unconventional shale systems a comparative study of the in source sweet spot developed in the lacustrine chang 7 shale and the marine barnett shale abstract the upper triassic chang 7 shale and the mississippian barnett shale two remarkable shale systems that are considered promising unconventional shale oil gas targets in china and the usa respectively have different kerogen types that formed under lacustrine and marine environments in this study seventy six and ninety seven drilled core samples were collected separately from the zhang 22 well ordos basin and the mesquite 1 well fort worth basin respectively for determining the hydrocarbon generation retention and migration behaviors as well as the intrasource fractionation in the two shale successions geochemical indications of potentially producible oil as indicated by the oil crossover effect and the tmax shift phenomenon as well as the geochemical and molecular compositions of retained hydrocarbon suggest two categories of intervals in the chang 7 and barnett shale systems namely the in source sweet spot and the generative unit oils retained in the in source sweet spot have migrated from the adjacent generative unit and have been subjected to stratigraphic fractionation over geological time these oils are proportionally rich in aliphatic fraction and moderate chain length moieties which significantly improve the fluid mobility and producibility the oil quality improved by fractionation the weak adsorptive affinity of oil to organic matter and the good shale frackability all provide promise for the exploration and production potential of shale oil from the chang 7 and the barnett shale systems

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

magmatic record of neoarchean arc polarity reversal from the dengfeng segment of the central orogenic belt north china craton abstract the dengfeng segment of the archean central orogenic belt in the north china craton ncc includes circa 2 66 2 5 ga metavolcanics metasedimentary and plutonic rocks all units are intruded by 2 5 2 4 ga mafic dykes and a suite of coeval circa 2 5 2 45 ga potassic granites and leucogranitic veins mafic dykes are characterized by mixed alkaline and calc alkaline affinities enriched lree and negative nb and zr anomalies indicating a lithospheric mantle source region potassic granitic dykes exhibit highly fractionated lree patterns and negative hfse anomalies and positive zircon ehf t values which are similar to those of ttg gneisses in the area leucogranitic veins cutting across the diorites are characterized by high contents of sio2 and k2o lree enriched patterns positive eu and sr anomalies negative nb and ti anomalies we suggest that the granitic dykes and leucogranitic veins were derived from partial melting of thickened older crusts combined with other reported 2 5 ga mafic dykes and potassic granites from the ncc we propose that they demonstrate an ancient example of arc polarity reversal and provide a record of the formation of a new plate boundary in the archean the underplating of the mafic dyke source beneath the newly accreted eastern block may have generated the widespread 2 5 2 47 ht lp metamorphism with anticlockwise p t paths in the eastern block

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

geochemistry and u pb zircon ages of metamorphic volcanic rocks of the paleoproterozoic luliang complex and constraints on the evolution of the trans north china orogen north china craton abstract the paleoproterozoic luliang complex is situated in the central part of the western margin of the trans north china orogen and consists of volcanic rocks sedimentary rocks and paleoproterozoic granitoid intrusions the volcanic rocks and earlier granitoid rocks were strongly deformed and metamorphosed into the greenschist to amphibolite facies these metamorphosed volcanic rocks are dominated by basalts to basaltic andesites the parental mafic magmas of these metamorphosed volcanic rocks were mainly derived from the 5 to 30 partial melting of spinel lherzolites to spinel garnet lherzolites which had been enriched by the subduction melts mafic magma experienced fractional crystallization and crustal assimilation u pb zircon dating on two metamorphosed volcanic rocks from the yejishan and luliang groups reveals that they formed at 2210 13 ma and 2213 47 ma respectively and were metamorphosed at 1832 ma this suggests that the metamorphosed volcanic rocks in the yejishan and luliang groups formed synchronously in the paleoproterozoic these new ages integrated with recently reported u pb zircon ages for the jiehekou group and paleoproterozoic granitoids suggest that all of the lithological assemblages of the luliang complex formed and were metamorphosed in the paleoproterozoic not in the neoarchean petrological geochronological and geochemical data suggest that the geodynamic evolution of the paleoproterozoic luliang complex was involved in the development of a magmatic arc system at an active continental margin generating widespread arc related magmatism at 2 2 ga the luliang complex then underwent intense deformation and metamorphism and was incorporated into the trans north china orogen during the 1 88 1 83 ga collisional event which was followed by post collision extension at 1 80 ga

