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

emergence of modern continental crust about 3 billion years ago the continental crust provides a record of earth s evolution analysis of the geochemical signature of continental crust formed since the hadean points to the initiation of plate tectonics about 3 billion years ago

fracture roughness and gouge distribution of a granite shear band 1 localization of deformation during fracture mechanical tests leads to the development of shear bands we performed triaxial tests using sidobre granite at four different confining pressures from 20 to 80 mpa we compared two sets of tests one set was stopped immediately after the formation of the shear band a second one included additional shear deformation from the analysis of thin sections of these laboratory samples we characterize the typical microstructures in the shear band mode i and ii cracks riedel cracks cataclastic flow statistical properties of rupture surface roughness and gouge grain size reveals scaling invariance using a mechanical profiler the fracture roughness is measured along parallel profiles and shown to be correctly described over up to 3 orders of magnitude by self affine geometry with a roughness exponent close to 0 80 this property is very similar to tensile crack even if local processes are different the influence of the slip is observed fracture surfaces are rougher along the slip direction 0 74 than perpendicular to it 0 80 the confining pressure is shown to have a weak effect on the fracture roughness it smoothes the surface slight increase of the roughness exponent gouge particles extracted from the shear band present a power law distribution with an exponent ranging from 1 44 to 1 91 this exponent appears to increase with the shearing displacement and the confining pressure when a significant shear of the band is combined with a high confining pressure i e impeded dilation of the band the hallmark of fragmentation is observed for the particle distribution and related to a smoothing of the band boundary

the origin of earth s first continents and the onset of plate tectonics the growth and recycling of continental crust has resulted in the chemical and thermal modification of earth s mantle hydrosphere atmosphere and biosphere for 4 0 b y however knowledge of the protolith that gave rise to the first continents and whether the environment of formation was a subduction zone still remains unknown here tonalite melts are formed in high p t experiments in which primitive oceanic plateau starting material is used as an analogue for eoarchean 3 6 4 0 ga oceanic crust generated at early spreading centers the tonalites are produced at 1 6 2 2 gpa and 900 950 c and are mixed with slab derived aqueous fluids to generate melts that have compositions identical to that of eoarchean continental crust our data support the idea that the first continents formed at ca 4 ga and subsequently through the subduction and partial melting of 30 45 km thick eoarchean oceanic crust modified earth s mantle and eoarchean environments and ecosystems

apatite composition tracing petrogenetic processes in transhimalayan granitoids apatites crystallized from different types of igneous rocks show significant variations in the abundances of some minor and trace elements in this study electron probe microanalysis and laser ablation inductively coupled plasma mass spectrometry were used to determine the concentrations of 25 minor and trace elements in apatite separated from three principal rock types of thetranshimalayan igneous plutonic suite s type granites the i type gangdese batholith and postcollisional adakites f mn sr and rare earth elements ree in apatite vary systematically with the composition of the host magma and thus have high potential as petrogenetic tracers more specifically the f and mn contents of apatite can be used as an indicator of magma aluminosity or differentiation index combined with sr and ree data which show significant variations in apatite from different rock types these elements are useful for constructing discrimination diagrams this study also reveals that apatite has the capacity to retain geochemical information about the host magma through the course of magmatic evolution systematic variations of sr and ree in apatite with bulk rock aluminosity are the results of partition competition with pre existing and coexisting major and accessory minerals in silicate melts and thus are useful for more detailed investigations of petrogenetic processes such as fractional crystallization and magma mixing which is signaled by inconsistent eu anomalies sr abundances and ree patterns relative to bulkrock compositions

igneous garnet and amphibole fractionation in the roots of island arcs experimental constraints on andesitic liquids to evaluate the role of garnet and amphibole fractionation at conditions relevant for the crystallization of magmas in the roots of island arcs a series of experiments were performed on a synthetic andesite at conditions ranging from 0 8 to 1 2 gpa 800 1 000 c and variable h2o contents at water undersaturated conditions and fo2 established around qfm garnet has a wide stability field at 1 2 gpa garnet amphibole are the high temperature liquidus phases followed by plagioclase at lower temperature clinopyroxene reaches its maximal stability at h2o contents 9 wt at 950 c and is replaced by amphibole at lower temperature the slopes of the plagioclase in boundaries are moderately negative in text t text x text h 2 text o space at 0 8 gpa garnet is stable at magmatic h2o contents exceeding 8 wt and is replaced by spinel at decreasing dissolved h2o the liquids formed by crystallization evolve through continuous silica increase from andesite to dacite and rhyolite for the 1 2 gpa series but show substantial enrichment in feo mgo for the 0 8 gpa series related to the contrasting roles of garnet and amphibole in fractionating fe mg in derivative liquids our experiments indicate that the stability of igneous garnet increases with increasing dissolved h2o in silicate liquids and is thus likely to affect trace element compositions of h2o rich derivative arc volcanic rocks by fractionation garnet controlled trace element ratios cannot be used as a proxy for slab melting or dehydration melting in the deep arc garnet fractionation either in the deep crust via formation of garnet gabbros or in the upper mantle via formation of garnet pyroxenites remains an important alternative despite the rare occurrence of magmatic garnet in volcanic rocks

3d numerical simulations of fault gouge evolution during shear grain size reduction and strain localization abstract strain localization has important implications for the mechanical strength and stability of evolving fault zones structural fabrics interpreted as strain localization textures are common in natural and laboratory faults however the dynamic microscale processes controlling localization and delocalization are difficult to observe directly discrete numerical models of faulting allow a degree of dynamic visualization at the grain scale not easily afforded in nature when combined with laboratory validation experiments and field observations they become a powerful tool for investigating the dynamics of fault zone evolution we present a method that implements realistic gouge evolution in 3d simulations of granular shear the particle based model includes breakable bonds between individual particles allowing fracture of aggregate grains that are composed of many bonded particles during faulting simulations particle motions and interactions as well as the mechanical behavior of the entire system are continuously monitored we show that a model fault gouge initially characterized by mono disperse spherical aggregate grains gradually evolves with accumulated strain to a wide size distribution the comminution process yields a highly heterogeneous textural signature that is qualitatively comparable to natural and laboratory produced fault gouges mechanical behavior is comparable to a first order with relevant laboratory data simulations also reveal a strong correlation between regions of enhanced grain size reduction and localized strain thus in addition to producing realistic fault gouge textures the model offers the possibility to explore direct links between strain partitioning and structural development in fault zones this could permit investigation of subtle interactions between high and low strain regions that may trigger localization delocalization events and therefore control macroscopic frictional stability and hence the seismic potential of evolving fault zones

