multichronometric evidence for an in situ origin of the ultrahigh pressure metamorphic terrane of dabieshan china abstract u pb zircon data on gneisses indicate a late proterozoic protolith at 700 ma at bixiling and a late archean to early proterozoic for the shuanghe sheet within the ultrahigh pressure metamorphic terrane of dabieshan china u pb zircon and sm nd dates on the gneisses of the shuanghe sheet constrain a metamorphic peak at 230 ma rb sr and 40ar 39ar analyses of phengite from five gneisses in the bixiling complex yield ages at 198 212 ma overlapping the 190 220 ma age on eclogites the host gneisses record a similar metamorphic evolution as the coesite bearing eclogites since both the peak and the retrograde evolution were contemporaneous this argues for an in situ tectonic relationship

common pb of uhp metamorphic rocks from the ccsd project 100 5000 m suggesting decoupling between the slices within subducting continental crust and multiple thin slab exhumation abstract in order to understand the vertical structure of the dabie sulu ultrahigh pressure metamorphic uhpm belt common pb isotopic compositions of omphacites in eclogites and feldspars in gneisses from the chinese continental scientific drilling ccsd project 100 5000 m have been investigated in this study samples from 0 to 800 m unit 1 in the drilling core have moderately high radiogenic pb isotopes with small variations of 206pb 204pb 16 82 17 38 207pb 204pb 15 37 15 49 and 208pb 204pb 37 21 37 72 indicating either high 238u 204pb or high initial pb isotope ratios of their protoliths in contrast the samples from 1600 to 2040 m unit 3 and most of samples from 3200 to 5000 m unit 5 have moderately or very unradiogenic pb unit 3 206pb 204pb from 16 05 to 16 46 207pb 204pb from 15 22 to 15 29 and 208pb 204pb from 36 68 to 37 48 unit 5 206pb 204pb from 15 52 to 15 69 207pb 204pb from 15 15 to 15 27 and 208pb 204pb from 36 48 to 37 20 indicating either low or low initial pb isotope ratios of their protoliths pb isotopes of samples from 800 to 1600 m unit 2 and from 2040 to 3200 m unit 4 in the drilling core with abundant ductile shear zones are intermediate between those of units 1 and 3 or 5 and display larger variations pb isotopes combined with the published oxygen isotope data of the ccsd samples reveal the original positions of the five units before the triassic continental subduction units 1 3 and 5 as three uhpm rock slabs could be derived from the subducted upper continental crust upper middle continental crust and lower middle continental crust respectively the ductile shearing zones in units 2 and 4 could be the interfaces where the detachment and decoupling took place between the upper upper middle and lower middle continental crusts the detachment between the upper slab and subducting continental lithosphere probably occurred during continental subduction and the upper slab unit 1 was uplifted to a shallow depth along the detachment surface by thrusting units 3 and 5 may be detached later from the subducted middle and lower crust and uplifted to a shallow level underneath unit 1 the low 18o values 4 0 to 7 4 xiao y l zhang z m hoefs j kerkhof a 2006 ultrahigh pressure metamorphic rocks from the chinese continental drilling project ii oxygen isotope and fluid inclusion distributions through vertical sections contribution mineral petrology 152 443 458 zhang z m xiao y l zhao x d shi c 2006 fluid rock interaction during the continental deep subduction oxygen isotopic profile of the main hole of the ccsd project acta petrologica sinica 22 7 1941 1951 in units 2 and 4 suggest that the detachment interfaces could be developed along an ancient fault zones which were the channels of meteoric water activity during the neoproterozoic

a high precision u pb age of metamorphic rutile in coesite bearing eclogite from the dabie mountains in central china a new constraint on the cooling history this paper first reports a high precision u pb age of 218 1 2 ma for rutile in coesite bearing eclogite from jinheqiao in the dabie mounteins east central china this work shows that the u pb mineral rutile omphacite isochron age of 218 2 5 ma and conventional rutile u pb concordia age of 218 1 2 ma obtained by common pb correction based on the pb isotopic composition of omphacite in the same eclogite sample are consistent proving that the omphacite with low u pb ratio 2 8 can be used for common pb correction in u pb dating of rutile oxygen isotope analysis of rutile aliquots gave the consistent 18o values of 6 1 0 1 demonstrating oxygen isotope homogenization in the rutile of different grains as inclusion in garnet and grain in matrix oxygen isotope thermometry yields temperatures of 695 35 and 460 15 c for quartz garnet and quartz rutile pairs respectively these oxygen isotopic observations suggest that the diffusion of oxygen in rutile as inclusion in garnet is not controlled by garnet according to field based thermochronological studies of rutile an estimate of the tc of about 460 c for u pb system in rutile under rapid cooling conditions 20 c ma was advised based on this u pb age as well as the reported chronological data with their corresponding metamorphic and or closure temperature an improved t t path has been constructed the t t path confirms that the uhpm rocks in south dabie experienced a rapid cooling following the peak metamorphism before 220 ma and a long isothermal stage from 213 to 180 ma around 425 c

variation in peak p t conditions across the upper contact of the uhp terrane dabieshan china gradational or abrupt the southern dabieshan terrane sdt has previously been divided into high pressure hp and ultrahigh pressure uhp terranes and its regional extent and the tectonic nature of its boundaries are hotly debated topics in this study an eclogite bearing area of 100 km2 near taihu is mapped in detail and divided into northern middle and southern zones on the basis of lithological characteristics the northern zone consists of epidote biotite gneiss and eclogite blocks the middle zone includes granitic gneiss biotite gneiss eclogites and amphibolite and the southern zone is composed mainly of garnet bearing mica schist the eclogites occur mainly as lens or blocks in the northern and middle zones the peak p t conditions for 61 eclogite samples across the area are estimated using the grt cpx fe2 mg thermometers and the grt cpx phe barometers the results indicate three different p t regions 2 82 4 09 gpa 759 942 c in the northern zone and 2 00 3 54 gpa 641 839 c in the granitic gneiss and 1 38 2 36 gpa 535 768 c in the biotite gneiss from the middle zone combined with the spatial distribution of eclogites across the area the p t values for eclogites increase continuously from the south to the north defining a reference geotherm of 5 c km 1 however some unreasonable apparent gradients can be established along two south north profiles across the area and display a p t difference between the northern and middle zones on the basis of the average p t data for eclogites across the area a gap of at least 0 3 gpa 20 c exists between the northern and middle zones by contrast the p t values of eclogites from the middle zone show a coherent pattern with transitional characteristics from hp in the south to uhp in the north we suggest that the sdt was a coherent slab during subduction and was broken up by a major fault during exhumation which was formed under uhp metamorphic conditions

upb zircon and rutile chronology of archean greenstone formation and gold mineralization in the val d or region quebec abstract u pb geochronology provides an absolute time framework for the evolution of the sigma gold deposit and surrounding rocks at val d or southern abitibi subprovince the bourlamaque batholith the largest pluton in the area gives a 2699 8 1 0 ma u pb zircon age this pluton cuts the val d or formation which hosts the mineralization a u pb zircon age of 2704 9 1 1 ma on a felsic volcanic rock the colombiere rhyolite 13 km east of the mine dates that formation the gold bearing quartz vein system at sigma is hosted by andesites and two generations of porphyry intrusions all metamorphosed to the greenschist facies the oldest porphyry porphyritic diorite shows the same deformation as the volcanic rocks and has a 2703 7 2 5 ma u pb zircon age the porphyritic diorite and volcanic rocks are cut by feldspar porphyry dykes which post date regional folding and have a 2694 0 2 2 ma u pb zircon age regional greenschist metamorphism has been dated directly with a u pb date of 2684 7 ma on rutile in the colombiere rhyolite the mineralization and hydrothermal alteration in the mine are superimposed on the metamorphic minerals hydrothermal rutile from an alteration halo around the veins in andesite has a 2599 9 ma u pb age textural evidence clearly indicates that the wall rock alteration and vein filling are contemporaneous and hence the vein system and gold mineralization appear to have developed at least 80 m y after the formation and metamorphism of host greenstones

sims u pb dating of vein hosted hydrothermal rutile and carbon isotope of fluids in the wulong lode gold deposit ne china linking gold mineralization with craton destruction abstract wulong is the largest lode gold deposit in the liaodong peninsula northeastern north china craton ncc gold mineralization mainly occurs in the quartz sulfide veins which are hosted in jurassic monzogranite and early cretaceous diorite dikes here rutile grains are firstly identified in the auriferous quartz pyrite vein the vein hosted rutiles have textural associations with quartz chlorite pyrite and bi sulfosalts and host fluid inclusions indicating that they are unequivocally of hydrothermal origin and co precipitate with gold the rutile grains are characterized by high w and v concentrations and u shaped rare earth elements pattern with the greatest depletion in mree due to excess zr incorporation during fast rutile crystal growth process from the f rich ore forming fluids zr in rutile thermometers provide unrealistic crystallization temperatures 550 c which are much higher than the previous homogenization temperatures data of fluid inclusions yu et al 2018 all these geochemical signatures also support that rutiles formed from the hydrothermal fluids the 13c values of co2 gases in low salinity aqueous carbonic fluid inclusions from the mineralized quartz range from 9 6 to 1 7 average 5 7 indicating mantle magmatic sources for the ore forming fluids sims u pb dating yields a more precise and robust age of 122 3 1 1 ma n 23 mswd 0 82 for the gold mineralization at wulong this new date is roughly coeval with the ore hosted diorite dikes and the sanguliu granitic pluton most in 125 120 ma such close temporal association provides additional weights of evidence for a magmatic affinity to the gold formation regionally the rutile u pb age indicates that the gold mineralization occurs in an extensional tectonic regime caused by the thinning and destruction of the lithosphere beneath the ncc meanwhile the timing of gold mineralization is the same as the peak age of ncc destruction therefore we propose that the wulong deposit has a genetic link with this craton destruction this is similar to numerous other early cretaceous gold deposits across the ncc such as those in the xiaoqinling and jiaodong districts our study indicates that rutile is a robust u pb chronometer for the lode gold deposit but may be not a good geothermometer in the case of f rich hydrothermal fluid system

