unexposed archean components and complex post archean accretion reworking processes beneath the southern yangtze block revealed by zircon xenocrysts from the paleozoic lamproites south china abstract integrated u pb dating and hf isotope analysis of zircon xenocrysts from the baifen lamproites of the zhenyuan area in the southeastern guizhou province offer a unique glimpse into the evolution of the deep continental crust beneath the southern part of the yangtze block south china a total of 148 zircons with 90 110 concordance yield u pb ages varying from 3458 9 ma to 252 2 ma they generally have high th u ratios 0 3 indicating a magmatic origin the two youngest magmatic zircons give a weighted mean 206pb 238u age of 253 0 2 8 ma which is interpreted as the emplacement age of the host baifen lamproites most zircons with pre eruption ages are xenocrysts derived from the deeper crust of the southern yangtze block they define two major paleoproterozoic age populations at 2 17 ga and 1 8 ga with similar hf model ages tcrust of 2 4 2 8 ga peak at 2 64 ga the oldest zircon has a 207pb 206pb age of 3458 9 ma with ehf t 1 47 and tcrust 3 7 ga this demonstrates the presence of archean crustal components or remnants as old as 3 7 ga beneath the southern yangtze block magmatic events involving significant addition of juvenile mantle derived materials mainly occurred at 2 2 2 1 ga and 2 0 1 8 ga but the latter event also involved reworking of the neoarchean crust and might represent the cratonization of the yangtze continent during the middle paleoproterozoic which could be correlated to the assembly of the columbia supercontinent this segment of the yangtze block became tectonically quiescent until 1 2 ga implying that the crustal stabilization lasted for ca 600 million years a few late mesoproterozoic zircon xenocrysts within the age range 1 3 1 0 ga suggest the occurrence of grenville aged events these neoproterozoic zircon xenocrysts 992 547 ma have a wide range of ehf t values 8 47 to 22 07 consistent with the zircon hf isotopic compositions of widespread coeval igneous rocks in the yangtze block the data show that the yangtze block has undergone important crustal growth and reworking during the neoproterozoic in response to the breakup of the rodinia supercontinent the early middle paleozoic zircon xenocrysts 513 357 ma have negative ehf t values 2 97 to 12 69 and tcrust hf model ages of 1 59 2 27 ga indicating reworking of the precambrian crustal basement without juvenile contributions during the caledonian orogeny the positive ehf t values 13 79 to 0 22 of twelve late paleozoic zircons 284 264 ma with a major age peak at ca 275 ma suggest that the precambrian basement beneath the southern yangtze block experienced an episode of strong rejuvenation during the permian we propose a possible linkage between the baifen lamproite magmatism 284 252 ma and the emeishan flood basalt volcanism

structural characteristic and origin of intra continental fold belt in the eastern sichuan basin south china block abstract the fold and thrust belt in the eastern sichuan basin is represented by a series of subparallel chevron anticlines under the orogenic tectonic setting within the south china block in meso cenozoic period and the influence of the multi layer detachment fault the deformation of the thrust belt exhibits remarkably layered and large scale intracontinental thrusting structural characteristics in this paper we focus on the structural geometry and deformational mechanisms using the latest two dimensional 2d and three dimensional 3d seismic reflection data in combination with well and outcrop data the multi layer detachment faults especially the upper gypsum bearing detachment in the middle triassic jialingjiang formation and lower detachment with gypsum or shale in the lower middle cambrian system directly control the deformational styles of the study area interpretation of seismic sections indicates that the fold and thrust belt has various deformational styles during folding including fault propagation fold fault bend fold and detachment fold with box fold or pop up structural geometry regional location and structural boundaries play significant roles in controlling the deformational styles and distinct differences exist among the different anticlines the huayingshan anticline located at the front of the thrust belt shows intense structural deformation with northwestward thrusting direction and a relatively weak opposite southeastward thrusting in addition the anticlines exhibit structural differences along strike and the fold and thrust belt in the northern segment is influenced by the north china block

petrologic and zircon u pb geochronological characteristics of the pelitic granulites from the badu complex of the cathaysia block south china abstract the recognition of the indosinian orogeny in the south china block has been controversial and difficult because of strong weathering and thick cover high temperature ht and high pressure hp metamorphic rocks related to this orogeny were considered to be absent from this orogenic belt until the recent discovery of eclogite and granulite facies meta igneous rocks occurring as lenses within the meta sedimentary rocks of the badu complex however metamorphic state of these meta sedimentary rocks is still not clear besides there have been no geochronological data of ht pelitic granulites previously reported from the badu complex this paper presents petrographic characteristics and zircon geochronological results on the newly discovered kyanite garnet gneiss pyroxene garnet gneiss and the ht pelitic granulites sillimanite garnet gneiss mineral assemblages are garnet sillimanite ternary feldspar plagioclase quartz biotite for the ht pelitic granulite kyanite ternary feldspar garnet sillimanite plagioclase quartz biotite for the kyanite garnet gneiss and garnet biotite pyroxene plagioclase ternary feldspar quartz for the pyroxene garnet gneiss respectively decompressional coronas around garnet grains can be observed in all these pelitic rocks typical granulite facies mineral assemblages and reaction textures suggest that these rocks experienced hp granulite facies metamorphism and overprinted decompression along a clockwise p t loop results from integrated u pb dating and ree analysis indicate the growth of metamorphic zircons from depleted heavy ree sources 100 50 chondrite compared with detrital zircons derived from granitic sources typically 1000 chondrite metamorphic zircons in hp granulite exhibit no or subdued negative eu anomalies which perhaps indicate zircon overgrowth under eclogite facies conditions the zircon overgrowth ages range from 250 to 235 ma suggesting that hp granulite eclogite to granulite facies metamorphism of these supracrustal rocks occurred in the early middle triassic based on the presence of hp granulite facies pelitic rocks it is inferred that significant underthrusting was involved during the indosinian orogeny which introduced these supracrustal rocks to lower crustal levels

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

tectonic feedback intraplate orogeny and the geochemical structure of the crust a central australian perspective the geological record of intraplate deformation in central australia implies that past tectonic activity basin formation deformation and erosion has modulated the response of the lithosphere during subsequent tectonic activity in particular there is a correspondence between the localization of deformation during intraplate orogeny and the presence of thick sedimentary successions in the preserved remnants of a formerly widespread intracratonic basin this behaviour can be understood as a kind of tectonic feedback effected by the longterm thermal and mechanical consequences of changes in the distribution of heat producing elements induced by earlier tectonism from a geochemical point of view one of the most dramatic effects of intraplate orogeny in central australia has been the exposure in the cores of the orogens of deep crustal rocks largely depleted in the heat producing elements the geochemical structuring of the crust associated with the erosion of the heat producing upper crust resulted in long term cooling of the deep crust and upper mantle with associated lithospheric strengthening this is illustrated here by mapping the consequences of deformation and associated tectonic responses onto the h qc plane where h is the characteristic length scale for heat production distribution and qc is the total crustal heat production because rates of intraplate deformation in central australia appear to be much slower than that typical of plate margin orogens it is possible that the ongoing geochemical structuring of the crust has played an important role in terminating intraplate orogeny in central australia by providing a thermal lock the diagnostic geophysical signature of this lock may be the extraordinary gravity anomalies of the central australian intraplate orogens central australia provides an intriguing record of intraplate deformation resulting in the development of a number of extraordinary orogens that expose deep crustal rocks in their cores forman 1971 teyssier 1985 goleby et al 1989 the spatial distribution of intraplate deformation during the alice springs and petermann orogenies suggests that the mechanical response of this region has varied through time consequently this response is important for our understanding of the processes controlling reactivation in intraplate settings in previous contributions it has been argued that the distribution of deformation in central australia has been strongly influenced by prior tectonic processes such as basin formation and erosion sandiford hand 1998a hand sandiford 1999 this paper focusses on the way in which intraplate deformation modifies the geochemical structure of the crust in this region in particular the distribution of heat sources recognizing that the distribution of heat sources provides a first order control on the thermal structure of the crust the consequences of intraplate deformation for the longterm thermal and mechanical evolution of the crust is then explored our aim is to demonstrate the relevance of a novel style of tectonic feedback which seems to have modulated the longterm tectonic behaviour of the central australian region and which may be relevant to continental interiors in general the notion of tectonic feedback is compatible with a highly temperature dependent lithospheric rheology as suggested by many experimental studies of natural rocks such feedback has profound implications for the longterm tectonic evolution of continental interiors particularly their potential for reactivation the record of intraplate deformation in

