neoproterozoic adakitic plutons in the northern margin of the yangtze block china partial melting of a thickened lower crust and implications for secular crustal evolution abstract numerous neoproterozoic felsic and mafic ultramafic intrusions occur in the hannan region at the northern margin of the yangtze block among these the wudumen and erliba plutons consist of granodiorites and have shrimp zircon u pb ages of 735 ma the rocks have high k 2 o 0 8 3 6 wt and na 2 o 4 4 6 4 wt and low mgo 0 4 1 7 wt they also have high sr y 32 209 and la yb n ratios 4 4 38 6 their e nd values range from 0 41 to 0 92 and zircon initial 176 hf 177 hf ratios from 0 282353 to 0 282581 these geochemical features are similar to those of adakitic rocks produced by partial melting of a thickened lower crust our new analytical results combined with the occurrence of voluminous arc related mafic ultramafic intrusions emplaced before 740 ma lead us to propose that the crustal evolution in the northern margin of the yangtze block during neoproterozoic involved 1 rapid crustal growth and thickening by underplating of mafic magmas from the mantle which was modified by materials coming from the subducting oceanic slab from 1 0 to 0 74 ga and 2 partial melting of the thickened lower crust due to a thermal anomaly induced by upwelling of asthenosphere through an oceanic slab window producing the 735 ma adakitic wudumen and erliba plutons our model suggests that the crustal thickness was more than 50 km at the northern margin of the yangtze block at 735 ma and rule out the possibility of a mantle plume impact causing the 735 ma magmatism in the region

an exotic mesoarchean microcontinent the coorg block southern india abstract sandwiched between the dharwar craton in the north and the neoarchean proterozoic crustal blocks to the south the coorg block in southern india is composed dominantly of a suite of arc magmatic rocks including charnockites ttg tonalite trondhjemite granodiorite related granitoid suite and felsic volcanic tuffs together with minor accreted oceanic remnants along the periphery of the block coeval mafic and felsic magmatism with magma mixing and mingling in an arc setting is well represented in the block here we present the petrology geochemistry zircon u pb geochronology and lu hf isotopes of all the major lithologies from this block computation of metamorphic p t conditions from mineral chemical data shows consistent granulite facies p t conditions of 820 870 c and up to 6 kbar our geochemical data from major trace and ree on representative samples of the dominant rock types from the coorg block corroborate an arc related signature with magma generation in a convergent margin setting the zircon data yield weighted mean 207pb 206pb ages of 3153 4 9 to 3184 0 5 5 ma for syenogranites 3170 3 6 8 ma for biotite granite 3275 5 1 ma for trondhjemite 3133 12 to 3163 8 6 9 ma for charnockites 3156 10 to 3158 3 8 2 for mafic enclaves 3161 16 ma for diorite and 3173 16 ma for felsic volcanic tuff an upper intercept age of 3363 59 ma and a lower intercept age of 2896 130 ma on zircons from a charnockite as well as an evaluation of the th u values of the zircon domains against respective 207pb 206pb ages suggest that the mesoarchean magma emplacement which probably ranged from 3 3 to 3 1 ga was immediately followed by metamorphism at ca 3 0 to 2 9 ga the ages of magmatic zircons from the charnockites and their mafic granulite enclaves as well as those from the volcanic tuff and biotite granite are all remarkably consistent and concordant marking ca 3 1 ga as the peak of subduction related crust building in this block within the tectonic milieu of an active convergent margin the majority of zircons from the coorg rocks show hf isotope features typical of crystallization from magmas derived from juvenile sources their hf crustal model ages suggest that the crust building might have also involved partial recycling of basement rocks as old as ca 3 8 ga the crustal blocks in the southern granulite terrane in india preserve strong imprints of major tectonothermal events at 2 5 ga 2 0 ga 0 8 ga and 0 55 ga associated with various subduction accretion collision or rifting events however the coorg block is exceptional with our data suggesting that none of the above events affected this block importantly there is also no record in the coorg block for the 2 5 ga pervasive regional metamorphism that affected all the other blocks in this region the geochronological data raise the intriguing possibility that this block is an exotic entity within the dominantly neoarchean collage in the northern domain of the southern granulite terrane of india the mesoarchean arc related rocks in the coorg block suggest that the magma factories and their tectonic architecture in the early earth were not markedly different from those associated with the modern style plate tectonics

alteration and geochemical patterns in the 3 7 3 8 ga isua greenstone belt west greenland abstract volcanic rocks of the 3 7 3 8 ga isua greenstone belt have undergone heterogeneous poly phase post magmatic alteration resulting in mobilisation of many elements including lree and th in contrast nb ta ti zr y and hree were relatively immobile during alteration processes the degree of element mobility appears to have been controlled by chemical composition primary volcanic structures carbonate and silica enrichment and proximity to ultramafic units and shear zones on the basis of the geochemical characteristics of the least altered samples two distinct associations have been recognised in structurally separated lithotectonic sequences these magmatic associations are a low hfse association tio2 0 20 0 40 wt zr 12 30 ppm nb 0 13 0 80 ppm y 6 14 ppm in the central arc sequence resembling tertiary boninites and a high hfse association tio2 0 50 1 14 wt zr 34 77 ppm nb 1 2 2 7 ppm y 11 28 ppm in the outer and inner arc sequences comparable to phanerozoic island arc basaltic to picritic volcanic rocks these two geochemically distinct sequences were likely to have been juxtaposed as a consequence of phanerozoic style plate tectonic processes operating in the early earth the isua volcanic rocks are characterised by sub chondritic nb ta 10 16 ratios post magmatic alteration can be ruled out as the cause of these low ratios rather these ratios reflect a compositional characteristic of the paleoarchean mantle source s the sub chondritic nb ta ratios may have resulted from either preferential sink of nb into the core as a weakly siderophile element or preferential retention of nb in the subducted oceanic crust during subduction zone petrogenetic processes alternatively the building blocks of the earth were not composed of ci like chondritic material

