an experimental study of the relationship between joint spacing and layer thickness abstract two methods for measuring joint spacing are described and compared the area method is a constant for a given outcrop area and is not affected by joint distribution within that area in contrast the line method depends on the location of the linear traverse two kinds of joint sets are distinguished on bedding surfaces 1 a poorly developed set represents the early stages of development when typical joint lengths are less than typical spacing 2 a well developed set represents later stages when lengths are much greater than spacing the area method is applicable to both poorly and well developed sets whereas the line method produces inconsistent results for poorly developed joint sets surface textures on many joints indicate point fracture origins and propagation parallel to bedding whereas most numerical model studies of spacing assume linear origins and propagation perpendicular to bedding the laboratory experiments described in this paper do not suffer from these restrictions an important concept confirmed during these experiments is fracture saturation when the applied strain reaches a certain value fracture spacing stops evolving and remains nearly constant these fracture sets are well developed spacing at saturation is a function of layer thickness but is independent of strain whereas spacing before saturation varies strongly with applied strain thus plotting spacing vs thickness and comparing the slopes of lines fit to such data for poorly developed joint sets in different layers is unlikely to be a diagnostic test for differences in material properties on the other hand spacing may be a sensitive indicator of strain for layers with poorly developed joint sets assessing fracture saturation is a lirst order consideration when gathering spacing data

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

paleo to mesoarchean ttg accretion and continental growth in the western dharwar craton southern india constraints from shrimp u pb zircon geochronology whole rock geochemistry and nd sr isotopes a multidisciplinary study involving field petrographic shrimp u pb zircon titanite ages whole rock geochemical and nd sr isotope data is presented for the peninsular gneisses and associated plutons forming core of the dharwar craton two major periods 3350 3280 ma and 3230 3200 ma of crustal growth through ttg accretion sub contemporaneous with greenstone volcanism are documented elemental and nd sr isotope data suggest that the ttgs originated by low and high pressure melting of heterogeneous mafic sources thickened island arc or oceanic plateau crust at different depths among the early accreted ttgs magmatic protoliths of low al gneisses formed by low pressure 10 12 kbar melting of a depleted mafic source at shallow levels island arc type crust with plagioclase amphibole ilmenite in the residue whilst high al gneisses were probably produced by high pressure melting 14 18 kbar of a chondritic or a less depleted mafic source with garnet amphibole pyroxene ilmenite in the residue base of island arc crust or thickened oceanic plateau the 3230 3200 ma trondhjemitic plutons marking the second accretion event derived by higher pressure 14 16 kbar melting of a mafic source island arc type crust with garnet plagiocalse hornblende ilmenite with a minor involvement of previously accreted ttgs this plutonism was coeval with a major phase of crustal scale diapiric overturn that had a major imprint on crustal reworking a later magmatic episode at ca 3100 ma produced granitic plutons derived from crustal melting and leucocratic gneisses derived from a depleted mafic source in the lower crust documentation of depleted shallow and deep mantle reservoirs during 3350 3280 ma magmatism implies massive differentiation of the mantle during an earlier episode of crust formation around 3800 600 ma as substantiated by nd model ages and published pb isotope data as well as u pb detrital zircon ages with in situ hf isotope data

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

a vestige of earth s oldest ophiolite a sheeted dike complex within the 3 8 billion year old isua supracrustal belt isb in southwest greenland provides the oldest evidence of oceanic crustal accretion by spreading the geochemistry of the dikes and associated pillow lavas demonstrates an intraoceanic island arc and mid ocean ridge like setting and their oxygen isotopes suggest a hydrothermal ocean floor type metamorphism the pillows and dikes are associated with gabbroic and ultramafic rocks that together make up an ophiolitic association the paleoarchean isua ophiolite complex these sheeted dikes offer evidence for remnants of oceanic crust formed by sea floor spreading of the earliest intact rocks on earth

continental growth and convergence related arc plutonism in the mesoarchaean evidence from the barberton granitoid greenstone terrain south africa u pb zircon ages from trondhjemitic gneisses of the badplaas domain located in the southwestern parts of the mesoarchaean barberton granitoid greenstone terrain of south africa document a hitherto largely unrecognized plutonic event the geochronological results indicate the addition of juvenile felsic crust over a period of 60 myr between ca 3290 and 3230ma and prior to the main collisional event in the granitoid greenstone terrain at 3230 ma the timing and duration of plutonism together with the structural and compositional heterogeneity of the badplaas domain suggest that the badplaas domain represents part of a convergence related magmatic arc on a regional scale the spatial and temporal relationships between plutonism metamorphism and deformation are interpreted to preserve an almost complete inventory of a mesoarchean arc trench system this includes 1 the 3290 3230ma convergence related magmatic arc 2 a largely coeval deposition of the back arc type volcano sedimentary 3260 3225ma fig tree group of the barberton greenstone belt 3 relics of the underplated crust preserved in high p low t rocks to the immediate east of the magmatic arc juxtaposed against 4 high t medium p rocks of the overlying plate along the major crustal structure of the inyoni shear zone these findings corroborate the notion that thermal and rheological conditions on the early earth supported at least locally crustal growth and the extraction of buoyant felsic crust along convergence related magmatic arcs

provenance of meso to neoproterozoic cover sediments at the ming tombs beijing north china craton an integrated study of u pb dating and hf isotopic measurement of detrital zircons and whole rock geochemistry abstract the north china craton ncc was subjected to an extensional regime after the luliang movement at 1 8 ga and then was covered by an extensive meso to neoproterozoic sedimentary succession namely the changcheng jixian and qingbaikou groups in ascending order we report age spectra for detrital zircons and monazites hf isotopic systematics of detrital zircons and whole rock chemical and nd isotopic compositions for sediments from the succession in the ming tombs area beijing one of the typical meso to neoproterozoic areas in the ncc detrital zircons of six sedimentary samples have two distinct age peaks at 2 52 ga and 1 85 ga there are some detrital zircons at 2 4 2 0 ga but none at 2 3 ga and only a few 2 7 ga the detrital zircon age spectra change with time sediments in the lower succession changcheng group and in the upper successions jixian and qinbaikou groups are dominated by significant detrital zircon populations of late neoarchean and late paleoproterozoic ages respectively the 2 5 ga detrital zircons of the changcheng group have ehf 2 5 ga values and tdm hf model ages mainly ranging from 2 to 7 and 2 8 to 2 7 ga respectively detrital monazites of a sample from the jixian group exhibit a major age peak between 1 95 and 1 80 ga with some data between 2 0 and 1 95 ga the sedimentary rocks of the changcheng group are characterized by high k2o contents mostly 7 09 15 20 and insignificant eu anomalies eu eu 0 71 1 16 they have tdm nd model ages ranging from 2 70 to 2 43 ga being older than the tdm nd ages 2 11 and 1 99 ga of sedimentary samples from the qingbaikou group based on a comparison with ages for the early precambrian 1 8 ga basement of the ncc it can be concluded that 1 the sediments of the meso to neoproterozoic cover were undoubtedly derived from the ncc itself or once neighboring terranes 2 variations in the detrital zircon age spectra from the lower to the upper successions reflect provenance evolution in that the lower crustal late paleoproterozoic rocks were exposed at the surface after the upper crustal late neoarchean rocks had already been eroded

