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

metamorphic evolution and zircon u pb geochronology of the mts huashan amphibolites insights into the palaeoproterozoic amalgamation of the north china craton abstract exposed in the southernmost terminal of the palaeoproterozoic trans north china orogen tnco the garnet bearing amphibolites in the mts huashan metamorphic complex preserve three generations of mineral assemblages including the prograde peak and retrograde metamorphic stages the prograde metamorphic stage m1 is recognized as inclusion type mineral assemblages hornblende plagioclase quartz biotite ilmenite preserved in the garnet porphyroblasts the peak metamorphic assemblages m2 are represented by the garnet porphyroblasts and matrix minerals hornblende plagioclase quartz biotite ilmenite the retrograde metamorphic assemblages m3 are characterized by the symplectitic intergrowth minerals hornblende plagioclase quartz orthopyroxene biotite ilmenite rimming decomposed relict garnet porphyroblasts known as the white eye socket reaction textures calculated by geothermometers and geobarometers the p t conditions of these three metamorphic stages are estimated to be of 5 7 6 0 kbar 660 760 c for the prograde stage m1 6 6 7 7 kbar 700 820 c for the peak stage m2 and 4 5 6 7 kbar 720 760 c for the retrograde stage m3 respectively therefore these three metamorphic stages define clockwise p t paths including nearly isothermal decompression itd processes geochronological studies of zircons from the amphibolites indicate that the protolith of the amphibolites formed at 2 3 ga and the metamorphism occurred at 1 87 1 85 ga and the metamorphic period was further deciphered to be of 1 96 1 80 ga when considering our previous reports it is therefore proposed that the southernmost terminal of the tnco was also involved in the subduction and collision processes between the eastern and western blocks of the north china craton at least as early as 1 96 ga and continued as late as 1 80 ga followed by rapid uplift and exhumation the available data suggest that the palaeoproterozoic tectono metamorphic event of the southernmost terminal of the trans north china orogen not only started earlier but also continued longer than the other terranes in the middle and northern sections elsewhere in the orogen

chemical processes affecting the mobility of major minor and trace elements during weathering of granitic rocks abstract the behaviour of 38 major and trace elements as well as changes in the mineralogy have been examined in 10 weathering profiles developing on some portuguese granitic rocks element mobilities are calculated from geochemical data normalized with respect to ti in the fresh parent rock chemical elements are divided into two groups immobile and mobile on the basis of their geochemical distribution during weathering elements that are immobile during weathering are zr hf fe al th nb sc and the ree very mobile are ca na p k sr ba rb mg and si mobile elements are derived mainly from leachable minerals such as feldspars micas and apatites whereas immobile elements are either concentrated in resistate phases or strongly adsorbed by secondary minerals the geochemical behaviour of mn cr v fe and ce is very dependent on redox conditions redox transformations of these elements can be used to set limits on the oxidation state of a weathering suite the ree are mobilized or fractionated during late stages of weathering but not during moderate stages of weathering this fractionation is caused by selective leaching of rocks composed of both stable and unstable minerals containing ree

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

geochemistry of ttg and ttg like gneisses from lushan taihua complex in the southern north china craton implications for late archean crustal accretion the late archean taihua complex mainly composed of amphibolite and ttg and ttg like gneisses is volumetrically most important metamorphic rock suites scattered along the southern margin of the north china craton ncc zircon shrimp u pb dating shows two episodes 2765 13 and 2723 9 ma of archean magmatism in the lushan area with distinct geochemical features the early 2765 13 ma suite ttg like gneisses has low sio2 52 5 66 1 wt high mg 0 47 0 68 low hree ybn 3 0 5 4 and y 8 07 13 9 ppm with low to moderate la yb n 6 7 37 1 and sr y 25 9 119 3 the younger 2723 9 ma suite ttg gneisses has high sio2 63 5 74 3 wt low mg 0 13 0 52 very low ree ybn 1 1 20 2 43 both suites show pronounced negative nb ta and ti anomalies on the primitive mantle normalized spidergram the ttg like gneiss suite has similar bulk rock nd and zircon hf model ages 3 0 ga with end t 0 0 26 1 46 and is interpreted as resulting from melt of mantle interactions with the melts derived from partial melting of subducted ocean crust with a residual assemblage of garnet clinopyroxene rutile amphibole which favors subducted slab model for the late archean ttg the ttg gneiss suite has abundant relic zircons 2 95 2 80 ga with end t 0 1 31 to 0 23 which is best interpreted as derived from partial melting of thickened lower continental crust with a garnet amphibolite residue garnet amphibole rutile significant high pressure fractional crystallization garnet amphibole and accumulation plagioclase are also required in the petrogenesis the lushan amphibolite with nearly flat primitive mantle normalized trace element pattern is interpreted to represent early ocean crust metamorphism these observations suggest a possible model of late archean crustal accretion from ocean crust to continental terrain in the southern north china craton

polarity reversal in the solomon islands arc mckenzie1 proposed that the direction of subduction beneath an island arc will reverse following arc continent collision dewey and bird2 suggested that such an arc polarity reversal occurred in northern new guinea johnson and molnar3 however proposed two alternative models of plate convergence that could explain the observed seismicity we present here a spatial seismicity study of the solomon islands region which has revealed the existence of two juxtaposed wadati benioff w b zones of opposite polarity shallow and intermediate foci define a north east dipping w b zone associated with active subduction along the new britain and san cristobal trenches and a south west dipping w b zone associated with the inactive north solomon trench nst these data provide the first direct seismic evidence of a reversal in subduction polarity at an island arc

