controls on hydrothermal alteration in a section of fast spreading oceanic crust abstract young oceanic crustal sections that formed at the fast spreading east pacific rise epr were mapped by submersible along steep fault scarps that bound the hess deep rift valley excellent exposures allowed the documentation of the spatial distribution of alteration within the lava sequence sheeted dike complex and uppermost plutonic sequence alteration assemblages record the initiation and subsequent cooling of an axial hydrothermal system s that was periodically perturbed by the injection of sheeted dikes mineralogical and geochemical data indicate that temperatures increased across the lava dike transition 50 180 c and that the grade of metamorphism was heterogeneous within the sheeted dike complex in that temperatures varied laterally from 100 300 c to 600 800 c within the upper dikes and were 400 450 c within the lower sheeted dikes during vigorous hydrothermal circulation at the epr the base of the hydrothermal system was located within or at the top of a sequence of gabbros that is interpreted to be the root zone of the sheeted dike complex the base of the hydrothermal cell did not penetrate into the subjacent plutonic sequence until the section of crust had migrated off axis and the magma lens had solidified the vigor of the hydrothermal system would have waned by this time and hydrothermal fluids migrated into the lower crust along broadly distributed microfracture networks at temperatures between 450 and 650 c although locally temperatures may have reached 800 c the highest temperature alteration assemblages within the sheeted dikes formed in response to the injection of a dike or swarm of dikes that initiated a short lived hydrothermal system the sheeted dikes and plutonic rocks experienced very little retrograde alteration and there is no evidence for a discrete off axis hydrothermal event s incipient alteration conditions in crustal sections that formed at mid ocean ridges with slower rates of spreading than the epr are similar to hess deep although there are differences in the distribution and mineralogical expression of incipient alteration in plutonic sequences the evolution of alteration as a section of crust cools and moves off axis reflects its particular magmatic tectonic history and thus may vary significantly in ridges with all spreading rates

rbsr and 40ar39ar laser probe dating of high pressure phengites from the sesia zone western alps underscoring of excess argon and new age constraints on the high pressure metamorphism abstract the combined use of the rb sr and 40ar 39ar laser probe methods allows confirmation of the existence of excess argon in phengites from eclogites from the sesia zone western alps despite systematic 40ar 39ar plateau ages 40ar 39ar phengite ages from two different areas show integrated age spreads between 65 4 0 3 ma and 109 5 0 4 ma the youngest 40ar 39ar plateau age 65 9 0 4 ma is concordant with two rb sr phengite whole rock isochron ages from the same outcrop which display a mean age at 64 2 2 5 ma and older than a rb sr biotite whole rock isochron age at 53 0 1 0 ma this age concordance being incompatible with the concept of isotopic closure temperature is probably fortuitous and probably results from an excess argon contamination however the concordance of the rb sr phengite isotopic closure temperature with the temperature reached during the eclogite facies event suggests that the age at 64 2 2 5 ma could be a crystallization age probably contemporaneous with the eclogite facies event in the sesia zone when compared to recent age data from the piemont zone and the dora maira massif the entire data set which needs to be confirmed by extra measurements support diachronous high pressure hp events in the western alps

eclogite occurrences in the southern tianshan high pressure belt xinjiang western china abstract eclogite pods and layers of eclogites in mafic blueschists are the most prominent witnesses of eclogite facies metamorphism in the southern tianshan high pressure belt of western china the p t evolution was deciphered in order to understand the geotectonic framework of these rocks geothermobarometric evaluation for matrix assemblages reveals peak metamorphic condition between 14 and 21 kbar at 500 to 600 c prograde growth zoning and mineral inclusions in garnet are evidence for a clockwise p t path the presence of intimate eclogite metabasic blueschist interlayering and the high p t estimates suggest that the eclogite facies rocks from the southern tianshan high pressure belt formed in a tectonic regime similar to the franciscan type therefore the southern tianshan high pressure belt is most important in order to resolve the paleozoic tectonic evolution of central asia

allochthonous oceanic basalts within the mesozoic accretionary complex of alexander island antarctica remnants of proto pacific oceanic crust the mesozoic lemay group accretionary complex of alexander island antarctica contains thrust bound slices of accreted ocean floor ocean islands and seamounts they represent fragments of proto pacific oceanic crust of which only a tiny remnant the phoenix plate remains off northern antarctic peninsula they therefore provide an excellent opportunity to sample the ancient oceanic crust that formerly occupied the southern pacific ocean all the basalts experienced sea floor and subduction accretion metamorphism ranging from zeolite to transitional blueschist facies on the basis of rare arth and other immobile trace element characteristics the basalts are divided into depleted morb n morb e morb and tholeiitic and alkaline oib oceanic basalts occur within two rock associations on alexander island basalt volcaniclastite chert and basalt volcaniclastite tuff the basalt volcaniclastite chert rock association is dominated by pillow lavas which have light ree depleted n morb geochemical characteristics and is interpreted as representing ocean floor formed at spreading centres locally sills of tholeiitic oib intrude the sequence the basalt volcaniclastite tuff rock association exposed in the lully foothills was formed in shallow water during the early jurassic it is geochemically varied consisting of basalts with n morb e morb and tholeiitic oib characteristics the association is interpreted to have been formed on a large seamount or ocean island

diamictic sediments within high arctic lake sediment cores evidence for lake ice rafting along the lateral glacial margin sediment cores from six small lake basins in the canadian high arctic reveal a gravel rich 30 by weight to gravel poor 2 diamict facies underlying massive post glacial clayey silt ten other lakes contain a second diamict facies within what are interpreted to be glaciolacustrine sedimentary assemblages the sedimentology clast fabrics and fossil remains diatoms ostracodes and chironomid head capsules within both diamict facies suggest that these deposits are not tills clast fabrics yielded low s1 0 41 0 57 and high s3 0 09 0 22 eigenvalues placing them within the range of ice rafted diamictons and glacigenic sediment flows the high percentage of clast dip angles 45 15 61 random clast azimuth and lower diamict contacts conformable to underlying current bedded sediment favours an origin as a rain out or settling deposit samples of the matrix and scrapings of clasts from the diamicts revealed a diatom assemblage dominated by littoral and planktonic forms such as are found in the littoral regions of the lakes today this contrasts sharply with the assemblages within the overlying clayey silt in which benthic forms predominate clasts are thus interpreted to have been rafted from the littoral areas of the lake the process proposed to explain this is rafting by the lake ice cover in a glacial marginal environment early season meltwater impounded along the lateral margin of retreating cold based glaciers would buoyantly lift the lake ice cover and any adfrozen lake sediment higher lake levels and increased areal extent of seasonal freeze on between the lake ice cover and the lake bed would allow the redeposition of littoral sediments to the benthic regions through greater lateral shifting of the ice cover as it broke up incision by meltwater streams into the lateral glacial margins would later isolate the lake allowing seasonal warming of lake water enough to support the growth and maturation of the ostracode and chironomid species found as fossils within the diamicts

late paleozoic arc magmatism in the southern yili block nw china insights to the geodynamic evolution of the balkhash yili continental margin central asian orogenic belt abstract the yili block represents the easternmost segment of the kazakhstan yili microcontinent late paleozoic magmatic rocks are extensively exposed in the yili block and are essential in understanding the tectonic development of the sw central asia orogenic belt caob we conducted field investigations zircon u pb dating whole rock geochemical and sr nd isotopic studies on the magmatic rocks from the wusun range southern yili block the late paleozoic volcanic and plutonic rocks of the wusun range show geochemical features consistent with that of calc alkaline series and they have positive end t values 0 9 to 5 6 neo to mesoproterozoic single stage nd model ages 0 80 1 33 ga and variable initial 87 sr 86 sr ratios 0 7041 0 7059 suggesting that these magmatic rocks are likely derived from partial melting of a moderately depleted mantle wedge with minor involvement of a precambrian continental crust these new data combined with previously published results put constraints on the late paleozoic magmatic and geodynamic evolution of the balkhash yili active continental margin the late paleozoic arc magmatism initiated from 386 ma along the northern yili block and migrated southward to have occurred since 340 ma along its southern margin subsequently the arc related magmatism moved back northward and terminated at 310 ma in the south and at 295 ma in the north the oib like geochemical features of some volcanic rocks indicate incipient marginal rifting occurred along the southern yili block during late carboniferous taking into account the regional structural and tectonic data the carboniferous yili magmatic arc may be laterally correlated with the balkhash yili active margin of the kazakhstan microcontinent which was likely resulted from the southward subduction of the junggar north tianshan ocean

geochemical discrimination of metabasalt rocks of the fan karategin transitional blueschist greenschist belt south tianshan tajikistan seamount volcanism and accretionary tectonics the fan karategin metamorphic belt south tianshan tajikistan is regarded to be an ancient subduction accretionary complex and is composed of three tectonostratigraphic units which display lithologies consistent with different tectonic settings the mafic schists which make up the major part of the older unit of the belt contain both alkali and tholeiitic metabasalts on the basis of rare earth and other immobile element characteristics the alkali metabasalts are akin to within plate ocean island basalts whereas the tholeiitic metabasalts resemble e type morb the association is interpreted to have been formed on seamount like structures under a within plate plume bedded cherts and marbles in the unit are regarded as ancient pelagic sediments and carbonate caps developed upon basaltic seamounts respectively dismemberment of the seamount related basalts and pelagic sediments and the high p low t prograde metamorphism of the unit rocks up to transitional blueschist greenschist facies was the result of paleoseamount submergence into a subduction zone this unit is tectonically overlain by arc derived metavolcanic unit and a disrupted mainly clastic unit of upper ordovician lower silurian age metavolcanic and metasedimentary rocks of the two upper units have geochemical characteristics compatible with subduction related origin the lithological assemblages of the individual units and their juxtaposition suggest an origin involving collision accretionary processes the fan karategin belt is a subduction accretionary complex which formed during subduction of oceanic crust under a volcanic arc and was subjected to tectonic juxtaposition and imbrication of seamount deep sea trench and volcanic arc sequences

