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

oxygen diffusion in amphiboles the kinetics of oxygen isotope self diffusion in natural samples of hornblende tremolite and richterite have been measured samples were run under hydrothermal conditions using 18o enriched water profiles of 18o 16o 18o vs depth into the crystal were obtained using an ion microprobe the depths of sputtered holes were measured using an optical interferometer at 1000 bars 100 mpa water pressure the activation energies q and pre exponential factors d0 for diffusion parallel to c are d0 cm2 sec q kcal gm atom t c hornblende 1 20 1 10 741 6 650 800 tremolite 2 30 2 10 8 39 5 650 800 richterite 3 5 2 10 4 57 2 650 800 the diffusion coefficient d for hornblende at 800 c and 1000 bars water pressure measured parallel to the c crystallographic direction is at least ten times greater than that parallel to the a or b directions an increase in water pressure from 200 to 2000 bars increases d by a factor of 2 7 for diffusion parallel to c at 800 c the d value for hornblende at 800 c is about 0 01 that for quartz and 0 001 that for anorthite as a result closure temperatures for oxygen exchange in natural primary amphiboles are significantly higher than for quartz or feldspars it is unlikely that amphiboles will exchange oxygen isotopes by diffusion under most crustal conditions

inherited smnd isotope components preserved in monazite inclusions within garnets in leucogneiss from east antarctica and implications for closure temperature studies abstract application of the sm nd dating technique using garnet can be hampered by the presence of minor microscopic to submicroscopic ree rich inclusions in particular monazite and to a lesser extent zircon an acid leaching method using a mixture of hno 3 hcl has been used to tackle the monazite contamination problem with this leaching technique we demonstrate that inherited monazite inclusions have been preserved in garnet formed under high grade metamorphic conditions even very low levels of monazite contamination 0 001 wt have a profound effect on the sm nd system in the garnet analysis of a garnet from a leucogneiss dyke in the northern prince charles mountains eastern antarctica reveals that unleached garnet concentrates and the monazite inclusions as represented by the leachates define an apparent isochron of 1150 30 ma mswd 1 88 which represents a mixing age of inclusions preserving their inherited sm nd isotopic systematics in contrast analyses of the leached garnet concentrates yield much younger ages of 630 to 555 ma model calculations show that the presence of as little as 0 01 wt of monazite inclusions would swamp the sm nd isotope signature of the garnet concentrates this very low level of contamination is beyond the reach of physical purification processes and without leaching contamination is inevitable in inclusion bearing garnets such contamination can lead to older apparent ages than those of garnet formation and thus has profound implications for sm nd as well as u pb garnet chronometry

palaeomagnetic study of llandovery lower silurian red beds in north west england high unblocking temperature magnetization components carried by haematite have been isolated from 12 sites of llandovery lower silurian red mudstones from nw england the principal deformation phase in this area is acadian mid devonian however some faults with related folds were also active during the later variscan orogeny the site mean magnetization directions pass a regional fold test at the 95 per cent confidence level implying that the magnetization components pre date acadian folding the overall mean direction declination 43 4d inclination 24 1d 95 12 4d gives a pole position at 13 6ds 313 9de and a palaeolatitude of 12ds the low palaeolatitude implies closure of the iapetus ocean between eastern avalonia and laurentia by late llandovery time and is consistent with a recently published palaeolatitude 13ds for the wenlock mid silurian of southern britain comparison of these values with the published palaeolatitude from nw england of 43ds for caradoc late ordovician time implies very rapid at least 15 cm yr 1 northward drift of eastern avalonia during late ordovician early silurian time the new pole position from the llandovery red beds lies close to late silurian and early devonian poles from scotland and wales but is significantly different from a group of mid silurian poles comprising data from scottish intrusions and from swedish sediments this discrepancy may be due to late silurian early devonian pre acadian remagnetization of our sampling sites or local clockwise rotation of the sampling sites during acadian and or variscan deformation however the positive fold test presence of reversals and local deformation history tend to mitigate against these possibilities the more likely interpretation is that although baltica scotland were one continental block from the mid silurian scandian orogeny eastern avalonia did not become part of this block until latest silurian

oxygen diffusion in quartz diffusional oxygen exchange between quartz and water was studied by determining 18o profiles in the quartz using an ion microprobe inverse error function plots of the data are linear as required for a diffusion mechanism one natural and two synthetic samples were used and although they had different impurity concentrations oh na and al per 106 si atoms are 7 4000 35 835 and 67 127 respectively the d values were the same within experimental uncertainty diffusion parallel to c is about 2 orders of magnitude faster than that normal to c at 700 c and diffusion normal to the rhombohedron is intermediate the data indicate a change in slope on an arrhenius plot at the boundary at 100 mpa 1 kbar water pressure the preexponential factors do in square centimeters per second and activation energies q in kilojoules per mole for quartz 500 550 c are as follows do 190 and q 284 parallel to c and do 8 10 2 and q 238 normal to the rhombohedron for quartz 600 800 c do 4 10 7 and q 142 parallel to c do 9 10 7 and q 155 normal to the rhombohedron and do 1 10 4 and q 234 normal to c d increases with ph 2 o and between 25 and 350 mpa log d versus log h 2 o has a slope of approximately 1 1 these d values are all several orders of magnitude larger than previously reported values for oxygen diffusion in water free systems

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

ultrahigh pressure metamorphic rocks from the chinese continental scientific drilling project i petrology and geochemistry of the main hole 0 2 050 m the main hole mh of the chinese continental scientific drilling project ccsd in southern sulu has penetrated into an ultrahigh pressure uhp metamorphic rock slice which consists of orthogneiss paragneiss eclogite ultramafic rock and minor schist recovered eclogites have a uhp metamorphic mineral assemblage of garnet omphacite rutile phengite kyanite coesite epidote ultramafic rocks contain garnet olivine clinopyroxene orthopyroxene ti clinohumite phlogopite gneisses and schists contain an amphibolite facies paragenesis but their zircons have coesite garnet omphacite or jadeite and phengite inclusions indicating that eclogites and gneisses have been subjected to in situ uhp metamorphism using available geothermobarometers p t estimates of 3 1 4 4 gpa and 678 816 c for eclogites were obtained if surface outcrops and neighboring shallow drill holes are considered together we suggest that a huge supracrustal rock slab 50 km long 100 km wide 5 km deep was subducted to a depth 100 km and then exhumed to the surface the depth interval 0 2 050 m of the ccsd mh can be divided into six lithological units unit 1 consists of alternating layers of quartz rich and rutile rich eclogites with thin interlayers of gneiss and schist eclogites of unit 1 are characterized by nb ta sr and ti depletions low mg number and general lree enrichment unit 2 comprises rutile and ilmenite rich eclogite and minor normal eclogite and is characterized by high tio2 total fe v co and sr and very low sio2 alkali zr ba nb ta and total ree contents and lree depleted ree patterns with slightly positive eu anomalies unit 3 contains ultramafic rock and minor mgo rich eclogite protoliths of uhp rocks from units 1 2 and 3 represent a layered mafic to ultramafic intrusion at crustal depth units 4 and 6 consist of interlayered eclogite and paragneiss the eclogites are characterized by th u nb ta and ti depletion and k enrichment and lree enriched ree patterns paragneisses show nb ta sr and ti depletions and lree enriched ree patterns occasionally with slightly negative eu anomalies indicating that their protoliths represent metamorphic supracrustal series unit 5 consists mainly of orthogneisses showing distinct nb ta sr and ti depletions and lree enriched ree patterns with pronounced negative eu anomalies suggesting granitic protoliths in conclusion it is proposed that the southern sulu uhp belt consists of a series of meta supracrustal rocks a layered mafic ultramafic complex and granites

