mid ocean ridge and island arc affinities in ophiolites from iran palaeographic implications complementary reference abstract fifty six samples of ophiolitic rocks from various late cretaceous eocene suture zones in iran have been analyzed for major and trace elements in order to assess their petrogenetic affinity and to determine their original tectonic setting in both southern baluchestan i e between the makran range and the lut block the fanuj maskutan sequence and northeastern baluchestan i e in the east iranian ranges the tchehel kureh sequence the ophiolitic complexes are well developed the rocks display affinities with mid ocean ridge tholeiites the basaltic rocks are comparable to respectively transitional fanuj maskutan and normal tchehel kureh morb in contrast the sequence along the zagros the sahneh sequence appears to have formed in an island arc setting in iranshar i e in southeastern baluchestan ophiolites are dismembered into a melange which includes cumulate intrusives similar to those from fanuj maskutan and lavas with island arc tholeiitic affinity a similar island arc origin may be inferred for the sabzevar ophiolitic sequence north of the lut block studied by other authors the chemical data thus help to elucidate the development of the jigsaw puzzle of microplates of gondwanian origin that during the mesozoic were intermediate in position between afro arabia india and eurasia and that now constitute the middle east ophiolite obduction in iran occurred during arc basin continent or ocean continent collisions in the late cretaceous early paleogene largely predating the final suturing between arabia and india with eurasia to which the intermediate microplates were already attached which did not take place before the neogene

40ar39ar ages and tectonic setting of ophiolite from the neyriz area southeast zagros range iran abstract an ophiolite considered to be an allochthonous fragment of tethyan oceanic crust and mantle crops out near neyriz in the zagros range iran 40 ar 39 ar ages ranging from 76 8 23 8 ma to 105 23 3 ma were measured on hornblende from five samples of plagiogranite and diabase from the ophiolite the most precise ages are 85 9 3 8 ma for a diabase and 83 6 8 4 ma for a plagiogranite the weighted mean age of hornblende from the five samples is 87 5 7 2 ma which indicates that the igneous part of the neyriz ophiolite formed during the early part of the late cretaceous pargasite from amphibolite below peridotite of the neyriz ophiolite has a 40 ar 39 ar age of 94 9 7 6 ma the pargasite age agrees within analytical uncertainty with the ages measured on diabase and plagiogranite comparable ages have been measured on igneous rocks from the samail ophiolite of oman and on amphibolite below peridotite of the samail ophiolite

the origin of rodingites from cassiar british columbia and their use to estimate t and p h2o during serpentinization abstract the cassiar serpentinite located in north central british columbia is a low grade alpine type serpentinite within the sylvester allochthon two stages of rodingitization have occurred at cassiar and these correlate with two stages of serpentinization that have occurred in the ultramafic rock the first stage is characterised by an increase in ca activity and decreases in mg and sio 2 activities in the rodingite culminating in the crystallization of grossular in the rodingite and by the formation of lizardite in the serpentinite at a later time a second event occurred characterised by a decrease in ca activity and increases in mg and sio 2 activities both in the rodingite and the serpentinite this was marked by the crystallization of diopside prehnite and lizardite in the rodingite and by the recrystallization of lizardite to chrysotile and antigorite in the serpentinite the serpentine recrystallization and the development of the second stage rodingite minerals indicate that the changes in rodingite and serpentine mineral assemblages were driven by similar changes in the bulk rock compositions this suggests that the second stage event in both rodingite and serpentinite was caused by an externally derived fluid measurements on fluid inclusions in the rodingite minerals clinozoisite grossular and diopside of the second event indicate that the externally derived fluid was co 2 poor but relatively saline 8 1 5 equiv wt nacl by combining the pressure dependant temperature measurements from fluid inclusions with the pressure independant temperature measurements on 18 o in coexisting serpentine magnetite pairs the pressure of formation can be estimated the second event of serpentine recrystallization and rodingitization was an isothermal process at 300 c 36 c p h 2 o

overview of tectonic settings related to the rifting and opening of mesozoic ocean basins in the eastern tethys oman himalayas and eastern mediterranean regions

