origin of the alxa block western china new evidence from zircon u pb geochronology and hf isotopes of the longshoushan complex abstract the nw se trending longshoushan is in the southwestern margin of the alxa block which was traditionally considered the westernmost part of the north china craton ncc precambrian crystalline basement exposed in the longshoushan area was termed the longshoushan complex this complex s formation and metamorphism are significant to understand the geotectonics and early precambrian crustal evolution of the western ncc in this study field geology petrology and zircon u pb and lu hf isotopes of representative orthogneisses and paragneisses in the longshoushan complex were investigated u pb datings reveal three paleoproterozoic magmatic episodes ca 2 33 ca 2 17 and ca 2 04 ga and two subsequent regional metamorphic events ca 1 95 1 90 ga and ca 1 85 ga for metamorphic granitic rocks in the longshoushan complex u pb dating of the detrital magmatic zircons from two paragneisses yields concordant 207pb 206pb ages between 2 2 ga and 2 0 ga and a small number of metamorphic zircon rims provide a ca 1 95 ga metamorphic age suggesting that the depositional time of the protolith was between 2 0 and 1 95 ga and that the sedimentary detritus was most likely derived from the granitic rocks in the longshoushan complex itself zircon lu hf isotopic analyses indicate that nearly all magmatic zircons from ca 2 0 ga to ca 2 17 ga orthogneisses have positive ehf t values with two stage hf model ages tdmc ranging from 2 45 to 2 65 ga peak at ca 2 5 ga indicating that these paleoproterozoic granitic rocks were derived from the reworking of the latest neoarchean early paleoproterozoic juvenile crust detrital magmatic zircons from two paragneisses yield scattered 176hf 177hf ratios ehf t and tdmc values further indicating that the sedimentary detritus was not only derived from these plutonic rocks but also from other unreported or denuded paleoproterozoic igneous rocks the ca 2 15 ga detrital magmatic zircons from one paragneiss have negative ehf t values with tdmc ranging from 2 76 to 3 04 ga indicating another important crustal growth period in the longshoushan region these data indicate that the longshoushan complex experienced neoarchean early paleoproterozoic crustal growth approximately ca 2 3 2 0 ga experienced multiphase magmatic events and approximately ca 1 95 1 90 ga and ca 1 85 ga experienced high grade metamorphic events the sequence of tectonothermal events is notably similar to that of the main ncc together with the datasets from an adjacent area we suggest that the western alxa block was most likely an integrated component of the ncc from the neoarchean to the paleoproterozoic

on the antarctic peninsula batholith the plutonic rocks of the antarctic peninsula magmatic arc form one of the major batholiths of the circum pacific rim the antarctic peninsula batholith is a 1350 km long by 210 km wide structure which was emplaced over the period 240 to 10 ma with a cretaceous peak of activity that started at 142 ma and waned during the late cretaceous early jurassic and late jurassic early cretaceous gaps in intrusive activity probably correspond to episodes of arc compression in a northern zone of the antarctic peninsula the batholith intrudes palaeozoic mesozoic low grade meta sedimentary rocks and in a central zone it intrudes schists and ortho and paragneisses which have late proterozoic nd model ages and were deformed during triassic to early jurassic compression in a southern zone the oldest exposed rocks are permian sedimentary rocks and deformed jurassic volcanic and sedimentary rocks all these pre batholith rocks formed a belt of relatively immature crust along the gondwana margin with few exceptions jurassic plutons crop out only within the central zone many are peraluminous having s like mineralogies and relatively high 87sr 86sri they are considered to consist largely of partial melts of upper crust schists and gneisses and components of mafic magmas that caused the partial fusion by contrast early cretaceous plutons crop out along the length of the batholith few magma compositions appear to have been affected by upper crust the bulk being compositionally independent of the type of country rock they intrude they are dominated by metaluminous calcic si oversaturated 1 type granitoid rocks with relatively low 87sr 86sri intermediate silicic compositions 5 mgo we interpret these to represent partial melts of basic to intermediate igneous locally garnet bearing lower crust contemporaneous mafic magmas e g syn plutonic dykes form a more alkaline si saturated series having higher 143nd 144nd at the same87sr 86sr than the intermediate silicic series to which they are not petrogenetically related the change from limited partial fusion of upper crust in jurassic times to widespread partial fusion of lower crust in early cretaceous times is considered to be a result of an increasing volume of basaltic intrusion into the crust with time

u pb trace element and lu hf properties of unique dissolution reprecipitation zircon from uhp eclogite in sw sulu terrane eastern china abstract a unique dissolution reprecipitation zircon has been identified for the first time in a qtz and phe bearing eclogite from the drill hole ccsd mh southwestern sulu uhp terrane by using laser raman and cathodoluminescence cl imaging combined with trace element analyses and u pb and lu hf isotope data relic inherited domains of partially dissolved zircon show magmatic features and preserve mineral inclusions of cpx pl qtz ap whereas newly formed zircons crystallized around or along cracks of the inherited magmatic domains and contain mineral inclusions of coe grt omp phe ap typical for uhp metamorphism in addition primary fluid inclusions were identified in the coesite bearing zircon domains characterized by a single h2o phase or co2 h2o phase indicating the presence of fluids during uhp conditions strongly dissolved inherited magmatic zircons yielded shrimp 206pb 238u ages of 782 588 ma with an upper intercept age of 780 14 ma new recrystallized or metamorphic zircons yielded consistent triassic uhp ages around 238 225 ma with a weighted mean of 229 2 2 3 ma zircon trace element and hf isotope compositions and mineral inclusions classify the protolith of the sulu eclogite as an intermediate to mafic neoproterozoic igneous rock which was subducted to mantle depth and experienced uhp metamorphism in middle triassic times the juvenile ehf 780 ma value of 7 4 2 9 in inherited magmatic zircon points to a significant input from the depleted mantle to the dabie sulu uhp terrane during middle neoproterozoic new uhp zircon domains have low lu hf th u and significantly higher 176hf 177hf ratios than the inherited magmatic zircons consistent with formation during recrystallization and fractionation of the lu hf system during uhp metamorphism

zircon u pb ages and hf isotope compositions of migmatite from the north dabie terrane in china constraints on partial melting migmatite gneisses are widespread in the dabie orogen but their formation ages are poorly constrained eight samples of migmatite including leucosome melanosome and banded gneiss were selected for u pb dating and hf isotope analysis most metamorphic zircon occurs as overgrowths around inherited igneous cores or as newly grown grains morphological and internal structure features suggest that their growth is associated with partial melting according to the hf isotope ratio relationships between metamorphic zircon and inherited cores three formation mechanisms for metamorphic zircon can be determined which are dissolution reprecipitation of pre existing zircon breakdown of zr bearing phase other than zircon in a closed system and crystallization from externally derived zr bearing melt four samples contain magmatic zircon cores yielding upper intercept u pb ages of 807 35 768 12 ma suggesting that the protoliths of the migmatites are neoproterozoic in age the migmatite zircon yields weighted mean two stage hf model ages of 2513 97 894 54 ma indicating reworking of both juvenile and ancient crustal materials at the time of their protolith formation the metamorphic zircons give u pb ages of 145 2 120 2 ma the oldest age indicates that partial melting commenced prior to 145 ma which also constrains the onset of extensional tectonism in this region to pre 145 ma the youngest age of 120 ma was obtained from an undeformed granitic vein indicating that deformation in this area was complete at this time two major episodes of partial melting were dated at 139 1 and 123 1ma the first episode of partial melting is obviously older than the timing of post collision magmatism corresponding to regional extension the second episode of partial melting is coeval with the widespread post collision magmatism indicating the gravitational collapse and delamination of the orogenic lithospheric keel of the dabie orogen which were possibly triggered by the uprising of the cretaceous mid pacific superplume

thickening and partial melting of the northern qinling orogen china insights from zircon u pb geochronology and hf isotopic composition of migmatites the widespread migmatites in the precambrian basement of the north qinling belt are products of partial melting of the earth9s crust and record the tectono thermal response to late paleozoic collision along the shangdan suture the morphology and trace element geochemistry of zircons from the migmatites suggest a metamorphic origin related to partial melting the zircons from leucosome melanosome and banded migmatites yield u pb weighted mean ages of 408 2 403 2 and 400 2 ma respectively constraining the timing of migmatization the granitic dyke intruded into the migmatites gives a zircon u pb weighted mean age of 396 2 ma constraining the minimum time of migmatization zircons from the melanosome and banded migmatite yield upper intercept ages of 1088 1061 ma representing the protolith ages of the basement of the north qinling belt the zircon rims of the melanosome and banded migmatite predominantly display negative and positive ehf t values respectively suggesting the variable input of mantle material together with the regional geology and previous geochronological data we propose that the thickening and partial melting in the north qinling belt was induced by the collision along the shangdan suture between the north and south china blocks from c 408 to 400 ma supplementary material la icp ms u pb data lu hf isotopic compositions and trace element geochemistry data of the zircons and ti in zircon thermometry are available at https doi org 10 6084 m9 figshare c 4521389

a mid cretaceous age for the palmer land event antarctic peninsula implications for terrane accretion timing and gondwana palaeolatitudes new structural and age data suggest that west gondwana may have been at lower palaeolatitudes than previously interpreted from albian sequences in gondwana marginal suspect terranes the palmer land event which juxtaposed mesozoic terranes on the gondwana margin deformed granitoids in the southern antarctic peninsula u pb shrimp dating of zircons from a microgranite dyke yields a crystallization age of 106 9 1 1 ma this result and re interpretation of the structural position of another granite pluton date the palmer land event and probable terrane collision as late early cretaceous and not latest jurassic as formerly interpreted

early jurassic magmatism on the antarctic peninsula and potential correlation with the subcordilleran plutonic belt of patagonia early jurassic silicic volcanic rocks of the chon aike province v1 187 182 ma are recognized from many localities in the southern antarctic peninsula and ne patagonia and are essentially coeval with the extensive karoo 182 ma and ferrar 183 ma large igneous provinces of pre breakup gondwana until recently plutonic rocks of this age were considered either rare in or absent from the antarctic peninsula batholith which was thought to have been mainly constructed during the middle jurassic and the mid cretaceous new u pb zircon geochronology from the antarctic peninsula and recently published u pb ages from elsewhere in the peninsula and patagonia are used to demonstrate the more widespread nature of early jurassic plutonism eight samples are dated here from the central and southern antarctic peninsula they are all moderately to strongly foliated granitoids tonalite granite granodiorite and locally represent the crystalline basement they yield ages in the range 188 181 ma which overlap with published ages of 185 180 ma from granitoids from elsewhere on the antarctic peninsula and from the subcordilleran plutonic belt of patagonia 185 181 ma whereas early jurassic plutons of the subcordilleran plutonic belt of patagonia are directly related to subduction processes along the proto pacific margin of gondwana coeval volcanic rocks of the chon aike province are interpreted to be directly associated with extension and plume activity during the initial stages of gondwana breakup this indicates that subduction was continuing when chon aike province volcanism started the early jurassic plutonism on the antarctic peninsula is transitional between subduction related and breakup related magmatism supplementary material cathodoluminescence images of analysed zircon grains from sites on the antarctic peninsula are available at https doi org 10 6084 m9 figshare c 3521973

zircon lu hf systematics and the evolution of the archean crust in the southern superior province canada a combined lu hf and u th pb isotopic study was made of 25 zircons and 2 whole rocks from the late archean crust 2 888 2 668 ma in the southern superior province canada the relative abundances of u th lu and hf in zircons from the low grade michipicoten and gamitagama greenstone belts show variable patterns which in part reflect the bulk compositional differences of their parent rocks zircons from the high grade lower crustal regions adjacent to these belts kapuskasing structural zone are distinguished from the low grade zircons by their strong depletions of lu and hf the low hf contents imply that the growth of metamorphic zircon involves a significant fractionation of the zr hf ratio

a reconnaissance ion probe study of hafnium isotopes in zircons a sims technique for the isotopic analysis of hafnium in zircons using the shrimp ion microprobe has been developed and a precision of typically 0 5 2 achieved in the mean reduced 176hf177hf ratio measured at several spots on a single grain unfractionated chondritic initial hf isotopic compositions have been measured on a number of archaean zircon populations these include the oldest known terrestrial minerals the 4 2 ga old mount narryer detrital zircons thereby confirming their antiquity in contrast positive initial hf relative to the chondritic model composition has been found in several post archaean zircon populations reflecting the increasing involvement of isotopically evolved depleted mantle sources in the formation of younger crust the 570 ma old sri lankan zircon standard sl7 yielded an exceptionally low initial hfof 23 implying a metamorphic origin as a reworked product of ancient crust shrimp u pb analyses of zircons from archaean tonalitic gneiss at watersmeet michigan yield a precise crystallization age of 3636 6 ma 2 and show that a previously reported correlation between 176hf177hf and u pb isotopic discordance in bulk zircon samples patchett 1983 was caused by the addition of radiogenic hf in discrete overgrowths of new zircon ca 2 7 ga ago the original 3 64 ga grains show no evidence of disturbance to their original chondritic hf isotopic composition there is presently no evidence for significant isotopic exchange of hf between zircon and other minerals in crustal rocks

inherited and magmatic zircon from neogene hoyazo cordierite dacite se spain anatectic source rock provenance and magmatic evolution

