zircon u pb ages and hf isotope compositions of migmatite from the north dabie terrane in china constraints on partial melting migmatite gneisses are widespread in the dabie orogen but their formation ages are poorly constrained eight samples of migmatite including leucosome melanosome and banded gneiss were selected for u pb dating and hf isotope analysis most metamorphic zircon occurs as overgrowths around inherited igneous cores or as newly grown grains morphological and internal structure features suggest that their growth is associated with partial melting according to the hf isotope ratio relationships between metamorphic zircon and inherited cores three formation mechanisms for metamorphic zircon can be determined which are dissolution reprecipitation of pre existing zircon breakdown of zr bearing phase other than zircon in a closed system and crystallization from externally derived zr bearing melt four samples contain magmatic zircon cores yielding upper intercept u pb ages of 807 35 768 12 ma suggesting that the protoliths of the migmatites are neoproterozoic in age the migmatite zircon yields weighted mean two stage hf model ages of 2513 97 894 54 ma indicating reworking of both juvenile and ancient crustal materials at the time of their protolith formation the metamorphic zircons give u pb ages of 145 2 120 2 ma the oldest age indicates that partial melting commenced prior to 145 ma which also constrains the onset of extensional tectonism in this region to pre 145 ma the youngest age of 120 ma was obtained from an undeformed granitic vein indicating that deformation in this area was complete at this time two major episodes of partial melting were dated at 139 1 and 123 1ma the first episode of partial melting is obviously older than the timing of post collision magmatism corresponding to regional extension the second episode of partial melting is coeval with the widespread post collision magmatism indicating the gravitational collapse and delamination of the orogenic lithospheric keel of the dabie orogen which were possibly triggered by the uprising of the cretaceous mid pacific superplume

u pb trace element and lu hf properties of unique dissolution reprecipitation zircon from uhp eclogite in sw sulu terrane eastern china abstract a unique dissolution reprecipitation zircon has been identified for the first time in a qtz and phe bearing eclogite from the drill hole ccsd mh southwestern sulu uhp terrane by using laser raman and cathodoluminescence cl imaging combined with trace element analyses and u pb and lu hf isotope data relic inherited domains of partially dissolved zircon show magmatic features and preserve mineral inclusions of cpx pl qtz ap whereas newly formed zircons crystallized around or along cracks of the inherited magmatic domains and contain mineral inclusions of coe grt omp phe ap typical for uhp metamorphism in addition primary fluid inclusions were identified in the coesite bearing zircon domains characterized by a single h2o phase or co2 h2o phase indicating the presence of fluids during uhp conditions strongly dissolved inherited magmatic zircons yielded shrimp 206pb 238u ages of 782 588 ma with an upper intercept age of 780 14 ma new recrystallized or metamorphic zircons yielded consistent triassic uhp ages around 238 225 ma with a weighted mean of 229 2 2 3 ma zircon trace element and hf isotope compositions and mineral inclusions classify the protolith of the sulu eclogite as an intermediate to mafic neoproterozoic igneous rock which was subducted to mantle depth and experienced uhp metamorphism in middle triassic times the juvenile ehf 780 ma value of 7 4 2 9 in inherited magmatic zircon points to a significant input from the depleted mantle to the dabie sulu uhp terrane during middle neoproterozoic new uhp zircon domains have low lu hf th u and significantly higher 176hf 177hf ratios than the inherited magmatic zircons consistent with formation during recrystallization and fractionation of the lu hf system during uhp metamorphism

variation in peak p t conditions across the upper contact of the uhp terrane dabieshan china gradational or abrupt the southern dabieshan terrane sdt has previously been divided into high pressure hp and ultrahigh pressure uhp terranes and its regional extent and the tectonic nature of its boundaries are hotly debated topics in this study an eclogite bearing area of 100 km2 near taihu is mapped in detail and divided into northern middle and southern zones on the basis of lithological characteristics the northern zone consists of epidote biotite gneiss and eclogite blocks the middle zone includes granitic gneiss biotite gneiss eclogites and amphibolite and the southern zone is composed mainly of garnet bearing mica schist the eclogites occur mainly as lens or blocks in the northern and middle zones the peak p t conditions for 61 eclogite samples across the area are estimated using the grt cpx fe2 mg thermometers and the grt cpx phe barometers the results indicate three different p t regions 2 82 4 09 gpa 759 942 c in the northern zone and 2 00 3 54 gpa 641 839 c in the granitic gneiss and 1 38 2 36 gpa 535 768 c in the biotite gneiss from the middle zone combined with the spatial distribution of eclogites across the area the p t values for eclogites increase continuously from the south to the north defining a reference geotherm of 5 c km 1 however some unreasonable apparent gradients can be established along two south north profiles across the area and display a p t difference between the northern and middle zones on the basis of the average p t data for eclogites across the area a gap of at least 0 3 gpa 20 c exists between the northern and middle zones by contrast the p t values of eclogites from the middle zone show a coherent pattern with transitional characteristics from hp in the south to uhp in the north we suggest that the sdt was a coherent slab during subduction and was broken up by a major fault during exhumation which was formed under uhp metamorphic conditions

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

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

triassic polyphase deformation in the feidong zhangbaling massif eastern china and its place in the collision between the north china and south china blocks the feidong zhangbaling massif is located between the dabieshan and sulu areas along the tan lu fault in eastern china five tectonic metamorphic events are distinguished there the earliest deformation d1 corresponds to a southward compression that occurred during subduction of the south china block below the north china block top to the south shearing is coeval with late permian early triassic blueschist facies metamorphism and possibly with the development of south verging recumbent folds in the neoproterozoic paleozoic sedimentary cover of the south china foreland the main ductile deformation d2 is an extensional one characterized by top to the north shearing coeval with the early stage of exhumation of the high pressure rocks a top to the nw ductile shearing and microfolds overturned to the northwest belong to a second deformational phase of exhumation d3 which is distinct from the main event d2 previous 40ar 39ar mica dates ranging between 245 and 212 ma suggest late permian early triassic ages for the d1 to d3 events the d4 event produced ne sw trending folds in the sedimentary cover interpreted as gravity collapse structures a late cretaceous brittle extensional event d5 controls the opening and infill of continental half grabens in the study area the tan lu fault is a cretaceous brittle normal fault the lack of ductile deformation presented along the tan lu fault suggests that it did not play a significant role in the exhumation of high pressure metamorphic rocks

biogeomorphology revisited looking towards the future although biogeomorphological research is well established with many studies on a range of two way interrelations between organisms and geomorphology in different environments there is little consensus over what constitutes biogeomorphology why it might be useful and where it is heading starting with definitions of core biogeomorphic processes we consider the need for future biogeomorphological studies to evaluate the crucial links between bioprocesses biological community dynamics and inorganic earth surface processes five key applications of biogeomorphological research are identified the roles of organisms in environmental reconstruction trace fossil analysis extraterrestrial geomorphology environmental engineering and the built environment some key research directions and methodological challenges for future biogeomorphological research include expanding the spatial and temporal coverage of datasets investigating the role of bioprocesses in landform development tackling scale issues investigating the relevance of nonlinear dynamical ideas to biogeomorphology and developing better sampling and monitoring techniques for bioprocesses

