tectonic evolution of the qinling orogenic belt central china new evidence from geochemical zircon u pb geochronology and hf isotopes abstract the qinling orogenic belt qob located between the north china craton ncc and the yangtze craton yzc is composed of the northern qinling belt nqb and the southern qinling belt sqb this study presents new geochemical data zircon u pb ages and hf isotopes from a suite of metamorphic rocks from the kuanping qinling danfeng douling and wuguan groups in the nqb and the sqb our results suggest that the qinling group in the nqb formed in an active continental margin related to the subduction of the songshugou ocean under the southern ncc during grenvillian period the kuanping group in the northern nqb was deposited in the within plate extension basin during late neoproterozoic to early paleozoic the detritus of sedimentary rocks of the kuanping group were mainly sourced from the nqb and subordinately from the basement rocks of the ncc the danfeng group in the southern nqb formed later than 827 ma with the sediments derived mainly from the qinling terrane the nqb witnessed multiphase tectono thermal events from mesoproterozoic to early paleozoic including several magmatic events and the early paleozoic metamorphic event the original douling group in the sqb represents the basement complex of the sqb which is composed of rocks with different formation ages the oldest rock in the complex is represented by 3 0 ga granitic gneiss which underwent at least two periods of metamorphism at 2 5 ga and 0 97 0 91 ga in this study the douling group is redefined as a suite of neoproterozoic 860 750 ma rocks formed in an extensional setting after the subduction of the songshugou ocean the wuguan group in the sqb formed in the middle late neoproterozoic later than 740 ma and was subjected to early paleozoic metamorphism our data indicate that the sqb experienced early neoproterozoic 970 865 ma and early paleozoic 444 ma metamorphism and strong middle neoproterozoic 870 730 ma magmatism the age spectra of zircons from the nqb trace major tectono thermal events at 1462 ma 927 ma and 412 ma which are different from those recorded in the southern ncc and northern yzc suggesting that the nqb had a distinct evolution during mesoproterozoic to early neoproterozoic grenvillian the hf isotopic compositions of zircons from the various rocks suggest that the nqb probably developed on the basement of the southern ncc the rocks in the sqb show zircon age spectra and hf isotope compositions similar to those in the northern yzc suggesting a close affinity we therefore interpret the sqb to have developed on the basement of the northern yzc integrating the new data in this study with those from previous works we propose a new tectonic model for the formation and evolution of the qob during late mesoproterozoic to early paleozoic involving the following major events 1 late mesoproterozoic to early neoproterozoic grenvillian northward subduction of the songshugou ocean 2 early middle neoproterozoic 870 800 ma bidirectional subduction and collision 3 middle neoproterozoic 800 710 ma post collision extension 4 middle late neoproterozoic 710 600 ma within plate extension 5 late neoproterozoic early paleozoic 600 520 ma opening of the shangdan ocean and 6 early paleozoic 520 420 ma subduction collision we thus trace at least two distinct wilson cycles in the qob

zircon u pb ages and hf isotope compositions of migmatite from the north dabie terrane in china constraints on partial melting migmatite gneisses are widespread in the dabie orogen but their formation ages are poorly constrained eight samples of migmatite including leucosome melanosome and banded gneiss were selected for u pb dating and hf isotope analysis most metamorphic zircon occurs as overgrowths around inherited igneous cores or as newly grown grains morphological and internal structure features suggest that their growth is associated with partial melting according to the hf isotope ratio relationships between metamorphic zircon and inherited cores three formation mechanisms for metamorphic zircon can be determined which are dissolution reprecipitation of pre existing zircon breakdown of zr bearing phase other than zircon in a closed system and crystallization from externally derived zr bearing melt four samples contain magmatic zircon cores yielding upper intercept u pb ages of 807 35 768 12 ma suggesting that the protoliths of the migmatites are neoproterozoic in age the migmatite zircon yields weighted mean two stage hf model ages of 2513 97 894 54 ma indicating reworking of both juvenile and ancient crustal materials at the time of their protolith formation the metamorphic zircons give u pb ages of 145 2 120 2 ma the oldest age indicates that partial melting commenced prior to 145 ma which also constrains the onset of extensional tectonism in this region to pre 145 ma the youngest age of 120 ma was obtained from an undeformed granitic vein indicating that deformation in this area was complete at this time two major episodes of partial melting were dated at 139 1 and 123 1ma the first episode of partial melting is obviously older than the timing of post collision magmatism corresponding to regional extension the second episode of partial melting is coeval with the widespread post collision magmatism indicating the gravitational collapse and delamination of the orogenic lithospheric keel of the dabie orogen which were possibly triggered by the uprising of the cretaceous mid pacific superplume

thickening and partial melting of the northern qinling orogen china insights from zircon u pb geochronology and hf isotopic composition of migmatites the widespread migmatites in the precambrian basement of the north qinling belt are products of partial melting of the earth9s crust and record the tectono thermal response to late paleozoic collision along the shangdan suture the morphology and trace element geochemistry of zircons from the migmatites suggest a metamorphic origin related to partial melting the zircons from leucosome melanosome and banded migmatites yield u pb weighted mean ages of 408 2 403 2 and 400 2 ma respectively constraining the timing of migmatization the granitic dyke intruded into the migmatites gives a zircon u pb weighted mean age of 396 2 ma constraining the minimum time of migmatization zircons from the melanosome and banded migmatite yield upper intercept ages of 1088 1061 ma representing the protolith ages of the basement of the north qinling belt the zircon rims of the melanosome and banded migmatite predominantly display negative and positive ehf t values respectively suggesting the variable input of mantle material together with the regional geology and previous geochronological data we propose that the thickening and partial melting in the north qinling belt was induced by the collision along the shangdan suture between the north and south china blocks from c 408 to 400 ma supplementary material la icp ms u pb data lu hf isotopic compositions and trace element geochemistry data of the zircons and ti in zircon thermometry are available at https doi org 10 6084 m9 figshare c 4521389

subduction collision and exhumation in the ultrahigh pressure qinling dabie orogen c abstract high pressure metamorphism and ophiolite emplacement songshugou ophiolite attended suturing of the yangtze craton to rodinia during the c l o ga grenvillian oro geny the qinling microcontinent then rifted from the yangtze craton at c 750ma the erlangping intraoceanic arc formed in the early ordovician was emplaced onto the qinling microcontinent in the ordovician silurian and then both units were accreted to the sino

tectonic evolution of a composite collision orogen an overview on the qinling tongbai hong an dabie sulu orogenic belt in central china abstract the formation of collisional orogens is a prominent feature in convergent plate margins it is generally a complex process involving multistage tectonism of compression and extension due to continental subduction and collision the paleozoic convergence between the south china block scb and the north china block ncb is associated with a series of tectonic processes such as oceanic subduction terrane accretion and continental collision resulting in the qinling tongbai hong an dabie sulu orogenic belt while the arc continent collision orogeny is significant during the paleozoic in the qinling tongbai hong an orogens of central china the continent continent collision orogeny is prominent during the early mesozoic in the dabie sulu orogens of east central china this article presents an overview of regional geology geochronology and geochemistry for the composite orogenic belt the qinling tongbai hong an orogens exhibit the early paleozoic hp uhp metamorphism the carboniferous hp metamorphism and the paleozoic arc type magmatism but the three tectonothermal events are absent in the dabie sulu orogens the triassic uhp metamorphism is prominent in the dabie sulu orogens but it is absent in the qinling tongbai orogens the hong an orogen records both the hp and uhp metamorphism of triassic age and collided continental margins contain both the juvenile and ancient crustal rocks so do in the qinling and tongbai orogens in contrast only ancient crustal rocks were involved in the uhp metamorphism in the dabie sulu orogenic belt without involvement of the juvenile arc crust on the other hand the deformed and low grade metamorphosed accretionary wedge was developed on the passive continental margin during subduction in the late permian to early triassic along the northern margin of the dabie sulu orogenic belt and it was developed on the passive oceanic margin during subduction in the early paleozoic along the northern margin of the qinling orogen three episodes of arc continent collision are suggested to occur during the paleozoic continental convergence between the scb and ncb the first episode of arc continent collision is caused by northward subduction of the north qinling unit beneath the erlangping unit resulting in uhp metamorphism at ca 480 490 ma and the accretion of the north qinling unit to the ncb the second episode of arc continent collision is caused by northward subduction of the prototethyan oceanic crust beneath an andes type continental arc leading to granulite facies metamorphism at ca 420 430 ma and the accretion of the shangdan arc terrane to the ncb and reworking of the north qinling erlangping and kuanping units the third episode of arc continent collision is caused by northward subduction of the paleotethyan oceanic crust resulting in the hp eclogite facies metamorphism at ca 310 ma in the hong an orogen and low p metamorphism in the qinling tongbai orogens as well as crustal accretion to the ncb the closure of backarc basins is also associated with the arc continent collision processes with the possible cause for granulite facies metamorphism the massive continental subduction of the scb beneath the ncb took place in the triassic with the final continent continent collision and uhp metamorphism at ca 225 240 ma therefore the qinling tongbai hong an dabie sulu orogenic belt records the development of plate tectonics from oceanic subduction and arc type magmatism to arc continent and continent continent collision

deformation paths in the shallow levels of an accretionary prism the eocene shimanto belt of southwest japan detailed structural analysis of coherent turbidites in a part of the eocene shimanto belt on the muroto peninsula shikoku island japan reveals a structural evolution that is typical of the shallow levels of accretionary prisms in style and sequence of deformational fabrics but anomalous in that it includes both a marked rotation in shortening direction and a major out of sequence thrust deformation throughout a history of progressive lithification and burial was dominated by subhorizontal shortening and seaward vergent shear structures evolved from predominantly mesoscale thrusts and folds to larger more brittle faults and finally to kilometer scale amplified fault bend folds with an associated pressure solution cleavage this structural progression is consistent with increasing strength of the rocks decreasing fluid pressures and increasing temperature and pressure as the sediments were dewatered lithified and accreted to the prism in contrast to this straightforward structural evolution the prism scale deformation of the eocene shimanto belt is difficult to reconcile with simple models of prism developmentthat assume orthogonal subduction and seaward stepping imbrication a 50 counterclockwise rotation in the shortening direction and a major out of sequence or intraprism thrust are recorded by two distinct structural domains in both the earliest structures are clastic dikes and faults indicative of north south shortening one domain is characterized by east striking regional scale folds and cleavage that are transected by an east northeast trending cleavage however the other is characterized by northeast striking folds and cleavage that are transected by a slightly more north striking cleavage the different deformation histories of the two do mains require that they be separated by a major intraprism fault this fault carries tectonic melange containing blocks of pillow basalt and chert indicating that it rooted in the decollement zone beneath the prism the tectonostratigraphy and deformation histories of the two domains suggest that both before and after the out of sequence fault developed higher structural levels of the prism experienced significant subhorizontal shortening whereas lower structural levels bypassed this zone of deformation

a pressure solution creep law for quartz from indentation experiments 1 indenter experiments have been performed on quartz crystals in order to establish a pressure solution creep law relevant at upper to middle crustal conditions this deformation mechanism contributes to earth s crust geodynamics controlling processes as different as fault permeability strength and stress evolution during interseismic periods or mechanochemical differentiation during diagenesis and metamorphism indenter experiments have been performed at 350 c and 20 120 mpa during months with differential stress varying from 25 to 350 mpa several experimental parameters were varied nature of quartz synthetic or natural nature of fluid manner in which the solid solution solid interface was filled and orientation of the indented surfaces versus quartz crystallographic c axis significant strain rates could only be obtained when using high solubility solutions naoh 1 mol l 1 displacement rates of the indenter were found activated by differential stress with exponential dependence as theoretically predicted the mean thickness of the trapped fluid phase below the indenter was estimated in the range 2 10 nm moreover the development of this trapped fluid phase was relatively fast and allowed fluid penetration into previously dry contact regions by marginal dissolution the indenter displacement rate was driven by differential stress and its kinetics was controlled by diffusion along the trapped fluid and the development of a morphological roughness along the interface conversely marginal strain energy driven dissolution was observed around the indenter and its kinetics was controlled by free surface reaction these experimental results are applied to model the interactions between pressure solution and brittle processes in fault zones providing characteristic time scales for postseismic transitory creep and sealing processes in quartz rich rocks

protracted fluid induced melting during barrovian metamorphism in the central alps the timing and dynamics of fluid induced melting in the typical barrovian sequence of the central alps has been investigated using zircon chronology and trace element composition multiple zircon domains in leucosomes and country rocks yield u pb ages spanning from 32 to 22 ma the zircon formed during alpine melting can be distinguished from the inherited and detrital cores on the basis of their age th u 0 1 and trace element composition ti in zircon thermometry indicates crystallization temperatures around 620 700 c their composition allows discriminating between 1 zircon formation in the presence of early garnet 2 zircon in equilibrium with abundant l mree rich accessory phases allanite titanite and apatite typical of metatonalites and 3 zircon formed during melting of metasediments in feldspar dominated assemblages the distribution of zircon overgrowths and ages indicate that repeated melting events occurred within a single barrovian metamorphic cycle at roughly constant temperature that in the country rocks zircon formation was limited to the initial stages of melting whereas further melting concentrated in the segregated leucosomes that melting occurred at different times in samples a few meters apart because of the local rock composition and localized influx of the fluids and that leucosomes were repeatedly melted when fluids became available the geochronological data force a revision of the temperature time path of the migmatite belt in the central alps protracted melting over 10 my followed the fast exhumation of alpine eclogites contained within the same region and preceded fast cooling in the order of 100 c ma to upper crustal levels

