high-U Zircon Research Articles

Preferential dissolution of uranium-rich zircon can bias the hafnium isotope compositions of granites

Abstract Hafnium (Hf) isotopes in zircon are important tracers of granite petrogenesis and continental crust evolution. However, zircon in granites generally shows large Hf isotope variations, and the reasons for this are debated. We applied U-Pb geochronology, trace-element, and Hf isotope analyses of zircon from the Miocene Himalayan granites to address this issue. Autocrystic zircon had εHf values (at 20 Ma) of–12.0 to–4.3 (median =–9). Inherited zircon yielded εHf values (at 20 Ma) of–34.8 to +0.3 (median =–13); the majority of εHf values were lower than those of autocrystic zircon. The εHf values of inherited zircon with high U concentrations resembled those of autocrystic zircon. Geochemical data indicates that the granites were generated during relatively low-temperature (<800 °C) partial melting of metasedimentary rocks, which, coupled with kinetic hindrance, may have led to the preferential dissolution of high-U zircon that could dissolve more efficiently into anatectic melt due to higher amounts of radiation damage. Consequently, Hf values of autocrystic zircon can be biased toward the values of U-rich zircon in the source. By contrast, literature data indicate that granites generated at high temperatures (>820–850 °C) generally contain autocrystic and inherited zircons with comparable Hf isotope values. During higher-temperature melting, indiscriminate dissolution of source zircon until saturation is reached will result in near-complete inheritance of Hf isotope ratios from the source. Our results impose an extra layer of complexity to interpretation of the zircon Hf isotope archive that is not currently considered.

Constraints on the timing of multiple thermal events and re-equilibration recorded by high-U zircon and xenotime: Case study of pegmatite from Piława Górna (Góry Sowie Block, SW Poland)

The application of zircon and xenotime geochronometers requires knowledge of their potential and limitations related to possible disturbance of the age record. The alteration of the intergrown zircon and xenotime in pegmatite from the Góry Sowie Block (SW Poland) was studied using the electron microprobe analysis, X-ray WDS compositional mapping, micro-Raman analysis, and LA-ICP-MS U-Pb dating of zircon and xenotime, as well as the U-Th-total Pb dating of uraninite. These microanalytical techniques were applied to understand the formation mechanisms of the secondary textures related to post-magmatic processes in the zircon and xenotime intergrowth, and to constrain their timing. Textural and compositional features combined with U-Pb data indicate that the pegmatite-related crystallization of the zircon and xenotime intergrowth occurred ca. 2.09 Ga (2086 ± 35 Ma for zircon and 2093 ± 52 Ma for xenotime), followed by the re-equilibration of zircon and xenotime ca. 370 Ma (373 ± 18 Ma and 368 ± 6 Ma, respectively) during the formation of the younger pegmatite. The zircon and xenotime were most likely derived from Precambrian basement rocks and emplaced in the pegmatite as a restite. The zircon preserved textures related to diffusion-reaction processes that affected its high-U core (up to ca. 9.6 wt% UO2), which underwent further metamictization and amorphization due to self-radiation damage. The zircon rim and xenotime were affected by coupled dissolution-reprecipitation processes that resulted in patchy zoning, age disturbance and sponge-like textures. Xenotime was also partially replaced by fluorapatite or hingganite-(Y) and Y-enriched allanite-(Ce). The termination of the low-temperature alteration was constrained by the U-Th-total Pb age of the uraninite inclusions that crystallized in zircon at 281 ± 2 Ma, which is consistent with the age of 278 ± 15 Ma obtained from the youngest cluster of U-Pb ages in the re-equilibrated high-U zircon domains. This study demonstrates the importance of the careful examination of compositional, microtextural and geochronological data obtained using microanalytical techniques to reconstruct the complex thermal histories recorded by accessory minerals.

Alteration and chemical U-Th-total Pb dating of heterogeneous high-uranium zircon from a pegmatite from the Aduiskii massif, middle Urals, Russia

The U-Th-Pb isotope system in the accessory mineral zircon may be disturbed, as for instance by the secondary loss of radiogenic lead. The recognition of such alteration is crucial for the sound interpretation of geochronology results, in particular for chemical dating by means of an electron probe micro-analyser (EPMA). Here we present the example of high-U zircon samples from a granite pegmatite from the Aduiskii Massif, Middle Urals, Russia. The structural and chemical heterogeneity of samples was characterised by EPMA, including joint probability distribution (JPD) analysis of back-scattered electrons (BSE), cathodoluminescence (CL) and U M β images, and by Raman and photoluminescence (PL) spectroscopy. We found a high-U interior region (U up to 11.4 wt%) without any obvious indication of alteration. This domain has stoichiometric composition, and its Raman spectrum is similar to that of amorphous ZrSiO4. In addition, altered lower-U regions are present that are non-stoichiometric and contain non-formula elements such as Ca, Al, Fe, and water up to several wt%. Their Raman spectra yielded a band near 760–810 cm−1 which is not related to any ZrSiO4 vibration; we assign it tentatively to the symmetric stretching of (UO2)2+ groups. This assignment is supported by the observation of a fairly intense PL phenomenon whose spectral position and vibrational-coupling structure strongly indicates a uranyl-related emission. Altered zones were formed by both fluid-driven diffusion reaction and coupled dissolution-reprecipitation processes. The variation of BSE and CL intensities in amorphous high-U zircon is controlled by its chemical composition and the presence of water and uranyl groups. We have determined a weighted mean EPMA age of 246 ± 2 Ma, which agrees reasonably well with previous dating results for the Aduiskii Massif.