mesoarchean assembly and stabilization of the eastern kaapvaal craton a structural thermochronological perspective 1 understanding the construction and stabilization of archean continental lithosphere has important implications for models of tectonic and thermal regimes in the early earth as well as for the subsequent evolution of continents we provide new constraints for the amalgamation and stabilization of the eastern kaapvaal craton in the vicinity of the barberton greenstone belt circa 3 3 3 1 ga isotope dilution thermal ionization mass spectrometry u pb geochronology and thermochronology are combined with mapping and structural analysis around the margins of this belt in order to constrain movement on major shear zones that separate high grade orthogneiss terranes from low grade supracrustal rocks and to relate this movement to the timing of differential exhumation new and existing data are consistent with accretion at 3 23 ga within an oblique subduction plate boundary manifested in the bgb as an asymmetric flower like structural geometry for 100 ma following terrane assembly transform boundary tectonics manifested partially as transtension on reactivated faults led to large scale juxtaposition of middle to lower crustal basement orthogneiss complexes against upper crustal greenstone sequences and episodic emplacement of granitic batholiths stabilization of this portion of archean lithosphere involved at least a three stage process 1 creation of a thick and rigid mantle lithosphere during crustal growth subduction and terrane assembly from circa 3 30 to 3 23 ga 2 generation of a rigid crust during strike slip and transtensional tectonics coupled with migration of granites and heat producing elements into the upper crust between circa 3 2 and 3 1 ga and 3 erosion and removal of the uppermost crust during regional peneplanation that lasted until circa 2 9 ga

evolution of archaean crust in the dharwar craton the nd isotope record abstract a synthesis of the available nd isotope data from the dharwar craton puts important constrains on antiquity of crust extent of basement terranes and events marking juvenile crustal addition and crustal recycling the craton is divided into two blocks namely eastern dharwar craton edc and western dharwar craton wdc nd model ages trace crust extraction as far as 3 5 ga back in both the blocks although rock record of such antiquity is yet to be found in the craton in wdc 3 35 3 0 ga is the most significant period of juvenile crustal addition as well as crustal recycling significantly depleted mantle 1 5 to 6 4 existed below the wdc as early as 3 35 ga which was probably refertilized in a later event from the resultant chondritic to slightly enriched mantle 0 3 to 0 4 juvenile mafic crust was added to the wdc during 2 9 2 6 ga a widespread crustal recycling event at 2 6 ga marks the last major event in this block in edc 3 3 3 0 ga granitoids occur as vestiges and show nd model ages higher than crystallizations ages suggesting recycling of palaeoarchaean crust during 2 7 2 5 ga extensive juvenile magmatism took place in edc from variably depleted mantle 1 4 to 5 6 which is later than the global 2 7 ga peak of crustal growth attendant crustal recycling destroyed most of the earlier crust distribution of nd model ages suggests in contrast to general believe that palaeoarchaean crust was not only abundant in the wdc but also widespread in an area extending from the wdc edc boundary to further 60 150 km east up to the hungund ramagiri kolar hrk belt this belt appears to be a major crustal boundary along which terranes of different nd isotope signatures were amalgamated during the late neoarchaean by horizontal accretion major neoarchaean crustal recycling events documented in the dharwar craton are missing from nd isotope record of the eastern block eb of the north china craton this fact does not support correlation between the two cratons as suggested by some authors

dating the oldest terrestrial rocks fact and fiction abstract this paper is a review of well documented geochronological and related isotopic evidence for the age and total crustal residence time of the oldest known terrestrial rocks mainly using rb 1bsr sm 1bnd and u 1bpb decay schemes the oldest known stable sialic crust of true continental character predominantly composed of calc alkaline orthogneisses was produced during major mantle differentiation some 3 7 3 5 ga ago and is represented on several continents enclaves of older supracrustal rocks as well as a major remnant of a volcano sedimentary sequence of greenstone belt affinity dated at 3 8 ga are preserved within the surrounding gneisses in southern west greenland recent ionmicroprobe evidence for 4 1 ga old detrital zircon grains in a younger early archaean metasedimentary sequence in western australia is also discussed several cases are reviewed where claims for early archaean 3 4 ga dates are believed to be erroneous and to result from oversimplistic and incorrect interpretation of isotopic data some of the geological and geochemical mechanisms which have given rise to claims for spuriously old sm 1bnd u 1bpb and pb pb dates are critically discussed to supplement published data we present several new sets of whole rock sm 1bnd and pb pb isotopic data from west greenland labrador zimbabwe and eastern india relevant to the overall topic of this review

sm nd isotopic and geochemical study of the archean tonalite amphibolite association from the eastern indian craton we report the results of a sm nd isotopic major element and rare earth element ree study of the older metamorphic group omg tonalite amphibolite association of the eastern indian craton the older metamorphic tonalite gneisses omtg have been previously dated to be 3 8 ga using sm nd isotope systematies and 3 2 3 4 ga by rb sr and pb pb dating the results of this study indicate that the protoliths of the omg amphibolites are 3 3 ga isochron age 3 30 0 06 ga nd 0 9 0 7 and therefore the omtg which intrude into the associated amphibolites cannot be any older than 3 3 ga the amphibolites display light ree enrichment ce yb n 2 2 6 7 la 30 100 x chondrite and nearly flat heavy ree patterns tb lu n 1 2 1 9 the basaltic parents of the amphibolites were probably generated by the partial melting of a spinel lherzolite mantle strong linear relationships between the amphibolites and tonalites in 147sm 144nd 143nd 144nd space isochron age 3 29 0 04 ga nd 0 8 0 8 imply that they are genetically related the tonalites display fractionated ree patterns la 100 300 x chondrite with moderate heavy ree depletions tb lu n 1 9 3 4 the isotopic major element and ree data are consistent with the derivation of the omtg from partial melting of omg amphibolites or equivalent rocks at amphibolegarnet stabilization depths an initial nd t value of 0 9 0 7 for the amphibolites indicates the presence of a slightly depleted mantle source at 3 3 ga with 147sm 144nd between 0 20 and 0 22 it is suggested that the growth of continental crust in the eastern indian craton occurred in response to magmatic underplating in a plume setting

