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

does neoproterozoic nam co formation in northwest vietnam belong to south china or indochina abstract the nam co belt is located to the northeast of song ma fault in northwestern vietnam the nature of the precambrian basement rocks in this area and its tectonic attribute have still been controversial in this study zircon u pb hf isotopic compositions and whole rock geochemical compositions of meta sedimentary rocks from the nam co formation in this belt have been integrated to reveal the components and tectonic setting of the formation of the basement rocks in the nam co belt zircon u pb dating results show that the nam co formation is of middle late neoproterozoic age the lower nam co formation probably formed between 821 and 726 ma and the upper formation was deposited later than 726 721 ma the sedimentary rocks from the lower and upper nam co formations are characterized by numerous neoproterozoic 900 800 ma detrital materials but the upper part contains more meso to paleoproterozoic sediments than the lower part detrital zircon age distribution and hf isotope compositions of the nam co formation are similar to those of neoproterozoic sedimentary rocks in the southern yangtze block both zircon u pb hf isotopic and whole rock geochemical data suggest that the source materials of the neoproterozoic nam co formation were dominated by felsic rocks with minor mafic materials and the detrital materials were mainly derived from the jiangnan orogenic belt in the southern yangtze block with minor contribution from the panxi belt in the western yangtze block ailaoshan song hong and phan si pan belts the lower nam co formation contains more sediments derived from the panxi belt than the upper part the middle late neoproterozoic nam co formation probably was deposited in a continental extensional basin and the sedimentation environment in later period

a simplified 3 d navier stokes numerical model for landslide tsunami application to the gulf of mexico 1 a simplified three dimensional navier stokes 3 d ns model for two fluids water and landslide material mudslide is presented and validated with standard laboratory experiments dubbed tsunami3d tsunami solution using navier stokes algorithm with multiple interfaces is applied to a 3 d full scale landslide scenario in the gulf of mexico gom i e the east breaks underwater landslide the simplified 3 d ns model is conceived to be computationally efficient for tsunami calculations the simplification is derived from the large aspect ratio of the tsunami waves wavelength wave height and the selected computational grid that has a smaller aspect ratio this allows us to assume a horizontal fluid surface in each individual cell containing the interface air water air mudslide and water mudslide the tracking of fluid interfaces is based on the volume of fluid method and the surfaces are obtained by integrating the fluxes of each individual fluid cell along the water column in the momentum equation the pressure term is split into two components hydrostatic and nonhydrostatic the internal friction is solved in a simplified manner by adjusting the viscosity coefficient despite the simplification to get an efficient solution the numerical results agree fairly well with standard landslide laboratory experiments required by the national tsunami hazard mitigation program for tsunami model validation the numerical effect caused by using a sharp versus a diffusive water mudslide interface for a full scale landslide tsunami scenario is also investigated observations from this experiment indicated that choosing a sharp or diffusive interface seems to have no remarkable effect at early stages of the tsunami wave propagation last a large scale 3 d numerical simulation is carried out for the ancient gom s east breaks landslide by using the simplified model to calculate the early stages of the tsunami wave propagation

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

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

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

thermochronological evidence for the movement of the ailao shan red river shear zone a perspective from vietnam in order to explore the timing of strike slip movement along the ailao shan red river shear zone an 40ar 39ar thermochronological study of the day nui con voi metamorphic massif in northern vietnam was undertaken the massif exposed in the southeastern segment of the ailao shan red river shear zone reveals a rapid cooling in the early miocene 25 21 ma after a very slow cooling ca 34 25 ma the slow cooling period most likely resulted from a geothermal perturbation in the lithosphere owing to the late paleogene 40 30 ma extension in eastern tibet and western yunnan china the rapid cooling consistent with evidence of a wider rapid cooling span from 27 to 17 ma compiled from the entire ailao shan red river shear zone constrains the duration of left lateral shearing which eventually offset indochina from south china by about 600 km the time constraints we have established reinforce the argument that the onset of the ailao shan red river shear zone postdates the opening of the south china sea that began ca 30 ma this result highlights the necessity for reevaluating the cenozoic tectonic models of southeast asia