orthorectification of 1960s satellite photographs covering greenland this article presents a rigorous high precision model for geometric orthorectification of declassified intelligence satellite photography disp imagery for the generation of a seamless full coverage mosaic of the greenland ice sheet this model integrates the bundle adjustment method and satellite orbital parameters solving for interior orientation including lens distortion and exterior orientation parameters simultaneously in addition the techniques of adaptive filtering bright strip removal radiometric balancing and mosaic postprocessing are discussed two full coverage mosaics of greenland using 24 disp images from eight orbits of the argon 9034a mission of may 1962 and 36 images from 14 orbits of the 9058a 59a mission of october 1963 were created the average planimetric accuracy relative to the synthetic aperture radar sar mosaic is about 168 m from statistical measurements of 182 points in topographically flat areas and 186 m from statistical measurements of 201 points in mountainous areas the two mosaic products have been delivered to the u s national snow and ice data center nsidc for use by the research community

a full coverage high resolution topographic model of greenland computed from a variety of digital elevation data in this paper a new high resolution elevation model of greenland including the ice sheet as well as the ice free regions is presented it is the first published full coverage model computed with an average resolution of 2 km and providing an unprecedented degree of detail the topography is modeled from a wide selection of data sources including satellite radar altimetry from geosat and ers 1 airborne radar altimetry and airborne laser altimetry over the ice sheet and photogrammetric and manual map scannings in the ice free region the ice sheet model accuracy is evaluated by omitting airborne laser data from the analysis and treating them as ground truth observations the mean accuracy of the ice sheet elevations is estimated to be 12 13 m and it is found that on surfaces of a slope between 0 2 and 0 8 corresponding to approximately 50 of the ice sheet the model presents a 40 improvement over models based on satellite altimetry alone on coastal bedrock the model is compared with stereo triangulated reference points and it is found that the model accuracy is of the order of 25 35 m in areas covered by stereo photogrammetry scannings and between 200 and 250 m elsewhere

u pb and hf isotope analysis of zircons in mafic xenoliths from fuxian kimberlites evolution of the lower crust beneath the north china craton mafic xenoliths from the paleozoic fuxian kimberlites in the north china craton include garnet granulite and minor pyroxene amphibolite metagabbro anorthosite and pyroxenite the formation conditions of the amphibolites are estimated at 745 820 c and 7 6 8 8 kb 25 30 km the granulites probably are derived from greater depths in the lower crust lam icpms u pb dating of zircons from four granulites reveals multiple age populations recording episodes of magmatic intrusion and metamorphic recrystallisation concordant ages and upper intercept ages interpreted as minimum estimates for the time of magmatic crystallisation range from 2 620 to 2 430 ma in three granulites two amphibolites and two metagabbros lower intercept ages represented by near concordant zircons are interpreted as reflecting metamorphic recrystallisation and range from 1 927 to 1 852 ma one granulite contains two metamorphic zircon populations dated at 1 927 55 ma and 600 700 ma separated minerals from one granulite and one amphibolite yield sm nd isochron ages of 1 619 48 ma 143nd 144nd i 0 51078 and 1 716 120 ma 143nd 144nd i 0 51006 respectively these ages are interpreted as recording cooling following metamorphic resetting model ages for both samples are in the range 2 40 2 66 ga lam mc icpms analyses of zircon show a range in 176hf 177hf from 0 28116 to 0 28214 corresponding to a range of ehf from 34 to 12 the relationships between 207pb 206pb age and ehf show that 1 the granulites amphibolites and metagabbro were derived from a depleted mantle source at 2 6 2 75 ga 2 zircons in most samples underwent recrystallisation and pb loss for 100 200 ma after magmatic crystallisation consistent with a residence in the lower crust 3 metamorphic zircons in several samples represent new zircon growth incorporating hf liberated from breakdown of silicates with high lu hf 4 in other samples metamorphic and magmatic zircons have identical 176hf 177hf and the younger ages reflect complete resetting of u pb systems in older zircons the fuxian mafic xenoliths are interpreted as the products of basaltic underplating derived from a depleted mantle source in neoarchean time an important period of continental growth in the north china craton paleoproterozoic metamorphic ages indicate an important tectonic thermal event in the lower crust at 1 8 1 9 ga corresponding to the timing of collision between the eastern and western blocks that led to the final assembly of the north china craton the growth of metamorphic zircon at 600 700 ma may record an asthenospheric upwelling in neoproterozoic time related to uplift and a regional disconformity in the north china craton

lu hf and u pb isotope systematics of zircons from the storgangen intrusion rogaland intrusive complex sw norway implications for the composition and evolution of precambrian lower crust in the baltic shield abstract the storgangen orebody is a concordantly layered sill like body of ilmenite rich norite intruding anorthosites of the rogaland intrusive complex ric sw norway 17 zircon grains were separated from ca 5 kg of sand size flotation waste collected from the on site repository from ilmenite mining these zircons were analysed for major and trace elements by electron microprobe and for u pb and lu hf isotopes by laser ablation microprobe plasma source mass spectrometry eight of the zircons define a well constrained mswd 0 37 concordant population with an age of 949 7 ma which is significantly older than the 920 930 ma ages previously reported for zircon inclusions in orthopyroxene megacrysts from the ric the remaining zircons interpreted as inherited grains show a range of 207pb 206pb ages up to 1407 14 ma with an upper intercept age at ca 1520 ma the concordant zircons have similar trace element patterns and a mean initial hf isotope composition of 176hf 177hf949 ma 0 28223 5 ehf 2 2 this is similar to the hf isotope composition of zircons in a range of post tectonic sveconorwegian granites from south norway and slightly more radiogenic than expected for mid proterozoic juvenile crust the older inherited zircons show lu hf crustal residence ages in the range 1 85 2 04 ga one undated zircon plots well within the field of hf isotope evolution of paleoproterozoic rocks of the baltic shield these findings indicate the presence of paleoproterozoic components in the deep crust of the rogaland area but do not demonstrate that such rocks or a sveconorwegian mantle derived component contributed significantly to the petrogenesis of the ric if the parent magma was derived from a homogeneous lower crustal mafic granulite source the lower crustal protolith must be at least 1 5 ga old and it must have an elevated rb sr ratio this component would be indistinguishable in sr nd and hf isotopes from some intermediate mixtures between sveconorwegian mantle and paleoprotoerzoic felsic crust but it cannot account for the initial 143nd 144nd of the most primitive late sveconorwegian granite in the region without the addition of mantle derived material

anatexis of ultrahigh pressure eclogite during exhumation in the north qaidam ultrahigh pressure terrane constraints from petrology zircon u pb dating and geochemistry plagioclase rich leucosomes and k feldspar rich veins are widely distributed within retrogressed eclogite in the xitieshan area of the north qaidam ultrahigh pressure metamorphic terrane western china in this contribution a combined study of petrology zircon internal structure zircon u pb age zircon trace element composition whole rock geochemistry and sr nd isotopes in plagioclase rich leucosomes and k feldspar rich veins provides insight into the nature and timing of partial melting in these rocks petrological evidence for partial melting is provided by elongated highly cuspate feldspars quartz along grain boundaries e g muscovite garnet and clinopyroxene and felsic veinlets composed of quartz and feldspar along the boundaries of garnet clinopyroxene and clinopyroxene plagioclase symplectite in the metabasite whole rock geochemistry suggests that the plagioclase rich leucosomes have lower contents of both rare earth elements rees and high field strength elements hfses but higher large ion lithophile elements liles e g rb ba k sr and pb than the metabasite hosts the plagioclase rich leucosomes may be divided into two subgroups according to their distinct ree patterns 1 higher total ree content with or without weak negative eu anomalies and 2 lower total ree content with conspicuous positive eu anomalies the eu rich group also shows higher sr content than those in the eu poor group however the k feldspar rich veins show higher ree hfse and lile concentrations than the plagioclase rich leucosomes and metabasites the inherited cores of k feldspar rich veins are of typical magmatic origin oscillatory zoning with high th u ratios enriched heavy h rees and negative eu anomalies yielding 206 pb 238 u ages ranging from 904 8 ma to 915 28 ma the zircon cores of plagioclase rich leucosomes and mantles of k feldspar rich veins exhibit no zoning or weak zoning with flat hree patterns and no negative eu anomalies and they contain mineral inclusions of garnet and clinopyroxene implying that these zircon domains were formed during an episode of eclogite facies metamorphism with 206 pb 238 u ages ranging from 444 10 ma to 452 9 ma internal textures and mineral inclusions quartz plagioclase and k feldspar and trace element systematics steep hree patterns with negative eu anomalies and low th u ratios of the zircon rims from plagioclase rich leucosomes and k feldspar rich veins are similar to anatectic zircon these anatectic zircon domains yielded 206 pb 238 u ages ranging from 430 10 ma to 440 12 ma and 406 12 ma to 430 13 ma respectively an integrated study of petrology geochronology and geochemistry demonstrated that the metabasites of the north qaidam ultrahigh pressure terrane likely experienced initial partial melting with plagioclase rich leucosome formation under eclogite facies conditions and partial melt crystallization during the granulite facies stage triggered by dehydration melting involving zoisite and rare muscovite the k feldspar rich veins within the metabasites may be the result of injected melts that were derived from partial melting of nearby gneisses

zircon u pb ages ree concentrations and hf isotope compositions of granitic leucosome and pegmatite from the north sulu uhp terrane in china constraints on the timing and nature of partial melting granitic leucosome and pegmatite are widely distributed within biotite bearing orthogneiss in the northern part of the sulu ultrahigh pressure uhp metamorphic terrane eastern china a combined study of mineral inclusions cathodoluminescence cl images u pb shrimp dates and in situ trace element and lu hf isotope analyses of zircons provided insight into the nature and timing of partial melting in these rocks zircon grains separated from biotite bearing orthogneiss typically have three distinct domains 1 pre metamorphic magmatic cores with qtz kfs pl ap inclusions which record a neoproterozoic protolith age of 790 ma 2 mantles with coe phe ap inclusions that record triassic uhp age at 227 3 ma and 3 narrow rims with quartz inclusions that record hp granulite facies retrograde metamorphism at 210 3 ma in contrast zircons separated from granitic leucosome have only two distinct domains 1 the central uhp areas with coe phe ap inclusions record triassic uhp age of 227 3 ma and 2 outer magmatic areas with qtz kfs ab ap inclusions that record partial melting time of 212 2 ma zircons separated from pegmatite contain mineral inclusions of qtz kfs ap and show regular magmatic zoning from centre to edge the centres record partial melting time of 212 2 ma in line with the outer domains of granitic leucosome whereas the edges give a younger age of 201 2 ma related to pb loss and partial recrystallization during late triassic regional amphibolite facies retrogression these data indicate that partial melting in the north sulu uhp gneissic rocks took place during post uhp retrograde hp granulite facies metamorphism pre metamorphic magmatic zircon cores from biotite bearing orthogneiss give uniform 176hf 177hf of 0 28187 0 00003 2 sd standard deviation corresponding to ehf 790 and hf model ages tdm2 of about 16 3 and 2 41 ga respectively this is consistent with the generation of its protolith by reworking of paleoproterozoic to late archean crust in contrast uhp zircon domains from biotite bearing orthogneiss and granitic leucosome are characterized by distinct trace element composition with low lu hf 0 006 low th u 0 1 and considerably higher 176hf 177hf 0 28233 0 00002 2 sd than the pre metamorphic cores the uniform but significantly different hf isotope composition between the uhp ehf 227 14 6 0 8 2 sd and pre metamorphic ehf 227 27 7 domains indicates equilibration of the lu hf isotope system only within the uhp metamorphic mineral assemblage the disequilibrium between whole rock and uhp zircon suggests that about two thirds of the whole rock hf retained in the pre metamorphic zircon domains zircon domains crystallized during partial melting at 212 ma in granitic leucosome and pegmatites have a hf isotope composition indistinguishable from that of the uhp zircon domains this suggests that only hf and zr equilibrated during uhp metamorphism was remobilized during partial melting while pre metamorphic zircon remained stable or was not accessible in contrast the magmatic zircon edges from pegmatite have somewhat lower 176hf 177hf 0 28216 and ehf t 17 6 1 2 2 sd indicating some release of less radiogenic hf for instance by dissolution of pre metamorphic zircon during late regional amphibolite facies retrogression

evidence for a two phase palmer land event from crosscutting structural relationships and emplacement timing of the lassiter coast intrusive suite antarctic peninsula implications for mid cretaceous southern ocean plate configuration 1 new analysis of the relationships between geological structural data and radiometric ages for the lassiter coast intrusive suite indicate that the collisional mid cretaceous palmer land event orogeny in the antarctic peninsula has had two kinematic phases forming an intersection orocline one of which can be related to cretaceous southern ocean plate motions both are compressional phases along the eastern palmer land shear zone phase 1 occurred at 107 ma with a principal paleostrain axis of 341 and is best expressed in southern palmer land although evident elsewhere on the antarctic peninsula phase 2 occurred at 103 ma with a principal paleostrain axis of 259 5 but is confined to between 68 s and 74 s a peak in lassiter coast intrusive suite magma emplacement rate was coeval with phase 1 whereas phase 2 may have coincided with a lull during phase 1 the allochthonous central and western domain terranes may have been transported to the gondwana margin represented by the para autochthonous eastern domain on board the phoenix plate or on board the south american plate the variable provenance indicators from the central and western terranes can be cited to support either or a combination of these scenarios the convergence direction evident from phase 2 structures cannot easily be reproduced in regional plate kinematic models this and the localized evidence for superposition of phase 2 structures on phase 1 structures suggests that phase 2 is unlikely to have occurred solely in response to changes in the kinematics of the large plates considered

