tectonics and geodynamics of south china an introductory note highlights the neoproterozoic jiangnan collision of cathaysia and yangtze is responsible for the formation of the south china block scb during the early paleozoic the scb was welded to the north china block along the qinling belt and also experienced an intracontinental orogeny that partly reworked the jiangnan belt during the middle triassic the scb was welded to indochina the qinling belt was reworked by n directed intracontinental subduction the xuefengshan belt is also a middle triassic intracontinental orogen

timing and duration of partial melting and magmatism in the variscan montagne noire gneiss dome french massif central unravelling the detailed pressure temperature time deformation p t t d evolution of magmatic and metamorphic rocks provides essential insights into the timing and duration of partial melting and related plutonism during crustal flow and migmatitic dome formation the montagne noire axial zone mnaz is a migmatitic dome located within the variscan orogen in the southern french massif central the timing of the main thermal event that was responsible for intense partial melting is still highly debated in this study we present new laser ablation inductively coupled plasma mass spectrometry la icp ms age data on micaschists migmatites and granites that clarify the p t t d evolution of the mnaz structurally controlled samples were collected in order to constrain the timing of metamorphism migmatization and plutonism regarding the main structural pattern d1 d2 and d3 d1 and d2 correspond to nappe stacking and dextral transpression respectively d3 is related to vertical shortening and coaxial thinning with a preferential ne sw to e w directed stretching la icp ms analyses on the syntectonic angles soulie and martys granites yielded u th pb monazite ages of 305 1 5 306 1 9 and 314 2 ma respectively five migmatitic rocks sampled in the eastern and central espinouse area yielded in situ ages ranging between 312 2 and 301 2 ma along the dome envelope two garnet staurolite bearing micaschists near saint pons de thomieres village gave in situ u th pb ages of 312 1 2 1 and 309 0 3 1 ma a fine grained gneiss with a d3 fabrics in the eastern dome envelope yield a 208pb 232th mean age at 305 7 3 9 ma all ages obtained in this study for the micaschists migmatites and granites range between 315 and 301 ma we interpret this time span as the record of the high thermal event responsible for intense crustal partial melting within the lower and middle crust the onset of partial melting occurred at ca 315 ma that marked the beginning of transpressional deformation d2 based on structural and petrological studies our new u th pb results suggest that 1 partial melting may have started at ca 315 ma and lasted 15 10 myr and 2 d2 et d3 developed between 315 and 300 ma and were synchronous d1 deformation ended at 315 ma the onset and duration of d1 related to nappe stacking and crustal thickening is still uncertain

structural and geochronological constraints on the tectonic evolution of the dulong song chay tectonic dome in yunnan province sw china abstract the dulong song chay tectonic dome lies on the border of china se yunnan province and northern vietnam and consists of two tectonic and lithologic units a core complex and a cover sequence separated by an extensional detachment fault these two units are overlain unconformably by late triassic strata the core complex is composed of gneiss schist and amphibolite shrimp zircon u pb dating results for the orthogneiss yield an age of 799 10 ma which is considered to be the crystallization age of its igneous protolith formed in an arc related environment a granitic intrusion within the core complex occurred with an age of 436 402 ma which probably formed during partial closure of paleotethys within the core complex metamorphic grades change sharply from upper greenschist low amphibolite facies in the core to low greenschist facies in the cover sequence there are two arrays of foliation within the core complex detachment fault and the cover sequence s 1 and s 2 the pervasive s 1 is the axial plane of intrafolial s 0 folds d 1 deformation related to this foliation is characterized by extensional structures the strata were structurally thinned or selectively removed along the detachment faults indicating exhumation of the dulong song chay tectonic dome the major extension occurred at 237 ma determined by shrimp zircon u pb and 39 ar 40 ar isotopic dating techniques regionally simultaneous tectonic extension was associated with pre indosinian collision between the south china and indochina blocks the s 2 foliation appears as the axial plane of nw striking s 1 buckling folds formed during a compressional regime of d 2 d 2 is associated with collision between the south china and indochina blocks along the jinshajiang ailao shan suture zone and represents the indosinian deformation the dulong granites intruded the dulong song chay dome at 144 2 140 2 and 116 10 ma based on 39 ar 40 ar measurement on muscovite and biotite the dome was later overprinted by a conjugate strike slip fault and related thrust fault which formed a vortex structure contemporaneously with late cenozoic sinistral movement on the ailao shan red river fault

permian rifting related fayalite syenite in the panxi region sw china abstract within the panxi rift of the permian emeishan large igneous province elip sw china a 20 km 2 quartz bearing fayalite syenitic pluton intrudes the 260 ma baima igneous complex bic the fayalite syenite is dated using shrimp zircon u pb techniques at 252 2 5 ma the panxi fayalite syenite is mainly composed of alkali feldspar ferroan augite fayalite quartz and fe ti oxides and contains 61 8 63 7 wt sio 2 5 0 7 3 wt fe 2 o 3 t and 9 9 12 1 wt total alkalis belonging to an a type variety of ferroan alkalic metaluminous granitoids the fayalite syenite displays positive eu anomalies eu eu 0 9 8 9 and is enriched in lree la yb n 6 6 11 5 the nd values and 87 sr 86 sr initial ratios of the fayalite syenite range from 1 3 to 1 9 and from 0 7035 to 0 7068 respectively the fayalite syenite and the bic had the same mantle source however the 8 myr age gap between them suggests the fayalite syenite formed by remelting of underplated mafic magmas injected into the crust during elip magmatism the ages of the fayalite syenite and bic are coincident with two permian mass extinction events and suggest that elip may have influenced both

geochronological and geochemical constraints on mesozoic suturing in east central tibet 1 this paper reports isotopic major and minor element geochemistry of igneous and metamorphic rocks from the kokoxili and yushu regions of central and eastern tibet the first region lies along the kunlun suture which separates the bayan har songpan ganze songpan terrane from the tarim and qaidam blocks two kokoxili granitoids yield u pb zircon dates of 217 10 and 207 3 ma late triassic which represent the time of emplacement and rb sr isochron dates of 195 3 and 190 3 ma early jurassic which are interpreted as cooling ages the geochemical signatures of these granitoids suggest that they are related to subduction continuing into the late triassic in the yushu area three samples help constrain the age of the jinsha suture which separates the songpan terranes from the qiangtang blocks a leucocratic granite and an orthogneiss in the suture zone yield u pb zircon dates of 206 7 and 204 1 ma respectively and a paragneiss south of it a u pb monazite date of 244 4 ma the existence of coeval magmatism in both the jinsha and kunlun sutures suggests that the two subduction zones were simultaneously active combining isotopic dating with structural evidence on subduction polarity and paleomagnetic reconstructions we propose that the kunlun and qinling block boundaries which were distinct in the permian subsequently formed a continuous late triassic northward subducting plate margin our data suggest that the jinsha suture correlates with the benzilan and nan uttaradit sutures which together belong to a major late triassic subduction zone

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

petrogenesis and tectonic implications of early cretaceous s and a type granites in the northwest of the gan hang rift se china the gan hang rift trending at least 450 km in a ne sw direction is a part of a mesozoic basin and range province in southeastern china detailed shrimp zircon u pb chronology major and trace element and sr nd hf isotope data of three granitic plutons and a diabasic dike in the northwest of the gan hang rift are used to explore the origin of these granites and their relationship to the evolution of the gan hang rift shrimp zircon u pb dating shows that the granitic plutons and diabasic dike were emplaced in the early cretaceous 122 129 ma the tongshan and damaoshan plutons close to the gan hang rift consist mainly of weakly peraluminous granitic rocks which show a2 subtype affinity these granites have initial 87sr 86sr ratios of 0 7080 0 7103 end t values of 1 4 to 5 6 and ehf t in situ zircon values of 3 8 to 1 2 detailed elemental and isotopic data suggest that they were formed by partial melting of granulitized mesoproterozoic metamorphic basement including metasedimentary and metaigneous rocks in the shallow 30 km crust at a high temperature 823 c these pure crustal melts underwent crystal fractionation of biotite plagioclase and k feldspar forming the more felsic end member of granitic rocks the ehu pluton more distant from the gan hang rift is composed of two mica granites which are strongly peraluminous and are s type granites these granites have higher initial 87sr 86sr ratios 0 7162 0 7191 and lower end t 7 1 to 7 4 and ehf t 6 8 in situ zircon values than the tongshan and damaoshan granites detailed elemental and isotopic data suggest that the ehu granites were formed by partial melting of paleoproterozoic metasedimentary basement in the deep 40 50 km crust at a relatively low temperature 757 c with no fractional crystallization and mantle derived magma mixing in their evolution the coeval diabasic dike is calc alkaline and has initial 87sr 86sr ratio of 0 7085 end t value of 0 9 and ehf t in situ zircon value of 7 1 suggesting that it was derived from the asthenospheric mantle wedge that had been metasomatised via addition of subducted sediment derived melt with monazite in the residual phase the association of early cretaceous 122 129 ma s and a type granites in the northwest of the gan hang rift marks the onset of back arc extension or intra arc rift with ongoing extension the crust and lithospheric mantle became progressively thinned the upwelling of asthenosphere triggered partial melting of both metasedimentary and metaigneous rocks in the more thinned crust close to the gan hang rift forming the a type granitic magmas such as tongshan and damaoshan whereas partial melting of metasedimentary rocks in the less thinned crust farther from the gan hang rift formed the s type granitic magmas such as ehu the red sediments with the total thickness more than 10 000 m have been successively deposited in the gan hang rift valley since the late early cretaceous 105 ma suggesting that this region experienced the most back arc extension

indosinian deformation of the songpan garze fold belt northeast tibetan plateau abstract deformation of the songpan garze fold belt southwest china occurred progressively during the indosinian orogeny ca 230 200 ma two phases of south directed deformation are identified d 1 associated with pervasive thickening of the songpan garzi sedimentary pile and detachment of that portion of the fold belt onlapping the yangtze passive margin and d 2 a localized phase of deformation associated with transpressive shortening of the fold belt s southeast margin an arcuate form surface in the southeast fold belt reflects the distribution of d 1 and d 2 structural phases and the diachronous migration of these phases into the fold belt this migration accommodated lateral growth and cumulative sinistral transpression within the fold belt s eastern margin an enigmatic metamorphic complex the so called danba antiform is constrained to have formed during the early stages of the d 2 deformation this was associated with diapiric upwelling of buoyant migmatised basement and domal distortion of the overlying metasedimentary pile

ar ar ages in phlogopites from marble hosted ruby deposits in northern vietnam evidence for cenozoic ruby formation abstract ruby growth in phlogopite bearing marbles has been indirectly dated using the 40 ar 39 ar laser stepwise heating technique on purified syngenetic phlogopite and other micas from ruby deposits in yen bai luc yen and quy chau mining districts in northern vietnam the principal results indicate the following 1 across the red river shear zone the phlogopites from the yen bai deposits yielded miocene cooling ages between 23 2 and 24 4 ma identical to those previously published using the same dating method on magmatic and metamorphic rocks from the day nui con voi range 2 luc yen ruby deposits in the lo gam zone on the eastern flank of the red river shear zone yielded oligocene cooling 40 ar 39 ar mica ages between 30 8 and 34 0 ma regarding the age of ruby crystallisation itself the most plausible hypothesis is that all rubies in both zones formed during the period 40 to 35 ma diachronism of cooling in adjacent zones leads to the conclusion that around 35 ma the ductile deformation in the lo gam zone ended and the ruby bearing marbles cooled rapidly while the high temperature deformation remained in the red river shear zone resulting in cooling through blocking temperature some 15 ma later 3 the quy chau ruby deposit is restricted to the quy chau shear zone that bounds the eastern part of the oligocene miocene bu khang dome phlogopite and biotite samples reveal miocene cooling ages between 21 and 22 5 ma which are minimum ages for ruby formation these ages could be linked with the end of the extension of the bu khang dome vietnamese ruby formation is linked to cenozoic tectonics resulting from continental collision between the asian and eurasian plates as for other marble hosted ruby deposits in central and southeast asia

north directed triassic nappes in northeastern vietnam east bac bo abstract a careful re examination of the previous tectonic descriptions and the acquisition of new structural and kinematic data have been undertaken in northeastern vietnam the structure of the area consists of a system of slightly metamorphosed but ductilely deformed nappes including recumbent folds formed during the triassic prior to the unconformable deposition of the upper triassic terrigenous sediments these results confirm the previous interpretation of preyunnanaises nappes represented by middle upper paleozoic foliated limestone resting through a flat mylonitic contact over an intermediate song mien unit deprat 1915 the latter made essentially of deformed lower triassic sedimentary and volcanic formations which previously filled the song hien rift bourret 1922a bourret 1922b in the external part of the belt the triassic strata conformably overlie slightly deformed and unmetamorphosed upper paleozoic rocks forming an autochthonous domain alternatively the same triassic succession can be seen resting tectonically through decollement zones directly over the middle paleozoic marbles to form a distinct but subsidiary allochthonous unit our data show that the song chay orthogneiss and its lower paleozoic sedimentary country rocks belong to the main nappe they also demonstrate that this nappe was transported to the n ne a newly discovered ophiolitic melange of supposed oceanic nature and triassic age lying along the song chay fault north of the nui con voi could hypothetically represent the rooted zone of the nappe a tertiary overprinting event in particular accommodated by strike slip movements likely accounts for the present orocline of ne vietnam

