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

triassic paragonite and garnet bearing epidote amphibolite from the hida mountains japan paragonite and garnet bearing high grade epidote amphibolite pgea in the ise area of the hida mountains japan is characterized by the high pressure hp epidote amphibolite facies parageneses m1 garnet hornblende clinozoisite paragonite quartz rutile paragonite and garnet of the peak m1 stage are locally replaced by retrograde albite oligoclase and chlorite m2 respectively phase equilibria constrain peak metamorphic conditions of p 1 1 1 4 gpa and t 530 570 c and a decompressional p t path for this rock mineral parageneses of prograde epidote amphibolite facies are comparable to some hp rocks from the hongan region of western dabie but differ from other hp mafic schists with cooling ages of c 330 ma in the hida mountains new paragonite k ar dating for the pgeayields a triassic cooling event at 210 ma that is coeval with regional cooling and exhumation of the sulu dabie qinling sdq belt both petrological and geochronological data of the triassic hp epidote amphibolite in hida mountains support our earlier hypothesis that the sdq belt extends across the korean peninsula to sw japan

tectonic evolution of the tancheng lujiang tan lu fault via middle triassic to early cenozoic paleomagnetic data the north striking tancheng lujiang tan lu fault is a conspicuous and controversial feature of the eastern asian landscape near the southeast extremity of the fault in anhui province we collected paleomagnetic samples at 17 middle triassic t2 and 10 upper cretaceous k2 to lower cenozoic e1 sites t2 remanent magnetizations are interpreted as primary in two of three areas the three areas are rotated 37 to 137 counterclockwise with respect to the south china block scb reference direction k2 e1 remanent magnetization directions pass regional fold and reversals tests and are not rotated with respect to surrounding areas counterclockwise rotation of t2 strata therefore ended before k2 and is attributed to left lateral shear acting along tan lu during the north china block ncb scb collision in shandong province 700 km north of the anhui sites four areas containing 33 upper jurassic j3 and cretaceous sites have negligible declination differences except for one which has dispersed directions the fold test is inconclusive for this latter area and positive for the other three regional concordance of the j3 e1 paleomagnetic data including paleolatitudes together with observed deformation patterns suggest that an extensional regime prevailed in the late cretaceous and cenozoic euler pole positions that constrain the north south china collision and account for tan lu motion suggest at least 500 km of sinistral shear took place along the fault and either 1 subduction and related ultrahigh pressure uhp metamorphism occurred near the present location of the qinling dabieshan and sulu uhp belts while tan lu acted as a transform fault that connected the two subduction zones or 2 tan lu and sulu were parts of the same transform fault system and no uhp rocks formed in situ at sulu in either case uhp rocks originally exhumed near dabieshan could have been transported by plate capture toward sulu along tan lu after north and south china impacted near dabieshan the tan lu fault grew within the scb as the dabieshan corner indented the scb causing folds in scb cover rocks to conform to the ncb margin late cretaceous to cenozoic reactivation of tan lu with both right lateral strike slip and normal fault motion occurred as the scb extruded east relative to the ncb under the influence of the india asia collision

refolded syn ultrahigh pressure thrust sheets in the south dabie complex china field evidence and tectonic implications the exhumation mechanisms of ultrahigh pressure uhp rocks have been a controversial issue ever since the discoveries of coesite and diamond in metamorphic rocks from several orogenic belts the lack of consensus principally reflects the limited number of combined structural petrologic and geochronologic studies that have been carried out in great detail within these terrains in this study we derive a complex structural history for the uhp south dabie terrain based on detailed 1 10000 mapping in the eastern dabie shan and use this to place new constraints on the exhumation process structural analysis demonstrates that syn uhp south vergent thrusting d 1 was followed by north vergent folding under amphibolite facies conditions d 2 and a third generation of folding at shallower level d 3 this supports a two stage exhumation model in which initially rapid exhumation from mantle depth to lower crustal levels is followed by slower exhumation from the lower crust level to the earth surface the remarkable structural concordance between lithological units containing uhp mineral assemblages and the enclosing gray gneiss units implies that the apparent difference in peak pressures recorded by different lithologies is not a consequence of tectonic juxtaposition but a result of differential reequilibration during decompression

exhumation of the dabie shan ultra high pressure rocks and accumulation of the songpan ganzi flysch sequence central china the presence of coesite and diamond bearing ultra high pressure uhp metamorphic rocks in the dabie and sulu regions central china suggests that a 100 km thick crustal section 4 x 106 km6 in volume has been denuded this volume is comparable to that represented by the 10 15 km thick middle to upper triassic flysch rocks in the songpan ganzi region regional geology and radiometric dates are compatible with an interpretation that the majority of these sedimentary rocks were derived from denudation of the orogenic belt between north and south china blocks following their latest paleozoic to triassic collision this correlation implies a high denudation rate of 4 mm yr that may have been in part promoted by tropical precipitation and extreme topographic relief for as long as 25 m y

geochronology and isotopic character of ultrahigh pressure metamorphism with implications for collision of the sino korean and yangtze cratons central china the collision zone between the sino korean and yangtze cratons in central china is marked by subduction related metamorphic rocks composed of crustal protoliths these range in metamorphic grade from ultrahigh pressure eclogite to low greenschist facies the paragenesis of the ultrahigh pressure rocks coesite and diamond bearing indicates subduction to depths greater than 120 km as a result of continental collision despite extreme metamorphic conditions zircons have not undergone pb loss but rather illustrate two periods of growth crystallization between 700 and 800 ma and ultrahigh pressure metamorphism at 218 2 5 ma previous studies have predicted a time transgressive collision but age data presented in this study indicate that the collision was approximately coeval along the length of the suture u pb zircon ages of these ultrahigh pressure metamorphic rocks are 218 4 2 5 ma and 217 1 8 7 ma for the dabie mountains and shandong peninsula respectively the similar ages imply that the tan lu fault which offsets the ultrahigh pressure metamorphic rocks is a secondary feature not related to collision or subduction whole rock isotopic data show that both rb sr and u pb were open systems during metamorphism but that sm nd probably remained closed nd initial isotopes display crustal signatures indicating that the crust subducted as a coherent slab rather than becoming intercalated with mantle material at depth sm nd data in conjunction with u pb zircon dating indicate that the ultrahigh pressure rocks are not typical yangtze craton basement 2 9 ga but that they originally crystallized in a rift environment between 700 and 800 ma discrepancies between 40ar 39ar plateaus sm nd isochrons and u pb zircon ages render calculation of a p t t path unrealistic

crustal structure beneath the dabie orogenic belt from ambient noise tomography abstract the qinling dabie sulu orogenic belt in east central china is the largest high and ultrahigh pressure hp and uhp metamorphic zone in the world the dabie mountains are the central segment of this orogenic belt between the north china and yangtze cratons this work studies the nature of the crustal structure beneath the dabie orogenic belt to better understand the orogeny to do that we apply ambient noise tomography to the dabie orogenic belt using ambient noise data from 40 stations of the china national seismic network cnsn between january 2008 and december 2009 we retrieve high signal noise ratio snr rayleigh waves by cross correlating ambient noise data between most of the station pairs and then extract phase velocity dispersion measurements from those cross correlations using a spectral method taking those dispersion measurements we obtain high resolution phase velocity maps at 8 35 second periods by inverting rayleigh wave phase velocity maps we construct a high resolution 3d shear velocity model of the crust in the dabie orogenic belt the resulting 3d model reveals interesting crustal features related to the orogeny high shear wave velocities are imaged beneath the hp uhp metaphoric zones at depths shallower than 9 km suggesting that hp uhp metaphoric rocks are primarily concentrated in the upper crust underlying the high velocity hp uhp metamorphic zones low shear velocities are observed in the middle crust probably representing ductile shear zones and or brittle fracture zones developed during the exhumation of the hp uhp metamorphic rocks strong high velocities are present beneath the northern dabie complex unit in the middle crust possibly related to cooling and crystallization of intrusive igneous rocks in the middle crust resulting from the post collisional lithosphere delamination and subsequent magmatism a north dipping moho is revealed in the eastern dabie with the deepest moho appearing beneath the northern dabie complex unit consistent with the model of triassic northward subduction of the yangtze craton beneath the north china craton

oxygen and hydrogen isotope geochemistry of gneisses associated with ultrahigh pressure eclogites at shuanghe in the dabie mountains the oxygen and hydrogen isotope compositions of minerals and whole rock were determined for two types of gneiss biotite gneiss and granitic gneiss associated with ultrahigh pressure uhp eclogites in the shuanghe district of the eastern dabie mountains there are significant differences in 18o between the two gneisses the uhp biotite gneiss varying from 4 3 to 10 6 similar to the associated eclogites whereas the non uhp granitic gneiss ranges only from 3 8 to 1 2 the d values are similar in the two gneisses with 37 to 64 for epidote zoisite 92 to 83 for amphibole and 63 to 109 for biotite phengite hydrogen isotope disequilibrium among the coexisting hydroxyl bearing minerals is ascribed to retrograde exchange subsequent to amphibolite facies metamorphism oxygen isotopic equilibrium has been preserved among various minerals in both gneisses regardless of the large variation in rock 18o oxygen isotopic geothermometers yield different but regular temperatures corresponding to the closure temperatures of oxygen diffusion in the minerals the metamorphic temperatures of both eclogite facies and amphibolite facies have been recovered in mineral pairs from the biotite gneiss the isotopic temperatures for the granitic gneiss are mostly in accordance with amphibolite facies metamorphism however high temperatures of 550 to 650 c are obtained from those minerals resistant to retrograde oxygen isotope exchange implying that the granitic gneiss may have experienced higher temperature metamorphism than expected from petrologic thermometers the 18o depletion of both gneisses is interpreted to result from meteoric hydrothermal exchange before during plate subduction therefore the measured 18o values of the gneisses reflect the oxygen isotope compositions of their protoliths prior to the uhp metamorphism it is inferred that the uhp unit is in foreign contact with the non uhp unit like a tectonic melange but both of them experienced the two common stages of geodynamic evolution 1 18o depletion prior to the uhp metamorphism 2 uplifting since the amphibolite facies metamorphism

constraints on the jurassic swing of the apparent polar wander path for the north china block summary in order to better constrain the mesozoic apparent polar wander path for the north china block late triassic and early jurassic sandstones in the ordos basin and early jurassic sandstones in the datong area were collected at 37 sites thermal demagnetization isolated a high temperature magnetization component with unblocking temperatures of 500 580 c from five sites of the early jurassic yongdingzhuang formation in the datong area palaeomagnetic directions from the five sites yielded a positive fold test at the 95 per cent confidence level these directions combined together with previously reported ones give a new early jurassic mean direction d 358 0 i 50 6 95 4 4 and corresponding palaeomagnetic pole 81 5 n 302 5 e a95 5 5 the palaeomagnetic pole is regarded as representative of the north china block because this pole is derived from two localities in the undisputed part of the north china block comparisons of the early jurassic pole for the north china block with coeval ones for the south china block identify significant differences between the poles from the two regions at early jurassic times the north china block was at the peak of rotational motion associated with the collision of the north and south china blocks the north china block underwent a counterclockwise rotation of 12 32 with respect to the south china block since the early jurassic at least 17 per cent of the total rotational motion remaining at that time

thermochronological constraints on two stage extrusion of hp uhp terranes in the dabie sulu orogen east central china along the qinling dabie sulu orogenic belt in china crops out the world s largest terrane composed of ultrahigh pressure uhp metamorphic rocks differences in the timing and mechanisms of oceanic and continental subductions are assumed to be responsible for different ages of high pressure hp and uhp slices in different parts of the belt the western part of the dabie orogen western dabie terrane holds a key to understanding of the transition from oceanic to continental subduction this paper reports geochronological results to test a two stage tectonic model for the exhumation of hp uhp rocks in western dabie this model involves two different stages and types of extrusion for exhumation of the hp uhp rocks in east central china mica ar ar ages ranging from 241 to 231 ma indicate a general middle triassic cooling probably driven by early upward extrusion during the collision between the north and south china blocks late triassic early jurassic cooling was associated with later eastward extrusion ranging from 200 to 184 ma the second event is recorded also in mica in the region that was not affected by later deformation and magmatism the lateral movement along lithosphere scale faults resulted in the eastward extrusion of the hp uhp metamorphic terrane which was followed in the late triassic early jurassic time by a major compressive event these two extrusion events are correlative with the two stages of triassic exhumation of the western dabie hp uhp rocks respectively wintin the framework of the qinling dabie sulu orogenic belt it is suggested for western dabie that the subduction exhumation of blueschist facies unit is related to the mianlue suture whereas the subduction exhumation of hp uhp eclogite facies units is related to the shangdan suture