geochemistry of ttg and ttg like gneisses from lushan taihua complex in the southern north china craton implications for late archean crustal accretion the late archean taihua complex mainly composed of amphibolite and ttg and ttg like gneisses is volumetrically most important metamorphic rock suites scattered along the southern margin of the north china craton ncc zircon shrimp u pb dating shows two episodes 2765 13 and 2723 9 ma of archean magmatism in the lushan area with distinct geochemical features the early 2765 13 ma suite ttg like gneisses has low sio2 52 5 66 1 wt high mg 0 47 0 68 low hree ybn 3 0 5 4 and y 8 07 13 9 ppm with low to moderate la yb n 6 7 37 1 and sr y 25 9 119 3 the younger 2723 9 ma suite ttg gneisses has high sio2 63 5 74 3 wt low mg 0 13 0 52 very low ree ybn 1 1 20 2 43 both suites show pronounced negative nb ta and ti anomalies on the primitive mantle normalized spidergram the ttg like gneiss suite has similar bulk rock nd and zircon hf model ages 3 0 ga with end t 0 0 26 1 46 and is interpreted as resulting from melt of mantle interactions with the melts derived from partial melting of subducted ocean crust with a residual assemblage of garnet clinopyroxene rutile amphibole which favors subducted slab model for the late archean ttg the ttg gneiss suite has abundant relic zircons 2 95 2 80 ga with end t 0 1 31 to 0 23 which is best interpreted as derived from partial melting of thickened lower continental crust with a garnet amphibolite residue garnet amphibole rutile significant high pressure fractional crystallization garnet amphibole and accumulation plagioclase are also required in the petrogenesis the lushan amphibolite with nearly flat primitive mantle normalized trace element pattern is interpreted to represent early ocean crust metamorphism these observations suggest a possible model of late archean crustal accretion from ocean crust to continental terrain in the southern north china craton

archean upper crust transition from mafic to felsic marks the onset of plate tectonics the archean eon witnessed the production of early continental crust the emergence of life and fundamental changes to the atmosphere the nature of the first continental crust which was the interface between the surface and deep earth has been obscured by the weathering erosion and tectonism that followed its formation we used ni co and cr zn ratios in archean terrigenous sedimentary rocks and archean igneous metaigneous rocks to track the bulk mgo composition of the archean upper continental crust this crust evolved from a highly mafic bulk composition before 3 0 billion years ago to a felsic bulk composition by 2 5 billion years ago this compositional change was attended by a fivefold increase in the mass of the upper continental crust due to addition of granitic rocks suggesting the onset of global plate tectonics at 3 0 billion years ago

sm nd studies of archaean metasediments and metavolcanics from west greenland and their implications for the earth s early history metasedimentary and metavolcanic rocks from the archaean of west greenland have been examined for evidence of crustal components greater than 3 8 ga in age and for their compatibility with the presently adopted bulk earth sm nd parameters sm nd isotopic data have been obtained for the garbenschiefer metagabbro unit metasediments from the isua supracrustal belt gneisses interior to the isua belt and metasediments from the malene supracrustal belt using estimates of emplacement age t of between 3 77 and 3 67 ga for the parental volcanics to the garbenschiefer unit initial143nd 144nd ratios yield positiveendt values between 1 0 and 3 1 relative to the chur parameters for seven out of eight samples model sm nd ages for the isua gneisses and metasediments are only compatible with their estimated stratigraphic ages if their sources were ca 2end relative to chur at those times similarly model sm nd ages for the malene samples are only compatible with stratigraphic age constraints when based on a source evolution with positiveendt implications of these results for the early development of the earth s mantle are discussed

high sr y and la yb ratios the meaning of the adakitic signature abstract the name of adakite is used to describe a far too large group of rocks whose sole common feature is high sr y and la yb ratios however such a signature can be achieved via different processes melting of a high sr y and la yb source deep melting with abundant residual garnet fractional crystallization or afc or interactions of felsic melts with the mantle causing selective enrichment in lree and sr over hree a database of the compositions of adakitic rocks including high silica and low silica adakites martin h smithies r h rapp r p moyen j f champion d c 2005 an overview of adakite tonalite trondhjemite granodiorite ttg and sanukitoid relationships and some implications for crustal evolution lithos 79 1 2 1 24 continental adakites and archaean adakites was assembled geochemical modeling of the potential processes is used to interpret it and reveals that 1 the classical model of slab melting provides the best explanation for the genesis of high silica adakites 2 low silica adakites are explained by garnet present melting of an adakite metasomatized mantle i e at depths greater than 2 5 gpa 3 continental adakites is a term encompassing a huge range of rocks with a corresponding diversity of petrogenetic processes and most of them are different from both low and high silica adakites 4 archaean adakites show a bimodal composition range with some very high sr y examples similar to part of the ttg suite reflecting deep melting 2 0 gpa of a basaltic source with a relatively high sr y while lower sr y rocks formed by shallower 1 0 gpa melting of similar sources comparison with the archaean ttg suite highlights the heterogeneity of the ttgs whose composition spreads the whole combined range of hsa and archaean adakites pointing to a diversity of sources and processes contributing to the ttg suite

