highly fractionated i type granites in ne china ii isotopic geochemistry and implications for crustal growth in the phanerozoic ne china is the easternmost part of the central asian orogenic belt caob the area is distinguished by widespread occurrence of phanerozoic granitic rocks in the companion paper part i we established the jurassic ages 184 137 ma for three granitic plutons xinhuatun lamashan and yiershi we also used geochemical data to argue that these rocks are highly fractionated i type granites in this paper we present sr nd o isotope data of the three plutons and 32 additional samples to delineate the nature of their source to determine the proportion of mantle to crustal components in the generation of the voluminous granitoids and to discuss crustal growth in the phanerozoic despite their difference in emplacement age sr nd isotopic analyses reveal that these jurassic granites have common isotopic characteristics they all have low initial 87 sr 86 sr ratios 0 7045f0 0015 positive end t values 1 3 to 2 8 and young sm nd model ages 720 840 ma these characteristics are indicative of juvenile nature for these granites other late paleozoic to mesozoic granites in this region also show the same features sr nd and oxygen isotopic data suggest that the magmatic evolution of the granites can be explained in terms of two stage processes 1 formation of parental magmas by melting of a relatively juvenile crust which is probably a mixed lithology formed by pre existing lower crust intruded or underplated by mantle derived basaltic magma and 2 extensive magmatic differentiation of the parental magmas in a slow cooling environment the widespread distribution of juvenile granitoids in ne china indicates a massive transfer of mantle material to the crust in a post orogenic tectonic setting several recent studies have documented that juvenile granitoids of paleozoic to mesozoic ages are ubiquitous in the central asian orogenic belt hence suggesting a significant growth of the continental crust in the phanerozoic d 2003 elsevier science b v all rights reserved

an isotopic mixing model for the origin of granitic rocks in southeastern australia a simple mixing process was a significant element in the genesis of many southeastern australian granitic rocks major and trace element abundances are near linearly correlated nd and sr initial isotopic compositions define a simple hyperbolic trend and many initial 87sr 86sr ratios are a regular function of rb sr ratios geological arguments and regression of geochemical variation diagrams place limits on the nature of the end members the high si end member is meta sedimentary derived magma comparable in composition to the cooma granodiorite the one pluton in the region surrounded by and clearly derived from regional metamorphic rocks the low si end member is basaltic material with similarities to rare gabbros also found in the area isotope dilution calculations based upon initial sr and nd isotopic compositions deduced for the end members yield realistic hypothetical sr and nd concentrations for the end members in a demonstration of the internal consistency of the model all of the granitic rocks from hornblende tonalites to cordierite granodiorites are a single broad family products of variable mixing between distinct batches of basaltic material and a uniform partial melt of the regional basement

depleted mantle source for the ulungur river a type granites from north xinjiang china geochemistry and nd sr isotopic evidence and implications for phanerozoic crustal growth abstract the ulungur river granites from north xinjiang china display two petrological trends 1 a peralkaline trend composed of alkali amphibole and alkali pyroxene bearing granites and dykes and 2 an aluminous trend represented by alkali granites monzogranites and aluminous granite dykes within the peralkaline granites rb sr whole rock isochron dating yielded an age of 300 ma for the emplacement of the peralkaline granites and 270 ma for the aluminous granites the ulungur river granites share all the geochemical features common to a type granites of the world the ulungur river granites are post orogenic and emplaced in a post collisional extensional environment the peralkaline and aluminous rocks have indistinguishable nd isotopic compositions they have unusually positive and relatively uniform nd t values ranging from 5 1 to 6 7 for the peralkaline rocks at 300 ma and from 5 5 to 5 9 for the aluminous rocks at 270 ma the initial 87sr 86sr ratios cannot be determined precisely by the whole rock isochron technique mainly because most samples are highly radiogenic and have high rb sr ratios in addition the large variation in sr isotopic compositions can partly be explained by crustal contamination during magma differentiation the nd isotopic data clearly indicate that both the peralkaline and aluminous rocks were derived from a long lived depleted mantle reservoir the formation of the ulungur river a type granites is hypothesized as follows 1 a depleted mantle was first metasomatised then subjected to partial melting leading to the generation of basic magmas now represented by the kalatongke mafic ultramafic complex in the ulungur river area 2 the basic magmas have undergone high degrees of fractional crystallisation with separation of biotite hornblende plagioclase potassium feldspar and other accessory phases such as apatite and ilmenite with the residual magmas forming the peralkaline granites during the magmatic differentiation the liquids were probably contaminated by crustal materials as evidenced by nb depletion and nd sr isotopic systematics on the other hand the formation of aluminous granites involved a replenishment of new mantle derived magmas into the differentiating magma chamber and followed by fractional crystallisation with retention of hydrous phases hornblende and biotite the petrological evolution of the ulungur river granites is thus from peralkaline to calc alkaline and not from calc alkaline to peralkaline as more often observed elsewhere the ulungur river granites in common with the voluminous granitoids occurring in the central asian orogenic belt represent a significant juvenile addition of the continental crust in the phanerozoic

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

origin of late variscan granitoids from ne bavaria germany exemplified by ree and nd isotope systematics sm nd isotopic results are presented for late and post tectonic granitoids comprising peraluminous granites and subordinate metaluminous dioritic rocks redwitzites from the fichtelgebirge fg and northern oberpfalz nop ne bavaria the data combined with a number of earlier geochromological studies on these granitoids place severe constraints on the crustal evolution of this region during the carboniferous and pre carboniferous epoch redwitzites range in nd t from 4 to 0 inclusive data from p k holl and coworkers the nd t values of the granites s s are restricted to overlapping ranges of 8 to 3 fg and 8 to 2 nop in both domains the older granites 330 325 ma are characterized by nd t values of 4 whilst the younger granites 315 305 and 290 ma have nd t values of 4 the diversity observed in nd isotopic characteristic is interpreted in terms of different source material the nd isotopic compositions of the redwitzites exhibit source heterogeneity suggesting mixing between mantle magma and crust the nd isotopic features of the older granites are consistent with the magmatic precursors having been generated by partial melting of pre existing mature crust variably contaminated by mantle material or alternatively by melting of chemically less evolved crust resembling paragneisses of teh zev structural unit the origin of the younger granites can be confidently linked to anatexis of common moldanubian and saxothuringian metasediments two stage nd model ages tdm of the granitoids are in the range 1 1 1 7 ga these ages provide further support of substantial involvement of pre phanerozoic crust in the generation of the granitoids the leuchtenberg granite and the g2 and g3 granites yield late variscan sm nd isochron ages which are concordant with previous rb sr data on these granites by contrast the nd isotopic data for barnau and flossenburg define pseudo isochrons suggestive of mixing between contrasting felsic melts

