the growth reworking and metamorphism of early precambrian crust in the jiaobei terrane the north china craton constraints from u th pb and lu hf isotopic systematics and ree concentrations of zircon from archean granitoid gneisses abstract archean granitoid tonalite trondhjemite granodiorite ttg gneisses are widely distributed in the early precambrian metamorphic basement of the jiaobei terrane the north china craton ncc and record the formation and evolution of the continental crust in the early precambrian in this study zircon cathodoluminescence cl images have assisted in obtaining u th pb and lu hf isotopic systematics and rare earth element ree concentrations from distinct zircon domains of complex zircons and the results have been integrated in order to unravel the growth reworking and metamorphism of the early precambrian crust in the jiaobei terrane zircons from seven representative archean granitoid samples collected from the jiaobei terrane record three groups of magmatic ages of 2 9 2 7 and 2 5 ga and two groups of metamorphic ages of 2 5 and 1 86 ga the 2 5 ga metamorphic event is thought to be linked to underplating of large amounts of mantle derived magma and the 1 86 ga metamorphic event to arc or continent continent collision magmatic zircons have positive hf t values 0 7 to 12 58 average 5 5 and two stage hf model ages cluster at 2 7 2 8 and 2 9 3 0 ga this indicates that the archean granitoid rocks were derived from partial melting of juvenile crust that was formed mainly during 2 7 2 8 and 2 9 3 0 ga which represent two periods of growth of juvenile crust the most significant period of crustal growth is 2 7 2 8 ga whereas 2 5 ga represents a period of reworking of the 2 7 2 8 ga juvenile crust in the jiaobei terrane as well as other areas in the ncc

destruction of ancient lower crust through magma underplating beneath jiaodong peninsula north china craton u pb and hf isotopic evidence from granulite xenoliths abstract zircons from granulite xenoliths entrained in a late cretaceous mafic dike in the jiaodong peninsula north china craton ncc show three distinct u pb age populations part of the old zircon grains yield discordant data that project to ages of about 2 4 to 2 5 ga a few grains indicate growth at about 2 0 ga and a third group yield cretaceous ages with peaks at 120 and 90 ma the oldest zircons give hf t dm model ages of 2 6 2 8 ga these results demonstrate the existence of original archean lower crust in the jiaodong region zircons of 2 0 ga have similar hf t dm model ages as the neoarchean paleoproterozoic grains suggesting that these zircons were products of metamorphic recrystallization due to thermal event without juvenile input early cretaceous zircons yield ehf t values of 21 to 12 and late cretaceous zircons large variable ehf t from 4 to 50 these data suggest that magmatic underplating occurred in the neoarchean to earliest proterozoic lower crust of the ncc both in the early and late cretaceous it is suggested that the mesozoic magma underplating which also provided the heat source for the voluminous mesozoic magmatism in the ncc significantly modified the composition of the archean to paleoproterozoic lower crust of the ncc

two stage collision related extrusion of the western dabie hp uhp metamorphic terranes central china evidence from quartz c axis fabrics and structures abstract the western dabie orogen also known as the hong an block forms the western part of the dabie sulu hp uhp belt central china rocks of this orogen have been subjected to pervasive ductile deformation and include numerous quartz schists and felsic mylonites cropping out in ductile shear zones quartz textures in these mylonites contain important clues for understanding the movement sense of late collisional extrusion and exhumation of high pressure ultrahigh pressure hp uhp rocks from the lower crustal level to the upper crustal level during middle triassic and early jurassic the orientation and distribution of quartz crystallographic axes were used to confirm the regional shear sense across the orogen the asymmetry of c axis patterns consistently indicates top to the southeast thrusting across the orogen in early structural stages later stages of deformation show different senses of movement in northern and southern parts of the orogen with top to the northwest sinistral shearing recorded in rocks north of the xinxian hp uhp eclogite facies belt and top to the southeast dextral shearing south of the same unit based on our study on quartz c axis fabrics and marco to micro scale structures simultaneous southeastward shearing within a large part of the orogen and normal faulting north of the xinxian hp uhp unit is explained by upward extrusion in early stages of deformation the extrusion process has been attributed to syn and late collisional processes accounting for some earlier deformation in the western dabie orogen such as metamorphic sequences around the core of the xinxian hp uhp eclogite facies unit much higher pressure of deformation is also indicated in the aligned glaucophane and omphacite from blueschist and eclogite in the field an orogen parallel eastward extrusion of the xinxian hp uhp eclogite facies unit however occurred diachronously in later stages of deformation therefore a tectonic model combining an early upward extrusion with a later eastward extrusion is presented two different stages and types of extrusion for exhumation of hp uhp rocks are suitable to all of east central china geochronological data shows that the first upward extrusion occurred during middle triassic the second eastward extrusion occurred during late triassic to early jurassic these two extrusions are correlative with two stages of rapid exhumation of the dabie hp uhp rocks respectively these two stage late collisional middle triassic to early jurassic extrusion events bridge the gap between syn collisional early to middle triassic vertical extrusion and post collisional cretaceous eastward directed lateral escape and provide vital clues to understanding the more detailed processes of exhumation of hp uhp rocks

komatiites from west shandong north china craton implications for plume tectonics the mengyin komatiites are located at the base of the taishan complex 2 7 ga in the western shandong greenstone belt of the eastern block of north china craton the mengyin komatiites have high magnesian low titanium and aluminum undepleted chemical characteristics although the ultramafic rocks underwent amphibolite to greenschist facies metamorphism they preserve remnant igneous spinifex textures some komatiite samples even preserve rare fresh olivine according to the mass equilibrium between the olivine and melt the potential eruption temperature of the mengyin komatiites at 1 atm is about 1270 c which might indicate a plume tectonic setting it is inferred that komatiites in mengyin west shandong may be part of a plume related global magmatic event related to the break up of a supercontinent that included the eastern block of the north china craton 2006 international association for gondwana research published by elsevier b v all rights reserved

origin of paired high pressure ultrahigh temperature orogens a ridge subduction and slab window model introductionmetamorphism at elevated pressure temperature conditions and distinct uid regimes generates ultrahigh pres sure uhp high pressure hp andultrahigh temperature uht rocks harley 2004 brown 2006 2007 2009 kelsey 2008 santosh andomori 2008a b santos et al 2009 santosh et al 2009a zhang et al 2009 although previous studies havecorrelated these di erent orogens todistinct tectonic settings recent inves tigations record high pressure anduht conditions from the same tec tonic environment within the sameorogenic belt and in some cases evenwithin the same samples mostoftheseexamples come from major collisionalzones along which continental blocksweresuturedatdi erenttimesinearthhistory including from the neoarch ean hp uht rocks of the limpopobeltinsouthafrica tsunogaeandvanreenen 2006 the palaeoproterozoichp guo et al 2004 obrien et al 2005 and uht santosh et al 2007a b 2008 2009b belts in thenorth china craton and the hp uht belts in the neoproterozoiceastern ghats belt as well as thecambrian gondwana suture in south ern india shimpo et al 2006 tsuno gaeet al 2008a santoshet al 2009c notably the thermal regimes for thesepaired hp and uht rocks are similar 1000 c and they also share acommon uid history characterizedby co

aeromagnetic study of the hengshan wutai fuping region unraveling a crustal profile of the paleoproterozoic trans north china orogen abstract an integrated crustal profile of the intervening trans north china orogen tnco is one of the key issues to understanding the tectonic evolution of the north china craton however the existing geological studies focus only on the surface mapping based petrological geochemical and structural analysis but lack subsurface geophysical evidence and thus make the crustal profile interpretations ambiguous in contrast the current geophysical data covers a very large scale lithospheric mantle and fails to image the detailed structural pattern of the orogenic crust to achieve this goal we present high resolution aeromagnetic data for the hengshan wutai fuping region the largest exposure of the central tnco the reduced to pole magnetic anomaly map firstly verifies the regional tectonic subdivision that the high grade metamorphic terranes i e hengshan and fuping complexes are consistent with high magnetic responses and long wavelength anomalies intervened by a low grade terrane wutai complex characterized by low magnetic responses and short wavelength anomalies 3d euler deconvolution reveals that the tendencies of the clustered solutions show large consistence with the major structural pattern of the region which is characterized by a fan shaped doubly vergent orogenic wedge upward continuation further shows that the northwest part of the orogen yields a thicker crust and is most likely located closer to the paleosubduction zone the new aeromagnetic data combined with structural petrological and metamorphic data indicate that an eastward dipping subduction zone was most possibly active before the collision of the western and eastern blocks leading to the formation of the tnco and the final amalgamation of the north china craton

aspects of the tectonic evolution of china the subject of this special publication is one of the most interesting in global geoscience the tectonic evolution of china the assemblage of terranes that underlie this part of the world provides outstanding opportunities to elucidate global processes and many of the factors that shape the earth s lithosphere are best exemplified by the geology of china and its immediately adjacent areas in addition there are geological features that are particular and unique to the region some have been the focus of recent attention and have attracted international interest because of their global importance this volume provides accounts of up to date research by chinese and international geological teams on key aspects of the tectonic evolution of china and its surrounding areas the papers describe the formation of the geological terranes that make up this part of east asia place constraints on plate tectonic models for their assembly and provide accounts of unique geological feature of the subcontinent

new evidence from seismic imaging for subduction during assembly of the north china craton comment the frozen in information in the crust plays an important role in improving our understanding of cratonic formation and evolution and plate tectonics in the precambrian the trans north china orogen is a continent continent collision belt generated by the assembly of the north china craton the mechanism and modality of the collision are disputed here we present a seismic image of the western block and the trans north china orogen of the north china craton derived using receiver function analysis of the teleseismic records from a dense array a low velocity zone extending from the middle crust to the moho is interpreted as a remnant of upper middle crustal material associated with westward dipping subduction beneath the western block of the north china craton crustal uplift and magmatic underplating resulting from subsequent tectonic events were responsible for modifying the remaining subduction architecture the western boundary of the trans north china orogen is located west of the boundary earlier identified by surface investigation the results combined with previous seismic imaging in the eastern north china craton provide insight into the amalgamation of the eastern and western blocks and the subsequent tectonic deformation of the north china craton

