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Mat ic dyke is a sign of regional extension, and thus has important tectonic significance. A great amount of mafic dykes occur in the Kongling terrain of the Yangtze Craton, which have great bearing on the early evolution of the Yangtze Craton. Their ages, however, have not been well constrained. In this paper we report an integrated study of zircon U-Pb age and Hf isotope compositions for a mafic dyke in the Kongling terrain. The zircons yielded a weighted mean 207Pb/206Pb age of 1852±11Ma, which represents its intrusion age. They have ϵHf(∫) values of -6.3 to 0.5, with a weighted mean of -3.06±0.88, suggesting that the mat ic dyke came from metasomatic mantle. The results indicate that the Yangtze Block had transformed into post-collisional extensional regime at Ca. 1850 Ma. In the same period, the Yangtze Craton shows enough rigidity to produce brittle rupture, and thus has the characteristics of a craton. [ABSTRACT FROM AUTHOR]

Gemstones are minerals of gem qualities used for adornment and decoration with the attributes of beauty, durability and rarity. Traditionally, although China has been regarded as the most important source for nephrite, over the past decades, a large variety of gemstone resources have been newly discovered in China owing to continuous exploration works. The vast land with various geological and geochemical backgrounds is rich in gemstone resources with potential for new deposits discoveries. In pegmatites, gemstones are related to granitic magma events and mainly occur in pegmatitic cavities, such as tourmaline, aquamarine, spodumene, spessartine, moonstone, quartz, apatite, and topaz. The eruption of Tertiary basaltic magma provides gem-quality sapphire, spinel, olivine, garnet, and zircon. The supergene oxidation zones of some copper and iron deposits in Hubei and Anhui province host gem-quality turquoise and malachite. Moreover, the formation of the nephrite deposit in China is mostly related to the carbonatite and serpentinite rocks involved in the metamorphic-metasomatic processes. This paper comprehensively introduces the distribution of gemstones deposits, as well as the gemological and mineralogical characteristics of gemstones in China. Our present investigation provides insights into the gemstone potential of China for further exploitation. [ABSTRACT FROM AUTHOR]

This paper evaluates the analytical precision, accuracy and long-term reliability of the U-Pb age data obtained using inductively coupled plasma–mass spectrometry (ICP-MS) with a frequency quintupled Nd-YAG (λ = 213nm) laser ablation system. The U-Pb age data for seven standard zircons of various ages, from 28 Ma to 2400 Ma (FCT, SL13, 91500, AS3, FC1, QGNG and PMA7) were obtained with an ablation pit size of 30 μm diameter. For 207Pb/206Pb ratio measurement, the mean isotopic ratio obtained on National Institute of Standards and Technology (NIST) SRM610 over 4 months was 0.9105 ± 0.0014 ( n = 280, 95% confidence), which agrees well with the published value of 0.9096. The time-profile of Pb/U ratios during single spot ablation showed no significant difference in shape from NIST SRM610 and 91500 zircon standards. These results encouraged the use of the glass standard as a calibration standard for the Pb/U ratio determination for zircons with shorter wavelength (λ = 213 nm) laser ablation. But 206Pb/238U and 207Pb/235U ages obtained by this method for seven zircon standards are systematically younger than the published U-Pb ages obtained by both isotope dilution–thermal ionization mass spectrometry (ID-TIMS) and sensitive high-resolution ion-microprobe (SHRIMP). Greater discrepancies (3–4% younger ages) were found for the 206Pb/238U ages for SL13, AS3 and 91500 zircons. The origin of the differences could be heterogeneity in Pb/U ratio on SRM610 between the different disks, but a matrix effect accuracy either in the ICP ion source or in the ablation-transport processes of the sample aerosols cannot be neglected. When the 206Pb/238U (= 0.2302) newly defined in the present study is used, the measured 206Pb/238U and 207Pb/235U ages for the seven zircon standards are in good agreement with those from ID-TIMS and SHRIMP within ±2%. This suggests that SRM610 glass standard is suitable for ICP-MS with laser ablation sampling (LA-ICP-MS) zircon analysis, but it is necessary to determine the correction factor for 206Pb/238U by measuring several zircon standards in individual laboratories. [ABSTRACT FROM AUTHOR]

Sedimentary strata in the Lhasa terrane of southern Tibet record a long but poorly constrained history of basin formation and inversion. To investigate these events, we sampled Palaeozoic and Mesozoic sedimentary rocks in the Lhasa terrane for detrital zircon uranium–lead (U–Pb) analysis. The >700 detrital zircon U–Pb ages reported in this paper provide the first significant detrital zircon data set from the Lhasa terrane and shed new light on the tectonic and depositional history of the region. Collectively, the dominant detrital zircon age populations within these rocks are 100–150, 500–600 and 1000–1400 Ma. Sedimentary strata near Nam Co in central Lhasa are mapped as Lower Cretaceous but detrital zircons with ages younger than 400 Ma are conspicuously absent. The detrital zircon age distribution and other sedimentological evidence suggest that these strata are likely Carboniferous in age, which requires the existence of a previously unrecognized fault or unconformity. Lower Jurassic strata exposed within the Bangong suture between the Lhasa and Qiangtang terranes contain populations of detrital zircons with ages between 200 and 500 Ma and 1700 and 2000 Ma. These populations differ from the detrital zircon ages of samples collected in the Lhasa terrane and suggest a unique source area. The Upper Cretaceous Takena Formation contains zircon populations with ages between 100 and 160 Ma, 500 and 600 Ma and 1000 and 1400 Ma. Detrital zircon ages from these strata suggest that several distinct fluvial systems occupied the southern portion of the Lhasa terrane during the Late Cretaceous and that deposition in the basin ceased before 70 Ma. Carboniferous strata exposed within the Lhasa terrane likely served as source rocks for sediments deposited during Cretaceous time. Similarities between the lithologies and detrital zircon age-probability plots of Carboniferous rocks in the Lhasa and Qiangtang terranes and Tethyan strata in the Himalaya suggest that these areas were located proximal to one another within Gondwanaland. U–Pb ages of detrital zircons from our samples and differences between the geographic distribution of igneous rocks within the Tibetan plateau suggest that it is possible to discriminate a southern vs. northern provenance signature using detrital zircon age populations. [ABSTRACT FROM AUTHOR]

Monazite dating is an important technique in geochronological studies. However, monazite U-Pb dating by SHRIMP is much less popular than zircon in geochronological applications. This paper compares the results of SHRIMP U-Pb dating of monazites and zircons separated from two granite samples, indicating that monazite SHRIMP U-Pb dating at the Beijing SHRIMP Centre is feasible and provides identical results within error. [ABSTRACT FROM AUTHOR]

This paper presents detrital zircon ages obtained in rocks of the Embu Complex, southeastern São Paulo State, Brazil. The Embu Complex encompasses a Paleoproterozoic basement represented by migmatitic hornblende and biotite orthogneisses covered by (kyanite)-(sillimanite)-(garnet) bearing biotite-muscovite schists and paragneisses with decametric intercalations of quartzites and calcsilicate rocks. In the studied area this metasedimentary sequence is intruded by the porphyritic Serra do Quebra Cangalhagranite. Through field and microstructural studies, four ductile deformational phases wereidentified. Metamorphic events related to the Brasiliano Orogeny that affected the studied rocks were dominantly under medium temperature and pressure conditions, from greenschist to middle amphibolite facies. Detrital zircon crystals from a ∼10 m thick quartzite layer were analyzed by LA-ICP-MS resulting in a wide range of ages between 2100 and 600 Ma that fall mainly in four groups: the first group between 2.1 and 1.6 Ga, with apex at 1.7 Ga; two less expressive Mesoproterozoic groups with values between 1.6 and 1.2 Ga; and a fourth group with values between 1.2 and 0.6 Ga. Considering the geochronological data, the sedimentation of the basin began after 852 ± 40 Ma (the youngest igneous grain) and finished before ∼786 Ma (metamorphic rim). The age of the intrusive Serra do Quebra Cangalha granite (∼680 Ma) is consistent with this minimum age. The opening of the basin could be related to the break-up of Rodinia, which resulted in several small continents, among them the Paranapanema and São Francisco paleocontinents. Comparing these data with similar provenance data from the Apiaí terrane, itseems probable that the Embu Complex was physically connected with it during most of their evolution. [ABSTRACT FROM AUTHOR]

