Concordia Intercept Age Research Articles

Microstructural and isotopic analysis of shocked monazite from the Hiawatha impact structure: development of porosity and its utility in dating impact craters

U–Pb geochronology of shocked monazite can be used to date hypervelocity impact events. Impact-induced recrystallisation and formation of mechanical twins in monazite have been shown to result in radiogenic Pb loss and thus constrain impact ages. However, little is known about the effect of porosity on the U–Pb system in shocked monazite. Here we investigate monazite in two impact melt rocks from the Hiawatha impact structure, Greenland by means of nano- and micrometre-scale techniques. Microstructural characterisation by scanning electron and transmission electron microscopy imaging and electron backscatter diffraction reveals shock recrystallisation, microtwins and the development of widespread micrometre- to nanometre-scale porosity. For the first time in shocked monazite, nanophases identified as cubic Pb, Pb3O4, and cerussite (PbCO3) were observed. We also find evidence for interaction with impact melt and fluids, with the formation of micrometre-scale melt-bearing channels, and the precipitation of the Pb-rich nanophases by dissolution–precipitation reactions involving pre-existing Pb-rich high-density clusters. To shed light on the response of monazite to shock metamorphism, high-spatial-resolution U–Pb dating by secondary ion mass spectrometry was completed. Recrystallised grains show the most advanced Pb loss, and together with porous grains yield concordia intercept ages within uncertainty of the previously established zircon U–Pb impact age attributed to the Hiawatha impact structure. Although porous grains alone yielded a less precise age, they are demonstrably useful in constraining impact ages. Observed relatively old apparent ages can be explained by significant retention of radiogenic lead in the form of widespread Pb nanophases. Lastly, we demonstrate that porous monazite is a valuable microtexture to search for when attempting to date poorly constrained impact structures, especially when shocked zircon or recrystallised monazite grains are not present.

Indosinian W-Sn mineralization event in the Xikeng deposit, southern Jiangxi Province, Nanling Range: Evidence from U-Pb geochronology and Hf isotopes

The Chongyi–Shangyou–Dayu ore-concentrated district represents one of the most important wolframite production bases in China, and most of the W-Sn deposits formed in the early Yanshanian period. The Xikeng quartz vein-type W-Sn deposit is located spatially not far from the Triassic Keshuling granitic pluton. The deposit contains 43 ore-bearing quartz veins, in which the ore minerals are mainly cassiterite and wolframite. These ore veins are concentrated in the Sinian metasandstone and sandy slate of the Laohutang Formation and extend down to the top of the concealed granite. The concealed intrusion is composed of medium- to fine-grained muscovite granites, which relate to the Indosinian Keshuling granite, according to geophysical data. Zircon grains from the concealed Xikeng granitic intrusion have a concordant U-Pb age of 232.1 ± 1.1 Ma (MSWD = 0.9). The zircon εHf(t) values range from −11.0 to −7.9, indicating primary reworking of the ancient crustal materials. Wolframite and cassiterite grains yield Tera-Wasserburg concordia intercept ages of 228.4 ± 0.9 Ma (MSWD = 1.2) and 227.8 ± 1.6 Ma (MSWD = 0.3) for the Xikeng deposit, respectively. These data demonstrate that the Indosinian W-Sn mineralization events were genetically related to granitic magma activities that share similar magmatic sources with the Yanshanian W-Sn mineralization-associated granitic intrusions. Combined with previous robust granitic and metallogenic ages, this paper identifies a new Indosinian W-Sn mineralization event in southern Jiangxi Province, Nanling Range.

DQPB: software for calculating disequilibrium U–Pb ages

Abstract. Initial radioactive disequilibrium amongst intermediate nuclides of the U decay chains can have a significant impact on the accuracy of U–Pb ages, especially in young samples. For samples that can reasonably be assumed to have attained radioactive equilibrium at the time of analysis, a relatively straightforward correction may be applied. However, in younger materials where this assumption is unreasonable, it is necessary to replace the familiar U–Pb age equations with more complete expressions that account for growth and decay of intermediate nuclides through time. DQPB is software for calculating U–Pb ages while accounting for the effects of radioactive disequilibrium among intermediate nuclides of the U decay chains. The software is written in Python and distributed as both a pure Python package and a stand-alone graphical user interface (GUI) application that integrates with standard Microsoft Excel spreadsheets. The software implements disequilibrium U–Pb equations to compute ages using various approaches, including concordia intercept ages on a Tera–Wasserburg diagram, U–Pb isochron ages, Pb*/U ages based on single aliquots, and 207Pb-corrected ages. While these age-calculation approaches are tailored toward young samples that cannot reasonably be assumed to have attained radioactive equilibrium at the time of analysis, they may also be applied to older materials where disequilibrium is no longer analytically resolvable. The software allows users to implement a variety of regression algorithms based on both classical and robust statistical approaches, compute weighted average ages and construct customisable, publication-ready plots of U–Pb age data. The regression and weighted average algorithms implemented in DQPB may also be applicable to other (i.e. non-U–Pb) geochronological datasets.

Two-stage exhumation of deeply subducted continental crust: Insight from zircon, titanite, and apatite petrochronology, Sulu belt of eastern China

