are there large bathymetric highs near 73 n 139 w in the arctic basin comment on acoustic backscattering from the basin and margins of the arctic ocean by i dyer et al

influence of water insoluble content on the short term strength of bedded rock salt from three locations in china unlike the presence of thick domes and thick layers of rock salt deposits in europe and the usa rock salt in china is mainly composed of thin layers 80 300 m characterized by a high content of water insoluble impurities thus the influence of these impurities on the strength of rock salt is barely known and has become an important topic for basic research in the construction of underground salt caverns for energy storage in china in this paper the influence of water insoluble content on the short term strength of bedded rock salt based on a series of combined laboratory tests including solubility uniaxial and triaxial compression tests on bedded rock salt samples from three locations in china namely the huai an ha site in jiangsu province the pingdingshan pds site in henan province and the anning an site in yunnan province is presented samples in which the water insoluble content was less than 50 were defined as rock salt and the remainder as impure rocks with salt analysis of the experimental results for rock salt samples leads to the conclusion that the uniaxial strength including the error in its measurement increases with increase in the water insoluble content in the sample meanwhile the triaxial strength of rock salt will hardly vary with changes in the water insoluble content it can also be concluded that application of confining pressure will suppress the impact of the water insoluble content on the short term strength of rock salt i e the inhibition will increase with increasing confining pressure application of hou s strength model to bedded rock salt from the three regions yields comparable results with the measured data nevertheless with a bearable small degree of uncertainty the modified hou s strength model has been proved to be more suitable for the determination of strength of bedded rock salt from the three study locations in china

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

multiple 3 8 3 1 ga tectono magmatic events in a newly discovered area of ancient rocks the shengousi complex anshan north china craton abstract near anshan city in the north china craton the oldest rocks in asia 3300 ma have been thoroughly documented at two localities baijiafen and dongshan in this paper we report the full geological context for more ancient rocks from a third anshan locality the polyphase migmatite shengousi complex shrimp u pb zircon dating indicates a protracted tectono magmatic history for the shengousi complex the oldest recognised component is banded trondhjemitic gneiss 3773 6 ma which is veined by strongly deformed granitic pegmatite these occur with a second generation of trondhjemitic rocks 3454 8 and 3448 9 ma the next generation of plutonic rocks is a composite suite of iron enriched mafic dykes 3332 6 and 3331 8 ma with broadly coeval felsic veins 3311 4 ma finally there was intrusion of monzogranite 3129 6 ma strong deformation has generally brought these 3773 3129 ma plutonic phases into close concordance to form banded rocks however locally cross cutting relationships are preserved in small lower strain domains that give a relative chronology agreeing with the absolute zircon u pb chronology all of this complex history is recorded in a single some of the the 3450 ma igneous phases discovered in the shengousi complex are new ages for igneous rocks in the anshan area the precambrian geology of the anshan area is thus marked by polyphase eoarchaean orthogneisses the newly recognised 3450 ma phases widespread 3360 3300 magmatic activity found in association with mesoarchaean plutonic rocks and the neoarchaean anshan group and palaeoproterozoic liaohe group metasedimentary rocks this shows that the geology of the anshan area has a strong similarity with the narryer gneiss complex of western australia which contains the polyphase eoarchaean meeberrie gneisses the 3490 3440 ma eurada gneiss association 3300 ma granites and migmatisation and is intercalated with younger precambrian metasedimentary rocks of different ages containing hadean detrital zircons no other ancient gneiss complexes in the world show such a close match in their history with the anshan area rocks this means that the anshan area is prospective for locating new occurrences of hadean crustal components intercalated with younger sedimentary rocks

acoustic backscattering from the basin and margins of the arctic ocean sound waves created by high energy explosives were used to measure reverberation and backscattering in the arctic ocean from an ice camp in the beaufort sea signals were backscattered from the continental margin and other major features of the basin an acoustic array was used to analyze the signals in azimuth based on these data we constructed charts of normalized backscatter level which can be compared with known topography of the arctic ocean resolution of this remote sensing technique for our experimental conditions analysis frequency 9 hz averaging time 20 s array size 600 600 m is about 8 in azimuth 15 km in radial extent and 1 km in depth we obtained interpretable signals out to 2700 km for 1000 km or less the backscatter charts provide continuous coverage of the margins and major submerged features strong returns from about 73 2 n 139 0 w indicate one or more prominent features that many contemporary topographic charts do not show but which we believe to be real other strong returns are evident from the northwind escarpment and from the continental slopes of alaska and the canadian archipelago via a backscatter model we estimate the roughness product rms height times correlation radius of these features to be about 4500 m2 on average

new evidence for neoarchean ca 2 7 ga crustal growth in the north china craton abstract the north china craton ncc is characterized by major ca 2 5 ga tectonothermal events which is markedly different from most other typical cratons worldwide whereas ca 2 7 ga rocks are sporadically exposed in the whole craton and only have been identified in several localities in this contribution we report new discovery of ca 2 7 ga rocks from the western huixian in the ncc zircon u pb dating reveals their protoliths were emplaced between 2714 and 2704 ma the rocks are tonalitic gneisses in composition and present similar geochemical characteristics to the archean ttg suites and phanerozoic high sio2 adakites worldwide zircons from the three tonalitic gneisses yield variable positive ehf t values of 2 3 6 7 4 2 8 7 and 3 3 6 3 with hf model average ages of ca 2848 ma ca 2838 ma and ca 2840 ma respectively the measured zircon 18o values range from 5 5 to 6 2 5 1 to 6 1 and 5 2 to 6 2 with weighted mean values of 5 7 5 7 and 5 8 respectively all of them overlap with the mantle range within two sigma uncertainty limits the hf o isotope compositions in zircons from the tonalitic gneisses indicate that their protoliths originated in a juvenile crust forming event at ca 2 7 ga available data suggest ca 2 7 ga is a major period of crustal growth in the ncc geochemical signatures of the ca 2 7 ga tonalitic gneiss reflect that their protoliths formed by partial melting of hydrated mafic rocks with variable proportions of the amphibole and garnet residues to be left behind in the residuum but absence of plagioclase and rutile in their source the rocks are classified as hp ttg their protolith formed under amphibolite facies conditions it is noted that the ca 2 7 ga tonalitic gneisses in huixian display low mgo

a palaeoarchean mesoarchean micro continent entrained in the jiao liao ji belt at the southeastern north china craton evidence from the zircon record in the bengbu area the bengbu area in the southeastern north china craton ncc consists predominantly of archean palaeoproterozoic gneissic granitoids with minor supracrustal rocks the fengyang and wuhe groups this study presents new zircon laser ablation inductively coupled plasma mass spectrometry u pb and lu hf isotopic data and trace element contents for these granitoids which improve understanding the archean palaeoproterozoic crustal evolution of the ncc magmatic zircon u pb data reveal that zircons in the gneissic granitoids were generated by multi stage events at 2 93 2 73 2 53 2 52 and 2 18 2 13 ga metamorphic zircon u pb data obtained from these rocks show two distinct metamorphic ages of 2 49 2 52 and 1 84 ga suggesting that the bengbu area experienced a regional metamorphic event at the end of the neoarchean era and encountered reworking by a tectonothermal event associated with the formation of the palaeoproterozoic jiao liao ji belt trace element compositions of magmatic zircons reveal the highest ti concentrations 8 08 3 38 ppm and growth temperatures 718 44 c for the zircons aged 2 13 2 17 ga and an increase in zircon u yb ratios from 2 93 ga 0 34 0 12 through 2 73 ga 0 96 0 42 to 2 53 ga 1 05 0 46 but an evident decrease at 2 17 2 13 ga 0 61 0 40 ppm similar palaeoarchean xenocrystic and detrital zircons with negative hf t values late mesoarchean magmatic zircons with juvenile hf isotopic features early neoarchean magmatic zircons with model ages of 2 9 3 0 ga and two regional metamorphic events at 2 52 2 48 and 1 88 1 80 ga in the bengbu and jiaobei areas indicate a palaeoarchean mesoarchean micro continent entrained in the jiao liao ji belt at the southeastern ncc