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

geochronology geochemistry and nd hf isotopes of the xiaokouzi granite from the helanshan complex constraints on the paleoproterozoic evolution of the khondalite belt north china craton abstract the khondalite belt is one of the three paleoproterozoic mobile belts in the north china craton and was formed by a continental collision between the yinshan and ordos blocks at ca 1 95 ga although the metamorphism geochemistry and geochronology of the khondalite belt have been extensively studied the provenance and depositional setting of the khondalite protoliths remain controversial in this study u pb zircon ages and geochemistry of the xiaokouzi granite in the southern helanshan complex part of the khondalite belt are investigated zircon u pb dating indicates that the xiaokouzi granite was formed between 2023 14 ma and 2034 16 ma the granite shows variable sio2 64 33 74 14 wt moderate al2o3 14 75 17 23 wt and a cnk values 1 09 1 66 low cao 0 33 2 35 wt tio2 0 30 0 66 wt tfe2o3 2 58 5 76 wt mgo 0 82 2 32 wt and k2o 2 57 4 96 wt contents these features indicate that the xiaokouzi granite is a peraluminous s type granite the xiaokouzi granite has cr and ni contents of 32 7 84 5 ppm and 10 7 19 3 ppm respectively the granite is enriched in cs k pb and zr and depleted in nb ta p and ti the granite displays variable nd t values 1 7 to 3 5 with a corresponding two stage nd model age of 2 2 2 5 ga in addition the granite has high initial 176hf 177hf ratios 0 281569 0 281789 and positive hf t values 1 1 to 8 1 with a two stage hf model age of 2 2 2 6 ga suggesting a juvenile source the crystallization age of the xiaokouzi granite is synchronous with the peak in the detrital zircon age population from metasedimentary rocks in the khondalite belt in addition hf isotope values from the xiaokouzi granite are consistent with the detrital zircons from metasedimentary rocks together with other paleoproterozoic igneous rocks in the khondalite belt it is inferred that paleoproterozoic magmatic rocks in the khondalite belt may be the potential provenance for khondalite protoliths combined with previous geological data from the khondalite belt we propose that the helanshan khondalite protoliths were most likely deposited in an active continental margin near the ordos block

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

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

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

indentation tectonics of the fanshi block in the trans north china orogen abstract the trans north china orogen tnco is a tectonic zone that records significant magmatic metamorphic and tectonic events during the paleoproterozoic the central part of the tnco comprises several precambrian blocks the hengshan block the wutai block the fuping block and the fanshi block that are separated by three paleoproterozoic ductile shear zones as tectonic boundaries the hengshan block and the fuping block are represented by a group of large scale gneissic domes whereas the wutai block characterized by the ene wsw trending synclinorium and the framework of the fanshi block is dominated by strike slip ductile shear zones superimposed on tight folds we present detailed structural analysis of the zhujiafang ductile shear zone in the central hengshan the wangjiahui ductile shear zone at the northern margin of the wutaishan and the metamorphic complex between these two shear zones the macro and microstructures indicate that the ductile shear zones show different kinematics the zhujiafang ductile shear zone is e w trending and record sinistral strike slip shearing and the wangjiahui ductile shear zone is wsw ene trending and record dextral strike slip shearing both of shear zones have similar deformation ages according to the 40ar 39ar dating results and they cut through all the gneissosity and the fold developed in these metamorphic rocks the kinematic difference between the zhujiafang and the wangjiahui ductile shear zones clearly define the fanshi block sandwiched between them which is characterized by movement from west to east the fanshi block differs from the hengshan block and the wutai block in terms of deformation history metamorphism and magmatism in addition a series of ne trending ductile brittle thrust nappes inside the wutai block under the influence of the indention of the fanshi block inside therefore we propose that the fanshi block was indented to the tnco from west to east accompanied by strong strike slip ductile shearing and thrusting during the late paleoproterozoic

introduction to tectonics of china abstract the continental crust of china is a mosaic of cratonic blocks and orogenic belts containing small cratons and terranes with various tectonic settings they have diverse origins and complex histories of amalgamation and often suffered repeated reworking after multiple episodes of amalgamation in the last three decades extensive geological geochemical and geophysical investigations have been carried out on these cratonic blocks and intervening orogenic belts producing an abundant amount of new data and competing interpretations this provides important insights into understanding the formation and evolution of the chinese continents the papers assembled in this volume present a timely and comprehensive overview on major advancements and controversial issues related to the formation and evolution of continental crust in china complex tectonic histories were experienced not only by the large scale cratonic blocks and orogenic belts but also by small scale terranes and orogens between and inside these blocks nevertheless our understanding of lithotectonic units and geological processes has been greatly advanced by recent studies of zirconology and geochemistry for various rock types from major petrotectonic units in china it has been further advanced from integrated interpretations of geochemical and petrological data for petrogenesis of magmatic rocks an overview of these observations and interpretations provides new insights into understanding the continental plate tectonics and the chemical geodynamics of subduction zones

