Zircon of Triassic Age in the Stuttgart Formation (Schilfsandstein)—Witness of Tephra Fallout in the Central European Basin and New Constraints on the Mid-Carnian Episode

The Carnian Stuttgart-Formation (Schilfsandstein) of the Central European Basin contains relics of Triassic volcanic detritus in form of euhedral zircon grains and authigenic analcime. Multiple LA-ICP-MS spot analyses of single zircon crystals from an outcrop near Heilbronn (SW Germany) yielded weighted average 206Pb/238U ages between 250 and 230 Ma, providing evidence for tephra fallout in the southern part of the Central European Basin related to Olenekian, Anisian–Ladinian and Carnian volcanic activity. The tephra was probably transported by monsoonal circulations from volcanic centres of the NW Tethys to the Central European Basin. The four youngest zircon crystals gave a weighted average 206Pb/238U age of 231.1 ± 1.6 Ma (10 analyses), which is interpreted to date syn-depositional tephra fallout into the fluvial Lower Schilfsandstein Member of the Stuttgart Formation. This new maximum depositional age provides the first evidence that deposition of the Stuttgart Formation, which represents the type-example of the mid-Carnian episode, a global episode of enhanced flux of siliciclastic detritus and related environmental perturbations, occurred during the Tuvalian 2 substage at ca. 231 Ma, about 3 million years later than suggested by previous correlations. Zircon grains with weighted average 206Pb/238U ages of 236.0 ± 1.2 Ma (n = 17) and 238.6 ± 1.5 Ma (n = 6) and 206Pb/238U ages between 241 ± 6 and 250 ± 3 Ma point to the presence of tephra in early Carnian to Olenekian strata of the Keuper to Buntsandstein Groups. Traces of these reworked tephra were incorporated into the Stuttgart Formation due to fluvial erosion in the southern Central European Basin and at its margins.

Technological investigations of alluvial garnet sands from the Kola Peninsula

The paper presents the investigation results on two technological samples of alluvial garnet sands from Yavr area located in the southwest of the Kola Peninsula. The first sample TP-11/2 (mass 250 kg) was used to study the granulometric, mineralogical and chemical composition of garnet sands. The second sample TP-12 (mass about 6.5 ton) was used to manufacture some garnet concentrates for the waterjet cutting testing. It has been established that the main material of TP-11/2 sample is concentrated in the size of -0.63 + 0.10 mm. The larger particles make 5.0%, the smaller particles make 7.0%. The size distribution in the TP-12 sample is similar. The main minerals of the sands are quartz, feldspar, garnet, amphiboles, pyroxenes, mica, rutile and ilmenite. The percentage of secondary minerals represented by sillimanite, kyanite, apatite, tourmaline, graphite, magnetite, and single zircon grains is less than 2%. Garnet in Javr area sands is 9–-97% in the free phase. The mass concentration of garnet in the sands is 15–19%, the mass concentration of rutile is 1.0%. Garnet corresponds to almandine in composition. A small part of garnets (about 15–20%) contains a pyrope component. The prevailing shape of garnet grains is angular; the profile of the grains is predominantly isometric and elongated. The garnet sands were dressed at the pilot concentration plant of the Mining Institute, in the gravitymagnetic-electric circuit using the following equipment: a fine screen, a spiral sluice, a concentration table, a weak field magnetic separator, a high-intensity magnetic separator, an electrostatic separator for scavenging of rough garnet concentrates. According to the test results, all garnet concentrates with the size of –0.63 + 0.315 mm, –0.315 + 0.18 mm, –0.18 + 0.10 mm were assumed suitable for the waterjet cutting.

Tectonic significance of the Variscan suture between Brunovistulia and the Bohemian Massif

The Velké Vrbno Dome crops-out at the boundary between the Brunovistulian Terrane and the internal parts of the Bohemian Massif. Here, eclogite boudins occur within an Ediacaran volcano-sedimentary sequence. Strong Nb depletion (Nb/Nb* = 0.19 – 0.82) combined with moderately positive Nd isotopic compositions (εNd(i) = +3.89 – +5.77) are used to argue for emplacement of the eclogite protoliths in a transitional supra-subduction to continental-rift setting. Conversely, heterogeneously enriched large ion lithophile elements and highly radiogenic Sr isotopic ratios (87Sr/86Sr = 0.705–0.720) are interpreted to have been modified following fluid infiltration subsequent to eclogite-facies metamorphism.U-Pb laser ablation inductively coupled plasma mass spectrometry dating of magmatic zircon from the rift-type eclogite indicates Early Cambrian emplacement (c.535 Ma) following episodic Ediacaran volcanic arc activity. Moreover, a continental setting is emphasised by zircon dating of a mylonitic orthogneiss, revealing a fragment of Palaeoproterozoic (c.2000 Ma) basement, the first such finding within the Brunovistulian Terrane sensu stricto.The new data from eclogite confirm that rifting in this segment of Gondwana pre-dated the Ordovician opening of the Rheic Ocean and therefore that the suture between Brunovistulia and the rest of the Bohemian Massif likely represents the vestige of an older hyperextended basin or oceanic tract.Supplementary material: Previously unpublished single zircon evaporation ages from Ediacaran orthogneiss from the Velké Vrbno Dome (supplement A); detailed analytical methodology (supplement B); whole rock geochemical data (supplement C); and U-Pb LA-ICP-MS zircon data (supplement d). https://doi.org/10.6084/m9.figshare.c.5233079

