biostratigraphic and palaeoenvironmental controls on the trilobite associations from the lower ordovician fezouata shale of the central anti atlas morocco abstract trilobites are a major component of both biomineralized and soft bodied assemblages of the fezouata biota central anti atlas morocco trilobite taxa of the fezouata shale tremadocian floian in the zagora area are typical of lower ordovician communities from shallow shelf environments and show strong affinities with early ordovician assemblages from the montagne noire southern france dramatic variations in both taxonomic composition and taphonomic attributes together with the sedimentological facies changes allow us to discriminate the palaeoenvironmental versus stratigraphic control on trilobite assemblages three communities are described along an onshore offshore profile namely the pilekiid bavarillid agerina and raphiophorid biofacies additionally three stratigraphic assemblages are identified assemblage 1 is confined to the late tremadocian assemblage 2 ranges from late tremadocian to middle floian and assemblage 3 is restricted to the floian the higher diversity of some levels is explained by storm induced inputs of debris from taxa living in adjacent environments additionally control of relative seal level variation on some peculiarities of the trilobite fauna specific endemicity lack of some expected taxa is evidenced in the fezouata shale finally temporary dysoxic events are highlighted by preliminary geochemical analyses probably allowing episodic exceptional preservation

unconformity of red sandstones in north vietnam field evidence for indosinian orogeny in northern indochina we re examine several unconformity sites in northwestern vietnam where the indosinian orogenesis has been defined field observations in the black river song da region near son la demonstrate that red sandtones and conglomerates reportedly of cretaceous age occur unconformably above schistosed epimetamorphic middle triassic sediments a mesozoic unconformity thus really exists in this region though the age of deformation is poorly contrained upper triassic to upper cretaceous in most places however the unconformity is strongly deformed folded triassic limestones are often thrusted onto the cretaceous rocks this implies that tertiary deformations probably due to the india asia collision have been strong in the northern part of the indochina block

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

a condensed lower to middle ordovician graptolite succession at matane quebec canada the highly condensed lower to middle ordovician deep water succession from the cap des rosiers group at matane bears three conspicuous graptolite faunas of late tremadoc to mid darriwilian age the tremadoc fauna belongs to the aorograptus victoriae zone in the castlemainian isograptus victoriae maximodivergens zone fauna the earliest pseudisograptus species pseudisograptus initialis n sp appears together with the ancestral arienigraptus gracilis a darriwilian fauna with holmograptus spinosus and bergstroemograptus crawfordi represents the youngest pre nemagraptus gracilis zone fauna described from the quebec appalachians

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

geochemical and sr nd isotopic characteristics of granitic rocks from northern vietnam abstract five major felsic igneous suites from northern vietnam with ages from mid proterozoic to early cenozoic were studied representative granitic rocks from the posen complex mid proterozoic and the dienbien complex late permian to early triassic show geochemical characteristics similar to those of calc alkaline to high k calc alkaline i type granites however the former located in the south china block has significantly higher initial nd isotopic ratios end t 0 7 to 1 5 and older nd isotopic model ages tdm 1 7 ga than the latter end t 4 7 to 9 7 tdm 1 3 1 5 ga which were emplaced south of the song ma suture and thus in the indochina block the generation of both complexes may be attributed to subduction related processes that occurred in two distinct crustal provenances with different degrees of mantle inputs on the other hand jurassic to cretaceous granitic rocks from the phusaphin complex contemporaneous rhyolites from the tule basin and late paleogene granitic rocks from the yeyensun complex all exposed in the south china block between the ailao shan red river shear zone and the song ma suture display geochemical features similar to those of a type granites with intermediate end t values 0 6 to 2 8 and younger tdm ages 0 6 1 1 ga these magmas are suggested to have been generated as a consequence of intraplate extension in the western part of the south china block yunnan and to have been transported to their present position by mid tertiary continental extrusion along the ailao shan red river shear zone related to the india asia collision overall the isotopic and model age data reported in this study indicate that in northern vietnam the most important crust formation episode took place in the proterozoic likewise repeated mantle inputs have played a role in the petrogenesis of phanerozoic granitic rocks

early palaeozoic palaeobiogeography and palaeoecology of stylophoran echinoderms abstract stylophorans cornutes mitrates represent one of the most diverse classes of cambro ordovician echinoderms they were free living benthic non radiate forms closely related to asterozoans and crinoids taphonomic sedimentological and palaeosynecological data provide useful information on key aspects of stylophoran palaeoecology such a combined approach suggests that the rarity of stylophorans in proximal environments above storm wave base was probably original and does not exclusively result from the possession of a loosely articulated polyplated calcitic test conversely stylophorans were relatively abundant in deeper settings below storm wave base where they frequently occur in low diversity dense beds several tens to hundreds of individuals per square metre such assemblages were either largely dominated by stylophorans only middle cambrian darriwilian or by the association of both stylophorans and ophiuroids latest darriwilian silurian comparison with ecological requirements of similar modern and fossil dense populations of ophiuroids suggests that stylophoran beds occurred in well oxygenated oligotrophic waters with low predatory pressure and that sea temperature played a key role in the palaeobiogeographic distribution of stylophorans seven main stylophoran biofacies are identified and described the associated environments and tropical distribution of primitive heavily plated forms suggest that the initial radiation of the class probably took place in warm to temperate shallow settings earliest middle cambrian late cambrian however from this primitive stock stylophorans radiated several times towards deeper and or colder environments possibly because of the lower predation pressure in such settings cothurnocystid cornutes in the middle cambrian mitrates in the late cambrian hanusiids in the tremadocian as a consequence the fact that most cambro ordovician cornutes and mitrates were psychrospheric i e cold water adapted probably explains their relatively wide palaeobiogeographic distribution in high palaeolatitudes cambro ordovician stylophorans were abundant and diverse in shallow to deep environments but they were generally restricted to deep settings in low palaeolatitudes the great caradocian transgression favoured the extension of stylophorans from circum polar regions e g western margin of gondwana towards external settings at lower palaeolatitudes avalonia baltica and eastern margin of laurentia where one group of mitrates underwent a major radiation anomalocystitids and extended towards shallower and warmer palaeoenvironments in contrast all silurian stylophoran assemblages known so far are rare poorly diversified and restricted to shallow warm to temperate environments in tropical regions this distribution partly results from 1 the probable eradication of psychrospheric forms e g cornutes because of the prolonged anoxia of deep environments in the late hirnantian early silurian of west gondwana 2 the radiation of some mitrates anomalocystitids in shallow tropical settings in the late ordovician and 3 of strong taphonomic and or sampling biases numerous ghost lineages