halaqin volcano sedimentary succession in the central northern margin of the north china craton products of late paleoproterozoic ridge subduction abstract the halaqin volcano sedimentary succession hvss is tectonically situated on top of the largely late archean to early paleoproterozoic yinshan terrane in the central northern margin of the north china craton it comprises greenschist to amphibolite facies schists quartzites marbles and amphibolites which were originally pelites pebbly sandstones sandstones limestones and volcanic rocks the presence of tight to isoclinal folds and thrusts of the hvss indicates tectonic transport from southeast to northwest present coordinates u pb ages of detrital zircon grains in the clastic rocks suggest provenance from both the fengzhen belt in the south and the yinshan terrane in the north zircon u pb ages of the hvss volcanic rocks e g 1909 10 ma 1894 6 ma 1887 6 ma and 1884 8 ma indicate volcanism in the period 1910 1880 ma detrital zircon age study of the neighbouring majiadian group reveals significant 2700 ma crust growth and 2500 ma metamorphism events in the yinshan terrane the hvss volcanic rocks have a trimodal geochemical distribution demonstrated by sio2 contents of 45 50 wt basalts 53 60 wt andesites and 70 76 wt dacites rhyolites the basalts and andesites are tholeiitic in chemistry and show distinct depletion in high field strength elements hfse e g nb zr ti compared to neighbouring elements in spider diagrams specifically low mg basalts and andesites mgo 5 wt have more or less flat ree patterns la ybn 0 45 1 76 and lile ndt t 1890 ma values of the basalts and andesites vary from 4 8 to 0 4 the rhyolites and dacites have strongly enriched light trace element patterns la ybn 8 49 47 9 and lile but depleted hfse e g nb zr ti the basalts and basaltic andesites most likely originated from the asthenosphere contaminated by melts from the lithosphere whereas the dacites and rhyolites likely originated from the crust we propose that the basalts and andesites are extrusive counterparts of the coeval and geochemically similar xuwujia gabbronorites and the dacites and rhyolites are upper crustal equivalents of the contemporaneous chemically comparable liangcheng granitoids which are in the originally underlying fengzhen belt we suggest that the hvss was most likely generated by ridge subduction below active continental margin just prior to the final amalgamation of the north china craton this ridge subduction contributed to the ultra high temperature metamorphism in this area terminal collision tectonics was responsible for the regional metamorphism and high deformation of the halaqin succession when it was transported northwestwards onto the yinshan terrane

spatial distribution of 1950 1800 ma metamorphic events in the north china craton implications for tectonic subdivision of the craton abstract great progress has been made in the last twenty years in understanding the unification of the north china craton according to a prevailing model it developed by amalgamation of four sub blocks by three spatially and temporally separate orogenic belts the first at 1950 ma formed the western block the second at 1900 ma created the eastern block and the third at 1850 ma unified the eastern and western blocks however new data show that the 1950 ma orogen was affected by granulite facies reequilibration at 1800 ma and parts of the 1850 ma orogen have evidence of metamorphism at 1950 1900 ma new zircon u pb ages enable the paleoproterozoic metamorphic events to be statistically sub divided into four at 1955 ma m 1 1920 ma m 2 1885 ma m 3 and 1850 ma m 4 the m 1 2 events always appear together mainly in two belt areas including the helanshan qianlishan northern ordos yinshan liangcheng huai an lushan jiaobei and liaodong regions whereas the m 3 4 events are distributed over larger areas and were superimposed on the m 1 2 activities this age distribution is inconsistent with previous models of three separate orogens it should also be noted that the igneous rocks contemporary with the regional ultra high temperature high pressure granulite facies metamorphisms m 1 2 were previously described as arc related series i e the gabbro norites i s type granites and trimodal volcanics based on the spatial distribution of the paleoproterozoic metamorphic events m 1 4 and of the coeval igneous rocks we speculate that the assembly of the ncc was created by one orogen which formed between the eastern and western cratons in two tectonic stages two marginal arcs the korean and xuwujia developed against both eastern sides of the two sub cratons and were both accreted and deformed at 1965 1900 ma m 1 2 subsequently the amalgamation of the two sub cratons resulted in metamorphism on the western side of the eastern craton and in metamorphism that was superimposed on both accreted arcs at 1900 1790 ma m 3 4

new age ca 2970 ma mantle source composition and geodynamic constraints on the archean fiskenaesset anorthosite complex sw greenland the archean fiskenaesset complex sw greenland consists of an association of ca 550 meter thick layered anorthosite leucogabbro gabbro and ultramafic rocks peridotite pyroxenite dunite hornblendite the complex was intruded by tonalite trondhjemite and granodiorite ttg sheets now orthogneisses during thrusting that was followed by several phases of isoclinal folding the trace element systematics of the fiskenaesset complex and associated volcanic rocks are consistent with a supra subduction zone geodynamic setting the fiskenaesset anorthosites leucogabbros gabbros and ultramafic rocks collectively yield an sm nd errorchron age of 2973 28 ma mswd 33 with an average initial epsilon nd 3 3 0 7 consistent with a long term depleted mantle source regression of pb isotope data define an age of 2945 36 ma mswd 44 and the regression line intersects the average growth curve at 3036 ma slightly lower pb pb errorchron age is interpreted as reflecting partial disturbance of the u pb system in gabbros leucogabbros and ultramafic rocks during intrusion of ttgs complex internal structures in zircons from orthogneisses reveal several episodes of zircon growth and recrystallization taking place between ca 3200 and 2650 ma zircon ages peak at about 3200 3100 3000 2950 2820 and 2750 ma the 3200 3000 ma zircon cores are interpreted as inherited xenocrysts from older reworked crustal rocks 2950 ma is considered as an approximate intrusion age of sampled ttgs the 29402650 ma ages are attributed to metamorphic overgrowth and recrystallization in response to multiple tectonothermal events that affected the fiskenaesset region on the basis of recently published trace element data and new nd and pb isotope and u pb zircon age data a three stage geodynamic model is proposed to explain the evolution of the fiskenaesset complex stage 1 represents the formation of depleted shallow mantle source 3000 ma epsilon nd 3 3 0 7 for the complex stage 2 corresponds to the development of an intra oceanic island arc between 3000 2950 ma stage 3 is characterized by the collision of the island arc with either a passive continental margin or with an older arc between 2950 2940 ma c 2010 elsevier b v all rights reserved less

paleoproterozoic accretionary orogenesis in the north china craton a shrimp zircon study abstract the inner mongolia suture zone imsz and the trans north china orogen tnco incorporate the major paleoproterozoic accretionary orogens in the north china craton ncc with the jiao liao ji belt jljb representing the third one here we investigate the paleoproterozoic tectonics of the imsz and tnco through zircon shrimp geochronology on a representative suite of rocks comprising metasediments and arc magmatic rocks shrimp analysis of zircons with textures indicating extreme recrystallization under ultrahigh temperature uht conditions from the metapelites at heling er in the southern domain of the imsz reveals a single population with a weighted mean 207 pb 206 pb age of 1913 17 ma the zircons in another uht granulite from this locality yield a weighted mean 207 pb 206 pb age of 1910 18 ma these data correlate with the ca 1 92 ga age reported from zircons in sapphirine bearing uht granulites further north and confirm the regional extent of the paleoproterozoic uht metamorphism within the imsz zircons in a charnockite from the southern margin of the khondalite belt fringing the uht granulites in the imsz show two distinct age groups an older population with a magmatic paragenesis and a weighted mean 207 pb 206 pb age of 1932 24 ma and a younger group of metamorphic zircons with an age of 1858 26 ma we also report zircon ages from charnockites in two localities around xing he in the huangtuyao belt belonging to the huai an complex within the westernmost domain of the tnco at the junction with the imsz the charnockite from first locality carries two distinct zircon populations with the older group yielding a weighted mean 207 pb 206 pb age of 2477 2 ma and the younger population showing an age of 1807 38 ma the internal structure as revealed from cl images and the overall high th u values up to 2 42 of the older zircons suggest their magmatic affinity whereas the younger group with extremely low th u 0 02 0 09 is of metamorphic origin zircons from the charnockite in the second locality also define two distinct age clusters with a dominant older magmatic group having a weighted mean 207 pb 206 pb age of 2147 11 ma and a minor younger group with an age of 1958 25 ma the range of ages from 2477 to 2147 ma from magmatic zircons in the charnockites from the eastern periphery of the imsz within the western margin of the tnco in combination with similar ages reported in recent studies from zircons in magmatic complexes within the imsz suggest a prolonged history of subduction related arc magmatism and accretionary tectonics analogous to those in some of the phanerozoic belts such as the central asian orogenic belt and the western pacific subsequent progressive collision and suturing of the continental blocks were accompanied by the exhumation of high pressure hp and uht metamorphic rocks the available data from the imsz and tnco suggest long lived convergent margins associated with the southward subduction of the yinshan block and westward subduction of the eastern block in a double sided subduction realm prior to the final amalgamation of the ncc and its incorporation within the columbia supercontinent in the late paleoproterozoic

prolonged anatexis of paleoproterozoic metasedimentary basement first evidence from the yinchuan basin and new constraints on the evolution of the khondalite belt north china craton abstract discerning the history of burial residence at depth and exhumation of high grade metamorphic rocks from an orogen is essential to understanding crustal differentiation and orogenesis such high grade rocks are inferred to exist beneath the yinchuan basin which is overlain by a thick covering of younger sediments and is located at the inferred southwest boundary of the paleoproterozoic khondalite belt in the northwestern north china craton the metamorphic basement sampled from a recently completed deep borehole in the yinchuan basin is migmatitic and contains an assemblage of garnet biotite sillimanite plagioclase quartz minor k feldspar and local occurrences of muscovite phase equilibrium modelling in the na2o cao k2o feo mgo al2o3 sio2 h2o tio2 o system constrains the peak p t conditions of anatexis to 6 10 kbar at 760 810 c followed by final crystallization at 4 5 5 5 kbar and 690 710 c ti in zircon thermometry for the narrow metamorphic zircon rims provides a temperature range of 687 753 c demonstrating formation during retrogression to the solidus nanosims u pb analyses on the metamorphic overgrowths of zircon grains yields an upper intercept age of 1895 36 ma chronological and chemical analyses of monazite indicate irregular bright gray domains with relatively high th and low y and hree contents developed during prograde metamorphism 1962 8 ma conversely dark domains with low th and high y and hree overgrew during retrograde cooling 1892 14 ma that was synchronous with the development of metamorphic zircon rims la icp ms u pb analyses of detrital zircons reveal that the metamorphic basement beneath the yinchuan basin has a sedimentary protolith of paleoproterozoic age the results directly confirm the existence of paleoproterozoic basement beneath the yinchuan basin and the well constrained p t t path supports the involvement of the basement as part of the western khondalite belt in burial and exhumation processes during the 1 96 1 89 ga period during which anatexis occurred for 70 ma

