formation and palaeozoic evolution of the gorny altai altai mongolia suture zone south siberia zircon u pb constraints on the igneous record abstract the altai sayan fold belt epitomises how phanerozoic island arc continental crust contributed to the growth of the palaeo eurasian continent its formation is dominated by the closure of the palaeo asian ocean pao when island arc systems and gondwana derived terranes accreted to the siberian margin relics of the pao related tectonic units and associated granitoids occur along the ophiolitic charysh terekta ulagan sayan suture ctuss between gorny altai and altai mongolia zircon la icp ms u pb dating of this igneous record constrains the multi stage geodynamic pao evolution primitive kuznetsk altai island arc crust formed at the siberian margin during the ediacaran early cambrian 525 555 ma this island arc matured during the middle late cambrian 510 ma and was consumed by pao subduction in the late cambrian early ordovician 480 490 ma forming the siberian early caledonian accretion collision belt south of the ctuss granitic magmatism occurred within gondwana derived altai mongolia during the middle late ordovician 450 470 ma palaeo kazakhstan assembly and during a silurian early devonian 400 425 ma andean type collision middle late devonian 360 395 ma granitoids were emplaced as a result of the collision of altai mongolia with siberia during final pangaea amalgamation the suture was strike slip reactivated associated magmatism 295 ma the youngest 220 255 ma sampled granitoids originated in an intra plate setting

evolution of the archean and paleoproterozoic lower crust beneath the trans north china orogen and the western block of the north china craton abstract we report u pb ages and hf isotopes of zircons in granulites from two continental suture zones in the north china craton ncc from granulite xenoliths entrained in the cenozoic hannuoba basalts within the trans north china orogen tnco that amalgamates the western and eastern blocks and from a mafic granulite associated with ultrahigh temperature uht granulites within the inner mongolia suture zone imsz that welds the yinshan and ordos blocks the zircons in these rocks together with the inherited zircons from mesozoic subalkalic intrusives of these regions show several distinct u pb age populations and provide constraints on the evolution of the lower crust beneath the ncc parts of the older zircons from the tnco yield discordant data that project to ages of about 2 5 to 2 7 ga these old zircons show hf tdm model ages of 2 6 2 9 ga suggesting the existence of archean lower crust beneath the tnco which is coupled with the neoarchean metamorphic basement and greenstone belt a major zircon population grew at 1 75 2 0 ga consistent with the timing of the paleoproterozoic collision between the various crustal blocks within the ncc suggesting that this event had a significant effect on the archean lower crust the older inherited zircons from the western block also yield discordant data that project to younger ages of about 1 8 to 1 9 ga this age is consistent with the timing of intrusion of the mafic magmatic protoliths of the granulites associated with the uht rocks in the khondalite belt within the imsz these older zircons yield hf tdm model ages of 2 0 2 5 ga suggesting that the lower crust beneath the western block in the ncc to be dominantly paleoproterozoic and decoupled with the archean metamorphic basement of the region yet another population of zircons from the hannuoba banded granulite xenoliths garnet bearing pyroxenite cumulate and dark pyroxenite xenoliths yield a large spread of phanerozoic concordant ages ranging from 470 ma to 40 ma with peaks at 315 ma 220 230 ma 120 ma and 46 ma the late paleozoic zircons predominantly occur in the garnet bearing pyroxenite xenoliths and yield variable ehf t values of 24 to 18 triassic cretaceous zircons mainly occur in the banded granulite xenoliths and yield highly variable ehf t values ranging from 48 to 16 the tertiary zircons occur only in the dark pyroxenite xenoliths and show a restricted ehf t with a dominant range from 0 to 18 these data suggest episodic magmatic underplating in the ancient lower crust of the ncc lasting continuously throughout phanerozoic producing zircons from the underplated magmas or providing the heat source for the recrystallization of zircons from the ancient crust our data clearly demonstrate the important events of phanerozoic magmatic underplating beneath the neoarchean paleoproterozoic crust of the ncc

structural architecture of the northern composite terrane the eastern ghats mobile belt india implications for gondwana tectonics abstract new data from structural mapping and tectonic evaluation in the northern parts of the eastern ghats mobile belt egmb north involving the interpretation of satellite images field traverses critical outcrop mapping and kinematic studies of macro as well as microstructures of the shear zone rocks together with the geometry and disposition of gondwana basins led to for the first time the elucidation of post grenvillian structural architecture of the terrane this helps in assessing the sequence of successive tectonothermal events that were responsible for the origin and progressive evolution of the permo carboniferous coal bearing sediments along the mahanadi rift that forms significant in the reconstruction models of east gondwana the composite terrane of high grade metamorphic rocks egmb north strikes e w in contrast to the regional ne sw trend of the egmb the structural architecture obtained from this study is controlled by the boundary shear zones and associated link shear zones the dextral kinematic displacements along the northern boundary shear zone nbsz as well as the mahanadi shear zone msz and koraput sonapur rairakhol shear zone ksrsz were derived from multi scale field based structural observations a n s structural cross section presents a crustal scale flower structure across the composite terrane exposing different domains displaying distinctive internal structures with widely varying different geological evolution history and strain partitioning separated by crustal scale shear zones deep seismic imaging and gravity signatures support flower structure model the pervasive first formed gneissic fabrics were continuously reworked and partitioned into a series of e w crustal scale shear zones the neoproterozoic regional dextral transpressional tectonics along the shear zones and their repeated reactivation could be responsible for initiation and successive evolution of gondwana basins and different episodes of sedimentation available geochronological data shows that the structural architecture presented here is post grenvillian which has been repeatedly reactivated through long lived transpressional tectonics the composite terrane is characterized by all the typical features of an oblique convergent orogen with transpressional kinematics in the middle to lower crust the kinematic changes from transpression to transtensional stresses were found to be associated with global geodynamics related to the transformation from rodinia to gondwana configuration

the chimalpahad anorthosite complex and associated basaltic amphibolites nellore schist belt india magma chamber and roof of a proterozoic island arc abstract new major and trace element data on the proterozoic chimalpahad layered anorthositic complex and associated basaltic amphibolites of the nellore schist belt of south india provide new constraints on their petrogenesis and geodynamic setting the complex consists of layered anorthosites leucogabbros gabbros ultramafic rocks and is spatially associated with basaltic amphibolites despite deformation and metamorphism primary cumulate textures and igneous layering are locally well preserved throughout the complex whereas the amphibolites display diverse ree systematics the chimalpahad anorthositic gabbroic rocks are characterized by moderately depleted to strongly enriched lree patterns and by flat to depleted hree patterns the field relations major and trace element compositions of the basaltic amphibolites suggest that they are petrogenetically related to the anorthositic gabbroic rocks by fractional crystallization the anorthositic rocks and the basaltic amphibolites share the depletion of nb relative to th and la on primitive mantle normalized diagrams they exhibit signatures of arc magmatic rocks such as high lile and lree relative to the hfse and hree as well as high ba nb ba zr sr y la yb ratios that mimic chondrite normalized ree and primitive mantle normalized trace element patterns of arc magmas similarly on log transformed tectonic discrimination diagrams the chimalpahad rocks plot within the field of phanerozoic magmatic arcs consistent with a subduction zone origin on the basis of field relations and geochemical characteristics the chimalpahad complex is interpreted as a fragment of a magma chamber of an island arc which is tectonically juxtaposed against its original volcanic cover a new preliminary sm nd date of anorthosite from the chimalpahad complex indicates a model age of 1170 ma

late paleoproterozoic 1900 1800ma nappe stacking and polyphase deformation in the hengshan wutaishan area implications for the understanding of the trans north china belt north china craton abstract the hengshan wutaishan area forms part of the trans north china belt developed in response to the final amalgamation of the north china craton the hengshan wutaishan domain and the adjacent fuping massif constitute the largest and the most representative rock exposure of the belt a structural study allows us to redefine the lithological and tectonic units that compose the architecture of the hengshan wutaishan domain and to propose that the trans north china belt was built up through a polyphase tectonic evolution within the period 1900 1800 ma the first event d1 corresponds to the emplacement of lower and upper nappes herein called the orthogneiss and volcanites unit ovu and the low grade mafic unit lgmu respectively the ovu mainly consists of rocks formed in an arc system and metamorphosed under amphibolite facies conditions whereas the lgmu represents oceanic crust rocks with an arc component that underwent greenschist facies metamorphism the syn metamorphic d1 deformation is characterized by a nw se stretching and mineral lineation with a top to the se sense of shear that corresponds to the sense of nappe motion u th pb epm chemical dating on monazite from three metapelites of the ovu gives isochron ages of 1884 11 ma 1886 5 ma and 1887 4 ma interpreted as the age of the prograde amphibolite facies metamorphism coeval with nappe stacking during the d1 event the d2 event corresponds to a widespread crustal melting of the ovu and underlying basement rocks in hengshan related to the exhumation of deep structures about 20 30 ma after nappe emplacement the migmatite contains blocks of retrograded eclogite or hp granulitic restites and is characterized by the development of a pervasive foliation and lineation a late d2 event represents the folding stage of the migmatitic foliation during the completion of a dome the orthogneiss and volcanics unit and the low grade mafic unit are unconformably covered by weakly metamorphosed to unmetamorphosed sediments belonging to the hutuo supergroup these rocks were deformed by a ductile d3 event characterized by a nw se trending stretching lineation south verging folds associated with an axial planar slaty cleavage and an e w crenulation lineation lastly a left lateral wrenching along the zhujiafang shear zone separates the northern and southern parts of hengshan our structural study shows that there is no significant difference between the southern part of hengshan and the lower part of wutaishan both areas belong to ovu and show a structural continuity we suggest that this polyphase deformation developed in response to a north westward subduction of an old ocean named the luliang ocean beneath the western block followed by collision with a micro continent named the fuping block at 1890 1880 ma

origin of paired high pressure ultrahigh temperature orogens a ridge subduction and slab window model introductionmetamorphism at elevated pressure temperature conditions and distinct uid regimes generates ultrahigh pres sure uhp high pressure hp andultrahigh temperature uht rocks harley 2004 brown 2006 2007 2009 kelsey 2008 santosh andomori 2008a b santos et al 2009 santosh et al 2009a zhang et al 2009 although previous studies havecorrelated these di erent orogens todistinct tectonic settings recent inves tigations record high pressure anduht conditions from the same tec tonic environment within the sameorogenic belt and in some cases evenwithin the same samples mostoftheseexamples come from major collisionalzones along which continental blocksweresuturedatdi erenttimesinearthhistory including from the neoarch ean hp uht rocks of the limpopobeltinsouthafrica tsunogaeandvanreenen 2006 the palaeoproterozoichp guo et al 2004 obrien et al 2005 and uht santosh et al 2007a b 2008 2009b belts in thenorth china craton and the hp uht belts in the neoproterozoiceastern ghats belt as well as thecambrian gondwana suture in south ern india shimpo et al 2006 tsuno gaeet al 2008a santoshet al 2009c notably the thermal regimes for thesepaired hp and uht rocks are similar 1000 c and they also share acommon uid history characterizedby co