contrasting zircon hf and o isotopes in the two episodes of neoproterozoic granitoids in south china implications for growth and reworking of continental crust abstract the genetic links among rift magmatism crustal growth and water rock interaction are an important issue about mass and heat transfer between mantle and crust during supercontinent breakup a combined study of hf and o isotopes in zircons from neoproterozoic granitoids in south china provides evidence for growth and reworking of juvenile and ancient crusts with different styles of water rock interactions along rift tectonic zones two generations of the granitoids show contrasting features in both zircon hf and o isotope compositions indicating their distinct petrogenesis the 825 ma granitoids exhibit negative ehf t values of 3 4 0 8 to 1 6 0 8 with old model hf ages of 1 81 0 07 to 1 92 0 10 ga and high 18o values of 8 7 to 10 4 these indicate that the source material of granitoid magmas was derived from melting of paleoproterozoic basement that has the hf isotope signature similar to the enriched mantle but experienced chemical weathering process before anatexis reworking of ancient crust is demonstrated to occur at 825 ma in the orogenic collapse zone with overprinting of subsolidus hydrothermal alteration during magma emplacement in contrast the 760 750 ma bimodal intrusives are characterized by positive ehf t values of 3 5 0 8 to 9 9 0 8 with young model hf ages of 0 94 0 06 to 1 18 0 06 ga and both low and high 18o values of 4 2 to 6 2 relative to 5 3 0 3 for the normal mantle zircon prompt reworking of juvenile crust is suggested to occur at 750 ma in the rifted tectonic zone with occurrence of supersolidus hydrothermal alteration and local low 18o magmatism during supercontinent breakup contributions of the depleted mantle to their magma sources are contrasting in the two episodes of magmatism in association with breakup of the supercontinent rodinia while the change in melt source from the crust to the mantle keeps pace with the advance from continental rifting to supercontinent breakup significant transport of both heat and material from the depleted mantle to the continental crust only occurred along the active rifting zone in either case the growth and reworking of continental crust are episodically associated with rift magmatism

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

study of the relationship between surface subsidence and internal pressure in salt caverns because of the ultra low permeability and favorable rheological properties of rock salt salt caverns are globally considered to be an ideal medium for energy storage the prediction and control of surface subsidence above storage caverns are the key problems that require steady attention to ensure their long term safety and stable operation for salt caverns already in existence factors such as depth shape and geological conditions cannot be changed therefore the operating condition i e internal pressure becomes a key factor in influencing surface subsidence however cyclic pressure change during the operation phase has not been seriously considered by analytical investigation yet in this paper theoretical models for the volume convergence rates in spherical and cylindrical caverns have been derived thereby developing a new concept equivalent internal pressure that of rock salt is considered

shrimp u pb zircon geochronology of the fuping complex implications for formation and assembly of the north china craton abstract the fuping complex located within the central zone of the north china craton is composed of amphibolite to granulite facies orthogneisses interleaved with minor supracrustal rocks at similar metamorphic grade the oldest components recognised are hornblende gneiss enclaves within the predominant biotite orthogneiss which have a shrimp u pb zircon age of 2708 8 ma we consider these enclaves to represent fragments of 2 7 ga continental materials incorporated in the biotite gneiss the biotite gneiss has a shrimp u pb zircon age of 2513 12 ma interpreted to be time of magmatic crystallisation of the igneous precursor based on the igneous characteristics of the zircons this indicates a major magmatic episode at 2 52 ga in the fuping complex identical to the age of felsic volcanism within the low grade wutai complex which crops out immediately to the west a gneissic granite that intrudes the biotite gneisses has a poorly defined 207pb 206pb age of 2045 64 ma this is within error of the age of 2097 46 ma obtained from a fine grained gneiss interlayered with amphibolite of the wanzi supracrustal suite wss interpreted to be volcanic in origin zircons from both these samples have strong oscillatory zoning and provide the first indication of a palaeoproterozoic magmatic event in the area again similar in age to magmatic events recently recognised in the adjacent wutai complex these data indicate a comparable history for the fuping and wutai complexes and support geochemical evidence that they had a common origin and formed part of a late archaean arc affected by later palaeoproterozoic re activation low uranium zircons without oscillatory zoning separated from a sample of biotite gneiss yield data clustered at 1817 26 ma which is interpreted to reflect a period of new zircon growth during a major metamorphic event these data support the recently proposed tectonic model that amalgamation of the north china craton occurred due to collision of the eastern and western blocks along the central zone at 1 8 ga ago

2 7 ga crustal growth in the north china craton evidence from zircon u pb ages and hf isotopes of the sushui complex in the zhongtiao terrane abstractthe major precambrian crustal growth in the north china craton ncc has been thought to have occurred at 2 5 ga here we present secondary ion mass spectroscopy sims and laser ablation inductively coupled mass spectrometry la icp ms zircon u pb ages and hf isotope compositions from dioritic and tonalite trondhjemite granodiorite ttg gneisses of the neoarchean paleoproterozoic metamorphic terrane in the central ncc zircons from the trondhjemitic samples yield emplacement ages at and ma those from the dioritic gneiss yield an age of ma the ttg gneisses are characterized by a na rich and k poor nature strongly fractionated rare earth element patterns high sr 418 318 and sr y 78 49 low y 5 4 6 4 and ybn 3 0 3 6 and low mgo 0 89 1 10 mg 47 42 cr 39 20 and ni 11 8 nb ta 16 12 which suggests that magmas were derived from the partial melting of a flatly subducted oceanic slab the dioritic gneiss shows higher mgo 3 17 mg 50 cr 44

timing of destruction of the north china craton abstract the north china craton ncc is the typical example of destruction of an ancient craton however the destruction mechanism and geodynamic controlling factors still remain enigmatic due to controversy on the timing of destruction which is the key to understanding the destruction processes based on temporal and spatial distributions of the igneous rocks and their sources and tectonic settings it is recognized that six stages of tectono magmatism occurred in the ncc during the phanerozoic i e carboniferous to early permian late permian to middle triassic late triassic jurassic early cretaceous and cenozoic among them cenozoic magmatism mainly consists of alkali basalts and is found only occasionally in the eastern ncc the first four stages of magmatism and tectonism related to the southward subduction of the paleo asian plate and the assembly of sino korean and yangtze cratons are locally distributed in limited parts of the ncc reflecting a multiple stage modification of the ncc from the late carboniferous to jurassic however the intensive development of early cretaceous magmatism extensional deformation and associated gold mineralization with significant continental crustal growth indicate that the eastern ncc was destroyed during this time period this destruction was the result of paleo pacific subduction beneath the eastern asian continent with lithospheric removal and or replacement of an ancient cratonic lithosphere by a juvenile oceanic lithosphere

