sulfur isotopes in diamonds reveal differences in continent construction neoproterozoic west african diamonds contain sulfide inclusions with mass independently fractionated mif sulfur isotopes that trace archean surficial signatures into the mantle two episodes of subduction are recorded in these west african sulfide inclusions thickening of the continental lithosphere through horizontal processes around 3 billion years ago and reworking and diamond growth around 650 million years ago we find that the sulfur isotope record in worldwide diamond inclusions is consistent with changes in tectonic processes that formed the continental lithosphere in the archean slave craton diamonds that formed 3 5 billion years ago do not contain any mif sulfur younger diamonds from the kaapvaal zimbabwe and west african cratons do contain mif sulfur which suggests craton construction by advective thickening of mantle lithosphere through conventional subduction style horizontal tectonics

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

middle archean continent formation by crustal delamination the processes that created the first large cratonic areas such as the pilbara and the kaapvaal remain poorly understood models based on the uniformitarian extrapolation of present day arc volcanic processes to a hotter early earth have not adequately explained the observations in these terranes here we propose an alternative mechanism for the formation of the earliest continental crust the formation of continental crust may be achieved by delamination of the lower eclogitic part of an oceanic plateau like protocrust such delamination results in uplift extension and the production of tonalite trondhjemite and granodiorite ttg suites as recorded in middle archean cratons the available geologic and geophysical observations in combination with model calculations permit this scenario as an alternative to subduction based hypotheses

petrology and geochemistry of mafic rocks in the acasta gneiss complex implications for the oldest mafic rocks and their origin abstract the acasta gneiss complex located in the western part of the slave province canada is widely recognized as the oldest eoarchean terrane in addition to felsic gneisses with the ages of 3 6 4 0 ga minor mafic rocks occur as rounded to elliptical enclaves and inclusions within the felsic gneisses despite serving as potential sources of geochemical information on the hadean mantle the mafic rocks have received less attention in previous studies thus we conducted a comprehensive geological petrological and geochemical investigation on the acasta mafic rocks to constrain their petrogenesis and geodynamic setting the mafic rocks comprise massive to weakly foliated amphibolite garnet amphibolite and hornblendite with variable abundances of hornblende plagioclase chlorite and quartz and subordinate clinopyroxene garnet and cummingtonite they commonly underwent high grade metamorphic recrystallization under amphibolite to upper amphibolite facies conditions the observed variations in mineral assemblages abundances and compositions reflect large differences in whole rock compositions likely caused by crustal anatexis during the eoarchean thermal events responsible for the generation of the surrounding felsic gneisses infiltration or extraction of felsic melts formed due to partial melting of precursor rocks can account for an overall negative correlation between al 2 o 3 and mgo contents and variable enrichments in the incompatible elements despite the widespread influence of anatexis on the geochemistry of acasta mafic rocks we identified the precursor compositions of the least modified amphibolites as basaltic magmas they are characterized by intermediate al 2 o 3 and mgo contents on the observed array and by near chondritic patterns for incompatible trace elements except for slightly negative nb and ta anomalies we considered two scenarios to explain the origin of eoarchean basaltic rocks with nb ta anomalies 1 generation of nb ta deficient basaltic magma in a suprasubduction setting analogous to modern arcs derived magmas and 2 generation of nb ta deficient basaltic magma from the melting of a nb ta deficient primitive mantle possible if the core contains significant proportions of the earth s nb and ta budget although the operation of plate tectonics and the presence of subduction zones at the end of hadean may be an attractive explanation for the observed nb ta depletions the chondritic relative proportions of other immobile trace elements for acasta mafic rocks leave open the possibility of their formation from an nb ta deficient primitive mantle

missing aragonitic biota and the diagenetic evolution of heterozoan carbonates a case study from the oligo miocene of the central mediterranean abstract several previous studies have addressed the diagenetic evolution of heterozoan carbonate assemblages generally it is assumed that early diagenetic processes in heterozoan settings are mainly destructive including abrasion and dissolution on the sea floor constructive diagenesis cementation is delayed to later stages in the burial environment with pressure solution of calcitic grains acting as a cement source this paper presents a study of oligo miocene inner to outer ramp heterozoan carbonates from the central mediterranean maltese islands and sicily indicating that early diagenetic processes are more important than previously assumed four to five different cement types including fibrous two types of epitaxial bladed and blocky cement are distinguished based on transmitted light microscopy cathodoluminescence microscopy allowed a differentiation between primary high mg calcitic fibrous and epitaxial cement i and primary low mg calcitic epitaxial cement ii bladed and blocky cements stable isotope data indicate cement precipitation from marine marine derived and meteoric waters trace element analyses point to cementation in an open system maltese islands and a closed system sicily our investigations show that the majority of constructive diagenetic processes in these rocks occurs rather early in the shallow marine burial environment which is transitional between the marine seafloor and the deep burial diagenetic environment the main cement source in this environment is assumed to be aragonite we suggest careful consideration of the importance of aragonitic components in fossil heterozoan settings which seem to be more abundant than previously assumed and can act as a major early cement source due to the low preservation potential of these components detailed geochemical studies are necessary to detect aragonite as the cement source our findings also have implications when considering the reservoir qualities of these rocks because primary porosity can be occluded early and secondary porosity is not preserved