A scanning ion imaging investigation into the micron-scale U-Pb systematics in a complex lunar zircon

The full U-Pb isotopic systematics in a complex lunar zircon ‘Pomegranate’ from lunar impact breccia 73235 have been investigated by the development of a novel Secondary Ion Mass Spectrometry (SIMS) scanning ion imaging (SII) technique. This technique offers at least a four-fold increase in analytical spatial resolution over traditional SIMS analyses in zircon. Results from this study confirm the hypothesis that the Pomegranate zircon crystallized at 4.302±0.013Ga and experienced an impact that formed, U-enriched zircon around primary zircon cores at 4.184±0.007Ga (2σ, all uncertainties). The increase in spatial resolution offered by this technique has facilitated targeting of primary zircon that was previously inaccessible to conventional spot analyses. This approach has yielded results indicating that individual grains with a diffusive distance of less than ~4μm have been reset to the young impact age, while individual grains with a diffusive distance larger than ~6μm have retained the old crystallization age. Assuming a broad range in cooling rate of 0.5–50°C/year, which has been observed in a suite of similar lunar breccias, a maximum localized temperature generated by the impact that reset small primary zircon and created new, high-U zircon is estimated to be between 1100 and 1280°C.

Ra’s Abdah of the north Eastern Desert of Egypt: the role of granitic dykes in the formation of radioactive mineralization, evidenced by zircon morphology and chemistry

Syenogranitic dykes in the north of Egypt’s Eastern Desert are of geological and economic interest because of the presence of magmatic and supergene enrichment of radioactive mineralization. Zircon crystal morphology within the syenogranitic dykes allows precise definition of sub-alkaline series granites and crystallized at mean temperature of about 637 °C. The growth pattern of the zircons suggest magmatic and hydrothermal origins of radioactive mineralization. Hydrothermal processes are responsible for the formation of significant zircon overgrowth; high U-zircon margins might have occurred contemporaneously with the emplacement of syenogranitic dykes which show anomalous uranium (eU) and thorium (eTh) contents of up to 1386 and 7330 ppm, respectively. Zircon chemistry revealed a relative increase of Hf consistent with decreasing Zr content, suggesting the replacement of Zr by Hf during hydrothermal activity. Visible uranium mineralization is present and recognized by the presence of uranophane and autunite.

Behaviour of geochronometers and timing of metamorphic reactions during deformation at lower crustal conditions: phase equilibrium modelling and U–Pb dating of zircon, monazite, rutile and titanite from the Kalak Nappe Complex, northern Norway

AbstractThis study investigates the behaviour of the geochronometers zircon, monazite, rutile and titanite in polyphase lower crustal rocks of the Kalak Nappe Complex, northern Norway. A pressure–temperature–time–deformation path is constructed by combining microstructural observations with P–T conditions derived from phase equilibrium modelling and U–Pb dating. The following tectonometamorphic evolution is deduced: A subvertical S1 fabric formed at ~730–775 °C and ~6.3–9.8 kbar, above the wet solidus in the sillimanite and kyanite stability fields. The event is dated at 702 ± 5 Ma by high‐U zircon in a leucosome. Monazite grains that grew in the S1 fabric show surprisingly little variation in chemical composition compared to a large spread in (concordant) U–Pb dates from c. 800 to 600 Ma. This age spread could either represent protracted growth of monazite during high‐grade metamorphism, or represent partially reset ages due to high‐T diffusion. Both cases imply that elevated temperatures of >600 °C persisted for over c. 200 Ma, indicating relatively static conditions at lower crustal levels for most of the Neoproterozoic. The S1 fabric was overprinted by a subhorizontal S2 fabric, which formed at ~600–660 °C and ~10–12 kbar. Rutile that originally grew during the S1‐forming event lost its Zr‐in‐rutile and U–Pb signatures during the S2‐forming event. It records Zr‐in‐rutile temperatures of 550–660 °C and Caledonian ages of 440–420 Ma. Titanite grew at the expense of rutile at slightly lower temperatures of ~550 °C during ongoing S2 deformation; U–Pb ages of c. 440–430 Ma date its crystallization, giving a minimum estimate for the age of Caledonian metamorphism and the duration of Caledonian shearing. This study shows that (i) monazite can have a large spread in U–Pb dates despite a homogeneous composition; (ii) rutile may lose its Zr‐in‐rutile and U–Pb signature during an amphibolite facies overprint; and (iii) titanite may record crystallization ages during retrograde shearing. Therefore, in order to correctly interpret U–Pb ages from different geochronometers in a polyphase deformation and reaction history, they are ideally combined with microstructural observations and phase equilibrium modelling to derive a complete P–T–t–d path.

