Concordia Diagram Research Articles

Trace Element Composition of Discordant Zircon as a Reflection of the Fluid Regime of Paleoproterozoic Granulite Metamorphism (Khapchan Terrane, Anabar Shield)

New data on the U–Pb age (SHRIMP-II) and trace element composition (SIMS) of zircon from gneisses of the Khapchan Group of the Khapchan terrane of the Anabar Shield are presented. Zircon grains contain relicts of magmatic zircon, the protolith and source areas of which are difficult to specify. The only zircon in this group with the least altered core preserved the 207Pb/206Pb protolith age of 1971 ± 19 Ma. During granulite metamorphism, the zircon was subjected to the impact of fluid enriched in incompatible elements. The disurbances affected both the U–Pb isotope system (zircon age was “reset” at metamorphic age of 1920–1930 Ma) and composition of zircon, magmatic cores of which were significantly recrystallized in a solid state or dissolved by fluid up to the practically complete disappearance of primary zircon. In both the cases, zircon was sharply enriched in incompatible elements (Ca, Ti, Pb, Sr, Ba, and some others) owing to fluid effect, while preserved cores seemed to be rimmed by a new zircon population (CL-black). The REE distribution patterns in the recrystallized cores acquired a “bird’s wing” profile atypical for zircon. When the fluid lost its reactivity, the main part of the zircon grain crystallized, typical of granulite zircon. This zircon population is less enriched in incompatible element than the cores are. The horizontal pattern of HREE distribution is consistently repeated, which indicates the co-crystallization of zircon and garnet. The estimated crystallization temperature of the main part of zircon varies in a narrow range of 800–830°C. All zircon domains in the Wetherill concordia diagram form a single trend with a zero lower intercept and an upper intercept confirmed by concordant cluster with an age around 1920–1930 Ma. This value corresponds to the age of regional Paleoproterozoic granulite metamorphism. A unique feature of zircon from the Khapchan gneisses is that its cores did not retain the age marks of the protolith, but were completely reset during metamorphism both in terms of the U-Pb system and the trace element composition, which can be explained by the extremely high intensity of fluid impact during the granulite facies metamorphism superimposed on the rocks of the Khapchan terrane of the Anabar Shield.

U–Pb Age and Mineral Inclusions in Zircon from Diamondiferous Garnet–Pyroxene Rocks of the Kumdy-Kol Microdiamond Deposit in Northern Kazakhstan

Abstract ––This paper describes the results of studying inclusions and determining the U–Pb age in zircon from the diamondiferous garnet–pyroxene rock of the Kumdy-Kol metamorphogenic diamond deposit, located in the Kokchetav subduction-collision zone. The distribution of rare earth elements in garnet and clinopyroxene is used as a basis for estimating the equilibrium pressure (5.5 ± 0.3 GPa) and temperature (993 ± 24 °C), which correspond to the diamond stability field. The composition of mineral inclusions in zircon indicates its formation at both the progressive and the regressive stage of metamorphism. The concordia diagram shows that the figurative points of zircon lie on a discordia with an upper intersection at 1953 ± 139 Ma and a lower intersection at 512 ± 4 Ma. The main peak on the graph of the probability density distribution of zircon ages corresponds to an age of 519 Ma. The presence of grossular–almandine garnet inclusions in zircon confirms the previous assumption that the basement rocks of the Kokchetav massif act as protoliths of garnet–pyroxene rocks.

Uranium–lead geochronology applied to pyrope garnet with very low concentrations of uranium

AbstractWe present U–Pb dates from peridotitic pyrope-rich garnet from four mantle xenoliths entrained in a kimberlite from Bultfontein, South Africa. Garnet dates magmatic emplacement due to the high mantle residence temperatures of the source material prior to eruption, which were most likely above the closure temperature for the pyrope U–Pb system. We determine a U–Pb date of 84.0 ± 8.1 Ma for the emplacement of the Bultfontein kimberlite from garnet in our four xenolith samples. The date reproduces previous dates obtained from other mineral-isotope systems (chiefly Rb–Sr in phlogopite). Garnet can be dated despite extremely low concentrations of U (median ∼0.05 μg/g), because concentrations of common Pb are often low or non-detectable. This means that sub-concordant garnets can be dated with moderate precision using very large laser-ablation spots (130 μm) measured by quadrupole inductively coupled plasma – mass spectrometry (LA-Q-ICP-MS). Our strategy demonstrates successful U–Pb dating of a U-poor mineral due to high initial ratios of U to common Pb in some grains, and the wide spread of isotopic compositions of grains on a concordia diagram. In addition, the analytical protocol is not complex and uses widely available analytical methods and strategies. This new methodology has some advantages and disadvantages for dating kimberlite emplacement versus established methods (U-based decay systems in perovskite and zircon, or Rb- or K-based systems in phlogopite). However, this method has unique promise for its potential application to detrital diamond prospecting and, more speculatively, to the dating of pyrope inclusions in diamond.