the mamonia complex sw cyprus revisited remnant of late triassic intra oceanic volcanism along the tethyan southwestern passive margin upper triassic volcanic and sedimentary rocks of the mamonia complex in southwestern cyprus are exposed in erosional windows through the post cretaceous cover where the mamonia complex is tectonically imbricated with the troodos and akamas ophiolitic suites most of these upper triassic volcanic rocks have been considered to represent remnants of triassic oceanic crust and its associated seamounts new nd and pb isotopic data show that the whole mamonia volcanic suite exhibits features of oceanic island basalts oib four rock types have been distinguished on the basis of the petrology and chemistry of the rocks volcanism began with the eruption of depleted olivine tholeiites type 1 and oceanic island tholeiites type 2 associated with deep basin siliceous and or calcareous sediments the tholeiites were followed by highly phyric alkali basalts type 3 interbedded with pelagic halobia bearing limestones or white reefal limestones strongly lree enriched trachytes type 4 were emplaced during the final stage of volcanic activity nd and pb isotopic ratios suggest that tholeiites and mildly alkali basalts derived from partial melting of heterogeneous enriched mantle sources fractional crystallization alone cannot account for the derivation of trachytes from alkaline basalts the trachytes could have been derived from the partial melting at depth of mafic material which shares with the alkali basalts similar trace element and isotopic compositions this is corroborated by the rather similar isotopic compositions of the alkali basalts and trachytes the correlations observed between incompatible elements nb th and varepsilon nd and pb isotopic initial ratios suggest that the mamonia suite was derived from the mixing of a depleted mantle dmm and an enriched component of high 238u 204pb himu type models using both nd and pb isotopic initial ratios suggest that the depleted tholeiites type 1 derived from a dmm source contaminated by an enriched mantle type 2 component em2 and that the oceanic tholeiites type 2 alkali basalts type 3 and trachytes type 4 were derived from the mixing of the enriched mantle source of the depleted tholeiites with a himu component none of the mamonia volcanic rocks show evidence of crustal contamination the upper triassic within plate volcanism likely erupted in a small southerly neotethyan basin located north of the eratosthenes seamount and likely floored by oceanic crust

well documented travel history of mesozoic pelagic chert in japan from remote ocean to subduction zone the mino tanba belt in southwest japan a segment of the cordilleran type orogenic chain of jurassic east asia is composed mainly of a middle upper jurassic subduction accretion complex in which triassic and lower jurassic bedded radiolarian cherts occur as large allochthonous units structurally interlayered with middle upper jurassic clastic rocks high resolution microfossil conodont and radiolaria research has identified very low average sedimentation rates of about 0 5 g cm 1000 yr in the chert units similar to those of modern pelagic sediments accumulated in open ocean environments judging from the low average sedimentation rate high purity of biogenic silica long duration of continuous deposition 50 m y and wide along strike extent 1000 km the bedded radiolarian cherts in the mino tanba belt are best understood as ancient pelagic sediments that accumulated in open ocean environments accordingly the alleged origin in smaller marginal basins is untenable upward lithologic change from bedded chert to overlying siliceous mudstone in the uppermost portion of chert sequences suggests the gradual landward approach of the oceanic plate toward a trench the tectonic interlayering of these cherts and coarse grained terrigenous elastics is a secondary feature that was added through duplexing underplating in the subduction zone on the basis of the primary stratigraphy and field occurrence of triassic bedded chert in the mino tanba belt newly proposed are an idealized oceanic plate stratigraphy and a generalized travel history of a cordilleran type bedded chert from its birth at a mid oceanic ridge to its demise at a subduction zone

geochemistry and u pb zircon ages of metamorphic volcanic rocks of the paleoproterozoic luliang complex and constraints on the evolution of the trans north china orogen north china craton abstract the paleoproterozoic luliang complex is situated in the central part of the western margin of the trans north china orogen and consists of volcanic rocks sedimentary rocks and paleoproterozoic granitoid intrusions the volcanic rocks and earlier granitoid rocks were strongly deformed and metamorphosed into the greenschist to amphibolite facies these metamorphosed volcanic rocks are dominated by basalts to basaltic andesites the parental mafic magmas of these metamorphosed volcanic rocks were mainly derived from the 5 to 30 partial melting of spinel lherzolites to spinel garnet lherzolites which had been enriched by the subduction melts mafic magma experienced fractional crystallization and crustal assimilation u pb zircon dating on two metamorphosed volcanic rocks from the yejishan and luliang groups reveals that they formed at 2210 13 ma and 2213 47 ma respectively and were metamorphosed at 1832 ma this suggests that the metamorphosed volcanic rocks in the yejishan and luliang groups formed synchronously in the paleoproterozoic these new ages integrated with recently reported u pb zircon ages for the jiehekou group and paleoproterozoic granitoids suggest that all of the lithological assemblages of the luliang complex formed and were metamorphosed in the paleoproterozoic not in the neoarchean petrological geochronological and geochemical data suggest that the geodynamic evolution of the paleoproterozoic luliang complex was involved in the development of a magmatic arc system at an active continental margin generating widespread arc related magmatism at 2 2 ga the luliang complex then underwent intense deformation and metamorphism and was incorporated into the trans north china orogen during the 1 88 1 83 ga collisional event which was followed by post collision extension at 1 80 ga

a review of the western part of the altaids a key to understanding the architecture of accretionary orogens abstract the altaids is one of the largest accretionary collages in the world and the tectonic styles of the accretionary processes have been interpreted in several ways including as an amalgamation of multiple terranes as a result of oroclinal bending of a long single arc or as a caledonian continental collision based on recent tectonostratigraphic analyses together with paleomagnetic data the tectonic styles of the neoproterozoic to paleozoic accretionary processes of the altaids are discussed the western altaids is the main focus of the study which was mainly composed of several independent linear components such as arcs and microcontinents with proterozoic basement and cover rocks various kinds of arcs existed in the paleo asian ocean including a complicated type of arc alaskan type which is a combination of the japan and mariana type intra oceanic arcs and the cordillera type continental arcs these linear components rotated and collided with each other with multiple subduction polarities which could have been an important result of multiple linear element amalgamation and which has contributed greatly to the architecture of the eurasian continent the basic tectonic styles of the altaids can be summarized as arc arc collision oroclinal bending and large scale rotation and multiple subductions with a complicated archipelago paleogeography these basic features of accretionary orogens in general can be attributed to the amalgamation of complicated multiple linear elements some mesozoic to cenozoic accretionary orogens in the world are also characterized by processes of multiple linear element amalgamation more attention should be paid to the multiple linear element amalgamation of ancient accretionary orogens which will shed light on lateral and vertical continental growth

subduction zone metamorphism calc alkaline magmatism and convergent margin crustal evolution abstract at the dawn of the age of plate tectonics akiho miyashiro published a seminal paper in which he drew attention to significant differences in regional metamorphic phase assemblages described contrasting geologic occurrences and inferred their characteristic ranges of physical conditions he advanced the paired metamorphic belt concept involving an oceanward narrow high p low t blueschist zone intimately intermixed with ophiolites and a landward broad low p high t realm associated with arc volcanic plutonic rocks more recent studies have illuminated the following 1 two main types of convergent plate junction can be distinguished pacific underflow of thousands of km of oceanic lithosphere and alpine closure of an intervening oceanic basin leading to continental collision 2 plate descent carries mafic felsic lithologies to depths of 35 120 km or more at relatively low temperatures producing high pressure hp pacific type and ultrahigh pressure uhp alpine type metamorphic terranes respectively 3 exhumed hp uhp complexes display low aggregate bulk densities reflecting chiefly buoyancy driven ascent of quartzofeldspathic allochthons 4 at a depth of 35 km in a warm lithosphere subduction zone sialic crust should melt if an aqueous fluid were abundant it does not hence the activity of h2o must be low at such depths 5 global observations indicate that volcanic plutonic arcs are sited 100 20 km above subducting oceanic plates devolatilization of sinking hydrated oceanic crust mantle promotes the generation of calc alkaline arc melts in this magmagenic zone 6 pacific type underflow of basaltic crust capped plates produces new and recycled continental crust whereas alpine type convergence reshuffles collided terranes but does not generate juvenile sialic crust

an experiment with the seismic crosshole method in an iron mine seismic techniques have so far been only sparsely employed by the mining industry for ore prospecting in the last few years however growing attention has been paid to the seismic reflection method which is the most powerful geophysical tool in terms of resolution and depth penetration for hydrocarbon exploration at least in sweden there is an increasing interest in geophysical techniques for ore prospecting which allow depth penetration of 1 km or more the known bodies in sweden usually dip however very steeply and the standard seismic reflection method will probably have to be modified substantially in order to perform well for such formations

anatomy of zircons from an ultrahot orogen the amalgamation of the north china craton within the supercontinent columbia abstract we report shrimp u pb rare earth element hf isotope and laser raman spectroscopic data on zircons from ultrahigh temperature uht granulites from the jining complex of the khondalite belt in the western block of the north china craton ncc these uht rocks form part of an ultrahot orogen that formed along the collisional margin of the ncc associated with the tectonics of assembly of the paleoproterozoic supercontinent columbia despite the core rim textures displayed by some of the uht zircons their age values sharply converge within error yielding weighted mean 207pb 206pb ages of ca 1 92 ga indicating growth by recrystallization under extreme thermal conditions in general the zircons show moderate heavy rare earth element enrichment with sharp positive ce and negative eu anomalies the hf isotope data from the uht zircons also display a fairly uniform character with the majority of them characterized by positive ehf values without any indication of mixing between reworked crust and

paleoproterozoic accretionary orogenesis in the north china craton a shrimp zircon study abstract the inner mongolia suture zone imsz and the trans north china orogen tnco incorporate the major paleoproterozoic accretionary orogens in the north china craton ncc with the jiao liao ji belt jljb representing the third one here we investigate the paleoproterozoic tectonics of the imsz and tnco through zircon shrimp geochronology on a representative suite of rocks comprising metasediments and arc magmatic rocks shrimp analysis of zircons with textures indicating extreme recrystallization under ultrahigh temperature uht conditions from the metapelites at heling er in the southern domain of the imsz reveals a single population with a weighted mean 207 pb 206 pb age of 1913 17 ma the zircons in another uht granulite from this locality yield a weighted mean 207 pb 206 pb age of 1910 18 ma these data correlate with the ca 1 92 ga age reported from zircons in sapphirine bearing uht granulites further north and confirm the regional extent of the paleoproterozoic uht metamorphism within the imsz zircons in a charnockite from the southern margin of the khondalite belt fringing the uht granulites in the imsz show two distinct age groups an older population with a magmatic paragenesis and a weighted mean 207 pb 206 pb age of 1932 24 ma and a younger group of metamorphic zircons with an age of 1858 26 ma we also report zircon ages from charnockites in two localities around xing he in the huangtuyao belt belonging to the huai an complex within the westernmost domain of the tnco at the junction with the imsz the charnockite from first locality carries two distinct zircon populations with the older group yielding a weighted mean 207 pb 206 pb age of 2477 2 ma and the younger population showing an age of 1807 38 ma the internal structure as revealed from cl images and the overall high th u values up to 2 42 of the older zircons suggest their magmatic affinity whereas the younger group with extremely low th u 0 02 0 09 is of metamorphic origin zircons from the charnockite in the second locality also define two distinct age clusters with a dominant older magmatic group having a weighted mean 207 pb 206 pb age of 2147 11 ma and a minor younger group with an age of 1958 25 ma the range of ages from 2477 to 2147 ma from magmatic zircons in the charnockites from the eastern periphery of the imsz within the western margin of the tnco in combination with similar ages reported in recent studies from zircons in magmatic complexes within the imsz suggest a prolonged history of subduction related arc magmatism and accretionary tectonics analogous to those in some of the phanerozoic belts such as the central asian orogenic belt and the western pacific subsequent progressive collision and suturing of the continental blocks were accompanied by the exhumation of high pressure hp and uht metamorphic rocks the available data from the imsz and tnco suggest long lived convergent margins associated with the southward subduction of the yinshan block and westward subduction of the eastern block in a double sided subduction realm prior to the final amalgamation of the ncc and its incorporation within the columbia supercontinent in the late paleoproterozoic