pb and nd isotopic composition of the jigongshan granite constraints on crustal structure of tongbaishan in the middle part of the qinling tongbai dabie orogenic belt central china abstract the qinling dabie sulu belt is the world s largest ultrahigh pressure uhp metamorphic belt the uhp metamorphism is well dated at 220 245 ma in the dabie sulu belt but at 507 ma in the qinling belt the tongbaishan is located between the qinling orogenic belt to the west and the dabie sulu uhp metamorphic belt to the east it is the key area for studying the tectonic relation between the qinling and dabie sulu belts and the diachronous uhp metamorphism the jigongshan granitic pluton t 128 ma with a total area of 1200 km 2 composed of monzogranite was mostly emplaced into the tongbai complex an exposed basement in the tongbaishan the jigongshan granites have sio 2 69 85 72 35 k 2 o na 2 o 0 87 1 13 a cnk 0 91 1 03 rb sr 0 14 0 25 and th u 3 3 12 their ree compositions show strongly fractionated patterns with la yb n 14 58 and eu eu 0 79 1 05 the granites are characterized by low radiogenic pb isotopic composition the present day whole rock pb isotopic ratios are 20 6 pb 204 pb 16 707 17 055 207 pb 204 pb 15 239 15 326 and 208 pb 204 pb 37 587 37 853 which are similar to that of the continental lower crust their e nd t values range from 16 to 20 and depleted mantle nd model ages t dm from 1 8 to 2 2 ga the above evidence indicates that the magma of the jigongshan granites was derived from the partial melting of the continental crust the pb and nd isotopic compositions of the jigongshan granites resemble those of the dabie core complex in the dabieshan but are distinct from those of the tongbai complex in the tongbaishan thus the dabie core complex would be the magma source of the jigongshan granites the result implies that the dabie core complex is extended to the west and constitutes the unexposed basement underlaying the tongbai complex in the tongbaishan

the early cretaceous orogen scale dabieshan metamorphic core complex implications for extensional collapse of the triassic hp uhp orogenic belt in east central china the dabieshan massif is famous as a portion of the world s largest hp uhp metamorphic belt in east central china that was built by the triassic north south china collision the central domain of the dabieshan massif is occupied by a huge migmatite cored dome i e the central dabieshan dome cdd origin of this domal structure remains controversial synthesizing previous and our new structural and geochronological data we define the cretaceous dabieshan as an orogen scale metamorphic core complex mcc with a multistage history onset of lithospheric extension in the dabieshan area occurred as early as the commencement of crustal anatexis at the earliest cretaceous ca 145 ma which was followed by primary early stage detachment during 142 130 ma the central dabieshan complex in the footwall and surrounding detachment faults recorded a consistently top to the nw shearing it is thus inferred that the primary detachment was initiated from a flat lying detachment zone at the middle crust level removal of the orogenic root by delamination at ca 130 ma came into the extensional climax and subsequently isostatic rebound resulted in rapid doming along with exhumation of the footwall the mid crustal detachment zone had been warped as shear zones around the cdd after 120 ma the detachment system probably experienced a migration accommodated to the crustal adjustment which led to secondary late stage detachment with localized ductile shearing at ca 110 ma the migmatite gneiss with hp uhp relicts in the cdd i e the central dabieshan complex was product of the cretaceous crustal anatexis that consumed the deep seated part of the hp uhp slices and the underlying para autochthonous basement compared with the contemporaneous mccs widely developed along the eastern margin of the eurasian continent we proposed that occurrence of the dabieshan mcc shares the same tectonic setting as the destruction of the north china craton however geodynamic trigger of the cretaceous continental scale nw se extension in eastern asia is still a mystery

continental subduction intracrustal shortening and coeval upper crustal extension p t evolution of subducted south iberian paleomargin metapelites betics se spain abstract the nevado filabride complex represents the basement and permo triassic cover of the south iberian passive margin metamorphosed during miocene continental subduction under the alboran crustal domain it forms a nappe stack with the thick calar alto unit 6 km thick sandwiched between the ragua unit below and the bedar macael unit above the nappe stacking occurred by ductile flow along basal syn metamorphic shear zones structural analysis of the calar alto unit together with thermobarometric data of samples from both the bottom and top of the unit served us to understand the tectonic significance of this episode of intracrustal shortening in the context of a strongly extended orogen multiequilibrium thermobarometry shows that the calar alto schist underwent a prograde p t path at mantle depths from 1 6 gpa at 450 500 c to 1 0 gpa at 550 570 c during the growth of the schistosity s1 parallel to the compositional layering after detachment from the downgoing slab and underplating the unit underwent nearly isobaric cooling by up to 250 c from peak metamorphic conditions the main crenulation cleavage s2 and the mylonitic foliation sm developed within the crustal nappe stack during decompression and heating sm at the base of the unit grew between 0 8 gpa at 400 c and 0 4 gpa at 500 c while s2 at the top of the unit developed between 0 6 gpa at 300 c and 0 2 gpa at 400 c these shortening structures developed simultaneously to the activity of overlying middle miocene extensional detachments intracrustal shortening and coeval upper crustal extension resulted in a mode of exhumation where the material rise entailed an important horizontal component of displacement at rates of approximately 15 mm year being exhumed along an s shaped p t path after peak metamorphic conditions and its detachment from the subducting slab

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

upb zircon age and geochemical constraints on tectonic evolution of the paleozoic accretionary orogenic system in the tongbai orogen central china abstract the tongbai orogen is located in a key tectonic position linking the qinling orogen to the west and the dabie sulu orogen to the east this orogen comprises a paleozoic accretionary orogenic system in the north and a permo triassic collisional orogenic system in the south hence it may serve as an ideal place to unravel the tectonic evolution from the initial oceanic subduction accretion to the final continental collision between the sino korean and yangtze cratons u pb zircon geochronology of metasedimentary and metaigneous rocks and geochemical characterization of metabasalts indicate a close genetic relationship between the individual tectonic units of the paleozoic accretionary orogenic system 1 the kuanping group as a neoproterozoic ordovician subduction accretion complex which received sediments from the qinling microcontinent 2 the erlangping group and huanggang diorite granodiorite complex as an ordovician silurian island arc evolving from backarc basin 3 the qinling group as a precambrian microcontinent converted into continental arc during the ordovician 4 the guishan complex as a mixture of the qinling continental arc and devonian forearc sediments and 5 the nanwan flysch as a devonian forearc sequence deposited on the newly accreted active margin of the sino korean craton consequently the northern and southern boundaries of the paleozoic accretionary orogenic system represent two sutures of the qinling microcontinent with the sino korean and yangtze cratons the new and previously published data reveal that the tectonic evolution of the tongbai orogen involved a series of events from ordovician early silurian ca 490 440 ma oceanic subduction and arc magmatism silurian early devonian ca 440 400 ma arc continent collision carboniferous ca 340 310 ma oceanic subduction and accretion late permian triassic ca 260 200 ma continental subduction and collision to cretaceous ca 140 90 ma extension and lateral eastward extrusion the entire accretionary orogenic processes until the final collision between the sino korean and yangtze cratons may have lasted more than 200 ma

timing tectonic implications and genesis of gold mineralization in the xincheng gold deposit china c h o isotopes pyrite rb sr and zircon fission track thermochronometry abstract the xincheng gold deposit located at the northwestern edge of the jiaodong peninsula in the north china craton has a reserve of 96 t au and is among the largest in the jiaodong gold province here disseminated and stockwork style ores are hosted in mesozoic granitoids the mineralization and alteration are largely controlled by the regional jiaojia fault the c h o isotope values in the ores indicate that the ore forming fluids were derived mainly from magmatic sources new rb sr geochronological data on hydrothermal pyrite from the xincheng deposit presented in this study indicate that the main gold mineralization occurred between 126 ma and 122 ma the zircon fission track of convergent ages and decomposed divergent ages from the ores and altered rocks show four groups of ages 135 ma 124 ma 96 75 ma and 66 52 ma the 135 ma is related to the exhumation and cooling of the linglong granitoids the 124 ma is considered to represent the principal stage of gold mineralization and is consistent with the pyrite rb sr ages the younger ages of 96 75 ma and 66 52 ma resulted from subsequent annealing caused by later tectonic activity the timing of gold mineralization in the xincheng gold deposit broadly coincides with the switch in stress field in eastern china from extension to transpression resulting from the change in subduction direction of the pacific plate from ne sw to nw se and marked by extension related voluminous magmatism associated with the cretaceous giant igneous event in eastern china

