nd isotopic and geochemical constraints on the provenance and tectonic setting of the low grade meta sedimentary rocks from the trans north china orogen north china craton abstract in the trans north china orogen tnco of the north china craton low grade supracrustal successions extensively occur in the wutai luliang zanhuang zhongtiao and dengfeng complexes from north to south meta sedimentary samples from the wutai and zhongtiao complexes were collected for geochemical and nd isotopic studies and several samples from the zanhuang and dengfeng complexes were also analyzed for nd isotopic studies for comparative purpose most of the meta siltstones and meta sandstones from the wutai and zhongtiao complexes are characterized by depletions in mobile elements like cao sr and na2o high chemical index of weathering ciw values and strong positive correlation between al2o3 and tio2 indicating intense weathering conditions significant post depositional k metasomatism is indicated in the a cn k diagrams for most of the analyzed samples and the relatively high pre metasomatized chemical index of alteration cia values 43 87 also imply a high degree of source weathering depleted transitional trace elements ni cr co and sc fractionated light rare earth elements patterns mild negative eu anomalies in the majority of these meta sedimentary samples point toward felsic source rocks including the 2 5 ga granitics ttg gneisses and the paleoproterozoic granitics in the tnco minor contribution from mafic rocks is evidenced from relative high contents of mgo fe 2 o 3 t sc and lower la ybn ratios in some older sequence set samples from the zhongtiao complex our geochemical and nd isotopic data combined with previous studies in the lithostratigraphic sequence provenance and depositional age suggest that the older and younger sequence sets of the low grade supracrustal successions in the tnco were deposited in the different depositional environments the older sequence set was deposited in a back arc basin between the andean type continental margin arc and the eastern block after 2 1 ga whereas the younger sequence set formed in a foreland basin between 1 88 and 1 80 ga which is consistent with the model that the collision between the eastern and western blocks to form the trans north china orogen occurred at 1 85 ga

petrology of high pressure granulites from the eastern himalayan syntaxis the eastern himalayan syntaxis situated at the eastern terminus of the himalayas is the least known segment of the himalayas recent research in this area has revealed that the syntaxis consists of the gangdise the yarlung zangbo and the himalayan units each of which is bounded by faults the himalayan unit the northernmost exposed part of the indian plate mainly contains amphibolite facies rocks marked by the assemblages staurolite kyanite plagioclase biotite muscovite sillimanite and garnet amphibole plagioclase in the south to the north low to medium pressure granulite grade pelitic gneisses and marbles are present and are characterized by the assemblages garnet sillimanite k feldspar plagioclase or antiperthite biotite quartz spinel cordierite orthopyroxene in gneisses and anorthite diopside wollastonite and plagioclase diopside quartz phlogopite calcite in marbles within this unit the namula thrust system is a series of moderately north dipping structures that displaced the granulite facies rocks southwards over the amphibolite facies rocks high pressure granulites occur as relics within these granulite facies rocks and contain garnet kyanite granulite and garnet clinopyroxenite the peak assemblage of the garnet kyanite granulite includes garnet core part kyanite ternary feldspar quartz rutile sillimanite garnet rim part k feldspar oligoclase ilmenite biotite and spinel albite biotite or spinel cordierite orthopyroxene which are coronas around sillimanite and garnet are retrograde products of this peak assemblage another peak assemblage includes very high ca garnet cao 32 34 wt alm10 grs 80 and diopside cao 22 24 wt scapolite meionite quartz and accessory al bearing titanite al2o3 4 4 5 wt the diopside has kink bands partial or complete breakdown of ca rich garnet during post peak metamorphism produced pseudomorphs and coronas consisting of fine grained symplectic intergrowths of hedenbergite and anorthite thermobarometric estimates in combination with reaction textures mineral compositions and recent experimental studies indicate that these peak assemblages were formed at p c 1 7 1 8 gpa t c 890 c and the retrograde assemblages experienced near isothermal decompression to p 0 5 0 1 gpa t 850 50 c the whole rock compositions indicate that marble and pelite are plausible candidates for the protoliths these facts suggest the following 1 sedimentary rocks were transported to upper mantle depths and equilibrated at those conditions to form these high pressure granulites which were then emplaced into the crust quickly during the rapid exhumation of these rocks the earlier high pressure assemblages were overprinted by the later low to medium pressure assemblages that is the high pressure granulite belt formed in the syntaxis 2 the namula thrust system is an important tectonic boundary in the syntaxis or even in the higher himalaya more generally

paleoproterozoic high pressure high temperature pelitic granulites from datong in the north china craton and their geological implications constraints from petrology and phase equilibrium modeling abstract paleoproterozoic granulite facies metapelites are widespread in northern part of the north china craton ncc and their metamorphic conditions and p t paths provide important constraints for tectonic evolution of the ncc however it is still controversial on the metamorphic evolution of these rocks due to variations of retrograde modification in combination with detailed petrological studies geological mapping and geochronology we here present new evidence from gushan datong conjunction zone of the ji ning complex and huai an complex to constrain p t evolution of the metapelites field investigations reveal that the pelitic granulites are spatially associated with high pressure hp mafic granulites showing concordant characteristics of deformation and metamorphism fine mineralogical and microstructural studies reveal that kyanite bearing hp granulite facies indicators preserved in garnet porphyroblasts of the pelitic granulites it suggests the previously assumed moderate to low pressure mp lp and high to ultrahigh temperature ht uht pelitic granulites have experienced hp granulite facies metamorphism four metamorphic generation assemblages m1 m4 are recognized in the studied samples the pre pmax assemblages m1 are characterized by mineral inclusions within garnet core including bt qz rt ms liq representing the late stage of prograde metamorphism the pmax assemblages m2 hp granulite facies are represented by garnet mantle and its inclusion type minerals i e grt ky kfs bt rt liq qz ms zn spl it is followed by heating until the maximum of temperature stage m3 tmax featured by disappearance of biotite and transition from kyanite into sillimanite with mineral assemblages of grt sil kfs liq qz ilm zn spl the cooling stage m4 is featured by breakdown of garnet to bt qz pl sil ilm phase equilibrium modeling and titanium in biotite thermometer are combined to constrain the p t evolution of the studied samples the p t conditions for the m1 m4 are respectively estimated around 11 11 5 kbar 770 790 c 10 3 13 8 kbar 820 835 c 6 5 9 5 kbar 850 925 c 6 kbar 645 761 c which defined a clockwise p t path zircon u pb ages of two representative samples demonstrated comparable age clusters of 1950 ma and 1850 ma which are interpreted to represent the timing of hp and mp lp granulite facies stages respectively combining with other studies of the ncc we propose that the pelitic granulites and associated hp mafic granulites have experienced the common metamorphic processes from peak stage hp ht granulite facies to retrograde metamorphism due to intensive and pervasive overprint of sillimanite type granulite facies metamorphism the hp granulite facies mineral assemblages of the pelitic granulites are partially or completely reworked the metamorphic peak conditions and relatively high thermal gradients of these granulite facies rocks suggest that their formation is probably in response to a crustal scale subduction collision process during the late paleoproterozoic

petrogenesis p t t path and tectonic significance of high pressure mafic granulites from the jiaobei terrane north china craton abstract high pressure hp mafic granulites in the jiaobei terrane are composed predominantly of garnet mafic granulites garnet hypersthene granulites and garnet amphibolites and they are found as irregular lenses or deformed dike swarms within tonalitic trondhjemitic granodioritic gneisses and granitic gneisses the hp mafic granulites contain four distinct metamorphic assemblages of which the early prograde assemblage m 1 is represented by the cores of garnets together with mineral inclusions of clinopyxene plagioclase quartz and it formed at 740 770 c and 0 90 1 00 gpa in contrast the peak assemblage m 2 consists of high ca cores in garnet high al cores in clinopyroxene and high na cores in plagioclase in the matrix which formed under p t conditions of 850 880 c and 1 45 1 65 gpa the peak metamorphism was followed by near isothermal decompression m 3 which resulted in the development of orthopyroxene clinopyxene plagioclase quartz amphibole magnetitie symplectites or coronas surrounding some garnet grains with p t conditions of 780 830 c and 0 65 0 85 gpa surrounding some garnet grains are symplectites of amphibole plagioclase quartz magnetitie which formed during a cooling retrograde stage m 4 with p t conditions of 590 650 c and 0 62 0 82 gpa an integrated study involving laser raman analysis of mineral inclusions cathodoluminescence imaging and in situ u pb dating of zircons shows that the protolith ages of the hp mafic granulites are mainly 2550 2500 ma and that the timing of the peak metamorphism of the hp mafic granulites ranges from 1900 to 1860 ma as recorded by the cores of metamorphic zircons on the other hand the medium to low pressure granulite amphibolite facies retrogression occurred mainly at 1860 1820 ma as recorded by the rims of some single zircon grains and the zircon grains that contain inclusions of garnet orthopyroxene plagioclase sphene the combination of petrography mineral compositions metamorphic reaction history thermobarometry and geochronology defines a near isothermal decompressional clockwise p t t path for the jiaobei hp mafic granulites suggesting that the jiaobei terrane underwent initial crustal thickening during 1950 1860 ma followed by relatively rapid exhumation cooling and retrogression in the period 1860 1820 ma this tectonothermal path was probably generated by subduction and collision related tectonic processes

metamorphic evolution and shrimp u pb geochronology of mafic granulites with double symplectites in the fuping metamorphic complex middle palaeoproterozoic trans north china orogen abstract garnet bearing mafic granulites occur as lenses within ttg gneiss or metapelite in the fuping complex middle palaeoproterozoic trans north china orogen they are composed mainly of garnet clinopyroxene orthopyroxene hornblende plagioclase quartz and accessary zircon and titanite four stages of metamorphic mineral assemblages have been found in these mafic granulites a the prograde assemblages m1 are inclusions preserved in the garnet porphyroblasts including clinopyroxene hornblende plagioclase and quartz b the metamorphic peak assemblages m2 consist of garnet porphyroblasts plus matrix minerals clinopyroxene orthopyroxene hornblende plagioclase quartz zircon titanite c the first retrograde assemblages m3 are represented by clinopyroxene orthopyroxene plagioclase quartz symplectites rimming the garnets and 4 the final retrograde assemblages m4 are hornblende plagioclase quartz symplectites rimming or adjacent to the garnets the retrieved metamorphic p t paths pass from 530 600 c 6 5 7 0 kbar m1 through 680 820 c 7 2 11 7 kbar m2 to 730 760 c 3 2 3 9 kbar m3 and finally to 600 640 c 3 2 3 7 kbar m4 all the p t paths of the mafic granulites are clockwise two of these even contain near isothermal decompression itd and subsequent near isobaric cooling ibc segments shrimp u pb dating of metamorphic zircons of the mafic granulites yields a mean weighted age of 1849 9ma which is consistent with the metamorphic time of surrounding ttg gneisses therefore the fuping complex records the collision event between the eastern and western blocks of the north china craton that led to the final assembly of this craton during the late paleoproterozoic