early mesozoic tectonics of the south china block insights from the xuefengshan intracontinental orogen abstract intracontinental orogens remain less understood than accretionary or collisional orogens that are related to plate margin interactions in the center of the south china block the xuefengshan belt provides a well exposed example of such an intracontinental orogen of early mesozoic age detailed field tectonic observations indicate that the xuefengshan belt can be divided into a western outer zone characterized by km scale box fold structures and an eastern zone separated from the western outer zone by the se dipping main xuefengshan thrust in the eastern zone nw verging folds coeval with a pervasive slaty cleavage and a nw se trending lineation are the dominant structures from west to east the dip of the cleavage surface exhibits a fan like pattern the bulk architecture of the xuefengshan belt results from polyphase deformation d 1 is characterized by a top to the nw ductile shearing d 2 corresponds to se directed back thrusting and folding d 3 consists of upright folds with vertical cleavage and lineation at depth a high strain zone characterized by greenschist facies metamorphic rocks and a top to the nw ductile shearing corresponds to a ductile decollement zone that accommodated the deformation of the neoproterozoic to early triassic sedimentary series kinematic compatibility suggests that the synmetamorphic ductile shearing was coeval with the d 1 event in the sedimentary cover the xuefengshan belt is interpreted as an early mesozoic intracontinental orogen which possibly originated from the se directed continental subduction of a piece of the south china block in response to northwestwards subduction of the pacific plate

post glacial rebound and transient lower mantle rheology summary although post glacial rebound data have been conventionally interpreted as being governed by the steady state component of the mantle viscosity spectrum the radial profile of this parameter which is then inferred by fitting a model to observations is characterized by the fact that it exhibits rather slight variation with depth this disagrees with expectations based upon microphysical models of the solid state creep process it also disagrees with very recent inferences of the viscosity stratification based on isostatic geoid anomalies expected on the basis of the internal lateral heterogeneity of mantle density obtained from seismic tomographic analyses the new calculations of the signatures of post glacial rebound reported here show that these two types of information are easily reconciled if the previously inferred value of the lower mantle viscosity is interpreted as a transient value as originally suggested by weertman on the basis of qualitative considerations in these new models considered here the steady state creep resistance of the lower mantle is not constrained at all by post glacial rebound observations it can be futed only by an appeal to other geophysical data whether such models are actually required by the data should become clear in the very near future

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

incremental emplacement of the late jurassic midcrustal lopolith like qitianling pluton south china revealed by ams and bouguer gravity data modes of magma emplacement may provide key information on geodynamic tectonic settings of magmatism when the threshold of magma volume and duration of emplacement process is crossed brittle ductile crustal deformation behavior and volumes and rates of magma transfer in the crust the majority of upper crustal granitic intrusions are incrementally emplaced as laccoliths or sills while midcrustal or deeper level granitic intrusions appear to be predominantly emplaced as sills or lopoliths but relatively few examples have been studied we present a case study on the jurassic qitianling pluton from south china which was constructed in the upper part of the middle crust at a depth of 10 15 km to characterize its emplacement mechanism and evolution we have combined field and microscopic observations anisotropy of magnetic susceptibility measurements and bouguer gravity anomaly modeling the main findings are 1 the generally undeformed granite country rock contacts with a narrow thermal aureole 2 irregularly wavy and gradational contacts between the main granite facies 3 dominant subhorizontal magnetic fabrics in the granites with a scattered magnetic foliation 4 rare occurrence of dykes and 5 a subcircular surface and lopolith like pluton geometry accordingly we suggest that the qitianling pluton forms an 25 km wide and 5 km thick lopolith that was incrementally emplaced during a period of tectonic quiescence most likely by under accretion of multiple magma sheets deflected from magma pulses that ascended through pipe like channels the mineralogical and compositional variation of the pluton may be the result of in situ fractionation fractionation at deeper crustal level or variation in source derived melt compositions

tibetan tectonic evolution inferred from spatial and temporal variations in post collisional magmatism cenozoic magmatism on the tibetan plateau shows systematic variations in space and time that must be considered in models concerning tibetan tectonic evolution after the india asia collision which started in the early tertiary and terminated the gangdese arc magmatism in the lhasa terrane made of the southern tibetan plateau widespread potassium rich lavas and subordinate sodium rich basalts were generated from f50 to 30 ma in the qiangtang terrane of northern tibet subsequent post collisional magmatism migrated southwards producing ultrapotassic and adakitic lavas coevally between f26 and 10 ma in the lhasa terrane then potassic volcanism was renewed to the north and has become extensive and semicontinuous since f13 ma in the western qiangtang and songpan ganze terranes such spatial temporal variations enable us to elaborate a geodynamic evolution model which depicts when and how the indian continental lithospheric mantle started thrusting under asia by involving rollback and breakoff of the subducted neo tethyan slab followed by removal of the thickened lhasa lithospheric root we propose that only after the lithospheric removal which occurred at f26 ma could the indian mantle lithosphere have commenced its northward underthrusting and henceforth served as a pivotal control to the himalayan tibetan orogenesis consequently the tibetan plateau is suggested to have risen diachronously from south to north whereas the southern part of the plateau may have been created and maintained since the late oligocene the northern plateau would have not attained its present day elevation and size until the mid miocene when the lower part of the western qiangtang and songpan ganze lithospheres began to founder owing to the push of the underthrust indian mantle lithosphere d 2004 elsevier b v all rights reserved

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

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

origin and tectonic significance of the huangling massif within the yangtze craton south china as the oldest exposed basement and the typical sedimentary cover of the yangtze craton the huangling massif is a suitable place to decipher the tectonics of south china block structural analysis shows that the huangling massif has an elliptic domal shape with n s striking long axis an asymmetric antiform with a steep western flank and a gentle eastern flank there three litho tectonic units are recognized from inner to outer parts 1 archean paleoproterozoic metamorphic rocks intruded by neoproterozoic granitoids 2 neoproterozoic to jurassic sedimentary envelope around the dome core 3 cretaceous terrigeneous alluvial fluvial deposits unconformably overlying the dome flanks coeval with the uplifting of the massif the pre cretaceous strata on the western and eastern flanks of the huangling massif were involved in a series of folds with nearly n s axes and layer parallel slip structures with top to the w and top to the e motion respectively the subsequent brittle normal faulting controlled the deposition of the graben or half graben basins on both flanks cooling history reveals that the huangling massif underwent uplifting between 160 ma and 110 ma with an average cooling rate of 2 3 c ma moreover the huangling area was not significantly affected by the early paleozoic and triassic orogenies of south china comparable with the contemporaneous extensional structures such as metamorphic core complexs syntectonic plutons bounded by ductile normal faults and rift related basins in eastern china it is proposed that the huangling massif might be an extensional structure controlled by a weak crustal extension in this case it will represent the western front of the late mesozoic lithospheric thinning in entire eastern china however the compressional model cannot be ruled out

the typical large scale superposed folds in the central south china implications for mesozoic intracontinental deformation of the south china block abstract the south china block has experienced polyphase intensive intracontinental deformation in the mesozoic time large scale superposed folds in central south china document the mesozoic tectonic events within the south china block here we present new structural data related to synfolding deformation coupling with chronological data to reconstruct the deformation sequence and tectonic regime during crustal shortening these data indicate that the superposed folds dominated by a two stage tectonic compressive regime experienced two phases of superposed buckle folding leading to the orthogonal superposition of ne sw trending folds onto wnw ese trending folds between the late middle jurassic and the earliest early cretaceous our structural analysis together with geochronological data for this area suggests that the south china block predominately underwent two phases of intracontinental deformation during the mesozoic the early phase of tectonism d1 is characterized by a late middle triassic to earliest early jurassic ne sw compression causing the occurrence of an orogeny perpendicular shortening accompanying with evident magmatism this tectonic event was most likely associated with progressive clockwise collision of the south china block toward the north in the indosinian event subsequent tectonic activity d2 between the late middle jurassic and the earliest early cretaceous contributed to a phase of nw se contraction that overprinted the early ne sw shortening in the interior of the south china block generating a large scale nw convex fold belt and the typical large scale superposed folds within the central south china block the later tectonism was probably driven by the nw directed subduction of the paleo pacific plate beneath the eastern part of the asian continent

u pb and 40ar 39ar geochronology of the baiyunshan gneiss central guangdong south china constraints on the timing of early palaeozoic and mesozoic tectonothermal events in the wuyun wuyi yunkai orogen composite sensitive high resolution ion microprobe shrimp u pb zircon and 40 ar 39 ar step heating biotite hornblende ages are used to provide constraints on the timing and origin of the felsic gneissic rocks in the baiyunshan mountains region and to elucidate their tectonothermal history shrimp dating and cl imaging of zircons give magmatic zircon crystallization ages between late ordovician and early silurian c 453 5 ma 446 ma and 439 ma for three representative felsic gneisses suggesting that most of the baiyunshan gneiss cannot represent basement rocks of the cathaysia block as previously thought including the present age information a synthesis of available age data for regional wuyun wuyi yunkai events reflects the emergence of orogen wide magmatism that could be syn orogenic and have occurred mainly between 460 and 420 ma in the south china block inherited zircons are abundant with ages clustering at late mesoproterozoic 1189 1017 ma and middle mesoproterozoic 772 ma which reveals that the baiyunshan orthogneiss samples a crustal basement containing significant igneous or recycled components related to the rodinia amalgamation and break up a shrimp date of 212 12 ma from a white rim of zircon provides evidence for metamorphic overprinting of an indosinian tectonothermal event on the baiyunshan gneiss incremental heating experiments with six biotite samples and one hornblende sample from a variety of metamorphic rocks yielded two distinct 40 ar 39 ar age groups 150 155 and 94 98 ma the older ages are similar to zircon u pb dates for widespread granitic intrusions in central guangdong we attribute them to late jurassic magmatism induced thermal resetting of the biotite k ar system on the other hand the younger age group is interpreted to record either cooling through the biotite closure temperature of 300 350 c or a second resetting of biotite ar isotopes at c 94 98 ma due to contemporaneous magmatic activity our present age data suggest that the maofengshan orthogneiss was exhumed to 8 to 10 km crustal levels at c 150 ma whereas the eastward components of gneissic rock masses appear to have passed upward through the same crustal depth synchronously or later by c 94 ma exhumation of middle crustal level rocks in the study area since c 155 ma is roughly coeval with exhumation of gneissic rocks from elsewhere in the wuyun orogen suggesting a large scale mechanism for the exhumation pulse related to the yanshanian extensional tectonic regime

