synorogenic melting of mafic lower crust constraints from geochronology petrology and sr nd pb and o isotope geochemistry of quartz diorites damara orogen namibia quartz diorites represent the earliest ca 540 ma and most primitive plutonic rocks in the pan african damara belt and they pre date the main phase of high t regional metamorphism two suites of synorogenic quartz diorites are unusual among damaran intrusive rocks in their elemental and isotopic features comparison of the diorite compositions with melts from amphibolite dehydration melting experiments points to a garnet bearing meta tholeiite probably enriched in k2o as a likely source rock partial melting processes generated mafic ca 50 wt sio2 quartz diorites in the deep crust at temperatures of between 1 000 and 1 100 c based on comparison with experimental results and similar temperature estimates based on p2o5 solubility in mafic rocks subsequently the quartz diorites evolved by multistage polybaric differentiation processes including fractional crystallization of mainly hornblende and plagioclase and assimilation of felsic basement gneisses although their chemical characteristics high lile low hfse resemble those of other quartz diorites with calc alkaline affinities they differ in their enriched sr initial 87sr 86sr 0 70943 0 71285 nd initial e nd 9 1 to 15 2 and o 18o 6 8 8 1 isotope compositions neodymium model ages tdm that range from 1 7 to 2 2 ga and large variation in 207pb 204pb relative to 206pb 204pb indicates involvement of ancient crustal material lead 206pb 204pb 17 08 17 23 207pb 204pb 15 53 15 62 208pb 204pb 37 71 38 16 isotope compositions are strongly retarded indicating that the source underwent a pre pan african u pb fractionation and u depletion it is proposed that the quartz diorites originated by synorogenic high temperature melting of mafic lower crust this contrasts with previous suggestions favouring an origin of these rocks by melting of an enriched mantle during pan african times with characteristics modified by subduction of oceanic crust and sedimentary rocks

melting of newly formed mafic crust for the formation of neoproterozoic i type granite in the hannan region south china abstract neoproterozoic magmatism in the hannan region at the northwestern margin of the yangtze block is characterized by numerous felsic plutons associated with minor mafic ultramafic intrusions the felsic plutons are either adakitic or normal arc granitic in composition the adakitic plutons are 735 ma in age and are interpreted as having formed by partial melting of a thickened lower mafic crust among the normal arc related felsic plutons the tianpinghe pluton is the largest and has a shrimp zircon u pb age of 762 4 ma older than the adakitic plutons in the region rocks from the tianpinghe pluton have relatively high sio2 67 1 70 1 wt and k2o na2o 7 8 8 6 wt and relatively low mgo 0 7 1 3 wt and al2o3 contents 14 5 15 6 wt with al2o3 cao k2o na2o a cnk values ranging from 0 95 to 1 08 they have arc affinity trace element compositions that are characterized by enrichment of large ion lithophile elements and depletion of high field strength elements nb ta with strong

neoproterozoic adakitic plutons in the northern margin of the yangtze block china partial melting of a thickened lower crust and implications for secular crustal evolution abstract numerous neoproterozoic felsic and mafic ultramafic intrusions occur in the hannan region at the northern margin of the yangtze block among these the wudumen and erliba plutons consist of granodiorites and have shrimp zircon u pb ages of 735 ma the rocks have high k 2 o 0 8 3 6 wt and na 2 o 4 4 6 4 wt and low mgo 0 4 1 7 wt they also have high sr y 32 209 and la yb n ratios 4 4 38 6 their e nd values range from 0 41 to 0 92 and zircon initial 176 hf 177 hf ratios from 0 282353 to 0 282581 these geochemical features are similar to those of adakitic rocks produced by partial melting of a thickened lower crust our new analytical results combined with the occurrence of voluminous arc related mafic ultramafic intrusions emplaced before 740 ma lead us to propose that the crustal evolution in the northern margin of the yangtze block during neoproterozoic involved 1 rapid crustal growth and thickening by underplating of mafic magmas from the mantle which was modified by materials coming from the subducting oceanic slab from 1 0 to 0 74 ga and 2 partial melting of the thickened lower crust due to a thermal anomaly induced by upwelling of asthenosphere through an oceanic slab window producing the 735 ma adakitic wudumen and erliba plutons our model suggests that the crustal thickness was more than 50 km at the northern margin of the yangtze block at 735 ma and rule out the possibility of a mantle plume impact causing the 735 ma magmatism in the region

balancing crustal thickening in arcs by tectonic vs magmatic means igneous intrusions were emplaced prior to and contemporaneous with horizontal shortening of the crust in the late cretaceous to late eocene magmatic arc in north chile 21 45 22 30 s temporally changing major and trace elements of magmatic rocks from this paleo arc system chronicled gradual crustal thickening prior to and substantial crustal thickening contemporaneously with crustal shortening balanced structural cross sections indicate a minimum of 9 km of arc normal shortening that occurred simultaneously with dextral arc parallel movements accounting for orogen parallel lengthening of 10 km this shortening produced 5 4 km of tectonic crustal thickening and resulted in a minimum of 42 km late eocene andean crustal thickness temporal and spatial geochemical changes diagnostic of crustal thickening indicate that the remainder 2 6 km was accommodated by basaltic underplating at or near the base of the arc crust prior to and during transpression the ratio of tectonic to magmatic crustal thickening is 2 1 whole crustal magmatic addition rates during the 12 m y duration of arc transpression are 35 km 3 per kilometer of model arc length per million years mafic underplating may have thickened the andean crust considerably but most pre neogene crustal thickening was due to discrete episodes of tectonic shortening

dehydration melting of amphibolite at 10 kbar the effects of temperature and time we have simulated the dehydration melting of a natural low k calcic amphibolite 67 4 hornblende 32 5 anorthite in piston cylinder experiments at 10 kbar and 750 1000 c for 1 9 days the solidus temperature is lower than 750 c garnet appears at 850 c the overall reaction is hb p l cpx al hb ca hb ga opx three stages of reaction are 1 melting dominated by the growth of clinopyroxene and garnet with little change in composition of liquid or garnet 2 a reversal of this reaction between 875 c and 900 c with decreases in the amounts of liquid and garnet and 3 a large increase in liquid along with the loss of hornblende and decrease of plagioclase while clinopyroxene and garnet increase garnet is enriched in pyrope and zoned from fe cores to mg edges range 3 mol pyrope liquid composition is enriched first in an to 950 c and then in ab the liquids are more calcic and aluminous than natural tonalites which is attributed to the plagioclase composition an90 the formation of peraluminous liquid from the metaluminous amphibolite is caused by anorthite not h2o saturated conditions the results are consistent with an amphibolite phase diagram with relatively high solidus temperatures in the garnet absent field 900 1000 c but with a solidus backbend at 7 9 kbar coincident with the garnet in boundary hornblende breakdown due to garnet formation in a closed system must make h2o available for h2o undersaturated melting right down to the h2o saturated solidus below 700 c which defines a large low temperature pt area where hydrous granitoid melts can be generated with residual garnet and hornblende

tectonic evolution of the qinling orogenic belt central china new evidence from geochemical zircon u pb geochronology and hf isotopes abstract the qinling orogenic belt qob located between the north china craton ncc and the yangtze craton yzc is composed of the northern qinling belt nqb and the southern qinling belt sqb this study presents new geochemical data zircon u pb ages and hf isotopes from a suite of metamorphic rocks from the kuanping qinling danfeng douling and wuguan groups in the nqb and the sqb our results suggest that the qinling group in the nqb formed in an active continental margin related to the subduction of the songshugou ocean under the southern ncc during grenvillian period the kuanping group in the northern nqb was deposited in the within plate extension basin during late neoproterozoic to early paleozoic the detritus of sedimentary rocks of the kuanping group were mainly sourced from the nqb and subordinately from the basement rocks of the ncc the danfeng group in the southern nqb formed later than 827 ma with the sediments derived mainly from the qinling terrane the nqb witnessed multiphase tectono thermal events from mesoproterozoic to early paleozoic including several magmatic events and the early paleozoic metamorphic event the original douling group in the sqb represents the basement complex of the sqb which is composed of rocks with different formation ages the oldest rock in the complex is represented by 3 0 ga granitic gneiss which underwent at least two periods of metamorphism at 2 5 ga and 0 97 0 91 ga in this study the douling group is redefined as a suite of neoproterozoic 860 750 ma rocks formed in an extensional setting after the subduction of the songshugou ocean the wuguan group in the sqb formed in the middle late neoproterozoic later than 740 ma and was subjected to early paleozoic metamorphism our data indicate that the sqb experienced early neoproterozoic 970 865 ma and early paleozoic 444 ma metamorphism and strong middle neoproterozoic 870 730 ma magmatism the age spectra of zircons from the nqb trace major tectono thermal events at 1462 ma 927 ma and 412 ma which are different from those recorded in the southern ncc and northern yzc suggesting that the nqb had a distinct evolution during mesoproterozoic to early neoproterozoic grenvillian the hf isotopic compositions of zircons from the various rocks suggest that the nqb probably developed on the basement of the southern ncc the rocks in the sqb show zircon age spectra and hf isotope compositions similar to those in the northern yzc suggesting a close affinity we therefore interpret the sqb to have developed on the basement of the northern yzc integrating the new data in this study with those from previous works we propose a new tectonic model for the formation and evolution of the qob during late mesoproterozoic to early paleozoic involving the following major events 1 late mesoproterozoic to early neoproterozoic grenvillian northward subduction of the songshugou ocean 2 early middle neoproterozoic 870 800 ma bidirectional subduction and collision 3 middle neoproterozoic 800 710 ma post collision extension 4 middle late neoproterozoic 710 600 ma within plate extension 5 late neoproterozoic early paleozoic 600 520 ma opening of the shangdan ocean and 6 early paleozoic 520 420 ma subduction collision we thus trace at least two distinct wilson cycles in the qob