crustally derived granites in the panzhihua region sw china implications for felsic magmatism in the emeishan large igneous province abstract in the panxi region of the late permian 260 ma emeishan large igneous province elip there is a bimodal assemblage of mafic and felsic plutonic rocks most emeishan granitic rocks were derived by differentiation of basaltic magmas i e mantle derived or by mixing between crustal melts and primary basaltic magmas i e hybrid the yingpanliangzi granitic pluton within the city of panzhihua intrudes sinian 600 ma marbles and is unlike the mantle derived or hybrid granitic rocks the shrimp zircon u pb ages of the yingpanliangzi pluton range from 259 8 ma to 882 22 ma younger ages are found on the zircon rims whereas older ages are found within the cores field relationships and petrography indicate that the yingpanliangzi pluton must be pm anomalies low end 260 ma values 3 9 to 4 4 and high i sr 0 71074 to 0 71507 consistent with a crustal origin the recognition of a crustally derived pluton along with mantle derived and mantle crust hybrid plutons within the panxi region of the elip is evidence for a complete spectrum of sources as a consequence the types of panxi granitoids can be distinguished according to their asi eu eu end t ehf t t zr c and nb ta pm values the diverse granitic magmatism during the evolution of the elip from 260 ma to 252 ma demonstrates the complexity of crustal growth associated with lips

understanding mesozoic accretion in southeast asia significance of triassic thermotectonism indosinian orogeny in vietnam results from a zircon u pb study of the metamorphic basement of vietnam reveal that a large part of the continental crust was affected by a short lived episode of ductile deformation and high temperature metamorphism between 258 6 ma and 243 5 ma although coincident with final stages of north south china collision qinling orogenesis the thermotectonism in vietnam was caused by accretion of sibumasu to indochina south china this accretion event indosinian orogeny has regional significance because it contributed to the final stages of north south china collision an aspect not explicitly recognized in qinling orogenic models

structural petrological and thermal evolution of a tertiary ductile strike slip shear zone diancang shan yunnan the diancang shan a horst massif within the red river fault zone in yunnan people s republic of china preserves a structural petrological and thermal record of two distinct phases of tectonic activity a left lateral ductile shear that terminated between 20 and 17 ma and a ductile to brittle phase of normal faulting which began at 4 7 ma and remains active mylonitic rocks in the core of the range display an early steep high temperature ht schistosity and a horizontal stretching lineation that are both parallel to the trend of the belt kinematic indicators indicate that shear was left lateral the complex shape of the ht schistosity at the southern termination of the massif likely results from a large scale oblique left lateral c shear plane that dismembered the shear zone and separated the ailao shan and the diancang shan as left lateral deformation terminated thermochronological and thermobarometric results suggest that the gneisses were partially unroofed during this event along the eastern edge of the diancang shan the ht fabrics were overprinted by low temperature structures during activation of east dipping normal faults cooling associated with this normal right lateral faulting along the diancang shan and perhaps activation of the right lateral normal movement on the range front fault farther south along the ailao shan began at 4 7 0 1 ma these results tend to support the view that extrusion of indochina occurred along the left lateral red river shear zone between 35 and 19 17 ma initiation of right lateral normal slip during the late miocene may relate to eastward extensional collapse of the thickened tibetan crust or more probably to initiation of the second phase of extrusion

mesozoic cenozoic evolution of the danba dome songpan garze east tibet as inferred from la icpms u pb and fission track data abstract the eastern margin of the tibetan plateau is a key region to understand the formation and tectonic evolution of the plateau most studies concentrated on the longmen shan range that corresponds to the tectonically very active boundary between the tibetan plateau and the sichuan basin this work addresses the deformation and exhumation pattern in the poorly understood metamorphic danba dome situated at the conjunction between the southern longmen shan range and the major strike slip xian shui he fault the early mesozoic and cenozoic evolution of the dome are then compared to the regional evolution of eastern tibet the about 830 ma zircon u pb ages obtained on the gezong and gongcai gneiss as well as u pb ages on inherited zircons in the early mesozoic granites confirm the occurrence of neo proterozoic magmatic episodes in the danba basement at 800 900 ma and 650 750 ma following the closure of the palaeotethys ocean granitoids were emplaced in the danba metamorphic basement and triassic cover from about 230 ma to about 170 ma the western part of the lian he kau granitic massif emplaced between 210 and 200 ma within the triassic cover sequence of the dome was only slightly affected by the early mesozoic deformation however in the eastern deeper level compartment of the massif ductile deformation occurred between 200 and 190 ma contemporaneous with the onset of the barrovian type metamorphism dated at 200 180 ma that deformation is compatible with the early mesozoic deformation observed in the longmen shan range and with the development of the large scale decollement level in the songpan garze area apatite fission track data then indicate that following the oligocene middle eocene onset of eastward motion of the songpan garze terrane the danba dome developed as a crustal scale antiform during the late miocene early pliocene 10 4 ma the exhumation was controlled by the lian he kau fault a major transpressive structure parallel to the xian shui he fault and the associated manai thrust faults regionally while the oligocene middle miocene exhumation phase seems to have been of regional extent the late miocene to present day phase is localised along the longmen shan range thrusts as well as along the back thrusts formed by the lian he kau manai and associated faults

brittle modification of triassic architecture in eastern tibet implications for the construction of the cenozoic plateau abstract in the songpan ganzi fold belt east tibetan plateau post triassic deformation is localised within northwest trending sinistral and southwest trending dextral brittle fault zones between these zones large tracts of the fold belt preserving late triassic indosinian architecture have been transported effectively intact this transport was almost exclusively strike slip and did not accommodate significant differential exhumation or uplift right dihedra paleostress reconstructions from fault plane kinematic data indicate a rotation of the principal compressive stress from ne sw trending in the north to a dominant e w trend throughout the eastern songpan ganzi fold belt exceptions occur adjacent to competent proterozoic basement complexes and other rigid bodies these stress solutions are consistent with a compressive stress regime that radiates about the eastern himalayan syntaxis a secondary n s compressive paleostress identified in southern regions may record late southwards transportation of the songpan ganzi fold belt relative to the eastern himalayan syntaxis the lack of pervasive cenozoic recent upper crustal shortening in this region suggests that thickening of the east tibetan crust was predominantly accommodated by viscous deformation at depth we envisage decoupling of the lower and upper crust and partitioning of an e w compressive stress between 1 a laterally mobile thin skinned veneer and 2 a homogeneously thickened tectonic basement

late permian to middle triassic sedimentary records in shiwandashan basin implication for the indosinian yunkai orogenic belt south china abstract the shiwandashan basin is an elongate region of the late permian middle triassic potential sediment accommodation that formed on continental crust between the yunkai orogen and the adjacent yangtze craton block south china several terms such as remanent marine trough structural window rift and pull apart basin have been proposed studies of the late permian middle triassic sedimentary sequences and the tectonics lithofacies and paleogeography of the shiwandashan basin indicate that the late permian middle triassic succession is composed of thick coarse molasse deposits and fine clastics organized into coarsening upward and fining upward stratigraphic sequences the shiwandashan basin went through the process of growth and disappearance from molasse to turbidite deposition and finally to volcanic eruption during late permian middle triassic to some extent the facies evolution reflects the tectonic controls on the sedimentation of the foreland basin the coarsening upward sequences hundreds to thousands of metres scale occurring immediately adjacent to the thrust front demonstrate that important episodic thrusting orogenesis and piedmont flexuring subsidence with low velocity took place in the northwestern part of the yunkai orogen whereas fining upward sequences are the indicator of continued deformation with high thrust velocity the following four facts have been demonstrated 1 a foreland basin existed in late permian middle triassic 2 the development of the foreland basin was controlled by the evolution the yunkai orogen 3 the basin and mountains continuously changed and migrated 4 the shiwandashan basin was a key demarcation between an active region in southeastern south china cathaysian block and a relatively stable region in northwestern yangtze block it was a convergent boundary in late permian middle triassic and its tectonic nature changed in the wake of the tectonic evolution of the surrounding area

structural evidence for the permo triassic tectonic evolution of the yidun arc eastern tibetan plateau abstract deformation across the yidun arc occurred as a two phase process i early triassic deformation of paleozoic sequences resulting from closure of the jinsha jiang suture and ii late triassic deformation of middle to late triassic rocks as a result of final basin closure deformation in the western yidun arc is dominantly west vergent a north south striking west directed thrust the jinsha thrust forms the boundary between the western yidun arc and the eastern qiangtang block the jinsha thrust incorporates within it ophiolitic material of the jinsha jiang suture and the structural geometry across the jinsha thrust suggests that early triassic subduction was east dipping along the jinsha jiang suture deformation within middle to late triassic sequences of the eastern yidun arc eastern qiangtang block and western songpan garze fold belt is thin skinned being restricted largely to a single generation of upright folds and associated thrusts late triassic westwards subduction underneath the combined western yidun arc qiangtang block resulted in arc volcanism in the eastern yidun arc coeval arc volcanics in the northern margin of the qiangtang block suggest that the suture along the northern margin of the qiangtang block may be correlated with the garze litang suture rather than the jinsha jiang suture

new constraints on the structure thermochronology and timing of the ailao shan red river shear zone se asia new structural petrographic and 40ar 39ar data constrain the kinematics of the asrr ailao shan red river shear zone in the xuelong shan xls geochronological data reveal triassic early tertiary and oligo miocene thermal events the latter event 33 26 ma corresponds to cooling during left lateral shear in the fansipan fsp range thrusting of the sapa nappe linked to left lateral deformation and cooling of the fsp granite occurred at 35 ma rapid cooling resumed at 25 29 ma as a result of uplift within the transtensive asrr in the daynuiconvoi dncv foliation trends nw se but is deflected near large scale shear planes stretching lineation is nearly horizontal on steep foliations shear criteria indicate left lateral shear sense zones with flatter foliations show compatible shear senses petrographic data indicate decompression from 6 5 kbar during left lateral shear temperatures 700 c 40ar 39ar data imply rapid cooling from above 350 c to below 150 c between 25 and 22 ma without diachronism along strike along the whole asrr cooling histories show two main episodes 1 rapid cooling from peak metamorphism during left lateral shear 2 rapid cooling from greenschist conditions during right lateral reactivation of the asrr in the nw part of the asrr xls diancang shan we link rapid cooling 1 to local denudations in a transpressive environment in the sw part ailao shan and dncv cooling 1 resulted from regional denudation by zipper like tectonics in a transtensive regime the induced cooling diachronism observed in the ailao shan suggests left lateral rates of 4 to 5 cm yr from 27 ma until 17 ma dncv rocks always stayed in a transtensive regime and do not show cooling diachronism the similarities of deformation kinematics along the asrr and in the south china sea confirms the causal link between continental strike slip faulting and marginal basin opening

geochemical characterization using geophysical data and markov chain monte carlo methods a case study at the south oyster bacterial transport site in virginia 1 the study demonstrates the use of ground penetrating radar gpr tomographic data for estimating sediment geochemical parameters using data collected at the department of energy south oyster bacterial transport site in virginia by exploiting the site specific mutual dependence of gpr attenuation and extractable fe ii and fe iii concentrations on lithofacies we develop a statistical model in which lithofacies and fe ii and fe iii concentrations at each pixel between the boreholes are considered as random variables the unknown variables are estimated by conditioning to the colocated gpr data and the lithofacies measurements along boreholes using a markov chain monte carlo method cross validation results show that the geophysical data constrained by lithofacies have the potential for providing high resolution multidimensional information on extractable fe ii and fe iii concentrations at the south oyster site

precise age determination of mafic and felsic intrusive rocks from the permian emeishan large igneous province sw china abstract radiogenic isotopic dating using different mainly micro beam analyses techniques of rocks from the emeishan large igneous province elip has yielded a range of ages from the capitanian to the early triassic in contrast estimates based on paleontological and paleomagnetic data suggest the elip was emplaced rapidly 2 ma and that volcanism ended by 257 ma new zircon ca tims u pb ages that have superior precision compared to ages from micro beam analyses were obtained from strategic intrusive rocks of the panxi region inner zone of the elip the huangcao 258 9 0 7 ma daheishan 259 1 0 5 ma woshui 259 6 0 5 ma and cida 258 4 0 6 ma syenitic and granitic plutons and three mafic dykes 259 2 0 4 ma 259 5 0 8 ma 257 6 0 5 ma yielded a narrow range of ages between 257 ma and 260 ma the new results are consistent with the estimates from magneto biostratigraphic data and a rapid emplacement model the new wuchiapinqian ages of the huangcao and daheishan plutons suggests that magmatism may have lasted significantly less than 10 ma as previously suggested and the precise ages of the mafic dykes refute the hypothesis that emeishan high ti basalts represent the waning stages of the elip magmatism the short duration of magmatism also implies that degassing of the host sediments mostly carbonates and thus releasing of greenhouse gasses occurred within a substantially shorter interval than previously thought which significantly changes the loading of atmosphere per time unit the latter corollary potentially has severe environmental effects that may have contributed to causing the end guadalupian mass extinction