high pressure low temperature metamorphism in northern hubei province central china the qinling dabie accretionary fold belt in east central china represents the e w trending suture zone between the sino korean and yangtze cratons a portion of the accretionary complex exposed in northern hubei province contains a high pressure low temperature metamorphic sequence progressively metamorphosed from the blueschist through greenschist to epidote amphibolite eclogite facies the hongan metamorphic belt can be divided into three metamorphic zones based on progressive changes in mineral assemblages zone in the south is characterized by transitional blueschist greenschist facies zone is characterized by greenschist facies zone in the northern most portion of the belt is characterized by eclogite and epidote amphibolite facies sequences changes in amphibole compositions from south to north as well as the appearance of increasingly higher pressure mineral assemblages toward the north document differences in metamorphic p t conditions during formation of this belt preliminary p t estimates for zone i metamorphism are 5 7 kbar 350 450 estimates for zone iii eclogites are 10 22 kbar 500 50 the petrographic chemical and structural characteristics of this metamorphic belt indicate its evolutionin a northward dipping subduction zone and subsequent uplift prior to and during the final collisionbetween the sino korean and yangtze cratons

u pb zircon ages constrain the architecture of the ultrahigh pressure qinling dabie orogen china new shrimp and tims zircon ages 40ar 39ar ages and eclogite locations contribute significantly to our understanding of the ultrahigh pressure dabie shan 1 the geographic extent of the yangtze craton that was subducted to ultrahigh pressure extends to the northern edge of the dabie shan 2 the northern half of the dabie shan is a magmatic complex intruded over a 10 myr interval between 137 and 126 ma that accommodated 100 n s stretching of the pre existing collisional architecture 3 granitic orthogneisses and enclosing ultrahigh pressure paragneisses have indistinguishable zircon populations the population of triassic zircon ages ranges from 219 to 245 ma leading us to question the prevailing assumption that 219 ma zircons formed at ultrahigh pressure and to propose instead that they reflect late retrogression at crustal pressures following the bulk of exhumation

preliminary phanerozoic polar wander paths for the north and south china blocks polar wander paths for the north and south china blocks suggest that 1 both were parts of gondwana in the palaeozoic 2 the north china block accreted to siberia in the late permian and 3 the south china block accreted to the north china block in the middle triassic to the early jurassic comparison of the polar wander path for the south china block with that for northern eurasia suggests that relative motion of over 4 000 km has occurred between them

triassic high pressure metamorphism in the huwan shear zone tracking the initial subduction of continental crust in the whole dabie orogen abstract la mc icpms trace element u pb and hf isotope analyses were performed on zircons from continental derived retrograde high pressure metamorphic rocks which were attested by the occurrence of hp mineral inclusions in metamorphic zircons from the huwan shear zone in the western dabie orogen zircons in a schist sample are mostly detrital and gave u pb ages from 1212 to 2312 ma suggesting that the detritus were derived from multiple sources or suffered partial pb loss during subsequent metamorphism magmatic zircons in an amphibolite sample yielded an upper intercept u pb age of 1806 63 ma which was interpreted as its protolith formation age the zircons in the amphibolite show a weighted e hf t value of 16 3 and a weighted hf model age of 3455 ma suggesting that the protolith was derived from reworking of ancient continental materials metamorphic zircons in a quartz rich leucosome sample and the amphibolite sample gave weighted mean 206 pb 238 u ages of 241 1 and 243 4 ma respectively these metamorphic zircons contain mineral inclusions of garnet and omphacite and are characterized by relatively flat hree patterns with slight negative eu anomalies this indicates that they formed in the presence of garnet but the absence of plagioclase and thus under eclogite facies conditions the u pb age of ca 243 ma is consistent with eclogite facies metamorphic ages of the juvenile oceanic crust in the western dabie orogen suggesting that the initial continental subduction for high pressure metamorphism of the whole dabie orogen can be traced back to ca 243 ma in addition the western dabie hp uhp orogenic belt might have suffered multi slice subduction and exhumation during the triassic

ages of ultrahigh pressure metamorphism and protolith orthogneisses from the eastern dabie shan u pb zircon geochronology the dabie shan contains two rare geologic features one the occurrence of ultra high pressure metamorphic rocks peak p 2 7 gpa uhp in the east central part of this belt and two some of these uhp rocks are characterized by anomalously 18o depleted oxygen isotopic compositions geochronological data are presented that constrain the age of the uhp metamorphism the ages of protoliths and potentially the age of hydrothermal alteration believed to be responsible for the anomalous oxygen isotopic compositions the age of uhp metamorphism in the eastern dabie shan is dated at 218 5 1 7 ma using u pb in zircon separated from host gneisses this age is identical within uncertainty with a less precise age using u pb in zircon separated from uhp eclogite 225 5 3 6 ma and with previously published ages from uhp eclogites of the dabie shan map relationships strongly support an in situ as opposed to exotic orgin of the uhp eclogites in this region the identical ages of uhp metamorphism in the gneisses and eclogites provides additional support for this interpretation the age of the protolith gneisses is dated as 772 5 9 5 ma based on the upper intercept of the zircons which accords well with other dates of granitic gneisses in the qinling belt the association of mafic and felsic magmatism is interpreted to reflect rifting along the northern margin of the yangtze block at this time these upper intercept ages may date the hydrothermal circulation of 18o depleted meteoric water which gave rise to the anomalously 18o depleted uhp protoliths of the dabie shan and sulu region the coincidence of these protolith ages with the early sinian glaciation of south china is consonant with the suggestion that the highly anomalous oxygen isotopic compositions in this region date from this time the upper intercept age of the eclogite is imprecisely determined at 447 82 79 ma but is younger than the host gneisses and is not characterized by anomalous oxygen isotopic compositions

the origin and pre cenozoic evolution of the tibetan plateau different hypotheses have been proposed for the origin and pre cenozoic evolution of the tibetan plateau as a result of several collision events between a series of gondwana derived terranes e g qiangtang lhasa and india and asian continent since the early paleozoic this paper reviews and reevaluates these hypotheses in light of new data from tibet including 1 the distribution of major tectonic boundaries and suture zones 2 basement rocks and their sedimentary covers 3 magmatic suites and 4 detrital zircon constraints from paleozoic metasedimentary rocks the western qiangtang amdo and tethyan himalaya terranes have the indian gondwana origin whereas the lhasa terrane shows an australian gondwana affinity the cambrian magmatic record in the lhasa terrane resulted from the subduction of the proto tethyan ocean lithosphere beneath the australian gondwana the newly identified late devonian granitoids in the southern margin of the lhasa terrane may represent an extensional magmatic event associated with its rifting which ultimately resulted in the opening of the songdo tethyan ocean the lhasa northern australia collision at similar to 263 ma was likely responsible for the initiation of a southward dipping subduction of the bangong nujiang tethyan oceanic lithosphere the yarlung zangbo tethyan ocean opened as a back arc basin in the late triassic leading to the separation of the lhasa terrane from northern australia the subsequent northward subduction of the yarlung zangbo tethyan ocean lithosphere beneath the lhasa terrane may have been triggered by the qiangtang lhasa collision in the earliest cretaceous the mafic dike swarms ca 284 ma in the western qiangtang originated from the panjal plume activity that resulted in continental rifting and its separation from the northern indian continent the subsequent collision of the western qiangtang with the eastern qiangtang in the middle triassic was followed by slab breakoff that led to the exhumation of the qiangtang metamorphic rocks this collision may have caused the northward subduction initiation of the bangong nujiang ocean lithosphere beneath the western qiangtang collision related coeval igneous rocks occurring on both sides of the suture zone and the within plate basalt affinity of associated mafic lithologies suggest slab breakoff induced magmatism in a continent continent collision zone this zone may be the site of net continental crust growth as exemplified by the tibetan plateau c 2012 international association for gondwana research published by elsevier b v all rights reserved

subduction ultrahigh pressure metamorphism and regurgitation of buoyant crustal slices implications for arcs and continental growth abstract intracontinental collisional belts that retain mineralogic relics of phanerozoic ultrahigh pressure uhp metamorphism are increasingly being recognized adjacent regions generally lack evidence of coeval calcalkaline volcanism plutonism following consumption of intervening oceanic lithosphere each orogen marks the site of a deeply subducted microcontinental promontory fragments of which later returned to midcrustal levels mafic and ultramafic rocks are volumetrically minor pressures in uhp complexes approached or exceeded 2 8 gpa at 600 900 c subduction zones involve low t prograde gradients and constitute the only plate tectonic realm where such conditions exist although internal portions of descending plates may have even lower geothermal gradients crustal upper margins are typified by p t trajectories of 5 10 c km attending underflow clinoamphibole rich metabasaltic rocks dehydrate at pressures 2 2 2 5 gpa this accounts for the kinetically favorable solid state production of mafic eclogites and partial fusion of morbs to form hydrous andesitic melts devolatilization of partly serpentinized mantle lithosphere also introduces h 2 o to the magmagenic zone as large masses of continental crust enter deeper portions of subduction zones mineralogic transformations slow or cease because micas the major oh bearing phases in such rocks retain volatile contents to pressures 4 gpa persistence of hydroxyl bearing layer silicates may explain the metastable preservation of low density assemblages in continental crust subjected to uhp conditions and the dearth of calcalkaline magmatism in most collision belts exhumed uhp terranes are intensely deformed and disaggregated all consist dominantly of quartzofeldspathic compositions mafic eclogites garnet peridotites constitute only a few volume percent eventual decoupling of the relatively low density subducted crust promotes buoyancy propelled transport to shallow depths along the subduction channel with the hanging wall lithosphere acting as a stress guide extensional collapse and erosional unroofing of rising masses result in the continued ascent of deeply subducted but buoyant crustal material although equidimensional massifs must return surfaceward large volume to surface areas render the contained uhp phase assemblages virtually unquenchable surviving uhp terranes thus consist of relatively thin slabs of continental crust allochthons that retain uhp relics apparently lost heat by conduction across both upper normal fault and lower reverse fault contacts based on present exposures estimated thicknesses of uhp sheets are qinling dabie sulu belt 5 km kokchetav complex 1 3 km dora maira massif 1 2 km western gneiss region 1 km slices evidently rose to midcrustal levels at exhumation rates approaching 10 mm per year back reaction attending decompression in all cases was nearly complete where uhp mineral inclusions persisted retrogression apparently was limited by the relative impermeability of the rocks to catalytic aqueous fluids and by declining temperatures the coarse grain size of host phases also disfavored complete retrogression

coesite bearing eclogites from the dabie mountains central china petrogenesis p t paths and implications for regional tectonics coesite and kyanite bearing eclogites are abundant in the southern part of the dabie mountains southern dabie terrane sdt two types of eclogites from the sdt types iii and iv were selected for detailed paragenetic study type iii eclogites most abundant in the northern part of the sdt occur as blocks in gneisses and marble and contain eclogitic assemblages of omphacite garnet phengite epidote coesite kyanite carbonate rutile ilmenite these minerals exhibit weak compositional zoning and contain few mineral inclusions type iv eclogites mostly in the southern part of the sdt occur as coherent layers interbedded with gneisses and amphibolites and have assemblages of omphacite garnet glaucophane kyanite epidote phengite quartz rutile ilmenite garnets of type iv eclogites exhibit a prograde compositional zoning and have mineral inclusions of paragonite phengite epidote quartz and rutile in the core and omphacite barroisite and mg katophorite in the rim prograd

felsic magmatic phases and the role of late stage aplitic dykes in the formation of the world class cantung tungsten skarn deposit northwest territories canada abstract a field and petro chemical classification of felsic magmatic phases fmps at the world class cantung w skarn deposit was undertaken to document the evolution of magmatism and the relationships between different fmps metasomatism and mineralization early fmps include moderately differentiated zr hf 18 26 ti zr 14 15 biotite monzogranitic plutons and early biotite rich granitic dykes and compositionally similar quartz feldspar porphyry dykes late highly fractionated zr hf 8 17 ti zr 3 13 fmps sourced from a deeper monzogranitic intrusion include 1 leucocratic biotite or tourmaline bearing dykes derived from localized entrapments of residual magma and 2 sub vertical ne trending aplitic dykes derived from a larger segregation of residual fluid and incompatible element enriched magma the aplitic dykes have textures morphologies spatial associations and a pervasive calcic metasomatic mineral assemblage ca plagioclase quartz or clinozoisite indicative of syn mineralization emplacement very late stage overpressuring and initiation of sub vertical fractures into the overlying plutonic carapace and country rocks by supercritical magmatic fluid led to an interaction with calcareous country rocks that resulted in an increased aca2 in the fluid and the concurrent precipitation of w skarn residual magma also ascended with and quenched in equilibrium with the magmatic fluid to from the aplitic dykes then was metasomatized by the fluid as it interacted with calcareous country rocks overall highly fractionated and moderately to very highly undercooled fmps at cantung provide evidence for a large and evolving felsic magmatic system at depth that segregated and maintained a stable fluid and incompatible element enriched residual magma until the latest stages of crystallization the detailed study of fmps associated with magmatic hydrothermal mineral deposits allow us to refine our understanding of these mineralizing systems and better define metallogenic and exploration models for intrusion related mineralization