melting of continental crust during subduction initiation a case study from the chaidanuo peraluminous granite in the north qilian suture zone the chaidanuo granite batholith is a similar to 500 km 2 homogeneous high level intrusion in the north qilian oceanic suture zone three types of enclaves have been recognized including 1 supracrustal xenoliths 2 biotite gneiss that represents restite of the upper crustal melting and 3 coeval mantle derived mafic magmatic enclave mme the batholith consists dominantly of peraluminous biotite monzogranite with sio2 69 73 wt and a cnk 1 05 1 28 and shows geochemical affinity with the upper continental crust e g enrichment of large ion lithophile elements liles k rb u th pb depletion of high field strength elements hfses nb ta ti p eu and sr and enriched sr sr 87 sr 86 i 0 731 to 0 744 nd epsilon nd t 6 0 to 7 1 isotopes zircon u pb dating indicates that this batholith formed at 516 505 ma coeval with the mmes 510 ma which represent the early stage of seafloor subduction in the north qilian suture zone the hf isotopic composition of the mme epsilon hf t 6 0 to 2 1 is more depleted than that of the host granite epsilon hf t 14 8 to 4 7 suggesting mixing of mantle and crust derived melts the inherited zircon cores in the host granite yield an age peak at similar to 750 ma with a few detritals of 591 1683 ma zircons from a biotite gneiss enclave yield a protolith age of 744 ma and a metamorphic melting event at 503 ma sr nd isotopic modeling reveals that the batholith was generated by melting of neoproterozoic granitic rocks with the addition of 10 18 mantle derived magma therefore peraluminous granite can be produced by melting of crustal materials heated by mantle derived magmas during the early stage of subduction initiation at the site of a prior passive continental margin c 2014 elsevier ltd all rights reserved

zircon hf o isotopic constraints on the origin of late mesozoic felsic volcanic rocks from the great xing an range ne china abstract the voluminous late mesozoic magmatism was related to extensive re melting of juvenile materials that were added to the central east asia continent in phanerozoic time the most favoured magma generation mechanism of late mesozoic magmas is partial melting of underplated lower crust that had radiogenic hf nd isotopic characteristics but this mechanism faces difficulties when interpreting other isotopic data the tectonic environment controlling the generation of the late mesozoic felsic magmas is also in dispute in this study we obtained new u pb ages and geochemical and isotopic data of representative jurassic 154 4 1 5 ma and cretaceous 140 2 1 5 ma felsic volcanic samples the jurassic sample has inherited zircon cores of permian age with depleted mantle like ehf t of 7 4 8 5 which is in contrast with those of the magmatic zircons ehf t 2 4 0 7 whereas the inherited cores and the magmatic zircons have identical mantle like 18o composition ranges 4 25 5 29 and 4 69 5 54 respectively these hf o isotopic characteristics suggest a mixed source of enriched mantle materials rather than ancient crustal components and a depleted mantle source represented by the inherited permian zircon core this mechanism is manifested by the eruption of jurassic alkaline basalts originated from an enriched mantle source the cretaceous sample has high ehf t of 7 0 10 5 suggesting re melting of a mafic magma derived from a depleted mantle source however the sub mantle zircon 18o values 3 70 4 58 suggest the depleted mantle derived mafic source rocks had experienced high temperature hydrothermal alteration at upper crustal level therefore the cretaceous felsic magma if not all could be generated by re melting of down dropped supracrustal volcanic rocks that experienced high temperature oxygen isotope alteration the two processes enriched mantle contribution and supracrustal juvenile material re melting are new generation mechanisms of the late mesozoic magmas from central east asia rift settings may have controlled these processes throughout crustal and mantle levels

two phase southward subduction of the mongol okhotsk oceanic plate constrained by permian jurassic granitoids in the erguna and xing an massifs ne china abstract geodynamics of the mongol okhotsk oceanic plate southward subduction are still pending problems this paper presents new zircon la icp ms u pb age and whole rock geochemical data for the middle permian to middle jurassic granitoids in the western erguna and central xing an massifs 267 264 ma 241 ma and 173 ma i type granites and 216 ma a type granites were identified in the erguna and xing an massifs ne china the i type granites were produced by partial melting of the lower mafic crust the 216 ma a type granites were derived from partial melting of crustal materials with tonalitic to granodioritic compositions the 267 264 ma and 241 ma i type granites were generated in an andean type arc setting wheras the 216 ma a type and 173 ma granites were formed in supra subduction extensional setting we summarized previous age data of the middle permian to middle jurassic magmtaic rocks in the erguna and xing an massifs and identified two isolated phases of magmatic activity including the ca 267 225 ma and ca 215 165 ma periods with a significant magmatic gap at ca 225 215 ma these middle permian to middle jurassic magmatic rocks are closely related to the southward subduction of the mongol okhotsk ocean a two stage tectonic evolutionary model was proposed to account for these geological observations in the erguna and xing an massifs involving permian to middle triassic continuous southward subduction of the mongol okhotsk oceanic plate and late triassic to jurassic slab rollback and supra subduction extension

crustal evolution of the hercynian belt of western europe evidence from lower crustal granulitic xenoliths french massif central abstract granulite facies lower crustal xenoliths scavenged by the tertiary basaltic pipes of bournac and roche pointue massif central france have been analysed for major and trace elements and radiogenic sr nd isotope ratios they display wide variations in major elements rare earth elements and isotopes 87 sr 86 sr 0 7041 0 7216 nd 11 9 to 1 5 which tend to correlate with the three different rock types encountered basic and acidic meta igneous and metasediments the basic liquids are of calc alkaline affinity and cumulates are abundant there is a gap in silica content between the basic and acid meta igneous xenoliths the isotopic compositions suggest mixing of old pre existing continental crust represented by the metasediments and mantle derived basic melts which represent juvenile addition to the crust during the hercynian orogeny the basic meta igneous rocks have been contaminated during their emplacement into the crust whereas the surrounding metasediments have suffered partial fusion and contamination by the same liquid producing the hercynian granitoids which have nd and sr initial ratios identical to those of the lower crustal xenoliths the residue from melting may be represented by the acid meta igneous rocks some of which have positive eu anomalies these results indicate that an important basaltic underplating event occurred during the evolution of hercynian orogenic belt although the exact timing of the underplating requires further constraint

