Zircon Domains Research Articles

Application of Ti-in-zircon and Zr-in-rutile thermometers to constrain high-temperature metamorphism in eclogites from the Dabie orogen, central China

Ultrahigh-pressure (UHP) eclogites from the northern Dabie orogen, central China underwent a complex metamorphic evolution during Triassic continental deep subduction and subsequent exhumation. The eclogites were strongly affected by multiple decompression and re-crystallization processes during exhumation, thus making the determination of peak metamorphic conditions particularly challenging. The recently calibrated Ti-in-zircon and Zr-in-rutile thermometers provide new tools to estimate the peak and post-peak temperatures. Rutiles within UHP mineral-bearing domains of zircon have Zr contents of 1030−4310ppm and record a temperature range of 880−1080°C, with a peak at 970°C. Rutiles within high-pressure (HP) mineral-bearing domains contain Zr contents of 800−5800ppm and define a temperature range of 780−1030°C with an average value of 873°C. Rutiles in granulite-facies mineral-bearing rims of zircon yield a temperature of 845°C. In contrast, rutile inclusions within garnet have lower Zr contents of 100−800ppm, yielding lower temperatures of 600−850°C. The UHP and HP mineral-bearing mantle domains of zircon have Ti contents of 2.15–97.8ppm, yielding Ti-in-zircon temperatures of 620−1020°C. Ti contents in the rim domains of zircon are low (<7ppm), indicating a temperature of 650−700°C, except for two granulite-facies overgrowth rims which yield a temperature of 793−840°C. Thus, most of the temperatures for different domains of metamorphic zircons are <800°C and generally lie around 650±50°C. Few measures of temperature are the >900°C for the UHP and HP domains of zircon. Therefore, this study combined with the previously published data suggests that the eclogites in the region experienced a protracted high-temperature (HT) metamorphic evolution from UHP eclogite-facies to HP granulite-facies metamorphic stages. This long-lived HT evolution coupled with slow cooling and partial melting may explain the rare preservation of UHP traces in the area. HP/UHP mineral inclusions in garnet and zircon experienced variable degrees of decompression breakdown or retrogression. Therefore, also Ti concentrations in zircons and Zr concentrations in rutiles within garnet and zircon may have been strongly modified by re-crystallization or partial melting. As a result, only few zircons record their actual crystallization temperatures. In comparison, the rutile inclusions in zircon generally define higher temperatures. We therefore suggest that rutile inclusions in zircon are the most suitable candidates for HT estimates, especially in strongly retrogressed eclogites.

An exotic Mesoarchean microcontinent: The Coorg Block, southern India

Sandwiched between the Dharwar Craton in the north and the Neoarchean–Proterozoic crustal blocks to the south, the Coorg Block in southern India is composed dominantly of a suite of arc magmatic rocks including charnockites, TTG (tonalite–trondhjemite–granodiorite)-related granitoid suite and felsic volcanic tuffs together with minor accreted oceanic remnants along the periphery of the block. Coeval mafic and felsic magmatism with magma mixing and mingling in an arc setting is well represented in the block. Here we present the petrology, geochemistry, zircon U–Pb geochronology and Lu–Hf isotopes of all the major lithologies from this block. Computation of metamorphic P–T conditions from mineral chemical data shows consistent granulite-facies P–T conditions of 820–870°C and up to 6kbar. Our geochemical data from major, trace and REE on representative samples of the dominant rock types from the Coorg Block corroborate an arc-related signature, with magma generation in a convergent margin setting. The zircon data yield weighted mean 207Pb/206Pb ages of 3153.4±9 to 3184.0±5.5Ma for syenogranites, 3170.3±6.8Ma for biotite granite, 3275±5.1Ma for trondhjemite, 3133±12 to 3163.8 ±6.9Ma for charnockites, 3156±10 to 3158.3±8.2 for mafic enclaves, 3161±16Ma for diorite and 3173±16Ma for felsic volcanic tuff. An upper intercept age of 3363±59Ma and a lower intercept age of 2896±130Ma on zircons from a charnockite, as well as an evaluation of the Th/U values of the zircon domains against respective 207Pb/206Pb ages suggest that the Mesoarchean magma emplacement which probably ranged from >3.3 to 3.1Ga was immediately followed by metamorphism at ca. 3.0 to 2.9Ga. The ages of magmatic zircons from the charnockites and their mafic granulite enclaves, as well as those from the volcanic tuff and biotite granite, are all remarkably consistent and concordant marking ca. 3.1Ga as the peak of subduction-related crust building in this block, within the tectonic milieu of an active convergent margin. The majority of zircons from the Coorg rocks show Hf isotope features typical of crystallization from magmas derived from juvenile sources. Their Hf crustal model ages suggest that the crust building might have also involved partial recycling of basement rocks as old as ca. 3.8Ga. The crustal blocks in the Southern Granulite Terrane in India preserve strong imprints of major tectonothermal events at 2.5Ga, 2.0Ga, 0.8Ga and 0.55Ga associated with various subduction–accretion–collision or rifting events. However, the Coorg Block is exceptional with our data suggesting that none of the above events affected this block. Importantly, there is also no record in the Coorg Block for the 2.5Ga pervasive regional metamorphism that affected all the other blocks in this region. The geochronological data raise the intriguing possibility that this block is an exotic entity within the dominantly Neoarchean collage in the northern domain of the Southern Granulite Terrane of India. The Mesoarchean arc-related rocks in the Coorg Block suggest that the magma factories and their tectonic architecture in the Early Earth were not markedly different from those associated with the modern-style plate tectonics.

