revisiting the lushan taihua complex new perspectives on the late mesoarchean early neoarchean crustal evolution of the southern north china craton abstract the taihua complex in the lushan area was divided into gneisses series and supracrustal rocks the former is mainly composed of ttg gneisses and amphibolites and records the late mesoarchean early neoarchean crustal evolution of the southern north china craton ncc late mesoarchean amphibolites 2 84 ga are enriched in light rare earth elements lrees and large ion lithophile elements liles with negative nb ta and ti anomalies derived from partial melting of a metasomatic mantle source late mesoarchean ttg gneisses 2 81 ga have high la yb and sr y ratios and low mgo cr and ni contents and are interpreted as resulting from partial melting of a thickened lower crust with residual garnet and amphibole in the source early neoarchean amphibolites 2 80 2 73 ga show flat ree patterns but slightly negative nb ta and ti anomalies due to the fractionation of ti rich minerals and have depleted hf nd isotopes and were derived from large degree partial melting of a depleted mantle source most of the early neoarchean ttg gneisses show pronounced positive eu anomalies and have relatively lower trace element concentrations and more depleted nd hf isotopes than the late mesoarchean ttg gneisses resulted from the partial melting of thickened juvenile lower crust with residual garnet and amphibole in the source and subsequent plagioclase accumulation in addition some of the early neoarchean ttg gneisses have high mgo cr and ni contents which were likely derived from partial melting of delaminated lower crust or a thickened lower crust interacting with mafic melt late mesoarchean amphibolites and ttg gneisses are restricted in the center of lushan area denoting an ancient continental nucleus in contrast early neoarchean amphibolites and ttg gneisses outcrop in peripheral of the ancient nucleus exhibiting as an elongated belt along the dangzehe river which is evidence for the subduction overall the lushan amphibolites and ttg gneisses show slightly negative to positive whole rock end t and ehf t values and zircon ehf t values indicating a dominant crustal accretion with subordinate crustal reworking therefore the continental growth recorded in the lushan taihua complex was caused by the subduction accretion process suggesting a convergent margin in the southern ncc during the late mesoarchean and early neoarchean

clastic sedimentation in a late archaean accretionary terrain midlands greenstone belt zimbabwe abstract late archaean 2 68 ga clastic sedimentary rocks of the shamvaian group form a shear zone bounded lithotectonic unit in the midlands greenstone belt mgb of zimbabwe the sedimentary rocks are wedged in between mid archaean granitoid gneisses and late archaean mafic and bimodal volcanic rocks the 2000 m thick sedimentary sequence consists of marine turbidite deposits overlain by high energy shelf sandstone fluvial braid plain pebbly sandstone and alluvial fan conglomerate deposition took place in a northward deepening basin with fluvial systems draining a source dominated by granitoid gneiss and banded chert basin formation was associated with a west directed thrusting event and the basin fill was incorporated into the thrust stack soon after deposition the shamvaian group is similar to modern foreland and forearc successions and possibly accumulated during collision of a continental fragment with an island arc like complex

continental growth at convergent margins facing large ocean basins a case study from mesozoic convergent margin basins of baja california mexico abstract mesozoic rocks of the baja california peninsula form one of the most areally extensive best exposed longest lived 160 my least tectonized and least metamorphosed convergent margin basin complexes in the world this convergent margin shows an evolutionary trend that may be typical of arc systems facing large ocean basins a progression from highly extensional phase 1 through mildly extensional phase 2 to compressional phase 3 strain regimes this trend is largely due to the progressively decreasing age of lithosphere that is subducted which causes a gradual decrease in slab dip angle and concomitant increase in coupling between lower and upper plates as well as progressive inboard migration of the arc axis this paper emphasizes the usefulness of sedimentary and volcanic basin analysis for reconstructing the tectonic evolution of a convergent continental margin phase 1 consists of late triassic to late jurassic oceanic intra arc to backarc basins that were isolated from continental sediment sources new progressively widening basins were created by arc rifting and sea floor spreading and these were largely filled with progradational backarc arc apron deposits that record the growth of adjacent volcanoes up to and above sea level inboard migration of the backarc spreading center ultimately results in renewed arc rifting producing an influx of silicic pyroclastics to the backarc basin rifting succeeds in conversion of the active backarc basin into a remnant backarc basin which is blanketed by epiclastic sands phase 1 oceanic arc backarc terranes were amalgamated by late jurassic sinistral strike slip faults they form the forearc substrate for phase 2 indicating inboard migration of the arc axis due to decrease in slab dip phase 2 consists of early cretaceous extensional fringing arc basins adjacent to a continent phase 2 forearc basins consist of grabens that stepped downward toward the trench filled with coarse grained slope apron deposits phase 2 intra arc basins show a cycle of 1 arc extension characterized by intermediate to silicic explosive and effusive volcanism culminating in caldera forming silicic ignimbrite eruptions followed by 2 arc rifting characterized by widespread dike swarms and extensive mafic lavas and hyaloclastites this extensional rifting cycle was followed by mid cretaceous backarc basin closure and thrusting of the fringing arc beneath the edge of the continent caused by a decrease in slab dip as well as a possible increase in convergence rate phase 2 fringing arc terranes form the substrate for phase 3 which consists of a late cretaceous high standing compressional continental arc that migrated inboard with time strongly coupled subduction resulted in accretion of blueschist metamorphic rocks with development of a broad residual forearc basin behind the growing accretionary wedge and development of extensional forearc trench slope basins atop the gravitationally collapsing accretionary wedge inboard of this ongoing phase 3 strongly coupled subduction together with oblique convergence resulted in development of forearc strike slip basins upon arc basement the modern earth is strongly biased toward long lived arc trench systems which are compressional therefore evolutionary models for convergent margins must be constructed from well preserved ancient examples like baja california this convergent margin is typical of many others where the early to middle stages of convergence phases 1 and 2 create nonsubductable arc ophiolite terranes and their basin fills in the upper plate these become accreted to the continental margin in the late stage of convergence phase 3 resulting in significant continental growth

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

a high resolution14c dated sediment sequence from southwest sweden age comparisons between different components of the sediment a macrofossil rich glaciomarine marine lacustrine sediment from a soft water lake in southwestern sweden has provided an opportunity to 14c date different components of its sediments bulk sediment dates are 100 to 500 yr older than fragile terrestrial macrofossils of corresponding levels with a mean age difference of ca 300 yr this is explained by the presence of old and reworked organic material in the sediment five age comparisons between terrestrial macrofossils and periostraca of marine bivalves probably arctica islandica of allerod age give a mean age difference of 380 yr i e slightly greater than the present reservoir age of 340 30 yr for the swedish west coast this difference is roughly the same as for the marine macroalga desmarestia aculeata a date from shell carbonate fragments of mytilus edulis yields an age that is more than 1000 yr older than corresponding periostraca and terrestrial macrofossils and 500 1000 yr older than the age of the supposed deglaciation of the site altogether this indicates a larger marine reservoir effect during the allerod than at present 1998 john wiley sons ltd

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

late paleoproterozoic 1900 1800ma nappe stacking and polyphase deformation in the hengshan wutaishan area implications for the understanding of the trans north china belt north china craton abstract the hengshan wutaishan area forms part of the trans north china belt developed in response to the final amalgamation of the north china craton the hengshan wutaishan domain and the adjacent fuping massif constitute the largest and the most representative rock exposure of the belt a structural study allows us to redefine the lithological and tectonic units that compose the architecture of the hengshan wutaishan domain and to propose that the trans north china belt was built up through a polyphase tectonic evolution within the period 1900 1800 ma the first event d1 corresponds to the emplacement of lower and upper nappes herein called the orthogneiss and volcanites unit ovu and the low grade mafic unit lgmu respectively the ovu mainly consists of rocks formed in an arc system and metamorphosed under amphibolite facies conditions whereas the lgmu represents oceanic crust rocks with an arc component that underwent greenschist facies metamorphism the syn metamorphic d1 deformation is characterized by a nw se stretching and mineral lineation with a top to the se sense of shear that corresponds to the sense of nappe motion u th pb epm chemical dating on monazite from three metapelites of the ovu gives isochron ages of 1884 11 ma 1886 5 ma and 1887 4 ma interpreted as the age of the prograde amphibolite facies metamorphism coeval with nappe stacking during the d1 event the d2 event corresponds to a widespread crustal melting of the ovu and underlying basement rocks in hengshan related to the exhumation of deep structures about 20 30 ma after nappe emplacement the migmatite contains blocks of retrograded eclogite or hp granulitic restites and is characterized by the development of a pervasive foliation and lineation a late d2 event represents the folding stage of the migmatitic foliation during the completion of a dome the orthogneiss and volcanics unit and the low grade mafic unit are unconformably covered by weakly metamorphosed to unmetamorphosed sediments belonging to the hutuo supergroup these rocks were deformed by a ductile d3 event characterized by a nw se trending stretching lineation south verging folds associated with an axial planar slaty cleavage and an e w crenulation lineation lastly a left lateral wrenching along the zhujiafang shear zone separates the northern and southern parts of hengshan our structural study shows that there is no significant difference between the southern part of hengshan and the lower part of wutaishan both areas belong to ovu and show a structural continuity we suggest that this polyphase deformation developed in response to a north westward subduction of an old ocean named the luliang ocean beneath the western block followed by collision with a micro continent named the fuping block at 1890 1880 ma