evidence from neodymium isotopes for mantle contributions to phanerozoic crustal genesis in the canadian cordillera sm nd and rb sr isotopic studies of several phanerozoic orogenic belts have shown that much of the crust in these belts is composed of reworked pre existing continental crust in contrast isotopic data collected from two of the largest terranes in the canadian cordilleran orogenic belt the alexander and stikine terranes show that these tectonic fragments are composed of mantle derived continental crust in many respects the style and rate of crustal accretion of these and related terranes appear similar to those of some proterozoic crustal regions

rapid growth of some major segments of continental crust some major segments of continental crust display a narrow range of sm nd crustal formation ages the sizes of the canadian shield the svecokarelian province of northern europe the west central united states and the arabian nubian shield suggest rapid crustal growth island arc accretion models rank among the most favored tectonic models for the formation of these areas a quantitative comparison of the growth rates of these crustal segments to mesozoic cenozoic arc addition rates shows however that island arc accretion alone seems insufficient to account for the amount of crust that was produced in each of these terrains other additional mechanisms such as hot spot volcanism and underplating may have been active in addition to arc accretion alternatively large amounts of preexisting basement have gone so far undetected

paleozoic crustal growth and metallogeny of central asia evidence from magmatic hydrothermal ore systems of central kazakhstan abstract the ore deposit spectrum of central kazakhstan reflects a range of lithospheric magmatism controlled by the polycyclic geodynamic evolution of an active continental margin which develops from back arc oceanic settings volcanic hosted massive sulphide cu au ore deposits to subduction controlled calcalkaline magmatism copper porphyries with subsequent stages of crust differentiation of low degree partial melting and extended intramagmatic fractionation molybdenum porphyries on to continental rifting peralkaline ree zr nb rich systems mesothermal gold deposits mark transcrustal shear zone controlled fluid conduits formed during arc slicing and thermal relaxation in a post orogenic setting the ore deposits characterize successive and spatially overlapping stages of crustal growth and consolidation the general subduction setting of this range of ore deposits is reflected by calc alkaline trends in the associated subalkaline high k felsic rocks with a wide range in silica contents the predominant metal spectrum of au cu and mo indicates relatively oxidized magmatic hydrothermal systems in accordance with the metaluminous i type nature of their igneous host rocks i e oxidized igneous precursor material despite of presumably highly variable ratios of mantle crust components in individual magmatic systems gabbros to leucogranites most rocks have very similar positive initial e nd values of 0 to 5 5 and depleted mantle model ages in the range of 500 800 ma the ubiquitous relatively young mantle extraction age is likely to characterize the lower crust of central kazakhstan this basement is interpreted as back arc oceanic crust that formed behind oceanwards drifting continental margin slivers during the initial stages of the active continental margin evolution of the late precambrian angara baltica supercontinent the early back arc oceanic crust was transformed into the present lower crust during the paleozoic orogenic evolution in which the back arc crust was buried by magmatic arc and sedimentary material and recurrently affected by high grade metamorphism basaltic melt injection underplating and granitic melt extraction anorogenic rift related permian peralkaline riebeckite granites with ree zr nb mineralization have very high positive e nd of 5 to 8 which is probably inherited from young subcrustal lithosphere at the mantle crust boundary that was affected by partial remelting due to asthenospheric upwelling during rifting

two mineralization events in the baiyinnuoer zn pb deposit in inner mongolia china evidence from field observations s pb isotopic compositions and u pb zircon ages abstract the xing mong orogenic belt xmob is located in the eastern part of the central asian orogenic belt caob and has experienced multiple tectonic events the baiyinnuoer pb zn deposit may be a rare case that documents two periods of mineralization in the tectonically complex xmob there are two types of pb zn mineralization in the deposit 1 skarn type ore hosted by the skarn in the contact zone between marble and granodiorite and within the marble and 2 vein type ore hosted by crystal tuff and feldspar porphyry this study revealed that the host rocks mineral assemblages mineralization occurrences s pb isotopes and ages between the two types of ore are notably different zircon u pb dating indicates that the granodiorite was emplaced in the early triassic 244 1 to 242 1 ma the crystal tuff was deposited in the early cretaceous 140 1 to 136 1 ma and the feldspar porphyry was intruded in the early cretaceous 138 2 to 136 2 ma the first skarn mineralization occurred at 240 ma and the second vein type pb zn mineralization took place between 136 and 129 ma thus the triassic orebodies were overprinted by early cretaceous mineralization the sphalerite and galena from the skarn mineralization have higher 34s values 4 7 to 0 3 than the sphalerite galena and aresenopyrite from the vein type mineralization 7 5 to 4 2 indicating different sulfur sources or ore forming processes for the two types of mineralization the pb isotopic compositions of the two types of ore are very similar suggesting similar lead sources geochemistry and nd pb hf isotopic systematics of the igneous rocks in the region show that the triassic granodiorite was generated from hybridization of mafic and felsic magmas due to strong crust mantle interaction under the collisional setting that resulted following the closure of the paleo asian ocean and the collision of north china and siberian cratons at the end of the permian while the cretaceous igneous rocks at baiyinnuoer originated from the partial melting of a juvenile lower crust with minor input from the crust caused by the underplating of mafic magma in an extensional setting

highly fractionated late triassic i type granites and related molybdenum mineralization in the qinling orogenic belt geochemical and u pb hf and re os isotope constraints abstract we present new data on the highly fractionated late triassic i type liyuantang granite which is located in the middle segment of the south qinling subzone of central china and is associated with molybdenum mineralization zircon u pb dating indicates that the granite was emplaced at 210 1 1 9 ma with a single zircon containing an inherited core that yielded an age of 449 8 7 1 ma magmatic zircons from the granite have e hf t values of 4 0 to 1 5 whereas the inherited zircon core has a e hf t value of 5 3 calculated hf model ages of crust formation are indicative of substantial contributions from melting of proterozoic crust that ranges in age from 1501 to 1155 ma the granite contains high concentrations of si al na and k is enriched in rb th and u has elevated rb sr and ga al ratios and is depleted in ti fe mn mg ca and p with significantly negative eu anomalies eu 0 33 0 50 similar to other highly fractionated i type granites these data indicate that the magmas that formed the liyuantang pluton were produced during partial melting of proterozoic garnet absent quartz amphibolites the magmas then fractionated apatite feldspar ti bearing phases biotite and hornblende prior to emplacement re os isotope analysis of molybdenite from the study area yields a mineralization age of 200 9 6 2 ma suggesting that the liyuantang molybdenum deposit formed during a previously unrecognized mineralization event the present results together with previous data demonstrate that highly fractionated i type granites associated with the second pulse of magmatism in the south qinling subzone should be considered highly prospective for mineral exploration focusing on triassic early jurassic granitoids