parentage of archean basement within a paleoproterozoic orogen and implications for on craton diamond preservation slave craton and wopmay orogen northwest canada the wopmay orogen is a paleoproterozoic accretionary belt preserved to the west of the archean slave craton northwest canada reworked archean crystalline basement occurs in the orogen and new bedrock mapping u pb geochronology and sm nd isotopic data further substantiate a slave craton parentage for this basement detrital zircon results from unconformably overlying paleoproterozoic supracrustal rocks also support a slave craton provenance rifting of the slave margin began at ca 2 02 ga with a second rift phase constrained between ca 1 92 and 1 89 ga resulting in thermal weakening of the archean basement and allowing subsequent penetrative deformation during the calderian orogeny ca 1 88 1 85 ga the boundary between the western slave craton and the reworked archean basement in the southern wopmay orogen is interpreted as the rifted cratonic margin which later acted as a rigid backstop during compressional deformation age isotopic characteristics of plutonic phases track the extent and evolu

genesis of adakitic granitoids by partial melting of thickened lower crust and its implications for early crustal growth a case study from the huichizi pluton qinling orogen central china abstract adakitic rocks are often considered as a key to deciphering the genesis of archean ttgs and the early crustal growth granites from the huichizi pluton in the north qinling nq unit have high sr y and la yb n ratios similar to adakites their relatively high sio 2 k 2 o and na 2 o and very low mgo cr and ni contents are in the range of high sio 2 adakites and early archean ttgs and are compositionally similar to experimental melts derived from metabasalt sources new sims zircon u pb dating constrains the emplacement age of the huichizi pluton at 422 5 ma rock samples from the huichizi pluton have e nd t and zircon e hf t values similar to the neoproterozoic metabasalts in the nq unit in combination with their normal mantle like 18 o zir values these adakites are best explained by partial melting of the neoproterozoic mafic crustal root due to subduction of the shangdan ocean regional geological data suggest that the crust was probably thickened by a ca 490 ma arc collision process prior to the emplacement of the huichizi pluton our results confirm that underplating of mafic magma and its subsequent fusion triggered by slab subduction under high pressure conditions could be an important mechanism for the formation of early continental crust

generation of eoarchean continental crust from altered mafic rocks derived from a chondritic mantle the 3 72 ga aktash gneisses tarim craton nw china abstract our understanding of crust mantle evolution and geodynamics of the early earth has been hampered by the scarcity of hadean to eoarchean rocks and by the almost ubiquitous isotopic disturbance of the oldest rocks here we present in situ zircon u pb ages o oh and lu hf isotopes as well as whole rock sr nd hf pb isotopes for the recently identified eoarchean aktash gneisses in the tarim craton nw china the new age data confirm that the aktash gneisses were formed at least by 3 72 ga and were affected by two metamorphic events at 3 56 and 2 0 ga respectively whole rock isotopic data show that the sr nd hf pb isotopic systematics have all been variously disturbed by 2 0 ga regional granulite facies metamorphism resulting in decoupling of nd hf isotopic systematics that is in some samples similar to that observed in sw greenland the least disturbed sample 16alt67 however yielded chondritic to slightly sub chondritic whole rock end 0 3 and ehf 0 9 this is consistent with the in situ magmatic zircon ehf weighted mean 0 2 to 1 8 for most samples of the aktash gneisses suggesting a mafic source derived from chondritic mantle with a short crustal residence time our data thus provide no evidence for the presence of an eoarchean depleted mantle or a hadean enriched crust implying that early earth crust mantle differentiation was either limited or balanced by large scale crustal recycling in situ zircon o oh data show that metamorphism and later alteration tend to decrease the zircon 18 o of the aktash gneisses and the weighed mean 18 o of pristine magmatic zircons 6 2 6 3 from the least disturbed samples indicate that the mafic source was mildly altered at low temperature by the early hydrosphere facilitating partial melting and formation of some of the earliest preserved continental crust known on earth