Abstract The rates and mechanisms by which deeply subducted continental crust was exhumed back to the surface are not well understood, but can be better characterized using multimineral petrochronology. Here, we combine zircon, titanite, and apatite U-Pb ages from leucogranite and phengite gneiss with a pressure–temperature (P–T) path from eclogite to provide robust quantitative constraints on cooling and exhumation of the Sulu belt, a large ultrahigh-pressure metamorphic terrane in eastern China. The leucogranite, which formed during exhumation, is enriched in light rare earth elements (REE) relative to heavy REE and in large ion lithophile elements relative to high field strength elements, similar to hydrous crustal melts. Whole-rock Sr-Nd isotope compositions indicate that the leucogranite was not directly derived from the host phengite gneiss, but was more likely sourced from deeper in the exhuming crust. For the gneiss, mantles on inherited zircon yield an age of 230 ± 2 Ma and a temperature of 802 ± 36 °C based on a minimum pressure of 2.9 GPa, which records the minimum timing and P–T of initial decompression. Overgrowths on inherited zircon from the leucogranite constrain crystallization to 224 ± 1 Ma, coeval with the growth of zircon rims in the gneiss, at a temperature of 764 ± 42 °C and a pressure within the quartzeclogite facies. Titanite and apatite define single populations with lower concordia intercept ages of 222 ± 3 Ma and 198 ± 7 Ma, at temperatures of 720 ± 30 °C and ∼450 ± 100 °C, respectively, recording the timing of passage through the quartz-eclogite to the amphibolite facies and then the transition to the upper greenschist facies. Although the data yield a nearly constant cooling rate of 10.9−3.6+4.5 °C/m.y., exhumation was completed in two stages. The first stage from coesiteeclogite facies to ∼1.2 GPa, corresponding to the depth of the Moho, occurred at a rate of 7.5−2.6+5.8 km/m.y. Thereafter, exhumation into the mid-crust occurred at a much slower rate of 0.87−0.71+0.86 km/m.y. The first stage of faster exhumation was accompanied by migration of leucogranite melt along foliation in the gneiss, which would have decreased the average density and weakened the crust, enhancing the rate of return flow.

High-temperature fluids in granites during the Neoarchaean-Palaeoproterozoic transition: Insight from Closepet titanite chemistry and U-Pb dating (Dharwar craton, India)

The aim of this paper is to determine the composition and age of high-temperature (HT) hydrothermal fluids in Closepet granites (Dharwar craton, India). The magmatic origin of the Neoarchaean Closepet batholith has been recognized and dated many times, whereas a HT fluid system, strong evidence for which has been partly investigated, has never been dated. Titanite was chosen for a geochemical study on its pristine and altered domains to illustrate the fluid composition; the domains were dated to recognize the timing of fluid activity.Titanite presents distinct trace element enrichments in non-altered domains and strong depletions in trace elements in marginal, irregular, patchy zones. The high total rare earth element (REE) concentrations and the marked negative Eu anomalies suggest crystallization from a residual melt. The negative Eu anomalies diminish progressively in the REE patterns of altered parts and become positive. Trace element ratios (Nb/Ta, Y/Ho, and U/Th) indicate fluid-crystal interactions. Strong U/Th fractionation might be linked to the presence of CO2 and Cl−. Zr-in-titanite thermometry yields a crystallization/recrystallization temperature of ~700 °C for both magmatic and altered domains. The titanite factor Ti+4/(Al+3 + Fe+3) reveals an affinity with significant mantle input to the fluid system.Pristine and altered titanite domains have been successfully dated by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Reversely discordant apparent ages suggest interaction with fluids, which modified the Pb composition of the magmatic titanite. The oldest semi-concordant analyses of titanite cores indicate an age of approximately 2500 Ma, which we interpret as the minimum age of magmatic titanite crystallization. Narrow dark rims of titanite with considerably lower Th/U ratios form a more coherent group of analyses defining an upper concordia intercept age of 2345 ± 0.016 Ma, interpreted as the best estimate for the time of complete titanite resetting associated with the metasomatic event. The HT fluid system appears to have remained active into the Palaeoproterozoic.Confirmation by dating that the tested fluids belonged to the Archaean hydrothermal system gives us information about this system at the time, but the information that it worked at least until the Palaeoproterozoic provides additional valuable information about the hydrothermal systems during the Neoarchaean-Palaeoproterozoic transition.

Reconsidering initial Pb in titanite in the context of in situ dating

Abstract In situ U-Pb dating of titanite, which can preserve trace-element records of various petrologic processes but also incorporates significant initial Pb, has proliferated in recent years. The widespread use of titanite data to construct tectonic P-T-t paths warrants careful assessment of the available dating techniques, as well as attention to the assumptions that underpin the U-Pb data analysis. This contribution provides the first direct comparison of the two major analytical methods [SHRIMP (SIMS) and LA-ICP-MS] for in situ U-Pb titanite dating. A set of well-characterized titanite grains from Harrisville, New York, in the Adirondack Mountains were analyzed for U-Th-Pb isotopes along the same cross-grain traverses by Sensitive High Resolution Ion Microprobe (SHRIMP) and LA-ICP-MS. Both LA-ICP-MS and SHRIMP data sets define approximately linear arrays on the Tera-Wasserburg Concordia (semi-total Pb/U) diagram and would commonly be interpreted as representing a single date population with minor scatter. However, previous studies have suggested that Adirondack titanite actually records two regionally well-defined thermal events, ~50–100 m.y. apart. When titanite data arrays are treated in detail, attempts to determine concordia-intercept ages by robust three-dimensional linear regression produce large uncertainties and/or poor fit statistics that suggest that the data are not, in fact, isochronous. Grain-by-grain analysis of U-Pb titanite data shows that different subsets of titanite (determined by additional geochemical and microstructural data) show different patterns of U-Pb data. By comparing predictions for Pb-ingrowth evolution paths in Tera-Wasserburg diagrams with observed data, it is possible to recognize both a change in initial Pb composition and Pb loss in the Adirondack titanite U-Pb data set. This study provides an example of how greater geochronologic detail can be extracted from large in situ U-Pb titanite data sets. Even when precise dates are not recovered, geological processes and events that cause data scatter can be recognized through analysis of U-Pb data patterns using the Tera-Wasserburg diagram.