ar ar and fission track ages in the song chay massif early triassic and cenozoic tectonics in northern vietnam the song chay massif is the northeasternmost metamorphic complex in vietnam to the east of the red river shear zone it shows a large antiformal structure involving orthogneisses and migmatites overlain on its northern flank by muscovite bearing marbles an e w striking fault bounds the dome to the south kinematic indicators along a s n section reveal top to the n shear sense along the interface between the orthogneissic core and the overlying metasediments radiometric ages were obtained by the 40ar 39ar method using purified mica separates across the dome ages range from 236 ma at the southern edge to 160 ma in the core attesting to a strong imprint in the early triassic time a clear difference is seen between these mesozoic ages and the eocene to miocene ages from 40 to 24 ma that obtained in the nearby red river shear zone using the same method these data show that the song chay massif was already high in the crust when the high temperature deformation of the red river shear zone took place the final exhumation of the song chay orthogneiss constrained by fission track analysis on samples along the same transect occurred during the early miocene and could be interpreted as the consequence of a first normal sense of motion along the fault which bounds the massif to the south timing is similar to that of exhumation in the red river shear zone

early paleozoic or early middle triassic collision between the south china and indochina blocks the controversy resolved structural insights from the kon tum massif central vietnam abstract in central vietnam the kontum massif is subdivided from north to south into i kham duc ii ngoc linh iii kan nack complexes the kham duc complex consists of metapelite metapsammite paragneiss metagabbro amphibolite serpentinized ultramafic and orthogneiss blocks the southern part of this complex experienced an early paleozoic crustal melting event responsible for migmatites and anatectic granites the ngoc linh complex the largest one is formed by metatexite and granitoid enclosing blocks of eclogite high pressure hp granulite orthogneiss metagabbro amphibolite granodiorite and migmatite the kan nack complex is formed by migmatites with ultra high temperature u ht granulite charnockites and enderbites the three metamorphic complexes are intruded by per aluminous granite and subordinate mafic plutons in spite of local variations the foliation of the ngoc linh complex defines a bulk domal structure and the stretching lineation consistently trends nw se the nw se lineation is also observed in the tectonically overlying the kan nack complex the ngoc linh and kan nack complexes are interpreted here as parts of a metamorphic core complex mcc or ngoc linh mcc exhumed by a top to the nw detachment fault along the northern margin of the ngoc linh mcc upright folds with e w striking axes and e w stretching lineation deform the foliation these structures are developed in a transpressive regime coeval with the activity of plurikilometer scale dextral strike slip faults zircon and monazite u pb radiometric datings document a 250 240 ma age for the crustal melting and the top to the nw ductile shearing coeval with the formation of the ngoc linh mcc this extensional tectonics is followed by a dextral strike slip faulting at ca 240 230 ma before the emplacement of the two mica granitic plutons at ca 240 224 ma furthermore in the ngoc linh and kan nack complexes zircon and monazite yield u pb early paleozoic ages in the kham duc complex a ca 460 ma mp mt garnet biotite staurolite kyanite metamorphism followed by migmatites at ca 450 ma is also recognized the 420 400 ma age of the dai loc plutonic suite and the 450 425 ma age of the dien binh calc alkaline granodiorite document an early paleozoic event the early paleozoic mafic and ultramafic masses of the hiep duc complex are ophiolites included into a metasedimentary matrix thus the kham duc complex is interpreted as a tectonic melange formed during a collisional orogeny that subsequently underwent crustal melting giving rise to the migmatites a part of the early paleozoic rocks are enclosed as xenoliths in the triassic migmatite of the ngoc linh mcc these petrologic structural and chronological features allow us to propose the following geodynamic evolution for the kontum massif during the early paleozoic a continental collision which was accommodated by a north directed subduction of a southern block s vietnam or viet cambodia block below a northern block n vietnam or viet lao block occurred along the tam ky phuoc son suture during the early triassic at ca 250 245 ma the s vietnam block experienced a crustal melting giving rise to the ngoc linh mcc that reworked the early paleozoic orogen lastly during the middle triassic the dextral faulting superimposed upon the ngoc linh migmatite the geodynamic significance of the kontum massif is discussed in the general framework of indochina south china relationships the early paleozoic collision in vietnam is viewed as the driving force responsible for the development of the intracontinental orogeny of s china in order to account for the important heat flow required for the formation of uht metamorphic rocks a possible interaction between the scb indochina collision and the emeishan mantle plume is proposed

the south china block indochina collision where when and how this study uses new field observations and existing studies to shed new light on the origin and significance of two nw se striking orogens in nw and ne vietnam we conclude that the architecture of each belt is a stack of ne directed nappes formed either under deep ductile synmetamorphic conditions or under shallow depth in the sw and ne parts respectively the song ma zone and song chay ophiolitic melange represent two ophiolitic sutures however the late permian song da and babu mafic rocks are not ophiolites but intraplate basalts related to the emeishan plume a late triassic unconformity the 225 205 ma postorogenic plutonism and the 250 230 ma syntectonic metamorphism support an early to middle triassic age for these tectonic events both nw and ne belts are due to sw directed subduction with arc magmatism ocean closure and continental collision though two contemporary s dipping subduction zones might explain the structural evolution of the two belts a single convergent system offset by the tertiary red river fault is preferred as this s directed subduction better accounts for the late permian intraplate magmatism this scenario is discussed in the general geodynamic framework of se asia

late miocene provenance change on the eastern margin of the yinggehai song hong basin south china sea evidence from u pb dating and hf isotope analyses of detrital zircons abstract understanding the provenance of the reservoir in a sedimentary basin is of great importance in hydrocarbon exploration and production the yinggehai song hong basin is one of the most important cenozoic petroliferous basins in the south china sea to better understand the provenance characteristics of the upper miocene huangliu formation tortonian messinian on the eastern margin of the basin and test the influence of uplift of the tibetan plateau in the south china sea 172 single zircons from two sandstone samples in the lingtou gas field were dated by u pb chronometer and 54 of these grains were spot analyzed for hf isotopes the results indicate that the upper member of the huangliu formation in this gas field shows two major peaks of u pb ages at ca 250 ma and ca 432 ma along with four subordinate peaks at ca 757 ma ca 1926 ma ca 2529 ma and ca 2775 ma the initial hf isotope ratios ehf t of these zircons are negative to positive for each age group suggesting that existing crustal materials were mixing with new mantle melts in some magmatic episodes in contrast the lower member of the huangliu formation show two major age peaks at ca 98 ma and 248 ma with a subordinate peak at ca 1453 ma the ehf t values are concentrated between 11 6 and 3 4 revealing that they were derived from older crust comparing with the source characteristics of tectonic units surrounding the basin the source of the upper member in the gas field was mainly from the southern yangtze block whereas the lower member was derived from the hainan uplift together with the change of climate and sedimentation rate after 10 ma on the northern margin of the south china sea we attribute the provenance change to the uplift of the tibetan plateau at the same time

40ar 39ar geochronology of the charnockites and granulites of the kan nack complex kon tum massif vietnam abstract the truong son belt forms the eastern rim of the indochina block in southeast asia the age of the metamorphism mainly along nw se mylonitic shear zones that affects this belt has been formerly determined at about 240 250 ma this age corresponds to the indosinian tectonometamorphic episode the kon tum massif situated to the south of this belt comprises high temperature rocks the kan nack complex including charnockites and granulites the main charnockitic outcrops restricted to the song ba valley establish the intrusive nature of these magmatic rocks within granulite facies material basic charnockitic rocks are mainly quartz enderbites to norites and hornblende pyroxene granulite facies rocks the 40 ar 39 ar age of intrusion cooling of charnockitic magmas is determined from primary magmatic biotites at about 245 ma in the east of the kan nack complex some granulite facies rocks exhibit relicts of primary granulite facies parageneses whereas others show evidence of overprinting by a retrogressive low grade metamorphism ar ar dating confirm this evolution giving ages of 400 ma for primary relict granulite facies phases and 260 270 ma from the most retrogressed samples establishing the youngest limit for the granulite facies metamorphism granulites intruded by charnockites in the song ba valley yield ages of about 250 ma equivalent to the ages of the charnockites and have evidently been completely reset by these high temperature intrusions therefore the kan nack complex of the kon tum massif is not an independent unit with respect to the indosinian orogen but represents the deep crustal part of this belt

the endogenetic metallogeny of northern laos and its relation to the intermediate felsic magmatism at different stages of the paleotethyan tectonics a review and synthesis abstract northern laos is at the junction of the paleo tethys convergence and assemblages of several crustal blocks such as the sibumasu south china and the indochina blocks and also comprises the houayxay sukhothai the simao phongsaly the louang phrabang loei the truong son and the song da terranes during the amalgamation of the terranes derived from gondwana calc alkaline magmatic rocks peraluminous magmatic rocks sub alkaline volcanic intrusive complexes and alkaline rocks and related endogenetic metallogenic systems were emplaced in multiple stages during subduction collision and post collision events according to chronological order the tectonic magmatic development processes and the metallogenic assemblages can be divided as follows 1 subduction arc related magmatic hydrothermal metallogenic systems formed in calc alkaline magmatic rocks due to the initial subduction during the late carboniferous including porphyry cu au deposits skarn and epithermal cu au deposits 2 arc continent collisions and back arc extensional settings and associated magmatic hydrothermal mineralization with calc alkaline porphyry cu au deposits as well as alkaline porphyry and epithermal cu au deposits during the early permian 3 collisional orogenic magmatic hydrothermal metallogenic systems formed in the late permian to the early to middle triassic comprising high alumina magmatic rocks with au pb zn ag fe cu deposits associated with both orogenic type and hydrothermal vein type and 4 post orogenic extensional tectonic hydrothermal metallogenic assemblages which were formed in calc alkaline to alkaline rocks within a extensional tectonic setting during the late triassic early jurassic including porphyry cu mo sn deposits porphyry skarn and epithermal cu au deposits and also subalkaline volcanic intrusive complex related hydrothermal stratabound cu pb zn and au sb hg deposits further detailed studies and classification of metallogenic assemblages in laos is of significance to understand the nature and timing of different mineralization styles in relation the evolution of the paleo tethys

nanpanjiang basin a window on the tectonic development of south china during triassic assembly of the southeastern and eastern asia abstract as the largest siliciclastic sink in the south china block the triassic nanpanjiang basin is generally thought of as a foreland basin related to indosinian collision between the south china and indochina blocks our basin analysis spanning an eight years period provides details of lopingian to middle triassic stratigraphic fill and tectonic subsidence of the basin field observation and sedimentological analyses reveal that the basin was not filled by a monotonous turbidite system and suggest instead the development of five primary depositional units constituted by deep water turbidite systems mass transport deposits condensed drapes shallow water isolated carbonate platforms and volcano volcaniclastic rocks the stratigraphic completeness of the basin the coexistence of widespread but localized carbonate platforms within axially filled deep water basin tectonically generated cyclothems of transverse mass transport deposits along intrabasinal faults and basin fill of extrusive rocks of bimodal composition together apparently call in question the claim that crustal shortening prevailed during the late permian to pre norian triassic patterns of basin fill differential tectonic subsidence and very high subsidence rate of the region adjacent to master fault are more closely match the pattern expected for extensional to transtensional basin in a back arc setting the crustal extension in the southwestern south china was not consistent with hypothesized indosinian collision between south china and indochina blocks but more likely developed in response to northwestward subduction of the paleo pacific plate our results combined with regional considerations lead to a new model for the triassic tectonic evolution of south china during triassic assembly of the southeastern and eastern asia

intrusion related lang vai gold antimony district northeastern vietnam geology mineralogy geochemistry and 40ar 39ar age abstract the lo gam domain is one of the most important areas for metallogenic studies in northeastern vietnam the lo gam domain comprises numerous pb zn deposits and occurrences which account for 80 of the pb and zn resources in vietnam furthermore within the lo gam domain there are several gold antimony and base metal deposits the largest of these are the lang vai khoun puc and lung luong deposits which are included in the lang vai ore district each of the deposits includes several ore zones and ore bodies with peculiar mineral compositions and geochemical features the geological and structural settings show that the khoun puc lang vai and lung luong deposits represent a uniform ore system the metal zoning in the lang vai district occurs laterally away from a granitic intrusion a general paragenetic sequence for the three deposits may be represented by the following mineral associations from early to late i arsenopyrite pyrite native gold sphalerite ii bi rich pb sb sulfosalts iii bi poor pb sb sulfosalts iv sb sulfosalts v sb rich minerals native antimony and vi pb rich sb sulfosalts as a distal mineral association among the different types of mineralizations there is exists a transient mineralization that is characterized by a complex mineral composition bi bearing minerals have been found only in the khoun puc deposit which is located near a granite stock whereas bi mineralization is absent away from the deposit the general outward sequence of zoning is from au as bi to au as sb to sb au as to pb sb at the periphery the ages of mineralization at the lang vai and lung luong deposits have been estimated to be ca 240 ma and ca 236 ma respectively and are slightly younger than the earlier reported ages ca 254 245 ma of the phia bioc granite complex indicating a possible genetic relationship between the mineralization and the intrusions the geological characteristics metal zoning patterns and new geochronological data suggest that the gold antimony ores in the lang vai district are genetically related to the nearby granitic intrusions