tectonics of se china new insights from the lushan massif jiangxi province in south china the lushan massif forms a topographic high of the south china block south of the qinling dabie belt the lushan massif consists of two main lithotectonic units separated by a major tectonic contact a neoproterozoic upper sinian paleozoic unit comprising primarily unmetamorphosed sandstones overlies a paleoproterozoic unit mainly composed of low pressure high temperature gneisses and micaschists both units are cut by cretaceous granitic intrusions three primary tectono metamorphic and magmatic events are recognized the eastern part of the lushan massif is cut by a nne ssw trending ductile normal fault d3 deformation coeval to the emplacement of a 100 110 ma leucogranite dated by 40ar 39ar laserprobe on biotite and muscovite d2 deformation is responsible for the formation of a decakilometer scale ne sw trending upright anticline characterized by ne sw stretching and nw se shortening the age of this folding event is defined by a 127 1 2 ma u pb titanite date obtained for a syntectonic granodiorite and 40ar 39ar ages of 133 ma for amphibole this cretaceous age also corresponds to the 40ar 39ar ages of 126 ma found on syntectonic muscovites at the base of the sinian unit an older deformation event d1 characterized by a top to the nw extensional decollement of the sinian paleozoic series above proterozoic metamorphic rocks is related to the triassic tectonics of the dabieshan lastly in the lower part of sinian rocks the occurrence of kyanite cataclased during d1 documents an older poorly preserved late paleozoic early mesozoic tectonometamorphic event dx related to a blind thrust in the continental crust of the south china block in the southern foreland of the dabieshan

highly evolved archean basement beneath the western cathaysia block south china abstract how has the earth s continental crust evolved most of our knowledge comes from surface exposures but zircon xenocrysts in volcanic rocks can provide samples of deeper crustal levels the u pb and hf isotope systematics of xenocrystic zircons brought to the surface by the cenozoic 48 49 ma pingnan basaltic rocks and the mesozoic 166 ma pingle minettes in guangxi province south china suggest the presence of unexposed relict archean basement beneath the western cathaysia block where the oldest exposed rocks are neoproterozoic phanerozoic in age this basement has provided zircons with several distinct age populations 3 85 3 55 3 3 3 2 and mainly 2 9 2 5 ga these have hf depleted mantle model ages t dm of 2 5 to 3 9 ga the oldest t dm 3 9 ga shows the existence of paleo to eoarchean components in this area this relict basement experienced complex modification including the addition of juvenile mantle material with e hf up to 12 7 at ca 3 6 3 2 2 5 1 0 and 0 5 ga the zircons also record thermal events that reworked remelted the older crustal components of the block at ca 2 0 1 8 1 6 1 5 ga and 80 ma although these younger events have modified the archean nature of the basement it seems that they do not represent significant post archean crustal growth

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

234u 238u activity ratios in freshwaters as tracers of hydrological processes the strengbach watershed vosges france abstract 234u 238u and 87sr 86sr isotopic ratios as well as major and trace u ba sr rb element concentrations were analyzed in dissolved loads of the strengbach stream vosges france in order to constrain the sources of u isotopes 234u 238u to river waters the variations of the 234u 238u activity ratios along the stream indicate a clear dependence between the 234u 238u activity ratios of the surface waters and the different types of rocks forming the watershed with near equilibrium values for the waters draining granites and high 234u 238u disequilibria 1 4 for those flowing over carbonates the high 234u 238u disequilibria are related to a supply of 234u enriched groundwaters located within the carbonate rocks the 234u 238u activity ratios of the waters at the outlet of the catchment collecting the sources of the stream on the granitic lithology decrease from 1 02 to 0 96 when the discharge of the stream increases such a tendency requires mixing between a water body enriched in 234u which weathered the granitic bed rock at secular equilibrium and a water with a 234u 238u activity ratio below unity representing a mobilization of u from material that has already been weathered comparison of the geochemical characteristics of waters collected during the year and those collected during a flood event reveals the involvement of two different weathered end members depending on the hydrological conditions during the year the dissolved u transported by the river originates from bed rock and deep horizons of the weathering profile whereas a significant part of u during the flood event is supplied by superficial horizons of soils probably complexed by organic colloids these results outline the potential of 234u 238u activity ratios to investigate hydrological processes and emphasize that their use as tracers of rock weathering should be made cautiously

the hengshan low angle normal fault zone structural and geochronological constraints on the late mesozoic crustal extension in south china abstract we document in this paper a significant low angle normal fault namely the hengshan detachment fault which extends more than 150 km long in nne orientation and bounds an elongated early cretaceous basin in hunan province central south china detailed structural and geochronological analyses have been conducted along its southern segment where a well exposed 3 km thick flat ductile shear zone develops along the western margin of the hengshan granitic massif this zone is featured by shallow dipping foliations of varied trend from ne to nw which bear penetrative stretching lineations varying from nw to sw trend shear sense criteria indicate top to the nw and top to the sw motions along its northern and southern parts respectively quartz c axis orientations of mylonitic rocks from the shear zone exhibit asymmetric single or crossed girdle patterns and the distributions of fabric point maxima suggest a simultaneous operation of basal and prism slips indicating a deformation temperature ranging from 400 c to 550 c one zircon u pb age of the sheared albitite and three muscovite 40 ar 39 ar ages of the mylonitic rocks indicate that the ductile shearing initiated at 136 ma and lasted till 97 ma the zircon u pb dating results of five granitic samples from the hengshan granitic pluton yield two phases of crystallization ages of 232 228 ma and 150 151 ma respectively indicating two preceding magmatic events prior to the ductile shearing in the lower plate our structural and geochronological data allow defining the hengshan massif as an extensional dome rather than a metamorphic core complex as previously predicted due to the lack of syn tectonic plutonism and the origin of detachment fault associated with the reactivation of the inherited thrust this study provides crucial evidence for understanding how the mid upper crust deformed during the process of extension which testifies to a coeval occurrence of non coaxial deformation of the ductile flow in the middle crust and brittle faulting in the upper crust it also sheds new insights into the timing of the regional crustal extension in south china and permits to constrain its onset time to be ca 136 ma

ar ar geochronology of ultrahigh pressure metamorphism in central china the dabie shan of china contain the largest areal exposure of ultrahigh pressure regional metamorphic rock known on earth the thermal history of these unusual rocks is central to understanding the tectonic processes responsible for their creation preservation and exhumation published ages of ultrahigh pressure metamorphism and subsequent cooling range from archean to jurassic by analyzing 21 dabie hornblende phengite and biotite samples by the 40ar 39ar method we find that 1 cooling from peak metamorphic temperatures to 300 c occurred between about 206 and 178 ma 2 widespread cretaceous ages reflect reheating by a postultrahigh pressure magmatic extensional episode 3 40ar 39ar and k ar ages older than 230 ma and perhaps 210 ma are geologically meaningless due to the incorporation of excess 40ar and 4 dabie ultrahigh pressure metamorphic rocks are temporally related to blueschist facies rocks farther west in the suture zone

mesozoic large scale magmatism and mineralization in south china preface abstract south china is famous for its polymetallic deposits with more than 50 of the world s w and sb reserves and 20 of the sn reserves as well as abundant cu mo pb zn au ag ta nb hree and u this special issue reports recent progress on the tectonic evolution magmatism and ore deposits of south china which leads to a more comprehensive understanding of the mesozoic large scale magmatism and mineralization events 1 south china has been located in backarc settings during multiple subductions since the paleozoic which may have contributed to pre enrichments of the ore forming materials 2 the pacific plate subduction was a principle factor that controlled the late mesozoic large scale magmatism and mineralization in south china through flat subduction ridge subduction slab tearing and slab rollbacks

