dehydration melting of amphibolite at 10 kbar the effects of temperature and time we have simulated the dehydration melting of a natural low k calcic amphibolite 67 4 hornblende 32 5 anorthite in piston cylinder experiments at 10 kbar and 750 1000 c for 1 9 days the solidus temperature is lower than 750 c garnet appears at 850 c the overall reaction is hb p l cpx al hb ca hb ga opx three stages of reaction are 1 melting dominated by the growth of clinopyroxene and garnet with little change in composition of liquid or garnet 2 a reversal of this reaction between 875 c and 900 c with decreases in the amounts of liquid and garnet and 3 a large increase in liquid along with the loss of hornblende and decrease of plagioclase while clinopyroxene and garnet increase garnet is enriched in pyrope and zoned from fe cores to mg edges range 3 mol pyrope liquid composition is enriched first in an to 950 c and then in ab the liquids are more calcic and aluminous than natural tonalites which is attributed to the plagioclase composition an90 the formation of peraluminous liquid from the metaluminous amphibolite is caused by anorthite not h2o saturated conditions the results are consistent with an amphibolite phase diagram with relatively high solidus temperatures in the garnet absent field 900 1000 c but with a solidus backbend at 7 9 kbar coincident with the garnet in boundary hornblende breakdown due to garnet formation in a closed system must make h2o available for h2o undersaturated melting right down to the h2o saturated solidus below 700 c which defines a large low temperature pt area where hydrous granitoid melts can be generated with residual garnet and hornblende

earliest paleoproterozoic supracrustal rocks in the north china craton recognized from the daqingshan area of the khondalite belt constraints on craton evolution abstract the upper wulashan subgroup at daqingshan in the khondalite belt of the western block of the north china craton contains an early paleoproterozoic 2 5 2 45 ga supracrustal sequence incorporating banded iron formations which we refer to as the daqingshan supracrustal rocks they contain rounded to elliptical zircon grains that commonly show core rim or core mantle rim structures and precise shrimp u pb dating of these domains allows the discrimination of a series of tectono thermal events that straddle the archean proterozoic boundary detrital zircon cores with oscillatory zoning have formation ages ranging from 2 55 to 2 50 ga indicating derivation from late neoarchean magmatic rocks as a result of exhumation and erosion these became incorporated in earliest paleoproterozoic sediments that were deposited between 2 50 and 2 45 ga and then underwent high grade metamorphism at 2 45 2 40 ga this event variously recrystallized the cores into two main domains a dark inner domain and a gray outer domain as imaged in cathodoluminescence the gray domains commonly show sector zoning and have th u ratios of 0 1 0 5 a feature commonly noted in zircons recrystallized under high grade metamorphic conditions metamorphic mantles are generally more homogeneous in structure and have lower th u ratios commonly

multiple mafic magmatic and high grade metamorphic events revealed by zircons from meta mafic rocks in the daqingshan wulashan complex of the khondalite belt north china craton abstract meta mafic rocks in the daqingshan wulashan complex in the central segment of the paleoproterozoic khondalite belt of the north china craton are composed predominantly of garnet bearing mafic granulites mafic granulites and amphibolites they occur mainly as irregular lenses and as a set of deformed dyke sill swarms within dioritic granitic orthogneisses and al rich paragneisses an integrated study involving laser raman and scanning electron microscope analyses of mineral inclusions from zircons cathodoluminescence imaging and la icp ms u pb dating of zircons shows that magmatic zircons in these meta mafic rocks record four age groups of 2500 2450 2300 2100 1970 1930 and ca 1850 ma indicating multiple mafic magmatic events occurred within the daqingshan wulashan complex in addition four groups of metamorphic zircons with ages of ca 2450 ca 1950 ca 1900 and ca 1850 ma are present within these meta mafic rocks zircons of the first oldest group contain medium to low pressure mlp granulite facies mineral inclusions of orthopyroxene clinopyoxene amphibole plagioclase quartz and yield ages of ca 2450 ma representing the timing of the early mlp granulite facies metamorphic event within the daqingshan wulashan reworked basement complex in contrast zircons of the second group are free of mineral inclusions and yield ages of ca 1950 ma which is considered to be the timing of the regional high grade metamorphic and structural event probably caused by the collision of the yinshan and ordos blocks zircons of the third group contain mlp granulite facies mineral inclusions of orthopyroxene clinopyoxene amphibole plagioclase quartz similar to the zircons of the first group but yield younger ages of ca 1900 ma interpreted as the timing of the regional mlp granulite facies metamorphism that most likely occurred during the early stage of post orogenic exhumation of the daqingshan wulashan complex zircons of the fourth group contain mlp amphibolite facies mineral inclusions of clinopyoxene amphibole plagioclase quartz and yield the youngest ages of ca 1850 ma which probably represent the timing of the regional amphibolite facies metamorphism during the late stage of post orogenic exhumation of the daqingshan wulashan complex combined with previous geological data of the khondalite belt it is speculated that the multiple mafic magmatism and contemporaneous high grade metamorphism of the daqingshan wulashan complex from the khondalite belt were most likely generated by an orogenic process involving early subduction collision and late exhumation in an active continental margin arc

the 1 75 1 68ga anorthosite mangerite alkali granitoid rapakivi granite suite from the northern north china craton magmatism related to a paleoproterozoic orogen abstract the amgrs is a suite of anorthosite mangerite alkali granitoid rapakivi granite in the northern north china craton ncc new zircon shrimp and laser ablation inductively coupled plasma mass spectrometry la icpms u pb dating of the damiao anorthosite changsaoying k feldspar granite lanying anorthosite and quartz syenite and the gubeikou k feldspar granite yielded emplacement ages of 1726 9 ma 1753 23 ma 1739 43 ma 1712 15 ma and 1692 19 ma respectively combined with previous geochronological data from the damiao norite and mangerite and shachang rapakivi granite it indicates that the whole amgrs was intruded between 1750 ma and 1680 ma the damiao anorthosites and norites exhibit a high mg and this decreases from mafic up to 63 to felsic t dm c ages ranging from 2 32 ga to 2 43 ga and 2 70 ga to 2 88 ga respectively this suggests that these rocks are the fractional crystallization products of one parental magma zircon hf and whole rock sr nd pb isotopic systematics combined with high la yb 10 and intermediate th ta ratios suggest that the parental magma of the amgrs could have been derived from em i lithosphere mantle enriched by recycled archean continent crustal materials accompanied by some assimilation of lower crustal components the mafic to felsic compositions of the amgrs are different from typical bimodal rocks formed in an anorogenic intra continent rift setting but resemble those emplaced during post collisional post orogenic extensional tectonic setting this is compatible with their age of