petrology and u pb geochronology of mafic high pressure metamorphic coronites from the tshenukutish domain eastern grenville province abstract a suite of mafic and ultramafic rocks from the tshenukutish domain eastern grenville province have been the subject of a detailed petrographic and geochemical study combined with u pb geochronology the tshenukutish domain is the highest tectonic level in the recently discovered eclogite bearing manicouagan imbricate zone the mafic and ultramafic rocks are characteristic of two of the lithotectonic segments in the domain the lac espadon segment and the baie du nord segment in the lac espadon segment cumulate textures and geochemical characteristics suggest that the units represent a layered mafic and ultramafic complex and an unrelated apatite rich fe ti mafic pegmatite nelsonite body in contrast the samples from the baie du nord segment represent fairly homogeneous fe ti gabbros and anorthosites the samples studied show abundant petrographic evidence for high pressure metamorphism for instance the widespread development of garnet clinopyroxene and kyanite bearing metamorphic assemblages estimates of the conditions of high pressure metamorphism in previous studies and melting textures in deformed and hydrated gabbro margins are in the range 750 900 c and 1600 1800 mpa the gabbroic end members from both segments suggest within plate tholeiite petrotectonic signatures ree patterns of gabbros and cumulates in both segments suggest fractionation from a tholeiitic source cumulate textures fractionation and inherited zircons suggest high crustal level emplacement of the magmatic protoliths in the tshenukutish domain in the lac espadon segment u pb ages of igneous zircon and baddeleyite give an age range of 1643 1629 ma for the emplacement of the layered complex the nelsonite in the les gives a poorly constrained igneous zircon age of 1596 126 99 ma in the baie du nord segment baddeleyite gives an age of 1170 5 ma for emplacement of the fe ti gabbros this is the youngest intrusion in the td and thus gives a maximum limit for the onset of grenvillian metamorphism ages of high p metamorphism are further constrained by zircon formed during partial melting of deformed and hydrated fe ti gabbro margins 1046 3 ma overgrowth of igneous zircon 1042 22 28 ma in the nelsonite saccharoidal zircon replacing baddeleyite 1030 10 7 ma and 1030 12 ma and new growth of equant zircon 1012 12 ma from the undeformed interior of the gabbros these ages indicate that the high p metamorphism of the tshenukutish domain was late grenvillian ottawan retrogression to amphibolite facies is constrained by the ages of titanite 1006 997 ma in deformed gabbro margins rutile ages range between 960 and 929 ma and are interpreted as cooling ages the difference between the ages of titanite and rutile suggest slow cooling of the tshenukutish domain after initial high pressure and temperature metamorphism and exhumation

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

u pb geochronology of the mazinaw terrane an imbricate segment of the central metasedimentary belt grenville province ontario the mazinaw terrane in the central metasedimentary belt of the grenville province comprises volcanic sedimentary and plutonic rocks that were intensely folded and faulted and metamorphosed to as high as upper amphibolite facies u pb geochronology establishes an early period of magmatism and sedimentation at about 1280 1240 ma probably in a marginal basin setting and a multistage rhetamorphic evolution in the period between 1100 and 980 ma which was probably related to crustal thickening by imbrication during compression and wedging of the terrane some of the earliest magmatism formed calc alkalic volcanic rocks of kashwakamak formation at 1276 2 ma an associated sedimentary assemblage was intruded by the helena trondhjemite stock at 1267 5 ma a younger succession formed between 1250 and 1240 ma includes the bimodal volcanic mazinaw lake formation alaskitic granites of the norway lake and abinger plutons and a granodioritic gneiss phase of the cross lake pluton these units were covered

young ages from old garnet in the polyphase metamorphic terrane of the limpopo belt south africa abstract the p t t path of high grade metamorphic rocks is significant for understanding vertical motions and heating of the crust in general garnet is used to constrain p t paths and metamorphic zircon is used to obtain t for different metamorphic stages however this approach may be complicated by polyphase tectonothermal events the central zone of the limpopo belt in south africa is a complex precambrian metamorphic terrane that has experienced three distinct high grade metamorphic events at 3 22 ga m1 2 62 ga m2 and 2 02 ga m3 based on zircon dating a previous investigation of a metapelite enclave in the bulai pluton in the central zone showed that metamorphic zircon in the enclave recorded mainly m2 granulite facies metamorphism and was protected by the bulai pluton however titanite and rutile u pb ages of the same sample record the m3 event here we investigated garnet sm nd isotopic systematics of four metapelite samples from the evelyn enclave in the bulai pluton we constructed single mineral sm nd isochrons for garnet using micro samples in order to avoid the risk of isotopic disequilibrium between different minerals or whole rocks metamorphic zircon with sillimanite inclusions has yielded a granulite facies metamorphic age of ca 2 6 ga however the sm nd isotopic data for the garnet record much younger ages of ca 2 03 ga these ages are consistent with the m3 event in this area but were rarely identified in previous zircon dating studies this suggests that the m3 event reached sufficiently high temperatures to totally reset the rutile and titanite u pb and garnet sm nd systems moreover the decoupling of the age of garnet formation and the date it records needs to be carefully considered when reconstructing p t t paths and again highlights the usefulness of garnet in the study of polyphase metamorphic terranes

constraints on timing of peak and retrograde metamorphism in the dabie shan ultrahigh pressure metamorphic belt east central china using u th pb dating of zircon and monazite abstract the dabie shan ultrahigh pressure metamorphic uhpm belt occupies the suture between the yangtze and sino korean blocks in east central china the timing of uhpm in the dabie belt is controversial and most recent data come from dating of zircons monazite has recently been recognized as useful for dating of multiple tectonic events due to its preservation of multiple growth zones and monazite growth has been documented to occur during prograde peak and retrograde metamorphism zircons and monazites from uhp mafic rocks from maowu and a uhp jadeite quartzite from shuanghe were imaged in thin sections and in mineral separates and dated using a high resolution ion microprobe maowu mafic rocks are unique in that they contain high light rare earth element concentrations and therefore contain abundant monazite maowu eclogites and garnet pyroxenites contain zircons with mean 206 pb 238 u age of 230 4 ma and monazites from a clinopyroxenite have 208 pb 232 th ages of 209 4 ma multiple lines of evidence suggest that the measured ages represent the timing of new growth and recrystallization and are not cooling ages i e they do not correspond to the cessation of diffusional pb loss as rocks cooled below the closure temperature during exhumation shuanghe jadeite quartzites contain zircons with cores that define a discordia with an upper intercept age 1921 22 ma and lower intercept age of 236 32 ma interpreted to represent the ages of the source rocks and of peak metamorphism respectively zircon rims contain jadeite inclusions that restrict growth to pressures greater than 1 5 gpa their pooled age of 238 3 ma agrees well with the lower intercept defined by cores suggesting that pb loss from cores and growth of new rims occurred during uhpm at 235 240 ma monazite records multiple events during retrograde metamorphism maowu clinopyroxenite and shuanghe jadeite quartzite experienced monazite growth at 209 ma interpreted to reflect regional amphibolite facies overprinting resulting from pervasive retrograde fluid infiltration only the jadeite quartzite records growth at 223 1 ma estimated exhumation rates for maowu are 7 8 km ma

in situ rutile u pb dating based on zircon calibration using la icp ms geological applications in the dabie orogen china abstract rutile u pb dating by laser ablation inductively coupled plasma mass spectrometry la icp ms has received increasing attention in geological applications previous natural rutile reference materials are insufficient to meet the growing demand of working standards for laser u pb dating in present study we have developed a new calibration strategy for la icp ms rutile dating using the most popular zircon as reference material the matrix dependent elemental fractionation of rutile and zircon could effectively decrease to a better level by optimized analytical parameters and with large ablation spot size 44 90 m the acceptable pb u correction coefficient between rutile and zircon could be obtained while the diameter depth ratio of the crater is above 1 after assessing elemental fractionation of different ablation size five rutile were analyzed by zircon calibration and their ages show strong agreement with those obtained by secondary ion mass spectrometry sims or thermal ionization mass spectrometry tims analysis with both precision and accuracy within 2 two rutiles of eclogitic rocks from the dabie orogen yield the ages of 207 0 2 1 ma and 205 6 2 4 ma respectively both ages are conceivably younger than that previously reported rutile ages 218 ma for eclogites the quite different u pb ages of rutile from different localities most likely indicate the different exhumation history of the subducted continental crust la icp ms rutile u pb dating using zircon as an external calibration reference material would assuage rutile reference material insufficient supply and improve its widespread application

experimental mechanical compaction of reconstituted shale and mudstone aggregates investigation of petrophysical and acoustic properties of sw barents sea cap rock sequences abstract this study investigates petrophysical and acoustic properties of experimentally compacted reconstituted samples of seal sequences from the southwestern barents sea the aggregates were collected from drill cuttings of mudstone and shale formations of two exploration wells 7220 10 1 salina discovery and 7122 7 3 goliat field the washed and freeze dried samples were characterized for grain size distributions geochemical analyses and mineralogical compositions a total of 25 compaction tests 12 dry and 13 brine saturated were performed with a maximum effective vertical stress of 50 mpa the laboratory measurements demonstrated that petrophysical and acoustic properties of argillaceous sediments can change within a sedimentary basin and even within a given formation the results show that the collected aggregates from goliat field are compacted more compared to salina discovery the maximum and minimum compaction are measured in samples collected from snadd and fuglen formations respectively the final porosity of brine saturated specimens varies between 5 and 22 the ultrasonic velocity measurements depict that samples with the same porosity values can have a broad range of velocity values the resulting compaction trends in this study were compared to published compaction curves for synthetic mixtures of quartz and clay all compaction trends show higher porosity reduction than the silt fraction with 100 quartz comparison of experimental compaction result of each mudstone and shale aggregate with its corresponding acquired well log data helps to delineate the burial history and exhumation in the study area a net exhumation of 950 m and 800 m is estimated at salina and goliat wells respectively the outcomes of this study can provide insights for hydrocarbon prospect discovery in a pre mature sedimentary basin in terms of exploration and production and also for geological co2 storage sites the experimental results may provide information for well log and seismic interpretation basin modeling and seal integrity of investigated horizons

