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

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

a new model for the indochina and south china collision during the late permian to the middle triassic article i nfo a comparative analysis of indochina and south china during the early paleozoic indicates the former may be extended to include north vietnam part of the qinzhou tectonic zone and southern hainan island these three regions were traditionally regarded as parts of south china separated from indochina by the song ma suture in central north vietnam a new suture called the dian qiong suture is proposed here approximately along the southern margin of the present nanpanjiang basin this suture is linked to its eastern counterpart in hainan island through a nne trending dextral transform fault zone along the eastern margin of the nanpanjiang basin the conventional song ma suture originally constitutes its western extension and was translated to its present location by sinistral displacement along the red river fault zone during the tertiary upper paleozoic deep water turbidites and associated mid oceanic ridge basalts along the suture in the nanpanjiang basin illustrate a coeval ocean between indochina and south china the ocean was consumed by south directed subduction beneath indochina during the late permian to the middle triassic this suture zone was finally re shaped by the indentation between the active margin of indochina and the irregular passive continental margin of south china during the late triassic as well as displacement along the red river fault zone during the cenozoic

zircon u pb ages and geochemistry of granitoids in the truong son terrane vietnam tectonic and metallogenic implications abstract truong son terrane one of the most important tectonic and metallogenic terranes in indochina block is composed of many volcano plutonic complexes reported here is geochronological and geochemical data obtained from six different volcano plutonic complexes the new data reveals that the granite from the hai van complex is 438 ma in age and shows collision related geochemical characteristics whereas another five samples from five volcano plutonic complexes present consistent emplacement and crystallization ages ranging from 261 to 242 ma dien bien granodiorite phia bioc monzogranite and dong trau rhyolite display typical subduction related calc alkaline affinity e g depletion in nb ta and ti and enrichment in rb and la while monzogranite from song ma complex displays collision related shoshoniteseries and granites from bengiang queson complex are related to post collision calc alkaline series based on these observations in combination with the previous published geochronological data we propose that at least four major stages of magmatic activities occurred during the paleozoic and the early mesozoic through the truong son terrane ordovician silurian 420 470 ma late carboniferous early permian 280 300 ma late permian to mid triassic 245 270 ma and middle late triassic 200 245 ma these magmatic activities are not only attributed to the tethyan song ma ocean southwestward subduction but also related to paleo tamky phuoc son oceanic bidirectional subduction synthesized with regional metallogenic data we identified three metallogenic epochs 1 late carboniferous early permian 280 300 ma arc magmatic hydrothermal cu au fe polymetallic metallogenic system related to the tamky phuoc son ocean north dipping subduction 2 late permian middle triassic 245 280 ma arc magmatic hydrothermal cu au fe and orogenic w sn au polymetallic metallogenic system which linked to both paleo tethyan song ma ocean south dipping subduction and tamky phuoc son ocean closure 3 middle to late triassic 200 245 ma syn collisional magmatic epithermal fe au w sn and post collisional or orogenic au system

the devonian vertebrates placodermi sarcopterygii from central vietnam and their bearing on the devonian palaeogeography of southeast asia abstract new vertebrate remains are described from the terrigenous facies of the givetian dong tho formation in quang binh province central vietnam they are referred to three placoderm groups the antiarchi arthrodira and acanthothoraci and to a sarcopterygian the antiarchs are represented by a new form vukhuclepis lyhoaensis n g n sp which is regarded here as closely related to the yunnanolepididae an endemic group from the lower devonian of the south china block additional material of the phlyctaeniid arthrodire lyhoalepis duckhoai tong dzuy thanh janvier and doan nh t truong is described the sarcopterygian is regarded here as a dipnomorph and may be closely related to the lower middle devonian youngolepididae of the south china block this material confirms the close ties between the indochina and south china blocks as early as the middle devonian

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

neoproterozoic to early triassic tectono stratigraphic evolution of indochina and adjacent areas a review with new data the neoproterozoic to early triassic tectono stratigraphic evolution of indochina and adjacent areas can be divided into six mega episodes 1 the neoproterozoic early cambrian mega episode was characterized by shallow marine environment in the sino vietnam composite terrane meanwhile the different blocks of the indochina composite terranes were still separated by branches of the proto tethys sino vietnam and part of indochina amalgamated along the proto song ma suture by the end of the early cambrian 2 during the middle cambrian early ordovician mega episode shallow shelf carbonate deposition prevailed north of the proto song ma suture with associated strata containing a rich benthic fauna this contrasts with deep marine and volcanic arc setting in the south of the proto song ma suture 3 the middle ordovician silurian mega episode was characterized by subduction of the tam ky phuoc son ocean beneath the viet lao terrane in the north and the kon tum terrane in the south this was followed by a collisional orogeny that sutured the kon tum terrane with the viet lao and viet cambodia terranes along the tam ky phuoc son suture the hypothetic po ko and chu sinh sutures are discussed the collision led to an intra continental orogeny in the sino vietnam composite terrane by the end of the mega episode a unified sino vietnam indochina super terrane was established during the latest silurian early devonian benthic fauna and flora of the sino vietnam and indochina composite terranes shared cathaysian affinity 4 the devonian tournaisian mega episode was characterized by an early middle devonian transgression and a late devonian earliest carboniferous regression except for parts of northern indochina where deep marine conditions remained 5 during the visean middle permian the song ma ocean reopenned in the north of indochina meanwhile in the west a volcano plutonic belt formed due to eastward subduction of the paleo tethys under the indochina composite terrane 6 the late permian early triassic mega episode was characterized by emeishan mantle plume related continental rifting and the indosinian orogeny resulting from resuturing of the indochina and sino vietnam composite terranes along the song ma suture

