precise sm nd and u pb isotopic dating of the supergiant shizhuyuan polymetallic deposit and its host granite se china the supergiant shizhuyuan w sn bi mo deposit is hosted by the qianlishan granite a small highly fractionated granitic pluton 10 km 2 with multiple phases of intrusions within the early yanshanian granitoid province of se china strong alteration of skarn and greisen that formed in the contact zone between the first and second phases of granite intrusions and devonian limestone is responsible for the polymetallic mineralizations shrimp u pb zircon analysis indicates that the two early phases of the qianlishan granite formed contemporaneously at 152 2 ma metasomatic minerals garnet fluorite and wolframite separated from the skarn and greisen yield a sm nd isochron age of 149 2 ma that is interpreted as the formation age of the shizhuyuan deposit therefore the mineralization of the supergiant shizhuyuan polymetallic deposit formed contemporaneously with or very shortly after the intrusion of the small highly fractionated qianlishan granite

forms and processes of two highly contrasting rivers in arid central australia and the implications for channel pattern discrimination and prediction tooth s nanson g c 2004 forms and processes of two highly contrasting rivers in arid central australia and the implications for channel pattern discrimination and prediction geological society of america bulletin 116 7 8 802 816 rae2008

is stoping a volumetrically significant pluton emplacement process discussion glazner and bartley 2006 1 suggest that stoping is an insignificant to potentially nonexistent process in the emplacement and evolution of magmatic systems we strongly disagree with this conclusion and present here a number of alternative perspectives which we group into three categories

hafnium neodymium isotope systematics of the 2 7 ga gadwal greenstone terrane eastern dharwar craton india implications for the evolution of the archean depleted mantle abstract the neoarchean gadwal greenstone belt in the eastern dharwar craton india hosts a well preserved metavolcanic sequence that is dominated by tholeiitic and calc alkaline basalt andesite dacite rhyolite series which includes boninitic geochemical varieties bulk rock lu hf and sm nd isotope systematics of these apparently arc related volcanic rocks yield indistinguishable ages of 2 701 0 024 ga and 2 702 0 026 ga respectively on the basis of the close spatial association and identical ages of the different rock types we suggest 2 70 0 03 ga as the age of crystallization of the different rock types within the gadwal metavolcanic sequence in contrast bulk rock pb pb isotope systematics of the same samples yield a significantly younger and less precise age of 2 466 ga 0 068 0 110 ga we tentatively interpret this younger age to represent a metallogenic and crustal reworking event in the dharwar craton which disturbed the u pb system but not the lu hf or sm nd systems the gadwal metavolcanic rocks have positive initial ehf 2 70ga 1 6 to 8 7 and slightly negative to positive end 2 70ga 0 1 to 3 0 values consistent with an origin from a long term depleted source relative to a chondritic reservoir at 2 7 ga lack of correlation between initial isotopic compositions and major or trace element indices of fractionation and alteration suggest that the observed isotope variability probably reflects compositional variation in the gadwal source similar to that observed in modern day island arcs two boninitic samples of the gadwal sequence have ehf 8 3 and 8 7 and are more radiogenic than average depleted mantle for the time period 3 2 to 2 5 ga ehf 4 to 6 early perhaps hadean differentiation events that led to a depleted and heterogeneous mantle are apparent in the nd and hf isotope systematics of 3 7 3 8 ga isua supracrustal rocks the radiogenic hf isotopes of the gadwal boninites and the hf nd isotope systematics of rocks from other locations in the 3 4 to 2 5 ga time period are consistent with the survival of fragments of an early depleted mantle later in the archean from 2 0 ga to present the time integrated 176lu 177hf and 147sm 144nd of the depleted mantle appears nearly constant and similar to the present day average morb source these data indicate that progressive elimination of early 4 5 ga formed heterogeneities in the depleted mantle dominated the history of the archean mantle and that portions of early depleted reservoirs survived through the mesoarchean these results have implications for the mixing scales for the early terrestrial mantle and the timing of the initiation of present day plate tectonics

constraints from zircon u pb ages o and hf isotopic compositions on the origin of neoproterozoic peraluminous granitoids from the jiangnan fold belt south china zircon textures and micro chemical compositions precisely record the origin and petrogenesis of granitoids which are crucial for evaluating crustal growth and reworking thermal and geodynamic evolution zircons in peraluminous granitoids from the three largest 820 ma complexes guibei yueyang and jiuling in the jiangnan fold belt in south china are used to constrain their sources and petrogenetic processes zircons in the guibei granitoids have complex internal structures nearly all magmatic and inherited zircons have similar ehf 6 8 to 5 6 and 18o values 8 8 11 6 and dominantly lie between ehf evolution vectors for a crust created between 1 7 and 2 1 ga suggesting that the guibei granitoids were produced by partial melting of recycled heterogeneous supracrustal material however the yueyang granitoids contain zircons with high ehf 0 5 to 9 7 and relatively low 18o values 5 9 8 4 and two stage model ages of 1 1 1 8 ga and thus may have been formed by melting of mafic rocks from the lower crust the jiuling granitoids and their enclaves contain more complex zircons with more variable ehf 7 2 to 9 7 and 18o values 7 0 10 6 and lie along the mixing trend between the above proposed infracrustal and supracrustal granitoids therefore the neoproterozoic peraluminous granitoids in the jiangnan fold belt were produced by melting and mixing of continental crust compared with extremely low 4 and negative 18o values of neoproterozoic igneous zircons formed in its northern active continental margin the high 18o peraluminous granitoids in the southeastern yangtze block are considered to have been formed by melting of hydrothermally unaltered continental crust triggered by asthenosphere upwelling in the nanhua back arc basin

significant effect of sediment cohesion on delta morphology delta morphology is thought to be controlled by factors such as river discharge tides and waves numerical modelling shows that sediment cohesion also strongly influences the development of a delta s characteristics the morphologies of the world s deltas are thought to be determined by river discharge tidal range and wave action 1 more recently sea level rise 2 3 and human engineering4 have been shown to shape delta evolution the effects of factors such as sediment type and the overall amount of sediment carried by rivers are considered secondary4 5 6 in particular the role of sediment cohesion which is controlled by sediment size and type of vegetation is unclear here we use a numerical flow and transport model7 8 9 10 to show that sediment cohesiveness also strongly influences the morphology of deltas we find that holding all other factors constant highly cohesive sediments form bird s foot deltas with rugose shorelines and highly complex floodplains whereas less cohesive sediments result in fan like deltas with smooth shorelines and flat floodplains in our simulations sediment cohesiveness also controls the number of channels that form within the deltas and the average angle of bifurcation of those channels as vegetation generally acts as a cohesive agent we suggest that deltas that formed before the expansion of land plants in the devonian period should show fan like characteristics a finding consistent with the limited data from the sedimentological record11

anabranching rivers their cause character and classification anabranching rivers consist of multiple channels separated by vegetated semi permanent alluvial islands excised from existing floodplain or formed by within channel or deltaic accretion these rivers occupy a wide range of environments from low to high energy however their existence has never been adequately explained they occur concurrently with other types of channel pattern although specific requirements include a flood dominated flow regime and banks that are resistant to erosion with some systems characterized by mechanisms to block or constrict channels thereby triggering avulsion the fundamental advantage of an anabranching river is that by constructing a semi permanent system of multiple channels it can concentrate stream flow and maximize bed sediment transport work per unit area of the bed under conditions where there is little or no opportunity to increase gradient on the basis of stream energy sediment size and morphological characteristics six types of anabranching river are recognized types 1 3 are lower energy and types 4 6 are higher energy systems type 1 are cohesive sediment rivers commonly termed anastomosing with low w d ratio channels that exhibit little or no lateral migration they are divisible into three subtypes based on vegetative and sedimentary environment type 2 are sand dominated island forming rivers and type 3 are mixed load laterally active meandering rivers type 4 are sand dominated ridge forming rivers characterized by long parallel channel dividing ridges type 5 are gravel dominated laterally active systems that interface between meandering and braiding in mountainous regions type 6 are gravel dominated stable systems that occur as non migrating channels in small relatively steep basins anabranching rivers represent a relatively uncommon but widespread and distinctive group that because of particular sedimentary energy gradient and other hydraulic conditions operate most effectively as a system of multiple channels separated by vegetated floodplain islands or alluvial ridges