Petrogenesis of the Lijiagou spodumene pegmatites in Songpan-Garze Fold Belt, West Sichuan, China: Evidence from geochemistry, zircon, cassiterite and coltan U-Pb geochronology and Hf isotopic compositions

Spodumene and other pegmatites in the Lijiagou area are located in the central Songpan-Garze Fold Belt (SGFB), West Sichuan. These pegmatites are hosted within the giant Songpan-Garze polymetallic belt. A lack of precise geochronological constraints on the magmatic crystallization of the pegmatite suite and nearby plutonic granitoids has precluded definitive statements on genetic links between spodumene ore formation and the magmatic suite. Systematic zircon, cassiterite and coltan U-Pb dating, Hf isotope and whole-rock geochemical analysis were undertaken. Two-mica granite and muscovite albite granite have S-type granite affinities and chemical compositions suggesting a post-orogenic setting, and the albite spodumene pegmatites belong to the LCT (Li-Cs-Ta) group of pegmatites. Zircon LA-MC-ICP-MS U-Pb dating of the two-mica granite, muscovite albite granite, albite pegmatite and albite spodumene pegmatite give crystallization ages of 219.2 ± 2.3 Ma (MSWD = 0.55), 217 ± 2.8 Ma (MSWD = 0.47), 202.8 ± 4.9 Ma (MSWD = 3.9) and 200.1 ± 4.6 Ma (MSWD = 3.1), respectively. Zircons from the same units yield εHf(t) values of −39.17 to 6.81, −22.73 to −2.83, −11.17 to 8.14, and −4.92 to −2.4, respectively, consistent with mixed crustal sources for the pegmatite and granite magmas. Cassiterite from albite spodumene pegmatite yields a concordia intercept age of 211.4 ± 3.3 Ma (MSWD = 2.9), while coltan yields a weighted mean age of 211.1 ± 1.0 Ma (MSWD = 0.97). The U-Pb zircon ages of the two-mica granite and muscovite albite granite are interpreted as ages of magmatic crystallization. Metamictization of zircon in both albite and albite spodumene pegmatites makes their U-Pb zircon ages liable to inaccuracy due to Pb loss. The coltan U-Pb age is regarded as an accurate measure of the magmatic crystallization age of the albite spodumene pegmatite. Given the differences in magmatic ages and εHf(t) ranges between albite spodumene pegmatite and both the two-mica granite and the muscovite albite granite, the albite spodumene pegmatites probably represent anatectic melts and not fractionation products of either of the granitic magmas. U-Pb coltan and U-Pb cassiterite dating are more likely to provide accurate crystallization ages of albite spodumene pegmatites than U-Pb zircon ages.

The Tongkuangyu Cu Deposit, Trans-North China Orogen: A Metamorphosed Paleoproterozoic Porphyry Cu Deposit

AbstractThe Tongkuangyu copper deposit in the Zhongtiaoshan region, southern Trans-North China orogen, is hosted by a poorly constrained sequence of Paleoproterozoic volcano-sedimentary (quartz-sericite schist and biotite schist) and granitic rocks that have been metamorphosed to lower greenschist facies and variably deformed. The deposit has previously been proposed to be either a porphyry-type or a sediment-hosted stratiform Cu deposit, and its age of formation has been debated.The quartz-sericite schist is interpreted to be a felsic crystal tuff and consists of angular quartz crystals in a fine-grained sericite-altered matrix. Two quartz-sericite schist samples yielded zircon U-Pb upper concordia intercept ages of 2512 ± 12 (2σ, mean square of weighted deviates [MSWD] = 0.19) and 2335 ± 16 Ma (2σ, MSWD = 0.80). Biotite schist, which is interleaved locally with the quartz-sericite schist and is interpreted to be a basaltic-andesitic sill, yielded a younger zircon U-Pb upper concordia intercept age of 2191 ± 10 Ma (2σ, MSWD = 1.7). Five samples of granodiorite and granodiorite porphyry that intruded the schist sequence yielded similar zircon U-Pb ages, with a weighted mean upper concordia intercept age of 2182 ± 7 Ma (2σ, MSWD = 1.3). These results suggest that the volcanic sequence was deposited between ∼2.5 and 2.3 Ga and was intruded by basaltic-andesitic sills and a suite of granodiorite and granodiorite porphyry intrusions at ∼2.19 to 2.18 Ga.Two stages of copper mineralization are interpreted to have formed after pervasive sericite alteration of the felsic volcanic rocks. Stage 1 mineralization includes disseminated and deformed quartz veinlets containing chalcopyrite ± pyrite ± magnetite ± molybdenite associated with biotite ± K-feldspar alteration in granodiorite porphyry and schist. Stage 2 comprises undeformed quartz-chlorite-carbonate veins with bornite ± chalcopyrite ± magnetite associated with local chlorite and silicic alteration. Allanite crystals intergrown with chalcopyrite in the granodiorite porphyry yielded an approximate concordia U-Pb age of 2115 ± 31 Ma (2σ, MSWD = 2.3). Two molybdenite samples in a deformed quartz-chalcopyrite-molybdenite vein yielded Re-Os model ages of 2106 ± 9 and 2089 ± 9 Ma (2σ), consistent with previously published results.Hydrothermal monazite grains with Cu-Fe sulfide inclusions in the granodiorite porphyry, quartz-sericite schist, and undeformed chlorite-bornite-quartz veins yielded much younger U-Pb upper concordia intercept ages of 1832 ± 16 (2σ, MSWD = 0.48), 1810 ± 14 (2σ, MSWD = 0.92), and 1809 ± 12 Ma (2σ, MSWD = 0.38), respectively. The results are in agreement with four Re-Os model ages for pyrite mineral separates from undeformed quartz-sulfide veins, which yielded a weighted mean age of 1807 ± 4 Ma (2σ, n = 4, MSWD = 0.42). In contrast, hydrothermal rutile crystals in the quartz-sericite schist and biotite schist yielded a range of roughly concordant ages between 2.1 and 1.8 Ga, reflecting isotopic disturbance.We interpret these results to indicate original copper mineralization at ∼2.1 Ga that is significantly later than the granodiorite (∼2.18 Ga) and schists (∼2.5–2.2 Ga), followed by hydrothermal remobilization and metamorphism at ∼1.8 Ga. The metavolcanic and granodiorite porphyry host rocks, alteration styles, and disseminated and veinlet form of the earlier mineralization are strongly reminiscent of porphyry Cu deposits, and ages of ∼2.1 Ga have been reported for one intrusion and three volcanic rock samples from the district. The Tongkuangyu, therefore, represents one of the oldest known porphyry copper deposits. Remobilization of copper occurred at ∼1.8 Ga during the Zhongtiao orogeny.