discovery of sapphirine bearing mg al granulites in the north china craton implications for paleoproterozoic ultrahigh temperature metamorphism abstract we report here for the first time the occurrence of mg al granulites within the khondalite belt of the north china craton and provide robust evidence for extreme crustal metamorphism at ultrahigh temperature uht conditions in this region the uht indicator assemblages in these rocks include equilibrium sapphirine quartz high alumina orthopyroxene sillimanite quartz and high temperature perthites the surrounding khondalites also carry spinel quartz equilibrium assemblage the sapphirines occurring in various microstructural associations are markedly magnesian x mg 0 69 0 77 with a composition varying from 7 9 3 to 2 2 1 end members spinel is a solid solution of mg spinel and hercynite x mg 0 41 0 54 with zno content varying from 0 48 to 1 79 wt garnet shows a compositional range between pyrope and almandine x mg 0 39 0 48 with minor grossular 2 6 3 8 mol and spessartine 0 5 1 8 mol the orthopyroxenes display marked compositional variations in terms of al 2 o 3 content with the core of the medium grained orthopyroxene adjacent to sillimanite in the matrix showing the highest al 2 o 3 content 8 4 wt the history of prograde to peak metamorphism of these rocks is indicated by inclusions of sapphirine spinel quartz and sillimanite within garnet sapphirine formed during the peak stage through the reaction spl qtz sil spr occurrence of sapphirine and quartz inclusions trapped in sillimanite suggests the reaction spr qtz grt opx sil a series of retrograde reactions are observed in these rocks as corona and symplectite textures around early coarse grained minerals such as opx sil qtz crd grt sil qtz crd grt qtz crd opx grt o 2 crd qtz fe 2 o 3 in mag or ilm and finally opx kfs h 2 o bt qtz whereas the stability of sapphirine quartz and orthopyroxene sillimanite quartz in the matrix assemblage suggests t 1000 c and p 10 kbar peak metamorphism conventional geothermobarometric estimates confirm the ultrahigh temperature nature and also trace the retrograde conditions 930 970 c at p 10 kbar al in opx 900 1000 c perthite 930 990 c grt opx and 910 940 c sapphirine spinel electron microprobe dating of monazites in textural association with the uht assemblages yield a sharply defined isochron age of 1917 48 ma which correlates with the 1927 11 ma obtained in a previous study from monazites in the surrounding khondalites carrying spinel quartz assemblage an evaluation of the assemblages and textures using appropriate petrogenetic grids indicate that following peak ultrahigh temperature conditions the rocks underwent initial isobaric cooling and subsequent isothermal decompression and we interpret these trajectories to be part of an overall anti clockwise p t evolution the study area located within the northern margin of the north china craton defines a collisional orogenic belt where continental collision and deep subduction was possibly followed by extensional collapse of the orogen evidence for the extensional phase is provided by the several mafic magmatic intrusives of paleoproterozoic age occurring in the region which suggest a possible link between the thermal anomaly associated with extreme crustal metamorphism in this region with mantle derived magmas the uht metamorphism in north china craton coincided with the assembly of the paleoproterozoic supercontinent columbia

the cauvery shear zone southern granulite terrain india a crustal scale flower structure abstract the cauvery shear zone csz is a crustal scale shear system within the southern granulite terrain along the southern margin of the archaean dharwar craton structural interpretation of satellite data and field observations reveal four major shear zones within the csz system they show dextral shear kinematics synchronous with a major neoproterozoic tectono metamorphic event d2 associated with intracrustal melting and migmatisation the disposition geometry and contemporaneity of shear fabrics of the csz system are modelled in terms of a crustal scale flower structure akin to transpressional and collisional orogens in the light of recent seismic evidence for a displaced moho structure and a mid to lower crustal low velocity zone the flower structure across the csz may extend to mantle depths

development of the north china craton during the late archaean and its final amalgamation at 1 8 ga some speculations on its position within a global palaeoproterozoic supercontinent abstract new data obtained over the past few years indicate that by the end of the archaean the north china craton consisted of two major crustal entities identified as the eastern and western blocks the formation of island and magmatic arcs along the western margin of the eastern block record the latest archaean events so far identified the location of the western block at this time is unknown however the occurrence of palaeoproterozoic passive margin sediments similar in age to those occurring in the eastern block indicates essentially similar conditions and possible separation by an ocean of unknown dimensions re initiation of east dipping subduction beneath the western margin of the eastern block resulted in closure of this palaeoproterozoic ocean bringing the western block into close proximity continued subduction resulted in a major continental continental collision and the western margin of the eastern block was again reactivated leading to extensive thrusting high pressure metamorphism and the generation of crustal melts this occurred between 1 88 and 1 79 ga ago resulting in the formation of the trans north china orogen and final amalgamation of the north china craton although information is sparse to non existent we have reviewed the possible location of the north china craton at the time of collision and note a similarity in terms of age lithology and distribution of major crustal components between the north china craton and baltica although speculative we present this model in the hope that it will stimulate debate and lead to further research in these areas

la icp ms u pb zircon ages of the qianlishan complex constrains on the evolution of the khondalite belt in the western block of the north china craton abstract the qianlishan complex is located in the westernmost part of the khondalite belt a continent continent collisional belt along which the yinshan block in the north and the ordos block in the south amalgamated to form the western block which then collided with the eastern block along the trans north china orogen to form the north china craton the complex is dominated by high grade supracrustal rocks and minor s type granites of which the supracrustals consist of graphite bearing sillimanite garnet gneiss garnet quartzite felsic paragneiss calc silicate rock and marble cl images reveal the existence of detrital and metamorphic zircons in major rocks of the qianlishan complex in most cases detrital zircons occur as either single grains with oscillatory zoning or oscillatory zoning cores typical of igneous origin which are surrounded by metamorphic overgrowth rims that are structureless high bright and low in th u ratio detrital zircons from the qianlishan complex yield nearly concordant 207 pb 206 pb ages ranging from 2 3 to 2 0 ga suggesting that the protoliths of the high grade supracrustal rocks in the qianlishan complex were deposited at some time after 2 0 ga metamorphic zircons yield two age populations with one at 1 95 ga and another at 1 92 ga of which the former is interpreted as the timing of the collision between the yinshan and ordos blocks to form the western block whereas the later is considered to be the age of subsequent post orogenic extensional event minor s type granites were emplaced at 1 88 ga as a result of partial melting of supracrustals at the stage of the exhumation of the qianlishan complex these new zircon ages combined with structural and metamorphic considerations enable resolution of the tectonothermal events involving the collision between the yinshan and ordos blocks to form the western block followed by the post collisional extension and subsequent exhumation of the khondalite belt

structural anatomy of a dismembered ophiolite suite from gondwana the manamedu complex cauvery suture zone southern india abstract detailed geological and structural mapping of the manamedu ophiolite complex moc from the south eastern part of the cauvery suture zone csz within the gondwana collisional suture in southern india reveals the anatomy of a dismembered ophiolite succession comprising pyroxenite actinolite hornblendite hornblendite gabbro norite gabbro anorthosite amphibolite plagiogranite mafic dykes and associated pelagic sediments such as chert magnetite bands and carbonate horizons the magmatic foliation trajectory map shows inward dipping foliations and a variety of fold structures structural cross sections of the moc reveal gentle inward dips with repetition and omission of different lithologies often marked by curvilinear hinge lines the succession displays imbricate thrust sheets and slices of dismembered ophiolite suites distributed along several localities within the csz the moc can be interpreted as a deformed large duplex structure associated with south verging back thrust system consistent with crustal scale flower structure the nature and distribution of ophiolitic rocks in the csz suggest supra subduction zone setting associated with the lithospheric subduction of the neoproterozoic mozambique ocean followed by collision and obduction during the final stage of amalgamation of the gondwana supercontinent in the end precambrian

mesoproterozoic ophiolitic melange from the se periphery of the indian plate u pb zircon ages and tectonic implications abstract the kanigiri melange within the proterozoic nellore khammam schist belt in southern peninsular india includes ophiolitic fragments that represent the remnants of an oceanic plate the ophiolitic units were accreted along a ne trending suture that juxtaposes the proterozoic eastern ghats granulite belt eggb against the archean nellore schist belt of the dharwar craton the ophiolite components in the kanigiri melange include plagiogranites and gabbros which show mutually intrusive relations indicating their coeval nature we report laser ablation icp ms age data and ree geochemistry of zircons from the gabbro and granite the zircons from both gabbro and granite show high ree contents prominent hree enrichment and a conspicuous negative eu anomaly suggesting a common melt source zircon ree abundances and normalized patterns show little intersample and intrasample variations u pb dating of the zircons reveals prominent mesoproterozoic ages for the plagiogranite with the ca 1 33 ga age of the kanigiri ophiolitic melange offering important clues for arc continent collision during the final stages of amalgamation of the columbia derived fragments within the neoproterozoic supercontinent assembly

evolution of subduction zone curvature and its dependence on the trench velocity and the slab to upper mantle viscosity ratio 1 three dimensional laboratory models of upper mantle subduction are presented investigating the effect of the trench velocity vt and the slab to upper mantle viscosity ratio sp um on trench curvature and slab curvature one set of experiments varies sp um from 66 to 1375 another set of experiments modifies vt through applying different velocities at the trailing plate the results show that the radius of trench curvature rtc progressively decreases with continuous trench retreat due to quasi toroidal mantle return flow from the subslab region around the lateral slab edges and toward the mantle wedge region but starts to increase when trench retreat changes to trench advance due to quasi toroidal return flow in the opposite direction furthermore rtc increases with increasing sp um i e progressively stronger slabs are progressively less curved following a cubic root function the force required to curve the slab and trench varies with progressive subduction but is only 0 1 2 5 of the total negative buoyancy force of the slab while the viscous dissipation rate due to progressive slab curvature is only 0 01 0 60 of the potential energy release rate comparison of trench curvature in the models scaled slab width w 750 km with that of the scotia subduction zone w 800 km indicates that for the scotia subduction zone the effective sp um is of the order 1 2 102 finally the elastic sphere indentation model for arc curvature is revisited demonstrating that four main predictions are not met by observations implying that the model should be rejected