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

pressure temperature time paths of regional metamorphism i heat transfer during the evolution of regions of thickened continental crust the development of regional metamorphism in areas of thickened continental crust is investigated in terms of the major controls on regional scale thermal regimes these are the total radiogenic heat supply within the thickened crust the supply of heat from the mantle the thermal conductivity of the medium and the length and time scales of erosion of the continental crust the orogenic episode is regarded as consisting of a relatively rapid phase of crustal thickening during which little temperature change occurs in individual rocks followed by a lengthier phase of erosion at the end of which the crust is at its original thickness the principal features of pressure temperature time ptt paths followed by rocks in this environment are a period of thermal relaxation during which the temperature rises towards the higher geotherm that would be supported by the thickened crust followed by a period of cooling as the rock approaches the cold land surface the temperature increase that occurs is governed by the degree of thickening of the crust its conductivity and the time that elapses before the rock is exhumed sufficiently to be affected by the proximity of the cold upper boundary for much of the parameter range considered the heating phase encompasses a considerable portion of the exhumation decompression part of the ptt path in addition to the detailed calculation of ptt paths we present an idealized model of the thickening and exhumation process which may be used to make simple calculations of the amount of heating to be expected during a given thickening and exhumation episode and of the depth at which a rock will start to cool on its ascent path an important feature of these ptt paths is that most of them lie within 50 c of the maximum temperature attained for one third or more of the total duration of their burial and uplift and for a geologically plausible range of erosion rates the rocks do not begin to cool until they have completed 20 to 40 per cent of the total uplift they experience considerable melting of the continental crust is a likely consequence of thickening of crust with an average continental geotherm a companion paper discusses these results in the context of attempts to use metamorphic petrology data to give information on tectonic processes 1984 oxford university press

two stage decompression in garnet bearing mafic granulites from sostrene island prydz bay east antarctica mafic garnet bearing granulites from sostrene island 150 km southwest of davis station on the coast of prydz bay east antarctica exhibit two stage symplectic coronas on garnet formed after peak metamorphic conditions m1 an outer corona of opx mg66 pl an94 97 minor hbl mantles a finer grained inner corona of opx mg67 pl an95 96 spl mg36 both symplectites contain minor ilmenite magnetite intergrowths the finer grained symplectite also occurs along a fracture cleavage in the garnet the outer corona originated during a second metamorphic event m2 via the reaction grt cpx hbl sio2 opx pl 1 whereas the inner corona formed later in response to decompression and minor deformation resulting in the fracture cleavage in the garnet according to the reaction grt opx pl spl 2 the grossular content of the garent xgrs 0 168 is almost exactly that which is required for the stoichiometric breakdown by reaction 2 calculated xgrs 0 167 the mafic rocks are silica undersaturated and the sio2 for reaction 1 was most probably derived externally from the surrounding felsic gneisses preferred p t estimates for m1 based on garnet core prp40alm42grs17sps1 matrix opx cpx hbl pairs are c 10 kbar at 980 c the fine grained symplectite formed post peak m2 at c 7 kbar and 850 c the enclosing felsic gneisses yield pressure estimates of between 5 and 7 kbar which compare with conditions of c 6 kbar and 775 c in the nearby bolingen islands these lower p t estimates are considered to be representative of the widespread 1100 ma metamorphic event recognized in outcrops along the prydz bay coast the high p high t estimates derived from the garnet relics provide evidence for an earlier possibly archaean high grade metamorphic event

petrogenesis of taxitic dioritic tonalitic gneisses and neoarchean crustal growth in eastern hebei north china craton abstract the taxitic dioritic tonalitic gneisses in the zunhua qinglong microblock of eastern hebei north china craton are composed of dioritic quartz dioritic tonalitic and trondhjemitic gneisses they show taxitic or banded structures and were subjected to amphibolite to local granulite facies metamorphism based on their whole rock geochemistry the dioritic and quartz dioritic gneisses dq group can be classified into a high ni subgroup hndq subgroup and a low ni subgroup lndq subgroup and the tonalitic and trondhjemitic gneisses tt group into a low fractionated subgroup lftt subgroup and a high fractionated subgroup hftt subgroup la icp ms zircon u pb isotopic dating reveals that the magmatic precursors of the hndq lndq and lftt subgroups were emplaced from 2522 13 to 2533 14 ma whereas those of the hftt subgroup emplaced at 2499 4 ma studies in petrogenesis revealed that the magmatic precursors of the hndq subgroup were derived from partial melting of mantle peridotites metasomatized chiefly by slab melts while those of the lftt subgroup were formed by fractional crystallization of the magmas of the hndq subgroup with clinopyroxene magnetite plagioclase and apatite as the major fractionated phases magmatic precursors of the lndq subgroup were derived from partial melting of mantle peridotites metasomatized in a dominant manner by slab derived fluids with melts experiencing fractionation of hornblende magnetite plagioclase and apatite during the magmatic evolution further magmatic precursors of the hftt subgroup were derived from partial melting of volcanic sedimentary rocks formed in the fore arc to trench belt which were subducted along with oceanic slabs with the melts contaminated by mantle peridotites during their ascent through the mantle wedge these new data combined with previous investigations on metavolcanic rocks and granitoid orthogneisses suggest that the magmatic precursors of these taxitic dioritic tonalitic gneisses were formed in an arc to back arc tectonic setting from north to south furthermore lu hf isotopic data from the late neoarchean metavolcanic rocks and granitoid gneisses in the zunhua qinglong microblock reveal that the zunhua qinglong microblock experienced three major periods of juvenile crustal growth during neoarchean at 2 72 2 70 ga 2 64 2 56 ga and 2 53 2 50 ga respectively with the latter two periods coupled with reworking of old crustal materials

fluid influence on mineral reactions in ultrahigh pressure granulites a case study in the śnieznik mts west sudetes poland small tectonic slices of undeformed eclogites and ultrahigh pressure granulites occur in three tectonic units of the nieznik mts sw poland ultrahigh pressure granulite eclogite transitions with peak metamorphic conditions between 21 and 28 kbar at 800 to 1000 c occur only in the zlote unit conventional u pb multigrain analyses of zircons from a mafic granulite provided 207pb 206pb ages between 360 to 369 ma which are interpreted to approximate timing of original crystallisation from a melt diffusion kinetics and the restricted availability of a fluid phase mainly controlled the conversion from granulite to eclogite although some bulk chemical differences were also recognised the ultrahigh pressure granulites from the zlote unit exclusively contain h2o rich inclusions with variable salinities which distinguishes them from high temperature ht granulites world wide this is also in contrast to the fluid regime h2o n2 co2 recognised in the lower temperature eclogites 600 800 c from the closely associated miedzygorze and nieznik units the variation in fluid composition between the lower temperature eclogites and ultrahigh pressure granulites on the one hand and ultrahigh pressure granulites and ht granulites on the other hand probably indicates contrasting p t t paths as a result of different tectonic environments

p t t path and tectonic significance of pelitic migmatites from the luliang complex in xiyupi area of trans north china orogen north china craton abstract in order to reveal the metamorphic evolution of the luliang complex a combined study of petrography phase equilibria modeling and zircon u pb dating is carried out on two mesosome of pelitic migmatites from the jiehekou group in the luliang complex sample 15xyp 02 preserved the thermal peak assemblage of garnet biotite k feldspar sillimanite quartz and ilmenite without plagioclase it recorded two near isothermal decompressional indicators that rutile was moated by ilmenite and garnet was replaced by cordierite small plagioclase occurred in the rim of k feldspar and garnet four stages m1 m4 of metamorphic evolution have been identified based on the mineral assemblages and p t pseudosection it is interpreted to record pre tmax decompression from rutile stability field m1 represented by rutile relicts in matrix and biotite sillimanite quartz inclusions in garnet porphyroblasts at 8 9 kbar to the tmax stage m2 characterized by garnet rim with ilmenite and biotite k feldspar sillimanite quartz in matrix of 815 838 c the following suprasolidus post tmax cooling comprises m3 stage represented by appearance of cordierite in the assemblage of garnet biotite k feldspar sillimanite ilmenite quartz of 780 838 c 5 0 6 3 kbar and m4 stage defined by the final mineral assemblage of garnet biotite k feldspar plagioclase sillimanite cordierite ilmenite quartz of 775 c 5 0 kbar another sample 15xyp 21 preserved the thermal peak assemblage of garnet biotite plagioclase k feldspar sillimanite quartz it also records pre tmax decompression from rutile stability field at 9 0 kbar to the tmax stage of 796 821 c 6 4 7 1 kbar followed by suprasolidus post tmax cooling stage of 778 803 c 5 0 5 7 kbar the clockwise p t paths are consistent with those of orogenic belt furthermore the zircon u pb dating on mesosome and leucosome of pelitic migmatite yield metamorphic ages of 1 95 ga and 1 91 ga respectively in which the former represents the early stage of cooling after tmax nearly close to the time of pressure peak metamorphism combined with those granulites in the trans north china orogen a common long lived continent continent collision processes was proposed which may culminated at 1 95 ga and followed by a prolonged cooling process

high pressure granulites from the sudetes south west poland evidence of crustal subduction and collisional thickening in the variscan belt two high grade gneissic complexes of the western sudetes the gory sowie block and the nieznik area complex contain small predominantly felsic granulitic inliers with minor cpx bearing intercalations the p t conditions of the granulite facies events and of the subsequent re equilibration are estimated using the ternary feldspar thermometer and the geo calc computer program version twq jan 92 in the gory sowie granulites the peak granulitic event occurred at c 18 20 kbar and 900 c and the late decompressive re equilibration within a range of 4 10 kbar and temperatures decreasing to 600 700 c the latter event is thought to have coincided with the main metamorphic phase in the surrounding gneisses the p t estimates are more scattered in the nieznik granulites but the peak conditions for the granulitic event are estimated at pressure over 22 kbar possibly around 30 kbar and temperature exceeding 900 c the analysed samples from the nieznik area bear no significant evidence of lower pressure re equilibration integrating the thermobarometric data and some age constraints indicates that the gory sowie granulites belong to the early stage type i granulites of the variscan belt c 400 ma old which are interpreted as fragments of continental crustal materials subducted to mantle depths in the earliest stages of the variscan orogeny the nieznik granulites are more problematic they may belong to a younger high p suite c 350 ma old widespread in the southern and eastern parts of the bohemian massif and possibly related to the climax of the variscan continent continent collision

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

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

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

detrital zircon u pb dating hf isotopes and whole rock geochemistry from the songshan group in the dengfeng complex constraints on the tectonic evolution of the trans north china orogen abstract the dengfeng taihua and zhongtiao complexes are located in the southernmost segment of the trans north china orogen along which the western and eastern blocks amalgamated to form the north china craton the dengfeng complex consists of the neoarchean granitoids and supracrustal rocks of which the latter are subdivided into the dengfeng and songshan groups the songshan group consists of basal conglomerates quartzites schists with minor dolomites and phyllites geochemistry of the songshan schists and quartzites are indicative of felsic source rocks and display continental island arc and passive continental margin signatures respectively u pb ages of detrital zircons from the songshan group yielded three age populations of 3700 3100 ma 3000 2550 ma and 2550 1950 ma the dominant 3000 2550 ma detrital zircons were most likely sourced from the ttg gneisses of the dengfeng and taihua complexes the subordinate 2550 1950 ma detrital zircons were probably derived from the paleoproterozoic granitoid plutons and meta rhyolites in the dengfeng and zhongtiao complexes and minor amounts of 3700 3100 ma detrital zircons may have been derived from the paleoarchean and mesoarchean crust of the eastern block the youngest detrital zircon age peaks in the luohandong and wuzhiling formations constrain their maximum depositional ages at 2350 ma and 1960 ma respectively in combination with the 1775 ma undeformed granitoids that cut the songshan group the depositional age of the songshan group can be constrained in the period between 2350 and 1780 ma considering the lithostratigraphic feature provenance and depositional age the songshan group is interpreted as having deposited in a retroarc foreland basin which is consistent with the model that the collision between the eastern and western blocks to form the trans north china orogen occurred at 1 85 ga after long lived subduction

zircon u pb and trace element data from rocks of the huai an complex new insights into the late paleoproterozoic collision between the eastern and western blocks of the north china craton abstract the trans north china orogen tnco is interpreted as a continent continent collisional belt along which the discrete archean eastern and western blocks amalgamated to form the basement of the north china craton the timing of its formation has not been precisely constrained yet six samples including five high grade metamorphic rocks and a pegmatite dike from the huai an complex in the northern segment of the tnco were selected for zircon u pb dating and trace element analysis a tonalite trondhjemite granodiorite ttg gneiss sample and a high pressure hp mafic granulite sample contain magmatic zircon cores yielding protolith formation ages of 2503 17 and 1964 38 ma respectively detrital zircons of magmatic origin in two metasedimentary rocks show concordant or moderately discordant u pb ages of 2683 22 to 1964 13 ma indicating that they were mainly derived from the tnco with some possibly derived from the archean continental rocks of the north china craton metamorphic zircons in the ttg gneiss and two metasedimentary rocks exhibit low hree contents flat hree patterns and negative eu anomalies implying their formation coeval with garnet and feldspar under grannulite facies conditions metamorphic zircons in an hp mafic granulite and a sample of leucosome from a ttg gneiss also have depleted hree contents flat hree patterns but only weak negative eu anomalies which may also indicate formation under granulite facies conditions whereas the weak eu anomalies may be due to the whole rock composition all these metamorphic zircons have formation temperatures of 702 749 c approaching the peak metamorphic temperatures of hp granulite rocks in this area they have identical ages of ca 1845 ma within analytical errors which is the best estimate age for hp granulite facies metamorphism of the huai an complex these results confirm previous models proposing late paleoproterozoic collision between the eastern and western blocks of the north china craton along the tnco on the other hand some metamorphic zircons in the leucosome sample show planar or polygonal zoning high th u contents and high th u ratios high ree contents and steep hree patterns indicating that they crystallized during partial melting with abundant garnet decomposition they yielded a weighted mean age of 1819 13 ma which we interpret to register the time of retrograde metamorphism combined with previously published data there are three age peaks at 1876 6 1849 2 and 1814 4 ma for metamorphic zircons from high grade metamorphic rocks in the tnco these may indicate either a protracted event or at least three peaks of metamorphism a pegmatite dike cutting the foliation of a mafic granulite boudin was dated at 1806 15 ma which signifies termination of collision between the eastern and western blocks in the huai an area

