rutile solubility and tio2 activity in silicate melts an experimental study abstract sixty eight new experiments on rutile solubility in synthetic silicate melts of a wide compositional range have been conducted at ambient pressure p at temperature t from 1230 to 1500 c the results support the previous findings namely 1 at fixed t addition of silica and alumina results in essential decrease of rutile solubility 2 at fixed composition increasing t results in essential increase in rutile solubility based on the new and literature experimental data 350 data points new equations describing rutile solubility in silicate melts as a function of t p and melt composition both empirical and consistent with the thermodynamic model of melts ghiorso and sack 1995 are derived the new equation predicts log xtio2 tio2 mole fraction in the melts with a standard error of 0 066 which is equivalent to ca 15 error in a wide range of melts major element composition and the xtio2 value in the range from 0 0008 to 0 39 tio2 from 0 1 to 44 8 wt covered by the experiments the equations are applied to calculate the tio2 activity values relative to rutile saturation atio2 m rut in a variety of well documented natural magmatic rocks the resulting atio2 m rut values for different tested samples vary from 0 1 to close to 1 simultaneously solving the new equations and the equation for ti in zircon geothermometer ferry and watson 2007 gives more reliable t estimates than those based on the assumption of constant atio2 m rut 0 6 or 1

dehydration melting of amphibolite at 10 kbar the effects of temperature and time we have simulated the dehydration melting of a natural low k calcic amphibolite 67 4 hornblende 32 5 anorthite in piston cylinder experiments at 10 kbar and 750 1000 c for 1 9 days the solidus temperature is lower than 750 c garnet appears at 850 c the overall reaction is hb p l cpx al hb ca hb ga opx three stages of reaction are 1 melting dominated by the growth of clinopyroxene and garnet with little change in composition of liquid or garnet 2 a reversal of this reaction between 875 c and 900 c with decreases in the amounts of liquid and garnet and 3 a large increase in liquid along with the loss of hornblende and decrease of plagioclase while clinopyroxene and garnet increase garnet is enriched in pyrope and zoned from fe cores to mg edges range 3 mol pyrope liquid composition is enriched first in an to 950 c and then in ab the liquids are more calcic and aluminous than natural tonalites which is attributed to the plagioclase composition an90 the formation of peraluminous liquid from the metaluminous amphibolite is caused by anorthite not h2o saturated conditions the results are consistent with an amphibolite phase diagram with relatively high solidus temperatures in the garnet absent field 900 1000 c but with a solidus backbend at 7 9 kbar coincident with the garnet in boundary hornblende breakdown due to garnet formation in a closed system must make h2o available for h2o undersaturated melting right down to the h2o saturated solidus below 700 c which defines a large low temperature pt area where hydrous granitoid melts can be generated with residual garnet and hornblende

middle archean continent formation by crustal delamination the processes that created the first large cratonic areas such as the pilbara and the kaapvaal remain poorly understood models based on the uniformitarian extrapolation of present day arc volcanic processes to a hotter early earth have not adequately explained the observations in these terranes here we propose an alternative mechanism for the formation of the earliest continental crust the formation of continental crust may be achieved by delamination of the lower eclogitic part of an oceanic plateau like protocrust such delamination results in uplift extension and the production of tonalite trondhjemite and granodiorite ttg suites as recorded in middle archean cratons the available geologic and geophysical observations in combination with model calculations permit this scenario as an alternative to subduction based hypotheses

rutile solubility in albite h2o and na2si3o7 h2o at high temperatures and pressures by in situ synchrotron radiation micro xrf abstract rutile is an important mineral host for high field strength elements so its solubility in geologic fluids at high pressure and temperature plays an important role in the crustal and mantle processes that control the terrestrial cycling of these elements however experimental measurements of rutile solubility are in conflict by a factor of more than 100 at most studied conditions we performed new measurements of rutile solubility in h 2 o albite and h 2 o na 2 si 3 o 7 ns3 fluids by in situ synchrotron radiation x ray fluorescence spectroscopy using modified bassett type hydrothermal diamond anvil cells minimum detection limits were 1 9 and 2 3 ppm ti by weight for the two cells three albite h 2 o experiments at starting bulk compositions of 2 7 6 7 and 10 3 wt albite involved spectral acquisition at rutile saturation in the presence of albite crystals melt or a single homogeneous fluid phase after accounting for the additional phases corrected fluid compositions were 0 6 to 7 5 wt dissolved silicate over the run conditions at 2 7 wt albite rutile dissolution rate was slow and steady state was not achieved at 600 800 c however at higher dissolved albite contents constant solubility with time was observed rutile solubilities in the presence of a single fluid phase at 700 c 0 79 gpa 5 4 wt albite and at 800 c 1 10 gpa 6 7 wt albite were 37 2 and 156 6 ppm respectively these data agree with results acquired using hydrothermal piston cylinder methods with long run times and suppression of new crystal growth but not with data derived from visual observation in hydrothermal diamond anvil cells this discrepancy is likely due to lack of equilibrium in the latter approach two experiments in 10 and 30 wt ns3 at 660 800 c 0 5 0 1 gpa show extreme concentration dependent enhancement of rutile solubility to 4500 ppm the data indicate a strong positive correlation between rutile solubility and na al because na k al is likely to be greater than unity in aqueous fluids at high pressure and temperature due to incongruent dissolution of albite and micas the increase in rutile solubility along the albite ns3 join points to the possibility of significant ti transport by silicate bearing aqueous fluids in the lower crust and upper mantle