triassic southeastward subduction of north china block to south china block insights from new geological geophysical and geochemical data abstract lithospheric subduction prior to the assembly of the south china and north china blocks is traditionally considered to be directed northward however some critical geological and geochemical data cannot be reconciled with this northward subduction this paper presents new lines of evidence against the traditional models and proposes a new and revolutionary tectonic model to explain the distribution and exhumation of high pressure hp ultrahigh pressure uhp metamorphic rocks of the dabie sulu belt we emphasize the following 1 the triassic tectonic environment of the southern margin of the north china block was passive not active based on the stratigraphy 2 in the southern margin of the north china block no arc magmatism was recorded 3 many paleoproterozoic slices of jiaobei affinity of the jiao liao ji belt in the north china block were located in the triassic sulu orogen 4 many 1 85 ga metamorphic zircons are preserved in the dabie sulu high pressure ultra high pressure hp uhp metamorphic rocks 5 the geometric asymmetry of many structural patterns in the hp uhp slices indicates top to the northwest thrusting during the exhumation of hp uhp slices 6 blueschists occur in the south of the uhp eclogite slices 7 in the eastern segment of the north qinling orogen no components with an affinity of the south china block have been found along the shangdan suture of the qinling orogen has been recorded an apparent northward subduction we consider that the suture is just a lateral subduction zone rather than a major collisional zone along the shangdan suture the rarity of i type plutonism can be attributed to a transform type continental margin the bureya jiamusi khanka block has an affinity to the south china block based on its similarity regarding the paleozoic history of deformation and triassic blueschist metamorphic facies metamorphism the bureya jiamusi khanka block could be the northern extension of the dabie sulu belt and this gigantic belt could be interpreted as an orocline related to the southeastward subduction of the north china block beneath the greater south china block

early cretaceous extensional reworking of the triassic hp uhp metamorphic orogen in eastern china corresponding to the early mesozoic continental subduction between the north china block ncb and the south china block scb the tongbaishan hong an dabieshan sulu massifs are famous for their hp uhp metamorphism more than 50 of the hp uhp orogenic belt was significantly reworked by early cretaceous extensional tectonics this early cretaceous event with a fast cooling period at 130 120 ma superimposed on the early mesozoic hp uhp orogenic belt and intensively changed the architecture of this orogen each individual segment documents different early cretaceous extensional structures namely the central tongbaishan domain is a metamorphic core complex mcc represented by an a type non cylindrical antiform the central dabieshan domain is a typical cordilleran type migmatite cored mcc the southern sulu uhp domain is a wedge shaped structure exhumed by a simple detachment fault these late stage extensional structures expose the previous hp uhp orogenic belt as fragments along the ncb scb boundary the geodynamic setting of this early cretaceous extensional tectonics along the hp uhp orogen is a part of a 1000km scale crustal extension belt that is widespread in eastern eurasia continent from trans baikal to the central part of scb convective erosion or delamination of the mantle lithosphere might be considered as a possible mechanism for mantle removal

crustal structure and exhumation of the dabie shan ultrahigh pressure orogen eastern china from seismic reflection profiling comment and reply comment yuan et al 2003 1 published a deep seismic reflection profile of the dabie shan ultrahigh pressure uhp orogen acquired by the chinese geological survey the seismic profile provides new insight into the crust structures of the orogen and gives us a better understanding of this specific uhp

a multidisciplinary study on the emplacement mechanism of the qingyang jiuhua massif in southeast china and its tectonic bearings part i structural geology ams and paleomagnetism during the cretaceous the south china block scb experienced a widely distributed extensional event including numerous plutons emplacement and basin opening investigations on the tectonic regime coeval with pluton emplacement and emplacement mechanism of the pluton remain relatively rare in the scb in order to address these questions a multidisciplinary approach including field structural and petrographic observations anisotropy magnetic susceptibility ams and paleomagnetic analyses was carried out on the qingyang jiuhua granitic massif which intrudes into the lower yangtze fold belt in the northeastern part of scb the qingyang jiuhua massif is composed of the granodioritic qingyang and monzogranitic jiuhua plutons dated by zircon u pb method at ca 142 ma and ca 131 ma respectively our structural observations show that the intrusion of the qingyang jiuhua massif does not modify the fold strike a weak ductile deformation of the country rocks and granitoid can be only observed in the boundary zone with limited contact metamorphism in the contact aureole of the massif the foliation follows the pluton contour and the mineral lineation is rare when present it exhibits a down dip attitude field and microstructural observations indicate isotropic magmatic textures in most parts of the massif the ams analysis of 93 sites reveals weak values for the anisotropy degree pj 0 for most of the measured samples two principal foliation patterns are identified horizontal foliations in the center of the plutons and vertical foliations on the boundaries magnetic lineation strike is largely scattered and weakly inclined at the scale of the entire massif the paleomagnetic investigations indicate that a the younger jiuhua pluton did not produce a remagnetization in the older qingyang pluton b no relative movement can be observed between these two plutons c the entire massif did not experience any important relative movement with respect to south china considering the paleomagnetic uncertainties integrating the newly obtained results with previous observations our study favors a permissive emplacement mechanism for the two plutons i e vertical magma intrusion into an opening space controlled by the nw se brittle stretching of the upper crust which is in agreement with a weak extensional regional tectonic framework of the scb

where is the north china south china block boundary in eastern china the qinling dabieshan belt results from the collision of the north china and south china blocks the eastern extension of the belt the sulu area consists of several stacked units from top to bottom these are 1 a weakly metamorphosed slate sandstone unit of neoproterozoic paleozoic age 2 a gneiss quartzite unit metamorphosed under high pressure p conditions 3 a marble amphibolite unit 4 an ultrahigh p unit that was very deeply subducted and then exhumed and 5 a migmatitic dome resulting from the melting of ultrahigh p rocks all these units underwent the same deformation which was characterized by a top to the northwest extensional ductile shearing during their exhumation granulite facies restites in the migmatite are not significantly different from granulitized eclogites derived from the ultrahigh p rocks the migmatite exhibits the same structural and petrologic evolution as that determined for the other units the resemblance of petrologic and structural features and the lack of ocean basin rock shows that the boundary between the north china block and south china block must be placed north of the sulu area

zircon u pb dating of water rock interaction during neoproterozoic rift magmatism in south china abstract a combined study of zircon u pb dating and mineral o isotope analysis was carried out for neoproterozoic granite gabbro and gabbrodiorite from the beihuaiyang zone that marks a rifted margin between the south china block and the supercontinent rodinia the results provide precise constraints on timing of water rock interaction resulting in unusual 18 o depletion in igneous minerals cl imaging reveals that zircons mostly show clear oscillatory and or sector zoning with or without thin alteration rims no metamorphic zircon occurs in the igneous intrusives although they suffered greenschist facies metamorphism during triassic continental collision la icpms zircon u pb dating yields consistent ages of 739 8 to 754 5 ma which are interpreted as timing of magma crystallization mineral separates have highly variable 18 o values of 0 6 to 6 8 for zircon 1 2 to 12 5 for quartz and 0 3 to 8 4 for plagioclase oxygen isotope heterogeneities exist within single intrusions and between different intrusions magmatic zircons with low 18 o values and a weighted mean u pb age of 748 6 ma crystallized from low 18 o magmas that were derived from melting of hydrothermally altered low 18 o rocks both low and normal 18 o intrusives in the beihuaiyang zone are shown to form at the same time contemporaneous with the rodinia breakup thus tectonic evolution from supercontinental rift to breakup is proposed to create conditions for alternating processes of high t hydrothermal alteration and low 18 o magmatism such processes are a characteristic feature of high t water rock interaction in volcanic rift margins associated with localized water saturated melting while the occurrence of low 18 o magmas demonstrates penetration of meteoric water into the rift tectonic zone of crustal melting the existence of normal 18 o magmas indicates that their emplacement may serve as a thermal engine to drive the second stage of meteoric water rock interaction resulting in the unusual 18 o depletion in adjacent rocks the concurrent occurrence of both low and normal 18 o magmas appears to indicate highly heterogeneous high t water rock interaction during rift magmatism