Abstract: Clasts of red siltstone with Glossopteris from moraine around Mt Rymill in the southern Prince Charles Mountains, East Antarctica, can be traced upslope in the Lambert Graben system to the nearby Gamburtsev Subglacial Mountains (GSM). The clasts contain zircons with SHRIMP U–Pb ages of 620–460 Ma and 1300–970 Ma from host rocks of intermediate to mafic rocks, and a clay fraction with a T DM Nd model age of 2.72 Ga and εNd of −18.3, derived from upper continental crust. East of the GSM, at Lake Vostok, clasts of siltstone in accreted ice can be traced to the Vostok Subglacial Highlands (VSH). The clasts contain zircons and monazites with SHRIMP U–Pb ages [Leitchenkov, G.L., Belyatsky, B.V., Rodionov, N.V., Sergeev, S.A., 2007. Insight into the geology of the East Antarctic hinterland: a study of mineral inclusions from ice cores of the Lake Vostok borehole. In: Cooper, A.K., Raymond, C.R. (Eds.), Online Proceedings of the 10th ISAES, USGS Open-file Report 2007-1047, Short research Paper 014, 4 pages.] broadly similar to those from Mt Rymill, and a T DM Nd model age of 1.88 Ga and εNd of −15 [Delmonte, B., Petit, J.R., Basile-Doelsch, I., Lipenkov, V., Maggi, V., 2004. First characterization and dating of East Antarctic bedrock inclusions from subglacial Lake Vostok accreted ice. Environmental Chemistry 1, 90–94.]. Other Mesozoic and Paleozoic sediments deposited in a radial pattern about central Antarctica contain detrital zircons dominated by populations aged 700–500 Ma and 1300–1000 Ma, which we interpret as reflecting a GSM-VSH provenance dominated by rocks generated during the Pan-Gondwanaland and Grenville events of supercontinental assembly. [Copyright &y& Elsevier]

Abstract: The Forsmark area, in the western part of the Svecofennian orogen, central Sweden, is situated between two major Palaeoproterozoic tectonic domains that show contrasting histories with respect to timing of igneous activity, ductile deformation and metamorphism. Geological features common to both the adjacent domains are found at Forsmark, which, consequently, is a key area for understanding the tectonic evolution of the Svecofennian orogen in this region. New SIMS and TIMS geochronological data from several samples in a limited area at Forsmark constrain the timing of igneous activity, ductile deformation and metamorphism in this area. U/Pb zircon dating, in combination with field studies, reveals the existence of two calk-alkaline igneous suites at Forsmark. The older and most voluminous plutonic suite intruded at 1.89–1.87Ga and is affected by penetrative, yet variably intense ductile deformation, with the development of a ductile grain-shape fabric under amphibolite facies metamorphic conditions and folding. The younger and less voluminous hypabyssal suite intruded at 1.86–1.85Ga, during the waning stages of this penetrative deformational phase. Furthermore, U/Pb titanite dating shows that the area has been affected by one or more tectonothermal events after regional fabric development, and possibly even after 1.83Ga. The deformation during these events involved more restricted strain along discrete zones. The Forsmark data, in combination with a compilation and evaluation of available geochronological data from the time interval 1.91–1.84Ga in central Sweden, points to the existence of at least two major tectonic cycles in this region. Each cycle is characterised by igneous activity associated at least partly with extension, a short interval of compressional deformation and migration of the tectonic activity. In this paper, we discuss two highly contrasting tectonic models that may explain the cyclic tectonic evolution of the western part of the Svecofennian orogen in central Sweden. The favoured model involves continuous subduction, with a constant polarity, beneath a single active continental margin, combined with alternating subduction hinge retreat and advance. The model involves migration of what has been described as tectonic switching in the younger, accretionary orogenic systems of eastern Australia (Palaeozoic Lachlan orogen) and New Zealand. [Copyright &y& Elsevier]

Huge diabase occurs within the Proterozoic strata in the southern margin of the North China Block, geographically in the northern parts of Anhui and Jiangsu provinces, where no strata younger than Neoproterozoic have been found. Zircons from the diabase show three different structures from cathodoluminesecence imaging, but they are all of magmatic origin. SHRIMP U-Pb dating yields a range of 206Pb/238U ages from 2600 to 124 Ma, but two geological events can clearly be identified from both concordant and discordant ages. One gives two weighted mean 207Pb/206Pb ages of 976 ± 24 and 1038 ± 26 Ma which are interpreted as timing of diabase crystallization, and the other has a weighted mean 207Pb/206Pb age of 2523 ± 52 Ma that represents formation time of inherited zircon from the old crust. The dating results indicate that the mafic magmatic event occurred in a transition period from the latest Mesoproterozoic to the earliest Neoproterozoic, consistent with global subduction of Grenvill ocean crust to result in the assembly of supercontinent Rodinia. Therefore, the North China Block needs to be taken into account when reconstructing the position of China continents within the framework of Rodinia supercontinent. [ABSTRACT FROM AUTHOR]

Abstract: This paper describes a strategy for effective discrimination and provenance evaluation of sandstones transported by different sediment dispersal systems using an integrated programme of heavy mineral analysis, mineral chemistry and zircon age dating. Sandstones derived from different source regions are initially discriminated using provenance–sensitive heavy mineral parameters (ratios of abundances of stable, hydraulically equivalent minerals). Differences between the heavy mineral populations are reinforced by single-grain mineral chemical analysis, which also provides further information on the nature of the source terrains. Zircon age dating of representative samples places further constraints on the location of the source areas, by identifying the main crust-forming events. In addition, the validity of the mineralogical discrimination can be tested by the zircon age data. The value of this integrated approach is illustrated by a case study of deep marine Late Cretaceous–Paleocene sandstones in the Norwegian Sea. Potential sources of coarse clastic sediment lie both to the east, in mid-Norway, and the west, in East Greenland. Three main sand types (MN1, MN2 and MN3) have been identified, each of which has a unique combination of heavy mineral ratios (rutile/zircon, monazite/zircon, chrome spinel/zircon and apatite/tourmaline) and mineral chemistry (garnet and tourmaline). Sand type MN1 has a simple zircon age spectrum, with a large Early–Middle Proterozoic group and a small Early Paleozoic group, indicating derivation from northern mid-Norway. Sand type MN2 has a complex age structure including Archaean and early Proterozoic zircons. It can be divided into two subtypes, MN2a and MN2b, and can be ascribed to a source in East Greenland. Sand type MN3 has a simple age structure dominated by Middle Proterozoic zircons, and it was sourced from southern mid-Norway. Since reservoir presence is a major area of uncertainty in the deep water Norwegian Sea, the use of heavy mineral, mineral chemical and zircon age data to discriminate sandstones of eastern and western origin is crucial to hydrocarbon exploration of the area. [Copyright &y& Elsevier]

Abstract: This paper presents a new approach to quantify sediment mixing based on the mixing of U–Pb zircon age distributions within sediment. Two statistical techniques are presented to determine the proportion in which two known age distributions combine to create a known mixed age distribution. These techniques are then used to determine relative erosion rates between adjacent drainage basins above and below the Main Central Thrust (MCT) in the central Nepal Himalaya. The MCT region is coincident with an abrupt north–south change in geomorphic character and mineral cooling ages that are thought to represent an erosional response to higher rock uplift rates north of the MCT zone. However, it is unclear whether the ongoing deformation responsible for the differential uplift rates is: (1) focused on the MCT; (2) at depth along a crustal scale ramp; or (3) along newly mapped thrust faults south of the MCT. Our study explores this issue by comparing modern erosion rates with longer-term erosion rates determined from mineral cooling ages. Zircons were separated from modern river sand and dated by LA-MC-ICPMS before the measured isotopic ratios and ages were used in 1-d and 2-d mixing calculations. The 1-d technique creates probability density functions of zircon ages for each sample and then uses both an iterative and inverse approach to estimate mixing between samples. In contrast, the 2-d technique estimates mixing between probability “fields” defined by the measured 238U/206Pb and 207Pb/206Pb ratios. Given a finite mixture with perfect sample representation, both techniques produce perfect mixing estimates across a range of mixing proportions. Modeling results demonstrate that given imperfect subsample representation of the complex parent age distribution, differing degrees of subsample smoothing may be required to achieve an accurate mixing estimate. Using mixing of zircon ages as a quantitative proxy for sediment mixing requires a correction for the concentration of zircon in the river sediment. Two new methods for establishing zircon concentration in river sediment are presented demonstrating the existence of 2- to 5-fold differences in zircon concentration between adjacent drainages. Relative erosion rates are estimated by determining the zircon mixing ratio between adjacent drainages which are then normalized by the ratio of zircon concentrations and the ratio of drainage areas. Results show ∼3 times higher modern erosion rates south of the MCT in the northernmost Lesser Himalaya. Future applications of this new technique may include reach-scale sediment transport dynamics, improved sedimentary basin analysis, and better interpretation of foreland mineral cooling ages. [Copyright &y& Elsevier]

