Articles de revues : « Pb zircon age »

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Pidgeon, R. T., et W. Compston. « A SHRIMP ion microprobe study of inherited and magmatic zircons from four Scottish Caledonian granites ». Earth and Environmental Science Transactions of the Royal Society of Edinburgh 83, n^o 1-2 (1992) : 473–83. http://dx.doi.org/10.1017/s0263593300008142.

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ABSTRACTUsing the ion microprobe SHRIMP we have analysed zircons from the Ben Vuirich, Glen Kyllachy, Inchbae and Vagastie Bridge granites from the Scottish Caledonides, in an attempt to resolve the ages of inherited zircons shown to be present in these granites by previous conventional multigrain analyses. Middle Proterozoic age components were found in inherited zircons from all four granites. Late Proterozoic (900–1,100 Ma) components have been identified in zircons from the Glen Kyllachy and Ben Vuirich granites in the Grampian Highlands. A Late Archaean age has only been detected in one zircon from the Glen Kyllachy granite. The distribution of inherited components in the granite zircon populations could reflect fundamental divisions in the age composition of granite source rocks; however, detailed assessment of this possibility must await further ion microprobe analyses on zircons from many more granites.SHRIMP isotopic and U, Th and Pb analyses were made on successive shells of zoned zircon surrounding inherited cores from the Glen Kyllachy granite to monitor chemical changes during magmatic zircon growth. Results show that zircon shells have characteristic but significantly different Th, U and Pb concentrations. Magmatic zircon from the Vagastie Bridge granite also forms as clearly defined oscillatory zoned shells around unzoned nuclei of inherited zircon. However, the distinction between magmatic and inherited zircon in zircons from the Inchbae granite is less clear. Zircons from the Ben Vuirich granite occur as euhedral, magmatic zircons, or as rounded, subhedral, inherited zircon grains. A SHRIMP age of 597 ± 11 (2σ) Ma for euhedral magmatic zircon from this granite is identical, within the uncertainty, to the conventional multigrain zircon age of 590 ± 2 (2σ) Ma reported by Rogers et al. (1989) and confirms the conclusions of those authors that sedimentation of the Dalradian sequence took place in the Precambrian.

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SHUMLYANSKYY, L. V., V. KAMENETSKY et B. V. BORODYNYA. « Age and Composition of Zircons From the Devonian Petrivske Kimberlite Pipe of the Azov Domain, the Ukrainian Shield ». Mineralogical Journal 43, n^o 4 (2021) : 50–55. http://dx.doi.org/10.15407/mineraljournal.43.04.050.

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Results of a study of U-Pb and Hf isotope systematics and trace element concentrations in five zircon crystals separated from the Devonian Petrivske kimberlite are reported in the paper. Four zircons have yielded Paleoproterozoic and Archean ages, while one zircon grain gave a Devonian age of 383.6±4.4 Ma (weighted mean 206Pb/238U age). The Precambrian zircons have been derived from terrigenous rocks of the Mykolaivka Suite that is cut by kimberlite, or directly from the Precambrian rock complexes that constitute continental crust in the East Azov. The Devonian zircon crystal has the U-Pb age that corresponds to the age of kimberlite emplacement. It is 14 m.y. younger than zircon megacrysts found in the Novolaspa kimberlite pipe in the same area. In addition, Petrivske zircon is richer in trace elements than its counterparts from the Novolaspa pipe. Petrivske and Novolaspa zircons crystallized from two different proto-kimberlite melts, whereas the process of kimberlite formation was very complex and possibly included several episodes of formation of proto-kimberlite melts, separated by extended (over 10 M.y.) periods of time.

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Waight, Tod, Mikael Stokholm, Benjamin Heredia et Tonny B. Thomsen. « U-Pb zircon and titanite age of the Christiansø granite, Ertholmene, Denmark, and correlation with other Bornholm granitoids ». Bulletin of the Geological Society of Denmark 70 (23 mars 2022) : 27–38. http://dx.doi.org/10.37570/bgsd-2022-70-03.

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A granitic sample from the Danish island of Christiansø in the Ertholmene island group north of Bornholm is described petrographically and geochemically, and dated using U-Pb in zircon and titanite. Zircon systematics in the sample are complicated by abundant Pb loss and a large population of zircons interpreted as being inherited. Removal of highly disturbed zircons, imprecise analyses, and assumed inherited zircons yield an upper intercept date of 1500 ± 18 Ma (MSWD = 13, n = 58). Removal of zircons with high common Pb from this population yields an identical result of 1500 ± 22 Ma (MSWD = 8, n = 34). Zircons that are ≤3% discordant give a weighted average 206Pb/238U age of 1458 ± 12 Ma (MSWD = 3.0, n = 18), and a weighted average 207Pb/206Pb age of 1495 ± 14 Ma (MSWD = 4.7, n = 19). Titanites from the sample yield a lower intercept age of 1448 ± 15 Ma (MSWD = 6.8, n = 45). The sample contains a significant number of inherited grains indicative of ages around 1.7–1.8 Ga. The relatively large MSWDs for these age determinations indicate geological complexity, likely reflecting Pb loss, and the possible presence of inherited zircons which suffered major Pb loss during incorporation in the granitic magma. The zircon and titanite dates agree reasonably well with previous age determinations on felsic lithologies from the Bornholm mainland, as well as from the Blekinge Province of southern Sweden. Petrographically and geochemically, the Christiansø granite is indistinguishable from, and can be correlated with, the A-type granites and gneisses which occur on Bornholm. The high abundance of disturbed and inherited zircons (c. 1.7–1.8 Ga) may indicate that the granite was intruded into and assimilated a nearby region of unexposed Transscandinavian Igneous Belt rocks. The somewhat altered nature of the rock, and overall disturbance of U-Pb zircon systematics, suggest alteration associated with fluid-flow along nearby faults defining the northern margin of the Sorgenfrei–Tornquist Zone.

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Kusiak, Monika A., Daniel J. Dunkley, Richard Wirth, Martin J. Whitehouse, Simon A. Wilde et Katharina Marquardt. « Metallic lead nanospheres discovered in ancient zircons ». Proceedings of the National Academy of Sciences 112, n^o 16 (6 avril 2015) : 4958–63. http://dx.doi.org/10.1073/pnas.1415264112.

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Zircon (ZrSiO4) is the most commonly used geochronometer, preserving age and geochemical information through a wide range of geological processes. However, zircon U–Pb geochronology can be affected by redistribution of radiogenic Pb, which is incompatible in the crystal structure. This phenomenon is particularly common in zircon that has experienced ultra-high temperature metamorphism, where ion imaging has revealed submicrometer domains that are sufficiently heterogeneously distributed to severely perturb ages, in some cases yielding apparent Hadean (>4 Ga) ages from younger zircons. Documenting the composition and mineralogy of these Pb-enriched domains is essential for understanding the processes of Pb redistribution in zircon and its effects on geochronology. Using high-resolution scanning transmission electron microscopy, we show that Pb-rich domains previously identified in zircons from East Antarctic granulites are 5–30 nm nanospheres of metallic Pb. They are randomly distributed with respect to zircon crystallinity, and their association with a Ti- and Al-rich silica melt suggests that they represent melt inclusions generated during ultra-high temperature metamorphism. Metallic Pb is exceedingly rare in nature and previously has not been reported in association with high-grade metamorphism. Formation of these metallic nanospheres within annealed zircon effectively halts the loss of radiogenic Pb from zircon. Both the redistribution and phase separation of radiogenic Pb in this manner can compromise the precision and accuracy of U–Pb ages obtained by high spatial resolution methods.

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Hervé, Francisco, Víctor Faúndez, Manfred Brix et Mark Fanning. « Jurassic sedimentation of the Miers Bluff Formation, Livingston Island, Antarctica : evidence from SHRIMP U–Pb ages of detrital and plutonic zircons ». Antarctic Science 18, n^o 2 (juin 2006) : 229–38. http://dx.doi.org/10.1017/s0954102006000277.

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Detrital zircon populations from two sandstone samples from the lower member (Johnsons Dock Member) of the Miers Bluff Formation at Hurd Peninsula have been dated by the Sensitive High Resolution Ion Microprobe (SHRIMP) U–Pb method. In one of the samples, zircons as young as early Middle Jurassic (Bajocian) age are present. In the second sample, the youngest detrital zircons are Middle Triassic in age. The detrital zircon age spectra indicate that Permian, early Palaeozoic and Meso- to Neoproterozoic zircon bearing rocks were present in the source areas of the Miers Bluff Formation. The sedimentary rocks are intruded by the Hespérides Point Intrusive diorite stock which yielded a U–Pb zircon crystallization age of 137.7 ± 1.4 Ma (Early Cretaceous, Valanginian). These results indicate that sedimentation of the Johnsons Dock Member of the Miers Bluff Formation is bracketed in time between the Bajocian and the Valanginian. The Miers Bluff Formation has been correlated with the Trinity Peninsula Group from the Antarctic Peninsula, based on sedimentological and structural similarity. Since the Trinity Peninsula Group is older than Middle Jurassic a direct chronological correlation is not supported by our new U–Pb zircon data. However, we suggest that the tectonic setting may have migrated in time with deposition of the pre-Middle Jurassic TPG on the peninsula, to Livingston Island where the maximum age for deposition of the MBF is Bajocian (about 170 Ma).

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Compston, W. « Interpretation of SHRIMP and isotope dilution zircon ages for the Palaeozoic time-scale : II. Silurian to Devonian ». Mineralogical Magazine 64, n^o 6 (décembre 2000) : 1127–46. http://dx.doi.org/10.1180/002646100549931.

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AbstractIon probe data are documented for zircons from tuffs within the early Llandovery, the mid-Caradoc and the Ludlow. 206Pb/238U ages for tuff magmatism have been interpreted using mixture-modelling to distinguish inheritance and Pb loss. Comparisons with the reference zircon SL13 have been improved through a direct determination of the component of secondary ion discrimination caused by changes in target potential.Interpretation of the SHRIMP data for the Birkhill ash (Scotland, Llandovery) is ambiguous. The more conservative possibility is that most zircons are 439 Ma, in close agreement with the previous isotope dilution ages for the same zircon concentrate. The other is that the 439 Ma group should be split into an inherited population at ˜447 Ma, with a minority at ˜434 Ma that corresponds with the ash volcanism. Although imprecise, the latter is detectably younger than the multi-grain MSID age, which itself might be a composite of the same two ages.Most zircon analyses from the mid Caradoc Pont-y-ceunant Ash, Wales, fit an age-group at 452.5 Ma, similar to its published 206Pb/238U age by MSID, with a definite older age group at ˜476 Ma but none showing Pb loss. By contrast, those from the Millbrig bentonite (Virginia) of the same age mainly fall in a well-defined post-eruption age group at 435 Ma, while the remainder give 456 Ma. Most zircon analyses from the Kinnekulle bentonite, Sweden, fall into an apparent 464 Ma group which exceeds other estimates for the mid-Caradoc magmatism. It is interpreted to be a composite age, caused by an inability to resolve it into a younger magmatic and older inherited group owing to the larger analytical errors of the Kinnekulle data. The best SHRIMP estimate for the mid-Caradoc volcanism is 452.6±1.7 Ma found by combining the ages for the three volcanic units. During unmixing of the combined ages, the Kinnekulle ages are redistributed and the 464 Ma ‘group’ vanishes. Precambrian grains are present in all the above volcanics.The original and new zircon analyses from the Laidlaw Volcanics (Canberra, Australia) of Ludlow age, are dominated by three groups of inherited zircons at ˜436 Ma, ˜450 Ma and ˜476 Ma, which makes it unfavourable for time-scale definition using zircons. The youngest zircon age group is 417.5 Ma (˜30%), but this is not associated with overgrowths on older grains or with wholly younger grains. Instead, it is composed of sporadic low ages within older grains suggestive of Pb loss rather than magmatic zircon growth. Nevertheless, the age for volcanism is 420.7±1.1 Ma based on published Rb-Sr and K-Ar dating, so that the youngest zircon group does appear to be associated with volcanism.One zircon U-Pb age for the Frasnian by MSID is much older than a precise age by other decay schemes, and another for the Lochkovian is significantly older than a recent SHRIMP age for the same Stage. By small changes in the common Pb composition, both MSID ages can be changed from single volcanic ages affected by Pb loss to an inherited and younger volcanic age, which removes the conflict with the other determinations.A zircon-based geological time-scale is constructed from the Ordovician to the Carboniferous using the time-points presented and discussed in Parts I and II of this paper.