retraction notice to ca 2 5 billion year old mafic dykes in western shandong province implications for hybridization between subducted continental crust and the north china craton china

structural pattern of the wutai complex and its constraints on the tectonic framework of the trans north china orogen abstract wutai complex is a key to test controversial models for the tectonic evolution of the trans north china orogen tnco along which the eastern and western blocks amalgamated to form the north china craton three stages of deformation d 1 to d 3 were identified in the wutai complex d 1 is characterized by ne sw trending foliations s 1 and nw se orientated lineations l 1 indicating an initial nw se directed compression associated with d 1 was prograde metamorphism m 1 d 2 progressively overprinted d 1 and generated z shaped folds and nee sww trending ductile shear zones d 2 resulted in a fan shaped structural pattern of the complex with top to the nw thrusting in the northwestern part and opposite top to the se thrusting in the southeastern part during d 2 the crust was doubled through thickening leading to peak metamorphism m 2 d 3 developed during post collision exhumation and deformed the early fabrics to be open folds resulting in near isothermal decompression m 3 and late cooling metamorphism m 4 the overall structure of the wutai complex is characterized by a fan shaped pattern with structural divergence structural features of the wutai complex combined with those of the adjacent hengshan and fuping complexes indicate that the tnco is characterized by a fan shaped orogenic wedge with opposite senses of thrust this orogenic wedge could have resulted from an eastward subduction and collision between the western and eastern blocks to finally form the tnco at 1 85 ga

spatial distribution of 1950 1800 ma metamorphic events in the north china craton implications for tectonic subdivision of the craton abstract great progress has been made in the last twenty years in understanding the unification of the north china craton according to a prevailing model it developed by amalgamation of four sub blocks by three spatially and temporally separate orogenic belts the first at 1950 ma formed the western block the second at 1900 ma created the eastern block and the third at 1850 ma unified the eastern and western blocks however new data show that the 1950 ma orogen was affected by granulite facies reequilibration at 1800 ma and parts of the 1850 ma orogen have evidence of metamorphism at 1950 1900 ma new zircon u pb ages enable the paleoproterozoic metamorphic events to be statistically sub divided into four at 1955 ma m 1 1920 ma m 2 1885 ma m 3 and 1850 ma m 4 the m 1 2 events always appear together mainly in two belt areas including the helanshan qianlishan northern ordos yinshan liangcheng huai an lushan jiaobei and liaodong regions whereas the m 3 4 events are distributed over larger areas and were superimposed on the m 1 2 activities this age distribution is inconsistent with previous models of three separate orogens it should also be noted that the igneous rocks contemporary with the regional ultra high temperature high pressure granulite facies metamorphisms m 1 2 were previously described as arc related series i e the gabbro norites i s type granites and trimodal volcanics based on the spatial distribution of the paleoproterozoic metamorphic events m 1 4 and of the coeval igneous rocks we speculate that the assembly of the ncc was created by one orogen which formed between the eastern and western cratons in two tectonic stages two marginal arcs the korean and xuwujia developed against both eastern sides of the two sub cratons and were both accreted and deformed at 1965 1900 ma m 1 2 subsequently the amalgamation of the two sub cratons resulted in metamorphism on the western side of the eastern craton and in metamorphism that was superimposed on both accreted arcs at 1900 1790 ma m 3 4

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

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 algoma type banded iron formations from eastern hebei north china craton shrimp u pb age origin and tectonic setting abstract algoma type bif hosted iron deposits occur in the shirengou xingshan erma lingshan and sijiaying areas of eastern hebei the deposits are hosted in archean rocks and their formation was related to volcanic and sedimentary processes in small volcano sedimentary basins supracrustal rocks of the qianxi and dantazi groups were metamorphosed from granulite facies to amphibolites facies available data indicate that these bif deposits mainly formed during paleoarchean and neoarchean times the paleoarchean bif hosted iron deposits are represented by the xingshan deposit that yielded a shrimp u pb zircon age of 3389 5 7 6 ma the other bif hosted iron deposits formed in the late neoarchean represented by the shirengou shuichang lingshan erma and sijiaying deposits that formed between 2605 ma and 2503 ma the late neoarchean bif iron deposits are considered to have formed under a mantle plume environment the eu anomalies of the ore samples from eastern hebei show that the bif ores formed in a hydrothermal related volcanic submarine environment where submarine hydrothermal effluents associated with volcanic activities may have supplied both the iron and silica for the bifs from eastern hebei