crustal structure of the southern dabie ultrahigh pressure orogen and yangtze foreland from deep seismic reflection profiling a new 140 km long seismic reflection profile provides a high resolution crustal scale image of the southern dabieshan high pressure hp metamorphic belt and the yangtze foreland fold and thrust belt the seismic image of the stacked section shows that the southern dabieshan metamorphic terrane and yangtze foreland belt are separated by a large north dipping fault in the foreland the upper crust is dominated by a series of folds and thrusts formed during the collisional stage in the mid triassic it was reworked by crustal extension resulting in the formation of a late jurassic and cretaceous red bed basin the southern dabieshan profile shows stacked crustal slabs developed along the margin of the collisional orogenic belt the moho reflectors at 10 11 s 30 33 km are seismically prominent and segmented by a number of south verging thrusts that were probably developed by foreland directed thrusting of the deeply subducted continental crust during exhumation the seismic reflection profile suggests that structures related to the triassic jurassic subduction and exhumation of the yangtze plate are preserved despite the severe crustal extension superimposed during the late mesozoic and cenozoic

role of partial melting in the evolution of the sulu eastern china ultrahigh pressure terrane strongly deformed potassium feldspar rich dikes are widely distributed in the northern part of the sulu ultrahigh pressure uhp metamorphic terrane eastern china the fact that the crystallization ages of these dikes overlap with the age of peak uhp metamorphic conditions implies the presence of melt during metamorphism sr isotopic ratios of the dikes are compatible with their origin as partial melts of the dominant felsic sulu gneiss partial melting may be the key to solving several unusual features of the sulu and other uhp terranes such as the almost complete lack of mineralogical evidence for uhp conditions and the limited growth of zircon during uhp conditions in the dominant felsic gneiss in addition because partial melting will cause a drastic reduction in the strength of the uhp gneisses the most likely exhumation mechanism is diapiric rise of a low viscosity partially molten mass containing entrained blocks of eclogite and not a thin sheet as usually proposed

recognition of yanshanian magmatic hydrothermal gold and polymetallic gold mineralization in the laowan gold metallogenic belt tongbai mountains new evidence from structural controls geochronology and geochemistry abstract the laowan metallogenic belt in china is an important metallogenic belt within the tongbai orogenic belt and contains the medium sized laowan and shangshanghe gold deposits the small huangzhuyuan lead zinc silver gold deposit and some gold and cu pb occurrences these deposits are hosted in mesoproterozoic plagioclase amphibolite or schist and mica quartz schist the gold ores are mainly quartz veins and veinlets and disseminated altered ores subordinate ore types include massive sulfides and breccias the laowan gold deposit is characterized by three right stepping en echelon fracture controlled alteration zones that dip gently to the south and includes disseminated sheeted and stockwork ores these lodes were formed by the interaction of ore forming fluid with foliated to laminated cataclasite within the transpressional faults the shangshanghe gold deposit is characterized by parallel ore lodes that dip steeply to the north and includes quartz veins and breccias in addition to ores in altered wallrocks these lodes were formed by focusing of fluids into transtensional faults these ore controlling faults displaced early barren quartz veins 10 m horizontally with a dextral sense of motion the ore hosting structures at the laowan and shangshanghe deposits correspond to the p and r type shears of a brittle dextral strike slip fault system respectively which make angles of about 15 and 15 to the laowan and songpa boundary faults the ore controlling fault system post dated formation of a ductile shear zone and peak regional metamorphism this precludes a genetic relationship between hydrothermal mineralization and regional metamorphism and ductile shear deformation these gold deposits are not typical orogenic gold deposits the metallogenic belt displays district scale zoning of mo cu pb zn ag au relative to songpa granite porphyry dike zone suggesting the mineralization may be closely related to the granite porphyry measured 34 s of sulfides and 18 o and d of fluid inclusion waters in auriferous quartz also are consistent with a magmatic source for sulfur and ore fluids the similarity of pb isotope ratios between the ores and yanshanian granitoids suggests a similar source as the age 139 3 ma of granite porphyry obtained by zircon u pb isotope overlaps the mineralization age 138 1 ma zhang et al 2008a the gold and polymetallic metallogenesis of the laowan gold belt has close spatial temporal and possibly genetic relationships with yanshanian high level magmatism

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

a multidisciplinary study of the emplacement mechanism of the qingyang jiuhua massif in southeast china and its tectonic bearings part ii amphibole geobarometry and gravity modeling abstract the late mesozoic geology of the lower yangtze area is characterized by extensional sedimentary basins numerous granitic plutons and several world class ore deposits regionaly the coeval tectonic and geodynamic framework remains inadequately investigated in order to provide a more comprehensive understanding of the regional tectonic context we performed a multidisciplinary study of the emplacement mechanism of the qingyang jiuhua massif it consists of the granodioritic qingyang and the monzogranitic jiuhua plutons u pb dated at 142 1 0 ma and 131 2 6 ma respectively biotite and amphibole cooling ages are 5 8 myr younger results deduced from field structural observation petrographic and magnetic fabrics paleomagnetism show that this massif was probably vertically emplaced by permissive intrusion coeval with weak regional extension however detailed information on the characteristics of this tectonic event and its emplacement depth was not documented therefore we carried out an investigation of amphibole geobarometry and gravity modeling in order to address the emplacement mode of the plutons within their regional tectonic framework amphibole data show that the qingyang jiuhua massif was emplaced in the upper crust at a depth between 5 and 12 km gravity modeling indicates that the massif is laccolithic it is characterized by several ne sw striking linear thickening zones that we interpret as the feeder roots of the massif these results led us to conclude that 1 the qingyang jiuhua massif was fed by vertical ne sw striking tension gashes 2 consequently the late early cretaceous tectonics in the study area was characterized by nw se extension

