low altitude magnetic field measurements by messenger reveal mercury s ancient crustal field magnetized rocks can record the history of the magnetic field of a planet a key constraint for understanding its evolution from orbital vector magnetic field measurements of mercury taken by the mercury surface space environment geochemistry and ranging messenger spacecraft at altitudes below 150 kilometers we have detected remanent magnetization in mercury s crust we infer a lower bound on the average age of magnetization of 3 7 to 3 9 billion years our findings indicate that a global magnetic field driven by dynamo processes in the fluid outer core operated early in mercury s history ancient field strengths that range from those similar to mercury s present dipole field to earth like values are consistent with the magnetic field observations and with the low iron content of mercury s crust inferred from messenger elemental composition data

propagation tectonics and multiple accretionary processes of the qinling orogen abstract the qinling orogen was built through collision between the north china and south china blocks previous detailed geological geochemical and geochronological investigations revealed that the mountain range can be divided into four tectonic units with distinct tectono lithostratigraphy which are from north to south the southern sector of the north china block north qinling belt south qinling belt and northern sector of the south china block separated by the kuanping shangdan and mianlue sutures according to the petrology geochemistry and geochronology of ophiolitic melanges and related magmatic rocks as well as the features of sedimentary units we think that the north china block the north qinling belt and the south china block were originally independent continental units while the south qinling belt had been the northern part of the south china block these units experienced three episodes of accretionary tectonic processes and amalgamation from south to north the neoproterozoic accretion took place along the luonan luanchuan fault and kuanping ophiolitic melange belt as a result of southward subduction and subsequent collision between the north qinling and north china blocks during ca 1 0 0 8 ga related to the formation of the supercontinent of rodinia the paleozoic accretion occurred along the shangdan suture resulted from northward subduction of oceanic lithosphere in the early paleozoic and subsequent continental subduction in the late paleozoic late triassic accretion took place along the mianlue suture between the south qinling and south china blocks due to northward subduction of the mianlue oceanic lithosphere during the permian early triassic and subsequent collision in the late triassic after the late triassic collision along the mianlue suture the whole qinling mountain range entered the phase of intense intracontinental deformation

a 1 78 ga large igneous province in the north china craton the xiong er volcanic province and the north china dyke swarm abstract the 1 78 ga xiong er volcanic province xvp and coeval north china giant mafic dyke swarm ncds are the most important magmatic events occurring after the amalgamation of the north china craton ncc the xvp consists of 3 7 km of extrusive volcanics and some feeder dykes sills located along the southern margin of the ncc and extending over an area 0 06 m km2 compositions vary from basalt to rhyolite but are predominantly intermediate in terms of silica content there are also minor sedimentary intercalations and pyroclastic units the sedimentary interlayers indicate an environment changing from continental facies to oceanic facies up section the xvp is characterized by fractional crystallization from an em i type mantle source and both continental arc andean type and rift environments have been proposed the ncds is widespread in the central ncc with an outcrop area 0 1 m km2 and are exposed at variable depths up to 20 km deepest in the north dyke compositions vary from basalt to andesite and dacite but are dominantly mafic and comprise two series of magmatism previous studies revealed that the ncds recorded assimilation and fractional crystallization of an em i type magma source with a minor dm contribution in the younger magmas both syn collisional and intra continental anorogenic environments have been proposed spatial and petrogenic correlations suggest a cogenetic relationship between the ncds and xvp and considered together they define a large igneous province lip of 0 1 m km2 in area and 0 1 m km3 in volume which is also notable for its continuous compositional range from mafic to felsic with no gap at intermediate compositions the petrology is explained by a common magma source that undergoes a silica poor and iron enriched fractionation trend at depth followed by a silica rich and iron poor fractionation trend in shallow level magma conduits dykes and surface lavas a mantle plume is favored as the cause of this 1 78 ga north china lip

genesis of the dianfang breccia hosted gold deposit western henan province china constraints from geology geochronology and geochemistry abstract the dianfang gold deposit located in songxian western henan province china is one of two breccia hosted gold deposits in the xiong ershan waifangshan region previous studies suggest that the breccia and host volcanic rocks of the xiong er group formed coevally the deposit consists of seven vein type orebodies occurring within the southern and hanging wall margins of the breccia pipe as well as in fractures in the rhyolites of the jidanping formation and are structurally controlled by ene trending faults their mineral assemblage includes k feldspar quartz sericite chlorite epidote carbonate adularia and sulfides formed from hydrothermal alteration related to mineralization pyrite is the dominant ore mineral and is associated with minor galena sphalerite and chalcopyrite the trace minerals include magnetite rutile cervelleite matildite and bismuthinite gold occurs as native gold and electrum inclusions in pyrite or along microfractures in sulfides and quartz four stages of hydrothermal mineralization have been identified quartz k feldspar i quartz pyrite ii polymetallic sulfide iii and carbonate adularia iv zircon u pb geochronology of the granitic matrix of the breccia constrains the timing of breccia pipe formation to 145 8 ma indicating it formed at the end of the late jurassic rather than the paleoproterozoic as suggested by previous studies hydrothermal sulfides from the main orebody yield a rb sr isochron age of 121 5 1 7 ma placing gold mineralization at the end of the early cretaceous the sulfur and lead isotope data suggest that the ore forming materials have a close connection with magmatic fluid the initial 87 sr 86 sr value of the sulfides is 0 71056 0 00012 suggesting an ancient crustal source likely the archean taihua group mixed with a juvenile mantle component the trace element characteristics of pyrite indicate the mineralizing fluid was magmatic in origin sourced from a magma derived from the partial melting of the taihua complex and may have been affected by water rock interaction at high temperatures under reducing conditions geological geochronological and geochemical data suggest the dianfang ore deposit is genetically related to a hidden late mesozoic granite and forms part of an intrusion related auriferous system gold deposits of similar ages are widespread at the southern margin of the north china craton and are coeval with the emplacement of felsic and mafic intrusions exhumation of a metamorphic core complex and formation of extensively faulted basins this suggests that the dianfang gold deposit formed under conditions of lithospheric extension caused by crustal thinning and the destruction of the lithosphere mantle beneath the craton during the late jurassic to early cretaceous

crustal thickness and support of topography on venus united states national aeronautics and space administration nasa planetary geology and geophysics program grant

lithification mechanisms for planetary regoliths the glue that binds it is understood how rocks are made on earth however on the moon mercury and to a lesser extent mars and venus there are distinct rock forming processes that we do not fully comprehend the surfaces and crusts of the inner planetary bodies may retain a history of disruption by hypervelocity impact resulting in the generation of disaggregated materials to several kilometers depth the uppermost component of this is called regolith typically 20 m thick on the moon which is part of a more extensive megaregolith that is up to tens of kilometers thick and which in places may pervade the entire crust of a planetary body it is from these pulverized materials that new rocks are reaggregated to form so called breccias this work reviews regolith and megaregolith structure for the inner planetary bodies and investigates how extraterrestrial breccias are produced three principal formation mechanisms are explored thermal sintering shock sintering and the dynamic interaction of impact generated melt w

structural evolution of the paleoproterozoic trans north china orogen evidence from the xiaoqinling region central china abstract high grade metamorphic rocks of archean to paleoproterozoic age collectively called the taihua complex are well exposed in the xiaoqinling region in the western part of the southern segment of the trans north china orogen tnco central china however the evolution of orogenic events in this part of the tnco remains unclear here we present structural and zircon u pb data to constrain the structural evolution of the rocks in the xiaoqinling region the first phase of deformation d1 produced penetrative ductile fabrics and localized ductile shear belts and it was accompanied by a regional high grade metamorphism the d1 structures were the result of wnw directed thrusting movements that have been dated at 1863 1840 ma and the microstructures suggest temperatures of 600 650 c which correspond to the conditions of exhumation of these orogenic rocks these data support the ca 1 85 ga collision model for the western and eastern blocks that involves the se directed subduction of the western block the second phase of deformation d2 produced localized e w striking normal shear belts and the d2 structures provide reliable evidence for n s extension during the late paleoproterozoic intense magmatic activity accompanied this period of extension zircon u pb dating constrains the extension to the period between 1829 and 1703 ma post orogenic extension in the tnco accounts for d2 and the associated magmatism and orogen parallel extension is indicated by the structures thus the paleoproterozoic structural evolution of the rocks in the xiaoqinling region of the tnco records orogenic processes from exhumation to post orogenic extension

