Dating Of Zircon Grains Research Articles

The Mongol–Okhotsk Belt in Mongolia — An appraisal of the geodynamic development by the study of sandstone provenance and detrital zircons

The Mongol–Okhotsk Belt formed in a late stage of Jurassic orogeny in the composite Central Asian Orogenic Belt. The present paper investigates the Late Palaeozoic–Mesozoic sandstones associated with the belt in Mongolia, aimed at reconstructing the time and mode of ocean formation, subduction and collision. We apply provenance analysis including (1) heavy mineral and sandstone framework grain analysis, (2) U/Pb laser ablation ICP-MS dating of detrital zircons for identifying contemporaneous volcanic arc activity and the recycling of older crustal material, and (3) on dated zircon grains, we analyze trace element contents and Hf isotopic ratios in order to characterize the rock types in which they crystallized and the magma sources, respectively. The investigated samples are derived from (1) the Adaatsag and Doschgol terranes, which represent the suture zone, (2) the Hangai–Hentei belt to the northwest, and (3) the Ereendavaa terrane and the Middle Gobi volcanic belt to the southeast of the suture. The latter two are concurrent with the northern and southern margins of the Mongol–Okhotsk Ocean. Tectono-stratigraphic arguments suggest that the Mongol–Okhotsk ocean opened during the Silurian within the Early Palaeozoic collage. In the suture zone, Permian syn-sedimentary zircon trace element contents confirm mafic rock sources, and the mantle involvement in the magmatism (epsilon Hf(t) from + 13 up to + 20). N and S directed, bi-vergent subduction developed as revealed by contemporaneous zircons: (1) along the northern margin (Hangai–Hentei), from Silurian–Early Carboniferous, subduction and accretion prevailed (epsilon Hf(t) from + 3 up to + 12 in associated zircons), which was re-initiated during the Permian, and (2) the contemporaneous Silurian–Devonian southern margin (Ereendavaa–Middle Gobi) still represented an extensional continental margin showing reworking of Neoproterozoic–Early Palaeozoic zircons from the basement. It turned into an active continental margin with starting arc magmatism in the Carboniferous (zircon mean epsilon Hf(t) + 5.75).

Initiation of left-lateral deformation along the Ailao Shan–Red River shear zone: new microstructural, textural, and geochronological constraints from the Diancang Shan metamorphic massif, SW Yunnan, China

The Diancang Shan metamorphic massif, the northwestern extension of the Ailao Shan Massif, is a typical metamorphic complex situated along the NW–SE-trending Ailao Shan–Red River shear zone. Diancang Shan granitic and amphibolitic mylonites collected from sheared high-grade metamorphic rocks were studied using petrographic and electron-backscatter diffraction techniques. Sensitive high-resolution ion microprobe U–Pb dating of zircon grains from the granitic mylonites constrains the timing of shearing. Macro- and microstructural and textural analysis reveals intense plastic deformation of feldspar, quartz, and amphibole under amphibolite-facies conditions, all consistently document left-lateral shearing. Porphyritic monzogranitic mylonite within the shear zone possesses evidence supporting a sequential, progressive process from crystallization during magma emplacement, through submagmatic flow to solid-state plastic deformation. We suggest that the early-kinematic pluton subsequently underwent strong left-lateral strike–slip shearing. The development of complex textures of quartz, feldspar, and amphibole from the granitic and amphibolitic mylonites apparently records successive variation of conditions attending coherent, solid-state high-temperature ductile deformation during regional left-lateral shearing. All magmatic zircons from the mylonitized porphyritic monzogranite give U–Pb ages of 30.95 ± 0.61 million years for the crystallization of the granite. This age provides the timing of onset of left-lateral shearing along the Ailao Shan–Red River shear zone in the Diancang Shan high-grade metamorphic massif.

Lu–Hf isotope evidence for the provenance of Permian detritus in accretionary complexes of western Patagonia and the northern Antarctic Peninsula region