dilemma of late palaeozoic mixed floras in gondwana abstract during carboniferous and permian periods the plant fossil assemblages of northern and southern hemispheres were distributed in four floral provinces and each flora had its own characteristics pteridosperms were dominant in the euramerian floras of europe and america cordaitales and equisetales along with angaridium angaropteris represent the angara flora of russia russian federation and far east provinces gigantopteris flora characterizes the cathaysian province of china japan and south east asian countries and glossopteris flora are present in southern hemispheric land masses of gondwana which includes india australia africa south america and antarctica well defined floral provinces quite often show the intermixing of different floristic elements exemplifying the presence of mixed floras the occurrence of mixed floras suggests that they have two modes of distribution one distribution pattern shows mixed floras along the marginal basins of the tethyan region during the middle and late permian another pattern is evident in the gondwana floras where glossopteris floras contain a number of extra gondwanic elements likewise some euramerian cathaysian and angaran floras show the possible presence of glossopterid related elements having comparative morphological features as part of the flora mixed floral elements of northern and southern floras have been examined in detail and it is suggested that in all probability the so called elements of the mixed floras had their ancestry or affiliation with carboniferous floras and during the early permian they duped due to severity of climatic conditions but developed again during late permian when climate became more hospitable for their development the cathaysian and euramerian forms discovered with gondwanan forms in the mixed floras of tethyan region i e hazro saudi arabia new guinea oman jordan kashmir central tibet and southern tibet belong to middle and upper permian sequences and during this time the warm humid and temperate climate helped to revive the northern hemispheric forms which had their endurance in a relatively similar climate of the late carboniferous the morphological conservatism flexibility radiation relationship and analogous features of mixed floral elements have been examined and attempts are made to sort out their disputed presence in different floral provinces

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

the structural setting of the song ma region vietnam and the indochina south china plate boundary problem abstract the song ma song da region of northern vietnam contains the song ma anticlinorium a polydeformed polymetamorphosed early palaeozoic island arc forearc terrane accreted to the south china plate in siluro devonian times the song ma anticlinorium is not an indosinian subduction zone and nor is the post triassic post cretaceous song ma fault the song ma fault is one of many northwest trending post cretaceous high angle reverse oblique slip faults and thrusts responsible for shortening and strike slip transposition of northern vietnam folds penecontemporaneous with this faulting include the song ma anticlinorium which is formed probably of thrust ramp folds the faulting and penecontemporaneous folding developed during oligocene sinistral strike slip on the song hong fault and was reactivated during pliocene quaternary times we conclude that the present plate boundary is a broad deformation zone between da nang and the song hong and that parts of the south china plate extend well to the south of the song hong fault

the boundary between the simao and yangtze blocks and their locations in gondwana and rodinia constraints from detrital and inherited zircons abstract the simao block which is the northern extension of the indochina block and the yangtze block were amalgamated by the closure of a paleo tethys branch ocean commonly referred to as the ailaoshan ocean the exact location of the suture between these two continental blocks has been contested some researchers believed that it is located within the cenozoic ailaoshan shear zone whereas other researchers thought that it is present 50 km farther to the west the detrital and inherited xenocrystic zircons from paleozoic sedimentary rocks and a triassic granite dyke respectively in the laowangzhai mojiang suspect terrane which occurs between the two suggested locations for the same suture show that neither of these previously suggested locations is accurate the age distribution pattern of detrital and inherited xenocrystic zircons from the laowangzhai mojiang suspect terrane is similar to that of detrital zircons from the simao indochina block but different from that of those from the jianshui area in the western margin of the yangtze block this indicates that the laowangzhai mojiang suspect terrane belongs to the simao indochina block the actual location of the suture between the simao indochina and yangtze blocks is suggested to superimpose the ailaoshan late devonian to early carboniferous ophiolite belt within the suspect terrane the new interpretation is also supported by hf isotopes of detrital zircons from the related continental blocks the age distribution pattern of precambrian detrital zircons from the western part of the yangtze block is different from that of those from australia or tethyan himalaya the northern part of the indian continent indicating that the yangtze block was not an integral part of australia or india in rodinia the age distribution pattern of pre carboniferous detrital zircons from the simao indochina block is remarkably similar to that of those from the tethyan himalaya indicating that the simao indochina block was derived from indian gondwana

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

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

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

structural development of the lower paleozoic belt of south china genesis of an intracontinental orogen abstract an extensive structural study of wuyishan and surrounding areas south china brings data on the structures formed prior to the devonian unconformity building the lower paleozoic belt an ordovician tectonic event was responsible for i south directed structures in the wuyishan proper and its southern border related to both thin and thick skinned tectonics accompanied by metamorphism and crustal thickening ii north directed structures to the west of ganjiang fault and north of jiangshan shaoxing fault where only thin skinned tectonics is visible the southward deformation accommodating the shortening includes emplacement of thrust sheets involving deep crustal material and some mantle peridotite and likely repetition of continental material both responsible for crustal thickening the orogeny was due to the underthrusting of the southern part of the south china block beneath the northern part of this block closing the pre existing nanhua rift created at around 850 800 ma and involving the rift sedimentary fill in the southward thrusting the syn metamorphic tectonic piling was followed by anatexis and granite emplacement at around 440 390 ma our new u th pb epma monazite dating gives ages of 433 9 ma and 437 5 ma early silurian for the main anatectic event and a younger age of 412 5 ma late silurian early devonian for a late re heating process the lower paleozoic belt is an intracontinental orogen without oceanic suture record it shows some similarities with the cenozoic european pyrenees chain with a strong asymmetry