arc generated metavolcanic rocks in the anshan benxi greenstone belt north china craton constraints from geochemistry and zircon u pb hf isotopic systematics abstract the anshan benxi granite greenstone belt abgb in the eastern block of the north china craton mainly comprises supracrustal rocks of amphibole plagioclase gneisses plagioclase amphibolites chlorite schists biotite schists quartzites and banded iron formations bifs and intrusive monzogranitic to syenogranitic gneisses based on geology spatial distributions lithologies and zircon u pb geochronology the metavolcanic rocks of the southern abgb can be divided into the 2527 ma southern plagioclase gneiss group 2536 ma southern amphibolite group 2574 to 2560 ma northern chlorite schist group and northern amphibolite group from south to north these rocks outcrop as stratified and stratoid beds whereas some occur as enclaves within later intrusive k rich granitic gneisses the southern plagioclase gneiss group exhibits calc alkaline andesite compositions with high mgo cr and ni fractionated ree patterns and depletion in nb ta which resemble high magnesian andesite hma and are derived from the partial melting of sub arc mantle peridotite that was previously metasomatized by slab melts the southern amphibolite group has tholeiitic basalt compositions with slight enrichment in lrees and high nb ti concentrations but low nb la pm ratios which coincide with nb enriched basalt and are generated by the remelting of residues of sub arc mantle after the extraction of hma melts the northern schist group shows high k calc alkaline andesite dacite compositions and have highest silica and potassium and obvious enrichment in lrees and liles and depletion in nb ta which are comparable to the sanukitoid series rocks these rocks probably formed by low degree partial melting of a sub arc mantle wedge that was metasomatized by slab derived fluids the northern amphibolite group exhibits tholeiite compositions with unfractionated ree patterns and low nb la pm ratios which are consistent with back arc basin basalts babb and are produced by partial melting of depleted mantle combined with lithological associations of the hma neb sanukitoid like rocks and babb like rocks the anshan benxi granite greenstone belt was produced under a convergent plate environment both the zircon ehf values and petrogenesis indicate that the four groups of the metamorphic volcanic rocks directly contributed to continental crustal growth integrating with the widespread 2 5 ga syenogranites and subordinate k rich granitoids that were derived from reworking of ancient crust we propose that the abgb experienced a significant episode of continental growth dominated by a subduction related lateral accretion accompanied with contemporaneously strong reworking of ancient crustal materials

ludlow and pridoli upper silurian graptolite biostratigraphy of the central arctic islands a preliminary report a fauna comprising slightly more than 30 species of upper silurian graptolites has been recognized from the arctic islands the following preliminary biostratigraphic biozones are recognizable l

aeromagnetic study of the hengshan wutai fuping region unraveling a crustal profile of the paleoproterozoic trans north china orogen abstract an integrated crustal profile of the intervening trans north china orogen tnco is one of the key issues to understanding the tectonic evolution of the north china craton however the existing geological studies focus only on the surface mapping based petrological geochemical and structural analysis but lack subsurface geophysical evidence and thus make the crustal profile interpretations ambiguous in contrast the current geophysical data covers a very large scale lithospheric mantle and fails to image the detailed structural pattern of the orogenic crust to achieve this goal we present high resolution aeromagnetic data for the hengshan wutai fuping region the largest exposure of the central tnco the reduced to pole magnetic anomaly map firstly verifies the regional tectonic subdivision that the high grade metamorphic terranes i e hengshan and fuping complexes are consistent with high magnetic responses and long wavelength anomalies intervened by a low grade terrane wutai complex characterized by low magnetic responses and short wavelength anomalies 3d euler deconvolution reveals that the tendencies of the clustered solutions show large consistence with the major structural pattern of the region which is characterized by a fan shaped doubly vergent orogenic wedge upward continuation further shows that the northwest part of the orogen yields a thicker crust and is most likely located closer to the paleosubduction zone the new aeromagnetic data combined with structural petrological and metamorphic data indicate that an eastward dipping subduction zone was most possibly active before the collision of the western and eastern blocks leading to the formation of the tnco and the final amalgamation of the north china craton

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

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

heterogeneous zircon cargo in voluminous late paleozoic rhyolites hf o isotope and zr hf records of plutonic to volcanic magma evolution voluminous rhyolitic lavas and ignimbrites c 34 000 km3 were formed in the ne german basin in a post collisional setting at c 295 ma trace elements ehf and 18o have been measured in dated magmatic and inherited rhyolitic zircons from three drill cores across the basin magmatic zircons crystallized in two stages the first stage in less differentiated and isotopically heterogeneous magmas and the second stage in more differentiated and isotopically homogeneous magmas the first and second stages can be related to the crystallization of zircons within a heterogeneous pluton and in an evolved silicic magma that was later erupted some zircons crystallized entirely during the evolved magma stage and provide good resolution for identifying and characterizing processes happening shortly before eruption the isotopic compositions of the magmatic zircons constrain the proportions of juvenile and reworked materials involved in the formation of voluminous silicic magmas in a post collisional tectonic setting the advantage of studying the volcanic rocks from the ne german basin is that their petrogenesis involved a relatively simple two source interaction during magma production which permits quantitative estimation of the amount of each source component afc modelling shows that the first stage zircons crystallized from magmas containing 5 80 of the juvenile component whereas the final rhyolitic magmas contained 30 40 of this material the inherited zircons have hf model ages of 1 9 2 2 ga suggesting that much of the local basement was initially derived from the mantle at that time and that it was subsequently reworked at around 1 5 ga similar model ages are a feature of baltica derived sediments and the implication is that such sediments underlie large areas of the ne german basin the lack of any record of avalonian basement in the ne german basin may indicate that both the sedimentary cover and the underlying basement are part of baltica

crustal structure transition from oceanic arc to continental arc eastern aleutian islands and alaska peninsula abstract the aleutian island arc crosses from the pacific ocean to the north american continent at the island of unimak 3 d finite difference traveltime inversion of our onshore offshore seismic reflection refraction data gives a velocity model of the crust and uppermost mantle the arc crust is on average 30 km thick but thickens to almost 40 km under the western alaska peninsula the transition from oceanic arc to continental arc is characterised by a decrease in average velocity in the upper crust from about 6 5 km s to less than 6 0 km s with no systematic change in the velocity of the lower crust throughout our study area in the upper 15 km of the crust the highest velocities are observed in the fore arc just south of the volcanic line in the lower crust the lowest velocities of just 6 2 km s are found close to the volcanic line the uppermost mantle is quite heterogeneous with velocities ranging from 7 6 to 8 2 km s in part due to the thermal gradient from cold fore arc to hot back arc whereas the aleutian oceanic fore arc has higher seismic velocities than average continental crust throughout the crust the peninsula section is close to the continental average in the upper c 20 km of the crust we infer that repeated episodes of arc magmatism can produce a felsic to intermediate upper crust as is observed in the continents but arc magmatism produces a thicker mafic lower crust than the average continent retains some of the excess mafic material in the island arc crust can be attributed to pre existing oceanic crust which is less evident or absent in a continental arc

crustal growth and intracrustal recycling in the middle segment of the trans north china orogen north china craton a case study of the fuping complex the most important crustal growth on earth occurred at 2 7 ga but the north china craton ncc is characterized by prevalent development of 2 5 ga juvenile crust with relatively rare records of 2 7 ga crustal growth the fuping complex in the middle segment of the trans north china orogen tnco between the eastern and western blocks of the ncc is composed mainly of 2 5 ga fuping tonalitic trondhjemitic granodioritic ttg gneisses and longquanguan augen gneisses 2 1 ga nanying granitic gneisses and the wanzi supracrustal rocks previous studies have suggested one major phase of crustal growth at 2 5 ga possible intracrustal recycling at 2 1 ga and the presence of older rocks in the fuping complex but there has been no record of 2 7 ga crustal growth the fuping ttg gneisses are dominated by stromatic migmatite and new u pb dating of magmatic zircons from two stromatic migmatite samples yielded three different ages 1 2 75 ga which is the oldest age obtained from the fuping ttg gneisses 2 2 54 ga which just falls in the published zircon u pb age range of 2 53 to 2 47 ga for the fuping ttg gneisses and 3 2 11 ga which is almost the same as the age of the nanying granitic gneisses therefore there are two generations of ttg gneisses in the fuping complex importantly both of the 2 75 and 2 54 ga zircons have the highest e hf t values almost equal to the contemporaneous depleted mantle this indicates high contributions of juvenile material to the two generations of ttg gneisses in contrast the 2 11 ga zircons have apparently low e hf t values of 0 47 to 2 04 just falling in between 2 55 and 2 75 ga continental crust values this strongly suggests the reworking of the two generations of ttg gneisses at 2 1 ga zircon u pb and hf isotopes convincingly reveal two major phases of crustal growth in the fuping complex at 2 7 and 2 5 ga the same as in the northern and southern segments of the tnco and also confirm one major phase of intracrustal recycling at 2 1 ga which may be responsible for the nanying granitic gneisses

