numerical models of fluid flow in carbonate platforms implications for dolomitization abstract the role of fluid flow in dolomitization is evaluated through hydrologic simulations designed to capture and test numerically the essential elements of some common dolomitization models a two dimensional finite difference code bethke 1985 was used to simulate single phase flow of variable salinity fluids driven by compaction convection and topography in carbonate platforms and adjacent basinal sediments a simple hydrostratigraphic model consisting of a platform reef complex surrounded by basin filling shale was constructed to examine the magnitude and direction of compaction flow during the initial phase of subsidence most pore fluids in the shale migrate to the surface at velocities less than 0 1 mm yr and basin derived fluids are focused into the reef from only a limited distance of 10 km relatively porous and permeable basinal limestones and sandstones may act as conduits for updip fluid flow into the platform reef complex magnesium mass balance constraints coupled with hydrologic simulations suggest that burial compaction is not an efficient mechanism for regional dolomitization two layer hydrostratigraphic models involving variable salinity fluids show that evaporation of seawater results in downward and seaward fluid migration the magnitude of flow is a function of fluid density even fluids at salinities marginally elevated above normal seawater values are capable of penetration several hundreds of meters into the underlying sediments albeit at rates of less than 1 cm yr paleohydrologic simulations coupled with thermodynamic arguments support that reflux of surface derived waters evaporated past gypsum saturation is a viable dolomitization mechanism calculations utilizing rates of mg mass flux show that dolomitization of large carbonate platforms 10 km3 by fluids below halite saturation requires millions of years of reflux driven flow p thermal convection of seawater through carbonate platforms has been simulated by varying the basal heat flow the thermal pump produces a convection cell in which lateral and vertical flow achieve rates of 75 0 and 13 0 mm yr respectively the potential for dolomitization is high because a large reservoir of mg seawater is tapped elevated temperatures drive fluids further into the stability field for dolomite and kinetic inhibition of dolomite nucleation and growth is decreased simulations of high frequency glacio eustatic sea level oscillations suggest that a dynamic hydrologic regime may be established a sea level fall of 2 m lasting for 10 000 yr can drive lateral flow at rates up to 0 15 m yr dolomitization is possible because marine waters migrate through the carbonate platform in response to gradients of hydraulic head created by eustatic sea level fluctuations

mantle flow through the northern cordilleran slab window revealed by volcanic geochemistry the northern cordilleran slab window formed beneath western canada concurrently with the opening of the californian slab window beneath the southwestern united states beginning in late oligocene miocene time a database of 3530 analyses from miocene holocene volcanoes along a 3500 km long transect from the northern cascade arc to the aleutian arc was used to investigate mantle conditions in the northern cordilleran slab window using geochemical ratios sensitive to tectonic affi nity such as nb zr we show that typical volcanic arc compositions in the cascade and aleutian systems derived from subduction hydrated mantle are separated by an extensive volcanic fi eld with intraplate compositions derived from relatively anhydrous mantle this chemically defi ned region of intraplate volcanism is spatially coincident with a geophysical model of the northern cordilleran slab window we suggest that opening of the slab window triggered upwelling of anhydrous mantle and displacement of the hydrous mantle wedge which had developed during extensive early cenozoic arc and backarc volcanism in western canada high heat fl ow throughout the western canadian cordillera is broadly coincident with the fi eld of intraplate volcanism and is linked to slab window induced mantle upwelling

textural and geochemical alternations in late cenozoic bahamian dolomites dolomitized intervals of a core from san salvador island bahamas exhibit variations of two texturally and geochemically distinct end members in the pliocene section of the dolomitized interval the two end members alternate in a pattern that may reflect originally and or diagenetically modified depositional facies formerly mud free intervals locally capped by exposure surfaces are massive crystalline mimetic dolomites cm muddier sediments are replaced by friable microsucrosic dolomites ms cm and ms dolomites also differ in porosity 30 permeability 100 md mol mgco3 44 9 vs 47 7 mol oxygen isotopic composition 1 7 vs 2 7 and strontium content 241 vs 106 ppm these data indicate that depositional and diagenetic fabric are the principal controls governing the distribution of dolomite types differences in texture and geochemistry are suggested as arising through differential rates of crystallization produced as a result of variations in permeability and reactivity of the precursor sediments and rocks

petrogenesis of neoarchean ttg rocks in the yangtze craton and its implication for the formation of archean ttgs abstract the yangtze craton is one of the most important cratonic blocks in eastern asia however its early evolution has not been well constrained yet because of the poor exposure of archean basement rocks in this contribution a combined study of zircon u pb age hf o isotope compositions as well as whole rock element compositions was carried out for gray gneiss rocks from the douling complex in the northwestern part of the yangtze craton zircon u pb dating for two gneiss samples yielded weighted mean ages of 2509 21 and 2496 15 ma the gray gneisses have high sio2 63 64 73 74 na2o 3 66 6 21 but low k2o 0 76 2 93 contents and belong to the trondhjemite tonalite and granodiorite series rocks they are characterized by enrichments of liles and lrees but notable depletions in nb ta and ti as well as heavy rees and y all the samples plot into the fields of typical archean ttg rocks in the la yb n vs yb n and sr y vs y diagrams therefore the gray gneisses in the douling complex are neoarchean ttg rocks the relatively high nb ta and zr sm ratios of 19 6 46 0 and 14 5 143 4 suggest their formation under eclogite facies conditions they have low hf t values of 3 21 and 3 39 and hf model ages of 3194 26 and 3205 24 ma indicating their derivation from ca 3 2 ga juvenile mafic rocks their 18o values of 6 41 0 12 and 5 44 0 12 suggest that surface rocks were differently recycled into their sources collectively the ttg rocks in the douling complex were generated by partial melting of thickened mafic crust rather than direct partial melting of oceanic crust in a convergent plate boundary a compilation of available zircon u pb and hf isotope compositions of the yangtze craton reveals that there are quite differences of hf isotope compositions in archean the ca 3 8 3 4 ga zircons exhibit hf t values of 5 8 to 0 0 these chondritic to slightly enriched hafnium isotope compositions imply that their host rocks were formed by protracted intra crustal reworking of ancient protoliths without juvenile input by contrast zircons with ages younger than ca 3 2 ga register both negative and positive hf t values up to the depleted mantle providing clear evidence for the addition of juvenile materials these imply that the subduction driven plate tectonics might have been operated since ca 3 2 ga in the yangtze craton

break away of south china from gondwana insights from the silurian high nb basalts and associated magmatic rocks in the diancangshan ailaoshan fold belt sw china abstract the diancangshan ailaoshan fold belt yunnan sw china represents the remnants of the once vast ailaoshan ocean a branch or back arc basin of the eastern paleotethys however when and how this ocean back arc basin opened and rifted south china away from gondwana are still unresolved in this study ordovician silurian high nb basalts hnbs 420 ma mg rich andesite 435 ma high mg basalt 446 ma and i type granite 423 ma were newly identified in the diancangshan ailaoshan fold belt the hnbs are alkaline with high tio2 1 8 1 9 wt and nb 16 9 23 1 ppm contents high nb u 36 8 39 1 and nb th pm 1 17 1 21 ratios positive nb ta anomalies primitive mantle normalized and positive end t values 3 2 the hnbs were likely derived from an oib arc mixed mantle source in the garnet stability field the mg rich andesites are medium k calc alkaline andesite with negative end t values 10 1 and negative nb ta anomalies the magma was likely derived from a mixture of depleted mantle peridotites and subducted sediments under low pressure and high h2o conditions the high mg basalts are high k calc alkaline and characterized by high mgo contents 9 3 9 4 wt mg 70 high cr 369 379 ppm and low al2o3 13 6 13 9 wt contents they were likely derived from an enriched mantle source metasomatized by marine sediment derived fluids the i type granites show medium sio2 content 63 6 wt low a cnk ratio 0 9 and negative end t value 9 8 and may have formed by the partial melting of meta basalts these hnb mg rich andesite high mg basalt and i type granite were likely formed in an intracontinental back arc rift setting integrated with our published detrital zircon u pb age and lu hf isotope evidence we propose that south china may have commenced its break up from gondwana in response to the late ordovician silurian 446 420 ma proto tethyan subduction which opened a back arc basin along the ailaoshan fold belt

multistage dolomitization in late jurassic early cretaceous platform carbonates berdiga formation basoba yayla trabzon ne turkey implications of the generation of magmatic arc on dolomitization abstract late jurassic early cretaceous berdiga formation which is widely distributed in an e w trending belt in the eastern pontides ne turkey is composed of platform carbonates the formation shows varying lithofacies changing from supratidal to platform margin reef both laterally and vertically the studied basoba yayla trabzon ne turkey area comprises one of the typical exposures of formation in the northern zone of eastern pontides in this area the lower parts of the formation are pervasively dolomitized by fabric destructive and fabric preserving replacive dolomites that are ca rich and nonstoichiometric ca56 60mg40 44 replacement dolomites rd are represented by 18o values of 15 3 to 4 2 vpdb 13c values of 3 7 1 5 vpdb and 87sr 86sr ratios of 0 70675 0 70731 petrographic and geochemical evidence implies that rd dolomites are formed before compaction at shallow moderate burial depths from late jurassic early cretaceous seawater and or partly modified seawater as a consequence of rock water interaction and they are recrystallized at elevated temperatures during subsequent burial during subsequent late cretaceous diagenetic process when the region became a magmatic arc rd1a and rd3 dolomite were recrystallized by hydrothermal fluids of marine origin the dolomite cements cd overlying the low amplitude stylolites have similar geochemical charasteristics than rd1a and rd3 dolomite and are characterized by high homogenization temperatures th this might indicate that cd dolomite was precipitated from hydrothermal fluids of marine origin that recrystallized the rd1a and rd3 dolomite the fact that cd dolomite comprising the latest phase of dolomitization is overlain by coarsely crystalline mosaic calcite and silica cements may indicate that diagenetic fluids were undersaturated with respect to dolomite and oversaturated with respect to calcite and silica

geochemistry of holocene volcanic rocks associated with ridge subduction in baja california mexico although the geochemistry of magmatism associated with the subduction of oceanic lithosphere is well understood1 6 the geochemical signature of lavas produced shortly after subduction of an oceanic spreading centre has not been characterized the baja california peninsula in mexico provides an ideal scenario to study the latter process geophysical models indicate that in response to oblique collision of the ancestral east pacific rise epr with the western seaboard of north america at 29 myr subduction of the farallon plate was succeeded by the progressive southwards development of a transform fault system7 9 at 12 5 myr a considerable length of the epr was simultaneously subducted beneath baja thus terminating subduction processes along this segment10 by 3 5 myr the locus of transcurrent faulting had switched from the west of baja to within the developing gulf of california coupling baja to the pacific plate11 13 we present here data for holocene volcanic rocks from baja california norte which show that post ridge subduction volcanism has a distinctive geochemistry these results have important consequences for the interpretation of ancient orogenic belts

sediment dynamics of the severn estuary and inner bristol channel net sediment transport pathways have been determined in many environments by studying grain size trends this approach is extended here to an inner shelf environment with improved statistical techniques patterns of net surficial sediment transport are proposed indicating that the area is dominated by up estuary transport exceptions exist in the form of radial mud transport at the river mouths and clockwise sand circulation around linear sandbanks six different sedimentary environments are defined on the basis of the shape of the transfer function and its relationship to the grain size distributions erosion takes place along the axis of the estuary dynamic equilibrium is reached around the linear sandbanks whilst accretion occurs at the head of the estuary and where there are riverine inputs the results are compared with numerical model outputs representing water movement and sediment transport over the seaward part of the study area there is agreement between the two approaches contradictions increase however towards the upper reaches of the estuary the discrepancies can be attributed to 1 inherent differences between patterns of water and sediment movement 2 difficulties in modelling intertidal boundary conditions 3 inadequate representation of river influences in the models and 4 time scale differences associated with sediment transport processes

subduction of diverging plates and the principles of slab window formation consumption of an ocean basin by subduction commonly brings a sea floor spreading ridge toward a deep sea trench if plate divergence and convergence continue after the ridge intersects the subduction zone a slab window forms between the subducted parts of the diverging oceanic plates producing anomalous thermal physical and chemical effects in the surrounding asthenospheric mantle in turn these conditions alter the tectonic and magmatic evolution of the overriding plate usually disturbing ordinary fore arc and arc regimes differential lithospheric stresses on opposite sides of the triple junction contribute to disturbances in the overriding plate anomalous magmatism from fore arc to back arc may be accompanied by fore arc metamorphism strike slip faulting uplift extension and in extreme cases rifting the shape and size of the window are controlled mainly by the pre subduction ridge transform trench configuration slab dip angles and vectors of plate convergence subducted ridge segments expand into windows whose margins approximately parallel the motion vectors between the triple junction and the subducting plates subducted transform faults continue to be active usually as oblique slip faults until the plates separate as transform faults subduct they become longer on the plate which occupies the acute angle between ridge and trench and shorter on the other plate trains of isolated windows produced by subduction of a segmented ridge transform system progressively expand during descent commonly merging together to form a composite slab window oblique subduction of a highly segmented ridge is likely to produce two or more fraternal slab windows one at each site of ridge trench intersection above a slab window arc volcanism diminishes and may be replaced by volcanism of mid ocean ridge or rift affinity the change in chemical character reflects various processes including elevated heat flow decreasing hydration of the upper mantle juxtaposition of supra and sub slab mantle reservoris asthenospheric upwelling and melting of the trailing plate edges if the slab window migrates the anomalous magmatic regime may be replaced by renewed arc volcanism identifying the effects of slab windows in ancient convergent margin assemblages requires an understanding of slab window principles and implications

dolomitization of atolls by sea water convection flow test of a hypothesis at niue south pacific the core of the former atoll on niue island south pacific is extensively dolomitized a detailed stable isotope study reveals that the dolomites are uniformly enriched in 18o and 13c 18o 2 8 0 5 13c 2 0 0 3 relative to their carbonate precursor the sr isotope ratios are equally constant with depth yielding 87sr 86sr values between 0 70912 and 0 70916 the observed isotopic compositions point to sea water as the dominant dolomitizing fluid the sr age of the dolomites is younger than the biostratigraphic age mid to late miocene placing the dolomitization event at plio pleistocene time a model of sea water convection is proposed for atoll dolomitization on the basis of a thermal gradient between the atoll and the ambient ocean water sea water is drawn through the atoll margin and transferred upward by convective flow delivering mg to the sites of dolomite precipitation transported with the sea water are volcano derived metals fe cu zn mn as evidenced by the chemical gradient in