THE SPECIFIC URANIUM-LEAD ISOTOPIC SYSTEM OF MONAZITE FROM THE LEUCOCRATE GNEISS OF PRIAZOVIA

The uranium-lead isotopic system of monazites from leucocratic gneiss (Pryazovsky megablock) was studied and the age of their formation was determined. Monazites from leucocratic gneiss are characterized by significant discordance, which is obviously due to the radiogenic component of impurity lead. In our case, the reverse discordance is to a large extent obviously due to the addition of radiogenic lead, which was captured by monazite during crystallization and is a component of impurity (abnormal composition) of lead. Calculating the age of monazite using a concordia diagram (Arens-Wetherill method) gave the following results: at the upper intersection of the regression line, the age was 2029 ± 145 and 1106 ± 337 Ma, at the lower one MSWD = 35. Taking into account the rather large contents of ordinary lead in the monazite samples studied and the significant dependence of the isotopic age determination results on the adequately introduced correction for ordinary (impurity lead), the isotopic composition of which may not correspond to the isotopic composition of ordinary lead according to Stacey and Kramers, calculated the relationship between numerical age values obtained from the 207Pb/206Pb isotopic ratio and the 206Pb/204Pb ratio, which sufficiently characterizes the content of ordinary lead. Therefore, the numerical values of the age, calculated using different models, according to the obtained analytical data, are quite different: 2027 Ma, 2037 ± 18, 2029 ± 145, and 2099 Ma. In our opinion, the isotopic date, 2037 ± 18 Ma, calculated by the isotopic composition of lead, taking into account the errors in the determination of lead isotopic ratios, corresponds to the time of monazite crystallization to a greater extent. Within the margin of error, it coincides with the date of 2029 ± 145 Ma, obtained from the upper intersection of the discordia with the concordia in the coordinates of the Ahrens-Weatherill method.

U-PB AND LU-HF IN ZIRCON DECIPHERING MAGMATIC PULSES OF THE JUÍNA KIMBERLITE: IMPLICATIONS FOR THE LITHOSPHERIC MANTLE ISOTOPIC COMPOSITION OF THE AMAZONIAN CRATON

This work aims to analyze U-Pb and Lu-Hf ages in zircons formed together with diamonds from the kimberlite province of Juína, in Mato Grosso state, Brazil. Fourteen megacryst (2-3 cm) of zircon were imaged by cathodoluminescence to evaluate the internal structure of the zircon grains and zoning features related to structures of reactions and resorption indicative of magma mixture during evolution. According to the taken images, most of the investigated crystals have a non-metamict structure, with rare fractures, inclusions, and overgrowths, which generates a small degree of heterogeneity. Analyses by Laser Ablation- Multi-Collector- Inductively Coupled Plasma Mass Spectrometry (LA-MC-ICP-MS) were performed to determine grain ages. Concordia diagrams indicate ages ranging from 99.8 ± 1.6 Ma to 89.5 ± 5.6 Ma with two clusters indicating different magmatic pulses. The analyzed zircons displayed total concentrations of U and Th ranging from 108 to 673 mg kg-1 and 26 to 194 mg kg-1, respectively, and U/Th values varying between 4 and 0.3. The concentrations of U and Pb suggest the occurrence of magmas with two distinct signature components. One hundred and twenty Lu-Hf isotopic results obtained by LA-MC-ICP-MS indicate homogeneity in the isotopic composition, with εHf values ranging from +2.2 to -1.9, and depleted mantle model ages (TDM) between 1.45 Ga and 2.55 Ga. The results suggest that the kimberlite magma derives from a mantle source enriched with an Hf isotopic signature equivalent to the chondritic unfractionated reservoir (CHUR).

Geochronology of the Cordillera Real granitoids, the inner magmatic arc of Bolivia

The Cordillera Real granitoids are a suite of Triassic and Oligocene plutons in the core of the Eastern Cordillera of the Central Andes of Bolivia. Their tectonic setting, chemical and ore compositions make them part of the so-called “Inner Magmatic Arc”, which differs from the current “Magmatic Arc” located immediately to the west. U-Pb SHRIMP data were obtained in order to constrain crystallization ages. The Triassic group yielded the following results: 239±2 Ma for the Huato granite, 231±1 Ma for the Illampu granodiorite, 221±3 Ma for the Huayna Potosí granite and 223±2 Ma for the Taquesi granodiorite. For the Oligocene group we obtained ages of 27 Ma for two samples of the Quimsa Cruz granite. Secondary processes related to regional thermal anomalies and magmatic melt-enrichment, reset the K-Ar and U-Pb isotopic systems, producing: a) younger ages by Ar loss and b) anomalous data plot in the Concordia diagram by reorganization of U-Pb isotopic ratios. As noted in previous studies, most zircon analysed from the Zongo/Kuticucho Triassic granite exhibited extremely high U enrichment, producing reverse discordia curves that obscure the true crystallization age. Relatively abundant zircon inheritance was found in these “cold” granitoids, with ages suggesting provenance from early Paleozoic metapelites that also contained recycled older sources. This relatively abundant xenocrystic inheritance probably records the influence of the subduction process acting during the Gondwanide orogeny (336-205 Ma) as an overall subduction arc environment, punctuated at its final stage with the imprint of a continental rifting (245-220 Ma).

