highly evolved juvenile granites with tetrad ree patterns the woduhe and baerzhe granites from the great xing an mountains in ne china abstract in ne china voluminous granitoids were emplaced in late paleozoic and mesozoic times we report here sr nd o isotopic and elemental abundance data for two highly evolved granitic plutons woduhe and baerzhe from the great xing an mountains they show a rather juvenile sr nd isotopic signature and a spectacular tetrad effect in their ree distribution patterns as well as non charac charge and radius controlled trace element behavior the emplacement ages are constrained at 130 4 ma for the woduhe and 122 5 ma for the baerzhe granites by rb sr and sm nd isotope analyses both granites are also characterized by low but imprecise initial 87sr 86sr ratios of about 0 703 the nd sr isotope data argue for their generation by melting of dominantly juvenile mantle component with subordinate recycled ancient crust this is largely compatible with the general scenario for much of the phanerozoic granitoids emplaced in the central asian orogenic belt the parental magmas for both the woduhe and baerzhe granites have undergone extensive magmatic differentiation during which intense interaction of the residual melts with aqueous hydrothermal fluids probably rich in f and cl resulted in the non charac trace element behavior and the tetrad effect of ree distribution both the woduhe and baerzhe granites show the characteristic trace element patterns of rare metal granites but their absolute abundances differ by as much as two orders of magnitude the oxygen isotope compositions of the two granites have been severely disturbed significant 18o depletion in feldspar but not so much in quartz suggests that the hydrothermal alteration took place in a temperature condition of 300 500 c this subsolidus hydrothermal alteration is decoupled from the late stage magma fluid interaction at higher temperatures despite the two distinct and intense events of water rock interaction the rb sr and sm nd geochronological systems seem to have maintained closed hence suggesting that the two events occurred shortly after the plutonic emplacements

climatology of barrier jets along the alaskan coast part i spatial and temporal distributions abstract this paper investigates the temporal and spatial climatology of coastal barrier jets in the gulf of alaska the jets are divided into two categories based upon the origin of the air involved classic barrier jets fed primarily by onshore flow and hybrid jets fed primarily by gap flow from the continental interior the analyses are compiled from five years 1998 2003 of synthetic aperture radar images from the radarsat 1 satellite totaling 3000 images thermodynamic and kinematic data from the ncep reanalysis is used in the interpretation of the results the majority of coastal barrier jets occur during the cool season with the coastline near mount fairweather and the valdez cordova mountains experiencing the greatest number of barrier jets hybrid jets are even more strongly restricted to the cool season and are commonly found to the west of cross sound yakutat bay and icy bay some interannual variability in the total number of jets is observed coastal barrier jet formation is associate

new shrimp u pb zircon ages from the heilongjiang high pressure belt constraints on the mesozoic evolution of ne china the heilongjiang complex is a high pressure accretionary belt composed mainly of serpentinite glaucophane bearing metabasalt epidote glaucophane albite schist greenschist marble two mica schist muscovite albite schist quartz schist and quartzite chert located along the suture zone separating the jiamusi and songliao blocks in ne china shrimp u pb zircon analyses of two samples of granitic gneiss from yilan that are interleaved with the heilongjiang complex record diverse protolith ages that are consistent with derivation from regional permian granitoids 263 7 ma and the adjacent mashan complex 492 6 ma indicating tectonic incorporation of older granitic components into the accretionary complex the age of the basaltic protolith of two samples of glaucophane albite schist from yilan is 245 ma and is slightly younger than the protolith age of two samples of amphibole schist from mudanjiang located about 200 km south of yilan that define weighted mean 206pb 238u ages of 257 4 and 257 5 ma all four samples contain a few older inherited zircon grains and also show the effects of an isotopic disturbance at 200 to 190 ma most likely related to the subsequent high pressure metamorphism the timing of which has been determined in previous studies the basaltic rocks record rifting and ocean development from 260 to 210 ma based on previous studies in which both e morb and oib protolith signatures were identified detrital zircons in two samples of mica schist from mudanjiang show distinct populations at 700 ma 500 to 450 ma 350 to 213 ma peak at 255 ma 290 to 200 ma and 200 ma all except the youngest ages can be matched with rocks from the older components of the jiamusi block and with other areas in the central asia orogenic belt caob supporting the view that the jiamusi block was most likely the eastern segment of the caob prior to rifting the youngest concordant magmatic detrital grain with an age of 207 3 ma defines the maximum age of sedimentation this also limits the timing of high pressure metamorphism in the heilongjiang complex to post late triassic consistent with argon data obtained in previous studies final closure between the jiamusi and songliao blocks took place in the latest triassic early jurassic with the two blocks accreted as a result of pacific ocean subduction this suggests that the heilongjiang complex records the time when north south closure of the caob in china gave way to westward movement related to the pacific ocean subduction which has dominated the tectonics of ne china and far east russia since that time

transformation from paleo asian ocean closure to paleo pacific subduction new constraints from granitoids in the eastern jilin heilongjiang belt ne china abstract the eastern jilin heilongjiang belt ejhb of ne china is a unique orogen that underwent two stages of evolution within the tectonic regimes of the paleo asian and paleo pacific oceans 158 available zircon u pb ages including 26 ages obtained during the present study and 132 ages from the literature were compiled and analyzed for the mesozoic and cenozoic granitoids from the ejhb and the adjacent russian sikhote alin orogenic belt saob to examine the temporal spatial distribution of the granitoids and to constrain the tectonic evolution of the east asian continental margin five stages of granitic magmatism can be identified early triassic 251 240 ma late triassic 228 215 ma latest triassic to middle jurassic 213 158 ma early cretaceous 131 105 ma and late cretaceous to paleocene 95 56 ma the early triassic granitoids are restricted to the yanbian region along the changchun yanji suture and show geochemical characteristics of magmas from a thickened lower crust source probably due to the final collision of the combined ne china blocks with the north china craton the late triassic granitoids with features of a type granites represent post collisional magmatic activities that were related to post orogenic extension marking the end of the tectonic evolution of the paleo asian ocean the latest triassic to paleocene granitoids with calc alkaline characteristics were ne trending emplaced along the ejhb and saob and young towards the coastal region and represent continental marginal arc magmas that were associated with the northwestwards subduction of the paleo pacific plate two periods of magmatic quiescence 158 131 and 105 95 ma correspond to changes in the subduction direction of the paleo pacific plate from oblique relative to the continental margin to subparallel taking all this into account we conclude that 1 the final closure of the paleo asian ocean occurred along the changchun yanji suture during the early triassic 2 the onset of the subduction of the paleo pacific plate beneath the east asian continental margin probably took place during the latest triassic ca 215 ma 3 changes in the drifting direction of the paleo pacific plate were responsible for the intermittent magmatic activities and 4 roll back of the subducted plate resulted in the oceanwards migration of the magmatic arc and large scale back arc extension throughout ne china during the early cretaceous

end permian to mid triassic termination of the accretionary processes of the southern altaids implications for the geodynamic evolution phanerozoic continental growth and metallogeny of central asia the altaids is one of the largest accretionary orogenic collages in the world with the highest rate of phanerozoic continental growth and significant metallogenic importance it is widely accepted that subduction related orogenesis of the altaids started in the late precambrian and gradually migrated southward present coordinates however it is uncertain when and how the building of the altaids was finally completed based on structural geology geochemical geochronological and paleomagnetic data this paper presents late paleozoic to early mesozoic accretionary tectonics of two key areas north xinjiang in the west and inner mongolia in the east together with neighboring mongolia the late paleozoic tectonics of north xinjiang and adjacent areas were characterized by continuous southward accretion along the wide southern active margin of siberia and its final amalgamation with the passive margin of tarim which may have lasted to the end permian to early mid triassic in contrast in inner mongolia and adjacent areas two wide accretionary wedges developed along the southern active margin of siberia and the northern active margin of the north china craton which may have lasted to the mid triassic the final products of the long lived accretionary processes in the southern altaids include late paleozoic to permian arcs late paleozoic to mid triassic accretionary wedges composed of radiolarian cherts pillow lavas and ophiolitic fragments and high pressure ultrahigh pressure metamorphic rocks permian alaskan type zoned mafic ultramafic complexes intruded along some major faults of the tien shan we define a new tarim suture zone immediately north of the tarim craton that is probably now buried below the tien shan as a result of northward subduction of the tarim block in the cenozoic the docking of the tarim and north china cratons against the southern active margin of siberia in the end permian to mid triassic resulted in the final closure of the paleoasian ocean and terminated the accretionary orogenesis of the southern altaids in this part of central asia this complex geodynamic evolution led to formation of giant metal deposits in central asia and to substantial continental growth