two stage collision related extrusion of the western dabie hp uhp metamorphic terranes central china evidence from quartz c axis fabrics and structures abstract the western dabie orogen also known as the hong an block forms the western part of the dabie sulu hp uhp belt central china rocks of this orogen have been subjected to pervasive ductile deformation and include numerous quartz schists and felsic mylonites cropping out in ductile shear zones quartz textures in these mylonites contain important clues for understanding the movement sense of late collisional extrusion and exhumation of high pressure ultrahigh pressure hp uhp rocks from the lower crustal level to the upper crustal level during middle triassic and early jurassic the orientation and distribution of quartz crystallographic axes were used to confirm the regional shear sense across the orogen the asymmetry of c axis patterns consistently indicates top to the southeast thrusting across the orogen in early structural stages later stages of deformation show different senses of movement in northern and southern parts of the orogen with top to the northwest sinistral shearing recorded in rocks north of the xinxian hp uhp eclogite facies belt and top to the southeast dextral shearing south of the same unit based on our study on quartz c axis fabrics and marco to micro scale structures simultaneous southeastward shearing within a large part of the orogen and normal faulting north of the xinxian hp uhp unit is explained by upward extrusion in early stages of deformation the extrusion process has been attributed to syn and late collisional processes accounting for some earlier deformation in the western dabie orogen such as metamorphic sequences around the core of the xinxian hp uhp eclogite facies unit much higher pressure of deformation is also indicated in the aligned glaucophane and omphacite from blueschist and eclogite in the field an orogen parallel eastward extrusion of the xinxian hp uhp eclogite facies unit however occurred diachronously in later stages of deformation therefore a tectonic model combining an early upward extrusion with a later eastward extrusion is presented two different stages and types of extrusion for exhumation of hp uhp rocks are suitable to all of east central china geochronological data shows that the first upward extrusion occurred during middle triassic the second eastward extrusion occurred during late triassic to early jurassic these two extrusions are correlative with two stages of rapid exhumation of the dabie hp uhp rocks respectively these two stage late collisional middle triassic to early jurassic extrusion events bridge the gap between syn collisional early to middle triassic vertical extrusion and post collisional cretaceous eastward directed lateral escape and provide vital clues to understanding the more detailed processes of exhumation of hp uhp rocks

u pb age trace element and hf isotope compositions of zircon in a quartz vein from eclogite in the western dabie mountains constraints on fluid flow during early exhumation of ultrahigh pressure rocks quartz veins in high pressure hp to ultrahigh pressure uhp rocks are the products of fluid rock interaction and thus provide insight into fluid processes in subduction zones in this paper we report an integrated study of mineral inclusion trace element u pb age and lu hf isotope compositions of hydrothermal zircon grains from a quartz vein within an uhp eclogite outcrop from the hong an area western dabie mountains these data are used to decipher the age conditions of formation and source of fluid for zircon formation during the exhumation of uhp rocks zircon grains from the vein have perfect euhedral shape and show sector zoning or weak zoning indicating that they precipitated from the aqueous fluid responsible for the vein formation raman spectroscopy analysis reveals that the zircon grains contain inclusions of garnet omphacite rutile quartz and h2o implying that they crystallized from aqueous fluid under hp eclogite facies conditions the zircon grains show low th u and lu hf ratios nearly flat hree patterns absent eu anomalies and low lree contents these characteristics are consistent with their precipitation in the presence of garnet and epidote and absence of feldspar and thus suggest that trace element concentrations in hydrothermal zircon are controlled by co precipitation of mineral assemblages crystallization temperatures of 670 to 712 c which were calculated using the ti content of zircon are consistent with their formation under eclogite facies conditions and may correspond to the temperature of the infiltrating fluid the weighted mean 206pb 238u age of 224 7 1 3 ma is taken as the best estimate for the age of quartz vein formation and records aqueous fluid flow during the early exhumation stage of uhp rocks the zircon grains in the quartz vein have hf compositions similar to those in the host eclogite which demonstrates isotopic equilibrium between fluid and rocks and that the fluid rock ratio was likely low

metallogeny structural evolution post mineral cover distribution and exploration in concealed areas of the northern chilean andes abstract mineral exploration of prospective areas concealed by extensive post mineralization cover is growing being very complex and expensive the projection of rich and giant paleocene to early oligocene porphyry cu mo belts in northernmost chilean andes 17 5 19 5 s has major exploration potential but only a few minor deposits have been reported to date due to the fact that the area is largely covered by post mineral strata we integrate the cenozoic stratigraphic structural and metallogenic evolution of this sector in order to identify the most promising regions related to lesser post mineral cover and the projection of different metallogenic belts the paleocene to early eocene metallogenic belt extends along the precordillera with ca 30 km wide and includes porphyry cu prospects and small cu mo au ag vein and breccia pipe deposits geochronological data indicate an age of 55 5 ma for an intrusion related to one deposit and ages from 69 5 to 54 5 ma for hydrothermal alteration in one porphyry cu prospect and largest known cu deposits the middle eocene to early oligocene porphyry belt in the western cordillera farther east is associated with 46 44 ma intrusions it is estimated to be 40 km wide but is largely concealed by thick post mineral cover the youngest miocene to early pliocene metallogenic belt also in the western cordillera is well exposed and includes au ag epithermal and polymetallic veins and manto type deposits the oligocene holocene cover consists of a succession of continental sedimentary and volcanic rocks that overall increase in thickness from 0 to 5000 m from west to east these strata are subhorizontal in the west and folded faulted towards the east miocene gentle anticlines and monocline flexures extend along strike for 30 60 km in the precordillera and were generated by propagation of high angle east dipping blind reverse faults with at least 300 900 m of oligocene bedrock offset the thickness of cover exceeds 2000 m in the eastern central depression whereas it is generally less than 1000 m in the precordillera along the paleocene to early eocene porphyry cu belt and it can reach locally up to 5000 m in the western cordillera above the middle eocene to early oligocene belt in the studied andean segment the miocene to early pliocene metallogenic belt is superimposed on the paleocene to oligocene belts in a 40 50 km wide zone this overlap may be explained by an accentuated migration of the magmatic front from east to west since ca 25 ma as a consequence of subduction slab steepening after a period of magmatic lull and flat subduction from ca 30 35 to 25 ma the identified areas of lesser cover thickness are prone to exploration for concealed deposits especially along the projection of major porphyry cu mo belts

timing of the north south china collision from new middle to late mesozoic paleomagnetic data from the north china block we collected paleomagnetic samples from 43 sites of middle and upper jurassic and cretaceous rocks along a 250 km wide east west trending belt located north of the suture between the north and south china blocks in each case laboratory treatment isolates dual polarity antipodal magnetizations that are demonstrably pre folding the paleomagnetic poles show very good coherence with coeval poles from other localities from the north china block ncb suggesting the apparent polar wander path apwp from the ncb is not distorted by post middle jurassic local vertical axis block rotations a comparison of the ncb apwp with that from the south china block suggests that collision continued after the early jurassic and ended no later than the late jurassic a cusp in the ncb apwp coincides with a marked angular unconformity that separates middle jurassic from upper jurassic and younger sediments together these observations indicate that the scissor like collision and associated deformation ceased at the middle to late jurassic boundary 159 ma the time of rapid 1 ma relative angular velocity between the two plates 225 to 190 ma coincides with a peak in u pb and ar ar dates obtained from metamorphic rocks in the suture the combined late jurassic and cretaceous paleomagnetic data from both blocks suggest that the united china plate traveled 2000 km north relative to eurasia during the late jurassic before docking in the cretaceous this younger collision event is also marked by a cusp on the cretaceous part of the eurasian apwp

ar ar geochronology of ultrahigh pressure metamorphism in central china the dabie shan of china contain the largest areal exposure of ultrahigh pressure regional metamorphic rock known on earth the thermal history of these unusual rocks is central to understanding the tectonic processes responsible for their creation preservation and exhumation published ages of ultrahigh pressure metamorphism and subsequent cooling range from archean to jurassic by analyzing 21 dabie hornblende phengite and biotite samples by the 40ar 39ar method we find that 1 cooling from peak metamorphic temperatures to 300 c occurred between about 206 and 178 ma 2 widespread cretaceous ages reflect reheating by a postultrahigh pressure magmatic extensional episode 3 40ar 39ar and k ar ages older than 230 ma and perhaps 210 ma are geologically meaningless due to the incorporation of excess 40ar and 4 dabie ultrahigh pressure metamorphic rocks are temporally related to blueschist facies rocks farther west in the suture zone

experimental investigations of blueschist greenschist transition equilibria pressure dependence ofal2o3 contents in sodic amphiboles a new geobarometer

u pb dating on single detrital zircon grains from the triassic songpan ganze flysch central china provenance and tectonic correlations the songpan ganze flysch belt central china covers a huge triangular area of more than 200 000 km and is bounded by the continental blocks of south china north china and the tibetan plateau detrital zircons extracted from three flysch samples collected in the central part of the belt were analyzed grain by grain using the u pb method two samples of middle triassic sandstones collected at different locations in the belt provide identical results which suggests similar source regions the detrital zircons yield a wide range of ages and indicate their principal derivation from mid proterozoic 1 8 2 0 ga source rocks with minor contribution from late archean ca 2 5 2 6 ga material the discordance and pb loss patterns from low u zircons indicate disturbances during a subsequent event which may be of caledonian age as suggested by concordant zircon grains at ca 420 and 450 ma one sample collected within the palang shan pass zone provides concordant zircon grains at around 230 ma 231 1 ma and 233 1 ma these triassic ages are synchronous to flysch deposition and suggest intense geological activity calc alkaline volcanism at that time in the area close to the basin the data support an origin of the clastic material mainly from a northeastern landmass corresponding to the southern margin of the sino korean craton to a lesser degree inputs from the yangtze craton and possibly from the northern margin of the basin kunlun arc are also detected the age spectrum from the upper triassic sandstone is significantly different and shows predominance of sinian ca 760 ma grains probably derived from the yangtze craton this change in the source region is interpreted as reflecting the tectonic evolution of this area and in particular as being linked to the late triassic collision between south china and north china in the middle triassic while subduction of the songpan sea northward beneath the north china plate was still taking place continental subduction of south china in the dabie region was responsible for uplift of the overriding plate i e the sino korean craton which supplied large volumes of sediments during the late triassic clockwise rotation of the south china block uplifted the indo sinian part of the qinling belt and closed the basin as the accretionary wedge was thickening along the southern margin of north china detritus derived from this source region were unable to reach the flysch basin the age spectrum presented by detrital zircons indicates

transient hot channels perpetrating and regurgitating ultrahigh pressure high temperature crust mantle associations in collision belts abstract two dimensional numerical modeling of early continental collision associated with subduction of the lithospheric mantle shows that the formation and exhumation of coesite and diamond bearing rocks metamorphosed at 700 to 900 c in the presence of dense supercritical silicate fluids and melts may be explained by a transient hot channel effect anomalously high temperature is caused by intense viscous and radiogenic heating in the channel composed of deeply subducted radiogenic upper crustal rocks especially sediments of passive margin origin and mantle rocks heating is associated with partial melting of crustal rocks caused by pervasive flow of aqueous fluids relieved by rapid dehydration deserpentinization of the overriding mantle lithosphere that has been hydrated during earlier subduction stages the channel penetrates the plate interface to the bottom of the lithosphere of the overriding plate 150 200 km and is characterized by metamorphic temperatures reaching 700 to 900 c low effective viscosity of rocks subsequent to increased temperature partial melting and fluid infiltration permits profound mixing of hydrated mantle and crustal rocks the hot channel exists only during early collision but rapidly produces large amounts of ultrahigh pressure high temperature rocks further collision closes the channel by squeezing rheologically weak partially molten buoyant rocks between the strong lithospheric mantles of the two colliding plates assemblages of complicated p t paths with repetitive loops characterize exhumation of ultrahigh pressure rocks in the convoluted flow pattern of the hot channel