unexposed archean components and complex post archean accretion reworking processes beneath the southern yangtze block revealed by zircon xenocrysts from the paleozoic lamproites south china abstract integrated u pb dating and hf isotope analysis of zircon xenocrysts from the baifen lamproites of the zhenyuan area in the southeastern guizhou province offer a unique glimpse into the evolution of the deep continental crust beneath the southern part of the yangtze block south china a total of 148 zircons with 90 110 concordance yield u pb ages varying from 3458 9 ma to 252 2 ma they generally have high th u ratios 0 3 indicating a magmatic origin the two youngest magmatic zircons give a weighted mean 206pb 238u age of 253 0 2 8 ma which is interpreted as the emplacement age of the host baifen lamproites most zircons with pre eruption ages are xenocrysts derived from the deeper crust of the southern yangtze block they define two major paleoproterozoic age populations at 2 17 ga and 1 8 ga with similar hf model ages tcrust of 2 4 2 8 ga peak at 2 64 ga the oldest zircon has a 207pb 206pb age of 3458 9 ma with ehf t 1 47 and tcrust 3 7 ga this demonstrates the presence of archean crustal components or remnants as old as 3 7 ga beneath the southern yangtze block magmatic events involving significant addition of juvenile mantle derived materials mainly occurred at 2 2 2 1 ga and 2 0 1 8 ga but the latter event also involved reworking of the neoarchean crust and might represent the cratonization of the yangtze continent during the middle paleoproterozoic which could be correlated to the assembly of the columbia supercontinent this segment of the yangtze block became tectonically quiescent until 1 2 ga implying that the crustal stabilization lasted for ca 600 million years a few late mesoproterozoic zircon xenocrysts within the age range 1 3 1 0 ga suggest the occurrence of grenville aged events these neoproterozoic zircon xenocrysts 992 547 ma have a wide range of ehf t values 8 47 to 22 07 consistent with the zircon hf isotopic compositions of widespread coeval igneous rocks in the yangtze block the data show that the yangtze block has undergone important crustal growth and reworking during the neoproterozoic in response to the breakup of the rodinia supercontinent the early middle paleozoic zircon xenocrysts 513 357 ma have negative ehf t values 2 97 to 12 69 and tcrust hf model ages of 1 59 2 27 ga indicating reworking of the precambrian crustal basement without juvenile contributions during the caledonian orogeny the positive ehf t values 13 79 to 0 22 of twelve late paleozoic zircons 284 264 ma with a major age peak at ca 275 ma suggest that the precambrian basement beneath the southern yangtze block experienced an episode of strong rejuvenation during the permian we propose a possible linkage between the baifen lamproite magmatism 284 252 ma and the emeishan flood basalt volcanism

geochemical constraints on the provenance and depositional setting of neoproterozoic volcaniclastic rocks on the northern margin of the yangtze block china implications for the tectonic evolution of the northern margin of the yangtze block abstract neoproterozoic volcanic and sedimentary rocks cropping out along the chengkou fengxi fault on the northern margin of the yangtze block in china consist primarily of volcaniclastic deposits and sandstone conglomerate successions the volcaniclastic deposits termed the longtanhe formation are composed of volcanogenic sedimentary rocks with several primary volcaniclastic rock interlayers the longtanhe formation provides a good opportunity to investigate the neoproterozoic tectonic evolution of the northern margin of the yangtze block in this paper geochemistry and zircon u pb geochronology are used to infer the provenance and tectonic setting of the longtanhe formation volcaniclastic rocks two volcanic sandstone samples collected from the base and top part of the longtanhe formation respectively and one tuff sample were studied for zircon u pb dating they show three consistent age populations with peaks at 825 ma 770 ma and 703 ma indicating that the longtanhe formation was deposited after 703 ma and there were no changes in the sources during the period of sedimentation chondrite normalized ree patterns are uniform with slight lree enrichment la n yb n average 3 8 and flat hree patterns without a distinct eu anomaly eu eu average 0 99 indicating a magmatic arc source sc v enrichment high la th average 9 4 cr ni average 2 1 and low th sc commonly

modeling basin scale internal waves in a stratified lake basin scale internal waves provide the driving forces for vertical and horizontal fluxes in a stratified lake below the wind mixed layer thus correct modeling of lake mixing and transport requires accurate modeling of basin scale internal waves examining this capability with a hydrostatic z coordinate three dimensional 3d numerical model at coarse grid resolutions is the focus of this paper it is demonstrated that capturing the correct thermocline forcing with a 3d mixed layer model for surface dynamics results in a good representation of low frequency internal wave dynamics the 3d estuary and lake computer model elcom is applied to modeling lake kinneret israel and is compared with field data under summer stratification conditions to identify and illustrate the spatial structure of the lowest mode basin scale kelvin and poincarewaves that provide the largest two peaks in the internal wave energy spectra the model solves the unsteady reynolds averaged navier stokes equations using a semi implicit method similar to the momentum solution in the trim code with the addition of quadratic euler lagrange dis cretization scalar e g temperature transport using a conservative flux limited approach and elimination of vertical diffusion terms in the governing equations a detailed description is provided of turbulence closure for the vertical reynolds stress terms and vertical turbulent transport using a 3d mixed layer model parameterized on wind and shear energy fluxes instead of the convential eddy viscosity diffusivity assumption this approach gives a good representation of the depth of the mixed layer at coarse vertical grid resolutions that allows the internal waves to be energized correctly at the basin scale wind stresses surface heating and density currents form the driving energy fluxes of a stratified lake the basin scale energy flux from the wind is of particular interest because of its dominant role in setting the thermocline in motion which in the absence of inflows and outflows is the primary energy store for transport and mixing below the wind mixed layer thus modeling the basin scale internal wave behavior is an a priori requirement to modeling and quantifying the flux paths of nutrients in a stratified lake imberger 1994 this paper takes a first step in this direction by analyzing our ability to model basin scale internal waves that are seen in lake kinneret israel energy flux path in a stratified lake energy flux through a stratified lake has a fundamental dependence on forced and free baroclinic motions the wind imparts both momentum and turbulent kinetic energy tke to the water in the sur face layer the tke distributes momentum vertically in the

internal waves in a small reservoir horizontally distributed temperature time series recorded in the thermocline of a small reservoir during autumn are used to identify both basin scale and higher frequency internal waves stationary and nonstationary spectral analyses are used to identify dominant frequencies the horizontal distribution of data allows a map of the basin scale wave energy to be determined three layer normal mode modeling incorporating damping emulates the basin scale behavior well this provides estimates of velocity shear and hence the internal stability the nonstationary frequency analysis identifies times of high frequency internal wave activity wave periods of 2 10 min this is definitely linked to the basin scale wave driven shear and hence to deceases in stability estimates of dissipation for the thermocline and hypolimnion are 3 10 8 and 6 10 9 m 2 s 3 respectively the inferred variability in the hypolimnetic value is demonstrated

petrogenesis and geodynamic implications of neoproterozoic gabbro diorites adakitic granites and a type granites in the southwestern margin of the yangtze block south china abstract the occurrence of a type granites following adakitic granites can provide unique insights into the tectonic evolution of subduction zone this paper presents a comprehensive study of zircon u pb hf isotopes and whole rock geochemistry on the neoproterozoic gabbro diorites biotite granites and k feldspar granites in the southwestern margin of the yangtze block south china zircon u pb dating of three distinctive intrusions yield concordant ages of 810 4 2 0 ma for gabbro diorites 800 3 2 1 ma for biotite granites and 749 1 2 0 ma for k feldspar granites the gabbro diorites are sodic and calc alkaline rocks with low sio2 52 62 53 87 wt medium mgo 2 67 3 41 wt high fe2o3t 7 18 7 49 wt and cao 5 68 7 50 wt contents they display high th zr and rb y ratios but low nb zr and nb y ratios together with their positive whole rock end t 1 0 to 1 5 and zircon ehf t values 3 66 to 8 18 we suggest that these gabbro diorites were derived from the partial melting of depleted lithospheric mantle modified by subduction related fluids the biotite granites display relatively high sr contents 335 395 ppm sr y ratios 38 9 54 3 low y 7 04 9 71 ppm and yb contents 0 78 1 08 ppm resembling the adakitic granites their low mg 36 41 values cr 2 94 3 59 ppm and ni 1 32 1 55 ppm contents as well as positive end t 0 5 to 0 6 and ehf t values 1 62 to 8 07 indicate that they were formed by partial melting of thickened juvenile mafic lower crust probably within the existence of garnet the k feldspar granites have high sio2 76 61 77 14 wt and alkalis na2o k2o 8 55 9 69 wt contents high 10 000 ga al 2 56 2 80 ratios and differentiation index 95 3 96 9 indicating the affinity to highly fractionated a type granites we suggest that these a type granites were mainly generated by the partial melting of felsic crustal rocks under low pressure condition and subsequently extensive fractional crystallization is significant taking into account previous studies from the western yangtze block south china we propose that the gabbro diorites adakitic granites and a type granites in this study were formed in persistent subduction process during the neoproterozoic

compositional and dynamic controls on mafic silicic magma interactions at continental arc volcanoes evidence from cordon el guadal tatara san pedro complex chile

where was the ailaoshan ocean and when did it open a perspective based on detrital zircon u pb age and hf isotope evidence abstract the indochina simao and yangtze blocks were separated by a branch of the paleo tethys ocean commonly referred as the ailaoshan ocean in the paleozoic remnants of this ailaoshan ocean have been variably suggested to locate along from east to west the ailaoshan fault jiujia anding fault and amojiang lixianjiang fault in order to test these models we have carried out comprehensive detrital zircon u pb dating and hf isotope analyses on the cambrian to devonian sedimentary units in the ailaoshan belt and its adjacent western margin of the yangtze block our results indicate marked detrital zircon provenance variation on different sides of the ailaoshan tengtiaohe fault detrital zircons from east of the fault display a diagnostic age peak at 730 900 ma which is characterized by both positive and negative e hf t values with a hf model age t dm c peak at 1 8 ga whereas detrital zircons from west of the fault display two major age populations of 400 500 and 900 1000 ma both characterized by mainly negative e hf t values with a hf model age t dm c peak at 2 1 ga our new data indicate that detritus from east and west of the ailaoshan tengtiaohe fault may have been mainly derived from respectively the yangtze block and indochina simao blocks thus suggesting the fault may represent the actual suture between the two blocks our study also reveals that the ailaoshan ocean may have started its early continental rift in the early silurian

