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

mesozoic collision extrusion tectonics in eastern asia abstract the eastern part of the asian continent was intensively deformed in early cretaceous time deformation has been interpreted to be related to subduction tectonics along the eastern continental margin we propose an alternative tectonic framework of collision extrusion tectonics resulting from collision of the indochina block with the eurasian continent this event controlled the jurassic to cretaceous tectonic history in eastern asia and the evolution of the subduction zone along the eastern margin of the continent

geochronological and geochemical constraints on aolunhua porphyry mo cu deposit northeast china and its tectonic significance abstract the aolunhua porphyry mo cu deposit is located in the northern margin of the north china craton ncc and belongs to the northern part of the xilamulun metallogenic belt more than 90 of the mineralization occurs within the aolunhua monzogranite porphyry a small part is hosted within quartz veins that crosscut late permian strata the syenogranite occurring as dikes and cutting through the aolunhua monzogranite porphyry is radially distributed in the mining district zircon u pb ages show that the aolunhua monzogranite porphyry and the post ore syenogranite have concordant 206pb 238u ages of 138 7 1 2 ma and 131 4 2 8 ma respectively based on analyses of major trace elements and hf isotopes the aolunhua porphyry is characterized by high sr low y with high la yb and sr y ratios typical of adakitic granites whereas the post ore syenogranite has lower sr and higher y values showing apparently negative eu anomalies eu 0 26 to 0 31 the hf isotopic composition of the aolunhua porphyry ehf t 3 6 to 9 2 and the post ore syenogranite ehf t 3 6 to 8 7 indicates that both juvenile crustal sources and depleted mantle contributed to their origin the regional geological setting together with the discrepancy of geochemistry between the aolunhua porphyry and the post ore syenogranite probably indicates that they formed in different tectonic regimes the aolunhua porphyry is derived from partial melting of the thickened crust due to underplating of the basaltic magma under the transformation tectonic regime while the post ore syenogranite comes from the crustal root melting during the lithospheric delamination stage under the lithosphere thinning regime of northeast china

plate tectonic history basin development and petroleum source rock deposition onshore china abstract china comprises a mosaic of distinct continental fragments separated by fold belts these fold belts are suture zones resulting from the accretion of various fragments formerly separated by intervening areas of oceanic crust the major sedimentary basins onshore china can be classified into four groups those in western china are flexural developing as a result of north south compression in contrast those in the east are extensional and related to development of the pacific oceanic margin in central china basins have a more problematic origin those of north central china ordos sichuan are flexural basins controlled by eastward directed thrusting along their western margin in contrast basins further south chuxiong shiwandashan are predominantly extensional and related to major strike slip movements by synthesizing basin stratigraphies across china in tectonostratigraphic terms and in particular comparing the nature and timing of unconformities it is possible to formulate a coherent model for the palaeoreconstruction of china we identify five major tectonostratigraphic breaks which equate with the collision of the following continental fragments tarim north china carboniferous permian south china block permian triassic qiantang late triassic early jurassic lhasa block late jurassic early cretaceous and india early tertiary prior to permian times the southern margin of eurasia ran approximately along the northern border of modern china the late carboniferous collision of tarim north china with eurasia resulted in the development of a flexural basin junggar and deposition of non marine clastics to the south of the suture shallow marine deposition continued in the late permian early triassic the progressive collision from east to west of the south china block with the north china block resulted in a change to fluvial lacutrine sedimentation across the entire north china tarim block open marine carbonate deposition in the north of the south china block passed southward into a deeper marine clastic sequence deposited in a backarc basin further south a subduction zone existed along the southeastern margin of the south china block in western china northward subduction throughout the triassic resulted in the development of the songban ganzi accretionary prism with retroarc thrusting resulting in flexure and the first development of the tarim basin oblique collision of the qiantang block in the late triassic along the east of the south china block resulted in east west directed thrusting which initiated the suchuan and ordos basins continued strike slip deformation along the south western margin of the south china block resulted in the development of basins with a significant extensional component such as chuxiong the collision of the qiantang block with the southern edge of the tarim basin early jurassic resulted in a renewed clastic influx in both the tarim and junggar basins along the eastern pacific margin a compressional arc and retroarc basin in the south passed northwards into an extensional arc system subduction rollback of the extensional arc initiated rifting in the late jurassic in the eren and songliao basins the late jurassic early cretaceous collision of the lhasa block in the west rejuvenated the thrust systems bordering the western basin and resulted in a renewed clastic influx in the southeast the compressional arc phase culminated in widespread thrusting and folding of early cretaceous age in the northeast extension continued with the progressive migration of the rift system southward with time the arrival of the indian block in the early tertiary rejuvenated the bounding thrust belts of all the western basins in the east the change in convergence of the pacific plate to a more westerly direction is marked by extension and widespread rifting along the entire length of eastern china throughout most of china mesozoic and cenozoic deposition occurred in predominantly non marine environments source rocks in such settings comprise principally mudstones deposited in lakes organic rich mudstones these can accumulate in both deep and shallow lakes in order to accumulate substantial volumes the lake must be significant in space and time in china lacustrine orms occur in both rift and flexural basins lacustrine orms deposited under humid climatic conditions are restricted to the period of maximum tectonic subsidence in the flexural basins of western china source rock deposition follows basin initiation by 20 30 ma in the extensional basins of eastern china source rock deposition takes place 5 15 ma after basin initiation by contrast semi arid and arid climate lacustrine orms whilst being best developed during the period of maximum tectonic subsidence occur at all stages in the basin history

great jurassic thrust sheets in beishan north mountains gobi areas of china and southern mongolia abstract jurassic thrust sheets with minimum displacements of 120 180 km have been discovered within the hercynian indosinian orogenic belt of the beishan of china and south gobi area the thrusts strike e w extend over 1200 km in length and carried meso proterozoic massive dolomitic limestones over strata ranging from neoproterozoic cryogenian and terminal proterozoic to lower middle jurassic slip linear plots based on kinematic indicators such as slickenlines and groove lineations fiber lineations and drag folds adjacent to the fault surface vergence of folds and imbricated thrusts in the upper plate indicate northward movement in the beishan area to the west and southward movement in the south gobi area to the east the two major thrust faults the beishan thrust and south gobi thrust are presumably separated by a major tear fault the ruo shui fault the major thrust faults were later deformed into a series of e w antiforms and synforms and the sheets are separated due to erosion into a number of klippen mainly located on synforms of the major faults the yagan metamorphic core complex which is a result of an extensional event that postdates the thrust event yields an 40 ar 39 ar plateau age of 155 1 10 ma and rb sr isochron age of 153 6 2 ma the thrust sheets formed during the late middle jurassic long after the closure of any oceans in the study area previously reported for this region and are ascribed to a phase of intracontinental deformation the closing of the jurassic tethys or retroarc deformation behind an active continental margin at the southern edge of asia prior to the tethyan collision or and the closing of mongolo okhotsk oceans might be responsible for this event

adakites from continental collision zones melting of thickened lower crust beneath southern tibet adakites are geochemically distinct intermediate to felsic lavas found exclusively in subduction zones here we report the first example of such magmas from southern tibet in an active continental collision environment the tibetan adakites were emplaced from ca 26 to 10 ma and their overall geochemical characteristics suggest an origin by melting of eclogites and or garnet amphibolites in the lower part 50 km of thickened tibetan crust this lower crustal melting required a significantly elevated geotherm which we attribute to removal of the tectonically thickened lithospheric mantle in late oligocene time the identification of collision type adakites from southern tibet lends new constraints to not only the himalayan tibetan orogenesis how and when the indian lithosphere started underthrusting asia can be depicted but also the growth of the early continental crust on earth that consists dominantly of the tonalite trondhjemite granodiorite suites marked by adakitic geochemical affinities