Lu–Hf isotope data are presented for dated Permian zircon grains from six samples of the latest Palaeozoic to Jurassic low-grade metasedimentary rocks of western Patagonia and the northern Antarctic Peninsula, as well as from potential source rocks in the North Patagonian Massif. The results for the metasedimentary rocks yield eHf values mostly between −15 and +4 (130 analyses), with a dominant range (more than 85%) of −6 to +1, indicating provenance from Permian magmatic rocks that incorporated continental crust with a significant residence time. Other zircon grains record more negative eHf values indicating derivation from yet more mature crustal sources. Permian subvolcanic granites in the North Patagonian massif appear to be the closest large source area and dated zircon grains from eight samples of these granites yield initial of eHf values of −12 to +4 (45 measurements), 84% of which fall between −6 and +1, the range shown by the metasediments. However, the North Patagonian massif also contains some more juvenile Permian–Carboniferous components not seen in the metasediments, so that this may not be the primary or unique source. These granites are considered to represent the southernmost extension of the Choiyoi igneous province, which contains abundant Permian rhyolites that crop out on the eastern side of the Andes in central Argentina, for which unpublished Hf isotope data yield a very similar range to that of the metasediments. The widespread nature of the Choiyoi volcanic rocks and the predominance of Permian zircon could make this a more favoured source for the detrital grains. Hf isotope data reinforce the uniformity of the provenance of the turbidite detrital zircons and confirm the Choiyoi igneous province and the Permian granitic rocks of the North Patagonian Massif as feasible sources. They further confirm the dominantly crustal origin of the Permian magmas. A source region involving mixing of, for example, crustal materials of Panafrican/Brasiliano and Grenvillian ages, together with a minor but significant subduction-related magmatic input, is an isotopically feasible explanation and is broadly consistent with the provenance of pre-Permian crust in this region, but the proportions of such a mixture must have remained relatively constant. This supports the proposal that recently recognised but widespread Permian magmatism in Patagonia represents voluminous crustal melting in response to subducted slab break-off. The results are also consistent with the premise that the Antarctic Peninsula and southern Patagonia were closely located from Permian to Jurassic times, receiving detritus from the same source.

First data on the age of rocks from the central part of the Vitoria-Trindade Ridge (Brazil Basin, South Atlantic)

Micropaleontological and isotope-geochronological investigations of calcareous sedimentary rocks and volcanites dredged out from the central portion of the submarine Vitoria-Trindade Ridge during the 24th cruise of R/V Akademik Vavilov have been conducted. It has been established based on micropaleontological analysis, which included determination of the species composition of foraminifera and nannoplankton, that the sequence of sedimentary rocks having a pelagic nature formed on the slopes of the volcanic seamounts in the central portion of the Vitoria-Trindade Ridge from the Early to Mid-Miocene to the Holocene; a good correlation between the degree of lithification of these rocks and their age is observed. It has also been established that the carbonate platforms on the abraded tops of the Davis Seamount and the Dogaressa Bank, which are located in the east-central portion of the Vitoria-Trindade Ridge, started forming in the Early Miocene (19–24 Ma). It has been determined using local U-Pb dating of zircon grains with a SHRIMP-II high resolution secondary ion mass spectrometer that the volcanites forming the upper portion of the volcanic rock sequence of the Jaseur Seamount (29.8 ± 6.6 Ma) located in the west-central portion of the Vitoria-Trindade Ridge date to the Oligocene. The investigations conducted have confirmed the opinion that the Vitoria-Trindade Ridge formed in general because of the activity of the hot spot located under the volcanic Trindade and Martin Vaz Islands. However, separate extended lenticular segments of this ridge existed for a long time as single structures, within which the age of the seamounts was not linearly dependent on the distance from the location of the hot spot. Lenses of hot mantle matter that form at the sublithospheric level as a result of impulses of plume activity and move along with the lithospheric plate play a defining role in the development of individual segments forming the Vitoria-Trindade Ridge.

A Laurentian provenance for the Dalradian rocks of north Mayo, Ireland, and evidence for an original basement–cover contact with the underlying Annagh Gneiss Complex

Abstract: Metasediments of the early Dalradian Grampian Group (Erris Group) and probable equivalents (Inishkea Division) structurally overlie Palaeoproterozoic to Neoproterozoic Annagh Gneiss Complex orthogneisses in NW Mayo, Ireland. Sm–Nd isotopic data suggest a Palaeoproterozoic source for the metasediments. K-feldspar and granitoid clasts from the Doonamo Formation, Erris Group, record U–Pb zircon ages of c . 1740 Ma and c . 980 Ma, respectively. These ages are within error of the c . 1730–1750 Ma Mullet gneisses and c . 990 Ma Grenvillian migmatitic leucosomes in the underlying Annagh Gneiss Complex. U–Pb detrital zircon data reveal that the Erris Group was deposited after c . 955 Ma, with predominant input from c . 1640, c . 1500 and c . 990 Ma interpreted Laurentian sources (Labradorian, Pinwarian and Grenvillian terranes, respectively). Limited detrital zircon data from the Inishkea Division yield similarly aged detritus and a tentative maximum depositional age of c . 1005 Ma. Correlation of both sequences with the Scottish Dalradian Grampian Group is considered valid based on detrital zircon U–Pb and whole-rock Sm–Nd data. The clast ages support the hypothesis that the Dalradian unconformably overlies the Annagh Gneiss Complex in north Mayo, whereas the detrital zircon data imply more distal Laurentian sources. Dalradian deposition is thereby tied to the margins of Laurentia. Supplementary material: U–Pb zircon (secondary ion mass spectrometry) data and descriptions of dated zircon grains are available at http://www.geolsoc.org.uk/SUP18414 .