multiple partial melting events in the ailao shan red river and gaoligong shan complex belts se tibetan plateau zircon u pb dating of granitic leucosomes within migmatites abstract thin layers and lenses of granitic leucosomes are widely distributed within migmatites from the ailao shan red river asrr and gaoligong shan gs complex belts se tibetan plateau they are parallel to or cross cut foliations in the host rocks at different scales and show evidence of coalescence and migration to form centimetre to decimetre scale segregations a combined study of mineral inclusions cathodoluminescence cl images u pb sims dates and in situ trace element compositions of zircons provides clear evidence on the nature and timing of partial melting in these rocks most zircons from the granitic leucosomes occur as distinct overgrowths around inherited detrital cores or as new euhedral crystals the inherited detrital cores are rounded or irregular contain felsic mineral inclusions of qtz kfs ab ap with abundant opaque impurities and show clear dissolution textures in contrast the overgrowths and new crystals commonly show perfectly euhedral shapes have pronounced oscillatory zoning contain felsic mineral inclusions such as qtz kfs ab ap and show pronounced high u 1419 6129 ppm and relatively low th 58 301 ppm contents with extremely low th u ratios 0 02 0 07 the chondrite normalized ree patterns show steep rising slopes from the hree to lree with evidently negative eu anomalies eu eu 0 10 0 31 and pronounced positive ce anomalies ce ce 35 145 these data suggest that the new zircons are anatectic in origin and that they grew during partial melting of the metamorphic rocks abundant u pb spot analyses of the magmatic zircons from granitic leucosomes reveal three discrete and meaningful ages of partial melting within the asrr and gs complex belts anatectic zircons from four granitic leucosomes in weakly deformed migmatites from the gs complex belt yield consistent mean u pb ages of 40 6 0 3 38 4 0 2 ma in contrast anatectic zircons from six granitic leucosomes in weakly deformed migmatites from the asrr complex belt record younger mean u pb ages of 33 7 0 3 30 9 0 3 ma however anatectic zircons from another six granitic leucosomes in mylonitic migmatites from both belts yield the youngest mean u pb ages of 27 8 0 2 26 3 0 3 ma these data imply that both the asrr and gs complex belts experienced multiple partial melting events the differences in the oldest u pb ages of zircons from granitic leucosomes indicate that the partial melting event related to the peak amphibolite granulite facies metamorphism within the gs complex belt occurred 7 10 ma earlier than that in the asrr complex belt the youngest u pb ages of zircons from granitic leucosomes within mylonitic migmatites in both the asrr and gs belts are genetically related to post peak isothermal decompressional retrogression accompanied by regional high temperature ductile shearing the 40 ar 39 ar age data 27 8 22 2 ma of biotites and amphiboles from migmatites strongly support this hypothesis

multistage anatexis during tectonic evolution from oceanic subduction to continental collision a review of the north qaidam uhp belt nw china abstract the north qaidam ultra high pressure uhp metamorphic belt in the northern tibetan plateau is considered as a typical alpine type uhp metamorphic belt due to the early paleozoic subduction of the qaidam block beneath the qilian block the well preserved paleozoic metatexite migmatite diatexite migmatite felsic sheet dyke and anatectic granite in the north qaidam uhp metamorphic belt provide us with an excellent opportunity to study the generation transport and final fate of crustal melt and geodynamic processes associated with orogenesis based on structural relationships petrology geochronology whole rock geochemistry and sr nd isotope data the early paleozoic anatexis during oceanic subduction to continental collision in the north qaidam uhp metamorphic belt can be further divided into three stages of development 470 ma 446 428 ma and 432 420 ma the first stage granitic leucosomes are rich in k poor in na with low sr y and enrichment of large ion lithophile elements liles which were probably derived from partial melting of ancient felsic gneiss in continental arc circumstances during oceanic subduction the second stage na rich leucosomes and tonalite plutons are characterized by high na sr sr y and la yb and low heavy rare earth elements hrees with positive end t values of 0 1 4 3 and zircon ehf t values of 8 3 10 2 similar to typical tonalite trondhjemite granodiorites ttgs these ttg like magmas were produced through partial melting of newly emplaced gabbroic rocks with arc affinity under high pressure hp granulite facies conditions in a thickened lower crust during continental collision the volumetrically significant migrated plutons evolved from ttg like melt will become segments of continental crust and contribute to crustal growth with partial residual products of hp granulite and or garnet pyroxenite to the mantle by delamination the third stage of anatexis was preserved in both migmatitic uhp gneiss and eclogite in the xitieshan and luliangshan terranes the kfs rich felsic leucosomes inside uhp gneisses in the third stage are characterized by high alkali contents and low mafic component with feot mgo tio2 2 0 2 high la yb and sr y and low y and hrees 3 high al2o3 and low mg values and 4 enriched in liles e g rb ba k sr pb and poor in high field strength elements hfses partial melting of uhp eclogite and felsic gneiss during the initial retrogression stage with pl rich and kfs rich leucosome formation was triggered by dehydration melting involving predominant zoisite and muscovite the anatectic melts from partial melting of deeply subducted uhp gneiss have accumulated and migrated outside the deeply subducted crustal slice in the form of syn collisional plutons the melt evolution from the leucosomes produced at the early exhumation stage to syn collisional granitoids produced at the late exhumation might contribute greatly to the exhumation of the north qaidam uhp metamorphic belt from mantle depths to the lower crustal levels

zircon u pb age trace element and hf isotope composition of kongling terrane in the yangtze craton refining the timing of palaeoproterozoic high grade metamorphism u pb age trace element and hf isotope compositions of zircon were analysed for a metasedimentary rock and two amphibolites from the kongling terrane in the northern part of the yangtze craton the zircon shows distinct morphological and chemical characteristics most zircon in an amphibolite shows oscillatory zoning high th u and 176lu 177hf ratios high formation temperature high trace element contents clear negative eu anomaly as well as hree enriched patterns suggesting that it is igneous the zircon yields a weighted mean 207pb 206pb age of 2857 8 ma representing the age of the magmatic protolith the zircon in the other two samples is metamorphic it has low th u ratios low trace element concentrations variable hree contents 33 8 lun 2213 14 7 lun smn 354 and 176lu 177hf ratios 0 000030 0 001168 the data indicate that the zircon formed in the presence of garnet and under upper amphibolite facies conditions the metamorphic zircon yields a weighted mean 207pb 206pb age of 2010 13 ma these results combined with previously obtained palaeoproterozoic metamorphic ages suggest a c 2 0 ga palaeoproterozoic collisional event in the yangtze craton which may result from the assembly of the supercontinent columbia the zircon in two samples yields weighted mean two stage hf model tdm2 ages of 3217 110 and 2943 50 ma respectively indicating that their protoliths were mainly derived from archean crust

u pb dating of zircons from granitic leucosomes in migmatites of the jiaobei terrane southwestern jiao liao ji belt north china craton constraints on the timing and nature of partial melting abstract in the jiaobei terrane of the jiao liao ji belt eastern china layers and irregular lenses of granitic leucosomes are widely distributed within high pressure hp mafic granulites medium pressure mp granulites amphibolites ttg gneisses and hp and mp pelitic granulites the layers and lenses may be parallel to or cross cut the foliation in the host rocks on different scales and they show evidence of coalescence and migration in the formation of centimeter to decimeter scale segregations the migmatized rocks extend over an area of at least 120 km 75 km from laixi through laiyang and qixia to sw yantai a combined study of mineral inclusions cathodoluminescence cl images and zircon u pb la icp ms dates provides clear evidence on the nature and timing of the partial melting in these granulite facies metamorphic rocks most zircons from the granitic leucosomes are either inherited crystals igneous or detrital with distinct overgrowths or are simply new euhedral crystals both the overgrowths and the new crystals commonly exhibit perfect euhedral shapes pronounced oscillatory zoning and contain mineral inclusions of qtz kfs pl ap in contrast the inherited cores are rounded or irregular in shape and exhibit signs of dissolution these observations suggest that the new zircons are anatectic in origin and that they crystallized from melts derived from dehydration melting reactions and partial melting of the host granulite facies rocks numerous u pb spot analyses of anatectic zircons reveal that the jiaobei terrane experienced a major episode of partial melting in the paleoproterozoic with crystallization of the melt occurring in two distinct periods from 1860 9 2 2 to 1852 1 2 8 ma and 1839 9 2 9 to 1833 3 5 3 ma as represented respectively by the magmatic cores and rims of the anatectic zircons the first age group probably represents the initial crystallization of melts derived from partial melting within the jiaobei terrane while the second age group may represent the completion of migmatization and crystallization of granitic leucosomes this significant partial melting event was genetically associated with the paleoproterozoic mp granulite facies hot stage of exhumation of the southwestern part of the jiao liao ji belt and it induced extensive retrogression of the ttg gneisses and mafic and pelitic granulites as well as the formation of regional migmatites

s type granitic magmas petrogenetic issues models and evidence abstract despite a perception that it represents a perverse divergence it is perfectly possible to believe in the existence of s and i type granites and the implications for the nature of their protoliths and to disbelieve in the applicability of the restite unmixing model for chemical variation in granitic magmas white and chappell erected the s i classification with impeccable validity the isotopic evidence demands contrasting source reservoirs for s and i type granitic magmas however the major advance was not the classification but the recognition that highly contrasting parental materials must be involved in the genesis of granitic magmas the restite unmixing model is commonly seen as a companion to the s i classification but it is really a separate issue this model implies that the compositions of granites image those of their source rocks in a simple way however there are other equally valid models that can explain the data and none of them represents a unique solution the most cogent explanation for the high grade metasedimentary enclaves in most s type granites is that they represent mid crustal xenoliths restitic enclaves are either rare or absent inherited zircons in s type rocks are certainly restitic however the occurrence of a substantial restitic zircon population does not imply an equally substantial restitic component in the rest of the rock zircon and zirconium behaviours are controlled by disequilibrium and kinetics and zr contents of granitic rocks can rarely be used to infer magma temperatures since the dominant ages among inherited zircons in lachlan fold belt lfb s type granites are ordovician and proterozoic it seems likely that crust of this age but geochemically different from the exposed rocks not only underlies much of the lfb but also forms a component in the granite magma sources the evidence is overwhelming that the dark microgranular enclaves that occur in both s and i type granites are igneous in origin they represent globules of quenched more mafic magma mingled and modified by exchange with the host granitic magma however magma mixing does not appear to be a significant process affecting the chemical evolution of the host magmas likewise the multicomponent mixing models erected for some granitic rock suites are mathematically nonunique and in some cases violate constraints from isotopic studies s and i type magmas commonly retain their distinct identities this suggests limited source mixing limited magma mixing and limited wall rock assimilation though intermediate types certainly exist they are probably relatively minor in volume crystal fractionation probably plays the major role in the differentiation of very many granitic magmas including most s types especially those emplaced at high crustal levels or in the volcanic environment minor mechanisms include magma mixing wall rock assimilation and restite unmixing isotopic variations within plutons and in granite suites could be caused by source heterogeneities magma mixing assimilation and even by isotopic disequilibrium however source heterogeneity coupled with the inefficiency of magma mixing is probably the major cause of observed heterogeneity normal geothermal gradients are seldom sufficient to provide the necessary heat for partial melting of the crust and crustal thickening likewise fails to provide sufficient heat generally the mantle must be the major heat source this might be provided through mantle upwelling and crustal thinning and possibly through the intra and underplating of mafic magmas upper crustal extension seems to have been common in regions undergoing granitic magmatism migmatites probably provide poor analogues of granite source regions because they are mostly formed by fluid present reactions granitic magmas are mostly formed by fluid absent processes where we do see rare evidence for arrested fluid absent partial melting the melt fraction is invariably concentrated into small shear zones veinlets and small dykes thus it seems likely that dyking is important in transporting granitic magma on a variety of scales and at many crustal levels however one major missing link in the chain is the mechanism by which melt fractions in small scale segregations occurring over a wide area can be gathered and focused to efficiently feed much wider spaced major magma conduits answers may lie in the geometry of the melting zones and in the tendency of younger propagating fractures to curve toward and merge with older ones self organization almost certainly plays a role

hafnium isotopic heterogeneity in zircons from granitic rocks geochemical evaluation and modeling of zircon effect in crustal anatexis abstract we carried out a geochemical evaluation and modeled the mechanism responsible for varied hf isotopic ratios in magmatic zircons of a single granitic rock specimen five representative granitic samples were selected from southern china based on preliminary hf isotopic data our new dataset of zircon hf isotopes confirmed significant hf isotope variations 5 9 epsilon units for each sample and these zircons show roughly positive th u versus t crystallizing temperature correlations while the lu hf variation is independent from t in addition some zircons show significantly higher ti concentrations in the rims compared to the interiors implying reverse thermal zonation based on the ti in zircon thermometry these geochemical features in zircons suggest open system processes which may have resulted from frequent replenishment we modeled zircon dissolution during crustal anatexis to reveal the hf isotopic evolution in the extracted melts the model suggests that the extracted melts may have extremely variable hf isotope compositions 20 epsilon units if the bulk zr concentration in the source is initially above 100 ppm and the melting is rapid 10 4 yr 1 the decoupled release of zircon hf and non zircon hf from a single crust derived magma source can lead to significant hf isotope variations in a solidified granitic body this work provides an alternative explanation for the hf isotopic heterogeneity in magmatic zircons of granitic rocks which is in contrast to the common interpretation by mixing with mantle derived magmas

