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

a middle silurian early devonian magmatic arc in the qinling mountains of central china a discussion lerch et al 1995 documented an early to middle paleozoic magmatic arc in the qinling orogenic belt of central china by detailed field mapping geochemistry and geochronology in the heihe area of north qinling their u pb analyses of zircons from i type granites developed in the heihe area suggested short lived arc activity from middle silurian to early devonian time this result is very interesting the termination of calc alkaline magmatism in the devonian is plausible and consistent with our studies on geochemistry and geochronology of a mafic pluton developed in lerch et al s 1995 zone 2 we reported a sm nd mineral isochron pyr 2 plag wr age of 402 6 17 4 ma for the lajimiao noritegabbro that intruded into the metabasaltic lava developed in the south of the shangxian area li et al 1989 then demonstrated its island arc origin using trace elements and sr nd isotopes li et al 1994a along the south marigin of zone 2 metamorphic andesite interbedded with metabasaltand intruded by mafic and granitic plutons was found recently in the heihe area their major and trace element compositions show island arc andesite characteristics w sun et al 1995 the 40ar 39ar age of 426 0 1 3 ma li et al unpub age for an amphibole from a metabasalt sample shows that these rocks underwent low amphibolite facies metamorphism in the middle silurian the caledonian metamorphic age was regarded as the time of collision between this volcanic arc and the north china craton li et al 1992 the tectonic system of the north qinling area between 426 and 400 ma might be similar to that of the andes in south america i e an island arc that had already accreted to the continent while the subduction of oceanic crust continued

hot orogens tectonic switching and creation of continental crust comment and reply reply this reply touches on several issues that reflect brown s and my different emphasis on the tectonic setting of granulites essentially brown suggests that the tectonic switching model should produce counterclockwise pressure temperature time p t t paths then argues that most documented

la icp ms monazite u pb age and trace element constraints on the granulite facies metamorphism in the tongbai orogen central china abstract granulite facies metamorphic rocks in the tongbai orogen have great implications for the evolution of the whole qinling tongbai dabie orogenic belt however the age and tectonic setting of the granulite facies metamorphism have not been well constrained yet in this contribution bse imaging trace element analysis and u pb dating of monazite were undertaken for three felsic granulites a gneiss and a leucosome from the tongbai orogen most monazite grains from the granulite and gneiss samples exhibit no zoning relatively low th u ratios significant depletion in hrees and y and negative eu anomalies indicating their formation under granulite facies metamorphic conditions their formation ages range from 423 2 to 430 3 ma with a weighted mean of 426 5 which is taken as the timing of the peak granulite facies metamorphism in the leucosome sample the monazite shows a euhedral to subhedral shape coupled with a core and rim structure besides being depleted in hrees and y both of them have significantly negative eu anomalies and variable u contents 1220 13323 ppm and th u ratios 7 2 136 7 this suggests that their formation was coeval with the crystallization of feldspar metamorphic zircon and garnet the monazites yield a weighted mean age of 426 3 ma suggesting nearly coeval partial melting during granulite facies metamorphism the relationship between metamorphic monazite and zircon within the same metamorphic sample is complex as revealed by metamorphic monazite and zircon u pb ages for the gneiss granulite and leucosome samples in the other two granulite samples the monazite yields u pb ages of 423 2 and 425 3 ma whereas only little overgrowth or recrystallization of metamorphic zircon this indicates that monazite is more sensitive than zircon in recording of metamorphic age even at the granulite facies conditions combined with previous metamorphic zircon u pb age results the prograde peak and retrograde stages of the granulite facies metamorphism in the tongbai orogen can be constrained at ca 440 426 and 415 ma respectively therefore the granulite facies metamorphism would have lasted for more than 20 myr the prolonged granulite facies metamorphism might result from the continuous northward subduction of the paleotethyan oceanic crust beneath the north qinling terrane

subduction collision and exhumation in the ultrahigh pressure qinling dabie orogen c abstract high pressure metamorphism and ophiolite emplacement songshugou ophiolite attended suturing of the yangtze craton to rodinia during the c l o ga grenvillian oro geny the qinling microcontinent then rifted from the yangtze craton at c 750ma the erlangping intraoceanic arc formed in the early ordovician was emplaced onto the qinling microcontinent in the ordovician silurian and then both units were accreted to the sino

tectonic evolution of a composite collision orogen an overview on the qinling tongbai hong an dabie sulu orogenic belt in central china abstract the formation of collisional orogens is a prominent feature in convergent plate margins it is generally a complex process involving multistage tectonism of compression and extension due to continental subduction and collision the paleozoic convergence between the south china block scb and the north china block ncb is associated with a series of tectonic processes such as oceanic subduction terrane accretion and continental collision resulting in the qinling tongbai hong an dabie sulu orogenic belt while the arc continent collision orogeny is significant during the paleozoic in the qinling tongbai hong an orogens of central china the continent continent collision orogeny is prominent during the early mesozoic in the dabie sulu orogens of east central china this article presents an overview of regional geology geochronology and geochemistry for the composite orogenic belt the qinling tongbai hong an orogens exhibit the early paleozoic hp uhp metamorphism the carboniferous hp metamorphism and the paleozoic arc type magmatism but the three tectonothermal events are absent in the dabie sulu orogens the triassic uhp metamorphism is prominent in the dabie sulu orogens but it is absent in the qinling tongbai orogens the hong an orogen records both the hp and uhp metamorphism of triassic age and collided continental margins contain both the juvenile and ancient crustal rocks so do in the qinling and tongbai orogens in contrast only ancient crustal rocks were involved in the uhp metamorphism in the dabie sulu orogenic belt without involvement of the juvenile arc crust on the other hand the deformed and low grade metamorphosed accretionary wedge was developed on the passive continental margin during subduction in the late permian to early triassic along the northern margin of the dabie sulu orogenic belt and it was developed on the passive oceanic margin during subduction in the early paleozoic along the northern margin of the qinling orogen three episodes of arc continent collision are suggested to occur during the paleozoic continental convergence between the scb and ncb the first episode of arc continent collision is caused by northward subduction of the north qinling unit beneath the erlangping unit resulting in uhp metamorphism at ca 480 490 ma and the accretion of the north qinling unit to the ncb the second episode of arc continent collision is caused by northward subduction of the prototethyan oceanic crust beneath an andes type continental arc leading to granulite facies metamorphism at ca 420 430 ma and the accretion of the shangdan arc terrane to the ncb and reworking of the north qinling erlangping and kuanping units the third episode of arc continent collision is caused by northward subduction of the paleotethyan oceanic crust resulting in the hp eclogite facies metamorphism at ca 310 ma in the hong an orogen and low p metamorphism in the qinling tongbai orogens as well as crustal accretion to the ncb the closure of backarc basins is also associated with the arc continent collision processes with the possible cause for granulite facies metamorphism the massive continental subduction of the scb beneath the ncb took place in the triassic with the final continent continent collision and uhp metamorphism at ca 225 240 ma therefore the qinling tongbai hong an dabie sulu orogenic belt records the development of plate tectonics from oceanic subduction and arc type magmatism to arc continent and continent continent collision

wave energy gradients across a maldivian atoll implications for island geomorphology exposure to wave energy has been used to account for a range of ecological geological and geomorphic processes in coral reef systems but few attempts have been made to quantify spatial variations in energy at the atoll scale this study presents results of measurements of wave energy on reef platforms across south maalhosmadulu atoll maldives and their implications for island geomorphology the atoll has a perforated rim 37 effective aperture and experiences predictable shifts in monsoon winds from the west 8 months and northeast 4 months results show that wave energy affecting the atoll is considerably greater during the westerly monsoon atoll structure promotes significant changes in wave energy and wave characteristics across the atoll short period 3 8 s monsoon driven wave energy which is significant on windward reefs is dissipated on the peripheral reef network and the density of lagoonal patch reefs limits development of locally generated wind wave energy across the lagoon however longer period swell 8 20 s propagates through the lagoon to leeward reefs a windward to leeward decay in wave energy is evident in the westerly monsoon but not in the northeast monsoon when long period swell from the southwest remains significant on western reefs net energy calculations that account for seasonal changes in wave energy across the atoll identify a steep west east gradient that has geomorphic significance for island building western reefs are dominated by westerly flowing energy that is 4 5 7 times the total energy input elsewhere in the atoll wave energy on central reefs is balanced whereas net energy on eastern reef platforms is dominated by eastward propagating waves this steep energy gradient provides a physical explanation for the presence and distribution of islands on reef platforms across the atoll and provides quantitative support for the theory of gardiner gardiner j s 1903 the fauna and geography of the maldives and laccadives archipelagoes cambridge university press london to explain the prevalence of islands on the eastern atoll reef rim

granulites in the tongbai area qinling belt china geochemistry petrology single zircon geochronology and implications for the tectonic evolution of eastern asia the tongbai area of the eastern qinling belt in china includes granulite grade metamorphic assemblages qinling complex which were previously regarded as archean to early proterozoic in age and belonging to the southern margin of the north china plate craton our petrological and geochemical data characterize these rocks as two pyroxene granulites and garnet granulites which formed at temperatures of 757 840 c and pressures of about 9 5 kbar and are now found as xenoliths in granodioritic gneisses the protoliths of these rocks were granodiorites and tholeiitic basalt or gabbro the 207pb 206pb ratios derived from evaporation of single zircons yield ages of 470 20 and 470 14 ma respectively for the basic granulites which we interpret to reflect the time of protolith emplacement these are intruded by a 435 14 ma granodioritic gneiss post dating granulite formation a metaquartzite sample contains detrital zircons as old as 2555 8 ma two samples of granitoid gneiss from the tongbai complex s of the qinling granulites have single zircon 207pb 206pb evaporation ages of 776 8 and 746 10 ma respectively and document late proterozoic igneous activity we suggest that the qinling granulites document an important and hitherto unknown phase of early silurian crustal thickening following subduction and continental collision and that both the qinling and tongbai complexes were part of the southern margin of the north china craton prior to this event and record late proterozoic igneous activity