la icp ms u pb zircon ages of the qianlishan complex constrains on the evolution of the khondalite belt in the western block of the north china craton abstract the qianlishan complex is located in the westernmost part of the khondalite belt a continent continent collisional belt along which the yinshan block in the north and the ordos block in the south amalgamated to form the western block which then collided with the eastern block along the trans north china orogen to form the north china craton the complex is dominated by high grade supracrustal rocks and minor s type granites of which the supracrustals consist of graphite bearing sillimanite garnet gneiss garnet quartzite felsic paragneiss calc silicate rock and marble cl images reveal the existence of detrital and metamorphic zircons in major rocks of the qianlishan complex in most cases detrital zircons occur as either single grains with oscillatory zoning or oscillatory zoning cores typical of igneous origin which are surrounded by metamorphic overgrowth rims that are structureless high bright and low in th u ratio detrital zircons from the qianlishan complex yield nearly concordant 207 pb 206 pb ages ranging from 2 3 to 2 0 ga suggesting that the protoliths of the high grade supracrustal rocks in the qianlishan complex were deposited at some time after 2 0 ga metamorphic zircons yield two age populations with one at 1 95 ga and another at 1 92 ga of which the former is interpreted as the timing of the collision between the yinshan and ordos blocks to form the western block whereas the later is considered to be the age of subsequent post orogenic extensional event minor s type granites were emplaced at 1 88 ga as a result of partial melting of supracrustals at the stage of the exhumation of the qianlishan complex these new zircon ages combined with structural and metamorphic considerations enable resolution of the tectonothermal events involving the collision between the yinshan and ordos blocks to form the western block followed by the post collisional extension and subsequent exhumation of the khondalite belt

development of the north china craton during the late archaean and its final amalgamation at 1 8 ga some speculations on its position within a global palaeoproterozoic supercontinent abstract new data obtained over the past few years indicate that by the end of the archaean the north china craton consisted of two major crustal entities identified as the eastern and western blocks the formation of island and magmatic arcs along the western margin of the eastern block record the latest archaean events so far identified the location of the western block at this time is unknown however the occurrence of palaeoproterozoic passive margin sediments similar in age to those occurring in the eastern block indicates essentially similar conditions and possible separation by an ocean of unknown dimensions re initiation of east dipping subduction beneath the western margin of the eastern block resulted in closure of this palaeoproterozoic ocean bringing the western block into close proximity continued subduction resulted in a major continental continental collision and the western margin of the eastern block was again reactivated leading to extensive thrusting high pressure metamorphism and the generation of crustal melts this occurred between 1 88 and 1 79 ga ago resulting in the formation of the trans north china orogen and final amalgamation of the north china craton although information is sparse to non existent we have reviewed the possible location of the north china craton at the time of collision and note a similarity in terms of age lithology and distribution of major crustal components between the north china craton and baltica although speculative we present this model in the hope that it will stimulate debate and lead to further research in these areas

paleoproterozoic crustal growth in the western block of the north china craton evidence from detrital zircon hf and whole rock sr nd isotopic compositions of the khondalites from the jining complex detrital zircons from the khondalites in the western block of the north china craton were mainly derived from a 1 9 to 2 1 ga paleoproterozoic provenance most of the more than 200 detrital zircons analyzed in this study have positive initial ehf values and a wide range of hf depleted mantle model ages tdmc that range from 2 08 ga to 2 74 ga with a peak at 2 24 to 2 40 ga the whole rock nd isotopic analyses for the khondalite samples give quite uniform 143nd 144nd ratios from 0 511188 to 0 511858 corresponding to initial end 2 10ga values ranging from 1 6 to 0 6 in contrast sr isotopic compositions of these samples are more varied with initial 87sr 86sr ratios at 2 10 ga ranging from 0 7017 to 0 7076 in the end versus 87sr 86sr correlation diagram most samples plot near the mixing hyperbola of the depeleted mantle and crust with the contribution of depleted mantle about 65 to 75 percent these results suggest that a large amount of juvenile materials from the depleted mantle was accreted to the western block at 2 1 ga a period of magmatic quiescence in many cratons of the world

determining precambrian crustal evolution in china a case study from wutaishan shanxi province demonstrating the application of precise shrimp u pb geochronology

petrogenesis of ca 1 95 ga meta leucogranites from the jining complex in the khondalite belt north china craton water fluxed melting of metasedimentary rocks abstract anatexis occurs in response to changing p t fluid conditions during orogenesis is thus crucial for understanding the tectonic evolution of orogenic belts here we report an integrated petrological geochemical whole rock oxygen isotopic and zircon u pb hf o isotopic study of meta leucogranites from the jining complex north china craton to unravel its petrogenesis and constrain the tectonic evolution of the paleoproterozoic khondalite belt the meta leucogranites are weakly peraluminous with homogeneous minimum melt compositions and are characterized by low rb contents and rb sr ratios high ba and sr contents and sr y ratios and moderately fractionated ree patterns without obvious eu anomalies these trace element characteristics differ from that of leucogranites derived from fluid absent melting considering their low whole rock zr saturation temperatures 746 780 c and comparable compositions with experimentally generated melts produced by water fluxed melting we infer that the meta leucogranites are low temperature granites and were generated by water fluxed melting of metasedimentary rocks at 6 8 kbar sims zircon u pb analyses suggest that the meta leucogranites were emplaced at 1 95 ga or slightly earlier and were subsequently metamorphosed and cut by a pegmatite dike at 1 92 ga the abundant inherited zircons yield nearly concordant 207 pb 206 pb ages ranging from 2 24 to 1 98 ga which are consistent with the detrital zircon age spectra of the metasedimentary rocks from the khondalite belt the hf isotopic compositions of inherited and magmatic zircons e hf t 0 2 to 7 7 closely overlap with the time evolved e hf range of detrital zircons from metasediments in the khondalite belt different types of zircons inherited magmatic and metamorphic zircons have similar 18 o values of 7 2 9 2 which is lower than that of metamorphic zircons 10 0 13 6 from surrounding metasediments thus we propose that the meta leucogranites were formed by water fluxed melting of metasedimentary rocks from the khondalite belt at 1 95 ga during the prograde upper amphibolite facies metamorphism

