detrital zircon u pb dating of low grade metamorphic rocks in the sulu uhp belt evidence for overthrusting of the north china craton onto the south china craton during continental subduction the shiqiao pingshang low grade metasedimentary rocks sporadically crop out in the sulu ultrahigh pressure uhp belt in east central china major and trace element data indicate that they were deposited in a passive margin basin probably located at the southern margin of the north china craton sensitive high resolution ion microprobe shrimp u pb dating of detrital zircons from a quartzite sd53 from pingshang and a quartz schist sd54 from shiqiao records ages ranging from 2800 to 1900 ma three age populations are defined at 2660 2500 2350 2150 and 2100 1900 ma with peak ages at 2522 2212 and 2020 ma respectively muscovite ar ar dating of the quartz schist sd54 yields a plateau age of 265 9 1 2 ma these data support the view that the shiqiao pingshang low grade metasedimentary rocks originated from the north china craton and underwent deformation in the permian during subduction of the south china craton yangtze block beneath the north china craton this requires that the low grade metasedimentary rocks were overthrust for several tens of kilometres onto the south china craton explaining why source materials from the north china craton are common in triassic to jurassic sedimentary basins adjacent to the dabie sulu orogenic belt

pb isotopes of granitoids suggest devonian accretion of yangtze south china craton to north china craton feldspar separates of 30 granitoid intrusions ranging in age from neoproterozoic to mesozoic were analyzed for pb isotopic compositions together with associated precambrian basement whole rocks from the north and south qinling orogenic belts the two belts constitute the southern active margin of the north china craton and the northern passive margin of the yangtze craton respectively proterozoic basement rocks from the north and south qinling belts are clearly distinguished in terms of both present day pb isotopes and calculated initials at the time of granite magmatism with the former having considerably more radiogenic pb granitoids of the south qinling belt irrespective of age and specific granitoid rock type show a fairly homogeneous pb isotopic composition that is similar to that of the south qinling basement in contrast in the north qinling belt the pb isotopic composition of granitoids is correlated with their age neoproterozoic and early paleozoic granitoids have radiogenic pb isotopic compositions like those of the north qinling proterozoic basement rocks but late paleozoic and mesozoic granitoids show significantly less radiogenic pb isotopic compositions similar to those of the south qinling basement rocks the unusual change is interpreted to reflect underthrusting of the south qinling belt beneath the north qinling belt at ca 360 ma the process resulted in a change of the source for younger north qinling granitoids from the radiogenic north qinling to the less radiogenic south qinling basement rocks thus the yangtze craton accreted to the north china craton no later than 360 ma

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

nd and sr isotopes from diamondiferous eclogites udachnaya kimberlite pipe yakutia siberia evidence of differentiation in the early earth nd and sr isotopic data from diamond bearing eclogites found in the udachnaya kimberlite yakutia siberia are interpreted as indicating an early 1 4 ga differentiation event whereby the mantle split into complementary depleted and enriched reservoirs reconstructed whole rock 87sr s6sr ratios present day range from 0 70151 to 0 70315 and are consistent with a mantle origin for these rocks the nd isotopic evolution lines of four samples u 5 u 37 u 41 and u 79 converge at 2 2 2 7 ga sample u 5 is unique in exhibiting the most enriched signature of any of the samples yet analyzed present day end of 20 and this sample points unequivocally to an old enriched component a complementary depleted mantle component is suggested by two of the eclogite samples u 86 and u 25 which yield end values at 2 2 ga of 13 and 7 respectively the two mantle reservoirs possibly formed prior to 4 ga and evolved separately until 2 2 2 7 ga at that time the reservoirs were melted forming eclogites both as residues from the enriched reservoir and as partial melts of peridotite from the depleted reservoir resulting in demonstrably different histories for eclogites from the same locality

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

upb zircon age and geochemical constraints on tectonic evolution of the paleozoic accretionary orogenic system in the tongbai orogen central china abstract the tongbai orogen is located in a key tectonic position linking the qinling orogen to the west and the dabie sulu orogen to the east this orogen comprises a paleozoic accretionary orogenic system in the north and a permo triassic collisional orogenic system in the south hence it may serve as an ideal place to unravel the tectonic evolution from the initial oceanic subduction accretion to the final continental collision between the sino korean and yangtze cratons u pb zircon geochronology of metasedimentary and metaigneous rocks and geochemical characterization of metabasalts indicate a close genetic relationship between the individual tectonic units of the paleozoic accretionary orogenic system 1 the kuanping group as a neoproterozoic ordovician subduction accretion complex which received sediments from the qinling microcontinent 2 the erlangping group and huanggang diorite granodiorite complex as an ordovician silurian island arc evolving from backarc basin 3 the qinling group as a precambrian microcontinent converted into continental arc during the ordovician 4 the guishan complex as a mixture of the qinling continental arc and devonian forearc sediments and 5 the nanwan flysch as a devonian forearc sequence deposited on the newly accreted active margin of the sino korean craton consequently the northern and southern boundaries of the paleozoic accretionary orogenic system represent two sutures of the qinling microcontinent with the sino korean and yangtze cratons the new and previously published data reveal that the tectonic evolution of the tongbai orogen involved a series of events from ordovician early silurian ca 490 440 ma oceanic subduction and arc magmatism silurian early devonian ca 440 400 ma arc continent collision carboniferous ca 340 310 ma oceanic subduction and accretion late permian triassic ca 260 200 ma continental subduction and collision to cretaceous ca 140 90 ma extension and lateral eastward extrusion the entire accretionary orogenic processes until the final collision between the sino korean and yangtze cratons may have lasted more than 200 ma

multi stage metasomatism of diamondiferous eclogite xenoliths from the udachnaya kimberlite pipe yakutia siberia the primary garnet pyrope almandine omphacite cpx 1 6 5 7 wt na2o sulfide fe ni co mss assemblage of the two diamondiferous eclogite xenoliths studied u33 1 and ux 1 experienced two mantle metasomatic events the metasomatic event i is recorded by the formation of platy phlogopite 10 wt k2o prior to incorporation of the xenoliths in the kimberlite the bulk of the metasomatic alteration consisting of spongy textured clinopyroxene cpx 2a 1 3 wt na2o coarser grained clinopyroxene cpx 2b 2 5 wt na2o pargasitic amphibole 0 8 wt k2o 3 3 5 wt na2o kelyphite cpx 3 mostly 1 wt na2o and zoned mg fe al spinel sodalite calcite k feldspar djerfisherite k5 95na0 02fe18 72ni2 36co0 01cu4 08s26cl and a small amount of k ca fe mg glass is ascribed to the metasomatic event ii that occurred also in the upper mantle but after the xenoliths were incorporated in the kimberlite a pervasive chloritic alteration mainly clinochlore magnetite that overprints earlier assemblages probably took place in the upper crustal environment the diamonds are invariably associated with secondary clinopyroxene and chlorite but the diamonds formed before the entrainment of the xenoliths in the udachnaya kimberlite