petrogenesis of the late mesozoic qijinfeng granite complex in the tongbai orogen geochronological geochemical and sr nd pb hf isotope evidence abstract the tongbai orogen is located in the northern hubei and southern henan provinces together with the qinling hong an and dabie orogens they constitute one of the most important orogenic belt separated the north china block and yangtze block among these orogens the tongbai receives less attention and its magmatic and tectonic evolution is still unclear in this paper we present a detailed study on geochronology geochemistry and sr nd pb hf isotopes of the five plutons daxianduo jinziling sanguanduo sanhedian and zhoulou that composes the qijianfeng granite complex in suizao area in order to better understanding the late mesozoic magmatic and tectonic evolution of the tongbai orogen the la icp ms u pb dating of zircon suggests an early cretaceous emplacement age 135 139 ma for the qijianfeng granite complex all the five plutons of the qijianfeng granite complex are weakly peraluminous and belong to high k calc alkaline series and post collisional i type granites the chemical and isotopic compositions of the qijianfeng granite complex show sio2 contents of 69 84 74 36 wt al2o3 of 13 85 15 62 wt mgo of 0 16 0 52 wt n2o k2o of 0 99 1 73 506 1205 ppm sr 3 4 12 9 ppm y 0 49 1 25 ppm yb eu eu values of 0 7 1 0 lree hree values of 3 1 9 5 87sr 86sr values of 0 7057 0 7080 and end t values of 16 59 to 6 18 which share similarities with the c type high k adakite the low mgo ni and cr contents further suggest that the qijianfeng granite complex belongs to adakite derived from partial melting of thickened lower continental crust combined with the geochemical characteristics of other contemporaneous intrusions in the whole orogen our study shows that the extensive early cretaceous adakitic magmatic activity is triggered by lower crustal thickened mountain basin coupling and crust mantle interaction in the tongbai orogeny

linking granulites silicic magmatism and crustal growth in arcs ion microprobe zircon u pb ages from the hidaka metamorphic belt japan there is no consensus as to how the extreme metamorphic conditions required to form granulites are attained or how these rocks relate to crustal growth and differentiation processes studying young granulites offers two advantages the tectonic setting is likely to be well constrained and the ambiguities that result from overprinting by younger metamorphic events are potentially avoided we report the first ion microprobe u pb zircon ages for orthopyroxene bearing granulites tonalites and gabbros of the cenozoic hidaka metamorphic belt hokkaido japan to clarify the magmatic metamorphic connection in this area the data support a two stage evolution for this terrane which is attributed to episodes of supra subduction zone magmatism late eocene and back arc extension early miocene we relate granulite facies metamorphism and garnet orthopyroxene tonalite generation to mafic magma under accretion and lithosphere thinning due to the opening of the japan sea at 19 ma the hidaka granulites are thus among the youngest exposed granulites on earth and manifest the thermal response to continental growth

wave transformation over coral reefs ocean wave attenuation on coral reefs is discussed using data obtained from a preliminary field experiment and from the seasat altimeter marked attenuation of the waves is observed the rate being consistent with existing theories of bottom friction and wave breaking decay in addition there is a significant broadening of the spectrum during propagation across reefs three dimensional effects such as refraction and defraction can also lead to substantial wave height reduction for significant distances adjacent to coral reefs as a result a matrix of such reefs provides significantly more wave attenuation than may initially be expected

wave set up wave run up and beach water table interaction between surf zone hydraulics and groundwater hydraulics the relationship between wave set up wave run up and beach water table is reviewed starting from the work and ideas of nielsen and using laboratory observations of wave set up and beach water table in beaches formed from various beach materials under similar wave conditions a new more extensive analysis of wave set up on impermeable fixed plane beaches has been made using all available experimental data scaling parameters for set up and water depth based upon both deep water and breaking wave conditions and incorporating appropriate forms of the surf similarity parameter are developed the dimensionless set up parameters are of the form n long right leg n long right leg b h0 0 n where 0 3 n 0 5 for deep water wave parameters and n long right leg n long right leg b hb tan ib n where n 0 4 for breaking wave conditions 0 tan ho lo and ib hb 1 2 g 1 2 t tan

zircon u pb age trace element and hf isotope composition of kongling terrane in the yangtze craton refining the timing of palaeoproterozoic high grade metamorphism u pb age trace element and hf isotope compositions of zircon were analysed for a metasedimentary rock and two amphibolites from the kongling terrane in the northern part of the yangtze craton the zircon shows distinct morphological and chemical characteristics most zircon in an amphibolite shows oscillatory zoning high th u and 176lu 177hf ratios high formation temperature high trace element contents clear negative eu anomaly as well as hree enriched patterns suggesting that it is igneous the zircon yields a weighted mean 207pb 206pb age of 2857 8 ma representing the age of the magmatic protolith the zircon in the other two samples is metamorphic it has low th u ratios low trace element concentrations variable hree contents 33 8 lun 2213 14 7 lun smn 354 and 176lu 177hf ratios 0 000030 0 001168 the data indicate that the zircon formed in the presence of garnet and under upper amphibolite facies conditions the metamorphic zircon yields a weighted mean 207pb 206pb age of 2010 13 ma these results combined with previously obtained palaeoproterozoic metamorphic ages suggest a c 2 0 ga palaeoproterozoic collisional event in the yangtze craton which may result from the assembly of the supercontinent columbia the zircon in two samples yields weighted mean two stage hf model tdm2 ages of 3217 110 and 2943 50 ma respectively indicating that their protoliths were mainly derived from archean crust

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

deep subduction of continental crust in accretionary orogen evidence from u pb dating on diamond bearing zircons from the qinling orogen central china abstract accretionary orogenic belts are considered the most important sites for the generation of continental crust however it is still not very clear whether in such settings the continental crust has been significantly transported back to the mantle accompanied with its lateral growth continental uhp rocks the best indicators of the recycling of the continental crust have rarely been discovered in accretionary orogenic belts which hinder the exploration of this essential process the possible occurrence of early paleozoic uhp metamorphism in the qinling orogen provides an excellent opportunity to address this question this article reports an integrated study of u pb age trace element and mineral inclusion of zircon from an amphibolite sample in the qinling orogen the zircon crystals show typical metamorphic growth zoning low th u ratios flat hree patterns and insignificant eu anomalies they give a weighted mean u pb age of 490 4 5 8 ma most importantly an in situ diamond inclusion was identified from one of the zircon crystals therefore the obtained 490 4 5 8 ma age was taken as registering the peak uhp metamorphism the discovery of in situ diamond inclusion provides important evidence for the uhp metamorphism of the qinling orogen and indicates that the north qinling microcontinent was subducted to mantle depths of 120 km when it collided with the erlangping arc attached to the southern margin of the north china block combining with previous results we suggested that the north qinling uhp terrane underwent fast exhumation from 120 km to ca 30 km within 10 myr intervals as a few examples of uhp metamorphism have been discovered in some accretionary orogens more attention needs to be paid to the occurrence of continental uhp metamorphism during accretionary orogenic process furthermore evident crustal growth has been documented in the north qinling orogen during the early paleozoic it is inferred that the accretionary orogenic belts may play important roles in recycling of continental crust into the mantle besides generation of continental crust

mid paleozoic collision in the north qinling sm nd rb sr and 40 ar 39 ar ages and their tectonic implications abstract different isotopic geochronometers have been used to study the nature of the mid paleozoic tectonothermal event in the north qinling arc system at the southern margin of the north china block 40ar 39ar age of amphibole from sample qh 3 426 2 ma rb sr mineral isochron age 411 5 ma of sample qsp 10 of the danfeng group and rb sr whole rock isochron age 406 22 ma of a greenschist facies pillow lava sample of the erlangping group indicate a widespread late silurian tectonothermal event in the north qinling area the data also suggest a limited uplift in the north qinling during the late silurian tectonothermal event this event is restricted to the north qinling area and the southern margin of north china block and not observed in the south qinling belt and the northern margin of the south china block consequently it is related to the collision between the north china block and a north qinling arc system which resulted in the closure of the north qinling back arc basin s

monsoonally influenced circulation around coral reef islands and seasonal dynamics of reef island shorelines synchronous measurements of reef flat waves and nearshore currents were made around the perimeter of three coral reef islands in the central maldives nearshore current velocities were shown to be tidally modulated in most instances with maximum velocities associated with lower tidal stages peak currents were consistently observed on the windward and lateral flanks of island shorelines 0 3 0 4 ms 1 the relatively low elevation of reef surfaces and medium to low energy incident wave energy hs 0 2 0 3 m accounts for low tidal stage peaks in velocity as this corresponds to the period when wave setup induced current processes are maximised significantly all nearshore currents were unidirectional and alongshore the generation of unidirectional alongshore flow patterns results from island configuration which deflects flow and variations in reef geometry that control spatial variations in wave setup around the reef platform edge and at the island shoreline which can drive alongshore currents the presence of a nearshore moat that channelizes flow in the nearshore and variations in reef geometry where topographic low points act as preferential drainage points results show that changes in monsoon conditions that modulate wind and wave patterns from the west to northeast can force substantial changes in reef platform and nearshore current flows but that the degree of change is dependent on reef platform shape and position in the atoll the circular study island was found to exhibit a near complete reversal in unidirectional flow from northeast flow in the westerly monsoon to southwest flow in the northeast monsoon however the elongate and triangular shaped reef platforms exhibited more localised changes in flow patterns significantly those shorelines that exhibit the greatest changes in shoreline currents correspond to the sections of coast that exhibit the largest morphological adjustments between seasons results provide significant insights for the morphodynamics of reef platform islands in particular the nearshore process regime is dominated by alongshore processes furthermore reef platform shape and the magnitude of change in direction of wave approach to reef platforms provide critical indicators of the morphodynamic behaviour of reef platform islands

