generation of leucogranites via fractional crystallization a case study of the jurassic bengbu granite in the southeastern north china craton abstract the magmatic processes that form leucogranites are closely related to plate collision as the products of crustal anatexis leucogranites provide clues to intracrustal differentiation and the regional tectonic evolution the bengbu area is located along the southeastern margin of the north china craton ncc 150 km north of the dabie orogen and is bounded by the tancheng lujiang fault zone to the east late jurassic leucogranites are widely distributed in the bengbu area we collected samples of the bengbu leucogranite blg and analyzed data on whole rock geochemistry zircon u pb geochronology zircon in situ lu hf isotopes quartz in situ o isotopes and total bulk sr nd pb isotopes zircon u pb dating yielded formation ages for these leucogranites of 160 ma the samples are characterized by high sio2 72 4 wt 76 9 wt alkali 7 3 wt 9 4 wt and al2o3 13 4 wt 15 5 wt contents in addition to two samples exhibiting the rare earth element tetrad effect all rocks have high differentiation index values 95 99 extremely low iron magnesium titanium oxide contents total fe2o3

mesozoic orogenic gold mineralization in the jiaodong peninsula china a focused event at 120 2 ma during cooling of pregold granite intrusions

destruction of ancient lower crust through magma underplating beneath jiaodong peninsula north china craton u pb and hf isotopic evidence from granulite xenoliths abstract zircons from granulite xenoliths entrained in a late cretaceous mafic dike in the jiaodong peninsula north china craton ncc show three distinct u pb age populations part of the old zircon grains yield discordant data that project to ages of about 2 4 to 2 5 ga a few grains indicate growth at about 2 0 ga and a third group yield cretaceous ages with peaks at 120 and 90 ma the oldest zircons give hf t dm model ages of 2 6 2 8 ga these results demonstrate the existence of original archean lower crust in the jiaodong region zircons of 2 0 ga have similar hf t dm model ages as the neoarchean paleoproterozoic grains suggesting that these zircons were products of metamorphic recrystallization due to thermal event without juvenile input early cretaceous zircons yield ehf t values of 21 to 12 and late cretaceous zircons large variable ehf t from 4 to 50 these data suggest that magmatic underplating occurred in the neoarchean to earliest proterozoic lower crust of the ncc both in the early and late cretaceous it is suggested that the mesozoic magma underplating which also provided the heat source for the voluminous mesozoic magmatism in the ncc significantly modified the composition of the archean to paleoproterozoic lower crust of the ncc

u pb and hf isotope analysis of zircons in mafic xenoliths from fuxian kimberlites evolution of the lower crust beneath the north china craton mafic xenoliths from the paleozoic fuxian kimberlites in the north china craton include garnet granulite and minor pyroxene amphibolite metagabbro anorthosite and pyroxenite the formation conditions of the amphibolites are estimated at 745 820 c and 7 6 8 8 kb 25 30 km the granulites probably are derived from greater depths in the lower crust lam icpms u pb dating of zircons from four granulites reveals multiple age populations recording episodes of magmatic intrusion and metamorphic recrystallisation concordant ages and upper intercept ages interpreted as minimum estimates for the time of magmatic crystallisation range from 2 620 to 2 430 ma in three granulites two amphibolites and two metagabbros lower intercept ages represented by near concordant zircons are interpreted as reflecting metamorphic recrystallisation and range from 1 927 to 1 852 ma one granulite contains two metamorphic zircon populations dated at 1 927 55 ma and 600 700 ma separated minerals from one granulite and one amphibolite yield sm nd isochron ages of 1 619 48 ma 143nd 144nd i 0 51078 and 1 716 120 ma 143nd 144nd i 0 51006 respectively these ages are interpreted as recording cooling following metamorphic resetting model ages for both samples are in the range 2 40 2 66 ga lam mc icpms analyses of zircon show a range in 176hf 177hf from 0 28116 to 0 28214 corresponding to a range of ehf from 34 to 12 the relationships between 207pb 206pb age and ehf show that 1 the granulites amphibolites and metagabbro were derived from a depleted mantle source at 2 6 2 75 ga 2 zircons in most samples underwent recrystallisation and pb loss for 100 200 ma after magmatic crystallisation consistent with a residence in the lower crust 3 metamorphic zircons in several samples represent new zircon growth incorporating hf liberated from breakdown of silicates with high lu hf 4 in other samples metamorphic and magmatic zircons have identical 176hf 177hf and the younger ages reflect complete resetting of u pb systems in older zircons the fuxian mafic xenoliths are interpreted as the products of basaltic underplating derived from a depleted mantle source in neoarchean time an important period of continental growth in the north china craton paleoproterozoic metamorphic ages indicate an important tectonic thermal event in the lower crust at 1 8 1 9 ga corresponding to the timing of collision between the eastern and western blocks that led to the final assembly of the north china craton the growth of metamorphic zircon at 600 700 ma may record an asthenospheric upwelling in neoproterozoic time related to uplift and a regional disconformity in the north china craton

the age isotopic signature and significance of the youngest mesozoic granitoids in the jiaodong terrane shandong province north china craton abstract most cretaceous granitoids in the jiaodong terrane of shandong province north china craton were intruded between 130 and 122 ma however four granitoid batholiths are significantly younger with an average zircon u pb shrimp emplacement age of 116 2 ma these are the sanfoshan batholith 118 1 ma the aishan batholith 116 1 ma the yashan pluton 113 2 ma and the laoshan batholith 115 2 ma these granitoids post date proposed delamination that occurred prior to 125 ma in the eastern ncc and their geochemistry thus provides information about the nature of the continental crust at this time the rocks are mostly monzogranites and syenogranites of i type affinity with local alkali feldspar granite of a type affinity in addition a monzogranite sample from the aishan batholith records an age of 125 3 ma indicating a direct association with the slightly older granitoids in the area indeed they all contain inherited zircons with ages of 125 120 ma indicating interaction with these earlier granitoids most samples are metaluminous enriched in light rare earth elements and depleted in high field strength elements paleo to neoarchean t dm2 model ages ranging from 3900 ma to 2491 ma and negative e hf t values imply the involvement of archean basement and older recycled continental crustal material in the magma source region they were likely derived from partial melting of the lower or middle crust due to mafic magma underplating followed by various degrees of interaction between mantle derived mafic magma and felsic crustal magma accompanied by fractional crystallization we propose that lithospheric thinning and extension in the early cretaceous due to roll back of the pacific plate beneath the eastern north china craton led to asthenospheric upwelling and basaltic magmatism this underplating in turn led to crustal melting and evolution of the granitic intrusions described here

geochemistry geochronology and sr nd hf isotopes of two types of early cretaceous granite porphyry dykes in the sulu orogenic belt eastern china late mesozoic granitic rocks are widely distributed in the sulu orogenic belt but the source tectonic affinity and associated geodynamic setting that produced the respective magmas remain contro

