helium isotopes in hydrothermal volcanic fluids of the azores archipelago abstract we present the first helium isotope data for thermal waters and gas emissions on the islands of terceira graciosa faial pico and flores as well as new data for sao miguel the results allow us to track current mantle degassing associated with the azores hot spot to delineate its spatial distribution and to discuss its possible origin as a general rule we find that free gases tend to display somewhat higher 3 he 4 he ratio than groundwaters we argue that this difference is likely due to radiogenic helium inputs to aquifers during water rock interactions and therefore that gas phases are the fluid carriers with the most representative of mantle source signature the measured 3 he 4 he ratios normalized to the air ratio r a range from lower than morb values 5 23 6 07 r a on central sao miguel to morb values on faial 8 53 r a and flores 8 04 r a located on either side of the mid atlantic ridge and to plume type values on graciosa 11 2 r a and terceira 13 5 r a where free gases also display ten times higher than morb co 2 3 he ratios 1 8 2 6 10 10 such a wide he isotopic range and its spatial distribution corroborate with available data for volcanic rocks indicating that plume s head presently underlies the central part of the archipelago the plume type 3 he 4 he ratios on terceira and graciosa agree with geochemical and seismic evidence of a deep rooted mantle plume feeding the azores hot spot our finding that high 3 he 4 he ratios correspond to low 3 he concentrations and high arc type co 2 3 he values exclude a simple plume supply of 3 he rich primitive mantle instead the simultaneity of both elevated co 2 3 he and 3 he 4 he ratios is best explained by a 3 he rich contribution from the lower mantle diluted in a co 2 rich feeding plume that contains a recycled altered oceanic plate component the alternative possibility of an enhanced time integrated 3 he u th ratio in the azores plume due to a greater compatibility of helium relative to u and th during melting events is difficult to reconcile with the enriched pattern of volcanic rocks from the central islands in any case the azores plume should derive from a mantle reservoir that could escape convective homogenization for a very long period of time in agreement with sub chondritic osmium isotopic ratios in volcanic rocks from the central islands of the archipelago

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

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

the composition and petrogenesis of the lower crust a xenolith study granulite facies lower crustal xenoliths from a single basaltic vent hill 32 in the mcbride volcanic province north queensland australia illustrate the extreme lithologic diversity of the deep crust these xenoliths are dominantly mafic but intermediate and felsic granulites represent a significant proportion 20 of the xenolith population all xenoliths have high grade mineralogies and generally well equilibrated textures as well as decompression features which are indicative of derivation from deep crustal levels 0 7 1 0 gpa major and trace element chemistry for 12 xenoliths chosen to span the observed lithologic range are used to constrain the petrogenesis of these rocks in the smaller layered samples metamorphic differentiation may lead to nonrepresentative sampling however in most instances such secondary processes can be identified and the original chemical characteristics delineated the mafic xenoliths formed through a variety of processes including crystallization of mafic magma crystal accumulation from mafic and felsic magmas and partial melting of intermediate compositions to yield a mafic residuum the two analyzed intermediate xenoliths are metasediments based upon high alumina to alkali ratios and rare earth element patterns whereas the two felsic xenoliths have compositions similar to igneous rocks comparisons of noncumulate and or restite xenoliths with unmetamorphosed rock types show that k and rb are variably depleted whereas th and u are strongly depleted in all rock types the large ion lithophile element depletion patterns for xenoliths are similar to those of rocks from granulite facies terrains suggesting similar processes were operative suggestions of an anomalously ba and sr rich lower crust are not supported by the data by using the observed lithologic proportions of xenoliths at hill 32 and the chemical analyses presented here a weighted mean composition of the lower crust can be obtained and compared with recently proposed lower crustal compositions the weighted mean composition is mafic and formed through a combination of basaltic underplating crystal accumulation tectonic underplating and partial melting no simple model of lower crust formation through basaltic or andesitic underplating nor intracrustal melting is sufficient to explain lower crust formation in phanerozoic continental margin settings

