uhp metamorphism and exhumation of the dabie orogen china evidence from shrimp dating of zircon and monazite from a uhp granitic gneiss cobble from the hefei basin a shrimp u pb study of zircons separated from an ultrahigh pressure uhp granitic gneiss cobble sample ds12 from the upper jurassic fenghuangtai formation of the hefei basin north of the dabie orogen has identified three different domains 1 cores some of which show straight boundaries and strong oscillatory zoning with th u ratios of 0 12 0 70 and an imprecise upper intercept age of 777 220 ma mswd 2 4 2 mantles variable in shape and cathodoluminescence cl intensity and containing many uhp mineral inclusions including coesite and omphacite and with th u ratios of 0 02 0 26 and a weighted mean 206pb 238u age of 244 5 ma mswd 4 7 and 3 rims which are more homogeneous and luminescent in cl than the mantles and contain fewer uhp mineral inclusions with th u ratios of 0 01 0 05 and a weighted mean 206pb 238u age of 226 2 ma mswd 0 65 monazites from the same sample do not contain coesite and omphacite inclusions but they do show sector zones and are composed of two contrasting domains the light domains in backscattered electron bse images have a th u ratio of 7 3 18 9 and a weighted mean 206pb 238u age of 221 2 ma mswd 0 94 the dark domains in bse have a th u ratio of 2 8 7 2 and have a weighted mean 206pb 238u age of 218 2 ma mswd 0 96 based on these data and a review of the literature it can be concluded that 1 the protolith of gneiss sample ds12 is a granitoid of neoproterozoic age 2 the ages of 244 and 226 ma for the mantle and rim domains of the zircons are time records of two discrete stages in the uhp evolution of dabieshan 3 the ages of 221 and 218 ma for the light and dark gray domains respectively of the monazites record the times of two stages of retrogressive metamorphism during exhumation of the uhp granitic gneiss to a higher crustal level 4 the exhumation rate of the gneiss is estimated to be 6 km ma and 5 the uhp metamorphic blocks of the dabie orogen provided significant detritus to the hefei basin in the jurassic

850 790 ma bimodal volcanic and intrusive rocks in northern zhejiang south china a major episode of continental rift magmatism during the breakup of rodinia abstract shrimp u pb zircon ages and geochemical and nd isotopic data are reported for mid neoproterozoic igneous rocks in northern zhejiang province south china the shenwu dolerite dykes the daolinshan granite diabase complex and the shangshu bimodal basalt rhyolite volcanic rocks were dated at 849 7 ma 794 9 ma and 792 5 ma respectively all the studied mafic rocks are tholeiitic in compositions and exhibit overall moderate enrichments in most incompatible trace elements resembling intraplate basaltic rocks in continental rifts the 850 ma shenwu dolerites are the earliest anorogenic basaltic rocks identified so far after the ca 1 1 0 9 ga sibao orogeny but seem to predate the neoproterozoic rifting our results are clearly at variance with the alternative tectonic model that an active continental margin existed along the southern margin of the yangtze block until ca 800 ma the intensive 790 ma bimodal volcanic and plutonic rocks part of a major phase of mid neoproterozoic rifting are concurrent with the widespread mid neoproterozoic intraplate magmatism throughout south china they are most likely related to the proposed mantle superplume that led to the breakup of the supercontinent rodinia

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

long term sea level fluctuations driven by ocean basin dynamics earth9s long term sea level history is characterized by widespread continental flooding in the cretaceous period 145 to 65 million years ago followed by gradual regression of inland seas however published estimates of the late cretaceous sea level high differ by half an order of magnitude from 40 to 250 meters above the present level the low estimate is based on the stratigraphy of the new jersey margin by assimilating marine geophysical data into reconstructions of ancient ocean basins we model a late cretaceous sea level that is 170 85 to 270 meters higher than it is today we use a mantle convection model to suggest that new jersey subsided by 105 to 180 meters in the past 70 million years because of north america9s westward passage over the subducted farallon plate this mechanism reconciles new jersey margin based sea level estimates with ocean basin reconstructions