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

single zircon ages for granitoid gneisses in the central zone of the limpopo belt southern africa and geodynamic significance abstract ion microprobe evaporation and vapour digestion single zircon emplacement ages for granitoid rocks throughout the central zone cz of the limpopo mobile belt south africa define three broad age groups namely at 3188 3314 2637 2734 and ca 2500 ma respectively all these granitoids including the youngest group display polydeformational features suggesting that much of the tectonic history of the cz occurred in the late archaean and early proterozoic the oldest group of predominantly tonalitic trondhjemitic granodioritic gneisses was found in relatively small domains from the extreme w to the e of the cz and probably represents remnants of a once more widespread early archaean terrain the most abundant granitoid gneisses belong to the 2 6 2 7 ga age group and were emplaced into already ductilely deformed gneisses of the oldest group several of these rocks were derived from intracrustal melting rare metamorphic zircons with ages of 2560 2575 ma reflect a high grade metamorphic event in the cz that has also been postulated by other workers a strongly deformed orthogneiss with a protolith emplacement age of 2510 ma proves beyond doubt that at least part of the structural history in the cz occurred in the early proterozoic an anatectic granite in the extreme w of the cz intruded at 2022 ma and is probably related to the peak of granulite facies metamorphism previously dated at 2027 6 ma in the messina area the second period of granitoid magmatism is broadly contemporaneous with charnockite and enderbite emplacement and granulite facies metamorphism in the northern and southern marginal zones of the limpopo belt broad contemporaneity of extensive granitoid magmatism in all three zones raises doubts about the exotic nature of the cz and suggests a revised evolutionary model for the limpopo belt

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

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

episodic crustal growth in the southern segment of the trans north china orogen across the archean proterozoic boundary abstract the dengfeng and taihua complexes are well exposed neoarchean to paleoproterozoic units in the southern segment of the trans north china orogen tnco zircon u pb dating shows that the dengfeng complex records two episodes 2568 11 ma and 2306 6 ma of tonalite trondhjemite granodiorite ttg magmatism all of the ttg rocks are characterized by high sio2 66 7 75 4 wt na2o 3 20 5 06 wt and relatively low mgo 0 40 1 78 wt the late neoarchean ttg gneisses have very low contents of hree ybn 0 69 2 75 and y 1 73 7 07 ppm with moderate la yb n 24 1 53 8 and high sr y 65 1 291 3 ratios the early paleoproterozoic ttg gneisses have low contents of hree ybn 2 93 6 37 and y 6 7 11 0 ppm with moderate la yb n 10 1 27 0 and sr y 10 6 52 1 ratios both suites show pronounced negative nb ta p and ti anomalies but positive sr and pb anomalies the late neoarchean ttg gneisses all have similar bulk rock nd and zircon hf model ages with mainly positive nd t and are interpreted as resulting from the melting of dominantly juvenile thickened lower crust with residual garnet and amphibole the early paleoproterozoic ttg gneisses have extremely variable nd t 6 23 to 4 23 and heterogeneous zircon hf t 3 3 to 3 1 which are also best interpreted as resulting from the partial melting of thickened lower crust with residual amphibole and garnet the taihua complex in the xiaoqinling area records three episodes of early paleoproterozoic ttg magmatism 2 48 ga at caotan 2 31 ga at houjiacun and 2 16 ga at bayuan younger than the taihua complex in the lushan area 2 85 2 72 ga all rocks have relatively low contents of hree ybn 1 03 8 32 and y 2 84 24 ppm with moderate la yb n 8 7 88 4 and sr y 19 8 125 8 ratios and show negative ta nb and ti anomalies and positive sr and pb anomalies the caotan gneisses at 2 48 ga and the houjiacun ttg gneisses at 2 31 ga have low mg 0 14 0 45 low cr the geochronology of the dengfeng complex in the dengfeng area and the taihua complex in the lushan xiong er and xiaoqinling areas reveals at least four magmatic episodes in the southern segment of the tnco from the late mesoarchean to early paleoproterozoic 2 85 2 72 ga 2 57 2 48 ga 2 34 2 30 ga and 2 20 2 07 ga the rocks of the two early episodes are dominantly of juvenile compositions with mostly positive whole rock nd t and zircon hf t values suggesting two episodes of crustal growth formed in a subduction tectonic setting the magmatic rocks of the third episode consist of both the juvenile and pre existing crustal materials with variable whole rock nd t and zircon hf t values which were generated in a subduction zone during the initial assembly of the ncc within the columbia supercontinent cycle the final episode of magmatism lacks juvenile materials with whole rock nd t and zircon hf t values being consistently negative these may have resulted from the orogenic collapse the episodic continental growth recorded in the southern segment of the tnco was caused by subduction and consequent orogeny consistent with global supercontinent cycles within the late archean and early paleoproterozoic

crustal growth in the north china craton at 2 5ga evidence from in situ zircon u pb ages hf isotopes and whole rock geochemistry of the dengfeng complex abstract the dengfeng complex is located on the southern margin of the north china craton ncc and belongs to the southern portion of the trans north china orogen this terrane is important to understand the formation and evolution of ncc during late neoarchean 2 5 ga the dengfeng complex is well exposed in the junzhao region and comprises two distinct lithologic units supracrustal assemblage and plutonic rocks la icpms magmatic zircon u pb age data shows that the rocks formed within the range of 2547 2504 ma the available hf isotope data indicate that the majority of ca 2 5 ga zircons from the dengfeng complex have high e hf t values close to the initial hf isotope ratios of the contemporaneous depleted mantle these data indicate that the rocks in the dengfeng represent juvenile crust the ttg gneisses in the dengfeng complex display low mg 41 48 mgo 59 69 mgo 3 5 6 6 wt ni 82 130 ppm and cr 148 237 ppm abundances elevated sr 1759 1927 ppm and ba 1742 2289 ppm concentrations and high lree la n 38 487 such geochemical features are similar to archean sanukitoids a two stage model is applied here to explain the genesis of metadiorites of dengfeng complex 1 firstly the mantle is metasomatized either by melts or by aqueous fluids from a subducted slab 2 subsequently sanukitoid magmas were produced by partial melting of the hybridized mantle furthermore the amphibolites of supracrustal rocks have a mixture of morb and arc like geochemical affinities suggesting the development of a back arc in the southern ncc at ca 2 5 ga the contemporary late neoarchean ttgs sanukitoids and morb back arc association may represent a late neoarchean tectonic melange implying a neoarchean subduction accretion process which would suggest that modern style plate tectonics processes was probably initiated in the southern ncc by 2 5 ga

zircon u pb ages and lu hf isotope compositions from clastic rocks in the hutuo group further constraints on paleoproterozoic tectonic evolution of the trans north china orogen abstract the hutuo group as one of the classic examples of paleoproterozoic strata plays an important role in establishing evolutionary processes in the trans north china craton in this contribution we present petrologic detrital zircon u pb ages and lu hf isotopes from three subgroups of the hutuo group sandstones in the doucun and dongye subgroups are dominated by q monocrystalline and polycrystalline quartz and f k feldspar and plagioclase with minor lithic fragments suggesting that the detrital components were mainly derived from both the continental block and recycled orogen in contrast clastic components in the guojiazhai subgroup are mainly q and lithic fragments indicating they were derived predominantly from the recycled orogen the ages of detrital zircons from sandstones in the doucun and dongye subgroups are mainly concentrated at ca 2 5 ga and 2 2 2 1 ga with minor 2 7 ga zircons also present which indicates they were dominantly sourced from 2 5 ga wutai fuping and zanhuang complexes and paleoproterozoic intrusives the guojiazhai subgroup displays a different zircon age population of ca 2 4 ga 2 2 2 1 ga and 2 0 1 9 ga which indicates likely derivation from paleoproterozoic intrusives in the wutai luliang and hengshan areas based on the observation that sandstone clastic components in the doucun and dongye subgroups are dominantly quartz feldspar and sedimentary lithic fragments but not volcanic lithics we propose that they were deposited in a rift related setting zircons from the lower sequence of the hutuo group yielded a young population of 2140 ma considering the volcanic rocks of 2140 14 ma at the base of the group and 2 2 2 0 ga magmatism along the tnco we propose that doucun and dongye subgroups formed at 2 2 2 0 ga the youngest ca 1 9 ga zircons in the guojiazhai subgroup indicate this group was deposited during closure of the rift at 1 9 1 8 ga the two stage model ages of the detrital zircons mainly range from 2 6 to 2 9 ga with a minor 2 5 ga contribution therefore we infer that 2 6 2 9 ga represents a period of intensive crustal growth in the trans north china orogen but that some degree of crustal growth continued to 2 5 ga

determining an age for the inararo tuff eruption of mt pinatubo based on correlation with a distal ash layer in core md97 2142 south china sea abstract the largest known eruption of mt pinatubo in the late quaternary was the inararo tuff formation itf eruption roughly estimated as five times larger than the 1991 eruption the precise age of the itf eruption has been uncertain here a correlative of the itf eruption layer d is identified in marine sediments and an age obtained tephras were identified in core md97 2142 of leg ii of the images iii cruise in northern offshore of palawan southeastern south china sea 12 41 33 n 119 27 90 e on the basis of the geochemical and isotopic fingerprints layer d can be correlated with the itf eruption of the modern pinatubo eruption sequence by means of the md97 2142 specmap chronology layer d was dated at around 81 2 ka this estimated age of the itf eruption and tephra layer d coincides with an anomalously high so4 2 spike occurring within the 5 millennia from 79 to 84 ka in the gisp2 ice core record

