Zircon Core Ages Research Articles

Effect of chemical abrasion of zircon on SIMS U–Pb, δ18O, trace element, and LA-ICPMS trace element and Lu–Hf isotopic analyses

Abstract. This study assesses the effect of chemical abrasion on in situ mass spectrometric isotopic and elemental analyses in zircon. Chemical abrasion improves the U–Pb systematics of SIMS (secondary ion mass spectrometry) analyses of reference zircons, while leaving other isotopic systems largely unchanged. SIMS 206Pb/238U ages of chemically abraded reference materials TEMORA-2, 91500, QGNG, and OG1 are precise to within 0.25 % to 0.4 % and are within uncertainty of chemically abraded TIMS (thermal ionization mass spectrometry) reference ages, while SIMS 206Pb/238U ages of untreated zircons are within uncertainty of TIMS reference ages where chemical abrasion was not used. Chemically abraded and untreated zircons appear to cross-calibrate within uncertainty using all but one possible permutation of reference materials, provided that the corresponding chemically abraded or untreated reference age is used for the appropriate material. In the case of reference zircons QGNG and OG1, which are slightly discordant, the SIMS U–Pb ages of chemically abraded and untreated material differ beyond their respective 95 % confidence intervals. SIMS U–Pb analysis of chemically abraded zircon with multiple growth stages is more difficult to interpret. Treated igneous rims on zircon crystals from the S-type Mount Painter Volcanics are much lower in common Pb than the rims on untreated zircon grains. However, the analyses of chemically abraded material show excess scatter. Chemical abrasion also changes the relative abundance of the ages of zircon cores inherited from the sedimentary protolith, presumably due to some populations being more likely to survive the chemical abrasion process than others. We consider these results from inherited S-type zircon cores to be indicative of results for detrital zircon grains from unmelted sediments. Trace element, δ18O, and εHf analyses were also performed on these zircons. None of these systems showed substantial changes as a result of chemical abrasion. The most discordant reference material, OG1, showed a loss of OH as a result of chemical abrasion, presumably due to dissolution of hydrous metamict domains or thermal dehydration during the annealing step of chemical abrasion. In no case did zircon gain fluorine due to exchange of lattice-bound substituted OH or other anions with fluorine during the HF partial dissolution phase of the chemical abrasion process. As the OG1, QGNG, and TEMORA-2 zircon samples are known to be compositionally inhomogeneous in trace element composition, spot-to-spot differences dominated the trace element results. Even the 91500 megacrystic zircon pieces exhibited substantial chip-to-chip variation. The light rare earth elements (LREEs) in chemically abraded OG1 and TEMORA-2 were lower than in the untreated samples. Ti concentration and phosphorus saturation ((Y + REE) / P) were generally unchanged in all samples.

Two episodes of Early Palaeozoic high-pressure metamorphism in North Balkhash ophiolite zone (Central Kazakhstan, western Central Asian Orogenic Belt): Evidence for tectonic evolution of Junggar–Balkhash Ocean

The structure of serpentinite mélange of the North Balkhash ophiolite zone (Central Kazakhstan; west Central Asian Orogenic Belt) is ascertained to be high-grade formations assigned to epidote-glaucophane eclogites and garnet blueschists, and their varieties. The rocks follow a clockwise ‘subduction-type’ evolution with the estimated near–peak metamorphic conditions of 1.6–1.9 GPa and 500–560 °C. 40Ar39Ar phengite ages of ∼491 Ma and ∼ 465 Ma obtained for the eclogites and garnet blueschists, respectively, are interpreted to reflect the near-peak to shortly retrograde stages of rock evolution and to record the late Cambrian and Middle Ordovician episodes of high-pressure re-equilibration in the North Balkhash zone. Protoliths of the eclogites were N-MORB-like mafic rocks, which comprised structurally different (from the lower gabbroic to the upper dolerite/basalt) parts of pre-late Cambrian oceanic crust and were formed in the spreading centre setting at the expense of depleted mantle source. Protoliths of the garnet blueschists and associated rocks were represented by volcanogenic–sedimentary, predominantly mafic, complexes (tuffaceous sandstones, greywackes), an accumulation of which and subsequent involvement into subduction occurred at ∼478–465 Ma in the intra-oceanic (fore-arc) setting. Zircon core ages indicate formation of the source of the protoliths of the garnet blueschists occurred in the 509–478 Ma range. A comprehensive correlation of the metamorphic and igneous formations of the North Balkhash zone with those assigned to the adjacent complexes of the southern part of the West Junggar area (NW China) suggested their mutual Early Palaeozoic tectonic evolution within the Junggar–Balkhash Ocean.