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

partial equilibrium of radiogenic and stable isotope systems in garnet peridotite during ultrahigh pressure metamorphism a critical question in radiometric dating of metamorphic rocks is whether minerals used to define internal isochrons have achieved isotopic equilibrium during a given metamorphic event followed by preservation of the equilibrium afterward a garnet peridotite at zhimafang in the sulu terrane of eastern china shows incongruent u pb rb sr and sm nd ages zircon shrimp u pb dating yielded an isochron age of 224 8 ma pointing to zircon growth during triassic uhp metamorphism two mineral assemblages gave consistent rb sr internal isochron ages of 201 4 and 205 4 ma but much older sm nd internal isochron ages of 376 16 and 378 24 ma apparently the sm nd radiometric system fails to reequilibrate among the dated minerals during the triassic uhp metamorphic event for the garnet peridotite this result is confirmed by the state of oxygen isotope equilibrium or disequilibrium among the constituent minerals in the garnet peridotite in particular garnet is not in oxygen isotope equilibrium with any other of the analyzed minerals the degree of oxygen isotope disequilibria among the other minerals varies from pair to pair oxygen isotope equilibrium is observed only between orthopyroxene and olivine for both samples and between phlogopite and clinopyroxene for one sample it appears that the u pb rb sr sm nd and o isotope systems in minerals of the garnet peridotite are in partial equilibrium during uhp metamorphism i e equilibrium with respect to some isotopes and minerals but not all based on available experimental diffusion data our study of the combined u pb rb sr sm nd and o isotope systems in the garnet peridotite demonstrates that the time scale for the uhp metamorphism and subsequent hp eclogite facies recrystallization is possibly in the range of about 12 to 26 ma which was not long enough for the reequilibration of the sm nd and o isotope systems but just long enough for the reequilibration of the rb sr isotope systems therefore the rate of sr diffusion in phlogopite thus sr isotope reequilibration between the mafic minerals is constrained to be faster than rates of nd and o diffusion in garnet thus sm nd and o isotope reequilibration between the same minerals under conditions of subduction zone metamorphism

multicomponent magnetization in the helmsdale granite north scotland geotectonic implication abstract a palaeomagnetic study of the helmsdale granite u pb 420 m y k ar 400 m y northeast scotland has revealed a multicomponent remanence dominated by two characteristic axes of magnetization the suggested oldest of these magnetizations the direction of which is nearly horizontal and directed n s is thought to have been acquired in upper silurian lower devonian times the existence of this shallow direction of magnetization discounts a recent hypothesis of a ca 2000 km sinistral offset along the great glen fault the second component of magnetization appears to be partly carried by haematite that apparently formed through disintegration of biotite and or plagioclase this secondary magnetization has a direction that can be associated with a permian early mesozoic age similar overprinted magnetizations are characteristic features also in the devonian sedimentary sequences north of helmsdale

smnd isotopic systematics of a gabbro eclogite transition abstract sm nd isotopic systems have been studied in 1 a sequence of coronitic olivine gabbros showing different degrees of transition to eclogite 2 coronitic leuco gabbro norite and 3 country rock eclogite all from the nordoyane brattvag area western norway isochrons defined by combinations of whole rock samples and relict igneous phases give ages of 1198 56 and 1289 48 ma olivine gabbro and 926 70 ma leuco gabbro norite and have been interpreted in terms of two different episodes of igneous intrusion in gabbro with advanced corona formation relict augite tends to retain its original nd isotopic composition while plagioclase may show selective disturbance related to recrystallization and partial replacement by garnet in completely eclogitized samples there is a correlation between the degree of isotopic equilibrium and microstructural equilibrium whole rock eclogites occurring within gabbro are characterized by pseudomorphs after igneous phases and preserve original igneous nd isotopic composition results for eclogite minerals suggest caledonian ages but these phases have not achieved complete isotopic equilibrium in contrast nd isotopic systems in texturally equilibrated and strongly deformed eclogite within adjacent gneisses have been disturbed on a whole rock scale minerals from an external eclogite define a sm nd isochron with an age of 400 16 ma which is interpreted to date synkinematic eclogite equilibration at high p and t conditions

ultrahigh pressure metamorphic rocks from the chinese continental scientific drilling project ii oxygen isotope and fluid inclusion distributions through vertical sections in order to reconstruct the formation and exhumation mechanisms of uhp metamorphic terrains the chinese continental scientific drilling program ccsd has been carried out in donghai of the dabie sulu ultrahigh pressure uhp metamorphic belt east china eclogite gneiss amphibolite retrograded from eclogite ultramafic rocks and minor schist and quartzite have been drilled aiming to reveal the fluid behaviour in a vertical sequence of an uhp slab we investigated fluid inclusion and oxygen isotope characteristics of selected drillcores from the main hole and the pilot holes pp2 and zk 703 of the ccsd more than 540 laser ablation oxygen isotope analyses on garnet omphacite quartz kyanite amphibole phengite rutile epidote amphibole plagioclase and biotite from various rocks in the depth range of 0 3 000 m mainly eclogite and gneiss show that the investigated rocks can be divided into two groups 18o depleted rocks as low as 18o 7 4 for garnet indicate interaction with cold climate meteoric waters whereas 18o normal rocks with bulk 18o 5 6 have preserved the o isotopic compositions of their protoliths meteoric water rock interaction has reached depths of at least 2 700 m oxygen isotope equilibrium has generally been achieved isotopic compositions of mineral phases are homogeneous on a mm to cm scale regardless of lithology but heterogeneous on the scale of a few metres oxygen isotope distributions in the vertical sections favour an in situ origin of the uhp metamorphic rocks the very negative 18o eclogites usually have higher hydroxyl mineral contents than the normal 18o rocks indicating higher water content during uhp metamorphism fluid inclusion data suggest that rocks with depleted 18o compositions have had different fluid histories compared to those with normal 18o values rocks with depleted 18o mainly have primary medium to high salinity inclusions in omphacite kyanite and quartz and abundant secondary low salinity or pure water inclusions in quartz indicating a high salinity brine dominated fluid system during peak uhp metamorphism no carbonic inclusions have been identified in these rocks by contrast primary very high density co2 inclusions are commonly found in the rocks with normal 18o values these observations suggest that fluid and oxygen isotope composition of minerals are related and reflect variable degrees of alterations of the dabie sulu uhp metamorphic rocks

sm nd and rb sr dating of pyroxene garnetite from north dabie in east central china problem of isotope disequilibrium due to retrograde metamorphism abstract oxygen isotopes may serve as a guide to evaluate whether equilibria are preserved in the sm nd and or rb sr systems in eclogites that experience retrograde metamorphism this study demonstrates that sm nd rb sr and o isotopes for minerals from two portions of a clinopyroxene garnetite from north dabie in east central china record both equilibrium peak metamorphic conditions and disequilibrium conditions resulting from anhydrous retrograde metamorphism the minerals are homogeneous in terms of major elements and show no variation across the sample but clinopyroxene shows variable degrees of alteration pure primary mineral separates that are in o isotope equilibrium yield triassic sm nd isochron ages of 219 11 and 229 13 ma that match previously published zircon u pb ages for peak metamorphism thus the sm nd system in these minerals equilibrated during peak metamorphism and remained unaffected by retrograde metamorphism in contrast altered and impure clinopyroxenes that display oxygen isotope disequilibria fall off the sm nd isochron defined by the pure minerals implying that like oxygen the sm nd systems in these clinopyroxene were disturbed during retrogression no meaningful isochrons are found from the rb sr dating even in the pure primary minerals implying that these systems were disturbed during retrograde metamorphism and the diffusion rate of sr is faster than those of sm nd and o in mafic minerals our results suggest that the oxygen isotopes in metamorphic minerals can be used to evaluate the validity of sm nd mineral isochrons in dating peak metamorphic events in addition variations in the sm nd concentration and isotopic compositions of altered and impure clinopyroxenes from the two portions suggest heterogeneous effects on the minerals from external and internal fluids