the role of floodplain width and alluvial bar growth as a precursor for the formation of anabranching rivers abstract anabranching rivers are defined as a system of multiple channels separated by stable alluvial islands while substantial progress has been made in developing a physics based understanding of what drives the differences between meandering and braided river channels anabranching rivers have not been well integrated into these models here we propose that alluvial bar growth on the floodplain may be a precursor for the formation of anabranching rivers alluvial bar growth strongly depends on the aspect ratio of the flow width divided by depth and rivers with wide floodplains have flows with a large aspect ratio consistent with this idea remotely sensed measurements of floodplain width of four rivers from different climatic and tectonic settings show that anabranching river patterns are often associated with relatively wide floodplains to explore the physics behind that empirical relationship we carried out two sets of morphodynamic numerical simulations using boundary conditions from field scale modern anabranching rivers spanning different climatic and geologic settings as well as hypothetical floodplain widths results from the simulations show that for a given flood discharge widening the floodplain changes the river pattern from single channel to multi threaded with mobile bars to multi channeled with immobile islands multi channeled patterns arise because the emergence of bars causes flow bifurcation subsequent bifurcation instability leads to the emergence of multiple stable channels as the channels increase their cross sectional area shields stresses on intervening bars are reduced until the bars stabilize into islands we suggest that the presence of stable islands allows vegetation to grow or cohesive sediment to accumulate leading to enhanced channel bank strength and a stable anabranching pattern our results suggest that alluvial bar growth can be a precursor to formation of anabranching rivers compared with field measurements our simulations accurately predict the number of active channels to within 20 which seems to support the idea that some anabranching rivers may originate from alluvial bar growth

hafnium isotopic heterogeneity in zircons from granitic rocks geochemical evaluation and modeling of zircon effect in crustal anatexis abstract we carried out a geochemical evaluation and modeled the mechanism responsible for varied hf isotopic ratios in magmatic zircons of a single granitic rock specimen five representative granitic samples were selected from southern china based on preliminary hf isotopic data our new dataset of zircon hf isotopes confirmed significant hf isotope variations 5 9 epsilon units for each sample and these zircons show roughly positive th u versus t crystallizing temperature correlations while the lu hf variation is independent from t in addition some zircons show significantly higher ti concentrations in the rims compared to the interiors implying reverse thermal zonation based on the ti in zircon thermometry these geochemical features in zircons suggest open system processes which may have resulted from frequent replenishment we modeled zircon dissolution during crustal anatexis to reveal the hf isotopic evolution in the extracted melts the model suggests that the extracted melts may have extremely variable hf isotope compositions 20 epsilon units if the bulk zr concentration in the source is initially above 100 ppm and the melting is rapid 10 4 yr 1 the decoupled release of zircon hf and non zircon hf from a single crust derived magma source can lead to significant hf isotope variations in a solidified granitic body this work provides an alternative explanation for the hf isotopic heterogeneity in magmatic zircons of granitic rocks which is in contrast to the common interpretation by mixing with mantle derived magmas

in situ u pb and nd hf sr isotopic investigations of zirconolite and calzirtite abstract although the calcium and titanium bearing zirconsilicates zirconolite and calzirtite are common minerals in carbonatites little is known regarding their trace element or isotopic characteristics available data indicate that they can have significant contents of u pb nb ta zr hf rare earth elements rees minor sr and low rb sr and lu hf ratios their compositions indicate that they have potential for u pb age determination together with nd hf and possibly sr isotopic analyses in this study zirconolite and calzirtite from carbonatites occurring at phalaborwa south africa prairie lake canada afrikanda and kovdor kola russia guli siberia russia and jacupiranga brazil were selected for an isotopic analysis by in situ techniques ion microprobe analyses using a cameca 1280 ion microprobe show that reliable pb pb ages can be obtained from these minerals as they have extremely high u concentrations coupled with negligible common pb although these minerals have low rb sr ratios their low sr concentrations in most cases render in situ sr isotopic analyses difficult in contrast their high sm nd and hf concentrations with generally low lu hf ratios permit precise in situ isotopic analysis of nd and hf

rates of magma differentiation and emplacement in a ballooning pluton recorded by u pb tims tea adamello batholith italy abstract geochemical structural field and geochronological data have been used to arrive at very different models for the construction of upper crustal batholiths models for pulsed intrusion of small magma batches over long timescales 1 ma versus transfer of larger magma bodies on shorter timescales predict a different thermal metamorphic and rheological state of the crust highlighting the importance of robust time constraints this study focuses on a well characterized upper crustal intrusion the 15 km2 lago della vacca complex lvc adamello batholith n italy previous studies used structural and petrologic data to argue that the lvc was emplaced through pulsed magma injection and in situ expansion or ballooning of a short lived 105 yr magma chamber we test this model using a dense sampling strategy and high precision id tims u pb geochronology of zircon and titanite combined with hafnium isotope and trace element analyses of the same volume of dated mineral u pb tims tea these data show that the marginal mafic pulses of magma crystallized zircon with primitive hf isotopes and negligible eu anomalies during fractional crystallization and ascent through the crust on 10 30 ka timescales subsequent more felsic pulses yield individual zircon dates spanning as much as 200 ka within single handsamples and restrict the total construction time of the lvc to

fore arc basalts and subduction initiation in the izu bonin mariana system recent diving with the jamstec shinkai 6500 manned submersible in the mariana fore arc southeast of guam has discovered that morb like tholeiitic basalts crop out over large areas these fore arc basalts fab underlie boninites and overlie diabasic and gabbroic rocks potential origins include eruption at a spreading center before subduction began or eruption during near trench spreading after subduction began fab trace element patterns are similar to those of morb and most izu bonin mariana ibm back arc lavas however ti v and yb v ratios are lower in fab reflecting a stronger prior depletion of their mantle source compared to the source of basalts from mid ocean ridges and back arc basins some fab also have higher concentrations of fluid soluble elements than do spreading center lavas thus the most likely origin of fab is that they were the first lavas to erupt when the pacific plate began sinking beneath the philippine plate at about 51 ma the magmas were generated by mantle decompression during near trench spreading with little or no mass transfer from the subducting plate boninites were generated later when the residual highly depleted mantle melted at shallow levels after fluxing by a water rich fluid derived from the sinking pacific plate this magmatic stratigraphy of fab overlain by transitional lavas and boninites is similar to that found in many ophiolites suggesting that ophiolitic assemblages might commonly originate from near trench volcanism caused by subduction initiation indeed the widely dispersed jurassic and cretaceous tethyan ophiolites could represent two such significant subduction initiation events

channel patterns braided anabranching and single thread a new channel pattern classification is presented based on theoretically derived channel pattern discriminant functions the thresholds are formulated as power laws that relate the critical slope associated with a change in channel pattern to dimensionless discharge and relative bank strength one threshold demarcates the boundary between stable single thread channels both straight and meandering and stable multiple thread channels anabranching another threshold separates anabranching from braided channels where braided channels are considered to be fundamentally unstable the exponents of the thresholds are nearly identical to that in the threshold equation originally proposed by leopold and wolman 1957 and the coefficients are quite similar as well an analysis of their data set using our dimensionless approach reveals that no fundamental difference exists between meandering and straight patterns and thus data from both types are grouped together under the more general heading of single thread channels furthermore we demonstrate that over a limited range of conditions an unstable single thread channel can form stable multiple threads but that for systems far from the threshold bounding the single thread channels the number of divisions required to produce stable anabranches grows geometrically this motivates a separation of multiple thread channels into anabranching and braided types our theoretical thresholds are then compared against several large data sets of field data and the results broadly confirm our proposed thresholds