geological and isotopic geochemical constraints on the evolution of the fuping complex north china craton abstract the neoarchean to paleoproterozoic fuping complex together with the adjacent hengshan and wutaishan complexes form the middle part of the central zone that separates the north china craton into the eastern and western archean continental blocks the fuping complex comprises three major lithological assemblages the 2 50 ga fuping trondhjemite tonalite granodiorite ttg gneisses the wanzi paragneisses and the 2 08 and 2 02 ga nanying granitic gneisses the ttg gneisses contain abundant enclaves of highly metamorphosed supracrustal rocks including 2 70 ga mafic granulites and amphibolites with endt of 3 29 to 3 96 and tdm of 2 72 2 82 ga and are characterized by tdm of 2 76 3 04 ga and a linear correlation between endt 1 64 to 0 96 and 1 nd indicative of mixing of melts derived from the 2 70 ga metabasic rocks and 3 0 ga crustal materials mafic dikes along the chengnanzhuang shear zone were most likely emplaced at 2 31 ga and have endt of 2 24 to 4 78 and tdm of 2 34 2 59 ga the nanying granitic gneisses with endt of 4 04 to 5 24 were most likely related to partial melting of the 2 70 ga supracrustal rocks and or the 2 50 ga ttg gneisses these geological relationships and isotopic data suggest that the fuping complex probably formed from the following five major tectonomagmatic events 1 mafic magmatism and early crustal growth at 2 7 ga with the possible existence of older 3 0 ga crust s 2 collision related crustal thickening underplating of basaltic magma high grade metamorphism crustal anatexis and ttg magmatism at 2 5 ga 3 extensional deformation and emplacement of the mafic dikes and the nanying granitic plutons along the chengnanzhuang shear zone 2 5 to 2 0 ga 4 s type granitic magmatism and associated amphibolite facies metamorphism 2 0 to 1 80 ga and 5 emplacement of granitic pegmatite dikes at 1 80 ga

volcanic history and magmatic evolution of seguam island aleutian island arc alaska new 40 ar 39 ar dating coupled with detailed field mapping stratigraphy and chemical analyses have established an eruptive chronology that reveals and constrains the compositional and volumetric evolution of seguam island in the aleutian island arc alaska sixty new 40 ar 39 ar ages from lavas domes and pyroclastic deposits were obtained using furnace incremental heating techniques on replicate samples of whole rock and ground mass separates and they constrain the duration of pleistocene to holocene subaerial volcanism to 318 k y the 40 ar 39 ar plateau ages indicate that over 85 of the complex 68 km 3 of material was erupted almost continuously between 318 ka and 9 ka at ca 9 ka a stratocone on the eastern half of the island partially collapsed producing a 4 km wide crater rhyolitic dome forming eruptions followed from vents in the newly created crater and were likely contemporaneous with 8 0 km 3 of basaltic and basaltic andesitic effusions from pyre peak and a 1 4 km 3 basaltic eruption from a monogenetic cone on the far eastern end of the island geochemical changes over the last 318 k y are subtle most notably the earliest eruptions from 318 to 142 ka produced no andesite and basalt from this period has larger ranges in zr rb and la yb than younger basalts small volumes of dacitic to rhyolitic magma were produced from basalt by a monotonic crystal liquid fractionation process that varied only slightly in successive eruptive phases over 318 k y we identified minor geochemical changes in magma composition during each of the three main stages of volcanism but overall the monotonic variations in major and trace element compositions of basaltic andesitic to rhyolitic lavas are consistent with an origin via closed system fractional crystallization of basalt using the 40 ar 39 ar geochronology and estimates of individual flow volumes we calculated a time averaged eruptive rate at seguam that is similar to growth rates of other well dated arc volcanoes in the cascades and chilean southern volcanic zone but less than that of mount katmai and mount mageik which are located on the alaska peninsula the eruptive flux at seguam has been highly variable fluctuating more than an order of magnitude from 0 07 km 3 k y during the early history of bimodal volcanism to 1 18 km 3 k y over the past 9 k y

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

zircon u pb geochronology and hf isotopes of major lithologies from the jiaodong terrane implications for the crustal evolution of the eastern block of the north china craton abstract as an important component of the western shandong complex in the eastern block of the north china craton the yishui terrane consists of neoarchean high grade supracrustal rocks granitoid gneisses and charnockites la icp ms zircon u pb dating and hf isotopic analyses on these lithologies have been carried out and the results provide new insights into the neoarchean crustal evolution of the eastern block of the north china craton new zircon dating results reveal that the magmatic precursors of the supracrustal metapelites and granitoid gneisses were generated at 2 54 2 53 ga and 2 57 2 55 ga respectively and the charnockites were emplaced contemporaneously around 2 56 2 53 ga single metamorphic zircon grains and overgrowth rims from these rocks document consistent metamorphic ages at 2 50 ga suggesting that the yishui terrane experienced a high grade metamorphic event at the end of the neoarchean detrital zircons from pelitic gneisses yield 207pb 206pb ages of 2 89 2 65 ga with a major age peak at 2 53 ga and a subordinate age peak at 2 70 ga implying that the sedimentary protoliths of the pelitic gneisses may have been sourced from 2 53 ga rocks in the yishui terrane and 2 70 ga rocks in the adjacent luxi granite greenstone terrane hf isotopic compositions show that the neoarchean magmatic zircons have positive ehf t values ranging from 1 4 to 7 8 and depleted mantle model ages of 2 92 2 60 ga with a prominent peak at 2 8 2 7 ga suggesting that the neoarchean crust was derived mainly from juvenile sources and partly from the recycling of old continental crust in the yishui area combined with previous data from the adjacent luxi granite greenstone terrane and other complexes in the eastern block it is concluded that the neoarchean was an important period of crust accretion in the eastern block of the north china craton

magmatic affinity of modern and ancient subalkaline volcanic rocks determined from trace element discriminant diagrams when dealing with ancient subalkaline volcanic rocks the alkali total iron magnesium afm diagram is of limited use in assigning a tholeiitic versus calc alkaline affinity because these eleme