first application of the revised ti in zircon geothermometer to paleoproterozoic ultrahigh temperature granulites of tuguiwula inner mongolia north china craton the revised titanium in zircon geothermometer was applied to paleoproterozoic ultrahigh temperature uht granulites at tuguiwula inner mongolia north china craton the tuguiwula granulites contain diagnostic uht mineral assemblages such as sapphirine quartz and high alumina orthopyroxene sillimanite quartz suggesting formation under temperatures of ca 1 000 c and pressures of up to 10 kbar here we report detailed petrographic studies and icp ms data on titanium concentration in zircons associated with the uht assemblages the zircons associated with sapphirine spinel sillimanite magnetite assemblages have the highest ti concentration of up to 57 ppm yielding a temperature of 941 c and suggesting that the growth of zircons occurred under ultrahigh temperature conditions the maximum temperatures obtained by the revised ti in zircon geothermometer is lower than the equilibrium temperature of sapphirine quartz indicating an interval of cooling history of the granulites from uht condition to ca 940 c many of the zircons have ti concentrations ranging from 10 to 33 ppm indicating their growth or recrystallization under lower temperatures of ca 745 870 c these zircons are interpreted to have recrystallized during the retrograde stage indicated by microstructures such as cordierite rim or corona between spinel and quartz and orthopyroxene cordierite symplectite around garnet previous geochronological study on the zircons of the tuguiwula uht granulites gave a mean u pb shrimp age of 1 92 ga however based on the ti in zircon geothermometer results reported in this work and considering the relatively slow thermal relaxation of these rocks we infer that the timing of peak uht metamorphism in the tuguiwula area could be slightly older than 1 92 ga

assembling north china craton within the columbia supercontinent the role of double sided subduction abstract an evaluation of recent s wave receiver functions s wave velocities and two versions of p wave tomographic images along various transects in the north china craton provides some clues on the subduction collision history of the different crustal blocks and their final amalgamation within the paleoproterozoic columbia supercontinent interpretation of a n s seismic section of the craton suggests thick slab debris sinking to various depths in the mantle the w e seismic corridors show the preservation of a thick 200 km lithospheric root tectosphere beneath the ordos block and its variable and extensive erosion towards the yanliao block eastern block this zone is characterized by layers with marked velocity contrast and suggests repeated stacking of the remnants of underplated and accreted paleoproterozoic oceanic lithosphere the present day lithosphere asthenosphere boundary beneath this region probably marks the erosional plane along which decratonization occurred through subduction erosion from the east and thermal and material erosion by upwelling asthenosphere from below resulting in the partial destruction of the tectosphere and its thinning towards the east within the asthenosphere below the yanliao block younger and thinner slabs predominate in the absence of any prominent thick high velocity layers these younger slabs define a westward polarity and constitute a mega scale duplex formed by underplating through phanerozoic subduction process particularly the pacific plate subduction from the east the lithologic associations within the inner mongolia suture zone dividing the yinshan block to the north and ordos block to the south correspond to an accreted ocean plate stratigraphic sequence with the tonalite trondhjemite granodiorite ttg gneisses charnockites and calc alkaline granites representing a continental arc built up through subduction from the north the seismic transects bring out a contrasting polarity in the subduction regime with an oblique east to southward subduction of the yinshan block and a westward subduction of the yanliao block here i propose a double sided subduction history for the ncc similar to the ongoing subduction process in the western pacific such double sided subduction is considered to promote rapid amalgamation of continental fragments within supercontinents and the subduction polarities and mantle dynamics of ncc are therefore considered to be critical in evaluating the final assembly of the paleoproterozoic supercontinent columbia

new insight into a subduction related orogen a reappraisal of the geotectonic framework and evolution of the japanese islands abstract the geotectonic framework and the evolutionary history of the japanese islands need revision in accordance with the various geophysical geological evidence gathered by new methodologies in the recent years including seismic tomography vibroseis ground breaking seismic experiments and detrital zircon chronology these investigations have addressed various themes such as 1 seismic profile of the crust and mantle beneath the japanese islands 2 high precision ages of the protoliths of high p t metamorphic rocks and 3 provenance of terrigenous clastics the results have led to a number of important findings including 1 detection of a large mass of slab around the mantle boundary layer suggesting the long term oceanic subduction beneath japan 2 confirmation of the subhorizontal piled nappe structure for the entire crust of japan 3 finding a new high p t metamorphosed accretionary complex unit that represents the youngest blueschist in japan 4 finding of the oldest early cambrian arc batholith and cover sediments and 5 the identification of plural arc batholiths which have already been erased from the surface based on a synthesis of these new data this article presents a re evaluation of the conventional geotectonic subdivision of the subduction related orogen in japan re definition of the elements and their mutual boundaries and re consideration of the geotectonic evolution of the japanese islands in particular the historical change in provenance suggests that proto japan has experienced large scale tectonic erosion in multiple stages and the corresponding large amounts of continental crust materials were subducted for understanding the orogenic growth of japan during the last ca 500 million years the significance of tectonic erosion coupled with continental contraction as well as the oceanward accretionary growth requires further attention

shrimp u pb zircon geochronology of palaeoproterozoic metasedimentary rocks in the north china craton evidence for a major late palaeoproterozoic tectonothermal event abstract although metasedimentary successions are widely distributed across the north china craton those of palaeoproterozoic age are restricted to three orogenic belts namely the trans north china orogen the khondalite belt and the jiao liao ji belt prior to this work some of those successions were considered to be archaean in age the palaeoproterozoic metasedimentary rocks contain detrital igneous zircon grains with ages ranging from 3 0 to 2 1 ga with the majority recording an age between 2 3 and 2 0 ga this latter group define a series of magmatic events that occurred between formation of the archaean basement and deposition of the metasedimentary rock protoliths the metasedimentary rocks occur as two associations older volcano sedimentary units that formed between 2 37 and 2 0 ma and younger sedimentary units that formed mainly between 2 0 and 1 88 ga all these rock associations were then subjected to metamorphism during the late palaeoproterozoic 1 88 and 1 85 ga tectonothermal event named the luliang movement in early chinese literature which was a continent continent collision that led to the amalgamation of the north china craton

in mine seismic delineation of mineralization and rock structure significant progress has been made towards the goal of generating detailed seismic images as an aid to mine planning and exploration at the kambalda nickel mines of western australia crosshole and vertical seismic profiling instrumentation including a slimline multi element hydrophone array three component geophone sensors and a multishot detonator sound source have been developed along with special seismic imaging software to map rock structure seismic trials at the hunt underground mine established that high frequency 1 khz signals can be propagated over distances of tens of meters tomographic as well as novel 3 d multicomponent reflection imaging procedures have been applied to the data to produce useful pictures of the ore stope geometry and host rock tomogram interpretation remains problematic because velocity changes not only relate to differing rock types and or the presence of mineralisation but can also be caused by alteration weathering and other rock condition variations ultrasoni

decoding multiple tectonothermal events in zircons from single rock samples shrimp zircon u pb data from the late neoarchean rocks of daqingshan north china craton abstract a complex sequence of tectonothermal events shaped the neoarchean paleoproterozoic crustal evolution in the north china craton ncc here we present a systematic shrimp zircon u pb investigation on a suite of metamorphic rocks from the hademengou area of daqingshan within the khondalite belt of the inner mongolia suture zone we analyzed zircons from two meta volcanic rocks of the sanggan group and four meta igneous rocks of late neoarchean to early paleoproterozoic zircons from these rocks display typical core rim or core mantle rim structures the relict magmatic zircons record crystallization ages of ca 2 5 ga followed by variable pb loss showing a range of ages in the concordia up to 2 1 ga the recrystallized domains of zircons show sector zoning or are composed of inner dark domains and outer lighter domains the overgrowth mantles are homogeneous the recrystallized zircons and overgrowth mantles show similar age data distribution along a concordia from 2 5 to 2 1 ga the overgrowth rims are poorly developed but record ages 1 96 1 84 ga similar to those displayed by metamorphic zircons in the late paleoproterozoic upper wulashan subgroup as well as the charnockites and uht granulites reported in previous studies from this region a meta syenogranite intruding the sanggan group and hypersthene dioritic gneiss contains 1 95 ga magmatic and 1 85 ga metamorphic zircons magmatic zircons of the late neoarchean early paleoproterozoic metavolcanic rocks have e hf t of 1 2 to 7 0 t dm1 hf model ages of 2687 to 2532 ma and t dm2 cc hf model ages of 2837 to 2559 ma the recrystallized zircons and mantle domains display hf isotope composition similar to that of the magmatic domains our study is the first report from this region that identifies multiple tectonothermal events ranging from late neoarchean through early to late paleoproterozoic from zircons within single rock samples in the ncc a strong overprint of late paleoproterozoic metamorphism traced commonly in all the rock units indicates that these rocks were involved in the tectonothermal event associated with the final amalgamation of the ncc

tectonics and surface effects of the supercontinent columbia abstract assembly of the supercontinent columbia at about 1 85 1 90 ga coincided with several events that affected the entire earth the oldest worldwide network of orogenic belts formed at the same time although some granite granodiorite gg suites had formed earlier the gg suites became common in the 1 8 1 9 ga orogenic belts these suites succeeded the older tonalite trondhjemite granodiorite ttg suites which were not produced after 1 8 ga changes on the earth s surface at 1 8 1 9 ga include rapid increase in the concentration of oxygen in the atmosphere and oceans and probably the evolution of eukaryotes all of these surface changes occurred as columbia accreted and the assembly of columbia may have contributed to the drastic changes in the earth s surface environment as well as to the evolution of primitive life forms

deformation history of the hengshan wutai fuping complexes implications for the evolution of the trans north china orogen abstract the trans north china orogen separates the north china craton into two small continental blocks the eastern and western blocks as one of the largest exposure in the central part of the orogen the hengshan wutai fuping complexes consist of four lithotectonic units the wutai hengshan and fuping complexes and the hutuo group the hengshan complex contains high pressure mafic granulites and retrograded eclogites structural analysis indicates that most of the rocks in these complexes underwent three distinct episodes of folding d 1 to d 3 and two stages of ductile thrust shearing stz 1 between d 1 and d 2 and stz 2 after d 3 the d 1 deformation formed penetrative axial planar foliations s 1 mineral stretching lineations l 1 and rarely preserved small isoclinal folds f 1 in the hengshan and fuping complexes in the wutai complex however large scale f 1 recumbent folds with sw vergence are displayed by sedimentary compositional layers penetrative transposition resulted in stacking of thrust sheets which are separated by ductile shear zones stz 1 the kinematic indicators of stz 1 in the hengshan and wutai complexes show top to the s230 w thrusting likely related to northeastward oblique pre collisional subduction d 1 resulted in crustal thickening with resultant prograde peak metamorphism the hutuo group did not undergo the d 1 deformation either because sedimentation was coeval with the d 1 deformation or because it was at a high structural level and was not influenced directly by the early deformation the d 2 deformation produced nw verging asymmetric and recumbent folds the d 2 deformation is interpreted to have resulted from collision between the eastern and western blocks of the north china craton in the hutuo group and the fuping complex the development of ese verging asymmetric tight folds is associated with d 2 the structural pattern resulting from superimposition of d 1 and d 2 is a composite synform in the hengshan wutai fuping complexes all four lithotectonic units were superposed during the later d 3 deformation the d 3 deformation developed nw trending open upright folds ongoing collision led to development of transpressional ductile shearing stz 2 forming the transpressional zhujiafang dextral ductile shear zone between the northern hengshan complex and the southern hengshan complex and generating the sinistral longquanguan ductile shear zone between the fuping complex and the wutai complex respectively the stz 1 and d 2 deformation were possibly responsible for fast syn collisional exhumation of the high pressure mafic granulites and retrograded eclogites the structural patterns and elucidation of the deformation history of the hengshan wutai fuping complexes places important constraints on the tectonic model suggesting that an oceanic lithosphere between the eastern and western blocks underwent northeastward directed oblique subduction beneath the western margin of the eastern block and that the final closure of this ocean led to collision between the two blocks to form the coherent basement of the north china craton