Linking Time to the Pressure-Temperature Path for Ultrahigh-Pressure Rocks

Establishing the timing and duration of ultrahigh-pressure metamorphism (UHP) for crustal rocks subducted to mantle depths of over 100 km requires high-precision geochronology directly coupled with pressure-sensitive indicators. The best links between UHP conditions and an age estimate are inclusions of the UHP indicator minerals coesite and/or diamond in datable zircon or garnet. Lu–Hf and Sm–Nd garnet ages define the prograde and peak portions of the pressure–temperature path for cold ( 1 cm/y) UHP systems. UHP metamorphism in hotter (>800 °C) and slower (<1 cm/y) terranes is best dated by U–Pb analysis of coesite-bearing zircon domains coupled with Sm–Nd and Lu–Hf garnet analysis.

The origin and evolution of low-δ18O magma recorded by multi-growth zircons in granite

We for the first time reported low-δ18O granites within craton in Bengbu uplift, southeast margin of North China Craton. Integrated ion microprobe study of δ18O and U–Pb age on single zircon grains and zircon populations, gives direct evidence for the origin and later magmatic evolution of the low-δ18O granites. Three types of zircon domains are recognized: inherited magmatic core (Type I), inherited metamorphic core/mantle (Type II) and overgrown magmatic rim (Type III). Type I zircons were formed in Neoproterozoic, and have the average δ18O values of 5.3±0.6‰ around mantle value. Triassic Type II zircons have extremely low and highly heterogeneous δ18O values ranging from −9.4‰ to 8.6‰, consistent with the values and variations exhibited by metamorphic zircons from Dabie–Sulu orogen. This feature argues for a Triassic metamorphic origin for the 18O-depletion signature of zircons from ultrahigh pressure metamorphic rocks in Dabie–Sulu orogen. Jurassic Type III zircons have isotopically homogeneous δ18O values of 3.3±0.5‰. Collectively, zircon populations recorded the δ18O variations from source to low-δ18O granite. The primary protolith of Jurassic granites in Bengbu uplift is a Neoproterozoic granitic intrusion in South China Block. Later high-temperature meteoric hydrothermal alteration has lowered the whole-rock δ18O values in various degrees. The low-δ18O protolith was then buried to the middle-lower crust by Triassic continental subduction and Jurassic anatexis produced the low-δ18O granitic magma. Moreover, δ18O values of Type III zircons are isotopically lighter than previously reported values of magmatic garnets in Jurassic granites, indicating later magmatic evolution of low-δ18O magma toward high δ18O values probably as a result of infiltration of high-δ18O fluid from wall-rocks.

Timing and mechanisms of multiple episodes of migmatization in the Korla Complex, northern Tarim Craton, NW China: Constraints from zircon U–Pb–Lu–Hf isotopes and implications for crustal growth

Formation of ubiquitous migmatites in high-grade metamorphic terranes may involve both closed- and open-system processes, e.g., in situ anatexis, infiltration of external melts and fluids. Coupled CL-imaging, in situ U–Th–Pb and Lu–Hf isotopic analyses of zircons from various migmatites and their components, i.e., leucosomes and melanosomes, enabled us to determine the time and mechanism(s) of three episodes of migmatization in the Korla Complex, northern Tarim Craton. The first episode of migmatization took place at ca. 1.85Ga and probably resulted from in situ partial melting due to a regional high-grade metamorphic event. The second migmatization event was marked by the presence of abundant ca. 830Ma leucogranitic veins, dykes and small plutons. Zircon Hf isotopic data indicate that these leucogranites were probably derived from anatexis of a relatively juvenile crustal source, rather than their immediate country rocks, implying large-scale melt migration and infiltration. These two episodes of migmatization might have resulted from two regional orogenic events at ca. 1.85Ga and 830Ma, respectively. In contrast, the third episode of migmatization at ca. 660Ma was probably a local remelting event induced by intrusion of small quartz syenite plutons into previously migmatized rocks. In addition, zircon domains with extremely low Th (0.003–10ppm) and U (3–30ppm) contents were found in samples related to the ca. 830Ma migmatization event. These domains generally occur as CL-bright rims that penetrate the primary zoned zircons without changing their morphology, and are ascribed to strong hydrothermal alteration via the interface-coupled dissolution–reprecipitation mechanism. A compilation of published zircon Hf isotopic data reveals three episodes of crustal growth during ca. 3.1–3.4Ga, 2.5–2.8Ga and 1.6–1.9Ga in the northern Tarim Craton. However, these ‘peaks’ of crustal growth may be biased by selective sampling and magma mixing. Zircon Hf isotopic data from a ca. 2.29Ga orthogneiss and the least recrystallized detrital cores from the migmatized sedimentary rocks suggest that continental crust older than ca. 3.3Ga might have existed in the northern Tarim Craton.