an early paleoproterozoic high k intrusive complex in southwestern tarim block nw china age geochemistry and tectonic implications abstract systematic geochronologic geochemical and nd isotopic analyses were carried out for an early paleoproterozoic high k intrusive complex exposed in southwestern tarim nw china the results provide a better understanding of the paleoproterozoic tectonic evolution of the tarim block zircon u pb age dating indicates two paleoproterozoic magmatic episodes occurring at ca 2 41 ga and ca 2 34 ga respectively which were followed by a ca 1 9 ga metamorphic event the 2 41 ga granodiorite adamellite suite shares characteristics of late to post orogenic metaluminous a type granites in its high alkalinity na2o k2o 7 6 9 3 total ree 410 788 ppm zr 370 660 ppm and y 21 7 58 4 ppm contents end t values for the suite range from 3 22 to 4 71 and accordingly the nd modal ages t2dm vary between 3 05 ga and 3 17 ga based on geochemical data the 2 34 ga suite can be subdivided into two sub suites namely a type and s type however both types have comparable nd isotope compositions end t 0 41 to 2 08 and similar narrow t2dm ranges 2 76 2 91 ga geochemical and nd isotopic data for the high k intrusive complex in conjunction with the regional geological setting suggest that both the 2 41 ga suite and the 2 34 ga a type sub suite might have been produced by partial melting of the archean mafic crust in a continental rift environment the s type sub suite is thought to have formed by partial melting of felsic pelites and or metagreywackes recycled from archean crust ttg gabbro enclaves with positive end t value 2 15 have been found to be intermingling within the 2 34 ga suite ca 2 34 2 36 ga gabbroic dykes and adamellites have previously been documented in eastern tarim these observations indicate that the high k intrusions may reflect the emergence of depleted mantle upwelling beneath the tarim block at that time we suggest a three stages model for the precambrian crustal evolution in the tarim block 1 the formation of proto crust ttg by ca 2 5 ga 2 episodes of felsic magmatism possibly occurring in continental rift environments at ca 2 41 ga and ca 2 34 2 36 ga and 3 ca 1 9 ga metamorphism that may represent the solidification of the precambrian basement of the tarim block

anatomy of zircons from an ultrahot orogen the amalgamation of the north china craton within the supercontinent columbia abstract we report shrimp u pb rare earth element hf isotope and laser raman spectroscopic data on zircons from ultrahigh temperature uht granulites from the jining complex of the khondalite belt in the western block of the north china craton ncc these uht rocks form part of an ultrahot orogen that formed along the collisional margin of the ncc associated with the tectonics of assembly of the paleoproterozoic supercontinent columbia despite the core rim textures displayed by some of the uht zircons their age values sharply converge within error yielding weighted mean 207pb 206pb ages of ca 1 92 ga indicating growth by recrystallization under extreme thermal conditions in general the zircons show moderate heavy rare earth element enrichment with sharp positive ce and negative eu anomalies the hf isotope data from the uht zircons also display a fairly uniform character with the majority of them characterized by positive ehf values without any indication of mixing between reworked crust and

shrimp u pb zircon ages of granitoid rocks in the luliang complex implications for the accretion and evolution of the trans north china orogen abstract the luliang complex is situated in the central segment of the trans north china orogen tnco a continent continent collisional belt along which the discrete eastern and western blocks amalgamated to form the basement of the north china craton the complex consists of supracrustal and granitoid rocks of which the latter can be subdivided into pre tectonic ttg gneisses including the yunzhongshan guojiazhuang and chijianling guandishan gneisses syn tectonic gneissic granites represented by the huijiazhuang gneissic granite and post tectonic granites including the luyashan charnockite luchaogou porphyritic granite and tangershang guandishan massive granite the pre tectonic ttg gneisses are mostly calc alkaline and considered to have formed in a magmatic arc environment shrimp u pb analyses reveal that the yunzhongshan gneisses were emplaced at 2499 9 ma representing the earliest arc related magmatic event in the luliang complex this was followed by the intrusion of the guojiazhuang gneisses at 2375 10 ma the most widespread arc related magmatic event in the region was the emplacement of the chijianling guandishan granitoid gneisses of which the tonalitic granodioritic and monzogranitic gneisses were emplaced at 2199 11 ma 2180 7 ma and 2173 7 ma respectively metamorphic zircon overgrowth rims from a tonalitic gneiss yielded a weighted mean 207 pb 206 pb age of 1872 7 ma consistent with the metamorphic age range of 1880 1820 ma defined by metamorphic zircons from various high grade rocks in the tnco thus the evolution of the luliang complex involved emplacement of the yunzhongshan granitoids at 2499 ma the guajiazhuang granitoids at 2375 ma and the chijianling guandishan granitoids at 2199 2173 ma with the final collision between the eastern and western blocks in this area occurring at 1872 7 ma the whole series of magmatic events lasting nearly 650 ma this suggests that the trans north china orogen represents a long lived magmatic arc the new shrimp zircon data combined with structural and petrological studies also reveal the existence of both syn and post collisional granites in the luliang complex the former is represented by the huijiazhuang gneissic granite that was emplaced at 1832 11 ma whereas the latter include the 1815 5 ma luyashan charnockite 1807 10 ma luchaogou porphyritic granite 1798 11 ma guandishan massive granite and 1790 14 ma tangershan massive granite

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

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

large scale vertical eddy diffusion in the main pycnocline of the central north pacific abstract indirect procedures are used to estimate the latitudinal distribution of the large scale vertical eddy diffusivity coefficient in the main pycnocline from the interannual change in t structure of the water column in the central midlatitude north pacific from 35 44 n 150 170 w using t s data from two identical hydrographic surveys one made in june 1976 and the other in may 1977 the interannual change in potential temperature is recorded on three potential density surfaces i e 26 6 27 0 and 27 2 that lie in the upper middle and lower part of the main pycnocline respectively by relating these interannual changes to the vertical gradients of the average potential temperature on each surface i e through the heat conservation equation the large scale vertical eddy diffusivity coefficient and its vertical gradient were determined by least squares estimate procedures the vertical eddy diffusivity coefficient k is found to have been positive over the entire region with magnit