geochronology geochemistry and petrogenesis of the late palaeoproterozoic a type granites from the dunhuang block se tarim craton china implications for the break up of the columbia supercontinent the discovery of c 1 77 ga a type granite in the tarim craton tc provides the first evidence that supports an extensional event related to fragmentation of the columbia supercontinent in the late palaeoproterozoic we present laser ablation inductively coupled plasma mass spectrometry la icp ms zircon u pb ages lu hf isotopic data and the whole rock geochemical and nd isotopic data of a type granites in the dunhuang area in the se tarim craton zircon u pb dating for three granite samples indicate that they were emplaced at c 1 77 ga zircons from these granites have e hf t values ranging from 5 9 to 8 7 corresponding to two stage model ages of 1 9 2 7 ga these granites exhibit the following petrological geochemical characteristics that are typical of a type granite a high content of sio 2 and alkalis i e high k 2 o na 2 o with k 2 o na 2 o 1 enrichment of high field strength elements hfse and rare earth elements ree except for eu and extreme depletion of ba sr p ti and eu b 10000 ga al ratios in the dunhuang granites of 3 5 4 4 with an average value of 3 79 which is similar to the global average of 3 75 for a type granites c the presence of characteristic minerals such as amphibole sphene and perthite and d zirconium saturation temperature results indicate that the dunhuang granites have high initial magmatic temperatures in the range 887 950 c similar to those of typical of a type granites whole rock e nd t values range from 2 5 to 6 2 and t dm model ages from 2 3 to 2 7 ga nd hf isotopic and whole rock geochemical data indicate that these granites were most likely derived from the late archean crustal source in a post collisional post orogenic extensional tectonic environment the late palaeoproterozoic a type granites in the tc could be correlated with those of the north china craton ncc india and the canadian shield thus demonstrating extensional tectonics and break up of the columbia supercontinent

discovery of sapphirine bearing mg al granulites in the north china craton implications for paleoproterozoic ultrahigh temperature metamorphism abstract we report here for the first time the occurrence of mg al granulites within the khondalite belt of the north china craton and provide robust evidence for extreme crustal metamorphism at ultrahigh temperature uht conditions in this region the uht indicator assemblages in these rocks include equilibrium sapphirine quartz high alumina orthopyroxene sillimanite quartz and high temperature perthites the surrounding khondalites also carry spinel quartz equilibrium assemblage the sapphirines occurring in various microstructural associations are markedly magnesian x mg 0 69 0 77 with a composition varying from 7 9 3 to 2 2 1 end members spinel is a solid solution of mg spinel and hercynite x mg 0 41 0 54 with zno content varying from 0 48 to 1 79 wt garnet shows a compositional range between pyrope and almandine x mg 0 39 0 48 with minor grossular 2 6 3 8 mol and spessartine 0 5 1 8 mol the orthopyroxenes display marked compositional variations in terms of al 2 o 3 content with the core of the medium grained orthopyroxene adjacent to sillimanite in the matrix showing the highest al 2 o 3 content 8 4 wt the history of prograde to peak metamorphism of these rocks is indicated by inclusions of sapphirine spinel quartz and sillimanite within garnet sapphirine formed during the peak stage through the reaction spl qtz sil spr occurrence of sapphirine and quartz inclusions trapped in sillimanite suggests the reaction spr qtz grt opx sil a series of retrograde reactions are observed in these rocks as corona and symplectite textures around early coarse grained minerals such as opx sil qtz crd grt sil qtz crd grt qtz crd opx grt o 2 crd qtz fe 2 o 3 in mag or ilm and finally opx kfs h 2 o bt qtz whereas the stability of sapphirine quartz and orthopyroxene sillimanite quartz in the matrix assemblage suggests t 1000 c and p 10 kbar peak metamorphism conventional geothermobarometric estimates confirm the ultrahigh temperature nature and also trace the retrograde conditions 930 970 c at p 10 kbar al in opx 900 1000 c perthite 930 990 c grt opx and 910 940 c sapphirine spinel electron microprobe dating of monazites in textural association with the uht assemblages yield a sharply defined isochron age of 1917 48 ma which correlates with the 1927 11 ma obtained in a previous study from monazites in the surrounding khondalites carrying spinel quartz assemblage an evaluation of the assemblages and textures using appropriate petrogenetic grids indicate that following peak ultrahigh temperature conditions the rocks underwent initial isobaric cooling and subsequent isothermal decompression and we interpret these trajectories to be part of an overall anti clockwise p t evolution the study area located within the northern margin of the north china craton defines a collisional orogenic belt where continental collision and deep subduction was possibly followed by extensional collapse of the orogen evidence for the extensional phase is provided by the several mafic magmatic intrusives of paleoproterozoic age occurring in the region which suggest a possible link between the thermal anomaly associated with extreme crustal metamorphism in this region with mantle derived magmas the uht metamorphism in north china craton coincided with the assembly of the paleoproterozoic supercontinent columbia

mesozoic crust mantle interaction beneath the north china craton a consequence of the dispersal of gondwanaland and accretion of asia we present evidence from zircons entrained within lower crustal xenoliths in the cenozoic hannuoba basalt of multiple melting events beneath the north china craton in the late mesozoic peak activity was between 180 and 80 ma the upper crustal signature of which was the generation of voluminous granitoids and related volcanic rocks emplacement of dioritic and lamprophyric dikes and widespread gold mineralization the process involved partial loss of mantle lithosphere accompanied by wholesale rising of asthenospheric mantle beneath eastern china we correlate these events with lithospheric thinning resulting from the breakup and dispersal of gondwanaland accompanied by a major mantle overturn fueled by the destruction of oceanic lithosphere and triggered by its sinking into the lower mantle during the subsequent accretion of asia

zircon u pb ages and hf isotopes of the huai an gneisses from the tianzhen xinpingbu area implications for the tectonic evolution of the trans north china orogen abstract one of the major advancements in understanding the formation and evolution of the north china craton was the recognition of a himalayan type collisional belt in the central part of the craton called the trans north china orogen tnco along which two micro continental blocks named the eastern block and western block joined together to form a coherent craton however controversy still remains about the pre collisional tectonic setting and architectures of the tnco in this contribution field based zircon u pb and lu hf isotopic investigations were carried out on the huai an gneisses from the tianzhen xinpingbu area in the northern part of the tnco the huai an gneisses consist predominantly of tonalitic trondhjemitic granodioritic ttg gneisses with minor gabbroic dioritic and granitic gneisses our zircon u pb results show that the plutonic protoliths of the huai an gneisses were emplaced at 2 51 2 40 ga and experienced a granulite facies metamorphic event at 1 86 1 80 ga the magmatic zircons of these samples yield e hf t values from 1 9 to 7 0 and t dm c ages of 2862 2619 ma these positive e hf t values associated with the lack of old inherited zircons suggest the derivation of their precursor magmas from a juvenile crust without contamination of old continental crust the t dm c model ages indicate that 2 8 2 6 ga is the best estimation of the mantle extraction time for the source rocks in the huai an gneisses revealing a major period of juvenile crustal growth thus it can be inferred that the partial melting of subducted oceanic crust slab produced the magmas of the huai an gneisses which subsequently interacted with overlying mantle wedge during the ascent suggesting an intra oceanic arc environment for the huai an gneiss complex taken together the development of the huai an gneiss complex was involved in a subduction related magmatic event in an intra oceanic arc at 2 5 ga followed by collision between the eastern and western blocks at 1 85 ga

timing and tectonic setting of the sijiaying banded iron deposit in the eastern hebei province north china craton constraints from geochemistry and sims zircon u pb dating the north china craton ncc one of the oldest continental blocks in asia has a complicated evolutionary history with the age of the old crust up to 3 8 ga and records the important geological events of the earth the sijiaying bif the largest banded iron formation located in eastern hebei province in the east part of ncc records an important thermo tectonic event previous studies focused on the geological description but the timing and tectonic setting have been unclear so far the sijiaying bif is hosted in neoarchean metamorphic rocks which includes biotite leptynite hornblende plagioclase gneiss and biotite plagioclase gneiss using major element contents and ratios of the host rocks the protoliths of the hornblende plagioclase gneiss and biotite plagioclase gneiss are shown to be dacite whereas those of the biotite leptynites are sedimentary rocks based on geology and geochemistry of the host rocks we infer that the sijiaying bif is an algoma type deposit sims zircon u pb dating shows igneous zircons from the biotite plagioclase gneiss and hornblende plagioclase gneiss have u pb ages of 2535 8 ma and 2543 14 ma respectively zircons from the biotite leptynite have an age of 2537 13 ma we infer that the ages of 2543 2535 ma represent the time of the sijiaying bif paas normalized rey profiles of the ore samples are characterized by lree depletion and hree enrichment positive la and eu anomalies and an average y ho weight ratio of 32 indicating a mixture of submarine hydrothermal fluids and deep seawater ree data was combined with the 18o analyzed by previous researcher of the ore from individual magnetite bands to infer that the sijiaying bif precipitated from hydrothermal fluids discharging on the sea floor the hornblende plagioclase gneiss and biotite plagioclase gneiss that are enriched in lile and lree and depleted in high field strength elements hfse nb ta and ti have geochemical signatures that are similar to those of island arc volcanic rocks the biotite leptynite shows the characteristics of continental island arc based on the geology and geochemistry of associated wall rocks and ore samples we suggest that the sijiaying bif formed in a back arc basin in neoarchean