cathodoluminescence guided zircon hf isotope depth profiling mobilization of the lu hf system during u hp rock exhumation in the woodlark rift papua new guinea abstract cathodoluminescence image guided hf isotope depth profiling by laser ablation of zircons from two quartzofeldspathic host gneisses constrains the lu hf system s behavior during rapid exhumation of u hp rocks in the woodlark rift papua new guinea investigation of the depth profiling technique in individual and composite zircon standards demonstrates that it is possible to resolve 8 m thick domains in which ehf present differs by as little as 4 units in a metasedimentary gneiss 2 89 0 29 ma zircon overgrowths on cretaceous aged inherited cores have radiogenic ehf present indicating growth in a medium that was originally in equilibrium with garnet undergoing recrystallization the garnet effect of zheng et al 2005 in a separate gneiss sample that originated as an exhumation related anatectic melt 3 66 0 13 ma zircons lacking inheritance contain sub domains that differ from each other by 15 ehf present some of these sub domains are radiogenic and can be explained by the garnet effect whereas others also contain highly elevated lu and yb in addition to their radiogenic hf compositions thus necessitating a medium derived from the complete breakdown of garnet zircons in this sample also contain non radiogenic sub domains that grew in the presence of hf mobilized from the surrounding rocks of the subducted and metamorphosed remnants of the australian continental margin the results confirm that rapid exhumation of u hp rocks can result in the following 1 transmission of radiogenic hf and sometimes lu and the other hree from garnet bearing mafic lithologies into the quartzofeldspathic gneisses and 2 mobilization and transport of unradiogenic hf present within the quartzofeldspathic remnants of subducted continental crust

hidden archaean and palaeoproterozoic crust in nw ireland evidence from zircon hf isotopic data from granitoid intrusions the presence of major crystalline basement provinces at depth in nw ireland is inferred from in situ hf isotope analysis of zircons from granitoid rocks that cut structurally overlying metasedimentary rocks granitoids in two of these units the slishwood division and the tyrone central inlier contain complex zircons with core and rim structures in both cases cores have average hf values that differ from the average hf values of the rims at 470 ma the time of granitoid intrusion the hf data and similarity in u pb age between the inherited cores and detrital zircons from the host metasedimentary rocks suggests local contamination during intrusion rather than transport of the grains from the source region at depth rims from the slishwood division intrusions have average hf 470 values of 7 7 consistent with a derivation from juvenile palaeoproterozoic crust such as the annagh gneiss complex or rhinns complex of nw ireland implying that the deep crust underlying the slishwood division is made of similar material rims from the tyrone central inlier have extremely negative hf 470 values of approximately 39 this isotopic signature requires an archaean source suggesting rocks similar to the lewisian complex of scotland or sediment derived wholly from it occurs at depth in nw ireland

greenland ice sheet surface properties and ice dynamics from ers 1 sar imagery c band synthetic aperture radar sar imagery from the european space agency s ers 1 satellite reveals the basic zonation of the surface of the greenland ice sheet the zones have backscatter signatures related to the structure of the snowpack which varies with the balance of accumulation and melt at various elevations the boundaries of zones can be accurately located with the use of this high resolution imagery the images also reveal a large flow feature in northeast greenland that is similar to ice streams in antarctica and may play a major role in the discharge of ice from the ice sheet

recycled metaigneous crustal sources for s and i type variscan granitoids from the spanish central system batholith constraints from hf isotope zircon composition abstract the spanish central system scs is one of the largest granite batholiths in the european variscan belt zircons from five granitic intrusions from the eastern scs have been separated and analysed for hf isotopes by laser ablation mc icpms in order to evaluate the nature of granitic magma source two samples are cordierite bearing s type granites the rest are amphibole bearing i type monzogranites hf isotope composition of zircons from these granites defines a narrow range of negative hf values 1 1 to 5 8 typical of crustal protoliths the within sample variation of zircon hf isotope composition is slightly above the analytical uncertainty 1 5 units suggesting the mixing of different magma batches during pluton assembly the s and i type granites display the same range of initial hf values that is in agreement with previous data showing that both scs granite types have similar sr nd o pb isotope signatures these isotopic data suggest that they derived from similar sources while the variation in peraluminosity could be related to different partial melting conditions the presence of a lower ordovician inherited zircon population 478 462 ma at least in i type granites suggests the involvement of cambro ordovician orthogneisses as components in the source of some scs granites inherited zircon population of similar age has also been identified in the scs lower crustal granulites the hf isotope composition of the analysed granite zircons is within that of zircons from the above granulites moreover some evidence of a juvenile input at 560 595 ma is recorded by hf isotope data in zircons from both the scs granites and granulite xenoliths hf isotope signature in conjunction with other chemical isotopic sr nd o pb and geochronological evidences suggests that scs granites were probably derived from metaigneous lower crustal sources the variscan magmatism in central spain is dominantly a crustal reworking event

in situ u pb geochronology and hf isotope analyses of the rayner complex east antarctica in situ zircon u pb and hf isotopic analysis via laser ablation microprobe inductively coupled plasma mass spectrometer lam icpms of samples from kemp and macrobertson lands east antarctica suggests that the kemp land terrane evolved separately from the rest of the rayner complex prior to the ca 940 ma rayner structural episode several archaean metamorphic events in rocks from western kemp land can be correlated with events previously reported for the adjacent napier complex recently reported ca 1 600 ma isotopic disturbance in rocks from the oygarden group may be correlated with a charnockitic intrusion in the stillwell hills before ca 1 550 ma despite being separated by some 200 km t hf dm ages indicate felsic orthogneiss from rippon point the oygarden group havstein island and the stillwell hills share a ca 3 660 3 560 ma source that is indistinguishable from that previously reported for parts of the napier complex more recent additions to this crust include proterozoic charnockite in the stillwell hills and the vicinity of mawson station these plutons have distinct 176hf 177hf ratios and formed via the melting of crust generated at ca 2 150 2 550 ma and ca 1 790 1 870 ma respectively

the latest neoarchean paleoproterozoic evolution of the dunhuang block eastern tarim craton northwestern china evidence from zircon u pb dating and hf isotopic analyses abstract the dunhuang block in the easternmost segment of the tarim craton exposes precambrian mafic granulite and felsic gneiss to better understand the poorly known formation and evolution of the tarim craton we carried out detailed field based petrological and geochronological investigations on the major lithologies of the dunhuang block u pb dating and hf isotopic analyses on the mafic granulite and felsic gneiss ttg gneiss of the dunhuang block reveal two distinct age populations the latest neoarchaean 2 5 ga and the late palaeoproterozoic 1 82 1 85 ga the zircon magmatic cores and metamorphic rims of tonalitic gneisses yield similar ages of 2 5 ga supporting the existence of archean rocks in the dunhuang block the short time interval between latest neoarchean magmatism and high grade metamorphism suggests that they were related to the same latest neoarchean tectonothermal event the 2 5 ga zircons show hf depleted mantle model ages between 2 55 ga and 2 8 ga with a peak at 2 7 ga suggesting that 2 7 ga juvenile crust of the dunhuang block was reworked during a 200 300 m y period at the end of the neoarchean the age of 1 85 ga obtained from the mafic granulites and felsic gneisses is interpreted as the age of high pressure granulite facies metamorphism combined with the hf isotopic data these data suggest that the dunhuang block experienced 2 7 ga crustal growth a 2 5 ga magmatic metamorphic event and a 1 82 1 85 ga hp granulite facies metamorphic event this sequence of events is very similar to that of the north china craton implying that the evolutionary history of the dunhuang block was consistent with that of the north china craton from the neoarchean to paleoproterozoic

formation and stabilization of the dunhuang block nw china constraints from the late paleoproterozoic a type granites of the dunhuang complex abstract the geochemical composition and temporal evolution of a type granites may hold key information regarding critical geodynamic processes that occurred during the evolution of the continental crust moreover these aspects may provide an indication of when orogenic processes ended and craton formation stabilization began in early precambrian blocks in this study we report the results of zircon u pb ages hf isotope data and whole rock geochemistry for the proterozoic huoyanshan potassic granites in the dunhuang block northwest china zircon u pb dating reveals that the huoyanshan granites were emplaced at ca 1791 1786 ma representing a late paleoproterozoic magmatism event the granites have high sio2 70 71 72 20 wt total alkali contents k2o na2o 8 81 8 96 wt k2o na2o 1 57 1 66 tio2 mgo ratios 2 19 2 57 low cao 0 77 1 27 wt mgo contents 0 12 0 16 wt and mg values 7 72 8 67 with a cnk ratios of 0 96 1 04 pointing to alkaline and metaluminous to weakly peraluminous affinities these granites are characterized by high total rare earth element concentrations ree 419 755 ppm and variable la yb n values 8 24 with significantly negative eu anomalies eu eu 0 31 0 42 the primitive mantle normalized trace element patterns suggest that the huoyanshan plutons are depleted in ba nb ta sr p and ti but enriched in rb th and pb additionally the granites have a high zirconium saturation temperature of 855 897 c with an average of 878 c these geochemical characteristics indicate that huoyanshan potassic granites are an aluminous a type granite and further classified as a2 type together with the zircon hf isotope data ehf t 11 21 to 2 76 indicates that these granites were generated from a crustal source the heterogeneous hf isotope compositions in zircons are principally caused by the disequilibrium melting of zr bearing sources in addition two stage hf model ages of zircons from the granites are ca 2 6 3 1 ga and plot on the average crustal evolution trend of archean tonalite trondhjemite granodiorite ttg gneiss 2 5 to 3 0 ga in the dunhuang block thus we propose that a type granites in the study area were generated from the disequilibrium partial melting of archean tonalitic to granodioritic rocks under high temperature and low pressure conditions according to their low al2o3 cao na2o k2o molar values and high 10000 ga al values 2 9 subsequently the magma experienced prominent fractional crystallization of k feldspar plagioclase orthopyroxene and or magnetite our results with available data suggest that the huoyanshan granites formed in a tectonic transform regime within a collisional to extensional setting and serve as a precursor for the formation and stabilization of the dunhuang block during the late paleoproterozoic furthermore it is probable that the post collisional magmatism event at ca 1 79 ga in the dunhuang block was closely related to the assembly of the columbia supercontinent

in situ hafnium and lead isotope analyses of detrital zircons from the devonian sedimentary basin of ne greenland a record of repeated crustal reworking an intramontane collapse basin developed within the hanging wall above the large scale extensional fjord regional detachment of ne greenland in middle to late devonian times the continental clastic sediments within the basin are derived locally from laurentian source rocks which makes them well suited for a study of the crustal evolution of the source terrain this is the first integrated in situ pb and hf isotope study to be presented and zircon data on a selected sandstone from the basin are combined with sm nd whole rock data on sand siltstones nd whole rock ages of two samples of sandstones and a siltstone are 2 0 2 1 ga peak frequencies of zircon 207pb 206pb ages at 1 764 1 912 ma and 176hf 177hf values at 0 28142 0 28163 tdm 2 47 to 2 06 for the sandstone suggest the generation of a considerable volume of juvenile continental crust in the ultimate zircon provenance at 1 9 2 0 ga the hf isotopic compositions of archaean zircons in the sandstone are distinct from those of the source materials of proterozoic protocrust at 1 9 2 0 ga but zircons with elevated hf tdm ages of up to 2 47 ga can be related to a component of archaean crust or reworked archaean material in the ultimate zircon source area zircon 207pb 206pb ages are also recorded at 1 480 1 572 1 318 and 1 014 ma grenvillian the hf isotope compositions of these zircons are consistent with reworking of the proterozoic protocrust at these times with little or no juvenile input the proterozoic zircons form two distinct groups defined by 176yb 177hf 0 05055 and 176yb 177hf 0 03301 and the latter group overlaps with yb hf isotope data on the archaean zircons the two groups may represent zircons derived from evolved granites and intermediate to mildly felsic rocks respectively the repeated reworking of the continental crust also comprised erosion and deposition of sediments in the proterozoic the krummedal sequence and the eleonore bay supergroup ebs and intrusion of caledonian anatectic granites in the ebs which both represent provenance components to the devonian sediments no discrete caledonian pb pb zircon ages are recorded but caledonian magmatism may be represented by strongly discordant zircons which form arrays with a lower intercept age at ca 400 ma and an upper intercept at 1 600 2 000 ma one undated zircon records a 176hf 177hf ratio of 0 282218 higher than that of the proterozoic protocrust in caledonian late caledonian times 380 450 ma which may represent a caledonian mantle contribution

shrimp u pb dating trace elements and the lu hf isotope system of coesite bearing zircon from amphibolite in the sw sulu uhp terrane eastern china abstract in this study we link mineral inclusion data trace element analyses u pb age and hf isotope composition obtained from distinct zircon domains of complex zircon to unravel the origin and multi stage metamorphic evolution of amphibolites from the sulu ultrahigh pressure uhp terrane eastern china zircon grains separated from amphibolites from the ccsd mh drill hole g12 and niushan outcrop g13 were subdivided into two main types based on cathodoluminescence cl and laser raman spectroscopy big dusty zircons with inherited cores and uhp metamorphic rims and small clear zircons weakly zoned grey white luminescent inherited cores preserve mineral inclusions of cpx pl ap qtz indicative of a mafic igneous protolith dark grey luminescent overgrowth rims contain the coesite eclogite facies mineral inclusion assemblage coe grt omp phe ap and formed at t 732 839 c and p 3 0 4 0 gpa in contrast white luminescent small clear zircons preserve mineral inclusions formed during retrograde hp quartz eclogite to lp amphibolite facies metamorphism t 612 698 c and p 0 70 1 05 gpa inherited zircons from both samples yield shrimp 206 pb 238 u ages of 695 520 ma with an upper intercept age of 800 31 ma the uhp rims yield consistent triassic ages around 236 225 and 239 225 ma for g12 and g13 with weighted means of 229 3 and 231 3 ma respectively small clear zircons from both samples give 206 pb 238 u ages around 219 210 ma with a weighted mean of 214 3 ma interpreted as the age of retrograde quartz eclogite facies metamorphism matrix amphibole from both samples indicate ar ar ages of 209 0 7 and 207 0 7 ma respectively probably dating late amphibolite facies retrogression the data suggest subduction of neoproterozoic mafic igneous rocks to uhp conditions in middle triassic 230 ma times and subsequent exhumation to an early hp 214 ma and a late lp stage 208 ma over a period of 16 and 6 myr respectively thus early exhumation from a mantle depth of 120 100 km to about 60 km occurred at an average rate of 0 3 cm y while subsequent exhumation to a middle crustal level took place at approximately 0 54 cm y these exhumation rates are considerably slower than those obtained for uhp rocks in the dora maira and kokchetav massifs 2 3 cm y based on similar p t estimates and trace element and hf isotope compositions sulu amphibolites can be identified as retrograde uhp eclogites the ehf 800 of 8 implies a significant input from the depleted mantle to the sulu dabie terrane during the middle neoproterozoic overgrown rims are characterized by a distinct trace element composition with low lu hf and th u and significantly higher 176 hf 177 hf ratios than inherited cores consistent with formation during after garnet re crystallization and fractionation of the lu hf system during uhp metamorphism the combined dataset suggests homogenization of the 176 hf 177 hf ratio within the metamorphic mineral assemblage and during protolith formation observed variations are explained by mixing of material from both domains during laser ablation e g due to partial recrystallization of inherited cores