petrogenesis of taxitic dioritic tonalitic gneisses and neoarchean crustal growth in eastern hebei north china craton abstract the taxitic dioritic tonalitic gneisses in the zunhua qinglong microblock of eastern hebei north china craton are composed of dioritic quartz dioritic tonalitic and trondhjemitic gneisses they show taxitic or banded structures and were subjected to amphibolite to local granulite facies metamorphism based on their whole rock geochemistry the dioritic and quartz dioritic gneisses dq group can be classified into a high ni subgroup hndq subgroup and a low ni subgroup lndq subgroup and the tonalitic and trondhjemitic gneisses tt group into a low fractionated subgroup lftt subgroup and a high fractionated subgroup hftt subgroup la icp ms zircon u pb isotopic dating reveals that the magmatic precursors of the hndq lndq and lftt subgroups were emplaced from 2522 13 to 2533 14 ma whereas those of the hftt subgroup emplaced at 2499 4 ma studies in petrogenesis revealed that the magmatic precursors of the hndq subgroup were derived from partial melting of mantle peridotites metasomatized chiefly by slab melts while those of the lftt subgroup were formed by fractional crystallization of the magmas of the hndq subgroup with clinopyroxene magnetite plagioclase and apatite as the major fractionated phases magmatic precursors of the lndq subgroup were derived from partial melting of mantle peridotites metasomatized in a dominant manner by slab derived fluids with melts experiencing fractionation of hornblende magnetite plagioclase and apatite during the magmatic evolution further magmatic precursors of the hftt subgroup were derived from partial melting of volcanic sedimentary rocks formed in the fore arc to trench belt which were subducted along with oceanic slabs with the melts contaminated by mantle peridotites during their ascent through the mantle wedge these new data combined with previous investigations on metavolcanic rocks and granitoid orthogneisses suggest that the magmatic precursors of these taxitic dioritic tonalitic gneisses were formed in an arc to back arc tectonic setting from north to south furthermore lu hf isotopic data from the late neoarchean metavolcanic rocks and granitoid gneisses in the zunhua qinglong microblock reveal that the zunhua qinglong microblock experienced three major periods of juvenile crustal growth during neoarchean at 2 72 2 70 ga 2 64 2 56 ga and 2 53 2 50 ga respectively with the latter two periods coupled with reworking of old crustal materials

the genesis of early carboniferous adakitic rocks at the southern margin of the alxa block north china abstract adakite is a highly debated petrologic term that was originally proposed to describe igneous rocks formed by slab melting subsequent studies reported other ways to generate adakitic signatures such as the melting of mafic lower continental crust and fractional crystallization of basaltic magma we studied adakitic rocks from the taohuala mountain at the southern margin of the alxa block north china these rocks are characterized by high sr concentrations 300 450 ppm high sr y 20 75 ppm and la yb n 25 67 ppm ratios and low y

geochronology geochemical and sr nd hf pb isotopic compositions of the granitoids in the yemaquan orefield east kunlun orogenic belt northern qinghai tibet plateau implications for magmatic fractional crystallization and sub solidus hydrothermal alteration abstract a general consensus has emerged that high field strength elements hfse can mobile to some extent in a hydrothermal fluid however there are hot debates on whether sub solidus hydrothermal alteration can lower the nb ta ratio in evolved melts in this study we present petrography geochronology and geochemistry of the barren and mineralized rocks in the yemaquan skarn iron deposit northern qinghai tibet plateau to probe magmatic hydrothermal transition the barren rocks consist of diorites granodiorites granites and syenogranites whereas the porphyritic granodiorites are associated with mineralization for an excellent consistency between the magmatic zircon u pb age 225 2 ma and the hydrothermal phlogopite 40 ar 39 ar age 225 1 5 ma the sr nd hf pb isotopic data demonstrate that the yemaquan granitoids are originated from a relatively homogenous enriched mantle with different degrees of crust contamination assimilation fractional crystallization afc trace elements signatures indicate that the porphyritic granodiorites related to mineralization display amphibole crystallization for high water contents whereas the barren granites have gone through biotite crystallization due to potassium enrichment by continuous upper crust contamination both of which are responsible for their nb ta ratios respectively modeling results suggest that a basaltic melt with nb ta ratio of 15 3 can reach a minimum nb ta ratio of 12 in the producing granodioritic melt by amphibole fractional crystallization based on partition coefficients of nb and ta between amphibole and melts from previous experiments this may explain the average nb ta ratio 13 7 of the barren granodiorites while it cannot account for the average nb ta ratio 8 4 of the mineralized porphyritic granodiorites and it is even lower than that of the granites 10 3 with biotite fractional crystallization exsolution of a magmatic hydrothermal fluid is inevitable when a water saturated magma emplaced in shallow crust leading to a transportation of certain chemical components from the magmatic melts to exsolved fluids because nb seems more mobile than ta in fluorine bearing fluids we contend that a preferentially transport nb over ta by sub solidus hydrothermal alteration can further lower the nb ta ratios of the mineralized porphyritic granodiorites which may also result in a broad range of hfse contents and their ratios in the altered porphyritic granodiorites formed in a post magmatic process

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

dehydration melting and water saturated melting of basaltic and andesitic greenstones and amphibolites at 1 3 and 6 9 kb

phase relationships of gabbro tonalite granite water at 15 kbar with applications to differentiation and anatexis phase relationships have been determined at 15 kbar with variable h 2o contents for three rocks a gabbro a tonalite and a muscovite granite representing the compositional trend of the calc alkaline rock series experiments were conducted in 0 5 in piston cylinder apparatus using ag pd or pt capsules the results obtained are acceptable representations of the phase relationships for elucidation of petrological processes selected runs with 5 h 2o were completed using au capsules to overcome the problem of iron loss up to the temperature limit of au for reliable electron microprobe analyses of glass garnet clinopyroxene amphibole and plagioclase no glasses could be analyzed in the gabbro the data provide k d values as a function of temperature for pairs of coexisting minerals and liquid the chemical variations are used to evaluate 1 the fractionation trends in hydrous magmas at 55 km in thickened continental crust or uppermost mantle and 2 the products of anatexis of the thickened crust at this depth fractionation of hydrous basalt or anatexis of gabbro and tonalite or their metamorphosed equivalents appears to produce liquids diverging from the calc alkaline trend anatexis of granite produces a liquid less siliceous than itself with some chemical characteristics of syenite

