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

petrogenesis p t t path and tectonic significance of high pressure mafic granulites from the jiaobei terrane north china craton abstract high pressure hp mafic granulites in the jiaobei terrane are composed predominantly of garnet mafic granulites garnet hypersthene granulites and garnet amphibolites and they are found as irregular lenses or deformed dike swarms within tonalitic trondhjemitic granodioritic gneisses and granitic gneisses the hp mafic granulites contain four distinct metamorphic assemblages of which the early prograde assemblage m 1 is represented by the cores of garnets together with mineral inclusions of clinopyxene plagioclase quartz and it formed at 740 770 c and 0 90 1 00 gpa in contrast the peak assemblage m 2 consists of high ca cores in garnet high al cores in clinopyroxene and high na cores in plagioclase in the matrix which formed under p t conditions of 850 880 c and 1 45 1 65 gpa the peak metamorphism was followed by near isothermal decompression m 3 which resulted in the development of orthopyroxene clinopyxene plagioclase quartz amphibole magnetitie symplectites or coronas surrounding some garnet grains with p t conditions of 780 830 c and 0 65 0 85 gpa surrounding some garnet grains are symplectites of amphibole plagioclase quartz magnetitie which formed during a cooling retrograde stage m 4 with p t conditions of 590 650 c and 0 62 0 82 gpa an integrated study involving laser raman analysis of mineral inclusions cathodoluminescence imaging and in situ u pb dating of zircons shows that the protolith ages of the hp mafic granulites are mainly 2550 2500 ma and that the timing of the peak metamorphism of the hp mafic granulites ranges from 1900 to 1860 ma as recorded by the cores of metamorphic zircons on the other hand the medium to low pressure granulite amphibolite facies retrogression occurred mainly at 1860 1820 ma as recorded by the rims of some single zircon grains and the zircon grains that contain inclusions of garnet orthopyroxene plagioclase sphene the combination of petrography mineral compositions metamorphic reaction history thermobarometry and geochronology defines a near isothermal decompressional clockwise p t t path for the jiaobei hp mafic granulites suggesting that the jiaobei terrane underwent initial crustal thickening during 1950 1860 ma followed by relatively rapid exhumation cooling and retrogression in the period 1860 1820 ma this tectonothermal path was probably generated by subduction and collision related tectonic processes

the growth reworking and metamorphism of early precambrian crust in the jiaobei terrane the north china craton constraints from u th pb and lu hf isotopic systematics and ree concentrations of zircon from archean granitoid gneisses abstract archean granitoid tonalite trondhjemite granodiorite ttg gneisses are widely distributed in the early precambrian metamorphic basement of the jiaobei terrane the north china craton ncc and record the formation and evolution of the continental crust in the early precambrian in this study zircon cathodoluminescence cl images have assisted in obtaining u th pb and lu hf isotopic systematics and rare earth element ree concentrations from distinct zircon domains of complex zircons and the results have been integrated in order to unravel the growth reworking and metamorphism of the early precambrian crust in the jiaobei terrane zircons from seven representative archean granitoid samples collected from the jiaobei terrane record three groups of magmatic ages of 2 9 2 7 and 2 5 ga and two groups of metamorphic ages of 2 5 and 1 86 ga the 2 5 ga metamorphic event is thought to be linked to underplating of large amounts of mantle derived magma and the 1 86 ga metamorphic event to arc or continent continent collision magmatic zircons have positive hf t values 0 7 to 12 58 average 5 5 and two stage hf model ages cluster at 2 7 2 8 and 2 9 3 0 ga this indicates that the archean granitoid rocks were derived from partial melting of juvenile crust that was formed mainly during 2 7 2 8 and 2 9 3 0 ga which represent two periods of growth of juvenile crust the most significant period of crustal growth is 2 7 2 8 ga whereas 2 5 ga represents a period of reworking of the 2 7 2 8 ga juvenile crust in the jiaobei terrane as well as other areas in the ncc

mineral inclusions in zircons of para and orthogneiss from pre pilot drillhole ccsd pp1 chinese continental scientific drilling project the pre pilot drillhole ccsd pp1 chinese continental scientific drilling project ccsd with depth of 432 m is located in the donghai area in the southwestern sulu terrane the core samples are mainly comprised of paragneiss orthogneiss and ultramafic rock with minor intercalated layers of eclogite and phengite bearing kyanite quartzite all analyzed paragneiss and orthogneiss samples were overprinted on amphibolite facies retrograde metamorphism coesite and coesite bearing ultrahigh pressure uhp mineral assemblages were identified by raman spectroscopy and electron microprobe analysis as inclusions in zircons separated from paragneiss eclogite and phengite bearing kyanite quartzite samples in the paragneiss samples uhp mineral inclusion assemblages mainly consist of coe omp grt phe coe jd phe ap preserved in the mantles m and rims r of zircons these uhp mineral inclusion assemblages yield temperatures of 814 852 c and pressures of 28 kbar presenting the p t condition of uhp peak metamorphism of these country rocks according to the mineral inclusions and cathodoluminescence images of zircons the orthogneisses can be divided into two types uhp og1 and non uhp og2 in og1 orthogneisses low pressure mineral inclusion assemblage mainly consisting of qtz phe ab ksp ap were identified in zircon cores c while coesite or coesite bearing uhp mineral inclusions were identified in the mantles m and rims r of the same zircons these features suggest that the og1 orthogneisses together with the paragneisses phengite bearing kyanite quartzite and eclogite experienced widespread uhp metamorphism in the sulu terrane however in the zircons of og2 orthogneiss samples no uhp mineral inclusions were found inclusions mainly comprised qtz phe ap and were identified in cores c mantles m and rims r of og2 zircons the cathdoluminescence images of all analyzed zircons showed clear zonings from cores to rims these features indicate that the og2 orthogneisses in pre pilot drillhole ccsd pp1 did not experience uhp metamorphism therefore we should not rule out the possibility that some orthogneisses in sulu terrane might represent relatively low pressure granitic intrusives emplaced after the uhp event

ultrahigh pressure minerals and metamorphic terranes the view from china abstract ultrahigh pressure uhp metamorphism refers to mineralogical modifications of continental and oceanic crustal protoliths associated mafic ultramafic rocks initially formed or emplaced in shallow levels of the lithosphere but which subsequently have experienced p t conditions within or above the coesite stability field 2 7 gpa 700 c typical products include eclogite garnet peridotite and uhp varieties of metapelite quartzite marble paragneiss and orthogneiss uhp metamorphic assemblages require relatively cold lithospheric subduction to mantle depths some recrystallization even occurs under forbidden p t conditions characterized by a geotherm of the current explosion of research on continental uhp terranes reflects their significance for mantle dynamics and the tectonics of continental subduction collision exhumation mantle slab interactions and geochemical recycling a further characterization of uhp phases and positive identification of uhp minerals requires new experimental studies coupled with state of the art analyses for example the very rare occurrence of microdiamond inclusions in zircons from dabie sulu uhp rocks may reflect high f o 2 attending recrystallization inasmuch as epidote is rather common rutile needles within garnets from sulu uhp eclogitic rocks may not be the result of exsolution so in such cases the apparent uhp pressure may have been over estimated hadean igneous microdiamond inclusions in jack hills detrital zircons could have originated from mantle xenoliths whereas abundant detrital phanerozoic diamonds containing inclusions of coesite and other eclogitic minerals from new south wales might have been derived from unexposed uhp metamorphic terranes micro mineral intergrowth and nano size minerals may hold important key to deciphering the actual p t paths of subduction and mantle return flow although most exhumed terranes have returned surfaceward relatively rapidly after short time of uhp condition the long duration of storage at great depth and slow exhumation for the largest uhp terranes remain as major problems