Eoarchean ultra-depleted mantle domains inferred from ca. 3.81 Ga Anshan trondhjemitic gneisses, North China Craton

Early mantle differentiation is yet to be well constrained because isotopic systems recorded in many ancient rocks are arguably susceptible to later disturbances. Although refractory zircon is of singular importance in deciphering early Earth history, a growing body of work on ancient zircons indicates that interpretations of their U–Pb ages and Hf–O isotopes are neither straightforward nor unique. Owing to these complexities, the question of whether the Anshan Complex of North China Craton (NCC) really preserves 3.8Ga rocks remains controversial. To better understand these issues, we conducted a systematic in situ zircon U–Pb, Hf and O isotopic study guided by detailed field mapping (1:50 scale) and sampling at Guodishan, northern Anshan Complex. Three rock units were recognized and precisely dated: a ca. 3.13Ga massive trondhjemitic gneiss that dominates the outcrop, a ca. 3.36–3.30Ga migmatite complex that is intruded by the trondhjemitic gneiss, and a ca. 3.81Ga massive to weakly-banded trondhjemitic gneiss that is enclosed by the migmatite complex.U abundances, U–Pb discordances, apparent 207Pb/206Pb ages and δ18O values of a single generation of magmatic zircons reveal that only low-U (generally U<500ppm), concordant zircons can preserve magmatic oxygen isotopes. High-U zircons generally tend towards lower δ18O values, possibly due to secondary alterations facilitated by strong metamictization. Concordant ca. 3.81Ga oscillatory zoned zircons from the oldest unit give ɛHf(t)=−0.7 to 6.2 and δ18O=5.2–7.0‰. These data combined with whole-rock geochemistry suggest that this Eoarchean trondhjemitic geniss was either formed by magma mixing or probably derived from a heterogeneous basaltic source at ca. 13–14kbar, 950–1000°C. The highest initial ɛHf value of 6.2 at ca. 3.81Ga exceeds those recorded by contemporaneous zircons worldwide and the modeled depleted mantle value. This provides new isotopic evidence for the existence of an Eoarchean ultra-depleted mantle domain underlying NCC. A literature survey reveals that this Eoarchean ultra-depleted signature was not registered in younger zircons worldwide, suggesting that this ultra-depleted mantle signature may be transient, possibly erased by subsequent mantle re-homogenization processes.

Reliability of LA-ICP-MS U-Pb dating of zircons with high U concentrations: A case study from the U-bearing Douzhashan Granite in South China

Granite-hosted uranium ore deposits are the most important commercial U-producers in South China. These U-bearing granites generally contain high-U-concentration zircons. Zircons in the U-bearing Douzhashan Granite show black rims in cathodoluminescence images, which have very high U concentrations of up to >25,000ppm. The high-U black rim domains of zircons show similar REE patterns to those of low-U mantle domains with oscillatory zoning, but have higher REE concentrations and weaker positive Ce anomalies. The high-U black rim domains of zircons may have formed from late-stage evolved U-rich and H2O-rich magma. We carried out a SHRIMP U-Pb dating for these zircons and the results show that the high-U (12,007–26,706ppm) rim domains of the zircons always yield older 206Pb/238U ages (268±9Ma, n=5, MSWD=2.7) than the low-U (558–3667ppm) mantle domains (228±11Ma, n=4, MSWD=3.1), and both of these ages are older than the emplacement age (211±3Ma) of the U-barren Xiangchaoping Granite, which was intruded by the Douzhashan Granite. Hence, these results demonstrate a matrix effect associated with SHRIMP U-Pb analyses of high-U zircons. The reverse discordance for SHRIMP results of high-U zircons was produced by analytical artifact. However, our LA-ICP-MS analyses show that the black rim domains (12,840–26,018ppm U) of the zircons have the same U-Pb ages as the low-U mantle domains (1215–4075ppm U). Both domains yield a weighted mean 206Pb/238U age of 203±4Ma (n=13, MSWD=1.4), which is consistent with the 40Ar-39Ar age (207±4Ma) of muscovite from this granite. It thus indicates that LA-ICP-MS U-Pb analyses for high-U zircons (up to 26,000ppm U) show an insignificant matrix effect and likely yield more reliable U-Pb ages for the high-U zircons than SHRIMP analyses. In order to get reliable U-Pb ages using the LA-ICP-MS method for the high-U zircons, a pulse-analog cross calibration for the ICP-MS collector should be performed. Our precise U-Pb dating and geochemical studies indicate that the U-bearing Douzhashan Granite was emplaced during the late Indosinian stage and formed in a post-collisional setting of the Indosinian Orogeny. It is distinct from most U-bearing Granites in South China, which were largely emplaced during the early Indosinian stage and formed in a syn-collisional setting.