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

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

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

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

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

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

uplift denudation history of the qinling orogen constrained from the detrital zircon u pb geochronology abstract this paper presents a great number of detrital zircon u pb ages from the middle triassic to the middle jurassic sediments in the jiyuan basin southern north china the results represent age spectra from 2 9 ga to 216 ma with five peaks at 2 5 ga 1 9 ga 840 ma 440 ma and 270 ma and two grains of 220 ma the ages of 2 5 ga and 1 9 ga are mainly derived from the precambrian basement of the north china block whereas the others are typical characteristics of the qinling orogenic belt an important observation is that the qinling sourced detrital zircons become older as the strata get younger samples from the middle triassic to early late triassic strata are characterized by the age peak at 270 ma whereas the late late triassic to early middle jurassic samples are dominated by age peaks at 840 ma and 440 ma and minor grains within 800 650 ma two grains of 220 ma are preserved in the late middle jurassic sample which may be contributed by the carnian deep plutons these signatures indicate that the unroofing pattern of the qinling orogenic belt developed by the denudation of materials from young covers to old basements and the carnian deep plutons integrated with the data reported from the hefei basin it is well established that the intensity of unroofing increased from the qinling to the dabie orogen in the early jurassic and the denudation timing of the ultra high pressure uhp and high pressure hp rocks or carnian plutons changed successively from the early jurassic in the dabie to the late middle jurassic in the qinling orogen

melting of enriched archean subcontinental lithospheric mantle evidence from the ca 1760ma volcanic rocks of the xiong er group southern margin of the north china craton abstract the xiong er group is an important geologic unit in the southern margin of the north china craton it is dominated by the volcanic rocks dated at 1763 15 ma that have sio2 contents ranging from 52 10 wt to 73 51 wt these volcanic rocks are sub alkaline and can be classified into three subgroups basaltic andesites andesites and rhyolites they unexceptionally show enrichment of light rare earth elements lree and share similar trace element patterns depletions in nb ta sr p and ti relative to the adjacent elements are evident for all the samples the volcanic rocks are evolved with low mgo contents 0 29 5 88 wt and accordingly low mg values of 11 53 the nd isotopes are enriched and show a weak variation with nd t 7 12 to 9 63 zircon hf isotopes are also enriched with hf t 12 02 0 45 the volcanic rocks of the xiong er group are interpreted to represent fractional crystallization of a common mantle source the volcanic rocks might have been generated by high degree partial melting of a lithospheric mantle that was originally modified by the oceanic subduction in the late archean this brings a correlation with the subduction modified lithospheric mantle in an extensional setting during breakup of the columbia supercontinent in the late paleoproterozoic rather than in an arc setting the elevated sio2 contents and evolved radiogenic isotope features indicate the possible incorporation into their source of lower crustal materials that have similar nd isotopic characteristics to the subcontinental lithospheric mantle the existence of extensive xiong er volcanic rocks 60 000 km2 indicates an early large scale subduction related metasomatism in the area and probably suggest a flat subduction model for the plate margin magmatism in the late archean

late mesozoic felsic magmatism and mo au pb zn mineralization in the southern margin of the north china craton a review abstract the widespread large scale mo au pb zn mineralization in the southern margin of the north china craton ncc is spatially and temporally associated with the voluminous late mesozoic granitic magmatism relationship between the granitic magmatism and ore mineralization has been extensively investigated in the last decade however controversy still exists regarding the magma source metallogeny and geodynamic settings this paper systematically reviews the major features of the mesozoic granitic magmatism and associated mo au pb zn mineralization in the southern margin of the north china craton including temporal and spatial variations of magmatic rocks composition of ore fluids sources of ore metals and metallogeny the granitic rocks were mainly formed in late mesozoic and emplaced in two stages the early stage 158 128 ma granites are of typical i type derived from remelting of ancient crust of the southern margin of the ncc with input of mantle material under lithospheric thinning tectonic regime the late stage 127 112 ma granites include a type and or highly fractionated i type granites which commonly contain more fractions of mantle materials as exhibited by more depleted isotopic signatures suggestive of an intracontinental extensional setting mo mineralization 155 130 ma related to the early stage granitic magmatism includes mainly of porphyry or porphyry skarn type with ore metals mainly derived from the mixed source of the mantle and crust whereas the late stage mo mineralization 127 116 ma is mainly of porphyry type with the ore metals from similar sources with more input of mantle materials pb zn deposits 144 130 ma associated with the early stage granitic magmatism are mainly of hydrothermal vein and skarn types whereas those 125 117 ma related to the late stage granites are of hydrothermal vein type the pb zn deposits are most likely distal products of the mo deposits together forming a metallogenic system au deposits are located in the xiaoqinling and xiong ershan areas and were formed during middle jurassic to early cretaceous 163 118 ma mostly from 132 to 120 ma the ores mainly occur in quartz veins and altered rocks and are hydrothermal magmatic in origin with au derived from a mixed source of crust and mantle

metallogeny during continental outgrowth in the columbia supercontinent isotopic characterization of the zhaiwa mo cu system in the north china craton abstract the eastern qinling molybdenum belt eqmb in central china represents the largest mo province in the world located at the southern margin of the north china craton this mo province is largely associated with mesozoic granitoids here we report re os isochron age of 1761 33 ma 2 mswd 2 9 from molybdenite in the recently discovered zhaiwa porphyry mo cu system within the eqmb this age coincides with the peak eruption period 1 78 1 75 ga of the xiong er group volcanic rocks which unconformably overlie the taihua supergroup and are interpreted to have developed on a continental arc the re concentrations 1 to 43 ppm of the zhaiwa molybdenites are similar to those of typical porphyry cu mo au systems developed on continental arcs the 34 s values of the zhaiwa ores 2 7 7 3 average 5 2 suggest that the sulfur was mainly sourced from a magmatic system the sulfides from the ores yield sr i average of 0 70533 similar to that of the xiong er group 0 70547 albeit higher than that of the kuanping group 0 70334 and lower than the taihua supergroup 0 70763 suggesting a mixed source the average e nd 1 76 ga of the sulfides from the ores 0 9 falls between the averages of the taihua supergroup ca 6 8 and the kuanping group ca 7 4 the pb isotope ratios of the ores are similar to but slightly higher than those of the taihua supergroup reflecting preferential leaching of radiogenic pb from the taihua supergroup by magmatic fluids integrated data and interpretations constrain the zhaiwa mo cu deposit to be a porphyry mineral system developed within a ca 1 76 ga continental arc related to oceanic plate subduction beneath the southern margin of the north china craton a correlation between tectonic settings and metallogenic types clearly indicate that the xiong er group was formed in a volcanic arc confirming paleo mesoproterozoic continental outgrowth along the margins of the columbia supercontinent the isotopic characterization of the zhaiwa deposit provides new insights into the enigma of the widespread distribution of large and super large mo deposits of eqmb along the southern margin of the north china craton

a morb arc basalt adakite association in the 2 5 ga wutai greenstone belt late archean magmatism and crustal growth in the north china craton abstract the wutai fuping and hengshan complexes form part of a major 2 5 ga late archean arc system in the northern part of the north china craton the wutai complex is a well preserved greenstone belt dominated by mafic intermediate and felsic volcanic rocks with a mid ocean ridge basalt morb arc back arc basalt adakite association multi element plots for morb type mafic volcanic rocks are characterised by depletion of th nb ta and la nearly flat or lree depleted ree patterns and nb u and nb th ratios greater than primitive mantle values back arc basin basalt babb type rocks have nearly flat trace element patterns with nb ti and y troughs and lree depleted or near flat ree patterns island arc basalt iab type rocks are characterized by negative anomalies of nb ta zr hf ti and y and by lree enrichment the felsic volcanic rocks conform to most compositional criteria for cenozoic adakites including low y and yb high la smn la yb and zr sm ratios and similar trace element and ree patterns trends defined by the association are consistent with magma mixing between morb like melts and subduction slab related melts intra oceanic subduction coupled with contemporary morb type mantle upwelling related to back arc basin extension can account for the interaction between the two components subduction accretion or back arc basin collapse led to accretion of the wutai intra oceanic arc back arc complexes with the magmatic arcs of the fuping and hengshan terranes that are dominated by high grade tonalite trondhjemite granodiorite ttg gneisses this geodynamic interpretation is different from many tectonic models for archean greenstone belts in which magmatism and crustal growth were dominated by mantle plumes or interaction between subduction zones and mantle plumes the final disposition of the arc complexes was controlled by the collision of the eastern and western blocks of the north china craton during the paleoproterozoic resulting in extensive tectonic interleaving within a major orogenic belt that extended throughout the central portion of the north china craton