a neoarchean arc back arc system in eastern hebei north china craton constraints from zircon u pb hf isotopes and geochemistry of dioritic tonalitic trondhjemitic granodioritic dttg gneisses and felsic paragneisses abstract dioritic tonalitic trondhjemitic granodioritic dttg gneisses are widely exposed in the zunhua qinglong microblock of eastern hebei north china craton ncc la icp ms zircon u pb isotopic dating for the dttg gneisses in the qinglong area reveals that their magmatic precursors were emplaced between 2517 10 and 2505 23 ma zircons of these dttg gneisses exhibit positive hf t values of 0 0 to 6 5 with adakite like geochemical features represented by relatively high mgo mg and la yb n ratios petrogenetic studies reveal that dioritic magmas were derived from depleted mantle peridotites that were strongly modified by slab derived melts fluids and that the tonalitic trondhjemitic granodioritic ttg magmas were formed by a high degree of fractional crystallization in the dioritic magmas with hornblende as the primary fractionated phase la icp ms zircon u pb isotopic analyses reveal that the protoliths of the felsic paragneisses in the northern zunhua area were deposited in the late neoarchean with a maximum depositional age of 2517 20 ma most of the detrital zircon grains display apparent 207pb 206pb ages from 3432 20 to 2502 21 ma with negative hf t values of 15 4 to 2 4 and calculated tdmc ages between 3995 and 3368 ma suggesting that they were formed from the recycling of eo to paleoarchean crustal materials closely related to the southeastern ancient continental crust of the interior eastern block these data together with previous investigations reveal that the basement rocks of zunhua qinglong microblock formed in a neoarchean arc back arc system with the subduction direction from present northwest to southeast

observation of fault gouge development in laboratory see through experiments abstract we describe the results of a set of brittle sliding and wear experiments on thin sheets of polycrystalline sodium nitrate we have built a new brittle deformation apparatus that allowed us to simultaneously observe the microstructural evolution under the microscope and measure the mechanical response of the material the microstructural history was recorded using a video microscopy technique and selected frames were then digitized and analysed fault gouge in these experiments was the direct product of frictional wear and was produced dominantly by intense intergranular fracturing and accompanying grain surface erosion although an increase in displacement was accompanied by an increase in gouge zone width the development of a gouge zone is not a constant process either with respect to displacement or with respect to the spatial distribution along the fault zone

multiphase deformation and displacement within a basement complex on a continental margin the wudang complex in the qinling orogen china abstract the structural evolution of the wudang complex in the foreland of the southern qinling has been resolved through detailed field mapping and microstructural analysis four major deformational episodes were developed within the complex these involved 1 1 accretion of preliminary continental nuclei to form the basement of the south china block in the late mesoproterozoic early neoproterozoic 2 2 extension and subsidence of the continental margin associated with accumulation of thick sediments from the end of the neoproterozoic to the mid triassic 3 3 thrust stacking of basement and cover during collision detachment orogenesis in the late triassic to early cretaceous and 4 4 development of intermontane basins since the late cretaceous the wudang complex including the wudang and yaolinghe groups is considered mesoproterozoic in age based on 1 recent isotopic data such as a 1304 ma zircon age and 2 recognition of new stratigraphie and structural relationships such as the supposed discontinuous sedimentary contact between the yaolinghe group and sinian rocks which actually is a detachment zone wide ranging isotopic ages in the wudang complex could be due to 1 emplacement of new material associated with magma during extension of the continental margin and 2 reworking and overprinting of early fabrics during basement subsidence and burial the mesozoic orogeny resulted in renewed intense deformation of the basement complex but preservation of early fabrics in these rocks allowed analysis of the kinematics of proterozoic accretion on the northern margin of the preliminary continental nuclei the foliation intersection axes fia preserved in porphyroblasts within the wudang complex display a consistent 115 trend independent of the strike of bedding and late foliations this suggests that the bulk orientation of material movement may have been about n25 e to s25 w during that accretion event

formation age and tectonic setting of the shirengou neoarchean banded iron deposit in eastern hebei province constraints from geochemistry and sims zircon u pb dating abstract the shirengou iron deposit a typical bif deposit located in eastern hebei province in the north china craton is hosted in neoarchean metamorphic rocks the metamorphic rocks include hornblende plagiogneiss plagioclase amphibolite and magnetite quartzite we use contents and ratios of major elements for protolith reconstruction showing that protoliths of the hornblende plagiogneiss and plagioclase amphibolite should be dacite and basaltic andesite respectively based on geology and geochemistry of the hosting rocks we infer that the shirengou bif is an algoma type deposit formed in an arc related basin in the neoarchean constrains from sims u pb dating and oxygen isotopic analysis of zircon from wall rocks and interlayer of the shirengou iron bodies can be summarized as follow 1 igneous zircons from two samples of amphibolite and gneiss yield u pb ages of 2541 21 ma and 2553 31 ma respectively providing the age of the neoarchean protolith because the shirengou bif is an algoma type deposit determined u pb age 2541 2553 ma for zircons from hosting volcanic rocks can constrain the bif forming time 2 metamorphic overgrowth zircons from amphibolite and gneiss give two u pb ages of 2512 13 ma and 2510 21 ma respectively corresponding to the neoarchean matemorphism 3 the zircons with 18o values of 5 9 7 6 also indicate magmatic origin the zircons have 18o values of 6 8 9 9 suggesting that exotic high 18o fluids were involved during metamorphism based on the geology and geochemistry of bif and combing with zircon age and oxygen isotopic systematics we suggest that the shirengou iron deposit was formed in a submarine volcanic setting related to subduction of an oceanic slab