crustal evolution of the north qinling terrain of the qinling orogen china evidence from detrital zircon u pb ages and hf isotopic composition abstract the north qinling terrain reveals a different geological history from the north china block to the north and the south qinling micro continental block to the south the timing of events and geochemical features of the north qinling terrain are essential for understanding the evolution of the qinling orogenic belt that joins the north and south china blocks this study presents detrital zircon ages and hf isotopic compositions from the guandaokou group a cover sequence of the north china block and from the kuanping group an upper basement sequence of the north qinling terrain the youngest detrital zircon u pb ages constrain the initiation of sedimentation in the guandaokou group to 1 85 ga and the maximum depositional age of the kuanping group to 640 ma zircon u pb ages and hf data testify to a major phase of magmatism in the north qinling terrain at 1000 900 ma the magmatic rocks were largely produced from reworking of early paleoproterozoic 2 5 2 0 ga crust with minor contribution of late paleoproterozoic 1 7 ga juvenile material this age record differs in time from the two important thermal events of 1 85 ga and 2 5 ga widely manifested in the north china block the north qinling terrain can thus be regarded as a separate micro continent with a pre neoproterozoic evolution different from that of the north china block the final assembly of the north qinling terrain to the north china block took place after about 640 ma

archean paleoproterozoic crustal evolution in the eastern north china craton zircon u th pb and lu hf evidence from the jiaobei terrane abstract the precambrian metamorphic basement of the north china craton ncc records a complex evolutionary history during the neoarchean and the paleoproterozoic the jiaobei terrane provides one of the best regions to evaluate the early precambrian crustal growth of the eastern ncc here we report the results of u pb zircon geochronology and hf isotopes of tonalite trondhjemite granodiorite ttg gneiss and associated mafic rocks from the jiaobei terrane sims dating of zircons from the ttg gneiss shows that the protolith was formed at 2912 6 ma and subjected to anatexis at 2488 9 ma zircons from the mafic rocks yield three age populations at 2484 7 ma 1855 5 ma and 1836 3 ma based on the differences in morphology internal texture and the u th compositions between magmatic and metamorphic zircons in the mafic suite we interpret the 2484 7 ma and 1836 3 ma ages obtained from two meta mafic intrusions as the crystallization age of their magmatic precursors and the 1855 5 ma from a garnet amphibolite as the metamorphic age magmatic zircons from ttg gneiss and meta gabbro yield hf model ages ranging from 3 2 to 2 8 ga suggesting an important period for crustal growth during mesoarchean the earlier mafic magmatic event 2 49 ga involved magma generation from a depleted mantle whereas the later phase 1 84 ga was derived from differentiation of a parent magma that originated from the partial melting of the mesoarchean lower curst the newly grown metamorphic zircons in the garnet amphibolite show initial 176hf 177hf and t dm c values from 0 281211 to 0 281343 and from 3 11 to 3 42 ga indicating that the mesoarchean and older crustal materials reworked at 1 86 ga the data reported in this study coupled with previous geochronological and lu hf isotopic data from other localities of the eastern ncc on the widespread emplacement of the mafic magmas and the coeval high grade metamorphism or melting events at 2 50 2 48 ga suggest that these regions were already part of a coherent tectonic unit with a common crustal evolution history during the end of neoarchean

extensional collapse and uplift in a polymetamorphic granulite terrain in the archaean and palaeoproterozoic of north china abstract the archaean granulite terrain in the datong huai an area north china comprises a basement complex of felsic and mafic granulite overlain by a sedimentary sequence dominated by metapelite and metapsammite both lithological associations are separated by a tectonic contact and structural textural and stratigraphic data indicate that the terrain was affected by two prominent tectono thermal cycles c a and c b associated with two granulite facies events m1 and m2 which probably occurred 2500 ma and 1800 ma ago respectively of these m1 events were dominant and associated with all major structures in the area in the basement gneiss m1 reached peak metamorphic conditions of 12 14 kbar and 800 900 c during a foliation forming event d 2ba which transposed an earlier s 1ba foliation of unknown age and origin in the allochthonous cover sediments m1 peak conditions of about 8 kbar and 800 c were reached after a prograde thickening event d 1co associated with the formation of a bedding parallel foliation while large amounts of s type granites were emplaced the localised d 2ba and d 1co structures are interpreted as spatially distinct responses at different crustal levels to one metamorphic event juxtaposition of basement and allochthonous cover along a large low angle normal shear zone occurred during d 3 this detachment zone accommodated extension after crustal thickening and allowed unloading of the footwall resulting in a rise of the basement rocks through 4 6 kbar as deduced from syn d 3 decompression textures coeval cooling textures in the allochthonous cover sequence indicate that lateral rather than vertical displacements occurred in the hanging wall of the detachment during c b m2 peak conditions of about 4 6 kbar and 650 700 c were reached coeval with the development of discrete left lateral strike slip zones d 4 that transect both basement and allochthonous cover and accommodated some uplift the deduced c a and c b p t paths have equivalents in many precambrian granulites in general decompressional paths like the one recorded in the basement rocks are linked to processes involving crustal thickening followed by extensional collapse in the datong huai an area it can be shown that low angle detachments may have played an important role in the uplift and cooling history of granulite belts as long ago as the archaean proterozoic boundary

petrogenesis of neoarchean metavolcanic rocks in changyukou northwestern hebei implications for the transition stage from a compressional to an extensional regime for the north china craton abstract the 2 5 ga metavolcanic rocks in changyukou northwestern hebei can be classified into three groups based on major and trace elements high mg basalts tholeiitic basalts and the calc alkaline series basaltic andesites andesites and dacites rhyolites both high mg basalts and tholeiitic basalts have negative anomalies of nb zr ti and heavy rare earth elements hree as well as enrichments of sr k pb ba and light rare earth elements lree and show typical subduction zone affinities the petrogenesis of high mg basalts can be ascribed to high degree partial melting of an enriched mantle source in the spinel stability field that was previously enriched in large ion lithophile elements lile and lree by slab derived hydrous fluids melts supercritical fluids as well as the subsequent magma mixing processes of different sources at different source depths with little or no influence of polybaric fractional crystallization the flat hree of tholeiitic basalts indicates they may also originate from the spinel stability field but from obviously shallower depths than the source of high mg basalts they may form at a later stage of the subduction process when rapid slab rollback leads to extension and seafloor spreading in the upper plate we obtain the compositions of the archean lower crust of the north china craton based on the archean wutai jining section by compiling the average tonalite trondhjemite granodiorite ttg components average mafic granulite components and average sedimentary rock components the modeling results show that the generation of high al basalts basaltic andesites and andesites can be attributed to assimilation by high mg basalts primary basalts of relatively high al 2 o 3 thickened lower crust and the subsequent crystallization of prevailing mafic mineral phases while al 2 o 3 rich plagioclase crystallization is suppressed under high pressure and nearly water saturated conditions dacites and rhyolites may be the result of further fractional crystallization of basaltic andesites high al basalts and andesites mixing of magmas at various stages along the fractionation course of basaltic andesites high al basalts toward rhyolites promotes the trend of the calc alkaline series to reconcile the 2 55 to 2 5 ga ttgs derived from overthickened crust the 2 51 to 2 50 ga calc alkaline volcanic rocks derived from thickened crust tholeiitic basalts representing low pressure and an extensional tectonic setting 2493 ma leucosyenogranites derived from overthickened crust 2437 ma biotite monzogranites derived from slightly thinner crust than leucosyenogranites but still thickened as well as the clockwise hybrid itd and ibc p t paths of the hp granulites and widespread extension and rifting setting within the ncc from 2300 ma we propose a model of an evolving subduction process among them the composition of the 2 5 ga changyukou volcanic rocks and potassic granites as well as the clockwise hybrid itd and ibc p t paths of the hp granulites may reveal that the tectonic setting in northwest hebei was in a transition stage from a subduction related compressional regime to an extensional regime related to plate rollback

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

high pressure temperature metamorphism in the st leonhard granulite massif austria evidence from intermediate pyroxene bearing granulites the st leonhard granulite massif lower austria is one of the small occurrences of high pressure granulite found in the gfohl unit at the highest tectono stratigraphic level of the moldanubian zone although predominantly composed of extremely deformed acidic garnet kyanite bearing rocks thin conformable layers of intermediate garnet clinopyroxene bearing granulites are seen pressure temperature estimates for the peak metamorphic assemblage of garnet clinopyroxene ternary feldspar quartz in these rocks are 15 19 kbar 950 1050 c a close coherence between results obtained from a combination of independent geothermobarometers and those derived from an internally consistent thermobarometric method indicate the retention of high pressure temperature equilibrium mineral compositions even though there is a wealth of petrographic evidence for significant post peak metamorphic decompression pressure temperature estimates for the orthopyroxene bearing intermediate pressure decompression stage obtained from discrete reaction textures are 8 12 kbar and 800 900 c post decompressive cooling from 800 to 500 c at ca 5 8 kbar is recorded by the final amphibolite facies biotite bearing assemblage together with petrological constraints from the enclosing acid granulites

geochemical and isotopic study of the xiong er volcanic rocks at the southern margin of the north china craton petrogenesis and tectonic implications abstract the xiong er volcanic rocks constitute a large paleo mesoproterozoic volcanic belt along the southern margin of the north china craton the volcanic rocks can be petrographically and geochemically classified into the basaltic andesite andesite and dacite rhyolite series with the former being the dominant component of the xiong er volcanic rocks the volcanic rocks of the dacite rhyolite series have been derived from the wet and low temperature differentiation of the basaltic andesite andesite series at shallow depth suggested by regional geological lithological and geochemical studies in addition crustal assimilation was involved in the magmas to form volcanic rocks of the dacite rhyolite series as suggested by low nd t values 8 09 to 9 99 and high th yb ratios 1 7 2 7 some of volcanic rocks from the basaltic andesite andesite series show features of high mg andesites hma being characterized by high mgo contents and mg numbers representing the most primitive end member of the

garnet orthopyroxene plagioclase quartz barometry refinement and application to the english river subprovince and the minnesota river valley recent experimental theoretical and thermodynamic studies permit better calibration of two reactions for geobarometry grossular pyrope quartz anorthite enstatite grossular almandine quartz anorthite ferrosilite if both reactions are applied using the same thermodynamic data and activity models they should yield the same pressure for a given garnet pyroxene plagioclase quartz assemblage application to a variety of high grade terrains generally yielded excellent results however poor results have been obtained for high fe rocks which can be traced to errors in activity models for garnet and or pyroxene either a two site ideal mixing model for orthopyroxene cf wood and banno 1973 underestimates enstatite activity for high fe orthopyroxenes or the ganguly and saxena 1984 model overestimates pyrope activity in low mg garnets

paleomagnetism of ca 1 35ga sills in northern north china craton and implications for paleogeographic reconstruction of the mesoproterozoic supercontinent abstract we report paleomagnetic data on precisely dated mesoproterozoic sills in zircon and baddeleyite 207pb 206pb ages of 1345 12 and 1353 14 ma respectively intruding the xiamaling and wumishan formations in north china with an aim of evaluating how the north china craton ncc was involved in the evolution of the columbia supercontinent from ca 1 78 to 1 35 ga after systematic thermal demagnetization we isolate a high temperature characteristic remanence from 18 sites with a tilt corrected site mean direction of d 294 4 i 31 7 and 95 4 3 corresponding to a mean paleomagnetic pole at 5 9 n 359 6 e n 18 with angular standard deviation of 10 0 and a95 4 3 rock magnetic experiments light microscopy and scanning electron microscopy all indicate that the main magnetic minerals in the sampled sill rocks are medium sized titanomagnetites the positive fold test and fresh titanomagnetite grains suggest that the characteristic remanence is likely to be a primary acquired at ca 1 35 ga the preliminary paleogeography reconstruction based upon the well matched apparent polar wander paths between the laurentia baltica siberia united block and the ncc supports the interpretation that the ncc may have drifted together with this united block before ca 1 35 ga during the late history of the columbia supercontinent we argue that the ncc was located in low latitudes and kept tight connections with the siberian and indian cratons during the interval between ca 1 8 and 1 35 ga and that the ca 1 35 ga sills widely developed in the mesoproterozoic xiamaling and wumishan formations as well as the ca 1 3 1 2 ga magma events well developed in siberia and northern margin of the ncc may represent the complete fragmentation of the ncc with the neighboring cratons