precise determination of phanerozoic zircon pb pb age by multicollector sims without external standardization 1 zircon has long been recognized as the best geochronometer and the most important timekeeper in geosciences modern microbeam techniques such as sims and la icpms have been successfully applied to in situ u pb zircon age determinations at spatial resolutions of 20 30 m or less matrix matched calibration by external standardization of well characterized natural zircon references is a principal requirement for precise microbeam u pb zircon age determination due to fractionation effects between pb and u which usually result in an external age error exceeding 1 alternatively zircons with a closed u pb system can be directly dated by measurement of 207pb 206pb isotopic ratio without external standardization which has been a common practice for zircons older than 1 0 ga but not for relatively young 1 0 ga and particularly phanerozoic ones because of limitations of analytical precision we describe in this paper a method of 207pb 206pb measurement on phanerozoic zircons using a new generation of large radius magnetic sector multicollector cameca ims 1280 sims in combination with multicollector mode a nuclear magnetic resonance nmr magnet controller and oxygen flooding techniques we achieve precisions of 207pb 206pb ratio of 0 1 and 0 1 0 2 propagating to pb pb age errors 0 4 and 1 3 excluding u decay constant uncertainties for zircons of latest neoproterozoic and late paleozoic to mesozoic age respectively therefore the multicollector sims is capable of direct determination of zircon pb pb ages as young as mesozoic age with uncertainties of geological significance this technique is useful for direct dating of zircons in thin sections moreover it has significance for dating of some other u rich minerals i e baddeleyite and zirconolite that are not suitable for sims u pb dating by external standardization

tectonics of the zagros orogenic belt of iran new data and interpretations abstract the zagros orogenic belt of iran is the result of the opening and closure of the neo tethys oceanic realm and consists from northeast to southwest of three parallel tectonic subdivisions 1 1 the urumieh dokhtar magmatic assemblage 2 2 the sanandaj sirjan zone and 3 3 the zagros simply folded belt recent detailed and reconnaissance mapping and stratigraphie structural analyses in various parts of the belt indicate that the tectonics of the sanandaj sirjan zone is dominated by either large scale composite duplex structures or low and high angle ne dipping imbricate systems these thrust systems have transported numerous horses and sheets nappes of metamorphosed and non metamorphosed phanerozoic rocks for tens or possibly hundreds of kilometers stratigraphic evidence and synorogenic conglomerates indicate that thrusting was initiated in the late cretaceous and has continued episodically since that time shear sense indicators in various parts of the zone reveal a ne sw direction of transport thrust systems generated in each episode are superimposed upon the previous structures cutting across and displacing them and producing complicated stacks of thrust sheets southwestward transport and stacking of the thrust sheets has resulted in an increase of about 10 15 km in the thickness of the continental crust in the southwestern part of this zone the so called z main zagros thrust which has been traditionally considered as the boundary between the sanandaj sirjan zone and the zagros simply folded belt is by no means a single high angle reverse fault nor is it a narrow zone of crushed rocks but a series of low angle thrusts locally cut by normal faults these thrusts are indistinguishable from those mapped in the sanandaj sirjan zone and are geometrically part of the sanandaj sirjan zone structures there is no single thrust among them that can be singled out and referred to as the main one the boundary of the sanandaj sirjan zone with the urumieh dokhtar magmatic assemblage to the northeast is characterized by a series of structural depressions formed by compression rock sequences including ophiolite complexes exposed along this boundary are strongly dismembered sheared and mylonitized shearing planes and foliations are either subvertical or dipping at high angles to the northeast this zone is suggested to be the suture between the afro arabian and the iranian plates overall the intensity of deformation increases northeastward from the persian gulf mesopotamian basin where strata are undeformed and horizontal through the zagros simply folded belt which is characterized by en echelon doubly plunging flexural slip folds gentle to open in the southwest and close to locally sw overturned in the northeast the intensity of deformation continues to increase further northeastward through the sanandaj sirjan zone where thrust faulting is dominant this pattern suggests that the deformational front has migrated through time from the northeastern part of the sanandaj sirjan zone i e the suture zone to its present position in the central portion of the persian gulf mesopotamian foreland basin