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

crustal growth in the north china craton at 2 5ga evidence from in situ zircon u pb ages hf isotopes and whole rock geochemistry of the dengfeng complex abstract the dengfeng complex is located on the southern margin of the north china craton ncc and belongs to the southern portion of the trans north china orogen this terrane is important to understand the formation and evolution of ncc during late neoarchean 2 5 ga the dengfeng complex is well exposed in the junzhao region and comprises two distinct lithologic units supracrustal assemblage and plutonic rocks la icpms magmatic zircon u pb age data shows that the rocks formed within the range of 2547 2504 ma the available hf isotope data indicate that the majority of ca 2 5 ga zircons from the dengfeng complex have high e hf t values close to the initial hf isotope ratios of the contemporaneous depleted mantle these data indicate that the rocks in the dengfeng represent juvenile crust the ttg gneisses in the dengfeng complex display low mg 41 48 mgo 59 69 mgo 3 5 6 6 wt ni 82 130 ppm and cr 148 237 ppm abundances elevated sr 1759 1927 ppm and ba 1742 2289 ppm concentrations and high lree la n 38 487 such geochemical features are similar to archean sanukitoids a two stage model is applied here to explain the genesis of metadiorites of dengfeng complex 1 firstly the mantle is metasomatized either by melts or by aqueous fluids from a subducted slab 2 subsequently sanukitoid magmas were produced by partial melting of the hybridized mantle furthermore the amphibolites of supracrustal rocks have a mixture of morb and arc like geochemical affinities suggesting the development of a back arc in the southern ncc at ca 2 5 ga the contemporary late neoarchean ttgs sanukitoids and morb back arc association may represent a late neoarchean tectonic melange implying a neoarchean subduction accretion process which would suggest that modern style plate tectonics processes was probably initiated in the southern ncc by 2 5 ga

continental growth and the crustal record abstract the continental crust is the archive of earth history the spatial and temporal distribution of the earth s record of rock units and events is heterogeneous with distinctive peaks and troughs in the distribution of ages of igneous crystallisation metamorphism continental margins and mineralisation this distribution reflects the different preservation potential of rocks generated in different tectonic settings rather than fundamental pulses of activity and the peaks of ages are linked to the timing of supercontinent assembly in contrast there are other signals such as the sr isotope ratios of seawater mantle temperatures and redox conditions on the earth where the records are regarded as primary because they are not sensitive to the numbers of samples of different ages that have been analysed new models based on the u pb hf and o isotope ratios of detrital zircons suggest that at least 60 70 of the present volume of the continental crust had been generated by 3 ga the growth of continental crust was a continuous rather than an episodic process but there was a marked decrease in the rate of crustal growth at 3 ga this appears to have been linked to significant crustal recycling and the onset plate tectonics the 60 70 of the present volume of the continental crust estimated to have been present at 3 ga contrasts markedly with the

petrogenesis of neoarchean ttg rocks in the yangtze craton and its implication for the formation of archean ttgs abstract the yangtze craton is one of the most important cratonic blocks in eastern asia however its early evolution has not been well constrained yet because of the poor exposure of archean basement rocks in this contribution a combined study of zircon u pb age hf o isotope compositions as well as whole rock element compositions was carried out for gray gneiss rocks from the douling complex in the northwestern part of the yangtze craton zircon u pb dating for two gneiss samples yielded weighted mean ages of 2509 21 and 2496 15 ma the gray gneisses have high sio2 63 64 73 74 na2o 3 66 6 21 but low k2o 0 76 2 93 contents and belong to the trondhjemite tonalite and granodiorite series rocks they are characterized by enrichments of liles and lrees but notable depletions in nb ta and ti as well as heavy rees and y all the samples plot into the fields of typical archean ttg rocks in the la yb n vs yb n and sr y vs y diagrams therefore the gray gneisses in the douling complex are neoarchean ttg rocks the relatively high nb ta and zr sm ratios of 19 6 46 0 and 14 5 143 4 suggest their formation under eclogite facies conditions they have low hf t values of 3 21 and 3 39 and hf model ages of 3194 26 and 3205 24 ma indicating their derivation from ca 3 2 ga juvenile mafic rocks their 18o values of 6 41 0 12 and 5 44 0 12 suggest that surface rocks were differently recycled into their sources collectively the ttg rocks in the douling complex were generated by partial melting of thickened mafic crust rather than direct partial melting of oceanic crust in a convergent plate boundary a compilation of available zircon u pb and hf isotope compositions of the yangtze craton reveals that there are quite differences of hf isotope compositions in archean the ca 3 8 3 4 ga zircons exhibit hf t values of 5 8 to 0 0 these chondritic to slightly enriched hafnium isotope compositions imply that their host rocks were formed by protracted intra crustal reworking of ancient protoliths without juvenile input by contrast zircons with ages younger than ca 3 2 ga register both negative and positive hf t values up to the depleted mantle providing clear evidence for the addition of juvenile materials these imply that the subduction driven plate tectonics might have been operated since ca 3 2 ga in the yangtze craton

rates of generation and destruction of the continental crust implications for continental growth less than 25 of the volume of the juvenile continental crust preserved today is older than 3 ga there are no known rocks older than approximately 4 ga and yet a number of recent models of continental growth suggest that at least 60 80 of the present volume of the continental crust had been generated by 3 ga such models require that large volumes of pre 3 ga crust were destroyed and replaced by younger crust since the late archaean to address this issue we evaluate the influence on the rock record of changing the rates of generation and destruction of the continental crust at different times in earth9s history we adopted a box model approach in a numerical model constrained by the estimated volumes of continental crust at 3 ga and the present day and by the distribution of crust formation ages in the present day crust the data generated by the model suggest that new continental crust was generated continuously but with a marked decrease in the net growth rate at approximately 3 ga resulting in a temporary reduction in the volume of continental crust at that time destruction rates increased dramatically around 3 billion years ago which may be linked to the widespread development of subduction zones the volume of continental crust may have exceeded its present value by the mid late proterozoic in this model about 2 6 2 3 times of the present volume of continental crust has been generated since earth9s formation and approximately 1 6 1 3 times of this volume has been destroyed and recycled back into the mantle this article is part of a discussion meeting issue earth dynamics and the development of plate tectonics9