regional granulite facies metamorphism in the ivrea zone is the mafic complex the smoking gun or a red herring one widely accepted paradigm for the development of continental lower crust is that regional granulite facies metamorphism is caused by intrusion of mafic magma beneath or into the crust magmatic accretion the amphibolite to granulite facies supracrustal section exposed in the ivrea zone southern alps northern italy is commonly cited as a classic example establishing this postulated genetic relationship our interpretation of the pattern of metamorphic isograds compositional trends in high grade metasedimentary rocks and textural evidence in metapelite however indicates that final emplacement of the mafic plutonic rocks the mafic complex occurred subsequent to the regional thermal maximum field and petrographic relations suggest that a spatially restricted contact melting event in crustal rocks accompanied the emplacement of the mafic complex this inference is consistent with leucosome compositions in migmatites and a low pressure high temperature metamorphic overprint recorded by mineral assemblages in wall rocks proximal to the intrusion therefore evidence of anatexis and metamorphism of crustal rocks associated unequivocally with emplacement of the mafic complex is found only within an 2 km wide contact aureole overlying the intrusion the narrow aureole associated with emplacement of the mafic complex demonstrates that in some cases emplacement of large volumes of mafic magma within the crust does not inexorably lead to regional scale granulite facies metamorphism and large ion lithophile element depletion by melt loss

age and nature of eclogites in the huwan shear zone and the multi stage evolution of the qinling dabie sulu orogen central china abstract in situ la icpms u pb trace element and hf isotope data in zircon demonstrate a carboniferous age for eclogite facies metamorphism in siluro devonian protoliths in the huwan shear zone dabie mountains central china this age contrasts with the more prevailing triassic age for high to ultrahigh pressure hp to uhp metamorphism in the qinling dabie sulu orogen metamorphic zircon in two eclogite samples from sujiahe is characterized by low th u ratios small negative eu anomalies flat hree patterns and low 176 lu 177 hf ratios these geochemical signatures suggest that the zircon crystallized in the presence of garnet and in the absence of plagioclase feldspar furthermore temperatures of 655 and 638 c calculated using the ti content of zircon are consistent with their formation during eclogite facies metamorphism the weighted mean 206 pb 238 u age of 309 4 ma 2 for this zircon improves previous age estimates for eclogite facies metamorphism in the huwan shear zone ranging from 420 to 220 ma metamorphic zircon from one eclogite sample from hujiawan most likely formed during prograde metamorphism yields an equivalent age estimate of 312 11 ma magmatic zircon cores in the three samples yield ages for the magmatic protoliths of the eclogites ranging from 420 7 to 406 5 ma and post dating the middle paleozoic collision of the north china and the qinling terrain the zircon crystals in the three eclogite samples display a large variation of e hf t values of 4 9 to 21 3 the metamorphic zircon overgrowths show the same range of e hf t values as those of the inherited magmatic crystal interiors this suggests that the metamorphic zircon overgrowths may have formed by dissolution reprecipitation of pre existing magmatic zircon thereby preserving their original hf isotopic composition the high e hf t values suggest that the protoliths were derived from depleted mantle sources most likely paleotethyan oceanic crust while the low e hf t values are attributed to crustal contamination some eclogites in the huwan shear zone have a distinctive signature of continental crust most probably derived from the yangtze craton the coexistence of paleozoic oceanic crust and neoproterozoic continental crust with similar metamorphic ages in the huwan shear zone implies that paleozoic paleotethyan oceanic crust was produced within a marginal basin of the northern yangtze craton the opening of the paleo tethyan ocean was slightly younger than the collision of the north china craton and the qinling terrain during the late paleozoic in the qinling dabie sulu orogen subduction of the paleo tethyan oceanic crust and associated continental basement resulted in the 309 2 ma 2 eclogite facies metamorphism in the huwan shear zone the subsequent triassic continent continent collision led to the final coalescence of the yangtze and sino korean cratons amalgamation of the yangtze and north china cratons was therefore a multistage process extending over at least 200 ma

qinling gneiss domes and implications for tectonic evolution of the early paleozoic orogen in central china abstract the qinling arc terrane qat at the southern margin of the north qinling early paleozoic orogen central china is composed of gneiss domes overridden by cold allochthon nappes the gneiss domes are characterized by migmatitic and plutonic cores of granite or gabbro and overlying upper amphibolite to granulite facies metamorphic sequences with pervasive injection of leucogranitic veins the gneiss domes display a high temperature domal foliation dipping radially away the core and a down dipping stretching lineation cascading folds occur in the mantling metamorphic rocks the new data of zircon monazite u pb dating suggest that the gabbroic and granitic plutons formed at 514 476 ma and 495 441 ma respectively whereas the high grade metamorphism of the mantling rocks took place at 469 412 ma and the leucogranitic veins formed at two main episodes 472 440 ma and 416 413 ma mica 40ar 39ar dating yields 402 328 ma for the cooling ages of the leucogranitic veins and gneissic rocks the basaltic magma underplated into the qat lower crust resulted in partial melting producing granitic magma at the late cambrian to middle ordovician during northward subduction of the shangdan proto tethyan oceanic slab buoyant upwelling of the migmatitic and plutonic core was promoted by constriction in the lower crust and decompression extension in the upper crust during the silurian since the closure of the shangdan ocean and the collision between the north china block and south china block 400 ma the qinling gneiss domes experienced rapid uplift and were finally overthrust by the upper allochthon transported from the north qinling

zircon u pb and trace element data from rocks of the huai an complex new insights into the late paleoproterozoic collision between the eastern and western blocks of the north china craton abstract the trans north china orogen tnco is interpreted as a continent continent collisional belt along which the discrete archean eastern and western blocks amalgamated to form the basement of the north china craton the timing of its formation has not been precisely constrained yet six samples including five high grade metamorphic rocks and a pegmatite dike from the huai an complex in the northern segment of the tnco were selected for zircon u pb dating and trace element analysis a tonalite trondhjemite granodiorite ttg gneiss sample and a high pressure hp mafic granulite sample contain magmatic zircon cores yielding protolith formation ages of 2503 17 and 1964 38 ma respectively detrital zircons of magmatic origin in two metasedimentary rocks show concordant or moderately discordant u pb ages of 2683 22 to 1964 13 ma indicating that they were mainly derived from the tnco with some possibly derived from the archean continental rocks of the north china craton metamorphic zircons in the ttg gneiss and two metasedimentary rocks exhibit low hree contents flat hree patterns and negative eu anomalies implying their formation coeval with garnet and feldspar under grannulite facies conditions metamorphic zircons in an hp mafic granulite and a sample of leucosome from a ttg gneiss also have depleted hree contents flat hree patterns but only weak negative eu anomalies which may also indicate formation under granulite facies conditions whereas the weak eu anomalies may be due to the whole rock composition all these metamorphic zircons have formation temperatures of 702 749 c approaching the peak metamorphic temperatures of hp granulite rocks in this area they have identical ages of ca 1845 ma within analytical errors which is the best estimate age for hp granulite facies metamorphism of the huai an complex these results confirm previous models proposing late paleoproterozoic collision between the eastern and western blocks of the north china craton along the tnco on the other hand some metamorphic zircons in the leucosome sample show planar or polygonal zoning high th u contents and high th u ratios high ree contents and steep hree patterns indicating that they crystallized during partial melting with abundant garnet decomposition they yielded a weighted mean age of 1819 13 ma which we interpret to register the time of retrograde metamorphism combined with previously published data there are three age peaks at 1876 6 1849 2 and 1814 4 ma for metamorphic zircons from high grade metamorphic rocks in the tnco these may indicate either a protracted event or at least three peaks of metamorphism a pegmatite dike cutting the foliation of a mafic granulite boudin was dated at 1806 15 ma which signifies termination of collision between the eastern and western blocks in the huai an area

timing of collision of the north and south china blocks controversy and reconciliation the qinling orogen was formed by the joining of the north and south china blocks but the timing of their integration has been debated for more than a decade the controversies obviously stem from different approaches to reconstruction of the integration history two contrasting lines of evidence yield two different ages for collision of the north and south china blocks middle paleozoic and late triassic the shangdan suture within the qinling was regarded in previous studies as the trace along which the north and south china blocks collided our studies however demonstrate that there are two sutures within the qinling the well documented shangdan suture and the newly discovered mianlue suture we show in this paper that the late proterozoic to early mesozoic evolution of the qinling involved interactions between the north china block the north and south qinling orogens and the south china block the middle paleozoic collision along the shangdan suture as constrained by some evidence accreted only the south qinling orogen to the southern part i e the north qinling of the north china block contemporaneous rifting of the south china block and subsequent drifting separated the south qinling from the south china block during the middle to late paleozoic the separation of the south from the north china blocks is supported by other evidence in particular geomagnetic data evidently it was the late triassic collision of the south china block with the south qinling orogen along the mianlue suture that led to final integration of the north and south china blocks

paleozoic metamorphism in the qinling orogen tongbai mountains central china the collision of the sino korean and yangtze blocks to form a significant part of china is recorded in the qinling tongbai and dabie mountains radiometric ages of the ultrahigh pressure metamorphic rocks in the south qinling orogenic belt suggest that subduction and collision took place during the triassic period our new 40 ar 39 ar geochronology of units in the north qinling orogenic belt confirms that high grade metamorphism and deformation took place also during the silurian devonian and carboniferous periods these results imply that the amalgamation of eastern china was a multistage process extending over at least 200 m y

high temperature geochronology constraints on the tectonic history and architecture of the ultrahigh pressure dabie sulu orogen 1 new u pb zircon and th pb monazite ages are presented from the giant sulu ultrahigh pressure uhp terrane combined with sm nd ages rb sr ages inclusion relationships and geologic relationships they help define the timing of peak recrystallization the timing of subsequent amphibolite facies metamorphism and the architecture of the dabie sulu suture zone between the collided sino korean and yangtze cratons the data indicate a 15 myr record of uhp recrystallization the first clearly documented for a giant uhp terrane this requires that continental subduction in the dabie sulu orogen involved multiple uhp tectonic or recrystallization events a 244 236 ma precursor uhp event seen only in the dabie shan was followed by a second 230 220 ma main uhp event which was itself terminated by a 220 205 ma amphibolite facies overprint older eclogite facies events seen in the qinling segment of this orogenic belt raise the possibility that these rocks have undergone u hp metamorphism three or four times but at present there is no geochronological evidence in the dabie sulu area to support this the subduction of the lower yangtze plate did not proceed in a simple fashion the ages of inherited zircon cores demonstrate that a ribbon continent of yangtze affinity escaped subduction and became wedged against the sino korean plate hanging wall