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

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

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

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

structural deformation and mineralization in the early proterozoic liaojitite suite eastern liaoning province china abstract the liaojitite suite located in eastern liaoning province china formed in the eugeosyncline of an early proterozoic mobile zone structural deformation occurred during several orogenic episodes especially during the second deformation of the third episode the luliang orogeny 1900 2000 ma ago superposed dome shaped structures and their unevenly distributed groups formed during the luliang orogeny constitute the principal tectonic style in the mobile zone the tectonic style may be considered transitional in the evolution from an oval style in archean granite greenstone terrains to a linear style in phanerozoic terrains the superposed dome shaped structures contain the liaoji granite in the cores ringed successively by the boron bearing series and the outer turbidite series the simple appearances of the dome shaped structures mask their complicated multiply deformed nature which is revealed by tracing their formation and evolution from primary volcanic domes to granitic diapirs and finally to superposed dome shaped structures the evolution of the superposed dome shaped structures represents a change in crustal activity from simple shear to horizontal compression mineralization in the liaojitite suite was closely related to the three deformations of the luliang orogeny metallogenic elements were concentrated during the first deformation physical re emplacement of orebodies occurred during the second and third deformations

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

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

u pb and hf isotopic study of detrital zircons from the luliang khondalite north china craton and their tectonic implications two types of metasedimentary rocks occur in the trans north china orogen of the north china craton one type consists of highly metamorphosed supracrustal rocks with protoliths of mature cratonic shale called khondalites as found in the luliang complex rocks of the other type are also highly metamorphosed but less mature as represented by the wanzi supracrustal assemblage in the fuping complex u pb isotopic data for detrital zircons from khondalites show a provenance dominated by 1 9 2 1 ga palaeoproterozoic rocks these detrital zircons display a wide range of e hf values from 16 0 to 9 2 and give hf isotopic model ages mostly around 2 3 ga the high positive e hf values approach those for the depleted mantle at 2 1 ga highlighting a juvenile crustal growth event in palaeoproterozoic times hf isotopic data also imply that c 2 6 ga old crustal material was involved in the palaeoproterozoic magmatic event these data are similar to those for the khondalitic rocks from the interior of the western block of the north china craton suggesting a common provenance in contrast other metasedimentary rocks in the trans north china orogen such as the wanzi supracrustal assemblage in the fuping complex have a source region with both palaeoproterozoic and archaean rocks their detrital zircon hf isotopic data indicate reworking of old crustal material and a lack of significant juvenile palaeoproterozoic magmatic input these rocks are similar to the coevally deposited meta sedimentary rocks in the interior of the eastern block we propose that the luliang khondalites were deposited on the eastern margin of the western block in a passive continental margin environment and were thrust eastward later during collision with the eastern block other metasedimentary rocks in the trans north china orogen were deposited on the western margin of the eastern block in a continental arc environment our data support the eastward subduction model for the palaeoproterozoic tectonic evolution of the north china craton

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

decoding multiple tectonothermal events in zircons from single rock samples shrimp zircon u pb data from the late neoarchean rocks of daqingshan north china craton abstract a complex sequence of tectonothermal events shaped the neoarchean paleoproterozoic crustal evolution in the north china craton ncc here we present a systematic shrimp zircon u pb investigation on a suite of metamorphic rocks from the hademengou area of daqingshan within the khondalite belt of the inner mongolia suture zone we analyzed zircons from two meta volcanic rocks of the sanggan group and four meta igneous rocks of late neoarchean to early paleoproterozoic zircons from these rocks display typical core rim or core mantle rim structures the relict magmatic zircons record crystallization ages of ca 2 5 ga followed by variable pb loss showing a range of ages in the concordia up to 2 1 ga the recrystallized domains of zircons show sector zoning or are composed of inner dark domains and outer lighter domains the overgrowth mantles are homogeneous the recrystallized zircons and overgrowth mantles show similar age data distribution along a concordia from 2 5 to 2 1 ga the overgrowth rims are poorly developed but record ages 1 96 1 84 ga similar to those displayed by metamorphic zircons in the late paleoproterozoic upper wulashan subgroup as well as the charnockites and uht granulites reported in previous studies from this region a meta syenogranite intruding the sanggan group and hypersthene dioritic gneiss contains 1 95 ga magmatic and 1 85 ga metamorphic zircons magmatic zircons of the late neoarchean early paleoproterozoic metavolcanic rocks have e hf t of 1 2 to 7 0 t dm1 hf model ages of 2687 to 2532 ma and t dm2 cc hf model ages of 2837 to 2559 ma the recrystallized zircons and mantle domains display hf isotope composition similar to that of the magmatic domains our study is the first report from this region that identifies multiple tectonothermal events ranging from late neoarchean through early to late paleoproterozoic from zircons within single rock samples in the ncc a strong overprint of late paleoproterozoic metamorphism traced commonly in all the rock units indicates that these rocks were involved in the tectonothermal event associated with the final amalgamation of the ncc