continental growth of northwest china the northwest corner of china is the most interior part of central asia and includes parts of the siberian tarim and kazakhstan plates where the junggar and tarim basins are separated by the east west trending tianshan range a preliminary terrane map divides the area into thirteen separate areas which are classified into four types l continental 2 oceanic crust 3 island arc 4 composite the boundaries for these terranes are faults that have been previously defined either by geologic mapping or holocene activity passive consolidation of the southern margin of paleo asia is marked by accretion and subduction of the paleo tethys oceanic basins and by development of volcanic arcs paleomagnetic data indicate that the major plates and associated smaller terranes did not reach final consolidation until the permian or later the stratigraphic record reveals development of intracontinental basins in the late carboniferous followed by the pan asian thermal event that gave rise to widespread intrusion of a type granites of late paleozoic age the frontal collision of india along the southern border of asia in the paleocene reactivated nearly all of the older major strike slip and thrust faults formed during the late paleozoic consolidation of paleo asia the complex nature of the continental growth of central asia has produced and destroyed many varieties of sutures and reconstruction of the original plate and terrane configurations therefore is enigmatic

palaeozoic collisional tectonics and magmatism of the chinese tien shan central asia abstract the chinese tien shan range is a palaeozoic orogenic belt which contains two collision zones the older southern collision accreted a north facing passive continental margin on the north side of the tarim block to an active continental margin on the south side of an elongate continental tract the central tien shan collision occurred along the qinbulak qawabulak fault southern tien shan suture the time of the collision is poorly constrained but was probably in in the late devonian early carboniferous we propose this age because of a major disconformity at this time along the north side of the tarim block and because the youshugou ophiolite is imbricated with middle devonian sediments a younger probably late carboniferous early permian collision along the north tien shan fault northern tien shan suture accreted the northern side of the central tien shan to an island arc which lay to its north the north tien shan arc this collision is bracketed by the middle carboniferous termination of arc magmatism and the appearance of late carboniferous or early permian elastics in a foreland basin developed over the extinct arc thrust sheets generated by the collision are proposed as the tectonic load responsible for the subsidence of this basin post collisional but palaeozoic dextral shear occurred along the northern suture zone this was accompanied by the intrusion of basic and acidic magmas in the central tien shan late palaeozoic basic igneous rocks from all three lithospheric blocks represented in the tien shan possess chemical characteristics associated with generation in supra subduction zone environments even though many post date one or both collisions rocks from each block also possess distinctive trace element chemistries which supports the three fold structural division of the orogenic belt it is unclear whether the chemical differences represent different source characteristics or are due to different episodes of magmatism being juxtaposed by later dextral strike slip fault motions because the southern collision zone in the tien shan is the older of the two the tarim block sensu stricto collided not with the eurasian landmass but with a continental block which was itself separated from eurasia by at least one ocean the destruction of this ocean in late carboniferous early permian times represented the final elimination of all oceanic basins from this part of central asia

a hot spring in granite of the western tianshan china the western tianshan range is a major cenozoic orogenic belt in central asia exposing predominantly paleozoic rocks including granite ongoing deformation is reflected by very rugged topography with peaks over 7000 m high active tectonic deformation is tied to an e w trending fracture and fault system that sections the mountain chain into geologically diverse blocks that extend parallel to the orogen in the muzhaerte valley upwelling hot water follows such a fault system in the muza granite about 20 l min 1 na so4 cl water with a temperature of 55 c having a total mineralization of about 1 g l 1 discharge from the hot spring the water is used in a local spa that is frequented by the people of the upper ili river area its waters are used for balneological purposes and the spa serves as a therapeutic institution the major element composition of the hot water is dominated by na and by so4 and cl ca is a minor component dissolved silica 1 04 mmol l 1 corresponds to a quartz saturation temperature of 116 c and a corresponding depth of the source of the water of about 4600 m this temperature is consistent with na k and na li geothermometry the water is saturated with respect to fluorite and contains 7 5 mg l 1 f as a consequence of the low ca concentration the water is undersaturated with respect to the primary minerals of the reservoir granite at reservoir temperature causing continued irreversible dissolution of granite the waters are oversaturated with respect to ca zeolite minerals such as stilbite and mesolite and it is expected that zeolites precipitate in the fracture pore space and in alteration zones replacing primary granite the stable isotope composition of o and h supports a meteoric origin of the water the cl br mass ratio of 1500 suggests that the salinity results from halite dissolution salts leached from powders of muza granite show the same cl br signature as the hot spring water sodium chloride is stored in fluid and solid inclusions in the granite which have been introduced to quartz by ductile shearing and faulting related to ongoing orogenesis the hot water remobilizes the salt that is continuously liberated by the tectonic deformation water granite interaction contributes a thenardite component na2so4 to the major element composition by albite dissolution in h2so4 the water rock interaction along faults and fractures transforms and alters muza granite to a low temperature epigranite

sedimentary processes of canadian arctic fjords abstract conceptual models of circulation and sedimentation in arctic fjords are presented to illustrate the current understanding of these environments circulation is a vigorous process in these fjords and is enhanced by the loss of the freshwater cap during the long winter period of near zero inflow the process of brine rejection during the formation of sea ice and the role of glaciers and icebergs injecting freshwater at depth on the other hand the presence of sea ice blocks reduces the mixing action of wind which may otherwise represent the largest input of energy to fjord circulation it also provides a freshwater layer on melting and may in some circumstances damp the amplitude of tides during summer large loads of glacial sediment entering arctic fjords are separated into streams of overflowing suspended load and bed load in the presence of salt water as the concentrations of suspended load in the freshwater are normally much less than required to overcome the density of the salt water deposition occurs throughout the fjord by rapid settling of flocculated particles from the overflowing brackish water and from grain flows in the proximal environment created as the fluvial traction carpet moves over the delta foreset slope after sediment has been deposited it may be remobilized by slides which in turn may develop into fully turbulent flows as they move down the fjord floor a range of deposits from slump debris to minor turbidites are postulated to occur with the flocculated muds although not all have yet been examined in the arctic fjord stratigraphic record sea ice is an important sedimentological agent in arctic fjords it passively carries fluvial aeolian and colluvial material which is subsequently released from the ice during the stage of puddle draining at break up sea ice also actively incorporates fine and coarse sediments up to large boulders during freezing along the shores in winter and by wave and tidal current washing controlled by the presence of the ice during break up

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

40ar 39ar and rb sr geochronology of high pressure metamorphism and exhumation history of the tavsanli zone nw turkey geochronological investigations in high and ultra high pressure metamorphic rocks are problematic since firstly the low temperatures lead to fine grain size and disequilibrium assemblages and secondly the problem of excess argon affects 40ar 39ar systematics the most commonly used isotopic system the tavsanli zone is a belt of high pressure low temperature hp lt rocks spanning nw turkey and is one such region where previous geochronological studies have produced a range of estimates for the age of hp lt metamorphism raising the question of whether they are geologically significant this study presents new data from the tavsanli zone 40ar 39ar ages are in the range 60 ma to 175 ma whilst rb sr ages are restricted to 79 7 ma to 82 8 ma confirming the presence of excess argon detailed ultra violet laser ablation microprobe uvlamp studies have revealed younger 40ar 39ar ages in the cores of coarser white micas which in conjunction with 40ar 39ar ages from the finest grained lithologies and the rb sr white mica crystallisation ages constrain the post hp lt metamorphism exhumation rates of these rocks petrological and regional constraints suggest that syn subduction exhumation and cooling took place initially by synchronous subduction and exhumation by underplating this is followed by a phase of syn continent continent collision at a rate of approximately 1 5 mma 1 and exhumation to the surface via thrusting the 40ar 39ar hornblende data from a granodiorite intruding the hp lt rocks constrain the later parts of exhumation path this study highlights the importance of a multi system geochronological approach when attempting to determine the history of hp lt rocks

geochemical and geochronological studies of granitoid rocks from the western tianshan orogen implications for continental growth in the southwestern central asian orogenic belt abstract the western tianshan orogen in northwestern china is part of the southwestern margin of the central asian orogenic belt caob which represents the largest tectonic assembly of accretionary complexes and the most significant area of phanerozoic continental growth in the world granitoid rocks have a widespread occurrence and occupy about 30 of the whole exposure of the orogen ages obtained for the granitoids vary from 896 ma to 247 ma while initial 87 sr 86 sr ratios vary from 0 70329 to 0 72070 and e nd values from 14 1 to 7 3 the occurrence of neoproterozoic granitic gneisses implies the presence of precambrian basement in this area during oceanic subduction continental growth occurred as a result of several combined processes i e by addition of oceanic crustal melts adakites the intrusion of basaltic magmas derived by partial melting of the metasomatized depleted mantle wedge and the upwelling of granitic magma derived from a mixed source of basaltic magmas and old continental basement however during the post collisional period vertical accretion of underplated juvenile mantle material may have been accomplished during slab breakoff delamination this study suggests a two stage model of continental growth of syn subduction lateral accretion of arc complexes and post collisional vertical accretion of underplated mantle material which may also be relevant for the western segment of the caob in the phanerozoic the reworking and involvement of old crust are more obvious in the western tianshan orogen compared to other parts of the caob comprising the western junggar eastern junggar alataw and altay terranes