diverse p t paths of the northern dabie complex in central china and its reworking in the early cretaceous abstract the northern dabie complex ndc in the dabie orogen central china is a high temperature migmatite terrane it is composed mainly of banded gneiss migmatite and minor amphibolite granulite retro eclogite marble and spinel peridotite and intruded by the early cretaceous massive granitoid plutons and subordinate mafic ultramafic rocks whether or not the ndc experienced a high pressure or ultrahigh pressure hp uhp metamorphic history has been a long standing issue however in the last 10 years an increasing number of retrograde uhp eclogite relics and marbles and hp granulites have been identified throughout the ndc this together with the discovery of microdiamond preserved as inclusions in zircons within eclogite relics and gneisses and numerous age dates of 220 240 ma and 130 ma from the ndc supports the notion that at least part of the ndc was subjected to triassic continental subduction and hp uhp metamorphism the youngest ages being related to an intense overprinting of ultrahigh temperature uht granulite facies metamorphism in the early cretaceous distinct peak uhp conditions 2 5 gpa and 800 c 3 5 gpa and 750 800 c 4 gpa and 900 c or 750 c have been inferred from oriented qtz or ab kfs qtz needles in ca na clinopyroxene of retrograde eclogite ti chu bearing assemblage preserved in marble rod like cpx rt ap inclusion in garnet of retrograde eclogite and microdiamond inclusions in zircon of tonalitic gneiss and retrograde eclogite from the ndc the overprinted uht 900 c or 1000 c metamorphism is indicated by osm growth in garnet pgt exsolution in clinopyroxene of retrograde eclogite and spl crd qtz symplectite in felsic granulites diverse p t paths of an overall clockwise evolution are suggested with post peak decompression or heating followed by decompression cooling and consist of two distinct paths opx pl inclusions in garnet and zircon in retrograde eclogite relics indicate that the lower continental crust in the ndc underwent triassic subduction the ndc shows overprinted uht granulite facies metamorphism thus consists of two metamorphic events in contrast to the single event of the central dabie uhp eclogite belt cdb distinct peak p t conditions may be attributed to the lack of suitable mineral assemblages for geobarometry the overall clockwise p t paths consist of a peak hp uhp event with overprinted uht event suggesting that the ndc experienced triassic subduction and intense intracontinental reworking in the early cretaceous the latter stage may be associated with a heat pulse due to asthenospheric upwelling and lithospheric thinning as a result of cretaceous mantle superwelling and the westward subduction of the pacific plate

orogen scale l tectonite domain in the tongbai orogenic belt central china geological setting and origin abstract l tectonite is well developed and widely distributed in the tongbai orogenic belt in central china the orogenic belt as a whole has an antiformal geometry and the hinge of the antiform is subhorizontal and trends nw se the l tectonite occurs in the core of the antiform in a zone that is 10 30 km wide and over 100 km long lineations in the l tectonite are sub horizontal parallel to the hinge of the antiform sheath folds are also well developed associated with the l tectonite with the hinges parallel to the lineations migmatite occurs in the core and structurally below the l tectonite and has a gradational relationship with the l tectonite the domain of l tectonite is bounded by three ductile shear zones on the north at the top and on the south respectively well developed shear sense indicators indicate that the southern the overlying and the northern shear zones have a dextral top to nw and sinistral sense of shear respectively these geometrical and kinematic data indicate that the three shear zones are likely part of a single shear zone that wraps around the l tectonite domain the l tectonite zone in the core moves southeast relative to the hanging wall the development of the tectonite is interpreted to be a result of this special geometry and kinematics and reflects a post collisional orogen parallel extension synchronous with migmatization and the continuing convergence between the yangtze block and the north china block in the early cretaceous

argon redistribution during a metamorphic cycle consequences for determining cooling rates 40ar 39ar thermochronology is commonly used to constrain the rates and times of cooling in exhumed metamorphic terranes with ages usually linked to temperature via dodson s closure temperature tc formulation whilst many metamorphic 40ar 39ar data are consistent with the timing of crystallisation or cooling within a chronological framework defined by other higher temperature chronometers other 40ar 39ar data are more difficult to interpret we report white mica and biotite single grain fusion and laser ablation 40ar 39ar ages from felsic gneisses from the western gneiss region norway the rocks record isothermal decompression from peak eclogite facies conditions white mica stable to amphibolite facies conditions biotite stable at c 700 c white mica and biotite yield dispersed single grain fusion dates from 416 to 373 ma and 437 to 360 ma respectively in situ laser ablation analyses provide a similar range with white mica spot ages ranging from 424 to 370 ma and biotite spot ages ranging from 437 to 370 ma the dates span the duration of the metamorphic cycle suggested by previous studies and cannot be reconciled with the results of simple models of ar loss by diffusion during cooling samples that show evidence for different physical processes such as the chemical breakdown of white mica partial melting and fluid ingress generated different age populations to samples that did not experience or record obvious petrological evidence for these processes samples that record significant recrystallization and deformation yielded younger white mica but older biotite single grain fusion ages than more pristine samples amphibolite facies gneisses that preserve evidence for significant partial melting generated younger biotite ages than samples that recorded evidence for significant hydration our data support other reported observations that high temperature metamorphic mica 40ar 39ar dates cannot be assumed to record the timing of cooling through a specific temperature window careful assessment of the petrographic context of the dated minerals and consideration of their post crystallisation history may provide a more robust insight into whether age links to stage in a temporally meaningful way

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

late mesozoic compressional to extensional tectonics in the yiwulushan massif ne china and its bearing on the evolution of the yinshan yanshan orogenic belt part i structural analyses and geochronological constraints abstract with a cratonic nucleus the north china craton ncc experienced a complex tectonic evolution with multiphase compressional and extensional events during mesozoic times along the northern part of the ncc the yinshan yanshan fold and thrust belt was a typical intraplate orogen jurassic and cretaceous continental sedimentation magmatism widespread intraplate characterize the yinshan yanshan orogenic belt the geodynamic significance of these tectonic events is still in dispute in the western part of the liaoning province the yiwulushan massif crops out at the eastern end of the yinshan yanshan orogenic belt the yiwulushan massif presents an elliptical domal shape with a ne sw striking long axis the structural evolution of this massif brings new insights for the understanding of the mesozoic plutonic tectonic history of the ncc a multidisciplinary study involving structural geology geochronology anisotropy of magnetic susceptibility ams and gravity modeling have been carried out the presentation of the new results splits into two parts part i this paper deals with field and laboratory structural observations and presents the main geochronological results the ams gravity modeling data will be provided in a companion paper part ii the early compressional deformation d1 corresponds to a late jurassic to early cretaceous southward thrusting the subsequent deformation is related to the early cretaceous exhumation of the yiwulushan massif a detailed structural analysis allows us to distinguish several deformation events d2 d3 and d4 the cretaceous extensional structures such as syntectonic plutons bounded by ductile normal faults metamorphic core complexes and half graben basins are recognized in many places in east asia these new data from the yiwulushan massif constitute a link between transbaikalia mongolia north china and south china indicating that nw se extensional mesozoic tectonics occurred throughout the entire region

cretaceous cenozoic tectonic history of the jiaojia fault and gold mineralization in the jiaodong peninsula china constraints from zircon u pb illite k ar and apatite fission track thermochronometry the jiaojia fault jjf in the jiaodong area of eastern china is an important nne trending structure that is subsidiary to the regional tancheng lujiang tan lu fault zone and hosts 1200 t of gold reserves contained in disseminated and stockwork ore dominantly in the footwall of the fault we present new zircon u pb apatite fission track and illite k ar data along the jjf and have delineated its tectonic history focusing on its formation and reactivation zircon u pb dating shows that the shangzhuang granite is a composite body with ages between 132 1 and 127 1 ma illite k ar ages for the fault s gouge range from 83 2 to 68 2 ma and the measured apatite fission track ages for ores are between 55 and 21 ma previous zircon u pb geochronology and structural studies suggest that the jjf was originally activated in the jurassic during 160 150 ma as a sinistral fault the jjf was a normal fault in the early cretaceous due to nw se orientated tension and ne sw compression which lasted from 135 to 120 ma this was followed by sinistral strike slip faulting due to nw se compression and ne sw tension during 120 110 ma and it changed to normal displacement at ca 110 ma our apatite fission track data analysis and thermal modeling of representative samples suggest that there was a subsequent dextral reactivation of the fault at ca 55 ma previous age data of ca 130 110 ma for gold mineralization along the jjf coincides with the early cretaceous magmatism and is coeval with the transition from normal faulting to sinistral strike slip faulting of the jjf in early cretaceous which is interpreted to be due to changing direction of the subducting pacific plate