geochemistry isotope systematics and petrogenesis of the volcanic rocks in the zhongtiao mountain an alternative interpretation for the evolution of the southern margin of the north china craton abstract the xiong er volcanic rocks constitute a large paleo mesoproterozoic volcanic belt along the southern margin of the north china craton which are composed primarily of basaltic andesites and andesites with minor dacites and dacitic rhyolites the basaltic andesites and andesites from the xiong er volcanic rocks in the zhongtiao mountain show consistent nd t la nb and th nb values irrespectively of sio 2 which preclude significant crustal contamination during ascent both of these volcanic rocks share similar incompatible ratios e g la sm zr nb la nb indicating that the basaltic andesites and andesites originated from the same source the basaltic andesites are characterized by high mg number 45 55 precluding the possibility of their derivation from the melting of the lower crust variable ti eu zr sm and nb la ratios for the basaltic andesites with higher partial melting degree suggest that amphiboles remained as residues in the magma source indicating that the xiong er volcanic rocks were derived from hydrous magma non radiogenic nd isotopes for the xiong er volcanic rocks are inherited from a mantle source which had been enriched by the subduction related crustal recycling during archean to paleoproterozoic time basaltic andesites show hfse enrichments especially nb 4 7 ppm and high fe ti contents comparable with nb enriched basalts suggesting that the xiong er volcanic rocks were derived from a metasomatized mantle source similar to that of nb enriched basalts therefore the mantle source of the xiong er volcanic rocks in the zhongtiao mountain has been enriched in fe ti oxides and hfse by equilibrium reactions with the slab melts during late archean and paleoproterozoic subduction processes as evidenced by nd isotopes in the primitive mantle normalized trace element diagrams the xiong er volcanic rocks show enrichments in the lile and lree and negative anomalies on the nb ta ti similar to arc related volcanic rocks produced by the hydrous melting of the metasomatized mantle wedge in the paleo mesoproterozoic the xiong er volcanic rocks in the zhongtiao mountain were produced by the slab dehydration induced melting of an existing metasomatized mantle source and the fluids from the slab dehydration introduced significant lile and lree to the source masking its inherent hfse enriched characteristics the arc related characteristics of the xiong er volcanic rocks in the zhongtiao mountain suggest the xiong er volcanic belt constructs a subduction zone along the southern margin of the north china craton in the paleo mesoproterozoic implying that the craton may have recorded the outbuilding history of the columbia supercontinent during paleo mesoproterozoic time

petrogenesis and tectonic setting of volcanic rocks in the xiaoshan and waifangshan areas along the southern margin of the north china craton constraints from bulk rock geochemistry and sr nd isotopic composition abstract as part of the xiong er volcanic belt along the southern margin of the north china craton volcanic rocks in the xiaoshan and waifangshan areas have a compositional range from the basaltic andesite andesite dacite to rhyolite which display consistent variation trends in terms of their major and trace elements and sr nd isotopic compositions the variable yb contents with nearly constant la yb and tb yb ratios of volcanic rocks in two areas suggest that the fractional crystallization may have played an important role in the differentiation from the basaltic andesite through andesite and dacite to rhyolite the volcanic rocks in these two areas are characterized by the lile and lree enrichments and negative hfse anomalies implying hydrous melting of a mantle wedge in a subduction zone variable sr nd ratios of the basaltic andesite and andesite are interpreted as a result of the fluid addition from a subducting slab non radiogenic nd isotopic compositions as well as high zr y and nb y ratios suggest that the volcanic rocks in these areas were derived from an enriched mantle source on the other hand the volcanic rocks of the basaltic andesite and andesite possess markedly higher fe ti and hfse concentrations than those of typical intra oceanic arcs implying that the mantle source from which the volcanic rocks were derived was metasomatised by siliceous melts during the archean to paleoproterozoic subduction collision in the trans north china orogen these data suggest that in the paleo mesoproterozoic the southern margin of the north china craton was most likely an andean type continental arc in which slab dehydration not only induced the melting of a pre existing metasomatised mantle source but also released lile enriched fluids into the mantle source masking the inherent hfse enriched characteristics of the volcanic rocks along the southern margin of the craton the results of this study indicate that the north china craton like many other continental components e g north america greenland baltica amazonia australia etc of the supercontinent columbia nuna also underwent a subduction related outgrowth along its southern margin during the paleo mesoproterozoic time

la icp ms u pb zircon ages of the liaohe group in the eastern block of the north china craton constraints on the evolution of the jiao liao ji belt abstract as a major lithostratigraphic unit of the palaeoproterozoic jiao liao ji belt in the eastern block of the north china craton the liaohe group consists of an assemblage that is transitional from a lower arkose and volcanic rich sequence langzishan li eryu and gaojiayu formations through a middle carbonate rich sequence dashiqiao formation to an upper argillaceous sequence gaixian formation tectonically associated with the liaohe group are large volumes of granitoid rocks named the liaoji granite whose emplacement age has been dated at about 2 1 2 2 ga regionally the liaohe group and the associated liaoji granite extend along a nee trending belt that is flanked by 2 45 2 55 ga basement gneisses u pb zircon data and cathodoluminescence cl imaging are used to constrain the maximum depositional and metamorphic ages of the liaohe group which are critical in understanding the tectonic setting and history of the jiao liao ji belt cathodoluminescence images reveal that most detrital zircon grains from the liaohe group are characterized by concentric oscillatory zoning with comparatively low luminescence and high th u ratios suggesting magmatic origin but minor zircon grains have structureless overgrowth or recrystallization rims that are characterized by low th u ratio indicating metamorphic origin results of this study show that the detrital zircon grains of the langzishan formation the lowest sequence of the liaohe group have concordant u pb ages ranging from 2 05 to 2 24 ga suggesting that the liaohe group was deposited at some time after 2 05 ga although most overgrowth or recrystallization rims are too narrow to be analyzed some rims yield concordant or upper intercept ages around 1 93 ga interpreted as the peak metamorphic age of the liaohe group detrital zircon grains from the dashiqiao formation in the middle sequence of the liaohe group have concordant u pb ages ranging from 2 25 to 2 53 ga some of which are similar to those of the basement gneisses in the area this study indicates that the liaohe group was deposited after the emplacement of the liaoji granite and the latter may be a major component of the provenance for the langzishan formation in contrast the protoliths of the middle sequence of the liaohe group were most probably derived from the basement gneisses these new zircon ages for the liaohe group and recent data for the liaoji granite combined with lithological structural and metamorphic considerations support a rifting model for the evolution of the jiao liao ji belt

the origins of granulites a metamorphic perspective although many recent reviews emphasize a uniformity in granulite pressure temperature p t conditions and paths granulites in reality preserve a spectrum of important petrogenetic features which indicate diversity in their modes of formation a thorough survey of over 90 granulite terranes or occurrences reveals that over 50 of them record p t conditions outside the 7 5 1 kbar and 800 50 c average granulite regime preferred by many authors in particular an increasing number of very high temperature 900 1000 c terranes are being recognized both on the basis of distinctive mineral assemblages and geothermobarometry petrogenetic grid and geothermobarometric approaches to the determination and interpretation of p t histories are both evaluated within the context of reaction textures to demonstrate that the large range in p t conditions is indeed real and that both near isothermal decompression itd and near isobaric cooling ibc p t paths are important amphibolite granulite transitions promoted by the passage of co 2 rich fluids as observed in southern india and sri lanka are exceptional and not representative of fluid related processes in the majority of terranes it is considered on the contrary that fluid absent conditions are typical of most granulites at or near the time of their recorded thermal maxima itd granulites are interpreted to have formed in crust thickened by collision with magmatic additions being an important extra heat source erosion alone is not however considered to be the dominant post collisional thinning process instead the itd paths are generated during more rapid thinning 1 2 mm yr exposure related to tectonic exhumation during moderate rate or waning extension ibc granulites may have formed in a variety of settings those which show anticlockwise p t histories are interpreted to have formed in and beneath areas of voluminous magmatic accretion with or without additional crustal extension ibc granulites at shallow levels very rapid 5 mm yr extensional thinning subsequent to collision it is suggested that the preservation of ibc paths rather than itd paths in many granulites is primarily related to the rate and timescale of extensional thinning of thickened crust and that hybrid itd to ibc paths should also be observed most ibc granulites and probably many itd granulites have not been exposed at the earth s surface as a result of the tectonic episodes which produced them but have resided in the middle and lower crust for long periods of time 100 2000 ma following these events the eventual exhumation of most granulite terranes only occur through their incorporation in later tectonic and magmatic events unrelated to their formation

2090 2070 ma a type granitoids in zanhuang complex further evidence on a paleoproterozoic rift related tectonic regime in the trans north china orogen abstract the xuting pluton located at the north section of the zanhuang complex contains potassic and sodic granite in both of which shallow level emplacement is shown by graphic texture feldspar and quartz intergrowth two potassic granite and one sodic granite samples yielded zircon 207 pb 206 pb ages of 2066 17 ma 2092 14 ma and 2071 14 ma respectively suggesting that the xuting pluton emplaced between 2070 ma and 2090 ma which is coeval with the paleoproterozoic gantaohe group dominated by mafic volcanic and sedimentary rocks the potassic granite is enriched in sio 2 total feo rees except eu zr nb ga and y and depleted in mgo cao al 2 o 3 sr v cr and ni with high tfeo mgo ratios consistent with the features of a type granite except for the high mgo and low k 2 o contents the sodic granite also presents most of the features of a type granites the zircon saturation temperatures from the potassic and sodic granites range from 855 to 998 c and 853 to 933 c respectively indicating that they are from the high temperature magma the potassic granites have the e hf t ranging from 5 9 to 1 33 and hf model ages of 2 5 2 8 ga which are identical with the ages of neoarchean ttg gneisses in the zanhuang complex we propose that the potassic granite in the xuting pluton derived from the partial melting of neoarchean ttg gneisses the sodic granites also present negative e hf t values from 4 81 to 0 76 and hf model ages of 2 5 2 8 ga suggesting that they come from the partial melting of the neoarchean crust however with relative enrichment in mgo and total feo and lower k 2 o and rb the sodic granite was derived from a mafic source in the lower crust the likely heat source was high heatflow associated with mantle upwelling the a type xuting pluton and mafic volcanic rocks in the gantaohe group constitute a bimodal lithologic association from a paleoproterozoic continental rift environment 2 2 2 0 ga magmas are widely distributed along the trans north china orogen tnco and also present a bimodal character we propose that 2 2 2 0 ga bimodal magmas throughout the tnco were likely formed in a paleoproterozoic rifting setting considering the widespread 2 2 2 0 ga magmatic rocks in the north china craton ncc we infer that the magmatism likely occurs in tensional environment after the initial amalgamation of the ncc in the late archean

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

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

two new geobarometers for garnet amphibolites with applications to southeastern vermont two new geobarometers for the assemblage garnet hornblende plagioclase quartz have been calibrated on the basis of the equilibrium 6 anorthite 3 tremolite 2 grossular 1 pyrope 3 tschermakite 6 quartz and its fe end member equivalent data representing 37 natural samples that equilibrated at conditions of 2 5 to 13 kbar and 500 to 800 c were fitted to the general equation rt in keq a bt p i c by using a weighted least squares method multiple correlation coefficients are high 0 98 and 0 97 lines of constant keqhave extremely shallow slopes in p t space 2 to 8 bars per degree suggesting that pressures may be deduced precisely even where temperatures are only poorly constrained typical analytical errors and temperature imprecisions propagate to very small errors in pressure about 500 bars caution is advised in applying the barometers outside the range of calibrant mineral compositions application of the calibrations to samples from southeastern vermont near the strafford chester and athens domes documents relatively high pressure metamorphism 7 to 10 kbar for these structures