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

possible southwestward extrusion of the ordos block in the late paleoproterozoic constraints from kinematic and geochronologic analysis of peripheral ductile shear zones abstract the paleoproterozoic is a pivotal period of the tectonic evolution of the north china craton ncc various models have been postulated by previous researchers through the study of petrology and metamorphism but the lack of structural analyses has hampered further understanding of the tectonic evolution of the ncc a series of well exposed ductile shear zones developed on the northern northwestern and eastern edges of the ordos block in the western part of the ncc we carried out detailed field studies along with isotopic dating focused on the kinematics and geochronology of the ductile shear zones on the north margin of the ordos block the wulashan daqingshan ductile shear zone is characterized by an e w trending shearing foliation that steeply dips 75 80 to the north or south with subhorizontal stretching lineation plunging 10 15 toward the east or west and dextral strike slip shearing kinematics on the west margin of the ordos block is the zongbieli ductile shear zone with top to the nw thrusting kinematics on the east margin of the ordos block is the xueling ductile shear zone with top to the se thrusting accompanied by sinistral strike slip shearing kinematics the 40ar 39ar ages of deformed minerals from mylonitic high grade metamorphic rocks and la icp ms u pb dating results of zircons from syn tectonic anatectic granites reflect that the ductile shear zones surrounding the ordos block deformed between 1 85 and 1 81 ga the similar deformation ages indicate that the ductile shear zones might be genetically correlated the geometry kinematics and geochronology characteristics of the wulashan daqingshan ductile shear zone on the north margin the zongbieli ductile shear zone on the west margin and the xueling ductile shear zone combined with the zhujiafang sinistral ductile shear zone on the east margin defined the possible southwestward extrusion of the ordos block in the late paleoproterozoic which might have resulted from the westward subduction collision of the eastern block under the western block at approximately 1 85 ga and which is correlated with the amalgamation of the columbia supercontinent

formation and tectonic evolution of the khondalite series at the southern margin of the north china craton geochronological constraints from a 1 85 ga mo deposit in the xiong ershan area abstract the taihua complex is an important metamorphic rock suite at the southern margin of the trans north china orogen and hosts important mineral deposits its protolith and metamorphic ages are still controversial the 1 85 ga longmendian mo deposit in the xiong ershan is hosted by the khondalite series of the duangou formation taihua complex three kinds of rocks are collected for shrimp cameca u pb dating including the meta sedimentary rock that occurred as wallrock the mo mineralized meta gabbro that intruded the meta sedimentary rock and the leucosome that cut both the meta sedimentary rock and the meta gabbro zircon u pb analyses reveal that the precursor of the meta sedimentary rock was crystallized between 2200 4 and 2742 18 ma with most before 2500 ma whereas the protolith of the meta gabbro was formed at 2178 6 ma anatexis zircons in the leucosome yield a weighted mean age of 1943 8 ma that is indistinguishable with the metamorphic age of the meta gabbro 1952 4 ma in the intensively altered meta gabbro hydrothermal titanites were formed by at least two pulses of crystallization at 1940 7 ma and 1916 9 ma respectively the new data reveal that the khondalite series of the duangou formation were formed between 2200 ma and 2178 ma the results are inconsistent with previous subdivision of the taihua complex into the upper taihua and lower taihua subgroup instead they fit well with the tripartitite divison into the beizi group 3 0 2 55 ga the dangzehe group 2 55 2 3 ga and the shuidigou group 2 3 2 1 ga compiling zircon u pb ages indicate that in the trans north china orogen the 1 95 ga and 1 85 ga metamorphic events are widespread and prominent