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

geochemistry zircon u pb geochronology and lu hf isotopes of metavolcanics from eastern hebei reveal neoarchean subduction tectonics in the north china craton abstract the neoarchean metavolcanic rocks of saheqiao occur within the central domain of the precambrian terrane in eastern hebei within the north china craton these rocks consist mainly of garnet two pyroxene granulites garnet clinopyroxene amphibolites amphibole plagioclase gneisses and biotite plagioclase gneisses these metavolcanic rocks are exposed as several kilometers scale of massive blocks or several tens of centimeters scale of enclaves within precambrian granitoid gneisses based on whole rock geochemistry we classify these intermediate mafic volcanic rocks into four groups group 1 exhibits low sio2 yb and nb ta ratios and high ni and cr and shows n morb like normal mid ocean ridge basalts geochemical features suggesting that the magmatic precursors of these rocks were derived through partial melting of depleted mantle compared to group 1 group 2 shows slightly right declined ree and nearly flat multi element patterns with variable negative nb anomalies matching with primitive arc basaltic rocks formed by partial melting of the mantle wedge group 3 exhibits chemical compositions of basaltic and andesitic rocks and is characterized by pronounced lree enrichment but hree depletion and shows distinctly negative nb and ta anomalies and weakly negative zr and hf anomalies these rocks may compare with island arc volcanic rocks and were probably generated by partial melting of the mantle wedge modified by dehydration of subducted oceanic slab the magmatic precursors of the andesites were derived from the crystallization differentiation of the basaltic magma group 4 is characterized by clearly right declined ree patterns and slightly positive or no nb ti anomalies which are analogous to nb enriched basalts neb and originated from melting of hfse enriched mantle sources la icp ms u pb dating of magmatic zircons from five samples from the three groups reveals that group 1 n morb like rocks formed at 2525 7 ma group 2 primitive arc basaltic rocks at 2518 15 ma and group 3 typical arc volcanic rocks at 2523 6 ma to 2614 18 ma zircons from all the dated samples also preserve signatures of 2450 2370 ma metamorphisms or thermal events the zircon lu hf data from the three dated samples in group 3 show ehf t values ranging from 1 4 to 9 0 and tdm values of 2627 ma to 2965 ma suggesting a significant episode of 2 6 2 9 ga crustal growth in this region the 2525 ma n morb like basaltic rocks are younger than the arc related rocks 2614 2518 ma suggesting that the morb like basaltic rocks were generated in a back arc environment on the basis of the rock association of back arc morb basalts group 1 arc related andesites basalts groups 2 and 3 and neb like rocks group 4 we suggest that the neoarchean volcanic rocks of the eastern hebei region formed in a subduction related setting within a convergent plate margin

upb zircon geochronology of late archaean metamorphic rocks in the taihangshan wutaishan area north china abstract a geochronological study has been made in the taihangshan wutaishan area shanxi province on a sequence of precambrian supracrustal rocks and stratigraphically related intrusives these constitute part of the north china basement block the oldest rocks of the fuping group consist predominantly of mafic granulites paragneisses quartzites and amphibolites this group was always considered archaean and early k ar measurements yielded some minimum ages as old as 2300 ma and others as young as 1500 1900 ma the unconformably overlying wutai group is considered as the base of the proterozoic succession and consists of amphibolite grade paragneisses carbonates and amphibolites in its lower part and greenschist grade clastic sediments and volcanics in its upper part it has given apparent k ar mineral ages between 1500 and 2000 ma present investigations have employed the u pb zircon approach on selected parts of the sequences to provide direct depositional ages or age constraints for the fuping and wutai groups and to find the age s of subsequent metamorphic processes a minimum age for the fuping group and maximum age for the wutai group has been derived from zircon age measurements on the lanzhishan granite 2560 9 ma which intrudes the fuping group and is unconformably overlain by the wutai group this late archaean result can be interpreted as the age of an igneous crystallization event and therefore a maximum for the depositional age of the wutai group the above result is well substantiated by zircon dating of the wutai group itself zircons from a low grade metamorphosed keratophyric lava in the upper part of the group give a volcanic crystallization age of 2520 17 ma this provides close constraints for the age of deposition of the wutai group at between 2560 and 2520 ma ago if the wutai group is indeed basal proterozoic the archaean proterozoic boundary in this part of china is also dated at between 2560 and 2520 ma further evidence as to the antiquity of the wutai group comes from a u pb zircon age of 2520 30 ma for the ekou granite which intrudes the wutai group sequence the age of the underlying late archaean rocks in the tainhangshan wutaishan area the fuping group could not be directly determined because of the paucity of volcanic markers and the higher grade of metamorphism its minimum age is 2560 ma see above but only an approximate maximum age can be gleaned from the zircon systematics in granulite and amphibolite grade paragneiss samples these data indicate a degree of pb inheritance which combined with metamorphic disturbance and more recent pb loss inhibit any further interpretation other than that they indicate an approximate maximum age of around 2800 ma several of the paragneisses and one amphibolitic schist contain zircon systems which are interpreted as having completely responded to a well defined amphibolite facies metamorphic event at 2500 ma ago a younger but less well defined metamorphism caused complete pb loss from zircon and recrystallization in the amphibolite and granulite facies terrains at between 2000 and 1800 ma ago rocks which show evidence of this younger metamorphic event were not recognized from the field or structural data base but from the distribution of the few such samples analyzed it seems possible that this younger terrain may now be interpretable as a cross cutting proterozoic mobile belt this proterozoic metamorphic province is also reflected by k ar mica ages between 1600 and 2000 ma and some preliminary rb sr total rock ages at 1700 ma on nearby paragneisses whose u pb zircon age is 2500 ma

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

radiocarbon dating of tephra layers recent progress in japan abstract a large number of tephra layers deposited over the past 40 000 years in japan have been studied by means of newly developed techniques as well as the traditional radiocarbon 14 c method the samples for 14 c dating were commonly charcoal and wood directly covered or involved in tephra fall and pyroclastic flow deposits recently the 14 c dates measured for paleosols directly underlying a tephra layer have proved to represent the accurate eruption age of the tephra and have successfully established the high resolution eruptive histories of several volcanoes in addition oceanic and lacustrine sediments have preserved a large number of tephra layers and accelerator mass spectrometry 14 c measurements on terrestrial macrofossils from varved sediments have provided highly accurate ages for tephra layers wiggle matching of 14 c ages from several horizons of peat sediments to intcal98 calibration data that was established by dendrochronology as well as by varve chronology has enabled us to obtain highly accurate ages of tephras intercalated in the peat deposits

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

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

a model for continental crust genesis by arc accretion rare earth element evidence from the irish caledonides abstract the formation of continental crust is a complex problem with a paradox at its center continental material is believed to form by arc magmatism a model that does not reconcile the bulk mafic and light rare earth element lree depleted composition of intra oceanic arcs with the andesitic lree enriched composition of continents we present evidence supporting an arc origin for continental crust by demonstrating significant changes in magmatic chemistry during arc continent collision we use as an example the connemara region of the western irish caledonides where metamorphism and orogenesis of dalradian sedimentary sequences record the collision of the oceanic lough nafooey arc with the passive margin of laurentia at 475 ma arenig this arc was mafic and lree depleted during its oceanic activity during collision rhyolites more lree enriched than the surrounding continental crust were erupted while the mid crust was intruded by similarly lree enriched gabbro and diorite crystal fractionation caused lree enrichment to exceed that expected by assimilation of continental crust alone we propose that subsequent loss of the corresponding lower crustal cumulates into the mantle removed this lree depleted material from the net crust added to the continent the combined processes of crystal fractionation and lower crustal loss during arc continent collision drive the average bulk arc composition toward that of average continental crust

rb sr and sm nd isotopic dating of an early precambrian spilite keratophyre sequence in the wutaishan area north china preliminary evidence for nd isotopic homogenization in the mafic and felsic lavas during low grade metamorphism the spilite keratophyre sequence in the wutaishan area north china has been dated by zircon from the quartzkeratophyre and a volcanic crystallization age of 2522 ma liu dunyi et al 1985 was obtained this volcanic rock sequence experienced a low grade metamorphism of greenschist facies during 1800 2000 ma however combined sm nd whole rock data from the volcanic rock sequence if interpreted in terms of an isochron gives a younger age of 2250 182 ma if separately calculated sm nd isochrons for quartz keratophyre and spilite yield two consistently younger ages of 1977 385 ma and 1981 178 ma these sm nd isochron ages are consistent with a rb sr age of 1871 98 ma and with the time of regional greenschist facies metamorphism consequently we interpret the 2250 ma alignment on the sm nd diagram as an errorchron the two sm nd metamorphic ages suggest that partial homogenization of nd isotopic composition in water rich spilite keratophyric meta volcanics can be achieved during low grade metamorphism as a result of ree mobility using the nd values at the time of metamorphism and the average 147sm 144nd ratios of the spilite and the quartzkeratophyre initial nd values of 2 6 3 5 and 3 0 6 1 respectively are estimated at the time of volcanic eruption 2522 ma similar positive initial nd values for the spilites and quartz keratophyres suggest that they might have been derived from the same depleted mantle source or that the quartz keratophyres are derived from a short lived mafic source