two subduction events in a polycyclic basement alpine and pre alpine high pressure metamorphism in the suretta nappe swiss eastern alps this study is essentially based on coupling macrostructures microstructures and metamorphic petrology in polymetamorphic mafic rocks from the swiss eastern alps suretta nappe penninic domain petrographic criteria are used in conjunction with structural analysis and microprobe work to define crystallization deformation relationships and to establish a relative but precise sequence of tectono metamorphic events a first eclogite facies overprint and related exhumation occurred before emplacement of late palaeozoic intrusives during the alpine cycle the suretta nappe was part of the thinned european continental margin the tertiary burial due to subduction and collision is responsible for d1 ductile thrusting and blueschist facies metamorphism late deformation phases related to exhumation are responsible for the development of extensional structures under greenschist facies conditions quantitative metamorphic petrology based on gibbs free energy minimization domino by de capitani gives a constraint on the p t conditions during the polymetamorphic and polycyclic evolution the first high p metamorphic event related to pre alpine structures occurred at c 700 c and at least 2 0 gpa these conditions are compatible with pre alpine high p re equilibration already described in several alpine units the alpine high p metamorphism occurred under blueschist facies conditions at c 400 450 c and 1 0 gpa similar high p low t conditions have already been described in the mesozoic and permian rock types the two high p events are clearly related to two different geothermal regimes and geodynamic environments

the mineralogy petrology metamorphic ptdt trajectory and exhumation mechanism of blueschists south tianshan northwestern china abstract the south tianshan blueschist belt extends for about 200 km along the south tianshan suture from changawuzhi to the upper kekesu river the high pressure low temperature high p low t metamorphic rocks contain sodic amphibole garnet phengite albite quartz epidote chlorite and stilpnomelane these blueschists occur as small blocks lenses laminae and bedded slabs or sheets in greenschist facies country rocks a few dismembered ophiolitic nappe slices and marble lenses are interlayered in the high p t metamorphic belt metabasite blueschists are mainly e type morb less commonly n type the protoliths of the meta sedimentary rocks are graywackes which contain volcaniclastic material of e type morbs and probably some continental basement materials derived from the tarim plate these protoliths may represent subduction complexes or meta melanges which formed within an accretionary wedge on the southern side of the yili central tianshan plate yctp at the end of the early paleozoic the mineral assemblages include sodic amphibole mainly crossite garnet phengite epidote chlorite albite estimated to have crystallized at 450 c and 9 10 kbar these conditions correspond to the epidote blueschist facies and are the peak metamorphic conditions the prograde metamorphism is represented by lawsonite blueschist facies and the retrograde is greenschist facies the metamorphic ptdt trajectory of the blueschists runs clockwise and reveals that the metamorphism revolved from laumontite facies lawsonite blueschist facies epidote blueschist facies 408 ma to greenschist facies 345 ma during the creation and exhumation of the blueschists in the south tianshan suture we have recognized six stages of deformation in the blueschists the earlier three stages were concomitant with metamorphism the later three postdated the metamorphism on the basis of the deformation characteristics of the blueschists and coherent rocks the regional geology and the metamorphic ptdt path we discuss the exhumation mechanism of the blueschists the post collisional ductile thrust within the suture probably exhumed the blueschists and coherent rocks allowing them to escape retrograde overprinting in the greenschist facies the influence of erosion on the exhumation of blueschists could not be considered directly and quantitatively because of the absence of devonian strata and the few exposures of early carboniferous clastic rocks all without detritus of blueschists

changes in dip of subducted slabs at depth petrological and geochronological evidence from hp uhp rocks tianshan nw china article high resolution seismic imaging has previously revealed sudden changes in the dip of subducted oceanic plates this kinking feature is crucial because in many subduction zones it is thought to coincide with the disappearance of the low velocity layers associated with subducted oceanic crust in this study we present petrological evidence for this phenomenon derived from oceanic blueschist and eclogite facies rocks from the chinese tianshan the investigated samples span a large range of peak metamorphic conditions with temperatures between 330 and 580 c at 1 5 to 2 3 gpa such variable peak metamorphic conditions together with the intimate interlayering of high and ultrahigh pressure rocks have also been reported from other tianshan localities these observations suggest that the rocks were derived from varying depths within the subduction zone and then juxtaposed during exhumation in the subduction channel multi point lu hf isochrons from four high pressure rocks yield consistent garnet growth ages of 313 12 315 8 2 9 313 9 4 8 and 315 2 1 6 ma these inconjunctionwiththe 311 maclusterof published 40

effect of surficial disturbance on exchange between groundwater and surface water in nearshore margins 1 low permeability sediments situated at or near the sediment water interface can influence seepage in nearshore margins particularly where wave energy or currents are minimal seepage meters were used to quantify flow across the sediment water interface at two lakes where flow was from surface water to groundwater disturbance of the sediment bed substantially increased seepage through the sandy sediments of both lakes seepage increased by factors of 2 6 to 7 7 following bed disturbance at seven of eight measurement locations at mirror lake new hampshire where the sediment representing the greatest restriction to flow was situated at the sediment water interface although the veneer of low permeability sediment was very thin and easily disturbed accumulation on the bed surface was aided by a physical setting that minimized wind generated waves and current at lake belle taine minnesota where pre disturbance downward seepage was smaller than at mirror lake but hydraulic gradients were very large disturbance of a 20 to 30 cm thick medium sand layer resulted in increases in seepage of 2 to 3 orders of magnitude exceptionally large seepage rates some exceeding 25 000 cm d were recorded following bed disturbance since it is common practice to walk on the bed while installing or making seepage measurements disruption of natural seepage rates may be a common occurrence in nearshore seepage studies disturbance of the bed should be avoided or minimized when utilizing seepage meters in shallow nearshore settings particularly where waves or currents are infrequent or minimal

submerged early holocene coastal and terrestrial landforms on the inner shelves of atlantic canada coastal and terrestrial landforms that formed with lowered relative sea levels during the early postglacial period in atlantic canada were submerged during the holocene transgression however these landforms are seldom seen on sea floor imagery factors that contribute to their destruction include the brevity of sea level lowstands and high wave energy on shallow modern shelves we identify one situation within which preservation has been relatively good large coastal lakes that existed for many thousands of years before being connected to the ocean by rising sea level in the mid holocene we describe bedford basin near halifax nova scotia and deal more exhaustively with the bras d or lakes an inland sea in cape breton nova scotia the preservation of shore platforms barrier beaches and spits and fluvial systems was due to the rapid onset of the transgression and the relatively low wave energy in the subsequent marine phases the well preserved early holocene coastlines are highly favourable targets in the search for evidence of human occupation in the early to mid holocene

early neoproterozoic granitic gneisses in the chinese eastern tianshan petrogenesis and tectonic implications abstract early neoproterozoic granitic gneisses are known from several locations such as the xingxingxia alatage and baluntai areas in the central tianshan arc terrane of the chinese eastern tianshan which is one of several precambrian microcontinents within the southern central asian orogenic belt caob the petrogenesis and the tectonic significance of the early neoproterozoic granitic gneisses have not been well constrained yet resulting in an ambiguous understanding of the early tectonothermal evolution of the chinese eastern tianshan geochronological results suggest that the protoliths of these granitic gneisses formed during a relatively short time period between ca 970 and 915 ma mainly clustering at ca 940 ma these peraluminous to strongly peraluminous granitic rocks show a common geochemical s type granite affinity involving low cao na 2 o th and y contents zircon saturation thermometry and ti in zircon thermometry yielded a relatively low crystallization temperature of approximately 750 800 c for the magmatic protolith of the granitic gneisses zircon hf isotopic compositions of the xingxingxia and alatage granitic gneisses show e hf t values of 5 4 to 4 2 with t dm2 model ages of 1 53 2 14 ga indicating that their protoliths were derived by the recycling of older continental crust we suggest that the early neoproterozoic granitic rocks in the chinese eastern tianshan were formed along an active continental margin furthermore we propose that the central tianshan arc terrane was located at the periphery of rodinia and separated from the tarim craton terrane in the early neoproterozoic