Radiation damage to Kokchetav UHPM diamonds in zircon: Variations in Raman, photoluminescence, and cathodoluminescence spectra

We conducted detailed in-situ Raman, photoluminescence (PL) and cathodoluminescence (CL) studies on microdiamonds in a tourmaline-rich quartzofeldspathic rock from the Kokchetav Massif, Kazakhstan. The microdiamonds occur as inclusions in the cores of K-tourmaline and in zoned zircons with varying U contents. The results of 2D Raman mapping of zircon showed that the U-rich parts were more metamictized than the U-poor parts. All the diamonds showed a strong Raman band at approximately 1332 cm − 1 , however, the features of the Raman bands were distinctly different depending on the host minerals. On the one hand, diamonds in tourmaline had a sharp Raman band that is similar to that of kimberlite diamonds [full width at half maximum (FWHM): 2–3 cm − 1 ]. On the other hand, diamonds in zircon had a broad and downshifted band compared to those in tourmaline. In particular, diamonds in U-rich cores of zircons (up to 0.15 wt.% UO 2 ) showed broader and more downshifted Raman bands (FWHMs and peak positions varied up to 9.3 cm − 1 and 1328 cm − 1 , respectively), with additional small bands at approximately 1490 cm − 1 and 1630 cm − 1 . A negative correlation was observed between the peak position and the FWHM of the principal Raman band of microdiamonds. Furthermore, the PL and CL spectra showed systematic variations. Diamonds in zircons with low to moderate U-concentrations had very strong PL and CL compared to diamonds in U-rich zircons and in tourmalines. Several characteristic peaks appeared in the PL and CL spectra, indicating the presence of irradiation and/or nitrogen-related point defects in the diamonds. PL and CL of microdiamonds in high-U zircon were weak, but still showed irradiation-related peaks. The relationship between the occurrence of microdiamonds (i.e., corresponding to the estimated total α-dose since crystallization) and the Raman, PL, and CL spectral characteristics of microdiamonds strongly suggests that radiation damage predominantly caused by α-particle emission from radioactive decay of actinides (mainly U) in zircon is a principal factor for the spectral variations. Radiation damage (metamictization) is probably a common phenomenon among microdiamonds in zircons in various diamond-bearing rocks from the Kokchetav Massif. To discuss the genesis of diamonds based on their Raman, PL, or CL spectra, it is highly recommended that microdiamonds included in actinide-bearing minerals (e.g., zircon) should be treated with the most careful attention, because post-crystallization radiation damage significantly influences the spectra. • We compared Raman, PL, and CL spectra of microdiamonds in tourmaline and zircon. • The Raman peak of diamonds in zircon is downshifted and broad. • Diamonds in zircon show PL and CL that are characteristic of irradiated diamonds. • Radiation damage is important when genesis of diamond is discussed by spectroscopy.

Screening criteria for reliable U–Pb geochronology and oxygen isotope analysis in uranium-rich zircons: A case study from the Suzhou A-type granites, SE China

Zircon is widely used in geochronology and as an isotopic tracer in igneous processes. However, the use of isotopic data obtained from zircon requires evaluating of processes that might compromise the robustness of its isotopic systems. In this study we have assessed how different U contents and degrees of metamictization have affected the SIMS U–Pb and oxygen-isotope signatures of zircon–quartz pairs from the U-rich A-type granites from Suzhou (southern China). Our SIMS results show that zircons with low U concentrations define a U–Pb age of 126.1±0.5Ma and homogeneous mantle δ18O values of 5.34±0.46‰ (2SD). In contrast, zircons with high U contents show a positive correlation between U concentration and U–Pb ages (123–157Ma) and a negative correlation between U concentration and δ18O values (range from 5.3 to 3.7‰). These observations are integrated with Raman spectra and oxygen-isotope analyses of coexisting quartz to reveal that the anomalously high U–Pb ages and low δ18O values reflect the degree of radiation damage to the zircons (quantified by radiation dose in displacements-per-atom, Ddpa). The older SIMS U–Pb ages are a direct result of the matrix effect caused by metamictization, whereas the low δ18O values result from interaction with OH-bearing fluids, the infiltration and diffusion of which was facilitated in the radiation-damaged areas.Coexisting quartz preserves primary O-isotope signatures of the magma better than high-U zircon, and thus provides a constraint for zircon oxygen-isotope values. This study demonstrates that it is essential to evaluate the degree of radiation damage before/after carrying out dating or oxygen-isotope analysis on zircons with high U contents. The results indicate that for the Suzhou pluton, Ddpa<0.03 is a robust discriminant threshold to identify zircons with primary oxygen-isotope ratios, and Ddpa<0.08 is a robust discriminant to screen for reliable U–Pb dating. These different screening values reflect the evidence that oxygen-isotope compositions can be affected at a lower level of post-crystallization disturbance of the zircon lattice than the U–Pb system.