high pressure blueschists and eclogites in hong an a framework for addressing the evolution of high and ultrahigh pressure rocks in central china the hong an region in the qinling dabie collisional zone in eastern china hosts a series of metamorphic rocks exposing a south to north distribution from blueschist blueschist greenschist amphibolite eclogite kyanite free and kyanite eclogite to coesite eclogite facies rocks that represent progressively deeper levels of the mesozoic subduction collision complex the hong an area is interesting for three reasons 1 it escaped the thermal and structural overprint imparted on much of the dabie mountains during early cretaceous intrusion of voluminous granites and granodiorites 2 the high pressure hp hong an eclogites are widely distributed often preserve prograde crystallization histories and can be directly linked in time and space to the blueschist blueschist greenschist rocks exposed to the south 3 the blueschist blueschist greenschist facies rocks are generally better exposed than their equivalents in the southeastern dabie mountains and offer some opportunity for simultaneous structural and metamorphic analysis the hong an area hp rocks offer perhaps the closest approximation to a preserved snapshot of mesozoic pressure temperature p t conditions attending early subduction exhumation in the region and are thus essential to generating a coherent picture of the dynamics attending both metamorphism and exposure of the coeval ultrahigh pressure uhp rocks the purpose of this contribution is twofold 1 to document previously unpublished metamorphic and structural data characterizing these hp sequences and their relative continuity in hong an 2 to incorporate these data with recent geochronologic structural and paleomagnetic information in the context of protracted late paleozoic through mesozoic subduction collision and exhumation metamorphism and exhumation of some of the southern hong an hp sequences appear to have occurred concomitant with oceanic subduction immediately to the west and thus may have preceded widespread continental subduction collision moreover all of the hp uhp sequences in the region were exhumed before the end of collision between the sino korean and yangtze cratons at ca 160 ma exhumation of hp uhp rocks both before and during continental plate collision is neither novel for central china nor for other hp uhp zones but is important to take into account when reconstructing the evolution of such orogens

triassic continental subduction in central tibet and mediterranean style closure of the paleo tethys ocean the qiangtang metamorphic belt qmb in central tibet is one of the largest and most recently documented high pressure hp to near ultrahigh pressure near uhp belts on earth lu hf ages of eclogite and blueschist facies rocks within the qmb are 244 223 ma indistinguishable from the age of uhp metamorphism in the qinling dabie orogen results of a u pb detrital zircon study suggest that protoliths of the qmb include upper paleozoic qiangtang continental margin strata and sandstones that were derived from a paleozoic arc terrane that developed within the paleo tethys ocean to the north we attribute qmb hp metamorphism to continental collision between the qiangtang terrane and a paleo tethys arc terrane this collision and the coeval south china north china collision may have slowed convergence between laurasia and gondwana derived terranes and initiated mediterranean style rollback and backarc basin development within much of the remnant paleo tethys ocean realm

cassiterite la mc icp ms u pb and muscovite 40 ar 39 ar dating of tin deposits in the tengchong lianghe tin district nw yunnan china the tengchong lianghe tin district in northwestern yunnan china is an important tin mineralization area in the sanjiang tethyan metallogenic domain there are three n s trending granite belts in the tengchong lianghe area with emplacement ages ranging from early cretaceous to late cretaceous and early cenozoic tin mineralization is spatially associated with these granitic rocks however the petrogenetic link between the tin deposits and the host granites is not clear because of the lack of age data for the tin mineralization we investigate the possibility of direct dating of cassiterite from three typical tin deposits in the tengchong lianghe tin district using laser ablation multicollector inductively coupled plasma mass spectrometry la mc icp ms in situ la mc icp ms dating of seven cassiterite samples from the lailishan lls 1 and lls 2 xiaolonghe xlh wds dsp and hjs and tieyaoshan tys tin deposits yielded well defined 206pb 207pb 238u 207pb isochron ages to assess the accuracy of the in situ u pb dating of cassiterite 40ar 39ar dating of coexisting muscovite in samples lls 1 dsp and tys was also performed the cassiterite in situ u pb ages 47 4 2 0 71 9 2 3 and 119 3 1 7 ma respectively are in excellent agreement with the coexisting muscovite 40ar 39ar ages 48 4 0 3 71 9 1 4 and 122 4 0 7 ma respectively the u pb ages of cassiterite combined with the 40ar 39ar ages of muscovite indicate that there are three tin mineralization events in this district the lailishan tin deposit at 47 4 2 0 to 52 2 7 ma the xiaolonghe tin deposit at 71 6 2 4 to 3 9 2 0 ma and the tieyaoshan tin deposit at 119 3 1 7 to 122 5 0 7 ma these ages are highly consistent with the zircon u pb ages of the host granites it is su ggested that the cretaceous tin mineralization might have taken place in a subduction environment while the early tertiary tin metallogenesis was in a postcollisional geodynamic setting

assembly of the lhasa and qiangtang terranes in central tibet by divergent double subduction abstract integration of lithostratigraphic magmatic and metamorphic data from the lhasa qiangtang collision zone in central tibet including the bangong suture zone and adjacent regions of the lhasa and qiangtang terranes indicates assembly through divergent double sided subduction this collision zone is characterized by the absence of early cretaceous high grade metamorphic rocks and the presence of extensive magmatism with enhanced mantle contributions at ca 120 110 ma two jurassic cretaceous magmatic arcs are identified from the caima duobuza rongma kangqiong amdo magmatic belt in the western qiangtang terrane and from the along tso yanhu daguo baingoin daru tso magmatic belt in the northern lhasa terrane these two magmatic arcs reflect northward and southward subduction of the bangong ocean lithosphere respectively available multidisciplinary data reconcile that the bangong ocean may have closed during the late jurassic early cretaceous most likely ca 140 130 ma through arc arc soft collision rather than continent continent hard collision subduction zone retreat associated with convergence beneath the lhasa terrane may have driven its rifting and separation from the northern margin of gondwana leading to its accretion within asia

the relation between cu au ratio and formation depth of porphyry style cu au mo deposits constraints on gold and copper ore grades in porphyry style cu au mo deposits are re examined with particular emphasis on published fluid pressure and formation depth as indicated by fluid inclusion data and geological reconstruction defining an arbitrary subdivision at a molar cu au ratio of 4 0 104 copper gold deposits have a shallower average depth of formation 2 1 km compared with the average depth of copper molybdenum deposits 3 7 km based on assumed lithostatic fluid pressure from microthermometry the correlation of cu au ratio with depth is primarily influenced by the variations of total au grade despite local mineralogical controls within some ore deposits the overall cu au ratio of the deposits does not show a significant correlation with the predominant type of cu fe sulfide i e chalcopyrite or bornite primary magma source probably contributes to metal endowment on the province scale and in some individual deposits but does not explain the broad correlation of metal ratios with the pressure of ore formation by comparison with published experimental and fluid analytical data the observed correlation of the cu au ratio with fluid pressure can be explained by dominant transport of cu and au in a buoyant s rich vapor coexisting with minor brine in two phase magmatic hydrothermal systems at relatively shallow depth approximately 3 km and greater confining pressure is likely to precipitate copper molybdenum only while sulfur complexed gold remains dissolved in the relatively dense vapor upon cooling this vapor may ultimately contract to a low salinity epithermal liquid which can contribute to the formation of epithermal gold deposits several kilometers above the au poor porphyry cu mo deposit these findings and interpretations imply that petrographic inspection of fluid inclusion density may be used as an exploration indicator low pressure brine vapor systems are favorable for coprecipitation of both metals leading to au rich porphyry copper gold deposits epithermal gold deposits may be associated with such shallow systems but are likely to derive their ore forming components from a deeper source which may include a deeply hidden porphyry copper molybdenum deposit exposed high pressure brine vapor systems or stockwork veins containing a single type of intermediate density inclusions are more likely to be prospective for porphyry copper molybdenum deposits

thermobarometry of phengite bearing eclogites in the dabie mountains of central china pressure temperature conditions for formation of the peak metamorphic mineral assemblages in phengite bearing eclogites from dabieshan have been assessed through a consideration of fe2 mg2 partitioning between garnet omphacite and garnet phengite pairs and of the reaction equilibrium celadonite pyrope grossular muscovite diopside which incorporates an evaluation of the extent of the strongly pressure dependent inverse tschermak s molecule substitution in the phengites for the latter equilibrium the calibration and recommended activity composition models indicated by waters martin 1993 have been employed and importantly yield results consistent with petrographic evidence for the stability at peak conditions of coesite in certain samples and quartz in others confirmation that in some phengite eclogite samples peak silicate mineral assemblages have equilibrated at confining pressures sufficient for the stability of coesite and in some cases even diamond rather negates previous suggestions that coesite may have been stabilized in only very localized possibly just intracrystalline domains inherent difficulties in the evaluation of peak metamorphic temperatures from fe2 mg2 partitioning between mineral phases due to uncertainties over fe3 fe2 ratios in the minerals especially omphacites and to re equilibration during extensive retrograde overprinting in some samples are also assessed and discussed our results indicate the existence in south central dabieshan of phengite eclogites with markedly different equilibration conditions within two structurally distinct tectonometamorphic terranes thus our data do not support earlier contentions that south central dabieshan comprises a structurally coherent continental crust terrane with a regional p t gradient signalling previous deepest level subduction in the north instead we recognize the central dabie ultra high pressure coesite eclogite bearing terrane to be structurally overlain by a southern dabie high pressure quartz eclogite bearing terrane at a major southerly dipping shear zone along which late orogenic extensional collapse appears to have eliminated at least 20 km of crustal section

what brought them up exhumation of the dabie shan ultrahigh pressure rocks metamorphic coesite and diamond in the dabie shan eastern china testify to subduction of continental crust to 100 km depth exhumation of these ultrahigh pressure rocks through the crust encompassed two stages 1 south dipping foliation southeast plunging stretching lineation lineation parallel isoclinal folds and boudins indicate extreme subhorizontal shortening and subvertical extension during top to northwest shearing at 200 180 ma syntectonic recrystallization occurred at eclogite and amphibolite facies temperatures and at pressures below coesite stability 2 northwest southeast subhorizontal extension from 133 to 122 ma was concentrated within an asymmetric structural dome in a magmatic complex that forms the northern half of the dabie shan pluton cores at deep structural levels have weak hypersolidus fabrics and pluton carapaces are mylonitic gneisses formed at upper amphibolite facies conditions deformation is concentrated in greenschist facies mylonites and ultramylonites along the xiaotian mozitang detachment fault at the northern topographic limit of the dabie shan our preferred exhumation model involves two stages triassic indentation vertical extrusion and erosion followed by cretaceous plate margin transtension

early palaeozoic orogenesis along the indian margin of gondwana tectonic response to gondwana assembly abstract shrimp u pb dating of zircons from a peralkaline s type lesser himalayan granite from the kathmandu region nepal indicate an age of emplacement of 475 ma the granites along with metasedimentary xenoliths show a similar signature of inherited detrital zircon ages ranging from archaean to early palaeozoic with prominent late mesoproterozoic grenvillian and neoproterozoic pan african age peaks and have a maximum age of 500 ma based on the youngest detrital grains deformation structures in xenoliths are truncated by the granite and along with the granites are assigned to a cambro ordovician orogenic event herein termed the bhimphedian orogeny that can be traced across the himalaya from pakistan to the eastern himalaya and possibly extends west into afghanistan we interpret the orogeny as being related to andean type orogenic activity on the northern margin of the indian continent following gondwana assembly the magmatic arc was associated with andesitic and basaltic volcanism and was active from c 530 to 490 ma the arc activity overlaps with and is succeeded by regional deformation crustal melting and s type granite emplacement that extended to 470 ma orogenic activity was driven by coupling across the plate margin either during on going subduction or through accretion of microcontinental ribbons possibly represented by the lhasa and qiangtang blocks it represents the termination of an orogenic cycle termed the north indian orogen that commenced with neoproterozoic rifting and passive margin development and terminated with the bhimphedian orogeny this was succeeded by a return to passive margin setting along the northern indian margin of gondwana which continued until the cenozoic himalayan orogeny

u pb zircon geochemical and sr nd hf isotopic constraints on age and origin of jurassic i and a type granites from central guangdong se china a major igneous event in response to foundering of a subducted flat slab abstract the mesozoic geology of se china is characterized by widespread jurassic to cretaceous igneous rocks consisting predominantly of granites and rhyolites and subordinate mafic lithologies however the tectonic regime responsible for the inland jurassic granites remains controversial we report here u pb zircon ages geochemical and sr nd hf isotopic data for the nankunshan alkaline granite and the fogang granitic batholith in central guangdong mineralogical and geochemical features suggest that the fogang and nankunshan rocks are i and aluminous a type granites respectively shrimp u pb zircon analyses yield consistent ages ranging from 159 3 ma to 165 2 ma for four samples from the fogang batholith and an age of 158 5 ma for the nankunshan granite the fogang granites having i sr 0 7098 0 7136 e nd t 4 3 to 12 2 and e hf t 11 5 to 3 1 for the magmatic zircons were derived from paleoproterozoic mafic intermediate igneous protolith with minor addition of mantle derived magmas the nankunshan rocks have relatively lower i sr 0 706 0 708 higher e nd t 0 3 to 2 4 and e hf t 5 7 to 1 1 for the magmatic zircons and some oib like trace element ratios they were likely generated through extensive fractional crystallization of mantle derived alkaline parental magma associated with crustal assimilation these 160 ma i and a type granites in central guangdong were emplaced coeval with the widespread 165 155 ma i and a type granites and syenites following the initiation of intraplate basaltic and or bimodal igneous magmatism at 180 170 ma in the adjacent regions we interpret these jurassic igneous rocks as anorogenic magmatism formed during a major igneous event in response to foundering of an early mesozoic subducted flat slab beneath se china continent