An Iapetus origin for a layered eclogite complex in the northern Western Gneiss Region, Scandinavian Caledonides

AbstractThe Western Gneiss Region (WGR) is a Precambrian basement domain in the Scandinavian Caledonides and one of the world's largest high‐ and ultrahigh‐pressure terranes. The south–central WGR underwent regional eclogite facies metamorphism 415–400 Ma ago when Baltica subducted beneath Laurentia, during the Scandian orogeny. Eclogites in the WGR group into two traditional types: (1) Precambrian mafic intrusions metamorphosed in situ during Scandian continental subduction and (2) eclogites, garnet peridotites and garnet pyroxenites within ultramafic complexes derived from the subcontinental mantle beneath Laurentia. We document, using field relations, petrography, whole‐rock geochemistry and secondary ion mass spectrometry (SIMS) zircon geochronology, a hitherto unrecognized third type of eclogite in the WGR that places new constraints on its tectonic architecture: an eclogitized fragment of oceanic crust from the Iapetus Ocean. The Kråkfjord eclogite complex is a km2‐sized body with an interior consisting of kyanite eclogite (meta‐troctolite) and subordinate layers and lenses of garnet peridotite, garnet websterite and kyanite–garnet leucotonalite. This interior is capped by Fe–Ti‐rich eclogite, which locally contains subordinate pockets of migmatitic aluminous gneiss. The elemental abundances and isotopic compositions of the Fe–Ti‐rich eclogites resemble those of mid‐ocean ridge basalt (MORB). In contrast, the interior kyanite eclogites, peridotites and pyroxenites have compositions similar to the gabbroic cumulates in the lower oceanic crust of slow‐spreading ridges. U–Pb SIMS dating of igneous zircon cores from a leucotonalite pod in the interior of the Kråkfjord complex yields Cambro‐Ordovician igneous ages of 500–440 Ma, with the ~500 Ma age interpreted as the isotopically undisturbed age. This age matches those of Iapetan oceanic rocks exposed elsewhere in the mountain belt. Metamorphic zircon from an Fe–Ti‐rich eclogite in the carapace of the Kråkfjord complex dates the eclogite facies metamorphism at 421.9 ± 2.2 Ma, synchronous with the continental collision. Zircon from a leucosome in Fe–Ti‐rich retro‐eclogite indicates an age of 408.5 ± 2 Ma for the crystallization of partial melt following the decompression. Detrital zircon core ages from a pocket of aluminous migmatitic gneiss in the carapace indicate derivation of sediment from the Baltic crust. Collectively, the data show that the eclogite complex (1) originated at an Iapetus spreading centre near the continent Baltica, (2) subducted to eclogite conditions during Scandian continental collision and (3) was tectonically intercalated with the Precambrian Baltica basement of the WGR.

Vodorazdelny Granite Massif (Subpolar Urals) and Correlation Problems of the Pre-Ordovician Granitoids and Volcanic Rocks from the Northern Part of the Lyapinsky Anticlinorium

Petrogeochemical features of the Vodorazdelny massif granites (Subpolar Urals, Lyapinsky anticlinorium) indicate these rocks are close to I-granites and were formed in suprasubduction environment. The ratios of the key elements (Rb, Ba, Th, Sr, Y, Nb) suggest that the basites of the melting slab and the fluid separated during their dehydration could participate in the granite generation. The U–Pb age of the main population of magmatogenic zircons is 593 ± 4 Ma and corresponds to the Vendian (Ediacarian). It coincides with the age of the granites from the nearby Vangyr massif (598 ± 5 Ма), as well as with the age of zircon cores from the granites of the Kozhim massif located to the north. Values of εHf(t) from –2 to 0 in magmatogenic zircons with an age corresponding to the age of Vodorazdelny granites indicate a heterogeneous source of melts. The zircons of these granites also contain ancient cores with U–Pb ages from 2200 to 700 Ma, where the values of εHf(t) from +0.8 to +13 indicate the presence of a crust component in the substrate (the substance of the ancient platform basement). Petrogeochemical and isotope-geochronological parameters of granites (and their zircons) do not confirm the validity of attributing of Vodorazdelny granite massif (and its analogues – the Vangyr and Kozhim massifs) to the Cambrian Salner-Mankhambo complex. We consider there are the possibility of separating of independent complex (about 598 Ma, Vendian?) during geological mapping. The presence in the Lyapinsky anticlinorium of several stages of granite generation (Middle Riphean–Vendian–Cambrian), accompanied by metamorphism, and complicate composition of ancient metamorphic strata from this structure basement led to varying isotopic parameters characterizing the heterogeneity of the source of melts, on the one hand, and the convergence of a number of geochemical features, on the other.

The effect of crystal fractionation on the geochemical composition of syn-exhumation magmas: Implication for the formation of high δ56Fe granites in collisional orogens

Syn-exhumation magmatism in collisional orogens is an important process for crustal differentiation and crust-mantle interaction at convergent plate boundaries. It is intriguing to elucidate which factors control the geochemical composition of such magmatic products. To answer this question, a combined study of whole-rock major-trace elements and Nd-Fe isotopes, mineral O isotopes, and zircon U–Pb ages and trace elements was carried out for syn-exhumation granites from the Sulu ultrahigh-pressure (UHP) metamorphic belt in east-central China. Several granitic plutons of Triassic age were newly found in this typical collisional orogen, with a total exposure area of ∼10 km2, suggesting large-scale partial melting of the deeply subducted continental crust in the terminal stage of continental collision. Field observations indicate that the massive granites show little deformation. The granites are high-K calc-alkaline and exhibit enrichment in LILE and LREE but depletion in HFSE relative to HREE in trace element distribution patterns. Zircons in these granites contain relict magmatic cores characterized by steep HREE patterns with strong negative Eu anomalies. These cores show middle Neoproterozoic U-Pb ages of 724–779 Ma, consistent with the protolith age of UHP metaigneous rocks in the Dabie-Sulu orogenic belt. Zircon rims exhibit oscillatory zoning, and are characterized by Triassic U-Pb ages of 210–216 Ma, low LREE contents and steep HREE patterns with significant negative Eu anomalies, suggesting their growth from granitic magmas during exhumation of the deeply subducted continental crust. The granites have low δ18O values of 2.0–6.1‰ for quartz, 1.1–5.2‰ for whole-rock, and 0.9–3.2‰ for zircon cores. The low δ18O values and Neoproterozoic U-Pb ages of zircon cores in the granites are characteristic feature of the deeply subducted continental crust in the northern margin of the South China Block. One coesite inclusion was found in the anatectic zircon rim, pointing to the deep origin of anatectic melts at a subarc depth of >80 km. Whole-rock SiO2 contents are correlated with major and trace elements, which are ascribed to crystal fractionation of mainly biotite and plagioclase during magma evolution. In particular, these granites exhibit highly variable δ56Fe values from 0.05 to 0.30‰, which are correlated with Fe3+/ΣFe, SiO2, Al2O3, Nb/Ta and Eu/Eu*. This suggests that the crystal fractionation of biotite would have controlled the Fe isotope variation in the granites. Therefore, the syn-exhumation granites experienced significant geochemical differentiation mainly through the fractional crystallization during the magma evolution. A compilation of syn-exhumation granites in the typical collisional orogens of the world shows that such granites were mainly formed through dehydration melting of hydrous minerals during decompressional exhumation. They fall into the magnesian group with relatively low REE and HFSE (like Ti, Nb and Zr) contents, distinct from the geochemical compositions of A-type granites. Therefore, syn-exhumation granites record crustal differentiation in the terminal stage of continental collision.