unraveling the complexity of zircons from the 4 0 2 9 ga acasta gneiss complex abstract the acasta gneiss complex agc of the northwest territories canada contains some of the oldest evolved terrestrial crust and has a sustained record of more than a billion years of magmatism 4 0 2 9 ga these rocks provide an opportunity to investigate the nature of continental crust formation on the early earth because complexities in zircons and bulk rocks are characteristic of the early earth record including the agc strategies are needed to extract accurate and meaningful age and isotopic information in order to evaluate the accuracy of the early earth hf isotope record we examined agc zircons from a range of lithologies using paired chemical abrasion isotope dilution u pb age and solution lu hf isotope analysis and compared these with previous results obtained using laser ablation split stream lass analysis we describe an approach whereby lass is used to identify rocks with the least complex zircons and when appropriate solution methods are then used to refine the age and hf isotopic composition to the highest precision this two pronged analytical approach results in a more robust determination of the age and hf isotopic record of complex rocks and zircons and allows identification of complexity in the hf isotopic record that would not be apparent by solution analysis alone thereby refining the record of magmatic evolution on the early earth despite the better precision solution techniques are unsuitable for rocks with complexly zoned zircons in particular zircons from the two oldest agc rocks in this study have positive initial ehf values when determined by solution analysis 2 8 and 4 9 at 4 0 and 3 9 ga respectively but have negative ehf values when determined by lass 3 4 and 4 7 respectively this is attributable to the presence of later radiogenic overgrowths on the zircon grains which are incorporated in solution analyses but can be avoided using lass this provides important clarity to the agc hf isotope record in total all but one of the agc rocks we analyzed including the oldest samples have negative ehf values and indicate derivation from an enriched reservoir none of these samples in contrast to whole rock nd isotope compositions have sufficiently positive ehf values to indicate their derivation from a depleted mantle reservoir despite the presence of ancient agc crust there is no record of corresponding mantle depletion this implies that extraction of hadean crust in this region did not happen in sufficient volume to result in widespread mantle depletion in the agc source by the eoarchean our results underscore the importance of identifying different components in ancient zircons and the rocks that contain them and accurately determining the age and isotopic composition of those components this is critically important for clarifying the record of the formation of enriched crust and development of the depleted mantle in earth s early history

formation of paleoarchean continental crust through infracrustal melting of enriched basalt abstract paleoarchean rocks of the tonalite trondhjemite granodiorite ttg series require a basaltic source region more enriched in k lile th and lree than the low k tholeiites typical of archean supracrustal sequences most ttg of the pilbara craton in northwestern australia formed between 3 5 and 3 42 ga through infracrustal melting of a source older than 3 5 ga basaltic to andesitic rocks of the 3 51 ga coucal formation at the base of the pilbara supergroup are amongst the only well preserved remnants of pre 3 5 ga supracrustal material on earth and may have formed a large proportion of pre 3 5 ga pilbara crust these rocks are significantly enriched in k lile th and lree compared to post 3 5 ga paleoarchean basalts and andesites and form a compositionally suitable source for ttg enrichment in these basalts was not the result of crustal assimilation but was inherited from a mantle source that was less depleted than modern morb source and was enriched in recycled crustal components we suggest that the formation of paleoarchean ttg and of their voluminous mafic source regions reflects both a primitive stage in the thermal and compositional evolution of the mantle and a significant prehistory of crustal recycling

occurrence and geochronology of the eoarchean 3 9 ga iqaluk gneiss in the saglek block northern labrador canada evidence for the oldest supracrustal rocks in the world abstract understanding earth s early evolution requires decoding the cryptic geological record that is preserved in a few extremely rare terrains on earth the saglek hebron block in the north atlantic craton is one of the oldest terrains in the world to better understand the eoarchean history of the saglek hebron block we undertook a comprehensive geological and geochronological investigation of this terrain the saglek hebron locality contains orthogneisses and supracrustal rocks which are classified into two groups of the eoarchean and mesoarchean suites based on field associations defined by intrusion of the mesoarchean mafic saglek dykes the eoarchean suites contain the nulliak supracrustal rocks and uivak gneisses in this paper we report cathodoluminescence cl observations and u pb ages of zircons from the eoarchean orthogneisses to constrain the evolution of the saglek hebron block we made detailed sketch maps 1 20 of critical outcrops to establish relative chronological relationships between lithologies at least seven generations of mafic and felsic units were recognized in an outcrop of the st john s harbour south area the first and second generations are mafic supracrustal rocks whereas the third to seventh generations are tonalitic gneisses which are intruded into the supracrustal units we study cl observation geochemistry and u pb dating of zircons from the oldest and youngest tonalitic gneisses the cl observation of internal structures of the zircons showed they typically comprise three domains of core mantle and rim the cores have clear oscillatory zoning whereas the mantles lack oscillatory zoning and exhibit dark cl emission and structureless cl images the obvious correlation between the ages and chemical compositions of u contents and th u ratios indicates that the cores with low u contents and high th u ratios preserve the protolith age the cores with clear oscillatory zoning low u contents and high th u ratios of zircons from the oldest suite plot on a concordia line with ages ranging from 3953 to 3797 ma the oldest and average ages of the six oldest spots on the concordia line are 3953 54 and 3920 49 ma respectively on the other hand the average age of the four old spots of zircons from a younger generation of orthogneiss is 3869 63 ma the geological and geochronological relationships between the third and seventh generation orthogneisses are consistent with each other confirming an eoarchean age of ca 3 9 ga for the oldest suite of the uivak gneisses we name this oldest suite the iqaluk gneiss as the protolith of the iqaluk gneiss was clearly intruded into the nulliak supracrustal rocks the nulliak supracrustals must have a minimum age of ca 3 9 ga indicating that they are the oldest supracrustal rocks on earth but further studies should be necessary to obtain more precise age of the iqaluk gneiss and nulliak supracrustal rocks

