thermochronology of the posen complex northern vietnam implications for tectonic evolution in se asia abstract the posen complex located closely adjacent to the southwestern margin of the red river shear zone represents the uplifted basement of north vietnam and may record the motion of the shear zone however its thermochronological history has not been fully examined yet here we applied u pb and 40 ar 39 ar dating methods to reveal its thermochronological history u pb analysis of composite zircon grains by tims yielded an average age of 760 25 ma clustering on the concordia line twelve shrimp u pb analyses also yielded a consistent result of 751 7 ma along with the geochemical features the u pb dating results suggest the posen complex was a late proterozoic magmatic complex which could correspond to the chengjiang orogeny a widespread thermal event in southwest china results of 40 ar 39 ar dating of micas and k feldspars were in the range of 36 30 ma revealing a rapid cooling and exhumation history of the posen complex during the late paleogene the time span of cooling and exhumation of the posen complex is slightly older than the main cooling phases of the ailao shan red river asrr metamorphic massifs 28 17 ma but is synchronous with the early igneous activity stage in the eastern indo asian collision zone of southeast china and north vietnam owing to the ongoing debate about the initiation and offset of the asrr shear zone the tectonic force for the late paleogene cooling of the posen complex is still inconclusive the rapid exhumation of the posen complex could be in response to either the detachment of the neo tethyan slab or a transpressional phase of continental subduction along the asrr shear system in the eastern indo asian collision zone

intraplate extension prior to continental extrusion along the ailao shan red river shear zone left lateral movement of the ailao shan red river shear zone lends support to the hypothesis of continental extrusion resulting from the collision of india with asia our new observations from northwestern yunnan china and northwestern vietnam on different sides of the shear zone demonstrate that the sinistral offset was 600 km according to correlations of permian triassic flood basalt successions and late paleogene highly potassic mafic magmas we conclude that the shear was propagating on the south china continental margin and does not correspond to a suture between south china and indochina furthermore the highly potassic magmas were emplaced from ca 40 to 30 ma before the shear movement which was caused by the late oligocene to early miocene ca 27 22 ma extrusion activity this suggests that a late eocene to early oligocene intraplate extension possibly induced by delamination of thickened continental lithosphere took place in northwestern yunnan or eastern tibet as a response to the india asia collision this extension and sea floor spreading of the south china sea that began ca 30 ma could have accounted for the initiation of the ailao shan red river shear zone

fe cu deposits in the kangdian region sw china a proterozoic iocg iron oxide copper gold metallogenic province numerous fe cu deposits are hosted in the late paleoproterozoic dongchuan and dahongshan groups in the kangdian region sw china the dongchuan group is composed of siltstone slate and dolostone with minor volcanic rocks whereas the dahongshan group has undergone lower amphibolite facies metamorphism and consists of quartz mica schist albitite quartzite marble and amphibolite with local migmatite deposits in the dongchuan group are commonly localized in the cores of anticlines in fault bends and intersections and at lithological contacts orebodies are closely associated with breccias which are locally derived from the host rocks fe oxides magnetite and or hematite and cu sulfides chalcopyrite bornite form disseminated vein like and massive ores and typically fill open spaces in the host rocks the deposits have extensive albite alteration and local k feldspar alteration overprinted by quartz carbonate sericite and chlorite deposits in the dahongshan group have orebodies sub parellel to stratification and show crude stratal partitioning of metals fe oxide ores occur as massive and or banded replacements within the breccia pipes whereas cu sulfide ores occur predominantly as disseminations and veinlets within mica schists and massive magnetite ores ore textures suggest that cu sulfides formed somewhat later than fe oxides but are possibly within the same mineralization event both ore minerals predated regional neoproterozoic metamorphism both orebodies and host rocks have undergone extensive alteration of albite scapolite amphibole biotite sericite and chlorite silica and carbonate alterations are also widespread ore hosting strata have a la icp ms zircon u pb age of 1681 13 ma and a dolerite dyke cutting the fe oxide orebodies has an age of 1659 16 ma thus the mineralization age of the dahongshan deposit is constrained at between the two all ores from the two groups have high fe and low ti with variable cu contents locally they are rich in mo co v and ree but all are poor in pb and zn sulfides from the fe cu deposits have 34s values mostly in the range of 2 to 6 per mil suggesting a mix of several sources due to large scale leaching of the strata with the involvement of evaporites isotopic dating and field relationships suggest that these deposits formed in the late paleoproterozoic ore textures mineralogy and alteration characteristics are typical of iocg type deposits and thus define a major iocg metallogenic province with significant implications for future exploration

land subsidence due to groundwater withdrawal potential damage of subsidence and sea level rise in southern new jersey usa land subsidence due to groundwater withdrawal combined with a global sea level rise creates a serious environmental problem in the coastal region groundwater withdrawal results in fluid pressure change in the layers the pressure change in the layers induces both elastic and inelastic land compaction the elastic compaction can be recovered if the water level rises again and inelastic compaction becomes permanent groundwater response to barometric pressure change is used to estimate the elastic compaction in this study the storativity specific storage and other layer and hydrological information are used to estimate the inelastic compaction of the layers due to fluid withdrawal the discussed methods are applied to estimate and predict the subsidence potentials resulting from overdrafting of the groundwater in the southern new jersey the estimated subsidence is about 2 3 cm near the location of monitoring wells in atlantic camden cumberland and cape may counties over the past 20 years if the current trend of water level drop continues the average subsidence in southern new jersey in the vicinity of some monitoring wells will be about 3 cm in the next 20 years the rise of global sea level is about 2 mm year on average because of the very gentle slope in southern nj the combination of subsidence and sea level rise will translate into a potentially substantial amount of land loss in the coastal region in each 20 year period this combination will also accelerate the coastal flooding frequency and the erosion rate of the new jersey coastal plain and pose a serious threat to the coastal economy

subsurface characterization and prediction of land subsidence for hcm city vietnam abstract the demands of domestic and industrial water supply in ho chi minh city hcm city have surged rapidly since 1990s with the opening up of vietnam s economy in the same time groundwater extraction has continuously increased and is suspected to have caused land subsidence which in turn would affect the growing infrastructure of the city by now only a few monitoring stations have been installed and both pore pressure and settlement observation data are not yet available as an alternative we proposed an approach to estimate land subsidence based on the available data of groundwater drawdown in the pumped aquifer and geotechnical properties of the adjacent clay layers in this study a large amount of borehole data was collected to construct a computer based subsoil database as a result a complete characterization of a six aquifer system up to 360 m deep and that of the shallower subsoil profile up to 140 m deep was done based on data from more than 3000 boreholes examples of land subsidence prediction were done for three cases i e i at a borehole location that has sign of well protrusion ii along the new mrt line no 1 and iii over a profile crossing the city from nw to se the first two cases were to show the procedure of land subsidence prediction while the third case was to compare the fem analysis results with those estimated by satellite image analysis the results of this study showed that the subsoil in hcm city with thick soft clay layers is vulnerable to the groundwater extraction it is expected that the maps of drawdown and the subsoil database developed by this study in combination with a 1d fem consolidation program will provide a useful tool to estimate land subsidence of hcm city while the pore pressure and settlement data are not yet available

ages and compositions of primary and secondary allanite from the lala fe cu deposit sw china implications for multiple episodes of hydrothermal events numerous fe cu deposits in southwestern china form the kangdian iron oxide copper gold iocg metallogenic province these deposits have a close association of fe oxides and cu sulfides formed at different stages which are possibly related to multiple hydrothermal events in this paper u pb dating and chemical analyses on allanite from different stages of the lala deposit were used to constrain timing and origin of such events allanite occurs as disseminated grains or patches in fe cu ores and is closely associated with chalcopyrite molybdenite calcite and minor titanite postdating magnetite and apatite high resolution backscattered electronic bse imaging electron microprobe compositions and x ray scanning profiles demonstrate that ree rich primary allanite was replaced by later relatively porous and ree poor secondary allanite such a replacement was promoted by interaction between primary allanite and fluid fluxes infiltrating the minerals following an exchange scheme of ree3 fe2 ca2 al3 the secondary allanite has higher fe3 fe3 fe2 ratios and u contents indicating involvement of relatively oxidized fluids during alteration the alteration has also produced unidentified secondary ree minerals in fractures indicating re deposition of some of the removed rees the primary and secondary allanites are dated by in situ la icp ms technique and have u pb ages of 1 067 41 ma and 880 850 ma respectively the 1 07 ga primary allanite was contemporaneous with the main mo cu lree mineralization with a molybdenite re os age of 1 08 ga the 880 850 ma secondary allanite is comparable with the ar ar ages 890 830 ma of biotite from hosting schists and undeformed sulfide veins occurring throughout the kangdian province suggesting that such an event was possibly syn deformational and represents a younger hydrothermal event occurrences of both primary and secondary allanites suggest that the mineralization may have involved multiple tectonothermal events including the 1 05 1 1 ga intra plate and subsequent 960 740 ma arc magmatism in the kangdian region