episodic mantle melting crustal reworking in the late neoarchean of the northwestern north china craton zircon ages of magmatic and metamorphic rocks from the yinshan block abstract the yinshan block in the northwestern north china craton comprises late neoarchean greenstones high grade metamorphic rocks granulite charnockite enderbite and amphibole gneiss diorites sanukitoid and mg adakite and granitoids predominantly trondhjemite tonalite a greenstone terrane comprises a volcanic sequence of predominant basalt and dacite with minor andesite zircons from a high mg andesite and an andesite have weighted mean 207pb 206pb ages of 2533 5 ma and 2510 7 ma respectively which reflect the timing of two distinct volcanic episodes these two either predated or postdated a major volcanic episode that formed a basalt dacite bimodal suite during 2515 10 basalt to 2516 10 ma dacite magmatic zircons from the high grade metamorphic rocks yielded weighted mean 207pb 206pb ages of 2544 5 ma for a mafic granulite 2521 6 ma for a garnetiferous gneiss 2535 5 ma for a charnockite and 2526 5 ma for an amphibole gneiss these ages reflect the minimum emplacement time of rocks that make up the protoliths of the high grade metamorphic complex the metamorphic ages of the high grade complex were determined at 2515 18 ma for mafic granulite and 2503 12 ma for garnetiferous gneiss zircons from a high mg diorite i e sanukitoid have a weighted mean age of 2520 6 ma which is significantly younger than the 2556 14 ma age of a magnesian diorite mg adakite from the same diorite massif these ages reflect distinctive magmatic pulses that generated the diorite massif a tonalite has a zircon age of 2516 7 ma indistinguishable from the 2520 9 ma age of a trondhjemite from the same trondhjemite tonalite dominated batholith a granitic compositional layer in the 2516 7 ma tonalite has zircon ages of 2515 6 ma high u zircon domains and 2512 22 ma low u zircon domains which reflect an intracrustal melting event the presence of an older crustal remnant ca 2600 ma prior to the formation of the yinshan block is indicated by a 2594 4 ma leucogranite xenolith in the diorite massif and by the widespread presence of ca 2583 3485 ma mostly ca 2700 ma zircon xenocrysts in a variety of volcanic and plutonic rocks an undeformed pegmatite dike in high mg andesite has an emplacement age of 2508 17 ma zircon an undeformed trondhjemite dike that cuts the supracrustal greenstone sequence yielded a zircon age of 2502 6 ma similar to that of the pegmatite dike and representing the latest archean magmatic event in the yinshan block these ages of undeformed granitoid dikes delimit the termination of thermo tectonic development of the yinshan block based on these new ages and published geological and geochronological data we conclude that the yinshan block is a typical neoarchean cratonic block not a segment of the so called paleoproterozoic inner mongolia north hebei orogen episodic mantle melting and resultant crustal reworking account for the thermo tectonic development of the yinshan block in a brief period ca 2556 2502 ma in the late neoarchean and based on their regional chronological correlations we argue for their craton scale significance

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

subduction polarity reversal at the junction between the western alps and the northern apennines italy abstract the controversial relationship between the orogenic segments of the western alps and the northern apennines is here explored integrating recently published 3d tomographic models of subduction with new and re interpreted geological observations from the eclogitic domain of the voltri massif ligurian alps italy where the two belts joint each other the voltri massif is here described as an extensional domain accommodating the opposing outward migration of the alpine and apennine thrust fronts since about 30 35 ma using tomographic images of the upper mantle and paleotectonic reconstructions we propose that this extensional setting represents the surface manifestation of an along strike change in polarity of the subducted oceanic slab whose polarity changed laterally in space and in time our tectonic model suggests that the westward shift of the alpine thrust front from the oligocene onward was the consequence of the toroidal asthenospheric flow induced by the retreat of the apenninic slab

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

geochemistry of neoarchean mafic volcanic rocks and late mafic dikes in the zanhuang complex central orogenic belt north china craton implications for geodynamic setting abstract an archean melange belt has been recognized in the zanhuang complex central orogenic belt cob north china craton ncc all units of the melange belt are intruded by a ca 2 5 ga granite pluton and cross cut by undeformed 2 5 ga pegmatites which constrains the archean formation age of the melange some exotic mafic ultramafic blocks are tectonically dispersed in the metasedimentary matrix to the melange and mainly consist of metabasaltic rocks deformed pillow lavas with epidosite cores gabbros and ultramafic rocks both metabasalts and epidosites are characterized by enrichment in light rare earth elements lree and pronounced negative nb and zr anomalies on chondrite and primitive mantle normalized diagrams these geochemical characteristics are consistent with supra subduction zone geochemical signatures based on the pillow structures and seafloor hydrothermal alteration combined with the geochemical characteristics an intra oceanic arc forearc setting is proposed for the formation of these exotic blocks furthermore a west dipping intra oceanic subduction zone is proposed to have formed in an ocean between the eastern block and an arc terrane prior to 2 5 ga the collision between the arc and the eastern block of the ncc happened at ca 2 5 ga resulting in the formation of the archean melange belt in the zanhuang complex all of the units of the melange belt were intruded by late mafic dikes which are present as boudinaged or lenticular shapes in the melange and surrounding gneisses and are intruded by younger ca 2 5 ga pegmatites geochemical characteristics of the late mafic dikes are consistent with an arc related mantle source region rather than an oib like source accordingly pending determination of their precise age an arc polarity reversal event is proposed to have occurred following the arc continent collision resulting in east dipping subduction beneath the newly accreted arc and eastern block forming the swarm of arc affinity mafic dikes the new subduction zone further resulted in ponded magma below the accreted arc block which induced the partial melting of thickened crust and the intrusion of ca 2 5 ga granitic and pegmatitic melts into the melange and the mafic dikes the mafic dikes were then deformed and metamorphosed when the remaining open part of the ocean between the arc collision modified margin of the eastern block collided with the western block of the ncc

ice covered water volcanism on ganymede eruption of liquid h2o magmas along extensional fractures and graben bounding normal faults may have played a critical role in the development of ganymede s grooved terrain the resurfacing potential of a water magma is dependent on a variety of factors including the areal extent of the source region the rate of discharge the thickness of the flow and the time that it takes the flow to completely freeze to its base in this paper the thermal evolution of such a flow is considered in detail the minimum unfrozen lifetime of a 5 m flow is approximately 12 5 days while a 10 m flow would survive for at least 50 days heating resulting from factional head loss could reasonably extend these lifetimes by 50 or more with a discharge rate of the order of 1 10 km3 d 1 an individual volcanic water flow could flood an area of 104 km2 before freezing as the flow solidifies its volume will increase thus lifting and arching its protective ice cover extensional fractures may then develop in the ice subparallel to the graben walls these fractures could result in grooves directly given a sufficiently thick 1 km flow or they may simply act to concentrate various tectonic forces that could initiate groove producing faults