peralkaline granitoid magmatism in the mongolian transbaikalian belt evolution petrogenesis and tectonic significance article i nfo the central asian orogenic belt caob is well known for its massive generation of juvenile crust in the phanerozoic in eastern caob voluminous peralkaline and alkaline alkali feldspar granitoids and genetically related bimodal volcanics were emplaced in three stages from early permian to late mesozoic 295 270 ma 230 190 ma and 150 120 ma they occur as huge granitoid belts in extensional tectonic settings and form a complex network of about 12 million km 2 in area among them the 2500 km long mongolian transbaikalian belt mtb is the most spectacular and it comprises more than 350 granite syenite plutons and stocks with numerous co genetic volcanic fields the three stages of granitoids have similar chemical compositions but show temporal variation in nd isotopic composition initial end t values range from 1t o 5 for the early permian 0 to 4 for the early mesozoic and 2t o 3 5 for the late mesozoic granitoids the negative end t values observed in the mtb are not typical of the caob granitoids which are generally characterized by positive values however several pieces of evidence suggest that the mtb peralkaline and alkali feldspar granitoids were produced from enriched mantle derived sources the evidence includes 1 felsic and mafic rocks formed in the same stage have similar initial nd sr isotopic ratios and sm nd model ages tdm 2 the granitoid belts extend over thousands of kilometers and intersect distinct crustal provinces but no correlation is found in chemical compositions between the granitoids and country rocks 3 the abundant syenites are considered to be cogenetic or in some cases parental to the granites experimental and isotope data argue for the derivation of syenites from an enriched mantle source thus the granites and comendites are regarded as mantle derived 4 a study of melt inclusions in quartz phenocrysts of comendite indicates a high liquidus temperature of 1000 1100 c for the magma generation this suggests that the silicic magma was generated in an unusually high temperature condition which is likely produced by basaltic magma that underplated the lower crust the generation of voluminous peralkaline and alkaline alkali feldspar granitoids and genetically related bimodal volcanic rocks represents an important addition of juvenile crustal mass to the earth s continental crust during a time span of about 150 ma from late paleozoic to late mesozoic

derivation of mesozoic adakitic magmas from ancient lower crust in the north china craton three mesozoic plutons were intruded into the archean granulite terrains in northern margin of the north china craton adjacent to the granulite xenolith bearing tertiary hannuoba basalts they are triassic guzuizi and honghualiang granites and early cretaceous zhuanzhilian diorite rocks of the three intrusions are characterized by high sr 576 1216 ppm and na2o 4 0 low y 18 ppm and heavy rare earth elements yb 1 8 ppm these features are similar to modern adakites siliceous rocks with high sodium aluminum and strontium and low heavy rare earth element and yttrium contents from island arcs and archean high al tonalite trondhjemite granodiorite ttg however they are distinctly potassium rich and their evolved strontium neodymium and hafnium isotopic compositions and inherited zircon ages coincide with those of the archean granulites and the hannuoba granulite xenoliths such features cannot readily be explained with previous models of the origin of adakites and ttgs we propose that these adakitic rocks are formed by partial melting of ancient lower crust and that the restites are represented by some of the hannuoba granulite xenoliths we further argue that this could be one major mechanism to generate voluminous mesozoic adakitic magmas in the eastern north china craton our hypothesis can also explain the discrepancies in ages and compositions between the granulite terrains and xenoliths and implies that crustal anatexis may be one of the major processes controlling the chemical differentiation of the continental crust

a new model for the dynamic evolution of chinese lithosphere continental roots plume tectonics abstract chinese continental lithosphere comprises three tectonic domains 1 eastern china a region characterized by rifting extensional basins and voluminous basaltic volcanism 2 central china a plexus of cratonic terrains with low heat flow 40 50 mw m2 including the tarim erdos and yangtze blocks welded by pre cenozoic orogenic belts and 3 western china a region comprising the qinghai tibet himalaya orogen the relatively thin crust 35 km and lithosphere 70 km of eastern china is believed to reflect mantle upwelling while near normal crust 45 km and thickened lithosphere 200 km in central china suggest a mantle lithosphere root resembling those inferred for the kaapvaal and siberian cratons thickened crust 70 km and lithosphere 150 km in western china reflect the advanced development of an orogen root the lower density and relative buoyancy of central cratonic mantle roots appear to have contributed to the long term tectonic stability of the central chinese lithospheric blocks in contrast gravitational instability produced by thickening of the denser western chinese lithosphere and the resulting subsidence and eventual delamination of orogenic roots are believed to have led to postorogenic extensional collapse although the geological relationships suggest compressional forces caused both mountain building and orogen root development extensional stress resulting from gravitational collapse is believed to have induced lithospheric and crustal thinning with concomitant reduction of topography the term continental roots plume tectonics has been adopted to describe the configuration and dynamic condition of subcontinental lithosphere and upper mantle beneath china accordingly it is proposed that supracrustal tectonic forms represent the surface expressions of and responses to deep continental roots plume tectonics while the prevailing view is that the western orogenic belt is not genetically related to eastern continental rifting such a relationship is inherent to the continental roots plume tectonics model it is further proposed that the formation of orogenic lithosphere roots triggered eastward extrusion of the asthenosphere along the 400 km depth mantle interface and in response to subduction at the eastern margin produces plume like upwelling beneath eastern china we suggest that processes involved in continental roots plume tectonics are directly responsible for the formation and evolution of the eurasian supercontinent

carboniferous and cretaceous mafic ultramafic massifs in inner mongolia china a shrimp zircon and geochemical study of the previously presumed integral hegenshan ophiolite abstract we present shrimp zircon ages and whole rock geochemical data to identify mafic ultramafic massifs of early carboniferous and early cretaceous ages from northern inner mongolia china the early carboniferous massifs chongenshan xiaobaliang wusnihei comprise predominantly lherzolite and minor gabbroic rocks zircons from a microgabbro e nd t 9 8 and a plagiogranite e nd t 9 yielded weighted mean 206 pb 238 u ages of 354 7 ma and 333 4 ma that we interpret as reflecting the time of zircon and magma crystallization a volcanic breccia resting unconformably on the ultramafic rocks serpentinites has an eruption age of 300 2 ma and provides a younger minimum time limit for the emplacement of the early carboniferous mafic ultramafic massifs in contrast the early cretaceous mafic ultramafic massifs hegenshan chaogenshan consist predominantly of harzburgite a pegmatoid gabbro and a rodingite e nd t 10 6 from the hegenshan massif have magmatic zircon ages of 139 2 ma and 132 7 ma respectively the chaogenshan massif is concentrically zoned with an ultramafic core dominantly harzburgite dunite subordinate lherzolite surrounded by gabbro with minor hornblendite and an outermost volcano sedimentary sequence metabasaltic flows metapelites we dated zircons from a leuocogabbro 125 2 ma e nd t 10 6 a coarse grained gabbro 142 4 ma and a metabasalt 134 2 ma zircon xenocrysts ca 152 492 ma are ubiquitous in these and other associated rocks a coarse grained gabbro and a diabase as in the hegenshan rocks ca 152 468 ma the youngest detrital zircon in outer zone garnet schist i e metapelite has an age of 132 2 ma accordingly we conclude that the northern inner mongolia lherzolite dominant and harzburgite dominant massifs are temporally and genetically unrelated their emplacement is attributed to asthenospheric upwelling and consequent lithospheric extension during periods of orogenic quiescence in the southern central asian orogenic belt