precise determination of phanerozoic zircon pb pb age by multicollector sims without external standardization 1 zircon has long been recognized as the best geochronometer and the most important timekeeper in geosciences modern microbeam techniques such as sims and la icpms have been successfully applied to in situ u pb zircon age determinations at spatial resolutions of 20 30 m or less matrix matched calibration by external standardization of well characterized natural zircon references is a principal requirement for precise microbeam u pb zircon age determination due to fractionation effects between pb and u which usually result in an external age error exceeding 1 alternatively zircons with a closed u pb system can be directly dated by measurement of 207pb 206pb isotopic ratio without external standardization which has been a common practice for zircons older than 1 0 ga but not for relatively young 1 0 ga and particularly phanerozoic ones because of limitations of analytical precision we describe in this paper a method of 207pb 206pb measurement on phanerozoic zircons using a new generation of large radius magnetic sector multicollector cameca ims 1280 sims in combination with multicollector mode a nuclear magnetic resonance nmr magnet controller and oxygen flooding techniques we achieve precisions of 207pb 206pb ratio of 0 1 and 0 1 0 2 propagating to pb pb age errors 0 4 and 1 3 excluding u decay constant uncertainties for zircons of latest neoproterozoic and late paleozoic to mesozoic age respectively therefore the multicollector sims is capable of direct determination of zircon pb pb ages as young as mesozoic age with uncertainties of geological significance this technique is useful for direct dating of zircons in thin sections moreover it has significance for dating of some other u rich minerals i e baddeleyite and zirconolite that are not suitable for sims u pb dating by external standardization

a possible buried paleoproterozoic collisional orogen beneath central south china evidence from seismic reflection profiling abstract the south china craton consisting of the yangtze and cathaysia blocks has figured importantly in the neoproterozoic supercontinent of rodinia however its lack of grenville age structures and metamorphism and the presence of paleoproterozoic high grade metamorphism and s type magmatism raise questions about the timing and nature of the yangtze cathaysia collision here we present results of a 400 km long high resolution seismic reflection profile across the purported suture between the yangtze and cathaysia blocks the seismic profile reveals folded and thrust imbricated seismic reflectors which we interpret to represent the relics of the yangtze cathaysia collisional orogen the inferred orogen was extended in the neoproterozoic resulting in its burial by neoproterozoic flysch strata geochronological data suggest that this buried orogen was formed in the paleoproterozoic 2 0 1 9 ga likely associated with the assembling of the columbia supercontinent these results call for major revision of the models for the formation of the south china craton and its role in the assembling of the rodinia and the columbia supercontinents

petrogenesis and geodynamic significance of the 850 ma dongling a type granites in south china abstract the ongoing controversy on the timing of amalgamation of the yangtze and cathaysia blocks either prior to 890 ma or later than 830 ma impedes a proper understanding of geological evolution of the south china block and its role in the breakup of rodinia supercontinent in this study we report 850 ma la icp ms zircon u pb ages and whole rock geochemistry of the dongling granites from the southeastern yangtze block that did not receive much geoscientific attention as compared to the better studied 830 760 ma sedimentary and igneous rocks in the south china block the studied dongling granites have high k2o na2o 7 44 9 09 wt and low mgo 0 07 0 54 wt and cao 0 16 1 21 wt contents their high feot feot mgo 0 82 0 94 and ga al 2 6 values and hfse abundance zr nb ce y 443 965 ppm allow their grouping as a type granite in the chondrite normalized diagrams these granites display right inclined ree patterns and strong to moderate negative eu anomalies eu eu 0 32 0 70 the dongling granites have variable and negative whole rock end t 14 6 to 8 6 and zircon ehf t 12 8 to 4 4 with paleoproterozoic two stage nd 2 20 2 69 ga and hf model ages 2 01 2 41 ga zircon saturation parameters tzr suggest 850 c temperature and other geochemical characteristics reveal low melting pressure 350 to

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

tectonics of the dabieshan eastern china and possible exhumation mechanism of ultra high pressure rocks the dabieshan is divided into three tectonic domains the southern dabieshan is a stack of allochtons from top to bottom i unmetamorphosed cambrian early triassic series unconformably covered by jurassic sandstone ii weakly metamorphosed proterozoic slate iii hp rocks mostly retrogressed into greenschist facies iv uhp allochton v uhp free gneisses these units exhibit a nw se lineation and top to the nw shearing reworked by s verging folds the central dabieshan is a migmatitic dome superimposed on uhp metamorphism and bounded by a detachement fault responsible for the exhumation of the uhp unit during its retrogression into amphibolite facies in the northern dabieshan early foliation and n s trending compressional lineation are deformed by n verging folds coeval to the syn exhumation ductile structures of the central dabieshan a geodynamic model involving coeval thrusting and normal faulting is discussed

origin and tectonic significance of a mesozoic multi layer over thrust system within the yangtze block south china abstract in the yangtze block south china a well developed mesozoic thrust system extends through the xuefeng and wuling mountains in the southeast to the sichuan basin in the northwest the system comprises both thin and thick skinned thrust units separated by a boundary detachment fault the dayin fault to the northwest the thin skinned belt is characterized by either chevron anticlines and box synclines to the northwest or chevron synclines to the southeast the former structural style displays narrow exposures for the cores of anticlines and wider exposures for the cores of synclines thrust detachments occur along silurian fs and lower cambrian fc strata and are dominantly associated with the anticlines to the southeast this style of deformation passes gradually into one characterized by chevron synclines with associated principal detachment faults along silurian fs cambrian fc and lower sinian fz strata there are however numerous secondary back thrusts therefore the thin skinned belt is like the valley and ridge province of the north american applachian mountains the thick skinned belt structurally overlies the thin skinned belt and is characterized by a number of klippen including the xuefeng and wuling nappes it is thus comparable to the blue ridge province of appalachia the structural pattern of this thrust system in south china can be explained by a model involving detachment faulting along various stratigraphic layers at different stages of its evolution the system was developed through a northwest stepwise progression of deformation with the earliest delamination along lower sinian strata fz analyses of balanced geological cross sections yield about 18 1 21 total 88 km shortening for the thin skinned unit and at least this amount of shortening for the thick skinned unit the compressional deformation from southeast to northwest during late jurassic to cretaceous time occurred after the westward progressive collision of the yangtze block with the north china block and suggests that the orogenic event was intracontinental in nature

textures and trace element signatures of pyrite and arsenopyrite from the gutaishan au sb deposit south china the gutaishan deposit is a slate hosted lode au sb deposit in the xiangzhong sb au metallogenic province south china the deposit has a proven reserve of ca 9 t gold at an average grade of 13 g t and contains 2500 t sb the deposit represents the product of a complex ore forming process comprising four different stages of which the third stage is the main gold stibnite bearing stage scanning electron microscopy electron probe microanalysis and laser ablation inductively coupled plasma mass spectrometry were used to investigate the occurrence of visible gold and the distribution of invisible gold in pyrite and arsenopyrite visible gold displays textural complexity with four distinct categories defined and assumed to have formed directly precipitated from ore forming fluid and remobilized from pyrite and arsenopyrite pyrite and arsenopyrite occur as ten and four texturally distinct subtypes all host invisible gold which is lattice bound trace element mapping of selected pyrite and arsenopyrite grains reveals a marked compositional zoning with respect to au and other elements in both minerals observed decoupled behavior between au and as in some grains contradicts the paradigm of coupled au and as substitution evidence suggests that the gutaishan deposit likely represents an evolving au sb mineralization event recorded by the complex texture and geochemical signatures of visible gold sulfides and sulfosalts

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

u pb and hf isotopic study of zircons from migmatised amphibolites in the cathaysia block implications for the early paleozoic peak tectonothermal event in southeastern china abstract amphibolites occur in a number of localities in the cathaysia block some of them have been migmatised and their protoliths represent basaltic magmas erupted in various tectonic settings four migmatised amphibolites were collected from jiangxi and fujian provinces cathodo luminescence images of zircons extracted from the representative amphibolites show unzoned or sector zoned structure la icp ms analysis indicates that most zircons have high th u ratios and yield u pb zircon ages of 446 5 435 2 434 4 and 423 2 ma respectively lu hf isotopic analysis on these zircons gives hf model ages ranging from 900 to 1200 ma based on lithological observations and previously published geochronological data we interpret that these u pb ages record an important tectonothermal event that led to the migmatization this early paleozoic caledonian tectonothermal event in the southeastern china has a great tectonic implication for the evolutionary history of the cathaysia block

infrared microthermometric and noble gas isotope study of fluid inclusions in ore minerals at the woxi orogenic au sb w deposit western hunan south china abstract the woxi au sb w deposit hosted by the neoproterozoic low grade metamorphic clastic rocks is located in a brittle ductile shear zone within the xuefengshan range south china orebodies are predominantly banded quartz veins which are strictly controlled by bedding faults and display significant vertical extents up to 2 km without obvious vertical metal zoning fluid inclusions hosted in quartz scheelite and stibnite from quartz scheelite and quartz sulfide gold veins have been studied using conventional and infrared microscopy respectively four types of fluid inclusions were identified based on petrography including type i two phase liquid rich aqueous inclusions type ii two or three phase co 2 rich inclusions type iii two phase vapour rich aqueous inclusions and type iv single phase aqueous inclusions the fluid inclusions in ore minerals scheelite and stibnite and their coexisting quartz largely share similar characteristics in terms of their types homogenization temperatures and salinities this is consistent with the fact that these ore minerals are always intergrown with quartz microthermometric and laser raman data indicate a low to moderate temperature 140 240 c low salinity 2 rich n 2 bearing aqueous ore forming fluid such fluid is further identified as a deeply non magmatic crustal fluid rather than a mantle source fluid by the significantly low 3 he 4 he ratios 0 002 0 281 ra and a small amount of meteoric water or host rock buffered fluid could be involved w ore precipitation was probably associated with mixing between a deeply originated crustal fluid and host rock buffered fluid based on the fluid inclusion features in scheelite and quartz i however au and sb ore deposition probably resulted from boiling which was caused by the marked pressure drop geological features such as banded structure and crack sealing structure also indicate that fluid pressure fluctuation induced by fault valve mechanism occurred during ore precipitation these characteristics of the ore forming fluids in the woxi deposit are in good agreement with the definition of orogenic gold deposits and the woxi au sb w deposit is probably an atypical orogenic gold deposit for its unique ore forming element association