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

sm nd dating and cooling history of scourian granulites sutherland the scourian gneisses of the lewisian complex of north west scotland have been the subject of many geochronological investigations sm nd whole rock measurements suggest that the protoliths of the lewisian complex differentiated 2 92 0 05 gyr ago from an approximately chrondritic mantle1 rb sr and pb pb whole rock ages between 2 8 and 2 6 gyr reflect subsequent depletion in rb and u refs 2 3 which has generally been associated with granulite facies metamorphism zircons which are thought to have crystallized during this metamorphism have also given u pb ages of 2 66 gyr ref 4 this granulite facies metamorphism involving pressures in excess of 10 kbar and peak temperatures estimated between 820 50 c ref 5 and 1 250 c refs 6 7 affects a terrain including supracrustal rocks7 and thus implies a major tectonic event we report here the first geochronological data measured on mineral assemblages that have also provided estimates of the conditions of metamorphism such measurements are possible because the major minerals of granulite facies assemblages show an unusually favourable spread in sm nd ratios garnet especially is strongly enriched in sm ref 8 data for three garnetiferous basic gneiss assemblages suggest that closure of the sm nd system occurred significantly later than the peak of the metamorphism

mineral and fluid inclusions in zircon of uhp metamorphic rocks from the ccsd main drill hole a record of metamorphism and fluid activity abstract the chinese continental scientific drilling ccsd main drill hole 0 3000 m in donghai southern sulu orogen consists of eclogite paragneiss orthogneiss schist and garnet peridotite detailed investigations of raman cathodoluminescence and microprobe analyses show that zircons from most eclogites gneisses and schists have oscillatory zoned magmatic cores with low pressure mineral inclusions of qtz pl kf and ap and a metamorphic rim with relatively uniform luminescence and eclogite facies mineral inclusions of grt omp phn coe and rt the chemical compositions of the uhp metamorphic mineral inclusions in zircon are similar to those from the matrix of the host rocks similar uhp metamorphic p t conditions of about 770 c and 32 kbar were estimated from coexisting minerals in zircon and in the matrix these observations suggest that all investigated lithologies experienced a joint in situ uhp metamorphism during continental deep subduction in rare cases magmatic cores of zircon contain coesite and omphacite inclusions and show patchy and irregular luminescence implying that the cores have been largely altered possibly by fluid mineral interaction during uhp metamorphism abundant h 2 o co 2 h 2 o or co 2 dominated fluid inclusions with low to medium salinities occur isolated or clustered in the magmatic cores of some zircons coexisting with low p mineral inclusions these fluid inclusions should have been trapped during magmatic crystallization and thus as primary only few h 2 o and or co 2 dominated fluid inclusions were found to occur together with uhp mineral inclusions in zircons of metamorphic origin indicating that uhp metamorphism occurred under relatively dry conditions the diversity in fluid inclusion populations in uhp rocks from different depths suggests a closed fluid system without large scale fluid migration during subduction and exhumation

late orogenic extensional tectonics at the nw margin of the caledonides in scotland abstract a major ductile extensional structure throwing down several kilometres to the ese probably marks the southern portion of the nw caledonian orogenic margin in scotland this structure the sound of iona fault is probably related to the intensely mylonitized zones that cut the lewisian and adjacent iona group torridonian metasedimentary cover on iona the fault related mylonites predated the 414 3 ma ross of mull granite and are inferred to have postdated the c 425 ma moine thrust it is concluded that the sound of iona fault is one of the many late caledonian faults in scotland but is exposed at a deeper structural level than normal its position at the orogenic margin of the caledonides is similar to that of the loch gruinart fault on islay hinting that a major steep fault s may be a significant feature of the largely concealed caledonian margin south of skye

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

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

siluro devonian palaeomagnetism terrane emplacement and rotation in the caledonides of western ireland summary the palaeomagnetism of a silurian succession lough mask formation lavas and sediments and intrusive rocks kilbride dolerites and andesites and microgranodiorites killary harbour area in the connemara mayo segment of the irish caledonides is described magnetizations are related to the regional tectonic history with the aid of fold conglomerate and contact tests they describe sequences of shallow to intermediate inclinations in both the eastern and western sectors of this caledonian inlier the shallow components appear to pre date siluro devonian folding while the intermediate ones post date this episode the change in palaeofield inclination is identical to the transition described by contemporaneous deep level silurian to shallow and surface level lower devonian bodies in the british and scandinavian caledonides but declinations are rotated consistently to the west the lower middle silurian vector is rotated clockwise by c 100 this difference is progressively reduced to c 50 by the time of the siluro devonian d2 folding showing that this fold generation now oriented e w developed in parallelism with contemporaneous folds on the ne strike continuation of the caledonides post folding remanences continue to deviate in a clockwise sense from the remainder of the caledonides showing that rotation continued into middle upper devonian times and finally ceased during carboniferous times the integrated effect of this rotation is recognized in ordovician magnetizations from the dalradian metamorphic terrane of south connemara it can be explained by block rotations within a zone of distributed deformation according to the mckenzie jackson model and illustrated by many neovolcanic zones palaeomagnetism defines a tectonic regime commencing with rotations of c 3 myr and lasting from mid silurian to carboniferous times western ireland is sited within a closure gap along the iapetus suture extending from ireland to newfoundland and these terranes were apparently emplaced by progressive elimination of this gap during sinistral transpression across the caledonian orogen

oxygen isotope geothermometers for metamorphic rocks the chicago mineral carbonate oxygen isotope fractionation curves have been combined with mineral water fractionation data for jadeite zoisite and rutile and new data for grossular water to provide a set of self consistent mineral pair calibrations the a coefficients in the equation 1000 in a 106t 2 of the new mineral pair fractionations are jadeite zoisite grossular rutile quartz 1 69 2 00 3 03 5 02 jadeite 0 31 1 34 3 33 zoisite 1 03 3 02 grossular 1 99 the isotopic fractionation properties of natural pyralspite garnet ca fe mg mn 3al2si3o12 can be approximated by those of the grossular end member appropriate substitutions also yield coefficients for the solid solution minerals sodic pyroxene and epidote e g aquartz sodic pyroxene 2 75 1 06xjd aquartz epidote 2 00 0 75xps where xjd and xps are the mole fractions of the jadeite and pistacite components respectively the new data set is particularly suitable for the geothermometry of metamorphic rocks 18o data from minerals of the high pressure metamorphic rocks of the sesia zone of italy and cyclades complex of greece yield well constrained mean temperatures of 572 and 478d c respectively type iii blueschist metabasalts of the franciscan formation of california give mean quartz garnet temperatures of 354d c

