generation of leucogranites via fractional crystallization a case study of the jurassic bengbu granite in the southeastern north china craton abstract the magmatic processes that form leucogranites are closely related to plate collision as the products of crustal anatexis leucogranites provide clues to intracrustal differentiation and the regional tectonic evolution the bengbu area is located along the southeastern margin of the north china craton ncc 150 km north of the dabie orogen and is bounded by the tancheng lujiang fault zone to the east late jurassic leucogranites are widely distributed in the bengbu area we collected samples of the bengbu leucogranite blg and analyzed data on whole rock geochemistry zircon u pb geochronology zircon in situ lu hf isotopes quartz in situ o isotopes and total bulk sr nd pb isotopes zircon u pb dating yielded formation ages for these leucogranites of 160 ma the samples are characterized by high sio2 72 4 wt 76 9 wt alkali 7 3 wt 9 4 wt and al2o3 13 4 wt 15 5 wt contents in addition to two samples exhibiting the rare earth element tetrad effect all rocks have high differentiation index values 95 99 extremely low iron magnesium titanium oxide contents total fe2o3

the origin and evolution of low δ18o magma recorded by multi growth zircons in granite we for the first time reported low 18o granites within craton in bengbu uplift southeast margin of north china craton integrated ion microprobe study of 18o and u pb age on single zircon grains and zircon populations gives direct evidence for the origin and later magmatic evolution of the low 18o granites three types of zircon domains are recognized inherited magmatic core type i inherited metamorphic core mantle type ii and overgrown magmatic rim type iii type i zircons were formed in neoproterozoic and have the average 18o values of 5 3 0 6 around mantle value triassic type ii zircons have extremely low and highly heterogeneous 18o values ranging from 9 4 to 8 6 consistent with the values and variations exhibited by metamorphic zircons from dabie sulu orogen this feature argues for a triassic metamorphic origin for the 18o depletion signature of zircons from ultrahigh pressure metamorphic rocks in dabie sulu orogen jurassic type iii zircons have isotopically homogeneous 18o values of 3 3 0 5 collectively zircon populations recorded the 18o variations from source to low 18o granite the primary protolith of jurassic granites in bengbu uplift is a neoproterozoic granitic intrusion in south china block later high temperature meteoric hydrothermal alteration has lowered the whole rock 18o values in various degrees the low 18o protolith was then buried to the middle lower crust by triassic continental subduction and jurassic anatexis produced the low 18o granitic magma moreover 18o values of type iii zircons are isotopically lighter than previously reported values of magmatic garnets in jurassic granites indicating later magmatic evolution of low 18o magma toward high 18o values probably as a result of infiltration of high 18o fluid from wall rocks

formation and evolution of the tancheng lujiang wrench fault system a major shear system to the northwest of the pacific ocean abstract in the northwestern peri pacific tectonic belt on the continental margin of east asia there is a series of major faults trending ne nne i e the tancheng lujiang wrench fault system this fault system extends more than 5000 km with a width of 800 1000 km in the continent and a general strike of n30 e on the basis of the results of special research on the major faults and a lot of new information about the fault system the authors discuss early mesozoic triassic jurassic period sinistral transcurrent faulting in the different regions it is suggested that in the eastern continent of east asia the e w trending tectonic pattern of the paleozoic and beginning of the mesozoic appear in the nuclei of the sino korean continent massif and the yangtze continent massif these ancient continent massifs accreted both southwards and northwards which finally led to their collision and suturing to the angura continent massif later this tectonic framework was changed greatly by the tancheng lujiang wrench fault system along which left lateral horizontal displacement of 1000 1500 km took place since the mesozoic the dislocation and deformation of horizontal displacement of the tancheng lujiang fault system produced a series of structures trending ne nne in the continental margin which have greatly transformed the ancient structures of the preceding mesozoic this is an important diktyogenese in the continent of east asia during the horizontal displacement motion of these faults the folds nappes and decollement structures pull apart basins and subductions within the continent occurred in the continent at the same time some large sinistral ductile shear zones of horizontal displacement appeared the depth of the deformed horizon of ductile shear which belongs to simple shear was controlled by the tectonic element and distance away from the continental margin whilst the ductile shear deformation occurred the fault dynamo thermal metamorphism and the fault remelting took place and magma activity appeared within the continent in the mesozoic the fault zones used to be high energy zones and recently became the seismic belts within the continent formation of the tan lu fault system was due to the collision between the southern and northern blocks its large sinistral horizontal displacement in the mesozoic was closely related to the kula plate moving northwards therefore the continent disintegrated by shearing which produced some new terranes the shear edge belt was formed between the continent and ocean the change of relative motion between the continent and pacific ocean plate transformed the tan lu fault system into first extension and then compression clarifying the characteristics of the tan lu fault system is an important key in understanding the evolution of the east asia continent since the mesozoic

ductile fabrics in the zone of active oblique convergence near the alpine fault new zealand identifying the neotectonic overprint the mid crustal alpine schist in central southern alps new zealand has been exhumed during the past similar to3 m y on the hanging wall of the oblique slip alpine fault these rocks underwent ductile deformation during their passage through the similar to 150 km wide pacific australia plate boundary zone likely to be cretaceous in age peak metamorphism predates the largely pliocene and younger oblique convergence that continues to uplift the southern alps today late cenozoic ductile deformation constructively reinforced a pre existing fabric that was well oriented to accommodate a dextral transpressive overprint quartz microstructures below a recently exhumed brittle ductile transition zone reflect a late cenozoic increment of ductile strain that was distributed across deeper levels of the pacific plate deformation was transpressive including a dextral normal shear component that bends and rotates a delaminated panel of pacific plate crust onto the oblique footwall ramp of the alpine fault progressive ductile shear in mylonites at the base of the pacific plate overprints earlier fabrics in a dextral reverse sense a deformation that accompanies translation of the schists up the alpine fault ductile shear along that structure affects not only the 12 km thick section of alpine mylonites but is distributed across several kilometres of overlying nonmylonitic rocks c 2001 elsevier science ltd all rights reserved

shisha pangma leucogranite south tibetan himalaya field relations geochemistry age origin and emplacement the shisha pangma pluton forming most of the xixabangma 8027 m massif in south tibet is one of the 20 larger leucogranite intrusives that mark the highest structural levels of the himalayan metamorphic core the pluton occurs immediately below the shisha pangma detachment a strand of the south tibetan detachment std system a low angle 30 north dipping normal fault placing paleozoic black slates atop sillimanite grade pelites and calc silicate rocks k feldspar augen gneisses containing fibrolite and sillimanite paragneisses along the footwall show strong internal s c fabrics indicative of down to the north extension the shisha pangma leucogranite is a heterogeneous polyphase intrusion with an earlier foliated biotite rich phase and a later tourmaline muscovite rich phase typically containing the assemblage kfs pl qtz ms tur gt bt sil ap the highly peraluminous granites have high 87sr 86sr ratios 0 738 0 750 typical of pelite derived anatectites nd depleted mantle model