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

eclogites with oceanic crustal and mantle signatures from the bellsbank kimberlite south africa part 2 sr nd and o isotope geochemistry the sr nd and o isotopic compositions of garnet and clinopyroxene mineral separates from nine eclogite xenoliths from the bellsbank kimberlite erupted at 120 ma define three groups group a eclogites considered to be mantle cumulates are characterized by 6180 and 87sr srsr values typical of mantle derived materials 5 1 to 5 6 o and 0 7042 0 7046 respectively and very low sm nd ratios apparent rb sr ratios and nallz01 values 0 057 0 078 0 00005 0 00136 and 14 to 16 respectively the ree and isotopic data for these eclogites can be modelled in terms of crystallization from a group ii kimberlite magma at 1 1 5 ga group b and c eclogites believed to be the metamorphosed products of ancient subducted oceanic crust are characterized by low 3180 2 9 to 4 7 extremely high nd a20 40 to 219 and radiogenic 87sr s6sr ratios 0 708 0 710 the sm nd ratios of the group b eclogites are very high up to 1 6 the data for group b and c eclogites define a linear correlation on sm nd and 1 nd vs nal 201 diagrams these relationships are consistent with mixing of the bellsbank kimberlite cno 1201 10 sm nd 0 10 with a depleted eclogite end member end 120 219 sm nd 1 6 during a cryptic metasomatic event the sr isotopic variations in group b and c eclogites cannot be generated by simple two component mixing the st nd and o isotope data for group b and c eclogites probably reflect a complex sequence of depletion and enrichment events in both crust and mantle settings enrichments which possibly affected the group b and c eclogites include seawater alteration of a morb like protolith which lowered the 6180 and raised the svsr 86sr ratio but left the nd isotopic compositions unchanged and cryptic metasomatism by the magmatism that produced the bellsbank kimberlite the high sm nd ratio of the depleted eclogite end member cannot be generated by extraction of a melt from a modern morb composition rather it is argued that such high sm nd ratios are produced as a result of partitioning during the recrystallization of a morb like component to eclogite during subduction nd model ages suggest that this process occurred 2 3 2 4 ga

neocrystallization or cooling 40ar 39ar ages of white micas from low grade mylonites abstract 40 ar 39 ar data for white micas from quartzite mylonites from a thrust system are evaluated with respect to whether the minerals record cooling ages or alternatively ages of deformation induced neocrystallization to address these interpretations three types of information are needed 1 the effective diffusion length scale controlling volume diffusion of argon in white micas 2 an estimate of the diffusion parameters for volume diffusion of argon and 3 the thermal history completely recrystallized quartzites exhibit smooth grain size distributions for white micas with good exponential or power law fits suggesting that the grains grew under similar conditions the microstructures indicate that the grains grew over a narrow range in time near the end of the deformation history measured grain size distributions provide a maximum limit for the diffusion length scale thermal histories are estimated by 40 ar 39 ar diffusion experiments on k feldspars using the multi diffusion domain method of lovera lovera o m richter f m harrison t m 1989 the 40 ar 39 ar thermochronometry for slowly cooled samples having a distribution of diffusion domain sizes j geophys res 94 17917 17935 these results are consistent with the expected temperature time path for the thrusting environment and predict maximum temperatures of 250 350 c during the time span recorded by the white micas experimentally and empirically calibrated diffusion parameters for white micas are applied to a cylindrical diffusion geometry to explore the question of cooling ages versus neocrystallization ages forward modeling of the passage of the white micas through the closure window for argon diffusion using the thermal histories derived from the k feldspars indicates that the white micas record neocrystallization rather than cooling and supports the idea that the white micas record the timing of when ductile deformation ceased

40ar 39ar thermochronology from the northwestern dabie shan constraints on the evolution of qinling dabie orogenic belt east central china abstract seven structural units with distinct kinematics have been recognized in the northwestern dabie shan 114 e 115 e and 31 n 32 n east central china they are from north to south 1 the north verging mafan fold belt mfb involving early paleozoic metamorphic rocks 2 the liangting melange belt lmb which is separated from the mfb by a south dipping thrust and consists of gneissic and marble blocks surrounded by highly sheared greenschist 3 the north verging laoshan ductile fold belt lfb which was thrust northward over the liangting melange belt and involves proterozoic schists 4 the suhe thrust sheet sts which consists of phyllite and slate above a folded thrust fault dipping both to the north and south 5 the 5 8 km wide huwan shear belt hsb in the footwall of the folded thrust consisting of mylonitic gneisses and sheared ecologite 6 the kafang eclogite bearing gneiss dome kgd which is defined by broadly warped foliations and 7 the 10 km wide pendian ductile shear belt psb characterized by dextral strike slip deformation nine phengite and muscovite samples in eclogites mylonitic shear zones and metamorphic gneisses from four of the seven structural units have been dated by the 40ar 39ar step heating method phengites from deformed eclogites in the hsb yield complex age spectra that we interpret as most likely indicating closure with respect to ar loss between 350 and 430 ma the rock samples collected from the eclogite bearing gneiss display a significant static recrystallization of quartz which suggests temperatures in excess of ca 350 400 c the ages of 350 430 ma from samples 9669b and 9669c are therefore interpreted as retrograde metamorphic ages for the eclogites six additional phengites sampled from mylonitic rocks yield two groups of ages 263 249 and 236 214 ma the older group samples 96 5 9667 and 9692 were collected from mylonitic rocks with top to north shear sense in the lmb hsb and kgd respectively 9667 and 9692 exhibit a strong static recrystallization which means that ductile deformation took place at a temperature in excess of 350 400 c we therefore adopt the age of 263 249 ma as the cooling ages of white micas considering sample 96 5 shows microfabrics formed at lower temperature around or slight below the closure temperature of white mica there is a strong possibility that the age of 254 1 ma represents a crystallization age rather than a cooling age because these samples are taken from the top to north shear zones we also interpret the 263 249 ma event as a result of thrusting induced denudation the younger group samples 96106 96124 and 96113 collected from dextral shear belts in the hsb and psb exhibit a strong static recrystallization at a temperature in excess of 350 400 c their ages 236 214 ma are therefore interpreted as the cooling ages related to later dextral strike slip faulting sample 96101 collected from weakly foliated rock without penetrative deformation on the north limb of the kgd shows poorly recovered quartz microfabrics at the temperature around or slightly below the closure temperature of white mica its age of 187 1 ma is therefore interpreted as crystallization age during the downdip detachment event related to doming the 40ar 39ar data from the northwestern dabie shan suggest four distinctive cooling events on the basis of the local structural framework and regional tectonic relationships we suggest that the 410 350 ma cooling event was related to either subduction of an oceanic plate beneath the sino korean plate or collision between the sino korean plate and a microcontinental block in the middle paleozoic the three younger episodes of cooling between the late permian and the late triassic may all have been related to the collision between the sino korean and yangtze plates specifically the 263 249 ma event could have been a result of thrusting induced denudation whereas the 236 214 ma cooling event is likely related to strike slip faulting the younger ages of 195 187 ma obtained from the gneiss dome are interpreted to be a result of regional denudation related to doming during the late stage of collision between the sino korean and yangtze plates