The trace element and U-Pb systematics of metamorphic apatite

Abstract Apatite is a common accessory mineral in igneous, sedimentary and metamorphic rocks. It has potential as a provenance indicator in sedimentary systems, as it can host a wide variety of trace elements in its crystal structure and can yield thermochronological age information. However, the processes controlling the trace element and U-Pb systematics of metamorphic apatite remain poorly understood, and metamorphic apatite remains significantly under-represented in compositional provenance databases linking apatite trace-element chemistry to its corresponding parent rock type. We investigate the trace-element and U-Pb systematics of metamorphic apatite from a suite of 22 bedrock samples of diverse metamorphic grade and protolith type, sampled from a variety of metamorphic terranes. Metamorphic apatite from low- to medium-grade metapelites and metabasites can be easily distinguished from granitic apatite as it is significantly depleted in Th, REE, and Y. Depletion in Th and REE + Y is attributed to growth of co-genetic epidote, which is the dominant carrier phase of the REE + Y, Th, and U in all the low- to medium-grade samples mapped by laser ablation quadrupole inductively coupled plasma mass spectrometry (LA-Q-ICP-MS) and energy dispersive X-ray spectroscopy (EDS). Apatite U contents in low- to medium-grade metapelites and metabasites are more variable than the Th and REE + Y contents, but are typically low. Consequently, grains from these rock types are often undateable by the U-Pb LA-ICP-MS method and thus are underrepresented in apatite U-Pb detrital datasets, but can still be identified as low- to medium-grade metamorphic apatite by their trace-element characteristics. Low-grade metapelite apatite is difficult to distinguish from low-grade metabasite apatite, which is likely due to the growth of U-, Th-, and REE-rich epidote in both lithologies. Detrital (granitic) apatite is stable in very low-grade (e.g. pumpellyite-actinolite facies) metasedimentary samples, while neo- or re-crystallized metamorphic apatite is widespread by the upper-greenschist facies. LA-Q-ICP-MS imaging demonstrates that low REE + Y, Th, and U metamorphic apatite rims can nucleate on detrital igneous apatite precursors with high REE + Y, Th, and U. With increasing metamorphic grade, 1) relict detrital apatite is consumed, 2) the coherence of the U-Pb concordia systematics dating metamorphism improves, and 3) the degree of dispersion on metamorphic apatite multi-element plots decreases. The highest-grade metamorphic apatite samples investigated are paragneisses, some of which have locally undergone anatexis. Apatites from these samples yield well constrained TW concordia intercept ages and minor dispersion on multi-element plots (which is attributed to the absence of epidote) and closely resemble apatite from S-type granites in their trace element characteristics.

New SIMS U-Pb age on zircon from the Epembe carbonatite dyke, NW Namibia: Implications for Mesoproterozoic evolution of carbonatites at the southern margin of the Congo Craton

The Epembe carbonatite dyke is situated approximately 80 km north of Opuwo, NW Namibia. The 10 km long dyke is dominated by massive and banded sövitic carbonatite intrusions. Two types of sövite have been recognized: (1) coarse-grained light grey Sövite I which occurs in highly brecciated and fractured zones and (2) medium to fine-grained Sövite II which hosts significant concentrations of pyrochlore and apatite. The contact zone between the carbonatite and basement gneisses is marked by K-feldspar fenite. Zircons from Sövite II were dated with Secondary Ion Mass Spectroscopy (SIMS) yielding a concordia intercept age of 1184 ± 10 Ma (MSWD = 0.65, Pof = 0.42) corresponding with the weighted mean 207Pb/206Pb age of 1173 ± 12 Ma (N = 20, MSWD = 1.2) as the crystallization age for the Epembe carbonatite. Our result presents a slightly older age than previously published data for the Swartbooisdrif ferrocarbonatites about 35 km to the east of Epembe. As calcitic carbonatites usually represent the early magmatic phase during the evolution of carbonatite magmas, the calcitic and ferrocarbonatite veins linking the two carbonatite suites at the southern margin of the Kunene Intrusive Complex (KIC), suggests that the spatially associated sövitic and ferrocarbonatite dykes are genetically related and form part of carbonatite melts emplaced about 180 Ma after the KIC magmatic event.

Zircon growth in (U)HP quartzo-feldspathic host gneisses exhumed in the Woodlark Rift of Papua New Guinea

To understand zircon behavior as a function of bulk composition and metamorphic grade in the world's youngest (U)HP terrane, we report U-Pb SIMS spot-mode and depth-profiling analyses for quartzo-feldspathic gneisses. Zircons from two gneisses contain Cretaceous inherited cores, with μm sized metamorphic rims requiring depth profiling for reliable dating. Linear regression of the common-Pb uncorrected data for rims yield 206Pb/238U ages of 2.89 ± 0.29 Ma and 2.77 ± 0.99 Ma (concordia intercept ages at 95% confidence). The older age is within two-sigma error of previously reported 206Pb/238U ages on zircons from mafic eclogite within the gneiss, indicating that rims formed on inherited cores within host gneisses during eclogite facies metamorphism. At the (U)HP locality zircons from host gneiss lack inheritance and yield a 206Pb/238U age of 3.66 ± 0.13 Ma. These results are younger than previously reported 206Pb/238U ages on zircons from coesite eclogite within the gneiss, but are within error of the youngest reported LA-ICP-MS 206Pb/238U zircon ages on retrogressed mafic eclogite. We also report intragrain geochemical heterogeneity, indicated by zircon Hf, Y, and Ti variations in depth profiles which suggest chemical disequilibrium over the interval of zircon growth. Collectively, these results indicate that zircon recrystallization and new growth of zircon rims on relict grains occurred during eclogite facies metamorphism and during subsequent retrogression, but not at (U)HP conditions. Comparison between results from PNG and other (U)HP terranes bolsters previous suggestions that the PNG (U)HP terrane evolved rapidly.