late triassic bimodal igneous rocks in eastern heilongjiang province ne china implications for the initiation of subduction of the paleo pacific plate beneath eurasia abstract this paper reports new zircon la icp ms and sims u pb ages and hf isotope data and whole rock major and trace element data for late triassic igneous rocks of eastern heilongjiang province ne china these data provide new insights into the timing of the initiation of subduction of the paleo pacific plate beneath the eurasian continent the zircon u pb age data indicate that a suite of late triassic 228 202 ma igneous rocks is present within the songnen zhangguangcai range massif and within the western margin of the khanka massif the late triassic igneous rocks within the songnen zhangguangcai range massif consist of basalts basaltic andesites gabbro diabases and rhyolites whereas coeval igneous rocks in the western margin of the khanka massif consist of hornblende gabbros and syenogranitic porphyries these late triassic rocks constitute a geochemically bimodal igneous rock association that contains mafic rocks enriched in light rare earth elements lree and large ion lithophile elements lile and depleted in heavy rare earth elements hree and high field strength elements hfse such as nb ta zr hf and ti zircons from these mafic rocks have ehf t values and tdm1 ages of 2 8 to 9 8 and 477 733 ma respectively suggesting that they formed from a primary magma generated by the partial melting of depleted lithospheric mantle material that had been previously modified by subduction related fluids the coeval felsic rocks are characterized by enrichments in lrees and liles and depletions in hrees and hfses including nb ta and ti and their zircons have ehf t values and t dm2 ages of 0 6 to 7 9 and 766 to 1461 ma respectively implying that these rocks were generated during the partial melting of juvenile crustal material the late triassic bimodal igneous rocks in eastern heilongjiang province combined with the regional geologic information therefore record a post orogenic extensional environment related to the final late permian early triassic closure of the paleo asian ocean in addition the presence of late triassic bimodal igneous rocks within the eastern margin of the eurasian continent suggests that the subduction of the paleo pacific plate beneath the eurasian continent began after the late triassic

final amalgamation of the central asian orogenic belt in ne china paleo asian ocean closure versus paleo pacific plate subduction a review of the evidence abstract the central asian orogenic belt caob evolved through complex closure of the paleo asian ocean from the neoproterozoic to the late phanerozoic this caused the chinese cratons to collide with eurasia and led to the formation of the world s largest phanerozoic orogenic belt ocean closure commenced in the west and was completed in the east near changchun closure of the paleo asian ocean in ne china was along the solonker xar moron changchun yanji suture and this was likely completed in the late permian although associated activity continued into the triassic there was an overlap in the latest permian early triassic between terminal activity associated with paleo asian ocean closure and the onset of tectonism associated with subduction of the paleo pacific plate this switch in geodynamic setting occurred at 260 250 ma and is reflected by a relaxing of north south directed compression and the onset of east west directed processes related to paleo pacific subduction by the early jurassic events associated with the westward advance of the paleo pacific plate dominated leading to extensive development of i type granites as far inland as the great xing an range from 140 ma the paleo pacific plate retreated eastward resulting in an extensional setting in the early cretaceous the effects of which were enhanced by regional thinning of the lithosphere commonly attributed to delamination throughout this period the eastern asian margin was tectonically complex the north south oriented jiamusi khanka bureya block was rifted away from the eastern margin of the caob in the late triassic but was then re united in the jurassic by westward advancing subduction that affected both the western and eastern margins of the block accretionary complexes continued to evolve in the cretaceous along the whole eastern margin of asia with final accretion of the nadanhada terrane part of the sikhote alin accretionary terrane with the caob at 130 ma followed by the emplacement of s type granites

nd hf o isotopic evidence for subduction induced crustal replacement in ne china abstract convergent plate margins represent an ideal setting to understand the formation and evolution of the earth s crust nevertheless the geodynamic processes involved in crustal evolution remains highly debatable based on a large geochemical dataset of permian cretaceous granitic rocks from the yanbian area in ne china we propose a new geodynamic model involving crustal replacement to decipher crustal evolution during a superimposed multi stage subduction system these phanerozoic granitic rocks from two adjacent zones show contrasting hf nd isotopic variation trends with decreasing age of magmatism whole rock end t and zircon ehf t decrease gradually in the north granite zone ngz but show the opposite trends in the south granite zone sgz zircon 18o shows a slight rise in the ngz but it initially increases from permian to jurassic and then decreases to cretaceous in the sgz the nd hf o isotopic compositions of the cretaceous granitic rocks in both zones converge four crustal components are identified in the source of granites across the yanbian area including 1 juvenile crust 2 subduction accretionary complex 3 new arc crust and 4 precambrian crust the contributions made to the bulk crustal melting source of granites by the juvenile crust component in the ngz and by the precambrian crust in the sgz were both gradually decreased whereas the proportion of the new arc crust increased in both zones pre existing crustal components were extensively eroded destructed diluted and replaced by the new arc crust formed during paleo pacific slab subduction considering the prevalence of these two nd hf o isotopic variation trends in many orogenic belts subduction induced crustal replacement represents an important mechanism for crustal evolution in convergent plate margins

measurements of natural uranium concentration and isotopic composition with permil level precision by inductively coupled plasma quadrupole mass spectrometry a new analytical technique using inductively coupled plasma quadrupole mass spectrometry icp qms has been developed that produces permil level precision in the measurement of uranium concentration u and isotopic composition 234u in natural materials a 233u 236u double spike method was used to correct for mass fractionation during analysis to correct for ratio drifting samples were bracketed by uranium standard measurements a sensitivity of 6 7 108 cps ppm was generated with a sample solution uptake rate of 30 l min with a measurement time of 15 20 min standards of 30 ng uranium produced a within run precision better than 3 2 r s d for 234u and better than 2 for u replicate measurements made on standards show that a between run reproducibility of 3 5 for 234u and 2 for u can be achieved icp qms data of 234u and u in seawater coral and speleothem materials are consistent with the data measured by other icp ms and tims techniques advantages of the icp qms method include low cost easy maintenance simple instrumental operation and few sample preparation steps sample size requirements are small such as 10 14 mg of coral material the results demonstrate that this technique can be applied to natural samples with various matrices

triassic volcanism in eastern heilongjiang and jilin provinces ne china chronology geochemistry and tectonic implications abstract with the aim of constraining the early mesozoic tectonic evolution of the eastern section of the central asian orogenic belt caob we undertook zircon u pb dating and geochemical analyses major and trace elements sr nd isotopes of volcanic rocks of the luoquanzhan formation and daxinggou group in eastern heilongjiang and jilin provinces china the analyzed rocks consist mainly of dacite and rhyolite with sio2 contents of 68 52 76 65 wt three samples from the luoquanzhan formation and one from the daxinggou group were analyzed using laser ablation inductively coupled plasma mass spectrometry la icp ms u pb zircon techniques three zircons with well defined oscillatory zoning yielded weighted mean 206pb 238u ages of 217 1 214 2 and 208 1 ma and one zircon with oscillatory zoning yielded a weighted mean 206pb 238u age of 201 1 ma these ages are interpreted to represent the timing of eruption of the volcanic rocks the triassic volcanic rocks are characterized by high sio2 and low mgo concentrations enrichment in large ion lithophile elements liles and light rare earth elements lrees depletion in high field strength elements hfses and heavy rare earth elements hrees 87sr 86sr i 0 7040 0 7050 luoquanzhan formation and 0 7163 0 7381 daxinggou group and end t 1 89 3 94 luoquanzhan formation and 3 42 3 68 daxinggou group these geochemical features indicate an origin involving the partial melting of juvenile lower crust nd model ages tdm2 of 651 821 ma and that compositional variation among the volcanic rocks arose from mineral fractionation and minor assimilation these volcanic rocks formed within an extensional environment following collision of the ncc and jiamusi khanka massif during the late paleozoic early triassic