geochemistry zircon u pb geochronology and lu hf isotopes of metavolcanics from eastern hebei reveal neoarchean subduction tectonics in the north china craton abstract the neoarchean metavolcanic rocks of saheqiao occur within the central domain of the precambrian terrane in eastern hebei within the north china craton these rocks consist mainly of garnet two pyroxene granulites garnet clinopyroxene amphibolites amphibole plagioclase gneisses and biotite plagioclase gneisses these metavolcanic rocks are exposed as several kilometers scale of massive blocks or several tens of centimeters scale of enclaves within precambrian granitoid gneisses based on whole rock geochemistry we classify these intermediate mafic volcanic rocks into four groups group 1 exhibits low sio2 yb and nb ta ratios and high ni and cr and shows n morb like normal mid ocean ridge basalts geochemical features suggesting that the magmatic precursors of these rocks were derived through partial melting of depleted mantle compared to group 1 group 2 shows slightly right declined ree and nearly flat multi element patterns with variable negative nb anomalies matching with primitive arc basaltic rocks formed by partial melting of the mantle wedge group 3 exhibits chemical compositions of basaltic and andesitic rocks and is characterized by pronounced lree enrichment but hree depletion and shows distinctly negative nb and ta anomalies and weakly negative zr and hf anomalies these rocks may compare with island arc volcanic rocks and were probably generated by partial melting of the mantle wedge modified by dehydration of subducted oceanic slab the magmatic precursors of the andesites were derived from the crystallization differentiation of the basaltic magma group 4 is characterized by clearly right declined ree patterns and slightly positive or no nb ti anomalies which are analogous to nb enriched basalts neb and originated from melting of hfse enriched mantle sources la icp ms u pb dating of magmatic zircons from five samples from the three groups reveals that group 1 n morb like rocks formed at 2525 7 ma group 2 primitive arc basaltic rocks at 2518 15 ma and group 3 typical arc volcanic rocks at 2523 6 ma to 2614 18 ma zircons from all the dated samples also preserve signatures of 2450 2370 ma metamorphisms or thermal events the zircon lu hf data from the three dated samples in group 3 show ehf t values ranging from 1 4 to 9 0 and tdm values of 2627 ma to 2965 ma suggesting a significant episode of 2 6 2 9 ga crustal growth in this region the 2525 ma n morb like basaltic rocks are younger than the arc related rocks 2614 2518 ma suggesting that the morb like basaltic rocks were generated in a back arc environment on the basis of the rock association of back arc morb basalts group 1 arc related andesites basalts groups 2 and 3 and neb like rocks group 4 we suggest that the neoarchean volcanic rocks of the eastern hebei region formed in a subduction related setting within a convergent plate margin

precise determination of phanerozoic zircon pb pb age by multicollector sims without external standardization 1 zircon has long been recognized as the best geochronometer and the most important timekeeper in geosciences modern microbeam techniques such as sims and la icpms have been successfully applied to in situ u pb zircon age determinations at spatial resolutions of 20 30 m or less matrix matched calibration by external standardization of well characterized natural zircon references is a principal requirement for precise microbeam u pb zircon age determination due to fractionation effects between pb and u which usually result in an external age error exceeding 1 alternatively zircons with a closed u pb system can be directly dated by measurement of 207pb 206pb isotopic ratio without external standardization which has been a common practice for zircons older than 1 0 ga but not for relatively young 1 0 ga and particularly phanerozoic ones because of limitations of analytical precision we describe in this paper a method of 207pb 206pb measurement on phanerozoic zircons using a new generation of large radius magnetic sector multicollector cameca ims 1280 sims in combination with multicollector mode a nuclear magnetic resonance nmr magnet controller and oxygen flooding techniques we achieve precisions of 207pb 206pb ratio of 0 1 and 0 1 0 2 propagating to pb pb age errors 0 4 and 1 3 excluding u decay constant uncertainties for zircons of latest neoproterozoic and late paleozoic to mesozoic age respectively therefore the multicollector sims is capable of direct determination of zircon pb pb ages as young as mesozoic age with uncertainties of geological significance this technique is useful for direct dating of zircons in thin sections moreover it has significance for dating of some other u rich minerals i e baddeleyite and zirconolite that are not suitable for sims u pb dating by external standardization

neoarchean subduction a case study of arc volcanic rocks in qinglong zhuzhangzi area of the eastern hebei province north china craton abstract we report on volcanic rocks of the qinglong zhuzhangzi area in northeastern eastern hebei province north china craton the supracrustal rock associations in the study area consist of amphibole biotite plagioclase gneiss mica plagioclase gneiss schist biotite k feldspar gneiss pyroxene plagioclase amphibolite and amphibolite twenty five new mafic intermediate to felsic samples can be geochemically classified into four groups group 1 is similar to fore arc low ti tholeiite and is characterized by low sio 2 49 7 wt and tio 2 0 43 wt slightly depleted lree and mree pattern with la yb n ratio of 0 74 gd yb n ratio of 0 82 it most likely derived from depleted mantle in a subduction initiation environment group 2 is akin to primitive arc tholeiites with low sio 2 49 7 51 8 wt slight enrichments of lree la sm n 1 06 1 53 and negative nb anomalies nb la pm 0 50 0 76 group 3 resembles evolved arc tholeiites and displays pronounced enrichments of lree la yb n of 2 38 11 3 la sm n of 1 70 2 86 and negative nb ta anomalies nb la pm 0 14 0 34 except sample 11jd28 4 with that ratio of 1 00 group 2 and 3 formed by partial melting of sub arc depleted mantle wedge modified by differing quantities of slab flux addition group 4 comprises k rich andesite dacites which are distinguished by the highest sio 2 54 0 71 8 wt k contents up to 5 03 wt strong enrichments of lree la yb n ratios of 4 83 19 and negative nb ta anomalies analogous to shoshonitic rocks marking late stage of subduction la icpms zircon u pb and lu hf analyses of evolved arc basalts and k rich andesite dacites display crystallization ages of 2511 2604 ma and single stage hf model ages of 2534 2927 ma most focus on 2650 2800 ma in contrast to 2510 2560 ma arc associations 2604 ma shoshonitic dacites most plausibly represent an older orogenic event collectively supracrustal rocks in the qinglong zhuzhangzi area constitute a series of volcanic arc associations that correspond to an entire subduction cycle ranging from orogen to primitive arc evolved arc and back to an orogenic setting which further demonstrate that by the late archean plate tectonic and subduction processes already closely resembled modern counterparts