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

constraints of volcanic rocks of the wutai complex shanxi province northern china on a giant late neoarchean intra oceanic arc system in the trans north china orogen abstract the wutai complex is located in the central part of the trans north china orogen the complex consists of neoarchean to paleoproterozoic granitoids and low grade metamorphosed volcanic and sedimentary rocks of the wutai group the group is subdivided into the shizui taihuai and gaofan subgroups meta volcanic rocks from the taihuai subgroup have igneous zircon u pb ages of 2548 2538 ma and e hf t values of 4 10 to 8 84 with xenocrystic zircons of 3 6 2 7 ga geochemically the meta volcanic rocks can be classified into four groups group 1 low ti tholeiites have the highest nb la pm ratios and the lowest la sm cn ratios flat to slightly light ree depleted chondrite normalized patterns and lower tio 2 and yb contents compared to v they were most likely derived from partial melting of depleted mantle in a fore arc setting group 2 island arc tholeiites displays flat to mildly right declined ree patterns and moderately negative nb ta zr and ti anomalies group 3 mature arc calc alkaline basalts display strongly fractionated chondrite normalized ree patterns and evidently negative nb ta and ti anomalies group 2 and 3 basalts formed by partial melting of sub arc depleted mantle wedge modified by different degrees of slab derived melts group 4 meta andesites and rhyolites have high la and low yb contents and strongly fractionated ree patterns and may have formed by fractionational crystallization of magma of group 3 basalts with clinopyroxene as a major mineral phase the 2 55 2 52 ga wutai group represents an intra oceanic arc underlain in part by a 2 7 ga continental nucleus derived fragment

iron formations a global record of neoarchaean to palaeoproterozoic environmental history abstract iron formations if represent an iron rich rock type that typifies many archaean and proterozoic supracrustal successions and are chemical archives of precambrian seawater chemistry and post depositional iron cycling given that if accumulated on the seafloor for over two billion years of earth s early history changes in their chemical mineralogical and isotopic compositions offer a unique glimpse into environmental changes that took place on the evolving earth perhaps one of the most significant events was the transition from an anoxic planet to one where oxygen was persistently present within the marine water column and atmosphere linked to this progressive global oxygenation was the evolution of aerobic microbial metabolisms that fundamentally influenced continental weathering processes the supply of nutrients to the oceans and ultimately diversification of the biosphere and complex life forms many of the key recent innovations in understanding if genesis are linked to geobiology since biologically assisted fe ii oxidation either directly through photoferrotrophy or indirectly through oxygenic photosynthesis provides a process for if deposition from mineral precursors the abundance and isotope composition of fe ii bearing minerals in if additionally suggests microbial fe iii reduction a metabolism that is deeply rooted in the archaea and bacteria linkages among geobiology hydrothermal systems and deposition of if have been traditionally overlooked but now form a coherent model for this unique rock type this paper reviews the defining features of if and their distribution through the neoarchaean and palaeoproterozoic this paper is an update of previous reviews by bekker et al 2010 2014 that will improve the quantitative framework we use to interpret if deposition in this work we also discuss how recent discoveries have provided new insights into the processes underpinning the global rise in atmospheric oxygen and the geochemical evolution of the oceans

shrimp u pb zircon ages of the fuping complex implications for late archean to paleoproterozoic accretion and assembly of the north china craton the fuping complex is situated in the central part of the north china craton and consists of four major lithological assemblages fuping tonalitic trondhjemitic granodioritic gneisses longquanguan augen gneisses wanzi supracrustal assemblage and nanying granitic gneisses shrimp u pb geochronology combined with u th and cathodoluminescence cl imaging of zircon enables resolution of magmatic and metamorphic events that can be directed towards understanding the late archean to paleoproterozoic history of the fuping complex cl images reveal the coexistence of magmatic and metamorphic zircons in nearly all rock types of the fuping complex the metamorphic zircons occur as either single grains or overgrowth or recrystallization rims surrounding and truncating oscillatory zoned magmatic zircon cores and are all characterized by nebulous zoning or being structureless with extremely high luminescence and very low th contents these features make them distinct from magmatic zircons that are characterized by concentric oscillatory zoning comparatively low luminescence and high th and u contents shrimp u pb analyses on magmatic zircons reveal that the tonalitic trondhjemitic and granodioritic plutons of the fuping gneisses were emplaced at 2523 14 ma 2499 10 ma and 2486 8 ma respectively whereas the monzogranitic and granitic plutons of the longquanguan augen gneisses were intruded respectively at 2510 22 ma and 2507 11 ma prismatic and oscillatory zoned zircons dominate in the pelitic rocks of the wanzi supracrustal assemblage and are interpreted as detritus from igneous source rocks the concordant and discordant u pb ages of 2502 5 ma and 2507 14 ma obtained from two pelitic rock samples indicate these rocks must have been deposited no earlier than 2507 ma ago in addition a zoned zircon grain in one pelitic rock sample has a near concordant age of 2109 5 1 ma which may provide a maximum depositional age for the wanzi supracrustal rocks shrimp results also reveal that granitic magmatism assigned to the nanying granitic gneisses occurred over a protracted interval from 2077 13 ma to 2024 21 ma the nebulously zoned zircon grains and overgrowth recrystallization zircon rims from different rocks yielded similar concordant 207pb 206pb ages in the range 1875 to 1802 ma interpreted as approximating the age of regional metamorphism of the fuping complex timing of primary zircon crystallization and regional metamorphism of the fuping complex is in general agreement with recent u pb zircon ion probe results for the wutai and hengshan complexes that bound the fuping complex to the northwest these areas are characterized by the emplacement of major granitoid bodies at around 2 50 ga to 2 48 ga ago deposition of supracrustal rocks in the paleoproterozoic intrusion of paleoproterozoic granitic bodies at 2 1 to 2 0 ga and regional metamorphism at 1 875 to 1 802 ga these data indicate that the fuping and hengshan complexes do not represent an older crystalline basement to the wutai complex as suggested in previous tectonic models but together with the wutai complex represent elements of a single late archean to paleoproterozoic magmatic arc system that has been subsequently tectonically disrupted and juxtaposed during the collision of the eastern and western north china blocks at 1 85 ga which resulted in the final assembly of the north china craton

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

paleoproterozoic p t t evolution in the hengshan wutai fuping area north china craton evidence from petrological and geochronological data abstract this contribution carries out zircon dating and a review of the previous metamorphic and geochronological work on the hengshan wutai fuping area in the central trans north china orogen for elucidating its paleoproterozoic orogenic evolution the hengshan complex can be subdivided into north hengshan and south hengshan complexes separated by the strike slip zhujiafang shear zone high pressure hp mafic granulite and garnet absent amphibolite collected from the north hengshan complex yield metamorphic ages of 1963 1769 ma and 1882 1793 ma respectively both with weighted mean ages of 1 85 ga in the hp mafic granulite zircons of 1 90 ga and those of

history of sedimentary infilling and faulting in subic bay philippines revealed in high resolution seismic reflection profiles abstract subic bay sediments and faults identified in seismic reflection profiles were dated using sea level curves the oldest sedimentary packages are marine sediments subaerially exposed and eroded 20 ka fluvio marine to wholly marine sediments were deposited during the ensuing transgression and prograding units were deposited during stillstands or minor sea level falls faults within the bay have three age ranges the oldest set cuts through the pre 18o stage 2 rock units 18 ka a second disrupts 10 2 11 3 ka sediments and the youngest which cut the uppermost sedimentary package show that movements occurred about every 2 ky most recently about 3 ka northwest southeast faults that parallel onshore structures associated with paleogene emplacement of the zambales ophiolite complex to the west and north likely represent rejuvenated tectonism the northern coastline and north south trending axial bay islands appear related to a lineament that dissects mt pinatubo farther northeast a breach in the caldera of mt natib is the most likely source of a presumed pyroclastic deposit in the eastern bay that is associated with sediments about 11 3 18 ka indicating that a natib eruption occurred much more recently than previously documented for this volcano

archean geodynamics in the central zone north china craton constraints from geochemistry of two contrasting series of granitoids in the fuping and wutai complexes abstract the archean to paleoproterozoic central zone of the north china craton is situated between the eastern and western archean continental blocks and contains two contrasting series of neoarchean granitoids the 2523 2486 ma tonalite trondhjemite granodiorite ttg gneisses in the fuping complex and the 2555 2525 ma calc alkaline granitoids tonalite granodiorite granite and monzogranite in the wutai complex the fuping ttg gneisses most likely formed from partial melting of 2 7 ga basalts at 50 km with an involvement of 3 0 ga crustal material the wutai granitoids have higher k 2 o lile and rb sr but lower sr y and la n yb n than the fuping ttg gneisses are characterized by nd t dm from 2 5 to 2 8 ga and e nd t from 0 49 to 3 34 and are derived from partial melting of a juvenile source at the geochemistry of these two contrasting series of neoarchean granitoids provides further evidence that the wutai complex originated and evolved separately from the fuping complex the wutai complex most likely formed as an oceanic island arc with volcanism and synvolcanic granitoid intrusions at 2555 2525 ma the wutai complex was subsequently accreted onto the eastern archean continental block and was probably responsible for crustal thickening and ttg magmatism at 2523 2486 ma in the fuping complex as part of the taihangshan hengshan block at the western margin of the eastern archean continental block

a two stage model for the formation of the granite greenstone terrains of the kalgoorlie norseman area western australia abstract accurate dating of granites and greenstones from the kalgoorlie norseman area shows that there is considerable overlap in time between the formation of granites and greenstones however within any given area the oldest granites are always younger than the oldest greenstones it is suggested that the long lived 100 ma thermal anomaly in the uppermost asthenosphere that gave rise to the greenstones is also responsible for regional metamorphism and widespread crustal melting leading to the production of granites the time lag between greenstone and granite formation reflects the time taken to conduct heat from the mantle to the lower crust anatexis of the lower crust produces a layer of light granitic magma that may temporarily stop the ascent of basaltic magma where this happens the basaltic magma ponds at the base of the crust supplying additional heat and accelerating the melting process eventually the granitic melt layer becomes unstable and rises within the crust areas intruded by granite become topographic highs with topographic lows forming above areas where light melt has been withdrawn from the lower crust when the topographic highs protrude above sea level they are eroded to shed sediments into the topographic lows which contain the basaltic greenstones thus the change from a basalt dominated lower greenstone succession to a sediment and felsic volcanic dominated upper whitestone sequence seen in many granite greenstone terrains results from the intrusion of granitic magma into the upper crust the crust of the kalgoorlie norseman area formed in two distinct stages separated by a time break of up to 700 ma the first stage involved the formation of primitive crust probably above subduction zones this primitive crust was characterized by low rb sr and high sm nd ratios the second stage resulted from a major change in mantle convection that produced thermal upwellings in the mantle below the continents this led to the eruption of basaltic magmas and to metamorphism and anatexis of the lower crust to produce granitic magmas melting reset the rb sr sm nd and u pb systems giving the impression that the second event which occurred at 2 7 ga was a major crust forming event it was in fact a crustal reworking process although some basalt was added to the crust at this time although the two stage history of the kalgoorlie norseman area is clearly seen in the lead isotopes and in zircons it is not readily discernible from the rb sr and sm nd systematics due to mantle like ratios of those elements in the primitive crust