early cretaceous extensional reworking of the triassic hp uhp metamorphic orogen in eastern china corresponding to the early mesozoic continental subduction between the north china block ncb and the south china block scb the tongbaishan hong an dabieshan sulu massifs are famous for their hp uhp metamorphism more than 50 of the hp uhp orogenic belt was significantly reworked by early cretaceous extensional tectonics this early cretaceous event with a fast cooling period at 130 120 ma superimposed on the early mesozoic hp uhp orogenic belt and intensively changed the architecture of this orogen each individual segment documents different early cretaceous extensional structures namely the central tongbaishan domain is a metamorphic core complex mcc represented by an a type non cylindrical antiform the central dabieshan domain is a typical cordilleran type migmatite cored mcc the southern sulu uhp domain is a wedge shaped structure exhumed by a simple detachment fault these late stage extensional structures expose the previous hp uhp orogenic belt as fragments along the ncb scb boundary the geodynamic setting of this early cretaceous extensional tectonics along the hp uhp orogen is a part of a 1000km scale crustal extension belt that is widespread in eastern eurasia continent from trans baikal to the central part of scb convective erosion or delamination of the mantle lithosphere might be considered as a possible mechanism for mantle removal

an empirical evaluation of the argon diffusion geometry in muscovite the temperature time significance of 40ar 39ar muscovite ages is difficult to evaluate quantitatively because of ambiguities in the choice of an appropriate model diffusion geometry the effective diffusion dimension and large uncertainties in experimentally determined diffusion parameters laser 40ar 39ar studies of natural 40ar concentration gradients suggest that argon loss in muscovite occurs primarily via cylindrical volume diffusion commonly with an effective diffusion dimension equal to the crystal diameter the present study tests the cylinder model geometry and the control of physical grain size on the effective diffusion dimension by measuring the anisotropy of a natural40ar diffusion gradient parallel and perpendicular to 001 in a single euhedral muscovite phenocryst the new data combined with previous studies clearly indicate loss of 40ar via cylindrical volume diffusion and strongly suggest that loss of 40ar across the 001 sheets slab diffusion geometry is minimal in undeformed crystals

archean mantle heterogeneity and the origin of diamondiferous eclogites siberia evidence from stable isotopes and hydroxyl in garnet data are presented for the o isotopic composition of clinopyroxene and garnet the c isotopic composition of diamond and the oh content of garnet from eclogite xenoliths brought to the surface by the udachnaya kimberlite pipe yakutia siberia radiogenic isotopic data suggest that the eclogites could have been derived from an ultradepleted mantle at approximately 2 9 ga pearson et al 1995 snyder et al in preparation o isotopic compositions generally show equilibration between the eclogitic minerals cpx grt 0 11 0 41 and have 18 o smow for both garnet and clinopyroxene that lie near the range of accepted mantle values of 5 7 0 7 however several eclogites indicate significant deviations from this range at higher values of 6 8 7 0 also two eclogites lie at the lower end of the mantle range at values of 4 8 and 5 0 all in clinopyroxene c isotopic compositions of diamonds all have 13 c pdb in the range of 1 to 7 and are centered at approximately 5 also within the range of accepted mantle values the oh contents of the garnet are generally between 0 and 22 ppm as h 2 0 although two samples exceed 70 ppm this range of oh is similar to eclogitic garnet from the kaapvaal craton of southern africa the mantle c isotopic values of associated diamonds the majority of o isotopic data and the low oh content of the minerals although suggesting a general lack of crustal participation in the formation of the udachnaya eclogites do not rule out the participation of some ancient crustal material that these eclogites include both 18 o enriched and 18 o depleted types suggests that the protoliths may have been overprinted by both low and high temperature hydrothermal events cf jacob et ai 1994 a positive correlation between 18 o and 87 sr 86 sr allows the interpretation of these eclogites as representing a cross section of an archean ophiolite however the lack of a single coherent grouping on a plot of 18 o vs 87 sr 86 sr suggests that if the udachnaya eclogites were derived from oceanic crust they cannot be cogenetic and must represent at least two separate ophiolite sequences conversely if the eclogites are found to be cogenetic then a totally different process affected their formation and a probable metasomatic mechanism was operative because of the total lack of correlation of 18 o with other geochemical parameters we find no compelling evidence that all eclogites are derived ultimately from oceanic crust

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

nd and sr systematics in an eclogite xenolith from tanzania evidence for frozen mineral equilibria in the continental lithosphere combined isotopic nd sr and petrographic analyses are reported for an eclogitic xenolith from the vissury mine tanzania an exceedingly large difference in measured nd values in garnet and clinopyroxene 200 units suggest a major mineral equilibration at 1 75 0 014 ga nd isotopic heterogeneities are demonstrated in different generations of garnet and zonation in sr and nd isotope compositions is present within the clinopyroxene the variation in measured nd isotope compositions in different generations of garnet corresponds to 16 units exsolution of garnet from clinopyroxene occurred in an essentially closed system with respect to sr and nd petrographically different generations of garnet are present one garnet with rutile inclusions as well as the rims of clinopyroxene are clearly off the isochron features like this can originate either if a system is not closed e g affected by a migrating fluid or if a remanent isotopic heterogeneity survived the homogenisation at 1 75 ga the subsolidus partitioning of sm and nd between garnet and clinopyroxene results in higher fractionation of the smnd ratio in garnet than in the liquid crystal partitioning since the closure temperature for the sm nd exchange between garnet and clinopyroxene may be around 850 c in a dry system many of the lithospheric ultramafic rocks may exhibit frozen mineral equilibria the isotopic data suggest that this eclogite was emplaced at 1 75 ga into the lithosphere mantle at temperatures of about 1040 c and then cooled as a closed system at a rate of 6 50 c my down to 840 c where all the exchange reactions were efficiently frozen in at 1 75 ga ago the coincidence of the emplacement of granites 1 7 1 8 ga into the central part of the tanzanian craton with the age of the eclogite suggests that lower crustal melting was triggered by a thermal event in the upper lithosphere

reos and upb geochronological constraints on the eclogite tonalite connection in the archean man shield west africa abstract mantle derived eclogite xenoliths and tonalite trondhjemite granodiorites ttg that occur in the man shield west africa sample different levels of archean lithosphere chemical and oxygen isotope systematics indicate that low mgo eclogites from the koidu kimberlite are ancient remnants of subducted oceanic crust that may have been involved in regional archean crust formation re os whole rock isotopic data for these eclogites scatter about a line with slope corresponding to an archean age of 3 44 0 76 ga 2 with re os model ages ranging from 1 4 to 3 9 ga this wide range of model ages overlaps with the age range for crust formation and metamorphism in the man shield in situ u pb ages of zircons from crustal rocks have been measured by laser ablation icp ms a tonalitic gneiss has discordant zircons with rare old cores 3 6 ga and an upper concordia intercept at 2890 9 ma 2 zircons from a mafic lower crustal granulite xenolith are concordant at 2686 32 ma our results together with previously published ages for man shield rocks indicate an early archean crust formation event followed by major crustal growth at 2 9 3 0 ga and a last major metamorphic event at 2 7 ga these data show that the eclogites and the continental crust of the west african craton overlap in time of formation but only at the very broad age uncertainty provided by the eclogite re os results they are permissive of archean crustal growth by melting of the protoliths of the materials now sampled as the koidu eclogite xenoliths if so this suggests that archean crustal growth in the man shield occurred in a convergent margin setting