rutile solubility in hydrous rhyolite melts at 750 900 c and 2 kbar with application to titanium in quartz titaniq thermobarometry abstract the solubility of rutile in water saturated haplogranite melts with molar al na k ratios ranging from 0 84 to 1 25 was determined at 750 900 c and 2 kbar in cold seal pressure vessel experiments due to the low diffusivity of ti at these conditions a new method was developed to determine tio2 solubility in this method glasses with tio2 gradients were used as starting material and after the experiments the tio2 content was measured at the contact between rutile bearing and rutile free glass the glasses were either directly equilibrated at the desired p t conditions i e crystallization experiments or first treated at 50 150 c lower temperatures and then subjected to the desired conditions i e dissolution experiments the results obtained in crystallization and dissolution experiments agree well with each other suggesting that equilibrium was attained rutile solubility in peralkaline melts strongly increases with temperature and the amount of excess alkalies according to the relation log tio 2 wt 1 8 ank 0 53 10 000 t 12 8 ank 4 3 in which ank is the deviation of the molar al na k ratio from unity and t is given in kelvin excess alumina does not seem to promote tio2 solubility for natural melt compositions it was found that if ca mg and fe are assumed to be 10 times less effective in promoting tio2 solubility than excess na and k good fits to previous solubility data in silicic 70 wt sio2 melts at upper crustal pressures 10 kbar are obtained application of this extended tio2 solubility model to natural melt inclusions in quartz phenocrysts from five silicic volcanic systems returns tio2 activities that are similar to those calculated with a previous experiment based model and to those calculated from pairs of coexisting fe ti oxides but are up to 2 9 times higher than those calculated with melts and rhyolite melts pressures calculated from ti concentrations in the host quartz using the most recent calibration of the titaniq thermobarometer are in good agreement with independent pressure estimates suggesting that at upper crustal conditions this calibration is valid

zircon u pb chronology hf isotope analysis and whole rock geochemistry for the neoarchean paleoproterozoic yudongzi complex northwestern margin of the yangtze craton china abstract the archean paleoproterozoic high grade basement of yudongzi complex is a key to understanding the early precambrian crustal evolution of the yangtze craton it comprises mainly orthogneiss paragneiss and amphibolite whose protoliths are tonalitic trondhjemitic granitic ttg sedimentary and basic intermediate volcanic rocks respectively the ttg gneiss amphibole plagiogneiss and biotite plagiogneiss yield magmatic zircon la icp ms u pb ages of 2815 18 ma mswd 0 92 2692 26 ma mswd 0 59 and 2449 4 ma mswd 0 94 respectively metamorphic overgrowths on zircon from amphibolite have an age of 1848 5 ma mswd 0 71 ttg gneisses show medium sr y and variable high la yb n ratios with low y and yb contents they are characterized by positive eu anomaly and distinct depletion of hree together with negative nb ta and ti implying amphibole garnet and minor rutile as residual phases their positive e hf t values of 2 1 to 8 1 with t dm2 of ca 2 80 3 10 ga suggest significant reworking of juvenile crust amphibole plagiogneisses display a strong enrichment of lrees and depletion of nb ta and ti additionally a relative enrichment of ba rb pb and zr as well as high cr and ni contents and mg values imply a mantle source with the addition of continental crust material zircon e hf t values vary between 0 9 and 3 9 suggesting a proportionally significant input of juvenile material and therefore interaction between the mantle and pre existing continental crust biotite plagiogneisses show negative e hf t values between 3 4 and 0 1 with a few positive e hf t values ranging from 0 1 to 1 5 together with t dm2 ages of ca 2 80 3 00 ga these e hf t values suggest that these rocks were mainly generated by reworking of ancient crust thus the yudongzi complex exposed in the northern part of the yangtze craton has experienced significant reworking of juvenile crust at ca 2 80 ga and subsequent crustal growth at ca 2 70 ga followed by a second stage of reworking of ancient crust at ca 2 45 ga during the late paleoproterozoic the yudongzi complex was probably involved in the amalgamation of the paleoproterozoic supercontinent columbia and affected by a post collisional metamorphic event at ca 1 85 ga

zircon u pb ages geochemistry and sr nd pb hf isotopes of the nuri intrusive rocks in the gangdese area southern tibet constraints on timing petrogenesis and tectonic transformation abstract abundant magmatic rocks of various ages are exposed in gangdese southern tibet these rocks play an important role in understanding the tectonic transformation from the subduction of neo tethyan oceanic crust to the collision of the indian and asian continents based on zircon u pb ages geochemistry and sr nd pb hf isotopic data of the late cretaceous to early oligocene 96 30 ma intrusive rocks in the nuri cu w mo deposit we discuss the late cretaceous to early oligocene tectonic transformation of the region and the origin of oligocene cu w mo mineralization in southern gangdese the nuri intrusive rocks represent three magmatic episodes 96 91 56 52 and 33 30 ma the 96 91 and 56 52 ma rocks have relatively low 87sr 86sr i 0 7041 to 0 7060 and high end t 3 1 to 3 5 and ehf t values 3 7 to 15 the 33 30 ma rocks have relatively high 87sr 86sr i 0 7061 to 0 7063 and pb isotopes and low end t 3 8 to 1 8 and ehf t values 0 6 to 10 1 the three stages of intrusive rocks have geochemical characteristics that are similar to those of coeval rocks in gangdese the 96 91 and 33 30 ma rocks are adakitic whereas the 56 52 ma rocks have characteristics of arc calc alkaline magmatic rocks the 96 91 ma rocks were produced by the partial melting of neo tethyan basaltic oceanic crust and minor sediments whereas the 56 52 ma rocks were generated by the partial melting of juvenile crust and the 33 30 ma rocks were formed by the melting of indian plate lower crust contaminated with overlying mantle materials on the basis of the regional tectonic and magmatic characteristics we suggest that neo tethyan oceanic slab subduction and slab roll back occurred from 100 to 65 ma collision between the indian and asian continents occurred at 65 to 40 ma neo tethyan oceanic slab break off took place at 50 ma and the indian continent subducted northwards beneath the asian continent at 30 ma from the late cretaceous 96 91 ma to oligocene 30 ma the geodynamic setting of gangdese was changed from neo tethyan oceanic subduction to indian continental subduction in contrast to the miocene metallogenic rocks the magmatic source of oligocene metallogenic rocks contains old indian continental crust materials which result in the formation of intense unique cu w mo mineralization especially w mineralization in the southern gangdese