U–Pb LA–ICPMS dating using accessory mineral standards with variable common Pb

Precise and accurate U–Pb LA–ICPMS dating of many U-bearing accessory minerals (e.g. apatite, allanite, titanite and rutile) is often compromised by common Pb. LA–ICPMS dating of these U-bearing accessory phases typically requires a matrix-matched standard, and data reduction is often complicated by variable incorporation of common Pb not only into the unknowns but also particularly into the reference material. We present here a general approach to common Pb correction in U–Pb LA–ICP–MS dating using a modified version of the VizualAge U–Pb data reduction package for Iolite (VizualAge_UcomPbine). The key feature of the method is that it can correct for variable amounts of common Pb in any U–Pb accessory mineral standard as long as the standard is concordant in the U/Pb (and Th/Pb) systems following common Pb correction. Common Pb correction of the age standard can be undertaken using either the 204Pb, 207Pb or 208Pb(no Th) methods, and the approach can be applied to raw data files from all widely used modern multi-collector and single-collector ICPMS instruments.VizualAge_UcomPbine first applies a common Pb correction to the user-selected age standard integrations and then fits session-wide “model” U–Pb fractionation curves to the time-resolved U–Pb standard data. This downhole fractionation model is applied to the unknowns and sample-standard bracketing (using a user-specified interpolation method) is used to calculate final isotopic ratios and ages. 204Pb- and 208Pb(no Th)-corrected concordia diagrams and 204Pb-, 207Pb- and 208Pb(no Th)-corrected age channels can be calculated for user-specified initial Pb ratio(s). All other conventional common Pb correction methods (e.g. intercept or isochron methods on co-genetic analyses) can be performed offline.The approach was tested on apatite and titanite age standards (for which there are independent constraints on the U–Pb crystallization age) using a Thermo Scientific iCAP-Qc (Q–ICP–MS) coupled to a Photon Machines Analyte Excite 193nm ArF Excimer laser. Madagascar apatite, OLT1 titanite and R10 rutile were used as primary standards and were corrected for variable common Pb using the new VizualAge_UcomPbine DRS. The secondary Durango (31.44±0.18Ma) apatite standard yielded a U–Pb TW concordia intercept age of 31.97±0.59Ma (MSWD=1.09; primary standard corrected by the 207Pb-method) and a U–Pb concordia age of 31.82±0.40Ma (MSWD=1.4; primary standard corrected by the 204Pb-method). McClure Mountain (523.51±1.47Ma) yielded a U–Pb TW concordia intercept age of 524.5±3.7Ma (MSWD=0.72) while the Fish Canyon Tuff (28.201±0.046Ma) and Khan (522.2±2.2Ma) titanite standards yielded U–Pb TW concordia intercept ages of 28.78±0.41Ma (MSWD=1.4) and 520.9±3.9Ma (MSWD=4.2) respectively. The suitability of the 208Pb(no Th)-correction is demonstrated by the agreement between a U–Pb TW concordia intercept age of 452.6±4.7Ma (MSWD=0.89) and a 208Pb(no Th)-corrected TW concordia age of 448.6±4.5Ma (MSWD=1.4) on a c. 450Ma rutile which exhibits variable incorporation of common Pb.A range of LA–ICPMS U–Pb dating applications are presented and include U–Pb dating of apatite from >3.8Ga gneisses from Akilia, SW Greenland. These apatites host 13C-depleted graphite inclusions that are interpreted as biogenic in origin and representing the oldest indications of life on Earth. The U–Pb age profiles on single apatite grains presented here are characteristic of Pb loss by volume diffusion with core–rim age differences of up to 300Ma. These data explain the scatter and poor precision of earlier U–Pb apatite age determinations on Akilia apatite. Other LA–ICPMS dating applications include U–Pb apatite dating as a rapid method for determining the age of mafic intrusions, U–Pb titanite and apatite dating of ash fall tuffs, determining temperature–time histories using multiple U–Pb thermochronometers and improving concordance in LA–ICPMS primary zircon standard datasets by analysing young, common Pb-bearing primary zircon standards that have not accumulated significant radiation damage.

Mesoproterozoic juvenile mafic–ultramafic magmatism in the SW Amazonian Craton (Rio Negro-Juruena province): SHRIMP U–Pb geochronology and Nd–Sr constraints of the Figueira Branca Suite