late triassic post collisional slab break off along the ailaoshan suture insights from oib like amphibolites and associated felsic rocks what dynamic processes the south china indochina collision had gone through is still a pending problem our recent investigations identified a late triassic gabbroic intrusion in mengdong village and several granitic plutons near wana village yunnan province sw china both have underwent strong metamorphism and been altered to amphibolite and granitic gneiss respectively we carried out sims and la icpms zircon u pb dating for the amphibolites and granitic gneisses respectively dating results yield weighted mean 206pb 238u ages of 221 5 5 3 ma for the amphibolites and 224 0 1 8 and 235 4 0 6 ma for the granitic gneisses the amphibolite samples have low mg 41 7 42 8 high tio2 3 7 wt and na2o na2o k2o 1 89 2 68 contents and depleted sr nd isotopic compositions 87sr 86sr i 0 707647 and e nd t 1 17 they exhibit oib like ree patterns and spidergrams with strongly enriched lree contents insignificant eu anomalies eu 0 99 1 03 and moderate negative sr anomalies sr 0 46 0 56 the protolith gabbro of the mengdong amphibolites is derived from low degree partial melting of a homogeneous oib type mantle source in the garnet stability field and experienced significant fractionation of olivine clinopyroxene and plagioclase during magma evolution the wana samples are strongly peraluminous a cnk 1 1 and k enriched k2o na2o 2 93 3 63 they show enriched sr nd isotopic compositions with 87sr 86sr i 0 718589 0 719754 and e nd t 11 34 to 10 92 the wana plutons are typical s type granite and product of the dehydration melting of meta sedimentary rocks we summarized late triassic age data in the literature along the south china indochina suture zone and recognized a significant late triassic magmatic flare up in combination with previous studies on the late triassic hp uhp metamorphic rocks we proposed that during the late triassic the south china indochina suture zone had transformed into post collisional setting and accompanied slab break off triggered the oib type asthenospheric mantle upwelling and provided the heat source for the late triassic magmatic flare up

fe iii reducing bacteria in deeply buried sediments of the atlantic coastal plain the possibility that microorganisms are catalyzing the ongoing reduction of fe iii in the sediments of deep 20 250 m aquifers was investigated acetate oxidizing fe iii reducing bacteria were recovered from deep subsurface sediments but only from sediments in which it appeared that fe iii reduction was the terminal electron accepting process for oxidation of organic matter the fe iii reducing microorganisms were capable of reducing ferric oxides present in deep subsurface sediments although fe iii reduction in subsurface sediments is frequently regarded as an abiological reaction the enzymatic reduction of fe iii by microorganisms reported here is the first mechanism of any kind actually shown to have the potential to couple the oxidation of organic matter to carbon dioxide with the reduction of fe iii under the environmental conditions typically found in deep aquifers we propose that microbially catalyzed fe iii reduction is responsible for such late postdepositional phenomena as the formation of variegated red beds and the release of high concentrations of dissolved iron into anaerobic ground waters

p t t constraints on the development of the doi inthanon metamorphic core complex domain and implications for the evolution of the western gneiss belt northern thailand abstract the western gneiss belt in northern thailand is exposed within two overlapping cenozoic structural domains the extensional doi inthanon metamorphic core complex domain located west of the chiang mai basin and the mae ping strike slip fault domain located west of the tak batholith new p t estimates and u pb and 40ar 39ar age determinations from the doi inthanon domain show that the gneiss there records a complex multi stage history that can be represented by a clockwise p t t path u pb zircon and titanite dating of mylonitic calc silicate gneiss from the mae wang area of the complex indicates that the paragneissic sequence experienced high grade medium pressure metamorphism m1 in the late triassic early jurassic ca 210 ma in good agreement with previously determined zircon ages from the underlying core orthogneiss exposed on doi inthanon late cretaceous monazite ages of 84 and 72 ma reported previously from the core orthogneiss are attributed to a thermal overprint m2 to upper amphibolite facies in the sillimanite field u pb zircon and monazite dating of granitic mylonite from the doi suthep area of the complex provides an upper age limit of 40 ma late eocene for the early stage s of development of the actual core complex by initially ductile low angle extensional shearing under lower amphibolite facies conditions m3 accompanied by near isothermal diapiric rise and decompression melting 40ar 39ar laserprobe dating of muscovite from both doi suthep and doi inthanon provided miocene ages of ca 26 15 ma representing cooling through the ca 350 c isotherm and marking late stage development of the core complex by detachment faulting of the cover rocks and isostatic uplift of the sheared core zone and mantling gneisses in the footwall similarities in the thermochronology of high grade gneisses exposed in the core complex and shear zone domains in the western gneiss belt of northern thailand and also in northern vietnam laos yunnan and central myanmar suggest a complex regional response to indentation of southeast asia by india

pb2 and zn2 adsorption by a natural aluminum and iron bearing surface coating on an aquifer sand pb2 and zn2 adsorption was studied in batch experiments with material collected from a shallow unconfined aquifer of glacial outwash sand and gravel in falmouth massachusetts usa the aquifer solids contain primarily quartz 95 w w with minor amounts of alkali feldspars and ferromagnetic minerals pb2 and zn2 adsorption experiments with various grain size and mineral fractions of the aquifer solids showed that 1 zn2 adsorption was independent of grain size but pb2 was preferentially adsorbed by the 64 m size fraction and 2 pb2 adsorption decreased after removal of the paramagnetic fe bearing mineral fraction but zn2 adsorption was unaffected pb2 and zn2 adsorption on mineral separates from the aquifer material compared with metal adsorption on a purified quartz powder indicated that adsorption of both metal ions was dominated by coatings on the quartz fraction of the sediment characterization of the coatings by aes sem eds and tof sims demonstrated that the natural quartz grains were extensively coated with al and fe bearing minerals of variable composition thin sections of quartz grains examined by tem showed that the coatings contained both polycrystalline regions and single mineral crystals the coating thickness varied from 10 nm up to 30 m the coatings were mostly resistant to dissolution by an extraction protocol designed to dissolve noncrystalline phases the effect on metal adsorption of dissolving surface coatings from the sediment by chemical extraction was also measured a hydroxylamine hc extraction designed to dissolve crystalline fe oxide phases decreased pb2 and zn2 adsorption relative to untreated sediment extracted fe al 1 but pb2 and zn2 adsorption were not appreciably changed after sediment was extracted with dithionite citrate extracted fe al 5 overall the results suggest that pb2 preferred to form complexes with iron hydroxyl sites while aluminol sites were more important for zn2 adsorption however a definitive understanding of adsorption reactions in groundwaters will require detailed studies of the extensive coatings formed at mineral water interfaces by chemical weathering processes

development of the khao khwang fold and thrust belt implications for the geodynamic setting of thailand and cambodia during the indosinian orogeny abstract the indosinian orogeny in thailand is often viewed as having developed between strongly linear terranes which today trend approximately n s the terranes were subsequently disrupted by later tectonics particularly nw se trending cenozoic strike slip faults the ene wsw to ne sw striking thrusts and folds in the khao khwang platform area of the saraburi group on the sw margin of the indochina terrane are not easily explained in the context of this traditional view reversal of the clockwise rotation shown to have affected the block north of the mae ping fault zone only enhances the e w orientation of structures in the fold and thrust belt and moves the belt further east towards cambodia one solution for the trend that fits better with regional understanding from hydrocarbon exploration of the khorat plateau is that the indochina terrane was actually a series of continental blocks separated by permian rifting during the early triassic the early stages of collision south china cathaysian terrane collision with vietnam indochina resulted in the amalgamation of disparate blocks that now form the indochina terrane by closure along the rifts at the same time or following on from the collision there was closure of the back arc area between indochina and the sukhothai zone the rift basins were thrusted and inverted during the early stages of the indosinian orogeny and only underwent minor reactivated when later sibumasu collided with sukhothai zone indochina terrane margin during the late triassic the scenario described above requires the presence of a minor e w trending suture in nw cambodia evidence for this suture is suggested by the presence of permo triassic calc alkaline volcanism

low rates of bedrock outcrop erosion in the central appalachian mountains inferred from in situ 10be bedrock outcrops are common on central appalachian mountain ridgelines because these ridgelines define watersheds the rate at which they erode influences the pace of landscape evolution to estimate ridgeline erosion rates we sampled 72 quartz bearing outcrops from the potomac and susquehanna river basins and measured in situ produced 10 be ridgeline erosion rates average 9 1 m m y 1 median 6 m m y 1 similar to 10 be derived rates previously reported for the region the range of erosion rates we calculated reflects the wide distribution of samples we collected and the likely inclusion of outcrops affected by episodic loss of thick slabs and periglacial activity outcrops on main ridgelines erode slower than those on mountainside spur ridges because ridgelines are less likely to be covered by soil which reduces the production rate of 10 be and increases the erosion rate of rock ridgeline outcrops erode slower than drainage basins in the susquehanna and potomac river watersheds suggesting a landscape in disequilibrium erosion rates are more similar for outcrops meters to tens of meters apart than those at greater distances yet semivariogram analysis suggests that outcrop erosion rates in the same physiographic province are similar even though they are hundreds of kilometers apart this similarity may reflect underlying lithological and or structural properties common to each physiographic province average 10 be derived outcrop erosion rates are similar to denudation rates determined by other means sediment flux fission track thermochronology u th he dating indicating that the pace of landscape evolution in the central appalachian mountains is slow and has been since post triassic rifting events

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

indosinian nw trending shear zones within the truong son belt vietnam 40ar39ar triassic ages and cretaceous to cenozoic overprints abstract several nw trending ductile and partly mylonitic shear zones cross the truong son belt of central vietnam along the song ma and song ca valleys and north of the kontum block from da nang to aluoi and khe sanh high grade metamorphic rocks of amphibolite facies showing a retrograde evolution and consisting of ortho and paragneisses metavolcanics amphibolites marbles and quartzites are exposed along these structures they display a homogeneous deformation pattern characterized by a generally steeply dipping foliation and a near horizontal to gently plunging mineral stretching lineation indicating a strike slip tectonic regime of deformation along the southernmost fault zone at least various and consistent kinematic indicators including s c structures asymmetric tails of porphyroclasts prove that the strain and metamorphism have been generated by a phase of dominantly non coaxial deformation with a dextral sense of shear 40ar 39ar dating applied on the high grade metamorphic rocks minerals occurring along these zones provide plateau cooling ages closely around 245 ma establishing that this event took place in the lowermost triassic as an early phase of the indosinian orogeny the existence of indosinian movements in vietnam as they have been defined by previous authors during the early century is now accurately confirmed and this is the first insight in the occurrence of ductile strike slip tectonics of indosinian age along nw se fault zones well expressed in central vietnam is a cretaceous thermal and deformational overprint marked by epimetamorphism which took place between 90 and 120 ma as attested by low temperature degassing ages this cretaceous event is not found further north in the song ma zone where younger ages appear as a result of the influence of cenozoic shear movements along the red river fault which displays 20 30 ma ages on the northern flank of song ca in the bu khang phu hoat core complex comparable oligocene lower miocene ages 20 35 ma yielded by biotite and phlogopite reflect a strong overprinting and attest to a rapid uplift of the basement

the 40ar 39ar dating of biotite in ore veins and zircon u pb dating of porphyrtic granite dyke in the nanyangtian tungsten deposit in se yunnan china abstract the nanyangtian tungsten deposit is located in the laojunshan sn w polymetallic metallogenic province in se yunnan china tectonically it is located in the conjunction of the cathaysia and indochina blocks the orebodies primarily hosted within the stratiform like skarn and marble in the paleoproterozoic nanyangtian formation three ore forming stages have been identified based on mineral assemblages and on the intercalated relationships between the veins they include skarn scheelite stage quartz scheelite vein stage and carbonate scheelite vein stage in this paper we present 40ar 39ar laser incremental heating analyses of hydrothermal biotite intergrowth with scheelite from quartz scheelite vein with two yielded well defined 40ar 39ar plateau ages of 97 49 0 97 ma and 99 48 0 97 ma and two isochronal ages of 97 01 1 39 ma and 99 58 0 98 ma respectively representing the age of second mineralization stage of the nanyangtian tungsten deposit the ages are well consistent with the emplacement age of the laojunshan granite 96 ma to 117 ma u pb geochronological data for magmatic zircons separated from porphyritic granite dyke in the mine pit indicate that the granite body was emplaced in the post ore stage 89 88 0 53 ma integrated with previously high resolution diagenetic and metallogenic geochronological data two significant magmatic mineralization events are presented in se yunnan china and north vietnam the first event took place from 97 ma to 124 ma which is closely related to the tungsten mineralization the second event occurred from 75 ma to 95 ma which is closely related to the sn cu ag zn mineralization according to the exhibited data we preliminarily conclude that the late cretaceous magmatic mineralization activity in se yunnan and north vietnam is the product of post collision of the cathaysia and indochina blocks and should have formed under a same geodynamic setting