spinup of a submesoscale eddy in the toga coare intensive flux array during the spindown of an intense eastward jet abstract during the toga coare intensive observing period an energetic surface intensified submesoscale cyclonic eddy was observed in the near equatorial western pacific warm pool the eddy appeared to have been generated as part of the spindown of a strong eastward surface jet forced by the december 1992 westerly wind burst because of its potential impacts on the long term heat salt and momentum budgets of the warm pool the authors provide a thorough description of the evolution of the surface jet and the development of the eddy in the present study both the isopycnal surface fit and the zeroth order dynamic balance confirm the existence of the eddy surface layer convergence and northward inertial motion are suggested to be the main causes of the negative eddy vorticity and it is likely that the eddy drew its energy from the decaying surface jet this study indicates that in the near equatorial region the inertial motion has a decreasing meridional spatial scale with time t 1 due to the

water transportation from the subducting slab into the mantle transition zone using a recently developed petrogenetic grid for morb h2o we propose a new model for the transportation of water from the subducting slab into the mantle transition zone depending on the geothermal gradient two contrasting water transportation mechanisms operate at depth in a subduction zone if the geothermal gradient is low lawsonite carries h2o into mantle depths of 300 km with further subduction down to the mantle transition depth approximately 400 km lawsonite is no longer stable and thereafter h2o is once migrated upward to the mantle wedge then again carried down to the transition zone due to the induced convection at this depth hydrous phase olivine is stable and plays a role as a huge water reservoir in contrast if the geothermal gradient is high the subducted slab may melt at 700 900 c at depths shallower than 80 km to form felsic melt into which water is dissolved in this case h2o cannot be transported into the mantle below 80 km between these two endmember mechanisms two intermediate types are present in the high pressure intermediate type the hydrous phase a plays an important role to carry water into the mantle transition zone water liberated by the lawsonite consuming continuous reaction moves upward to form hydrous phase a in the hanging wall which transports water into deeper mantle this is due to a unique character of the reaction because phase a can become stable through the hydration reaction of olivine in the case of low pressure intermediate type the presence of a dry mantle wedge below 100 km acts as a barrier to prevent h2o from entering into deeper mantle 2006 published by elsevier b v on behalf of international association for gondwana research

geochronological framework of mesozoic volcanic rocks in the great xing an range ne china and their geodynamic implications abstract mesozoic volcanic rocks are widespread throughout the great xing an range ne china however precise data constraining the exact eruption ages are limited especially for those from the southern great xing an range which severely hampers our understanding of the petrogenesis and geodynamics of these rocks in this paper we report precise in situ la icpms zircon u pb age measurements for these volcanic rocks volcanic rocks in the southern great xing an range were divided into four units from bottom to top namely the manketouebo manitu baiyingaolao and meiletu formations the previous studies suggested that these volcanic rocks were mainly formed in the late jurassic our data demonstrate that the manketouebo formation erupted during late jurassic to early cretaceous time whereas the other formations are all of cretaceous age the southern great xing an range age dataset along with recently obtained precise ages for volcanic rocks from the northern great xing an range indicate that mesozoic volcanism throughout the great xing an range commenced in late jurassic but peaked during the cretaceous they formed under an extensional tectonic setting which resulted from closure of the mongol okhotsk ocean and subsequent orogenic collapse the globally elevated mantle temperature in cretaceous may provided thermal contributions to the generation of the volcanisms

permian bimodal volcanism in the zhangguangcai range of eastern heilongjiang province ne china zircon u pb hf isotopes and geochemical evidence abstract in this paper we report on zircon u pb dating hf isotopes major and trace elements and sr nd isotope data with the aim of constraining the petrogenesis and regional tectonic evolution of late paleozoic volcanic rocks in the zhangguangcai range of eastern heilongjiang province ne china located in the eastern segment of the central asian orogenic belt caob between the siberian and north china cratons the late paleozoic volcanic rocks in the zhangguangcai range are composed mainly of basalt basaltic andesite rhyolite and minor dacite the results of la icp ms zircon u pb dating for two basaltic rocks three rhyolites and one dacite indicate that they formed in the early permian ca 292 ma the mafic rocks have sio2 50 13 53 80 wt k2o 0 98 2 28 wt mg 0 51 0 71 cr 144 541 ppm ni 74 260 ppm 87sr 86sr i 0 7044 and end t 4 28 whereas the felsic rocks have sio2 69 12 77 98 wt k2o 3 09 5 33 wt mg 0 17 0 36 87sr 86sr i 0 7032 and end t 4 32 these data are typical of bimodal volcanism the mafic volcanic rocks are characterized by a strong enrichment in large ion lithophile elements liles such as rb ba sr and pb depletion in high field strength elements hfses such as nb ta and ti depletion in heavy rare earth elements hrees and weak negative eu anomalies eu eu 0 88 0 94 on the other hand the felsic rocks show a strong depletion in nb ta sr p and ti enrichment in th u and k and relatively large negative eu anomalies eu eu 0 28 0 95 taken together these data suggest that the mafic magma was derived from the partial melting of a depleted lithospheric mantle modified by subducted slab derived fluids and that the felsic magma originated by partial melting of newly accreted crust the early permian bimodal volcanic rocks together with the coeval a type granites indicate an extensional environment similar to a back arc basin such a setting was possibly related to subduction of the paleo asian oceanic plate beneath the jiamusi and songnen zhangguangcai range massifs