Sr-Nd-Hf Isotopic Disequilibrium During the Partial Melting of Metasediments: Insight From Himalayan Leucosome

Radiogenic isotopes of granitoids are widely applied to fingerprint the source of granitoids and study the magma mingling and assimilation processes, aiming to decipher the planetary differentiation. This weapon is based on the assumption that crustal melts inherit the radiogenic isotopes of protoliths. However, complicated melting processes in the crust would drive the radiogenic isotopes of melt away from the source, thus calling for a clear understanding of the behaviors of radiogenic isotopes during crustal melting. This study investigated the joint behavior of Rb-Sr, Sm-Nd, and Lu-Hf isotope systems during the melting of metasediments. Leucosome, schist, and leucogranite samples were collected from the Nyalam, South Tibet, where the leucosome was produced by muscovite dehydration melting of metapelites. Results show that the leucosome has a broad range of Sr-Nd-Hf isotopes (87Sr/86Sri: 0.763,48–0.875321, εNd(t): 14.6–11.6, εHf(t): 15.0–4.4) that deviate from the metasediments. We attributed it to the non-modal or disequilibrium melting of metapelites. The variation of Sr isotopes of leucosome is formed due to preferential entry into the melt of muscovite relative to plagioclase during melting. The changing 87Sr/86Sri and 87Rb/86Sr of leucosome define an errorchron at ∼ 500 Ma indicating an early Paleozoic tectono-thermal event supported by the U-Pb age of zircon cores (∼460 Ma) in the leucosome. The Nd isotopes of leucosome are mainly controlled by the preferential dissolution of apatite relative to monazite. The inadequate dissolution of zircon caused more depleted initial Hf isotopes compositions in the leucosome than the source. However, the leucosome with a higher crystallization temperature has radiogenic isotopic composition closer to the source. It indicates that the melting temperature greatly affects the isotope equilibrium between source and melt.

Metamorphic evolution of a Tonian eclogite associated with an island arc of the southern Brasiliano Orogen

The early evolution of the southern Brasiliano Orogen is recorded in the São Gabriel Terrane, which consists of two Neoproterozoic (Tonian and Cryogenian) juvenile arcs, ophiolite complexes and supracrustal rocks. This study aims at providing new insights into the geodynamics processes and subduction process of this orogen by investigating the first Tonian eclogite (named Três Vendas eclogite) discovered in the southern Brasiliano Orogen. To determine the tectono-metamorphic evolution, we integrated field work with petrography, scanning electron microscopy, electron microprobe analyses of minerals and in situ LA-ICP-MS U-Pb geochronology with Lu-Hf and rare earth elements in zircon. The preserved high-grade metamorphic assemblage in these rocks has garnet, rutile, phengite, and omphacite, indicating that these rocks are (retro) eclogites. The presence of omphacite with up to 50% jadeite indicates minimum burial pressures of ≥15 kbar that are consistent with forward modeling of the garnet compositions. Omphacite was altered to fine-grained symplectites of hornblende, oligoclase, and diopside or augite with minor quartz during rapid exhumation. The second stage of retrogression involved reaction of garnet and diopside to plagioclase and hornblende symplectites (kelyphites) at amphibolite facies conditions. Kelyphitic hornblende-garnet pairs indicate peak temperatures of 710–760 °C. Whole rock composition and Zr–Y characteristics are consistent with tholeiitic island arc basalts, while their garnet compositions point to subduction zone eclogites. U-Pb ages of zircon cores yielded 909.5 ± 5.3 Ma and metamorphic rims 891.9 ± 8.2 Ma. Zircon age-corrected εHf isotope compositions range between +4 and +10, reflecting the moderately juvenile nature of these rocks. Zircon cores are distinguished by higher rare earth element (REE) contents and steeper middle REE to heavy REE (MREE-HREE) patterns, higher Th/U ratios and higher Sr and Ca contents than the metamorphic rims, and the rims are likely coeval with the kelyphites. The combined data indicate that the Três Vendas eclogites belong to a Tonian arc and were buried to >45 km, most likely in a subduction channel. The data from the first eclogite in southern Brazil have significant consequences for the interpretation of the Tonian evolution of the Brasiliano Orogen and, by extension, the Rodinia framework.