phanerozoic magma underplating and crustal growth beneath the north china craton evidence for post archaean crustal growth via magma underplating is largely based on u pb dating of zircons from granulite facies xenoliths however whether the young zircons from such xenoliths are genetically related to magma underplating or to anatexis remains controversial the lower crustal xenoliths carried by igneous rocks in the chifeng and ningcheng north china craton have low sio2 and high mgo indicating that parental melts of their protoliths were of unambiguous mantle origin the xenoliths contain abundant magmatic zircons with late palaeozoic ages and have more radiogenic zircon hf isotope compositions and hence younger model ages than ancient crustal magmas and the reworking array of the basement rocks our data suggest that the granulites represent episodic magmatic underplating to the lower crust of this craton in phanerozoic time considering the observation that regional lowermost crust 5 km is mafic and characterized by phanerozoic zircons this work reports an example of post archaean crustal growth via magma underplating

mesozoic crust mantle interaction beneath the north china craton a consequence of the dispersal of gondwanaland and accretion of asia we present evidence from zircons entrained within lower crustal xenoliths in the cenozoic hannuoba basalt of multiple melting events beneath the north china craton in the late mesozoic peak activity was between 180 and 80 ma the upper crustal signature of which was the generation of voluminous granitoids and related volcanic rocks emplacement of dioritic and lamprophyric dikes and widespread gold mineralization the process involved partial loss of mantle lithosphere accompanied by wholesale rising of asthenospheric mantle beneath eastern china we correlate these events with lithospheric thinning resulting from the breakup and dispersal of gondwanaland accompanied by a major mantle overturn fueled by the destruction of oceanic lithosphere and triggered by its sinking into the lower mantle during the subsequent accretion of asia

hannuoba intermediate mafic granulite xenoliths revisited assessment of a mesozoic underplating model abstract the intermediate mafic granulite xenoliths from tertiary hannuoba alkali basalts at the northern margin of the north china craton have been commonly considered to be formed by mesozoic basaltic underplating due to their abundant mesozoic zircons and compositionally higher mg molecular mg mg fe than rocks of the adjourning archean granulite terrain however such a mesozoic underplating model cannot account for the presence of precambrian zircons in the xenoliths and their evolved sr nd isotopic compositions which are dissimilar to expected basaltic melts from both the lithospheric and asthenospheric mantle instead their sr nd isotopic compositions resemble those of the granulite terrain and of the mesozoic low mg granitoids here newly acquired data and published data are combined to assess the mesozoic basaltic underplating model and to discuss the origin of the hannuoba intermediate mafic granulite xenoliths in terms of their relationships with the mesozoic magmatism and the granulite terrain zircons from a mafic granulite xenolith jn0724 which has geochemical and sr nd isotopic compositions indistinguishable from other mafic granulite xenoliths yield 2 5 ga u pb ages providing unambiguous evidence that the mafic xenoliths represent portions of ancient lower crust combined data reveals that the pyroxene rich granulite xenoliths can be best explained as restites left after partial melting of the ancient lower crust to produce the mesozoic low mg granitoids hf isotope analysis on mesozoic zircons from the hannuoba feldspar rich granulite xenoliths demonstrates that the zircons crystallized from mesozoic granitic magmas that were derived from ancient lower crust rather than from mesozoic underplated magmas the proposed scenarios successfully explain the age and compositional differences between granulite xenoliths and the granulite terrain and also provide new insights into derivation and evolution of granitic magmas it is concluded that most of the hannuoba intermediate mafic granulite xenoliths and the mesozoic granitoids reflect reworking of ancient lower crust rather than juvenile crust addition it provides strong evidence that crustal anatexis may be one of the major processes controlling the chemical differentiation of the continental crust our data imply that whether younger zircon ages in granulite xenoliths from many other archean cratons register post archean basaltic underplating could be tested by integrated analysis of u pb age and hf isotopes