Zircon U–Pb ages and Lu–Hf isotopes of Paleoproterozoic metasedimentary rocks in the Korla Complex, NW China: Implications for metamorphic zircon formation and geological evolution of the Tarim Craton

Widespread Paleoproterozoic supracrustal rocks in the northern Tarim Craton contain important information about its geological evolution and correlation with adjacent blocks. We present new in situ LA-(MC-)ICP-MS zircon U–Pb and Lu–Hf isotopic data for six mica schist samples from the Korla Complex. Field and petrological studies indicate a pelitic to semi-pelitic protolith and a high pressure upper amphibolite-facies peak metamorphic condition (T=690±50̊C and P=11±2kbar) for these samples. CL-images reveal that zircons in these samples are dominantly metamorphic origin and only a few detrital zircons occur as relics in sample T1, the ages of which suggest a maximum deposition age of ca. 2.0Ga and a sedimentary provenance from the Tarim Craton itself. All metamorphic zircons consistently record a metamorphic age of ca. 1.85Ga, despite of various degrees of discordance probably due to later Pb-loss. Both recrystallization and new zircon growth are recognized for the genesis of these metamorphic zircons. The metamorphic zircon domains in sample T1 show a relatively large range of initial 176Hf/177Hf ratios similar to the detrital cores, whereas those in the other samples show similar initial 176Hf/177Hf ratios (ca. 0.28140±0.00010, 2σ) regardless of their internal structures and degrees of discordance. The former is interpreted as a result of complete U–Pb resetting through fluid-mediated recrystallization, whereas the later probably implies a large-scale Hf isotopic homogenization during new zircon growth. Petrological and zircon isotopic evidence supports that the new zircon growth and Hf isotopic homogenization probably resulted from the mixing of Hf–Zr derived from dissolution of tiny detrital zircons and decomposition of garnet to chlorite in hydrothermal fluids during retrograde metamorphism. Accordingly, the ages of these new zircon growths may postdate the peak metamorphism, which was probably related to a late Paleoproterozoic collisional orogenic event in the northern Tarim Craton. A compilation of available geological and geochronological data enables us to identify two Late Paleoproterozoic orogenic belts: the ca. 1.9–1.8Ga North Tarim Orogen and the ca. 2.0–1.9Ga South Tarim Orogen. It is suggested that the Tarim Craton, including the Dunhuang and Quanji Blocks, was correlative with the Alxa–Yinshan Block of the North China Craton, and they probably formed a coherent massif during the Neoarchean–early Paleoproterozoic, which collided with the Ordos Block and its western extension along the ca. 1.95Ga Khondalite Belt–South Tarim Orogen to form a larger landmass in the Columbia Supercontinent.

Zirconological tracing of transition between aqueous fluid and hydrous melt in the crust: Constraints from pegmatite vein and host gneiss in the Sulu orogen

Zircon can grow from aqueous fluid and hydrous melt in crustal rocks when they undergo metamorphic dehydration and partial melting under high-pressure (HP) to ultrahigh-pressure (UHP) conditions. A distinction between fluid and melt has important bearing on the development of crustal anatexis in collisional orogens. A genetic transition between fluid and melt is recorded by zircons from pegmatite vein and host UHP gneiss in the Sulu orogen. This transition is elucidated by an integrated study of SIMS U–Pb and O isotope analyses with LA-ICPMS U–Pb isotope and trace element analyses in the zircons. Pegmatite veins yield zircon U–Pb ages of 147–153Ma for new growths, and 700–800Ma for relict cores. A one meter-width pegmatite vein exhibits two episodes of zircon growth at 153±3Ma for inner domain and at 147±2Ma for outer domain. The two types of domains have a series of differences in CL image, inclusion type, REE content and pattern, trace element contents and ratios, and Ti-in-zircon temperature. The inner domain grew from the hydrous melt at 730–840°C, whereas the outer domain grew from the aqueous fluid at 520–650°C. Nevertheless, they have similarly low δ18O values of 1.0–2.3‰, suggesting their growth from O isotope homogeneous media despite the transition from melt to fluid. The host gneiss of this pegmatite vein exhibits three generations of zircon growth, with 180–205Ma at 700–770°C during amphibolite-facies metamorphism, 157±3Ma at 610–670°C for fluid-assisted growth, and 147±2Ma at 780–860°C for melt-assisted growth. The last two types of growth record the development of crustal anatexis from hydration melting to dehydration melting. The other decimeter-width pegmatite vein exhibits only one episode of zircon growth from melt at 149±2Ma and 660–860°C. Its host gneiss exhibits a residual melt texture but no significant growth of zircon, indicating low degree of dehydration melting. Therefore, the zircon domains in the host gneisses record their growth during a transition from aqueous fluid to hydrous melt along a temperature-increasing path, whereas the zircon domains in the pegmatite veins record their growth during a transition from hydrous melt to aqueous fluid along a temperature-decreasing path. Despite the difference in the direction of fluid/melt evolution, the all zircon domains in these gneisses and pegmatite veins record the same event of crustal anatexis in the UHP gneisses. The present study also provides insights into a genetic definition of anatectic melt and thus anatectic zircon.