high pressure metamorphic rocks from tongbaishan central china u pb and 40ar 39ar age constraints on the provenance of protoliths and timing of metamorphism abstract the tongbaishan area is situated at the junction between the paleozoic north qinling orogen and the mesozoic south qinling dabie sulu orogen the area contains an important geological section for understanding the assembly of the sino korean and yangtze cratons two eclogite zones were recently recognized in this area the peak metamorphic conditions are estimated to be 530 610 c and 17 20 kbar for the northern eclogite zone and 460 560 c and 13 19 kbar for the southern eclogite zone shrimp u pb analyses on zircons of eclogite metagabbro and orthogneiss indicate that their protoliths were formed during the neoproterozoic and experienced high pressure hp recrystallization during the permian triassic period therefore the tongbaishan area belongs to the western extension of the dabieshan hp uhp terrane in addition u pb ages of detrital zircons from quartzite paragneiss and schist show three age populations of 2 49 ga 1 93 ga and 1 85 1 82 ga suggesting that extensive paleoproterozoic orogeny or tectonothermal events took place in the northern margin of the yangtze craton the shrimp dating and complementary 40ar 39ar dating of muscovite from quartzofeldspathic gneiss and quartzite reveal that the crustal rocks were subducted to great depths and underwent hp eclogite facies metamorphism at ca 255 ma slightly earlier than the timing of deep subduction of the uhp slice the hp rocks were later exhumed to mid to upper crustal levels and suffered from retrograde recrystallization at ca 238 ma the dating also discloses a still later phase of deformation and recrystallization at ca 215 ma the available age data throughout the tongbai dabie sulu orogen reflect diachronous exhumation from ca 238 ma for the hp slice to ca 213 ma for the uhp slice such a phenomenon could be explained by successive subduction and exhumation of the broken slices of the subducting yangtze slab

depth limited design wave heights in very flat regions abstract engineers working on the flat tops of offshore coral reefs or offshore banks require design criteria for depth limited wave heights over horizontal beds particularly in regions leeward of the bank or reef edge surf zone this paper presents convincing evidence of the fact that in engineering practice both in the laboratory and in the field the value of 0 55 is the largest wave height to water depth ratio realizable for stable shallow water oscillatory waves propagating in water of constant depth horizontal bed this value is much lower than that often used in engineering practice namely a value of about 0 8 which is historically based on the theoretical considerations and laboratory experiments associated with solitary waves this result based on a review of laboratory data and the original results of a major field experiment undertaken on the great barrier reef of australia leads to a significant reduction in design wave forces design crest levels and wave induced scour velocities at the bed

discovery of metamorphic diamonds in central china an indication of a 4000 km long zone of deep subduction resulting from multiple continental collisions the central orogenic belt cob of china is a major continental collision zone that contains extensive outcrops of deeply subducted and exhumed rocks at both the eastern and the western end of the belt here we report discovery of microdiamonds from both eclogites and felsic gneisses in the north qinling zone in the central portion of the cob this discovery demonstrates that the country rocks of continental affinity shared in the ultra high pressure metamorphic uhpm event and provides a bridge connecting the two previously recognized uhpm terranes thereby establishing the existence of a uhpm belt extending more than 4000 km geochronological dating yields early palaeozoic ages in the west and early mesozoic ages in the east recording two separate continental collisions overprinted within the cob occurrence of uhp metamorphism during recurrent continental collision here and in the alps suggests that deep subduction of continental material during such collisions is probably common rather than exceptional with significant implications for processes of plate tectonic reorganization and mantle mixing over time

u pb zircon ages and geochemistry of the wuguan complex in the qinling orogen central china implications for the late paleozoic tectonic evolution between the sino korean and yangtze cratons abstract the tectonic evolution of the qinling orogen central china is the key to understanding the assembly of the sino korean and yangtze cratons the wuguan complex between the early paleozoic north qinling and mesozoic south qinling tectonic belts can provide important constraints on the late paleozoic evolutionary processes in the qinling orogen u pb zircon analyses using laser ablation multicollector inductively coupled plasma mass spectrometry reveal protolith ages of 446 2 ma for a garnet amphibolite 368 3 ma for a meta andesite and 351 2 ma for a mylonitized granitic dike from the wuguan complex elemental geochemistry indicates typical island arc affinities for all the above rocks but some amphibolites of unknown age have e morb signatures detrital zircons from a metaquartzite have an age spectrum with a major peak at 462 ma two subordinate peaks at 828 and 446 ma and a youngest weighted mean age of 423 5 ma this suggests that at least some of metasedimentary rocks from the wuguan complex belong to the part of the devonian turbidite sequence of the liuling group which was deposited in a fore arc basin along the southern accreted margin of the sino korean craton whereas the late ordovician precursors of the amphibolite might be derived from the danfeng group the occurrence of late devonian early carboniferous arc related rocks in the wuguan complex implies penecontemporaneous oceanic subduction and therefore the paleo qinling ocean was not finally closed until the early carboniferous on the other hand metamorphic zircon grains from two amphibolites yielded ages of 321 2 and 318 3 ma hence the wuguan complex in the qinling orogen and the guishan complex in the tongbai orogen constitute a medium pressure carboniferous metamorphic belt that is more than 500 km long and which was formed in the hanging wall of a subduction zone

evolution of the qinling orogenic belt detailed field mapping geophysics and geochemistry show that the qinling orogenic belt is a southward vergent tectonic wedge consisting of six regional thrust sheets the lushan luanchun qinling douling wudang and zhuqi thrust sheets these regional thrusts underwent displacement at the end of the mid triassic and the end of the late jurassic due to the collision between the north and south china blocks and subsequent detachment the qinling orogenic belt is thus an indosinian yanshanian collision detachment zone the so called caledonian orogeny in the qinling orogenic belt records a succession of extensional and partial melting events in the continental crust the basic lavas in the lower paleozoic were erupted into rifting zones both in the northern qinling subzone and southern qinling subzone at the same time local melting of the transitional continental crust occurred and resulted in formation of s type granites

paleozoic peralkaline a type magmatism of the tongbai orogen central china petrogenesis and tectonic implications abstract the precise timing petrogenesis and geodynamic significance of the paleozoic magmatic rocks in the southern qinling tongbai orogenic belt qtob central china are poorly constrained we performed an integrated study of petrology zircon u pb ages geochemistry and hf o nd isotopic compositions of the huangyangshan pluton from the southern qtob to constrain its genesis and tectonic implications a granite sample from the pluton yields a weighted mean zircon 206pb 238u age of 445 6 ma representing the age of the pluton the huangyangshan pluton is dominated by alkaline granite and quartz syenite has a peralkaline signature a nk 0 82 0 96 and belongs to the shoshonite and high k calc alkaline series both the alkaline granite and quartz syenite have geochemical signatures similar to a type granite with high sio2 and total alkali na2o k2o contents high fe numbers and tio2 mgo and ga al ratios and high field strength elements hfses along with low mgo cao and p2o5 contents these samples are enriched in light rare earth elements lrees and have flat heavy rare earth element hree patterns and strongly negative eu anomalies they are enriched in most large ion lithophile elements liles and hfses with significant negative eu ba sr and ti anomalies in primitive mantle normalized trace element diagrams the samples have relatively high zr contents 175 776 ppm yielding minimum magmatic temperatures of 768 c 891 c the huangyangshan pluton has initial zircon ehf t values of 6 6 to 10 0 and whole rock end t values of 2 86 to 3 62 the analyzed zircons yield a mean 18o value of 5 14 0 26 indicating negligible incorporation of hydrothermally altered crustal materials the geochemical signatures of the huangyangshan pluton indicate that the magmas were derived from prompt partial melting of juvenile crust rocks combined with previous results we suggest that the paleozoic huangyangshan peralkaline a type granites resulted from the rifting of a passive continental margin in the northern south china block followed by the opening of the paleotethyan ocean

geochronology geochemistry and isotope compositions of piaochi s type granitic intrusion in the qinling orogen central china petrogenesis and tectonic significance abstract s type peraluminous granite can crop out in various tectonic settings the paleozoic piaochi granitic intrusion is a large s type peraluminous intrusion located in the north qinling orogen and has important implication for its tectonic evolution though intensively studied the age and tectonic setting of this intrusion are still controversial in this study we carried out an integrated study of zircon u pb age major and trace elements and sr nd hf o isotope compositions for the piaochi intrusion with aims to constrain its petrogenesis and tectonic significance sims and la icpms u pb zircon data obtained from magmatic zircons indicate that the piaochi intrusion was emplaced at 473 4 ma and contained ca 490 ma inherited zircons the granite samples from the piaochi intrusion show typical s type characteristics such as the relatively moderate aluminum saturation index 1 00 1 08 low zirconium saturation temperatures occurrences of muscovite and high zircon 18 o values 8 70 to 9 54 the relatively low sm nd 0 13 0 20 and feo t mgo ratios 2 21 3 9 suggest that the granites are unfractionated the geochemical compositions reveal that they were derived from partial melting of a clay poor but plagioclase and biotite rich psammitic source similar to metasedimentary rocks outcropping in the north qinling unit moreover the piaochi granites are moderately depleted in eu and heavy rare earth elements and have relatively high la yb n values 22 8 56 8 implying a high pressure plagioclase poor and garnet rich residual assemblage the piaochi granites have high and variable initial 87 sr 86 sr ratios 0 72167 0 72511 and low e nd t from 10 23 to 8 71 values with two stage nd model ages t dm2 of 1 91 2 03 ga which are comparable with those of supracrustal materials in the north qinling unit in addition our results show that the ca 470 ma magmatic zircons display a narrow e hf t array within those of the inherited cores these imply that the piaochi granites inherited their isotopic characteristics wholly from their sources and the hf isotopes might have been differently homogenized during the partial melting our new zircon u pb ages indicate that the piaochi granites were emplaced after the climax of the uhp metamorphism coeval with later retrograde metamorphism during exhumation therefore we suggest that the formation of the early paleozoic piaochi intrusion was triggered by the exhumation of the uhp rocks