the geochemistry of young volcanism throughout western panama and southeastern costa rica an overview oblique aseismic subduction below western panama and southeastern costa rica has produced recent arc related volcanism the aseismicity is probably related to the subduction of relatively hot oceanic lithosphere the volcanism throughout this region over the past 2 ma has been quite distinct consisting of felsic magmas andesites to rhyolites but mainly dacites with geochemical signatures suggesting a metamorphosed basaltic source it is believed that the subduction of young oceanic crust sets up conditions under which the slab melts rather than the overlying mantle wedge rocks with slab melt geochemistries and associated with young subducted crust have been termed adakites elsewhere the young adakite melts are sometimes associated with a few rare young high nb basalts but there is no obvious genetic link between them through differentiation high nb basalts may also be derived from the partial melting of the subducted oceanic crust high nb basalt migmatites have been found with pegmatites of adakite compositions in the exposed subduction terrain of the catalina schist california alternatively the high nb basalts may be partial melts of phlogopite rich mantle that has previously reacted with adakite magmas eruption of adakites and high nb basalts was preceded by a 2 3 ma period of relative quiescence prior to this there was a 7 ma period of calc alkaline volcanism typical of the present day magmatism associated with a distinct benioff zone found throughout the central american arc the abrupt transition in volcanism with time from an early calc alkaline sequence to a later adakite high nb basalt sequence may record a change in the tectonic setting of western panama and southeastern costa rica over the past 12 ma

isotope signatures associated with early meteoric diagenesis the environments in which carbonate diagenesis proceeds have been documented in previous studies of holocene and late pleistocene sediments and limestones on barbados west indies variations in the carbon and oxygen isotopic composition of limestones produced during early freshwater diagenesis have been observed in this study to occur in specific patterns six potentially useful patterns emerge when one views stable isotope data within a stratigraphic framework 1 the subaerial exposure surface is characterized by strongly 12c enriched limestones 13c compositions of underlying limestones grow progressively heavier with increasing depth 2 the subaerial exposure surface may also be marked by slight 18o enrichment 3 an abrupt shift in 18o values may differentiate sediments above the exposure surface from those below 4 sediments altered in the marine meteoric mixing zone may be characterized by positive covariance between their 18o and 13c compositions 5 the vadose phreatic boundary may be marked by a sharp increase in 13c values in the seaward portions of a fresh groundwater system and 6 samples contemporaneously altered in a single fresh groundwater system within an areally restricted region should display a narrow range of 18o and a wide range of 13c compositions analysis of samples from five palaeozoic and mesozoic formations which contained petrographic evidence of early freshwater diagenesis showed that isotope patterns similar to those observed in barbados limestones have been preserved in rocks as old as mississippian these isotope patterns could prove to be useful for identifying diagenetically induced porosity trends in carbonate rocks they might be used to identify limestones diagenetically altered in meteoric environments to identify mixing zone cements and dolomites and to trace the regional and vertical distributions of early meteoric groundwater systems in ancient carbonate formations

mount st helens potential example of the partial melting of the subducted lithosphere in a volcanic arc mount st helens 50 km to the west of mount adams and the main cascade volcanic chain is only 80 km above the subducting oceanic lithosphere the elevated temperatures off the subducting slab because of the close proximity of the juan de fuca ridge to the trench may induce slab melting at a depth of 80 km dacites from mount st helens have geochemical compositions off magmas that are derived by direct partial melting of metamorphosed basalts at high pressure i e relatively high ai al2o3 15 at 70 sio2 low y and yb because of garnet and amphibole stability in the source low sc and high sr and eu trace element modeling of the partial melting of mid oceanic ridge basalt morb from the juan de fuca ridge that yields a hornblende eclogite residue can reproduce the mount st helens data results off the model are quite distinct from data derived from the mount adams volcanic rocks in contrast mount adams is 135 km above the subducting slab and is associated with normal arc magmatism believed to be derived from the mantle above the subducting plate the cascade are has been active in its present locality because of oblique subduction for the past 7 m y the major volcanoes along the arc have existed for at least 500 ka but mount st helens has existed for 40 ka we suggest that the subducting plate may have reached elevated temperatures because of the approach of north america to the juan de fuca ridge at 40 ka which initiated melting of the slab

niobium enriched basalts from the wabigoon subprovince canada evidence for adakitic metasomatism above an archean subduction zone abstract late archean niobium enriched basalts from the central sturgeon lake assemblage and neepawa group of the western wabigoon subprovince have mantle normalized nb la between 0 8 and 1 3 and zr y between 4 and 7 they are compositionally similar to basalts attributed to adakite metasomatism of mantle wedge regions in cenozoic subduction zones sajona et al j petrol 37 1996 693 726 in detail their sc ree systematics suggest the archean basalts were generated above the garnet stability field an association with adakite like volcanic rocks an absence of komatiites and the arc like attributes of their host sequences suggest a subduction related origin for the basalts if current models of adakite and niobium enriched basalt genesis are valid then additional examples of these rocks should be relatively common in other archean greenstone belts

cretaceous ridge subduction along the lower yangtze river belt eastern china the lower yangtze river belt is one of the most important metallogenic belts in china the mechanisms responsible for ore genesis and the formation of related cretaceous igneous rocks such as adakite a type granitoid and nb enriched basalt remain controversial mesozoic granitoids in the lower yangtze river belt were mostly formed in the early cretaceous 140 125 ma and three granitoid belts the inner the south and the north have been defined according to petrological and geochemical characteristics previously based mainly on negative end and high initial sr isotope values the adakitic rocks were generally attributed to partial melting of thickened or delaminated lower crust both of which require crustal thickening mesozoic crustal thickening however is not supported by the development of extensional basins in the region from the late jurassic to cretaceous eastern china was closely associated with subduction of the pacific plate in the south and the izanagi plate in the north the midocean ridge mor between these two plates was drifting toward and likely subducting under the lower yangtze river belt a ridge subduction model can therefore explain the distribution of different magmatic rocks and ore deposits in the belt partial melting of subducting young hot oceanic slabs close to the ridge formed adakitic rocks the negative end values of adakitic rocks can be plausibly interpreted by mixing between adakitic magmas and enriched components in the lithospheric mantle and or crustal materials through afc process a slab window opened during ridge subduction as indicated by a type granitoids in the center of the inner belt nb enriched basalt found in the belt was likely formed by partial melting of a mantle wedge metasomatized by fluids released from the subducting slab at shallow depths

tracing a subducted ridge transform system in a late carboniferous accretionary prism of the southern altaids orthogonal sanukitoid dyke swarms in western junggar nw china abstract two sets of late paleozoic orthogonal dioritic dyke swarms are defined in the eastern part of the western junggar accretionary collage in the southern altaids dioritic dykes hornblende plagioclase are characterized by relatively high sio 2 elevated mgo na 2 o k 2 o mg ni si and cr contents la yb ratios very low y contents and enrichments in large ion lithophile elements lile and light rare earth elements lree which are all comparable with those of sanukitoids these dioritic dykes have low 87 sr 86 sr ratios mostly between 0 70366 and 0 70381 and high positive e nd t 6 6 to 8 4 values suggesting they were sourced from a depleted mantle accordingly these dioritic dykes were probably derived by mixing of depleted mantle components that originated from upwelling of asthenosphere through a slab window with melts that were derived by dehydration of a subducted slab detailed mapping and structural data of the dyke swarms indicate that the nw se trending set was earlier than the ne sw trending dykes paleostress analysis and inversion of the dyke swarms indicate that an early ne sw extension was associated with a nw se trending slab window and was followed by a slightly later nw se extension that was associated with a ne sw trending slab window field relationships together with published age constraints suggest that these two dyke swarms were intruded in the late carboniferous therefore two orthogonal slab windows were opened one ne sw after the other nw se during this time period a subducted ridge transform system could well account for and is consistent with these relationships we propose that in the late carboniferous a nw se trending mid oceanic ridge was subducted beneath the darbut trench generating large scale closely spaced nw se trending dykes that were intruded through a slab window that event was followed by subduction of a ne sw trending transform fault fracture zone that connected with the earlier nw se trending ridge producing widely spaced ne sw trending dykes that were intruded through a ne sw trending vertical slab window this new approach to the relationships between dyke swarms adakitic sanukitoid magmatism ridge subduction and slab windows has broad implications for tectonic reconstruction of ancient accretionary orogens

origin of atoll dolomites distinguished by geochemistry and crystal chemistry kita daito jima northern philippine sea abstract kita daito jima is a carbonate island located at the northwestern region of the philippine sea 25 55 6 57 6 n 131 16 9 19 8 e dolomites extend from the island surface to a depth of 100 m below the ground surface mbgs strontium isotope stratigraphy indicates that subsurface units c1 0 49 7 mbgs and c2 49 7 103 4 mbgs were dolomitized at 5 5 ma and 2 0 ma respectively and that island surface dolomites are products of dolomitization at 1 6 2 0 ma x ray diffraction analysis indicates that the island surface and borehole dolomites comprise variable mixtures of four and three dolomite crystal phases respectively each of these phases is distinguished by a different ca and mg content three textural types can be recognized in the kita daito jima dolomites fabric preserving crystalline nonmimetic cnm crystalline mimetic cm and fabric preserving microsucrosic ms cnm dolomites contain more calcian phases whereas ms dolomites commonly are richer in more stoichiometric phases backscattered electron images indicate that calcian dolomite phases were precipitated earlier than the more stoichiometric dolomite phases and that there is no significant hiatus between the phases although they are diachronous both the island surface and borehole cores dolomites show linear relationships between whole rock 18o composition and mg contents and between whole rock trace element concentrations sr and na and mg contents these covariances result from phase mixing not stoichiometric effects deconvolution of whole rock isotopic and elemental compositions based on the relative abundance of phases reveals that each phase has a distinct chemical and isotopic composition oxygen isotopic compositions of the island surface and borehole dolomites suggest that all dolomite phases formed in seawater and that dolomitization primarily occurred during glacio eustatic sea level lowstands and cooler ocean temperatures

residual sediment transport paths on a tidal sand bank a comparison between the modified mclaren model and bedform analysis abstract in order to evaluate the validity of the modified mclaren model for sand transport on a tidal sand bank 42 grab samples taken from the northern part of the kwintebank were decalcified sieved and analyzed for statistical parameters the areal distribution of mean grain size standard deviation and skewness were defined and used to determine the residual transport directions the grain size trends taken into consideration were that the sediment becomes finer better sorted and more negatively skewed along the transport pathways f b and the sediment becomes coarser better sorted and more positively skewed along the transport pathways c b these were compared with the transport paths inferred from bedform asymmetries obtained by side scan sonar records there were major differences between the sediment transport paths resulting from the two techniques when the f b trend and small dune asymmetries are compared only 5 out of 24 cases showed agreement 30 whereas in 9 cases the trends were 180 30 different there was even less agreement between the c b trend and the asymmetry of the small dunes only 1 out of 24 cases agreed 30 and almost half of the cases were 90 30 different the sediment transport directions deduced from the morphological analysis of the large dunes all point in the direction of the peak flood current the large dunes seem to be more suitable indicators of the sediment transport paths than the small ones however they conflict with the vectors resulting from the f b trend only one out of 20 cases agrees 30 while in 13 cases the vectors are 180 30 different and in three cases 90 30 neither do the transport directions from the c b trend agree with those deduced from the large dunes only one case agrees 30 while in four out of 20 cases the vectors point in opposite directions in four cases the vectors are 90 30 different the asymmetric cross section of the sand bank suggests sediment transport in a northwest direction it does not seem probable however that sediment transport occurs by migration of the whole sand bank the transport directions inferred from the bank morphology are quite different from the f b trend in 12 out of 42 cases the vectors agree 30 in four cases the vectors point in opposite directions and in 15 cases they are 90 30 different there is slightly more agreement between the c b trend and the transport directions deduced from the sand bank 16 out of 42 cases agree 30 in nine cases the vectors point in opposite directions and three cases are 90 30 different from this study it seems that the residual sediment transport directions can best be deduced from the morphological characteristics of the large dunes the departures between the results of the sediment trend analysis according to the modified mclaren model and the morphology of the subaqueous bedforms are partly explained by the location of the study area near the edge of the sand bank probably better results of the sediment trend analysis would be obtained if samples over the whole length of the sand bank were analyzed and compared perhaps a smaller spacing between the samples would improve the results as well and maybe the samples should not have been decalcified possibly the different transport patterns are attributed to different sediment transport processes which makes a comparison virtually impossible

cenozoic dolomites of carbonate islands their attributes and origin abstract dolomites found on and below carbonate islands atolls and oceanic platforms provide useful insights into the origin of dolomite insights that may not be attainable from the study of more ancient cratonic dolomites for this reason they have been the subject of study for decades a critical mass of case studies now exist and some significant conclusions can be drawn from the cumulative data in most cases the association with an island is causal and not genetic nevertheless these dolomite occurrences are referred to herein as island dolomites one type of dolomite on carbonate islands is penecontemporaneous dolomite a phase that forms while the host sediment is in its original depositional setting island examples are holocene in age occur in holocene sediments and originate as a direct precipitate from either normal or evaporated seawater these are microcrystalline poorly ordered ca rich 18o enriched and geochemically unstable phases that are susceptible to recrystallization in the setting in which they formed post depositional dolomite is the other type of dolomite associated with carbonate islands this type of dolomite replaces older precursors and forms as cement distinctive characteristics include a dominance of fabric preserving texture pore lining cement rims that may exhibit micron scale banding with low mg calcite and formation in association with precursor dissolution sr isotopic dating indicates that all examples are neogene or quaternary in age those formed during the middle miocene through pliocene are massive laterally continuous and often multigenerational in contrast younger examples tend to be localized partial replacements of a single generation sr isotope ages also suggest global synchroneity in many dolomitization events which suggests a connection between dolomitization global eustacy and or global climatic factors geochemical attributes of post depositional island dolomites are ca enrichment positive 18o and 13c low sr contents 150 300 ppm and low fe global similarity in petrography and geochemistry of replacive island dolomites argues for a similar origin inferred origins however depend primarily on how 18o and sr data are interpreted values for 18o and dsr must be assumed due to uncertainties in oxygen isotope fractionation and sr partitioning there is no consensus or uniformity in those assumptions thus interpretations can vary and be biased to a desired result covariant trends in 13c and 18o some negative 13c values and high sr all favor a mixing zone origin but examples with these attributes are few lack of covariance in the isotopes mean 18o of 2 0 to 3 5 and low sr future studies should standardize the type of data collected and the analytical techniques employed multiple geochemical attributes should be measured on microsampled components and quantitative modeling should be employed in order to constraint interpretations as much as possible also needed are a better understanding of the kinetic processes that form these dolomites more careful assessment of their recrystallization status and an improved understanding of dsr and 18o at low temperature