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

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

high sr y and la yb ratios the meaning of the adakitic signature abstract the name of adakite is used to describe a far too large group of rocks whose sole common feature is high sr y and la yb ratios however such a signature can be achieved via different processes melting of a high sr y and la yb source deep melting with abundant residual garnet fractional crystallization or afc or interactions of felsic melts with the mantle causing selective enrichment in lree and sr over hree a database of the compositions of adakitic rocks including high silica and low silica adakites martin h smithies r h rapp r p moyen j f champion d c 2005 an overview of adakite tonalite trondhjemite granodiorite ttg and sanukitoid relationships and some implications for crustal evolution lithos 79 1 2 1 24 continental adakites and archaean adakites was assembled geochemical modeling of the potential processes is used to interpret it and reveals that 1 the classical model of slab melting provides the best explanation for the genesis of high silica adakites 2 low silica adakites are explained by garnet present melting of an adakite metasomatized mantle i e at depths greater than 2 5 gpa 3 continental adakites is a term encompassing a huge range of rocks with a corresponding diversity of petrogenetic processes and most of them are different from both low and high silica adakites 4 archaean adakites show a bimodal composition range with some very high sr y examples similar to part of the ttg suite reflecting deep melting 2 0 gpa of a basaltic source with a relatively high sr y while lower sr y rocks formed by shallower 1 0 gpa melting of similar sources comparison with the archaean ttg suite highlights the heterogeneity of the ttgs whose composition spreads the whole combined range of hsa and archaean adakites pointing to a diversity of sources and processes contributing to the ttg suite

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

la icp ms u pb geochronology of detrital zircons from the jining complex north china craton and its tectonic significance abstract la icp ms u pb zircon dating and cathodoluminescene cl image analysis were carried out to determine the protolith and metamorphic ages of high grade al rich gneisses named as khondalites from the jining complex of the north china craton ncc the analytical results of more than 200 detrital zircon grains from the khondalites show three main age populations 2060 ma 1940 ma and 1890 ma these data indicate that the provenance of the jining khondalites is paleoproterozoic in age but not archean as previously suggested and the sediments were derived from a provenance different from the eastern block and the yinshan terrane of the ncc the nearly concordant youngest age of 1842 16 ma 207pb 206pb age for the detrital zircons is interpreted as the maximum depositional age of the khondalites overgrowth rims of detrital zircons yield an age of 1811 23 ma which we interpret as the metamorphic age the new age data are consistent with the recent three fold tectonic subdivision of the ncc and support that the eastern and western blocks collided at 1 8 ga to form the coherent ncc

a hybrid origin for the qianshan a type granite northeast china geochemical and sr nd hf isotopic evidence abstract major and trace element whole rock sr and nd isotope and zircon hf isotope data are reported for a suite of a type granites and mafic microgranular enclaves from the early cretaceous 126 2 ma qianshan pluton liaodong peninsula northeast china with the aim of investigating the sources and petrogenesis of a type granites the qianshan pluton includes hornblende alkali feldspar granite graphic biotite granite and mafic microgranular enclaves the hornblende alkali feldspar granites have high sio 2 fe 2 o 3t mgo k 2 o na 2 o rb zr and lree contents and low ba and sr concentrations with strongly negative eu anomalies their high rb sr 87 rb 86 sr 16 76 24 15 and initial 87 sr 86 sr ratios 0 7215 to 0 7283 negative e nd t values 14 1 to 16 5 and zircon e hf t values 18 9 to 11 5 indicate they were mainly derived from a crustal source but with involvement of high e nd t and e hf t materials graphic biotite granites have similar geochemical features and sr nd hf isotopic compositions to enclaves indicating they were the result of crystal fractionation of evolved mafic magmas but with involvement of low e nd t and e hf t materials the mafic enclaves have an igneous texture and contain acicular apatite suggesting quenching of mafic magmas that have co mingled with the host granites they have low initial 87 sr 86 sr ratios 0 7097 0 7148 negative e nd t 14 5 to 11 9 and zircon e hf t 17 1 to 6 9 values and are enriched in liles and lrees and depleted in hfses when coupled with the high mgo mg up to 54 this indicates derivation from an enriched lithospheric mantle source but contaminated by crustal materials geochemical and sr nd and zircon hf isotopic compositions rule out simple crystal liquid fractionation or restite unmixing as the major genetic link between enclaves and host rocks instead magma mixing of mantle derived mafic and crustal derived magmas coupled with crystal fractionation is compatible with the data this example shows that at least some a type granites formed through a complex process involving mantle and crustal derived magma mixing crystal fractionation and infracrustal melting

generation of sodium rich magmas from newly underplated basaltic crust sodium rich rocks of trondhjemite tonalite dacite ttd or granodiorite ttg suites form much of precambrian continental crust1 they are thought to have formed by partial melting of subducted oceanic crust2 3 a process that would have been much more widespread early in earth history than at present owing to the higher thermal gradients prevailing at that time4 phanerozoic ttd suites do exist however and seem also to relate to subduction zones5 defant and drummond6 proposed that these suites form where young 25 myr hot oceanic lithosphere is subducted and melts thus locally simulating the conditions that led to widespread crustal growth in the archaean here we describe plutonic and volcanic rocks from the cordillera blanca complex in peru which have characteristics of the high al ttd suite but which were produced above a subduction zone containing a 60 myr old slab we present evidence that the complex formed by partial melting of newly underplated basaltic crust and argue that this mechanism should be considered more generally as an additional way of generating sodium rich arc magmas

late paleoproterozoic crustal evolution in the daqingshan area evidences from adakitic and a type granitoids in the guyang changshengqu goldfield khondalite belt north china craton abstract granitic magmatism related to the collisional orogenic belt provides important information for the evolution of continental crust we performed petrological geochemical and zircon u pb hf isotopic studies on four granitoid samples from the guyang changshengqu goldfield daqingshan area khondalite belt north china craton the zircon u pb dating results indicate that the granitoids emplaced at 1950 11 ma 1935 12 ma 1846 11 ma and 1812 8 6 ma represent two periodic granitic magmatism in the daqingshan area the 1 95 and 1 93 ga granites exhibit high sio2 k2o and na2o contents with high sr y and la yb n ratios but low mgo cr and ni contents indicating that they were produced by the partial melting of a thickened lower crust under relatively high pressure conditions the granites show negative dominated zircon ehf t values 5 7 1 9 and their zircon tdm2 hf values range between 2 9 and 2 5 ga indicating an ancient crust source with minor juvenile crustal material involved the 1 85 and 1 81 ga quartz monzonites exhibit high sio2 k2o na2o zr nb ce and y but low mgo cr and ni contents with high ga al and feot feot mgo ratios indicating an a type granite affinity the whole rock geochemistry suggests that they were formed by dehydration re melting of ttg and quartz diorite at high temperatures and shallow depths the quartz monzonites show homogeneous ehf t values 10 6 6 1 and the zircon tdm2 hf values range between 3 0 and 2 8 ga suggesting that they were derived from a neoarchean ancient crust the 1 95 1 93 ga adakitic magmatism is associated with the 1 93 1 92 ga mafic magmatism and uht metamorphic in the eastern part of the khondalite belt this suggests a tectonic transition from collisional compression to post collisional extension the 1 85 1 81ga a type granitic magmatism indicates evident continental crust thinning and asthenospheric upwelling the two unrelated magmatism are possibly induced by two collisional events from the khondalite belt and the trans north china orogen respectively the collision of the western and eastern blocks at ca 1 85 ga may have played an important role in the a type quartz monzonites origin and influenced the crustal evolution as far as the daqingshan area in the khondalite belt