granulites in the tongbai area qinling belt china geochemistry petrology single zircon geochronology and implications for the tectonic evolution of eastern asia the tongbai area of the eastern qinling belt in china includes granulite grade metamorphic assemblages qinling complex which were previously regarded as archean to early proterozoic in age and belonging to the southern margin of the north china plate craton our petrological and geochemical data characterize these rocks as two pyroxene granulites and garnet granulites which formed at temperatures of 757 840 c and pressures of about 9 5 kbar and are now found as xenoliths in granodioritic gneisses the protoliths of these rocks were granodiorites and tholeiitic basalt or gabbro the 207pb 206pb ratios derived from evaporation of single zircons yield ages of 470 20 and 470 14 ma respectively for the basic granulites which we interpret to reflect the time of protolith emplacement these are intruded by a 435 14 ma granodioritic gneiss post dating granulite formation a metaquartzite sample contains detrital zircons as old as 2555 8 ma two samples of granitoid gneiss from the tongbai complex s of the qinling granulites have single zircon 207pb 206pb evaporation ages of 776 8 and 746 10 ma respectively and document late proterozoic igneous activity we suggest that the qinling granulites document an important and hitherto unknown phase of early silurian crustal thickening following subduction and continental collision and that both the qinling and tongbai complexes were part of the southern margin of the north china craton prior to this event and record late proterozoic igneous activity

tracking partial melting and protolith nature by zircon u pb and hf o isotope compositions of migmatites in the north dabie terrane with emphasis on paleozoic low δ18o magmatism

genesis of adakitic granitoids by partial melting of thickened lower crust and its implications for early crustal growth a case study from the huichizi pluton qinling orogen central china abstract adakitic rocks are often considered as a key to deciphering the genesis of archean ttgs and the early crustal growth granites from the huichizi pluton in the north qinling nq unit have high sr y and la yb n ratios similar to adakites their relatively high sio 2 k 2 o and na 2 o and very low mgo cr and ni contents are in the range of high sio 2 adakites and early archean ttgs and are compositionally similar to experimental melts derived from metabasalt sources new sims zircon u pb dating constrains the emplacement age of the huichizi pluton at 422 5 ma rock samples from the huichizi pluton have e nd t and zircon e hf t values similar to the neoproterozoic metabasalts in the nq unit in combination with their normal mantle like 18 o zir values these adakites are best explained by partial melting of the neoproterozoic mafic crustal root due to subduction of the shangdan ocean regional geological data suggest that the crust was probably thickened by a ca 490 ma arc collision process prior to the emplacement of the huichizi pluton our results confirm that underplating of mafic magma and its subsequent fusion triggered by slab subduction under high pressure conditions could be an important mechanism for the formation of early continental crust

experimental compaction of quartz sand at low effective stress and temperature conditions experiments have been carried out on dry and fluid saturated quartz sands to investigate the behaviour during compaction under diagenetic conditions and to evaluate the importance of stress induced solution transfer 9pressure solution9 as a compaction mechanism the experiments were performed at temperatures t in the range 150 350 c applied effective stresses e up to 20 7 mpa and pore fluid pressures p 1 of 12 5 and 15 5 mpa using material with a grain size d in the range 20 100 m and an initial porosity of 45 52 dry quartz sands underwent significant compac tion during the loading stage but showed very little compaction creep once full load was achieved i e essentially time independent compaction in contrast fluid saturated material at constant applied effective stress showed substantial time dependent compaction i e creep with increasing temperature there is a decreasing number of grains in the wet compacted sand which show intragranular cracks and an increasing number of grains which show dissolution features at contacts with adjacent grains in addition there is a decreasing dependence of the volumetric strain rate on e with both increasing volumetric strain e v and increasing e and a decreasing dependence of on e v with increasing temperature these observations suggest that for wet quartz sand a gradual change might occur from compaction creep controlled by time dependent microcracking at t 250 300 c to compaction creep controlled by stress induced intergranular solution transfer at t 300 350 c

episodic growth of zircon in uhp orthogneisses from the north dabie terrane of east central china implications for crustal architecture of a collisional orogen zircon u pb dating of three orthogneiss samples from the north dabie terrane ndt is undertaken in order to reconstruct their formation and evolutionary histories and also the crustal architecture of the dabie orogen after triassic subduction and exhumation shrimp zircon u pb dating in combination with back scattered electron bse imaging and laser raman spectrometry provides accurate identification of the core mantle and rim structure for zircon growth during protolith formation and overgrowth during subduction exhumation and post collisional metamorphism concordant u pb ages of 760 730 ma and high th u ratios of 0 4 are obtained for relict oscillatory zoning fields of inherited cores that were not metamictized these features suggest that these ages represent the time of magmatic protolith formation during the breakup of rodinia the overgrown mantle domains around the metamictized cores are clean with few mineral inclusions e g quartz garnet and apatite mantle domains have low th u ratios of 0 719 similar to the values obtained from uhp orthogneisses in the cdt it is concluded that as with the cdt the orthogneisses with episodic zircon growths from the ndt should also be a part of the exhumed slice following the continental deep subduction however the orthogneisses in this study were buried at a lower level in the orogenic crust compared with those of the cdt prior to the cretaceous magmatism therefore the orthogneisses from the ndt were affected by the cretaceous magmatism whereas the cdt orthogneisses were not affected

zircon u pb ages ree concentrations and hf isotope compositions of granitic leucosome and pegmatite from the north sulu uhp terrane in china constraints on the timing and nature of partial melting granitic leucosome and pegmatite are widely distributed within biotite bearing orthogneiss in the northern part of the sulu ultrahigh pressure uhp metamorphic terrane eastern china a combined study of mineral inclusions cathodoluminescence cl images u pb shrimp dates and in situ trace element and lu hf isotope analyses of zircons provided insight into the nature and timing of partial melting in these rocks zircon grains separated from biotite bearing orthogneiss typically have three distinct domains 1 pre metamorphic magmatic cores with qtz kfs pl ap inclusions which record a neoproterozoic protolith age of 790 ma 2 mantles with coe phe ap inclusions that record triassic uhp age at 227 3 ma and 3 narrow rims with quartz inclusions that record hp granulite facies retrograde metamorphism at 210 3 ma in contrast zircons separated from granitic leucosome have only two distinct domains 1 the central uhp areas with coe phe ap inclusions record triassic uhp age of 227 3 ma and 2 outer magmatic areas with qtz kfs ab ap inclusions that record partial melting time of 212 2 ma zircons separated from pegmatite contain mineral inclusions of qtz kfs ap and show regular magmatic zoning from centre to edge the centres record partial melting time of 212 2 ma in line with the outer domains of granitic leucosome whereas the edges give a younger age of 201 2 ma related to pb loss and partial recrystallization during late triassic regional amphibolite facies retrogression these data indicate that partial melting in the north sulu uhp gneissic rocks took place during post uhp retrograde hp granulite facies metamorphism pre metamorphic magmatic zircon cores from biotite bearing orthogneiss give uniform 176hf 177hf of 0 28187 0 00003 2 sd standard deviation corresponding to ehf 790 and hf model ages tdm2 of about 16 3 and 2 41 ga respectively this is consistent with the generation of its protolith by reworking of paleoproterozoic to late archean crust in contrast uhp zircon domains from biotite bearing orthogneiss and granitic leucosome are characterized by distinct trace element composition with low lu hf 0 006 low th u 0 1 and considerably higher 176hf 177hf 0 28233 0 00002 2 sd than the pre metamorphic cores the uniform but significantly different hf isotope composition between the uhp ehf 227 14 6 0 8 2 sd and pre metamorphic ehf 227 27 7 domains indicates equilibration of the lu hf isotope system only within the uhp metamorphic mineral assemblage the disequilibrium between whole rock and uhp zircon suggests that about two thirds of the whole rock hf retained in the pre metamorphic zircon domains zircon domains crystallized during partial melting at 212 ma in granitic leucosome and pegmatites have a hf isotope composition indistinguishable from that of the uhp zircon domains this suggests that only hf and zr equilibrated during uhp metamorphism was remobilized during partial melting while pre metamorphic zircon remained stable or was not accessible in contrast the magmatic zircon edges from pegmatite have somewhat lower 176hf 177hf 0 28216 and ehf t 17 6 1 2 2 sd indicating some release of less radiogenic hf for instance by dissolution of pre metamorphic zircon during late regional amphibolite facies retrogression

deformation processes at the down dip limit of the seismogenic zone the example of shimanto accretionary complex in order to constrain deformation processes close to the brittle ductile transition in seismogenic zone we have carried out a microstructural study in the shimanto accretionary complex japan the fossil equivalent of modern nankai accretionary prisms the hyuga tectonic melange was sheared along the plate interface at mean temperatures of 245 c 30 c as estimated by raman spectroscopy of carbonaceous material rscm it contains strongly elongated quartz ribbons characterized by very high fluid inclusions density as well as micro veins of quartz both fluid inclusion planes and micro veins are preferentially developed orthogonal to the stretching direction furthermore crystallographic preferred orientation cpo of quartz c axes in the ribbons has maxima parallel to the stretching direction recrystallization to a small grain size is restricted to rare deformation bands cutting across the ribbons in such recrystallized quartz domains cpo of quartz c axes are orthogonal to foliation plane the evolution of deformation micro processes with increasing temperature can be further analyzed using the foliated morotsuka a slightly higher grade metamorphic unit 342 30 c by rscm from the shimanto accretionary complex in this unit in contrast to hyuga tectonic melange recrystallization of quartz veins is penetrative cpo of quartz c axes is concentrated perpendicularly to foliation plane these variations in microstructures and quartz crystallographic fabric reflect a change in the dominant deformation mechanism with increasing temperatures above 300 c dislocation creep is dominant and results in intense quartz dynamic recrystallization in contrast below 300 c quartz plasticity is not totally activated and pressure solution is the major deformation process responsible for quartz ribbons growth in addition the geometry of the quartz ribbons with respect to the phyllosilicate rich shear zones shows that bulk rheology is controlled by quartz behavior consequently below 300 c the application of quartz pressure solution laws based on realistic geometry derived from hyuga microstructures results in strongly lowering the overall strength of the plate interface with respect to the classical brittle envelop

shrimp u pb dating of coesite bearing zircon from the ultrahigh pressure metamorphic rocks sulu terrane east china this paper provides further evidence for the ongoing discussion as to whether the dabie uhpm belt formed in triassic or palaeozoic time and whether the sulu uhpm belt formed in triassic or neoproterozoic time combined use of laser raman spectrometer lr cathodoluminescence imaging cl and ion probe u pb in situ dating shrimp provided accurate ages of uhpm from rocks collected from weihai ne sulu uhpm belt lr was used to identify coesite and other uhp minerals as inclusions in zircon separates from an amphibolized peridotite and an eclogite cl was used to examine the zoning structure of these zircon and shrimp dating was performed on specific spots on zircon to obtain ages of different geological events an age of 221 12 ma was obtained for coesite bearing zircon from the amphibolized peridotite an age of 228 29 ma for eclogite was obtained from the lower intercept of a concordia plot these ages are interpreted as the time of uhpm in the weihai region ultramafic rocks to the east of weihai yield a magmatic age at 581 44 ma the zircon in the ultramafic rocks possibly also records a thermal event at c 400 ma but no independent geological evidence for this event has been found the eclogite protolith formed in the middle proterozoic 1821 19 ma which is similar to the age of country rock gneisses of 1847 1744 ma the new geochronological data confirm that uhpm occurred in the triassic in the sulu area when subduction took the ultramafic body and the eclogite protolith together with the adjacent supracrustal rocks to mantle depths

deep subduction of continental crust in accretionary orogen evidence from u pb dating on diamond bearing zircons from the qinling orogen central china abstract accretionary orogenic belts are considered the most important sites for the generation of continental crust however it is still not very clear whether in such settings the continental crust has been significantly transported back to the mantle accompanied with its lateral growth continental uhp rocks the best indicators of the recycling of the continental crust have rarely been discovered in accretionary orogenic belts which hinder the exploration of this essential process the possible occurrence of early paleozoic uhp metamorphism in the qinling orogen provides an excellent opportunity to address this question this article reports an integrated study of u pb age trace element and mineral inclusion of zircon from an amphibolite sample in the qinling orogen the zircon crystals show typical metamorphic growth zoning low th u ratios flat hree patterns and insignificant eu anomalies they give a weighted mean u pb age of 490 4 5 8 ma most importantly an in situ diamond inclusion was identified from one of the zircon crystals therefore the obtained 490 4 5 8 ma age was taken as registering the peak uhp metamorphism the discovery of in situ diamond inclusion provides important evidence for the uhp metamorphism of the qinling orogen and indicates that the north qinling microcontinent was subducted to mantle depths of 120 km when it collided with the erlangping arc attached to the southern margin of the north china block combining with previous results we suggested that the north qinling uhp terrane underwent fast exhumation from 120 km to ca 30 km within 10 myr intervals as a few examples of uhp metamorphism have been discovered in some accretionary orogens more attention needs to be paid to the occurrence of continental uhp metamorphism during accretionary orogenic process furthermore evident crustal growth has been documented in the north qinling orogen during the early paleozoic it is inferred that the accretionary orogenic belts may play important roles in recycling of continental crust into the mantle besides generation of continental crust

