high angle normal faulting along the southern segment of the tan lu fault system an implication for the onset of eastern china rift systems abstract high angle normal faulting in eastern china was an important tectonic process responsible for the rifting of the eastern asian continental margin along the southern segment of the tan lu fault system part of the eastern china rift system 55 70 east dipping normal faults are the oldest structures within this rift system chlorite pseudotachylite and fault breccia are found in fault zones which are characterized by microstructures and syn deformation chlorite minerals aligned parallel to a down dip stretching lineation 40ar 39ar dating of syn deformation chlorite and k feldspar from the fault gouge zone yields cooling ages of 75 70 ma interpreted as the timing of slip along the normal faults this age is older than that of opening of the japanese sea and back arc extension in the west pacific but similar to the onset of the indo asian soft collision

faster spreading or greater ridge length in the archean dissipation of more heat from a hotter early earth by the plate tectonics mechanism would require subduction of on average younger lithosphere in principle this could be achieved equally well by faster spreading by having greater ridge length or some combination thereof present day evidence however suggests that younger oceanic lithosphere subducts more slowly than older if this applied in the archean then the greater ridge length alternative prevailed a quantitative expression is derived which indicates that oceanic heat loss is proportional to the cube root of ridge length this relationship means that if archean heat flow was 3 times that of the present 27 times as much ridge would have been required which would suggest that the archean earth was covered by many small plates moving slowly

upb zircon geochronology of late archaean metamorphic rocks in the taihangshan wutaishan area north china abstract a geochronological study has been made in the taihangshan wutaishan area shanxi province on a sequence of precambrian supracrustal rocks and stratigraphically related intrusives these constitute part of the north china basement block the oldest rocks of the fuping group consist predominantly of mafic granulites paragneisses quartzites and amphibolites this group was always considered archaean and early k ar measurements yielded some minimum ages as old as 2300 ma and others as young as 1500 1900 ma the unconformably overlying wutai group is considered as the base of the proterozoic succession and consists of amphibolite grade paragneisses carbonates and amphibolites in its lower part and greenschist grade clastic sediments and volcanics in its upper part it has given apparent k ar mineral ages between 1500 and 2000 ma present investigations have employed the u pb zircon approach on selected parts of the sequences to provide direct depositional ages or age constraints for the fuping and wutai groups and to find the age s of subsequent metamorphic processes a minimum age for the fuping group and maximum age for the wutai group has been derived from zircon age measurements on the lanzhishan granite 2560 9 ma which intrudes the fuping group and is unconformably overlain by the wutai group this late archaean result can be interpreted as the age of an igneous crystallization event and therefore a maximum for the depositional age of the wutai group the above result is well substantiated by zircon dating of the wutai group itself zircons from a low grade metamorphosed keratophyric lava in the upper part of the group give a volcanic crystallization age of 2520 17 ma this provides close constraints for the age of deposition of the wutai group at between 2560 and 2520 ma ago if the wutai group is indeed basal proterozoic the archaean proterozoic boundary in this part of china is also dated at between 2560 and 2520 ma further evidence as to the antiquity of the wutai group comes from a u pb zircon age of 2520 30 ma for the ekou granite which intrudes the wutai group sequence the age of the underlying late archaean rocks in the tainhangshan wutaishan area the fuping group could not be directly determined because of the paucity of volcanic markers and the higher grade of metamorphism its minimum age is 2560 ma see above but only an approximate maximum age can be gleaned from the zircon systematics in granulite and amphibolite grade paragneiss samples these data indicate a degree of pb inheritance which combined with metamorphic disturbance and more recent pb loss inhibit any further interpretation other than that they indicate an approximate maximum age of around 2800 ma several of the paragneisses and one amphibolitic schist contain zircon systems which are interpreted as having completely responded to a well defined amphibolite facies metamorphic event at 2500 ma ago a younger but less well defined metamorphism caused complete pb loss from zircon and recrystallization in the amphibolite and granulite facies terrains at between 2000 and 1800 ma ago rocks which show evidence of this younger metamorphic event were not recognized from the field or structural data base but from the distribution of the few such samples analyzed it seems possible that this younger terrain may now be interpretable as a cross cutting proterozoic mobile belt this proterozoic metamorphic province is also reflected by k ar mica ages between 1600 and 2000 ma and some preliminary rb sr total rock ages at 1700 ma on nearby paragneisses whose u pb zircon age is 2500 ma

elements of the archean thermal history and apparent polar wander of the eastern kaapvaal craton swaziland from single grain dating and paleomagnetism abstract single grains of zircon hornblende biotite and feldspar have been dated to define the thermal history of the archean mbabane pluton swaziland coincident207pb 206pb zircon and40ar 39ar hornblende ages suggest that the pluton underwent rapid cooling to about 450 c at the time of emplacement at 2690 ma because the mbabane pluton is one of a suite of granites which represent the last major archean intrusive event in the eastern kaapvaal craton this time of emplacement marks the end of cratonization much younger biotite and feldspar ages indicate argon loss perhaps due to several low temperature events related to dike intrusion at 2000 2300 ma from such a thermal history the age of magnetization of a paleomagnetic pole polelatitude 19 7 n polelongitude 105 7 e a95 9 4 k 67 obtained from 5 sites can be determined based on the unblocking temperature of the magnetic carrier and the consistent direction seen in several sites it appears that the hornblende age and coincidently the207pb 206pb zircon age most accurately represents this age of magnetization thus the mbabane pluton yields a precisely dated archean 2687 6 ma 2 hornblende plateau age paleomagnetic pole which is different than other archean poles such as that from the nearby usushwana complex 2875 40ma although the two intrusions are separated by less than 12 km and there is no evidence of relative structural rotation between them the paleomagnetic poles from the two bodies are 65 apart this implies that there was significant apparent polar wander during the late archean