sm nd and shrimp u pb zircon geochronology of high pressure granulites in the sanggan area north china craton timing of paleoproterozoic continental collision abstract high pressure hp mafic granulites are widely distributed as sheet like and lenticular bodies within highly deformed tonalitic gneisses in the sanggan area north china craton they have clockwise isothermal decompression and isobaric cooling p t paths implying continental collision tectonics whole rock analyses of 15 samples define a sm nd isochron yielding an age of 2547 87 ma with e nd t 2 7 corresponding nd depleted mantle model ages show a similar range and are interpreted as the age of the magmatic protolith to the high pressure granulites a combined sm nd mineral whole rock isochron from one of these samples yields an age of 1813 23 ma taken to represent the metamorphic age at an early stage on the cooling path shrimp u pb zircon analyses of three hp mafic granulite samples give uniform weighted mean 207 pb 206 pb ages of 1817 12 1817 12 and 1819 16 ma these ages are interpreted to date metamorphism of the hp granulites and with the sm nd mineral whole rock result strongly suggest that the time of continental collision was at ca 1 82 ga an additional sample records two age populations one at 1803 9 ma and an older one at 1872 16 ma the latter may indicate the timing of either an earlier metamorphic event or possibly the earliest stage of a single prolonged metamorphic episode that peaked at 1 82 ga these results confirm the paleoproterozoic age for assembly of the north china craton

isopach and borehole data as indicators of rifted margin evolution in southwestern africa abstract isopach and borehole data are used to document post rifting sediment accumulation along the western margin of southern africa the isopach data reveal an increasing rate of sediment accumulation up to the end of the eocene but the borehole data generally show the opposite trend an integrated analysis of both types of data however is consistent with a general model of post rifting drainage development in which the area supplying sediment expanded significantly after breaching of a linear upwarp along the margin allowed a dominant drainage network to develop the borehole data suggest that such a major sediment outlet from the interior of the sub continent the palaeo orange became established in the late cretaceous coniacian in the vicinity of the present orange river mouth after conversion to equivalent rock volumes the isopach data can be combined with the general drainage model to infer the likely spatial and temporal patterns of erosion over southern africa during the accumulation of four offshore sedimentary sequences beginning with the onset of rift related sedimentation and extending up to the present this analysis using three variant forms of the general model to encompass a wide range of drainage development possibilities indicates that the average rate of erosion for catchment areas draining to the western margin has declined from 41 82 m ma 1 during the first sedimentary sequence 152 to 113 ma to about 7 9 m ma 1 during the last sequence 37 to 0 ma maximum depths of post rifting erosion of several kilometres appear to have occurred in a narrow zone bordering the margin primarily during its early development while depths of erosion in some interior areas have probably been an order of magnitude less the average depth of post rifting erosion for the present day catchment as a whole is estimated to be about 1 8 km and the erosion rate over this period has averaged about 12 m ma 1

u pb zircon and rutile ages from granulite xenoliths in the slave province evidence for mafic magmatism in the lower crust coincident with proterozoic dike swarms zircon and rutile in two garnet bearing mafic granulite xenoliths from a kimberlite in the archean slave province record proterozoic ages similar to the 1 27 ga mackenzie dike swarm or similar to the ca 2 23 2 21 ga malley mckay diabase dike swarms the xenolith sample site currently overlies thick lithosphere more than 700 km to the southeast of the proposed mackenzie plume center because lower crust was effectively shielded from the thermal effects of an upwelling plume by thick lithospheric mantle heat was most likely advected into the lower crust by lateral intrusion of mafic sills over distances of hundreds of kilometres from magma generation zones nearer the plume center

age and composition of granulite and pyroxenite xenoliths in hannuoba basalts reflect paleogene underplating beneath the north china craton abstract in situ u pb and hf isotope systematics of zircons mineral chemistry of silicates and whole rock major element trace element and platinum group element pge data are reported for five mafic xenoliths granulites and pyroxenite in the hannuoba cenozoic basalts north china temperature estimates of 859 886 c suggest that these xenoliths are derived from the lower crust the whole rock compositions of the xenoliths show weakly negative eu anomalies eu 0 67 0 95 high sio 2 al 2 o 3 5 80 13 8 low ir 0 10 0 29 ppb and wide ranges of ni 4 5 235 ppm and la nb 1 70 7 64 most zircons in these samples are structureless and give paleogene 44 5 47 3 ma u pb ages minor populations have early mesozoic 210 220 ma late mesozoic 90 ma and neogene 14 ma ages the fractionation of pyroxene and plagioclase controlled the petrogenesis of the xenoliths which are interpreted as the products of basaltic underplating and fractionation cumulates in paleogene time 45 47 ma with assimilation of some older intermediate crustal components the paleogene underplating corresponds in time to lithosphere scale extension in the north china craton leading to the widespread formation of sedimentary basins most paleogene zircons have positive e hf up to 13 2 with uniform t dm 0 46 0 50 ga whereas most mesozoic zircons have negative e hf this evolution suggests a temporal decrease in the degree of crustal assimilation during the extension of the lithosphere in addition to complex modification in early and late mesozoic times the thermal event related to the eruption of the host magmas ca 14 ma also reheated the lower crust beneath the north china craton