Copyright of Canadian Journal of Earth Sciences is the property of Canadian Science Publishing and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The Mt Bruce Supergroup of Western Australia was laid down between ca 2,8 Ga and ca 2.2 Ga in the Hamersley Basin, unconformably aver a basement at the alder, granite-greenstone, component of the Pilbara Craton.The Mt Bruce Supergroup consists of three groups: the Fartescue Graup, Hamersley Group and Turee Creek Group in upward sequence. The Hamersley Group, which is divided into eight formations, has a general thickness of -2.5 km, and is characterised by major banded iron-formation (BIF) units. Reported here are SHRIMP U-Pb zircon results (406 grain analyses) tram 13 samples taken from the Hamersley Group and near the top of the underlying Fortescue Group. In combination with SHRIMP results previously published from 12 Hamersley Group samples, the present results provide significant new constraints on the depositional chronology of the group, and suggest that the overage Compacted) depositional rates of each of the main depositional lithologies (BIF, carbonate, shale) were -180 m per million years, 12 m per million years and 5 m per million years, respectively. Some recently published SHRIMP ages from the Joffre Member differ slightly from those that are interpreted from the present data, and it is suggested that the two datasets may be reconciled it non-zero-age Pb loss is taken into account. The total body of zircon U-Pb age data tram the Fortescue and Hamersley Groups is consistent with a model involving continuous accumulation of basin-fill far at least 330 million years, from ca 2780 Mo to the tap of the Hamersley Group at ca 2449 Ma, The word 'continuous' in this context means that there may have been no breaks in deposition longer thon 1 million years. However, this model is not proven, and a major challenge far future work is to measure the length of any proposed non-depositional intervals. [ABSTRACT FROM AUTHOR]

In this article the authors' respond to a comment on their article "New zircon shock phenomena and their use for dating and reconstruction of large impact structures revealed by electron nanobeam (EBSD, CL, EDS) and isotopic U-Pb and (U-Th)/He analysis of the Vredefort dome," in the 2011 issue. Accordingly, the authors H.M. Rajesh and M.W. Knoper objected to the interpretation of two zircon U-Pb isotopic data from a pegmatite dyke sample. However, they argue that the zircons were high quality.

Here, we document a detailed characterisation of two zircon gemstones, GZ7 and GZ8. Both stones had the same mass at 19.2 carats (3.84 g) each; both came from placer deposits in the Ratnapura district, Sri Lanka. The U‐Pb data are in both cases concordant within the uncertainties of decay constants and yield weighted mean 206Pb/238U ages (95% confidence uncertainty) of 530.26 Ma ± 0.05 Ma (GZ7) and 543.92 Ma ± 0.06 Ma (GZ8). Neither GZ7 nor GZ8 have been subjected to any gem enhancement by heating. Structure‐related parameters correspond well with the calculated alpha doses of 1.48 × 1018 g−1 (GZ7) and 2.53 × 1018 g−1 (GZ8), respectively, and the (U‐Th)/He ages of 438 Ma ± 3 Ma (2s) for GZ7 and 426 Ma ± 9 Ma (2s) for GZ8 are typical of unheated zircon from Sri Lanka. The mean U mass fractions are 680 μg g−1 (GZ7) and 1305 μg g−1 (GZ8). The two zircon samples are proposed as reference materials for SIMS (secondary ion mass spectrometry) U‐Pb geochronology. In addition, GZ7 (Ti mass fractions 25.08 μg g−1 ± 0.18 μg g−1; 95% confidence uncertainty) may prove useful as reference material for Ti‐in‐zircon temperature estimates. Key Points: We document a detailed characterisation of the Sri Lankan zircon gemstones GZ7 and GZ8 and propose them as SIMS U‐Pb reference materials. Both specimens have high mean U mass fractions and concordant Cambrian (GZ7) and Ediacaran (GZ8) U‐Pb ages. In addition, GZ7 is proposed as a reference material for Ti‐in‐zircon temperature estimates. [ABSTRACT FROM AUTHOR]

Copyright of Canadian Journal of Earth Sciences is the property of Canadian Science Publishing and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

U–Pb ages of detrital zircons from the Wedington Sandstone member in northwest Arkansas provide evidence for Late Mississippian westward transcontinental sediment transport from the Appalachian foreland. The Late Mississippian Wedington Sandstone member of the Fayetteville Shale is a fine- to medium-grained quartzarenite. It separates the Fayetteville Shale into informal lower and upper intervals, and was deposited as a small constructive delta complex that prograded towards the south and southeast during the Late Mississippian. As a major influx of clastic sediments, the Wedington Sandstone member records the sediment source and dispersal in the mid-continent during the Late Mississippian. A total of 559 detrital zircon grains from six Wedington samples were recovered for U–Pb detrital zircon geochronological analysis. Results show that age distributions can be subdivided into six groups: ~350–500, ~900–1350, ~1360–1500, ~1600–1800, ~1800–2300, and > ~2500 Ma, and are characterized by a prominent peak for the age group of ~900–1350 Ma, a major peak at ~1600–1800 Ma, and a few other minor age clusters. Regional correlation and geological evidence from surrounding areas suggest that the transcontinental sediment dispersal started as early as the Late Mississippian. U–Pb detrital zircon age distribution suggests that the Wedington Sandstone member was likely derived from the Appalachian foreland with contributions from the Nemaha Ridge to the west where the Yavapai–Mazatzal sources were exposed during the Late Mississippian. Sediment was likely transported westward through or around the Illinois Basin, merged with mid-continent sediment, and then entered into its current location in northwest Arkansas. Transportation of this sediment from mixed sources continued along its course to the south, forming a delta on the Northern Arkansas Structural Platform. [ABSTRACT FROM PUBLISHER]

Highly restitic garnet-kyanite-phlogopite metapelitic schists from the Goshen Dome of western Massachusetts contain: a population of prograde monocrystalline, megacrystic garnet, some with significant P in substitution for Si; precipitates of hydroxylapatite and rutile; and <1 μm zircon crystals of undetermined origin and abundance on the order of 105/mm3. The unusual P content and the abundant internal precipitate suite are similar to features reported in garnet from ultrahigh-pressure (UHP) and mantle settings, suggesting a potential (U)HP origin for the garnet megacrysts. Zircon included in megacrysts is surrounded by radial fractures, indicating in situ volumetric expansion or new growth. Cores display rare earth element (REE) profiles and cathodoluminescence (CL) zoning consistent with magmatic growth, and yield only Paleozoic dates (447-404 Ma). The embayed core-rim boundary is marked by a several micrometers wide band of CL-dark zircon enriched in Y, P, U, and Th that is interpreted as the accumulation of redistributed xenotime component from the original zircon rim during metamorphism. Outside of this band, the rim has elevated Hf, Th/U << 1, and steep heavy REE profiles. The metamorphic rims yield concordant dates from 400 to 381 Ma. Matrix zircon grains have magmatic cores (1726-415 Ma) with similar core-rim boundaries enriched in Y, P, U, and Th. Metamorphic rims on matrix zircon yield slightly younger dates (393-365 Ma) and are compositionally heterogeneous. The difference between the youngest core and oldest rim indicates a short interval (ca. 4 Ma) between deposition of detrital zircon and the onset of metamorphism in the earliest Acadian. The oldest zircon rim dates are found within phosphatic garnet megacrysts of possible very high-pressure origin. The compositional uniformity of these rims indicates equilibrium with a single source; the anomalous composition suggests a combination of dissolution-reprecipitation and new growth of zircon that is derived from garnet. The range in both composition and dates indicates that matrix zircon rims formed in response to local changes in mineralogy and fluid/melt composition and/or availability. New growth of zircon on these grains cannot be confirmed, suggesting that dissolution-reprecipitation reactions during continued metamorphism may be the dominant mechanism that formed these rims. The data collectively suggest that dissolution-reprecipitation may be a common mechanism for producing metamorphic rims on zircon that does not require additional Zr and Hf, which are limited within most metamorphic settings. [ABSTRACT FROM AUTHOR]

We report newly obtained U-Pb SHRIMP ages of detrital zircons from metagreywackes in the Hiriyur Formation (Chitradurga Group, Dharwar Supergroup) from the central eastern part of the Chitradurga greenstone belt. U-Pb analyses yield three major Neoarchean age populations ranging from 2.70-2.54 Ga with some minor age population of Mesoarchean. The maximum age of deposition is constrained by the youngest detrital zircon population at 2546 Ma. This is the first report of the occurrence of supracrustal rocks less than 2.58 Ga in the central part of Chitradurga greenstone belt. Close evaluation of detrital ages with the published ages of surrounding igneous rocks suggest that the youngest detrital zircons might be derived from rocks of the Eastern Dharwar craton and the inferred docking of the western and eastern Dharwar cratons happened prior to the deposition of the Hiriyur Formation. The Chitradurga shear zone, dividing the Dharwar craton into western and eastern blocks, probably developed after the deposition. Furthermore, the lower intercept is interpreted as evidence for the Pan-African overprints in the study area. [ABSTRACT FROM AUTHOR]