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Doughty, P. Ted, et K. R. Chamberlain. « Protolith age and timing of Precambrian magmatic and metamorphic events in the Priest River complex, northern Rockies ». Canadian Journal of Earth Sciences 45, n^o 1 (1 janvier 2008) : 99–116. http://dx.doi.org/10.1139/e07-067.

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U–Pb dating of detrital zircons, magmatic zircons, and metamorphic zircon overgrowths and titanites from the Priest River complex, USA, reveal the ages of high-grade metasedimentary rocks, intrusive ages of associated igneous rocks, and the timing of Precambrian metamorphic events. Sensitive high-mass resolution ion microprobe (SHRIMP) U–Pb dates of detrital zircons from the Hauser Lake gneiss and Gold Cup Quartzite are predominately Paleoproterozoic with some Archean grains. The Hauser Lake gneiss contains detrital zircons with nearly concordant ages as young as 1511 Ma. Isotope dilution – thermal ionization mass spectrometry (ID–TIMS) and chemical abrasion – thermal ionization mass spectrometry (CA–TIMS) dating of magmatic zircon from amphibolites in the Hauser Lake Gneiss yield 1470–1430 Ma crystallization ages based on discordant data, with Cretaceous lower intercepts. The U–Pb zircon systematics are very complex and reflect multiple periods of magmatic and metamorphic growth. A combination of mechanical abrasion, chemical abrasion (CA–TIMS), and SHRIMP methods were required to identify the major geochronological components. These data, in addition to one Nd model age, strengthen correlations between the Hauser Lake Gneiss and the lower Belt–Purcell Supergroup and the Gold Cup Quartzite with the Neihart Quartzite, which underlies the Belt–Purcell Supergroup. Four SHRIMP-dated metamorphic zircon overgrowths give concordant Grenville dates with a weighted average of 1127 ± 110 Ma. This Grenville-aged metamorphic event is recorded by many isotopic systems in the Belt–Purcell basin and reflects a static thermal event, possibly driven by magmatism at depth.

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Liu, Chao, Xiangdong Chang, Beilei Sun et Fangui Zeng. « New Insight into the Depositional Age of No. 6 Coal in Heidaigou Mine, Late Paleozoic Jungar Coalfield, Inner Mongolia, China ». Sustainability 14, n^o 10 (21 mai 2022) : 6297. http://dx.doi.org/10.3390/su14106297.

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Coal deposits can provide novel stratigraphic markers for reconstructing the evolution history of a sedimentary basin and correlating sedimentary successions. Age dating was conducted on zircons harvested from the No. 6 coal seam within the Heidaigou Mine, Inner Mongolia. Two-kilogram samples were taken, and the recovered zircons were analyzed for U–Pb isotopic and rare earth elements (REE). The REE results of the zircon grains showed that all the zircon grains were enriched in heavy rare earth elements (HREE) but depleted in light rare earth elements (LREE). In addition, zircons from the No. 6 coal seam had strongly positive Ce (Ce/Ce* = 2.4–224.6) and strongly negative Eu anomalies (Eu/Eu* = 0.1–0.6). Combined with the clear oscillatory zones in the cathodoluminescence images, all the zircon grains of the No. 6 coal were characteristic of zircons with magmatic origins. The 206Pb/238U ages of 34 zircon grains produced a narrow age population of 303–286 Ma, with a weighted average age of 293.0 ± 1.5 Ma (mean-squared weighted deviation = 1.5). Therefore, we infer that the No. 6 coal in the Heidaigou Mine was deposited during the Early Permian, and the Carboniferous–Permian boundary should be located stratigraphically lower than the No. 6 coal. The zircon U–Pb geochronology is a useful tool to determine the depositional ages of non-marine-influenced coal.

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Shi, Yu, Xi Jun Liu et Zuo Hai Feng. « Formation Age of the Qinling Complex and the early Paleozoic Tectonic Event ». Advanced Materials Research 734-737 (août 2013) : 60–70. http://dx.doi.org/10.4028/www.scientific.net/amr.734-737.60.

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The Qinling orogenic belt (QOB) located between the North China Craton (NCC) and the South China Craton (SCC) is composed of the Northern Qinling Belt (NQB) and the Southern Qinling Belt (SQB). This study presents new geochemical data, zircon U-Pb ages and Hf isotopes from two rocks from the Qinling complex in the NQB. LA-ICP-MS zircon U-Pb dating results suggest that the Qinling complex was formed in early Neoproterozoic and experienced the early Paleozoic metamorphism. HighεHf(t) values of 9.0-12.0 for the early Paleozoic zircons indicated that there is mantle-derived magma intruding into the Qinling complex in the early Paleozoic.

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Skublov, Sergey G., Aleksey V. Berezin, Xian-Hua Li, Qiu-Li Li, Laysan I. Salimgaraeva, Veniamin V. Travin et Dmitriy I. Rezvukhin. « Zircons from a Pegmatite Cutting Eclogite (Gridino, Belomorian Mobile Belt) : U-Pb-O and Trace Element Constraints on Eclogite Metamorphism and Fluid Activity ». Geosciences 10, n^o 5 (21 mai 2020) : 197. http://dx.doi.org/10.3390/geosciences10050197.

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This report presents new data on U-Pb geochronology, oxygen isotopes, and trace element composition of zircon from a pegmatite vein crosscutting an eclogite boudin on Stolbikha Island, Gridino area, Belomorian mobile belt (BMB). The zircon grains occur as two distinct populations. The predominant population is pegmatitic and shows dark cathodoluminescence (CL); about a third of this population contains inherited cores. The second zircon population is typical of granulite and exhibits a well-defined sectorial (mosaic) zoning in CL. Both the inherited cores and sectorial in CL zircons appear to have been captured from metabasites as xenocrysts during the pegmatite vein formation. A U-Pb age of 1890 ± 2 Ma for the main zircon population is interpreted as the age of the pegmatite injection. This value is close to the age threshold for the BMB eclogites (~1.9 Ga) and unambiguously defines the upper age limit for the eclogite metamorphism. The pegmatite formation is thus related to partial melting events that occurred during the retrograde amphibolite-facies metamorphism shortly after the eclogitization. A U-Pb date of 2743 ± 10 Ma obtained for the sectorial in CL zircons is considered as the age of the granulite-facies metamorphism established previously within the BMB. The values of δ18O in the zircon populations overlap in a broad range, i.e., δ18O in the pegmatitic zircons varies from 6.1‰ to 8.3‰, inherited cores show a generally higher δ18O of 6.7–8.8‰, and in the captured granulitic zircons δ18O is 6.2–7.9‰. As a result of fluid attack during the final stage of the pegmatite vein formation, the composition of the pegmatitic zircons in terms of non-formula elements (REE, Y, Ca, Sr, Ti) has become anomalous, with the content of these elements having been increased by more than tenfold in the alteration zones. Our data provide new constraints on the timing of eclogite metamorphism within the BMB and show that the late-stage pegmatite-related fluids exerted a very pronounced influence on trace element abundances in zircon, yet had no significant impact on the isotopic composition of oxygen.

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Vozárová, Anna, Miloš Šmelko et Ilya Paderin. « Permian single crystal U-Pb zircon age of the Rožňava Formation volcanites (Southern Gemeric Unit, Western Carpathians, Slovakia) ». Geologica Carpathica 60, n^o 6 (1 décembre 2009) : 439–48. http://dx.doi.org/10.2478/v10096-009-0032-1.

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Permian single crystal U-Pb zircon age of the Rožňava Formation volcanites (Southern Gemeric Unit, Western Carpathians, Slovakia)Zircon populations from the Rožňava Formation volcanic rock complex have been analysed. Euhedral zircons from the 1stvolcanogenic horizon with fine oscillatory growth zoning, typical of magmatic origin, gave the average concordia age of 273.3 ± 2.8 Ma, with Th/U ratios in the range of 0.44-0.73. The Permian ages ranging from 266 to 284 Ma were identified in the wider, zoned or unzoned, central zircon parts, as well as in their fine-zoned oscillatory rims. The average concordia age of 275.3 ± 2.9 was obtained from the euhedral zircon population of the 2ndvolcanogenic horizon of the Rožňava Formation. The analyses were performed on zoned magmatic zircons in the age interval from 267 to 287 Ma, with Th/U ratios in the range of 0.39-0.75. In the later zircon population two inherited zircon grains were dated giving the age of 842 ± 12 Ma (Neoproterozoic) and 456 ± 7 Ma (Late Ordovician). The magmatic zircon ages document the Kungurian age of Permian volcanic activity and contemporaneous establishment of the south-Gemeric basin. The time span of volcanic activity corresponds to the collapse of the Western Carpathian Variscan foreland which expanded southward.

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STEPANYUK, L. M. « Zircon and Monazite as Geochronometers ». Mineralogical Journal 44, n^o 1 (2022) : 41–55. http://dx.doi.org/10.15407/mineraljournal.44.01.041.