Impact and Correction of Analytical Positioning on Accuracy of Zircon U-Pb Dating by SIMS

Secondary ion mass spectrometry (SIMS) is one of the most important analytical tools for geochronology, especially for zircon U-Pb dating. Due to its advantages in spatial resolution and analytical precision, SIMS is the preferred option for multi-spot analyses on single zircon grain with complex structures. However, whether or how much the relative positions of multiple analytical spots on one zircon grain affect the U-Pb age accuracy is an important issue that has been neglected by most researchers. In this study, we carried out a series of investigation on the influence of relative analytical position during zircon U-Pb age analyses, using Cameca IMS 1280-HR instrument. The results demonstrated a significant influence on the second spot, with apparent U-Pb age deviation as high as around 10% especially on the left and right side with overlap in the raster area. Nevertheless, a linear correlation between a secondary ion centering parameter (DTCA-X) and age deviation in percentage terms was found, and a calibration method was established to correct this position effect. Four zircon standards (91500, M257, TEMORA-2, and Plešovice) were measured to prove the reliability of the established procedure. The original U-Pb apparent data show inconsistent deviation on four directions relative to the datum, while the final U-Pb age results is calibrated to be consistent with their recommended values, within uncertainties of ~1%. This work calls for re-examination for the previous SIMS U-Pb dating results on core-rim dating strategy, and provides a calibration protocol to correct the relative position effect.

Photoluminescence Imaging of Whole Zircon Grains on a Petrographic Microscope—An Underused Aide for Geochronologic Studies

The refractory nature of zircon to temperature and pressure allows even a single zircon grain to preserve a rich history of magmatic, metamorphic, and hydrothermal processes. Isotopic dating of micro-domains exposed in cross-sections of zircon grains allows us to interrogate this history. Unfortunately, our ability to select the zircon grains in a heavy mineral concentrate that records the most geochronologic information is limited by our inability to predict internal zonation from observations of whole grains. Here we document the use of a petrographic microscope to observe and image the photoluminescence (PL) response of whole zircon grains excited under ultraviolet (UV) light, and the utility of this PL response in selecting grains for geochronology. While zircon fluorescence has long been known, there is limited documentation of its utility for and application to geochronologic studies. Our observations of zircon from an un-metamorphosed igneous rock, two meta-igneous rocks, and a placer deposit show that variations in the PL color are readily observable in real-time, both among grains in a population of zircons and within single grains. Analyses of cross-sections of the same grains demonstrate that the changes in PL correlate with zoning in backscattered electron (BSE) and cathodoluminescence (CL) images as well as with changes in U + Th concentration and spectroscopic proxies for radiation damage. In other words, the whole grain PL provides a low-resolution preview of the U + Th zoning expected in a cross-sectioned grain. We demonstrate the usefulness of this “preview” in identifying and selecting the subset of zircon grains in a heavy mineral separate that has metamorphic rims of sufficient width to date by secondary ionization mass spectrometry (SIMS). The data are also used to place preliminary constraints on the age and U + Th concentrations at which a yellow PL response is observed in natural samples. The PL response of zircon is well-known among spectroscopists, and these simple applications demonstrate several ways in which the response might be more effectively used by geochronologists.