dolomite abundance and stratigraphic age constraints on rates and mechanisms of phanerozoic dolostone formation increasing dolomite abundance with age relative to limestone has long been accepted as one of the general truisms of sedimentary geology a relationship which suggests that dolomitization is a cumulative process occurring slowly over the full burial history of a carbonate sequence however reevaluation of data in support of this assumption indicates that although relative dolomite abundances do change significantly throughout phanerozoic carbonate sequences no data set demonstrates any direct correlation between dolomite content and age whereas tabulation of available data does suggest greater amounts of dolomite formation during periods of continental flooding global mass balance calculations of potential magnesium sources also indicate that most dolomite has formed from marine or marine derived fluids at the earth9s surface this conclusion is indirectly supported by significant short term period scale changes in phanerozoic dolomite abundance a variation which would be more attenuated if dolomitization were dominantly a burial process dolomitization may be enhanced during times of global transgression higher atmospheric pco 2 and lower calcite saturation state in shallow marine settings

classification of the australian continental shelf based on predicted sediment threshold exceedance from tidal currents and swell waves abstract estimates of significant wave height and period together with tidal current speed over a semi lunar cycle were used to predict the area on the australian continental shelf over which unconsolidated sediment was mobilised threshold exceedance these sediment entraining processes were examined independently to quantify their relative importance on the continental shelf using observed grain size data mobilisation from swell waves occurred on 31 and tidal currents on 41 of the continental shelf swell wave energy is sufficient to mobilise fine sand 0 1 mm diameter to a water depth of 142 m on the otway shelf near the western entrance to bass strait tidal currents in king sound northwest shelf are capable of mobilising large areas of medium sand 0 35 mm diameter 100 of the time superimposing the distribution of threshold exceedance by wave and tidal currents indicates that there are areas on the shelf where either wave induced or tidal currents dominate some areas where waves and tides are of relatively equal importance and still other areas where neither is significant we define six shelf regions of relative wave and tidal energy zero no mobility waves only wave dominated mixed tide dominated and tides only our results provide a predictive process based understanding of the shelf sedimentary system that has applications to marine engineering projects and to regional studies of pollution dispersal and accumulation where significant shelf sediment mobilisation is a factor

origin of nb enriched basalts and adakites in baja california mexico revisited reply maury et al 2009 present new and detailed field observations and petrologic analyses to argue that the new model that i propose as an alternative explanation for postsubduction magmatism in baja california mexico castillo 2008 1 is incorrect specifically they claim that their new

geochemical sr nd and zircon u pb hf isotopic studies of late carboniferous magmatism in the west junggar xinjiang implications for ridge subduction abstract voluminous granitic intrusions are distributed in the west junggar nw china and they can be classified as the dioritic rocks charnockite and alkali feldspar granite groups the dioritic rocks sio 2 50 4 63 8 wt are calc alkaline and mg enriched average mgo 4 54 wt mg 0 39 0 64 with high sr y ratios average 21 2 weak negative eu average eu eu 0 80 and pronounced negative nb ta anomalies their sr nd and zircon hf isotopic compositions 87 sr 86 sr i 0 7035 0 7042 e nd t 4 5 7 9 e hf t 14 1 14 5 show a depleted mantle like signature these features are compatible with adakites derived from partial melting of subducted oceanic crust that interacted with mantle materials the charnockites sio 2 60 0 65 3 wt show transitional geochemical characteristics from calc alkaline to alkaline with weak negative eu average eu eu 0 75 but pronounced negative nb ta anomalies sr nd and zircon hf isotopic compositions 87 sr 86 sr i 0 7037 0 7039 e nd t 5 2 8 0 e hf t 13 9 14 7 also indicate a depleted source suggesting melts from a hot juvenile lower crust alkali feldspar granites sio 2 70 0 78 4 wt are alkali and fe enriched and have distinct negative eu and nb ta anomalies average eu eu 0 26 low sr y ratios average 2 11 and depleted sr nd and zircon hf isotopic compositions 87 sr 86 sr i 0 7024 0 7045 e nd t 5 1 8 9 e hf t 13 7 14 2 these characteristics are also comparable with those of rocks derived from juvenile lower crust despite of the differences in petrology geochemistry and possibly different origins zircon ages indicate that these three groups of rocks were coevally emplaced at 305 ma a ridge subduction model can account for the geochemical characteristics of these granitoids and coeval mafic rocks as the slab window opened upwelling asthenosphere provided enhanced heat flux and triggered voluminous magmatisms partial melting of the subducting slab formed the dioritic rocks partial melting of the hot juvenile lower crust produced charnockite and alkali feldspar granite and partial melting in the mantle wedge generated mafic rocks in the region these results suggest that subduction was ongoing in the late carboniferous and thus support that the accretion and collision in the central asian orogenic belt took place in north xinjiang after 305 ma and possibly in the permian

sediment transport under waves and tidal currents a case study from the northern bristol channel u k bed load transport under tidal currents and waves and waves alone is predicted from near bed self recording current meter data and observations from a wave rider buoy transport rates and paths are derived using seven different unidirectional transport formulae and those of bagnold 1963 bijker 1967 and madsen and grant 1976 for combined flows predicted rates are compared with the results of a fluorescent sand tracer study the data are available from swansea bay a high tidal and wave energy embayment on the british continental shelf sediment transport paths in this region had been determined previously using a variety of techniques including bedform orientation and sea bed drifter recovery patterns transport is enhanced under the superimposed effect of waves and in some circumstances the direction of the resultant vector is altered such storm induced catastrophic transport is different to that predicted under tidal currents alone this could provide a mechanism for long term sediment supply in some areas there is correspondence between bedform orientation and storm induced transport paths in others there is opposition

holocene evolution of bottom sediment distribution on the continental shelves of the bohai sea yellow sea and east china sea abstract coastline shifts due to transgression and regression can remodel tidal fields of continental shelves and this can control transport of sediments and modulate sediment distribution accordingly tidal currents have become the dominant hydrodynamic processes on the continental shelves of the bohai sea yellow sea and east china sea byecs since the transgression after the last glacial maximum lgm to examine the evolution of the bottom sediment distributions on the continental shelves of the byecs we simulated patterns of tides and tidal currents sediment transport and bottom sediment types sand mud and mixed sediments for five periods corresponding to sea level lowstands of 80 m 52 m and 30 m below present the holocene transgression maximum htm and the present the simulation shows that both sediment transport and shelf sediment distribution patterns were controlled by the strength type and asymmetry of tidal currents in the byecs since the lgm evolution of shelf sediment distribution patterns occurred in two stages 1 sediment emplacement and formation stage before the htm and 2 local adjustment after the htm the marked changes in coastline configuration since the lgm are the dominant factor controlling tide and tidal current evolution distribution of shelf sediment types in the byecs is closely related to tidal current fields during transgression after the lgm

stabilization of early formed dolomite a tale of divergence from two mississippian dolomites abstract several large hydrocarbon accumulations in alberta canada are hosted in dolomitized successions of stacked thin sabkha capped cycles of visean age porosity is micro intercrystalline and occurs in dolomitized restricted subtidal and intertidal muds that have their fine primary fabric preserved two such fields are here considered that illustrate divergent and contrasting modes of dolomite stabilization despite initial similarities in facies and textures the dolomite in the upper debolt formation of the dunvegan field nw alberta forms planar e or microsucrosic fabrics with crystals in the 1 20 m range the dolomite is non ferroan ca rich average of 58 mol caco 3 and poorly ordered its stable isotopic signatures range from 0 12 to 3 4 vpdb for 18 o mean 1 3 and 0 9 to 4 3 vpdb for 13 c mean 2 6 the average radiogenic 87 sr 86 sr ratio for this dolomite is 0 7077 both sets of values are consistent with dolomite precipitation from mississippian marine or modified marine evaporated seawater these parameters are strongly reminiscent of holocene protodolomites and hence suggestive of a sabkha dolomitization process shallow seepage reflux or evaporitive pumping this dolomite with its high associated porosity average of 15 and up to 38 relatively unaltered mineralogical and chemical signatures both preserved despite 4 km of burial depth suggests a very unique set of relatively non reactive physico chemical conditions during burial likely a closed system in contrast the dolomite from the mount head formation of the shell waterton field sw alberta has undergone measurable neomorphic alteration in several stages in deeper burial environments open system such alteration has affected its crystal size range 18 o ranges between 1 5 in the least altered dolomite and 13 2 vpdb 13 c ranges from 3 9 to 1 5 vpdb and 87 sr 86 sr ranges from 0 7078 to 0 7090 the dolomite has retained a degree of non stoichiometry with an average ca content of 55 mol caco 3 the mount head dolomites clearly indicate ongoing reactivity between the rocks and basinal fluids during burial and are in this respect representative of the norm for dolostones in the western canada sedimentary basin

triassic nb enriched basalts magnesian andesites and adakites of the qiangtang terrane central tibet evidence for metasomatism by slab derived melts in the mantle wedge new chronological geochemical and isotopic data are reported for triassic 219 236 ma adakite magnesian andesite nb enriched basaltic rock associations from the tuotuohe area central qiangtang terrane the adakites and magnesian andesites are characterized by high sr y 25 45 la yb 14 42 and na2o k2o 12 49 ratios high al2o3 15 34 18 28 wt and moderate to high sr concentrations 220 498 ppm and end t 0 86 to 1 21 values low enrichments of th rb relative to nb and subequal normalized nb and la contents and enrichments of light rare earth elements combine to distinguish a group of nb enriched basaltic rocks nebs they have positive end t 2 57 to 5 16 values positive correlations between th la and nb and an absence of negative nb anomalies on mantle normalized plots indicate the nebs are products of a mantle source metasomatized by a slab melt rather than by hydrous fluids a continuous compositional variation between adakites and magnesian andesites confirms slab melt interaction with mantle peridotite the spatial association of the nebs with adakites and magnesian andesites define an adakitic metasomatic volcanic series recognized in many demonstrably subduction related environments e g mindanao arc philippines kamchatka arc russia and southern baja california arc mexico the age of the touhuohe suite and its correlation with triassic neb to the north indicates that volcanism derived from subduction modified mantle was abundant prior to 220 ma in the central qiangtang terrane

paleoproterozoic subduction within the yangtze craton constraints from nb enriched mafic dikes in the kongling complex abstract niobium enriched mafic rocks constitute a rare suite of rocks formed in the subduction related settings and provide important insights into mantle sources and geodynamic processes associated with plate subduction in this contribution we report newly defined nb enriched mafic rocks from the archean paleoproterozoic kongling complex within the northern yangtze craton which shed new light on the paleoproterozoic subduction process of the craton la icp ms zircon u pb dating reveals two episodes of paleoproterozoic intermediate mafic magmatism 1 the ca 2 15 ga dioritic dikes and the younger nb enriched mafic dikes formed in 2 05 2 03 ga the ca 2 15 ga diorites have high nb contents of 14 7 18 0 ppm but low nb lapm 0 27 0 31 nb u 9 0 to 13 5 and nb thpm values 0 24 to 0 31 the enriched zircon hf isotope hf t 4 8 1 1 and whole rock nd isotope nd t 4 2 3 7 of the older diorites are similar to those of the coeval high mg andesites 2 12 ga from the northern huangling dome indicate an enriched mantle source the geochemical and isotopic data collectively suggest that they were derived from partial melting of a metasomatized sub arc lithospheric mantle at the initial stage of subduction the younger mafic rocks ca 2 05 2 03 ga possess high nb contents 10 9 24 2 ppm and lower magnitude of negative nb anomalies with nb u 17 2 51 6 nb thpm 0 40 0 88 and nb lapm 0 49 0 97 which is different from those of normal island arc rock suites the mafic dikes have negative zircon hf t values 12 6 10 5 dominantly negative nd t values 4 9 to 1 0 except one with 2 5 indicate an enriched mantle source the nb enriched mafic rocks were likely produced by partial melting of mantle wedge that has been modified by the recycled continental sediments along the subduction zone we infer that a slab window regime likely prevailed in the kongling complex at ca 2 05 ga during the subduction termination the opening of a slab window and asthenosphere upwelling possibly triggered partial melting of the enriched mantle wedge and account for the generation of the younger nb enriched mafic dikes

origin of island dolostones a case study from the cayman formation miocene cayman brac british west indies abstract cayman brac the easternmost of the cayman islands is 19 km long 1 5 to 3 km wide and rises 40 m above sea level at its eastern end geographically isolated by the deep oceanic waters of the caribbean sea this island with its thick succession of pervasively dolomitized tertiary rocks is a natural laboratory for assessing the origin of island dolostones dolostones in the miocene cayman formation 100 m thick with most crystals 55 mol caco3 geochemically they are characterised by 13c values of 1 6 to 3 5 18o values of 2 3 4 0 80 279 ppm sr 52 340 ppm fe and 9 82 ppm mn the inverse correlation between 18o and mol caco3 and positive correlation between sr and mol caco3 largely reflect kinetic effects on oxygen isotope fractionation and the sr partition behaviour between dolomite and water dolomite stoichiometry indicates formation from waters with a high mg ca ratio and 18o values corrected for biases associated with dolomite stoichiometry and phosphoric acid fractionation indicate formation from normal seawater like fluids under near surface conditions the 87sr 86sr ratios point to two phases of dolomitization phase i in the late miocene 6 8 ma caused partial dolomitization of the basal part of the cayman formation phase ii during the pliocene to early pleistocene 1 5 ma completed dolomitization of the cayman formation both phases of dolomitization were mediated by similar fluids although available information suggests that dolomitization was probably linked to sea level fluctuations the timing of dolomitization relative to the transgressive regressive cycle remains open to debate