Zircon and titanite recording 1.5 million years of magma accretion, crystallization and initial cooling in a composite pluton (southern Adamello batholith, northern Italy)

The southern part of the Adamello batholith (the so-called “Re di Castello unit”) is an example of a composite pluton, ranging from gabbro to granodiorite in composition. U–Pb dating of single-zircon crystals from four tonalitic to granodioritic lithologies reveals that zircon crystallization is protracted in all studied lithologies, showing apparent durations of growth between 90 and 700 ka. The youngest zircons crystallized near the solidus and yield identical or slightly older ages than titanite. The formation of these autocrystic zircons is considered to approximate the age of emplacement of the melt and its final crystallization, in contrast to antecrystic zircons present in the same sample, which had formed earlier in the magmatic column or were derived from re-mobilized earlier magma. The autocryst-derived “emplacement” ages range from 42.43 ± 0.09 Ma to 40.90 ± 0.05 Ma, recording 1.5 Ma of intrusion and crystallization history. We anticipate that extended periods of zircon crystallization may be common in silicic rocks, whereas the zircons from residual melts from initially undersaturated mafic liquids should yield far more precise emplacement ages within our present analytical uncertainties of 0.1–0.2% in 206Pb/ 238U age. Decreasing Th/U ratios of dated zircons within one melt batch document the depletion of the residual melt portion in Th due to the contemporaneous crystallization of titanite. Preliminary Hf isotopic compositions of the dated zircon grains suggest that the early stage melts of the southern Re di Castello unit represent hybrid melts with an important crustal component ( ε Hf between − 2.8 and +3.0). Subsequently emplaced melts are more juvenile at ε Hf values at +6.4 to +8.9 and may thus reflect the addition of large volumes of mafic melt to the magmatic system.

Genesis of metapelitic migmatites in the Adirondack Mountains, New York

AbstractOxygen isotope ratios and rare earth element (REE) concentrations provide independent tests of competing models of injection v. anatexis for the origin of migmatites from amphibolite and granulite facies metasedimentary rocks of the Adirondack Mountains, New York. Values of δ18O and REE profiles were measured by ion microprobe in garnet–zircon pairs from 10 sample localities. Prior U–Pb SIMS dating of zircon grains indicates that inherited cores (1.7–1.2 Ga) are surrounded by overgrowths crystallized during the Grenville orogenic cycle (∼1.2–1.0 Ga). Cathodoluminescence imaging records three populations of zircon: (i) featureless rounded ‘whole grains’ (interpreted as metamorphic or anatectic), and rhythmically zoned (igneous) cores truncated by rims that are either (ii) discordant rhythmically zoned (igneous) or (iii) unzoned (metamorphic or anatectic). These textural interpretations are supported by geochronology and oxygen isotope analysis. In both the amphibolite facies NW Adirondacks and the granulite facies SE Adirondacks, δ18O(Zrc) values in overgrowths and whole zircon are highly variable for metamorphic zircon (6.1–13.4‰; n = 95, 10 μm spot). In contrast, garnet is typically unzoned and δ18O(Grt) values are constant at each locality, differing only between leucosomes and corresponding melanosomes. None of the analysed metamorphic zircon–garnet pairs attained oxygen isotope equilibrium, indicating that zircon rims and garnet are not coeval. Furthermore, REE profiles from zircon rims indicate zircon growth in all regions was prior to significant garnet growth. Thus, petrological estimates from garnet equilibria (e.g. P–T) cannot be associated uncritically with ages determined from zircon. The unusually high δ18O values (>10‰) in zircon overgrowths from leucocratic layers are distinctly different from associated metaigneous rocks (δ18O(Zrc) < 10‰) indicating that these leucosomes are not injected magmas derived from known igneous rocks. Surrounding melanosomes have similarly high δ18O(Zrc) values, suggesting that leucosomes are related to surrounding melanosomes, and that these migmatites formed by anatexis of high δ18O metasedimentary rocks.