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

neoarchean to paleoproterozoic continental growth in the southeastern margin of the north china craton geochemical zircon u pb and hf isotope evidence from the huoqiu complex abstract the huoqiu complex in the southeastern margin of the north china craton ncc is dominated by neoarchean gray gneisses amphibolites and voluminous metasediments here we report the occurrence of neoarchean to paleoproterozoic rocks from drill core samples the gneisses are similar to ttg tonalite trondhjemite granodiorite in composition and show close spatial association with amphibolites geochemical characteristics such as high sr y and la yb n with steep ree patterns and trace element modeling suggest that these rocks were generated by partial melting of hydrous meta basalts amphibolites at the base of a thickened mafic continental crust leaving a rutile bearing eclogite residue la mc icp ms u pb age data from magmatic zircon grains show protolith emplacement ages of 2 76 2 71 ga subsequently widespread migmatization took place at 1 91 1 82 ga generating voluminous migmatites and high k granites hf isotopic compositions of zircon grains from the amphibolite and gneiss show ehf t values of 2 4 15 5 and 3 0 1 5 respectively the tdm2 hf model ages of the gneisses range from 2 87 to 3 14 ga and are identical to the tdm1 hf ages of amphibolites 2 84 3 16 ga within analytical uncertainty suggesting that the gneisses formed by partial melting of amphibolite and attest to large scale reworking of the ancient continental crust during neoarchean the zircon grains from the granites define two groups with regard to their hf isotopic composition the older group 1916 42 ma has ehf t values and tdm2 hf ages of 10 5 2 4 and 2 40 3 20 ga respectively whereas the younger one 1823 41 ma shows a large variation in ehf t values ranging from 18 1 to 12 5 with tdm2 hf model ages of 1 70 3 59 ga a couple of zircon grains from the younger group display consistent u pb ages and tdm2 indicating accretion of juvenile crust from depleted mantle sources during 1 82 to 1 91 ga however the dominant hf isotope features are consistent with the reworking of preexisting continental crust we therefore infer that only limited accretion of juvenile crust occurred during this time and that the paleoproterozoic 1 82 to 1 91 ga tectonics in the southeastern margin of the ncc witnessed extensive reworking of older continental crust

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

lithotectonic elements and geological events in the hengshan wutai fuping belt a synthesis and implications for the evolution of the trans north china orogen the hengshan wutai fuping belt is located in the middle segment of the trans north china orogen a palaeoproterozoic continental collisional belt along which the eastern and western blocks amalgamated to form the north china craton the belt consists of the medium to high grade hengshan and fuping gneiss complexes and the intervening low to medium grade wutai granite greenstone terrane and most igneous rocks in the belt are calc alkaline and have affinities to magmatic arcs previous tectonic models assumed that the hengshan and fuping gneiss assemblages were an older basement to the wutai supracrustal rocks but recent studies indicate that the three complexes constitute a single long lived neoarchaean to palaeoproterozoic magmatic arc where the wutai complex represents an upper crustal domain whereas the hengshan and fuping gneisses represent the lower crustal components forming the root of the arc the earliest arc related magmatism in the belt occurred at 2560 2520 ma marked by the emplacement of the wutai granitoids which was followed by arc volcanism at 2530 2515 ma forming the wutai greenstones extension driven by widespread arc volcanism led to the development of a back arc basin or a marginal sea which divided the belt into the hengshan wutai island arc japan type and the fuping relict arc at 2520 2480 ma subduction beneath the hengshan wutai island arc caused partial melting of the lower crust to form the hengshan tonalitic trondhjemitic granodioritic ttg suites whereas eastward directed subduction of the marginal sea led to the reactivation of the fuping relict arc where the fuping tonalitic trondhjemitic granodioritic suite was emplaced in the period 2360 2000 ma sporadic phases of isolated granitoid magmatism occurred in the hengshan wutai fuping region forming 2360 ma c 2250 ma and 2000 2100 ma granitoids in the hengshan complex the c 2100 ma wangjiahui and dawaliang granites in the wutai complex and the 2100 2000 ma nanying granitoids in the fuping complex atc 1920 ma the hengshan wutai island arc underwent an extensional event possibly due to the subduction of an oceanic ridge leading to the emplacement of pre tectonic gabbroic dykes that were subsequently metamorphosed together with their host rocks to form medium to high pressure granulites at 1880 1820 ma the hengshan wutai fuping arc system was juxtaposed intensely deformed and metamorphosed during a major and regionally extensive orogenic event the l uliang orogeny which generated the trans north china orogen through collision of the eastern and western blocks the hengshan wutai fuping belt was finally stabilized after emplacement of a mafic dyke swarm at 1780 1750 ma

fossil oceanic subduction zone beneath the western margin of the trans north china orogen magnetotelluric evidence from the luliang complex abstract the trans north china orogen tnco is a major collisional belt along which the western and eastern blocks of the north china craton were amalgamated through subduction accretion collision tectonics during paleoproterozoic the luliang complex along the western margin of the tnco is thought to be the trace of an ancient suture zone here we employ magnetotelluric array to build a lithosphere scale high resolution 3 d resistivity model for this region between 36 n and 38 n e w trending slices of the resistivity model show relatively high conductivity 10 100 m lithospheric mantle of the western block that extends eastward beneath the relatively high resistivity lithosphere of the tnco at depths from 80 to 180 km we interpret this east dipping upper mantle conductivity contrast as the lithosphere scale suture between the western block and tnco that formed through eastward subduction the enhanced conductivity of the fossil oceanic subduction zone might be a reflection of the reduction in grain size of olivine grains in mantle peridotite by the localized shearing stress associated with the subduction combined with the interconnecting high conductivity minerals along grain boundaries formed through metasomatism by slab derived fluids similar subduction related boundaries are absent in the resistivity models beneath the northern luliang complex near 39 n where prominent high conductivity anomalies indicating asthenosphere upwelling and lower crustal underplating are seen such mantle architecture could result from the subduction of an oceanic ridge or slab window as also proposed for the origin of the subduction related luyashan charnockite suite in this region our data also suggest that the phanerozoic tectonic and magmatic events in this region only locally modified the precambrian lithospheric tectonic framework without total destruction we propose a 3 d model to illustrate the contrasting paleoproterozoic tectonic evolution between the northern and southern domains of the luliang complex and the tnco during the transition stage between the termination of oceanic subduction and the subsequent continental collision

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

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

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

the wutaishan orogenic belt within the shanxi province northern china a record of late archaean collision tectonics abstract the archaean crust of northeastern shanxi province northern china is dominated by three distinct terranes the high grade gneiss terranes hengshan in the north and fuping in the southeast and the intervening low grade terrane of the wutaishan belt the 2 5 ga beitai arc complex which constitutes the major part of the wutaishan belt is interpreted as a pre collision magmatic arc there was a marginal sea basin between the beitai arc and the fuping terrane at the fore arc collision stage the passive margin deposits which occurred along the western margin of the fuping terrane became a foreland fold thrust belt after the closure of the marginal sea basin the remnants of sea floor occur as a melange zone along the suture zone between the beitai arc and fuping terrane nearly at the same time at 2 5 ga the back arc oceanic lithosphere started to subduct beneath the hengshan terrane then the hengshan terrane came into collision with the arc and the fuping terrane some dismembered sea floor rocks are preserved as a back arc melange zone between the hengshan terrane and the beitai arc the term wutaishan orogeny should be reserved for the processes which lasted from 2600 to 2450 ma

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

neoarchean paleoproterozoic magmatic arc evolution in the wutai hengshan fuping area north china craton new perspectives from zircon u pb ages and hf isotopic data abstract the wutai granite greenstone belt which formed in the neoarchean paleoproterozoic with the adjoining high grade metamorphic hengshan fuping region record important transitions in crustal and tectonic evolution although these evolutionary processes have still been in dispute in this study new u pb and lu hf la icp ms zircon analyses were obtained from eight metamorphosed magmatic rocks in the wutai hengshan fuping area in north china craton ncc the results reveal that most of wutai granitoids yield emplacement ages of 2 56 2 52 ga with corresponding ehf t2 of 4 7 to 8 3 the tonalitic gneiss from dongshandi intrusion however gives a new age of 2482 ma and ehf t2 of 2 5 to 4 6 which are similar with yixingzhai pluton marked by ages of 2501 2499 ma and ehf t2 of 3 2 to 4 4 also 2 7 ga and 2 5 ga continental crust generation are further revealed but 2 5 ga crustal growth is more complicated due to the involvement of old crustal components which can be identified by variable ehf t2 values ranging from 1 8 to 7 5 thus based on the distinctions of hf isotopes compositional and spatial features two consecutive groups of 2 56 2 52 ga and 2 52 2 48 ga are recognized and a two step tectonic evolution model for the wutai hengshan fuping area is proposed during 2 56 2 52 ga intra oceanic subduction led to the formation of wutai oceanic island arc accompanied mainly by the development of wutai granitoids ehf t2 of 4 7 to 8 3 and coeval volcanics predominant arc tholeiitic chemical affinity corresponding to the juvenile crustal growth between 2 52 ga and 2 48 ga with intervening oceanic crust between intra oceanic arc and continent consumed the wutai intra oceanic arc then collided and was obducted onto the western edge of the hengshan fuping continental block large amounts of ttg gneisses produced in both the wutai and hengshan fuping ehf t of 1 8 to 7 5 along with calc alkaline volcanism in wutai possibly in response to the thickening through arc accretion and commencing the continental arc this proposed two step tectonic evolution model combined with a late neoarchean 2 6 2 5 ga intra oceanic arc system along the northwestern of the eastern block eb probably provides a better explanation for the fundamental geodynamic mechanism for continental arc development at the proterozoic dawn