timing of collision of the sino korean and yangtse cratons u pb zircon dating of coesite bearing eclogites the suture zone between the sino korean and yangtse cratons in central china is marked by a series of fault bounded belts composed of crustal protoliths in the dabie mountains some of these blocks contain coesite and diamond indicating subduction to minimum depths of 120 km as a result of continental collision the blocks range from the structurally lowest ultrahigh pressure metamorphic belt and decrease in metamorphic grade upsection to the structurally highest low greenschist facies belt furthermore the faults mark large contrasts in metamorphic grade these features form a structure that is similar to metamorphic core complexes indicating extension as part of the exhumation process timing of the continental collision has previously been inferred as late triassic from regional geologic relations or caledonian from 40 ar 39 ar dating of the adjacent and possibly related qinling metamorphic belt u pb dating of zircon from ultra high pressure eclogites yields a metamorphic age of 209 2 ma this age coincides with later stages of collision and is therefore a minimum estimate of the time of collision

apparent activation energy and rate limiting process estimation from natural shale deformed by pressure solution in shallow subduction zone pressure solution is one of the main deformation mechanisms for lithify and stress release in the rocks from shallow subduction zones we observed temperature dependent pressure solution development in naturally deformed shale in shimanto accretionary complex in southwest japan the pressure solution develops with shear dominated or co axial dominated deformation we evaluated apparent activation energy by applying the constitutive equation of pressure solution creep to the temperature dependent relations the activation energy of each deformation type was estimated as 18 kj mol 1 for shear dominated and 45 kj mol 1 for co axial dominated shale the energies enable us to speculate rate limiting processes of pressure solution i e dissolution diffusion and precipitation by comparing the energies obtained in this study with energies had been measured by experiments the lower activation energy estimated here was similar with that of diffusion the similarity indicates that possible rate limiting process of shale deformation in shallow subduction zone would be diffusion the difference of energy between deformation types can be explained by distinctive grain boundary structure

age and nature of eclogites in the huwan shear zone and the multi stage evolution of the qinling dabie sulu orogen central china abstract in situ la icpms u pb trace element and hf isotope data in zircon demonstrate a carboniferous age for eclogite facies metamorphism in siluro devonian protoliths in the huwan shear zone dabie mountains central china this age contrasts with the more prevailing triassic age for high to ultrahigh pressure hp to uhp metamorphism in the qinling dabie sulu orogen metamorphic zircon in two eclogite samples from sujiahe is characterized by low th u ratios small negative eu anomalies flat hree patterns and low 176 lu 177 hf ratios these geochemical signatures suggest that the zircon crystallized in the presence of garnet and in the absence of plagioclase feldspar furthermore temperatures of 655 and 638 c calculated using the ti content of zircon are consistent with their formation during eclogite facies metamorphism the weighted mean 206 pb 238 u age of 309 4 ma 2 for this zircon improves previous age estimates for eclogite facies metamorphism in the huwan shear zone ranging from 420 to 220 ma metamorphic zircon from one eclogite sample from hujiawan most likely formed during prograde metamorphism yields an equivalent age estimate of 312 11 ma magmatic zircon cores in the three samples yield ages for the magmatic protoliths of the eclogites ranging from 420 7 to 406 5 ma and post dating the middle paleozoic collision of the north china and the qinling terrain the zircon crystals in the three eclogite samples display a large variation of e hf t values of 4 9 to 21 3 the metamorphic zircon overgrowths show the same range of e hf t values as those of the inherited magmatic crystal interiors this suggests that the metamorphic zircon overgrowths may have formed by dissolution reprecipitation of pre existing magmatic zircon thereby preserving their original hf isotopic composition the high e hf t values suggest that the protoliths were derived from depleted mantle sources most likely paleotethyan oceanic crust while the low e hf t values are attributed to crustal contamination some eclogites in the huwan shear zone have a distinctive signature of continental crust most probably derived from the yangtze craton the coexistence of paleozoic oceanic crust and neoproterozoic continental crust with similar metamorphic ages in the huwan shear zone implies that paleozoic paleotethyan oceanic crust was produced within a marginal basin of the northern yangtze craton the opening of the paleo tethyan ocean was slightly younger than the collision of the north china craton and the qinling terrain during the late paleozoic in the qinling dabie sulu orogen subduction of the paleo tethyan oceanic crust and associated continental basement resulted in the 309 2 ma 2 eclogite facies metamorphism in the huwan shear zone the subsequent triassic continent continent collision led to the final coalescence of the yangtze and sino korean cratons amalgamation of the yangtze and north china cratons was therefore a multistage process extending over at least 200 ma

modeling fluid transfer along california faults when integrating pressure solution crack sealing and compaction processes 1 several models pertaining to earthquake cycles imply intermittent fluid flow through fault during the interseismic period increase in fluid pressure from hydrostatic to lithostatic values is a crucial parameter in mechanisms leading to earthquakes to achieve such pressures geodynamic processes gouge compaction fluid flow and changes in permeability are required previous models have postulated that changes in permeability by self healing are faster than the effects of geodynamic processes we consider the different mechanisms and rates of crack sealing near active fault on the examples of uplifted californian faults we find that natural crack sealing is normally not achieved by a rapid self healing process pressure solution with mass transfer from solution cleavage to cracks appears to be a more important mechanism for crack sealing and creep during postseismic deformation the geometry of transfer path and experimental data have been used to model crack sealing rates by pressure solution which are estimated to be rather slow similar to the recurrence time of some earthquakes such slow changes in permeability may be crucial factors in controlling the increase in fluid pressure and consequently the mechanism of critical failure in faults then numerical modeling of fluid pressure and transfer around active faults has been performed integrating a slow change in permeability by crack sealing gouge compaction and fluid flow from depth this modeling shows various location and evolution of fluid overpressure during the interseismic period depending on these processes and allows one to estimate the amount of fluid transferred from depth during interseismic periods

field and modelling investigations of fresh water plume behaviour in response to infrequent high precipitation events sydney estuary australia abstract runoff from the urban environment is a major contributor of non point source contamination for many estuaries yet the ultimate fate of this stormwater within the estuary is frequently unknown in detail the relationship between catchment rainfall and estuarine response within the sydney estuary australia was investigated in the present study a verified hydrodynamic model environmental fluid dynamics computer code was utilised in concert with measured salinity data and rainfall measurements to determine the relationship between rainfall and discharge to the estuary with particular attention being paid to a significant high precipitation event a simplified rational method for calculating runoff based upon daily rainfall subcatchment area and runoff coefficients was found to replicate discharge into the estuary associated with the monitored event determining fresh water supply based upon estuary conditions is a novel technique which may assist those researching systems where field measured runoff data are not available and where minor field measured information on catchment characteristics are obtainable the study concluded that since the monitored fresh water plume broke down within the estuary contaminants associated with stormwater runoff due to high precipitation events daily rainfall 50 mm were retained within the system for a longer period than was previously recognised

paleozoic metamorphism in the qinling orogen tongbai mountains central china the collision of the sino korean and yangtze blocks to form a significant part of china is recorded in the qinling tongbai and dabie mountains radiometric ages of the ultrahigh pressure metamorphic rocks in the south qinling orogenic belt suggest that subduction and collision took place during the triassic period our new 40 ar 39 ar geochronology of units in the north qinling orogenic belt confirms that high grade metamorphism and deformation took place also during the silurian devonian and carboniferous periods these results imply that the amalgamation of eastern china was a multistage process extending over at least 200 m y

timing of collision of the north and south china blocks controversy and reconciliation the qinling orogen was formed by the joining of the north and south china blocks but the timing of their integration has been debated for more than a decade the controversies obviously stem from different approaches to reconstruction of the integration history two contrasting lines of evidence yield two different ages for collision of the north and south china blocks middle paleozoic and late triassic the shangdan suture within the qinling was regarded in previous studies as the trace along which the north and south china blocks collided our studies however demonstrate that there are two sutures within the qinling the well documented shangdan suture and the newly discovered mianlue suture we show in this paper that the late proterozoic to early mesozoic evolution of the qinling involved interactions between the north china block the north and south qinling orogens and the south china block the middle paleozoic collision along the shangdan suture as constrained by some evidence accreted only the south qinling orogen to the southern part i e the north qinling of the north china block contemporaneous rifting of the south china block and subsequent drifting separated the south qinling from the south china block during the middle to late paleozoic the separation of the south from the north china blocks is supported by other evidence in particular geomagnetic data evidently it was the late triassic collision of the south china block with the south qinling orogen along the mianlue suture that led to final integration of the north and south china blocks

paleothermal structure of the shimanto accretionary prism shikoku japan role of an out of sequence thrust vitrinite reflectance study on the cretaceous shimanto accretionary prism of southwest japan shows that the vitrinite reflectance varies from 0 6 to 3 0 corresponding to the estimated maximum paleotemperature variation of 90 210 c the thermal structure shows a sharp discontinuity across an out of sequence thrust separating it into two thermal blocks in each block vitrinite reflectance values increase systematically toward the prism younging direction and the thermal structure is clearly overprinted on the tectonic imbrication of turbidite and melange units this provided us with a set of data to calculate the geometry and displacement of the out of sequence thrust the fission track zircon partial annealing ages indicate that this thrust was active about 43 ma probably in response to the accretion of eocene rocks this study shows that initial thickening of the accretionary prism in an order of several kilometres was mainly controlled by out of sequence thrust

coupled fluid flow and sediment deformation in margin scale salt tectonic systems 2 layered sediment models and application to the northwestern gulf of mexico 1 in paper 1 we described a methodology to model coupled fluid flow and deformation in composite salt and siliciclastic tectonic systems and investigated their compaction and overpressuring behavior prior to and during continental margin scale gravitational spreading compaction driven darcy fluid flow in clastic sediments is coupled through the effective pressure to their frictional plastic yield and mechanical deformation viscous flow of the underlying salt is independent of fluid pressure paper 1 presented prototype models that are limited to single uniform sediment lithologies either sandstone type or shale type that undergo mechanical and volumetric viscous compaction in this paper we present models with layered sandstone type and shale type lithologies designed to better approximate the more complex stratigraphy of the gulf of mexico our natural example a first set of models demonstrates that layered lithologies can produce fluid pressure regimes similar to those observed in sedimentary basins we then introduce an improved formulation of viscous compaction that includes a stronger dependence on porosity and depth used as proxy for temperature thereby more effectively self limiting viscous compaction a second set of models with the improved viscous compaction formulation demonstrates that the onset of gravity spreading is mainly controlled by overpressuring in the landward end of the salt basin and that resulting shortening in the distal part is partly accommodated by horizontal compaction models with moderately high fluid pressure best reproduce conditions considered to have been necessary for large scale gravitational spreading in the northwestern gulf of mexico which led to the formation of the perdido fold belt

geochronology in migmatites a sm nd u pb and rb sr study from the proterozoic damara belt namibia implications for polyphase development of migmatites in high grade terranes sm nd garnet u pb monazite and rb sr biotite ages from a composite migmatite sample damara orogen namibia constrain the time of high grade regional metamorphism and the duration of regional metamorphic events sm nd garnet whole rock ages for a strongly restitic melanosome and an adjacent intrusive leucosome yield ages of 534i5 528i11 and 539i8 ma these results provide substantial evidence for pre 500 ma pan african regional metamorphism and melting for this segment of the orogen other parts of the migmatite yield younger sm nd ages of 488i9 ma for melanosome and 496i10 492i5 and 511i16 ma for the corresponding leucosomes garnet from one xenolith from the leucosomes yields an age of 497i2 ma major element compostions of garnet are different in terms of absolute abundances of pyrope and spessartine components but the flat shape of the elemental patterns suggests late stage retrograde equilibration rare earth element compositions of the garnet from the different layers are similar except for garnet from the intrusive leucosome suggesting that they grew in different environments monazite from the leucosomes is reversely discordant and records 207 pb 235 u ages between 536 and 529 ma indicating that this monazite represents incorporated residual material from the first melting event monazite from the mesosome mes 2 and the melanosome mel 3 gives 207 pb 235 u ages of 523 and 526 ma and 529 and 531 ma respectively which probably indicates another thermal event previously published 207 pb 235 u monazite data give ages between 525 and 521 ma for composite migmatites and 521 and 518 ma for monazite from neosomes monazite from granitic to granodioritic veins indicates another thermal event at 507 505 ma these ages are also recorded in 207 pb 235 u monazite data of 508 ma from the metasediment met 1 from the migmatite and also in the sm nd garnet ages obtained in this study taken together these ages indicate that high grade metamorphism started at c 535 ma or earlier and was followed by thermal events at c 520 ma and c 505 ma the latter event is probably connected with the intrusion of a large igneous body donkerhoek granite for which so far only imprecise rb sr whole rock data of 520i15 ma are available rb sr biotite ages from the different layers of the migmatite are 488 469 and 473 ma these different ages indicate late stage disturbance of the rb sr isotopic system on the sub sample scale nevertheless these ages are close to the youngest sm nd garnet ages indicating rapid cooling rates between 13 and 20u cm a x1 and fast uplift of this segment of the crust similar sm nd garnet and u pb monazite ages suggest that the closure temperatures for both isotopic systems are not very different in this case and are probably similar or higher than the previously estimated peak metamorphic temperatures of 730i30uc the preservation of restitic monazite in leucosomes indicates that dissolution of monazite in felsic water undersaturated peraluminous melts can be sluggish this study shows that geochronological data from migmatites can record polymetamorphic episodes in high grade terranes that often contain cryptic evidence for the nature and timing of early metamorphic events