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

titanite rutile thermochronometry across the boundary between the archaean craton in karelia and the belomorian mobile belt eastern baltic shield abstract u pb isotopic dating has been carried out on titanites and rutiles from the karelian protocraton the belomorian mobile belt and the intervening junction zone these are some of the principal archaean crustal units in the baltic shield which have undergone regeneration to various degrees during the palaeoproterozoic palaeoproterozoic resetting of u pb titanite ages was complete in the belomorian belt and almost complete in the junction zone while it hardly affected the karelian protocraton in the latter major crustal cooling occurred at 2 71 2 69 ga after a major igneous event at 2 74 2 72 ga but a tectonothermal event at 2 65 2 64 ga was less comprehensive in the belomorian belt a northeastern marginal zone immediately underlying the collisional thrusting suture of the lapland kola orogen has somewhat higher titanite ages of ca 1 94 1 87 ga than the central zone where these ages range between 1 87 and 1 82 ga comparison between the titanite and rutile u pb ages suggests a postorogenic cooling rate between 2 and 4 ma in these parts of the belt the neoarchaean junction zone between the karelian and belomorian provinces was a zone of particularly intense tectonic magmatic and hydrothermal activity during or after the palaeoproterozoic lapland kola orogeny dominant newly grown titanites in that zone have ages as young as 1 78 1 75 ga and the age differences between the titanite and rutile u pb ages are substantially smaller than elsewhere

u pb zircon age dating of a rapakivi granite batholith in rangnim massif north korea rapakivi granites and several small leucogabbroic and gabbroic bodies are located in the rangnim massif north korea the largest batholith in the myohyang mountains covers an area of 300 km 2 and was intruded into precambrian metamorphosed rocks it has a shrimp u pb zircon weighted mean 207 pb 206 pb age of 1861 7 ma the country rocks of rapakivi granites are neoarchaean orthogneisses and palaeo mesoproterozoic graphite bearing metasedimentary rocks of granulite facies and they are similar to those of the rapakivi granites and anorthosites exposed in south korea and in the north china craton we conclude that the three massifs in the korean peninsula commonly record an identical palaeo mesoproterozoic anorogenic magmatic event indicating that they have a common precambrian basement with the north china craton

oxygen isotope equilibrium between eclogite minerals and its constraints on mineral sm nd chronometer abstract sm nd and oxygen isotope analyses were carried out for mineral separates of ultrahigh pressure eclogites from the sulu terrane in eastern china the results show a direct correspondence in equilibrium or disequilibrium state between the oxygen and sm nd isotope systems of eclogite minerals the omphacite garnet pairs of oxygen isotope equilibrium at eclogite facies conditions yield meaningful triassic sm nd isochron ages whereas those of oxygen isotope disequilibrium give non triassic ages of geological meaninglessness this can be reasonably interpreted by the fact that the rates of oxygen diffusion in garnet and pyroxene are lower than or close to those of nd diffusion and thus attainment of isotopic equilibrium in the omphacite garnet o system suggests achievement of nd isotope equilibrium in the same mineral pairs the presence or absence of fluid in the eclogite protoliths is a major rate controlling factor for isotopic equilibration during high grade metamorphism it appears that the state of oxygen isotope equilibrium between cogenetic minerals can provide a critical test for the validity of the sm nd mineral chronometer in addition the exact timing of the ultrahigh pressure metamorphism in the dabie sulu terranes is constrained at early triassic rather than late triassic

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

calculation of oxygen isotope fractionation in metal oxides abstract a theoretical model calculating the thermodynamic isotope factors of oxygen in metal oxides of geochemical interest is presented for temperatures between 0 and 1200 c on the basis of the modified increment method the following order of 18o enrichment is obtained cassiterite rutile magnetite ilmenite ulvospinel hematite uraninite chromite corundum spinel the results are expressed as 103 in a 106 t2 b 103 t c which compare well with existing theoretical experimental and empirical calibrations on quartz oxides and oxides water systems the present calculations enable not only estimation of the oxygen isotope composition of fluids from which the metal oxides were precipitated when independent temperature information is available but can also be applied as oxygen isotope geothermometers to igneous and metamorphic rocks

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

a precise u pb age on cassiterite from the xianghualing tin polymetallic deposit hunan south china we report the first precise u pb isotope data on cassiterite from the large xianghualing tin polymetallic deposit in the central nanling district south china the results show that four separates from sample xf 51 have a relatively narrow range of 206pb 238u apparent ages varying from 152 to 157 ma and the three 206pb 238u apparent ages yield a weighted average value of 156 4 ma mswd 0 32 separates from two other cassiterite samples do not have sufficient radiogenic pb to generate a reliable 206pb 238u age seven separates from the above three cassiterite samples define a well constrained 238u 206pb isochron corresponding to an age of 157 6 ma mswd 34 a comparison of the u pb cassiterite ages with published ar ar dates on muscovite from this deposit and k ar age data on biotite from the pluton genetically related to the tin mineralization in this area demonstrates that the u pb isotope system of cassiterite is a potential geochronometer combined with the ar ar dates of muscovite from this deposit we can constrain the absolute age of tin polymetallic mineralization in xianghualing at 154 157 ma the dates obtained in this study consistent with the published geochronological results from other important deposits in this region reveal that the large scale tungsten tin mineralization in the central nanling region was predominantly emplaced during 150 161 ma

sm nd and rb sr isotopic chronology and cooling history of ultrahigh pressure metamorphic rocks and their country rocks at shuanghe in the dabie mountains central china abstract ultrahigh pressure metamorphic uhpm rocks at shuanghe area in the dabie mountains occur as a uhpm block within the regional granitic gneiss a sm nd isochron defined by garnet omphacite rutile from coesite bearing eclogite yield an age of 226 3 3 2 ma another sm nd isochron defined by garnet two phengites form uhp gneiss yield a similar age of 226 5 2 3 ma the two consistent sm nd ages defined by three uhpm minerals suggest that nd isotopic equilibrium between uhpm minerals in these rocks during uhp metamorphism has been achieved this may correspond to the age of peak metamorphism with an average metamorphic temperature of 800 c it is demonstrated that the retrograde metamorphism of uhpm rocks occurred in an open chemical system whereas the nd and sr isotopic systematics of uhpm minerals may have remained closed nd and sr isotopic disequilibrium between uhpm minerals and retrograde metamorphic minerals has been observed therefore the tie line of garnet or phengite and whole rock containing retrograde metamorphic minerals gives an old sm nd age and a young rb sr age respectively with no geologic significance however a rb sr ages of 219 0 6 6 ma defined by phengite garnet from a uhpm gneiss indicates that the uhpm rocks at shuanghe cooled down to 500 c at that time it suggests that the uhpm rocks at shuanghe experienced the first rapid cooling during 226 219 ma on the other hand the rb sr ages of 174 7 8 to 169 2 3 3 ma defined by retrograde minerals amphibole or biotite with closure temperature ranging from 450 to 300 c and intensely retrograded metamorphic rocks reflect a second rapid cooling during this time interval this is consistent with the rapid cooling time of 190 170 ma of the orthogneiss in the dabie su lu uhpm belt obtained by ar ar dating method in contrast to uhpm rocks the nd isotopic composition of the garnet in granitic gneiss has been reset during retrograde metamorphism and is in equilibrium with those of retrograde epidote and biotite which yields a sm nd isochron age of 213 5 ma indicating the retrometamorphic time corresponding to amphibolite facies in addition the biotites from the granitic gneiss yield rb sr ages of 171 173 ma similar to those of the uhpm rocks these data suggest that the country rocks granitic gneiss may have a similar cooling history to the uhpm rocks at shuanghe two stages of rapid cooling of uhpm rocks at shuanghe may correspond two stages of fast uplift the initial rapid uplifting and cooling of uhpm rocks during 226 219 ma may be caused by compression tectonics during subducting time of the continental crust whereas the later rapid cooling may reflect the exhumation of the entire subducted continental crust by extension during the early middle jurassic