archean paleoproterozoic crustal evolution of the ordos block in the north china craton constraints from zircon u pb geochronology and hf isotopes for gneissic granitoids of the basement abstract the ordos block has been considered as an archean micro block in north china craton however its formation and evolution are poorly understood for a long time due to lack of available basement rocks our la icpms and sims zircon u pb dating of gneissic granitoids from the ordos block basement identified two distinct periods of granitic magmatism at 2 5 ma and 2 2 2 0 ga and two phases of metamorphisms at 1 95 ga and 1 85 ga respectively of which most zircons of 2 5 ga granitoids show positive hf t values ranging from 0 1 to 4 1 with a peak t dm of 2 7 ga minor zircons have negative hf t values from 3 8 to 0 02 and t hf c of 2 8 3 1 ga moreover the 3 40 ga inherited zircon and zircons with t hf c of 3 1 4 2 ga were found from 2 2 ga to 2 0 ga granitoids these suggest that there existed a neoarchean crust in the ordos block which underwent a major crustal growth at 2 7 ga and granitic magmatism at 2 5 ga highly consisting with 2 7 ga crustal growth and 2 5 ga magmatism resulted from juvenile crustal reworking in north china craton the zircons from 2 2 ga to 2 0 ga granitoids in ordos block basement display variable hf isotopic composition of positive to negative hf t values much similar to those of 2 2 2 0 ga granitoids related to continental margin arc setting in the trans north china orogen they were subsequently subjected to 1 95 and 1 85 ga metamorphisms which were widespread throughout the trans north china orogen based on the highly coherent zircon geochronology and hf isotopic composition between ordos block and other micro blocks of north china craton we propose that there is a neoarchean crust in ordos block which underwent a tectonic evolution similar to those in east block in later neoarchean and then experienced 2 2 2 0 ga granitic magmatism related to continental margin arc and 1 95 1 85 ga metamorphism resulted from collision between the east and west blocks along tnco

first application of the revised ti in zircon geothermometer to paleoproterozoic ultrahigh temperature granulites of tuguiwula inner mongolia north china craton the revised titanium in zircon geothermometer was applied to paleoproterozoic ultrahigh temperature uht granulites at tuguiwula inner mongolia north china craton the tuguiwula granulites contain diagnostic uht mineral assemblages such as sapphirine quartz and high alumina orthopyroxene sillimanite quartz suggesting formation under temperatures of ca 1 000 c and pressures of up to 10 kbar here we report detailed petrographic studies and icp ms data on titanium concentration in zircons associated with the uht assemblages the zircons associated with sapphirine spinel sillimanite magnetite assemblages have the highest ti concentration of up to 57 ppm yielding a temperature of 941 c and suggesting that the growth of zircons occurred under ultrahigh temperature conditions the maximum temperatures obtained by the revised ti in zircon geothermometer is lower than the equilibrium temperature of sapphirine quartz indicating an interval of cooling history of the granulites from uht condition to ca 940 c many of the zircons have ti concentrations ranging from 10 to 33 ppm indicating their growth or recrystallization under lower temperatures of ca 745 870 c these zircons are interpreted to have recrystallized during the retrograde stage indicated by microstructures such as cordierite rim or corona between spinel and quartz and orthopyroxene cordierite symplectite around garnet previous geochronological study on the zircons of the tuguiwula uht granulites gave a mean u pb shrimp age of 1 92 ga however based on the ti in zircon geothermometer results reported in this work and considering the relatively slow thermal relaxation of these rocks we infer that the timing of peak uht metamorphism in the tuguiwula area could be slightly older than 1 92 ga

nd isotope evidence for crust mantle interaction in the generation of a type granitoid suites in labrador canada early proterozoic a type granite suites in labrador northeastern canada have initial nd isotopic compositions that lie between those of a depleted mantle reservoir and the local basement rocks one of these a type granite suites consists of a linear array of discrete plutons that crosses the boundary between an archean craton and an adjoining early proterozoic crustal block the nd of these granites at formation t 1720 ma changes from 8 to 4 across this boundary reflecting changes in the nature and antiquity of the continental crust however in both domains the granites cannot be derived entirely by anatexis of the local basement rocks and apparently require significant amounts of juvenile mantle derived material a type granites have been widely interpreted as anatectic derivatives of lower crustal rocks that were depleted by previous melt extraction the nd isotopic data suggest that these labrador examples were instead mixtures of mantle derived magmas and overlying lower crustal rocks and that new material may have been the volumetrically dominant component 50 90 in most cases as an alternative to the prevalent residual source model it is proposed that the distinctive geochemical characteristics of these a type granites are a function of the high liquidus temperatures and primitive mg rich character of subcrustal magma fluxes in post orogenic and anorogenic environments where such magmas are emplaced into stabilized continental crust further nd isotope studies of other a type granite suites are required to ascertain if models invoking crust mantle interaction are more widely applicable