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

u pb dating of detrital zircons in the eastern guangdong basin south china and constraints on the tectonic transformation from the early to middle jurassic controversies exist regarding the mechanism of formation of basins located on the continental margin of south china as well as when they formed it was ascertained based on clastic petrology geoch

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

multiple partial melting events in the ailao shan red river and gaoligong shan complex belts se tibetan plateau zircon u pb dating of granitic leucosomes within migmatites abstract thin layers and lenses of granitic leucosomes are widely distributed within migmatites from the ailao shan red river asrr and gaoligong shan gs complex belts se tibetan plateau they are parallel to or cross cut foliations in the host rocks at different scales and show evidence of coalescence and migration to form centimetre to decimetre scale segregations a combined study of mineral inclusions cathodoluminescence cl images u pb sims dates and in situ trace element compositions of zircons provides clear evidence on the nature and timing of partial melting in these rocks most zircons from the granitic leucosomes occur as distinct overgrowths around inherited detrital cores or as new euhedral crystals the inherited detrital cores are rounded or irregular contain felsic mineral inclusions of qtz kfs ab ap with abundant opaque impurities and show clear dissolution textures in contrast the overgrowths and new crystals commonly show perfectly euhedral shapes have pronounced oscillatory zoning contain felsic mineral inclusions such as qtz kfs ab ap and show pronounced high u 1419 6129 ppm and relatively low th 58 301 ppm contents with extremely low th u ratios 0 02 0 07 the chondrite normalized ree patterns show steep rising slopes from the hree to lree with evidently negative eu anomalies eu eu 0 10 0 31 and pronounced positive ce anomalies ce ce 35 145 these data suggest that the new zircons are anatectic in origin and that they grew during partial melting of the metamorphic rocks abundant u pb spot analyses of the magmatic zircons from granitic leucosomes reveal three discrete and meaningful ages of partial melting within the asrr and gs complex belts anatectic zircons from four granitic leucosomes in weakly deformed migmatites from the gs complex belt yield consistent mean u pb ages of 40 6 0 3 38 4 0 2 ma in contrast anatectic zircons from six granitic leucosomes in weakly deformed migmatites from the asrr complex belt record younger mean u pb ages of 33 7 0 3 30 9 0 3 ma however anatectic zircons from another six granitic leucosomes in mylonitic migmatites from both belts yield the youngest mean u pb ages of 27 8 0 2 26 3 0 3 ma these data imply that both the asrr and gs complex belts experienced multiple partial melting events the differences in the oldest u pb ages of zircons from granitic leucosomes indicate that the partial melting event related to the peak amphibolite granulite facies metamorphism within the gs complex belt occurred 7 10 ma earlier than that in the asrr complex belt the youngest u pb ages of zircons from granitic leucosomes within mylonitic migmatites in both the asrr and gs belts are genetically related to post peak isothermal decompressional retrogression accompanied by regional high temperature ductile shearing the 40 ar 39 ar age data 27 8 22 2 ma of biotites and amphiboles from migmatites strongly support this hypothesis

tectono stratigraphy of late carboniferous to triassic successions of the khorat plateau basin indochina block northeastern thailand initiation of the indosinian orogeny by collision of the indochina and south china blocks abstract the late paleozoic and early mesozoic indosinian orogeny in southeast asia was characterized by subduction in the paleo tethys and collisions among continental blocks and fragments the khorat plateau basin in northeastern thailand covers much of the indochina block and was subjected to complex tectonic activity during the late paleozoic and mesozoic the mesozoic sequence of the basin comprises thick gently folded non marine sediments that include red beds and is separated from the underlying upper paleozoic sequence by the major angular unconformity of the permo triassic boundary which we refer to as the indosinian i event in the khorat plateau basin seismic and well data acquired for oil and gas exploration indicate that the upper paleozoic sequence is heavily faulted and structurally complex our study showed that in some areas about 2000 m of the upper paleozoic sequence has been eroded at the indosinian i unconformity although thicknesses of up to 6000 m of upper paleozoic rocks remain in some areas previous interpretations of oil and gas exploration data attribute the erosional event to back arc closure due to collision of the indochina and sibumasu blocks during subduction in the paleo tethys ocean however our detailed structural mapping indicated that the indosinian i unconformity was a consequence of the collision of the indochina and south china blocks during the late permian to middle triassic we propose that this collision initiated the indosinian orogeny in mainland southeast asia

the habberley formation youngest tremadoc in the welsh borderlands the habberley formation overlies the shineton shale formation in the shelve inlier shropshire and includes the youngest tremadoc strata known from the welsh basin it probably represents deposition during the interval cut out by the sub arenig unconformity in north wales and removed by the sub caradoc unconformity in the wrekin district there is no angular unconformity between tremadoc and arenig in the shelve inlier the habberley formation is likely to correlate in part with the hunnebergian stage of scandinavia it was deposited probably in a largely stagnant basin and records a shallowing upwards sequence prior to the arenig transgression of the stiperstones quartzite a sparse trilobite fauna proves the presence of the angelina sedg vickii biozone and a succeeding interval is characterized by asaphellus cf graffi the trilobites are briefly described they include the first undistorted material of angelina sedgwvickii the brachiopod lingulella bella walcott hitherto known only from eastern newfoundland is described by a w a rushton