common pb isotope mapping of uhp metamorphic zones in dabie orogen central china implication for pb isotopic structure of subducted continental crust abstract we report pb isotopic compositions for feldspars separated from 57 orthogneisses and 2 paragneisses from three exhumed uhpm slices representing the north dabie zone the central dabie zone and the south dabie zone of the dabie orogen central east china the feldspars from the gneisses were recrystallized during triassic continental subduction and uhp metamorphism precursors of the orthogneisses are products of neoproterozoic bimodal magmatic events those in north dabie zone emplaced into the lower crust and those in central and south dabie zones into middle or upper crust respectively on a 207 pb 204 pb vs 206 pb 204 pb diagram almost all orthogneisses data lie to the left of the 0 23 ga paleogeochron and plot along the model mantle evolution curve with the major portion of the data plotting below it on a 208 pb 204 pb vs 206 pb 204 pb diagram the most of data of north dabie zone extend in elongate arrays along the lower crustal curve and others extend between the lower crustal curve to near the mantle evolution curve for the plumbotectonics model this pattern demonstrates that the pb isotopic evolution of the feldspars essentially ended at 0 23 ga and the orthogneiss protoliths were principally dominated by reworking of ancient lower crust with some addition of juvenile mantle in the neoproterozoic rifting tectonic zone according to geological evolution history of the locally dabie orogen a four stage pb isotope evolution model including a long time evolution between 2 0 and 0 8 ga with a lower crust type u pb ratio 5 6 suggests that magmatic emplacement levels of the protoliths of the orthogneisses in the dabie orogen at 0 8 ga also play an important role in the pb evolution of the exhumed uhpm slices corresponding to their respective pb characters at ca 0 8 0 23 ga for example north dabie zone requires low values 3 4 9 6 while central and south dabie zones require high values 10 9 17 2 on the other hand pb isotopic mixing between north and central or south dabie zones during retro grade metamorphism enhanced by the extensive magmatism in the cretaceous has also been observed in the 207 pb 204 pb vs 206 pb 204 pb and 208 pb 204 pb vs 206 pb 204 pb diagrams a combined study of common pb isotopic compositions of dabie orthogneisses and sulu uhpm rocks from the chinese continental scientific drilling project demonstrates that a slab marked by extremely unradiogenic pb observed in the main hole was absent in the dabie orogen however occurrence of some mesozoic granitoids with such unradiogenic character in the dabie orogen suggests that their source may be a buried unradiogenic unit underlying below north dabie zone this case study clearly shows that whether the position of the dabie data relative to the orogen curve of the plumbotectonic model is helpful in understanding the pb isotopic structure and evolution of subducted continental crust

are the alteration halos of massive sulfide deposits syngenetic evidence from u pb dating of hydrothermal rutile at the kidd volcanic center abitibi subprovince canada the kidd volcanic complex is composed of felsic volcanic and subvolcanic rocks of archean age the felsic rocks are intercalated with sedimentary rocks and are in contact with komatiitic ultramafic rocks large diorite plutons were emplaced into the rhyolite and the complex is overlain by younger basalts metasomatic events affecting the lithology of the kidd volcanic complex include silicification extensive co 2 metasomatism carbonate k metasomatism sericite fuchsite and chlorite and minor carbonate alterations petrographic evidence supported by stable isotope and fluid inclusion studies suggests that silicification and early carbonate alteration were synvolcanic and therefore related to ore deposition during subsequent extensive k metasomatism sericite precipitated in the rhyolite and fuchsite precipitated in the ultramafic rocks although chlorite post dates k metasomatism the micas and chlorite are both found in anastomosing microfissures commonly occupying the same set of fractures hydrothermal rutile formed by the breakdown of magnetiteilmenite during k metasomatism and chlorite alteration gives an age of 2624 62 ma 95 confidence level by pb pb isotopic measurements it is therefore approximately 100 m y younger than syngenetic massive sulfide mineralization 2717 2 ma sulfide stringers within sericite and chlorite veins suggest some remobilization of the ores during these later events this alteration assemblage generally thought to be intimately associated with mineralization is identical to that found associated with many lode gold deposits in the superior province recent dating of micas and rutile associated with gold deposits in the abitibi subprovince gives comparable ages to the rutile in the kidd volcanic complex which must therefore record a widespread late hydrothermal event affecting mineralized rocks

compaction of lithic sands experimental results and applications compaction experiments showed that the degree of physical compaction of a lithic sand is related to the amount and type of lithic material present these experiments produced a product that is texturally indistinguishable from natural lithic sandstones generally sedimentary lithic fragments are more ductile than metamorphic lithic fragments volcanic lithic fragments when altered to phyllosilicate minerals by weathering or diagenesis are extremely ductile mineralogy and intragranular microporosity influence ductility ratios based on weight percent of soft ductile minerals clay and micas to hard brittle minerals quartz and feldspar within the lithic fragments provides a good indication of ductility a ratio greater than 1 indicates the lithic material will be h ghly ductile microporous lithic fragments are more ductile than dense lithic fragments physical compaction models were developed based on experimental data from well sorted lithic sandstones for which the percentage of preserved porosities were functions of the effective stress the type of lithic material and the quantity of lithic material these models are derived from laboratory sandstones with at least 25 lithic grains in the following categories moderately ductile metamorphic lithic fragments highly ductile shale lithic fragments and extremely ductile altered volcanic lithic fragments application of the compaction models of the experimental compaction techniques to modern subsurface sand samples or outcrop equivalents can provide valuable estimates of the preserved porosities i e reservoir potential for lithic sandstones that may be exploratory objecti es in frontier basins examples using the models and experimental compaction techniques for exploration are given experimental data showed that precompaction cement and overpressure may be beneficial to porosity preservation an early formed partial cement can retard compaction and preserve porosity by stabilizing the sand pack a later cement does not have this beneficial effect overpressure when developed early retards compaction by reducing the effective stress of course a late developing overpressure is not effective at preserving porosity because compaction is an irreversible process

diffusion effects on oxygen isotope temperatures of slowly cooled igneous and metamorphic rocks recently obtained data on oxygen diffusion in feldspars quartz and hornblende permit the prediction of the apparent18o16o temperatures that would be measured in a rock that consisted only of those three minerals and cooled slowly from high temperature the computed temperatures would be based on the differences in the18o16o ratios between coexisting pairs of minerals the present calculation takes into account the diffusion rates for oxygen as a function of temperature the cooling rate of the rock the mineral grain sizes and the mode of the rock for mineral grains 1 mm in radius and a cooling rate of 10 c m y the minimum difference in apparent temperature between quartz feldspar and feldspar hornblende pairs will be 115 c despite the assumption of a normal uneventful slow cooling history to room temperature further the apparent quartz hornblende temperature will range over 30 c 590 620 c depending on the mode of the rock for a cooling rate of 1000 c m y the apparent difference in temperature can be as much as 400 c consequently consistency in temperatures obtained by oxygen isotope analysis should not be expected in most high grade metamorphic rocks or igneous rocks which are cooled slowly departures from the pattern of temperatures obtained in this model would imply a very rapid quench from high temperature or a complex history for the rock for some minerals including hornblende the relation between temperature and the equilibrium fractionation of oxygen isotopes between coexisting phases has been derived from observed relations in natural specimens the choice of the specimens used for such calibrations needs to be re evaluated in light of these findings this may result in a change in the equilibrium equation constants an example from the literature the san jose tonalite baja california mexico was modelled and yields 18o concentrations in the minerals that correspond closely with the measured values this suggests that the model used is appropriate that the rock has had a simple thermal history and that it cooled at 100 200 c m y over the temperature range 800 500 c the set of paleotemperatures obtained for a rock will in general yield neither the mineral closure temperatures nor the formation or crystallization temperatures on the other hand the cooling rate of the rock may be derived from the data this in turn may have important tectonic implications with regard to denudation and uplift rates

u pb zircon and rutile ages from granulite xenoliths in the slave province evidence for mafic magmatism in the lower crust coincident with proterozoic dike swarms zircon and rutile in two garnet bearing mafic granulite xenoliths from a kimberlite in the archean slave province record proterozoic ages similar to the 1 27 ga mackenzie dike swarm or similar to the ca 2 23 2 21 ga malley mckay diabase dike swarms the xenolith sample site currently overlies thick lithosphere more than 700 km to the southeast of the proposed mackenzie plume center because lower crust was effectively shielded from the thermal effects of an upwelling plume by thick lithospheric mantle heat was most likely advected into the lower crust by lateral intrusion of mafic sills over distances of hundreds of kilometres from magma generation zones nearer the plume center

excess argon in phengite from eclogite evidence from dating of eclogite minerals by smnd rbsr and 40ar39ar methods the existence of excess ar in phengites from coesite bearing ultrahigh pressure eclogites of the dabie mountains and su lu terrain china has been confirmed by dating of minerals using sm nd rb sr and 40ar39ar methods 40ar39ar ages of phengites for two samples studied are much older than the sm nd and rb sr mineral isochron ages sample ql 1 has rb sr age of 219 5 0 5 ma sm nd age of 226 3 4 5 ma and phengite 40ar39ar age of 877 5 15 9 ma the corresponding ages for sample zb 4 are 223 9 0 9 ma rb sr 228 4 6 0 ma sm nd and 943 3 12 4 ma 40ar39ar phengite an age of 225 ma based on the sm nd and rb sr mineral isochron ages is consistent with the time inferred from paleomagnetic and geological data for collision of the north china and yangtse blocks and formation of these eclogites we call for greater caution in 40ar39ar dating of phengites from other occurrences of ultrahigh pressure environments