palaeozoic landscapes shaped by plant evolution fluvial landscapes diversified markedly over the 250 million years between the cambrian and pennsylvanian periods the diversification occurred in tandem with the evolution of vascular plants and expanding vegetation cover in the absence of widespread vegetation landscapes during the cambrian and ordovican periods were dominated by rivers with wide sand beds and aeolian tracts during the late silurian and devonian periods the appearance of vascular plants with root systems was associated with the development of channelled sand bed rivers meandering rivers and muddy floodplains the widespread expansion of trees by the early pennsylvanian marks the appearance of narrow fixed channels some representing anabranching systems and braided rivers with vegetated islands we conclude that the development of roots stabilized the banks of rivers and streams the subsequent appearance of woody debris led to log jams that promoted the rapid formation of new river channels our contention is supported by studies of modern fluvial systems and laboratory experiments in turn fluvial styles influenced plant evolution as new ecological settings developed along the fluvial systems we suggest that terrestrial plant and landscape evolution allowed colonization by an increasingly diverse array of organisms

reactive bulk assimilation a model for crust mantle mixing in silicic magmas bulk assimilation of small millimeters to 1 km fragments of crust driven and ultimately masked by reactions during xenolith melting and magma crystallization is an important mechanism for crust mantle mixing xenoliths containing mica or amphibole undergo dehydration melting when incorporated into a host magma yielding mainly plagioclase pyroxene fe ti oxides and hydrous melt the xenolith is physically compromised by partial melting and begins to disintegrate xenolithic melt and crystals are mixed into the host magma xenocrystic zircon is liberated at this stage the cryptic character of assimilation is greatly enhanced in any hydrous magma by hydration crystallization reactions the reverse of dehydration melting all pyroxenes and oxides phenocrysts xenocrysts or crystals having a hybrid signature will be subject to these reactions producing feldspars amphiboles and micas that incorporate material from several sources a particularly effective mixing mechanism implicit in the model is a reduced energy penalty for bulk assimilation much of the assimilant remains in solid form compared to melt assimilation models a large role for bulk assimilation supports stoping as a credible mechanism for the ascent of magmas while the assimilation of low density crust and concomitant fractionation provide the isostatic impetus for ascent the wholesale incorporation and processing of crustal rocks in the magma chamber helps create the room for ascent

an investigation of dissolution methods for lu hf and sm nd isotope studies in zircon and garnet bearing whole rock samples 1 the distinct chemical behavior of lu and hf can limit complete sample dissolution and spike sample equilibration here we investigate the effects that four dissolution methods hotplate low temperature microwave high temperature microwave and steel jacketed parr bomb have on the sm nd and lu hf isotope compositions of rocks any of the dissolution methods produce high quality sm nd isotope data and hotplate dissolution of young volcanic rocks produces adequate lu hf data however low temperature microwave and hotplate dissolution methods produce significantly different lu hf and 176hf 177hf ratios as compared to parr bomb dissolution data for old samples or those that contain garnet rutile or zircon a characteristic feature of incomplete spike sample equilibration and sample dissolution for garnet bearing samples is anomalously high 176lu 177hf ratios relative to 176hf 177hf ratios our results show that for lu hf isotope studies of rocks of any significant age the errors in calculated initial hf values may exceed 10 hf units if complete dissolution and spike sample equilibration are not attained for garnet or zircon bearing rocks high temperature and pressure dissolution such as that which can be achieved through parr bomb dissolution is critical to ensure complete dissolution of refractory phases such as garnet or zircon

zircon growth and recycling during the assembly of large composite arc plutons abstract concordant u pb zircon dates have been interpreted traditionally to date the crystallization ages of plutons because until recently analytical uncertainties have generally been large enough to encompass the anticipated duration of pluton growth advances in zircon tims analysis and geochronological studies by sims on zircon from young volcanic rocks along with evolving views of the evolution of magmatic systems makes it necessary to revaluate this interpretation assignment of unique pluton crystallization ages based on zircon dating is complicated by the propensity of zircon to survive multiple intrusive events that culminate in a large pluton distinctions must be made between true inherited zircon xenocrystic zircon derived from host rocks during magma ascent and or emplacement and multi stage growth of zircon during waxing and waning of magma reservoirs that solidify to form large plutons these complications are exemplified by recent high precision u pb zircon age dating from the cretaceous tuolumne and mt stuart batholiths which clearly show that the timeframe of pluton assembly was long 5 ma for mt stuart and 8 ma for tuolumne zircon crystals from samples from both batholiths exhibit appreciable concordant age dispersion for zircon several 10 5 yr up to 2 10 6 yr and assignment of statistically valid rock solidification ages is not possible from these data low zr in the rocks of each batholith indicates that magmas were initially strongly undersaturated in zircon when emplaced and inherited zircon is rare or absent recycling of zircon antecrysts during successive magmatic injections is the primary cause of the modest age dispersion of concordant zircon ages and is compatible with progressive growth of a large long lived crystal mush body because eruption of magma requires low to moderate crystal fraction and high temperatures relative to stored crystal mushes zircon crystals are more likely to be eradicated during thermal rejuvenation preceding major eruptions

a catchment scale assessment of anabranching in the 143 000 km2 fitzroy river catchment north eastern australia this paper presents a catchment scale investigation of anabranching in the moderately large 143 000 km2 fitzroy river catchment in north eastern australia the primary aim is to determine whether mapped and remotely sensed data can provide useful information about the characteristics of anabranching in a catchment of this scale anabranching comprises 6 of the total channel network by length and 24 of higher order channels those with catchment areas over 100 km2 three anabranching planform morphologies are described which occur in geographically distinct regions and add to previous descriptions of anabranching rivers sinuosity and link length are calculated for all channels of the mapped channel network a link is a stretch of river between two stream junctions slope mean floodplain width mean annual rainfall and underlying and catchment geology parameters are calculated for links of a stream network derived from a digital elevation model dem in which each link has either a single channel or anabranching morphology anabranching and single channel links do not occupy different ranges of attribute values and a logistic regression analysis was unable to predict anabranching however slope catchment area mean floodplain width length and sinuosity parameters all have significantly different means when comparing single channel with anabranching links although it is shown that the difference in mean floodplain widths is the result of its correlation with catchment area anabranching channels have a tendency towards shorter link lengths lower sinuosities and lower valley slopes and occur at larger catchment areas than single channels these differences are discussed in the context of published hypotheses regarding the cause of anabranching however the spatial resolution and precision of our data limit our ability to investigate controls on anabranching which will require detailed measurement of variables at a reach scale

why some alluvial rivers develop an anabranching pattern anabranching rivers have been identified globally but a widely accepted and convincing theoretical explanation for their occurrence has remained elusive using basic flow and sediment transport relations this study analyzes the mechanisms whereby self adjusting alluvial channels can anabranch to alter their flow efficiency sediment transport capacity per unit of stream power it shows that without adjusting channel slope an increase in the number of channels can produce a proportional decrease in flow efficiency a finding particularly relevant to understanding energy consumption in some braided rivers however anabranching efficiency can be significantly increased by a reduction in channel width as occurs when vegetated alluvial islands or between channel ridges form the counteracting effects of width reduction and an increasing number of channels can cause with no adjustment to slope an otherwise unstable system underloaded or overloaded to achieve stability as with other river patterns anabranching can be characterized by stable equilibrium or accreting disequilibrium examples copyright 2007 by the american geophysical union