Significant Zr isotope variations in single zircon grains recording magma evolution history

Zircons widely occur in magmatic rocks and often display internal zonation finely recording the magmatic history. Here, we presented in situ high-precision (2SD <0.15‰ for δ94Zr) and high–spatial-resolution (20 µm) stable Zr isotope compositions of magmatic zircons in a suite of calc-alkaline plutonic rocks from the juvenile part of the Gangdese arc, southern Tibet. These zircon grains are internally zoned with Zr isotopically light cores and increasingly heavier rims. Our data suggest the preferential incorporation of lighter Zr isotopes in zircon from the melt, which would drive the residual melt to heavier values. The Rayleigh distillation model can well explain the observed internal zoning in single zircon grains, and the best-fit models gave average zircon–melt fractionation factors for each sample ranging from 0.99955 to 0.99988. The average fractionation factors are positively correlated with the median Ti-in-zircon temperatures, indicating a strong temperature dependence of Zr isotopic fractionation. The results demonstrate that in situ Zr isotope analyses would be another powerful contribution to the geochemical toolbox related to zircon. The findings of this study solve the fundamental issue on how zircon fractionates Zr isotopes in calc-alkaline magmas, the major type of magmas that led to forming continental crust over time. The results also show the great potential of stable Zr isotopes in tracing magmatic thermal and chemical evolution and thus possibly continental crustal differentiation.

Detailed timescale of magma-chamber assembly and eruption revealed by ultra-high precision zircon U-Pb geochronology on a Permian caldera plumbing system, Sesia Magmatic System (southern Alps, Italy)

&lt;p&gt;In recent years, technical developments in isotope dilution thermal ionization mass spectrometry technique (ID-TIMS) have pushed the precision of single zircon U-Pb geochronology to new limits. The use of interlaboratory calibrated U-Pb tracer solutions for isotopic dilution [1] paired with using newly developed high ohmic resistors (10&lt;sup&gt;13&lt;/sup&gt;Ohm) in Faraday cup amplifiers, allow the determination of single zircon dates with precision and accuracy at the 0.02 % level [2].This level of analytical precision makes the ID-TIMS technique a geochronological tool able to unravel the detailed temporal evolution of magmatic plumbing systems older than the Mesozoic Era.&lt;/p&gt;&lt;p&gt;In the southern Alps, a thick sliver of continental crust, tilted and exhumed during the Alpine orogeny, is exposed as a complete crustal cross-section (Ivrea crustal section). This section preserves a transcrustal magmatic system, developed in an extensional environment in &lt;em&gt;ca.&amp;#160;&lt;/em&gt;4 My during the Early Permian [3]. Its upper crustal portion consists of a zoned granitic intrusion (Valle Mosso pluton) overlaid by a dominantly rhyolitic caldera-related volcanic field (Sesia Caldera).&lt;/p&gt;&lt;p&gt;To obtain a time-integrated view of the petrological evolution of this plumbing system, we combine a new ultra high precision ID-TIMS zircon U-Pb dataset with zircon geochemistry from samples collected in compositionally and texturally different units of the Valle Mosso pluton and Sesia Caldera. All the analyzed units are coeval within 700 ky and the overall trends in zircon trace elements (Eu*/Eu, Zr/Hf, Sm/Yb) suggest an evolution of the reservoir dominated by fractional crystallization. The data show a ca. 200 ky gap in zircon crystallization, following the injection of recharge magma that triggered the eruption of the crystal-rich rhyolite followed by caldera collapse [3]. This suggests mass addition and rejuvenation of a partly crystallized mush, which temporarily hindered zircon crystallization. On the other hand, crystal-poor rhyolites, characterized by a younger eruption age and evolved zircon composition, likely represent late stage evacuation of evolved melt lenses extracted from a mostly crystalline framework.&lt;/p&gt;&lt;p&gt;[1] Condon, D. J., et al., 2015, Geochim. Cosmochim. Acta, &lt;strong&gt;164&lt;/strong&gt;, 464-480.&lt;/p&gt;&lt;p&gt;[2] Wotzlaw, J. F., et al., 2017, J. Anal. At. Spectrom., &lt;strong&gt;32&lt;/strong&gt;, 579-586.&lt;/p&gt;&lt;p&gt;[3] Karakas, O., et al., 2019, Geology, &lt;strong&gt;47&lt;/strong&gt;, 1-5.&lt;/p&gt;

Magmas of the El Quemado Complex (Chon Aike Silicic Igneous Province, Patagonia): Elevated Oxygen Isotope Signatures Across Space and Time