timing and evolution of jurassic cretaceous granitoid magmatisms in the mongol okhotsk belt and adjacent areas ne asia implications for transition from contractional crustal thickening to extensional thinning and geodynamic settings abstract the mongol okhotsk belt and adjacent areas are key areas to study the relationship between the okhotsk and paleo pacific tectonic regimes and their superposition on the older paleo asian regimes during late mesozoic times this paper summarizes the spatial temporal evolution of late mesozoic jurassic cretaceous granitoids and related intrusions in these areas and interprets the magmatic evolution in terms of a transition from contractional crustal thickening to extensional thinning according to 407 published zircon ages these granitoids were mainly emplaced during the intervals 200 180 ma 180 165 ma 165 145 ma 145 135 ma and 135 100 ma jurassic granitoids 200 145 ma predominately occur in the baikal ne mongolia bnem and great xing an range early cretaceous 145 100 ma granitoids are mainly occur in the great xing an range and display a southward younging migration significantly early cretaceous granitoids also extend into the trans baikal area across the mongol okhotsk suture far away from the paleo pacific plate margin in ne china thus they were more plausibly related to post orogenic collapse of the mongol okhotsk orogen from the late jurassic to early cretaceous the granitoids evolved compositionally from calc alkaline and high k calc alkaline i type with some adakite like features to high k calc alkaline and shoshonitic highly fractionated i transitional i a or a type characterized by a significant decrease in their sr y ratios this evolution coincided with a tectonic transition from contractional crustal thickening to extensional thinning combined with regional geology we speculate that the jurassic granitoids were likely derived from melting of the deep seated thickened lower continental crustal lcc sources whereas the cretaceous granitoids produced through crustal melting from an extensional thinning setting our results provide a case study demonstrating that the petrogenesis of granitic magmatism was closely associated with crustal tectonics early jurassic granitoids along the okhotsk belt formed in a subduction collision setting related to closure of the mongol okhotsk ocean whereas late jurassic granitoids in the great xing an range and in the northern north china craton may have formed in a syn or post collisional setting superposed by far field affects of subduction of the paleo pacific plate early cretaceous granitoids in these areas formed in response to post orogenic extensional collapse of the mongol okhotsk belt coupled with back arc extension related to paleo pacific plate subduction

origin of a mesozoic granite with a type characteristics from the north china craton highly fractionated from i type magmas we report geochronological geochemical and isotopic data for the mesozoic shangshuiquan granite from the northern margin of the north china craton the granite is highly fractionated with sio2 74 occurrence of annitic biotite high contents of alkalis k2o na2o rb y nb and heavy rare earth elements high feot mgo low contents of cao al2o3 ba and sr and large negative eu anomalies makes it indistinguishable from typical a type granites a mantle derived origin for the rocks of the granite is not favored because their high initial 87sr 86sr 0 706 and low end t 15 are completely different from either those of the lithospheric or asthenospheric mantle in fact their sr nd isotopes fall within the range of sr nd isotopic compositions of the archean granulite terrains and are comparable to those of mesozoic crustal derived i type granitoids in the region therefore the shangshuiquan granite is considered to be dominantly derived from partial melting of the ancient lower crust its parental magmas prove to be similar to i type magmas and to have undergone extensive fractionation during its ascent this is supported by the fact that some of the nearby hannuoba feldspar rich granulite xenoliths can be indeed regarded as the early cumulates in terms of their mineralogy chemistry sr nd isotopes and zircon u pb ages and hf isotopes it is furthermore argued that some of highly fractionated granites worldwide especially those with a type characteristics and lacking close relationship with unfractionated rocks may in fact be fractionated i type granites this suggestion can explain their close temporal and spatial associations as well as similar sr nd isotopes with i type granites our study also sheds new light on the petrogenesis of deep crustal xenoliths

neoproterozoic to paleozoic geology of the altai orogen nw china new zircon age data and tectonic evolution abstract we present a synthesis and a new account of the geological and tectonic history of the terranes of the chinese paleozoic altai orogen together with new single zircon ages for granitic and rhyodacitic rocks a central terrane consists of neoproterozoic to silurian amphibolite facies metasedimentary rocks and abundant devonian carboniferous granites the presence of precambrian basement is indicated by sinian fossils our xenocryst ages and published nd mean crustal residence ages of granites felsic arc type lavas on the southern margin of the terrane have a mean 207pb 206pb zircon age of 505 ma reflecting the time of arc volcanism and the presence of xenocysts with ages between 614 and 921 ma suggests derivation by intracrustal melting accordingly we suggest that a cambro ordovician continental magmatic arc was built on the southern margin of the central terrane by northward subduction a low grade ordovician andean type arc with a continental basement is situated above a normal fault on

lower crustal processes leading to mesozoic lithospheric thinning beneath eastern north china underplating replacement and delamination abstract our recent studies show that the lower crust and lithospheric mantle in the eastern north china craton ncc was thinned in the mesozoic the lower crust played a key role in the tectonic mechanism that led to lithospheric thinning geochemical data from volcanic rocks and xenoliths support the idea that delamination of the lower crust and lithospheric mantle took place only along the northern and perhaps the southern margins of the ncc and this was linked to post collisional orogenic processes however it is difficult for the delamination process to act on the whole eastern ncc and remove 80 120 km of the lithosphere lower crustal xenoliths in mesozoic and cenozoic basalts within the ncc are of two kinds one is granulite facies metamorphosed two pyroxene gabbros and eclogite facies metamorphosed garnet pyroxenites formed at 140 120 ma the other is precambrian granulites that were strongly overprinted by mesozoic metamorphism large scale lower crustal replacement took place in the mesozoic beneath the ncc in other words some or perhaps most part of the present lower crust of the ncc is not precambrian the present lowermost crust is mostly composed of mesozoic meta gabbros and pyroxenites abundant late jurassic high sr granites in the ncc which are compositionally similar to a dakite probably formed by partial melting of the lowermost crust leaving a residue of eclogite or garnet amphibolite a hot upwelling mantle was necessary to partially melt the pre existing lowermost crust and underplate magma in order to form the present lower crust although eclogites could sink into the mantle it is hard to imagine that there was a thick enough eclogite layer that could drag 80 120 km of the lithosphere into the asthenosphere beneath the ncc magmatic underplating seems to be a better alternative mechanism thus the formation of the present lower crust through the underplating and replacement of the pre existing one was closely related to lithospheric thinning and represented a continuation of the same geodynamic process

discovery of archaean crust within the akitkan orogenic belt of the siberian craton new insight into its architecture and history abstract the akitkan orogenic belt separates the aldan and anabar superterranes of the siberian craton and was previously suggested to consist exclusively of palaeoproterozoic juvenile crust foliated granitoids in the baikal basement salient within the akitkan belt were studied geochronologically and geochemically thermal ionisation mass spectrometry tims analysis of six zircons of a tonalite sample yielded a discordia line with an upper concordia intercept age of 2884 12 ma which is interpreted as the time of tonalite emplacement geochemical data of the foliated granitoids confirm their affinity to the tonalite trondhjemite granodiorite ttg series they are characterized by high al2o3 contents 14 4 17 2 wt high sr y ratios 18 99 and show significant rare earth element ree fractionation with lan ybn 11 89 the nd t values vary from 1 6 to 0 8 and nd mean crustal residence ages range from 3 23 to 3 07 ga geochemical and isotopic data confirm their genesis through interaction of mantle and crustal material under high pressure near the base of thickened crust either after accretion of ancient continental crust and an oceanic plateau or after underplating of oceanic plateau like basaltic magmas the akitkan orogenic belt is a composite crustal unit of archaean and palaeoproterozoic age we propose two possible scenarios of its evolution in the first scenario palaeoproterozoic complexes developed along the active margin of the archaean anabar superterrane at 2 1 2 0 ga alternatively archaean and palaeoproterozoic components of the akitkan belt could have a distal origin and were accreted to the anabar superterrane these accretional processes were followed by a major collision of the aldan and anabar superterranes at 1 9 ga finalised by post collisional granitoid magmatism at 1 87 1 84 ga

the enigmatic yinshan fold and thrust belt of northern china new views on its intraplate contractional styles the east to east northeast trending yinshan belt lies within north china extending westward at least 1100 km from china s eastern coast to inner mongolia this intraplate jurassic cretaceous belt underwent contractional and normal faulting folding and contemporaneous terrestrial sedimentation and magmatism current views on its contractional deformational style favor relatively limited thick skinned faulting of archean basement and cover units these views are challenged however by recent discoveries in the eastern part of the belt of south directed ductile nappe formation and large displacement 40 45 km thin skinned northward thrust faulting both involving archean and younger rock units collision of the siberian and north china plates upon closure of a jurassic and early cretaceous mongolo okhotsk ocean more than 800 1100 km to the north may have been responsible for yinshan north south contraction some patterns of contraction e g jurassic cretaceous ductile nappe formation appear to have been influenced by a superposed magmatic regime related to westward subduction of a pacific basin plate beneath the north china plate