early jurassic mafic magmatism in the lesser xing an zhangguangcai range ne china and its tectonic implications constraints from zircon u pb chronology and geochemistry abstract la icp ms zircon u pb dating and geochemical data have been obtained from five representative mafic ultramafic intrusions in the lesser xing an zhangguangcai range ne china with the aim of improving our understanding of the mesozoic tectonic evolution in the region and in particular determining the time of initiation of the circum pacific tectonic system the selected zircons exhibit striped absorption in cathodoluminescence cl images and have high th u ratios 0 20 3 16 indicating a magmatic origin the zircon u pb dates indicate that most of these magmatic zircons other than a few relics that were captured and entrained in the magma formed in the late early jurassic 186 182 ma and not as previously supposed in the middle caledonian the five mafic ultramafic plutons are composed of olivine gabbro hornblendite gabbro hornblende gabbro and gabbro diorite the olivine gabbro and hornblendite display cumulate textures implying that fractional crystallization of olivine and plagioclase took place during magma evolution these mafic ultramafic igneous rocks have sio2 37 3 55 7 mgo 3 05 13 3 al2o3 11 8 23 8 mg 42 69 mg 100mg mg fe2 total and eu 0 88 1 32 and they display three types of rare earth element ree distribution patterns right slipped flat patterns or dome like the trace element spider diagrams show that the rocks are enriched in large ion lithophile elements liles such as ba k and sr and depleted in high field strength elements hfses such as nb ta zr and hf the zircons have ehf 186 182 ma 2 7 to 12 0 and tdm1 366 732 ma the geochemical data indicate that the early jurassic mafic magma originated in an extensional environment from the partial melting of a depleted mantle wedge that had been metasomatized by fluids released from a fossil subducted slab these data combined with information on the spatial variation of coeval igneous rocks indicate that the formation of the early jurassic mafic ultramafic rocks in the lesser xing an zhangguangcai range was related to the subduction of the paleo pacific plate beneath the eurasian continent and this event would mark the beginning of the circum pacific tectonic system

emplacement ages geochemical and sr nd hf isotopic characterization of mesozoic to early cenozoic granitoids of the sikhote alin orogenic belt russian far east crustal growth and regional tectonic evolution abstract the sikhote alin range of the russian far east is an important accretionary orogen of the western pacific orogenic belt in order to study the formation and tectonic evolution of the orogen we performed zircon u pb dating as well as geochemical and sr nd hf isotopic analyses on 24 granitoid samples from various massifs in the primorye and khabarovsk regions the zircon dating revealed that the granitoids were emplaced from 131 to 56 ma cretaceous to paleogene in the primorye region granitoids in the coastal sikhote alin intruded the cretaceous taukha accretionary terrane from ca 90 to 56 ma whereas those along the central sikhote alin fault zone intruded the jurassic samarka accretionary terrane during ca 110 75 ma the oldest monzogranite 131 ma was emplaced in the lermontovka area of the nw primorye region granitoid massifs along the central sikhote alin fault zone in the khabarovsk region formed from 109 to 58 ma thus the most important tectonothermal events in the sikhote alin orogen took place in the cretaceous geochemical analysis indicates that most samples are i type granitoids they have initial 87sr 86sr ratios ranging from 0 7040 to 0 7083 and initial nd isotopic ratios expressed as end t values from 3 0 to 5 0 mostly 0 to 5 the data suggest that the granitoid magmas were generated by partial melting of sources with mixed lithologies including the subducted accretionary complex hidden paleozoic proterozoic basement rocks based on whole rock nd isotopic data we estimated variable proportions 36 77 of juvenile component mantle derived basaltic rocks in the generation of the granitic magmas furthermore zircon hf isotopic data ehf t 0 to 15 indicate that the zircon grains crystallized from melts of mixed sources and that crustal assimilation occurred during magmatic differentiation the quasi continuous magmatism in the sikhote alin orogen suggests that the paleo pacific plate subduction was very active in the late cretaceous the apparently regular progression of granitic intrusion ages from 80 to 56 ma in the taukha terrane may reflect oblique underflow of the paleo pacific plate beneath the eurasian continental margin subduction was not only manifested by granitic intrusion but also by abundant silicic volcanism the late cretaceous paleo pacific plate motion probably changed from parallel or sub parallel to oblique relative to the continental margin of the sikhote alin leading to the change of magmatic source region and geochemical characteristics of the derived igneous rocks late cretaceous rapid sea floor spreading at ca 100 ma induced highly active subduction and led to voluminous magmatism in the entire circum pacific realm finally the present age and isotopic study lends support to the hypothesis of geologic and tectonic correlation between sikhote alin and sw japan

late triassic intrusive complex in the jidong region jiamusi khanka block ne china geochemistry zircon u pb ages lu hf isotopes and implications for magma mingling and mixing abstract whole rock major and trace element geochemistry together with zircon u pb ages and lu hf isotope compositions are reported for a late triassic intrusive complex in the jidong region jiamusi khanka block ne china zircon u pb dating yields ages between 211 and 208 ma for enclaves of microgranular diorite and quartz diorite and between 211 and 209 ma for the host granitoids these ages correlate with a previously established intensive late triassic magmatic event along the eastern asian margin field observations together with petrographic features geochemistry and zircon hf isotope data preclude simple crystal fractionation or restite unmixing as a genetic link for the various rock types within the intrusive complex the syenogranite suite has high sio 2 75 5 76 3 wt and low mgo 0 15 0 19 wt and yields enriched lile and lree patterns most of the zircons in the syenogranites have two stage model ages of 766 and 1461 ma together with positive e hf t values of 0 6 to 9 1 these results indicate that the granitoid magmas were generated by partial melting of meso to neoproterozoic lower crust the gabbro suite has a restricted range of sio 2 46 1 51 9 wt together with high mg values 49 70 and high concentrations of ni co and cr zircons from two diorite samples have single stage hf model ages of 557 787 ma and e hf t values of 1 9 to 8 3 that are consistent with the coeval gabbros previously studied in the jidong region these features together with the observation that all the gabbros are enriched in lree and lile suggest that the mafic magmas were derived from melting of depleted neoproterozoic lithospheric mantle that had been metasomatized by slab derived fluids it is concluded that the dominant igneous suites within the late triassic intrusive complex formed by mingling mixing of felsic and mafic magmas the geochemical data combined with regional geological investigations indicate that the late triassic intrusive complex formed during lithospheric extension caused by slab break off of the yuejinshan oceanic plate that was subducting westward beneath the jiamusi khanka block

a review of the relationships between granitoid types their origins and their geodynamic environments abstract granitoids are divided into several types according to their mineral assemblages their field and petrographical features and their chemical and isotopic characteristics this typology complements most of the recent classifications because it is not based solely on chemical and isotopic criteria but also on the field petrographical and mineralogical criteria it thus has the advantage of distinguishing the various granitoid types in the field in most cases the proposed classification shares many similarities with the twenty most used genetic classifications of granitoids both types of peraluminous granitoids are of crustal origin the tholeiitic alkaline and peralkaline granitoids are of mantle origin and both types of calc alkaline granitoids are of mixed origin and involve both crustal and mantle materials each granitoid type is generated and emplaced in a very specific tectonic setting each stage of the wilson cycle is characterised by typical associations of granitoids well typed and precisely dated granitoids can then complement structural approaches and indicate on the geodynamic environment with reference to some case studies the use of granitoids as tracers of the geodynamic evolution is also proposed and discussed