neoarchean to paleoproterozoic continental evolution and tectonic history of the north china craton a review rocks of archean age are widely distributed in the north china craton ncc and can be divided into high grade regions and granite greenstone belts even though most underwent amphibolite to granulite facies metamorphism quartzo feldspathic gneisses occupy 70 to 85 of the total exposure of archean rocks in the craton supracrustal rocks commonly consist of intercalated volcano sedimentary rocks including metabasites meta intermediate to acid rocks banded iron formations metapelites and minor marbles the oldest rocks outcrop over a small area of 4 km2 and are a quartzite amphibolite sequence that yield 3 8 3 5 ga isotopic ages chemical and geochronological data constrain the earliest main crust forming episode at ca 2 9 2 7 ga when most rocks were metamorphosed to upper amphibolite or granulite facies at moderate pressures mafic granulites can be classified into two textural types referred to as near isobaric cooling ibc and near isothermal decompression itd types high pressure granulites and retrograded eclogites occur mainly near the junction of the provinces of hebei shanxi and inner mongolia in the central ncc and in eastern shandong province of the eastern ncc as well as locally in henan province in the southern ncc these high pressure rocks show clockwise p t paths indicating decompression from 14 kb to 5 6 kb an exposed section interpreted as representing precambrian continental lower crust has also been identified in the northern part of the craton although some mafic ultramafic metamorphic rocks have been interpreted as remnants of ancient oceanic crust and mantle by a number of geologists geochemical and petrological evidence demonstrates that many supracrustal orthogneiss associations have island arc affinities therefore several tectonic models have been proposed including continent continent collision and island arc terrain accretion achieved by arc arc or arc microcontinent collision the ncc underwent multiple high grade metamorphic events of which the two most important took place during the intervals 2600 2450 ma and 1950 1750 ma which are considered to be related to early precambrian supercontinental cycles

isotope geochemistry and geochronology of the qiubudong silver deposit central north china craton implications for ore genesis and lithospheric dynamics abstract the qiubudong silver deposit is a breccia type silver deposit in the fuping ore cluster region within the taihang mountain region of the central north china craton ncc the cylindrical breccia pipe occurs at the conjunction of a ne trending fracture and a nw trending fracture and along the latter a number of quartz porphyry and felsite dikes are emplaced the breccia clasts include quartz porphyry leptite and felsites cemented mainly by felsic magmatic and hydrothermal components the ore formation process can be divided into five stages magnetite quartz stage i biotite quartz stage ii quartz pyrite stage iii quartz polymetallic sulfide stage iv and quartz carbonate stage v hydrothermal alteration is characterized by pyritization sericitization silicification carbonatization potash feldspathization epidotization and chloritization with a systematic zoning from the ore body to the margin with transitional contact boundaries we present s pb he and ar isotope compositions of the ore minerals and evaluate the source of the ore forming materials and fluids the 34 s values of the pyrite range from 0 5 to 3 4 with an average of 1 6 consistent with a mantle input the pb isotope compositions from the sulfide minerals range from 15 799 to 16 049 for 206 pb 204 pb 15 143 to 15 241 for 207 pb 204 pb and 36 999 to 37 590 for 208 pb 204 pb suggesting a lower crustal origin helium isotopic compositions of the fluid inclusions trapped in pyrite vary from 0 45r a to 1 65r a with an average of 0 98r a r a is the 3 he 4 he ratio of air equal to 1 4 10 6 tending to crustal contribution with calculated mantle helium proportions ranging between 5 and 20 average at 12 six pyrite samples from the silver deposit yield a rb sr isochron age of 102 04 0 61 ma whereas crystallization of the quartz porphyry dike is dated by zircon u pb la icp ms chronology at 107 95 0 87 ma these ages suggest a close link between the silver mineralization and magmatism that generated the quartz porphyry combined with previous age data we infer that the magmatism and metallogenesis in the central ncc took place during 140 100 ma with a duration of 40 ma accompanied by mantle upwelling and lithospheric thinning

assembling north china craton within the columbia supercontinent the role of double sided subduction abstract an evaluation of recent s wave receiver functions s wave velocities and two versions of p wave tomographic images along various transects in the north china craton provides some clues on the subduction collision history of the different crustal blocks and their final amalgamation within the paleoproterozoic columbia supercontinent interpretation of a n s seismic section of the craton suggests thick slab debris sinking to various depths in the mantle the w e seismic corridors show the preservation of a thick 200 km lithospheric root tectosphere beneath the ordos block and its variable and extensive erosion towards the yanliao block eastern block this zone is characterized by layers with marked velocity contrast and suggests repeated stacking of the remnants of underplated and accreted paleoproterozoic oceanic lithosphere the present day lithosphere asthenosphere boundary beneath this region probably marks the erosional plane along which decratonization occurred through subduction erosion from the east and thermal and material erosion by upwelling asthenosphere from below resulting in the partial destruction of the tectosphere and its thinning towards the east within the asthenosphere below the yanliao block younger and thinner slabs predominate in the absence of any prominent thick high velocity layers these younger slabs define a westward polarity and constitute a mega scale duplex formed by underplating through phanerozoic subduction process particularly the pacific plate subduction from the east the lithologic associations within the inner mongolia suture zone dividing the yinshan block to the north and ordos block to the south correspond to an accreted ocean plate stratigraphic sequence with the tonalite trondhjemite granodiorite ttg gneisses charnockites and calc alkaline granites representing a continental arc built up through subduction from the north the seismic transects bring out a contrasting polarity in the subduction regime with an oblique east to southward subduction of the yinshan block and a westward subduction of the yanliao block here i propose a double sided subduction history for the ncc similar to the ongoing subduction process in the western pacific such double sided subduction is considered to promote rapid amalgamation of continental fragments within supercontinents and the subduction polarities and mantle dynamics of ncc are therefore considered to be critical in evaluating the final assembly of the paleoproterozoic supercontinent columbia

geochronology mineralogy and geochemistry of alkali feldspar granite and albite granite association from the changyi area of jiao liao ji belt implications for paleoproterozoic rifting of eastern north china craton abstract paleoproterozoic granites divided into alkali feldspar granite and albite granite were newly discovered in the changyi area of the jiao liao ji belt from the eastern block of the north china craton la icp ms zircon u pb dating shows that the alkali feldspar granite was emplaced at 2193 ma whereas the albite granite intruded at 2171 ma these granites have high sio2 na2o k2o zr nb ga y and hree contents and show remarkably low cao eu ba and sr concentrations strongly suggesting their a type affinities zircon saturation thermometry and alt in biotite geobarometer indicate that the alkali feldspar granite crystallized at temperatures of 823 856 c averaging at 839 c and pressures of 1 54 2 73 kbar averaging at 2 05 kbar while the albite granite was cooled at the temperatures of 851 853 c and the pressures of 2 34 2 64 kbar averaging at 2 45 kbar combined with such high temperatures and low pressures the crustal like nd hf isotopic compositions nd t 5 3 to 4 4 hf t 7 0 to 3 8 support their derivation from the shallow melting of archean crustal rocks under high heat flow however the contrasted k2o and na2o compositions between the alkali feldspar granite k2o 4 76 5 14 wt na2o 4 76 5 14 wt and the albite granite k2o 0 29 0 44 wt na2o 6 15 6 56 wt imply that they may have been derived from distinct source rocks or have undergone different magmatic processes genetic modeling suggests that the alkali feldspar granite was originated from melting of granodioritic rocks the albite granite however was most likely derived from the reworking of plagioclase rich layers after the extraction of a type melts coupled with other studies on sedimentary and volcanic successions in the jiao liao ji belt the unusual association of the a type alkali feldspar granite and albite granite indicates that the southern segment of the jiao liao ji belt experienced continental rifting during 2 1 2 2 ga

age and genesis of the neoarchean algoma type banded iron formations from the dengfeng greenstone belt southern north china craton geochronological geochemical and sm nd isotopic constraints abstract banded iron formations bifs are important for understanding the composition and redox condition of paleo oceans as well as the evolution of the precambrian crust the neoarchean dengfeng greenstone belt ca 2 54 2 50 ga in the southern north china craton ncc consists of metamorphosed igneous and sedimentary assemblages including some bifs here we present mineralogical geochronological geochemical and sm nd isotopic data of the dengfeng bifs for the first time to constrain their age origins and source characteristics and then discuss their paleo environmental implications the bifs are mainly associated with metabasaltic rocks and quartz rich schists and can be classified as algoma type bifs zircon u pb dating of quartz rich schists interbedded with the bifs coupled with ages of wall rock metabasites demonstrate that the dengfeng bifs formed at 2 53 2 51 ga most bif samples have high contents of sio2 and fe2o3t and extremely low contents of al2o3 tio2 and high field strength elements e g zr hf th u indicating that they are detrital free deposits in the post archean australia shale normalized multi element diagram the detritus free bif samples are characterized by depleted light rare earth elements positive la and y anomalies and suprachondritic y ho ratios resembling geochemical features of modern seawater the absence of negative ce anomaly indicates an anoxic condition positive eu anomalies and relatively high concentrations of transition metal elements suggest that high temperature hydrothermal fluids contributed to the source of bifs isotopically all bif samples have positive end t values 0 12 to 4 53 consistent with the range of those 0 10 to 4 31 of ca 2 66 2 50 ga dengfeng metabasites and ttg gneisses suggesting input of possible juvenile crustal fluxes the elemental and isotopic characteristics of the dengfeng bifs collectively suggest that they precipitated from anoxic seawater with various input of hydrothermal fluids and oceanic crust derived fluxes

ree geochemistry of carbonates from the guanmenshan formation liaohe group ne sino korean craton implications for seawater compositional change during the great oxidation event abstract the worldwide 2 33 2 06 ga positive 13 c carb excursion has been correlated with the great oxidation event goe and termed as the lomagundi event the 2 3 1 85 ga guanmenshan formation in the liaohe group of the northeastern sino korean craton is characterized by positive 13 c carb excursion and is a potential candidate to evaluate the lomagundi event using rey rare earth element and yttrium ree y chemical fingerprints here we present major and trace element analysis of 42 samples from the guanmenshan formation which are pure marine chemical sediments and use the data to trace the seawater composition during 2 3 1 85 ga 15 least altered dolomicrite samples 600 m strata have ree values of 0 739 4 175 ppm 2 414 1 184 ppm and the y ho ratios of 34 5 56 6 44 1 5 7 they show uniform positive la sn la sn 1 04 0 27 and gd sn gd sn 1 64 0 40 anomalies and notable lree depletions indicated by nd sn yb sn values of 0 24 0 92 average 0 56 0 19 these features are consistent with the geochemistry of well oxygenated shallow ambient seawater and suggest that these samples provide a robust record of the primary rey signature of seawater during the lomagundi event the rey patterns of 15 silicified dolomites marbles locally with veinlets from the pb zn mining camps in the region with average eu cn eu cn 1 56 0 95 are identical to those of high temperature hydrothermal fluids 250 c characterized by a flat pattern and marked positive eu anomalies indicating that these rocks were subjected to metasomatism by hydrothermal fluids the guanmenshan formation shows average ce sn ce sn of 0 93 0 09 and sm cn yb cn of 1 which are higher than those of the archean 2 33 ga chemical sediments generally f o 2 and p co 2 the rey in the dolomicrite were mainly sourced from fluxes of solutes from terrestrial weathering and also from seafloor hydrothermal processes on a subordinate scale the guanmenshan dolomicrites have eu sn eu sn values of 1 34 2 55 i e around 1 53 indicating that they were deposited during 2 33 2 06 ga as the eu sn eu sn 1 53 can be used as a proxy for the 2 33 2 06 ga marine chemical sediments our study shows that the guanmenshan formation was formed at a critical turning point in earth history when the global atmosphere hydrosphere system witnessed a dramatic change from reducing to oxidizing conditions