slab breakoff as a mechanism for flipping of subduction polarity in taiwan we propose a model that involves breakoff of the east dipping eurasian slab beneath the taiwan orogen to account for the flipping of subduction polarity in northern taiwan the breakoff was initiated in southern ryukyu in the early pliocene and then propagated southwestward into taiwan like an opening zipper detachment of the eurasian slab has created a mantle window for the north dipping philippine sea plate to move in laterally causing a switch in the subduction polarity slab breakoff not only provides a testable model for the lithospheric structure of taiwan but also accounts for the high heat flow rapid uplift synorogenic extension deep focus earthquakes and north diminishing crustal contraction in the taiwan mountain belt

evolutionary model for the taiwan collision based on physical modelling abstract 2 d and 3 d physical modelling of lithospheric convergence in the luzon taiwan ryukyu region is performed with properly scaled laboratory models the lithospheric model consists of two parts continental the asian plate ap and oceanic the philippine sea plate psp the oceanic lithosphere has one layer while the continental lithosphere includes both mantle and crustal layers the continental margin is covered by sediments a low viscosity asthenosphere underlies the lithosphere the opposing luzon and ryukyu subduction zones are initiated by inclined cuts made within the psp the subduction collision is driven by a piston pre collisional intraoceanic subduction along the luzon and ryukyu boundaries results in the formation of a transform zone between them with two tear faults at the ends the psp undergoes strong compression along this zone subduction of the chinese margin under the luzon boundary further increases the compression compressive stresses reach the yield limit of the psp in the arc area which is a weak zone in the experiments the plate fails at the western side of the arc along an eastward dipping fault the longitudinal valley fault underthrusting of the frontal wedge of the psp along this fault results in the closure of the fore arc basin and is then blocked the psp fails at the opposite side of the luzon arc along the westward dipping fault the failure releases lithospheric compression in this region and results in the initiation of southward propagating subduction of the psp under northeastern taiwan the incipient subduction zone becomes part of the southeastward retreating ryukyu subduction zone which allows the okinawa back arc rift to propagate into taiwan the taiwan collision thus includes the following succession of major processes over time or from south to north 1 an e w shortening of the psp in the luzon arc 2 a failure of this plate at the western side of the arc and the formation of the eastward dipping longitudinal valley fault the transient plate boundary 3 a closure of the fore arc basin and a rapid uplift of the orogen 4 a failure of the psp at the eastern side of the luzon arc partly overthrusting the orogen and the initiation of westward wn ward subduction of the psp 5 and finally back arc rifting in the rear of this incipient subduction zone i e in northern taiwan all these processes commence with some delay with respect to the preceding ones and propagate southwards

geochronology and geochemistry of 2 5 to 2 4 ga granitic plutons from the southern margin of the north china craton implications for a tectonic transition from arc to post collisional setting abstract the xutai and lujiagou plutons exposed in the songshan jishan area in the southern margin of the north china craton ncc are studied to constrain the tectonic regime of the ncc between 2 5 and 2 4 ga the xutai pluton dated at 2509 33 ma mainly consists of tonalite and monzogranite with a calc alkaline affinity rittmann index

behavior of major and trace elements including ree during paleoproterozoic pedogenesis and diagenetic alteration of an archean granite near ville marie quebec canada abstract this paper presents a complete set of chemical data from a pre 2 0 ga weathering profile that formed on the archean granite near ville marie quebec examination of data from two sections of the paleosol indicates that na ca p and sr were lost from the paleosol whereas ti nb th zr hf and ta remained immobile during chemical weathering and all subsequent alteration rare earth mobility during genesis of the ville marie paleosol closely follows the pattern of early stages of chemical weathering in modern profiles average concentration of ree in both profiles is 20 40 lower than what is found in the fresh granite suggesting mobilization during pedogenesis hrees seem to be less mobilized than the lrees la sm n ratios gradually decrease toward the top of the paleosol but the values remain very close to that of the protolith gd yb n values remain unchanged in the weathering profile la yb n values in the weathering profile are quite variable and generally lower than that of the protolith eu eu and ce ce values are essentially the same as those of the protolith but two samples from the uppermost part of the profile have developed positive cerium anomaly of 0 3 relative to the protolith

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

upper plate deformation induced by subduction of a volcanic arc the snellius plateau molucca sea indonesia and mindanao philippines abstract the northern molucca sea shows the incipient subduction of a composite oceanic and arc volcanic block the snellius halmahera block shb multi beam reflectivity seismic and gravity data obtained during the modec marine survey showed that the shb disappears beneath the accretionary wedge and the outer ridge of the sangihe arc to the north in mindanao island ongoing convergence generated shortening of the forearc basin and the backthrusting of the shb meanwhile a classical system of paired subduction philippine trench and strike slip fault philippine fault was installed the transition from lithospheric subduction to crustal overthrusting is located where the philippine trench sensu stricto begins and also coincides with the off shore extension of the philippine fault we observe a reversal of the thrusts from an eastward vergence in the molucca sea to a westward vergence in mindanao island this reversal takes place at the latitude where the forearc area emerges by uplift and the downgoing crust shb deepens resulting in a strong gravity low centered above the accretionary wedge the philippine fault initiated in a place where the crust was sliced off by a transfer zone which marked the northern termination of the molucca sea and drags northward a sliver of the previously accreted shb the northward drifting of this sliver created an extension which however cannot account for the gravity low we propose that the shortening and the uplift of the upper plate were induced by the buoyancy of the subducted unit shb and triggered the thrust reversal