New interpretations of the ages and origins of the Hawkeye Granite Gneiss and Lyon Mountain Granite Gneiss, Adirondack Mountains, NY: Implications for the nature and timing of Mesoproterozoic plutonism, metamorphism, and deformation

The Hawkeye Granite Gneiss and Lyon Mountain Granite Gneiss are widespread Mesoproterozoic plutonic rocks that occur in the amphibolite- to granulite-facies Adirondack Highlands of northern New York, USA. The strongly deformed Hawkeye Granite Gneiss, previously dated by zircon multi-grain thermal ionization mass spectrometry (TIMS) U-Pb analyses at about 1100 Ma, was intruded by the weakly deformed Lyon Mountain Granite Gneiss. Previous sensitive high resolution ion microprobe (SHRIMP) analyses of Lyon Mountain Granite Gneiss zircon rims were considered to record the time of igneous emplacement at about 1.05 Ga, whereas the ages of zircon cores (~1.15 Ga) were interpreted as being inherited from nearby metaigneous country rocks. This interpretation has formed the basis of numerous models for the Mesoproterozoic structural and tectonic evolution of the Adirondacks Highlands. New U-Pb spot analyses (~15–20 µm diameter) by SHRIMP from four samples of Hawkeye Granite Gneiss and eight samples of Lyon Mountain Granite Gneiss challenge the historically accepted ages of the rocks. Using a combination of high-resolution CL imagery of oscillatory zoned cores and weakly zoned to unzoned rims, SHRIMP U-Pb geochronology, SHRIMP trace element geochemistry, and SEM petrography, we conclude that: (1) the Hawkeye Granite Gneiss was emplaced at about 1160–1155 Ma; (2) the vast majority of Lyon Mountain Granite Gneiss zircon cores yield ages of 1150–1145 Ma and are mainly primary, not inherited; and (3) the Hawkeye Granite Gneiss and Lyon Mountain Granite Gneiss zircon rims (~1080–1000 Ma) are metamorphic in origin, not igneous. Thus, both the Hawkeye Granite Gneiss and Lyon Mountain Granite Gneiss are considered herein to be late members of the mangerite-charnockite-granite (MCG) plutonic suite, emplaced during the waning stages of the Shawinigan orogeny. Zircon cores from two small plutons of largely undeformed fayalite granite of the Lyon Mountain Granite Gneiss are about 1142 Ma, providing a minimum age constraint for the termination of the Shawinigan. Zircon rim ages can be deconvoluted into multiple metamorphic events of the Ottawan and Rigolet tectonothermal events. These new interpretations have profound implications for structural, tectonic, and ore deposit models of the Adirondacks Highlands.

Geochemistry of two types of Palaeoproterozoic granites, and zircon U–Pb dating, and Lu–Hf isotopic characteristics in the Kuandian area within the Jiao‐Liao‐Ji Belt: Implications for regional tectonic setting

Palaeoproterozoic granites accompanied with granulite‐facies gneisses can be recognized in the Kuandian area, northern Jiao‐Liao‐Ji Belt (JLJB). This study highlights two episodes of Palaeoproterozoic magmatism, identified by granites and biotite monzogranitic gneisses herein. Zircon grains from biotite monzogranitic gneiss with homogeneous oscillatory zoning yielded a U–Pb age of 2,153 ± 16 Ma, in accordance with the age of zircon cores from the monzogranitic gneiss. Zircon grains from the two granites yielded upper intercept ages of 1,892 ± 38 Ma and 1,859 ± 36 Ma respectively. Monzogranitic gneiss samples are enriched in Th, Rb, Pb, U, and REE, but depleted in Sr and P. They have high 1,000 × Ga/Al ratios versus Zr, Ce, Nb, Y contents, which are usually interpreted as the features of classical A‐type granite that originated in a lithospheric extension setting. The ~1.89–1.86 Ga granite samples have relatively high Al, Rb, Th, U and low Sr, Zr, and Hf contents. These features, combined with the observation of minor apatite grains under the microscope are similar with the I‐type granites. In situ zircon Lu–Hf isotopic analyses revealed that ~1.89–1.86 Ga granite had a similar range and TDM2 ages with the monzogranitic gneiss, but with much lower εHf(t) values. The monzogranitic gneiss may have originated from an Archean crust, while the later granite may be derived from melting of the partial melting of the monzogranitic gneiss or the Archean crust that generated the monzogranitic gneiss. The ~1.89–1.86 Ga granites, together with the coeval S‐type granites in the JLJB, represent an orogeny process that may involve a Palaeoproterozoic subduction.