boninite like volcanic rocks in the 3 7 3 8 ga isua greenstone belt west greenland geochemical evidence for intra oceanic subduction zone processes in the early earth abstract the 3 7 3 8 ga isua greenstone belt of southwest greenland is characterized by variably metamorphosed metasomatised and deformed lithotectonic successions of volcanic and sedimentary rocks the voluminous mafic volcanic rocks are composed primarily of pillow basalts intercalated with ultramafic units the sedimentary rocks consist mainly of banded iron formation cherts conglomerates and siliciclastic turbidites the least altered metavolcanic amphibolites the garbenschiefer unit from the central tectonic domain of the isua greenstone belt are characterized by high mg number 0 60 0 80 mgo 7 18 wt al2o3 14 20 wt ni 60 645 ppm and cr 60 1920 ppm contents but low tio2 0 20 0 40 wt zr 12 30 ppm y 6 14 ppm and rare earth element ree concentrations these compositional features collectively represent a coherent mafic to ultramafic suite chondrite normalized ree patterns are concave upward on the primitive mantle normalized extended trace element diagrams they are characterized by relative depletion of nb but with an enrichment of zr relative to neighboring rees alteration deformation and crustal contamination can be ruled out as the cause of the distinct and coherent composition the average initial end value of these metavolcanic rocks is 2 collectively these geochemical characteristics are comparable to those of phanerozoic boninites given the observation that in the tertiary boninites are exclusively associated with intra oceanic subduction environments e g izu bonin mariana subduction system this suggests that intra oceanic subduction zone like geodynamic processes were operating as early as 3 7 3 8 ga

geochemical and nd isotopic compositions of the palaeoproterozoic metasedimentary rocks in the kongling complex nucleus of yangtze craton south china block implications for provenance and tectonic evolution

ridge collision slab window formation and the flux of pacific asthenosphere into the caribbean realm mantle wedge derived arc volcanism ceased in southern costa rica after ca 8 ma because of subduction of the aseismic cocos ridge beneath the central american arc and the subsequent opening of a slab window geochemical and isotopic compositions of small volumes of adakitic and alkalic backarc lavas erupted between 5 8 and 2 ma identify a source derived from the galapagos plume the presence of this source is explained by an influx of pacific upper mantle into the caribbean mantle wedge through a slab window where the alkalic rocks form by melting of the upwelling mantle and the adakites result from melting of the leading edge of the subducted cocos ridge by using geochemical and isotopic signatures we trace this upper mantle flow beneath central america from southern costa rica northward at a rate of 40 mm yr

the 2 65 ga a type granite in the northeastern yangtze craton petrogenesis and geological implications abstract the presence of old residual continental crustal rocks in cratons is crucial for investigation of their early evolution due to the poor exposure of archean rocks the early evolution of the yangtze craton has not been well constrained yet in this study a combined study of zircon u pb and hf o isotopes as well as whole rock major and trace elements and nd isotopes have been conducted for a granite pluton at huji town in the northern part of the yangtze craton u pb dating for magmatic zircons yielded a weighted mean age of 2656 6 ma revealing another archean outcrop in the yangtze craton besides the kongling terrane the granite samples show relatively evaluated k 2 o na 2 o cao tfeo mgo ratios and zr nb ce ga and y contents sharing the same features as the a type granite they have pronounced eu sr and ba negative anomalies on the primitive mantle normalized trace element spidergram implying the residual of plagioclase in the magma sources their high zr contents of 440 529 ppm correspond to initial magma temperatures of 930 962 c the zircons have negative hf t values of 2 7 to 0 1 and hf model ages of ca 3 2 ga respectively the 18 o values of the zircons range between 4 94 and 5 72 and give an average of 5 26 0 14 suggesting no contribution of supracrustal rock into the host magma therefore the hf model ages indicate the generation of the huji granite by reworking of ca 3 2 ga juvenile crustal rocks combined with previous studies the finding of the neoarchean huji granites with ca 3 2 ga hf model ages suggests the widespread of archean basement in the yangtze craton the occurrence of the ca 2 65 ga huji granites likely records the thermally stable lithosphere and thus heralds the craton stabilization of the eastern part of the yangtze block

sr c and o isotope geochemistry of ordovician brachiopods a major isotopic event around the middle late ordovician transition here we present sr c and o isotope curves for ordovician marine calcite based on analyses of 206 calcitic brachiopods from 10 localities worldwide these are the first ordovician wide isotope curves that can be placed within the newly emerging global biostratigraphic framework a total of 182 brachiopods were selected for c and o isotope analysis and 122 were selected for sr isotope analysis seawater 87sr 86sr decreased from 0 7090 to 0 7078 during the ordovician with a major quite rapid fall around the middle late ordovician transition most probably caused by a combination of low continental erosion rates and increased submarine hydrothermal exchange rates mean 18o values increase from 10 to 3 through the ordovician with an additional short lived increase of 2 to 3 during the latest ordovician due to glaciation although diagenetic alteration may have lowered 18o in some samples particularly those from the lower ordovician maximum 18o values which are less likely to be altered increase by more than 3 through the ordovician in both our data and literature data we consider that this long term rise in calcite 18o records the effect of decreasing tropical seawater temperatures across the middle late ordovician transition superimposed on seawater 18o that was steadily increasing from 3 standard mean ocean water smow by contrast 13c variation seems to have been relatively modest during most of the ordovician with the exception of the globally documented but short lived latest ordovician 13c excursion up to 7 nevertheless an underlying trend in mean 13c can be discerned changing from moderately negative values in the early ordovician to moderately positive values by the latest ordovician these new isotopic data confirm a major reorganization of ocean chemistry and the surface environment around 465 to 455 ma the juxtaposition of the greatest recorded swings in phanerozoic seawater 87sr 86sr and 18o at the same time as one of the largest marine transgressions in phanerozoic earth history suggests a causal link between tectonic and climatic change and emphasizes an endogenic control on the o isotope budget during the early paleozoic better isotopic and biostratigraphic constraints are still required if we are to understand the true significance of these changes we recommend that future work on ordovician isotope stratigraphy focus on this outstanding middle late ordovician event

implications of volcanism in coastal california for the neogene deformation history of western north america 1 the geologic record of coastal california includes evidence of numerous volcanic centers younger than 30 ma that do not appear to have erupted in an arc setting by correlating these volcanic centers with specific slab windows predicted from analysis of magnetic anomalies on the pacific plate we add new constraints to tectonic reconstructions since 30 ma our correlations such as erupting the morro rock islay hill complex south of the pioneer fracture zone and the iversen basalt south of the mendocino fracture zone require larger displacements within western north america than advocated by most previous authors specifically we infer at least 315 km of motion between the sierra nevada and rigid north america at an azimuth of about n60 w and at least 515 km between baja california and rigid north america in a similar direction a consequence of inferring a large displacement of baja california is that the pacific north american plate boundary must have developed most of its current form prior to 10 ma we interpret a slab window developing between cocos and monterey plates after 19 ma that reconstructs under nearly all of the southern california volcanic centers dated at 18 14 ma most of the sedimentary basins associated with volcanic rocks show brief periods of rapid subsidence synchronous with volcanism followed by slow subsidence of variable but often extended duration consistent with rapid extension of cold lithosphere over recently introduced hot asthenosphere

origin of dolomite in the late cretaceous paleocene limestone turbidites eastern pontides turkey abstract the late cretaceous paleocene tonya formation of the hacimehmet region in trabzon eastern pontides turkey is composed of limestone turbidites these limestone turbidites were deposited at water depths of around 500 to 600 m and remained buried until uplifted in late paleocene time the lower part of this formation which is made up of clast supported calcirudites and pebbly calcarenites is partially to completely dolomitized two morphological types of dolomite are identified 1 fine to medium crystalline dolomite rd1 2 coarse to very coarse crystalline dolomite rd2 each one of the dolomite types is non stoichiometric ca rich ca 57 61 mg 43 39 and has similar geochemical features and uniform dull red brown luminescence these dolomites have 18 o pdb values from 2 6 to 3 3 average 3 0 13 c pdb values from 2 3 to 3 1 average 2 7 and 87 sr 86 sr ratios from 0 707719 to 0 707772 fluid inclusion data of rd2 dolomites yield homogenization temperatures t h of 48 58 c and salinities of 20 4 to 21 7 wt nacl equivalent the sr concentration in dolomites varying from below a detection limit of 20 to 360 ppm is not homogeneous within each dolomite crystal the na concentrations of dolomites are similar varying from 600 to 760 ppm average 680 ppm and are higher than those for dolomites formed in normal marine environments the fe and mn concentrations range from 2500 to 7000 ppm average 3800 ppm and from 50 to 850 ppm average 330 ppm respectively considering the petrography paragenesis trace element and stable isotope data the dolomites in the tonya formation are interpreted to have formed in the subsurface during mechanical compaction at shallow to intermediate burial depths dolomitization in the tonya formation was most probably driven by seawater that was slightly modified through rock water interaction and in part at elevated temperatures

genesis of fabric destructive dolostones a case study of the brac formation oligocene cayman brac british west indies abstract the origin of fabric destructive and fabric retentive textures in dolostones of all ages remains open to debate evolution of the fabric destructive textures which effectively destroys all traces of the original limestones remains particularly controversial on cayman brac a small island located in the caribbean sea the brac formation lower oligocene is formed of limestones and fabric destructive dolostones the top of this formation is defined by the brac unconformity which developed as a karst surface during the late oligocene and early miocene the overlying cayman formation lower to middle miocene is pervasively dolomitized and characterized by fabric retentive textures that have preserved the depositional textures of the original limestones the stratigraphic juxtaposition of these two formations provides an ideal opportunity to compare the factors that controlled the development of fabric destructive and fabric retentive fabrics in dolostones the dolostones in the brac formation are formed of dolomite that replaced the limestone and dolomite cements typically zoned that were precipitated in pores throughout the dolostones interpretations based on the composition of the dolomite 13 c values 18 o values and sr fe and mn contents indicate that dolomitization of both the brac formation and the cayman formation was probably controlled by similar slightly modified seawater with the distribution of the dolomite in the limestones and the evolution of dolostone textures being controlled largely by permeability pathways that governed circulation patterns of the dolomitizing fluids the large sucrosic dolomite crystals which are commonly zoned probably developed as a result of repeated cycles of limestone matrix dissolution and dolomite precipitation

comparisons of measured and predicted sediment transport rates in tidal currents abstract this paper describes bedload transport measurements in a large scale sedimentary system on the british continental shelf steady flow sediment transport theories are applied to oscillatory tidal currents and various equations are compared with measured transport rates from tracer dispersion experiments the effects of waves on both measured and predicted rates are briefly considered and the predicted directions of sediment movement compared with those inferred from sedimentological data

new distribution coefficient for the incorporation of strontium into dolomite and its implications for the formation of ancient dolomites late tertiary dolomites form the bahamas exhibit a strong inverse correlation between sr and mol mgco sub 3 it is proposed that this relation is a result of a distribution coefficient dependent on the concentration of ca in dolomite because these dolomites precipitated from water with a sr sup 2 ca sup 2 ratio similar to that of seawater a distribution coefficient d sub sr sup dolomite of 0 0118 is calculated for sr in stoichiometric dolomite this coefficient increases by about 0 0039 for every additional mol caco sub 3 in nonstoichiometric dolomite these data have three important implication first the new d sub sr sup dolomite is substantially lower than previously suggested second a relation has been established among the sr sup 2 ca sup 2 ratio of dolomite forming waters the major element composition of dolomites and the sr content of dolomites allowing the determination of the sr sup 2 ca sup 2 ratio of fluids involved in the formation of ancient dolomites third some published data sets on dolomites show a similar relation between sr content and major element composition suggesting that these dolomites may also have formed from seawater other data sets deviate from the correlation defined by little bahama bank dolomites indicating either dolomitization in fluids other than seawater or changes ofmore the sr sup 2 ca sup 2 ratio of seawater through time less

hydrothermal dolomitization of jurassic cretaceous limestones in the southern alps italy relation to tectonics and volcanism dolomitization has affected up to 750m of the jurassic and cretaceous pelagic carbonate sequence of the southern continental margin of the alpine tethys the sequence crops out in the southern alps of italy monti lessini late paleocene to miocene extrusion of basaltic tuffs breccias and lavas was contemporaneous with the dolomitization was was associated with extensive tectonism in an ancient back arc basin more than 200 samples were analyzed by x ray diffraction cathodoluminescence scanning electron microscopy stable isotope ratios carbon oxygen strontium and clay mineralogy the dolomite contains 40 to 50 mgco sub 3 in thin sections the crystal size distribution is unimodal about 100 mu m possibly indicating a single nucleation for the main crystallization phase the delta sup 13 c of the dolomite is not appreciably different from the undolomitized pelagic limestone 1 0 per thousand to 2 0 per thousand peedee belemnite pdb the delta sup 18 o variation 5 0 per thousand to 13 0 per thousand pdb is due to temperature variation in the system the sup 87 sr sup 86 sr ratio in the dolomite 0 70839 0 70867 is consistent with the ratio in late oligocene miocene marine water clay minerals in limestone and dolomite differ in the presence of neoformed mg chlorite indicating a maximum temperature of about 150c for dolomitization the dolomite is suggested more to have a hydrothermal origin the heat flow associated with the volcanism allowed marine water to penetrate the system and circulate in convective cells through the tectonic breccias locally dolomitizing the limestone less

sediment transport predictions for the english channel using numerical models output from an hydrodynanmic numerical model salomon breton 1991 a b has been combined with various empirical formulae to derive sediment transport rates and directions the predictions are compared with radioactive sand tracer experiments demonstrating the applicability of gadd et al s 1978 expression the derived bedload transport pattern for the area is consistent with sedimentological and geomorphological evidence but provides improved resolution a bedload parting zone is identified for the central english channel this is well defined for sand movement but seabed material present over this area gravel is not predicted to move significantly the area is non erosive over which fine grained sediments are being transported a bedload convergence is located to the southwest of the dover strait not in the strait itself as proposed previously a transport pathway bypassing the convergence along the french coastline which is supported by radioactive tracer experiments could be responsible for sand transfer from the english channel to the southern north sea

volcanic record of the subduction of an active spreading ridge taitao peninsula southern chile during the pliocene the taitao peninsula was the locus of the subduction of the chile active spreading ridge beneath the south american margin new field data allow us to distinguish within the taitao ophiolite a genuine ophiolitic body of undetermined age and two volcanic and volcaniclastic units one unit shows evidence for deposition in a shallow water environment and it unconformably overlies the chilean continental basement its established pliocene age demonstrates that it was deposited during the subduction of the chile ridge lavas from both units include enriched mid oceanic ridge basalts morbs together with nb depleted basalts andesites and dacites la nb ratios vary randomly from 1 to 4 in both units and are positively correlated with sio 2 content these chemical characteristics are consistent with upper crustal contamination of morb type basalts by the chilean crust as already suggested from isotopic data we consider these volcanic suites to be the result of eruptions of morb magmas that originated from the downgoing shallowly buried active spreading center and were contaminated during their rise through the chilean continental basement