consequences of progressive eclogitization on crustal exhumation a mechanical study summary the very low water content of the granulitic unit of holsnoy in the bergen arcs norway caused its partial metastable preservation throughout caledonian burial end exhumation leading to the observed mixture of completely eclogitized and uneclogitized rocks the eclogitization of the granulite facies protolith led to a density increase 10 but also to a large rheological weakening that resulted in the localization of all deep ductile deformation in the eclogite fraction we address the consequences on subduction dynamics of the concomitant evolution of large scale density and rheology during progressive eclogitization of crust by comparing the behaviour of buried crust with various properties in the channel flow model large scale buoyancy and density are integrated in a single adimensional parameter the exhumation number which describes the capacity of the crust to exhume for a crust whose eclogitization causes a very large viscosity decrease burial resistant end member goes through a maximum in the partially eclogitized zone that is there is a given proportion of eclogite for which the crust is strongly weakened but still significantly lighter than surrounding mantle for this partially eclogitized crust with max the maximum possible downward flow is very low this zone acts as a bottleneck and all incoming crust in excess is forced backwards and starts exhuming this return flow zone is bounded downwards in the partially eclogitized zone by max while its upper limit propagates upwards at a constant rate the curve eclogite fraction controls the maximum proportion of eclogite of the crust that can be exhumed we also demonstrate that large exhumation rates reached by certain uhp units are incompatible with any stationary flow regime whatever the nature of subducted crust the additional presence of a weak layer on top of the channel favours burial for narrow channels and exhumation for large ones but does not qualitatively change the dynamics of the subduction channel this study shows that deep crustal circulation in subduction zones and exhumation from large depths are controlled by the evolution of large scale crustal properties with progressive eclogitization

structural and stratigraphical constraints on the kinematics history of the southern tan lu fault zone during the mesozoic anhui province china abstract the tan lu fault zone tlfz extends in a nne ssw direction for more than 2000 km in eastern china it has been considered either as a major sinistral strike slip fault as a suture zone or as a normal fault we have conducted a structural analysis of the southern segment of this fault zone stlfz in the anhui province the ages triassic to palaeocene of the formations affected by the faults have been re appraised taking into account recent stratigraphical studies to better constraint the ages of the successive stages of the kinematics of the stlfz subsequently the kinematics of the faults is presented in terms of strain stress fields by inversion of the striated fault set data finally the data are discussed in the light of the results obtained by previous workers we propose the following history of the stlfz kinematics during the mesozoic at the time of collision a nne orientated tan lu margin probably connected two margins located north of the dabie and sulu collision belts during the middle late triassic the scb has been obliquely subducted below the ncb along this margin which has acted as a compressional transfer zone between the dabie and sulu continental subduction zones the stlfz has been initiated during the early jurassic and has acted as a sinistral transform fault during the jurassic following which the ncb scb collision stopped a nw trending extension related to metamorphic domes was active during the basal early cretaceous 135 130 ma it has been followed by a nw se compression and a ne sw tension during the middle late early cretaceous 127 to 105 ma possibly 95 ma at that time the tlfz was a sinistral transcurrent fault within the eastern part of the asian continent during the late cretaceous palaeocene the stlfz was a normal fault zone under a wnw ese tension

strain analysis of the xuefengshan belt south china from internal strain variation to formation of the orogenic curvature abstract the early mesozoic xuefengshan belt located in the central part of the south china block east asia developed as an intracontinental belt during the middle triassic analyses of structural and strain patterns in neoproterozoic conglomerates constrain the internal deformation and three dimensional kinematic evolution of this fold and thrust belt finite strain data rxz range from 1 1 to 3 2 and generally increase eastwards towards the deep units of the orogenic core our results also show that at basal decollement zone the d1 schistosity forming deformation has a gradient of stronger fabrics higher strain ratios and a dominant thrust normal flattening mechanism high tectonic levels exhibit lower strain ratios and later structural overprinting that locally modified the d1 strain patterns parallel to the belt enhanced strain and tectonic uplifting along the northern and southern sections of the belt indicate an along strike structural variation that corresponds to the formation of the orogenic curvature during the thrusting propagation large igneous intrusions escaped deformation due to their stiffness and facilitated strain accumulation on the peripheries whereas stress in the central part migrated progressively and formed a tectonic salient towards the west without significant strain localization

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

meso cenozoic uplift of the taihang mountains north china evidence from zircon and apatite thermochronology the taihangshan mountain belt in the central north china craton represents an important crustal and tectonic boundary to explore the complex tectonic evolution of this area during the mesozoic cenozoic we gathered zircon and apatite u th he thermochronology data along a vertical transect elevation of 630 1584 m of the northern part of the taihang mountain belt from our data we observed three separate rapid cooling phases that occurred at 100 ma 50 40 ma and 27 ma combined with previously published geochronological ages we suggest that the uplift of the taihang mountain belt initiated during the jurassic and experienced multiphase rapid uplift from the cretaceous to the cenozoic the early cretaceous rapid cooling uplifting events are widespread in the north china craton and are caused by the collision between the okhotomorsk block and the east asian continental margin the eocene and oligocene rapid cooling events correspond to the initial rifting and thermal subsidence of the bohai bay basin indicating a coupling between the creation of basins and mountains

collision and rotation of the south china block and their role in the formation and exhumation of ultrahigh pressure rocks in the dabie shan orogen terra nova 24 339 350 2012 abstract studies of regional plate scale processes that may influence the exhumation of ultra high pressure uhp metamorphic rocks are few despite their potential importance here we review and summarize available age constraints along with the qinling dabie shan orogen which separates the north china block ncb from the south china block scb we find that collision related events in the west and extension related events in the east where the large dabie shan uhp terrane is located were contemporaneous available palaeomagnetic data indicate clockwise rotation of the scb relative to the ncb during this time previous work indicated that the collision propagated from east to west where indentation of the scb into the qinling dabie shan orogen was active in the hannan dome area at 165 3 ma as constrained by new 40ar 39ar dates on synkinematic sericite in an indentation related mylonitic shear zone while this contraction was occurring in the west exhumation of the uhp terrane was occurring in the dabie shan in the east we propose that the collision of the scb with the ncb occurred not only in a scissor like fashion as previously suggested but also involved a later rotation about a pole west of the dabie shan during the closure of the mianlue ocean as a consequence of this rotation extension occurred between the south china block and north china block east of the pole of rotation leading to extensional exhumation of the dabie shan orogen this microcontinent scale rotation of the scb may have provided an important component in the exhumation of the largest uhp terrane in the world

mesozoic intracontinental underthrust in the se margin of the north china block insights from the xu huai thrust and fold belt the xu huai thrust and fold belt located in the southeastern margin of the north china block consists mainly of thrust and folded pre mesozoic strata its geodynamic evolution and tectonic setting are topics of long debate this paper provides new evidence from geological mapping structural analysis and making balance cross sections with restoration of cross sections results suggest that this belt was subjected to two phase deformation including an early phase regional scale nw ward thrust and fold and a late phase extension followed by the emplacement of dioritic monzodioritic porphyrites dated at 131 135 ma and locally strike slip shearing according to the mapping field observations and drill hole data three structural units were distinguished namely 1 the pre neoproterozoic crystalline basement in the eastern segment 2 the nappe unit or the thrust and fold zone in the central segment which is composed of neoproterozoic to ordovician carbonate rocks and carboniferous permian coal bearing rocks about 2600 m thick and 3 the western frontal zone a major decollement fault has also been identified in the base of the nappe unit on which dozen meter to km scale thrust and fold bodies were commonly developed all pre mesozoic depositional sequences were involved into a widespread thrust and fold event six uncompetent rock layers with biostratigraphic ages nanjing university 1996 have been recognized and each uncompetent rock layer occurred mainly in the top of the footwall playing an important role in the development of the xu huai thrust and fold belt geometry of the major decollement fault suggests that the nappe unit of this belt was rooted in its eastern side near the tan lu fault zone two geological cross sections were chosen for structural balancing and restoration from the balanced cross sections ramp flat and imbricated faults as well as fault related folds were identified a shortening of 20 6 29 6 km was obtained from restoration of balanced sections corresponding to a shortening rate of 43 6 46 4 this shortening deformation was likely related to the se ward intracontinental underthrust of the north china block beneath the south china block during the mesozoic