3 d x ray tomography of diamondiferous mantle eclogite xenoliths siberia a review abstract diamonds form over billions of years hundreds of kilometers beneath the earth s surface and in combination with inclusions trapped within provide important constraints on the evolution of the mantle over geological time diamonds are generally studied as individual crystals sourced from highly explosive kimberlite pipes which entrain and subsequently disaggregate mantle fragments xenoliths en route to the surface this has resulted in a general absence of robust textural descriptions of diamonds relative to their hosting mantle protolith the textural associations of diamonds within their mantle host rocks are reviewed here on the basis of a compilation of x ray tomographic data for 17 diamondiferous eclogite xenoliths from siberian kimberlites this review represents a comprehensive description of diamonds relative to their host silicates the lack of such descriptions in previous studies is largely due to the rarity of these xenoliths the difficulty in preparing petrographic thin sections containing diamonds and their high monetary value high resolution computed x ray tomography hrcxt produces up to 1200 sequential 2 d slices through individual xenoliths each of which represents a pseudo thin section with a resolution on the order of 5 20 m the improved resolution of x ray imaging in recent studies allows for the identification of not only primary minerals but metasomatic minerals assemblages including spongy textured clinopyroxene phlogopite k richterite and hercynitic spinel allowing for the delineation of distinct metasomatic pathways through the xenoliths and their relationship to diamonds diamonds are observed in three distinct textural settings potentially representing several temporally distinct diamond growth events these setting includes 1 diamonds completely enclosed in garnet 2 diamonds associated with highly embayed silicate grain boundaries and 3 diamonds contained within distinct metasomatic plumbing systems diamonds observed completely enclosed in garnets suggest an early diamond forming event prior to major re crystallization and eclogite formation during subduction the occurrence of diamond in association with embayed garnets suggests that diamond grew at the expense of the hosting silicate protolith in addition the spatial relationships of diamonds with metasomatic pathways which are generally interpreted to result from late stage proto kimberlitic fluid percolation indicate a period of diamond growth occurring close to but prior to the time of kimberlite emplacement furthermore the paragenesis of sulfides within eclogite xenoliths are described using 3 d models for entire xenoliths volumes providing important constraints of the timing of sulfide mobilization within the mantle three d animations created using x ray tomography data for ten of the xenoliths can be viewed at the following link http eps utk edu faculty taylor tomography php

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

ages of deformation from k ar and 40ar 39ar dating of white micas a structural and isotopic dating study of white micas from mylonites in the ruby gap duplex off central australia shows that mylonitic deformation occurred during the paleozoic alice springs orogeny deformation of the white micas took place under greenschist facies conditions in the approximate temperature range 250 350 c two distinct populations of white mica have been found 1 porphyroclasts of original muscovite and 2 neocrystallized phengites that define the fabric associated with deformation 40ar 39ar age spectra of neocrystallized phengites yield plateau like spectra of paleozoic age in contrast 40ar 39ar age spectra of muscovite porphyroclasts exhibit proterozoic apparent ages indicating that white micas were not open to significant argon diffusion during formation of the ruby gap duplex we conclude that the ages measured on neocrystallized phengites record the cessation of ductile deformation and migration of tectonic activity toward the internal zones of the thrust system

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

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

timing and kinematics of caledonian thrusting and extensional collapse southern norway evidence from 40ar 39ar thermochronology 40ar 39ar dating methods have been applied to rocks across the caledonian orogen in southwestern norway in the eastern nappe area k feldspar thermochronological modeling microfabric characteristics and conodont color alteration together indicate that temperatures were not above the closure temperature of muscovite for any significant period of time two groups of muscovite and biotite ages from this area 415 408 for most samples with top to the se fabrics and 402 394 for samples with top to the nw fabrics are therefore interpreted as ages of contractional thrusting and extensional hinterland directed nappe translation deformation respectively in the west hinterland peak caledonian temperatures were higher and 40ar 39ar plateau ages are generally interpreted as cooling ages the western basement and lower nappes cooled rapidly through 500 c basement at 404 ma and 350 c basement and lower nappes shortly after i e during extensional top to the nw transport of the orogenic wedge in addition tectonostratigraphically higher nappes in the hinterland show evidence of earlier cooling probably following ordovician orogenic activity prior to the main collisional stage the new 40ar 39ar data conform to kinematic observations that contraction and extension in the caledonian nappe region were sequential and that the change from contraction convergence to extension divergence was quick between 408 and 402 ma

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

the magma ocean concept and lunar evolution the model of lunar evolution in which the anorthositic plagioclase rich oldest crust of the moon is formed over a period of 300 myr or less by crystallization as it floats on a global ocean of magma tens or hundreds of km thick is examined in a review of petrological and theoretical studies consideration is given to the classification of lunar rocks the evidence for primordial deep global differentiation constraints on the depth of the molten zone the effects of pressure on mineral stability relationships mainly liquid vs mainly magmifer ocean models and the evidence for multiple ancient differentiation episodes a synthesis of the model of primordial differentiation and its aftereffects is presented and the generalization of the model to the earth and to mars mercury venus and the asteroids is discussed

tracing exhumation of the dabie shan ultrahigh pressure metamorphic complex using the sedimentary record in the hefei basin china jurassic rocks in the hefei basin were deposited by braided fluvial and alluvial fan systems characterized by a general coarsening upward sequence multiproxy provenance analyses demonstrate that the sediment source areas for the hefei basin are composed of a variety of rocks including ultrahigh pressure uhp and high pressure hp metamorphic rocks and yangtze basement rocks of the axial dabie shan metamorphic complex the luzhenguan complex granite low and medium grade metamorphic rocks and the yangshan group sandstone in the north huaiyang fold and thrust belt a middle jurassic section in the western part of the basin is characterized by relatively high e nd values at 176 ma ranging from 13 8 to 11 3 whereas a section in the middle part of the basin has higher 147 sm 144 nd ratios from 0 1168 to 0 1266 and somewhat lower e nd values at 176 ma from 15 0 to 14 5 sediments in a section in the eastern part of the basin have the lowest e nd values at 176 ma ranging from 22 0 to 14 6 the highest t dm values from 1 8 to 2 4 ga and low 147 sm 144 nd ratios from 0 0937 to 0 1067 provenance analyses of detrital compositions and nd isotopic compositions of the sediments in the hefei basin clearly demonstrate that the depth of exhumation in the dabie shan orogen increases from the west to the east the unroofing ages of the uhp and hp metamorphic rocks change from early jurassic to late jurassic westward the exhumation rate during the late triassic and jurassic is inferred to have increased eastward from 1 4 mm a to 2 5 mm a on average the sediments in the basin record the episodic thrusting events and periodic unroofing in the orogen