Abstract The Figueira Branca Suite (FBS) comprises a layered mafic–ultramafic complex which together with mafic–felsic plugs makes up a string of NW-trending intrusive bodies that are emplaced into the Jauru domain (Rio Negro-Juruena province; 1.80–1.60 Ga). This domain comprises Orosirian calc-alkaline rocks and coeval metamorphic volcanic–sedimentary associations, intruded by voluminous granitoid plutons resulted from outboard Cachoeirinha (1587–1522 Ma) and Santa Helena (1485–1420 Ma) accretionary orogens that eventually created the Rondonian-San Ignacio province along the SW margin of the proto-Amazonian Craton. SHRIMP U–Pb age in zircon for one cumulatic gabbro from the FBS yielded a concordia intercept age of 1425.5 ± 8.0 Ma (MSWD = 1.11). Another gabbroic plug which crops out to the East gives a similar within error concordia intercept zircon age of 1415.9 ± 6.9 Ma (MSWD = 0.25), whereas a nearby monzogranite yields a concordia intercept zircon age of 1428.9 ± 2.8 Ma (MSWD = 1.30). All these results are crystallization ages and constrain an important intraplate magmatic event within the Orosirian continental crust at the time of outboard Santa Helena orogen. On the other hand, igneous titanite from another gabbro located to the West of the FBS yielded a weighted mean 207 Pb/ 206 Pb crystallization age of 1541 ± 23 Ma (MSWD = 0.74). Therefore this rock is not genetically associated with the FBS, as previously suggested by the field information. Additional Nd–Sr isotopic analyses of the FBS mafic–ultramafic rocks and coeval gabbro showed comparable ɛ Nd(1.42Ga) values (+3.0 to +4.7) and variable ɛ Sr(1.42Ga) ones (−39.1 to −8.1). These data plot in the depleted field quadrant of the Nd–Sr diagram, indicating a significant influence of the MORB end-member reservoir in the magma genesis. This interpretation is similarly supported by comparison of the Nd evolutionary path of the FBS with those that characterize the isotopic evolution of the Jauru crust and the Cachoeirinha and Santa Helena intrusive plutons. The bulk isotopic signature allows the genetic relationship among the Santa Helena Suite and the FBS and coeval rocks. The anorogenic character of the FBS is supported by the geologic framework of the Jauru domain, given that the emplacement took place under an extensional regime, associated with the recognized regional NW-trending structures. 40 Ar– 39 Ar analyses were carried out in mafic rocks that are assigned to the FBS. Biotite from a gabbronorite yielded an ideogram age of 1222 ± 5 Ma while a nearby troctolite yielded plateau biotite ages of 1275 ± 4 Ma and 1268 ± 4 Ma. These ages are minimum estimates of the regional cooling of the FBS. One gabbro of the Alto Jauru Group yields a 40 Ar– 39 Ar plateau age of 1781 ± 15 Ma, interpreted as the time of regional cooling that succeeded continental accretion and metamorphism at Orosirian times. Later on heterogenous crustal thickening and uplift took place from West to East, as response from the Cachoeirinha and Santa Helena orogen dynamics, which is marked by the 1539–1510 Ma and 1452–1322 Ma 40 Ar– 39 Ar age-patterns, respectively. The available apparent ages suggest that Cachoeirinha crust was subjected to a fast exhumation rate (ca. 50 m.y.), as estimated by the zircon–titanite–hornblende–biotite time-path, while the Santa Helena crust seems to display a lower exhumation rate. Particularly, the youngest 40 Ar– 39 Ar biotite age (1322 Ma) available for the Santa Helena crust signals the post-tectonic phase of the Rondonian-San Ignacio province.

Single crystal U–Pb zircon age and Sr–Nd isotopic composition of impactites from the Bosumtwi impact structure, Ghana: Comparison with country rocks and Ivory Coast tektites

The 1.07Myr old Bosumtwi impact structure (Ghana), excavated in 2.1–2.2Gyr old supracrustal rocks of the Birimian Supergroup, was drilled in 2004. Here, we present single crystal U–Pb zircon ages from a suevite and two meta-graywacke samples recovered from the central uplift (drill core LB-08A), which yield an upper Concordia intercept age of ca. 2145±82Ma, in very good agreement with previous geochronological data for the West African Craton rocks in Ghana. Whole rock Rb–Sr and Sm–Nd isotope data of six suevites (five from inside the crater and one from outside the northern crater rim), three meta-graywacke, and two phyllite samples from core LB-08A are also presented, providing further insights into the timing of the metamorphism and a possibly related isotopic redistribution of the Bosumtwi crater rocks. Our Rb–Sr and Sm–Nd data show also that the suevites are mixtures of meta-greywacke and phyllite (and possibly a very low amount of granite). A comparison of our new isotopic data with literature data for the Ivory Coast tektites allows to better constrain the parent material of the Ivory Coast tektites (i.e., distal impactites), which is thought to consist of a mixture of metasedimentary rocks (and possibly granite), but with a higher proportion of phyllite (and shale) than the suevites (i.e., proximal impactites). When plotted in a Rb/Sr isochron diagram, the sample data points (n=29, including literature data) scatter along a regression line, whose slope corresponds to an age of 1846±160Ma, with an initial Sr isotope ratio of 0.703±0.002. However, due to the extensive alteration of some of the investigated samples and the lithological diversity of the source material, this age, which is in close agreement with a possible “metamorphic age” of ∼1.8–1.9Ga tentatively derived from our U–Pb dating of zircons, is difficult to consider as a reliable metamorphic age. It may perhaps reflect a common ancient source whose Rb–Sr isotope systematics has not basically been reset on the whole rock scale during the Bosumtwi impact event, or even reflect another unknown geologic event.

Discovery of Archaean crust within the Akitkan orogenic belt of the Siberian craton: New insight into its architecture and history

The Akitkan orogenic belt separates the Aldan and Anabar superterranes of the Siberian craton and was previously suggested to consist exclusively of Palaeoproterozoic juvenile crust. Foliated granitoids in the Baikal basement salient within the Akitkan belt were studied geochronologically and geochemically. Thermal ionisation mass spectrometry (TIMS) analysis of six zircons of a tonalite sample yielded a discordia line with an upper concordia intercept age of 2884 ± 12 Ma which is interpreted as the time of tonalite emplacement. Geochemical data of the foliated granitoids confirm their affinity to the tonalite–trondhjemite–granodiorite (TTG) series. They are characterized by high Al 2O 3 contents (14.4–17.2 wt.%), high Sr/Y ratios (18–99) and show significant rare-earth element (REE) fractionation with La n /Yb n = 11–89. The ɛNd (T) values vary from −1.6 to +0.8 and Nd mean crustal residence ages range from 3.23 to 3.07 Ga. Geochemical and isotopic data confirm their genesis through interaction of mantle and crustal material under high pressure near the base of thickened crust either after accretion of ancient continental crust and an oceanic plateau, or after underplating of oceanic plateau-like basaltic magmas. The Akitkan orogenic belt is a composite crustal unit of Archaean and Palaeoproterozoic age. We propose two possible scenarios of its evolution. In the first scenario Palaeoproterozoic complexes developed along the active margin of the Archaean Anabar superterrane at 2.1–2.0 Ga. Alternatively Archaean and Palaeoproterozoic components of the Akitkan belt could have a distal origin and were accreted to the Anabar superterrane. These accretional processes were followed by a major collision of the Aldan and Anabar superterranes at ∼1.9 Ga finalised by post-collisional granitoid magmatism at 1.87–1.84 Ga.