formation and stabilization of the dunhuang block nw china constraints from the late paleoproterozoic a type granites of the dunhuang complex abstract the geochemical composition and temporal evolution of a type granites may hold key information regarding critical geodynamic processes that occurred during the evolution of the continental crust moreover these aspects may provide an indication of when orogenic processes ended and craton formation stabilization began in early precambrian blocks in this study we report the results of zircon u pb ages hf isotope data and whole rock geochemistry for the proterozoic huoyanshan potassic granites in the dunhuang block northwest china zircon u pb dating reveals that the huoyanshan granites were emplaced at ca 1791 1786 ma representing a late paleoproterozoic magmatism event the granites have high sio2 70 71 72 20 wt total alkali contents k2o na2o 8 81 8 96 wt k2o na2o 1 57 1 66 tio2 mgo ratios 2 19 2 57 low cao 0 77 1 27 wt mgo contents 0 12 0 16 wt and mg values 7 72 8 67 with a cnk ratios of 0 96 1 04 pointing to alkaline and metaluminous to weakly peraluminous affinities these granites are characterized by high total rare earth element concentrations ree 419 755 ppm and variable la yb n values 8 24 with significantly negative eu anomalies eu eu 0 31 0 42 the primitive mantle normalized trace element patterns suggest that the huoyanshan plutons are depleted in ba nb ta sr p and ti but enriched in rb th and pb additionally the granites have a high zirconium saturation temperature of 855 897 c with an average of 878 c these geochemical characteristics indicate that huoyanshan potassic granites are an aluminous a type granite and further classified as a2 type together with the zircon hf isotope data ehf t 11 21 to 2 76 indicates that these granites were generated from a crustal source the heterogeneous hf isotope compositions in zircons are principally caused by the disequilibrium melting of zr bearing sources in addition two stage hf model ages of zircons from the granites are ca 2 6 3 1 ga and plot on the average crustal evolution trend of archean tonalite trondhjemite granodiorite ttg gneiss 2 5 to 3 0 ga in the dunhuang block thus we propose that a type granites in the study area were generated from the disequilibrium partial melting of archean tonalitic to granodioritic rocks under high temperature and low pressure conditions according to their low al2o3 cao na2o k2o molar values and high 10000 ga al values 2 9 subsequently the magma experienced prominent fractional crystallization of k feldspar plagioclase orthopyroxene and or magnetite our results with available data suggest that the huoyanshan granites formed in a tectonic transform regime within a collisional to extensional setting and serve as a precursor for the formation and stabilization of the dunhuang block during the late paleoproterozoic furthermore it is probable that the post collisional magmatism event at ca 1 79 ga in the dunhuang block was closely related to the assembly of the columbia supercontinent

evolution of fixed channel alluvial plains in response to carboniferous vegetation the establishment of terrestrial plants during the palaeozoic era was one of the most significant changes to the earth system during the phanerozoic eon the continuing evolution and expansion of land plants irrevocably altered the alluvial landscape the broad unconfined and unconsolidated blankets of coarse sediment found at the start of the cambrian period were replaced by a diverse array of braided and meandering channel styles and stable floodplains by the end of the devonian period here we show that the first appearance of a sedimentary facies suite attributed to low energy organic rich river systems with multiple channels and stable alluvial islands known as anabranching or anastomosing rivers occurred during the carboniferous period our field studies and literature review demonstrate that the appearance of these rivers coincides with the continuing evolution of tree like plants we suggest that increased floodplain stability and new triggers for channel avulsion were provided by the increase in complexity and diversity of root assemblages density of floodplain forests and production of woody debris associated with expansion of arborescence we conclude that the expansion of tree habitats led to the crossing of a threshold in vegetative control of floodplain and river morphology during the carboniferous

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

petrogenesis and geochemical diversity of late mesoproterozoic s type granites in the western yangtze block south china co entrainment of peritectic selective phases and accessory minerals abstract deciphering the geochemical diversity of s type granites is crucial for obtaining more profound insight into their petrogenesis we therefore undertook an integrated study of whole rock geochemistry sr nd isotopes and zircon u pb hf isotopes for newly recognized late mesoproterozoic s type granites including two mica biotite garnet bearing two mica granites from the western yangtze block south china the crystallization ages of these granites are ca 1040 ma weighted mean 206pb 238u age 1036 4 4 5 ma to 1042 2 1 1 ma they are peraluminous to strongly peraluminous a cnk molar ratio of al2o3 cao na2o k2o 1 02 1 67 high k calc alkaline rocks and display high concentrations of normative corundum 0 54 7 04 wt as well as positive correlations of a cnk and feot mgo values which are characteristics of s type granites these s type granites are characterized by enriched in rb th k and pb depleted in ba sr ti and eu with negative whole rock end t 0 3 to 6 8 and predominantly negative zircon ehf t values 8 09 to 5 70 indicating the affinity of middle upper crustal trends and a heterogeneous metasedimentary source compared with the geochemical diversity of s type granites around the world the variably negative end t values as well as positive and negative ehf t values of our s type granites may be caused by source heterogeneity and disequilibrium melting processes more importantly similar to typical more mafic s type granites from the cape granite suite south africa and north queensland australia the high and variable feot mgo contents 2 21 6 64 wt are significantly attributed to coupled co entrainment of peritectic and accessory minerals e g garnet ilmenite zircon and monazite evidenced by positive relationships between feot mgo and tio2 cao zr th hf y yb light rare earth elements lrees in conjunction with the existence of coeval rift related a type igneous rocks depleted asthenosphere derived mafic rocks marine sedimentary sequences and subsequent extensive neoproterozoic arc magmatism along the western yangtze block we infer that the ca 1040 ma s type granites studied here were produced in a continental rifting basin that was transformed into a compression setting during the neoproterozoic

late triassic intrusive complex in the jidong region jiamusi khanka block ne china geochemistry zircon u pb ages lu hf isotopes and implications for magma mingling and mixing abstract whole rock major and trace element geochemistry together with zircon u pb ages and lu hf isotope compositions are reported for a late triassic intrusive complex in the jidong region jiamusi khanka block ne china zircon u pb dating yields ages between 211 and 208 ma for enclaves of microgranular diorite and quartz diorite and between 211 and 209 ma for the host granitoids these ages correlate with a previously established intensive late triassic magmatic event along the eastern asian margin field observations together with petrographic features geochemistry and zircon hf isotope data preclude simple crystal fractionation or restite unmixing as a genetic link for the various rock types within the intrusive complex the syenogranite suite has high sio 2 75 5 76 3 wt and low mgo 0 15 0 19 wt and yields enriched lile and lree patterns most of the zircons in the syenogranites have two stage model ages of 766 and 1461 ma together with positive e hf t values of 0 6 to 9 1 these results indicate that the granitoid magmas were generated by partial melting of meso to neoproterozoic lower crust the gabbro suite has a restricted range of sio 2 46 1 51 9 wt together with high mg values 49 70 and high concentrations of ni co and cr zircons from two diorite samples have single stage hf model ages of 557 787 ma and e hf t values of 1 9 to 8 3 that are consistent with the coeval gabbros previously studied in the jidong region these features together with the observation that all the gabbros are enriched in lree and lile suggest that the mafic magmas were derived from melting of depleted neoproterozoic lithospheric mantle that had been metasomatized by slab derived fluids it is concluded that the dominant igneous suites within the late triassic intrusive complex formed by mingling mixing of felsic and mafic magmas the geochemical data combined with regional geological investigations indicate that the late triassic intrusive complex formed during lithospheric extension caused by slab break off of the yuejinshan oceanic plate that was subducting westward beneath the jiamusi khanka block

magmatic evolution and crustal recycling for neoproterozoic strongly peraluminous granitoids from southern china hf and o isotopes in zircon zircons can retain a grain scale record of granitoid compositional evolution that is accessible through microanalysis in situ u pb hf and o isotope data yield new insights into the petrogenesis and evolution of the neoproterozoic strongly peraluminous granitoids of the jiangnan orogen jo southern china a negative correlation of th u versus d 18 o is found for most analyses some zircons from eastern jo granitoids show d 18 o variations of 3 6 from core to rim indicating a dramatic shift toward higher oxygen isotope values by voluminous partial melting of supracrustal rocks and signaling a transition from i type like to s type like magmas during the later stage of magmatic evolution this mechanism provides a reasonable explanation why some granitoids have intermediate geochemical features between s type and i type granites hf isotope trends indicate that a larger proportion of mature continental crust was incorporated into the magma sources of the western jo granitoids whereas more juvenile arc crust was incorporated into the eastern jo magmas no significant depleted mantle derived mafic magma was injected into the jo granitoid magmas instead radiogenic hf and nd signatures in jo granitoids reflect incorporated juvenile arc crust and document crustal growth in southern china during the early neoproterozoic ca 900 ma thus our zircon data suggest that strongly peraluminous granitoids which are widely regarded as the products of orogenesis that primarily recycle evolved crust can also record important information about early crustal growth