neoproterozoic crustal growth and reworking of the northwestern yangtze block constraints from the xixiang dioritic intrusion south china abstract neoproterozoic magmatism along the northwestern margin of the yangtze block south china produced voluminous adakitic and normal arc felsic intrusions associated with many mafic ultramafic bodies dioritic plutons are rare but are important for understanding the crustal growth and secular evolution of the region the xixiang dioritic intrusion at hannan on the northwestern margin of the yangtze block provides a good opportunity to investigate the petrogenesis of intermediate plutons and to document the neoproterozoic continental crustal evolution of the region the pluton has a zircon u pb age of 764 9 ma rocks from the pluton have relatively low sio 2 59 3 63 0 wt and k 2 o na 2 o 5 8 6 2 wt and high mgo 2 3 3 1 wt and al 2 o 3 15 8 17 1 wt they have chondrite normalized ree patterns enriched in lree la ybn 7 2 14 7 with negative eu anomalies their primitive mantle normalized trace element patterns are characterized by enrichment of lile and depletion of hfse nb and ta with positive pb and negative ti anomalies they have a narrow range of initial 87 sr 86 sr ratios 0 7033 to 0 7035 with positive end values 1 94 to 3 16 their zircon ehf values range from 5 5 to 9 8 with an average of 7 32 these isotopic data are similar to those of the neoproterozoic mafic ultramafic intrusions in the hannan region suggesting that the diorites were generated by partial melting of newly emplaced basaltic rocks the diorites have mg values 50 3 54 4 higher than those of experimental melts of mafic igneous rocks in addition they contain fe rich clinopyroxene suggesting that partial melts of the newly formed mafic lower crust interacted with the mafic residue based on its arc related geochemical affinity and emplacement preceding voluminous adakitic magmatism the xixiang intrusion is proposed to have been generated in a neoproterozoic arc setting during crustal growth and reworking that lasted from 950 ma to

turbulence and internal waves at the equator part ii details of a single event abstract in the low richardson number shear flow above the pacific equatorial undercurrent a single vertical microstructure profile intersected the overturning crest of a packet of high horizontal wavenumber waves the observed dissipation rates within the overturning wave were so high that if they were representative of the volume averaged rate the total wave energy would have been dissipated within a single buoyancy period the chaotic structure and temperature fluctuations with horizontal scales less than 2 m of the two wave crests and troughs west of the overturning wave crest suggest that recent mixing had occurred there wave crests and troughs east of the overturning wave crest showed little or no sign of turbulent mixing similar high horizontal wavenumber waves believed to be shear instability waves have been observed in low richardson number regions of the midlatitude seasonal thermocline although the equatorial waves have a horizontal wavelength appropriate for shear instability waves t

petrogenesis and tectonic significance of the baoxing granitic and mafic intrusions southwestern china evidence from zircon u pb dating and lu hf isotopes and whole rock geochemistry abstract voluminous neoproterozoic gabbros gabbro diorites and granites are exposed in the baoxing area along the western margin of the yangtze block we present here zircon u pb and lu hf isotope data as well as whole rock geochemical data for these rocks which help constrain the tectonic characteristics and evolution of the western margin of the yangtze block zircons from these rocks have high th and u contents and th u ratios 0 2 and show striped absorption patterns growth striations or zoning indicating a typical magmatic origin zircon u pb dating and lu hf isotope results show that the gabbros formed at ca 850 ma with e hf 5 44 to 11 94 and a single stage model age t hf of 1 04 ga suggesting a depleted mantle derived source the gabbro diorites formed at ca 800 ma with major e hf 3 99 to 14 62 and t hf 0 97 ga and minor e hf 0 68 to 5 66 and t hf 1 27 ga again suggesting a depleted mantle derived source the granites were emplaced at ca 770 ma with e hf 2 53 to 10 58 and t hf 1 07 ga indicating that they were derived from the partial melting of juvenile crust the geochemical data show that the gabbros and gabbro diorites are characterized by low sio 2 contents high mg values and cr co and ni contents significant enrichment in light rare earth elements and large ion lithophile elements depletion in high field strength elements e g nb ta zr and hf and relatively high nb u ta u and ce pb ratios these data suggest that the rocks were derived from the partial melting of depleted lithospheric mantle that had been metasomatized by subduction derived fluids or melts and which underwent very minimal crustal contamination during magma evolution and ascent the granites have high sio 2 and k 2 o and low mgo contents exhibit enrichment in rb ba th and pb and depletion in nb ta and ti and have low sr y and la yb n ratios and high y and yb contents which together with their hf isotopic compositions imply a juvenile crust source taking into account regional pre 750 ma arc related magmatism and the subsequent extensional setting it is proposed that a long lived active continental margin setting may have evolved along the western margin of the yangtze block during the neoproterozoic and that the coeval granitic and mafic rocks in the baoxing area were formed by the subduction of the western yangtze block

neoproterozoic tectonic evolution of south qinling china evidence from zircon ages and geochemistry of the yaolinghe volcanic rocks abstract the south qinling terrain is located at the northern margin of the yangtze block neoproterozoic volcanic sedimentary sequences of the yaolinghe group play the most significant role in studying the tectono magmatic evolution of the south qinling based on temporal and spatial geochemical variations the yaolinghe group can be divided into three volcanic cycles basaltic lavas of cycle 1 crystallized at 847 8 ma and exhibit typical island arc basalt characteristics with enrichment of liles th u and lrees depletion of hfses nb ta zr hf ti and homogeneous initial 143nd 144nd ratios ranging from 0 51162 to 0 51172 corresponding to initial nd values of 1 6 to 3 5 these features suggest that they were derived from a depleted mantle source during a subduction period volcanic rocks of cycle 2 show a complete lithological evolution from basalt through andesite to rhyolite without a significant gap of sio2 content they formed between 731 11 ma and 768 7 ma and display trace element signatures similar to arc volcanics with positive lile and negative hfse anomalies these basalts have lower initial 87sr 86sr ratios from 0 7030 to 0 7052 and higher initial 143nd 144nd ratios from 0 51172 to 0 51194 with positive initial nd values ranging from 0 9 to 5 3 whereas intercalated felsic rocks show lower initial 143nd 144nd ratios from 0 51150 to 0 51177 with initial nd values varying from 4 5 to 1 9 suggestive of metasomatic alteration of lithospheric mantle by slab derived fluids melts cycle 3 lavas show undepleted hfses nb ta zr hf and slightly enriched liles th u and ti concentrations they were generated in a rift related setting between 680 ma and 650 ma geochronological and geochemical evidence from lavas of the yaolinghe group suggest that the panxi hannan arc system extended to northern south qinling and the magmatism in the northern margin of the yangtze block evolved from arc to rift magmatism during the neoproterozoic

neoproterozoic high k granites produced by melting of newly formed mafic crust in the huangling region south china abstract the huangling igneous complex in the central yangtze block south china consists of the 850 ma tonalite and trondhjemite which were intruded by the 826 13 ma high k granite the granites are composed of plagioclase 30 60 alkali feldspar 10 30 quartz 20 30 biotite 2 63 1 73 3 wt k 2 o 2 58 4 79 wt and na 2 o 3 90 5 89 wt and low mgo 0 49 1 78 wt and are magnesian alkali calcic in composition their chondrite normalized ree patterns are characterized by enrichment of lree la yb n 14 5 32 8 and mostly negative eu anomalies eu eu 0 5 1 1 they have moderately high sr 202 698 ppm and low y 6 13 18 7 ppm with sr y ratios of 14 64 these large elemental variations probably resulted from fractional crystallization they also have negative whole rock nd 9 75 to 11 73 and zircon hf values 7 1 to 15 8 which are similar to those of the neoproterozoic mafic rocks in the same region suggesting that the granites may have been produced by melting of newly emplaced mafic rocks tonalites and trondhjemites of the huangling complex previously were considered as products by differentiation of water rich basaltic magmas and melting of the thickened archean continental crust in an arc setting respectively therefore the crustal growth and reworking of the yangtze block were probably wholly controlled by the subduction system which resulted in formation of the stable continent during the neoproterozoic

magmatism along the southeast margin of the yangtze block precambrian collision of the yangtze and cathysia blocks of china the time and the environment of formation of igneous rocks along the southern margin of the yangtze continental block in southern anhui province and northeastern jiangxi province china are discussed sm nd dating for samples from two ophiolite suites give ages of about 1 ga late proterozoic granodiorites in the study area have ages of about 0 9 ga and have normal intrusive relations against banxi group metamorphic country rocks trace element abundances and initial nd isotope ratios for ophiolite and proterozoic granodiorites are consistent with formation in an island arc environment in contrast sr and nd isotopes for mesozoic granitoids of the region suggest that they are products of remelting of crustal sources similar to the basement of the yangtze block these relations do not support a recently proposed mesozoic overthrust tectonic model for southeast china but rather favor a late precambrian collision history for the yangtze and cathysia blocks

dislocation formation and albitization in alkali feldspars from the shap granite orthoclase rich alkali feldspars in the lower devonian shap granite northwest england contain two generations of albite rich feldspar these have partially replaced earlier exsolution microtextures consisting of albite lamellae coarse semicoherent albite films and fine coherent albite platelets in tweed orthoclase the earlier generation of replacive albiterich feldspar ab90an9ori occurs together with orthoclase rich feldspar ab10or90 i n veins that crosscut exsolution microtextures throughout grain interiors this episode of recrystallization was mediated by magmatic fluids at 410 8c estimated from two feldspar geothermometry and was driven by stored elastic strain energy which was relatively homogeneously distributed throughout the microtextures the later generation of replacive albite rich feldspar which is restricted to grain margins and is compositionally pure ab 99 was produced by magmatic hydrothermal fluids at 370 8c this generation of albite rich feldspar does not crosscut exsolution microtextures and has selectively replaced volumes of highly elastically strained feldspar surrounding edge dislocations along semicoherent albite films marked differences in controls of the localization of the two generations of replacive albite rich feldspar by pre existing exsolution microtextures indicate that significant numbers of edge dislocations developed along albite films after the first phase of fluid feldspar interaction and associated albitization but before the second phase this relation indicates that edge dislocations formed between 410 and 370 8c these observations have important implications for understanding the factors that control the interaction of alkali feldspars with fluids both in cooling igneous rocks and in clastic sedimentary rocks during diagenesis