compression to extension switch during the middle triassic orogeny of eastern china the case study of the jiulingshan massif in the southern foreland of the dabieshan abstract the jiulingshan massif is an e w trending anticlinorium in the south china block scb that forms the southern foreland of the qinling dabie orogen the jiulingshan consists of middle proterozoic ca 1 ga 800 ma metamorphic and plutonic rocks unconformably overlain by sinian to triassic rocks in these cover rocks two episodes of deformation of pre late triassic age are recognized namely a n s compressional phase characterized by south verging thrusts upright folds and vertical cleavage followed by an extensional phase characterized by layer parallel slip collapse folds and extensional allochthons of proterozoic slate on top of carboniferous to middle triassic carbonates the extensional tectonics appears to be a mechanical consequence of the compressional tectonics on a regional scale the jiulingshan massif forms one of the domed crustal scale shear zones of the scb that accommodated the continuing convergence that immediately followed the qinling dabie collision

variable crustal structure of strike slip fault zones as observed on deep seismic reflection profiles seismic reflection profiling across strike slip faults has revealed a distinct variation in deep crustal structure profiles across the san andreas great glen and bray faults depict a near vertical reflection free zone that appears to truncate and in some cases offset reflections interpreted to have originated from the base of the crust a near vertical crustal penetrating fault zone is suggested for these continental transform faults in contrast reflection profiles across intra plate strike slip fault systems such as the northern walker lane and garlock faults suggest that they are decoupled in the middle crust by subhorizontal detachments these detachments are inferred from reflections that are traceable below the surface faults without disruption such detachments in the middle crust may act to ease the rotation of upper crustal blocks adjacent to strike slip fault zones as observed in paleomagnetic studies furthermore moderately dipping reflections interpreted to be from the fault zone project to the surface traces of the northern walker lane faults and the frontal faults of the wichita uplift indicating that these strike slip faults with large components of dip slip displacement are not vertical but dip moderately into the mid crust although the available data sets are still few they suggest that there is a primary variation in fault geometry that is related to the different behavior of transform versus intra plate strike slip fault systems

plastic flow of coesite eclogite in a deep continent subduction regime microstructures deformation mechanisms and rheologic implications abstract ultrahigh pressure uhp metamorphic rocks are continental crust that once has been subducted to mantle depth and subsequently exhumed to the surface in collisional orogenic belts despite the long distance transport of the uhp rocks in a deep subduction channel and the fast strain rate required for rapid exhumation little evidence for uhp deformation is found in the exhumed uhp terrains so far identified some uhp rocks have even preserved primary igneous and volcanic structures these observations have led to the postulation that the stress level is low a few mpa in the deep subduction zones deformation microstructures of some eclogite mylonites from a ductile shear complex in the sulu uhp metamorphic belt eastern china provide for the first time convincing evidence that dislocation creep is the predominant deformation mechanism for the major constituent minerals of the uhp rocks in a highly strain localized shear zone at the uhp conditions omphacite phengite and kyanite are the main strain accommodating phases and are dynamically recrystallized through progressive subgrain rotation and grain boundary migration coesite is either fractured with irregular extinction in the moderately deformed eclogite or pseudomorphed by polycrystalline quartz aggregates that have a foam structure garnet does not show optically discernable plastic deformation p t conditions for the deformation are estimated to be p 3 3 gpa and t 624 c using garnet omphacite kyanite phengite coesite quartz thermobarometry suggesting that deeply subducted continental crust remains cold at the upper mantle depths large part of the deeply subducted crust remains relatively undeformed strain is strongly localized into narrow shear zones stress level in the deep subduction channel is extrapolated on the order of tens of mpa much higher than it was previously thought

tectonic setting of the low grade metamorphic rocks of the dabie orogen central eastern china abstract the tectonic setting on both the northern and southern sides of the dabie mountains reveals that low grade metamorphic rocks are important constituents produced by the subduction of the oceanic crust prior to collision between the sino korean and yangtze cratons the zhangbaling group mulanshan schist is a pre ordovician oceanic crust the sujiahe and xinyang foziling groups are trench sediments of the ordovician devonian age and constitute an accretionary prism associated with subduction the yangshan coal measures meishan group was a forearc basin sediment of carboniferous age and was overthrust by the accretionary prism during collision the susong group is composed of passive continental margin sediments of the yangtze craton backarc basin sediments are postulated to be concealed by mesozoic cenozoic sediments to the north of the dabie mountains high ultrahigh pressure terrains are exotic tectonic slices exhumed from depths located between low grade metamorphic rocks and disturb the integrity of the earlier subduction orogen subduction occurred during the ordovician to devonian periods and collision initiated at the beginning of the permian

dating of subduction and differential exhumation of uhp rocks from the central dabie complex e china constraints from microfabrics rb sr and u pb isotope systems abstract the correlation of deformation fabrics and metamorphic reactions with geochronologic data of uhp metamorphic rocks demonstrate that the multistage subduction and exhumation history of the central dabie complex requires rapid subduction and rapid initial exhumation moreover these data show that volume diffusion is not the major resetting mechanism of radiogenic isotope systems thus our age data do not simply reflect a thermal cooling history in the investigated section the maximum age for uhp is given by the 244 3 ma 2 u pb age of a pre uhp titanite phenocryst that survived uhp metamorphism and subsequent tectonometamorphic events a minimum age for uhp is set by the 238 1 ma 2 238 u 206 pb mineral isochron age of titanite and cogenetic epidote these minerals formed from local partial melts during ascent and their age suggests fast exhumation and emplacement in the middle crust in the period of ca 238 218 ma the uhp terrain records ht metamorphism local partial melting and extensive pervasive strain below the eclogite jd grt stability field exhumation was polyphase with a first phase of fast exhumation succeeded by episodes of ht metamorphism and concomitant deformation at deep mid crustal level between 238 and 218 ma slow exhumation related to the final emplacement of tectonic units along greenschist facies shear zones did not cease before ca 209 204 ma the resetting and homogenization of radiogenic isotope systems were aided by dissolution precipitation creep which was the dominant deformation mechanism in quartz feldspar rocks in combination with fluid influx