exhumation of the ultrahigh pressure continental crust in east central china cretaceous and cenozoic unroofing and the tan lu fault the orogenic architecture of the world s largest ultrahigh pressure exposure the hong an dabie mountains of the triassic qinling dabie orogenic belt is dominated by cretaceous and cenozoic structures that contributed to its exhumation from 30 km depth cretaceous magmatic crustal recycling 50 for the entire dabie and heating 250 to 700 c were most prominent in dabie and exhumation magmatism and cooling were all controlled by cretaceous transtension exhumation was accomplished principally by an asymmetric cordilleran type extensional complex in the northern dabie northern orthogneiss unit between 140 and 120 ma at rates as fast as 2 mm yr and average horizontal stretching rates of up to 6 mm yr cretaceous reactivation occurred within a regional transtensional strain field as a result of far field collisions and pacific subduction the onset of crustal extension was preceded and possibly facilitated by a reheating of the hong an dabie crust 140 ma coeval with the onset of voluminous magmatism in eastern china 145 ma which resulted from a change in pacific subduction from highly oblique to orthogonal the tan lu continental scale fault was a normal fault zone in the mid cretaceous 110 90 ma and underwent 5 4 km dip slip and 4 km throw in the cenozoic during the india asia collision the qinling dabie belt acted as the structural discontinuity between the strike slip dominated escape tectonics south of the qilian qinling dabie belt and the rifting dominated tectonism north of it the most prominent cretaceous and cenozoic structures of the hong an dabie the xiaotian mozitang and the jinzhai fault zones respectively reactivated major lithospheric structures of the triassic orogen i e the huwan detachment zone and the suture

chronology and geochemistry of mesozoic granitoids in the bengbu area central china constraints on the tectonic evolution of the eastern north china craton abstract we performed zircon u pb dating and analyses of major and trace elements and sr nd pb isotopes for granitoids in the bengbu area central china with the aim of constraining the magma sources and tectonic evolution of the eastern north china craton ncc the analyzed zircons show typical fine scale oscillatory zoning indicating a magmatic origin zircon u pb dating reveals granitoids of two ages late jurassic and early cretaceous 206pb 238u ages of 160 ma and 130 110 ma respectively the late jurassic rocks jingshan intrusion consist of biotite syenogranite whereas the early cretaceous rocks huaiguang xilushan nushan and caoshan intrusions are granodiorite syenogranite and monzogranite the late jurassic biotite syenogranites and early cretaceous granitoids have the following common geochemical characteristics sio2 70 35 74 56 wt k2o na2o 0 66 1 27 mainly the occurrence of neoproterozoic magmatic zircons 850 ma and inherited early mesozoic 208 228 ma metamorphic zircons within the late jurassic biotite syenogranites together with the occurrence of neoproterozoic magmatic zircons 657 and 759 ma and inherited early mesozoic 206 231 ma metamorphic zircons within the early cretaceous nushan and xilushan granitoids suggests that the primary magmas were derived from partial melting of the yangtze craton yc basement in contrast the occurrence of paleoproterozoic and paleoarchean inherited zircons within the huaiguang granitoids indicates that their primary magmas mainly originated from partial melting of the ncc basement the occurrence of yc basement within the lower continental crust of the eastern ncc indicates that the yc was subducted to the northwest beneath the ncc along the tan lu fault zone during the early mesozoic

crustal structure and exhumation of the dabie shan ultrahigh pressure orogen eastern china from seismic reflection profiling crustal penetrating seismic reflection data across the ultrahigh pressure uhp dabie shan orogen show that the yangtze craton was subducted beneath the sino korean craton forming a bivergent orogenic fabric strikingly similar to normal non uhp collision belts hence preservation of uhp minerals coesite and diamond may be unrelated to differences between collisional histories but may be due to opportunities for subsequent intracrustal uplift of material that is routinely returned from mantle depths into the lower crust in most collisional orogens only a narrow channel 5 km now exists from the mantle into the crust through which uhp material was returned to lower crustal depths as thin slabs we image the dabie shan as a crustal scale dome formed during postcollisional intracrustal uplift by core complex type exhumation of the lower crust tectonically unrelated to the earlier exhumation from 100 km to 35 40 km

tectonic evolution of the tancheng lujiang tan lu fault via middle triassic to early cenozoic paleomagnetic data the north striking tancheng lujiang tan lu fault is a conspicuous and controversial feature of the eastern asian landscape near the southeast extremity of the fault in anhui province we collected paleomagnetic samples at 17 middle triassic t2 and 10 upper cretaceous k2 to lower cenozoic e1 sites t2 remanent magnetizations are interpreted as primary in two of three areas the three areas are rotated 37 to 137 counterclockwise with respect to the south china block scb reference direction k2 e1 remanent magnetization directions pass regional fold and reversals tests and are not rotated with respect to surrounding areas counterclockwise rotation of t2 strata therefore ended before k2 and is attributed to left lateral shear acting along tan lu during the north china block ncb scb collision in shandong province 700 km north of the anhui sites four areas containing 33 upper jurassic j3 and cretaceous sites have negligible declination differences except for one which has dispersed directions the fold test is inconclusive for this latter area and positive for the other three regional concordance of the j3 e1 paleomagnetic data including paleolatitudes together with observed deformation patterns suggest that an extensional regime prevailed in the late cretaceous and cenozoic euler pole positions that constrain the north south china collision and account for tan lu motion suggest at least 500 km of sinistral shear took place along the fault and either 1 subduction and related ultrahigh pressure uhp metamorphism occurred near the present location of the qinling dabieshan and sulu uhp belts while tan lu acted as a transform fault that connected the two subduction zones or 2 tan lu and sulu were parts of the same transform fault system and no uhp rocks formed in situ at sulu in either case uhp rocks originally exhumed near dabieshan could have been transported by plate capture toward sulu along tan lu after north and south china impacted near dabieshan the tan lu fault grew within the scb as the dabieshan corner indented the scb causing folds in scb cover rocks to conform to the ncb margin late cretaceous to cenozoic reactivation of tan lu with both right lateral strike slip and normal fault motion occurred as the scb extruded east relative to the ncb under the influence of the india asia collision

magmatism along the southeast margin of the yangtze block precambrian collision of the yangtze and cathysia blocks of china the time and the environment of formation of igneous rocks along the southern margin of the yangtze continental block in southern anhui province and northeastern jiangxi province china are discussed sm nd dating for samples from two ophiolite suites give ages of about 1 ga late proterozoic granodiorites in the study area have ages of about 0 9 ga and have normal intrusive relations against banxi group metamorphic country rocks trace element abundances and initial nd isotope ratios for ophiolite and proterozoic granodiorites are consistent with formation in an island arc environment in contrast sr and nd isotopes for mesozoic granitoids of the region suggest that they are products of remelting of crustal sources similar to the basement of the yangtze block these relations do not support a recently proposed mesozoic overthrust tectonic model for southeast china but rather favor a late precambrian collision history for the yangtze and cathysia blocks

how did the foreland react yangtze foreland fold and thrust belt deformation related to exhumation of the dabie shan ultrahigh pressure continental crust eastern china during the triassic collision of the yangtze and sino korean cratons the leading edge of the yangtze crust subducted to mantle depths and was subsequently exhumed as a penetratively deformed coherent slab capped by a normal shear zone this geometry requires a reverse shear zone at the base of the slab and we suggest that the yangtze foreland fold and thrust belt constitutes this zone lower triassic rocks of the eastern foreland record nw se compression as the oldest compressional stress field onset of related deformation is indicated by middle triassic clastic sedimentation subsequent jurassic stress fields show a clockwise change of compression directions based on nearly coeval onset and termination of deformation and on a common clockwise change in the principal strain stress directions we propose that the foreland deformation was controlled by the extrusion of the ultra high pressure slab widespread cretaceous cenozoic reactivation occurred under regional extension to transtension which characteristically shows a large scale clockwise change of the principal extension directions during the lower cretaceous