Metamorphism of the Buchan type-area, NE Scotland and its relation to the adjacent Barrovian domain

The metamorphism of the Buchan block in northeast Scotland, and its relation to the Barrovian domain to its west, have been reassessed from consideration of mineral assemblages, microstructures, phase equilibrium modelling and monazite U–Pb geochronology. Zones of increasing metamorphic grade surround a central low-grade domain (biotite zone) and define a northward-opening, U-shaped metamorphic map pattern. The eastern and southern parts of the Buchan domain show the classic Buchan-type prograde sequence of cordierite – andalusite – sillimanite – migmatite/gneiss, representing a metamorphic field gradient of gently increasing pressure between (2.5–3.0 kbar, c. 550°C) and (3.5–4.0 kbar, c. 750°C). A slightly lower pressure metamorphic field gradient (by less than c. 0.5 kbar) is interpreted for the northeastern Buchan domain. The west Buchan domain shows a prograde sequence of staurolite+andalusite – sillimanite – gneiss/migmatite, representing a metamorphic field gradient of higher pressure by c. 1 kbar or less than the classic sequence over a similar temperature range. Unique to the northwestern Buchan domain is a staurolite+cordierite+andalusite domain in which staurolite-bearing schists have been overprinted by cordierite + andalusite assemblages. Monazite U–Pb geochronology of schists and gneisses from the Buchan block, including the Cowhythe and Ellon gneisses, yields ages in the range 470 ± 5 Ma, supporting geological evidence that the gneisses are metamorphosed Dalradian strata rather than older basement gneisses. The metamorphic ages are similar to the ages of mafic igneous intrusions in the Buchan block, even though many of the exposed intrusions post-date the regional metamorphic zones. The Buchan metamorphic zones are truncated to the west by the Portsoy–Duchray Hill Lineament, a ductile shear zone that juxtaposes the Buchan rocks against higher pressure, lower temperature (kyanite-bearing) Barrovian schists to the west ( c . 5.5–7.0 kbar, c. 580–650 °C). A 2–15 km wide corridor of andalusite pseudomorphed by kyanite occurs between the Portsoy–Duchray Hill Lineament and the Keith shear zone to its west. Monazite geochronology of the Barrovian rocks west of Portsoy shows little evidence of the c. 470 Ma signature of the Buchan block, instead yielding a dominant cluster of ages at c. 450 Ma and a more poorly defined grouping at c. 500 Ma. Supplementary materials: Analytical procedures; Thermodynamic modelling; Tables S1 to S13 (mineral assemblages, whole rock compositions, mineral compositions, isotopic analyses); Figures S1 to S5 (supplementary mineral assemblage map, graphs of mineral composition vs. grade, supplementary thermodynamically calculated phase diagrams, back-scattered electron (BSE) images of monazite grains, concordia diagrams for monazite U–Pb results) are available at https://doi.org/10.6084/m9.figshare.c.5536745

LA-ICPMS, TEM and Raman study of radiation damage, fluid-induced alteration and disturbance of U-Pb and Th-Pb ages in experimentally metasomatised monazite

Experimentally metasomatised monazite was studied in terms of preservation of U-Pb and Th-Pb ages during alkali-bearing fluid-induced alteration over a broad range of temperature conditions 250–750 °C. Starting materials for experiments included Burnet monazite (Concordia age 1100.5 ± 11.6 Ma, 2σ), albite, K-feldspar, biotite, muscovite, SiO2, CaF2, Na2Si2O5 and H2O. Monazite from experiments at 250–550 °C is partially replaced by secondary REE-rich fluorapatite [(Ca,LREE,Si,Na)5(PO4)3F], fluorcalciobritholite [(Ca,REE)5(SiO4,PO4)3F] and REE-rich steacyite [K1−x(Na,Ca)2(Th,U)Si8O20], and developed patchy zoning, whereas partial replacement by fluorcalciobritholite and cheralite [CaTh(PO4)2] occurred at 650 and 750 °C, with no signs of compositional alteration based on EPMA data and BSE imaging. Raman microspectroscopy results show narrowing of the ν1(PO4) stretching band in unaltered domains, which indicates advancing annealing of the monazite structure with increasing temperature, and narrow ν1(PO4) band with low FWHM values in altered domains. TEM investigations revealed that unaltered domains of monazite from experiments at 250–550 °C have mottled diffraction contrast, similar to the starting Burnet monazite, which indicates low to moderate degree of metamictization. On the contrary, the altered domains of monazite (patchy zones) show no mottled contrast, suggesting an ordered crystalline structure. TEM imaging demonstrated low degree of metamictization in monazite from the experiment at 650 °C; fluid-aided alteration along the cleavage planes resulted in the development of nanoporosity or partial replacement by fluorcalciobritholite and cheralite. Monazite from the experiment at 750 °C has crystalline structure with no signs of metamictization and shows significant development of nanoporosity and formation of secondary cheralite nanocrystals across the grain. For comparison, TEM and Raman evaluation of xenotime from similar experiments at 350 and 650 °C revealed that both starting xenotime and xenotime from experimental products are crystalline with no signs of radiation damage or fluid-induced alteration affecting internal domains on submicron scale, which could result in compositional alteration of the xenotime.The unaltered domains of monazite from runs at 250–550 °C yielded U-Pb and Th-Pb dates similar to the age of Burnet monazite, whereas altered domains yielded discordant dates due to various degree of Pb-loss (up to 99.4%). Linear regressions on the Concordia diagrams show lower intercept ages from −266 ± 160 Ma (run 350 °C, 200 MPa) to −1 ± 48 Ma (450 °C, 800 MPa), which reflect the “true age” of experimental alteration. The monazite from runs at 650 and 750 °C yielded data indicating initial disturbance of the U-Th-Pb system, ranging from 8.4% Pb-gain to 18.6% Pb-loss. Linear regressions with lower intercepts of −53 ± 420 Ma and −55 ± 610 Ma roughly correspond to the timing of the experiments. Furthermore, LA-ICPMS results demonstrate discrepancy between Th-Pb and U-Pb dates suggesting higher mobility of 208Pb than that of 207Pb and 206Pb.To summarize, TEM and Raman data indicate increasing annealing of the radiation damaged monazite with increasing temperature. Alteration processes induced by alkali-bearing fluid can result in recrystallization of monazite and various degrees of the age disturbance at temperatures 250–550 °C, whereas isotopic U-Th-Pb microanalysis provide an opportunity to constrain the age of the metasomatic processes as the lower intercept in the Concordia diagram. The particular importance of this study lies in submicron alteration of monazite at 650–750 °C induced by alkali-bearing fluid and/or melt, which remains unnoticed using common electron microscopy BSE imaging. Such alteration, however, induces substantial disturbance of U-Pb and Th-Pb ages, which can cause misinterpretations in reconstructions of geological processes.