episodic slab rollback and back arc extension in the yunnan burma region insights from cretaceous nb enriched and oceanic island basalt like mafic rocks one highly debated topic related to the nature of the cretaceous mantle and geodynamic processes in se asia is whether they were dominated by the tethyan or pacific ocean subductions the main reason for this puzzle is the lack of geochemical studies on mantle derived mafic rocks in se asia in this paper we report early cretaceous nb enriched diabases ca 139 ma and late cretaceous nb enriched ca 68 ma and oceanic island basalt oib like ca 73 ma amphibolites from the ailaoshan tectonic zone in the yunnan burma region the ca 139 ma diabases and ca 68 ma amphibolites display similar geochemical features with typical nb enriched basalts e g high na 2 o tio 2 and nb 7 ppm contents and high nb th pm nb la pm and nb u ratios unlike adakite associated nb enriched basalts our nb enriched diabases and ampibolites were derived through mixing between oib like and arc like mantle source components in the spinel stability field 2 and nb 20 0 26 0 ppm contents and oib like rare earth element patterns and trace element spidergrams the oib like rocks were generated by a low degree of partial melting of garnet facies peridotite 80 km in combination with previous cretaceous to early eocene age data from granitoids three significant magmatic flare ups are identified at 140 115 ma 75 64 ma and 55 50 ma with two magmatic gaps at 115 75 ma and 64 55 ma given the distances of the studied area from the pacific and neotethyan trenches 2400 5000 km and 320 km respectively these cretaceous igneous rocks more likely formed in a neotethyan subduction related tectonic setting rather than a pacific ocean related setting we propose a tectonic model involving episodic flat slab subduction slab rollback and intracontinental back arc extension for neotethyan subduction in the yunnan burma region

initiation of subduction and the generation of slab melts in western and eastern mindanao philippines adakite found in both the eastern and western parts of mindanao island philippines is a rare rock type characterized by low heavy rare earth elements and y contents together with high sr y ratios and is considered to be the result of the melting of young subducted oceanic crust which leaves an eclogite residue pliocene quaternary adakites from western mindanao zamboanga peninsula are probably derived from the melting of the young miocene sulu sea crust which is currently subducting beneath zamboanga associated nb enriched basalts are thought to come from mantle metasomatized through interaction with adakitic melts in eastern mindanao pliocene quaternary cones and plugs of typical adakitic composition mark the trace of the philippine fault in surigao and north davao the underlying philippine sea crust is of eocene age and therefore cannot melt under normal subduction thermal conditions thermal models indicate that melting at the start of subduction can occur subduction of the philippine sea plate began 3 to 4 ma beneath eastern mindanao and probably accounts for the presence of adakites along the philippine fault

late miocene adakites and nb enriched basalts from vizcaino peninsula mexico indicators of east pacific rise subduction below southern baja california a typical slab melt association was emplaced from 11 to 8 ma in the santa clara volcanic field vizcaino peninsula baja california sur it includes adakitic domes and associated pyroclastic flow deposits together with lava flows of niobium enriched basalts the trace element and isotopic sr nd pb signatures of adakites are consistent with melting of altered mid ocean ridge basalts and the sources of the nb enriched basalts contain an enriched mantle wedge component such associations commonly form at depths of 70 80 km during low dip subduction of very young oceanic crust however the santa clara field is relatively close 100 km to the paleotrench which suggests that the genesis of its adakites and nb enriched basalts occurred in a very high thermal regime linked to the subduction of the then active guadalupe spreading center of the east pacific rise our data suggest that the asthenospheric window documented below northern baja california also developed beneath the south of the peninsula during the neogene this hypothesis is consistent with the spatial distribution and the ages of adakites and magnesian andesites from this region

mantle and slab contributions in arc magmas destructive plate margins are major sites of terrestrial magmatism that have long had a key role in models for the generation of continental crust and the development of chemical heterogeneities in the upper mantle in these models it is important to evaluate the nature and size of contributions from preexisting crust in the generation of magmas above subduction zones and to constrain the size of the fluxes of different elements from the subducted crust and the overlying mantle wedge in arc magmas kay 1 980 hawkesworth ellam 1 989 hawkesworth et al 1 99 1 a mcculloch gamble 1 99 1 the more mobile components of the subducted oceanic crust are the products of element fractionation processes in the upper continental crust the ocean basins and hydrothermal systems on the ocean floor they therefore have distinctive isotope and even trace element ratios that in principle should be readily distinguished from those in peridotite in the mantle wedge prior to subduction in practice however element ratios are almost certainly fractionated during shallow level dehydration of sub ducted material and at greater depths where material from the slab appears to contribute in the generation of arc magmas moreover the isotope signal of slab derived material may be heavily diluted by exchange with

dolomitization of the waulsortian limestone lower carboniferous in the irish midlands the waulsortian limestone lower carboniferous of the southern irish midlands is dolomitized pervasively over a much larger region than previous studies have documented this study indicates a complex multistage multiple fluid history for regional dolomitization partially and completely dolomitized sections of waulsortian limestones are characterized by finely crystalline 0 01 0 3 mm planar dolomite planar replacive dolomite is commonly followed by coarse 0 5 mm nonplanar replacive dolomite and pervasive void filling saddle dolomite cement is frequently associated with zn pb mineralization planar dolomite has average 18o and 13c values pdb of 4 8 and 3 9 respectively these are lower oxygen and slightly higher carbon isotope values than averages for marine limestones in the waulsortian 18o 2 2 13c 3 7 mean c and o isotope values of planar replacive dolomite are also distinct from those of nonplanar and saddle dolomite cement 7 0 and 3 3 7 4 and 2 4 respectively fluid inclusions indicate a complex history involving at least three chemically and thermally distinct fluids during dolomite cementation the petrography and geochemistry of planar dolomites are consistent with an early diagenetic origin possibly in equilibrium with modified carboniferous sea water where the waulsortian was exposed to hydrothermal fluids 70 280 c planar dolomite underwent a neomorphic recrystallization to a coarser crystalline planar and nonplanar dolomite characterized by lower 18o values void filling dolomite cement is isotopically similar to nonplanar replacive dolomite and reflects a similar origin from hydrothermal fluids this history of multiple stages of dolomitization is significantly more complex than earlier models proposed for the irish midlands and provides a framework upon which to test competing models of regional vs localized fluid flow

late carboniferous high end t ehf t granitoids enclaves and dikes in western junggar nw china ridge subduction related magmatism and crustal growth abstract we report results of petrologic geochronological and geochemical investigation of the late carboniferous diorites granodiorites amphibole am bearing granites and associated dioritic and monzonitic enclaves and mafic and granitic dikes in the keramay area of the western junggar region of central asian orogenic belt caob zircon u pb dating suggests that they were generated in the late carboniferous 316 304 ma the diorite and granodiorite compositions extend over a wide range of sio2 53 70 wt sr 240 602 ppm and mg 41 58 values and are characterized by moderately fractionated rare earth element ree patterns nb ta depletion and relatively low y and yb contents the mafic dikes consist of dolerites diorite porphyries and minor granodiorite porphyries and have variable sio2 51 59 wt and high mg cr and ni values with the exception of two samples with relatively high heavy ree hree contents the mafic dikes exhibit trace element characteristics similar to diorites and granodiorites the am bearing granites and a granite porphyry dike sample have high levels of sio2 73 77 wt hrees e g yb 3 46 15 7 ppm and low mg cr and ni contents along with clearly negative eu ba and sr anomalies similar to typical a type granites all granitoids enclaves and dikes in this region have high positive end t 7 13 to 9 74 and zircon ehf t 10 to 16 values and moderate initial 87sr 87sr ratios 0 7004 0 7049 mineral composition data suggest that the parental magmas for mafic dikes are similar to cenozoic sanukitoids in the setouchi arc area japan and were possibly generated under water rich and high oxygen fugacity nno 1 5 to nno 2 7 conditions they most likely originated from partial melting of a mantle source variably modified by subducted oceanic crust derived melts and minor fluids and subsequently underwent fractional crystallization the diorites and granodiorites were possibly generated by magma mixing between enriched lithospheric mantle and juvenile lower crust derived magmas coupled with minor crystal fractionation the am bearing granites and granite porphyry dike were produced by partial melting of juvenile crustal materials at shallow crustal levels taking into account widespread contemporaneous magmatism including morb type basalts and slab derived adakites in western junggar we suggest that the keramay intrusive rocks were generated in a special arc setting related to ridge subduction and resultant slab window which played an important role in the crustal growth of the caob

ridge subduction and slab windows in the central asian orogenic belt tectonic implications for the evolution of an accretionary orogen abstract cenozoic ridge subduction and the resultant slab windows have been well documented worldwide especially along the western margins of north and south america the principal characteristics of ridge subduction which can be used to recognize the process in ancient orogens include intrusion of ridge generated magmas into a forearc in a near trench position this can be regarded as the hallmark of ridge subduction the key magmatic products are adakites that include plutonic and volcanic rocks with intermediate to high sio2 high sr y and la yb ratios elevated mgo na2o k2o ni and cr contents and high lile and lree elements they range from hornblende biotite bearing high mg diorites and granodiorites to hornblende pyroxene high mg andesites they are typically associated with a wide variety of coeval and compositionally diverse volcanic and plutonic rocks such as high ca boninites tholeiitic high t basalts alkaline basalts charnockites peraluminous ttgs i type granites and mid ocean ridge mor not supra subduction ophiolites the trace element chemistry of these rocks demonstrates that they were generated by partial melting of dehydrated subducted oceanic crust and of lower continental crust contaminated and metasomatized by juvenile melts that originated by upwelling of asthenospheric mantle it is this combination of mixed sources that gives rise to the diagnostic and distinctive features of coeval and associated ridge subduction rocks in a near trench environment other important features of ridge subduction are high temperature metamorphism closely associated with near trench plutons created by heat released through a slab window and porphyry gold copper zinc mineralization associated with mor ophiolites and generated in hydrothermal vents at a mor when a ridge subducts under a continental margin the diverging plates associated with the ridge continue to separate creating a slab window that forms between the separating plates adakitic melts tend to be generated close to the hot plate margins whereas a type granites and porphyry cu au deposits form in the centre of the opening slab window most of the above features have been described and analyzed in many parts of the central asian orogenic belt caob particularly in west junggar chinese altai russian altai east junggar beishan chinese tianshan inner mongolia kyrgyzstan and at alxa prominent in one of two slab windows defined in west junggar are two swarms of late carboniferous high mg hornblende biotite diorite dykes with sanukitoid chemistry that intruded through the slab window one generation was orthogonal to an ophiolite strewn paleotrench suture and the second was parallel to a transform fault that was parallel to the paleo trench the dikes provide the best evidence so far for the orientation and opening of a slab window in the caob and for the relative movement directions of the downgoing and overriding plates during the subduction and closure of the paleo asian ocean however these are early days because most parts of the caob have yet to be investigated for evidence of ridge subduction and slab windows so the prospects are high

estimating global shelf sediment mobility due to swell waves global climate models gcms such as those produced at the european centre for medium range weather forecasts ecmwf generate data that are relevant to the study of marine sedimentary processes we used estimates of significant wave height and period derived from an ecmwf gcm to predict for the first time the area of continental shelves over which sediment is mobilised the wave climatology was such that 0 1 mm diameter quartz sand had the potential to be mobilised on at least one occasion between july 1992 and july 1995 over 41 6 of the earth s continental shelves the north atlantic region has the most energetic global wave climate strong enough to mobilise 0 1 mm diameter quartz sand to water depths of up to 234 m at least once over a 3 year period

geochemical and zircon u pb hf o isotopic evidence for a coherent paleoproterozoic basement beneath the yangtze block south china abstract the early history of the yangtze block is not well constrained owing to the scarce outcrops of the archean to paleoproterozoic rocks here we report an integrated study of zircon u pb and hf o isotopes as well as whole rock major and trace elements and nd isotopes for the orthogneisses of the dongling complex in the eastern yangtze block la icp ms zircon u pb dating yields a 207 pb 206 pb age of ca 1 85 ga for the protolith age of the orthogneiss revealing a reliable paleoproterozoic basement in the eastern yangtze block zircons from the orthogneiss have e hf t values ranging from 13 4 to 9 0 corresponding to t hf dm2 model ages of 3 03 2 80 ga the zircon 18 o values range between 6 5 and 8 8 with an average of 7 8 in addition the whole rock e nd t values of the orthogneisses range from 9 3 to 8 0 and t nd dm2 model ages from 2 98 ga to 3 05 ga the orthogneisses have relatively high sio 2 k 2 o na 2 o rb ga zr nb y and ce contents and show low cao eu ba and sr concentrations their 10 000 ga al ratios range between 3 0 and 3 5 all these geochemical and nd hf o isotopic signatures suggest a type granite protolith for orthogneisses in the dongling complex they were generated by partial melting of archean crustal materials in a post collisional tectonic setting when our data is combined with the existing geochronological data in other parts of the yangtze block it demonstrates a coherent paleoproterozoic basement beneath the yangtze block representing an important part of the columbia supercontinent the ca 1 85 ga a type magmatism may have led to craton stabilization over all of the yangtze block in addition a potential precambrian dual basement structure may exist in the eastern yangtze block which is supported by paleoproterozoic metamorphic basement and overlaying neoproterozoic sedimentary cover

paleoproterozoic porphyries and coarse grained granites manifesting a vertical hierarchical structure of archean continental crust beneath the yangtze craton abstract granites generated by anatexis are ideal crustal probes to decipher the differentiation processes within deep continental crust in this study the successive emplacement of granites including the porphyries and the coarse grained granites was found in the nucleus the kongling complex of the yangtze craton the porphyries intruding at 1840 17 ma shrimp ages occur as veins and contain phenocrysts of k feldspar 15 quartz 5 plagioclase 4 and garnet 1 setting in a fine grained groundmass 75 of k feldspar plagioclase quartz and biotite the coarse grained granites emplacing at 1821 8 ma shrimp ages are outcropped as pluton and comprise k feldspar 60 quartz 30 biotite 5 plagioclase 5 they are similar to a type granites with relatively high sio2 69 2 71 5 wt 72 6 74 5 wt high fe2o3t mgo 21 5 27 7 5 9 19 5 104 ga al ratios 3 6 4 3 2 7 3 5 respectively and enriched hfses th u pb ga the porphyries contain higher total trace element contents and zirconium saturation temperature mean 892 c than those of the coarse grained granites mean 803 c magmatic zircons from the porphyries and coarse grained granites have ehf t values of 17 6 to 11 7 and 21 0 to 14 8 which yield two stage hf model ages of 3 6 3 3 ga and 3 8 3 4 ga respectively both falling into the evolutionary trend of the 2 9 2 8 ga kongling ttgs it implies both granites are anatectic crustal reworking derivatives from partial melting of the archean ttgs under sustaining extension in paleoproterozoic applying trace element modelling we further identify a unique tonalitic source for both granites which is analogous to the exposed natural kongling tonalite and find the porphyries slightly affected by the juvenile crust based on the intrusion of porphyry veins into granitic gneisses we envisage the crustal structure beneath the craton is layered from older 2 94 ga tonalites upward to younger 2 87 ga granitic gneisses such hierarchical structure could shed new light on the imperceptible vertical differentiation process as cratonization