solar and tropical ocean forcing of late holocene climate change in coastal east asia abstract in this study paleoclimate signals were extracted from high resolution sediment records very recently produced from the mulyoungari swamp on jeju island south korea and examined to determine the main factors driving late holocene climate change in east asia the high resolution of the mulyoungari records allows the detection of multidecadal to centennial scale climate change with various cycles this enables comparisons with important paleoclimate proxies such as total solar irradiation tsi and sea surface temperatures ssts in the western tropical pacific wtp spectral analyses of pollen and magnetic susceptibility ms data identify significant periodicities of 780 470 135 145 and 50 70 years the mulyoungari swamp responded sensitively to global climate change during the holocene showing well known holocene cycles in the results our tree pollen index of warmness tpiw shows important late holocene cold events associated with low sunspot periods such as oort wolf sporer and maunder minimum comparisons among standard z scores of filtered tpiw tsi and other paleoclimate records from central and northeastern china off the coast of northern japan southern philippines and peru all demonstrate significant relationships this suggests that solar activity drove holocene variations in both east asian monsoon eam and el nino southern oscillation enso in particular the latter seems to have predominantly controlled the coastal climate of east asia to the extent that the influence of precession was nearly muted during the late holocene

uranium lead isotopic ages of the samail ophiolite oman with applications to tethyan ocean ridge tectonics plagiogranites are a minor but widespread component of the samail ophiolite plutonic member they crystallized from the most fractionated melts generated by magmatic crystallization and differentiation of a steady state magma chamber beneath the tethyan spreading ocean ridge and their ages are thought to mark the time of ocean crust formation isotopic u pb ages of zircons from 13 plagiogranites collected along a 270 km segment of the samail ophiolite subparallel to the regional trend of the sheeted dike complex the former spreading ridge axis direction define a narrow time interval of 93 5 97 9 m y with a pronounced clustering about 95 m y the zircon ages of the plagiogranites agree remarkably well with the early cenomanian to early turonian biostratigraphic ages of sediments that are intercalated within the ophiolite pillow lavas and that lie just above them tippit et al 1981 the agreement of radiometric and biostratigraphic ages provides strong support for the conclusion that the plagiogranite u pb ages closely date the time span of ocean crust formation no step changes in age patterns are observed along the ridge axis sheeted dike direction suggesting that there are no major internal offsets of the ophiolite by transform or other faults along most of the traverse one possible exception occurs at the southeastern end of the sampled interval ibra area where a 3 m y discontinuity might be caused by an unmapped fault assuming that the regional trend of the sheeted dikes n10 25 w marks the direction of the former spreading ridge axis the present array of sample localities spans a distance of 130 to 195 km normal to that axis i e in the spreading direction the data as a whole do not define a clear cut age trend normal to the spreading axis but by eliminating samples that may be aberrant due to faulting the data array suggests a pattern of increasing ages from east to west this would indicate that the samail ophiolite is derived from the western southwestern side of the tethyan spreading system a conclusion reached independently from geological evidence also the gentle age gradient in the spreading direction i e between 130 and 195 km of ocean crust were formed within an age span of no more than 2 m y suggests a very fast spreading rate this conclusion also is supported by independent geologic evidence

the western central asian orogenic belt a window to accretionary orogenesis and continental growth abstract the architecture of accretionary orogens is a key to understand continental growth here we present an overview of the orogenic components and their amalgamation in the western central asian orogenic belt caob the caob records the convergence and interactions among various types of orogenic components including the japan type mariana type and alaska aleutian type arc systems as well as the active marginal sequences of the siberia craton which incorporated wide accretionary complexes and accreted arcs and terranes during construction of the caob the kazakhstan arc chain was characterized by multiple subduction whereas the northern fringe of the tarim craton remained mostly as a passive margin the multiple convergence and accretions among these various orogenic components generated huge orogenic collages in the late paleozoic and even in the early triassic involving parallel amalgamation circum microcontinent amalgamation and oroclinal bending the preservation of trapped basins played a significant role in orogenesis with some parts of the oceanic plate being subducted and others behaving as rigid units the orogenesis in the caob was long lived lasting for more than 800 m y involving multiple subduction and long continuous accretion and featuring the complexity of accretionary orogenesis and continent growth