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

connections between the pacific ocean tropics and midlatitudes on decadal timescales abstract connections between the tropical and midlatitude pacific on decadal timescales are examined using a 137 yr run of a fully coupled ocean atmosphere general circulation model it is shown that the model does a credible job of simulating both enso scale and decadal scale variability and that there are statistically significant correlations between the midlatitudes and tropics on decadal timescales three physical mechanisms linking the regions are examined 1 oceanic advection along isopycnal surfaces from the midlatitude subduction regions to the tropics 2 coastally trapped or kelvin wave propagation between the tropics and midlatitudes and 3 near simultaneous communication between the regions affected by changes in the atmosphere it is found that communication via the atmosphere explains the strongest correlations found in the model further evidence is presented that is consistent with the idea that midlatitude sea surface temperature anomalies drive changes in the trade wind system that a

zircon u pb ages and geochemistry of the huai an ttg gneisses terrane petrogenesis and implications for 2 5ga crustal growth in the north china craton abstract the huai an gneiss terrane is a typical archean terrane within the trans north china orogen of the north china craton ncc and is composed of tonalite trondhjemite granodiorite ttg suite and dioritic granulite facies gneisses precise shrimpii and cameca ims1280 zircon u pb ages demonstrate that the huai an gneisses formed around 2 55 2 50 ga and underwent granulite facies metamorphism at 1 85 1 80 ga the ttg gneisses are compositionally equivalent to high sio2 adakites with high sio2 and al2o3 concentrations strongly fractionated rare earth element ree patterns la yb n 10 54 with positive or negligible eu anomalies eu eu 0 9 3 4 low heavy ree hree and y concentrations high sr 435 938 ppm concentrations and high sr y ratios 48 261 high mg numbers 33 60 and high compatible element concentrations e g cr 11 183 ppm and ni 11 82 ppm indicate that the magmas that formed these gneisses were derived from interaction between adakitic melts and mantle material the dioritic gneisses have compositions similar to high mg diorites or sanukitoids with high al2o3 14 46 17 61 wt mgo 3 0 6 4 wt cr 100 304 ppm and ni 57 109 ppm concentrations and fractionated ree patterns with weak negative or no eu anomalies eu eu 0 8 1 0 all the ttg and dioritic gneiss samples analysed during this study have negative nb ta and ti anomalies compared to neighbouring elements in spider diagrams with the majority having high positive nd t and hf t values close to values of the depleted mantle these geochemical characteristics strongly suggest that protoliths of the huai an ttg and dioritic gneisses originated in a juvenile crust forming event at 2 55 2 50 ga through partial melting of subducted oceanic crustal material these magmas also assimilated mantle peridotite material during their ascent this indicates that 2 5 ga mantle derived magmatism led to significant crustal growth in the north china craton and was probably associated with subduction

origin and evolution of granulites in normal and thickened crusts rocks buried in the upper part of a doubly thickened crust during orogeny can show prograde reactions with transitional features from lower grade rocks up to granulite facies they can be exposed at the earth9s surface within this one erogenic cycle for example the granulites exposed in british columbia have concordant u pb zircon ages of 65 85 ma in contrast rocks buried in the lower part of a thickened crust will crystallize granulites that typically show only retrograde relations with younger lower grade rocks at the cessation of orogeny such rocks will undergo near isobaric cooling from the peak metamorphism and can have very long residence times at the base of a crust of normal thickness these granulites require a second orogeny to uplift and expose them at the surface as an example the archean granulites of enderby land antarctica were metamorphosed at 3070 ma at 1000 c at 8 10 kbar pressure they then followed at least a 400 c isobaric cooling path they also had a prolonged residence time of 2000 m y near the base of the crust before uplift no doubt a continuum exists between these extremes but it is proposed that many intermediate and high pressure granulites form in the lower parts of thickened crust and therefore require a two stage cycle of tectonism to be uplifted and exposed at the earth9s surface although most granulites probably form in thickened crust at active plate margins underplating of normal 30 40 km thickness crust by mantle melts during extension can also produce granulites these granulites would also undergo isobaric cooling and require a later orogeny to expose them an example is the paleozoic lachlan fold belt of eastern australia which has undergone only minor uplift since the major ultrametamorphism at about 400 ma however isotopic petrologic and geochemical differences are to be expected between these extremes of granulite formation

thermobarometry and geotectonic significance of high pressure granulites examples from the moldanubian zone of the bohemian massif in lower austria

zircon u pb geochronology of the zanhuang metamorphic complex reappraisal of the palaeoproterozoic amalgamation of the trans north china orogen amphibolites and metapelites exposed in the zanhuang metamorphic complex situated in the south middle section of the trans north china orogen tnco underwent upper amphibolite facies metamorphism and record clockwise p t paths including retrograde isothermal decompression high resolution zircon u pb geochronological analyses indicate that the metamorphic peak occurred during 1840 1860 ma which is in accordance with the ubiquitous metamorphic ages of 1850 ma retrieved by miscellaneous geochronologic methods throughout the metamorphic terranes of the northern tnco confirming that the south middle section of the tnco was involved in the amalgamation of the eastern and western blocks of the north china craton during the palaeoproterozoic

u pb dating and hf isotope study of detrital zircons from the zhifu group jiaobei terrane north china craton provenance and implications for precambrian crustal growth and recycling abstract the zhifu group exposed in the northeastern jiaobei terrane is subdivided from base to top into the laoyeshan bingying and dongkou formations and comprises mainly quartzites and muscovite and tourmaline bearing quartz schists in order to define its provenance and depositional age and to constrain the crustal growth and recycling of the source regions we have conducted la icp ms u pb dating and in situ hf isotopic study of detrital zircons from three quartz schists samples collected from the zhifu group most of the analyzed zircon grains have oscillatory zoning and th u ratios 0 4 suggesting igneous origins 270 detrital zircons u pb analyses yield u pb ages of 1709 3679 ma that cluster into three major age populations of 1 7 1 9 ga with a peak at 1844 ma 1 9 2 0 ga with a peak at 1936 ma and 2 4 2 55 ga with a peak at 2465 ma and two minor age populations of 2 2 2 35 and 2 6 2 7 ga the peaking age of 1844 ma for the youngest age population indicates that the zhifu group must be deposited after 1844 ma detrital zircons with ages of 1 7 2 35 ga can be linked to pre and post tectonic polyphase magmatic events along the jljb and most of the zircons have negative hf t values from 1 76 to 18 02 and two stage hf model ages t dm2 concentrating at 2 7 3 0 ga indicating that source rocks of these zircons were derived from recycling of ancient crusts detrital zircons with ages of 2 4 2 9 ga may have been sourced from the archean granitoid mainly ttg gneisses in the jiaobei terrane and these zircons have hf t values from 6 4 to 7 53 and t dm2 of 2 6 3 4 ga peak at 2 7 3 0 ga suggesting that source rocks of these zircons were mainly derived from juvenile crusts associated with a major crustal growth at 2 7 3 0 ga and a limited amount of ancient crustal component the two detrital zircons with ages of 3631 and 3679 ma have negative hf t values of 4 42 and 1 51 and t dm2 of 4137 and 4000 ma respectively implying the recycling of ancient continental crust 3 6 ga and crustal growth prior to 4 1 ga in the ncc

petrogenesis and tectonic implications of the iron rich tholeiitic basalts in the hutuo group of the wutai mountains central trans north china orogen abstract the hutuo group located in the wutai mountains area is characteristic of paleoproterozoic strata in the north china craton and plays an important role in establishing evolutionary processes in the trans north china orogen we present petrological observations along with geochemical and nd isotopic data for basalts in the qingshicun and hebiancun formations of the hutuo group these basalts are enriched in tfeo 11 97 18 01 wt consistent with occurrences of iron rich basalt world wide they also display relatively high mgo 4 95 12 25 wt ni 79 121 ppm and cr 37 101 ppm and low in sio 2 41 37 51 95 wt therefore the iron rich character of the hutuo group basalts originated with their parental magma rather than via shallow crustal fractionation at low oxygen fugacity as inferred for many high iron basalts the hutuo group basalts exhibit weak to moderate differentiation of light ree over heavy ree la lu cn 2 61 6 79 with minor or slight eu anomalies eu eu 0 77 1 41 and also show the nb and ti troughs without obvious zr and hf anomalies the nd t values of the basalts range from 1 23 to 0 81 with t dm of 2489 2664 ma the basalts most plausibly originated from the fertile subcontinental lithosphere mantle and in the case of the qingshicun formation basalts also underwent the crustal contamination basalts in the hutuo group also have the high zr hf ratios and present high ti v values of 24 55 and zr y ratios ranging from 4 6 to 13 6 suggesting that they were emplaced in a within plate setting considering the sedimentary associations and detrital fragments in the hutuo and gantaohe groups and bimodal magmatism along the trans north china orogen we propose that 2 2 2 0 ga geological events within the orogen were rift related but did not result from subduction or island arc processes

the xiong er volcanic belt at the southern margin of the north china craton petrographic and geochemical evidence for its outboard position in the paleo mesoproterozoic columbia supercontinent abstract the xiong er volcanic belt covering an area of more than 60 000 km 2 along the southern margin of the north china craton has long been considered an intra continental rift zone and recently interpreted as part of a large igneous province formed by a mantle plume that led to the breakup of the paleo mesoproterozoic supercontinent columbia however such interpretations cannot be accommodated by lithology mineralogy geochemistry and geochronology of the volcanic rocks in the belt lithologically the xiong er volcanic belt is dominated by basaltic andesite and andesite with minor dacite and rhyolite different from rock associations related to continental rifts or mantle plumes which are generally bimodal and dominated by mafic components however they are remarkably similar to those rock associations in modern continental margin arcs in some of the basaltic andesites and andesites amphibole is a common phenocryst phase suggesting the involvement of h 2 o rich fluids in the petrogenesis of the xiong er volcanic rocks geochemically the xiong er volcanic rocks fall in the calc alkaline series and in most tectono magmatic discrimination diagrams the majority of the xiong er volcanic rocks show affinities to magmatic arcs in the primitive mantle normalized trace element diagrams the xiong er volcanic rocks show enrichments in the lile and lree and negative nb ta ti anomalies similar to arc related volcanic rocks produced by the hydrous melting of metasomatized mantle wedge nd isotope compositions of the xiong er volcanic rocks suggest that 5 15 older crust has been transferred into the upper lithospheric mantle by subduction related recycling during archean to paleoproterozoic time available shrimp and la icp ms u pb zircon age data indicate that the xiong er volcanic rocks erupted intermittently over a protracted interval from 1 78 ga through 1 76 1 75 ga and 1 65 ga to 1 45 ga though the major phase of the volcanism occurred at 1 78 1 75 ga such multiple and intermittent volcanism is inconsistent with a mantle plume driven rifting event but is not uncommon in ancient and existing continental margin arcs taken together the xiong er volcanic belt was most likely a paleo mesoproterozoic continental magmatic arc that formed at the southern margin of the north china craton similar paleo mesoproterozoic continental magmatic arcs were also present at the southern and southeastern margins of laurentia the southern margin of baltica the northwestern margin of amonzonia and the southern and eastern margins of the north australia craton which are considered to represent subduction related episodic outbuilding on the continental margins of the paleo mesoproterozoic supercontinent columbia therefore in any configuration of the supercontinent columbia the southern margin of the north china craton could not have been connected to any other continental block as proposed in a recent configuration but must have faced an open ocean whose lithosphere was subducted beneath the southern margin of the north china craton

paleoproterozoic evolution of the eastern alxa block westernmost north china evidence from in situ zircon u pb dating and hf o isotopes abstract the alxa block is the westernmost part of the north china craton ncc in the past it has been considered to be part of the archean ncc however formation and evolution of this block remain poorly understood and this has hampered a broader understanding of the ncc in this study we analyzed the in situ zircon u pb age and hf o isotopes for the two oldest rock units in the eastern alxa block namely the bayanwulashan and diebusige complexes the bayanwulashan complex consists mainly of metamorphic rocks with mafic and felsic protoliths sims u pb zircon dating results indicate that the primary magmatic ages of the mafic and felsic igneous rocks are ca 2 34 ga and ca 2 32 2 30 ga respectively and both sets of rocks were overprinted by two metamorphic events at ca 1 89 ga and ca 1 79 ga geochemical and zircon hf o isotopic data suggest that the ca 2 34 ga amphibolites within the bayanwulashan complex are characteristically high in tio 2 up to 3 2 zr up to 394 ppm and ti v 30 resembling the basaltic rocks formed within continental rifts the ca 2 32 2 30 ga felsic rocks were generated by remelting of dominant meta igneous rocks that have zircon hf model ages of 2 92 2 81 ga the diebusige complex is composed of amphibolites mafic gneisses and paragneisses it was intruded by granites dated at ca 1 97 1 98 ga and subjected to high grade metamorphic events at ca 1 89 ga and ca 1 79 ga re examination of u pb ages for detrital and metamorphic zircons indicate that the depositional ages of protoliths of the diebusige paragneisses may be considered to be between ca 2 45 and 2 0 ga these u pb zircon age data do not support the existence of exposed archean rocks in the eastern alxa block although the possibility of archean rocks at deeper crustal levels cannot be ruled out entirely the eastern alxa block contrasts to the neighboring yinshan block and khondlite belt as well as the trans north china orogen in terms of the timing and evolution of magmatism and metamorphism thus we conclude that the alxa block is a separated paleoproterozoic terrane from the western block of the ncc rather than the western extension of the yinshan block or part of the khondalite belt as previously proposed