geochronology and isotopic character of ultrahigh pressure metamorphism with implications for collision of the sino korean and yangtze cratons central china the collision zone between the sino korean and yangtze cratons in central china is marked by subduction related metamorphic rocks composed of crustal protoliths these range in metamorphic grade from ultrahigh pressure eclogite to low greenschist facies the paragenesis of the ultrahigh pressure rocks coesite and diamond bearing indicates subduction to depths greater than 120 km as a result of continental collision despite extreme metamorphic conditions zircons have not undergone pb loss but rather illustrate two periods of growth crystallization between 700 and 800 ma and ultrahigh pressure metamorphism at 218 2 5 ma previous studies have predicted a time transgressive collision but age data presented in this study indicate that the collision was approximately coeval along the length of the suture u pb zircon ages of these ultrahigh pressure metamorphic rocks are 218 4 2 5 ma and 217 1 8 7 ma for the dabie mountains and shandong peninsula respectively the similar ages imply that the tan lu fault which offsets the ultrahigh pressure metamorphic rocks is a secondary feature not related to collision or subduction whole rock isotopic data show that both rb sr and u pb were open systems during metamorphism but that sm nd probably remained closed nd initial isotopes display crustal signatures indicating that the crust subducted as a coherent slab rather than becoming intercalated with mantle material at depth sm nd data in conjunction with u pb zircon dating indicate that the ultrahigh pressure rocks are not typical yangtze craton basement 2 9 ga but that they originally crystallized in a rift environment between 700 and 800 ma discrepancies between 40ar 39ar plateaus sm nd isochrons and u pb zircon ages render calculation of a p t t path unrealistic

dehydration melting and water saturated melting of basaltic and andesitic greenstones and amphibolites at 1 3 and 6 9 kb

turbulence and internal waves at the equator part i statistics from towed thermistors and a microstructure profiler abstract high correlations between turbulent dissipation rates and high wavenumber internal waves and the high values of turbulent dissipation associated with internal wave activity suggest that internal waves are the main direct source of mixing in the thermocline above the core of the equatorial undercurrent an extensive dataset obtained using a microstructure profiler and thermistor chain towed along the equator was analyzed to examine the correspondence between turbulent mixing and high wavenumber internal waves in the low richardson number ri thermocline below the mixed layer but above the core of the equatorial undercurrent and when winds were moderate and steadily westward it was found that the spectrum of vertical isotherm displacement was dominated by a narrow wavenumber band corresponding to 150 250 m zonal wavelength of internal waves both turbulence and internal waves varied diurnally hourly averaged values of turbulent dissipation rate and wave potential energy were greater by a

neoproterozoic quartz monzodiorite granodiorite association from the luding kangding area implications for the interpretation of an active continental margin along the yangtze block south china block abstract neoproterozoic magmatism along the western margin of the yangtze block can provide important information about the evolution of the supercontinent rodinia for this reason this study is focused on neoproterozoic high mg quartz monzodiorite and granodiorite from the luding kangding area detailed zircon la icp ms u pb dating indicates that the high mg quartz monzodiorite and the granodiorite have identical ages of 754 10 ma mswd 0 51 2 and 748 11 ma mswd 0 33 2 respectively the high mg quartz monzodiorite are characterized by low sio2 contents 60 76 63 78 and high tio2 contents 0 41 0 56 and they are sodic and meta aluminous they are enriched in lree with la yb n values of 4 14 8 51 and without significant negative eu anomalies eu eu 0 79 0 92 the high mg quartz monzodiorite have relative low initial 87sr 86sr i ratios of 0 703513 0 704519 and positive nd t values of 2 4 to 4 8 and are depleted in hfses nb and ta thus it can be inferred that these high mg quartz monzodiorite were formed by high degree 40 partial melting of newly formed mafic lower crust and that their high mg values were the result of assimilation of some residual mafic minerals in their source region the granodiorite are characterized by higher sio2 contents 65 32 67 59 and they are also sodic and meta aluminous they have more evolved sr nd isotopic compositions and higher sr 425 537 ppm and ba 705 1074 ppm contents than the quartz monzodiorite suggesting that they were derived from a plagioclase rich source region based on their field occurrence and geochemical features we propose that both the neoproterozoic high mg quartz monzodiorite and the granodiorite were formed in an active continental arc setting but they derived from two distinct source regions

neoproterozoic arc related mafic intrusions along the northern margin of south china implications for the accretion of rodinia abstract south china has been considered as part of the rodinian supercontinent during neoproterozoic time although its paleogeographic position within this supercontinent is still a matter of debate the wangjiangshan and bijigou complexes along the northern margin of south china are among the largest mafic intrusions in china new shrimp zircon u pb results indicate that these two intrusions have crystallization ages of 820 ma and 780 ma respectively enrichment of large ion lithophile and light rare earth elements and depletion of high field strength elements in these intrusions suggest derivation from an active arc along a continental margin this interpretation of these intrusions as part of a continental arc assemblage is in contrast with the previous view that they were products of a neoproterozoic mantle plume that initiated the breakup of rodinia the presence of a neoproterozoic magmatic arc suggests that rodinian oceanic lithosphere was subducted beneath the present northern margin of south

neoproterozoic post collisional extension of the central jiangnan orogen geochemical geochronological and lu hf isotopic constraints from the ca 820 800 ma magmatic rocks abstract neoproterozoic magmatic rocks in south china contain important information on evaluating crustal reworking during the yangtze cathaysia collision and post collisional extension we present here geochemical geochronological and lu hf isotopic data of a suite of granitoids and diorites from the jiuling and meixian plutons of the central jiangnan orogen the granitoids display intermediate geochemical features between s type and i type granites they exhibit arc like patterns of ree and trace element partition reflecting inheritance of juvenile arc derived crust igneous zircons from the jiuling and meixian granitoids yield u pb ages of 822 819 ma and 816 815 ma respectively inherited zircons from the granitoids commonly yield a major age population of 980 860 ma consistent with the time of the shuangxiwu volcanic arc the igneous and inherited zircons have highly variable ehf t values ranging from 9 to 12 2 we interpret the granitoids to have been generated by remelting and binary mixing of 1 juvenile arc derived crustal material and 2 supracrustal material of the shuangqiaoshan meta sedimentary rocks the previous crustal thickening achieved by the yangtze cathaysia collision is a prerequisite for their generation the two granitoids together with regional 825 815 ma s type granites indicate incipient post collisional collapse of thickened crust heralding tectonic switch from compression to extension the meixian diorites display arc like geochemical characteristics and yield zircon u pb ages of 805 804 ma and neutral to positive zircon ehf t values 0 1 7 5 the diorites were generated by decompression melting of arc derived lower crust associated with asthenospheric mantle upwelling during continental rifting such rifting may be part of global neoproterozoic rifting events that finally led to breakup of the rodinia supercontinent

origin and tectonic significance of a mesozoic multi layer over thrust system within the yangtze block south china abstract in the yangtze block south china a well developed mesozoic thrust system extends through the xuefeng and wuling mountains in the southeast to the sichuan basin in the northwest the system comprises both thin and thick skinned thrust units separated by a boundary detachment fault the dayin fault to the northwest the thin skinned belt is characterized by either chevron anticlines and box synclines to the northwest or chevron synclines to the southeast the former structural style displays narrow exposures for the cores of anticlines and wider exposures for the cores of synclines thrust detachments occur along silurian fs and lower cambrian fc strata and are dominantly associated with the anticlines to the southeast this style of deformation passes gradually into one characterized by chevron synclines with associated principal detachment faults along silurian fs cambrian fc and lower sinian fz strata there are however numerous secondary back thrusts therefore the thin skinned belt is like the valley and ridge province of the north american applachian mountains the thick skinned belt structurally overlies the thin skinned belt and is characterized by a number of klippen including the xuefeng and wuling nappes it is thus comparable to the blue ridge province of appalachia the structural pattern of this thrust system in south china can be explained by a model involving detachment faulting along various stratigraphic layers at different stages of its evolution the system was developed through a northwest stepwise progression of deformation with the earliest delamination along lower sinian strata fz analyses of balanced geological cross sections yield about 18 1 21 total 88 km shortening for the thin skinned unit and at least this amount of shortening for the thick skinned unit the compressional deformation from southeast to northwest during late jurassic to cretaceous time occurred after the westward progressive collision of the yangtze block with the north china block and suggests that the orogenic event was intracontinental in nature

spatial temporal variability in surface layer deepening and lateral advection in an embayment of lake victoria east africa vertical and horizontal exchanges in pilkington bay a shallow 9 m embayment of lake victoria were determined from a surface energy budget time series measurements of temperature and quasi synoptic transects of conductivity temperature and depth conducted over a 2 d period the surface energy budget is the first from a tropical lake over a diurnal timescale strong stratification developed during morning and early afternoon 40 cycles h 21 but was eroded beginning in the afternoon by the combination of wind and heat loss surface heat losses contributed 70 of the energy for surface layer deepening 82 of the time from midafternoon until midmorning circulation times of the surface layer were 2 min as it deepened to 1 5 m in the afternoon and were 12 min at night even when mixing extended to the lake bottom spatial differences in the rates of heating and cooling and in the depth of wind mixing caused fronts to develop on spatial scales of kilometers within the bay convergence of these fronts led to downwelling of surface waters and upwelling of deep waters during the stratified period horizontal pressure gradients due to differential heating contributed to thermocline downwelling lateral movement of deep anoxic waters and generation of high frequency internal waves all of which contribute to vertical and horizontal transports although wind and heat loss at one location generally determine the depth of the surface layer and thermocline the depths of these key features may be strongly influenced by rates of heating and cooling elsewhere in a basin the temporal and spatial scales of advection and vertical mixing determine the flux paths of particles and solutes in aquatic ecosystems and ultimately lacustrine productivity time series measurements of surface meteorology from which surface energy budgets can be calculated and water column temperatures can be used as an indirect approach to determine these pathways the data also can be used to determine internal wave dynamics the energetics of mixing and the role of intrusions and density driven flows in advection with this information the rates of circulation within the upper mixed layer and vertical and horizontal transports can be assessed the diurnal cycle of stratification and mixing is deter