archean crustal evolution in the southeastern north china craton new data from the huoqiu complex abstract the southeastern margin of the north china craton ncc is characterized by zircon ages of dominantly 2 6 ga and differs significantly from most other regions in the ncc with mostly 2 5 ga zircons in order to better understand the precambrian crustal evolution of the ncc we conducted a study of in situ zircon u pb dating trace element and hf isotope analyses as well as whole rock geochemistry on a comprehensive set of core samples from three drilling holes intersecting the basement rocks of the huoqiu area at the southeastern margin of the ncc our data reveal three magmatic events at 3 02 ga 2 77 ga and 2 71 ga in addition seven analyses of possibly inherited zircon cores plot along a discordant line with an upper intercept age of 3262 35 ma indicating possible existence of a paleoarchean crystalline basement the 3 02 ga zircons have positive hf t values close to the depleted mantle growth curve and their u pb ages are only slightly younger than the tdm1 model ages hence recording a mesoarchean crust forming event the 2 77 ga zircons also register a period of crustal growth but probably with some involvement of older crust the 2 71 ga zircons have lower hf t values and older tdm1 model ages than the 2 77 ga zircons suggesting that the former were mainly derived from older continental crust in contrast to the traditional idea that 2 7 ga was a major period of crustal growth in the ncc

zircon the metamorphic mineral a mineral that forms under conditions as variable as diagenesis to deep subduction melt crystallization to low temperature alteration and that retains information on time temperature trace element and isotopic signatures is bound to be a useful petrogenetic tool the variety of conditions under which zircon forms and reacts during metamorphism is a great asset but also a challenge as interpretation of any geochemical data obtained from zircon must be placed in pressure temperature deformation fluid context under which condition and by which process zircon forms in metamorphic rocks remains a crucial question to answer for the correct interpretation of its precious geochemical information in the last 20 years there has been a dramatic evolution in the use of zircon in metamorphic petrology with the advent of in situ dating techniques zircon became relevant as a mineral for age determinations in high grade metamorphic rocks since then there has been incredible progress in our understanding of metamorphic zircon with the documentation of growth and alteration textures its capacity to protect mineral inclusions zircon thermometry trace element patterns and their relation to main mineral assemblages solubility of zircon in melt and fluids and isotopic systematics in single domains that go beyond u pb age determinations metamorphic zircon is no longer an impediment to precise geochronology of protolith rocks but has become a truly indispensable mineral in reconstructing pressure temperature time fluid paths over a wide range of settings an obvious consequence of its wide use is the rapid increase of literature on metamorphic zircon and any attempt to summarize it can only be partial in this chapter reference to published works are intended as examples and not as a compilation this chapter approaches zircon as a metamorphic mineral reporting on its petrography and texture deformation structure and mineral chemistry including trace element and isotopic systematics linking this information together highlights the potential of zircon as a key mineral in petrochronology

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

long range backscatter from the mid atlantic ridge acoustic backscatter returns measured with a vertical source and horizontal receiving array are beamformed and charted to respective scattering sites on the western flank of the mid atlantic ridge mar well defined patterns of high level backscatter resembling bottom morphology occur within the direct path and at convergence zone ranges measured backscatter is compared with modeled two way transmission loss using the wide angle parabolic equation pe and high resolution supporting bathymetry the level of backscatter is found to be strongly dependent upon two way transmission loss tl specifically tl selects prominent bathymetric features from which high backscatter is returned with backscatter maxima corresponding to tl minima scattering strength estimated for the wide area surveyed shows low variance and relatively minimal spatial variation apparently distinct scattering contributions are so well mixed in long range backscatter that dominant variations are generally due to propagation these

reply to are there large bathymetric highs near 73 n 139 w in the arctic basin comment on acoustic backscattering from the basin and margins of the arctic ocean by i dyer et al

origin of high magnesian andesites in the setouchi volcanic belt southwest japan i petrographical and chemical characteristics high magnesian andesites of middle miocene age occur in southwest japan forming an obvious volcanic belt these andesites have low feo mgo ratios 0 546 0 931 and are rich in ni 101 312 ppm co 30 0 45 1 ppm and cr 208 756 ppm they are relatively aphyric phenocrysts 10 vol and the phenocrysts of magnesian olivine fo88 are in equilibrium with the host high magnesian andesite magmas on the basis of the fe mg exchange partitioning these features suggest that the high magnesian andesites are not differentiated or accumulative they appear to represent primary andesites generated in the upper mantle these southwest japanese high magnesian andesites are rich in incompatible elements and show light rare earth enrichment relative to boninites suggesting that the former is derived from a less depleted mantle source than the latter

age and evolution of a late archean to paleoproterozoic upper to lower crustal section in the wutaishan hengshan fuping terrain of northern china abstract the taihangshan wutaishan area forms part of the central north china craton and consists of three main components the hengshan and fuping complexes containing predominantly ductilely deformed late archean to paleoproterozoic high grade granitoid orthogneisses intruded by mafic dykes of gabbroic composition and the low grade late archean wutai greenstone belt developed between the high grade terrains and consisting of bimodal volcanic rocks and metasediments associated with coeval granitoids zircon dating shows that both the hengshan and fuping complexes were intruded by major granitoid bodies between 2 52 and 2 48 ga with rare occurrences of 2 7 ga gneisses wutai granitoids adjacent to the fuping complex also contain 2 7 ga xenocrysts all possibly reflecting the same older basement rare granitic gneisses in the hengshan have emplacement ages of 2 35 ga and show the same structural features as the older rocks indicating that the main deformation occurred after 2 3 ga deposition of supracrustal rocks and emplacement of mafic dykes occurred in the paleoproterozoic together with intrusion of granitic bodies at 2 1 2 0 ga and all underwent regional metamorphism at 1 88 1 85 ga we interpret the hengshan and fuping granitoid gneisses as the lower plutonic part of a late archean to early paleoproterozoic japan type magmatic arc with the upper volcanic part represented by the wutai complex components of this arc may have evolved at a continental margin as indicated by the 2 7 ga zircons virtually all magmatic events are recorded in all three complexes however the 1 85 ga metamorphic event is only recorded in the higher grade portions of the wutai since low grade and presumably high crustal level precluded metamorphic zircon growth shear zones in the hengshan are interpreted as major lower crustal discontinuities post dating the peak of high pressure granulite metamorphism we suggest that they formed during orogenic collapse and uplift of the hengshan complex in the late paleoproterozoic 1800 ma

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

high pressure granulites in the sanggan area north china craton metamorphic evolution p t paths and geotectonic significance high pressure basic granulites are widely distributed as enclaves and sheet like blocks in the huaian ttg gneiss terrane in the sanggan area of the central zone of the north china craton four stages of the metamorphic history have been recognised in mineral assemblages based on inclusion exsolution and reaction textures integrated with garnet zonation patterns as revealed by compositional maps and compositional profiles the p t conditions for each metamorphic stage were obtained using thermodynamically and experimentally calibrated geothermobarometers the low ca core of growth zoned garnet along with inclusion minerals defines a prograde assemblage m1 of garnet clinopyroxene plagioclase quartz yielding 700 c and 10 kbar the peak of metamorphism at about 750 870 c and 11 14 5 kbar m2 is defined by high ca domains in garnet interiors and inclusion minerals of clinopyroxene plagioclase and quartz kelyphites or coronas of orthopyroxene plagioclase magnetite around garnet porphyroblasts indicate garnet breakdown reactions m3 at conditions around 770 830 c and 8 5 10 5 kbar garnet exsolution lamellae in clinopyroxene and kelyphites of amphibole plagioclase around garnet formed during the cooling process at about 500 650 c and 5 5 8 kbar m4 these results help define a sequential p t path containing prograde near isothermal decompression itd and near isobaric cooling ibc stages the clockwise hybrid itd and ibc p t paths of the hp granulites in the sanggan area imply a model of thickening followed by extension in a collisional environment furthermore the relatively high pressures 6 14 5 kbar of the four metamorphic stages and the geometry of the p t paths suggest that the hp granulites together with their host huaian ttg gneisses represent the lower plate in a crust thickened during collision the corresponding upper plate might be the tectonically overlying khondalite series which was subjected to medium to low pressure mp lp 7 4 kbar granulite facies metamorphism with a clockwise p t path including an itd segment both the hp and the mp lp granulite facies events occurred contemporaneously at c 1 90 1 85 ga in a collisional environment created by the assembly process of the north china craton