do hf isotopes in magmatic zircons represent those of their host rocks abstract lu hf isotopic system in zircon is a powerful and widely used geochemical tracer in studying petrogenesis of magmatic rocks and crustal evolution assuming that zircon hf isotopes can represent initial hf isotopes of their parental whole rock however this assumption may not always be valid disequilibrium partial melting of continental crust would preferentially melt out non zircon minerals with high time integrated lu hf ratios and generate partial melts with hf isotope compositions that are more radiogenic than those of its magma source dissolution experiments with hotplate bomb and sintering procedures of zircon bearing samples demonstrate this disequilibrium effect where partial dissolution yielded variable and more radiogenic hf isotope compositions than fully dissolved samples a case study from the neoproterozoic jiuling batholith in southern china shows that about half of the investigated samples show decoupled hf isotopes between zircons and the bulk rocks this decoupling could reflect complex and prolonged magmatic processes such as crustal assimilation magma mixing and disequilibrium melting which are consistent with the wide temperature spectrum from 630 c to 900 c by ti in zircon thermometer we suggest that magmatic zircons may only record the hf isotopic composition of their surrounding melt during crystallization and it is uncertain whether their hf isotopic compositions can represent the primary hf isotopic compositions of the bulk magmas in this regard using zircon hf isotopic compositions to trace crustal evolution may be biased since most of these could be originally from disequilibrium partial melts

waterhole form and process in the anastomosing channel system of cooper creek australia one of the most distinctive features of cooper creek s anastomosing channel system is the preponderance of waterholes enlarged segments of channel ranging in length from 100 m to over 20 km enlargement occurs in both the width and depth dimensions to give bankfull cross sectional areas four to eight times the average size of individual anastomosing channels the anastomosing system is cut into a floodplain of subtle relief but depths of incision can be highly variable giving rise to changes in the degree of anastomosis as different sets of channels are activated during the filling stage of a flood also cross sectional form and bed topography can vary markedly over short channel distances which has implications for local flow conditions and erosive potential hydraulic geometry analyses suggest that cross sectional area adjusts much more readily than velocity to increasing discharge in waterholes particularly if there is limited lateral confinement consequently velocities tend to be rather modest across a wide spectrum of flows with average values rarely reaching 1 m s 1 nevertheless observations made during a 1 20 year flood reveal a deeply penetrative band of relatively high velocity in one waterhole suggesting that localized values of bed shear could be quite large even when sectional averages are moderate waterholes appear to be a product of the present regime since they concentrate flow from both feeder channels and the floodplain during flood discharges in several respects they resemble chains of ponds morphology eyles r j 1977 birchams creek the transition from a chain of ponds to a gully aust geogr stud 15 146 157 and could represent the discontinuous sections of a present day channel which cannot be maintained in that form over its entire length that the one waterhole to accommodate the entire cross valley flow at bankfull stage meringhina waterhole has cross sectional dimensions similar to those predicted by appropriate regime equations lends support to this argument essentially waterholes are self maintaining scour features which play a significant hydrological and geomorphological role in the cooper creek anastomosing system they provide long term water storage in an arid environment and their fixed position helps to maintain the stability of the multi channel pattern

the flaw in the crustal zircon archive mixed hf isotope signatures record progressive contamination of late stage liquid in mafic ultramafic layered intrusions

timing and origin of migmatitic gneisses in south karakoram insights from u pb hf and o isotopic record of zircons abstract the timing and origin of partial melting in collision belts is crucial to understand the thermotectonic evolution and the relationship between ht metamorphism and magmatism in over thickened crust in the present study we used the in situ isotopic hf o and u pb record of zircons to investigate the timing and origin of migmatitic gneisses exposed in the core of the dassu dome in south karakoram the new u pb zircon dating identified the proterozoic inherited cores 1 8 1 9 ga and 2 3 2 5 ga surrounded by a neogene overgrowth with ages ranging from 6 to 20 ma these ages imply that the partial melting in the karakoram metamorphic complex lasted from 20 ma to 6 ma and can be correlated with the miocene magmatism in the adjacent baltoro region oxygen isotopic data from proterozoic inherited cores 1 8 1 9 ga and neogene overgrowths are indistinguishable and generally vary from 8 to 9 5 these values are slightly higher than the most igneous zircons 6 5 8 valley et al 2005 indicating an igneous precursor with heavy initial o composition that later might have equilibrated with low temperature environment or some involvement of supracrustal material is likely however a few low u th relatively old inherited cores 2 3 2 5 ga showed mantle like 18o 5 3 0 6 valley et al 2005 values of 18o 5 5 2 7 the present day weighted mean ehf 0 of the proterozoic inherited cores ranges from 50 1 0 to 44 3 1 2 in contrast the neogene rims are 15 20 e units higher than the inherited core with present day ehf 0 30 6 0 9 this implies that the hf composition of the neogene overgrowth is not controlled exclusively by the dissolution of the inherited cores and that contamination by external melts is likely we suggest a contribution from the neogene less evolved magmatism in the baltoro region ehf 0 4 to 10 the elevated oxygen composition is not consistent with the contribution from pristine mantle derived magmas the observed homogeneous and uniform hf o isotopic composition of the proterozoic inherited cores suggest their derivation from mildly evolved infracrustal sources with minor input from supracrustal material the older inherited zircons 2 3 2 5 ga were precipitated from juvenile mantle derived magmas

the sedimentology and alluvial architecture of a large braid bar rio parana argentina abstract this paper presents 30 km of ground penetrating radar gpr data from a mid channel bar in the sixth largest river in the world the rio parana argentina gpr profiles with depth of penetration up to 12 m below the bar surface were collected from a sandy braid bar 3 km long by 1 km wide on a grid with a 200 to 400 m spacing two facies were found to dominate the sedimentary architecture the principal facies 83 of total facies comprises trough and planar cross strata related to the migration of dunes with the thickness of the cross strata decreasing towards the bar surface the second significant facies 15 is high angle generally 10 20 strata that typically form by accretion at the bar margins or bartail clay drapes the rio parana gpr surveys are compared with other gpr studies of sandy braid bars of different sizes from the south saskatchewan wisconsin and jamuna rivers the dominance of dune deposits is ubiquitous to all rivers with each also possessing a significant proportion of large scale high angle strata however two differences were found to exist between the deposits of these rivers 1 the compound bar deposits of smaller rivers contained greater proportions of the fills of cross bar channels which suggests a potential role for discharge as a factor in shaping the alluvial architecture through its impact on the frequency of sediment reworking over the bar tops and 2 the thickness of large scale high angle sets decreases with the age of the bar which suggests that the deposits of older bars may provide more useful geometrical analogues for interpreting ancient successions than smaller transient or recent bar forms that have undergone only limited modification

protracted near solidus storage and pre eruptive rejuvenation of large magma reservoirs building super eruptive magma reservoirs in the cold upper parts of earth s crust requires a significant influx of magma over an extended period sufficient to allow the magma to accumulate differentiate and periodically erupt some models favour magma storage in a cold non eruptible state requiring extensive reactivation of the reservoirs before eruption whereas others suggest storage at higher temperature and lower crystallinity implying that magma in such reservoirs is readily eruptible consequently constraining volcanic hazards requires observations directly linking magma residence timescales to the thermal state and crystallinity of storage here we simultaneously determine crystallization temperatures and ages of magmatic crystals of zircon and titanite in the 900 km3 kneeling nun tuff new mexico usa which allows us to place tight constraints on the long term thermal evolution of the magma reservoir we show that zircon and titanite crystals record more than 600 000 years of magma assembly and constrain the dominant storage conditions to low temperatures set between the granitic solidus 680 to 700 c and the temperature of the onset of titanite crystallization about 720 to 730 c we apply the zircon titanite systematics to a suite of other super eruptions and suggest that protracted low temperature storage culminating in late stage reheating is a widespread feature of large crystal rich eruptions super eruptions are fed by large magma reservoirs geochemical analyses of volcanic rocks erupted in new mexico suggest the magma was stored under cool conditions in the crust for 600 000 years before late stage heating triggered an eruption