geochemical and numerical constraints on neoarchean plate tectonics this paper discusses early neoarchean 2 8 2 7 ga plate tectonics by integrating knowledge from new geochemical observations and numerical models based on a geochemical dataset of 295 granitoid samples from the karelian and kola cratons of the fennoscandian shield we divide neoarchean juvenile extracted from oceanic crust or mantle granitoids into three groups 1 low hree heavy rare earth elements ttgs tonalite trondhjemite granodiorite high sio2 low mg low hree higher sr lower ybn and higher nb ta 2 high hree ttgs slightly lower range of sio2 larger range of mgo contents and higher cr and ni contents high hree lower sr higher ybn and lower nb ta and 3 high ba sr sanukitoids medium hree high mg and high k ba sr the main difference between the low and high hree groups lies in their pressure sensitive element contents which indicates high pressure melting conditions for the low hree group and low pressure conditions for the high hree group a possible tectonic scenario for the genesis of the two groups is an incipient hot subduction underneath a thick oceanic plateau protocrust melting in the lower part of thick basaltic oceanic crust could produce ttgs of the low hree type whereas low pressure melting of subducting slab and possible interactions with the mantle wedge at shallow depths would be capable of generating high hree ttgs the third group of archean high ba sr sanukitoids was formed after the ttgs their low sio2 and high mg k ba sr contents suggest an origin by melting in an enriched metasomatized mantle source probably as a result of a slab breakoff following a continental collision or attempted subduction of a thick oceanic plateau ttg protocontinent such hypothesis is supported by numerical modeling results that suggest an increased occurrence of slab breakoff in the archean which locally increased temperatures within the mantle wedge more frequent breakoff resulted because subducting plates were weaker due to rheologically thinner lithosphere and a thicker basaltic crust and tensile stresses within the subducting plate were larger due to a thick crust that transforms from buoyant basalt to dense eclogite

hf isotopic measurements on barberton komatiites effects of incomplete sample dissolution and importance for primary and secondary magmatic signatures komatiites from a single lava flow carl s flow in the schapenburg greenstone remnant near barberton in south africa yielded very different hf isotope compositions depending on the manner in which the samples were treated analytically data from samples dissolved in savillex beakers define a statistically significant mswd 0 77 isochron with an age of 3 4 0 2 ga which is identical within error to the probable eruption age and an initial ehf of 1 3 analyses of samples dissolved in bombs on the other hand plot on a linear array which if interpreted as an isochron gives an age of 2 9 ga their initial 176hf 177hf calculated using the probable eruption age of 3 45 ga are positive and variable ranging from ehf of 1 3 to 6 3 hf contents are much lower and lu hf ratios systematically higher in the savillex beaker dissolved batch of komatiites indicating that a hf rich phase zircon or baddeleyite remained undissolved in these samples the negative ehf of the savillex beaker dissolved samples is attributed to interaction with crust derived fluids that affected the komatiite soon after its eruption the anomalously low isochron age of the bomb dissolved samples is due to the presence of a single olivine cumulate sample that appears to have lost hf during post emplacement alteration without this sample initial ehf values cover a much narrower range between 4 3 and 6 3 which is believed to be that of the mantle source of the komatiites the combination of highly positive ehf and lower than chondritic lu hf indicates that the mantle source of the carl s flow was garnet enriched for a substantial period of time prior to komatiite formation and that garnet was retained in the source during the melting event that eventually produced the komatiite magma

hf nd isotope evidence for a transient dynamic regime in the early terrestrial mantle modern basalts have seemingly lost all memory of the primitive earth s mantle except for an ambiguous isotopic signal observed in some rare gases1 2 although the earth is expected to have reached a thermal steady state within several hundred million years refs 3 4 of accretion it is not known how and when the initial chemical fractionations left over from planetary accretion and perhaps a stage involving a magma ocean were overshadowed by fractionations imposed by modern style geodynamics because of the lack of samples older than 4 gyr this early dynamic regime of the earth is poorly understood here we compare published hf nd isotope data on supracrustals from isua greenland with similar data on lunar rocks and the snc martian meteorites and show that about 3 8 gyr ago the geochemical signature of the archaean mantle was partly inherited from the initial differentiation of the earth the observed features seem to indicate that the planet at that time was still losing a substantial amount of primordial heat the survival of remnants from an early layering in the modern deep mantle may account for some unexplained seismological thermal and geochemical characteristics of the earth as observed today

nd isotopic constraints on crustal formation in the north china craton abstract recent tectonic analysis suggests that the north china craton consists of two archean continental blocks called the eastern and western blocks separated by the paleoproterozoic trans north china orogen although the published geochronological data are not sufficient to constrain the detailed tectonothermal evolution of the craton the available nd isotopic data show some important differences in nd model ages between the tectonic units the eastern block shows two main nd model age peaks one between 3 6 and 3 2 ga and the other between 3 0 and 2 6 ga limited nd isotopic data from the western block show a large range of model ages between 3 2 and 2 4 ga these differences are consistent with the recently proposed model the nd isotopic data from mantle derived mafic rocks indicate that the mantle beneath the north china craton was depleted in the archean consistent with major crustal growth during this period in the paleoproterozoic however the mantle derived mafic rocks show negative end t values implying crustal contamination this may have resulted from subduction and collision between the eastern and western block implying that the mechanisms of crustal formation and evolution may have been different between the archean and paleoproterozoic the north china craton was re activated by addition of mantle derived magma into the lower crust in the late mesozoic resulting in rejuvenation of the lower crust this indicates that underplating is also an important mechanism for continental addition although in this case it may not equate to crustal growth since it was preceded by removal of lithospheric mantle and possible some lower crust

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

207pb 206pb zircon ages and the evolution of the singhbhum craton eastern india an ion microprobe study1this paper is dedicated to prof a k saha one of the senior co authors who passed away after submission of the paper 1 abstract we used an ion microprobe to measure 207 pb 206 pb ages of single zircons from four key lithologies from the singhbhum craton of eastern india detrital zircons from the oldest recognized sedimentary sequence of the craton the older metamorphic group yielded 3 5 3 6 ga ages indicating that crustal formation had already been initiated by 3 6 ga in this region this data also established an older limit of 3 5 ga for the age of sediment deposition the first appearance of extensive continental crust in the singhbhum region marked by the emplacement of the older metamorphic tonalite gneisses is dated at 3 44 ga our data from both of these units reveal that a 3 2 ga event induced overgrowths within each of these pre existing zircon populations we determine an age of 3328 7 ma for the singhbhum granite phase ii the most widespread of this granite s three phases the emplacement of the two phases of the mayurbhanj granite marking the youngest major episode of acid plutonism in this craton took place in rapid succession at 3 1 ga marking the broad scale stabilization of this terrain

formation of paleoarchean mesoarchean na rich ttg and k rich granitoid crust of the singhbhum craton eastern india constraints from major and trace element geochemistry and sr nd hf isotope composition abstract the singhbhum craton in eastern india is underlain by paleo to mesoarchean tonalites trondhjemites granodiorites ttgs and granites that formed in two pulses at 3 45 3 44 ga and 3 35 3 32 ga followed by potassic granite activity at 3 1 ga the ttgs are metaluminous to slightly peraluminous and highly evolved with low ni cr and mgo low heavy ree concentrations sr y and eu gd and high nb ta and y concentrations indicate they belong to the low hree medium to low pressure ttg type their compositions are similar to ttgs derived from chondritic source with amphibolitic residue consistent with rutile free and garnet bearing metabasic source the granites have similar ree and multielement patterns as the ttgs albeit with pronounced negative eu sr and ti anomalies high k2o na2o higher heavy ree concentration low sr y and eu gd and negative eu anomalies of the 3 05 ga mayurbhanj potassic granites are consistent with shallow plagioclase bearing crustal sources the increasing k2o na2o of the granitoids from 3 45 ga to 3 05 ga is related to gradual thickening of the continental crust and increasing contributions of intracrustal melts in younger granitoids neodymium and hf isotope data indicate the 3 45 3 44 ga granitoids have juvenile as well as eoarchean ttg crust like signature the 3 35 3 32 ga granitoids were produced by partial melting of the same mafic crust that gave rise to the 3 45 3 44 ga ttgs with some components requiring up to 50 crustal contribution the mayurbhanj granites represent intracrustal melts of more potassic members of the paleoarchean ttgs the trace element characteristics of the granitoids reflect melt generation at different depths in a tectonic setting producing both ttgs as well as granites contemporaneously plausible geodynamic settings could be very hot subduction or dripduction regime involving shallow melting of delaminated mafic crust producing the 3 45 ga juvenile ttgs and minor granites large scale delamination and melting of mafic lower crust and felsic proto crust in a thickened crustal pile in a subduction like regime could have produced granitoids with juvenile as well as recycled isotopic signatures the nd hf isotope data indicate that the paleoarchean upper mantle was moderately depleted in response to a spurt in paleoarchean continental growth