For over a century, the ichnogenusOldhamiahas been used as an index fossil for Cambrian deep-marine siliciclastic successions. New specimens from the Cambrian of Selwyn Basin, eastern Yukon, Canada, provide an opportunity to constrain the chronostratigraphic range ofOldhamiaand test its potential for more detailed biozonation. The upper part of Narchilla Formation (Arrowhead Lake Member) yieldsO. antiqua, O. curvata, O. flabellata, andO. radiata. The overlying Gull Lake Formation yields a less diverse assemblage ofO. antiqua, O. curvata, andO. flabellata. The upward disappearance ofO. radiatais consistent with models that infer increasing complexity ofOldhamiaichnospecies through time but may also be a function of limited sampling. Regional stratigraphic correlations suggestOldhamiaappeared in Arrowhead Lake Member during Cambrian Stage 3. Archaeocyathans and trilobites from Gull Lake Formation confirm thatOldhamiawas present during Cambrian Stages 3 to 4. A younger age limit onOldhamiain Selwyn Basin is provided by syn-volcanic detrital zircons recovered from the base of the overlying Old Cabin Formation, which yield an age of 499.89 ± 0.14 Ma (earliest Guzhangian).Oldhamiaappears to be a reliable index fossil for Cambrian Series 2 and Series 3 in Selwyn Basin. [ABSTRACT FROM AUTHOR]

The Neoproterozoic Hedmark Basin in the Caledonides of South Norway was formed at the western margin of the continent Baltica by rifting 750–600 Ma ago. The margin was destroyed in the Caledonian Orogeny and sedimentary basins translated eastwards. This study uses provenance analysis to map the crustal architecture of the pre-Caledonian SW Baltican margin. Conglomerate clasts and sandstones were sampled from submarine fan, alluvial fan and terrestrial glacigenic sedimentary rocks. Samples were analysed for U–Pb isotopes and clast samples additionally for Lu–Hf isotopes. The clasts are mainly granites c. 960 Ma and 1680 Ma old, coeval with the Sveconorwegian Orogeny and formation of the Palaeoproterozoic Transscandinavian Igneous Belt (TIB). Mesoproterozoic (Sveconorwegian) ages are abundant in the western part of the basin, whereas Palaeoproterozoic ages are common in the east. Lu–Hf isotopes support crustally contaminated source for all clasts linking them to Fennoscandia. Detrital zircon ages of the sandstones can be matched with those from the granitic clasts except for ages within the range 1200–1500 Ma. These ages are typically found in the present-day Telemark, SW Norway. The sandstones and conglomerate clasts in the western part of the Hedmark Basin were sourced from the Sveconorwegian domain in the present SW Norway or its continuation to the present-day NW. The conglomerate clasts in the eastern part of the Hedmark Basin were sourced mainly from the TIB domain or its northwesterly continuation. The Hedmark Basin was initiated within the boundary of two domains in the basement: the TIB and the Sveconorwegian domains. [ABSTRACT FROM AUTHOR]

Prior to this work, the existence of crustal materials older than 4.0 Ga has not been reported from the North China Craton (NCC) – one of the few global terrains where crustal rocks from ∼3.8 Ga have been identified. Here we report the first occurrence of a xenocrystic zircon with a207Pb/206Pb age of 4174 ± 48 Ma, from the Anshan–Benxi Archaean supracrustal greenstone belt, based on laser ablation–inductively coupled plasma–mass spectrometry. The 4.17 Ga zircon xenocryst is hosted within ∼2523 ± 12 Ma massive fine-grained amphibolites which were subsequently metamorphosed at ∼2481 ± 19 Ma. The xenocryst age is ca. 350 million years, older than the oldest zircon previously identified in the NCC, and is consistent with prior zircon Lu–Hf isotopic studies. Documentation of 4.17 Ga xenocrystal zircon not only provided a geochronological record of the oldest known crustal materials in the NCC, but also identified the geologic environment for further search for the rocks that formed during Earth’s earliest recorded evolution. [ABSTRACT FROM AUTHOR]

The shifts in wavenumber of the v3(SiO4) (~1008 cm-1) Raman band of fully crystalline synthetic zircon with changing pressure (P) and temperature (T) were calibrated for application as a Raman spectroscopic pressure sensor in optical cells to about 1000 °C and 10 GPa. The relationship between wavenumber (v) of this band and T from 22 to 950 °C is described by the equation v (cm-1) = 7.54⋅10-9⋅T³ - 1.61⋅10-5⋅T² - 2.89⋅10-2⋅T + 1008.9, where T is given in °C. The pressure dependence is nearly linear over the studied range in P. At ~25 °C, the ∂v/∂P slope to 6.6 GPa is 5.69 cm-1/GPa, and that to 2 GPa is 5.77 cm-1/GPa. The ∂v/∂P slope does not significantly change with temperature, as determined from experiments conducted along isotherms up to 700 °C. Therefore, this pressure sensor has the advantage that a constant ∂v/∂P slope of 5.8 ± 0.1 cm-5/GPa can be applied in experiments to pressures of at least about 6.6 GPa without introducing a significant error. The pressure sensor was tested to determine isochores in experiments with H2O+Na2Si2O2 and H2O+NaAlSi3O8 fluids to 803 °C and 1.65 GPa. These pressures were compared to pressures calculated from the equation of state (EoS) of H2O based on the measured vapor dissolution or ice melting temperature for the same experiment. Pressures determined from the zircon sensor in runs in which NaAlSi3O8 melt dissolved in aqueous fluid were close to or lower than the pressure calculated from the EoS of H2O using the vapor dissolution or ice melting temperature. In experiments with H2O+Na2O+SiO2 fluids, however, the pressure obtained from the Raman spectrum of zircon was often significantly higher than that estimated from the EoS of H2O. This suggests that the pressures along some critical curves of water-silicate melt pseudobinary systems should be revised.v [ABSTRACT FROM AUTHOR]

The OH stretching vibration frequencies of crystalline zircon that contains water have been investigated by quantum mechanical calculation using CRYSTAL09 and several hybrid functionals. Incorporation mechanisms considered for H in zircon include: (1) hydrogarnet and partial hydrogarnet-type substitution; (2) H-compensated trivalent substitution at the Si site; and (3) H-compensated trivalent substitution at the Zr site. The results provide a clear picture of the H locations in zircon and their associated IR stretching vibrations. Based on the results of structural relaxation and corresponding OH stretching calculations, we can assign the 3420 cm-1 band to a hydrogarnet or partial hydrogarnet-type substitution with polarization dependent on the extent of local relaxation (i.e., closer to Ellc for partial hydrogarnet and less polarized for hydrogarnet). These results also show that the 3385 cm-1 (E⊥c) IR band originates from H incorporation that accompanies a trivalent cation substitution at the Si-site. As an example of such a substitution, the [AlO4/H] defect was investigated. H-incorporation coupled with a trivalent cation substitution at the Zr-site appears to be responsible for generating the 3180 cm-1 (E⊥c) IR band. The ab initio calculation on [YO8/H] and [LaO8/H] structures show that the 3180 cm-1 band represents an average between two distinct bands at ~3220-3250 and ~3100 cm-1 attributed to H locations close to or further away from the Zr-substituted atom. The energetics of these substitutions demonstrate that the overlapping of the two IR bands depends on the substituting cation, and this is controlled, in part, by the thermal history of the sample and/or by the experimental conditions under which the IR spectra are obtained. Furthermore, these results predict for the first time the existence of a peroxy-type O bond associated with the partial hydrogarnet substitution in the zircon structure that now accounts for the ~800 cm-1 bands. [ABSTRACT FROM AUTHOR]

The large southern Zhuguangshan granitic batholith composite consists of granites with ages varying from the Caledonian through Indosinian to Yanshanian. Based on K-Ar dating data, the ages of the major parts of this composite were previously regarded as Yanshanian. In this study, the SHRIMP zircon U-Pb dating method has been adopted for six plutons, Ledong, Longhuashan, Dawozi, Zhaidi, Baiyun and Jiangnan, in the southern Zhuguangshan composite, in which the four plutons other than Baiyun and Jiangnan were previously regarded as Yanshanian granites. Magmatic zircons from these six plutons, dated by this study, have yielded ages of 239±5 Ma (MSWD = 2.5), 239±5 Ma (MSWD = 2.5), 239±2 Ma (MSWD = 1.7), 239±4 Ma (MSWD = 3.2), 231±2 Ma (MSWD = 0.81) and 231±3 Ma (MSWD = 1.8), respectively. The results indicate that these plutons were formed by early Indosinian magmatism. Geochemical characteristics suggest that these granites were formed in an extensional tectonic environment. Therefore, the Indosinian period granites in the southern Zhuguangshan composite were formed by partial melting of the Paleo-Mesoproterozoic crustal components during the collapse of thickened lithosphere after the collision between the South China and Indosinian plates. [ABSTRACT FROM AUTHOR]