geochronology and geochemistry of the giant qian echong mo deposit dabie shan eastern china implications for ore genesis and tectonic setting abstract the qian echong mo deposit is a giant porphyry deposit in the dabie shan of eastern china molybdenum mo mineralization mainly occurs as numerous veinlets in altered schists with the development of potassic phyllic argillic and propylitic alteration assemblages a buried granite porphyry stock and locally exposed dikes are spatially and genetically related to the mo mineralization zircon crystals from exposed quartz porphyry rhyolitic porphyry and granite porphyry dikes as well as a buried granite porphyry dike yield weighted average 206 pb 238 u ages of 128 9 1 1 ma mswd 0 30 2 n 13 127 42 0 94 ma mswd 0 58 2 n 17 127 44 0 98 ma mswd 1 6 2 n 16 and 126 6 1 4 ma mswd 0 67 2 n 15 respectively the buried granite porphyry stock yields a zircon u pb weighted average age of 124 7 1 6 ma mswd 1 5 2 n 16 which is slightly younger than the exposed dikes but within error of the age of the buried granite porphyry dike five molybdenite samples from the ores yield re os isotope ages of 123 31 1 02 to 128 49 1 40 ma which are consistent with the u pb ages for the igneous rocks these age data constrain magmatic hydrothermal activity at the qian echong mo deposit to a period of about six million years from 129 to 123 ma this period overlaps the recognized regional mo mineralization event in the dabie shan of 142 111 ma which took place during the post collisional tectonism subsequent to amalgamation of the yangtze and north china blocks the qian echong granite porphyry stock and dikes have high contents of sio 2 k 2 o and al 2 o 3 and low contents of tio 2 mgo and cao showing a peraluminous high k calc alkaline to shoshonite affinity with obvious lree enrichment and a negative eu anomaly the rocks have a high initial 87 sr 86 sr of 0 70729 to 0 71788 and highly negative e nd t values of 16 2 to 26 1 with t dm2 nd ages of 2 24 to 3 03 ga their 206 pb 204 pb t 207 pb 204 pb t and 208 pb 204 pb t values range from 16 017 to 16 701 15 252 to 15 368 and 37 095 to 37 578 respectively this sr nd pb isotopic signature indicates that the causative granitic intrusions for the qian echong deposit were mainly melts with components of both the northern dabie complex and the taihua and xiong er groups that are part of the precambrian basement of the north china block the melts were generated during the tectonic extrusion of the dabie shan and their emplacement onto the basement of the north china block the qian echong mo deposit is unique based upon the crustal source for the causative porphyry localization of most mineralization in the surrounding schist country rocks and the co 2 rich ore forming fluid thus we propose the qian echong deposit to represent a new sub type of porphyry mo deposit herein termed collisional or dabie type porphyry deposit

re os and sr nd pb isotope constraints on source of fluids in the zhifang mo deposit qinling orogen china abstract the zhifang mo deposit is located in the northeastern qinling orogen along the southern margin of the north china craton the deposit represents a quartz vein system hosted in the mesoproterozoic xiong er group volcanic rocks we identify three hydrothermal stages early middle and late characterized by veinlets of quartz pyrite quartz molybdenite pyrite chalcopyrite galena sphalerite and quartz carbonate assemblages respectively five molybdenite samples from the zhifang deposit yield re os ages ranging from 241 2 1 6 ma to 247 4 2 5 ma with an isochron age of 246 0 5 2 ma 2 mswd 7 4 and a weighted mean age of 243 8 2 8 ma 2 mswd 5 5 the re os age shows that the mo mineralization occurred during the indosinian orogeny and suggests that the mineralization is unrelated to the yanshanian magmatism or the paleo mesoproterozoic volcanic hydrothermal event this study also reports a new sr nd pb isotope dataset from ore sulfides in an attempt to constrain the source of the ore forming fluids ten sulfide samples from middle stage of the zhifang mo deposit yield i sr t ratios of 0 710286 0 711943 with an average of 0 711004 e nd t values between 19 5 and 14 8 with an average of 16 7 and 206 pb 204 pb i 207 pb 204 pb i and 208 pb 204 pb i ratios of 17 126 17 535 15 374 15 466 and 37 485 37 848 with averages of 17 380 15 410 and 37 631 respectively one pyrite from the early stage yield i sr t of 0 722711 0 722855 with an average of 0 722783 which is higher than those of the middle stage sulfides and suggests equilibration with wallrocks the e nd t values are in the range of 17 3 to 16 6 with a mean at 17 0 and 206 pb 204 pb i 207 pb 204 pb i and 208 pb 204 pb i ratios are 17 386 15 405 and 37 622 respectively the ore sulfides show higher pb isotope ratios higher e nd t and lower i sr t values than the host rocks the results suggest that the ore forming fluids had lower i sr t and higher e nd t values than the ore sulfides and were possibly sourced from the dengfeng complex the southward subduction of the north china craton beneath the huaxiong block during the triassic was possibly responsible for the formation of the waifangshan orogenic mo system

genesis of the banxi sb deposit south china constraints from wall rock geochemistry fluid inclusion microthermometry rb sr geochronology and h o s isotopes abstract the banxi sb deposit is located in the world class central western hunan sb belt in south china with quartz vein type sb only mineralization hosted by neoproterozoic epimetamorphic clastic sedimentary rocks in this contribution microthermometric rb sr geochronological and h o isotopic analyses on fluid inclusions trapped in quartz s isotopic studies on stibnite and major trace elemental measurements on fluid altered and unaltered wall rocks are presented to constrain the genesis of the deposit the fluid inclusions are characterized by low homogenization temperature 170 260 c and low salinity 3 7 wt nacl equiv with a low fluid density of 0 72 0 93 g cm3 and a ph of 5 59 the low dw fluid 2h 140 to 107 whereas positive 18ov smow 15 9 17 2 and 18ow fluid 18o 5 9 8 4 values in quartz indicate that the mineralizing fluid was generated from the low grade metamorphic basement rocks and received a certain input of meteoric water the rb sr isotopic compositions of quartz samples yielded an isochron age of 196 4 ma 1 mswd 0 70 for quartz stibnite ores a highly radiogenic 87sr 86sr value 0 72663 0 74002 initial 87sr 86sr 0 72640 indicates that ore forming materials are derived mainly from basement rocks and a direct material contribution from local granitic rocks can be ruled out the 34sv cdt values of stibnite range narrowly from 4 81 to 6 72 and are in the 34sv cdt range of the metamorphic rocks indicating that the sulfur was sourced from the basement rocks by sulfate reduction compositional differences between fluid altered decolorized and unaltered wall rocks are obvious the migrated sio2 so3 lree hf nb zr ta th u mo sn w into altered wall rocks suggests a deep basement metamorphic rocks derived si rich alkaline brine for the fluid rock interaction while the leached fe2o3 mgo mno tio2 ba cs rb as co v ni sb sc cu pb and zn from the altered rocks imply a material contribution of banxi group wall rocks during the sb mineralization process consequently we conclude that the quartz stibnite stage of the banxi sb deposit was formed during the late triassic orogeny in the xuefeng mountain region heated by granitic magmatism sb and s were leached from the basement rocks forming an initial si rich and alkaline ore bearing fluid precipitation of stibnite from the fluid resulted from interacting with wall rocks and mixing of meteoric water coupled by releasing of stress decreasing of temperature and lowering of the ph at higher crustal levels

th u pb monazite geochronology of the luliang and wutai complexes constraints on the tectonothermal evolution of the trans north china orogen abstract recent lithological metamorphic and geochronological studies show that the basement of the north china craton ncc formed by collision of two discrete archean to paleoproterozoic blocks eastern and western blocks along a paleoproterozoic orogen named the trans north china orogen however the timing of the collision is controversial and post collisional exhumation has not been precisely dated this study applies the electron microprobe th u pb monazite dating technique to determine the ages of the peak metamorphic event and subsequent exhumation of the low to medium grade luliang and wutai complexes in the trans north china orogen electron microprobe th u pb monazite data for the luliang complex reveal five tho 2 pbo age ranges 1 1940 1938 ma 2 1880 1847 ma 3 1795 1755 ma 4 1720 1703 ma and 5 1648 ma of these age ranges the first four have also been recorded in the wutai complex which yields tho 2 pbo ages of 1 1930 ma 2 1838 1822 ma 3 1793 ma and 4 1719 ma the oldest age range of 1940 1930 ma is interpreted to record the widespread emplacement of mafic dikes in rocks of the luliang and wutai complexes the age range of 1882 1822 ma is in accord with the shrimp u pb ages of metamorphic zircons and mineral sm nd and 40 ar 39 ar ages obtained for other complexes in the orogen and is interpreted as the age of the major metamorphic event caused by amalgamation between the eastern and the western blocks the age range of 1795 1755 ma is consistent with emplacement of large scale unmetamorphosed mafic swarms at 1800 1765 ma interpreted as the time of post orogenic extension the 1720 1703 ma and 1648 ma ages are considered to date later multiple stages of hydrothermal alteration since monazites with such young ages only occur along fractures in older monazite grains and at the rims these new monazite tho 2 pbo ages support the tectonic model for the evolution of the north china craton that envisages discrete eastern and western blocks colliding along the trans north china orogen at 1 85 ga and then undergoing post collisional extension in the period 1795 1755 ma