continental collision and accretion recorded in the deep lithosphere of central china abstract basaltic andesitic volcaniclastic diatremes erupted in the mesozoic 160 ma in the xinyang area henan province of central china the locality lies on the western margin of the eastern block of the north china craton and the southern margin of this craton the diatremes carry a suite of xenoliths including lower crustal mafic to felsic granulites eclogites meta gabbros pyroxenites and mantle peridotites the petrological and geochemical data show that the crust in the area is temporally and compositionally zoned exposed rocks up to ca 2 85 ga old underlain by felsic granulites and rare pyroxenites with zircon ages of 3 6 3 4 ga to ca 30 km depth deeper ca 30 45 km parts of the lower crust consist of high pressure mafic to felsic granulites and meta gabbro which give paleoproterozoic 2 0 1 8 ga zircon ages our data show the significance of underplating vertical crustal growth in the paleoproterozoic which was related to the amalgamation of the eastern and western block of this craton and a global columbia supercontinent assembly hf isotope data indicate that both juvenile material and remelting of older 3 8 3 0 ga crustal rocks were involved in this important event paleozoic 440 260 ma and early mesozoic 228 219 ma zircons are also found in xenoliths from the deeper part of the lower crust and the uppermost mantle i e eclogite and peridotite they are interpreted as reflecting geodynamic processes related to the continental collision between this craton and the yangtze craton respectively the xenoliths from xinyang diatremes thus record the growth and modification of the old paleo mesoarchean continental lithosphere by magma underplating during several tectonic events assembly of the southern and northern parts of the eastern block in neoarchean time collision of the western and eastern blocks along the trans china orogen in the paleoproterozoic and subduction and collision of the yangtze craton with the north china craton in early paleozoic and triassic times respectively

neoarchean to paleoproterozoic continental growth in the southeastern margin of the north china craton geochemical zircon u pb and hf isotope evidence from the huoqiu complex abstract the huoqiu complex in the southeastern margin of the north china craton ncc is dominated by neoarchean gray gneisses amphibolites and voluminous metasediments here we report the occurrence of neoarchean to paleoproterozoic rocks from drill core samples the gneisses are similar to ttg tonalite trondhjemite granodiorite in composition and show close spatial association with amphibolites geochemical characteristics such as high sr y and la yb n with steep ree patterns and trace element modeling suggest that these rocks were generated by partial melting of hydrous meta basalts amphibolites at the base of a thickened mafic continental crust leaving a rutile bearing eclogite residue la mc icp ms u pb age data from magmatic zircon grains show protolith emplacement ages of 2 76 2 71 ga subsequently widespread migmatization took place at 1 91 1 82 ga generating voluminous migmatites and high k granites hf isotopic compositions of zircon grains from the amphibolite and gneiss show ehf t values of 2 4 15 5 and 3 0 1 5 respectively the tdm2 hf model ages of the gneisses range from 2 87 to 3 14 ga and are identical to the tdm1 hf ages of amphibolites 2 84 3 16 ga within analytical uncertainty suggesting that the gneisses formed by partial melting of amphibolite and attest to large scale reworking of the ancient continental crust during neoarchean the zircon grains from the granites define two groups with regard to their hf isotopic composition the older group 1916 42 ma has ehf t values and tdm2 hf ages of 10 5 2 4 and 2 40 3 20 ga respectively whereas the younger one 1823 41 ma shows a large variation in ehf t values ranging from 18 1 to 12 5 with tdm2 hf model ages of 1 70 3 59 ga a couple of zircon grains from the younger group display consistent u pb ages and tdm2 indicating accretion of juvenile crust from depleted mantle sources during 1 82 to 1 91 ga however the dominant hf isotope features are consistent with the reworking of preexisting continental crust we therefore infer that only limited accretion of juvenile crust occurred during this time and that the paleoproterozoic 1 82 to 1 91 ga tectonics in the southeastern margin of the ncc witnessed extensive reworking of older continental crust

archean blocks and their boundaries in the north china craton lithological geochemical structural and p t path constraints and tectonic evolution an examination of lithological geochemical geochronological structural and metamorphic p t path data suggests that the basement of the north china craton can be divided into eastern and western blocks separated by major crustal boundaries that roughly correspond with the limits of a 300 km wide zone called the trans north china orogen the eastern block consists predominantly of late archean domiform tonalitic trondhjemitic granodioritic ttg batholiths surrounded by anastomosing networks and linear belts of open to tight synforms of minor volcanic and sedimentary rocks metamorphosed from greenschist to granulite facies at 2 5 ga with anticlockwise p t paths some early to middle archean rocks are locally present in the eastern block but their tectonic history is unclear due to reworking by the 2 5 ga tectonothermal event the western block has a late archean assemblage structural style and metamorphic history similar to that of the eastern block but it differs in the absence of early to middle archean assemblages and in being overlain by and interleaved with paleoproterozoic khondalites which were affected by a 1 8 ga metamorphic event involving clockwise p t paths a mantle plume model is proposed for the formation and evolution of late archean basement rocks in the eastern and western blocks based on a combination of extensive exposure of ttg gneisses affinities of mafic rocks to continental tholeiitic basalts presence of voluminous komatiitic rocks dominant diaprism related domiform structures anticlockwise p t paths and a short time span from the primary emplacement of ttg and ultramafic to mafic rocks until the onset of regional metamorphism between the two blocks is the trans north china orogen which is bounded by two major fault systems and is composed of late archean to paleoproterozoic ttg gneisses and granitoids interleaved with abundant sedimentary and volcanic rocks that are geochemically interpreted as having developed in magmatic arc and intra arc basin environments these rocks underwent multiple phases of compressional deformation and peak high pressure metamorphism followed by rapid exhumation during the late paleoproterozoic at 1 8 ga as a result of collision between the eastern and western blocks resulting in the amalgamation of the north china craton

early neoarchean 2 7 ga tectono thermal events in the north china craton a synthesis abstract the north china craton ncc is characterized by major 2 5 ga tectono thermal events and is thus different from many other cratons worldwide where 2 7 ga events are well developed however whole rock nd and hf in zircon isotopic compositions of 2 5 ga crustally derived granitoids reveal that they mainly formed through reworking of late mesoarchean to early neoarchean continental material we review the spatial distribution rock types geochemical and nd hf isotopic compositions of 2 7 ga granitiods that are widely identified in the ncc including eastern shandong western shandong huoqiu hengshan fuping zanhuang zhongtiao and wuchuan these granitoids are mainly tonalitic in composition and frequently underwent strong metamorphism deformation and anatexis they show large variations in sio 2 total feo mgo and cao and can be subdivided into two types in terms of their ree patterns more importantly whole rock nd and hf in zircon isotopic compositions indicate that the strong 2 7 ga tectono thermal event mainly involved juvenile additions to the continental crust the early neoarchean was the most important period for rapid production of new continental crust in the ncc the main difference of the ncc from many other cratons worldwide is a strong and widespread superimposed tectono thermal event at 2 5 ga that stabilized the cratonic assemblage