shrimp u pb dating of zircons from late yanshanian granitic complex in xiaoxinancha gold rich copper orefield of yanbian and its geological implications the shrimp u pb dating of zircons from the granitic complex in the xiaoxinancha gold rich copper orefield was conducted for the first time and as a result three concordant ages of zircons were obtained from medium fine grained biotite monzonitic granite medium fine grained biotite granodiorite and porphyraceous hornblende granodiorite respectively i e 98 4 109 0 ma weighted average value being 104 6 2 5 ma nequal to 10 mswd equal to 0 71 confidence level being 0 70 98 3 108 3 ma weighted average value 106 7 1 6 ma n 12 and 104 4 118 8 ma weighted average value 111 7 2 8 ma n 12 suggesting that magmatic emplacement took place in 112 104 ma in order of porphyraceous hornblende granodiorite medium fine grained biotite granodiorite and medium fine grained biotite monzonitic granite taking into account their relationships with mineralization in ore forming elements and geological characteristics and the extensional torsional features of the ore controlling fractures the authors hold that large scale gold copper mineralization must have occurred after the formation of porphyraceous hornblende granodiorite medium fine grained biotite granodiorite and medium fine grained biotite monzonitic granite but before the emplacement of the associated dioritic porphyrite i e between 104 6 and 102 1 ma and the metallogenic environment was the subduction and extinction of izanaqi plate towards the ancient asian continent in late early cretaceous and the conversion process of slip or extension structures in eastern china continental margin during the late yanshanian orogenic movement

evolution of a permian intraoceanic arc trench system in the solonker suture zone central asian orogenic belt china and mongolia abstract the identification of a fossil arc trench system from the ophiolite decorated solonker suture zone in the southernmost central asian orogenic belt caob enables us to constrain the timing of pre subduction extension ca 299 290 ma subduction initiation ca 294 280 ma ridge trench collision ca 281 273 ma and slab break off ca 255 248 ma in the permian a fraction of proto arc crust ca 45 km long up to 8 km wide is preserved as a volcanic plutonic sequence and is juxtaposed against a wide ca 30 80 km forearc melange this proto arc crust comprises two distinct magma series island arc tholeiite iat and mid ocean ridge basalt morb both of which have strong supra subduction zone ssz geochemical signatures zircons from a gabbro and a plagiogranite yielded weighted mean 206pb 238u ages of 284 0 4 0 and 288 0 6 0 ma the forearc melange consists of numerous ophiolite fragments and continental margin derived olistoliths blocks that predate the ophiolite the olistoliths are best represented by a gabbroic block 291 8 2 3 ma that contains granite xenoliths 312 6 1 8 and 313 6 3 1 ma other dated blocks include a trondhjemite 323 9 2 7 ma a gabbro 296 6 1 7 ma and a tonalite 294 9 2 4 ma small bodies of diabase andesite and diorite in the forearc melange exhibit a wide variety of geochemical signatures we dated zircons from an n morb like diabase 274 4 2 5 ma an e morb like diabase 252 5 2 3 ma a transitional sanukitoid adakite andesite 250 2 2 4 ma a sanukitoid high mg diorite 251 8 1 1 ma and an anorthosite 252 2 1 7 ma the n morb like diabase contains ca 301 394 ma zircon xenocrysts suggesting assimilation of trench sediments when a spreading ridge intersected a trench the other dated rocks simultaneously formed near the permian triassic boundary and captured abundant zircon xenocrysts ca 269 295 ma ca 301 495 ma and ca 923 2501 ma our new formation ages constrain a magmatic episode in response to slab break off beneath a fossil forearc in a young post collisional setting and the youngest xenocryst ages ca 269 273 ma may define the maximum depositional age of trench sediments

pattern and kinematic polarity of late mesozoic extension in continental ne asia perspectives from metamorphic core complexes 1 late mesozoic extension in ne asia resulted in the development of a large extensional province metamorphic core complexes mccs are the major features in this province and have 40ar 39ar ages of 130 110 ma for the mylonites and u pb zircon ages of 150 110 ma for the integral granitic intrusions based on this and previous studies this paper summarizes major characteristics of these mccs and recognizes a regional kinematic shear sense most mccs in the transbaikalia region sino mongolia border tract and the northwest central portion of the north china craton ncc show a top to the southeast se shear whereas those in the eastern and southern ncc locally underwent top to the northwest nw shear the three largest basins songliao huabei and ordos in north china are located in the transitional zone between domains of opposing shear sense we interpret the extension in the transbaikalia sino mongolia tract and northwestern part of the ncc to reflect late orogenic collapse of thickened crust following middle late jurassic collision along the okhotsk suture the southeastward extension is probably controlled by crustal scale top to the se tangential shear the transition from contraction to extension is marked by detachment faults that nucleated as extensional crenulation cleavage ecc i e c in sub horizontal ductile shear zones late in orogenic crustal thickening the combined effect of gravitational loading and thermal uplifting is considered to be the origin of the late or post orogenic collapse the top to the nw extension in the ne of the ncc might reflect antithetic sub extensional zones or mesozoic back arc extension as a far field effect of cretaceous pacific plate subduction

magma mixing origin for the aolunhua porphyry related to mo cu mineralization eastern central asian orogenic belt abstract many cu mo au deposits are considered to be related to adakitic porphyries formed in non arc settings e g in collisional orogenic zones and intra plate environments but their genesis is still under discussion the aolunhua porphyry complex and its related mo cu deposit from the eastern central asian orogenic belt caob provide important insights into this issue the porphyries are characterized by high sr 496 705 ppm and sr y and la yb ratios similar to those of typical adakitic rocks and low i sr ratios 0 7049 0 7052 and positive e nd t 0 5 to 1 4 and e hf t 3 5 to 9 8 values these features along with the occurrence of mafic microgranular enclaves mmes compositional and textural disequilibrium of plagioclase phenocrysts and relatively high mg values 45 52 indicate that they were derived from mixing of felsic magma from partial melting of a juvenile arc type lower crust and mafic magma from a lithospheric mantle previously metasomatized by subduction zone fluids melts high sr y and la yb ratios are indicative of contribution from enriched mantle derived materials with high liles e g sr la which were strengthened by subsequent fractionation of ferromagnesian phases such as pyroxene and hornblende mmes hosted by the ore bearing porphyry have zircon u pb ages of ca 132 ma similar to those of the host rocks the enclaves have elevated mg 56 63 liles e g sr 660 891 ppm lree la n 68 150 la sm n 3 0 4 0 la yb n 12 0 19 6 and ratios of radiogenic isotopes of nd and hf e nd 0 7 to 1 6 e hf 3 3 to 10 9 suggesting that their parental magmas were derived from the metasomatized mantle source the mo cu mineralization was probably related to the high water content high oxygen and sulfur fugacity of hybrid magma formation of the adakitic porphyries and related mo cu deposits of the eastern caob could be related to the early cretaceous lithospheric extension caused by the subduction of the paleo pacific plate and its induced reactivation of juvenile arc type lower crust

accretion subduction erosion and tectonic extrusion during late paleozoic to mesozoic orogenesis in ne china abstract the high pressure heilongjiang complex records a long and complicated evolution from the late paleozoic to mesozoic in ne china which is still subject to controversy in this paper we present a new detailed study including structural geochemical and geochronological analyses ar ar dating on metamorphic minerals u pb sims dating on zircons two main periods of oceanward accretionary growth are identified between 305 ma and 195 ma and around 112 101 ma these periods of accretionary growth were associated with the emplacement of voluminous magmatism slab roll back with continental back arc extension and growth of a forearc accretionary prism between these periods of accretionary growth a major phase of regional subduction erosion took place between 195 ma and 142 ma at the eastern edge of the songliao block the main effects of this subduction erosion phase were that regional jurassic early cretaceous extrusion overlapped with mesozoic accretion and erosion in ne china which disturbed the initial orogenic architecture with a review of relevant zircon ages of this accretionary orogen we unravel the complicated geodynamic history of accretion subduction erosion and tectonic extrusion that shaped ne china from the late paleozoic to mesozoic