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

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

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

are the south and north liaohe groups of north china craton different exotic terranes nd isotope constraints abstract the liaohe group is an important paleoproterozoic stratigraphic unit in the northeastern part of the north china craton and is traditionally subdivided into the north and south liaohe groups associated with both the north and south liaohe groups are voluminous paleoproterozoic granitoid rocks named the liaoji granitoids different tectonic models including terrane amalgamation continent arc collision and rift closure have been proposed to interpret the tectonic setting and evolution of the north and south liaohe groups and associated liaoji granitoids at the centre of the controversy between these models is whether or not the north and south liaohe groups developed on the same archean basement nd isotopic geochemistry of the liaoji granitoids provides important constraints on this controversial issue the liaoji granitoids associated with the north and south liaohe groups display similar e nd values restricted to a narrow range from 0 to 2 implying that these granitoid rocks were derived from the same or a similar magma source moreover the liaoji granitoids associated with the north and south liaohe groups have similar nd model ages t dm ranging from 2 4 to 2 6 ga suggesting that the protoliths of the liaoji granitoids associated with both groups may have formed simultaneously and that the basement rocks underneath the liaoji granitoids and associated north and south liaohe groups belong to the same continental block rather than two different blocks combining lithological structural and geochronological considerations we interpret the north and south liaohe groups as having developed on a single late archean basement that underwent paleoproterozoic rifting associated with the intrusion of the liaoji granitoids and the formation of the liaohe group and closed upon itself in the paleoproterozoic

structural evolution and 40ar 39ar dating of the zanhuang metamorphic domain in the north china craton constraints on paleoproterozoic tectonothermal overprinting abstract the zanhuang domain in the hinterland of the north china craton consists of archaean paleoproterozoic mesoproterozoic and palaeozoic rocks and was previously considered a mesozoic metamorphic core complex our reappraisal of regional structural history and mineral 40 ar 39 ar geochronology shows that the domain was subjected to a number of deformation events and had a heterogeneous uplift cooling history during the paleoproterozoic four primary deformation events have been recognized in the domain the first event reflects the earliest deformation in the region however only closed and rootless folds are preserved d2 was dominated by wnw ese shortening and top to ese thrusting in a collisional environment d3 was a divergent extensional and ductile shearing event driven by the post collisional extensional collapse and exhumation of the thickened crust in the domain d4 was characterized by localized ductile shearing with the development of three nne trending ductile shear zones the 40 ar 39 ar plateau ages derived from the thermochronology analyses of the biotite concentrates from the metamorphic rocks are interpreted as cooling ages through biotite 40 ar 39 ar closure temperature 300 c and this allows us to constrain the timing of deformation events in conjunction with the evidence from structural analyses and previous geochronological data we conclude that 1870 and 1870 1826 ma define the timing of d1 and d2 events respectively while 1826 1793 ma represents the age of d3 the last event d4 occurred at 1689 1633 ma as constrained by 40 ar 39 ar plateau ages of biotite concentrates from the mylonites within the ductile shear zones these data together with k feldspar 40 ar 39 ar geochronology allows us to depict the cooling history of the region during the paleoproterozoic and mesoproterozoic it appears that the uplift cooling rate varies temporally and spatially in the region

the koras sinclair ghanzi rift in southern africa volcanism sedimentation age relationships and geophysical signature of a late middle proterozoic rift system abstract a number of relatively undeformed late middle proterozoic volcano sedimentary basins are aligned along the western and northern margins of the kalahari craton although the assemblages of these basins have been compared and lithologically correlated by many authors radiometric ages partly contradict these interpretations attempts to incorporate all of these basins into a regional structural interpretation have been rare watters magmatic arc interpretation is contradicted by the overall geochemistry of the volcanic rocks the coarse continental red bed sediments and the structural style of the basins the depositional troughs were narrow fault bounded continental rift grabens with a structural style and sedimentation pattern controlled by strong vertical tectonism the development of the basins along two branches of a propagating continental rift system the koras sinclair ghanzi rift ksg rift is proposed radiometric ages reveal a distinct younging along the rift from the southern toward the northeastern branch the geophysical signature of the rift is dominated by positive bouguer gravity anomalies flanked by gravity lows which follow a linear trend the basins contain distinctly bimodal volcanic rocks with some intermediate volcanics in the sinclair basin contemporaneous with the extrusion of the volcanics granitic high level intrusions were emplaced along the rift the possible existence of a northern coastal branch of the ksg rift and the location of a triple junction is discussed the rift process possibly continued and led to the development of the early damara rift in swa namibia the african plate kalahari and congo cratons migrated southward between 1050 and 950 ma ago a stationary mantle plume probably caused the development of both the ksg and damara rift a comparison with rift migration in kenya is drawn the regional trends and structures of the ksg rift were reactivated several times undergoing both extension and compression the most prominent of these events was the development of the damara orogen which followed the regional trend of the late middle proterozoic rift but fault structures have been reactivated up to recent times

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

kilometer scale topographic roughness of mercury correlation with geologic features and units we present maps of the topographic roughness of the northern circumpolar area of 30 mercury at kilometer scales the maps are derived from range profiles obtained by the 31 mercury laser altimeter mla instrument onboard the mercury surface space 32 environment geochemistry and ranging messenger mission as measures of 33 roughness we used the interquartile range of profile curvature at three baselines 0 7 km 34 2 8 km and 11 km the maps provide a synoptic overview of variations of typical 35 topographic textures they show a dichotomy between the smooth northern plains and 36 rougher more heavily cratered terrains analysis of the scale dependence of roughness 37 indicates that the regolith on mercury is thicker than on the moon by approximately a 38 factor of three roughness contrasts within northern volcanic plains of mercury indicate a 39 younger unit inside goethe basin and inside another unnamed stealth basin these new 40 data permit interplanetary comparisons of topographic roughness

shrimp u pb zircon geochronology of the liaoji granitoids constraints on the evolution of the paleoproterozoic jiao liao ji belt in the eastern block of the north china craton abstract the paleoproterozoic jiao liao ji belt lies at the eastern margin of the eastern block of the north china craton and is composed mainly of the liaohe group and the liaoji granitoids the liaoji granitoids are divisible into pre tectonic magnetite and hornblende biotite monzogranitic gneisses and post tectonic or anorogenic porphyritic monzogranites and alkaline syenites shrimp u pb zircon geochronology combined with cathodoluminescence cl imaging has enabled for resolution of magmatic and metamorphic events that can be directed towards understanding the history of the paleoproterozoic jiao liao ji belt shrimp u pb zircon analyses reveal that the pre tectonic magnetite monzogranitic gneisses were emplaced in the period 2176 2166 ma and metamorphosed at 1914 ma and the pre tectonic hornblende biotite monzogranitic gneisses were emplaced at 2150 2143 ma these data demonstrate that the pre tectonic monzogranitic gneisses of the liaoji granitoids were not younger than the deposition of the liaohe group as previously considered since the latter contains a large amount of 2100 2000 ma detrital zircons in contrast the pre tectonic monzogranitic gneisses of the liaoji granitoids or similar aged granitoids may have been an important component of the provenance for the liaohe group shrimp u pb zircon analyses also reveal that the post tectonic porphyritic monzogranites and granites in the jiao liao ji belt were emplaced at 1875 10 and 1856 31 ma respectively simultaneously with the emplacement of the alkaline syenites that was dated at 1857 20 and 1843 23 ma similar post tecotnic or anorogenic granites are not only limited to the jiao liao ji belt but are also found in the north and south korea suggesting that the 1 85 ga post tectonic or anorogenic granitic magmatic event may have occurred in both the korean peninsula and the eastern block of the north china craton the shrimp zircon ages of this study combined with lithological structural metamorphic and geochemical data is consistent with a rift closure model which suggests that the archean complexes on the northern and southern sides of the jiao liao ji belt were originally situated on a single continental block that underwent paleoproterozoic rifting associated with the formation of the liaohe group and liaoji granitoids and closed upon itself at 1 9 ga