permian continental paleoenvironments in southeastern asia new insights from the luang prabang basin laos in laos southeastern asia late paleozoic sediments were identified by early french explorations across indochina during the late 19th century pavie missions but little work was undertaken to characterize the sedimentological and stratigraphical context until now from detailed sedimentological and paleontological studies we propose an interpretation of the depositional environment and of the stratigraphic context of series located on the right bank of the mekong river in the luang prabang basin where three main formations were described the silicoclastic red claystone formation attributed to alluvial plain environment contains large fragments of unidentified dicynodonts the limestones and sandstones formation preserves a new macrofloral assemblage displaying affinities with middle to late permian cathaysian floras of south china this assemblage occurs as an intercalation within marine calcareous sandstones that have yielded a marine fauna including the ammonoid pseudotirolites sp which indicates a late permian changhsingian age the well developed purple claystones formation yielded an abundant and well preserved late permian fauna composed of a carnivorous amphibian and numerous dicynodon cranial and postcranial elements this formation shows a vertical evolution from braided river to alluvial plain with sheet flood sand bed and bed load rivers with a constant supply of volcanic clasts results from the analysis of the paleontological associations in the luang prabang basin suggest that a continental communication between laurussia and the indochina block existed during the permian allowing for migration of the terrestrial dicynodon fauna

tectonics and metallogeny of mainland southeast asia a review and contribution abstract the southeast asia region is endowed with a diversity of mineral resources notably porphyry related skarn epithermal and sediment hosted orogenic gold deposits formation of these deposits was associated with a long and complex tectonic history of gondwana supercontinent break up arc magmatism backarc basin development together with arc continent and continent continent collisions that created the present day mainland se asia this paper summarizes historical and current se asian geological research and ore deposit studies here we present a new tectonic and metallogenic model for phanerozoic mainland se asia from this model we conclude that incipient arc backarc basin magmatism is the key to the formation of many important ore deposits in the truong son and loei fold belts the two major metallogenic belts in mainland se asia in addition triassic to cenozoic arc continent and continent continent collisions have led to the formation of many sediment hosted orogenic gold deposits in the sukhothai and the sibumasu terranes oblique cretaceous to recent subduction along the andaman sunda trench was responsible for gold and copper gold molybdenum porphyry and epithermal mineralization along the kawlin wutho mt popa arc in myanmar in the north and the sumatran volcanic arc in the south we anticipate that the present emphasis on exploring for shallower level epithermal deposits will increasingly lead to exploration for deeper level porphyry and porphyry related skarn systems in the coming decades

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

intracontinental subduction a possible mechanism for the early palaeozoic orogen of se china the early palaeozoic orogen of se china consists of three lithotectonic elements from top to bottom a sedimentary upper unit a metamorphic lower unit and a gneissic basement the boundaries between these units are flat lying south directed ductile decollements the lower one is coeval with an amphibolite facies metamorphism m1 the belt is reworked by migmatite granite domes high temperature metamorphism m2 and granitic plutons related to post orogenic crustal melting we date here the syn m1 ductile shearing at 453 7 ma by u th pb method on monazite previous ages and our new 40 ar 39 ar ages of biotites and muscovites show that the metamorphic rocks experienced syn m2 exhumation from 440 to 400 ma the early palaeozoic orogen of se china is an intracontinental belt in which decollements accommodated the north directed subduction of the cathaysian continent this orogen is an example of intracontinental subduction that was not preceded by oceanic subduction

devonian fish placodermi antiarcha from tra ban island bai tu long bay quang ninh province vietnam and the question of the age of the do son formation abstract remains of an asterolepis like antiarchan placoderm fish are reported from the top part of the sandstone series of the tra ban island quang ninh province vietnam which are referred to the do son formation this sandstone series is overlain by the well dated late givetian frasnian trang kenh formation which constrains the top part of the do son formation to the middle devonian and most probably the givetian as suggested by the presence of a large asterolepis like antiarch this would agree with the fish and plant assemblage previously recorded from the upper part of the do son formation at do son the question of the age of the base of the do son formation is raised by the discovery of late silurian or early lochkovian plant arthropod and fish remains at do son the do son formation now appears to be composed of two members late silurian or early lochkovian and givetian in age respectively recent field observations suggest that the two members are separated by an unconformity the meaning of the devonian terrigenous deposits in the eastern part of vietnam and south china is briefly discussed

permo triassic granitoids in the northern part of the truong son belt nw vietnam geochronology geochemistry and tectonic implications abstract the northern segment of the truong son belt of northwestern vietnam is composed of three granitoid complexes chieng khuong dien bien and song ma respectively our new la icp ms u pb dating results of magmatic zircons from five granitoid samples in the dien bien and the chieng khuong complexes reveal the existence of three age populations of magma emplacement and crystallization from 276 to 202 ma the new data show that the quartz diorite of the chieng khuong complex is dated to be 271 ma however the previously named dien bien complex exposed at different localities has various ages from 281 to 202 ma of magmatic crystallization the chieng khuong quartz diorite exhibits high mg value 60 7 and enriched sr nd isotopic features i sr 0 713822 end t 11 34 with subduction related geochemical features e g depletion in nb ta and ti and enrichment in rb and la the dien bien gabbroic diorite at muong lay is characterized by relatively depleted isotopic data i sr 0 705050 end t 0 09 and extensive depletion in nb and ta by contrast the dien bien granodiorite and quartz monzonite at dien bien are high k calc alkaline to calc alkaline with high mg values 45 5 to 54 4 and they display enriched sr nd isotopic data i sr 0 714912 to 0 716430 end t 9 58 to 8 85 and post collisional geochemical characteristics e g enrichment in total alkali lile and lree and depletion in eu sr p ti and nb under the framework of regional tectonic evolution the permo triassic magmatism along the northern truong son belt is thus grouped into an early stage from ca 280 to 250 ma and a late stage from 229 to 202 ma the early permian early triassic arc magmatism 280 270 ma and 250 245 ma which is widely distributed in the truong son belt provides reliable information on the continuous subduction of the song ma oceanic plate beneath the indochina block furthermore the magmatism of the dien bien complex at dien bien was formed during post collisional extension at ca 229 to 202 ma diachronous suturing of the eastern tethyan evolution is characterized by the differences in the ages of subduction post collisional evolution along the ailaoshan and the song ma belts