chlorite illite muscovite interlayered and interstratified crystals a tem stem study phyllosilicates in rocks which are transitional from mudstone to slate from lehigh gap pa have been studied by a variety of techniques including high resolution transmission electron microscopy and analytical electron microscopy the principal minerals are white mica which is transitional from illite in mudstone to ordered twolayer mica in slate and chlorite 7a berthierine occurs more rarely dioctahedral and trioctahedral layers are shown to be interleaved in individual crystals at all scales between the following two end members 1 both random and regular 1 1 interlayering at the scale of individual layers as shown in part by lattice fringe images 2 packets of trioctahedral and dioctahedral layers up to a few thousand angstroms or microns in thickness detectable with ordinary optical techniques the complete range of intermediate structures is represented in samples which are in transition to slate bulk analytical empa x ray diffraction or other measurements are shown to result in averages over both kinds of layers when tem techniques are not used

great slave lake shear zone canadian shield reconstructed vertical profile of a crustal scale fault zone abstract the great slave lake shear zone gslsz of the northwestern canadian shield a 25 km wide early proterozoic transcurrent dextral mylonite zone is a type example of a crustal scale fault zone profile with time 1 1 the metamorphic grade decreased from granulite to greenschist facies 2 2 the locus of high strain narrowed and jumped laterally abandoning relatively older mylonites progressively younger mylonites represent both cooler temperatures and lower pressures thus the sequence of narrowing mylonite belts represents a series of progressively shallower seated sections through the gslsz 3 3 deformation evolved through the brittle ductile transition initially producing non dilational penetrative breccias followed first by discrete non dilational faulting and subsequently by dilational quartz stockworks developing crustal scale shear zones grow to an optimum width dictated by the requirement of minimum geologically reasonable strain rates for given conditions of wall rock rheology and displacement rate independent of variations in relative plate motion mature shear zones change width and migrate laterally as a function of inferred thermally activated rheological changes within the wall rocks

multi chronometric ages and origin of archaean tonalitic gneisses in finnish lapland a case for long crustal residence time the tojottamanselka gneisses of the koitelainen region northern finland have been dated by the sm nd and the common pb methods the sm nd data of seven samples from a small area 100 m 100 m define an isochron oft 3 06 0 12 2 ga with correspondingind 0 50848 9 2 orend t 3 7 1 8 this age is in good agreement with the zircon u pb discordia age 3 1 ga reported by kroner et al 1981 and is interpreted as the time of magmatic emplacement the distinctly negativeend t value is found for the first time for archean tonalitic gneisses and implies derivation of these magmas by remelting of continental material with a long 200 500 ma crustal residence time a few samples on the other hand possessend t values close to zero hence they are thought to be derived by partial melting of basaltic sources with nearchondritic ree distribution patterns

low grade metamorphism around the down dip limit of seismogenic subduction zones example from an ancient accretionary complex in the shimanto belt japan abstract reactions involving clay minerals during low grade metamorphism at the depth of an ancient accretionary complex in the shimanto belt kyushu japan were studied by integrated transmission electron microscopy energy dispersive x ray spectroscopy and x ray diffraction analyses of the bulk rock and clay fraction the analyzed metasediment the kitagawa unit contain an incipient sub horizontal slaty cleavage illite crystallinity data and mica b dimensions indicate that the conditions of metamorphic deformation were anchizone epizone grade and intermediate pressure cleavage formation was linked to two reactions involving clay minerals 1 the recrystallization of 1m dominant matrix mica inherited from the original sedimentary fabric into thick defect free 2m1 packets along cleavage planes and 2 the formation of chlorite from 7 a berthierine balanced equations among the clay phases based on compositional data and their relative abundance suggest that the decomposition of matrix mica resulted in the formation of paragenetic mica and chlorite along the cleavage planes without significant elemental outflux although a modal increase in phyllosilicates is not indicated by the data the growth of chlorite and mica along cleavage planes may have a large influence on the rheological properties of a decollement and may be related to the occurrence of the seismic aseismic transition at 350 c

3820 ma zircons from a tonalitic armisoq gneiss in the godthab district of southern west greenland abstract zircons from a sample of the tonalitic amitsoq gneisses near godthhb west greenland exhibit several stages of crystal growth measurements of the u th pb isotopic composition of 30 m areas within individual grains using the ion microprobe shrimp reveal that components of magmatic zircon from the original igneous protolith are at least 3822 5 ma l old slightly but significantly older than the age of zircons from a fclsic volcanic unit in the isua supracrustal belt as measured at high precision by the same technique and older than all ages previously determined by mineral and whole rock isotopic techniques for amitsoq gneisses at godthab and at isukasia overgrowths of younger zircon formed at ca 3630 ma coinciding with an episode of major pb loss from the older grains these results indicate that previous whole rock isochron ages refer to later isotopic redistribution and or that other phases of the amitsoq gneisses intruded during this later period and dominate the isotopic systematics comparison of the ion probe results with previous conventional u th pb data for the same zircon sample demonstrates the strong influence of discordant high uranium parts of zircon on the mean isotopic composition of multi grain samples