late paleoproterozoic sedimentary and mafic rocks in the hekou area sw china implication for the reconstruction of the yangtze block in columbia abstract the paleoproterozoic hekou group in the western yangtze block is a volcano sedimentary succession that is intruded by gabbroic plutons sedimentary rocks in the group include slates marble and meta siltstones interlayered with felsic metavolcanic rocks metabasalts and metatuffs both the volcanic rocks of the hekou group and gabbros have undergone upper greenschist to lower amphibolite facies metamorphism metatuff samples from different layers have average zircon u pb ages of 1697 ma slightly older than the intruding gabbroic plutons with zircon u pb ages of 1684 ma both metabasalts and metagabbros have similar elemental and isotopic compositions indicative of a co magmatic origin they are rich in tio2 mostly 2 5 wt zr 94 5 347 ppm ta 0 48 3 00 ppm and th 1 05 7 61 ppm with high nb y ratios mostly 0 6 and lree enriched chondrite normalized ree patterns resembling within plate mafic rocks their positive whole rock nd t 0 2 to 3 4 and zircon hf t values 3 3 to 8 4 suggest contributions from a depleted mantle source the large range of nd t and hf t values and variable degrees of nb ta anomalies ta lapn 0 17 1 75 are indicative of crust contamination during magma ascending both the metabasalts and metagabbros are considered to have formed in a continental rift setting detrital zircon grains from meta siltstones in the hekou group have u pb age populations mainly at 2070 1880 ma 2330 2250 ma and 2900 2700 ma these age populations are comparable to those of the north australian and north china cratons in the columbia supercontinent however in terms of geochemical features the 1 7 ga within plate mafic rocks at hekou are similar to those from the 1 7 ga leichhardt and calvert superbasins of the north australian craton but different from those from the 1 7 1 2 ga zhaertai bayan obo rift zone of the north china craton it is thus suggested that the yangtze block was more likely linked with the north australian craton in columbia during the late paleoproterozoic these rifting basins and mafic rocks may record the initial break up of the columbia supercontinent

areal subsidence under pumping well curtain interaction in subway foundation pit dewatering conceptual model and numerical simulations subway foundation pit dewatering may contribute to the regional land subsidence in a built up area when aquifers are too thick to be cut off by a curtain the land subsidence induced by subway foundation pit dewatering was divided into local subsidence and areal subsidence the former was managed by construction organizations while the latter was managed by a land resource urban management and hazard prevention department the areal subsidence could not be recovered after dewatering because of its vast influence area and contribution to regional land subsidence the boundary between local subsidence and areal subsidence was defined as the location of three times the foundation pit depth to the foundation pit boundary the subsidence within the boundary belonged to local subsidence whereas the subsidence outside the boundary belonged to areal subsidence with shanghai as background the conceptual and mathematical models that considered hydrogeological conditions curtain depth and pumping well screens were established and numerical simulations were performed when the pumping well screens were enveloped by a diaphragm wall the best vertical distance of about 1 0 4 0 m was obtained between the screen bottoms and curtain bottom the decreasing rate of areal subsidence was the highest under the interaction of diaphragm wall and pumping well screens further increasing the penetration depth of the diaphragm wall and shortening the length of the pumping well screen were both less efficient

using elemental and boron isotopic compositions of tourmaline to trace fluid evolutions of iocg systems the worldclass dahongshan fe cu deposit in sw china abstract the worldclass dahongshan iron oxide copper gold iocg deposit is hosted within the late paleoproterozoic dahongshan group in the kangdian region sw china the orebodies are spatially associated with sub volcanic intrusions and are structurally controlled extensive metamorphic overprinting during the neoproterozoic makes it difficult to determine the initial compositions of the ore fluids tourmaline however is a useful phase for this purpose because of its refractory nature and chemical diversity it occurs widely in the four stages of mineralization and alteration of the dahongshan deposit including pre mineralization sodic alteration stage i iron oxide mineralization stage ii sulfide mineralization stage iii and late quartz calcite veining stage iv the temporal association between tourmaline and hydrothermal alteration provides an excellent opportunity to investigate the fluid evolution tourmaline from all four stages belongs to the alkali group and ranges from dravite to schorl in composition in situ b isotope analyses of tourmaline conducted by la mc icpms yielded a total range of 11 b values from 14 7 to 5 9 and the values show significant variations among the different stages tourmaline of stage i albitization has 11 b values from 14 7 to 5 7 stage ii tourmaline from fe oxide ores shows 11 b values of 11 6 to 6 1 similar to those of stage i whereas stage iii tourmaline from cu sulfide ores displays significantly heavier isotopes with 11 b values from 4 4 to 0 6 tourmaline of stage iv represented by barren quartz calcite tourmaline veins in intensely altered wall rocks has positive 11 b values from 2 9 to 5 9 the observed variations of b isotopes cannot be produced by fractionation of the ore fluids alone mixing of b from at least two distinct sources a magmatic source and external sources with highly positive 11 b values are hence proposed on the basis of geological constraints and modal calculations the calculated oxygen isotope compositions of fluids in equilibrium with hydrothermal minerals that associated with tourmaline from stage ii stage iii and stage iv are 7 4 to 10 2 8 4 to 8 6 and 3 8 to 8 0 the decreasing trend of 18 o of fluids from stage ii to stage iv is consistent with the incursion of external basinal brines by fluid mixing our dataset thus demonstrates that ore fluids of dahongshan iocg system were initially of magmatic origin stages i and ii and were later progressively modified by mixing with external brines stages iii and iv

global changes in intensity of the earth s magnetic field during the past 800kyr recent advances in palaeomagnetic and dating techniques have led to increasingly precise records of the relative intensity of the earth s past magnetic field at numerous field sites the compilation and analysis of these records can provide important constraints on changes in global magnetic field intensity and therefore on the earth s geodynamo itself a previous compilation for the past 200 kyr integrated 17 marine records into a composite curve1 with the geomagnetic origin of the signal supported by an independent analysis of 10be production made on different cores2 the persistence of long term features in the earth s magnetic intensity or the existence of long term periodic changes cannot however be resolved in this relatively short time span here we present the integration of 33 records of relative palaeointensity3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 into a composite curve spanning the past 800 kyr we find that the intensity of the earth s dipole field has experienced large amplitude variations over this time period with pronounced intensity minima coinciding with known excursions in field direction reflecting the emergence of non dipole components no stable periodicity was found in our composite record and therefore our data set does not support the hypothesis that the earth s orbital parameters have a direct and strong influence on the geodynamo

multiple episodes of mineralization revealed by re os molybdenite geochronology in the lala fe cu deposit sw china the lala fe cu deposit is one of the largest iron oxide copper gold iocg deposits in the kangdian copper belt southwest china the paragenetic sequence of the lala deposit includes six hydrothermal stages pre ore pervasive na alteration i magnetite stage with k feldspar and apatite ii polymetallic disseminated massive magnetite sulfide stage iii banded magnetite sulfide stage iv sulfide vein stage v and late quartz carbonate vein stage vi fifteen molybdenite separates from stages iii to vi were analyzed for re os dating our new re os data together with previous studies identify four distinct hydrothermal events at the lala deposit molybdenite from the stage iii disseminated to massive chalcopyrite magnetite ores yielded a weighted average re os age of 1306 8 ma mswd 1 1 n 6 which represents the timing of main ore formation molybdenite from the stage iv banded magnetite chalcopyrite ores yielded a weighted average re os age of 1086 8 ma mswd 2 2 n 7 i e a second ore forming event molybdenite from the stage v sulfide veins yielded a weighted average re os age of 988 8 ma mswd 1 3 n 7 which represents the timing of a third hydrothermal event molybdenite from the quartz carbonate veins stage vi yielded a weighted average re os age at 835 4 ma mswd 0 66 n 10 and documented the timing of a late hydrothermal event our results indicate that the lala deposit formed during multiple protracted mineralization events over several hundred million years the first three mesoproterozoic mineralization events are coeval with intra continental rifting breakup of the supercontinent nuna and share a temporal link to other iocg style deposits within the kangdian copper belt and the last neoproterozoic hydrothermal event is coeval with the sibao orogeny which culminated with the amalgamation of the yangtze block with the cathaysia block at 860 815 ma