successive zoning of amphiboles during progressive oxidation in the daito yokota granitic complex san in belt southwest japan abstract clinoamphibole present in shallow intrusive masses of quartz diorite in the daito yokota area of the san in belt southwest japan is strongly zoned successively from a magnesio homblende core to an actinolite rim systematically decreasing in fe 2 fe 2 mg towards the rim the features of compositional variation and cation substitution are similar to those of amphiboles which have been altered by fluid separated during secondary boiling in porphyry copper bearing granitoids mineralogie data and textural relations of amphibole and the accompanying minerals indicate that the zoned amphibole has crystallized or recrystallized from a subsolidus fluid phase under progressively oxidizing conditions truly magmatic amphibole is restricted to small patches in the core of strongly zoned amphibole grains progressive oxidation is responsible for reequilibration of early formed fe ti oxide to yield extensive development of secondary magnetite and sphene the decreasing fe trend of amphibole has resulted from progressive fe depletion of the fluid phase by abundant magnetite precipitation the amphibole grains usually show an alternation of chemically distinct zones with unconformable boundaries indicating repeated growth and corrosion by multiple circulation of fluids

subduction related origin of the 750 ma xuelongbao adakitic complex sichuan province china implications for the tectonic setting of the giant neoproterozoic magmatic event in south china abstract the xuelongbao plutonic complex in the eastern margin of the tibetan plateau is dated at 748 7 ma using the shrimp zircon u pb method and represents part of the neoproterozoic igneous assemblage of south china rocks in the complex include tonalite and granodiorite and have sio2 ranging from 62 0 to 74 8 wt and al2o3 from 14 3 to 20 9 wt their na2o contents range from 4 2 to 6 7 wt and k2o from 0 47 to 1 96 wt indicating that they belong to the na series these rocks show chondrite normalized ree patterns depleted in hree and variably enriched in lree they have positive sr and negative nb and ti anomalies in the primitive mantle normalized trace elemental spider diagram their sr contents range from 320 to 780 ppm and y contents are lower than 10 ppm resulting in high sr y ratios 52 320 characteristic of typical adakites their end t and initial sr isotopic compositions range from 0 36 to 2 88 and from 0 7033 to 0 7054 respectively the geochemical features of the xuelongbao plutonic complex are consistent with an origin from adakitic magmas that were likely derived from partial melting of a subducted oceanic slab together with arc signatures of other granites and mafic intrusions in the region the 750 ma xuelongbao adakitic complex provides evidence for a major subduction related neoproterozoic magmatic event of south china

mixed layer dynamics in a lake exposed to a spatially variable wind field1 field measurements are described detailing the formation of frontal structures in the surface mixed layer near the entrance of an embayment in a medium sized lake the observed evolution of the fronts is interpreted in terms of three well defined dynamical regimes the first differential heating is defined as the variable heat capture due to decreased surface heat losses and warmer surface water in the sheltered embayments the second variable deepening of the mixed layer is induced by a sudden increase in wind speed in the open central areas of the lake with only minimal mixing in the sheltered parts of the lake the third is the horizontal gravitational motion driven by the horizontal density gradients set up by the active deepening of the mixed layer in the exposed parts of the lake although detailed wind data were not available observations of the temperature and salinity fields show the existence of fronts in lakes and strongly suggest the importance of the above mechanisms in determining the horizontal variability of these fields in the surface mixed layer of medium sized lakes

estimation of the frictional strength of faults from inversion of fault slip data a new method abstract the conventional stress inversion methods estimate only four of the six independent parameters of the tectonic stress tensor using the coulomb navier failure criterion as an additional constraint it is possible to estimate the fifth parameter characterized by the normalized critical stress difference i e the critical stress difference divided by the effective overburden pressure this parameter is related to the average friction coefficient of faults if the stress field is uniform faults with different orientations and at different depths have the same normalized critical stress difference on this basis a new method is proposed to estimate the average friction coefficient and the normalized critical stress difference from inversion of a population of faults of measured orientations and slip directions this method is applicable both to newly formed faults and to reactivated faults this method is applied to four data sets in three cases an average friction coefficient gm 0 0 64 0 70 and 0 88 is obtained one case shows a relatively low average friction coefficient gm 0 0 22 but this value is of poor quality due to the effect of a possibly nonuniform stress field these results are in agreement with the average value of friction coefficient gm 0 0 75 derived from laboratory experiments

the evolution of thermal structure and water chemistry in lake nyos abstract we collected a time series of physical and chemical data to gain a better understanding of the dynamics of lake nyos measurements of water and gas chemistry and temperature made during january march and may 1987 are compared to data taken in september 1986 just after the initial co 2 gas release there is no pattern of change in overall heat content of the lake although heat input to bottom waters 185 208 m has occurred at a rate of 1600 mw m 2 this increase in heat content translates to a change from 23 38 to 24 12 c at 200 m and can be explained by geothermal heat flow and addition of thermal spring water concentrations of ca 2 mg 2 na k fe 2 and alkalinity have increased only in bottom waters in situ lake processes such as sulfate and iron reduction are unable to account for the changes in alkalinity observed chemical changes are consistent with a scenario where slightly thermal soda water is being input to the bottom of the lake measurements of p co 2 at depth ranged from 18 to 28 of saturation and exhibited horizontal variability overall recharge of co 2 in bottom waters is negligible mainly because of increasing ion concentrations in bottom water total stability of the water column increased 33 from 48 800 j m 2 in september 1986 to 64 700 j m 2 in may 1987 as long as co 2 concentrations remain the same this level of stability is higher than could be disrupted by common limnologic or meteorologic processes there is thermal and chemical evidence that a buildup of dissolved iron and co 2 in bottom waters must have preceded the august 1986 gas release in addition a survey of all crater lakes in cameroon indicates that only lakes nyos and monoun contain high concentrations of dissolved iron and co 2 thus there is a low probability of any other cameroonian lake releasing a substantial volume of co 2

exposure of a late cretaceous layered mafic felsic magma system in the central sierra nevada batholith california new u pb zircon ages for the lamarck granodiorite associated synplutonic gabbro and diorite plutons and two large mafic intrusive complexes that underlie them in the sierra nevada batholith are 92 1 ma these ages establish the late cretaceous as a period of extensive mafic felsic magmatism in the central part of the batholith and confirm the significance of mafic magmatism in the evolution of the voluminous silicic plutions in the sierran arc the lack of significant zircon inheritance in any of the units analyzed supports isotopic evidence that the lamarck and other late cretaceous sierran plutons were derived predominantly from young crust recognition of an extensive mafic felsic magma system in the sierra nevada batholith emphasizes the importance of basaltic liquids in the evolution of continental crust in arc settings

sedimentary provenance constraints on the jurassic to cretaceous paleogeography of sichuan basin sw china abstract the provenance of jurassic to cretaceous sediments in the sichuan basin is constrained by sandstone petrology and detrital zircon u pb geochronology which provide critical insights into the mid late mesozoic paleogeography of the sichuan basin the petrography of 22 sandstone samples suggests moderately to highly mature sediments which are primarily derived from cratonic or recycled sources detrital zircon u pb ages from middle jurassic to late cretaceous are classified into age groups of 130 200 ma 200 330 ma 400 490 ma 680 890 ma 1730 1960 ma and 2360 2600 ma the middle jurassic sediments contain a relatively high density of 260 ma 1 85 ga and 2 5 ga zircons as well as a low density of 800 ma zircons consistent with the derivation from the emeishan large igneous province the songpan ganzi terrane and the south qinling belt and to a lesser extent from the western jiangnan orogen the late jurassic and early cretaceous sediments with scattered age distribution share common multiple source to sink systems which were predominantly draining toward the south and southeast with southward drainage being higher subsequently they were disrupted by a synchronous northeastward drainage capture late cretaceous sediments have a distinct reduction in zircons younger than 213 ma suggesting southeastward and southwestward transport of sediments that were likely derived from the songpan ganzi terrane the sanjiang orogen the south segment of the longmenshan thrust belt and the western yangtze craton and areas of uplift in the northern and northeastern sichuan basin changes in the provenance during the mid late mesozoic coincide with the spatiotemporal variations in the depocenter migration and paleogeographic evolution of the sichuan basin which are closely related to the multistage intracontinental subduction associated with the clockwise rotation of the south china block

internal wave weather in a stratified lake thermistor chain data collected in lake biwa japan during a period of strong stratification were used to determine the nature of the water column response to wind forcing the evolution of the basin scale waves in the lake were reproduced in three dimensional numerical simulations and the time variability of the internal wave spectrum is discussed based on spectral analysis of the thermistor chain records severe winds associated with the passage of three typhoons during the sampling period excited basin scale kelvin and poincare waves which by interaction with the topography of the lake and non linear effects generate trains of high frequency internal waves time frequency analysis of the temperature signals suggests that the 2 slope of the internal wave spectrum is probably the result of the composition of groups of waves with high intermittence travelling through the wave guide the observed rapid decay of internal wave energy after severe wind events seemed to be due mostly to high dissipation at the sloping boundaries of the lake