in situ u pb geochronology and hf isotope analyses of the rayner complex east antarctica in situ zircon u pb and hf isotopic analysis via laser ablation microprobe inductively coupled plasma mass spectrometer lam icpms of samples from kemp and macrobertson lands east antarctica suggests that the kemp land terrane evolved separately from the rest of the rayner complex prior to the ca 940 ma rayner structural episode several archaean metamorphic events in rocks from western kemp land can be correlated with events previously reported for the adjacent napier complex recently reported ca 1 600 ma isotopic disturbance in rocks from the oygarden group may be correlated with a charnockitic intrusion in the stillwell hills before ca 1 550 ma despite being separated by some 200 km t hf dm ages indicate felsic orthogneiss from rippon point the oygarden group havstein island and the stillwell hills share a ca 3 660 3 560 ma source that is indistinguishable from that previously reported for parts of the napier complex more recent additions to this crust include proterozoic charnockite in the stillwell hills and the vicinity of mawson station these plutons have distinct 176hf 177hf ratios and formed via the melting of crust generated at ca 2 150 2 550 ma and ca 1 790 1 870 ma respectively

resetting of smnd systematics during metamorphism of 3 7 ga rocks implications for isotopic models of early earth differentiation abstract new sm nd isotopic results for sixteen samples from the garbenschiefer amphibolite unit of the 3700 ma isua supracrustal belt of west greenland are reported the data illustrate the difficulty in identifying primary nd t values in poly metamorphic and poly deformed rock sequences the investigated samples are interpreted as derived from a mg and al rich igneous protolith which can be argued to have been essentially isotopically homogeneous at least on the outcrop scale at the time of its emplacement however the data do not give uniform nd t values even from single outcrops the wide variation of nd t which we obtain 1 0 to 5 0 demonstrates that the sm nd systematics of the samples were disturbed by one or more metamorphic event s which took place some considerable time after the original crystallization of the isua belt a sm nd isochron age of 2849 116 ma has been obtained on plagioclase and amphibole separates which represent the oldest metamorphic assemblage now preserved at isua this result indicates that the earliest amphbolite facies metamorphism of the isua belt for which pressure and temperature conditions can be obtained p 5 kbar t 500 600 c is late archaean and not early archaean as had been assumed earlier by some authors 2850 ma is also thought to represent the main time of disturbance of the sm nd system in the garbenschiefer amphibolite unit a consideration of available isotopic data suggests that many of the rocks from isua underwent some nd isotopic re equilibration and or fractionation of their 147 sm 144 nd ratio during the 2850 ma metamorphic episode in these rocks the computed nd t are apparent values only and need not represent either the primary isotopic composition of their protoliths or early archaean mantle values only nd rich rocks 20 ppm such as some felsic schists and the grey gneisses adjacent to the belt are arguably sufficiently robust to retain nd t values which are close to their original composition these average 2 0 which may represent a close approximation of the nd value of the earth s mantle at 3800 ma the results presented in this paper show that great care should be exercised in the use of sm nd data from very early archaean rocks as constraints for isotopic models of early earth s evolution particularly in terranes which have been affected by several metamorphic episodes

length displacement scaling and fault growth abstract following an earthquake in a fault zone commonly the co seismic rupture length and the slip are measured similarly in a structural analysis of major faults the total fault length and displacement are measured when possible it is well known that typical rupture length slip ratios are generally orders of magnitude larger than typical fault length displacement ratios so far however most of the measured co seismic ruptures and faults have been from different areas and commonly hosted by rocks of widely different mechanical properties which have strong effects on these ratios here we present new results on length displacement ratios from 7 fault zones in holocene lava flows on the flanks of the volcano etna italy as well as 10 co seismic rupture length slips and compare them with fault data from iceland the displacement and slip data from etna are mostly from the same fault zones and hosted by rocks with largely the same mechanical properties for the co seismic ruptures the average length is 3657 m the average slip 0 31 m and the average length slip ratio 19 595 for the faults the average length is 6341 m the average displacement 73 m and the average length displacement ratio 130 thus the average rupture slip ratio is about 150 times larger than the average length displacement ratio we propose a model where the differences between the length slip and the length displacement ratios can be partly explained by the dynamic young s modulus of a fault zone being 101 2 times greater than its static modulus in this model the dynamic modulus controls the length slip ratios whereas the static modulus controls the length displacement ratio we suggest that the common aseismic slip in fault zones is partly related to adjustment of the short term seismogenic length slip ratios to the long term length displacement ratios fault displacement is here regarded as analogous to plastic flow in which case the long term displacement can be very large so long as sufficient shear stress concentrates in the fault

priscoan 4 00 4 03 ga orthogneisses from northwestern canada by samuel a bowring and ian s williams discussion

secular changes in tonalite trondhjemite granodiorite composition as markers of the progressive cooling of earth archean tonalite trondhjemite granodiorite associations ttg are classically thought to generate through partial melting of hydrous metabasalts however the chemical composition of the least differentiated ttg parental magmas evolved from 4 0 to 2 5 ga during this interval the mg as well as the ni and cr contents increased which is interpreted as reflecting increased interactions between felsic melts generated by metabasalt melting and mantle peridotite similarly cao na2o and sr also increased over time thus reflecting an increase in the abundance of plagioclase in the melt residue the presence or absence of residual plagioclase is interpreted in terms of melting depth the demonstrated interaction between ttg parental magmas and the mantle rules out their genesis by fusion of previously underplated metabasalt and favors the melting of subducted slab material at 4 0 ga earth s geothermal gradient was sufficiently high to allow slab melting at shallow depths where plagioclase was stable consequently due to the small thickness of the overlying mantle wedge felsic magmas interacted little with the mantle at 2 5 ga however owing to lower geothermal gradients the melting depth was greater and plagioclase became no longer stable in the thick mantle wedge overlying the subducted slab as a result felsic magmas reacted strongly with the mantle peridotite the changes of ttg composition during archean time can be thus interpreted as reflecting the progressive cooling of earth