Prograde metamorphism, decompressional partial melting and subsequent melt fractional crystallization in the Weihai migmatitic gneisses, Sulu UHP terrane, eastern China

In order to decipher fluid/melt activity during subduction and subsequent exhumation of felsic continental crust, we carried out a combined study of zircon internal structure, zircon U–Pb age, zircon trace element composition and whole-rock geochemistry on migmatitic gneisses from the Weihai region in the Sulu ultra-high pressure (UHP) metamorphic terrane, eastern China. The Weihai migmatitic gneisses are composed of intercalated compositional layers of melanosome and plagioclase (Pl)-rich leucosome and K-feldspar (Kfs)-rich pegmatite veins. Whole-rock geochemistry suggests that MgO, Fe2O3T, CaO, TiO2, P2O5, rare earth elements (REE), and large ion lithophile elements (HFSE) are nearly completely retained in the melanosome, whereas significant amounts of LILE (e.g., Rb, Ba, K, Sr, Pb) and U are partitioned into the Pl-rich leucosome and the Kfs-rich pegmatite veins. The Kfs-rich pegmatite vein has much higher K2O, U, Pb and Rb, but lower CaO and Na2O contents than those of the Pl-rich leucosome layer. Protolith magmatic zircon domains, with an upper intercept age of about 780Ma, are common in the cores of zircon grains from the melanosome and the Pl-rich leucosome layers, indicating that the protolith of the migmatitic gneiss is Mid-Neoproterozoic magmatic rock. Metamorphic zircons with concordant ages ranging from 243 to 256Ma occur as overgrowth mantles on the protolith magmatic zircon cores. They are characterized by remarkably flat heavy rare earth element (HREE) patterns without obviously negative Eu anomalies, and have low Y and Th contents, low Th/U and high Hf/Y ratios, indicating a prograde metamorphism in the HP eclogite-facies during subduction. Recrystallized rims of zircons (228±2Ma) in the Pl-rich leucosome layer are similar to magmatic zircons in terms of their HREE enriched patterns with negative Eu anomalies, high Y and low Th contents, and low Hf/Y and Th/U ratios. This indicates that the gneiss has experienced decompressional partial melting and the Pl-rich leucosome layers were formed by fractional crystallization of the primary melt during the early stage hot exhumation. Zircon grains (219±2Ma) in the Kfs-rich veins are characterized by aqueous fluid/melt-related signatures with strongly HREE enriched patterns, high U, Pb and low Th contents, and low Th/U and high Lu/Hf ratios, indicating that the pegmatite vein was formed by crystallization of residual aqueous melt after the crystallization of Pl-rich leucosome layers during later stage cooling during exhumation. Our results suggest that the migmatitic gneiss successively recorded information concerning the protolith, prograde HP metamorphism during subduction, fractional crystallization of primary melt and crystallization of the residual melt during exhumation.

New isotopic constraints on age and magma genesis of an embryonic oceanic crust: The Chenaillet Ophiolite in the Western Alps

The Chenaillet Ophiolite is one of the best-preserved remnants of the Piemont-Liguria oceanic basin, a branch of the Central Atlantic that opened during the separation of Adria/Africa from Europe. Despite numerous studies of structure, petrology, geochemistry and isotope geochronology, the timing and genesis of various magmatic rocks within the Chenaillet Ophiolite are still controversial. We provide in this study integrated in situ analyses of zircon U–Pb age and O–Hf isotopes for the troctolite and albitite within the Chenaillet Ophiolite. Our new results indicate that the troctolite and albitite crystallized synchronously at ~165Ma. Zircons from the troctolite have homogeneous Hf and O isotopic compositions, with εHf(T)=+13.5±1.0 (2SD) and δ18O=5.4±0.4‰ (2SD), indicating crystallization from magmas that were derived from a depleted, MORB-like mantle. The albitite zircons give consistent εHf(T) values (+13.0 to +13.5) within errors with those of troctolite zircons, but variable δ18O values. The altered zircon domains have relatively low δ18O values of 4.7±0.6‰ (2SD) due to subsolidus hydrothermal alteration, whilst the least-altered zircon domains give δ18Ozir values of 5.1±0.4‰ (2SD), indistinguishable within errors with the troctolite zircons and the igneous zircons from the Mid-Atlantic and Southwest Indian Ridges gabbros, norites, and plagiogranites of modern oceanic crust. In situ zircon O–Hf isotopic data suggest that the troctolites and albitites are most likely cogenetic, with the albitites being formed by extreme fractional crystallization from the basaltic magma.Based on our new age results and compilation of the reliable literature U–Pb age data, the ophiolitic gabbros from Eastern, Central and, Western Alps, Liguria and Corsica crystallized nearly synchronously at 158–166Ma, suggesting a short life span of <11m.y. for the formation of the Piemont-Liguria oceanic domain, rather than a ~30m.y. life span as previously thought. The Chenaillet ophiolite is likely a remnant of embryonic oceanic crust, rather than a piece of “mature” oceanic crust. Provided the spreading velocities of <3cm/yr full rate for magma-poor MOR sequences, the maximum width to the Piemont-Liguria oceanic floor would have been in the order of 300km.