geodynamic estimates of the viscosity of the earth s inner core recent seismological studies1 2 have suggested that the inner core is rotating relative to the bulk of the earth a situation which according to numerical simulations3 may be sustained by convective flow in the liquid outer core on the other hand large gravitational forces due to the heterogeneous distribution of mass in the earth s mantle should be sufficient to keep the inner core aligned with the mantle4 here i show that the differential rotation of the inner core can be reconciled with these strong gravitational forces by allowing the shape of the inner core to adjust as it rotates so permitting an estimate of the effective viscosity of this innermost region of the earth the inferred rotation rate constrains the viscosity of the inner core to be less than 1016 pa s or greater than 1020 pa s as two different dynamical regimes are possible the viscosity estimates for these two regimes have very different implications for the origin of seismic anisotropy in the inner core

core mantle coupling including a viscoelastic inner core an application to the axial rotation associated with the quaternary glacial cycles abstract the mass redistribution associated with the quaternary glacial cycles with a cycle of approximately 90 kiloyears glaciation and 10 kiloyears deglaciation phases causes differential rotations of the outer and inner cores relative to the mantle the differential rotations are examined for an earth model composed of incompressible maxwell viscoelastic mantle inviscid outer core and incompressible maxwell viscoelastic inner core a quasi rigid rotation is assumed for both cores and viscous and electromagnetic torques at the core mantle boundaries cmb and inner core boundaries icb are taken into account the effect of the inner core viscosity is negligibly small but the lower mantle viscosity l influences significantly on the predictions in a weak frictional coupling case the differential rotation of both cores amounts to 5 10 10 rad s 1 0 9 per year at the end of the deglaciation its magnitude gradually increases with increasing lower mantle viscosity and generally approaches to a constant value at l 1022 pa s the predictions are sensitive to the frictional torques at the cmb and icb if the strength of the poloidal field at the icb bp is larger than the half at the cmb then both cores rotate at a similar rate the critical poloidal strength bpc of course depends on an uncertain value of the conductivity at the top of the inner core in the case of bp 3 10 21 pa s l 6 10 21 pa s the magnitude of the outer and inner cores is however less than 5 10 11 and 1 10 10 rad s 1 respectively the response to rapid potential variations corresponding to a pulse like sea level change such as meltwater pulse observed at 14 kiloyears bp is dominantly elastic and takes a maximum value of 10 10 rad s 1 0 2 per year the acceleration for this response however reaches 10 20 rad s 2 significantly larger than the maximum estimate of 10 21 rad s 2 for an average potential variation model time series of the predicted differential rotation may be grouped into three distinctive types from the degree of the coupling at the cmb in a relatively weak coupling case the predictions are approximately similar to an average sea level change of the quaternary glacial cycles and the spectrum evaluated from the time series has a sharp peak at the period of about 100 kiloyears in a strong coupling case the dominant signals are due to the pulse like potential variations resulting in a white spectrum

two episodes of rees mineralization at the sin quyen iocg deposit nw vietnam abstract the sin quyen iron oxide copper gold iocg deposit in northwestern vietnam contains a proven reserve of 52 8 mt ores at 14 fe 0 91 cu 0 7 rees rare earth elements and 0 44 g t au rees mineralization is hosted within ca silicate alteration zone and associated with fe cu ore bodies in the gneiss mica schist and marble of the upper sin quyen formation two mineralization episodes are identified based on the field and petrographic relations the first episode includes sodic alteration stage i 1 calcic alteration and associated ree mineralization stage i 2 and localized k fe alteration stage i 3 the second episode is main stage mineralization and contains four main stages of alteration pre ore na alteration stage ii 1 ca fe alteration and the associated fe ree mineralization stage ii 2 k fe alteration and associated cu au mineralization stage ii 3 and the final carbonate stage of quartz carbonate veins stage ii 4 crosscutting the above mentioned alteration phases and ores primary allanite and type 1 titanite tnt 1 from episode 1 yield weighted mean 206pb 238u ages of 882 3 ma and 881 8 ma 2 respectively which are interpreted as the timing of ree mineralization in episode 1 secondary allanite and type 2 titanite tnt 2 associated with iron oxide and cu au ores however have u pb ages of 840 7 ma and 838 5 ma 2 respectively these ages are hence considered to represent the timing of polymetallic iocg mineralization in episode 2 880 ma titanite grains have end t values ranging from 7 00 to 5 43 which are intermediate between the neoproterozoic felsic intrusions and ore hosting meta sedimentary rocks in the region and likely reflect interaction between magmatic fluids and ore hosting rocks in contrast 840 ma titanite have end t values ranging from 4 31 to 1 97 similar to those of the neoproterozoic felsic intrusions indicating a possible magmatic fluid source for the episode ii iocg mineralization multiple episodes of mineralization at sin quyen were likely formed during the prolonged tectonothermal events related to backarc extension during the neoproterozoic subduction along the western margin of the yangtze block

multiple stages of hydrothermal ree remobilization recorded in fluorapatite in the paleoproterozoic yinachang fe cu ree deposit southwest china abstract the yinachang deposit in the kangdian region southwest china contains large amounts of fe cu and ree and formed at 1700 ma in this deposit there are three stages of alteration and mineralization including pre ore na fe alteration fe ree mineralization and cu ree mineralization in the fe ree mineralization stage ree rich fluorapatite with total ree concentrations ranging from 10 700 to 34 000 ppm formed together with magnetite in the following cu ree mineralization stage large amounts of ree especially lree were leached out of the ree rich fluorapatite due to the interaction between fluorapatite and cl f co2 and ca rich but ree unsaturated fluids the leaching of ree was associated with the removal of si na th u pb and ba and modification of the oxygen isotope signature in the fluorapatite during a 840 ma tectonothermal event ree rich fluorapatite underwent the second interaction with oxidized f co2 and possibly cl rich but na and ca deficient fluids due to fluid fluorapatite interaction ree were removed from the fluorapatite but were immediately reincorporated into new phases within the fluorapatite thus the altered fluorapatite contains abundant ree mineral inclusions including bastnasite ce monazite ce and minor xenotime y a very small portion of the lree were transported out of the fluorapatite and formed bastnasite ce and monazite ce grains in the vicinity of the altered fluorapatite in addition to the metasomatism of fluorapatite allanite and primary synchysite ce from the cu ree mineralization stage were also altered with variable replacement of allanite by an assemblage of synchysite ce chlorite bastnasite ce and primary synchysite ce by bastnasite ce these styles of alteration were possibly synchronous with the second alteration phase of the fluorapatite this study demonstrates that ree can be mobilized during multiple stages of hydrothermal activities and highlights the significant controls fluid compositions have on ree transportation and deposition in hydrothermal system

sulfide re os and rb sr isotope dating of the kangdian iocg metallogenic province southwest china implications for regional metallogenesis the kangdian iron oxide copper gold iocg metallogenic province contains a number of economically important fe cu deposits in south china and northern vietnam due to the lack of precise mineralization ages the metallogenesis of this province has long been a matter of debate in this study molybdenite re os pyrite re os and rb sr isotope methods are used to date the yinachang and e touchang fe cu deposits in the central part of this province fine grained 0 1 mm molybdenite occurs as veinlets along bandings or fractures of magnetite ores and hydrothermal veins in the yinachang deposit molybdenite separates from magnetite ores have highly reproducible re os model ages of 1654 7 ma whereas those from hydrothermal veins have a much younger re os model age of 1451 6 ma euhedral pyrite grains occur locally along foliations of hematite ores and their proximal alteration assemblages in the e touchang deposit pyrite separates have an re os isochron age of 1487 110 ma and a single grain rb sr isochron age of 1453 28 ma both of which are compatible with the molybdenite re os age of hydrothermal veins from the yinachang deposit the molybdenite age of magnetite ores is in good agreement with zircon u pb ages of dolerite intrusions in the region and is considered to represent the timing of a major ore formation event at 1655 ma the younger re os and rb sr ages likely record a secondary hydrothermal overprint event at 1450 ma which has not been recognized before the mineralization potential and implications of this event need to be investigated in future studies our new isotopic data set hence demonstrates that the kangdian iocg deposits were likely formed at 1655 ma synchronous with the late paleoproterozoic intracontinental rifting event of the western yangtze block