geochemistry and sr nd pb hf isotopes of early cretaceous basalts from the great xinggan range ne china implications for their origin and mantle source characteristics abstract late mesozoic volcanism is widespread throughout northeastern china mainly clustering in the great xinggan range where the major period of volcanic eruption was during the early cretaceous the spatial distribution of volcanic rocks shows a nne extending pattern the lithology of mesozoic basaltic rocks in the great xinggan range ranges from trachytic basalts basalts to basaltic andesites on the basis of lithological associations and spatial relationships we divide the basalts into four groups i e tahe genhe zalute and xiwu the early cretaceous volcanism in the great xinggan range is characterized by significant large ion lithophile elements lile and light rare earth elements lree enrichment and high field strength elements hfse depletion with a slightly enriched to depleted nd hf and weakly enriched sr isotopic compositions initial 87sr 86sr 0 7046 0 7079 end t 3 6 to 6 9 and ehf t 8 9 to 4 6 the pb isotopic compositions are also variable with 206pb 204pb 207pb 204pb and 208pb 204pb values of 17 791 18 530 15 477 15 597 and 37 761 38 497 respectively the sr nd pb hf isotopic data indicate that the magmatic sources are heterogeneous showing an enriched to depleted signature of continental lithospheric mantle the magmatic sources of the genhe and zalute groups are characterized by mixing among pm em ii and dm whereas the tahe and xiwu groups derive from enriched lithospheric mantle that had experienced metasomatism by fluids or melts from subducted paleo asian and mongol okhotsk ocean slabs and possible influence of precambrian blocks the late mesozoic volcanic rocks in the great xinggan range resemble those of the cenozoic calc alkaline magmatism in the basin and range province of the united states we therefore suggest that the widespread mesozoic volcanic rocks in the great xinggan range were generated in an extensional tectonic setting

geological setting and controls on the development of graphite sillimanite and phosphate mineralization within the jiamusi massif an exotic fragment of gondwanaland located in north eastern china abstract china is the world s biggest producer and exporter of graphite and also possesses the world s largest reserves of crystalline graphite one of the main production areas is in northeastern china around jixi in heilongjiang province where graphite sillimanite and phosphate form in original sedimentary horizons within the mashan group the mashan group is composed of khondalitic metasediments and tectonically interleaved orthogneisses all metamorphosed to granulite facies the peak conditions have been calculated at 800 850 c at 5 3 6 2 kb and the p t t path defines a tight clockwise loop involving simultaneous decrease in temperature and pressure falling to 410 490 c at less than 3 kb this metamorphic event is interpreted as resulting from continent continent collision with emplacement of mantle derived mafic magma and possible delamination at the base of the crust metamorphism has been precisely dated at 500 ma using u pb zircon shrimp techniques it appears likely that this terrane formed part of an originally more extensive orogenic belt of late pan african age rocks of similar age are recorded from east antarctica western australia india and sri lanka suggesting that the mashan group was most likely juxtaposed with these areas within gondwanaland the mechanism by which the mashan group arrived at its present position between the siberian and north china cratons is poorly constrained but it appears likely that along with the north china south china and tarim blocks it underwent a northward drift from an initial location off northern australia to finally dock with the siberian craton in the late permian to late jurassic