Different zircon recrystallization types in carbonatites caused by magma mixing: Evidence from U–Pb dating, trace element and isotope composition (Hf and O) of zircons from two Precambrian carbonatites from Fennoscandia

Zircon grains from two Precambrian carbonatites from Fennoscandia (Siilinjärvi and Tiksheozero) were studied by in-situ geochemical and isotope methods. Zircon domains which preserved primary mantle signatures were identified by a combination of microscopic investigations of thin sections of the carbonatite rocks and separated zircon grains (optical microscopy, optical microscopy combined with cathodoluminescence (OM–CL), and scanning electron microscopy). All studied samples show evidence for alteration processes caused by infiltration of late-stage carbonatite melt/s or fluid/s. This led to different changes in the geochemistry and isotope composition of altered zircon domains. For the 2.6Ga old Siilinjärvi carbonatite complex, zircon grains underwent solid state recrystallization mainly at their rims. Zircon cores often preserved primary mantle signatures and register high HREE/LREE ratios, undisturbed U–Pb ages, εHf values close to CHUR (chondritic uniform reservoir), and δ18O values typical for the mantle. The solid state recrystallization of zircon occurred mainly in contact with a late-stage carbonatite melt or fluid and led to (i) diffusion driven loss of HREE, Th, and U, (ii) partial disturbance of the U–Pb system, (iii) a small shift of the δ18O toward lower values, and (iv) higher CL intensities in such zircon domains. Contrarily, zircons from the 2.0Ga old Tiksheozero complex underwent a coupled dissolution–reprecipitation process. Dissolved–reprecipitated zircon domains typically have a distinctive patchy texture in BSE images and sometimes contained abundant micro-inclusions of calcite, phlogopite, apatite, and baddeleyite. The EDS spectra documented that these non-luminescent zircon regions have much higher concentrations of Ca (and sometimes Fe) indicating an “impure” zircon composition. In addition, many trace elements are highly enriched in such “impure” zircon. Enrichment of Th (and to a lower degree of U) resulted in high Th/U ratios (>1) and disturbance of the U/Pb ages. Hf and O isotope values varied widely even within a single zircon grain. Isotope signatures attest a complex crystallization history of carbonatites and can best be interpreted as melt mixing processes of different carbonatite melt pulses. The non-luminescent reprecipitated zircon domains in Tiksheozero are often rimmed by zircon with high CL-intensity recrystallized in a solid state as well as by new zircon overgrowths. These zircon regions consist of pure zircon without elevated concentrations of Ca and Fe. Solid state recrystallization was accompanied by annealing that caused volume loss resulting in fractures that are now frequently filled with calcite (often containing a minor proportion of apatite) and phlogopite which sampled the withdrawn elements from the recrystallized zircon. Co-crystallizing calcite may thus lead to HREE depletion in newly grown zircon. It is expected that interaction with carbonatite melts can lead to similar changes in zircon grains from other sub-continental mantle rocks. Then, the primary geochemical and isotope systems of zircon may be changed despite its chemical and physical robustness. Infiltration of late-stage carbonatite melts seems to be a common process within many carbonatitic rocks that may hamper to determine the primary geochemical and isotope signatures of carbonatitic zircons.

Geochronology and geochemistry of the enderbite series in the Lapland Granulite Belt: generation, tectonic setting, and correlation of the belt1NORDSIM Publication No. 322.

The Lapland Granulite Belt of the northeastern Fennoscandian Shield is dominated by blastomylonitic migmatitic metagreywackes and a deformed, structurally conformable norite–enderbite series. The belt contains minor granodiorites and one major anorthosite massif and is crosscut by post-tectonic granite plutons and diabase dykes. The major and trace element composition of the enderbite series is similar to that of arc-related igneous rocks. Magmatically zoned zircon yield U–Pb secondary ion mass spectrometry ages of 1.91–1.92 Ga, while homogeneous metamorphic zircon domains are aged 1.88–1.90 Ga. Major enderbite bodies reveal only a few older inherited, xenocrystic zircon cores or grains, mainly Archaean in age, while a tonalite body and a narrow quartz norite vein have abundant inherited zircons comparable with the detrital zircon population in the metagreywackes. The intrusive series has slightly positive initial εNdvalues, while the tonalite body has a slightly negative initial εNd, consistent with the occurrences of inherited zircon grains. The major intrusive series, interpreted to have been emplaced in an arc environment, has juvenile character and assimilated only a minor amount of older crustal material during ascent, intrusion, and cooling. Narrow veins and more evolved bodies had more interaction with the metasediments and other crustal rocks. The enderbite series of the Lapland Granulite Belt may be correlated with arc-related plutonism in the Nagssugtoqidian and Torngat orogens, but metamorphic and structural evolution took place considerably later in Greenland and the northeastern Canadian Shield, with the exception of the Inglefield Belt of northwestern Greenland.