high temperature geochronology constraints on the tectonic history and architecture of the ultrahigh pressure dabie sulu orogen 1 new u pb zircon and th pb monazite ages are presented from the giant sulu ultrahigh pressure uhp terrane combined with sm nd ages rb sr ages inclusion relationships and geologic relationships they help define the timing of peak recrystallization the timing of subsequent amphibolite facies metamorphism and the architecture of the dabie sulu suture zone between the collided sino korean and yangtze cratons the data indicate a 15 myr record of uhp recrystallization the first clearly documented for a giant uhp terrane this requires that continental subduction in the dabie sulu orogen involved multiple uhp tectonic or recrystallization events a 244 236 ma precursor uhp event seen only in the dabie shan was followed by a second 230 220 ma main uhp event which was itself terminated by a 220 205 ma amphibolite facies overprint older eclogite facies events seen in the qinling segment of this orogenic belt raise the possibility that these rocks have undergone u hp metamorphism three or four times but at present there is no geochronological evidence in the dabie sulu area to support this the subduction of the lower yangtze plate did not proceed in a simple fashion the ages of inherited zircon cores demonstrate that a ribbon continent of yangtze affinity escaped subduction and became wedged against the sino korean plate hanging wall

high pressure metamorphic rocks from tongbaishan central china u pb and 40ar 39ar age constraints on the provenance of protoliths and timing of metamorphism abstract the tongbaishan area is situated at the junction between the paleozoic north qinling orogen and the mesozoic south qinling dabie sulu orogen the area contains an important geological section for understanding the assembly of the sino korean and yangtze cratons two eclogite zones were recently recognized in this area the peak metamorphic conditions are estimated to be 530 610 c and 17 20 kbar for the northern eclogite zone and 460 560 c and 13 19 kbar for the southern eclogite zone shrimp u pb analyses on zircons of eclogite metagabbro and orthogneiss indicate that their protoliths were formed during the neoproterozoic and experienced high pressure hp recrystallization during the permian triassic period therefore the tongbaishan area belongs to the western extension of the dabieshan hp uhp terrane in addition u pb ages of detrital zircons from quartzite paragneiss and schist show three age populations of 2 49 ga 1 93 ga and 1 85 1 82 ga suggesting that extensive paleoproterozoic orogeny or tectonothermal events took place in the northern margin of the yangtze craton the shrimp dating and complementary 40ar 39ar dating of muscovite from quartzofeldspathic gneiss and quartzite reveal that the crustal rocks were subducted to great depths and underwent hp eclogite facies metamorphism at ca 255 ma slightly earlier than the timing of deep subduction of the uhp slice the hp rocks were later exhumed to mid to upper crustal levels and suffered from retrograde recrystallization at ca 238 ma the dating also discloses a still later phase of deformation and recrystallization at ca 215 ma the available age data throughout the tongbai dabie sulu orogen reflect diachronous exhumation from ca 238 ma for the hp slice to ca 213 ma for the uhp slice such a phenomenon could be explained by successive subduction and exhumation of the broken slices of the subducting yangtze slab

cohesive strengthening of fault zones during the interseismic period an experimental study 1 there is widespread evidence indicating that faults regain a portion of their strength during the interseismic period here we present experiments designed to understand and quantify the interseismic cohesive strengthening resulting from fluid rock reactions in fault zones the triaxial experiments consisted of fracturing cores of fontainebleau sandstone under dry conditions forming a localized shear failure zone stage 1 the specimens were then reacted hydrothermally under isostatic conditions allowing the fault damage zone to compact consolidate and strengthen stage 2 following reaction the specimens were then reloaded to failure under nominally dry conditions so that the increase in cohesive strength of the fault could be measured stage 3 experiments show that cohesion increase is positively correlated to temperature and pore pressure during reaction after 6 hours of reaction at the highest temperatures 927 c and pore pressures 200 mpa cohesion increases by as much as 35 mpa microstructural examination of the specimens showed that the gouge particles within the fault compacted and cemented together exhibiting textures typical of pressure solution and that fractures in the surrounding damage zone had healed a theoretical treatment of the data was conducted using these experiments in combination with results on time dependent changes in fault cohesion presented by tenthorey et al 2003 we find that the rate controlling process in our experiments has an activation energy q of approximately 70 kj mol 1 we use this information to develop a model for time dependent cohesive strengthening in fault zones within the continental seismogenic regime we conclude that significant cohesive strengthening of fault zones can occur during the interseismic period of medium to large earthquakes given the presence of reactive pore fluids

timing relationships between pegmatite emplacement metamorphism and deformation during the intra plate alice springs orogeny central australia in the harts range central australia the upper amphibolite facies to lower granulite facies c 480 460 ma harts range metamorphic complex hrmc and the upper amphibolite facies c 340 320 ma entia gneiss complex are cut by numerous generally peraluminous pegmatites and their deformed equivalents the pegmatites have previously been interpreted as locally derived partial melts however shrimp u pb monazite and zircon dating of 29 pegmatites or their deformed equivalents predominantly from the hrmc reveal that they were emplaced episodically throughout almost the entire duration of the polyphase c 450 300 ma intra plate alice springs orogeny episodes of pegmatite intrusion correlate with the age of major alice springs age structures and with deposition of syn orogenic sedimentary rocks in the adjacent centralian superbasin similar alice springs ages have not been obtained from anatectic country rocks in the hrmc suggesting that the pegmatites were not locally derived instead they are interpreted as highly fractionated granites and imply that much larger parental alice springs age granites exist at depth the mechanism to allow repeated felsic magmatism in an intraplate setting where all exposed rock types had a previous high temperature history is enigmatic however we suggest that episodic underthrusting and dehydration of unmetamorphosed centralian superbasin sedimentary rocks allowed crustal fertility to maintained over a c 140 ma interval during the intra plate alice springs orogeny

discovery of metamorphic diamonds in central china an indication of a 4000 km long zone of deep subduction resulting from multiple continental collisions the central orogenic belt cob of china is a major continental collision zone that contains extensive outcrops of deeply subducted and exhumed rocks at both the eastern and the western end of the belt here we report discovery of microdiamonds from both eclogites and felsic gneisses in the north qinling zone in the central portion of the cob this discovery demonstrates that the country rocks of continental affinity shared in the ultra high pressure metamorphic uhpm event and provides a bridge connecting the two previously recognized uhpm terranes thereby establishing the existence of a uhpm belt extending more than 4000 km geochronological dating yields early palaeozoic ages in the west and early mesozoic ages in the east recording two separate continental collisions overprinted within the cob occurrence of uhp metamorphism during recurrent continental collision here and in the alps suggests that deep subduction of continental material during such collisions is probably common rather than exceptional with significant implications for processes of plate tectonic reorganization and mantle mixing over time

tectonic controls on metamorphism partial melting and intrusion timing and duration of regional metamorphism and magmatism in the nigde massif turkey protracted heating and melting of orogenic middle crust followed by emplacement of a syn extensional pluton during decompression and cooling are demonstrated for a metamorphic core complex in central turkey new geochronologic data for the nigde massif of the central anatolian crystalline complex document an interval of melting crystallization and pb loss at elevated ambient temperature 85 92 ma determined by u pb shrimp within a single sample of peraluminous granite crustal melting was syn to post peak metamorphism 725 c dated at 91 0 2 0 ma by shrimp analysis of zircon rims in sillimanite schist that is part of the country rock to the pluton metamorphic monazite in sillimanite schist yields a younger age of 84 7 0 7 ma for matrix grains and inclusions in garnet hornblende 40ar 39ar ages from amphibolites interlayered with the sillimanite schists coincide with the timing of emplacement of magma into metamorphic country rock that had experienced cooling and decompression to 12 km and 550 c the similarity of 40ar 39ar ages for biotite in the pluton 79 ma and hornblende 79 81 ma biotite 74 76 ma and muscovite 78 83 ma in the metamorphic country rock document rapid cooling 100 c my following intrusion the coincidence of the youngest u pb magmatic zircon ages and the oldest 40ar 39ar ages combined with structural evidence for normal sense shearing and tectonic unroofing in the late cretaceous suggests that intrusion was driven by extension we propose that crustal melting occurred in the middle crust during prograde metamorphism and or initial decompression and that ascent and emplacement of magma as a pluton were only triggered once extension occurred and removed upper crustal rocks

evolution of the qinling orogenic belt detailed field mapping geophysics and geochemistry show that the qinling orogenic belt is a southward vergent tectonic wedge consisting of six regional thrust sheets the lushan luanchun qinling douling wudang and zhuqi thrust sheets these regional thrusts underwent displacement at the end of the mid triassic and the end of the late jurassic due to the collision between the north and south china blocks and subsequent detachment the qinling orogenic belt is thus an indosinian yanshanian collision detachment zone the so called caledonian orogeny in the qinling orogenic belt records a succession of extensional and partial melting events in the continental crust the basic lavas in the lower paleozoic were erupted into rifting zones both in the northern qinling subzone and southern qinling subzone at the same time local melting of the transitional continental crust occurred and resulted in formation of s type granites

numerical modelling of the sydney harbour estuary new south wales lateral circulation and asymmetric vertical mixing abstract a fully calibrated three dimensional hydrodynamic model of the sydney harbour estuary was used to determine the dominant forcing of regulating estuarine circulation in a sinuous channel under conditions of low river discharge during dry weather the system characterized by stratified flood and mixed ebb during spring tides and stratified flood and ebb during neap tides was analysed using model results of current profiles density field eddy viscosity and gradient richardson number at a mooring station and cross channel transect the magnitude and direction of lateral circulation was modulated in the flood ebb and spring neap cycles due to changes in the interactions between the tides and the complex geometry during spring tides a differential advection induced lateral baroclinic pressure gradient tends to drive the lateral circulation and stratifies the water column when the along estuary circulation is strong enough to break the stratification induced by the lateral baroclinic pressure gradient centrifugal force works in concert with nonlinear advection establishing a classical two layer helical flow across the channel channel bends also induce vertical mixing as a result of the overturning of the density fields the lateral circulation redistributes the differential advection induced lateral shear of the along estuary momentum and is transferred to either reinforce or cancel the along estuary circulation the lateral bathymetry variability in a channel is therefore the key factor causing the intratidal asymmetries in the along estuary circulations stratification suppresses lateral circulation during neap tides and generates a three layer lateral circulation structure in the water column during stratified ebb

geochronology geochemistry and isotope compositions of piaochi s type granitic intrusion in the qinling orogen central china petrogenesis and tectonic significance abstract s type peraluminous granite can crop out in various tectonic settings the paleozoic piaochi granitic intrusion is a large s type peraluminous intrusion located in the north qinling orogen and has important implication for its tectonic evolution though intensively studied the age and tectonic setting of this intrusion are still controversial in this study we carried out an integrated study of zircon u pb age major and trace elements and sr nd hf o isotope compositions for the piaochi intrusion with aims to constrain its petrogenesis and tectonic significance sims and la icpms u pb zircon data obtained from magmatic zircons indicate that the piaochi intrusion was emplaced at 473 4 ma and contained ca 490 ma inherited zircons the granite samples from the piaochi intrusion show typical s type characteristics such as the relatively moderate aluminum saturation index 1 00 1 08 low zirconium saturation temperatures occurrences of muscovite and high zircon 18 o values 8 70 to 9 54 the relatively low sm nd 0 13 0 20 and feo t mgo ratios 2 21 3 9 suggest that the granites are unfractionated the geochemical compositions reveal that they were derived from partial melting of a clay poor but plagioclase and biotite rich psammitic source similar to metasedimentary rocks outcropping in the north qinling unit moreover the piaochi granites are moderately depleted in eu and heavy rare earth elements and have relatively high la yb n values 22 8 56 8 implying a high pressure plagioclase poor and garnet rich residual assemblage the piaochi granites have high and variable initial 87 sr 86 sr ratios 0 72167 0 72511 and low e nd t from 10 23 to 8 71 values with two stage nd model ages t dm2 of 1 91 2 03 ga which are comparable with those of supracrustal materials in the north qinling unit in addition our results show that the ca 470 ma magmatic zircons display a narrow e hf t array within those of the inherited cores these imply that the piaochi granites inherited their isotopic characteristics wholly from their sources and the hf isotopes might have been differently homogenized during the partial melting our new zircon u pb ages indicate that the piaochi granites were emplaced after the climax of the uhp metamorphism coeval with later retrograde metamorphism during exhumation therefore we suggest that the formation of the early paleozoic piaochi intrusion was triggered by the exhumation of the uhp rocks

diffusion in zircon despite its low abundance in most rocks zircon is extraordinarily useful in interpreting crustal histories the importance of u th pb isotopic dating of zircon is well and long established davis et al this volume parrish et al this volume zircon also tends to incorporate trace elements useful as geochemical tracers such as the ree y and hf a number of characteristics of zircon encourage the preservation of internal isotopic and chemical variations often on extremely fine scale which provide valuable insight into thermal histories and past geochemical environments the relative insolubility of zircon in crustal melts and fluids as well as its general resistance to chemical and physical breakdown often result in the existence of several generations of geochemical information in a single zircon grain the fact that this information is so frequently retained as evidenced through backscattered electron or cathodoluminescence imaging that often reveal fine scale zoning down to the sub micron scale has long suggested that diffusion of most elements is quite sluggish in zircon in this chapter we present an overview of the findings to date from laboratory measurements of diffusion of cations and oxygen in zircon because of its importance as a geochronometer attempts have been made to measure diffusion especially of pb for over 30 years but only in the last decade or so have profiling techniques with adequate depth resolution been employed in these studies resulting in a plethora of new diffusion data these findings have important implications for isotopic dating interpretation of stable isotope ratios closure temperatures and formation and preservation of primary chemical composition and zoning in zircon efforts have been made for some time to quantify and characterize diffusion in zircon most notably of pb in deference to its significance in interpreting pb isotopic signatures and refining understanding of thermal histories as is evident from