zircon u pb hf isotopes and geochemistry of neoarchean dioritic trondhjemitic gneisses eastern hebei north china craton constraints on petrogenesis and tectonic implications abstract dioritic trondhjemitic gneisses of neoarchean age are widely distributed throughout the gualanyu shangying area in eastern hebei north china craton these orthogneisses consist of dioritic trondhjemitic and quartz rich trondhjemitic gneisses which were subjected to amphibolite to granulite facies metamorphism la icp ms zircon u pb isotopic dating reveals that the magmatic precursors of these rocks were contemporaneously emplaced from 2535 23 to 2513 8 ma reflecting an important neoarchean dioritic trondhjemitic magmatic event followed by sequential proterozoic tectonothermal events between 2472 60 and 2307 50 ma dioritic and trondhjemitic gneisses exhibit similar zircon hf t values of 1 5 to 5 2 and 2 9 to 5 3 respectively indicating their depleted mantle affinity geochemical analysis reveals that the dioritic gneisses have low sio2 contents 55 4 63 0 various mgo contents 2 07 4 78 and mg 36 07 63 56 and high la yb n ratios 5 45 15 19 in contrast the trondhjemitic gneisses exhibit higher sio2 contents 67 4 73 8 and la yb n ratios 12 86 76 73 and lower mgo contents 0 66 2 67 although their mg 36 29 61 41 are similar to those of dioritic gneisses these geochemical and zircon lu hf isotopic features together with geochemical modeling suggest that the dioritic magmas were derived mainly from the partial melting of a depleted mantle wedge that was metasomatized by slab derived melts or fluids and that the trondhjemitic magmas were formed by fractional crystallization of the dioritic magmas quartz rich trondhjemitic gneisses exhibit extremely high sio2 contents 77 4 84 6 with zircon hf t values ranging from 3 4 to 4 1 suggesting that their precursors formed after the partial melting of a mixed magmatic source it is suggested that the precursors comprised a mafic member derived from a mantle wedge altered by oceanic slab melts or fluids and a felsic member derived from the partial melting of ancient crustal material these new data combined with previous studies of contemporaneous metavolcanic rocks in gualanyu shangying area and surrounding areas suggest that the magmatic precursors of these neoarchean dioritic trondhjemitic gneisses formed in a subduction related tectonic environment at an e w strike convergent plate margin furthermore the eastern block of the north china craton had a major lateral accretion of crustal growth in the north margin in the neoarchean whereas vertical crustal growth and recycling under plume tectonic backgrounds occurred in the interior of the eastern block

grain fracture in 3d numerical simulations of granular shear we present a new method to implement realistic grain fracture in 3d numerical simulations of granular shear we use a particle based model that includes breakable bonds between individual particles allowing the simulation of fracture of large aggregate grains during shear grain fracture simulations produce a comminuted granular material that is texturally comparable to natural and laboratory produced fault gouge our model is initially characterized by monodisperse large aggregate grains and gradually evolves toward a fractal distribution of grain sizes with accumulated strain comminution rate and survival of large grains is sensitive to applied normal stress the fractal dimension of the resultant grain size distributions 2 3 0 3 and 2 9 0 5 agree well with observations of natural gouges and theoretical results that predict a fractal dimension of 2 58 copyright 2005 by the american geophysical union

earth s first stable continents did not form by subduction phase equilibria modelling of rocks from western australia confirms that the ancient continental crust could have formed by multistage melting of basaltic parents along high geothermal gradients a process incompatible with modern style subduction tim johnson et al perform phase equilibria modelling of the coucal basalts from western australia and confirm their suitability as parent rocks of the archaean continental crust the authors suggest that these early crustal rocks were produced by 20 30 per cent melting along high geothermal gradients they conclude that the production and stabilization of the first continents required a protracted multistage process when coupled with the high geothermal gradients this suggests that the continents did not form by subduction instead it favours a stagnant lid regime in the early archaean eon in which a single rigid plate lay over the mantle the geodynamic environment in which earth s first continents formed and were stabilized remains controversial1 most exposed continental crust that can be dated back to the archaean eon 4 billion to 2 5 billion years ago comprises tonalite trondhjemite granodiorite rocks ttgs that were formed through partial melting of hydrated low magnesium basaltic rocks2 notably these ttgs have arc like signatures of trace elements and thus resemble the continental crust produced in modern subduction settings3 in the east pilbara terrane western australia low magnesium basalts of the coucal formation at the base of the pilbara supergroup have trace element compositions that are consistent with these being source rocks for ttgs these basalts may be the remnants of a thick more than 35 kilometres thick ancient more than 3 5 billion years old basaltic crust4 5 that is predicted to have existed if archaean mantle temperatures were much hotter than today s6 7 8 here using phase equilibria modelling of the coucal basalts we confirm their suitability as ttg parents and suggest that ttgs were produced by around 20 per cent to 30 per cent melting of the coucal basalts along high geothermal gradients of more than 700 degrees celsius per gigapascal we also analyse the trace element composition of the coucal basalts and propose that these rocks were themselves derived from an earlier generation of high magnesium basaltic rocks suggesting that the arc like signature in archaean ttgs was inherited from an ancestral source lineage this protracted multistage process for the production and stabilization of the first continents coupled with the high geothermal gradients is incompatible with modern style plate tectonics and favours instead the formation of ttgs near the base of thick plateau like basaltic crust9 thus subduction was not required to produce ttgs in the early archaean eon

petrogenesis of adakitic porphyries in an extensional tectonic setting dexing south china implications for the genesis of porphyry copper mineralization the dexing adakitic porphyries quartz diorite granodiorite porphyries associated with giant porphyry cu deposits are located in the interior of a continent south china they exhibit relatively high mgo cr ni and sr contents high la yb and sr y ratios but low yb and y contents similar to adakites produced by slab melting associated with subduction however they are characterized by bulk earth like nd sr isotope compositions end t 1 14 to 1 80 and 87 sr 86 sr i 0 7044 0 7047 and high th 12 6 27 2ppm contents and th ce 0 19 0 94 ratios which are different from those of cenozoic slab derived adakites sensitive high resolution ion microprobe shrimp geochronology studies of zircons reveal that the dexing adakitic porphyries have a crystallization age of 171 3ma this age is contemporaneous with middle jurassic extension within the shi han rift zone and within plate magmatism elsewhere in south china indicating that the dexing adakitic porphyries were probably formed in an extensional tectonic regime in the interior of the continent rather than in an arc setting their high th contents and th ce ratios and middle jurassic age argue against an origin from a neoproterozoic 1000ma stalled slab in the mantle taking into account available data for the regional metamorphic magmatic rocks and the present day crustal thickness 31km in the area we suggest that the dexing adakitic porphyries were most probably generated by partial melting of delaminated lower crust which was possibly triggered by upwelling of the asthenospheric mantle due to the activity of the shi hang rift zone moreover the dexing adakitic magmas must have interacted with the surrounding mantle peridotite during their ascent which elevated not only their mgo cr and ni contents but also the oxygen fugacity fo2 of the mantle the high fo2 could have induced oxidation of metallic sulfides in the mantle and mobilization of chalcophile elements which are required to produce associated cu mineralization therefore the cu metallogenesis associated with the dexing adakitic porphyries is probably related to partial melting of delaminated lower crust similar to the metallogenesis accompanying slab melting