geochemistry and zircon u pb hf isotopic systematics of the neoarchean yixian fuxin greenstone belt northern margin of the north china craton implications for petrogenesis and tectonic setting abstract the yixian fuxin greenstone belt in the western liaoning metamorphic terrane is situated in the middle segment of the northern margin of the north china craton ncc and is composed mainly of plagioclase amphibolites pyroxene amphibolites and hornblende plagioclase gneisses la icpms zircon u pb isotopic dating of three representative samples of hornblende plagioclase gneisses reveals that the magmatic precursors of the yixian fuxin greenstone belt were formed during 2589 ma 2534 ma and subjected to subsequent 2490 ma metamorphism lu hf isotopic analyses of the zircons dated from the three samples yield positive ehf t values of 8 3 to 2 7 and tdm from 2 76 ga to 2 57 ga the youngest tdm value is close to their crystallization ages suggesting that they were mainly derived from partial melting of the depleted mantle materials these zircon chronological data and lu hf isotopic features indicate an important 2 5 ga episode of crustal growth in the western liaoning metamorphic terrane based on whole rock geochemical characteristics these meta volcanic rocks in the yixian fuxin greenstone belt may be subdivided into four groups samples in group 1 show high mgo contents from 7 19 to 7 93 and cr and ni contents of 208 395 ppm and 113 210 ppm with mg numbers mg of 53 4 to 55 1 and are characterized by slight lree depleted to flat ree patterns la sm n 0 77 1 05 flat patterns on the primitive mantle normalized multiple trace elements diagram and nb ta ratios between 12 3 and 15 9 implying that they were derived from partial melting of depleted mantle source samples in group 2 also display higher mgo 7 52 14 51 cr 372 570 ppm and ni 240 320 ppm contents with high mg 53 5 70 2 low tio2 contents 0 52 0 64 high nb ta ratios 15 8 19 9 and have right declined ree patterns with la sm n ratios from 1 73 to 3 6 and negative nb ta and ti anomalies indicative of their derivation from partial melting of a metasomatized refractory mantle source samples in group 3 show mgo content from 1 81 to 3 56 with mg of 35 9 to 43 5 and high cr 1179 1280 ppm and ni 712 721 ppm contents lower y and yb contents 6 7 9 42 ppm and 0 64 0 88 ppm respectively and higher nb ta ratios 18 4 21 8 and are characterized by the highest la yb n ratios 14 91 22 67 and negative nb ta and ti anomalies revealing that they formed from partial melting of garnet bearing oceanic slab and the slab melts were contaminated by mantle peridotite and subjected to crustal contamination during their ascent samples in group 4 exhibit high mgo content 5 98 10 52 with high mg numbers 52 4 70 8 cr 198 376 ppm and ni 112 177 ppm contents and lower nb ta ratios between 12 7 and 14 6 and have la yb n ratios ranging from 4 12 to 6 78 lower than those of the group 3 and strongly negative nb ta and ti anomalies suggesting that they were derived from partial melting of mantle peridotite metasomatized by slab derived fluids thus the yixian fuxin greenstones display geochemical features of the normal mid ocean ridge basalts n morbs group 1 boninite like rocks group 2 adakite like rocks group 3 and high magnesium andesites hma group 4 suggesting that the protoliths of the greenstone belt might have formed in an active continental margin similar to the andean type orogen

formation age and tectonic setting of the shirengou neoarchean banded iron deposit in eastern hebei province constraints from geochemistry and sims zircon u pb dating abstract the shirengou iron deposit a typical bif deposit located in eastern hebei province in the north china craton is hosted in neoarchean metamorphic rocks the metamorphic rocks include hornblende plagiogneiss plagioclase amphibolite and magnetite quartzite we use contents and ratios of major elements for protolith reconstruction showing that protoliths of the hornblende plagiogneiss and plagioclase amphibolite should be dacite and basaltic andesite respectively based on geology and geochemistry of the hosting rocks we infer that the shirengou bif is an algoma type deposit formed in an arc related basin in the neoarchean constrains from sims u pb dating and oxygen isotopic analysis of zircon from wall rocks and interlayer of the shirengou iron bodies can be summarized as follow 1 igneous zircons from two samples of amphibolite and gneiss yield u pb ages of 2541 21 ma and 2553 31 ma respectively providing the age of the neoarchean protolith because the shirengou bif is an algoma type deposit determined u pb age 2541 2553 ma for zircons from hosting volcanic rocks can constrain the bif forming time 2 metamorphic overgrowth zircons from amphibolite and gneiss give two u pb ages of 2512 13 ma and 2510 21 ma respectively corresponding to the neoarchean matemorphism 3 the zircons with 18o values of 5 9 7 6 also indicate magmatic origin the zircons have 18o values of 6 8 9 9 suggesting that exotic high 18o fluids were involved during metamorphism based on the geology and geochemistry of bif and combing with zircon age and oxygen isotopic systematics we suggest that the shirengou iron deposit was formed in a submarine volcanic setting related to subduction of an oceanic slab

the application of zircon cathodoluminescence imaging th u pb chemistry and u pb ages in interpreting discrete magmatic and high grade metamorphic events in the north china craton at the archean proterozoic boundary abstract two distinct late neoarchean to early paleoproterozoic magmatic events are recognized at the northern margin of the eastern block of the north china craton based on interpretation of zircon structure cathodoluminescence signature th u pb chemistry and age the first event is represented by emplacement of the chaihe trondhjemite and xianjinchang tonalite with magmatic zircon core ages ranging from 2559 2553 7 ma respectively these rocks experienced a subsequent high grade metamorphic event constrained by partial recrystallization of primary magmatic zircon and growth of new zircon rims between 2517 and 2490 ma a second magmatic episode generated by partial melting of crustal rocks during this metamorphic event is represented by the hongshilazi syenogranite with an age of 2502 11 ma the short time interval between initial magmatism and high grade regional metamorphism signifies they were related to the same major tectonothermal event when considered along with evidence of a similar sequence of events at this time 800 km to the south in eastern hebei and the previously established 2500 ma age of granulite facies metamorphism in mafic rocks across the craton it is proposed that a large mantle plume developed beneath the north china craton at the close of the archean