u pb and hf isotope records in detrital and magmatic zircon from eastern and western dharwar craton southern india evidence for coeval archaean crustal evolution abstract u pb and lu hf isotopic records in detrital and magmatic zircons from the eastern and the western dharwar craton edc and wdc suggest presence of 3 4 ga crustal components in both the blocks magmatic zircon from the western block has ages ranging between 3 1 and 3 2 ga while those from the eastern block show a bi modal age distribution with records of 3 0 3 2 ga protolith components as well as 2 7 and 2 5 ga metamorphic overgrowths detrital and magmatic zircons from granitoid conglomerate clasts of the western dharwar block show initial hf values consistent with models for the early archaean mantle suggesting their juvenile character lower hf values in younger as well as detrital zircons from eastern dharwar block reflect importance of reworking of 3 2 ga crust the combined u pb and hf isotope study strengthens suggestions for contemporaneous crust formation processes in both the western and eastern parts of the dharwar craton

a smnd and pb isotope study of archaean greenstone belts in the southern kaapvaal craton south africa abstract a sm nd and pb study on a wide variety of lithologies in archaean greenstone belt fragments in the southern kaapvaal craton reveals a complex petrogenetic history the fragments are important because they represent a 350 km transect through the craton south of barberton to its southern margin the commondale greenstone belt yields a precise sm nd age of 3334 18ma on an exceptionally well preserved peridotite suite of komatiitic affinity the wide range of sm nd from 0 6 to 1 0 is attributed to the unusual occurrence of orthopyroxene in the spinifex bearing rocks a considerably younger age of about 3 2 ga is suggested for the nondweni greenstone belt close to the southern margin of the craton on the basis of separate sm nd isochrons on individual lithologies ranging from komatiite through komatiitic basalt and basalt to felsic volcanic rocks the most precise sm nd age for the greenstone belt of 3208 87ma is based on a combined mineral whole rock isochron for komatiitic basalts but the possibility cannot be excluded that this represents a later metamorphic age on the basis of the present study the greenstone belts appear to have been emplaced at progressively younger ages towards the southern margin of the craton of greater significance is that irrespective of age the individual lithologies of the nondweni sequence have isotopically distinct initial ratios combining isotopic data from these lithologies invariably yields apparent older ages the isotopic characteristics are consistent with a model of progressive contamination by older sialic crust thought to be the basement to the nondweni sequence

the role of friction and secondary flaws on deflection and re initiation of hydraulic fractures at orthogonal pre existing fractures summary in this study we explore the nature of plane strain hydraulic fracture growth in the presence of pre existing fractures such as joints without or with secondary flaws the 2 d plane strain fracture studied can be taken as a cross section through the short dimensions of an elongated 3 d fracture or as an approximate representation of the leading edge of a 3 d fracture where the edge curvature is negligible the fluid driven fracture intersects a pre existing fracture to which it is initially perpendicular and is assumed not to immediately cross but is rather deflected into the pre existing fracture the intersection results in branching of the fracture and associated fluid flow into the pre existing fracture further growth results in opening and frictional sliding along the pre existing fracture fracture propagation in an impermeable homogeneous elastic medium and fluid invasion into a pre existing fracture are both driven by an incompressible newtonian fluid injected at a constant rate the frictional stress on the surfaces of pre existing fractures is assumed to obey the coulomb law the governing equations for quasi static fluid driven fracture growth are given and a scaling is introduced to help identify important parameters the displacement discontinuity method and the finite difference method are employed to deal with this coupling mechanism of rock fracture and fluid flow in order to account for fluid lag a method for separately tracking the crack tip and the fluid front is included in the numerical model numerical results are obtained for internal pressure frictional contact stresses opening and shear displacements and fluid lag size as well as for fracture re initiation from secondary flaws after fracture intersection the hydraulic fracture growth mode changes from tensile to shearing this contributes to increased injection pressure and to a reduction in fracture width in the presence of pre existing fractures the fluid driven cracks can be arrested or retarded in growth rate as a result of diversion of fluid flow into and frictional sliding along the pre existing fractures frictional behaviour significantly affects the ability of the fluid to enter or penetrate the pre existing fracture only for those situations where the fluid front is within a certain distance from the intersecting point importantly fluid penetration requires higher injection pressure for frictionally weak pre existing fractures fracture re initiation from secondary flaws can reduce the injection pressure but re initiation is suppressed by large sliding on pre existing fractures that are frictionally weak

quantification of fracture networks in non layered massive rock using synthetic and natural data sets abstract two quantitative measures correlation dimension and maximum lyapunov exponent are investigated for quantifying fracture spacing in massive non layered rock the correlation dimension a measure of fractal dimension characterizes some aspects of fracture distribution relative to other fractal methods used in structural geology the correlation dimension provides a more rigorous mathematical calculation of the fractal dimension and is particularly robust in cases where data is limited the maximum lyapunov exponent calculation also provides quantification of observed distributions of fracture spacing emphasizing variations in adjacent spacing measurements the two methods are tested on two different synthetic data sets to elucidate their usefulness in distinguishing between different distributions and their sensitivity to population size average fracture spacing and standard deviation both methods provide accurate results for small population sizes and for a range of average fracture spacings these methods are then used together to quantify two different fracture sets in the granitoids of the sierra nevada batholith the results indicate that the different fracture sets inferred to form from different tectonic processes have distinctive and quantifiable differences in patterns of fracture spacing

archaean ttgs as sources of younger granitic magmas melting of sodic metatonalites at 0 6 1 2 gpa two natural low k2o na2o ttg tonalitic gneisses one hornblende bearing and the other biotite bearing were partially melted at 0 8 1 2 gpa fluid absent the chief melting reactions involve the breakdown of the biotite and hornblende the hornblende tonalite is slightly less fertile than the biotite tonalite but melt volumes reach around 30 at 1 000 c this contrasts with results of most previous work on more potassic ttgs which generally showed much lower fertility though commonly producing more potassic melts garnet is formed in biotite bearing tonalitic protoliths at p 0 8 gpa and at 1 0 gpa in hornblende bearing tonalitic protoliths all fluid absent experiments produced peraluminous granitic to granodioritic melts typically with sio2 70 wt for the biotite tonalite increasing t formed progressively more melt with progressively lower k2o na2o however the compositions of melts from the hornblende tonalite do not vary significantly with t with increasing p melts from the biotite tonalite become less potassic due to the increasing thermal stability of biotite for the hornblende tonalite again there is no consistent trend fluid absent melting of sodic ttgs produces melts with insufficient k2o to model the magmas that formed the voluminous late potassic granites that are common in archaean terranes reconnaissance fluid present experiments at 0 6 gpa imply that h2o saturated partial melting of ttgs is also not a viable process for producing magmas that formed these granites the protoliths for these must have been more potassic and less silicic nevertheless at granulite facies conditions sodic ttgs will produce significant quantities of broadly leucogranodioritic melt that will be more potassic than the protoliths upward abstraction of this melt would result in some lile depletion of the terrane younger k rich magmatism is unlikely to represent recycling of ttg crust on its own and it seems most likely that evolved crustal rocks and or highly enriched mantle must be involved

possible juvenile palaeoarchaean ttg magmatism in eastern india and its constraints for the evolution of the singhbhum craton high precision shrimp u pb zircon dating yields a late palaeoarchaean age 3290 8 6 ma for a large unmetamorphosed weakly peraluminous ttg body the keonjhargarh bhaunra pluton in the singhbhum craton of eastern india one inherited subhedral zircon grain gave a concordant age of 3495 9 5 3 ma and nd isotope characteristics show a juvenile trend with end t 0 and t dm 3395 3453 ma the data support a model of typical archaean crustal evolution until late palaeoarchaean times for the singhbhum craton which is in contrast to the more southerly bastar craton where palaeoarchaean non ttg granites have been identified these data demonstrate the diachronous development of continental crustal blocks now forming the basement of the eastern and central peninsular of india