Structure, age and mode of emplacement of the Hercynian Bordères-Louron pluton (Central Pyrenees, France)

The Hercynian Borderes-Louron pluton (20 km2) in the Central Pyrenees intrudes Devonian and Carboniferous metasediments. It shows a concentric zoning and consists of a significant proportion of (quartz) gabbros in its periphery, and of granodiorites, biotite monzogranites and biotite-muscovite monzogranites in its core. AMS study shows that the pluton corresponds to an elongated dome with a N100°E-trending axis. The anisotropy intensity Ppara% is high in the south and in the core of the pluton, whereas it is low in the north. The shape parameter T indicates that the fabric is strongly planar in a large central band oriented NW-SE, whereas it is strongly linear in the western and eastern tips of the pluton. These characteristics suggest that the Borderes-Louron pluton emplaced in two episodes: (1) intrusion of mafic magmas along a N100°E sill parallel to the regional foliation of the host metasediments; and (2) injection of three successive silicic batches (granodiorite, biotite monzogranite, two-micas monzogranite) which pushed aside the early mafic injections. In situ U-Pb dating of zircon grains indicates that the emplacement age of the biotite monzogranite is 309 ± 4 Ma, synchronous with the D 2 dextral transpressive event and close to the ages of the eastern Pyrenean plutons, may be slightly older.

U-Pb Zircon Dating of Regional Deformation in the Lower Crust of the Kohistan Arc

Felsic veins several cm thick to bodies several hundreds of m wide are sporadically distributed in the Kamila amphibolites of the Kohistan arc sequence. Some small dikes (less than one m thick) were deformed together with ductile deformation of wall-rock amphibolites; other small undeformed dikes crosscut foliation in the amphibolites. Large granitic intrusive bodies include gneissose and massive varieties. Ion-microprobe U-Pb dating of zircon grains reveal ages as follows: three deformed dikes, 107.7 ± 1.8 Ma, 94.0 ± 1.9 Ma, 81.0 ± 1.6 Ma; a single gneissose granitic body, 89.5 ± 4.2 Ma; three undeformed dikes, 82.0 ± 2.0 Ma, 80.6 ± 4.5 Ma, 75.8 ± 1.7 Ma; a massive granitic body, 75.7 ± 2.2 Ma; and a foliated amphibolite xenolith in the massive granitic body, 110.7 ± 4.9 Ma. Ages of the deformed dikes, the gneissose granitic body, and the amphibolite xenolith indicate that ductile deformation in the lower crust of the Kohistan arc took place episodically or successively during the period from 107 to 81 Ma. Ages of the undeformed dikes and the massive granitic body indicate that ductile deformation ceased prior to 80 Ma. Collision between Asia and the Kohistan arc elsewhere was inferred to be before 80 Ma. We conclude that the ductile deformation events did not extend beyond the time of Asia-Kohistan collision.

Probability density plot for fission-track grain-age samples

This paper outlines the statistical basis, construction and interpretation of probability-density (PD) plots as applied to the analysis of mixed distributions of fission-track (FT) grain ages. Such distributions can be viewed as being composed of a mixture of elemental component distributions, with each component characterized by a unique average FT age. The original PD plot of Hurford et al. is shown to have a clear statistical basis, as defined by the Gaussian-kernel method of density estimation. This background is used to develop and justify an improved version of the PD plot. Three modifications are recommended: (1) as originally suggested by Galbraith, the PD plot should be constructed using the transform variable z, which is approximately proportional to the logarithm of FT age. By using z i to represent the FT grain ages τ i , each component distribution within a mixed FTGA sample becomes approximately Gaussian-distributed. This approximation is shown to work well for FT dating of zircon grains, which generally have relatively high uranium content and high track densities, and less so for FT dating of apatite grains; (2) estimation of the PD plot is optimized by setting the width of the Gaussian-kernel equal to α( SE( z ̂ i) , where α is a scaling factor with an optimal value of ∼ 0.6 and SE( z ̂ i) is the standard error of the FT estimate for z for the ith grain. This arrangement ensures the best compromise between resolution and smoothness for the final PD plot; (3) probability density is estimated as a function of z but it is best presented as a function of FT age τ. This objective is accomplished by transforming the z coordinates of the PD plot to τ and plotting τ on a logarithmically-scaled axis, which ensures that the original scaling of the PD plot is preserved. With these modifications, the component distributions in a PD plot will appear as symmetric Gaussian-shaped peaks and the area beneath each peak will be proportional to the relative size of the component. Several examples are given that illustrate the general concepts behind the PD plot and the advantages of the recommended modifications.