tectonic evolution of the hengshan wutai fuping complexes and its implication for the trans north china orogen abstract the hengshan wutai fuping complexes comprise the central segment of the trans north china orogen tnco in the north china craton ncc the tectonic evolution of these complexes is regarded as representative for the entire tnco although the timing of the events has been controvesial on the basis of new nd isotope and structural data and a re appraisal of published geochemical and geochronological data a refined model for the tectonic evolution in the neoarchean to paleoproterozoic has been proposed a major ocean named the proto north china ocean developed between the western and eastern blocks prior to 2565 ma northwest ward intra oceanic subduction resulted in the formation of a proto arc as represented by the 2565 2540 ma wutai granitoids the proto arc was rifted and a back arc basin propagated and extended to form an intra oceanic arc basin system at about 2540 2525 ma the main products during the development of this system were a morb arc back arc basalt adakite association in the wutai greenstone belt and synchronous granitoids in the wutai complex the main subduction accretion process involving multiple subduction zones occurred at 2525 2475 ma resulting in closure of the wutai back arc basin and the formation of diverse arc magmas although the tectonic history between 2 5 and 1 8 ga remains uncertain final collision between the eastern and western blocks and the buildup of the tnco most likely took place at 1 9 1 8 ga the tectonic evolution of the tnco is characterized by subduction accretion processes and the development of arc basin systems thus it is more likely a cordilleran type orogen than a simple continent continent collisional orogen

neoarchean subduction a case study of arc volcanic rocks in qinglong zhuzhangzi area of the eastern hebei province north china craton abstract we report on volcanic rocks of the qinglong zhuzhangzi area in northeastern eastern hebei province north china craton the supracrustal rock associations in the study area consist of amphibole biotite plagioclase gneiss mica plagioclase gneiss schist biotite k feldspar gneiss pyroxene plagioclase amphibolite and amphibolite twenty five new mafic intermediate to felsic samples can be geochemically classified into four groups group 1 is similar to fore arc low ti tholeiite and is characterized by low sio 2 49 7 wt and tio 2 0 43 wt slightly depleted lree and mree pattern with la yb n ratio of 0 74 gd yb n ratio of 0 82 it most likely derived from depleted mantle in a subduction initiation environment group 2 is akin to primitive arc tholeiites with low sio 2 49 7 51 8 wt slight enrichments of lree la sm n 1 06 1 53 and negative nb anomalies nb la pm 0 50 0 76 group 3 resembles evolved arc tholeiites and displays pronounced enrichments of lree la yb n of 2 38 11 3 la sm n of 1 70 2 86 and negative nb ta anomalies nb la pm 0 14 0 34 except sample 11jd28 4 with that ratio of 1 00 group 2 and 3 formed by partial melting of sub arc depleted mantle wedge modified by differing quantities of slab flux addition group 4 comprises k rich andesite dacites which are distinguished by the highest sio 2 54 0 71 8 wt k contents up to 5 03 wt strong enrichments of lree la yb n ratios of 4 83 19 and negative nb ta anomalies analogous to shoshonitic rocks marking late stage of subduction la icpms zircon u pb and lu hf analyses of evolved arc basalts and k rich andesite dacites display crystallization ages of 2511 2604 ma and single stage hf model ages of 2534 2927 ma most focus on 2650 2800 ma in contrast to 2510 2560 ma arc associations 2604 ma shoshonitic dacites most plausibly represent an older orogenic event collectively supracrustal rocks in the qinglong zhuzhangzi area constitute a series of volcanic arc associations that correspond to an entire subduction cycle ranging from orogen to primitive arc evolved arc and back to an orogenic setting which further demonstrate that by the late archean plate tectonic and subduction processes already closely resembled modern counterparts

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

zircon u pb hf isotopes and geochemistry of neoarchean dioritic trondhjemitic gneisses eastern hebei north china craton constraints on petrogenesis and tectonic implications abstract dioritic trondhjemitic gneisses of neoarchean age are widely distributed throughout the gualanyu shangying area in eastern hebei north china craton these orthogneisses consist of dioritic trondhjemitic and quartz rich trondhjemitic gneisses which were subjected to amphibolite to granulite facies metamorphism la icp ms zircon u pb isotopic dating reveals that the magmatic precursors of these rocks were contemporaneously emplaced from 2535 23 to 2513 8 ma reflecting an important neoarchean dioritic trondhjemitic magmatic event followed by sequential proterozoic tectonothermal events between 2472 60 and 2307 50 ma dioritic and trondhjemitic gneisses exhibit similar zircon hf t values of 1 5 to 5 2 and 2 9 to 5 3 respectively indicating their depleted mantle affinity geochemical analysis reveals that the dioritic gneisses have low sio2 contents 55 4 63 0 various mgo contents 2 07 4 78 and mg 36 07 63 56 and high la yb n ratios 5 45 15 19 in contrast the trondhjemitic gneisses exhibit higher sio2 contents 67 4 73 8 and la yb n ratios 12 86 76 73 and lower mgo contents 0 66 2 67 although their mg 36 29 61 41 are similar to those of dioritic gneisses these geochemical and zircon lu hf isotopic features together with geochemical modeling suggest that the dioritic magmas were derived mainly from the partial melting of a depleted mantle wedge that was metasomatized by slab derived melts or fluids and that the trondhjemitic magmas were formed by fractional crystallization of the dioritic magmas quartz rich trondhjemitic gneisses exhibit extremely high sio2 contents 77 4 84 6 with zircon hf t values ranging from 3 4 to 4 1 suggesting that their precursors formed after the partial melting of a mixed magmatic source it is suggested that the precursors comprised a mafic member derived from a mantle wedge altered by oceanic slab melts or fluids and a felsic member derived from the partial melting of ancient crustal material these new data combined with previous studies of contemporaneous metavolcanic rocks in gualanyu shangying area and surrounding areas suggest that the magmatic precursors of these neoarchean dioritic trondhjemitic gneisses formed in a subduction related tectonic environment at an e w strike convergent plate margin furthermore the eastern block of the north china craton had a major lateral accretion of crustal growth in the north margin in the neoarchean whereas vertical crustal growth and recycling under plume tectonic backgrounds occurred in the interior of the eastern block