chlorite mica aggregates morphology orientation development and bearing on cleavage formation in very low grade rocks abstract chlorite mica aggregates in slates from northern spain have been investigated in very thin thin sections specimens from various parts of a fold show different morphologies that are dependent on the operating deformation mechanism from the limb to the hinge rigid body rotation becomes less important and folding and intragranular kinking are more common this is associated with an increasing aspect ratio width length for the aggregates chlorite mica aggregates were in part present before cleavage development and are statistically parallel to the bedding enveloping surface variation in degree of distortion of chlorite suggests continuous solution and growth during deformation muscovite on the other hand is generally more highly deformed suggesting that it was all early splitting of micas along 001 planes occurs creating extension sites where chlorite growth can take place thus an early diagenetic metamorphic formation with mimetic growth of chlorite on detrital micas is favoured for the origin of the aggregates the behaviour of the aggregates during deformation provides information on a mechanism for cleavage development as a result of intragranular kinking high angle boundaries are created with parts of the deformed grains at a low angle to the cleavage plane mechanical rotation appears to be the important mechanism for cleavage development in these rocks modification by recrystallization solution and growth processes can account for the observed microstructure

ree geochemistry and isotopic data of archean silicic volcanics and granitoids from the pilbara block western australia implications for the early crustal evolution eighteen silicic volcanic rocks of the warrawoona group and ten associated plutonic rocks from the pilbara block western australia have been chosen for geochemical and isotopic studies silicic volcanics of the unsb upper member of north star basalt are dated at 3 56 3 57 by both the rb sr and the sm nd methods the respective 1 initial isotopic composition values are 0 7005 5 sr and 0 50810 39 nd this age is consistent with the stratigraphic interpretation that the talgatalga subgroup in which the north star basalt occupies the lowermost position is overlain by the duffer formation whose age was earlier established at 3 45 by the zircon u pb method the new rb sr data on six silicic lava samples from the duffer formation yield an isochron of 3 23 0 28 2v though imprecise this age agrees with the zircon age within error limits rb sr ages of 2 3 2 4 obtained for the panorama rocks and the wyman formation do not correspond to their initial eruption ages chemical arguments suggest that these ages represent the time of metasomatism associated with the widespread thermal event in this region about 2 3 2 4 ago geochemically most of these analyzed rocks volcanic and plutonic are of tonalite trondhjemitegranodiorite ttg composition a typical feature found in many other archean terrains they generally show fractionated ree patterns except the panorama formation rocks furthermore the wyman formation rhyolites and the post tectonic adamellites show significant negative eu anomalies suggesting a similar mode of magma generation and a probable genetic link theoretical considerations suggest that most of these ttg rocks could have been generated by partial melting of amphibolitic or basaltic sources followed by fractional crystallization although the archean granitic gneisses often possess mantle like isr values the trace element data indicate that they could not have been derived by direct melting of upper mantle materials the immediate tectonic implication is that in any archean terrain the formation of na rich continental crust of ttg suite must be preceded by the presence of basaltic crust the occurrence of this basaltic crust is a matter of controversy such crust might have been totally destroyed by repeated melting processes or its remnants are now represented by some of the mafic ultramafic enclaves within the tonalite trondhjemite batholiths

the interaction of deformation and metamorphism in slates abstract the distribution microchemistry and internal structure of phases present in a slate are reported and used as a basis for a discussion of the interaction between deformation and metamorphism in slates the study reveals details of the interactions between mechanical and chemical processes involved in slaty cleavage evolution evidence for the involvement of mechanical rotation solution and crystallisation recrystallisation and metamorphic reactions is presented and amalgamated into a model of cleavage development the model involves a synchronous variation in the type location and rate of cleavage evolution processes and a change in the importance of particular processes during the total history the early stages of cleavage development are dominated by mechanical rotation associated with crenulation and possibly accompanied by grain boundary sliding and solution processes later stages of cleavage evolution are more heterogeneous and occur by a complex interaction of deformation and metamorphic growth processes which are concentrated at the interfaces between domains generated earlier in the strain history the variations in grain chemistry and orientation together with changes in the strain rate or stress level are suggested to control the different deformation behaviour of different grains along these strain accommodation zones and to dictate whether grains bend kink dissolve recrystallise or react and undergo growth evidence for the involvement of reactions ranging from ionic exchange to crystallisation from solution and solid state reordering is presented and the influence of deformation on the location and rates of these reactions discussed approximately ten potential reactions are involved in the cleavage evolution of the slate studied the model presented incorporates these reactions into the deformation processes and accounts for the concentration of phengite and iron poor chlorites in to one microstructural domain and paragonite quartz and fe rich chlorites into the other domain two transformations appear to be fundamental in the slate studied 1 1 the transformation of illite to phengite 2 2 the transformation of fe rich chlorite to fe poor chlorites and both can be related to particular responses of grains to the reorientation processes at domain interfaces

ridge collision at convergent margins implications for archean and post archean crustal growth abstract growth of continental crust today occurs dominantly at convergent plate margins through magmatic addition and tectonic accretion lateral and vertical related to 1 1 steady state subduction 2 2 non steady state episodic processes ridge forearc collision is one episodic process that can cause significant highly diachronous thermal and tectonic effects when a buoyant bathymetrically high and thermally active oceanic spreading ridge approaches collides with and potentially subducts below a continental margin near field effects within 100 km of collision site in the forearc may include anomalous heat anomalous magmatism ophiolite obduction forearc deformation and tectonic erosion of the base of the forearc far field effects generally postdate collision and may include changes in the thermal structure of the continent changes in the composition of arc magmas a reduction or hiatus in arc magmatism uplift and or subsidence of the continental margin marginal basin formation underplating of subducted slabs and intraplate deformation effects of ridge collision can vary depending on the triple junction geometry the chile margin triple junction cmtj where the actively spreading chile ridge is colliding with the south american forearc is the best modern example of ridge collision near field effects of pliocene recent ridge collisions near the cmtj include 1 1 emplacement of the pliocene taitao ophiolite less than 15 km from the trench 2 2 intrusion of pliocene granitic plutons and 3 3 forearc deformation formation of a pull apart basin on the continental shelf and late tertiary recent movement on a major fault along the arc forearc boundary far field effects include reduction in subduction related seismicity and volcanism chemical changes in arc magmatism and possibly uplift and or subsidence ridge collison features in southern chile are similar to features of archean greenstone belts suggesting that some greenstone sequences may have formed in a ridge collision setting and that ridge collision may have been an important process in archean crustal growth these features include granitic rocks closely associated with ophiolitic rocks formed near a continental margin mafic ultramafic section overlain by basaltic to andesitic volcanic rocks of mixed calc alkaline and morb character intercalated with volcaniclastic sedimentary rocks differences between archean and phanerozoic tectonic processes mainly related to higher heat production in the past probably have affected the nature and importance of ridge collision in archean crustal growth during the archean greater ridge length and smaller plates probably resulted in more ridge collisions and related processes contributing to a higher continental growth rate in particular underplating of subducted slabs and direct slab melting may have been important processes in archean continental growth but are of relatively minor importance today