Radiogenic Pb mobilization induced by shock metamorphism of zircons in the Apollo 72255 Civet Cat norite clast

In situ U-Pb radiometric dating of zircons is regarded as one of the most widely used and reliable methods to acquire geochronologic ages. However, it has been recently reported that radiogenic Pb (Pb*) mobilization within zircon may, in some cases, cause inaccurate age determinations with no geological significance. Such Pb* mobilization can be caused by deformation, α-coil damage, fluid-assisted annealing, and recrystallization. In this study, we report an investigation of Pb* mobilization in shock metamorphosed lunar zircons. NanoSIMS (nanoscale secondary ion mass spectrometry) and IMS 1280HR ion microprobe dating, EBSD (electron backscatter diffraction) and CL (cathodoluminescence) mapping, and scanning ion imaging (SII) were applied to micro-zircon grains from the Apollo 72255 Civet Cat norite clast. Based on the large number of grains with similarities in internal zoning, habit and trace element geochemistry, and host mineral context, the Civet Cat norite zircons are interpreted to be primary, igneous grains. The chronology obtained for three consecutive surfaces (at different depths) by NanoSIMS, SII, and IMS 1280HR, respectively, indicates that the radiogenic Pb distribution of the Civet Cat norite zircons is heterogeneous among different polished or sputtering surfaces. Forty-two NanoSIMS U-Pb ages (beam size of 5 μm) are concordant on a Wetherill Concordia diagram, and their corresponding 207Pb/206Pb ages spread from 4015 Ma to 4459 Ma. More notably, the six oldest spots of the 42 define a concordant U-Pb age of 4460 ± 31 Ma (2σ, MSWD = 0.47, P = 0.92) and a weighted mean 207Pb/206Pb age of 4453 ± 34 Ma (MSWD = 0.056, P = 0.998). These dates are among the oldest in the lunar highland rocks. However, the 207Pb/206Pb ages of repolished surfaces of these zircons by IMS 1280HR (beam size of 5 μm) do not reproduce the NanoSIMS results (up to 300 Ma younger). The SII (spatial resolution of 2 μm) confirms a heterogeneous distribution of radiogenic Pb within single grains. The EBSD mapping of these zircon grains shows that they have 3–20° of cumulative lattice misorientation. It is proposed that shock-related deformation has facilitated Pb* migration after primordial crystallization. With currently available data, we cannot preclude the possibility that the large errors of the U-Pb ages obscure reverse discordance that would bias our oldest 207Pb/206Pb ages to older values. Conversely, our data could be explained by mixing of Pb-retention and Pb-loss nanodomains as seen in shocked terrestrial zircon such that U-Pb date of 4460 ± 31 Ma approximates the norite formation.

In situ low-U garnet U-Pb dating by LA-SF-ICP-MS and its application in constraining the origin of Anji skarn system combined with Ar-Ar dating and Pb isotopes

Garnet becomes an important tool to analyze the timing and genesis of metamorphic rocks (especially skarn deposits), clastic sediments, and igneous rocks due to its common occurrence. However, for in-situ U-Pb dating of garnet, is there any difference to use 91500 or Willsboro as a primary standard and can a robust age be obtained for low-U (≤ 10 ppm) garnet as Willsboro and Mali in small spot sizes of 32–16 μm? Can garnet U-Pb dating be used to exactly identify the ore-related intrusive unit in an intrusive complex and its related distal skarn system setting? In this contribution, Willsboro, Mali, QC04, and three other andradite samples WMQ-2(3), WSG-1, and YJ-4(2) from the Anji polymetallic skarn deposit were analyzed to discuss these questions. In our analysis, zircon 91500 is confirmed as reliable reference material for these garnets in spot sizes of 32–16 μm. Notably, both 91500 and Willsboro are suitable as primary standards for the garnets as Mali and QC04, which have extremely low common lead and are mainly plotted at the lower intersection in the Tera-Wasserburg Concordia diagram; Willsboro is not suitable to calibrate the garnets as WMQ-2(3), WSG-1, and YJ-4(2), which contain relatively high common lead and fall along a mixing line between initial common Pb and radiogenic components in the Tera-Wasserburg Concordia diagram. In the Anji polymetallic system, based on garnet U-Pb, mica Ar-Ar and previous zircon U-Pb dating, proximal Pb-Zn-Ag-Cu and distal Fe and Fe-Zn-Cu skarn deposits were formed by the same magmatic-hydrothermal-mineralization event at 137–138 Ma. These ages, geological evidences, our and previous lead isotopic compositions confirm that Fe, Fe-Zn-Cu, Pb-Zn-Ag-Cu, Mo, and fluorite-chalcedony mineralization have genetic relationships with fine-grained granite. Compared with other units of the Wushanguan complex, the lead isotopic compositions indicate that fine-grained granite supplies most ore-forming materials, and the increased mantle-sourced materials may play an important role in the formation of the Anji skarn system.