discrepancy between bulk rock and zircon hf isotopes accompanying nd hf isotope decoupling abstract zircon is an important accessory mineral for studying the crust mantle interaction and crustal growth through time because zircon crystals not only allow precise dating but also record initial hf isotope ratios of the host magma our study on a suite of gabbronorite mafic diorite cumulate diorite and granite from the kekeli batholith in the north qilian orogenic belt northern tibetan plateau shows 1 a significant hf isotope discrepancy between zircon and their bulk rocks and 2 bulk rock nd hf isotope decoupling these observations thus demonstrate that zircons do not always capture the full history of magmatic system the significant positive correlation between bulk rock hf isotope ratios and tio2 content r2 0 94 indicates that ti rich minerals e g ilmenite amphibole are likely important hf hosts the early formed ti rich minerals possibly record different hf isotopes from those of zircons crystallized subsequently thus causing discrepancy between zircon and bulk rocks and leading to bulk rock nd hf isotope decoupling correlations between bulk rock tio2 content mg isotopes and hf isotopes indicate a mixing process with granite and gabbronorite representing two compositional endmembers because ti minerals have higher crystallization temperatures than zircons when the mixing melts have contrasting isotopes or from heterogeneous sources have a strong crustal contamination the bulk rock and zircon hf isotope discrepancies reflect mineral crystallization sequence during mafic and felsic magma mixing it is thus imperative to consider early formed minerals such as ti rich minerals and the bulk rock composition not just zircons when using hf isotopes to track melt evolution and precisely constrain mantle contribution to granitoid petrogenesis

pb diffusion in zircon abstract chemical diffusion of ti under anhydrous conditions at 1 atm and under fluid present elevated pressure 1 1 1 2 gpa conditions has been measured in natural zircon the source of diffusant for 1 atm experiments was a zro2 tio2 zircon mixture with experiments run in crimped pt capsules diffusion experiments done in the presence of h2o co2 fluid were run in a piston cylinder apparatus using a source of ground tio2 zrsio4 and sio2 with oxalic acid added to produce h2o co2 vapor and partially melt the solid source material yielding an assemblage of rutile zircon melt vapor nuclear reaction analysis nra with the resonant nuclear reaction 48ti p 49v was used to measure diffusion profiles for both sets of experiments the following arrhenius parameters were obtained for ti diffusion normal to cover the temperature range 1350 1550 c at 1 atm d ti 3 33 10 2 exp 754 56 kj mol 1 r t m 2 s 1 ti diffusivities were found to be similar for experiments run under fluid present conditions a fit to all of the data yields the arrhenius relation d 1 34 102 exp 741 46 kj mol 1 rt m2 s 1 these data suggest that zircon should be extremely retentive of ti chemical signatures indicating that the recently developed ti in zircon crystallization geothermometer watson e b harrison t m 2005 zircon thermometer reveals minimum melting conditions on earliest earth science 308 841 844 will be quite robust in preserving temperatures of zircon crystallization titanium diffuses somewhat faster in zircon than larger tetravalent cations u th and hf but considerably more slowly than pb the ree and oxygen hence ti crystallization temperatures may be retained under circumstances when radiometric ages or other types of geochemical information are lost

coupled zircon lu hf and u pb isotopic analyses of the oldest terrestrial crust the 4 03 ga acasta gneiss complex abstract the acasta gneiss complex of the northwest territories canada contains some of the earliest terrestrial continental crust and thus provides a critical sample set for characterization of crust forming processes on the early earth here we report the results of a combined lu hf and u pb isotopic study of zircons from predominantly felsic orthogneisses from the acasta gneiss complex that crystallized between 4 0 and 2 9 ga many of which contain complex zoning and therefore require an analytical treatment suited to distinguish amongst compositionally distinct age and hf isotopic domains to ensure the reliability of the analyses and of subsequent geologic interpretations we employed the laser ablation split stream lass technique to concurrently measure the lu hf and u pb isotopic systems in zircon our results confirm prior findings of precursor hadean crust 4 0 ga in the source of these rocks and the continued involvement of this reservoir until 3 6 ga we present evidence for the input of relatively more juvenile material at 3 6 ga which we suggest corresponds to a fundamental change in the source of the magmas this study extends the lower bound of the published acasta hf isotopic record from 3 6 ga to 2 9 ga and demonstrates that the 3 6 ga 2 9 ga interval is largely represented by reworking of relatively juvenile 3 6 ga crust and the diminution of the 4 0 ga crustal signal significantly there is no evidence that rocks within the acasta gneiss complex were derived from a strongly depleted mantle

multi scale factors controlling the pattern of floodplain width at a network scale the case of the rhone basin france abstract in this study the floodplain width fw was assessed for the entire rhone catchment river network 32 160 km long to highlight controlling factors the fw data which is measured every kilometre on a digital elevation model dtm is aggregated into spatial homogenous segments using a pettitt break detection test resulting in aggregated geographical objects ago based on these agos an analysis of the variability of the floodplain width was performed the general pattern for the rhone network shows a downstream increase in fw related to the concept of river continuum this general pattern explains ca 30 of the variation in fw deviations from this general trend are however large values for floodplain widths are normalized for catchment scale to study deviations that do not depend on scale effects based on a conceptual model the main environmental parameters influencing fw were identified these parameters include lithology and different parameters that influence the river profile such as base level changes and tectonics sediment load and discharge an analysis of catchment wide patterns shows that the main variations on the scaling effect are caused by lithology superposed on this general scaling pattern disturbed by lithological variations are the variations in floodplain width caused by disturbances in the geomorphic system which influence floodplain slope sediment and water discharge and which include inherited landscapes although large data sets become increasingly available and the applied methodology reduces data noise data availability and quality still hampers studies at a larger scale nevertheless such information sources are becoming crucial because they are more and more accessible and diverse and the analysis is now easier with the increasing capacities of gis technologies as a result they are becoming very complementary to field studies to understand geomorphic processes at wider spatial and longer temporal scales in a sense the tradition of geomorphology may evolve in the next years with image data providing additional evidence of earth evolution

assessing impacts of future potential climate change scenarios on aquifer recharge in continental spain abstract climate change will modify the availability of groundwater resources in the future thus the evaluation of average aquifer recharge from precipitation and its uncertainty becomes a key subject in determining suitable countrywide water policies the confident prediction of renewable groundwater resources requires an accurate evaluation of aquifer recharge over time and space especially in large territories with varied conditions for aquifer recharge such as continental spain this study asses impacts of future potential climatic change scenarios on distributed net aquifer recharge nar from precipitation over continental spain for this the used method 1 generates future time series of climatic variables precipitation temperature spatially distributed over the territory for potential aquifer recharge par and 2 simulates them within previously calibrated spatial par or nar recharge models from the available historical information to provide distributed par or nar time series the information employed comes from the spain02 project for the historical climatic data from alcala and custodio 2014 alcala and custodio 2015 for the historical spatial nar and from the cordex eu project 2013 regional climate models rcms simulations for the future climate scenarios a distributed empirical precipitation recharge model is defined by using a regular 10 km 10 km grid and assuming that precipitation p and temperature t are the most important climatic variables determining par while their spatiotemporal variabilities determine the impacts of future potential climatic scenarios on renewable groundwater resources potential plausible pictures of future climate scenarios are defined by combining information coming from different rcms and general circulation models gcms downscaling techniques and ensemble hypothesis these scenarios were simulated within the used precipitation recharge model to estimate impacts on nar the results show that global mean nar decreases by 12 on average over continental spain over 99 8 of the territory a variable degree of recharge reduction is obtained the reduction is quite heterogeneously distributed in line with the variety of conditions for aquifer recharge over continental spain the standard deviation of annual mean nar will increase by 8 on average in the future the dependence of these changes regarding potential explanatory variables such as elevation and latitude was also analysed