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

major types and time space distribution of mesozoic ore deposits in south china and their geodynamic settings the ore deposits of the mesozoic age in south china can be divided into three groups each with different metal associations and spatial distributions and each related to major magmatic events the first event occurred in the late triassic 230 210 ma the second in the mid late jurassic 170 150 ma and the third in the early mid cretaceous 120 80 ma the late triassic magmatic event and associated mineralization is characterized by peraluminous granite related w sn nb ta mineral deposits the triassic ore deposits are considerably disturbed or overprinted by the later jurassic and cretaceous tectono thermal episodes the mid late jurassic magmatic and mineralization events consist of 170 160 ma porphyry skarn cu and pb zn ag vein deposits associated with i type granites and 160 150 ma metaluminous granite related polymetallic w sn deposits the late jurassic metaluminous granite related w sn deposits occur in a ne trending cluster in the interior of south china such as in the nanling area in the early mid cretaceous from about 120 to 80 ma but peaking at 100 90 ma subvolcanic related fe deposits developed and i type calc alkaline granitic intrusions formed porphyry cu mo and porphyry epithermal cu au ag mineral systems whereas s type peraluminous and or metaluminous granitic intrusions formed polymetallic sn deposits these cretaceous mineral deposits cluster in distinct areas and are controlled by pull apart basins along the south china continental margin based on mineral assemblage age and space time distribution of these mineral systems integrated with regional geological data and field observations we suggest that the three magmatic mineralization episodes are the result of distinct geodynamic regimes the triassic peraluminous granites and associated w sn nb ta mineralization formed during post collisional processes involving the south china block the north china craton and the indo china block mostly along the dabie sulu and songma sutures jurassic events were initially related to the shallow oblique subduction of the izanagi plate beneath the eurasian continent at about 175 ma but i type granitoids with porphyry cu and vein type pb zn ag deposits only began to form as a result of the breakup of the subducted plate at 170 160 ma along the nne trending qinzhou hangzhou belt also referred to as qin hang or shi hang belt which is the neoproterozoic suture that amalgamates the yangtze craton and cathaysia block a large subduction slab window is assumed to have formed in the nanling and adjacent areas in the interior of south china triggering the uprise of asthenospheric mantle into the upper crust and leading to the emplacement of metaluminous granitic magma and associated polymetallic w sn mineralization a relatively tectonically quiet period followed between 150 and 135 ma in south china from 135 ma onward the angle of convergence of the izanagi plate changed from oblique to parallel to the coastline resulting in continental extensional tectonics and reactivation of regional scale ne trending faults such as the tan lu fault this widespread extension also promoted the development of ne trending pull apart basins and metamorphic core complexes accompanied by volcanism and the formation of epithermal cu au deposits granite related polymetallic sn w deposits and hydrothermal u deposits between 120 and 80 ma with a peak activity at 100 90 ma

palaeomagnetic constraints on the collision and rotation of north and south china the eastern half of china is dominated by the north china block ncb and south china block scb separated by a complex fold system that trends roughly east west interpretation of the geology of this fold belt has led to a variety of estimates of the age of amalgamation of these two blocks1 7 palaeomagnetic results are few and have also been subject to conflicting interpretations8 11 here we present our own palaeomagnetic results for late permian rocks for both the scb and ncb and summarize other relevant palaeomagnetic data by reinterpreting the polarity given in some previous studies we are able to synthesize the results for the widespread emeishan basalts of the scb and to suggest a new model for the collision and relative rotation of the two blocks

diamond from the dabie shan metamorphic rocks and its implication for tectonic setting diamond occurs in ultrahigh pressure metamorphic rocks from dabie shan anhui province eastern china diamond bearing rocks include eclogite gamet pyroxenite and jadeitite diamond occurs in a mineral assemblage with coesite and jadeite the diamonds and diamondiferous rocks of dabie shan are interpreted to be the products of ultrahigh pressure metamorphism in the underthrust basement of the yangtze continental plate during the early mesozoic at greater than 4 0 gigapascals and 900 c this interpretation is based on the distribution of rock units the stability field of diamond and isotopic data indicating a crustal origin for the rocks most diamonds occur as euhedral inclusions in garnets and are 10 to 60 micrometers across although some are up to 700 micrometers across

two stage exhumation of ultrahigh pressure metamorphic rocks from the western dabie orogen central china abstract zircon trace element u pb age and lu hf isotope composition were determined for two eclogite samples from the xinxian ultrahigh pressure uhp unit in the western dabie orogen to constrain its exhumation processes cathodoluminescence imaging reveals that most zircons are metamorphic while some have inherited magmatic cores in one sample the magmatic cores have high th u and lu hf ratios high trace element contents and a clear negative eu anomaly consistent with their igneous genesis they yield an upper intercept age of ma and a positive ehf t value of 8 0 arguing for reworking of early mesoproterozoic crust during the middle neoproterozoic in the western dabie orogen the metamorphic zircons contain mineral inclusions of garnet omphacite rutile and quartz they are characterized by low nb ta y and heavy rare earth elements hree contents a nearly flat hree pattern and an insignificant negative eu anomaly these indicate that the metamorphic zircons formed under high pressure

different origins of garnet in high pressure to ultrahigh pressure metamorphic rocks abstract garnet in high pressure hp to ultrahigh pressure uhp metamorphic rocks in subduction zone commonly shows considerable zonation in major and trace elements as well as mineral inclusions which bears information on its growth mechanism via metamorphic or peritectic reactions in coexistence with relic minerals and metamorphic fluids or anatectic melts at subduction zone conditions it provides an important target to retrieve physicochemical changes in subduction zone processes including those not only in pressure and temperature but also in the durations of metamorphism and anatexis garnet from different compositions of hp to uhp metamorphic rocks may show different types of major and trace element zonation as well as mineral inclusions discrimination between the different origins of garnet provides important constraints on pressure and temperature and the evolution history for the hp to uhp metamorphic rocks magmatic garnet may occur as relics in granitic gneisses despite metamorphic modification at subduction zone conditions with spessartine increasing or flat major element profiles from inner to outer core and exceptionally higher contents of trace elements than metamorphic mantle and rim metamorphic garnet can grow at different metamorphic stages during prograde subduction and retrograde exhumation with spessartine decreasing from core to rim if the intracrystalline diffusion is not too fast the compositional profiles of metamorphic garnet in the abundances of grossular almandine and pyrope are variable depending on the composition of host rocks and co existing minerals peritectic garnet grows through peritectic reactions during partial melting of hp to uhp rocks with the composition of major elements to be controlled by anatectic p t conditions and the compositions of parental rocks and anatectic melts trace element profiles in garnet with different origins are also variable depending on the coexisting mineral assemblages the garnet forming reactions and the property of metamorphic fluids or anatectic melts mineral inclusions not only present key clues to identify the different origins of garnet but also serve as sound candidates for the temporal constraint on garnet growth

magmatic evolution and related w mo mineralization in the larong deposit eastern tibet evidence from zircon u pb ages geochemistry and sr nd hf isotopes abstract the late cretaceous ca 92 ma larong porphyry w mo deposit 39 44 mt wo3 0 1541 is the first discovered giant w mo deposit in the leiwuqi zogang metallogenic belt sanjiang region multi stage granitoids including the dacite porphyry dp granodiorite porphyry gp biotite granite porphyry bgp monzogranite porphyry mp biotite granite bg and granite aplite ga have been identified in the larong deposit however the magmatic evolution and related w mo mineralization in this deposit are still poorly understood la icp ms zircon u pb dating of the dp gp bgp mp bg and ga yield weighted mean 206pb 238u ages of 214 0 0 7 ma 213 8 1 3 ma 104 4 0 9 ma 93 9 1 3 ma 91 7 0 5 ma and 36 2 0 6 ma respectively the late triassic dp and gp should be classified as peraluminous a cnk values of 0 95 1 57 s type granitoids they are characterized by pronounced negative end t values 10 01 to 13 92 and ehf t values 8 5 to 16 6 with old tdm2 nd and tdm2 hf ages 1 81 2 12 ga and 1 79 2 30 ga respectively indicating that the dp and gp were mainly derived from partial melting of ancient crustal materials after break off of the paleo tethyan oceanic slab the late cretaceous bgp mp and bg and late eocene ga have low a cnk ratios 0 92 1 06 but high sio2 70 57 77 34 and differentiation index di 87 55 98 06 similar to the characteristics of highly fractionated i type granite detailed elemental e g depletion in nb ta ti sr ba eu and p and isotopic data 87sr 86sr i 0 71192 0 72932 end t 11 81 to 9 11 ehf t 10 4 to 1 4 demonstrate that they were the result of magma mixing between ancient crust derived melts and mantle derived melts the bgp was produced during the final stage of bangong nujiang tethyan ocean bnto subduction the mp and bg were formed under a post collisional tectonic setting related to slab break off of the bnto and the ga was likely associated with delamination of the thickened eclogitic lower crust together with lithospheric mantle the w mo mineralization in the larong deposit is genetically associated with the mp with four favorable factors that can account for the formation of this deposit 1 high w background 2 multi stage magmatism 3 highly evolved relatively oxidized and f rich magmatic system and 4 sufficient calcium obtained by intense fluid rock interaction

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

thermobaric structure of a traverse across western dabieshan implications for collision tectonics between the sino korean and yangtze cratons as the best preserved high and ultrahigh pressure hp and uhp metamorphic terrane in the qinling dabieshan sulu orogen western dabieshan is divided into six lithotectonic units along a traverse across the orogen i e from north to south the nanwan balifan huwan xinxian hong an and mulanshan units in this terrane five eclogite bearing zones i v are developed the garnet and clinopyroxene in eclogites from these zones exhibit chemical zoning suggesting that the rims record general peak temperature and pressure thermobarometric study indicates that the peak p t conditions of eclogite are 550 570 c and 21 kbar for zone i 470 500 c and 14 17 kbar for zone ii 620 670 c and 26 29 kbar for zone iii 530 560 c and 20 22 kbar for zone iv and 490 510 c and 19 20 kbar for zone v the symmetrical thermobaric pattern in conjunction with structural and geochronological data demonstrates that the huwan and hong an units belong to the same hp slice overlying the uhp slice this pattern together with the mulanshan lt hp blueschist greenschist belt in the south roughly constitutes a normal metamorphic zonation however clear metamorphic gaps occur between different slices it is inferred that the lt hp hp and uhp slices were broken up from the downgoing slab during subduction and reached different depths along different geothermal gradients the successive subduction of underlying slices leads to a nearly concomitant uplift of overlying slices whereas exhumation of the deepest uhp slice was effected by underthrusting of the lower crust of the yangtze craton

carbon isotope anomaly in marbles associated with eclogites from the dabie mountains high 13c values up to 5 7 pdb were measured in marbles associated with ultrahigh pressure eclogites in the dabie mountains east china such a carbon isotope anomaly is interpreted to represent the carbon isotope composition of premetamorphic limestone i e the protolith of the marbles because there is no known process to enrich carbonate in 13c during regional metamorphism the high 13c also corresponds to high 18o of up to 22 9 smow suggesting that premetamorphic 13c values would be about 5 to 6 for their protolith limestones the carbon isotope anomaly in the protolith of the marbles implies a local change in the redox state of water during deposition of the limestones preservation of the unusually high positive 13c values in the marbles supports the previous conclusion that there was no significant isotopic exchange with the mantle during subduction of supracrustal rocks to mantle depths of greater than 100 km