precambrian evolution of the quanji block northeastern margin of tibet insights from zircon u pb and lu hf isotope compositions abstract the quanji block a cratonic fragment has been discovered between the north qaidam hp uhp metamorphic belt and the qilian orogenic belt in northwestern china this fragment has a pre neoproterozoic crystalline basement overlain unconformably by neoproterozoic paleozoic and mesozoic sedimentary strata pervasive anatexis of the basement produced extensive leucosomes in mafic and felsic migmatites zircons from the leucosomes yielded upper intercept and weighted mean 207 pb 206 pb ages of 1950 ma indicating that anatexis occurred in the late paleoproterozoic during regional high grade metamorphism hf model ages t dm2 for these zircons suggest that protoliths of the felsic and mafic migmatites had crustal residence time of 2 80 and 2 65 ga respectively the basement rocks of the quanji block are compositionally similar to those of the yangtze craton and may have been involved in assembly of the late paleoproterozoic global supercontinent columbia

a new analytical method for estimating antarctic ice flow in the 1960s from historical optical satellite imagery ice flow velocity is used to estimate ice mass changes in glaciers and is a significant indicator of the stability of the antarctica ice sheet in global change studies the existing regional antarctica ice flow speed maps are usually derived from radar or optical satellite observations of modern satellites since the 1970s this paper presents a new analytical photogrammetric method for estimating antarctica ice flow velocity fields by using film based stereo argon photographs collected in the 1960s the key of the proposed innovative method is a parallax decomposition that separates the effect of the terrain relief from the ice flow motion an innovative implementation strategy is developed by using a framework that involves key techniques of hierarchical stereo image matching ice flow direction determination parallax decomposition and ice flow speed estimation this method is applied in the rayner glacier in eastern antarctica by using two sets of argon images with a two month interval in 1963 the produced digital terrain model and speed map achieved a ground position accuracy of 61 m and a speed accuracy of 70 m text a 1 a comparison with recent products from 2000 to 2010 shows no significant topographic changes in the study area furthermore the speed around the grounding line remained at the same level while the speed in the ice shelf front decreased by 73 m text a 1 the ice shelf front advanced by approximately 7 km over more than 40 years overall the observation results indicate favorable conditions for the stability of the rayner glacier ice shelf system

geochronology and geochemistry of the neoarchean lulong complex in the eastern hebei province north china craton implications on regional crustal evolution abstract the lulong complex exposed in the southeastern part of the eastern hebei province in the north china craton ncc is mainly composed of meta basalt biotite plagioclase gneiss migmatite amphibolite and granitoids mineral assemblages indicate the rocks in this complex underwent upper amphibolite facies metamorphism zircon u pb and lu hf data constrain that both protolith of the biotite plagioclase gneiss and the homogeneous domain of the migmatite formed at 2 53 ga and were mainly derived from the reworking of 2 8 2 7 ga crustal source the similar mineral association zircon u th pb systematics and geochemical features suggest that their protolith should be dominated by felsic volcanic sequences with limited contributions from older crust the protolith of the amphibolite gabbro or diabase was metamorphosed at 2477 20 ma geochemistry and zircon lu hf systems indicate that these meta mafic rocks originated from an enriched subcontinental lithospheric mantle source the granitoids exposed in the lulong area yield zircon u pb ages of 2507 21 ma to 2473 38 ma and record 2 5 ga regional metamorphic event the initial zircon 176hf 177hf ratios of the granite are in the range of 0 280640 0 281678 and suggest a complicated source where as those from the granodiorite indicate melting of juvenile crustal materials geochemically the felsic meta volcanics of the lulong complex are comparable to rhyodacite dacite with calc alkaline affinities the granitoids suite comprises 2 47 ga peraluminous granite felsic dikes and 2 51 ga metaluminous granodiorite all these felsic intrusions are characterized by moderate differentiation of ree patterns and low sr y ratios

cretaceous arc volcanism of palmer land antarctic peninsula zircon u pb geochronology geochemistry distribution and field relationships abstract the cretaceous marks an episode of enhanced magmatism sedimentation and tectonic processes along the entire proto pacific convergent margin of west gondwana cretaceous magmatism across the antarctic peninsula is dominated by the intrusive lassiter coast intrusive suite which developed as episodic flare up events during the mid cretaceous at a time of increased convergence volcanic rocks of this age are poorly defined as a consequence of limited field observations and an absence of accurate geochronology recent field mapping combined with unpublished field observations has identified a region 10 000 km2 of dominantly subaerial rhyolitic pyroclastic and epiclastic successions from northern palmer land of the antarctic peninsula volcanic successions up to 1500 m in thickness consist of dominantly silicic ignimbrites lavas heterolithic breccias and lahar deposits fed by caldera forming eruptions the volcanic rocks of this region were widely considered to be early cretaceous in age based on field relationships and early geochronology new u pb zircon ages identify three distinct volcanic episodes during the late cretaceous early cenozoic at 108 ma 93 ma and 64 ma lu hf data indicate significant proportions of ancient crust in the petrogenesis of the volcanic rocks and the slightly negative ehf values are consistent with a mid position 120 km within the magmatic arc relative to more juvenile compositions close to the trench

mantle and crustal sources of archean anorthosite a combined in situ isotopic study of pb pb in plagioclase and lu hf in zircon isotopic analyses of ancient mantle derived magmatic rocks are used to trace the geochemical evolution of the earth s mantle but it is often difficult to determine their primary initial isotope ratios due to the detrimental effects of metamorphism and secondary alteration we present in situ analyses by la mc icpms for the pb isotopic compositions of igneous plagioclase an75 89 megacrysts and the hf isotopic compositions of bse imaged domains of zircon grains from two mantle derived anorthosite complexes from south west greenland fiskenaesset and nunataarsuk which represent two of the best preserved archean anorthosites in the world in situ la icpms u pb geochronology of the zircon grains suggests that the minimum crystallization age of the fiskenaesset complex is 2 936 13 ma 2 mswd 1 5 and the nunataarsuk complex is 2 914 6 9 ma 2 mswd 2 0 initial hf isotopic compositions of zircon grains from both anorthosite complexes fall between depleted mantle and a less radiogenic crustal source with a total range up to 5 ehf units in terms of pb isotopic compositions of plagioclase both anorthosite complexes share a depleted mantle end member yet their pb isotopic compositions diverge in opposite directions from this point fiskenaesset toward a high more radiogenic pb crustal composition and nunataarsuk toward low less radiogenic pb crustal composition by using hf isotopes in zircon in conjunction with pb isotopes in plagioclase we are able to constrain both the timing of mantle extraction of the crustal end member and its composition at fiskenaesset the depleted mantle melt interacted with an eoarchean 3 700 ma mafic crust with a maximum 176lu 177hf 0 028 at nunataarsuk the depleted mantle melt interacted with a hadean 4 200 ma mafic crust with a maximum 176lu 177hf 0 0315 evidence from both anorthosite complexes provides support for the long term survival of ancient mafic crusts that although unidentified at the surface to date could still be present within the fiskenaesset and nunataarsuk regions

u pb ion microprobe zircon geochronology from the basement inliers of eastern graham land antarctic peninsula new geological mapping combined with u pb ion microprobe zircon geochronology on the isolated but locally extensive exposures of crystalline basement inliers of eastern graham land has greatly improved our understanding of the region s early crustal evolution and has allowed a more thorough evaluation of patagonia antarctic peninsula connections prior to gondwana break up at eden glacier diorite gneisses yield early ordovician protolith ages of 487 3 and 485 3 ma and represent the oldest in situ rocks recorded on the antarctic peninsula and indicate a significant spatial extension of famatinian age magmatism of patagonia zircon overgrowths in the early ordovician protoliths and granitic leucosomes developed within them record two phases of permian metamorphism at c 275 and c 257 ma coincident in part with diorite plutonism of the area at 272 2 ma at adie inlet granitic leucosomes from paragneiss have been dated at 276 3 ma and these are in turn cut by 257 3 ma xenolith rich diorite gneiss the diorite intruded during a second phase of deformation which folded the paragneiss leucosomes into tight folds this whole assembly is cut by intensely brecciated megacrystic granodiorite which yielded a 259 3 ma age south of cabinet inlet a very different sequence of events is evident with triassic magmatism at c 236 ma extensive along the joerg peninsula migmatitic leucosomes are dated at c 224 ma and magmatism and deformation events apparently continued to c 209 ma at cape casey our data indicate that the devonian and carboniferous magmatism at target hill considered to represent the classic basement complex of the antarctic peninsula is not representative regionally the target hill crustal block contains a major break along cabinet inlet to the north ordovician and permian protoliths were variably migmatized during two episodes of permian deformation and metamorphism whereas to the south triassic protoliths and triassic metamorphism are encountered supplementary material u pb isotopic data are available at http www geolsoc org uk sup18526

a rare earth element study of complex zircons from early archaean amitsoq gneisses godthabsfjord south west greenland we present high spatial resolution ion microprobe rare earth element ree data for discrete growth phases of complex polyphase zircons from early archaean amitsoq gneisses outer godthabsfjord sw greenland in matsuda diagrams the two major growth phases 3 8 ga cores and ca 3 65 ga rims have steep positive slopes from la to lu prominent positive ce anomalies and negative eu anomalies that are consistent with growth in a melt exceptions to this are non cathodolurnmescent zircon developed between the cores and rims sometimes truncating zoning in the cores and late archaean prismatic tip overgrowths both of which exhibit flatter light ree lree patterns and have small or no eu anomaly which we interpret as the result of metamorphism and or small degree isolated partial melting our data support previous interpretations that the ca 3 65 ga zircon phase was generated in a melt with the 3 8 ga phase representing either original protolith zircons in a large degree partial melt or inherited zircons in an introduced magma regardless which of these two interpretations is correct for these and similar rocks in the outer godthabsfjord the 3 65 ga event will have profoundly affected isotopic systems and obscured beyond recognition any earlier igneous features such as cross cutting relationships which may only be assigned a minimum 3 65 ga age c 2003 elsevier science b v all rights reserved

u pb age and hf isotope geochemistry of zircon from felsic volcanic rocks of the paleoproterozoic aillik group makkovik province labrador abstract u pb zircon shrimp geochronology and lu hf zircon la mc icpms isotope geochemistry are used to constrain the ages of paleoproterozoic felsic magmatism and their crustal sources in the aillik group makkovik province of labrador this information is important for understanding the extent of crustal reworking vs juvenile crustal growth in subduction related magmatism during the paleoproterozoic and improves framework knowledge for uranium exploration in the central mineral belt of labrador u pb shrimp zircon geochronology from four felsic tuff samples collected from two different areas of the aillik group yield magmatic 207pb 206pb ages of 1852 7 ma 2 1854 7 ma 2 1861 7 ma 2 and 1862 7 ma 2 the ages fall within the range of previous u pb zircon dates reported for felsic volcanic rocks of the aillik group altogether these dates indicate that felsic volcanism spanned at least 30 m y and was perhaps most intense during the last 10 m y of that interval a foliated syn deformational monzogranite in the aillik group cross lake granite yields a 207pb 206pb zircon date of 1805 6 ma 2 which indicates that development of a pervasive planar fabric associated with northwestward thrusting of the aillik group continued after this time initial e h f s t values for magmatic zircons from the four felsic tuffs 4 8 to 1 9 1 7 to 2 2 11 8 to 3 8 9 0 to 4 6 and monzogranite 4 8 to 1 5 exhibit significant variability which indicates heterogeneous sources most of the zircon grains have negative initial e h f s t values and calculated crust formation hf isotope tdm model ages point to substantial contributions from melts of ancient crust ranging in age from ca 2200 to 2900 m y and clustering at ca 2500 m y inherited zircon grains in the felsic tuffs and monzogranite mainly have u pb ages of ca 1880 1920 ma though others are older up to ca 2743 ma hf isotope tdm model ages for the inherited zircons are similar to those of the magmatic zircons the hf isotope results indicate that subduction related felsic magmatism in the makkovik orogen involved significant crustal reworking 80 during formation of the aillik group