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

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

boninite like volcanic rocks in the 3 7 3 8 ga isua greenstone belt west greenland geochemical evidence for intra oceanic subduction zone processes in the early earth abstract the 3 7 3 8 ga isua greenstone belt of southwest greenland is characterized by variably metamorphosed metasomatised and deformed lithotectonic successions of volcanic and sedimentary rocks the voluminous mafic volcanic rocks are composed primarily of pillow basalts intercalated with ultramafic units the sedimentary rocks consist mainly of banded iron formation cherts conglomerates and siliciclastic turbidites the least altered metavolcanic amphibolites the garbenschiefer unit from the central tectonic domain of the isua greenstone belt are characterized by high mg number 0 60 0 80 mgo 7 18 wt al2o3 14 20 wt ni 60 645 ppm and cr 60 1920 ppm contents but low tio2 0 20 0 40 wt zr 12 30 ppm y 6 14 ppm and rare earth element ree concentrations these compositional features collectively represent a coherent mafic to ultramafic suite chondrite normalized ree patterns are concave upward on the primitive mantle normalized extended trace element diagrams they are characterized by relative depletion of nb but with an enrichment of zr relative to neighboring rees alteration deformation and crustal contamination can be ruled out as the cause of the distinct and coherent composition the average initial end value of these metavolcanic rocks is 2 collectively these geochemical characteristics are comparable to those of phanerozoic boninites given the observation that in the tertiary boninites are exclusively associated with intra oceanic subduction environments e g izu bonin mariana subduction system this suggests that intra oceanic subduction zone like geodynamic processes were operating as early as 3 7 3 8 ga

provenance and depositional age of paleoproterozoic metasedimentary rocks in the kuluketage block northern tarim craton implications for tectonic setting and crustal growth abstract precambrian geology of the tarim craton especially for the crustal evolution is poorly constrained paleoproterozoic metasedimentary rocks are extensively exposed in the kuluketage block northern tarim craton and thus have recorded abundant geological information of its tectonic history supracrustal rocks within the archean basement tuoge complex and the paleoproterozoic xingditage group were collected from the kuluketage block for geochemical study and zircon u pb dating the tuoge supracrustal rocks have low cia values between 45 and 63 indicating weak chemical weathering although the xingditage group shows a large variation of cia values 37 67 their low average value suggests that they underwent similar weathering to the rocks from the basement this similarity is also supported by their respective pia values mostly 43 57 for the basement rocks and mostly 37 60 for the xingditage group the high icv values mostly 1 of both rock suites indicate an immature source that lacked alumina rich minerals although these metasedimentary rocks show different ree patterns they are all enriched in light rare earth element lree and display relatively flat heavy rare earth element hree patterns the low values of ferromagnesian e g cr co and ni and high field strength elements e g zr hf nb ta and y are similar to those of graywackes from active continental margins therefore the kuluketage block most likely evolved in an active continental margin setting during the late paleoproterozoic detrital zircons from a slate sample of the tuoge supracrustal rocks are dominated by igneous zircons which mostly yield paleoproterozoic 207pb 206pb ages between 1 72 ga and 2 08 ga other igneous grains give paleoproterozoic 207pb 206pb ages at 2 2 ga and 2 32 ga all these igneous zircons show large variations of hf t values mostly between 17 and 3 and have neoarchean crustal model ages tdmc 2 6 3 7 ga schist and quartzite samples of the upper xingditage group show similar age spectra to the slate and contain a major igneous zircon population with paleoproterozoic formation ages mostly 1 8 2 1 ga and paleo to mesoarchean tdmc ages 2 7 3 1 ga a schist sample of the lower xingditage group shows a much different age spectrum detrital zircons from this sample are dominated by neoarchean grains 2 5 2 7 ga with paleoarchean tdmc ages 3 14 3 51 ga and two of them yield the youngest ages at 1 94 and 2 09 ga the youngest zircons from these samples therefore define the maximum depositional ages at 1 63 1 94 and 1 77 ga for the tuoge supracrustal rocks the upper xingditage group and the lower xingditage group respectively on basis of these age spectra we suggest that the tuoge supracrustal rocks are equivalents of the upper xingditage group instead of parts of the archean basement overall the hf isotopic compositions of these detrital zircons indicate three early precambrian crustal growth events in the tarim craton which occurred in the late paleoarchean the early neoarchean and at the beginning of the paleoproterozoic

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

geochronology and geochemistry of the late paleoproterozoic aluminous a type granite in the xiaoqinling area along the southern margin of the north china craton petrogenesis and tectonic implications abstract the occurrence of extensional related a type granites with a high k nature in some early precambrian blocks hints a possible link with the orogeny and may imply the ending of the orogenic processes and the beginning of craton stabilization the newly recognized late paleoproterozoic a type granite the guijiayu pluton composed of monzogranites in the xiaoqinling area of the southern margin of the north china craton ncc is a promising locus to investigate the late paleoproterozoic tectonic evolution of the ncc la icp ms zircon u pb dating shows that the guijiayu pluton was emplaced at 1 80 ga petrologically the guijiayu granites contain perthite annite and calcic amphibole but are lack of alkali mafic minerals geochemically all the rocks are enriched in silicon and total alkali and depleted in mgo cao and p2o5 with high feot mgo tio2 mgo and 10 000 ga al ratios and high hfses i e zr nb ce y concentrations as well as negative eu sr p and ti anomalies they are also characterized by high melting temperatures and metaluminous to weakly peraluminous natures taken together the guijiayu granites show an affinity to aluminous a type granite their geochemical characteristics e g obviously negative eu and sr anomalies along with the low zircon ehf t values 13 9 to 7 0 and whole rock end t values 5 81 to 5 65 indicate that the guijiayu aluminous a type granite was derived from partial melting of the crustal calc alkaline granitoids dominated by the granodiorite of the taihua group in the study area combined with the regional data especially for the identification of the clockwise p t paths of paleoproterozoic metamorphism in the southern margin of the ncc even coeval granites and regional retrograde metamorphism in other areas in the ncc it is most likely that the peak of continent continent collision had finished in the study area at least before 1 80 ga and that the generation of the guijiayu aluminous a type granite was related to the underplating of mafic magmas in a post collisional regime

u pb geochronology and hf isotope geochemistry of detrital zircons from the zhongtiao complex constraints on the tectonic evolution of the trans north china orogen abstract the zhongtiao complex is located in the south segment of the trans north china orogen a continent continent collisional belt along which the discrete eastern and western blocks amalgamated to form the north china craton the complex consists of the neoarchean to paleoproterozoic granitoid plutons and low grade supracrustal rocks of which the latter are subdivided into the songjiashan jiangxian lower zhongtiao upper zhongtiao and danshanshi groups the songjiashan jiangxian and lower zhongtiao groups are composed of metaclastic rocks carbonates and metavolcanic rocks interpreted as the products of a back arc basin whereas the upper zhongtiao and danshanshi groups consist of metaconglomerates and metasandstones interpreted as molasses type siliciclastic rocks deposited in a retro arc foreland basin the u pb ages of detrital zircons from the songjiashan lower zhongtiao upper zhongtiao and danshanshi groups form four populations of 3600 3100 ma 2850 2750 ma 2630 2200 ma and 2150 1850 ma of which the population of 2630 2200 ma is the most dominant and its major sources are considered to be derived from the 2560 to 2300 ma granitoids and metavolcanics in the zhongtiao complex the source rocks of the subordinate 3600 3100 ma detrital zircons were probably derived from the paleo mesoarchean crust of the eastern block since similar aged rocks have not been found in the western block and the trans north china orogen whereas the 2850 2750 ma detrital zircons are interpreted to be derived from the 2850 to 2752 ma ttg gneisses and amphibolites in the adjacent taihua complex the age population of 2150 1850 ma is coincident with the ages of paleoproterozoic granitoids in the trans north china orogen the youngest detrital zircons in the songjiashan and lower zhongtiao groups constrain their maximum depositional ages at 2253 ma and 2168 ma respectively whereas the maximum depositional age of the overlying upper zhongtiao and danshanshi groups is defined at 1848 ma the late paleoproterozoic shift from a back arc basin to a retro arc foreland basin is consistent with the model that the collision between the eastern and western blocks to form the trans north china orogen occurred at 1 85 ga

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

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

u pb dating of baddeleyite and zircon from the shizhaigou diorite in the southern margin of north china craton constrains on the timing and tectonic setting of the paleoproterozoic xiong er group abstract the xiong er volcanism marks an important thermal event in the southern margin of the north china craton during the paleoproterozoic precise u pb ages of zircon and baddeleyite from the shizhaigou diorite which intruded the majiahe formation of the upper part of the xiong er group were determined in this study by multi collector sims the mean 207 pb 206 pb ages are 1789 3 5 ma for baddeleyite and 1780 11 ma for zircon from shizhaigou diorite respectively the major and trace elements and sr nd isotopic compositions indicate an em i mantle type signature and crustal contamination features combining with previous chronological data it is proposed that the age of the xiong er volcanism is between 1770 ma and 1800 ma with a peak at 1780 ma our result supports the view that the xiong er volcanic rocks formed in an extensional rift environment