timing duration and role of magmatism in wide rift systems insights from the jiaodong peninsula china east asia in east asia widespread extensional sedimentary basins together with the close association of metamorphic core complexes and magmatism are prominent features of the large scale wide rift system developed during late mesozoic times this region thus appears as a proper place to study continental extension as well as questions about the link between continental extension and magmatism this paper primarily provides pioneer exhumation time constraints on the linglong mcc jiaodong peninsula including cooling and deformation ages mcc cooling at mid crustal levels occurred at ca 143 ma besides last ductile deformation occurred at ca 134 ma while final exhumation stages under brittle 2 conditions occurred as late as ca 128 ma continuous crustal stretching were then recorded by the emplacement of a synkinematic pluton in the upper crust at ca 128 ma that cooled fast below an intracrustal shear zone crossing the ductile brittle transition at ca 123 ma the ca 120 118 ma age cluster is ascribed to the fast cooling of undeformed plutons marking the end of extension that lasted in the area over a minimum period of ca 30 my combining other mcc exhumation constraints and the onset of subsidence in the sedimentary basins total duration of late mesozoic extension in east asia could be estimated at ca 60 my related with a rather long process for extension from 160 to at least 100 ma east asian continental extension is heterogeneously distributed in space and time as revealed by fundamental differences between two end member classes of migmatite cored mcc already described in other wide rift systems extension seems to have first favoured partial melting which subsequently in turn maintained continental extension

rb sr sm nd and pb isotope systematics of pyrite implications for the age and genesis of lode gold deposits lode gold deposits commonly consist of pyrite and lesser amounts of lead and zinc sulfides accompanied by quartz and calcite their exact origin remains controversial mainly because of the scarcity of reliable geochronological data here we present direct rb sr dating of pyrite from a lode gold deposit and propose the method as a useful geochronological technique for gold mineralization for which age data are poor a positive correlation between present day 87 sr 86 sr and 87 rb 86 sr ratios of pyrite fes 2 subsamples from the linglong gold mine of china corresponds to an age of 122 123 ma which dates the age of gold mineralization the sr nd and pb isotopic compositions of pyrites and the associated rocks suggest that the hydrothermal fluids responsible for the pyrite and gold mineralization were probably derived from a mixed source i e degassing of mafic magmas and meteoric water that had leached the country rocks

do s type granites commonly sample infracrustal sources new results from an integrated o u pb and hf isotope study of zircon in contrast to i type granites which commonly comprise infracrustal and supracrustal sources s type granites typically incorporate predominantly supracrustal sources the initial aim of this study was to identify the sources of three scottish caledonian 460 ma s type granites kemnay cove and nigg bay by conducting oxygen u pb and hf isotope analyses in zircon in order to characterise one potential end member magma involved in the genesis of the voluminous late caledonian 430 400 ma i type granites field whole rock geochemical and isotopic data are consistent with the generation of the s type granites by melting their dalradian supergroup country rocks while hf isotope compositions of magmatic zircon u pb data of inherited zircons and high mean zircon 18o values of 9 0 2 7 2sd and 9 8 2 0 for the kemnay and cove granites support this model the nigg bay granite contains zircons with much lower 18o values 6 8 2 1 similar to those found in scottish i type granites this suggests that the nigg bay granite contains low 18o material representing either altered supracrustal material or more likely an infracrustal source component with mantle like 18o mixing trends in plots of 18o vs ehf for s type granite zircons indicate involvement of at least two sources in all three granites this pilot study of scottish caledonian s type granites demonstrates that while field and whole rock geochemical data are consistent with local melting of only supracrustal sources the oxygen isotopic record stored in zircon reveals a much more complex petrogenetic evolution involving two or more magma sources

timing of destruction of the north china craton abstract the north china craton ncc is the typical example of destruction of an ancient craton however the destruction mechanism and geodynamic controlling factors still remain enigmatic due to controversy on the timing of destruction which is the key to understanding the destruction processes based on temporal and spatial distributions of the igneous rocks and their sources and tectonic settings it is recognized that six stages of tectono magmatism occurred in the ncc during the phanerozoic i e carboniferous to early permian late permian to middle triassic late triassic jurassic early cretaceous and cenozoic among them cenozoic magmatism mainly consists of alkali basalts and is found only occasionally in the eastern ncc the first four stages of magmatism and tectonism related to the southward subduction of the paleo asian plate and the assembly of sino korean and yangtze cratons are locally distributed in limited parts of the ncc reflecting a multiple stage modification of the ncc from the late carboniferous to jurassic however the intensive development of early cretaceous magmatism extensional deformation and associated gold mineralization with significant continental crustal growth indicate that the eastern ncc was destroyed during this time period this destruction was the result of paleo pacific subduction beneath the eastern asian continent with lithospheric removal and or replacement of an ancient cratonic lithosphere by a juvenile oceanic lithosphere

lower crustal processes leading to mesozoic lithospheric thinning beneath eastern north china underplating replacement and delamination abstract our recent studies show that the lower crust and lithospheric mantle in the eastern north china craton ncc was thinned in the mesozoic the lower crust played a key role in the tectonic mechanism that led to lithospheric thinning geochemical data from volcanic rocks and xenoliths support the idea that delamination of the lower crust and lithospheric mantle took place only along the northern and perhaps the southern margins of the ncc and this was linked to post collisional orogenic processes however it is difficult for the delamination process to act on the whole eastern ncc and remove 80 120 km of the lithosphere lower crustal xenoliths in mesozoic and cenozoic basalts within the ncc are of two kinds one is granulite facies metamorphosed two pyroxene gabbros and eclogite facies metamorphosed garnet pyroxenites formed at 140 120 ma the other is precambrian granulites that were strongly overprinted by mesozoic metamorphism large scale lower crustal replacement took place in the mesozoic beneath the ncc in other words some or perhaps most part of the present lower crust of the ncc is not precambrian the present lowermost crust is mostly composed of mesozoic meta gabbros and pyroxenites abundant late jurassic high sr granites in the ncc which are compositionally similar to a dakite probably formed by partial melting of the lowermost crust leaving a residue of eclogite or garnet amphibolite a hot upwelling mantle was necessary to partially melt the pre existing lowermost crust and underplate magma in order to form the present lower crust although eclogites could sink into the mantle it is hard to imagine that there was a thick enough eclogite layer that could drag 80 120 km of the lithosphere into the asthenosphere beneath the ncc magmatic underplating seems to be a better alternative mechanism thus the formation of the present lower crust through the underplating and replacement of the pre existing one was closely related to lithospheric thinning and represented a continuation of the same geodynamic process

large scale phreatomagmatic silicic volcanism a case study from new zealand abstract pyroclastic deposits of the 20 000 year old wairakei formation occur in the region surrounding lake taupo in the central north island of new zealand recent advances in the correlation of both proximal and distal equivalents of the wairakei deposits show that they were the result of one large eruption the ash layers once covered most of new zealand they are also present in deep sea cores from the sw pacific and on the chatham islands the dispersal and grain size parameters of the ash fall members indicate that their vent area was within the taupo volcano tectonic depression which for a long period has contained a large lake the wairakei eruption is interpreted to have been phreatomagmatic throughout each phase of the eruption sequence generated its own characteristic deposits the sequence of events can be summarized as follows 1 eruption and deposition of a white fine grained bedded fall deposit 2 eruption and deposition of a medium grained pumice fall deposit 3 generation of ultra fine wet ash clouds with a fall deposit accumulating largely as aggregates and accretionary lapilli accompanied by base surges 4 formation of water cooled pyroclastic flows and deposition of an extensive ignimbrite 5 return to a dominantly vertical eruption column and deposition of a medium grained bedded ash fall immediately followed by another wet ultra fine fall deposit charged with accretionary lapilli finally 6 an ignimbrite less widespread than that of phase 4 was produced co ignimbrite fall deposits of fine vitric dust and medium grained pumice were laid down over a huge area there may have been a short hiatus in activity between phases 4 and 5