147sm143nd 146sm142nd systematics of early archaean rocks and implications for crust mantle evolution combined high precision 147sm 143nd 146sm 142nd data have been obtained for two early archaean pre 3 8 ga samples a 4 0 0 2 ga meta komatiite and an approximately 3 9 ga orthogneiss from the northern labrador segment of the north atlantic craton the samples have t143nd values at 3 8 ga of 4 0 and 1 4 respectively however t142nd142nd for both samples is indistinguishable from the bulk earth value for the sample with t143nd 4 0 t142nd is 0 03 0 14 which suggests that it was derived from a source which differentiated from the primitive mantle at between 4 1 and 4 3 ga 14sm 143nd and 146sm 142nd data are also presented for a kyanite eclogite xenolith from a 350 ma kimberlite from yakutia siberia 142nd 144nd for this sample is also within error of the bulk earth value eclogite xenoliths such as those from kimberlite pipes are therefore unlikely to be residues of crystallization of a pre 4 4 ga magma ocean but instead may represent fragments of post 4 3 ga oceanic crust incorporated into the subcontinental lithospheric mantle the absence of widespread variations in 142nd144nd in early archaean rocks supports models in which the volume of the depleted mantle has grown through time continuous growth of the depleted mantle and continental crust at the expense of the primordial mantle at between 4 56 and 4 0 ga can generate variations in t143nd and account for the absence of detectable variations in 142nd144nd in most early archaean rocks

sm nd k ar and petrologic study of some kimberlites from eastern united states and their implication for mantle evolution we provide new data on sm nd systematics k ar dating and the major element chemistry of kimberlites from the eastern united states mostly from central new york state and their constituent mineral phases of olivine clinopyroxene garnet phlogopite and perovskite in addition we report nd isotopes in a few kimberlites from south africa lesotho and from the eastern part of china the major element compositions of the new york dike rocks and of their constituent minerals including a xenolith of eclogite are comparable with those from the kimberley area in south africa the k ar age of emplacement of the new york dikes is further established to be 143 ma

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

field calibration of sediment flux dependent river incision 1 bed erosion and sediment transport are ubiquitous and linked processes in rivers erosion can either be modeled as a detachment limited function of the shear stress exerted by the flow on the bed or as a transport limited function of the sediment flux capacity of the flow these two models predict similar channel profiles when erosion rates are constant in space in time but starkly contrasting behavior in transient settings traditionally detachment limited models have been used for bedrock rivers whereas transport limited models have been used in alluvial settings in this study we demonstrate that rivers incising into a substrate of loose but very poorly sorted relict glacial sediment behave in a detachment limited manner we then develop a methodology by which to both test the appropriate incision model and constrain its form specifically we are able to tightly constrain how incision rates vary as a function of the ratio between sediment flux and sediment transport capacity in three rivers responding to deglaciation in the ladakh himalaya northwest india this represents the first field test of the so called tools and cover effect along individual rivers

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

eclogitic inclusions in diamonds evidence of complex mantle processes over time the first ion probe trace element analyses of clinopyroxene garnet pairs both included within diamonds and from the eclogite host xenoliths are reported these diamondiferous eclogites are from the udachnaya and mir kimberlite pipes yakutia russia the major and trace element analyses of these diamond inclusion and host rock pairs are compared in order to determine the relative ages of the diamonds confirm or deny genetic relationships between the diamonds and the eclogites evaluate models of eclogite petrogenesis and model igneous processes in the mantle before during and after diamond formation the most striking aspect of the chemical compositions of the diamond inclusions is the diversity of relationships with their eclogite hosts no single distinct pattern of variation from diamond inclusion minerals to host minerals is found for all four samples garnet and clinopyroxene inclusions in the diamonds from two samples u 65 3 and u 66 3 have lower mg s lower mg and higher fe contents and lower lree than those in the host eclogite we interpret such variations as due to metasomatism of the host eclogite after diamond formation one sample u 41 3 shows enrichment in diamond inclusion mree enrichment relative to the eclogite host and may indicate a metasomatic event prior to or during diamond formation bulanova 2 found striking differences between inclusions taken from within different portions of the very same diamond clinopyroxene inclusions taken from the central early portions of yakutian diamonds were lower in mg and mg contents by up to 25 than those later inclusions at the rims of diamonds these trends are parallel to those between diamond inclusions and host eclogites determined for four of the five samples from the present study and may merely represent changing magmatic and or p t conditions in the mantle garnet trace element compositions are similar in relative proportions but variable in abundances between diamond inclusions and host eclogites this is probably due to the rapid diffusion of trace elements in garnet under mantle temperatures and consequent alteration of the garnet and not due to juvenile diamonds locking in source heterogeneities c f 3 trace element compositions of clinopyroxenes included in diamonds are generally similar to those in the host eclogite however one host clinopyroxene does show enrichment in the lree compared to that in the inclusion and may be attributed to mantle metasomatism not related to kimberlite transport in another eclogite m 46 the host clinopyroxene is depleted in the lree and fe and enriched in the hree and mg relative to the inclusion and is consistent with partial melting of the eclogite subsequent to diamond formation smnd ratios in clinopyroxenes appear to be little affected by these processes for most samples allowing sm nd isotopic studies to yield important information about ancient protoliths eclogitic mineral inclusions in yakutian diamonds appear consanguineous with the diamonds a contention supported by the observations of bulanova 2 therefore re os whole rock and smnd clinopyroxene age determinations of the udachnaya eclogites also yield the time of diamond formation approximately 2 9 ga 32 33

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

the origins of yakutian eclogite xenoliths owing to the association with diamonds eclogite xenoliths have samples were derived from typical depleted mantle however two groups of three samples each indicate both enriched mantle and received disproportionate attention given their low abundance in kimberlites several hypotheses have been advanced for the origin of possible ultra depleted mantle present beneath yakutia during the late archean and early proterozoic the vast majority of eclogites eclogite xenoliths from the subduction and high pressure melting of oceanic crust to cumulates and liquids derived from the upper studied from the obnazhennaya pipe also exhibit characteristics of mantle we have amassed a comprehensive data set including group a eclogites and are probably derived directly from the mantle majorand trace element mineral chemistry carbon isotopes in however the eclogites from the mir kimberlite are more typical of diamonds and rb sr sm nd re os and oxygen isotopes in other eclogites world wide and show convincing evidence of a recycled ultrapure mineral and whole rock splits from eclogites of the udoceanic crustal affinity we concur with the late ted ringwood achnaya kimberlite pipe yakutia russia furthermore eclogites that eclogites can be formed in a variety of ways both within the from two other yakutian kimberlite pipes mir and obnazhennaya mantle and from oceanic crustal residues have been studied in detail and offer contrasting images of eclogite protoliths relative to eclogites from southern africa and other yakutian localities udachnaya eclogites are notable in the absence of chemical zoning in mineral grains as well as the degree of light

a middle silurian early devonian magmatic arc in the qinling mountains of central china an early to middle paleozoic magmatic arc was discovered in the qinling orogenic belt of central china that marks the boundary between the north china craton and the south china craton the arc was built on the deformed southern passive continental margin of the north china craton which has been modified by the accretion of an oceanic island arc in early paleozoic time u pb analyses of zircons from granites indicate short lived arc activity from middle silurian to early devonian time trace element analyses as well as ree analyses of i type granites are consistent with the interpretation that the magmatic arc developed above a north dipping subduction zone termination of magmatic activity was probably caused by the initiation of collision between the north china craton and the south china craton this interpretation is consistent with faunal data and places new constraints on the paleozoic tectonic evolution of the qinling belt