SHRIMP U/Pb Zircon Ages of Acid Volcanic Rocks in the Chitradurga and Sandur Groups, and Granites Adjacent to the Sandur Schist Belt, Karnataka

Abstract SHRIMP U/Pb dating of zircon grains from a schistose acid volcanic rock from the Daginkatte Formation in the Chitradurga Group of the Late Archaean Dhwar Supergroup in western Karnataka has yielded a precise concordant age of 2614 ± 8 Ma which indicates the time of melt crystallisation. In contrast, zircons from similar schistose acid volcanic rocks in the Vibhuti Gudda Formation of the Sandur Group in the Sandur schist belt of eastern Karnataka yielded less precise concordia intercept ages of 2658±14 and 2691±18 Ma. Zircons from high-strain grey gneisses which appear to be the oldest recognisable component of the Late Archaean granite complex adjacent to the Sandur schist belt have an imprecise age of 2719 ± 40 Ma. Zircons from the youngest recognisable granite adjacent to the belt have also yielded an imprecise, but younger, age of 2570 ± 62 Ma. The ages show that granite emplacement and deformation adjacent to the Sandur schist belt took pIace in a period of c.150 Ma. The imprecise ages of zircon in the acid volcanic rocks in the Sandur Group and the adjacent granites are related to Neoproterozoic loss of lead which may have been an effect of either weathering or a regional thermal event. We favour the latter in the light of the record of Pan-African thermal effects in the east and south of southern Peninsular India.

Age of xenocrystic zircon from diatremes of western Canada

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.

Zircon from sediments: A combined OSL and TL auto-regenerative dating technique

Auto-regenerative dating of zircon grains from fired materials has been shown to be a very powerful technique. The high internal dose rate in zircon gives a measurable auto-regenerated TL signal a few months after the natural TL measurement, allowing the determination of age without dosimetry. The technique has been extended to zircon grains from sediments, using optically stimulated luminescence (OSL) techniques to measure the natural signal. The auto-regenerated signal is only a few hundred photons and cannot be measured above background using present OSL techniques, so it is counted using the highly sensitive TL reader developed for fired zircon grains. Preliminary measurements on fired zircons are in good accord with TL dating and the known ages, but precision becomes poor for samples <1 ka in age. Possible improvements using auto-regenerative phototransfer TL dating are discussed.

Properties of Portland pozzolana cements

Massive dolostones pervasively occur in the Lower Paleozoic strata in Tarim Basin, NW China. This study focuses on the hydrothermally-altered dolostones in the Lower Ordovician at Sancha, Bachu County where the hydrothermally-altered dolostones, marbles and diabase intrusions co-occur within a large-scale, NW trending strike-slip fault zone. Within this zone, the intrusive diabases mostly occur along the master border faults to the east and west, respectively. The limestones that got in contact with the intrusions commonly were marbleized and the antecedent dolostones were subject to intense hydrothermal alteration, particularly along fractured portions. Based on field investigations and petrographic examinations on the hydrothermally-altered dolostones, five distinctive fabrics of matrix and cement dolomites are distinguished. Two episodes of vein-filling calcites postdate the hydrothermally-precipitated saddle dolomites. Integrated isotopic geochemistry (C, O and Sr) and fluid inclusion microthermometry suggest that dolomite recrystallization and precipitation were associated with the hydrothermal activity induced by fracturing/faulting and intrusive magmatism. CAMECA SIMS U–Pb isotopic dating of zircon grains from the diabases yields an age of 290.5 ± 2.9 Ma in the Early Permian, which reconciles the relative timing revealed by the paragenetic history. The magmatic emplacement was thus linked to the large igneous province (LIP) of Early Permian in Tarim Basin. In this case, the widespread magmatic activities could have caused the thermal anomaly and fracturing in the strata that were penetrated by the intrusions, enhancing the thermal diffusion and hydrothermal fluid migration (although minor volumes of thermal fluids from the basic magmas) through the fracture/fault systems, thereby resulting in contact metamorphism (or marbleization), recrystallization of matrix dolomites and dolomite precipitation commonly in primary dolostone successions. This study provides a useful analogue to understand the dolomitization and relevant issues for the deeply-buried dolostones associated with tectono-magmatism within the Tarim Basin and elsewhere.