Mineralogy and U–Th–Pb age of a uranium-bearing jasperoid vein, Sunshine Mine, Coeur d'Alene district, Idaho, USA

The age of the Mesoproterozoic Belt Supergroup hosted silver and base metal vein deposits of the Coeur d'Alene mining district in northern Idaho has long been controversial. Evidence for Precambrian mineralization comes largely from many ∼ 1400 Ma Pb model ages for galena throughout the district, and several problematic 1100–1300 Ma 207Pb/ 206Pb ages reported in the 1950s on uraninite from the Sunshine Mine. Field studies, on the other hand, have generally related mineralization to Mesozoic and Cenozoic tectonic and igneous events. A possible explanation of the galena model age enigma is that lead from metal-enriched Belt rocks was remobilized with minimal isotopic modification into younger deposits. The availability of a new occurrence of the uranium-bearing jasperoid vein exposed in the 5700-foot level of the Sunshine Mine allows reinvestigation of the uraninite using modern electron microprobe and isotope analysis techniques. Most of the uranium occurs in a prismatic UTi 2O 6 mineral, commonly in aggregates of 5–50 µm long lath-like crystals, identified as originally being brannerite. Subsequently, the brannerite has exsolved into extremely fine-grained uraninite and a TiO 2 polymorph. Rare, small crystals of zircon discovered with the electron microprobe, however, proved to be an even more useful geochronometer in unraveling the vein's complex history. Three morphological varieties of zircon were identified: (1) rounded, purple grains of obvious detrital nature, (2) simple euhedral crystals inferred to have originated in an igneous(?) rock or quartziferous vein precursory to the present jasperoid vein, and (3) crystallographically complex crystals attributed to hydrothermal growth in the jasperoid vein itself. The first two varieties lend themselves to rather straightforward age interpretations, while the third held the key to resolving the problematic uraninite ages. Four detrital zircon grains give discordant but projected concordia ages of 1.5–1.8 Ga, which indicate derivation from Paleoproterozoic crystalline rocks that supplied sediments to the Belt basin. Two zircon crystals yield essentially concordant ages of ∼ 1350 Ma that significantly postdates deposition of the host St. Regis Formation, and appear to have crystallized during a widespread orogenic event occurring soon after deposition of the uppermost Belt Supergroup. Unlike the detrital and ∼ 1350 Ma-old zircon, six grains of the younger, presumably hydrothermal zircon have a relatively high common lead content and give no consistent age pattern. For example, a Coeur d'Alene-type initial lead ( 206Pb/ 204Pb = 16.20, 207Pb/ 204Pb = 15.36, 208Pb/ 204Pb = 35.90) yields highly discordant results that range in 207Pb/ 206Pb age from 556–170 Ma and suggest variable 20–80% radiogenic lead loss. Similar discordance with an even greater range in 207Pb/ 206Pb age is shown by brannerite crystal aggregates, a euhedral pyrite crystal, and the whole-rock. Despite the uncertainty in initial lead isotopic composition, the rather tight clustering of 206Pb/ 238U ages around 120–150 Ma combined with the much older 207Pb/ 206Pb ages strongly hints at an anomalous, old radiogenic lead inherited by younger crystals. Indeed, some choice of initial lead lying along a secondary isochron defined by radiogenic lead grown between 1350 Ma and ∼ 130 Ma with an original Coeur d'Alene isotopic composition could be selected for every mineral analysis so as to make it concordant at between 124 and 139 Ma. Alternatively, a 3-D plane can be fit to the mineral analyses on a 238U/ 206Pb– 207Pb/ 206Pb– 204Pb/ 206Pb plot, yielding concordia intercept ages of 1344 ± 77 Ma and 136 ± 2 Ma (MSWD = 0.54). A scenario is proposed whereby a siliceous, uranium-enriched rock of uncertain but probable igneous or hydrothermal origin was introduced at ∼ 1350 Ma into the St. Regis Formation, and later contributed an isochronously related initial lead to a second generation of minerals at 136 ± 2 Ma. This enigmatic rock appears to have initially been emplaced during a contemporaneous episode of Mesoproterozoic faulting and metamorphism well-documented to the south in the Central Idaho thrust belt. The degree to which the precursor rock was altered and remobilized in forming the present jasperoid vein is poorly understood, but thorough re-crystallization of its original minerals seems to be required. The remobilization is most likely associated with renewed tectonism along the Lewis and Clark line caused by the Early Cretaceous docking of exotic terranes to the North American continent. Silver and base metal veins in the Sunshine mine are even younger that the reconstituted jasperoid veins, and, thus, cannot be assigned a Mesoproterozoic age.

Aspectos tectônicos das intrusões dos granitos do cerne, passa três e Rio Abaixo, sudeste do Pré-cambriano Paranaense. Estudo baseado em datações 39Ar-40Ar em Micas

The Cerne, Passa Três and Rio Abaixo granitic stocks are NE-SW-striking elliptic bodies intrusive in the metavolcano-sedimentary sequences of the southeastern portion of the Ribeira Belt (Paraná State, Brazil). These stocks are mainly biotite syenogranite, monzogranite and quartz-syenite with porphyritic textures containing microcline megacrystals. They are usually isotropic, but may also contain magmatic flow foliation and deformation restricted to the contact with the host rocks. These granites present petrological, geochemistries and geocchronological similarities, with many A-type characteristics, but they are enriched in Ba Sr and Zr. 40Ar-39Ar analyses of the Cerne Granite biotites indicate an integrated age of 557 ± 2 Ma, which is quite close to the lower Concordia intercept age interpreted as the crystallization age of about 563 Ma obtained by TIMS analysis of zircon from the Cerne and Rio Abaixo intrusions by Cury (2003). The 40Ar-39Ar determinations in the host muscovite schists from the vicinity of the Cerne granite are older (800-1200 Ma) than crystallization ages, indicating that the temperatures of the granite intrusion and of the regional metamorphic event that took place during the Brasiliano Cycle (c.a. 600 Ma) were not high enough to erase the isotopic record from these muscovites (T < 300ºC). These granitic stocks represent a late-orogenic magmatism (~560 Ma) in the context of the deformations phases registered in the Apiaí Domain, being different and subsequent to the event of Três Córregos - Cunhaporanga magmatic arc (c.a. 630-590 Ma). Their emplacement was closely associated with transcurrent shear zones and the development of the major antiforms and synforms during the Upper Proterozoic/Cambrian, associated with the final adjustments of the agglutination of the Gondwana Supercontinent.