caledonian early paleozoic veins overprinted by yanshanian late mesozoic gold mineralization in the jiangnan orogen a case study on gold deposits in northeastern hunan south china abstract the jiangnan orogen situated between the yangtze and cathaysia blocks in south china is an important gold polymetallic metallogenic belt in china like many gold deposits hosted in precambrian rocks in eastern china the timing of gold mineralization in the jiangnan orogen mostly hosted by neoproterozoic rocks has been controversial a number of previous geochronological studies have suggested that mineralization ages range from neoproterozoic to cretaceous with caledonian early paleozoic and yanshanian jurassic cretaceous being considered most important as caledonian is the most important period of regional metamorphism and yanshanian is the most important period of granitic magmatism the timing of mineralization is closely related to the genetic nature orogenic versus post orogenic of the gold mineralization this paper addresses these problems by studying the ages and characteristics of ore forming fluids of two major gold deposits wangu and huangjindong in the central part of the jiangnan orogen in northeastern hunan muscovite ar ar dating suggests that the bulk of the quartz veins in the wangu and huangjindong deposits were formed at ca 399 397 ma i e during the caledonian orogeny whereas gold mineralization and associated veining took place at ca 142 130 ma i e during the yanshanian orogeny fluid inclusions in the barren caledonian veins are characterized by the h2o nacl co2 system homogenization temperatures of 266 276 c and salinities of 5 5 4 wt nacl equivalent similar to those found in many orogenic type gold deposits whereas the fluid inclusions in the gold bearing yanshanian veins with abundant sulfides overprinting the barren veins have a h2o nacl composition system homogenization temperatures of 169 to 256 c and salinities from 8 7 to 15 2 wt nacl equivalent it is proposed that both the caledonian and yanshanian veining events are controlled by the same fracture systems with the former being formed in a relatively deep orogenic deformation metamorphic environment and the latter in a relatively shallow extensional environment accompanied by extensive granitic magmatism reactivation of the caledonian structures during the yanshanian orogeny together with availability of sulfur and gold possibly associated with granitic intrusions played a key role in the formation of the gold deposits in northeastern hunan and possibly elsewhere in the jiangnan orogen

multiple emplacement and exhumation history of the late mesozoic dayunshan mufushan batholith in southeast china and its tectonic significance 1 structural analysis and geochronological constraints the south china block scb experienced a polyphase reworking by the phanerozoic tectono thermal events to better understand its late mesozoic tectonics an integrated multidisciplinary investigation has been conducted on the dayunshan mufushan composite batholith in the north central scb this batholith consists of two major intrusions that recorded distinct emplacement features according to our structural analysis two deformation events in relation to batholith emplacement and subsequent exhumation are identified the early one d1 was observed mostly at the southern border of the batholith characterized by a top to the sw ductile shearing in the early stage intrusion and along its contact zone this deformation chiefly associated with the pluton emplacement at ca 150 ma was probably assisted by far field compression from the yangtze foreland belt the second but main event d2 involved two phases 1 ductile shearing d2a prominently expressed along the dayunshan detachment fault at the western border of the batholith where the syntectonic late stage intrusion and minor metasedimentary basement in the footwall suffered mylonitization with top to the nw kinematics 2 subsequent brittle faulting d2b further exhumed the entire batholith that behaved as rift shoulder with half graben basins developed on its both sides geochronological constraints show that the crustal ductile extension occurred during 132 95 ma such a cretaceous nw se extensional tectonic regime as indicated by the d2 event has been recognized in a vast area of east asia this tectonism was responsible not only for the destruction of the north china craton but also for the formation of the so called southeast china basin and range tectonics

cenozoic exhumation history of south china a case study from the xuefeng mt range abstract new apatite fission track aft dating was applied to the xuefeng mt range and yuanma basin to constrain the cenozoic exhumation process of the southeastern yangtze block south china the analyzed samples in this study have aft ages ranging from 27 9 2 5 to 61 5 5 9 ma which are younger than the deposition or crystallization ages of the host rocks the aft analysis and thermal history modeling indicate that both the xuefeng mt range and the yuanma basin underwent significant exhumation during the early cenozoic ca 60 40 ma these samples were rapidly exhumed to near the surface during this period our results suggest that an important tectonic event occurred along or near regional fault zones e g the qinhang fault in south china during the early cenozoic ca 60 40 ma however it is difficult to relate this event to the eastern sichuan fold belt which is much older and is characterized by large scale folding and thrusting combined with fieldwork in the yuanma xupu and xinning basins we refute the cretaceous pan yangtze basin that was proposed to have been separated by the uplifted xuefeng mt range after the late cretaceous the exhumation stage from ca 60 ma to 40 ma was an important period during which plate movements across the eastern asian and pacific regions were reorganized the early cenozoic tectonothermal event in south china can be attributed to a change in the direction and speed of the subduction of the pacific plate beneath the eurasian plate an oligocene miocene cooling event was also recorded in the eastern xuefeng mt range which we tentatively attribute to the activity of dextral faults in this area as a far field effect of the collision between the indian and eurasian plates

tectonics of the middle triassic intracontinental xuefengshan belt south china new insights from structural and chronological constraints on the basal decollement zone in orogenic belts a basal decollement zone often develops at depth to accommodate the shortening due to folding and thrusting of the sedimentary cover in the early mesozoic intracontinental xuefengshan belt of south china such a decollement zone is exposed in the core of anticlines formed by the emplacement of the late orogenic granitic plutons our detailed multi scale structural analysis documents a synmetamorphic ductile deformation in the basal decollement the neoproterozoic pelite and sandstone and the intruding early paleozoic granites were deformed and metamorphosed into mylonites and orthogneiss respectively the metamorphic foliation contains a nw se stretching lineation associated with top to the nw kinematic indicators the ductile shearing of these high strained rocks can be correlated with nw verging folds and thrusts recognized in the neoproterozoic to early triassic sedimentary cover monazite u th pbtot chemical dating and zircon sims u pb dating provide age constraints of the ductile shearing between 243 and 226 ma and late orogenic granite emplacement around 235 215 ma in agreement with recent geochronological data these new results show that the xuefengshan belt is an early mesozoic orogen dominated by the nw directed shearing and thrusting at the southeastern boundary of the xuefengshan belt the chenzhou linwu fault separates the early mesozoic domain to the nw from the early paleozoic domain to the se the tectonic architecture of this belt was possibly originated from the continental underthrusting to the se of the south china block in response to northwest directed subduction of the paleo pacific plate

intraplate deformation in central australia the link between subsidence and fault reactivation abstract central australia has experienced two intraplate orogenic events involving significant north south shortening the late neoproterozoic to early cambrian petermann orogeny and the devonian to carboniferous alice springs orogeny in each event pre existing structures inherited from mesoproterozoic terrain amalgamation were reactivated and basement rocks exhumed from beneath thick sedimentary successions accumulated in the centralian superbasin the pattern of fault reactivation during these events shows a striking similarity to the pattern of subsidence in the overlying basin immediately prior to the petermann orogeny the centralian superbasin was thickest in the vicinity of the musgrave block the region in which deformation was subsequently localised at the same time crustal scale faults elsewhere in central australia that were covered by a relatively thin sheet of sediment remained inactive despite being favourably oriented to accommodate the north south shortening between the petermann and alice springs orogenies subsidence patterns shifted such that fault systems in the arunta block and also those in the southern musgrave block were buried by significant thicknesses of sediment whereas the major structures that were exhumed during the petermann orogeny were not significantly buried during the alice springs orogeny reactivation once again occurred along the most deeply buried faults even in the instances where those faults had remained inactive during the earlier petermann orogeny importantly the major petermann aged structures that were not buried during renewed subsidence remained inactive during the alice springs orogeny the record of reactivation implies that the presence of pre existing crustal scale faults alone was insufficient to localise deformation rather fault reactivation appears to have required a priming process that modulated the strength of the lithosphere on a regional scale the correspondence between the distribution of basement fault reactivation and subsidence patterns during both the petermann and alice springs orogenies implies a link between relatively thick sedimentation and long term lithospheric weakening we show that this link is compatible with the thermal effects of a thick sedimentary blanket in the context of central australia the mechanical impact of basin formation is likely to be enhanced by the presence of regionally elevated heat production in the proterozoic basement