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

geochronology and geochemistry of leucosomes in the north dabie terrane east china implication for post uhpm crustal melting during exhumation migmatites are widespread in the north dabie ultrahigh pressure metamorphic terrane ndt of dabie orogen east china idiomorphic and poikilitic amphibole grains in both leucosome and melanosome contain inclusions of plagioclase quartz and biotite suggesting formation of leucosome by fluid present melting of biotite plagioclase quartz bearing protoliths at p 5 7 kbar t 700 800 c precise sims zircon u pb dating indicates that migmatization of dabie orogen initiated at 140 ma and lasted for 10 ma coeval with the formation of low mg adakitic intrusions in dabie orogen based on mineralogical petrographic and geochemical data leucosomes in ndt can be subdivided into three groups 1 high la yb n medium sr y group group i whose high dy yb n but medium sr y ratios are caused by amphibole and plagioclase residual during partial melting of dioritic to granodioritic gneisses 2 low la yb n low sr y group group ii whose flat hree patterns are produced by entrainment of peritectic amphiboles into melts derived from partial melting of dioritic gneiss 3 high la yb n high sr y and eu group group iii whose extremely high sr and eu but low other ree concentrations are caused by accumulation of plagioclase and quartz although group i and iii fall in the adakitic fields on la yb n yb n and sr y y diagrams they are chemically distinct from contemporary high pressure adakitic intrusions in dabie orogen in a series of geochemical indexes for example lower dy yb n and or sr y ratios at given la yb n ratio lower sr cao ratios lower rb concentration but higher k rb ratios therefore leucosomes are produced by anatexis of the exhumed ultrahigh pressure metamorphic rocks at middle crustal level instead of partial melting of thickened lower crust with garnet rich and plagioclase poor residual the coeval occurrence of migmatites and high pressure adakitic intrusions in dabie orogen indicates large scale partial melting of middle to thickened lower crustal column in the early cretaceous the required heat source may be the mantle heat conducting through the lithospheric mantle whose lower parts have been convectively removed

ion microprobe u pb age and zr in rutile thermometry of rutiles from the daixian rutile deposit in the hengshan mountains shanxi province china the daixian rutile deposit is located in the hengshan mountains in the trans north china orogen it is con sidered to be one of the largest rutile deposits in china with 6 million metric tons mt of contained titanium most of the rutile ores are hosted in garnet free anthophyllite gneiss with minor mg hornblende feldspar quartz phlogopite rutile zircon and titanite rutile grains are euhedral 0 02 to 0 50 mm in size contain 98 649 to 99 784 wt tio 2 and form chains thin layers along the foliation and dense aggregates rutiles are compositionally homogeneous and contain no detectable mineral inclusions except local ilmenite lamellae and zircon crystallization temperatures of the rutile are estimated at 640 c at 0 7 gpa and 647 c without pres sure calibration according to the zr in rutile thermometer recording amphibolite facies metamorphism of an intermediate p t ratio series variations in nb versus cr in rutiles indicate a connection of the ores to mafic protolith not a pelitic rock derived from aluminous sedimentary rocks sims u pb analyses of rutiles from the deposit yield a mean 207 pb 207 pb age of 1780 2 9 6 ma considering the closure temperature up to 650 c grain sizes and recrystallization of the rutile this age is more likely to represent closure and or recrystallization time rather than peak metamorphism period so the rutile deposit formed not younger than 1780 ma this unique garnet free rutile deposit was metamorphosed from mafic rocks in amphibolite facies during the paleoproterozoic or archean being distinct from any other metamorphic rutile deposits such as the known eclogite related types

metamorphic evolution mineral chemistry and thermobarometry of schists and orthogneisses hosting ultra high pressure eclogites in the dabieshan of central china as is typical of ultra high pressure uhp terrains the regional extent of the uhp terrain in the dabieshan of central china is highly speculative since the volume of eclogites and paragneisses preserving unequivocal evidence of coesite and or diamond stability is very small by contrast the common garnet xmn 0 18 0 45 phengite si 3 2 3 35 zoned epidote ps38 97 biotite titanite two feldspars quartz assemblages in the more extensive orthogneisses have been previously thought to have formed under low p t conditions of ca 400 50 c at 4 kbar however certain orthogneiss samples preserve garnets with xca up to 0 50 rutile inclusions within titanite or epidote and relict phengite inclusions within epidote with si contents p f u of up to 3 49 overlapping with the highest values 3 49 3 62 recorded for phengites in samples of undoubted uhp schists these and other mineral composition features such as a site deficiencies in the highest si phengites na in garnets linked to y yb substitution and al f ti 1 o 1 substitution in titanites are taken to be pointers towards the orthogneisses having experienced a similar metamorphic evolution to the associated uhp schists and eclogites re evaluated garnet phengite and garnet biotite fe mg exchange thermometry and calculated 5 rutile 3 grossular 2sio2 h2o 5 titanite 2 zoisite equilibria indicate that the orthogneisses may indeed have followed a common subduction related clockwise p t path with the uhp paragneisses and eclogites through conditions of pmax at ca 690 c 715 c and 36 kbar to tmax at ca 710 c 755 c and 18 kbar prior to extensive re crystallisation and re equilibration of these ductile orthogneisses at ca 400 c 450 c and 6 kbar the consequential conclusion that it is no longer necessary to resort to models of tectonic juxtapositioning to explain the spatial association of these dabieshan orthogneisses with undoubted uhp lithologies has far reaching implications for the interpretation of controversial gneiss eclogite relationships in other uhp metamorphic terrains

the growth interval of garnet in the uhp eclogites from the dabie orogen china zircon inclusions extracted from garnet from an ultrahigh pressure eclogite in the eastern part of the dabie orogen were investigated with high spatial resolution u pb geochronology the unzoned zircon rims have garnet and omphacite inclusions and show weak cathodoluminescence low th u ratios 0 02 0 18 flat high rare earth elements patterns and an absence of eu anomalies which suggest metamorphic growth in the presence of garnet and in the absence of feldspar dispersed concordia u pb ages ranging from 205 1 3 3 to 255 3 6 1 ma defined a weighted mean 206 pb 238 u age of 223 8 ma mean square of weighted deviation mswd 38 which is significantly younger than the lu hf garnet whole rock age of 242 2 2 6 ma the u pb age probability distribution indicates two age populations at ca 241 and ca 216 ma the zircon rims with younger ages are distinguished from those with older ages by the occurrence of garnet and omphacite inclusions well preserved prograde major and trace element zoning in garnet and the significant contribution of older garnet cores in bulk isotopic digestion suggest that the lu hf age mostly reflects an early phase of garnet growth likely at amphibolite facies conditions the new lu hf age combined with the u pb ages of the zircon inclusions in garnet suggest that garnet growth was sustained over a time interval of 25 m y

nb and zr behavior in rutile during high grade metamorphism and retrogression an example from the ivrea verbano zone abstract detailed textural observations and in situ analyses emp sims and la icp ms are used to characterize trace element behavior during prograde and retrograde metamorphic reactions involving rutile the ivrea verbano zone is a classic granulite area and rocks from the strona and d ossola valleys are an example of the amphibolite to granulite facies transition although different rock types occur in the area detailed sampling and petrographic work show that rutile only occurs in granulite facies paragneisses these rocks show a rich inventory of textures that allow not only for the investigation of trace element behavior in response to prograde rutile growth but also for the effect of post peak processes on rutile chemistry nb concentrations in rutile from lower grade samples show a larger spread from 500 to 5000 ppm within one sample when compared to those from higher grades this pattern can be modeled using prograde rutile growth formed from biotite breakdown zr concentrations in rutile are characterized by an anomalously large spread and a bimodal distribution maximum zr concentrations increase according to the general metamorphic gradient known for this area temperatures from zr in rutile thermometry although feasible are considerably higher than previous calculations increasing from ca 850 to 930 c a second cluster of zr concentrations in rutile occurs at rather constant concentrations ca 1000 ppm for all localities and is interpreted to be related to intense fluid influx at high temperature and or to post peak diffusional resetting favored by slow cooling rates alteration textures characterized by a complex network of micro veins are evidence for the late fluid influx the fluid strongly affected the rutiles which is evidenced by corrosion of older rutile grains and formation of rutile veinlets

temperature time paths in the high p manicouagan imbricate zone eastern grenville province abstract the manicouagan imbricate zone miz eastern grenville province is a series of lithotectonic units assembled from different crustal levels during the grenvillian orogeny structural evidence along with u pb dating suggests that two separate stages of metamorphism occurred at ca 1050 and 1020 ma respectively the first stage is characterised by high pressure and temperature high pt metamorphism up to eclogite facies with a range of maximum recorded pressures between 14 and 20 kbar consistent with variations in burial depth maximum t conditions recorded from apparently different structural levels are rather similar 800 900 c suggesting high heat flow during eclogite facies metamorphism evidence of high temperatures 700 800 c recorded during the early stage of tectonic exhumation indicate near isothermal decompression the second stage is a high t event coeval with the emplacement of syn metamorphic granite and is recorded by rocks in the upper most tectonic slice boundary zone in the south east miz this event may also be responsible for amphibolite facies overprinting across the rest of the miz using the u pb and pt data sets along with published diffusion rates for pb in monazite titanite and rutile tt paths have been modelled for several of the lithotectonic units in most of the units suggested cooling rates appear to have been relatively fast during initial exhuamtion but show a prolonged period of slower cooling 5 10 c ma for 30 40 ma apparently at odds with the steep near isothermal pt paths preservation of eclogitic textures and preservation of garnet zoning in the highest grade rocks on the other hand in the boundary zone the recorded cooling rates are much faster 20 c ma indicative of tectonic exhumation this phase of rapid cooling in the boundary zone corresponds in time with the slow cooling rates at 1030 1000 ma in the rest of the miz thus thrusting of the boundary zone over the rest of the miz appears to have caused a thermal overprint which was superimposed on the tt paths taken as a whole the tt paths for the miz are interpreted as representing two separate metamorphic events which were both relatively short in duration

differential exhumation and cooling history of north qaidam uhp metamorphic rocks nw china constraints from zircon and rutile thermometry and u pb geochronology abstract we analyze zircon and rutile u pb ages and trace elements using la icp ms and sims of eclogites from the north qaidam ultra high pressure metamorphic belt in nw china zr in rutile and ti in zircon thermometries give consistent peak metamorphic temperatures for two fresh eclogite samples zircon and rutile u pb dating from the yuka eclogite yielded ages of 430 2 3 7 ma and 407 16 ma respectively zircons from north dulan eclogite record two distinct eclogite facies metamorphic events at 446 6 4 1 and 430 5 4 7 ma with t ti in zircon 684 16 c and 649 26 c respectively while rutile u pb dating for the same north dulan eclogite sample yielded an age of 416 10 ma these rutile u pb ages are interpreted to record the time when these rocks cooled down to the closure temperature of pb in rutile here taken to be ca 570 c thus we obtain a slow cooling rate of 3 4 c ma for yuka and north dulan terranes for one granulite facies overprinted south dulan eclogite sample rutile u pb dating gave a younger age of 383 10 ma combined with literature chronological and temperature data for south dulan the data imply later exhumation than the other samples with a faster cooling rate of 6 6 c ma after the granulite facies event the t t paths obtained show that the north qaidam eclogites experienced considerably slower cooling history than previously assumed and also that while the thermal histories of the yuka and north dulan eclogites resemble one another the south dulan eclogite is quite distinct so a differential exhumation and emplacement of uhp metamorphic rocks occurred in the north qaidam