first evidence for archean continental crust in northern vietnam and its implications for crustal and tectonic evolution in southeast asia southeast asia or in a more strict sense the indochinese continent was previously considered to be composed entirely of proterozoic to phanerozoic rocks and reliable evidence for archean crust was lacking gneisses from the cavinh complex south of the red river shear zone northern vietnam however show archean nd model ages of 3 4 3 1 ga zircon separates from the rocks yielded u pb dates of 2 8 2 5 ga the first convincing evidence for the presence of a late archean complex in southeast asia given that the red river shear zone was propagating in the south china block with a left lateral offset of 600 km the cavinh complex can be correlated with the late archean kangding complex in the western margin of the yangtze craton southwestern china the cavinh complex therefore represents one of the oldest crustal nuclei of the south china block

petrogenesis and tectonic implications of neoproterozoic highly fractionated a type granites from mianning south china abstract the origin of neoproterozoic intrusions ca 860 750 ma along the western part of the yangtze craton has been the subject of debate in recent years with two competing models proposed the plume model argues for an extensional setting and emphasizes the role of a superplume in the rodinia breakup whereas the arc model argues for the presence of a subduction zone in the yangtze craton as a contribution to this animated dispute geochronologic and geochemical analyses have been carried out on the mianning granite which is the largest pluton 700 km2 in the northern kangdian rift of the western yangtze craton it is shown that the mianning granites were emplaced at ca 780 ma and display highly fractionated feature i e sio2 75 wt eu eu 0 03 0 50 enrichment of k rb th u zr hf y and rees depletion of nb ta ba sr p eu and ti they are metaluminous to strongly peraluminous a cnk 0 93 1 55 and contain abundant perthite and minor alkali riebeckite and sphene sharing the petrological and geochemical characters of a2 type granites positive nd t 2 97 5 24 and zircon hf t 9 2 12 1 values are consistent with a derivation by partial melting of a relatively young crust formed about 1000 900 ma given the general absence of a type granites in arc settings the mianning a type granites are suggestive of an anorogenic crustal extensional environment for the western yangtze craton during the neoproterozoic the data presented in this study are therefore consistent with an intracontinental rift model but are not sufficient to identify plume involvement in the neoproterozoic magmatism

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

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

petrogenesis of late permian silicic rocks of tu le basin and phan si pan uplift nw vietnam and their association with the emeishan large igneous province abstract major and trace elements and whole rock sr nd isotope data of late permian silicic plutonic and volcanic rocks from the phan si pan tu le region in nw vietnam were collected in order to establish their petrogenetic relationship with the magmatic rocks of the song da zone nw vietnam and the alkaline silicic rocks of the panxi area of the emeishan large igneous province elip in sw china the granites and rhyolites have geochemical characteristics of anorogenic granites e g high fe and high ga al and are further subdivided based on mineralogy the phu sa phin and muong hum granites contain sodic to sodic calcic amphiboles and sodic pyroxene while the phan si pan granite does not the phu sa phin and muong hum granites occasionally show peralkaline to metaluminous compositions while the phan si pan granites and the tu le rhyolites have metaluminous to peraluminous compositions the chondrite normalized ree patterns la yb n 3 6 51 3 gd yb n 1 4 3 4 and the primitive mantle normalized spidergrams enrichment in high field strength elements are similar to those of the song da silicic rocks and the panzhihua and taihe granites in the panxi area rhyolite melts modeling suggest that the silicic magmas were likely generated by fractionation of the song da high ti basalts the end t values range from weakly negative to moderately positive values 2 2 to 2 2 suggesting the silicic rocks may be assimilated by basement rocks during magma emplacement permian magmatic rocks in the phan si pan tu le region are petrologically geochemically and geochronologically comparable to those of the inner zone of elip although permian lithospheric mantle and lower crustal structure of the phan si pan tu le region may be destroyed by a lithospheric removal in response to cenozoic india eurasia collision our results along with recent geophysical analyses in nw vietnam suggest that the upper to middle crust of phan si pan tu le region represent a remnant upper to middle crust of inner zone of elip