원동 중석 스카른대에서의 관입암류와 스카른광물에 대한 U-Pb(SHRIMP) 및 K-Ar 연대

The geology of the weondong deposit area consists mainly of Cambro-Ordovician and Carboniferous-Triassic formations, and intruded quartz porphyry and dyke. The skarn mineralized zone in the weondong deposit is the most prospective region for the useful W-mineral deposits. To determine the skarn-mineralization age, U-Pb SHRIMP and K-Ar age dating methods were employed. The U-Pb zircon ages of quartz porphyry intrusion (WD-A) and feldspar porphyry dyke (WD-B) are 79.37 Ma and 50.64 Ma. The K-Ar ages of coarse-grained crystalline phlogopite (WD-1), massive phlogopite (WDR-1), phlogopite coexisted with skarn minerals (WD-M), and vein type illite (WD-2) were determined as Ma, Ma, Ma, and Ma, respectively. And the ages of the high uranium zircon of hydrothermally altered quartz porphyry (WD-C) range from 59.7 to 38.7 Ma, which dependson zircon`s textures affected by hydrothermal fluids. It is regarded as the effect of some hydrothermal events, which may precipitate and overgrow the high-U zircons, and happen the zircon`s metamictization and dissolution-reprecipitation reactions. Based on the K-Ar age datings for the skarn minerals and field evidences, we suggest that the timing of W-skarn mineralization in weondong deposit may be about 50 Ma. However, for the accurate timing of skarn mineralization in this area, the additional researches about the sequence of superposition at the skarn minerals and geological relationship between skarn deposits and dyke should be needed in the future.

Episodic mantle melting-crustal reworking in the late Neoarchean of the northwestern North China Craton: Zircon ages of magmatic and metamorphic rocks from the Yinshan Block

The Yinshan Block in the northwestern North China Craton comprises late Neoarchean greenstones, high-grade metamorphic rocks (granulite, charnockite, enderbite and amphibole gneiss), diorites (sanukitoid and Mg-adakite) and granitoids (predominantly trondhjemite-tonalite). A greenstone terrane comprises a volcanic sequence of predominant basalt and dacite, with minor andesite. Zircons from a high-Mg andesite and an andesite have weighted mean 207Pb/206Pb ages of 2533±5Ma and 2510±7Ma, respectively, which reflect the timing of two distinct volcanic episodes. These two either predated or postdated a major volcanic episode that formed a basalt-dacite bimodal suite during 2515±10 (basalt) to 2516±10Ma (dacite). Magmatic zircons from the high-grade metamorphic rocks yielded weighted mean 207Pb/206Pb ages of 2544±5Ma for a mafic granulite, 2521±6Ma for a garnetiferous gneiss, 2535±5Ma for a charnockite, and 2526±5Ma for an amphibole gneiss. These ages reflect the (minimum) emplacement time of rocks that make up the protoliths of the high-grade metamorphic complex. The metamorphic ages of the high-grade complex were determined at 2515±18Ma for mafic granulite and 2503±12Ma for garnetiferous gneiss. Zircons from a high-Mg diorite (i.e. sanukitoid) have a weighted mean age of 2520±6Ma, which is significantly younger than the 2556±14Ma age of a magnesian diorite (Mg-adakite) from the same diorite massif. These ages reflect distinctive magmatic pulses that generated the diorite massif. A tonalite has a zircon age of 2516±7Ma, indistinguishable from the 2520±9Ma age of a trondhjemite from the same trondhjemite/tonalite-dominated batholith. A granitic compositional layer in the 2516±7Ma tonalite has zircon ages of 2515±6Ma (high-U zircon domains) and 2512±22Ma (low-U zircon domains), which reflect an intracrustal melting event. The presence of an older crustal remnant (ca. >2600Ma) prior to the formation of the Yinshan Block is indicated by a 2594±4Ma leucogranite xenolith in the diorite massif and by the widespread presence of ca. 2583–3485Ma (mostly, ca. 2700Ma) zircon xenocrysts in a variety of volcanic and plutonic rocks. An undeformed pegmatite dike in high-Mg andesite has an emplacement age of 2508±17Ma (zircon). An undeformed trondhjemite dike that cuts the supracrustal greenstone sequence yielded a zircon age of 2502±6Ma, similar to that of the pegmatite dike, and representing the latest Archean magmatic event in the Yinshan Block. These ages of undeformed granitoid dikes delimit the termination of thermo-tectonic development of the Yinshan Block. Based on these new ages and published geological and geochronological data, we conclude that the Yinshan Block is a typical Neoarchean cratonic block, not a segment of the so-called “Paleoproterozoic Inner Mongolia-North Hebei Orogen”. Episodic mantle melting and resultant crustal reworking account for the thermo-tectonic development of the Yinshan Block in a brief period (ca. 2556–2502Ma) in the late Neoarchean, and based on their regional chronological correlations we argue for their craton-scale significance.