40ar 39ar age constraints on deformation and metamorphism in the main central thrust zone and tibetan slab eastern nepal himalaya in eastern nepal structural and petrologic observations suggest that movement on the main central thrust mct the development of an inverted metamorphic sequence and leucogranite plutonism were genetically related samples from across the mct zone and its hanging wall the tibetan slab were collected from four locations in the everest region for 40ar 39ar studies 1 the mct zone 2 the lower tibetan slab 3 the upper tibetan slab away from intrusive rocks and 4 the upper tibetan slab adjacent to and including leucogranitic material within the mct zone muscovite and hornblende yield cooling ages of 12 0 0 2 ma and 20 9 0 2 ma respectively k feldspar gives a minimum age of 8 0 0 2 ma closure temperature tc 220 15 c in the lower tibetan slab a sample from a sheared pegmatite yields muscovite and biotite isochron ages of 7 7 0 4 ma and 7 5 0 6 ma and a k feldspar minimum age of 6 4 0 8 ma tc 225 20 c an adjacent gneiss yields a 9 1 0 2 ma biotite isochron and a k feldspar minimum age of 3 6 0 2 ma tc 210 50 c in the upper tibetan slab samples collected 200 m from visible intrusives yield a biotite isochron age of 20 2 0 2 ma and a complex hornblende age spectrum suggestive of excess argon at the fourth location an augen gneiss a pegmatite and a tourmaline bearing leucogranite all in mutual contact yield indistinguishable mineral ages the average biotite and muscovite isochron ages for these samples are 17 0 1 4 and 16 6 0 2 ma respectively the minimum age for k feldspar from the leucogranite is 15 5 1 8 ma as thermobarometric data from the mct zone in this area suggest synmetamorphic deformation at temperatures of 500 550 c the mct hornblende tc 500 c dates this event at 21 ma diffusion experiments on hornblendes from the mct zone provide data which support a maximum duration of peak metamorphic temperatures of 2 ma biotite ages as old as 20 ma from the upper tibetan slab near leucogranites indicate that leucogranite intrusion was essentially coeval with deformation and metamorphism in the mct zone very young ages in a ductile shear zone in the lower tibetan slab suggest that there has been deformation within the tibetan slab that postdates major movement on the mct

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

diagenetic evolution of clay minerals in oil bearing neogene sandstones and associated shales mahakam delta basin kalimantan indonesia the clay fraction of the mahakam delta basin eastern kalimantan indonesia consists of mixed layer illite smectite kaolinite dickite detrital illite and chlorite kaolinite formed early as vermicular particle aggregates and thick dickite booklets formed late in pore spaces the dickite particles probably grew in deeper intervals of the sequence in contact with brines acidified by decarboxylation of organic matter during dissolution of k feldspar and upward migration of potassium in the handil field on the nearshore anticline evolution of the mixed layer illite smectite is characterized by a decrease in expandability with depth in the tunu field located on the next offshore anticline variable amounts of expandable layers suggest a different evolution the conversion process of smectite to illite layers in the mixed layers depends on the lithology of the host rocks transformation in the shales and dissolution precipitation in the sandstones sample location also played a role in the conversion process because the types of illite clays in the two locations differ when they are compared at similar degrees of evolution present day temperatures and paleotemperatures vitrinite reflectance and illitization of the mixed layer illite smectite all suggest that organic matter and clay particles evolved differently in the handil field but similarly in the tunu field integration of the analytical results obtained here and comparison with a two dimensional numerical model suggest that hydrocarbon generation took place in the deeper synclinal zones of the sequence and that oil migrated upward with brines probably inducing most of the illitization in the upper sequence both models emphasize the role of faults in channelizing fluid flow during the final stages of evolution of the basin

detrital zircon constraint on the timing of amalgamation between alxa and ordos with exploration implications for jinchuan type ni cu ore deposit in china abstract the jinchuan ni cu deposit which is the largest single magmatic sulfide ore deposit in the world is hosted by a mafic ultramafic intrusion in the alxa block northwestern china the timing of amalgamation between the alxa block and the adjacent ordos block which is important for precambrian reconstruction of north china craton and the search for another jinchuan type ore deposit is contentious the age distribution patterns of detrital zircon crystals with precambrian ages from the devonian and older sedimentary rocks of the alxa and ordos blocks are similar this supports the traditional view that the alxa and ordos blocks were amalgamated in the paleoproterozoic along the 1 95 ga khondalite belt long before the emplacement of the 830 ma jinchuan intrusion the u pb lu hf isotope data of comagmatic zircon crystals from the jinchuan intrusion together with its whole rock sm nd isotopes and trace element compositions given previously by other researchers indicate that the jinchuan ore bearing intrusion is the product of continental rift related basaltic magmatism which took place during the early stage of rodinia breakup based on these observations we conclude that in addition to the alxa block the adjacent regions in the north china craton such as the ordos basin the khondalite belt and the yinshan terrane are most promising for new discoveries of jinchuan type ni cu ore deposits

u pb dating of minerals in alteration halos of superior province massive sulfide deposits syngenesis versus metamorphism u pb geochronology of igneous zircon from rhyolitic host rocks to the archean kidd creek geco and winston lake massive sulfide deposits in the superior province of ontario shows that volcanism which accompanied mineralization occupied a narrow time span 2717 2 ma 2720 2 ma and 2723 2 ma respectively precise ages of hydrothermal monazite allanite and rutile from alteration zones surrounding the above deposits indicate that these minerals crystallized 40 70 million years after volcanism monazite from kidd creek mine is 2659 3 ma old in agreement with spatially associated 2664 25 ma old rutile monazite from a biotite schist at geoco mine gives a similar age of 2661 1 ma however monazite from a sericite schist which hosts the ore at geco mine is 2675 2 ma old abraded large monazite grains from three units in the winston lake deposit are coeval with biotite crystallization and record an age of 2677 2 ma approximately the same as monazite in the sericite schist at geco data points from allanite fractions from both the winston lake and geco deposits fall on a pb pb isochron that gives an age of 2672 5 ma rutile from winston lake gives a younger age of 2651 6 2 ma and may date retrograde alteration of biotite to chlorite the ca 2676 ma age of monazite from winston lake and in the sericite schist at geco mine probably dates a regional metamorphic event that affected most of the southern superior province the ca 2660 ma old monazite in the biotite schist at geco mine and in the chlorite sericite alteration at kidd creek may date later k metasomatism caused by metamorphically derived fluids that were focussed along old fault structures such fluids were also responsible for local sulfide remobilization monazite and rutile are spatially associated with chlorite and sericite alterations at kidd creek their young ages indicate that these originally syngenetic mineral assemblages may have been significantly affected by regional metamorphism formation of monazite at all three deposits studied was a result of significant ree remobilization during metamorphism the discrete character of syn metamorphic hydrothermal activity in different units of the same deposit as well as its synchroneity among different widely separated deposits requires a mechanism for episodic injection of heat and fluid into the crust on a regional scale these activities are broadly coeval with and probably related to plutonism within adjacent metasedimentary subprovinces and middle to lower crustal metamorphism in the superior province

constraints on the thermal evolution of the adriatic margin during jurassic continental break up u pb dating of rutile from the ivrea verbano zone italy the ivrea verbano zone ivz northern italy exposes an attenuated section through the permian lower crust that records high temperature metamorphism under lower crustal conditions and a protracted history of extension and exhumation associated partly with the jurassic opening of the alpine tethys ocean this study presents shrimp u pb geochronology of rutile from seven granulite facies metapelites from the base of the ivz collected from locations spanning 35 km along the strike of paleozoic fabrics rutile crystallised during permian high temperature metamorphism and anatexis yet all samples give jurassic rutile u pb ages that record cooling through 650 550 c rutile age distributions are dominated by a peak at 160 ma with a subordinate peak at 175 ma both 160 and 175 ma age populations show excellent agreement between samples indicating that the two distinctive cooling stages they record were synchronous on a regional scale the 175 ma population is interpreted to record cooling in the footwall of rift related faults and shear zones for which widespread activity in the lower jurassic has been documented along the western margin of the adriatic plate the 160 ma age population postdates the activity of all known rift related structures within the adriatic margin but coincides with extensive gabbroic magmatism and exhumation of sub continental mantle to the floor of the alpine tethys west of the ivrea zone we propose that this 160 ma early post rift age population records regional cooling following episodic heating of the distal adriatic margin likely related to extreme lithospheric thinning and associated advection of the asthenosphere to shallow levels the partial preservation of the 175 ma age cluster suggests that the post rift 160 ma heating pulse was of short duration the regional consistency of the data presented here which is in contrast to many other thermochronometers in the ivz demonstrates the value of the rutile u pb technique for probing the thermal evolution of high grade metamorphic terrains in the ivz a significant decoupling between zr in rutile temperatures and u pb ages of rutile is observed with the two systems recording events 120 ma apart