erratum to magnesium sulphate salts on monuments in saxony germany regional geological and environmental causes a record of mortar and salt analyses from the last 20 years on various monuments in saxony demonstrates the close relation between the occurrence of magnesium sulphate salts and the use of dolomitic lime mortars in a regional frame the regional distribution scheme of dolomitic lime in historic mortars depends on the geological setting of the territory i e the natural occurrence of dolomite deposits sulphation of these mortars with the formation of magnesium sulphate and gypsum is mainly due to environmental pollution as can be shown by sulphur stable isotope analyses exposure of the sulphated mortar surfaces to rain leads to the preferential dissolution and advective transport of the novel formed magnesium sulphate salts which effloresce or subfloresce on the mortar surface or can be accumulated in the structure of adjoining porous building materials like natural stones these processes may cause severe damages in mortars by the weakening of the binder and in porous stones by salt crystallisation of phases like epsomite and hexahydrite

the relationship between adakitic calc alkaline volcanic rocks and ttgs implications for the tectonic setting of the karelian greenstone belts baltic shield abstract two types of coeval acid intermediate rocks with different petrological geochemical and isotopic features have been discovered among volcanic rocks and surrounding synkinematic tonalite trondhjemite granodiorite ttg plutons of late archaean greenstone belts in the karelian granite greenstone terrane type 1 rocks comprise trondhjemites and sub volcanic occasionally volcanic dacite rhyolite rocks they are characterized by high sr low y and hree contents high sr y ratios and strongly fractionated ree patterns with no significant positive or negative eu anomaly initial nd is positive indicating a generation from juvenile source with little or no contribution of ancient continental crust type 2 is represented by diorite granodiorites and calc alkaline basalt andesite dacite rhyolite badr series as compared to type 1 these rocks differ by their lower sr higher y and hree contents lower sr y ratios and less fractionated hree patterns with negative eu anomalies initial nd varies from negative to positive values thus indicating a variable contribution of sialic crust geochemistry of the two magmatic series suggests their formation in a convergent plate margin setting the type 1 rocks resemble phanerozoic adakites which represent slab derived melts contaminated by overlying mantle wedge the type 2 rocks resemble badr series which were derived from a mantle wedge metasomatized by slab derived fluids and melts with subsequent variable crustal contamination the spatial distribution of these two types of magmatic series defines the asymmetry of the studied granite greenstone structures which presumably reflects the primary lateral zoning of island arc formed under specific thermal conditions in the archaean mantle adakite melts upraised to the surface in the frontal part of the island arc where mantle wedge was thin showing no or little interaction with metasomatized mantle and formed adakite type plutonic and sub volcanic rocks at greater depths adakitic melts and fluids interacted with the overlying mantle wedge and caused its partial melting with generation of calc alkaline badr volcanic rocks and diorite granodiorite plutons in the rear part of the island arc our data suggest that greenstone belt volcano plutonic arcs were initiated on different types of crust which presumably determined the petrogenetic and isotope variations of the studied badr and adakite type island arc complexes

growth and reworking of the early precambrian continental crust in the north china craton constraints from zircon hf isotopes abstract we synthesize more than 2600 hf isotope data on the archean paleoproterozoic zircons from the north china craton ncc recalculation of the data based on single stage and two stage hf model ages of the eastern block of the ncc shows peak ages of 3902 13 ma and 3978 18 ma respectively and also small peaks at 3 5 4 0 ga the majority of zircon ehf t values are positive suggesting the possibility of the crust and the mantle differentiation at ca 3 9 4 0 ga in the eastern block of the ncc most magmatic zircons from the whole of ncc have their hf model age range of 2 4 2 9 ga and the single stage model ages is cluster at 2698 4 ma whereas the two stage model ages concentrate at 2714 5 ma implying that the protoliths were juvenile crustal rocks the most prominent peak at 2 7 ga indicates that this period marks the most important stage of the crust mantle differentiation and crust formation of the ncc the widespread 2 5 ga rocks in the ncc and the absence of the 2 5 ga peaks in hf model ages are consistent with the partial melting and reworking of the juvenile rocks at 2 5 ga furthermore the 2 5 1 7 ga zircon hf isotope features are also related to the reworking of the crustal rocks our results from the integration of a large database suggest that the eastern block and the trans north china orogen have undergone similar crust mantle differentiation and magmatism leading to the conclusion that the essential cratonization of the north china took place at the end of neoarchean

episodic earth evolution abstract u pb ages of zircons from precambrian granitoids and major rivers are grouped into a series of major peaks at about 2 7 2 5 2 1 1 9 and 1 1 ga recently these peaks have been interpreted as times of enhanced preservation of the continental crust associated with the assembly of supercontinents an older interpretation which we support is that they correspond instead to periods of accelerated crustal growth related to episodic convection of the mantle in this paper we use fluid mechanics experiments to develop a new model of mantle convection and crustal growth a dense layer at the base of the mantle persists until 2 7 ga when it destabilizes and generates large domes that rise into the upper mantle there they cause a large increase in the rate of subduction which leads to enhanced granite magmatism at convergent margins and thus to a pulse of crustal growth the domes heat the upper mantle which partially melts at mid ocean ridges to produce thick oceanic crust that resists subduction the subsequent period of subdued plate motion is broken by the next generation of mantle domes before and after the archean proterozoic period of episodic crustal growth plate tectonics operated quasi continuously

durability assessment of the basalts used in the diyarbakir city walls turkey the diyarbakir city walls dcw which were recently added to unesco s world heritage list are among the largest and one of the most impressive monuments from ancient times the history of the dcw stretches back more than 4000 years therefore making the extant city walls a combination and reflection of influences of the various civilizations that settled in the region basalts having such different textural properties as massive and vesicular were employed as the principal material in the construction of the dcw like many other historical structures the dcw are suffering from stone deterioration a large variety of weathering forms can be observed on the basalts used in different sections of the dcw the behavior of the basalt under different cyclic environmental conditions is not known in this study deterioration mechanisms of the basalts were investigated with field and laboratory studies it is found that iddingsite is a common weathering product developed through the crystal boundaries of olivine accelerated weathering tests show that salt crystallization is the most effective deterioration mechanism the field and laboratory studies indicate that both massive and vesicular basalts are durable however the massive basalts are more durable than the vesicular ones it is also observed that although chemical processes slightly trigger the deterioration mechanisms of the rocks most of the weathering forms on the dcw are controlled by the physical factors affecting the parameters of the basalts such as porosity water absorption and uniaxial compressive strength