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The paper discusses the results of U-Pb isotope dating of monazites and zircons from granitoids formed under PT conditions of granulite and amphibolite facies and gneisses, and crystalline shales that are like xenoliths in these granitoids of the Ukrainian Shield. In some cases, such as the Berdychiv-type granitoids and in the Zhezhelivsky quarry, the age values of monazites and outer edges of zircons coincide well; in others, such as the granites of the Ivanivsky quarry, they differ by more than the error. In the Odessa quarry, even greater differences were found between the age of zircon (1.95-1.96 ± give errors Ga) from mafic granulite, and the age of zircon (1.99 Ga) and the age of monazite (1.89-1.83 ± Ga) of a crosscutting enderbite. The U-Pb isotope system of monazite from a vein of biotite granite common in this quarry does not meet the conditions of the Ahrens-Wetherill model. The 207Pb / 206Pb age of monazite ranges from 1909.5 to 1867.0 ± 5 Ma, i.e. by more than 40 Ma. For the Sabarivsky quarry, significant differences in the 207Pb / 206Pb age values (2015.8-1984.8 Ma), which significantly exceed the analytical error, do not allow the use of the concordia U-Pb diagram to interpret the analytical data obtained for zircons from the enderbite. It is concluded that monazite is the better mineral geochronometer, compared to zircon, to determine the age of palingenetic-anatectic granitoids. Unlike zircon, which usually inherits the relict nuclei of parent rocks, relict nuclei in monazites are an exception. However, monazite is not an ideal mineral geochronometer for determining the time of endogenous geological processes. Since the crystallization of monazite is depends on the activity of calcium in the environment, in the same rock association in rocks of different composition, monazite may crystallize at different times. The combination of radiometric studies of zircon and monazite, together with zoning of their crystals, provides ample opportunities to determine the time and duration of endogenous geological processes.

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Davies, Joshua H. F. L., Jörn-Frederik Wotzlaw, Alexander P. Wolfe et Larry M. Heaman. « Assessing the age of the Late Cretaceous Danek Bonebed with U–Pb geochronology ». Canadian Journal of Earth Sciences 51, n^o 11 (novembre 2014) : 982–86. http://dx.doi.org/10.1139/cjes-2014-0136.

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An ash-rich volcaniclastic sandstone immediately underlying dinosaur-rich material from the Danek Bonebed in the Horseshoe Canyon Formation (HCF), Edmonton, Alberta, Canada, contains accessory zircon, which have been dated employing U–Pb geochronology. Both laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) and chemical abrasion isotope dilution thermal ionization mass spectrometry (ID-TIMS) U–Pb analyses have been conducted. The zircon age distributions are complex with U–Pb dates ranging from Precambrian to Cretaceous. We consider the youngest ID-TIMS 206Pb/238U date of 71.923 ± 0.068 Ma as the maximum deposition age of the ash-rich sandstone, placing the overlying Danek bonebed in the early Maastrichtian. This age is compatible with the paleontological assemblage from the Danek Bonebed and the regional stratigraphy. The zircon age distribution also implies that the HCF had a complex provenance of the detritus with some Archean and Proterozoic zircons, a group of Mesozoic, and a large compliment of Cretaceous grains. The results highlight the importance of high precision geochronology in constraining the age of important fossil deposits such as the Danek Bonebed.

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Williams, Ian S. « Some observations on the use of zircon U-Pb geochronology in the study of granitic rocks ». Earth and Environmental Science Transactions of the Royal Society of Edinburgh 83, n^o 1-2 (1992) : 447–58. http://dx.doi.org/10.1017/s0263593300008129.

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ABSTRACTIn situ, microscale, U-Pb isotopic analyses of zircon using the SHRIMP ion microprobe demonstrate both the potential and the limitations of zircon U-Pb geochronology. Most zircons, whether from igneous or metamorphic rocks, need to be considered as mixed isotopic systems. In simple, young igneous rocks the mixing is principally between isotopically disturbed and undisturbed zircon. In polymetamorphic rocks, several generations of zircon growth can coexist, each with a different pattern of discordance. A similar situation exists for igneous rocks rich in inherited zircon, as these contain both melt-precipitated zircon and inherited components of several different ages. Microscale analysis by ion probe makes it possible to sample the record of provenance, age and metamorphic history commonly preserved within a single zircon population. It also indicates how the interpretation of conventionallymeasured bulk zircon isotopic compositions might be improved.

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Brenhin Keller, C., Patrick Boehnke, Blair Schoene et T. Mark Harrison. « Stepwise chemical abrasion–isotope dilution–thermal ionization mass spectrometry with trace element analysis of microfractured Hadean zircon ». Geochronology 1, n^o 1 (11 décembre 2019) : 85–97. http://dx.doi.org/10.5194/gchron-1-85-2019.

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Abstract. The Hadean Jack Hills zircons represent the oldest known terrestrial material, providing a unique and truly direct record of Hadean Earth history. This zircon population has been extensively studied via high-spatial-resolution high-throughput in situ isotopic and elemental analysis techniques, such as secondary ionization mass spectrometry (SIMS), but not by comparatively destructive, high-temporal-precision (<0.05 % two-sigma) thermal ionization mass spectrometry (TIMS). In order to better understand the lead loss and alteration history of terrestrial Hadean zircons, we conduct stepwise chemical abrasion–isotope dilution–thermal ionization mass spectrometry with trace element analysis (CA-ID-TIMS-TEA) on manually microfractured Hadean Jack Hills zircon fragments previously dated by SIMS. We conducted three successive HF leaching steps on each individual zircon fragment, followed by column chromatography to isolate U–Pb and trace element fractions. Following isotopic and elemental analysis, the result is an independent age and trace element composition for each leachate of each zircon fragment. We observe ∼50 Myr of age heterogeneity in concordant residues from a single zircon grain, along with a protracted history of post-Hadean Pb loss with at least two modes circa ∼0 and 2–4 Ga. Meanwhile, stepwise leachate trace element chemistry reveals enrichments of light rare earth elements, uranium, thorium, and radiogenic lead in early leached domains relative to the zircon residue. In addition to confirming the efficacy of the LREE-I alteration index and providing new insight into the mechanism of chemical abrasion, the interpretation and reconciliation of these results suggest that Pb loss is largely driven by low-temperature aqueous recrystallization and that regional thermal events may act to halt – not initiate – Pb loss from metamict domains in the Hadean Jack Hills zircons.

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KRISTOFFERSEN, MAGNUS, TOM ANDERSEN et ARILD ANDRESEN. « U–Pb age and Lu–Hf signatures of detrital zircon from Palaeozoic sandstones in the Oslo Rift, Norway ». Geological Magazine 151, n^o 5 (29 octobre 2013) : 816–29. http://dx.doi.org/10.1017/s0016756813000885.

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AbstractU–Pb and Lu–Hf isotope analyses of detrital zircon from the latest Ordovician (Hirnantian) Langøyene Formation, the Late Silurian Ringerike Group and the Late Carboniferous Asker Group in the Oslo Rift were obtained by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Overall the U–Pb dating yielded ages within the range 2861–313 Ma. The U–Pb age and Lu–Hf isotopic signatures correspond to virtually all known events of crustal evolution in Fennoscandia, as well as synorogenic intrusions from the Norwegian Caledonides. Such temporally and geographically diverse source areas likely reflect multiple episodes of sediment recycling in Fennoscandia, and highlights the intrinsic problem of using zircon as a tracer-mineral in ‘source to sink’ sedimentary provenance studies. In addition to its mostly Fennoscandia-derived detritus, the Asker Group also have zircon grains of Late Devonian – Late Carboniferous age. Since no rocks of these ages are known in Fennoscandia, these zircons are inferred to be derived from the Variscan Orogen of central Europe.

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Huang, Yong-Shu, Qiu-Li Li, Yu Liu, Ping-Ping Liu, Sun-Lin Chung et Xian-Hua Li. « 238U–206Pb dating of U-series disequilibrium zircons by secondary ion mass spectrometry ». Journal of Analytical Atomic Spectrometry 36, n^o 5 (2021) : 999–1006. http://dx.doi.org/10.1039/d0ja00510j.

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Theoretical and analytical methods for ²³⁸U–²⁰⁶Pb dating of U-series disequilibrium zircons by secondary ion mass spectrometry are developed. U–Pb dates improve age precision 3 times better than U–Th dating for zircon >150 ka.

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Schiøtte, L., W. Compston et D. Bridgwater. « U–Th–Pb ages of single zircons in Archaean supracrustals from Nain Province, Labrador, Canada ». Canadian Journal of Earth Sciences 26, n^o 12 (1 décembre 1989) : 2636–44. http://dx.doi.org/10.1139/e89-224.

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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.

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Parrish, R. R., et I. Reichenbach. « Age of xenocrystic zircon from diatremes of western Canada ». Canadian Journal of Earth Sciences 28, n^o 8 (1 août 1991) : 1232–38. http://dx.doi.org/10.1139/e91-110.

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Numerous diatremes of middle and late Paleozoic age intrude miogeoclinal middle and lower Paleozoic strata in the Canadian Cordillera. In addition to abundant crustal xenoliths and conspicuous mantle-derived mineral xenocrysts, rare zircon grains are present. U–Pb dating of single zircon crystals from many of these diatremes has failed to identify the presence of cogenetic (magmatic) zircons. All dated zircon grains are interpreted as xenocrysts derived from the crust. Their morphologies range from euhedral to very rounded, and their ages range from early Paleozoic to Archean. Most ages fall between 1.8 and 2.1 Ga, with subordinate age groupings in the late Archean (ca. 2.6 Ga), Middle Proterozoic (1.0–1.1 Ga), and early Paleozoic (ca. 470 Ma, 530 Ma). The Proterozoic and Archean zircons could have been derived from either the crystalline basement or its overlying sedimentary cover of Late Proterozoic to early Paleozoic age. Paleozoic zircons were probably derived from either intrusions within the basement or sills that intrude the early Paleozoic sedimentary cover, and they signify magmatic activity possibly related to rifting of the continental margin.

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Nutman, A. P., et F. Kalsbeek. « Search for Archaean basement in the Caledonian fold belt of North-East Greenland ». Rapport Grønlands Geologiske Undersøgelse 162 (1 janvier 1994) : 129–33. http://dx.doi.org/10.34194/rapggu.v162.8254.

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SHRIMP U-Pb isotope data on zircon crystals from a gneiss sample near Danmarkshavn, where the presence of Archaean rocks has earlier been documented, show that the rock has undergone a complex history of igneous and metamorphic zircon growth. At least three generations of zircon are present with ages of c. 3000 Ma, c. 2725 Ma and 1967 ±8 Ma (2 α). Apparently the rock was formed from an Archaean protolith which underwent high grade metamorphism during the early Proterozoic. Another sample from the easternmost exposures of the Caledonian basement, collected further north, yielded only early Proterozoic zircons with an age of 1963 ± 6 Ma. Together with a SHRIMP U-Pb zircon age of 1974 ± 17 Ma reported earlier, these results give evidence of a major igneous and metamorphic event in North-East Greenland about 1965 Ma ago.

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Jackson, William T., Matthew P. McKay, Donald A. Beebe, Carolyn Mullins, Adelie Ionescu, Barry Shaulis et David L. Barbeau. « Late Cretaceous sediment provenance in the eastern Gulf Coastal Plain (U.S.A.) based on detrital-zircon U-Pb ages and Th/U values ». Journal of Sedimentary Research 91, n^o 10 (8 octobre 2021) : 1025–39. http://dx.doi.org/10.2110/jsr.2020.177.