rutile saturation in hydrous siliceous melts and its bearing on ti thermometry of quartz and zircon the tio2 solubility of rutile saturated hydrous siliceous melts has been investigated at p 1 gpa and t 650 1000 c for several representative felsic compositions the dissolution of a rutile crystal into a tio2 undersaturated melt provides information on both tio2 solubility and ti diffusion results of this study confirm that tio2 solubility is strongly dependent on both temperature and melt composition but not on the amount of h2o present for a given t t io2 content decreases as the melts become more felsic the solubility of tio2 is given by log ti ppm 7 95 5305 t 0 124fm

very low solubility of rutile in h2o at high pressure and temperature and its implications for ti mobility in subduction zones the solubility of rutile in h2o has been measured at 1000 1100 c 1 2 gpa the data indicate that solubility is very low over the investigated range with a maximum of 4 7 millimol kg h2o at 1100 c 2 gpa the data were fit with the equation log m ti 4 892 10470 t 0 1923 p where m ti is ti molality t is in kelvins and p in gpa when compared to previous results the new data indicate substantially lower solubility opposite pressure dependence and thermodynamic properties of the reaction rutile tio2 aq that are now consistent with other oxide hydrolysis reactions calculations of ti transport during mantle metasomatism by h2o in subduction zone environments predict much lower ti mobility at all conditions these results offer strong support for models of ti retention in eclogites during slab devolatilization and require that examples of significant ti mass transfer be explained by complexing agents in solution most likely aluminosilicate complexes

carbon elemental and isotopic composition in mantle xenoliths from spain insights on sources and petrogenetic processes abstract the carbon elemental concentration c wt and isotopic 13c composition of mantle xenoliths from the tallante and calatrava volcanic occurrences in south east and central spain respectively have been investigated to identify carbon sources and processes occurring in distinct geodynamic settings of the iberian peninsula the peridotitic mantle xenoliths from calatrava show elemental c ranging from 0 11 to 2 87 wt which is coupled with a continuous isotopic variation from very negative values 13c 26 1 to typical mantle values 13c 5 9 on the other hand the tallante mantle xenolith suite displays lower c contents 0 06 0 15 wt showing a tighter variation with 13c depleted values ranging between 20 1 and 23 7 higher elemental c up to 0 41 wt displaying distinctly less negative isotopic values 13c between 13 8 and 11 9 have been recorded in veins crosscutting tallante peridotites plausibly representing the product of metasomatic reactions the data from the two investigated xenolith suites invariably display a good correlation between elemental and isotopic composition suggesting a mantle origin for carbon and rayleigh type fractionation as the process responsible for the observed c 13c variation however the correlation between the carbon isotopic data with other isotopic tracers e g 87sr 86sr 3he 4he used to identify distinct mantle components and metasomatic reactions indicates systematic differences between the two xenolith suites suggesting that beneath the betic cordillera where tallante is located the deep c cycle involves recycling via subduction preceding accompanying continental collision of crustal components back in the mantle coherently geochemical trends observed in the tallante xenoliths seem to be influenced by metasomatic agents generated by melting of crustal lithologies that according to the analysis of a metasedimentary xenolith can contain c up to 1 2 wt having 13c of ca 18 0

trace element evidence for growth of early continental crust by melting of rutile bearing hydrous eclogite the early continental crust is composed dominantly of archean tonalite trondhjemite granodiorite ttg gneisses and is generally explained as the product of melting of metabasalts in the subducted crust however whether the melting occurs in shallow level amphibolite facies or in relatively deep eclogite facies is debated here i present experimental partition coefficients ds for 27 trace elements between garnet amphibole and tonalitic melts they are used together with published mineral melt trace element ds to model the melting of metabasalt in order to delimit the conditions for ttg production the results clearly show that model melts with trace element characteristics that completely mimic the ttg are in equilibrium with rutile bearing anhydrous and hydrous amphibole bearing eclogitic residues but not rutile free amphibole dominated residues rutile appears to be a necessary residual phase to account for the characteristic negative nb ta anomaly in the ttg these results thus suggest that the early continental material was produced under eclogite facies conditions based on the modeling results obtained using appropriate partition coefficients and the archean geotherm the preferred process for the ttg production is the melting of rutile bearing hydrous eclogite triggered by the release of h2o from the progressive breakdown of amphibole the pressure temperature p t conditions for ttg production via this process are constrained to 1 5 2 5 gpa 50 80 km and 850 1050 c by the p t stability boundaries of amphibole and rutile in the basalt system

on the emergence of plate tectonics at present young oceanic lithosphere is positively buoyant and it does not become negatively buoyant until it is older than about 20 m y if in the past the mantle was hotter the oceanic crust would have been thicker and the lithosphere9s age of neutral buoyancy greater on the other hand the hotter mantle would have had a lower viscosity and convected faster so the average age of oceanic plates at subduction would have been less than the present 100 m y at some time in the past average oceanic lithosphere would have only just become negatively buoyant as it reached a subduction zone it is estimated here that this condition occurred when the mantle was only about 50 c hotter than at present and as recently as 0 9 1 4 b y ago transformation of basaltic crust to eclogite might have enhanced the vigor of the plates but possibly only modestly and intermittently in earlier times plate tectonics could have operated but more slowly so that it could not by itself have accomplished the necessary rate of heat removal from the earth a different tectonic mode must also have operated perhaps subcrustal delamination or dripping plate tectonics would have gradually taken over from the earlier mode plumes are a omplementary mode of mantle convection driven by a lower thermal boundary layer and so they could have operated in conjunction with both regimes of the upper boundary layer