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

refining the timing of eclogite metamorphism a geochemical petrological sm nd and u pb case study from the pohorje mountains slovenia eastern alps high pressure metamorphism in the pohorje mountains of slovenia austroalpine unit eastern alps affected n morb type metabasic and metapelitic lithologies thermodynamic calculations and equilibrium phase diagrams of kyanite phengite bearing eclogites reveal pt conditions of 2 1 gpa at t 750 c but within the stability field of quartz metapelitic eclogite country rocks contain the assemblage garnet phengite kyanite quartz for which calculated peak pressure conditions are in good agreement with results obtained from eclogite samples the eclogites contain a single population of spherical zircon with a low th u ratio combined constraints on the age of metamorphism come from u pb zircon as well as garnet whole rock and mineral mineral sm nd analyses from eclogites a coherent cluster of single zircon analyses yields a 206pb 238u age of 90 7 1 0 ma that is in good agreement with results from sm nd garnet whole rock regression of 90 7 3 9 and 90 1 2 0 ma end 8 for two eclogite samples the agreement between u pb and sm nd age data strongly suggests an age of approximately 90 ma for the pressure peak of the eclogites in the pohorje mountains the presence of garnet omphacite and quartz inclusions in unfractured zircon indicates high pressure rather than ultrahigh pressure conditions the analysed metapelite sample yields a sm nd garnet whole rock scatterchron age of 97 15 ma these data probably support a single p t loop for mafic and pelitic lithologies of the pohorje area and a late cretaceous high pressure event that affected the entire easternmost austroalpine basement including the koralpe and saualpe eclogite type locality in the course of the complex collision of the apulian microplate and europe

palaeomagnetism of the ross of mull granite complex western scotland lower palaeozoic apparent polar wander of the orthotectonic caledonides the ross of mull granite complex was emplaced near the western margin of the orthotectonic caledonides of scotland in early mid silurian times anisotropy of magnetic susceptibility reflects the post tectonic character of the pluton and contrasts with magnetic fabrics in adjoining country rocks which record the regional d3 deformation palaeomagnetic study identifies two dominant magnetizations an a component is magnetite resident with distributed blocking temperatures 100 500 c and normal polarity d i 339 50 30 samples 95 5 6 palaeomagnetic pole at 12 e 1 s dp dm 5 0 7 5 it was probably acquired during initial cooling a b magnetization unblocking temperatures 500 580 c d i 195 21 40 samples 95 4 3 palaeomagnetic pole at 339 e 22 s dp dm 2 4 4 5 is of dual polarity and comparable to magnetizations widely recorded in the younger granite suite of the scottish caledonides it does not correlate with the timing of regional isotopic closure as defined by k ar thermochron ages and appears to have been imparted during a regional thermochemical event at low temperatures 250 150 c and a late stage of cooling of the orogen 420 410 ma metamorphic facies in the aureole and adjoining country rocks record examples of both a and b magnetizations together with a low blocking temperature 0 350 c component acquired during late palaeozoic early mesozoic rifting in the hebridean basin palaeomagnetic poles from the orthotectonic caledonides define a late to post tectonic magnetization record equivalent to a general east to west apparent polar wander trend of late ordovician lower devonian age a short term reversal of this trend is identified during the earlier part of silurian times so that mid late silurian poles are located in a similar position to late ordovician poles the overall path coincides with the contemporaneous record from the paratectonic caledonides of england and wales from c 455 ma it is therefore concluded that the three plate model of the caledonides incorporating the collision of laurentia and baltica in mid ordovician times to produce the orthotectonic caledonides and the closure of the iapetus ocean between baltica laurentia and avalonia in late ordovician times to produce the paratectonic caledonides was unified prior to the oldest magnetizations preserved in the orthotectonic caledonides certain second order discrepancies remain and may either reflect the incompleteness of the present database or the effects of subsequent regional rotation and strike slip movements prior to full welding of the orogen by late lower devonian times

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

palaeomagnetism of caledonian intrusive suites in the northern highlands of scotland constraints to tectonic movements within the caledonian orogenic belt abstract to evaluate the scale of tectonic movements within the northern sector of the 500 400 ma caledonian orogenic belt and its precambrian foreland zone between the great glen fault ggf zone to the southeast and the laurentian block to the northwest we have studied the palaeomagnetism of minor intrusive rocks within the northern highlands terrain these rocks include 1 1 amphibolites and other metamorphic rocks predating deformation 2 2 microdiorities dolentes and related suites emplaced and probably magnetised between 450 and 420 ma and 3 3 lower middle devonian lamprophyres a range of predominantly nne negative and ssw positive components are resolved by cleaning treatment with a dispersion of declinations towards a minority wnw ese axis isolated southerly negative directed hematite held components suggests limited but no widespread remagnetisation in devonian carboniferous times comparison is made with data from other tectonic divisions in the caledonian orogenic belt and the bordering forelands palaeopoles from the northern highlands closely conform in part with north american ordovician poles and in part with the post ordovician palaeopoles from britain south of the ggf the definitive motions of the british caledonides to emerge from the palaeomagnetic analysis are an anticlockwise rotation of the caledonian terrain in early ordovician times small relative motions during the remainder of ordovician times followed by large clockwise and then anticlockwise rotations during late ordovician to early silurian times contemporary with the last major movements on the moine thrust ca 430 ma late silurian devonian movements along the ggf were probably below the limits of palaeomagnetic detectability the collective data require that apparent polar wander movements and concomitant continental movements have currently been incompletely recovered by north american studies and the path for lower palaeozoic times is more complex than recognised hitherto

comments about the age of the canada basin abstract the canada basin appears to have formed over a 60 ma interval during the cretaceous period the opening process can be divided into two parts continental breakup widespread in hauterivian through aptian time about 113 131 ma ago along what was about to become the polar margin of north america and seafloor formation active during the extended cretaceous interval of normal geomagnetic polarity to marine magnetic anomaly 33 time about 118 79 ma ago presently available age controls indicate that all parts of the north american polar margin could have formed at about the same time two styles of margin formation are evident from brock to axel heiberg island there appears to be an extended 70 ma stratigraphic hiatus and northeastward from ellef ringnes island extensive tholeiitic igneous activity associated with continental breakup from banks island to northeastern alaska the breakup interval is abbreviated 20 30 ma and few igneous rocks are observed a progressive difference may be indicated in the amount and or rate of crustal stretching during margin formation with relatively less slower stretching having occurred along the segment southwest of brock island and relatively more faster stretching having taken place along the segment to the northeast

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

palaeomagnetism of the foyers and strontian granites scotland abstract palaeomagnetic data from the newer granites of foyers and strontian show similar directions of magnetization with southerly declinations and horizontal to intermediate mostly downward dipping inclinations these palaeomagnetic directions which accord with previous data from the helmsdale granite are thought to be of siluro devonian age the present results are not sufficiently precise to justify discussion of possible lateral displacements along the great glen fault of the order of a few hundred kilometres but it is clear that the recent idea of a c 15 20 latitudinal offset along the fault in the carboniferous has to be disregarded

zircon ages and nd isotopic and chemical compositions of orthogneisses from the black forest germany evidence for a cambrian magmatic arc single zircon u pb dating combined with 207pb 206pb ages obtained by the evaporation method constrains the emplacement of tonalitic trondhjemitic and granodioritic orthogneisses of the moldanubian zone in the black forest between 500 and 510 ma two detrital zircon populations of 1 9 and 1 6 ga indicate early middle proterozoic material in the former setting of the basement the initial end values range from 0 1 to 3 4 and mean crustal residence ages of 1 0 1 4 ga are consistent with involvement of early middle proterozoic crust and a subordinate juvenile component probably originating from subduction related melting of the mantle the orthogneisses have fractionated ree patterns and slightly higher k2o na2o ratios than typical low k tonalite trondhjemite granite suites the chemical data are interpreted as evidence for melting of amphibolite and contributions from evolved crust the emplacement of the orthogneisses was superceded by a high temperature metamorphic event at 480 ma which we interpret as a result of lithospheric thinning in a marginal basin behind a cambrian magmatic arc