geochemical and isotopic studies of the sedimentary and granitic rocks of the altai orogen of northwest china and their tectonic implications the altai orogen northwest china represents the southwestern margin of the central asian orogenic belt geochemical and nd sr isotope analyses were carried out on the palaeozoic sedimentary and granitic rocks in order to trace their sources and to evaluate the pattern of continental growth of the orogen nd isotopic data for both the granites and sediments suggest a significant proportion of middle proterozoic crust beneath the altai orogen however addition of juvenile material arc back arc oceanic crust during palaeozoic times is also significant trace elements and isotopic data of sediments suggest their sources were immature they represent mixtures between a palaeozoic juvenile component and an evolved continental crust the early palaeozoic sediments show e nd t 3 4 to 5 0 t dm 1 5 1 8 ga and i sr 0 710 0 712 they represent a passive margin setting with a predominance of evolved crustal material in the source the devonian sequences however might have been deposited in a back arc basin setting produced by subduction of the junggar oceanic crust along the irtysh fault a significant addition of arc material into the sedimentary basin is responsible for the highly variable e nd values 6 to 0 and i sr 0 711 0 706 the carboniferous rocks were also deposited in a back arc basin setting but with predominantly arc material in the source as suggested by an abrupt increase in e nd t 6 to 3 and decrease in i sr 0 7045 0 7051 voluminous syn orogenic granitoids have e nd t 2 1 to 4 3 i sr 0 705 0 714 and t dm 0 7 1 6 ga they were not derived by melting of local metasedimentary rocks as suggested by previous workers but by melting of a more juvenile source at depth post orogenic granites have higher e nd t 4 4 than the syn orogenic granitoids indicating their derivation from a deeper crustal level where juvenile crust may predominate

sources of hercynian granitoids from the french massif central inferences from nd isotopes and consequences for crustal evolution the french massif central is characterized by a great abundance and variety of granitoids emplaced during hercynian times 360 280 ma distinct nd i and 87sr86sr i are not associated with particular petrographic types however lower nd i values 8 5 occur among the younger leucogranites from the western part of the massif intermediate nd i 6 correlate with aluminous monzogranites and higher values 6 to 3 with calc alkaline granodiorites in the eastern massif central nd i are homogeneous at the regional scale in the western massif central the leucogranites and the monzogranite granodiorites display different trends of nd i with emplacement ages from these results extreme models involving a depleted mantle or an old archean or early proterozoic source may be ruled out the trend of increasing nd i with decreasing age observed in the monzogranite granodiorite group suggests either an increasing contribution of a mantle component or the progressive melting of more mafic more refractory crustal sources or both the reverse trend displayed by the leucogranites might be explained in terms of the upward migration of partial melting across an isotopically zoned metasedimentary pile the large amount of granites may be the result of favourable processes during the collision tectonics combined with the unusual abundance of fertile materials enriched in water and heat producing elements in the pre carboniferous crust

devonian alaskan type ultramafic mafic intrusions and silicic igneous rocks along the southern altai orogen implications on the phanerozoic continental growth of the altai orogen of the central asian orogenic belt abstract in this contribution we have carried out an integrated study of the petrography geochronology geochemistry and mineral chemistry for the devonian alaskan type ultramafic mafic complexes and granitoids widely distributed along the southern altai orogen in order to provide a better understanding of the continental growth process and the regional tectonic evolution la icpms zircon u pb dating of three mafic samples one acidic volcanic rock sample and one granitic sample yielded the concordia ages of ca 400 ma olivine chemical compositions indicate these mafic intrusions crystallized from evolved mafic magmas and compositions of the clinopyroxene and hornblende show typical arc affinities whole rock compositions reveal their tholeiitic signature with enriched lrees and liles and intensive depleted hfses nb ta zr and hf leading to low hfse lree ratios nb la 0 1 0 4 isotopically these mafic intrusions have positive zircon e hf t 7 6 to 14 3 mostly 10 and positive whole rock e nd t 0 8 to 5 3 values which suggest that their parental magmas were derived from a time integrated depleted mantle source these features together with the presence of primary water bearing minerals biotite and or hornblende suggest that their primitive magmas were derived from a time integrated depleted lithosphere mantle which were metasomatized by fluids released from subduction slab these findings in combination with coeval voluminous granitoids and arc signature volcanic rocks indicate that the underplating of mafic magmas at the continental arc root leads to the partial melting of the continental crust it is inferred that mafic magmatic underplating in the altai orogenic belt significantly contributed to the phanerozoic vertical continental crust growth in the altai orogen

high resolution 3 d crustal structure beneath ne china from joint inversion of ambient noise and receiver functions using necessarray data abstract we used ambient noise data recorded by the necessarray a portable seismic array deployed in northeast china to construct rayleigh wave phase velocity dispersion curves and obtain phase velocity maps for periods from 6 to 40 s s wave velocity structure beneath ne china is obtained by joint inversion of the phase velocities and receiver functions our results show good correlation between crustal velocity structure and geological features at shallow depths the songliao basin is separated into two small sub basins by a fast velocity anomaly corresponding to a southwest uplifted horst and the southern sub basin is associated with the kailu depression a low velocity anomaly extending from the lower crust to uppermost mantle beneath the changbaishan mountain range is consistent with p and s wave tomography models the mantle low velocity has been interpreted as a local mantle upwelling attributed to the subducting pacific slab induced return flow finally the s wave velocity in the mid and lower crust is lower beneath the xing an mongolia orogenic belt xmob than beneath the changbaishan mountain range this was not expected since the changbaishan mountain range has experienced more extensive volcanic activity in the cenozoic including the currently active changbaishan and jingpohu volcanoes than the xmob we infer that the mid and lower crust of the xmob is mainly felsic in composition and propose that the low shear wave velocity of the mid and lower crust beneath the xmob could be the consequence of removal of deep mafic crust through a continental accretion and subsequent delamination event in the paleozoic or via convection removal induced by the rollback of the flat subducted paleo pacific slab during the middle mesozoic