evolution in composition of granitic rocks controlled by time dependent changes in petrogenetic processes examples from the archaean of eastern finland abstract the grey gneisses of eastern finland form the basement on which the archaean greenstone belts were developed they are composed of tonalites trondhjemites and granodiorites ttg emplaced during two distinct magmatic episodes at 2 86 ga kivijarvi gneisses and 2 65 ga naavala petrogenetic models based on trace element behaviour indicate that the ttg are ultimately derived from the mantle by a mechanism involving two stages 1 1 melting of the mantle to form a tholeiitic crust and 2 2 re melting of these tholeiites transformed into garnet amphibolites to yield ttg rocks the low yb contents of archaean ttg are explained by the influence of residual garnet and hornblende in the source rocks involved in magma genesis after 2 5 ga the role of these minerals becomes progressively less important and this is interpreted in terms of a cooling earth model at present the subducted oceanic crust is relatively cold and the geothermal gradient at its interface with the overlying mantle wedge is low in addition the descending lithospheric slab undergoes dehydration before partial melting is able to occur and the liberated fluids pass upward into the mantle wedge or base of the continental crust where partial melting may begin these magmas have calc alkaline affinities and have equilibrated with residual olivine and pyroxene in the source consequently the liquids derived from partial melting have high yb contents during the archaean subducted oceanic crust was relatively hot and the geothermal gradients in the interface zone with the mantle were high c 20 25 c km 1 thus partial melting of the subducted oceanic material was possible at temperatures between 650 and 700 c before dehydration would take place residual phases in the source would include garnet and hornblende thus explaining the low yb contents of the ttg association it is concluded that there has been a change in the site of cale alkaline magma genesis since archaean times

deformation of biotite and muscovite tem microstructure and deformation model abstract growth and deformation defects have been observed in 1m and 2m 1 biotites and 2m 1 muscovites using transmission electron microscope tem techniques these have been related to optical observations in deformed biotite the dislocation density increases as the strain increases the density of stacking faults also increases with strain the spatial ordering of stacking faults decreases as the strain increases in muscovites selected from the same areas as the biotites stacking fault and dislocation densities show little or no variation with deformation the different responses of biotite and muscovite to deformation are explained in part by the physical and chemical differences between 1m biotite and 2m 1 muscovite there is strong evidence in both the biotites and muscovites that stacking faults may be result of deformation as well as a crystal growth process the grain size of the deformed mica aggregates is reduced by the nucleation of new micas there is also evidence for a grain size reduction mechanism segmentation in biotites this mechanism appears to achieve the same result as recovery recrystallization mechanisms but not by the same method segmentation may be a mechanism which is favoured in strongly anisotropic minerals with a limited number of slip systems a model is proposed that attempts to correlate response of the biotite to deformation the deformation is synchronous with and post dates the nucleation of the new metamorphic micas

deformation of chlorite in naturally deformed low grade rocks the intracrystalline deformation of chlorite in naturally deformed low grade rocks was investigated with transmission electron microscopy tem as in other phyllosilicates the deformation of chlorite is dominated by the 001 slip plane slip along this plane is very easy through the generation and movement of partial dislocations with burgers vector b 1 n uv0 after initial bending of the 001 planes through a flexural slip mechanism accommodated by slip along 001 the misorientation is concentrated in tiltwalls subnormal to 001 which are formed by partial dislocations with b 1 n uv0 tiltwalls subparallel to 001 are formed by arrays of dislocations with a total burgers vector 001 these boundaries are either initiated at the grain boundary and move inwards by dislocation climb or form by movement of free dislocations towards the tip of propagating boundaries by glide along planes inclined to 001 once a boundary is formed the increasing misfit is accommodated by climb of these dislocations along 001 due to the combination of very easy slip along 001 and the ability to form subgrain boundaries subparallel to 001 by dislocation climb and activation of hard slip systems chlorite deforms plastically even at temperatures below 300 c