Dating multiple generation of zircons from granites and gneiss from Thailand: Implication for the crustal evolution of the Sibumasu terrane

The widespread granitic rocks of the Sibumasu terrane were emplaced during the collision of the Sibumasu and Indochina in the Triassic. Previous studies have suggested that each granite had a simple, single-step magmatic evolution. In situ LA-MC-ICP-MS U-Pb zircon dating of granites and gneiss from Thailand revealed the evolution of these rocks: Inherited zircon core ages (515 ± 130 Ma) from a granite of the Chonburi pluton in the Central-Granitoid Belt are coeval with an orthogneiss protolith age (513.6 ± 9.2 Ma) from Hua Hin area. Similar Cambrian ages were also reported from the Khao Tao area in southern Thailand. Moreover, there are a series of inherited zircon core ages from Neo-Proterozoic (1017 ± 43 and 984 ± 56 Ma), Ordovician (449 ± 26 Ma), Devonian (385 ± 15 Ma), Carboniferous (316 ± 12 Ma) to Permian (276 ± 31 and 266 ± 12 Ma). Taken together, these Neo-Proterozoic to Permian protolith ages are interpreted to represent the basement of the Sibumasu terrane in Thailand. Two magmatic zircon growth events can be distinguished at approximately (a) 241.8 ± 8.2 to 235 ± 12 Ma and (b) 230.6 ± 3.4 to 213.4 ± 3.0 Ma. We concluded that the Sibumasu and Indochina collision during the Triassic caused a two-step magmatic evolution of the granites of the Sibumasu terrane. The first magmatic crystallization during the Middle Triassic corresponded to the Sibumasu and Indochina collision, and the second magmatic crystallization during the Late Triassic was related to the late-collisional stage.

Timing of metamorphism and exhumation in the Nordøyane ultra-high-pressure domain, Western Gneiss Region, Norway: New constraints from complementary CA-ID-TIMS and LA-MC-ICP-MS geochronology

We present the results of a combined CA-ID-TIMS and LA-MC-ICP-MS U–Pb geochronology study of zircon and associated rutile and titanite from the Nordøyane ultra-high-pressure (UHP) domain in the Western Gneiss Region (WGR) of Norway. The dated samples include 4 eclogite bodies, 2 host-rock migmatites, and 2 cross-cutting pegmatites and leucosomes, all from the island of Harøya. Zircon from a coesite eclogite yielded an age of ca. 413 Ma, interpreted as the time of UHP metamorphism in this sample. Zircon data from the other eclogite bodies yielded metamorphic ages of ca. 413 Ma, 407 Ma, and 406 Ma; zircon trace-element data associated with 413 Ma and 407 Ma ages are consistent with eclogite-facies crystallization. In all of the eclogites, U–Pb dates from zircon cores, interpreted as the times of protolith crystallization, range from ca. 1680–1586 Ma, consistent with Gothian ages from orthogneisses in Nordøyane and elsewhere in the WGR. A zircon core age of ca. 943 Ma from one sample agrees with Sveconorwegian ages of felsic gneisses and pegmatites in the western part of the area. Migmatites hosting the eclogite bodies yielded zircon core ages of ca. 1657–1591 Ma and rim ages of ca. 395–392 Ma, interpreted as the times of Gothian protolith formation and Scandian partial melt crystallization, respectively. Pegmatite in an eclogite boudin neck yielded a crystallization age of ca. 388 Ma, interpreted as the time of melt crystallization. Rutile and titanite from 3 samples (an eclogite and two migmatites) yielded concordant ID-TIMS ages of 378–376 Ma. The results are similar to existing U–Pb data from other Nordøyane eclogites (415–405 Ma). In combination with previous pressure-temperature data from the coesite eclogite, these ages indicate that peak metamorphic conditions of ~3 GPa/760 °C were reached ca. 413 Ma, followed by decompression to ~1 GPa/810 °C by ca. 397 Ma and cooling below ca. 600 °C by ca. 375 Ma. The results are compatible with protracted UHP metamorphism followed by relatively slow exhumation. The question of whether partial melting began at UHP conditions is not resolved by this study.

Detrital zircon and igneous protolith ages of high-grade metamorphic rocks in the Highland and Wanni Complexes, Sri Lanka: Their geochronological correlation with southern India and East Antarctica

The high-grade metamorphic rocks of Sri Lanka place valuable constraints on the assembly of central parts of the Gondwana supercontinent. They are subdivided into the Wanni Complex (WC), Highland Complex (HC) and Vijayan Complex (VC), but their correlation with neighbouring Gondwana terranes is hindered by a poor understanding of the contact between the HC and WC. Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) U–Pb dating of remnant zircon cores from 45 high-grade metamorphic rocks in Sri Lanka reveals two domains with different age characteristics that correlate with the HC and WC and which help constrain the location of the boundary between them. The HC is dominated by detrital zircon ages of ca. 3500–1500 Ma from garnet–biotite gneiss, garnet–cordierite–biotite gneiss, some samples of garnet–orthopyroxene–biotite gneiss and siliceous gneiss (interpreted as paragneisses) and igneous protolith ages of ca. 2000–1800 Ma from garnet–hornblende–biotite gneiss, other samples of garnet–orthopyroxene–biotite gneiss, garnet–two-pyroxene granulite, two-pyroxene granulite and charnockite (interpreted as orthogneisses). In contrast, the WC is dominated by detrital zircon ages of ca. 1100–700 Ma from paragneisses and igneous protolith ages of ca. 1100–800 Ma from orthogneisses. This clearly suggests the HC and WC have different origins, but some of our results and previous data indicate their spatial distribution does not correspond exactly to the unit boundary proposed in earlier studies using Nd model ages. Detrital zircon and igneous protolith ages in the HC suggest that sedimentary protoliths were eroded from local 2000–1800 Ma igneous rocks and an older Paleoproterozoic to Archean craton. In contrast, the WC sedimentary protoliths were mainly eroded from local late Mesoproterozoic to Neoproterozoic igneous rocks with very minor components from an older 2500–1500 Ma craton, and in the case of the WC precursor sediments there was possibly additional detritus derived from early to middle Neoproterozoic metamorphic rocks. The relic zircon core ages in the HC are comparable with those of the Trivandrum Block and Nagercoil Block of southern India. In contrast, those ages in the WC match the Achankovil Shear Zone and Southern Madurai Block of southern India. These comparisons are also supported by Th/U ratios of detrital zircon cores from paragneisses (Th/U ratios of >0.10 for the former and not only >0.10 but also ≤0.10 for the latter). Comparisons with the Lützow-Holm Complex of East Antarctica indicate that the geochronological characteristics of the HC and WC broadly match those of the Skallen Group, and the Ongul and Okuiwa Groups, respectively.