U–Pb geochronology of two discrete Ordovician high-pressure metamorphic events in the Seve Nappe Complex, Scandinavian Caledonides

U–Pb geochronology for eclogites in two different areas of the Seve Nappe Complex (SNC) in Sweden confirms previous indications of discrete Ordovician high-pressure events affecting various parts of the complex. In Norrbotten, just north of the Arctic circle, eclogites from the Tsakkok and Vaimok Lenses yield identical metamorphic zircon ages of 482 ± 1 Ma. Titanite in a metagabbro from the Vaimok Lens retains an older age of 607 ± 2 Ma, which may date a protolith coeval with mafic dikes in the overlying Sarek Lens; high-U zircon cores in one of the eclogites also indicate a similar age. Farther south, in Jamtland, the Tjeliken eclogite yields a significantly younger metamorphic age of 446 ± 1 Ma. Although they support the age discrepancy between the Norrbotten and Jamtland eclogites, the U–Pb ages of both eclogite suites are ca. 20 m.y. younger than previously reported Sm–Nd ages. The latter may either represent early prograde growth or be spuriously too old due to isotopic disequilibrium. The SNC has traditionally been taken to represent the outermost margin of Baltica, linking the Early Caledonian eclogite–forming events to subduction of Baltica below an offshore arc. Alternatively, the coincidence of these eclogite-forming events with orogenic phases recorded on the Laurentian margin may point to an origin from other regions of the Iapetus Ocean.

In-situ U-Pb analyses of highly altered zircon from sediments overlying the Bangombé natural fission reactor, Gabon

In-situ isotopic analyses of Pb and U in the highly altered zircons found in the clay and black shale layers above the Bangombé natural reactor, Republic of Gabon, were analyzed using a sensitive high resolution ion microprobe (SHRIMP) to understand the geological history in this area. The analyzed zircons widely vary in the U contents ranging from <100 to 59000 ppm. To confirm the problem on insufficient Pb/U calibration of high-U zircon in the SHRIMP technique, 206Pb/238U isotopic ratios from the SHRIMP analysis were compared with those from the analytical combination of U/Pb elemental ratios by electron probe micro analyzer (EPMA) with Pb isotopic ratios by SHRIMP. A significant deviation was found in the 206Pb/238U data of high-U zircons between two analytical techniques. Then, we concluded that the analytical data with U concentrations less than 2500 ppm can be used as reliably calibrated SHRIMP U-Pb data and those with high U concentration greater than 2500 ppm were determined by the analytical combination by EPMA and SHRIMP. The U-Pb data of zircons provide chronological information on the igneous activity associated with the basement rock formation at ca. 2.8 Ga. Moreover, nearly constant 207Pb/206Pb data corresponding to the mean age of 1959 ± 12 Ma and wide variation of 238U/206Pb data suggest that U and Pb were originated from 2.05-Ga old uraninite and migrated into zircon grains independently under the recent severe weathering.

SHRIMP U–Pb Zircon geochronology of the Altai No. 3 Pegmatite, NW China, and its implications for the origin and tectonic setting of the pegmatite

The “Altai No. 3 granitic pegmatite” in NW China is the largest pegmatite in the world that has been found in an orogenic belt; it forms one of the largest muscovite deposits in Asia and is most famous for its concentric-ring structure. Although it has been mined for more than 70 years, the age of formation and mineralization of the pegmatite has not been well determined, despite numerous previous geochronological studies. Here we present new results of SHRIMP U–Pb zircon dating. Three weighted mean 206Pb/ 238U ages of 220 ± 9 Ma, 198 ± 7 Ma and 213 ± 6 Ma were obtained for pegmatite zones 1, 5, and 7, respectively. The analyses were made on clear, recrystallized domains and overgrowth rims of large, opaque high-U zircon grains. We interpret the age of 220 ± 9 Ma for zone 1 to approximately represent the initial time of formation for No. 3 pegmatite. An additional date of 213 ± 2 Ma was determined on zircon from a small subordinate vein formed in the late-stage phase of the No. 3 pegmatite. This age set the late age of emplacement of the pegmatite. The time span of 213 to 198 Ma may reflect late stage hydrothermal alteration. In addition, a weighted mean 206Pb/ 238U age of 277 ± 6 Ma was obtained from small, clear, transparent low-U zircon grains which are more probably xenocrysts. Our new age data indicate that the pegmatite has no genetic relationship with the wall granitic rocks (409 Ma), but is likely related to the early Mesozoic (220 Ma) granitic magmatism that was just recognized. The No. 3 pegmatite was formed in an early Mesozoic anorogenic extensional regime. A stable tectonic setting was undoubtedly required for the formation of the large pegmatite.