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

ca 825 ma komatiitic basalts in south china first evidence for 1500 c mantle melts by a rodinian mantle plume mantle plume or superplume activities have repeatedly been invoked as a cause for the breakup of the neoproterozoic supercontinent rodinia with supportive evidence including radiating dike swarms globally synchronous anorogenic igneous activity large scale lithospheric doming and unroofing and geochemical signatures similar to recent plume related magmatism however high temperature magmas such as picrite or komatiite a requisite product of mantle plume activities have not previously been identified for those events we present here geochronological and geochemical data from a suite of pillow lavas in central south china sensitive high resolution ion microprobe shrimp u pb dating of zircons from an evolved member of andesitic composition within the suite indicates that the lavas were erupted at 823 6 ma all but a few highly evolved crust contaminated basaltic rocks are characteristically high in mgo 10 2 17 5 ni 183 661 ppm and cr 677 1672 ppm but low in tio2 0 5 0 7 al2o3 10 6 12 7 and feot total fe as feo 7 4 10 5 typical of komatiitic basalts our geochemical modeling which removes the effect of olivine crystallization suggests that their primary magma has typical komatiitic compositions with mgo 20 feot 11 sio2 47 tio2 0 48 al2o3 10 ni 860 ppm and cr 1780 ppm such a high mgo content in the primary melts implies a melt temperature of 1500 c suggesting that the yiyang komatiitic basalts were generated by melting of an anomalously hot mantle source with potential temperature t p 260 50 c higher than the ambient mid oceanic ridge basalt morb source mantle similar to that of modern mantle plumes the yiyang komatiitic basalts are thus the first solid petrological evidence for the proposed ca 825 ma mantle plume that played a key role in the breakup of the supercontinent rodinia

clay mineral assemblages of the townsend tuff bed lower old red sandstone south wales and the welsh borders suites of samples from the townsend tuff bed possess widely varying clay mineralogies which may be divided into 5 major assemblages showing a general increase in diagenetic grade from smectite illite kaolinite to chlorite illite to the w this trend appears to be related to depth of burial fold style and proximity to basement illite crystallinity closely parallels cleavage development

zircon u pb and hf isotope constraints from the ailao shan red river shear zone on the tectonic and crustal evolution of southwestern china abstract the ailao shan red river asrr shear zone one of the most prominent geologic strike slip shear zones in southeast asia consists of high grade metamorphic complexes that provide a rare opportunity to sample the mid crustal rocks along the western margin of the yangtze block of south china here we report combined in situ analyses of zircon u pb and lu hf isotopes of eight gneisses from the diancang shan and ailao shan segments of the asrr shear zone our zircon u pb data indicate that the rocks contain abundant magmatic zircons ranging in age from 1785 to 25 ma with peak ages at ca 770 350 and 240 ma suggesting three major periods of protolith formation the 350 ma zircons observed only in an orthogneiss from the diancang shan show uniform initial hf isotopic ratios marked by high and positive e hf t values from 16 to 10 this is in contrast with the other two zircon populations that are more common occurring in both the diancang shan and ailao shan and overall delineate very heterogeneous hf isotopic compositions with e hf t values ranging from 15 to 16 many zircons reveal distinctive core rim age variations zircon rims formed between ca 34 and 26 ma show significant variations in th u ratios 5 0 01 and e hf t values 14 to 10 that suggest complicated magmatic and metamorphic zircon overgrowth during the oligocene the presence of both metamorphic and magmatic overgrowths on zircons suggests that the metamorphism reached upper amphibolite facies corresponding to mid crustal level p t conditions as also suggested by structural and petrologic data our data furthermore suggest that the asrr gneisses were not produced solely by shear heating during the tertiary strike slip faulting but are uplifted mid crustal basement rocks that formed essentially during two major stages of magmatism represented by the neoproterozoic kangding complex and late permian emeishan large igneous province as a result of the india asia collision these basement rocks underwent regional magmatic and metamorphic overprinting in the oligocene that based on our relevant work searle et al 2010 geosphere 6 1 23 predated the initial left lateral movement along the asrr shear zone the 350 ma protolith which requires a dominant depleted mantle input in the petrogenesis cannot be linked to any major magmatic events in south china but may be interpreted as part of the paleotethys remnants that were later incorporated into the asrr shear zone