multiple sources for the origin of granites geochemical and nd sr isotopic evidence from the gudaoling granite and its mafic enclaves northeast china geochemical and sr and nd isotopic data have been determined for mafic to intermediate microgranular enclaves and host granitoids from the early cretaceous gudaoling batholith in the liaodong peninsula ne china the rocks include monzogranite porphyric granodiorite and quartz diorite monzogranites have relatively high 87rb 86sr ratios 0 672 0 853 low initial 87sr 86sr ratios 0 7052 0 7086 and e nd t values 18 5 to 20 9 indicating that they were mainly derived from a newly underplated crustal source with a short crustal residence time quartz diorites have high initial 87sr 86sr ratios 0 7118 0 7120 and negative e nd t values 13 2 to 18 1 coupled with high al2o3 and mgo contents indicating they were derived from enriched lithospheric mantle with contributions of radiogenic sr from plagioclase rich metagreywackes or meta igneous rocks i e ancient lower crust two groups of enclaves with igneous textures and abundant acicular apatites are distinguished dioritic enclaves and biotite monzonitic enclaves dioritic enclaves have low al2o3 13 5 16 4 wt and high mgo mg 72 3 concentrations low initial 87sr 86sr ratios 0 7058 0 7073 and negative e nd t values 7 2 and are enriched in liles and lrees and depleted in hfses suggesting they were derived from an enriched lithospheric mantle source biotite monzonitic enclaves have sr and nd isotopic compositions similar to the monzogranites indicating they were crystal cumulates of the parental magmas of these monzogranites granodiorites have transitional geochemistry and nd and sr isotopic compositions intermediate between the monzogranites quartz diorites and the enclaves geochemical and sr and nd isotopic compositions rule out simple crystal liquid fractionation or restite unmixing as the major genetic link between enclaves and host rocks instead magma mixing of mafic mantle derived and juvenile crustal derived magmas coupled with crystal fractionation and assimilation of ancient lower crust is compatible with the data this example shows that at least some calc alkaline granitoids are not produced by pure intracrustal melting but formed through a complex multi stage hybridization process involving mantle and crustal derived magmas and several concomitant magmatic processes crystal fractionation crustal assimilation and crustal anatexis

a hybrid origin for the qianshan a type granite northeast china geochemical and sr nd hf isotopic evidence abstract major and trace element whole rock sr and nd isotope and zircon hf isotope data are reported for a suite of a type granites and mafic microgranular enclaves from the early cretaceous 126 2 ma qianshan pluton liaodong peninsula northeast china with the aim of investigating the sources and petrogenesis of a type granites the qianshan pluton includes hornblende alkali feldspar granite graphic biotite granite and mafic microgranular enclaves the hornblende alkali feldspar granites have high sio 2 fe 2 o 3t mgo k 2 o na 2 o rb zr and lree contents and low ba and sr concentrations with strongly negative eu anomalies their high rb sr 87 rb 86 sr 16 76 24 15 and initial 87 sr 86 sr ratios 0 7215 to 0 7283 negative e nd t values 14 1 to 16 5 and zircon e hf t values 18 9 to 11 5 indicate they were mainly derived from a crustal source but with involvement of high e nd t and e hf t materials graphic biotite granites have similar geochemical features and sr nd hf isotopic compositions to enclaves indicating they were the result of crystal fractionation of evolved mafic magmas but with involvement of low e nd t and e hf t materials the mafic enclaves have an igneous texture and contain acicular apatite suggesting quenching of mafic magmas that have co mingled with the host granites they have low initial 87 sr 86 sr ratios 0 7097 0 7148 negative e nd t 14 5 to 11 9 and zircon e hf t 17 1 to 6 9 values and are enriched in liles and lrees and depleted in hfses when coupled with the high mgo mg up to 54 this indicates derivation from an enriched lithospheric mantle source but contaminated by crustal materials geochemical and sr nd and zircon hf isotopic compositions rule out simple crystal liquid fractionation or restite unmixing as the major genetic link between enclaves and host rocks instead magma mixing of mantle derived mafic and crustal derived magmas coupled with crystal fractionation is compatible with the data this example shows that at least some a type granites formed through a complex process involving mantle and crustal derived magma mixing crystal fractionation and infracrustal melting

the focus thermal study around the spreading center of southwestern okinawa trough abstract voluminous hydrothermal systems are corresponding to the heat flow anomaly discovered from previous geophysical surveys in the southern okinawa trough sot which is an area with a high sedimentation rate supplied by the east china sea shelf and the island of taiwan our latest fifteen field measurements around the geolin mound glm 24 36 6 n 122 53 e are located at a sedimentary basin west of the yonaguni rift which is nearby the southwestern tip of a ne sw trending volcano trail in the sot the heat flow results illustrate a concentric decrease from 31 477 to 180 mw m2 in a 1 km radius around the glm evidently proving the focus fluid migration along the major fluid conduit the ultrahigh temperature observations revealed the existence of magmatic components in shallow sediments combining the subsurface profiles with the temperature fields around the glm a 0 16 km2 acoustic blanking area of the g1 fluid conduit was recognized from the chirp sonar image and numerous interpreted igneous reflections with fluids related features from seismic profiles as a result we proposed a conceptual model which simultaneously illustrates the hydrothermal circulation pattern from the previous general surveys and the enormous thermal anomaly from current detailed measurements the sill to sill feeding sedimentary basin of the glm is capable of demonstrating the enormous thermal anomaly around the spreading center of the sot and appropriately represents the potential hydrothermal discharge and recharge by heat flow distribution around the glm

geochronology and geochemistry of early middle triassic magmatism in the erguna massif ne china constraints on the tectonic evolution of the mongol okhotsk ocean abstract in this paper we present new zircon u pb dating hf isotope major and trace elements and sr nd isotope data for early middle triassic intrusive rocks in the erguna massif of ne china in the eastern segment of the central asian orogenic belt caob our aim is to elucidate the tectonic evolution of the mongol okhotsk ocean and its influence on ne china zircons from five representative intrusions in the erguna massif are euhedral subhedral in shape display oscillatory growth zoning in cl images and have th u ratios of 0 17 1 54 and in combination these features indicate that the zircons are of igneous origin zircon u pb dating results demonstrate that the early middle triassic magmatism in the erguna massif took place between 241 and 247 ma the early middle triassic rocks are composed mainly of a suite of diorites quartz diorites granodiorites monzogranites and syenogranites they have sio 2 57 71 72 86 wt mg 19 52 al 2 o 3 14 27 17 23 wt and na 2 o k 2 o 0 67 1 59 chemically they are metaluminous to peraluminous and belong to the high k calc alkaline series moreover they are enriched in light rare earth elements and large ion lithophile elements and depleted in heavy rare earth elements and high field strength elements e g nb ta and ti the whole rock e nd t and zircon e hf t values of these samples range from 4 68 to 3 42 and from 2 0 to 6 6 respectively indicative of formation from primary magmas generated by partial melting of juvenile crustal material with a contribution from mantle derived mafic magmas these magmas subsequently underwent fractional crystallization contamination and magma mixing geochemically these early middle triassic intrusive rocks have affinities with intrusive rocks from active continental margin settings we conclude therefore that the early middle triassic magmatism in the erguna massif was generated within an active continental margin setting related to the southwards subduction of the mongol okhotsk oceanic plate beneath the erguna massif this conclusion is supported by the occurrence of early mesozoic porphyry and skarn deposits along the southeastern side of the mongol okhotsk suture belt