rheology and deformation mechanisms of an isotropic mica schist we have investigated the transitional semibrittle deformation of a mica schist 75 biotite by shortening cylinders cored at 0 45 and 90 to foliation to varying strains at confining pressures pc to 500 mpa constant strain rates e from 1 5 10 7 to 1 6 10 4 s 1 and temperatures t from 25 to 400 c deformation is concentrated within one or more throughgoing millimeter wide shear zones at all conditions these localize at low strains e 2 through the nucleation and coalescence of dense sets of intragranular microkink bands mkbs despite distinct differences in the relative number of mica grains oriented favorably for slip and kinking in different loading directions the differential stresses required for shear zone development vary little with fabric orientation biotite schist undergoes a transition from strain softening to steady strength mechanical response at confining pressures in the range 75 to 150 mpa the pressure sensitivity of strength characterized by the slope of the mohr envelope decreases from 0 5 at pc 100 mpa to 0 1 at pressures greater than 200 mpa reflecting the increasing contribution of glide and kinking in biotite at higher pressures however dilatancy associated with microcracking and void formation along mkb boundaries persists to at least 500 mpa within the pressure insensitive regime 200 pc 500 mpa temperature and strain rate dependencies of strength determined in stepping tests reveal a strong history dependence to flow that cannot strictly be described by a steady state constitutive law samples deformed in steps from low to high temperatures or fast to slow strain rates consistently exhibit stronger temperature and strain rate sensitivities than those deformed along t decreasing or e increasing paths path dependent effects may reflect differences in the degree to which inherited dislocation substructures are utilized or overprinted during later deformation increments by assuming an exponential relationship between differential stress d and strain rate e of the form e c exp d exp q rt we fit the data with two end member flow laws with a single activation energy q 89 kj mol and exponential constants ss 0 15 0 01 mpa 1 and ws 0 55 0 04 mpa 1 that account for the different responses observed along stepping paths that are strongly sensitive or weakly sensitive to t and e respectively application of the results to crustal deformation suggests that mica rich aggregates are weaker than other common rock types throughout a broad midcrustal depth range supporting the inference that retrograde reaction to phyllosilicates may be important in localizing crustal deformations within large faults and shear zones

early archean foredeep sedimentation related to crustal shortening a reinterpretation of the barberton sequence southern africa abstract the upper dominantly terrigenous sedimentary successions of the early archean barberton sequence kaapvaal province record an evolution similar to that of phanerozoic foredeeps and may represent the oldest known foredeep sequence rather than a passive continental margin sequence as previously proposed pre foredeep ocean crust the 3 5 ga old onverwacht group consists of bimodal volcanics with intercalated shallow water sediments gravity modeling and boundary shear zones indicate that deformed onverwacht crust was obducted northward onto sialic continental crust immediately before during or after burial by the synorogenic terrigenous clastic fill foredeep basin deepening due to crustal loading from the south is recorded by basinal iron formation chert and mudstone of the basal fig tree group which accumulated paraconformably on the onverwacht overlying sediments of the fig tree group consist of 2 km of submarine fan greywackes and mudstones shoaling of the basin is indicated by the transition to the overlying braided alluvial and shallow marine sediments of the ca 3 3 ga old moodies group 3 km thick changing ree patterns and relative abundances of ni cr sr th and ur in the sediments of the fig tree and moodies groups indicate a significant ultramafic mafic volcanic component diluted upward in the stratigraphic sequence by an increasing granitic contribution a similar trend is reflected in sandstone petrography in which the ratio of extrusive to intrusive rock fragments decreases upwards in the fig tree group whereas the moodies group sediments are enriched in quartz and potassium feldspar a persistent chert component in both units suggests that the tectonic setting of the basin is analogous to that of phanerozoic recycled orogens the distribution of facies in the fig tree and moodies groups and paleocurrent petrographic and geochemical data indicate progressive unroofing of a southerly source terrane consisting of the onverwacht volcanics and intercalated sediments and the 3 5 3 2 ga old ancient gneiss complex agc in addition older components of the fig tree group were recycled into overlying stratigraphie units the agc consists of mantle derived 3 5 ga old bimodal metavolcanics and metasediments a tonalite gneiss batholith metabasite dikes and meta anorthosites its structures record progressive northwestward uplift beginning around 3 4 ga ago intense homogeneous ductile strains were overprinted by northwest verging major folds rotated by large scale northwestward simple shear followed by imbricate faulting and pseudotachylite formation by 3 3 ga ago the fig tree and moodies groups had also shortened with similar kinematics to those of the agc by partly syndepositional folding and volume loss during the formation of transecting cleavage further folding and slide formation followed during subsolidus diapiric rise of the initially tabular kaap valley tonalite by 3 2 ga and emplacement of trondhjemite plutons from 3 3 to 3 0 ga ago

nd and sr isotope systematics of clastic metasediments from isua west greenland identification of pre 3 8 ga differentiated crustal components recent field work on the isua metasediments has established that there are two lithological sequences a and b which are divided into a number of distinct formations each clastic metasedimentary unit has a relatively unique trace element signature the depleted mantle sm nd model ages of samples from all major units range from 3 59 to 3 98 ga whereas rb sr model ages cover a very wide range 1 9 4 6 ga the samples from the sequence a felsic gneiss formation a6 yield a relatively good rb sr isochron age of t 3 71 0 07 ga the sequence b samples do not yield a rb sr isochron however some of the samples plot close to the a6 rb sr isochron and sequences a and b appear to have essentially the same deposition age the sm nd data for both sequences yield initial end values in the range 1 to 3 for a deposition age tstrat 3 64 ga if tstrat 3 81 ga the range is 0 4 this range appears to reflect real variability of initial end values corresponding to the time of deposition data trends in end versus 1 nd diagrams suggest at least three distinct components in each of sequences a and b model age calculations suggest that some of these components must be derived from continental crustal sources that predate the time of deposition of these sediments by as much as 0 4 b y other components appear to be derived from volcanic sources that are roughly coeval with sediment deposition the very high initial end values implied for early depleted mantle sources 2 to 4 suggest a very large pre 3 8 ga continental crust alternatively these high values could reflect an early terrestrial mantle with highly fractionated cumulate layers that formed in a 4 5 ga magma ocean

archean oceanic flake tectonics assuming that the archean mantle was hotter and the oceanic crust thicker than at present we estimate that the archean oceanic lithosphere had a sandwich rheology with the lower crust behaving as a soft ductile layer accordingly the archean oceanic lithosphere had a rheological structure similar to present day continental lithosphere but unlike present day oceanic lithosphere the thickness of the ductile layer is a decreasing function of age while its strength is an increasing function of age the buoyancy problem in archean subduction could be overcome by delamination of oceanic upper crust but mass balance calculation indicates that most archean oceanic crust must have been recycled where the right dynamic conditions existed for delamination oceanic flake tectonics may have been an important process leading to the preservation of oceanic upper crust in greenstone belts that are characteristic of archean terrains