early paleozoic metamorphic rocks of the erguna block in the great xing an range ne china evidence for the timing of magmatic and metamorphic events and their tectonic implications abstract the erguna block is a crustal unit located along the eastern margin of the central asian orogenic belt caob in the far north of china it contains a variety of khondalitic rocks that include sillimanite and garnet bearing gneiss biotite plagioclase gneiss and carbonate in association with hornblende plagioclase gneiss and a variety of granitic orthogneisses these rocks are collectively referred to as the mohe complex and make up the basement rocks in this poorly exposed terrane four samples were analyzed for this study including two samples of biotite plagioclase gneiss one sample of garnet sillimanite gneiss and a sample of hornblende plagioclase gneiss all samples provide evidence of metamorphism in the form of discrete zircon grains or metamorphic rims around detrital cores and they record remarkably consistent 206 pb 238 u ages between 495 2 ma and 497 6 ma detrital zircon cores with oscillatory zoning from the two biotite plagioclase gneiss samples yield magmatic ages that range from 608 8 ma to 1015 12 ma whereas those in the garnet sillimanite gneiss range from 678 8 to 1373 17 ma these results establish that a range of meso to neoproterozoic magmatic rocks were present in the source area and contributed to the sedimentary protoliths the youngest zircon core with an age of 608 8 ma limits the maximum possible age of deposition indicating that the rocks of the mohe complex cannot be late archean to early proterozoic as previously considered the 500 ma age for the granulite facies metamorphic event in the mohe complex is identical to that recorded in the mashan complex of the jiamusi block and the hutou complex of the khanka block 1000 km to the south southeast implying that an extensive early paleozoic khondalite belt extended across much of the eastern caob rocks of similar age and metamorphic grade have also been recorded from the sayang baikal orogen along the southern margin of the siberia craton 1000 km to the west it is unclear at present whether these two belts are directly related but whatever the relationship it is evident that extensive tracts of khondalitic rocks were present in the eastern caob they were subsequently affected by high grade metamorphism in the early paleozoic forming collisional accretionary terranes that were accreted during the late pan african global event

yardea dacite large volume high temperature felsic volcanism from the middle proterozoic of south australia the yardea dacite is a large volume felsic volcanic unit from the middle proterozoic gawler range volcanics of south australia it has been previously described as an ignimbrite however some samples contain no petrographic evidence for a pyroclastic origin but have characteristics compatible with final crystallization from a nonfragmented magma these samples may have erupted as lavas but others are likely to be extremely densely welded ignimbrites suggesting a compound nature for the unit geothermometry and phase equilibria indicate that the yardea dacite originated from a high temperature 1000 c felsic magma with a low water content 2 the yardea dacite is not associated with a known caldera of the valles type and shares many characteristics of recently described cenozoic felsic volcanic rocks from the western united states interpreted as rheoignimbrites or as unusually extensive lavas

tectonic evolution of the anuy metamorphic rocks sikhote alin russia and their place in the mesozoic geodynamic framework of east asia abstract in sikhote alin the anuy river provides a representative cross section of the pre eocene structure of the area east of the central sikhote alin fault the mesozoic rocks are divided into the following groups from bottom to top anuy metamorphic rocks the tithonian berriasian samarkinsk accretionary complex early cretaceous zhuravlevsk tumnin turbidite and mid cretaceous al rich khungariisk granitoids the anuy metamorphic rocks have a postfolial ne sw trending antiformal structure from bottom to top they consist of 1 low grade schist and sandstone 2 gneiss and micaschist 3 migmatite and 4 ophiolitic nappe east west trending stretching lineation isoclinal folds and top to the east shear criteria are found in the ophiolite and migmatite units these structures are related to ophiolitic thrusting that likely occurred in early cretaceous times 40 ar 39 ar dating of migmatite and metagabbro indicates mid cretaceous ages about 110 ma close to those of the khungariisk granites the main deformation coeval to a high temperature metamorphism is characterized by ne sw trending stretching lineation with divergent senses of shear 40 ar 39 ar dating of biotite and muscovite from several lithologies give late cretaceous to paleogene ages 73 58 ma this tectono thermal event is related to ne sw trending strike slip faulting along the central sikhote alin fault system the unroofing of the anuy metamorphic rocks is likely related to granite emplacement the collision of the anuy microcontinent with asia can be compared to that of the southern japan microcontinent

on ferroan a type granitoids their compositional variability and modes of origin we recognize eight types of ferroan granitoid that can be distinguished on the basis of major element chemistry these include alkalic granitoids that may be metaluminous or peralkaline alkali calcic granitoids that may be metaluminous peraluminous or peralkaline calc alkalic granitoids that may be metaluminous or peraluminous and rare calcic ferroan granitoids these granitoids may form by two distinct end member processes extreme differentiation of basaltic melts results in ferroan granitoids that are either peralkaline alkalic and alkali calcic or metaluminous alkalic alkali calcic and calc alkalic with alkalinity increasing with increasing pressure of differentiation partial melting of tonalitic to granodioritic crust produces alkali calcic to calc alkalic granitoids that are metaluminous at low pressures and peraluminous at high pressures it is likely that a combination of these two processes plays some role in the formation of most ferroan granitoids most granitoids that are referred to as ferroan however the term has become more confusing than edifying because it has been applied to a broad spectrum of granitoid compositions with varying petrogenesis for this reason we recommend that the term be discontinued and ferroan used in its stead