rate laws for water assisted compaction and stress induced water rock interaction in sandstones mineral water interactions under conditions of nonhydrostatic stress play a role in subjects as diverse as ductile creep in fault zones phase relations in metamorphic rocks mass redistribution and replacement reactions during diagenesis and loss of porosity in deep sedimentary basins as a step toward understanding the fundamental geochemical processes involved using naturally rounded st peter sand we have investigated the kinetics of pore volume loss and quartz water reactions under nonhydrostatic hydrothermal conditions in flow through reactors rate laws for creep and mineral water reaction are derived from the time rate of change of pore volume sand water dissolution kinetics and flow rate independent steady state silica concentrations and reveal functional dependencies of rates on grain size volume strain temperature effective pressure confining minus pore pressure and specific surface areas together the mechanical and chemical rate laws form a self consistent model for coupled deformation and water rock interaction of porous sands under nonhydrostatic conditions microstructural evidence shows a progressive widening of nominally circular and nominally flat grain grain contacts with increasing strain or equivalently porosity loss and small quartz overgrowths occurring at grain contact peripheries the mechanical and chemical data suggest that the dominant creep mechanism is due to removal of mass from grain contacts termed pressure solution or solution transfer with a lesser component of time dependent crack growth and healing the magnitude of a stress dependent concentration increase is too large to be accounted for by elastic or dislocation strain energy induced supersaturations favoring instead the normal stress dependence of molar gibbs free energy associated with grain grain interfaces

constraining the role of the qinling orogen in the assembly and break up of rodinia tectonic implications for neoproterozoic granite occurrences abstract new shrimp and id tims dating results in this study reveal that the widespread early neoproterozoic granites in the qinling orogen were emplaced mainly between 960 and 900 ma major elements mineral compositions and trace elements data also indicate that all these early neoproterozoic granites are characterized by the features of crust derived and typical continent continent syn collisional granites this imply that continent continent collisional event could have occurred in the qinling orogen during the period of 960 to 900 ma which was closely related to the formation of rodinia thus our study argues that the final assembly of rodina occurred at 900 ma on the other hand k feldspar granite dq05 a type granite and granodiorite dq07 from the study area are dated at 711 11 and 717 10 ma respectively and are considered to have generated in an extensional setting this extensional tectonic regime originated from 810 ma when large scale continental bimodal volcanic rocks were formed in south qinling and lasted at least until the time of 710 ma which is consistent with the break up of rodinia this study also suggests that the north china craton did not collide and join together with the south china craton in the neoproterozoic and both of them were probably far away from each other

early crustal evolution in the western yangtze block evidence from u pb and lu hf isotopes on detrital zircons from sedimentary rocks abstract in situ u pb and lu hf data on detrital zircons from paleo to neoproterozoic sediments have been used to gain a clearer picture of the regional tectonic background and crustal evolution in the western part of the yangtze block the youngest concordant zircon ages for sedimentary units suggest maximum depositional ages of 1014 ma for the kunyang group 750 649 ma for the sinian sequences and 525 ma for the cambrian sediments the yinmin formation previously assigned to the middle part of the kunyang group was actually deposited after 1667 ma and contains zircons whose ages are dominantly from late archean to paleoproterozoic 2 8 2 7 ga 2 5 2 3 ga and 1 85 ga the heishantou formation in the lower part of the kunyang group has two major age populations of 1 0 ga and 1 8 1 6 ga the sinian and cambrian sedimentary rocks are dominated by neoproterozoic zircons with age peaks at 760 ma and 825 ma consistent with the ages of widespread igneous rocks around the yangtze block the lu hf isotope data suggest that a significant juvenile input took place during archean and neoproterozoic times respectively while crustal reworking was dominant during the paleoproterozoic time the archean paleoproterozoic detritus is isotopically distinct from the archean paleoproterozoic basement exposed in the northern part of the yangtze block suggesting that a subarea of old crust lies beneath the young sediments covering the craton the presence of abundant 1 85 ga zircons suggests that the yangtze block was probably part of the columbia supercontinent during paleoproterozoic time comparisons to the other parts of columbia suggest that the yangtze block could have been adjacent to the north china craton and or australia

the timing of partial melting barrovian metamorphism and granite intrusion in the naxos metamorphic core complex cyclades aegean sea greece abstract the relationship between partial melting and granite intrusion in a classic barrovian metamorphic terrane has been assessed thirteen samples were dated by shrimp u pb zircon geochronology from the island of naxos greece one of the aegean metamorphic core complexes the effect of partial melting during peak barrovian metamorphism on naxos is recorded by fine

variations in the provenance of the late neogene red clay deposits in northern china abstract the voluminous loess red clay deposits in northern china forming part of the chinese loess plateau clp are valuable terrestrial archives of climatic evolution for the late cenozoic era fundamental in reconstructing the late miocene and pliocene wind patterns and aridification history is a detailed knowledge of the provenance of these deposits this paper provides end member modelling of bulk grain size distributions and u pb dating of detrital zircons for three distant red clay sequences in the northeastern baode southern lantian and western dongwan clp data show that these different sections each display a distinctive compositional structure indicating variable depositional processes but they also share two significant zircon age populations of 200 300 ma and 400 500 ma while the permian triassic 200 300 ma group accounts for a larger proportion of zircons ages in the northeastern ne clp the ordovician silurian 400 500 ma component is dominant in the southern and western clp it is suggested that the red clay in the southern and western clp was mainly derived from the northern tibetan plateau ntp and the taklimakan desert by low level westerly winds samples of the ne clp show an increased signature of sediments transported by near surface northwesterly winds from the broad area of the central asian orogen belt caob this spatial transport and deposition pattern is supported by the results from the backtrace trajectory modelling of the dominant dust transport pathways in the clp it is noted that the red clay sample of around 3 6 ma obtained from the ne clp shows increased detrital contributions from its west possibly indicating an intensified westerly wind strength and or aridity of the ntp and taklimakan desert due to the uplift of the tibetan plateau and tianshan mountains in the pliocene the onset of enhanced drainage of the yellow river caused by the increased denudation of the netp since 3 6 ma could also have contributed to this

palaeozoic pre and post collision calc alkaline magmatism in the qinling orogenic belt central china as documented by zircon ages on granitoid rocks based on large scale reconnaissance mapping we identified two calc alkaline plutonic assemblages from the northern qinling orogenic belt central china the older assemblage of intrusions closely associated and deformed coevally with their host volcanic arc sequences seems to represent the fractionation product of basaltic arc magma it therefore predates the collision of the north china block with the central qinling island arc system that developed in a sw pacific type oceanic domain south of the north china block single zircon 207pb 206pb evaporation dating yielded early to middle ordovician ages for this assemblage with a relatively small range from 487 2 1 1 to 470 2 1 3 ma intrusions of the younger assemblage are largely undeformed and truncate structures shown in rocks of the older assemblage they are interpreted as post collisional calc alkaline granitoids single zircon dating provided an age of 401 8 0 8 ma for the younger assemblage consistent with earlier work that defines an age range from c 420 to 395 ma our data favour a tectonic model involving formation and amalgamation of island arc and microcontinent terranes between ca 490 and 470 ma ago to create the central qinling zone which subsequently collided with the north china block prior to c 400 ma ago a late precambrian age of 762 0 0 7 ma for a granitoid gneiss at the northern margin of the yangtze block supports a gondwana affinity for this large continental block

the paleozoic metamorphic history of the central orogenic belt of china from 40ar 39ar geochronology of eclogite garnet fluid inclusions abstract the pressure temperature time evolution of the uhp eclogites of dabie sulu in the eastern sector of the central orogenic belt of china shows a complex pattern of predominantly triassic and to a lesser extent early paleozoic ages 40 ar 39 ar stepwise crushing analysis of eclogitic garnets yielded consistently paleozoic ages because our initial results from the bixiling eclogite were of limited areal extent their significance required further testing data for garnet bearing gneiss and eclogites from new locations in northern and southern dabieshan shuanghe ganghe and hongmiao confirm the paleozoic garnet results obtained from bixiling eclogite we also tested the technique on a garnet sample of known paleozoic age from altyn tagh in the western central orogenic belt these new results indicate that light colored garnets from dabieshan and altyn tagh yield concordant 40 ar 39 ar ages ranging from 445 465 ma by in vacuo crushing and confirm that a paleozoic u hp metamorphic event took place in the central orogenic belt from altyn tagh in western china to dabieshan in eastern china in contrast dark colored garnets from mafic metavolcanics do not yield 40 ar 39 ar plateaux or isochrons ages

dynamic role of tectonic melange during interseismic process of plate boundary mega earthquakes abstract tectonic melange has a key role in subduction zones because its thick pile forms the plate boundary itself therefore the plate boundary process is nearly identical to the melange forming process we examined three tectonic melanges in the shimanto belt in southwest japan to decipher their progressive deformation process with subduction especially within the seismogenic zone here we report detailed observation of sandstone blocks and the strain history of the shale matrix of a sediment dominated tectonic melange the necessity of tectonic melange accompanying fossil seismogenic decollement is unveiled several deformation processes continue by turn until the depth of the down dip limit of the seismogenic zone is reached the results support the space time partition of deformation in terms of seismic behavior and suggest a possible candidate for a geological consequence of recently observed slow earthquakes

evidence of localized failure along altered basaltic blocks in tectonic melange at the updip limit of the seismogenic zone implications for the shallow slow earthquake source

u pb hf and o isotope evidence for two episodes of fluid assisted zircon growth in marble hosted eclogites from the dabie orogen abstract a combined study of internal structure u pb age and hf and o isotopes was carried out for metamorphic zircons from ultrahigh pressure eclogite boudins enclosed in marbles from the dabie orogen in china cl imaging identifies two types of zircon that are metamorphically new growth and recrystallized domain respectively the metamorphic zircons have low th and u contents with low th u ratios yielding two groups of 206 pb 238 u age at 245 3 to 240 2 ma and 226 4 to 223 2 ma respectively anomalously high 18 o values were obtained for refractory minerals with 9 9 to 21 4 for garnet and 16 9 for zircon this indicates that eclogite protolith is sedimentary rocks capable of liberating aqueous fluid for zircon growth during continental subduction zone metamorphism most of the zircons are characterized by very low 176 lu 177 hf ratios of 0 000001 0 000028 indicating their growth in association with garnet recrystallization a few of them falling within the older age group have comparatively high 176 lu 177 hf ratios of 0 000192 0 000383 suggesting their growth prior to the formation of garnet in the late stage of subduction the variations in the lu hf ratios for zircons can thus be used to correlate with garnet growth during eclogite facies metamorphism in either case the zircons have variable e hf t values for individual samples suggesting that their protolith is heterogeneous in hf isotope composition with localized fluid availability in the bulk processes of orogenic cycle nevertheless a positive correlation exists between 206 pb 238 u ages and lu hf isotope ratios for the metamorphically recrystallized zircons suggesting that eclogite facies metamorphism in the presence of fluid has the identical effect on zircon lu hf and u th pb isotopic systems we conclude that the zircons of the older group grew in the presence of fluid during the subduction prior to the onset of peak ultrahigh pressure metamorphism whereas the younger zircons grew in the presence of fluid released during the initial exhumation toward high pressure eclogite facies regime

rheology of the plate interface dissolution precipitation creep in high pressure metamorphic rocks abstract subduction zone models invoke deformation to be concentrated along the plate interface in a region of particularly low temperature geophysical observations do not provide constraints on temperature stress and deformation patterns with desired resolution in contrast the record of high pressure metamorphic rocks exhumed from subduction zones provides details on p t history deformation mechanisms and stress state albeit not readily correlated with the former dynamic situation on larger scale here we review available information on dissolution precipitation creep dpc in high pressure metamorphic rocks which if representative for subduction zones in general can pose constraints on conditions rheology and flow patterns along the plate interface the key observations and conclusions are that 1 deformation is typically highly inhomogeneous and localized into shear zones 2 stresses are generally too low to drive crystal plastic deformation 3 microfabrics suggest dissolution precipitation creep to be the predominant deformation mechanism 4 an aqueous fluid at quasi lithostatic pressure is available throughout allowing for tensile fracturing and crack healing or sealing 5 low stress combined with high strain rates required for localized deformation at typical subduction rates implies low viscosity and 6 contribution of shear heating to the thermal budget of subduction zones should be moderate the dominant deformation mechanism dpc is reviewed in some detail including experimental and theoretical approaches various examples of dpc in high pressure metamorphic rocks are illustrated emphasizing the role of interphase boundaries as sites of dissolution rheology governed by dpc is proposed to control interplate coupling and development of a subduction channel with return flow being a likely candidate for rapid exhumation of high pressure metamorphic rocks

melange and its seismogenic roof decollement a plate boundary fault rock in the subduction zone an example from the shimanto belt japan 1 the mugi melange located in western shikoku of the shimanto belt shows systematic y p deformation fabrics formed by microshear and pressure solution that penetrate throughout the melange pile magnetic susceptibility ellipsoids obtained from the anisotropy of magnetic susceptibility ams are highly oblate maximum and minimum axes of the ellipsoids are consistent with the shear orientation of the melange and the mean pole of p surfaces respectively this agreement suggests that the mugi melange was formed as a result of underthrusting of trench filling sediment vitrinite reflectance ranges from 2 52 to 3 08 which corresponds to a maximum paleotemperature of 180 200 c pseudotachylyte evidence of a seismogenic slip was found in the upper boundary roof fault of the mugi melange however there is not a thermal gap between the melange and the overlying coherent piles and the temperature from vitrinite reflectance gradually rises downward from the coherent piles to the melange beyond the boundary fault which suggests that paleoisotherms parallel the boundary fault orientation the isotherms in the seismogenic zone are estimated as subparallel to the plate boundary decollement therefore the setting of the cataclastic boundary fault which includes pseudotachylyte appears to be a major plate boundary thrust or a subhorizontal splay fault a probable geologic setting that accounts for the mugi melange and the seismogenic roof fault is partitioning of the slip along the plate boundary fault in space and time interseismic slip in the melange and seismic slip along the roof fault