mechanisms of mass and heat transport during barrovian metamorphism a discussion based on field evidence from the central alps switzerland northern italy 1 tectonic and metamorphic data for the central alps switzerland italy are used to discuss this classic example of a barrovian metamorphic terrain notably the evolution of its thermal structure in space and time available p t t data indicate variable contributions of advective and conductive heat transport during collision and subsequent cooling and exhumation some areas experienced a prolonged period of partial melting while other areas at the same time show but moderate heating the barrow type metamorphic field gradient observed in the final orogen is the result of two distinct tectonic processes with their related advective and conductive heat transport processes the two tectonic processes are 1 accretion of material within a subduction channel related to decompression and emplacement of high pressure units in the middle crust and 2 wedging and related nappe formation in the continental lower plate the second process postdates the first one wedging and underthrusting of continental lower plate material produces heat input into lower crustal levels and this process is responsible for predominantly conductive heat transport in the overlying units the interacting processes lead to different maximum temperatures at different times producing the final barrovian metamorphic field gradient the south experienced rapid cooling whereas the north shows moderate cooling rates this discrepancy principally reflects differences in the temperature distribution in the deeper crust prior to cooling differences in the local thermal gradient that prevailed before the cooling also determined the relationships between cooling rate and exhumation rate in the different areas citation berger a s m schmid m engi r bousquet and m wiederkehr 2011 mechanisms of mass and heat transport during barrovian metamorphism a discussion based on field evidence from the central alps switzerland northern italy tectonics 30 tc1007 doi 10 1029 2009tc002622

new paleomagnetic results from northern china collision and suturing with siberia and kazakhstan abstract we report new results from our paleomagnetic studies of the inner mongolia autonomous region in northeastern china and the xinjiang autonomous region in northwestern china primary magnetizations are isolated in tertiary cretaceous late jurassic and late permian rocks and secondary components of magnetization of late jurassic and recent ages are seen in strata of inner mongolia western junggar basin and part of the tarim block suffered widespread complete tectono thermal remagnetization at about 280 ma permian the paleomagnetic evidence suggests that the sampled areas in inner mongolia have been part of the north china block at least since late permian time and the junggar block collided with the tarim block by late permian time our own paleomagnetic results together with available data from other parts of eastern eurasia suggest a tectonic model in which kazakhstan europe and siberia collided with each other and formed the europe siberia kazakhstan plate eur sib kaz in late permian time to the east mongolia outer mongolia inner mongolia and the north china block were also wedged together forming the mongolia north china plate mon nc by this time these two composite continental plates were not a stable unit in the late permian the geometrical relationship of the paleomagnetic poles for these two plates suggests that collision initially occurred in the late permian near the western end 50 n 75 e of the southern siberian fold belt and progressed eastward as the mon nc plate rotated and clockwise about 117 relative to the eur sib kaz plate during the early mesozoic suturing and rotation were probably complete by the late jurassic significant relative motion between the tarim and siberia blocks continued from early triassic to late jurassic as tarim moved northeastward 1400 km relative to siberia

paleo pacific subduction accretion evidence from geochemical and u pb zircon dating of the nadanhada accretionary complex ne china the nadanhada terrane located along the eastern margin of eurasia contains a typical accretionary complex related to paleo pacific plate subduction accretion the yuejinshan complex is the first stage accretion complex that consists of meta clastic rocks and metamafic ultramafic rocks whereas the raohe complex forms the main parts of the terrane and consists of limestone bedded chert and mafic ultramafic rocks embedded as olistolith blocks in a weakly sheared matrix of clastic meta sedimentary rocks geochemical data indicate that the yuejinshan metabasalts have normal mid ocean ridge basalt n morb affinity whereas the raohe basaltic pillow lavas have an affinity to ocean island basalts oib sensitive high resolution ion microprobe shrimp u pb zircon analyses of gabbro in the raohe complex yield a weighted mean 206pb 238u zircon age of 216 5 ma whereas two samples of granite intruded into the complex yield weighted mean 206pb 238u zircon ages of 128 2 and 129 2 ma laser ablation inductively coupled plasma mass spectrometry la icpms u pb zircon analyses of basaltic pillow lava in the raohe complex define a weighted mean age of 167 1 ma two sandstone samples in the raohe complex record younger concordant zircon weighted mean ages of 167 17 and 137 3 ma these new data support the view that accretion of the raohe complex was between 170 and 137 ma and that final emplacement of the raohe complex took place at 137 130 ma the accretion of the yuejinshan complex probably occurred between the 210 and 180 ma suggesting that paleo pacific plate subduction was initiated in the late triassic to early jurassic

palaeomagnetism of the west sakhalin basin evidence for northward displacement during the cretaceous summary a significant margin parallel translation of terranes is postulated by all models for tectonic evolution of the east asian continental margin although the timing and magnitude of displacements of individual elements are poorly constrained as yet the west sakhalin basin a forearc basin associated with the cretaceous volcanic arcs is one of the displaced tectonic elements our palaeomagnetic study of the basin rocks provides the first quantitative constraints on its cretaceous history we identified a characteristic magnetization in the berriasian valanginian basalt representing the oceanic basement of the basin and four characteristic magnetizations in a continuous sequence of the albian to maastrichtian siliciclastic fill of the basin a combination of the positive fold conglomerate and reversals tests supports the primary origin of the remanence in the sediments palaeomagnetic data indicate a gradual shift of the west sakhalin basin from subequatorial latitudes during the early cretaceous to about 40on by the late cretaceous the main phase of the margin parallel migration took place during the early cretaceous and ended by the cenomanian the continuous sedimentary sequence records 50 degrees of progressive clockwise rotation during the late cretaceous

geochronology and isotope geochemistry studies of an epithermal gold deposit in the northern lesser khingan range ne china the gaosongshan example abstract the gaosongshan epithermal gold au deposit is located in the northern margin of the lesser khingan range polymetallic ore belt northeast ne china and contains au resources of more than 23 tons at an average grade of 6 3 g t gold orebodies are mainly hosted in intensely altered volcanic rocks of the lower cretaceous banzifang formation and as exploration has continued gold has also been discovered in extensively altered porphyritic intrusions with the highest au grade of 70 g t however the ages of metallogenesis and related magmatism have not been well constrained yet which have hindered the summary of metallogenic regularities in this study laser ablation inductively coupled plasma mass spectrometry la icp ms zircon u pb dating from the granite porphyry yields a weighted mean 206pb 238u age of 101 6 1 2 ma slightly older than the rb sr isotope age of 98 1 6 ma for the auriferous quartz samples indicating that the gaosongshan au mineralization is genetically related to the granite porphyry the fluid d and 18oh2o values vary from 136 to 111 and from 9 3 to 2 5 respectively and the 4he and 40ar isotope compositions range from 15 7 10 9 to 35 5 10 9 cm3 stp g and 13 4 10 7 cm3 to 34 1 10 7 cm3 stp g respectively the obtained isotopic data show that the ore forming fluids were mixtures of magmatic and meteoric water but were dominated by meteoric water the 206pb 204pb 207pb 204pb and 208pb 204pb ratios of ores in the gaosongshan deposit are 17 752 18 856 15 514 15 674 and 38 005 39 921 respectively which are similar to those of the gaosongshan granite porphyry implying that pb and possibly au originated primarily from late early cretaceous magmatic intrusions combining these results with data from previous studies we propose that the gaosongshan au deposit formed in an extensional tectonic setting during the rollback of the paleo pacific oceanic plate in ne china