age and evolution of a late archean to paleoproterozoic upper to lower crustal section in the wutaishan hengshan fuping terrain of northern china abstract the taihangshan wutaishan area forms part of the central north china craton and consists of three main components the hengshan and fuping complexes containing predominantly ductilely deformed late archean to paleoproterozoic high grade granitoid orthogneisses intruded by mafic dykes of gabbroic composition and the low grade late archean wutai greenstone belt developed between the high grade terrains and consisting of bimodal volcanic rocks and metasediments associated with coeval granitoids zircon dating shows that both the hengshan and fuping complexes were intruded by major granitoid bodies between 2 52 and 2 48 ga with rare occurrences of 2 7 ga gneisses wutai granitoids adjacent to the fuping complex also contain 2 7 ga xenocrysts all possibly reflecting the same older basement rare granitic gneisses in the hengshan have emplacement ages of 2 35 ga and show the same structural features as the older rocks indicating that the main deformation occurred after 2 3 ga deposition of supracrustal rocks and emplacement of mafic dykes occurred in the paleoproterozoic together with intrusion of granitic bodies at 2 1 2 0 ga and all underwent regional metamorphism at 1 88 1 85 ga we interpret the hengshan and fuping granitoid gneisses as the lower plutonic part of a late archean to early paleoproterozoic japan type magmatic arc with the upper volcanic part represented by the wutai complex components of this arc may have evolved at a continental margin as indicated by the 2 7 ga zircons virtually all magmatic events are recorded in all three complexes however the 1 85 ga metamorphic event is only recorded in the higher grade portions of the wutai since low grade and presumably high crustal level precluded metamorphic zircon growth shear zones in the hengshan are interpreted as major lower crustal discontinuities post dating the peak of high pressure granulite metamorphism we suggest that they formed during orogenic collapse and uplift of the hengshan complex in the late paleoproterozoic 1800 ma

multiple tectonothermal events in the granulite blocks of southern india revealed from epma dating implications on the history of supercontinents abstract we report age data on zircon monazite uraninite and huttonite from a suite of 29 samples covering four major granulite blocks in southern india using an electron microprobe technique the rocks analysed in this study cover all of the major lithounits in these terrains and include garnet bearing and garnet free charnockites garnet biotite gneisses khondalites calc silicate rocks and a suite of orthogneisses biotite gneiss biotite hornblende gneiss two pink metagranites representing the magmatic phase were also analysed zircons from the madras block yield well defined isochrons at 2 5 2 6 ga core to rim analyses of single zircon grains show age zoning with 2 6 2 9 ga igneous cores mantled by 2 4 2 5 ga rims detrital zircons show age up to ca 3 2 ga monazites in this block have cores and rims with 2 5 2 3 ga a suite of 19 samples from the madurai block brings out the multiple tectonothermal events in this terrain zircons from an orthogneiss yield well defined isochrons at 1 7 0 1 ga 0 82 0 05 ga and 0 58 0 04 ga from core inner rim and rim portions respectively zircon grains in other rocks preserve either core or secondary growth ages at 0 8 1 0 ga zircons in a pink metagranite from this block show sharply defined isochrons of 0 68 0 03 ga for the core and 0 57 0 01 ga for the secondary portion a late pan african overprint is observed throughout this block with zircon rims monazite uraninite and huttonite yielding age values in the range of 0 45 0 60 ga zircons from both the trivandrum and nagercoil blocks show a major tectonothermal event at 0 55 ga with faint indications of previous tectonothermal events during 0 8 1 0 and 1 7 2 0 ga monazite data from both the trivandrum and nagercoil blocks are essentially similar to those from the madurai block except for presence of relic monazite in the former our study confirms the notion that the palghat cauvery shear zone marks the major terrain boundary between an archean craton in the north and proterozoic terrains in the south it also strengthens the view of paleoproterozoic accretion and pan african reworking of blocks south of this shear zone the ages of production accretion and reworking in the terrains of southern india yield important information for the histories of columbia 1 8 1 5 ga rodinia 1 1 1 0 ga and gondwana 0 6 0 5 ga supercontinents the southern indian terrains formed part of a worldwide network of orogenic belts that is centered around 1 8 ga and outlines the configuration of columbia accretion of terrains in the madurai block to archean rocks north of the palghat cauvery shear zone at this time is consistent with the ages suggested to be the oldest metamorphic event in the eastern ghats orogen of eastern india and the rayner belt of coastal east antarctica our data confirm earlier evidence that all of southernmost india underwent resetting of isotopic systems during the final accretion of gondwana at the time of the pan african orogeny 0 5 ga the possibility that the trivandrum and nagercoil terrains accreted to the madurai block after 0 7 ga suggests that this pan african zone may also have involved accretion and closure of ocean basins if that happened then this zone may be the long sought suture between east and west gondwana

a late archean foreland fold and thrust belt in the north china craton implications for early collisional tectonics abstract the archean north china craton is divided into the western and eastern blocks along the central orogenic belt a 1600 km long archean foreland basin and thrust belt fringes the eastern side of the central orogenic belt rocks in the orogen form tectonically stacked east vergent fold and thrust sheets including foreland basin sediments 2 50 ga ophiolitic melange and an island arc complex foreland basin sediments overlie a passive margin sequence and include a 2 50 ga deep water turbidite sequence that grades upward and westward into shallow water molasse now disposed in structurally imbricated east verging thrusts and asymmetric folds that gradually migrated craton ward with deformation uplift and erosion of the orogen there is a strong linked relationship of the formation of the foreland basin to collision of the east and west blocks of the north china craton along the central orogenic belt at 2 50 ga the qinglong foreland basin and central orogenic belt of the north china craton represents one of the best preserved archean orogen to craton transitions in the world its classic internal to external zonation and flexural response to loading demonstrate that convergent tectonics in the archean were broadly similar to phanerozoic convergent margin processes

tectonic and magmatic responses to the subduction of high bathymetric relief subduction of high bathymetric relief such as aseismic ridges and magmatic plateaus is considered to be responsible for dramatic changes in the dynamics and kinematics of the subduction zone for example the buoyancy of high bathymetric relief is thought to flatten the dip of the subducting slab modifying the structural and magmatic evolution of the overriding plate and terminating arc volcanism in addition the effect of ridge subduction in retreating plate boundaries can inhibit subduction rollback a process that could locally pin the subduction hinge and lead to the development of cusps and slab tearing here we discuss the tectonic response to subduction of high bathymetric relief using examples from the circum pacific subduction systems we demonstrate that flattening of the subduction dip angle is only significant in the eastern pacific where the average slab dip angle is relatively shallow in the western pacific in contrast the average subduction dip angle is steeper and there is no significant flattening of the dip angle in areas of ridge subduction subduction of high bathymetric relief in the circum pacific is commonly associated with reduced arc volcanism and in many cases the area of ridge subduction coincides with a volcanic gap in the overriding plate ridge subduction is associated with pronounced changes in the style of deformation involving uplift reactivation of basement thrusts development of orogen perpendicular tear faults and block rotations leading to oroclinal bending the discussed characteristic patterns associated with ridge subduction provide important guidelines for reconstructing past plate tectonic processes and could help constraining the geodynamics of ancient subduction systems

the precambrian khondalite belt in the daqingshan area north china craton evidence for multiple metamorphic events in the palaeoproterozoic era high grade pelitic metasedimentary rocks khondalites are widely distributed in the northwestern part of the north china craton and were named the khondalite belt prior to the application of zircon geochronology a stratigraphic division of the supracrustal rocks into several groups was established using interpretative field geology we report here shrimp u pb zircon ages and hf isotope data on metamorphosed sedimentary and magmatic rocks at daqingshan a typical area of the khondalite belt the main conclusions are as follows 1 the early precambrian supracrustal rocks belong to three sequences a 2 56 2 51 ga supracrustal unit the previous sanggan group a 2 51 2 45 ga supracrustal unit a portion of the previous upper wulashan group and a 2 0 1 95 ga supracrustal unit including the previous lower wulashan group a portion of original upper wulashan group and the original meidaizhao group the units thus do not represent a true stratigraphy 2 strong tectono thermal events occurred during the late neoarchaean to late palaeoproterozoic with four episodes recognized 2 6 2 5 2 45 2 37 2 3 2 0 and 1 95 1 85 ga with the latest event being consistent with the assembly of the palaeoproterozoic supercontinent columbia 3 during the late neoarchaean to late palaeoproterozoic 2 55 2 5 2 37 and 2 06 ga juvenile mantle derived material was added to the crust

lithotectonic elements and geological events in the hengshan wutai fuping belt a synthesis and implications for the evolution of the trans north china orogen the hengshan wutai fuping belt is located in the middle segment of the trans north china orogen a palaeoproterozoic continental collisional belt along which the eastern and western blocks amalgamated to form the north china craton the belt consists of the medium to high grade hengshan and fuping gneiss complexes and the intervening low to medium grade wutai granite greenstone terrane and most igneous rocks in the belt are calc alkaline and have affinities to magmatic arcs previous tectonic models assumed that the hengshan and fuping gneiss assemblages were an older basement to the wutai supracrustal rocks but recent studies indicate that the three complexes constitute a single long lived neoarchaean to palaeoproterozoic magmatic arc where the wutai complex represents an upper crustal domain whereas the hengshan and fuping gneisses represent the lower crustal components forming the root of the arc the earliest arc related magmatism in the belt occurred at 2560 2520 ma marked by the emplacement of the wutai granitoids which was followed by arc volcanism at 2530 2515 ma forming the wutai greenstones extension driven by widespread arc volcanism led to the development of a back arc basin or a marginal sea which divided the belt into the hengshan wutai island arc japan type and the fuping relict arc at 2520 2480 ma subduction beneath the hengshan wutai island arc caused partial melting of the lower crust to form the hengshan tonalitic trondhjemitic granodioritic ttg suites whereas eastward directed subduction of the marginal sea led to the reactivation of the fuping relict arc where the fuping tonalitic trondhjemitic granodioritic suite was emplaced in the period 2360 2000 ma sporadic phases of isolated granitoid magmatism occurred in the hengshan wutai fuping region forming 2360 ma c 2250 ma and 2000 2100 ma granitoids in the hengshan complex the c 2100 ma wangjiahui and dawaliang granites in the wutai complex and the 2100 2000 ma nanying granitoids in the fuping complex atc 1920 ma the hengshan wutai island arc underwent an extensional event possibly due to the subduction of an oceanic ridge leading to the emplacement of pre tectonic gabbroic dykes that were subsequently metamorphosed together with their host rocks to form medium to high pressure granulites at 1880 1820 ma the hengshan wutai fuping arc system was juxtaposed intensely deformed and metamorphosed during a major and regionally extensive orogenic event the l uliang orogeny which generated the trans north china orogen through collision of the eastern and western blocks the hengshan wutai fuping belt was finally stabilized after emplacement of a mafic dyke swarm at 1780 1750 ma