the origins of granulites a metamorphic perspective although many recent reviews emphasize a uniformity in granulite pressure temperature p t conditions and paths granulites in reality preserve a spectrum of important petrogenetic features which indicate diversity in their modes of formation a thorough survey of over 90 granulite terranes or occurrences reveals that over 50 of them record p t conditions outside the 7 5 1 kbar and 800 50 c average granulite regime preferred by many authors in particular an increasing number of very high temperature 900 1000 c terranes are being recognized both on the basis of distinctive mineral assemblages and geothermobarometry petrogenetic grid and geothermobarometric approaches to the determination and interpretation of p t histories are both evaluated within the context of reaction textures to demonstrate that the large range in p t conditions is indeed real and that both near isothermal decompression itd and near isobaric cooling ibc p t paths are important amphibolite granulite transitions promoted by the passage of co 2 rich fluids as observed in southern india and sri lanka are exceptional and not representative of fluid related processes in the majority of terranes it is considered on the contrary that fluid absent conditions are typical of most granulites at or near the time of their recorded thermal maxima itd granulites are interpreted to have formed in crust thickened by collision with magmatic additions being an important extra heat source erosion alone is not however considered to be the dominant post collisional thinning process instead the itd paths are generated during more rapid thinning 1 2 mm yr exposure related to tectonic exhumation during moderate rate or waning extension ibc granulites may have formed in a variety of settings those which show anticlockwise p t histories are interpreted to have formed in and beneath areas of voluminous magmatic accretion with or without additional crustal extension ibc granulites at shallow levels very rapid 5 mm yr extensional thinning subsequent to collision it is suggested that the preservation of ibc paths rather than itd paths in many granulites is primarily related to the rate and timescale of extensional thinning of thickened crust and that hybrid itd to ibc paths should also be observed most ibc granulites and probably many itd granulites have not been exposed at the earth s surface as a result of the tectonic episodes which produced them but have resided in the middle and lower crust for long periods of time 100 2000 ma following these events the eventual exhumation of most granulite terranes only occur through their incorporation in later tectonic and magmatic events unrelated to their formation

palaeozoic pre and post collision calc alkaline magmatism in the qinling orogenic belt central china as documented by zircon ages on granitoid rocks based on large scale reconnaissance mapping we identified two calc alkaline plutonic assemblages from the northern qinling orogenic belt central china the older assemblage of intrusions closely associated and deformed coevally with their host volcanic arc sequences seems to represent the fractionation product of basaltic arc magma it therefore predates the collision of the north china block with the central qinling island arc system that developed in a sw pacific type oceanic domain south of the north china block single zircon 207pb 206pb evaporation dating yielded early to middle ordovician ages for this assemblage with a relatively small range from 487 2 1 1 to 470 2 1 3 ma intrusions of the younger assemblage are largely undeformed and truncate structures shown in rocks of the older assemblage they are interpreted as post collisional calc alkaline granitoids single zircon dating provided an age of 401 8 0 8 ma for the younger assemblage consistent with earlier work that defines an age range from c 420 to 395 ma our data favour a tectonic model involving formation and amalgamation of island arc and microcontinent terranes between ca 490 and 470 ma ago to create the central qinling zone which subsequently collided with the north china block prior to c 400 ma ago a late precambrian age of 762 0 0 7 ma for a granitoid gneiss at the northern margin of the yangtze block supports a gondwana affinity for this large continental block

u pb ages and trace elements of detrital zircons from early cretaceous sedimentary rocks in the jiaolai basin north margin of the sulu uhp terrane provenances and tectonic implications abstract the dabie sulu orogen in east central china is the largest uhp belt in the world many studies have dealt with detrital minerals from sedimentary basins surrounding the dabie belt in order to constrain the tectonic unroofing of this terrane on the other hand still few are the works focused on detritals from the basins associated with the sulu belt the jiaolai basin adjacent to the sulu belt is made of mesozoic sedimentary rocks two sandstone samples collected from the early cretaceous laiyang group in the northern and southern margins of the jiaolai basin were selected for zircon u pb dating and trace element analysis the zircons show main u pb age clusters at ca 2500 2400 2000 1700 850 700 and 330 130 ma with a minor peak at ca 1100 ma and a few archean ages most zircons show texture features reflecting a magmatic origin whereas several late paleoproterozoic and triassic zircons show homogeneous cl images and low th u ratios indicating a metamorphic origin the neoproterozoic ages are consistent with the protolith ages of the metamorphic rocks in the sulu terrane implying that the sulu tarrane may be a major source for this component whereas most neoproterozoic zircons are euhedral to subhedral or angular in shape and are not surrounded by overgrowths this implies that low grade metamorphic neoproterozoic metaigneous rocks ocurring along the northern margin of the sulu terrane or metasediments overlying the hp uhp rocks in the sulu terrane might provide source materials to these detrital zircons the late and early paleoproterozoic age peaks are consistent with the thermal events occurred in the jiaobei terrane slightly younger than those in the yangtze block these suggest that the jiaobei terrane was a potential source for the paleoproterozoic to archean detrital zircon components only two early paleozoic zircon grains were found in the studied samples indicating the early paleozoic arc did not extend into the sulu terrane according to the distribution of different kinds of rock adjacent to the sulu terrane the early cretaceous detrital zircons may be potentially derived from cretaceous plutons from the sulu belt eight detrital zircons including six metamorphic grians and two metamorphic overgrowth rims yield the triassic ages which might be derived from hp uhp rocks from the sulu terrane a metamorphic overgrowth rim yields a metamorphic age of 246 3 ma probably recording the timing of the prograde metamorphism of the sulu terrane one metamorphic grain yields an age of 212 2 ma which is interpreted to record a retrograde metamorphic event based on the ree patterns the remaining six triassic analyses can be subdivided into two groups four of them record a weighted mean age of 230 6 ma and show nearly flat hree patterns and slight negative or positive eu anomalies the last two grains are enriched hree considering their ages and the ree patterns it is suggested that they may originate from other diverse uhp rocks the triassic metamorphic zircons were detected in the laiyang group indicating that the sulu uhp rocks were exposed to the surface before the early cretaceous

u pb hf and o isotope evidence for two episodes of fluid assisted zircon growth in marble hosted eclogites from the dabie orogen abstract a combined study of internal structure u pb age and hf and o isotopes was carried out for metamorphic zircons from ultrahigh pressure eclogite boudins enclosed in marbles from the dabie orogen in china cl imaging identifies two types of zircon that are metamorphically new growth and recrystallized domain respectively the metamorphic zircons have low th and u contents with low th u ratios yielding two groups of 206 pb 238 u age at 245 3 to 240 2 ma and 226 4 to 223 2 ma respectively anomalously high 18 o values were obtained for refractory minerals with 9 9 to 21 4 for garnet and 16 9 for zircon this indicates that eclogite protolith is sedimentary rocks capable of liberating aqueous fluid for zircon growth during continental subduction zone metamorphism most of the zircons are characterized by very low 176 lu 177 hf ratios of 0 000001 0 000028 indicating their growth in association with garnet recrystallization a few of them falling within the older age group have comparatively high 176 lu 177 hf ratios of 0 000192 0 000383 suggesting their growth prior to the formation of garnet in the late stage of subduction the variations in the lu hf ratios for zircons can thus be used to correlate with garnet growth during eclogite facies metamorphism in either case the zircons have variable e hf t values for individual samples suggesting that their protolith is heterogeneous in hf isotope composition with localized fluid availability in the bulk processes of orogenic cycle nevertheless a positive correlation exists between 206 pb 238 u ages and lu hf isotope ratios for the metamorphically recrystallized zircons suggesting that eclogite facies metamorphism in the presence of fluid has the identical effect on zircon lu hf and u th pb isotopic systems we conclude that the zircons of the older group grew in the presence of fluid during the subduction prior to the onset of peak ultrahigh pressure metamorphism whereas the younger zircons grew in the presence of fluid released during the initial exhumation toward high pressure eclogite facies regime

the northern boundary of the proto tethys ocean constraints from structural analysis and u pb zircon geochronology of the north qinling terrane abstract the north qinling terrane including the kuanping erlangping and qinling groups is located in the northern part of the qinling orogen here we present results from structural analyses of the north qinling terrane and its surrounding regions three episodes of deformation d 1 d 2 and d 3 are recognized the d 1 deformation is characterized by nne dipping and ssw dipping penetrative foliations s 1 the s 1 foliations are mainly marked by a strong preferred orientation of hornblende crystals and they were subsequently overprinted by the d 2 deformation the d 2 deformation is marked by tight asymmetric folds with nne dipping and ssw dipping axial plane foliations s 2 we propose that the d 1 and d 2 deformations mainly occurred during 440 400 ma after the peak ultra high pressure metamorphic event 520 480 ma and so these two deformation events could constrain the collision and exhumation processes of the north qinling terrane which probably indicates the closure of the kuanping ocean located between the north qinling terrane and the north china block in the early paleozoic these two deformations are overprinted by the d 3 deformation characterized by ene trending upright open folds two detrital zircon age spectra for the kuanping group with the youngest zircon of 640 ma and 510 ma respectively are different from those recorded for the north china block which may reveal the continuous southward subduction of the kuanping ocean during the neoproterozoic to early paleozoic interval we propose that the kuanping ocean marks the northern boundary of the proto tethys ocean in the study area

near isothermal conditions in the middle and lower crust induced by melt migration simple one dimensional thermal models that include the effects of melt migration have been developed the models show that long lived plutonism results in a quasi steady state geotherm with a rapid temperature increase in the upper crust and nearly isothermal conditions in the middle and lower crust the thermal structure of the crust strongly influences deformation metamorphism and plutonism1 2 3 models for the geothermal gradient in stable crust predict a steady increase of temperature with depth this thermal structure however is incompatible with observations from high temperature metamorphic terranes exhumed in orogens1 4 5 6 global compilations7 of peak conditions in high temperature metamorphic terranes define relatively narrow ranges of peak temperatures over a wide range in pressure for both isothermal decompression and isobaric cooling paths here we develop simple one dimensional thermal models that include the effects of melt migration these models show that long lived plutonism results in a quasi steady state geotherm with a rapid temperature increase in the upper crust and nearly isothermal conditions in the middle and lower crust the models also predict that the upward advection of heat by melt generates granulite facies metamorphism and widespread andalusite sillimanite metamorphism in the upper crust once the quasi steady state thermal profile is reached the middle and lower crust are greatly weakened due to high temperatures and anatectic conditions thus setting the stage for gravitational collapse8 exhumation and isothermal decompression after the onset of plutonism near isothermal conditions in the middle and lower crust result from the thermal buffering effect of dehydration melting reactions that in part control the shape of the geotherm