composite nature of the north china granulite facies belt tectonothermal and geochronological constraints granulite facies rocks are intermittently exposed in a roughly e w trending belt that extends for approximately 2000 km across the north china craton from the helanshan qianlishan wulashan daqingshan guyang and jining complexes in the western block through the huai an hengshan xuanhua and chengde complexes in the trans north china orogen to the jianping western liaoning eastern hebei northern liaoning and southern jilin complexes in the eastern block the belt is generally referred to as the north china granulite facies belt previously interpreted as the lowest part of an obliquely exposed crust of the north china craton recent data indicate that the north china granulite facies belt is not a single terrane instead it represents components of three separate terranes the eastern and western blocks and trans north china orogen each of these units records different metamorphic histories and reflect the complex tectonic evolution of the ncc during the late archean and paleoproterozoic mafic granulites in the eastern block and the yinshan terrane western block underwent medium pressure granulite facies metamorphism at about 2 5 ga with anticlockwise p t paths involving near isobaric cooling following peak metamorphism reflecting an origin related to intrusion and underplating of mantle derived magmas pelitic granulites in the khondalite belt western block underwent medium pressure granulite facies metamorphism at about 2 0 1 9 ga with clockwise p t paths which record the paleoproterozoic amalgamation of the yinshan and ordos terranes to form the western block mafic and pelitic granulites in the trans north china orogen experienced high to medium pressure granulite facies metamorphism at 1 85 ga with clockwise p t paths involving nearly isothermal decompression following peak metamorphism which are in accord with the final collision between the eastern and western blocks to form the north china craton at 1 8 ga the ncgb cannot therefore represent a separate unique terrane instead it reflects the amalgamation of three separate granulite terranes that evolved independently and at different times d 2005 international association for gondwana research published by elsevier b v all rights reserved

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

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

large scale geomorphological mapping of tors proposal of a key and landform interpretation abstract isolated bedrock outcrops amidst regolith covered slopes often named as tors have been the object of interest in many geomorphological studies usually as a part of long term landscape evolution inquiries however detailed presentation of tor morphology on maps in publications is very uncommon in this paper we propose a new comprehensive method of detailed large scale geomorphic mapping of tors it is tested on selected tor groups located in the sudetes in southwestern poland they are diversified in terms of lithology granite greenstone and sandstone bedrock and morphological features solitary tor rock city cluster of tors rock cliffs etc the method consists of three stages preparation of digital spatial data field mapping and final legend and map design the legend consists of 40 symbols which allows to show tor morphology in considerable detail assisting thereby in the interpretation of controls behind tor formation and evolutionary pathways of different types of tors using detailed maps one can extract information about major vertical joint sets and the patterns of tor decay weathering of rock surfaces in subaerial conditions structure of slope covers and internal morphological differentiation within large clusters of tors the latter being a possible manifestation of certain stages in the development of these landforms

fluid flow during exhumation of deeply subducted continental crust zircon u pb age and o isotope studies of a quartz vein within ultrahigh pressure eclogite quartz veins in high pressure to ultrahigh pressure metamorphic rocks witness channelized fluid flow that transports both mass and heat during collisional orogenesis this flow can occur in the direction of changing temperature pressure during subduction or exhumation shrimp u pb dating of zircon from a kyanite quartz vein within ultrahigh pressure eclogite in the dabie continental collision orogen yields two age groups at 212 7 and 181 13 ma which are similar to two groups of la icpms age at 210 4 and 180 5 ma for the same sample these ages are significantly younger than zircon u pb ages of 224 2 ma from the host eclogite thus the two age groups from the vein date two episodes of fluid flow involving zircon growth the first due to decompression dehydration during exhumation and the second due to heating dehydration in response to a cryptic thermal event after continental collision laser fluorination o isotope analyses gave similar 18o values for minerals from both vein and eclogite indicating that the vein forming fluid was internally derived synchronous cooling between the vein and eclogite is suggested by almost the same quartz mineral fractionation values with regularly decreasing temperatures that are in concordance with rates of o diffusion in the minerals while the quartz veining was caused by decompression dehydration at 700 650 c in a transition from ultrahigh pressure to high pressure eclogite facies retrogression the postcollisional fluid flow was retriggered by heating dehydration at 500 c without corresponding metamorphism in either case the kyanite quartz vein formed later than the peak ultrahigh pressure metamorphic event at the middle triassic pointing to focused fluid flow during exhumation rather than subduction the growth of metamorphic zircon in the eclogite appears to have depended on fluid availability so that their occurrence is a type of geohygrometer besides geochronological applicability to dating of metamorphic events in orogenic cycles

deformation history of the paleoproterozoic liaohe assemblage in the eastern block of the north china craton the paleoproterozoic liaohe assemblage and associated liaoji granitoids represent the youngest basement in the eastern block of the north china craton various structural elements and metamorphic reaction relations indicate that the liaohe assemblage has experienced three distinct deformational events d1 to d3 and four episodes of metamorphism m1 to m4 the earliest greenschist facies event m1 is recognized in undeformed or weakly deformed domains wrapped by the s1 schistosity suggesting that m1 occurred before d1 the d1 deformation produced small mostly meter scale isoclinal and recumbent folds f1 an associated penetrative axial planar schistosity s1 a mineral stretching lineation l1 and regional scale ductile shear zones concurrent with d1 was m2 metamorphism which occurred before d2 and produced low to medium pressure amphibolite facies assemblages regionally divergent motion senses reflected by the asymmetric f1 folds and other sense of shear indicators together with the radial distribution of the l1 lineation surrounding the liaoji granitoids imply that d1 represents an extensional event the d2 deformation produced open to tight f2 folds of varying scales s2 axial crenulation cleavages and ene ne striking thrust faults involving broadly nw se compression following d2 was m3 metamorphism that led to the formation of sillimanite and cordierite in low pressure type rocks and kyanite in medium pressure rocks the last deformational event d3 formed nw wnw trending folds f3 axial planar kink bands spaced cleavages s3 and strike slip and thrust faults which deflect the earlier d1 and d2 structures d3 occurred at a shallow crustal level and was associated with or followed by a greenschist facies retrograde metamorphic event m4 the liaohe assemblage and associated liaoji granitoids are considered to have formed in a paleoproterozoic rift the late spreading of which led to the occurrence of the early extensional deformation d1 and the m1 and m2 metamorphism and the final closing of which was associated with the d2 and d3 phases of deformation and m3 and m4 metamorphism

u pb dating of zircons from granitic leucosomes in migmatites of the jiaobei terrane southwestern jiao liao ji belt north china craton constraints on the timing and nature of partial melting abstract in the jiaobei terrane of the jiao liao ji belt eastern china layers and irregular lenses of granitic leucosomes are widely distributed within high pressure hp mafic granulites medium pressure mp granulites amphibolites ttg gneisses and hp and mp pelitic granulites the layers and lenses may be parallel to or cross cut the foliation in the host rocks on different scales and they show evidence of coalescence and migration in the formation of centimeter to decimeter scale segregations the migmatized rocks extend over an area of at least 120 km 75 km from laixi through laiyang and qixia to sw yantai a combined study of mineral inclusions cathodoluminescence cl images and zircon u pb la icp ms dates provides clear evidence on the nature and timing of the partial melting in these granulite facies metamorphic rocks most zircons from the granitic leucosomes are either inherited crystals igneous or detrital with distinct overgrowths or are simply new euhedral crystals both the overgrowths and the new crystals commonly exhibit perfect euhedral shapes pronounced oscillatory zoning and contain mineral inclusions of qtz kfs pl ap in contrast the inherited cores are rounded or irregular in shape and exhibit signs of dissolution these observations suggest that the new zircons are anatectic in origin and that they crystallized from melts derived from dehydration melting reactions and partial melting of the host granulite facies rocks numerous u pb spot analyses of anatectic zircons reveal that the jiaobei terrane experienced a major episode of partial melting in the paleoproterozoic with crystallization of the melt occurring in two distinct periods from 1860 9 2 2 to 1852 1 2 8 ma and 1839 9 2 9 to 1833 3 5 3 ma as represented respectively by the magmatic cores and rims of the anatectic zircons the first age group probably represents the initial crystallization of melts derived from partial melting within the jiaobei terrane while the second age group may represent the completion of migmatization and crystallization of granitic leucosomes this significant partial melting event was genetically associated with the paleoproterozoic mp granulite facies hot stage of exhumation of the southwestern part of the jiao liao ji belt and it induced extensive retrogression of the ttg gneisses and mafic and pelitic granulites as well as the formation of regional migmatites