u pb dating of high temperature metamorphic episodes in the kon tum massif vietnam abstract in vietnam recent studies demonstrated that the kon tum massif was part of the indosinian mountain system together with the truong son belt one sector of the kon tum massif song ba valley is a granulite facies terrain for which ar ar and u pb ages cluster around 245 250 ma in the area of song bien river also metamorphosed under granulite facies conditions ar ar ages of 410 ma are displayed by biotite in this study u pb dating of monazite and zircon has been used to understand the reason for these two groups of ages taking into account the metamorphic mineral assemblages related to granulite facies metamorphism in the two districts for granulites of song ba valley ages of 245 250 ma are confirmed from monazites in the song bien district ages of 465 470 ma displayed by monazites are interpreted to correspond to the metamorphic climax zircons from one of these samples record the age of anatexis that has affected the granulitic rocks the younger age recorded in the song ba valley is considered to reflect high temperature related to charnockitic intrusions emplaced near the end of the indosinian orogeny overprinting the earlier formed granulites and resetting the u pb and ar ar ages for the first time two ht events are dated in the kon tum massif one occurring during the ordovician and the other during the permo triassic

petrogenesis and tectonic significance of a mesozoic granite syenite gabbro association from inland south china article i nfo a granite syenite gabbro association from inland south china has been studied for zircon u pb ages and hf isotopic compositions as well as whole rock elemental and sr nd isotopic compositions to constrain their sources and tectonic settings these rocks have distinctive crystallization ages the longyuanba biotite granites have a mean age of 240 ma the longyuanba two mica granites pitou alkali feldspar granites and amphibole bearing alkali feldspar granites tabei and huangbu syenites and chebu and chenglong gabbros have a similar age of 178 ma whereas the dafengnao syenite gives a mean age of 165 ma the longyuanba biotite granites and two mica granites exhibit s type characteristics probably derived from a neoproterozoic crustal source the chenglong gabbro has oib like trace element and highly depleted isotope compositions ehf t 10 0 1 3 end t 5 2 suggesting its parental melt to be of asthenospheric origin with insignificant crustal assimilation however crustal contamination is required to explain the isotopic compositions of the chebu gabbro the syenites are shoshonitic in composition and have depleted sr nd hf isotopic signatures which we interpret to have resulted from a mixed source of asthenospheric mantle and metasomatized lithospheric mantle as expected the signals of crustal assimilation are conspicuous in the petrogenesis of the more evolved syenites the pitou alkali feldspar granite and amphibole bearing alkali feldspar granite exhibit i type and a type characteristics respectively they are isotopically more depleted than the s type longyuanba granites we suggest that they may have formed through magma mixing of predominantly mantle derived melts with the neoproterozoic crust the jurassic granite syenite gabbro association was the product of asthenosphere lithosphere crust interactions which records the primary role of asthenospheric mantle upwelling in magma generation both in the mantle and in the crust in the early yanshanian time in south china they probably occurred in an intraplate rift like environment as a tectonic response to far field stress at plate margins during the early stage of the paleo pacific plate subduction the tectonic transition from the tethys orogenic regime to the paleo pacific regime was accomplished in the early jurassic and the early yanshanian magmatism should be genetically associated with the paleo pacific tectonic regime

changes in the sorption capacity of coastal plain sediments due to redox alteration of mineral surfaces chemical characteristics of grain coatings in a coastal plain sandy aquifer on the eastern shore of virginia were investigated where sediments have been exposed to distinct groundwater redox conditions dissolved o2 was 5 0 to 10 6 mg l 1 in the regionally extensive aerobic groundwater whereas in a narrow leachate plume it was only 0 001 to 0 9 mg l 1 the amount of dissolved fe in the aerobic groundwater was only 0 005 to 0 01 mg l 1 but it was 12 to 47 mg l 1 in the anaerobic zone the amount of extractable fe was an order of magnitude higher for the aerobic sediments than for the anaerobic sediments indicating that reductive dissolution removed the oxide coatings the capacity for anion sorption on the sediment surfaces as indicated by the sorption of 35so42 was an order of magnitude higher in the aerobic vs anaerobic sediments the presence of anaerobic groundwater did not significantly alter the amount of extractable al oxides on the surface of the sediments and those coatings helped to maintain a high surface area the removal of the fe oxides from the grain surfaces under anaerobic conditions was solely responsible for the significant reduction of so4 sorption observed this loss of capacity for anion sorption could lead to more extensive transport of negatively charged constituents such as some contaminant chemicals or bacteria that may be of concern in groundwater

natural organic matter in sedimentary basins and its relation to arsenic in anoxic ground water the example of west bengal and its worldwide implications in order to investigate the mechanism of as release to anoxic ground water in alluvial aquifers the authors sampled ground waters from 3 piezometer nests 79 shallow 80 m wells in an area 750 m by 450 m just north of barasat near kolkata calcutta in southern west bengal high concentrations of as 200 1180 mug l 1 are accompanied by high concentrations of fe 3 13 7 mgl 1 and po4 1 6 5 mg l 1 ground water that is rich in mn 1 5 3 mg l 1 contains 50 mug l 1 of as the composition of shallow ground water varies at the 100 m scale laterally and the metre scale vertically with vertical gradients in as concentration reaching 200 mug l 1 m 1 the as is supplied by reductive dissolution of feooh and release of the sorbed as to solution the process is driven by natural organic matter in peaty strata both within the aquifer sands and in the overlying confining unit in well waters thermotolerant coliforms a proxy for faecal contamination are not present in high numbers 10 cfu 100 ml in 85 of wells showing that faecally derived organic matter does not enter the aquifer does not drive reduction of feooh and so does not release as to ground water arsenic concentrations are high much greater than50 mug l 1 where reduction of feooh is complete and its entire load of sorbed as is released to solution at which point the aquifer sediments become grey in colour as feooh vanishes where reduction is incomplete the sediments are brown in colour and resorption of as to residual feooh keeps as concentrations below 10 mug l 1 in the presence of dissolved fe sorbed as released by reduction of mn oxides does not increase as in ground water because the as resorbs to feooh high concentrations of as are common in alluvial aquifers of the bengal basin arise because himalayan erosion supplies immature sediments with low surface loadings of feooh on mineral grains to a depositional environment that is rich in organic mater so that complete reduction of feooh is common c 2004 published by elsevier ltd

high resolution 5 μm u th pb isotope dating of monazite with excimer laser ablation ela icpms abstract monazite lree po 4 a common accessory mineral in magmatic and metamorphic rocks is complementary to zircon in u th pb geochronology because the mineral can record successive growth phases it is useful for unravelling complex geological histories a high spatial resolution is required to identify contrasted age domains that may occur at the crystal scale bulk mineral techniques such as id tims applied to single monazite grains recording multiple overgrowths or isotope resetting can result in partly scattered discordant analytical points that produce inaccurate intercept ages laser ablation la icpms has been demonstrated to be a useful technique for u th pb dating of zircons and this study tests its analytical capabilities for dating monazite a sector field high resolution icpms coupled with a 193 nm arf excimer laser ablation microprobe is capable of achieving a high spatial resolution and producing stable and reliable isotope measurements the u th pb systematic was applied to monazite grains from several samples a lower palaeozoic lens from high grade terrains in southern madagascar neogene hydrothermal crystals from the western alps a palaeoproterozoic very high temperature granulite from central madagascar and a variscan leucogranite from spain directly on a polished thin section the major aim was to compare and or reproduce tims and emp ages of monazite from a variety of settings and ages the three independent 206 pb 238 u 207 pb 235 u and 208 pb 232 th ratios and ages were calculated isotope fractionation effects mass bias laser induced fractionation were corrected using a chemically homogeneous and u pb concordant monazite as external standard this study demonstrates that excimer laser ablation ela icpms allows u th pb dating of monazite with a high level of repeatability accuracy and precision as well as rapidity of analysis a spatial resolution almost comparable to that of emp in terms of crater width 5 m produced precise 208 pb 232 th 206 pb 238 u and 207 pb 235 u ratios for dating palaeozoic to precambrian monazites the advantages of ela icpms isotope dating are precision accuracy and the ability to detect discordance in the case of late miocene hydrothermal monazites from the alps a larger spot size of 25 m diameter is required and precise and accurate ages were obtained only for 208 pb 232 th systematics results from the variscan granite show that in situ u th pb dating of monazites with ela icpms is possible using a 5 m spot directly on thin sections so that age data can be placed in a textural context

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

the early triassic indosinian orogeny in vietnam truong son belt and kontum massif implications for the geodynamic evolution of indochina new structural field data at various scale and 40ar 39ar geochronological results from the basement rocks in the truong son belt and kontum massif of vietnam confirm that ductile deformation and high temperature metamorphism were caused by the early triassic event of the indosinian orogeny in the range of 250 240 ma a compilation of isotopic data obtained in other countries along the sibumasu indochina boundary broadly indicates same interval of ages this tectonothermal event is interpreted as the result of a synchronous oblique collision of indochina with both sibumasu and south china inducing dextral and sinistral shearing along e w to nw se and n s fault zones respectively the collision along song ma follows the northwards subduction of indochina beneath south china and the subsequent development of the song da zone which in turn was affected by the late triassic indosinian phase of shortening within the indochina plate internal collisions occurred coevally in the early triassic as along the poko suture at the western border of the kontum massif

closure of the east paleotethyan ocean and amalgamation of the eastern cimmerian and southeast asia continental fragments abstract the phanerozoic record of southeast asia preserves the history of opening and subsequent consumption of the paleotethyan ocean and the assembly of continental fragments into asia as part of the broader scale reconstruction of pangea however uncertainty remains as to which of the many suture zones in southeast asia represents the relict of the main ocean when final ocean closure occurred and the assembly history of the eastern cimmerian and southeast asia continental fragments our overview of the geological features of the suture zones the bounding continental fragments and their magmatic metamorphic and sedimentary records resolves many of these key issues the sedimentary biogeographical structural lithological geochemical and geochronological data from the changning menglian inthanon and bentong raub suture zones argue for their linkage with the longmu co shuanghu suture zone in central tibet and together constitute the main east paleotethyan ocean relict the eastward subduction of the ocean resulted in the development of a series of magmatic arc back arc basin and continental fragments in southeast asia including from west to east the lincang sukhothai east malaya arc the jinghong nan sa kaeo back arc basin the simao west indochina fragment the luang prabang loei back arc basin the south indochina fragment the wusu and truong son back arc basins the north indochina fragment the jinshajiang ailaoshan song ma branch back arc basin and the south china block assembly of these fragments resulted in indosinian high temperature and high pressure metamorphism and related tectonothermal event available data indicate a switch from subduction of the main east paleotethyan ocean to the collision of the sibumasu with simao indochina blocks at 237 ma with subsequent syn and post collisional events at 237 230 ma and 230 200 ma respectively along the changning menglian inthanon and bentong raub suture zones the timing of initial syn and post collision events along the jinshajiang ailaoshan song ma suture zone with its record of back arc basin closure is at 247 ma 247 237 ma and 237 200 ma generally 10 ma older than that along the changning menglian inthanon and bentong raub suture zones our synthesis of all available data enables establishment of a comprehensive geodynamic model for the east paleotethyan evolution this model links the spatial temporal pattern across southeast asia into a series of tectonic events including ocean back arc basin opening subduction closure subsequent assemblage and orogenic collapse along with associated igneous metamorphic and sedimentary activities

predominance of high th u magmatic zircon in brazilian shield sandstones the strong predominance of magmatic th u 0 2 zircon grains in quartz sandstones from 10 locations in the brazilian shield as measured by sensitive high resolution ion microprobe shrimp introduces a strong bias in the tectonic interpretation of source terrains because the metamorphic sources remain undetected in the detrital population zircon grains from many mature and some immature sedimentary terrains show th u ratios 0 2 ratios commonly considered magmatic complemented by the near absence of metamorphic th u ratios 0 1 metamorphic rims on magmatic zircon grains from the brazilian shield are nearly absent from quartz sandstones only being present in shielded young cores of a few crystals high u metamorphic portions of zircon grains are more defective and liable to undergo comminution by abrasion during weathering and transport and they do not remain in the sand fraction ages obtained from detrital zircon in mature sandstones reflect magmatic sources not the full spectrum of orogenic events only less mature sedimentary rocks preserve a full record of orogenic rocks in the source mostly magmatic events were characterized in previous zircon provenance studies brazilian shield detrital zircon may be a proxy for equivalent investigations in other provinces of the world