orogen parallel westward oblique uplift of the qinling basement complex in the core of the qinling orogen china an example of oblique extrusion of deep seated metamorphic rocks in a collisional orogen abstract both vertical uplift and lateral extrusion of rocks in convergent orogens have been widely documented this article using the basement core of the qinling orogen as an example demonstrates orogen parallel westward uplift with both vertical and horizontal displacement components the qinling complex of the qinling orogen is a tectonic slice of a precambrian crystalline basement it is an elongate lens between younger terranes bounded by the shangdan suture to the south and by the zhuxia zone to the north during the middle paleozoic the complex underwent large scale orogen parallel westward oblique ductile uplift supporting evidence for this includes the following 1 opposite shear senses of the two bounding shear zones i e the thrust sinistral shear on the south and thrust dextral shear on the north 2 well developed eastward plunging orogen parallel a type structures in the complex such as stretching lineation and sheath folds 3 strong decompression metamorphism of the complex f

femg partitioning between coexisting garnet and phengite at high pressure and comments on a garnet phengite geothermometer abstract the fe mg exchange reaction between coexisting garnet and phengite has been studied by reacting a natural phengite mg 67 in the presence of quartz and water at pressures of 20 35 kb and temperatures of 800 1000 c compositions of coexisting garnet and mica indicate a linear relation between both the inkd fe mg garnet fe mg phengite and temperature and inkd and pressure in the above p t range this fe mg exchange reaction between garnet and phengite is shown to be dependent on the ca content of the garnet and on the mg number of the bulk composition these two composition effects have been studied by usin phengitic mica mixes with mg numbers of 23 and 46 and by using a synthetic basaltic composition the overall results allow broad empirical calibration of separate geothermometers for pelitic and basaltic systems respectively however because of non ideality in the exchange reaction this geothermometer should not be used in any practical application outside the composition ranges studied also careful consideration of the presence of fe3 in phengite must be made if the fe3 content of the natural phengite is unknown then the temperatures obtained will be maximum temperatures only

the primodial terrestrial magma ocean and its implication for stratification of the mantle fractional crystallization behaviour of a magma ocean extending to lower mantle depths was deduced from estimations of melting relations for the deep mantle and the density relationships between ultrabasic liquid and mantle minerals the accretional growth of the earth necessarily involves a molten zone magma ocean in the outer layer of the growing earth the fractionation by melting during accretion results in primary stratification composed of a molten ultrabasic upper mantle magma ocean a perovskite rich lower mantle and an iron core a certain amount of al2o3 and cao was removed from the magma ocean and retained in the lower mantle due to eclogite fractionation in the early stage of accretion and the perovskite fractionation in the later stage of accretion models of the stratification of the upper mantle arising from fractional crystallization of the magma ocean and subsequent convective disturbance were deduced on the basis of estimations of melting relations for the deep mantle and the density relationships between the ultrabasic liquid and mantle minerals the stratification of the mantle which is consistent with geophysical constraints is as follows the upper mantle is composed of two layers the upper olivine rich layer and the lower garnet rich layer with a thickness around 200 km and the lower mantle with a perovskite rich composition in this model both the 400 and 650 km discontinuities are the chemical boundaries

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

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

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

nature of diamonds in yakutian eclogites views from eclogite tomography and mineral inclusions in diamonds abstract we have performed dissections of two diamondiferous eclogites ux 1 and u33 1 from the udachnaya kimberlite yakutia in order to understand the nature of diamond formation and the relationship between the diamonds their mineral inclusions and host eclogite minerals diamonds were carefully recovered from each xenolith based upon high resolution x ray tomography images and three dimensional models the nature and physical properties of minerals in direct contact with diamonds were investigated at the time of diamond extraction polished sections of the eclogites were made containing the mould areas of the diamonds to further investigate the chemical compositions of the host minerals and the phases that were in contact with diamonds major and minor element compositions of silicate and sulfide mineral inclusions in diamonds show variations among each other and from those in the host eclogites oxygen isotope compositions of one garnet and five clinopyroxene inclusions in diamonds from another udachnaya eclogite u51 span the entire range recorded for eclogite xenoliths from udachnaya in addition the reported compositions of almost all clinopyroxene inclusions in u51 diamonds exhibit positive eu anomaly this feature together with the oxygen isotopic characteristics is consistent with the well established hypothesis of subduction origin for udachnaya eclogite xenoliths it is intuitive to expect that all eclogite xenoliths in a particular kimberlite should have common heritage at least with respect to their included diamonds however the variation in the composition of multiple inclusions within diamonds and among diamonds from the same eclogite indicates the involvement of complex processes in diamond genesis at least in the eclogite xenoliths from yakutia that we have studied

ultradeep xenoliths from african kimberlites sr and nd isotopic compositions suggest complex history abstract rare kimberlite xenoliths have chemical characteristics that indicate an origin deep in the upper mantle in the vicinity of the transition zone the nd isotopic compositions of minerals separated from several of these xenoliths suggest that these materials were emplaced into the african lithosphere at times ranging from approximately the time of kimberlite volcanism to many hundreds of millions of years earlier possibly by periodic plume transport they also indicate that these ultradeep samples have a complex chemical history involving both enrichment and depletion and assuming that there has not been re equilibration within the lithosphere that they sample an isotopically heterogeneous deep upper mantle this would be expected if this part of the mantle contains a heterogeneous assemblage of subducted crustal and lithospheric components

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

the thermal history of leaky chronometers above their closure temperature 1 a theory is presented which describes the simultaneous radiogenic ingrowth and loss of a radiogenic isotope by volume diffusion in crystals cooling slowly above their closure temperatures when the diffusion parameters of a given radiogenic isotope in the host mineral are known the age deficit resulting from the open system behavior can be simply related to the cooling rate an application to the apparent 147sm 143nd ages of pyrope garnets in archean eclogite xenoliths attests to the slow cooling rate of the subcontinental lithosphere

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

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

archaean re os age for siberian eclogites and constraints on archaean tectonics considerable uncertainty surrounds the nature and evolution of the earth s mantle in archaean times 2 5 gyr ago mantle derived eclogite xenoliths erupted by kimberlites provide important clues in this regard because their basaltic composition suggests that they may be remnants of an early 4 gyr magma ocean1 2 subducted archaean oceanic crust3 5 or crystallized high pressure mantle melts6 9 better constraints on the age and origin of mantle eclogites are thus important for our understanding of early earth processes here we present rhenium osmium isotope data for diamond bearing eclogites from the udachnaya kimberlite pipe in siberia which indicate formation in the archaean 2 9 0 4 gyr this age is too young for the eclogites to be remnants of early earth differentiation but overlaps the age range for crust generation and craton stabilization on both the aldan and anabar shields of the siberian craton 2 85 3 2 gyr 10 11 these data together with 3 1 gyr re os model ages for diamond bearing udachnaya peridotites12 show that the siberian craton lithosphere was at least 150 km thick the minimum required for diamond stability by the mid archaean