silicic magma petrogenesis in iceland by remelting of hydrothermally altered crust based on oxygen isotope diversity and disequilibria between zircon and magma with implications for morb terra nova 24 227 232 2012 abstract petrogenesis of silicic magmas in iceland has fundamental significance for understanding the relative importance of fractional crystallization of mantle derived basalt and partial melting of hydrothermally altered basaltic crust in formation of the earliest continental crust first results of in situ oxygen isotope investigation of zircons in large volume silicic eruptive products of three volcanoes in iceland askja torfajokull and hekla demonstrate isotope diversity and disequilibria and long u th zircon pre eruptive residence of 103 104 year this suggests that zircons did not grow from their host melts but instead were inherited from older magma batches and leftover cumulates with generally low and variable 18o values this study demonstrates that segregation of cubic kilometres of silicic magma is faster than mineral diffusive or recrystallization time scales estimated at 103 years and it suggests that partial melting of hydrothermally altered and oxidized oceanic crust is the mechanism that best explains silicic rocks in iceland and early earth environments

low δ18o rhyolites from yellowstone magmatic evolution based on analyses of zircons and individual phenocrysts the yellowstone plateau volcanic field is one of the largest centers exchange time to form zoned zircons is between a few hundred and a few thousand years which reflects the residence time of lowo of rhyolitic magmatism on earth major caldera forming eruptions are followed by unusual lowo rhyolites new oxygen isotope magmas after formation and before eruption petrologic and geochemical data from rhyolites belonging to the 2 0 my eruptive history of yellowstone are presented with emphasis on the genesis of lowo magmas erupted after the huckleberry ridge tuff 2 0 ma 2500 km and lava creek tuff 0 6 ma

low 18o silicic magmas why are they so rare low 180 silicic magmas are reported from only a small number of localities e g yellowstone and iceland yet petrologic evidence points to upper crustal assimilation coupled with fractional crystallization afc during magma genesis for nearly all silicic magmas the rarity of 10w l o magmas in intracontinental caldera settings is remarkable given the evidence of intense 10w l o meteoric hydrothermal alteration in the subvolcanic remnants of larger caldera systems in the platoro caldera complex regional ignimbrites 150 1000 km3 have plagioclase 6180 values of 6 8 0 1 whereas the middle tuff a small volume est 50 100 km3 post caldera collapse pyroclastic sequence has plagioclase 8 80 values between 5 5 and 6 8 o on average the plagioclase phenocrysts from the middle tuff are depleted by only 0 3 0 relative to those in the regional tuffs at yellowstone small volume post caldera collapse intracaldera rhyolites are up to 5 5 o depleted relative to the regional ignimbrites two important differences between the middle tuff and the yellowstone 10w 180 rhyolites elucidate the problem middle tuff magmas reached water saturation and erupted explosively whereas most of the 10w l 80 yellowstone rhyolites erupted effusively as domes or flows and are nearly devoid of hydrous phenocrysts comparing the two eruptive types indicates that assimilation of 10w 180 material combined with fractional crystallization drives silicic melts to water oversaturation water saturated magmas either erupt explosively or quench as subsurface porphyrins bejiire the magmatic 180 can be dramatically lowered partial melting of low 180 subvolcanic rocks by near anhydrous magmas at yellowstone produced small volume 10w 180 magmas directly thereby circumventing the water saturation barrier encountered through normal afc processes

cyclic formation and stabilization of archean lithosphere by accretionary orogenesis constraints from ttg and potassic granitoids north china craton this study is financially supported by the national natural science foundation of china grant nos 41502179 41530207 41472165 and central university basic scientific research business expenses of china university of geosciences beijing grant no 2652015038 pac acknowledges support from australian research council grant fl160100168

in situ evidence for continental crust on early mars understanding of the geologic evolution of mars has been greatly improved by recent orbital 1 3 in situ 4 5 and meteorite 6 8 data but insights into the earliest period of martian magmatism 4 1 to 3 7 billion years ago remain scarce 9 the landing site of nasa s curiosity rover gale crater which formed 3 61 billion years ago 10 within older terrain 11 provides a window into this earliest igneous history along its traverse curiosity has discovered light toned rocks that contrast with basaltic samples found in younger regions 12 here we present geochemical data and images of 22 specimens analysed by curiosity that demonstrate that these light toned materials are feldspar rich magmatic rocks the rocks belong to two distinct geochemical types alkaline compositions containing up to 67 wt sio2 and 14 wt total alkalis na2o k2o with fine grained to porphyritic textures on the one hand and coarser grained textures consistent with quartz diorite and granodiorite on the other hand our analysis reveals unexpected magmatic diversity and the widespread presence of silica and feldspar rich materials in the vicinity of the landing site at gale crater combined with the identification of feldspar rich rocks elsewhere 9 13 14 and the low average density of the crust in the martian southern hemisphere 15 we conclude that silica rich magmatic rocks may constitute a significant fraction of ancient martian crust and may be analogous to the earliest continental crust on earth