ca 2 9 ga granitoid magmatism in eastern shandong north china craton zircon dating hf in zircon isotopic analysis and whole rock geochemistry abstract eastern shandong is one of the important early precambrian areas in the north china craton where extensive 2 5 2 7 and 2 9 ga magmatic activities have been identified followed by very strong late neoarchean and paleoproterozoic tectono thermal events we carried out shrimp zircon dating hf in zircon isotope analysis and a whole rock geochemical study on ca 2 9 ga mesoarchean magmatic rocks these include gneissic diorite gneissic quartz diorite gneissic tonalite and gneissic high si trondhjemite and were identified in five new areas all these rocks contain ca 2 5 ga metamorphic zircons the dioritic and tonalitic rocks are enriched in large ion lithosphile lil elements k rb ba and depleted in nb and p with moderate to highly fractionated ree patterns similar in chemical compositions to archean ttg rocks worldwide the high si trondhjemites are characterized by very high sio2 and very low feot mgo the magmatic zircon grains from all these rocks are similar in their hf isotopic compositions and have hf t values ranging from 1 3 to 9 9 combined with previous work our main conclusions are 1 eastern shandong underwent a major magmatic event at ca 2 9 ga 2 diorites and tonalites formed by melting of thickened lower crust or the root of an oceanic plateau whereas the high si trondhjemites were derived from melting of intermediate to felsic rocks 3 the mesoarchean ca 2 9 ga was a major period of juvenile crustal addition from the depleted mantle earlier than the major crustal growth period at 2 7 2 8 ga in the north china craton 4 the 2 7 2 9 ga rocks underwent strong metamorphism at ca 2 5 ga probably as a result of magmatic underplating

fluid regime in continental subduction zones petrological insights from ultrahigh pressure metamorphic rocks abstract intensive studies of ultrahigh pressure uhp metamorphic rocks in continental collision zones have provided petrological insights into the fluid regime during subduction and exhumation recent studies in the dabie sulu orogen of china have made important contributions to understanding fluid action in such systems hence these rocks are the main focus of this review sources of metamorphic fluid can be deduced from study of hydrous minerals structural hydroxyl and molecular water in nominally anhydrous minerals fluid and solid inclusions of uhp metamorphic minerals are also a record of fluid action in deep subduction zones the presence or absence of metamorphic fluid during subduction and exhumation dictates zircon growth and recrystallization making available materials for u pb dating isotopic dating demonstrates that after peak uhp metamorphism retrograde fluid with deuteric origin became active the decomposition of hydrous minerals and the exsolution of molecular water and structural hydroxyl during exhumation can provide sufficient amounts of aqueous fluid for quartz veining and amphibolite facies retrogression within uhp slabs aqueous fluid released by the exsolution of molecular water and structural hydroxyl from uhp nominally anhydrous minerals is characterized by low salinity and light hydrogen and oxygen isotope compositions therefore one part of the uhp slab is a source of aqueous fluid whereas another part is a sink in both cases the fluid action during subduction and exhumation is episodic rather than continuous supplementary material a description of the dabie sulu uhp metamorphic rocks analytical methods la icpms zircon u pb isotope data and rare earth element compositions are available at http www geolsoc org uk sup18355

permo triassic and paleoproterozoic metamorphism related to continental collision in yangpyeong south korea abstract gneisses and migmatites exposed in the yangpyeong area in the northern gyeonggi massif provide insight into the paleoproterozoic and triassic metamorphic events in south korea garnet biotite gneiss and sillimanite garnet biotite gneiss in the western part of the area reveal paleoproterozoic metamorphism 1888 1871 ma at p t conditions of 760 820 c and 8 10 kbar and 710 750 c and 5 7 kbar respectively these rocks were overprinted by low p t type metamorphism 590 650 c 3 4 kbar during the triassic ca 237 ma in contrast a cordierite rich migmatite near the post collisional triassic igneous complex in the eastern part of the area was strongly metamorphosed during the triassic ca 235 ma at 750 790 c and 7 8 kbar the similar triassic ages in the western and eastern areas suggest that low p t type metamorphism occurred as a second stage of regional metamorphism which is characterized by the production of cordierite with an irregularly shaped garnet the metamorphic grade of the triassic metamorphism decreases spatially towards the west from granulite facies to amphibolite facies metamorphic conditions and the paleoproterozoic metamorphism is well preserved in the western part with low grade triassic metamorphism the new discovery of the triassic metamorphic event in the yangpyeong area in addition to the previously reported triassic post collision igneous event supports the idea that the continental collision belt between the north and south china blocks extends from the hongseong area into the odesan area through the yangpyeong area in south korea

regional ultrahigh pressure coesite bearing eclogitic terrane in central china evidence from country rocks gneiss marble and metapelite the dabie mountains represent a fault bounded coesite bearing eclogite gneissic terrane within the collisional belt between the sino korean and yangtze cratons the following geologic evidence indicates that this terrane has been subjected to regional ultrahigh pressure metamorphism 1 the country rocks have a metamorphic pressure temperature evolution compatible with that of the eclogites on the basis of the occurrence of quartz pseudomorphs after coesite calcite pseudomorphs after aragonite relict high pressure assemblages in different rock types of the country rocks and consistent comparable compositional zonations of garnets in both country rocks and eclogites 2 peak metamorphic pressure temperature conditions consistently decrease both from east to west in the collisional belt and from north to south in the dabie mountains and clockwise pressure temperature paths are common for eclogites over the region 3 layered eclogites interbedded with gneisses and amphibolites contain omphacite kyanite and garnet indicating a minimum pressure of 20 kbar at 600 c for formation of these rocks

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

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

fission track analysis and thermotectonic history of the main borehole of the chinese continental scientific drilling project abstract the chinese continental scientific drilling ccsd project part of the international continental drilling program icdp has completed drilling a 5158 m hole in the eastern part of the dabie sulu ultrahigh pressure metamorphic belt this study reports on an apatite fission track analysis of core samples from 0 to 4000 m depth in the ccsd main hole ccsd mh we determined the fission track ages of 38 apatite samples from different depths the ages range between 98 6 17 0 and 3 2 1 3 ma showing a general decreasing trend with depth from 87 1 11 2 ma at the surface to 3 2 1 3 ma at 3899 m depth as a first approximation an average uplift rate of 35 m ma is calculated for the period 90 30 ma the trend in ages within the borehole shows some fluctuations and indicates movements along major faults it is inferred that the highest level major normal fault occurs at a depth of 350 m recording a vertical displacement of 400 m movement along another prominent normal fault at a depth of 2150 m occurred subsequent to 25 ma three major reverse faults occur at about 2450 3050 and 3250 m depth testing geological constrains against the fission track data set indicated an agreement with a reheating of the area during the late cretaceous and eocene followed by cooling to 80 c during the eocene and a low cooling until the samples reached their present day position in the donghai area