dolomitization of the pedro castle formation pliocene cayman brac british west indies abstract the pedro castle formation pliocene on cayman brac is variably dolomitized by texture preserving but non mimetic and texture destructive replacive dolomite mimetic replacement of skeletal grains is limited to echinoderm plates and with few rare exceptions there is no mimetic replacement of red algae foraminifera green algae or any other type of skeletal grain the lack of mimetic dolomite is atypical of island dolostones found in the caribbean sea and the pacific ocean dolostones in the pedro castle formation are formed entirely of high ca calcian dolomite average of 57 4 mol caco 3 oxygen isotopes mean 1 25 pdb from the dolomite indicate that dolomitization was mediated by seawater or modified seawater carbon isotopes in the dolomite which range from 1 81 to 1 42 pdb were probably inherited from the precursor limestone the average sr content in the dolomite 360 ppm is higher than that found in most other island dolomites the sediments that now form the pedro castle formation were deposited in shallow water on an open bank during the early pliocene pre dolomitization diagenesis of those sediments included syntaxial overgrowths around echinoderm fragments dissolution of aragonitic bioclasts stabilization to low magnesium calcite and local precipitation of vadose cements thus the limestones had been extensively stabilized by the time that dolomitization took place during the late pliocene the general paucity of mimetic replacement in these dolostones can probably be attributed to the calcite stabilization that took place before dolomitization

boundary layer dynamics and sediment transport under storm and non storm conditions on the scotian shelf abstract near bed measurements of waves currents seabed responses and ripple migration rates were obtained using an instrumented tripod at a water depth of 39 m on the scotian shelf during the winter of 1992 93 these data and the grant and madsen s 1986 grant w d madsen o s 1986 the continental shelf bottom boundary layer annu rev fluid mech 18 265 305 combined flow boundary layer model are used to examine wave current interaction various sediment dynamic thresholds and sediment transport on an exposed high energy continental shelf the seabed was found to be rippled for most time of the deployment and thus the ripple enhanced combined skin friction shear velocity had to be used to determine the initiation of bedload transport under combined flows this indirectly indicates adequate predictions of the skin friction shear velocities inside the wave boundary layer by the grant and madsen model at high transport stages bedload roughness must be added to the grain size roughness to obtain a transportrelated combined shear velocity in order to predict correctly the onsets of saltation suspension and sheet flow transports otherwise the initiation of suspended load transport and the total sediment transport rates will be severely under estimated the comparison between the prediction by the grant and madsen 1986 model and the calculation of a quadratic stress law suggests that the total current shear velocity was enhanced by a factor of 2 3 due to the wave current interaction during storms while the apparent bottom roughness was increased by more than an order of magnitude the non linear coupling between waves and currents also causes a 20 increase of the combined skinfriction shear velocity inside the wave boundary layer this non linear coupling is most important when waves and currents are roughly equal in magnitude and the angles between them are less than 30 four sediment transport formulae were tested while the engelund hansen total load and yalin bedload methods did not perform well the einstein brown and bagnold formulae were found to respectively give reasonable predictions of the bedload and total load sediment transport rates under the observed combined flow conditions the predicted sediment transport direction is also in good agreement with the observed ripple migration direction the gsc sediment transport model sedtrans92 predicts that the net daily transport rates during the storms reached 822 kg m 1 day 1 and were 2 3 orders of magnitude higher than the non storm transport suggesting the dominance of storm processes in sediment transport on the scotian shelf these values are in good agreement with the results from sand tracer experiments conducted in this region we thus conclude that sedtrans92 can properly simulate boundary layer dynamics and sediment transport on continental shelves

dolomitization of the late cretaceous paleocene platform carbonates golkoy ordu eastern pontides ne turkey abstract late cretaceous paleocene agillar formation in the eastern pontide zone ne turkey is composed of shallow marine platform carbonates that were buried until uplift in late paleocene time the formation which is represented by 100 m thick reefal limestones around golkoy is completely dolomitized on the basis of petrographic and geochemical features four different replacive and cement dolomite phases are present the replacive dolomites which are cut by low amplitude stylolites include 1 fine crystalline planar s dolomite rd1 and 2 coarsely crystalline planar s e dolomite rd2 two types of dolomite cements were developed in minor amounts 1 a coarse dolomite cement cd1 that mostly fills molds vugs and fractures in rd1 dolomites and 2 rim dolomite cement cd2 that commonly develops on the space facing surfaces of rd2 dolomite the replacive dolomites are nearly stoichiometric ca 49 51 mg 48 50 and have 18 o values of 3 5 to 1 6 vpdb 13 c values of 1 1 to 0 8 vpdb and 87 sr 86 sr ratios of 0 707104 to 707458 sr na mn and fe concentrations of replacive dolomites are 102 171 ppm bdl 371 ppm 244 9941 ppm and 2643 8951 ppm respectively petrographic and geochemical data indicate that the replacive dolomites were formed prior to chemical compaction at shallow to intermediate burial depths from late cretaceous paleocene seawater and or seawater slightly modified by water rock interaction process and then recrystallized by meteoric waters at increasing burial depths and temperatures compared with replacement dolomites cd1 dolomites yield lower 18 o values 4 9 to 5 3 vpdb similar 13 c values 1 2 to 0 5 vpdb and higher 87 sr 86 sr ratios 0 707660 0 707609 the cd1 dolomites overlie the low amplitude stylolites and have homogenization temperatures between 94 and 112 c indicating that they were deposited from basinal brines although replacive dolomites and dolomite cements have similar na bdl 413 ppm mn 259 6990 ppm and fe 2184 6690 ppm concentrations the cements have lower sr contents 63 114 ppm like replacive dolomites cd1 dolomite is nearly stoichiometric ca 49 50 mg 47 49 but cd2 dolomite is slightly ca rich ca 51 52 mg 47 48 the cd2 dolomite is overlain by coarsely crystalline mosaic calcite cement indicates that diagenetic fluids eventually evolved to undersaturation with respect to dolomite and oversaturation with respect to calcite

sand transport under the combined influence of waves and tidal currents an assessment of available formulae abstract various formulae are used to predict the transport of sand under the combined influence of waves and currents these approaches include the use of the unidirectional formulae of einstein 1950 frijlink 1952 yalin 1963 engelund and hansen 1967 sternberg 1972 ackers and white 1973 and gadd et al 1978 modified for the presence of waves in accordance with the techniques described by bijker 1967 and swart 1976 and those of bagnold 1963 madsen and grant 1976 and vincent et al 1981 predicted rates are compared with measurements based upon fluorescent sand tracer studies in an area of high tidal current and wave energy northern bristol channel u k in general predicted rates are lower than those measured and only the formulae of madsen and grant 1976 and the modified formula of sternberg 1972 appear to provide realistic estimates of transport under wave and tidally induced currents in comparison with the field measurements

petrogenesis and geochronology of paleoproterozoic magmatic rocks in the kongling complex evidence for a collisional orogenic event in the yangtze craton abstract the early precambrian crustal evolution of the yangtze craton especially the paleoproterozoic orogenic event has drawn great attention the early precambrian magmatism in the archean palaeoproterozoic basement kongling complex is critical for understanding the early precambrian tectonic evolution of the yangtze craton we report the presence of two distinct groups of paleoproterozoic felsic rocks in the kongling complex providing new insights into the paleoproterozoic orogenic event in the northern yangtze craton group 1 is composed mainly of deformed or weakly deformed monzogranite syenite and quartz monzonite and formed at 2 0 1 95 ga they have high sr y 60 71 and la ybcn 32 464 ratios and are inferred to have equilibrated with garnet rich residues suggesting that they have been generated by partial melting of thickened continental crust group 2 is represented mainly by undeformed to weakly deformed k feldspar granitic rocks with minor rhyolites and was emplaced at 1 87 1 84 ga in contrast to group 1 group 2 rocks display flat hree patterns and large negative eu sr and ti anomalies with low sr y zircons from group 1 rocks have ehf t values ranging from 9 4 to 14 9 whereas zircons from group 2 rocks have lower ehf t values ranging from 14 2 to 20 6 both groups have similar two stage hf model ages ranging between 3 2 and 3 6 ga the production of the two groups of granitoids in the huangling dome is ascribed to the reworking dehydration melting of archean rocks in the paleoproterozoic widespread paleoproterozoic magmatism and metamorphism indicate that the yangtze craton experienced an orogenic event at ca 2 0 ga and subsequent orogenic collapse caused by lithospheric extension at ca 1 85 ga

petrography and geochemistry of early stage fine and medium crystalline dolomites in the middle devonian presqu ile barrier at pine point canada the petrography and geochemistry of fine and medium crystalline dolomites of the middle devonian presqu ile barrier at pine point western canada sedimentary basin are different from those of previously published coarse crystalline and saddle dolomites that are associated with late stage hydrothermal fluids fine crystalline dolomite consists of subhedral to euhedral crystals ranging from 5 to 25 m mean 8 m the dolomite interbedded with evaporitic anhydrites that occur in the back barrier facies in the elk point basin fine crystalline dolomite has 18 values between 1 6 to 3 8 pdb and 87sr 86sr ratios from 0 7079 0 7081 consistent with derivation from middle devonian seawater its sr concentrations 55 225 p p m mean 105 p p m follow a similar trend to modern little bahama seawater dolomites its rare earth element ree patterns are similar to those of the limestone precursors these data suggest that this fine crystalline dolomite formed from middle devonian seawater at or just below the sea floor medium crystalline dolomite in the presqu ile barrier is composed of anhedral to subhedral crystals 150 250 m mean 200 m some of which have clear rims toward the pore centres this dolomite occurs mostly in the southern lower part of the barrier medium crystalline dolomite has 18o values between 3 7 to 9 4 pdb mean 5 9 pdb and 87sr 86sr ratios from 0 7081 0 7087 mean 0 7084 sr concentrations from 30 to 79 p p m mean 50 p p m and mn content from 50 to 253 p p m mean 161 p p m and negative ce anomalies compared with those of marine limestones the medium crystalline dolomite may have formed either 1 during shallow burial at slightly elevated temperatures 35 40 c from fluids derived from burial compaction or more likely 2 soon after deposition of the precursor sediments by middle devonian seawater derived from the elk point basin these results indicate that dolomitization in the middle devonian presqu ile barrier occurred in at least two stages during evolution of the western canada sedimentary basin the geochemistry of earlier formed dolomites may have been modified if the earlier formed dolomites were porous and permeable and water rock ratios were large during neomorphism

origin of the adakite high nb basalt association and its implications for postsubduction magmatism in baja california mexico constraining the origin of the adakite high nb basalt hnb association in baja california mexico is critical to a better understanding of global arc magmatism currently the preferred explanation for the close spatial and temporal association of the two rock suites is through melting of the basaltic portion of the subducted farallon cocos plate thus providing support for the slab melting origin of adakites elsewhere moreover a tectono magmatic model involving the production of both adakite and hnb from slab melts offers a comprehensive explanation for the origin of the atypical arc related postsubduction magmatism in baja california this paper proposes alternative models for the origin of hnb and postsubduction magmatism in baja california wherein the unusual geologic setting of western mexico and westward movement of north america permitted the influx of pacific asthenosphere beneath the adjacent gulf of california after the cessation of subduction unlike the previous tectono magmatic model the new models propose that the asthenosphere provided a direct source for postsubduction tholeiitic and rare alkali magmas that were erupted in baja california as tholeiites and hnb respectively fractional crystallization of some of the hnb magmas plus assimilation of tholeiitic materials produced nb enriched basalts neb the influx of pacific asthenosphere after the cessation of subduction also provided thermal energy to melt the mafic lower baja california crust producing adakite rocks and the preexist ing metasomatized mantle wedge producing bajaites and calc alkaline magmas

geochemical confirmation of the kula farallon slab window beneath the pacific northwest in eocene time plate tectonic models indicate subduction of the kula farallon spreading ridge and thus imply formation of the kula farallon slab window beneath western north america from late cretaceous to middle eocene time seafloor magnetic anomalies however pro vide few constraints on the configuration of the subducted ridge and the location of the slab window we use geochemical data from eocene igneous rocks to provide an estimate of the slab window s position at 50 ma contouring of the trace element ratios k 2o sio2 rb zr and ta ce for lavas of the eocene challis kamloops volcanic belt demonstrates a southward trend toward enriched geochemical character this trend is consistent with a pacific northwest position for the slab window this is supported by the presence of adak ites slab melts near the canada united states border and by ca 51 ma forearc intru sions and volcanic rocks on vancouver island that are regarded as products of ridge subduction a new reconstruction of the slab window s geometry over time indicates that its chemically defined position is kinematically viable

dolomite crystal architecture genetic implications for the origin of the tertiary dolostones of the cayman islands abstract dolostones in the cayman formation on grand cayman and cayman brac which are the product of two and possibly three phases of dolomitization are formed of very small the recognition that individual dolomite crystals irrespective of their size may be architecturally heterogeneous carries important implications for dolomite genesis many interpretations of finely crystalline dolostones for example have tacitly assumed that they formed through one phase of dolomitization in the cayman formation however dolomite crystals as small as 10 m are characterized by internal heterogeneities that record their development through multiple time separated growth phases any model developed to explain dolomitization must take this into account the interpretation of chemical data for example must consider the multiple stage development of the dolostones that is evident from their internal zoning and discontinuities