fluid inclusion characteristics and u pb rutile age of late hydrothermal alteration and veining at the musoshi stratiform copper deposit central african copper belt zaire hydrothermal veining at the musoshi stratiform copper deposit in zaire has caused extensive alteration in the arkosic footwall sediments and in the mineralized ore shale argillaceous siltstone horizon pervasive growth of secondary rutile biotite and muscovite in the wall rocks accompanied this activity whereas hematite biotite calcite k feldspar barite and accessory rutile occur with quartz in footwall veins in contrast veins cutting the ore shale carry cu fe sulfides uraninite and molybdenite with quartz calcite biotite k feldspar and minor rutile linear zones of particularly intense alteration and veining probably represent structurally controlled fluid conduits fluid inclusions in vein quartz from such zones identify an apparently halite saturated liquid at 397 degrees or 5 degrees c with approximately 39 wt percent nacl 15 wt percent kcl and minor amounts of co 2 and other components rutile is found pervasively both as a wall rock alteration phase and as large euhedra up to 3 cm in the quartz veins the hydrothermal growth of this normally very insoluble mineral is explained by the high temperature and salinity of the aqueous phase the widespread occurrence of rutile in the deposit makes it suitable for determining the age of hydrothermal alteration six samples of rutile picked from footwall and ore shale veining and alteration give a u pb concordia upper intercept of 514 or 2 m y 2 sigma this value is confirmed on a 207 pb 204 pb vs 206 pb 204 pb plot four of these samples contained common pb concentrations in excess of blank which yields a slope age of 513 or 5 m y 2 sigma a seventh sample from an unusual footwall hematite vein gave a slightly younger 207 pb 206 pb age 496 or 1 m y the 514 or 2 m y alteration event is considerably younger than the minimum age of the host sediments 602 or 20 m y and probably also postdates ore deposition believed to be older than 600 m y no unambiguous evidence was seen at musoshi to support a relationship between hydrothermal alteration and stratiform cu mineralization although sulfide textures were modified at this time as shown by intergrowths with rutile dated here at 514 m y the hydrothermal activity is concluded therefore to be a late overprint unrelated to cu ore deposition but perhaps related to compressional deformation and metamorphism associated with the lufilian orogeny

u pb and pb pb age constraints on paleoproterozoic magmatism deformation and gold mineralization in the omai area guyana shield abstract the omai intrusion centred au quartz vein system the largest au producer presently operating in the guyana shield was sampled for detailed u pb and pb pb geochronology and petrological investigation the age of a metavolcanic sub volcanic unit in the host rock sequence is 2120 2 ma zircon analyses from the main body dioritic rocks give u pb ages of 2094 6 2092 11 and 2096 11 10 ma magmatic titanite and apatite that grew in hornblende rich peripheral phases of the intrusion define a consistent pb pb age of 2094 1 ma in agreement with the zircon data colourless titanite and rutile from strongly altered phases of the intrusion along with low u apatite and feldspar define a significantly younger pb pb isochron age of 2002 5 ma the igneous ages agree with data from similar units in the guyana shield and west africa showing that 2100 ma was a time of significant intrusive activity the ages obtained for the deformed metavolcanic and undeformed intrusion at omai define a 26 2 ma bracket for trans amazonian deformation in central guyana previous fluid inclusion studies indicate that the mineralizing solutions at omai were too co 2 rich to form titanite and the titanite bearing sample is unmineralized suggesting that it was not altered by gold bearing solutions therefore the 2002 5 ma age is interpreted as a late hydrothermal overprint that formed titanite and reset rutile zircon and baddeleyite from a thick gabbro dyke of the avanavero suite which cuts the omai pluton define an age of 1794 4 ma ruling out the dyke as a source for the late thermal effects the hydrothermal age may record the passage of fluids released by deep crustal metamorphism due to late stage tectonic underplating as previously proposed for the superior province

mechanical compaction and ultrasonic velocity of sands with different texture and mineralogical composition this study presents the results of experimental compaction while measuring ultrasonic velocities of sands with different grain size shape sorting and mineralogy uniaxial mechanical compaction tests up to a maximum of 50 mpa effective stress were performed on 29 dry sand aggregates derived from eight different sands to measure the rock properties a good agreement was found between the gassmann saturated bulk moduli of dry and brine saturated tests of selected sands sand samples with poor sorting showed low initial porosity while sands with high grain angularity had high initial porosity the sand compaction tests showed that at a given stress well sorted coarse grained sands were more compressible and had higher velocities vp and vs than fine grained sands when the mineralogy was similar this can be attributed to grain crushing where coarser grains lead to high compressibility and large grain to grain contact areas result in high velocities at medium to high stresses the angular coarse to medium grained sands both sorted sands and un sorted whole sands showed high compaction and velocities vp and vs the small grain to grain contact areas promote higher deformation at grain contacts more crushing and increased porosity loss resulting in high velocities compaction and velocities vp and vs increased with decreasing sorting in sands however at the same porosity the velocities in whole sands were slightly lower than in the well sorted sands indicating the presence of loose smaller grains in between the framework grains quartz poor sands containing less than 55 quartz showed higher velocities vp and vs compared to that of quartz rich sands this could be the result of sintering and enlargement of grain contacts of ductile mineral grains in the quartz poor sands increasing the effective bulk and shear stiffness tests both from wet measurements and gassmann brine substitution showed a decreasing vp vs ratio with increasing effective stress the quartz rich sands separated out towards the higher side of the vp vs range the gassmann brine substituted vp and vs plotted against effective stress provide a measure of the expected velocity range to be found in these and similar sands during mechanical compaction deviations of actual well log data from experimental data may indicate uplift the presence of hydrocarbon overpressure and or cementation data from this study may help to model velocity depth trends and to improve the characterization of reservoir sands from well log data in a low temperature 80 100o c zone where compaction of sands is mostly mechanical

seismic velocities of unconsolidated sands part 1 pressure trends from 0 1 to 20 mpa knowledge of the pressure dependences of seismic velocities in unconsolidated sands is necessary for the remote prediction of effective pressures and for the projection of velocities to unsampled locations within shallow sand layers we have measured the compressional and shear wave velocities and bulk shear and pwave moduli at pressures from 0 1 to 20 mpa in a series of unconsolidated granular samples including dry and water saturated natural sands and dry synthetic sand and glass bead samples the shear wave velocities in these samples demonstrate an average pressure dependence approximately proportional to the fourth root of the effective pressure vsp 1 4 as commonly observed at lower pressures for the compressional wave velocities the exponent in the pressure dependence of individual dry samples is consistently less than the exponent for the shear wave velocity of the same sample averaging 0 23 for the dry sands and 0 20 for the glass bead samples these pressure dependences are generally consistent over the entire pressure range measured a comparison of the empirical results to theoretical predictions based on hertz mindlin effective medium models demonstrates that the theoretical models vastly overpredict the shear moduli of the dry granular frame unless the contacts are assumed to have no tangential stiffness the models also predict a lower pressure exponent for the moduli and velocitiesvp1 6 than is generally observed in the data we attribute this discrepancy in part to the inability of the models to account for decreases in the amount of slip or grain rotation occurring at grain to grain contacts with increasing pressure

relationships between o isotope equilibrium mineral alteration and rb sr chronometric validity in granitoids implications for determination of cooling rate a combined study of mineral o and rb sr isotopes was carried out for a number of mesozoic granitoids in china in order to compare the degree of o isotope equilibrium between coexisting minerals with the validity of mineral rb sr isochrons for granitoids a scrutiny of both o isotope geothermometry and rb sr internal isochron dating for corresponding minerals indicates that equilibrium o isotope fractionation between rb sr isochron minerals corresponds to geologically meaningful isochron ages if the variation in 87rb 86sr ratio is big enough to provide reasonably small uncertainties in age significant deviation of the rb sr isochron age from the actual age appears to depend on the difference in sr isotopic composition between an external fluid and the igneous minerals as a result o isotope disequilibrium is often caused by interaction between the rock and the external fluid that results in mineral alteration post magmatic alteration can cause isotope exchange between the minerals and an internally buffered fluid that is isotopically identical to the host rock the o isotope composition of coexisting minerals in studied samples changed principally due to a decrease in temperature both rb and sr concentrations and the sr isotope ratios of isochron minerals also changed due to the mixing of different sr reservoirs nevertheless the isochron age can remain unchanged if the mixing took place along the isochron chord between the internal fluid and the minerals from that newly altered minerals formed this provides an insight into the effect of internal and external fluids on the validity of mineral rb sr chronometry in addition an alternative approach is proposed to construct the cooling curve by a combined use of o isotope temperature and mineral isotope age for the granitoids of interest comparing with the traditional method using the empirical closure temperature for rb sr chronometry the proposed approach utilizes fewer variables with smaller uncertainties than the traditional way

constraints on pb closure temperature in titanite based on rocks from the ungava orogen canada implications for u pb geochronology and p t t path determinations on the basis of multiequilibrium metamorphic pressure temperature p t estimates we have determined that 660 700 c is a reasonable minimum estimate of the closure t of pb diffusion in titanite a temperature significantly higher than previously suggested activity corrected reactions that produce titanite define narrow t stability fields 40 c that correspond closely to metamorphic p t determinations derived for coronitic garnet clinopyroxene hornblende samples growth of titanite occurring between 1789 and 1814 ma in the underthrust archean basement of the paleoproterozoic ungava orogen northern quebec is associated with retrograde metamorphism during underthrusting of the archean basement approx 1 b y after formation of the original granulitic assemblages