shrimp u pb dating trace elements and the lu hf isotope system of coesite bearing zircon from amphibolite in the sw sulu uhp terrane eastern china abstract in this study we link mineral inclusion data trace element analyses u pb age and hf isotope composition obtained from distinct zircon domains of complex zircon to unravel the origin and multi stage metamorphic evolution of amphibolites from the sulu ultrahigh pressure uhp terrane eastern china zircon grains separated from amphibolites from the ccsd mh drill hole g12 and niushan outcrop g13 were subdivided into two main types based on cathodoluminescence cl and laser raman spectroscopy big dusty zircons with inherited cores and uhp metamorphic rims and small clear zircons weakly zoned grey white luminescent inherited cores preserve mineral inclusions of cpx pl ap qtz indicative of a mafic igneous protolith dark grey luminescent overgrowth rims contain the coesite eclogite facies mineral inclusion assemblage coe grt omp phe ap and formed at t 732 839 c and p 3 0 4 0 gpa in contrast white luminescent small clear zircons preserve mineral inclusions formed during retrograde hp quartz eclogite to lp amphibolite facies metamorphism t 612 698 c and p 0 70 1 05 gpa inherited zircons from both samples yield shrimp 206 pb 238 u ages of 695 520 ma with an upper intercept age of 800 31 ma the uhp rims yield consistent triassic ages around 236 225 and 239 225 ma for g12 and g13 with weighted means of 229 3 and 231 3 ma respectively small clear zircons from both samples give 206 pb 238 u ages around 219 210 ma with a weighted mean of 214 3 ma interpreted as the age of retrograde quartz eclogite facies metamorphism matrix amphibole from both samples indicate ar ar ages of 209 0 7 and 207 0 7 ma respectively probably dating late amphibolite facies retrogression the data suggest subduction of neoproterozoic mafic igneous rocks to uhp conditions in middle triassic 230 ma times and subsequent exhumation to an early hp 214 ma and a late lp stage 208 ma over a period of 16 and 6 myr respectively thus early exhumation from a mantle depth of 120 100 km to about 60 km occurred at an average rate of 0 3 cm y while subsequent exhumation to a middle crustal level took place at approximately 0 54 cm y these exhumation rates are considerably slower than those obtained for uhp rocks in the dora maira and kokchetav massifs 2 3 cm y based on similar p t estimates and trace element and hf isotope compositions sulu amphibolites can be identified as retrograde uhp eclogites the ehf 800 of 8 implies a significant input from the depleted mantle to the sulu dabie terrane during the middle neoproterozoic overgrown rims are characterized by a distinct trace element composition with low lu hf and th u and significantly higher 176 hf 177 hf ratios than inherited cores consistent with formation during after garnet re crystallization and fractionation of the lu hf system during uhp metamorphism the combined dataset suggests homogenization of the 176 hf 177 hf ratio within the metamorphic mineral assemblage and during protolith formation observed variations are explained by mixing of material from both domains during laser ablation e g due to partial recrystallization of inherited cores

assembling north china craton within the columbia supercontinent the role of double sided subduction abstract an evaluation of recent s wave receiver functions s wave velocities and two versions of p wave tomographic images along various transects in the north china craton provides some clues on the subduction collision history of the different crustal blocks and their final amalgamation within the paleoproterozoic columbia supercontinent interpretation of a n s seismic section of the craton suggests thick slab debris sinking to various depths in the mantle the w e seismic corridors show the preservation of a thick 200 km lithospheric root tectosphere beneath the ordos block and its variable and extensive erosion towards the yanliao block eastern block this zone is characterized by layers with marked velocity contrast and suggests repeated stacking of the remnants of underplated and accreted paleoproterozoic oceanic lithosphere the present day lithosphere asthenosphere boundary beneath this region probably marks the erosional plane along which decratonization occurred through subduction erosion from the east and thermal and material erosion by upwelling asthenosphere from below resulting in the partial destruction of the tectosphere and its thinning towards the east within the asthenosphere below the yanliao block younger and thinner slabs predominate in the absence of any prominent thick high velocity layers these younger slabs define a westward polarity and constitute a mega scale duplex formed by underplating through phanerozoic subduction process particularly the pacific plate subduction from the east the lithologic associations within the inner mongolia suture zone dividing the yinshan block to the north and ordos block to the south correspond to an accreted ocean plate stratigraphic sequence with the tonalite trondhjemite granodiorite ttg gneisses charnockites and calc alkaline granites representing a continental arc built up through subduction from the north the seismic transects bring out a contrasting polarity in the subduction regime with an oblique east to southward subduction of the yinshan block and a westward subduction of the yanliao block here i propose a double sided subduction history for the ncc similar to the ongoing subduction process in the western pacific such double sided subduction is considered to promote rapid amalgamation of continental fragments within supercontinents and the subduction polarities and mantle dynamics of ncc are therefore considered to be critical in evaluating the final assembly of the paleoproterozoic supercontinent columbia

la icp ms u pb zircon ages of the liaohe group in the eastern block of the north china craton constraints on the evolution of the jiao liao ji belt abstract as a major lithostratigraphic unit of the palaeoproterozoic jiao liao ji belt in the eastern block of the north china craton the liaohe group consists of an assemblage that is transitional from a lower arkose and volcanic rich sequence langzishan li eryu and gaojiayu formations through a middle carbonate rich sequence dashiqiao formation to an upper argillaceous sequence gaixian formation tectonically associated with the liaohe group are large volumes of granitoid rocks named the liaoji granite whose emplacement age has been dated at about 2 1 2 2 ga regionally the liaohe group and the associated liaoji granite extend along a nee trending belt that is flanked by 2 45 2 55 ga basement gneisses u pb zircon data and cathodoluminescence cl imaging are used to constrain the maximum depositional and metamorphic ages of the liaohe group which are critical in understanding the tectonic setting and history of the jiao liao ji belt cathodoluminescence images reveal that most detrital zircon grains from the liaohe group are characterized by concentric oscillatory zoning with comparatively low luminescence and high th u ratios suggesting magmatic origin but minor zircon grains have structureless overgrowth or recrystallization rims that are characterized by low th u ratio indicating metamorphic origin results of this study show that the detrital zircon grains of the langzishan formation the lowest sequence of the liaohe group have concordant u pb ages ranging from 2 05 to 2 24 ga suggesting that the liaohe group was deposited at some time after 2 05 ga although most overgrowth or recrystallization rims are too narrow to be analyzed some rims yield concordant or upper intercept ages around 1 93 ga interpreted as the peak metamorphic age of the liaohe group detrital zircon grains from the dashiqiao formation in the middle sequence of the liaohe group have concordant u pb ages ranging from 2 25 to 2 53 ga some of which are similar to those of the basement gneisses in the area this study indicates that the liaohe group was deposited after the emplacement of the liaoji granite and the latter may be a major component of the provenance for the langzishan formation in contrast the protoliths of the middle sequence of the liaohe group were most probably derived from the basement gneisses these new zircon ages for the liaohe group and recent data for the liaoji granite combined with lithological structural and metamorphic considerations support a rifting model for the evolution of the jiao liao ji belt

shrimp u pb zircon geochronology of palaeoproterozoic metasedimentary rocks in the north china craton evidence for a major late palaeoproterozoic tectonothermal event abstract although metasedimentary successions are widely distributed across the north china craton those of palaeoproterozoic age are restricted to three orogenic belts namely the trans north china orogen the khondalite belt and the jiao liao ji belt prior to this work some of those successions were considered to be archaean in age the palaeoproterozoic metasedimentary rocks contain detrital igneous zircon grains with ages ranging from 3 0 to 2 1 ga with the majority recording an age between 2 3 and 2 0 ga this latter group define a series of magmatic events that occurred between formation of the archaean basement and deposition of the metasedimentary rock protoliths the metasedimentary rocks occur as two associations older volcano sedimentary units that formed between 2 37 and 2 0 ma and younger sedimentary units that formed mainly between 2 0 and 1 88 ga all these rock associations were then subjected to metamorphism during the late palaeoproterozoic 1 88 and 1 85 ga tectonothermal event named the luliang movement in early chinese literature which was a continent continent collision that led to the amalgamation of the north china craton