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 composite archaean grey gneisses petrological significance and evidence for a non unique tectonic setting for archaean crustal growth abstract the geodynamic context of formation of the archaean continental crust is a matter of debate the crust is largely made of grey gneiss complexes a composite rock assemblage dominated by granitoids that are generally regarded as belonging to the ttg tonalite trondhjemite granodiorite series using a large database of published ttg and grey gneiss compositions it is possible to show that the granitoids forming grey gneiss complexes actually belong to at least four main geochemical groups i a potassic component made of granodiorites and formed by melting of existing crustal lithologies and ii three sodic groups ttg proper that comprise low medium and high pressure groups the geochemistry of the low pressure group is consistent with derivation from a plagioclase and garnet amphibolite the medium pressure group was formed in equilibrium with a garnet rich plagioclase poor amphibolite whereas the high pressure group derived from a rutile bearing eclogite as the temperature of melting of metamafic rocks is largely independent from pressure this corresponds to melting along a range of contrasting geothermal gradients in turn reflecting a range of tectonic sites for the formation of the archaean continental crust

la sio2 and yb sio2 systematics in mid ocean ridge magmas implications for the origin of oceanic plagiogranite analytical expressions for the variation in d la and d yb with increasing liquid sio2 for olivine plagioclase augite hornblende orthopyroxene magnetite and ilmenite brophy in contrib mineral petrol 2008 online first have been combined with numerical models of hydrous partial melting of mid ocean ridge mor cumulate gabbro melting and fractional crystallization of slightly hydrous mid ocean ridge basalt morb magma to assess a melting versus fractionation origin for oceanic plagiogranite for felsic magmas 63 wt sio2 the modeling predicts the following mor cumulate gabbro melting should yield constant or decreasing la and constant yb abundances with increasing liquid sio2 the overall abundances should be similar to those in associated mafic magmas morb fractional crystallization should yield steadily increasing la and yb abundances with increasing sio2 with overall abundances significantly higher than those in associated mafic magmas application to natural occurrences of oceanic plagiogranite indicate that both mor cumulate gabbro melting and morb fractionation are responsible application of the model results to icelandic rhyolites strongly support a fractional crystallization rather than a crustal melting origin

crustal evolution and the temporality of anorthosites abstract types of rock such as komatiite which formed entirely or dominantly during restricted time periods in solar system history are important indicators of how planetary bodies evolved in deep time in this paper we characterize three different types of temporally restricted anorthosites and discuss their significance to the broad scale evolution of planetary processes primordial anorthosites which constitute the bulk of the lunar crust were sampled during apollo and luna missions and have subsequently been identified in our meteorite collections they are characterized by very calcic plagioclase an93 98 and all have magmatic crystallization ages 4 3 ga suggesting that the earliest planetary crust forming processes on the moon and possibly elsewhere involved substantial if not total melting the magma ocean hypothesis which has endured for nearly 50 years argues that the lunar anorthositic rocks represent global flotation cumulates of plagioclase which crystallized after extensive precipitation and sinking of olivine and pyroxene another possibility revitalized by critical filtering of age data involves serial magmatism whereby the lunar crust was constructed by younger smaller episodic magmatic events on earth two distinct types of temporally restricted anorthosites offer opportunities to understand the evolution of terrestrial geodynamics tectonics and magmatic processes archean megacrystic anorthosites formed only between 3 73 and 2 49 ga as small bodies

zircon u pb hf isotopes and geochemistry of two contrasting neoarchean charnockitic rock series in eastern hebei north china craton implications for petrogenesis and tectonic setting abstract the zunhua qinglong microblock in eastern hebei north china craton exposes two contrasting charnockitic rock series in the yuhuzhai taipingzhai and cuizhangzi areas charnockitic plagioclase gneiss series with chemical compositions of dioritic and tonalitic rocks and charnockite series showing tonalitic and granodioritic components petrographic observation reveals that two types of orthopyroxenes are preserved in these rocks medium fine grained anhedral orthopyroxenes are preserved in both series and coarse grained subhedral orthopyroxenes are preserved only in the charnockite series in situ trace element analyses reveal that the coarse grained subhedral orthopyroxenes exhibit typical chemical features of magmatic orthopyroxene whereas the medium fine grained anhedral orthopyroxenes display lower medium to heavy rare earth elements ree and no obvious eu anomalies resulting in left inclined ree patterns combined with petrographic features these new data indicate that the medium fine grained anhedral orthopyroxenes were formed by granulite facies metamorphism caused by dehydration reactions whereas the coarse grained subhedral orthopyroxenes were directly crystallized from their magmatic precursors laser ablation inductively coupled plasma mass spectrometry la icp ms zircon u pb isotopic dating reveals that the magmatic precursors of the charnockitic plagioclase gneiss series were emplaced between 2530 17 and 2523 12 ma and those of the charnockite series were emplaced between 2527 28 and 2515 22 ma we conclude that these magmatic precursors are contemporaneous emplacements of two neoarchean granitoid magmatic events which were followed by regional granulite facies metamorphism at 2 45 ga the charnockitic plagioclase gneiss series and charnockite series exhibit similar hf t values of 2 4 to 5 3 and 3 2 to 5 5 indicating either depleted mantle or juvenile crust affinity and involvement of ancient crustal components the charnockitic plagioclase gneiss series exhibit relatively low sio2 contents 54 1 65 2 with various mgo contents 1 93 5 36 mg 0 36 0 55 and la yb n values 3 49 56 00 the charnockite series exhibit higher sio2 contents 60 1 69 2 with similar la yb n values 14 61 42 69 mgo contents 2 10 3 99 and mg 0 29 0 52 these geochemical and zircon lu hf isotopic features together with chemical modeling suggest that the magmatic precursors of the charnockitic plagioclase gneiss series may have been primarily derived from the partial melting of a depleted mantle that was metasomatized by slab derived melts fluids and experienced fractional crystallization processes with hornblende and plagioclase as primary fractionated phases the aforementioned features further suggest that the charnockite series originated from a partial melting of the subducted slab and it was strongly contaminated by mantle peridotites these new data combined with the previous studies of neoarchean basement rocks in the zunhua qinglong microblock in eastern hebei suggest that the magmatic precursors of these two contrasting charnockitic rock series were formed in a back arc related tectonic environment at a convergent plate margin