In-situ U-Pb geochronology and trace element analysis for the scheelite and apatite from the deep seated stratiform-like W(Cu) ore of the Zhuxi tungsten deposit, northeastern Jiangxi Province

钨和铜由于地球化学行为存在明显差异，导致二者通常很难同时发生大规模的成矿作用。然而，江南钨矿带却出现了以石门寺和朱溪为代表的钨（铜）矿床。本文对朱溪钨（铜）矿床中最为重要的铜矿化作用开展了研究，即对形成于新元古代浅变质岩与古生代碳酸盐岩不整合界面附近的似层状钨（铜）矿体进行了精细的矿物学微区原位测试分析。厘定了朱溪矿床深部似层状钨（铜）矿体中铜矿化的形成时代为150.2±2.4Ma，与朱溪矿床的钨矿化时代及钨矿化相关岩体的成岩时代近于一致；揭示了深部似层状钨（铜）矿体中白钨矿、磷灰石的形成与朱溪矿床成矿相关岩浆高程度结晶分异形成的残余岩浆热液流体相关；结合朱溪矿床高分异残余岩浆（流体）具有高度还原的特征，初步提出朱溪矿床成矿过程中，钨来自于高分异残余岩浆热液流体，而铜来自于岩浆热液流体对基底地层中铜元素的萃取。;Due to the obvious differences in geochemical behavior for tungsten and copper in magmatic systems, it is usually difficult to occurring large-scale Cu- and W-mineralization at the same time. However, tungsten (copper) deposits represented by Shimensi and Zhuxi deposits appear in the Jiangnan tungsten belt. This study focuses on the most important copper mineralization occurred in the Zhuxi W (Cu) deposit, which formed the stratiform-like W(Cu) ore body near the unconformity interface between the Neoproterozoic metamorphic rock and the Paleozoic carbonate rock. In situ micro-zone analysis have been performed on the scheelite and apatite from the stratiform-like W(Cu) ore body. The LA-ICP-MS U-Pb data on the apatite show a Lower intercept age of 150.2±2.4Ma on the Tera-Wasserburg concordia diagrams, which represents the copper mineralization in the stratiform-like W(Cu) ore body and is closely consistent with the tungsten mineralization age and the diagenetic age of the granitic rocks associated with tungsten mineralization. The geochemical characteristics of scheelite and apatite reveal that the formation of scheelite and apatite grains in the stratiform-like W(Cu) ore body is genetically associated with the residual magmatic hydrothermal fluids formed from the highly fractionated magma system that induced the tungsten mineralization of the Zhuxi deposit. In view of the highly fractionated residual magma system of the Zhuxi deposit is characterized by highly reductive features. This study preliminarily proposed that during the mineralization process of the Zhuxi deposit, tungsten is derived from the highly fractionated magmatic system, while copper comes from the extraction of the basement strata by the magmatic hydrothermal fluid.

Unraveling a hidden Rhyacian magmatic arc through provenance of metasedimentary rocks of the Crixás greenstone belt, Central Brazil

The Crixás-Goiás Domain, Central Brazil, comprises an association of Archean TTG (tonalite-trondhjemite-granodiorite) and Archean-Paleoproterozoic greenstone belts. The Crixás greenstone belt, located in the northern part of the domain, is constituted of Mesoarchean metakomatiites and metabasalts overlaid by Paleoproterozoic carbonaceous phyllites and metagraywackes that host important gold deposits. In this work, we present new integrated petrographical, geochemical and isotopic data to investigate the provenance and tectonic setting of the metasedimentary rocks of the Crixás greenstone belt. Mineral and textural immaturities, particularly of the metagraywackes, are consistent with a proximal source area; which is also evidenced by the chemical aspects that denote absence of significant sediment sorting and recycling. Geochemical data indicate provenance from mixed mafic and felsic sources. U-Pb detrital zircon geochronology shows unimodal age spectra, with a dominant peak at ca. 2.2–2.1 Ga and minor contribution of ca. 2.7 and 2.4 Ga grains. Concordia diagrams yielded upper intercepts between 2213.4 ± 5.6 and 2162.2 ± 9.2 Ma, interpreted as the age range of the main source area. The rocks present relatively homogeneous Sm-Nd model ages (2.44 to 2.34 Ga) and slightly negative to positive ƐNd, ranging from −0.27 to +1.25. We argue that the original sedimentary protoliths were deposited in a trench or forearc basin related to a ca. 2.2–2.1 Ga island arc formed adjacent to an Archean paleocontinent constituted of TTG-greenstone terranes. Rapid erosion of the rising intraoceanic arc resulted in syn-orogenic sedimentation that started with deep ocean black shale deposition in an anoxic environment followed by episodic turbidite currents of poorly-sorted immature sands. Sedimentary rock layers were tectonically transported to the continent and preserved in the elongated keels of Mesoarchean mafic–ultramafic volcanic rocks. This orogeny can be correlated with the amalgamation of the Goiás Massif during the Siderian-Rhyacian crustal growth related to successive collage of arc-related terranes in the formation of the western margin of the São Francisco paleocontinent. Sediment provenance of the Crixás greenstone belt proved to be crucial to the recognition and characterization of Paleoproterozoic crust that are apparently no longer preserved in outcrops.