Age spectra of detrital zircons from metamorphosed quartzites of the Krkonoše–Jizera Massif in the northeastern part of the Saxothuringian Domain were obtained by U–Pb laser ablation inductively coupled plasma mass spectrometry dating. The zircon ages cluster in the intervals of 450–530 Ma and 550–670 Ma, and show individual data between 1.6 and 3.1 Ga. Zircons in the analysed samples are predominantly of Cambrian–Ordovician and Neoproterozoic age, and the marked peak at c. 525–500 Ma suggests a late Cambrian maximum age for the sedimentary protolith. Detritus of the quartzites probably originated from the erosion of Cambrian–Ordovician granitoids and their Neoproterozoic (meta)sedimentary or magmatic country rocks. The lack of Neoproterozoic (meta)sedimentary rocks in the central and eastern part of the Krkonoše–Jizera Massif suggests that the country rocks to voluminous Cambrian–Ordovician magmatic bodies were largely eroded during the formation of early Palaeozoic rift basins along the southeast passive margin of the Saxothuringian Domain. The detrital zircon age spectra confirm the previous interpretation that the exposed basement, dominated by Neoproterozoic to Cambrian–Ordovician granitoids, was overthrust during Devonian–Carboniferous subduction–collision processes by nappes composed of metamorphosed equivalents of the uppermost Cambrian–Devonian passive margin sedimentary formations. Only a negligible number of Mesoproterozoic ages, typically from the Grenvillian event, supports the interpretation that the Saxothuringian Neoproterozoic basement has an affinity to the West African Craton of the northwestern margin of Gondwana. [ABSTRACT FROM PUBLISHER]

Integral model of Ti-Zr placer formation is proposed for the Voronezh anteclise in the following system: primary source-weathering crust-intermediate collector-placer. It is shown that zircon, the most important useful component in this system, can be zirtolitized intensely during crust formation. This process is reflected ultimately in the resource potential of placers. Creation of the integral model took into consideration abundance of the major lithostructural complexes (LSC) in the crystalline basement and their productivity, hiatus and crust formation intervals, paleofacies conditions and mechanism of the formation of intermediate collectors and placers, as well as migration direction and other factors of placer localization. The most productive Cenomanian placers were formed as triple washout products according to the panning mechanism. It is suggested that the resource potential of placers should be revised with the consideration of variation in the density characteristics of the zirtolitized zircon varieties. [ABSTRACT FROM AUTHOR]

Detrital zircon U-Pb ages and Rb-Sr metamorphic ages from low-grade Torlesse Supergroup metasedimentary rocks from North Canterbury and Marlborough provide preliminary evidence for a continuation of the Kaweka Terrane of the central North Island into the Torlesse Composite Terrane of the South Island. This would extend from the south side of the Wairau Fault in the upper Wairau River valley southwards to the Lake Tennyson and Lake Sumner areas, and as far as Hawarden. Rb-Sr ages indicate Jurassic metamorphism, 170±24 Ma, with initial 87Sr/86Sr ratios at that time 0.7073±0.0007 i.e. similar to their North Island counterparts. These Kaweka Terrane rocks have detrital zircon ages that follow the distinctive pattern of the Torlesse rocks in general, i.e. substantial (>30%) Permian-Triassic and Precambrian-Early Palaeozoic groupings, but they also have minor youngest age components c. 175-165 Ma which constrain a maximum Early-Middle Jurassic depositional age. In detail, significant, older zircon components are Late Permian-Early Triassic (260-240 Ma) and Late Triassic (220-210 Ma), but the Precambrian-Early Palaeozoic detrital zircon ages are more scattered. The Kaweka Terrane thus forms a linear belt (c. 500 km) along the length of the Torlesse Composite Terrane in both North and South Islands. In the South Island it coincides with the Esk Head Belt, i.e. east of the Rakaia Terrane (Triassic-Late Permian) and west of the Pahau Terrane (Early Cretaceous-Late Jurassic). [ABSTRACT FROM AUTHOR]

Late Cretaceous calc-alkaline granites in the Gyeongsang Basin evolved through the mixing of mafic and felsic magmas. The host granites contain numerous mafic magmatic/microgranular enclaves of various shapes and sizes. New SHRIMP-RG zircon U-Pb ages of both granite and mafic magmatic/microgranular enclaves are 75.0 ± 0.5 Ma and 74.9 ± 0.6 Ma, respectively, suggesting that they crystallized contemporaneously after magma mixing. The time of injection of mafic melt into the felsic magma chamber can be recognized as approximately 75 Ma by field relations, petrographic features, geochemical evolution, and SHRIMP-RG zircon dating. This Late Cretaceous magma mixing event in the Korean Peninsula was probably related to the onset of subduction of the Izanagi (Kula)-Pacific ridge. [ABSTRACT FROM AUTHOR]

A fluctuation electron microscopy study of amorphous zircons is reported. A natural metamict zircon sample was compared with amorphous samples prepared by irradiating a gem-quality zircon crystal with a 1.0 MeV beam of N2+ ions over a range of fluences up to 1.5 × 1017 ions/cm2. The two types of amorphous material exhibit similar selected-area diffraction patterns. However, the statistics of the scattering from 1 nm wide volumes indicates that they have significantly different normalised intensity variance plots. Modelling of the variance suggests that the natural metamict zircon consists of paracrystalline grains of zircon embedded within an amorphous ZrSiO4 matrix. The ion-irradiated samples are better modelled as segregated paracrystalline grains of ZrO2 composition within an amorphous SiO2-rich matrix. Although both samples have been rendered disordered by radiation damage, there are two important differences. First, the ion-irradiated materials experience significantly higher fluence rates compared with the metamict materials. Second, the metamict zircon has had geologic time to anneal. The experiments show that zircon, freshly-amorphised by ion-irradiation, exhibits different medium-range order to that in natural metamict zircon. [ABSTRACT FROM AUTHOR]

natural, altered zircon crystal from an alkaline pegmatite from the Zomba-Malosa Complex of the Chilwa Alkaline Province in Malawi has been studied by a wide range of analytical techniques to understand the alteration process. The investigated zircon shows two texturally and chemically different domains. Whereas the central parts of the grain (zircon I) appear homogeneous in backscattered electron images and are characterised by high concentrations of trace elements, particularly Th, U, and Y, the outer regions (zircon II) contain significantly less trace elements, numerous pores, and inclusions of thorite, ytttrialite, and fergusonite. Zircon II contains very low or undetectable concentrations of non-formula elements such as Ca, Al, and Fe, which are commonly observed in high concentrations in altered radiation-damaged zircon. U-Pb dating of both zircon domains by LA-ICPMS and SHRIMP yielded statistically indistinguishable U-Pb weighted average ages of 119.3 ± 2.1 (2σ) and 118 ± 1.2 (2σ) Ma, respectively, demonstrating that the zircon had not accumulated a significant amount of self-irradiation damage at the time of the alteration event. Electron microprobe dating of thorite inclusions in zircon II yielded a Th-U-total Pb model age of 122 ± 5 (2σ) Ma, supporting the age relationship between both zircon domains. The hydrothermal solution responsible for the alteration of the investigated zircon was alkaline and rich in CO, as suggested by the occurrence of REE carbonates and CO-bearing fluid inclusions. The alteration of the crystalline, trace element-rich zircon is explained by an interface-coupled dissolution-reprecipitation mechanism. During such a process, the congruent dissolution of the trace element-rich parent zircon I was spatially and temporally coupled to the precipitation of the trace element-poor zircon II at an inward moving dissolution-precipitation front. The driving force for such a process was merely the difference between the solubility of the trace element-rich and -poor zircon in the hydrothermal solution. The replacement process and the occurrence of mineral inclusions and porosity in the product zircon II is explained by the thermodynamics of solid solution-aqueous solution systems. [ABSTRACT FROM AUTHOR]

The Neoproterozoic Atud diamictite in Wadi Kareim and Wadi Mobarak in the Eastern Desert of Egypt and the Nuwaybah formation in NW Saudi Arabia consist of poorly sorted, polymictic breccia, with clasts up to 1 m of granitoid, quartz porphyry, quartzite, basalt, greywacke, marble, arkose, and microconglomerate in fine-grained matrix. Stratigraphic relations indicate that the diamictite was deposited in a marine environment. Integrated field investigation, petrographic study and U–Pb SHRIMP zircon ages demonstrate that the Atud and Nuwaybah are correlative. The distribution of zircon ages indicate that ~750 Ma ages are dominant with a significant component of older materials, characterized by minor Mesoproterozoic and more abundant Paleoproterozoic and Neoarchean ages. Some matrix and metasedimentary clast zircons yield ages that are a few 10s of Ma younger than the age of the youngest clast (754 ± 15 Ma), suggesting Atud/Nuwaybah diamictite deposition ~750 Ma or slightly later, broadly consistent with being deposited during the Sturtian glaciation (740–660 Ma). The Paleoproterozoic and Neoarchean clasts have no source within the ensimatic Arabian–Nubian Shield. The distribution of the pre-Neoproterozoic ages are similar to the distribution of the pre-Neoproterozoic ages in Yemen and Saharan Metacraton, suggesting that these clasts have been transported hundreds of kilometers, maybe by ice-rafting. The Atud diamictite may represent important evidence for Cryogenian “Snowball Earth” in the Arabian–Nubian Shield. [ABSTRACT FROM AUTHOR]