u pb zircon ages field geology and geochemistry of the kermanshah ophiolite iran from continental rifting at 79ma to oceanic core complex at ca 36ma in the southern neo tethys abstract the geodynamic evolution of the zagros mountains of iran remains obscure in particular the time of formation of the zagros ophiolites and the closure of the neo tethys ocean are highly controversial here we present new precise zircon u pb ages that show that the younger part sahneh kamyaran of the kermanshah ophiolite formed at 35 7 0 5 ma and the older part harsin at 79 3 0 9 ma field relations and geochemical evidence show that the younger sahneh kamyaran part is probably a fossil oceanic core complex and the older sahneh part is probably a continental oceanic transition complex both the sahneh kamyaran and sahneh parts were later emplaced into an accretionary complex we conclude and infer that the final closure time of the southern neo tethys ocean was after the late eocene our data and tectonic model have crucial implications for the geodynamic evolution of the zagros region

sulfur and oxygen isotopes in italian marine sulfates of permian and triassic ages evaporitic sulfates of permian and triassic ages from italian occurrences were analyzed for sulfur and oxygen isotopes in order to supplement the corresponding isotope age curves and to use isotopic data to trace the origin of the sulfate in hydrologic systems the sulfur isotopic composition age relationships obtained in this study generally agree with those found throughout the world in two cases i e in samples from the carniola di bovegno formation bergamo and brescia alps and from the lower part of the gracilis formation recoaro vicenza sulfur isotopes suggest different ages for the rocks than the formerly ascribed ones oxygen isotopic compositions show the same range from 10 5 to 18 5 without any consistent relationship with sulfur these ranges of values are almost identical to those published previously for sulfates of permian and triassic ages from other locations

middle to late miocene middle eastern climate from stable oxygen and carbon isotope data southern alborz mountains n iran abstract the alborz mountains of northern iran intercept and divert the northern hemisphere westerlies carrying moisture from the mediterranean and black sea and form an orographic barrier to moisture sourced to the north in the caspian sea this implies that terrestrial deposits along the southern alborz mountains leeward side of northerly winds and windward side of westerlies with the respect to tibet and other mountainous terrain in the himalayan karakoram realm potentially track changes in past moisture and erosional regimes and mirror rainfall patterns here we present results of a stable isotope analysis and clay mineral study of the miocene ca 17 5 7 6 ma upper red formation in the foreland of the southern alborz mountains the changes recorded by stable oxygen and carbon isotope data from pedogenic and lacustrine palustrine carbonate in the southern alborz mountains suggest 1 an increase in aridity possibly related to the evolution of the alborz orographic rain shadow which became more efficient between 17 5 and 13 2 ma 2 a steady increase in precipitation between 13 2 and 10 3 ma with a significant increase in rainout along the southern slope of the alborz mountains between 11 and 10 3 ma possibly related to perturbations in atmospheric circulation pattern in the northern hemisphere as suggested by coeval wetter phases in southern europe and 3 a decrease in aridity from ca 9 6 to 7 6 ma possibly reflecting an increase in the seasonality of precipitation based on environmental and climatic changes observed across southern asia and india starting from ca 10 ma we speculate that the topographic evolution of the himalayan tibetan system might have affected the late miocene climate in middle east