sm nd and shrimp u pb zircon geochronology of high pressure granulites in the sanggan area north china craton timing of paleoproterozoic continental collision abstract high pressure hp mafic granulites are widely distributed as sheet like and lenticular bodies within highly deformed tonalitic gneisses in the sanggan area north china craton they have clockwise isothermal decompression and isobaric cooling p t paths implying continental collision tectonics whole rock analyses of 15 samples define a sm nd isochron yielding an age of 2547 87 ma with e nd t 2 7 corresponding nd depleted mantle model ages show a similar range and are interpreted as the age of the magmatic protolith to the high pressure granulites a combined sm nd mineral whole rock isochron from one of these samples yields an age of 1813 23 ma taken to represent the metamorphic age at an early stage on the cooling path shrimp u pb zircon analyses of three hp mafic granulite samples give uniform weighted mean 207 pb 206 pb ages of 1817 12 1817 12 and 1819 16 ma these ages are interpreted to date metamorphism of the hp granulites and with the sm nd mineral whole rock result strongly suggest that the time of continental collision was at ca 1 82 ga an additional sample records two age populations one at 1803 9 ma and an older one at 1872 16 ma the latter may indicate the timing of either an earlier metamorphic event or possibly the earliest stage of a single prolonged metamorphic episode that peaked at 1 82 ga these results confirm the paleoproterozoic age for assembly of the north china craton

petrography and zircon u pb isotopic study of the bayanwulashan complex constrains on the paleoproterozoic evolution of the alxa block westernmost north china craton abstract the bayanwulashan metamorphic complex bmc exposes along the eastern margin of the alxa block the westernmost part of the north china craton ncc bmc is principally composed of metamorphic rocks with amphibole plagiogneiss biotite plagioclase gneiss and granitic gneiss our research has been focused on the petrography and zircon u pb geochronology of the bmc to better understand the evolution of the alxa block and its relationship with the ncc evidences from field geology petrography and mineral chemistry indicate that two distinct metamorphic assemblages the amphibolite and greenschist facies had overprinted the preexisting granitic gneiss and suggest that the bmc experienced retrograde metamorphic episodes the la icp ms zircon u pb ages reveal that the primary magmatic activities of bmc were at ca 2 30 2 24 ga and the two metamorphic events were at ca 1 95 1 91 ga and ca 1 88 1 85 ga respectively these ages indicate that bmc initially intruded during paleoproterozoic not as previously suggested at archean period the early paleoproterozoic metamorphic records and the magmatic thermochronological data in bmc exhibit different evolution paths between the alxa block and the ncc the alxa block was most likely an independent early paleoproterozoic terrain following different amalgamation processes the alxa block combined with western block at ca 1 95 ga and then united with ncc at ca 1 85 ga

shrimp u pb zircon geochronology of palaeoproterozoic metasedimentary rocks in the north china craton evidence for a major late palaeoproterozoic tectonothermal event abstract although metasedimentary successions are widely distributed across the north china craton those of palaeoproterozoic age are restricted to three orogenic belts namely the trans north china orogen the khondalite belt and the jiao liao ji belt prior to this work some of those successions were considered to be archaean in age the palaeoproterozoic metasedimentary rocks contain detrital igneous zircon grains with ages ranging from 3 0 to 2 1 ga with the majority recording an age between 2 3 and 2 0 ga this latter group define a series of magmatic events that occurred between formation of the archaean basement and deposition of the metasedimentary rock protoliths the metasedimentary rocks occur as two associations older volcano sedimentary units that formed between 2 37 and 2 0 ma and younger sedimentary units that formed mainly between 2 0 and 1 88 ga all these rock associations were then subjected to metamorphism during the late palaeoproterozoic 1 88 and 1 85 ga tectonothermal event named the luliang movement in early chinese literature which was a continent continent collision that led to the amalgamation of the north china craton

tsunami early warning using earthquake rupture duration and p wave dominant period the importance of length and depth of faulting summary after an earthquake rapid real time assessment of hazards such as ground shaking and tsunami potential is important for early warning and emergency response tsunami potential depends on seafloor displacement which is related to the length l width w mean slip d and depth z of earthquake rupture currently the primary discriminant for tsunami potential is the centroid moment tensor magnitude mcmtw representing the seismic potency lwd and estimated through an indirect inversion procedure the obtained mcmtw and the implied lwd value vary with the depth of faulting assumed earth model and other factors and is only available 30 min or more after an earthquake the use of more direct procedures for hazard assessment when available could avoid these problems and aid in effective early warning here we present a direct procedure for rapid assessment of earthquake tsunami potential using two simple measures on p wave seismograms the dominant period on the velocity records td and the likelihood that the high frequency apparent rupture duration t0 exceeds 50 55 s t0 can be related to the critical parameters l and z while td may be related to w d or z for a set of recent large earthquakes we show that the period duration product tdt0 gives more information on tsunami impact and size than mcmtw and other currently used discriminants all discriminants have difficulty in assessing the tsunami potential for oceanic strike slip and backarc or upper plate intraplate earthquake types our analysis and results suggest that tsunami potential is not directly related to the potency lwd from the seismic faulting model as is assumed with the use of the mcmtw discriminant instead knowledge of rupture length l and depth z alone can constrain well the tsunami potential of an earthquake with explicit determination of fault width w and slip d being of secondary importance with available real time seismogram data rapid calculation of the direct period duration discriminant can be completed within 6 10 min after an earthquake occurs and thus can aid in effective and reliable tsunami early warning

partial melting of subducting oceanic crust the conditions under which partial melting of subducting oceanic crust occurs can be determined by combining a partial melting model for basaltic compositions with two dimensional thermal models of subduction zones for porosities of 1 containing h20 the amount of partial melt generated at the wet basaltic solidus is limited to 100 mpa can be maintained by rocks close to or above their melting temperatures in the absence of high shear stresses substantial melting of the oceanic crust will only occur during subduction of very young 5 ma oceanic lithosphere partial melting of hydrated basalt amphibolites derived from the mid ocean ridge has been proposed e g 1 3 as being responsible for the generation of certain recent high al andesitic to dacitic volcanic rocks adakites three of these volcanic suites mount st helens southern chile and panama occur in volcanic arcs where oceanic crust 25 ma is being subducted at rates of 1 3 cm yr and the calculated thermal regime is several hundreds of degrees hotter than more typical subduction zone environments however oceanic lithosphere is not currently being subducted beneath baja and new guinea where recent adakites are also present suggesting that some adakite magmas may form by water undersaturated partial melting of underplated mafic lower crust or previously subducted oceanic crust further experimental work on compositions representative of oceanic crust is required to define the depth of possible adakite source regions more accurately