normal faulting block tilting and decollement in a stretched crust extensional tectonics cover a wide range of crustal structures from narrow continental rifts to large continental areas if we except continental and oceanic rifts stretched portions of the crust exhibit complex faulting patterns at upper levels with high angle and low angle normal faults very little is known concerning the geometry of structures and physical processes at depth it is therefore difficult to extrapolate surface observations to the crustal scale five general models are proposed that combine continuous and discontinuous deformation and geometrical and geological tests are proposed two natural examples the west armorican atlantic margin and eldorado mountains basin and range province are presented they are characterized by an association of high angle normal faults tilted blocks and low angle normal faults field and geometrical arguments are given to demonstrate that low angle faults are controlled by decollement surfaces along preexisting interfaces geometrical implications of the tilted block pattern are presented and the gravity gliding hypothesis is proposed as a possible explanation of block tilting when associated with low angle normal faults in such cases the amount of stretching cannot be related to bulk thinning of the crust

carboniferous and triassic eclogites in the western dabie mountains east central china evidence for protracted convergence of the north and south china blocks shrimp u pb dating and laser ablation icp ms trace element analyses of zircon from four eclogite samples from the north western dabie mountains central china provide evidence for two eclogite facies metamorphic events three samples from the huwan shear zone yield indistinguishable late carboniferous metamorphic ages of 312 5 307 4 and 311 17 ma with a mean age of 309 3 ma one sample from the hong an group 1 km south of the shear zone yields a late triassic age of 232 10 ma similar to the age of ultra high pressure uhp metamorphism in the east qinling dabie orogenic belt ree and other trace element compositions of the zircon from two of the huwan samples indicate metamorphic zircon growth in the presence of garnet but not plagioclase namely in the eclogite facies an interpretation supported by the presence of garnet omphacite and phengite inclusions zircon also grew during later retrogression zircon cores from the huwan shear zone have ordovician to devonian 440 350 ma ages flat to steep heavy ree patterns negative eu anomalies and in some cases plagioclase inclusions indicative of derivation from north china block igneous and low pressure metamorphic source rocks cores from hong an group zircon are neoproterozoic 780 610 ma consistent with derivation from the south china block in the western dabie mountains the first stage of the collision between the north and south china blocks took place in the carboniferous along a suture north of the huwan shear zone the major triassic continent continent collision occurred along a suture at the southern boundary of the shear zone the first collision produced local eclogite facies metamorphism in the huwan shear zone the second produced widespread eclogite facies metamorphism throughout the dabie mountains sulu terrane and a lower grade overprint in the shear zone

rb sr and sm nd isotopic variations in dissected crustal xenoliths abstract four crustal xenoliths from scotland have been dissected in order to evaluate the effects of magma xenolith interaction on rb sr and sm nd isotopic systematics and hence the use of the xenoliths for accurate determination of source compositions each xenolith was sampled from the edge to the center the nd isotopic compositions are virtually uniform across each xenolith however there are significant variations in rb sr and ree concentration as well as rb sr and sm nd ratios and sr isotopic composition most of this variation appears to be inherited from the protolith however one sample shows systematic variations in rb and sr concentration and sr isotopic composition with distance from the xenolith edges due to interaction with the host magma this sample displays petrographic evidence for grain boundary melt infiltration these data are consistent with theoretical considerations of transport times and diffusivities that place limits on the amount of modification that is possible via diffusion alone the calculated effects of transport of even large amounts of crustal xenoliths on the compositions of xenoliths and host magmas will be small provided transport times are short overall the isotopic variations within the xenolith suite mirror the source rocks from which they were extracted and are inferred to reflect variations in the deep crust of scotland however small samples such as most crustal xenoliths may be of little use in defining meaningful crustal residence ages because of modal variations in metamorphic minerals and isotopic re equilibration the one xenolith studied has extremely variable sm nd ratios due to the presence of garnet and yields a broad range of geologically meaningless nd model ages but yields an sm nd isochron age of 396 49 ma the loch roag xenolith has a large range of rb sr ratio and the sr isotopic data define an isochron age of 416 28 ma both ages are taken to represent the timing of metamorphism and or cooling from 600 c at the end of the caledonian orogeny in scotland

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

diamondiferous eclogites from yakutia siberia evidence for a diversity of protoliths major element and ree compositions of 14 diamondiferous eclogites from the udachnaya kimberlite in yakutia siberia have been determined by electron microprobe and secondary ion mass spectrometer sims based on previous clinopyroxene classification schemes e g taylor and neal 1989 all of these eclogite xenoliths belong to group b c although some of the garnet compositions and mineral ree abundances are inconsistent with the indicated groups this demonstrates the inadequacy of the classification scheme based on african eclogites for application to siberian samples because of the coarse grain size of the udachnaya nodules meaningful modal abundances could not be obtained however reconstructed ree compositions using various garnet clinopyroxene ratios demonstrate relative insensitivity to changes in mode for common eclogitic assemblages many of these reconstructed ree compositions show lree depletions some depletions are consistent with an origin either directly or through partial melting as normal or type i ocean floor basalt others however require material of eclogitic or pyroxenitic affinities to undergo partial melting this facilitates the depletion of lree while leaving the hree at nearly original levels many of the eclogites of south africa are consistent with a protolith of anomalous or type ii ocean floor basalt this fundamental difference between the two regions is the likely cause of the inconsistencies with the chemicallybased classification

mesozoic evolution of the hefei basin in eastern china sedimentary response to deformations in the adjacent dabieshan and along the tanlu fault this paper presents a study of jurassic early cretaceous sedimentary evolution of the hefei basin in eastern china and explores the relationship between clastic sedimentation and coeval deformation of the dabieshan to the south and the tanlu fault to the east the hefei basin experienced a two stage evolution the basin was initiated in the early jurassic and expanded in the middle and late jurassic the synsedimentary jinzhai normal fault is considered to be a border fault of the basin because it controlled middle to upper jurassic proximal alluvial deposits persistent n to ne directed paleo flows in the southern hefei basin indicate that sediments came from the dabieshan and the presence of triassic coesite bearing detrital zircon in lower jurassic sediments documents exhumation of ultrahigh pressure rocks of the dabieshan to the surface as early as the early jurassic occurrences of eclogite clasts in an upper jurassic unit indicate continued denudation of the dabieshan at that time thickening of jurassic clastic units to the southern and southeast parts of the basin suggests that basin subsidence and depositional loci were under the coupled control of the jinzhai normal fault on the south and the ne striking left lateral transtensional tanlu fault on the east jurassic extensional subsidence of the hefei basin is in marked contrast to the coeval development of a contractional foreland basin south of the dabieshan which combines to indicate contemporaneous extension and contraction on the north and south sides of the dabieshan respectively vigorous volcanism and uplift of the southern hefei basin characterized the second stage of development of the hefei basin in the early cretaceous and this led to a synchronous shift of its main depocenter to the north this younger depocenter is characterized by lacustrine and fluvial deltaic sedimentation where alluvial and fan deltaic coarse grained deposition mainly occurred along the eastern edge of the basin early cretaceous subsidence is attributed to e w extension across the middle segment of the tanlu fault and the zhangbaling massif on the east acted as a footwall and provided a source for sediment to the northern basin a model is accordingly advanced to account for how the hefei basin developed in response to the tectonic exhumation of the dabieshan and the deformation of the tanlu fault in the mesozoic it illustrates that the hefei basin initiated and evolved during jurassic time in an extensional setting that was triggered by southerly upward extrusion of ultra high pressure rocks of the dabieshan early cretaceous development of the basin was controlled by magmatism related uplift of the dabieshan on the south and orthogonal normal faulting of the middle segment of the tanlu fault on the east this study provides an independent constraint upon the exhumation processes of ultrahigh pressure rocks of the dabieshan