the pre devonian tectonic framework of xing an mongolia orogenic belt xmob in north china abstract a new tectonic division of the xing an mongolia orogenic belt xmob in north china has been presented according to our research and a lot of new data of tectonics geochronology and geochemistry four blocks and four sutures have been recognized in the xmob including the erguna eb xing an airgin sum xab songliao hunshandake shb and jiamusi jb and xinlin xiguitu xxs xilinhot heihe xhs mudanjiang ms and ondor sum yongji sutures oys the framework of the xmob is characterized by a tectonic collage of the blocks and orogenic belts between them different precambrian basements have been found in the blocks including the neoproterozoic metamorphic rocks and plutons in the eb the neoproterozoic metamorphic rocks in western and eastern of segments of the xab mesoproterozoic and neoproterozoic metamorphic rocks in middle segments of the xab respectively the neoproterozoic metamorphic rocks and mesoproterozoic volcanic rocks and plutons in the shb and neoproterozoic metamorphic rocks in the jb the xxs resulted from a northwestward subduction of the xab beneath the eb during the cambrian which was followed by the forming of the xhs and oys in the northwest and south margins of the shb in the silurian respectively the ms was caused by a westward subduction of the jb beneath the east margin of the shb during the middle devonian the three cambrian silurian and middle devonian events indicate that the xmob belongs to a pre middle devonian multiple orogenic belt in the central asian orogenic belt caob forming of the xmob suggests that the southeast part of the paleo asian ocean closed before the middle devonian

mesozoic terrane accretion in northeast china sikhote alin and japan regions abstract the nadanhada range in northeast china is composed of upper paleozoic limestone greenstone middle to late triassic bedded chert and middle jurassic siliceous shale these rocks are enclosed in post middle jurassic clastic rocks the lithologic association ages radiolarian assemblages and geologic structure of the nadanhada range are very similar to those of the tamba mino ashio terrane in southwest japan this indicates that the nadanhada range is the northern extension of the tamba mino ashio terrane mesozoic tectonostratigraphic terranes identified in northeast china sikhote alin and southwest japan are the khanka terrane a micro continent composed of precambrian metamorhic rocks and paleozoic to mesozoic sedimentary and volcanic covers the nadanhada western sikhote alin terrane a disrupted terrane composed of upper paleozoic to jurassic sedimentary rocks the eastern sikhote alin terrane a terrane composed mainly of early cretaceous clastics the tetyukhe terrane a disrupted terrane composed of upper paleozoic to early cretaceous sedimentary rocks the hida terrane a continent type stratigraphic terrane composed of precambrian and upper paleozoic basement covered with jurassic to cretaceous coarse clastics and the tamba mino ashio terrane a disrupted terrane composed of upper paleozoic to earliest cretaceous sedimentary rocks accretionary history of these terranes is discussed on the basis of paleomagnetic and paleobiogeographic data

zircon u pb age and sr nd hf isotope geochemistry of permian granodiorite and associated gabbro in the songliao block ne china and implications for growth of juvenile crust abstract post orogenic granitic and associated mafic rocks from northeastern ne china consist of granodiorite and gabbro intrusions we report here upon new u pb zircon ages geochemical data and sr nd hf isotopic data for these rocks la icp ms u pb zircon analysis yields an age of 262 8 1 0 ma for the granitic rocks and a uniform age of 262 1 0 7 ma for the gabbro most of the granitic and mafic rocks are characterised by low k 2 o na 2 o and pertain to the subalkaline series in the total alkali silica diagram the granodiorite samples show low 87 sr 86 sr i ranging from 0 700 to 0 705 positive e nd t values from 0 3 to 0 8 and large variation in e hf t values of between 4 0 and 2 5 indicating that both newly underplated basalt 70 80 and ancient lower crustal sources 20 30 contributed to their origin furthermore positive e hf t values with two stage model ages t dm2 of 1123 1260 ma together with nd model ages 960 1000 ma suggest an important episode of crustal growth during the meso neoproterozoic beneath the songliao block in contrast the investigated gabbro is characterised by relatively high 87 sr 86 sr i ratios 0 707 0 708 negative e nd t 5 9 to 5 3 and e hf t values 5 0 to 2 3 implying that this was derived from an enriched mantle source the geochemical data indicate that the granitic magmas underwent separation of clinopyroxene hornblende k feldspar plagioclase ti bearing phases e g rutile ilmenite titanite apatite and zircon during their evolution whereas the gabbro is characterised by low mgo 2 92 3 92 wt mg 35 41 and compatible elements content such as cr 10 68 ppm co 16 31 ppm and ni 5 7 33 ppm features of a more evolved mafic magma there is no evidence that the granitic and mafic rocks were affected by crustal contamination during emplacement our interpretation is that the two coeval intrusive suites were both formed in a post orogenic extensional setting related to lithospheric delamination or collapse of the central asian orogenic belt caob xingmeng orogenic belt in china

collision belt between the khanka block and the north china block in the yanbian region northeast china the pre jurassic geotectonics in northeast china and its adjacent regions are characterized by collision of microcontinental blocks the yanbian region lies between the north china block ncb and the khanka block kb along the dashanzui antu kaishantun collision belt the yanbian pre jurassic fold belt can be divided into two parts one part represents the western margin of the kb which consists of late proterozoic metamorphic basement and a permian active continental margin the other one is located at the eastern margin of the ncb and is composed of proterozoic passive margin deposits where the seluohe and qinglongcun groups are developed and rocks of the permian passive margin collision of these two parts took place along the dashanzui antu kaishantun collision zone during the late permian to early triassic as evidenced by the emplacement of syncollision granites and a type granites in northeast china and adjacent regions a series of microcontinental blocks collided with one other from the end of early paleozoic to the early triassic the collision event that took place in the yanbian region may represent the latest collision event in the paleo asian oceanic domain c 2003 elsevier ltd all rights reserved

geochronology and geochemistry of middle permian middle triassic intrusive rocks from central eastern jilin province ne china constraints on the tectonic evolution of the eastern segment of the paleo asian ocean abstract to constrain the permian early mesozoic tectonic evolution of the eastern segment of the paleo asian ocean we conducted zircon u pb dating and whole rock geochemical analyses on six middle permian middle triassic intrusive plutons in central eastern jilin province ne china zircons from the six plutons display distinct oscillatory zoning and striped absorption in cathodoluminescence cl images and th u ratios of 0 11 1 41 indicating a magmatic origin zircon u pb dating indicates that the intrusive rocks formed in the middle permian ca 260 ma to middle triassic ca 245 ma in central eastern jilin province the middle permian and middle triassic plutons are composed mainly of strongly deformed monzogranites with affinities to adakitic rocks which are formed from partial melting of thickened mafic lower crust in contrast the late permian the early triassic plutons in central eastern jilin province consist of a bimodal association including gabbros and granitoids and deformed monzonites which typically form in extensional settings these observations along with the results of previous studies on early middle permian granitoids in western jilin province indicate that the eastern segment of the paleo asian ocean underwent the initial closure in central western jilin province during the middle permian and the final closure in eastern jilin province in the middle triassic

evolution of the solonker suture zone constraints from zircon u pb ages hf isotopic ratios and whole rock nd sr isotope compositions of subduction and collision related magmas and forearc sediments abstract the solonker zone in northern inner mongolia china is considered as the suture between the north china craton and the south mongolian microcontinent two magmatic belts are recognized along the suture zone a subduction related magmatic belt represented by the baolidao arc rocks and a younger collision related granite belt represented by the halatu granites we use zircon u pb ages zircon in situ hf isotopic analyses and whole rock nd sr isotopic data of the two magmatic belts and related forearc sediments the xilinhot metamorphic complex to constrain timing of the suturing and to discuss the petrogenesis of the magmatic rocks a gabbroic diorite bld 1 of the baolidao arc was dated at 310 5 ma by shrimp this sample shows an e nd t value of 2 5 and i sr of 0 7052 hf isotopic analyses on 25 zircons from the same sample show e hf t 5 4 to 11 5 another diorite sample xh 2 of the same arc from south of xilinhot displays even more depleted isotopic compositions with e nd t 5 6 and i sr 0 7037 the main population of zircons from this sample have highly variable and depleted hf isotopic compositions e hf t 0 18 3 the large variation in hf isotopic composition of zircons with largely the same crystallization age from a single pluton is explained by a mixing process between depleted mantle derived magma and continental crust in an active continental arc setting the halatu granite hlt 2 was dated at 234 7 ma by shrimp zircons from the granite also show a large variation of e hf t values 9 1 to 26 despite most samples having whole rock e hf t 2 the large variation in e hf t values suggests that the granite formed probably by partial melting of two source regions a dominant juvenile crust and a subordinate old continental crust most zircons from the xilinhot metamorphic complex show ages comparable with those of the baolidao arc rocks suggesting that the protolith of the metamorphic complex was probably deposited during or after arc magmatism some zircons however show precambrian ages that fall into two groups one with ages of 780 900 ma resembling those from the south mongolian microcontinent and the other with ages of 1524 2900 ma similar to those of the north china craton thus the protolith of the metamorphic complex probably formed in a forearc basin during convergence of the two continents and metamorphosed subsequently during collision in the late paleozoic our zircon age data thus constrain timing of collision between the south mongolian microcontinent and the north china craton to have been between 296 and 234 ma