Constraining timing and tectonic implications of Neoproterozoic metamorphic event in the Cathaysia Block, South China

The Cathaysia Block of the South China Craton includes a Proterozoic basement that experienced a prolonged Precambrian crustal evolution but to date lacks evidence of Proterozoic metamorphic ages. At Lichuan and Jianning, in the Wuyi Domain of the eastern Cathaysia Block, Proterozoic rock units include migmatized paragneiss of the Wanyuan Group and minor amphibolite of the Tianjingping Formation, which are enveloped by schist of Mayuan Group, and all are intruded by Paleozoic and Mesozoic igneous rocks. Detrital zircon grains from the Wanyuan paragneiss display metamorphic rims that yield concordant weighted average 206Pb/238U ages of 860±6Ma and 435±5Ma, along with variably disconcordant ages with lower intercept ages of 442±41Ma. The zircon core ages range from 3015Ma to 851Ma, with three major age populations at 930–865Ma, 1850–1200Ma and 2650–2400Ma. Detrital zircon grains from Mayuan schist samples at Jianning generally lack core-rim structures and yield three main age populations at 860–736Ma, 1835–1775Ma and 2720–2500Ma. Metamorphic ages of ca. 860Ma and ca. 435Ma for the Wanyuan paragneiss along with the youngest detrital zircon constrain the depositional age of the protolith to ca. 865–860Ma, whereas the Mayuan Group is younger and probably deposited after ca. 736Ma. Characteristics of detrital zircon age populations along with regional geological data suggest accumulation of the Wanyuan Group in a convergent and/or collisional setting. Metamorphism and a possible subduction-collision process within the Cathaysia Block at around 860Ma suggest it was not a unified block in early Neoproterozoic. The growth of ca. 440Ma metamorphic rims is likely related to granitic magmatism, such as that exposed in the Lichuan region. The sparse evidence for early Neoproterozoic metamorphism likely reflects widespread overprinting by the Paleozoic tectonothermal event at around 440Ma.

Trondhjemite-granodiorite intrusive magmatism of the Losevo lithotectonic zone of the Voronezh crystalline massif

The mineralogical-petrographic, petrochemical, and geochemical characteristics of the trondhjemite-granodiorite intrusions widespread among the volcanoterrigenous rocks of the eastern part of the Voronezh crystalline massif are studied. The rocks are characterized by cotectic quartz-plagioclase differentiation and differentiated (La/Yb = 31) REE pattern and are enriched in LILE with a decrease in the K/Rb and Zr/Hf ratios and an increase in the Th/U ratio by 1.5–2 times relative to subduction trondhjemites and their volcanic analogs from the modern and young island arcs. These data allow us to conclude that generation of the trondhjemite magma is a result of melting of the rocks of the continental crust during collision. The concordant (SHRIMP) age of trondhjemites is 2047 ± 11 Ma, and the age of zircon cores probably entrapped from the host rocks by the trondhjemitic melt is 2172 ± 17 Ma.

Zircon from the East Orebody of the Bayan Obo Fe–Nb–REE deposit, China, and SHRIMP ages for carbonatite-related magmatism and REE mineralization events

Extremely U-depleted (<1 ppm) zircons from H8 banded ores in the East Orebody of the Bayan Obo REE–Nb–Fe deposit are presented, with mineral compositions, textures, 232Th–208Pb SHRIMP ages and petrological context. Cores of East Orebody zircon contain up to 7 wt% HfO2 and are zoned, depicting bipyramidal crystal forms. A distinct generation of patchy, epitaxial rim zircon, similarly depleted in U, is intergrown with rare earth ore minerals (bastnasite, parisite, monazite). Overprinting aegirine textures indicate paragenetically late, reactive Na-rich fluids. Chondrite-normalized REE patterns without Eu anomalies match closely with those from the Mud Tank and Kovdor carbonatitic zircons. Increased HREE in rims ((Lu/Gd)N 43–112) relative to cores ((Lu/Gd)N 6–7.5) and the localized presence of xenotime are attributable to reactive, mineralizing fluid compositions enriched in Y, REE and P. Cathodoluminescence further reveals HREE fractionation in rims, evidenced by a narrow-band Er3+ emission at 405 nm. The extreme depletion of U in core and rim zircon is characteristic for this mineral deposit and is indicative of a persistent common source. U depletion is also a characteristic for zircons from carbonatitic or kimberlitic systems. 232Th–208Pb (SHRIMP II) geochronological data reveal the age of zircon cores as 1,325 ± 60 Ma and a rim-alteration event as 455.6 ± 28.27 Ma. The combined findings are consistent with a protolithic igneous origin for zircon cores, from a period of intrusive, alkaline–carbonatitic magmatism. Fluid processes responsible for the REE–Nb mineralizations affected zircon rim growth and degradation during the widely reported Caledonian events, providing a new example in a localized context of HREE enrichment processes.