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

luminescence dating indicates radiocarbon age underestimation in late pleistocene fluvial deposits from eastern england robust geochronological control is crucial for interpreting and correlating the environmental record from fragmentary terrestrial sequences from the last glacial period traditional terrestrial stratigraphies are typically reliant on radiocarbon dating although estimates of antiquity for samples older than 40 ka may be unreliable optically stimulated luminescence osl dating of sediments is an alternative for dating the whole of the last glacial period and also provides the opportunity to independently test radiocarbon based chronologies this paper presents a set of nine directly comparable paired osl and accelerator mass spectrometry ams radiocarbon ages from multiple sites within devensian fluvial sediments in lowland britain and shows that it is possible to obtain robust osl age estimates throughout this time period the two techniques agree well for ages younger than ca 29 14c ka bp ca 35 cal ka bp but disagree beyond ca 35 14c ka bp ca 40 cal ka bp this is thought to be due to radiocarbon ages on older organic material having been contaminated by low levels of modern carbon it is therefore suggested that all conventionally pre treated radiocarbon ages greater than 35 14c ka bp should be treated with extreme caution copyright 2009 john wiley sons ltd

the cretaceous duimiangou adakite like intrusion from the chifeng region northern north china craton crustal contamination of basaltic magma in an intracontinental extensional environment abstract zircon u pb ages major and trace element and sr nd and pb isotope compositions of the duimiangou dmg quartz monzonite from the chifeng region on the northern north china craton ncc were studied to investigate its derivation evolution and geodynamic significance laser ablation inductively coupled plasma mass spectrometry la icp ms zircon u pb dating yields an emplacement age of 128 1 ma for this intrusion with numerous mesozoic inherited zircons clustering at 219 12 ma and 161 3 ma along with some ancient zircons with ages of 2 5 ga 1 77 ga and 324 ma bulk rock analyses show that this intrusion is characterized by variable sio 2 63 4 69 4 wt al 2 o 3 14 5 16 3 wt na 2 o k 2 o 8 01 8 95 wt and mg 41 3 48 0 they are enriched in large ion lithophile elements and light rare earth elements without significant eu anomalies mostly between 0 89 1 10 and depleted in heavy rare earth elements and high field strength elements with high sr y 63 7 101 7 and la yb n 20 5 31 0 ratios the dmg intrusion formed in an intracontinental extensional setting contemporaneous with the formation of pull apart basins metamorphic core complexes and intense magmatism rather than in a convergent margin it has homogeneous sr 87 sr 86 sr i 0 7059 0 7066 nd e nd t 6 2 to 7 2 and pb 206 pb 204 pb i 17 289 17 375 207 pb 204 pb i 15 359 15 463 208 pb 204 pb i 37 130 37 472 isotope compositions sr nd isotope modeling results plus relatively young nd model ages 1522 1618 ma and the presence of relict zircons suggest that this intrusion could have originated from crustal contamination of newly formed basaltic melts derived from asthenospheric mantle accompanied by fractional crystallization of k feldspar biotite apatite fe ti oxides and minor hornblende and plagioclase thus the dmg adakite like intrusion may record the magmatic event associated with underplating of asthenospheric magma in an intracontinental extensional environment and both asthenospheric magma underplating and lithospheric delamination may have played important roles in the lithospheric thinning of the northern ncc

age and evolution of late mesozoic metamorphic core complexes in southern siberia and northern mongolia numerous cretaceous metamorphic core complexes mccs extend from transbaikalia in russia to northern mongolia within the central asian orogenic belt we investigated the buteel and zagan mccs in detail shear sense indicators in mylonitized rocks show footwall to the nw tectonic transport single zircon dating of footwall rocks in the buteel mcc establishes the emplacement of granitoid orthogneiss precursors at 240 211 ma a felsic metavolcanic rock at 265 0 1 2 ma a syenite at 265 5 1 2 ma and a metarhyolite of the pre granitoid basement at 553 6 2 9 ma a peralkaline granite intruding orthogneisses of the zagan mcc has a new u pb zircon age of 151 6 0 7 ma 40 ar 39 ar ages of 133 5 1 8 ma of hornblende from amphibolite and 122 6 1 8 ma of biotite from mylonitized gabbro dolerite of the buteel mcc are interpreted as cooling ages representing the time of deformation in the footwall geological data suggest that the mccs in transbaikalia and northern mongolia formed as a result of extension in a crust that had previously been thickened by abundant calc alkaline magmatism in an andean type setting on the border of the closing mongol okhotsk ocean by widespread collisional to post collisional thrusting and by extensive alkaline peralkaline magmatism

new u pb and re os age data and the geodynamic setting of the xiaojiayingzi mo fe deposit western liaoning province northeastern china abstract the xiaojiayingzi deposit located in the western liaoning province in northeastern china is a skarn type mo fe deposit with an average mo grade of 0 23 wt the deposit is temporally and spatially associated with fine grained diorite mo fe mineralization is present mainly as veins lenses and layers that are hosted by skarn new shrimp u pb zircon and re os molybdenite age determinations yielded a crystallization age for the diorite of 169 9 1 4 ma and a molybdenite re os isochron age of 165 5 4 6 ma the u pb and re os ages are identical within error the mineralization is therefore considered to be genetically related to diorite emplacement in the middle jurassic a high re content of 21 75 to 163 0 ppm with an average of 64 31 ppm in the molybdenite together with 34 s values between 3 3 and 8 0 of sulfides in the ores indicates that the metal source was a mixed crust mantle system from these geochronological data and previously presented regional geological relationships we propose that diorite emplacement and mo fe mineralization in the xiaojiayingzi mine resulted from an early stage of the subduction of the paleo pacific plate beneath the eurasian block

timing and processes of late mesozoic mid lower crustal extension in continental ne asia and implications for the tectonic setting of the destruction of the north china craton mainly constrained by zircon u pb ages from metamorphic core complexes abstract late mesozoic large scale crustal extension occurred across the northeastern section of continental asia and metamorphic core complexes mccs are the major features in this region this paper presents new zircon u pb and ar ar ages of biotite and hornblende for pre or syn mylonitized and post kinematic unmylonitized intrusions from these mccs and attempts to determine the timing of initiation duration and termination of the mid lower crustal ductile extension by integrating with geochronological data from previous studies these mccs have similar 40 ar 39 ar biotite and hornblende ages 130 to 110 ma mainly from 130 to 120 ma which are interpreted as the cooling ages and represent the timing of the late stage formation of the mccs zircon u pb ages for syn to post kinematic intrusions from some mccs suggest that the mid lower crustal extension probably initiated at ca 150 145 ma with a peak occurring 145 130 ma and continued to 130 120 ma which is older than that suggested by 40 ar 39 ar ages by 10 20 ma the early ductile extension most likely initiated in mongolia and northern china during the late jurassic 150 145 ma at a deeper level then propagated southwards 140 130 ma and finally developed throughout the entire continental ne asia 130 120 ma based on the timing and processes of the extension a geodynamic model and tectonic setting of the destruction of the north china craton ncc are proposed

magma mixing controlling the origin of the early cretaceous fangshan granitic pluton north china craton in situ u pb age and sr nd hf and o isotope evidence abstract in situ u pb age and sr nd hf and o isotope analysis of accessory minerals zircon apatite and titanite have been obtained for mafic microgranular enclaves and their host granites from the early cretaceous fangshan pluton in the north china craton in order to constrain their sources and petrogenesis the fangshan pluton is composed of quartz monzodiorite at the margins and granodiorite with abundant mafic microgranular enclaves in the core zircons from the marginal quartz monzodiorite record u pb ages of 130 to 133 ma identical to those of granodiorite in the core the enclaves have zircon u pb ages of 133 to 134 ma identical within error to those of the host granodiorite establishing that the mafic and felsic magmas were coeval furthermore apatite and titanite from the enclaves and host granodiorite have the same u pb ages as the zircons indicating rapid cooling of the fangshan pluton the granodiorites have relatively high sio 2 and low mgo concentrations they have variable negative e hf t values of 25 1 to 18 2 and 18 o values for zircon of 6 2 to 8 3 the negative e nd t values of 19 7 to 14 0 for titanite and apatite with initial 87 sr 86 sr ratios of 0 7052 0 7058 for apatite indicate that they were derived from an ancient lower crustal source with addition of materials with higher e hf t and e nd t values and low 18 o values the enclaves have relatively high mgo contents up to 15 5 wt at intermediate silica concentrations indicating that their parental magmas were derived from a mantle source their zircons have strongly negative hf isotopic compositions e hf t 22 9 to 14 9 the nd isotopic composition of titanite and apatite is also strongly negative e nd t 17 6 to 12 1 all these features combined with the 18 o zircon values of 4 8 8 2 for zircons and relatively low initial sr isotopic compositions of apatite initial 87 sr 86 sr 0 7051 0 7058 indicate that they were derived from an enriched and or depleted mantle source but with involvement of crustal materials the marginal quartz monzodiorites have transitional geochemical features between those of the enclaves and host granodiorites in situ sr nd hf and o isotopic data of accessory minerals coupled with field observations and whole rock geochemical data rule out simple crystal liquid fractionation or wall rock contamination as the major genetic link between the enclaves and host rocks instead mixing of mantle derived mafic magmas with crustal derived felsic magmas coupled with assimilation of wall rocks is compatible with the data our study shows that in situ hf o nd and sr isotopic analyses of accessory minerals are powerful geochemical tracers that provide unique petrogenetic information regarding the origin of granitoids