two linear granite belts in the central western north china craton and their implication for late neoarchaean palaeoproterozoic continental evolution abstract precambrian granitoid rocks are widely distributed in the north china craton ncc the dominant granitic rocks in the central western ncc are trondhjemitic tonalitic and granodioritic ttg gneisses as well as some calc alkaline granitic gneisses and minor charnockites two linear granite belts in this area are different from other early precambrian granitic rocks in the ncc in terms of their occurrence and chemistry the liang tu lt garnet granite belt and the sanggan sg granite belt the lt belt occurs along two large scale ductile shear zones in metamorphosed al rich sedimentary country rocks khondalitic series in the central part of the lt belt are metagabbroic granulite lenses that were originally mafic dykes in the outer part of the belt garnet granites grade into khondalitic rocks lt belt granites have geochemical characteristics of s type granites the sg granite belt is 400 km long and composed of many deformed gneissic granitic bodies and granitic sills and veins with without gneissosity they contain various lenses including high pressure granulites and retrograded eclogites the sg granite belt trends east northeast parallel to the regional gneissosity of the country rocks major and trace element geochemistry of these granites shows characteristics of syn collisional and post collisional granite the country rocks beside the sanggan granite belt are different including the huai an fengzhen terrain hft to the northwest and the wutai fuping terrain wft to the southeast the lt and sg granite belts formed within an old continental craton and an old orogenic zone between two blocks respectively

metallogeny of the north china craton link with secular changes in the evolving earth abstract the north china craton ncc has experienced a complex geological evolution since the early precambrian and carries important records of secular changes in tectonics and metallogeny here we synthesize the salient geological and tectonic features of the evolution and destruction of the ncc vis a vis major metallogenic events and the formation of potential ore deposits we identify a close relationship between the major geological events in the ncc and those reported elsewhere on the globe we trace the records of a regular change in the pattern of metallogeny mineral deposit character spatial distribution and genetic mechanisms which closely match the timing and styles of the major geological and tectonic events in this craton the ncc went through five major tectonic cycles 1 neoarchean crustal growth and stabilization 2 paleoproterozoic rifting subduction accretion collision with imprints of the great oxidation event goe 3 late paleoproterozoic neoproterozoic multi stage rifting 4 paleozoic orogenesis at the margins of the craton and 5 mesozoic extensional tectonics associated with lithospheric thinning and decratonization coinciding with these major geological events are five major metallogenic systems identified as follows 1 an archean bif system 2 paleoproterozoic cu pb zn and mg b systems 3 a mesoproterozoic ree fe pb zn system 4 a paleozoic orogenic cu mo system and 5 mesozoic intracontinental au and ag pb zn and mo systems the ore deposit types in each of these metallogenic systems show distinct characteristics and tectonic affinities from early precambrian through late precambrian to paleozoic and mesozoic the ncc records a transition from primitive to modern style plate tectonics evidence for imbricated oceanic plate stratigraphy in a subduction accretion setting and collisional orogenesis along at least three major zones of ocean closure are documented major transitions in tectonic style and surface environmental changes recorded in other parts of the world are also reflected in the geological history and metallogenic events in the ncc large scale gold deposits formed through intraplate tectonics during the mesozoic provide important insights into mantle dynamics and crust mantle interaction associated with lithospheric thinning and craton destruction the ncc provides one of the best examples for documenting secular changes in the geological history and metallogenic epochs of an evolving earth

geophysical and geological tests of tectonic models of the north china craton abstract the geometry and timing of amalgamation of the north china craton have been controversial with three main models offering significantly different interpretations of regional structure geochronology and geological relationships one model suggests that the eastern and western blocks of the ncc formed separately in the archean and an active margin was developed on the eastern block between 2 5 and 1 85 ga when the two blocks collided above an east dipping subduction zone a second presumes the eastern block rifted from an unknown larger continent at circa 2 7 ga and experienced a collision with an arc perhaps attached to the western block above a west dipping subduction zone at 2 5 ga and the 1 85 ga metamorphism is related to a collision along the northern margin of the craton when the ncc joined the columbia supercontinent a third model suggests two collisions in the central orogenic belt at 2 1 and 1 88 ga but recognizes an early undated deformation event recent seismic results reveal details of the deep crustal and lithospheric structure that support both the second and third models showing that subduction beneath the central orogenic belt was west directed and that there is a second west dipping paleosubduction zone located to the east of the cob dipping beneath the western block ordos craton the boundaries identified through geophysics do not correlate with the boundaries of the trans north china orogen suggested in the first model and the subduction polarity is opposite that predicted by that model high pressure granulite facies metamorphism at 1 85 ga is not restricted to the tnco as suggested by the first model but is documented across the ncc as predicted by the second model suggesting a major continent continent collision along the north margin of the craton at 1 85 ga further it has recently been shown that in the southern tnco there is no record of metamorphism at circa 1 85 ga but only at 2 7 2 5 ga showing that the tnco as defined as a circa 1 85 ga orogen does not exist this is further confirmed by recent re os isotopic studies which show that the subcontinental lithospheric mantle beneath the southern cob is late archean in age and that a province in the northern ncc is circa 1 8 ga correlating with the proposed collision belt of the ncc with the columbia supercontinent across the entire ncc the cob is an archean convergent belt re worked in the paleoproterozoic and the paleoproterozoic tectonism is widespread across the ncc as predicted by the model whereby the previously amalgamated eastern and western blocks experienced a continental collision with columbia at circa 1 85 ga but uplift exhumation rates are slow necessitating a re evaluation of the tectonic models of the ncc

redefinition of depositional ages of neoarchean supracrustal rocks in western shandong province china shrimp u pb zircon dating abstract western shandong province is an important terrane in the eastern part of the north china craton where 2 75 2 5 ga tectono thermal events are well recognized supracrustal rocks named the taishan group experienced greenschist to amphibolite facies metamorphism and occur as lenses and strips in granitoids and are subdivided into the yanlingguan shancaoyu and liuhang formations the yanlingguan formation is mainly composed of amphibolite but also contains fine grained biotite hornblende gneiss komatiitic rocks and schist the liuhang formation is similar in rock association to the yanlingguan formation but contains more metasedimentary rocks the shancaoyu formation is mainly composed of fine grained biotite gneiss leptynite preserving bedding and is interlayered with schist its protoliths are mainly sedimentary rocks with some mafic to felsic volcano sedimentary varieties we undertook shrimp zircon dating on eighteen metasedimentary and volcanic samples mainly from the shancaoyu formation and the upper part of the liuhang formation ages of detrital and volcanic zircons vary from 2 75 to 2 54 ga and 2 55 to 2 525 ga respectively and the age distribution patterns vary in different samples we conclude that the shancaoyu formation and the upper part of the liuhang formation were deposited in the late neoarchean 2 55 2 525 ga and not in the early neoarchean 2 8 2 7 ga as thought before western shandong is the only terrane in the north china craton where both early and late neoarchean supracrustal rocks have been identified a magmatic arc model may be applied to explain the tectono thermal evolution at the end of the neoarchean