permo triassic collision subduction zone metamorphism and tectonic exhumation along the east asian continental margin abstractconvergent plate motion at 320 210 ma generated the tongbai dabie sulu east central china imjingang gyeonggi central korea renge suo southwestern japan sikhote alin orogen along the paleo pacific edge of cratonal asia this amalgamated belt reflects collision between the sino korean and yangtze cratons on the sw portion and accretion of outboard oceanic arcs sialic fragments against the ne margin subducted proterozoic paleozoic continental and oceanic crustal complexes underwent high and ultrahigh pressure metamorphism at low to moderate temperatures tectonic slices of sialic crust episodically disengaged from the downgoing plate and driven by buoyancy ascended rapidly to midcrustal levels from depths exceeding 90 200 km after continental collision in east central china plus or minus korea and from 30 50 km after arrival of far traveled oceanic terranes in sw japan and the russian far east on achieving neutral buoyancy and stalling out at 10 20 km depth later doming gravitation

zircon u pb ages and hf isotope compositions of migmatites from the north qinling terrane and their geological implications migmatites are predominant in the north qinling nq orogen but their formation ages are poorly constrained this paper presents a combined study of cathodoluminescence imaging u pb age trace element and hf isotopes of zircon in migmatites from the nq unit in the migmatites most zircon grains occur as new homogeneous crystals while some are present as overgrowth rims around inherited cores morphological and trace element features suggest that the zircon crystals are metamorphic and formed during partial melting the inherited cores have oscillatory zoning and yield u pb ages of c 900 ma representing their protolith ages the early neoproterozoic protoliths probably formed in an active continental margin being a response to the assembly of the supercontinent rodinia the migmatite zircon yields hf model ages of 1911 20 to 990 22 ma indicating that the protoliths were derived from reworking of palaeoproterozoic to neoproterozoic crustal materials the anatexis zircon yields formation ages ranging from 455 5 to 420 4 ma with a peak at c 435 ma combined with previous results we suggest that the migmatization of the nq terrane occurred at c 455 400 ma the migmatization was c 50 ma later than the c 490 ma ultra high p uhp metamorphism indicating that they occurred in two independent tectonic events by contrast the migmatization was coeval with the granulite facies metamorphism and the granitic magmatism in the nq unit which collectively argue for their formation due to the northward subduction of the shangdan ocean uhp rocks were distributed mainly along the northern margin and occasionally in the inner part of the nq unit indicating that they were exhumed along the northern edge and detached from the basement by the subsequent migmatization process

multiple partial melting events in the sulu uhp terrane zircon u pb dating of granitic leucosomes within amphibolite and gneiss thin layers and lenses of granitic leucosome are widely distributed within amphibolites paragneisses and orthogneisses of the sulu uhp terrane they are parallel to or cross cut foliations in the host rocks at different scales and show evidence of coalescence and migration to form centimetre to decimetre scale segregations variously migmatized rocks extend at least 350 km from sw sulu maobei to ne sulu weihai in a band at least 50 km wide a combined study of mineral inclusions cathoduluminescence cl images u pb la icp ms dates and in situ trace element compositions of zircon provide clear evidence on the nature and timing of partial melting in these uhp rocks most zircon from the granitic leucosomes occurs as distinct overgrowths around inherited igneous or metamorphic cores or as new euhedral crystals the overgrowths and new crystals commonly show perfectly euhedral shapes have pronounced oscillatory zoning and contain felsic mineral inclusions such as kfs pl qtz ilm monazite mon in contrast the inherited igneous or metamorphic cores are rounded or irregular contain low p or uhp mineral inclusions and show clear dissolution textures these data suggest that the new zircon is anatectic in origin and that it grew during partial melting of the uhp rocks the ree patterns of the anatectic zircon show steep slopes from the hree to lree with strongly to moderately negative eu anomalies eu eu 0 31 0 72 and pronounced positive ce anomalies ce ce 6 8 26 5 abundant u pb spot analyses of the anatectic zircon reveal two discrete and meaningful ages of partial melting within the sulu uhp terrane anatectic zircon from 12 granitic leucosomes within amphibolites paragneisses and orthogneisses from sulu uhp slices ii and iii yields consistent mean u pb ages of 219 0 1 2 to 218 3 1 6 ma 218 8 2 0 to 217 3 1 7 ma and 218 2 1 4 to 215 0 1 5 ma respectively in contrast anatectic zircon from six granitic leucosomes within paragneisses and orthogneisses from sulu uhp slice iii records younger mean u pb ages of 151 9 1 3 to 151 1 1 8 ma and 155 9 1 8 to 153 7 1 7 ma respectively these data imply that the sulu uhp terrane experienced two mesozoic partial melting events the first partial melting event 219 215 ma was probably associated with a late triassic granulite facies stage of hot exhumation whereas the second 156 151 ma is interpreted as the result of middle late jurassic extension and thinning of the previously thickened crust of the sulu uhp terrane both partial melting events induced extensive retrograde metamorphism of the eclogites and their country rocks

deformation by solution deposition and re equilibration of fluid inclusions in crystals depending on temperature internal pressure and stress abstract changes of shape and density of fluid inclusions in crystals were demonstrated when these crystals were subjected to temperatures and internal pressures greater than those of their growth experimental relations have been established between the rate of length change of the inclusions and the different parameters of the experiment nature of the crystal temperature and internal pressure state of stress on the crystal geometric characters of the inclusion length width orientation of the inclusion with respect to the optical axes of host mineral it is essentially the effect of the variation of surface energy with the curvature around the cavity which causes a mass transfer of the host crystal by diffusion through the fluid under pressure the rate of length change is limited by the rate of dissolution at the median part of elongate inclusions this change of shape and density is known to occur in naturally deformed crystals an example of such a behaviour is given which allowed us to use the inclusions as markers of the conditions of temperature and pressure subsequent to the growth of crystal

silurian granulite facies metamorphism and coeval magmatism and crustal growth in the tongbai orogen central china abstract it is intriguing whether the formation of granulite in an orogenic belt is related to magmatism and crustal growth the tongbai orogen is one of the particular areas in the qinling tongbai dabie sulu orogen that contains both granulites and associated arc magmatic rocks and thus is an optimal place to address this issue however the formation age and genesis of the granulites and their relations to the magmatic rocks have not yet been well constrained in situ la mc icpms u pb dating and trace element and lu hf isotope analyses were undertaken on zircon grains from gneiss granulite gabbro and granodiorite samples most zircons in three granulite samples and a gneiss sample have core rim structures the cores have oval or rounded shapes oscillatory zones high th u ratios and variable u pb ages suggesting a detrital genesis the youngest group of the detrital zircons yields a weighted mean 206 pb 238 u age of 450 5 ma which is interpreted as the maximum depositional age of their protoliths a major age cluster is ca 450 490 ma corresponding to prolonged arc magmatism in the northern tongbai and qinling orogens another group of ages range from 660 to 950 ma implying that the qinling group of the tongbai orogen belongs to the south china block scb metamorphic zircons in the granulites have weak or no zoning low th u ratios and ree contents negative eu anomalies and relatively flat hree patterns which indicate that they formed contemporaneously with garnet and plagioclase and thus under granulite facies metamorphic conditions their weighted mean age of 424 4 ma is taken as the best estimated age of the granulite facies metamorphism zircons in a leucosome sample have significant negative eu anomalies and relatively flat hree patterns with a formation age of 428 4 ma indicating partial melting coeval with the granulite facies metamorphism metamorphic zircons in the gneiss show apparently negative eu anomalies and variable th u ratios and hree contents they yield a weighted mean 206 pb 238 u age of 438 4 ma which was suggested to record the timing of the prograde metamorphism zircon grains in a gabbro and a granodiorite yield u pb ages of 432 4 and 424 4 ma respectively which are interpreted as their formation ages and identical to the ages of the granulite facies metamorphism they have positive e hf t values of up to 5 1 showing clear evidence for crustal growth during their formation the appearance of the silurian granulite facies metamorphism and magmatism in the tongbai orogen provides a good paradigm of coeval granulite facies metamorphism magmatism and crustal growth in an arc continent collision orogenic belt

zircon u pb ages and hf o isotopic composition of migmatites from the zanjan takab complex nw iran constraints on partial melting of metasediments abstract we study migmatites and other metamorphic rocks in the zanjan takab region of nw iran and use these results to report the first evidence of oligocene core complex formation in iran four samples of migmatites associated with paragneisses including leucosomes and associated para amphibolite melanosomes were selected for u pb dating and hf o isotopic analysis zircon cores interpreted as originally detrital zircons have variable ages that peak at ca 100 110 ma but their sedimentation age indicated by the youngest 206 pb 238 u ages is ca 35 40 ma new zircons associated with incipient melting occur as overgrowths around zircon cores and or as newly grown grains morphologies and internal structures suggest that rim growth and formation of new zircons were associated with partial melting all four samples contain zircons with rims that yield 206 pb 238 u ages of 28 25 ma indicating that partial melting occurred in late oligocene time 18 o values for zircon rims vary between 8 2 and 12 3 significantly higher than expected for mantle inputs 18 o 6 and consistent with equilibrium with surface materials zircon rims yield ehf t between 2 2 and 12 4 and two stage hf model ages of 448 562 ma indicating that the region is underlain by cadomian caledonian crust according to the hf o isotopic values the main mechanism forming zircon rims was dissolution of pre existing detrital zircons with reprecipitation of new zircon shortly thereafter oligocene ages indicate that partial melting accompanied core complex formation in the zanjan takab region extension melting and core complex formation in south central iran are eocene in age but younger ages of oligocene miocene in nw iran and turkey indicate that extension was distributed throughout the region during cenozoic time

relationships between zircon u pb shrimp ages and leucosome type in migmatites of the halls creek orogen western australia field relations and zircon zoning and shrimp u pb patterns introduction from an outcrop of deformed pelitic migmatites reveal relationships both the amount and isotopic variability of accessory between leucosome type and the amount of zircon growth during minerals incorporated into granitic magmas and their migmatization zircons from the oldest stromatic migmatites show constituent trace elements are a complex function of several pb pb intragrain analyses with 1850 1840 ma source and crystallization processes bea et al 1994 anatectic ages and distinctive cathodoluminescence cl patterns brown 1994 ayres et al 1995 barbey et al 1995 bea although the sample is dominated by inferred pre 1855 ma detrital 1996 it is recognized that initial bulk compositions x zircons subsequent thick contact sheath leucosomes and backand pressure p temperature t conditions of anatexis veins developed at 1845 1830 ma in response to intrusion of are major controls on the chemistry and subsequent mafic dykes into the stromatic migmatites zircons show cl evidence behaviour of granitic magmas e g clemens 1990 for substantial anatectic growth and overgrowths with post 1850 brown 1994 also there is increasing awareness that ma ages and a single population age 20 my younger than the deformation in the source region can also control the early other migmatites zircons from the structurally youngest shearand physical pathways and chemical evolution of magmas e g vein hosted leucosomes show few overgrowths and retain a large sawyer 1991 hand dirks 1992 brown et al 1995 detrital population correlation between shear zones and minimal rushmer 1995 watt et al 1996 oliver barr 1997 anatectic zircon growth is inferred to reflect rapid deformationwhich in turn may have a profound impact on subsequent induced melt extraction from the protolith and or rapid quenching granitic magma evolution for a given bulk source comthe earlier contact sheath and stromatic leucosomes probably had position magma extracted at low melt fractions from a combination of slow melt segregation larger interacting melt an actively deforming partially molten zone may differ volume and longer residence times at high temperatures results substantially in composition from one extracted at high suggest that rapid melt extraction by deformation will increase the melt volumes from rocks undergoing little deformation tendency for preservation of source rock isotopic patterns in some rushmer 1995 the link between frozen migmatites granites and complete granite bodies may not be direct clemens

tectonic evolution of the east qinling mountains china in the palaeozoic a review and new tectonic model east asia is a composite assemblage built up from various plates and the qinling mountains are prominent among the orogenic belts dividing these plates because of its complex and prolonged history interpretation of the geology of the qinling mountains has led to a variety of tectonic models we review and summarize recently published data and previous work and develop a new tectonic model for the eastern part of this mountain range four tectonic zones are recognized from north to south the lesser qinling the north qinling the central qinling and the south qinling zones the lesser and north qinling zones are interpreted as a late proterozoic to early palaeozoic passive continental margin bordering the north china block the central qinling separated from the lesser north qinling zone by a suture zone mainly consists of volcanic arcs and a high grade metamorphic complex the south qinling zone forms part of the northern margin of the yangtze block and has a long latest proterozoic to middle triassic depositional history as revealed by its sedimentary cover available data do not support the hypothesized existence of a major suture divide between the central and the south qinling zones these two zones seem to have collectively formed an early palaeozoic destructive continental margin on the northern edge of the yangtze block with the central qinling as an island arc system and the south qinling as a wide highly attentuated marginal basin late ordovician to early silurian collision between the north china and the yangtze block built up the palaeozoic orogenic belt in the northern part of the qinling and gave rise to the embryonic east asian continent yet the south qinling was transformed to a remnant basin and remained undeformed until the late triassic when the palaeo tethys collision to the south of the yangtze block drove the yangtze block to move northeastwards deformation in a transpressive regime during this late event largely reconstructed the qinling belt and hence further complicated its tectonics