building the zagros collisional orogen timing strain distribution and the dynamics of arabia eurasia plate convergence he zagros mountains are the result of the arabia eurasia collision initiated at 35 ma as the rifted arabian lithosphere was underthrusted beneath the iranian plate due to its negative buoyancy the onset of crustal thickening started at 25 ma as recorded by the hinterland exhumation and foreland clastic deposition deformation throughout the arabia eurasia collision zone and the uplift of the iranian plateau occurred after 15 12 ma as a result of shortening thickening of the thin iranian crust we emphasize that only 42 of the post 35 ma convergence is partitioned by shortening within central iran tomographic constraints show ongoing slab steepening or breakoff in the nw zagros whereas underthrusting of the arabian plate is observed beneath central zagros the current subduction dynamics can be explained by the original lateral difference in the buoyancy of the distal margin that promoted slab sinking in nw zagros and underthrusting in central zagros critical wedge approach applied to the zagros favors the hypothesis of strong brittle crust detached above a viscous lower crust in contrast the weak sedimentary cover deforms by buckling of a thick multilayered cover thrust faulting associated with folding occurs in the competent layers and is responsible for most of the earthquakes there is evidence that the role of the slab pull force in driving the arabian plate motion was reduced after 12 ma large scale mantle flow induced by mantle upwelling at the afar plume appears to be the main driver of the arabia plate motion we stress that the main kinematic change in the zagros region occurred at 15 12 ma as the zagros uplifted before the arabian slab detached the zagros appears key to investigate coupling between continental rheology plate driving forces and mountain building in which the role of rift inheritance appears to be central

fluid evolution in an oceanic core complex a fluid inclusion study from iodp hole u1309 d atlantis massif 30 n mid atlantic ridge in the detachment mode of slow seafloor spreading convex upward detachment faults take up a high proportion of the plate separation velocity exposing gabbro and serpentinized peridotite on the seafloor large long lived hydrothermal systems such as tag are situated off axis and may be controlled by fluid flow up a detachment fault with the source of magmatic heat being as deep as 7 kmbsf the consequences of such deep circulation for the evolution of fluid temperature and salinity have not previously been investigated microthermometry on fluid inclusions trapped in diabase gabbro and trondjhemite recovered at the atlantis massif oceanic core complex 30 n mid atlantic ridge reveals evidence for magmatic exsolution phase separation and mixing between hydrothermal fluids and previously phase separated fluids four types of fluid inclusions were identified ranging in salinity from 1 4 to 35 wt nacl although the most common inclusions have salinities close to seawater 3 4 wt nacl homogenization temperatures range from 160 to 400 c with the highest temperatures in hypersaline inclusions trapped in trondjhemite and the lowest temperatures in low salinity inclusions trapped in quartz veins the fluid history of the atlantis massif is interpreted in the context of published thermochronometric data from the massif and a comparison with the inferred circulation pattern beneath the tag hydrothermal field to better constrain the pressure temperature conditions of trapping and when in the history of exhumation of the rocks sampled by iodp hole u1309d fluids have been trapped

convergence history across zagros iran constraints from collisional and earlier deformation the zagros orogen provides a unique opportunity within the alpine system to evaluate the interplay between a young tertiary collision and earlier subduction obduction processes within the crush zone and the sanandaj sirjan internal zone separating the zagros fold belt from central iran we document several major tectonic events taking place at the end of the cretaceous of the eocene and from the mio pliocene onwards ca 20 15 ma contrary to recent interpretations our data cross sections and description of the overall deformation style strongly suggest that the main zagros thrust mzt is deeply rooted possibly to moho depths and that the suture zone effectively runs along the mzt field observations show that the final resorption of the oceanic domain took place slightly after 35 ma and that collision must have started before ca 23 25 ma in northern zagros the shortening rate across the crush zone since the mid miocene 20 15 ma is estimated at a minimum 3 4 mm year shear movements in the crush zone during the eocene oligocene period and extensional strike slip movements in the internal zones during the late cretaceous point to an oblique setting early in the convergence history a geotectonic scenario for convergence from the time of obduction to the present is finally proposed