late triassic paleomagnetic result from the baoshan terrane west yunnan of china implication for orientation of the east paleotethys suture zone and timing of the sibumasu indochina collision abstract in order to better understand the paleogeographic position of the baoshan terrane in the northernmost part of the sibumasu block during formation of the pangea supercontinent a paleomagnetic study has been conducted on late triassic basaltic lavas from the southern part of the baoshan terrane in the west yunnan region of southwest china following detailed rock magnetic investigations and progressive thermal demagnetization stable characteristic remanent magnetizations chrms were successfully isolated from late triassic niuhetang lava flows the chrms are of dual polarity and pass fold and reversal tests with magnetic carriers dominated by magnetite and subordinate oxidation induced hematite we thus interpret them as a primary remanence this new paleomagnetic result indicates that the baoshan terrane was located at low paleolatitudes of 15 n in the northern hemisphere during late triassic times together with available paleomagnetic data from the baoshan terrane and surrounding areas a wider paleomagnetic comparison supports the view that the east paleotethys ocean separated the sibumasu and indochina blocks and closed no later than late triassic times we argue that the currently approximately north to south directed changning menglian suture zone is very likely to have been oriented nearly east to west at the time of the sibumasu indochina collision

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

carboniferous triassic felsic igneous rocks and typical mineral deposits in the truong son orogenic belt se asia implications for paleo tethyan tectonic evolution and metallogeny abstract the truong son orogenic belt tsob is part of the eastern tethyan mineral deposit domain and preserves a near complete record of paleo tethyan orogeny related magmatism and mineralization by compiling and evaluating the data of plutonic and sub volcanic rocks and associated deposits on a regional scale and combining with new data of typical igneous rocks and mineral deposits we establish a unified paleo tethyan evolutionary model to help understanding the metallogeny in the tsob the geochronological data reveal the felsic magmatism and associated mineralization occurred in carboniferous triassic ca 317 202 ma with a magmatic lull at ca 270 263 ma before the lull most igneous rocks are high mg sodic metaluminous i type granitoids or volcanic equivalents with depleted hf isotopes which were derived from a juvenile crustal source with significant mantle contribution after the lull the igneous rocks ca 262 235 ma consist of metaluminous i type granitoids and abundant s type granitoids with geochemical features of low mg potassic peraluminous and enriched hf isotopes suggesting their derivation from ancient continental crust without obvious mantle contribution this magmatic lull corresponds to the tectonic transition from subduction to initial collision which reasonably accounts for the changing of petrogenetic types and magma sources of the associated igneous rocks in the tsob the slightly depleted hf isotopes of the ca 234 202 ma rocks possibly suggest another episode of tectonic transition in late triassic the distinct mineralization styles and s isotopes of ore deposits of each episode indicate that mineralization was related to the tectonic evolution and magmatism in each stage here we propose an evolutionary model for the paleo tethys orogeny in the tsob as follows 1 low angle subduction beneath the tsob during late carboniferous late permian favored the formation of juvenile crust derived adakitic rocks and high mg i type felsic rocks as well as associated porphyry cu au skarn fe cu au and epithermal au ag mineralization 2 subsequent collision during late permian middle triassic not only accounted for the generation of precambrian crust derived i type granitoids and intrusion related au ag deposits but also resulted in the formation of low mg s type rocks minor volcanic rocks and associated porphyry and or skarn sn zn pb deposits 3 possible post collisional extension during late triassic may be favorable for the formation of metaluminous high k high mg i type granitoids and volcanic rocks and carlin au porphyry and or skarn mineralization

palaeogeographical and palaeoecological aspects of the cambro ordovician radiation of echinoderms in gondwanan africa and peri gondwanan europe abstract ecology and tempo of the lower palaeozoic radiation of echinoderms are discussed in this paper based on comparison of the diversity patterns observed in cambro ordovician faunas from laurentia and the northern gondwana margin the cambrian agronomic revolution triggered a global radiation of echinoderms with the progressive disappearance of biomat related lifestyles and the colonisation of new environments both in laurentia and on the northern gondwana margin soft substrate echinoderm assemblages related to cold and or deep environments were dominated by blastozoans and stylophorans these assemblages show a pattern of continuous diversification from the middle cambrian to the middle ordovician the major radiation of crinoids in the lower to middle ordovician of laurentia is only a local diversification correlated with the development of hardgrounds in warm shallow environments no comparable major ordovician radiation event occurred in the cooler environments of the northern gondwana margin

lower ordovician ibexian trilobites from the tribes hill formation central mohawk valley new york state the early ordovician tribes hill formation of east central new york state is a sequence of peritidal to subtidal carbonates and minor shales that rests disconformably on late cambrian carbonates and is in turn succeeded disconformably by middle ordovician strata more than 800 trilobites from 24 collections are assigned to six species bellefontia gyracantha raymond clelandia parabola cleland hystricurus ellipticus cleland hystricurus cf hystricurus oculilunatus ross symphysurina convexa cleland and symphysurina cf symphysurina woosteri ulrich two distinct biofacies are present the bellefontia biofacies in subtidal shales with thin storm generated bioclastic interbeds and the gastropod rostroconch biofacies in shallow carbonate bank lithofacies the trilobites of the tribes hill formation are assigned to a single informal biostratigraphic unit the clelandia parabola fauna which is correlative with trilobite zone b of the garden city formation of utah and with the bellefontia frank