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ABSTRACT Detrital-zircon U-Pb geochronology documents a regional- to continental-scale drainage reorganization along the eastern Gulf Coastal Plain (USA) from the Late Cretaceous (Cenomanian) to the Paleocene–Eocene. We present detrital-zircon U-Pb ages and Th/U values from the Maastrichtian Ripley Formation to determine the sedimentary provenance and to provide spatiotemporal resolution of drainage reorganization. The Ripley Formation contains a 12.7% overall average abundance of detrital zircons with low (< 0.1) Th/U values relative to the underlying Cenomanian Tuscaloosa Group (3.6%), the overlying Paleocene–Eocene Wilcox Group (2.8%), an Appalachian foreland composite (2.1%), and the laterally equivalent McNairy Sandstone in the northern Mississippi Embayment (3.8%). Multidimensional scaling of detrital-zircon U-Pb spectra shows that the Ripley Formation is dissimilar from underlying and overlying Gulf Coastal Plain units, the McNairy Sandstone, and an Appalachian foreland composite sample because of differences in proportions of Appalachian (490–270 Ma) and Grenville (1250–900 Ma) zircons. We interpret the southern Appalachian Piedmont province as the principal sediment source region for the Ripley Formation to account for the elevated abundance of grains with low (< 0.1) Th/U values and unique detrital-zircon U-Pb age spectra. Results suggest a regional-scale (105 km2) drainage network, which delivered sediment to the Maastrichtian coast followed by northwestward littoral transport and eventual mixing with Appalachian foreland-derived sediment in the northern Mississippi Embayment. This study further brackets drainage reorganization along the eastern Gulf Coastal Plain and demonstrates how simple chemical–age relationships, such as zircon Th/U values coupled with U-Pb ages, can be used to evaluate sediment provenance.

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Røhr, Torkil S., Tom Andersen, Henning Dypvik et Ashton F. Embry. « Detrital zircon characteristics of the Lower Cretaceous Isachsen Formation, Sverdrup Basin : source constraints from age and Hf isotope data ». Canadian Journal of Earth Sciences 47, n^o 3 (mars 2010) : 255–71. http://dx.doi.org/10.1139/e10-006.

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Detrital zircons from the Lower Cretaceous Isachsen Formation of the Sverdrup Basin, Canadian Arctic Archipelago, have been dated by the U–Pb method and analyzed for Hf isotopes by laser ablation microprobe – inductively coupled plasma – mass spectrometry (LAM–ICP–MS). Five samples from four locations on Ellesmere and Axel Heiberg islands display similar ranges of U–Pb ages, with significant zircon populations at 2.8–2.6, 1.9–1.8, 1.7–1.6, and 1.2–1.0 Ga. Major hiatuses occur between 2.4 and 2.0 Ga and from 0.96 to 0.5 Ga. Low initial εHfvalues indicating recycled crust components are significant in Palaeoproterozoic (1.9–1.8 Ga) and Neoarchaean (2.8–2.6 Ga) zircon populations. Other U–Pb age populations in the studied samples are dominated by zircon with positive εHfvalues, indicating a significant contribution from mantle-derived protoliths. The εHfvalues seen within a given U–Pb age population are generally consistent, with only minor scatter among the different samples. U–Pb and Hf data closely resemble previously published data from Lower Cretaceous rocks in northern Greenland, suggesting they have the same origin. The data are also largely consistent with the East Greenland Caledonides and the Precambrian basement of Greenland and northern Canada as predominant sources of zircon for the studied sandstones. However, based on the level of similarity between data from the Wandel Sea Basin and Sverdrup Basin sediments and on previous Nd isotope work in the Sverdrup Basin, it is likely that the sediments represent redeposited lower and middle Palaeozoic sediments.

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SANTOSH, M., A. S. COLLINS, T. MORIMOTO et K. YOKOYAMA. « Depositional constraints and age of metamorphism in southern India : U–Pb chemical (EMPA) and isotopic (SIMS) ages from the Trivandrum Block ». Geological Magazine 142, n^o 3 (mai 2005) : 255–68. http://dx.doi.org/10.1017/s0016756805000506.

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We report U–Pb electron microprobe (zircon and monazite) and Secondary Ion Mass Spectrometry (SIMS) U–Pb (zircon) ages from a granulite-facies metapelite and a garnet–biotite gniess from Chittikara, a classic locality within the Trivandrum Block of southern India. The majority of the electron-microprobe data on zircons from the metapelite define apparent ages between 1500 and 2500 Ma with a prominent peak at 2109±22 Ma, although some of the cores are as old as 3070 Ma. Zircon grains with multiple age zoning are also detected with 2500–3700 Ma cores, 1380–1520 mantles and 530–600 Ma outer rims. Some homogeneous and rounded zircon cores yielded late Neoproterozoic ages that suggest that deposition within the Trivandrum Block belt was younger than 610 Ma. The outermost rims of these grains are characterized by early Cambrian ages suggesting metamorphic overgrowth at this time. The apparent ages of monazite grains from this locality reveal multiple provenance and polyphase metamorphic history, similar to those of the zircons. In a typical case, Palaeoproterozoic cores (1759–1967 Ma) are enveloped by late Neoproterozoic rims (562–563 Ma), which in turn are mantled by an outermost thin Cambrian rim (∼515 Ma). PbO v. ThO*2 plots for monazites define broad isochrons, with cores indicating a rather imprecise age of 1913±260 Ma (MSWD=0.80) and late Neoproterozoic/Cambrian cores as well as thin rims yielding a well-defined isochron with an age of 557±19 Ma (MSWD=0.82). SIMS U–Pb isotopic data on zircons from the garnet–biotite gneiss yield a combined core/rim imprecise discordia line between 2106±37 Ma and 524±150 Ma. The data indicate Palaeoproterozoic zircon formation with later partial or non-uniform Pb loss during the late Neoproterozoic/Cambrian tectonothermal event. The combined electron probe and SIMS data from the metapelite and garnet–biotite gneiss at Chittikara indicate that the older zircons preserved in the finer-grained metapelite protolith have heterogeneous detrital sources, whereas the more arenaceous protolith of the garnet–biotite gniess was sourced from a single-aged terrane. Our data suggest that the metasedimentary belts in southern India may have formed part of an extensive late Neoproterozoic sedimentary basin during the final amalgamation of the Gondwana supercontinent.

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Prytchin, M. E., E. I. Soroka et V. N. Puchkov. « Novel U-Pb isotopic zircon data on the rhyolite of the Saf’yanovskoe Cu-Zn deposit (Middle Urals) ». LITHOSPHERE (Russia) 21, n^o 6 (29 décembre 2021) : 884–93. http://dx.doi.org/10.24930/1681-9004-2021-21-6-884-893.

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Research subject. Zircons from the Saf’yanovskoe Cu-Zn deposit rhyolite (Middle Urals). For the first time, zircon U-Pb dating for the rhyolite of the ore-bearing volcanic-sedimentary rocks of the Saf’yanovskoe deposit was performed. The volcanites are characterized by an andesite-rhyodacite composition and are localized at the southern edge of the Rezhevskaya structural-formation zone (SFZ) of the Eastern Ural megazone. A number of publications assign these rocks either to the basalt-rhyolite formation of the Middle Devonian, or to the basalt-andesite-dacite-rhyolite formation of the Lower-Middle Devonian.Aim. To estimate the age of the ore-bearing volcanic rocks under study using the U-Pb SHRIMP-II isotop ic system of zircon from the rhyolite of the eastern side of the Saf’yanovskoe deposit. By its chemical composition, the rhyolite belongs to the silicic varieties of subvolcanic rocks. Methods and results. The U-Pb isotopic system of zircon was studied by 5-collector mass-spectrometer of high precision and emission of the secondary ions SHRIMP-II (ASI, Australia) in the VSЕGEI Institute. U-Pb relations were investigated by a procedure developed by I.S. Williams. The U-Pb data obtained based on 13 zircon grains showed the age of 422.8 ± 3.7 Ma. Conclusions. The U-Pb dating of zircon obtained previously from the lens-shaped andesite bodies of the western side of the Safyanovskoe deposit gave the age of 422.8 Ma, which corresponds to the Przydoli series epoch of the Upper Silurian. We established that, among the volcanic rocks of the Saf’yanovskoe deposit, the effusive formations of the Upper Silurian are present.

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WANG, JIALIN, CHAODONG WU, ZHUANG LI, WEN ZHU, TIANQI ZHOU, JUN WU et JUN WANG. « The tectonic evolution of the Bogda region from Late Carboniferous to Triassic time : evidence from detrital zircon U–Pb geochronology and sandstone petrography ». Geological Magazine 155, n^o 5 (16 janvier 2017) : 1063–88. http://dx.doi.org/10.1017/s0016756816001217.

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AbstractField-based mapping, sandstone petrology, palaeocurrent measurements and zircon cathodoluminescence images, as well as detrital zircon U–Pb geochronology were integrated to investigate the provenance of the Upper Carboniferous – Upper Triassic sedimentary rocks from the northern Bogda Mountains, and further to constrain their tectonic evolution. Variations in sandstone composition suggest that the Upper Carboniferous – Lower Triassic sediments displayed less sedimentary recycling than the Middle–Upper Triassic sediments. U–Pb isotopic dating using the LA-ICP-MS method on zircons from 12 sandstones exhibited similar zircon U–Pb age distribution patterns with major age groups at 360–320 Ma and 320–300 Ma, and with some grains giving ages of > 541 Ma, 541–360 Ma, 300–250 Ma and 250–200 Ma. Coupled with the compiled palaeocurrent data, the predominant sources were the Late Carboniferous volcanic rocks of the North Tianshan and Palaeozoic magmatic rocks of the Yili–Central Tianshan. There was also input from the Bogda Mountains in Middle–Late Triassic time. The comprehensive geological evidence indicates that the Upper Carboniferous – Lower Permian strata were probably deposited in an extensional context which was related to a rift or post-collision rather than arc-related setting. Conspicuously, the large range of U–Pb ages of the detrital zircons, increased sedimentary lithic fragments, fluvial deposits and contemporaneous Triassic zircon ages argue for a Middle–Late Triassic orogenic movement, which was considered to be the initial uplift of the Bogda Mountains.

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Breemen, O. van, et L. Corriveau. « UPb age constraints on arenaceous and volcanic rocks of the Wakeham Group, eastern Grenville Province ». Canadian Journal of Earth Sciences 42, n^o 10 (1 octobre 2005) : 1677–97. http://dx.doi.org/10.1139/e05-079.

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Combined sensitive high-resolution ion microprobe (SHRIMP) and thermal ionization mass spectrometry (TIMS) UPb zircon data from a tightly constrained stratigraphic context of the Wakeham Group provide a precise depositional age for sedimentation within this extensive basin of the Grenville Province. Metavolcanic rocks at the eastern exposure of the Wakeham Group yield ages of 1511 ± 13, 1506 ± 11, 1502 ± 9, and 1491 ± 7 Ma. A crosscutting 1493 ± 10 Ma porphyry vein marks the end of volcanism. The older two volcanic rocks rest stratigraphically above metasediments, with a 1517 ± 20 Ma maximum age of sedimentation derived from the youngest detrital zircons of an arenite. Five 1.611.55 Ga inherited zircons in the volcanics, reinforced by coeval inheritance in nearby plutons, indicate a Labradorian basement source to the supracrustals. The predominant arenite detrital zircons dates are in the 1.951.75 Ga range, however, and feature both trace element and morphological evidence for metamorphism in the source terrane. Together with zircons as old as 2.95 Ga, the detrital age spectrum is consistent with a circum-Superior provenance. The ages obtained imply that Wakeham Group volcanism and sedimentation were, at least in part, coeval with the onset of 1.521.46 Ga Pinwarian plutonism along the southeastern margin of Laurentia. UPb zircon analyses record a late Grenvillian metamorphic event around 1019 Ma. UPb monazite analyses from one sample yield 10101000 Ma ages, and the end of Grenvillian metamorphism is marked by 990 Ma UPb titanite ages.