sources of phanerozoic granitoids in the transect bayanhongor ulaan baatar mongolia geochemical and nd isotopic evidence and implications for phanerozoic crustal growth abstract the central asian orogenic belt caob is renowned for massive generation of juvenile crust in the phanerozoic mongolia is the heartland of the caob and it has been subject to numerous investigations particularly in metallogenesis and tectonic evolution we present new petrographic geochemical and sr nd isotopic analyses on phanerozoic granitoids emplaced in west central mongolia the data are used to delineate their source characteristics and to discuss implications for the phanerozoic crustal growth in central asia our samples come from a transect from bayanhongor to ulaan baatar including three tectonic units the baydrag cratonic block late archean to middle proterozoic the eo cambrian bayanhongor ophiolite complex and the hangay hentey basin of controversial origin the intrusive granitoids have ages ranging from ca 540 to 120 ma the majority of the samples are slightly peraluminous and can be classified as granite s s including monzogranite syenogranite and alkali feldspar granite most of the rocks have initial 87sr 86sr ratios between 0 705 and 0 707 late paleozoic to mesozoic granitoids 250 ma are characterized by near zero end t values 0 to 2 whereas older granitoids show lower end t values 1 5 to 7 the data confirm the earlier observation of kovalenko et al geochemistry international 34 1996 628 who showed that granitoids emplaced outside of the pre riphean basement rocks are characterized by juvenile positive end t values whereas those within the pre riphean domain and the baydrag cratonic block as for the present case show a significant effect of contamination by precambrian basement rocks nevertheless mass balance calculation suggests that the granitoids were derived from sources composed of at least 80 juvenile mantle derived component despite our small set of new data the present study reinforces the general scenario of massive juvenile crust production in the caob with limited influence of old microcontinents in the genesis of phanerozoic granitoids

late paleozoic final closure of the paleo asian ocean in the eastern part of the xing meng orogenic belt constrains from carboniferous permian meta sedimentary strata and meta igneous rocks abstract zircon u pb dating and whole rock geochemical data for carboniferous permian meta sedimentary sequences igneous rocks and precambrian amphibolite in the eastern xing meng orogenic belt xmob were used to constrain the final stage evolution and the position of closure of the paleo asian ocean pao detrital zircons from late paleozoic strata range in age from archean to late paleozoic with major age groups at 1 0 0 7 ga and 0 5 0 25 ga in the northern part and at 2 7 2 5 ga 2 1 1 8 ga and 0 5 0 25 ga in the southern part of the xmob striking changes in zircon age distribution patterns indicate inputs to these strata were separated by the paleo asian ocean pao in the late paleozoic the pao closed along the solonker linxi suture the late paleozoic formation ages 346 ma 303 ma and 269 ma of the igneous rocks with arc like geochemical features south to the pao together with previously published data on the regional igneous rocks and ophiolites indicate double side subduction of the pao in the late paleozoic

permian geodynamic setting of northeast china and adjacent regions closure of the paleo asian ocean and subduction of the paleo pacific plate abstract northeast china and adjacent regions are located in the central east asian continent and consist tectonically of both the paleo asian and paleo pacific orogens between the siberian platform and sino korean north china block this paper discusses some hotly debated issues concerning the permian geodynamic setting of these regions based on a comprehensive analysis of available geological geochemical paleobiogeographical and paleomagnetic data spatial and temporal distribution of ophiolites and associated continental marginal sequences permian sedimentary sequences spatial distribution and geochemistry of permian magmatic rocks and the evolution of paleobiogeographical realms imply 1 that the permian marine basins in northeast china and adjacent regions include remnants of the paleo asian ocean in southeastern inner mongolia and central jilin province and active continental margins of the paleo pacific ocean 2 that the suture between the siberian and sino korean paleoplates was finally emplaced in the permian and is located in areas from suolunshan solonker eastwards through regions north to the xar moron river in southeastern inner mongolia and then central jilin province to the yanji area and 3 that the permian crustal evolution of northeast china and adjacent regions as well as parts of the siberian paleoplate was influenced by subduction of the paleo pacific oceanic plate finally the permian tectonic framework and paleogeography of northeast china and adjacent regions in central east asia are discussed briefly and early and late permian palinspastic reconstruction maps are provided

spatial temporal relationships of mesozoic volcanic rocks in ne china constraints on tectonic overprinting and transformations between multiple tectonic regimes abstract la icp ms zircon u pb ages and geochemical data are presented for the mesozoic volcanic rocks in northeast china with the aim of determining the tectonic settings of the volcanism and constraining the timing of the overprinting and transformations between the paleo asian ocean mongol okhotsk and circum pacific tectonic regimes the new ages together with other available age data from the literature indicate that mesozoic volcanism in ne china can be subdivided into six episodes late triassic 228 201 ma early middle jurassic 190 173 ma middle late jurassic 166 155 ma early early cretaceous 145 138 ma late early cretaceous 133 106 ma and late cretaceous 97 88 ma the late triassic volcanic rocks occur in the lesser xing an zhangguangcai ranges where the volcanic rocks are bimodal and in the eastern heilongjiang jilin provinces where the volcanics are a type rhyolites implying that they formed in an extensional environment after the final closure of the paleo asian ocean the early middle jurassic 190 173 ma volcanic rocks both in the erguna massif and the eastern heilongjiang jilin provinces belong chemically to the calc alkaline series implying an active continental margin setting the volcanics in the erguna massif are related to the subduction of the mongol okhotsk oceanic plate beneath the massif and those in the eastern jilin heilongjiang provinces are related to the subduction of the paleo pacific plate beneath the eurasian continent the coeval bimodal volcanic rocks in the lesser xing an zhangguangcai ranges were probably formed under an extensional environment similar to a backarc setting of double direction subduction volcanic rocks of middle late jurassic 155 166 ma and early early cretaceous 145 138 ma age only occur in the great xing an range and the northern hebei and western liaoning provinces limited to the west of the songliao basin and they belong chemically to high k calc alkaline series and a type rhyolites respectively combined with the regional unconformity and thrust structures in the northern hebei and western liaoning provinces we conclude that these volcanics formed during a collapse or delamination of a thickened continental crust related to the evolution of the mongol okhotsk suture belt the late early cretaceous volcanic rocks widely distributed in ne china belong chemically to a low to medium k calc alkaline series in the eastern heilongjiang jilin provinces i e the eurasian continental margin and to a bimodal volcanic rock association within both the songliao basin and the great xing an range the volcanics in the eastern heilongjiang jilin provinces formed in an active continental margin setting related to the subduction of the paleo pacific plate beneath the eurasian continent and the bimodal volcanics formed under an extensional environment related either to a backarc setting or to delamination of a thickened crust or both late cretaceous volcanics limited to the eastern heilongjiang jilin provinces and the eastern north china craton ncc consist of calc alkaline rocks in the eastern heilongjiang jilin provinces and alkaline basalts in the eastern ncc suggesting that the former originated during subduction of the paleo pacific plate beneath the eurasian continent whereas the latter formed in an extensional environment similar to a backarc setting taking all this into account we conclude that 1 the transformation from the paleo asian ocean regime to the circum pacific tectonic regime happened during the late triassic to early jurassic 2 the effect of the mongol okhotsk suture belt on ne china was mainly in the early jurassic middle late jurassic and early early cretaceous and 3 the late early cretaceous and late cretaceous volcanics can be attributed to the subduction of the paleo pacific plate beneath the eurasian continent