isotopic evidence from the ivrea zone for a hybrid lower crust formed by magmatic underplating the large mafic complex overlying mantle peridotites in the ivrea zone of northern italy originated by underplating of mantle derived magma onto the base of continental crust and may be a unique outcrop of typical mafic lower crust sampled elsewhere only by mafic xenoliths neodymium and strontium isotope data show that the magmas assimilated large and roughly uniform proportions of crustal material the resulting hybrid magmas have the same isotopic signature as and may represent the lower crustal roots of upper crustal hercynian granitoid intrusions

formation and tectonic evolution of southeastern china and taiwan isotopic and geochemical constraints abstract the southern part of china consists of the proterozoic yangtze craton and the phanerozoic south china foldbelts including the himalayan foldbelt of taiwan models for continental growth have been many and controversial isotopic and geochemical data from mesozoic and younger granitoids and sediments are used here to place constraints on the tectonic evolution of southeastern china and to evaluate whether the young phanerozoic foldbelts are representative of old rejuvenated landmass whether they characterize crustal accretion through successive subduction processes hence suggesting a net growth of continental mass or whether they represent some intermediate situation available sm nd isotopic data for phanerozoic granitoids and metasediments from the south china foldbelts and taiwan invariably show proterozoic model ages t dm ranging from 1 to 2 5 ga with a mean of 1 54 0 30 1 ga all rocks have negative nd t values 2 to 15 suggesting variable but important contributions of old continental materials to the sources of the phanerozoic rocks obviously they do not represent wholesale growth of continental mass the granitoids of the south china foldbelts have multiple origins as viewed from their chemical and isotopic characteristics secular geochemical variation has been established for inland granitoids of proterozoic to mesozoic age their high i sr values 0 710 0 737 suggest that most granitoids are essentially anatectic products of ancient continental crust this is compatible with remelting via microcontinental collision or terrane accretion repeated intracrustal reprocessing by partial fusion and differentiation has undoutedly contributed to important mineralization mesozoic granitoids excluding a type alkaline granites in the coastal region and taiwan show relatively low i sr 0 705 0 710 and high nd values implying that greater amounts of mantle components have been added to ancient continental material in the generation of these rocks this means that mesozoic magmatism in the coastal region resulted from subduction of a paleo pacific plate kula beneath the asian continent the overall geochemical evidence seems to be consistent with a model of serial collisions between microcontinental blocks for the bulk of the south china foldbelts the latest arc continent collision began about 4 ma ago in eastern taiwan

late permian tectonic evolution at the southeastern margin of the songnen zhangguangcai range massif ne china constraints from geochronology and geochemistry of granitoids abstract this paper reports the la icp ms u pb zircon dating and geochemical data for the huangqigou xiaobeihu and lalagou granitic plutons at the southeastern margin of the songnen zhangguangcai range massif which will not only place important constraints on the rock forming ages source characteristics and tectonic setting of these granitic plutons but will also provide insights into understanding the tectonic evolution of the eastern segment of the central asian orogenic belt zircons from three plutons have distinct oscillatory zoning structures in their cathodoluminescene cl images which combined with the th u ratios of 0 28 1 06 indicate their magmatic origins zircon u pb dating results for these granitic plutons indicate that the granitic magmatism occurred mainly in the late permian 256 252 ma the granites have sio2 of 58 0 74 3 na2o of 2 74 3 76 k2o of 2 75 3 95 na2o k2o of 0 77 1 13 mg 100 mg mg fe2 of 21 8 39 9 and a cnk of 0 88 1 08 with ree patterns characterized by lree hree 7 75 12 82 la yb n 7 88 14 9 and eu 0 56 0 87 the trace element spider diagrams indicate that these granites are enriched in large ion lithosphile elements rb ba th u etc and relatively depleted in high field strength elements ti nb ta with sr and pb ranging from 205 to 350 ppm and from 2 02 to 3 60 ppm respectively the in situ hf isotope analysis reveals that ehf t values range from 2 22 to 11 65 and the two stage hf model ages tdm2 range between 641 and 1274 ma these geochemical characteristics indicate that the late permian granitic plutons at the se margin of the songnen zhangguangcai range massif can be assigned to metaluminous and weakly peraluminous high potassium calc alkaline i type granites with low sr and high yb features and that the magmas to have formed these granites were originated from the partial melting of mafic igneous rocks in the lower crust the source rocks are composed mainly of the juvenile crust with small contributions of ancient continental crust considering the existence of coeval mafic intrusive rocks and sedimentary basins we propose that the late permian magmatism in the study area occurred in a post collisional extensional setting following the collision of the xingkai block and the united jiamusi and songnen zhangguangcai range massifs

retrodictions of mid paleogene mantle flow and dynamic topography in the atlantic region from compressible high resolution adjoint mantle convection models sensitivity to deep mantle viscosity and tomographic input model abstract although mantle convection at earth like vigor is a chaotic process it has been shown by conceptual studies that one can constrain its flow history back in time for periods comparable to a mantle overturn 100 million years if knowledge of the tangential surface velocity field and estimates on the present day heterogeneity state are available such retrodictions enabled through computationally demanding adjoint methods are a promising tool to improve our understanding of deep earth processes and to link uncertain geodynamic modeling parameters to geologic observables here we present the first mantle flow retrodictions for geodynamically plausible compressible high resolution earth models with 670 million finite elements going back in time to the mid paleogene our retrodictions involve the dynamic effects from a low viscosity zone lvz in the upper mantle assimilate a past plate motion model for the tangential surface velocity field and probe the influence from uncertain modeling parameters by using two different estimates for the present day heterogeneity state of the mantle as imaged by two recent seismic tomographic studies and two different values for deep mantle viscosity focusing on the african hemisphere we find that our retrodictions produce a spatially and temporally highly variable asthenosphere flow with faster than plate velocities and a history of dynamic topography characterized by local doming events in agreement with considerations on plate driving forces and regional scale uplifts reported in the geologic literature our results suggest that improved constraints on non isostatic vertical motion of earth s surface provided for instance by basin analysis seismic stratigraphy landform studies or the sedimentation record will play an important role in our understanding of the recent mantle flow history