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

petrogenesis of the early cretaceous intermediate and felsic intrusions at the southern margin of the north china craton implications for crust mantle interaction abstract new major and trace element whole rock sr and nd isotopes and zircon u pb ages and hf isotope data are presented for rocks from the early cretaceous tianqiaogou dioritic and taishanmiao granitic plutons at the southern margin of the north china craton ncc in order to investigate their petrogenesis and geological evolution la icp ms u pb analyses for zircons from these two plutons yield similar 206 pb 238 u ages of 122 ma and 115 125 ma respectively monzodiorites from the tianqiaogou pluton have whole rock e nd t values ranging from 6 2 to 1 3 and zircon e hf t values from 2 9 to 6 2 they are variably enriched in ra ba and sr and depleted in nb ta zr hf and ti indicating that they were derived from a depleted mantle and underwent subsequent magma differentiation and crustal contamination the taishanmiao pluton is composed of metaluminous to peraluminous highly fractionated i type granites that have high sio 2 na 2 o k 2 o rb th and u and low p ba sr ti and eu contents the granites have strong negative whole rock e nd t values 16 1 to 7 5 and zircon e hf t values 20 9 to 6 1 their nd t dm ages 1 19 to 2 01 ga and zircon hf t c dm ages 1565 to 2490 ma are much younger than the basement rocks beneath the southern margin of the ncc suggesting derivation from an ancient crustal source with minor involvement of mantle derived components therefore rocks from the tianqiaogou dioritic pluton were partial melts of the mantle source underplating of the mafic magmas initiated partial melting of the ancient continental crust resulting in the formation of the taishanmiao granitic pluton their complex petrogenesis reflects a strong crust mantle interaction process related to lithospheric thinning beneath the southern margin of the ncc in early cretaceous

origin of the late paleoproterozoic low δ18o a type granites on the southern margin of the north china craton and their geodynamic mechanism abstract a type granites with 18o depletion signatures are rather enigmatic as regards not only their typology and petrogenesis but also their rock forming geodynamic mechanism here we report a suite of late paleoproterozoic low 18o a type granites from the longwangzhuang pluton on the southern margin of the north china craton sims zircon u pb dating showed that the longwangzhuang pluton was emplaced at 1613 4 ma combined with previous data it is clear that the mineral components of longwangzhuang granites are variable and consist of perthite quartz plagioclase biotite and amphibole calcic hastingsite ferroedenite and sodic ferrogedrite geochemically they are metaluminous to peraluminous and have high alkali contents k2o na2o 8 76 wt k2o na2o 1 feot feot mgo 0 95 ratios zr nb ce y 993 ppm contents and 10000 ga al 4 31 ratios resembling a type granites in situ oxygen isotope analysis displayed low zircon 18o values between 4 0 and 4 9 with a weighted average of 4 4 based on the archean two stage zircon hf and whole rock nd model ages coupled with the low la yb cn ratios negative anomalies of eu and sr and high zr saturation temperatures and the presence of low pressures 3 3 5 7 kbar amphiboles the longwangzhuang a type granites were generated by self cannibalizing pre existing low 18o old rocks i e the taihua complex and emplaced at shallow depth combined with regional geological observations i e coeval tuffites a passive rift setting was the most plausible explanation for the generation of the late paleoproterozoic low 18o a type granites on the southern margin of the north china craton such a rifting could likely have lasted at least till 1 6 ga

u pb and re os isotopic systematics and zircon ce4 ce3 ratios in the shiyaogou mo deposit in eastern qinling central china insights into the oxidation state of granitoids and mo au mineralization article i nfo the newly discovered shiyaogou molybdenum deposit is located in the eastern qinling metallogenic belt in central china the deposit contains at least 152 000 t of mo metal and bears typical porphyry type features in terms of its concentric alteration zonation quartz vein hosted mo mineralization veining sequence and the spatial association with concealed granite porphyries re os isotope analyses of molybdenite from the de posit yield an ore forming age of 132 3 2 8 ma la icp ms u pb zircon dating of ore related porphyries yields crystallization ages from 135 ma to 132 ma indicating a temporal link between granitic magmatism and mo mineralization a population of captured magmatic zircons indicates another pulse of magmatism at 143 ma a barren granite intrusion near the deposit gives a zircon u pb age of 148 1 1 1 ma these magmatic activities were concurrent with the emplacement of the nearby heyu granitic batholith a largely ore barren intrusive complex formed from 148 ma to 127 ma zircon ce 4 ce 3 ratios of ore related por phyries are obviously higher than those of contemporaneous barren granitoids implying an affinity between mo mineralization and highly oxidized magmas moreover zircons from these granitoids overall have de creasing ce 4 ce 3 ratios from 148 ma to 132 ma reflecting decreasing oxygen fugacities during magma evolution available geological radiometric and stable isotopic evidence suggests that the decrease of magma oxygen fugacity was probably associated with an increase of mantle contribution to granitic magmatism and metallogenesis which probably gave rise to successive mineralization of mo and au in the eastern qinling the intense magmatic metallogenic events in the eastern qinling during late jurassic to early cretaceous times are interpreted as a response to the large scale lithosphere thinning and subsequent asthenosphere upwelling beneath the eastern part of the north china craton

geochemistry of silurian devonian volcanic rocks in the coastal volcanic belt machias eastport area maine evidence for a pre acadian arc the coastal volcanic belt in easternmost maine consists of silurian devonian volcanic rocks related to the convergence of avalonia with the leading edge of composite laurentia just before and during early stages of the acadian orogeny new major and trace element analyses from the machias eastport area indicate that the early to late silurian volcanic rocks in the dennys edmunds and leighton formations formed in a continental arc environment in contrast to some earlier interpretations these rocks have a continuous compositional range from basalt through andesite to rhyolite the mafic rocks in these formations are calc alkaline and show subduction related geochemical signatures on various discrimination diagrams whereas the felsic rocks have geochemical characteristics of volcanic arc granites in contrast the thick volcanic deposits of the younger late silurian eastport formation are bimodal and tholeiitic and have transitional to within plate geochemical signatures thus the transition over time as the acadian seaway closed from subduction and arc settings to continental extension can been seen in this rock sequence and it constrains the time of avalonia s docking regionally volcanic rocks in the kingston terrane of southern new brunswick are likely correlatives of the earlier arc volcanic rocks dennys edmunds and leighton formations while eastport formation analogues are found in the coastal maine magmatic province cranberry island series and probably the passamaquoddy bay volcanic series which similarly appear to have formed after the transition to an extensional environment occurred although other extensional settings cannot be entirely ruled out basalts in the post acadian perry formation indicate continental extension was also active in the late devonian

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

geochemical ar 40 ar 39 geochronological and sr nd isotopic constraints on the origin of paleoproterozoic mafic dikes from the southern taihang mountains and implications for the ca 1800 ma event of the north china craton abstract the wide occurrence of paleoproterozoic mafic dikes in the basement of the trans north china orogen provides important constraints on the major 1800 ma tectonothermal event of the north china craton these mafic dikes form a tholeiitic series dominated by basalts and basaltic andesites and can be geochemically classified into three groups the groups 1 and 2 rocks are characterized by high total feo contents 12 69 15 61 of which the group 1 rocks are more enriched in lile and lree contents and depleted in hfse th nb n 2 5 3 8 la yb cn 3 0 5 8 nb la n 0 27 0 38 and have more radiogenic nd isotopic ratios nd t 3 40 to 5 14 than the group 2 rocks th nb n 0 8 1 2 la yb cn 1 5 2 1 nb la n 0 65 0 87 nd t 0 60 to 1 67 in contrast the group 3 rocks are typified by low feot 7 86 11 04 and high mgo 5 62 9 56 contents higher la yb cn 4 22 7 13 ratios less radiogenic nd isotopic ratios nd t 2 75 to 5 52 and more significant th u and nb ta depletion are also apparent features 40ar 39ar geochronology of three representative samples from each group yielded plateau ages of 1780 7 0 5 ma 1765 3 1 1 ma and 1774 7 0 7 ma respectively these elemental and isotopic data suggest that the geochemical variations of these rocks cannot be simply explained by crystallization from a common parental magma involving crustal contamination during emplacement instead they most likely originated from variable sources under different degrees of partial melting the group 1 rocks were derived from relatively low degrees of partial melting of a refractory lithospheric mantle previously metasomatised by subduction related fluids whereas the group 2 rocks originated from a hybridized source involving ca 40 subduction modified lithospheric mantle similar to the group 1 and ca 60 n morb component in contrast the group 3 rocks show significant th u depletion that may mark the involvement of portions of gabbroic lower crust trapped in the source region during subduction based on all the available data we propose a tectonic model of a full subduction syncollision postcollision rifting cycle for the evolution of the ncc between 1870 and 1765 ma a variety of source regions beneath the southern taihang mountains initially developed during the period of subduction collision and subsequent extension rifting resulted in the melting of the subduction modified lithospheric mantle which produced widespread paleoproterozoic mafic dikes on the trans north china orogen in response to upwelling of convective mantle