Petrogenesis of Late Permian sodic metagranitoids in southeastern Korea: SHRIMP zircon geochronology and elemental and Nd–Hf isotope geochemistry

One of the striking tectonomagmatic features recently found in southeastern Korea is the occurrence of ca. 250Ma high-silica adakite. Sodic metagranitoids mainly consisting of tonalitic–trondhjemitic–granodioritic gneisses occur in the Andong-Cheongsong area adjacent to the Yeongdeok adakite site. To investigate temporal and petrogenetic relationships of these orthogneisses with the adakite, we conducted SHRIMP zircon U–Pb dating as well as elemental and Nd–Hf isotopic analyses. Zircon core ages of the orthogneisses (ca. 262–251Ma) confirm the widespread occurrence of arc-related Late Permian magmatism in southeastern Korea. The Late Triassic (ca. 230Ma) zircon overgrowths reflect a thermal overprint probably related to the initiation of another subduction system. The analyzed orthogneisses have major element compositions comparable to the Phanerozoic adakites and Archean TTG suite, such as high SiO2 (58.7–65.5wt.%) and Al2O3 (17.1–19.1wt.%) contents and Na2O/K2O ratios (1.83–4.95). However, their moderate Sr/Y (35–43) and La/Yb (14–53) ratios and negative Eu anomalies (Eu/Eu*=0.75–0.95) are incompatible with the key features reported from the Yeongdeok adakite. Moreover, initial whole-rock εNd (−7.9 to −3.3) and zircon εHf (−0.3±2.4) values of the orthogneisses negate a direct derivation from the subducted slab. Our elemental and Nd–Hf isotopic data collectively suggest that the protoliths of the tonalitic–trondhjemitic–granodioritic gneisses were generated by partial melting of mafic lower crust at depths shallower than the garnet stability field. Our Nd and Hf model ages of the gneisses, together with those previously reported from the Mesozoic granitoids indicate a selective involvement of young source materials along the margin of the Yeongnam massif. The Hf isotopic compositions of zircons from a trondhjemitic gneiss attest to the involvement of primitive melts during their crystallization. The ridge subduction and consequent development of a slab window may have facilitated partial melting of the subducted oceanic lithosphere and the lower crust.

Further geochronological and paleomagnetic constraints on Malani (and pre-Malani) magmatism in NW India

article i nfo At 750 Ma India was part of a larger fragment of eastern Gondwana blocks that included the Seychelles-Mauritia, Madagascar, Sri Lanka and the Enderby Land-Prydz Bay region of East Antarctica. Sub- duction of the Mozambique Ocean beneath Seychelles-Mauritia, northern Madagascar and northwestern India formed a lengthy continental arc that remained active during the formation of Gondwana. Paleomag- netic data from the Malani rhyolites and associated dykes provide a robust paleomagnetic pole constraining India's position at this time. The rhyolitic and granitic rocks associated with the Malani Igneous Suite (MIS) have robust age constraints; however, the ages of the mafic dykes were inferred solely on the basis of simi- larity in paleomagnetic directions to the rhyolitic units. Here we present new geochronological data from the Malani mafic dykes that yield a minimum age of 704 Ma. The 207 Pb/ 206 Pb ages obtained for the dykes are less-likely to be affected by Pb-loss and yield a more reliable estimate for the age of the mafic dykes of ~750 Ma. We argue that intrusion of these mafic (and minor felsic) dykes represents the final pulse of MIS magmatism. Many of the granitic rocks in the region are reported as 'unclassified' due to limited geochemical data and/or geochronological ages. Some of these 'unclassified' granites are intruded by the mafic dykes sampled in this study near the town of Bilara. The granites yielded zircon core ages of ~1100 Ma with younger rims averaging ~1020 Ma. We argue that this provides further evidence for a significant orogenic event ~1000 Ma that may relate to the collision of the Marwar block with the Banded Gneiss Complex/Bundelkhand craton in north-central India. Other ~1000 Ma orogenesis is also known along the Central Indian Tectonic Zone (CITZ) and the Eastern Ghats Mobile Belt. Globally, this same time interval is thought to represent the amal- gamation of the supercontinent Rodinia and may also have resulted in the closure of the major Purana basins in India.

Isotope-geochronological (U-Th-Pb, Lu-Hf) study of the zircons from the archean magmatic and metasedimentary rocks of the Podolia domain, Ukrainian shield

Zircons from the oldest magmatic and metasedimentary rocks of the Podolia domain of the Ukrainian shield were studied and dated by U-Pb method on a NORDSIM secondary-ion mass spectrometer. The age of zircon cores in the enderbite gneisses taken in the Kazachii Yar and Odessa quarries on the opposite banks of the Yuzhnyi Bug River reaches 3790 Ma. Cores of the terrigenous zircons in the quartzites from the Odessa quarry as well as in the garnet gneisses from the Zaval’e graphite quarry have an age within 3650–3750 Ma. Zircon rims record two metamorphic events at 2750–2850 Ma and around 1900–2000 Ma. Extremely low U content in the zircons of the second age group indicates conditions of the granulite-facies metamorphism in the Paleoproterozoic within the Podolia domain. Obtained data on the orthorocks (enderbite-gneiss) and metasedimentary rocks unambiguously suggest the existence of ancient Paleoarchean crust in the Podolia (Dniester-Bug) domain of the Ukrainian shield. They contribute in our knowledge of scales of the formation and geochemical features of the primordial crust.