zircon ages and geochemistry of late neoarchean syenogranites in the north china craton a review abstract at the end of the neoarchean continental evolution voluminous syenogranites were emplaced in the north china craton together with other magmatic rocks trondhjemite tonalite granodiorite ttg monzogranite diorite gabbro syenogranites are widely distributed in anshan benxi qinhuangdao and western shandong and also occur in southern jilin northern liaoning northwestern hebei and central henan based on geological relationships degree of metamorphism deformation and magmatic zircon ages two phases of syenogranite magmatism are recognized rocks produced during the first phase show a gneissic texture and were formed between 2 53 and 2 52 ga and locally comprise abundant ttg rocks of the second phase cut late neoarchean ttg and supracrustal rocks display a massive structure and mainly formed between 2 52 and 2 50 ga all syenogranites share the same features in major element compositions being high in sio2 and low in cao total feo mgo tio2 and p2o5 however they are different in trace and ree compositions and can be subdivided into three types 1 type 1 shows a large variation in total ree contents low la yb n ratios strong negative eu eu anomalies and ba depletion 2 type 2 is similar to type 1 but has higher la yb n ratios 3 type 3 shows a large variation in total ree and la yb n ratios and significantly do not show strongly negative eu eu anomalies and ba depletion whole rock sm nd isotopic compositions show large variations in nd t values and tdm nd modal ages ranging from 9 49 to 4 72 and 3 70 to 3 25 ga type 1 0 55 1 03 and 2 77 2 71 ga type 2 and 2 35 to 1 23 and 2 93 2 66 ga type 3 respectively hf isotopic compositions of zircons from three samples have hf t values and tdm1 hf ages of 0 7 7 2 and 2 84 2 56 ga type 1 2 6 7 4 and 2 74 2 56 ga type 2 and 2 1 6 3 and 2 76 2 60 ga type 3 it is concluded that syenogranites were generated by melting of continental crust with different mean crustal residence ages and most of them were emplaced during the second phase 2 52 2 50 ga in an extensional tectonic regime the formation of these voluminous syenogranites marks a tectono magmatic event resulting in stabilization of the north china craton at the end of the neoarchean

a neoarchean subduction polarity reversal event in the north china craton abstract subduction polarity reversal events following arc continent arc arc or continent continent collisions have been well documented from cenozoic mesozoic and paleozoic orogens but not from the archean we here document a neoarchean subduction reversal event after an arc continent collision between the eastern block of the north china craton ncc and the fuping arc using field geochemical and geochronological data we focus our work on the wangjiazhuang granite in the zanhuang massif located along the eastern margin of the central orogenic belt cob of the ncc and a regional tectonic comparison with other granitic rocks with similar ages geochemical and petrogenetic characteristics the ca 2 5 ga a type wangjiazhuang granite intrudes the neoarchean zanhuang melange belt and contains mafic and felsic inclusions it has positive end t values 0 12 to 1 13 and tdm2 ages between 2784 ma and 2869 ma this work shows clearly from field structural relationships geochemistry and geochronology that the wangjiazhuang granite formed after an arc continent collision between the eastern block which is defined as a continental block and the fuping arc after a subduction polarity reversal event placed a new slab beneath the collisionally modified margin of the eastern block and converted it to an andean type margin the subduction polarity reversal event at ca 2 5 ga resulted in melting of the enriched mantle meanwhile the rising magma induced partial melting of the old and thickened ttg crust leading to the intrusion of ca 2 5 ga wangjiazhuang granite into the neoarchean zanhuang melange there are other granitic rocks with similar ages and geochemical and petrogenetic features in the central orogenic belt and eastern block of the north china craton suggesting that they formed in a similar tectonic setting as the circa 2 5 ga granites across the eastern block the neoarchean subduction polarity reversal event and prior arc continent collision provide strong evidence that plate tectonics was operating by the end of the neoarchean

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

archean crustal evolution in china u pb geochronology of the qianxi complex abstract the qianxi complex formerly the qianxi group in eastern hebei province is a key terrain for studying the archean crustal evolution of the sino korean craton in china the evolution of the complex involved several stages of supracrustal deposition and magmatic intrusion except for a few post tectonic intrusives all rock assemblages have been metamorphosed to high grade granulite and amphibolite facies the evolution of the complex has spanned more than 1 ga since about 3 5 ga ago in the present work two areas in the qianan region qianan county have been investigated for u pb age study 1 the caozhuang area in the south and 2 the shuichang yangyashan area in the north important tectonothermal events and magmatic episodes of the qianxi complex were identified by the u pb isotopic analyses of zircons extracted from various rock types the results of age determination and the most likely interpretation in terms of temporal evolution of the complex are summarized schematically the oldest zircons 3 65 3 67 ga of china have been found in a fuchsite bearing quartzite in association with the 3 5 ga sm nd isochron age amphibolites in the caozhuang area this emphasizes the existence of early archean sialic crust in the qianxi complex however no ages older than 2 9 ga have been revealed for grey gneisses enclosing the ca 3 5 ga amphibolite enclaves in the caozhuang area but a leptynite sample of dacitic composition gave a minimum 207pb 206pb age of 3 3 ga obtained by a single zircon stepwise evaporation analysis this age and the 3 5 ga sm nd isochron age for amphibolites suggest that the earliest deposition of supracrustal rocks took place about 3 5 3 3 ga ago the deposition of fuchsite quartzite could have taken place contemporaneously or in an earlier period up to 3 6 ga but its precise stratigraphic positions remains to be determined nevertheless the presence of very ancient zircons clearly advocate for the existence of sialic crust at least 3 6 ga in china in the shuichang yangyashan area a supracrustal sequence containing abundant banded iron formations was intruded by a granitic gneiss dated at 2 96 ga by the conventional multigrain zircon method sm nd model ages of about 3 3 ga were obtained for two amphibolite enclaves in a charnockitic gneiss it appears that supracrustal deposition of the qianxi complex might have migrated to the north and took place in the period of 3 3 3 0 ga in addition to supracrustal rocks the qianxi complex is characterized by polyphase granitoid intrusions except for the contaminated grey gneisses sm nd tdm 3 2 3 3 ga in the caozhuang area and the ca 3 ga granitic gneiss in the yangyashan area most granitoid intrusions are of late archean ages the period of 2 7 2 5 ga has undoubtedly recorded the most important tectonothermal events in the evolution of the sino korean craton the present zircon age study and available age data in the literature demonstrate that the archean crustal development in china culminated towards the end of the archean