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Leary, Ryan J., M. Elliot Smith et Paul Umhoefer. « Mixed eolian–longshore sediment transport in the late Paleozoic Arizona shelf and Pedregosa basin, U.S.A. : A case study in grain-size analysis of detrital-zircon datasets ». Journal of Sedimentary Research 92, n^o 8 (22 août 2022) : 676–94. http://dx.doi.org/10.2110/jsr.2021.101.

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ABSTRACT Detrital-zircon U–Pb geochronology has revolutionized sediment provenance studies over the last two decades, and zircon has been successfully analyzed from nearly all sedimentary lithologies, depositional environments, and sediment grain sizes. However, despite the ubiquity of this method and the far-reaching interpretations supported by detrital-zircon data, few studies have investigated the potential role of zircon grain size on age spectra and provenance interpretation. In this study, we investigate the connections between sample grain size, zircon grain size, U–Pb age spectra, and interpreted provenance using 18 detrital-zircon samples (4999 individual grains) collected from Pennsylvanian–Permian strata in central and southern Arizona, USA. In these samples, there is no clear correlation between sample grain size and zircon grain size and no clear correlation between sample grain size and age spectra. However, when all grains are grouped by zircon minimum long-axis dimension, the abundance of some age groups is correlated to zircon grain size. In Pennsylvanian samples, < 400 Ma grains and 2500–3000 Ma zircons are more abundant in the finer fractions, and 1400–1900 Ma zircons are more abundant in coarser fractions of both Pennsylvanian and Permian samples. In Permian samples, 500–800 Ma zircons are most abundant in the finer fractions, and 2500–3000 Ma grains are concentrated in the coarser fractions. Based on changes in abundance and grain-size distribution of 500–800 Ma grains, we interpret a change in zircon provenance across the Pennsylvanian–Permian boundary that reflects regional climate and paleogeographic changes driven in part by the northward drift of Laurentia across the equator. Specifically, we interpret the concentration of 500–800 Ma zircons in Permian samples in central and southern Arizona to indicate that these grains were: 1) sourced from Gondwana, 2) deposited in, and subsequently eroded (recycled) from, Mississippian–Pennsylvanian strata in the Arkoma, Anadarko, and Fort Worth basins at the margins of Laurentia, and 3) finally transported into the Arizona study area as loess by easterly trade winds. This study serves as a case study in the value and interpretive power of basic grain-size characterization of detrital-geochronology datasets.

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ZECK, H. P., M. J. WHITEHOUSE et J. M. UGIDOS. « 496 ± 3 Ma zircon ion microprobe age for pre-Hercynian granite, Central Iberian Zone, NE Portugal (earlier claimed 618 ± 9 Ma) ». Geological Magazine 144, n^o 1 (27 octobre 2006) : 21–31. http://dx.doi.org/10.1017/s0016756806002718.

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Ion microprobe (SIMS) dating of zircon from the Miranda do Douro orthogneiss, Central Iberian Zone, Hercynian Iberian belt, defines an Early Ordovician U–Pb age of 496.0 ± 2.6 Ma (95 % conf., MSWD = 1.14) for magmatic zircon crystallization in its granitic protolith. The age contrasts with an earlier, conventional (ID-TIMS) U–Pb zircon age of 618 ± 9(95 % conf.) Ma, now thought to be an artefact of the complex zircon population. Individual SIMS ages for zircon from the rock range from 2700 to 180 Ma and comprise inherited and magmatic zircon, both concordant and common Pb-enriched, and younger, reset ages. The ID-TIMS study seems to have misinterpreted this heterogeneous population as a cogenetic suite consisting of magmatic zircon and its age-reset equivalents produced by recent Pb loss. The 496 ± 3 Ma SIMS age represents the weighted average for 26 magmatic zircon domains located by careful inspection of cathodoluminescence, secondary electron and optical microscopy images of ∼ 700 individual zircon crystals. Inherited zircon is widespread, ranging in age from 2700 to 550 Ma, with age clusters, which are statistically indistinguishable from those known from pre-Hercynian granitic basement material elsewhere in the Central Iberian Zone. Including the present age information, 582 ± 4 Ma (95 % conf., MSWD = 1.02, n = 13) and 619 ± 9 Ma (95 % conf., MSWD = 0.93, n = 7) appear as prevailing inherited zircon age components in basement intrusions in the Central Iberian Zone.

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Anfinson, Owen A., Andrew L. Leier, Rich Gaschnig, Ashton F. Embry et Keith Dewing. « U–Pb and Hf isotopic data from Franklinian Basin strata : insights into the nature of Crockerland and the timing of accretion, Canadian Arctic Islands ». Canadian Journal of Earth Sciences 49, n^o 11 (novembre 2012) : 1316–28. http://dx.doi.org/10.1139/e2012-067.

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New detrital zircon uranium–lead (U–Pb) ages and initial epsilon hafnium (εHf(i)) data from the Devonian clastic succession of the Canadian Arctic Islands refines the provenance of strata within the Franklinian Basin and provides constraints on the geologic evolution of the landmass responsible for the Ellesmerian Orogen. This study contributes more than 500 U–Pb ages and 32 εHf(i) values from the Blackley Formation and the Parry Islands Formation. The Middle Devonian Blackley Formation represents the onset of clastic sedimentation into the Franklinian Basin during the Devonian period. Detrital zircon from two samples yield U–Pb age populations of 380–470, 500–700, 900–2100, and 2550–3000 Ma. The population of 500–700 Ma U–Pb ages indicates a source exotic to the northern Laurentian margin and is attributed to a continental landmass located north of the present Canadian Arctic Islands (often referred to as Crockerland). This is some of the earliest evidence of 500–700 Ma detrital zircon deposition onto the northern Laurentian margin and indicates this northern landmass is at least partially accreted to Laurentia by early-Eifelian time. The Late Devonian Parry Islands Formation is the uppermost succession of Ellesmerian Orogen foreland basin sedimentation in the Franklinian Basin. Detrital zircon from four samples yield U–Pb age populations of 370–450, 470–750, 930–2100, and 2300–3200 Ma. The U–Pb ages suggest the Parry Islands Formation is derived from the northern source terrane (Crockerland) and indicate this landmass contains rocks similar to that of the east Greenland Caledonides, Pearya, and northeastern Baltica. Rim and core U–Pb double dates from the 500–700 Ma detrital zircon population and εHf(i) values from the 380–450, 520–550, and 650–710 Ma detrital zircon populations help constrain magma generation processes within Crockerland and suggest the zircons are derived from a juvenile lithosphere.

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Takehara, Lucy, Farid Chemale Júnior, Léo A. Hartmann, Ivo A. Dussin et Koji Kawashita. « U-Pb dating by zircon dissolution method using chemical abrasion ». Anais da Academia Brasileira de Ciências 84, n^o 2 (juin 2012) : 399–405. http://dx.doi.org/10.1590/s0001-37652012000200011.

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Chemical abrasion was carried out on zircons grains of the Temora II standard for U-Pb dating prior to analyses using in situ Laser Ablation-MultiCollector Ion Coupled Plasma Mass Spectrometer (LA-ICPMS) followed by the Isotope Dissolution Thermal Ionization Mass Spectrometer (ID-TIMS) method. The proposed methodology was herein applied in order to reduce primarily the effects of secondary Pb loss, the presence of common lead and/or silicate impurities. Nine Temora II zircon grains were analyzed by the laser ablation method yielding an age of 418.3±4.3 Ma. Zircon grains of a same population were separated for chemical abrasion before dissolution and mass spectrometry analyses. Six fractions of them were separated for isotope dissolution using 235U-205Pb mixed spike after we have checked and assured the laboratory conditions of low blank values for total Pb of less than 2 pg/g. The obtained U-Pb zircon age by the ID-TIMS method was 415.7±1.8 Ma (error 0.43 %) based on four successful determinations. The results are consistent with the published ages for the Temora diorite (Temora I â€“ 416.75±1.3 Ma; Temora II â€“ 416.78±0.33 Ma) and established as 416±0.33 Ma. The technique is thus recommended for high precision U-Pb zircon analyses (error < 1 %), mainly for high resolution stratigraphic studies of Phanerozoic sequences.

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Steshenko, Ekaterina N., Tamara B. Bayanova et Pavel A. Serov. « The Paleoproterozoic Kandalaksha-Kolvitsa Gabbro-Anorthosite Complex (Fennoscandian Shield) : New U–Pb, Sm–Nd, and Nd–Sr (ID-TIMS) Isotope Data on the Age of Formation, Metamorphism, and Geochemical Features of Zircon (LA-ICP-MS) ». Minerals 10, n^o 3 (10 mars 2020) : 254. http://dx.doi.org/10.3390/min10030254.

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The paper provides new U–Pb, Sm–Nd, and Nd–Sr isotope-geochronological data on rocks of the Paleoproterozoic Kandalaksha-Kolvitsa gabbro-anorthosite complex. Rare earth element (REE) contents in zircons from basic rock varieties of the Kandalaksha-Kolvitsa area were analyzed in situ using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Plots of REE distribution were constructed, confirming the magmatic origin of zircon. Temperatures of zircon crystallization were estimated using a Ti-in-zircon geochronometer. The U–Pb method with a 205Pb artificial tracer was first applied to date single zircon grains (2448 ± 5 Ma) from metagabbro of the Kolvitsa massif. The U–Pb analysis of zircon from anorthosites of the Kandalaksha massif dated the early stage of the granulite metamorphism at 2230 ± 10 Ma. The Sm–Nd isotope age was estimated on metamorphic minerals (apatite, garnet, sulfides) and whole rock at 1985 ± 17 Ma (granulite metamorphism) for the Kolvitsa massif and at 1887 ± 37 Ma (high-temperature metasomatic transformations) and 1692 ± 71 Ma (regional fluid reworking) for the Kandalaksha massif. The Sm–Nd model age of metagabbro was 3.3 Ga with a negative value of εNd = 4.6, which corresponds with either processes of crustal contamination or primary enriched mantle reservoir of primary magmas.

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Sun, Xilin, Klaudia F. Kuiper, Yuntao Tian, Chang’an Li, Zengjie Zhang et Jan R. Wijbrans. « Comparison of Detrital Zircon U-Pb and Muscovite 40Ar/39Ar Ages in the Yangtze Sediment : Implications for Provenance Studies ». Minerals 10, n^o 7 (20 juillet 2020) : 643. http://dx.doi.org/10.3390/min10070643.