could iceland be a modern analogue for the earth s early continental crust for the last two decades iceland and other oceanic plateaux have been considered as potential analogues for the formation of the early earth s continental crust this study examines the compositions of silicic rocks from modern oceanic plateaux revealing their differences to archaean continental rock types trondhjemite tonalite granodiorite or ttg and thereby emphasising the contrasted mechanisms and or sources for their respective origins in most oceanic plateaux felsic magmas are thought to be formed by fractional crystallization of basalts in iceland the interaction between mantle plume and the mid atlantic ridge results in an abnormally high geothermal gradient and melting of the hydrated metabasaltic crust however despite the current archaean like high geothermal gradients melting takes place at a shallow depth and is unable to reproduce the ttg trace element signature consequently oceanic plateaux are not suitable environments for the genesis of the archaean continental crust however their subduction could account for the episodic crustal growth which has occurred throughout the earth s history

a yilgarn seed to the pilbara craton australia evidence from inherited zircons knowledge of the age and compositional architecture of archean cratonic lithosphere is critical for models of geodynamics and continental growth on early earth but can be difficult to unravel from

magmatic complexity on early mars as seen through a combination of orbital in situ and meteorite data abstract until recently mars was considered a basalt covered world but this vision is evolving thanks to new orbital in situ and meteorite observations in particular of rocks of the ancient noachian period in this contribution we summarise newly recognised compositional and mineralogical differences between older and more recent rocks and explore the geodynamic implications of these new findings for example the msl rover has discovered abundant felsic rocks close to the landing site coming from the wall of gale crater ranging from alkali basalt to trachyte in addition the recently discovered martian regolith breccia nwa 7034 and paired samples contain many coarse grained noritic monzonitic clasts demonstrably noachian in age and even some clasts that plot in the mugearite field olivine is also conspicuously lacking in these ancient samples in contrast to later hesperian rocks the alkali suite requires low degree melting of the martian mantle at low pressure whereas the later hesperian magmatism would appear to be produced by higher mantle temperatures various scenarios are proposed to explain these observations including different styles of magmatic activity i e passive upwelling vs hotspots a second petrological suite of increasing interest involves quartzo feldspathic materials that were first inferred from orbit in local patches in the southern highlands and in the lower units of valles marineris however identification of felsic rocks from orbit is limited by the low detectability of feldspar in the near infrared on the other hand the msl rover has described the texture mineralogy and composition of felsic rocks in gale crater that are granodiorite like samples akin to terrestrial ttg tonalite trondhjemite granodiorite suites these observations and the low average density of the highlands crust suggest the early formation of continental crust on mars although the details of the geodynamic scenario and the importance of volatiles in their generation are aspects that require further work

geology of the eoarchean 3 95 ga nulliak supracrustal rocks in the saglek block northern labrador canada the oldest geological evidence for plate tectonics the earth is a unique planet which has been highly evolved diversified and complicated through geologic time and underwent many key events including giant impact magma ocean core formation large scale mantle differentiation and late heavy bombardment especially in its dawn but our knowledge of early earth is limited due to the lack of the hadean supracrustal rocks the supracrustal rocks with the eoarchean ages provide key evidence for the earth s early evolution but few supracrustal rocks have been comprehensively investigated therefore we mapped in seven areas of the saglek block northern labrador where ancient supracrustal sequences are interleaved with a diverse assemblage of orthogneisses early studies suggested that some of them have the mesoarchean ages because of the lack of the mesoarchean saglek dyke but we found the saglek dykes in the areas to recognize the eoarchean nulliak supracrustal rocks and uivak gneiss in all the areas recent reassessment of u pb dating and cathodoluminescence observation of zircons from the oldest suites of the uivak gneiss showed that the uivak gneiss has the eoarchean age 3 95 ga and forms the iqaluk uivak gneiss series because our geological survey clearly showed that the iqaluk uivak gneisses were intruded into the nulliak supracrustal belts the nulliak supracrustal rocks are the oldest supracrustal rock in the world the supracrustal belts consist of piles of fault bounded blocks which are composed of the ultramafic rocks mafic rocks and sedimentary rocks in ascending order similar to modern ocean plate stratigraphy ops in addition small scale duplex structures are found over the areas the presence of duplex structure and ops indicates that the 3 95 ga nulliak supracrustal belts originate from an accretionary complex the presence of the accretionary complex ophiolite and granitic continental crust provides the oldest evidence for the plate tectonics on the early earth

earth s earliest evolved crust generated in an iceland like setting the setting in which earth s first continental crust formed is unclear because few rocks survive from that time geochemical analysis of 4 03 billion year old rocks discovered in the acasta gneiss complex canada reveals rocks that are strikingly similar to those formed in iceland today implying earth s first continental crust was generated in an iceland like setting