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

dating the lower crust by ion microprobe ion microprobe uthpb ages of zircons from granulite facies lower crustal xenoliths from north queensland australia correlate well with the ages of major orogenic episodes manifest at the earth s surface about half of the xenoliths contain proterozoic zircons which are similar in age to the episodes of high grade metamorphism of the older surface rocks all the xenoliths contain late paleozoic zircons which show a real 100 ma range in206pb238 u ages from 320 to 220 ma which is attributed to granulite facies metamorphism followed by slow cooling in the deep crust the paleozoic zircon ages coincide in time with the prolonged episode of eruption of voluminous felsic ash flows and intrusion of high level granites in this region 320 270 ma mineral and melt inclusions in the zircons provide clues to the origin of some of the xenoliths and coupled with the age information can be used to infer the geological processes operating in the lower crust the zircons from two mafic xenoliths contain felsic and intermediate melt inclusions implying at least a two stage history for these rocks involving either partial melting of a more felsic protolith or crystal accumulation from an evolved melt some of the zircons from the felsic xenoliths contain co2 rich fluid inclusions indicating that those zircons grew during high grade metamorphism the isotopic and chemical data for the whole rock xenoliths show that they originate from a segment of the lower crust which is a heterogeneous mixture of supracrustal and mafic mantle derived lithologies the major orogenic event responsible for the formation of that crust occurred in the late paleozoic when proterozoic supracrustal rocks were emplaced into the lower crust possibly along thin skinned thrust slices this was accompanied by intrusion of high temperature mantle derived melts which caused partial melting of pre existing crust the most likely setting for such tectonism is a continental margin subduction zone

proterozoic zircon growth in archean lower crustal xenoliths southern superior craton a consequence of matachewan ocean opening granulite grade anorthositic and mafic xenoliths recovered from a jurassic kimberlite pipe near kirkland lake ontario are fragments of the lower crust that underlies the ca 2 7 ga abitibi greenstone belt of the superior craton cathodoluminescence imaging and or backscatter electron microscopy of zircon from four individual xenoliths reveals a complex crystallization history characterized by two main stages of zircon growth the age of the two stages has been constrained by combining imaging results with isotope dilution u pb dating of grain fragments and single grains minimum ages for the first crystallization stage in individual xeno liths are 2584 7 ma 2629 8 ma 2633 3 ma whereas an approximate crystallization age for a fourth sample is 2788 57 ma the second main stage of growth consists of chemically and isotopically distinct metamorphic zircon overgrowths times of solid state zircon growth are most broadly constrained in three samples to the interval between 2 52 ga to 2 40 ga and most precisely dated in a meta anorthosite at 2416 30 ma these complex zircons are intergrown with garnet and clinopyroxene of the host granulite facies assemblage and thus the paleoproterozoic ages of the metamorphic overgrowths are interpreted to reflect an interval of isobaric granulite grade metamorphism of the lower crust beneath the greenstone belt approximately 150 million years after craton formation this interval of metamorphism is broadly coeval with the intrusion of the matachewan dyke swarm across the southern superior craton and with mafic magmatism and deposition of huronian rift margin sediments 200 km to the south during the opening of the matachewan ocean it is proposed that a significant volume of magma intruded the crust mantle interface during rifting promoting isobaric metamorphism and zircon growth in the deep levels of the superior craton subsequent major rifting events along this margin apparently failed to produce a similar lower crustal response the results have important implications for the structure of lithosphere beneath archean continental crust