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

a late archean tectonic melange in the central orogenic belt north china craton abstract laterally extensive belts of melange characterize phanerozoic convergent plate margins but are rare in archean terranes we document a late archean melange in the zanhuang massif of the north china craton ncc the zanhuang melange separates a passive margin to foreland basin sequence developed on the western edge of the eastern block of the ncc from an arc terrane consisting of trondhjemitic tonalitic and granodioritic ttg gneisses in the central orogenic belt cob of the ncc the melange belt contains a structurally complex tectonic mixture of metapelites metapsammites marbles and quartzites mixed with exotic tectonic blocks of ultramafic and metagabbroic rocks metabasalts that locally include relict pillow structures and ttg gneisses all units in the melange have been intruded by mafic dikes that were subsequently deformed and are now preserved as garnet amphibolite boudins we interpret the melange to mark the suture zone between the eastern block and the arc terrane in the cob field relationships and geochemistry suggest that the exotic ultramafic metagabbroic metabasaltic blocks are possible slivers of an intra oceanic arc or fore arc ophiolite incorporated into the melange during the arc continent collision process a circa 2 5 ga granitic pluton intrudes the melange and undeformed circa 2 5 ga pegmatites cut the melange tectonic models for the evolution of the cob are varied but include models that favor collision at 2 5 ga 2 1 ga and 1 8 ga this work shows clearly from field structural relationships and geochronology that the first collision must have occurred prior to 2 5 ga consistent with late archean suturing of the western margin of the eastern block with an arc terrane fuping terrane during an arc continent collision the presence of an archean melange with exotic blocks in a suture zone between an archean arc and continental margin is clear evidence for the operation of plate tectonics at circa 2 5 ga

palaeoproterozoic assembly of the north china craton the basement of the north china craton consists of the eastern and western blocks separated by the central zone both the eastern and western blocks are dominated by late archaean tonalitic trondhjemitic granodioritic gneiss complexes interdigitated with minor supracrustal rocks metamorphosed at 2 5 ga with anticlockwise p t paths the central zone is composed of reworked late archaean components and palaeoproterozoic juvenile crustal materials that underwent regional metamorphism at 1 85 ga with clockwise p t paths involving isothermal decompression as a result of collision between the eastern and western blocks which resulted in the final assembly of the north china craton

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

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

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

kinematic wave solutions for dam break floods in non uniform valleys abstract in non uniform valleys dam break flood waves can be significantly affected by downstream variations in river width slope and roughness to model these effects we derive new analytical solutions to the kinematic wave equation applicable to rating curves in the power law form and hydrographs of generic shape as long as they produce a single shock at the wave front new results are first obtained for uniform channels using the gauss green theorem applied to characteristic bounded regions of the x t plane the results are then extended to non uniform valleys using a change of variable that homogenizes river properties by rescaling the distance coordinate the solutions are illustrated and validated for idealized cases then applied to three historical dam failures the 2008 breaching failure of the tangjiashan landslide dam the 1976 piping failure of teton dam and the 1959 sudden failure of malpasset dam in spite of the much reduced computational cost and data requirements the results agree well with the field data and with more elaborate simulations they also clarify how both river and hydrograph properties affect flood propagation and attenuation

geochemistry of ultramafic rocks and hornblendite veins in the fiskenaesset layered anorthosite complex sw greenland evidence for hydrous upper mantle in the archean abstract the fiskenaesset complex sw greenland contains the world s best preserved archean 2970 ma layered anorthosite leucogabbro gabbro and ultramafic association the complex was emplaced into archean oceanic crust distal from continental lithosphere and later intruded by tonalites trondhjemites and granodiorites ttg constituting archean continental crust the complex and bordering ttg intrusions were variably affected by granulite facies metamorphism and retrogressed under amphibolite facies conditions this study presents new whole rock major and trace element petrographic and sem scanning electron microscope bse backscatter electron image data for a 45 60 m thick ultramafic sill and cross cutting hornblendite veins the ultramafic sill is composed of 75 olivine pyroxene hornblendite 15 pyroxene hornblendite and 10 hornblende pyroxenite despite granulite facies metamorphism and multiple phases of deformation the fiskenaesset ultramafic sill shares many petrographic characteristics of unmetamorphosed mafic to ultramafic layered intrusions and alaskan type ultramafic complexes hornblende appears to have originated as an igneous mineral but underwent extensive recrystallization during granulite facies metamorphism formation of orthopyroxene magnetite symplectitic vermicular intergrowths mainly at the expense of olivine is attributed to chemical reactions between late stage residual hydrous melts and olivine field geochemical and petrographic observations suggest that the ultramafic sill did not undergo significant fractional crystallization following its intrusion high mgo 16 31 wt contents suggest that the sill was emplaced as a crystal mush rather than as melt only petrographic observations and bse images indicate that olivine orthopyroxene and clinopyroxene were the major crystal phases whereas hornblende represents the main interstitial liquid phase in the crystal mush the sill exhibits three different ree patterns corresponding to three cryptic layers although the origin of these ree patterns is not fully understood they may reflect the intrusion of three batches of magma containing different hornblende forming melt crystal ratios geochemical data indicate that the cross cutting hornblendite veins likely originated from evolved late stage melts but were not related to the melts of the ultramafic sill large negative nb anomalies nb nb 0 04 0 66 suggest that the fiskenaesset hornblendites hornblende pyroxenites and hornblendite veins were derived from a hydrous sub arc mantle peridotite phanerozoic hornblendites are typically associated with supra subduction zone ophiolites and magmatic arcs accordingly it is suggested that water was recycled to the source of the fiskenaesset hornblendites through subduction of altered oceanic crust recycling of water to the upper mantle via subduction not only resulted in the generation of hornblendites but also played an important role in the formation of ttg dominated archean continental crust therefore the field petrographic and geochemical data presented in this study suggest that the origin of the mesoarchean to neoarchean terrane in the fiskenaesset region sw greenland is consistent with subduction zone geodynamic processes rather than non uniformitarian density driven vertical crustal overturn and diapiric processes