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

granulite facies metamorphism and crust melting in the huai an terrane at 1 95ga north china craton new constraints from geology zircon u pb lu hf isotope and metamorphic conditions of granulites abstract geological implications of 1 95 ga and 1 85 ga metamorphic ages for hp granulites in the huai an terrane are still controversial which have affected the crustal evolution models for the north china craton this study reports two geological sections in which distinct geological relationships of a quartz dioritic and a tonalitic dike cutting through mafic hp granulite and granulite facies meta sedimentary rocks are described in these two sections magmatic dikes mafic hp granulites and meta sedimentary rocks preserve conformable foliation to indicate their similar deformation history the u pb dating and lu hf isotopic analysis of zircons from metamorphic and meta igneous rocks provide new constraints on the timing and mechanism of thermal events associated with metamorphic and deformation events both magmatic dikes were formed around 1 95 ga and overprinted by later metamorphic event during 1 85 ga the s type granite at geological section gs 1 revealed its formation age was formed around 1959 34 ma n 3 mswd 1 5 and overprinted by later metamorphic event at 1840 8 ma n 10 mswd 1 2 additionally zircons from the mafic hp granulite in the gs2 are dominated by metamorphic ages of 1972 22 ma n 6 mswd 0 03 with few grains recording an age of 1856 29 ma n 3 mswd 0 17 in situ lu hf analyses on zircons from the quartz dioritic dike tonalitic dike mafic hp granulite and s type granite reveal that the magmatic dikes were derived from partial melting of 2 0 2 2 ga meta basic rocks and the s type granite was generated by melting of meta sediments beyond 840 50 c it suggested that those meta sediments were taken into the lower crust in a certain way at granulite facies conditions ca 1 95 ga ago a clockwise p t path acquired from mafic hp granulite revealed peak metamorphic conditions of 880 890 c and 11 13 kbar the temperature results recorded in the 1 95 ga zircons of mafic hp granulite similar to those of high pressure mineral assemblage in matrix suggest that the hp granulite metamorphism was approximately 1 95 ga ago according to the results above this contribution concludes that mafic and meta sedimentary rocks were metamorphosed together under high pressure granulite metamorphic conditions 1 95 ga ago followed by high temperature granulite facies melting event that gave rise to granite and s type granite at 1 95 ga the granulite facies metamorphic rocks and magmatic dikes in the huangtuyao graphite mine experienced the same evolution history after 1 95 ga the mafic and pelitic hp granulites in the huai an terrane are interpreted to be collisional products of the paleoproterozoic cratonization of the ncc

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

interactions between topographic airflow and cloud precipitation development during the passage of a winter storm in arizona abstract a case study showing comparisons between observations and numerical simulations of the passage of a winter storm over complex terrain is presented the interactions between the mesoscale and cloud environments and the microphysical and dynamical processes are addressed using both observations and numerical simulations a three dimensional time dependent nested grid model was used to conduct numerical simulations of the three dimensional airflow and cloud evolution over the mogollon rim and adjacent terrain in arizona the modeling results indicated that the flow patterns and cloud liquid water clw were closely linked to the topography to a large extent gravity waves excited by the flow over the mountains determine the distribution of clouds and precipitation the waves extend through deep layers of the atmosphere with substantial updrafts and downdrafts at times exceeding 5 m s 1 the simulated vertical velocities and horizontal wavelengths of about 20 km were in good agreement with the air

u pb and hf isotopic study of zircons of the helanshan complex constrains on the evolution of the khondalite belt in the western block of the north china craton abstract the helanshan complex is located at the westernmost segment of the khondalite belt a continent continent collisional belt along which the yinshan block in the north collided with the ordos block in the south to form the western block of the north china craton the complex consists of s type granites and high grade pelitic granulite gneiss felsic paragneiss quartzite calc silicate rock and marble together called the khondalite series rocks la q icp ms u pb geochronology and la mc icp ms lu hf isotopic data of zircons combined with cathodoluminescence cl imaging enable the resolution of magmatic and metamorphic events that can be directed towards understanding the formation and evolution of the khondalite belt in the western block of the north china craton cl images reveal the coexistence of magmatic type detrital zircons and metamorphic zircons in most of the khondalite series rocks of which the metamorphic zircons occur as either single grains or overgrowth rims surrounding and truncating magmatic type detrital zircon cores la q icp ms u pb analyses on magmatic type detrital zircons reveal two distinct age populations with one in the archaean 2 85 2 53 ga and the other in the palaeoproterozoic 2 2 2 0 ga suggesting that the sedimentary protoliths of the khondalite series rocks in the helanshan complex must have been deposited at some time after 2 0 ga the hf analyses show that the archaean detrital zircons possess negative ehf t values from 7 64 to 0 15 with depleted mantle model ages ranging from 3 34 to 3 10 ga this implies the existence of a paleo mesoarchaean 3 34 3 10 ga crust in the western block which underwent a recycling event in the period 2 85 2 53 ga nearly all palaeoproterozoic 2 2 2 0 ga detrital zircons possess positive ehf t values 0 86 9 38 with depleted mantle model ages of 2 45 2 15 ga suggesting a significant crustal growth event in the western block in the palaeoproterozoic metamorphic zircons yield two groups of age with one at 1 95 ga and the other at 1 87 ga of which the former is considered as the timing of the collision between the yinshan and ordos blocks to form the western block whereas the latter is consistent with the age of 1 86 ga from two s type granites which are considered to have formed by the partial melting of pelitic granulites gneisses at the stage of the exhumation of the helanshan complex

pressure temperature time paths and a tectonic model for the evolution of granulites petrologic studies and application of well calibrated mineralogic thermometers and barometers reveal several important features common to many granulite terrains 1 peak metamorphic pressure and temperature conditions cluster around values of 7 5 pm 1 kbar and 800 pm 50 c implying average geothermal gradients of 30 35 c km 2 in comparison with upper amphibolite facies rocks especially in paired amphibolite granulite terrains it is evident that granulites are distinguished from amphibolites by the former s higher metamorphic temperatures but not higher pressure 3 initial cooling of granulite terrains from peak conditions is nearly isobaric for garnet closure temperatures of 600 pm 50 c the dp dt retrograde paths inferred from compositionally zoned garnet rims are 2 8 b c in most granulites 4 primary coarse sillimanite is found in most granulites and retrograde kyanite is known in a few these data suggest that granulites form as a result of anomalous thermal gradients

p t t paths and tectonic history of an early precambrian granulite facies terrane jining district south east inner mongolia china abstract the widespread khondalite series of south east inner mongolia consists largely of biotite sillimanite garnet gneiss and quartzo feldspathic gneiss with some marble and mafic granulite layers it has experienced two metamorphic events at c 2500 and 1900 2000 ma a pre peak stage of the first metamorphism at t 600 700 c and p 6 7 kbar is recognized by the relict amphibolite facies assemblage ky grt bt pl qtz and protected inclusions of biotite hornblende sodic plagioclase and quartz in garnet or orthopyroxene the peak stage with t c 800 50 c and p 8 10 kbar is characterized by the widespread granulite facies assemblages sil grt bt kfs pl qtz in gneiss and opx cpx pl hbl grt in granulite the p t t path suggests that the supracrustal sequence was buried in the lower crust by tectonic thickening during d1 d2 the beginning of the second metamorphism is characterized by further temperature rise to 700 c or more at lower pressure this stage is manifested by the appearance of cordierite after garnet fibrolite sil2 after biotite in gneiss and transformation of hbl1 into opx2 and cpx2 in granulite coronas of symplectitic opx2 pl2 surrounding grt1 and cpx1 in mafic granulite are interpreted as products of near isothermal decompression the p t t path may be related tectonically to waning extension of the crust by the end of the early proterozoic

late neoarchean monzogranitic syenogranitic gneisses in the eastern hebei western liaoning province north china craton petrogenesis and implications for tectonic setting abstract the medium grained monzogranitic syenogranitic gneisses expose as roughly northeast southwest trending and cover 1500 km 2 in the southern part of the eastern hebei western liaoning province north china craton these plutonic gneisses have leucocratic features and weakly gneissic to massive structures and were metamorphosed under greenschist to lower amphibolite facies la icp ms zircon u pb isotopic dating reveals that the magmatic precursors of these monzogranitic syenogranitic gneisses were emplaced during 2527 2511 ma they are characterized by high contents of sio 2 71 64 78 08 and k 2 o 3 55 6 40 and low contents of cao 0 25 1 71 mgo 0 01 0 42 and na 2 o 2 29 4 96 according to their chondrite normalized ree patterns the monzogranitic syenogranitic gneisses can be subdivided into a high ree group and a low ree group studies of petrogenesis reveal that magmatic precursors of the high ree group samples were derived from partial melting of the regionally widespread porphyritic or medium grained dioritic quartz monzonitic granodioritic monzogranitic gneisses under low pressure conditions less than ca 10 kbar the magmatic precursors of the low ree group were formed by fractionated crystallization of the high ree group magma with titanite allanite epidote apatite and zircon as the major crystallization fractionation phases combined with previous investigations on the late neoarchean metavolcanic rocks and granitoid orthogneisses in the northwestern part of the eastern hebei western liaoning province the magmatic precursors of these monzogranitic syenogranitic gneisses were most likely formed in a back arc basin along the north of the eastern block north china craton

neoarchean sanukitoids and associated rocks from the tengzhou pingyi intrusive complex north china craton insights into petrogenesis and crust mantle interactions abstract quartz monzodiorites granodiorites and monzogranites constitute the dominant lithological assemblage in the neoarchean tengzhou pingyi intrusive complex tpic in the southwestern part of western shandong province wsp in the eastern part of the north china craton ncc and mafic magmatic enclaves mmes are widely developed only in granodiorites of the tpic la icp ms zircon u pb isotopic dating reveals that the tpic was emplaced at 2 54 2 52 ga the 2 54 ga monzogranites display geochemical affinities with high k granites and they were likely derived from the partial melting of metamorphic greywackes at lower crustal levels the 2 54 2 52 ga quartz monzodiorites and granodiorites show significant geochemical similarities to the archean sanukitoids of the western karelian province of finland with higher mg 43 57 values cr 24 88 ppm sr 547 1154 ppm and ba 662 1961 ppm contents and k2o na2o 0 42 1 00 ratios therefore we define the quartz monzodiorites and granodiorites as neoarchean sanukitoids based on their lithological assemblages and geochemical characteristics these sanukitoids can be subdivided into sanukitoid quartz monzodiorites sqms and porphyritic sanukitoid granodiorites psgs the sqms exhibit higher positive hf t2 values from 1 9 to 3 6 with tdm values of 2 70 to 2 77 ga and are strongly enriched in light rare earth elements lrees and large ion lithophile elements liles implying a mantle source that was enriched by either fluids or melts the psgs have a wide range of hf t2 values from 3 4 to 3 8 suggesting that they were formed by crust mantle magma mixing this interpretation is also supported by the petrological textural and compositional disequilibrium of plagioclase phenocrysts from the psgs moreover the mmes and their host psgs have identical crystallization ages 2 54 ga similar mineral chemistries and zircon lu hf isotopic compositions indicating that the mmes are cumulates from the earlier host psgs magma petrogenetic studies and zircon lu hf isotopic features of these lithological assemblages indicate that the interval of 2 55 2 52 ga in the tpic represents an important period of crustal growth and crust mantle interactions considering the spatial temporal relationships and the petrogenesis of the diverse lithological assemblages we propose that the neoarchean tpic was most likely formed in an active continental margin setting