age and geochemistry of the early mesoproterozoic a type granites in the southern margin of the north china craton constraints on their petrogenesis and tectonic implications abstract a type granites are a spectrum of especial felsic rocks that are produced commonly within an extensional environment they are rare before middle paleoproterozoic but present abundantly since the late paleoproterozoic probably indicating a causal change of tectonics and geodynamic mechanism in the continental evolution of the earth an early mesoproterozoic a type granite belt was recognized in the southern margin of the north china craton ncc in recent decades the occurrence of this type of rocks indicates a possible spatial temporal link between the tectonic evolution of the ncc and the breakup of the columbia supercontinent during the mesoproterozoic the zhangjiaping granitic pluton is located in the southern margin of the ncc and one part of the a type granite belt it can be divided into three zones according to their textural and compositional differences of the rocks with a gradually increasing degree of evolution inwards wall zone intermediate zone and core zone la icp ms zircon u pb dating shows that it was emplaced at ca 1 53 ga petrologically the zhangjiaping granites contain perthite and calcic amphibole but are lack of alkali mafic minerals geochemically all the zhangjiaping granitic rocks show an affinity to fractionated aluminous a type granites with enrichment in silicon and total alkali and depletion in mgo cao and p2o5 with negative eu sr and ti anomalies as well as high feot mgo tio2 mgo and ga al ratios and high hfses i e zr nb ce y concentrations such signatures along with their low ehf t values 12 5 to 4 0 of all the granites from the zhangjiaping pluton obviously indicate that the zhangjiaping aluminous a type granites were derived from partial melting of the crustal calc alkaline granitoids i e the granodiorites of the taihua complex in the study area and experienced fractional crystallization during magmatic evolution their generation would be related to the underplating of mafic magmas in an extensional regime integrated with regional data the zhangjiaping aluminous a type granites were the products of anorogenic magmatism likely corresponding to the breakup of the columbia supercontinent

ejecta megaregolith accumulation on planetesimals and large asteroids megaregolith accumulation can have important thermal consequences for bodies that lose heat by conduction as vacuous porosity of the kind observed in the lunar megaregolith lowers thermal conductivity by a factor of 10 i have modeled global average ejecta accumulation as a function of the largest impact size with no explicit modeling of time in conjunction with an assumed cratering size distribution exponent b the largest crater constrains the sizes of all other craters that significantly contribute to a megaregolith the largest impactor mass ratio is a major fraction of the catastrophic disruption mass ratio and in general the largest crater s diameter is close to the target s diameter total accumulation is roughly 1 5 of and proportional to the target s radius global accumulations estimated by this approach are higher than in the classic housen et al 1979 study by a factor of roughly 10 this revision is caused mainly by higher typical largest crater size for b 2 the single largest crater typically contributes close to 50 of the total of new nonrecycled ejecta megaregolith can be destroyed by sintering a process whose pressure sensitivity makes it effective at lower temperature on larger bodies planetesimals 100 km in diameter may be surprisingly well suited about as well suited as bodies two to three times larger in diameter for attaining temperatures conducive to widespread melting a water rich composition may be a significant disadvantage in terms of planetesimal heating as the shallow interior may be densified by aqueous metamorphism and will have a low sintering temperature

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

shrimp and la icp ms zircon geochronology of the xiong er volcanic rocks implications for the paleo mesoproterozoic evolution of the southern margin of the north china craton the paleo mesoproterozoic xiong er volcanic rocks along the southern margin of the north china craton are lithologically and geochemically similar to those formed in subduction related continental margin volcanic arcs the volcanic rocks are primarily composed of basaltic andesites and andesites with minor dacites and dacitic rhyolites traditionally the xiong er volcanic rocks have been divided from lower to upper into the xushan jidanping and majiahe formations but the ages of volcanic rocks in these formations have not been well constrained which has hindered further understanding the tectonic significance of the xiong er volcanic belt at the southern margin of the north china craton shrimp and la icp ms u pb zircon analyses combined with cathodeluminescence cl images have enabled resolution of xenocrystic and magmatic zircons that can be directed toward determination of the ages of the xiong er volcanic rocks shrimp and la icp ms u pb analyses on magmatic zircons from two basaltic andesite samples one dacite sample and one rhyolite sample of the xushan formation known as the lowest sequence of the xiong er volcanic rocks indicate that the volcanic eruption of this formation occurred at 1 78 ga but most xenocrystic inherited zircons in these samples yielded 207pb 206pb ages ranging from 2 55 ga to 1 91 ga of three samples collected from the jidanping formation two rhyolite samples 05xe015 and 05xe100 yielded weighted mean 207pb 206pb ages of 1778 5 5 ma and 1751 14 ma respectively similar to the ages of the volcanic rocks in the xushan formation whereas one dacite sample 05xe066 gave a weighted mean 207 pb

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 geochemistry of young volcanism throughout western panama and southeastern costa rica an overview oblique aseismic subduction below western panama and southeastern costa rica has produced recent arc related volcanism the aseismicity is probably related to the subduction of relatively hot oceanic lithosphere the volcanism throughout this region over the past 2 ma has been quite distinct consisting of felsic magmas andesites to rhyolites but mainly dacites with geochemical signatures suggesting a metamorphosed basaltic source it is believed that the subduction of young oceanic crust sets up conditions under which the slab melts rather than the overlying mantle wedge rocks with slab melt geochemistries and associated with young subducted crust have been termed adakites elsewhere the young adakite melts are sometimes associated with a few rare young high nb basalts but there is no obvious genetic link between them through differentiation high nb basalts may also be derived from the partial melting of the subducted oceanic crust high nb basalt migmatites have been found with pegmatites of adakite compositions in the exposed subduction terrain of the catalina schist california alternatively the high nb basalts may be partial melts of phlogopite rich mantle that has previously reacted with adakite magmas eruption of adakites and high nb basalts was preceded by a 2 3 ma period of relative quiescence prior to this there was a 7 ma period of calc alkaline volcanism typical of the present day magmatism associated with a distinct benioff zone found throughout the central american arc the abrupt transition in volcanism with time from an early calc alkaline sequence to a later adakite high nb basalt sequence may record a change in the tectonic setting of western panama and southeastern costa rica over the past 12 ma

isotopic characteristics of subduction fluids in an intra oceanic setting izu bonin arc japan new radiogenic isotope and trace element data are presented for the volcanic sequences along 600 km of the active izu bonin arc the oligocene izu arc and their associated rift basins as with many intra oceanic island arcs the pliocene recent izu bonin frontal arc lavas are highly depleted in zr nb and the rare earth elements relative to typical mid ocean ridge basalt morb indicating that the mantle wedge source has undergone a previous episode of melting ratios between these elements such as nb zr and la sm as well as 143nd 144nd do not vary significantly along the length of the frontal arc these parameters suggest that each of the arc volcanoes is derived from similar melt fractions of the mantle wedge however ba zr ba rb and 87sr 86sr increase along the frontal arc to the north this leads us to propose that a variable enrichment in ba and radiogenic sr is superimposed on the mantle wedge sr nd and pb nd isotope variation indicate that both sr and pb become more radiogenic after fluid addition however pb isotope ratios do not correlate with increases in pb concentration or ratios such as ba zr and nb pb in other words the pb isotopic composition of the arc lavas appears to be independent of the amount of pb introduced by subduction fluids into the mantle source this buffering of pb isotopes along the frontal arc means that the isotopic composition of the lavas is indistinguishable from that of the fluid isotopic mixing models presented for the arc are only illustrative of the many plausible combinations of components and quantities despite this we are able to determine that the mantle wedge has isotopic characteristics similar to indian ocean morb and that the subduction fluid solute is primarily derived from subducted oceanic basalt with a 2 contribution from subducted sediment lavas in the oligocene izu arc and fore arc basin were derived from a mantle wedge of similar composition to the active arc despite levels of pb enrichment comparable to those of the modern arc the pb isotopes of the oligocene volcanics indicate a lower sediment input into the melting region