late jurassic early cretaceous closure of the mongol okhotsk ocean demonstrated by new mesozoic palaeomagnetic results from the trans baikal area se siberia summary we present new palaeomagnetic results from the transba ikal area se siberia from the mongol okhotsk suture zone the boundary between the amuria and siberia blocks in order to better constrain the time of closure of the mongol okhotsk ocean in the mesozoic we col lected 532 rock samples at 68 sites in six localities of basalts trachy basalts and andesites from both sides of the mongol okhotsk suture at unda river j 3 51 7 n 117 4 e kremljevka peak k1 51 8 n 117 5 e and torey lakes k 1 50 1 n 115 9 e on the southern side of the suture and at monostoy river j 1 51 1 n 106 8 e ingoda river k 1 51 2 n 112 2 e and bichura town k 1 50 6 n 107 6 e on the northern side progressive thermal demagnetization enabled us to resolve low ltc and high htc temperature components of magnetization at most sites jurassic palaeopoles computed from the htcs show a large discrepancy with respect to the apparent polar wander path of eurasia which we interpret in terms of 1700 2700 km of post late jurassic northward movement of amuria with respect to siberia although ge ological data suggest a middle jurassic closure of the mongol okhotsk ocean in the west trans baikal region our data give evidence of a large remaining palaeolatitude difference between the amuria and siberia blocks in contrast early cretaceous sites cluster remarkably well along a small circle which is centred on the average site location this implies the absence of post early cretaceous northward motion of amuria relative to siberia and demonstrates the pre early cretaceous closure of the mongol okhotsk ocean finally we interpret the very large tectonic rotations about local vertical axes evidenced by the small circle distribution of poles as arising both from collision processes and from left lateral shear movement along the suture zone due to the eastward extrusion of amuria under the effect of the collision of india into asia

late mesozoic calc alkaline volcanism of post orogenic extension in the northern da hinggan mountains northeastern china abstract late mesozoic calc alkaline volcanism in the northern da hinggan mountains ndhm ne china exhibits geochemical and sr nd isotopic characteristics similar to those of cenozoic calc alkaline volcanism in the basin and range province usa whole rock k ar dating results show that these volcanic sequences were erupted during 138 116 ma composed of basaltic andesites trachyandesites group 1 hornblende andesites trachytes group 2 and rhyolite lavas group 3 they are characterized by low mgo contents 4 20 lile lree enrichment and significant nb ta depletion as well as a little depleted to slightly enriched nd and weakly enriched sr isotopic ratios group 1 initial 87sr 86sr 0 70502 0 70572 nd t 0 78 to 0 91 group 2 initial 87sr 86sr 0 70497 0 70518 nd t 0 86 to 1 26 group 3 initial 87sr 86sr 0 70510 0 70635 nd t 0 41 to 0 25 the systematic variations in major and trace elements homogeneous sr nd isotope data and temporal consistency among three volcanic groups indicate that they were derived from a similar mantle source metasomatized by fluids related to the closure of the paleo asian and or mongolia okhotsk oceans and were produced through different degrees of fractional crystallization of the primary melts group 1 basaltic rocks were formed through removal of olivine and pyroxene of the primary melts while group 2 trachytes which contain the lowest lree contents e g la 24 28 ppm and relatively less enriched sr and higher nd isotope ratios were generated after removal of a few percent of lree rich minerals such as hornblende clinopyroxene and apatite of melts like group 1 group 3 rhyolite lavas exhibiting the highest abundances of strongly incompatible elements such as rb and k moderate lree contents e g la 28 53 ppm as well as apparently negative eu and sr anomalies represent the final crystallized products following a plagioclase predominant fractionation of melts like group 2 the low mgo contents and evolved affinities of the volcanic rocks imply that beneath the ndhm there existed many crustal magma reservoirs throughout the eruption episodes in which mantle derived primary melts had experienced intense differentiation these facts in combination with the contemporaneous basin and range tectonic regime suggest that the extensive calc alkaline volcanism in the ndhm was attributed to post orogenic diffuse extension rather than either an upwelling mantle plume or mesozoic oceanic plate subduction

early jurassic extensional basin formation in the daqing shan segment of the yinshan belt northern north china block inner mongolia abstract structural and stratigraphic studies of the western daqing shan segment of the yinshan belt have recognized an early jurassic extensional episode supported by several lines of evidence first normal faults cut the lowermost jurassic sequence and are overlapped by younger lower jurassic rocks second lower jurassic rocks include growth strata in small scale graben at the base of the jurassic basin third rapid lateral facies changes are mapped from boulder conglomerates along the basin bounding faults to lacustrine and meandering fluvial rocks in the basin center fourth paleodrainage systems provided sediment input from three sides of the basin two transverse and one axial finally there is a strongly asymmetric distribution of coarse proximal and fine distal facies within the basin the early jurassic extensional episode was responsible for formation of an east trending half graben basin in the western daqing shan in which at least 1800 m of syn extensional nonmarine sediments accumulated and are preserved previous studies in the triassic and jurassic of other parts of northwest and north central china have concluded that the early mesozoic was a time of continental amalgamation and contractile deformation the recognition of an early jurassic extensional episode along the northern margin of the north china block is problematic in this context at least superficially we propose two possible explanations for the early jurassic extension transtension associated with strike slip tectonics or gravitational collapse of a pre existing late paleozoic early mesozoic contractile orogenic belt the first possibility transtensional deformation is problematic because specific strike slip faults have not been identified that could control extension in the western daqing shan however several lines of evidence allow the possibility of such a driving mechanism including documented early jurassic transtensional systems in the southwestern north china block several candidate strike slip faults along the china mongolia border region structural discontinuity between the daqing shan and southern mongolia and along strike changes in structural style within the yinshan belt the second possibility orogenic collapse is similarly difficult to establish because of the limited amount of data concerning the regional distribution and orientation of pre jurassic contractile structures and early jurassic extensional structures however documented late paleozoic triassic contractile deformation as well as the close temporal and spatial association and parallelism between early jurassic extensional structures and older contractile structures in the western daqing shan requires consideration of gravitational collapse as a driving mechanism for early jurassic extension in the yinshan belt