nd and sr isotopic compositions of lower crustal xenoliths from north queensland australia implications for nd model ages and crustal growth processes abstract sr and nd isotopic analyses of granulite facies lower crustal xenoliths from northern queensland australia fall on a mixing array on a 87 sr 86 sr vs 143 nd 144 nd diagram at 300 ma this was a time of intense orogeny in northern queensland attended by granitic magmatism in the upper crust and granulite facies metamorphism in the lower crust ion microprobe zircon dating indicates that most of the xenoliths formed at this time the sr and nd isotopic compositions of these rocks are therefore interpreted to have resulted from large scale mixing in the lower crust between mantle derived basaltic melts and isotopically evolved early proterozoic continental crust both end members are represented in the xenolith suite depleted mantle nd model age tdm ages for meta igneous granulite xenoliths having appropriate 147 sm 144 nd i e between 0 12 and 0 15 range from 1 9 to 2 2 ga for mafic melt like xenoliths and 1 2 to 1 4 ga for felsic melt like xenoliths however zircon ages on these same samples show that their protoliths crystallized during the permo triassic orogeny thus the old tdm ages for the mafic xenoliths do not accurately reflect their crustal residence ages possibly due to crustal assimilation or their derivation from an enriched mantle source this is a common feature of mafic granulite xenoliths the world over and highlights the difficulty in determining the age of mafic underplates hence the timing of crustal growth from tdm ages because the felsic melt like xenoliths lie on the mixing array their mid proteozoic tdm ages are likely to have resulted from mixing and therefore represent an average crustal residence age the combined data suggest that significant crustal growth and reworking occurs in the lower crust during phanerozoic crustal orogenies

deep probe imaging the roots of western north america analysis of the lithoprobe deep probe and southern alberta refraction experiment data sets focusing on the region between deep probe shots 43 and 55 has resulted in a continental scale velocity structural model of the lithosphere of platformal western laurentia reaching depths of 150 km three major lithospheric blocks were investigated i the hearne province a typical continental archean cratonic province lying beneath the western canada sedimentary basin ii the wyoming province an even older block of phanerozoic modified archean crust with an enigmatic lower lithosphere and iii the yavapai mazatzal province proterozoic terranes underlying the colorado plateau and southern rocky mountains in this study the northern two of these regions are investigated with a modified ray theoretical traveltime inversion routine that respects the spherical geometry of the earth the resulting crustal velocity structure combined with supporting geological and geophysical data reveals that the medicine hat block mhb lying between the hearne and wyoming provinces is a third independent archean crustal block the subcrustal lithosphere along the profile is homogeneous in velocity structure but two significant northward dipping reflectors are apparent and interpreted as relic subduction zones associated with sutures between the three archean blocks the hearne crust is typical of an archean shield or platform both in its thickness of 34 50 km and its seismic velocity structure the crust of the archean mhb and wyoming province which ranges in thickness from 49 to 60 km includes a 10 30 km thick high velocity layer interpreted to be proterozoic in age such a feature is unexpected beneath archean crustal provinces but if the region is considered to be the remanent marginal portion of a larger archean continent then the interpreted proterozoic underplating and lack of an archean lithospheric root can be explained the variable topography along the reflective upper and lower boundaries of this layer especially within the mhb suggests considerable variability in its emplacement and subsequent tectonic history