wave transformation on a coral reef wave transformation of regular waves was measured in a laboratory model of a fringing reef with a steep face and an outer reef top slope gradually decreasing in the landward direction data was obtained for various wave conditions and water levels a nonlinearity parameter fco g1 25ho 0 5t2 5 hc 1 75 based upon one proposed by swart and loubser 1979 is proposed as a suitable parameter for classifying wave transformation regimes on this reef in particular when fco 150 waves plunge on the reef edge and the amount of wave energy reaching a shore or structure is small 16 when fin co 100 waves spill on the reef top but the greater part of their energy is transmitted over the reef top the maximum values of the wave height to water depth ratio on the reef top were found to be consistent with nelson s analyses for laboratory and field data which indicate that the maximum stable wave height to depth ratio h d on a horizontal bottom never exceeds 0 55 for shallow water waves fc 500 the experimental data confirms that the maximum value of h d decreases when fc decreases but that it also increases when the bottom slope increases

tectonic evolution of the east qinling mountains china in the palaeozoic a review and new tectonic model east asia is a composite assemblage built up from various plates and the qinling mountains are prominent among the orogenic belts dividing these plates because of its complex and prolonged history interpretation of the geology of the qinling mountains has led to a variety of tectonic models we review and summarize recently published data and previous work and develop a new tectonic model for the eastern part of this mountain range four tectonic zones are recognized from north to south the lesser qinling the north qinling the central qinling and the south qinling zones the lesser and north qinling zones are interpreted as a late proterozoic to early palaeozoic passive continental margin bordering the north china block the central qinling separated from the lesser north qinling zone by a suture zone mainly consists of volcanic arcs and a high grade metamorphic complex the south qinling zone forms part of the northern margin of the yangtze block and has a long latest proterozoic to middle triassic depositional history as revealed by its sedimentary cover available data do not support the hypothesized existence of a major suture divide between the central and the south qinling zones these two zones seem to have collectively formed an early palaeozoic destructive continental margin on the northern edge of the yangtze block with the central qinling as an island arc system and the south qinling as a wide highly attentuated marginal basin late ordovician to early silurian collision between the north china and the yangtze block built up the palaeozoic orogenic belt in the northern part of the qinling and gave rise to the embryonic east asian continent yet the south qinling was transformed to a remnant basin and remained undeformed until the late triassic when the palaeo tethys collision to the south of the yangtze block drove the yangtze block to move northeastwards deformation in a transpressive regime during this late event largely reconstructed the qinling belt and hence further complicated its tectonics

zircon u pb ages and hf isotope compositions of migmatites from the north qinling terrane and their geological implications migmatites are predominant in the north qinling nq orogen but their formation ages are poorly constrained this paper presents a combined study of cathodoluminescence imaging u pb age trace element and hf isotopes of zircon in migmatites from the nq unit in the migmatites most zircon grains occur as new homogeneous crystals while some are present as overgrowth rims around inherited cores morphological and trace element features suggest that the zircon crystals are metamorphic and formed during partial melting the inherited cores have oscillatory zoning and yield u pb ages of c 900 ma representing their protolith ages the early neoproterozoic protoliths probably formed in an active continental margin being a response to the assembly of the supercontinent rodinia the migmatite zircon yields hf model ages of 1911 20 to 990 22 ma indicating that the protoliths were derived from reworking of palaeoproterozoic to neoproterozoic crustal materials the anatexis zircon yields formation ages ranging from 455 5 to 420 4 ma with a peak at c 435 ma combined with previous results we suggest that the migmatization of the nq terrane occurred at c 455 400 ma the migmatization was c 50 ma later than the c 490 ma ultra high p uhp metamorphism indicating that they occurred in two independent tectonic events by contrast the migmatization was coeval with the granulite facies metamorphism and the granitic magmatism in the nq unit which collectively argue for their formation due to the northward subduction of the shangdan ocean uhp rocks were distributed mainly along the northern margin and occasionally in the inner part of the nq unit indicating that they were exhumed along the northern edge and detached from the basement by the subsequent migmatization process

silurian granulite facies metamorphism and coeval magmatism and crustal growth in the tongbai orogen central china abstract it is intriguing whether the formation of granulite in an orogenic belt is related to magmatism and crustal growth the tongbai orogen is one of the particular areas in the qinling tongbai dabie sulu orogen that contains both granulites and associated arc magmatic rocks and thus is an optimal place to address this issue however the formation age and genesis of the granulites and their relations to the magmatic rocks have not yet been well constrained in situ la mc icpms u pb dating and trace element and lu hf isotope analyses were undertaken on zircon grains from gneiss granulite gabbro and granodiorite samples most zircons in three granulite samples and a gneiss sample have core rim structures the cores have oval or rounded shapes oscillatory zones high th u ratios and variable u pb ages suggesting a detrital genesis the youngest group of the detrital zircons yields a weighted mean 206 pb 238 u age of 450 5 ma which is interpreted as the maximum depositional age of their protoliths a major age cluster is ca 450 490 ma corresponding to prolonged arc magmatism in the northern tongbai and qinling orogens another group of ages range from 660 to 950 ma implying that the qinling group of the tongbai orogen belongs to the south china block scb metamorphic zircons in the granulites have weak or no zoning low th u ratios and ree contents negative eu anomalies and relatively flat hree patterns which indicate that they formed contemporaneously with garnet and plagioclase and thus under granulite facies metamorphic conditions their weighted mean age of 424 4 ma is taken as the best estimated age of the granulite facies metamorphism zircons in a leucosome sample have significant negative eu anomalies and relatively flat hree patterns with a formation age of 428 4 ma indicating partial melting coeval with the granulite facies metamorphism metamorphic zircons in the gneiss show apparently negative eu anomalies and variable th u ratios and hree contents they yield a weighted mean 206 pb 238 u age of 438 4 ma which was suggested to record the timing of the prograde metamorphism zircon grains in a gabbro and a granodiorite yield u pb ages of 432 4 and 424 4 ma respectively which are interpreted as their formation ages and identical to the ages of the granulite facies metamorphism they have positive e hf t values of up to 5 1 showing clear evidence for crustal growth during their formation the appearance of the silurian granulite facies metamorphism and magmatism in the tongbai orogen provides a good paradigm of coeval granulite facies metamorphism magmatism and crustal growth in an arc continent collision orogenic belt

tectonics of the qinling central china tectonostratigraphy geochronology and deformation history abstract the qinling orogen preserves a record of late mid proterozoic to cenozoic tectonism in central china high pressure metamorphism and ophiolite emplacement songshugou ophiolite assembled the yangtze craton including the lower qinling unit into rodinia during the 1 0 ga grenvillian orogeny the lower qinling unit then rifted from the yangtze craton at 0 7 ga subsequent intra oceanic arc formation at 470 490 ma was followed by accretion of the lower qinling unit first to the intra oceanic arc and then to the sino korea craton subduction then imprinted a 400 ma andean type magmatic arc onto all units north of the northern liuling unit oblique subduction created silurian devonian wnw trending sinistral transpressive wrench zones e g lo nan shang dan and late permian early triassic subduction reactivated them in dextral transpression lo nan shang xiang shang dan and subducted the northern edge of the yangtze craton exhumation of the cratonal edge formed the wudang metamorphic core complex during dominantly pure shear crustal extension at 230 235 ma post collisional south directed shortening continued through the early jurassic cretaceous reactivation of the qinling orogen started with nw se sinistral transtension coeval with large scale early cretaceous crustal extension and sinistral transtension in the northern dabie shan it presumably resulted from the combined effects of the siberia mongolia sino korean and lhasa west burma qiangtang indochina collisions and pacific subduction regional dextral wrenching was active within a ne sw extensional regime between 60 and 100 ma an early cretaceous andean type continental magmatic arc with widespread early cretaceous magmatism and back arc extension was overprinted by shortening related to the collision of yangtze indochina block with the west philippines block strike slip and normal faults associated with eocene half graben basins record paleogene nne ssw contraction and wnw ese extension the neogene is characterized by normal faults and nne trending sub horizontal extension pleistocene quaternary nw se extension and ne sw contraction comprises sinistral strike slip faults and is part of the nw se extension imposed across eastern asia by the india asia collision

u pb zircon ages constrain the architecture of the ultrahigh pressure qinling dabie orogen china new shrimp and tims zircon ages 40ar 39ar ages and eclogite locations contribute significantly to our understanding of the ultrahigh pressure dabie shan 1 the geographic extent of the yangtze craton that was subducted to ultrahigh pressure extends to the northern edge of the dabie shan 2 the northern half of the dabie shan is a magmatic complex intruded over a 10 myr interval between 137 and 126 ma that accommodated 100 n s stretching of the pre existing collisional architecture 3 granitic orthogneisses and enclosing ultrahigh pressure paragneisses have indistinguishable zircon populations the population of triassic zircon ages ranges from 219 to 245 ma leading us to question the prevailing assumption that 219 ma zircons formed at ultrahigh pressure and to propose instead that they reflect late retrogression at crustal pressures following the bulk of exhumation