tectonics of the qinling central china tectonostratigraphy geochronology and deformation history abstract the qinling orogen preserves a record of late mid proterozoic to cenozoic tectonism in central china high pressure metamorphism and ophiolite emplacement songshugou ophiolite assembled the yangtze craton including the lower qinling unit into rodinia during the 1 0 ga grenvillian orogeny the lower qinling unit then rifted from the yangtze craton at 0 7 ga subsequent intra oceanic arc formation at 470 490 ma was followed by accretion of the lower qinling unit first to the intra oceanic arc and then to the sino korea craton subduction then imprinted a 400 ma andean type magmatic arc onto all units north of the northern liuling unit oblique subduction created silurian devonian wnw trending sinistral transpressive wrench zones e g lo nan shang dan and late permian early triassic subduction reactivated them in dextral transpression lo nan shang xiang shang dan and subducted the northern edge of the yangtze craton exhumation of the cratonal edge formed the wudang metamorphic core complex during dominantly pure shear crustal extension at 230 235 ma post collisional south directed shortening continued through the early jurassic cretaceous reactivation of the qinling orogen started with nw se sinistral transtension coeval with large scale early cretaceous crustal extension and sinistral transtension in the northern dabie shan it presumably resulted from the combined effects of the siberia mongolia sino korean and lhasa west burma qiangtang indochina collisions and pacific subduction regional dextral wrenching was active within a ne sw extensional regime between 60 and 100 ma an early cretaceous andean type continental magmatic arc with widespread early cretaceous magmatism and back arc extension was overprinted by shortening related to the collision of yangtze indochina block with the west philippines block strike slip and normal faults associated with eocene half graben basins record paleogene nne ssw contraction and wnw ese extension the neogene is characterized by normal faults and nne trending sub horizontal extension pleistocene quaternary nw se extension and ne sw contraction comprises sinistral strike slip faults and is part of the nw se extension imposed across eastern asia by the india asia collision

geologic framework and tectonic evolution of the qinling orogen central china abstract the geologic framework of the qinling orogen was built up through interplay of three blocks the north china block including the north qinling the south qinling and the south china block separated by the shangdan and mianlue sutures the shangdan suture resulted from middle paleozoic collision of the north china block and the south qinling the mianlue suture resulted from late triassic collision of the south qinling and the south china block present upper crust of the qinling is structured dominantly by thrust fold systems the north qinling displays thick skinned deformation with crystalline basement involved whilst the south qinling is characterized by thin skinned thrusts and folds detached above the lower sinian two types of precambrian basement crystalline and transitional are defined according to lithology and metamorphic grade and different in age stratigraphic and sedimentary architecture is characterized by distinct zonation the qinling orogen experienced a prolonged continental divergence and convergence between blocks during the period from late neoproterozoic to early paleozoic times the south qinling was the northern margin of the south china block and the north qinling was the southern margin of the north china block separated by a proto tethyan qinling ocean the north qinling evolved into an active margin when the proto tethyan qinling ocean subducted northward during ordovician time collision of the south and north qinling took place in middle paleozoic along the shangdan suture synchronous with the collision rifting occurred at the southern rim of the south qinling and was followed by the opening of the paleo tethyan qinling ocean during the late paleozoic resulting in the splitting of the south china block from the south qinling collision of the south qinling and the south china block came about in the late triassic along the mianlue suture the late triassic collisional orogeny caused extensive fold and thrust deformation and granitoid intrusions throughout the qinling and led to final amalgamation of the north and south china blocks

structural and magmatic responses to spreading ridge subduction an example from southwest japan subduction of a spreading ridge is expected to leave a geological signature upon the overriding accretionary prism distinct from that of conventional convergent plate boundaries the late oligocene early miocene rocks of the shimanto accretionary prism at cape muroto shikoku island record a tectonomagmatic fabric that is anomalous with respect to typical accretionary prisms these accreted strata have been imprinted with an unusual late stage event that involved 1 regional cusp like flexing of structural trends in the prism 2 near trench magmatism and 3 pervasive faulting this event overlaps in time with the opening of the shikoku back arc basin circa 26 14 ma the shikoku basin spreading ridge trends perpendicular to the strike of the accretionary prism and is now located immediately offshore of cape muroto the regional flexing of structural trends in the prism affects rocks as old as cretaceous although the flexure is most tightly developed in the late oligocene early miocene portion of the prism where it is steeply plunging kinematic analysis of the flexure and the pervasive fault system indicates that the flexure most likely resulted from indentation of the prism by a rigid feature at cape muroto the core of the flexure forms a locus for mafic dikes that are petrochemically equivalent to some mid ocean ridge basalts morb from the shikoku back arc basin preliminary paleomagnetic data suggest that the dikes were intruded during the later stages of flexure development these data combined with plate reconstructions for the area at 15 ma indicate that indentation of the prism most likely resulted from early miocene orthogonal subduction of the topographically elevated shikoku basin spreading ridge in the muroto area morb magmatic rocks in the core of the flexure appear to have emanated from the active spreading ridge during subduction

architectural evolution of the nojima fault and identification of the activated slip layer by kobe earthquake 1 evolutionary history of nojima fault zone is clarified by comprehensive examinations of petrological geophysical and geochemical characterizations on a fault zone in deep drilled core penetrating the nojima fault on the basis of the results we reconstruct a whole depth profile of the architecture of the nojima fault and identify the primal slip layer activated by 1995 kobe earthquake the deepest part 8 to 12 km depth of the fault zone is composed of thin slip layers of pseudotachylite 5 to 10 mm thick each 10 cm in total middle depth 4 to 8 km depth of the fault zone is composed of fault core 6 to 10 m thick surrounded by thick 100 m thick damage zone characterized by zeolite precipitation the shallow part of the fault zone 1 to 4 km depth is composed of distributed narrow shear zones which are characterized by combination of thin 0 5 cm thick each 10 cm in total ultracataclasite layers at the core of shear zones surrounded by thicker 1 to 3 m thick damage zones associated with carbonate precipitation an extremely thin ultracataclasite layer 7 mm thick activated by the 1995 kobe earthquake is clearly identified from numerous past slip layers overprinting one of the shear zones as evidenced by conspicuous geological and geophysical anomalies the nojima fault zone was 10 to 100 times thicker at middle depth than that of shallower and deeper depths the thickening would be explained as a combination of physical and chemical effects as follows 1 thickening of fault core at middle depth would be attributed to normal stress dependence on thickness of the shear zone and 2 an extreme thickening of damage zone in middle depth of the crust would result from the weakening of the fault zone due to super hydrostatic fluid pressure at middle depths the high fluid pressure would result from faster sealing with low temperature carbonate at the shallower fault zone

remnants of a neoproterozoic collisional orogenic belt in the core of the phanerozoic qinling orogenic belt china abstract nw se trending paleotectonics are recognized from the wnw ese trending qinling complex in the core of the qinling belt they consist of trending stratum boundary shear zones deformed linear plutons and ophiolite the field relationships and ages of the deformation magmatism and metamorphism suggest that the paleotectonics formed in neoproterozoic the ages of deformed and undeformed plutons approximately constrain the peak deformation during 958 889 ma both the two trending tectonics constitute an orogen scale tectonic replacement by s2 phanerozoic wnw ese tectonics of s1 nnw sse trending neoproterozoic similar to structural replacement in outcrop the strong contractional deformation high pressure metamorphism and granitic magmatism that evolved from syn collisional s type to post collisional i type and to post collisional a type as well as regional extension at 885 700 ma in qinling display a cycle of a collisional orogeny thus the nw se trending paleotectonics could be remnants of a collisional orogenic belt the belt originally could trend nnw sse if reworking and modification of paleozoic and or mesozoic nne ssw contractional deformation are eliminated accordingly assemblage and breakup of some continental blocks occurred in neoproterozoic in qinling which seems to correspond to assemblage and breakup of rodinia supercontinent in the world this study provides new insight into the evolution of the qinling orogenic belt and the reconstruction of proterozoic supercontinent of china and also presents a case study of recognition of an old orogen from a young orogen

eclogite origin and timings in the north qinling terrane and their bearing on the amalgamation of the south and north china blocks the amalgamation of south scb and north china blocks ncb along the qinling dabie orogenic belt involved several stages of high pressure hp ultra high pressure uhp metamorphism the new discovery of uhp metamorphic rocks in the north qinling nq terrane can provide valuable information on this process however no precise age for the uhp metamorphism in the nq terrane has been documented yet and thus hinders deciphering of the evolution of the whole qinling dabie sulu orogenic belt this article reports an integrated study of u pb age trace element mineral inclusion and hf isotope composition of zircon from an eclogite a quartz vein and a schist in the nq terrane the zircon cores in the eclogite are characterized by oscillatory zoning or weak zoning high th u and 176lu 177hf ratios pronounced eu anomalies and steep heavy rare earth element hree patterns the zircon cores yield an age of 796 13 ma which is taken as the protolith formation age of the eclogite and implies that the nq terrane may belong to the scb before it collided with the ncb the hf t values vary from 11 3 to 3 2 and corresponding two stage hf model ages are 2402 to 1495 ma suggesting the protolith was derived from an enriched mantle in contrast the metamorphic zircon rims show no zoning or weak zoning very low th u and 176lu 177hf ratios insignificant eu anomalies and flat hree patterns they contain inclusions of garnet omphacite and phengite suggesting that the metamorphic zircon formed under eclogite facies metamorphic conditions and their weighted mean 206pb 238u age of 485 9 3 8 ma was interpreted to date the timing of the eclogite facies metamorphism zircon in the quartz vein is characterized by perfect euhedral habit some oscillatory zoning low th u ratios and variable hree contents it yields a weighted mean u pb age of 480 5 2 5 ma which registers the age of fluid activity during exhumation zircon in the schist is mostly detrital and u pb age peaks at c 1950 to 1850 1800 to 1600 1560 to 1460 and 1400 to 1260 ma with an oldest grain of 2517 ma also suggesting that the nq terrane may have an affinity to the scb accordingly the amalgamation between the scb and the ncb is a multistage process that spans c 300 myr which includes the formation of the erlangping intra oceanic arc zone onto the ncb before c 490 ma the c 485 ma crustal subduction and uhp metamorphism of the nq terrane the c 430 ma arc continent collision and granulite facies metamorphism the 420 to 400 ma extension and rifting in relation to the opening of the palaeo tethyan ocean the c 310 ma hp eclogite facies metamorphism of oceanic crust and associated continental basement and the final 250 to 220 ma continental subduction and hp uhp metamorphism

the source of granitic gneisses and migmatites in the antarctic peninsula a combined u pb shrimp and laser ablation hf isotope study of complex zircons zircons gneisses and migmatites collected from the antarctic peninsula have different core rim hafnium isotope ratio relationships depending on whether evidence for zircon dissolution is present or absent two samples contain inherited zircon that is partially dissolved in these samples the 176hf 177hf rations of the inherited zircon and new magmatic zircon rims are on average indistinguishable and consistent with in situ melting in such cases the hafnium isotopic composition of the melt was probably strongly influenced by the dissolved zircon component at the source variation in 176hf 177hf within the magmatic zircon rims from grain to grain suggests that hf isotopes were only partially homogenized during melt migration alternatively zircon growth may have taken place within small volumes of partial melt other samples do not preserve textural evidence for zircon dissolution during melt generation in these samples the 176hf 177hf values of the inherited zircon and new magmatic zircon rims are different the zircon rims apparently suggest a source of less evolved hafnium than that contained within the inherited zircon whether this relates to a separate juvenile source or alternatively is derived from minerals other than zircon at the source cannot be resolved inherited zircon irrespective of age has been strongly influenced by the reworking of a juvenile late mesoproterozoic source suggesting that such crust underlies the antarctic peninsula our results therefore suggest that hf isotope analyses provide great potential for future studies investigating the source and processes involved in the generation of crustal melts

formation of paleoproterozoic eclogitic mantle slave province canada insights from in situ hf and u pb isotopic analyses of mantle zircons abstract in situ hf isotope analyses and u pb dates were obtained by laser ablation mc icp ms for a zircon bearing mantle eclogite xenolith from the diamondiferous jericho kimberlite located within the archean slave province nunavut canada the u pb zircon results yield a wide range of ages 2 0 to 0 8 ga indicating a complex geological history of importance one zircon yields a u pb upper intercept date of 1989 67 ma providing a new minimum age constraint for zircon crystallization and eclogite formation in contrast hf isotope systematics for the same zircons display an intriguing uniformity and corresponding hf depleted mantle model ages range between 2 1 0 1 and 2 3 0 1 ga the youngest hf model age is within error to the oldest u pb date the jericho eclogites have previously been interpreted as representing remnants of metamorphosed oceanic crust and their formation related to paleoproterozoic subduction regimes along the western margin of the archean slave craton during the wopmay orogeny hf isotope compositions and u pb results for the jericho zircons reported here are in good agreement with a paleoproterozoic subduction model suggesting that generation of oceanic crust and eclogite formation occurred between 2 0 and 2 1 ga the slightly older hf depleted mantle model ages 2 1 to 2 3 ga may be reconciled with this model by invoking mixing between crustal derived hf from sediments and more radiogenic hf associated with the oceanic crust during the 2 ga subduction event this results in intermediate hf isotope compositions for the jericho zircons that yield fictitiously older hf model ages