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

the early palaeozoic of the north atlantic region as a test case for the use of fossils in continental reconstruction abstract fossils have an important contribution to make in the reconstruction of early palaeozoic palaeogeography the north atlantic regions include well known early palaeozoic fossil faunas which should relate to the dispositions of continental plates during the period however different versions of ordovician palaeogeography based on fossils are described in the literature and in this paper we evaluate the influences which produce these discrepancies the most reliable results are obtained by combining analysis of faunal similarity especially shelf faunas with mapping the distribution of those biofacies which relate to former deep water sites planktic organisms may not be reliable palaeocontinent discriminants cladistic analyses of different groups of organisms living at the same early ordovician time point to the usefulness of trilobite data for palaeogeographic reconstruction such data indicate a three fold division of the north atlantic region into baltica laurentia and gondwana from the cambrian to at least the mid ordovician paris and robardet 1990 denied the existence of a tornquist s sea separating baltica from gondwana in the early ordovician it is considered that their different reconstruction was because the planktic organisms on which they based their maps were not affected by this important oceanic separation to the same extent as shelly benthos on the other hand there is no evidence of deeper water biofacies along the supposed line of rheic ocean separation of avalonia from gondwana until the late ordovician however there is some evidence of deeper biofacies along the nantes sardic line new term extending from southern armorica to sardinia

a revision of ordovician series and stages from the historical type area this paper is a revision of the standard chronostratigraphy of the ordovician of the historical type area in england and wales the revision is a response to the need for more precise definitions of series and stages especially for practical international correlation in some cases this has entailed moving horizons for series bases away from those in the classical sections the scheme will be used as the standard for a forthcoming revision of the correlation chart of the british ordovician system the ordovician is divided into five series tremadoc arenig llanvirn caradoc and ashgill the llanvirn is extended to include part of the classical llandeilo series which is included within it as a stage where stage subdivisions of standard series have not yet been defined as in the tremadoc they are proposed in this paper however bancroft s fine stadial divisions of the caradoc which have been criticized for their brevity and local utility are combined into four new stages which will have wider application the former caradoc subdivisions are retained as substages for the purposes of regional correlation

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

emeishan large igneous province sw china in recent years there have been major advances in our understanding of the emeishan large igneous province lip of sw china following publication of a number of lip focused investigations of the terrain and associated rocks this paper reviews the current state of knowledge the volcanic and upper intrusive portion of the province is relatively small 0 3 106 km3 even when offset fragments eroded sections and buried portions are included in the volume calculation the most reliable radiometric age dates zircon u pb shrimp from an associated layered intrusive body several kilometers in area indicate generation at 259 ma consistent with the end guadalupian end middle permian stratigraphic age in addition several ar ar dating studies have been carried out mainly on the volcanic rocks with a number of reported dates 253 251 ma late permian but a consensus is emerging that these ages are problematic because they are in conflict with the stratigraphic data possibly due to a monitor standard miscalibration the ar ar investigations have also yielded a large number of secondary ages which are clustered at 175 142 98 and 42 ma and these are inferred to record sub regional tectonic events that affected the western yangtze block as east asia was assembled and later deformed by india s collision indentation into asia magnetostratigraphic data and field observations suggest that the bulk of the volcanic sequence formed within 1 2 my the geochemistry of the volcanic rocks and bio lithostratigraphic studies of the underlying maokou formation suggests a mantle plume generated the province the basalts can be classified into low and high ti groups with different parental magmas the low ti basalts are confined to the western part of the province and are overlain by the high ti basalts the low ti magmas formed at shallow mantle depths in the spinel garnet field transition zone 60 80 km whereas the high ti magmas formed by low degrees of partial melting within the deeper garnet stability field this observation suggests a deepening of the melting column as the lithosphere thickened due to under plating and a transition from peak basalt generation to the waning stage outstanding issues which might focus future studies are also outlined

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

a triassic to cretaceous sundaland pacific subduction margin in west sarawak borneo metamorphic rocks in west sarawak are poorly exposed and studied they were previously assumed to be pre carboniferous basement but had never been dated new 40ar 39ar ages from white mica in quartz mica schists reveal metamorphism between c 216 to 220 ma the metamorphic rocks are associated with triassic acid and basic igneous rocks which indicate widespread magmatism new u pb dating of zircons from the jagoi granodiorite indicates triassic magmatism at c 208 ma and c 240 ma u pb dating of zircons from volcaniclastic sediments of the sadong and kuching formations confirms contemporaneous volcanism the magmatic activity is interpreted to represent a triassic subduction margin in westernmost west sarawak with sediments deposited in a forearc basin derived from the magmatic arc at the sundaland pacific margin west sarawak and nw kalimantan are underlain by continental crust that was already part of sundaland or accreted to sundaland in the triassic one metabasite sample also previously assumed to be pre carboniferous basement yielded early cretaceous 40ar 39ar ages they are interpreted to indicate resumption of subduction which led to deposition of volcaniclastic sediments and widespread magmatism u pb ages from detrital zircons in the cretaceous pedawan formation are similar to those from the schwaner granites of nw kalimantan and the pedawan formation is interpreted as part of a cretaceous forearc basin containing material eroded from a magmatic arc that extended from vietnam to west borneo the youngest u pb ages from zircons in a tuff layer from the uppermost part of the pedawan formation indicate that volcanic activity continued until c 86 to 88 ma when subduction terminated