This study documents the chemical and textural responses of zircon in the Elba igneous complex, with particular reference to the 7- to 7.8-Ma-old Monte Capanne pluton in relation to its coeval volcanic counterpart (Capraia), using BSE imaging and quantitative electron microprobe analyses. The Monte Capanne pluton displays multiple field and geochemical evidence for magma mixing. The samples we have investigated (including monzogranitic, mafic enclave and dyke samples) display similar zircon textures and are associated with an extremely large range of trace and minor element (Hf, Y, HREE, Th, U) compositions, which contrast with relatively simple textures and zoning patterns in zircons from a Capraia dacite. We have used a relatively simple textural classification (patchy zoning, homogenous cores, oscillatory zoning and unzoned zircon) as the basis for discussing the chemical composition and chemical variation within zircons from the Monte Capanne pluton. Based on these data and other works (Dini et al. in Lithos 78:101–118, 2004) , it is inferred that mixing between metaluminous and peraluminous melts occurred early in the evolution of the Monte Capanne magma chamber. In particular, mixing was responsible for the development of the patchy-zoning texture in the zircon cores, which was associated with reactions between other accessory phases (including monazite, apatite, allanite), which we infer to have significantly affected the Th distribution in zircon. Zircons from the MC pluton displaying “homogeneous cores” have chemical affinities with zircons in the coeval Capraia volcanic system, consistent with the participation of a Capraia-like mantle end-member during mixing. Further zircon growth in the MC pluton produced the oscillatory zoning texture, which records both long-term (crystal fractionation) and transient (recharge with both silicic and mafic magmas) events in a hybrid magma chamber. It is inferred that Hf and the Th/U ratio cannot be used alone to infer magmatic processes due to their dependency on temperature, nor are they a diagnostic feature of xenocrystic grains. This study shows that zircon chemistry coupled with detailed textural analysis can provide a powerful tool to elucidate the complex evolution of a magma system. [ABSTRACT FROM AUTHOR]

Variscan collision of peri-Gondwanan terranes led to a doubly vergent crustal wedge that was thicker than 55 km in the area of the Bohemian Massif. This crustal thickness resulted in a highly elevated Bohemian plateau with a topographic height >3–4 km. The Bohemian plateau was covered with unmetamorphic Paleozoic strata, all of which are today well preserved in the Tepla–Barrandian unit because of crustal-scale vertical slip along the Bohemian shear zone (BSZ). The BSZ forms a subvertical, ca. 500-km long and up to 2-km wide belt of dip–slip mylonites which show several 90° deflections in map view. Tepla-Barrandian-down movements were active under retrograde metamorphic conditions, starting with granulite and ceasing with greenschist facies conditions. As slip along the BSZ was largely vertical and led to a minimum throw of 10 km, this type of crustal-scale deformation is referred to as elevator tectonics. The elevator-style movements caused the juxtaposition of the supracrustal Tepla–Barrandian lid (the “elevator”) against high-grade rocks of the extruding orogenic root. The BSZ has further governed the foci of mantle-derived plutonism. New U–Pb zircon and monazite TIMS dating of six plutons suggest that emplacement of mantle-derived melts along the BSZ lasted for at least 20 m.y., starting with the emplacement of the Klatovy granodiorite at 347 +4/−3 Ma and ceasing with the emplacement of the Drahotin pluton at 328 ± 1 Ma. When taking into account the new ages of synkinematic plutons, the simultaneous vertical slip along the individual segments of the BSZ (North, West, and Central Bohemian shear zone) is bracketed to the period 343–337 Ma. Elevator tectonics was probably controlled by delamination of thickened mantle lithosphere that caused a dramatic thermal turnover and heating-up of the orogenic root. The overheated lower crust was thermally softened by anatexis and diffusion creep resulting in channel flow, vertical extrusion, fast uplift, and exhumation of the orogenic root. [ABSTRACT FROM AUTHOR]

Outgrowths of zircon and xenotime are abundant on detrital zircon in a greenschist facies regional metamorphic slate from the Scottish Highlands. Back-scattered electron images reveal that ca. 3-μm wide zircon outgrowths are porous, inclusion-rich and contain fine-grained intergrowths with xenotime. Focused ion beam milling, transmission electron microscopy and low-voltage scanning transmission electron microscopy show that the outgrowth microstructure is complex, composed of mis-orientated nano-crystalline zircon and a poorly crystalline zircon–xenotime complex. Well-defined micron-sized xenotime is also present within the outgrowth. Micro-textures show that the outgrowth developed in a polyphase history during cooling and exhumation. Accommodation space for the outgrowth is probably generated by a combination of thermal decompaction and the migration of defects in adjacent quartz driven by the force of crystallisation. Zircon may be unique in its ability to record textural evidence of events during uplift and exhumation that are not recorded in major silicate phases. [ABSTRACT FROM AUTHOR]

The large Variscan Karkonosze Granite in the West Sudetes, representative of the vast Variscan granite plutonism in Central Europe and located adjacent to regional tectonic suture and strike-slip-zones, has been difficult to date precisely; a range of published data varies between c. 304 and 328 Ma. However, the granite is cut by locally numerous lamprophyre and other dykes. Dating of the dyke rocks, emplaced shortly after the granite intrusion and cooled more rapidly, provides a promising tool for the verification of published SHRIMP results on the granite itself. SHRIMP zircon geochronology of a studied micromonzodiorite dyke indicates substantial admixture of inherited zircons of c. 2.0, 1.4 Ga (207Pb-206Pb minimum ages), and c. 570 (and 500?) Ma. The average concordia age of the main magmatic population of the zircons in the dyke is 313 ±3 Ma (2s); however, the true magmatic age might be older, around 318 Ma. This would constrain the age of the hypabyssal magmatism in the Karkonosze Massif and the minimum age of the host Karkonosze Granite. Thus, the Karkonosze Granite is confirmed as representative of an early phase of Variscan granite plutonic activity in the central-European Variscides. [ABSTRACT FROM AUTHOR]

Approximately 500-Ma-old orthogneisses are widespread in the eastern part of the Variscan belt and are commonly interpreted to have intruded mica-schist series of assumed Neoproterozoic age. New SHRIMP zircon ages of quartzofeldspathic metavolcanogenic rocks of the mica schist series in the eastern part of the Karkonosze-Izera Massif (SW Poland) indicate that they are late Cambrian/early Ordovician rather than Neoproterozoic in age, based on the zircon age spectra distributed mainly between ca. 500 and 660 Ma (with a few Proterozoic inherited minimum ages of ca. 970 and 1,825 Ma). Younger zircon dates, dispersed between ca. 412 and 464 Ma, are interpreted as a result of Pb-loss likely caused by subsequent metamorphism. Consequently, the felsic metavolcanogenic rocks appear to be roughly contemporaneous with the intrusion of ca. 500-Ma-old orthogneiss protoliths (with the pooled concordia age of 487 ± 8 Ma interpreted as the best approximation of the protolith intrusive age). Field relationships, petrological and geochemical features of the felsic and mafic rocks studied support a model in which the accompanying mica schist series are not the original country rocks to the ca. 500 Ma granite intrusions, and indicate that their recent close proximity is the result of tectonic juxtaposition. However, both the mica schists enclosing the bimodal metavolcanic rocks, and the orthogneisses, are interpreted to represent a Cambro-Ordovician passive continental margin sequence being part of the Saxothuringian domain. They are tectonically overlain to the east by HP/T metamorphic units, comprising MORB-type metaigneous rocks, and delineating a tectonic suture separating the Saxothuringian block in the west from an assumed continental block (Tepla-Barrandian) to the south-east. [ABSTRACT FROM AUTHOR]

New whole rock Rb-Sr and zircon U-Pb geochronological data and Sm-Nd isotopic data are presented from the central magmatic arc domain of the Antarctic Peninsula in the area of north-west Palmer Land and south-west Graham Land, Rb-Sr isochrons indicate an age of 169 ± 6 Ma for basement orthogneisses and 132 ± 9 to 71 ± 9Ma for plutons. A U-Pb age of 183 ± 2.1 Ma, with no detectable inheritance, on zircons from an orthogneiss from Cape Berteaux provides the first reliable age for the orthogneisses, which are interpreted as metamorphosed silicic volcanic rocks, and Sm-Nd data indicate derivation in a mature volcanic arc. The age indicates they may be correlatives of the Jurassic 'Chon Aike' volcanism of the eastern Antarctic Peninsula. A U-Pb zircon age of 107 ± 1.7 Ma on a terrestrial volcanic sequence overlying an uncomformity strongly suggests a mid-Cretaceous age for the extensive volcanic cover of north-west Palmer Land that was previously thought to be Jurassic. The unconformity is interpreted to have been a result of compressional uplift related to the Palmer Land event. This is the first date for the event in the western part of the central magmatic arc terrane of the Antarctic Peninsula. [ABSTRACT FROM AUTHOR]