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.

Zircons isotope dating and thermochronology of the migmatites from the Gondaray metamorphic complex of the Greater Caucasus

Results from isotope dating of the Greater Caucasus crystalline basement in the Elbrus subzone of the Main Caucasus Range inside of the HT gneiss-migmatite area of the Gondaray Metamorphic Complex is discussed herein. The measurements of the zircons isotope composition were performed on the ion microprobe SHRIMP-II at the CIR VSEGEI (St. Petersburg). All zircon crystals from the gneiss sample N 526 show chemical zoning and an old clastogenic core. Almost all U-Pb isotope dating points toile on the concordant line of the concordia diagram and show a wide age range from 320–1000 Ма, partially obtained from clastogenic grains of the zircon from the initial pelitic sediments. The youngest ages (320 Ма) belong to regenerating zones of the zircon grains recrystallized during stage of the anatexis and migmatization. The other part of the age range 540–1000 Ма belongs to detrital zircons from different magmatic sources that existed during accumulation of the proto-metamorphic sediments. Several clastogenic zircon grains show a Cambrian age, which is an evidence for the Early Paleozoic age of the metamorphic protolith. Traditionally the age of the Caucasus crystalline basement was suggested to be Precambrian. The ages of rim zones of the recrystallized zircons (320 Ма) have a direct correlation with postmetamorphic granite ages of the Greater Caucasus. It is shown by termochronological modeling that cooling of the Gondaray Metamorphic Complex during a retrogressive stage, from the temperature of migmatite crystallization (650оС) to the moment of biotite K-Ar isotope system closure temperature (350оС), was relatively fast (rate of cooling 8–10оС/Ma) at subisobaric conditions and during a time range about 30–40 Ма.

Columbite-group minerals and mica of peraluminous granite record the magmatic-hydrothermal processes that formed the Zhaojinggou Ta[sbnd]Nb deposit in the North China Craton

The newly discovered Zhaojinggou TaNb deposit in the northern margin of the North China Craton contains ore reserves of more than 38 Mt. at an average grade of 0.013 wt% Ta2O5 and 0.012 wt% Nb2O5. The TaNb mineralization is mainly hosted in a peraluminous, albite granite body, which is about 400 m long and 20 to 100 m wide, with a surface exposure of ~0.05 km2. Granitic pegmatites and greisen veins that range from several centimeters to meters in width, occur in the margin of the granite body, and wolframite-quartz veins intrude the wall rocks up to tens of meters away from the granite contact. LA-ICP-MS UPb dating for columbite of the granite yields the lower intercept ages between 131.8 ± 1.0 Ma and 134.6 ± 2.1 Ma on the Tera-Wasserburg concordia diagram, indicating that the mineralization occurred in the early Cretaceous, coeval with the peak stage of lithospheric thinning and destruction of the North China Craton. The columbite grains in the granite are commonly euhedral, and are surrounded or penetrated by irregular tantalite patches/rims containing Ta2O5 as high as 59.5 wt%. The Ta concentrations within single columbite crystals commonly increase slightly from the center outwards, consistent with a systematic decrease of Nb/Ta caused by crystallization of columbite from the silicic magma. Mica in the granite shows disordered bright and dark domains in their BSE images; bright domains are F-rich zinnwaldite crystallized from highly evolved melts that were relatively enriched in Fe, Li, Rb, Nb and Ta, whereas the dark domains are F-poor muscovite crystallized from later hydrothermal fluids that were relatively enriched in Al, Mn, Ba, Sn and W. It is thus likely that the tantalite patches/rims around the columbite grains crystallized from a Ta- and F-rich hydrosilicate melt in a magmatic-hydrothermal transitional stage. The wolframite-quartz veins adjacent to the granite body likely formed from a W-rich hydrothermal fluid.