shrimp u pb zircon dating of the neoproterozoic penglai group and archean gneisses from the jiaobei terrane north china and their tectonic implications abstract archean basement gneisses and supracrustal rocks together with neoproterozoic sinian metasedimentary rocks the penglai group occur in the jiaobei terrane at the southeastern margin of the north china craton shrimp u pb zircon dating of an archean ttg gneiss gave an age of 2541 5 ma whereas metasedimentary rocks from the neoproterozoic penglai group yielded a range in zircon ages from 2 9 to 1 8 ga the zircons can be broadly divided into three age populations at 2 0 1 8 ga 2 45 2 1 ga and 2 5 ga detrital zircon grains with ages 2 6 ga are few in number and there are none with ages

shrimp u pb zircon geochronology of the liaoji granitoids constraints on the evolution of the paleoproterozoic jiao liao ji belt in the eastern block of the north china craton abstract the paleoproterozoic jiao liao ji belt lies at the eastern margin of the eastern block of the north china craton and is composed mainly of the liaohe group and the liaoji granitoids the liaoji granitoids are divisible into pre tectonic magnetite and hornblende biotite monzogranitic gneisses and post tectonic or anorogenic porphyritic monzogranites and alkaline syenites shrimp u pb zircon geochronology combined with cathodoluminescence cl imaging has enabled for resolution of magmatic and metamorphic events that can be directed towards understanding the history of the paleoproterozoic jiao liao ji belt shrimp u pb zircon analyses reveal that the pre tectonic magnetite monzogranitic gneisses were emplaced in the period 2176 2166 ma and metamorphosed at 1914 ma and the pre tectonic hornblende biotite monzogranitic gneisses were emplaced at 2150 2143 ma these data demonstrate that the pre tectonic monzogranitic gneisses of the liaoji granitoids were not younger than the deposition of the liaohe group as previously considered since the latter contains a large amount of 2100 2000 ma detrital zircons in contrast the pre tectonic monzogranitic gneisses of the liaoji granitoids or similar aged granitoids may have been an important component of the provenance for the liaohe group shrimp u pb zircon analyses also reveal that the post tectonic porphyritic monzogranites and granites in the jiao liao ji belt were emplaced at 1875 10 and 1856 31 ma respectively simultaneously with the emplacement of the alkaline syenites that was dated at 1857 20 and 1843 23 ma similar post tecotnic or anorogenic granites are not only limited to the jiao liao ji belt but are also found in the north and south korea suggesting that the 1 85 ga post tectonic or anorogenic granitic magmatic event may have occurred in both the korean peninsula and the eastern block of the north china craton the shrimp zircon ages of this study combined with lithological structural metamorphic and geochemical data is consistent with a rift closure model which suggests that the archean complexes on the northern and southern sides of the jiao liao ji belt were originally situated on a single continental block that underwent paleoproterozoic rifting associated with the formation of the liaohe group and liaoji granitoids and closed upon itself at 1 9 ga

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

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

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

paleoproterozoic arc magmatism in the north china craton no siderian global plate tectonic shutdown abstract arc magmatism in convergent plate margins has been a major contributor to continental growth following arc arc and arc continent collisions in the archean leading to the amalgamation of micro blocks the north china craton ncc witnessed major pulses of continental arc magmatism during the paleoproterozoic in this study we present geochemistry zircon u pb geochronology and lu hf isotope data from a suite of magmatic rocks sampled from the region of confluence of two major paleoproterozoic suture zones in the ncc the inner mongolia suture zone imsz and the trans north china orogen tnco our zircon u pb geochronological data indicate new zircon growth during multiple tectonothermal events as displayed in the 207pb 206pb weighted mean ages of 2410 41 ma for metagranite 2480 12 ma 2125 18 ma 1946 8 ma 1900 15 ma and 1879 12 ma from metagabbros 2446 11 ma from charnockite and 1904 6 ma and 1901 9 ma from metatuffs the 207pb 206pb upper intercept age of zircons in the khondalite shows 2102 76 ma which is identical to the age obtained from the magmatic zircons in one of the metagabbros the khondalites also carry a group of concordant metamorphic zircons with 207pb 206pb mean age of 1881 20 ma metamorphic zircons in the gabbros and charnockites also yield similar ages of 1890 14 ma and 1852 19 ma respectively the age data suggest prolonged arc magmatism in a convergent margin setting during ca 2 48 to 1 9 ga followed by metamorphism at ca 1 89 1 85 ga associated with the final collision lu hf analyses reveal that the dominant populations of zircons from all the rock types are characterized by positive ehf values 1 9 to 6 8 mean 1 8 the ehf and tdmc data suggest that the magmas were mostly derived from neoarchean and paleoproterozoic juvenile components the salient geochemical features of these rocks attest to magma generation from heterogeneous sources involving subduction derived arc components with minor input from continental crust the results presented in this study together with those from previous investigations in different domains of the imsz and tnco suggest major paleoproterozoic arc magmatic events in the ncc lasting for nearly 600 million years associated with the final assembly of the crustal blocks into a coherent craton construction of the final cratonic architecture of the ncc thus witnessed not only the arc continent amalgamations at 2 7 2 5 ga but also major crust building events in the paleoproterozoic through melts generated from juvenile and recycled components in continental magmatic arc systems along an active convergent margin followed by intense deformation and metamorphism during the final collision at 1 85 1 80 ga the prominent paleoproterozoic magmatic records in the ncc do not support the proposal of global plate tectonic shutdown in the siderian and confirm vigorous convergent margin magmatism and crust building processes throughout the paleoproterozoic

paleoproterozoic structural evolution of the southern segment of the jiao liao ji belt north china craton abstract the paleoproterozoic jiao liao ji belt separates the eastern block of the north china craton into two small sub blocks the northern longgang and the southern rangrim blocks however it still remains unknown or controversial about the subduction polarity collisional deformation and kinematics between two sub blocks the southern segment of the belt consists of the paleoproterozoic fenzishan and jingshan groups and paleoproterozoic high pressure mafic granulites and serpentinites blocks which are located in the jiaodong complex all of which are separated from the jiaodong complex of neoarchean ttg gneisses by stz1 ductile shear zones structural analysis in this study indicates that most of the rocks in all the units of the southern segment of the jiao liao ji belt underwent three distinct episodes of folding d1 to d3 and two stage of ductile thrust shearing stz1 coeval to d1 and d2 stz2 between d2 and d3 the d1 deformation formed penetrative axial planar foliations s1 bedding parallel ductile shear zone mineral stretching lineations l1 and rarely preserved small isoclinal d1 folds in the jingshan and fenzishan groups in the jingshan group however penetrative deformational transposition resulted in stacking of sedimentary compositional layers which are separated by bedding parallel ductile shear zones stz1 at a period of about 1956 ma to 1914 ma the kinematic indicators of stz1 in the jingshan group with resultant prograde peak metamorphism up to granulite facies grade and the fenzishan group with peak metamorphism up to amphibolite facies grade indicate nw directed compression d2 resulted in crustal thickening with retrograded medium pressure granulite facies grade at about 1914 1893 ma the d2 deformation produced nw verging asymmetric and recumbent folds interpreted to have resulted from basement involved thicken skin structures the jiaodong complex was also involved into the development of wnw verging asymmetric tight folds associated with d2 in the jingshan and the fenzishan groups ongoing collision led to the development of transpressional ductile shearing stz2 forming the transpressional taipingzhuang dextral ductile shear zone between the jingshan group and the southern archean complex and the transpressional tading xiadian sinistral ductile shear zone between the jingshan group and the northern archean complex all three lithotectonic units were superposed during the late d3 deformation with amphibolite facies metamorphism the d3 deformation developed wnw trending open to tight upright folds at about 1893 1875 ma the structural pattern resulting from superimposition of d2 and d3 is a composite synform in the fenzishan and jingshan groups the structural events of d1 and stz1 and d2 and stz2 deformation were possibly responsible for fast syn collisional exhumation of the high pressure mafic granulites the structural patterns and deformational history of the fenzishan and jingshan groups suggest a southeastward directed oblique subduction beneath the northwestern margin of the rangrim block and that the final scissor shaped closure of the rift led to collision between the two blocks to form the coherent basement of the eastern block of the north china craton