Synexhumation anatexis of ultrahigh-pressure metamorphic rocks: Petrological evidence from granitic gneiss in the Sulu orogen

Petrological evidence is provided for partial melting of ultrahigh-pressure (UHP) metamorphic granitic gneiss in the Sulu orogen. Petrographic observations show the occurrence of elongated, highly cuspate feldspars in grain boundaries, interstitial cuspate feldspars in triple junctions, felsic veinlets mainly consisting of K-feldspar+quartz, and feldspar crystal faces against quartz. These features indicate that the feldspar and quartz would have grown from anatectic melts in the granitic gneiss. Zircon domains grown from these melts were identified based on CL images, mineral inclusions and REE patterns. Some large zircon grains (>100μm) contain small relict domains of magmatic origin, suggesting nearly complete dissolution of the protolith zircon during the anatexis. All newly grown zircon domains are categorized into two groups based on the presence or absence of coesite inclusions. One group of domains contain no coesite inclusion and exhibit high U contents but low Th/U ratios (<0.1), steep REE patterns with strong negative Eu anomalies, and UPb ages of 217±2 to 224±2Ma. The other group of domains contain coesite inclusions and exhibit low U contents and very low Th/U ratios (<0.01), steep REE patterns with strong negative Eu anomalies, and UPb ages of 221±5 to 226±3Ma. The two groups of zircon domains are thus interpreted as growing from the anatectic melts at different pressures because they exhibit marked negative Eu anomalies that are absent for metamorphic zircons grown from aqueous fluids. The zircon UPb ages of 217±2 to 226±3Ma are close to, but slightly younger than, known ages for major UHP metamorphism in the Sulu orogen. Therefore, the UHP gneiss experienced incipient melting during the initial exhumation subsequent to the peak UHP metamorphism and extensive anatexis later at lower pressures. Muscovite relicts coexist with cuspate feldspars, suggesting that the anatectic melts originate from dehydration melting due to decompression breakdown of phengitic muscovite in the UHP granitic gneiss. As the degree of partial melting increases with temperature, significant fractions of the anatectic melts would be produced in the regional gneiss provided that the UHP rocks experienced “hot” exhumation. Such melts can be gathered together, being eventually emplaced as synexhumation granitic intrusions.

The growth, reworking and metamorphism of early Precambrian crust in the Jiaobei terrane, the North China Craton: Constraints from U–Th–Pb and Lu–Hf isotopic systematics, and REE concentrations of zircon from Archean granitoid gneisses

Archean granitoid (tonalite–trondhjemite–granodiorite, TTG) gneisses are widely distributed in the early Precambrian metamorphic basement of the Jiaobei terrane, the North China Craton (NCC), and record the formation and evolution of the continental crust in the early Precambrian. In this study, zircon cathodoluminescence (CL) images have assisted in obtaining U–Th–Pb and Lu–Hf isotopic systematics, and rare earth element (REE) concentrations from distinct zircon domains of complex zircons, and the results have been integrated in order to unravel the growth, reworking and metamorphism of the early Precambrian crust in the Jiaobei terrane. Zircons from seven representative Archean granitoid samples, collected from the Jiaobei terrane, record three groups of magmatic ages of ∼2.9, ∼2.7 and ∼2.5Ga, and two groups of metamorphic ages of ∼2.5 and ∼1.86Ga. The ∼2.5Ga metamorphic event is thought to be linked to underplating of large amounts of mantle-derived magma, and the ∼1.86Ga metamorphic event to arc (or continent)–continent collision. Magmatic zircons have positive ɛHf(t) values (+0.7 to +12.58, average +5.5), and two-stage Hf model ages cluster at ∼2.7–2.8 and ∼2.9–3.0Ga. This indicates that the Archean granitoid rocks were derived from partial melting of juvenile crust that was formed mainly during ∼2.7–2.8 and ∼2.9–3.0Ga which represent two periods of growth of juvenile crust. The most significant period of crustal growth is ∼2.7–2.8Ga, whereas ∼2.5Ga represents a period of reworking of the ∼2.7–2.8Ga juvenile crust in the Jiaobei terrane, as well as other areas in the NCC.

Mineralogy, petrology, U-Pb geochronology, and geologic evolution of the Dabie-Sulu classic ultrahigh-pressure metamorphic terrane, East-Central China

The Dabie-Sulu Triassic collisional orogen in eastern Asia was created by northward subduction of the Yangtze continental-crust capped plate beneath the Sino-Korean craton. Eclogites, garnet peridotites, and surrounding country rock gneisses and marbles were all subjected to in situ UHP metamorphism, as indicated by the presence of rare but widespread coesite inclusions in eclogitic minerals and in zircon crystals in the country rocks, as well as by virtually identical metamorphic ages of various UHP rock types. Metamorphic P-T estimates, combined with investigations of mineral exsolution textures and high- P polymorphs, indicate that recovered depths of continental subduction may have exceeded 200 km. Parageneses of mineral inclusions in zoned zircon domains combined with U-Pb ages delineate a well-constrained P-T -time path, suggesting exhumation rates of 5–10 km/Myr. A similar P-T -time trajectory has been established for the microdiamond-bearing Kokchetav Massif. Thus far, however, diamond inclusions have not been confirmed from coesite-bearing zircon domains of Dabie-Sulu UHP rocks despite numerous detailed studies. Oxygen isotopes of minerals from many outcrop samples and the Chinese Continental Scientific Drilling (CCSD) project main hole cores indicate that Delta; 18 O depletion took place in a volume of Proterozoic protoliths exceeding 100 000 km 3 along the northern edge of the Yangtze craton. Evidently, passive-margin sediments and bimodal igneous rocks that had formed during rifting and breakup of the supercontinent Rodinia were subjected to extensive meteoric water-rock interactions attending terminal Neoproterozoic Snowball Earth conditions. Such hydrothermal alteration volatilized and depleted C from the relatively oxidized protoliths, accounting for the rare occurrences of graphite and apparent lack of microdiamond in Dabie-Sulu UHP rocks.