collision between the north and south china blocks a crustal detachment model for suturing in the region east of the tanlu fault a crustal detachment model based on the interpretation of linear aeromagnetic anomalies surface geological observations and deep seismic profiles is proposed for the continent continent collision between the north and south china blocks east of the tanlu fault the model suggests that during the mid mesozoic collision between the two continental blocks the upper crust of the south china block in the subei yellow sea region was detached from the lower crust and thrust over the north china block for 400 km whereas the lower part of the lithosphere was subducted under the north china block along a subsurface suture running east from nanjing the sinistral offset of the qinling suture by the tanlu fault is only 110 120 km in the deep crust much less than previously suggested

petrogenetic interpretation of granitoid rock series using multicationic parameters abstract granitoid rock compositions from a range of tectonic environments are plotted on a multicationic diagram devised by de la roche and his coworkers this shows that there is a systematic change through an orogenic cycle which leads progressively to the ultimate development of alkaline magmas possible source materials and mechanisms of magma generation are considered from analysis of mineral compositional vectors these suggest that most granitoid series result from a two stage process first fractional crystallisation of clinopyroxene olivine and calcic plagioclase from a basic source with tholeiitic affinities produces a magma of intermediate composition this magma then undergoes periodic mixing with a felsic magma followed by in situ fractionation to generate individual intrusions within granitoid series

generation of sodium rich magmas from newly underplated basaltic crust sodium rich rocks of trondhjemite tonalite dacite ttd or granodiorite ttg suites form much of precambrian continental crust1 they are thought to have formed by partial melting of subducted oceanic crust2 3 a process that would have been much more widespread early in earth history than at present owing to the higher thermal gradients prevailing at that time4 phanerozoic ttd suites do exist however and seem also to relate to subduction zones5 defant and drummond6 proposed that these suites form where young 25 myr hot oceanic lithosphere is subducted and melts thus locally simulating the conditions that led to widespread crustal growth in the archaean here we describe plutonic and volcanic rocks from the cordillera blanca complex in peru which have characteristics of the high al ttd suite but which were produced above a subduction zone containing a 60 myr old slab we present evidence that the complex formed by partial melting of newly underplated basaltic crust and argue that this mechanism should be considered more generally as an additional way of generating sodium rich arc magmas

zircon u pb ages major and trace elements and hf isotope characteristics of the tiantangzhai granites in the north dabie orogen central china tectonic implications cretaceous granites are widespread in the north dabie orogen central china but their emplacement sequence and mechanism are poorly known the tiantangzhai complex in the north dabie complex is the largest cretaceous granitic suite consisting of six individual intrusions in this study zircon u pb ages are used to constrain the crystallization and protolith ages of these intrusions the shigujian granite is a syn tectonic intrusion with an age of 141 ma this granite was emplaced under a compressional regime oscillatory rims of zircons have yielded two peaks at 137 1 ma and 125 1 ma the 137 1 ma peak represents the beginning of orogenic extension and tectonic collapse whereas the 125 1 ma peak represents widespread granitic magmatism zircon cores have yielded concordant ages between 812 and 804 ma which indicate a crystallization age for the protolith the tiantangzhai granites show relatively high sr contents and high la yb and sr y ratios the shigujian granite has positive eu anomalies resulting from partial melting of a plagioclase rich source in an over thickened crust correspondingly in situ lu hf analyses from zircons yield high negative e hf t values from 24 8 to 26 6 with two stage hf model ages from 2748 34 to 2864 40 ma suggesting that the magmas were dominantly derived from partial melting of middle to lower crustal rocks the dabie orogen underwent pervasive nw se extension at the beginning of the early cretaceous associated with subduction of the palaeo pacific plate beneath eastern china

evidence against subduction related magmatism for the jiaoziyan gabbro northern dabie shan china the largest gabbroic intrusion jiaoziyan in the northern orthogneiss of the dabie shan was investigated to test whether such intrusions represent arc related or collision related magma as postulated in recent regional tectonic models the jiaoziyan intrusions comprise a gabbro monzogabbro suite and are locally characterized by cumulus textured anhydrous silicates petrologic indications against arc magmatism include 1 coexisting olivine plagioclase compositions beyond the documented range of arc gabbros and 2 a compositional trend of al ti ratios in the gabbroic clinopyroxenes that is significantly different from subduction related gabbros worldwide in contrast the jiaoziyan rocks are mineralogically similar to rift gabbros geochronologic and geometric arguments against collision related magmatism i e as a consequence of slab breakoff include the following 1 crystallization of the jiaoziyan mafic intrusion postdated the triassic 245 240 ma subduction and peak orogenic metamorphism by 100 m y and 2 the rocks of the dabie cretaceous magmatic suite including jiaoziyan are on the footwall plate rather than on the hanging wall of the triassic subduction zone framework and do not show a linear distribution these observations argue that the voluminous early cretaceous magmatism in the northern orthogneiss is geodynamically unrelated to the qinling dabie orogeny but was caused by extensional tectonism related to regional magmatic underplating beneath the thinned dabie crust

evidence in cerro pampa volcanic rocks for slab melting prior to ridge trench collision in southern south america late miocene approx 12 ma hornblende bearing andesitic to dacitic documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage wasysym pagestyle empty declaremathsizes 10 9 7 6 begin document mathrm 63 to 68 sio 2 1 2 to 1 9 k 2 o end document adakite flows at the small cerro pampa center in patagonia 47 55 s 71 25 have some of the clearest slab melt geochemical signatures yet seen in a phanerozoic center on continental crust these magmas formed in response to melting of the hot thin slab that was subducting beneath south america prior to the collision of the chile rise at approx 6 ma or at approx 10 ma their n morb like 87 sr 86 sr 0 70285 0 70309 and low 206 pb 204 pb 18 44 18 59 ratios show that they could have been generated by

mineral leucosome trace element partitioning in a peraluminous migmatite a laser ablation icp ms study abstract low pressure peraluminous migmatites from the pena negra complex central spain stayed partially molten for 5 10 ma until the latest hercynian deformations allowed the segregation of melt as discordant leucosome veins due to long residence within its source the melt had time enough to equilibrate with residual phases other than accessories included within major refractory minerals we estimated crystal melt partition coefficients as the concentration ratios between leucosome samples appearing to be pure melts and mesosome minerals which were analyzed for trace elements with a laser probe coupled to an icp mass spectrometer our data reveal that when biotite is stable li rb cs tl sc v cr ni nb and ta become strongly compatible the role of biotite in fractionating y th u and the ree is insignificant cordierite strongly fractionates li and be and also has some effect on the hree and u garnet produces extreme fractionation of sc y and the hree the ree partition coefficients for ferromagnesian silicates increase with the atomic number this effect being progressively more important through biotite cordierite and garnet k feldspar strongly fractionates ba and pb but plays a secondary role to biotite for rb and cs k feldspar fractionation does not change the lree hree ratio but plagioclase fractionation produces a significant decrease in the lree hree ratio both feldspars greatly fractionate sr and eu monazite fractionation produces a dramatic depletion in ree th and u as well as a decrease in the lree hree and th u ratios apatite also fractionates the ree although in contrast with monazite it increases the lree hree ratio and does not affect the th u ratio zircon fractionation like apatite and garnet produces a strong depletion in the hree and a concomitant increase in the lree hree ratio in contrast with monazite zircon fractionation causes the th u ratio to increase