complete miscibility between silicate melts and hydrous fluids in the upper mantle experimental evidence and geochemical implications the phase relationships between silicate melts and hydrous fluids were studied by direct visual observation in an externally heated diamond anvil cell complete miscibility of silicate melt and water was observed for a wide range of melt compositions including nepheline jadeite dacite haplogranite and ca bearing granite some evidence for complete miscibility was also observed in the system basalt h2o the critical temperatures in all systems decrease rapidly with pressure at 15 kbar the critical temperature for nepheline is around 550 c for jadeite around 800 c and for granitic compositions it is close to 900 c in general the critical temperatures appear to increase with silica content in the system our results suggest that there is complete miscibility between silicate melts and water in most of the upper mantle except at very shallow depths this means that a water saturated solidus cannot be defined any more in the deeper parts of the upper mantle very silica rich melt inclusions found in spinel lherzolites associated with fluid inclusion are probably the result of the unmixing of a supercritical fluid containing comparable amounts of water and silicate components the decomposition of amphibole in a subducted slab occurs at conditions where the miscibility gap between fluid and silicate melt is not yet closed while the decomposition of phengite and lawsonite occurs far beyond the critical curve accordingly the fluids released by the breakdown of these minerals should have very different properties highly mobile hydrous fluids containing little dissolved silicate should be produced by amphibole breakdown while the decomposition of lawsonite and phengite will lead to much more silicate rich and less mobile fluid phases

formation of earth s early archaean continental crust abstract subduction of oceanic crust at an unusually low angle has been proposed as a model for the growth of continental crust older than about 2 5 ga at modern zones of low angle or flat subduction magmatic additions to new crust come from partial melting of both the subducting oceanic crust slab and the thin wedge of mantle above the slab evidence for both a slab and wedge source is preserved in most late archaean 3 0 2 5 ga terrains but we find little evidence that a mantle wedge contributed to crustal growth prior to 3 1 ga this lack of evidence in part reflects a dearth of exposed crust aged between 3 0 and 3 3 ga but also suggests that subduction enriched mantle source regions did not develop before 3 3 ga and possibly not before 3 1 ga in contrast to most modern terrains and some late archaean terrains early archaean 3 3 ga continental crust evolved through direct melting of thick mafic crust we invoke a process of subduction that does not include the development of a mantle wedge and call this process archaean flat subduction to distinguish it from modern low angle subduction

a neoarchean k rich granitoid belt in the northern north china craton abstract the archean metamorphic basement in the southeastern part of eastern hebei to western liaoning provinces northern north china craton is dominated by neoarchean k rich granitoids with a small amount of neoarchean supracrustal rocks and na rich dioritic tonalitic trondhjemitic granodioritic gneisses and minor paleo to mesoarchean rocks the paleo to mesoarchean lithological signatures include ttg gneisses and supracrustal rocks at the caozhuang and caochang areas the neoarchean k rich granitoids form a ca 350 km long belt extending in a ne direction and primarily consist of porphyritic monzodioritic quartz monzodioritic granodioritic monzogranitic mqgm gneisses and secondary monzogranite syenogranites and charnockites with minor neoarchean ttg gneiss enclaves based on their major mafic mineral phases the mqgm gneisses can be divided into an amphibole dominated group and a biotite dominated group zircon u pb isotopic dating reveals that the caozhuang trondhjemitic gneisses amphibole dominated mqgm gneisses and biotite dominated mqgm gneisses were emplaced at 2 90 ga 2 55 2 53 ga and 2 55 2 53 ga respectively the amphibole dominated porphyritic mqgm gneisses are geochemically analogous to sanukitoids and the sanukitoid magma was derived from the partial melting of lithospheric mantle metasomatized by dehydration fluids and melts from subducted slabs and related sediments following the fractionation of amphibole and clinopyroxene during their magmatic evolution the magmatic precursors of the biotite dominated mqgm gneisses were most likely generated by the partial melting of medium to high k mafic intermediate rocks and locally generated by the partial melting of metagreywackes at medium pressures within the k rich granitoid belt the 2 55 2 53 ga porphyritic mqgm gneisses experienced 2 53 2 51 ga e w extension and strike slip shearing deformation in the jinzhou xingcheng structural domain and diapiric upwelling by the formation of dome structures in the jielingkou anziling suizhong structural domain all these structures then suffered a top to the se nee thrusting in the northern margin during 2 51 2 50 ga the 2 53 2 51 ga dioritic tonalitic trondhjemitic granodioritic gneisses and charnockites in the taipingzhai santunying zunhua structural domain firstly underwent diapiric upwelling during 2 51 2 50 ga then together with the 2 52 2 50 ga volcanic sedimentary rocks in the shuangshanzi qian an lulong structural domain overprinted by nearly n s striking dextral strike slip shearing deformation during 2 50 1 80 ga the temporal spatial relationships petrogenesis and structural patterns of these various neoarchean lithological assemblages confirm that the neoarchean k rich granitoid belt in the northern north china craton was most likely developed in an active continental margin setting and our studies suggest this involved progressively southeastward multi stage retreating oceanic slab subduction and collisions between an oceanic arc and an ancient continental nucleus

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

a synthesis of geochemistry and sm nd isotopes of archean granitoid gneisses in the jiaodong terrane constraints on petrogenesis and tectonic evolution of the eastern block north china craton abstract the jiaodong terrane is situated in the northeastern part of the shandong province in eastern china experiencing multi stage magmatism during the mesoarchean 2 9 ga and neoarchean 2 7 ga and 2 5 ga it consists dominantly of granitoid gneisses with minor amphibolite enclaves or lenses this study presents new whole rock geochemical and sm nd isotopic data for the archean granitoid gneisses from the jiaodong terrane in order to constrain their petrogenesis and tectonic setting geochemical studies and petrological features suggest that the protoliths of most granitoid gneisses are typical archean tonalitic trondhjemitic granodioritic ttg suites which are high in sio2 60 9 76 8 wt al2o3 12 6 16 8 wt na2o 3 3 5 3 wt sr 135 604 ppm and sr y ratios 13 9 84 but low in mgo 0 4 4 5 wt k2o 0 7 3 4 wt tio2 0 2 0 9 wt cr