in situ sr isotope analysis of apatite by la mc icpms constraints on the evolution of ore fluids of the yinachang fe cu ree deposit southwest china apatite is a ubiquitous accessory mineral in a variety of rocks and hydrothermal ores strontium isotopes of apatite are well known to retain petrogenetic information and have been widely used to investigate the origin of igneous rocks but such attempts have rarely been made to constrain ore forming processes of hydrothermal systems we here report in situ la mc icpms sr isotope data of apatite from the 1660 ma yinachang fe cu ree deposit southwest china the formation of this deposit was coeval to the emplacement of regionally distributed doleritic intrusions within a continental rift setting the deposit has a paragenetic sequence consisting of sodic alteration stage i magnetite mineralization stage ii cu sulfide and ree mineralization stage iii and final barren calcite veining stage iv the stage ii and iii assemblages contain abundant apatite allowing to investigate the temporal evolution of the sr isotopic composition of the ore fluids apatite of stage ii apt ii is associated with fluorite magnetite and siderite whereas apatite from stage iii apt iii occurs intimately intergrown with ankerite and cu sulfides apt ii has 87sr 86sr ratios varying from 0 70377 to 0 71074 broadly compatible with the coeval doleritic intrusions 0 70592 to 0 70692 indicating that ore forming fluids responsible for stage ii magnetite mineralization were largely equilibrated with mantle derived mafic rocks in contrast apt iii has distinctly higher 87sr 86sr ratios from 0 71021 to 0 72114 which are interpreted to reflect external radiogenic sr likely derived from the paleoproterozoic strata some apt iii crystals have undergone extensive metasomatism indicated by abundant monazite inclusions the metasomatized apatite has much higher 87sr 86sr ratios up to 0 73721 which is consistent with bulk rock rb sr isotope analyses of cu ores with 87sr 86sri from 0 71906 to 0 74632 the elevated 87sr 86sr values of metasomatized apatite and bulk cu ores indicate that later fluids were dominated by highly radiogenic sr equilibrated with the paleoproterozoic country rocks results of this study highlight the utilization of in situ sr isotope analysis of apatite in unraveling the evolution of hydrothermal systems

gps measured land subsidence in ojiya city niigata prefecture japan abstract land subsidence caused by compression of clay layers in ojiya city japan was measured by global positioning system gps between 1 april 1996 and 31 december 1998 three baselines were selected in and around the city and height difference on a wgs 84 ellipsoid was measured by gps on each baseline the ground at the gps station in the city subsides and rebounds 7 cm every winter and spring respectively measurement accuracy was 9 5 mm standard deviation ground water level was observed at a well near the gps station regression analysis between total strain calculated as ratio of the height difference displacement to the total thickness of the clay layers and the layers effective stress change with ground water level change gave good correlation the slope of regression line 7 0 10 11 m2 n was obtained as an average apparent coefficient of volume compressibility of the layers

the effects of pulsed pumping on land subsidence in the santa clara valley california abstract land subsidence caused by pulsed and constant pumping schemes was calculated and the results were compared to determine the possible advantages of pulsed pumping in reducing land subsidence pulsed pumping refers to pumping strategies involving alternation between periods of pumping and recovery in a well subsidence was calculated using a numerical model based on the santa clara valley ca the model accounts for delay in the release of water from storage in compressible interbeds as well as flow within aquifers for these tests the ratio of pumping period length to recovery period length was either 1 1 or 1 3 pulsing interval length length of one pumping period plus one recovery period ranged from 4 to 90 days pumping rates were adjusted so that pulsed pumping and constant pumping removed the same volume of water over the period under consideration pulsed pumping has the potential to concentrate subsidence around the pumping well for all simulations pulsed pumping caused greater subsidence near the test well than constant pumping under some conditions a transition point was reached beyond which pulsed pumping caused less subsidence than steady pumping the distance from the test well to the transition point decreases for a decreasing aquifer transmissivity b decreasing ratio of pumping period to recovery period and c increasing pulsing interval length the concentration of subsidence was reduced considerably when delay in release of water from storage in compressible aquitards was neglected indicating the importance of including delay effects in subsidence studies all pulsed pumping strategies caused a greater total volume of subsidence than steady pumping

the oldest known rocks in south western china shrimp u pb magmatic crystallisation age and detrital provenance analysis of the paleoproterozoic dahongshan group abstract the paleoproterozoic dahongshan group outcrops along the present day southwest margin of the yangtze craton it is the oldest rock found in the region and forms the basement for the late mesoproterozoic sedimentary successions the dahongshan group is a sequence of volcanic and sedimentary rocks which have undergone structural deformation and metamorphism to upper greenschist lower amphibolite facies some of the original sedimentary features such as unimodal cross bedding in quartzite and rounded conglomerate clasts are still preserved suggesting deposition in a fluvial environment abundant carbonate rocks also found within the sedimentary sequence indicate periods of shallow marine or lagoonal conditions a weighted mean shrimp 206pb 207pb age of 1675 8 ma for a tuffaceous schist unit confirms the paleoproterozoic formation age of the group shrimp and lam icp ms u pb analysis of detrital zircon grains from metasedimentary units indicate a late archean to paleoproterozoic ca 2780 1860 ma source region the age spectrum of the detrital zircons is similar to that of the late archean and early paleoproterozoic basement rocks previously identified in the yangtze craton but includes a population of paleoproterozoic grains ca 2400 2100 ma which have no known source region in the exposed yangtze craton

groundwater derived land subsidence in the north china plain land subsidence usually bears strong relationship to abstraction of underground fluid land subsidence occurs commonly in the north china plain ncp and has become a major environmental factor hindering regional sustainable development this paper focuses on issues associated with mechanism of land subsidence in the ncp the analysis shows that multi layer aquifer systems with deep confined aquifers and the thick normally consolidated or unconsolidated compressible clay layers are the key of geological and hydrogeological conditions favorable for the development of land subsidence in the ncp groundwater withdrawal results in an increase in the distribution of effective stress within the strata and the compression of the aquifers and the confining layers and then triggers land subsidence in the middle east plain of the ncp the land subsidence volume approximately represents the amount of water released from compression of deep aquifers and aquitards in that land subsidence is primarily caused by excessive groundwater withdrawal in deep aquifer system the percentage of water released from compression of aquifers and aquitards in deep groundwater abstraction is significant but distinct in cangzhou city and in the whole middle east plain this is due to the difference in local lithological structure and recharge and discharge conditions of deep groundwater system the hysteresis of land subsidence is also discussed in typical areas and the results reveal that the time for completing the primary consolidation ranges from less than one year to tens of years and that the rate of secondary compression tends to increase with the moisture content

constraints of sr isotopic compositions of apatite and carbonates on the origin of fe and cu mineralizing fluids in the lala fe cu mo lree deposit sw china abstract numerous fe cu deposits with mineralization styles similar to iron oxide copper gold iocg deposits form the kangdian fe cu metallogenic province southwestern sw china as one of the largest deposits in the region the 1 0 ga lala fe cu deposit is hosted in a paleoproterozoic volcanic sedimentary succession named the hekou group which is alternately intruded by 1 0 ga doleritic plutons this deposit has a paragenetic sequence evolving from stage i of na alteration to stage ii of fe mineralization and finally to stage iii of cu mo ree mineralization coeval with mafic felsic intra plate magmatism in the region this study conducted in situ sr isotopic analyses on apatite and carbonate aiming to resolve the long controversial issue regarding the origin of the fe and cu mineralizing fluids in the deposit apatite of stage ii has 87sr 86sr ratios varying from 0 71380 to 0 72733 much higher than those of synchronous igneous rocks in the region 0 7074 to 0 7091 but similar to the paleoproterozoic host rocks 0 71368 to 0 71837 at 1 0 ga this similarity indicates that radiogenic sr of the fe mineralizing fluid was dominantly sourced from the host rocks apatite and calcites of stage iii have 87sr 86sr ratios 0 75758 0 79293 much higher than apatite of stage ii and the host rocks but similar to the archean basement rocks as high as 0 80 at 1 0 ga beneath the cover of the yangtze block suggesting that the highly radiogenic sr isotopic composition of the cu mineralizing fluid was mainly inherited from the old basement rocks in combination with previous c o s isotopic data indicating a magma hydrothermal origin it was suggested that the fe mineralizing fluid was exsolved from a mafic magma that generated the 1 0 ga doleritic plutons and inherited radiogenic sr from the host rocks during fluid rock interaction by contrast the cu mineralizing fluid might have been sourced from another pulse of magmatic cu mo ree and co2 rich fluid which have once interacted with archean basement rocks prior to mineralization the source of such a cu mo ree rich fluid was not well constrained in current study but was inferred to be exsolved from a hidden felsic magma we propose that intrusions of the bimodal magmas in kangdian are responsible for regional hydrothermal circulation which led to fe cu mo ree mineralization in the kangdian province