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

evolution of the archean continental crust some 20 years of plate tectonic theory combined with new insights into the fine structure of the lithosphere the application of multielement geo chemical and isotopic studies paleomagnetism and geophysical modeling of mantle processes have profoundly influenced present thinking on the origin and evolution of the earth s early continental crust previously our knowledge of the continental crust was based almost exclusively on field geological observations although there is now general agreement on how the earth worked for the last 200 m y because of observable evidence in the oceans and continents e g bird 1980 condie 1982 it has proved difficult to extend this history into more ancient times in view of the lost oceanic record and the ambiguity and complexity of the pre mesozoic rock relationships in the continents dewey 1982 however preserved characteristic rock as semblages uniquely identifying modern type wilson cycle processes i e opening and closure of oceans underlain by oceanic crust have now been recognized in continental terranes as old as 900 m y and these assemblages provide strong evidence for the conclusion that the present global tectonic regime has governed the evolution of the lithosphere at least since the late precambrian kroner 1977 1981a b goodwin 1981 profound disagreement on the older crustal history however prevails to the present day since typical features characterizing phanerozoic accretionary terranes such as obducted ophiolites blueschists and franciscan type melanges have not been found in more ancient regions thus two types of evolutionary models have been developed one type postulates uniformitarian development back to the earliest archean burke

disturbances in the sm nd isotope system of the acasta gneiss complex implications for the nd isotope record of the early earth abstract the whole rock 143nd isotope record of the acasta gneiss complex agc has been used to argue for the development of both depleted mantle and enriched crustal reservoirs during the hadean the 147sm 143nd isotope compositions of rocks from agc however fall on an array with an errorchron date of 3 3 ga despite the majority of samples yielding substantially older zircon u pb ages this has led to the suggestion of a major sm nd resetting event at this time in the agc to better understand the sm nd bulk rock systematics of the agc we investigate the u pb age and sm nd isotope compositions of apatite and titanite from the agc using the laser ablation split stream lass method apatite from all samples yield broadly similar u pb dates of 1 89 to 1 83 ga in agreement with resetting during the 1 9 ga wopmay orogen the sm nd isotope compositions of the apatite however lie off the wr errorchron and show a high degree of initial nd isotope heterogeneity conversely titanite sm nd systematics are consistent with the 3 3 ga whole rock errorchron further supporting sm nd modification at 3 3 ga taken together these show that the oldest rocks in the agc underwent at least two post crystallization events at 3 3 and 1 9 ga that modified the sm nd isotope system these results also demonstrate the relative mobility of the sm nd isotope system during orogenic events which may compromise the ability to determine robust bulk rock 143nd isotope signatures

kyanite garnet bearing cambrian rocks and grenville granulites from the ayer s cliff quebec canada lamprophyre dike suite deep crustal fragments from the northern appalachians granulite facies metamorphic xenoliths have been found in several monteregian age lamprophyric dikes at ayer9s cliff quebec canada from geothermobarometric calculations it is shown that middle to lower crustal metapelitic assemblages formed at 700 800 c and 8 10 kbar whereas mafic assemblages formed near 1200 100 c and 14 1 kbar the metapelitic assemblages are interpreted as cambrian metasediments similar to those found farther south in vermont the mafic assemblages are interpreted as grenville basement the occurrence of these lithologies at ayer9s cliff and in similar dikes in vermont confirms the seismic interpretation of grenville basement beneath a large part of the western appalachians in southern quebec and northern new england

late neoarchean subduction related crustal growth in the northern liaoning region of the north china craton evidence from 2 55 to 2 50 ga granitoid gneisses abstract the north china craton ncc dominated by 2 6 2 5 ga tectonothermal events provides a natural laboratory to study neoarchean crustal growth and geodynamic evolution late neoarchean granitoid gneisses are well exposed in the northern liaoning province located north of the ancient anshan benxi terrane along the northeastern margin of the eastern block eb of the ncc la icpms zircon u pb isotopic data reveal that granitoid gneisses in the qingyuan area can be grouped into two major episodes i e 2559 2534 ma strongly gneissic quartz dioritic and tonalitic to trondhjemitic gneisses and 2529 2495 ma weakly gneissic to massive quartz monzodioritic and monzogranitic gneisses with subordinate tonalitic to trondhjemitic gneisses the late magmatic episode was accompanied by regionally high grade metamorphism 2510 2495 ma most granitoid gneisses display highly depleted zircon ehf t2 values 4 2 to 8 1 whereas one monzogranitic gneiss shows negative values of 4 7 to 1 0 indicating late neoarchean crustal growth with minor involvement of ancient continental materials probably sourced from the anshan benxi terrane geochemical and petrogenetic studies reveal that the quartz dioritic magmas were derived from partial melting of plagioclase poor garnet amphibolites or eclogites metamorphosed from oceanic slab materials with slab melts contaminated by mantle wedge peridotites during ascent the tonalitic to trondhjemitic magmas stemmed from partial melting of mainly juvenile metabasaltic rocks with minor metagreywackes of lower arc crust in comparison the quartz monzodioritic and monzogranitic magmas were derived respectively from partial melting of depleted mantle sources metasomatized by slab derived fluids and metagreywackes with different crustal resident ages at middle to lower crustal levels combined with previous studies of supracrustal metavolcanic rocks the northern liaoning province records late neoarchean crustal growth evolving from mid ocean ridge through initiation and maturation of an intra oceanic arc to arc continent collision arc continent accretion and possibly slab rollback processes may have triggered reworking of both juvenile arc crust and minor ancient continental margin materials generating the magmatic precursors for the monzogranitic gneisses overall the intense late neoarchean crustal growth of the eb was controlled mainly by arc continent accretion possibly linked to global assembly of cratonic fragments