paleozoic intrusive rocks from the dunhuang tectonic belt nw china constraints on the tectonic evolution of the southernmost central asian orogenic belt abstract the dunhuang tectonic belt dtb is of great importance for understanding the tectonic evolution of the southernmost central asian orogenic belt caob in this study the temporal spatial distribution petrogenesis and tectonic setting of the paleozoic representative intrusive rocks from the dtb were systematically investigated to discuss crustal evolution history and tectonic regime of the dtb during paleozoic our results reveal that the paleozoic magmatism within the dtb can be broadly divided into two distinct episodes of early paleozoic and late paleozoic the early paleozoic intrusive rocks represented by a suite metaluminous slight peraluminous and medium to high k calc alkaline i type granitoids crystallized at silurian ca 430 410 ma are predominantly distributed along the northern part of the dtb they were probably produced with mineral assemblage of eclogite or garnet amphibole rutile in the residue and were derived from magma mixing source of depleted mantle materials with various proportions of archean mesoproterozoic continental crust the late paleozoic intrusive rocks can be further subdivided into two stages of late devonian stage ca 370 360 ma and middle carboniferous stage ca 335 315 ma the former stage is predominated by metaluminous to slight peraluminous and low k tholeiite to high k calc alkaline i type granitic rocks distributed in the central part of the dtb they were also generated with mineral assemblage of amphibolite to eclogite facies in the residue and originated from magma source of depleted mantle materials mixed with different degrees of old continental crust the later stage is represented by adakite and alkali rich granite exposed in the southern part of the dtb the alkali rich granites studied in this paper were possibly produced with mineral assemblage of granulite facies in the residue and were generated by partial melting of thickened lower continental crust zircon hf isotopes and field distribution of those paleozoic intrusive rocks reveal that both the silurian and the late devonian magmatic activities predominantly represent crustal growth processes in the dtb accompanied by different degrees of reworking of pre existing continental crust however the middle carboniferous ca 335 315 ma magmatic activity reflects a crustal reworking process the silurian and late devonian intrusive rocks were most likely formed in the arc related subduction zones whereas the middle carboniferous intrusive rocks were possibly formed in a transitional tectonic setting from compression to extension representing the final stage of paleozoic orogeny in the dtb these paleozoic magmatic rocks further suggest that the dtb has reactivated from a stable block to an orogen and undergone two episodes the early paleozoic and the late paleozoic of orogeny during paleozoic it represents a paleozoic accretionary orogen of the southernmost margin of the caob between the tarim craton and north china craton and tectonically extends northward to the beishan orogen and westward to the eastern south tianshan belt

exhumation of coesite bearing rocks in the dora maira massif western alps italy coesite bearing very high pressure rocks in the southern dora maira massif western alps italy emerged from a depth of 100 km during cretaceous time these rocks are now covered only by a thin fraction of the synmetamorphic overburden tectonic contacts above the coesite bearing rocks are retrograde ductile shear zones that operated with a normal sense of motion they cut out thick crustal sections from within the tectonometamorphic pile and caused the condensation of the eoalpine baric and thermal structure the result of this was the lateral dispersion of the eoalpine orogenic wedge and the facilitation of the unroofing of the very high p rocks extensional unroofing is consistent with the absence of deeply cutting cretaceous erosion and with pressure temperature paths that show that the very high p rocks cooled during decompression the very high p rocks reached relatively shallow crustal levels by ca 40 ma and the attenuated sequence overthrust a more external unit these indicate that the exhumation of the very high p rocks took place within an overall plate onvergence setting prior to the final suturing of europe and adria

formation of hp lt rocks and their tectonic implications in the western tianshan orogen nw china geochemical and age constraints the western tianshan high pressure low temperature metamorphic belt in northwestern china formed within a paleozoic accretionary wedge on the south side of the yili central tianshan plate it comprises various lithologies including mafic metavolcanic rocks metavolcaniclastic rocks metagreywacke marble and serpentinite the metavolcanics comprise eclogite omphacitite epidosite and blueschist the trace and rare earth element characteristics of these rocks indicate morb to oib affinities suggesting a seamount like setting for the protoliths of these rocks the end value of the eclogite 8 9 indicates that the protolith was derived from a long term depleted mantle i e an n morb source supporting the trace and rare earth element characteristics of these samples sm nd isochron dating of 343 44 ma omp gln grt wr and an age of 346 3 ma grt gln were determined using an eclogite boudin within blueschist a well defined 40ar 39ar plateau age of 344 1 ma was obtained for crossite from an omphacite phengite bearing blueschist and is concordant with the sm nd isochron age for the eclogite suggesting that neither argon loss nor excess argon affected the crossite a well defined 40ar 39ar plateau age of 331 2 ma was determined for phengite from the same sample the high pressure metamorphic rocks were formed by the b type subduction of a paleozoic south tianshan ocean about 344 ma ago they were however exhumed to higher crustal levels during the collision of the tarim and yili central tianshan plates about 331 ma ago similar to an alpine type tectonic regime

p t path of high pressure low temperature rocks and tectonic implications in the western tianshan mountains nw china the chinese western tianshan high pressure low temperature hp lt metamorphic belt which extends for about 200 km along the south central tianshan suture zone is composed of mainly metabasic blueschists eclogites and greenschist facies rocks the metabasic blueschists occur as small discrete blocks lenses bands laminae or thick beds in meta sedimentary greenschist facies country rocks eclogites are intercalated within blueschist layers as lenses laminae thick beds or large massive blocks up to 2 km2 in plan view metabasic blueschists consist of mainly garnet sodic amphibole phengite paragonite clinozoisite epidote chlorite albite accessory titanite and ilmenite eclogites are predominantly composed of garnet omphacite sodic calcic amphibole clinozoisite phengite paragonite quartz with accessory minerals such as rutile titanite ilmenite calcite and apatite garnet in eclogite has a composition of 53 79 mol almandine 8 5 30 mol grossular 5 24 mol pyrope and 0 6 13 mol spessartine garnet in blueschists shows similar composition sodic amphiboles include glaucophane ferro glaucophane and crossite whereas the sodic calcic amphiboles mainly comprise barroisite and winchite the jadeite content of omphacite varies from 35 54 mol peak eclogite facies temperatures are estimated as 480 580 c for a pressure range of 14 21 kbar the conditions of pre peak epidote blueschist facies metamorphism are estimated to be 350 450 c and 8 12 kbar all rock types have experienced a clockwise p t path through pre peak lawsonite epidote blueschist to eclogite facies conditions the retrograde part of the p t path is represented by the transition of epidote blueschist to greenschist facies conditions the p t path indicates that the high pressure rocks formed in a b type subduction zone along the northern margin of the palaeozoic south tianshan ocean between the tarim and yili central tianshan plates

temporal evolution of the kerguelen plume geochemical evidence from 38 to 82 ma lavas forming the ninetyeast ridge abstract basaltic basement has been recovered by deep sea drilling at seven sites on the linear ninetyeast ridge in the eastern indian ocean studies of the recovered lavas show that this ridge formed from 82 to 38 ma as a series of subaerial volcanoes that were created by the northward migration of the indian plate over a fixed magma source in the mantle the sr nd and pb isotopic ratios of lavas from the ninetyeast ridge range widely but they largely overlap with those of lavas from the kerguelen archipelago thereby confirming previous inferences that the kerguelen plume was an important magma source for the ninetyeast ridge particularly important are the 81 ma ninetyeast ridge lavas from dsdp site 216 which has an anomalous subsidence history coffin 1992 these lavas are feti rich tholeiitic basalts with isotopic ratios that overlap with those of highly alkalic upper miocene lavas in the kerguelen archipelago the isotopic characteristics of the latter which erupted in an intraplate setting have been proposed to be the purest expression of the kerguelen plume weis et al 1993a b despite the overlap in isotopic ratios there are important compositional differences between lavas erupted on the ninetyeast ridge and in the kerguelen archipelago the ninetyeast ridge lavas are dominantly tholeiitic basalts with incompatible element abundance ratios such as la yb and zr nb which are intermediate between those of indian ocean morb mid ocean ridge basalt and the transitional to alkalic basalts erupted in the kerguelen archipelago these compositional differences reflect a much larger extent of melting for the ninetyeast ridge lavas and the proximity of the plume to a spreading ridge axis this tectonic setting contrasts with that of the recent alkalic lavas in the kerguelen archipelago which formed beneath the thick lithosphere of the kerguelen plateau from 82 to 38 ma there was no simple systematic temporal variation of sr nd and pb isotopic ratios in ninetyeast ridge lavas therefore all of the isotopic variability cannot be explained by aging of a compositionally uniform plume although class et al 1993 propose that some of the isotopic variations reflect such aging we infer that most of the isotopic heterogeneity in lavas from the ninetyeast ridge and kerguelen archipelago can be explained by mixing of the kerguelen plume with a depleted morb like mantle component however with this interpretation some of the youngest 42 44 ma lavas from the southern ninetyeast ridge which have206pb 204pb ratios exceeding those in indian ocean morb and kerguelen archipelago lavas require a component with higher206pb 204pb such as that expressed in lavas from st paul island

carbonate reservoirs dominated by secondary storage space key issues and technical strategy carbonate reservoirs of the tarim basin in china are buried deeply with strong later diagenesis and seldom are controlled by geologic facies the effective storage spaces are mainly dissolution caves and fractures with few primary pores fluid distribution in the reservoir is extremely complicated all these characteristics make it a world class challenge for oil and gas exploration and production conventional poststack methods ignore the complexity of the reservoir medium with low accuracy of prediction results by which the location of the dissolution cave is determined roughly only faults and fracture zones can be described to a certain degree instead of quantifying the fracture detection aiming at the key problems of the complex carbonate reservoirs in the tarim basin a series of techniques has been developed to improve the successful rate of reservoir prediction including the following aspects the wide azimuth and high density data acquisition technique to improve the quality of the original seismic data a series of migration techniques including prestack reverse time migration and amplitude preserved q migration to improve the quality of seismic gathers and the imaging precision with the matching precision of avo gathers the target reservoir data dominant frequency increased by more than 15 and by nearly 20 hz in addition the effective dem gassmann rock physics model is proposed for velocity prediction by which relative error of estimated shearwave velocity reaches to less than 5 furthermore an advanced inversion algorithm is introduced to improve the accuracy of extracted elastic information by using new prestack inversion and frequency dependent avo inversion the tie rate of the dissolution cave reservoir and fluid prediction increased by more than 15 furthermore the fracture prediction tie rate can be enhanced by 20 through limited azimuth anisotropic inversion on the basis of which the quantitative identification of fracture fluid for actual seismic data can be realized