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

growth of granite greenstone terranes at convergent margins and stabilization of archean cratons abstract archean granite greenstone terranes represent juvenile continental crust formed in a variety of plate tectonic settings and metamorphosed through a complex series of structural and magmatic events most archean granite greenstone terranes appear to have acquired their first order structural and metamorphic characteristics at convergent plate margins where large accretionary wedges similar in aspect to the chugach makran and altaids grew through offscraping and accretion of oceanic plateaux oceanic crustal fragments juvenile island arcs rifted continental margins and pelagic and terrigenous sediments buoyant slabs of parts of archean oceanic lithosphere may have been underplated beneath these orogens forming thick crustal roots characterized by interleaving between the depleted slabs and undepleted asthenosphere back stepping of the subduction zones after accretion of plateaux and arcs caused the arcs magmatic fronts to migrate trenchward through the accretionary wedges dehydration of the subducting slabs hydrated the mantle wedges below the new arcs and generated magmas sanukitoid suite in the mantle wedge whereas other magmas tonalite trondhjemite granodiorite or ttg suite appear to have been generated by melting of hot young subducted slabs eventual collision of these juvenile orogens with other continental blocks formed anatectic granites then thickened the crust beyond its ability to support its own mass which initiated gravitational collapse and decompressional release of syn to late tectonic granitoids from wedges of fertile mantle trapped between underplated oceanic lithospheric slabs and aided in the cratonization of the granite greenstone terranes deeply penetrating structural discontinuities such as shear zones and sutures provided pathways for fluids and granitoids to migrate into the mid and upper crust forming ore deposits and plutons most preserved granite greenstone terranes have been tectonically stable since the archean and form the cratonic interiors of many continents

a review of hadean to neoarchean crust generation in the singhbhum craton india and possible connection with pilbara craton australia the geochronological perspective abstract the precambrian singhbhum craton sc of the eastern india holds a prolonged record of sporadic crust generation and evolution starting from as ancient as the hadean till late archean like every craton episodic nature of crust generation in the sc has been indicated by several workers based on numerous stratigraphic and geochronological evidence long before however it is only in the past decade that a growing zircon u pb hf database has resolved its evolution in detail such as the timing of juvenile crust versus reworking events integration of over a thousand geochronological data n 1558 from hadean 4 ga till the end of archean 2 5 ga exhibits multiple prominent crustal growth peaks during c 3320 ma 3300 ma and 2780 ma combined with moderate peaks at c 3460 ma 3380 ma and 2820 ma integral crust modelling of hf isotopezircon indicates major juvenile crust addition during the periods c 3760 3640 ma 3620 3440 ma while prominent age frequency peak at c 2780 ma defined by u thbaddeyelite ages of regional mafic ultramafic dyke swarm provides a direct proxy of large scale juvenile crust generation the interplay between the growth of new crust and reworking of older crust combined with multiphase deformation and metamorphic events till the end of archean constructed the critical juxtaposition of diverse crustal components of this important craton constraining crust generation and deformation events from an updated geochronological database of this craton reveal remarkable similarity in crustal growth pattern with the archean pilbara craton australia moreover paleomagnetic studies of the neoarchean mafic dyke swarms of both the cratons indicate similar paleo positions therefore compiled field geochronological paleomagnetic parameters invoke the possibility that the archean singhbhum and pilbara cratons might have been connected and shared similar crustal history at least till the neoarchean era

origin and early growth rate of continents abstract data suggest that early continents formed over mantle sinks and grew rapidly between 2 7 and 3 0 ga ago nd isotopic results indicate that archean tonalites which are largely responsible for early continental growth are derived chiefly from ancient depleted mantle sources trace element and oxygen isotope data however indicate enriched sources for such tonalites these constraints can be explained if the mantle is fractionated into depleted dmr and enriched emr reservoirs during early degassing of the earth 4 4 ga blocks of emr are recycled back into the mantle and the emr dmr mixtures serve as sources for archean basalts these basalts are enriched in incompatible elements by deep sea alteration metasomatism in sinks or by other mantle processes and then undergo wet partial melting to produce tonalite magmas enrichment must occur not more than 150 ma prior to tonalite production basalt plateaus accumulate at sinks and as cooling continues geotherms at sinks pass into the eclogite stability field dense eclogite roots form on the basalt plateaus which pulls them into the mantle between 2 7 and 3 0 ga ago resulting in rapid tonalite production and thus in rapid continental growth

magmatic and metamorphic history of paleoarchean tonalite trondhjemite granodiorite ttg suite from the singhbhum craton eastern india texturally controlled dating of zircon from paleoarchean tonalite trondhjemite granodiorites of the older metamorphic tonalitic gneisses and the singhbhum granite batholith phases i ii and iii from the singhbhum craton in eastern india reveals a polycyclic evolution of the archean crust the granitoid suites were emplaced in two pulses at 3 45 3 44 ga and 3 35 3 32 ga tonalites and trondhjemites of the older metamorphic tonalitic gneisses were emplaced at ca 3 45 3 44 ga together with phase iii of the singhbhum granite pluton while granites belonging to the older metamorphic tonalitic gneisses were emplaced at ca 3 35 3 32 together with phase i and phase ii of the singhbhum granite pluton both crustal units underwent an early phase of relatively high grade metamorphism at 3 30 3 28 ga followed by extensive fluid induced alteration during low grade metamorphism at 3 19 3 12 ga and 3 02 2 96 ga the two units have also been marginally affected at ca 2 52 ga and 1 06 ga by major metamorphic events in the north singhbhum mobile belt and the singhbhum shear zone at the northern margin of the craton the zircon grains in granites have inherited cores with ages of ca 3 61 ga and 3 46 3 41 ga and with well developed oscillatory growth zonation which suggests the granitic magmas were derived by partial melting of an igneous precursor or sedimentary rocks derived from an igneous source the emplacement of the expansive granitoids belonging to the older metamorphic tonalitic gneisses and the singhbhum granite was synchronous with the amphibolite facies metamorphism ca 3 32 ga of older meta igneous and metasedimentary rocks belonging to the older metamorphic group major felsic crust formation in the craton occurred in a narrow time interval between 3 46 and 3 32 ma with minor contributions of material as old as 3 6 ga the complex polycyclic evolution of the paleoarchean crust in the singhbhum craton can account for the wide range of often disparate ages obtained using whole rock isochron dating techniques with some of the isochron dates being geologically meaningful while others representing mixing lines or disturbance of the isotopic systems during metamorphism