granitoid pluton formation by spreading of continental crust the wiley glacier complex northwest palmer land antarctica abstract the emplacement mechanism geometry and isotope geochemistry of plutons of the wiley glacier complex suggest that new continental crust grew by multiple injection of tonalitic dykes during dextral transtension in the antarctic peninsula magmatic arc in early cretaceous times the suggested mechanism is analogous to basalt dyke injection during sea floor spreading during normal dextral shear the burns bluff pluton a sheeted moderately east dipping syn magmatically sheared tonalite granodiorite intruded syn magmatically sheared quartz diorite of the creswick gap pluton and 140 5 ma hornblende gabbro u pb dating of zircon and ar ar dating of hornblende and biotite suggest that both granite s l plutons were emplaced between 145 and 140 ma but that extensional shearing was active from the time of emplacement until ca 127 ma the burns bluff pluton is chilled at its margin and grades through mylonitised porphyritic tonalite granodiorite sheets and tonalite granodiorite sheets with minor chilling to a kilometre scale body of coarse grained hypidiomorphic tonalite granodiorite co magmatic microdiorite forms dykes and abundant synplutonic mafic enclaves these dykes opened as echelon veins during episodic dextral shear and were deformed to trains of enclaves during continued normal dextral shear pluton marginal porphyritic and hypidiomorphic tonalite granodiorite forms large fault hosted sheets emplaced progressively under extension with minor dextral shear kinematic indicators from pluton marginal granite s l dykes suggest that early in pluton accretion intrusive sheets cooled rapidly with simple shear prior to full crystallisation changing to ductile simple shear during cooling kinematic indicators towards the pluton core suggest that as the pluton grew and cooled more slowly emplacement switched from sheeting to in situ inflation with simple shear distributed across a broad zone prior to full crystallisation of magma cross cutting relationships with the coeval syn extensional creswick gap pluton suggest that the burns bluff pluton was emplaced in a steeper second generation shear structure like those in normal fault systems this suggests that the wiley glacier complex was emplaced above the base of the brittle ductile transition zone 15 18 km depth the burns bluff pluton has nd and sr isotope values that range from mantle dominated nd141 3 8 87 sr 86 sr 141 0 70468 to more crustally influenced nd141 1 7 87 sr 86 sr 141 0 70652 this range probably represents different degrees of mixing between mantle derived magma and lower crustal partial melts generated in the garnet stability zone 40 km depth addition of new crustal material by mafic underplating at the base of the crust and by redistribution of granitic s l and mafic modified underplated magma to mid crustal levels along extensional shear zones as the arc spread were the primary mechanisms of crustal growth

the role of crustal and mantle sources in the genesis of granitoids of the antarctic peninsula and adjacent crustal blocks abstract magmatic rocks from the antarctic peninsula show marked variations in isotope composition which reflect changes in the geodynamic evolution of the peninsula through time most antarctic peninsula granitoids formed as a result of subduction they fall on well defined trends on plots of nd 207 pb 204 pb and 18 o against 87 sr 86 sr i between a component derived from subduction modified mantle or juvenile basaltic underplate ndi 6 207 pb 204 pb 15 61 18 o 5 5 87 sr 86 sr nd 7 207 pb 204 pb 15 67 18 o 10 87 sr 86 sr 0 709 a small group of granitoids emplaced before or during gondwana break up plot on distinct trends towards high 87 sr 86 sr i compositions reflecting mixing between melts derived from proterozoic lower crust and melts of middle upper crustal rocks ndi 9 207 pb 204 pb 15 64 18 o 10 87 sr 86 sr 0 726 with little or no input of new material derived from the mantle or from juvenile basaltic underplate these granitoids are thought to have formed as a result of crustal attenuation during the initial rifting phase of gondwana break up similar trends are shown by data from granitoids of the adjacent crustal blocks of west antarctica the isotope data suggest that an enriched ferrar karoo type lithosphere was not involved in the genesis of granitoids of the antarctic peninsula or of the ellsworth whitmore mountains crustal block

episodicity within a mid cretaceous magmatic flare up in west antarctica u pb ages of the lassiter coast intrusive suite antarctic peninsula and correlations along the gondwana margin long lived continental margin arcs are characterized by episodes of large volume magmatism or flare ups that can persist for 30 m y before steady state arc conditions resume flare up events ar

basement chronology of the antarctic peninsula recurrent magmatism and anatexis in the palaeozoic gondwana margin abstract a revised pre jurassic chronology is presented for the antarctic peninsula on the basis of new u pb zircon dating using both conventional 11 samples and shrimp microprobe data 10 samples the age range for plutonism and high grade metamorphism is from 435 8 ma silurian to c 206 ma approximately the triassic jurassic boundary with peaks of activity in devonian carboniferous permian and mid triassic times although some are represented by rocks with limited outcrops the new data confirm the importance of mid and latest triassic magmatic events previously identified using rb sr geochronology the adie inlet gneiss previously thought to be neoproterozoic is recognized as a permian migmatite derived from paragneiss with a provenance dominated by cambrian granitoids granite gneiss from nw palmer land previously dated as cambrian is shown to be triassic detrital zircons in metasedimentary rocks and inherited zircons in granitoids are dominated by mesoproterozoic to cambrian components with sparse palaeoproterozoic to archaean grains suggesting sources within gondwana at times these sources were nearby as shown by an archaean cobble sized granite clast from a permo triassic conglomerate another clast in the same conglomerate previously thought to be devonian in age has an ordovician crystallization age

u pb zircon studies of mid crustal metasedimentary enclaves from the s type deddick granodiorite lachlan fold belt se australia

west antarctica in gondwanaland crustal blocks reconstruction and breakup processes abstract a combined bas usarp west antarctic tectonic project has provided new constraints on the crustal structure and geological evolution of west antarctica and its relationship to the rest of gondwanaland rb sr age dating has confirmed the presence of 1200 1000 m y old precambrian gneisses within west antarctica and aeromagnetic data have defined the extent of this precambrian crustal block paleomagnetic data have constrained the position of four of the five geologically distinctive west antarctic crustal blocks within gondwanaland they can be restored to their original position prior to breakup by 15 25 counter clockwise rotation with little or no relative displacement between the blocks a major period of within plate middle jurassic magmatism associated with an important crustal melting event that is clearly related to the breakup of the supercontinent has also been identified based on new isotopic and geochemical data

multiple partial melting events in the sulu uhp terrane zircon u pb dating of granitic leucosomes within amphibolite and gneiss thin layers and lenses of granitic leucosome are widely distributed within amphibolites paragneisses and orthogneisses of the sulu uhp terrane they are parallel to or cross cut foliations in the host rocks at different scales and show evidence of coalescence and migration to form centimetre to decimetre scale segregations variously migmatized rocks extend at least 350 km from sw sulu maobei to ne sulu weihai in a band at least 50 km wide a combined study of mineral inclusions cathoduluminescence cl images u pb la icp ms dates and in situ trace element compositions of zircon provide clear evidence on the nature and timing of partial melting in these uhp rocks most zircon from the granitic leucosomes occurs as distinct overgrowths around inherited igneous or metamorphic cores or as new euhedral crystals the overgrowths and new crystals commonly show perfectly euhedral shapes have pronounced oscillatory zoning and contain felsic mineral inclusions such as kfs pl qtz ilm monazite mon in contrast the inherited igneous or metamorphic cores are rounded or irregular contain low p or uhp mineral inclusions and show clear dissolution textures these data suggest that the new zircon is anatectic in origin and that it grew during partial melting of the uhp rocks the ree patterns of the anatectic zircon show steep slopes from the hree to lree with strongly to moderately negative eu anomalies eu eu 0 31 0 72 and pronounced positive ce anomalies ce ce 6 8 26 5 abundant u pb spot analyses of the anatectic zircon reveal two discrete and meaningful ages of partial melting within the sulu uhp terrane anatectic zircon from 12 granitic leucosomes within amphibolites paragneisses and orthogneisses from sulu uhp slices ii and iii yields consistent mean u pb ages of 219 0 1 2 to 218 3 1 6 ma 218 8 2 0 to 217 3 1 7 ma and 218 2 1 4 to 215 0 1 5 ma respectively in contrast anatectic zircon from six granitic leucosomes within paragneisses and orthogneisses from sulu uhp slice iii records younger mean u pb ages of 151 9 1 3 to 151 1 1 8 ma and 155 9 1 8 to 153 7 1 7 ma respectively these data imply that the sulu uhp terrane experienced two mesozoic partial melting events the first partial melting event 219 215 ma was probably associated with a late triassic granulite facies stage of hot exhumation whereas the second 156 151 ma is interpreted as the result of middle late jurassic extension and thinning of the previously thickened crust of the sulu uhp terrane both partial melting events induced extensive retrograde metamorphism of the eclogites and their country rocks

zircon u pb ages and hf isotope compositions of migmatites from the north qinling terrane and their geological implications migmatites are predominant in the north qinling nq orogen but their formation ages are poorly constrained this paper presents a combined study of cathodoluminescence imaging u pb age trace element and hf isotopes of zircon in migmatites from the nq unit in the migmatites most zircon grains occur as new homogeneous crystals while some are present as overgrowth rims around inherited cores morphological and trace element features suggest that the zircon crystals are metamorphic and formed during partial melting the inherited cores have oscillatory zoning and yield u pb ages of c 900 ma representing their protolith ages the early neoproterozoic protoliths probably formed in an active continental margin being a response to the assembly of the supercontinent rodinia the migmatite zircon yields hf model ages of 1911 20 to 990 22 ma indicating that the protoliths were derived from reworking of palaeoproterozoic to neoproterozoic crustal materials the anatexis zircon yields formation ages ranging from 455 5 to 420 4 ma with a peak at c 435 ma combined with previous results we suggest that the migmatization of the nq terrane occurred at c 455 400 ma the migmatization was c 50 ma later than the c 490 ma ultra high p uhp metamorphism indicating that they occurred in two independent tectonic events by contrast the migmatization was coeval with the granulite facies metamorphism and the granitic magmatism in the nq unit which collectively argue for their formation due to the northward subduction of the shangdan ocean uhp rocks were distributed mainly along the northern margin and occasionally in the inner part of the nq unit indicating that they were exhumed along the northern edge and detached from the basement by the subsequent migmatization process

zircon u pb ages and hf o isotopic composition of migmatites from the zanjan takab complex nw iran constraints on partial melting of metasediments abstract we study migmatites and other metamorphic rocks in the zanjan takab region of nw iran and use these results to report the first evidence of oligocene core complex formation in iran four samples of migmatites associated with paragneisses including leucosomes and associated para amphibolite melanosomes were selected for u pb dating and hf o isotopic analysis zircon cores interpreted as originally detrital zircons have variable ages that peak at ca 100 110 ma but their sedimentation age indicated by the youngest 206 pb 238 u ages is ca 35 40 ma new zircons associated with incipient melting occur as overgrowths around zircon cores and or as newly grown grains morphologies and internal structures suggest that rim growth and formation of new zircons were associated with partial melting all four samples contain zircons with rims that yield 206 pb 238 u ages of 28 25 ma indicating that partial melting occurred in late oligocene time 18 o values for zircon rims vary between 8 2 and 12 3 significantly higher than expected for mantle inputs 18 o 6 and consistent with equilibrium with surface materials zircon rims yield ehf t between 2 2 and 12 4 and two stage hf model ages of 448 562 ma indicating that the region is underlain by cadomian caledonian crust according to the hf o isotopic values the main mechanism forming zircon rims was dissolution of pre existing detrital zircons with reprecipitation of new zircon shortly thereafter oligocene ages indicate that partial melting accompanied core complex formation in the zanjan takab region extension melting and core complex formation in south central iran are eocene in age but younger ages of oligocene miocene in nw iran and turkey indicate that extension was distributed throughout the region during cenozoic time

presence of palaeoproterozoic and archean components in the granulite facies rocks of central iberia the hf isotopic evidence abstract hf isotope analyses in previously dated zircon crystals from four variscan granulite facies rocks from central iberia have been studied to examine crustal evolution processes two samples are from exposed migmatite terranes the other two are granulite xenoliths from lower crustal levels hf model ages show that lower crustal granulites display a bigger contribution of archean components than exposed high grade rocks 14 of the zircon population instead of 6 reinforcing ideas of preservation of ancient crustal material in the lower crust the distribution of age hf isotope data from zircon suggests two juvenile crust formation events in central iberia the main one during the neoproterozoic 540 620 ma and a possible second mesoproterozoic event around 1 0 1 2 ga magmatism appears in the central iberian zone exclusively related to that younger juvenile addition hf data of granulite zircons agrees with recent palaeogeographical reconstruction models which present central iberia as a peri gondwanan terrane located between the west african craton and the arabian shield far from the amazonia microplate