lake heimtjonna at dovre mid norway reveals remarkable late glacial and holocene sedimentary environments and the early establishment of spruce picea abies alder alnus cf incana and alpine plants with present centric distributions abstract two and a half meters of sediments from lake heimtjonna in the scandes mountains reveal changes in the vegetation climate and sediment environment since deglaciation the lake was deglaciated in the late glacial lg perhaps as early as 16 18 ka cal bp after deglaciation the sediment environment at the heimtjonna coring point was extraordinary and challenging to interpret the genesis of the 1 7 m basal unsorted sediments including large stones is discussed concluding that the layer was rafted by lake ice after the lg deglaciation the younger dryas yd established a semi perennial lake ice that stopped the deposition of the ice rafted stone rich sediments in the early holocene the unstratified and well sorted clayey silt shows fluvial origin in late holocene strong flood activity including the major flood disaster stor ofsen in ad 1789 caused a sediment hiatus at the coring point from ca 9 5 ka cal bp to ca 250 a cal bp the lg interstadial warming initiated the succession from pioneer plants on mineral soils towards local dwarf shrub heath july temperatures reached at least 7 8 oc in the lg and early holocene papaver radicatum artemisia norvegica and campanula cf uniflora today occurring at dovre and with centric scandinavian distributions locally established all have previously been connected to weichselian survival on nunatak refugia continuous pollen curves of spruce picea abies support the much debated lg and early holocene presence of spruce in the scandes mountains for the first time it is shown that both grey alder alnus incana and armeria occurred in the scandes in the lg and early holocene due to dating errors lg ages is exclusively based on stratigraphical correlations and considerations the lg chronology must be thoroughly tested by future studies providing reliable ams 14c dates of terrestrial fossils

p t evolution of glaucophane omphacite bearing hp lt rocks in the western tianshan orogen nw china new evidence for alpine type tectonics the late palaeozoic western tianshan high pressure low temperature belt extends for about 200 km along the south central tianshan suture zone and is composed mainly of blueschist eclogite and epidote amphibolite greenschist facies rocks p t conditions of mafic garnet omphacite and garnet omphacite blueschist which are interlayered with eclogite were investigated in order to establish an exhumation path for these high pressure rocks maximum pressure conditions are represented by the assemblage garnet omphacite paragonite phengite glaucophane quartz rutile estimated maximum pressures range between 18 and 21 kbar at temperatures between 490 and 570 c decompression caused the destabilization of omphacite garnet and glaucophane to albite ca amphibole and chlorite the post eclogite facies metamorphic conditions between 9 and 14 kbar at 480 570 c suggest an almost isothermal decompression from eclogite to epidote amphibolite facies conditions prograde growth zoning and mineral inclusions in garnet as well as post eclogite facies conditions are evidence for a clockwise p t path analysis of phase diagrams constrains the p t path to more or less isothermal cooling which is well corroborated by the results of geothermobarometry and mineral textures this implies that the high pressure rocks from the western tianshan orogen formed in a tectonic regime similar to alpine type tectonics this contradicts previous models which favour franciscan type tectonics for the southern tianshan high pressure rocks

evidence for excess argon during high pressure metamorphism in the dora maira massif western alps italy using an ultra violet laser ablation microprobe 40 ar 39 ar technique ultra high pressure eclogite amphibolite grade metamorphism of the dora maira massif in the western alps is a well established and intensively studied event however the age of peak metamorphism and early cooling remains controversial the 40ar 39 ar step heating and laser spot ages from high pressure phengites yield plateau ages as old as 110 ma which have been interpreted as the time of early cooling after the high pressure event recent u pb and sm nd results challenge this assertion indicating a much younger age for the event around 45 ma and hence a radically different timing for the tectonic evolution of the western alps in a new approach to the problem samples from the undeformed hercynian metagranite brossasco were studied using an ultra violet laser ablation microprobe technique for 40ar 39 ar dating the new technique allowed selective in situ analysis at a spatial resolution of 50 urn of quartz phengite biotite and k feldspar the results demonstrate the frequent occurrence of excess argon with high 40ar 39 ar ratios 1000 10000 and a strong relationship between apparent ages and metamorphic textures the highest excess argon ratios are always associated with high closure temperature minerals or large diffusion domains within single mineral phases the best interpretation of this relationship seems to be that excess argon was incorporated in all phases during the high pressure event then mixed with an atmospheric component during rapid cooling and retrogression producing a wide range of argon concentrations and 40ar 36ar ratios step heating analysis of minerals with this mixture would produce linear arrays on a40ar 36ar versus 40ar 36 ar correlation diagram leading to geologically meaningless plateau ages older than the true closure age in the present case some ages in the range 60 110 ma could be explained by the presence of excess argon incorporated around 40 50 ma ago similar results found in other high pressure terrains in the alps may reconcile the argon geochronometer with other systems such as rb sr u pb or sm nd this study therefore calls for an increasing use of high resolution in situ sampling techniques to clarify the meaning of 40ar 36 ar ages in many high pressure terrains

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

formes figures et facies sedimentaires glaciels des estrans vaseux des regions froides abstract frost and drift ice are two important processes creating distinctive features in tidal flats of cold regions original sedimentary forms figures and structures and a particular facies characterized by two opposite grain sizes fine and coarse together result from ice action in muddy tidal flats grooves or furrows polished and striated surfaces tracks and trails various drag roll and skip marks circular depressions chaotic microreliefs pushed ridges polygonal nets monroes lenticular bedding and deformations are common to intertidal zones along the st lawrence and the james hudson bay area canada they also occur elsewhere in most tidal flats and on continental shelves dominated by ice a better knowledge of drift ice features in modern tidal flats is needed it is a prerequisite to the discovery of similar features in consolidated rocks the finding of such indicators should improve greatly any interpretation of the paleoenvironmental conditions of deposition

the application of high resolution 3d seismic acquisition techniques for carbonate reservoir characterization in china this paper describes major challenges to seismic acquisition in china s tarim basin figure 1 and how high resolution 3d seismic surveys can improve reservoir imaging in this area

rheology of the plate interface dissolution precipitation creep in high pressure metamorphic rocks abstract subduction zone models invoke deformation to be concentrated along the plate interface in a region of particularly low temperature geophysical observations do not provide constraints on temperature stress and deformation patterns with desired resolution in contrast the record of high pressure metamorphic rocks exhumed from subduction zones provides details on p t history deformation mechanisms and stress state albeit not readily correlated with the former dynamic situation on larger scale here we review available information on dissolution precipitation creep dpc in high pressure metamorphic rocks which if representative for subduction zones in general can pose constraints on conditions rheology and flow patterns along the plate interface the key observations and conclusions are that 1 deformation is typically highly inhomogeneous and localized into shear zones 2 stresses are generally too low to drive crystal plastic deformation 3 microfabrics suggest dissolution precipitation creep to be the predominant deformation mechanism 4 an aqueous fluid at quasi lithostatic pressure is available throughout allowing for tensile fracturing and crack healing or sealing 5 low stress combined with high strain rates required for localized deformation at typical subduction rates implies low viscosity and 6 contribution of shear heating to the thermal budget of subduction zones should be moderate the dominant deformation mechanism dpc is reviewed in some detail including experimental and theoretical approaches various examples of dpc in high pressure metamorphic rocks are illustrated emphasizing the role of interphase boundaries as sites of dissolution rheology governed by dpc is proposed to control interplate coupling and development of a subduction channel with return flow being a likely candidate for rapid exhumation of high pressure metamorphic rocks

a distinction between ice pushed and ice lifted landforms on lacustrine and marine coasts study of a small lake second roach pond in maine u s a clarifies the distinction between shoreline features created by ice push which occurs on both lacustrine and marine coasts and those created by ice lift related to tidal action in the marine environment ice lifting occurred as littoral sediments were frozen into the lake ice followed by a rise in water level due to damming of the lake before the nival melt each spring in the period 1905 to 1969 prominent barricades were constructed by this means a classification based on this distinction is proposed ice pushed landforms include ramparts ridges in coarse and fine sediments stone pavements ice keel grooves pushed boulder grooves and tails off promontories ice lift features include boulder barricades perched stones stone garlands and ice keel depressions