experimentally based water budgets for dehydrating slabs and consequences for arc magma generation phase diagrams of hydrous mid ocean ridge mor basalts to 330 km depth and of hydrous peridotites to 250 km depth are compiled for conditions characteristic for subduction zones a synthesis of our experimentally determined phase relations of chlorite lawsonite epidote zoisite amphibole paragonite chloritoid talc and phengite in basalts and of phase relations from the literature of serpentine talc chlorite amphibole and phase a in ultramafics permits calculation of h2o contents in hydrous phase assemblages that occur in natural compositions this yields the information necessary to calculate water budgets for descending slabs starting from low grade blueschist conditions 10 20 km depth with h2o contents between 5 and 6 wt for hydrated oceanic crust complete dehydration is achieved between 70 and 300 km depth as a function of individual slab geotherms hydrous phases which decompose at depth below volcanic arcs are lawsonite zoisite chloritoid and talc phengite in mafic compositions and chlorite and serpentine in peridotite approximately 15 35 of the initially subducted h2o are released below volcanic arcs the contribution of amphibole dehydration to the water budget is small 5 20 and occurs at relatively shallow depth 65 90 km in any predicted thermal structure dehydration is a combination of a stepwise and a continuous process through many different reactions which occur simultaneously in the different portions of the descending slab such a dehydration characteristic is incompatible with single phase dehydration models which focus fluid flow through a unique major dehydration event in order to explain volcanic fronts as a consequence of continuously progressing dehydration water ascending from the slab will be generally available to depth of ca 150 200 km the fluid rising from the subducting lithosphere will cause partial melting in the hot portion of the mantle wedge we propose that the volcanic front simply forms above the mantle wedge isotherm where the extent of melting is sufficient to allow for the mechanical extraction of parental arc magmas thermal models show that such an isotherm ca 1300oc locates below volcanic fronts slab surface depths below such an isotherm are compatible with the observed depths of the slab surface below volcanic fronts 1998 elsevier science b v all rights reserved

ignimbrite correlation using whole rock geochemistry an example from the sulcis sw sardinia italy ignimbrites are useful chronological markers in the geological record at local and regional scales they also provide information on the dynamics of the eruption that produced them making their study of great importance in terms of volcanic hazard assessment however their study is usually hampered by their lateral variation and discontinuity when stratigraphic and lithologic criteria are not sufficient for correlation purposes the use of multiple complementary correlation tools may be necessary to correctly determine their areal extension volume and facies variations whole rock geochemistry is considered one of the less reliable correlation techniques due to the pyroclastic nature of these deposits and their emplacement dynamics these may introduce vertical and horizontal geochemical heterogeneity in the final deposit in addition the occurrence of zoned ignimbrites due to magma supply of changing composition is common in this work we show that if appropriately used whole rock geochemistry can be a valid and highly useful tool for ignimbrite correlation we provide an example from the study of an ignimbrite sequence containing 18 units sensu lato in the sulcis region sw sardinia italy a protocol has been developed for unit recognition based on successive simple binary diagrams where the whole rock composition of a problem sample can be plotted immobile trace elements have been preferentially used to minimize effects of element mobilization associated with alteration and weathering the diagrams provided here are designed for the sulcis but the methodology followed to develop them may be applied to other study areas

titanium solubility in coexisting garnet and clinopyroxene at very high pressure the significance of exsolved rutile in garnet exsolution microstructures including ilmenite garnet in clinopyroxene and rutile in garnet are common in clinopyroxenite and eclogite from the sulu ultrahigh pressure uhp terrane in order to understand the phase relations and ti solubility in both garnet and clinopyroxene in a natural tio2 bearing system several experiments at 5 15 gpa 1000 1400 c were carried out using the multianvil high pressure apparatus the hujianlin ilmenite rich garnet clinopyroxenite showing exsolution microstructure was selected as starting material because it closely approaches a composition lying in the tio2 cao mgo feo al2o3 sio2 system except for minor melt in one experiment at 1400 c and 5 gpa other run products contain majoritic garnet clinopyroxene ilmenite or rutile and exhibit neoblastic texture with increasing pressure ti and ca mg and si contents of neoblastic garnet increase with decreasing al the principal coupled substitutions are ca2 ti4 2al3 and si4 mg2 2al3 responding to majorite component increase titanium solubility 0 8 4 5 wt as tio2 in garnet and grtti cpxti ratio have a pronounced positive correlation with pressure between 5 and 15 gpa on the other hand the coexisting clinopyroxene contains low ti 0 17 0 53 wt as tio2 and shows no significant pressure effect rutile exsolution in garnet is coupled to that of pyroxene exsolution both are exsolved from majoritic garnet on decompression therefore the amount of such exsolved lamellae is a potential indicator of high pressure metamorphism in exhumed rocks whereas the tio2 content of clinopyroxene coexisting with garnet is not sensitive to pressure change

experimental constraints on rutile saturation during partial melting of metabasalt at the amphibolite to eclogite transition with applications to ttg genesis tio2 solubility in rutile saturated felsic melts and coexisting minerals was determined at 1 5 3 5 gpa 750 1250 c and 5 30 wt h2o tio2 solubility in the melt primarily increases with temperature and melt basicity it increases slightly with water content in the melt and it decreases with pressure a general tio2 solubility model was obtained and is expressed as ln tio2 melt ln tio2 rutile 1 701 9041 t 0 173 p 0 348fm 0 016h2o where tio2 and h2o are in wt t is in kelvin p in gpa and fm is the melt composition parameter given by fm 1 si na k 2 ca fe mn mg al in which the chemical symbols represent cation fractions tio2 solubility in amphibole garnet and clinopyroxene also increases with temperature and empirical equations describing this temperature dependence were derived these data were used to assess the protolith tio2 content required for rutile saturation during partial melting of hydrous metabasalt at the amphibolite to eclogite transition the results show that only 0 8 1 0 wt tio2 is required for rutile saturation during low degree 20 melting rutile is stable up to 1150 c with 1 6 wt tio2 in the protolith and 30 40 melting for dehydration melting and up to 1050 c and 50 60 melting for fluid present melting the data also show that 0 7 0 8 wt tio2 in the protolith is needed for rutile saturation during subsolidus dehydration therefore nearly all basaltic protoliths in deep crustal settings and subduction zones will be saturated with rutile during subsolidus dehydration and low degree melting at hydrous conditions archean tonalites trondhjemites granites ttg are widely accepted to be the products of low degree melting of metabasalts at the amphibolite to eclogite transition with rutile being present in the residue comparison of natural ttg compositions with our experimental rutile solubility data indicates that the dominant ttg magmas were produced at temperatures of 750 950 c which requires that the partial melting occurred at hydrous conditions models involving melting at the base of oceanic plateaus are inadequate to explain ttg genesis because the plateau root zones are likely dominated by anhydrous cumulates a slab melting model satisfies the requirement of a hydrous metabasalt which during subduction would melt to produce voluminous ttg melts under high archean geothermal gradients the geothermal gradients responsible are estimated to be between 10 and 19 c km based on a pressure range of 1 5 2 5 gpa for the amphibolite to eclogite transition