contrasting origins of late mesozoic adakitic granitoids from the northwestern jiaodong peninsula east china implications for crustal thickening to delamination two suites of granitoids the late jurassic 158 3 ma linglong suite and the early cretaceous 130 126 ma guojialing suite crop out in the northwestern jiaodong peninsula eastern china the linglong suite is a monzogranite comprising alkali feldspar plagioclase quartz and fe rich biotite the guojialing suite includes at least five plutonic bodies of both granodiorite and monzo granite the rocks are composed of plagioclase alkali feldspar quartz mg rich amphibole and mg rich biotite both the linglong and guojialing suites have adakitic affinity they are enriched in lree with high la yb ratios and show positive eu anomalies the rocks are also enriched in lile and depleted in hfse with high sr y ratios the linglong granite shows very uniform sr nd isotopic compositions with initial 87 sr 86 sr ratios of 0 7119 0 7126 and e nd t values of 21 3 to 21 6 which are similar to those of the local neoarchaean basement the guojialing suite has variable initial 87 sr 86 sr ratios 0 7108 0 7120 and e nd t values 10 8 to 17 2 which are distinct both from those of the neoarchaean basement and from those of the local enriched lithospheric mantle inferred from the coeval mafic dykes in the studied area detailed petrological and geochemical data indicate that the linglong suite was derived by partial melting of neoarchaean metamorphic lower crustal rocks at depth of 50 km with a eclogite residue whereas the guojialing suite was formed by the reaction of delaminated eclogitic crust derived melt with the upwelling asthenospheric mantle the petrogenesis of these two contrasting adakitic granitoids suggests intensive lower crustal delamination during early cretaceous times following a crustal thickening process from the late stage of the early jurassic to early stage of the late jurassic with crustal thickness of 50 km respectively

precise determination of phanerozoic zircon pb pb age by multicollector sims without external standardization 1 zircon has long been recognized as the best geochronometer and the most important timekeeper in geosciences modern microbeam techniques such as sims and la icpms have been successfully applied to in situ u pb zircon age determinations at spatial resolutions of 20 30 m or less matrix matched calibration by external standardization of well characterized natural zircon references is a principal requirement for precise microbeam u pb zircon age determination due to fractionation effects between pb and u which usually result in an external age error exceeding 1 alternatively zircons with a closed u pb system can be directly dated by measurement of 207pb 206pb isotopic ratio without external standardization which has been a common practice for zircons older than 1 0 ga but not for relatively young 1 0 ga and particularly phanerozoic ones because of limitations of analytical precision we describe in this paper a method of 207pb 206pb measurement on phanerozoic zircons using a new generation of large radius magnetic sector multicollector cameca ims 1280 sims in combination with multicollector mode a nuclear magnetic resonance nmr magnet controller and oxygen flooding techniques we achieve precisions of 207pb 206pb ratio of 0 1 and 0 1 0 2 propagating to pb pb age errors 0 4 and 1 3 excluding u decay constant uncertainties for zircons of latest neoproterozoic and late paleozoic to mesozoic age respectively therefore the multicollector sims is capable of direct determination of zircon pb pb ages as young as mesozoic age with uncertainties of geological significance this technique is useful for direct dating of zircons in thin sections moreover it has significance for dating of some other u rich minerals i e baddeleyite and zirconolite that are not suitable for sims u pb dating by external standardization

particle fallout thermal disequilibrium and volcanic plumes we develop a model of a volcanic eruption column that includes the effects of fallout of pyroclasts and thermal disequilibrium we show that clast fallout with no thermal disequilibrium has only a small effect upon the column however disequilibrium changes column behaviour significantly and can even induce collapse our results may explain the lower plume heights and less widely dispersed fallout of cone forming eruptions contrasted with sheet forming eruptions the model also predicts that the transition from the gas thrust to the convective region in a column results in an inflection in dispersal curves that some features of the stratigraphy common to many fall deposits may result from column velocity structure and that there may exist a region near the volcanic vent in which maximum pyroclast size does not decrease significantly with distance

the initial giant umbrella cloud of the may 18th 1980 explosive eruption of mount st helens abstract the initial eruption column of may 18th 1980 reached nearly 30 km altitude and released 10 17 joules of thermal energy into the atmosphere in only a few minutes ascent of the cloud resulted in forced intrusion of a giant umbrella shaped cloud between altitudes of 10 and 20 km at radial horizontal velocities initially in excess of 50 m s the mushroom cloud expanded 15 km upwind forming a stagnation point where the radial expansion velocity and wind velocity were equal the cloud was initiated when the pyroclastic blast flow became buoyant the flow reduced its density as it moved away from the volcano by decompression by sedimentation and by mixing with and heating the surrounding air observations indicate that much of the flow covering an area of 600 km 2 became buoyant within 1 5 minutes and abruptly ascended to form the giant cloud calculations are presented for the amount of air that must have been entrained into the flow to make it buoyant assuming an initial temperature of 450 c and a magmatic origin for the explosion these calculations indicate that the flow became buoyant when its temperature was approximately 150 c and the flow consisted of a mixture of 3 25 10 11 kg of pyroclasts and 5 0 10 11 kg of air if sedimentation is considered these figures reduce to 1 1 10 11 kg of pyroclasts and 1 0 10 11 kg of air

magmatic evolution and crustal recycling for neoproterozoic strongly peraluminous granitoids from southern china hf and o isotopes in zircon zircons can retain a grain scale record of granitoid compositional evolution that is accessible through microanalysis in situ u pb hf and o isotope data yield new insights into the petrogenesis and evolution of the neoproterozoic strongly peraluminous granitoids of the jiangnan orogen jo southern china a negative correlation of th u versus d 18 o is found for most analyses some zircons from eastern jo granitoids show d 18 o variations of 3 6 from core to rim indicating a dramatic shift toward higher oxygen isotope values by voluminous partial melting of supracrustal rocks and signaling a transition from i type like to s type like magmas during the later stage of magmatic evolution this mechanism provides a reasonable explanation why some granitoids have intermediate geochemical features between s type and i type granites hf isotope trends indicate that a larger proportion of mature continental crust was incorporated into the magma sources of the western jo granitoids whereas more juvenile arc crust was incorporated into the eastern jo magmas no significant depleted mantle derived mafic magma was injected into the jo granitoid magmas instead radiogenic hf and nd signatures in jo granitoids reflect incorporated juvenile arc crust and document crustal growth in southern china during the early neoproterozoic ca 900 ma thus our zircon data suggest that strongly peraluminous granitoids which are widely regarded as the products of orogenesis that primarily recycle evolved crust can also record important information about early crustal growth