syn and post collisional granitoids in the central tianshan orogen geochemistry geochronology and implications for tectonic evolution abstract the eastern part of the central tianshan belt in china is characterized by abundant granitoids which are not only indicators of the interaction between crust and mantle but also can be used for tracking the tectonic evolutionary history of the orogen four granitic plutons muscovite granites mg biotite monzonitic granites bmg biotite granites bg and alkali granites ag have been recognized to have intruded in the mishigou area central tianshan belt the mg has pronounced a s type affinity which is strongly peraluminous with a high aluminum saturation index and displays positive rb th u and lree anomalies with a strong negative ba nb sr p ti and eu anomalies therefore the magma of the mg is considered to have been derived from melting of thickened continental crust which is composed mainly of the sediments eroded from a pre silurian continental island arc the u pb zircon age of 424 5 2 6 ma constrains the age of mg and crust thickening the bg and bmg granitoids display i type geochemical features and have affinities to subduction arcs both of them are characterized by calc alkaline peraluminous granites with a significant lree enrichment and a negative eu anomaly as well as a depletion in ba nb ta sr p and ti and an enrichment in rb th u nd zr and sm these characteristics indicate that they were derived from a mixed magma source between lower continental crust and the input of components derived from the mantle wedge above the subduction zone the la icpms zircon u pb ages of 411 4 7 ma and 402 3 4 ma represent the formation ages of the bg and bmg granitoids respectively which also constrain that the subduction of the south tianshan oceanic crust also occurred during early devonian time the ag shows a typical a type granite affinity with depletion of nb ta sr p ti and y it is interpreted that ag was formed in a within plate tectonic setting and related to continental up doming and rifting zones as a consequence of the extensional collapse after the collision of the tianshan orogen the u pb zircon age of 290 5 1 ma represents the formation age of the ag and the time of the extensional collapse and rifting

primary fluids entrapped at blueschist to eclogite transition evidence from the tianshan meta subduction complex in northwestern china the tianshan is the first locality worldwide where primary fluids at blueschist to eclogite transition have been documented veins containing high pressure minerals in massive host eclogites transitional to blueschists or eclogite boudins surrounded by blueschists indicate that a free fluid phase was present at the time of eclogitization the high pressure veins are predominantly composed of omphacite fibers with minor quartz or calcite the transition from blueschist to eclogite facies parageneses occurs as dehydration halos around these veins clinozoisite paragonite glaucophane and omphacite inclusions preserved in garnet porphyroblasts in wall eclogites document the transformation of blueschist to eclogite c axis parallel non planar populations of fluid inclusions were trapped during the growth of omphacite in high pressure veins and dehydrated wall rocks low salinity h2o nacl solid bearing inclusions are preserved in omphacite fibers in veins and matrix omphacite of wall rocks none of the isochores of these low salinity aqueous fluid inclusions intersect peak eclogite facies metamorphic conditions suggesting that although the textural evidence constrains the entrapment of fluid inclusions to peak metamorphic conditions their densities must have been modified during exhumation the fluids are interpreted to have been derived from the host blueschist as a result of dehydration reactions such as 13 gln 5 czo 9 prp 26 jd 12 di 19 qtz 15 h2o and gln pg prp 3 jd 2 otz 2 h2o the similarity of vein and wall rock mineral compositions fluid inclusion characteristics and o isotope data also favor an internal source for the fluids the major element composition of veins indicate that si na and ca rich aqueous fluids were released during dehydration at a depth of 50 10 km within a paleozoic subduction zone

environmental assessment from the sedimentary record of high latitude fiords the sedimentary record of fiords at high latitude where global change may be occurring earliest and will be greatest has the potential for assessment of environmental change at both low resolution the scale of millennia and high resolution decadal to less than daily unlike the lacustrine record which has been used very successfully in these studies the sedimentary processes and thus the sedimentary deposits of fiords differ 1 in the role of salt water in inhibiting mixing and promoting flocculation and through the role of sea ice 2 in the exchange of mass and energy with the much larger ocean beyond and 3 in the role of benthic biota in the sedimentary environment the work reviewed in this paper shows that fiord sediments are being used to assess sedimentary and oceanic processes as well as glacial periglacial and geomorphic history and that they are important proxies for long term climate and hydrology recommendations for advancing this work include long term monitoring of conditions in fiords and their drainage basins so that the transfer relations can be more solidly constructed from the proxy of fiord sediment integration of results from fiords in opposite polar latitudes and among proxies especially from lacustrine and ice core records will produce valuable insights assessment of the range of conditions in fiords from the most polar to temperate regions is important to building models of their processes and understanding of the paleoenvironmental signals that can be interpreted from each type

tectonic history of subduction zones inferred from retrograde blueschist p t paths many phanerozoic convergent plate junctions are marked by discontinuous blueschist belts reflecting relatively high pressure p prograde trajectories common blueschist paragneisses such as those of the western alps exhibit widespread overprinting by greenschist and or epidote amphibolite facies assemblages for this type of high p belt retrograde metamorphism involved fairly rapid nearly isothermal decompression some terranes underwent continued heating during early stages of pressure release uplift probably occurred as a consequence of the entrance of an island arc oceanic plateau or segment of continental crust into the subduction zone collision resulting in marked deceleration or cessation of underflow and buoyant approximately adiabatic rise of the stranded recrystallized subduction complex other high p belts such as the franciscan of western california preserve metamorphic aragonite and lack a low p overprint retrogression approximately retraced the prograde p t temperature path or for early formed high grade blocks occurred at even higher p t ratios parts of this type of metamorphic belt evidently migrated slowly back up the subduction zone in response to isostatic forces during continued plate descent and refrigeration upward motion took place as tectonically imbricated slices as laminar return flow in melange zones and perhaps partly a lateral spreading extension of the underplated accretionary prism retrograde p t trajectories of high pmore belts therefore provide important constraints on the tectonic evolution of convergent plate junctions less

the movement of isolated boulders on tidal flats by ice floes the ability of ice floes to move isolated boulders on tidal flats by flotation and by pushing and or rolling them along the bed is considered in the standard format of sediment movement entrainment transport and deposition analysis shows that flotation competence depends on ice thickness and floe size and is sufficient to transport most of the boulders reported from eastern canadian tidal flats however a limiting factor in the transportation of large boulders by this mode is likely to be the entrainment mechanism analysis of movement of grounded boulders by lateral ice thrust shows that on both deformable and non deformable beds rolling rather than sliding will occur rolling competence depends on floe size ice roughness the ice rock coefficient of friction and current and or wind strength resistance to movement is a function of the embedment ratio and bed properties rolling could account for most of the observed short distance transport of large boulders

mapping deeply buried karst cavities using controlled source audio magnetotellurics a case history of a tunnel investigation in southwest china abstractkarst cavity mapping is attracting great interest from engineering geologists because of its relation to the dangerous geohazards faced during engineering construction ground based geophysical methods still face challenges in karst mapping and concealed karst cavities potentially pose threats to tunnel construction in southwest china given the significant contrast in electrical resistivity between karst cavities and their host rocks geoelectrical methods are widely used for mapping these cavities we have developed a successful case history of mapping karst cavities on a planned railway route using controlled source audio magnetotellurics csamt scalar csamt with frequencies ranging from 0 5 to 8192 hz was used for field data acquisition a full frequency domain apparent resistivity correction method was used for near field corrections electromagnetic array profiling emap filtering was used for topographic and static shift corrections and the bostick conversion was used for data interp

excess 40ar transport scale and mechanism in high pressure phengites a case study from an eclogitized metabasite of the dora maira nappe western alps application of the bulk separate furnace heating laser step heating and in situ laser agemapping 40ar 39ar techniques reveals that high pressure phengites from an alpine eclogite of the dora maira basement nappe western alps were pervasively contaminated with excess 40ar to yield ages 130 ma spot fusion dating of individual grains with the 40ar 39ar laserprobe reveals concentric radial patterns with core ages as high as 320 ma showing that this excess component was trapped at an early stage and subsequently remobilized by volume diffusion during a late greenschist thermal pulse the single grains and population from this hand sample retain major differences in apparent ages indicating that the extent of the excess 40ar contamination varies from grain to grain attempt to model these differences in terms of grain size dependent fractional loss falls short and demonstrates that strong cm scale argon isotopic gradients locally prevailed during syn kinematic high pressure metamorphism on petrostructural grounds a model of internal isotopic buffering is outlined whereby the excess 40ar is argued to have been internally derived from within the eclogite protolith itself low intergranular ar diffusion rates due to reduced water activity buffered below unity by the phengite assemblage resulted in the local segregation and partitioning of the excess 40ar along grain boundaries during ongoing ductile deformation supporting evidence for this model comes from the recent literature on fluid and stable isotope behaviour in alpine eclogites and very high pressure metasediments which shows that volatile recycling during high pressure metamorphism may occur on a very limited scale

paleozoic tectonic evolution of the tianshan orogen northwestern china abstract the chinese tianshan orogenic belt which extends west east for about 1500 km is one of the most important paleozoic orogenic belts in central asia it includes parts of the tarim yili central tianshan and junggar plates which are microcontinental blocks with precambrian basements and meso cenozoic depositional covers the tarim plate is separated from the yili central tianshan plate by the southern central tianshan suture early carboniferous ophiolitic melanges and blueschists are widespread in this suture and are believed to be produced by the collision between a passive continental margin on the north side of the tarim plate and an active continental margin on the south side of the yili central tianshan plate the collision occurred at the end of early carboniferous the yili central tianshan and the junggar plates are separated by the northern central tianshan suture also considered as an early carboniferous collisional zone along which the middle ordovician ophiolitic melanges blueschists and late devonian early carboniferous ophiolites are exposed in early paleozoic times the subduction of the paleo junggar ocean took place in an active margin on the north side of the yili central tianshan plate along the northern central tianshan suture at the end of early carboniferous the north side of the yili central tianshan plate collided with the north tianshan arc along this suture the paleo oceanic crust in the tianshan and adjacent region was entirely consumed at the end of early carboniferous meanwhile the tarim yili central tianshan and junggar plates were accreted to eurasia epicontinental sediments were then deposited throughout the tianshan region during the middle late carboniferous times in the early permian tianshan began to uplift with accompanying volcanic eruption and granitic intrusion the uplift was accelerated in the late permian and continental molasse was extensively deposited