Fluid-induced magmatic and post-magmatic zircon and monazite patterns in granitoid pluton and related rhyolitic bodies

Zircon and monazite from the Stolpen granite and related rhyolitic dyke show magmatic as well as post-magmatic textures. The magmatic textures are overprinted by post-magmatic fluid interaction which is indicated by elevated contents of Y, P, Th, and U in altered zircon domains. This can be observed both in the granite and rhyolite; however these processes were less intensive in the rhyolite. Patchy zoning in primary monazite from the granite also suggests fluid-induced alterations. High Ca, Pb, and Si contents and depletion in Th, as well as porous, spongy textures in monazite from the rhyolite are results of fluid-induced alterations that caused also crystallization of cheralite. Zircon and monazite from the granite are accompanied by fluorite, Y-rich and Nb-rich minerals. The alteration of primary, magmatic accessory minerals and formation of secondary minerals is interpreted as related to magmatic fluids, which origin cannot be defined precisely on the base of the present data, however their composition suggests some mantle contribution. The presence of fluorine and CO2 which promoted the transport of some HFSE and LILE accounts for their enrichment in the accessory minerals. Fluids that affected the accessory minerals from the rhyolite show different features (silica- and Ca-enrichment) and most probably came from another source.

Is the HP–UHP Hong’an–Dabie–Sulu orogen a piercing point for offset on the Tan–Lu fault?

The Tan–Lu fault is a major strike-slip fault in eastern China that appears to offset the high-grade rocks of the Hong’an–Dabie–Sulu orogen left-laterally ∼540km. We evaluate models for the collision between the South and North China blocks, published radiometric dates recording HP–UHP metamorphism and exhumation in the Hong’an–Dabie and Sulu terranes, and the timing of sinistral motion on the Tan–Lu fault to evaluate whether UHP rocks provide a piercing point for offset on the Tan–Lu fault. UHP metamorphism in Hong’an–Dabie was concurrent with Sulu based on U–Pb dating of coesite-bearing domains of zircon at 244±5–226±2Ma for Hong’an–Dabie and 243±4–225±2Ma for Sulu. Retrograde metamorphism began c. 220Ma for both Hong’an–Dabie and Sulu, but retrograde zircon growth ended c. 214Ma in Hong’an–Dabie and continued until c. 202Ma in Sulu based on U–Pb dating of zircon domains external to coesite-bearing domains. Structures in Sulu are rotated 25° counter-clockwise from, but are broadly similar to, Hong’an–Dabie suggesting the two areas have a common Triassic orogenic history that pre-dates motion on the Tan–Lu fault, and that is consistent with paleomagnetic studies. We constructed a pre-Cretaceous restoration of the Hong’an–Dabie–Sulu belt that moves the Sulu terrane south, aligning the suture and the eclogite-facies isograd, and rotates Sulu c. 25° clockwise to re-align structures with Hong’an–Dabie. Our restoration is supported by published data and shows that the Hong’an–Dabie–Sulu orogen is a piercing point for post-collisional offset on the Tan–Lu fault and that these regions shared a common subduction–exhumation history. The Tan–Lu fault did not play a significant role in the Hong’an–Dabie–Sulu collision and likely developed later, in the Early Cretaceous.

The origin of zircon and the significance of U–Pb ages in high-grade metamorphic rocks: a case study from the Variscan orogenic root (Vosges Mountains, NE France)

U–Pb zircon dating is combined with petrology, Zr-in-rutile thermometry and mineral equilibria modelling to discuss zircon petrogenesis and the age of metamorphism in three units of the Variscan Vosges Mountains (NE France). The monotonous gneiss unit shows results at 700–500 Ma, but no Variscan ages. The varied gneiss unit preserves ages between 600 and 460 Ma and a Variscan group at 340–335 Ma. Zircon analyses from the felsic granulite unit define a continuous array of ages between 500 and 340 Ma. In varied gneiss samples, zoned garnet includes kyanite and rutile and is surrounded by matrix sillimanite and cordierite. In a pseudosection, it points to peak conditions of ~16 kbar/850 °C followed by isothermal decompression to 8–10 kbar/820–860 °C. In felsic granulite samples, the assemblage K-feldspar–garnet–kyanite–Zr-rich rutile is replaced by sillimanite and Zr-poor rutile. Modelling these assemblages supports minimum conditions of ~13 kbar/925 °C, and a subsequent P–T decrease to 6.5–8.5 kbar/800–820 °C. The internal structure and chemistry of zircons, and modelling of zircon dissolution/growth along the inferred P–T paths are used to discuss the significance of the U–Pb ages. In the monotonous unit, inherited zircon ages of 700–500 Ma point to sedimentation during the Late Cambrian, while medium-grade metamorphism did not allow the formation of Variscan zircon domains. In both the varied gneiss and felsic granulite units, zircons with a blurred oscillatory-zoned pattern could reflect solid-state recrystallization of older grains during HT metamorphism, whereas zircons with a dark cathodoluminescence pattern are thought to derive from crystallization of an anatectic melt during cooling at middle pressure conditions. The present work proposes that U–Pb zircon ages of ca. 340 Ma probably reflect the end of a widespread HT metamorphic event at middle crustal level.