correlation between groundwater level and altitude variations in land subsidence area of the choshuichi alluvial fan taiwan one of the prominent features observed in the groundwater overdraft area is the land subsidence that creates a permanent damage to the land surface severe land subsidence has occurred in the western taiwan due to excessive extraction of groundwater in the past decades the areas of land subsidence have been gradually expanded and caused many problems such as flooding and building damage this study examines the elevation changes on surface which is the vertical component from the global positioning system in the choshuichi alluvial fan of central taiwan and describes the land subsidence phenomena quantitatively the choshuichi alluvial fan has a high density array of wells monitoring the groundwater level variations of various aquifers automatically in an hourly basis through an assumed linear relationship the inferred results show that the global positioning system data are closely correlated to the groundwater level variations of deep aquifers the highest correlations between elevation changes and groundwater level variations have been found in the land subsidence area even if a drought period is present the responses on the surface vertical displacements from the groundwater level changes of deep aquifers can be quantitatively estimated furthermore the long term subsidence trends can be derived and be used as useful reference for land and water resources management

velocity field of gps stations in the taiwan area abstract the 131 stations of the taiwan gps network were surveyed 4 6 times from 1990 to 1995 with dual frequency geodetic receivers the standard deviation of an observed baseline length with its linear trend removed is in the range of 6 10 mm for a 3 120 km long baseline the average rates of length change for all baselines of the network and those from nine continuously monitoring permanent stations are used in a least squares adjustment to estimate the velocities of the gps stations relative to paisha penghu situated at the chinese continental margin to the south of fengping in the northern coastal range the velocity vectors of stations in lanhsu lutao and the coastal range trend in the directions of 306 322 with rates of 56 82 mm yr in contrast there is a dramatic decrease in the rates to the north of fengping this may be caused by the motion along the ne sw trending thrusts which obliquely cut the northern coastal range a discontinuity of about 30 mm yr in the rates along with a remarkable change in the directions of station velocity is observed across the longitudinal valley then the moving directions gradually shift to the west for the stations in the western foothills in the kaohsiung pingtung coastal area the station velocities are even directed toward the southwest to the north of the peikang high the velocity vectors of the stations change direction from the west gradually to the north and finally to the east and southeast significant nw se extensional deformation is found in the ilan plain and northern taiwan in general the pattern of the velocity field for gps stations in the taiwan area is quite consistent with the directions of present day tectonic stress

mapping the upper mantle three dimensional modeling of earth structure by inversion of seismic waveforms a method is presented for the inversion of waveform data for the three dimensional distribution of seismic wave velocities the method is applied to data from the global digital networks international deployment of accelerometers global digital seismograph network the selected data set consists of some 2000 seismograms corresponding to 53 events and 870 paths the moment tensors of the earthquakes are determined through an iterative procedure which minimizes the corrupting influence of lateral heterogeneity a global model is constructed for shear wave velocity expanded up to degree and order 8 in spherical harmonics and described by a cubic polynomial in depth for the upper 670 km of the earth s mantle although no a priori information is incorporated the model predictions reproduce much of what is known about the dispersion of mantle waves for example high phase velocities for shields low velocities at ridges and a strong degree 2 pattern for rayleigh waves since the method makes use of complete waveforms overtone data are also included it is shown that the model is reproducible in that substantially the same model can be constructed from each half of the total data set considered independently the model shows that shields and ridges are major features in the depth interval 25 250 km the ridges of the southern pacific and the larger shields persist to 350 km but the southeast indian rise is underlain by a high velocity anomaly at this depth as is much of the mid atlantic ridge at 450 650 km the major features are a broad region of high velocities incorporating south america much of the south atlantic and parts of west africa a broad region of low velocities in the central and eastern pacific high velocities in the western pacific and a low velocity anomaly beneath the red sea and the gulf of aden in the absence of a crustal correction degrees 2 and 3 show a high positive correlation with the geoid paradoxically this is largely destroyed when the distribution in crustal thickness is taken into account spherical harmonic degrees 4 7 show a significant negative correlation

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

detrital zircon u pb geochronological and lu hf isotopic constraints on the precambrian magmatic and crustal evolution of the western yangtze block sw china abstract in situ u pb geochronological and lu hf isotopic studies of detrital zircon from a variety of precambrian strata along the western margin of the yangtze block of south china show that the rocks have maximum depositional ages of ca 1000 740 ma all of the strata probably formed in a subduction related tectonic setting the kunyang group heishantou formation and huili group tongan formation have maximum depositional ages of 960 1000 ma and contain zircons whose ages are dominantly paleoproterozoic 2500 2300 1900 1700 and 1650 1500 ma the yanbian group has a zircon population ranging mainly from 860 to 1000 ma different from that of the bikou group with ages from 745 to 890 ma ages of zircons from the lieguliu and guanyinya formations range from 740 to 900 ma the paleoproterozoic detrital zircons commonly have negative or slightly positive hf values from 8 0 to 2 3 whereas most of the 1000 740 ma zircons have moderately to significantly high positive hf values from 0 3 to 13 0 the new detrital zircon u pb and lu hf isotopic dataset suggests four major episodes of magmatism in the region 2500 2300 1900 1700 1650 1500 and 1000 740 ma and the presence of widespread archean basement of the western yangtze block the data also suggest an important period of juvenile magmatism at 1000 740 ma our results favor a model in which continuous subduction 1000 740 ma along the western margin of the yangtze block lasted for 260 million years

thirty year land elevation change from subsidence to uplift following the termination of groundwater pumping and its geological implications in the metropolitan taipei basin northern taiwan several levelling routes in the metropolitan taipei basin have been repeatedly conducted during the past decades mainly in order to monitor the anthropogenic ground subsidence due to massive pumping of groundwater we analysed the levelling data released from government and investigated the rate of ground level change from 1975 to 2003 which postdate the massive groundwater exploitation in taipei area based on the contour maps created from the levelling data of 406 benchmarks the overall subsidence rate in the taipei basin gradually decreased since 1975 and around 1989 the basin switched to slight uplift throughout a large part of the basin three mechanisms are proposed to be responsible for the observed land elevation changes including shallow soil compaction deformation within aquifer and tectonic subsidence the trend of the ground level change in 1975 2003 essentially demonstrated the effects of natural recharge to previously depleted aquifers and is explained by the hydro mechanical coupling of aquifer materials i e elastic rebound to the rising piezometric level the rate of shallow soil compaction is estimated about 1 8 mm yr throughout the basin according primarily to the shallow clay thickness asymmetric tectonic subsidence related to the shanchiao fault was estimated to be 1 75 mm yr and 0 9 mm yr in the western part and the central part of the basin respectively by subtracting the components of the soil compaction and tectonic subsidence from the surface land elevation change the rebound of aquifer strata was estimated to be about 6 7 cm and 16 cm in western margin and central taipei respectively the amount of rebound is approximately 10 in magnitude comparing to the amount of previous anthropogenic subsidence due to massive groundwater pumping totally about 2 m 2007 elsevier b v all rights reserved