giant mesozoic gold provinces related to the destruction of the north china craton abstract lode gold deposits in precambrian cratons represent the world s major gold source and were mostly generated during formation and stabilization of the cratons however there is an extraordinary exception in the north china craton ncc where lode gold deposits formed after prolonged stabilization of the craton molybdenite re os and hydrothermal sericite and biotite 40 ar 39 ar dating of major gold deposits from the xiaoqinling district southern ncc bracket their emplacement in the range of 154 1 1 1 to 118 9 1 2 ma n 23 postdating formation of the craton by more than 1 7 billion years fluid inclusions extracted from gold bearing pyrite have elevated 3 he 4 he ratios 1 52 0 22 ra and mantle like ne isotopes 20 ne 22 ne 10 02 9 22 and 21 ne 22 ne 0 033 0 027 indicating presence of mantle derived fluids in the ore system measured 34 s of pyrite and d and 18 o of hydrothermal micas and fluid inclusion waters in auriferous quartz further confirm a magmatic mantle source for sulfur and ore fluids gold deposits of similar ages also widely occur in the eastern and northern margins of the ncc which together with those in the xiaoqinling district have a total reserve of 2500 t gold forming the only known giant late mesozoic gold province in the world s precambrian cratons these deposits formed coevally with extensive felsic to mafic magmatism development of intracontinental rift basins and exhumation of metamorphic core complexes across the eastern ncc events interpreted as indicating thinning and destruction of the lithosphere beneath the craton rising of asthenosphere coupled with destruction of the lithosphere has generated voluminous mafic and felsic magmas that provided sufficient fluids sulfur and by inference other ore components to form the giant gold provinces

titanite scale insights into multi stage magma mixing in early cretaceous of nw jiaodong terrane north china craton abstract the early cretaceous guojialing type granodiorites in northwestern jiaodong terrane carry significant records for strong mantle crust interaction during the destruction of north china craton ncc however the definite petrogenetic mechanism and detailed magmatic process remain an enigma titanite in igneous rocks can serve as an effective petrogenetic indicator here we present integrated geochronological and geochemical studies on titanites from guojialing type granodiorites and their dioritic enclaves to constrain their petrogenesis titanites from granodiorites g type and plagioclase rich dioritic enclaves e type i present an identical u pb age 130 ma and an indistinguishable wide range of zr and total rees contents and th u ratios however these two types of titanites exhibit distinct micro scale textures and geochemical compositions g type titanites are characterized by oscillatory zonings with two light bse zones lbz and two or three dark bse zones whereas e type i titanites are marked by core mantle rim zonings drastic increase of lrees zr hf and fe and decrease of nb ta al and f contents are observed in lbz of g type titanites whereas remarkable reduction of lrees zr and hf and elevation of f contents are observed from the cores to the mantles of e type i titanites based on zr in titanite thermometry g type titanites are interpreted to have experienced twice notable temperature increase while e type i titanites are inferred to have undergone a rapid cooling process furthermore we suggest that the drastic chemical changes in g type and e type i titanites are ascribed to early stage magma mixing between a colder felsic magma and a fe ree rich hotter dioritic magma compared to g type and e type i titanites titanites from plagioclase poor dioritic enclaves e type ii are characterized by their occurrence in interstitial space and present a relatively younger u pb age 128 ma and much narrower and lower range of zr total rees contents and th u ratios but reveal high f contents 0 35 0 76 wt and extreme high nb ta ratios up to 65 6 such titanites are perceived to record late stage mingling during which f rich and ree poor hybrid granodioritic magma squeezed into the incompletely consolidated dioritic enclaves with accompanying fluid rock interaction combining our results with previous isotopic studies a new genetic model for guojialing type granodiorites is envisaged which involves multi stage magma mixing between archean lower crust derived felsic magma and mafic lower crust derived dioritic magma triggered by mantle derived mafic magma underplating during the course of asthenospheric upwelling in early cretaceous such process further implicates the reactivation of jiaodong lower crust during the destruction of ncc

u pb ages and trace elements of detrital zircons from early cretaceous sedimentary rocks in the jiaolai basin north margin of the sulu uhp terrane provenances and tectonic implications abstract the dabie sulu orogen in east central china is the largest uhp belt in the world many studies have dealt with detrital minerals from sedimentary basins surrounding the dabie belt in order to constrain the tectonic unroofing of this terrane on the other hand still few are the works focused on detritals from the basins associated with the sulu belt the jiaolai basin adjacent to the sulu belt is made of mesozoic sedimentary rocks two sandstone samples collected from the early cretaceous laiyang group in the northern and southern margins of the jiaolai basin were selected for zircon u pb dating and trace element analysis the zircons show main u pb age clusters at ca 2500 2400 2000 1700 850 700 and 330 130 ma with a minor peak at ca 1100 ma and a few archean ages most zircons show texture features reflecting a magmatic origin whereas several late paleoproterozoic and triassic zircons show homogeneous cl images and low th u ratios indicating a metamorphic origin the neoproterozoic ages are consistent with the protolith ages of the metamorphic rocks in the sulu terrane implying that the sulu tarrane may be a major source for this component whereas most neoproterozoic zircons are euhedral to subhedral or angular in shape and are not surrounded by overgrowths this implies that low grade metamorphic neoproterozoic metaigneous rocks ocurring along the northern margin of the sulu terrane or metasediments overlying the hp uhp rocks in the sulu terrane might provide source materials to these detrital zircons the late and early paleoproterozoic age peaks are consistent with the thermal events occurred in the jiaobei terrane slightly younger than those in the yangtze block these suggest that the jiaobei terrane was a potential source for the paleoproterozoic to archean detrital zircon components only two early paleozoic zircon grains were found in the studied samples indicating the early paleozoic arc did not extend into the sulu terrane according to the distribution of different kinds of rock adjacent to the sulu terrane the early cretaceous detrital zircons may be potentially derived from cretaceous plutons from the sulu belt eight detrital zircons including six metamorphic grians and two metamorphic overgrowth rims yield the triassic ages which might be derived from hp uhp rocks from the sulu terrane a metamorphic overgrowth rim yields a metamorphic age of 246 3 ma probably recording the timing of the prograde metamorphism of the sulu terrane one metamorphic grain yields an age of 212 2 ma which is interpreted to record a retrograde metamorphic event based on the ree patterns the remaining six triassic analyses can be subdivided into two groups four of them record a weighted mean age of 230 6 ma and show nearly flat hree patterns and slight negative or positive eu anomalies the last two grains are enriched hree considering their ages and the ree patterns it is suggested that they may originate from other diverse uhp rocks the triassic metamorphic zircons were detected in the laiyang group indicating that the sulu uhp rocks were exposed to the surface before the early cretaceous

linking lithospheric thinning and magmatic evolution of late jurassic to early cretaceous granitoids in the jiaobei terrane southeastern north china craton abstract late mesozoic granitoids are widely distributed in the jiaobei terrane southeastern north china craton ncc and crucial to understanding the processes and mechanisms of the lithospheric thinning here we present petro geochemical zircon u pb lu hf and o isotopic constrains on three stages of granitoids building an integrated model of lithospheric thinning from beginning through peak to waning stages the early stage linglong granite 166 158 ma is geochemically comparable to adakitic rocks with negative nd t 21 3 to 19 2 zircon hf t 26 8 to 18 7 and high zircon 18o 6 7 8 3 it was originated from the subduction thickened lower crust of the eastern ncc with minor contribution from the subducted yangtze block in an extensional setting representing the beginning of lithospheric thinning the middle stage guojialing granodiorite 130 128 ma characterized by higher sr y ba 898 5075 ppm sr 657 2256 ppm nd t 19 1 to 11 5 zircon hf t 19 0 to 11 6 and 18o 7 5 8 7 than the linglong granite addressing a fluid metasomatized lithospheric mantle and extensive crust mantle interaction in the paleoproterozoic crustal source is coeval with peak lithospheric thinning due to roll back of the subducted pacific plate the late stage aishan complex 118 115 ma which is composed of granodiorite syenogranite porphyritic quartz syenogranite alkali granite and monzodiorite exhibits slightly lower nd t 17 9 to 16 2 zircon hf t 21 4 to 15 3 and 18o 6 6 7 9 than the guojialing granodiorite it was derived from the melting of neoarchean paleoproterozoic crust with limited crust mantle interaction and experienced crustal assimilation and fractional crystallization indicating an extended process associated with the waning stage of lithospheric thinning we document the magmatic responses to the lithospheric thinning at different stages which account for the genesis and evolution of the continental adakitic rocks and decratonization of the ncc