a trilobite fauna from the highest shineton shales in shropshire and the correlation of the latest tremadoc a rich fauna dominated by trilobites and calcified chordates has been collected in shropshire from the arenaceous beds the highest member of the tremadoc shineton shale formation and hitherto regarded as poorly fossiliferous this fauna shows that shelf conditions persisted longer in shropshire than has been supposed it is likely that even younger tremadoc is cut out at the unconformity below the caradoc in the shineton inlier the correlation of the later tremadoc is reviewed and the nomenclature of british tremadoc biozones is revised the trilobites described here are a mixture of previously known and new forms the name shumardia conophrys salopiensis callaway is revived for british material traditionally assigned to the scandinavian species shumardia pusilla sars from which it is distinct the new taxa litagnostus meniscus sp nov apatokephalus sarculum sp nov and skljarella cracens sp nov are described the type species of asaphellus a homfrayi and leptoplastides l salteri are redescribed and geragnostus callavei pseudokainella impar and parapilekia sp are recorded new information on the ontogeny of s c salopiensis and a homfrayi is given litagnostus and skljarella are recorded from the british isles for the first time

palaeozoic and mesozoic tectonic evolution and palaeogeography of east asian crustal fragments the korean peninsula in context east and southeast asia comprises a complex assembly of allochthonous continental lithospheric crustal fragments terranes together with volcanic arcs and other terranes of oceanic and accretionary complex origins located at the zone of convergence between the eurasian indoaustralian and pacific plates the former wide separation of asian terranes is indicated by contrasting faunas and floras developed on adjacent terranes due to their prior geographic separation different palaeoclimates and biogeographic isolation the boundaries between asian terranes are marked by major geological discontinuities suture zones that represent former ocean basins that once separated them in some cases the ocean basins have been completely destroyed and terrane boundaries are marked by major fault zones in other cases remnants of the ocean basins and of subduction accretion complexes remain and provide valuable information on the tectonic history of the terranes the oceans that once separated them and timings of amalgamation and accretion the various allochthonous crustal fragments of east asia have been brought into close juxtaposition by geological convergent plate tectonic processes the gondwana derived east asia crustal fragments successively rifted and separated from the margin of eastern gondwana as three elongate continental slivers in the devonian early permian and late triassic late jurassic as these three continental slivers separated from gondwana three successive ocean basins the palaeo tethys meso tethys and cenotethys opened between these and gondwana asian terranes progressively sutured to one another during the palaeozoic to cenozoic south china and indochina probably amalgamated in the early carboniferous but alternative scenarios with collision in the permo triassic have been suggested the tarim terrane accreted to eurasia in the early permian the sibumasu and qiangtang terranes collided and sutured with simao indochina east malaya in the early middle triassic and the west sumatra terrane was transported westwards to a position outboard of sibumasu during this collisional process the permo triassic also saw the progressive collision between south and north china with possible extension of this collision being recognised in the korean peninsula culminating in the late triassic north china did not finally weld to asia until the late jurassic the lhasa and west burma terranes accreted to eurasia in the late jurassic early cretaceous and proto east and southeast asia had formed palaeogeographic reconstructions illustrating the evolution and assembly of asian crustal fragments during the phanerozoic are presented d 2005 international association for gondwana research published by elsevier b v all rights reserved

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 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

thermotectonic events from eparly proterozoic to miocene in the indochina craton implication of k ar ages in vietnam abstract synkinematic mineral separates from high grade metamorphic rocks which were collected from the day nui con voi red river shear zone and the pre mesozoic metamorphic belt south of it the da nang khe sanh shear zone and the kontum massif in vietnam were dated by k ar method twenty four analyses of hornblende bioitite and muscovite are clustered into four different groups early proterozoic ca 1700 2000 ma early triassic ca 240 258 ma early cretaceous 115 116 ma and late oligocene early miocene 24 29 ma the k ar age data in general indicate cooling ages of ductile deformations and metamorphisms in vietnam suggesting an overprinting of thermotectonic events in indochina from early proterozoic to tertiary which are generally consistent with tectono stratigraphic episodes in the region the data demonstrate that the indosinian orogeny in vietnam has occurred during earliest triassic times and paleozoic orogeny caledonian and or hercynian was not inferred by these k ar age data

geologic geochemical and geophysical consequences of plume involvement in the emeishan flood basalt province prevolcanic kilometer scale lithospheric doming in the emeishan large igneous province southwest china allows us to evaluate the spatial and temporal consequences of uplift on the paleogeography geology geochemistry and geophysics of the region systematic spatial variations are observed across the domal structure in the distribution and thickness of clastic and carbonate sediments the extent of erosion thickness and chemistry of volcanic rocks and the crust mantle structure these features which are best explained by a mantle plume may be used to track older plume sites in the geologic record

zircon u pb ages hf isotope data and tectonic implications of early middle triassic granitoids in the ailaoshan high grade metamorphic belt of southeast tibet the ailaoshan tectonic belt where the effects of the paleo tethyan ocean evolution and indian eurasian plate collision are superimposed is one of the most significant geological discontinuities in western yunnan province of southeast tibet an ailaoshan micro block within the belt is bounded by the ailaoshan suture zone to the west and the red river fault to the east and consists of low and high grade metamorphic belts late permian middle triassic granitoids that are widely distributed to the west of the ailaoshan suture zone and within the ailaoshan micro block may yield significant information on the tethyan tectonic evolution of the ailaoshan tectonic belt this study reports new la icp ms zircon u pb geochronology and hf isotope data of four granitoids from the ailaoshan high grade metamorphic belt zircon grains from the yinjie granitoid do not have inherited cores and yield a weighted mean u pb age of 247 1 2 0 ma the zircon e hf t values range from 7 8 to 12 1 and hf model ages from 775 to 546 ma indicating that the granitoid was derived from juvenile crust the rims of zircons from the majie and yuanjiang granitoids yield weighted mean u pb ages of 239 5 1 8 and 237 9 2 6 ma respectively whereas the cores yield ages of 1608 352 ma the e hf t values of zircon rims range from 20 4 to 5 3 yielding hf model ages from 2557 to 1606 ma and suggesting that the source magma of the majie and yuanjiang granitoids was derived from ancient crust an additional granitoid located near the majie village yields a zircon u pb age of 241 2 1 0 ma based on our geochronological and geochemical data combined with geological observations we propose that the ailaoshan micro block was derived from the western margin of the yangtze block and is comparable to the zhongzan and nam co micro blocks the presence of late permian mafic rocks with rift related geochemical characteristics within the ailaoshan micro block together with granitoids derived from partial melting of ancient juvenile crust indicates the presence of an ailaoshan rift this possible rift may correspond to the ganzi litang ocean to the northwest and the jinping song da rift to the southeast it is suggested that westward subduction of the jinshajiang ailaoshan song ma oceanic lithosphere triggered the separation of the zhongzan ailaoshan and nam co micro blocks from the western passive continental margin of the yangtze block through the opening of the ganzi litang ailaoshan jinping song da ocean rift this ocean rift may represent a subsidiary branch of the paleo tethyan ocean along the western margin of the yangtze block