timing of exhumation of the ereendavaa metamorphic core complex north eastern mongolia u pb and 40ar 39ar constraints abstract the occurrence of numerous ne sw trending rift basins that locally bound metamorphic core complexes in transbaikalia northern mongolia and north eastern china indicates that the eastern domain of the central asian orogenic belt was affected by a large scale nw se extensional tectonic event during late jurassic to early cretaceous times available geochronological information about the formation of metamorphic core complexes in the region define partly synchronous periods in transbaikalia 134 121 ma and north china 136 110 ma suggesting dome exhumation during the early cretaceous in north eastern mongolia the ereendavaa range lined by the onon shear zone was recently interpreted as one of these metamorphic core complexes of late jurassic to early cretaceous age on the basis of u pb zircons data from two synkinematic pegmatite dykes however this wide time range remains to be discussed the present work constrains the tectono thermal evolution of the range by providing new mc icp ms u pb zircons ages from pegmatite dykes and metamorphic igneous rocks and mica and amphibole 40 ar 39 ar laser step heating ages from metamorphic and magmatic rocks that crop out within the range the results show that 1 the ereendavaa range was the locus of a widespread magmatic activity prior to its exhumation from late jurassic times 2 cooling of the range began at least at 137 9 0 3 ma as revealed by a 40 ar 39 ar plateau ages obtained from an hornblende sampled within the range core and 3 exhumation along the onon shear zone occurred from 130 3 1 9 ma down to 124 8 0 3 ma as shown by ages of most of micas and amphiboles from the shear zone these more accurate ages are consistent with metamorphic core complex ages from adjacent areas furthermore they argue for an extensional dynamic paroxysm at the scale of eastern asia during a narrow time range in the early cretaceous between ca 130 ma and ca 120 ma

geodynamics of the western part of the mongolia okhotsk collisional belt trans baikal region russia and mongolia abstract after the western edge of the mongolian microcontinent joined the siberian continent in the region of central mongolia in the earliest permian these two continental blocks remained turned at an angle of about 120 with respect to each other and separated on greater extent of their present day boundary by an enormous gulf of the paleopacific called the mongolia okhotsk ocean closure of this ocean at the early middle jurassic boundary led to the complete collision of siberia and mongolia which by then had already become part of the mongolia north china continent this main collisional episode which lasted through the middle and late jurassic involved thrusting folding and magmatism and produced the mongolia okhotsk belt the onon island arc which was located in the mongolia okhotsk ocean was squeezed between the two major continents inasmuch as the third element the island arc was involved in the collision it is reasonable to distinguish two branches of the mongolia okhotsk suture these branches control the spatial distribution of gold mineralization in the trans baikal region on the southeastern periphery of siberia the crust thickened considerably after the collision and a plateau like uplift formed in the early cretaceous when compression ceased the collisional uplift collapsed and the thrusts were transformed into low angle normal faults the motions on which were responsible for the formation of rift basins and exhumation of metamorphic core complexes

origin of mesozoic adakitic intrusive rocks in the ningzhen area of east china partial melting of delaminated lower continental crust to the best of our knowledge modern adakites have not been documented in a nonarc environment we report geochemical and isotopic data for early cretaceous anjishan adakitic intrusive rocks that are in a continental setting unrelated to subduction the anjishan adakitic intrusive rocks which are exposed in the ningzhen area of east china have high sr y and la yb ratios coupled with low yb and y as well as relatively high mgo contents and mg numbers mg 0 4 0 6 similar to products from slab melting however low n d t values 6 8 to 9 7 and high 8 7 sr 8 6 sr i 0 7053 0 7066 are inconsistent with an origin by slab melting the tectonics and geochemistry lead us to conclude that adakitic magmas were most likely derived from partial melting of mafic material at the base of the continental crust high sr y and la yb ratios of the adakitic intrusive rocks suggest that garnet was stable as a residual phase during partial melting implying that the crustal thickness exceeded 40 km in the early cretaceous the present thickness of the crust in the ningzhen area is only 30 km and therefore the crust appears to have been thinned by at least 10 km since the early cretaceous the relatively high mgo contents and mg of the anjishan intrusive rocks suggest that adakitic magmas interacted with mantle rocks possibly coinciding with lower crustal delamination which would also account for the observed thinning

tracing magma mixing in granite genesis in situ u pb dating and hf isotope analysis of zircons in situ zircon u pb and hf isotopic data have been determined for mafic microgranular enclaves and host granitoids from the early cretaceous gudaoling batholith in the liaodong peninsula ne china in order to constrain the sources and petrogenesis of granites the zircon u pb age of the enclaves 120 1 ma is identical to that of the host monzogranite 120 1 ma establishing that the mafic and felsic magmas were coeval the hf isotopic composition of the enclaves ehf t 4 5 to 6 2 is distinct from the host monzogranite ehf t 15 1 to 25 4 indicating that both depleted mantle and crustal sources contributed to their origin the depleted mantle component was not previously revealed by geochemical and nd and sr isotopic studies showing that zircon hf isotopic data can be a powerful geochemical tracer with the potential to provide unique petrogenetic information some wall rock contamination is indicated by inherited zircons with considerably older u pb ages and low initial hf isotopic compositions hafnium isotopic variations in early cretaceous zircons rule out simple crystal liquid fractionation or restite unmixing as the major genetic link between enclaves and host rocks instead mixing of mantle derived mafic magmas with crustal derived felsic magmas coupled with assimilation of wall rocks is compatible with the data

petrogenesis of late triassic granitoids and their enclaves with implications for post collisional lithospheric thinning of the liaodong peninsula north china craton abstract major and trace element sr nd and hf isotope zircon u pb and hf isotope data are reported for mafic microgranular enclaves and host granitoids from the xiuyan pluton in the liaodong peninsula of the eastern north china craton in order to investigate their sources petrogenesis and tectonic implications the zircon u pb age of the enclaves 211 1 ma is identical to that of the host granite 210 1 ma establishing that the mafic and felsic magmas were coeval the enclaves have high mgo concentrations at low silica contents and positive ehf t values they are enriched in large ion lithophile and light rare earth elements and depleted in high field strength elements which can be explained as melts derived from a depleted asthenospheric mantle with some crustal contamination the host granitoids have high sio2 contents variable initial 87sr 86sr ratios and highly negative end t and ehf t values suggesting they were mainly derived from an ancient crustal source heated by the input of asthenospheric mantle derived magmas field observations geochronology geochemistry sr and nd isotopic and zircon hf isotopic compositions point to a complex petrogenesis where asthenospheric mantle and crust derived magma mixing was coupled with crystal fractionation thus explaining the genetic link between enclaves and host rocks identification of an asthenospheric mantle component in the xiuyan granitoids suggests asthenospheric mantle upwelling and decompressional melting of the mantle indicating that post collisional lithospheric thinning in the liaodong peninsula was initiated in the late triassic

evolution source and tectonic significance of early mesozoic granitoid magmatism in the central asian orogenic belt central segment abstract numerous early mesozoic granitoids have been recognized from the central segment of the central asian orogenic belt caob they can be broadly classified into two groups according to zircon u pb ages an early stage group covering the time span from early to middle triassic 250 230 ma and a late stage group emplaced during late triassic to early jurassic ca 230 190 ma early stage 250 230 ma granitoids are mainly distributed in the western central mongolia erguna belt cmeb the western altai belt ab the south mongolia xing an belt smxb and the beishan inner mongolia jilin belt bijb they consist mainly of quartz diorites granodiorites and monzogranites mostly of i type with minor mafic intrusions with some of them showing adakite like signatures and some with s type features late stage 230 190 ma granitoids mainly occur in the north mongolia transbaikalia belt nmtb the eastern cmeb erguna massif and the eastern altai belt ab they are predominately syenogranites monzogranites and syenites associated with many alkaline granites and mafic intrusions and are a type and transitional i a type or highly fractionated i type granites whole rock sr nd and zircon hf isotopic data have been compiled for regional isotopic mapping the end t values show large variations from 7 0 to 7 4 and nd model ages tdm from 0 46 ga to 1 43 ga as well as the initial sr isotopic ratios sri from 0 7023 to 0 7174 the zircon ehf t values vary from 4 6 to 15 3 and give two stage hf model ages tdm2 from 0 30 ga to 2 09 ga the extremely large variations of whole rock sr nd and zircon hf isotopes demonstrate strong isotopic heterogeneity of the source regions that are mainly dominated by juvenile components with significant old crustal participation furthermore the late stage granitoids in the nmtb the cmeb and the ab generally have more negative end t values and more variable zircon ehf t values than those of the early stage granitoids in the same belt implying an increasing crustal signature from early to late stage in the assumed heterogeneous source regions within the same belt which probably results from melting of shallower crust in parallel with a shift to more alkaline chemistry of the late granitoid magmas by contrast most late stage granitoids in the smxb and the bijb have more positive end t values and homogeneous zircon ehf t values than those of the early stage granitoids in the same belt indicating more juvenile contribution to the source of the these granitoids the generation of the early mesozoic granitoid magmas in the nmtb and the cmeb was dominated by the ongoing closure of the mongol okhotsk ocean and some was probably related to a mantle plume process they were possibly derived from subducted materials melting or juvenile components with some probable contributions from ancient continental crust early mesozoic granitoid magmas in the smxb the ab and the bijb were generated in a post non orogenic setting after the closure of the paleo asian ocean and were the results of partial melting of crustal components in response to underplating of mantle derived magmas most likely linked to lithospheric thickening and delamination and asthenospheric upwelling early mesozoic granitoid magmatism provides critical information on mesozoic post accretionary tectonic evolution of the paleo asian ocean and transitional tectonic regimes from early mesozoic subduction to late mesozoic closure of the mongol okhotsk ocean as well as post accretionary continental growth