mineralogical geochemical and isotopic evolution of two miocene mafic intrusions from the zagros iran abstract in the zagros mountain belt which represents in iran the collision zone between eurasian and arabian african plates large plutonic intrusions were emplaced during miocene times these massifs are composed of layered cumulates with sequences of fine grained texturally isotropic rocks from dominant gabbros to subordinated felsic rocks for this study two intrusions have been selected according to the geotectonic site of their emplacement the tah intrusion is located in the ophiolitic zone representing the remnants of the tethyan seafloor whilst the kharsareh intrusion is emplaced in the sanandaj sirjan metamorphic domain which corresponds to the border of the central iran plate starting with equivalent cumulates and gabbros the evolution of the two intrusions are clearly different the tah rock series comprises syenogabbros syenodiorites and syenites whereas silica oversaturated rocks appear in the karsareh series with the sequence of monzodiorites quartz monzonites and monzogranites the two intrusions show regular variations in their mineralogical and geochemical characters suggesting a direct genetic relationship within each rock series sr and nd isotopic compositions show smaller variations in the tah intrusion with sr and nd values respectively in the 6 7 to 12 4 and 3 3 to 5 4 ranges than in the karsareh intrusion with equivalent ranges of 16 to 99 and 0 4 to 5 2 the na k ba sr rb and ree contents of the gabbros although not truly representative of the primary magmas are typical of transitional basalts a two step combined wall rock contamination and crystal fractionation process would account for the evolution of the two series isotopic modelling suggests that during the first step which affects both intrusions the wall rocks would be basaltic rocks whereas during the second one only acting in the kharsareh intrusion the wall rock would be a hydrous continental crust the major differences between the two intrusions have been related to the geotectonic site of their emplacement

geochemical 40ar 39ar age and isotopic data for crustal rocks of the neyriz ophiolite iran trace element data including the rare earth elements in the crustal sequence of the neotethyan neyriz ophiolite in southwest iran indicate normal mid ocean ridge basalt n morb or island arc tholeiite chemistry for the tang e hana basalt the data suggest that the tang e hana rhyodacite basalt plagiogranite and gabbro derived from a low k tholeiitic parent magma trace element distributions in amphibolite clasts in the sole detachment of the ophiolite south of lake neyriz correlate well with distributions in basalt clasts in the melange and in the tang e hana basalt these trace elements suggest that the amphibolite originated from metamorphism and deformation of a tholeiitic basalt protolith new 40 ar 39 ar incremental heating plateau ages from two hornblende plagiogranite specimens in the crustal sequence in tang e hana are 92 07 1 69 and 93 19 2 48 ma isotopic data for five tang e hana basalts yield end values of 7 8 and 7 9 and 87 sr 86 sr values of 0 70368 to 0 70476 the isotopic compositions ophiolite tectonostratigraphy and correlation of the 40 ar 39 ar cooling plagiogranite and deformation amphibolite ages suggest emplacement of the neyriz ophiolite either into an accretionary prism through offscraping and subduction erosion and or formation in a supra subduction zone environment around 82 96 ma progressive accretion probably led to the de velopment of a fore arc basin and deposition of upper cretaceous eocene fore arc and arc derived sediments on the ophiolite resume des donnees sur les elements traces incluant les terres rares dans la sequence crustale de l ophiolite neo tethysienne de neyriz dans le sud ouest de l iran indiquent que le basalte de tang e hana est de type basalte oceanique de type n n morb ou une tholeiite d arc insulaire les donnees recueillies suggerent que la rhyodacite le basalte le plagiogranite et le gabbro de tang e hana proviennent d un magma tholeiitique parent a faible teneur potassique la repartition des elements traces dans les clastes amphiboliques dans l unique portion separee de l ophiolite au sud du lac neyriz est en bonne correlation avec celle trouvee dans les clastes basaltiques du melange ophiolitique et du basalte de tang e hana ces elements traces laissent penser que l amphibolite provient du metamorphisme et de la deformation d un protolithe de basalte tholeiitique de nouveaux ges 40 ar 39 ar par paliers de temperature pour deux specimens de plagiogranite a hornblende appartenant a la sequence crustale de tang e hana ont donne des valeurs de 92 07 1 69 ma et de 93 19 2 48 ma des donnees isotopiques de cinq basaltes de tang e hana ont donne des valeurs end de 7 8 et de 7 9 et des valeurs 87 sr 86 sr de 0 70368 a 0 70476 les compositions isotopiques la tectonostratigraphie ophiolitique et la correlation entre les ges 40 ar 39 ar de refroidissement plagiogranite et de deformation amphibolite laissent croire que l ophiolite de neyriz a ete mise en place dans le prisme d accretion sous l effet de l ecaillage et de l erosion par subduction et ou s est formee au sein d un environnement a zone de supra subduction il y environ 82 ma a 96 ma l accretion graduelle a probablement conduit au developpement d un bassin d avant arc et au depot de sediments provenant de l avant arc et de l arc sur l ophiolite au cours du cretace superieur et de l eocene