the longmenshan tectonic complex and adjacent tectonic units in the eastern margin of the tibetan plateau a review abstract the longmenshan tectonic complex lstc along the eastern margin of the tibetan plateau the site of devastating earthquakes such as the magnitude 8 0 wenchuan earthquake on 12 may 2008 preserves an exceptionally complete history of the tectonic evolution of the yangtze block and its relations to adjacent tectonic units due to sequential tectonic superposition and tectonic reactivation the tectonic nature of the lstc and in particularly the older history has been profoundly debated and many different tectonic models have been proposed herein we summarize the current understandings of the major tectonic events that have shaped this important tectonic complex highlighting problems left to be solved by future work including 1 the nature and constraints for at least 6 regional tectonic events i e building of the metamorphic basement art the columbia nuna supercontinent pt1t the rodinia supercontinent pt3t the paleozoic passive continental margin pzt the paleotethys orogeny mzt and the neotethys orogeny kzt 2 metamorphic basement exposures and their tectonic implications including rock types and geochronological constraints for the archean paleoproterozoic and neoproterozoic basements 3 nature of the present lstc and its affinity with adjacent tectonic units 4 consideration of the ne striking longmenshan thrust belt and arcuate shape yanyuan muli thrust belt as parts of a single tectonic feature 5 mountain basin coupled systems recording past tectonic eposides we draw the following conclusions and tectonic models based on published research combined with our own recent studies 1 the well preserved archean yudongzi gneiss group in the lstc has a genetic affinity with the kongling group and thus belongs to the yangtze block 2 the paleoproterozoic hejiayan group juxtaposed adjacent to the archean yudongzi group may represent a 2000 1800 ma orogenic belt which corresponds to the supercontinent nuna columbia amalgamation event 3 a neoproterozoic trench arc basin system which is reconstructed based on identification of a neoproterozoic ophiolite complex arc type magmatic rock assemblages and volcaniclastic basinal deposits along the western margin of the yangtze block and the lstc may represent the record of eastward subduction of the neoproterozoic mozambique oceanic lithosphere beneath the yangtze block during the assembly of the rodinia supercontinent 4 a complete bidirectional wilson cycle was reconstructed by the formation of the late permian to the middle late triassic back arc ganze litang rift and ocean following the early paleozoic mianlue continental rift and ocean and subsequent closure of the ocean basin by simultaneous bidirectional northward and southwestward subduction and later collision this relatively uncommon bidirectional wilson cycle might be attributed to the formation of the three armed rift system in the eastern paleotethys associated with the late permian eemeishan large igneous province in the lstc 5 a three stage tectonic sequence of in sequence imbricate thrust in the lstc during india eurasian collisional orogeny at 55 15 ma extrusion from 15 to 5 ma and the plateau uplift since 5 ma resulting from lower crustal channel flow is proposed for the formation of the present lstc

the yanbian terrane southern sichuan province sw china a neoproterozoic arc assemblage in the western margin of the yangtze block abstract the magmatic and tectonic history of the yangtze block and its possible affinity with other neoproterozoic arc terranes are important elements in the reconstruction of neoproterozoic plate tectonics the yanbian terrane in the western margin of the yangtze block is a typical arc assemblage composed of a flysch type sedimentary sequence intruded by gabbroic and granodioritic plutons the sedimentary sequence consists chiefly of tuffaceous material with interlayered chert sandstone and pillow basalts laser ablation icp ms u pb dating of detrital zircons from the sandstones yield ages as young as 840 ma the gaojiacun and lengshuiqing mafic intrusions are dated at 812 3 ma and 806 4 ma respectively using the shrimp zircon u pb technique geochemical data show that both the gaojiacun and lengshuiqing intrusions have arc signatures with nd t values of 1 5 to 6 0 initial 87 sr 86 sr ratios of 0 705 0 706 and pronounced negative nb ta and zr hf anomalies their geochemical variations are best explained by fractional crystallization without major crustal contamination the yanbian terrane represents a typical arc assemblage formed on the western edge of the yangtze block during neoproterozoic time the sedimentary sequence was deposited in an oceanic setting probably in a back arc basin environment the depleted subduction modified lithospheric mantle wedge above the subduction zone was the source of melts from which the mafic plutons were crystallized this scenario suggests subduction of oceanic lithosphere eastward present day orientation underneath the yangtze block

boundary versus internal diapycnal mixing in stratified natural waters using the fluorescent dye uranin tracer release experiments to study the contribution of bottom boundary mixing to diapycnal transport in stratified natural waters were performed in lake alpnach central switzerland during 1992 1995 a first experiment involved injecting the tracer from a point source into the center of the hypolimnion that part of the lake below the surface mixed layer an in situ fluorometer was then employed to detect the horizontal and vertical spreading of the tracer cloud allowing rates of diapycnal diffusivity to be determined as long as the tracer was confined to the interior water region the diapycnal diffusivity was relatively small however after the tracer cloud had reached the lake boundary the diapycnal diffusivity increased by approximately one order of magnitude in a second experiment the tracer was released near the sediment water interface in this case the dynamics of vertical tracer spreading were opposite during the first few hours after tracer release diapycnal diffusivities were large subsequently decreasing as the tracer cloud drifted away from the lake boundary basin wide diapycnal diffusivities calculated from heat flux measurements based on temperature profiles obtained from thermistor chains or conductivity temperature depth casts agreed well with the values obtained from the vertical tracer diffusion after horizontal homogenization the results of the tracer experiments corroborate the hypothesis that diapycnal fluxes are determined predominantly by mixing in the bottom boundary region

origin of mafic intrusions in the micangshan massif central china implications for the neoproterozoic tectonic evolution of the northwestern yangtze block abstract the mafic intrusions exposed in the micangshan massif preserve important information pertaining to the neoproterozoic tectonic evolution along the northwestern margin of the yangtze block in this paper we report new la icp ms zircon u pb ages lu hf isotopes and whole rock compositions for mafic sills in the micangshan massif to understand their petrogenesis and supplement the regional tectonic evolution xenocrystic zircon grains from two mafic sill samples yield weighted mean ages of 860 4 ma and 858 5 ma respectively suggesting the maximum emplaced age of the mafic sills was at ca 860 ma a granitic dyke sample which intruded into the mafic sills yields a crystallization age of 799 4 ma implying the minimum emplacement of the mafic sills was at ca 800 ma these results indicate that the mafic sills were formed within the span of ca 860 800 ma geochemical results show that most of these mafic sills are tholeiitic in composition and are characterized by low to moderate feototal and tio2 contents as well as moderate to high mgo contents and mg values they display distinct enriched light rare earth element lree patterns variable remarkable enrichment in large ion lithophile elements liles e g cs and ba and evident depletion of high field strength elements hfses e g nb ta and p resembling signatures of typical subduction related basaltic rocks in addition these sills exhibit comparatively high nb ta moderate nb yb as well as low nb la th yb and nb y ratios indicating that they probably originated from partial melting of a previously metasomatized mantle wedge source these observations together with regional tectonics suggest that these mafic sills in the micangshan massif might be formed in a subduction related continental arc setting therefore combining our new data with available results from previous studies in the adjacent regions our study supports the dominant subduction related tectonics along the northwestern margin of the yangtze block during the early neoproterozoic time

the behavior of a turbulent front in a stratified fluid experiments with an oscillating grid a simple laboratory model utilizes an oscillating grid to produce a turbulent front in a linearly stratified fluid the motion of the front is studied visually using laser fluorescence and neutrally buoyant tracer particles when the turbulence is vigorous near the grid stratification has no effect as the front propagates away vertical scales become limited by stratification as postulated by ozmidov the break up of the front can be identified at the nondimensional time nt 10 here a series of layers are formed which intrude into the surrounding nonturbulent fluid the vertical scale of these layers is also predicted using ozmidov s postulate it is approximately 7 times the estimated ozmidov scale at nt 10 the advance of individual intrusions comprising the front is characterized by a local froude number which decreases as the intrusions lengthen the mechanism of advance is intimately connected with wave motions produced at the turbulent end of the intrusions

heavy mineral analysis and detrital u pb ages of the intracontinental paleo yangzte basin implications for a transcontinental source to sink system during late cretaceous time

composition age and origin of two proterozoic diorite tonalite complexes in the arabian shield abstract large amounts of diorite tonalite magma were intruded into the island arc successions of the southern arabian shield between ca 900 and 700 ma ago major oxide trace element rare earth ree and isotopic data are presented for two plutons exemplifying older and younger members of this plutonic phase the thurrat pluton which was emplaced into virtually unmetamorphosed volcanics of sequence b has yielded a 10 point rb sr isochron indicating emplacement 744 22 ma ago and an initial 87 sr 86 sr ratio of 0 70281 it consists of gabbros diorites quartz diorites tonalites and low al 2 o 3 trondhjemites which are compositionally primitive with depleted lithophile element contents and flat ree patterns with a negative eu anomaly in the trondhjemites the magma was most probably mantle derived and analogies with other trondhjemitic plutons suggest that it was probably emplaced in an island arc setting the bidah pluton which was emplaced into a compositionally very immature succession of metamorphosed volcanics and volcaniclastics of sequence c has yielded a nine point near isochron mswd 2 86 indicating an age of 901 37ma and an initial 87 sr 86 sr ratio of 0 70246 this date is accepted as the age of emplacement the rocks are compositionally primitive gabbros diorites quartz diorites tonalites and granodiorites with depleted lithophile element contents and flat ree patterns with a negative eu anomaly in the more siliceous components the country rock volcanics were formed in an immature island arc environment and the composition of the bidah pluton is suggestive of a mainly mantle derived magma emplaced into that arc the data for these two plutons therefore provide further evidence that most of the rock material added to the arabian shield between 900 and 700 ma ago was derived from the mantle

impact of nonlinear waves on the dissipation of internal tidal energy at a shelf break the vertical and temporal structure of the dissipation of turbulent kinetic energy within the internal tide at a location 5 km shoreward of the shelf break on the malin shelf has been determined using a combination of the free falling light yo yo profiler and acoustic doppler current profilers two distinct internal wave regimes were encountered period i in which large amplitude high frequency nonlinear internal waves niws occurred around neap tides and period ii in which the internal wave spectral continuum was not dominated by any particular frequency band around spring tides empirical orthogonal function analysis shows that for the low frequency waves 76 of the variance was described by mode 1 rising to 95 for the high frequency waves during period i the dissipation and vertical mixing were characterized by the niws and 70 of the dissipation occurred in the bottom boundary layer during period ii the depth integrated dissipation was more evenly distributed throughout the tidal cycle whereas vertical mixing was greatly enhanced during a single hour long episode of elevated thermocline dissipation coincident with weakened stratification during both periods i and ii 30 of the total measured dissipation occurred within the thermocline when averaged over 12 4 hours the remainder occurred within the bottom boundary layer bbl tidal average values for depth integrated dissipation and vertical eddy diffusivity for period i ii were 1 1 10 2 w m 2 4 0 10 2 w m 2 and 5 cm2 s 1 12 cm2 s 1 respectively decay rates and internal damping are discussed and vertical heat fluxes are estimated observed dissipation rates are compared with a simple model for bbl dissipation