epma u th pb monazite and shrimp u pb zircon geochronology of high pressure pelitic granulites in the jiaobei massif of the north china craton the jiaobei massif constitutes the southern segment of the paleoproterozoic jiao liao ji belt in the eastern block of the north china craton and contains high pressure hp mafic and pelitic granulites the hp pelitic granulites contain four distinct mineral assemblages garnet kyanite biotite muscovite rutile m1 garnet kyanite ternary feldspar biotite muscovite rutile m2 garnet sillimanite biotite perthite anti perthite m3 and biotite muscovite sillimanite plagioclase m4 of which m2 is a typical hp pelitic assemblage the origin of these hp granulites is important for understanding the tectonic evolution of the jiao liao ji belt but the timing of the hp event still remains unknown this study applied the epma u th pb monazite and shrimp u pb zircon dating techniques to determine the metamorphic ages of the hp pelitic granulites in the jiaobei massif cl images and th u ratios show that all monazite and zircon grains in the analyzed samples are of metamorphic origin eighty two epma analyses on 16 monazite grains from two samples yielded three age groups at 1895 to 1882 ma 1814 to 1813 ma and 1706 to 1692 ma and the ages of 16 shrimp analyses on 14 zircon grains from one sample range from 1864 to 1803 ma the oldest ages of 1895 to 1882 ma were yielded from high y monazite grains enclosed in garnet and sillimanite pseudomorph after kyanite and thus interpreted as the time of the hp m2 granulite facies metamorphism the epma monazite ages of 1814 to 1813 ma and shrimp zircon ages of 1864 to 1803 ma are interpreted as the approximate ages of the post hp granulite facies metamorphism m3 most probably resulting from the exhumation of the hp granulites to the medium pressure granulite facies levels the youngest age group of 1706 to 1692 ma obtained from low y monazite grains in the matrix can be interpreted as the age of late cooling and retrograde metamorphism m4 that occurred when the hp pelitic granulites were exhumed to the upper crust the presence of paleoproterozoic hp mafic and pelitic granulites in the southern segment of the jiao liao ji belt suggests that the evolution of the jiao liao ji belt in the eastern block of the north china craton must have been involved in subduction or collision related tectonic processes

heat capacities and entropies of andalusite and sillimanite the influence of fibrolitization on the phase diagram of the al 2 sio 5 polymorphs

zircon u pb ages and hf isotope of gneissic rocks from the huai an complex implications for crustal accretion and tectonic evolution in the northern margin of the north china craton abstract the precambrian tectonic evolution of the north china craton ncc has long been a matter of debate the huai an complex situated at the conjunction of the khondalite belt and trans north china orogen tnco undoubtedly can provide some key constraints on the geological evolution of the khondalite belt and the tnco we report zircon u pb ages and hf isotope results for four gneissic rocks and one mesozoic mafic dyke in the huai an complex to constrain the precambrian tectonothermal evolution and crustal accretion in the northern margin of the ncc the zircons from three dioritic to granitic gneisses and inherited zircons from the mafic dyke all yield neoarchean to early paleoproterozoic protolith ages of 2445 2533 ma and register three later tectonomagmatic events with ages of 2360 2150 and 1850 ma the 2360 ma magmatism is recorded by a dioritic gneiss 2358 34 ma from the dongyanghe area and a granodioritic gneiss 2370 24 ma from the hunyuanyao area the 2150 ma magmatism is recognized in a dioritic gneiss 2172 7 ma granodioritic gneiss 2159 63 ma and the mesozoic mafic dyke 2144 57 ma the 1850 ma magmatism is represented by zircon populations with ages of 1877 48 ma in the dioritic gneiss 1844 69 ma in the granodioritic gneiss and 1800 ma in the mafic dyke these neoarchean to paleoproterozoic ages reveal that the huai an complex underwent a successive complicated tectonomagmatic evolution similar to the hengshan wutai fuping complexes which represent a long lived neoarchean to paleoproterozoic magmatic arc in contrast a sillimanite garnet gneiss from the kouzicun area yields completely different zircon u pb ages with peaks at 2056 32 ma 1952 25 ma and 1901 25 ma these three age groups suggest that the kouzicun area experienced a tectonothermal evolution diagnostic of the khondalite belt including protolith ages 2100 2000 ma peak metamorphism 1950 ma and post orogenic exhumation 1900 ma thus the geological boundary between the khondalite belt and the tnco can be constrained to a position between the kouzicun and hunyuanyao areas coupled with zircon u pb ages zircon hf isotopic results reveal that 2500 ma mantle derived mafic magmas not only provided enormous heat for partial melting of the 2 7 2 8 ga juvenile crust to form widespread ttg rocks but also contributed directly to the generation of dioritic rocks the huai an complex in the northern margin of the ncc witnessed two neoarchean episodes of continental crust growth at 2 7 2 8 ga and at 2 5 ga

crystal chemical evaluation of garnet and omphacite microprobe analyses its bearing on the classification of eclogites abstract the reliability of 920 clinopyroxene and 804 garnet microprobe analyses performed from 1964 through 1984 has been evaluated on the basis of crystal chemical constraints resulting from the latest structural investigations accepted were 343 pyroxenes and 28 garnets which were increased to 444 and 287 respectively after relaxation of some of the initial rigorous constraints so as to meet the number suitable for statistical processing the graphical portray of the accepted data permits the detection of the chemical elements significant in discriminating these eclogite minerals in three groups related to the geological settings of the enclosing rocks and therefore to their p t conditions of formation and to their small variations in bulk chemistry the best discriminant plots were found to be the ca mg na al vi na fe 3 triangle for clinopyroxenes essentially the di jd acm triangle and the ca mg fe 2 triangle for garnets essentially the gross pyr alm triangle their updated versions are reported in addition the data suggest that the p t regime acting during subduction metamorphism was remarkably constant in space and in time they hint for a future possible division of common regional metamorphic eclogites into three isofacial groups provided that the statistics will be increased and additional geological information becomes available finally they point out that the peculiar crystal chemistry of the minerals in kimberlite xenoliths may require reconsideration of the constraints inferred from crustal occurrences

crustal evolution of the eastern block in the north china craton constraints from zircon u pb geochronology and lu hf isotopes of the northern liaoning complex abstract the northern liaoning complex in northeastern china constitutes an important component of the eastern block in the north china craton the major lithologies consist of archean granitoid gneisses with minor supracrustal rocks occurring as tectonic lenses this study presents zircon lu hf isotopic data for the first time as well as new shrimp zircon u pb data of the major lithologies from the northern liaoning complex in order to elucidate the crustal evolution of the complex and provide new constraints on the neoarchean crustal evolution of the eastern block in the north china craton magmatic zircon u pb data from this study show that the protolith magmas of the supracrustal rocks and granitoid gneisses were generated during 2 55 2 50 ga metamorphic zircons document consistent metamorphic ages at 2 49 2 48 ga suggesting a regional metamorphic event immediately after the magmatism at the end of the neoarchean in the northern liaoning complex inherited detrital zircons of 2 79 2 60 ga suggest possible existence of ancient crust in this region zircon lu hf isotopic compositions show that the magmatic zircons have variable hf t values from 4 0 to 9 0 with depleted mantle model ages of 3 6 2 5 ga of which most hf t values are positive with a model age peak at 2 9 2 7 ga these zircon hf signatures reveal major juvenile crustal growth with additions of older crustal materials during 2 9 2 7 ga and a crustal reworking event with minor juvenile additions at 2 6 2 5 ga in the studied area integrated with previous data from other neoarchean complexes in the eastern block both the major juvenile crustal growth during 2 9 2 7 ga and the strong crustal reworking at 2 6 2 5 ga contribute to the extensive neoarchean crust formation of the eastern block in the north china craton the north china craton share similar neoarchean continental crustal evolution to other cratons in the world though it is distinctively featured by intensive tectonothermal overprinting at the end of the neoarchean

garnet zoning and the identification of equilibrium mineral compositions in high pressure temperature granulites from the moldanubian zone austria a detailed investigation of the compositional variation in garnet has been undertaken in a garnet pyroxene bearing granulite from the high grade gfohl unit moldanubian zone lower austria textural observations together with the interpretation of the preserved garnet chemistry enables the recognition of both prograde core and peak metamorphic garnet mantle growth stages an extremely rare feature in high p t granulite facies rocks initial thermobarometric calculations undertaken across whole garnet zoning profiles show how correct interpretation of a zoning profile is essential if the maximum peak metamorphic p t conditions are to be recovered the effect of retrograde decompression and cooling driven reactions on inclusion and host garnet compositions has also been assessed the results indicate that caution should be exercised when utilizing inclusion and adjacent garnet compositions for the thermobarometric evaluation of peak metamorphic equilibration conditions peak p t conditions were determined by the tweequ thermobarometric method utilizing the core compositions of matrix phases combined with the interpreted high p t garnet mantle composition to give 15 6 kbar and 1090 c consistent with previously determined results for moldanubian granulites similar high p t estimates are also provided by a re evaluation of previously published results for a granulite sample from the same lithological unit using a modified interpretation of garnet and plagioclase compositional data the new estimates presented confirm the previously disputed idea that the gfohl unit underwent a high pressure granulite facies stage and is therefore distinctly different from the underlying tectonostratigraphic units it is emphasized that any interpretation of the peak metamorphic conditions in high grade rocks must be based on detailed petrographic observations combined with a thorough understanding of the co existing equilibrium mineral compositions

calculation and application of clinopyroxene garnet plagioclase quartz geobarometers recently published thermodynamic and experimental data in a variety of chemical systems have been evaluated to derive gibbs free energies for hedenbergite and pyrope these were used to calculate the geobarometric equilibria

petrochemistry and sr pb and nd isotopic geochemistry of early precambrian rocks wutaishan and taihangshan areas china abstract in the wutaishan and taihangshan region china the lowermost high grade metamorphic complex of grey gneiss and amphibolite the fuping complex gives a 2 2 0 2 ga pb pb isochron age with a first stage growth 7 73 and a 2 37 0 07 ga sm nd isochron age with 143nd 144nd 0 0 50963 0 00005 or nd t 1 50 nd depleted mantle model dates tdm of amhibolites 2 48 to 2 60 ga those of gneisses are 2 43 to 2 46 ga the unconformably overlying medium to low grade metavolcanic complex the wutai complex gives a 2 0 0 1 ga rb sr isochron age with 87 86sr 0 0 7025 2 a 2 27 0 02 ga pb pb isochron date with afirst stage growth 7 73 and a 2 26 0 06 ga sm nd isochron first stage 143nd 144nd 0 0 5097 2 or nd t 1 08 individual sample nd depleted mantle model date are 2 5 ga metavolcanic samples from low grade metasedimentary rocks of the hutuo group all lie close to a 2 3 ga reference line on a rb sr isochron plot have nd tdm 2 32 2 34 and 2 62 ga from previous geological work published u pb zircon dating and ourisotopic and geochemical results we infer that the fuping rocks formed 2 6 ga ago in an environment like a modern island arc the wutai complex formed by 2 5 ga ago mostly formed in a tectonic setting similar to that of the fuping complex with the exception that one volcanic cycle formed in an environment like a modern mor and one unit formed in an environment transitional between modern within plate and plate margin settings the hutuo group formed 2 4 ga ago in a within plate tectonic environment there is no evidence for continental crust before 2 6 ga in the wutaishan and taihangshan region this contradicts previous assignment of the fuping complex to 2 8 ga ol older the fuping complex is not an early nucleus of the sino korean craton the new data also confirm that the wutai complex is older than its original assignment as early proterozoic the sm nd systems for metabasaltic rocks in the region either in amphibolite facies or in greenschist facies are all significantly disturbed in contrast with the undisturbed sm nd reported in previous studies of the sino korea craton this study provides further evidence that the archean igneous rocks in china formed at different times from heterogeneous and depleted mantle sources