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

geochronology of late caledonian magmatism in northern britain rb sr k ar ar ar and sm nd isotopic data are presented for mineral separates and whole rock samples from old red sandstone ors lavas associated minor intrusions and 9newer granite9 intrusive complexes from the cheviot hills north to shetland these data yield 16 internally consistent rb sr mineral ages and one each of ar ar sm nd and k ar ages when considered with precise and reliable published data it is clear that there are systematic differences in the age of late caledonian magmatism through the geological regions of scotland in the grampian highlands and the scottish midland valley magmatism was concentrated during the period 423 to 410 ma ors volcanism in the highlands occurred around 420 ma with plutonism continuing to about 408 ma volcanism and plutonism in the midland valley took place between 415 and 410 ma in the southern uplands magmatism occurred earlier in the north with all magmatism close to the southern uplands fault dated around 408 ma and that close to the iapetus suture dated at around 394 ma as no evidence exists for ocean closure prior to c 400 ma the age of igneous activity north of the southern uplands fault is consistent with chemical evidence for a relationship to active nw directed subduction the younger magmatism of the s part of the southern uplands cannot be subduction related and the c 408 ma plutons nearer the southern uplands fault can only be subduction related if tectonic construction of the southern uplands was a two stage process

strontium diffusion kinetics in amphiboles and significance to thermal history determinations abstract strontium tracer diffusion coefficients have been determined in hornblende and tremolite single crystals using experiments conducted at a confining pressure of 200 mpa at temperatures ranging from 700 to 960 c a quasi hydrothermal experimental technique was devised to avoid solution precipitation effects depth profile analyses were made with an ion microprobe the arrhenius relation for diffusion parallel to the c crystallographic direction in the gore mountain hornblende over this temperature interval has an activation energy q 260 12 kj mol and a pre exponential factor d 0 4 9 18 1 3 10 8 m 2 s at 800 c transport rates parallel to the c axis measured in the two tremolite samples amnh 30563 and barrie township ontario are 3 2 10 21 m2 s and 2 0 10 21 m2 s respectively these values are factors of 3 and 5 slower than those observed for the hornblende sample at the same temperature transport rates parallel and perpendicular to the c axis are the same within the estimated uncertainty for all three compositions experiments that differed in o 2 by a factor of 6 107 mno mn3o4 mno and magnetite wustite mw for the hornblende gave the same strontium diffusion rates within a factor of two crystals with cleavage faces or polished with alumina or diamond gave the same d values provided the crystals were annealed prior to the diffusion experiment using a combination of closed system rb sr systematics and strontium diffusion kinetics it is now possible to determine the thermal histories of hornblende bearing igneous and metamorphic rocks since the diffusion rates for strontium in amphiboles are relatively slow hornblende will be more retentive of sr than of other species for which transport rates are known ar o h and will be a useful tool to record higher temperature portions of cooling paths

palaeomagnetism of the middle upper devonian esha ness ignimbrite w shetland abstract the ignimbrite division of the esha ness volcanics of w shetland has been studied palaeomagnetically the major blocking temperature spectrum ranges between 650 and 680 c implying that haematite is the principal remanence carrier the magnetization build up is simple a minor low stability magnetization aligned along the direction of the present earth s field is superimposed on a well defined and highly stable palaeomagnetic component with dec 220 and inc 3 the esha ness volcanic sequence is folded into a gentle syncline trending nne the rock collection concerned covers 8 sites 7 from the western flank and 1 from the eastern flank of the syncline the 7 sites on the west flank are consistently reversely magnetized while the site on the east flank has a normally directed magnetization after structural unfolding the normal and reverse directions are almost perfectly antiparallel i e the stable magnetization most likely predates the tilting which is probably of devonian age esha ness is located some 12 km to the west of the walls boundary fault wbf which is considered as the northward continuation of the great glen fault ggf the shallow palaeomagnetic inclination found in old red sandstone rocks to the west of ggf wbf corresponds to that encountered in most other european old red formations discounting therefore megascale 1000 km late post devonian movements along this dislocation

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

rb and sr diffusion in alkali feldspars with implications for cooling histories of rocks volume diffusion of sr and rb have been measured in orthoclase feldspar as well as sr diffusion in albite using isotope exchange hydrothermal solutions and analysis by ion microprobe for water pressures of 1000 bars the kinetics of diffusion parallel to the c axis are given by the arrhenius relations strontium transport in the or is isotropic to within a factor of four the measurements use a new procedure in which only the elements in question are present in the starting fluid cooling rates of igneous rocks can now be determined from closure temperatures material balance and the rb sr dating method based on the departures of the mineral data from the whole rock isochron for cooling rates less than approximately 100 c my

zircon age and nd hf isotopic composition of the yunnan tethyan belt southwestern china the baoshan block of the tethyan yunnan southwestern china is considered as northern part of the sibumasu microcontinent basement of this block that comprises presumably greenschist facies neoproterozoic metamorphic rocks is covered by paleozoic to mesozoic low grade metamorphic sedimentary rocks this study presents zircon ages and nd hf isotopic composition of granites generated from crustal reworking to reveal geochemical feature of the underlying basement dating results obtained using the single zircon u pb isotopic dilution method show that granites exposed in the study area formed in early paleozoic about 470 ma pingdajie granite and in late yanshanian about 78 61 ma late cretaceous to early tertiary huataolin granite the early paleozoic granite contains archean to mesoproterozoic inherited zircons and the late yanshanian granite contains late proterozoic to early paleozoic zircon cores both granites have similar geochemical and nd hf isotopic charateristics indicating similar magma sources they have whole rock tdm nd values of around 2 000 ma and zircon tdm hf values clustering around 1 900 1 800 and 1 600 1 400 ma the nd hf isotopic data imply paleoproterozoic to mesoproterozoic crustal material as the major components of the underlying basement being consistent with a derivation from archean and paleoproterozoic terrains of india or nw australia both granites formed in two different tectonic events similarly originated from intra crustal reworking temporally the late yanshanian magmatism is probably related to the closure of the neotethys ocean the early paleozoic magmatism traced in the baoshan block indicates a comparable history of the basements during early paleozoic between the se asia and the western tethyan belt such as the basement outcrops in the alpine belt and probably in the european variscides that are considered as continental blocks drifting from gondwana prior to or simultaneously with those of the se asia

comparison of laser ablation icp ms and isotope dilution ree analyses implications for sm nd garnet geochronology abstract high precision garnet chronometry using the sm nd u pb rb sr or lu hf isotope systems is gaining widespread application for constraining the prograde and high temperature history of metamorphic belts however due to the low concentrations of the chronometrically important elements garnet analyses are often susceptible to incorporation of small quantities of accessory phases such as zircon monazite and allanite here we assess the extent to which such inclusions affect garnet analyses by comparing in situ analyses of sm and nd concentrations in the crystal lattice obtained by laser ablation inductively coupled mass spectrometry la icp ms with those obtained by isotope dilution id on separates of the same garnets the results show that the garnets studied here are largely free from inclusions in general however the low concentrations of sm and nd in the garnet lattice mean that the incorporation of even very small quantities of accessory phases will result in a significant change in the measured garnet sm and nd concentration simple modelling of the effect of such inclusions on age information shows that garnet inclusion disequilibrium and the extent to which inclusions decrease the apparent parent daughter ratio are critical such modelling demonstrates that the incorporation of old lree enriched minerals such as monazite will yield an apparently lower age than the true garnet age however this study indicates that erroneous age information can be identified if duplicate garnet analyses are performed in addition la icp ms is shown to be a useful technique in verifying the purity of samples for isotope analysis