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

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

chronology and geochemistry of neoarchean bif type iron deposits in the yinshan block north china craton implications for oceanic ridge subduction abstract the dongwufenzi iron deposit is a typical algoma type banded iron formation bif in the yinshan block of the north china craton the iron orebodies occur as layers interlayered with plagioclase amphibolite la icp ms u pb dating of zircons from the plagioclase amphibolite yielded protolith crystallization age of 2538 9 ma mswd 1 01 followed by metamorphism at 2452 7 ma mswd 0 68 the geochemical characteristics of the ores including low al 2 o 3 tio 2 and hfse contents pronounced positive eu anomalies and the y ho ratios 26 37 suggest that the bifs precipitated from hydrothermal fluids discharged on the seafloor the geochemical features of the plagioclase amphibolite suggest the protolith magma to be high fe basalt sm nd analyses of the plagioclase amphibolite yielded end t t 2 54 ga and single stage depleted mantle nd model ages t dm in the range of ca 1 8 to 3 0 and 2 68 to 2 81 ga respectively suggesting an enriched mantle source we link the source of fe in the bifs to the komatiitic complex in the same greenstone belt and the source of si with the high fe basalt integrated geochronological and geochemical data from the other archean rocks in this area suggest that the dongwufenzi bif formed at 2 5 ga coeval with the other bifs in the north china craton ncc as well as similar occurrences in other parts of the world we propose a geodynamic model involving ridge subduction and slab window for the formation of the bifs and the associated rock suites in the greenstone belt

episodic paleoproterozoic 2 45 1 95 and 1 85 ga mafic magmatism and associated high temperature metamorphism in the daqingshan area north china craton shrimp zircon u pb dating and whole rock geochemistry abstract in the daqingshan area within the khondalite belt of the north china craton paleoproterozoic gabbro and dolerite intrusions show amphibolite to granulite facies metamorphism u pb zircon dating and whole rock geochemistry were undertaken on these rocks in order to understand the timing of mantle magmatism and thermal processes ten samples yielded zircon u pb formation ages of 2 46 2 44 ga 1 97 1 92 ga and 1 84 ga and metamorphic ages of 1 95 1 83 ga they are variable in geochemical composition with some being enriched in light rare earth and large ion lithosphere elements but depleted in nb ta combined with previous results it is concluded that mafic magmatism at 2 45 2 37 ga and 1 97 1 92 ga indicates two paleoproterozoic extensional phases with the latter accompanied by ht uht metamorphism as a result of underplating of mantle magma

u pb and hf isotopic study of detrital zircons from the wulashan khondalites constraints on the evolution of the ordos terrane western block of the north china craton abstract detrital zircons from the wulashan khondalites in the ordos terrane western block of the north china craton give u pb ages between 1 84 and 2 32 ga showing that their provenance was dominated by paleoproterozoic rocks much younger than those exposed in the eastern block these zircons have oscillatory growth zoning and possess e hf values between 8 and 9 suggesting derivation of their precursor magmas from old crust and juvenile materials from the mantle the lowest e hf values of zircons with different ages give a line that intersects the depleted mantle line at about 2 6 ga in a e hf vs time diagram this may imply that the precursor magma source in the deep level of the crust was largely separated from the mantle 2 6 ga ago and therefore 2 6 ga may be an important crustal formation period for the terrane one khondalite sample has detrital zircons with a single age population at 2 0 ga and positive e hf values from 1 to 9 clearly recording the significant addition of juvenile materials at 2 0 ga in the source region this may inspire us to seek whether there was a connection between a mantle superevent and the formation of columbia supercontinent data of this study exclusively demonstrate that the ordos terrane was developed independently from the eastern block until assembly of the columbia supercontinent therefore tectonic models involving that the whole north china craton was cratonized 2 5 ga ago should be dismissed

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

single zircon grains record two paleoproterozoic collisional events in the north china craton abstract the western block of the north china craton consists of the yinshan block in the north and the ordos block in the south which were amalgamated along the east west trending khondalite belt at 1 95 ga the western block then collided with the eastern block to form the coherent basement of the north china craton along the north south trending trans north china orogen at 1 85 ga the huaian complex a high grade terrrane located at the conjunction of the khondalite belt and trans north china orogen records metamorphic events associated with both collisions the complex consists of lithologies from both the khondalite belt and trans north china orogen of which the former consist of graphite garnet sillimanite gneiss garnet quartzite felsic paragneiss calc silicate rock and marble together called the khondalite series zircons in the graphite garnet sillimanite gneiss can be divided into three types 1 spherical grains without internal structures 2 grains with a core and rim structure and 3 grains with a dark core surrounded by double rims except for the dark core in type 3 all other types of zircon domains are structureless and highly luminescent with very low th u ratios typical of a metamorphic origin analyses on the cores of type 2 and the inner rims of type 3 from two samples yield upper intercept ages of 1946 26 and 1947 22 ma similar to previously determined metamorphic ages from the khondalite belt and thus interpreted as the time of collision between the yinshan and ordos blocks analyses on type 1 zircons rims of type 2 and the outer rims of type 3 from the same two samples give ages of 1850 15 and 1857 16 ma interpreted as the time of collision between the eastern and western blocks thus zircons in the graphite garnet sillimanite gneiss of the huaian complex record both of the paleoproterozoic collisional events in the north china craton

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

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

genesis of the hengling magmatic belt in the north china craton implications for paleoproterozoic tectonics abstract the 2200 1880 ma igneous rocks in the central and eastern parts of the north china craton ncc constitute a new hengling magmatic belt hmb which includes the 2147 ma hengling mafic sill dyke swarm the 2060 ma yixingzhai mafic dyke swarm and the 1973 ma xiwangshan mafic dyke swarm the three swarms are contiguous and have experienced variable degrees of metamorphism from greenschist to low amphibolite facies hengling medium granulite facies yixingzhai and medium high pressure granulite facies xiwangshan they are all tholeiitic in composition typically with 47 52 wt sio 2 and 4 10 wt mgo and all show light rare earth element enrichments and nb and ta depletion their nd t dm ages are in the range of 2 5 3 0 ga specifically the hengling and yixingzhai dykes sills are depleted in th u zr hf and ti whereas the xiwangshan dykes are enriched in u and weakly depleted in other elements variable sr anomalies indicate significant feldspar accumulation positive anomalies or fractionation the end t values of the three swarms are 3 2 3 0 hengling 1 7 1 8 yixingzhai and 1 4 1 0 xiwangshan these mafic representatives of the hmb originated from the 2 5 ga sub continental lithospheric mantle of the ncc and with a type granites and other igneous associations in this belt they likely evolved in an intra continental rift the progressive changing compositions of the three swarms are interpreted in terms of their source regions at different depths i e shallower and shallower through time and the decrease in scale and size of the intrusions and their magma volumes indicate the progressive weakening of magmatism in this rift the rocks in this belt are different chronologically petrologically and chemically from those in the xuwujia magmatic belt xmb we propose that the two magmatic belts represent two different magmatic systems in different blocks of the ncc i e an eastern block with the hmb and a western block with the xmb terminal collision was possibly a result of ridge subduction between the two blocks which led to exhumation of the igneous rocks in the two belts from different crustal levels distinguishable by their different grades of metamorphism