Unifying the U–Pb and Th–Pb methods: joint isochron regression and common Pb correction

Abstract. The actinide elements U and Th undergo radioactive decay to three isotopes of Pb, forming the basis of three coupled geochronometers. The 206Pb ∕238U and 207Pb ∕235U decay systems are routinely combined to improve accuracy. Joint consideration with the 208Pb ∕232Th decay system is less common. This paper aims to change this. Co-measured 208Pb ∕232Th is particularly useful for discordant samples containing variable amounts of non-radiogenic (“common”) Pb. The paper presents a maximum likelihood algorithm for joint isochron regression of the 206Pb ∕238Pb, 207Pb ∕235Pb and 208Pb ∕232Th chronometers. Given a set of cogenetic samples, this total-Pb/U-Th algorithm estimates the common Pb composition and concordia intercept age. U–Th–Pb data can be visualised on a conventional Wetherill or Tera–Wasserburg concordia diagram, or on a 208Pb ∕232Th vs. 206Pb ∕238U plot. Alternatively, the results of the new discordia regression algorithm can also be visualised as a 208Pbc ∕206Pb vs. 238U ∕206Pb or 208Pbc ∕207Pb vs. 235U ∕206Pb isochron, where 208Pbc represents the common 208Pb component. In its most general form, the total-Pb/U-Th algorithm accounts for the uncertainties of all isotopic ratios involved, including the 232Th ∕238U ratio, as well as the systematic uncertainties associated with the decay constants and the 238U ∕235U ratio. However, numerical stability is greatly improved when the dependency on the 232Th ∕238U-ratio uncertainty is dropped. For detrital minerals, it is generally not safe to assume a shared common Pb composition and concordia intercept age. In this case, the total-Pb/U-Th regression method must be modified by tying it to a terrestrial Pb evolution model. Thus, also detrital common Pb correction can be formulated in a maximum likelihood sense. The new method was applied to three published datasets, including low Th∕U carbonates, high Th∕U allanites and overdispersed monazites. The carbonate example illustrates how the total-Pb/U-Th method achieves a more precise common Pb correction than a conventional 207Pb-based approach does. The allanite sample shows the significant gain in both precision and accuracy that is made when the Th–Pb decay system is jointly considered with the U–Pb system. Finally, the monazite example is used to illustrate how the total-Pb/U-Th regression algorithm can be modified to include an overdispersion parameter. All the parameters in the discordia regression method (including the age and the overdispersion parameter) are strictly positive quantities that exhibit skewed error distributions near zero. This skewness can be accounted for using the profile log-likelihood method or by recasting the regression algorithm in terms of logarithmic quantities. Both approaches yield realistic asymmetric confidence intervals for the model parameters. The new algorithm is flexible enough that it can accommodate disequilibrium corrections and intersample error correlations when these are provided by the user. All the methods presented in this paper have been added to the IsoplotR software package. This will hopefully encourage geochronologists to take full advantage of the entire U–Th–Pb decay system.

Combined Zircon, Molybdenite, and Cassiterite Geochronology and Cassiterite Geochemistry of the Kuntabin Tin-Tungsten Deposit in Myanmar

AbstractThe Kuntabin Sn-W deposit, located in southern Myanmar, is characterized by abundant greisen-type and quartz vein-type cassiterite and wolframite mineralization. We have conducted multiple geochronological methods and isotope and trace element analyses to reveal the age and evolution of the Kuntabin magmatichydrothermal system.Zircon U-Pb dating of the two-mica granite yielded a weighted mean 206Pb/238U age of 90.1 ± 0.7 Ma. Cassiterite U-Pb dating provided a lower intercept age of 88.1 ± 1.9 Ma in the Tera-Wasserburg U-Pb concordia diagram. Molybdenite Re-Os dating returned a weighted mean model age of 87.7 ± 0.5 Ma and an isochron age of 88.7 ± 2.7 Ma. These ages indicate a genetic relationship between granite and Sn-W mineralization in the Kuntabin deposit and record the earliest magmatism and Sn-W mineralization in the Sibumasu and Tengchong terranes related to subduction of the Neo-Tethys oceanic slab. Three generations of cassiterite have been identified with distinctive cathodoluminescence textures and trace element patterns, indicating the episodic input of ore-forming fluids and distinctive changes in the physical-chemical conditions of the Kuntabin magmatichydrothermal system. Sudden changes of fluid pressure, temperature, pH, etc., may have facilitated the deposition of Sn and W. Rhenium contents of molybdenite from the Kuntabin deposit and many other Sn-W deposits in Myanmar are characteristically low compared to porphyry Cu-Mo-(Au) deposits worldwide. In combination with zircon Hf isotope signatures, we infer that granites associated with Sn-W deposits in Myanmar were predominantly derived by melting of ancient continental crust and contain minimal mantle contribution.Subduction of the Neo-Tethys oceanic slab from west of the West Burma terrane reached beneath the Sibumasu terrane and led to magmatism and Sn-W mineralization at ~90 Ma when the Kuntabin deposit was formed. The Paleoproterozoic Sibumasu crust was activated during the subduction-related magmatism to form predominantly crust derived melts. After a high degree of fractional crystallization and fluid exsolution, physical-chemical changes of the hydrothermal fluid resulted in Sn and W precipitation to form the Kuntabin Sn-W deposit.