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

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

a 2 5 ga fore arc subduction accretion complex in the dengfeng granite greenstone belt southern north china craton abstract the dengfeng granite greenstone belt dggb located in the southern segment of the central orogenic belt cob of the north china craton ncc consists of a volcano sedimentary assemblage intruded by tonalite trondhjemite granodiorite ttg suite diorite granites and late mafic dikes the volcano sedimentary assemblage in the dggb mainly consists of tectonically imbricated basaltic amphibolites meta gabbroic rocks with minor ultramafic rocks and metagreywacke marble and quartzite consistent with characterisitics of typical phanerozoic subduction accretion complexes the basaltic amphibolites yield a metamorphic zircon 207pb 206pb age of 2507 26 ma interpreted to represent the peak age of amphibolite facies metamorphism that took place during subduction accretion of the basaltic protolith the basaltic amphibolites are characterized by a tholeiitic affinity and flat lree patterns with minor negative nb and zr anomalies based on mixed morb and arc affinities the basaltic amphibolites in the dggb are interpreted to have formed in a fore arc tectonic setting one late potassic granite dike cutting across the fabrics of the volcano sedimentary assemblage yields an intrusion age of 2492 35 ma constraining the minimum deformation age for tectonic assembly of the package the ttg gneisses and diorites intruded the western margin and the center of the subduction accretion complex respectively one ttg sample and one diorite sample yield igneous zircon 207pb 206pb ages of 2514 26 ma and 2518 36 ma respectively constraining their intrusion ages the ttg gneisses display high ratios of la yb cn and sr y and depletion in hfse with negative nb ta and ti anomalies consistent with those of typical archean ttgs the ttg gneisses are therefore considered to be generated from partial melting of a shallowly subducting oceanic slab and or accreted arc amphibolites the diorites have high concentrations of mgo 2 89 6 05 wt ni 148 178 ppm and cr 85 7 120 6 ppm and highly fractionated ree patterns and are depleted in hfses with negative nb ta zr hf and ti anomalies we suggest that the high mg diorites in the dggb may have been derived from a hydrated mantle wedge which was previously metasomatized by subduction derived melts and or fluids collectively a neoarchean subduction accretion collision event is therefore proposed to have generated the dggb the volcano sedimentary assemblage in the dggb represents a fore arc subduction accretion complex which is interpreted to be related to the suture zone of ca 2 5 ga arc continent collision between a ttg dominated arc terrane in the cob and the eastern block of the ncc we further propose a long n s striking neoarchean suture zone occurring in the eastern margin of the cob mainly consists of the ca 2 5 ga subduction accretion complex in the dggb to the south the ca 2 5 ga melange belt in the zanhuang complex in the central and the ca 2 5 ga ophiolitic melange belt in the zunhua dongwanzi structural belt to the north which separate an arc terrane in the cob from the eastern block of the ncc

zircon u pb geochronology and geochemistry of low grade metamorphosed volcanic rocks from the dantazi complex implications for the evolution of the north china craton abstract the late neoarchean witnessed the cratonization of the north china craton ncc through amalgamation of several micro blocks to form a coherent basement the archean orthogneisses and supracrustal rocks in this craton have experienced various grades of metamorphism ranging up to upper amphibolite and granulite facies at 2500 ma recently a suite of low grade metamorphosed greenschist to lower amphibolite facies volcanic rocks was discovered in the late neoarchean dantazi complex in northern hebei province these meta volcanic rocks consist of bimodal basalt andesite and trachyte dacite with a sio2 gap between 54 4 wt and 60 7 wt here we report shrimp zircon u pb ages of 2490 19 ma mswd 2 0 and 2502 8 ma mswd 0 83 from the meta mafic and meta felsic volcanics respectively representing the timing of igneous activity all the meta mafic volcanic rocks display coherent trace element and ree patterns which are characterized by enriched lile and lree but depleted hfse and hree la yb n 6 29 15 10 combining these trace element features with the positive zircon ehf t values 1 3 to 6 6 we propose that the mafic rocks were likely derived from partial melting of a previously metasomatized lithospheric mantle in the primitive mantle normalized diagram the felsic rocks display uniform patterns enriched in lile but depleted in nb and ta similar to those of lower crust furthermore their strongly fractionated ree la yb n 15 24 61 20 lower hree concentrations yb 0 47 1 65 ppm and positive zircon ehf t values 1 6 to 5 3 suggest that they were derived from partial melting of the lower crust with garnet in the residue this coeval occurrence of metasomatized mantle derived mafic magmas and potassic felsic magmas from different source regions reflects an intracontinental extensional setting during the late neoarchean to earliest paleoproterozoic following the cratonization of the ncc our new data combined with previous published data allow us to speculate that the intracontinental extensional regime might have prevailed in the ncc from late neoarchean onwards

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

the building blocks of continental crust evidence for a major change in the tectonic setting of continental growth at the end of the archean abstract oceanic arcs are commonly cited as primary building blocks of continents yet modern oceanic arcs are mostly subducted also lithosphere buoyancy considerations show that oceanic arcs even those with a felsic component should readily subduct with the exception of the arabian nubian orogen terranes in post archean accretionary orogens comprise 50 produced in continental arcs felsic igneous rocks in oceanic arcs are depleted in incompatible elements compared to average continental crust and to felsic igneous rocks from continental arcs they have lower th yb nb yb sr y and la yb ratios reflecting shallow mantle sources in which garnet did not exist in the restite during melting the bottom line of these geochemical differences is that post archean continental crust does not begin life in oceanic arcs on the other hand the remarkable similarity of incompatible element distributions in granitoids and felsic volcanics from continental arcs is consistent with continental crust being produced in continental arcs during the archean however oceanic arcs may have been thicker due to higher degrees of melting in the mantle and oceanic lithosphere would be more buoyant these arcs may have accreted to each other and to oceanic plateaus a process that eventually led to the production of archean continental crust after the archean oceanic crust was thinner due to cooling of the mantle and less melt production at ocean ridges hence oceanic lithosphere is more subductable widespread propagation of plate tectonics in the late archean may have led not only to rapid production of continental crust but to a change in the primary site of production of continental crust from accreted oceanic arcs and oceanic plateaus in the archean to primarily continental arcs thereafter

geochronology and petrogenesis of neoarchean potassic meta granites from huai an complex implications for the evolution of the north china craton abstract neoarchean potassic meta granites occurring in the huai an complex within the north central part of the north china craton ncc comprise leucocratic syenogranites biotite monzogranites and garnet bearing syenogranites high pressure garnet granulites occur as lenses sheets or dismembered dikes with the granitoids here we present shrimp zircon u pb data from the leucocratic syenogranites which show an age of 2493 6 ma mswd 1 and la icp ms u pb data from the biotite monzogranites and garnet syenogranites revealing their formation ages of 2437 10 ma mswd 2 0 and 1977 2003 ma respectively we also report metamorphic ages of ca 1800 ma from zircons in the garnet syenogranites geochemical studies show low mgo 0 5 0 06 wt cr 6 19 0 53 ppm and ni 27 66 2 01 ppm with varying sio2 62 3 to 76 8 wt contents indicating their derivation from a crustal source the leucosyenogranites exhibit elevated positive eu anomalies with decreasing sr th u and bulk ree concentrations and th u ratio whereas rb and ba values increase with decreasing th u ratio indicating fluid melt interaction the leucosyenogranites have high sr and ba contents and sr y and la yb n ratios and low y and hrees with zircon ehf t values ranging from 1 7 to 5 8 and hf model ages of 2 60 2 73 ga reflecting their derivation by partial melting of overthickened juvenile lower crust of eclogitic composition or an eclogitic mineral assemblage in the residue geochemical compositions of the biotite monzogranites with positive end t values 2 3 to 2 5 and model ages

petrogenesis and geodynamics of late archean magmatism in eastern hebei eastern north china craton geochronological geochemical and nd hf isotopic evidence abstract the eastern part of the north china craton contains large volumes of late archean hornblenditic tonalitic dioritic granodioritic and granitic rocks la icpms zircon u pb dating of three plutons from eastern hebei eastern north china craton record emplacement ages of 2526 2515 ma and metamorphic ages of 2500 2490 ma for the diorite granodiorite and monzogranite and 2440 ma for the k feldspar granite hornblendites have high mgo 8 6 10 6 wt cr 674 1126 ppm ni 125 159 ppm and v 224 315 ppm at low basic silica contents sio2 45 7 49 0 wt they are enriched in light rare earth elements lrees and large ion lithophile elements liles and depleted in high field strength elements hfses with positive nd t values 1 2 to 2 0 indicating they were derived from an enriched mantle source the monzogranites have high sr and ba contents and sr y and la yb ratios low y and hrees abundances and positive nd t and zircon hf t values with nd and hf model ages less than 3 0 ga reflecting that they were derived from partial melting of a juvenile lower crust at high pressure however the k feldspar granites with low sr and ba concentrations and high hree and y contents are considered to have originated from partial melting of juvenile crustal materials at shallow crustal levels the diorites and granodiorites have variable major and trace element concentrations magmatic zircons from these rocks have heterogeneous hf t values but record uniform homogeneous u pb ages indicating they resulted from mixing of mafic and felsic magmas coupled with fractional crystallization because of the widespread distribution of late archean magmatism and evidence for magma mixing possibly related to mafic magma underplating it appears likely that this event in the eastern north china craton may relfect mantle plume activity that resulted in significant growth of continental crust during the late archean