thermochronologic constraints on deformation and cooling history of high and ultrahigh pressure rocks in the qinling dabie orogen eastern china the hong an block is the best place to study the exhumation of high pressure hp and ultrahigh pressure uhp metamorphic rocks in the qinling dabie orogen of eastern china because it lacks the extensive cretaceous tectonic and thermal overprint observed in the dabie shan we measured timing of deformation and rate of cooling of the hp uhp rocks by 40ar 39ar analyses of synkinematic minerals from tectonites of key structural zones in the hong an and tongbai shan normal sense shear along the north dipping huwan detachment at the northern edge of the hong an block occurred between 237 and 231 ma this detachment facilitated the bulk of the exhumation of the hp uhp rocks our new 40ar 39ar ages combined with u pb zircon and sm nd ages of 245 240 ma suggest that exhumation of uhp rocks from mantle depths occurred at rates of 5 25 mm yr from 245 to 230 ma the mountain range is a warped extensional footwall within which white mica cooled from 225 to 205 ma locally younger extension is recorded by white mica recrystallization at 198 194 ma after which the entire block had cooled to below 300 c early cretaceous 40ar 39ar ages from the tongbai shear zone indicate that dextral shear along the southwest boundary of the orogen was contemporaneous with normal to sinistral oblique slip along the xiaotian mozitan fault along the northern boundary coeval dextral and sinistral shear zones along the northern and southwestern margins of the hong an and dabie shan would have caused eastward lateral extrusion of these two blocks perhaps driven by collision of the lhasa block with eurasia

are strongly peraluminous magmas derived from pelitic sedimentary sources igneous compositions that require the presence of a phase more aluminous than biotite are here designated as strongly peraluminous ps such compositions are common but subordinate in felsic rocks and exceedingly rare in more mafic rocks the term s type has been applied to many igneous rocks based largely or even solely upon their ps compositions neglecting or placing less emphasis on other criteria for sedimentary parentage in chappell and white s i and s type classification system the s type designation was in fact initially restricted to granites which were thought to have been derived from a chemically mature strongly weathered pelitic shaly source application of chappell s and white s criteria together with others based upon isotopic and phase equilibria considerations leads to the following conclusions about ps igneous rocks worldwide 1 very few ps rocks are derived entirely from pelitic sources 2 a majority are derived largely from intermediate to felsic crustal sources including

high pressure ultrahigh pressure eclogites from the hong an block east central china geochemical characterization isotope disequilibrium and geochronological controversy the hong an block western dabieshan exposes a series of hp uhp metamorphic rocks with a s to n distribution from blueschist greenschist kyanite free to kyanite and coesite bearing eclogites the available age data are inconclusive that hinder our understanding of the tectonic evolution of the block the metamorphic temperatures in the hong an block tmeta 700 to 500 c are lower by 50 150 c than that of the dabie and sulu terranes in this work we undertook new trace element and sr nd o isotopic analyses on minerals in order to gain more insight into the geochronological problems the results are as follows 1 trace element distribution patterns suggest that garnet and omphacite in many cases are out of chemical equilibrium and the presence of high temperature lree rich mineral inclusions e g epidote in garnet and omphacite has contributed to isotope disequilibrium 2 sm nd isotope analyses yielded no isochron ages for the hong an eclogites 3 rb sr isotope analyses gave mixed results in some cases coexisting minerals are completely out of isotope equilibrium and in others isochron relationship is established yielding ages from 210 ma to 225 ma the pattern of rb sr isotope disequilibrium appears to be independent of the petrological and o isotope temperatures 4 in contrast to the unequilibrated sm nd isotopic systems oxygen isotopes of the eclogite minerals seem to have attained isotope equilibrium or near equilibrium oxygen isotope temperatures are comparable with petrological temperatures however this is an apparent feature due to mass balance constraints 5 whole rock 18o values show a large variation from 10 to 8 suggesting that their protoliths have undergone very different processes of water rock interaction in view of the overall geochronological information we conclude that the hp uhp metamorphism in the hong an block took place in the triassic at about 220 230 ma as observed in the dabie and sulu terranes the significance of published paleozoic dates 450 300 ma for the xiongdian eclogite is not clear however any hypotheses advocating two periods of uhp metamorphic events for the same tectonic unit or in the same locality are not constrained by the geochronological data

phase relations of epidote blueschists phase diagrams have been computed for minerals in the system ncmash using the optimised thermodynamic data base of berman 1988 augmented by data on glaucophane and pumpellyite to illustrate the relationships between rocks containing epidote and sodic amphibole and those containing assemblages characteristic of neighbouring metamorphic facies the stoichiometries of stable bounding equilibria yield a simple list of mineral parageneses that serves to define an epidote blueschist facies and surrounding metamorphic facies the calculations suggest that when iron free the pairs zoisite glaucophane and clinozoisite glaucophane do not possess a stability field the natural fe bearing paragenesis of epidote sodic amphibole occupies a closed field in the pt diagram whose size increases dramatically as the ratio fe3 al in the minerals is increased where garnet is involved in bounding reactions such as with respect to eclogite and garnet bearing albite epidote amphibolite the epidote blueschist field decreases in size as the ratio fe2 mg is increased the classic blueschist to greenschist transition reaction is best developed in rocks with large fe3 and fe2 contents and is possibly metastable in mgal rich rocks in the latter a wedge of albite epidote actinolite barroisite amphibolite separates the epidote blueschist from the greenschits paragenesis field a first occurrence of garnet divides the epidote blueschist field into a lower temperature garnet free zone and a higher temperature garnet bearing zone although the temperatures calculated for this garnet isograd seem high the locations of all the facies boundaries are influenced by h2o activity however even minor amounts of co2 in the fluid phase x co2 0 005 render the epidote sodic amphibole paragenesis metastable so that facies boundaries cannot be shifted by varying the co2 content of the fluid phase below 600 c no mutual boundaries exist between the amphibolite facies and the epidote blueschist and eclogite facies respectively the calculated low pressure limit for the eclogite paragenesis pyralspite omphacite jd 50 quartz typical metabasite compositions is 12 to 14 kbars with h2o activity 0 9 or 1 5 to 2 kbar lower for jd 30

coesite bearing eclogite from the dabie mountains in central china coesite and coesite pseudomorphs are recognized in eclogite from the dabie mountains a collision zone between the sino korean and yangtze cratons in central china this third known occurrence of coesite in deep crustal rocks is within an archean gneiss terrane and is the first locality where coesite has been identified as an inclusion in both omphacite and garnet crystals at the same location coesite bearing eclogite is estimated to form at pressures of 28 kbar and temperatures from 600 to 710 c the dabie mountains eclogite underwent retrograde metamorphism to amphibolite facies after its formation the preservation of coesite in eclogite is found to be strongly controlled by the pressure temperature time path that coesite bearing eclogite followed during its uplift and retrograde metamorphism

origin of fluids in the hetaoping pb zn deposit baoshan narong dongzhi block metallogenic belt yunnan province sw china abstract the hetaoping skarn type pb zn deposit is located in the baoshan narong dongzhi block metallogenic belt bnd belt a belt between the tengchong terrane and the lanping basin the deposit is hosted by marble of the upper cambrian hetaoping formation and there are no outcrops of plutonic rocks present this deposit is one of two large pb zn deposits recently discovered in the bnd belt the hetaoping deposit is a high mn skarn four types of fluid inclusions were recognized in quartz from the deposit vapor rich inclusions type i liquid rich inclusions type ii pure vapor inclusions type iii and pure fluid inclusions type iv the coexistence of type i and type iii inclusions in stage i pre ore stage and stage ii main ore stage shows evidence of fluid boiling quartz hosted fluid inclusions stage i and stage ii display high homogenization temperatures and salinities 134 315 c 3 7 18 6 wt nacl equivalent but calcite hosted fluid inclusions in stage iii post ore stage record lower homogenization temperatures and salinities 85 214 c 0 5 5 4 wt nacl equivalent these data suggest a possible mixing between primary magmatic water and meteoric water based on chromatography data the fluid inclusions in quartz contain abundant co2 and o2 and subordinate co ch4 and c2h2 c2h4 suggesting an oxidizing environment based on their na k and cl so4 ratios fluids contained in fluid inclusions are similar to volcanic spring waters the low na k ratios 0 40 1 34 of the ore forming fluids may have resulted from interaction with a deep alkaline intermediate acid intrusion hydrogen and oxygen isotope determinations on quartz from different ore stages show low 18o and d values relative to vsmow 4 3 to 2 3 109 to 91 indicating that the ore forming fluids were diluted by external fluid sources as the skarn system cooled overall geological and geochemical interpretations suggest that the hetaoping deposit is a distal manganese skarn pb zn deposit related to concealed intrusions

kinematics of exhumation of high and ultrahigh pressure rocks in the hong an and tongbai shan of the qinling dabie collisional orogen eastern china

the origin and response of zircon in eclogite to metamorphism during the multi stage evolution of the huwan shear zone china insights from lu hf and u pb isotopic and trace element geochemistry abstract the huwan shear zone hsz is an eclogite bearing transpressive wrench zone located along the shangdan suture that juxtaposes the paleozoic qinling and mesozoic hong an dabie orogenic terrains the region preserves a complex history that bridges the gap between adjacent orogenic terrains simultaneous in situ trace element u th pb and lu hf isotope analysis of zircon grains from samples of the xiongdian and sujiahe eclogite identify a late carboniferous to early permian period of high pressure metamorphism ca 283 to 306 ma zircon grains are observed to respond to metamorphic overprint via a two stage process 1 an initial prograde stage of fluid catalyzed interface coupled dissolution reprecipitation involving exsolution of a non ideal solid solution thorite thsio 4 end member and loss of highly incompatible components lree and pb 2 a second stage of coupled zircon dissolution coarsening and new rim growth in equilibrium with garnet at high pressure conditions we identify proterozoic whole rock sm nd and zircon grain lu hf isotopic evidence which challenges the traditional interpretation that the xiongdian and sujiahe eclogite formed in response to early paleozoic mantle melting and oceanic crust generation we argue the huwan shear zone contains no conclusive evidence of early middle paleozoic oceanic crust but rather proterozoic crustal components analogous to those found in the northern qinling terrain and associated with formation of the shangdan suture we present a simpler geodynamic model involving continuous convergence and accretion of terrains onto the southern margin of the north china block during the paleozoic qinling and mesozoic dabie orogenies

zircon u pb and hf evidence for coupled subduction of oceanic and continental crust during the carboniferous in the huwan shear zone western dabie orogen central china both oceanic and continental hp rocks are juxtaposed in the huwan shear zone in the western dabie orogen and thus provide a window for testing the buoyancy driven exhumation of dense oceanic hp rocks the hp metamorphic age of the continental rocks in this zone has not been well constrained and hence it is not known if they are of the same age as the exhumation of the hp oceanic rocks in situ laser ablation multiple collector inductively coupled plasma mass spectrometry la mc icp ms u pb trace element and hf isotope analyses were made on zircon in a granitic gneiss and two eclogites from the huwan shear zone u pb age and trace element analysis of residual magmatic zircon in an eclogite constrain its protolith formation at 411 4 ma the zircon in this sample displays ehf t values of 6 1 to 14 4 the positive ehf t values up to 14 4 suggest that the protolith was derived from a relatively depleted mantle source most likely palaeotethyan oceanic crust a granitic gneiss and the other eclogite yield protolith u pb ages of 738 6 and 700 14 ma respectively which are both the neoproterozoic basement rocks of the yangtze block the zircon in the granitic gneiss has low ehf t values of 14 2 to 10 5 and old tdm2 ages of 2528 2298 ma suggesting reworking of palaeoproterozoic crust during the neoproterozoic the zircon in the eclogite has ehf t values of 1 0 to 7 4 and tdm1 ages of 1294 966 ma implying prompt reworking of juvenile crust during its protolith formation metamorphic zircon in both eclogite samples displays low th u ratios trace element concentrations relatively flat heavy rare earth element patterns weak negative eu anomalies and low 176lu 177hf ratios all these features suggest that the metamorphic zircon formed in the presence of garnet but in the absence of feldspar and thus under eclogite facies conditions the metamorphic zircon yields u pb ages of 310 3 and 306 7 ma therefore both the oceanic and continental type eclogites share the same episode of carboniferous eclogite facies metamorphism this suggests that high pressure continental type metamorphic rocks might have played a key role in the exhumation and preservation of oceanic type eclogites through buoyancy driven uplift