reworking of hadean crust in the acasta gneisses northwestern canada evidence from in situ lu hf isotope analysis of zircon abstract the acasta gneiss complex of northwestern canada comprises early archean orthogneisses and includes the oldest known terrestrial rocks 3 94 4 03 ga previous zircon geochronological studies revealed the presence of zircon xenocrysts with ages up to 4 2 ga in the oldest rocks indicating that the source of early archean granitoids contained hadean 4 03 ga crust in this study we have determined the lu hf isotopic compositions of zircon grains extracted from the early archean acasta gneisses to evaluate the extent of the hadean crust contribution to the formation of these granitoids analyses of the lu hf isotopes were carried out using in situ laser ablation inductively coupled plasma mass spectrometry combined with cathodoluminescence images of the internal structure of the grains two ca 3 59 ga granitoids have e hf t of 2 4 2 2 and 3 2 2 5 suggesting that the source of the granitoids was extracted from the mantle as far back as 3 8 ga this is consistent with the presence of abundant zircon xenocrysts with ages up to 3 9 ga the hf isotopic composition of a 3 72 ga granitoid is significantly less radiogenic with an e hf t of 6 1 2 5 this indicates that the granitoid formed by remelting of very old crust the e hf t values for the two oldest rocks the ca 3 97 ga tonalites are 1 2 3 3 and 3 3 1 7 respectively indicating that even the oldest known granitoids contain reworked older crustal materials these results suggest that hadean crust had significantly contributed to the genesis of some of the early archean acasta granitoids

crustal geodynamics from the archaean bundelkhand craton india constraints from zircon u pb hf isotope studies a comprehensive study based on u pb and hf isotope analyses of zircons from gneisses has been conducted along the western part babina area of the e w trending bundelkhand tectonic zone in the central part of the archaean bundelkhand craton 207 pb 206 pb zircon ages and hf isotopic data indicate the existence of a felsic crust at 3 59 ga followed by a second tectonothermal event at 3 44 ga leading to calc alkaline magmatism and subsequent crustal growth the study hence suggests that crust formation in the bundelkhand craton occurred in a similar time frame to that recorded from the singhbhum and bastar cratons of the north indian shield

tectonic history of the barberton greenstone belt south africa 490 million years of archean crustal evolution rocks of the 3570 3080 ma barberton greenstone belt record a geological history of at least 490 million years m y and were affected by regional thermal perturbations for over 1500 m y small tectonic wedges of 3570 3511 ma schistose tonalite are the oldest rocks yet recognized but little is known about their tectonothermal history the earliest well recognized tectonothermal events 3490 3450 ma represent mid ocean ridge like processes these were followed by two periods of arc related and trench related processes separated by 160 m y the first 3445 3416 ma recording an intraoceanic suprasubduction like environment and the second 3260 3225 ma intra arc and interarc like processes culminating in arc amalgamation accretion like convergent processes dominated between 3230 ma and 3080 ma overlapping with the assembly of the kaapvaal craton at 3080 ma the barberton greenstone belt experienced a sudden shift from dominantly convergent to dominantly transtensional tectonism and was mineralized with mesothermal gold along a regional shear system soon afterward transtension became distributed across the entire kaapvaal craton heralding the onset of formation of the witwatersrand basin by 3070 ma

the conundrum of coal bed thickness a theory for stacked mire sequences maximum thicknesses for modern peats have been documented only up to about 20 m whereas coal beds can often be as much as 90 m thick since peat is expected to compact appreciably during burial there appears to be no modern analogue for the processes which formed thick coal beds and this seems to challenge the law of uniformitarianism however the conundrum of coal bed thickness can be resolved by identification of discontinuities in coal beds coal beds are generally treated as single entities created by a continuous process of peat deposition it is more likely however that most thick coal beds are composed of multiple paleo peat bodies stacked one upon another rather than a single paleo peat body it is suggested that there are three types of bounding surfaces seen in modern peat bodies that can be used to distinguish individual paleo peats in coal beds these bounding surfaces can be recognized both in the field and microscopically each of these surfaces represents cessation or at least ext

a new technique for identifying rock avalanche sourced sediment in moraines and some paleoclimatic implications moraine chronologies are widely used to infer local climate change events and to correlate these events globally based on the assumption that moraines always reflect climatic drivers however this assumption is unreliable in tectonically active terrain because moraines can also be formed by large landslide rock avalanche deposits on glaciers these can affect glacier motion and cause moraines to form while requiring no climate variation and can thus cause significant errors in climatic signals extracted from moraine chronologies to eliminate such errors requires a method for identifying moraines that have been influenced by rock avalanches herein we present and test a new diagnostic technique that unambiguously identifies rock avalanche sediments using newly discovered fine sediment signatures characteristic of rapid high stress comminution we test this technique on holocene moraines in the southern alps new zealand which have previously been interpreted as climatic indicators and demonstrate that some of them unambiguously contain rock avalanche material so their climatic significance is questionable

40ar 39ar incremental heating study of mineral separates from the early archean east indian craton implications for the thermal history of a section of the singbhum granite batholithic complex 40ar 39ar incremental heating studies on mineral separates from three sets of rocks in the singbhum craton in eastern india have helped unravel the thermochronometric history of the terrane and explain an earlier discrepancy between sm nd 3800 ma and rb sr 3100 ma whole rock ages for the older metamorphic tonalitic gneiss omtg high precision plateau ages for hornblende separates from the older metamorphic group of rocks omg suggest that this unit is older than 3300 ma and that enclaves of both the omtg and omg within the batholithic complex cooled to 500 c at 3300 15 ma following engulfment in magma forming the singbhum granite sg results from a biotite separate from the omtg imply that slow cooling continued to a temperature of 300 c at 3160 ma study of a feldspar separate from the singbhum granite suggests final cooling uplift through the 150 c isotherm 660 ma ago we suggest that an earlier rb sr whole rock age of 3130 85 ma on the omtg did not yield the crystallisation a

generation and evolution of palaeoarchaean continental crust in the central part of the singhbhum craton eastern india abstract palaeoarchaean granitoids are exposed over wide area in the singhbhum craton whose origin and role in crustal evolution are not well constrained this study presents whole rock and mineral chemical data coupled with zircon u pb dating and hf isotope results on such granitoids from the central part of the craton to understand their petrogenesis tectonic setting and role in continental crustal evolution the first phase of granitoid magmatism in this area is represented by a 3 47 ga na rich gneissic tonalite belonging to the archaean ttg tonalite trondhjemite granodiorite suite this rock is characterized by high hree heavy rare earth elements negative eu anomaly low sr y ratio and positive zircon hf isotope signature ehft 2 1 to 4 8 it is interpreted to be formed by shallow melting of a juvenile mafic source at 3 35 ga a silicic ferroan porphyritic biotite granite formed it shows variable k na low y and high sr y moderately fractionated hree and positive zircon ehft 1 8 to 4 0 and is explained as a product of high temperature melting of a heterogeneous juvenile source consisting of tonalites and mafic rocks at lower crustal depth the final phase of granitoid magmatism is marked by a 3 30 ga non porphyritic ferroan silica rich biotite granite geochemical characteristics like moderately high k moderately fractionated hree low ca and sr y and zircon ehft 0 8 to 3 7 suggest that the granite was formed by high pressure melting of a tonalite dominated source with short crustal residence all the three granitoid phases display low mg ni and cr contents and magnesium number mg precluding direct involvement of mantle in their origin rather crustal reworking caused by episodic plume related mafic ultramafic magma underplating and intraplating in an oceanic plateau setting is suggested as the possible mechanism for formation of the granitoids successive crustal reworking events involved progressively greater amount of previously formed felsic crust generating more evolved k rich granitoids it appears to be a globally important process which led to effective crustal differentiation and maturing of the cratons during palaeoarchaean