structural and geochronological evidence for early cretaceous orogen parallel extension of the ductile lithosphere in the northern dabie orogenic belt east china abstract the wnw trending north dabie dome ndd in the northern dabie orogenic belt is bounded by the xiaotian mozitan ductile shear zone xmsz in the north and the wuhe shuihou shear zone wssz in the south fabric geometries and kinematics of both the xmsz and the wssz are similar they both show a top to nw or wnw sense of shear and display similar microstructures which indicate that the deformation temperature was about 600 650 c 40 ar 39 ar dating of hornblende and biotite from the mylonites of the xmsz suggests that the shear zone was developed prior to 142 ma u pb zircon la icpms dating of undeformed granite dikes that cut the xmsz suggests that the ductile shearing occurred before 130 ma the wssz was also developed in the early cretaceous the interior of the ndd is dominated by top to nw or wnw sense of shear in the migmatitic gneiss and deformed plutons of the earliest early cretaceous ages we suggest that the xmsz the wssz and the interior of the ndd were involved in the same early cretaceous deformation the xmsz and the wssz were originally a single connected more flat lying zone which we call the east dabie detachment zone eddz it developed at a crustal level of over 18 km in depth kinematics of the eddz suggest that the ductile crust and possibly part of the lithospheric mantle in the eastern dabie orogen underwent pervasive orogen parallel and ese ward extension at the beginning of the early cretaceous large scale magmatic intrusions following this deformation and the resultant increase in buoyancy led to the exhumation of the ndd and the warping of the eddz erosion separated the eddz into two zones the xmsz and the wssz as observed today many metamorphic core complexes of early cretaceous were developed in the eastern north china craton most showing top to nw or wnw sense of shear we suggest that such a widespread uniform shear sense reflects a uniform flow direction of the ductile lithosphere in the east china that has also been documented in the liaonan yiwulushan and yumenshan metamorphic core complexes in the northeastern part of the north china craton

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

mesozoic evolution of the hefei basin in eastern china sedimentary response to deformations in the adjacent dabieshan and along the tanlu fault this paper presents a study of jurassic early cretaceous sedimentary evolution of the hefei basin in eastern china and explores the relationship between clastic sedimentation and coeval deformation of the dabieshan to the south and the tanlu fault to the east the hefei basin experienced a two stage evolution the basin was initiated in the early jurassic and expanded in the middle and late jurassic the synsedimentary jinzhai normal fault is considered to be a border fault of the basin because it controlled middle to upper jurassic proximal alluvial deposits persistent n to ne directed paleo flows in the southern hefei basin indicate that sediments came from the dabieshan and the presence of triassic coesite bearing detrital zircon in lower jurassic sediments documents exhumation of ultrahigh pressure rocks of the dabieshan to the surface as early as the early jurassic occurrences of eclogite clasts in an upper jurassic unit indicate continued denudation of the dabieshan at that time thickening of jurassic clastic units to the southern and southeast parts of the basin suggests that basin subsidence and depositional loci were under the coupled control of the jinzhai normal fault on the south and the ne striking left lateral transtensional tanlu fault on the east jurassic extensional subsidence of the hefei basin is in marked contrast to the coeval development of a contractional foreland basin south of the dabieshan which combines to indicate contemporaneous extension and contraction on the north and south sides of the dabieshan respectively vigorous volcanism and uplift of the southern hefei basin characterized the second stage of development of the hefei basin in the early cretaceous and this led to a synchronous shift of its main depocenter to the north this younger depocenter is characterized by lacustrine and fluvial deltaic sedimentation where alluvial and fan deltaic coarse grained deposition mainly occurred along the eastern edge of the basin early cretaceous subsidence is attributed to e w extension across the middle segment of the tanlu fault and the zhangbaling massif on the east acted as a footwall and provided a source for sediment to the northern basin a model is accordingly advanced to account for how the hefei basin developed in response to the tectonic exhumation of the dabieshan and the deformation of the tanlu fault in the mesozoic it illustrates that the hefei basin initiated and evolved during jurassic time in an extensional setting that was triggered by southerly upward extrusion of ultra high pressure rocks of the dabieshan early cretaceous development of the basin was controlled by magmatism related uplift of the dabieshan on the south and orthogonal normal faulting of the middle segment of the tanlu fault on the east this study provides an independent constraint upon the exhumation processes of ultrahigh pressure rocks of the dabieshan

kinematic and thermochronological constraints on the xincheng huangpi fault and mesozoic two phase extrusion of the tongbai dabie orogen belt abstract the tongbai dabie orogenic belt dob is composed of several fault bounded terranes with different metamorphic and deformation intensities these tectonically stacked terranes were extruded along their boundary faults from depths of 120 km during the mesozoic systematic structural studies on these boundary faults can provide important information for the exhumation processes of the orogen in this paper we focus on the xincheng huangpi fault xhf which is located along the southern boundary of the central dob ultrahigh pressure complex unit uc comprehensive macro and micro structural u pb zircon and 40 ar 39 ar geochronological data are presented here to constraint the structural chronology of the xhf the xhf represents the lower detachment surface formed during extrusion of the uhp slab and experienced two episodes of ductile shearing the xhf experienced early southwestward thrusting around 234 ma and was exhumed to middle crustal levels during 234 195 ma with an average exhumation rates of ca 1 5 mm yr the uhp rocks then exhumed to the surface by erosion later nw se trending dextral strike slip shearing of the xhf occurred between 145 and 140 ma and was generally contemporaraneous with sinistral oblique slip of the xiaotian mozitan fault along the northern margin of the uc coeval dextral and sinistral oblique shearing along the southern and northern margins of the uc would have caused southeastern lateral extrusion this lateral extrusion has no relationship with the uhp exhumation

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

zircon u pb age and trace element evidence for paleoproterozoic granulite facies metamorphism and archean crustal rocks in the dabie orogen abstract zircon u pb ages and trace elements were determined for granulites and gneiss at huangtuling which are hosted by ultrahigh pressure metamorphic rocks in the dabie orogen east central china cl images reveal core rim structure for most zircons in the granulites the cores show oscillatory zoning relatively high th u ratios and hree enriched patterns consistent with a magmatic origin they gave a weighted mean 207pb 206pb age of 2766 9 ma interpreted as dating magma emplacement of the protolith the rims are characterized by sector or planar zoning low th u ratios negative eu anomalies and flat hree patterns consistent with their formation under granulite facies metamorphic conditions zircon u pb dating yields a weighted mean 207pb 206pb age of 2029 13 ma which is interpreted to record a metamorphic event possibly during assembly of the supercontinent columbia the gneiss has a protolith age of 1982 14 ma which is younger than the zircon age of the granulite facies metamorphism suggesting a generally delay between ht metamorphism and the intrusion of post collisional granites a few inherited cores with igneous characteristics have 207pb 206pb ages of 2 90 3 28 and 3 53 ga suggesting the presence of mesoarchean to paleoarchean crustal remnants in the yangtze craton a few cretaceous metamorphic ages were also obtained suggesting the influence of post collisional collapse in response to cretaceous extension of the dabie orogen it is inferred that the recently discovered archean basement of the yangtze craton occurs as far north as the dabie orogen

exhumation tectonics of the ultrahigh pressure metamorphic rocks in the qinling orogen in east china new petrological structural radiometric insights from the shandong peninsula 1 in eastern china the sulu area is recognized as the eastern extension of the qinling dabie belt which is famous for its ultrahigh pressure uhp metamorphism although numerous petrologic and geochemical works are available structural data are still rare this paper provides the first extensive study of bulk geometry and kinematic analysis of the shandong peninsula the study area is divided into three tectonic areas by cretaceous faults namely a southern uhp belt or sulu area a northern migmatite area and an eastern eclogite and migmatite area or weihai area conversely to the deeply entrenched idea that the later area belongs to the north china belt and the two others to the south china block scb we argue that all three areas are parts of the scb structural petrologic 40ar 39ar and u pb data comply with this new interpretation in the north shandong area mafic granulites enclosed as blocks within gneissic migmatites do not significantly differ from the sulu and weihai eclogites which also experienced a granulite facies overprint before migmatization the circa 210 200 ma age of the main ductile deformation is related to an extensional event during the triassic or indosinian orogeny this date corresponds to the temperature climax but the time of the pressure peak i e the real age of the uhp metamorphism is discussed