retrograde evolution of eclogite facies rocks from ne oman evidence from fluid inclusions and petrological data eclogite facies rocks from eastern saih hatat oman metamorphosed under peak conditions of p 12 kbar and t of 550 580 c have several types of fluid inclusions in quartz clinopyroxene and garnet most of these inclusions are two phase at room t and contain an aqueous fluid belonging to the system nacl mgcl2 h2o final melting temperatures for most inclusions range from 4 to 0 c but values as low as 13 c have been recorded indicating salinities of 490 c inclusions with three phases at room t also contain a predominantly low salinity aqueous fluid with xco2 12 kbar to 4 kbar or less at t 500 550 c this conclusion is supported by the scatter in homogenization temperatures for these inclusions and their large variety of shapes and sizes which range from irregular with hair like fractures surrounded by satellite inclusions to negative crystals the replacement of ferroan dolomite ankerite and quartz by calcite talc and of rutile calcite by titanite or rutile glaucophane by ilmenite albite in these eclogites are also consistent with a stage of isothermal decompression following near isothermal decompression to depths of 10 15 km brittle deformation prevailed leading to the formation of layer parallel and crosscutting fractures that were filled by quartz calcite this stage was followed by near isobaric cooling recorded by the isochores for fluid inclusions in vein quartz and by the occurrence of dendritic networks of decrepitated and imploded fluid inclusions in these veins

blueschists in major suture zones of china blueschist facies rocks and c type eclogites occur as allochthonous terranes or as small tectonic blocks in almost all the accretionary fold belts and major suture zones within the interior of china and along the circum pacific and tethys himalayan belts at least forty blueschist localities have been described blueschists around the pacific margins and the himalaya alpine belts are mostly mesozoic in age only those in taiwan and new caledonia are cenozoic intracratonal blueschists of china are older than late permian when the collision amalgamation presumably occurred several alleged precambrian blueschist localities ye 1987 are shown in the recently published metamorphic map of china a precambrian blueschist belt in anhui hubei central china extends for about 2300 km however except for the pre sinian blueschist terrane in aksu the age determination for precambrian blueschist metamorphism elsewhere has not been confirmed characteristic features of chinese blueschists include the following 1 blueschist facies metamorphism predates the continental collision amalgamation 2 most blueschists have undergone multistage metamorphism commonly with increasing temperature and or decreasing pressure for later events 3 assemblages include sodic amphibole epidote albite quartz phengite chlorite sphene lawsonite jadeitic pyroxene and aragonite are not common and 4 protoliths are mainly mafic pelagic and clastic rocks blueschist facies metamorphism in china may be divided into two types on the basis of the imposed geothermal gradient hence the progressive change in mineral assemblage the rare lawsonite bearing blueschists produced at lower temperatures and higher pressures are confined to the inner mongolia and yarlung zangbo suture zones where aragonite and possibly jadeitic pyroxene quartz occur and the blueschists are associated with subgreenschist facies rocks most other blueschists of china formed at higher temperatures along the transitional blueschist greenschist facies boundary where epidote sodic and calcic amphiboles are ubiquitous and the blueschists are interlayered with greenschist facies rocks the common occurrence of intracratonal blueschists in major suture zones of china and elsewhere in eurasia indicates pre mesozoic subduction accretion of oceanic crust and flysch sediments these zones of accretion represent growth around the periphery of major precambrian cratons this fact together with the distribution of ophiolite belts and available paleomagnetic data provides evidence for an extremely mobile history of plate movement in eurasia however the paleozoic intracontinental blueschists and ophiolites of china have formed before rather than during final closure between cratons they do not usually mark the location of terminal sutures but are the result of earlier accretion and continental growth by subduction underplating and imbrication of oceanic materials similar to the accretion history of westernmost north america

preservation of re os isotope signatures in pyrite throughout low t high p eclogite facies metamorphism pyrite in lt hp eclogites from the western tianshan orogenic belt yields a re os age of 378 1 8 9 ma which is 30 70 ma older than ages previously obtained for the same rocks using the rb sr sm nd ar ar u pb and lu hf isotope systems the tianshan lt hp eclogite experienced temperatures of up to 570 c combined with pressures of up to 2 1 gpa during metamorphism these conditions are below the transition of pyrite to pyrrhotite which defines both pyrite stability and possibly its closure temperature for re os pyrite can preserve re os signatures through eclogite facies peak metamorphic conditions and thus allow determination of the formation age of pyrite in the protolith

petrogenesis of late paleozoic diorites and a type granites in the central eastern tianshan nw china response to post collisional extension triggered by slab breakoff abstract geological evolution of the eastern tianshan in the late paleozoic is significant to understanding the multiple accretions and final formation of the southern central asian orogenic belt here we report new whole rock geochemical and sr nd isotopic data and in situ zircon u pb ages for three typical a type granitic intrusions syenogranite k feldspar granite and monzonitic granite and one dioritic pluton in the central eastern tianshan nw china in order to constrain their petrogenesis tectonic setting and geodynamic evolution zircon la icp ms u pb ages indicate that the syenogranites and k feldspar granites were both formed during a short time interval in the early permian 284 286 ma whereas the diorites were crystallized at 293 6 ma the monzonitic granites were generated at 307 3 ma earlier than the other two granitic intrusions the three kinds of granites syenogranite k feldspar granite and monzonitic granite have high hfse hree and y nb ratios 1 68 2 84 and remarkably low sr and ba contents with the absence of aluminium rich minerals and show metaluminous to weakly peraluminous natures indicating geochemical characteristics of aluminous a2 type granites these granites show negative end t 3 32 to 4 40 values and mesoproterozoic two stage nd model ages tdm2 1 36 to 1 43 ga which suggest a relatively old crustal source for these rocks the sr nd isotopic compositions imply that the mesoproterozoic metamorphic basement of the central tianshan block can be considered as a potential magmatic source for these aluminous a2 type granites in addition they have geochemical characteristics similar to the experimental melt that was derived from partial melting of crustal metaigneous rocks at the depth of middle to lower crust levels therefore the granites were most likely produced by partial melting of metaigneous rocks in a post collisional extensional setting the diorites have low sio2 and high mgo with uniform 87sr 86sr i values 0 7055 negative end t values 4 11 to 4 59 and mesoproterozoic two stage nd model ages tdm2 1 41 to 1 45 ga they have higher ti zr 42 8 46 7 and ti y 280 359 ratios than continental crust and show negative nb ta ti anomalies which can be explained by partial melting of a relatively fertile mantle modified by old crustal materials of the central tianshan block the formation of the dioritic pluton was induced by an upwelling asthenosphere in an extensional geodynamic setting based on the zircon saturation temperatures of these a type granites and the distribution of late carboniferous to early permian mantle derived intrusions a slab breakoff model is suggested to interpret the formation of the studied a type granites and diorites we infer that the slab breakoff was initiated in the late carboniferous and ended in the early permian

palaeozoic tectonics of the south western chinese tianshan new insights from a structural study of the high pressure low temperature metamorphic belt the south western chinese tianshan orogenic belt is famous for its omphacite bearing blueschists and associated eclogite facies metavolcanic rocks although numerous petrologic geochemical and geochronological studies are available structural data and interpretations are still rare this paper provides new structural data including bulk geometry of structures and kinematic analyses based on field and laboratory studies along the akyazhi keburt and muzaert rivers the study area is divided into three tectonic units namely 1 a southern unit composed of weakly metamorphosed sedimentary rocks of silurian age 2 a central hp lt unit composed of blueschist eclogite facies metamorphic rocks derived from basalts pelites and volcaniclastic rocks 3 a northern unit which consists of a carboniferous magmatic arc developed upon an amphibolite facies metamorphic continental basement our structural analysis documents a polyphase deformation the main event d1 is reflected by devonian to carboniferous top to the northwest ductile shearing coeval with hp lt metamorphism this is followed by north directed thrusting d2 of the southern unit over the central hp lt unit coeval with retrogression of the high pressure rocks a top to the s se deformation d3 overprints the earliest events and is observed in the northern and central units lastly permian dextral ductile brittle wrenching d4 overprints the older flat lying fabrics d4 is conspicuous along the nalati fault that separates the northern unit from the central hp lt unit the absolute timing of these deformation events is discussed in the light of available radiometric dating the structural metamorphic and geochronological data are integrated into a geodynamic model of the south western chinese tianshan that emphasizes south directed subduction of microcontinents located between tarim and junggar

pressure temperature time evolution of the high pressure metamorphic complex of sifnos greece model studies of continental collision tectonics and pressure temperature time p t t predictions quantitatively demonstrate the key role of synsubduction uplift in the preservation of blueschist and eclogite facies metamorphic rocks rheological stratification of the subducting lithosphere allows development of detachment faults at the compositional boundary between the upper and lower crust material of supracrustal continental affinity is subducted to depths in excess of 50 km and then delaminated from the downgoing slab subduction continues while the detached domain is uplifted independently from the newly active hanging walls and footwalls two dimensional thermal calculations indicate that the nearby active subduction provides the cooling mechanism required to preserve blueschists and eclogites whereas the differential motions of the independently uplifted domains can explain the observed heterogeneity in p t t pathways with some domains displaying a greenschist overprint