geophysical and geological tests of tectonic models of the north china craton abstract the geometry and timing of amalgamation of the north china craton have been controversial with three main models offering significantly different interpretations of regional structure geochronology and geological relationships one model suggests that the eastern and western blocks of the ncc formed separately in the archean and an active margin was developed on the eastern block between 2 5 and 1 85 ga when the two blocks collided above an east dipping subduction zone a second presumes the eastern block rifted from an unknown larger continent at circa 2 7 ga and experienced a collision with an arc perhaps attached to the western block above a west dipping subduction zone at 2 5 ga and the 1 85 ga metamorphism is related to a collision along the northern margin of the craton when the ncc joined the columbia supercontinent a third model suggests two collisions in the central orogenic belt at 2 1 and 1 88 ga but recognizes an early undated deformation event recent seismic results reveal details of the deep crustal and lithospheric structure that support both the second and third models showing that subduction beneath the central orogenic belt was west directed and that there is a second west dipping paleosubduction zone located to the east of the cob dipping beneath the western block ordos craton the boundaries identified through geophysics do not correlate with the boundaries of the trans north china orogen suggested in the first model and the subduction polarity is opposite that predicted by that model high pressure granulite facies metamorphism at 1 85 ga is not restricted to the tnco as suggested by the first model but is documented across the ncc as predicted by the second model suggesting a major continent continent collision along the north margin of the craton at 1 85 ga further it has recently been shown that in the southern tnco there is no record of metamorphism at circa 1 85 ga but only at 2 7 2 5 ga showing that the tnco as defined as a circa 1 85 ga orogen does not exist this is further confirmed by recent re os isotopic studies which show that the subcontinental lithospheric mantle beneath the southern cob is late archean in age and that a province in the northern ncc is circa 1 8 ga correlating with the proposed collision belt of the ncc with the columbia supercontinent across the entire ncc the cob is an archean convergent belt re worked in the paleoproterozoic and the paleoproterozoic tectonism is widespread across the ncc as predicted by the model whereby the previously amalgamated eastern and western blocks experienced a continental collision with columbia at circa 1 85 ga but uplift exhumation rates are slow necessitating a re evaluation of the tectonic models of the ncc

geochronology mantle source composition and geodynamic constraints on the origin of neoarchean mafic dikes in the zanhuang complex central orogenic belt north china craton abstract mafic granulitic and amphibolitic boudins dispersed in archean felsic gneisses are widely distributed in the central orogenic belt cob and the eastern block of the north china craton ncc and are considered to constitute deformed mafic dike swarms previous studies have demonstrated that the mafic dikes in the zanhuang complex of the ncc intruded the fabrics of an archean melange belt and were boudinaged during younger deformation igneous zircons from an undeformed mafic dike yield a 207 pb 206 pb age of 2535 30 ma which is interpreted as the crystallization age in addition pegmatites cutting across the mafic dikes in the field also yield an igneous zircon 207 pb 206 pb age of 2504 16 ma providing strong evidence that the mafic dikes in the ncc intruded during the neoarchean metamorphic zircons from one deformed mafic dike sample yield a metamorphic 207 pb 206 pb age of 2090 83 ma and another four samples from deformed mafic dikes have a consistent metamorphic zircon age of ca 1850 ma indicating that the mafic dikes underwent at least two generations of paleoproterozoic metamorphism of ca 2 1 ga and ca 1 85 ga previously reported trace element systematics of the mafic dikes are consistent with an arc related lithospheric mantle source region rather than an ocean island basalt oib like source region the new whole rock nd isotopic composition nd t 0 71 to 3 70 is relatively more evolved compared to that of the depleted mantle at 2 5 ga indicating an enriched lithospheric mantle source accordingly the mafic dikes are proposed to have been formed in a subduction related environment and their enriched mantle source was metasomatized by the melts and fluids derived from the subducted slab based on previous studies of the ncc and new geochronological and isotopic data in this contribution a new comprehensive tectonic model is proposed for the evolution of the ncc between 2 7 ga and 1 85 ga 1 from 2 7 to 2 5 ga an oceanic arc terrane belt developed in what is now the cob of the ncc while a passive margin sequence was deposited on the western edge of the eastern block on the margin of the intervening ocean 2 the oceanic arc terrane collided with the passive margin sequence at 2 5 ga above a west dipping subduction zone 3 the subduction polarity was reversed at ca 2 5 ga and a new east dipping subduction zone gave rise to the intrusion of the 2 5 ga mafic dikes of the ncc 4 the final collision between the western block and the eastern block occurred at ca 2 1 ga 5 the whole ncc then collided with the columbia supercontinent along the north margin of the craton at 1 9 1 8 ga