re os and u pb geochronology of the laochang pb zn ag and concealed porphyry mo mineralization along the changning menglian suture sw china implications for ore genesis and porphyry cu mo exploration numerous polymetallic volcanogenic massive sulfide vms vein and replacement deposits are distributed along the changning menglian suture zone in sanjiang tethyan metallogenic province sw china laochang is the largest pb zn ag vein and replacement deposit in this area with a proven reserve of 0 51 mt pb 0 34 mt zn and 1 737 t ag its age and relationship to magmatic events and vms deposits in the region however have long been debated in this paper we present pyrite re os and titanite u pb ages aiming to provide significant insights into the timing and genesis of the pb zn ag mineralization pyrite grains in textural equilibrium with galena sphalerite and chalcopyrite from stratabound pb zn ag and cu bearing pb zn ag orebodies have a re os isochron age of 45 7 3 1 ma 2 mean square weighted deviation mswd 0 45 whereas titanite grains intergrown with sulfide minerals yield a weighted mean 206pb 238u age of 43 4 1 2 ma 2 n 8 a mo mineralized granitic porphyry intersected by recent drilling below the laochang pb zn ag ores yields a zircon u pb age of 44 4 0 4 ma 2 n 12 within analytical uncertainties the ages of the pb zn ag deposit and the concealed mo mineralized porphyry are indistinguishable indicating that they are products of a single magmatic hydrothermal system the results show that laochang pb zn ag deposit is significantly younger than the host mafic volcanic rock zircon u pb age of 320 8 2 7 ma 2 n 12 and silurian vms deposits along the changning menglian suture zone arguing against its origin as a carboniferous vms deposit as many researchers claimed the initial 187os 188os ratio 0 540 0 012 obtained from the pyrite re os isochron suggests that metals were likely derived from the granitic porphyry that formed from a hybrid magma due to mixing of crustal and mantle derived melts rather than from the mafic volcanic host rocks as previously thought our results favor that the laochang pb zn ag deposit is the shallow product of a porphyry mo system thus there is potential for discovery of porphyry mo or cu mo deposits below laochang and similar pb zn ag deposits in the changning menglian suture zone

pre collisional high pressure metamorphism and nappe tectonics at active continental margins a numerical simulation the evolution of a subduction channel and orogenic wedge is simulated in 2d for an active continental margin with p t paths being displayed for selected markers in our simulation subduction erosion affects the active margin and a structural pattern develops within a few tens of millions of years with four zones from the trench into the forearc i an accretionary complex of low grade metamorphic sedimentary material ii a wedge of nappes with alternating upper and lower crustal provenance and minor interleaving of oceanic or hydrated mantle material iii a megascale melange composed of high pressure hp and ultra high pressure uhp metamorphic rocks extruded from the subduction channel and iv the upward tilted frontal part of the remaining lid the p t paths and time scales correspond to those typically recorded in orogenic belts the simulation shows that hp uhp metamorphism of continental crust does not necessarily indicate collision but that the material can be derived from the active margin by subduction erosion and extruded from the subduction channel beneath the forearc during ongoing subduction

the relationship between systematic metamorphic patterns and collisional processes along the qinling sulu odesan collisional belt between the north and south china cratons

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

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

geochronology petrogenesis and tectonic significance of the jitang granitic pluton in eastern tibet sw china abstract the jitang granitic pluton which is situated in the southern margin of the north qiangtang block in eastern tibet may provide critical information about the source and derivation of the north qiangtang block during gondwana breakup and dispersion in this paper we report relevant data such as zircon u pb age whole rock major and trace element abundances and sr nd isotopes for the jitang pluton the major rock types of the pluton are granodiorite and biotite granite whole rock major element data reveal that the jitang pluton is a peraluminous s type granitic pluton the u pb age of zircons from the pluton is 219 1 1 7 ma which is 10 ma younger than the age of high pressure metamorphism in the longmu co shuanghu collisional suture between the north and south qiangtang blocks the jitang granitoids show pronounced negative ba eu sr anomalies high initial 87sr 86sr ratios from 0 7266 to 0 7389 and low end t values for from 11 1 to 13 2 which are remarkably similar to the gneisses and meta sedimentary rocks from the indian craton as well as the north qiangtang block the results from this study indicate that the jitang granitoids formed by melts derived from a crustal source with sr nd isotopic compositions similar to those of the indian cratonic crust we concur with the previous interpretation based on detrital zircon records that the north qiangtang block was derived from the indian gondwana

amphibole composition as an indicator of subtle grade variation in epidote glaucophane schists the paragenetic relations of epidote glaucophane schists are described in terms of the system al2o3 fe2o3 fe2o3 mgo cao with excess of quartz albite and epidote if alkali amphibole is free from ca and aliv its composition when associated with epidote is invariant univariant or divariant at a given pressure and temperature on miyashiro s 1957 diagram of alkali amphibole solid solution if it is also associated respectively with three two or one additional minerals in the system using a group of epidote glaucophane schists from the kotu area of the sanbagawa metamor phic belt in shlkoku japan isophysical compositional univariant boundary lines were determined for the assemblages that in addition to the ubiquitous quartz albite phengitic mica contain hematite chlorite garnet chlorite and actinolite chlorite respectively the slopes of the univariant boundary lines obtained from petrographical data are in good agreement with those calculated in a model system the positions of isophysical univariant boundary lines on the amphibole compositional diagram serve to distinguish the grade of metamorphism among the rocks of the same mineral facies the hematite chlorite univariant boundary line can be used to divide the zone of epidote glaucophane schists of the sanbagawa metamorphic belt into three and the garnet chlorite paragonite invariant equilibrium can be used to divide the epidote zone of new caledonia into three

evolution of the bangong nujiang tethyan ocean insights from the geochronology and geochemistry of mafic rocks within ophiolites abstract the bangong nujiang suture zone bnsz is located in the central tibetan plateau and represents the relict of the bangong nujiang tethyan ocean bnto this paper presents zircon u pb ages and whole rock geochemical and sr nd isotope data for the ophiolitic rocks from the rutog dongco dongqiao amdo and dengqen areas from west to east in the bnsz zircon u pb ages obtained from five gabbros and one leucogabbro from the five areas are 169 2 167 2 187 2 184 2 177 3 and 164 2 ma respectively mafic rocks gabbros and basalts in the bnsz can be divided into morb like and oib type based on geochemical data the morb like rocks are tholeiitic and can be further divided into n and e morb subtypes marked by depleted and flat ree patterns respectively all morb like rocks show supra subduction zone ssz geochemical affinities such as mild enrichment in the large ion lithophile elements and depletion in the high field strength elements coupled with high and positive whole rock e nd t values 8 1 to 5 4 the oib type rocks which formed in the early cretaceous 132 108 ma are exposed in the dongco dongqiao and tarenben areas and they consist mostly of alkali basalts with within plate geochemical characteristics and positive e nd t values 5 9 to 4 5 on the basis of these new data combined with information from the literature we further support that the bnto was an intra oceanic arc backarc basin that existed between the north lhasa subterrane nlt and the south qiangtang subterrane sqt during the early middle jurassic this basin may have been active until the early cretaceous when the oib type basalts erupted

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

the luanchuan mo w pb zn ag magmatic hydrothermal system in the east qinling metallogenic belt china constrains on metallogenesis from c h o s pb isotope compositions and rb sr isochron ages abstract the luanchuan mo w pb zn ag polymetallic ore district is located in the east qinling metallogenic belt on the southern margin of the north china craton two ore fields nannihu and yuku are recognized in the district and three types of deposits are identified from the two ore fields as follows 1 the 6 proximal porphyry skarn type mo w deposits occurring at the inner contact zone of the granite porphyries 2 the 3 middle skarn hydrothermal type zn deposits and 3 the 8 distal hydrothermal type pb zn ag deposits at the periphery of the porphyry we present c h o isotope compositions of hydrothermal quartz and calcite s pb isotope compositions of sulfide minerals and sphalerite rb sr isochron ages from the 17 deposits the geochemical and geochronological data from the two ore fields all show systematic temporal and spatial variation and primarily lead to the following inferences 1 the temperatures and salinities of the ore forming fluids decreased during mineralization the ore forming fluids gradually evolved from magmatic water to mixed magmatic meteoric water 2 the metallogenic components were primarily derived from igneous rocks with increasing proportions of the materials from the ore bearing rocks 3 the mineralization ages of these deposits are close 147 136 ma which correspond to the emplacement of the granite intrusions 4 the three types of deposits and the ore related late mesozoic intrusives constitute a unified magmatic hydrothermal mineralization system finally we also suggest exploration strategies for the luanchuan ore district

fluid rock interactions during ultra high pressure metamorphism dabie shan china oxygen isotope analyses of garnet separates and whole rock samples of gneiss and marble hosted eclogites from the ultra high pressure uhp dabie shan terrain fall in the range 6 76 to 10 76 comparison of the isotopic compositions of the eclogites with those of their host rocks leads to the following conclusions 1 the basaltic protolith of the eclogites had isotopic compositions in the range 2 15 to 4 99 it is proposed that this is the result of contamination of the parent basaltic magma with hydrothermally altered18o depleted source materials 2 18o enriched eclogite compositions resulted from local scale 3 m interaction between this basaltic eclogite protolith and fluids produced by decarbonation of impure marble lithologies during uhp metamorphism 3 18o depleted eclogite and gneiss compositions resulted from regional scale meteoric hydrothermal alteration of the basalts and their host rocks prior to uhp metamorphism epidote separates and whole rocks from the cores and rims of mineralogically zoned eclogite boudins within amphibolite grade biotite gneisses show no distinct isotopic differences this suggests that the preferential recrystallisation of the gneisses and the associated amphibolite grade overprint of the rims of eclogite bodies could not have resulted from regional scale infiltration of significant volumes of fluid during the exhumation of the dabie shan terrain

40ar 39ar geochronology and exhumation of high pressure to ultrahigh pressure metamorphic rocks in east central china new 40 ar 39 ar ages from rocks in the high pressure and ultra high pressure hp uhp metamorphic complex in the hong9an and dabie mountains areas east central china document a two phase cooling and exhumation history following triassic continental collision and metamorphism phengite ages from blueschist through kyanite bearing eclogite facies rocks in hong9an record initial exhumation from the collision zone between 230 and 195 ma biotite and hornblende ages from migmatites and eclogite bearing gneisses from the dabie mountains record a cooling event between 128 and 117 ma corresponding to a regional episode of crustal anatexis and emplacement of granitic plutons the triassic through early jurassic 40 ar 39 ar cooling ages corroborate u pb and sm nd metamorphic ages from previous studies and suggest that initial exhumation of these rocks was rapid emplacement of granitic melts within the hp uhp sequences 80 m y after metamorphism suggests that the metamorphic rocks were either exhumed at slower rates or became arrested at depth subsequent to the initial rapid exhumation episode

tectonics of an ultrahigh pressure metamorphic terrane the dabie shan tongbai shan orogen china ultrahigh pressure metamorphic rocks with coesite and diamond form a tectonic slice over 20 km thick called the eclogite zone within the dabie shan complex in the qinling orogen in central china the orogen separates the sino korean block in the north from the yangtze block in the south the dabie shan complex is a composite terrane made up of eclogite facies and amphibolite facies gneiss slices and represents fragments of the lower continental crust of the yangtze block the dabie shan complex is bounded in the south by a triassic foreland fold thrust belt and in the north by a greenschist facies metaclastic unit the foziling group which probably represents the passive continental apron deposits of the yangtze block farther north is a granulite facies gneiss complex the qinling group which has ultramafic slivers and includes the remnants of an island arc with two bounding suture zones north of the qinling group are early paleozoic active margin deposits of the sino korean block the eclogite zone in the dabie shan complex is sandwiched between amphibolite facies gneiss slices dating by sm nd rb sr and ar ar of two eclogite samples from the eclogite zone gives early to middle triassic ages 236 246 ma the initial end values indicate reworking of a 2 11 and 1 55 ga continental crust a himalayan type tectonic evolution is envisaged for the qinling orogen with the creation of a 100 km thick crustal thrust wedge through continuous underplating during the subduction of the yangtze continental lithosphere exhumation of the ultrahigh pressure metamorphic rocks was chiefly achieved by the southward propagation of the thrust planes thereby isostatically uplifting and eroding the earlier deeply subducted parts of the orogen a total of 680 km of southward thrusting in front of dabie shan is inferred based on the abrupt termination of the tanlu fault normal faulting possibly caused by gravitational collapse probably also had a role in the exhumation process