origin of siderite deposits from the lombardy valleys northern italy a carbon oxygen and strontium isotope study abstract in the lombardy valleys siderite mineralization is widespread as stratiform orebodies in lower triassic terrigenous carbonate rocks as well as vein orebodies in permian volcanics and continental sequences and in the metamorphic basement a number of these orebodies and host limestone dolomite rocks have been analysed for c o and sr isotope compositions in order to constrain geochemically the origin of the siderite the intermediate 13c 18o and 87 sr 86 sr values of stratiform siderite relative to vein siderite and host limestone and the very good linear correlations between and 87 sr 86 sr data points concur to support a metasomatic origin for stratiform siderite after calcite in addition stratiform and vein siderite should have formed from the same kind of fluids and possibly are the products of the same mineralizing event the values of carbonate rocks are discussed in terms of recrystallization dolomitization effects during diagenesis these processes occurred earlier than siderite mineralization

timing of metamorphism in the paleoproterozoic jiao liao ji belt new shrimp u pb zircon dating of granulites gneisses and marbles of the jiaobei massif in the north china craton abstract the paleoproterozoic jiao liao ji belt lies in the eastern block of the north china craton with its southern segment extending across the bohai sea into the jiaobei massif high pressure pelitic and mafic granulites have been recently recognized in the paleoproterozoic jingshan group jiaobei massif new shrimp u th pb geochronology combined with cathodoluminescence cl imaging of zircon has been applied to the determination of the timing of the metamorphism of the high temperature and high pressure granulites and associated gneisses and marbles metamorphic zircons in these high pressure granulites gneisses and marbles occur as either single grains or overgrowth or recrystallization rims surrounding and truncating oscillatory zoned magmatic zircon cores metamorphic zircons are all characterized by nebulous zoning or being structureless with high luminescence and relatively low th u values metamorphic zircons from two high pressure mafic granulites yielded 207pb 206pb ages of 1956 41 ma and 1884 24 ma one metamorphic zircon from a garnet sillimanite gneiss also gave an apparent 207pb 206pb age of 1939 15 ma these results are consistent with interval of ages of c 1 93 1 90 ga already obtained by previous studies for the north and south liaohe groups and the laoling group in the northern segment of the jiao liao ji belt metamorphic zircons from a high pressure pelitic granulite and two pelitic gneisses yielded weighted mean 207pb 206pb ages of 1837 8 ma 1821 8 ma and 1836 8 ma respectively two diopside olivine phlogopite marbles yielded weighted mean 207pb 206pb ages of 1817 9 ma and 1790 6 ma these paleoproterozoic metamorphic ages are largely in accordance with metamorphic ages of c 1 85 ga produced from the ji an group in the northern segment of the jiao liao ji belt and c 1 86 1 80 ga obtained for the high pressure pelitic granulites from the jingshan group in the southern segment as this metamorphic event was coeval with the emplacement of a type granites in the jiao liao ji belt and its adjacent areas it is interpreted as having resulted from a post orogenic or anorogenic extensional event

single zircon grains record two paleoproterozoic collisional events in the north china craton abstract the western block of the north china craton consists of the yinshan block in the north and the ordos block in the south which were amalgamated along the east west trending khondalite belt at 1 95 ga the western block then collided with the eastern block to form the coherent basement of the north china craton along the north south trending trans north china orogen at 1 85 ga the huaian complex a high grade terrrane located at the conjunction of the khondalite belt and trans north china orogen records metamorphic events associated with both collisions the complex consists of lithologies from both the khondalite belt and trans north china orogen of which the former consist of graphite garnet sillimanite gneiss garnet quartzite felsic paragneiss calc silicate rock and marble together called the khondalite series zircons in the graphite garnet sillimanite gneiss can be divided into three types 1 spherical grains without internal structures 2 grains with a core and rim structure and 3 grains with a dark core surrounded by double rims except for the dark core in type 3 all other types of zircon domains are structureless and highly luminescent with very low th u ratios typical of a metamorphic origin analyses on the cores of type 2 and the inner rims of type 3 from two samples yield upper intercept ages of 1946 26 and 1947 22 ma similar to previously determined metamorphic ages from the khondalite belt and thus interpreted as the time of collision between the yinshan and ordos blocks analyses on type 1 zircons rims of type 2 and the outer rims of type 3 from the same two samples give ages of 1850 15 and 1857 16 ma interpreted as the time of collision between the eastern and western blocks thus zircons in the graphite garnet sillimanite gneiss of the huaian complex record both of the paleoproterozoic collisional events in the north china craton

archean blocks and their boundaries in the north china craton lithological geochemical structural and p t path constraints and tectonic evolution an examination of lithological geochemical geochronological structural and metamorphic p t path data suggests that the basement of the north china craton can be divided into eastern and western blocks separated by major crustal boundaries that roughly correspond with the limits of a 300 km wide zone called the trans north china orogen the eastern block consists predominantly of late archean domiform tonalitic trondhjemitic granodioritic ttg batholiths surrounded by anastomosing networks and linear belts of open to tight synforms of minor volcanic and sedimentary rocks metamorphosed from greenschist to granulite facies at 2 5 ga with anticlockwise p t paths some early to middle archean rocks are locally present in the eastern block but their tectonic history is unclear due to reworking by the 2 5 ga tectonothermal event the western block has a late archean assemblage structural style and metamorphic history similar to that of the eastern block but it differs in the absence of early to middle archean assemblages and in being overlain by and interleaved with paleoproterozoic khondalites which were affected by a 1 8 ga metamorphic event involving clockwise p t paths a mantle plume model is proposed for the formation and evolution of late archean basement rocks in the eastern and western blocks based on a combination of extensive exposure of ttg gneisses affinities of mafic rocks to continental tholeiitic basalts presence of voluminous komatiitic rocks dominant diaprism related domiform structures anticlockwise p t paths and a short time span from the primary emplacement of ttg and ultramafic to mafic rocks until the onset of regional metamorphism between the two blocks is the trans north china orogen which is bounded by two major fault systems and is composed of late archean to paleoproterozoic ttg gneisses and granitoids interleaved with abundant sedimentary and volcanic rocks that are geochemically interpreted as having developed in magmatic arc and intra arc basin environments these rocks underwent multiple phases of compressional deformation and peak high pressure metamorphism followed by rapid exhumation during the late paleoproterozoic at 1 8 ga as a result of collision between the eastern and western blocks resulting in the amalgamation of the north china craton

contrasted tectonic styles for the paleoproterozoic evolution of the north china craton evidence for a 2 1 ga thermal and tectonic event in the fuping massif structural analysis along with 40ar 39ar and u pb datings in the fuping massif provide new insight into the evolution of the eastern part of the trans north china belt north china craton from 2 7 ga to 1 8 ga d1 is responsible for the development of a dome and basin structure coeval with crustal melting giving rise to migmatite and nanying gneissic granites at 2 1 ga this dome and basin architecture resulted from the interference between a n s compression of a weak ductile crust and gravity driven vertical flow in a high thermal regime the next events involved flat lying ductile thrusting d2 and normal faulting d3 dated at around 1880 ma and 1830 ma respectively the d2 and d3 events belong to the trans north china orogeny that results in the final amalgamation of the north china craton the d1 deformation is considered as evidence for an earlier orogen developed around 2 1 ga prior to the trans north china orogeny the change in the deformation style between the 2 1 ga and 1 8 ga could be viewed as a consequence of the cooling of the continental crust in the north china craton

the paleoproterozoic north hebei orogen north china craton s collisional suture with the columbia supercontinent abstract understanding the geologic history and position of the north china craton in the paleoproterozoic columbia supercontinent has proven elusive paleoproterozoic orogenic episodes 2 00 1 85 ga are temporally associated with ultimate stabilization of the north china craton ncc followed by the development of extensive craton wide rift systems at 1 85 1 80 ga the age difference between the sedimentary cover and the metamorphic basement is up to 500 700 ma suggesting that uplift and doming of cratonic basement occurred in the latest paleoproterozoic mafic dike swarms 1 80 1 77 ga and anorogenic magmatism 1 80 1 70 ga record the extensional breakup and dispersal of the north china craton during this stage the late paleoproterozoic tectonic framework and geological events documented provide important constraints for reconstruction of the ncc within the late paleoproterozoic supercontinent of columbia an east west striking thousand kilometer long belt of khondalites granulite facies metapelites stretches along the northern margin of the north china craton on the cratonward side of the northern hebei orogenic belt this granulite belt includes mg al sapphirine bearing granulites that reached ultrahigh temperature peak metamorphic conditions of 1000 c at 10 kbars at 1927 11 ma following peak ultrahigh temperature conditions the rocks underwent initial isobaric cooling and subsequent isothermal decompression and these trajectories are interpreted to be part of an overall anti clockwise p t evolution indicating that the northern margin of the craton experienced continental collision at 1 93 1 92 ga the position of the khondalite belt south of the northern hebei orogenic belt makes it analogous to tibet a continental collision related plateau characterized by double crustal thicknesses and granulite facies metamorphism at depth we suggest that the tectonic evolution of the ncc during this period was closely related to the assembly and break up of the columbia supercontinent and that the ncc was adjacent to the baltic and amazonian cratons in the period 2 00 1 70 ga craton wide extension occurred within 100 150 ma of collision along the northern margin of the craton at 1 93 1 92 ga it is concluded that mantle upwellings are chiefly responsible for the breakup of the ncc from the paleoproterozoic supercontinent

u pb and hf isotopic study of zircons of the helanshan complex constrains on the evolution of the khondalite belt in the western block of the north china craton abstract the helanshan complex is located at the westernmost segment of the khondalite belt a continent continent collisional belt along which the yinshan block in the north collided with the ordos block in the south to form the western block of the north china craton the complex consists of s type granites and high grade pelitic granulite gneiss felsic paragneiss quartzite calc silicate rock and marble together called the khondalite series rocks la q icp ms u pb geochronology and la mc icp ms lu hf isotopic data of zircons combined with cathodoluminescence cl imaging enable the resolution of magmatic and metamorphic events that can be directed towards understanding the formation and evolution of the khondalite belt in the western block of the north china craton cl images reveal the coexistence of magmatic type detrital zircons and metamorphic zircons in most of the khondalite series rocks of which the metamorphic zircons occur as either single grains or overgrowth rims surrounding and truncating magmatic type detrital zircon cores la q icp ms u pb analyses on magmatic type detrital zircons reveal two distinct age populations with one in the archaean 2 85 2 53 ga and the other in the palaeoproterozoic 2 2 2 0 ga suggesting that the sedimentary protoliths of the khondalite series rocks in the helanshan complex must have been deposited at some time after 2 0 ga the hf analyses show that the archaean detrital zircons possess negative ehf t values from 7 64 to 0 15 with depleted mantle model ages ranging from 3 34 to 3 10 ga this implies the existence of a paleo mesoarchaean 3 34 3 10 ga crust in the western block which underwent a recycling event in the period 2 85 2 53 ga nearly all palaeoproterozoic 2 2 2 0 ga detrital zircons possess positive ehf t values 0 86 9 38 with depleted mantle model ages of 2 45 2 15 ga suggesting a significant crustal growth event in the western block in the palaeoproterozoic metamorphic zircons yield two groups of age with one at 1 95 ga and the other at 1 87 ga of which the former is considered as the timing of the collision between the yinshan and ordos blocks to form the western block whereas the latter is consistent with the age of 1 86 ga from two s type granites which are considered to have formed by the partial melting of pelitic granulites gneisses at the stage of the exhumation of the helanshan complex