geochronological framework of the xiadian gold deposit in the jiaodong province china implications for the timing of gold mineralization abstract the xiadian gold deposit located in the zhaoyuan laizhou belt of the jiaodong gold province is a typical jiaojia type gold deposit which is characterized by disseminated and stockwork ores enclosed by hydrothermally altered wall rocks the exact age and the genesis of the gold deposit remain controversial here we present precise in situ monazite u pb dating to constrain the age of the gold mineralization which we integrate with published geochronological work on the associated geological events to understand the genesis of the gold deposit the linglong granite represents the earliest magmatic event at xiadian and yielded a zircon la icpms u pb age of 159 5 0 9 ma 2 mswd 0 63 subsequent geologic events were recorded by minor amounts of quartz pyrite molybdenite veins which are dated at 124 8 2 1 ma 2 mswd 0 01 by molybdenite re os dating in a granitic pegmatite the formation of such veins was close to the emplacement of adjacent guojialing granodiorite 130 ma and thus may be considered as a product of post magmatic hydrothermal activity it is proposed that the molybdenum bearing hydrothermal fluids acted as a prelude to gold mineralization and participated in the formation of the latter ore fluids gold mineralization took place at 120 0 1 4 ma 2 mswd 0 59 determined by la icpms u pb dating on hydrothermal monazite from quartz polymetallic sulfide veins prior to and post mineralization voluminous hydrothermally altered porphyritic diorite and fresh quartz diorite porphyry dykes were emplaced which yielded u pb ages of 121 3 1 4 ma 2 mswd 0 28 and 115 8 1 9 ma 2 mswd 0 71 respectively based on these geochronological data we have established a comprehensive geochronologic framework which suggests that the genesis of the xiadian deposit might be related to the craton destruction and lithosphere thinning in the north china craton

the influence of buoyancy flux from estuaries on continental shelf circulation abstract the release of freshwater from a midlatitude estuary to the continental shelf is modeled numerically as a rossby adjustment problem using a primitive equation model as the initial salinity front is relaxed a first baroclinic mode kelvin wave propagates into the estuary while along the continental shelf the disturbance travels in the direction of coastally trapped waves but with a relatively slow propagation speed when a submarine canyon extends offshore from the estuary the joint effect of baroclinicity and bottom relief provides forcing for barotropic flow the disturbance now propagates along the shelf at the first coastally trapped wave mode phase speed and the shelf circulation is significantly more energetic and barotropic than in the case without the canyon for both the experiments with and without a canyon an anticyclonic circulation generated by the surface outflow and deeper inflow over changing bottom topography is formed off the mouth of the estuary as the deeper inflow enco

reactivation of the archean lower crust implications for zircon geochronology elemental and sr nd hf isotopic geochemistry of late mesozoic granitoids from northwestern jiaodong terrane the north china craton the late mesozoic granitoids widely distributed in the northwestern jiaodong terrane are important markers of cratonic destruction and lithospheric thinning of the eastern north china craton ncc here we investigate the late jurassic linglong and luanjiahe granites and report zircon u pb emplacement ages of 157 159 ma these rocks also contain abundant late archean paleoproterozoic neoproterozoic early paleozoic and triassic inherited zircons suggesting the involvement of continental crustal materials from both the ncc and yangtze craton in magma tectonics the rocks investigated in this study show high na2o k2o and low mgo and are peraluminous with enrichment in lrees and liles rb ba u and sr and depletion in hfses nb ta p and ti they also display low ehf t values and high sr y ratios comparable to adakitic rocks suggesting that the linglong and luanjiahe granitoids formed under relatively high pressure conditions and were likely derived from the partial melting of the thickened lower crust of the ncc the guojialing granodiorites were emplaced in the early cretaceous 129 ma and also contain abundant late archean and paleoproterozoic inherited zircons the rocks possess high cao tfe2o3 and mgo and are metaluminous with enrichment in lrees and liles and depletion in hfses they are also characterized by high sr y ratios and have higher end t and ehf t values than the late jurassic granitoids suggesting the involvement of mantle components in the magmatic source we correlate the magma tectonics with the processes accompanying the subduction of the pacific plate beneath the ncc and the associated asthenospheric upwelling

juvenile vs recycled crust in ne china zircon u pb geochronology hf isotope and an integrated model for mesozoic gold mineralization in the jiaodong peninsula abstract the continental crust of the north china craton ncc is a major reservoir of mineral resources with imprints of secular changes in tectonics and metallogeny the jiaodong peninsula located in the eastern margin of the north china craton ncc is currently one of the largest gold producers over the globe and preserves the records of multiple magmatic and metamorphic events here we characterize the timing and tectonics of the major mesozoic magmatism and the associated gold metallogeny in this region through a comprehensive u pb geochronological and hf isotope investigation of zircons in a suite of granitoids mafic magmatic enclaves melanocratic dikes and melted basement rocks the linglong granite hosting one of the major gold deposits in jiaodong shows emplacement ages between 150 and 160 ma and the dominantly negative e hf t values 34 0 to 23 8 of zircons from this intrusion suggest magma derivation from recycled components in the archean basement the guojialing granodiorite and its mafic magmatic enclaves show similar ages between 123 and 127 ma with negative e hf t values 19 3 to 16 8 corresponding to crustal magma source the melanocratic dikes belonging to pre and syn mineralization stages with u pb age range of 126 to 166 ma display large variation in their zircon e hf t values 25 7 and 2 3 suggesting the involvement of both recycled crustal and juvenile mantle components zircons in the melted basement rocks with ages in the range of ca 127 132 ma also display both positive and negative e hf t values 44 6 and 9 8 indicating a mixture of recycled ancient crust and juvenile magmas our study shows that although the peak of gold metallogeny coincided with the tectonics associated with pacific plate subduction which mobilized and concentrated the ores the source materials of gold mineralization and magmatism had multiple origins including from the precambrian basement rocks mesozoic granitoids and mantle derived mafic magmas with extensive mixing of crustal lithosphere mantle and asthenospheric components a combination of delamination mantle upwelling subduction related metasomatic enrichment and recycling of ancient components facilitated the gold metallogeny in this region our study provides a typical case of juvenile and recycled components in the formation and evolution of continental crust and associated mineral resources