partial melting of two amphibolites contrasting experimental results under fluid absent conditions a large portion of the lower continental crust may be amphibolitic in composition and without a free fluid phase as a consequence h2o undersaturated or fluid absent melting of amphibolites may be responsible for the formation of some granites and migmatites produced during major orogenic events in an attempt to determine the systematics of melting under fluid absent conditions a series of piston cylinder experiments was conducted on two natural amphibolites one a meta alkali basalt aba with a total water content of wt contained in hornblende and the other a meta island arc tholeiite iat which has 1 1 3 wt water contained in hornblende cummingtonite and biotite the experimentally determined melting ranges of the two amphibolites showed that the solidus temperatures and sta temperature interval over which amphibole was stable were controlled by the amphibolites different bulk compositions and their resulting metamorphic assemblages the volume of melt produced by melting of the two amphibolites were compared with estimated amounts based on burnham s 1979 water melt solubility model and the fluid absent melting model presented by clemens and vielzeuf 1987 the observed melt volumes were greater than estimated as the water content of melt largely detemines the volume of melt produced independent measurements of the water content of the glass formed during partial melting in the aba were made by thermogravimetric analyses the water content of the aba glass is 2 wt which is less than the assumed melt water content water content of the melt used in previous modeling of fluidabsent anatexis in mafic lithologies as a consequence more melt can be expected during fluid absent partial melting of mafic lower crust as is observed in the experiments a modification of the clemens and vielzeuf 1987 fluid absent melting model for mafic compositions has been made using the experimental data available on melting in basaltic systems and is presented here for pressures of 5 8 and 10 kbar tectonic scenarios in which the crust is thickened i e by collision then undergoes extension or where a previously thinned crust is later rethickened provide enough heat so that amphibolite melting under fluid absent conditions can become importan and hence responsible for some melts produced during post collisional magmatism the results may also have applications to melting in hydrated oceanic crust in subduction zones and in island arc terains

pressure effect on ti or p rich accessory mineral saturation in evolved granitic melts with differing k2o na2o ratios the solubility of ti and p rich accessory minerals has been examined as a function of pressure and k2o na2o ratio in two series of highly evolved silicate systems these systems correspond to a alkaline varying from alkaline to peralkaline with increasing k2o na2o ratio and b strongly metaluminous essentially trondhjemitic at the lowest k2o na2o ratio and remaining metaluminous with increasing k2o na2o ratio to 3 the experiments were conducted at a fixed temperature of 1000 c with water contents varying from 5 wt at low pressure 0 5 gpa increasing through 5 10 wt at 1 5 2 5 gpa to 10 wt at 3 5 gpa pressure was extended outside the normal crustal range so that the results may also be applied to derivation of hydrous silicic melts from subducted oceanic crust for the alkaline composition series the tio2 content of the melt at ti rich mineral saturation decreases with increasing pressure but is unchanged with increasing k content at fixed pressure the p2o5 content of the alkaline melts at apatite saturation increases with increased pressure at 3 5 gpa only but decreases with increasing k content and peralkalinity for the metaluminous composition series termed as trondhjemite based series t series the tio2 content of the melt at ti rich mineral saturation decreases with increasing pressure and with increasing k content at fixed pressure the p2o5 content of the t series melts at apatite saturation is unchanged with increasing pressure but decreases with increasing k content the contrasting results for p and ti saturation levels as a function of pressure in both compositions point to contrasting behaviour of ti and p in the structure of evolved silicate melts ti content at ti rich mineral saturation is lower in the alkaline compared with the t series at 0 5 gpa but is similar at higher pressures whereas p content at apatite saturation is lower in the t series at all pressures studied the results have application to a type granite suites that are alkaline to peralkaline and to i type metaluminous suites that frequently exhibit differing k2o na2o ratios from one suite to another

late neoarchean monzogranitic syenogranitic gneisses in the eastern hebei western liaoning province north china craton petrogenesis and implications for tectonic setting abstract the medium grained monzogranitic syenogranitic gneisses expose as roughly northeast southwest trending and cover 1500 km 2 in the southern part of the eastern hebei western liaoning province north china craton these plutonic gneisses have leucocratic features and weakly gneissic to massive structures and were metamorphosed under greenschist to lower amphibolite facies la icp ms zircon u pb isotopic dating reveals that the magmatic precursors of these monzogranitic syenogranitic gneisses were emplaced during 2527 2511 ma they are characterized by high contents of sio 2 71 64 78 08 and k 2 o 3 55 6 40 and low contents of cao 0 25 1 71 mgo 0 01 0 42 and na 2 o 2 29 4 96 according to their chondrite normalized ree patterns the monzogranitic syenogranitic gneisses can be subdivided into a high ree group and a low ree group studies of petrogenesis reveal that magmatic precursors of the high ree group samples were derived from partial melting of the regionally widespread porphyritic or medium grained dioritic quartz monzonitic granodioritic monzogranitic gneisses under low pressure conditions less than ca 10 kbar the magmatic precursors of the low ree group were formed by fractionated crystallization of the high ree group magma with titanite allanite epidote apatite and zircon as the major crystallization fractionation phases combined with previous investigations on the late neoarchean metavolcanic rocks and granitoid orthogneisses in the northwestern part of the eastern hebei western liaoning province the magmatic precursors of these monzogranitic syenogranitic gneisses were most likely formed in a back arc basin along the north of the eastern block north china craton