magnetostratigraphic constraints on two stage eruptions of the emeishan continental flood basalts temporal and spatial variations of high and low contents of ti basalts in the permian emeishan large igneous province elip of southwest china and vietnam are crucial for modeling of continental basalt formation and the nature of mantle lithosphere interaction a combined geochemical and magnetostratigraphic study was carried out at yanyuan in the inner zone of the elip geochemical results indicate that the basalts are low ti lt basalts ti y 484 2 511 4 in the luomapu section and high ti ht basalts ti y 558 2 658 9 in the gongmushan section systematic paleomagnetic sampling and stepwise thermal demagnetization demonstrate that the higher temperature component shows two distinct characteristic remanent magnetizations with northeasterly upward and sww downward directions these results indicate a simple magnetostratigraphic normal and reversed polarity pattern from the lower and upper parts of the section respectively by comparing with available magnetostratigraphic and paleomagnetic data carried out in the elip these results imply that the eruption of the emeishan basalts can be divided into early normal polarity subchron and late reversal subchron stages the early stage included lt basalts in the inner zone and most of the ht basalts in the intermediate and outer zones and the late stage contained a part of lt and ht basalts in the upper parts of the sections in the inner zone and a few in the intermediate zone of the elip correlating with available middle late permian magnetostratigraphy and the permian magnetic time scale the new magnetic polarity sequence result suggests that the elip formed over a very short time interval 1 myr

u pb zircon geochronology and geochemistry from ne vietnam a tectonically disputed territory between the indochina and south china blocks volcanoplutonic complexes in ne vietnam have recently been interpreted as intraplate products of the emeishan plume alternatively mafic ultramafic rocks have been considered as dismembered palaeotethyan ophiolites juxtaposed along a tectonic melange zone new u pb zircon geochronological and geochemical datasets presented here suggest a complex geological history that records collision between the indochina south china blocks mafic ultramafic rocks exposed within a tectonic melange song hien tectonic zone include sub alkaline pillow basalts that define two geochemically distinct ophiolitic suites sh 1 n morb like sh 2 transitional e morb like both suites have geochemical signatures suggestive of crustal contamination compatible with a volcanic passive margin rift setting we suggest that sh 1 basalts may correlate with the devonian carboniferous jinshajiang ailaoshan song ma branch of the palaeotethys and form part of the associated dian qiong belt whereas sh 2 basalts are co magmatic with middle late permian mafic ultramafic intrusive rocks dolerites gabbros peridotites that developed in a rift basin most likely on the margin of the down going south china plate during west vergent subduction beneath indochina during continental orogenesis and thrust stacking these ophiolitic rocks were juxtaposed with other lithotectonic blocks within the song hien tectonic zone post collisional relaxation led to the development of a rift basin song hien rift comprising late permian triassic volcano sedimentary strata including 270 265 ma terrigenous sandstones 252 ma mudstones and c 254 248 ma felsic effusives granites and granodiorites were emplaced across ne vietnam between c 252 and 245 ma in a syn to post collisional setting the late permian early triassic felsic magmatic rocks best correlate with coeval rocks in sw guangxi and the central and western ailaoshan fold belts china and the truong son fold belt vietnam together they signal the final to post collisional stages of indochina south china collision we demonstrate that the analysed magmatic rocks in the lo gam song hien domains of ne vietnam are not genetically linked to the emeishan large igneous province in the yangtze block of south china as has been previously widely proposed

geochemical and zircon u pb and hf isotopic study of the baijuhuajian metaluminous a type granite extension at 125 100 ma and its tectonic significance for south china abstract the early cretaceous baijuhuajian pluton is an a type granitic intrusion emplaced along the jiangshan shaoxing jssx fault zone in western zhejiang province se south china it intruded into a late jurassic volcanic basin bounded by proterozoic sandstone and siltstone the granite has a coarse grained porphyritic texture and is composed of alkali feldspar phenocrysts in a matrix of k feldspar quartz plagioclase and biotite the pluton has a clear a type geochemical signature e g it is metaluminous a cnk 1 0 and has high sio2 75 to 78 wt total alkalis na2o k2o 4 8 to 8 5 wt rare earth elements total ree 228 to 438 ppm hfse and fe feot feot mgo 0 87 to 0 97 on the other hand it is characterized by low cao 0 51 to 1 01 wt tio2 0 06 to 0 13 wt p2o5 below detection and sr 7 0 to 9 5 ppm the hypersolvus texture and fine grained groundmass indicate emplacement at a shallow depth shrimp u pb zircon dating yielded a weighted mean 206pb 238u age of 126 3 ma and zircons from this pluton give ehf t values between 0 52 and 4 24 suggesting the importance of juvenile material in the magma source most of the plutonic rocks in southeast china are calc alkaline subduction related intrusions of jurassic and cretaceous age previous studies suggested that the sparse a type granites in south china were emplaced in two episodes 190 155 ma in the inland area and 100 ma along the coastal area in se south china our study of the baijuhuajian granite together with the coeval suzhou a type granite 123 ma in jiangsu province defines an important extensional event between 125 and 100 ma considering the available data for the mesozoic igneous rocks in south china we suggest that subduction of the pacific plate in the mesozoic produced voluminous calc alkaline granitoids and volcanic rocks with a younging trend from west to east the a type granites were generated during periods of local extension at 190 155 ma and 125 100 ma perhaps corresponding to localized rollback of the pacific plate

mesozoic cenozoic tectonothermal evolution of the eastern part of the tibetan plateau songpan garze longmen shan area insights from thermochronological data and simple thermal modelling we present a synthesis of the tectonic and thermochronological evolution of the eastern tibet since the triassic the long term cooling histories obtained on magmatic and metamorphic rocks of the south songpan garze kunlun and yidun blocks are similar showing a very slow and regular cooling during late jurassic and cretaceous confirming the suspected lack of major tectonic events between c 150 and 30 ma the exhumation linked to the tertiary growth of the tibetan plateau initiated around 30 ma and concentrates at the vicinity of the major tectonic structures exhumation rates increased again from about 7 ma in the longmen shan to interpret this very slow cooling rate between late jurassic and early cenozoic from granites of this area we use a simple 1d thermal model that takes into account the thermal properties of both sediments and crust the results suggest that 1 high temperature 500 8c can be kept over a long period of time 2 during cretaceous cooling is mostly controlled by the thermal properties of sediments of continental origin and 3 the initial late triassic rapid cooling rate was caused by the large thermal contrast between the granite body and the sedimentary rocks rather than by a high exhumation rate

iron reduction in the sediments of a hydrocarbon contaminated aquifer abstract sediments sampled at a hydrocarbon contaminated glacial outwash sandy aquifer near bemidji minnesota were analyzed for sediment associated fe with several techniques extraction with 0 5 m hcl dissolved poorly crystalline fe oxides and small amounts of fe in crystalline fe oxides and extracted fe from phyllosilicates use of ti citrate edta bicarbonate results in more complete removal of crystalline fe oxides the average hcl extractable fe iii concentration in the sediments closest to the crude oil contamination 16 2 mol g has been reduced by up to 30 from background values 23 8 mol g as a result of fe iii reduction in contaminated anoxic groundwater iron ii concentrations are elevated in sediments within an anoxic plume in the aquifer iron ii values under the oil body 19 2 mol g are as much as 4 times those in the background sediments 4 6 mol g indicating incorporation of reduced fe in the contaminated sediments a 70 increase in total extractable fe at the anoxic oxic transition zone indicates reoxidation and precipitation of fe mobilized from sediment in the anoxic plume scanning electron microscopy detected authigenic ferroan calcite in the anoxic sediments and confirmed abundant fe iii oxyhydroxides at the anoxic oxic boundary the redox biogeochemistry of fe in this system is coupled to contaminant degradation and is important in predicting processes of hydrocarbon degradation

la icpms zircons u pb dating of permo triassic and cretaceous magmatisms in northern vietnam geodynamical implications abstract in northeastern vietnam the major tectonic episode responsible for nappes emplacement is triassic these allochtonous structures are intruded by granitic melts two post tectonic massifs showing no sign of deformation have been dated by the la icpms zircon u pb techniques dating reveals a multiphase history with zircon cores showing evidence of proterozoic magmatism the emplacement of the phia bioc granite intrusive in allochtonous units is 248 245 ma an age which assesses a younger limit for the major nappes tectonic this tectonic could be synchronous of the tectonometamorphic strike slip faulting events 250 245 ma defined in the truong son belt as the indosinian orogen the phia bioc intrusion is probably linked with the intra plate magmatism of the emeishan large igneous province or with magmatism associated with the paleotethys closure the age of the phia oac granite intrusion in displaced units is much younger at 87 3 1 2 ma this granite is probably linked to the magmatic activity produced during the paleo pacific plate subduction under the se asia continental plate during the mesozoic although the cenozoic red river fault system is close to these two plutons this last thermotectonic episode has not been strong enough to disturb the u pb system zircons rims do not show any tertiary magmatic or metamorphic overprint

triassic high strain shear zones in hainan island south china and their implications on the amalgamation of the indochina and south china blocks kinematic and 40ar 39ar geochronological constraints abstract a kinematic and geochronological study has been carried out on the triassic high strain shear zones in hainan island the southern south china block there are wnw and ne trending high strain shear zones with greenschist to amphibolite facies metamorphism in this island kinematic indicators suggest a dextral top to the nne thrust shearing for the wnw trending high strain shear zones and a sinistral top to the se thrust shearing for the ne trending shear zones the quartz c axis orientations of mylonitic rocks exhibit the domination of basal slip and some activation of a rhombohedra gliding system the timing of shearing for these shear zones has been constrained by the 40ar 39ar dating analyses of synkinematic minerals middle triassic 242 250 ma and late triassic early jurassic 190 230 ma have been identified for the wnw and ne trending shear zones respectively a synthesis of these kinematic and thermogeochronological data points to a two stage tectonic model for hainan island that is top to the nne oblique thrusting at 240 250 ma followed by top to the se oblique thrusting at 190 230 ma in combination with the available data from the southern south china and indochina blocks it is inferred that south hainan and north hainan have affinity to the indochina and south china blocks respectively the tectonic boundary between south hainan and north hainan lies roughly along the wnw trending changjiang qionghai tectonic zone probably linking to the song ma and ailaoshan zones the middle triassic structural pattern of hainan island is spatially and temporally compatible with those of the south china and indochina blocks and thus might be a derivation from the amalgamation of the indochina with south china blocks in response to the closure of the paleotethys ocean and subsequent subduction collision

geochronology and isotope analysis of the late paleozoic to mesozoic granitoids from northeastern vietnam and implications for the evolution of the south china block abstract in northeastern vietnam late paleozoic and permo triassic granitic plutons are widespread but their tectonic significance is controversial in order to understand the regional magmatism and crustal evolution processes of the south china block scb this study reports integrated in situ u pb hf o and sr nd isotope analyses of granitic rocks from five plutons in northeastern vietnam zircon sims u pb ages of six granitic samples cluster around in two groups 255 228 ma and 90 ma bulk rock end t ranges from 11 to 9 7 suggesting that continental crust materials were involved in their granitic genesis in situ zircon hf o isotopic measurements for the granitic samples yield a mixing trend between the mantle and supracrustal derived melts it is suggested that the granitic rocks were formed by re melting of the continental crust these new data are compared with the paleozoic and mesozoic granitic rocks of south china we argue that northeastern vietnam belongs to the south china block though still speculated an ophiolitic suture between ne vietnam and south china so called babu ophiolite appears unlikely the late paleozoic to mesozoic magmatism in the research area provides new insights for the magmatic evolution of the south china block

structural signatures and 40ar 39ar geochronology of the indosinian xuefengshan tectonic belt south china block abstract to advance our understanding of the kinematics of the xuefengshan tectonic belt xfstb and the mesozoic tectonics of the south china block scb this paper presents new data and interpretations from our structural studies and thermochronological dating the xfstb is characterized by greenschist facies metamorphism and development of s and s l tectonic fabrics an asymmetric positive flower structural pattern is composed of nw wnw and se ese dipping cleavages faults and shear zones kinematic indicators indicate a dominant top to wnw thrusting with a sinistral strike slip component on the ese dipping shear zones and top to ese back thrusting on the wnw dipping shear zones the quartz c axis orientations of mylonitic rocks exhibit monoclinic point maximum asymmetry indicative of a sinistral shear sense under the a basal gliding condition the timing of the major deformation event has been constrained to the middle triassic to early jurassic 244 195 ma on the basis of 40ar 39ar geochronology and other geological observations the structural characteristics of the xfstb are probably related to an indosinian oblique convergent zone in combination with tectonic wedging and associated back thrusting all above a low angle se dipping basal detachment oblique northwestward and upward movement along the basal detachment was partitioned into the nw wnw directed thrusting se ese directed back thrusting and subsidiary strike slip movement along the ne and nne trending faults and shear zones the xfstb may represent part of a huge structural fan between the yangtze and cathaysian blocks and can be interpreted as a product of the indosinian intracontinental collision involving a weak zone