granulite facies metamorphism and crust melting in the huai an terrane at 1 95ga north china craton new constraints from geology zircon u pb lu hf isotope and metamorphic conditions of granulites abstract geological implications of 1 95 ga and 1 85 ga metamorphic ages for hp granulites in the huai an terrane are still controversial which have affected the crustal evolution models for the north china craton this study reports two geological sections in which distinct geological relationships of a quartz dioritic and a tonalitic dike cutting through mafic hp granulite and granulite facies meta sedimentary rocks are described in these two sections magmatic dikes mafic hp granulites and meta sedimentary rocks preserve conformable foliation to indicate their similar deformation history the u pb dating and lu hf isotopic analysis of zircons from metamorphic and meta igneous rocks provide new constraints on the timing and mechanism of thermal events associated with metamorphic and deformation events both magmatic dikes were formed around 1 95 ga and overprinted by later metamorphic event during 1 85 ga the s type granite at geological section gs 1 revealed its formation age was formed around 1959 34 ma n 3 mswd 1 5 and overprinted by later metamorphic event at 1840 8 ma n 10 mswd 1 2 additionally zircons from the mafic hp granulite in the gs2 are dominated by metamorphic ages of 1972 22 ma n 6 mswd 0 03 with few grains recording an age of 1856 29 ma n 3 mswd 0 17 in situ lu hf analyses on zircons from the quartz dioritic dike tonalitic dike mafic hp granulite and s type granite reveal that the magmatic dikes were derived from partial melting of 2 0 2 2 ga meta basic rocks and the s type granite was generated by melting of meta sediments beyond 840 50 c it suggested that those meta sediments were taken into the lower crust in a certain way at granulite facies conditions ca 1 95 ga ago a clockwise p t path acquired from mafic hp granulite revealed peak metamorphic conditions of 880 890 c and 11 13 kbar the temperature results recorded in the 1 95 ga zircons of mafic hp granulite similar to those of high pressure mineral assemblage in matrix suggest that the hp granulite metamorphism was approximately 1 95 ga ago according to the results above this contribution concludes that mafic and meta sedimentary rocks were metamorphosed together under high pressure granulite metamorphic conditions 1 95 ga ago followed by high temperature granulite facies melting event that gave rise to granite and s type granite at 1 95 ga the granulite facies metamorphic rocks and magmatic dikes in the huangtuyao graphite mine experienced the same evolution history after 1 95 ga the mafic and pelitic hp granulites in the huai an terrane are interpreted to be collisional products of the paleoproterozoic cratonization of the ncc

age and origin of post collision baltoro granites south karakoram north pakistan insights from in situ u pb hf and oxygen isotopic record of zircons abstract origin of post collision plutonism is critical to understand the tectonothermal evolution of the over thickened continental crust in collision zones this has proven difficult to reconcile with the conventional whole rock geochemical and field based studies alone we report in situ study of zircon u pb hf and o isotopes from five samples of the baltoro plutonic unit bpu in south karakoram the plutonic unit is the western part of the southern asian margin of the india asia convergent zone baltoro granites and a biotite rich enclave yielded similar and overlapping u pb ages ranging from 26 to 15 ma hafnium isotopic composition ehf 0 is very heterogeneous ranging from 17 1 to 4 4 while the oxygen isotopic composition of the granites is homogeneous with mean 18 o ranging from 7 2 to 9 4 based on u pb geochronology and hf o isotopic composition the involvement of two main sources is suggested 1 cretaceous calc alkaline karakoram crust and 2 karakoram gneisses moreover possible involvement of metasomatized asian lithospheric mantle is supported by elevated oxygen composition of granites and identical hf composition of biotite rich enclave to the mantle derived baltoro lamprophyre however direct contribution from juvenile pristine mantle is unlikely as no juvenile mantle type hf and oxygen values were obtained this also precludes the involvement of southward juvenile arc related component of kohistan ladakh batholith our new u pb and hf data are comparable to the mesozoic karakoram batholith miocene two mica leucogranites in the pangong range and magmatism from the lhasa terrane in south tibet suggesting a genetic link between the karakoram and the rocks to the east this magmatic event is best explained by lower crust partial melting promoted by both thermal equilibration following crustal thickening and heat advection by ultrapotassic magmas associated with the breakoff of the indian continental margin

a review of the geochronological constraints on the evolution of the kaapvaal craton south africa abstract all the published u pb geochronological data from zircon titanite sphene and monazite and pb pb evaporation data applicable to the palaeoarchaean to neoarchaean geology of the kaapvaal craton are employed to reconstruct the history and crustal architecture of the ca 3600 2500 ma cratonic basement in south africa only data interpreted as representative of the age of crystallisation or the 207 pb 206 pb ages from zircon xenocrysts have been considered in constructing graphs of cumulative probability versus age of rock formation in this manner it has been possible to identify the main magmatic events that have contributed to the assembly of the craton for convenience of data handling the kaapvaal craton has been subdivided into eastern central northern and western geographic domains these domains are not intended as discrete geological terranes although each one has its own geological and chronological character although consanguinity of events is also evident the oldest rocks so far recognised are located in the swaziland barberton areas where ages 3600 ma have been recorded the early stages of shield development are best exposed in the barberton mountain land where it is now apparent that continent formation took place by magmatic accretion and tectonic amalgamation of small protocontinental blocks at barberton several diachronous blocks formed between 3600 and 3200 ma have been identified each of which represents a cycle of arc related magmatism and sedimentation this phase of crustal development was followed by a period of mesoarchaean cratonic magmatism particularly prevalent between approximately 3100 3000 ma and marked by the development of a major crescent shaped juvenile arc that was accreted onto the northern and western margins of the evolving kaapvaal shield cratonisation was accomplished by the emplacement of major granitoid batholiths which thickened and stabilised the continental crust during the early stages of this cycle subsequent evolution of the craton between 3000 and 2700 ma was associated with continent arc collision during which time the witwatersrand basin and its correlatives were deposited as foreland sequences followed by episodic extension and rifting when the gaborone kanye and ventersdorp sequences were developed

the source of granitic gneisses and migmatites in the antarctic peninsula a combined u pb shrimp and laser ablation hf isotope study of complex zircons zircons gneisses and migmatites collected from the antarctic peninsula have different core rim hafnium isotope ratio relationships depending on whether evidence for zircon dissolution is present or absent two samples contain inherited zircon that is partially dissolved in these samples the 176hf 177hf rations of the inherited zircon and new magmatic zircon rims are on average indistinguishable and consistent with in situ melting in such cases the hafnium isotopic composition of the melt was probably strongly influenced by the dissolved zircon component at the source variation in 176hf 177hf within the magmatic zircon rims from grain to grain suggests that hf isotopes were only partially homogenized during melt migration alternatively zircon growth may have taken place within small volumes of partial melt other samples do not preserve textural evidence for zircon dissolution during melt generation in these samples the 176hf 177hf values of the inherited zircon and new magmatic zircon rims are different the zircon rims apparently suggest a source of less evolved hafnium than that contained within the inherited zircon whether this relates to a separate juvenile source or alternatively is derived from minerals other than zircon at the source cannot be resolved inherited zircon irrespective of age has been strongly influenced by the reworking of a juvenile late mesoproterozoic source suggesting that such crust underlies the antarctic peninsula our results therefore suggest that hf isotope analyses provide great potential for future studies investigating the source and processes involved in the generation of crustal melts

the eastern palmer land shear zone a new terrane accretion model for the mesozoic development of the antarctic peninsula a major ductile fault zone the eastern palmer land shear zone has been identified east of the spine of the southern antarctic peninsula this shear zone separates newly identified geological domains and indicates that during late jurassic terrane accretion and collision two and possibly three separate terranes collided resulting in the palmer land orogeny the orogeny is best developed in eastern palmer land and eastern ellsworth land there shallow marine sedimentary rocks of the latady formation and a metamorphic and igneous basement complex of possible lower palaeozoic to pre early jurassic age are thrust and folded this forms an arcuate east directed foreland fold and thrust belt up to 100 km wide and 750 km long parallel to the axis of the antarctic peninsula the newly identified antarctic peninsula domains include 1 a parautochthonous eastern domain that represents part of the margin of the gondwana continent comparable to the western province of new zealand the ross province superterrane of marie byrd land the eastern series of south central chile the pampa de agnia and tepuel rocks of north patagonia and the cordillera darwin rocks of tierra del fuego 2 a suspect central domain that represents an allochthonous microcontinental magmatic arc terrane comparable to the median tectonic zone of new zealand the amundsen province superterrane of marie byrd land and coastal cordillera of north chile and 3 a suspect western domain with strong similarities to the eastern province of new zealand western series of south central chile and chonos metamorphic complex of north patagonia that represents either a subduction accretion complex to the central domain or another separate crustal fragment although an allochthonous terrane hypothesis for the antarctic peninsula remains to be fully tested this has much in common with models for the new zealand and south american parts of the pacific margin of gondwana the identification of a potential allochthonous terrane continent collision zone allows us to define the edge of the gondwana continent in the antarctic peninsula sector of the supercontinent margin which has implications for mesozoic reconstructions of gondwana

generation of i type granitic rocks by melting of heterogeneous lower crust extract granite generation is a fundamental process for the growth and evolution of earth s continental crust i type granitic rocks nominally derived from infracrustal sources are the most common granite type and are voluminously emplaced in convergent margin settings a puzzling feature is that many i type granites show isotopic evidence for reworking of older supracrustal material in conflict with the i type designation how the supracrustal component was incorporated by i type magmas is a matter of deduction particularly given difficulties in recognizing the putative infracrustal source region in the exposed geology we report a case study of i type granitic magma generation by hybridization between metasedimentary derived partial melt and intercalated mafic granulite units during extraction of silicic magma from the lower crust in the hidaka metamorphic belt hmb japan hammerli et al 2018 isotopic data nd hf o obtained by microanalysis of accessory minerals in former melt networks leucosomes suggest that hybridization operates on a sub grain scale where repeated injections of externally derived melt attempt to approach local equilibrium with the host mafic granulites during transfer through complex melt pathways see our figure 3 and also hasalova et al 2011 and references therein

zircon petrochronology reveals the timescale and mechanism of anatectic magma formation abstract igneous rocks of intermediate to acidic composition commonly exhibit considerable degrees of isotope variability preserved at the crystal and sub crystal scale as well as a significant u pb age range reflecting protracted timescales of zircon crystallization and long magma residence times the association of high precision u pb zircon dates with stable and radiogenic isotope data represents a powerful tool to unravel the petrological evolution of granitic rocks hence allowing a better understanding of the processes that led to the formation and reworking of the continental crust in this case study we combine high precision u pb dates with stable and radiogenic isotope data from zircon crystals in the larderello travale italy shallow level granites these rocks are peraluminous two mica cordierite bearing granites and represent pure crustal anatectic magmas generated in a post collisional extensional setting as such they are ideal candidates to investigate the timing rates and mechanisms of melt production during anatectic magma formation giving insights into the process of intracrustal differentiation magmatic zircon crystals from the larderello travale granites contain 18 o values ranging from 8 6 to 13 5 and crystals from individual samples exhibit inter and intra grain oxygen isotope variability exceeding 3 the analysed crystals have ehf values ranging between 7 4 and 12 4 with moderate intra sample ehf isotope variability all ca id tims chemical abration isotope dilution thermal ionization mass spectrometry 206pb 238u zircon ages range from 4 5 to 1 6 ma and suggest four pulses of magmatic activity at 3 6 3 2 2 7 and 1 6 ma more importantly zircon crystals from individual samples typically exhibit an age spread as large as 300 500 ka this age dispersion suggests that most of the zircon did not crystallize at the emplacement level but in the middle crust and were subsequently recycled and juxtaposed during ascent and emplaced at shallow level when plotted against age stable and radiogenic isotope data suggest the co existence of multiple and isotopically distinct magma batches produced by partial melting of different crustal domains this requires coeval magma batches that are physically separated and evolve independently for hundreds of thousands of years before coalescing during ascent and emplacement the involvement of multiple sources in the production of crustal anatectic magmas reflects the inherent heterogeneous nature of the continental crust and result from the interplay between the rise and evolution of the geotherms through the crust and the composition of the fertile source rocks finally the isotopically diverse zircon bearing magma batches mixed and assembled into shallow level intrusions generated during the four major magma pulses

early silurian granitic rocks and associated enclaves as evidence of rapid cooling in a cognate magma system the case of the xuehuading panshanchong pluton south china block

in situ zircon u pb oxygen and hafnium isotopic evidence for magma mixing and mantle metasomatism in the tuscan magmatic province italy in this study we have used in situ u pb hf and o isotopic analyses of zircon grains to gain insights into both magmatic processes and duration of magmatism in igneous rocks from the tuscan magmatic province 0 1 9 ma italy three plutonic centres have been investigated monte capanne and porto azzuro monzogranites in elba and the giglio monzogranite as well as capraia the only volcanic centre in the tuscan archipelago new ion microprobe zircon u pb data reveal a continuum of plutonic activity in elba over 2 ma 8 3 6 3 ma with coeval volcanic activity in capraia 7 1 7 6 ma and plutonic activity resuming in giglio 5 5 ma after a gap of 1 ma from these zircon data we also show that construction of the monte capanne pluton elba may have occurred over a period of c 0 5 ma a significant range of both 176hf 177hf determined by la mc icpms and 18o determined by ion microprobe in zircon 7 epsilon hf units and 5 respectively is present which together with zircon morphology and trace element data gagnevin et al 2010 emphasises the importance of mixing and replenishment involving magma batches with both metaluminous and peraluminous affinities inherited and xenocrystic zircons also occur but are scarce these have a wide range of 176hf 177hf and 18o values further emphasising that a variety of crustal components has contributed to the genesis of the tuscan magmas either as contaminants or magma sources while mixing undoubtedly occurred between mafic metaluminous and felsic peraluminous magmas the range of hf and o isotopic data suggests a diversity within the peraluminous component the unradiogenic hf composition ehf t 6 of the inferred mantle derived component represented by capraia volcanism and at least in part lamproitic in composition strongly supports the idea that the mantle source involved in tuscan magmatism was severely modified by subduction related crustal derived metasomatic fluids