oxygen isotope geochemistry of ultrahigh pressure metamorphic rocks from 200 4000 m core samples of the chinese continental scientific drilling abstract oxygen isotope studies were carried out for ultrahigh pressure metamorphic rocks from the main hole of the chinese continental scientific drilling in the sulu orogen the samples of interest include various types of lithology mainly eclogite and gneiss in a depth of 200 to 4000 m containing five continuous core segments between contrasting lithologies the results show a large variation in 18o value from 10 41 to 9 63 for constituent minerals distinct 18o depletions are observed in the frequently alternated layers of eclogite and gneiss in which the variations of 18o values are gradual regardless of lithologies both equilibrium and disequilibrium o isotope fractionations are observed between quartz and the other minerals special attention was paid to the relationship between distance petrography and 18o value of adjacent samples the results show o isotope heterogeneities between the different and same lithologies on scales of about 20 to 50 cm corresponding to the maximum scales of fluid mobility during the continental collision amphibolite facies retrograde metamorphism during exhumation caused mineral reactions and o isotope disequilibria between some of the minerals considerable changes occur in 18o and petrography at the contact between eclogite and gneiss suggesting that the contact between different lithologies is the most favorable place for fluid activity despite the widespread retrogression retrograde fluid was internally buffered in the stable isotope compositions the retrograde fluid is of deuteric origin and thus was derived from the decompression exsolution of structural hydroxyl although locally external fluids became available along fault zones and lithological layers it still acted within the exhumed slabs with internal origins fluid flow also took place after the amphibolite facies retrogression but it only affects the o isotope compositions of feldspar and mica premetamorphic protoliths are deduced to have heterogeneous 18o values due to varying degrees of meteoric water rock interaction before continental collision minimum depth of the 18o depletion is up to 3300 m together with areal 18o depletion in surface outcropped rocks along the dabie sulu orogenic belt at least 66 000 km3 of supracrustal rocks were interacted with meteoric water along the northern margin of the south china block during the neoproterozoic

foreland basin development and tectonics on the northwest margin of eastern avalonia the early palaeozoic convergence of avalonia and laurentia created a foreland basin at the suture zone of the former lapetus ocean sedimentological and stratigraphic evidence of shallowing and contemporaneous shortening suggests that the southern part of the basin the windermere group became detached from its basement in the late ludlow and began to invert the detachment beneath the basin rooted into a northwest dipping mid crustal thrust system contemporaneous uplift to the north of the late silurian basin involved shortening of the avalonian foreland basement by thrusting basin inversion occurred ahead of a southeastward advancing mountain front we postulate a foreland southeast prograding sequence of thrusting through the ludlow in the lake district the basin continued to migrate onto the avalonian foreland through the early devonian ahead of an advancing orogenic wedge finally coming to a stop in the emsian

crystallization and very rapid exhumation of the youngest alpine eclogites tauern window eastern alps from rb sr mineral assemblage analysis multimineral rb sr internal isochrons from eclogite facies rocks of the eclogite zone tauern window eastern alps consistently yield an early oligocene age of 31 5 0 7 ma this age has been obtained both for late prograde dehydration related eclogitic veins and for rocks variably deformed and recrystallized under eclogite facies conditions 2 0 2 5 gpa 600 c initial sr isotopic equilibria among all phases indicate absence of significant post eclogitic isotope redistribution processes therefore the ages date eclogite facies assemblage crystallization equilibria also prove that no prolonged pre eclogite facies history is recorded in the rocks instead subduction prograde mineral reactions and eclogitization proceeded rapidly fast exhumation immediately after eclogitization with minimum rates 36 mm a is inferred from a 31 5 0 5 ma internal mineral isochron age of a post eclogitic greenschist facies vein assemblage such rates equal typical subduction rates late eocene to early oligocene subduction of the european continental margin with subsequent rapid exhumation of high pressure nappe complexes has previously been recognized only in the western alps the new data signify synchronous continental collision all along the alpine belt our results demonstrate the unique potential of rb sr assemblage system analysis for precise dating of both eclogite facies and post eclogitic events thus for precisely constraining exhumation rates of deep seated rocks and for straightforward linkage of petrologic evidence with isotopic ages

tectonic rotation within the british paratectonic caledonides and early palaeozoic location of the orogen magnetizations of late ordovician caradoc early ashgill age in the paratectonic caledonides of wales and the lake district are directed westerly and northerly respectively they identify c 55 of relative rotation of probable acadian age southward subduction of iapetus ocean crust occurred in ordovician times beneath a latitudinally oriented orogen sited in mid southerly latitudes with the borrowdale and snowdonia volcanic provinces forming parallel fore arc and back arc lineaments respectively mid ordovician to mid devonian palaeomagnetic poles from the orogen correlate with contemporaneous gondwana data to identify a former location sited west of the south american perimeter and not near to north africa as assumed in previous reconstructions

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

geochemistry and origin of eclogites from the type locality koralpe and saualpe eastern alps austria abstract in the koralpe and saualpe polycrystalline basement which is now a part of the austroalpine nappe system eclogites and metagabbros occur intercalated in gneisses and kyanite bearing schists we report here the chemical as well as sr and o isotopic composition of such eclogites and one metagabbro a low ti kyanite bearing and a high ti kyanite free group can be distinguished which may be related by igneous differentiation the low ti group which comprises eclogites and the metagabbro has chemical characteristics indicating formation of the igneous precursors as cumulates by crystallization of pyroxene and plagioclase from a mafic magma in contrast the protoliths of the high ti eclogites appear to be basaltic liquids with normal to enriched type morb affinities which are likely to have formed at an oceanic ridge their sr and o isotope relationships strongly suggest that they have suffered both a phase of low temperature and a phase of high temperature hydrothermal alteration by reaction with seawater

calculation of oxygen isotope fractionation in hydroxyl bearing silicates abstract the modified increment method has been applied to the calculation of oxygen isotope fractionation factors for hydroxyl bearing silicate minerals the order of 18o enrichment obtained in common rock forming minerals is pyrophyllite kaolinite tourmaline talc prehnite topaz illite phengite lepidolite muscovite staurolite epidote glaucophane serpentine chlorite tremolite hornblende phlogopite biotite humite norbergite ilvaite hydroxyl bearing silicates are enriched in 18o relative to hydroxyl groups but depleted in 18o relative to anhydrous counterparts three sets of self consistent fractionation factors between quartz and the hydroxyl bearing silicate minerals between calcite and the silicate minerals and between the silicate minerals and water have been calculated for a temperature range of 0 1200 c the fractionation factors calculated for mineral pairs are applicable to isotopic geothermometry in igneous metamorphic and sedimentary petrology they can be used as a test of isotopic equilibrium or disequilibrium in natural mineral assemblages over all temperature ranges of geological interest the difference in oxygen isotope composition between the hydroxyl bearing mineral and the oh group is quantitatively demonstrated to be temperature dependent and therefore can be used as a single mineral geothermometer

faunal evidence for oceanic separations in the palaeozoic of britain the faunal criteria for the recognition of the relative separation of old continents are 1 the contrast between shallow not deep water faunas off opposite shorelines 2 the recognition of deeper water facies surrounding the edges of continents and 3 the recognition of the disposition of ancient climatic belts these criteria are defined and used to assess the relative position of the various parts of what is now britain in the palaeozoic the iapetus ocean is well characterized by faunal differences from the cambrian until the lower ordovician but from caradoc times onwards these differences dwindle until by the latest silurian only the distribution of the ostracods separates the two sides of the ocean on faunal grounds in the early ordovician s british faunas are comparable with those from bohemia france and elsewhere to the s indicating connection with gondwanaland and these faunas differ from those in the baltic area a distribution which we attribute to a true ocean tornquist9s sea between n and s europe tornquist9s sea probably closed in the upper ordovician and from ashgill times onwards the s british and baltic faunas remained similar during the middle and late silurian there is evidence of increasing faunal distinction between britain the baltic area and north america on the one hand and southern europe and gondwanaland on the other indicating an opening rheic ocean which appears to have persisted until late carboniferous times