geochronology and geochemistry of metamorphic rocks in the jiaobei terrane constraints on its tectonic affinity in the sulu orogen abstract tectonic affinity of tectono lithological units close to ultrahigh pressure metamorphic belt is a key issue for understanding the geodynamics of continental collision this is particularly so for the jiaobei terrane northeast of the dabie sulu orogenic belt in china new data from la icpms zircon u pb dating whole rock elements and nd sr isotopes and mineral o isotopes are presented for metamorphic rocks from this terrane the results place geochronological and geochemical constraints on their protolith nature and metamorphic timing and thus on its tectonic affinity to one of the two triassic collided continents the north and the south china blocks protolith ages for ttg gneiss amphibolite and mafic granulite are 2 7 2 5 and 2 4 ga respectively regional metamorphism took place extensively at 1 76 ga protolith of the ttg gneiss was generated by partial melting of mantle derived rocks at the root of a thickened crust protolith of the amphibolite was probably a product of arc like magmatism protolith of the mafic granulite was derived from a depleted mantle source both of them were locally contaminated by supracrustal materials protoliths of paragneiss and schist in the fenzishan group were mostly derived from supracrustal sources but protolith of amphibolite in the fenzishan group is of mantle derived signature unlike the uhp metaigneous rocks in the dabie sulu orogenic belt that show unusual 18 o depletion the jiaobei metamorphic rocks have basically preserved their original mantle like o isotope compositions in general the nature and timing of geological events recorded in the metamorphic rocks from the jiaobei terrane are comparable with those from the north china block rather than the south china block thus the jiaobei terrane is concluded to have tectonic affinity to the former but behave like a micro continent during the triassic continental collision the 1 76 ga regional metamorphism in the jiaobei terrane is likely related to reworking of the arc continent collisional orogen in the periphery of the north china block rather than the 1 85 ga collision event between the eastern and western north china blocks the present study lends support to the common assumption that the suture boundary between the north and south china blocks in the sulu orogen is located along the wulian yantai fault tectonic mingling along the wulian yantai fault is probably related to subduction erosion during the continental collision

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

time dependent failure of rock under cyclic loading abstract a simple model which describes the failure of brittle rock under cyclic compressive loading is presented it is assumed that the damage incurred by the material during each stress cycle is due to the extension of tensile microcracks and that the specimen will fail when the microcrack damage reaches a critical level two mechanisms for crack extension stress corrosion and cyclic fatigue are assumed to act simultaneously and independently the rate of crack growth due to stress corrosion was determined from an empirical relationship for the static fatigue of glass the equation describing the rate of crack growth due to cyclic fatigue was taken to be similar to those derived for structural metals by combining the crack extension rates due to these two processes the time to failure for a specimen subjected to a given cyclic loading program was computed the time to failure was found to depend strongly on the mean differential stress and the magnitude of the stress change during each cycle at high mean stress and low cycle amplitude stress corrosion dominates whereas at low mean stress and high cycle amplitude cyclic fatigue dominates the predictions of the model are in good agreement with existing experimental data further the model explains the observed breakdown of discrete memory in dilatant rock under certain stress conditions

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

phanerozoic high pressure eclogite and intermediate pressure granulite facies metamorphism in the gyeonggi massif south korea implications for the eastward extension of the dabie sulu continental collision zone abstract petrological analysis zircon trace element analysis and shrimp zircon u pb dating of retrogressed eclogite and garnet granulite from bibong hongseong area sw gyeonggi massif south korea provide compelling evidence for triassic 231 4 3 3 ma high pressure hp eclogite facies m1 metamorphisms at a peak pressure temperature p t of ca 16 5 20 0 kb and 775 850 c this was followed by isothermal decompression itd with a sharp decrease in pressure from 20 to 10 kb and a slight temperature rise from eclogite facies m1 to granulite facies m2 followed by uplift and cooling granitic orthogneiss surrounding the baekdong garnet granulite and the ophiolite related ultramafic lenticular body near bibong records evidence for a later silurian 418 8 ma intermediate high pressure ihp granulite facies metamorphism and a prograde p t path with peak p t conditions of ca 13 5 kb and 800 c k ar ages of biotite from garnet granulites amphibolites and granitic orthogneisses in and around the bibong metabasite lenticular body are 208 219 ma recording cooling to about 310 c after the early triassic metamorphic peak neoproterozoic zircon cores in the retrogressed eclogite and granitic orthogneiss provide evidence that the protoliths of these rocks were 800 and 900 ma old respectively similar to the ages of tectonic episodes in the central orogenic belt of china this and the evidence for triassic hp uhp metamorphism in both china and korea is consistent with a regional tectonic link within northeast asia from the time of rodinia amalgamation to triassic continent continent collision between the north and south china blocks and with an eastward extension of the dabie sulu suture zone into the hongseong area of south korea

u pb dating and hf isotope study of detrital zircons from the zhifu group jiaobei terrane north china craton provenance and implications for precambrian crustal growth and recycling abstract the zhifu group exposed in the northeastern jiaobei terrane is subdivided from base to top into the laoyeshan bingying and dongkou formations and comprises mainly quartzites and muscovite and tourmaline bearing quartz schists in order to define its provenance and depositional age and to constrain the crustal growth and recycling of the source regions we have conducted la icp ms u pb dating and in situ hf isotopic study of detrital zircons from three quartz schists samples collected from the zhifu group most of the analyzed zircon grains have oscillatory zoning and th u ratios 0 4 suggesting igneous origins 270 detrital zircons u pb analyses yield u pb ages of 1709 3679 ma that cluster into three major age populations of 1 7 1 9 ga with a peak at 1844 ma 1 9 2 0 ga with a peak at 1936 ma and 2 4 2 55 ga with a peak at 2465 ma and two minor age populations of 2 2 2 35 and 2 6 2 7 ga the peaking age of 1844 ma for the youngest age population indicates that the zhifu group must be deposited after 1844 ma detrital zircons with ages of 1 7 2 35 ga can be linked to pre and post tectonic polyphase magmatic events along the jljb and most of the zircons have negative hf t values from 1 76 to 18 02 and two stage hf model ages t dm2 concentrating at 2 7 3 0 ga indicating that source rocks of these zircons were derived from recycling of ancient crusts detrital zircons with ages of 2 4 2 9 ga may have been sourced from the archean granitoid mainly ttg gneisses in the jiaobei terrane and these zircons have hf t values from 6 4 to 7 53 and t dm2 of 2 6 3 4 ga peak at 2 7 3 0 ga suggesting that source rocks of these zircons were mainly derived from juvenile crusts associated with a major crustal growth at 2 7 3 0 ga and a limited amount of ancient crustal component the two detrital zircons with ages of 3631 and 3679 ma have negative hf t values of 4 42 and 1 51 and t dm2 of 4137 and 4000 ma respectively implying the recycling of ancient continental crust 3 6 ga and crustal growth prior to 4 1 ga in the ncc