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

pan african metamorphic and magmatic rocks of the khanka massif ne china further evidence regarding their affinity the khanka massif is a crustal block located along the eastern margin of the central asian orogenic belt caob and bordered to the east by late jurassic early cretaceous circum pacific accretionary complexes of the eastern asian continental margin it consists of graphite sillimanite and cordierite bearing gneisses carbonates and felsic paragneisses in association with various orthogneisses metamorphic zircons from a sillimanite gneiss from the hutou complex yield a weighted mean 206 pb 238 u age of 490 4 ma whereas detrital zircons from the same sample give ages from 934 610 ma magmatic zircon cores in two garnet bearing granite gneiss samples also collected from the hutou complex yield weighted mean 206 pb 238 u ages of 522 5 ma and 515 8 ma whereas their metamorphic rims record 206 pb 238 u ages of 510 500 ma these data indicate that the hutou complex in the khanka massif records early palaeozoic magmatic and metamorphic events identical in age to those in the mashan complex of the jiamusi massif to the west the older zircon populations in the sillimanite gneiss indicate derivation from neoproterozoic sources as do similar rocks in the jiamusi massif these data confirm that the khanka massif has a close affinity with other major components of the caob to the west of the dun mi fault based on these results and previously published data the khanka massif is therefore confirmed as having formed a single crustal entity with the jiamusi and possibly the bureya massif since neoproterozoic time

geochemistry geochronology and petrogenesis of a late precambrian 590 ma composite dike from the north eastern desert of egypt late precambrian 575 600 ma igneous activity in the north eastern desert of egypt produced large volumes of compositionally bimodal magmas a single composite dike was studied to further examine petrogenetic relationships between andesitic and rhyolitic melts the dike consists of 1 5 2 m of andesite on either side of a 5 6 m wide rhyolite core contact relations indicate that the andesite and rhyolite simultaneously existed as porphyritic liquids wholerock rb sr dating indicates an age of 591 ma but with considerable scatter andesite and rhyolite had similar initial87sr 86sr of 0 7032 2 indicating derivation from a low rb sr source either the upper mantle very young upper crust or depleted lower crust the composition of the andesites on either margin cannot be distinguished these are very similar to andesites of the dokhan volcanics of similar age correspondingly the rhyolite is compositionally similar to the epizonal pink granites this dike and others like it represent hypabyssal feeders for the extrusive and shallow intrusive members of the north eastern desert bimodal suite

relationships between calc alkaline acidic and basic mantle derived magmas in late archaean composite dykes kambalda goldfield western australia abstract composite dykes from the north dome area kambalda comprise basic spessartite hornblende plagioclase lamprophyre 56 sio 2 with mg numbers cr and ni contents typical of primitive mantle derived rocks and more acidic plagioclase phyric quartz andesite 66 sio 2 together with felsic net veins and hornblendic patches suggesting local differentiation the spessartites can be regarded chemically as volatile lile and incompatible element enriched versions of siliceous high magnesian basalts from the local greenstone sequence a pb pb isochron on one dyke yields an age of 2636 27 ma making this the oldest composite dyke so far dated field and geochemical evidence including identical pb initial ratios for the constituent rock types virtually indistinguishable ree spectra and tight linear trends on plots such as y versus ti and ce cr versus ti cr suggests that the two magmas lamprophyric and acidic were genetically related and were emplaced contemporaneously into the same fracture quantitative least squares modelling of whole rock and mineral compositions suggests that the acidic components resulted mainly from fractionation of hornblende from parental spessartite magmas together with a possible contribution from mixing of the same spessartite magma with crustally derived acidic e g trondjhemitic magma the latter is represented by separate dykes and stocks in the kambalda area close similarities between the kambalda composite dykes and archaean tertiary composite dykes in other parts of the world imply a persistent role for mantle derived magmas such as lamprophyres in the genesis of the dominant intermediate acidic e g granitoid magmas in many calc alkaline provinces