the timing of final closure along the changchun yanji suture zone constraints from detrital zircon u pb dating of the triassic dajianggang formation ne china abstract the dajianggang formation is located in the changchun yanji suture zone of central jilin province and unconformably overlies the changchun yanji accretionary complex cyac which is a melange resulting from subduction of the jiamusi khanka block jkb beneath the north china craton ncc la mc icp ms u pb dating of detrital zircon from four samples of the formation yields ages of 2516 to 216 ma zircons with u pb ages at 2516 2501 ma and 1897 1832 ma indicate a provenance from precambrian basement rocks of the ncc the 525 482 ma ages indicate a provenance from metamorphic rocks of late pan african age in the jkb that have a tectonic affinity to the central asian orogenic belt caob zircon grains with ages of 383 314 ma and 275 250 ma were likely derived from the underlying cyac the youngest population has a peak age of ca 225 ma which together with late triassic fossils suggests that deposition of the dajianggang formation was late triassic or younger this result supports the view that the final collision of the jkb and ncc along the changchun yanji suture took place before the late triassic furthermore this closure time is at least 10 20 ma later than closure along the solonker xar moron changchun suture in the late permian we thus establish that the changchun yanji suture is not related to the collision between the siberia craton sc and the ncc but was instead related to the paleo pacific plate subduction consequently the changchun yanji suture is not the eastward extension of the solonker xar moron changchun suture as previously considered but the southern margin of the jilin heilongjiang high pressure metamorphic belt ji hei hp belt and resulted from westward subduction of the paleo pacific ocean thus the late triassic marked the switch in subduction from the paleo asian ocean to the paleo pacific ocean in ne china

direct measurement of timing underpinning a reliable petroleum system model for the mid continent rift system the amoco 1 eischeid well sec 6 t83n r35w carroll county iowa was the first significant well drilled along the flank of the iowa horst providing an opportunity to evaluate the stratigraphic and structural history of the enigmatic mid continent rift system of the united states new thermal history data obtained from cutting and core samples show that three paleothermal events affected the drilled section at the amoco 1 eischeid well results show that three significant cooling events occurred sometime between 300 and 200 70 and 50 and 35 and 10 ma the first of these events was associated with a high paleogeothermal gradient 35c km demonstrably greater than the present day value 16c km these new results provide direct evidence of a significantly higher paleogeothermal gradient and by inference higher heat flow during the paleozoic in addition results show that maximum maturity of the rich precambrian source the nonesuch shale equivalent was reached during the 300 200 ma event and not 600 700 m y earlier as previously proposed previous work has been significantly influenced by the view that the craton was relatively stable or behaved uniformly throughout the phanerozoic previous models impose a proterozoic time of generation and entrapment for any precambrian source intervals with recognition of significant phanerozoic heating events the opportunity for later generation emerges thus reducing the preservation time and increasing the probability of entrapment and preservation needed for exploration success in the region furthermore these events mapped at the mid continent rift system may be of great regional extent and correlative in time to other well known events along the north american plate margins e g alleghanian laramide ancestral rockies orogenies thus offering insights to new possible exploration targets within the rift perhaps indicative of deep seated processes in the mantle beneath the north american craton during the phanerozoic

a type granites in northeastern china age and geochemical constraints on their petrogenesis abstract a type granites are widely distributed in northeastern china ne china they were emplaced during three major episodes the permian late triassic to early jurassic and early cretaceous and evolved in different tectonic regimes according to their mineralogical and geochemical characteristics two subgroups of a type granites aluminous and peralkaline can be recognized the peralkaline subgroup contains alkali mafic minerals such as riebeckite arfvedsonite and sodic pyroxene while the aluminous subgroup contains annite and fe rich calcic or sodic calcic amphibole with respect to the aluminous subgroup the peralkaline granites contain higher rb ga and total rare earth elements ree but lower mgo cao al 2 o 3 ba and sr based on the discrimination criteria of eby geology 20 1992 641 the permian and late triassic to early jurassic a type granites belong to the a 2 post orogenic type whereas the early cretaceous granites are of the a 1 anorogenic type nd isotopic compositions of these a type granites indicate their derivation from a dominantly juvenile crustal source their origin is thought to have involved partial melting of an underplated lower crustal source because the generation of a type granites requires high melting temperature we propose models involving slab break off lithospheric delamination and extension the permian a type granites have the same age range as those in eastern junggar southern mongolia and central inner mongolia they occur along a major suture and form a narrow belt between the north china and siberian cratons we suggest that their formation was associated with post collisional slab break off the late triassic to early jurassic a type granites are likely to be the product of lithospheric delamination after the final collision of the major crustal blocks in the late paleozoic to early triassic the early cretaceous a type granites have an anorogenic affinity and were possibly associated with rifting in eastern china at this time associated with the onset of paleo pacific subduction consequently we conclude that the a type granites in ne china were generated at three different times involving multiple processes operative in different tectonic environments

zircon u pb geochronology and petrogenesis of the late paleozoic early mesozoic intrusive rocks in the eastern segment of the northern margin of the north china block abstract zircon u pb dating and whole rock geochemical analysis have been undertaken on late paleozoic early mesozoic intrusive rocks of central jilin province ne china with the aim of constraining the tectonic evolution of the eastern segment of the northern margin of the north china block ncb during the late paleozoic early mesozoic zircon u pb dating indicates that late paleozoic early mesozoic magmatic events can be subdivided into four stages middle permian 270 ma late permian 259 255 ma early triassic 249 ma and late triassic 222 ma the middle permian magmatic event formed peraluminous garnet bearing monzogranites indicative of formation under a compressional tectonic regime related to collision between the ncb and the songnen zhangguangcai range massif the late permian magmatic event formed gabbros and syenogranites a bimodal association that is typical of magmatism within post collisional extensional setting the early triassic magmatic event formed adakitic monzogranites suggesting that they formed from magmas derived from partial melting of a thickened mafic lower crust the late triassic magmatic event formed a series of mafic ultramafic intrusive rocks together with coeval granitoids in adjacent regions and make up a typical bimodal association suggesting that they formed under an extensional environment related to the final amalgamation of the ncb and the songnen zhangguangcai range massif taken together we conclude that the northern margin of the ncb underwent multiple orogenic events between the middle permian and the late triassic with final amalgamation of the ncb and the songnen zhangguangcai range massif occurring in the early triassic an event that led to the final suturing of the solonker xra moron changchun zone

early cretaceous high mg diorites in the yanji area northeastern china petrogenesis and tectonic implications abstract mesozoic granitic rocks are widely distributed in northeast ne china however high mg dioritic rocks are considerably rare here we report a newly recognized high mg diorite the xintun diorite in the yanji area ne china to constrain its origin and implications for the tectonic evolution of eastern asian continental margin zircon u pb dating yields a crystallization age of 128 1 ma for the xintun diorite the diorites are characterized by high mgo 4 4 6 6 wt cr 119 239 ppm ba 419 514 ppm and sr 649 747 ppm contents and mg values 59 64 but low feototal mgo ratios 1 2 1 4 with geochemical features similar to those of sanukitic high mg andesites hmas they show moderate radiogenic sr isr 0 7047 0 7050 and nd end 0 3 1 1 with high la sm ratios which are indicative of contributions from sediment components the mineral assemblage of euhedral hornblende magnetite and titanite implies a water rich and oxidized signature for their primitive magmas these features suggest that the xintun high mg diorites were probably formed via partial melting of the subducting sediments and subsequent interaction of mantle peridotites with both melts and aqueous fluids geochemical modeling reveals that hornblende dominated fractional crystallization under water sufficient conditions enabled the evolved magmas to acquire adakitic signatures we believe that the paleo pacific subduction beneath eastern asian continental margin caused large scale back arc extension of ne china in the early cretaceous and consequently induced the asthenospheric flow toward the mantle wedge reheating subducting sediments enough to cause melting therefore the occurrence of the xintun high mg diorites signifies the onset of extensive back arc extension of eastern asian continental margin at ca 128 ma