fault controlled dolomitization at swan hills simonette oil field devonian deep basin west central alberta canada the partly dolomitized swan hills formation middle upper devonian in the simonette oil field of west central alberta underwent a complex diagenetic history which occurred in environments ranging from near surface to deep 2500 m burial five petrographically and geochemically distinct dolomites that include both cementing and replacive varieties post date stylolites in limestones depths 500 m these include early planar varieties and later saddle dolomites fluid inclusion data from saddle dolomite cements th 137 190 c suggest that some precipitated at burial temperatures higher than the temperatures indicated by reflectance data tpeak 160 c thus at least some dolomitizing fluids were hydrothermal fluorescence microscopy identified three populations of primary hydrocarbon bearing fluid inclusions and confirms that saddle dolomitization overlapped with upper cretaceous oil migration the source of early dolomitizing fluids probably was devonian or mississippian seawater that was mixed with a more 87sr rich fluid fabric destructive and fabric preserving dolostones are over 35 m thick in the swan hills buildup and basal platform adjacent to faults thinning to less than 10 cm thick in the buildup between 5 and 8 km away from the faults this plume like geometry suggests that early and late dolomitization events were fault controlled late diagenetic fluids were in part derived from the crystalline basement or palaeozoic siliciclastic aquifers based on 87sr 86sr values up to 0 7370 from saddle dolomite calcite and sphalerite cements and 206pb 204pb of 22 86 from galena samples flow of dolomitizing and mineralizing fluids occurred during burial greater than 500 m both vertically along reactivated faults and laterally in the buildup along units that retained primary and or secondary porosity

zircon u pb age trace element and hf isotope composition of kongling terrane in the yangtze craton refining the timing of palaeoproterozoic high grade metamorphism u pb age trace element and hf isotope compositions of zircon were analysed for a metasedimentary rock and two amphibolites from the kongling terrane in the northern part of the yangtze craton the zircon shows distinct morphological and chemical characteristics most zircon in an amphibolite shows oscillatory zoning high th u and 176lu 177hf ratios high formation temperature high trace element contents clear negative eu anomaly as well as hree enriched patterns suggesting that it is igneous the zircon yields a weighted mean 207pb 206pb age of 2857 8 ma representing the age of the magmatic protolith the zircon in the other two samples is metamorphic it has low th u ratios low trace element concentrations variable hree contents 33 8 lun 2213 14 7 lun smn 354 and 176lu 177hf ratios 0 000030 0 001168 the data indicate that the zircon formed in the presence of garnet and under upper amphibolite facies conditions the metamorphic zircon yields a weighted mean 207pb 206pb age of 2010 13 ma these results combined with previously obtained palaeoproterozoic metamorphic ages suggest a c 2 0 ga palaeoproterozoic collisional event in the yangtze craton which may result from the assembly of the supercontinent columbia the zircon in two samples yields weighted mean two stage hf model tdm2 ages of 3217 110 and 2943 50 ma respectively indicating that their protoliths were mainly derived from archean crust

magmatic affinity of modern and ancient subalkaline volcanic rocks determined from trace element discriminant diagrams when dealing with ancient subalkaline volcanic rocks the alkali total iron magnesium afm diagram is of limited use in assigning a tholeiitic versus calc alkaline affinity because these eleme

identification of ca 2 65 ga ttgs in the yudongzi complex and its implications for the early evolution of the yangtze block abstract in this contribution we carry out an integrated study of petrology whole rock geochemistry as well as zircon morphology trace element u pb and lu hf isotopes for granitic gneisses and leucogranites from the yudongzi complex a poorly studied archean outcrop in the northwestern yangtze block the granitic gneisses have the ttg affinity characterized by relatively high na2o k2o and sr y ratios and fractionated ree pattern with relatively high lree but low hree concentrations zircon grains in the granitic gneisses show magmatic cores and metamorphic rims u pb dating on zircon core of four granitic gneiss samples yielded their formation ages as 2660 2632 ma with a weighted average of 2650 21 ma mswd 1 6 n 4 their average ehf t values range from 0 6 to 0 5 and two stage model ages from 3 20 to 3 10 ga suggesting that the granitic gneisses were likely generated by remelting of thickened mesoarchean proto mafic crust the metamorphic zircon rims have relatively high th u ratios and identical hf isotope compositions to their corresponding cores indicating that the rims were formed via solid state recrystallization process the metamorphic zircons recorded a metamorphic event at ca 2 5 ga u pb dating and lu hf isotopes on magmatic zircons from a leucogranite sample revealed a magmatic event at 2477 18 ma the recognization of the 2 65 ga ttg magmatism and the 2 48 ga intercrustal remelting event in the yudongzi complex demonstrates the widespread neoarchean magmatism in the yangtze block combined with the age spectra of previous detrital zircons we show that the yangtze block has experienced a major tectonothermal event in the late archean the yudongzi complex has a distinct crustal evolution history compare with the paleoarchean continent nucleus in the eastern part of the yangtze block suggesting that the continent nucleus might have subjected to lateral growth by accretion processes

geochronology and geochemistry of igneous rocks from the kongling terrane implications for mesoarchean to paleoproterozoic crustal evolution of the yangtze block abstract the kongling terrane is a high grade metamorphic complex exposed in the northern part of the yangtze block it mainly consists of ttg gneisses amphibolites migmatites and metasedimentary rocks the amphibolites occur as enclaves or layers in the ttg gneisses and both the amphibolites and the ttg gneisses are intruded by granitic and diabasic dykes u pb ages of zircons from the amphibolite migmatite granite and diabase are determined by la icp ms igneous zircons from an amphibolite have an upper intercept age of 2945 13 ma mswd 1 2 while metamorphic zircons yield an upper intercept age of 2870 13 ma mswd 0 9 igneous zircons from a migmatite and a granite have weighted average 207pb 206pb age and upper intercept age of 1987 5 ma mswd 1 7 and 1985 6 ma mswd 1 3 respectively igneous zircons from a diabase dyke yield an upper intercept age of 1783 23 ma mswd 0 86 the protoliths of the amphibolites are tholeiitic basalts and exhibit slightly depleted to variably enriched lree patterns with moderate enrichment of rb and ba and depletion in nb and zr similar to oceanic island arc volcanics they have high cr co ni mgo and feot content with negative nd t values indicating that the tholeiitic basalts resulted from high degree batch melting of enriched mantle wedge the granites are strongly peraluminous with a cnk 1 1 and they have relatively low hree and y and negative nd t values detailed elemental and isotopic data suggest that they formed by partial melting of recycled products of the ttg gneisses amphibolites and older crustal material in a continental collisional setting the diabases are characterized by relatively high al2o3 feot and tio2 contents and low zr ba ratios their high initial 87sr 86sr ratios and negative nd t values indicate that they were derived from enriched lithospheric mantle in extensional setting therefore the kongling terrane occurred as an oceanic island arc at ca 2 95 ga and experienced earliest metamorphism at ca 2 87 ga which probably contributed to the formation of the high grade metamorphic terrane this terrane experienced a strong collisional event at 2 0 ga which may have led to cratonization over all of the yangtze block and also experienced lithospheric extension during 1 85 1 78 ga the global 2 0 ga paleoproterozoic collisional events were related to the assembly of supercontinent columbia

zircon u pb chronology hf isotope analysis and whole rock geochemistry for the neoarchean paleoproterozoic yudongzi complex northwestern margin of the yangtze craton china abstract the archean paleoproterozoic high grade basement of yudongzi complex is a key to understanding the early precambrian crustal evolution of the yangtze craton it comprises mainly orthogneiss paragneiss and amphibolite whose protoliths are tonalitic trondhjemitic granitic ttg sedimentary and basic intermediate volcanic rocks respectively the ttg gneiss amphibole plagiogneiss and biotite plagiogneiss yield magmatic zircon la icp ms u pb ages of 2815 18 ma mswd 0 92 2692 26 ma mswd 0 59 and 2449 4 ma mswd 0 94 respectively metamorphic overgrowths on zircon from amphibolite have an age of 1848 5 ma mswd 0 71 ttg gneisses show medium sr y and variable high la yb n ratios with low y and yb contents they are characterized by positive eu anomaly and distinct depletion of hree together with negative nb ta and ti implying amphibole garnet and minor rutile as residual phases their positive e hf t values of 2 1 to 8 1 with t dm2 of ca 2 80 3 10 ga suggest significant reworking of juvenile crust amphibole plagiogneisses display a strong enrichment of lrees and depletion of nb ta and ti additionally a relative enrichment of ba rb pb and zr as well as high cr and ni contents and mg values imply a mantle source with the addition of continental crust material zircon e hf t values vary between 0 9 and 3 9 suggesting a proportionally significant input of juvenile material and therefore interaction between the mantle and pre existing continental crust biotite plagiogneisses show negative e hf t values between 3 4 and 0 1 with a few positive e hf t values ranging from 0 1 to 1 5 together with t dm2 ages of ca 2 80 3 00 ga these e hf t values suggest that these rocks were mainly generated by reworking of ancient crust thus the yudongzi complex exposed in the northern part of the yangtze craton has experienced significant reworking of juvenile crust at ca 2 80 ga and subsequent crustal growth at ca 2 70 ga followed by a second stage of reworking of ancient crust at ca 2 45 ga during the late paleoproterozoic the yudongzi complex was probably involved in the amalgamation of the paleoproterozoic supercontinent columbia and affected by a post collisional metamorphic event at ca 1 85 ga

estimation of annual bedload flux in a macrotidal estuary bristol channel u k sedimentological data from the bristol channel indicate that mutually evasive ebb and flood dominant zones of net bedload transport are present to test this hypothesis long term observations of currents from the channel are combined with empirical formulae to estimate annual bedload transport rates under the combined influence of tidal currents and surface wind waves extrapolating these values over the ebb and flood dominant zones sand flux to the upper part of the channel is estimated at about 64 105 t yr 1 with supply and removal being of nearly equal magnitude such patterns of sand and water movement may be typical of tidally dominated estuaries and embayments

boninites from the neoarchaean gadwal greenstone belt eastern dharwar craton india implications for archaean subduction processes abstract boninites are rare in archaean arcs all over the world whilst there are some reports from the phanerozoic convergent margins in this paper boninites are reported for the first time from an archaean greenstone belt of india which have not been reported from any other indian greenstone belt as yet the greenstone belts of western dharwar craton are characterized by high mg basalts and komatiites whereas in the greenstone belts of eastern dharwar craton boninites represent the high mg lavas these boninites are characterized by 1 high mgo 12 24 wt 2 enriched in transition elements such as ni 423 1184 ppm cr 943 2765 ppm 3 low tio2 0 22 0 36 wt 4 high al2o3 tio2 ratio 29 50 5 gd ybn less than one 6 slight enrichment in lree depletion in mree compared to lree and hree resulting in u shaped ree patterns 7 negative nb ta zr hf ti and v anomalies despite the depletion in mree these characteristics differentiate them from deep mantle plume related magmas these boninites are compositionally similar to those found in modern and fossil island arcs from intraoceanic subduction settings and provide evidence to propose that two stage melting of the archaean mantle wedge generated this type of high mg magma the other high mg type magma komatiitic suite found in the greenstone belts of western dharwar craton has an enriched deep mantle plume source and its dry melting whereas boninites are generated by wet melting of the refractory depleted mantle wedge under high temperature and fluid pressure generated by dehydration of the slab the geological setting hfse and ree data show that possibly the 2 7 ga peak of geological activity had transitional characteristics between those of paleoarchaean and phanerozoic high thermal and fluid flux is inferred to generate these mg enriched and mree hfse lfse depleted suite this discovery of boninites from gadwal greenstone belt indicate that even in archaean subduction of small young and hot oceanic plates produced magmatic assemblage resembling to those of some of the cenozoic subduction complexes

mechanisms for linear sandbank formation and maintenance in relation to dynamical oceanographic observations abstract theories associated with flow in the vicinity of headlands and sandbanks together with proposed mechanisms for sandbank formation and maintenance are reviewed to exemplify water and sand movement around a sandbank on a tidally dominated shelf tidal currents in the vicinity of a headland associated linear sandbank are described these data are integrated with sedimentological information on bedforms and grain size distributions the sandbank under detailed investigation the scarweather sands is located along the northern coastline of the bristol channel uk which is an area susceptible to high energy inputs the maximum spring tidal range here is around 10m and a 6000km fetch is available for storm wave approach from the atlantic ocean but the area is 460km from the shelf break detailed observations are presented from 11 long term current meter stations located within 10km of the sandbank currents are described in terms of residual water circulation shallow water harmonics and tidal ellipse characteristics bedload transport paths are inferred from the distribution of bedforms grain size and m2 m4 tidal current interactions transport vectors are predicted from near bed current meter observations combined with empirically derived formulae it is demonstrated that water circulation and sand transport paths form part of a clockwise eddy system around the sandbank mechanisms controlling the location and the characteristics of this particular sandbank are shown to be explicable in terms of a combination of models for sandbank formation and maintenance the results of the present study are thought to be applicable directly to headland associated sandbanks elsewhere it is shown on the basis of theoretical considerations that the pattern of residual water movement and possibly sand transport around this sandbank could be extended to continental shelves on a global basis

geochemistry and zircon u pb geochronology of paleoproterozoic arc related granitoid in the northwestern yangtze block and its geological implications abstract the early history of the yangtze block has not been well constrained yet due to the scarce outcrops of archean to paleoproterozoic rocks in this study we report an integrated study of zircon u pb age major and trace element data and sr nd hf isotope compositions for gray gneisses from the houhe complex in the northwestern part of the yangtze block zircon u pb dating yields a weighted mean 207 pb 206 pb age of 2081 9 ma for a gneiss this age is interpreted as the formation age of the houhe complex and thus the houhe complex represents the oldest rocks found in the western part of the yangtze block the gray gneisses from the houhe complex range in sio 2 contents from 58 63 to 68 59 and na 2 o from 3 88 to 5 28 and have relatively high fe 2 o 3 contents of 2 86 6 69 al 2 o 3 of 16 01 18 88 and mgo of 0 97 2 65 these rocks show low sr 149 390 ppm cr 9 07 45 1 ppm and ni 4 97 21 3 ppm contents but high y 12 9 32 7 ppm and yb 0 95 2 25 ppm they are characterized by a relative enrichment in liles and lrees but a depletion in hfses these features are similar to those of calc alkaline granitoids suggesting that their formation might be related to a subduction related process or remelting of preexisted arc rocks compiled age spectra of archean to paleoproterozoic zircon grains reveal that the western and eastern parts of the yangtze block have similar archean and early paleoproterozoic age ranges indicative of the occurrence of an old continental nucleus in both parts on the other hand there are large amounts of ca 2050 2400 ma zircon grains revealed in the western part of the yangtze block implying that there might be a microcontinent with an active type continental margin during the paleoproterozoic times in the western part of the yangtze block because of the poor outcrops of archean to paleoproterozoic rocks in the yangtze block it is no possible to establish the exact nature of such an active margin yet the occurrence of ca 2 0 ga khondalitic and metasedimentary rocks has been documented in the eastern part of the yangtze block implying that there might be a passive type continental margin in the eastern part of the yangtze block at ca 2 0 ga the collision between the western microcontinent and the eastern part may have resulted in the final arc continental collision and the amalgamation of the yangtze block