Mid-Neoproterozoic crustal evolution of the northeastern Yangtze Block: Evidence from the felsic-gneiss xenoliths hosted in the Donghai Cenozoic basalts

Crustal xenoliths can provide new insights into the unexposed crust, and those from the northeastern Yangtze Block have rarely been studied. This paper reports U–Pb–Hf isotopes and trace-element compositions of zircons from six felsic xenoliths hosted by the Neogene alkali basalts in the Donghai region (i.e. Anfengshan and Pingmingshan) of the Sulu orogen in central eastern China. The xenoliths are mainly composed of orthoclase and quartz, or orthoclase and natrolite, with accessory minerals of Fe–Ti oxides and zircon. Most zircon grains show core-rim structures, with the cores and rims being magmatic and metamorphic in origin, respectively. The zircon cores mainly yield ages of ca. 827–794Ma, while the zircon rims give ages of ca. 232–212Ma. We interpret the zircon core ages as the time of an early Mid-Neoproterozoic magmatic event in the northeastern Yangtze Block and the zircon rim ages as the time of collision between the Yangtze and North China Blocks. Our data suggest that much more ca. 830–800Ma magmatic records are possibly preserved in the unexposed deep crust, and the early Mid-Neoproterozoic is an important era for the crust evolution of the northeastern Yangtze Block. The new zircon Hf isotopic analyses show that the Anfengshan sample (south of Donghai) has zircon εHf (820Ma) values ranging from −15.3 to −9.4, and two-stage Hf model ages of 2.66–2.30Ga; the Pingmingshan sample (southeast of Donghai) has zircon εHf (820Ma) values ranging from −1.4 to +3.8, and two-stage Hf model ages of 1.80–1.47Ga. These data suggest that ancient crust as old as Neoarchean to Mesoproterozoic was involved in the early Mid-Neoproterozoic magmatism. Combined with the previously reported zircon U–Pb–Hf results of the exposed rocks, it is highlighted that crustal recycling was dominant in the early Mid-Neoproterozoic (ca. 830–800Ma) magmatism, whereas both crustal recycling and addition of mantle-derived melts were significant in the late Mid-Neoproterozoic (ca. 800–720Ma) magmatism in the northeastern Yangtze Block.

The fast evolution of a crustal hot zone at the end of a transpressional regime: The Saint-Tropez peninsula granites and related dykes (Maures Massif, SE France)

Granite and dolerite occurring in the eastern unit of the Maures Massif (southern France Variscan belt) were emplaced in the waning stage of the evolution of a dextral shear zone along the Gondwana border with Laurasia. We present a geochronological and petro-geochemical study (major and trace elements, Sr–Nd isotopes) of (1) the foliated Moulin Blanc cordierite granite and related foliated dykes; (2) the equant Camarat granite and dyke; and (3) the andesite–basalt dolerite dykes (frequently mingled with leucogranite material at Pinet and Capon tips) and cross-cutting leucogranitic dykes. Zircon and monazite dating of the Moulin Blanc granite gives a crystallization age of 301±2Ma. SHRIMP dating on zircons from the leucogranite component of a Pinet composite dyke only reveals zircon core ages of mainly 310±10Ma (age of migmatization of the host gneiss). Undulating contacts of dolerite dykes within the c. 300Ma old Camarat granite suggest penecontemporary relationships between the two intrusions. The age interval between the Moulin Blanc granite and the dolerite dykes would thus be very short. The Moulin Blanc pluton is formed by mingling and mixing of melts of granitic and dioritic composition, the latter occurring mainly as microgranular mafic enclaves. Two other types of diorites are also identified in the pluton. The foliated dykes are similar in composition to the Moulin Blanc granite, which confirms their relationship to the main pluton. The Moulin Blanc granite is a typical example of a pluton formed by repeated recharges coming from a deeper magma chamber. Formation of the Camarat dykes is controlled by hydrothermal fluids that are responsible for the non-CHARAC behavior of trace elements. The Pinet and Capon leucogranite components of the dolerite dykes have distinct geochemical signatures, different from the Camarat granite and dykes. The cross-cutting leucogranitic dykes have much geochemical similarities with the Capon granite. The age of similar dykes in the Tanneron Massif supports the contemporaneity of Camarat granite and dolerites. The dolerite geochemistry was deeply modified by alteration processes, but an andesite–basalt composition is still recognizable by using immobile element ratios. In a εNd vs. Sri diagram, all rocks plot close to a mixing hyperbola between a depleted mantle component and a crustal component, characteristic of the Variscan crust in the French Massif Central. There is a broad correlation between the NdTDM 2-stage model ages and the degree of fractionation of the rocks. Modeling of dehydration melting in a range of P–T conditions reveals that a variety of lithologies were the source of the Saint-Tropez magmas: high-alumina and alkali-basalt amphibolites, metagraywackes, metapelites and meta-arkoses. These sources were melted almost simultaneously at different levels in the crust. The calculated critical dyke width required to transport the granitic melt by buoyancy is far too large compared to the observed granitic dyke thicknesses. Therefore, tectonic processes and/or melt overpressure must have played a role in the emplacement mechanism. Thermo-mechanical constraints on mixing between mafic and felsic melts suggest that, in the Pinet composite dykes, both melts should have mixed. The observed lack of mixing is interpreted as due to quick cooling. The region was affected by a hot zone at the end of a transpressional regime.