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

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 precambrian khondalite belt in the daqingshan area north china craton evidence for multiple metamorphic events in the palaeoproterozoic era high grade pelitic metasedimentary rocks khondalites are widely distributed in the northwestern part of the north china craton and were named the khondalite belt prior to the application of zircon geochronology a stratigraphic division of the supracrustal rocks into several groups was established using interpretative field geology we report here shrimp u pb zircon ages and hf isotope data on metamorphosed sedimentary and magmatic rocks at daqingshan a typical area of the khondalite belt the main conclusions are as follows 1 the early precambrian supracrustal rocks belong to three sequences a 2 56 2 51 ga supracrustal unit the previous sanggan group a 2 51 2 45 ga supracrustal unit a portion of the previous upper wulashan group and a 2 0 1 95 ga supracrustal unit including the previous lower wulashan group a portion of original upper wulashan group and the original meidaizhao group the units thus do not represent a true stratigraphy 2 strong tectono thermal events occurred during the late neoarchaean to late palaeoproterozoic with four episodes recognized 2 6 2 5 2 45 2 37 2 3 2 0 and 1 95 1 85 ga with the latest event being consistent with the assembly of the palaeoproterozoic supercontinent columbia 3 during the late neoarchaean to late palaeoproterozoic 2 55 2 5 2 37 and 2 06 ga juvenile mantle derived material was added to the crust

seismic evidence for significant lateral variations in lithospheric thickness beneath the central and western north china craton abstract we have migrated teleseismic s receiver functions to construct detailed lithospheric structure images that cover the three constituent parts of the north china craton ncc our images show that in contrast to the eastern ncc where significantly thinned lithosphere 60 100 km is widespread the central and western ncc are characterized by the coexistence of both preserved thick and dramatically thinned lithosphere the thick lithosphere 200 km is present beneath the stable ordos plateau and the thinned lithosphere up to 80 km is found in the late cenozoic yinchuan hetao and shaanxi shanxi rift areas with sharp changes occurring over a lateral distance of

episodic widespread magma underplating beneath the north china craton in the phanerozoic implications for craton destruction abstract a comprehensive synthesis of u pb geochronology and hf isotopes of zircons from granulite pyroxenite xenoliths entrained in phanerozoic magmatic rocks and inherited xenocrysts from the associated lower crust rocks from various domains of the north china craton ncc provides new insights into understanding the phanerozoic evolution of the lower crust in this craton episodic widespread magma underplating into the ancient lower crust during phanerozoic has been identified throughout the ncc from early paleozoic to cenozoic broadly corresponding to the caledonian hercynian indosinian yanshanian and himalayan orogenies on the circum craton mobile belts the early paleozoic 410 490 ma ages come from xenoliths in the northern and southern margins as well as the central domain of the eastern block of the craton which mark the first phase of phanerozoic magma underplating since the final cratonization of the ncc in the paleoproterozoic the magmatism coincided with the northward subduction of the paleotethysian ocean in the south and the southward subduction of the paleoasian ocean in the north the subduction not only triggered magma underplating but also led to the emplacement of the diamondiferous kimberlites on the craton marking the initiation of decratonization the late paleozoic event as represented by the 315 ma garnet pyroxenite and or lherzolite xenoliths in hannuoba was restricted to the northern and southern margins of the craton correlating with the arc magmatism continuous associated with the subduction of the paleotethysian and paleoasian oceans and resulting in the interaction between the melts from subducted slabs and the lithospheric mantle lower crust the early mesozoic event also dominantly occurred in the northern and southern margins and was related with the final closure of the paleotethysian and paleoasian oceans as well as the collisional orogeny between the ncc and the yangtze craton the late mesozoic ca 120 ma was a major and widespread magmatic event which manifested throughout the ncc associated with the geothermal overturn due to the giant south pacific mantle plume the cenozoic magmatism identified only in the dark clinopyroxenite xenoliths in the hannuoba was probably induced by the himalayan movement in eastern asia and might also have been influenced by the subduction of the pacific ocean to some extent these widespread and episodic magma underplating or rejuvenation of the ancient lower crust beneath the ncc revealed by u pb and hf isotope data resulted from the corresponding addition of juvenile materials from mantle to lower crust with a mixing of the old crust with melts the process inevitably resulted in the compositional modification of the ancient lower crust similar to the compositional transformation from the refractory lithospheric mantle to a fertile one through the refractory peridotite infiltrated melt reaction as revealed in the lithospheric mantle beneath the craton

is the ordos block archean or paleoproterozoic in age implications for the precambrian evolution of the north china craton the ordos block is a large but poorly exposed crustal segment in the western part of the north china craton little is known about its precambrian basement rocks and their evolution because of an extensive cover by younger sedimentary rocks we report shrimp zircon ages and hf in zircon isotopic compositions for five samples recovered from drill holes that penetrated into the ordos basement based on these data two age groups of rocks can be distinguished an early paleoproterozoic group that is represented by one sample from the northern margin of the ordos block and a late or middle late paleoproterozoic group consisting of the remaining four samples the early paleoproterozoic sample contains zircon cores and metamorphic rims with ages of 2 4 ga and 2 28 ga respectively the cores have ehf t values of 8 8 to 6 2 and hf crustal model ages of 2606 to 3221 ma the remaining rocks of late or middle late paleoproterozoic age contain zircons that may also show a core rim structure but with the cores having ages of 2 08 ga and the metamorphic rims recording ages of 1 9 ga the zircon cores have ehf t values of 9 1 to 10 1 and hf crustal model ages of 2039 to 3068 ma combined with data from earlier studies we draw the conclusion that late paleoproterozoic metasedimentary rocks are widespread in the ordos basement there is no evidence from our samples of an archean component and this calls into question the prevailing view that the ordos basement is an archean cratonic block the fact that the ordos basement was involved in a widespread late paleoproterozoic tectono thermal event indicates that most current models for the tectonic evolution of the western portion of the north china craton need to be revised