oxygen isotope exchange kinetics of mineral pairs in closed and open systems applications to problems of hydrothermal alteration of igneous rocks and precambrian iron formations the systematics of stable isotope exchange between minerals and fluids are examined in the context of modal mineralogical variations and mass balance considerations both in closed and in open systems on mineral pair 18 o plots samples from terranes that have exchanged with large amounts of fluid typically map out steep positively sloped non equilibrium arrays analytical models are derived to explain these effects these models allow for different exchange rates between the various minerals and the external fluids as well as different fluid fluxes the steep arrays are adequately modelled by calculated isochron lines that involve the whole family of possible exchange trajectories these isochrons have initially steep near vertical positive slopes that rotate toward a 45 equilibrium slope as the exchange process proceeds to completion the actual data point array is thus analogous to the hand of an isotopic clock that measures the duration of the hydrothermal episode the dimensionless ratio of the volumetric fluid flux to the kinetic rate parameter u k determines the shape of each individual exchange trajectory in a fluid buffered system u k 1 the solutions to the equations 1 are independent of the mole fractions of the solid phases 2 correspond to taylor s open system water rock equation and 3 yield straight line isochrons that have slopes that approach 1 f where f is the fraction reacted of the more sluggishly exchanging mineral the isochrons for this simple exchange model are closely congruent with the isochrons calculated for all of the more complex models thereby simplifying the application of theory to actual hydrothermal systems in nature in all of the models an order of magnitude of time in units of kt separates steep non equilibrium arrays e g slope 10 from arrays approaching an equilibrium slope of unity on a diagram because we know the approximate lifetimes of many hydrothermal systems from geologic and heat balance constraints we can utilize the 18 o 16 o data on natural mineral assemblages to calculate the kinetic rate constants k s and the effective diffusion constants d s for mineral h 2o exchange these calculated values k qtz 10 14 k feld 10 13 10 12 agree with experimental determinations of such constants in nature once the driving force or energy source for the external infiltrating fluid phase is removed the disequilibrium mineral pair arrays will either 1 remain frozen in their existing state if the temperatures are low enough or 2 re equilibrate along specific closed system exchange vectors determined solely by the temperature path and the mineral modal proportions thus modal mineralogical information is a particularly important parameter in both the open and closed system scenarios and should in general always be reported in stable isotopic studies of mineral assemblages these concepts are applied to an analysis of 18 o 16 o systematics of gabbros plagioclase clinopyroxene and plagioclase amphibole pairs granitic plutons quartz feldspar pairs and precambrian siliceous iron formations quartz magnetite pairs in all these examples striking regularities are observed on and plots but we point out that plots have many advantages over their equivalent diagrams as the latter are more susceptible to misinterpretation using the equations developed in this study these regularities can be interpreted to give semiquantitative information on the exchange histories of these rocks subsequent to their formation in particular we present a new interpretation indicating that precambrian cherty iron formations have in general undergone a complex fluid exchange history in which the iron oxide magnetite precursor has exchanged much faster with low temperature 400 c fluids than has the relatively inert quartz

early cretaceous lower crustal reworking in ne china insights from geochronology and geochemistry of felsic igneous rocks from the great xing an range this paper presents new zircon la icp ms u pb ages and whole rock geochemical data for two granitic plutons and rhyolites of the baiyingaolao formation in the western xing an range ne china the two syenogranite granitic plutons yield identical zircon u pb age of 142 1 ma and the baiyingaolao rhyolites yield zircon u pb age of 138 2 ma the granites contain some hornblendes and show low zr and zr nb ce y contents and low a cnk 0 98 1 11 mg 6 55 and feot mgo values rhyolite samples show similar geochemical characteristics with a cnk of 0 99 1 10 and mg of 14 21 in combination with the high k2o contents 4 43 5 61 wt and negative correlations between p2o5 and sio2 both the granites and rhyolites were classified as high k calc alkaline i type granitoids all samples give high zirconium saturation temperature of 794 964 c with few initially inherited zircons and belong to high temperature i type granitoids they were generated by dehydration melting of biotite muscovite from sub alkaline meta basalts in lower crust depth leaving garnet amphibole and plagioclase as the major residual minerals the syenogranites and rhyolites are likely formed in mongol okhotsk oceanic subduction setting incorporating other lower crust originated felsic rocks in erguna and xing an massifs and songliao basin it is argued that lower crustal reworking is pronounced in ne china during early cretaceous

subduction style magmatism in a non subduction setting the colville igneous complex ne washington state usa of the klondike mountain formation which show no decoupling the colville igneous complex is located within the eocene magof the lile and hfse i e no subduction signature precluding matic belt of the north american cordilleran interior it straddles the presence of a subduction slab beneath this part of the north the us canadian border in northeast washington and southern american cordillera during the eocene we propose the alternative british columbia the complex consists of three intrusive and two model that the colville igneous complex formed as a result of extrusive phases the first extrusive phase being contemporaneous decompression melting of crust and mantle during post laramide with the latter two intrusive phases as a consequence of suborogenic collapse of an overthickened crust solidus re equilibration in the plutonic rocks this study concentrates on the two extrusive phases the sanpoil volcanic formation and the klondike mountain formation the sanpoil volcanic formation consists of andesites dacites and rare trachyandesites sio2

late orogenic post orogenic and anorogenic granites distinction by major element and trace element chemistry and possible origins granites classified into four categories based solely on tectonics of occurrence and associated rock types also have compositional characteristics that are consistent within groups and different am

u pb ages of detrital zircon of the paleozoic sedimentary rocks new constraints on the emplacement time of the hegenshan ophiolite ne china abstract the hegenshan ophiolite in the solonker hegenshan belt is the largest ophiolite in the central asian orogenic belt caob despite its significance in constraining regional tectonic evolution the emplacement time of the hegenshan ophiolite is still under debate in this study we provide new detrital zircon ages of the paleozoic sedimentary rocks that unconformably overlie the wusinihei ophiolite northeastern part of the hegenshan ophiolite to constrain the lower limit emplacement time of the hegenshan ophiolite the zircon ages obtained from the paleozoic sedimentary rocks range from 298 8 to 363 7 ma and show bimodal distribution at 300 320 ma peak at 308 ma and 320 360 ma peak at 330 ma the age group of 300 320 ma coincides with the age range of the volcanic rocks of the late paleozoic gegenaobao formation the age group of 320 360 ma with a peak at 330 ma may be linked to local mafic ultramafic rocks of the hegenshan ophiolite accordingly we suggest that the emplacement time of the hegenshan ophiolite should have occurred earlier than the deposition of the gegenaobao formation most likely during the time between 308 and 330 ma instead of the silurian devonian or mesozoic as previously considered