cenozoic deformation and exhumation of the kampot fold belt and implications for south indochina tectonics latest mesozoic to earliest cenozoic deformation affected se asia s sundaland core the deformation event bridges the mesozoic se asian fusion with the cenozoic era of rifting translation basin formation and the creation of modern se asian oceans southern cambodia and vietnam are central to this shift but geological investigations of the region are in their infancy based on apatite and zircon fission track analyses afta and zfta stratigraphic and structural observations seismic data thermal maturity and igneous rock dating the geological evolution of southern cambodia and vietnam is investigated diverse depositional styles igneous activity structural deformation and subsurface unconformities testify to a highly variable phanerozoic tectonic setting major latest cretaceous to paleocene thrusting and uplift affected the kampot fold belt and surrounding regions and the associated up to 11 km exhumation probably exceeds earlier denudation events since at least permian time the present relief of the bokor mountains rising high above the kampot fold belt represents an artifact after differential erosion and only 2 5 4 5 km of erosion affected this area the latest cretaceous to paleocene orogenesis affected much of greater indochina probably owing to plate collision along eastern sundaland or a combination of collisions along both east and west sundaland afta and zfta data document protracted cooling of cretaceous granites and locally elevated thermal gradients persisting a few tens of million years after their emplacement the thermal gradient had stabilized by early miocene time and miocene cooling probably reflects a renewed denudation pulse driven by either regional tectonism or climate enhanced erosion

fish suggests continental connections between the indochina and south china blocks in middle devonian time a yunnanolepiform antiarch placoderm fish is recorded from the givetian dong tho formation of quang binh province central vietnam this and other fishes from the same locality occur in marginal marine detrital facies with plant remains and lingulid brachiopods that indicate a nearshore to deltaic environment yunnanolepiform antiarchs were hitherto known exclusively from the lower devonian of the south china block the new occurrence of this group well south of the song ma suture suggests close links between the indochina and south china blocks in middle devonian time the massive sandstone exposures of the dong tho formation may be a southern extension of the do son sandstone formation of the hai phong area which is located on the south china block

origin and assembly of the tethyside orogenic collage at the expense of gondwana land abstract the tethysides are a superorogenic complex flanking the eurasian continent to the south and consisting of the cimmerides and alpides products of palaeo and neo tethys respectively we here review their evolution mainly on the basis of new maps showing the distribution of sutures magmatic rocks certain palaeobiogeographically and palaeoclimatologically significant taxa and facies and fragments of pan african 900 450 ma orogenic system forming the basement of many tethyside blocks these are supplemented by palaeomagnetic data reported in the literature a fundamental tenet of this paper is that major sutures which contain ophiolite fragments represent tectonic sections between continental blocks where oceanic crust has been subducted palaeo tethys came into existence largely in late carboniferous time coevally it began to be consumed by both internal and peripheral subduction zones which continued into the permian some of these had been inherited from pre tethyan times in the later permian rifting subparallel with the northern margin of gondwana land began between the zagros and malaysia separating a cimmerian continent from n gondwana land and thus heralding the opening of neo tethys and other smaller oceans that were back arc basins of palaeo tethys this rifting possibly also extended farther west into crete and mainland greece however the north china block yangtze block huanan block the eastern moity of the qangtang block north tibet and annamia all originally pieces of the end proterozoic early palaeozoic gondwana land had already separated from it in pre late carboniferous times possibly during the devonian all of these blocks and the cimmerian continent were characterized by cathaysian floral elements in late palaeozoic time palaeomagnetic and palaeontological data showing the original gondwana land affinity of these continental blocks are supplemented by correlating late proterozoic early palaeozoic pan african sutures orogenic belts and sedimentary basin fragments across tethyside sutures late permian foraminiferal provinces are related to this palaeogeographical interpretation by triassic times most cimmeride subduction zones were already in existence the cimmerian continent accelerated its separation from gondwana land and locally in the late permian began disintegrating internally along the waser rushan pshart banggong co nu jiang mandalay ocean by late triassic time all of the chinese blocks except lhasa and annamia had collided with each other and with laurasia the resulting enormous orogenic collage had a soft cushion between itself and laurasia in the form of the enormous accretionary complex of the songpan ganzi this connection enabled laurasian land vertebrates to reach south east asia by late triassic time in late triassic to middle jurassic times most major cimmeride collisions were completed widespread aridity in central asia occurred in late jurassic time probably in the rain shadow of the newly formed cimmeride mountain wall neo tethyan subduction systems formed along the s margin of the cimmerides or within neo tethyan oceanic lithosphere during the jurassic most if not all were north or east dipping they continued the northerly migration of the tethyside blocks evolution of the tethysides influenced the distribution of marine and terrestrial organisms and affected sea level changes and patterns of atmospheric circulation during much of the mesozoic and cainozoic it is likely to have reflected the surface expression of a persistent trend in the large scale convective circulation in the mantle that continuously transported material northward into the tethyan domain