petrogenesis of cretaceous adakitic and shoshonitic igneous rocks in the luzong area anhui province eastern china implications for geodynamics and cu au mineralization both adakitic and shoshonitic igneous rocks in the luzong area anhui province eastern china are associated with cretaceous cu au mineralization the shaxi quartz diorite porphyrites exhibit adakite like geochemical features such as light rare earth element lree enrichment heavy ree hree depletion high al2o3 mgo sr sr y and la yb values and low y and yb contents they have low end t values 3 46 to 6 28 and high 87sr 86sr i ratios 0 7051 0 7057 sensitive high resolution ion microprobe shrimp zircon analyses indicate a crystallization age of 136 3 ma for the adakitic rocks most volcanic rocks and the majority of monzonites and syenites in the luzong area are k rich or shoshonitic and were also produced during the cretaceous 140 125 ma they are enriched in lree and large ion lithophile elements and depleted in ti and nb and ba and exhibit relatively lower end t values ranging from 4 65 to 7 03 and relatively higher 87sr 86sr i ratios varying between 0 7057 and 0 7062 the shoshonitic and adakitic rocks in the luzong area have similar pb isotopic compositions 206pb 204pb 17 90 18 83 207pb 204pb 15 45 15 62 and 208pb 204pb 38 07 38 80 geological data from the luzong area suggest that the cretaceous igneous rocks are distributed along ne fault zones e g tanlu and yangtze river fault zones in eastern china and were likely formed in an extensional setting within the yangtze block the shaxi adakitic rocks were probably derived by the partial melting of delaminated lower crust at pressures equivalent to crustal thickness of 50 km i e 1 5 gpa possibly leaving rutile bearing eclogitic residue the shoshonitic magmas in contrast originated mainly from an enriched mantle metasomatized by subducted oceanic sediments they underwent early high pressure 1 5 gpa fractional crystallization at the boundary between thickened 50 km lower crust and lithospheric mantle and late low pressure 1 5 gpa fractional crystallization in the shallow 50 km crust the adakitic and shoshonitic rocks appear to be linked to an intra continental extensional setting where partial melting of enriched mantle and delaminated lower crust was probably controlled by lithospheric thinning and upwelling of hot asthenosphere along ne fault zones e g tanlu and yangtze river fault zones in eastern china both the shoshonitic and adakitic magmas were fertile with respect to cu au mineralization

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

systematics of internal zircon morphology in major variscan granitoid types the internal morphologies of zircon crystals from different types of granitoids alkaline calcalkaline and anatectic are revealed by cathodoluminescence imaging and are described in terms of growth rates of the crystal faces relative to each other zircons in the alkaline granitoids are characterized by high and constant growth rates of 010 relative to the pyramidal forms and by symmetric grwoth of 011 zircons in the calcalkaline and anatectic granitoids are characterized by fluctuating or gradually decreasing relative growth rates of 010 by asymmetric and highly variable growth of 011 and by a tendency of 110 to become grwoth inhibited corrosion events are interspersed during zircon growth in the calcalkaline magmas in the calcalkaline and anatectic magmas a discontinuity breaks the morphological evolution at late stages of crystallization the discontinuity coincides with a sharp drop in cathodoluminescence the growth behaviour of each crystal form is analysed and compared with predictions made by the periodic bond chain pbc theory it is argued that the relative growth rate of 010 depends on supersaturation that the growth rates of 011 faces are changed in response to different ratios of adsorbing cations na k al and that 110 faces become growth inhibited by the adsorption of h2o or trace elements enriched in the residual liquid morphological and chemical discontinuities at late stages of crystallization are reasonably explained by the formation of larger growth units from smaller ionic entities in the residual liquid important factors controlling the zircon morphology in different types of granitoids are high cooling rates alkaline magmas magma mixing calcalkaline magmas enrichment of h2o and trace elements in residual liquids calcalkaline and anatectic magmas and the major element chemistry of the magma possibly the ratio of na and k to al agpaicity

crustal generation of the himalayan leucogranites abstract detailed studies of the himalayan two mica leucogranites such as the manaslu pluton indicate that they have very uniform mineralogical petrological and structural characteristics one can relate their occurrence to the thickest zones of the underlying tibetan slab in these zones migmatization attains its greatest development and vertical extension the granite is emplaced at first along the main disharmonic plane above the main central thrust mct at the top of the tibetan slab infrastructure ductile deformation of the granite is variable the granite being syn to late kinematic with regard to the functioning of the mct major elements are very homogeneous except for na and k implying that p t conditions of melting were relatively uniform the melted material was of a similar composition over a vast volume and the percentage of melting was small 10 15 trace elements are highly variable some are characteristic for very evolved material ta rb cs u or show the link with the tibetan slab ba sr whilst others are problematic th ree ree and th abundances being much less in the granite than in the tibetan slab imply that they have been extracted during one of the main stages of formation possibly by monazite radiogenic pb sr nd and stable o isotopes are consistent with the origin of the granite from the tibetan slab however the heterogeneous sr isotopic ratios make age dating difficult and imply poor mixing or little fluid interaction during its evolution h isotope data indicate that magmatic compositions of the main body of the manaslu granite have been preserved late or post magmatic alterations are extremely local in the main pluton to the north another belt of two mica granites occurs whose characteristics are very similar to the high himalaya belt they were probably generated in a similar way during this recent intracontinental evolution

two stages of zircon crystallization in the jingshan monzogranite bengbu uplift implications for the syn collisional granites of the dabie sulu uhp orogenic belt and the climax of movement on the tan lu fault abstract the detailed study of zircon can provide vital clues about the petrogenetic environment of granitoid rocks morphological and chemical studies of zircon grains from the jingshan monzogranite in the bengbu uplift anhui province eastern china identify three phases of growth zircon i is brown translucent ovoid and occurs as inherited cores it shows two groups of 206pb 238u ages corresponding to the country rock of the jingshan monzogranite and the basement of the bengbu uplift zircon ii is colorless transparent and idiomorphic hypidiomorphic with ipr 0 34 0 52 ipy 0 03 0 24 and iel 0 26 0 34 it shows a very bright cl due to high contents of trace elements e g y u and th and the oscillatory zoning associated with sector zoning it contains 1 40 1 66 wt hfo2 with a mean ehf t of 17 88 these features indicate the igneous crystallization of zircon ii from a peraluminous granite of mainly crustal origin based on the field geology petrography geochemical analysis and especially a weighted mean age of 222 ma for zircon ii we argue that the jingshan monzogranite is a syn collisional granite of the dabie uhp orogenic belt this provides new evidence for the northward subduction of the south china block beneath the north china block just before 222 ma and allows quantitative estimates of the rate of post uhp exhumation in the dabie orogenic belt zircon iii is also colorless and transparent but is totally idiomorphic with ipr 0 41 1 00 ipy 0 88 1 00 and iel 0 39 0 83 it occurs only as overgrowths on zircon ii and shows weak cl due to its depletion in trace elements zircon iii has widely variable contents of hfo2 1 12 to 3 01 wt and hf isotope compositions very similar to those of zircon ii these features suggest crystallization of zircon iii from small volumes of leucosome probably in the beginning stages of migmatisation zircon iii has a weighted mean age of 156 ma interpreted as representing the climax of movement on the tan lu fault this suggests that the compressive movement of the tan lu fault has decreased progressively since its climax at 156 ma these two major conclusions from the zircon studies contrast with previous interpretations of the jingshan monzogranite as a late jurassic post collisional granite in the dabie orogenic belt