cross borehole compressional wave imaging of unconsolidated sediments cross borehole elastic wave tomography offers great potential for nondestructive imaging of near surface sediments fluids and soils this method was used to create a high resolution tomographic image of velocity structure from simulated sedimentary layers obtained using compressional wave travel times the image shows three distinct layers and a water filled container located in the center of the middle layer an image obtained from computer simulated travel times shows good agreement with the image from laboratory data the high quality data and the resulting image were due in part to a novel transmitter and receiver design these were composed of cylindrical piezoelectric transducers individually sealed in oil filled teflon containers and stacked to form stringers the transmitters were extremely efficient at coupling energy into the medium the transmitter frequency was 135 khz velocities ranged from 1 4 to 1 8 km s and thus wavelengths were 1 0 1 4 cm the distance between boreholes was 30 cm with about 60 cm of vertical coverage

the central india tectonic zone a geophysical perspective on continental amalgamation along a mesoproterozoic suture abstract the central india tectonic zone citz is a prominent divide and a major suture zone between the north indian and south indian crustal blocks the resistive upper crust as modeled in the magnetotelluric data from citz suggests a dominant tonalite trdondhjemite granodiorite composition associated with an accretionary complex characterized by mainly felsic rock components the highly conductive bodies in this zone might represent mafic ultramafic layered intrusives derived from a deeper reservoir of underplated basaltic magma related to the formation of the cretaceous deccan flood basalts the uniformly thick mafic lower crust below the cratons on both sides of the suture is interpreted as the accreted remnants of archaean and paleoproterozoic subducted slabs we redefine the nature of deep faults traversing the citz which were described as steep and penetrating the moho by previous workers and classify them as listric faults with gentle dips at depth seismic reflection data from the eastern side of the suture suggest a northwestward subduction of the bhandara craton reflection data from the central part of the citz show northerly dip in the southern part suggesting northward subduction of the dharwar craton however an opposite trend is observed in the northern part of the suture with a southward dip of the bundelkhand craton based on these features and in conjunction with existing magnetotelluric models we propose a double sided subduction history along the citz this would be similar to the ongoing subduction accretion process in the western pacific region which possibly led to the development of paired collision type and pacific type orogens one important feature is the domal structure along the central part of the suture with a thick felsic crust occurring between mafic and intermediate crust the high resistivity felsic domain suggests underplated sediments felsic crust that would have caused the doming our model also accounts for the extrusion of regional metamorphic belts at the orogenic core and the occurrence of high pressure ultrahigh temperature paired metamorphic belts within the suture

zircon geochronology and metamorphic evolution of mafic dykes in the hengshan complex of northern china evidence for late palaeoproterozoic extension and subsequent high pressure metamorphism in the north china craton abstract magmatic and metamorphic zircons have been dated from ductilely deformed gabbroic dykes defining a dyke swarm and signifying crustal extension in the northern part of the hengshan complex of the north china craton these dykes now occur as boudins and deformed sheets within migmatitic tonalitic trondhjemitic granodioritic and granitic gneisses and are conspicuous due to relics of high pressure granulite or even former eclogite facies garnet pyroxene bearing assemblages shrimp ages for magmatic zircons from two dykes reflect the time of dyke emplacement at 1915 ma whereas metamorphic zircons dated by both shrimp and evaporation techniques are consistently in the range 1848 1888 ma the youngest granitoid gneiss yet dated in the hengshan has an emplacement age of 1872 17 ma these results complement recent geochronological studies from the neighbouring wutai and fuping complexes to the se of the hengshan showing that a crustal extension event occurred in the late palaeoproterozoic this preceded a major high pressure collision type metamorphic event in the central part of the north china craton that occurred in the palaeoproterozoic and not in the late archaean as previously thought our data support recent suggestions that the north china craton experienced a major craton wide orogenic event in the late palaeoproterozoic after which it became cratonized and acted as a stable block

possible influence of subducting ridges on the himalayan arc and on the ruptures of great and major himalayan earthquakes abstract subduction of bathymetric features such as ridges seamounts fractures etc on the subducting plate influences the arc morphology and earthquake ruptures we analyse their effect on the development of the arcuate shape of the himalayan arc and on the ruptures of great and major himalayan earthquakes besides the two most prominent ridges in the indian ocean namely the chagos laccadive deccan ridge and the 90 e ridge which are assumed to extend up to the himalayan arc at least three major subsurface ridges have been mapped on the underthrusting indian plate under the indo gangetic plains it appears that the subduction of the two most prominent ridges contributed to the development of the arcuate shape of the himalayan arc the interaction and subduction of the other subsurface ridges probably influenced the himalayan arc morphology by causing a localised cusp in the frontal topography also these ridges probably acted as barriers to the ruptures of the major and great himalayan earthquakes

seismic methods in mineral exploration and mine planning a general overview of past and present case histories and a look into the future abstractdue to high metal prices and increased difficulties in finding shallower deposits the exploration for and exploitation of mineral resources is expected to move to greater depths consequently seismic methods will become a more important tool to help unravel structures hosting mineral deposits at great depth for mine planning and exploration these methods also can be used with varying degrees of success to directly target mineral deposits at depth we review important contributions that have been made in developing these techniques for the mining industry with focus on four main regions australia europe canada and south africa a wide range of case studies are covered including some that are published in the special issue accompanying this article from surface to borehole seismic methods as well as petrophysical data and seismic modeling of mineral deposits at present high resolution 2d surveys mostly are performed in mining areas but there is a general increasing trend in the use of 3

configuration of the late paleoproterozoic supercontinent columbia insights from radiating mafic dyke swarms abstract remnants of 2 1 1 8 ga orogens can be recognized in nearly every craton assembled within reconstructions of the rodinia supercontinent although no particular pattern of laterally extensive orogenic belts emerges these cratons may be the fragments of an older supercontinent formed in response to the collision and accretion of even older cratons during the early earth history paleomagnetic constraints have played a limited role in many previous reconstructions of the early precambrian supercontinent mainly because of the poor age control and a large scatter of the paleomagnetic poles the geometry of giant radiating dyke swarms and orogenic belts provide important constraints for the reconstruction of the pre rodinian supercontinent some early unmetamorphosed and undeformed mafic dyke swarms in north china southern peninsular india and north america share coeval age of 1 85 ga if these continents are assembled the overall pattern exhibited by the mafic dyke swarms appears to constitute a giant radiating swarm with a piercing point between the cuddapah rift in south india and the xiong er aulacogen in north china this suggests that the north china craton indian craton and canadian shield may be fragmented from the same landmass the reconstruction of 1 85 ga giant radiating dyke swarm attempted in this paper suggests that the north china craton india craton and canadian shield were united together to form a landmass within the columbia supercontinent before its extension and break up the 1 90 1 85 ga andean style north orogenic belt suggests that a subduction zone existed on the northern margin of the north china craton wopmay orogenic belt developed in a 1 88 1 84 ga continental volcano plutonic arc which is interpreted as the product of eastward subduction of oceanic lithosphere the transantarctica orogenic belt in east antarctica may be another candidate to be linked with the subduction zone the north orogenic wopmay orogenic and transantarctica orogenic belts could be connected to form a continuous subduction zone the orogenic comparison strengthens the configuration of the columbia supercontinent proposed by the reconstruction of the giant radiating dyke swarm the laurentia west australia and east antarctica were relatively stable from 1 85 ga to 1 20 ga as inferred from the data on dyke swarms magmatism and the paleomagnetism these continents constituted the core of the columbia supercontinent in the late paleoproterozoic time the north china craton and indian craton were however separated from the columbia supercontinent during the mesoproterozoic time

extreme crustal metamorphism during columbia supercontinent assembly evidence from north china craton abstract a vast supracrustal belt of khondalites granulite facies metapelites occur along the northern margin of the north china craton we report here for the first time spinel quartz equilibrium assemblage from these rocks in two textural settings 1 high zno up to 14 47 wt spinel with quartz as inclusions within garnet porphyroblasts defining pressure above 12 kbar and temperature of 900 c and 2 low zno down to 1 2 wt spinel in association with quartz in the matrix assemblage formed during peak ultrahigh temperature conditions ca 975 c and 9 kbar we present a unique case of decompression where the metamorphic conditions of the rocks traversed mostly through the spinel quartz extended stability field monazite grains in textural association with both types of spinel quartz textures were analysed for age determination and the data define two age peaks at 1927 11 ma and 1819 11 ma since the peak thermal regime of the khondalites was close to or exceeded the theoretical closure temperature of pb in monazite we infer the 1819 11 ma age as the timing of ultrahigh temperature event in this craton our data lend support to the idea of ca 1 9 1 8 ga e w collisional orogen at the northern margin of the north china craton we correlate the extreme crustal metamorphism with tectonics associated with the assembly of the north china craton within the columbia supercontinent

geophysical and geological tests of tectonic models of the north china craton abstract the geometry and timing of amalgamation of the north china craton have been controversial with three main models offering significantly different interpretations of regional structure geochronology and geological relationships one model suggests that the eastern and western blocks of the ncc formed separately in the archean and an active margin was developed on the eastern block between 2 5 and 1 85 ga when the two blocks collided above an east dipping subduction zone a second presumes the eastern block rifted from an unknown larger continent at circa 2 7 ga and experienced a collision with an arc perhaps attached to the western block above a west dipping subduction zone at 2 5 ga and the 1 85 ga metamorphism is related to a collision along the northern margin of the craton when the ncc joined the columbia supercontinent a third model suggests two collisions in the central orogenic belt at 2 1 and 1 88 ga but recognizes an early undated deformation event recent seismic results reveal details of the deep crustal and lithospheric structure that support both the second and third models showing that subduction beneath the central orogenic belt was west directed and that there is a second west dipping paleosubduction zone located to the east of the cob dipping beneath the western block ordos craton the boundaries identified through geophysics do not correlate with the boundaries of the trans north china orogen suggested in the first model and the subduction polarity is opposite that predicted by that model high pressure granulite facies metamorphism at 1 85 ga is not restricted to the tnco as suggested by the first model but is documented across the ncc as predicted by the second model suggesting a major continent continent collision along the north margin of the craton at 1 85 ga further it has recently been shown that in the southern tnco there is no record of metamorphism at circa 1 85 ga but only at 2 7 2 5 ga showing that the tnco as defined as a circa 1 85 ga orogen does not exist this is further confirmed by recent re os isotopic studies which show that the subcontinental lithospheric mantle beneath the southern cob is late archean in age and that a province in the northern ncc is circa 1 8 ga correlating with the proposed collision belt of the ncc with the columbia supercontinent across the entire ncc the cob is an archean convergent belt re worked in the paleoproterozoic and the paleoproterozoic tectonism is widespread across the ncc as predicted by the model whereby the previously amalgamated eastern and western blocks experienced a continental collision with columbia at circa 1 85 ga but uplift exhumation rates are slow necessitating a re evaluation of the tectonic models of the ncc