the schmidt hammer in rock material characterization the schmidt hammer provides a quick and inexpensive measure of surface hardness that is widely used for estimating the mechanical properties of rock material however a number of issues such as hammer type normalization of rebound values specimen dimensions surface smoothness weathering and moisture content and testing data reduction and analysis procedures continue to influence the consistency and reliability of the schmidt hammer test results this paper presents a a critical review of these basic issues and b the results of tests conducted on granitic rocks of various weathering grades in the light of the conclusions of this review it was found that a very good correlation exists between l and n hammer rebound values and that both hammers are fairly sensitive to the physical properties particularly to dry density though less so to effective and total porosities the n hammer producing a lesser scatter in the data proved to be more efficient than the l hammer in predicting uniaxial compressive strength and young s modulus the exponential form of the correlation curves was found to reflect microstructural changes during the course of weathering and the differences in the probing scales or mechanisms in the means of measuring these mechanical properties and could be generalized to other crystalline igneous rocks the possibility of predicting weathering grades from rebound values was also explored the changes in the rebound values during multiple impacts at a given point produced a better indication of the weathering grade than a single impact value it was concluded that increasing the impact energy and plunger tip diameter should significantly reduce the scatter in coarse grained weathered rocks and hence improve the reliability of the schmidt hammer as a rock material characterization tool

the volcaniclastic series from the luang prabang basin laos a witness of a triassic magmatic arc abstract the paleogeographic evolution of south east asia sea during the early mesozoic is still poorly understood and a number of models have recently been put forward to account for the geodynamic evolution of sea the luang prabang basin north laos located in the core of a paleogeographic jigsaw in sea recorded a long lasting volcanism that spanned for 35 my from the earliest triassic up to late triassic as evidenced by combined stratigraphic and geochronological u pb zircon analyses performed on both volcanic and volcaniclastic series the volcanic rocks are arc tholeiites and calk alkaline andesites to dacites the volcaniclastic rocks contain in part volcaniclasts produced contemporaneously with sedimentation both the volcanic and volcaniclastic series display geochemical features characteristic of a subduction related volcanism therefore the luang prabang basin documents a magmatic arc in a good agreement with the recent recognition of neighboring ophiolitic rocks in the luang prabang area following a passive margin setting that prevailed from the late carboniferous to the late permian an active margin then initiated along the western margin of the indochina block this active magmatic arc developed as the result of an east dipping subduction below the indochina block during most of the triassic at least from ca 250 to 215 ma subsequently this oceanic subduction episode must have been followed by a continental collision of the indochina block with the eastern simao block at a period that remains to be defined

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

paleomagnetic analysis of eastern tibet implications for the collisional and amalgamation history of the three rivers region sw china abstract a comparative analysis of paleolatitudes and latitudinal displacements was undertaken for the south china simao baoshan shan thai indochina qiangtang lhasa and himalayan blocks by using available paleomagnetic data to determine the collisional and amalgamation history of the main blocks in the three rivers region western yunnan the results show that 1 the simao block is a south china derived block 2 the baoshan and shan thai blocks were subjected to rapid northward drift during the period from late carboniferous to late permian 3 the baoshan block collided with the simao block in the late permian and continued to drift rapidly northwards together with the south china and shan thai blocks until the late triassic 4 the paleo tethys separating the baoshan and simao blocks was possibly opened in the early silurian 5 the meso tethys ranged in age from early permian time to early cretaceous time and reached its greatest width of about 42 latitude in the late triassic

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

geochemistry and geochronology of the chatree epithermal gold silver deposit implications for the tectonic setting of the loei fold belt central thailand abstract the chatree deposit is the largest epithermal au deposit in mainland se asia despite its economic significance the stratigraphy age geochemistry and tectonic setting of the host volcanic sequence is poorly documented in the literature the chatree deposit is located between phichit and phetchabun provinces central thailand and is hosted by late permian to early triassic volcaniclastic and volcanogenic sedimentary rocks detailed field mapping u pb zircon dating geochemistry and stratigraphy suggest that the volcanic sequence can be subdivided into four stratigraphic units and two volcanic suites the late permian suite 1 volcanic units are sourced from a more depleted mantle relative to the overlying early triassic suite 2 volcanic units the late permian suite 1 units probably formed immediately after the beginning of subduction and at the creation of a new island arc the less depleted early triassic suite 2 units were erupted during ongoing subduction once the system had achieved a steady state the chatree au mineralization appears to have occurred during the switch between the two mantle sources and this switch of magma source is marked by a mixed volcano plutonic magmatism at the permo triassic boundary ca 250 ma