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

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

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

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

zircon geochronology and metamorphic evolution of mafic dykes in the hengshan complex of northern china evidence for late palaeoproterozoic extension and subsequent high pressure metamorphism in the north china craton abstract magmatic and metamorphic zircons have been dated from ductilely deformed gabbroic dykes defining a dyke swarm and signifying crustal extension in the northern part of the hengshan complex of the north china craton these dykes now occur as boudins and deformed sheets within migmatitic tonalitic trondhjemitic granodioritic and granitic gneisses and are conspicuous due to relics of high pressure granulite or even former eclogite facies garnet pyroxene bearing assemblages shrimp ages for magmatic zircons from two dykes reflect the time of dyke emplacement at 1915 ma whereas metamorphic zircons dated by both shrimp and evaporation techniques are consistently in the range 1848 1888 ma the youngest granitoid gneiss yet dated in the hengshan has an emplacement age of 1872 17 ma these results complement recent geochronological studies from the neighbouring wutai and fuping complexes to the se of the hengshan showing that a crustal extension event occurred in the late palaeoproterozoic this preceded a major high pressure collision type metamorphic event in the central part of the north china craton that occurred in the palaeoproterozoic and not in the late archaean as previously thought our data support recent suggestions that the north china craton experienced a major craton wide orogenic event in the late palaeoproterozoic after which it became cratonized and acted as a stable block

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

sedimentology diagenesis and reservoir quality of the upper abu gabra formation sandstones in the fula sub basin muglad basin sudan abstract successful hydrocarbon exploration of the lower cretaceous abu gabra formation has established the need to understand the diagenesis and reservoir quality of the sandstones reservoirs in the area therefore this study contains a subsurface facies analysis based on description of three conventional cores from the moga21 2 well methods include wireline log analysis a petrographic study with both thin sections and scanning electron microscope investigations grain size x ray diffraction xrd and heavy minerals analysis of the abu gabra sandstone sedimentary composition and facies sequences of the well logs indicate a complex fluvial channel paleo environment grain size analysis revealed that the sandstone is a mixture of mainly sand size petrographically the sandstones are classified based on standard rock classification schemes into subfeldspathic arenites and feldspathic arenite lithofacies quartz grains are the most abundant detrital components in the samples both polycrystalline and monocrystalline followed by feldspars mainly k feldspar micas and detrital and authigenic clays with carbonatic detritus although lithic fragments iron oxides and heavy minerals occur in minor amounts presence of sillimanite epidote and hornblende compared to zircon tourmaline and rutile as well as the presence of chlorite the dominance of polycrystalline quartz and high crystallinity of kaolinite could be responsible for the low grade metamorphism kaolinite is an authigenic component while quartz overgrowths occur in variable quantity and carbonates calcite and siderite in a relatively minor amount pore types are basically primary and secondary interparticle with few secondary intraparticle pores due to the dissolution of feldspars although porosity ranges from 9 20 to 31 20 the presence of kaolinite carbonate and quartz overgrowth cement reduce the porosity of the samples analyzed samples are characterized by poor to very good pore interconnectivity despite the fact that most of the sandstones display point concavo convex and long contacts with poor to good compaction few sutured grain contacts which reflect a higher degree of compaction still exist xrd and sem analyses show that kaolinite chlorite illite smectite and illite smectite mixed layer clay are the common clay minerals in the studied samples the presence of vermiform morphology besides the sharp peak pattern of the kaolinite in most of the xrd pattern indicates that part of the kaolinite was authigenically formed

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

is the ordos basin floored by a trapped oceanic plateau abstract the ordos basin in china has about 10 km of neoarchean to quaternary sediments covering an enigmatic basement of uncertain origin the basement is tectonically stable has a thick mantle root low heat flow few earthquakes and has been slowly subsiding for billions of years the basement has geophysical signatures that indicate it is dominantly intermediate to mafic in composition and is similar to some other cratons world wide and also to several major oceanic plateaus it was accreted to the amalgamated eastern block and central orogenic belt of the north china craton ncc in the paleoproterozoic then involved in several proterozoic tectonic events including being over thrust by an accretionary orogen and intruded by andean arc related magmas and then involved in a continent continent collision during amalgamation with the columbia supercontinent thus the basement rocks are deformed metamorphosed to granulite facies and determining their initial origin is difficult we suggest that the data is consistent with an origin as an oceanic plateau that accreted to the ncc and later experienced different episodes of differentiation associated with later subduction and collisions formation of cratonic lithosphere by accretion of oceanic plateaus may be one mechanism to create stable cratons other cratons that apparently formed by partial melting of underplated and imbricated oceanic slabs are stable in some cases but also re activated and de cratonized in some cases in asia where they have been affected by younger subduction hydration slab roll back and melt peridotite reactions this suggests that the initial mode of craton formation may be a factor in the preservation of stable cratons and de cratonization is not only influenced by younger tectonic activity