hotspots basalts and the evolution of the mantle the trace element concentration patterns of continental and ocean island basalts and of mid ocean ridge basalts are complementary the relative sizes of the source regions for these fundamentally different basalt types can be estimated from the trace element enrichment depletion patterns their combined volume occupies most of the mantle above the 670 kilometer discontinuity the source regions separated as a result of early mantle differentiation and crystal fractionation from the resulting melt the mid ocean ridge basalts source evolved from an eclogite cumulate that lost its late stage enriched fluids at various times to the shallower mantle and continental crust the mid ocean ridge basalts source is rich in garnet and clinopyroxene whereas the continental and ocean island basalt source is a garnet peridotite that has experienced secondary enrichment these relationships are consistent with the evolution of a terrestrial magma ocean

late proterozoic magmatism and metamorphism recorded in gneisses from the dabie high pressure metamorphic zone eastern china evidence from zircon u pb geochronology this study presents zircon ages and geochemical and nd isotopic data for metagranites and quartzites from the susong high pressure metamorphic zone in the dabie sulu ultrahigh pressure collisional belt this belt belongs to the eastern part of the qinling dabie orogenic belt that formed during early mesozoic collision of the north and south china blocks two metagranites give zircon u pb model ages of 785 7 and 205 12 ma likely representing a late proterozoic magmatic event and an early mesozoic overprint they have low initial end values 12 4 and 11 1 at 780 ma favouring a crustal origin flat heavy rare earth elements ree patterns gdn ybn 1 2 probably reflect that melting took place at a shallow crustal section where heavy ree bearing mineral phases are unstable all zircons of three quartzites yield young discordant u pb ages and define a discordia with u pb model ages of 784 6 and 213 3 ma identical to those of the metagranites we assume that all detrital zircons had lost radiogenic pb prior to the early mesozoic overprint probably facilitated by fluid participation during a metamorphic event contemporaneous with the intrusion of the metagranites this simultaneous metamorphic and magmatic event was probably related to a rift setting along the periphery of the yangtze south china block during late proterozoic

40ar 39ar laser probe studies of clinopyroxene inclusions in eclogitic diamonds abstract 40 ar 39 ar laser probe ages are reported for individual clinopyroxene inclusions in thirty two eclogitic diamonds from four localities jwaneng and orapa both in botswana udachnaya siberia and argyle australia ages obtained from cleaved diamonds from jwaneng 244 ma orapa 102 ma and udachnaya 425 ma are similar to those of their host kimberlites however there is considerable variation in age between individual laser extractions obtained from the same inclusion which we suspect is mainly due to partial retention of radiogenic argon that diffused to the inclusion diamond interface while the diamond was in the mantle some of this argon was lost when the diamonds were cleaved as part of the experimental procedure the presence of an ambient mantle 40 ar component trapped in inclusions may be an additional cause of scatter in ages however this cannot be validated with the present data in support of the diffusive loss process the results from an udachnaya inclusion completely enclosed within host diamond with the argon being released by laser drilling yielded an age of 1149 37 ma which is considerably older than the host kimberlite about 340 400 ma likewise the scatter in 40 ar k ratios for eclogitic diamonds from jwaneng and orapa indicates that they too are probably xenocrysts therefore a combination of sample preparation technique and laser drilling can be used to offer a useful extension of the original method enabling both diamond formation from buried inclusions and host rock emplacement ages from cleaved inclusions to be determined argyle clinopyroxenes similarly cleaved to the diamond surface also show a considerable range of inclusion ages 1159 1540 ma only in this case there is a relationship between age and k ca ratio of the inclusions such a relationship is difficult to reconcile in terms of partial loss of 40 ar and instead indicates that there was a protracted period of diamond growth

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

a feasibility study of u pb and pb pb dating of kimberlites using groundmass mineral fractions and whole rock samples abstract this paper describes partly successful attempts to determine emplacement ages of kimberlites by u pb and pb pb methods involving groundmass minerals with high u content notably perovskite and whole rock kimberlite samples u pb ratios in perovs kite in the matrix of kimberlites can be two orders of magnitude larger than in the rest of the kimberlite material and with simple mineral separation techniques moderate success was achieved in u pb dating of fresh samples of younger kimberlites around 100 ma the differences in u pb ratios between kimberlite samples from different parts of the same pipe have also been found to be large enough in some cases to allow reasonably accurate u pb age determination in older kimberlites the u pb ages obtained were mostly incompatible with geological constraints and results obtained by other methods however for such pipes use of pb pb systematics yields realistic age limits in some cases

silurian devonian provenance changes of south qinling basins implications for accretion of the yangtze south china to the north china cratons abstract the north china and yangtze south china cratons are the most important tectonic units of china the two cratons finally coalesced through collision with the formation of the e w trending qinling mountains that are considered to be the eastern extreme of the paleotethys the north qinling and south qinling belts constitute the southern active margin of the north china craton and the northern passive margin of the yangtze craton respectively we have studied chemical compositions of 573 fine grained clastic sedimentary rocks of all the early proterozoic to neogene ages from the north china craton the yangtze craton and the north and south qinling orogenic belts as well as their ten tectonostratigraphic subunits the purpose of this study is to investigate possible provenance changes that are expected to have been recorded in the phanerozoic clastic sediments of the south qinling basins if north and south china had approached and accreted because the north china craton north qinling belt and yangtze craton have distinct upper crustal and provenance compositions both the north china and yangtze cratons and the north qinling belt are characterized by a relatively uniform non evolutionary composition of clastic sediments in la th sc co systematics during post archean times they can be however distinguished from each other in provenance in terms of the la th sc th th co as well as k2o na2o ratios in contrast there is a significant provenance change in the silurian devonian clastic sediments from the baiyun zhashui liuba yunxian and western xichuan subunits of the south qinling belt the late sinian 700 600 ma cambrian and ordovician pelites show characteristics that are consistent with the yangtze craton being high in la th 3 5 10 and sc th 1 4 2 4 and low in th co 0 2 1 0 and lacking la th ratios 3 5 these are accompanied by the increasing immaturity of the silurian devonian clastic sediments illustrated by the decreasing k2o na2o ratio which is attributed to the near source of the north qinling belt dominated by low k2o island arc igneous suites and immature metagraywackes we propose that the south qinling basins except the eastern xichuan subunit had been in a close proximity and accreted to the north qinling belt in silurian devonian times the result agrees well with recent studies of benthic faunas granite distribution age of granulite and clastic provenance directions of the south qinling devonian sediments but it is apparently inconsistent with the age of coesite and diamond bearing ecologite in the dabie mountains to the east and with paleomagnetic data both suggesting an important triassic jurassic collision event between north and south china it is likely that the silurian devonian and the mesozoic represent two different episodes phases of accretion and collision between north and south china new models regarding the phanerozoic tectonic evolution of east china are needed to reconcile the contrasting evidence