A Cambrian age for the upper Bambuí Group, Brazil, supported by the first U-Pb dating of volcaniclastic bed

The Serra da Saudade Formation is part of the Bambuí Group, a cratonic cover from the western São Francisco craton. It is composed by siliciclastic sediments deposited in a foreland basin during a transgressive event. The Serra da Saudade Formation is mainly represented by rhythmites, siltstones (locally glauconitic) and rare sandstones. The glauconitic siltstone was deposited during a maximum flooding surface and sediment starvation, which allowed high concentrations of probable volcanic-sourced potassium. The interpretation of the depositional age of the Bambui Group had multiple turnovers in the last decades. Finding and dating synsedimentary volcanic rocks is essential to solve this conundrum. This paper presents the first U-Pb direct dating of the Bambuí Group from zircon grains recovered from a volcaniclastic layer interbedded with glauconitic siltstones from the upper Serra da Saudade Formation. A total of 107 U-Pb ages were obtained by LA-ICP-MS and plotted into a Concordia diagram. The provenance signature comprises a wide range of detrital zircon ages (1.0–2.8 Ga). These data represent inherited zircon grains incorporated into the magma during magmatic activity that produced the volcanic sediment, or to dilution and reworking of volcanic ashes that mixed to fine-grained clastic sediments prior to deposition. A significant amount of 10 highly concordant, prismatic zircon grains clustered into a well constrained age of 520.2 ± 5.3 Ma, which represent the explosive volcanic eruption age, and by extent, the depositional age of the upper Serra da Saudade Formation. Since the currently final thrust stacking ages from the Brasilia Belt and the Araçuaí Belt are dated ca. 600 and 540 Ma, respectively, one must carefully look for the real age of the deformation event that affected the Bambuí Group over the São Francisco craton.

Sources of Terrigenous Clastic Material of the Pechenga Ore-Bearing Structure from Data of Detrital Zircon Isotopic Analysis (SIMS SHRIMP-II, LA-ICPMS)

The research results were received from studying the Pechenga ore-bearing Paleoproterozoic structure, which is located on the northwestern part of the Kola region. The U–Th–Pb zircon age was determined by Secondary Ion Mass Spectrometry (SIMS) on the Sensitive High-Resolution Ion microprobe (SHRIMP-II) spectrometer and Laser Ablation Inductively Coupled Mass Spectrometry (LA-ICPMS) on the ThermoQuest Finnigan MAT Neptune instrument. Basalt conglomerates of the Televi Formation, red gritstones of the Luchlompolo Formation, and high-siliceous turbidites from the middle part of the Matert Formation were tested. On the U–Pb concordia diagram, the part of analytical points for zircon from conglomerates are approximated by the discordia with a top intercept at 2792 ± 7 Ma. All zircon grains from gritstone are located on the concordia. Most of the values of their ages lie within 2700–2820 Ma, and the lesser part of the age values lies within 2840–3000 Ma. Single grains have an age from 3037 ± 4 Ma to 3698 ± 8 Ma. The concordant value of the 207Pb/206Pb age, which is 2640 ± 16 Ma, was established for turbidite zircon. Rocks of the basement at the northern rim of the Pechenga structure: gneisses of the Kirkenes, Varanger, and Svanvik complexes (2715, 2803, and 2825 Ma) and high-aluminous gneisses (2798–2830 Ma) were sources of zircon for conglomerates and gritstones. Granitoids from the southern flank of the structure were sources of zircon for turbidites. The predominant role of the zircon group with an age of 2.7 Ga proves the global increment of the continental crust during this period. The ancient age of zircon (3.0–3.7 Ga) of red-colored gritstones is related to deeper erosion of the basement and exposure of the Eoarchaean and Mezoarchaean rocks at the northern rim during the period of the first global “Oxygen revolution” (2.4–2.3 Ga).

Modelling U-Pb discordance in the Acasta Gneiss: Implications for fluid–rock interaction in Earth's oldest dated crust

The U–Pb isotopic system in zircon is the tool of choice to interrogate high-temperature geological processes, yet this system has potential to investigate lower temperature fluid–rock interaction as well. In some cases, removal of radiogenic Pb is incomplete, potentially allowing regression of discordant U–Pb data on a concordia diagram to determine both the age of crystallization and the timing of fluid-driven isotopic disturbance. However, in rocks preserving more complex histories, simple regression is not effective at resolving multiple Pb loss events. Here, we use a ‘concordant–discordant comparison’ (CDC) test to establish the times of U–Pb disturbance in the Acasta Gneiss Complex (AGC), Canada. AGC c. 4.03 to c. 3.40 Ga orthogneisses experienced a long and complex post-crystallization history, for which U–Pb zircon data reflects not only the heterogeneous nature of the rock, but also the varying degrees and duration of crustal reworking that inevitably involved open system processes. The CDC test calculates the similarity between the concordant age structure and a modelled age structure, the latter inferred from discordant analyses, over a wide range of potential disturbance times. Our analysis reveals concordant zircon components implying new growth and/or recrystallization at 3992 ± 5, 3501 ± 6, 3442 ± 5 and 3126 ± 6 Ma. In addition, we establish episodes of radiogenic-Pb loss driven by fluid–rock interaction at 3150 ± 50 Ma, and probably at 2875 ± 50 Ma and c. 2590 Ma. These Pb-loss episodes correlate with previously recognised events recording growth of zircon rims during metamorphism, granite emplacement, and unroofing. Pb-loss within the AGC shows an antithetic relationship in different samples that are in close geographic proximity. We suggest that zircon alteration and associated new growth effectively rendered those grains that underwent Pb-loss at a particular time less susceptible to alteration during the next episode of fluid interaction.