tectonothermal history of the basement rocks in the western zone of the north china craton and its tectonic implications the basement of the north china craton can be divided into the eastern central and western zones based on lithological structural metamorphic and geochronological data the western zone comprises two different petrotectonic units archaean tonalitic trondhjemitic granodioritic ttg grey gneisses and metamorphic mafic rocks and palaeoproterozoic khondalite series the former is characterized by isobaric cooling ibc type anticlockwise p t paths in the north northwestern part of the zone and near isothermal decompression itd type clockwise p t paths in the eastern part adjacent to the central zone on the other hand the tectonothermal evolution of palaeoproterozoic khondalite series rocks is characterized exclusively by nearly isothermal decompression following the peak of metamorphism and then cooling defining clockwise p t paths the archaean ttg gneisses and associated mafic rocks with anticlockwise metamorphic p t paths reflects an origin related to underplating and intrusion of mantle derived magmas which may be derived from mantle plumes they represent a late archaean continental block in the western part of the north china craton the palaeoproterozoic khondalite series rocks represent passive continental margin deposits they were metamorphosed and deformed in the late palaeoproterozoic during the amalgamation of the western continental block with another continental block in the east part of the north china craton the itd type clockwise p t t paths of the palaeoproterozoic khondalite series rocks record the tectonothermal histories of the collision of the western and eastern continental blocks which resulted in the final assembly of the north china craton at c 1800 ma

collision leading to multiple stage large scale extrusion in the qinling orogen insights from the mianlue suture abstract the geologic framework of the phanerozoic qinling dabie orogen was built up through two major suturing events of three blocks from north to south these include the north china craton including the north qinling block the qinling dabie microblock and the south china craton including the bikou block separated by the shangdan and mianlue sutures the mianlue suture zone contains evidence for mesozoic extrusion tectonics in the form of major strike slip border faults surrounding basement blocks a late paleozoic ophiolite and a ca 240 200 ma thrust belt that reformed by 200 150 ma thrusts during a type intracontinental subduction the regional map pattern shows that the blocks are surrounded by complexly deformed devonian to early triassic metasandstones and metapelites forming a regional scale block in matrix melange fabric five distinct tectonic units have been recognized in the belt 1 basement blocks including two types of precambrian basement crystalline and transitional 2 continental margin slices including early paleozoic strata and late paleozoic fluviodeltaic sedimentary rocks proximal and distal fan clastics reflecting the development of a north facing rift margin on the edge of the south china plate 3 out of sequence oceanic crustal slices including strongly deformed postrift deep water sedimentary rocks sheeted dikes basalts and mafic ultramafic cumulates of a late paleozoic ophiolite suite developing independent of the rift margin in a separate basin 4 out of sequence island arc slices 5 accretionary wedge slices all the tectonic units were deformed during three geometrically distinct deformation episodes d 1 d 2 and d 3 during 240 200 ma units 2 4 involved southward thrusting and vertical then southward extrusion of about 20 km of horizontal displacement above the autochthonous basement during the d 1 episode thrust slices 20 km south of the mianlue suture are related to this vertical extrusion due to the same rock assemblages ages and kinematics the d 2 and d 3 episodes folded all the units in a thick skinned style about east west d 2 and west northwest d 3 axes in the mianlue suture zone an early foreland propagating sequence of accretion of late paleozoic rocks deposited above the yangtze craton is not involved in d 1 deformation but is temporally equivalent to the d 2 and d 3 deformation in the mianlue suture two stages of strike slip faulting mainly occurred at the end of d 2 and d 3 respectively during d 2 deformation the bikou block was obliquely indented to the ese into the mianlue suture rather than being thrust over the mianlue suture from the north as a part of the qinling dabie microblock during d 3 deformation however the bikou block was bounded by the south boundary fault of the mianlue suture and the yangpingguan fault on the south these faults are coeval strike slip faults but of opposite senses and accommodated minor southwestward extrusion of the bikou block into songpan ganze orogen the other basement blocks north of the mianlue suture were extruded eastward by about 20 km of lateral displacement based on the offset of the wudang dome during the d 3 episode due to the northeastward indentation of the hannan complex of the south china craton post d 3 emplacement of granite cutting across the strike slip faults such as the mianlue suture provides a minimum age of 200 ma for d 3 deformation therefore based on insights from the evolution of the mianlue suture the d 2 and d 3 episodes in the mianlue suture and its neighbors are not responsible for and associated with the two stage extrusion of the dabie uhp hp terranes from the foping dome to the present erosional surface more than 350 km

deformation history of the hengshan wutai fuping complexes implications for the evolution of the trans north china orogen abstract the trans north china orogen separates the north china craton into two small continental blocks the eastern and western blocks as one of the largest exposure in the central part of the orogen the hengshan wutai fuping complexes consist of four lithotectonic units the wutai hengshan and fuping complexes and the hutuo group the hengshan complex contains high pressure mafic granulites and retrograded eclogites structural analysis indicates that most of the rocks in these complexes underwent three distinct episodes of folding d 1 to d 3 and two stages of ductile thrust shearing stz 1 between d 1 and d 2 and stz 2 after d 3 the d 1 deformation formed penetrative axial planar foliations s 1 mineral stretching lineations l 1 and rarely preserved small isoclinal folds f 1 in the hengshan and fuping complexes in the wutai complex however large scale f 1 recumbent folds with sw vergence are displayed by sedimentary compositional layers penetrative transposition resulted in stacking of thrust sheets which are separated by ductile shear zones stz 1 the kinematic indicators of stz 1 in the hengshan and wutai complexes show top to the s230 w thrusting likely related to northeastward oblique pre collisional subduction d 1 resulted in crustal thickening with resultant prograde peak metamorphism the hutuo group did not undergo the d 1 deformation either because sedimentation was coeval with the d 1 deformation or because it was at a high structural level and was not influenced directly by the early deformation the d 2 deformation produced nw verging asymmetric and recumbent folds the d 2 deformation is interpreted to have resulted from collision between the eastern and western blocks of the north china craton in the hutuo group and the fuping complex the development of ese verging asymmetric tight folds is associated with d 2 the structural pattern resulting from superimposition of d 1 and d 2 is a composite synform in the hengshan wutai fuping complexes all four lithotectonic units were superposed during the later d 3 deformation the d 3 deformation developed nw trending open upright folds ongoing collision led to development of transpressional ductile shearing stz 2 forming the transpressional zhujiafang dextral ductile shear zone between the northern hengshan complex and the southern hengshan complex and generating the sinistral longquanguan ductile shear zone between the fuping complex and the wutai complex respectively the stz 1 and d 2 deformation were possibly responsible for fast syn collisional exhumation of the high pressure mafic granulites and retrograded eclogites the structural patterns and elucidation of the deformation history of the hengshan wutai fuping complexes places important constraints on the tectonic model suggesting that an oceanic lithosphere between the eastern and western blocks underwent northeastward directed oblique subduction beneath the western margin of the eastern block and that the final closure of this ocean led to collision between the two blocks to form the coherent basement of the north china craton

response of the cr isotope proxy to cretaceous ocean anoxic event 2 in a pelagic carbonate succession from the western interior seaway abstract chromium offers a redox sensitive isotopic proxy with potential for tracing past oxygen levels in the oceans we explore this potential in a pelagic succession of marine carbonate sediment deposited during cretaceous ocean anoxic event 2 oae 2 in the western interior seaway colorado wis using the usgs portland 1 core reduction of cr vi causes light isotopes of cr to preferentially partition into cr iii because cr vi is the thermodynamically favoured species in oxygenated seawater and cr iii is relatively insoluble under the same conditions increased removal of cr iii into anoxic marine sediment during ocean anoxic events should cause positive shifts in seawater 53cr values this assumes that isotopic fractionation associated with all cr removal fluxes from the oceans was constant during oae 2 and that there was no change in cr input fluxes to the oceans here we report findings that counter this prediction namely evidence for a negative shift in seawater 53cr during oae 2 of 1 1 in the wis the magnitude of the excursion depends on the speciation of cr removed in this setting over the duration of oae 2 as well as the fractionation factor accompanying the removal flux of chromate into carbonate sediment both of which are uncertain the 53cr excursion reaches values as low 0 09 during oae 2 which is indistinguishable from the high temperature igneous rock baseline value of 0 124 0 101 2 moreover the minima in the 53cr profile coincides with a peak in cr concentrations in the study core as well as anomalous enrichments of other trace metals of basaltic affinity building on previous studies linking oae 2 to a massive episode of submarine volcanic activity we attribute the decrease in carbonate 53cr values during oae 2 to the expansion of a hydrothermal superplume created during eruptions of the caribbean large igneous province lip this metal laden plume enriched in cr iii and probably anoxic moved from deep waters of the eruption site in the eastern pacific into deep waters of the proto north atlantic through an oceanic gateway in the central americas once inside anoxic metal rich waters upwelled against the surrounding continental margins where isotopically unfractionated cr iii whose solubility was improved by organic complexation and or oxidation to cr vi mixed with the isotopically heavy pool of oceanic chromate in surface waters above the redoxcline application of a simple mixing equation shows that hydrothermal cr contributed 90 of the authigenic cr in the trace metal enriched interval of the study core the increased flux of hydrothermal cr was large enough to mask the expected isotopic response of the ocean cr cycle to increasing anoxia we consider this interpretation to be robust in that it is not substantially affected by uncertainty in the speciation of hydrothermal cr nor uncertainty in the fractionation factor sw caco 3 associated with chromate removal into marine carbonate sediment which we measured in the modern caribbean sea to be 0 46 0 14 2