boron isotope systematics of tourmaline formation in the sullivan pb zn ag deposit british columbia canada abstract we report here the results of 54 boron isotope analyses of tourmaline associated with the giant sullivan pb zn ag deposit in southeastern british columbia canada the 11 b values range from 11 1 to 2 9 which is almost as great as the range found worldwide in tourmalines from 33 massive sulfide deposits and tourmalinites in dominantly clastic metasedimentary terranes the major control on the overall 11 b values of the sullivan tourmalinites is the boron source potential controls over the large range of the data also include 1 differences in formation temperatures of the tourmalinites 2 different stages of tourmaline formation 3 variations in the proportions of dissolved boron incorporated into the tourmaline rayleigh fractionation 4 seawater entrainment and 5 post depositional metamorphism the boron isotope data at sullivan are consistent with boron derivation from leaching of footwall clastic sediments however the great abundance of tourmaline in the sullivan deposit suggests that the local clastic sediments were not the sole source of boron and we argue that non marine evaporites buried deep below the orebody are the most viable source of this additional boron it is likely that some of the variation in tourmaline 11 b values reflect mixing of boron from these two sources comparison of the potential effects of these controls with geologic and other geochemical evidence suggests that major causes for the wide range of 11 b values measured at sullivan are seawater entrainment and rayleigh fractionation although in places post depositional alteration and thermal metamorphism were important in determining 11 b values of some of the recrystallized tourmalinites

avo examples of long offset 2 d data in the gulf of mexico for more than 17 years avo has been a primary tool for predicting a reservoir s rock type and pore fluid content however the success of an avo interpretation recognition and validation of an anomaly is constrained by the rock properties of the reservoir and its surrounding media class 3 and 4 anomalies are recognized as amplitude bright spots on stack data and pore fluid prediction is routinely accomplished by anomaly background amplitude analyses for class 2 anomalies avo interpretation of prestack data is often needed just to recognize potential reservoirs finally avo interpretation in class 1 environments is difficult because hydrocarbons don t yield bright spots or amplitude brightening with source receiver offset in short success in recognizing an anomaly and validating its composition is best for class 3 and 4 environments and poorest for class 1

change of tectono stratigraphic regime in the australian plate during the 99 ma mid cretaceous and 43 ma mid eocene swerves of the pacific the clockwise bend at 99 ma mid cretaceous in linear volcanic chains in the tropical pacific coincides with a change from pre 99 ma head on chilean type subduction of the pacific plate beneath eastern gondwana to 99 43 ma sinistral oblique mariana type subduction and strike slip breakup by simple sea floor spreading between australia and antarctica and by backarc spreading in the southwest pacific the 99 ma breakup of australia from antarctica is documented by a mid cretaceous unconformity this tectono stratigraphic change founded modern australia with a mountain chain along an upper plate margin in the east and lowlands on the lower plate margin in the south the counterclockwise bend at 43 ma mid eocene the emperor hawaiian bend coincides with the onset of structure in the challenger rift of new zealand the eromanga cooper basin of central australia and the oil shale grabens of coastal queensland

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 trace element composition of the lavas and dikes from a 3 km vertical section through the lava pile of eastern iceland the tertiary lava succession of eastern iceland dips at shallow angles towards and beneath the axial quaternary volcanic zone a 3 km vertical section through part of this tertiary pile studied during the iceland research drilling project contains flows ranging in composition from basalt to icelandite this succession can be divided into three distinct stratigraphic groups upper and lower groups 1 7 and 0 6 km thick with zr y ratios of about 4 5 and a 0 7 km thick middle group with a zr y ratio of about 6 5 the middle group also has a significantly higher ce yb ratio and higher elemental sr abundances the data on within group variation are equally compatible with high level fractionation of a plagioclase olivine clinopyroxene assemblage with accessory apatite and opaque minerals or with open system fractionation models dikes occur throughout the 3 km section and are comparable in trace element composition to the lavas of the upper group the dikes are part of a n s trending swarm originating from the breiddalur volcanic center some 15 km to the south these dikes were injected laterally northward into the reydarfjordur lava pile at the time of the eruption of the upper group lavas the lower two major lava groups may be related to similar but unidentified discrete volcanic centers perhaps buried downdip the new data on this deeper section corroborate earlier studies which indicated that on a gross scale the eastern iceland lava pile becomes less mafic with depth this vertical change occurs because each major volcanic center erupted more silicic lavas close to the center and more magnesium rich compositions toward the periphery the formation of the lava pile occurred by tilting toward the spreading center and overlapping repetition of such major units which resulted in the more mafic edges of the lava groups forming the topographically higher parts of the crustal section