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Detrital zircon U-Pb and muscovite 40Ar/39Ar dating are useful tools for investigating sediment provenance and regional tectonic histories. However, the two types of data from same sample do not necessarily give consistent results. Here, we compare published detrital muscovite 40Ar/39Ar and zircon U-Pb ages of modern sands from the Yangtze River to reveal potential factors controlling differences in their provenance age signals. Detrital muscovite 40Ar/39Ar ages of the major tributaries and main trunk suggest that the Dadu River is a dominant sediment contributor to the lower Yangtze. However, detrital zircon data suggest that the Yalong, Dadu, and Min rivers are the most important sediment suppliers. This difference could be caused by combined effects of lower reaches dilution, laser spot location on zircons and difference in closure temperature and durability between muscovite and zircon. The bias caused by sediment laser spot targeting a core or rim of zircon and zircon reworking should be considered in provenance studies.

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Corfu, F., T. E. Krogh et L. D. Ayres. « U–Pb zircon and sphene geochronology of a composite Archean granitoid batholith, Favourable Lake area, northwestern Ontario ». Canadian Journal of Earth Sciences 22, n^o 10 (1 octobre 1985) : 1436–51. http://dx.doi.org/10.1139/e85-150.

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U–Pb zircon and sphene data for several phases of composite batholiths in the Favourable Lake area of the Superior Province indicate two major periods of plutonism separated by a hiatus of over 200 Ma.A trondhjemite, which now forms a metamorphosed remnant within a batholith, was emplaced 2950 ± 5 Ma ago during an early plutonic event. The second and dominant plutonic period occurred between [Formula: see text] and 2711.0 ± 2.0 Ma ago; these are the ages of the earliest and the latest widespread phase of the batholiths, respectively. Ages of 2716.3 ± 1.4 and 2716 ± 4 Ma for zircons of two minor dioritic phases also fall within this interval. Metamorphic zircons about 2730 and 2715 Ma old from another unit of the batholith indicate a temporal correlation between metamorphism and main plutonic pulses.Sphenes from two samples near the margins of the batholiths yield the same age of 2711 ± 2 Ma as coexisting zircons and date the primary crystallization of the rocks. In contrast, sphenes from six samples from the interior of a batholith yield ages of 2680 ± 10 Ma, which are significantly younger than primary ages of 2950–2716 Ma of coexisting zircons. These sphene ages probably record cooling below about 500 °C during a long and complex cooling process.Several of the studied zircon populations exhibit complex discordance patterns reflecting multiple stages of Pb loss. Zircons in rocks predating the 2700–2730 Ma old metamorphic–plutonic event experienced Pb loss during this event, probably by annealing. A second low-grade event apparently caused chemical alteration of high-U zircon domains and Pb loss about 1750 Ma ago. A late stage of Pb loss affected near-surface zircon domains about 600–0 Ma ago.

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Skublov, Sergey G., Natalia A. Rumyantseva, Qiuli Li, Boris G. Vanshtein, Dmitriy I. Rezvukhin et Xianhua Li. « Zircon Xenocrysts from the Shaka Ridge Record Ancient Continental Crust : New U-Pb Geochronological and Oxygen Isotopic Data ». Journal of Earth Science 33, n^o 1 (février 2022) : 5–16. http://dx.doi.org/10.1007/s12583-021-1422-2.

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AbstractOver the past two decades, a significant number of discoveries of ancient zircon xenocrysts in ocean-floor magmatic rocks have been reported. These findings provide compelling evidence for the presence of ancient continental crust within young oceanic lithosphere. Almost all finds of ancient zircon xenocrysts within oceanic crust are from the Mid-Atlantic Ridge. For other localities, however, similar data are very limited. This report presents the first age determinations (U-Pb, SHRIMP-II) and isotope-geochemical data (oxygen, trace and rare earth elements) for zircon xenocrysts from gabbro-diorites of the Shaka Ridge, in the vicinity of the Shaka fracture zone, near the western end of the Southwest Indian Ridge. The work is based on a study of bottom rock material dredged during expeditionary research on the R/V “Akademik Fedorov” (Russia) in 2016. The U-Pb isotope system of the zircon xenocrysts recorded a crystallization age of ∼2.8 Ga at an upper discordia intercept and an age of ∼600 Ma interpreted as the timing of a superimposed thermal event at a lower discordia intercept. The zircon xenocrysts show geochemical signatures of magmatic origin, i.e., fractionated REE distribution spectra with an increase in chondrite-normalized values from light to heavy REE, positive Ce anomalies and negative Eu anomalies, and high Th/U ratios (0.59–7.77). In discrimination diagrams based on a series of inter-element relationships, zircon compositions fall into the fields of zircons from rocks of continental crust, mostly granitoids. The Li content of the zircons is high (1.8 ppm–50 ppm), adding further evidence to their derivation from rocks of continental crust. During their residence within young oceanic crust, the zircon xenocrysts experienced alterations under the influence of submarine high-temperature hydrothermal fluids, which selectively affected the distribution of trace elements in the zircons and reduced the δ18O values to 1.75‰–3.15‰. The presence of obviously older zircons in Shaka gabbro-diorites clearly demonstrates the presence of ancient continental fragments and their recycling into the mantle at the western end of the Southwest Indian Ridge.

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Xie, Lie-Wen, Jin-Hui Yang, Qing-Zhu Yin, Yue-Heng Yang, Jing-Bo Liu et Chao Huang. « High spatial resolution in situ U–Pb dating using laser ablation multiple ion counting inductively coupled plasma mass spectrometry (LA-MIC-ICP-MS) ». Journal of Analytical Atomic Spectrometry 32, n^o 5 (2017) : 975–86. http://dx.doi.org/10.1039/c6ja00387g.

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A new LA-MIC-ICP-MS analytical technique has been developed for the rapid measurement of ²⁰⁶Pb/²³⁸U zircon age (<1%, 2s) at a high spatial resolution. We show that this technique can be routinely employed to date U–Pb in small and/or complex zircons, providing a powerful tool for geochronology.

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Taylor, Richard J. M., Tim E. Johnson, Chris Clark et Richard J. Harrison. « Persistence of melt-bearing Archean lower crust for >200 m.y.—An example from the Lewisian Complex, northwest Scotland ». Geology 48, n^o 3 (17 décembre 2019) : 221–25. http://dx.doi.org/10.1130/g46834.1.

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Abstract Geochronological data from zircon in Archean tonalite–trondhjemite–granodiorite (TTG) gneisses are commonly difficult to interpret. A notable example is the TTG gneisses from the Lewisian Gneiss Complex, northwest Scotland, which have metamorphic zircon ages that define a more-or-less continuous spread through the Neoarchean, with no clear relationship to zircon textures. These data are generally interpreted to record discrete high-grade events at ca. 2.7 Ga and ca. 2.5 Ga, with intermediate ages reflecting variable Pb loss. Although ancient diffusion of Pb is commonly invoked to explain such protracted age spreads, trace-element data in zircon may permit identification of otherwise cryptic magmatic and metamorphic episodes. Although zircons from the TTG gneiss analyzed here show a characteristic spread of Neoarchean ages, they exhibit subtle but key step changes in trace-element compositions that are difficult to ascribe to diffusive resetting, but that are consistent with emplacement of regionally extensive bodies of mafic magma. These data suggest suprasolidus metamorphic temperatures persisted for 200 m.y. or more during the Neoarchean. Such long-lived high-grade metamorphism is supported by data from zircon grains from a nearby monzogranite sheet. These preserve distinctive trace-element compositions consistent with derivation from a mafic source, and they define a well-constrained U-Pb zircon age of ca. 2.6 Ga that is intermediate between the two previously proposed discrete metamorphic episodes. The persistence of melt-bearing lower crust for hundreds of millions of years was probably the norm during the Archean.

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LIU, YU, KUNGUANG YANG, ALI POLAT et XIAO MA. « Reconstruction of the Cryogenian palaeogeography in the Yangtze Domain : constraints from detrital age patterns ». Geological Magazine 156, n^o 07 (20 août 2018) : 1247–64. http://dx.doi.org/10.1017/s0016756818000535.

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AbstractDetrital zircons are often used to constrain the maximum sedimentary age of strata and sedimentary provenance. This study aimed at reconstructing the Cryogenian palaeogeography of the Yangtze Domain based on U–Pb ages and Lu–Hf isotopic signatures of detrital zircons from sandstones in the southeastern part of the Yangtze Domain. U–Pb ages of the youngest detrital zircon grains from the Niuguping, Gucheng and Datangpo formations yielded average ages of 712±24 Ma, 679.2±6.2 Ma and 665.1±7.4 Ma, respectively, which are close to the depositional ages of their respective formations. An integrated study of detrital zircon Lu–Hf isotopes and U–Pb ages from three samples revealed six main peak ages in the samples from the Anhua section atc. 680 Ma,c. 780 Ma,c. 820 Ma,c. 940 Ma,c. 2000 Ma andc. 2500 Ma. The characteristics of the U–Pb ages and Hf isotopes indicate a link between the north and southeast margins of the Yangtze Domain as early asc. 680 Ma, and the provenance of the coeval sedimentary sequences in the SE Yangtze Domain was the South Qinling Block on the northern margin of the Yangtze Domain. The provenance analysis on thec. 680 Ma detritus composing upper Neoproterozoic strata in the Yangtze Domain revealed that the detritus was transported southward from South Qinling to the southeast margin of the Yangtze Domain through the Exi Strait, but was hindered by the Jiangnan Orogenic Belt.

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Dinh, Sang Quang. « PETROGRAPHIC CHARACTERISTICS AND ZIRCON U-PB GEOCHRONOLOGY OF GRANITOID ROCKS IN THE SOUTHERN BÊN GIANG, QUNG NAM PROVINCE ». Science and Technology Development Journal 14, n^o 4 (30 décembre 2011) : 17–30. http://dx.doi.org/10.32508/stdj.v14i4.2024.

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The early Palaeozoic calc-alkaline granitoid association in south Nam Giang town is forming large area reaching hundreds of km2, along the east – west ductile deformation zones, which is studied in detail in diffirent geologic maps scales by the geologists (Huynh Trung and Nguyen Xuan Bao, 1981 at 1:500,000 scale or Nguyen Van Trang et al, 1986, at 1:200,000 scale). The six samples were studied in details, which are composed mainly quartz diorite and granodiorite. The samples were crushed and large zircons were extracted. The laser ablation microprobe-inductively coupled plasma mass spectrometer (LA-ICP MS) U-Pb zircon dating was carried out by standard methods at CODES, University of Tasmania-Australia. In-situ zircon U–Pb geochronology was conducted on six samples of age between 306 Ma and 278 Ma. A total of seventytwo zircons were analysed and the results exist a narrow spread in individual zircon ages between ca. 269 and 313 Ma, with significant form a coherent single age suite of 295 Ma (early Permian). As the six samples are all assigned to the same granitic suite, I have combined the individual isotopic data yields a weighted mean age from 72 analyses of 294 Ma, which represents the minimum age for the crystallisation of the south Ben Giang granite.

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Peterman, Emily M., Steven M. Reddy, David W. Saxey, David R. Snoeyenbos, William D. A. Rickard, Denis Fougerouse et Andrew R. C. Kylander-Clark. « Nanogeochronology of discordant zircon measured by atom probe microscopy of Pb-enriched dislocation loops ». Science Advances 2, n^o 9 (septembre 2016) : e1601318. http://dx.doi.org/10.1126/sciadv.1601318.