Zircon and apatite fission track (AFT) thermochronology was applied to the James Ross Basin sedimentary rocks from James Ross and Seymour islands. The probable sources of these sediments were generated in Carboniferous to Early Paleogene times (∼315 to 60 Ma). The total depths of individual James Ross Basin formations are discussed. The AFT data were modelled, and the thermal history model was reconstructed for samples from Seymour Island. The first stage after a period of total thermal annealing (when the samples were above 120°C) involved Late Triassic cooling (∼230 to 200 Ma) and is followed by a period of steady cooling through the whole apatite partial annealing zone (PAZ, 60-120°C) to minimum temperature in Paleocene/Early Eocene. The next stage was the maximum burial of sedimentary rocks in the Eocene (∼35 Ma, 1.1-1.8 km) and the final cooling and uplift of Seymour Island sedimentary rocks at ∼35 to 20 Ma. [ABSTRACT FROM AUTHOR]

Within individual plutons, the trace element concentrations in S-type granites generally increase with maficity (total iron and magnesium content and expressed as atomic Fe + Mg in this study); the degree of variability in trace element concentration also expands markedly with the same parameter. The strongly peraluminous, high-level S-type granites of the Peninsular Pluton (Cape Granite Suite, South Africa) are the product of biotite incongruent melting of a metasedimentary source near the base of the crust. Leucogranites within the suite represent close to pure melts from the anatectic source and more mafic varieties represent mixtures of melt and peritectic garnet and ilmenite. Trace elements such as Rb, Ba, Sr and Eu, that are concentrated in reactant minerals in the melting process, show considerable scatter within the granites. This is interpreted to reflect compositional variation in the source. In contrast, elements such as LREE, Zr and Hf, which are concentrated within refractory accessory phases (zircon and monazite), show well-defined negative correlations with increasing SiO2 and increase linearly with increasing maficity. This is interpreted to reflect coupled co-entrainment of accessory minerals and peritectic phases to the melt: leucocratic rocks cannot have evolved from the more mafic compositions in the suite by a process of fractional crystallisation because in this case they would have inherited the zircon-saturated character of this hypothetical earlier magma. Trace element behaviour of granites from the Peninsular Pluton has been modelled via both equilibrium and disequilibrium trace element melting. In the disequilibrium case, melts are modelled as leaving the source with variable proportions of entrained peritectic phases and accessory minerals, but before the melt has dissolved any accessory minerals. Thus, the trace element signature of the melt is largely inherited from the reactants in the melting reaction, with no contribution from zircon and monazite dissolution. In the equilibrium case, melt leaves the source with entrained crystals, after reaching zircon and monazite saturation. A significant proportion of the rocks of the Peninsular Pluton have trace element concentrations below those predicted by zircon and monazite saturation. In the case of the most leucocratic rocks all compositions are zircon undersaturated; whilst the majority of the most mafic compositions are zircon oversaturated. However, in both cases, zircon is commonly xenocrystic. Thus, the leucocratic rocks represent close to pure melts, which escaped their sources rapidly enough that some very closely match the trace element disequilibrium melting model applied in this study. Zircon dissolution rates allow the residency time for the melt in the source to be conservatively estimated at less than 500 years. [ABSTRACT FROM AUTHOR]

Two hypotheses have been offered to account for the transport and accretion history of the Yakutat terrane in southern Alaska. To investigate these two options, we deconvolved fission-track (FT) and U/Pb ages of detrital zircons from stratigraphically coordinated samples collected in the northern Robinson Mountains into component populations. The strata of the Yakutat terrane include the Middle Eocene Kulthieth Formation, the Lower Oligocene to Lower Miocene Poul Creek Formation, and the Miocene-Pleistocene Yakataga Formation. The Kulthieth and Poul Creek formations record erosion of a simple, uniform, long-lived, nonvolcanic source terrain that crystallized from ~50 to 220 Ma and cooled from ~40 to 110 Ma. Miocene cooling episodes recorded in the source to the Kulthieth and Poul Creek formations are likely associated with plutons in the northern Coast Plutonic Complex and the Kuiu-Etoilin belt. The Upper Miocene to Pleistocene Yakataga Formation records erosion of rocks that crystallized from ~50 to 53 Ma and cooled below the zircon FTclosure at ~70-20 Ma. Upper Miocene strata are likely derived from erosion of the Chugach--Prince William terranes and the superimposed Sanak-Baranof plutonic belt. The uniform provenance of the Kulthieth and Poul Creek formations, the overall FT grain age distribution, and the distinct lack of volcanic zircons favor a northern position of the Yakutat terrane since the Eocene. However, a far-traveled southern option for the basement rocks cannot be ruled out, but it is unlikely that the Eocene and younger cover strata were deposited far to the south. [ABSTRACT FROM AUTHOR]

Typological study, including paragenic, morphological, textural, and chemical characteristics of zircon from nine rare metal granitic stocks and associated greisens, was carried out in order to identify the metallogenic processes of their host granitoids. The investigated zircon-bearing granitoids and type occurrences can be categorized into magmatically and metasomatically specialized types. The magmatic type includes: (i) peralkaline, Zr + Nb-enriched, A1-granite (e.g. Um Hibal); (ii) metaluminous, Nb + Zr + Y-enriched, A2-type alkali granite (e.g. Hawashia and Ineigi); and (iii) peraluminous, Ta ≥ Nb + Sn + Be ± W-enriched, Li-albite granite (e.g. Nuweibi, Igla and Abu Dabbab). The metasomatized granites are Nb>>Ta + Sn + Zr + Y + U ± Be ± W-enriched and hydrothermally altered alkali feldspar granite (i.e. apogranite; e.g. Um Ara, Abu Rusheid, and Um Naggat). Zircon of peralkaline granite is characteristically equant with well-developed pyramidal faces and short prisms (i.e. pseudo-octahedral form) with length/width ratios in the range of 2:1–1:1. It is of Zr0.990Hf0.007SiO4 composition and is associated with hypersolvus assemblage consisting of alkali feldspar, quartz, aegirine and minor reibeckite. Zircon of metaluminous alkali granites is of Zr0.99Hf0.01SiO4 composition and is associated with sub- to transolvus assemblage of K-feldspar, quartz, plagioclase and annite-siderophyllite mica. It is prismatic with length/width ratios in the range of 5:1–3:1, doubly terminated with small pyramidal faces. Compositionally, zircon of Li-albite granite ranges between Zr0.925Hf0.075SiO4 and Zr0.705Hf0.295SiO4. It is idiomorphic with a simple combination of prism and bipyramidal terminations with a length/width ratio of 3:1–2:1. This zircon commonly exhibits a normal zoning with rims consistently higher in Hf than cores. The higher Hf content, of this zircon coupled with its association with topaz, tantalite and lithian micas (e.g. zinnwaldite and Li-white mica), indicates a higher solubility of Hf-fluoride complexes and their more stabilized state at lower temperature in Li- and F-rich sodic melts. Zircon of apogranite association ranges in composition between Zr0.967Hf0.013SiO4 in the lower unaltered alkali feldspar granite zone and Zr0.805Hf0.064(Y, U, Th, heavy rare-earth elements) [HREE])0.125SiO4 in the apical metasomatized (i.e. microclinized, albitized, and greisenized) apogranite zones. This compositional change appears to reflect a roofward increasing in μKF, μNaF, and μHF of the exsolved fluids. Columbite, xenotime, thorite, cassiterite, beryl and fluorite are common associates of this zircon. This zircon is of bipyramidal to typical octahedral form with complete absence of prism concurrently with conspicuous development of pyramid, thus the zircon crystals have a length/width ratio of 1:1–0.5:1. The neoformed metasomatic zircon commonly exhibits either normal or reverse zoning with rims consistently different in Hf, U, Y, and HREE than cores, reflecting disequilibrium conditions (e.g. sudden change in P, T, salinity, and pH) between the growing crystals and the exsolved fluids. [ABSTRACT FROM AUTHOR]

SHRIMP U-Pb zircon dating gives ages of 260.6 ± 3.5 Ma and 260.7 ± 5.6 Ma for serpentinised wehrlite and plag-hornblendite in the Jinbaoshan ultramafic intrusion, respectively. The results indicate that the Jinbaoshan intrusion was emplaced at ca.260 Ma and contemporaneous with the Emeishan continental flood basalts (ECFB), similar to other maficultramafic intrusions of the Emeishan large igneous province (LIP). The new ages provide a geochronological constraint on the origin of the Jinbaoshan ultramafic intrusion. It confirms that the Jinbaoshan ultramafic intrusion belongs to the Emeishan LIP that formed at ca. 260 Ma. [ABSTRACT FROM AUTHOR]

Illite crystallinity, K–Ar dating of illite, and fission-track dating of zircon are analyzed in the hanging wall (Sampodake unit) and footwall (Mikado unit) of a seismogenic out-of-sequence thrust (Nobeoka thrust) within the Shimanto accretionary complex of central Kyushu, southwest Japan. The obtained metamorphic temperatures, and timing of metamorphism and cooling, reveal the tectono-metamorphic evolution of the complex, and related development of the Nobeoka thrust. Illite crystallinity data indicate that the Late Cretaceous Sampodake unit was metamorphosed at temperatures of around 300 to 310°C, while the Middle Eocene Mikado unit was metamorphosed at 260 to 300°C. Illite K–Ar ages and zircon fission-track ages constrain the timing of metamorphism of the Sampodake unit to the early Middle Eocene (46 to 50 Ma, mean = 48 Ma). Metamorphism of the Mikado unit occurred no earlier than 40 Ma, which is the youngest depositional age of the unit. The Nobeoka thrust is inferred to have been active during about 40 to 48 Ma, as the Sampodake unit started its post metamorphic cooling after 48 Ma and was thrust over the Mikado unit at about 40 Ma along the Nobeoka thrust. These results indicate that the Nobeoka thrust was active for more than 10 million years. [ABSTRACT FROM AUTHOR]