on the relationships between slab dip back arc stress upper plate absolute motion and crustal nature in subduction zones 1 statistical analysis of modern oceanic subduction zone parameters such as the age of a downgoing plate or the absolute plate motions is performed in order to investigate which parameter controls the dip of a slab and conversely what the influence of slab geometry is on upper plate behavior for that purpose parameters have been determined from global databases along 159 transects from all subduction zones that are not perturbed by nearby collision or ridge plateau seamount subduction on the basis of tomographic images slabs that penetrate through or lie on the 670 km discontinuity are also identified the results of the statistical analysis are as follows 1 back arc stress correlates with slab dip i e back arc spreading is observed for deep dips deeper than 125 km larger than 50 whereas back arc shortening occurs only for deep dips less than 30 2 slab dip correlates with absolute motion of the overriding plate the correlation is even better when the slab lies on or even more penetrates through the 670 km discontinuity 3 slabs dip more steeply by about 20 on average beneath oceanic overriding plates than beneath continental ones 4 slabs dip more steeply on average by about 10 near edges 5 slab dip does not correlate with the magnitude of slab pull the age of subducting lithosphere at the trench the thermal regime of the subducting lithosphere the convergence rate or the subduction polarity east versus west the present study provides evidence that the upper plate absolute motion plays an important role on slab dip as well as on upper plate strain retreating overriding plates are often oceanic ones and thus may partially explain the steeper slab dips beneath oceanic upper plates one can infer that low slab dips correlate well with compression in continental advancing upper plates whereas steep dips are often associated with extension in oceanic retreating upper plates excess weight of old slabs is often counterbalanced by other forces probably asthenospheric in origin such as lateral mantle flow near slab edges or anchor forces to determine slab dip

testing the fixed hotspot hypothesis using ar 40 ar 39 age progressions along seamount trails abstract hotspots and their associated intra plate volcanism producing seamount trails have become an accepted fact in geology from a conceptual theory the azimuths and age progressions of these seamount trails provide the only means to determine absolute plate motions with respect to an independent reference frame of fixed hotspots however the presumed fixity of hotspots is in disagreement with recent paleomagnetic studies and global circuit plate reconstructions for the hawaiian emperor seamount trail in this study we provide independent evidence suggesting that hotspots are not fixed relative to each other we use a straightforward test that compares the observed 40ar 39ar age progressions along pacific seamount trails 0 140 myr with the pacific plate velocities as predicted by their poles of plate rotation i e euler poles in most of these comparisons the age progressions were found incompatible with published euler poles or with a new set of euler poles as derived in this study using discrete seamount locations digitized from the bathymetry maps of smith and sandwell eos 77 1996 315 science 277 1997 1956 1921 we conclude that the relative motion between hotspots may be required to reconcile the observed age progressions with the predicted plate velocities from their modeled euler poles on average the pacific hotspots may show motion at 10 60 mm yr over the last 100 myr partly attributed to individual hotspot motion whereas systematic motion of these hotspots due to true polar wander may account for the remainder

u pb zircon geochemical and sr nd hf isotopic constraints on age and origin of jurassic i and a type granites from central guangdong se china a major igneous event in response to foundering of a subducted flat slab abstract the mesozoic geology of se china is characterized by widespread jurassic to cretaceous igneous rocks consisting predominantly of granites and rhyolites and subordinate mafic lithologies however the tectonic regime responsible for the inland jurassic granites remains controversial we report here u pb zircon ages geochemical and sr nd hf isotopic data for the nankunshan alkaline granite and the fogang granitic batholith in central guangdong mineralogical and geochemical features suggest that the fogang and nankunshan rocks are i and aluminous a type granites respectively shrimp u pb zircon analyses yield consistent ages ranging from 159 3 ma to 165 2 ma for four samples from the fogang batholith and an age of 158 5 ma for the nankunshan granite the fogang granites having i sr 0 7098 0 7136 e nd t 4 3 to 12 2 and e hf t 11 5 to 3 1 for the magmatic zircons were derived from paleoproterozoic mafic intermediate igneous protolith with minor addition of mantle derived magmas the nankunshan rocks have relatively lower i sr 0 706 0 708 higher e nd t 0 3 to 2 4 and e hf t 5 7 to 1 1 for the magmatic zircons and some oib like trace element ratios they were likely generated through extensive fractional crystallization of mantle derived alkaline parental magma associated with crustal assimilation these 160 ma i and a type granites in central guangdong were emplaced coeval with the widespread 165 155 ma i and a type granites and syenites following the initiation of intraplate basaltic and or bimodal igneous magmatism at 180 170 ma in the adjacent regions we interpret these jurassic igneous rocks as anorogenic magmatism formed during a major igneous event in response to foundering of an early mesozoic subducted flat slab beneath se china continent