meteoric modification of early dolomite and late dolomitization by basinal fluids upper arbuckle group slick hills southwestern oklahoma 1 thick massive dolomite units from the upper arbuckle lower ordovician peritidal carbonate sequence slick hills southwestern oklahoma provide insight into two important problems 1 modification of early dolomite and 2 late dolomitization early dolomite volumetrically dominant is nonluminescent to dully luminescent 87 sr 86 sr ratios of most early dolomite samples are similar to both early ordovician seawater and associated limestones consistent with an early origin from coeval seawater all early dolomite samples are characterized by low delta 18 o values 5 1 to 10 0 o oo pdb indicating postdepositional modification meteoric water buffered by younger carbonates with lower 87 sr 86 sr ratios was probably responsible for modification because lower sr concentrations delta 18 o values and 87 sr 86 sr ratios characterize the more intensively modified early dolomite samples meteoric modification probably occurred during emergence of the carbonate platform in the ordovician late dolomite volumetrically minor is brightly luminescent and occurs mainly in transition zones between early dolomite and associated limestone late dolomite has high 87 sr 86 sr ratios delta 18 o values 5 9 to 2 5 o oo pdb and fe and mn concentrations relative to early dolomite and associated limestone 87 sr 18 o fe and mn enriched fluids derived from the anadarko basin during the late paleozoic are proposed to have formed the late dolomite the fluids migrated through preexisting early dolomite aquifers and dolomitized limestones near the early dolomite limestone contacts which existed at that time although early dolomite remained more porous than limestone during early burial burial alteration by solution compaction and cementation by late dolomite have reduced porosity considerably better reservoir quality is expected to be associated with late dolomite in the arbuckle group

bed load transport under waves and currents the data from the flume experiments of kalkanis 1964 and abou seida 1965 giving the rates of accumulation of sand in recessed traps in oscillating sand beds have been reassessed in contrast to the work of madsen and grant 1976 the accumulation rate is interpreted as a measure of the average concentration of bed load per unit area during the period of oscillation for which the shields number t exceeds the threshold shields number th for the experimental runs where the reynolds number 35 the volume concentration of bed load c t cm3 cm 2 was found to be directly proportional to the excess shields number ex t t th c t 0 09 0 03 ext and the volume transport rate q t under instantaneous flow velocity u t was therefore q t 0 09 0 03 ex t u t for t th the rate of transport predicted by this equation was tested against the data of manohar 1955 and good agreement found showing a closer match between calculated and observed transport at high rates than found by the equation of madsen and grant 1976 the form of the transport rate equation proposed here q t u2 t u2th u t is compared with the form of the bedload transport equation for unidirectional flow and is found to have a firmer physical basis than some other wave transport equations presently in use

the isotopic record of ocean chemistry and diagenesis preserved in non luminescent brachiopods from mississippian carbonate rocks illinois and missouri fossil brachiopod shells are commonly used as a faithful record of the chemical and isotopic composition of the ancient ocean water in which they grew owing to their stable mineralogy and their presence in marine strata throughout phanerozoic time the utility of these fossils for preserving ocean water compositions is evaluated through analyses of multiple specimens that grew in contemporaneous sea water at individual stratigraphic horizons in the mississippian age burlington keokuk formation of mid continent north america significant variations in sr c and o isotopic compositions are observed in contemporaneous non luminescent shells indicating that apparently pristine brachiopod shell material has been altered by post depositional processes of diagenesis the least altered compositions at each horizon are used to estimate secular variations of 87 sr 86 sr in early mississippian ocean water the estimated rates of change of ocean water 87 sr 86 sr are comparable in magnitude to the rapid sr isotopic changes documented for cenozoic ocean water for which high frequency glacial cycles are inferred as a driving mechanism the rigorous assessment of diagenetic effects on the isotopic signatures of marine phases is requisite for the advancement of high resolution reconstructions of paleo ocean chemistry

petrogenesis of carboniferous adakites and nb enriched arc basalts in the alataw area northern tianshan range western china implications for phanerozoic crustal growth in the central asia orogenic belt abstract carboniferous volcanic rocks in the alataw area northern tianshan range xinjiang consist of early carboniferous ca 320 ma adakites and nb enriched arc basalts and basaltic andesites nebs and late carboniferous ca 306 310 ma mainly high k calc alkaline andesites dacites and rhyolites the adakites are calc alkaline and characterized by high na2o k2o 1 52 3 32 ratios negligible to positive eu anomalies strong depletion of heavy rare earth elements e g yb 0 74 1 47 ppm and y 6 7 14 9 ppm positive sr and ba but negative nb and ti anomalies and relatively constant end t values 3 4 6 6 and 87sr 86sr i ratios 0 7035 0 7042 some andesitic and dacitic adakite samples exhibit high mgo contents similar to magnesian andesites the nebs are sodium rich na2o k2o 2 03 8 06 and differ from the vast majority of arc basalts in their higher nb zr tio2 and p2o5 contents and nb th nb la and nb u ratios and minor negative to positive anomalies in ba nb sr zr and ti they have the highest end t values 6 4 11 6 but varying 87sr 86sr i ratios 0 7007 0 7063 the high k calc alkaline suite is similar to typical normal arc volcanic rocks in terms of moderately fractionated rare earth abundance and distinctly negative eu nb sr and ti anomalies they have end t values 1 2 6 4 and 87sr 86sr i ratios 0 7018 0 7059 geochemically they are similar to coeval i type granitoids in the alataw area given the presence of early carboniferous ophiolites in the northern tianshan range and the isotopically inappropriate compositions of proterozoic metamorphic basement in the alataw area we argue that the alataw adakites were most probably related to the melting of young subducted crust of the northern tianshan ocean the nebs likely originated from mantle wedge peridotites metasomatized by adakites and minor slab derived fluids the later high k calc alkaline suite was generated by afc processes that acted on melts derived from a mantle wedge metasomatized by hydrous fluids the larger range of isotopic compositions exhibited by both the neb and high k suite relative to the adakites suggests that the mantle wedge was heterogeneous prior to slab or fluid mediated metasomatism continental crustal growth of the central asian orogenic belt was dominated by contributions of the juvenile materials from the depleted mantle prior to 270 ma and possibly afterwards the results of this study suggest that other carboniferous nb enriched basalts in the tianshan range were generated by subduction processes rather than by intraplate tectonics as previously proposed

preliminary study of the dynamic origin of the distribution pattern of bottom sediments on the continental shelves of the bohai sea yellow sea and east china sea the bottom sediment types in the bohai sea yellow sea and east china sea byecs are diversified and their distribution pattern is very complicated however the bottom sediment types can be simplified to be sandy sediment clayey sediment and mixed sediment which comprise the complicated distribution pattern of bottom sediment in the byecs the continental shelves of the byecs are broad with shallow water depths and tidal currents which are permanent and dominate the marine dynamics in the byecs based on numerical simulation of tidal elevations and currents in the byecs the rates of suspended load transport and bed load transport during a single tidal cycle for sediments of eight different grain size ranges are calculated the results show that any sediment whose threshold velocity is less than that of tidal current has the same transport trend suspended load transport rare bed load transport rate and the ratio of the former to the latter decrease with grain size becoming coarser and coarser the erosion accretion patterns of sediments with different grain sizes are determined by the sediment transport rate divergences and the results show that the patterns are the same for sediments with different grain sizes three main bottom sediment types i e sandy sediment mainly composed of fine sand clayey sediment mainly composed of silty clay and mixed sediment mainly composed of fine sand silt and clay are obtained by computation the three bottom sediment types and their distribution pattern are consistent not only with sediment transport field and the sea bed erosion accretion pattern obtained by simulation but also with field data of bottom sediment types and divisions in the byecs sand ridges form mainly in the areas with strong rectilinear tidal currents sand sheets form mainly in the areas dominated by strong rotatory tidal currents and clayey sediments i e mud patches form mainly in the areas with weak tidal currents hence not only the sandy sediments but also the clayey sediments in the byecs are formed under the control of the whole tidal current field of the byecs the three main bottom sediment types are not isolated respectively in fact they constitute a whole tidal depositional system under the condition with no cyclonic cold eddy the clayey sediments in the byecs can form in weak tidal current environments therefore a cold eddy is not necessary for the deposition of clayey sediments in the byecs c 2000 academic press

magnesian andesites nb enriched basalt andesites and adakites from late archean 2 7 ga wawa greenstone belts superior province canada implications for late archean subduction zone petrogenetic processes magnesian andesites ma occur with normal tholeiitic to calc alkaline basalt andesite suites in four greenstone belts of the 2 7 ga wawa subprovince canada collectively the magnesian andesites span ranges of sio2 56 64 wt mg number 0 64 0 50 with cr and ni contents of 531 106 and 230 21 ppm respectively relative to normal andesites the magnesian andesites form distinct trends on variation diagrams with relatively high th and lree contents uniform yb over a range of mgo more fractionated hree and lower nb thpm and nb lapm ratios niobium enriched basalts and andesites neba nb 7 16 ppm and an al enriched rhyolite adakite suite are associated in space and time with the magnesian andesites nb enriched basalts and andesites are characterized by high tio2 p2o5 th and zr contents variably high zr hf 36 44 ratios and more fractionated hree gd ybcn 1 3 4 1 compared to the normal tholeiitic to calc alkaline basalt andesite suites the adakite suite has the high al al2o3 16 18 wt high la ybcn 21 43 and low yb 0 4 1 2 ppm of archean tonalite trondhjemite granodiorite ttg suites and cenozoic adakites indicative of liquids derived mainly from slab melting the basalt andesite suites are not characterized by normal tholeiitic or calc alkaline fractionation trends of major or trace elements rather compositional trends can be accounted for by some combination of fractional crystallization and variable degrees of metasomatism of the source of basalt and or andesites by adakitic liquids the occurrence of magnesian andesites nb enriched basalts andesites and adakites has been described from certain phanerozoic arcs featuring shallow subduction of young and or hot oceanic lithosphere adakites likely represent slab melts magnesian andesites the product of hybridization of adakite liquids with mantle peridotite and nb enriched basalts andesites melts of the residue from hybridization geological similarities between the late archean wawa greenstone belts and certain cenozoic transpressional orogens with the ma neba adakite association suggest that subduction of young hot oceanic lithosphere may have played an important role in the production of this arc related association in the late archean

energetics of calcium rich dolomite abstract the enthalpy of formation of sedimentary ca rich dolomite has been determined by oxide melt calorimetry the results show that ca rich dolomite is energetically different from well ordered stoichiometric dolomite the enthalpy of formation from calcite and magnesite becomes strongly more endothermic with increasing excess ca content this supports the idea that the substitution of ca in the mg layer of dolomite is energetically unfavorable ca rich dolomite is unstable relative to well ordered stoichiometric dolomite and calcite the enthalpy behavior for excess ca substitution in dolomite is different from that of mg substitution in calcite the enthalpy change is much larger in magnitude in dolomite and is more strongly dependent on composition differences in cation order as well as the presence of a modulated structure and low symmetry domains in ca rich dolomite cannot be discerned from the enthalpy data the findings confirm that the growth and occurrence of ca rich dolomite in sedimentary environments must be attributed to kinetic factors rather than to equilibrium

contrasting geochemical and sm nd isotopic compositions of archean metasediments from the kongling high grade terrain of the yangtze craton evidence for cratonic evolution and redistribution of ree during crustal anatexis abstract twenty three clastic metasediments from the kongling high grade terrain of the yangtze craton south china were analyzed for major trace and rare earth elements and sm nd isotopic ratios associated dioritic tonalitic trondhjemitic dtt and granitic gneisses as well as amphibolites were also analyzed in order to constrain provenance the results show that the clastic metasediments can be classified into 3 distinct groups in terms of mineralogical geochemical and sm nd isotopic compositions group a is characterized by having no to slight negative eu anomalies eu eu 0 82 1 07 being high in cr 191 396 ppm and ni 68 137 ppm and low in th 3 3 7 8 ppm and ree ree 99 156 ppm these characteristics are similar to those of metasediments from archean greenstone belts in addition the group a metasediments have the value of the chemical index of alteration ciw close to felsic gneisses their sm nd isotopic ree and trace element compositions can be interpreted by mixtures of the dtt gneisses and amphibolites dating of detrital zircons from 2 group a samples by shrimp reveals a major concordant age group of 2 87 3 0 ga which is identical to the age of the trondhjemitic gneiss these results strongly suggest that group a was principally the first cycle erosion product of the local kongling dtt gneiss and amphibolite moreover the higher than amphibolite cr content and slight eu depletion exhibited by some samples from this group infer that ultramafic rocks like komatiite and granite of probably 3 0 3 3 ga in age also played a role group b is characterized by the presence of graphite and shows a more evolved composition similar to post archean shales with a prominent negative eu anomaly eu eu 0 48 0 77 and high ciw on paired cr th vs la co and co th plots group b samples conform to a two end member mixing line of the kongling granitic gneiss and amphibolite however data on nd model age and ciw suggest that the granite component should be younger than the sampled granitic gneiss and derived from a distal source both groups a and b exhibit a clear positive correlation between ciw and tdm and a negative one between ciw and eu eu these correlations point to the crustal evolution of the yangtze craton towards coupled increasing ciw and eu depletion with decreasing age this in turn reflects the change of granitoid magmatism from local na rich dioritic tonalitic trondhjemitic rocks to widespread k feldspar granite the change led to the intracrustal differentiation stabilization and growth of the craton group c is restite and contains abundant sillimanite and garnet and unusually high ilmenite 7 11vol which can be seen to be dehydration melting products of biotite under the microscope this group shows extremely varied ree distributions from lree enriched to depleted and from negative to strong positive eu eu 1 63 eu anomalies compared to groups a and b group c is severely depleted in na2o k2o lree rb and ba whereas tio2 co v sc and hree and y are considerably enriched this is accompanied by anomalous high sm nd 0 21 0 28 147sm 144nd 0 1361 0 1738 and 143nd 144nd 0 511589 0 511958 ratios tdm correlates clearly with sm nd ratio and 2 out of 3 samples give significantly older to unrealistic tdm 3 9 4 9 ga the results document redistribution of ree and an open behavior of the sm nd isotope system during the biotite dehydration melting of metasediments