a comprehensive upb smnd rbsr and 40ar39ar geochronological study on guidong granodiorite southeast china records of multiple tectonothermal events in a single pluton a complicated tectonothermal history of the guidong granodiorite from the huanan fold belt has been revealed by using combined dating techniques u pb single zircon and whole rock nd model ages suggest that the granodiorite was derived from an ancient crustal source which was probably composed of a mixture of 1 2 2 4 ga crustal materials with a nd model age of 1 6 ga the granodiorite emplacement age of 427 3 ma is defined by concordant u pb single zircon analysis plagioclase biotite hornblende apatite sphene and whole rock yield an internal sm nd isochron age of 428 12 ma consistent within the analytical errors with the u pb zircon age due to the presence of inherited zircon however the zircon data point deviates from the sm nd isochron four whole rocks and two mineral separates plagioclase and hornblende yield a rb sr isochron age of 422 14 ma which is coincident with zircon u pb and sm nd isochron ages hornblende 40ar 39ar step heating analysis gives a main plateau age of 371 4 1 3 ma which is considerably younger than the crystallization age this age is interpreted as recording a post crystallization folding event which produced the prominent unconformity between pre devonian and middle devonian strata and resulted in the deformation of all major minerals of the granodiorite since most deformed minerals have remained closed to sm nd and rb sr isotopic migration since emplacement of the granodiorite it can be suggested that this folding event took place without significant fluid circulation and contemporaneous metamorphism a low temperature heating step of hornblende yields an apparent 40ar 39ar age of 208 5 2 0 ma which may or may not have a connection to the minimum age of the huanan indosinian orogeny a biotite 40ar 39ar age spectrum yields much younger apparent ages of 145 and 92 ma which are coincident with two important yanshanian magmatic episodes

thermal evolution of the hengshan extensional dome in central south china and its tectonic implications new insights into low angle detachment formation abstract the hengshan massif is an exhumed mid crustal plutonic metamorphic dome formed during cretaceous crustal extension in the jiangnan orogenic belt central south china multiple thermochronometers mica 40ar 39ar apatite fission track and zircon u th he are applied to its footwall along a slip parallel transect to quantify its thermal history and cooling rate and the slip magnitude rate initial geometry and kinematic evolution of the low angle hengshan detachment fault our thermochronological data in conjunction with previous ages indicate that 1 footwall rocks cooled from 700 c to 60 c in less than 60 myr 136 80 ma at variable rates ranging from 50 c myr to 13 c myr 2 the hengshan detachment fault accommodated 8 12 km of total slip at variable slip rates from 0 14 to 1 mm yr during tectonic exhumation 3 the footwall has been tilted 26 50 to the east since slip began indicating that the low angle hengshan detachment fault initiated at a steep dip and was passively rotated to a more gentle orientation during subsequent normal slip this study provides compelling evidence supporting that the low angle detachment fault in the extensional dome can be generated by the reactivation and passive rotation of an initially steep reverse fault during normal slip in addition our thermochronological data constrain the time of extension in the hengshan dome between 136 and 80 ma which implies that the back arc extension within south china associated with the rollback of the paleo pacific slab might have lasted until at least 80 ma

isotopic and paleomagnetic constraints on the mesozoic tectonic evolution of south china in order to better constrain the paleogeographic evolution of south china we measured sm nd and rb sr isotopic compositions for 23 mesozoic granites that crop out throughout the area tightly grouped neodymium depleted mantle model ages 1 4 0 3 ga suggest the region is underlain by relatively homogeneous proterozoic crust and fail to define crustal provinces neither the isotopic nor geologic data suggest that a mesozoic suture exists however granites possessing anomalously high sm 8 ppm and nd 45 ppm concentrations relatively high initial epsilon neodymium 4 to 8 and high but variable initial 87sr 86sr 0 759 to 0 713 form a northeast trending zone that coincides with two prominent mesozoic basins southeast of the zone lie the majority of mesozoic intrusives and upper triassic to lower cretaceous extensional basins found in south china mesozoic paleomagnetic poles are well clustered northwest of the zone pre cretaceous poles southeast of it are discordant with respect to those from the northwest the only recognized tectonostratigraphic terrane in south china lies southeast of the zone the terrane is bordered by a northeast trending sinistral fault that was active in the mesozoic other faults in south china have similar attitudes ages and sense of shear together the observations suggest that the mesozoic tectonic regime in south china consisted of strike slip activity plus concomitant rifting as terranes or fragments of similar crust were transported north along sinistral faults the zone defined by the granites enriched in nd and sm demarcates displaced terranes to the southeast from relatively stable land to the northwest

extensional tectonics within a subduction type orogen the case study of the wugongshan dome jiangxi province southeastern china abstract in southeastern china the wugongshan massif consists of metamorphic rocks and granites generally considered as basement rocks formed during an early paleozoic orogeny our structural and radiometric reappraisal of this massif shows that it results from a mesozoic doming in the metamorphic rocks the foliation defines an e w elliptical shape and bears a n s stretching lineation the axial zone of the dome is occupied by oriented plutons the outer part of the dome consists of devonian quartzite carboniferous permian limestone and triassic jurassic sandstone ductile deformation observed in the permian carbonates and older units develops contrasting kinematics the northern and southern flanks present top to the north and top to the south motions respectively the same divergent pattern of motion is recorded from brittle structures up to the jurassic sandstone the late cretaceous red beds are undeformed in the axial zone coaxial flow indicated by symmetric pressure shadows and quartz fabrics predominates along the n and s flanks of the dome post folial recumbent folds are symmetrically overturned to the n and s respectively but at the two extremities of the dome the fold axes are parallel to the stretching lineation 40 ar 39 ar ages of biotite and muscovite from mylonite and granite suggest a late triassic age for the ductile deformation and an early cretaceous age for the final doming the wugongshan dome is similar to a metamorphic core complex formed in an extensional tectonic regime a model which emphasizes plutonism is discussed from permian to early cretaceous southeastern china was a subduction type orogen the crust of which was thickened by accretion of mantle derived magmas the emplacement of a large amount of mesozoic granitoids in southeastern china promoted thermal softening and gravitational instability that allowed the crust to extend the formation of the wugongshan dome and other metamorphic core complexes appears to be a the direct consequence of plutonic activity and crust rheology

mesozoic tectonic evolution of the southeast china block new insights from basin analysis abstract in order to better understand the mesozoic tectonic evolution of southeast china block secb in short this paper describes geological features of mesozoic basins that are widely distributed in the secb the analyzed data are derived from a regional geological investigation on various mesozoic basins and a recently compiled 1 1 500 000 geological map of mesozoic cenozoic basins two types of basin are distinguished according to their tectonic settings namely the post orogenic basin type i and the intracontinental extensional basin type ii the latter includes the graben and the half graben or faulted depression basins our studies suggest that the formation of these basins connects with the evolution of geotectonics of the secb the post orogenic basin type i was formed in areas from the piedmont to the intraland during the interval from late triassic to early jurassic and the formation of the intracontinental extensional basin type ii connects with an intracontinental crustal thinning setting in the late mesozoic the graben basin was generated during the middle jurassic and is associated with a bimodal volcanic eruption and the half graben or faulted depression basin filled mainly by the rhyolite tuff and sedimentary rocks during early cretaceous is occupied by the late cretaceous paleogene red colored terrestrial clastic rocks we noticed that the modern outcrops of numerous granites and basins occur in a similar level and the mesozoic granitic bodies contact with the adjacent basins by large normal faults suggesting that the modern landforms between granites and basins were yielded by the late crustal movement the modern basin and range framework was settled down in the cretaceous abundant sedimentary structures are found in the various basins from that the deposited environments and paleo currents are concluded during the late triassic early jurassic time the source areas were situated to the north and northeast sides of the outcrop region in this paper we present the study results on one geological and geographical separating unit and two separating fault zones the wuyi orogenic belt is a late mesozoic paleo geographically separating unit the ganjiang fault zone behaves as the western boundary of early cretaceous volcanic rocks and the zhenghe dapu fault zone separates the se china coastal late mesozoic volcanic sedimentary basins and the wuyi orogenic belt finally we discuss the geodynamic mechanisms forming various basins proposing a three stage model of the mesozoic sedimentary evolution

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

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

relict proterozoic basement in the nanling mountains se china and its tectonothermal overprinting 1 the basement beneath the caledonian belt in se china has been overprinted by indosinian and yanshanian events and is poorly known in situ u pb and hf isotope analysis of zircons from granites and surrounding rocks in the nanling mountains area of the fold belt reveal that the lanhe felsic gneiss is an outcrop of mesoproterozoic basement probably with a partly late archean protolith this is consistent with nd model ages of granites in this region and exposures of cathaysia basement in the coastal area of se china in the nanling mountains this basement was reactivated to generate granitoid rocks during the caledonian indosinian and yanshanian tectonothermal events the caledonian overprint is documented in extensive exposures of the zhuguang migmatites 425 470 ma and by previous u pb zircon dating of adjacent granites the jiangnan granite may be of similar age but strongly reworked the indosinian overprint resulted in the formation of granites such as the luxi and xiazhuang bodies 233 246 ma in the guidong complex the yanshanian overprint produced the most widely distributed granites circa 160 ma such as the jiufeng and fogang bodies and parts of the guidong complex hf isotope data show that all of the phanerozoic magmatic rocks were derived from the mesoproterozoic basement and minor archean components without obvious contributions from juvenile sources the presence of zircons with archean age 2 5 2 7 ga in both the lanhe gneiss and devonian sediments indicates that the badu and mayuan groups may not be the oldest basement of the cathaysia block some archean basement circa 2 5 2 7 ga may still be exposed but unrecognized in this region and cathaysia may include archean microcontinental fragments