a 100 ma bimodal composite dyke complex in the jiamusi block ne china an indication for lithospheric extension driven by paleo pacific roll back abstract a 125 m wide bimodal composite dyke complex consisting of rhyolite and dolerite dykes was emplaced into cretaceous volcanic strata of the songmuhe formation in the jiamusi block of ne china the dolerite dykes are sub vertical strike north south and intruded into both the country rocks and rhyolite dykes soon after the latter solidified shrimp zircon u pb dating shows that the rhyolite dykes were emplaced at 100 2 ma and the dolerite dykes were also most likely emplaced at 100 2 ma the rhyolite is characterized by enrichment in large ion lithophile elements lile and light rare earth elements lree and depletion in high field strength elements hfse it shows a significant negative eu anomaly and has e nd t values ranging from 0 49 to 1 66 and two groups of initial 87 sr 86 sr ratios at 0 7045 and 0 7061 the rhyolite displays the compositional signature of peraluminous ferroan granitoid indicating it was derived by either differentiation of basalt and or low pressure partial melting of crust the dolerite is also characterized by enrichment in lile and lree and depletion in hfse it has a weak negative eu anomaly and has e nd t 1 22 to 3 26 and 87 sr 86 sr i 0 7057 0 7074 the dolerite originated from partial melting of lithospheric mantle which was affected by subducted oceanic crust and experienced different amounts of crustal contamination such bimodal dyke complexes are an important indicator of crustal extension under the influence of mantle processes thus the dyke complex in the jiamusi block indicates mid cretaceous intra plate extension in ne china related to the subduction of the paleo pacific plate along the eastern eurasian continental margin when compared with mesozoic bimodal magmatism further to the west our new data support a temporal eastward migration of magmatism over a distance 1000 km from 160 ma to 100 ma this was most likely associated with roll back of the paleo pacific plate and consequent upwelling of asthenospheric mantle

intensive hydration of the mantle transition zone beneath china caused by ancient slab stagnation the mantle transition zone beneath china is anomalously hydrated geochemical analyses of basalts erupted above the mantle transition zone in northeast china indicate that water may have been added to this zone during the dehydration of subducted slabs on two separate occasions over the past one billion years

mantle dynamics of western pacific and east asia insight from seismic tomography and mineral physics abstract recent results of high resolution seismic tomography and mineral physics experiments are used to study mantle dynamics of western pacific and east asia the most important processes in subduction zones are the shallow and deep slab dehydration and the convective circulation corner flow processes in the mantle wedge the combination of the two processes may have caused the back arc spreading in the lau basin affected the morphology of the subducting philippine sea slab and its seismicity under southwest japan and contributed to the formation of the continental rift system and intraplate volcanism in northeast asia which are clearly visible in our tomographic images slow anomalies are also found in the mantle under the subducting pacific slab which may represent a small mantle plumes b upwellings associated with the slab collapsing down to the lower mantle or c sub slab dehydration associated with deep earthquakes caused by the reactivation of large faults preserved in the slab combining tomographic images and earthquake hypocenters with phase diagrams in the systems of peridotite water we proposed a petrologic model for arc volcanism arc magmas are caused by the dehydration reactions of hydrated slab peridotite that supply water rich fluids to the mantle wedge and cause partial melting of the convecting mantle wedge a large amount of fluids can be released from hydrated morb at depths shallower than 55 km which move upwards to hydrate the wedge corner under the fore arc and never drag down to the deeper mantle along the slab surface slab dehydration reactions at 120 km depth are the antigorite related 5 reactions which supply water rich fluids for forming the volcanic front phase a and mg surssasite breakdown reactions at 200 and 300 km depths below 700 c cause the second and third arcs respectively moreover the dehydration reactions of super hydrous phase b phases d and e at 500 660 km depths cause the fluid transportation to the mantle boundary layer mbl 410 660 km depth the stagnant slabs extend from japan to beijing china for over 1000 km long indicating that the arc trench system covers the entire region from the japan trench to east asia we propose a big mantle wedge bmw model herein where hydrous plumes originating from 410 km depth cause a series of intra continental hot regions fluids derived from mbl accumulated by the double sided subduction zones rather than the india asia collision and the subsequent indentation into asia are the major cause for the active tectonics and mantle dynamics in this broad region