trace element partitioning between apatite and silicate melts we present new experimental apatite melt trace element partition coefficients for a large number of trace elements cs rb ba la ce pr sm gd lu y sr zr hf nb ta u pb and th the experiments were conducted at pressures of 1 0 gpa and temperatures of 1250 c the rare earth elements la ce pr sm gd and lu y and sr are compatible in apatite whereas the larger lithophile elements cs rb and ba are strongly incompatible other trace elements such as u th and pb have partition coefficients close to unity

particle mechanics model for the effects of shear on solute retardation coefficient in rock fractures abstract damage on rock fracture surfaces during shear process changes the mechanical and hydrological properties of the fractures and therefore affects solute migration in fractured rocks laboratory experiments on this issue are rarely reported in the literature due to technical difficulties in measuring asperity damage and gouge generation to conceptually investigate the effects of rock fracture surface damage on solute sorption during shear we present a retardation coefficient model considering the effect of wear the particle mechanics model was employed to investigate the effects of gouge generation abrasive wear and microcrack development in the damaged zones on the solute retardation coefficient in rock fractures the results from demonstration examples show that the shear process significantly increases the retardation coefficients by offering more sorption surfaces in the factures due to gouge generation wear microcracking and crushing of gouge particles conceptually three damage zones are classified to characterize the various wear impacts on the solute transport in single fractures outstanding issues of the present model and suggestions for future study are also presented

fault structure wear mechanisms and rupture processes in pseudotachylyte generation abstract fault generated pseudotachylyte is found within both cataclastic and mylonitic host rocks suggesting that rapid catastrophic displacements have occurred at a variety of depths within paleoseismogenic zones pseudotachylyte bearing fault zones represent a composite of structural features associated with the process of earthquake rupture propagation and coseismic slip the development of multiple pseudotachylyte veins in fault linkages duplexes sidewall ripouts en echelon arrays and brittle zones suggests repeated rupturing during a series of characteristic earthquakes each earthquake as a coseismic slip event can be subdivided into initial rupture acceleration stable sliding and final deceleration stages these evolve through distinctive sequences of wear and deformation mechanisms that vary with sliding velocity duration of slip total displacement and the hydrodynamics of the developing fault zone slip is thought to proceed toward surface refinement and possible frictional melting following the propagation of leading shear fracture process zones the passage of the initial process zone of oblique fracturing would be followed by linkage to a throughgoing structure with asperity reduction through brecciation comminution and refined cataclastic flow for frictional melting in an abrasive wear dominant model at greater depths in the presence of mylonitic anisotropy slip would proceed through initial layer parallel surfaces and duplex linkages with rapid surface refinement through plastic smearing and laminar flow for frictional melting in an adhesive wear dominant model

a neoarchean subduction recorded by the eastern hebei precambrian basement north china craton geochemical fingerprints from metavolcanic rocks of the saheqiao shangying qinglong supracrustal belt abstract the saheqiao shangying qinglong supracrustal belt ssqb in the northern eastern hebei precambrian basement ehpb is located in the northern margin of the eastern block eb of the north china craton ncc the shangying terrane constitutes the middle segment of the ssqb and contains primarily metamorphic volcanics and plutonic tonalitic gneisses the metamorphic volcanics mainly consist of pyroxene plagioclase amphibolites garnet plagioclase amphibolites biotite plagioclase amphibole gneisses and amphibole plagioclase gneisses zircon u pb lu hf isotopic analyses reveal that the metavolcanic rocks from the shangying terrane crystallized at 2506 2613 ma with t dm hf values of 2541 2944 ma these metamorphic volcanic rocks are subdivided into four groups based on their lithological and chemical features group i consists chiefly of tholeiites that are characterized by slightly light rare earth element lree depleted patterns and flat multi element spider diagrams which are similar to back arc basin basalt babb like rocks and were derived from the partial melting of the depleted mantle the tholeiites in group ii have slightly fractionated rare earth element ree patterns without nb anomalies exhibit an affinity to nb enriched basalt neb like rocks and were produced by the partial melting of hfse enriched mantle peridotites group iii is composed of slightly lree enriched tholeiites with negative nb ta anomalies that resemble island arc tholeiites group iv comprises calc alkaline basalts and andesites with highly enriched lrees and evident nb ta and ti depletions that are geochemically similar to the products of island arcs the island arc tholeiites and calc alkaline basalt andesites originated from the partial melting of sub arc mantle peridotites that were previously metasomatized by slab derived fluids melts with the fractional crystallization of ferromagnesian minerals collectively the babb like rocks the nebs arc tholeiites and calc alkaline basalt andesites recorded magmatism from a subduction regime similarly neb arc tholeiite andesite dacites and back arc related rocks were recognized in the saheqiao and qinglong areas in the western and eastern parts of the ssqb r guo et al 2013 2015 which further supports the existence of a neoarchean convergent margin in the eastern hebei precambrian basement ehpb compared to the northwestern ehpb the southeastern ehpb underwent lower metamorphism contains arc dacites rhyolites and greater contributions of the ancient continental crust implying that the southeastern ehpb might have been or near a back arc setting thus we conclude that the ssqb experienced a southeastward dipping subduction which is consistent with studies of the adjacent regions e g western liaoning province when combining the 2 5 2 6 ga supracrustal rocks and plutonic granitoid gneisses with depleted mantle origins along the northern margin of the eb of the ncc we concluded that 2 5 2 6 ga was a significant period for juvenile crustal growth and that lateral accretion played a dominant role in crustal growth