the tam ky phuoc son shear zone in central vietnam tectonic and metallogenic implications abstract the tam ky phuoc son shear zone tkpssz is one of the most pronounced crustal structures in central vietnam and juxtaposes high grade rocks of the kontum massif against lower grade rocks of the truong son fold belt an internal zone comprises highly strained rocks including ultramafic mafic tectonites bounded by mylonitic zones that straddle tens to hundreds of kilometers an external zone comprises mainly high grade metamorphic complexes southern flank and weakly metamorphosed siliciclastic and carbonate rocks with subordinate greenstone northern flank detailed structural analysis reveals that the shear zone is a multi deformed terrane d1 produced a regional high grade schistosity and mylonitization d2 generated regional northwest southeast trending fold arrays and brittle ductile shear zones d3 locally produced northeast trending folds brittle faulting occurred during d4 d5 u pb dating of zircon and monazite suggests that d1 involved metamorphism and felsic magmatism at ca 430 ma recording part of a regional collisional orogeny monazite and titanite growth at ca 250 240 ma in basement rocks is synchronous with widespread syn to post d2 magmatism between ca 260 and 245 ma suggesting a second major collisional event during the indosinian orogeny d3 may have occurred as part of or soon after this permo triassic event deformation during post collisional stages perhaps in response to extrusion wedging and oroclinal rotation of terranes led to post d3 structures d4 d5 the e w trending tkpssz is here shown to be a continuation of the n s trending po ko shear zone pksz most of the significant lode gold occurrences in central vietnam occur along this tkpssz pksz structure and are associated with and controlled by d1 ductile to ductile brittle high strain zones mineralized later stage structures support remobilization and reconcentration of ore during subsequent events at ca 240 ma re os molybdenite age the tkpssz pksz d1 structure likely represents part of a paleosuture zone marking the closure of an ancient ocean basin through terrane assembly in the early paleozoic

linking the indochina block and gondwana during the early paleozoic evidence from u pb ages and hf isotopes of detrital zircons abstract to constrain the paleoposition of indochina within gondwana during the early paleozoic we performed in situ u pb and hf isotope analyses on detrital zircons from three river sediment samples in the truong son belt of the indochina block the age distributions yield dominant neoarchean 2 5 ga mesoproterozoic 1 7 1 4 ga grenvillian 0 95 ga and pan african 0 65 0 5 ga age groups and minor paleo to meso archean zircons hf isotope compositions of zircons for each age group exhibit large ranges of ehf t suggesting that the zircon host rocks have diverse sources the oldest hf model ages for zircons of neoarchean grenvillian and pan african age group yield 3 7 ga or older while those of mesoproterozoic age group show 3 3 ga the remarkable similarity of age distribution and hf isotope compositions among detrital zircons of indochina and those of tethyan himalaya western cathaysia and qiangtang suggests that indochina was located outboard of qiangtang and south of south china in the indian margin of gondwana during the early paleozoic our results are consistent with the paleontological correlations of east gondwana margin during the early paleozoic

the ban houayxai epithermal au ag deposit in the northern lao pdr mineralization related to the early permian arc magmatism of the truong son fold belt abstract the ban houayxai au ag deposit is located in the truong son fold belt in the northern lao pdr mainland se asia the deposit is hosted within an early permian volcano sedimentary unit which is a part of a late carboniferous early permian 310 270 ma volcanic plutonic sequence of the truong son fold belt the main au ag mineralization stages i e stages 2 and 3 at ban houayxai consist of hydrothermal veins breccias associated with a carbonate quartz sulfides pyrite sphalerite galena chalcopyrite sericite chlorite electrum native silver stephanite mineral assemblage geochronological studies from la icp ms zircon u pb and adularia ar ar and k ar dating identified early permian cretaceous and eocene ages the early permian age is related to the emplacement of the mineralized volcanic arc sequence whereas the cretaceous and eocene ages represent resetting probably related to yanshanian and or himalayan orogenies a regional tectonic reconstruction indicates that the au ag epithermal deposit at ban houayxai was formed as a result of southward subduction of an oceanic crust attached to the south china terrane beneath the indochina terrane during the early permian from the metallogenic point of view the recognition of the old epithermal system at ban houayxai implies that the truong son fold belt possesses high potential for the presence of deeper magmatic related deposits such as porphyry and porphyry related skarn deposits

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

advances in research of asian geology a summary of 1 5m international geological map of asia project abstract the international geological map of asia at a 1 5 000 000 scale igma5000 is the first digital asian geological map under the standard of the commission for the geological map of the world cgmw major advances that have been achieved in compiling the map are manifested in the following understandings 1 large amounts of mesozoic volcanic rocks occurring in the eastern asian coastal area are mainly cretaceous instead of upper jurassic lower cretaceous most of the carboniferous permian volcanic rocks in central asia seem not to be arc volcanics but the product of an extensional stage the basal boundary of the meso neoproterozoic jixian section in china is not dated at 1 8 ga as defined previously but less than 1 68 ga 2 the most significant neoarchean tectono thermal events in the sino korean craton and the indian craton took place at 2 5 ga rather than at 2 7 ga the basement of the yangtze craton was finally formed at 0 75 0 8 ga which is 0 2 0 3 ga later than the greenville orogenic cycle geologically south china is identified to be an early paleozoic caledonian foldbelt the qinling belt where no oceanic basin was developed in triassic times is not an indosinian collisional orogen but a continental crust subduction one when pangea was formed indo australian gondwana had been joined to paleo asia and between them there was no oceanic basin i e no paleo tethys which continued from paleozoic to mesozoic a huge indosinian orogenic belt existed on the southern margin of paleo asia to the north of the zagros himalayas 3 asia is a composite continent consisting of three major cratons the siberian indian and arabian and three huge orogenic belts with a number of minor cratons and numerous microcontinents included the main body of the asian continent took its shape during the mesozoic the orogenic belts belong respectively to three global tectonic domains the paleo asian tethyan and pacific the small cratons such as sino korea yangtze tarim and sibumasu are thought to be affiliated to the tectonic transform zone between gondwana and siberia they had been situated on the northern margin of gondwana before the disappearance of the paleo asian ocean and were lying on the southern margin of paleo asia after the closing of the paleo asian ocean and then the opening of the tethys the fact that ophiolites in asia appear to get progressively younger in age from north to south throws light on the phanerozoic evolutionary process of the dispersion of gondwana and the accretion of asia accompanied by a southward migration of its orogenic belts