sm nd disequilibrium in high pressure low temperature himalayan and alpine rocks in order to decipher the causes of sm nd isotopic disequilibrium in high pressure low temperature rocks sm nd isotopic analyses were carried out on minerals from four himalayan tso morari unit and four alpine dora maira monte viso sesia lanzo eclogitic rocks of different lithologies and different intensities of post eclogitic metamorphism in most of these

oxygen diffusion rates in quartz exchanged with co2 oxygen self diffusion rates were determined in quartz samples exchanged with18o enriched co2 between 745 and 900 c and various pressures and the diffusion profiles were measured using an ion microprobe the activation energy q and preexponential factor d0 at p co2 p tot 100 bar for diffusion parallel to the c axis are 159 13 kj g atom and 2 10 0 75 0 55 10 8 cm2 s this rate is approximately 100 times slower than that obtained from hydrothermal experiments and 100 times faster than a previous 1 bar quartz o2 exchange experiment the oxygen diffusion rate measured at 0 6 bar 888 c and at 900 c in vacuum is in agreement with the previous 1 bar exchange experiments with18o2 the effect of higher co2 pressures is small at 900 c the diffusion rate exchanged with co2 is 2 35 10 15 cm2 s at 100 bar 2 24 10 15 cm2 s at 3 45 kbar and 8 13 10 15 cm2 s at 7 2 kbar there is probably a diffusing species other than oxygen that enhances the oxygen diffusion rate in these quartz co2 systems relative to that occurring at very low pressures or in a vacuum the effect of this diffusing species however is not as strong as that associated with h2o preserved oxygen isotope fractionations between coexisting minerals in a slowly cooled high grade metamorphic terrane will vary depending upon whether a water rich phase was present or not closure temperatures will be approximately 100 c higher in rocks where no water rich phase was present during cooling the measured fractionations between coexisting minerals in metamorphic rocks may potentially be used as a sensor of water presence during retrogression

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

timing of high pressure metamorphism and exhumation of the eclogite type locality kupplerbrunn prickler halt saualpe south eastern austria constraints from correlations of the sm nd lu hf u pb and rb sr isotopic systems sm nd lu hf rb sr and sims u pb data are presented for meta gabbroic eclogites from the eclogite type locality hauy 1822 kupplerbrunn prickler halt and other areas of the saualpe se austria and pohorje mountains slovenia mg rich eclogites derived from early gabbroic cumulates are kyanite and zoisite rich whereas eclogites with lower mg contents contain clinozoisite kyanite calculated p t conditions at the final stages of high pressure metamorphism are 2 2 0 2 gpa at 630 740 c kyanite rich eclogites did not yield geologically meaningful sm nd ages due to incomplete nd isotope equilibration whereas sm nd multifraction garnet omphacite regression for a low mg eclogite from kupplerbrunn yields an age of 91 1 1 3 ma the sm nd age of 94 1 0 8 ma obtained from the fe rich core fraction of this garnet dates the initial stages of garnet growth zircon that also crystallized at eclogite facies conditions gives a weighted mean u pb sims age of 88 4 8 1 ma lu hf isotope analysis of a kyanite eclogite from kupplerbrunn yields 88 4 4 7 ma for the garnet omphacite pair two low mg eclogites from the gertrusk locality of the saualpe yield a multimineral sm nd age of 90 6 1 0 ma a low mg eclogite from the pohorje mountains 70 km to the se gives a garnet whole rock lu hf age of 93 3 2 8 ma these new age data and published sm nd ages of metasedimentary host rocks constrain the final stages of the eo alpine high pressure event in the saualpe pohorje part of the south easternmost austroalpine nappe system suggesting that garnet growth in the high pressure assemblages started at c 95 94 ma and ceased at c 90 88 ma probably at the final pressure peak zircon and amphibole crystallization was still possible during incipient isothermal decompression rapid exhumation of the high pressure rocks was induced by collision of the northern apulian plate with parts of the austroalpine microplate following jurassic closure of the permo triassic meliata back arc basin

isotopic disequilibrium in ultrahigh pressure and retrograde metamorphism of eclogite and gneiss from the chinese continental scientific drilling in the sulu orogen china evidence from mineral nd sr o isotopic composition diffusion rates of sr and o in minerals are often comparable while nd has a lower diffusion rate during thermal overprint s thus the o isotope systems between metamorphic minerals can serve as an indicator to evaluate whether equilibrium of rb sr and sm nd systems has been preserved in the metamorphic minerals that experienced retrograde metamorphism this study presents a combination of investigation on sm nd rb sr and o isotopic compositions of minerals separated from ultrahigh pressure eclogite and gneiss that were collected from the main hole of the chinese continental scientific drilling project located in the sulu orogen eastern china oxygen isotopic compositions of minerals from gneiss and eclogite yield two temperature groups of 620 740 and 460 590 c representing diffusion cessation of isotopic exchange during the eclogite facies recrystallization and later amphibolite facies retrograde overprint rb sr mineral regressions of two eclogite samples give consistent triassic ages of 244 ma corresponding to eclogite facies metamorphism while the same minerals do not yield meaningful sm nd isochron ages this phenomenon likely suggests that rb sr isotopic equilibrium was achieved during eclogite facies metamorphism and preserved during late amphibolite facies retrogression in contrast sm nd isotopic equilibrium between the minerals of eclogite was not achieved under uhp metamorphic conditions regressions of epidote and biotite of one gneiss sample give a triassic sm nd age of 243 34 ma corresponding to the time of the eclogite facies metamorphism and a jurassic mineral rb sr age of 187 5 1 8 ma these results imply that fluids have played an important role to achievement of the sm nd isotopic equilibrium during eclogite facies metamorphism and re equilibration of the rb sr isotopic system during later retrograde overprint

palaeomagnetism of the early devonian lower old red sandstones of south wales implications to variscan overprinting and differential regional rotations component structures of magnetic remanence in the old red sandstones of south wales are related to tectonic history by detailed study of three contrasting areas 1 remote from 2 bordering and 3 within the variscan orogenic belt a uniform southerly shallow directed component dominates remanence in each sector in area 1 where it is considered most representative of the regional field this magnetisation has a mean di 191 1 8 0 151 samples in situ 95 2 equivalent to a pole position at 341 1 e 41 2 s fold tests show that it was acquired at an intermediate stage of orogenesis and during the carboniferous permian reversed superchron it is interpreted as a pervasive magnetic overprint imparted by northward fluid migration consequent upon loading of the orogen to the south remote from the orogenic front a higher blocking temperature dual polarity remanence of probable lower devonian age has sporadically survived di 246 38 27 samples tilt adjusted 95 7 2 corresponding to a pole position at 297 e 3 n this remanence is entirely replaced close to the frontal zone of the orogen within the orogen low blocking temperature components are more prominent and chemical alteration obscures the highest blocking temperature portion of the remanence spectrum this region was deformed into upright folds before the magnetising event following remanence acquisition continuing deformation produced asymmetry in fold structure producing a plunge and steepening of the southern limbs structures in the south were already more deformed than those in the north at the time of magnetisation implying that the causative fluid front moved northwards with time the palaeomagnetic vectors are rotated clockwise to the west in south wales and in part record the effects of regional dextral transpression the largest rotation however is observed within the same thrust block and apparent differential rotation here is probably a consequence of strain