geologic framework and tectonic evolution of the qinling orogen central china abstract the geologic framework of the qinling orogen was built up through interplay of three blocks the north china block including the north qinling the south qinling and the south china block separated by the shangdan and mianlue sutures the shangdan suture resulted from middle paleozoic collision of the north china block and the south qinling the mianlue suture resulted from late triassic collision of the south qinling and the south china block present upper crust of the qinling is structured dominantly by thrust fold systems the north qinling displays thick skinned deformation with crystalline basement involved whilst the south qinling is characterized by thin skinned thrusts and folds detached above the lower sinian two types of precambrian basement crystalline and transitional are defined according to lithology and metamorphic grade and different in age stratigraphic and sedimentary architecture is characterized by distinct zonation the qinling orogen experienced a prolonged continental divergence and convergence between blocks during the period from late neoproterozoic to early paleozoic times the south qinling was the northern margin of the south china block and the north qinling was the southern margin of the north china block separated by a proto tethyan qinling ocean the north qinling evolved into an active margin when the proto tethyan qinling ocean subducted northward during ordovician time collision of the south and north qinling took place in middle paleozoic along the shangdan suture synchronous with the collision rifting occurred at the southern rim of the south qinling and was followed by the opening of the paleo tethyan qinling ocean during the late paleozoic resulting in the splitting of the south china block from the south qinling collision of the south qinling and the south china block came about in the late triassic along the mianlue suture the late triassic collisional orogeny caused extensive fold and thrust deformation and granitoid intrusions throughout the qinling and led to final amalgamation of the north and south china blocks

episodic growth of the gondwana supercontinent from hafnium and oxygen isotopes in zircon it is thought that continental crust existed as early as 150 million years after planetary accretion but assessing the rates and processes of subsequent crustal growth requires linking the apparently contradictory information from the igneous and sedimentary rock records for example the striking global peaks in juvenile igneous activity 2 7 1 9 and 1 2 gyr ago imply rapid crustal generation in response to the emplacement of mantle super plumes rather than by the continuous process of subduction yet uncertainties persist over whether these age peaks are artefacts of selective preservation and over how to reconcile episodic crust formation with the smooth crustal evolution curves inferred from neodymium isotope variations of sedimentary rocks detrital zircons encapsulate a more representative record of igneous events than the exposed geology and their hafnium isotope ratios reflect the time since the source of the parental magmas separated from the mantle these model ages are only meaningful if the host magma lacked a mixed or sedimentary source component but the latter can be diagnosed by oxygen isotopes which are strongly fractionated by rock hydrosphere interactions here we report the first study that integrates hafnium and oxygen isotopes all measured in situ on the same precisely dated detrital zircon grains the data reveal that crust generation in part of gondwana was limited to major pulses at 1 9 and 3 3 gyr ago and that the zircons crystallized during repeated reworking of crust formed at these times the implication is that the mechanisms of crust formation differed from those of crustal differentiation in ancient orogenic belts

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

wave set up on coral reefs 1 set up and wave generated flow on an idealised two dimensional horizontal reef wave set up may be significant in determining water levels on coral reefs particularly in microtidal environments and hence is an important factor for the design of reef top structures and for the stability of reef top islands laboratory experiments have been made on a two dimensional model of an idealised horizontal reef under two different conditions corresponding to a fringing reef or closed lagoon situation and a platform reef or open lagoon situation both wave set up on the reef top and the wave generated flow across the reef were measured and related to wave and tide level conditions all other factors being the same wave set up is greatest at low tide levels whereas wave generated flow is greater at higher tide levels the magnitude of the set up on a platform reef with a wave generated flow is less than on a fringing reef without any net flow by an amount equal to the velocity head of the flow across the reef dimensionless parameters ghrt o and q gho3 are found to be functions of relative submergence parameters hrho or r hr ho for values of ghr hr ho 1 waves break on the reef top and radiation stress theory can be used to calculate set up for ghr hr ho 0 7 waves break on the reef face and set up is determined by broadcrested weir control at the reef edge the symbols are defined as follows g is gravitational acceleration hr is still water depth over horizontal reef top ho is offreef wave height equivalent deep water value q is discharge per unit length of reef edge t is wave period and ghr is maximum wave set up on reef top

eclogite origin and timings in the north qinling terrane and their bearing on the amalgamation of the south and north china blocks the amalgamation of south scb and north china blocks ncb along the qinling dabie orogenic belt involved several stages of high pressure hp ultra high pressure uhp metamorphism the new discovery of uhp metamorphic rocks in the north qinling nq terrane can provide valuable information on this process however no precise age for the uhp metamorphism in the nq terrane has been documented yet and thus hinders deciphering of the evolution of the whole qinling dabie sulu orogenic belt this article reports an integrated study of u pb age trace element mineral inclusion and hf isotope composition of zircon from an eclogite a quartz vein and a schist in the nq terrane the zircon cores in the eclogite are characterized by oscillatory zoning or weak zoning high th u and 176lu 177hf ratios pronounced eu anomalies and steep heavy rare earth element hree patterns the zircon cores yield an age of 796 13 ma which is taken as the protolith formation age of the eclogite and implies that the nq terrane may belong to the scb before it collided with the ncb the hf t values vary from 11 3 to 3 2 and corresponding two stage hf model ages are 2402 to 1495 ma suggesting the protolith was derived from an enriched mantle in contrast the metamorphic zircon rims show no zoning or weak zoning very low th u and 176lu 177hf ratios insignificant eu anomalies and flat hree patterns they contain inclusions of garnet omphacite and phengite suggesting that the metamorphic zircon formed under eclogite facies metamorphic conditions and their weighted mean 206pb 238u age of 485 9 3 8 ma was interpreted to date the timing of the eclogite facies metamorphism zircon in the quartz vein is characterized by perfect euhedral habit some oscillatory zoning low th u ratios and variable hree contents it yields a weighted mean u pb age of 480 5 2 5 ma which registers the age of fluid activity during exhumation zircon in the schist is mostly detrital and u pb age peaks at c 1950 to 1850 1800 to 1600 1560 to 1460 and 1400 to 1260 ma with an oldest grain of 2517 ma also suggesting that the nq terrane may have an affinity to the scb accordingly the amalgamation between the scb and the ncb is a multistage process that spans c 300 myr which includes the formation of the erlangping intra oceanic arc zone onto the ncb before c 490 ma the c 485 ma crustal subduction and uhp metamorphism of the nq terrane the c 430 ma arc continent collision and granulite facies metamorphism the 420 to 400 ma extension and rifting in relation to the opening of the palaeo tethyan ocean the c 310 ma hp eclogite facies metamorphism of oceanic crust and associated continental basement and the final 250 to 220 ma continental subduction and hp uhp metamorphism

hot orogens tectonic switching and creation of continental crust comment and reply comment in a stimulating paper collins 2002 1 p 535 asserted that many granulite terrains were too hot to have formed during continental collision in spite of evidence to the contrary e g harley 1989 2 o brien and rotzler 2003 3 based on this assertion collins 2002 1 p

structural style and geochronology of ductile shear zones in the western north qinling orogenic belt central china implications for paleozoic orogeny in the central china orogeny abstract the qinling orogen which was formed by long lived subduction and collision between the north and south china blocks can be divided into the south qinling orogenic belt sqob and the north qinling orogenic belt nqob by the shangdan suture zone orogen scale shear zones developed on the boundaries within the nqob can be used to enhance our understanding of collisional orogeny that occurred during the paleozoic in the qinling orogen previous studies have identified several ductile shear zones on boundaries between different lithotectonic units in the eastern nqob however ductile shear zones related to orogenic processes in the western nqob have attracted relatively little attention herein we present structural and chronological data for the ductile shear zones in the tianshui region western nqob three sub parallel ductile shear zones dsz 1 dsz 2 and dsz 3 are recognised in the southern qinling complex or as boundaries between the qinling complex and adjacent lithotectonic units in the huamiao area tianshui city microstructural and fabric analyses indicate that these shear zones have kinematics of transpressive sinistral strike slip zircon u pb and 40ar 39ar dating data suggest that these ductile shear zones experienced a prolonged period of deformation from 420 to 370 ma during exhumation which was related to oblique convergence between the sqob and the nqob combining with the synchronous xinyang yuanlong dextral shear zone in the northern qinling complex we suggest that the transpressive sinistral shearing and dextral transpressive ductile shearing on both sides of the qinling complex subjected it to orogen parallel oblique slip extrusion during the middle paleozoic our results together with previously published data in the eastern nqob indicate the occurrence of orogen scale strike slip shearing along the whole nqob during the paleozoic orogeny

early paleozoic island arc accretion to the north china craton and the shang dan fault zone a major paleoplate boundary in eastern asia the early paleozoic tectonic evolution of part of the qinling dabie shandong orogenic belt is reconstructed on the basis of the results of field mapping geochemical and geochronological studies the qinling orogenic belt separates the archean proterozoic north china craton ncc from the south china craton scc also yangtze craton the study area is located about 90 km southwest of the city of xi an three lithologically distinctive zones separated from each other by fault zones can be distinguished passive continental margin deposits of the ncc zone 1 have end t values between 6 16 and 12 54 and tdm ages between 1 29 and 2 20 ga they are separated from an early paleozoic island arc sequence zone 2 by the sinistral chenche fault mafic volcanic lithologies in the southern part of zone 1 are associated with the development of a passive continental margin in late proterozoic time similiar lithologies of zone 2 have geochemical characteristics of an oceanic island arc accretion was accompanied by a strong deformation event between early ordovician and late silurian time magmatic intrusions in zone 2 and the southern part of zone 1 are associated with a short lived magmatic arc that developed above a north dipping subduction zone on the modified continental margin of the ncc in late silurian to early devonian time the end t values of felsic intrusions from the northern part of zone 2 decrease in a northern direction from 1 78 to 1 05 mafic to intermediate intrusions in the southern part of zone 2 have positive end t values from 1 28 to 2 04 as well as low 87sr 86sr i values between 0 705 and 0 706 indicating an increasing mantle component toward the south low grade metamorphic pelites are separated from the island arc sequence of zone 2 by a 2 km wide shang dan shear zone isotopic studies show strong similarities between a granite sample from zone 3 and samples from the scc 87sr 86sr i values increase from 0 709 to 0 713 whereas end t values decrease from 1 90 to 5 34 in a southern direction the shear zone that separates zone 2 from 3 is of fundamental importance and marks the boundary between the ncc and the scc the results presented in this paper document a previously unknown early paleozoic tectonic evolution of the qinling orogenic belt and delineate one of the fundamental paleoplate boundaries in eastern asia