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

tectonic evolution of high grade metamorphic terranes in central vietnam constraints from large scale monazite geochronology abstract several metamorphic complexes in southeast asia have been interpreted as precambrian basement characterized by amphibolite to granulite facies metamorphism in this paper we re evaluate the timing of this thermal event based on the large scale geochronology and compositional variation of monazites from amphibolite to granulite facies metamorphic terranes in central vietnam most of the samples in this study are from metamorphic rocks n 38 and granitoids n 11 in the kontum massif gneisses n 6 and granitoids n 5 from the hai van migmatite complex and the truong son belt located to the north of the massif were also studied two distinct thermal episodes 245 230 ma and 460 430 ma affected kontum massif gneisses while a single dominant event at 240 220 ma is recorded in the gneisses from the hai van complex and the truong son belt monazites from granitoids commonly yield an age of 240 220 ma mesoproterozoic ages 1530 1340 ma were obtained only from monazite cores that are surrounded by c 440 ma overgrowths thermobarometric results combined with concentrations of y 2 o 3 ce 2 o 3 and heavy rare earth elements in monazite and recently reported pressure temperature paths suggest that triassic ages correspond to retrograde metamorphism following decompression from high to medium pressure temperature conditions ordovician silurian ages reflect low pressure temperature metamorphism accompanied by isobaric heating during prograde metamorphism some samples were affected by both metamorphic events we conclude that high grade metamorphism observed in so called precambrian basement terranes in central vietnam occurred during both the permian triassic and the ordovician silurian while peraluminous granitoid magmatism is triassic additionally our preliminary analyses for u pb zircon age and whole rock chemistry of granitic gneisses from the truong song belt suggests the presence of the ordovician silurian volcanic arc magmatism in the region based on the pressure temperature time protolith evolutions metamorphic rocks from central vietnam provide a continuous record of subduction accretion collision tectonics between the south china and indochina blocks in the ordovician silurian the region was characterized by active continental margin tectonics followed by continental collision during the late permian to early triassic and subsequent exhumation during the late triassic the results also suggest that the timing of metamorphism and protolith formation as well as the geochemical features in other southeast asian terranes should be verified to achieve a better understanding of the precambrian to early mesozoic tectonic history in asia

the oldest flora of the south china block and the stratigraphic bearings of the plant remains from the ngoc vung series northern vietnam abstract several outcrops of the late silurian and devonian of the ngoc vung series northern vietnam yielded plant remains the late silurian localities delivered the earliest known flora of the south china block although the fossils are fragmentary they complement our knowledge about the global composition of the flora the major components of the flora are plants with dichotomous habit and terminal bivalvate sporangia which are close relatives to zosterophylls and zosterophylls plants with possible euphyllophyte affinities and bryophytes are occasionally present this floral composition is similar to that of the rich younger south china block assemblages from the posongchong and xujiachong formations of china considered pragian in age the south china block flora is therefore likely to have been dominated by zosterophylls and pre zosterophylls at least from the late silurian to the pragian i e a 20 million years long period it also strengthens the hypothesis that more derived plants were present on eastern gondwana earlier that elsewhere in the first steps of tracheophyte evolution the devonian localities of the ngoc vung series delivered a thick fibrous stem fragment and a basal euphyllophyte these latter plant remains provide some stratigraphic data the large stem fragment is consistent with an eifelian age for the duong dong formation part of the ngoc vung series as suggested by the brachiopod fauna the accompanying basal euphyllophyte displays a combination of characters axes 3 4 mm wide and lateral branchings that is also consistent with an eifelian age but possibly more characteristic of the emsian flora it is therefore suggested that the stratigraphic range of the duong dong formation might be extended down to the emsian

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

bar to bank flow convergence zones a contribution to the origin of alluvial placers three locations in two south african rivers containing abundant magnetite were surveyed and sampled five fold magnetite enrichments at two sites but significant enrichment lacking in the third flow within the first two sites was maximally constricted sediment input was matched by output in the third site the bar was still migrating toward the bank so that flow was not yet fully constricted most bar transported sediment was thus trapped in marginal foresets and not sorted by currents modified journal abstract

the chanthaburi terrane of southeastern thailand stratigraphic confirmation as a disrupted segment of the sukhothai arc a permo triassic volcanic arc system the sukhothai arc is recognised between the indochina and sibumasu continental blocks the chanthaburi terrane is here interpreted as a fault detached highly disrupted southern segment of the sukhothai arc occupying part of southeastern thailand and extending into cambodia the klaeng tectonic line is defined as the boundary between the chanthaburi terrane and sibumasu block the stratigraphy of the chanthaburi terrane is compared with that of the sukhothai terrane in northern thailand the late palaeozoic mesozoic sequences of these two volcanic arc terranes in the sukhothai zone share important similarities but show marked contrasts to those of the sibumasu and indochina blocks where the late permian triassic is largely absent due to the indosinian i unconformity western indochina or is dominantly carbonates with little terrigenous clastic input sibumasu there is no clear evidence of pre carboniferous sedimentary rocks for either the sukhothai or chanthaburi terranes late permian lyttoniid brachiopod shale near klaeng in the chanthaburi terrane was revisited the brachiopod previously reported as leptodus is re identified to oldhamina the genus previously known elsewhere in southeast asia only in the huai tak formation of the sukhothai terrane oldhamina in thailand is confined to the sukhothai arc the marine stratigraphy of the sukhothai arc is represented by a permian triassic lithological succession of mixed carbonates and siliciclastics with common volcanic material the late permian and triassic litho and biostratigraphy of the chanthaburi terrane are comparable with the upper ngao and lampang groups of the sukhothai terrane in particular they share similar successions from oldhamina brachiopod bearing shale to palaeofusulina colaniella foraminifer bearing limestone in the latest permian marine depositional conditions were terminated on the sukhothai arc by end triassic times later than on the indochina block late permian but earlier than on the sibumasu block jurassic cretaceous