pb diffusion in rutile diffusion of pb was measured in natural and synthetic rutile under dry 1 atmosphere conditions using mixtures of pb titanate or pb sulfide and tio2 as the sources of diffusant pb depth profiles were then measured with rutherford backscattering spectrometry rbs over the temperature range 700 1100 c the following arrhenius relation was obtained for the synthetic rutile d 3 9 10 10exp 250 12 kj mol 1 rt m2s 1 results for diffusion in natural and synthetic rutile were quite similar despite significant differences in trace element compositions mean closure temperatures calculated from the diffusion parameters are around 600 c for rutile grains of 100 m size this is about 100 c higher than rutile closure temperature determinations from past field based studies suggesting that rutile is more resistant to pb loss through volume diffusion than previously thought

linking the sulu uhp belt to the korean peninsula evidence from eclogite precambrian basement and paleozoic sedimentary basins abstract considerable debate on whether and how the sulu orogenic belt extends eastward to the korean peninsula has remained over the past decade new results reported here include the following 1 an eclogite and retrograded eclogite bearing complex hongseong complex is discovered in south korea in which the eclogite occurs as lenses in circa 810 820 ma granitic gneiss shrimp zircon dating of the eclogite yields 230 ma for the metamorphic age and 880 ma for the protolith age 2 the basement of the rangnim gyeonggi and yeongnam massifs have affinities to the basement of the north china block ncb however the gyeonggi massif encloses a minor amount of large or small slabs of the hongseong complex that are similar to the rocks of the sulu belt 3 two main paleozoic basins within the rangnim and gyeonggi massifs have a similar paleozoic tectono stratigraphy to the ncb 4 the imjingang and ogcheon belts do not exhibit any metamorphic characteristics of collisional orogenic belts based on these facts we propose a crustal detachment and thrust model and suggest that the collision belt between the yangtze block yb and ncb sino korea craton is preserved along the western margin of the korean peninsula the lower part of the uhp metamorphosed lithosphere of the yb was subducted under the korean peninsula and not uplifted to the surface the lower crust of the yb the hongseong complex was detached from the subducted lithosphere and thrust over the korean peninsula and inserted into the basement rocks of the gyeonggi massif the upper crust of the yb possibly was detached from the lower crust and overthrusted along the honam and chugaryong shear zones the imjingang and ogcheon belts possibly represent the detached upper crust of yb and their present occurrences are controlled by a mesozoic strike slip shear structure all these detached lower and upper crustal slabs were strongly deformed during the late jurassic and early cretaceous tectonic event leading to their present geological distribution and characteristics

temporal relationship between granite cooling and hydrothermal uranium mineralization at dalongshan in china a combined radiometric and oxygen isotopic study abstract a combined study using multi radiometric dating and oxygen isotopic geothermometry was carried out for mesozoic quartz syenite alkali feldspar granite and associated hydrothermal uranium mineralization at dalongshan in the middle lower yangtze valley of east central china radiometric dating of the quartz syenite yields a whole rock rb sr isochron age of 135 6 4 3 ma a zircon u pb isochron age of 132 9 2 2 ma and k ar ages of 126 2 118 3 and 94 4 ma for hornblende biotite and orthoclase respectively the alkali feldspar granite yields a whole rock rb sr isochron age of 117 3 3 3 ma a zircon u pb isochron age of 114 7 2 1 ma and k ar ages of 112 2 109 3 and 88 4 ma for hornblende biotite and orthoclase respectively oxygen isotope thermometry for both granites gives temperatures of 685 to 720 555 to 580 435 to 460 and 320 to 330 c for hornblende magnetite biotite and orthoclase respectively when paired with quartz the systematic differences among the ages by the different techniques on the different minerals are used to reconstruct the cooling history of the granite the results yield rapid cooling rates of 27 4 to 58 6 c ma from 800 to 300 c in the early stage but slow cooling rates of 6 3 to 7 2 c ma from 300 to 150 c in the late stage the regular sequence of oxygen isotope temperatures for the different quartz mineral pairs demonstrates that diffusion is a dominant factor controlling the closure of both radiometric and o isotopic systems during granite cooling pitchblende u pb isochron dating yields an uranium mineralization age of 106 4 2 9 ma which is younger than the age of the granite emplacement and thus considerably postdates the time of magma crystallization but is close to the closure time of the k ar system in the biotite this points to a close relationship between granite cooling and ore forming process it appears that hydrothermal mineralization took place in the stage of slow cooling of the granite whereas the rapid cooling of the granite was concurrent with the migration of hydrothermal fluids along fault structures therefore the activity of the ore forming hydrothermal system is temporally dictated by the cooling rates of the granite and may lag about 25 to 30 ma behind the crystallization timing of associated granite

hydrogen and oxygen isotope evidence for fluid rock interactions in the stages of pre and post uhp metamorphism in the dabie mountains abstract hydrogen and oxygen isotope studies were carried out on high and ultrahigh pressure metamorphic rocks in the eastern dabie mountains china the 18 o values of eclogites cover a wide range of 4 2 to 8 8 but the d values of micas from the eclogites fall within a narrow range of 87 to 71 both equilibrium and disequilibrium oxygen isotope fractionations were observed between quartz and the other minerals with reversed fractionations between omphacite and garnet in some eclogite samples the 18 o values of 4 to 1 for some of the eclogites represent the oxygen isotope compositions of their protoliths which underwent meteoric water rock interaction before the high to ultrahigh pressure metamorphism heterogeneous 18 o values for the eclogite protoliths implies not only the varying degrees of the water rock interaction before the metamorphism at different localities but also the channelized flow of fluids during progressive metamorphism due to rapid plate subduction retrograde metamorphism caused oxygen and hydrogen isotope disequilibria between some of the minerals but the fluid for retrograde reactions was internally buffered in the stable isotope compositions and could be derived from structural hydroxyls dissolved in nominally anhydrous minerals

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

sims u pb rutile age of low temperature eclogites from southwestern chinese tianshan nw china article i nfo the geological evolution in the southwestern chinese tianshan orogen in nw china is poorly understood because of controversial geochronological results of the low temperature eclogites it is likely due to sm nd and rb sr isotopic disequilibrium in different mineral fractions excess 40 ar in the high pressure k bearing minerals and multi stage growths of zircon in u pb system rutile as a typical eclogite facies mineral with high u pb ratio is a feasible candidate to constrain the eclogite facies metamorphic event in this study we measured u pb rutile age data using sims cameca ims 1280 for low temperature eclogites from southwestern chinese tianshan orogen nw china we demonstrate that sims is a powerful technique for in situ u pb dating of rutile with u content as low as 1 ppm sims u pb rutile dating yielded an age of 318 7 ma which may record the time when the rocks cooled down to the closure temperature of pb in rutile ca 500 c but should be very close to the time of eclogite facies metamorphism in terms of common fast exhumation of coesite bearing eclogite this new proof strongly suggests a late carboniferous collision between kazakhstan yili block and central tianshan microcontinent

in situ sims u pb dating of hydrothermal rutile reliable age for the zhesang carlin type gold deposit in the golden triangle region sw china the contiguous region between guangxi guizhou and yunnan commonly referred to as the golden triangle region in sw china hosts many carlin type gold deposits previously the ages of the gold mineralization in this region have not been well constrained due to the lack of suitable minerals for radiometric dating this paper reports the first sims u pb age of hydrothermal rutile crystals for the zhesang carlin type gold deposit in the region the hydrothermal u bearing rutile associated with gold bearing sulfides in the deposit yields an u pb age of 213 6 5 4 ma which is within the range of the previously reported arsenopyrite re os isochron ages 204 19 to 235 33 ma for three other carlin type gold deposits in the region our new and more precise rutile u pb age confirms that the gold mineralization was contemporaneous with the triassic w sn mineralization and associated granitic magmatism in the surrounding regions based on the temporal correlation we postulate that coeval granitic plutons may be present at greater depths in the golden triangle region and that the formation of the carlin type gold deposits is probably linked to the coeval granitic magmatism in the region this study clearly demonstrates that in situ rutile u pb dating is a robust tool for the geochronogical study of hydrothermal deposits that contain hydrothermal rutile

porosity depth trends in reservoir sandstones theoretical models related to jurassic sandstones offshore norway abstract theoretical models of compaction and porosity modification in sandstones during burial are discussed theory predicts that mechanical compaction is the dominant porosity reducing mechanism during burial from 0 to 2 5 3 km porosity depth trends are at this stage mainly controlled by the stability of the grain framework and the effective stress time and temperature also influence the porosity depth gradients at shallow to moderate depths but are probably less important than effective stress and framework grain composition at greater burial depths below 2 5 3 km chemical compaction including pressure solution can explain most of the porosity reduction in well sorted clean arenites theoretical modelling of the intergranular pressure solution suggests that sandstones may lose porosity rapidly by this mechanism below 2 5 3 km and that factors capable of retarding pressure solution and quartz cementation are required to maintain prospective porosities below 4 km this theory and comparisons with empirical data from jurassic sandstones from offshore norway indicate that high porosity is favoured by 1 overpressures approaching the lithostatic pressure gradient 2 a short residence time below 3 km burial 3 coatings on grains of clay or microcrystalline quartz which prevent quartz dissolution and precipitation and 4 a high content of feldspar and early diagenetic secondary pores early hydrocarbon emplacement may retard quartz cementation and preserve porosity but chemical compaction probably continues after oil emplacement except at very high hydrocarbon saturations