A structural, petrological and geochronological (U-Th-Pb of zircon and monazite) study reveals that the lower crust sequences of the Variscan high-grade basement cropping out between Solenzara and Porto Vecchio, south-east Corsica (France) have been tectonically juxtaposed along with middle crustal rocks during the extrusion of the orogenic root of the Variscan chain. We propose that a system of high-temperature, orogen-parallel shear zones that developed under a transpressive dextral tectonic regime caused the exhumation of the entire sequence. This tectonic complex is thus made up of rocks having undergone different P–T conditions (eclogite-?, high-pressure granulite facies and amphibolite facies) at different times, reflecting the progressive foreland migration of the orogenic front. The Solenzara granulites were derived from burial of continental crust to high-pressure (1.8–1.4 GPa) and high- to ultrahigh-temperature conditions (900–1000 °C) during the Variscan convergence: U–Pb ELA-ICPMS zircon dating constrained the timing of this metamorphism at c. 360 Ma. The gneisses cropping out at Porto Vecchio are middle crustal-level rocks that reached their peak temperature conditions (700–750 °C at <1.0 GPa) at c. 340 Ma. The diachronism of the metamorphic events, the foliation patterns and their geometry suggest that the granulites were exhumed to middle crustal levels through channel flow tectonics under continuous compression. The amphibolite facies gneisses of Porto Vecchio and the granulites of Solenzara were accreted through the development of a major dextral mylonitic zone forming under amphibolite facies conditions: in situ monazite isotope dating (ELA-ICPMS) revealed that this deformation occurred at c. 320 Ma and was accompanied by the emplacement of syntectonic high-K melts. A final HTLP static overprint, constrained at 312–308 Ma by monazite U-Th-Pb isotope dating, is related to the emplacement of the igneous products of the Sardinia-Corsica batholith and marks the transition from the Variscan orogenic event to the Permian extension. [ABSTRACT FROM AUTHOR]

Based on the detailed analyses, multi-proxies such as AMS 14C dating, grain size, component and morphology of heavy minerals, micromorphology of zircon, magnetic susceptibility, Rb/Sr and Hg content are used to research the paleoflood sediments archived in the Yuxi Site. The research indicates that, since 7.6 kaBP, at least 16 times of paleoflood with water level above Wusong Elevation 147.024 m (a.s.l.) left deposits in the Neolithic layers of unit T0403 from the Vuxi Site. The results are induced from the following aspects: (1) The plaeoflood sediments take on great similarities with modern flood sediments in the Yuxi Site and Zhongba Site in probability cumulative curves which mainly show a pattern of 3 segments in fluvial pattern. (2) There is some resemblance between the heavy mineral components and the zircon shape characteristics of paleoflood deposits and those of modern flood deposits. (3) Magnetic susceptibility values (40.44-70.1OSI) are lower than those of sediments from cultural layers (59.59-188.68Sl). (4) Hg values (290.71 -742.51 ng/g) are lower than those of sediments from cultural layers (344.16-1051 8.17 ng/g). (5) Rb/Sr values are higher than those of sediments from cultural layers, while those of the 4th, 5th, 6th, 7th, 8th cultural layers are high, which shows that they are inundated by paleoflood. The reason for many flood deposits existing is related to the site situated on the first terrace where the Yuxi River joins with the Yangtze River. As there are some similarities between the plaeoflood sediments and those of the Zhongba Site, it is feasible to confirm the existence of paleoflood sediments based on the above points. [ABSTRACT FROM AUTHOR]

Migmatite gneisses are widespread in the Dabie orogen, but their formation ages are poorly constrained. Eight samples of migmatite, including leucosome, melanosome, and banded gneiss, were selected for U–Pb dating and Hf isotope analysis. Most metamorphic zircon occurs as overgrowths around inherited igneous cores or as newly grown grains. Morphological and internal structure features suggest that their growth is associated with partial melting. According to the Hf isotope ratio relationships between metamorphic zircon and inherited cores, three formation mechanisms for metamorphic zircon can be determined, which are dissolution–reprecipitation of pre-existing zircon, breakdown of Zr-bearing phase other than zircon in a closed system and crystallization from externally derived Zr-bearing melt. Four samples contain magmatic zircon cores, yielding upper intercept U–Pb ages of 807 ± 35–768 ± 12 Ma suggesting that the protoliths of the migmatites are Neoproterozoic in age. The migmatite zircon yields weighted mean two-stage Hf model ages of 2513 ± 97–894 ± 54 Ma, indicating reworking of both juvenile and ancient crustal materials at the time of their protolith formation. The metamorphic zircons give U–Pb ages of 145 ± 2–120 ± 2 Ma. The oldest age indicates that partial melting commenced prior to 145 Ma, which also constrains the onset of extensional tectonism in this region to pre-145 Ma. The youngest age of 120 Ma was obtained from an undeformed granitic vein, indicating that deformation in this area was complete at this time. Two major episodes of partial melting were dated at 139 ± 1 and 123 ± 1Ma. The first episode of partial melting is obviously older than the timing of post-collision magmatism, corresponding to regional extension. The second episode of partial melting is coeval with the widespread post-collision magmatism, indicating the gravitational collapse and delamination of the orogenic lithospheric keel of the Dabie orogen, which were possibly triggered by the uprising of the Cretaceous mid-Pacific superplume. [ABSTRACT FROM AUTHOR]

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The Eoarchaean (>3,600 Ma) Itsaq Gneiss Complex of southern West Greenland is dominated by polyphase orthogneisses with a complex Archaean tectonothermal history. Some of the orthogneisses have c. 3,850 Ma zircons, and they vary from rare single phase metatonalites to more common complexly banded migmatites. This is due to heterogeneous strain, in situ anatexis and granitic veining superimposed during younger tectonothermal events. In the single-phase tonalites with c. 3,850 Ma zircon, oscillatory-zoned prismatic zircon is all 3,850 Ma old, but shows patchy ancient loss of radiogenic Pb. SHRIMP spot analyses and laser ablation ICP-MS depth profiling show that thin (usually < 10 μm) younger (3,660–3,590 Ma and Neoarchaean) shells of lower Th/U metamorphic zircon are present on these 3,850 Ma zircons. Several samples with this simple zircon population occur on islands near Akilia. In contrast, migmatites usually contain more complex zircon populations, with often more than one generation of igneous zircon present. Additional zircon dating of banded gneisses across the Complex shows that samples with c. 3,850 Ma igneous zircon are not just a phenomenon restricted to Akilia and adjacent islands. For example, migmatites from Itilleq (c. 65 km from Akilia) contain variable amounts of oscillatory-zoned 3,850 Ma and 3,650 Ma zircon, interpreted, respectively, as the rock age and the time of crustal melting under Eoarchaean metamorphism. With only 110–140 ppm Zr in the tonalites and likely magmatic temperatures of >850°C, zircon solubility–melt composition relationships show that they were only one-third saturated in zircon. Any zircon entrained in the precursor magmas would thus have been highly soluble. Combined with the cathodoluminesence imaging, this demonstrates that the c. 3,850 Ma oscillatory zoned zircon crystallised out of the melt and hence gives a magmatic age. Thus the rare well-preserved tonalites and palaeosome in migmatites testify that c. 3,850 Ma quartzo–feldspathic rocks are a widespread (but probably minor) component in the Itsaq Gneiss Complex. C. 3,850 Ma zircon with negative Eu anomalies (showing growth in felsic systems) also occurs as detrital grains in rare c. 3,800 Ma metaquartzites and as inherited grains in some 3,660 Ma granites ( sensu stricto). These demonstrate that still more c. 3,850 Ma rocks were present, but were recycled into Eoarchaean sediments and crustally derived granites. The major and trace element characteristics (e.g. LREE enrichment, HREE depletion, low MgO) of the best-preserved c. 3,850 Ma rocks are typical of Archaean TTG suites, and thus argue for crust formation processes involving important contributions from melting of hydrated mafic crust to the earliest Archaean. Five c. 3,850 Ma tonalites were selected as the best preserved on the basis of field criteria and zircon petrology. Four of these samples have overlapping initial ɛNd (3,850 Ma) values from +2.9 to +3.6± 0.5, with the fourth having a slightly lower value of +0.6. These data provide additional evidence for a markedly LREE-depleted early terrestrial mantle reservoir. The role of c. 3,850 Ma crust should be considered in interpreting isotope signatures of the younger (3,800–3,600 Ma) rocks of the Itsaq Gneiss Complex. [ABSTRACT FROM AUTHOR]