the kinematic history of the khlong marui and ranong faults southern thailand abstract the khlong marui fault kmf and ranong fault rf are major nne trending strike slip faults which dissect peninsular thailand they have been assumed to be conjugate to the nw trending three pagodas fault tpf and mae ping fault mpf in northern thailand which experienced a diachronous reversal in shear sense during india eurasia collision it follows that the kmf and rf are expected to show the opposite shear sense and a slip sense reversal at a similar time to the tpf and mpf new field data from the kmf and rf reveal two phases of ductile dextral shear separated by campanian magmatism paleocene to eocene post kinematic granites date the end of this phase while a brittle sinistral phase deforms the granites and has exhumed the ductile fault rocks the timing of these movements precludes formation of the faults in response to himalayan extrusion tectonics instead they formed near the southern margin of a late cretaceous paleocene orogen and may have been influenced by variations in the rate of subduction ahead of india and australia north south compression prior to reactivation of the subduction zone around southern sundaland in the eocene caused widespread deformation in the over riding plate including sinistral transpression on the kmf and rf

discussion on the role of the red river shear zone yunnan and vietnam in the continental extrusion of se asia journal vol 163 2006 1025 1036 p h leloup p tapponnier r lacassin write in his recent paper searle 2006 acknowledges that the 1000 km long ailao shan red river shear zone is a large miocene left lateral shear zone but speculates that left lateral slip started after 21 ma and claims that the total finite offset remains unknown from this he concludes that continental extrusion was only a relatively minor tectonic factor during the india asia collision as long argued by other workers e g england houseman 1986 cobbold davy 1988 dewey et al 1989 houseman england 1993 we summarize below the field and geochronological evidence that makes us maintain a viewpoint in better accordance with facts timing of left lateral shear along the ailao shan red river shear zone the ailao shan red river shear zone is composed mostly of high grade metamorphic rocks and deformed granitoids with ubiquitous evidence for left lateral shear parallel to the belt e g tapponnier et al 1986 1990 leloup et al 1993 1995 2001 the crystallization of the granitoids has been dated between 22 and 35 ma fig 1d f for example by scharer et al 1990 1994 and zhang scharer 1999 leading those workers to propose that left lateral shear started at least c 35 ma ago in contrast searle 2006 claims that all the deformed granitoids found within the shear zone predate left lateral shear and that their crystallization ages should thus be interpreted to provide an upper limit for the onset of deformation rather than a lower limit thus suggesting a maximum age of 21 ma for this deformation a clear understanding of the p t history and in situ deformation history of the intrusions as well as of their relationships with surrounding paragneisses is fundamental for interpreting correctly the geochronological

ar ar ages of gold deposits in the song hien domain ne vietnam tectonic settings and comparison with golden triangle in china in terms of a single metallogenic province abstract the song hien rift basin is an important metallogenic area in ne vietnam this domain consists mainly of triassic sulfide rich black shale beds which play a role as a sedimentary host for various mineral systems such as antimony mercury and gold sulfide deposits most of gold deposits are hosted in carbonaceous sedimentary rocks however some deposits which have similar characteristics are hosted in fine grained mafic magmatic rocks an ar ar isotopic dating of hydrothermal sericite from the sedimentary hosted bo va and khung khoang gold deposits and intrusion hosted orogenic hat han gold deposit yields plateau ages of 184 8 2 1 ma 211 63 2 3 ma and 209 12 2 3 ma respectively the obtained ar ar ages convincingly show that the orogenic gold deposits in the song hien domain were formed in late triassic to early jurassic while the age of the bo va deposit is at least older than 184 8 2 1 ma loss of argon by volume diffusion supported by previously reported mineralogical and isotopic features of the bo va deposit may suggest that the jurassic cretaceous yanshanian tectonothermal events overprinted some deposits in the song hien domain formation of gold deposits in the song hien domain is linked to the same tectonic event as the carlin like gold deposits in sw china and is associated with an extensional tectonic regime that followed continental collision between the indochina and south china blocks the similarity in geology setting and mineral composition of gold deposits of the song hien domain and the golden triangle region as well as timing and kinematics of deformation magmatic features and stratigraphic sequence and bulk architecture lead to conclusion that ne vietnam and sw china is a single metallogenic zone the study of gold deposits in vietnam will provide a new data on the metallogenic history of this important part of se asia

detrital zircon ages and heavy mineral composition along the gulf of tonkin implication for sand provenance in the yinggehai song hong and qiongdongnan basins abstract neogene reservoir sand in the yinggehai song hong ygh shb and qiongdongnan basins qdnb was derived from the red river hainan island and the vietnamese truong son belt we investigate the sand dispersal pathways through comparisons of detrital zircon geochronology and heavy mineral analyses of modern vietnamese river sand holocene to recent near coastal sand from hainan island and vietnam and offshore neogene sand and we evaluate the methodologies eleven distinct zircon age clusters characterize the gulf of tonkin provenances red river sand contains the most complex age pattern with 35ma 88ma 155ma 250ma 440ma peaks and broader pre cambrian clusters the 88ma peak uniquely occurs in red river sand northern truong son belt sand contains 35ma 250ma 440ma peaks and pre cambrian clusters comparable to the red river the lack of cretaceous zircons and a high proportion of 250ma relative to 440ma zircons distinguishes the northern truong son belt from the red river the southern truong son belt is characterized by low proportions of 250ma relative to 440ma zircons and the absence of cretaceous and cenozoic zircons hainan island sand is identified by the presence of 100 ma zircons and the absence of cenozoic and 88 ma zircons the zircon age patterns suggest simple although dynamic sand dispersal pathways with mixed sand sources over much of the basins likely resulting from sediment reworking and down stream river merging induced by neogene sea level fluctuations sand in the western ygh shb was mainly sourced from the truong son belt the red river sourced the northern and central ygh shb and qdnb and hainan sand was primarily deposited along the island margin heavy mineral compositions in the gulf of tonkin primarily reflect sediment compositional maturity epidote amphibole pyroxene and garnet indicate immaturity whereas a high zircon content tends to indicate high maturity the use of heavy minerals as provenance indicator should proceed with the utmost caution

emeishan basalts sw china and the end guadalupian crisis magnetobiostratigraphic constraints abstract a magnetostratigraphic investigation of the permian emeishan lip large igneous province was carried out on a composite section in ebian county close to the type area in sichuan sw china the main succession of twelve flows 175 m thick carries a normal polarity whilst the one reliable site from the overlying 30 m thick volcanic waning sequence is marked by a reverse polarity the data enable a correlation to be proposed with an emeishan basalt sequence in western guizhou c 400 km to the se from our knowledge of the geomagnetic field9s reversal behaviour during the permian it suggests that the main lava pile along the eastern half of the emeishan basalt outcrop belt was erupted within a half to one million years using magnetobiostratigraphic data from the adjacent south china platform the normal polarity magnetozone is correlated with the normal polarity chron associated with the upper part of the maokou limestones constrained by conodont data the main emei basalts appear to be at least two biozones older 1 1 5 ma than the mid late permian boundary it is possible however that the emei basalt waning zone sequence which represents an explosive volcanic phase might be coeval with the end guadalupian biotic crisis thus arguments implicating emei basalt volcanism as the causal mechanism behind this major global event have to accommodate the new relative age constraints

timing of formation and exhumation of the montagne noire double dome french massif central new u pb monazite zircon and xenotime ages date the late carboniferous crystallization of the anatectic vialais granite in the montagne noire axial zone and the high temperature deformation and metamorphism of the augen gneiss ordovician granite protolith from which the granite was likely derived the u pb monazite ages obtained from the augen gneiss 308 3 ma late kinematic vialais granite 303 4 ma and post kinematic leucogranite 298 2 ma bracket the high temperature deformation and metamorphism at 310 300 ma clearly postdating regional contraction and nappe emplacement 320 ma the planar linear and locally linear fabrics in the augen gneiss outline a regional scale double dome structure caroux and espinouse sub domes containing smaller km scale upright folds compared to the caroux sub dome the larger migmatitic espinouse sub dome contains abundant leucosome leucogranite bodies and late kinematic intrusions vialais granite the vialais granite displays a weak magmatic foliation that defines the main layering of the espinouse migmatite ellipsoidal quartz sillimanite nodules concentrated above the roof of the granite recorded an increment of strain vertical shortening e ne elongation during granite emplacement and crystallization consistent with the extension event that exhumed the domes these new geochronological and structural data suggest that the montagne noire double dome formed in a pull apart structure within a dextral strike slip system upright folding in the sub domes has been traditionally assigned to a regional contraction event alternatively we propose that local contraction is associated with the convergence of low viscosity crust beneath the upper crust pull apart dynamic models of extension of hot crust indicate that contraction at depth is generated by flow of low viscosity orogenic crust converging and rising to fill the gap created by upper crust extension this interpretation solves the long standing problem of apparent coeval contraction and extension in the montagne noire double dome and establishes migmatite dome emplacement in strike slip corridors as a style of late variscan tectonics

oligocene miocene bu khang extensional gneiss dome in vietnam geodynamic implications a large oligocene miocene extensional gneiss dome in vietnam has implications for the kinematics of the ailao shan red river shear zone and rifting of the south china sea a major northeast dipping extensional shear zone separates the bu khang dome from less metamorphosed units above shearing deformation occurred during the exhumation of high pressure rocks 40 ar 39 ar ages on micas reveal oligocene miocene ages from 36 to 21 ma after a high pressure episode of probable oligocene age the area was subjected to northeast southwest extension during the opening of the south china sea a kinematic model involving left lateral shear and block rotation within a north south dextral shear zone integrates our observations in the bu khang dome with south china sea rifting

zircon and monazite upb dating of the doi inthanon core complex northern thailand implications for extension within the indosinian orogen abstract the doi inthanon metamorphic core complex occupies a prominent central position within the northwestern thai gneiss belt which has been inferred to be as old as precambrian and as young as permo triassic u pb dating of zircon from core orthogneisses of the complex suggests that the orthogneisses were derived from granitic protoliths with essentially the same late triassic early jurassic age as the mae cham pluton 205 3 4 ma to the west of the doi inthanon complex monazite ages from the orthogneisses indicate that low pressure and high temperature metamorphism and migmatitization occurred in the late cretaceous 84 72 ma limited published radiometric age data from elsewhere in the migmatitic gneiss belt suggest that widespread minor plutonism accompanied this thermal event a younger u pb data of 26 8 0 5 ma for zircon and monazite from the mae klang granite which intruded mylonitic gneisses on the eastern side of the doi inthanon complex indicates that there was also significant plutonism in the late oligocene these u pb ages constrain the development of the complex and hence of the migmatitic gneiss belt as a whole to an interval between late cretaceous and early miocene ductile shearing uplift and tectonic denudation via detachment faulting chronologically overlap the initiation of extensional basin development to the east and southeast of the gneiss belt and imply a genetic connection thus oligocene early miocene thermal rifting from the gulf of thailand northward into mainland thailand may have been the direct product of crustal extension rather than the transtensional product of regional strike slip faulting

timing of granite emplacement and cooling in the songpan garze fold belt eastern tibetan plateau with tectonic implications abstract new u pb and rb sr geochronology on syn and post orogenic granites provide constraints on the timing of major tectonic events in the songpan garze fold belt west sichuan china the ma nai granite was probably syn kinematic with the main deformation and yields an age of 197 6 ma that is interpreted as an upper age limit of the indosinian event zircons and apatites from the post kinematic rilonguan granite also yield jurassic ages 195 6 and 181 4 ma the post orogenic markam massif gives two ages of 188 1 and 153 3 ma both granites are undeformed and cut structures in the triassic sedimentary rocks these results demonstrate that the major deformation and decollement tectonics in the songpan garze fold belt occurred prior to the early jurassic the wide range of ages obtained for post kinematic granites from early jurassic to late jurassic suggests that locally magmatic activity persisted for a long time at least 50 ma after the indosinian compressional tectonism no tertiary ages have been obtained suggesting that these granites were not affected strongly by the india asia collision

u pb zircon ages of granites from the southern margin of the yangtze block timing of neoproterozoic jinning orogeny in se china and implications for rodinia assembly abstract neoproterozoic jinningian bendong granodiorite and sanfang and yuanbaoshan granites occurred in northern guangxi southern margin of the yangtze block se china and were generally considered to be of orogenic origin that is the collision between the yangtze and huanan blocks during the jinning orogeny these granites were intruded into the mesoproterozoic sibao metamorphic basement and covered unconformably by the neoproterozoic danzhou metasediments they are dominantly peraluminous asi 1 18 1 29 and have low initial end values of 4 8 to 7 6 indicating derivation from a supracrustal source with strong enrichment in rb k and th and relatively low zr hf and higher rare earth elements they are similar to post collision granites zircon u pb ages of 819 9 826 10 and 824 4 ma were obtained for the bendong granodiorite and the sanfang and yuanbaoshan granites respectively by both shrimp and conventional dating methods the age result precludes the existence of any late mesoproterozoic grenville age granitic magmatism in the southern margin of yangtze block together with previous age determinations of ophiolites along the southern margin of the yangtze block the jinning orogeny is referred to the neoproterozoic orogeny which took place between 1 0 and 0 80 ga and likely ended at 0 82 ga the granite formation age of 0 82 ga also supports the basal boundary age of 0 80 ga for the post orogenic sinian system which is probably stratigraphically correlated with the adeladean system in south australia as the link between australia east antarctica and laurentia in the configuration of the supercontinent rodinia the collision between the yangtze and huanan blocks during the neoproterozoic jinning orogeny probably marks the termination of the rodinia assembly