timing of metamorphism in the paleoproterozoic jiao liao ji belt new shrimp u pb zircon dating of granulites gneisses and marbles of the jiaobei massif in the north china craton abstract the paleoproterozoic jiao liao ji belt lies in the eastern block of the north china craton with its southern segment extending across the bohai sea into the jiaobei massif high pressure pelitic and mafic granulites have been recently recognized in the paleoproterozoic jingshan group jiaobei massif new shrimp u th pb geochronology combined with cathodoluminescence cl imaging of zircon has been applied to the determination of the timing of the metamorphism of the high temperature and high pressure granulites and associated gneisses and marbles metamorphic zircons in these high pressure granulites gneisses and marbles occur as either single grains or overgrowth or recrystallization rims surrounding and truncating oscillatory zoned magmatic zircon cores metamorphic zircons are all characterized by nebulous zoning or being structureless with high luminescence and relatively low th u values metamorphic zircons from two high pressure mafic granulites yielded 207pb 206pb ages of 1956 41 ma and 1884 24 ma one metamorphic zircon from a garnet sillimanite gneiss also gave an apparent 207pb 206pb age of 1939 15 ma these results are consistent with interval of ages of c 1 93 1 90 ga already obtained by previous studies for the north and south liaohe groups and the laoling group in the northern segment of the jiao liao ji belt metamorphic zircons from a high pressure pelitic granulite and two pelitic gneisses yielded weighted mean 207pb 206pb ages of 1837 8 ma 1821 8 ma and 1836 8 ma respectively two diopside olivine phlogopite marbles yielded weighted mean 207pb 206pb ages of 1817 9 ma and 1790 6 ma these paleoproterozoic metamorphic ages are largely in accordance with metamorphic ages of c 1 85 ga produced from the ji an group in the northern segment of the jiao liao ji belt and c 1 86 1 80 ga obtained for the high pressure pelitic granulites from the jingshan group in the southern segment as this metamorphic event was coeval with the emplacement of a type granites in the jiao liao ji belt and its adjacent areas it is interpreted as having resulted from a post orogenic or anorogenic extensional event

2 7ga crust growth in the north china craton abstract numerous u pb zircon geochronological data show that rocks of the precambrian basement from the north china craton ncc have two age peaks at 1 8 1 9 and 2 5 ga respectively whether the most prominent peak at 2 5 ga really registered a period of major crustal growth in the ncc thus making it different from the global major crustal growth period of 2 7 ga is uncertain in this paper we carried out in situ u pb and hf isotopes of zircons and whole rock chemical and sr nd isotopic analyses for six precambrian samples from the ncc in order to understand crustal growth and reworking the samples have u pb zircon ages of 1 8 1 9 2 5 and 2 7 ga respectively the two samples with u pb zircon ages of 2 7 ga have zircon hf model ages t hf dm and or whole rock nd model ages t nd dm close to their u pb zircon ages it is clear that they represent juvenile crust generation thus providing direct evidence that 2 7 ga was a period of new crustal addition the samples with u pb zircon ages of 1 8 and 2 5 ga also have t hf dm and or t nd dm similar to those of the 2 7 ga samples indicating that these samples also have a mantle extraction time of 2 7 0 1 ga and their 1 8 and 2 5 ga ages may represent periods of reworking of the 2 7 0 1 ga crust rather than juvenile crust generation the reworking is manifested either as metamorphism or as lower crustal derived magmatism specifically the 2 5 ga tonalitic gneiss sample is thought to be derived by melting of the 2 7 ga lower crust since many 2 5 ga tonalite trondhjemite granodiorites ttgs and calc alkaline granitoids in the ncc contain 2 7 ga xenocrystic and inherited zircons we postulate that they are probably also derived mainly from the 2 7 0 1 ga crust our data imply that the quantity of 2 7 ga continental crust in the ncc is probably greater in extent than that exposed today

geochronological and sulfur isotopic evidence for the genesis of the post magmatic deeply sourced and anomalously gold rich daliuhang orogenic deposit jiaodong china the daliuhang gold deposit 20 t gold is located in the central penglai qixia belt of the giant jiaodong gold province eastern china the ore hosting guojialing granodiorite and pegmatite were formed at 129 0 0 6 ma and 126 2 0 6 ma respectively syn ore monazite with a u pb age of 120 5 1 7 ma represents the timing of gold mineralization given at least 5 m y between magmatism and mineralization a genetic relationship to magmatic hydrothermal activity is negated noble gas isotopes of pyrite have crust mantle mixed 3he 4he 1 13 to 1 50 ra and air like 40ar 36ar 327 574 together with the broadly positive correlation between 3he and 36ar it is inferred that the initial ore forming fluids were deeply sourced from the sedimentary wedge overlying the subducted plate and overlying mantle during early cretaceous paleo pacific plate subduction the pre ore and post ore pyrites have low 34s values 3 7 5 6 and 5 3 6 4 respectively whereas ore related pyrites have higher 34s values especially in as au rich domains 7 8 8 3 these positive 34s values also suggest that the initial ore fluid and some of the sulfur component were derived via subduction related devolatilization the elevated 34s values of the ore related pyrite are partly ascribed to mass fractionation and partly to a contribution from sulfur leached from crustal host rocks this interpretation is also supported by neodymium isotope ratios of monazite end 120 ma 13 7 to 11 6 which correlate well with the ore hosting guojialing granodiorite this study highlights the combined roles of deeply derived fluids and intense interaction with upper crustal rocks in the formation of jiaodong gold deposits

evolution of the lithospheric mantle beneath the southeastern north china craton constraints from mafic dikes in the jiaobei terrain abstract two stages of mafic dikes with ages of 117 116 ma and 95 87 ma respectively are identified in the jiaobei terrain based on zircon u pb ages the early cretaceous mafic intermediate dikes ecmds and late cretaceous mafic dikes lcmds show distinct geochemical and isotopic features the ecmds are enriched in large ion lithophile elements lile rb sr ba u and th and light rare earth elements lree la ce pr nd sm and eu with a strong depletion in high field strength elements hfse nb ta ti and p they show uniformly high 87sr 86sr i 0 70841 0 71387 low end t values 18 1 to 13 2 positive 206pb 204pb i 17 07 17 90 and negative ehf t 21 0 to 6 1 values these rocks were derived from partial melting of an enriched lithospheric mantle and the enrichment of the mantle source resulted from source metasomatism imparted by the melts generated from remelting of the subducted lower crust of the yangtze craton yc in contrast the lcmds show lower sio2 41 5 50 1 wt lack of hfse depletion and are depleted in whole rock nd and zircon hf isotope they are interpreted to have sourced from a newly accreted depleted lithospheric mantle generated through continuous erosion of older enriched lithospheric mantle by asthenospheric upwelling leaving out minor remnants of the older mantle in the newly accreted lithospheric mantle the geochemical and isotopic characteristics reported in our study demonstrate that the lithospheric mantle beneath the southeastern north china craton ncc was generally enriched during the early cretaceous 120 ma and the mechanism of the enrichment may be related to source metasomatism caused by the subducted lower crust of the yc however in late cretaceous the lithospheric mantle beneath southeastern ncc was highly heterogeneous which might be related to asthenospheric non uniform erosion the 116 87 ma marks the transition from enriched lithospheric mantle to depleted asthenospheric mantle