paleozoic magmatism and metamorphism in the central tianshan block revealed by u pb and lu hf isotope studies of detrital zircons from the south tianshan belt nw china as a major precambrian microcontinent in the southernmost central asian orogenic belt caob the central tianshan block cts in the chinese tianshan is essential for understanding the final assembly of the southern caob it experienced multistage paleozoic magmatism and metamorphism but the detailed processes are still controversial and far from being completely understood this paper reports coupled u pb and lu hf isotopic data of detrital zircons from late paleozoic meta sedimentary strata in the south tianshan belt which can provide new insight into deciphering the paleozoic evolution of the eastern segment of the cts block characterized by typical oscillatory zoning and high th u ratios 0 2 detrital zircons in the permian sedimentary samples yield dominant age populations at ca 505 490 ma 475 440 ma 430 400 ma and 340 250 ma pinpointing the development of four episodes of magmatism in the eastern cts block particularly ordovician silurian 475 440 ma zircons possessing low negative ehf t values predominate in sedimentary strata in and surrounding the cts block indicating that the 475 440 ma magmatic rocks probably constitute the main body of the cts block the origin of this early paleozoic episode of magmatism was most likely related to the southward subduction of the junggar ocean beneath the cts block carboniferous triassic 340 250 ma zircons have dominantly positive ehf t values probably derived from the post collisional juvenile rocks in the cts block combined with previous studies our data suggest that the single source terrane for the sampled strata was the cts block which had been a topographic high providing substantial detritus to the surrounding areas at least since the early permian in the metasedimentary sample detrital zircons mostly show partially fully recrystallized internal textures and low th u ratios 0 2 probably sourced from the amphibolite to granulite facies metamorphosed rocks in the eastern cts block the youngest zircons suggest that the sampled metasedimentary strata were carboniferous 341 305 ma in age rather than neoproterozoic or early sinian in chinese literatures as previously interpreted based on u pb hf isotopes and ti in zircon thermometer of metamorphic zircons in this study as well as previous investigations we suggest that the eastern cts block has experienced a a 400 370 ma collisional event related to the closure of the eastern segment of the south tianshan ocean and b a 360 340 ma extensional event as a result of post collisional mantle upwelling therefore the final assembly of the eastern tianshan took place during late carboniferous time

exhumation of high pressure rocks a review of concepts and processes the exhumation of high pressure metamorphic rocks requires either the removal of the overburden that caused the high pressures or the transport of the metamorphic rocks through the overburden exhumation cannot be achieved simply by thrusting or strike slip faulting it may be caused by erosion of shortened and thickened crust but this is unlikely to be the only mechanism for exhuming rocks from depths greater than about 20 km one or more of the following additional mechanisms may be involved 1 corner flow of low viscosity material trapped between the upper and lower plates in a subduction zone can cause upward flow of deeply buried rock and may explain some occurrences of high pressure tectonic blocks in melange this process does not however appear to be adequate to explain the exhumation of regional high pressure terrains 2 buoyancy forces acting directly on metamorphic rock bodies may cause them to rise relative to more dense surroundings this is likely to be the most important mechanism of exhumation of crustal rocks subducted into the mantle but cannot explain the emplacement of coherent tracts of high density metamorphic rock into shallow crustal levels some high pressure blocks emplaced at shallow levels in accretionary terrains may have been entrained in diapiric intrusions of low density mud or serpentinite 3 extension driven by the forces associated with contrasts in surface elevation may explain the exhumation and structural setting of many high pressure terrains extension may occur in the upper part of an accretionary wedge thickened by underplating or it may affect the whole lithosphere in a region of intracontinental convergence if surface elevation has been increased by the removal of a lithospheric root in the second case extension may be accompanied by magmatism and an evolution towards higher temperature during decompression of the metamorphic terrain

paleozoic accretion and cenozoic redeformation of the chinese tien shan range central asia the tien shan range in central asia contains two late paleozoic sutures the older southern suture marks the collision of a passive margin at the north of the tarim block and an active continental margin subduction under the latter was to the north the younger northern suture separates a northern carboniferous island arc from an active continental margin developed over a south dipping subduction zone the subduction direction under the island arc is unknown mesozoic elastics were deposited over the doubly sutured orogen rate and energy of sedimentation waned until deposition of oligocene conglomerates above a regional unconformity interpreted as marking the onset of deformation induced by the india asia collision molasse deposition accelerated in pliocene and quaternary time and deposition continues today as active thrusts generate relief paleozoic structures control the gross divergence of cenozoic thrusts across the orogen

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

late paleozoic tectonic amalgamation of northwestern china sedimentary record of the northern tarim northwestern turpan and southern junggar basins sedimentary rocks contained in basins adjacent to the tian shan provide a long and complex record of the late paleozoic continental amalgamation of northwestern china complementing that provided by rocks preserved within the range this record which comprises dramatic changes in sedimentary facies sediment dispersal patterns sandstone provenance and basin subsidence rates broadly supports previous interpretations of a two part evolution of the tian shan late devonian to early carboniferous collision of the tarim continental block with the central tian shan followed by collision of this combined block with island arcs in the north tian shan and bogda shan in late carboniferous early permian times the first collision resulted in widespread angular unconformities within the tarim basin continued convergence following the collision created a long lived flexural foredeep along the northern margin of the tarim block which received at least 2000 m of lower carboniferous through lower permian fluvial and marine sediment derived from the interior of tarim subsequent early permian continental extension of the northern tarim basin resulted in the deposition of interbedded nonmarine siliciclastic sedimentary rocks and mafic to felsicvolcanic rocks sandstone within this interval was derived from the paleo tian shan and is composed predominantly of lithic volcanic grains similar to the rhyolite in contrast to the tarim basin calc alkaline volcanic rocks and volcanogenic sedimentary rocks dominated carboniferous and permian sedimentation in the northern turpan and northwestern junggar basins volcanic arcs remained active in the north tian shan and bogda shan through the early late carboniferous depositing a kilometers thick interval of deep marine sediment gravity flows in the northwestern junggar basin major arc magmatism ceased in the late carboniferous in response to closure of the oceanic basin between the combined tarim central tian shan block and the north tian shan bogda shan arcs upper carboniferous through lower permian rocks in the northwestern junggar basin compose the sedimentary fill of a bathymetric basin of oceanic depth on the northern side of the volcanic arcs culminating in a 1000 m thick marine regressive sequence middle to upper permian sandstones were derived from the uplifted paleo tian shan and bear the distinctive provenance imprint of granitic rocks presently exposed within the range late permian subsidence of the junggar basin accommodated 5 km of nonmarine sediments however the cause of this subsidence and its relationship to regional tectonic events remain controversial

local origin of high pressure vein material in eclogite facies rocks of the zermatt saas zone switzerland prograde dehydration reactions within metabasalts at the blueschist eclogite facies transition produce kyanite veins usually with quartz sometimes with omphacite or chloritoid of up to several meters length and 20 cm width bulk rock analyses show clear depletion haloes for al2o3 and sio2 near the veins mass balance calculations support the suggestion that the vein material must have been derived from the immediate surroundings these veins are interpreted as local segregation phenom ena and not as fluid dehydration channels of the eo alpine subduction zone calcula tions have been performed for a stagnant vein formation model that considers the rate of the dehydration reactions k responsible for providing the h2o for al and si transport and the rate of diffusion d of these from wall rock to growing vein the model is satisfied for quite a wide range of values for k d and upper limits on the duration of diffusional vein formation of about 1 ma are indicated

geochronology and geochemistry of the borohoro pluton in the northern yili block nw china implication for the tectonic evolution of the northern west tianshan orogen abstract the closure of the north tianshan ocean between the junggar terrane and the yili block is a longtime debated issue in literature because of the different understanding of the carboniferous volcanic rocks in the northern margin of the yili block this study presents new geochronological and whole rock geochemical data for the granitic rocks from the borohoro pluton to provide constraints on the tectonic regime for the northern west tianshan during the carboniferous la icp ms u pb dating results reveal two magmatic phases for the borohoro pluton the former magmatic activity in the early carboniferous formed the fine grained granodiorite 332 ma the later magmatic activity occurred during the late carboniferous 305 300 ma forming a diversity of granitic rocks involving quartz diorite granodiorite and granite geochemical and mineralogical studies reveal that the studied granitic rocks from the borohoro pluton all belong to metaluminous to weakly peraluminous calc alkaline i type granites they are characterized by enrichment in liles relative to hfses and depletion of nb ti and p typical of continental arc type granites the intermediate sio2 high al2o3 and relatively low fe2o3t mgo and tio2 contents reflect that these granitic rocks are mainly crust derived but the high mg values for most samples and the occurrence of microgranular mafic enclaves indicate that their magma sources were mixed by mantle derived components especially the late carboniferous rocks define an elegant mixing trend in both the rb rb v and the 1 v rb v diagrams consistent with mixing between magmas from subcontinental lithospheric mantle and mafic lower crust taking into consideration of the facts that all the devonian to carboniferous granitoids belong to calc alkaline i type granites and granitoids of a type didn t appear until the early permian we suggest that the subduction of the north tianshan ocean continued to the late carboniferous generating the granitic rocks of the borohoro pluton