two episodes of paleoproterozoic mafic intrusions from liaoning province north china craton petrogenesis and tectonic implications abstract mafic dyke swarms constitute a common occurrence within the cratonic provinces in the world since the archean proterozoic boundary and their geneses harbor critical information for facilitating regional tectonic and supercontinental reconstruction this zircon u pb dating and geochemical study documents two episodes of paleoproterozoic mafic intrusions from liaoning province north china craton ncc the haicheng suite hcs with an emplacement age of ca 2125 ma from the liaodong peninsula and the guohua suite ghs with an age of ca 1816 ma from the western liaoning terrane the hcs meta gabbros are tholeiitic in composition with an sio 2 range from 46 to 52 modest enrichment in light rare earth elements lree and depletion in high field strength elements hfse and possess radiogenic whole rock nd nd t 1 91 to 4 21 and zircon hf hf t 4 02 to 8 48 isotopic compositions these features endow the hcs with a juvenile subduction fluid metasomatized lithospheric mantle source by contrast the ghs exhibits a meta gabbroic to dioritic composition and a transitional character from tholeiitic to calc alkali with variably enriched lrees and more evolved isotopic signatures nd t 0 20 to 5 82 zircon hf t 0 09 to 4 91 than the hcs while the relatively intact high fe gabbroic samples are supposed to represent partial melts from a fertile cratonic sub continental lithospheric mantle source in the garnet spinel stability field the low fe dioritic ones tend to bear the imprints of subsequent assimilation and fractional crystallization process apart from making an important contribution to the present archive of the paleoproterozoic mafic dyke swarms in the ncc the present two mafic magmatic suites witnessed a shift in tectonic setting from a subduction related continental arc environment to a post orogenic extensional regime synthesizing two suites with coeval felsic igneous episodes as well as concomitant episodic metamorphic events in the jiao liao ji belt leads to the characterization of a possible divergent double subduction system in the eastern ncc during the time span of ca 2 2 1 80 ga synchronized beating of episodic magmatic and metamorphic pulses in the constituent mobile belts of the ncc carries further implication of attesting to the ubiquity and importance of divergent double subduction system in the cratonic amalgamation and its incorporation within the supercontinent columbia

archean to paleoproterozoic continental crust growth in the western block of north china constraints from zircon hf isotopic and whole rock nd isotopic data abstract this study presents new zircon hf isotopic data which combined with previous zircon u pb geochronological in situ hf and whole rock nd isotopic data from the western block of the north china craton ncc provide a new insight into global scale continental crustal growth and evolution of early earth in particular igneous zircons and detrital zircons from the yinshan block yield 207 pb 206 pb ages of 2446 2733 ma with an age peak at 2524 ma of those 86 have positive e hf t values and archean hf model ages t dm c 2 6 2 9 ga peaking at 2840 ma this indicates that those basement rocks of the western block are most likely derived from a depleted mantle source implying a crust mantle differentiation event at 2 8 2 7 ga followed by extensive reworking at 2 5 ga more than 95 detrital zircons from the khondalite belt have ages converging on 2 2 2 0 ga and their e hf t values vary from 5 43 to 9 54 of which 80 are positive and yield hf model ages t dm c peaking at 2250 ma these characteristics of e hf t values in the khondalite belt are also documented in the basement rocks of the ordos block under same age frame of 2 2 2 0 ga this suggests that the ordos block may be a juvenile crust with addition of older materials the initiation of subduction under the southern margin of the ordos block leads to the formation of a 2 2 2 0 ga magmatic arc which is likely related to the assembly of the supercontinent columbia

genesis of the buntsandstein lower triassic in the western eifel germany abstract the buntsandstein lower triassic of the western eifel germany is composed of sediments of different origin these are alluvial fan fluviatile aeolian lacustrine and deltaic deposits the fluviatile deposits are subdivided into channel lag bar and topstratum deposits alluvial fan deposits occur in local depressions of the pre triassic morphology aeolian sediments build up the main part of the middle buntsandstein locally interfingering with fluviatile deposits paleowind and paleocurrent directions generally coincide the upper buntsandstein consists of fluviatile sediments only which can be subdivided into well developed small cyclothems average thickness of sedimentation units 3 8 m capped by paleosoils the uppermost buntsandstein is of deltaic origin the ingression of the lower muschelkalk sea terminates the terrestrial buntsandstein sequence