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

fluid inclusion and isotopic constraints on the origin of the paleoproterozoic yinachang fe cu ree deposit southwest china the yinachang deposit in the kangdian fe cu metallogenic province southwest china contains approximately 20 mt ore 41 9 to 44 5 wt fe and 15 mt ore 0 85 to 0 97 wt cu with potentially significant rees orebodies are hosted in the late paleoproterozoic dongchuan group and consist mainly of massive and banded replacement ores the paragenetic sequence of this deposit includes pre ore na fe alteration stage i fe ree mineralization dominated by magnetite and siderite with subsidiary apatite and fluorite stage ii and cu ree mineralization with chalcopyrite ankerite biotite and subordinate apatite fluorite allanite and synchysite stage iii this sequence is similar to those of many iron oxide copper gold iocg deposits elsewhere there are two distinct types of fluid inclusions within apatite of both stages ii and iii type 1 fluid inclusions with liquid and vapor phases have relatively low homogenization temperatures 98 345 c and salinities 4 7 16 2 wt nacl equiv whereas type 2 fluid inclusions with one or more solid phases a liquid phase and a vapor phase have higher homogenization temperatures 273 500 c and salinities 36 2 59 8 wt nacl equiv fluids in sulfur isotope equilibrium with chalcopyrite of stages iii have 34 s cdt values ranging from 2 8 to 2 7 consistent with a magmatic origin for the sulfur thus sulfur isotopie compositions imply that magmatic hydrothermal fluids probably represented by the hot and saline type 2 fluid inclusions were involved in the ore forming process although 34 s close to 0 might also mean that sulfur was leached from igneous rocks by nonmagmatic fluids siderite of stage ii and ankerite of stage iii have 13 c pdb values ranging from 12 4 to 7 5 and from 8 5 to 4 4 respectively the large variation of 13 c pdb values reveals a strong interaction between the magmatic hydrothermal fluids and the dolostone in the ore hosting sequence calculated 18 o smow values of the fluids in stages ii and iii are broadly similar and vary from 11 1 to 17 1 avg 13 5 such values are higher than that of magmatic hydrothermal fluids and thus may indicate extensive fluid wall rock interaction and or mixing of magmatic hydrothermal fluids with shallow level nonmagmatic fluids this interpretation is consistent with the radiogenic sr isotope compositions of bulk ores 87 sr 86 sr i 0 710404 0 734034 and fluorite separated from ores 87 sr 86 sr i 0 709851 0 723658 these nonmagmatic fluids are probably representative of the type 1 fluid inclusions bulk ores and mineral separates have nd t values ranging from 8 1 to 0 4 mostly 0 9 to 0 4 similar to the coeval mafic intrusions in the region 2 0 to 2 8 this similarity suggests that rees in the yinachang deposit might have been derived dominantly from mantle derived magmas on the other hand relatively low nd t values 5 5 and 8 1 of some samples indicate insignificant ree contributions from wall rocks the yinachang deposit is temporally associated with regional mantle derived mafic magmatism and may have a genetic relationship with it fluids derived from deep seated magmas may have supplied abundant sulfur and carbon as well as some of the ore metals in this deposit the mafic magmatism could also have provided heat to induce the shallow level nonmagmatic fluids to interact with the sedimentary volcanic sequence thus leaching additional ore metals into the ore forming system fluid mixing can effectively trigger saturation and deposition of ore minerals our work highlights that nonmagmatic fluids are not a prerequisite for cu au ree mineralization but may facilitate metal deposition in magmatic hydrothermal iocg systems

geology geochronology and geochemistry of the dahongshan fe cu au ag deposit southwest china implications for the formation of iron oxide copper gold deposits in intracratonic rift settings the large dahongshan fe cu au ag deposit in the kangdian iron oxide copper gold iocg metallogenic province southwest china contains approximately 458 3 mt of ore at 41 0 fe 1 35 mt cu metal at 0 78 cu and significant amounts of au 16 t ag 141 t co 18 156 t and pd pt 2 1 t the deposit consists mainly of two types of ores 1 lenses of massive or banded magnetite hematite hosted in extensively na metasomatized metavolcanic rocks metaarenite and brecciated rocks and 2 strata bound disseminated stockwork and banded magnetite chalcopyrite bornite in mica schist and marble both types of orebodies and country rocks underwent extensive hydrothermal alteration resulting in a similar paragenesis pervasive stage i sodic alteration formed widespread albite and local scapolite it was subsequently replaced by ca or k rich minerals represented by actinolite k feldspar biotite sericite and chlorite of stages ii and iii magnetite is slightly younger than and partly overlaps the sodic alteration assemblages hematite is texturally later than magnetite is locally abundant within the massive fe oxide orebody and is closely associated with sericite copper sulfides are coeval with quartz biotite sericite and chlorite in stage iii assemblages widespread siderite and ankerite predominate in stages ii and iii respectively quartz calcite veins mark the result of waning stage iv hydrothermal alteration in addition to widespread alteration during the major ore forming event the deposit has also undergone extensive overprinting and remobilization during post ore magmatic and metamorphic events the dahongshan orebodies are intimately associated with abundant doleritic dikes and sills that have hydrothermal mineral assemblages similar to those in the ore hosting rocks one dolerite sill that cuts a massive fe orebody has a laser ablation inductively coupled plasma mass spectrometry zircon u pb age of 1661 7 ma which is within uncertainty consistent with the age of 1653 18 ma determined for hydrothermal zircons from stockwork chalcopyrite magnetite ore the zircon u pb ages are thus considered to mark the timing of major mineralization that formed the dahongshan deposit post ore modification is recorded by an re os isochron age of 1026 22 ma for pyrite in discordant quartz carbonate sulfide veins and by younger neoproterozoic mineralization dated at ca 830 ma using re os isotopes on molybdenite the former age is contemporaneous with late mesoproterozoic magmatism in the region whereas the latter is coeval with regional neoproterozoic metamorphic events in southwest china carbon and oxygen isotope values of albitized marble are between those of mantle derived magmatic carbon and dolostone end members the ore forming fluids that equilibrated with stage ii magnetite have 18 o values of 9 1 to 9 5 whereas fluids linked to the deposition of quartz and ankerite during stages iii and iv have lower 18 o values of 2 9 to 7 3 the oxygen isotope data indicate that the ore forming fluids related to stage ii are chiefly magmatically derived and mixed with abundant basinal brine during stages iii and iv this interpretation is consistent with sulfur isotope values of sulfides in the deposits pyrite and chalcopyrite from the dahongshan deposit have a large range of 34 s values from 3 4 to 12 4 implying mixing of magmatic and external sulfur likely from basinal brines in sedimentary rocks the dahongshan deposit formed in an intracratonic rift setting due to underplating by mafic magmas that induced large scale fluid circulation and pervasive sodic calcic metasomatism in country rocks ore metals were derived mainly from a deep seated magma chamber and partly from country rocks hydrothermal brecciation of the country rocks formed at the top of the dolerite intrusions and along zones of weakness within the country rocks owing to overpressure imposed by the ore fluids magnetite and hematite precipitated early near the dolerite intrusions whereas cu sulfides formed later in country rocks where sulfide saturation was favored we propose that this genetic model may be widely applicable to precambrian iocg deposits elsewhere that formed in intracratonic rift settings

processes governing natural land subsidence in the shallow coastal aquifer of the ravenna coast italy abstract we identified the processes governing natural land subsidence in a shallow coastal aquifer near ravenna north eastern italy by analysing the relationships among different data set time series water table level rainfall drainage sea level etc and establishing the correlations with vertical ground motion observed at a high resolution settlement gauge for the first time we establish the relationships between water table fluctuations and vertical displacement in a real field dataset as well as demonstrate the important contribution of primary consolidation and aquifer stratigraphy to natural land subsidence our study highlights the presence of three deformation components related to different processes controlling land subsidence elastic delayed elastic and irreversible plastic components the elastic and delayed elastic components are closely related to water table fluctuations that change the effective stress in two portions of the coastal aquifer at a daily in the sandy unconfined portion and seasonal time scales in the finely layered clay rich semiconfined prodelta portion respectively the irreversible component represents the trend in the land subsidence time series and is due to primary consolidation pore water pressure dissipation of the fine grained prodelta levels above where the settlement gauge is located the amplitudes of the elastic component can be up to 0 2 0 3 mm whereas the amplitude of the delayed elastic component reaches 0 89 mm the primary consolidation rate of deformation is 0 9 mm year and constrains the likely age of prodelta sediments deposition to 1300 2800 years before present the average degree of consolidation for the prodelta sediments varies from 0 8 to 0 99 according to consolidation coefficients varying from 1 58 to 3 15 m2 year which are accepted values in the literature our analysis point out that primary consolidation in the shallow fine grained sediments of the shallow coastal aquifer is still ongoing the delayed elastic land subsidence rate has similar magnitude to that due to primary consolidation and is likely connected to poroelastic effects in the prodelta sequence following seasonal variations in water table our findings are important for planning land subsidence management and monitoring strategies especially where the surface aquifer structure is heterogeneous due to different depositional settings

components and episodic growth of precambrian crust in the cathaysia block south china evidence from u pb ages and hf isotopes of zircons in neoproterozoic sediments abstract detailed lam icpms u pb dating and lam mc icpms lu hf isotope analysis were carried out on zircons from nine samples of basement metamorphic rocks in the southern cathaysia block south china the chemical compositions of these metamorphic rocks and zircon morphology indicate that their protoliths were sedimentary zircon u pb dating results show they mainly formed during the late neoproterozoic the precambrian crust of the cathaysia block can be divided into two distinct tectonic domains the wuyishan terrane to the northeast and the nanling yunkai terrane to the southwest the nearly e w boundary between these two terranes is also supported by geophysical petrological and geochemical evidence and the basement rocks of the two terranes are comprised of different components the wuyishan terrane is characterized by dominant paleoproterozoic 1 86 ga and lesser neoarchean magmatism and the southern central part of the terrane suffered strong neoproterozoic and early paleozoic tectonothermal reworking the zircon data suggest that there were five important episodes of juvenile crust generation 3 6 ga 2 8 ga 2 6 2 4 ga 1 85 ga and 0 8 0 7 ga in the wuyishan terrane strong paleoproterozoic 1 85 ga magmatism in northern wuyishan and neoproterozoic 0 8 0 7 ga magmatism in central and southern wuyishan both principally involved the remelting of old crust with minor input of juvenile material in contrast the nanling yunkai terrane contains abundant neoarchean and grenvillian zircons some evidence of mesoproterozoic activity and rare paleo to meso archean and neoproterozoic zircons u pb ages and hf isotopic compositions of the detrital zircons from the late neoproterozoic to early paleozoic meta sedimentary rocks in the nanling yunkai area suggest that the growth of juvenile crust mainly occurred at 3 6 ga 3 3 ga 2 5 2 6 ga 1 6 ga 1 0 ga and 0 8 0 7 ga neoarchaean 2 6 2 5 ga grenvillian 1 0 ga and neoproterozoic 0 8 0 7 ga magmatism mainly involved the recycling of old crust minor remnants of 4 1 ga crust may remain in the terrane this crustal history is distinct from that of the wuyishan terrane and analogous to those recorded for the eastern india east antarctic domain of northern gondwanaland suggesting that they probably were once linked