the himalayas a modern analogue for archaean crustal evolution abstract present crustal evolution models fail to account for the generation of the large volumes of continental crust in the required time intervals if the oceanic crust was appreciably thicker in the archaean as geothermal models would indicate then oceanic crustal collision on the scale of the present day himalayan continental collision zone may have been a frequent occurrence in the archaean resulting in extensive partial melting of the hydrous underthrust oceanic crust to produce voluminous tonalite melts leaving a depleted stabilised basic residuum

structure and metamorphism of the archean murchison belt kaapvaal craton south africa critical examination of five popular prejudices applied to greenstone belts in general and the murchison belt in particular show that 1 the term murchison schist belt is preferred to that of murchison greenstone belt which differs significantly from the barberton greenstone belt 2 the murchison belt is tectonized such that no simple stratigraphy may be erected 3 the murchison belt lacks a synclinal keel 4 heterogeneous oblique to dip slip shear characterizes much of the murchison belt but strike slip deformation occurred in the letaba shear zone along the northeastern margin of the belt and 5 contact metamorphism was a limited phenomenon that was not responsible for the highest grade rocks of the murchison belt the belt comprises a number of domains each with its own characteristic lithologies structural style and metamorphic history bounded by tectonic breaks which have juxtaposed rocks from different crustal levels

regional slaty cleavage formation and fold axis rotation by re use and reactivation of pre existing foliations the fiery creek slate belt north queensland abstract a deformation history comprised of six separate deformation events of differing intensity has affected the rocks of the south palmer river region of the the hodgkinson province north queensland within this region a zone of pervasive slaty cleavage herein termed the fiery creek slate belt has developed as a result of the superposition of fabrics formed during several of these events the most important processes in the formation of this composite cleavage were the re use and reactivation of the favourably oriented steep n s trending s2 foliation by the intense fourth deformation event d4 this produced micro meso and macroscopic folds in an originally shallow s3 foliation produced during the intervening d3 deformation with an axial planar s2 s4 foliation the d4 stretching lineation l44 plunges subvertically to steeply north and indicates that shear during d4 was oriented steeply north south in the fiery creek slate belt d2 fold axes are interpreted to have formed in much shallower orientations than their present moderately n s plunging to subvertical orientations we consider this to be a result of d4 shear which caused variable degrees of rotation of d2 fold axes toward the d4 stretching lineation due to subparallelism of the bulk shortening directions of the d2 and d4 events near total destruction of the pre d4 foliations during slaty cleavage formation has produced a misleading impression of a simple deformation history there is no relationship between metamorphic grade and intensity of slaty cleavage development

petrology geochemistry and origin of a major australian 1880 1840 ma felsic volcano plutonic suite a model for intracontinental felsic magma generation in australia a major period of felsic volcanism and granite emplacement occurred between 1880 and 1840 ma towards the end of the early proterozoic these volcanics and granites covering at least 37 000 km2 are compositionally uniform over large areas the felsic magmas are predominantly i type being derived from infracrustal sources which have never been exposed to surficial weathering processes when compared with early archaean and phanerozoic i type granites the 1880 1840 ma suite has distinctively high levels of k2o rb zr la ce th u and low mgo and cao contents even at relatively low sio2 levels of 60 65 the 1880 1840 ma rocks are characterized by low initial 87sr 86sr ratios and negative nd values chemical and isotopic modelling on these volcanics and granites suggest that they formed from mantle derived sources that are around 2300 2000 ma old there is no evidence of a major archaean component in these granites the sources are interpreted to have formed as a result of polygonal small scale mantle convection which led to extensive underplating of archaean crust during this underplating event large volumes of incompatible element enriched mantle derived material accreted to the base of the lower crust and fractionated significantly the more fractionated part of this source was remelted between 1880 1840 ma to produce the voluminous felsic volcanics and comagmatic granites present in nearly every proterozoic domain of australia the actual underplating event between 2300 2000 ma is believed to have triggered the formation of early proterozoic sedimentary basins which occur in a polygonal pattern and appear to wrap around archaean nuclei the composition of these felsic melts and the tectonic setting operating during their emplacement precludes genesis by contemporaneous subduction of oceanic crust all are generated by processes of vertical accretion in an intracontinental environment

structural evolution of the harts range area and its implication for the development of the arunta block central australia abstract a division of the high grade metamorphics of the arunta block in the harts range area into two main groups on the basis of lithology structure metamorphism and igneous activity is suggested as a result of detailed mapping the lower group is regarded as a basement terrain and comprises major layered felsic or granitic gneiss minor amphibolite and mafic granulite while the structurally overlying group is regarded as a cover sequence irindina supracrustal assemblage and comprises predominantly metasediments pelitic and semi pelitic gneiss calc silicate and quartzite and amphibolite anorthosite and ultramafic rock of the harts range meta igneous complex the cover sequence might have been deposited in an aulacogen or adjacent to a continental margin which developed by rifting of a pre existing basement the two groups both show early major ductile tectonothermal stages with four major fold generations and a later discrete shear event in the basement and two major fold generations and a later series of stacked nappes developing in the cover the basement s early orogenic evolution with associated granulite facies metamorphism is typical of intensely reworked early precambrian terrains and developed spatially and or temporally separated from the cover cover deformation closely resembles continental margin cordillera type overthrusting and thus thin skinned tectonics associated with large scale crustal shortening during the later harts range remobilization the basement and cover were juxtaposed by displacement on a fundamental crustal dislocation recumbent contraction fault mylonites repeatedly developed along the dislocation which was also isoclinally folded intruded by a major megacrystic granitoid bruna gneiss sheet and repeatedly rejuvenated the history of development of the dislocation is documented and described as a process of underthrusting of basement beneath cover which led to the final termination of the orogenic activity