the late archean record a puzzle in ca 35 pieces abstract the global archean record preserves ca 35 large cratonic fragments and a less well defined number of smaller slivers most archean cratons display rifted margins of proterozoic age and therefore are mere fragments of supercratons which are defined herein as large ancestral landmasses of archean age with a stabilized core that on break up spawned several independently drifting cratons the tectonic evolution of individual archean cratons such as the slave craton of north america or the kaapvaal craton of southern africa should therefore always be considered in the context of their ancestral supercratons this is particularly true for many of the smaller cratons which are too limited in size to preserve the complete tectonic systems that led to their formation these limitations not only apply to the crustal geology of archean cratons but also to their underlying lithospheric mantle keels if these keels are archean in age as their broad correlation with ancient surface rocks suggests they also are rifted and drifted remains of larger keels that initially formed below ancestral supercratons the study of archean cratons and their lithospheric keels should thus be global in scope in the search for late archean supercratons a craton like the slave with three to four rifted margins has a ca 10 maximum probability of correlating with any of the ca 35 remaining cratons around the globe assuming wholesale recycling of archean cratons has been limited since ca 2 0 ga alternatively if the original number of independently drifting cratons was significantly larger the probability of successful correlations is much less due to repeated cycles of break up and plate tectonic dispersal since ca 2 0 ga the probability of correlation is probably independent of present day proximity unless there is independent evidence that two neighbouring cratons were only separated by a narrow ocean nevertheless most previously proposed craton correlations rely erroneously on present day proximity implicitly extrapolating relatively recent paleogeography back to the archean considering the fundamental differences between some of the better known cratons such as the slave superior and kaapvaal it seems likely that these cratons originated from independent supercratons sclavia superia vaalbara with distinct amalgamation and break up histories this is contrary to widely held opinion based largely on an idealized view of the supercontinent cycle that all cratons shared a common history in a single late archean supercontinent in contrast to the superior and kaapvaal cratons which are radically different from the slave craton and each other the dharwar craton of peninsular india the zimbabwe craton of southern africa and the wyoming craton of north america all show significant similarities to the slave craton it thus seems likely that at least some of these were nearest neighbours to the slave craton in the ca 2 6 2 2 ga sclavia supercraton

2 7ga juvenile crust formation in the north china craton taishan xintai area western shandong province further evidence of an understated event from u pb dating and hf isotopic composition of zircon abstract the evolution of the north china craton ncc is well known for a marked 2 55 2 50 ga tectonothermal event however supracrustal and intrusive rocks of 2 75 2 70 ga are in fact widely distributed in the western shandong province the most important area of archaean basement in the eastern part of the ncc this paper reports shrimp u pb dating and la icpms hf isotopic composition of zircons from 2 75 2 70 ga supracrustal and trondhjemite tonalite granodiorite ttg rocks in that area three fine grained hornblende biotite gneiss samples known locally as leptynite with meta volcanic or volcanosedimentary rock protoliths and five ttg samples have shrimp zircon u pb ages varying from 2 75 to 2 70 ga and 2 74 to 2 71 ga respectively zircons from most of the samples have high positive hf t values 4 7 to 10 0 and t dm hf ages 2 85 2 60 ga similar to their zircon u pb ages this indicates that the rocks represent largely juvenile crustal additions derived from depleted mantle only a short time before however some granitoids show hf t zircon values of 13 6 to 5 1 and t dm hf of 3 51 2 80 ga therefore the strong 2 75 2 70 ga tectonothermal event in the western shandong province involved not only juvenile addition to the continental crust but also intracrustal recycling of older components combined with craton wide data it is shown that the ncc is similar to many other cratons around the world where tectonothermal events of 2 7 ga are well developed however the main difference is that in the ncc superimposed 2 5 ga tectonothermal events were much stronger than in most other cratons

monostatic and bistatic reverberation statistics west of the mid atlantic ridge monostatic and bistatic reverberation of highly resolved signals from very rough bottoms at one site is statistically analyzed scattering at a mean frequency of about 230 hz from a large number of bottom footprints is considered the reverberation envelope is found to be non rayleigh with the degree of departure from rayleigh dependent upon the bottom grazing angle for two cases considered g 5 and 40 and upon the bistatic angle in the entire range 0 to 180 these rough bottom observations can be explained by adopting a continuous scattering model having a rayleigh envelope added to a discrete scattering model arising from a small number of individual features within the sonar footprint having a distinctly non rayleigh envelope these models plus a heuristic mechanism of self selection within the discrete scattering model arguably explain the observed angle dependence of the reverberation statistics

horizontal natural gas caverns in thin bedded rock salt formations natural gas storage caverns are usually built either in a rock salt dome or in a bedded rock salt formation in china the rock salt stratum has the following characteristics thin inter layered bedded and highly impure the height of a cavern in bedded rock salt deposit in china depends largely on the thickness of the salt formation 80 300 m and is much smaller than in thick domes and thick layers of rock salt deposits in europe and the usa where a storage cavern is typically cylindrically shaped this renders the diameter of a bedded salt cavern in china to be greater than its height to obtain sufficient storage volume the stability of horizontal salt caverns with different diameters and varied minimal internal pressures during its long term operation was numerically analyzed in this paper regarding convergence damage stress to peak or dilatancy strength ratio and effective strain the hou lux constitutive model including some relevant parameters from previous laboratory investigations were used for the numerical simulations the simulation results show that a horizontal cavern height length for underground gas storage is more efficient profitable and suitable because of its few times larger storage capacity than the most used vertical cavern height diameter or a prolate cavern height or diameter under the chinese special geological condition of thin bedded rock salt deposits provided with appropriate construction and operation parameters

partial melting of metabasic rocks and the generation of tonalitic trondhjemitic granodioritic ttg crust in the archaean constraints from phase equilibrium modelling abstract rocks of tonalitic trondhjemitic granodioritic ttg composition preserved in archaean terranes represent fragments of the earth s earliest formed continental crust and are thought to have formed via partial melting of hydrated metabasalt the geodynamic environments in which such high grade metamorphism and anatexis may have occurred in the early earth is strongly debated constraining the pressure p and temperature t conditions at which melts of appropriate composition can be derived from protoliths containing plausible mineral assemblages is central to addressing this question phase equilibrium modelling has been undertaken for an enriched archaean tholeiite bulk composition a suggested protolith for early earth ttg magmas using newly parameterised thermodynamic models that were specifically developed to evaluate the anatectic behaviour of metabasalt assuming minimal h 2 o saturation at the wet solidus the potential fertility of the studied metabasalt is greatest if the solidus is crossed at a pressure of 11 kbar where the solidus temperature reaches a minimum of 610 c major element compositions and proportions of calculated partial melts show the best correlation with those of natural archaean ttgs when in the p t range 800 950 c and 10 18 kbar which we suggest are optimal conditions for their petrogenesis normative geochemistry suggests that these melts would crystallise to broadly trondhjemitic or tonalitic lithologies calculated modal proportions of garnet plagioclase amphibole and rutile in the residuum which control diagnostic trace element signatures in the melt also show the best agreement with natural and experimental data at these conditions importantly although partial melts calculated outside of this p t range still produce ttg like major element compositions and normative mineralogies they would poorly match the diagnostic trace element signatures of natural archaean examples owing to the absences of key minerals in the residuum this optimal p t range of 800 950 c and 10 18 kbar defined herein is most likely to characterise metamorphism of hydrated basalt at the base of a 40 km thick archaean oceanic plateau overthickened crust or else tectonic underplating via shallow subduction which we therefore suggest are the most likely tectonic settings for the formation of the first voluminous continental crust on the early earth