neoproterozoic granitoids from the phan si pan belt northwest vietnam implication for the tectonic linkage between northwest vietnam and the yangtze block abstract several neoproterozoic granitic intrusions including the phin ngan intrusion and some smaller ones from the sin quyen cu mine have been identified in the phan si pan belt northwest vietnam whole rock geochemical and sm nd isotopic data and zircon u pb age and hf o isotopes have been determined for these intrusions in order to constrain their ages petrogenesis and tectonic implications the phin ngan intrusion mainly composed of syenogranite was emplaced at 824 4 ma rocks from this intrusion have high sio 2 72 1 73 5 wt and k 2 o 5 14 5 52 wt with a cnk values ranging from 1 02 to 1 06 they have negative whole rock e nd t 8 4 to 5 2 and zircon e hf t values 8 1 to 5 5 and high zircon 18 o values 9 7 10 9 these geochemical features suggest that the phin ngan intrusion was derived from ancient k rich crustal sources granitic intrusions from the sin quyen mine occur as stocks or dykes intruding ores and ore hosting rocks they are composed of monzogranite and granodiorite emplaced between 736 8 and 758 6 ma they have high sio 2 68 2 76 7 wt and k 2 o na 2 o 6 93 8 54 wt and low mgo 0 19 0 87 with a cnk values ranging from 0 91 to 1 14 their whole rock e nd t values range from 6 6 to 0 mostly clustered between 6 6 and 4 3 their zircon e hf t values vary from 6 7 to 11 1 mostly lower than 2 0 such geochemical data indicate that these intrusions were produced mainly through partial melting of ancient crustal materials coupled with subordinate involvement of mantle juvenile crust derived components the phin ngan intrusion and intrusions from the sin quyen mine are sub alkaline in nature and have arc like trace element compositions compiled with other broadly synchronous igneous rocks with arc like affinities in the region the neoproterozoic magmatism in the phan si pan belt was generated in a subduction related setting the neoproterozoic intrusions in the phan si pan belt are geochronologically and geochemically correlated with the neoproterozoic subduction related igneous rocks along the western yangtze block thus the phan si pan belt represents the southern most part of the giant arc system of the western yangtze block

preseismic deformation and coseismic displacements associated with the 1999 chi chi taiwan earthquake the destructive 1999 chi chi taiwan earthquake m w 7 5 produced an approximately 100 km long surface rupture mostly along the previously recognized north south trending chelungpu fault preseismic deformation in central taiwan is realized from annually repeated global positioning system gps data acquired during the 1992 1999 period the total wnw ese shortening rate in the vicinity of the epicentral region that is from the west coast to the western boundary of the central range is up to 25 mm yr the crustal deformation before the chi chi earthquake was essentially a uniaxial compressional strain of 0 36 strain yr in the direction of 114 the gps measurements taken 0 2 2 7 yr before and within 3 months after the mainshock were utilized to estimate the coseismic displacements horizontal movements of 1 1 9 1 m in the nw nnw directions are observed on the hanging wall eastern side of the fault there is a northward increasing trend in the magnitude of the displacement vectors and a dramatic change in the direction of about 50 toward the east along the fault strike in contrast much smaller se see movements of 0 1 1 5 m are found on the footwall western side of the fault the gps data show 2 4 10 1 m of total horizontal offsets across the chelungpu fault vertical offsets of 1 2 4 4 m with the eastern side up are also observed along the surface rupture the uplift on the hanging wall decreases rapidly toward the east it becomes subsidence at sun moon lake and in the puli wushe area the stations on the footwall show subsidence of 0 02 0 26 m the width of the uplift zone increases from about 10 km in the south to approximately 30 km in the north manuscript received 13 october 2000

high temperature metamorphism in a major strike slip shear zone the ailao shan red river people s republic of china abstract petrographic and thermobarometric analysis provides constraints on thept path of the mylonitic gneisses of the left lateral ailao shan red river shear zone which has accommodated the lateral extrusion of indochina during the tertiary two different paragenesis p1 and p2 are coeval with this deformation and correspond respectively to the amphibolite and greenschist facies microprobe analysis reveals that p1 garnets bear a chemical zonation from core to rim this zonation indicates a temperature increase during garnet growth conditions of formation of garnet rims p1b which are estimated using biotite garnet and plagioclase garnet thermobarometers are close to the granitic solidus 710 70 c and4 5 1 5 kbar p2 conditions are estimated to be approximately 500 c and

geology mineralization styles and age of ore hosting rocks of the proterozoic longbohe sin quyen fe cu belt implications for regional metallogeny abstract a number of proterozoic fe cu deposits or prospects are distributed along the ailao shan red river shear zone asrr in south yunnan china and northern vietnam defining the potentially important longbohe sin quyen fe cu belt this belt was tentatively considered to be initially part of the proterozoic kangdian fe cu province in southwestern yangtze block sw china before being southeastward displaced into the cenozoic asrr however such a model is still not confirmed convincingly due to poor understanding of ore genesis of the belt and lack of comprehensive comparisons in this paper we described the geology and mineralization styles of typical deposits the longbohe and sin quyen deposits in the longbohe sin quyen fe cu belt and obtained new zircon u pb ages of ore hosting rocks and intruding granites to evaluate if this belt is comparable with the kangdian fe cu province both the longbohe and sin quyen deposits which are located in northwestern south yunnan china and southeastern northern vietnam sections of the belt respectively are hosted in volcanic sedimentary rocks that were traditionally named as the longbohe and sin quyen groups respectively both the ores and hosting rocks in the two deposits are variably metamorphosed or deformed but our results reveal that both deposits have similar paragenetic sequences including pre ore na alteration and early fe ree mineralization magnetite and allanite followed by cu mineralization chalcopyrite such mineralization styles are broadly comparable to those of the fe cu deposits in the kangdian province our new zircon u pb dating on meta volcanic and sedimentary rocks of the ore hosting longbohe group and intruding granites indicate that this group consists of at least two parts forming at 1000 750 and 1700 ma and thus is termed as the upper and lower longbohe groups in this study on the other hand the new zircon ages of the ore hosting sin quyen group and intruding granites tightly constrain the depositional age of this group as 870 830 ma broadly synchronous with the upper longbohe group sedimentary rocks of the upper longbohe group contain detrital zircon with ages dominantly clustering at 2 9 2 7 2 3 2 2 1 9 1 7 1 5 1 3 and 1 2 1 05 ga broadly similar to those of the sin quyen group even though the latter also contains an additional spectrum of 1 0 0 87 ga in total these ages of detrital zircon are comparable with those of the synchronous strata in the kangdian fe cu province such as the kunyang huili and yanbian groups it is also important to note that our newly identified 1700 ma lower longbohe group is comparable to synchronous strata in the kangdian fe cu province such as the 1700 ma hekou and dahongshan groups such similarities of ore hosting rocks in both regions provide robust evidence to support that the longbohe sin quyen fe cu belt is initially part of the kangdian fe cu metallogenic province before being displaced into the cenozoic asrr shear zone this new recognition has important implications for regional fe cu exploration particularly in the asrr shear zone

timing of the last glacial maximum from observed sea level minima during the last glacial maximum ice sheets covered large areas in northern latitudes and global temperatures were significantly lower than today but few direct estimates exist of the volume of the ice sheets or the timing and rates of change during their advance and retreat here we analyze four distinct sediment facies in the shallow tectonically stable bonaparte gulf australia each of which is characteristic of a distinct range in sea level to estimate the maximum volume of land based ice during the last glaciation and the timing of the initial melting phase we use faunal assemblages and preservation status of the sediments to distinguish open marine shallow marine marginal marine and brackish conditions and estimate the timing and the mass of the ice sheets using radiocarbon dating and glacio hydroisostatic modelling our results indicate that from at least 22 000 to 19 000 calendar years before present land based ice volume was at its maximum exceeding today s grounded ice sheets by 52 5 x 10 exp 6 cu km a rapid decrease in ice volume by about 10 percent within a few hundred years terminated the last glacial maximum at 19 000 250 years