zircon u pb ages trace elements and nd hf isotopic geochemistry of guyang sanukitoids and related rocks implications for the archean crustal evolution of the yinshan block north china craton abstract the yinshan block is a unique coherent neoarchean basement terrane of the western block of the north china craton in this paper three archean plutons namely the dajitu banshentu and haolaigou plutons have been investigated based on the field observations the major difference among these plutons is that many ultramafic mafic enclaves are enclosed in the dajitu and banshentu plutons whereas none occurs in the haolaigou pluton geochemically the dajitu and banshentu plutons are characterized by low sio2 contents 56 67 wt high mgo 1 9 7 1 wt cr 10 947 ppm ni 2 274 ppm and mg 44 64 contents negative nb ta and ti anomalies high large ion lithophile elements liles with no or slightly negative eu anomalies similar to a typical archean sanukitoid new geochronological data indicate that the dajitu and banshentu plutons were emplaced at 2523 7 ma and 2480 20 ma with inherited ages of 2606 19 ma and 2647 19 ma respectively in contrast the haolaigou pluton shows high sio2 68 71 wt mgo mg 46 55 cr and ni contents negative nb ta and ti anomalies and high liles with strongly positive eu anomalies high sr and ba concentrations and sr y and la yb ratios and low y and hrees abundances it was emplaced at 2466 15 ma with an inherited age of 2638 21 ma the whole rock nd t and zircon hf t values of the haolaigou pluton are positive the whole rock tdm nd and zircon tdm1 hf values range between 2 6 and 2 8 ga with a peak at 2 7 ga indicating that the melt was generated by the partial melting of a subducted slab interacting with the overlying mantle wedge the dajitu pluton has nd and hf isotope characteristics identical to those of the haolaigou pluton implying that the two plutons have similar sources it should be noted however that the dajitu pluton displays sanukitoid characteristics which are the result of interaction with ultramafic mafic enclaves the whole rock nd t and zircon hf t values of the banshentu pluton are mostly negative the whole rock tdm nd and zircon tdm1 hf values of the pluton range between 2 7 and 2 9 ga with a peak at 2 8 ga indicating that the pluton was generated from the melting of a lower crust the sanukitoid characteristics of the pluton resulted from interactions with an ultramafic enclave taken together these results with the geochronological geochemical data for other archean rocks in this area suggest that the yinshan block experienced significant crustal growth at 2 8 2 6 ga and further crustal reworking that led to strong magmatism at 2 5 ga

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

extreme crustal metamorphism during columbia supercontinent assembly evidence from north china craton abstract a vast supracrustal belt of khondalites granulite facies metapelites occur along the northern margin of the north china craton we report here for the first time spinel quartz equilibrium assemblage from these rocks in two textural settings 1 high zno up to 14 47 wt spinel with quartz as inclusions within garnet porphyroblasts defining pressure above 12 kbar and temperature of 900 c and 2 low zno down to 1 2 wt spinel in association with quartz in the matrix assemblage formed during peak ultrahigh temperature conditions ca 975 c and 9 kbar we present a unique case of decompression where the metamorphic conditions of the rocks traversed mostly through the spinel quartz extended stability field monazite grains in textural association with both types of spinel quartz textures were analysed for age determination and the data define two age peaks at 1927 11 ma and 1819 11 ma since the peak thermal regime of the khondalites was close to or exceeded the theoretical closure temperature of pb in monazite we infer the 1819 11 ma age as the timing of ultrahigh temperature event in this craton our data lend support to the idea of ca 1 9 1 8 ga e w collisional orogen at the northern margin of the north china craton we correlate the extreme crustal metamorphism with tectonics associated with the assembly of the north china craton within the columbia supercontinent

metamorphic p t paths of the zanhuang amphibolites and metapelites constraints on the tectonic evolution of the paleoproterozoic trans north china orogen garnet bearing metapelites and amphibolites are exposed in the south and middle parts of the zanhuang complex which is located in the central segment of the nearly ns striking trans north china orogen these rocks preserve three metamorphic mineral assemblages forming at the prograde peak and post peak decompression stages the prograde metamorphic stage m1 is represented by mineral inclusions within garnet porphyroblasts the peak metamorphic stage m2 is represented by garnet rims and matrix minerals whereas the retrograde stage m3 is represented by amphibole plagioclase symplectite rimming garnet porphyroblasts in the amphibolites and biotite plagioclase symplectite rimming garnet porphyroblasts in the metapelites all garnet porphyroblasts in the metapelites preserve prograde chemical zoning except for the ubiquitous quite narrow zones from the underwent post peak decompression it has been determined through thermobarometric computation that the metamorphic conditions are 650 710 c at 8 2 9 2 kbar for the m1 inclusion assemblages 810 c at 12 5 kbar for the metamorphic peak m2 matrix assemblages and 660 680 c at 4 4 4 5 kbar for the retrograde m3 symplectite assemblages these rocks are thus determined to have undergone metamorphism with clockwise p t paths involving nearly isothermal decompression itd segments which is inferred to be related to the amalgamation of the eastern and western blocks to form the coherent basement of the north china craton along the trans north china orogen in the late paleoproterozoic 1 88 1 85 ga

a type granites in northeastern china age and geochemical constraints on their petrogenesis abstract a type granites are widely distributed in northeastern china ne china they were emplaced during three major episodes the permian late triassic to early jurassic and early cretaceous and evolved in different tectonic regimes according to their mineralogical and geochemical characteristics two subgroups of a type granites aluminous and peralkaline can be recognized the peralkaline subgroup contains alkali mafic minerals such as riebeckite arfvedsonite and sodic pyroxene while the aluminous subgroup contains annite and fe rich calcic or sodic calcic amphibole with respect to the aluminous subgroup the peralkaline granites contain higher rb ga and total rare earth elements ree but lower mgo cao al 2 o 3 ba and sr based on the discrimination criteria of eby geology 20 1992 641 the permian and late triassic to early jurassic a type granites belong to the a 2 post orogenic type whereas the early cretaceous granites are of the a 1 anorogenic type nd isotopic compositions of these a type granites indicate their derivation from a dominantly juvenile crustal source their origin is thought to have involved partial melting of an underplated lower crustal source because the generation of a type granites requires high melting temperature we propose models involving slab break off lithospheric delamination and extension the permian a type granites have the same age range as those in eastern junggar southern mongolia and central inner mongolia they occur along a major suture and form a narrow belt between the north china and siberian cratons we suggest that their formation was associated with post collisional slab break off the late triassic to early jurassic a type granites are likely to be the product of lithospheric delamination after the final collision of the major crustal blocks in the late paleozoic to early triassic the early cretaceous a type granites have an anorogenic affinity and were possibly associated with rifting in eastern china at this time associated with the onset of paleo pacific subduction consequently we conclude that the a type granites in ne china were generated at three different times involving multiple processes operative in different tectonic environments