Young porphyries, old zircons: new constraints on the timing of deformation and gold mineralisation in the Eastern Goldfields from SHRIMP U–Pb zircon dating at the Kanowna Belle Gold Mine, Western Australia

Abstract The Kanowna Belle Gold Mine is a Late Archaean orogenic lode-gold deposit hosted by felsic volcaniclastic and intrusive rocks (porphyries) of the Kalgoorlie Terrane, Western Australia. Rare gold occurs in fragments of veins and alteration that form clasts within the Black Flag Group volcaniclastic rocks at the Kanowna Belle mine, indicating that epithermal gold mineralisation accompanied Black Flag Group volcanism. The SHRIMP U–Pb zircon age of the volcaniclastic unit is 2668±9 Ma, and xenocrystic zircons with ∼2.68, 2.70 and 2.71 Ga age groupings are common. The Black Flag Group rocks are faulted by a D1 thrust, and ∼2670 Ma is thus an older limit for regional D1 deformation. Although SHRIMP U–Pb zircon ages of felsic porphyries commonly give the best constraints on the timing of deformation and structurally controlled gold mineralisation, the data are complex and dates from single samples can be ambiguous. Four Porphyry samples from the Kanowna Belle Gold Mine were analysed. Backscattered electron and cathodoluminescence imaging show that most magmatic zircon in the porphyries is either high-U and metamict, or restricted to rims on older xenocrysts that are too narrow to be dated by SHRIMP. Some porphyries appear to have been saturated with zircon at source and contain only xenocrystic zircons. Zircons that are interpreted to be magmatic in a sample of the mineralised Kanowna Belle Porphyry gives a mean age of 2655±6 Ma. The Kanowna Belle Porphyry is cross cut by regional D2 fabrics and ∼2655 Ma is thus the maximum age for regional D2 deformation. This is a maximum age for epigenetic lode-gold mineralisation. The age of resetting of high-U zircon grains (2.63 Ga) and the age of ore-related Pb–Pb galenas (2.63 Ga) serves as an approximate date for lode-gold mineralisation. If the complex zircon history of the felsic porphyries at Kanowna Belle is typical of this suite throughout the Eastern Goldfields Province, it is clear that existing single zircon dates from this Province require reevaluation, backed up by careful backscattered and cathodoluminescence imaging and textural studies.

Late Neoproterozoic/Early Palaeozoic events in central Dronning Maud Land and significance for the southern extension of the East African Orogen into East Antarctica

Abstract New SHRIMP zircon data from Gjelsvikfjella and Muhlig-Hofmann-Gebirge indicate that the metamorphic basement is composed of Grenville-age rocks that are most likely part of the northeastern continuation of the Namaqua-Natal-Maud Belt. High-grade overprinting occurred between c. 560 and 490 Ma. This period is considered to bracket the collision of E- and W-Gondwana along the East African/Antarctic Orogen (EAAO) and also the subsequent collapse of the orogen. The onset of collision probably predates our oldest dates. We can clearly differentiate between an early compressional stage (Pan-African I) and a later dilational episode (Pan-African II). The Pan-African I event is characterised by the intrusion of leucogranites, and the formation of medium-U metamorphic zircon overgrowth. A sample of a syn-tectonic leucogranite gave a well-constrained crystallisation age of 558.4±5.6 Ma. These rocks are intensely deformed and have formed a second gt-bearing leucosome after their emplacement. Identical ages come from low-U metamorphic zircons and low-U rims from another leucosome at Festninga (557±13 Ma). The Pan-African II event (c. 530 and 490 Ma) is characterised by widespread melting and only little deformation. It is associated with charnockite magmatism, charnockitisation and high-temperature metamorphism. Wide, very high-U zircon rims form during this period. The early Pan-African II overprint between c. 530 and 520 Ma is recorded in almost all samples. An undeformed mafic dyke with a zircon crystallisation age of 523.2±4.8 indicates that there is no or minor deformation after its intrusion. This strongly metasomatised dyke, however, also indicates that although deformation ceased, intense fluid circulation continued. The youngest crystallisation ages come from a post-tectonic granite sheet which yields a crystallisation age of 486.9±3.8 Ma. Final cooling to temperatures below c. 600 °C is recorded by a titanite age of c. 483±11 Ma from the Stabben gabbro. Late to post-tectonic granitoids are voluminous and intrude over a period of c. 30 Ma, between c. 520 and 490 Ma. They are alkaline in composition and can be classified as A2-type. This indicates that they are probably formed by melting of a tonalitic to granodioritic lower crust in the cause of crustal thickening, or from underplated lower crust.