role of the red river shear zone yunnan and vietnam in the continental extrusion of se asia the 1000 km long nw se striking left lateral ailao shan red river shear zone runs from the southeastern corner of tibet to the gulf of tonkin and the south china sea it has been used as the prime example of a lithospheric scale strike slip fault that has accommodated between 500 and 1000 km of southeastwards extrusion of indo china away from the indian plate indentor central to the model of continental extrusion is that such faults cut through the entire lithosphere that shear heating resulted in high grade metamorphism and local anatexis and that the ages of sheared granites along the fault also date the timing of strike slip shearing however structural data from the red river shear zone clearly show that vertical strike slip faulting post dated metamorphism and granite emplacement most granites along the shear zone are mantle related granodiorites or within plate alkali granites formed prior to shearing along the red river shear zone left lateral kinematic indicators are ubiquitous within the red river mylonites but they are always lower temperature fabrics formed after peak sillimanite metamorphism and after granite crystallization it is suggested that left lateral strike slip shearing along the red river shear zone started after 21 ma not at 35 ma as previously thought and the fault was purely a crustal structure none of the geological features used to propose the 500 1000 km offsets are robust and the total finite offset remains unknown

geodynamic significance of the kontum massif in central vietnam composite 40ar 39ar and u pb ages from paleozoic to triassic abstract the kontum massif of south central vietnam has long been classified as lower continental crust of precambrian archean age u pb zircon and 40ar 39ar biotite ages from charnockites and amphibolite facies rocks from this basement however yield much younger ages and imply at least two paleozoic thermal events syntectonic charnockites from the kannack complex and the southern kontum massif record permo triassic magmatism u pb documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage ot2 ot1 fontenc newcommand cyr renewcommand rmdefault wncyr renewcommand sfdefault wncyss renewcommand encodingdefault ot2 normalfont selectfont declaretextfontcommand textcyr cyr pagestyle empty declaremathsizes 10 9 7 6 begin document landscape 249 pm 2 end document 2 ma and docume

crude oil in a shallow sand and gravel aquifer iii biogeochemical reactions and mass balance modeling in anoxic groundwater abstract crude oil floating on the water table in a sand and gravel aquifer provides a constant source of hydrocarbons to the groundwater at a site near bemidji minnesota the degradation of hydrocarbons affects the concentrations of oxidized and reduced aqueous species in the anoxic part of the contaminant plume that developed downgradient from the oil body the concentrations of fe2 mn2 and ch4 eh measurements and the 13c ratios of the total inorganic c indicate that the plume became more reducing ver a 5 a period however the size of the contaminant plume remained stable during this time field data coupled with laboratory microcosm experiments indicate that benzene and the alkylbenzenes are degraded in an anoxic environment in anaerobic microcosm experiments conducted under field conditions almost complete degradation 98 was observed for benzene in 125 d and for toluene in 45 d concentrations of aqueous fe2 and mn2 increased in these experiments indicating that the primary reactions were hydrocarbon degradation coupled with fe and mn reduction mass transfer calculations on a 40 m flowpath in the anoxic zone downgradient from the oil body indicated that the primary reactions in the anoxic zone are oxidation of organic compounds precipitation of siderite and a ferroan calcite dissolution of iron oxide and outgassing of ch4 and co2 the major difference in the two models presented is the ratio of co2 and ch4 that outgasses both models indicate quantitatively that large amounts of fe are dissolved and reprecipitated as ferrous iron in the anoxic zone of the contaminant plume

permian tectonic framework and palaeogeography of se asia abstract the principal continental lithospheric terranes now comprising se asia and that existed in the permian are the south china indochina super terrane and the sibumasu simao west burma and sw borneo semitau terranes the simao terrane is here regarded a separate terrane derived from south china by back arc spreading in the lower carboniferous rather than as an extension of indochina the main palaeo tethys ocean in thailand is identified as being represented by the chiang mai and sra kaeo sutures and the eastern boundary of the sibumasu terrane is thus interpreted to lie farther west in thailand than previously inferred the nan uttaradit suture is interpreted as representing a marginal back arc basin and contiguous with the ailaoshan and jinshajiang sutures in sw china the south china indochina super terrane and the simao terrane were located within the palaeo tethys in equatorial latitudes during the permian where they together with north china exhibit cathaysian floras and faunas the sibumasu terrane was located on the nw australian margin of gondwana until the sakmarian major shifts in faunal affinities from gondwana province faunas to an independent sibumasu province and then to assimilation into the cathaysian province records the separation and rapid northward drift of sibumasu after the late sakmarian the principal permian palaeogeographic models for the region are assessed for internal consistencies and conflicts

long and complex thermal history of the song chay metamorphic dome northern vietnam by multi system geochronology multi system geochronology was used to unravel the long and complex thermal history of the song chay range northern vietnam a high grade granitic and metamorphic dome in the vicinity of the cenozoic ailao shan red river fault zone it was considered to be proterozoic south china basement but its geological history was basically unknown scattered field observations suggest three episodes of high temperature deformation firstly at the time of granite emplacement secondly a decollement with top to the north shear and thirdly anticlinal doming of the foliations formed during the two first stages p t estimates suggest that metamorphism coeval with the second deformation phase culminated at 580 ca nd 4 5 kbar 16 km depth multi system geochronology is applied to a two micas granite sample slightly deformed within the decollement u pb dating of zircon yields an age of 428 5m a 2s interpreted as the time of granite crystallization within the south china caledonian belt rb sr on white micas and biotite yields ages of 206 10 ma and 176 5 3 ma respectively 2s whereas 39ar 40 ar ages of the same minerals are 210 9a nd 190 8m a 2s these ages suggest an upper triassic episode of rapid cooling interpreted as due to doming a few million years after the end of movement on the decollement the k feldspar irregular 39ar 40ar age spectrum can to the first order be explained by a cooling history with two episodes of rapid cooling one at 140 ma and a second around 41 to 25 ma apatite fission tracks central age 33 6 3 6 ma 1s confirms a tertiary rapid cooling event interpreted as the final exhumation of the song chay dome 2000 elsevier science b v all rights reserved

triassic tectonics of the ailaoshan belt sw china early triassic collision between the south china and indochina blocks and middle triassic intracontinental shearing in se yunnan the ailaoshan belt has been extensively studied for the ductile shearing coeval with the left lateral cenozoic activity of the red river fault however the late triassic unconformity of the continental red beds upon metamorphic and ductilely deformed rocks demonstrates that the ailaoshan belt was already built up by early mesozoic tectonics from west to east the belt is subdivided into western central eastern ailaoshan and jinping zones the western ailaoshan and central ailaoshan zones correspond to a carboniferous permian magmatic arc and an ophiolitic melange respectively the eastern ailaoshan and the jinping zones consist of deformed proterozoic basement and paleozoic to early triassic sedimentary cover series both belonging to the south china block this litho tectonic zonation indicates that the ailaoshan belt developed through a sw directed subduction followed by the collision between indochina and south china blocks crustal thickening triggered per aluminous magmatism dated at ca 247 240 ma field and microscope scale top to the ne ductile shearing observed only in the pre late triassic formations but never in late triassic or younger formations complies with this geodynamic polarity furthermore the late collisional two mica granitoids and felsic per aluminous volcanites record a ductile deformation that argues for a continuing crustal shearing deformation after the early triassic collision up to the middle triassic therefore a two stage tectonic evolution accounts well for the documented structural and magmatic features the triassic architecture of the ailaoshan belt and its geodynamic evolution correlate well to the south and north with the north vietnam orogens and the jinshajiang belt respectively

origin of the intrusion related lang vai gold antimony district northeastern vietnam constraints from fluid inclusions study and c o s pb isotope systematics abstract the lang vai ore district located in the central part of the lo gam domain in the northeast vietnam contains at least three deposits khoun puc lang vai and lung luong the general paragenetic sequences for these deposits may be represented by the following mineral associations from early to late i arsenopyrite pyrite native gold sphalerite ii bi rich pb sb sulfosalts iii bi poor pb sb sulfosalts iv sb sulfosalts v sb rich minerals native antimony and vi pb rich sb sulfosalts as a distal mineral association three major types were recognized within the primary fluid inclusions based on the phases present at room temperature including v type vapor rich inclusions l type liquid rich inclusions and c type co2 rich inclusions the early au as stage quartz crystals contain the v and l type fluid inclusions the results indicate that the initial ore forming fluid belonged to a co2 ch4 h2o nacl system characterized by high temperatures 290 352 c and variable to moderate salinities 3 16 13 89 wt nacl which are analogous to reduced intrusion related ore forming fluids that exhibit high homogenization temperatures and moderate salinities the quartz crystals from the bi pb sb mineral paragenesis occurred only at the khoun puc deposit contain both v and l type fluid inclusions and characterized by high temperatures mainly 304 340 c and moderate salinities 6 28 8 22 wt nacl equiv the quartz crystals from the middle hydrothermal stage contain the v and l type fluid inclusions that yield moderate homogenization temperatures 240 290 c and variable salinities 3 3 8 2 wt nacl equiv these features indicate that the fluid evolved into an co2 ch4 h2o nacl system at moderate temperatures lower ch4 concentrations and variable salinities thus indicating changing physicochemical conditions in the last sb stage the lang vai deposit and pb sb stage the lung luong deposit the ore forming fluid belongs to co2 h2o nacl system which is characterized by low temperature 180 240 c enrichment of co2 and medium low salinity 1 12 9 5 wt nacl equiv that corresponded to typical fluids which formed the sb and distal ag pb zn veins of the irgs deposits in the tintina gold province of alaska usa and yukon canada the fluid inclusion studies and laser raman spectroscopy suggest that the early fluids were dominated by ch4 whereas the late fluids by co2 our study suggests that the fluid boiling and accompanying decrease in the solubility of gold bisulfide complex played an important role in gold deposition at the khoun puc and lang vai deposits the trapping pressures estimated from the boiling fluid inclusions were 123 343 mpa during the ore forming process this suggests an alternating lithostatic hydrostatic fluid system controlled by a fault valve activity at a depth of 12 9 15 2 km the characteristics of the s isotopes and fluid inclusions suggest that the ore forming fluids were of magmatic origin during mineralization process the fluid rock interaction between the fluids and host rocks resulted in the increase of 34s values of the fluids as reflected in the range of sulfur isotope from the different deposits on a plumbotectonic diagram the sulfides show correlations between 206pb 204pb and both 207pb 204pb and 208pb 204pb which suggest a two component mixing between a high radiogenic end member and another low radiogenic end member the 13cpdb vs 18osmow study clear show that c and o in the khoun puc ore forming fluids were most likely to be derived from the magma with limited contribution from the carbonate host rocks predominantly magmatic origin of hydrothermal fluids wherein the calcite separates from the lang vai and lung luong deposits have 18osmow values higher than those of the khoun puc suggesting that the carbonate host rocks may have provided more c and o for ore forming fluids in these deposits the results combined with existing data on the regional geology ore geology ore geochemistry age of mineralization and tectonic setting indicate that the lang vai ore district is an intrusion related gold system

mesozoic plutons of the yidun arc sw china u pb geochronology and hf isotopic signature abstract the yidun arc is a triassic volcanic arc located between the songpan garze fold belt and the qiangtang block southwest china to constrain the age of a number of the major granitic plutons from the yidun arc laser ablation icp ms u pb analysis of zircon was conducted hafnium isotope data was also acquired through laser ablation multicollector icpms analysis of zircon with the aim of gaining insight into the age and nature of the source region of the plutons three age groups have been identified from seven granite samples early middle triassic 245 to 229 ma late triassic 219 to 216 ma and cretaceous 105 to 95 ma hafnium analysis shows the triassic granites to have negative and variable e hf values and mesoproterozoic 1 6 ga depleted mantle model ages which is interpreted to reflect derivation from an isotopically heterogeneous largely crustal source the cretaceous granite shows higher and less variable e hf values and slightly younger model ages 1 3 ga and is interpreted to be derived from melting of a more homogeneous crustal source a depleted mantle model age of 1 5 ga is calculated from the pooled triassic and cretaceous samples the source region for these magmas may be tentatively correlated with mesoproterozoic material of the yangtze craton which has been suggested to underlie the yidun arc however further work is necessary to demonstrate this suggestion