oceanic plateau model for continental crustal growth in the archaean a case study from the kostomuksha greenstone belt nw baltic shield abstract field studies combined with chemical and isotope data indicate that the kostomuksha greenstone belt in the nw baltic shield consists of two lithotectonic terranes one mafic igneous and the other sedimentary separated by a major shear zone the former contains submarine komatiite basalt lavas and volcaniclastic lithologies and the latter is composed of shelf type rocks and bif komatiitic and basaltic samples yield sm nd and pb pb isochron ages of 2843 39 and 2813 78 ma respectively their trace element compositions resemble those of recent pacific oceanic flood basalts with primitive mantle normalized nb th of 1 5 2 1 and nb la of 1 0 1 5 this is in sharp contrast with island arc and most continental magmas which are characterized by nb th la n 1 calculated initial nd isotope compositions nd t 2 8 to 3 4 plot close to an evolution line previously inferred for major orogens momo which is also consistent with the compositions of recent oceanic plateaux the high liquidus temperatures of the komatiite magmas 1550 c and their al depleted nature require an unusually hot 1770 c mantle source for the lavas 200 c hotter than the ambient mantle at 2 8 ga and are consistent with their formation in a deep mantle plume in equilibrium with residual garnet this plume had the thermal potential to produce oceanic crust with an average thickness of 30 km underlain by a permanently buoyant refractory lithospheric mantle keel nb u ratios in the komatiites and basalts calculated on the basis of th u pb relationships range from 35 to 47 and are thus similar to those observed in modern morb and oib this implies that some magma source regions of the kostomuksha lavas have undergone a degree of continental material extraction comparable with those found in the modern mantle the mafic terrane is interpreted as a remnant of the upper crustal part of an archaean oceanic plateau when the newly formed plateau reached the active continental margin its upper part collided with the sedimentary terrane but was too buoyant to subduct as a result the volcanic section of the plateau was imbricated and obducted thus becoming a new segment of continental crust the deeper zones were delaminated and tectonically underplated or subducted

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

generation of felsic crust in the archean a geodynamic modeling perspective abstract as a consequence of secular cooling of the earth there is generally no modern analog to assist in understanding the tectonic style that may have operated in the archean higher mantle temperatures and higher radiogenic heat production in the archean earth would have impacted the thickness and composition of the crust for this reason well constrained numerical modeling based on the fragmentary evidence preserved in the geological record is the most appropriate tool to evaluate hypotheses of archean crust formation the main lithology of archean gray gneiss complexes is the sodic tonalite trondhjemite granodiorite ttg suite melting of hydrated basalt at garnet amphibolite granulite or eclogite facies conditions is considered to be the dominant process that generated the archean ttgs taking into account geochemical signatures of possible mantle contributions to some ttgs models proposed for the formation of archean crust include subduction melting at the bottom of thickened continental crust and fractional crystallization of mantle derived melts under water saturated conditions we evaluated these hypotheses using a 2d coupled petrological thermomechanical tectono magmatic numerical model with initial conditions appropriate to the eoarchean mesoarchean based on the result of our experiments we identify three tectonic processes by which intermediate to felsic melts may be generated from hydrated primitive basaltic crust 1 delamination and dripping of the lower mafic crust into the mantle 2 local thickening of the crust and 3 small scale crustal overturns in the context of a stagnant deformable lid tectono magmatic geodynamic regime that is terminated by short lived subduction we identify two distinct types of continental crust the first type is a pristine granite greenstone like crust with dome and keel geometry formed over delaminating upwelling mantle which is mostly subjected to vertical tectonics processes by contrast the second type is a reworked accreted crust comprising strongly deformed granite greenstone and subduction related sequences and subjected to both strong horizontal shortening and vertical tectonics processes thus our study has identified a possible spatial and temporal transition from pristine granite greenstone like crust with dome and keel geometry to reworked accreted crust forming more felsic gneiss terranes in the archean we suggest that the contemporaneity of the proposed mechanisms can explain the variety and complexity of the archean geological record

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

shrimp u pb zircon geochronology of 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