Different age response of zircon and monazite during the tectono-metamorphic evolution of a high grade paragneiss from the Ruhla Crystalline Complex, central Germany

Results of TIMS, SIMS and SEM analyses show that zircon and monazite in a high-grade paragneiss of the Ruhla Crystalline Complex, central Germany, were formed and/or altered during different stages of a tectono-metamorphic history between Early Devonian and Permian times. Detrital zircon cores of >460 Ma place an older limit on the age of anatexis, and show that the paragneiss sequence contains rocks at least as young as early Cambrian. Metamorphic zircon growth commenced at ~365 Ma, peaking at ~360–355 Ma at the same time that granite dykes were emplaced. In contrast, monazite in the paragneiss preserves little record of the metamorphic peak. Most monazite grains grew or were recrystallised in the Lower Carboniferous at ~339 Ma, contemporaneous with the emplacement of voluminous diorite and granite bodies. These intrusions and related tectonics caused some of the high-U zircon overgrowths to undergo moderate to severe Pb loss. A second Pb loss event, between 300 and 280 Ma, can be related to Late Carboniferous/Early Permian large-scale block faulting.

Relationships between magmatism, metamorphism and deformation in the western Mount Isa Inlier, Australia

A structural and geochronological study of a granitoid and two pegmatites is used to clarify age relationships between magmatism, deformation and metamorphism in the Mount Isa Inlier. Analyses of zircon UPb isotopic systems date the emplacement age of the Sybella Batholith at 1660 ± 4 to 1655 ± 5 Ma. Similar analyses of two pegmatite generations demonstrate high-U zircons with complex UPb systematics which result from alteration and multi-stage Pb loss. A concordant group of zircon analyses from the older pegmatite displays a bimodal distribution including groups at 1532 ± 7 and 1565 ± 5 Ma which are interpreted to represent the crystallisation age and inherited xenocrystic zircon, respectively. Replicate analyses of a less altered zircon grain from the younger pegmatite are near concordant at 1480 ± 14 Ma. This is interpreted as a minimum age for crystallisation of the younger pegmatite. Neither pegmatite is genetically related to the Sybella Batholith. Granoblastic quartz textures and cleavage overprinting relationships indicate the 1532 ± 7 Ma pegmatities intruded prior to or (at latest) during low-pressure ( P), high-temperature ( T) metamorphism. Partial melting during peak metamorphism provides a likely source for the pegmatitic melts. This implies that metamorphism and concurrent polyphase deformation occurred at ∼ 1532 Ma, ∼ 130 Myr after emplacement of the Sybella Batholith. Thus the batholith did not contribute to the high heat flow responsible for low- P, high- T metamorphism nor did it intrude during the associated period of regional crustal shortening. Age constraints instead suggest that the batholith was emplaced during, or soon after, extension and rift-related sedimentation and felsic volcanism.

U–Th–Pb ages of single zircons in Archaean supracrustals from Nain Province, Labrador, Canada

Ion-probe U–Pb dating of igneous zircons from an acid metavolcanite belonging to the Nulliak supracrustal association of the Saglek area, northern Labrador, suggests an original age of 3776 ± 8 Ma (2σ), interpreted as the deposition age of the volcanite. In accordance with the field model, this is slightly but significantly older than the previously obtained ion-probe U–Pb age of igneous zircons from the main phase of the Uivak gneisses, which carries inclusions of the supracrustal association. A secondary event of zircon growth recorded in the metavolcanite has a maximum age of 3065 ± 8 Ma (2σ).A metapelite belonging to the Weekes amphibolite supracrustal association of the Hopedale area south of Nain contains rounded, low-U zircons of 3258 ± 24 Ma (2σ) age. Occasional textural evidence suggests that some of these zircons are most likely detrital grains of igneous origin, in which case 3258 ± 24 Ma is a minimum age of the source from which the sediment was derived and a maximum age for the deposition of the sediment. However, the possibility that the age represents recrystallization of the sediment under high-grade conditions cannot be ruled out. A 2841 ± 5 Ma (2σ) age obtained on a younger generation of euhedrally zoned high-U zircons from the metapelite is within error of the published bulk zircon U–Pb age of the tonalitic component of the Kanairiktok plutonic suite that intrudes the older orthogneiss and supracrustal units in the area. These younger zircons are most likely to have grown during migmatization of the metasediment at the time of emplacement of the suite.The zircon data give no positive evidence to support correlation between the Nulliak supracrustals and the Weekes amphibolite association.