modeling postbreakup landscape development and denudational history across the southeast african drakensberg escarpment margin 1 we employ a numerical surface processes model to study the controls on postbreakup landscape development and denudational history of the southeast african margin apatite fission track data presented in the companion paper suggest that the drakensberg escarpment formed by rapid postbreakup river incision seaward of a preexisting drainage divide located close to its present position and subsequently retreated at rates of only 100 m m y 1 numerical modeling results support such a scenario and show that the prebreakup topography of the margin has exerted a fundamental control on subsequent margin evolution the rheology of the lithosphere lithological variations in the eroding upper crust and inland base level falls provided secondary controls a relatively low flexural rigidity of the lithosphere te 10 km is required to explain the observed pattern of denudation as well as the observed geological structure of the southeast african margin lithological variations have contributed to the formation of flat topped ridges buttressing the main escarpment as well as major fluvial knickpoints both these features have previously been interpreted as supporting significant cenozoic uplift of the margin an inland base level fall possibly related to back cutting of the orange river drainage system and occurring 40 50 m y after breakup explains the observed denudation inland of the escarpment as well as the development of inland drainage parallel to the escarpment our model results suggest that in contrast to widely accepted inferences from classical geomorphic studies the southeast african margin has remained tectonically stable since breakup and escarpment retreat has been minimal 25 km

tectonics of the qinling belt build up and evolution of eastern asia the structure and history of the qinling range which separates the north and south china cratons are a key to the tectonic evolution of eastern asia the mountain range is composed of a northern palaeozoic belt and a southern mesozoic belt the palaeozoic orogeny probably involved the closure of an oceanic basin along a north dipping subduction zone followed by south vergent obduction of ophiolites and intracontinental thrusting during the devonian the mesozoic belt appears to be the product of purely intracontinental shallow crustal deformation over a large decollement with at least 100 km of horizontal transport since the devonian both belts have been sliced by huge east west sinistral strike slip faults

hf nd o isotopic evidence for melting of recycled sediments beneath the sulu orogen north china article the mantle source of late mesozoic mafic magmas in the southeastern north china craton is generally con sidered to have been enriched by deeply subducted continental lithosphere of the yangtze block the mechanisms through which enrichment occurs are still unclear here we document an early cretaceous zircon u pb age at 115 2 ma dioritic intrusion with some geochemical features similar to sanukitoids i e al2o3 n 17 5 ba 1692 1819 ppm sr 1042 1111 ppm and 18 ozircon 6 2 0 2 f rom the sulu orogen in north china compared with the regional older 120 130 ma mafi cm agmas dykes the diorite has less radiogenic sr 87 sr

an integrated mineral system model for the gold deposits of the giant jiaodong province eastern china abstract the giant jiaodong gold province with more than 5000 t of proven gold resources is unique in that its mesozoic gold deposits are hosted in a precambrian basement block there has long been debate on the source of gold the mechanism for its extreme enrichment and a holistic genetic model for these spectacular deposits in order to improve understanding of these factors a mineral system model is proposed for the jiaodong gold deposits for the first time it is based on a detailed review of the architecture and composition of the underlying mantle lithosphere the geodynamic setting at the time of gold mineralization the geological and geochemical features of the deposits themselves and mechanisms of their preservation regional geodynamic reconstructions combined with the nature of mesozoic magmatic systems show that late mesozoic breakoff of the subduction slab and rollback of the paleo pacific plate caused asthenosphere upwelling and lithosphere extension and thinning robust geochronological studies demonstrate that the jiaodong gold deposits formed at ca 120 ma during a change of subduction direction of the paleo pacific plate and the synchronous transition of tectonic regime from regional compression to transpression or transtension prior to peak extension caused by thinning of the lithosphere geophysical exploration data and hf nd isotope mapping of the jiaodong peninsula reveal a relatively simple ancient crustal structure cut by near vertical shear or fault zones including the lithosphere scale tan lu and wulian yantai faults that provided the first order architecture for emplacement of mantle magmas and sub crustal fluids in terms of fertility the significant older pre mineralization crustal metamorphism combined with sulfur lead and strontium isotope ratios indicate a dominant sub crustal source for the ore components for the jiaodong gold deposits consistent with evidence for volatile and gold fertilized metasomatized mantle lithosphere beneath the north china block helium ar and c o isotope ratios implicate mantle sources of ore fluids with contamination by crustal components en route to crustal depositional sites in terms of crustal architecture combined geophysical structural mineralogical fluid inclusion and geochemical studies show that the deeply sourced ore fluids and metals advected upwards through lithosphere scale faults to deposit gold via sulfidation and phase separation in crustal scale nne ne trending fault systems at mid to upper crustal levels preservation of disseminated to stockwork style deposits and auriferous quartz pyrite vein deposits was enhanced by deep crustal formation combined with limited uplift and exhumation of the province the jiaodong gold deposits although commonly hosted in granite intrusions clearly post date them on the basis of robust and reliable geochronology the spatial but non temporal relationships of gold deposits with granites their strong structural control and their other characteristics clearly place them in the orogenic gold group it is suggested because of their uniqueness that they be termed jiaodong type orogenic gold deposits

the nature of orogenic crust in the central andes 1 the central andes 16 22 s are part of an active continental margin mountain belt and the result of shortening of the weak western edge of south america between the strong lithospheres of the subducting nazca plate and the underthrusting brazilian shield we have combined receiver function and surface wave dispersion results from the banjo seda project with other geophysical studies to characterize the nature of the continental crust and mantle lithospheric structure the major results are as follows 1 the crust supporting the high elevations is thick and has a felsic to intermediate bulk composition 2 the relatively strong brazilian lithosphere is underthrusting as far west 65 5 w as the high elevations of the western part of the eastern cordillera ec but does not underthrust the entire altiplano 3 the subcrustal lithosphere is delaminating piecemeal under the altiplano ec boundary but is not completely removed beneath the central altiplano the altiplano crust is characterized by a brittle upper crust decoupled from a very weak lower crust that is dominated by ductile deformation leading to lower crustal flow and flat topography in contrast in the high relief inland sloping regions of the ec and sub andean zone the upper crust is still strongly coupled across the basal thrust of the fold thrust belt to the underthrusting brazilian shield lithosphere subcrustal shortening between the altiplano and brazilian lithosphere appears to be accommodated by delamination near the altiplano ec boundary our study suggests that orogenic reworking may be an important part of the felsification of continental crust

tarawera 1886 new zealand a basaltic plinian fissure eruption abstract new zealand s biggest and most destructive volcanic eruption of historical times was that of tarawera in 1886 the resulting scoria fall has a dispersal very similar in extent to that of the vesuvius a d 79 pumice fall and is one of the few known examples of a basaltic deposit of plinian type a new estimate of the volume 2 km 3 is significantly greater than previous estimates the basalt came mainly from a 7 km length of fissure and emission and exit velocity were fairly uniform along at least 4 km of it this is one of the few documented examples of a plinian eruption from a fissure vent primary welding of the scoria fall resulted where the accumulation rate exceeded about 250 mm min 1 a model of the eruption dynamics is proposed which leads to an estimate of 28 km for the height of the eruption cloud and implies a magma volatile fraction of 1 5 3 violent phreatic explosions occurred in the southwestern extension of the fissure across the rotomahana geothermal field and it is thought that some of the water responsible for the power of the plinian eruption came from this source though its amount was not sufficient to turn the eruption into a phreatoplinian one