zircon behaviour during crustal anatexis evidence from the tibetan slab migmatites nepal abstract the morphology and internal structures of zircon populations from three distinct types of migmatites from central nepal low zr and high zr in situ migmatites and injection migmatite were studied to examine the behaviour of zircon during anatexis the zircons from mesosomes and melanosomes were all residual zircons characterized by rounded morphologies with little overgrowth and a small elongation ratio suggestive of a detrital or metamorphic origin zircons from the leucosomes showed distinctive features the low zr tonalitic leucosomes we characterized by zircon populations with the same features as those from mesosomes and melanosomes indicative of no or very few transformations during migmatization the high zr leucosome contained euhedral crystals with morphological features typical of magmatic zircons found in crustal peraluminous magmas internal zoning patterns suggested a complex evolution involving dissolution and growth the injected granitic leucosome contained well developed magmatic zircons the role of subsolidus differentiation partial melting and melt composition on the contrasted behaviour of zircons during anatexis is discussed the zircon populations from the tibetan slab migmatites are distinct from those of the manaslu granite therefore suggesting that the migmatite leucosomes are not the unsegregated equivalents to the melts which gave raise to the manaslu type granites

the nature origin and thermal influence of the granite source layer of central iberia abstract the central iberian zone comprises low grade metamorphic zones alternating with variscan thermal domes apparently equilibrated at the same pressure the variscan thermal domes are composed of authochtonous to allochtonous granites and anatectic complexes that represent remnants of the granite source layer the anatectic complexes consist of 90 high grade metasediments and 10 cadomian documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage ot2 ot1 fontenc newcommand cyr renewcommand rmdefault wncyr renewcommand sfdefault wncyss renewcommand encodingdefault ot2 normalfont selectfont declaretextfontcommand textcyr cyr pagestyle empty declaremathsizes 10 9 7 6 begin document landscape 543 pm 6 end document ma felsic metaigneous rocks the latter being extremely ra

depositional age and provenance of the marboo formation from shrimp u pb zircon geochronology implications for the early palaeoproterozoic tectonic evolution of the kimberley region western australia abstract the marboo formation is the oldest rock unit in the hooper complex in the west kimberley region of western australia the complex forms an elongate belt 350 km long and at least 100 km wide of deformed and metamorphosed palaeoproterozoic low to high grade metasedimentary rocks and mafic and felsic igneous rocks the marboo formation consists of a sequence of turbiditic metasedimentary rocks derived from the northwest an area of crust now buried beneath the unconformably overlying speewah and kimberley basins shrimp u pb dating of detrital zircons from a low grade quartz wacke indicate that the provenance terrain contributed zircons dated at around 2500 to 2400 ma 2300 ma 2200 to 2050 ma 1970 ma and 1910 ma the mount joseph migmatite formed by in situ melting of the marboo formation and zircons interpreted as detrital in origin give ages of 2500 to 2000 ma a maximum depositional age is provided by a population of zircon cores dated at 1872 9 ma growth of rims on zircons is dated at 1861 5 ma and is interpreted as the age of high grade metamorphism a minimum age for deposition is provided by the richenda microgranodiorite which forms a high level intrusive stock into the marboo formation and is dated at 1858 5 ma comparison with recent shrimp ages from the halls creek group in the east kimberley indicates that the turbidites of the marboo formation were deposited deformed metamorphosed and intruded by granitoid before the majority of the turbiditic metasedimentary rocks in the olympio formation were deposited this is not consistent with deposition of the turbiditic rocks in the same basin the assumption of contemporaneous turbidite deposition throughout the kimberley region has been taken as evidence of intracratonic palaeoproterozoic orogeny however these results provide support for plate tectonic models and the recognition of tectonostratigraphic terranes within the kimberley region detrital zircons from the marboo formation suggest that unexposed crust beneath the speewah and kimberley basins is largely early palaeoproterozoic in age rather than archaean as previously assumed the spread in ages indicates that the time period between formation of late archaean basement in northern australia and the formation of palaeoproterozoic orogens was marked by ongoing crust formation and was not a period of quiessence

peeling oceanic crust in subduction zones thin slabs of oceanic pillow basalt formed at ridge axes with associated pelagic sediments are common features in ancient accretionary complexes estimates of the average thicknesses of scraped off basaltic sections of oceanic crust in accretionary prisms vary from a few tens of metres to 300 m with a mean thickness of 80 100 m in the complexes studied the permeability contrast within the uppermost part of the oceanic crust is an important factor governing the thickness of delaminated oceanic crust this mechanism is an effective means of separating oceanic crust altered at low temperatures from the remainder of the downgoing slab and thus controlling geochemical budgets in subduction zones

the lanthanide tetrad effect and its correlation with k rb eu eu sr eu y ho and zr hf of evolving peraluminous granite suites lanthanide tetrad effects are often observed in ree patterns of more highly evolved variscan peraluminous granites of mid eastern germany central erzgebirge western erzgebirge fichtelgebirge and northern oberpfalz the degree of the tetrad effect te1 3 is estimated and plotted vs k rb sr eu eu eu y ho and zr hf the diagrams reveal that the tetrad effect develops parallel to granite evolution and significant tetrad effects are strictly confined to highly differentiated samples mineral fractionation as a cause for the tetrad effect is not supported by a calculated rayleigh fractionation which also could not explain the fractionation trends of sr eu and eu eu the strong decrease of eu concentrations in highly evolved rocks suggests that eu fractionates between the residual melt and a coexisting aqueous high temperature fluid mineral fractionation as a reason for the tetrad effect is even more unlikely as ree patterns of accessory minerals display similar tetrad effects as the respective host rocks the accessory minerals inherit the ree signature of the melt and do not contribute to the bulk rock tetrad effect via mineral fractionation these results point in summary to significant changes of element fractionation behavior in highly evolved granitic melts ionic radius and charge which commonly control the element distribution between mineral and melt are no longer the exclusive control the tetrad effect and the highly fractionated trace element ratios of y ho and zr hf indicate a trace element behavior that is similar to that in aqueous systems in which chemical complexation is of significant influence this distinct trace element behavior and the common features of magmatic hydrothermal alteration suggest the increasing importance of an aqueous like fluid system during the final stages of granite crystallization the positive correlation of te1 3 with bulk rock fluorine contents hints at the importance of ree fluorine complexation in generating the tetrad effect as the evolution of a ree pattern with tetrad effect m type implies the removal of a respective mirroring ree pattern w type the tetrad effect identifies open system conditions during granite crystallization