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Isotopic discordance is a common feature in zircon that can lead to an erroneous age determination, and it is attributed to the mobilization and escape of radiogenic Pb during its post-crystallization geological evolution. The degree of isotopic discordance measured at analytical scales of ~10 μm often differs among adjacent analysis locations, indicating heterogeneous distributions of Pb at shorter length scales. We use atom probe microscopy to establish the nature of these sites and the mechanisms by which they form. We show that the nanoscale distribution of Pb in a ~2.1 billion year old discordant zircon that was metamorphosed c. 150 million years ago is defined by two distinct Pb reservoirs. Despite overall Pb loss during peak metamorphic conditions, the atom probe data indicate that a component of radiogenic Pb was trapped in 10-nm dislocation loops that formed during the annealing of radiation damage associated with the metamorphic event. A second Pb component, found outside the dislocation loops, represents homogeneous accumulation of radiogenic Pb in the zircon matrix after metamorphism. The207Pb/206Pb ratios measured from eight dislocation loops are equivalent within uncertainty and yield an age consistent with the original crystallization age of the zircon, as determined by laser ablation spot analysis. Our results provide a specific mechanism for the trapping and retention of radiogenic Pb during metamorphism and confirm that isotopic discordance in this zircon is characterized by discrete nanoscale reservoirs of Pb that record different isotopic compositions and yield age data consistent with distinct geological events. These data may provide a framework for interpreting discordance in zircon as the heterogeneous distribution of discrete radiogenic Pb populations, each yielding geologically meaningful ages.

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Chen, Xiao-Dong, Bin Li, Chong-Bo Sun et Hong-Bing Zhou. « Protracted Storage for Calc-Alkaline Andesitic Magma in Magma Chambers : Perspective from the Nageng Andesite, East Kunlun Orogen, NW China ». Minerals 11, n^o 2 (13 février 2021) : 198. http://dx.doi.org/10.3390/min11020198.

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Calc-alkaline andesitic rocks are a major product of subduction-related magmatism at convergent margins. Where these melts are originated, how long they are stored in the magma chambers, and how they evolved is still a matter of debate. In this study, we present new data of whole-rock elemental and Sr-Nd-Pb isotope compositions, and zircon U-Pb-Th isotopes and trace element contents of Nageng (basaltic-)andesites in the East Kunlun Orogen (NW China). The similar age and whole-rock elemental and Sr-Nd-Pb isotope contents suggest that the Nageng andesite and basaltic andesite are co-magmatic. Their low initial 87Sr/86Sr (0.7084–0.7086) but negative εNd(t) values (−10.61 to −9.49) are consistent with a magma source from the juvenile mafic lower crust, possibly related to the mantle wedge with recycled sediment input. The U-Pb age gap between the zircon core (ca. 248 Ma) and rim (ca. 240 Ma) reveals a protracted magma storage (~8 Myr) prior to the volcanic eruption. When compared to the zircon rims, the zircon cores have higher Ti content and Zr/Hf and Nb/Ta ratios, but lower Hf content and light/heavy rare earth element ratios, which suggests that the parental magma was hotter and less evolved than the basaltic andesite. The plagioclase accumulation likely resulted in Al2O3-enrichment and Fe-depletion, forming the calc-alkaline signature of the Nageng (basaltic-)andesites. The magma temperature, as indicated by the zircon saturation and Ti-in-zircon thermometry, remained low (725–828 °C), and allowed for the magma chamber to survive over ~8 Myr. The decreasing εHf(t) values from zircon core (avg. 0.21, range: −1.28 to 1.32) to rim (avg. −3.68, range: −7.30 to −1.13), together with the presence of some very old xenocrystic zircons (268–856 Ma), suggest that the magma chamber had undergone extensive crustal contamination.

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Young, David N., et Lance P. Black. « U-Pb zircon dating of Proterozoic igneous charnockites from the Mawson Coast, East Antarctica ». Antarctic Science 3, n^o 2 (juin 1991) : 205–16. http://dx.doi.org/10.1017/s095410209100024x.

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We report ion-microprobe U-Pb zircon ages from charnockites of a large Proterozoic composite batholith, Mawson Coast, Australian Antarctic Territory. The charnockites crystallized from orogenic magmas of intermediate composition (mainly 54–68% SiO2) intruded into a granulite-facies metasedimentary gneiss sequence between the second and third recognized deformations. A sample of low-Ti charnockite provides an age of 954 ± 12 Ma and a high-Ti charnockite is dated at 985 ± 29 Ma (all ages quoted at 95% confidence). The age difference is not significant at the 95% confidence level. Both these ages were obtained from zircons with igneous zoning and/or morphology and thus are thought to date igneous crystallization. Zircons from a felsic gneiss xenolith within the charnockite have cores of various ages, many from 1.7 to 2.0 Ga, but with other grains between 1.0 and 1.5 Ga and a single 2.5 Ga zircon. These zircon cores are direct evidence for an early to middle Proterozoic age for the supracrustal basement sequence in this mobile belt. Many of these zircon cores are concordant but abundant discordant grains suggest a complex history of multiple Pb-loss events. Zircon rims grew at 921 ± 19 Ma, probably during the post-charnockite deformation (D3). Previously obtained Rb-Sr dates for charnockite of 886 ± 48 Ma and 910 ± 18 Ma were probably also rest during D3. A Rb-Sr isochron date of 1061 ± 36 Ma previously reported for high-Ti charnockite from Mawson Rock is thought to be erroneous, and a new date of 959 ± 58 Ma (consistent with both the igneous and reset dates above) is interpreted from those data.

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Corfu, F., et J. Wood. « U–Pb zircon ages in supracrustal and plutonic rocks ; North Spirit Lake area, Northwestern Ontario ». Canadian Journal of Earth Sciences 23, n^o 7 (1 juillet 1986) : 967–77. http://dx.doi.org/10.1139/e86-098.

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The North Spirit Lake greenstone belt in the Sachigo Subprovince of the Superior Province comprises parts of three sequences of volcanic and sedimentary rocks; the main (youngest) is separated from the older sequences by an unconformity to disconformity. The belt is bounded by large granitoid batholiths and was metamorphosed under greenschist to, locally, hornblende–cordierite facies. U–Pb zircon dating was performed on volcanic, sedimentary, and plutonic rocks in order to establish an absolute chronology for the evolution of the area.A tuff breccia in the lowermost supracrustal sequence is dated at 3023 ± 2 Ma. Zircons from a tuff in the middle sequence show complex U–Pb relationships; although they do not allow a precise age determination, the data suggest that the tuff formed sometime between 2950 and 2800 Ma ago. Zircons from a quartz arenite, also in the middle sequence, yield a simple data pattern and define an age of [Formula: see text], suggesting derivation of the sediment from a uniform source of this age. A zircon analysis from a tonalitic clast in a conglomerate at the base of the upper sequence yields a minimum age of 2975 Ma. The clast may have had the same origin as the zircons in the quartz arenite. Another tonalitic clast from the same conglomerate yields a slightly older age of 3001 ± 3 Ma. No remnant of these tonalites can be recognized in the field, suggesting that they have been largely removed during subsequent erosional processes.Two subvolcanic intrusions from the upper sequence yield zircon ages of 2743 ± 2 and 2731 ± 2 Ma, respectively. A crystal tuff, also in the upper sequence, contains two generations of zircons: newly formed magmatic zircons, which date the extrusion of the tuff at 2735 ± 10 Ma, and older grains with a minimum age of 2862 Ma, which represent inherited zircons. This tuff was thus generated at least in part by anatectic melting of >2862 Ma crust.A quartz diorite from MacDowell Lake in the adjacent Berens River Subprovince yields an age of 2744 ± 2 Ma. A mafic inclusion in the quartz diorite contains amoeboidal, strongly fractured zircons, which point to an intercept age of about 2727 Ma. This age could either reflect a metamorphic event or represent a geologically meaningless mixed age.

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van Schijndel, V., G. Stevens, C. Lana, T. Zack et D. Frei. « De Kraalen and Witrivier Greenstone Belts, Kaapvaal Craton, South Africa : Characterisation of the Palaeo-Mesoarchaean evolution by rutile and zircon U-Pb geochronology combined with Hf isotopes ». South African Journal of Geology 124, n^o 1 (1 mars 2021) : 17–36. http://dx.doi.org/10.25131/sajg.124.0011.

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Abstract The formation and evolution of Palaeoarchaean De Kraalen and Witrivier Greenstone Belts (DKGB and WGB) of the Kaapvaal Craton are poorly known. Here we report zircon and rutile in situ U-Pb ages and zircon Hf isotopic data from a variety of supracrustal rocks. The zircon cores from a metamafic amphibole-bearing gneiss from the DKGB give a protolith age of 3 441 ± 5 Ma, whereas the zircon mantle domains give a metamorphic age of 3 211 ± 16 Ma. The 176Hf/177Hft values for all zircon domains give a tight cluster around 0.280596 ± 0.00006 (2 SD). U-Pb analyses of zircon for an amphibolite intercalated with thin calc-silicate layers from the WGB give a single crystallisation age of 3 230 ± 3 Ma, but the Hf isotope ratios of these zircon grains define two different populations. The first population yields 176Hf/177Hf~3.23 Ga = 0.28064 ± 0.00004, corresponding to εHf~3.23 Ga = 2.4 ± 1.9 (2SD) and Hf model ages between ca. 3.51 to 3.30 Ga. These are Hf isotope characteristics for zircons from a relatively juvenile source extracted from a depleted mantle source ca. 0.28 to 0.07 Ga prior zircon crystallisation. The second population yields 176Hf/177Hf~3.23 Ga = 0.28093 ± 0.00004 with εHf~3.23 Ga = 8.1 ± 1.3 (2SD). These Hf data combined with the 206Pb/207Pb ages lead to isotope ratios that lie above those of Depleted Mantle. The unusually high Hf isotope signature for the cores of the zircons from the WGB amphibolite most likely represent a contribution from an early highly depleted mantle source. A rutile in situ U-Pb age of 3.085 Ga from a recrystallised quartzite indicate that the rocks from the DKGB experienced slow cooling following the 3.21 Ga metamorphic event or (partial) resetting due to elevated geothermal gradient caused by the ca. 3.1 Ga intrusions of the Vrede Granitiod Suite. The latter interpretation is preferred because ~145 Ma of slow cooling from the amphibolite facies conditions of peak metamorphism to the blocking temperature for mass diffusion of Pb in rutile is unlikely. While the Zr-in-rutile temperature of ca. 710°C at 7 kbar for DKGB most likely records the peak temperature of the ~3.23 to 3.21 Ga event. The trace element concentrations of the metamorphic rutile grains within the quartzite of the DKGB indicate that the source rock was enriched in Cr. Either due to silification during hydrothermal alteration of the (ultra)mafic country rock or during deposition in an atmosphere that allowed for chromite grains to be part of the sediment.

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Compston, W. « Interpretations of SHRIMP and isotope dilution zircon ages for the geological time-scale : I. The early Ordovician and late Cambrian ». Mineralogical Magazine 64, n^o 1 (février 2000) : 43–57. http://dx.doi.org/10.1180/002646100549120.