coupled zircon lu hf and u pb isotopic analyses of the oldest terrestrial crust the 4 03 ga acasta gneiss complex abstract the acasta gneiss complex of the northwest territories canada contains some of the earliest terrestrial continental crust and thus provides a critical sample set for characterization of crust forming processes on the early earth here we report the results of a combined lu hf and u pb isotopic study of zircons from predominantly felsic orthogneisses from the acasta gneiss complex that crystallized between 4 0 and 2 9 ga many of which contain complex zoning and therefore require an analytical treatment suited to distinguish amongst compositionally distinct age and hf isotopic domains to ensure the reliability of the analyses and of subsequent geologic interpretations we employed the laser ablation split stream lass technique to concurrently measure the lu hf and u pb isotopic systems in zircon our results confirm prior findings of precursor hadean crust 4 0 ga in the source of these rocks and the continued involvement of this reservoir until 3 6 ga we present evidence for the input of relatively more juvenile material at 3 6 ga which we suggest corresponds to a fundamental change in the source of the magmas this study extends the lower bound of the published acasta hf isotopic record from 3 6 ga to 2 9 ga and demonstrates that the 3 6 ga 2 9 ga interval is largely represented by reworking of relatively juvenile 3 6 ga crust and the diminution of the 4 0 ga crustal signal significantly there is no evidence that rocks within the acasta gneiss complex were derived from a strongly depleted mantle

petrogenesis and tectonics of the acasta gneiss complex derived from integrated petrology and 142nd and 182w extinct nuclide geochemistry abstract the timing and mechanisms of continental crust formation represent major outstanding questions in the earth sciences extinct nuclide radioactive systems offer the potential to evaluate the temporal relations of a variety of differentiation processes on the early earth including crust formation here we investigate the whole rock 182w 184w and 142nd 144nd ratios and zircon 17 o values of a suite of well studied and lithologically homogeneous meta igneous rocks from the acasta gneiss complex northwest territories canada including the oldest known zircon bearing rocks on earth in the context of previously published geochemical data and petrogenetic models the new 142nd 144nd data indicate that formation of the hadean eoarchean acasta crust was ultimately derived from variable sources both in age and composition although 4 02 ga crust was extracted from a nearly bulk earth source heterogeneous 142nd signatures indicate that eoarchean rocks of the acasta gneiss complex were formed by partial melting of hydrated hadean age mafic crust at depths shallower than the garnet stability field by 3 6 ga granodioritic granitic rocks were formed by partial melting of archean hydrated mafic crust that was melted at greater depth well into the garnet stability field our 182w results indicate that the sources to the acasta gneiss complex had homogeneous high 182w on the order of 10 ppm a signature ubiquitous in other eoarchean terranes no significant deviation from the terrestrial mass fractionation line was found in the triple oxygen isotope 16o 17o 18o compositions of acasta zircons confirming homogeneous oxygen isotope compositions in earth s mantle by 4 02 ga

tectonics changing of the plates the composition of earth s crust depends on the style of plate tectonics and of the melting regimes in the mantle analyses of the oldest identified rocks suggest that these styles and the resulting crust have changed over earth s history