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

in situ zircon u pb oxygen and hafnium isotopic compositions of jurassic granites from the north china craton evidence for triassic subduction of continental crust and subsequent metamorphism related 18o depletion abstract six samples of the linglong granitic pluton from the southeastern margin of the north china craton yield similar jurassic magmatic zircon u pb ages and evolved whole rock sr nd isotopic compositions they are weakly peraluminous to strongly peraluminous with a cnk molar al 2 o 3 cao na 2 o k 2 o ratios of 1 05 to 1 3 except one sample with an a cnk of 0 98 however they have contrasting inherited zircon ages and magmatic zircon hf and o isotopic compositions two samples have 2 5 ga inherited zircons and high 18 o magmatic zircons and are interpreted to be derived dominantly from the lower crust of the north china craton another two samples with dominantly triassic inherited zircons and low 18 o magmatic zircons are considered to be derived mainly from the subducted continental crust of the south china craton and the source was considered to be significantly depleted in 18 o during triassic metamorphism the rest two samples have dominantly neoproterozoic inherited zircons and intermediate 18 o magmatic zircons and are also considered to be derived mainly from the subducted continental crust of the south china craton but the source was not significantly depleted in 18 o during triassic metamorphism the different lithologies whole rock e nd t and zircon e hf t between the samples also strongly support that they are derived from different sources although magmatic zircons from the six samples have variable 18 o from 0 7 to 8 5 and e hf t from 27 6 to 11 8 individual samples have more limited e hf t and 18 o the 18 o and e hf t of magmatic zircons from the samples indicate no contribution of mantle derived magmas to the linglong granites this is supported by the fact of no coeval mantle derived rocks in the region the results suggest that large variations of e hf t and 18 o do not necessarily indicate mixing between mantle and crustal derived magmas instead they may reflect heterogeneity in the source the granites are interpreted to result from partial melting of the thickened crust caused by triassic subduction of the south china craton continental crust beneath the north china craton subsequent lithospheric extension and orogenic collapse are the most likely mechanism for the granites

distribution of displacement on and evolution of a young transform fault system the northern san andreas fault system california this paper presents a working model for the spatial and temporal distribution of dextral slip on the northern san andreas fault system of coastal california based primarily on field relations in the san francisco bay area and offers insight into the evolution of a young transform fault system a fundamental difference between this and previous models of slip distribution is that this model assigns negligible slip to the pilarcitos fault which has been suggested to have 120 to 250 km of post early miocene dextral slip in previous models because separation on the san francisco peninsula reach of the san andreas fault is about 25 km and displacement on the san andreas fault in central california is 310 to 320 km more than 250 km of late cenozoic dextral slip must be accommodated east of the san francisco peninsula the distribution of this dextral slip is constrained by offset late cenozoic and basement units slip distribution combined with ages of offset features and plate boundary kinematic constraints show that the distribution of slip rates on groups of faults along the transform boundary has changed in an irregular fashion through time in contrast to existing models that propose progressive eastward migration of active faulting in the san andreas system in addition to the shifting patterns of displacement a migrating transition zone connecting the eastern faults of the strike slip system to the mendocino triple junction may have resulted in distributed dextral faulting and shortening in the northernmost and youngest part of the transform fault system

structural and stratigraphical constraints on the kinematics history of the southern tan lu fault zone during the mesozoic anhui province china abstract the tan lu fault zone tlfz extends in a nne ssw direction for more than 2000 km in eastern china it has been considered either as a major sinistral strike slip fault as a suture zone or as a normal fault we have conducted a structural analysis of the southern segment of this fault zone stlfz in the anhui province the ages triassic to palaeocene of the formations affected by the faults have been re appraised taking into account recent stratigraphical studies to better constraint the ages of the successive stages of the kinematics of the stlfz subsequently the kinematics of the faults is presented in terms of strain stress fields by inversion of the striated fault set data finally the data are discussed in the light of the results obtained by previous workers we propose the following history of the stlfz kinematics during the mesozoic at the time of collision a nne orientated tan lu margin probably connected two margins located north of the dabie and sulu collision belts during the middle late triassic the scb has been obliquely subducted below the ncb along this margin which has acted as a compressional transfer zone between the dabie and sulu continental subduction zones the stlfz has been initiated during the early jurassic and has acted as a sinistral transform fault during the jurassic following which the ncb scb collision stopped a nw trending extension related to metamorphic domes was active during the basal early cretaceous 135 130 ma it has been followed by a nw se compression and a ne sw tension during the middle late early cretaceous 127 to 105 ma possibly 95 ma at that time the tlfz was a sinistral transcurrent fault within the eastern part of the asian continent during the late cretaceous palaeocene the stlfz was a normal fault zone under a wnw ese tension

timing of synorogenic granitoids in the south qinling central china constraints on the evolution of the qinling dabie orogenic belt abstract the ca 400 km long granitoid belt in the south qinling is believed to be a synorogenic product of the collision between the north and south china blocks along the qinling dabie orogenic belt in central china single and multigrain zircon u pb dating of six of these granitoid bodies indicate that the granitoids were formed between documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage ot2 ot1 fontenc newcommand cyr renewcommand rmdefault wncyr renewcommand sfdefault wncyss renewcommand encodingdefault ot2 normalfont selectfont declaretextfontcommand textcyr cyr pagestyle empty declaremathsizes 10 9 7 6 begin document landscape 220 pm 1 end document and documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage m

paleomagnetism of basic intrusions from the brock inlier northwest territories canada basic intrusions of the brock inlier of the northwest territories are shown by means of paleomagnetism to be related to the franklin diabases specifically the coronation suite of the canadian shi

paleocene eocene record of ophiolite obduction and initial india asia collision south central tibet 1 uppermost cretaceous to eocene marine sedimentary sequences occur both to the south and north of the yarlung zangbo suture in south central tibet they consist of indian margin strata of the northern tethyan himalaya and asian margin strata of the gangdese forearc both assemblages are characterized by major changes in depositional environment and sedimentary provenance at 65 ma and an appearance of detrital chromium rich spinel of ophiolite affinity tio2 generally 0 1 wt during the paleocene ophiolitic melange exposed along the suture could have provided a source for detrital spinel the melange occurs in the hanging wall of a north dipping south directed mylonitic shear zone which includes a tectonic sliver of mafic schist amphibole from the schist yields 40ar 39ar ages of 63 ma which we attribute to cooling during slip along the shear zone and southward obduction of the melange melange obduction was coeval with the development of an angular unconformity within the gangdese forearc basin to the north between late maastrichtian time and 62 ma upper paleocene to middle eocene sandstones in the northern tethyan himalaya yield 200 120 ma u pb detrital zircon ages and 190 170 ma 40ar 39ar detrital mica ages these detrital grains were most likely sourced from regions north of the yarlung zangbo suture suggesting that onset of india asia collision in south central tibet is middle eocene or older in age collectively our results support previous suggestions that oceanic rocks were obducted onto the northern margin of india during latest cretaceous earliest tertiary time coeval changes in gangdese forearc sedimentation raise the possibility that this obduction event marks onset of tectonic interaction between india and asia at 65 ma alternatively in concert with the conventional view of eocene collision initiation the obducted oceanic rocks may be of intraoceanic origin while coeval changes in gangdese forearc sedimentation may be a consequence of an increase in the rate of ocean continent convergence following the demise of the intraoceanic subduction zone

isotopic and paleomagnetic constraints on the mesozoic tectonic evolution of south china in order to better constrain the paleogeographic evolution of south china we measured sm nd and rb sr isotopic compositions for 23 mesozoic granites that crop out throughout the area tightly grouped neodymium depleted mantle model ages 1 4 0 3 ga suggest the region is underlain by relatively homogeneous proterozoic crust and fail to define crustal provinces neither the isotopic nor geologic data suggest that a mesozoic suture exists however granites possessing anomalously high sm 8 ppm and nd 45 ppm concentrations relatively high initial epsilon neodymium 4 to 8 and high but variable initial 87sr 86sr 0 759 to 0 713 form a northeast trending zone that coincides with two prominent mesozoic basins southeast of the zone lie the majority of mesozoic intrusives and upper triassic to lower cretaceous extensional basins found in south china mesozoic paleomagnetic poles are well clustered northwest of the zone pre cretaceous poles southeast of it are discordant with respect to those from the northwest the only recognized tectonostratigraphic terrane in south china lies southeast of the zone the terrane is bordered by a northeast trending sinistral fault that was active in the mesozoic other faults in south china have similar attitudes ages and sense of shear together the observations suggest that the mesozoic tectonic regime in south china consisted of strike slip activity plus concomitant rifting as terranes or fragments of similar crust were transported north along sinistral faults the zone defined by the granites enriched in nd and sm demarcates displaced terranes to the southeast from relatively stable land to the northwest