syn collisional channel flow and exhumation of paleoproterozoic high pressure rocks in the trans north china orogen the critical role of partial melting and orogenic bending within the paleoproterozoic trans north china orogen the high pressure belt hpb is made of high pressure 15 kbar mafic granulites hosted in migmatitic gneisses in this contribution we document a set of structural analyses acquired over the whole hpb we also proposed a morphological subdivision of the partially molten rocks that compose the hpb according to changes in melt fraction a compilation of the p t and radiochronological data carried out over the last 15 years is presented the results highlight the concurrent effect of oroclinal bending and partial melting in controlling the exhumation of the deeply buried continental crust during ongoing compression of the thickening orogenic root onset of partial melting at peak metamorphism is responsible for a first strength drop that enhanced an eastward lateral flow radiometric ages show that the deep crust was partially molten over a 50 ma lasting period during which it evolved in a diatexite core mantled by metatexites this was responsible for a second strength drop with strain concentrated along the diatexite metatexite boundaries as exemplified by the newly documented datong chengde shear zone a 400 km long normal shear zone with a sinistral strike slip component that accommodated the final uprise of the high pressure rocks

geochronology and petrogenesis of neoarchean potassic meta granites from huai an complex implications for the evolution of the north china craton abstract neoarchean potassic meta granites occurring in the huai an complex within the north central part of the north china craton ncc comprise leucocratic syenogranites biotite monzogranites and garnet bearing syenogranites high pressure garnet granulites occur as lenses sheets or dismembered dikes with the granitoids here we present shrimp zircon u pb data from the leucocratic syenogranites which show an age of 2493 6 ma mswd 1 and la icp ms u pb data from the biotite monzogranites and garnet syenogranites revealing their formation ages of 2437 10 ma mswd 2 0 and 1977 2003 ma respectively we also report metamorphic ages of ca 1800 ma from zircons in the garnet syenogranites geochemical studies show low mgo 0 5 0 06 wt cr 6 19 0 53 ppm and ni 27 66 2 01 ppm with varying sio2 62 3 to 76 8 wt contents indicating their derivation from a crustal source the leucosyenogranites exhibit elevated positive eu anomalies with decreasing sr th u and bulk ree concentrations and th u ratio whereas rb and ba values increase with decreasing th u ratio indicating fluid melt interaction the leucosyenogranites have high sr and ba contents and sr y and la yb n ratios and low y and hrees with zircon ehf t values ranging from 1 7 to 5 8 and hf model ages of 2 60 2 73 ga reflecting their derivation by partial melting of overthickened juvenile lower crust of eclogitic composition or an eclogitic mineral assemblage in the residue geochemical compositions of the biotite monzogranites with positive end t values 2 3 to 2 5 and model ages

carbonic metamorphism at ultrahigh temperatures evidence from north china craton abstract the north china craton ncc constitutes one of the fundamental precambrian nuclei of asia the paleoproterozoic khondalite belt along the northern margin of the ncc shows evidence for extreme crustal metamorphism at temperatures exceeding 1000 c and pressures of 10 12 kbar with the formation of diagnostic ultrahigh temperature uht mineral assemblages including sapphirine quartz high alumina orthopyroxene sillimanite and high temperature mesoperthites here we record for the first time the common occurrence of co2 rich fluid inclusions in peak uht minerals in the mg al granulites of ncc and trace the fluid evolution history from microthermometric and laser raman spectroscopic studies we show that the syn metamorphic fluid in the uht rocks was pure co2 while very high density carbonic fluids are preserved in some cases corresponding to entrapment at peak conditions many of the inclusions underwent density modification during the rapid and near isothermal exhumation of the rocks following peak uht metamorphism and initial cooling at depth the present study area within the northern margin of the ncc defines a collisional orogenic belt where continental collision and deep subduction were possibly followed by extensional collapse of the orogen evidence for the extensional phase is provided by the several mafic magmatic intrusions of paleoproterozoic age that are seen emplaced within the granulite facies supracrustals the thermal anomaly displayed by the uht granulites may thus correlate with asthenospheric upwelling and emplacement of mafic magmas possibly associated with plume activity which provided the heat and co2 input to achieve extreme crustal metamorphism and generation of dry granulite facies assemblages our study is the first report of syn metamorphic co2 rich fluids in uht granulites from a paleoproterozoic ca 1 92 ga terrane and provide evidence for carbonic metamorphism at ultrahigh temperatures as north china craton joined the columbia supercontinent amalgam

deformation history of the hengshan complex implications for the tectonic evolution of the trans north china orogen structural analysis indicates that the hengshan complex underwent five distinct episodes of deformation d1 d5 the d1 episode formed small isoclinal folds f1 penetrative axial planar foliations s1 and mineral stretching lineations l1 d1 fabrics were reworked by following d2 deformation associated with d2 was the development of nw verging asymmetric folds and accompanying thrust faults both d1 and d2 resulted from crustal thickening and coherent with prograde and peak metamorphism d2 deformation was followed by transpressional dextral shearing d3 including top to nw oblique slip shearing and nne ssw dextral strike slip shearing ongoing collision led to development of the zhujiafang ductile shear zone d4 a near e w trending high strain belt across the hengshan complex d5 deformation is characterized by f5 open folds and associated normal faults probably related to exhumation of the complex structural patterns of the hengshan complex can place important insights into a recently proposed tectonic model suggesting that an ocean between the eastern and western blocks underwent eastward directed subduction beneath the western margin of the eastern block and closure of this ocean finally led to collision between the two blocks to form the coherent north china craton

seismic tomographic imaging of a buried concrete target1 this paper describes a field evaluation of an algebraic reconstruction technique for the tomographic imaging of sub surface velocity anomalies the authors describe the construction of a three dimensional concrete model and the acquisition and processing of seismic traveltime data through the model image reconstructions of the data sets using an algebraic reconstruction technique and incorporating prior knowledge are presented and these are compared with the actual model reconstructions show that it is essential that accurate data are obtained as we demonstrate that relatively small errors in the traveltime data can seriously degrade the reconstruction the authors also show that raypath effects are very important limiting factors to the analysis

paleoproterozoic ultrahigh temperature granulites in the north china craton implications for tectonic models on extreme crustal metamorphism abstract ultrahigh temperature uht metamorphic rocks associated with the khondalite belt within the inner mongolia suture zone imsz provide robust evidence for extreme thermal metamorphism in the north china craton ncc the imsz marks the collisional suture between the yinshan block to the north and the ordos block to the south as the ncc was incorporated within the columbia supercontinent amalgam during paleoproterozoic here we present a synthesis of the salient features of the uht rocks from the ncc including petrologic indicators fluid characteristics and monazite and zircon chronometry on the extreme crustal metamorphism the granulites carry diagnostic uht mineral assemblages including sapphirine quartz low zn fe3 spinel quartz high alumina orthopyroxene sillimanite quartz and high temperature mesoperthite the stability fields of the typical mineral assemblages conventional geothermobarometry and phase equilibria modeling using pseudosections as reported in a number of recent studies converge to indicate that these uht rocks experienced metamorphic temperatures up to or in excess of 1000 c at ca 10 kbar followed by an isobaric cooling segment the rocks were exhumed along a near isothermal decompression path microstructures mineral reactions and phase equilibria modeling suggest an anti clockwise p t path similar to those displayed by metamorphic orogens developed in subduction collision settings the dominant category of fossil fluids preserved within the major uht minerals is co2 consistent with the stability of the broadly anhydrous mineral assemblage in these rocks both chemical and radiogenic isotopic ages from monazite and zircon chronometry suggest the timing of the uht event as around 1 92 ga the paleoproterozoic high grade metamorphism younging from 1 95 ga in the western domain to 1 92 ga in the eastern domain of the khondalite belt might suggest a scissor like closure of oblique collision between the yinshan and the ordos blocks the salient features of the uht metamorphism in the ncc include 1 extreme metamorphic temperatures at moderate pressures 2 dominantly anhydrous nature of the mineral assemblages typically the stability of orthopyroxene 3 common presence of co2 rich fluid inclusions as the trace of the ambient fluid 4 regional extent of the uht granulites and 5 the association of the uht orogen with an accretionary belt in a continental collisional suture we evaluate the diverse models on the generation of uht orogens including their formation in thickened and inverted back arcs orogen self heating through heat producing elements heat and co2 input by plume impingement below a carbonated tectosphere and asthenospheric upwelling through ridge subduction and slab window process or during post collisional slab break off the ultra hot and dry uht rocks in the ncc provide one of the well preserved examples from the paleoproterozoic globe for investigating extreme metamorphism and related tectonic processes within the plate tectonic paradigm

a dynamic model of the curvature of the mariana trench oceanic trenches are prominent features of the earth s surface they represent the convergent plate boundaries and generally possess convex curvatures towards the subducting plates based on this observation many static geometric models have been proposed for the origin of trench curvatures1 6 unfortunately many of these models have limited applicability7 alternatively a dynamic model has been proposed which suggests that trench curvatures are evolved from collisions between aseismic ridges and the trench axis8 we have developed a flow model to quantitatively evaluate this dynamic process for the mariana trench system the model is able to yield an excellent fit to the present trench curvature and determine past rotations for back arc volcanic islands such as saipan and guam with surprising accuracy when compared with palaeomagnetic observations

polyphase deformation of the fuping complex trans north china orogen structures shrimp u pb zircon ages and tectonic implications abstract the fuping complex is located in the central part of the trans north china orogen tnco a paleoproterozoic collisional belt along which the eastern and western blocks were assembled to form the north china craton three stages of deformation d 1 d 3 have been identified in the fuping complex d 1 formed tight to isoclinal folds f 1 penetrative foliation s 1 and mineral lineation l 1 which resulted from the early nw se orientated thrusting and shearing d 2 progressively overprinted d 1 and was dominated by mostly nw se shortening and top to the se e thrusting under a compressive environment d 1 and d 2 occurred in association with the peak metamorphism m 1 d 3 occurred during the exhumation after the crustal thickening producing regional scale nww see trending open folds f 3 and associated low angle detachment faults and was responsible for the isothermal decompression m 2 metamorphism of the complex following the d 3 deformation the complex was exhumed to shallow crustal levels where the rocks underwent the retrogressive cooling m 3 metamorphism syn tectonic leucocratic dykes are widespread in the fuping complex two leucocratic dykes that did not experience d 1 but underwent d 2 yield shrimp zircon ages of 1843 12 ma and 1844 18 ma which indicate that d 1 must have occurred at some time earlier than 1844 ma whereas d 2 must have occurred at some time later than this age two weakly deformed post d 2 leucocratic dykes yield shrimp zircon ages of 1817 14 ma and 1815 45 ma suggesting that the d 2 deformation occurred in the period 1843 1815 ma the new shrimp data indicate that the collision related deformation of the fuping complex must have occurred in the paleoproterozoic not in the late archean structural and geochronological data presented in this study in association with previous lithological and geochemical data suggest that the fuping complex may have been located in the hinterland of an eastward subduction zone and underwent intensive deformation and metamorphism involved in the final collision between the western and eastern blocks at 1 85 ga