stability of the oceanic tectosphere a model for early proterozoic intercratonic orogeny resistance of modern oceanic tectosphere to the initiation of subduction can be evaluated by considering its flexural rigidity shear resistance in the subduction zone and the negative buoyancy force after mckenzie 13 these depend on the effective elastic thickness the depth to which shear stresses are appreciable and the tectospheric thickness respectively evaluation of these thicknesses and their variation with age allows development of a model in which oceanic tectosphere becomes progressively less stable with increasing age and spontaneously subducts at an age of 200 ma the same model applied to early proterozoic oceanic tectosphere distinguished only by having sub tectosphere temperatures 100 c higher than in the modern rquires that proterozoic ocean tectosphere subduct spontaneously at ages of about 75 ma the subduction of universally young oceanic tectosphere inhibits the production of juvenile magmas due to the release of volatiles from the descending slab at low pressures near the trench the model therefore accounts for the scarcity of subduction related magmas in many early proterozoic orogens it also predicts that ocean basins opening in the early proterozoic should have generally limited width that blueschist metamorphism should be rare and that early proterozoic oceanic crust should rarely be preserved at the earth s surface with progressive cooling of the mantle the proterozoic tectonic scheme may have changed gradually into the modern one by early phanerozoic time although parts of the pan african system may exhibit the characteristics of still unstable oceanic tectosphere

structural and temporal evolution of a reactivated brittle ductile fault part i fault architecture strain localization mechanisms and deformation history abstract faults are by nature dynamic as their architecture and composition evolve progressively in space and through time steered by the interplay between strain weakening and hardening mechanisms this study combines structural analysis geochemistry and chlorite geothermometry to investigate deformation and strain localization mechanisms of the kvenklubben fault a paleozoic brittle ductile thrust in northern norway with the goal to constrain their temporal variations and the consequences thereof on fault architecture development and rheological behavior the fault evolved from an initially discrete brittle feature slipping mainly by seismogenic ruptures to a wide brittle ductile phyllonite deforming by aseismic creep the formation of mechanically weak phyllosilicates by decarbonation of footwall dolostones and carbonation of hanging wall metabasalts was the main weakening mechanism whereas partitioning of fluid flow and fracture sealing following transient high pore pressure driven embrittlement caused episodic and localized strain hardening the interplay between strain weakening and hardening mechanisms caused the fault core to widen we suggest that the ability for carbonate hosted faults to slip by seismogenic rupture is also a function of the faults structural evolutionary stage and that it decreases progressively with fault maturity this study demonstrates the importance of calibrating the present day fault anatomy against the dynamic character of faults which evolve geometrically compositionally and mechanically in space and through time

four zircon ages from one rock the history of a 3930 ma old granulite from mount sones enderby land antarctica ion microprobe u th pb analyses of zircons from a granulite grade orthogneiss from mount sones enderby land antarctica record the ages of four principal events in the history of the gneiss three of which already have been recognized through previous isotopic dating of other samples the structure of the zircons indicates at least four different stages of growth the several zircon ages were obtained by grouping the analyses according to the stage they represented in the observed stratigraphic succession of growth and thereby defining separate u pb discordance patterns for each stage the stratigraphically oldest zircon rare discrete cores is indistinguishable in age from the most common euhedrally zoned zircon both crystallized when the tonalitic precursor of the orthogneiss was emplaced into the crust 3927 10 ma ago making the orthogneiss currently the oldest known terrestrial rock the outer parts of most grains and some whole grains recrystallized at 2948 31 17 ma during or immediately after possibly 100 ma of high granulite grade metamorphism the recrystallized zircon was isotopically disturbed by tectonism associated with reactivation of the southern margin of the napier complex at 1000 ma in the intervening time at 2479 23 ma the cores and zoned zircon suffered a major isotopic disturbance involving movement of radiogenic pb which left most of the crystals with radiogenic pb deficiencies but produced local radiogenic pb excesses in others a new generation of zircon characterized by very high th u and low u grew at that time that event deformation and possibly a minor rise in temperature produced widespread perturbations of other isotopic systems throughout the napier complex

caterpillar tectonics in the harts range area a kinship between two sequential proterozoic extension collision orogenic belts within the eastern arunta inlier of central australia abstract we propose that in the harts range area of the arunta inlier of central australia a basement and cover relationship previously described by us represents a thick complexly deformed contact zone between an early middle proterozoic fold belt the strangways orogenic belt sob and a younger middle proterozoic orogenic belt the harts range orogenic belt both of which contain their own supracrustal sequences rocks of the sob were deposited and possibly metamorphosed to a maximum grade of granulite facies at 1800 ma during an early phase of crustal extension they were subsequently intruded by a significant proportion of acidic magmas at 1767 ma and compressed to form a distinctively nne ssw trending orogenic belt a wnw ese trending shallowly north dipping zone of intense and repeated deformation termed the harts range detachment zone hrdz formed initially during a period of nne ssw crustal extension as a major low angle normal shear detachment zone within and crosses at a high angle to the trend of the basement rocks of the sob the hrdz has been intermittently active as a zone of crustal weakness since its initiation the first extensional movement on this zone began the formation of the harts range mobile belt hrmb which displays many of the characteristics of modern tectonic zones activated by plate movement a supracrustal cover sequence the harts range cover hrc formed in the major geosynclinal basin which developed on the subsiding upper plate of the hrdz continued crustal extension juxtaposed the cover sequence which was then metamorphosed to a maximum grade of amphibolite facies and granulite facies basement of the hrdz before a fundamental change in movement sense led to crustal contraction and a collision related thrusting phase the hrmb which includes the hrdz and the hrc was then internally deformed over a relatively very short time span 15 20 ma before voluminous granitic magma intruded mainly along the hrdz during the harts range orogeny hro the intrusion of this major granitoid at 1747 ma precisely fixes the time scale of evolution for the earlier phases of the hro the hro is documented in detail as a peculiar orogenic style of crustal contraction which was largely taken up by reverse sense discrete thrusting and shearing on faults and shear zones associated with the hrdz displacement on the hrdz was a stick slip process with repeated punctuation of thrust slip movement by pinning at points located progressively to the south each period of cessation of thrust slip movement was followed by folding of discrete thrusts of captured basement lobes within the hrdz and of the overlying supracrustal sequence the individual discrete movement phases and folding episodes are each named and described as is a model of this unusual type of proterozoic tectonic activity