importance of the lu hf isotopic system in studies of planetary chronology and chemical evolution the 176lu 176hf isotope method and its applications in earth sciences are discussed greater fractionation of lu hf than sm nd in planetary magmatic processes makes 176hf177hf a powerful geochemical tracer in general proportional variations of 176hf177hf exceed those of 143ndl44nd by factors of 1 5 3 in terrestrial and lunar materials lu hf studies therefore have a major contribution to make in understanding of terrestrial and other planetary evolution through time and this is the principal importance of lu hf new data on basalts from oceanic islands show unequivocally that whereas considerable divergences occur in 176hf177hf 87sr86sr and 143ndl44nd 87sr86sr diagrams 176hf177hf and 143nd144nd display a single linear isotopic variation in the suboceanic mantle these discordant 87sr86sr relationships may allow with the acquisition of further hf nd sr isotopic data a distinction between processes such as mantle metasomatism influence of seawater altered material in the magma source or recycling of sediments into the mantle in order to evaluate the hf nd isotopic correlation in terms of mantle fractionation history there is a need for measurements of hf distribution coefficients between silicate minerals and liquids and specifically for a knowledge of hf behavior in relation to rareearth elements for studying ancient terrestrial hf isotopic variations the best quality hf isotope data are obtained from granitoid rocks or zircons new data show that very u pb discordant zircons may have upwardly biased 176hf177hf but that at least concordant to slightly discordant zircons appear to be reliable carriers of initial 176hf177hf until the controls on addition of radiogenic hf to zircon are understood combined zircon whole rock studies are recommended lu hf has been demonstrated as a viable tool for dating of ancient terrestrial and extraterrestrial samples but because it offers little advantage over existing methods is unlikely to find wide application in pure chronological studies

patterns of anabranching channels the ultimate end member adjustment of mega rivers abstract large fluvial systems adjust to a combination of controls to form distinctive channels which represent a dominant factor in the evolution of floodplain geomorphology and sedimentology fluvial geomorphology has commonly classified river channels into meandering straight and braiding patterns which are seen to represent a continuum of channel geometry anabranching patterns rivers with multiple channels however are characteristic of many rivers the identification of a combination of variables that discriminates specific channel patterns has been a significant focus of research in fluvial geomorphology the development of this body of knowledge however has been established from medium and small rivers and laboratory flume studies very few of these research ideas developed from analysis of large fluvial systems this paper assesses the pattern of channel adjustment of large fluvial systems by employing hydraulic geometry discharge w d slope grain size stream power specific stream power and froude number q mean 1000 m 3 s the study demonstrates that methods currently used to discriminate channel patterns are not useful when applied to very large rivers further with the exception of the lower mississippi alluvial rivers with mean annual discharges greater than 17 000 m 3 s here classified as mega rivers do not generate single thread meandering or typical braided patterns these mega rivers develop anabranching patterns

sediment transport part i bed load transport a method is presented which enables the computation of the bed load transport as the product of the saltation height the particle velocity and the bed load concentration the equations of motions for a solitary particle are solved numerically to determine the saltation height and particle velocity experiments with gravel particles transported as bed load are selected to calibrate the mathematical model using the lift coefficient as a free parameter the model is used to compute the saltation heights and lengths for a range of flow conditions the computational results are used to determine simple relationships for the saltation characteristics measured transport rates of the bed load are used to compute the sediment concentration in the bed load layer a simple expression specifying the bed load concentration as a function of the flow and sediment conditions is proposed a verification analysis using about 600 alternative data shows that about 77 of the predicted bed load transport rates are within 0 5 and 2 times the observed values

fluvial processes morphology and sediment budgets in the coon creek basin wi usa 1975 1993 sediment budget processes in coon creek wisconsin from 1975 to 1993 changed only moderately generally continuing the trends that were evident in 1975 perhaps most importantly sediment yield to the mississippi river appears to be unabated with much of this clearly coming from storage loss especially stream bank erosion vertical accretion continued on the lower new floodplains inset within their meander plains in tributaries and the upper main valley the lower main valley also continued to aggrade but at the rate of about 6 of that occurring in the 1920s and 1930s the major revision from earlier studies is that the upper main valley is a less significant net source of sediment than previously thought and the earlier prognostications of it being a significant net source of sediment in the future may be wrong perturbations have been caused by changes of riparian vegetation and much more importantly by the introduction of fish shelter structures and protected cut banks along the stream system reaching downstream to the end of the upper main valley because these structures do not permit natural stream migration bank erosion and downstream sediment transfer the coon creek basin has lost much of its suitability as a natural laboratory of fluvial processes

a possible linkage between highly fractionated granitoids and associated w mineralization in the mesozoic yaogangxian granitic intrusion nanling region south china abstract the yaogangxian ygx w deposit in the central nanling w province south china is one of the largest w deposits worldwide both vein type wolframite and skarn type scheelite orebodies were exposed in ygx these scheelite and wolframite mineralization are spatially associated with the ygx granitic intrusion the exposed intrusion contains two main intrusive phases lower zone coarse grained two mica granite and upper zone fine grained muscovite granite an integrated study of whole rock geochemistry la icp ms zircon u pb geochronology and zircon hf isotopic geochemistry of the ygx granitic intrusion was undertaken to constrain its magmatic source and petrogenesis cassiterite la icp ms u pb dating was also undertaken to constrain the connection between the intrusion and hydrothermal w mineralization zircon u pb dating yielded weighted mean ages of 161 3 and 157 2 ma for the two mica and muscovite granites respectively cassiterite crystals from wolframite quartz veins display growth zonation under cathodoluminescence and have a weighted mean zircon u pb age of 158 2 ma the formation of the muscovite granite and related w mineralization in the ygx deposit was thus initiated during the early yanshanian the ygx granites display typical metaluminous to strongly peraluminous characteristics with high sio2 73 5 79 1 wt and k2o 3 0 5 0 wt contents they are enriched in heavy rare earth elements rees comparative to light rees notably they exhibited strong negative eu anomalies and are depleted in sr ba p and ti in addition muscovite granites in ygx are more significantly enriched in heavy rees relative to light rees than two mica granites indicating that the former experienced higher degree fractionation than the latter zircon grains of the ygx granitoids have strongly negative ehf t 15 to 10 3 with two stage hf model ages of 2 15 1 86 ga indicating a magma source of reworked ancient crustal material without obvious involvement of juvenile materials the results of petrology and geochemistry indicate that the ygx granitic intrusion is a highly fractionated s type granite emplaced during widespread 160 150 ma granitic magmatism in the nanling region further evolution to late stage for multiphase ygx granitic intrusion is needed to enrich ore forming elements after magmatic emplacement it is suggested that the extensional environment of continental lithosphere caused by the northwest subduction of the paleo pacific plate may play an key role in the genesis of ygx s type granite w rich proterozoic basement rocks and fractional crystallization may provide material for the formation of the granite related tungsten mineralization

the late quaternary evolution of the negro river amazon brazil implications for island and floodplain formation in large anabranching tropical systems abstract the rio negro has responded significantly in the late pleistocene and holocene to lagged environmental changes largely associated with activity during the last glacial in the amazon basin on the basis of geological structure the rio negro can be divided into six distinct reaches that each reflects very marked differential processes and geomorphological styles no deposits of the upper pleniglacial were recognized in the field the oldest recognizable late pleistocene alluvial unit is the upper terrace of middle pleniglacial age ca 65 25 ka reach i tentatively correlated with the oldest terrace identified on the left bank of reach iii at that time the river was mainly an aggradational bed load system carrying abundant quartz sand a product of more seasonal conditions in the upper catchment the late glacial 14 10 ka is represented by a lower finer grained terrace along the upper basin reach i which was recognized in the tiquie curicuriari and vaupes rivers at that time the river carried abundant suspended load as a response to climatic changes associated with deglaciation since about 14 ka the river has behaved as a progradational system infilling in downstream series a sequence of structurally controlled sedimentary basins or compartments creating alluvial floodplains and associated anabranching channel systems reach ii was the first to be filled then reach iii both accumulating mainly sand fine deposits increase downstream in reach iii and become predominant in some anabranch islands of the distal reach the lowermost reaches of the negro v and vi have been greatly affected by a rising base level and associated backwater effect from aggradation of the amazon during late glacial and recent times reach v has acted almost entirely as a fine sediment trap the remarkable anavilhanas archipelago is the product of holocene deposition in the upper part of this sedimentary basin however suspended sediment load declined about 1 5 ka prior to the lower part of this basin becoming infilled the progradational behavior of the rio negro filling tectonic basins as successive sediment traps with sand in the upper basins and fines in the downstream ones illustrates how a large river system responses to profound changes in late quaternary base level and sediment supply the most stable equilibrium conditions have been achieved in the holocene in reaches iib and iiia where an anabranching channel and erosional relictual island system relatively efficiently convey water and sediment downstream reaches iiib and v never achieved equilibrium conditions during the holocene characterised as they are today with incomplete floodplains and open water