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AbstractIon probe data for zircons from tuffs within the Llfynant flags (Arenig) and the Serw Formation (lower Llanvirn) of north Wales have been revised using better statistical methods for separating detrital ages, making allowance for recently-found variability in radiogenic 206Pb/238U in the reference zircon SL13, and testing the sensitivity of the ages to the secondary ion discrimination slope. The revised ages are options of 469.2 ± 2.1 (σ) or 472.9 ± 2.9 Ma for the Llfynant flags dependent on mixture modelling, and 465.3 ± 1.4 Ma for the Serw Formation. All ages are within error of previous SHRIMP results and the Serw age now has the same numerical value as a previous MSID age for the same sample. It is shown that an MSID age of 483 ± 0.5 Ma with interpreted Pb loss for a late Tremadoc bentonite is dependent on the correction for common Pb, and that a slightly more radiogenic choice for the common Pb composition places nearly all data on Concordia. The latter would indicate that the bentonite might contain two zircon populations: inherited grains at 482 Ma and tuff magmatic grains at 473 Ma, which is more compatible with the SHRIMP Arenig result. Interpretations of other MSID zircon ages from the Ordovician are also sensitive to choice of common Pb, and raise the likelihood that many multigrain ages might be too old owing to admixture with slightly older inherited zircon. A supposed 1–2% technical bias of SHRIMP 206Pb/238U ages relative to MSID is refuted.

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Burda, Jolanta, Beata Woskowicz-Ślęzak, Urs Klötzli et Aleksandra Gawęda. « Cadomian protolith ages of exotic mega blocks from Bugaj and Andrychów (Western outer Carpathians, Poland) and their palaeogeographic significance ». Geochronometria 46, n^o 1 (22 février 2019) : 25–36. http://dx.doi.org/10.1515/geochr-2015-0102.

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Abstract This study presents the first zircon U-Pb LA-MC-ICP-MS ages and whole-rock Rb/Sr and Sm/Nd data from exotic blocks (Bugaj and Andrychów) from the Western Outer Carpathians (WOC) flysch. The CL images of the zircon crystals from both samples reveal typical magmatic textures characterized by a well-defined concentric and oscillatory growth zoning. A concordia age 580.1 ± 6.0 Ma of the zircons from the Bugaj sample is considered to represent the crystallization age of this granite. The zircon crystals from the Andrychów orthogneiss yield an age of 542 ± 21 Ma, interpreted as the uppermost Proterozoic, magmatic crystallization age of the granitoid protholith. The initial (at ca. 580 Ma) 87Sr/86Sr ratios of the Bugaj granitoids (0.72997 and 0.72874) are highly radiogenic, pointing to the assimilation of an older, possibly strongly Rb enriched source to the Bugaj melt. The Nd isotope systematics (εNd580 –1.4 and 0.4) also point to a significant contribution of such a distinct mantle source. On the basis of the sequence of magmatic events obtained from U-Pb zircon ages, we suggest that exotic mega blocks deposited to the WOC basins were related to the Brunovistulicum Terrane. They belong to the group of Vendian/Cambrian granitoids representing the latest, posttectonic expression of the Cadomian cycle.

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Bonev, Nikolay, Petyo Filipov, Raya Raicheva, Massimo Chiaradia et Robert Moritz. « Detrital zircon age and Sr isotopic constraints for a Late Palaeozoic carbonate platform in the lower Rhodope thrust system, Pirin, SW Bulgaria ». Geological Magazine 156, n^o 12 (29 octobre 2019) : 2117–24. http://dx.doi.org/10.1017/s0016756819001183.

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AbstractWe focused on the Pirin–Pangeon–Thasos carbonate sequence of the Rhodope thrust system, combining Sr isotopes from marble with U–Pb dating of detrital zircons from interlayered schists with outcrop near the villages of Ilindentsi and Petrovo in Bulgaria. The youngest zircon age at Ilindentsi is 266 Ma, i.e. Middle Permian, while the youngest zircon at Petrovo yielded an age of 290 Ma, i.e. Early Permian. Strontium isotopes range from 0.707420 to 0.707653, and are consistent with a Middle Permian maximum depositional age. Middle Permian sedimentation of this carbonate platform most likely occurred along the Eurasian margin rather than the Gondwana margin.

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Liu, Li, et Daniel F. Stockli. « U-Pb ages of detrital zircons in lower Permian sandstone and siltstone of the Permian Basin, west Texas, USA : Evidence of dominant Gondwanan and peri-Gondwanan sediment input to Laurentia ». GSA Bulletin 132, n^o 1-2 (23 mai 2019) : 245–62. http://dx.doi.org/10.1130/b35119.1.

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Abstract The Permian Basin of west Texas, one of the most economically significant hydrocarbon basins in the United States, formed along the southwest margin of Laurentia in the foreland of the Ouachita-Marathon orogen during the late Paleozoic. While its stratigraphic record temporally coincides with syn- and post-orogenic Ouachita-Marathon sedimentation, sediment provenance, sediment routing and dispersal, and paleo-drainage evolution have remained controversial. This study presents more than 2000 new detrital zircon U-Pb ages from 16 samples across the Permian Basin to elucidate early Permian sediment provenance and basin-fill evolution. The data show that Wolfcampian sandstones are dominated by 950–1070 Ma and 500–700 Ma detrital zircon U-Pb ages, whereas Leonardian sandstones and siltstones are dominated by 500–700 Ma and 280–480 Ma detrital zircon U-Pb ages. Most of these age clusters are not typical Laurentian basement ages, but rather indicative of a southern Gondwanan and peri-Gondwanan sources of Mexico and Central America. This interpretation is corroborated by zircons with peri-Gondwanan and Gondwanan rim-core relationships, as well as major age components of euhedral zircons, matching Maya block basement ages. Regional comparison of these new detrital zircon results with published data from Carboniferous and Permian sedimentary rocks in various terranes of Mexico and Central America, Appalachian foreland basins, Ouachita orogenic belt, midcontinent of United States, and Fort Worth Basin (Texas), indicates that most sediment influx to the Permian Basin during the early Permian (Wolfcampian and Leonardian) was derived from basement or recycled upper Paleozoic strata associated with Gondwanan and peri-Gondwanan terranes in modern Mexico and Central America. North American basements such as the Appalachian Grenville (950–1300 Ma), Granite-Rhyolite (1300–1500 Ma), and Yavapai-Mazatzal (1600–1800 Ma) provinces, appear to have provided only minor amounts of sediment. In light of depositional age constraints, the timing of Marathon-Ouachita collision, and careful detrital zircon U-Pb age spectra comparison, the sediment provenance shift from Wolfcampian to Leonardian points to a diachronous, oblique continent-continent collision between Gondwana/peri-Gondwanan terranes and Laurentia.

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Grozdev, Valentin, Rossitsa Vassileva et Irena Peytcheva. « The rare earth elements in zircons as pathfinder of geological events ». Review of the Bulgarian Geological Society 83, n^o 3 (décembre 2022) : 93–96. http://dx.doi.org/10.52215/rev.bgs.2022.83.3.93.

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The usual main purpose of the zircon investigation is to determine the precise age of particular rocks, applying the U-Pb isotope method. In this study we attempt to reveal the vast potential of zircons in understanding the magmatic and modification processes, as well as the structure of the deeper Earth’s interior, using the analysis of the zircon population of the major volcanic variety in the Ruen tectono-magmatic zone, Kyustendil area, SW Bulgaria. The ascending magmas crosscut different rock types and units along their path to the Earth’s surface. Using the zircon crystal analysis, we may make consideration about the age and chemical characteristics of these units. The trace and rare earth element (REE) content of the zircons brings insights for the rock formation time and discloses characteristics of the deeper parts of the Earth’s crust.

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STEPANYUK, L. M., L. V. SHUMLYANSKYY, S. I. KURYLO, V. O. SYOMKA, S. M. BONDARENKO, S. A. WILDE et A. A. NEMCHIN. « The U-Pb Zircon Geochronology (LA-ICP-MS) of Geological Processes in Granulites of Middle Bouh Area. Article 3. Rock association in the lower reaches of the Yatran river ». Mineralogical Journal 43, n^o 1 (2021) : 34–50. http://dx.doi.org/10.15407/mineraljournal.43.01.034.

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LA-ICP-MS method was applied to investigate U-Pb and Lu-Hf isotope systematics of zircon crystals from charnockitic gneiss and biotite-garnet-hypersthene enderbite that occur in the lower reaches of the Yatran river (Yatran block of the Bouh river area). According to the obtained isotope data, charnockitic gneiss hosts three zircon populations. The oldest one is represented by three crystals that have isotope age between 3125 and 3300 Ma, and εHf values between –2.3 and –7.5. The next population is well-defined, it has an age of 2038±25 Ma and large variations of Hf isotope composition: 176Hf/177Hf — from 0.28122 to 0.28261, εHf — from –9.3 до 4.6. However, the ages of most of the analyzed zircons spread along the concordia between 2300 and 2800 Ma. All zircons in this population have a similar Hf isotope composition 176Hf/177Hf = 0.28072 to 0.28092, which does not depend on the age. It is characteristic that the oldest (with preserved U-Pb isotope systematics) crystals have positive or slightly negative εHf values. Most of the U-Pb isotope analyses of zircons from enderbite fall on the discordia line that has an upper interception age of 2029 ± 18 Ma. A small number of discordant grains have 207Pb/206Pb ages up to 2500 Ma. Hafnium isotope composition in zircons from enderbite varies widely: 176Hf/177Hf = 0.28131 to 0.28151, and εHf from –6.2 to 1.8.

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Bortnikov, N. S., S. А. Silantiev, F. Bea, P. Montero, T. F. Zinger, S. G. Skolotnev et E. V. Sharkov. « U-Pb dating, oxygen and hafnium isotopic ratios of zircon from rocks of oceanic core complexes at Mid-Atlantic Ridge : evidence for an interaction of young and ancient crusts at spreading of the ocean floor ». Доклады Академии наук 489, n^o 5 (20 décembre 2019) : 483–89. http://dx.doi.org/10.31857/s0869-56524895483-489.

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U-Pb age, oxygen and hafnium isotopic ratios in zircon from rocks of ocean core complexes at Mid-Atlantic Ridge have been studied using SHRIMP and MC-LA-ICP-MS techniques. U-Pb dating revealed four group of zircons: 1) 0,6-1,7 Ma, 2) 6,7-11,2 Ma, 3) 12,9-17,6 Ma, 4) 200 to 2044 Ma. The 18O values range from 4,74 to 7,2 and are distinct for zircon grains of different ages. Hafnium isotopic ratio for zircon aged from 0,6 to 17,6 Ma corresponds or is close to that of MORB from Central Atlantic. The oxygen and hafnium isotopic compositions of zircon elder than 280 Ma correspond to those of the sialic continental crust. A hypothesis of involvement of the ancient pre-Atlantic sialic (280 млн лет) and old Atlantic (7-17 Ma) crusts in a generation of the contemporary (young) oceanic crust during formation of the slow-spreading Mid-Atlantic Ridge has been proposed.

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