reappraisal of the ages of neoproterozoic strata in south china no connection with the grenvillian orogeny the jiangnan fold belt separates the yangtze and cathaysia blocks in south china and has long been considered grenvillian in age in order to place south china in central rodinia it consists of deformed early neoproterozoic strata that are unconformably overlain by undeformed late neoproterozoic strata and intruded by undeformed and unmetamorphosed granitic plutons zircons from the early neoproterozoic strata yield u pb ages as young as 830 ma and one granitic pluton has a zircon u pb age of ca 827 ma the 830 ma mafic rocks along the southeastern margin of the yangtze block have arc affinity geochemical characters whereas mafic rocks younger than 830 ma have typical ocean island basalt oib like compositions thus we suggest that the early neoproterozoic strata were deposited on an active continental margin prior to amalgamation of the yangtze and cathaysia blocks at ca 830 ma the overlying late neoproterozoic strata were deposited in the intracontinental rifted nanhua basin at 820 730 ma and probably reflect backarc spreading above the long lived 950 735 ma oceanic subduction zone along the northern and western margin of the yangtze block this model is consistent with the secular tectonic evolution of south china during the neoproterozoic the jiangnan fold belt is therefore not a grenvillian feature as previously suggested and there is no evidence to place south china in central rodinia instead we believe that south china was located in a marginal position relative to this supercontinent

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

structure and shearing conditions in the day nui con voi massif implications for the evolution of the red river shear zone in northern vietnam 1 the day nui con voi massif bears a record of the red river shear zone rrsz activity in north vietnam it forms a large scale antiformal core complex type structure bounded by the song hong and song chay faults the kinematics of both faults are identical and reflect transtensional shear initiated under upper amphibolite facies conditions and propagated into greenschist facies microfabric analysis establishes that both extensional and strike slip shearing initiated between 700 and 500 c the rrsz evolved from a single subvertical fault which due to strike perpendicular extension underwent progressive dilation the created space was intruded by already metamorphosed and deformed ductile middle crust in the form of a gneissic dome both strike slip and extensional shearing were accommodated in the limbs of the antiform while its core was uplifted from midcrustal level bearing only a minor record of sinistral shear

zircon u pb and lu hf isotopic systematics of the daping plutonic rocks implications for the neoproterozoic tectonic evolution of the northeastern margin of the indochina block southwest china abstract the daping neoproterozoic plutonic rocks at the northeastern margin of indochina block in southwest china provide an ideal opportunity for studying the tectonic setting and relationship between the indochina and yangtze blocks la icp ms u pb dating on the zircon cores and rims of a hornblende gabbro yield 206pb 238u weighted means ages of 873 9 1 ma and 769 7 ma respectively and that for cores mantles and rims of a granodiorite yield 206pb 238u weighted means ages of 981 987 ma 829 10 ma and 761 11 ma respectively the zircon cores and mantles are interpreted as inherited from their source region the zircon rims are magmatic their ages represent the emplacement timing the zircon cores and rims from the hornblende gabbro have ehf t values ranging from 5 0 to 5 8 and 0 6 to 6 4 corresponding single stage model ages range from 1626 to 1662 ma and 1094 to 1311 ma respectively for the granodiorite the inherited mantles including cores show two groups 1 ehf t values of 3 3 to 12 3 with single stage hf model ages of 897 to 1235 ma and 2 ehf t values of 1 9 to 7 8 with single stage model ages of 1470 1667 ma the zircon rims are characterized by positive ehf t values 5 4 to 8 2 with single stage model ages ranging from 977 to 1108 ma whole rock geochemical data for the hornblende gabbro such as enrichment of lile and lree negative anomaly of nb and ta and high mg 52 1 65 4 suggest magma generation in a subduction related setting an island arc affinity is strongly supported by the features of high alumina basalt and abundant hornblende in a large hornblende gabbro sill the granodiorites are characterized by high sr contents and sr y ratios strong enrichment of lile and lree and negative anomaly of nb ta p and ti comparable with the features of subduction related plutonic rocks these data show that the hornblende gabbro was generated by the partial melting of a metasomatized mantle wedge peridotite with contribution from aqueous fluids derived from a subducted slab the granodiorite magma is a product of the mixing of mafic magma produced by partial melting of a slab fluid enriched metasomatized mantle wedge peridotite and felsic magma formed by the partial melting of crustal materials the emplacement ages and geochemical features of subduction related daping plutonic rocks are the same as those reported from the western margin of the yangtze block suggesting the presence of an oceanic crust in between with subduction to either side generating island arc magmatism in the neoproterozoic