neoarchean andean type active continental margin in the northeastern north china craton geochemical and geochronological evidence from metavolcanic rocks in the jiapigou granite greenstone belt southern jilin province abstract the jiapigou granite greenstone belt of southern jilin province is located at the northeastern margin of the north china craton ncc the neoarchean metavolcanic rocks of this region provide strong lines of evidence in petrology and geochemistry for neoarchean crust mantle geodynamic evolution and significant crustal growth events the metavolcanic rock assemblage is dominated by amphibolite hornblende garnet biotite plagioclase gneiss and minor felsic gneiss zircon la icp ms u pb isotopic analyses reveal that these supracrustal rocks may be formed by two episodes of magmatism the first magmatic episode occurred at 2688 ma and was recorded by minor felsic gneiss the second episode occurred at 2588 2536 ma and was marked by a major lithological assemblage of metavolcanic rocks in this area all the supracrustal rocks experienced amphibolite facies metamorphism at 2510 2480 ma except for the felsic gneiss seventeen metavolcanic rock samples can be geochemically classified into three groups group 1 has tholeiitic basaltic chemical composition and is characterized by slightly lree enriched chondrite normalized ree pattern with low la yb n ratio and weakly negative nb anomalies similar to those of primitive arc tholeiite their magmatic precursor was most likely generated by partial melting of upper sub arc mantle wedge slightly metasomatized by slab derived fluids or melts in a subduction initiation environment group 2 with basaltic composition is akin to typical calc alkaline basalt cab showing fractionated chondrite normalized ree patterns and evidently negative nb ta and ti anomalies the magmatic precursor of the group 2 was derived from partial melting of a sub arc mantle wedge at relatively deeper levels significantly metasomatized by slab derived fluids or melts group 3 show chemical composition of andesite to dacite and is distinguished by wide variations in sio2 mgo and mg values strongly fractionated chondrite normalized ree patterns with higher la yb n ratios low yb and y contents and negative nb ta ti and p anomalies analogous to the phanerozoic adakites they might be derived from magma mixing between the mantle wedge melts metasomatized by slab derived melts with perhaps an ancient crustal contribution the felsic gneiss shows positive zircon ehf t2 values 2 2 to 4 5 together with 2 7 ga metamorphic volcanic rocks in the neighboring area indicating a 2 7 ga crustal growth episode most of the metavolcanic rocks display positive ehf t2 values of 0 4 to 7 4 the highest of which is close to the value of contemporaneous depleted mantle suggesting a significant crustal growth event at 2 59 2 54 ga combined with numerous 2 83 2 65 ga inherited zircons and 2688 ma felsic gneiss the jiapigou granite greenstone belt evolved at an andean type active continental margin recording the transition from primitive arc to mature arc

paleoproterozoic arc magmatism in the north china craton no siderian global plate tectonic shutdown abstract arc magmatism in convergent plate margins has been a major contributor to continental growth following arc arc and arc continent collisions in the archean leading to the amalgamation of micro blocks the north china craton ncc witnessed major pulses of continental arc magmatism during the paleoproterozoic in this study we present geochemistry zircon u pb geochronology and lu hf isotope data from a suite of magmatic rocks sampled from the region of confluence of two major paleoproterozoic suture zones in the ncc the inner mongolia suture zone imsz and the trans north china orogen tnco our zircon u pb geochronological data indicate new zircon growth during multiple tectonothermal events as displayed in the 207pb 206pb weighted mean ages of 2410 41 ma for metagranite 2480 12 ma 2125 18 ma 1946 8 ma 1900 15 ma and 1879 12 ma from metagabbros 2446 11 ma from charnockite and 1904 6 ma and 1901 9 ma from metatuffs the 207pb 206pb upper intercept age of zircons in the khondalite shows 2102 76 ma which is identical to the age obtained from the magmatic zircons in one of the metagabbros the khondalites also carry a group of concordant metamorphic zircons with 207pb 206pb mean age of 1881 20 ma metamorphic zircons in the gabbros and charnockites also yield similar ages of 1890 14 ma and 1852 19 ma respectively the age data suggest prolonged arc magmatism in a convergent margin setting during ca 2 48 to 1 9 ga followed by metamorphism at ca 1 89 1 85 ga associated with the final collision lu hf analyses reveal that the dominant populations of zircons from all the rock types are characterized by positive ehf values 1 9 to 6 8 mean 1 8 the ehf and tdmc data suggest that the magmas were mostly derived from neoarchean and paleoproterozoic juvenile components the salient geochemical features of these rocks attest to magma generation from heterogeneous sources involving subduction derived arc components with minor input from continental crust the results presented in this study together with those from previous investigations in different domains of the imsz and tnco suggest major paleoproterozoic arc magmatic events in the ncc lasting for nearly 600 million years associated with the final assembly of the crustal blocks into a coherent craton construction of the final cratonic architecture of the ncc thus witnessed not only the arc continent amalgamations at 2 7 2 5 ga but also major crust building events in the paleoproterozoic through melts generated from juvenile and recycled components in continental magmatic arc systems along an active convergent margin followed by intense deformation and metamorphism during the final collision at 1 85 1 80 ga the prominent paleoproterozoic magmatic records in the ncc do not support the proposal of global plate tectonic shutdown in the siderian and confirm vigorous convergent margin magmatism and crust building processes throughout the paleoproterozoic

juvenile magmatism and crustal recycling at the end of the neoarchean in western shandong province north china craton evidence from shrimp zircon dating western shangdong province experienced major crustal growth as a result of 2 75 to 2 50 ga tectonothermal events different from other archean areas in the north china craton besides early neoarchean tonalite trondhjemite granodiorite ttg and supracrustal assemblages there are large volumes of late neoarchean gabbroic dioritic and granitic rocks in western shandong shrimp zircon dating of thirty one samples of different rock types from late neoarchean rocks yielded a narrow age range from 2560 to 2490 ma based on these data and previously published results the following conclusions can be drawn 1 the archean basement can be divided into three belts a late neoarchean 2525 2490 ma crustally derived granite belt in the northeast an early neoarchean 2 75 2 60 ga rock belt in the center and a late neoarchean 2550 2500 ma belt of juvenile rocks in the southwest 2 the tectonic regime in western shandong province probably changed from compressional to extensional at around 2525 ma between 2530 and 2520 ma 3 crustal recycling with addition from the mantle occurred at the end of the neoarchean 4 the late archean magmatic rocks were probably formed in an arc environment