what drove late mesozoic extension of the northern china mongolia tract the northern china mongolia tract exhibited a tectonic transition from contractional to extensional deformation in late mesozoic time late middle to early late jurassic crustal shortening is widely thought to have resulted from collision of an amalgamated north china mongolia block and the siberian plate but widespread late late jurassic early cretaceous extension has not been satisfactorily explained by existing models some prominent features of the extensional tectonics of the northern china mongolia tract are 1 late jurassic voluminous volcanism prior to early cretaceous large magnitude rapid extension 2 overlapping in time of contractional deformation in the yinshan yanshan belt with development of extensionrelated basins in the interior of the northern china mongolia tract and 3 widespread occurrence of alkali granitic plutonism extensional basins and metamorphic core complexes in the early cretaceous a new explanation is advanced in this study for this sequence of events the collision of amalgamated north china mongolia with siberia led to crustal overthickening of the northern china mongolia tract and formation of a high standing plateau subsequent breakoff at depth of the north dipping mongol okhotsk oceanic slab is suggested as the main trigger for late mesozoic lithospheric extension of that tract slab breakoff resulted in mantle lithospheric stretching of the adjacent northern china mongolia tract with subsequent ascent of hot asthenosphere and magmatic underplating at the base of the crust collectively these phenomena triggered gravitational collapse of the previously thickened crust leading to late late jurassic early cretaceous crustal extension and importantly coeval contraction along the southern margin of the plateau in the yinshan yanshan belt the proposed model provides a framework for interpreting the spatial and temporal relationships of distinct processes and reconciling some seemingly contradictory phenomena such as the synchronous extension of northerly terranes during major contraction in the neighboring yanshan yinshan belt d 2003 elsevier b v all rights reserved

the yingba shear zone on the sino mongolian border southwestern extension of the zuunbayan fault from mongolia to china and implications for late mesozoic intracontinental extension in eastern asia abstract major strike slip and normal faults are prominent features of eastern asia during the meso cenozoic and they are critical to understanding the intracontinental deformation among others the northeast striking zuunbayan fault in southeastern mongolia is usually documented as a strike slip fault but it may be a normal fault during the late mesozoic however its southwestern extension from mongolia to china is still uncertain the northeast striking yingba shear zone in western inner mongolia china just lies in the place where the zuunbayan fault probably strikes through southwestward the gentle northwest dipping foliation and lineation and top to the northwest shear sense in the granitic mylonite indicate that the yingba shear zone is a northwest dipping low angle normal fault the mylonitization occurred mainly at 400 500 c as a result of the early cretaceous intracontinental extension as bounded by the zircon u pb ages of 145 ma from two deformed pre tectonic pegmatite veins and of 134 ma from one undeformed post tectonic pegmatite vein all of which definitely crosscut the shear zone the yingba normal faulting was coeval with the late mesozoic extensional phase of the zuunbayan fault in southeastern mongolia which is characterized by northwest dip and nw se extension from at least 155 to 126 ma therefore the yingba shear zone is considered as the southwestern segment of the zuunbayan fault in china the late mesozoic intracontinental extension which was possibly triggered by the jurassic thrusting in the south gobi area of mongolia generated the yingba shear zone and the yagan metamorphic core complex on the sino mongolian border

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

temporal and spatial variations of mesozoic magmatism and deformation in the north china craton implications for lithospheric thinning and decratonization abstract mesozoic triassic cretaceous magmatic rocks and structural deformation are widely distributed in the north china craton ncc and are crucial to understanding the timing location and geodynamic mechanisms of lithospheric thinning and decratonization of the ncc our new geochronological geochemical and structural data combined with previously published results on mesozoic magmatic rocks and deformational structures in the ncc indicate a temporal and spatial migration of magmatism and deformation from its margins to its cratonal interior triassic and early jurassic igneous rocks are only distributed along the northern southern and eastern margins of the ncc in contrast cretaceous magmatic rocks are widely distributed in whole eastern and central parts of the ncc there is a younging trend for mesozoic magmatic rocks from the northern and eastern parts yanshan jiaodong peninsula and liaodong to the central part of the ncc taihangshan mesozoic deformation in the ncc exhibits a similar migration trend from craton margins to its inland areas triassic early jurassic deformation mainly occurred in the margins of the ncc and transformed from compression during the early middle triassic to extension during the late triassic to early jurassic in its northern margin middle late jurassic to earliest cretaceous deformation is widely distributed in the ncc and exhibited non unique contractional directions usually perpendicular to boundaries of the ncc and its ordos block indicating that it was likely controlled by multiple tectonic regimes during the middle late jurassic to earliest cretaceous early cretaceous deformation was characterized by near unique nw se extension that was likely controlled by unique geodynamic regime that probably related to the far field effect of cretaceous paleo pacific plate subduction the above mentioned temporal and spatial migrations of mesozoic magmatic rocks and deformation indicate that lithospheric thinning and decratonization of the ncc was diachronous and complex the lithospheric thinning and decratonization of the ncc initially started from its northern and eastern margins as a result of post collisional post orogenic lithospheric delamination during the middle late triassic and then spread to the interior of the craton during the late mesozoic interactions of the surrounding orogenesis and the small size of the ncc may have played important roles on its late mesozoic lithospheric thinning and decratonization

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

tectonic and stratigraphic evolution of the mongolian people s republic and its influence on hydrocarbon geology and potential abstract the stratigraphy of mongolia can be divided into five megasequences each characterized by a common tectonic and stratigraphic history the earliest megasequence 1 precambrian silurian records the amalgamation of precambrian early palaeozoic passive and active margin terranes and the resulting growth of a caledonian fold belt in western mongolia megasequence 2 devonian permian appears to be the most complex with a variety of active margin collisions in southern and eastern mongolia and the continued deformation of the caledonian fold belt in the west a substantial fold belt underpinned by a variety of granitic plutons existed across mongolia by the triassic there was widespread erosion and limited deposition in small foreland and intermontane basins megasequence 3 triassic early jurassic megasequence 4 mid jurassic cretaceous records extension across the partially eroded fold belt with deposition confined to a series of alluvial and lacustrine rift basins these basins later experienced transpression inversion and erosion followed by onlap and overstep giving more widespread sedimentation from large post rift alluvial systems during the late cretaceous and tertiary megasequence 5 megasequences 1 3 are usually extensively folded megasequences 4 and 5 have been involved in transpressive inversion and mongolia is an area of present day compressive and transpressive tectonics erosion and general uplift the late jurassic early cretaceous sequences megasequence 4 are the most prospective for finding extractable hydrocarbons oil has been successfully produced from megasequence 4 alluvial and lacustrine rift sequences with rich oil prone lacustrine mudrocks which can form both source and seal facies interbedded sandstones siltstones and mudstones form various combinations of reservoirs and seals a large number of hydrocarbon trap types are yet to be tested past drilling has been concentrated around surface seeps and on simple structures that can be mapped at the surface with usually only shallow tests