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

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

epma u th pb monazite and shrimp u pb zircon geochronology of high pressure pelitic granulites in the jiaobei massif of the north china craton the jiaobei massif constitutes the southern segment of the paleoproterozoic jiao liao ji belt in the eastern block of the north china craton and contains high pressure hp mafic and pelitic granulites the hp pelitic granulites contain four distinct mineral assemblages garnet kyanite biotite muscovite rutile m1 garnet kyanite ternary feldspar biotite muscovite rutile m2 garnet sillimanite biotite perthite anti perthite m3 and biotite muscovite sillimanite plagioclase m4 of which m2 is a typical hp pelitic assemblage the origin of these hp granulites is important for understanding the tectonic evolution of the jiao liao ji belt but the timing of the hp event still remains unknown this study applied the epma u th pb monazite and shrimp u pb zircon dating techniques to determine the metamorphic ages of the hp pelitic granulites in the jiaobei massif cl images and th u ratios show that all monazite and zircon grains in the analyzed samples are of metamorphic origin eighty two epma analyses on 16 monazite grains from two samples yielded three age groups at 1895 to 1882 ma 1814 to 1813 ma and 1706 to 1692 ma and the ages of 16 shrimp analyses on 14 zircon grains from one sample range from 1864 to 1803 ma the oldest ages of 1895 to 1882 ma were yielded from high y monazite grains enclosed in garnet and sillimanite pseudomorph after kyanite and thus interpreted as the time of the hp m2 granulite facies metamorphism the epma monazite ages of 1814 to 1813 ma and shrimp zircon ages of 1864 to 1803 ma are interpreted as the approximate ages of the post hp granulite facies metamorphism m3 most probably resulting from the exhumation of the hp granulites to the medium pressure granulite facies levels the youngest age group of 1706 to 1692 ma obtained from low y monazite grains in the matrix can be interpreted as the age of late cooling and retrograde metamorphism m4 that occurred when the hp pelitic granulites were exhumed to the upper crust the presence of paleoproterozoic hp mafic and pelitic granulites in the southern segment of the jiao liao ji belt suggests that the evolution of the jiao liao ji belt in the eastern block of the north china craton must have been involved in subduction or collision related tectonic processes

paleoproterozoic and triassic metamorphic events in the jiaobei terrane jiao liao ji belt china hidden clues on multiple metamorphism and new insights into complex tectonic evolution abstract the laixi complex located on the margin between the jiao liao ji orogenic belt jljb and the sulu ultra high pressure uhp belt preserves the records of complex tectonic evolution of both belts from the paleoproterozoic to the early middle triassic with an outcrop area of ca 240 km2 the complex is dominantly composed of metasedimentary rocks including grt ky st schists grt mus schists mus qtz schists quartzites and marbles intruded by amphibolite dykes and mus bearing granitic gneisses here we present an integrated study of petrography metamorphic paragenesis mineral inclusions and cathodoluminescence cl features of zircon grains together with u pb ages and muscovite ar ar geochronology our results constrain the depositional age of the metasedimentary rocks as ca 2 1 1 96 ga although paleoproterozoic magmatic protoliths including ca 1 87 1 85 ga amphibolites and 1 87 1 86 ga granitic gneisses are identified neoproterozoic ca 800 650 ma protolith ages that are common in the sulu uhp rocks is absent in this complex two discrete metamorphic events are distinguished for the first time from the metasedimentary rocks the first episode of granulite facies metamorphism was characterized by a mineral assemblage of grt sil bt kfs pl qtz as inclusions preserved in the metamorphic zircons in contrast the second phase of lower amphibolite facies metamorphism is recorded by grt ky st mus bt kfs pl qtz in the matrix of the host rocks metamorphic zircon u pb and muscovite ar ar dating constrains the two metamorphic events in the laixi complex as paleoproterozoic ca 1 94 1 88 ga and early middle triassic ca 252 244 ma the metamorphic units in the laixi complex show similar features of lithology geochronologic and metamorphic evolution to that of the paleoproterozoic jiaobei terrane jbt in the southwestern jljb and are clearly distinguished from the sulu uhp rocks we therefore consider the laixi complex to be an integral component of the jbt during the collisional orogeny between longgang and langrim blocks in paleoproterozoic the early middle triassic lower amphibolite facies metamorphic event identified for the first time in the laixi complex coincides with that 249 230 ma of the adjacent jiaodong terrane in the northeastern sulu uhp belt these findings suggest that the laixi complex was also involved in triassic collision subduction between the north china and south china blocks and experienced low grade metamorphism at the middle crustal level combined with the observations in previous studies we propose a multiphase metamorphic belt extending at least 300 km from yantai through laixi to wulian parallel to the yantai qingdao wulian fault

into the forbidden zone very low temperatures at high pressures have long been believed to constitute a forbidden zone not realized in earth however recent studies of ultrahigh pressure uhp terranes indicate that these rocks have experienced such forbidden zone conditions liou et al discuss the implications of these observations for plate tectonics

microblock amalgamation in the north china craton evidence from neoarchaean magmatic suite in the western margin of the jiaoliao block abstract the archaean earth is considered to have been characterized by microcontinents that formed dominantly through the accretion of oceanic arcs and plateaus the north china craton ncc provides a typical case where at least seven ancient microcontinental nuclei with distinct lithological features and independent tectonic histories were amalgamated into the cratonic framework at the end of the archaean here we investigate a suite of magmatic rocks developed at the periphery of one of these microblocks the jiaoliao block that forms part of the composite eastern block of the ncc we present petrological geochemical and zircon u pb geochronological data from the taipingzhai charnockite suite and associated amphibolites metagabbros and orthogneisses from the qianxi complex geochemically the rocks show a wide range of sio 2 charnockite suite 52 57 75 50 wt metagabbro 43 71 wt amphibolite 50 24 wt garnet bearing biotite 63 73 wt and mgo charnockite suite 0 89 5 01 wt metagabbro 3 99 wt amphibolite 6 23 wt garnet bearing biotite 2 08 wt the composition of the felsic units straddle from diorite through syeno diorite to granite with both alkalic and subalkalic affinity with dominantly magnesian composition and arc related features their immobile trace element relationships suggest calc alkaline affinity they show positive pb ba la nd and gd and negative nb ta sr th and ti anomalies with slightly negative anomalies of ce and y attesting to arc related features in tectonic classification diagrams the rocks plot in the vag syn colg field or the vag area suggesting subduction related origin the dominant population of zircons in all these rocks displays magmatic crystallization features including high th u values with core rims textures indicating subsequent thermal events the zircon u pb data yield upper intercept ages of 2587 10 ma to 2543 17 ma and 207 pb 206 pb mean ages of 2578 7 3 ma to 2536 8 ma for the charnockite suite marking the timing of emplacement of the arc magmas the overgrowth rims as well as discrete neoformed grains are interpreted as dating subsequent metamorphism and yield 207 pb 206 pb ages between 2533 ma to 2490 ma zircons in the metagabbro preserve upper intercept ages of 2556 20 ma representing the crystallization age of this rock the younger ages of 2449 58 ma upper intercept age and 1845 25 ma 207 pb 206 pb spot age are interpreted to represent subsequent multiple thermal events in this area zircons in the amphibolite preserve the 207 pb 206 pb mean age of 2539 9 ma representing the crystallization age of this rock the garnet bearing biotite gneiss shows an upper intercept age of 2562 10 ma mswd 0 66 n 36 and the 207 pb 206 pb mean age of 2561 9 ma mswd 0 63 n 33 which is taken to represent the crystallization age of this rock some inherited zircons are also identified with 207 pb 206 pb ages of 2664 26 ma and 2628 26 ma zircon lu hf data show dominantly positive ehf t values and combined with crustal residence ages the results suggest mesoarchean to neoarchean juvenile crust formation in the ncc we interpret the data presented here to represent a phase of major late neoarchaean arc magmatism along the western margin of the jiaoliao block related to the birth of microcontinental nuclei within the ncc our data suggest that the western and eastern blocks might not have existed as discrete crustal blocks and that the construction of the ncc is a result of the assembly of several microblocks or terranes at the end of archaean similar archean cratonic nuclei in other regions of the world might have formed part of a primitive supercontinent in the early earth