A search for thermal excursions from ancient extraterrestrial impacts using Hadean zircon Ti-U-Th-Pb depth profiles

Few terrestrial localities preserve more than a trace lithic record prior to ca. 3.8 Ga greatly limiting our understanding of the first 700 Ma of Earth history, a period inferred to have included a spike in the bolide flux to the inner solar system at ca. 3.85–3.95 Ga (the Late Heavy Bombardment, LHB). An accessible record of this era may be found in Hadean detrital zircons from the Jack Hills, Western Australia, in the form of μm-scale epitaxial overgrowths. By comparing crystallization temperatures of pre-3.8 Ga zircon overgrowths to the archive of zircon temperature spectra, it should, in principle, be possible to identify a distinctive impact signature. We have developed Ti-U-Th-Pb ion microprobe depth profiling to obtain age and temperature information within these zircon overgrowths and undertaken a feasibility study of its possible use in identifying impact events. Of eight grains profiled in this fashion, four have overgrowths of LHB-era age. Age vs. temperature profiles reveal a period between ca. 3.85–3.95 Ga (i.e., LHB era) characterized by significantly higher temperatures (approximately 840–875 °C) than do older or younger zircons or zircon domains (approximately 630–750 °C). However, temperatures approaching 900 °C can result in Pb isotopic exchange rendering interpretation of these profiles nonunique. Coupled age-temperature depth profiling shows promise in this role, and the preliminary data we report could represent the first terrestrial evidence for impact-related heating during the LHB.

First record and timing of UHP metamorphism from zircon in the Xitieshan terrane: Implications for the evolution of the entire North Qaidam metamorphic belt

The Xitieshan terrane is one of four metamorphic terranes in the North Qaidam metamorphic belt, which is an early Paleozoic high-pressure to ultrahigh-pressure (HP-UHP) metamorphic belt in NW China. However, conclusive evidence and precise timing of UHP metamorphism in the Xitieshan terrane have not been well documented. In this study, we report an integrated study of zircon grains from an amphibolite in the Xitieshan terrane in terms of mineral inclusions, trace elements, and U-Pb age systematics. The first record of coesite is reported as an inclusion in a metamorphic zircon, which provides unambiguous evidence for the UHP metamorphism of the Xitieshan terrane. The metamorphic zircon domains have weak or no zoning, very low Th/U ratios, insignificant Eu anomalies, and flat HREE patterns. Zircon grains contain inclusions of garnet, omphacite and rutile, in addition to the coesite inclusion. These inclusions indicate that the metamorphic zircon grains formed under UHP metamorphic conditions. The metamorphic zircon grains were dated by the SIMS and LA-ICPMS methods and yield weighted mean ages of 432 ± 14 and 441 ± 9 Ma, respectively. Combined with previous results, the HP-UHP metamorphism of the Xitieshan terrane may have lasted 460-440 Ma with the peak UHP metamorphism at 441 ± 9 Ma. A compilation of the reported geochronological data reveals that all four terranes of the North Qaidam metamorphic belt might have experienced coeval UHP metamorphism during the early Paleozoic (420-450 Ma), and thus may have suffered a coherent subduction, UHP metamorphism, and exhumation cycle.

Late Permian–Early Triassic igneous activity in the Attic Cycladic Belt (Attica): New geochronological data and geodynamic implications

Igneous rocks of the Attic Cycladic Belt (ACB) formed prior to the Alpine orogeny have received relatively little attention, especially regarding their crystallisation age. New U–Pb SHRIMP zircon data from mylonitised metagranitoid rocks within metapelites and metabasites of the Pentelikon mountain, lower tectonic unit (LTU) of Attica, in combination with internal zircon characteristics reveal the following: the metagranitoid precursor was very likely an I-type pluton crystallising in two successive stages, at 255±3Ma and 246±2Ma (weighted mean 206Pb/238U ages; error at 95% c.l.). These age results are respectively marginally older than and within uncertainty of the 240±4Ma protolith age reported for metagranitoids of the same unit in the area of Lavrion, Attica. They are in line with the protolith ages of several meta-igneous rocks within the Cycladic Blueschist Unit of Aegean islands. One core zircon domain and one single zircon grain yielded 206Pb/238U ages of 311±3Ma and 318±4Ma (1σ errors), providing evidence for inheritance from a previous Hercynian event. The new data, whilst interpreted to favour a rift-related geotectonic setting of formation (poly-episodic magmatism), do not provide additional strong arguments for a rifting versus a subduction scenario. Assuming that the dated rocks formed indeed in a rift setting, prominent at that time in the broad European area, they probably represent a higher level of the crust, compared to Permo-Triassic metagabbros of the Rhodope or the European Alps, which formed at deeper crustal levels (underplated gabbros). The new data, in combination with earlier petrological data on the type of the metamorphic path, favour the view that the LTU of Attica belongs geotectonically to the Cycladic Blueschist Unit of the ACB. This inference is thus filling an important missing link in the geotectonic configuration of the geology of Attica.