The application of laser ablation-inductively coupled plasma-mass spectrometry to in situ U–Pb zircon geochronology

This paper reports new developments in in situ U–Pb zircon geochronology using 266 and 213 nm laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). Standard spot ablation (spot diameters 40–80 μm) was employed, with no sampling strategies employed specifically to minimise elemental fractionation. Instead, He ablation gas and carefully replicated ablation conditions were employed to maintain constant ablation-related elemental fractionation of Pb and U between analyses. Combining these strategies with calibration on a new zircon standard (GJ-1) allows elemental fractionation and instrumental mass bias to be corrected efficiently, and accurate 206Pb/ 238U and 207Pb/ 235U ratios to be measured with short-term precision (2 r.s.d.) of 1.9% and 3.0%, respectively. Long-term precision (2 r.s.d.) of the technique (266 nm ablation), based on 355 analyses of the 91500 zircon (1065 Ma) standard over more than a year, was 3.8%, 4.0% and 1.4% for the 206Pb/ 238U, 207Pb/ 235U and 207Pb/ 206Pb ratios, respectively. Long-term precision (2 r.s.d.) for the 206Pb/ 238U, 207Pb/ 235U and 207Pb/ 206Pb ratios of the Mud Tank zircon (732 Ma) was 3.9%, 4.1% and 1.7%, respectively (359 analyses). Selective integration of time-resolved signals was used to minimise the effect of Pb loss and common Pb enrichments on the measured ages. The precision and accuracy of our data compare very favourably with those obtained using more involved procedures to correct or minimise ablation- and ICP-MS-induced biases. 213 nm laser ablation produced comparable precision to 266 nm ablation using generally smaller spot sizes (40–50 vs. 60–80 μm), and offered significant advantages in terms of ablation duration and stability, particularly for small zircons (<60 μm). For the 91500 zircon, but not the Mud Tank zircon, 213 nm ablation also produced significantly older and more accurate Pb/U ages. This suggests that shorter wavelength ablation may have reduced a matrix-dependent elemental fractionation difference between sample and standard. The accuracy and precision of the technique for young zircons are demonstrated by analysis of three zircon populations ranging in age from 417 to 7 Ma. In each case, the zircons have yielded concordant ages or common Pb discordia which give concordia intercept ages that are in agreement with independently determined ages for the same samples. Application of Tera–Wasserburg diagrams [Earth Planet. Sci. Lett. 14 (1972) 281] was found to be the most useful approach to handling common Pb contributions that were not removed by selective integration of signals.

A Back-arc Palaeotectonic Setting for the Augaro Neoproterozoic Magmatic Rocks of Western Eritrea

The Augaro volcano-sedimentary assemblages of western Eritrea are part of the Neoproterozoic, N-S trending belt of low-grade volcano-sedimentary and associated plutonic rocks. In contrast to the volcanic-dominated oceanic-arc assemblages in central Eritrea, the predominant rock types in the west are supracrustal sequences of sedimentary origin with subordinate volcanic rocks. These Augaro supracrustal rocks are overlain, unconformably, by a basin-fill metasedimentary succession known as the Gulgula Group. The Augaro metavolcanic rocks are tholeiitic and range in composition from basalt to basaltic andesite. Comparison of trace element characteristics and N-MORB-normalised spidergrams of these rocks with those of modern volcanic environments and age-comparable metavolcanic rocks of known tectonic association from the Arabian-Nubian Shield suggest that the volcanic assemblages from western Eritrea were generated in a back-arc tectonic setting. Single zircon Pb-Pb evaporation and vapour-transfer U-Pb analyses of magmatic zircons from pre/syn-tectonic granites yield a mean 207Pb/ 206Pb age of 849±20 Ma and an upper concordia intercept age of 849±26 Ma. These ages are interpreted to represent the time of major magmatism in western Eritrea and are comparable to ages of early arc magmatism in central and northern Eritrea and in the southern Nubian Shield. Initial eNd values and initial Sr isotope ratios of whole-rock samples of magmatic rocks calculated for an age of 850 Ma range from +4.0 to +7.1 and 0.7026 to 0.7037, respectively. Single zircon 207Pb/ 206Pb ages, initial eNd value and Sr isotope ratio for a granitic clast in the Gulgula metaconglomerate suggest that the source area for the Gulgula metasedimentary rocks is similar to the surrounding Neoproterozoic rocks of western Eritrea.

U–Pb zircon and rutile metamorphic ages of Dahomeyan garnet-hornblende gneiss in southeastern Ghana, West Africa

U–Pb dating of zircon and rutile from a Dahomeyan garnet-hornblende gneiss in Ghana/West Africa shows exclusively Neoproterozoic-age components. Zircon data define a short mixing line with concordia intercept ages of 600+9/−73 Ma and 689+127/−61 Ma. Rutile defines a 206Pb/ 238U age of 576±2 Ma. Morphology and zoning patterns in the zircon suggest that it formed during high grade metamorphism. Peak regional granulite facies metamorphism likely occurred close to the age of the youngest zircon (603±5 Ma). A rutile age of 576±2 Ma can be taken as the time when the cooling isograd passed through approximately 400 °C. The older component in the zircon may represent igneous inheritance or early growth during prograde eclogite facies metamorphism no later than 621±3 Ma.