first shrimp u pb zircon dating of granulites from the kontum massif vietnam and tectonothermal implications abstract the kontum massif in central vietnam represents the largest continuous exposure of crystalline basement of the indochina craton the central kontum massif is chiefly made of orthopyroxene granulites enderbite charnockite and associated rocks of the kannack complex mineral assemblages and geothermobarometric studies have shown that the kannack complex has severely metamorphosed under granulite facies corresponding to p t conditions of 800 850 c and 8 1 kbars twenty three shrimp ii u pb analyses of eighteen zircon grains separated from a granulite sample of the kannack complex yield ca 254 ma and one analysis gives ca 1400 ma concordant age for a zoned zircon core this result shows that granulites of the kannack complex in the kontum massif have formed from a high grade granulite facies tectonothermal event of indosinian age triassic the cooling history and subsequent exhumation of the kannack complex during indosinian times ranged from 850 c at ca 254 ma to 300 c at 242 ma with an average cooling rate of 45 c ma

geologic and metamorphic evolution of the basement complexes in the kontum massif central vietnam abstract this paper presents a regional scale observation of metamorphic geology and mineral assemblage variations of kontum massif central vietnam supplemented by pressure temperature estimates and reconnaissance geochronological results the mineral assemblage variations and thermobarometric results classify the massif into a low to medium temperature and relatively high pressure northern part characterised by kyanite bearing rocks 570 700 c at 0 79 0 86 gpa and a more complex southern part the southern part can be subdivided into western and eastern regions the western region shows very high temperature 900 c and pressure conditions characterised by the presence of garnet and orthopyroxene in both mafic and pelitic granulites 900 980 c at 1 0 1 5 gpa the eastern region contains widespread medium to high temperature and low pressure rocks with metamorphic grade increasing from north to south epidote or muscovite bearing gneisses in the north

2 9 2 36 and 1 96 ga zircons in orthogneiss south of the red river shear zone in viet nam evidence from shrimp u pb dating and tectonothermal implications abstract orthogneissic rocks coexisting with migmatites and containing small amphibolite lenses are exposed in the center of the metamorphic belt which runs parallel to the day nui con voi red river shear zone in northern viet nam the orthogneiss complex has given some radiogenic dates of early proterozoic and late archean which are the oldest ages ever registered for the southeast asian continent zircon grains separated from three samples of the orthogneiss complex have been dated to establish the protolith age and the timing of high grade tectonothermal events in the complex sixty five shrimp u th pb analyses of these zircons define three age groups of 2 84 2 91 2 36 and 1 96 ga the age groups correspond to three periods of zircon generation the oldest 2 9 ga cores indicate a minimum age for the protolith of the orthogneiss complex two younger generations including 2 36 ga outer cores and 1 96 ga rims probably grew during later high grade tectono metamorphic events which were previously suggested by k ar and 40 ar 39 ar cooling ages of 2 0 ga for synkinematic hornblendes an early thermal history of the orthogneiss complex has been constrained including a primary magma crystallization stage starting at 2 9 ga followed by two early proterozoic 2 36 and 1 96 ga high grade tectonothermal events the ca 2 9 ga protolith age of the orthogneiss complex documented in this study provides new convincing evidence for the presence of archean rocks in indochina and clearly indicates that the crustal evolution of northern viet nam started as early as late archean time

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

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

u pb dating and tectonic implication of ophiolite and metabasite from the song ma suture zone northern vietnam the song ma suture zone including the classical song ma suture and the nam co antiform between indochina and south china blocks in northern vietnam is a crucial area for understanding the tectonic evolution of southeast asia to explore their natures and evolutionary history this paper reports new petrological studies and sims zircon u pb age data for the song ma ophiolites metabasites and fine grained quartzite the song ma ophiolites consist mainly of peridotite fo90 91 en92 basalt and gabbro and experienced greenschist to lower amphibolite facies ocean floor metamorphism lenses of rodingite and altered pyroxenite in serpentinites formed during alternation of peridotite we identified two types of zircon in the altered pyroxenite i uniform metamorphic 70 vol and ii zircon having inherited magmatic cores and metamorphic rims 30 vol all u pb analyses of both zircon types yield an upper intercept age of 340 29 ma that we interpret as the protolith age type i zircon and type ii metamorphic rims together yield a mean 206pb 238u age of 280 2 ma with rodingite forming broadly coevally at 283 to 282 ma metabasalt plagioclase amphibole schist has a morb type geochemical affinity magmatic zircon from a plagioclase amphibole schist yields a protolith age of 315 4 ma high pressure eclogite garnet phengite quartz schist and garnet hornblende rock are distributed in the northwestern nam co antiform zircon crystals from both eclogite and garnet hornblende rock have low th u ratios of 0 01 to 0 07 and 0 02 to 0 06 and yield metamorphic ages of 231 8 ma and 228 3 ma respectively the ages of detrital zircon grains from fine grained banded quartzite from the nam co antiform range from paleozoic to proterozoic with a peak at 1 7 to 1 8 ga all available data indicate a three stage tectonic evolution 1 340 to 280 ma the paleotethys that separated the indochina and south china blocks was characterized by oceanic crust formation at 340 to 315 ma and ocean floor metamorphism at 283 to 280 ma 2

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