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

thermal evolution of two textural types of mafic granulites in the north china craton evidence for both mantle plume and collisional tectonics mafic granulites from the north china craton can be divided into two textural types referred to as a and b types a type mafic granulites display garnet quartz symplectic coronas and outcrop in the eastern and western zones of the craton whereas b type mafic granulites exhibit orthopyroxene plagioclase clinopyroxene symplectites or coronas and are mainly exposed in the central zone of the craton most a type mafic granulites preserve the prograde m 1 peak m 2 and post peak near isobaric cooling m 3 assemblages which are represented respectively by inclusions of hornblende plagioclase quartz a peak mineralogy of orthopyroxene clinopyroxene plagio clase quartz garnet and overprinted by garnet quartz symplectic coronas these mineral assem blages and their p t pressure temperature estimates define anticlockwise p t evolutionary paths the b type mafic granulites preserve the peak m 1 post peak near isothermal decompression m 2 and cooling m 3 assemblages which are represented by the peak assemblage of orthopyroxene clinopyroxene plagioclase quartz garnet hornblende post peak orthopyroxene plagioclase clinopyroxene symplectites or coronas and later hornblende plagioclase magnetite symplectites respectively these mineral assemblages and their p t estimates define clockwise p t paths the anticlockwise p t paths of the a type mafic granulites in the eastern and western zones of the north china craton are consistent with a model of underplating and intrusion of mantle derived magmas in combination with lithological structural and geochronological data the eastern and west ern zones of the north china craton are considered to represent two continental blocks that devel oped through the interaction of mantle plumes with the lithosphere from the palaeoarchaean to the neoarchaean era the b type mafic granulites and associated rocks in the central zone represent a magmatic arc that was metamorphosed and deformed during amalgamation of the eastern and west ern continental blocks in the late palaeoproterozoic era the mineral reaction relations and clockwise p t paths of the b type mafic granulites from the central zone record the tectonothermal history of the collision that resulted in the final assembly of the north china craton at c 1800 ma

archean granulite gneisses from eastern hebei province china rare earth geochemistry and tectonic implications the granulite gneisses and their retrograded products of the qianxi group from eastern hebei province china have been investigated for their isotope and trace element geochemistry a consistent age of about 2 5 ae has been obtained by the rb sr and sm nd whole rock isochron methods in agreement with the zircon u pb data pidgeon 1980 d y liu unpubl geochemical arguments from initial isotopic ratios isr and ind and elemental distribution patterns have led us to conclude that this age of about 2 5 ae represents the time of granulite facies metamorphism which must have followed closely the primary emplacement of their protoliths previous claims for early archean ages 3 5 ae of these granulites are not substantiated the mineral isotope systematics register an important thermal event at about 1 7 ae roughly corresponding to the time of the widespread luliang orogeny ma and wu 1981 or chungtiao movement huang 1978

a late archean foreland fold and thrust belt in the north china craton implications for early collisional tectonics abstract the archean north china craton is divided into the western and eastern blocks along the central orogenic belt a 1600 km long archean foreland basin and thrust belt fringes the eastern side of the central orogenic belt rocks in the orogen form tectonically stacked east vergent fold and thrust sheets including foreland basin sediments 2 50 ga ophiolitic melange and an island arc complex foreland basin sediments overlie a passive margin sequence and include a 2 50 ga deep water turbidite sequence that grades upward and westward into shallow water molasse now disposed in structurally imbricated east verging thrusts and asymmetric folds that gradually migrated craton ward with deformation uplift and erosion of the orogen there is a strong linked relationship of the formation of the foreland basin to collision of the east and west blocks of the north china craton along the central orogenic belt at 2 50 ga the qinglong foreland basin and central orogenic belt of the north china craton represents one of the best preserved archean orogen to craton transitions in the world its classic internal to external zonation and flexural response to loading demonstrate that convergent tectonics in the archean were broadly similar to phanerozoic convergent margin processes

geochronology and geochemistry of the paleoproterozoic yinyugou group in the southern north china craton implications for provenance and tectonic evolution abstract the paleoproterozoic yinyugou group in the southern north china craton is a littoral facies sedimentary sequence which is mainly composed of feldspar quartzites and mica schists with minor carbonate rocks u pb isotopic dating of detrital zircons bracket the depositional age of the yinyugou group at 2 43 2 30 ga chemical index of alteration cia values of most feldspar quartzite and schist samples are lower than 75 indicating generally weak to moderate source weathering conditions for the yinyugou rocks the majority of the studied yinyugou samples give a large range of index of compositional variability icv values of 0 93 2 59 revealing a chemically immature source which deposited in an active continental margin setting chondrite normalized ree patterns and discrimination diagrams involving la th hf co and sc reveal that the yinyugou group dominantly received detritus from felsic sources with minor contribution from mafic igneous rocks u pb ages of detrital zircons from the yinyugou group can be divided into two groups 2 90 2 40 ga and 3 50 3 20 ga with a predominant age peak at 2 50 ga the 3 50 3 20 ga detrital zircons have negative ehf t values ranging from 11 2 to 3 8 and old crustal model ages tdmc of 4246 to 3909 ma indicating a mature crustal source most 2 50 ga zircons show sub depleted mantle positive ehf t values and tdmc ages mainly ranging from 3 10 to 2 70 ga representing the time when juvenile materials were extracted from the depleted mantle taking all these data into account the yinyugou group was most likely deposited in a back arc setting in combination with previous studies on other paleoproterozoic volcanic sedimentary sequences and magmatic rocks in the ncc a cycle of subduction collision extension rift subduction processes is proposed to explain the tectonic evolution of the ncc during paleoproterozoic

lithotectonic elements and geological events in the hengshan wutai fuping belt a synthesis and implications for the evolution of the trans north china orogen the hengshan wutai fuping belt is located in the middle segment of the trans north china orogen a palaeoproterozoic continental collisional belt along which the eastern and western blocks amalgamated to form the north china craton the belt consists of the medium to high grade hengshan and fuping gneiss complexes and the intervening low to medium grade wutai granite greenstone terrane and most igneous rocks in the belt are calc alkaline and have affinities to magmatic arcs previous tectonic models assumed that the hengshan and fuping gneiss assemblages were an older basement to the wutai supracrustal rocks but recent studies indicate that the three complexes constitute a single long lived neoarchaean to palaeoproterozoic magmatic arc where the wutai complex represents an upper crustal domain whereas the hengshan and fuping gneisses represent the lower crustal components forming the root of the arc the earliest arc related magmatism in the belt occurred at 2560 2520 ma marked by the emplacement of the wutai granitoids which was followed by arc volcanism at 2530 2515 ma forming the wutai greenstones extension driven by widespread arc volcanism led to the development of a back arc basin or a marginal sea which divided the belt into the hengshan wutai island arc japan type and the fuping relict arc at 2520 2480 ma subduction beneath the hengshan wutai island arc caused partial melting of the lower crust to form the hengshan tonalitic trondhjemitic granodioritic ttg suites whereas eastward directed subduction of the marginal sea led to the reactivation of the fuping relict arc where the fuping tonalitic trondhjemitic granodioritic suite was emplaced in the period 2360 2000 ma sporadic phases of isolated granitoid magmatism occurred in the hengshan wutai fuping region forming 2360 ma c 2250 ma and 2000 2100 ma granitoids in the hengshan complex the c 2100 ma wangjiahui and dawaliang granites in the wutai complex and the 2100 2000 ma nanying granitoids in the fuping complex atc 1920 ma the hengshan wutai island arc underwent an extensional event possibly due to the subduction of an oceanic ridge leading to the emplacement of pre tectonic gabbroic dykes that were subsequently metamorphosed together with their host rocks to form medium to high pressure granulites at 1880 1820 ma the hengshan wutai fuping arc system was juxtaposed intensely deformed and metamorphosed during a major and regionally extensive orogenic event the l uliang orogeny which generated the trans north china orogen through collision of the eastern and western blocks the hengshan wutai fuping belt was finally stabilized after emplacement of a mafic dyke swarm at 1780 1750 ma

precambrian geology of china preface abstract the north china south china and tarim cratons compose the tectonic framework of china in the last decade extensive structural metamorphic geochemical geochronological and geophysical investigations have been carried out on these cratonic blocks producing an abundant amount of new data and competing interpretations which have resulted in refinement of and in some cases significant reinterpretations of the precambrian crustal formation and evolution of china this forms a justification for organizing this special issue in which we invited representative experts working in the north china south china and tarim cratons to present overviews or detailed studies on the formation and tectonic evolution of major precambrian blocks in china twenty four contributions collected in this special issue provide current knowledge of the components and tectonic evolution of major precambrian blocks in china and discuss some key issues based on new data and interpretations which will provide important insights into understanding the precambrian geology of china

two stage triassic exhumation of hp uhp terranes in the western dabie orogen of china constraints from structural geology abundant exposures of widely distributed hp uhp metamorphic rocks in the western part of the dabie orogen enable us to study the tectonic evolution of hp uhp terranes associated with the world s largest preserved continental subduction zone previous tectonic models for the dabie orogen were based largely on metamorphic studies most of them lacking significant structural constraints we present a comprehensive structural analysis based on detailed structural geology the results suggest that syn uhp d0 at 241 231 ma and syn hp d1 at 225 215 ma southeast vergent thrusting formed a series of stacked structural slices this was followed by southeast vergent folding under amphibolite facies conditions d2 at 215 205 ma then a third generation of flexural folding occurred at shallow levels d3 at 200 184 ma this leads us to proposes a two stage triassic exhumation model in which initially rapid vertical extrusion d0 d1 from uhp to hp conditions to lower crustal levels is followed by slow southeastward extrusion d3 from lower crustal levels to the earth s surface the tectonic model combines the early southeastward vertical extrusion with the later southeastward lateral extrusion revealing two different stages and thus different types of triassic extrusion for the exhumation of hp uhp rocks in the dabie orogen the first stage extrusion occurred in the middle triassic whereas the second stage extrusion lasted from the late triassic to early jurassic these two extrusion episodes correlate with the two stages of triassic exhumation of the dabie hp uhp rocks respectively during continental collision