terminal cambrian and lowest ordovician succession of mexican west gondwana biotas and sequence stratigraphy of the tinu formation the tinu formation of oaxaca state is the only fossiliferous lower palaeozoic unit between the laurentian platform in northwest mexico and gondwanan successions in andean south america the tinu traditionally has been referred to the lower ordovician tremadoc and regarded as having a provincially mixed fauna with laurentian avalonian and gondwanan elements bio and lithostratigraphic re evaluation demonstrates that the tinu is a gondwanan passive margin succession it includes a lower thin to 16 m condensed uppermost cambrian yudachica member new the yudachica nonconformably overlies middle proterozoic basement as a result of very high late late cambrian eustatic levels the yudachica changes from storm dominated but slightly dysoxic shelf facies fossil hash limestone and shale in the south to an upper slope facies with debris flows 50 km to the north three biostratigraphically distinct depositional sequences comprise the yudachica the yudachica has gondwanan aspect trilobites with low diversity conodonts characteristic of unrestricted marine temperate facies the upper tinu or rio salinas member new is a lower ordovician tremadoc depositional sequence that records strong early but not earliest tremadoc eustatic rise marked by graptolite and olenid bearing dysoxic mudstones higher strata shoal upward into shell hash limestones and proximal tempestite sandstones with upper lower tremadocian unrestricted marine temperate conodonts new taxa include orminskia rexroadae landing gen et sp nov from the cordylodus andresi zone this euconodont is related to hyaline coniform genera best known from ordovician tropical platform successions cornuodus clarkei landing sp nov resembles the coeval upper lower tremadoc tropical species scalpellodus longipinnatus ji barnes

correlation of triassic stratigraphy between the simao and lampang phrae basins implications for the tectonopaleogeography of southeast asia abstract based on our extensive fieldwork in southwestern yunnan and northern thailand followed by detailed stratigraphic and paleontological studies we propose that the triassic simao basin in yunnan can be correlated with the triassic lampang phrae basin in thailand strata equivalent to those in the southern lancangjiang sub basin have not been identified in northern thailand we consider that during the triassic the simao and the lampang phrae basins belonged to the same tectonopaleogeographic unit the orogenic belt to the east of this unit includes the nan uttaradit and ailaoshan sutures the shan thai block in northern thailand can be divided from east to west into the sukhothai the inthanon and the shan terranes according to tectonopaleogeographic correlation our results support the idea that the sukhothai terrane including the lampang phrae basin belongs to the cathaysian domain and not to gondwana domain and that the geosuture corresponding to the changning menglian suture in yunnan must lie to the west of the sukhothai terrane in thailand

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

new insights into the triassic sedimentary environment of the eastern parts of the song da and sam nua basins alongside the indosinian song ma suture northern vietnam this study documents the sedimentology structures and foraminifer content of triassic formations from the eastern part of the truong son belt northern vietnam sedimentary rocks in the eastern song da and sam nua basins occur on both sides of the middle triassic song ma suture zone that separates the indochina block from the south china block the main outcomes of our study include detailed stratigraphic successions that we have established for the two basins new foraminifer determinations indicate a middle to late triassic age for the carbonate deposition the depositional settings of the carbonate deposits and their deformation patterns support a southward subduction beneath the indochina block ending with the collision with the south china block during the middle triassic moreover our new biostratigraphic dating of the stratigraphic successions can be used to compare the triassic carbonate platforms from northern vietnam with other platforms elsewhere including those that have already been well documented in indonesia

the jinshajiang ailaoshan suture zone china tectonostratigraphy age and evolution abstract the jinshajiang suture zone is important for enhancing our understanding of the evolution of the paleo tethys and its age tectonic setting and relationship to the ailaoshan suture zone have long been controversial based on integrated tectonic biostratigraphic chemostratigraphic and isotope geochronological studies four tectono stratigraphic units can be recognized in the jinshajiang suture zone the eaqing complex the jinshajiang ophiolitic melange the gajinxueshan group and the zhongxinrong group isotope geochronology indicates that the redefined eaqing complex composed of high grade metamorphic rocks might represent the metamorphic basement of the jinshajiang area or a remnant micro continental fragment eaqing complex protolith rocks are pre devonian and probably of early middle proterozoic age and are correlated with those of the ailaoshan complex two zircon u pb ages of 340 3 and 294 3 ma separately dated from the shusong and xuitui plagiogranites within the ophiolitic assemblage indicate that the jinshajiang oceanic lithosphere formed in latest devonian to earliest carboniferous times the oceanic lithosphere was formed in association with the opening and spreading of the jinshajiang oceanic basin and was contiguous and equivalent to the ailaoshan oceanic lithosphere preserved in the shuanggou ophiolitic melange in the ailaoshan suture zone the latter yielded a u pb age of 362 41 ma from plagiogranite the re defined gajinxueshan and zhongxinrong groups are dated as carboniferous to permian and latest permian to middle triassic respectively on the basis of fossils and u pb dating of basic volcanic interbeds the gajinxueshan group formed in bathyal slope to neritic shelf environments and the zhongxinrong group as bathyal to abyssal turbidites in the jinshajiang ailaoshan back arc basin latest permian earliest middle triassic synorogenic granitoids with ages of 238 18 and 227 5 255 8 ma respectively and an upper triassic overlap molasse sequence indicate a middle triassic age for the jinshajiang ailaoshan suture formed by collision of the changdu simao block with south china