a change in the geodynamics of continental growth 3 billion years ago models for the growth of continental crust rely on knowing the balance between the generation of new crust and the reworking of old crust throughout earth s history the oxygen isotopic composition of zircons for which uranium lead and hafnium isotopic data provide age constraints is a key archive of crustal reworking we identified systematic variations in hafnium and oxygen isotopes in zircons of different ages that reveal the relative proportions of reworked crust and of new crust through time growth of continental crust appears to have been a continuous process albeit at variable rates a marked decrease in the rate of crustal growth at 3 billion years ago may be linked to the onset of subduction driven plate tectonics

a review of structural patterns and melting processes in the archean craton of west greenland evidence for crustal growth at convergent plate margins as opposed to non uniformitarian models abstract the archean craton of west greenland consists of many fault bounded eoarchean to neoarchean tectonic terranes crustal blocks these tectonic terranes are composed mainly of tonalite trondhjemite granodiorite ttg gneisses granitic gneisses metavolcanic dominated supracrustal belts layered anorthositic complexes and late to post tectonic granites rock assemblages and geochemical signatures in these terranes suggest that they represent fragments of dismembered oceanic island arcs consisting mainly of ttg plutons tholeiitic to calc alkaline basalts boninites picrites and cumulate layers of ultramafic rocks gabbros leucogabbros and anorthosites with minor sedimentary rocks the structural characteristics of the terrane boundaries are consistent with the assembly of these island arcs through modern style of horizontal tectonics suggesting that the archean craton of west greenland grew at convergent plate margins several supracrustal belts that occur at or near the terrane boundaries are interpreted as relict accretionary prisms the terranes display fold and thrust structures and contain numerous 10 cm to 20 m wide bifurcating ductile shear zones that are characterized by a variety of structures including transposed and redistributed isoclinal folds geometrically these structures are similar to those occurring on regional scales suggesting that the archean craton of west greenland can be interpreted as a continental scale accretionary complex such as the paleozoic altaids melting of metavolcanic rocks during tectonic thickening in the arcs played an important role in the generation of ttgs non uniformitarian models proposed for the origin of archean terranes have no analogs in the geologic record and are inconsistent with structural lithological petrological and geochemical data collected from archean terranes over the last four decades the style of deformation and generation of felsic rocks on outcrop scales in the archean craton of west greenland and the mesozoic sulu orogenic belt of eastern china are similar consistent with the formation of archean continental crust by subduction zone processes

hf isotopic compositions of the standard zircons and baddeleyites used in u pb geochronology abstract zircon and baddeleyite u pb geochronological dating is widely used in solid earth sciences and the advent of rapid in situ methods of analysis such sims and icp ms has re emphasized the importance of having uniform standards recently it has been shown that hf isotopic data can provide important information on these minerals since they contain high concentrations of hf but have low lu hf ratios which results in negligible age correction however the complex internal structures that result from multiple thermal events such as inherited cores and metamorphic overgrowths require that the hf isotopic data be measured with high spatial resolution however the isobaric interferences of 176 yb and 176 lu on 176 hf hamper the precise determination of the 176 hf 177 hf ratio during in situ laser ablation mc icpms analysis it is shown here that mass biases of yb yb and hf hf change with time during analyses and behave differently for solutions and solid material therefore it is suggested that the mean yb value of the individual spot be used to obtain the precise isotopic composition for in situ zircon and baddeleyite hf isotopic analyses for low yb hf 176 yb 177 hf yb value have little effect on the accuracy of the hf isotopic composition however the appropriate yb isotopic abundance is necessary for high yb hf 176 yb 177 hf 0 001 zircons and baddeleyites since the interference of 176 yb on 176 hf is significant using the mean yb value of the individual spot and newly published yb isotopic abundance data six standard zircons and two standard baddeleyites have been investigated using a neptune mc icpms with 193 nm laser for zircons the obtained 176 hf 177 hf ratios are 0 282307 31 2sd for 91500 0 282680 31 2sd for temora 0 281729 21 2sd for cz3 282177 17 2sd for cn92 1 0 282983 17 2sd for fm0411 and 281234 11 2sd for phalaborwa the baddeleyites from phalaborwa and sk10 2 have 176 hf 177 hf ratios of 0 281238 12 and 0 282738 13 2sd these results agree well with the values obtained by the solution method and indicate that these standards have different hf isotopic compositions in which the extremely low 176 lu 177 hf and 176 yb 177 hf values of cz3 zircon and phalaborwa baddeleyite make them excellent standards for machine calibration during in situ zircon hf isotopic measurement with the other standards being more suitable for the development of the correction method

a novel three dimensional discrete fracture network model for investigating the role of aperture heterogeneity on fluid flow through fractured rock masses abstract effect of anisotropic aperture on the hydraulic properties of single rock fractures has been systematically investigated yet the aperture variability of individual fractures in 3d discrete fracture networks dfns is commonly negligible by using parallel plate fractures the present study proposed a 3d dfn model with fractures having heterogeneous apertures to estimate the influence of fracture variability on fluid flow in total a set of 1280 3d models with increasing fracture densities and fracture lengths are generated and the fluid flow through the models is simulated using a developed numerical code the influences of aperture heterogeneity and network topology on the flow pattern and permeability of 3d dfns are estimated the results show that the network topology provides a first order frame of geometrical connectivity and the heterogeneous aperture further allows the flow to select some most transmissive channels within these connected fractures the dfn model with identical apertures generates a large number of medium flow rate regions whereas the dfn model with fractures having heterogeneous apertures results in extremely low and high flow rate regions the permeability ratio of the two models is widely spread in terms of a small variation in the average mechanical aperture as a result of strong dependence on the aperture distribution the average permeability ratio increases significantly first and then approaches to 1 0 with increasing the average mechanical aperture this allows for the definition of a critical mechanical aperture above which the permeability can be properly predicted using the dfn model with fractures having identical apertures and below which the permeability is much altered by the aperture variability and the dfn model with fractures having heterogeneous apertures should be employed