a type granite and adakitic magmatism association in songpan garze fold belt eastern tibetan plateau implication for lithospheric delamination abstract the songpan garze fold belt covers a huge triangular area 200 000 km2 confined by the south china yangtze north china and tibetan plateau continental blocks in the songpan garze fold belt triassic adakitic granitoids have been identified however whether there are triassic a type granites is unclear here we report our first finding of an a type granite nianbaoyeche which occurs in the central part of the songpan garze fold belt the nianbaoyeche granite 820 km2 is characterized by arfvedsonite in its mineral assemblage using both la icpms and tims u pb zircon dating methods we obtain a magma crystallization age of 211 1 ma which is slightly younger than triassic adakitic granitoids 216 221 ma in the songpan garze fold belt the nianbaoyeche granite is enriched in si k na rb ree hfse nb ta zr hf with elevated feotot feotot mgo and ga al ratios but is depleted in al mg ca ba and sr the ree compositions show moderately fractionated patterns with la yb n 2 67 7 54 and eu eu 0 09 0 34 these geochemical characteristics indicate that the nianbaoyeche granite has an a type affinity geochemical data and u pb zircon age combined with regional studies show that the nianbaoyeche granite formed in a post collisional tectonic setting sr nd isotopic data for the granite exhibit isr 0 7090 0 7123 and end t 2 72 to 4 26 with tdm 1 15 1 51 ga suggesting that the magma has a dominantly crustal source though a minor contribution from the mantle cannot be ruled out melting to produce an a type granite may have resulted from triassic lithospheric delamination after triassic crustal thickening of the songpan garze fold belt due to convergence between the yangtze north china and north tibet continental blocks the lithospheric delamination model also helps to explain the triassic adakitic magma generation in the songpan garze belt we conclude that association of a type granite and adakitic granitoids in post collisional environment could be a useful indicator of lithospheric delamination

paleoproterozoic ultrahigh temperature granulites in the north china craton implications for tectonic models on extreme crustal metamorphism abstract ultrahigh temperature uht metamorphic rocks associated with the khondalite belt within the inner mongolia suture zone imsz provide robust evidence for extreme thermal metamorphism in the north china craton ncc the imsz marks the collisional suture between the yinshan block to the north and the ordos block to the south as the ncc was incorporated within the columbia supercontinent amalgam during paleoproterozoic here we present a synthesis of the salient features of the uht rocks from the ncc including petrologic indicators fluid characteristics and monazite and zircon chronometry on the extreme crustal metamorphism the granulites carry diagnostic uht mineral assemblages including sapphirine quartz low zn fe3 spinel quartz high alumina orthopyroxene sillimanite quartz and high temperature mesoperthite the stability fields of the typical mineral assemblages conventional geothermobarometry and phase equilibria modeling using pseudosections as reported in a number of recent studies converge to indicate that these uht rocks experienced metamorphic temperatures up to or in excess of 1000 c at ca 10 kbar followed by an isobaric cooling segment the rocks were exhumed along a near isothermal decompression path microstructures mineral reactions and phase equilibria modeling suggest an anti clockwise p t path similar to those displayed by metamorphic orogens developed in subduction collision settings the dominant category of fossil fluids preserved within the major uht minerals is co2 consistent with the stability of the broadly anhydrous mineral assemblage in these rocks both chemical and radiogenic isotopic ages from monazite and zircon chronometry suggest the timing of the uht event as around 1 92 ga the paleoproterozoic high grade metamorphism younging from 1 95 ga in the western domain to 1 92 ga in the eastern domain of the khondalite belt might suggest a scissor like closure of oblique collision between the yinshan and the ordos blocks the salient features of the uht metamorphism in the ncc include 1 extreme metamorphic temperatures at moderate pressures 2 dominantly anhydrous nature of the mineral assemblages typically the stability of orthopyroxene 3 common presence of co2 rich fluid inclusions as the trace of the ambient fluid 4 regional extent of the uht granulites and 5 the association of the uht orogen with an accretionary belt in a continental collisional suture we evaluate the diverse models on the generation of uht orogens including their formation in thickened and inverted back arcs orogen self heating through heat producing elements heat and co2 input by plume impingement below a carbonated tectosphere and asthenospheric upwelling through ridge subduction and slab window process or during post collisional slab break off the ultra hot and dry uht rocks in the ncc provide one of the well preserved examples from the paleoproterozoic globe for investigating extreme metamorphism and related tectonic processes within the plate tectonic paradigm

application of zr in rutile thermometry a case study from ultrahigh temperature granulites of the khondalite belt north china craton zr in rutile thermometry was applied to ultrahigh temperature uht granulites from three localities dongpo tuguishan and dajing tuguiwula of the khondalite belt north china craton zr concentrations of analyzed rutiles were detected by la icp ms and emp which display a mutative composition zoning a large inter grain variation a bimodal distribution at around 1 500 and 6 000 ppm and no relationship with the textural setting matrix vs inclusion these characteristics were likely caused by post peak diffusional resetting associated with slow cooling rates and the presence of a co2 rich fluid the grains with lower zr concentrations 500 to 3 000 ppm and temperature estimates 650 to 850 c occur close to or in contact with zircon which was easily affected by post peak processes for example diffusion dissolution reprecipitation the lowest temperatures 650 to 700 c we obtained represent the closure temperature of zr in rutile rutiles with higher zr concentrations 3 000 to 8 000 ppm and calculated temperatures 850 to 1 000 c were least affected by late resetting giving near peak metamorphic temperatures these temperature results higher than 900 c even in excess of 1 000 c from the three localities reconfirm the presence of uht metamorphism our results also suggest that zr in rutile thermometry is valid for ultrahigh temperature estimates in addition there are positive correlations between concentrations of zr and hf nb and ta of the investigated rutiles but the correlations weaken as the concentrations increase especially for nb and ta implying fractionation of nb and ta