giant versus small porphyry copper deposits of cenozoic age in northern chile adakitic versus normal calc alkaline magmatism cenozoic magmatic activity in northern chile led to the formation of two contrasting porphyry copper belts 1 a paleocene early eocene belt comprising small porphyry copper deposits e g lomas bayas of normal calc alkaline affinity and 2 a late eocene early oligocene belt hosting huge porphyry copper deposits e g chuquicamata of adakitic affinity although the first belt comprises both volcanic and plutonic rocks andesitic basaltic and rhyolitic lavas and tuffs and associated sub volcanic porphyries and felsic stocks the latter only includes intrusions mostly granodioritic types including porphyry copper deposits we suggest that the late eocene early oligocene belt formed when fast and oblique convergence between the south america and farallon plates led to flat subduction and direct melting of the subducting plate hence giving rise to plutonic rocks of adakitic affinity the absence of volcanism under prevailing compressional conditions prevented the escape of so2 from the adakitic sulfur rich highly oxidized magmas closed porphyry system which allowed formation of huge mineral deposits on the contrary coeval volcanic activity during formation of the paleocene early eocene calc alkaline porphyries allowed development of open systems hence to outgassing and therefore to small mineral deposits

u pb zircon and titanite systematics of the fish canyon tuff an assessment of high precision u pb geochronology and its application to young volcanic rocks a large data set of single and multi grain zircon and titanite analyses from a sample of the oligocene fish canyon tuff fct a voluminous ash flow from the san juan mountains of colorado and widely used 40ar 39ar geochronological standard has been used to evaluate the influence of various sources of analytical and geological uncertainty on the calculated age of this tuff by the isotope dilution u pb zircon method twenty three single zircon grains and seven small multi grain fractions of the fct yield an inverse variance weighted mean 206pb 238u date of 28 402 0 023 ma 2 mswd 0 93 and a slightly older weighted mean 207pb 235u date of 28 529 0 030 ma mswd 0 74 which are insensitive to common pb corrections initial 230th disequilibria calculated from a newly measured th u 2 2 for fct pumice shards indicate th deficiency for these zircons its correction brings the weighted mean 206pb 238u date to 28 478 0 024 mswd 0 97 minimizing the discordance of the fct zircon analyses these calculations culminate in concordia ages ludwig 1998 for the crystallization of the fct zircons of 28 476 0 029 ma mswd 1 50 without propagating decay constant errors or 28 498 0 035 ma mswd 1 03 with propagated decay constant errors these results are in close agreement with a previous single grain 206pb 238u zircon date of 28 41 0 05 ma for the fct oberli et al 1990 however they do not corroborate the discordia lower intercept date of 27 52 0 09 put forth by lanphere and baadsgard 2001 the 28 476 0 064 ma concordia age for fct zircons including systematic decay constant errors and uncertainty in pb u tracer calibration is substantially older than all published 40ar 39ar sanidine ages which range from 27 5 to 28 05 ma lanphere and baadsgaard 1997 renne et al 1998 a lengthy period 400 k y of zircon residence in the magma chamber before eruption is commonly called upon to explain the age discrepancy however fct titanite which may remain open to pb diffusive exchange until quenching upon eruption yield a minimum 230th disequilibrium corrected weighted mean 206pb 238u age of 28 395 0 049 ma 0 078 with tracer uncertainty weakening residence time arguments and supporting recent calls for re evaluation of the 40k decay constant and the accepted ages for commonly used 40ar 39ar mineral standards the fct zircon concordia age also provides an empirical constraint on the decay constants of the u isotopes indicating the relative accuracy of the currently accepted decay constants to within the original counting statistic errors of jaffey et al 1971

partial melting of thickened or delaminated lower crust in the middle of eastern china implications for cu au mineralization abstract field relations isotope systematics and plate tectonic reconstructions require that felsic adakites in the yangtze block and the dabie orogen eastern china were not derived from a subducting slab despite the signature of a mantle component in the contemporaneous mafic adakite hosts of cu au deposits in the same areas the apparently contradictory requirements are accounted for by a a deep crustal melting origin for barren adakites and b a crustal delamination origin followed by ascent through lithospheric mantle for adakites associated with mineralization the crustal delamination process associated with the prospective porphyries duplicates the metallogenically essential aspects of the subduction environment the importance of adakitic magmas in the genesis of porphyry style cu au deposits is affirmed by these findings but the range of prospective tectonic environments is extended to include an important intraplate postsubduction setting the porphyries of eastern china demonstrate

u pb age and genetic significance of heterogeneous zircon populations in rocks from the favourable lake area northwestern ontario the study evaluates the relationships between measured u pb ages and zircon characteristics of five morphologically texturally and isotopically complex zircon populations and compares the zircon development stages to the orogenic evolution of the favourable lake area two distinct zircon types from a hornblendite xenolith in a granitoid batholith of the sachigo subprovince of the superior province yield u pb ages of 2729 0 6 8 ma and 2714 8 6 4 7 4 ma which date specific metamorphic phases coinciding with major plutonic pulses in the batholith zircons from a metamorphosed felsic dike crosscutting the hornblendite consist of an old zircon component with a minimum age of 2788 ma possibly reflecting igneous crystallization 2950 ma ago and a younger component with an inferred age of 2725 15 ma probably reflecting metamorphism during batholith emplacement in the berens river subprovince to the south granodiorite forms both a late tectonic phase in a large batholith and a post tectonic pluton intruded into the batholith yet zircons from these granodiorites have identical ages of 2697 3 1 7 ma and 2696 2 1 2 ma respectively the late tectonic granodiorite also contains inherited zircons with a minimum age of 2767 ma which are indirect evidence for the presence of old sialic crust in this subprovince zircons from a sheared monzonite near the boundary fault between the two subprovinces yield an upper intercept age of 2769 26 63 ma which we interpret as the intrusion age of the monzonite this rock is older than most dated units in the surrounding batholiths and suggests that the boundary is a long lived archean structure a lower intercept age of about 1750 ma for zircons of the hornblendite is the result of chemical alteration of the zircons this and a similar lower intercept age shown by the sheared monzonite zircons are thought to reflect increased fluid activity and possibly shearing during the early proterozoic hudsonian orogeny which occurred in the churchill province to the northwest a later pb loss mainly from near surface domains of the zircons is indicated by lower intercept ages of about 500 100 ma

os os dating of copper and molybdenum deposits along the middle and lower reaches of the yangtze river china the 186 os 187 os and 188 os 187 os dating techniques were modified and applied to molybdenites from two cu vein deposits and three mo skarn deposits along the middle and lower reaches of the yangtze river china the results show that these deposits formed at 138 to 133 ma identical to the ages of 140 to 130 ma for the host granitoids re os and os os ages of two of the samples agree within analytical uncertainties the close temporal relationship between the granitoid intrusions and the formation of the deposits supports a magmatic model for the origin of these deposits the results show that the novel os os dating technique is capable of resolving geologic problems although the accuracy obtained by the present work 1 2 is lower than that of the re os method