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

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

lateral variation of aeromagnetic anomaly in south china and its tectonic implications we present a series of images from the aeromagnetic data observed in south china areas and reveal the detailed structures of the south china block by using lateral variation of the aeromagnetic anomalies we characterize individual geological blocks and suggest the deep boundaries of the blocks the yangtze block and jiangnan orogenic belt are characterized by low aeromagnetic anomalies whereas high values of the aeromagnetic anomaly are observed in the cathaysia block the boundary between the yangtze and cathaysia blocks which remains unclear to date can be plotted along the large magnetic gradients the high magnetic anomalies significantly relate to volcanic distributions in south china areas these aeromagnetic observations seem to be useful to investigate the intracontinental tectonic evolution in this region

microtectonics and 39ar 40ar dating of high pressure metamorphic rocks of the south ryukyu arc and their bearings on the pre eocene geodynamic evolution of eastern asia abstract the pre eocene metamorphic rocks found in the yaeyama islands the southern part of the ryukyu arc have disputed relationships with the metamorphic formations of japan and taiwan they are divided into the tomuru and fusaki formations 1 1 the tomuru formation is composed of high pressure hp metamorphic rocks derived from oceanic sediments and the uppermost oceanic crust the first deformation phase is characterized by a synmetamorphic nw se trending stretching lineation l1 kinematic analysis shows that the deformation corresponds to a subhorizontal ductile shear directed along l1 from the northwest to the southeast 2 2 the fusaki formation or chaotic formation is an olistostrome of triassic or lower jurassic radiolarite olistoliths the age of the matrix is assumed to be late jurassic it is slightly schistose and only a s vergent deformation phase is observed it is assumed that the hp schists overthrust the mesozoic olistostrome 39ar 40ar step heating experiments have been carried out on phengite and na amphibole from the tomuru formation phengite and zoned barroisite crossite give well defined plateaus at 225 4 8 ma and 237 6 3 ma respectively they are interpreted as the age of the ductile deformation crossite yields an integrated data of 104 8 8 3 ma interpreted as the age of the thrusting of the hp schists above the chaotic formation the lithostratigraphy petrology microtectonics and 39ar 40ar ages of the hp schists and the chaotic formation are similar to the permian sangun hp schists and the jurassic tanba olistostrome of the inner belt of southwestern japan respectively there the sangun schists also overthrust the tanba olistostrome during the mesozoic orogeny the same tanba like mesozoic olistostrome is also found in taiwan but was intensely deformed and metamorphosed in miocene times the geodynamic collision model proposed to account for the late permian to early triassic orogen of southwestern japan is extended to the south ryukyu and taiwan conversely the mesozoic sanbagawa hp schists and the upper jurassic to lower miocene turbidites and olistostrome sanbosan shimanto zones of southwestern japan are neither exposed in the south ryukyu arc nor in taiwan hypotheses are discussed that call upon tectonic erosion or non deposition to account for this absence

genesis of zixi gold deposit in xuefengshan jiangnan orogen south china age geology and isotopic constraints abstract the medium sized zixi gold deposit is hosted in a meta gabbro diabase dike and located geologically in the xuefengshan belt of the jiangnan orogen in south china gold bearing quartz carbonate sulfide veins and the ore hosting meta gabbro diabase dike are structurally controlled by the ne trending first stage faults which are secondary faults to the regional xupu jingzhou fault metallic minerals at zixi include arsenopyrite and pyrite along with minor chalcopyrite sphalerite galena bournonite and native gold ore related alteration includes silicic carbonate sericite and chlorite based on vein crosscutting relationships and mineral paragenesis the alteration mineralization is divided into three stages 1 arsenopyrite pyrite native gold quartz mineralization 2 polymetallic sulfides native gold chlorite carbonate quartz mineralization and 3 post ore carbonate quartz alteration to constrain any temporal relationship between the gold mineralization and the ore hosting meta gabbro diabase dike we obtained a re os isochron age for the gold bearing arsenopyrite 425 28 ma n 5 mswd 0 47 which is 350 myr younger the meta gabbro diabase zircon u pb age 783 08 0 97 ma n 30 mswd 0 39 this precludes the dike from any direct metallogenetic link with the gold deposit the ore forming fluids have calculated 18oh2o and d values of 3 5 to 9 1 and 68 1 to 55 5 respectively indicating a metamorphic source probably through dehydration breakdown of chlorite sulfur isotope compositions of the gold bearing pyrite 5 8 to 3 6 are distinct from those of the lengjiaxi group metamorphic rocks but marginal overlap with those from the banxi group metamorphic rocks lead isotope compositions of the gold bearing pyrite 206pb 204pb 17 08 to 17 63 207pb 204pb 15 48 15 54 208pb 204pb 37 64 37 94 suggest that the lead was sourced from metamorphic rocks the h o s pb isotopic data indicate that ore forming materials were derived from a metamorphic source however the precise nature of that source is unclear the zixi gold mineralization displays many typical features of orogenic gold deposits such as being structurally controlled and have similar alteration and mineralization styles ore mineral assemblages and h o s stable isotopic features the zixi gold deposit was likely formed during the early paleozoic intracontinental orogeny of the jiangnan orogen

continental subduction and exhumation of uhp rocks structural and geochronological insights from the dabieshan east china in the dabieshan the available models for exhumation of ultrahigh pressure uhp rocks are poorly constrained by structural data a comprehensive structural and kinematic map and a general cross section of the dabieshan including its foreland fold belt and the northern dabieshan domain foziling and luzenguang groups are presented here south dabieshan consists from bottom to top of stacked allochtons 1 an amphibolite facies gneissic unit devoid of uhp rocks interpreted here as the relative autochton 2 an uhp allochton 3 a hp rock unit susong group mostly retrogressed into greenschist facies micaschists 4 a weakly metamorphosed proterozoic slate and sandstone unit and 5 an unmetamorphosed cambrian to early triassic sedimentary sequence unconformably covered by jurassic sandstone all these units exhibit a polyphase ductile deformation characterized by i a nw se lineation with a top to the nw shearing and ii a southward refolding of early ductile fabrics the central dabieshan is a 100 km scale migmatitic dome newly discovered eclogite xenoliths in a cretaceous granitoid dated at 102 ma by the u pb method on titanite demonstrate that migmatization post dates hp uhp metamorphism ductile faults formed in the subsolidus state coeval to migmatization allow us to characterize the structural pattern of doming along the dome margins migmatite is gneissified under post solidus conditions and mylonitic ultramylonitic fabrics commonly develop the north and west boundaries of the central dabieshan metamorphics i e the xiaotian mozitan and macheng faults are ductile normal faults formed before late jurassic early cretaceous a cretaceous reworking is recorded by synkinematic plutons north of the xiaotian mozitan fault the north dabieshan domain consists of metasediments and orthogneiss foziling and luzenguang groups metamorphosed under greenschist to amphibolite facies which never experienced uhp metamorphism a rare n s trending lineation with top to the south shearing is dated at 260 ma by the 40ar 39ar method on muscovite this early structure related to compressional tectonics is reworked by top to the north extensional shear bands the main deformation of the dabieshan consists of a nw se stretching lineation which wraps around the migmatitic dome but exhibits a consistently top to the nw sense of shear the central dabieshan is interpreted as an extensional migmatitic dome bounded by an arched top to the nw detachment fault this structure may account for a part of the uhp rock exhumation however the abundance of amphibolite restites in the central dabieshan migmatites and the scarcity of eclogites found only in a few places argue for an early stage of exhumation and retrogression of uhp rocks before migmatization this event is coeval to the n s extensional structures described in the north dabieshan domain recent radiometric dates suggest that early exhumation and subsequent migmatization occurred in triassic liassic times the main foliation is deformed by north verging recumbent folds coeval to the south verging folds of the south dabieshan domain an intense cretaceous magmatism accounts for thermal resetting of most of the 40ar 39ar dates a lithosphere scale exhumation model involving continental subduction synconvergence extension with inversion of southward thrusts into nw ward normal faults and crustal melting is presented

phanerozoic tectonothermal events of the xuefengshan belt central south china implications from upb age and luhf determinations of granites the xuefengshan belt characterized by large scale fold and thrust structures and widespread granites is a key area to decipher the tectonic evolution of the south china block in this belt two magmatic episodes are recorded by early paleozoic and early mesozoic granites in this paper we carried out precise sims zircon usingle bondpb dating and in situ lusingle bondhf isotopes measurements on these granitic plutons our study indicates that the early paleozoic and the early mesozoic granites are late orogenic products of the early paleozoic orogen and the middle triassic xuefengshan orogen respectively in the xuefengshan belt the early paleozoic event is poorly registered since this area corresponds to the outer zone of the early paleozoic orogen the silurian early devonian granites are late orogenic plutons emplaced after the main tectonic and metamorphic stage in the wuyi baiyun yunkai belt coeval with a widespread subsequent extensional event in eastern and southern south china on the other hand triassic granites are formed in an intracontinental environment with weakly to strongly peraluminous signatures zircon usingle bondpb ages presented here associated with a summary of newly acquired data in the same region suggest that the emplacement of anatectic granites occurred around 225 215 ma not in a rather wide range of ca 245 200 ma in situ zircon ehf t values indicate a crust derived source without a mantle derived input for the two generations of granites combining our data with recent studies we infer that the central area of the south china block has experienced two tectonothermal events the early paleozoic magmatism developed as a result of the collapse of the early paleozoic orogen while these early mesozoic granites can be the late orogenic products of the intracontinental xuefengshan orogen most likely manifesting the far field effect by the