zircon u pb hf isotopes and geochemistry of two contrasting neoarchean charnockitic rock series in eastern hebei north china craton implications for petrogenesis and tectonic setting abstract the zunhua qinglong microblock in eastern hebei north china craton exposes two contrasting charnockitic rock series in the yuhuzhai taipingzhai and cuizhangzi areas charnockitic plagioclase gneiss series with chemical compositions of dioritic and tonalitic rocks and charnockite series showing tonalitic and granodioritic components petrographic observation reveals that two types of orthopyroxenes are preserved in these rocks medium fine grained anhedral orthopyroxenes are preserved in both series and coarse grained subhedral orthopyroxenes are preserved only in the charnockite series in situ trace element analyses reveal that the coarse grained subhedral orthopyroxenes exhibit typical chemical features of magmatic orthopyroxene whereas the medium fine grained anhedral orthopyroxenes display lower medium to heavy rare earth elements ree and no obvious eu anomalies resulting in left inclined ree patterns combined with petrographic features these new data indicate that the medium fine grained anhedral orthopyroxenes were formed by granulite facies metamorphism caused by dehydration reactions whereas the coarse grained subhedral orthopyroxenes were directly crystallized from their magmatic precursors laser ablation inductively coupled plasma mass spectrometry la icp ms zircon u pb isotopic dating reveals that the magmatic precursors of the charnockitic plagioclase gneiss series were emplaced between 2530 17 and 2523 12 ma and those of the charnockite series were emplaced between 2527 28 and 2515 22 ma we conclude that these magmatic precursors are contemporaneous emplacements of two neoarchean granitoid magmatic events which were followed by regional granulite facies metamorphism at 2 45 ga the charnockitic plagioclase gneiss series and charnockite series exhibit similar hf t values of 2 4 to 5 3 and 3 2 to 5 5 indicating either depleted mantle or juvenile crust affinity and involvement of ancient crustal components the charnockitic plagioclase gneiss series exhibit relatively low sio2 contents 54 1 65 2 with various mgo contents 1 93 5 36 mg 0 36 0 55 and la yb n values 3 49 56 00 the charnockite series exhibit higher sio2 contents 60 1 69 2 with similar la yb n values 14 61 42 69 mgo contents 2 10 3 99 and mg 0 29 0 52 these geochemical and zircon lu hf isotopic features together with chemical modeling suggest that the magmatic precursors of the charnockitic plagioclase gneiss series may have been primarily derived from the partial melting of a depleted mantle that was metasomatized by slab derived melts fluids and experienced fractional crystallization processes with hornblende and plagioclase as primary fractionated phases the aforementioned features further suggest that the charnockite series originated from a partial melting of the subducted slab and it was strongly contaminated by mantle peridotites these new data combined with the previous studies of neoarchean basement rocks in the zunhua qinglong microblock in eastern hebei suggest that the magmatic precursors of these two contrasting charnockitic rock series were formed in a back arc related tectonic environment at a convergent plate margin

early neoarchean 2 7 ga tectono thermal events in the north china craton a synthesis abstract the north china craton ncc is characterized by major 2 5 ga tectono thermal events and is thus different from many other cratons worldwide where 2 7 ga events are well developed however whole rock nd and hf in zircon isotopic compositions of 2 5 ga crustally derived granitoids reveal that they mainly formed through reworking of late mesoarchean to early neoarchean continental material we review the spatial distribution rock types geochemical and nd hf isotopic compositions of 2 7 ga granitiods that are widely identified in the ncc including eastern shandong western shandong huoqiu hengshan fuping zanhuang zhongtiao and wuchuan these granitoids are mainly tonalitic in composition and frequently underwent strong metamorphism deformation and anatexis they show large variations in sio 2 total feo mgo and cao and can be subdivided into two types in terms of their ree patterns more importantly whole rock nd and hf in zircon isotopic compositions indicate that the strong 2 7 ga tectono thermal event mainly involved juvenile additions to the continental crust the early neoarchean was the most important period for rapid production of new continental crust in the ncc the main difference of the ncc from many other cratons worldwide is a strong and widespread superimposed tectono thermal event at 2 5 ga that stabilized the cratonic assemblage

qingyuan high grade granite greenstone terrain in the eastern north china craton root of a neoarchaean arc abstract the qingyuan high grade granite greenstone terrain is one of the first recognized granite greenstone terrains in the eastern north china craton similar to other archaean terrains its tectonic environment is debated was it an intra continental rift or a modern analogy of arc occurrence and petrogenesis of major igneous series the ultramafic mafic and felsic volcanic rocks 2510 ma and the plutons of the quartz diorite 2570 2510 ma ttg 2570 2510 ma and quartz monzodiorite 2510 2490 ma series in xinbin area are investigated the mafic intrusives and volcanic rocks have high mgo content 5 4 7 5 wt and mg number 48 61 they show slightly depleted to flat trace element patterns the ultramafic rocks serpentinite could be genetically related to the mafic rocks the meta dacite rhyolite is adakitic with enriched light and middle rees and liles but significantly depleted hfses the quartz diorite has high mg number 60 64 moderately enriched light and middle rees and liles the ttg shows distinct light but moderate middle ree enrichment prominent nb ta depletion but zr hf enrichment zr sm 100 the quartz monzodiorite has moderate light and middle ree enrichment significant nb ta depletion and negative eu sr anomalies the ttg has more depleted sr nd isotopes endt 2 6 87sr 86srt 0 700 than all the others endt 0 2 87sr 86srt 0 701 703 their petrogenesis can be explained by an archaean style subduction defined as a mantle wedge absent flat hot subduction with significant vertical tectonism in the overriding slab the ultramafic mafic rocks were originated from primitive mantle the meta dacite rhyolite was originated from the eclogite facies overriding crust the quartz diorite was a mixture of melts from mantle and the overriding crust the ttg was from the subducting slab under amphibolite to amphibole bearing eclogite facies and the quartz monzodiorite was from the subducting slab after the derivation of the ttg the qingyuan terrain represents remnants of the root of a neoarchaean arc its anticlockwise p t path recorded the initiation of the arc to its cession and exhumation cratonization at 2480 ma

biomass burning recorded in the gisp2 ice core a record from eastern canada combustion byproducts from biomass burning are preserved in annually stratified layers in the greenland ice sheet electrical conductivity ecm and ammonium records from the gisp2 ice core in central greenland have been used to develop a 6000 year record of biomass burning although there is some uncer tainty regarding the source area for the combustion byproducts it is likely that the majority of the material originates in eastern canada time periods of high biomass burning activity occurred in 0 150 bp 350 750 bp and 5000 to 6000 bp the period 1150 3250 bp is characterized by moderately elevated levels of biomass burning activity these periods are associated with drier conditions the 5000 to 6000 bp period of high biomass burning activity may also be related to forest successional sequences following the removal of the laurentide ice sheet this paper explains how ice cores can be used to develop records of biomass burning and offers suggestions for future studies