neoarchean greenstone volcanism and continental growth dharwar craton southern india constraints from sims u pb zircon geochronology and nd isotopes abstract we present sims u pb zircon ages and nd isotope data for the felsic volcanic rocks from seven neoarchean greenstone belts of the eastern dharwar craton edc and from the chitradurga greenstone belt in the western dharwar craton wdc zircon ages show bimodal age distribution of felsic volcanism the ca 2 70 2 65 ga felsic volcanic event is contemporaneous with 2 7 ga mafic greenstone volcanism and emplacement of juvenile tonalitic to granodioritic crust while 2 58 2 54 ga felsic volcanics are coeval and spatially and probably genetically in some cases linked to the major episode calc alkaline magmatic accretion in the edc the chitradurga and veligallu greenstone belts host felsic volcanics of the first generation the latter showing inheritance at ca 2 95 ga four of the five greenstone belts hosting the second generation of felsic volcanics chitradurga kolar kadiri hutti show crustal inheritance at ca 2 6 2 7 2 9 3 0 3 1 and 3 3 ga ndt indicate derivation of the felsic volcanics from juvenile sources or short lived crustal sources with minor influence of older crust the new and existing data are consistent with two stage growth of the dharwar craton in the neoarchean first stage accretion led to the growth of a 2 7 2 6 ga juvenile crustal province of mafic volcanics and felsic plutons along the eastern margin of the wdc second stage accretion 2 58 2 52 ga led to the emplacement of ttg and calc alkaline plutons and felsic volcanics throughout the eastern dharwar craton an active margin context could apply for the two stage accretion scenario considering a west dipping subducting slab beneath the craton in the framework of long lived ultra hot accretionary orogen but the great width of influence of magmatic accretion and or reworking particularly during the second stage would suggest large scale mantle flow reorganization that would have generated large plume s

isua supracrustal belt greenland a vestige of a 3 8 ga suprasubduction zone ophiolite and the implications for archean geology abstract the mafic ultramafic units of the 3 8 ga isua supracrustal belt isb in greenland occur in a two armed arcuate zone eastern and western arms and are grouped into two major tectonostratigraphic units based on their lithological and geochemical characteristics 1 undifferentiated amphibolites ua and 2 garbenschiefer amphibolites ga the ua contains all major lithological units of a typical penrose type complete ophiolite sequence the ga is composed dominantly of volcaniclastic and volcanic rocks commonly found in immature island arcs the available geochemical data from ua and ga show distinct differences between the two units compared with the geochemical evolution of some of the well known phanerozoic ophiolites the pillow lavas and associated dikes of the ua show a compositional range that is similar to typical morb type ligurian ophiolites in the western alps apennines and those displayed by lip type caribbean ophiolites the ga is characterized by island arc tholeiite iat to boninite like rocks and defines a magmatic evolution that is comparable to that of suprasubduction zone ssz ophiolites in the mediterranean region our proposed geodynamic model for the isb suggests that the ua was built by primary to differentiated mantle generated melts during seafloor spreading little to moderately affected by subduction processes and that the iat to boninitic like rocks of the ga formed at a later stage by melting from a strongly subduction affected depleted and hydrated mantle our interpretation of the isb is that the ua and ga represent early and late stages respectively in the formation of a ssz ophiolite this implies that phanerozoic type plate tectonic processes such as seafloor spreading and subduction were operating by 3 8 ga in the palaeoarchean

mafic to felsic magmatism and crustal recycling in the obonga lake greenstone belt western superior province evidence from geochemistry nd isotopes and u pb geochronology abstract the central wabigoon subprovince of the western superior province contains mesoarchean granitoid and supracrustal rocks 3 01 2 83 ga and 2 78 2 69 ga granitoid plutons and supracrustal sequences it is a key area for understanding the relationship between greenstone belts and surrounding granitoid rocks that may have acted as basement the obonga lake greenstone belt contains two distinct assemblages 1 a ta nb th they have end values of 0 7 to 2 4 a dacitic unit in the northern assemblage has high la yb high sr y low nb y and heavy ree and is interpreted to represent a mantle modified slab melt similar to adakites in cenozoic arcs the associated enriched basalts may represent melts from the mantle wedge modified by slab melt adakitic metasomatism or they may represent an enriched oib like asthenospheric source rocks of the southern assemblage mostly show th and light ree enrichment and negative nb and ta anomalies low end values down to 0 9 can be modelled through contamination of a mafic liquid by 3 3 3 2 ga sialic crust although rocks of that age have not been observed suggested mesoarchean basement to the southern assemblage may therefore have had a complex history spanning several hundred million years the southern assemblage volcanism may have occurred in a continental arc to continental back arc system where crustal recycling played an important role

dating the oldest greenstone in india a 3 51 ga precise u pb shrimp zircon age for dacitic lava of the southern iron ore group singhbhum craton abstract this article reports a precise 3506 8 2 3 ma u pb shrimp zircon age for dacitic lava in a well preserved low grade metamorphic and low strained greenstone belt succession of the southern iron ore group singhbhum craton india this age makes the succession the oldest known greenstone belt succession in india and one of the oldest low strain greenstone successions in the world after the 3 51 ga coonterunah group of the pilbara craton western australia and the moderately deformed 3 54 ga theespruit formation of the barberton greenstone belt kaapvaal craton south africa the geochemical composition of the dacitic lava and related volcanic rocks suggests that they formed in a volcanic arc setting the succession also contains a major 120 m thick oxide facies banded iron formation that distinguishes it from the slightly older successions of the pilbara and kaapvaal cratons this banded iron formation may well be one of the oldest and most well preserved and together with associated volcanics

petrogenesis of archaean trondhjemites tonalites and granodiorites from eastern finland major and trace element geochemistry

geochemical and ion microprobe u pb zircon constraints on the archaean evolution of singhbhum craton eastern india abstract geochemical and shrimp u pb zircon analyses were obtained for nine samples of the singhbhum craton to investigate major regional granite intrusion volcanism sedimentary deposition metamorphism and deformation episodes detrital zircons within a recrystallized sandstone enclave within tonalitic gneiss provided a maximum time for deposition of 3375 3 ma all uncertainties are at 95 confidence unless otherwise specified for the host sandstone igneous crystallization dates between 3380 and 3299 ma were obtained for tonalite gneiss and foliated trondhjemite samples from widely dispersed sites whereas unfoliated granitic samples provided igneous crystallization dates