formation of a cohesive floodplain in a dynamic experimental meandering river field studies suggest that a cohesive floodplain is a necessary condition for meandering in contrast to braided rivers however it is only partly understood how the balance between floodplain construction by overbank deposition and removal by bank erosion and chutes leads to meandering this is needed because only then does a dynamic equilibrium exist and channels maintain meandering with low width depth ratios our objective is to understand how different styles of floodplain formation such as overbank deposition and lateral accretion cause narrower channels and prevent chute cutoffs that lead to meandering in this study we present two experiments with a self forming channel in identical conditions but to one we added cohesive silt at the upstream boundary the effect of cohesive silt on bank stability was tested in auxiliary bank erosion experiments and showed that an increase in silt reduced erosion rates by a factor of 2 the experiment without silt developed to a braided river by continuous and extensive shifting of multiple channels in contrast in the meandering river silt deposits increased bank stability of the cohesive floodplain and resulted in a reduction of chute cutoffs and increased sinuosity by continuous lateral migration of a single channel overbank flow led to deposition of the silt and two styles of cohesive floodplain were observed first overbank vertical accretion of silt e g levee overbank sedimentation or splays and second lateral point bar accretion with silt on the scrolls and in the swales the first style led to a reduction in bank erosion while the second style reduced excavation of chutes we conclude that sedimentation of fine cohesive material on the floodplain by discharge exceeding bankfull is a necessary condition for meandering

evaluating the construction and evolution of upper crustal magma reservoirs with coupled u pb zircon geochronology and thermal modeling a case study from the mt capanne pluton elba italy abstract evaluating mechanisms and rates for magma transport and emplacement in the upper crust is important in order to predict the thermal and rheological state of the crust and understand the relationship between plutonism and volcanism u pb geochronology on zircon is commonly used to constrain magma emplacement and storage time in the crust but interpreting complex zircon age populations in terms of in situ crystallization versus crystallization at a deeper level is not trivial this study focuses on the mt capanne pluton in elba italy a well documented example of arc related laccolith emplaced in the upper continental crust previous studies proposed that the mt capanne intrusion was accreted in less than 10 000 yr by distinct and mappable magma pulses here we couple high precision id tims u pb zircon geochronology with numerical thermal simulations to evaluate emplacement rates test different emplacement models inform zircon age interpretations and evaluate the potential for melt storage during construction of the mt capanne pluton our results require that the mt capanne intrusion was built in at least 250 000 yr by multiple magma injections a variety of emplacement scenarios show that melt was preserved for

petrogenesis of granitoids from the lachlan fold belt southeastern australia the role of disequilibrium melting abstract s and i type granites from the lachlan fold belt southeastern australia have been investigated to assess the role of disequilibrium melting in their petrogenesis differences between the median initial ehf compositions of magmatic zircon populations and the host bulk rock ehfblk zrc range from 0 6 to 2 5 e units providing evidence for intra sample and hence inter phase hf isotopic heterogeneity linear variations on harker diagrams and o and hf isotope compositions of magmatic zircon preserved in many i and s type granites are inconsistent with assimilation or simple mixing hypotheses in contrast isotopic disequilibrium between the melt and a restite assemblage can explain the bulk rock versus zircon differences observed in these samples assuming that magmatic zircon records the melt composition differences between the bulk rock ehf and ehf of magmatic zircon ehfblk zrc values measured for i type granites 0 4 2 5 can largely be explained by disequilibrium amphibole dehydration melting of meta igneous protoliths that were either isotopically heterogenous at the time they were formed or perfectly homogeneous before being aged in the crust for 0 4 1 0 billion years prior to partial melting the currowong suite exhibits petrographic features and preserves geochemical and isotopic compositions that do not lend themselves to simple restite model or magma mixing explanations however these observations could be explained by the restite unmixing of magma batches generated from a single source rock if as modelling has suggested separate batches contain different melt compositions by investigating the application of disequilibrium melting to granite genesis this study demonstrates that isotopic heterogeneity at various sampling scales should actually be expected for the production of granites from a single source rather than necessitating the involvement of multiple sources and mixing processes as a result great care should be taken in the interpretation of isotope data from granitic bulk rocks or their zircons

the isotopic composition of yb and the precise and accurate determination of lu concentrations and lu hf ratios by isotope dilution using mc icpms 1 we report yb isotopic data for the purpose of accurately correcting measurements of lu and lu hf ratios during multicollector inductively coupled plasma mass spectrometer mc icpms analysis the yb isotopic ratios agree with recent results of chu et al 2002 and amelin and davis 2004 when corrected for mass fractionation to the same reference values protocols for lu measurement involving simultaneous subtraction of yb interference and application of a fractionation correction based on yb are described it is shown that lu concentrations and lu hf ratios determined by mc icpms using this methodology are more precise and potentially more accurate than results based on analysis of lu by thermal ionization mass spectrometry tims a number of mc icpms lu hf isotopic analyses of standard rocks and other geological samples previously analyzed by tims are presented these document reproducibility of lu hf within 0 2 where most discrepancies can be assigned to the unknown lu mass fractionation during tims analysis

dynamic single thread channels maintained by the interaction of flow and vegetation most rivers on earth today flow as a single channel in some cases with occasional islands and follow a more or less sinuous course however single thread channels have proven difficult to reproduce and study experimentally experimental self formed channels tend to widen and subdivide leading to a braided pattern cohesive sediment has been the main mechanism studied for stabilizing banks and producing a single thread channel we show how laboratory experiments using vegetation to stabilize banks can organize the flow and convert the planform morphology from braided to single thread our experimental strategy a repeated cycle of short periods of high water discharge alternating with longer periods of low discharge accompanied by plant seeding and growth leads to the evolution of a dynamic self maintaining single thread channel with well defined banks and floodplain by eliminating weak flow paths the vegetation corrals the water into a single dominant channel until the reduction in total wetted width leads to a new self organized state in which the flow removes vegetated area as fast as it is produced the new channel is deeper and has a broader distribution of depths than the braided one with channel size adjusted to carry almost all the flood flow the resulting system maintains a dynamic steady state via similar mechanisms to those that operate in meandering channels in the field specifically erosion at the outside of bends bend growth and bar development our methodology provides a basis for experimental development of self sustaining high amplitude meanders and has applications for river management and basic research purposes

rethinking the emplacement and evolution of zoned plutons geochronologic evidence for incremental assembly of the tuolumne intrusive suite california new u pb geochronologic data indicate that the tuolumne intrusive suite california was assembled over a period of at least 10 m y between 95 and 85 ma and that the half dome granodiorite intruded over a period approaching 4 m y simple thermal considerations preclude the possibility that a magma chamber the size of the half dome pluton could have existed as a liquid at shallow crustal depths for that long rather field evidence for sheeting along the margins of the suite the range of ages and the regular decrease of ages toward the center of the suite and within individual plutons suggest incremental assembly geochronologic evidence for incremental assembly is consistent with the failure of geophysical methods to detect large magma chambers with more than 20 melt even in active volcanic areas because it is unlikely that the individual plutons composing the tuolumne ever coexisted as liquid rich magmas the chemical evolution of the suite cannot be the result of simple fractionation and or mixing between exposed units but instead must reflect processes occurring during magma generation