&lt;p&gt;The generation and source of ~230,000 km&lt;sup&gt;3&lt;/sup&gt; of total erupted volume of the predominately silicic (&gt;90 %; Pankhurst et al., 2000) magmas which comprise the Jurassic Chon Aike Large Silicic Igneous Province (CASP) of Southern Patagonia is currently debated. In this study, we conducted a widespread sampling of multiple eruptive units, primarily ignimbrites and minor rhyolitic flows, along the Eastern Andean front (~47&amp;#176;S to 49&amp;#176;S), owning to the third and youngest eruptive episode of the CASP (El Quemado Complex; EQC). To determine the magmatic source and potential role of a significant crustal contribution proposed in the generation of these magmas, we analyzed the in-situ &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O composition of both quartz and zircon by SIMS. We combined these data with LA-ICP-MS U/Pb analyses on single zircon crystals to characterize the potential for changing oxygen isotopic values through time and space within the EQC units along this ~230 km long transect.&lt;/p&gt;&lt;p&gt;The northern-most units sampled have the lightest average &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O (relative for the EQC) analyzed in zircon and quartz (7.7 and 10.4 &amp;#8240;, respectively). Oxygen isotope values increase towards the South, with the highest &amp;#948;&lt;sup&gt; 18&lt;/sup&gt;O values previously reported in El Chalt&amp;#233;n, reaching up to 10.1 &amp;#8240; for zircon and 12.5 &amp;#8240; for quartz (Seitz et al., 2018). Eruptive units from the same locality appear to be homogeneous in their oxygen isotopic composition.&lt;/p&gt;&lt;p&gt;U/Pb zircon ages for the EQC range overall from ~148 to 155 Ma, though no obvious trend from North to South in zircon crystallization ages is noticeable. Multiple inherited zircon cores (at ~230, 460, 500, 1300 Ma) with Jurassic magmatic overgrowths were discovered. Isotopic compositions of these inherited magmatic cores are variable in their &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O values throughout time. However, and more significantly, most of these inherited cores record high &amp;#948;&lt;sup&gt; 18&lt;/sup&gt;O values, with the highest value at 9.5 &amp;#8240; measured for a ~460 Ma core. These high values measured within inherited cores are found at all locations sampled for the EQC.&lt;/p&gt;&lt;p&gt;The &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O values of the EQC rocks are significantly higher than what would be expected for silicic magmas formed by simple closed-system fractionation from any mantle-derived melt (6&amp;#173;-7&amp;#8240;; Valley, 2003). Thus, our oxygen isotope data support significant input of crustal material - of either a sedimentary origin or from hydrothermally altered crust - to generate these ignimbrites and rhyolites with elevated &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O values all along this transect.&lt;/p&gt;

The Paleoproterozoic Kandalaksha-Kolvitsa gabbro-anorthosite complex (Fennoscandian Shield): new U-Pb, Sm-Nd and Nd-Sr (ID-TIMS) isotope data on the age of formation, metamorphism and geochemical features of zircon (LA-ICP-MS)

&lt;p&gt;The paper provides new U-Pb, Sm-Nd and Nd-Sr isotope-geochronological data on rocks of the Paleoproterozoic Kandalaksha-Kolvitsa gabbro-anorthosite complex. REE contents in zircons from basic rock varieties of the Kandalaksha-Kolvitsa area have been defined in situ using LA-ICP-MS. Plots of REE distribution have been constructed, confirming the magmatic origin of zircon. Temperatures of zircon crystallization have been estimated, using a Ti-in-zircon geochronometer. For the first time, the U-Pb method with &lt;sup&gt;205&lt;/sup&gt;Pb artificial tracer has been applied to date single zircon grains (2448&amp;#177;5 Ma) from metagabbro of the Kolvitsa massif. The U-Pb analysis of zircon from anorthosites of the Kandalaksha massif has dated the early stage of the granulite metamorphism at 2230&amp;#177;10 Ma. The Sm-Nd isotope age has been estimated on metamorphic minerals (apatite, garnet, sulfides) and the rock at 1985&amp;#177;17 Ma (granulite metamorphism) for the Kolvitsa massif, 1887&amp;#177;37 Ma (high-temperature metasomatic transformations) and 1692&amp;#177;71 Ma (regional fluid reworking) for the Kandalaksha massif. The Sm-Nd model age of metagabbro is 3.3 Ga with negative value &amp;#949;Nd=4.6, which corresponds either with processes of crustal contamination, or with primary enriched mantle reservoir of primary magmas.&lt;/p&gt;&lt;p&gt;This research was funded by the Scientific Research Contract of GI KSC RAS No. 0226-2019-0053, grants of the Russian Foundation for Basic Research NoNo. 18-05-70082 &amp;#171;Arctic Resources&amp;#187;, 18-35-00246 mol_a, and the Presidium RAS Program No. 8.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;

The Paleoproterozoic Kandalaksha-Kolvitsa Gabbro-Anorthosite Complex (Fennoscandian Shield): New U–Pb, Sm–Nd, and Nd–Sr (ID-TIMS) Isotope Data on the Age of Formation, Metamorphism, and Geochemical Features of Zircon (LA-ICP-MS)

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