early cretaceous low mg adakitic granites from the dabie orogen eastern china petrogenesis and implications for destruction of the over thickened lower continental crust it is generally accepted that the low mg adakitic rocks were derived from the partial melting of metabasalts eclogites in this study we demonstrate that the early cretaceous low mg adakitic granites in the north dabie complex ndc were generated by the partial melting of the ndc orthogneisses here we present in situ u pb and lu hf isotopes in zircon with whole rock geochemical and sr nd isotopic compositions were carried out for the tiantangzhai porphyritic monzogranites from the dabie orogen eastern china the monzogranites are characterized by high sr 576 988 ppm low y 7 3 19 0 ppm and depletion in hree yb 0 50 1 78 ppm thus resulting in high sr y 34 3 135 2 and la yb n 17 0 105 2 ratios without a negative eu anomaly they also exhibit high sio2 66 5 73 5 wt and k2o 2 7 4 7 wt and low mgo 0 4 1 6 wt or mg 28 2 45 3 mostly 40 values whole rock geochemical compositions suggest that the monzogranites represent low mg adakitic rock with high si and rich k features equilibrated with residues rich in garnet sr nd isotopic compositions end t 16 2 to 20 3 87sr 86sr i 0 707798 0 708804 tdm2 nd 2 3 2 6 ga of the monzogranites are distinct from that of the eclogites and amphibolites in the dabie orogen but similar to that of the neoproterozoic 700 800 ma gneisses in the ndc u pb dating of zircons gives a consistent age of 130 0 3 4 ma with discordia upper intercept age of 716 34 ma for inherited cores identified by cl imaging correspondingly in situ lu hf analyses of early cretaceous young age spots from zircons yield initial 176hf 177hf ratios from 0 281898 to 0 282361 ehf t values from 28 1 to 17 6 and two stage crust hf model ages tdm2 from 2293 89 to 2949 108 ma which are generally in agreement with values of 0 281891 to 0 282218 28 2 to 11 7 and 1927 87 to 2963 92 ma for the pre mesozoic inherited cores respectively as for individual core rim pairs in zircon th u ratios increase from the inherited cores to the young growth rims possibly due to variable degrees of partial melting whereas 176lu 177hf ratios greatly decrease because of the garnet effect in residues thus we suggest that the early cretaceous low mg adakitic granites were derived from the partial melting of the ndc neoproterozoic 700 800 ma gneisses and the foundering of the garnet bearing residues could have caused the destruction of the over thickened lower continental crust

distinguishing climate in the soil record using chemical trends in a vertisol climosequence from the texas coast prairie and application to interpreting paleozoic paleosols in the appalachian basin u s a a suite of vertisols clay rich soils with high shrink swell potential were examined across a climosequence climatic transect in twelve soil pits from the coast prairie of texas in order to determine if mean annual precipitation map exerts a control on the chemistry of these soils and if the observed chemical trends are useful for interpreting paleoclimate records of paleovertisols in the geologic record the precipitation regime of the climosequence spans a range between 144 and 86 cm year with moisture regimes classified as udic udic ustic ustic and aridic ustic in a general northeast to southwest direction other soil forming factors such as soil age 35 40 ka parent material fluviodeltaic beaumont formation of late pleistocene age landscape low relief coastal plain and vegetation prairie or mixed woody shrubs are relatively constant across the climosequence climate sensitive chemical proxies of map identified include dithionite citrate extractable fe fedith acid oxalate extractable fe feoxal caco3 equivalent caco3equiv s and ammonium acetate extractable na k and mg naacet kacet and mgacet respectively which vary across the climosequence because of differences in effective depths of leaching and intensity of wetting and drying cycles these standard usda wet chemical climate proxies are related to bulk oxide or element chemistry of soils and paleosols measured using xrf which supports the use of geochemical climate proxies for interpreting the paleoclimate records of paleovertisols application of the chemical index of alteration minus potash cia k geochemical climofunction to late mississippian paleosols from the appalachian basin of the eastern u s demonstrates evidence for a shift from a lower to a higher map paleoclimate that is consistent with previous paleoclimate models and with observed morphological changes in the paleosols we advocate actualistic research using bulk chemistry of soils and paleosols as a means of obtaining soil information useful for interpreting paleosols in the geological record

zircon u pb and muscovite 40ar 39ar geochronology of the gold bearing tianger mylonitized granite xinjiang northwest china implications for radiometric dating of mylonitized magmatic rocks abstract this study focuses on zircon u pb and muscovite 40ar 39ar geochronology of gold bearing mylonitized granite in the north tianshan of china all zircon samples have euhedral hydrothermal rims which have replaced their igneous mantles igneous zircon mantles with narrow hydrothermal rims in sample ts06 give a weighted average u pb age of 403 7 5 6 ma 1 standard error mswd 1 5 n 18 zircons in ts277 show a complex texture with core detrital origin 1276 2215 ma indicating igneous mantle and a hydrothermal rim the igneous mantles give a weighted average u pb age of 443 4 3 3 ma mswd 3 1 n 10 two younger ages 394 and 400 ma have been obtained on hydrothermal rims the mantles plus rims of all zircons have positive ehf t values with an average of 3 99 0 86 mswd 2 8 n 23 the positive ehf t values suggest that the granite magma is derived from melting of juvenile crustal rocks the shearing deformation post dated granite intrusion and introduced fluid which replaced igneous zircons via dissolution and reprecipitation however the age data for zircon hydrothermal rims do not represent the time of the dissolution re precipitation the zircons crystallized in granite in the early silurian 443 ma have been replaced on their rims during middle triassic deformation 248 238 ma which was followed by brittle deformation accompanied by gold deposition at 220 ma based on 40ar 39ar dating of muscovite

record of multiple stage channelized fluid and melt activities in deeply subducted slab from zircon u pb age and hf o isotope compositions abstract geochemical features of island arc rocks require two slab derived components of a melt and a fluid added to their sources these two liquids may unite to an intermediate aqueous melt at the base of volcanic arc direct evidence of this process has been scarce yet here we present an integrated study of in situ u pb age trace element and o hf isotope for zircons from a suite of a quartz vein a felsic vein their host eclogite and surrounding gneiss at the xitieshan uhp terrane of the north qaidam metamorphic belt nqmb oscillatory zoned zircons from the quartz vein are enriched in u li nb and ree with a formation age of 442 6 ma consistent with the timing of the peak uhp metamorphism this demonstrates there is no temporal gap between the uhp metamorphism and fluid flow in the xitieshan terrane in particular the oscillatory zoned zircons of the quartz vein have similar hf isotope compositions to zircons from the surrounding gneiss but variable o isotope compositions between the host eclogite and surrounding gneiss the fluid from surrounded gneiss was thus an aqueous melt with high transport ability for lu and hf and o while the fluid from the host eclogite was an aqueous fluid only with the ability to transport o according to the similar behaviors of granitic rock and sediment it is inferred that a channelized hydrous melt can be formed in the downgoing sediment layer where an aqueous fluid can be formed from underlying basalt and or peridotite layers at the sub arc depth the two types of fluid are fed to the overlying mantle wedge from subducting slab to form island arc rock by contrast the zircons in the felsic vein yield a weighted mean 206pb 238u age of 420 4 ma which can be regard as the timing of partial melting during the exhumed stage for the xitieshan uhp rocks the zircon o and hf isotopic features of the felsic vein are between the country gneiss and host eclogite compatible with their emanating from hydrous melt of both the eclogite and gneiss during the exhumation stage of the uhp rocks this indicates that hydrous melt would be released from both the sediment and basalt or peridotite layers of the subducting slabs at relatively shallow depth

building mesoarchaean crust upon eoarchaean roots the akia terrane west greenland constraining the source genesis and evolution of archaean felsic crust is key to understanding the growth and stabilization of cratons the akia terrane part of the north atlantic craton west greenland is comprised of meso to neoarchaean orthogneiss with associated supracrustal rocks we report zircon u pb and lu hf isotope data and whole rock geochemistry from samples of gneiss and supracrustals from the northern akia terrane including from the finnefjeld orthogneiss complex which has recently been interpreted as an impact structure isotope data record two major episodes of continental crust production at ca 3 2 and 3 0 ga minor ca 2 7 and 2 5 ga magmatic events have more evolved ehf interpreted as reworking of existing crust perhaps linked to terrane assembly felsic rocks from the finnefjeld orthogneiss complex were derived from the same source at the same time as the surrounding tonalites but from shallower melting requiring any bolide driven melting event to have occurred almost simultaneously alongside the production of the surrounding crust a simpler alternative has the finnefjeld complex and surrounding tonalite representing the coeval genesis of evolved crust over a substantial lithospheric depth hafnium isotope data from the two major mesoarchaean crust forming episodes record a contribution from older mafic eoarchaean crust invoking the involvement of an eoarchaean root in the growth of younger mesoarchaean crust puts important constraints on geodynamic models of the formation of the discrete terranes that ultimately assembled to form earth s cratons

zircon as a metamorphic timekeeper a case study from the caledonides of central norway abstract during metamorphism inherited or detrital zircon grains commonly serve as a template onto which new generations of metamorphic zircon precipitates charting successive episodes in the thermal history of rocks in addition chemical modification of pre metamorphic zircon can leave an imprint in the u pb systematics we present a case study from a sample of migmatitic amphibole biotite gneiss from central norway that contains zircon grains that chart much of the history of an entire orogenic episode not only through generation of new zircon but also resetting of the u pb system via radiogenic pb loss the investigated gneiss from the helgeland nappe complex within the uppermost allochthon of the scandinavian caledonides records a prolonged metamorphic evolution phase equilibria modelling implies peak metamorphic conditions of 7 5 8 0 kbar and 650 675 c consistent with low degree partial melting and the presence of small volumes of leucosome laser ablation split stream analysis that simultaneously measures u pb ree and lu hf in the same volume of zircon yields a range of analyses that spread along concordia a zircon population between 485 and 443 ma is characterized by faded and blurred oscillatory zoned domains that have gained nonformula elements these analyses are interpreted to indicate the presence of reactive solutions during this time period that led to leaching and mobility of pb another texturally distinct zircon component yields an age of 478 4 ma has typical igneous ree zircon profiles and records low apparent alpha doses interpreted to reflect new zircon growth during migmitization a further population of zircon rims and discrete grains yields an age of 453 2 ma interpreted as a later episode of zircon growth during high grade metamorphism a final set of zircon overgrowths yield an age of 439 3 ma also interpreted to reflect new metamorphic growth many zircon growth phases in this rock have similar ree patterns implying broadly isochemical metamorphism consistent with the evidence for only limited partial melting furthermore the hf isotopic signature in the analyzed zircon grains suggests that there was no release of radiogenic hf during metamorphism the metamorphic zircon in the studied rock records several episodes of growth and or alteration over a period of 40 ma that was related to protracted destruction of the iapetus ocean in the context of regional data the new results suggest that the helgeland nappe complex which is generally considered to be exotic to baltica underwent geographically widespread long lived ordovician high temperature metamorphism this record of metamorphism is distinct from that recorded by tectonostratigraphically lower units which are conventionally regarded to have been derived from baltica the results shed light on the assembly of the scandinavian caledonides and show that zircon from individual samples may be used to unravel complex orogenic histories

zircon inheritance in diatexite granodiorites and its consequence on geochronology a case study in lusatia and the erzgebirge saxo thuringia eastern germany abstract we used detailed investigations of zircon morphology and internal structures for the interpretation of zircon age data of diatexite migmatites the fine and medium grained varieties of cadomian granodiorites from lusatia and the erzgebirge are interpreted on the base of their geological geochemical and isotope characteristics as well as zircon populations as migmatites where no segregation of melt from residuum occurred whereas coarse grained varieties contain a large proportion of new formed zircons with a limited age range 540 530 ma most zircons in medium and especially fine grained granodiorites seem to be inherited from their source rock with only a small proportion of new zircon material in form of rare single grains or overgrowths therefore we consider mean evaporation zircon ages from these rocks as mixed ages consequently the formerly inferred age span of magmatic activity from 587 17 to 542 9 ma and the interpretation of three successive magmatic episodes kroner et al geol rundsch 83 1994 357 reported values of 585 563 and 542 ma have to be reconsidered as a mixture of two successive magmatic events i the late cadomian event about 540 530 ma that formed the granodiorites and ii a former magmatic event about 580 570 ma in the source indicated by inherited zircons

metamorphic zirconology of continental subduction zones abstract zircon is widely used to date geological events and trace geochemical sources in high pressure hp to ultrahigh pressure uhp metamorphic rocks of continental subduction zones however protolith zircons may be modified by three different types of metamorphic recrystallization via mechanisms of solid state transformation metasomatic alteration and dissolution reprecipitation new zircon growth may be induced by dehydration reactions below the wet solidus of crustal rocks metamorphic zircon or peritectic reactions above the wet solidus peritectic zircon as a consequence there are different origins of zircon domains in high grade metamorphic rocks from collisional orogens thus determining the nature of individual zircon domains is substantial to correct interpretation of their origin in studies of isotopic geochronology and geochemical tracing we advocate an integrated study of zircon mineragraphy internal structure and external morphology u pb ages mineral inclusions trace elements and lu hf and o isotope compositions only in this way we are in a position to advance the simple zircon applications to metamorphic zirconology enabling discrimination between the different origins of zircon and providing constraints on the property of fluid activity at subduction zone conditions the metamorphic recrystallization of protolith zircons and the new growth of metamorphic and peritectic zircons are prominent in hp to uhp metamorphic rocks of collisional orogens these different types of recrystallized and grown zircons can be distinguished by their differences in element and isotope compositions while the protolith nature of metamorphosed rocks dictates water availability the p t conditions of subduction zones dictate the property of subduction zone fluids the fluids of different properties may be produced at different positions of subducting and exhuming crustal slices and they may physically and chemically mix with each other in continental subduction channels such fluids can act as an important agent not only for the physical transport of protolith zircons but also for the chemical transport of element zr and other fluid mobile incompatible trace elements from the subducted crust to the mantle wedge therefore the discrimination between the different types of zircons provides a powerful means to decipher the role of fluids in subduction zone processes