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

granitoid evolution in the late archean wutai complex north china craton abstract two temporally discrete episodes of granite magmatism have been identified in the wutai complex of the north china craton based on shrimp u pb zircon dating of all major granitoid bodies in the area the older period is latest archean and ranges in age from 2560 to 2515 ma whereas the younger episode is paleoproterozoic with magmatism at 2170 and 2120 ma archean magmatism can be further separated into two pulses based on geochemistry and the association of the younger plutons with extensive coeval volcanic activity in the wutai complex the early phase of archean activity is represented by the lanzhishan ekou and portions of the chechang betai granites with ages ranging from 2560 to 2540 ma the lanzhishan granite is the only one at wutaishan that contains inherited zircon with a number of cores and individual grains giving ages in excess of 2700 ma these are relatively evolved granitoids interpreted to be derived from older basement of the eastern block of the north china craton the shifo guangmingshi the main grey phase of the wangjiahui granite and the major components of the chechang betai granite have ages of 2540 2515 ma and are coeval with felsic volcanism in the wutai complex which is a consequence of subduction and related arc magmatism the dawaliang granite and pink phase of the wangjiahui granite are considerably younger with paleoproterozoic ages of 2170 and 2120 ma respectively these are also evolved granitoids most likely derived by partial melting of the older basement although we have found no evidence of inheritance the petrogenesis of the granitoids reflects the precambrian amalgamation history of the north china craton indicating derivation along the western edge of the eastern block during development and amalgamation of the wutai arc and prior to collision with the western block of the craton 1 8 ga ago

tephra identification and radiocarbon chronology of sediment from paitan lake at the northern part of luzon central plain philippines abstract in order to construct the tephrostratigraphy of the northern region of the philippines two sediment cores were obtained from paitan lake paitan lake is located in the northern part of luzon central plain 88 km northeast of pinatubo volcano it s a crater lake with a surface area of 0 8 km2 its drainage basin has an area of 38 5 km2 and has an outflow river with a width of only a few meters these cores consisted of two units 1 an upper alternating bed of mud and peaty layers from depths 0 6 m and 2 a lower dark gray to gray mud bed obtained from depths 6 28 m eight tephras were recognized on the split surface of the cores radiocarbon ages of seven plant fragments from 1 50 to 13 41 m depths and twelve organic sediments from 4 72 to 28 00 m depths were measured by the ams facilities of kigam nagoya university and institute of accelerator analysis ltd these results were 1 6 24 cal kbp these ages of tephras were correlated to the eruption periods of pinatubo and natib volcano

petrogenesis of the 2090 ma zanhuang ring and sill complexes in north china a bimodal magmatism related to intra continental process abstract it is widely debated as to whether the 2100 ma bimodal magmatic events in the eastern north china craton represent intra continental rifting or marginal processes the genetic relationship between the two components of bimodal suites is the key solution to this issue and this is investigated on a group of newly identified ring complexes as well as the apparently associated sills in zanhuang southern taihang mts these ring complexes comprise varied components pyroxenite gabbro anorthosite k rich a type granite and minor na rich a type granite u pb analyses on baddeleyite and zircon grains liberated from a gabbro sample both yield a 2090 ma age which represents crystallization timing and is contemporaneous with a type granites of the ring complexes as well as the sills in the gantaohe group the mafic rocks of the ring complexes and sills show similar chemical features they were characterized by enriched light rare earth elements compared to middle and heavy ones la ybn 1 4 2 8 enriched large lithosphile elements but depleted high field strength elements and enriched sr nd isotopes endt t 2090 ma 1 9 0 6 nd tdm 2 7 ga they were most likely originated from the same magma source resided in the sub continental lithospheric mantle which formed in the late archean the compositional variation of the mafic sills may be resulted from the fractionation of olivine and plagioclase the k rich a type granite as well as the dacitic sills could be originated from this same source with firstly olivine plagioclase dominated fractionation in the lower magma chamber to more felsic compositions and then with significant removal of clinopyroxene plagioclase in the upper magma chamber which at the same time produced pyroxenite gabbro and minor anorthosite cumulates in the ring complexes na rich a type granite was likely formed locally under relatively high water fugacity with fractionation of k feldspar plagioclase clinopyroxene from a parental magma similar to the k rich a type granite this bimodal zanhuang ring and sill complexes and other comparable bimodal suites widely distributed in the eastern north china craton were likely intra continental representing a craton scale extension at 2100 ma

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

seismically induced soft sediment deformation structures revealed by x ray computed tomography of boring cores abstract x ray computed tomography ct allows us to visualize three dimensional structures hidden in boring cores nondestructively we applied medical x ray ct to cores containing seismically induced soft sediment deformation structures ssdss obtained from the kanto region of japan where the 2011 off the pacific coast of tohoku earthquake occurred the ct images obtained clearly revealed various types of the seismically induced ssdss embedded in the cores a propagating sand dyke bent complexly by the preexisting geological structure deformed laminations of fluidized sandy layers and two types of downward mass movement ductile downward folding and brittle normal faulting as compensation for upward sand transport through sand dykes two advanced image analysis techniques were applied to the sand dyke ct images for the first time the growcut algorithm a specific digital image segmentation technique that uses cellular automata was used successfully to extract the three dimensional complex sand dyke structures embedded in the sandy sediments which would have been difficult to achieve using a conventional image processing technique local autocorrelation image analysis was performed to detect the flow pattern aligned along the sand dykes objectively the results demonstrate that x ray ct coupled with advanced digital image analysis techniques is a promising approach to studying the seismically induced ssdss in boring cores

depositional age and provenance of the wutai group evidence from zircon u pb and lu hf isotopes and whole rock geochemistry abstract the hengshan wutai fuping area is situated in the middle segment of the trans north china orogen and consists of the upper amphibolite to granulite facies hengshan and fuping complexes and the intervening greenschist to lower amphibolite facies wutai complex the wutai complex is composed of neoarchean to paleoproterozoic granitoids and low grade metamorphosed supracrustal rocks of which the latter are further divided into the wutai and hutuo groups geochemistry of the meta sedimentary samples from the wutai group indicates weak to moderate source weathering dominantly compositional variation trend and major felsic with few mafic source rocks on the tectonic discrimination plots most of the samples display continental island arc and active continental margin features u pb ages of detrital and xenocrystic zircons from the wutai group yield two major age populations of 2565 2465 ma and 2729 2676 ma and a subordinate age population of 2372 2331 ma the major 2522 ma and 2699 ma zircons were most likely derived from the wutai meta volcanic rocks the minor 2348 ma zircons are only found from the upper gaofan subgroup and most likely sourced from the 2360 2330 ma orthogneisses in the hengshan complex the maximum depositional ages of 2535 2516 ma from the meta sedimentary rocks are consistent with the eruption ages of 2548 2513 ma from meta volcanic rocks in the lower shizui and middle taihuai subgroups indicating coeval sedimentation and volcanism in a fore arc or intra arc basin this favors a tectonically juxtaposed contact rather than a layer cake model however the maximum depositional age of 2348 ma and the 2176 2161 ma felsic intrusions suggest a much younger formation age for the gaofan subgroup

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

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

reworking of atmospheric sulfur in a paleoarchean hydrothermal system at londozi barberton greenstone belt swaziland anomalous fractionation of the minor isotopes of sulfur 33s 36s in archean pyrite is thought to reflect photochemical reactions in an anoxic atmosphere with most samples falling along a reference array with 36s 33s 1 small deviations from this array record microbial sulfate reduction or changes in atmospheric source reactions here we argue that reworking of atmospheric sulfur with distinct minor sulfur isotope ratios 36s 33s 1 produced additional variability in sulfide 33s and 36s values in a 3 52 ga hydrothermal barite deposit at londozi barberton greenstone belt swaziland in situ measurement of the four stable sulfur isotopes in pyrite revealed 36s 33s relationships and a 36s 33s trend 3 2 0 4 which is significantly different from the co variation between 36s and 33s in the co existing barite that reflects ambient paleoarchean seawater sulfate this argues against biological or thermochemical sulfate reduction at the time of barite deposition and requires incorporation of sulfide generated in a chemically distinct atmosphere before 3 52 ga we propose a model that combines reworking of this sulfur by hydrothermal leaching deep mixing with juvenile sulfur and surface mixing with biogenic sulfide to explain the observed variation in 34s 33s and 36s these interactions between abiotic and biological processes in the londozi hydrothermal system complicate the interpretation of biosignatures based on deviations in 33s and 36s from the archean reference array