ela icp ms u pb zircon geochronology of regional volcanism hosting the bajo de la alumbrera cu au deposit implications for porphyry related mineralization ela icp ms u pb zircon geochronology has been used to show that the porphyritic intrusions related to the formation of the bajo de la alumbrera porphyry cu au deposit nw argentina are cogenetic with stratigraphically well constrained volcanic and volcaniclastic rocks of the late miocene farallon negro volcanic complex zircon geochronology for intrusions in this deposit and the host volcanic sequence show that multiple mineralized porphyries were emplaced in a volcanic complex that developed over 1 5 million years volcanism occurred in a multi vent volcanic complex in a siliciclastic intermontane basin the complex evolved from early mafic intermediate effusive phases to a later silicic explosive phase associated with mafic intrusions zircons from the basal mafic intermediate lavas have ages that range from 8 46 0 14 to 7 94 0 27 ma regionally extensive silicic explosive volcanism occurred at 8 0 ma 8 05 0 13 and 7 96 0 11 ma which is co temporal with intrusion of the earliest mineralized porphyries at bajo de la alumbrera 8 02 0 14 and 7 98 0 14 ma regional uplift and erosion followed during which the magmatic hydrothermal system was probably unroofed shortly thereafter dacitic lava domes were extruded 7 95 0 17 ma and rhyolitic diatremes 7 79 0 13 ma deposited thick tuff blankets across the region emplacement of large intermediate composition stocks occurred at 7 37 0 22 ma shortly before renewed magmatism occurred at bajo de la alumbrera 7 10 0 07 ma the latest porphyry intrusive event is temporally associated with new ore bearing magmatic hydrothermal fluids other dacitic intrusions are associated with subeconomic deposits that formed synchronously with the mineralized porphyries at bajo de la alumbrera however their emplacement continued from 7 10 0 06 to 6 93 0 07 ma after the final intrusion at bajo de al alumbrera regional volcanism had ceased by 6 8 ma 6 92 0 07 ma

constraints of hydrothermal and magmatic zircon on the origin of the yaogangxian tungsten deposit southern china abstract the yaogangxian tungsten deposit located in the central nanling range is a well known vein type tungsten deposit and has been mined for centuries this study focuses on the morphology geochronology and geochemistry of zircon crystals from wolframite bearing quartz veins and coexisting alkali feldspar granite to better understand the source of ore forming materials and the exact timing of tungsten mineralization all zircon grains separated from the muscovite alkali feldspar granite and the wolframite bearing quartz veins show core to rim structure the overgrowth rims of the zircon grains from the muscovite alkali feldspar granite and the wolframite bearing quartz veins have crystallization ages of 133 4 1 3 ma and 133 7 1 3 ma whereas the relict cores have ages of 155 3 1 6 ma and 154 8 1 6 ma respectively the ages of the cores are identical to the crystallization age of the hosting yaogangxian biotite monzogranite thus these cores were derived from the biotite monzogranite whereas the overgrowth rims should have crystallized during the intrusion of muscovite alkali feldspar granite and wolframite bearing quartz veins the geochemical consanguinity and similar ages of the muscovite alkali feldspar granite and the ore veins suggest that they were both formed from the highly fractionated granitic magma in an extensional setting this highly fractionated granitic melt might be extracted from the biotite monzogranitic mush in a long surviving deep seated magma chamber

pressure temperature time paths of regional metamorphism i heat transfer during the evolution of regions of thickened continental crust the development of regional metamorphism in areas of thickened continental crust is investigated in terms of the major controls on regional scale thermal regimes these are the total radiogenic heat supply within the thickened crust the supply of heat from the mantle the thermal conductivity of the medium and the length and time scales of erosion of the continental crust the orogenic episode is regarded as consisting of a relatively rapid phase of crustal thickening during which little temperature change occurs in individual rocks followed by a lengthier phase of erosion at the end of which the crust is at its original thickness the principal features of pressure temperature time ptt paths followed by rocks in this environment are a period of thermal relaxation during which the temperature rises towards the higher geotherm that would be supported by the thickened crust followed by a period of cooling as the rock approaches the cold land surface the temperature increase that occurs is governed by the degree of thickening of the crust its conductivity and the time that elapses before the rock is exhumed sufficiently to be affected by the proximity of the cold upper boundary for much of the parameter range considered the heating phase encompasses a considerable portion of the exhumation decompression part of the ptt path in addition to the detailed calculation of ptt paths we present an idealized model of the thickening and exhumation process which may be used to make simple calculations of the amount of heating to be expected during a given thickening and exhumation episode and of the depth at which a rock will start to cool on its ascent path an important feature of these ptt paths is that most of them lie within 50 c of the maximum temperature attained for one third or more of the total duration of their burial and uplift and for a geologically plausible range of erosion rates the rocks do not begin to cool until they have completed 20 to 40 per cent of the total uplift they experience considerable melting of the continental crust is a likely consequence of thickening of crust with an average continental geotherm a companion paper discusses these results in the context of attempts to use metamorphic petrology data to give information on tectonic processes 1984 oxford university press

silurian devonian provenance changes of south qinling basins implications for accretion of the yangtze south china to the north china cratons abstract the north china and yangtze south china cratons are the most important tectonic units of china the two cratons finally coalesced through collision with the formation of the e w trending qinling mountains that are considered to be the eastern extreme of the paleotethys the north qinling and south qinling belts constitute the southern active margin of the north china craton and the northern passive margin of the yangtze craton respectively we have studied chemical compositions of 573 fine grained clastic sedimentary rocks of all the early proterozoic to neogene ages from the north china craton the yangtze craton and the north and south qinling orogenic belts as well as their ten tectonostratigraphic subunits the purpose of this study is to investigate possible provenance changes that are expected to have been recorded in the phanerozoic clastic sediments of the south qinling basins if north and south china had approached and accreted because the north china craton north qinling belt and yangtze craton have distinct upper crustal and provenance compositions both the north china and yangtze cratons and the north qinling belt are characterized by a relatively uniform non evolutionary composition of clastic sediments in la th sc co systematics during post archean times they can be however distinguished from each other in provenance in terms of the la th sc th th co as well as k2o na2o ratios in contrast there is a significant provenance change in the silurian devonian clastic sediments from the baiyun zhashui liuba yunxian and western xichuan subunits of the south qinling belt the late sinian 700 600 ma cambrian and ordovician pelites show characteristics that are consistent with the yangtze craton being high in la th 3 5 10 and sc th 1 4 2 4 and low in th co 0 2 1 0 and lacking la th ratios 3 5 these are accompanied by the increasing immaturity of the silurian devonian clastic sediments illustrated by the decreasing k2o na2o ratio which is attributed to the near source of the north qinling belt dominated by low k2o island arc igneous suites and immature metagraywackes we propose that the south qinling basins except the eastern xichuan subunit had been in a close proximity and accreted to the north qinling belt in silurian devonian times the result agrees well with recent studies of benthic faunas granite distribution age of granulite and clastic provenance directions of the south qinling devonian sediments but it is apparently inconsistent with the age of coesite and diamond bearing ecologite in the dabie mountains to the east and with paleomagnetic data both suggesting an important triassic jurassic collision event between north and south china it is likely that the silurian devonian and the mesozoic represent two different episodes phases of accretion and collision between north and south china new models regarding the phanerozoic tectonic evolution of east china are needed to reconcile the contrasting evidence