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

magnesian andesites nb enriched basalt andesites and adakites from late archean 2 7 ga wawa greenstone belts superior province canada implications for late archean subduction zone petrogenetic processes magnesian andesites ma occur with normal tholeiitic to calc alkaline basalt andesite suites in four greenstone belts of the 2 7 ga wawa subprovince canada collectively the magnesian andesites span ranges of sio2 56 64 wt mg number 0 64 0 50 with cr and ni contents of 531 106 and 230 21 ppm respectively relative to normal andesites the magnesian andesites form distinct trends on variation diagrams with relatively high th and lree contents uniform yb over a range of mgo more fractionated hree and lower nb thpm and nb lapm ratios niobium enriched basalts and andesites neba nb 7 16 ppm and an al enriched rhyolite adakite suite are associated in space and time with the magnesian andesites nb enriched basalts and andesites are characterized by high tio2 p2o5 th and zr contents variably high zr hf 36 44 ratios and more fractionated hree gd ybcn 1 3 4 1 compared to the normal tholeiitic to calc alkaline basalt andesite suites the adakite suite has the high al al2o3 16 18 wt high la ybcn 21 43 and low yb 0 4 1 2 ppm of archean tonalite trondhjemite granodiorite ttg suites and cenozoic adakites indicative of liquids derived mainly from slab melting the basalt andesite suites are not characterized by normal tholeiitic or calc alkaline fractionation trends of major or trace elements rather compositional trends can be accounted for by some combination of fractional crystallization and variable degrees of metasomatism of the source of basalt and or andesites by adakitic liquids the occurrence of magnesian andesites nb enriched basalts andesites and adakites has been described from certain phanerozoic arcs featuring shallow subduction of young and or hot oceanic lithosphere adakites likely represent slab melts magnesian andesites the product of hybridization of adakite liquids with mantle peridotite and nb enriched basalts andesites melts of the residue from hybridization geological similarities between the late archean wawa greenstone belts and certain cenozoic transpressional orogens with the ma neba adakite association suggest that subduction of young hot oceanic lithosphere may have played an important role in the production of this arc related association in the late archean

configuration of the late paleoproterozoic supercontinent columbia insights from radiating mafic dyke swarms abstract remnants of 2 1 1 8 ga orogens can be recognized in nearly every craton assembled within reconstructions of the rodinia supercontinent although no particular pattern of laterally extensive orogenic belts emerges these cratons may be the fragments of an older supercontinent formed in response to the collision and accretion of even older cratons during the early earth history paleomagnetic constraints have played a limited role in many previous reconstructions of the early precambrian supercontinent mainly because of the poor age control and a large scatter of the paleomagnetic poles the geometry of giant radiating dyke swarms and orogenic belts provide important constraints for the reconstruction of the pre rodinian supercontinent some early unmetamorphosed and undeformed mafic dyke swarms in north china southern peninsular india and north america share coeval age of 1 85 ga if these continents are assembled the overall pattern exhibited by the mafic dyke swarms appears to constitute a giant radiating swarm with a piercing point between the cuddapah rift in south india and the xiong er aulacogen in north china this suggests that the north china craton indian craton and canadian shield may be fragmented from the same landmass the reconstruction of 1 85 ga giant radiating dyke swarm attempted in this paper suggests that the north china craton india craton and canadian shield were united together to form a landmass within the columbia supercontinent before its extension and break up the 1 90 1 85 ga andean style north orogenic belt suggests that a subduction zone existed on the northern margin of the north china craton wopmay orogenic belt developed in a 1 88 1 84 ga continental volcano plutonic arc which is interpreted as the product of eastward subduction of oceanic lithosphere the transantarctica orogenic belt in east antarctica may be another candidate to be linked with the subduction zone the north orogenic wopmay orogenic and transantarctica orogenic belts could be connected to form a continuous subduction zone the orogenic comparison strengthens the configuration of the columbia supercontinent proposed by the reconstruction of the giant radiating dyke swarm the laurentia west australia and east antarctica were relatively stable from 1 85 ga to 1 20 ga as inferred from the data on dyke swarms magmatism and the paleomagnetism these continents constituted the core of the columbia supercontinent in the late paleoproterozoic time the north china craton and indian craton were however separated from the columbia supercontinent during the mesoproterozoic time

the geology and geochronology of the waifangshan mo quartz vein cluster in eastern qinling china abstract the waifangshan mo quartz vein cluster occurs at the northernmost margin of the qinling orogen the orebodies occur as quartz veins and are controlled by low angle faults we identify three hydrothermal stages characterized by veinlets of quartz pyrite quartz molybdenite pyrite chalcopyrite galena sphalerite and quartz carbonate assemblages respectively the early stage quartz veins were structurally deformed brecciated and filled by non deformed middle or late stage veinlets suggesting that the mineralization is associated with a tectonic transition from compression to extension five molybdenite separates from the zhifang mo deposit yield individual re os ages from 241 2 1 6 to 247 4 2 5 ma with a weighted mean age of 243 8 2 8 ma individual re os isotope ages of four molybdenite samples from the badaogou mo deposit range from 238 9 2 1 to 255 8 2 2 ma with a weighted mean age of 246 10 ma four analyses of molybdenite separates from the xiangchungou mo deposit yield individual re os isotope ages of 243 8 8 8 to 247 7 4 4 ma with a weighted mean age of 246 0 1 1 ma all of these data yield a weighted mean age of 241 9 1 9 ma coeval with the accretionary orogeny we thus conclude that the waifangshan mo quartz veins formed during a transition from continental arc to back arc setting the waifangshan mo quartz veins share similar geological geochemical and geochronological features with the orogenic type systems we propose that the waifangshan mo quartz veins are an orogenic type mo mineralization and possibly represent the deepest member in the crustal continuum model of the orogenic type mineral systems the molybdenite samples from the mo quartz veins in the waifangshan area have low re contents 1 457 to 39 16 ppm suggesting a continental crust dominated source

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

petrogenesis of the early cretaceous funiushan granites on the southern margin of the north china craton implications for the mesozoic geological evolution abstract late mesozoic granitoids are ubiquitous in the southern margin of the north china craton and are keys to the understanding of the mesozoic geological evolution the early cretaceous funiushan granitic pluton in the southern margin of the north china craton is composed of porphyritic biotite monzogranites rocks from the funiushan pluton have high sio2 64 45 73 98 wt na2o 3 19 4 67 wt and k2o 3 76 7 95 wt and low mgo 0 11 1 34 wt they are enriched in rb ba th u and lree la yb n 9 63 45 0 and depleted in nb ta ti and p and have negative eu anomalies eu eu 0 29 0 72 this geochemical feature is similar to those of typical i type granites zircons from the granites were dated using la icp ms and sims and yielded 206pb 238u ages of 115 131 ma they have ehf t values mainly vary from 17 7 to 0 9 and t dm c ages mainly from 2301 to 1118 ma whole rock end t values range from 20 3 to 9 6 and tdm ages from 1 49 to 2 29 ga indicating that the magma was produced by partial melting of the neoarchean to paleoproterozoic crustal rocks as represented by the taihua group basement rocks with minor involvement of mantle derived melts the funiushan pluton is considered to have been formed through a strong crust mantle interaction process under a low pressure condition in an extensional setting due to the lithospheric thinning caused by westward subduction of the paleo pacific oceanic crust

late jurassic to early cretaceous magmatism in the xiong ershan gold district central china implications for gold mineralization and geodynamics the xiong ershan area is the third largest gold producing district in china the late jurassic to early cretaceous magmatism in the xiong ershan area can be divided into two episodes early 165 150 ma and late 138 113 ma laser ablation inductively coupled plasma mass spectrometry la icp ms zircon u pb dating yields ages of 160 7 0 6 ma and 127 2 1 0 ma for the wuzhangshan and huashan monzogranites in the xiong ershan area respectively representing the two magmatic episodes the wuzhangshan monzogranites exhibit adakite like geochemical features e g high sr y ratios low yb and y contents their sr nd hf isotopic compositions are consistent with those of the amphibolites of the taihua group indicating that the wuzhangshan monzogranites were formed from partial melting of the taihua group metamorphic rocks compared to the wuzhangshan rocks the huashan monzogranites have higher mgo cr co and ni contents but lower sr y and fe 3 fe 2 all the samples from the huashan monzogranites plot in the area between the taihua group amphibolite rocks and the mantle rocks in the 87 sr 86 sr t vs e nd t and age vs e hf t diagrams suggesting that the huashan monzogranites were probably generated by mixing of mantle derived magmas and the taihua group metamorphic basement melts the gold mineralization 136 110 ma is coeval with the emplacement of the late episode magmas implying that crustal mantle mixed magma might be a better target for gold mineralization compared to the ancient metamorphic basement melt the data presented in this study further indicate that the transformation of the lithosphere from thickening to thinning in the xiong ershan area probably occurred between 160 ma and 127 ma and that the gold mineralization in this area was probably related to lithospheric thinning

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