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

the paleoproterozoic north hebei orogen north china craton s collisional suture with the columbia supercontinent abstract understanding the geologic history and position of the north china craton in the paleoproterozoic columbia supercontinent has proven elusive paleoproterozoic orogenic episodes 2 00 1 85 ga are temporally associated with ultimate stabilization of the north china craton ncc followed by the development of extensive craton wide rift systems at 1 85 1 80 ga the age difference between the sedimentary cover and the metamorphic basement is up to 500 700 ma suggesting that uplift and doming of cratonic basement occurred in the latest paleoproterozoic mafic dike swarms 1 80 1 77 ga and anorogenic magmatism 1 80 1 70 ga record the extensional breakup and dispersal of the north china craton during this stage the late paleoproterozoic tectonic framework and geological events documented provide important constraints for reconstruction of the ncc within the late paleoproterozoic supercontinent of columbia an east west striking thousand kilometer long belt of khondalites granulite facies metapelites stretches along the northern margin of the north china craton on the cratonward side of the northern hebei orogenic belt this granulite belt includes mg al sapphirine bearing granulites that reached ultrahigh temperature peak metamorphic conditions of 1000 c at 10 kbars at 1927 11 ma following peak ultrahigh temperature conditions the rocks underwent initial isobaric cooling and subsequent isothermal decompression and these trajectories are interpreted to be part of an overall anti clockwise p t evolution indicating that the northern margin of the craton experienced continental collision at 1 93 1 92 ga the position of the khondalite belt south of the northern hebei orogenic belt makes it analogous to tibet a continental collision related plateau characterized by double crustal thicknesses and granulite facies metamorphism at depth we suggest that the tectonic evolution of the ncc during this period was closely related to the assembly and break up of the columbia supercontinent and that the ncc was adjacent to the baltic and amazonian cratons in the period 2 00 1 70 ga craton wide extension occurred within 100 150 ma of collision along the northern margin of the craton at 1 93 1 92 ga it is concluded that mantle upwellings are chiefly responsible for the breakup of the ncc from the paleoproterozoic supercontinent

palaeoproterozoic assembly of the north china craton the basement of the north china craton consists of the eastern and western blocks separated by the central zone both the eastern and western blocks are dominated by late archaean tonalitic trondhjemitic granodioritic gneiss complexes interdigitated with minor supracrustal rocks metamorphosed at 2 5 ga with anticlockwise p t paths the central zone is composed of reworked late archaean components and palaeoproterozoic juvenile crustal materials that underwent regional metamorphism at 1 85 ga with clockwise p t paths involving isothermal decompression as a result of collision between the eastern and western blocks which resulted in the final assembly of the north china craton

zircon u pb geochronology and hf isotopic composition of the hongqiyingzi complex northern hebei province new evidence for paleoproterozoic and late paleozoic evolution of the northern margin of the north china craton abstract the hongqiyingzi complex located at the northern margin of the north china craton was previously defined as a paleoproterozoic unit it consists of volcanic and sedimentary rocks which underwent greenschist to amphibolite facies metamorphism this study presents zircon u pb ages and hf isotopic composition and whole rock nd isotopic composition of metamorphosed igneous and sedimentary rocks from the hongqiyingzi complex the age dating results show that different gneisses of this complex contain mainly late paleoproterozoic 1800 ma and late paleozoic 275 to 300 ma zircons and minor late archean zircons late paleozoic zircons either occur as magmatic grains or as metamorphic overgrowth and both types of zircon can be distinguished by their th u ratios and internal structures most zircon grains of a paleozoic magmatic rock sample hq14 have high e hf t values of 1 5 to 10 8 suggesting significant contribution of mantle or juvenile crustal material to the magma source s the results imply that the hongqiyingzi complex contains precambrian supracrustal sequences and paleozoic rocks of magmatic origin that underwent strong metamorphism and deformation during the late paleozoic related to the closure of the paleo asian ocean between siberia and the north china craton

uppermost mantle structure beneath eastern china and its surroundings from pn and sn tomography the pn and sn residuals from regional events provide strong constraints on the structure and lithological characteristics of the uppermost mantle beneath eastern china and its surroundings with the dense chinese digital seismic network in eastern china separate pn and sn tomographic inversions have been exploited to obtain p and s velocities at a resolution of 2 2 or better the patterns of p velocities are quite consistent with the s velocities at depth of 50 and 60 km but the amplitude of p wave speed anomalies are a little larger than those of s wave speed the low p wave speed high s wave speed and low vp vs ratio beneath the northern part of ordos basin are related to upwelling hot material abrupt changes in material properties are indicated from the rapid variations in the vp vs ratio

tectonic units of the early precambrian basement within the north china craton constraints from gravitational and magnetic anomalies abstract the tectonic divisions of the north china craton ncc basement has always been a hot and yet unresolved topic we collected the latest gravity and magnetic data and then used a two dimensional discrete wavelet transform method to decompose the bouguer gravity anomaly of the ncc into the 1st to 8th approximations and details respectively we then compared the dynamic tectonic division schemes with the gravity and magnetic division schemes for the precambrian basement at the end of the neoarchean and archean our results show that the 2nd order detail field of the bouguer gravity anomalies is well consistent with the paleoproterozoic tectonic divisions of the ncc by zhao et al 2005 for it clearly outlines the 3 unit layout of the eastern and western blocks and the central trans north china orogen with some anomalous zones identically trending with the dike swarms faults and volcanic rocks generally produce magnetic anomalies but the main anomalous zones in the magnetic field are merely distributed around the plate boundaries of the ncc it may indicate that the amalgamation of the ncc was mostly completed in the paleoproterozoic the 5th order detail field of bouguer gravity anomalies of the ncc corresponds well with its tectonic division at the end of the neoarchean for it well reflects the general boundaries and axial trends of the archean units these archean units are mainly separated into three paleoproterozoic tectonic units by paleoproterozoic tectonic boundaries respectively the magnetic anomalies within the western and eastern blocks have separately similarities with the trans north china orogen the 6th order detail fields however match well the boundaries of the 10 continental nucleii of the archean ncc but the anomalous distributions in the 6th order detail are also divided by the paleoproterozoic tectonic borders in view of the gravity and magnetic anomalies it further reveals that the ncc basement may have experienced a dynamic evolutionary process from continental nucleation to micro continental amalgamation and to a final unified crystalline basement

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

the petrology of two distinct granulite types in the hengshan mts china and tectonic implications abstract the archean to proterozoic hengshan complex north china craton comprises tonalitic and granodioritic gneisses with subordinate mafic lenses pegmatites and granites amphibolite facies assemblages predominate although granulite facies relics are widespread and greenschist facies retrogression occurs in km wide shear zones mafic lenses locally abundant occur as strongly deformed amphibolite hornblende plagioclase boudins or sheets in contrast to previously published models we find two series of mafic rocks with distinctly different granulite facies evolutions in the north of the complex relict high pressure mafic granulites are garnet clinopyroxene bearing rocks with a secondary development of orthopyroxene around both garnet kelyphites and clinopyroxene coronas south of the newly defined central e w trending zhujiafang shear zone numerous mafic boudins and less deformed dykes exhibit a macroscopically visible magmatic texture with coronitic growth of metamorphic garnet full of quartz inclusions between the magmatic plagioclase and pyroxene domains additional orthopyroxene after magmatic augite and sodic rims to magmatic plagioclase clearly indicate medium pressure granulite facies metamorphism these findings suggest tectonic juxtaposition in this area of three different structural levels of the same proterozoic imprinted crust high pressure granulite grade in the northern hengshan medium pressure granulite grade in the southern hengshan and amphibolite to greenschist facies grade in the wutaishan to the se

seismic estimates of stress release five seismic estimates of stress release are compared for eight multiply recorded aftershocks of the 1975 oroville california earthquake the dynamic stress drop the arms stress drop and the apparent stress provide stable estimates which are strongly correlated across the data set the uncertainties of these estimates the standard error of the mean divided by the mean are approximately 15 estimates of the brune stress drop are uncorrelated with estimates of the average static stress drop calculated from the seismic moments and the rupture geometries of the aftershocks determined from the signal durations of the recorded s waves the brune stress drops are most strongly correlated with the arms stress drops implying that the brune stress drop provides an estimate of the average dynamic stress drop as brune originally proposed

zircon u pb ages geochemistry and nd hf isotopes of the ttg gneisses from the jiaobei terrane implications for neoarchean crustal evolution in the north china craton abstract the precambrian basement in the jiaobei terrane is largely composed of tonalite trondhjemite granodiorite ttg suite of rocks and offers important insights into the crustal evolution history of the north china craton ncc the la icp ms zircon u pb age data presented in this study show that the magmatic protoliths of the ttg gneisses formed during 2508 2547 ma and recorded the paleoproterozoic metamorphism 1905 ma the rocks are enriched in lile rb ba and sr and depleted in hfse nb ta zr and hf they are characterized by high sr contents 406 2906 ppm sr y ratios 31 3 355 and subchondritic nb ta ratios 18 5 68 9 the ttgs show relatively high ree contents 72 0 266 ppm with strongly enriched lree la yb n 11 5 121 and positive or negligible negative eu anomalies eu eu 0 84 1 89 these geochemical features suggest that the magma source might have been rutile bearing amphibole eclogite their high mg numbers 42 56 and high cr 153 285 ppm and ni contents 22 2 74 5 ppm indicate interaction with the mantle wedge during magma ascent the whole rock end t values 2 6 to 3 8 and most of the magmatic zircon ehf t values 1 3 to 7 6 suggest juvenile to evolved isotopic signatures all these lines of evidence suggest that the ttg rocks in this study formed through partial melting of subducted oceanic slab in a continental arc environment the drill holes in the jiaobei terrane are dominated by 2 5 ga ttg gneisses suggesting that the ttg magma at 2 5 ga is more widely distributed deep underground than that of 2 7 2 9 ga at least within the approachable depth range of our research some zircon grains from jiaobei ttgs give high ehf t values plotting above the curve of 0 75 ehf of dm and their tcdm ages are very close to the time of the zircon crystallization however the majority of the ehf t values fall below the curve of 0 75 ehf of dm and their tcdm ages are concentrated between 2 7 2 9 ga therefore combining with other related studies we propose that the 2 5 2 6 ga magmatism in the ncc probably represents a coherent event of crustal growth and major reworking remelting