continental tectonics in the aftermath of plate tectonics the success of plate tectonics required an acceptance of continental drift and thus a reinterpretation of the large scale geological history of most of the earth but the basic tenet of plate tectonics rigid body movements of large plates of lithosphere fails to apply to continental interiors where buoyant continental crust can detach from the underlying mantle to form mountain ranges and broad zones of diffuse tectonic activity

isobaric cooling of proterozoic high temperature metamorphites in the northern arunta block central australia implications for tectonic evolution abstract protoliths of metamorphites in the proterozoic reynolds range region northern arunta block central australia included mafic and felsic igneous rocks pelites calcareous rocks and quartzites metamorphism began with an early high t low p facies stage followed by early rehydration in higher grade areas isobaric cooling and late high t low p facies hydration in and near shear zones metamorphic grade increases southeastwards shown by the change from andalusite in metapelites in the northwest to sillimanite in the southeast and by the incoming of cordierite cordierite biotite cordierite k feldspar sillimanite and cordierite garnet migmatites kornerupine quartz spinel and orthopyroxene k feldspar assemblages occur in the highest grade rocks downwarping at near peak temperatures is shown by garnet sillimanite replacing cordierite and by sillimanite pseudomorphing andalusite early in the near isobaric cooling that followed peak temperatures freezing of partial melts released water and biotite formed the hydrated retrograde assemblages formed after temperatures had fallen considerably are also zoned from northwest to southeast with andalusite sillimanite or kyanite bearing assemblages developed in appropriate bulk compositions the early high t metamorphism is best explained by tectonic models based on extension crustal delamination and underplating the hydration and deformation were caused by processes unrelated to the early history occurring at least 200 ma later possibly as late as the mid palaeozoic

parallel evolution of archaean low and high grade terrane a view based on relationships between the abitibi wawa and kapuskasing belts abstract the kapuskasing structural zone ksz is a region of high grade archaean gneiss transverse to the regional east west structural trends of the adjacent lower grade abitibi wawa and quetico belts the structural zone is made up of granulite and amphibolite facies paragneiss tonalitic gneiss and anorthosite suite gneiss whereas adjacent regions are metavolcanic metasedimentary belts with gneissic and plutonic components foliations in the ksz strike northeast and dip moderately northwest to account for the development of contrasting lithological structural and metamorphic characteristics in adjacent areas a unique archaean history is postulated for the ksz psammitic sedimentary rocks possibly deposited in a fault bounded basin were intruded by anorthosite which may have had its erruptive equivalents in the tholeiitic volcanic rocks in the adjacent abitibi belt during the kenoran orogeny the ksz was metamorphosed and intruded at infrastructural levels while low grade regions outside underwent sub greenschist to amphibolite metamorphism at higher levels late ductile shear in the ksz reoriented gneissic structural elements into northeast directions proterozoic fault movements juxtaposed low and high grade regions

role of a thin komatiite rich oceanic crust in the archean plate tectonic process in the archean tectonic regime the time available for oceanic lithosphere to cool on its journey from ridge to convergent plate boundary was far less than at present under these conditions only an inherently dense lithosphere could have acquired the negative buoyancy necessary for it to subduct at this time plate tectonics could have operated only if 1 the oceanic crust was rich in komatiite lava and 2 the thickness of this crust was not much greater than the thickness of modern oceanic crust

lewisian gneiss geochemistry and archaean crustal development models abstract the geochemistry of lewisian amphibolite facies gneisses from northwest scotland is described with particular reference to the rare earth elements ree and compared with the geochemistry of lewisian granulite facies gneisses the results show that there are no significant differences between laxfordian amphibolite facies and scourian granulite facies gneisses in terms of ree and other immobile trace elements at equivalent silica levels although the mobile radioactive heat producing elements k rb th u are significantly lower in the granulites in both types the basic gneisses have moderately fractionated ree patterns while the intermediate and acid gneisses have strongly fractionated ree patterns with low heavy ree abundances and decreasing levels of total ree with increasing sio 2 the most silicic gneisses develop large positive europium anomalies these gross chemical similarities between gneisses from intermediate amphibolite facies and lower granulite facies crustal levels constrain models for the evolution of the archaean crust the depletion of k rb cs th and u in granulites but not other incompatible trace elements cannot be explained by magmatic processes the positive eu anomaly in the more siliceous gneisses of both facies is a function of the primary processes of crustal generation and not secondary processes such as intracrustal melting or fractional crystallisation fractionation of radioactive heat producing elements from other trace elements is a result of granulite facies metamorphism with these elements being removed by an active fluid phase the apparent lack of partial melting in lower crustal granulites suggests a model for archaean crustal growth largely through underplating by primary tonalitic magmas