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

accommodation of transpressional strain in the arabia eurasia collision zone new constraints from u th he thermochronology in the alborz mountains north iran the alborz range of n iran provides key information on the spatiotemporal evolution and characteristics of the arabia eurasia continental collision zone the southwestern alborz range constitutes a transpressional duplex which accommodates oblique shortening between central iran and the south caspian basin the duplex comprises nw striking frontal ramps that are kinematically linked to inherited e w striking right stepping lateral to obliquely oriented ramps new zircon and apatite u th he data provide a high resolution framework to unravel the evolution of collisional tectonics in this region our data record two pulses of fast cooling associated with sw directed thrusting across the frontal ramps at 18 14 and 9 5 7 5 ma resulting in the tectonic repetition of a fossil zircon partial retention zone and a cooling pattern with a half u shaped geometry uniform cooling ages of 7 6 ma along the southernmost e w striking oblique ramp and across its associated nw striking frontal ramps suggests that the ramp was reactivated as a master throughgoing n dipping thrust we interpret this major change in fault kinematics and deformation style to be related to a change in the shortening direction from ne to n nne the reduction in the obliquity of thrusting may indicate the termination of strike slip faulting and possibly thrusting across the iranian plateau which could have been triggered by an increase in elevation furthermore we suggest that 7 6 m y old s directed thrusting predated inception of the westward motion of the south caspian basin

styles of post subduction collisional orogeny influence of convergence velocity crustal rheology and radiogenic heat production abstract a number of models of continental collision already exist but the role of foregoing oceanic plate subduction or variable coupling between plates still remains to be explored in addition heat generation by radioactive decay may be quite variable for example our geochemical data from low to high grade metamorphic rocks of the lesser and higher himalayan sequences reveal measured heat production values on the order 4 5 w m 3 which is considerably higher than normal inferred upper crustal values 3 therefore we performed 2d numerical experiments of oceanic continental subduction followed by continental collision for varying continental lower crustal strength convergence velocity and radiogenic heating rate oceanic subduction produces thinning and upwarping of the overriding lithospheric wedge such as is observed in andean type collisional settings cascadia and central andes after collision two main orogenic styles develop depending on the values of key parameters either a slow convergence rates weak shear heating and consequently strong rheological coupling between plates and or a weak lower crust generate a two sided thick and narrow symmetrical collisional zone or b high convergence rates strong shear heating and therefore weak rheological coupling between plates and a strong lower crust produce a one sided thin and broad asymmetrical collisional zone in this case a vertical lithospheric body divides two upper crustal wedges that develop distinct deformation patterns in both cases enhanced radiogenic heating favors the formation of midcrustal partially melted channels that propagate toward the surface and the pro foreland partial melting triggers lateral extrusion of metamorphic rocks from different depths 15 23 kbar 700 c on top of coherent lp lt units the geometry of metamorphic isograds inverted shapes of p t paths at different grades and exhumation rates are similar to those observed in parts of the himalayan chain indicating that the models capture the essence of real systems the model results demonstrate that internal heat production rate is a crucial parameter in determining the timing of slab breakoff polarity reversal and exhumation of the inverted metamorphic sequence with respect to the timing of partial melting late stages of some numerical experiments showed interesting geodynamical evolution of the collisional zone with the appearance of polarity reversal roll over of both plates and roll back of the subducting slab irrespective of these details hundreds of km of continental lithosphere are recycled into the mantle in all models supporting the idea that continental crustal consumption is indeed a common and considerable process during collision this affects inferences about extents of convergence and or shortening during orogenesis moreover crustal recycling into the deep mantle during collisional orogeny needs to be considered in models of crustal growth and evolution

metamorphic history and tectonic evolution of the qinling complex eastern qinling mountains china abstract the metamorphic history and tectonic evolution of the qinling complex is divided into formation and modification stages during the proterozoic formation stage three deformational sequences are recognized andalusite muscovite sillimanite muscovite and sillimanite k feldspar zones of amphibolite facies regional metamorphism are earlier than or synchronous with the first or second phase of folding ductile shear zones were formed and caledonian granites were emplaced during the modification stage the granites superimposed contact aureoles garnet k feldspar zone on the regional metamorphic fabric metamorphic reactions p t conditions of metamorphism and p t t paths were estimated by analysis of mineral textures and standard thermobarometric techniques the p t t path of the proterozoic tectonometamorphic cycle shows prominent clockwise decompression the p t t path of the caledonian tectonometamorphic cycle is characterized by an early rise of pressure and temperature followed by isothermal decompression rapid uplift and finally with isobaric cooling the p t t paths of the two tectonometamorphic cycles reflect two major stages of collision and uplift in the evolution of the qinling orogenic belt during the proterozoic and caledonian hercynian periods respectively

early paleozoic island arc accretion to the north china craton and the shang dan fault zone a major paleoplate boundary in eastern asia the early paleozoic tectonic evolution of part of the qinling dabie shandong orogenic belt is reconstructed on the basis of the results of field mapping geochemical and geochronological studies the qinling orogenic belt separates the archean proterozoic north china craton ncc from the south china craton scc also yangtze craton the study area is located about 90 km southwest of the city of xi an three lithologically distinctive zones separated from each other by fault zones can be distinguished passive continental margin deposits of the ncc zone 1 have end t values between 6 16 and 12 54 and tdm ages between 1 29 and 2 20 ga they are separated from an early paleozoic island arc sequence zone 2 by the sinistral chenche fault mafic volcanic lithologies in the southern part of zone 1 are associated with the development of a passive continental margin in late proterozoic time similiar lithologies of zone 2 have geochemical characteristics of an oceanic island arc accretion was accompanied by a strong deformation event between early ordovician and late silurian time magmatic intrusions in zone 2 and the southern part of zone 1 are associated with a short lived magmatic arc that developed above a north dipping subduction zone on the modified continental margin of the ncc in late silurian to early devonian time the end t values of felsic intrusions from the northern part of zone 2 decrease in a northern direction from 1 78 to 1 05 mafic to intermediate intrusions in the southern part of zone 2 have positive end t values from 1 28 to 2 04 as well as low 87sr 86sr i values between 0 705 and 0 706 indicating an increasing mantle component toward the south low grade metamorphic pelites are separated from the island arc sequence of zone 2 by a 2 km wide shang dan shear zone isotopic studies show strong similarities between a granite sample from zone 3 and samples from the scc 87sr 86sr i values increase from 0 709 to 0 713 whereas end t values decrease from 1 90 to 5 34 in a southern direction the shear zone that separates zone 2 from 3 is of fundamental importance and marks the boundary between the ncc and the scc the results presented in this paper document a previously unknown early paleozoic tectonic evolution of the qinling orogenic belt and delineate one of the fundamental paleoplate boundaries in eastern asia

compaction creep of quartz sand at 400 600 c experimental evidence for dissolution controlled pressure solution intergranular pressure solution ips is an important compaction and deformation mechanism in quartzose rocks but the kinetics and rate controlling process remain unclear the aim of the present study is to test microphysical models for compaction creep by ips against isostatic hot pressing experiments performed on quartz sand under conditions expected to favor pressure solution confining pressure 300 mpa pore water pressure 150 250 mpa temperature 400 600 c microstructural observations revealed widespread intergranular indentation features and confirmed that intergranular pressure solution was indeed the dominant deformation mechanism under the chosen conditions for porosities down to 15 the mechanical data agree satisfactorily with a microphysical model incorporating a previously determined kinetic law for dissolution of loose granular quartz suggesting that the rate limiting mechanism of ips was dissolution the model also predicts ips rates within one order of magnitude of those measured in previous experiments at 150 350 c and thus seem robust enough to model sandstone compaction in nature such applications may not be straightforward however as the present evidence for dissolution control implies that the compositional variability of natural pore fluids may strongly influence ips rates in sandstones

high paleogeothermal gradient with ridge subduction beneath the cretaceous shimanto accretionary prism southwest japan paleotemperature in the jurassic chichibu and the cretaceous shimanto accretionary complexes southwest japan increases southward generally without regard to the early phase geologic structure there is an exceptional peak in the okitsu melange and the thermal structure probably formed after accretion of the complex the estimated geothermal gradient is more than 90 c km in the okitsu melange and more than 50 c km elsewhere in the complex the ophiolitic okitsu melange formed concurrently with eocene subduction of the kula pacific ridge under the cretaceous accretionary prism the previous thermal structure was overprinted by this event

mass transfer and pressure solution in deformed shale of accretionary complex examples from the shimanto belt southwestern japan abstract this study investigates volume changes in the accretionary complex in southeastern shikoku japan using mesoscopic and microscopic observations and chemical analyses of shale three types of deformation in shale were recognized 1 coherent type compacted 2 type i melange less deformed and 3 type ii melange sheared pressure solution seams pss are common and reflect large volume reductions for coherent type 13 50 and type ii melange 17 54 positive correlations exist between pss density and concentration of the immobile chemical component tio2 and between pss density and paleotemperature for type ii melange and coherent type under constant temperature conditions the pss density is higher for type ii melange than for coherent type indicating more efficient generation of pss for type ii melange based on the analysis of rheological equations we conclude that generation of pss may be controlled by differences in differential stress and or the duration of deformation the positive correlation between pss density and tio2 concentration indicates that volume change in accretionary complexes can be described by a simple equation using pss density

palaeozoic polymetamorphism in the north qinling orogenic belt central china insights from petrology and in situ titanite and zircon u pb geochronology abstract the qinling orogenic belt experienced multiple phases of orogenesis during the palaeozoic unraveling the timing and p t conditions of these events is the key to understanding the convergence processes between the south china and the north china blocks the songshugou complex located in the southern part of the north qinling orogenic belt has registered multistage metamorphism in palaeozoic and thus potentially provides insights into the tectonic evolution of the qinling orogenic belt in this study three metabasic rocks a garnet pyroxenite a garnet amphibolite and a gneissic amphibolite from the songshugou complex were selected for petrological study and zircon and titanite u pb dating our results show that the metabasic rocks experienced three metamorphic events during the palaeozoic the first metamorphic event m1 is characterized by high pressure conditions two zircon grains in equilibrium with garnet and in absence of plagioclase were recognized from the garnet pyroxenite sample they yielded ti in zircon temperatures of 660 851 c at 12 0 kbar and a weighted mean age of 498 15 ma providing the constraints on the temperature and timing of prograde or peak metamorphism m1 1 zircons that are inequilibrium with garnet from the garnet pyroxenite and the garnet amphibolite gave u pb ages of 494 9 ma and 484 4 ma and ti in zircon temperatures of 793 33 c and 738 18 c respectively thus these zircons were formed on the retrograde amphibolite facies conditions at 8 0 kbar m1 2 titanite inclusions were found in actinolite cores of zoned amphibole from the garnet amphibolite they yielded a u pb age of 470 ma and zr in titanite temperature of 676 23 c at pressure of 7 0 kbar suggesting that the amphibolite facies retrogression perhaps persisted to 470 ma weakly zoned zircons from the garnet amphibolite and inclusion free titanites from the garnet pyroxenite gave consistent u pb ages of 418 5 ma and 423 10 ma and ti in zircon temperature of 742 26 c and zr in titanite temperature of 764 18 c at 7 0 kbar respectively it is suggested that a heating event m2 is registered by a subsequent phase of amphibolite facies metamorphism the ilmenite bearing titanite crystals from the garnet pyroxenite yielded a u pb age of 352 4 ma recording a late thermal event m3 on the basis of combined petrological and geochronological results we propose a revised tectonic model for the north qinling orogeny in palaeozoic the high pressure granulites were formed by the northward subduction of the shangdan oceanic slab and the arc continent collision at ca 500 ma their exhumation happened at ca 494 484 ma as a result of slab breakoff subsequent amphibolite facies metamorphism dated at ca 440 420 ma are coeval with the widespread magmatism in the north qinling terrane which are likely caused by the reinitiation northward subducted of shangdan oceanic slab at ca 350 ma the north qinling terrane was likely affected by another thermal overprinting event

timing of uhp metamorphism in the hong an area western dabie mountains china evidence from zircon u pb age trace element and hf isotope composition the hong an area western dabie mountains is the westernmost terrane in the qinling dabie sulu orogen that preserves uhp eclogites the ages of the uhp metamorphism have not been well constrained and thus hinder our understanding of the tectonic evolution of this area la icpms u pb age trace element and hf isotope compositions of zircons of a granitic gneiss and an eclogite from the xinxian uhp unit in the hong an area were analyzed to constrain the age of the uhp metamorphism most zircons are unzoned or show sector zoning they have low trace element concentrations without significant negative eu anomalies these metamorphic zircons can be further subdivided into two groups according to their u pb ages and trace element and lu hf isotope compositions one group with an average age of 239 2 ma show relatively high and variable hree contents 527 lun 14 and 176lu 177hf ratios 0 00008 0 000931 indicating their growth prior to a great deal of garnet growth in the late stage of continental subduction the other group yields an average age of 227 2 ma and shows consistent low hree contents and 176lu 177hf ratios suggesting their growth with concurrent garnet crystallization and or recrystallization these two groups of age are taken as recording the time of prograde hp to uhp and retrograde uhp hp stages respectively a few cores have high th u ratios high trace element contents and a clear negative eu anomaly these features support a magmatic origin of these zircon cores the upper intercept ages of 771 86 and 752 70 ma for the granitic gneiss and eclogite respectively indicate that their protoliths probably formed as a bimodal suite in rifting zones in the northern margin of the yangtze block young hf model ages tdm1 of magmatic cores indicate juvenile mantle derived materials were involved in their protolith formation