field occurrences and petrology of eclogites from the dabie mountains anhui central china four distinct types of eclogites are recognized according to their field occurrences and mineral parageneses in a gneiss terrane of the dabie mountains a collision zone between the sino korean and yangtze cratons in central china some eclogites contain coesite and its quartz pseudomorphs enclosed in garnet and omphacite type i eclogites occur as layers in serpentinites and contain garnet clinopyroxene orthopyroxene phengite rutile and coesite pseudomorph type ii eclogites occur as lenticular bodies inside serpentinites and contain garnet clinopyroxene quartz rutile and edenitic hornblende type iii eclogites occur as blocks of 2 cm to 20 m in size in a matrix of hornblende gneiss and biotite gneiss and type iv eclogites occur as thin layers interbedded with amphibolites p t estimates for these different eclogites indicate that they were formed under different physical conditions all the eclogites were affected by later regional metamorphism for which the p t conditions are estimated this paper provides an introduction to the abundant eclogites from central china which have not been reported previously in western literature specifically the mode of field occurrence petrography mineral chemistry and formation conditions of the four types of eclogites are described the paper is thus designed to establish a petrological framework for future detailed studies of the eclogites and their country rocks in an ancient zone of collision

cretaceous and cenozoic cooling history across the ultrahigh pressure tongbai dabie belt central china from apatite fission track thermochronology abstract the crystalline terrane of the tongbai dabie region central china comprising the earth s largest ultrahigh pressure uhp exposure was formed during triassic collision between the sino korean and yangtze cratons new apatite fission track aft data presented here from the uhp terrane extends over a significantly greater area than reported in previous studies and includes the eastern dabie the hong an northwestern dabie and tongbai regions the new data yield ages ranging from 44 3 to 142 36 ma and mean track lengths between 10 and 14 4 m thermal history models based on the aft data taken together with published 40 ar 39 ar k ar apatite and zircon u th he and u pb data exhibit a three stage cooling pattern that is similar across the study region commencing with an early cretaceous rapid cooling event followed by a period of relative thermal stability during which rocks remained at temperatures within the aft partial annealing zone 60 110 c and ending with a possible renewed phase of accelerated cooling during pliocene to recent time the first cooling phase followed large scale transtensional deformation between 140 and 110 ma and is related to early cretaceous eastward tectonic escape and pacific back arc extension between this phase and the subsequent slow cooling phase a transition period from 120 to 80 ma to 70 to 45 ma along the tan lu fault was characterised by a relatively low cooling rate 3 5 c ma this transition is likely related to a tectonic response associated with the mid cretaceous subduction of the izanagi pacific plate as well as lithospheric extension and thinning in eastern asia the present regional aft age pattern is therefore basically controlled by the early cretaceous rapid cooling event but finally shaped through active cenozoic faulting following the transition phase the subsequent slow cooling phase pattern implies a net reduction in horizontal compressional stress corresponding to increased extension rates along the continental margin due to the decrease in plate convergence modelling of the aft data suggests a possible pliocene recent cooling episode which may be supported by increased rates of sedimentation observed in adjacent basins this cooling phase may be interpreted as a response to the far field effects of the frontal india eurasia collision to the west approximate estimates suggest that the total amount of post 120 ma denudation across the uhp orogen ranged from 2 4 to 13 2 km for different tectonic blocks and ranged from 0 8 to 9 7 km during the cretaceous to between 1 7 and 3 8 km during the cenozoic

multistage growth of garnet in ultrahigh pressure eclogite during continental collision in the dabie orogen constrained by trace elements and u pb ages abstract major and trace elements for garnet and other minerals and u pb ages and trace elements for zircon as inclusions in garnet or as matrix were determined on thin sections of uhp eclogite from the western part of the dabie orogen the results provide temporal constraints on growth of garnet during continental subduction zone metamorphism major elements show homogeneous distribution across garnet grains with weak or no zonation of ca and almost flat distribution patterns of mg and fe however their ree compositions display two groups of patterns relative to chondrite group i garnet is characterized by steep patterns from lree to hree whereas group ii garnet shows enrichment of mree relative to lree and hree these two groups of garnet also have different parageneses of mineral inclusions group i garnet always contains abundant inclusions of amphibole epidote allanite rutile plagioclase quartz zircon whereas group ii garnet occasionally contains inclusions of omphacite rutile apatite zircon la icpms u pb dating and trace element analysis of the zircon provide direct constraints on the growth time of the two groups of garnet four types of zircon were identified based on their cl images ree distributions and u pb ages types i and ii zircons are metamorphosed after protolith magmatic zircon with type i being modified by solid state recrystallization and type ii by dissolution recrystallization types iii and iv zircons are metamorphic and grew in the stability field of garnet they gave consistent 206pb 238u ages of 239 6 ma and 222 4 ma respectively in association with groups i and ii garnets this provides a temporal constraint on the two stage growth of garnet respectively predating and postdating the peak uhp metamorphic event therefore we conclude that there are two stages of zircon and garnet growth during continental collision which occurred during the final subduction from hp to uhp phase and the early exhumation from uhp to hp eclogite facies respectively

u pb zircon ages of granitoid rocks in the north dabie complex eastern dabie shan china the north dabie complex ndc comprises a high temperature metamorphic suite dominated by amphibolite facies granitoid orthogneiss intruded by voluminous granites u pb zircon dating on these rocks yields the following new results 1 two of the granitoid orthogneiss samples have early cretaceous protolith emplacement ages of 133 7 2 3 ma and 134 0 2 8 ma 2 an undeformed granodiorite intrusive into granitoid gneiss yields an emplacement age at 125 6 0 3 ma these early cretaceous ages entirely post date late triassic early jurassic continental subduction and exhumation evident in the neighboring ultrahigh and high pressure metamorphic terrains to the south in the dabie shan hence they do not support previous tectonic models that infer paleozoic or triassic protolith emplacement ages for these granitoid orthogneisses 3 a mylonitic granite from the northern boundary of the ndc yields a concordant age of 756 6 0 8 ma which is interpreted as magma emplacement age the concordance of this late

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

tectonic setting of the low grade metamorphic rocks of the dabie orogen central eastern china abstract the tectonic setting on both the northern and southern sides of the dabie mountains reveals that low grade metamorphic rocks are important constituents produced by the subduction of the oceanic crust prior to collision between the sino korean and yangtze cratons the zhangbaling group mulanshan schist is a pre ordovician oceanic crust the sujiahe and xinyang foziling groups are trench sediments of the ordovician devonian age and constitute an accretionary prism associated with subduction the yangshan coal measures meishan group was a forearc basin sediment of carboniferous age and was overthrust by the accretionary prism during collision the susong group is composed of passive continental margin sediments of the yangtze craton backarc basin sediments are postulated to be concealed by mesozoic cenozoic sediments to the north of the dabie mountains high ultrahigh pressure terrains are exotic tectonic slices exhumed from depths located between low grade metamorphic rocks and disturb the integrity of the earlier subduction orogen subduction occurred during the ordovician to devonian periods and collision initiated at the beginning of the permian

late carboniferous high pressure metamorphism of the kassan metamorphic complex kyrgyz tianshan and assembly of the sw central asian orogenic belt abstract high pressure hp metamorphism of the kassan metamorphic complex kmc in the western kyrgyz tianshan has been related to either late ordovician or late carboniferous permian subduction processes we report sm nd ages for retrogressed eclogite samples and 40ar 39ar cooling ages for enclosing garnet muscovite samples from the kmc as new age constraints on hp metamorphism and rock exhumation these data will be used for an upgraded paleogeographic model for late paleozoic crustal consolidation in the southwestern central asian orogenic belt the retrogressed eclogite samples have transitional alkaline to tholeiitic affinity and trace element patterns consistent with protoliths derived from garnet bearing mantle sources at rifting plate margins geothermobarometric data for a retrogressed eclogite sample indicate peak metamorphic conditions of 540 30 c at 1 6 0 1 gpa samples from different lithotectonic units of the kmc provide coherent sm nd garnet whole rock ages of 317 4 ma and 316 3 ma 2 the prograde major element zoning in the mm sized garnets in combination with the moderate peak metamorphic temperature support our interpretation of the sm nd garnet ages as unambiguous evidence for late carboniferous hp metamorphism the sm nd garnet growth ages overlap within error with the 40ar 39ar mica cooling ages of 314 2 ma and 313 2 ma 2 indicating rapid uplift of the subduction complex after peak metamorphism the ca 317 313 ma hp exhumation event of the kmc is contemporaneous with those of the atbashi and akeyazi ca 500 km east in nw china hp complexes and implies similar collision histories at the south tianshan suture to the east and west of the talas fergana fault tff the exhumation of the kmc and atbashi hp complexes overlaps with the initiation of the tff rolland et al 2013 suggesting incipient separation of the chatkal and atbashi complexes during rock exhumation and early plate collision

crustal radial anisotropy beneath the dabie orogenic belt from ambient noise tomography summary in this study we continue our previous work of imaging isotropic shear velocity to study the crustal radial anisotropy beneath the dabie orogen and surrounding regions by jointly analysing rayleigh wave and love wave dispersion curves in addition to processing the vertical component data to retrieve rayleigh waves we collect horizontal component continuous ambient noise data and retrieve love waves from cross correlations of horizontal component noise data between station pairs then we measure love phase velocity dispersion curves at periods from 8 to 35 s using a spectral method and perform surface wave tomography to obtain high resolution phase velocity maps finally we determine crustal radial anisotropy by inverting the local phase velocity dispersion curves of rayleigh and love waves our model of crustal radial anisotropy reveals complex anisotropic patterns associated with the dynamic processes of continent continent collisions and post collision reworking between the north china craton and the yangtze craton in the huabei basin the radial anisotropy is positive throughout the entire crust corresponding to areas with extensional tectonics developed since the middle mesozoic in the jianghan basin strong positive radial anisotropy in the middle and lower crust is observed most likely resulting from subhorizontal alignment of seismic anisotropic materials under the crustal extensional deformation in the eastern dabie orogenic belt the radial anisotropy beneath northern dabie complex ndc appears positive in the upper and lower crust and negative in the middle crust the positive radial anisotropy in the upper crust is probably because of the presence of igneous rocks with anisotropic minerals aligned subhorizontally under the action of the extensional tectonics during igneous extrusion and emplacement whereas the positive radial anisotropy in the lower crust is probably because that subhorizontal alignment of anisotropic materials induced by magmatic underplating formed after the delamination of the orogeny in the middle crust finite strains associated with the vertical intrusion of deep magma might cause crystalline anisotropic minerals aligned vertically resulting in observed negative radial anisotropy in the western dabie orogenic belt positive radial anisotropy is present in the whole crust except beneath the hong an ultrahigh pressure uhp area where radial anisotropy is negative in the middle crust similar to ndc the hong an uhp probably experienced similar thermal geodynamic processes as the eastern dabie and the negative radial anisotropy might be also the result of vertical alignment of anisotropic minerals caused by the vertical intrusion of deep magma strong localized negative radial anisotropy in the middle crust beneath the xinyang area is imaged most likely caused by the vertical alignment of anisotropic minerals crystallized from the vertically intruding mafic magma originated from the upper mantle during the volcanism

empirical calibration of the clinopyroxene garnet magnesium isotope geothermometer and implications the large equilibrium mg isotope fractionation between clinopyroxene and garnet observed in eclogites makes it a potential high precision geothermometer but calibration of this thermometer by natural samples is still limited here we report mg isotopic compositions of eclogite whole rocks as well as mg and o isotopic compositions of clinopyroxene and garnet separates from 16 eclogites that formed at different temperatures from the dabie orogen china the whole rock 26mg values vary from 1 20 to 0 10 among them 11 samples display limited 26mg variations from 0 36 to 0 17 similar to those of their protoliths the mineral separates exhibit very different 26mg values from 0 39 to 0 39 for clinopyroxenes and from 1 94 to 0 81 for garnets the clinopyroxene garnet mg isotope fractionation 26mgclinopyroxene garnet 26mgclinopyroxene 26mggarnet varies from 1 05 to 2 15 the clinopyroxene garnet o isotope fractionation 18oclinopyroxene garnet 18oclinopyroxene 18ogarnet varies from 1 01 to 0 98 equilibrium mg isotope fractionation between clinopyroxene and garnet in the investigated samples is selected based on both the 26mgclinopyroxene versus 26mggarnet plot and the state of o isotope equilibrium between clinopyroxene and garnet the equilibrium 26mgclinopyroxene garnet and corresponding temperature data obtained in this study together with those available so far in literatures for natural eclogites are used to calibrate the clinopyroxene garnet mg isotope thermometer this yields a function of 26mgclinopyroxene garnet 0 99 0 06 106 t2 where t is temperature in kelvin the refined function not only provides the best empirically calibrated clinopyroxene garnet mg isotope thermometer for precise constraints of temperatures of clinopyroxene and garnet bearing rocks but also has potential applications in high temperature mg isotope geochemistry