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

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

palaeoproterozoic metamorphic evolution and geochronology of the wugang block southeastern terminal of the trans north china orogen abstract amphibolites of the wugang block in the taihua metamorphic complex which is located in the southeastern terminal of the palaeoproterozoic trans north china orogen tnco preserve three generations of metamorphic mineral assemblages i e the prograde m 1 peak m 2 and retrograde m 3 assemblages forming during the palaeoproterozoic tectono metamorphic event these amphibolites record metamorphic p t conditions of 4 8 6 7 kbar 720 750 c m 1 9 0 10 6 kbar 710 780 c m 2 and 7 1 8 5 kbar 700 780 c m 3 which define clockwise p t paths involving isothermal decompression itd the metamorphic peak reached upper amphibolite facies and high resolution in situ sims u pb dating of metamorphic zircons suggests that the peak metamorphism took place at 1 96 1 92 ga in the wugang block the present new data suggest that the wugang block was also involved in the continental collision between the eastern and western blocks of the north china craton ncc in the palaeoproterozoic combining metamorphic and geochronological data concerning the tnco obtained by previous scholars it may be anticipated that the middle and southern sections of the tnco were involved in a long lived continental collision and the collision event can be possibly traced back to as early as 1 96 ga and lasted to as late as 1 80 ga

ca 2 5 billion year old coeval ultramafic mafic and syenitic dykes in eastern hebei implications for cratonization of the north china craton abstract a group of extremely rare coeval ultramafic mafic and syenitic dykes was discovered in the eastern hebei region of the north china craton these dykes intrude the 3 8 2 55 ga old caozhuang complex an olivine gabbro dyke and syenite dyke yield respectively shrimp zircon u pb ages of 2516 26 ma and 2504 11 ma interpreted as the magmatic crystallization age their zircons have single stage hf model ages 2677 ma and 2705 ma the olivine gabbros have mg values of 59 63 similar to high magnesian tholeiitic basalt they show relatively lree enriched patterns without eu anomalies la yb n 9 28 9 78 gd yb n 2 8 eu 0 96 1 00 with enrichment in liles and depletion in hfses they also contain high cr and ni and have zr hf and nb ta ratios similar to those of the primitive mantle the syenites are alkaline in composition with 8 67 8 88 wt na 2 o k 2 o and show high total ree contents 543 854 ppm and strong lree enriched patterns with minor eu negative anomalies la yb n 50 101 gd yb n 5 2 5 3 eu 0 80 0 82 trace element diagrams show strong lile enrichment relative to hfse zr hf and nb ta ratios are 47 49 and 26 76 respectively which are much higher than those of primitive mantle and higher than the average nb ta ratio of post archean continental crust the petrological and geochemical features indicate that these dykes were derived from a deep subcontinental lithospheric mantle source which implies that the ncc probably had continental crust of considerable thickness at this time combined with evidence for 2 5 ga granite intrusion and metamorphism in the eastern hebei region and adjacent areas we propose that the ncc has been a present scale craton at the end of archean

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

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

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

arc continent collision and subduction reversal in the cenozoic evolution of the northwest pacific an example from kamchatka ne russia abstract the principal new feature in the cenozoic evolution of the northwest pacific is based on the fact that two arc terranes of upper cretaceous paleogene age were incorporated in the kamchatka orogen during the early tertiary and at the end of the miocene respectively these two arc terranes so called achaivayam valaginskaya campanian lower paleocene and kronotskaya coniacian eocene arcs are separated by a suture zone that contains deformed slices of young paleocene lower eocene oceanic crust paleomagnetic data on the original paleolatitudes of the arc positions as well as geochemical data characterized the arcs as intraoceanic originated in the northwest pacific the arcs were separated by the vetlovka oceanic plate suggested to be a trapped fragment of the kula plate in which a new spreading zone developed in the early tertiary the principal event in the reconstructed evolution is the eocene collision of the extended achaivayam valaginskaya arc terrane with the continental margin of asia the margin is interpreted as the southeastern facing the pacific cretaceous accretionary boundary of the okhotsk microplate docked against the asian continent at 55 65 ma the arc continent collision developed progressively northward from the end of the paleocene early eocene in southern kamchatka to the middle eocene in the olutorka region the different stages of the collision occurred simultaneously along strike of the achaivayam valaginskaya arc according to the geodynamic model based on geological data for southern kamchatka collision included 1 the entrance of the continental margin in the subduction zone deformation of the overriding plate and subduction of fore arc block 2 arc obduction on the continental margin blocking of continental margin subduction and 3 subduction reversal with onset of oceanic subduction at the eastern side of the deformed arc this subduction reversal produced deformation and reverse faulting along the eastern side of the arc thus allowing accretion of tectonic slices of young oceanic crust of the vetlovka plate which began to subduct during the subduction reversal the subducted plate dipping to the southeast was broken off and detached at depth under the arc area the subduction polarity reversal the most characteristic feature of the collision model is strongly supported by structural and geologic data of southern kamchatka and explains the main tectonic metamorphic and magmatic events related to and postdating the collision

thrust emplacement of the hispaniola peridotite belt orogenic expression of the mid cretaceous caribbean arc polarity reversal articulo sobre el emplazamiento fallero en la zona de peridotitas de la hispaniola republica dominicana estudiando si es la expresion orogenica de la polaridad reversa en la zona del caribe en el cretacico medio

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