tracking deep crust by zircon xenocrysts within igneous rocks from the northern alxa china constraints on the southern boundary of the central asian orogenic belt abstract the southern boundary of the central segment of the central asian orogenic belt caob with the precambrian alxa block is not well constrained due to poor recognition of deep crust statistical analysis of zircon xenocrysts within igneous rocks from the northern alxa and its adjacent regions was applied to resolve this problem we compiled new and previously published geochronological zircon age data obtained by shrimp sims tims and la icp ms for 316 igneous rock samples of which 61 samples contain zircon xenocrysts new and previously published hf isotopic compositions of these zircon xenocrysts are combined with zircon ages in this study the zircon xenocrysts are mainly contained within permian rocks from the yabulai nurgong honggueryulin ynh zone and igneous rocks from the northwestern margin of the north china craton wncc a few xenocrysts were also found in permian igneous rocks from the zongnaishan shalazhashan zs zone neo proterozoic and paleo proterozoic intrusive rocks in the ynh zone as well as devonian to carboniferous granitoids and volcanic rocks from southern mongolia altogether we analyzed more than 270 zircon xenocrysts and considered only ages that are less than 10 discordant xenocryst ages within the permian igneous rocks from the zs zone are mainly around ca 350 ma ca 600 ma and ca 1400 ma the oldest age of zircon xenocrysts in this zone is similar to those of zircon xenocrysts from the caob 1 1 ga by contrast abundant zircon xenocrysts within permian igneous rocks from the ynh zone show highly variable age populations at ca 2 6 2 1 ga 1 8 1 6 ga 930 750 ma and 460 300 ma zircon xenocrysts from the zs zone have positive e hf t values of 6 3 to 13 9 whereas those from the ynh zone display highly variable e hf t values from 16 1 to 11 6 from the southern caob and the zs zone to the ynh zone the zircon xenocrysts show a significant shift from juvenile to crustal hf isotopic compositions suggesting that the area between the zs and the ynh zones constitutes the southern boundary of the caob in the deep crust within this region our study indicates that statistical analysis or isotopic mapping of zircon xenocrysts is an effective method to trace the nature of the deep continental crust and to separate between newly formed orogenic domain and ancient cratons

long term preservation of early formed mantle heterogeneity by mobile lid convection importance of grainsize evolution abstract the style of tectonics on the hadean and archean earth particularly whether plate tectonics was in operation or not is debated one important albeit indirect constraint on early earth tectonics comes from observations of early formed geochemical heterogeneities 142nd and 182w anomalies recorded in hadean to phanerozoic rocks from different localities indicate that chemically heterogeneous reservoirs formed during the first 500 myrs of earth s history survived their remixing into the mantle for over 1 gyrs such a long mixing time is difficult to explain because hotter mantle temperatures expected for the early earth act to lower mantle viscosity and increase convective vigor previous studies found that mobile lid convection typically erases heterogeneity within 100 myrs under such conditions leading to the hypothesis that stagnant lid convection on the early earth was responsible for the observed long mixing times however using two dimensional cartesian convection models that include grainsize evolution we find that mobile lid convection can preserve heterogeneity at high mantle temperature conditions for much longer than previously thought because higher mantle temperatures lead to larger grainsizes in the lithosphere these larger grainsizes result in stronger plate boundaries that act to slow down surface and interior convective motions in competition with the direct effect temperature has on mantle viscosity our models indicate that mobile lid convection can preserve heterogeneity for 0 4 1 gyrs at early earth mantle temperatures when the initial heterogeneity has the same viscosity as the background mantle and 1 4 gyrs when the heterogeneity is ten times more viscous than the background mantle thus stagnant lid convection is not required to explain long term survival of early formed geochemical heterogeneities though these heterogeneities having an elevated viscosity compared to the surrounding mantle may be essential for their preservation

eoarchean to paleoproterozoic crustal evolution in the north china craton evidence from u pb and hf o isotopes of zircons from deep crustal xenoliths abstract granulite facies xenoliths entrained within igneous rocks shed a light on poorly understood yet critical questions about age origin and history of the lower continental crust here we present the first coupled in situ u pb lu hf and o isotope data for the precambrian zircons from fourteen deep crustal xenoliths from the north china craton the geochronological data demonstrates that the oldest lower crustal remnants formed at 3 82 ga and related magmatism continued until 3 55 ga the eo paleoarchean zircons with exception of one analysis have subchondritic initial hf isotopes with negative ehf t and are characterized by normal mantle to elevated 18o values 5 37 6 90 indicating a development of a low lu hf reservoir and hydrosphere crust interactions in the early earth magmatic zircons from lower crustal xenoliths of north china define a strongly episodic distribution of ages at 3 82 3 55 ga 2 7 ga 2 5 ga and 1 95 1 85 ga and a non linear hf isotope age array for nearly 2 gyr indicating episodic crustal generation and reworking through time the lower crustal xenoliths record a change in zircon o isotope compositions from a restricted range at 3 8 2 6 ga to more variable after 2 5 ga both heavier 18o up to 10 25 and lighter 18o low to 2 16 o isotope compositions than the normal mantle are observed in 2 5 ga magmatic zircons the secular change in zircon o isotopes documents an increase in recycling rate of surface derived materials including high 18o sediments weathered and altered rocks and low 18o altered oceanic crust into magma sources at the end of archean which in turn is possibly linked to modern style subduction processes and maturation of the crust at that time

early earth a new recipe for old crust the processes responsible for the growth of earth s first continents are enigmatic the geochemical signature of 4 billion year old rocks discovered in canada points to a key role for shallow magmatic processes above upwelling mantle rocks