2 7ga juvenile crust formation in the north china craton taishan xintai area western shandong province further evidence of an understated event from u pb dating and hf isotopic composition of zircon abstract the evolution of the north china craton ncc is well known for a marked 2 55 2 50 ga tectonothermal event however supracrustal and intrusive rocks of 2 75 2 70 ga are in fact widely distributed in the western shandong province the most important area of archaean basement in the eastern part of the ncc this paper reports shrimp u pb dating and la icpms hf isotopic composition of zircons from 2 75 2 70 ga supracrustal and trondhjemite tonalite granodiorite ttg rocks in that area three fine grained hornblende biotite gneiss samples known locally as leptynite with meta volcanic or volcanosedimentary rock protoliths and five ttg samples have shrimp zircon u pb ages varying from 2 75 to 2 70 ga and 2 74 to 2 71 ga respectively zircons from most of the samples have high positive hf t values 4 7 to 10 0 and t dm hf ages 2 85 2 60 ga similar to their zircon u pb ages this indicates that the rocks represent largely juvenile crustal additions derived from depleted mantle only a short time before however some granitoids show hf t zircon values of 13 6 to 5 1 and t dm hf of 3 51 2 80 ga therefore the strong 2 75 2 70 ga tectonothermal event in the western shandong province involved not only juvenile addition to the continental crust but also intracrustal recycling of older components combined with craton wide data it is shown that the ncc is similar to many other cratons around the world where tectonothermal events of 2 7 ga are well developed however the main difference is that in the ncc superimposed 2 5 ga tectonothermal events were much stronger than in most other cratons

a comparison of u pb and hf isotopic compositions of detrital zircons from the north and south liaohe groups constraints on the evolution of the jiao liao ji belt north china craton the jiao liao ji orogenic belt is one of the important paleoproterozoic mobile belts in the north china craton ncc and its central segment consists of the north and south liaohe groups and the liaoji granitoids the north and south liaohe groups consist of sedimentary and volcanic successions metamorphosed from greenschist to lower amphibolite facies and the liaoji granitoids are divisible into 2 2 2 1 ga pre tectonic monzogranitic gneisses and 1 88 1 85 ga post tectonic porphyritic monzogranites and alkaline syenites combined with cathodoluminescence cl imaging we report la icp ms laser ablation inductively coupled plasma mass spectrometry u pb and hf isotopic data for detrital zircons from the north and south liaohe groups zircons grains from both groups have igneous zircon cores that yielded two u pb age populations at 2 0 2 2 ga major and 2 5 ga minor indicating that their provenances were similarly dominated by paleoproterozoic liaoji granitoids and late archean basement rocks some structureless zircon rims in both groups yield concordant or upper intercept ages 1 9 ga interpreted as the peak metamorphic age of the two groups these results suggest that both groups were deposited in the period 2 0 1 9 ga shortly after the emplacement of the liaoji granitoids moreover zircons from both groups have similar hf model age peaks t c dm peaks at 2 66 2 75 ga and 2 80 3 00 ga

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

discovery of neoarchean suprasubduction zone ophiolite suite from yishui complex in the north china craton abstract the archean tectonic realm of the north china craton ncc is considered in recent models as a collage of several microblocks which were amalgamated along zones of ocean closure during late neoarchean here we report the finding of a dismembered ophiolite suite from the southern margin of the jiaoliao microblock in the interior of the unified eastern block of the ncc the suite is composed of lherzolite pyroxenite noritic and hornblende gabbro and hornblendite intruded by veins and sheets of leuco granite together with transposed layers and bands of metavolcanics and amphibolites banded iron formation bif and diabase dykes in the adjacent locations the yishui complex corresponds well with a dismembered suprasubduction zone ophiolite suite clinopyroxene in the pyroxenite and gabbroic rocks is mg rich and range in composition from augite to diopside among orthopyroxenes those in lherzolite show the highest xmg of 0 84 0 85 plagioclase in hornblende gabbro shows high anorthite content an50 64 calcic amphiboles in the gabbroic rocks range in composition from ferropargasite to ferro edenite edenite and pargasite spinel inclusions in lherzolite are cr rich magnesiospinel geochemically the mafic rocks from yishui complex show subalkaline basaltic source whereas the granitoids show volcanic arc affinity the hornblende gabbro and gabbro lherzolite and hornblendite show compositional similarity to e morb and n morb respectively the lherzolite and hornblendite possess arc related ultramafic cumulate nature with overall features straddling the fields of iat and iat morb the geochemical features are consistent with evolution in a suprasubduction regime with no significant crustal contamination the majority of zircon grains in the yishui suite exhibit magmatic texture and high th u ratios zircon grains from hornblendite define 207pb 206pb upper intercept age of 2538 30 ma zircons from the granite show ages of 2538 16 ma and 2503 21 ma and those from the gabbros yield ages of 2503 16 ma and 2495 10 ma the well defined major age peak at 2500 ma is broadly coeval with neoarchean ages reported from the microblocks in the north china craton the zircon lu hf data from the yishui suite display ehf t values between 2 5 and 5 0 with corresponding model ages suggesting magma derivation from neoarchean juvenile sources together with limited reworked paleo mesoarchean crustal components our study is the first report of neoarchean suprasubduction type ophiolites from a locality far from the margins of the major crustal blocks and suture zones in the ncc and strengthens the concept that the craton is a mosaic of several microblocks with intervening oceans that closed along multiple subduction zones we envisage that the amalgamation between the xuhuai and the jiaoliao microblocks resulted in the accretion of the yishui suprasubduction zone ophiolitic assemblages onto the southern margin of the jiaoliao microblock the neoarchean microblock amalgamation in the north china craton provides new insights into continental growth in the early earth and confirms that modern style plate tectonics might have been initiated early in the history of our planet

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

paleozoic ages and excess 40ar in garnets from the bixiling eclogite in dabieshan china new insights from 40ar 39ar dating by stepwise crushing abstract the 40 ar 39 ar stepwise crushing technique is applied for the first time to date garnet from ultra high pressure metamorphic uhpm eclogites three garnet samples from the bixiling eclogites analyzed by 40 ar 39 ar stepwise crushing yield regular predictable age spectra and a clear separation between excess 40 ar and concordant plateau and isochron ages all three age spectra begin with high apparent ages followed by step by step decreasing ages and finally age plateaux with apparent ages in the range from 427 20 to 444 10 ma the data points constituting the age plateaux yield excellent isochrons with radiogenic intercept ages ranging from 448 34 to 459 58 ma corresponding to initial 40 ar 36 ar ratios from 292 1 4 5 to 294 5 6 7 statistically indistinguishable from the modern air the high initial ages are interpreted to derive from secondary fluid inclusions containing excess 40 ar whereas the plateau ages are attributed to gas from small primary fluid inclusions without significant excess 40 ar the plateau ages are interpreted to approximate the time of garnet growth during initial uhpm metamorphism phengite analyzed by laser stepwise heating yielded a complicated two saddle age spectrum with a scattered isochron corresponding to age of 463 116 ma and initial 40 ar 36 ar ratio of 1843 1740 indicative of the presence of extraneous 40 ar within phengite these concordant isochron ages measured on minerals diagnostic of eclogite grade metamorphism strongly suggest that dabie uhpm eclogites were first formed in the early paleozoic during the same event that caused the qinling northern qaidam basin altyn tagh eclogites

exhumation tectonics of the ultrahigh pressure metamorphic rocks in the qinling orogen in east china new petrological structural radiometric insights from the shandong peninsula 1 in eastern china the sulu area is recognized as the eastern extension of the qinling dabie belt which is famous for its ultrahigh pressure uhp metamorphism although numerous petrologic and geochemical works are available structural data are still rare this paper provides the first extensive study of bulk geometry and kinematic analysis of the shandong peninsula the study area is divided into three tectonic areas by cretaceous faults namely a southern uhp belt or sulu area a northern migmatite area and an eastern eclogite and migmatite area or weihai area conversely to the deeply entrenched idea that the later area belongs to the north china belt and the two others to the south china block scb we argue that all three areas are parts of the scb structural petrologic 40ar 39ar and u pb data comply with this new interpretation in the north shandong area mafic granulites enclosed as blocks within gneissic migmatites do not significantly differ from the sulu and weihai eclogites which also experienced a granulite facies overprint before migmatization the circa 210 200 ma age of the main ductile deformation is related to an extensional event during the triassic or indosinian orogeny this date corresponds to the temperature climax but the time of the pressure peak i e the real age of the uhp metamorphism is discussed