petrogenesis of cretaceous granitoids in the bengbu wuhe area southeastern north china craton implications for gold mineralization abstract sinistral slip along the tancheng lujiang fault zone tlfz during the mesozoic resulted in an offset of approximately 400 km between the dabie and sulu orogens the bengbu wuhe gold belt bwgb represents an extension of the zhaoyuan yexian gold belt to the west of the tlfz early cretaceous intrusions are widely distributed throughout the bwgb gold deposits in the bwgb are commonly associated with intermediate acid dykes that are roughly contemporaneous with mineralization and some orebodies are hosted in granitoids this paper investigates granitoids within the bwgb to constrain their petrogenesis tectonic setting and metallogenic significance using systematic whole rock elemental geochemistry apatite geochemistry zircon u pb dating zircon lu hf isotopes and whole rock sr nd and pb isotopes the timing of au mineralization 134 113 ma in the area is consistent with the age of intrusive adakite like rocks 130 115 ma and normal calc alkaline granitoids 114 109 ma the adakite like samples are characterized by pronounced enrichment in light rare earth elements ree depletion in heavy ree la yb n 30 5 66 2 an absence of eu anomalies high sr contents and sr y ratios low y and yb contents and fractionated ree patterns dy yb n 1 22 1 69 they have relatively low mg values 38 42 and variable sr nd pb hf isotopic compositions 87sr 86sr t 0 7083 0 7111 end t 22 2 to 14 9 206pb 204pb t 17 219 18 010 zircon ehf t 21 0 to 12 3 these features indicate that the parental magmas of the adakites were derived from partial melting of thickened lower crust of the south china plate and or the north china craton the normal calc alkaline granitoids have low sr y 6 0 40 5 la ybn 6 8 24 7 and dy ybn 0 90 1 28 ratios exhibit negative eu anomalies and have variable sr nd pb hf isotopic compositions 87sr 86sr t 0 7082 0 7085 end t 22 7 to 17 7 206pb 204pb t 16 686 16 713 zircon ehf t 24 9 to 19 0 indicating they were derived from the middle to lower crust of the north china craton compared with the normal calc alkaline granitoids the adakite like rocks have higher zircon ce4 ce3 higher apatite so3 and lower apatite mno contents lower and more variable zircon crystallization temperatures and higher apatite molar oh f and cl f ratios indicating that the adakitic magmas were more enriched in volatiles and more highly oxidized taken together field observations and geochemical data suggest that the adakite like rocks related to the subducted south china block are the ore causative rocks

shrimp u pb zircon dating of the wulian complex defining the boundary between the north and south china cratons in the sulu orogenic belt china abstract neoproterozoic metasedimentary and metavolcanic rocks of the wulian group are associated with gneissic granitoids at the northern margin of the sulu uhp belt adjacent to the wulian yantai fault and are collectively referred to as the wulian complex shrimp u pb dating of detrital zircon from the metasedimentary rocks of the wulian group indicates a range in age from 3478 to 625 ma based on zircons 3000 ma with the oldest age being 3339 ma 2700 2400 ma with a peak at 2436 ma 2200 1800 ma with a peak at 2156 ma and at 770 ma these results indicate that deposition of the wulian group was after 770 ma and that some detrital material most likely came from neoproterozoic sources in the south china craton scc shrimp u pb zircon dating of a metavolcanic rock defines a weighted mean 206 pb 238 u age of 761 8 ma which is consistent with other rocks of south china affinity and can be correlated with syn rift magmatism during the breakup of the supercontinent rodinia the wulian group together with neoproterozoic granites of the wulian complex constrains the location of the triassic suture zone between the north china and south china cratons based on our new data the suture must lie north of the wulian complex and is marked by the baichihe yantai fault

hf isotopic compositions of the standard zircons and baddeleyites used in u pb geochronology abstract zircon and baddeleyite u pb geochronological dating is widely used in solid earth sciences and the advent of rapid in situ methods of analysis such sims and icp ms has re emphasized the importance of having uniform standards recently it has been shown that hf isotopic data can provide important information on these minerals since they contain high concentrations of hf but have low lu hf ratios which results in negligible age correction however the complex internal structures that result from multiple thermal events such as inherited cores and metamorphic overgrowths require that the hf isotopic data be measured with high spatial resolution however the isobaric interferences of 176 yb and 176 lu on 176 hf hamper the precise determination of the 176 hf 177 hf ratio during in situ laser ablation mc icpms analysis it is shown here that mass biases of yb yb and hf hf change with time during analyses and behave differently for solutions and solid material therefore it is suggested that the mean yb value of the individual spot be used to obtain the precise isotopic composition for in situ zircon and baddeleyite hf isotopic analyses for low yb hf 176 yb 177 hf yb value have little effect on the accuracy of the hf isotopic composition however the appropriate yb isotopic abundance is necessary for high yb hf 176 yb 177 hf 0 001 zircons and baddeleyites since the interference of 176 yb on 176 hf is significant using the mean yb value of the individual spot and newly published yb isotopic abundance data six standard zircons and two standard baddeleyites have been investigated using a neptune mc icpms with 193 nm laser for zircons the obtained 176 hf 177 hf ratios are 0 282307 31 2sd for 91500 0 282680 31 2sd for temora 0 281729 21 2sd for cz3 282177 17 2sd for cn92 1 0 282983 17 2sd for fm0411 and 281234 11 2sd for phalaborwa the baddeleyites from phalaborwa and sk10 2 have 176 hf 177 hf ratios of 0 281238 12 and 0 282738 13 2sd these results agree well with the values obtained by the solution method and indicate that these standards have different hf isotopic compositions in which the extremely low 176 lu 177 hf and 176 yb 177 hf values of cz3 zircon and phalaborwa baddeleyite make them excellent standards for machine calibration during in situ zircon hf isotopic measurement with the other standards being more suitable for the development of the correction method

rapid exhumation of the northern jiaobei terrane north china craton in the early cretaceous insights from al in hornblende barometry and u pb geochronology abstract the guojialing and aishan plutons in the northern jiaobei terrane represent the magmatic products of the lithospheric thinning of the north china craton in the early cretaceous the exhumation history of the terrane in response to the thinning process and its effect on the formation and preservation of coeval gold deposits remain poorly constrained granodiorites from the guojialing pluton and monzogranites from the aishan pluton contain zircon grains with u pb ages of ca 130 ma and ca 116 ma respectively using the al in hornblende barometry the guojialing and aishan plutons had crystallization pressures of 3 2 5 4 kbar 12 km and 1 5 2 6 kbar 5 km respectively using the hornblende plagioclase thermometry the guojialing and aishan plutons had crystallization temperatures of 628 697 c and 702 745 c respectively fluid inclusion microthermometric data show that auriferous quartz veins in the daliuhang deposit formed at temperatures between 243 and 322 c at a depth of 7 11 km which was similar to the emplacement depth of the aishan pluton these thermobarometric and geochronological data reveal 7 km uplift between 130 ma and 116 ma but negligible exhumation for the gold deposits after its formation until 116 ma combining previous results we envisage that the exhumation magnitude of the guojialing intrusions were nearly the same in the wide area with slight differences between the core and margin of a single pluton numerous gold deposits in the jiaobei terrane formed during or after the late stage of rapid exhumation of granitoids and were uplifted with similar depths to the aishan pluton since ca 116 ma the rapid exhumation of the guojialing pluton was with the regional extension and might be genetically associated with the intensive delamination of destabilized lithospheric root and dense residues