rapid emplacement of the oman ophiolite thermal and geochronologic constraints in understanding of ophiolite emplacement requires knowledge of the elapsed time between igneous crystallization and intraoceanic thrusting and the rate and duration of that thrusting hornblende 4oar 39ar ages demonstrate that the igneous oceanic crust in oman crystallized and cooled to 825 k in 1 2 m y hornblende ages from metamorphic rocks and from cross cutting dikes require that the basal metamorphic thrust fault beneath the ophiolite also cooled below 825 k in 1 2 m y motion along the sole thrust accounted for 200 km of displacement at a rate of 100 200 mm yr on the basis of age relationships and thermal considerations we favor a two stage model for the initial stages of samall ophiolite emplacement intraoceanic thrusting over 2 m y old lithosphere at 150 km m y parallel to a spreading ridge for 1 2 m y followed by equally rapid and brief thrusting over cold and old lithosphere preservation of the samail ophiolite is a direct result of its young age and positive buoyancy at the time of ocean closure and we propose that all ophiolites that originated near spreading centers and were emplaced onto continents were young at the time of intraoceanic thrusting

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

structures and microstructures related to steady state mantle flow in the neyriz ophiolite iran abstract structural analysis in the well exposed and well preserved neyriz ophiolite suggests that it is a relatively undisturbed piece of oceanic lithosphere detailed structural mapping of high t deformation mantle flow revealed the presence of three elliptical shaped diapirs trending nw se these diapirs are characterized by vertical mantle foliations associated with vertical plunging stretching lineations which progressively incline toward parallelism with the gently ne dipping moho the nw se direction of asthenospheric upwelling of diapirs is parallel with the orientations of the vertical sheeted dike complex this suggests that the neyriz ophiolite was created by two nw se palaeo ridge axes these palaeo ridges are classified as fast spreading ridges these ridges are segmented by a dextral palaeo transform fracture zone this is consistent with fast spreading ridges comparison between the neyriz and oman ophiolites reveals that they show similar characteristics most of the oman palaeo ridge systems are oriented nw se and nnw se they also display similar sheeted dike complex orientations and crustal thickness variations these two ophiolites originally were part of the neo tethyan oceanic lithosphere and afterwards were separated by the oman line during continental collision of the iranian micro continent and afro arabian continent

insights from the talysh of azerbaijan into the paleogene evolution of the south caspian region the age and mode of formation of the south caspian basin are disputed an 10 km thick predominantly middle eocene clastic and volcanic succession is exposed in the talysh mountains of azerbaijan at its western margin here high k alkali basalts pass laterally to the east and southeast into volcanogenic sandstone dominated turbidity current and debris flow deposits these southeasterly directed depositional systems accumulated in water depths generally greater than 200 m and fed directly into the western south caspian basin new ar ar ages cluster around 39 ma with an upper 1400 m thick volcanic interval being deposited in 2 2 0 2 m y we interpret that this rapid deposition and magmatism records a major back arc extensional transtensional event in the talysh north of the north dipping neotethyan subduction zone this event is recognized across much of southwest asia and may indicate a period of significant basin formation within the adjacent south caspian basin a transition into upper eocene lower oligocene strata dominated by fine grained turbidity current and hemipelagic sediments with slope instability features is interpreted to mark the end of rifting and volcanism in the talysh and the start of the arabia eurasia collision overlying oligocene coarse clastic rocks are interpreted as the erosional products of localized topography created by the further propagation of compressional deformation into the talysh region