pressure temperature time paths and a tectonic model for the evolution of granulites petrologic studies and application of well calibrated mineralogic thermometers and barometers reveal several important features common to many granulite terrains 1 peak metamorphic pressure and temperature conditions cluster around values of 7 5 pm 1 kbar and 800 pm 50 c implying average geothermal gradients of 30 35 c km 2 in comparison with upper amphibolite facies rocks especially in paired amphibolite granulite terrains it is evident that granulites are distinguished from amphibolites by the former s higher metamorphic temperatures but not higher pressure 3 initial cooling of granulite terrains from peak conditions is nearly isobaric for garnet closure temperatures of 600 pm 50 c the dp dt retrograde paths inferred from compositionally zoned garnet rims are 2 8 b c in most granulites 4 primary coarse sillimanite is found in most granulites and retrograde kyanite is known in a few these data suggest that granulites form as a result of anomalous thermal gradients

dehydration melting of a basaltic composition amphibolite at 1 5 and 2 0 gpa implications for the origin of adakites this study presents the results of dehydration melting experiments on a basaltic composition amphibolite under conditions appropriate to a hot slab geotherm 1 5 and 2 0 gpa and temperatures of 850 to 1150 c dehydration melting produces an omphacitic augite and garnet bearing residue coexisting with rhyolitic to andesitic composition melts at 1 5 gpa the amphibolite melts in two stages between 800 and 1025 c the 2 0 gpa data also define two melting stages at 2 0 gpa the first stage involves nearly modal melting of the original amphibolite minerals qtz pl amp to produce melt cpx grt during the second stage the eclogite restite melts non modally 0 86 cpx 0 14 grt 1 melt the experimental results were combined with data from the literature to generate a composite p t phase diagram for basaltic composition amphibolites over the 800 to 1100 c temperature range for pressures up to 2 0 gpa comparison of the major element compositions of the experimentally produced melts with compositions of presumed slab melts adakites shows that partial melting of amphibolite at conditions appropriate to a hot slab geotherm produces melts similar to andesitic and dacitic adakites except for significant mgo and cao depletions trace element modelling of amphibolite dehydration melting using the 2 0 gpa melting reactions produces ree abundances similar to those of adakites at 10 15 wt batch melting but the models do not reproduce the high sr y ratios characteristic of adakites taken together the major and trace element results are not consistent with the derivation of adakites by dehydration melting of the subducted slab with little or no interaction with the mantle wedge or crust if adakites are partial melts of the subducted slab they must undergo significant interaction with the mantle and or crust during which they acquire a number of their distinctive characteristics

prescriptions for heat flux and entrainment rates in the upper ocean during convection abstract a detailed investigation of the upper ocean during convection reveals the vertical structure of potential temperature to be steady in time and the current shear to vanish in the bulk of the mixed layer these imply that a slab type model may be an adequate representation of the convective ocean boundary layer obl in contrast when convection is not the dominant forcing mechanism the obl is stratified and can support a significant current shear this indicates the inadequacy of slab models for the nonconvective obl two independent estimates of the vertical heat flux profile in the convective obl were made the first estimate results from heat conservation and the steadiness of the vertical structure of potential temperature the second estimate is based on the turbulent kinetic energy tke balance and the vertical profiles of tke dissipation rate the estimates are consistent and suggest that the nondimensional vertical heat flux due to turbulence has a linear depth dependence o

a cryptic record of magma mixing in diorites revealed by high precision sims oxygen isotope analysis of zircons high precision in situ ion microprobe sims oxygen isotope analysis of zircons from two diorite intrusions associated with the late caledonian lochnagar pluton in scotland has revealed large differences in the degree of heterogeneity in zircon 18o between the diorites zircon crystals from the cul nan gad diorite cng show a unimodal distribution of oxygen isotope values 18o 6 0 0 6 2 and no or only minor grain scale variation those from the allt darrarie diorite ad1 show a large range in 18o and an apparent bimodal distribution with modes of 6 6 0 4 and 7 3 0 4 variations of up to 1 2 occur between and within grains both an increase and decrease in 18o with zircon growth has been observed the 18o composition of growing zircon can only change if open system processes affect the magma composition i e if material of contrasting 18o composition is added to the magma the variability in ad1 is interpreted to represent a cryptic record of magma mixing a deep crustal hot zone is a likely site for generation of the dioritic magmas which developed by mixing of residual melts and crustal partial melts or by melting of mafic lower crustal rocks the overall small number of zircons with mantle like 18o values 5 3 0 6 2 in the lochnagar diorites is largely the product of crustal differentiation rather than crustal growth the 18o of quartz from the cng and ad1 diorites shows only minor variation cng 10 9 0 5 2 ad1 11 7 0 6 2 within single populations with no evidence of mixing quartz zircon isotopic disequilibrium is consistent with later crystallisation of quartz from late magmatic fluids and in case of the ad1 diorite after the inferred magma mixing from a homogenised higher 18o melt high precision sims oxygen isotope analysis of zircon provides a new approach to identifying and resolving previously undetected early stage magma mixing and constraining the compositions and origins of the component magmas a combination of zircon quartz and whole rock data has proven to be a powerful tool in reconstructing the petrogenetic evolution of diorite from early crystallisation to late alteration

neoproterozoic active margin in the northwestern yangtze block south china new clues from detrital zircon u pb geochronology and geochemistry of sedimentary rocks from the hengdan group the yangtze block in south china constitutes an important precambrian landmass in the present east asian continent the neoproterozoic sedimentary successions of the hengdan group in the nw yangtze block record essential information for deciphering the neoproterozoic tectonics along the nw margin however its depositional age provenance and tectonic properties remain uncertain here a combined analysis of detrital zircon u pb dating and geochemistry is performed on representative samples from the hengdan group concordant dating results of samples from the bottom and upper parts constrain the maximum depositional age at c 720 ma detrital zircon age patterns of samples reveal a uniformly pronounced age peak at c 915 720 ma which is consistent with the magmatic pulses in domains at the nw end of the yangtze block in addition these samples display left sloping post archaean australian shale paas normalized rare earth element patterns and variable trace element patterns resembling sediments accumulated in a basin related to an active continental margin geodynamic setting provenance analysis reveals that the main sources featured intermediate to felsic components which experienced rapid erosion and sedimentation these integrated new investigations along with previous compilations indicate that the hengdan group might have been deposited in a fore arc basin controlled by subduction beneath the bikou terrane thus such interpretation further supports proposals for subduction related tectonics along the western margin of the yangtze block during the early neoproterozoic

neoproterozoic adakitic plutons and arc magmatism along the western margin of the yangtze block south china abstract neoproterozoic adakitic plutons that crop out along the western margin of the yangtze block south china from kangding on the north to panxi on the south provide constraints on the origin of the giant jinningian magmatic event of south china representative plutons include the xuelongbao 750 ma datian 760 ma and dajianshan intrusions the latter two bodies consist mainly of granodiorite with relatively high sio2 51 0 73 4 wt and mg s 0 36 0 55 they have fractionated rare earth element patterns with la yb n ratios ranging from 2 6 to 101 8 and are characterized by high sr 344 1018 ppm and low y 4 3 17 9 ppm yielding sr y ratios ranging from 27 to 111 on primitive mantle normalized trace element diagrams these rocks show enrichment of large ion lithophile elements and depletion of high field strength elements nb ta with positive zr hf and negative ti anomalies consistent with an arc related setting they have relatively constant initial whole rock 87sr 86sr ratios 0 70

early paleozoic tectonic evolution of the north qinling orogenic belt in central china insights on continental deep subduction and multiphase exhumation abstract the north qinling orogenic belt is an important component of qinling composite orogen in central china high pressure and ultra high pressure hp uhp rocks occur as lenses or layers surrounded by gneissic rocks in the northern central and southern qinling complex of the north qinling belt considered to have formed through deep continental subduction the magmatic protoliths of the hp uhp metamorphic rocks show formation ages ca 800 ma geochemical characteristics and pb nd isotopic compositions similar to those of the neoproterozoic igneous rocks of the south qinling belt continental materials of the south qinling belt were dragged down to mantle depths by the shangdan oceanic crust subducted and subjected to hp uhp metamorphism at ca 500 ma the erlangping and kuanping backarc basin formed in response to subduction of the shangdan ocean and might have developed into a limited small ocean basin at ca 500 ma the hp uhp rocks yielded retrograde metamorphic ages of ca 470 450 ma and ca 420 400 ma these ages are identical to the age of magmatic events in the north qinling hp uhp belt at 500 ma 450 ma and 420 ma related to deep subduction collision slab breakoff and crustal thinning during post collisional extension the dominant ca 500 400 ma ages of detrital zircons from the liuling group of the south qinling belt match well with those from the three stages of magmatic rocks and hp uhp rocks in the qinling complex this correlation suggests that the magmatic rocks and hp uhp metamorphic rocks in the north qinling belt initially exhumed to the surface eroded and were then deposited in the liuling basin in a post orogenic extensional setting during middle to late devonian new evidence suggests that the qinling complex is a tectonic complex rather than a uniform stratigraphic unit or a microcontinent as previously believed and is mainly composed of the exhumed hp uhp metamorphic rocks deep subduction exhumation related magmatic rocks and the early neoproterozoic granites together with the host rocks from the over riding plate at an active continental margin the early paleozoic tectonic history of the nqb includes oceanic slab subduction and formation of arc backarc spreading continental deep subduction arc continent collision break off and multi stage exhumation of the deep subducted slab as well as extension and thinning and associated erosion and deposition