2 7ga crust growth in the north china craton abstract numerous u pb zircon geochronological data show that rocks of the precambrian basement from the north china craton ncc have two age peaks at 1 8 1 9 and 2 5 ga respectively whether the most prominent peak at 2 5 ga really registered a period of major crustal growth in the ncc thus making it different from the global major crustal growth period of 2 7 ga is uncertain in this paper we carried out in situ u pb and hf isotopes of zircons and whole rock chemical and sr nd isotopic analyses for six precambrian samples from the ncc in order to understand crustal growth and reworking the samples have u pb zircon ages of 1 8 1 9 2 5 and 2 7 ga respectively the two samples with u pb zircon ages of 2 7 ga have zircon hf model ages t hf dm and or whole rock nd model ages t nd dm close to their u pb zircon ages it is clear that they represent juvenile crust generation thus providing direct evidence that 2 7 ga was a period of new crustal addition the samples with u pb zircon ages of 1 8 and 2 5 ga also have t hf dm and or t nd dm similar to those of the 2 7 ga samples indicating that these samples also have a mantle extraction time of 2 7 0 1 ga and their 1 8 and 2 5 ga ages may represent periods of reworking of the 2 7 0 1 ga crust rather than juvenile crust generation the reworking is manifested either as metamorphism or as lower crustal derived magmatism specifically the 2 5 ga tonalitic gneiss sample is thought to be derived by melting of the 2 7 ga lower crust since many 2 5 ga tonalite trondhjemite granodiorites ttgs and calc alkaline granitoids in the ncc contain 2 7 ga xenocrystic and inherited zircons we postulate that they are probably also derived mainly from the 2 7 0 1 ga crust our data imply that the quantity of 2 7 ga continental crust in the ncc is probably greater in extent than that exposed today

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

age diversity of the deep crust in northern mexico zircons extracted from five chemically diverse granulite facies crustal xenoliths from la olivina cinder cone chihuahua mexico were analyzed by shrimp ion microprobe in order to determine the age of the deep crust in northern mexico the results demonstrate the extreme age diversity of the deep crust in this region zircon populations of ca 1 25 37 ca 200 350 1100 and 1370 ma are present nearly all samples contain 0 20 ma zircons reflecting recent granulite facies conditions in the deep crust in addition high grade metamorphism may also have occurred at 200 ma and 1 10 ga grenville the paucity of carboniferous permian ages in the xenoliths suggests that the ouachita collision event provided little thermal input to the deep crust beneath la olivina

petrology and geochemistry of the shuiquangou syenitic complex northern margin of the north china craton the palaeozoic shuiquangou complex is a major syenitic intrusion along the northern margin of the north china craton that hosts a number of gold deposits despite the heterogeneity of the complex in terms of texture and modal mineralogy mineral compositions whole rock chemistry as well as radiogenic isotope compositions show a well constrained magmatic evolutionary trend the whole rock major element and ree trends are broadly consistent with fractional crystallization dominated by clinopyroxene amphibole and accessory minerals initial nd sr isotopic compositions suggest that the parent magmas to the complex were ultimately derived from an enriched mantle with relatively high initial 87 sr 86 sr ratios and negative nd values the enrichment of the mantle source was probably inherited from an older lithospheric mantle beneath the north china craton sr isotopic data indicate that the rb sr system was disturbed by later alteration processes whereas nd isotopes are essentially unaffected and exhibit a broad trend consistent with simultaneous fractional crystallization and country rock assimilation afc quantitative modelling indicates that a very high bulk distribution coefficient for nd and low ratios of assimilation to crystallization are needed to produce the observed nd isotope evolutionary trend

proterozoic zircon ages from lower crustal granulite xenoliths kola peninsula russia evidence for crustal growth and reworking lower crustal garnet granulite xenoliths from beneath the kola karelian domain of the fennoscandian shield russia were brought to the surface in devonian lamprophyre diatremes to establish the relationships between processes in the lower crust and geological events recorded in the upper crust zircons from two of these xenoliths were dated by ion microprobe one xenolith contains mainly primary magmatic zircon grains that yielded an average age of 2 47 ga the kola karelia domain formed part of a large basaltic igneous province in the earliest proterozoic c 2 45 ga thus such xenoliths probably represent deep seated equivalents of this magmatism a second garnet granulite xenolith contained only internally featureless zircons most of these yielded concordant ages between 1 77 1 61 ga this age distribution indicates that at c 1 7 ga the protolith was migmatized during an event that may be related to emplacement of granites and pegmatites in the region however two zircon grains in the second xenolith gave much younger ages of 1 47 and 1 45 ga this spread in the ages is related to an unknown younger event around 1 4 ga or perhaps later when the c 1 7 ga migmatite zircons were reset to variable degrees these zircon ages are evidence for the complex evolution of the lower crust recording multiple events some of which are not currently expressed on the surface