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

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

muscovite 40ar 39ar and in situ sulfur isotope analyses of the slate hosted gutaishan au sb deposit south china implications for possible late triassic magmatic hydrothermal mineralization abstract the gutaishan deposit is a slate hosted au sb deposit in the xiangzhong metallogenic province south china the deposit has proven reserves of ca 9 tonnes t of gold with an average grade of 13 g t and 2 500 t of sb with an average grade of 10 in this study muscovite 40ar 39ar and in situ sulfur isotope analyses of pyrite and arsenopyrite were performed to determine the mineralization age and the plausible source of s and au the 40ar 39 ar dating result of muscovite limits the formation age of the gutaishan deposit to ca 223 6 5 3 ma coeval with the adjacent baimashan granite 223 204 ma the in situ 34 s values of pyrite and arsenopyrite coexisting with native gold lie in a range between 3 7 and 2 1 with a mean value of 0 8 and these values are markedly different from those of sedimentary pyrite 7 0 to 23 3 indicating a contribution from magmatic sulfur a magma derived genetic model may therefore be suitable for the gutaishan deposit integrating the available ages of the au sb and sb au deposits and granites in the xiangzhong metallogenic province we propose that a widespread late triassic magmatic hydrothermal au sb mineralization event occurred

metallogenesis of the hengjiangchong gold deposit in jiangnan orogen south china abstract the hengjiangchong gold deposit is located in northeastern hunan of the central jiangnan orogen south china distribution of auriferous sulfide calcite quartz vein type orebodies are controlled by nw wnw trending ductile shear zones and hosted in the lengjiaxi group gp low grade metamorphic sequences and the hengjiangchong granite ore minerals include mainly pyrite arsenopyrite pyrrhotite chalcopyrite sphalerite galena and native gold whilst the major alteration styles include silicic sericite carbonate and chlorite alteration mineralization can be divided into three stages quartz calcite pyrite arsenopyrite mineralization stage 1 quartz calcite polymetallic sulfide mineralization stage 2 and quartz calcite ore barren alteration stage 3 two types of fluid inclusion fi are present in the auriferous sulfide calcite quartz ore veins co2 bearing c and h2o rich w type petrographic and microthermometric analyses of the fis yielded homogenization temperatures for stage 1 2 and 3 to be 254 377 191 339 and 134 223 c respectively with corresponding salinities of 2 22 10 37 2 23 9 98 and 1 56 4 94 wt naclequiv pressures of stage 1 and 2 mineralization are estimated to be 280 370 and 170 300 mpa respectively 18o and d values are determined to be 9 8 10 1 and 70 2 to 68 7 stage 1 7 4 8 1 and 72 4 to 71 2 stage 2 and 2 7 to 2 9 and 79 1 to 73 0 stage 3 respectively these results indicate that the primary ore forming fluids were derived from a metamorphic source for the auriferous sulfides their 34s values are of 15 4 to 7 5 whilst their 208pb 204pb 207pb 204pb and 206pb 204pb values are of 38 663 44 861 15 637 15 769 and 18 301 20 936 respectively both the stable and radiogenic isotopic data indicate that ore forming fluids and metals were derived from a deeper and higher metamorphic grade source e g underlying metamorphosed rocks fluid immiscibility and fluid rock interactions were likely critical for the gold ore precipitation the hengjiangchong deposit exhibits many features of orogenic gold deposits such as the structural control on orebody distribution alteration and mineralization styles and fi microthermometric and h o s pb isotopic features therefore the hengjiangchong is best classified as an orogenic gold deposit

late permian early triassic orogeny in japan piling up of nappes transverse lineation and continental subduction of the honshu block abstract field surveys in the oga atetsu and yamaguchi areas of southwest japan have been conducted in order to precise the structure of the permian orogen a stack of nappes is recognized comprising from top to bottom 1 the oga nappe which is considered to be a seamount complex 2 hp sangun metamorphics 3 the permian yakuno ophiolite and 4 the permian detrital maizuru group which is interpreted as the sedimentary cover of a continental block called here the honshu block outcropping as the older granite this stack of nappes is overthrust by the paleozoic hida basement consisting of ht gneisses granites and late carboniferous shallow water sediments microtectonic analysis of the sangun schists shows that the subhorizontal schistosity bearing a submeridian lineation was formed during the synmetamorphic phase asymmetric pressure shadows shear bands and sigmoidal minerals show that the synmetamorphic deformation corresponds to a ductile shear from north to south the permian early triassic orogeny is interpreted as the result of a collision between the hida gneiss or south china block and the honshu block the intervening oceanic area gave rise to southward directed nappes the permian orogenic belt extends at least from taiwan to central japan

hf isotopic compositions of the standard zircons and baddeleyites used in u pb geochronology abstract zircon and baddeleyite u pb geochronological dating is widely used in solid earth sciences and the advent of rapid in situ methods of analysis such sims and icp ms has re emphasized the importance of having uniform standards recently it has been shown that hf isotopic data can provide important information on these minerals since they contain high concentrations of hf but have low lu hf ratios which results in negligible age correction however the complex internal structures that result from multiple thermal events such as inherited cores and metamorphic overgrowths require that the hf isotopic data be measured with high spatial resolution however the isobaric interferences of 176 yb and 176 lu on 176 hf hamper the precise determination of the 176 hf 177 hf ratio during in situ laser ablation mc icpms analysis it is shown here that mass biases of yb yb and hf hf change with time during analyses and behave differently for solutions and solid material therefore it is suggested that the mean yb value of the individual spot be used to obtain the precise isotopic composition for in situ zircon and baddeleyite hf isotopic analyses for low yb hf 176 yb 177 hf yb value have little effect on the accuracy of the hf isotopic composition however the appropriate yb isotopic abundance is necessary for high yb hf 176 yb 177 hf 0 001 zircons and baddeleyites since the interference of 176 yb on 176 hf is significant using the mean yb value of the individual spot and newly published yb isotopic abundance data six standard zircons and two standard baddeleyites have been investigated using a neptune mc icpms with 193 nm laser for zircons the obtained 176 hf 177 hf ratios are 0 282307 31 2sd for 91500 0 282680 31 2sd for temora 0 281729 21 2sd for cz3 282177 17 2sd for cn92 1 0 282983 17 2sd for fm0411 and 281234 11 2sd for phalaborwa the baddeleyites from phalaborwa and sk10 2 have 176 hf 177 hf ratios of 0 281238 12 and 0 282738 13 2sd these results agree well with the values obtained by the solution method and indicate that these standards have different hf isotopic compositions in which the extremely low 176 lu 177 hf and 176 yb 177 hf values of cz3 zircon and phalaborwa baddeleyite make them excellent standards for machine calibration during in situ zircon hf isotopic measurement with the other standards being more suitable for the development of the correction method

mirror image thrusting in the south china orogenic belt tectonic evidence from western fujian southeastern china abstract the mesozoic orogenic belt in south china is readily divided into two fold and thrust belts and a central multiply deformed zone of ductile deformation dominated by folds with steeply dipping axial planes southeast directed thrusting in the eastern belt and northwest directed thrusting in the western belt resulted in a mirror image of one another during the mesozoic the eastern fold and thrust belt can be further divided into three domains that overlap one another and exhibit a southeast verging wedge like geometry and kinematics deviatoric stress has been calculated from quartz dislocation density at 91 5 mpa 74 5 mpa and 73 8 mpa decreasing toward the southeast upper devonian and lower carboniferous sandstones have been used to constrain bed length for cross section balancing to estimate crustal shortening from three typical areas these indicate 45 41 6 and 37 3 shortening and suggest that 110 km of contraction has been achieved as a result of thrusting this orogen correlates with the ogcheon belt in south korea to the northeast to the southwest it appears to correlate with a thrust belt in western guangdong this southeast verging thrusting may be at least partially controlled by cathaysia to the east