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

microblock amalgamation in the north china craton evidence from neoarchaean magmatic suite in the western margin of the jiaoliao block abstract the archaean earth is considered to have been characterized by microcontinents that formed dominantly through the accretion of oceanic arcs and plateaus the north china craton ncc provides a typical case where at least seven ancient microcontinental nuclei with distinct lithological features and independent tectonic histories were amalgamated into the cratonic framework at the end of the archaean here we investigate a suite of magmatic rocks developed at the periphery of one of these microblocks the jiaoliao block that forms part of the composite eastern block of the ncc we present petrological geochemical and zircon u pb geochronological data from the taipingzhai charnockite suite and associated amphibolites metagabbros and orthogneisses from the qianxi complex geochemically the rocks show a wide range of sio 2 charnockite suite 52 57 75 50 wt metagabbro 43 71 wt amphibolite 50 24 wt garnet bearing biotite 63 73 wt and mgo charnockite suite 0 89 5 01 wt metagabbro 3 99 wt amphibolite 6 23 wt garnet bearing biotite 2 08 wt the composition of the felsic units straddle from diorite through syeno diorite to granite with both alkalic and subalkalic affinity with dominantly magnesian composition and arc related features their immobile trace element relationships suggest calc alkaline affinity they show positive pb ba la nd and gd and negative nb ta sr th and ti anomalies with slightly negative anomalies of ce and y attesting to arc related features in tectonic classification diagrams the rocks plot in the vag syn colg field or the vag area suggesting subduction related origin the dominant population of zircons in all these rocks displays magmatic crystallization features including high th u values with core rims textures indicating subsequent thermal events the zircon u pb data yield upper intercept ages of 2587 10 ma to 2543 17 ma and 207 pb 206 pb mean ages of 2578 7 3 ma to 2536 8 ma for the charnockite suite marking the timing of emplacement of the arc magmas the overgrowth rims as well as discrete neoformed grains are interpreted as dating subsequent metamorphism and yield 207 pb 206 pb ages between 2533 ma to 2490 ma zircons in the metagabbro preserve upper intercept ages of 2556 20 ma representing the crystallization age of this rock the younger ages of 2449 58 ma upper intercept age and 1845 25 ma 207 pb 206 pb spot age are interpreted to represent subsequent multiple thermal events in this area zircons in the amphibolite preserve the 207 pb 206 pb mean age of 2539 9 ma representing the crystallization age of this rock the garnet bearing biotite gneiss shows an upper intercept age of 2562 10 ma mswd 0 66 n 36 and the 207 pb 206 pb mean age of 2561 9 ma mswd 0 63 n 33 which is taken to represent the crystallization age of this rock some inherited zircons are also identified with 207 pb 206 pb ages of 2664 26 ma and 2628 26 ma zircon lu hf data show dominantly positive ehf t values and combined with crustal residence ages the results suggest mesoarchean to neoarchean juvenile crust formation in the ncc we interpret the data presented here to represent a phase of major late neoarchaean arc magmatism along the western margin of the jiaoliao block related to the birth of microcontinental nuclei within the ncc our data suggest that the western and eastern blocks might not have existed as discrete crustal blocks and that the construction of the ncc is a result of the assembly of several microblocks or terranes at the end of archaean similar archean cratonic nuclei in other regions of the world might have formed part of a primitive supercontinent in the early earth

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

early earth a new recipe for old crust the processes responsible for the growth of earth s first continents are enigmatic the geochemical signature of 4 billion year old rocks discovered in canada points to a key role for shallow magmatic processes above upwelling mantle rocks

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

geochemistry of the paleoproterozonic nanying granitic gneisses in the fuping complex implications for the tectonic evolution of the central zone north china craton abstract the nanying granitic gneisses represent a major 2 0 ga magmatic event in the fuping complex of the central zone of the north china craton they occur in the chengnanzhuang shear zone which is chiefly developed along the contact between the neoarchean fuping tonalite trondhjemite granodiorite ttg gneisses and the wanzi paragneisses the nanying granitic gneisses were derived from monzogranite and syenogranite with minor amounts of quartz diorite and granodiorite and belong to the metaluminous to peraluminous high potassium calc alkaline series correlations between major and trace elements and similarities in chondrite normalized rare earth element ree patterns support field relationships e g gradational contacts indicating that the nanying granitic gneisses originated from similar magmas the granodioritic monzogranitic and syenogranitic gneisses have similar ranges of initial 87sr 86sr values 0 7030 0 7093 0 7021 0 7193 and 0 7049 0 7111 respectively although these variations are partly attributable to local resetting of the isotopic system during late deformation hydrothermal alteration the whole rock 208pb 204pb 206pb 204pb and 207pb 204pb values of the nanying granitic gneisses are 37 32 76 8 15 24 23 82 and 14 92 17 92 respectively these sr pb isotope systematics together with previously published nd isotope compositions suggest that the nanying granitic gneisses were derived from partal melting of the 2 50 ga fuping ttg gneisses with local assimilation involving the wanzi paragneisses the nanying granitic gneisses are geochemically similar to the cca and ccl groups of granitoids except for a few samples which plot in the hlo group the nanying granitic gneisses exhibit depletion in rb sr ba ti and u and enrichment in th nb zr hf and ree similar to those of syn shear high potassium calc alkaline granitoids and late shear alkaline granitoids they therefore represent the product of a syn collisional to post collisional magmatic event this collisional event which is significantly older than the 1 84 1 80 ga event that has been proposed for the juxtaposition of the eastern and western archean continental blocks in the final stabilization of the north china craton supports a tectonic model involving multiple collisional events for the amalgamation of the internal lithotectonic domains within the central zone and the assembly of the north china craton as a whole