determination of the fluid absent solidus and supersolidus phase relationships of morb derived amphibolites in the range 4 14 kbar experiments on a morb mid ocean ridge basalt derived amphibolite have been conducted in order to determine fluid absent solidus and supersolidus phase relationships the solidus has been determined in the range 4 14 kbar by using the incremental heating technique tested against classical procedures peritectic clinopyroxene was produced below 10 kbar and epidote was the stable peritectic phase at pressures greater than 10 kbar for temperatures near the solidus garnet was the peritectic phase for temperatures above 800 c and pressures of 10 to 14 kbar the shape of the solidus was similar to previous estimations but the temperature interval between the high and low pressure regions was strongly reduced with these new estimations the possibilities for melting of the oceanic crust at p below 10 kbar are increased and consequently the number of tectonic scenarios needed for magma generation from amphibolite sources is enlarged in the absence of free water the solidus in the pressure region above 10 kbar is at higher t than previous estimations

dehydration melting of a basaltic composition amphibolite at 1 5 and 2 0 gpa implications for the origin of adakites this study presents the results of dehydration melting experiments on a basaltic composition amphibolite under conditions appropriate to a hot slab geotherm 1 5 and 2 0 gpa and temperatures of 850 to 1150 c dehydration melting produces an omphacitic augite and garnet bearing residue coexisting with rhyolitic to andesitic composition melts at 1 5 gpa the amphibolite melts in two stages between 800 and 1025 c the 2 0 gpa data also define two melting stages at 2 0 gpa the first stage involves nearly modal melting of the original amphibolite minerals qtz pl amp to produce melt cpx grt during the second stage the eclogite restite melts non modally 0 86 cpx 0 14 grt 1 melt the experimental results were combined with data from the literature to generate a composite p t phase diagram for basaltic composition amphibolites over the 800 to 1100 c temperature range for pressures up to 2 0 gpa comparison of the major element compositions of the experimentally produced melts with compositions of presumed slab melts adakites shows that partial melting of amphibolite at conditions appropriate to a hot slab geotherm produces melts similar to andesitic and dacitic adakites except for significant mgo and cao depletions trace element modelling of amphibolite dehydration melting using the 2 0 gpa melting reactions produces ree abundances similar to those of adakites at 10 15 wt batch melting but the models do not reproduce the high sr y ratios characteristic of adakites taken together the major and trace element results are not consistent with the derivation of adakites by dehydration melting of the subducted slab with little or no interaction with the mantle wedge or crust if adakites are partial melts of the subducted slab they must undergo significant interaction with the mantle and or crust during which they acquire a number of their distinctive characteristics

the effect of water on accessory phase solubility in subaluminous and peralkaline granitic melts abstract the solubilities of columbite tantalite wolframite rutile zircon and hafnon were determined as a function of the water contents in peralkaline and subaluminous granite melts all experiments were conducted at 1035 c and 2 kbar and the water contents of the melts ranged from nominally dry to approximately 6 wt h 2 o accessory phase solubilities are not affected by the water content of the peralkaline melt by contrast solubilities are affected by the water content of the subaluminous melt where the solubilities of all the accessory phases examined increase with the water content of the melt up to 2 wt h 2 o at higher water contents solubilities are nearly constant it can be concluded that water is not an important control of accessory phase solubility although the water content will affect diffusivities of components in the melt thus whether or not accessory phases will be present as restite material the solubility behaviour in the subaluminous and peralkaline melts supports previous spectroscopic studies which have observed differences in the coordination of high field strength elements in dry vs wet subaluminous granitic glasses but not for peralkaline granitic glasses lastly the fact that wolframite solubility increases with increasing water content in the subaluminous melt suggests that tungsten dissolved as a hexavalent species

zircon u pb ages geochemistry and nd hf isotopes of the ttg gneisses from the jiaobei terrane implications for neoarchean crustal evolution in the north china craton abstract the precambrian basement in the jiaobei terrane is largely composed of tonalite trondhjemite granodiorite ttg suite of rocks and offers important insights into the crustal evolution history of the north china craton ncc the la icp ms zircon u pb age data presented in this study show that the magmatic protoliths of the ttg gneisses formed during 2508 2547 ma and recorded the paleoproterozoic metamorphism 1905 ma the rocks are enriched in lile rb ba and sr and depleted in hfse nb ta zr and hf they are characterized by high sr contents 406 2906 ppm sr y ratios 31 3 355 and subchondritic nb ta ratios 18 5 68 9 the ttgs show relatively high ree contents 72 0 266 ppm with strongly enriched lree la yb n 11 5 121 and positive or negligible negative eu anomalies eu eu 0 84 1 89 these geochemical features suggest that the magma source might have been rutile bearing amphibole eclogite their high mg numbers 42 56 and high cr 153 285 ppm and ni contents 22 2 74 5 ppm indicate interaction with the mantle wedge during magma ascent the whole rock end t values 2 6 to 3 8 and most of the magmatic zircon ehf t values 1 3 to 7 6 suggest juvenile to evolved isotopic signatures all these lines of evidence suggest that the ttg rocks in this study formed through partial melting of subducted oceanic slab in a continental arc environment the drill holes in the jiaobei terrane are dominated by 2 5 ga ttg gneisses suggesting that the ttg magma at 2 5 ga is more widely distributed deep underground than that of 2 7 2 9 ga at least within the approachable depth range of our research some zircon grains from jiaobei ttgs give high ehf t values plotting above the curve of 0 75 ehf of dm and their tcdm ages are very close to the time of the zircon crystallization however the majority of the ehf t values fall below the curve of 0 75 ehf of dm and their tcdm ages are concentrated between 2 7 2 9 ga therefore combining with other related studies we propose that the 2 5 2 6 ga magmatism in the ncc probably represents a coherent event of crustal growth and major reworking remelting