supra subduction zone ophiolites from inner mongolia north china implications for the tectonic history of the southeastern central asian orogenic belt abstract the recently discovered diyanmiao ophiolite in the eastern part of the erenhot hegenshan ophiolite belt forms part of the southeastern domain of the central asian orogenic belt caob the ophiolite suite is composed of the northern baiyinbulage ophiolite and the southern naolaiketu ophiolite each extending for nearly 100 km and a width of around 6 km the suite preserves a well exposed and a nearly complete ophiolite stratigraphy and includes from bottom to top harzburgite bedded gabbro isotropic gabbros anorthosite spilite pillow basalt keratophyre baschtauite with overlying chert and plagiogranite veins locally intruding the pillow basalt and gabbro the pillow basalts possess the geochemical characteristics of hahajima island arc tholeiite iat indicating that the diyanmiao ophiolite formed in the incipient arc setting of the supra subduction zone la icp ms u pb dating of zircon grains from three gabbro samples yield ages of 361 13 ma 356 7 3 8 ma and 340 14 ma indicating an early carboniferous age for the diyanmiao ophiolite we suggest that the intra oceanic subduction initiation of the erenhot hegenshan paleo ocean during the early carboniferous resulted in the formation of an incipient arc oceanic crust and that the diyanmiao ophiolite represents an important north dipping subduction event of the paleo asian oceanic crust beneath the southern margin of the siberian plate after early carboniferous and early permian our study provides further insights into the evolution of the paleo asian ocean and eventual construction of caob the globe s largest phanerozoic orogen

gold distribution and source of the j4 gold bearing breccia pipe in the qiyugou district north china craton constraints from ore mineralogy and in situ analysis of trace elements and s pb isotopes abstract the qiyugou district is one of the most important gold deposit distribution areas in the xiong ershan region north china craton these deposits are hosted in archean taihua supergroup rocks as gold bearing breccia pipes here we report a detailed in situ analysis of trace elements and s pb isotopes in sulfides from different ore stages in combination with electron probe microanalysis of ore minerals from the gold bearing j4 breccia pipe in the qiyugou district to unravel the gold distribution and their textural relationships to pyrite the metal sources and s pb isotopic composition variations during the ore forming processes the qiyugou breccia pipes hosted gold ore over 70 t au average grade of 2 5 g t that formed in four mineralization stages i e an early k feldspar biotite magnetite quartz pyrite py 1 stage i that can be further divided into two substages namely the ia k feldspar biotite magnetite stage and the ib quartz pyrite stage a gold pyrite py 2 quartz stage ii a gold quartz polymetallic sulfide py 3 stage iii and a late quartz carbonate stage iv the visible native gold and electrum grains occur as small inclusions and disseminations in the fractures within py 2 and py 3 tellurides and bi pb minerals are widespread in the stage ii stockworks and veins as is intergrowth with visible native gold and py 2 in situ la icp ms trace element analysis reveals the presence of invisible gold hosted in three types of pyrite with extremely low au content py 1 0 05 ppm py 2 0 03 ppm py 3 0 02 ppm concentrations of te bi and pb in pyrite are high and correlate well with au concentrations but as content is low therefore the te bi and pb played a significant role in gold mineralization in as deficient ore fluids in the j4 gold bearing breccia pipe from the qiyugou district high precision in situ laser ablation multiple collector inductively coupled mass spectrometry la mc icp ms lead isotope analyses indicate that the sulfides have 206pb 204pb 207pb 204pb and 208pb 204pb ratios from 17 250 to 17 629 from 15 401 to 15 621 and from 37 649 to 38 290 respectively which are similar to the granite porphyry in the district and indicative of their similar lead sources high precision in situ la mc icp ms sulfur isotope analyses of pyrite show a large 34s variation e g from 18 1 to 11 1 per mil for py 1 from 12 2 to 6 6 per mil for py 2 and from 4 9 to 1 6 per mil for py 3 with a remarkable increasing trend from stage i stage iito stage iii this trend may record a decreasing oxygen fugacity of the ore forming fluids during mineralization

combined u pb hafnium and oxygen isotope analysis of zircons from meta igneous rocks in the southern north china craton reveal multiple events in the late mesoarchean early neoarchean abstract we report the results of a comprehensive isotopic investigation of zircons from metamorphosed tonalites and amphibolites from lushan in the far south of the trans north china orogen of the north china craton that reveal the oldest rocks yet dated from the orogen a detailed investigation of the internal structures of zircons combined with targeted sims u pb and oxygen isotope analyses and icp ms hf zircon analyses has resulted in the discovery of previously unknown geological events in the north china craton magmatic zircon from the tonalite and amphibolite was found to be essentially coeval with a small spread of ages from 2829 18 ma to 2832 11 ma in the tonalites and from 2838 35 ma to 2845 23 ma in amphibolite hafnium isotope data suggest derivation of both tonalite and amphibolite from a depleted mantle source with minor crustal contamination oxygen isotope ratios in zircons range from 5 1 to 7 3 vsmow with high 18 o results recorded only from metamorphic domains the earliest stage of metamorphic zircon growth present in all four samples analyzed in this study occurred between 2772 17 22 in the tonalites and 2776 20 and 2792 12 ma in the amphibolites a younger metamorphic event at 2638 61 in the tonalite and between 2671 13 and 2651 13 ma in the amphibolites is recognized in zircons from three of the four samples investigated this is the first time that metamorphic events at either of these times have been recorded in the north china craton importantly there is no evidence for the 1 8 ga high grade metamorphic event that is manifest in the northern and central parts of the trans north china orogen

3 5 ga old amphibolites from eastern hebei province china field occurrence petrography sm nd isochron age and ree geochemistry amphibolite blocks frequently occur in association with grey gneisses within a small area near the villages of caozhuang and huangbaiyu in eastern hebei they are interpreted as remnants of an earlier bimodal suite now engulfed in late archaean k rich granitic magma and are further cut by pegmatite dykes they are considered together as giant enclaves within late archaean granite plutons u pb zircon ages 2 5 2 6 ga the amphibolites have been dated by the sm nd isochron method the results yielded t 3470 107 ma ind 0 50827 11 and nd t 2 7 0 6 this age represents the time of protolithic basalt eruption and possibly suggests a period of formation of primitive mafic crust this is the oldest reliable age so far obtained for the archaean rocks of china the sm nd results of grey gneiss samples yielded model ages tdm of about 3 24 ga it is not clear whether this younger age in comparison with that of the amphibolites indicates the intrusive nature of these rocks or is a result of contamination by later granite about 2 5 ga ago the rb sr and common pb isotopic systems have been highly disturbed and no isochron ages could be established the common pb data show retarded growth of radiogenic pb and hence imply reduction of u pb ratios in the past this is consistent with the petrographic study which suggests that the amphibolites have undergone an earlier period of granulite facies metamorphism and were subsequently retrogressed to the present amphibolite facies mineral assemblages the positive nd t value of 2 7 for the amphibolites implies that the protolithic basalt magmas were generated from mantle sources which had undergone long term depletion in light rare earth elements lree the depletion event must have taken place prior to 4 ga ago if their source had a chemical nature similar to the modern oceanic mantle it could have occurred as early as 4 5 ga ago if their source was less impoverished in lree if the mantle depletion was related to extraction of continental or sialic material the sm nd data of the caozhuang amphibolites lend support to the existence of sialic crust if not of a continental size before 4 ga ago

neoarchean crust mantle interactions in the yishui terrane south eastern margin of the north china craton constraints from geochemistry and zircon u pb hf isotopes of metavolcanic rocks and high k granitoids abstract the neoarchean yishui terrane yst is situated in the east of western shandong province wsp south eastern margin of the north china craton ncc the metavolcanic rocks of the yst are fine grained hornblende plagioclase gneisses group 1 and fine grained amphibolites group 2 in the yangzhuangzhen area and fine to medium grained amphibolites group 3 in the leigushan area the high k granitoids associated with groups 1 and 2 are dominated by fine to medium grained monzogranitic gneisses zircon la icp ms u pb dating reveals that the magmatic precursors of groups 1 and 2 were formed at 2641 ma and the magmatic precursors of concomitant monzogranitic gneisses were emplaced from 2615 to 2575 ma whereas group 3 represents a later 2500 ma volcanic eruption and all these metamorphic volcanic rocks and monzogranitic gneisses were subjected to subsequent 2470 2460 ma metamorphism the metamorphic volcanic rock samples in group 1 exhibit the chemical compositions of calc alkaline andesites showing fractionated chondrite normalized ree patterns la yb n 10 48 19 30 and negative nb ta and ti anomalies nb la pm 0 13 0 22 which are akin to those of typical high mg andesites hmas in the subduction related settings the magmatic precursors of the group 1 samples were derived from partial melting of a fluid or melt metasomatized depleted mantle wedge at deep levels in the upper mantle samples in group 2 show calc alkaline chemical compositions with less fractionated chondrite normalized ree patterns la yb n 2 24 3 34 and negative nb ta and ti anomalies nb la pm 0 47 0 76 which are consistent with those of the volcanic rocks in the aleutian island arc the magmatic precursors of group 2 were generated by partial melting of a fluid metasomatized depleted mantle wedge at shallow levels in the upper mantle the monzogranitic gneisses exhibit high sio2 and k2o contents with high k calc alkaline affinities and peraluminous characteristics based on their distinct hree contents and chondrite normalized ree patterns these granitoid samples are subdivided into low yb monzogranitic gneisses lymgs and high yb monzogranitic gneisses hymgs the lymg magma was derived from partial melting of a mixed source of juvenile two mica pelites and minor basic intermediate igneous rocks at lower crustal levels with pyroxene amphibole garnet as the main residual phases and the hymg magma was derived from partial melting of multi sourced juvenile two mica pelites at middle to lower crustal levels with pyroxene amphibole and subordinate plagioclase and garnet as the main residual phases in addition group 3 resembles tholeiitic back arc basalts in the okinawa trough and displays flat chondrite normalized ree patterns la yb n 1 22 2 08 and slightly negative nb and ta anomalies nb la pm 0 35 0 59 this group was most likely derived from partial melting of a depleted mantle source that had been modified by the addition of subducted slab derived fluids at shallow levels in the upper mantle these metavolcanic rocks and concomitant high k granitoids record important neoarchean crust mantle interactions involving the first modification and partial melting of the lithospheric mantle induced by oceanic crust subduction then upwelling and underplating of mantle derived magmas triggered partial melting of the middle to lower crust and mixing between crust and mantle derived magmas these processes imply that neoarchean crust mantle interaction played a crucial role in the evolution of the southeastern margin of the ncc available whole rock sm nd and zircon lu hf isotopic data from metamorphic volcanic rocks and plutonic granitoids from this study and previous studies reveal that yst experienced three crucial juvenile crustal growth events from 2 78 2 69 ga 2 64 2 56 ga and 2 54 2 50 ga