hot orogens tectonic switching and creation of continental crust many granulite terrains were too hot to have formed during continental collision rather along with many high grade metamorphic terrains that typify continental crust most formed in accretionary orogens during tectonic switching when prolonged lithospheric extension was interrupted by intermittent transient contraction based on modern and ancient examples tectonic switching occurs when slab retreat induces upper plate extension causing arc splitting formation of microcontinent slivers and backarc basins then intermittent arrival of buoyant oceanic plateaus induces transient flat subduction or slab flip and crustal thickening during slab retreat basaltic magmas produced from decompressed asthenosphere advect into the extending orogen causing granulite facies metamorphism and granite generation but subsequent thickening during flat subduction cools the region thickening is focused in the thermally softened backarc region and if sediment filled a hot short lived 10 m y narrow 50 100 km orogenic belt forms such thickening is often misleadingly ascribed to arc or microcontinent collision once slab retreat mode is reestablished lithospheric extension recommences and a new arc backarc system forms generally outboard arrival of another plateau will reverse the procedure and another short lived hot orogen will form within the orogenic system cycles of tectonic switching efficiently produce continental crust

wave attenuation on an offshore coral reef the wave climate along the northeastern tropical coastline of australia is controlled by the great barrier reef gbr however the processes by which the gbr attenuates and transforms waves are little understood as the first part of an on going study of the interaction between waves and coral reefs a field experiment was conducted to study the processes that occur as waves break on an offshore reef and proceed across the reef flat into the lagoon eighteen wave water level and current measuring instruments were deployed and data for a wide range of tide and wave conditions were collected preliminary results for wave attenuation are presented results for wave attenuation across the reef show that wave heights on the reef flat and in its lagoon are controlled by the depth of water over the windward reef flat as the waves travel across the reef flat the ratio of significant wave height to water depth reduces to a value of 0 40 and the ratio of maximum wave height to water depth reduces to a value of 0 6 to 0 8 in the deeper water in the middle of the reef lagoon both the ratios of significant wave height to the depth over the reef flat and maximum wave height to the depth over the reef flat remain in the above ranges however at the mid lagoon position these results are less general as wave heights inside a lagoon are also dependant on wind speed direction and fetch length inside the lagoon

oblique closure of the northeastern paleo tethys in central china a branch of the paleo tethys ocean once separated the north china plate from the south china plate however the mode of closure of the northeastern paleo tethys ocean during the late paleozoic to early mesozoic has been debated one reason for this debate is that the collisional suture zone was later buried by large scale thrust faults in the southern qinling dabieshan orogen which made it difficult to reconstruct the amalgamation of the supercontinent in central china new regional geologic mapping provides stratigraphic and structural constraints on the mechanism of this ocean closure our results indicate that dextral transpressional suturing in the southern qinling dabieshan foreland fold thrust belt resulted in the formation of the northern yangtze foreland basin where the stratigraphy precisely shows the time transgressive closure of the ocean and the orogenic sediments shed over 1000 km westward from eastern china to the closing paleo tethys therefore we propose an oblique subduction model to describe the closure of the paleo tethys ocean our findings suggest that prolonged slab pull during the oblique subduction of the oceanic plate continued to drive deep continental subduction thereby forming high and ultrahigh pressure metamorphic rocks and leading to sustained ocean closure

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

variability of depth limited waves in coral reef surf zones wave breaking and transformation on coral reef flats is an important process protecting tropical coastlines and regulating the energy regimes of coral reefs however the high hydraulic roughness shallow water and steep bathymetries of coral reefs may confound common surf zone assumptions such as a depth limited and saturated surf zone with a constant wave height to water depth ratio here we examine wave transformation across a coral reef flat during three separate swell events on both a time averaged and a wave by wave basis we use the relationship between significant wave height and water depth to examine the change in surf saturation across the reef flat and compare the measured wave height decay to results of modelled wave energy dissipation in the surf zone our results show that was not cross reef constant and varied according to location on the reef flat and local water depth on average was greatest at the outer reef flat near the reef crest and progressively reduced towards the inner reef flat near the reef lagoon this was most pronounced in shallow water with large values 0 85 at the outer reef flat and small values 0 1 at the inner reef flat this indicates that there is an increase in wave energy dissipation in shallow water most likely due to increased breaker and bed frictional dissipation the measured wave energy dissipation across the entire reef flat could on average be modelled accurately however this required location specific calibration of the free parameters the wave friction factor f and and further suggests that there is no value for either parameter that is universally applicable to coral reef flats despite model calibration inaccuracies were still observed primarily at the outer reef flat these inaccuracies reflected the observed cross reef variation of on the reef flat and potentially the limitations of random wave breaker dissipation models in complex surf zones our results have implications for the use of wave energy dissipation models in predicting breaker dissipation and subsequent benthic community change on coral reef flats and suggest that careful consideration of the free parameters in such models such as f and is required

palaeozoic polymetamorphism in the north qinling orogenic belt central china insights from petrology and in situ titanite and zircon u pb geochronology abstract the qinling orogenic belt experienced multiple phases of orogenesis during the palaeozoic unraveling the timing and p t conditions of these events is the key to understanding the convergence processes between the south china and the north china blocks the songshugou complex located in the southern part of the north qinling orogenic belt has registered multistage metamorphism in palaeozoic and thus potentially provides insights into the tectonic evolution of the qinling orogenic belt in this study three metabasic rocks a garnet pyroxenite a garnet amphibolite and a gneissic amphibolite from the songshugou complex were selected for petrological study and zircon and titanite u pb dating our results show that the metabasic rocks experienced three metamorphic events during the palaeozoic the first metamorphic event m1 is characterized by high pressure conditions two zircon grains in equilibrium with garnet and in absence of plagioclase were recognized from the garnet pyroxenite sample they yielded ti in zircon temperatures of 660 851 c at 12 0 kbar and a weighted mean age of 498 15 ma providing the constraints on the temperature and timing of prograde or peak metamorphism m1 1 zircons that are inequilibrium with garnet from the garnet pyroxenite and the garnet amphibolite gave u pb ages of 494 9 ma and 484 4 ma and ti in zircon temperatures of 793 33 c and 738 18 c respectively thus these zircons were formed on the retrograde amphibolite facies conditions at 8 0 kbar m1 2 titanite inclusions were found in actinolite cores of zoned amphibole from the garnet amphibolite they yielded a u pb age of 470 ma and zr in titanite temperature of 676 23 c at pressure of 7 0 kbar suggesting that the amphibolite facies retrogression perhaps persisted to 470 ma weakly zoned zircons from the garnet amphibolite and inclusion free titanites from the garnet pyroxenite gave consistent u pb ages of 418 5 ma and 423 10 ma and ti in zircon temperature of 742 26 c and zr in titanite temperature of 764 18 c at 7 0 kbar respectively it is suggested that a heating event m2 is registered by a subsequent phase of amphibolite facies metamorphism the ilmenite bearing titanite crystals from the garnet pyroxenite yielded a u pb age of 352 4 ma recording a late thermal event m3 on the basis of combined petrological and geochronological results we propose a revised tectonic model for the north qinling orogeny in palaeozoic the high pressure granulites were formed by the northward subduction of the shangdan oceanic slab and the arc continent collision at ca 500 ma their exhumation happened at ca 494 484 ma as a result of slab breakoff subsequent amphibolite facies metamorphism dated at ca 440 420 ma are coeval with the widespread magmatism in the north qinling terrane which are likely caused by the reinitiation northward subducted of shangdan oceanic slab at ca 350 ma the north qinling terrane was likely affected by another thermal overprinting event

baroclinic instability in an environment of small stability to slantwise moist convection part i two dimensional models abstract in the semigeostrophic system the growth rate of baroclinic waves varies with the inverse square root of the potential vorticity which acts as the effective static stability recent observations in the ascent regions of middle latitude cyclones show that the effective potential vorticity for saturated air is very near zero in this paper we examine the structure and rate of growth of baroclinic cyclones when the effective potential vorticity is small for upward saturated displacements but large in regions of descent analytic solutions for two dimensional disturbances in a two layer semigeostrophic model and numerical simulations using a multilevel semigeostrophic model show that when the effective potential vorticity is small in regions of upward motion growth rates are modestly increased and the region of ascent intensifies and collapses onto a thin ascending sheet in the limit of zero moist potential vorticity the fastest growing wave has a finite growth rate which is about 2 5 times the

timing of uhp metamorphism in the hong an area western dabie mountains china evidence from zircon u pb age trace element and hf isotope composition the hong an area western dabie mountains is the westernmost terrane in the qinling dabie sulu orogen that preserves uhp eclogites the ages of the uhp metamorphism have not been well constrained and thus hinder our understanding of the tectonic evolution of this area la icpms u pb age trace element and hf isotope compositions of zircons of a granitic gneiss and an eclogite from the xinxian uhp unit in the hong an area were analyzed to constrain the age of the uhp metamorphism most zircons are unzoned or show sector zoning they have low trace element concentrations without significant negative eu anomalies these metamorphic zircons can be further subdivided into two groups according to their u pb ages and trace element and lu hf isotope compositions one group with an average age of 239 2 ma show relatively high and variable hree contents 527 lun 14 and 176lu 177hf ratios 0 00008 0 000931 indicating their growth prior to a great deal of garnet growth in the late stage of continental subduction the other group yields an average age of 227 2 ma and shows consistent low hree contents and 176lu 177hf ratios suggesting their growth with concurrent garnet crystallization and or recrystallization these two groups of age are taken as recording the time of prograde hp to uhp and retrograde uhp hp stages respectively a few cores have high th u ratios high trace element contents and a clear negative eu anomaly these features support a magmatic origin of these zircon cores the upper intercept ages of 771 86 and 752 70 ma for the granitic gneiss and eclogite respectively indicate that their protoliths probably formed as a bimodal suite in rifting zones in the northern margin of the yangtze block young hf model ages tdm1 of magmatic cores indicate juvenile mantle derived materials were involved in their protolith formation