first discovery of early palaeozoic bathysiphon foraminifera test structure and habitat of a living fossil the giant agglutinated foraminiferan bathysiphon sars previously triassic recent occurs in much older sedimentary rock early ordovician late early tremadocian of avalonia the genus extends back to c 485 ma based on its discovery in platform mudstone of the chesley drive group in cape breton island nova scotia elongate up to 60 mm epibenthic bathysiphon tubes occur in wave rippled green grey mudstone with a low diversity probably dysoxic fauna the mudstone is coeval with and lithologically similar to the shineton formation in shropshire and the welsh borderlands scanning microscopy of the bathysiphon walls shows imbricated mica grains that parallel the long axis of the tests the lumen has a melange of packed sediment grains some of which are spherical structures of siliciclastic mud studded with tetrahedral pyrite crystals a felt like agglutinated test a lumen packed with spherical structures probable stercomata and the domal ends of some specimens are consistent with modern bathysiphon this report is the first time that cytoplasmic activity and stercomata formation have been used to refer fossil protists to a modern group bathysiphon differs from the cambrian foraminiferan platysolenites pander which has an open lumen without stercomata but support a comparable sediment deposit feeding niche bathysiphon is truly a living fossil with a mode of test construction cytoplasmic activity that formed stercomata and a niche unchanged for almost 500 million years foraminiferans have not been found prior to the cambrian period and the early cambrian appearance of agglutinated foraminiferans is part of the radiation of phanerozoic communities

the song ma anticlinorium northern vietnam the structure of an allochthonous terrane containing an early palaeozoic island arc sequence abstract the song ma anticlinorium displays evidence for four deformations the first two accompanied by high grade to greenschist facies metamorphism and the third by production of syn low grade metamorphic sheath folds the fourth occurred during post cretaceous folding and thrusting which produced a thrust fold complex across northern vietnam whilst the song ma anticlinorium may contain a metamorphosed island arc volcano sedimentary sequence this palaeozoic sequence cannot represent the active plate boundary in triassic cenozoic or recent times the song ma anticlinorium is not an indosinian subduction zone but is an allochthonous terrane which accreted to the south china plate in siluro devonian times

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

geology and geochemistry of the shuanggou ophiolite ailao shan ophiolitic belt yunnan province sw china evidence for a slow spreading oceanic basin origin abstract the early carboniferous shuanggou ophiolite lies in the middle segment of the ailao shan orogenic belt between the south china block to the north and the indochina block to the south the ophiolite consists of meta peridotite gabbro diabase and basalt capped by radiolarian bearing siliceous rocks no layered gabbros or sheeted dikes have been observed the meta peridotite underwent low degrees of partial melting consistent with the low magma budget of this oceanic lithosphere whole rock rare earth element analyses of gabbro indicate a geochemical affinity with normal mid ocean ridge basalts consistent with the crystallization order of plagioclase followed by clinopyroxene recognized in the gabbros the ophiolite is believed to have formed in a small slow spreading oceanic basin collision of the indochina block with the south china block in the late paleozoic was responsible for the closure of the oceanic basin and emplacement of the ophiolite in the ailao shan orogenic belt

an alternative plate tectonic model for the palaeozoic early mesozoic palaeotethyan evolution of southeast asia northern thailand burma abstract an alternative model for the geodynamic evolution of southeast asia is proposed and inserted in a modern plate tectonic model the reconstruction methodology is based on dynamic plate boundaries constrained by data such as spreading rates and subduction velocities in this way it differs from classical continental drift models proposed so far the different interpretations about the location of the palaeotethys suture in thailand are revised the tertiary mae yuam fault is seen as the emplacement of the suture east of the suture we identify an indochina derived terrane for which we keep the name shan thai formerly used to identify the cimmerian block present in southeast asia now called sibumasu this nomenclatural choice was made on the basis of the geographic location of the terrane eastern shan states in burma and central thailand and in order not to introduce new confusing terminology the closure of the eastern palaeotethys is related to a southward subduction of the ocean that triggered the eastern neotethys to open as a back arc due to the presence of late carboniferous early permian arc magmatism in mergui burma and in the lhasa block south tibet and to the absence of arc magmatism of the same age east of the suture in order to explain the presence of carboniferous early permian and permo triassic volcanic arcs in cambodia upper triassic magmatism in eastern vietnam and lower permian middle permian arc volcanites in western sumatra we introduce the orang laut terranes concept these terranes were detached from indochina and south china during back arc opening of the poko song ma system due to the westward subduction of the palaeopacific this also explains the location of the cathaysian west sumatra block to the west of the cimmerian sibumasu block