lower crustal rejuvenation and growth during post thickening collapse insights from a crustal cross section through a variscan metamorphic core complex the variscan lower crust of europe acquired its main features seismic layering intrusion of mantle derived magmas granulite grade metamorphism when the upper crust underwent post thickening extension through gravitational collapse we suggest that post thickening collapse is a geodynamic process that contributes to the growth and differentiation of continental lithosphere in collapsed mountain belts

sr nd isotopic and geochemical constraints on provenance of late paleozoic to early cretaceous sedimentary rocks in the western hills of beijing north china implications for the uplift of the northern north china craton late paleozoic to mesozoic strata which were deposited in the western hills of beijing north china recorded the contemporary tectonic evolution of the northern part of the north china craton ncc fine grained sandstones and siltstones of these strata were analyzed for their major and trace element concentrations and sr nd isotope compositions their la sc th sc la co th co and cr th ratios are similar to those of sediments derived from felsic source rocks indicating that the original source materials were mostly intermediate to acidic rocks their end 0 values show dramatic shifts implying changes in provenances from the late paleozoic to the early cretaceous the end 0 values of the late paleozoic and the late jurassic to early cretaceous formations are very low whereas the end 0 values of the xingshikou and the xiaoyaopo formations are much higher indicating that a juvenile component has been added into their sources based on the sr nd isotope compositions the late paleozoic and the late jurassic to early cretaceous samples fall in the paleozoic to mesozoic magmatic rocks distributed in the northern part of the ncc however the sr nd isotope compositions of the xingshikou and the xiayaopo formations vary between the xing meng orogenic belt xmob and the north part of the ncc components these changes can be interpreted by the complex uplift and exhumation history of the northern ncc from the late paleozoic to the early cretaceous during the late carboniferous the uplift of the inner mongolia paleo uplift impu which probably commenced at the late carboniferous might supply detrital materials to the late paleozoic strata in the late triassic the uplift of the xmob and impu was responsible for the mixing source of the xingshikou formation after that the northern xmob became the main source of the xiayaopo formation resulting from the uplift of the xmob due to the collision of siberia and north china mongolia plates during the middle to late jurassic finally the uplift of the yftb which occurred from the late jurassic to early cretaceous provided the source materials and blocked the input of the xmob components

petrogenesis and tectonic implications of late cretaceous highly fractionated i type granites from the qiangtang block central tibet abstract the tectonic evolution of the lhasa and qiangtang collision zone is poorly understood due to a lack of pivotal magmatic records and complex geology in central tibet in this study we present zircon u pb ages whole rock major and trace elements and sr nd pb isotopic data for the newly discovered chuburi pluton in the southern qiangtang block zircon u pb data reveal that the chuburi pluton was emplaced during the late cretaceous ca 73 ma bulk rock analyses show that this intrusion is characterized by high sio2 na2o and k2o but low mgo cao and p2o5 the granites are enriched with large ion lithophile elements and light rare earth elements with marked eu anomalies and there are few heavy rare earth elements and high field strength elements these geochemical features indicate that the chuburi pluton is a highly fractionated i type granite the nine granitic samples show high 87sr 86sr i ratios 0 7064 0 7089 negative end t values 1 94 to 5 59 and have nd isotopic model ages ranging from 0 76 to 0 97 ga the initial pb isotopic ratios of the chuburi pluton is uniform 206pb 204pb t 18 774 18 884 207pb 204pb t 15 695 15 713 208pb 204pb t 39 362 39 613 based on geochemical characteristics of the chuburi pluton and consideration of the region geological setting we suggest that the parent magma was generated by magmatic mixing of mantle and lower crust and then undergoing extensive fractional crystallization with removal of k feldspar plagioclase mafic mineral and minor accessory mineral the crust beneath the central tibet may have been significantly thickened following the final lhasa qiangtang amalgamation we argue the generation of the chuburi pluton can be interpreted as a result of regional lithospheric delamination

the formation of the suyunhe large porphyry mo deposit in the west junggar terrain nw china zircon u pb age geochemistry and sr nd hf isotopic results abstract newly discovered suyunhe mo deposit 0 57 mt mo metal located in the barluk mountains of the west junggar terrain xinjiang nw china is the largest porphyry mo deposit discovered in xinjiang so far the suyunhe mo deposit is associated with the suyunhe intrusive complex which was emplaced into the volcano sedimentary sequences of the middle devonian barluk group the suyunhe complex consists of the deep granite intrusion and the shallow granite porphyry granodiorite porphyry monzonitic granite porphyry and tonalite porphyry stocks which are cut by the felsic dikes dominant mo mineralization is hosted in the barluk group volcano sedimentary rocks but is minor in the granite granite porphyry granodiorite porphyry and monzonitic granite porphyry of the suyunhe complex we report in situ zircon u pb ages whole rock geochemistry sr nd isotopic and zircon lu hf isotopic data for the ore bearing intrusive rocks from the suyunhe deposit secondary ion mass spectrometry sims zircon u pb dating indicates that the deep granite emplaced at 298 4 1 9 ma 295 3 3 ma and the shallow granodiorite porphyry intruded during 294 7 2 1 ma 293 7 2 3 ma the suyunhe granite intrusion including fine and medium grained granite belongs to the high k calc alkaline series most of them have low zr nb y ce 2 7 values showing a geochemical affinity to i type granite these granites are enriched in light rare earth elements lree and depleted in heavy rare earth elements hree and eu ba sr nb p and ti indicating that they have experienced advanced fractional crystallization they have low initial 87sr 86sr ratios 0 70318 0 70380 high whole rock end t values 4 4 to 6 0 high zircon ehf t values 10 8 to 14 6 and young whole rock nd and zircon hf model ages they also show low mg 0 35 these features demonstrate that the ore bearing granite formed by partial melting of the juvenile lower crust and experienced advanced fractional crystallization the suyunhe granite porphyry and monzonitic granite porphyry are compositionally similar to the granite indicating that they share a common magma source and process with the granite the suyunhe granodiorite porphyries belong to slightly high k calc alkaline series and have lree enrichment and hree nb and ti depletion they have low 87sr 86sr i ratios 0 70421 0 70444 high end t values 4 9 to 6 2 high zircon ehf t values 9 6 to 15 2 and young whole rock nd and zircon hf model ages they also show low mg