provenance and tectonic setting of the neoproterozoic yanbian group western yangtze block sw china abstract tectonic evolution of the yangtze block of south china during neoproterozoic time has been a major focus of debate and is important in the reconstruction of the rodinian supercontinent the yanbian group in the western margin of the yangtze block is a well preserved neoproterozoic volcanic sedimentary sequence and provides a rare opportunity to examine the provenance and tectonic setting it consists of a lower part composed of 1500 m thick basaltic lavas and an upper part of 3500 m thick flysch deposits the flysch sequence contains typical deep marine turbidites consisting mainly of fine grained volcaniclastic sandstones and mudstones indicating a submarine fan depositional system detrital zircon dating yields ages ranging from ca 865 ma to 1000 ma with two peaks at ca 900 ma and 920 ma respectively the yanbian group is intruded by the 860 ma guandaoshan dioritic pluton constraining its depositional age at ca 870 ma the sandstones contain abundant feldspar and lithic fragments with minor amounts of quartz and have an average composition of q16f35l49 suggesting a proximal source and an undissected to transitional arc setting the lithic fragments in the sandstones point to andesitic and felsic volcanic rock sources the sandstones and mudstones have intermediate sio2 al2o3 typically 3 6 high fe2o3 mgo contents 5 12 wt and moderate to high k2o na2o ratios generally 0 1 1 and 1 10 for sandstones and mudstones respectively in comparison with average upper continental crust they show strong negative nb ta anomalies slight depletion of zr hf la and th but moderate enrichment of v cr ni and sc these rocks show lree enrichment la ybn 5 3 7 4 with flat hree and pronounced negative eu anomalies eu eu 0 6 0 74 in chondrite normalized ree patterns similar to post archean shales geochemical data for these rocks suggest an arc setting and intermediate felsic volcanic source consistent with a back arc basin scenario for the basaltic lavas in the lower yanbian group together with regional geological evidence the turbidites of the yanbian group strongly support a model in which the western margin of the yangtze block was a major magmatic arc active probably from 920 ma to 740 ma a period of more than 180 million years

paragenesis stable isotopes and molybdenite re os isotope age of the lala iron copper deposit southwest china the lala fe cu deposit in the kangdian iron oxide copper gold iocg metallogenic province sw china is hosted in the paleoproterozoic hekou group this volcanic sedimentary sequence has undergone upper greenschist lower amphibolite facies metamorphism at 850 ma the deposit contains more than 200 million metric tons mt of fe cu ores with an average grade of 13 wt fe and 0 92 wt cu orebodies are roughly strata bound but occur along foliation lithologic contacts and shear zones the paragenetic sequence of the lala deposit includes metamorphosed i ii and iii and unmetamorphosed iv and v stages stage i is characterized by extensively pervasive na alteration forming abundant albite but minor scapolite stage ii occurs as massive and banded replacements consists of ti v poor magnetite and apatite with minor disseminated sulfide minerals and was associated with pervasive k fe alterations forming k feldspar chlorite and actinolite stage iii occurs predominantly as bands and disseminations confined along foliations with minor veinlets cross cutting hosting rocks this stage is characterized by abundant fe cu mo sulfides intergrown with angular magnetite and minor titanite and allanite and is associated with alteration of carbonate quartz fluorite and mica stage iv has an assemblage dominated by chalcopyrite with variable pyrite and bornite occurring as veins of sulfides quartz calcite biotite muscovite fluorite stage v occurs chiefly as veins of hematite calcite quartz and also results in extensive hematization stages ii and iii have fluids with high 18 o 7 2 11 7 and 34 s values 0 4 indicative of a magmatic hydrothermal origin however the stage iii fluids have 34 c values 0 5 close to the least altered marble 0 2 of the hosting rocks indicating a possible contribution of strata sourced carbon the stage iv fluids have 34 c values similar to and 18 o 4 1 6 7 lower than the stage iii fluids and a wider range of 34 s values 9 0 to 10 5 consistent with significant contributions of strata sourced sulfur fluids of the late barren vein stage v have relatively low 34 c 4 0 to 2 4 and 18 o values 2 9 to 0 6 indicative of significant contributions of meteoric and oxidized fluids molybdenite from stage iii has an re os isotope age of 1086 8 ma providing tight constraints on the timing of the main stage of mineralization this age indicates that the lala deposit is coeval with the 1100 ma intraplate magmatism in the western yangtze block we propose that the majority of the fe cu deposits in the kangdian iocg metallogenic province may have formed in an intraplate extensional setting although it is possible that some of the deposits may have involved in multiple events including modification by the neoproterozoic metamorphism forming the mineral assemblages of stage iv

in situ pb pb isotopic dating of sulfides from hydrothermal deposits a case study of the lala fe cu deposit sw china precisely dating different generations of mineralization is important for understanding the origin and tectonic environment of ore formation in this study we have dated two generations of sulfides from the lala fe cu deposit sw china using in situ pb pb isotopic data obtained by the la mc icpms technique the lala deposit is composed of fe cu ore bodies hosted in sedimentary volcanic rocks of the paleoproterozoic hekou group it has a paragenetic sequence of pre ore banded pyrite stage i and na alteration stage ii followed by early fe stage iii and late cu stage iv mineralization the sulfides in the deposit have dominantly formed in two stages including pyrite from stratiform pyrite bands in the host rocks i e stage i and epigenetic pyrite chalcopyrite in fe cu ore i e stage iv the sulfides of both stages have highly radiogenic pb isotopic ratios which form linear trends in the 207pb 204pb versus 206pb 204pb diagram the regression trend for stage i pyrite grains defines an errorchron age of 1669 110 ma mswd 32 undistinguishable from zircon la icpms u pb ages of volcanic rocks in the hekou group i e 1700 ma the regressions for stage iv pyrite and chalcopyrite yield younger errorchron ages of 1088 88 ma mswd 93 and 1053 220 ma mswd 15 respectively taking pyrite and chalcopyrite together an errorchron age of 1053 75 ma mswd 108 is obtained these errorchron ages agree well with the molybdenite re os age of the fe cu ore 1086 8 ma despite the large error and mswd values that are interpreted to be related to the small variation of initial ratios or post ore open system behavior the good agreement with the precise zircon u pb and molybdenite re os ages indicates that the pb pb errorchron ages for stage i and iv sulfides are geologically meaningful therefore this study shows that the in situ pb isotopic analyses can yield geologically meaningful pb pb ages for radiogenic pb rich sulfide minerals we also propose that in situ pb pb isotopic dating may be more promising for precambrian deposits containing sulfides with ingrown pb particularly for those involving multiple mineralization hydrothermal events but without relevant minerals for precise re os or u pb dating

geomorphological controls on soil fertility of semi arid alluvial fans a case study of the joghatay mountains northeast iran abstract alluvial fans are ideal locations to examine the effect of geomorphic processes and landforms on the soil characteristics the rate of soil development and fertility differs between active surfaces exposed to aggradation and old or relict surfaces subject to degradation the aim of this research is to evaluate the effect of landforms and processes on soil fertility based on the studies on three alluvial fans of different ages relict old and young a total of 36 soil samples were collected from these three alluvial fan surfaces the samples were gathered from different locations of fans apex and toe and landforms gully beds interfluves swales and bars and for each sample nine variables including soil texture acidity ph electrical conductivity ec total neutralizing value tnv available phosphorus ap available potassium ak total nitrogen tn total organic carbon toc and hydraulic conductivity k were measured the results show that the values of tnv ap ak tn and toc increase as clay increases in the relict and old fans the values of tnv toc tn ap ak clay and silt are higher in the interfluves than in the gully beds also in the relict and old fans the rates of clay silt toc tn and ap are higher in the apexes than in the toes in the young fan the values of most variables are higher in the toe compared to the apex the wide and relatively flat interfluves of the apex of old and relict fans have soils that are more fertile compared to the extensively entrenched surfaces of relict and old fan toes with relatively steep sloped interfluves the less fertile soils in the apex of young fan can be attributed to its coarse textured soils and also to repeated flooding and depositional processes overall this study reveals that geomorphological landforms and processes such as aggradation and degradation play an important role in the spatial variation of soil fertility