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

palaeoecological aspects of the diversification of echinoderms in the lower ordovician of central anti atlas morocco abstract echinoderms are one of the major components of benthic faunas in the lower ordovician sequence near zagora central anti atlas morocco the fezouata shale tremadocian late floian has yielded numerous exquisitely preserved echinoderm assemblages ranging through several stratigraphic levels and palaeoenvironmental conditions these associations offer a unique opportunity to document both evolutionary and palaeoecological aspects of echinoderm diversification in high latitude siliciclastic dominated western gondwana sediments where rapid in situ burials facilitated excellent faunal census conditions lower shoreface deposits of the fezouata shale provide the most complete record of successive echinoderm faunas in late tremadocian times these relatively shallow shelf deposits show the progressive replacement of low diversity opportunistic cambrian like dwarfed communities dominated by cornute stylophorans in unhospitable dysoxic environmental conditions by higher diversity benthic assemblages more typical of the great ordovician biodiversification event and dominated by blastozoans on well oxygenated sea floors the turnover of the ordovician radiation was apparently slightly delayed in more proximal settings eocrinoid meadows persisted in shallower environmental conditions up to the middle floian in the late floian they were replaced by diploporite dominated communities typical of later ordovician high latitude peri gondwanan faunas from a palaeobiogeographic point of view low diversity assemblages display relatively strong affinities with cosmopolitan late cambrian echinoderm faunas whereas high diversity communities are dominated by peri gondwanan taxa

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

crustal anatexis and formation of two types of granitic magmas in the kontum massif central vietnam implications for magma processes in collision zones abstract asia grew in the late permian by the collision of a number of micro continents syn to post collisional magmatism occurred along the continental collision zones in this study we report two types of granitic rocks garnet granite grt granite and orthopyroxene granite opx granite from the kontum massif central vietnam which is situated on the continental collision zone between the south china and indochina cratons these granitic rocks were formed at ca 250 ma when high temperature ht and ultrahigh temperature uht metamorphism took place in the same zone based on the petrological and geochemical features compared with previously reported experimental results garnet bearing granite is derived from pelitic gneisses by partial melting whereas orthopyroxene bearing granite is produced by the partial melting of garnet bearing mafic granulites we inferred that a significantly high geothermal gradient is required to produce vietnamese granitic magmatism and related ht to uht metamorphism this geotherm may be attributed to upwelling mantle plume beneath the kontum massif during the late permian

tectonic framework and phanerozoic evolution of sundaland abstract sundaland comprises a heterogeneous collage of continental blocks derived from the india australian margin of eastern gondwana and assembled by the closure of multiple tethyan and back arc ocean basins now represented by suture zones the continental core of sundaland comprises a western sibumasu block and an eastern indochina east malaya block with an island arc terrane the sukhothai island arc system comprising the linchang sukhothai and chanthaburi blocks sandwiched between this island arc formed on the margin of indochina east malaya and then separated by back arc spreading in the permian the jinghong nan uttaradit and sra kaeo sutures represent this closed back arc basin the palaeo tethys is represented to the west by the changning menglian chiang mai inthanon and bentong raub suture zones the west sumatra block and possibly the west burma block rifted and separated from gondwana along with indochina and east malaya in the devonian and were accreted to the sundaland core in the triassic west burma is now considered to be probably cathaysian in nature and similar to west sumatra from which it was separated by opening of the andaman sea basin south west borneo and or east java west sulawesi are now tentatively identified as the missing argoland which must have separated from nw australia in the jurassic and these were accreted to se sundaland in the cretaceous revised palaeogeographic reconstructions illustrating the tectonic and palaeogeographic evolution of sundaland and adjacent regions are presented

u pb zircon geochronology of early permian to late triassic rocks from singapore and johor a plate tectonic reinterpretation article i nfo the results of u pb zircon geochronology of permian to middle triassic granitoids from singapore and neighbouring johor are presented rhyolite from pulau sibu is 276 5 ma gabbros from singapore give ages of 260 2 to 249 2 ma granites from singapore give ages from 249 2 to 230 6 ma and rhyolite from telkuma gives an age of 238 2 ma u pb detrital zircon age spectra from a late triassic conglomerate from singapore show carboniferous mid dle triassic and late triassic spikes at 350 245 and 217 ma respectively these new data when combined with published u pb zircon ages allow a revision of the plate tectonic evolution of the malaysian peninsula region a tin bearing magmatic arc evolved in the eastern malaysian peninsula part of indochina from the early permian to the middle triassic when the palaeo tethys ocean was subducted and sibumasu continental lithosphere was rifted from pangea this opened up the neo tethys ocean that the granitoid ages show a progressive decrease in age from pulau sibu 276 5 ma to singapore 230 6 ma suggests that during this andean type orogenic phase the subduction zone rolled back and steepened causing the locus of mainly i type magmatism to migrate from east to west in present day coordinates across the eastern and central belts and stretch the lithosphere as it did so in the late triassic sibumasu collided with indochina causing the western belt sibumasu to over thrust the central belt indochina along the bentong raub suture the lower half of the thickened crust formally the tin bearing indochina plate was partially melted and pre dominantly s type tin bearing granites were intruded into the upper crust formally sibumasu this late triassic collisional phase is the indosinian orogeny in peninsular malaysia

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

geodynamics of southeast asia in relation to the evolution of ocean basins abstract the phanerozoic geodynamics of southeast asia has been closely connected with the cyclic development of large oceanic basins these basins were paleotethys in the middle paleozoic to early mesozoic tethys from the end of the paleozoic to the beginning of the cenozoic and the indian and pacific oceans together with their marginal seas in the late mesozoic and cenozoic the opening of those basins was mainly due to the fragmentation and moving apart of blocks of continental crust and it was accompanied by the simultaneous closing of other earlier basins and by local accretion and compression of sialic crust thus the relative compensation of constructive and destructive processes seems to have existed in southeast asia the most impressive example of such interconnected geodynamics systems was the opening of tethys the indus tsangpo zone central burma sumatra and synchronous closing of paleotethys upper yangtse ailaoshan north and central thailand the malay peninsula in the permian triassic the opening of the indian ocean and simultaneous closing of tethys north of it occurred in the same manner from the late jurassic till the early neogene all events mentioned are clearly recorded in present and ancient continental margins and collision zones these structures can be studied by applying lateral structural stratigraphic analysis which helps in restoring existing and past geodynamics