genesis of post collisional granitoids and basement nature of the junggar terrane nw china nd sr isotope and trace element evidence the post collisional magmatism of the junggar terrane is characterized by intrusion of large amounts of granitoids and minor basic ultrabasic rocks the granitoids comprise two magmatic suites calc alkaline and alkaline which were emplaced contemporaneously at about 294 ma the calc alkaline rocks are typically sodium rich na2o k2o 1 1 3 5 and metaluminous a nk 1 0 a cnk 0 78 1 04 they show mildly fractionated ree patterns la yb n 15 and spidergrams with strong depletion of nb ti and to a lesser extent sr the alkaline granites have high contents of sio2 75 78 alkalis nb hree y sn f and high feo mgo ratios and huge sr and eu depletion in the spidergrams moreover they display characteristic tetrad ree patterns and non charac trace element behaviour the two rock suites have similar initial nd and sr isotopic compositions with end t in the range 5 2 to 7 1 and isr mostly in the range 0 7031 0 7041 this points to a predominance of juvenile components in their sources the calc alkaline rocks are most probably derived by dehydration melting of a basic lower crust leaving behind a granulite residue the process was probably triggered by underplating of mantle derived basic magmas in an extensional regime the alkaline granites are considered to have formed by differentiation of the calc alkaline granitoids our study argues for a juvenile continental crust for the basement of the junggar terrane which is likely dominated by early paleozoic oceanic crust and arc complex that were deeply buried during the late paleozoic subduction and accretion

mafic crustal xenoliths and the origin of the lower continental crust abstract the la nb ratio 30 ppm at a mg number of 40 in oceanic plateau basalts and morb provide a useful index to distinguish them from arc basalts morb can generally be distinguished from plateau basalts by their lower nb contents

detrital zircons reveal no jurassic plateau in the eastern north china craton abstract u pb hf isotopic analyses have been carried out on detrital zircons from late permian jurassic strata in the western shandong bohai bay wsbb basins with aims of constraining the paleogeography and evolution of the eastern north china craton ncc four major groups of u pb ages namely 3 0 2 3 ga 2 2 1 4 ga 1250 430 ma and 410 150 ma are recognized for zircons from the fine grained sandstones the post paleoproterozoic zircons of the late permian sample show an age cluster of 344 255 ma with e hf t values of 18 to 8 being similar to those extracted from late paleozoic igneous rocks from the yinshan yanshan orogenic belt yyob on the northern margin of the ncc however the devonian jurassic zircons 390 150 ma from the jurassic sediments have e hf t values from 45 to 14 very similar to the mixture of phanerozoic zircons from the xing meng orogenic belt xmob and the yyob but significantly different from those of the liaodong peninsula jiaodong peninsula and the western shandong uplift in addition cambrian silurian zircons 535 430 ma of the jurassic samples have e hf t values from 27 to 7 resembling the early paleozoic zircons extracted from the northern qinling orogen nqo the u pb age spectra and hf isotope of devonian jurassic zircons as well as bulk nd isotopes suggest that the central eastern ncc received some juvenile materials from the xmob the yyob and the nqo during the jurassic this indicates widespread prototype basins and a low elevation during the jurassic arguing against the presence of a paleo plateau in the eastern ncc at that time the lack of evidence predicted by lithospheric delamination e g large scale elevation or doming after late triassic makes the delamination unlikely to be a viable mechanism of lithospheric thinning of the ncc the subsidence rather than elevation of the eastern ncc after late triassic might be related to thermo chemical erosion of the lithosphere by underlying convective mantle in a back arc setting

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

geodynamic settings of mineral deposit systems mineral deposits represent extraordinary metal concentrations that form by magmatic magmatic hydrothermal or hydrothermal processes in geodynamic environments typified by anomalously high thermal and or mechanical energy near plate boundaries as they require the conjunction of specific environmental conditions to form particular mineral deposit types tend to occupy specific geodynamic niches the temporal distributions of mineral deposit types reflect both formational and preservational processes in the archaean and palaeoproterozoic these were linked because of preservation in continental crust connected to thick buoyant subcontinental lithospheric mantle sclm but were decoupled by the neoproterozoic and phanerozoic as a result of evolution to thinner increasingly dense sclm the transition marks a change from mantle plume influenced plate tectonics to modern style plate tectonics with broadly coincident environmental changes and a major impact on the nature and abundance of preserved mineral deposit types as mineral deposits represent an integral part of tectonic process they are essential indicators of that process and geodynamic settings and should be incorporated into any holistic tectonic terrane analysis their distribution also provides a particularly critical test on ancient continental reconstructions derived from palaeomagnetic data conversely such reconstructions provide a first order targeting tool for the conceptual exploration required to discover new mineral provinces and deposits under cover

ultrahigh ϵnd eclogites from an ultrahigh pressure metamorphic terrane of china the su lu ultrahigh pressure uhp metamorphic terrane is part of the qinling dabie suture zone formed by the collision between the sino korean and yangtze cratonic blocks geochemical and isotopic investigation of the uhp metamorphic rocks has revealed a spectacular history for some eclogitic rocks retrograded coesite bearing eclogites from the weihai area in the eastern end of the su lu terrane have recorded an extraordinary nd isotopic growth with a world record high nd value of 264 range 170 to 264 such isotopic characteristics are consistent with the observed extreme lree fractionation and very high sm nd ratios the sm nd isotopic data allow us to calculate the minimum age of protolith formation at 1 7 ga assuming their evolution from a depleted or a chondritic mantle geochemical data indicate that the eclogite protoliths were of basaltic compositions originally but underwent metasomatism water rock interaction at high pressures and temperatures leading to the present apparent andesitic composition and extreme lree depletion this process took place in the proterozoic 1 7 ga and was not related to the mesozoic continental collision and ultrahigh pressure metamorphism at 210 220 ma ago because of a protracted retrograde metamorphism garnet sm nd systems were not closed until 160 ma ago 50 ma after the peak eclogite metamorphism this implies that the exhumation history is significantly different in different parts of the dabie and su lu uhp metamorphic terranes from geochemical analyses and isotopic age determinations we argue that at least two ancient tectonic events in the middle and late proterozoic can be identified prior to the last mesozoic continental collision and accompanying uhp metamorphism in the su lu and dabie terrane