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

paleomagnetic results from the peterhead granite scotland implication for regional late caledonian magnetic overprinting abstract palaeomagnetic results from the peterhead granite silurian ne grampian highlands define a characteristic magnetization with overall parameters of dec 180 4 inc 22 3 and a 95 5 2 these results agree with other newer granite data from north scotland an east west trending dyke cutting the peterhead granite shows an extremely well defined paleomagnetic direction with dec 192 5 inc 19 1 and a 95 1 3 suggesting a permo carboniferous age the peterhead granite results are seen in conjunction with other palaeozoic palaeomagnetic data from scotland it is concluded that the observed directional spread of characteristic remanence directions previously reported from the aberdeenshire gabbros are not related to slow post orogenic cooling watts and briden but is rather seen as the result of late caledonian magnetic overprinting

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

origin of retrograde fluid in ultrahigh pressure metamorphic rocks constraints from mineral hydrogen isotope and water content changes in eclogite gneiss transitions in the sulu orogen abstract by taking advantage of having depth profiles between contrasting lithologies from core samples of the chinese continental scientific drilling ccsd project a combined study was carried out to examine changes in mineral h isotope total water and hydroxyl contents in garnet and omphacite across the contacts between ultrahigh pressure uhp eclogite and gneiss in the sulu orogen east central china the samples of interest were from two continuous core segments from the ccsd main hole at depths of 734 21 737 16 and 929 67 932 86 m respectively the results show d values of 116 to 64 for garnet and 104 to 82 for omphacite consistent with incorporation of meteoric water into protoliths of uhp metamorphic rocks by high t alteration both equilibrium and disequilibrium h isotope fractionations were observed between garnet and omphacite suggesting fluid assisted h isotope exchange at local scales during amphibolite facies retrogression while bulk water analysis gave total h2o concentrations of 522 1584 ppm for garnet and 1170 20745 ppm for omphacite structural hydroxyl analysis yielded h2o contents of 80 413 ppm for garnet and 228 412 ppm for omphacite it appears that significant amounts of molecular h2o are present in the minerals pointing to enhanced capacity of water storage in the uhp eclogite minerals hydrogen isotope variations in the transition between eclogite and gneiss show correlations with variations in their water contents petrographically the degree of retrograde metamorphism generally increases with decreasing distance from the eclogite gneiss boundary thus retrograde metamorphism results in mineral reactions and h isotope variation because hydroxyl solubility in nominally anhydrous minerals decreases with dropping pressure significant amounts of water are expected to be released from the minerals during decompression exhumation decompression exsolution of structural hydroxyl from 1 m3 volume of eclogite composed of only garnet and omphacite results in release of a quantitative estimate of 3 07 3 44 kg water that can form 140 156 kg amphibole during exhumation therefore it is concluded that fluid for retrogression of the eclogites away from the eclogite gneiss boundary was derived from the decompression exsolution of structural hydroxyl and molecular h2o in nominally anhydrous minerals for the eclogites adjacent to gneiss in contrast the retrograde metamorphism was principally caused by aqueous fluid from the gneiss which is relatively rich in water consequently both the origin and availability of metamorphic fluid during exhumation of deeply subducted continental crust are deciphered by this combined study focusing on the transitions and the retrograde processes between the felsic and mafic uhp rocks

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

palaeomagnetic resetting in the barrovian zones of scotland and its relationship to the late structural history the haematite schists in the lowest kyanite and the staurolite barrovian zones exposed in glen lethnot and glen esk angus scotland have a characteristic magnetization with blocking temperatures generally greater than 600 c and less than 695 c and coercivity greater than 200 mt the mean palaeomagnetic direction derived from 54 samples collected from n 10 sites isd 159 i 45 k 47 95 7 1 with the associated south pole position oflat 3 long 22 e dp 5 7 anddm 9 0 this magnetization is younger than folds of d3 generation and older than folds of d4 generation the pole position falls on the aberdeenshire polar wander track in a place consistent with an average kar biotite chrontour age of 455 m y a comparison of the blocking temperatures of magnetization to the estimated peak temperature of metamorphism suggest the magnetization is a chemical remanent magnetization crm possibly acquired during the metamorphic chemical reactions

contribution of sediment oxygen demand to hypoxia development off the changjiang estuary abstract this study evaluates the contribution of sediment oxygen demand sod to hypoxia development off the changjiang estuary based on sod measurements from onboard chamber incubations and numerical experiments onboard core incubations were conducted for samples collected from four stations in the yellow sea ys and the east china sea ecs during cruises in june august and october 2006 the measured sod ranges widely from 9 1 to 62 5 mmol o 2 m 2 d 1 with a mean rate of 22 6 16 4 mmol o 2 m 2 d 1 and the maximum occurring in august the rates in regions experiencing hypoxia vary from 13 5 to 24 0 mmol o 2 m 2 d 1 with a mean of 18 9 4 6 mmol o 2 m 2 d 1 additionally the vertical oxygen flux was calculated from profile measurements at a hypoxic mooring station adjacent to the changjiang estuary on september 3rd and 4th 2006 the oxygen flux across the pycnocline was about 3 1 mmol o 2 m 2 d 1 much lower than the sod implying that the low oxygen supply associated with stratification promotes the formation and maintenance of hypoxia in bottom waters the one dimensional numerical experiments configured for stratified and well mixed water columns further confirm that stratification is necessary for the development and persistence of hypoxia while sod is a major oxygen sink below the pycnocline integrated over june to august the amount of oxygen loss beneath the pycnocline due to sod is equal to 116 148 80 142 and 82 179 of the total net oxygen loss in the southern hypoxic region northern hypoxic region and the middle shelf respectively during june to august when hypoxia develops sod in the northern and southern regions has similar magnitude the changjiang diluted water cdw promotes hypoxia formation in both regions while the oxygen advection caused by the taiwan warm current twc would alleviate hypoxia formation especially for the southern hypoxic region

polarity stratigraphy and duration of the tertiary igneous activity of mull scotland previous work left unresolved whether the tertiary igneous activity had spanned a magnetic polarity succession r n r n r or if overprinting had occurred only r n r new work is reported that demonstrates that thermal overprinting has occurred in a narrow zone c 600 m wide bordering the outermost major intrusions of centre 2 criteria for the recognition of overprinting are examined the polarity sequence is now believed to be r n r in conjunction with radiometric dates which show that activity occurred from c 60 ma to c 57 ma our data show that the sequence corresponds to chrons 26r 26 25r of the polarity time scale of harland et al 1982 or 25r 25 and 24r of breggren et al 1985 upper and lower limits to the duration of the activity are respectively 4 67 ma and 0 67 ma or 4 07 ma and 0 60 ma a duration of about 3 5 ma is thought probable the magnetic evidence supports the view that the loch scridain sills correspond to the early acid and intermediate cone sheets of centre 1

magnetic properties of the lower old red sandstone lavas in the midland valley scotland palaeomagnetic and tectonic considerations abstract thermal demagnetization studies of lavas in the strathmore area of the midland valley scotland support overall palaeomagnetic data found in previous studies of these rocks reduced directional scatter as compared to some earlier studies is attributed to more effective demagnetization resolving some of the directional complexity of previous studies combined magnetic fabric and directional analysis suggest that at least some deviating directions may be explained by local tectonism the existence of almost antiparallel directional groups and field tests give supporting evidence for a primary deuteric origin of the main magnetization of these rocks additionally a second remanence component having shallow reverse directions of magnetization is attributed to later remagnetization in old red sandstone time the midland valley results are seen in conjunction with other palaeozoic palaeomagnetic results and possible geodynamic implications are discussed