electron spin resonance esr dating of pre quaternary faults in the sichuan basin sw china abstract the electron spin resonance esr dating method is thought to be limited to the quaternary period in this study we attempt to expand the esr dating range to the pre quaternary using three fault zone samples collected from faults in the sichuan basin and its surroundings we adopt a ray radiation dose of up to 2 104 gy which is much greater than that typically employed and find that the esr intensity of the e1 center a type of signal for esr dating in quartz continuously increases with the ray dose and is slightly influenced by the radiation dose rate this indicates the potential of esr to date pre quaternary faults by simulating environmental radiation under geological burial conditions we use esr to date the latest extension event in the western hubei and hunan thrust belt to 9 17 0 76 ma and the thrusting of the longmenshan fault zone toward the sichuan basin to approximately 6 14 0 73 ma conversely the saturated esr intensity of fault zone samples from the daba fault zone suggests a lack of strong tectonic activity in the southern daba fault zone in the last 10 ma our results are consistent with fission track ages and the regional tectonic evolution history therefore esr is a practical method for dating pre quaternary faults in structural and petroleum geology applications

paleomagnetic constraints of the lower triassic strata in south qinling belt evidence for a discrete terrane between the north and south china blocks

secular changes of water chemistry in shallow water ediacaran ocean evidence from carbonates at xiaofenghe three gorges area yangtze platform south china ediacaran carbonates from the shallow water section near xiaofenghe three gorges area hubei province on the yangtze platform south china have been studied to understand post marinoan changes in seawater chemistry major and trace element abundances and sr nd isotopic compositions were obtained on acetic acid leachates of carbonate sediments from both the doushantuo formation members d1 d4 and the overlying dengying formation c and o isotopic compositions of these samples were analysed using the phosphoric acid method with the exception of the d1 cap carbonates 87sr 86sr 0 7088 0 713 most samples show low mn sr and 87sr 86sr 0 7078 in d2 d4 and 0 709 in the dengying formation similar to seawater values inferred from ediacaran carbonates elsewhere these data together with the absence of a correlation between 87sr 86sr and 18ocarb indicate minor diagenetic fluid overprinting the initial nd values of the carbonate leachates show a limited variation range from 4 3 to 7 5 with the highest values in d1 these data overlap with published data for doushantuo sediments in south china and possibly indicate an isotopically homogenous nd source in the source area during most of the ediacaran d1 samples of cap carbonates display negative 13ccarb values of 2 0 to 3 6 vpdb and 18ocarb values ranging from 6 5 to 7 3 vpdb whereas the overlying d2 strata show positive 13ccarb values around 6 carbonate rocks from d3 and d4 show a large variation of 13ccarb from 1 4 to 9 4 with some samples having unusually high 13ccarb values coupled to low toc contents most carbonate leachates at the base of the doushantuo display superchondritic y ho and variable hree enrichment pr yb 1 similar to modern seawater this suggests a similar seawater chemistry as under modern open ocean conditions at the beginning of the ediacaran whereas lower y ho in d3 correlate with flat ree y patterns indicating a change to variable freshwater seawater mixing and suppression of hree lree fractionation as in modern estuaries carbonate rock samples from d4 are rich in clay minerals up to 15 of possible authigenic origin such as nontronite and saponite formation of these clay minerals may only be possibly under extreme alkaline and saline conditions the d4 carbonate leachates show the highest y ho and lowest ce ce values and have unusually high 13ccarb values which may also support the conditions of extreme shallowing and perhaps temporarily restricted basins that led to a rise in salinity due to high evaporation rates ce anomalies in carbonate leachates show strong variation throughout the profile a secular increase of seawater oxygenation through the ediacaran is not obvious however the most pronounced ce anomalies can be found in the upper doushantuo ce ce as low as 0 64

neoproterozoic accretionary tectonics along the northwestern margin of the yangtze block china constraints from zircon u pb geochronology and geochemistry abstract the neoproterozoic mafic intrusions and granitoids of the hannan and micangshan massifs located at the northwestern margin of the yangtze block in south china are composed mainly of subduction related plutons here we present la icpms zircon u pb data on the granitoids from wudumen xishenba xixiang and hanshan within the hannan massif which show crystallization ages of 718 9 ma 706 9 ma 707 20 ma and 777 8 ma respectively the arc related mujiaba gabbros display crystallization age of 746 4 ma in the micangshan massif the crystallization ages of the granodiorite at daheba and the granites at xihe and guangwushan are 871 77 ma 832 5 ma and 838 17 ma respectively whereas the shatan diorite xihe and zhengyuan gabbroic intrusions yield crystallization ages of 840 6 ma 824 4 ma and 857 46 ma respectively the tonalites in the wudumen pluton display adakitic affinity characterized by strong depletion of nb ta p and ti and enrichment of large ion lithophile elements lile i e cs rb ba th u and k as well as positive pb and sr anomalies suggesting an origin by the melting of a thickened mafic lower crust at ca 718 ma the syenogranite at xixiang is characterized by depletion of nb ta sr p and ti and marked enrichment of lile k and pb indicating a post collisional orogenic collapse at ca 707 ma the results from our study suggests that the arc related magmatism migrated from micangshan area during ca 870 820 ma through huijunba xixiang in ca 840 820 ma and to hannan during 825 706 ma based on which we propose an accretionary tectonic model for the northwestern margin of the yangtze block

wind forced motions in stratified lakes and their effect on mixed layer shear the solution for the response of a rectangular n layered stratified lake to an arbitrarily varying in time wind stress is presented the case n 3 is considered in detail for a three layered lake the response is made up of the individual responses of one external and two internal modes when the upper layer is shallow relative to the other two layers the mixed layer shear is due almost entirely to the slower second mode response moreover the upper interface can tilt quite severely while the lower interface remains nearly flat calculations based on the three layer model are compared with measurements made in wellington reservoir western australia

complex precambrian crustal evolution beneath the northeastern yangtze craton reflected by zircons from mesozoic volcanic rocks of the fanchang basin anhui province abstract our knowledge of the precambrian accretion and reworking history of deep crust beneath the northeastern yangtze craton is still sparse zircons from volcanic rocks can not only give accurate crystallization ages for zircons within the host magmas but also provide unique information on the history of deep crustal rocks sampled during magma ascent in situ u pb ages and hf isotopes were analyzed on zircons from two trachyandesites from the late mesozoic zhongfencun and kedoushan formations in the fanchang basin anhui province the zircons exhibit relatively complex internal structures four zircons in the zhongfencun trachyandesite yield a lower intercept 206 pb 238 u age of 133 0 3 2 ma with hf model ages of 2 8 2 4 ga t crust and significantly negative hf t 25 15 to 18 72 demonstrating that this lava formed in early cretaceous time zircons in the kedoushan biotite trachyandesite yield a 206 pb 238 u age of 126 1 ma as well as negative but relatively high hf t values up to 4 34 revealing it formed roughly coeval with the cretaceous a type granites and bimodal magmatism in this region more significantly u pb ages and hf isotopes for proterozoic zircon xenocrysts in the fanchang volcanic rocks reveal that the northeastern yangtze deep crust probably contains unexposed archean e g 3 4 2 9 ga and 2 8 2 5 ga components rarely identified from the outcrops in this area and has undergone complex evolutional history including the addition of juvenile mantle derived materials at 3 4 2 9 ga ca 2 65 ga and ca 0 8 ga and reworking of the precambrian crust at ca 2 0 1 8 ga ca 0 8 ga and 133 126 ma the history of the continental crust s accretion and reworking of the northeastern yangtze craton may not fully coincide with the major episodes of crustal evolution in the wider yangtze craton especially during the archean to paleoproterozoic periods

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

petrology of the chilliwack batholith north cascades washington generation of calc alkaline granitoids by melting of mafic lower crust with variable water fugacity calc alkaline granitoid rocks of the oligocene pliocene chilliwack batholith north cascades range from quartz diorites to granites 57 78 sio2 and are coeval with small gabbroic stocks modeling of major element trace element and isotopic data for granitoid and mafic rocks suggests that 1 the granitoids were derived from amphibolitic lower crust having ree rare earth element and sr nd isotopic characteristics of the exposed gabbros 2 lithologic diversity among the granitoids is primarily the result of variable water fugacity during melting the main effect of fh 2 o variation is to change the relative proportions of plagioclase and amphibole in the residuum the ree data for intermediate granitoids quartz diorite granodiorite eu eu 0 84 0 50 are modeled by melting with fh 2 o 1 kbar leaving a plagioclase pyroxene residuum in contrast data for leucocratic granitoids leuco granodiorites and granites eu eu 1 0 0 54 require residual amphibole in the source and are modeled by melting with fh 2 o 2 3 kbar consistent with this model isotopic data for the granitoids show no systematic variation with rock type 87sr 86sri 0 7033 0 7043 end 0 3 3 to 5 5 and overlap significantly with data for the gabbroic rocks 87sr 86sri 0 7034 0 7040 end 0 3 3 to 6 9 the fh 2 o variations during melting may reflect additions of h2o to the lower crust from crystallizing basaltic magmas having a range of h2o contents chillwack gabbros document the existence of such basalts one dimensional conductive heat transfer calculations indicate that underplating of basaltic magmas can provide the heat required for large scale melting of amphibolitic lower crust provided that ambient wallrock temperatures exceed 800 c based on lithologic and geochemical similarities this model may be applicable to other cordilleran batholiths

on the response of a reservoir sidearm to diurnal heating and cooling during the day the shallower regions of a reservoir sidearm absorb more heat per unit volume than the deeper parts leading to a horizontal pressure gradient that drives a circulation in the sidearm at night the shallow regions cool more rapidly leading to a circulation in the opposite direction since the spin up time of a typical sidearm is at least of the same order as a day the flow within a diurnally forced sidearm is principally an inertia buoyancy balance in this paper a diurnally forced sidearm is modelled by periodically forced natural convection in a triangular cavity the periodic forcing enters the model via an internal heating cooling term in the temperature equation reservoir sidearms typically have small bottom slopes and this fact can be exploited to obtain asymptotic solutions of the resulting equations these solutions clearly demonstrate the transition from the viscous dominated flow in the shallows to the inertia dominated flow in the deeper parts in the inertia dominated region the flow response significantly lags the forcing numerical solutions of the full nonlinear problem are consistent with the asymptotic solutions