zircon ages and nd hf isotopic composition of the zhaertai group inner mongolia evidence for early proterozoic evolution of the northern north china craton the mesoproterozoic rifting and accompanying magmatism and sedimentation in the north china craton are interpreted to be related to the breakup of the paleoproterozoic supercontinent columbia this study presents zircon ages and nd hf isotopic data of the low grade mesoproterozoic zhaertai group exposed along the northern margin of the north china craton sandstones of the zhaertai group contain detrital zircon populations of about 2500 ma which originated from the late archean basement rocks underlying this group a close match between the u pb ages and the tdm hf ages with high initial ehf values at 2500 ma for the detrital zircon grains imply a juvenile origin for protoliths of the basement rocks sedimentation of the zhaertai group likely began around 1750 ma tentatively constrained by the formation time of an intercalated volcanic layer the volcanic rocks have low initial end values of 8 0 to 8 5 at 1750 ma similar to those of contemporaneous mafic dyke swarms within the north china craton which have been interpreted to be related either to the breakup of the supercontinent columbia or to the post collisional collapse of the trans north china orogen a progressive increase of end values of whole rock samples upwards in the stratigraphic sequence likely indicates an increase of juvenile material with time probably related to the contribution of mesoproterozoic mantle derived rocks 2007 elsevier ltd all rights reserved

palaeoproterozoic metamorphic evolution and geochronology of the wugang block southeastern terminal of the trans north china orogen abstract amphibolites of the wugang block in the taihua metamorphic complex which is located in the southeastern terminal of the palaeoproterozoic trans north china orogen tnco preserve three generations of metamorphic mineral assemblages i e the prograde m 1 peak m 2 and retrograde m 3 assemblages forming during the palaeoproterozoic tectono metamorphic event these amphibolites record metamorphic p t conditions of 4 8 6 7 kbar 720 750 c m 1 9 0 10 6 kbar 710 780 c m 2 and 7 1 8 5 kbar 700 780 c m 3 which define clockwise p t paths involving isothermal decompression itd the metamorphic peak reached upper amphibolite facies and high resolution in situ sims u pb dating of metamorphic zircons suggests that the peak metamorphism took place at 1 96 1 92 ga in the wugang block the present new data suggest that the wugang block was also involved in the continental collision between the eastern and western blocks of the north china craton ncc in the palaeoproterozoic combining metamorphic and geochronological data concerning the tnco obtained by previous scholars it may be anticipated that the middle and southern sections of the tnco were involved in a long lived continental collision and the collision event can be possibly traced back to as early as 1 96 ga and lasted to as late as 1 80 ga

deformation history of the hengshan complex implications for the tectonic evolution of the trans north china orogen structural analysis indicates that the hengshan complex underwent five distinct episodes of deformation d1 d5 the d1 episode formed small isoclinal folds f1 penetrative axial planar foliations s1 and mineral stretching lineations l1 d1 fabrics were reworked by following d2 deformation associated with d2 was the development of nw verging asymmetric folds and accompanying thrust faults both d1 and d2 resulted from crustal thickening and coherent with prograde and peak metamorphism d2 deformation was followed by transpressional dextral shearing d3 including top to nw oblique slip shearing and nne ssw dextral strike slip shearing ongoing collision led to development of the zhujiafang ductile shear zone d4 a near e w trending high strain belt across the hengshan complex d5 deformation is characterized by f5 open folds and associated normal faults probably related to exhumation of the complex structural patterns of the hengshan complex can place important insights into a recently proposed tectonic model suggesting that an ocean between the eastern and western blocks underwent eastward directed subduction beneath the western margin of the eastern block and closure of this ocean finally led to collision between the two blocks to form the coherent north china craton

on the formation of granulites the tectonic settings for the formation and evolution of regional granulite terranes and the lowermost continental crust can be deduced from pressure temperature time p t time paths and constrained by petrological and geophysical considerations p t conditions deduced for regional granulites require transient average geothermal gradients of greater than 35 c km 1 implying minimum heat flow in excess of 100 mwm 2 such high heat flow is probably caused by magmatic heating tectonic settings wherein such conditions are found include convergent plate margins continental rifts hot spots and at the margins of large deep seated batholiths however particular p t time paths do not allow specific tectonic settings to be distinguished at this time under different conditions both clockwise cw pmax attained before tmax and anticlockwise acw pmax attained slightly after tmax paths are possible in the same tectonic setting both cw and acw end member paths can yield nearly isobaric cooling ibc paths such cooling paths are clearly not an artefact of thermobarometry but can be constrained by solid solid and devolatilization equilibria and geophysical modelling in terms of understanding the evolution of the deep crust a potentially significant group of regional granulite terranes are those that show evidence for acw ibc paths such paths are the likely result of i episodic igneous activity resulting in intrusions within all levels of the crust ii thickening of the crust by magmatic underplating iii slow uplift as a result of the formation of a deep garnet rich crustal root and iv excavation resulting from a later tectonic event unrelated to that resulting in the formation of the granulites the later event might be triggered by the delamination of the garnet rich lowermost crust

3800 to 2500 ma crustal evolution in the anshan area of liaoning province northeastern china shrimp u pb zircon geochronology has revealed several generations of granitoids and gneisses from the anshan area of northeastern china two samples of the baijiafen granite sensu lato 3804 5 and 3812 4 ma chentaigou granite 3306 13 ma lishan granite 3142 7 ma east and west anshan granites 3001 4 and 2994 8 ma tiejiashan granite 2962 4 and 2964 6 ma and qidashan granite 2475 ma the 3800 ma baijiafen granite consists of banded migmatitic trondhjemitic gneisses and forms only a small part of the area nonetheless they are the oldest known rocks in asia and are one of very few locations in the world where rocks of this antiquity have been found the younger suites are predominantly granite sensu stricto in composition and although foliated they have not been transformed into banded gneisses nd isotopic data suggest that the 3800 ma granites were produced at least in part by remelting of very ancient lree enriched sources perhaps like the exposed remnants of the 3800 ma rocks the chentaigou supracrustals contain layered mafic and ultramafic rocks banded iron formation bif and siliceous layered rocks of probable volcanogenic origin a sample of the latter has yielded a concordant u pb zircon age of 3362 5 ma interpreted as the age of deposition a trondhjemitic sheet cutting these rocks has yielded a u pb zircon age of 3342 6 ma the chentaigou supracrustals are the oldest recognised in the area younger supracrustal sequences collectively known as the anshan group contains the main economic bif units in the area bif is intruded by the 2475 ma qidashan granite indicating that the anshan group is archaean or earliest proterozoic in age

p t t evolution of orogenic belts and the causes of regional metamorphism barrow 1893 introduced three important ideas that furthered understanding of metamorphic processes i the use of critical index minerals in argillaceous rocks to define metamorphic zones and elucidate spatial features of regional metamorphism ii the concept of progressive metamorphism and iii the concept of magmatic advection of heat as a possible cause of regional metamorphism this article expands upon these themes by reviewing our understanding of the dynamic evolution of orogenic belts as interpreted from the p t t paths of metamorphic rocks and by considering the likely causes of the different kinds of regional metamorphism that we observe within orogenic belts understanding metamorphic rocks allows the distinction of two fundamentally different types of orogenic belt defined by relative timing of maximum t and maximum p orogenic belts characterized by clockwise p t paths achieved maximum p before maximum t the metamorphic peak normally post dated early deformation within the belt and additional heating above the normal conductive flux has been related to the amount of overthickening by contrast orogenic belts characterized by counterclockwise p t paths achieved maximum t before maximum p the metamorphic peak normally pre dated or was synchronous with early deformation within the belt and additional heating above the normal conductive flux has been related to the emplacement of plutons techniques used to constrain portions of p t t paths include the use of mineral inclusion suites in porphyroblasts and reaction textures thermobarometry the use of fluid inclusions thermodynamic approaches such as the gibbs method radiogenic isotope dating fission track studies and numerical modelling we can utilize specific mineral parageneses in suitable rocks to determine individual p t t paths and a set of p t t paths from one orogenic belt allows us to interpret the spatial variation in dynamic evolution of the metamorphism recent advances are reviewed with reference to collision metamorphism high temperature low pressure metamorphism granulite metamorphism and subduction zone metamorphism and some important directions for future work are indicated

polyphase deformation of the fuping complex trans north china orogen structures shrimp u pb zircon ages and tectonic implications abstract the fuping complex is located in the central part of the trans north china orogen tnco a paleoproterozoic collisional belt along which the eastern and western blocks were assembled to form the north china craton three stages of deformation d 1 d 3 have been identified in the fuping complex d 1 formed tight to isoclinal folds f 1 penetrative foliation s 1 and mineral lineation l 1 which resulted from the early nw se orientated thrusting and shearing d 2 progressively overprinted d 1 and was dominated by mostly nw se shortening and top to the se e thrusting under a compressive environment d 1 and d 2 occurred in association with the peak metamorphism m 1 d 3 occurred during the exhumation after the crustal thickening producing regional scale nww see trending open folds f 3 and associated low angle detachment faults and was responsible for the isothermal decompression m 2 metamorphism of the complex following the d 3 deformation the complex was exhumed to shallow crustal levels where the rocks underwent the retrogressive cooling m 3 metamorphism syn tectonic leucocratic dykes are widespread in the fuping complex two leucocratic dykes that did not experience d 1 but underwent d 2 yield shrimp zircon ages of 1843 12 ma and 1844 18 ma which indicate that d 1 must have occurred at some time earlier than 1844 ma whereas d 2 must have occurred at some time later than this age two weakly deformed post d 2 leucocratic dykes yield shrimp zircon ages of 1817 14 ma and 1815 45 ma suggesting that the d 2 deformation occurred in the period 1843 1815 ma the new shrimp data indicate that the collision related deformation of the fuping complex must have occurred in the paleoproterozoic not in the late archean structural and geochronological data presented in this study in association with previous lithological and geochemical data suggest that the fuping complex may have been located in the hinterland of an eastward subduction zone and underwent intensive deformation and metamorphism involved in the final collision between the western and eastern blocks at 1 85 ga

garnet zoning and reaction textures in overprinted eclogites bohemian massif european variscides a record of their thermal history during exhumation eclogites from different parts of the variscan basement of the bohemian massif show a variation in temperatures recorded at peak pressures as well as during subsequent exhumation these differences are reflected in the extent of modification of pre syn and post eclogite facies garnet compositions and by the types of reactions taking place at garnet rims munchberg massif eclogites formed at conditions of 600 650 c 20 kbar were mainly overprinted in the amphibolite facies field and contain garnets with an old homogeneous core and often well preserved idiomorphic growth zoning to mg rich rims in the marianske lazn complex the eclogite facies stage 640 715 c 15 kbar many eclogites show well developed granulite facies overprints with widespread orthopyroxene growth in garnet and omphacite domains spinel and or sapphirine after kyanite and in some cases even a low pressure high temperature overprint that produced fe rich olivine the latter rocks clearly endured higher temperatures during breakdown but remarkably strong growth zoning and sharp overgrowth features are preserved in some cases diffusion modeling shows that the preservation of zoning in garnet of rocks that have experienced granulite facies temperatures is only possible if the time spent at these conditions is short 1 ma this time scale is inconsistent with a smooth p t t path as expected from thermal modeling for erosion or extension driven exhumation and implies that these variscan rocks experienced short lived thermal events during their exhumation history

ttg and potassic granitoids in the eastern north china craton making neoarchean upper continental crust during micro continental collision and post collisional extension as the major component archean granitoids provide us with an insight into the formation of the early continental crust we report the study of a series of neoarchean granitoids including tonalite trondhjemite granodiorite ttg and potassic granitoids in the xingcheng region of the eastern north china craton zircon u pb dating shows that the ttg granitoids were emplaced in the neoarchean within a 75 myr period 2595 2520 ma with coeval mafic magmatic enclaves followed by intrusion of potassic granitoids the geochemistry of the ttg granitoids is consistent with partial melting of mesoarchean enriched mafic crustal sources at different depths up to 10 12 kbar equivalent pressure during a continental collision event the potassic granitoids are derived from either low degree melting of mesoarchean enriched mafic crustal sources or remelting of mesoarchean ttgs in response to post collisional extension and were hybridized with neoarchean mantle derived mafic melts to various degrees the ttg and potassic granitoids in the xingcheng region record the evolution from collision of micro continental blocks to post collisional extension consistent with other studies suggesting that the amalgamation of micro continental blocks is what gave rise to the cratonization of the north china craton at the end of the archean the rock assemblage of these granitoids resembles those of syn and post collisional magmatism in phanerozoic orogenic belts and the estimated average composition is similar to that of the present day upper continental crust suggesting that a prototype upper continental crust might have been developed at the end of the archean from a mixture of ttg and potassic granitoids together with concurrent high grade metamorphism in the north china craton we conclude that collisional orogenesis is responsible for continental cratonization at the end of the archean in the north china craton