Pb Isotopic Ages Research Articles

The first U–Pb isotope age data of island arc and continental–margin magmatism in the central part of the Koryak Highlands and U–Pb age of ore-controlling granitoids of the talyayigin ore field

Precision U–Pb (SHRIMP-II) isotope geochronological data, obtained for the first time, make it possible to suggest that sediments of the Neocomian primitive island arc sequence are missed or poorly developed in the South-Western part of the Mainitskii terrane of the Koryak Highlands. However, Late Albian mature island arc tuff and tuffaceous–turbidite formations are common. This enables us to extend the age range of the Mainitskii island arc from the Early Neocomian to the Late Albian and to suggest a two-stage pattern of its development. The isotope-geochronological data obtained for plagiogranite and moderately acid subvolcanics, previously attributed to the Koryak–Western Kamchatka volcanoplutonic belt, indicate that it is possible to combine them into the Middle Miocene postsubduction? polygenic complex. In addition, owing to modern high-precision isotope–geochronological methods, it has become possible to determine the age of gold–sulfide mineralization of the Talyaigin ore field, paragenetically related to the manifestations of the Middle Miocene Vilyuneiveem complex.

The provenance of terrigenous rocks of the basal horizons of the Uralides of the Baidarata Allochthon (Polar Urals) according to the U–Pb isotope age of detrital zircons

The U–Pb (LA–ICP–MS) age of detrital zircons from the Upper Cambrian–Lower Ordovician terrigenous rocks of the Baidarata Allochthon, which is located in the northern part of the Polar Urals, is determined. The analysis of the youngest zircon population indicates a broad occurrence of the Uralides in this area rather than Pre-Uralides, as was considered previously. The Bedamel island-arc rocks (rather than Timan orogen) were probably the major provenance for the studied sequences. The results of statistical processing of the U–Pb ages of zircons from coeval rocks of Arctic regions suggest similar provenances for the Baidarata Allochthon and Novaya Zemlya and Severnaya Zemlya archipelagoes.

First results of U–Pb dating of detrital zircons from the Ordovician clastic sequences of the Sol-Iletsk Block, East European Platform

The first LA–ICP–MS U–Pb isotopic ages of detrital zircons from the Ordovician sandstones of the Sol–Iletsk Block (well 2–Ordovician), located at junction of the East European Platform with the Pre-Caspian Basin and the Pre-Uralian foredeep, are presented. Two detrital zircons with well-defined ages of 561 ± 4 and 570 ± 5 Ma were found in sample K15–501. They confirm the Ordovician age of the sandstones, which earlier had been defined on the basis of seismic–stratigraphic and lithological correlations. The age distribution of the detrital zircons indicates the significant role of Late Precambrian rocks as provenance sources. However, those rocks still remain unknown in the Early Precambrian basement of the Volga–Ural part of the EEP.

The results of geochronological and isotope–geochemical study of zircons from tuff of the Sylvitsa Group (western slope of the Middle Urals): The origin of ash layers in Vendian rocks of the East European Platform

New LA-ICP-MS data on the U–Pb isotope age, Hf isotope characteristics, and concentrations of trace elements in zircons from volcanic tuff of the Upper Vendian Sylvitsa Group on the western slope of the Middle Urals are reported.

Early island-arc granitoids of the Shchuchinskaya zone of the Polar Urals: U–Pb (SIMS) zircon isotope data

Granitoids of the Rechnoy and Yalya-Pe paleovolcanoes, which were ascribed to the Silurian Khoimpe complex during a geological mapping, and granitoids of the Nganotsky-1 and Nganotsky-2 plutons that were ascribed to the Early Devonian Yunyaga complex were studied in the Shchuchinskaya zone of the Polar Urals. It was established that according to the mineral and chemical compositions the rocks of the plutons studied correspond to island-arc granitoids of I-type. Zircons from granitoids of the Rechnoy and Yalya-Pe paleovolcanoes and the Nganotsky-1 pluton yielded concordant U–Pb (SIMS) isotope ages of 456 ± 6, 454 ± 4, and 463 ± 3 Ma, respectively, which indicates the existence of an island arc within the Shchuchinskaya zone starting from the Middle–Late Ordovician. Based on the obtained zircon ages of granitoids, the country volcanics were ascribed to the Syaday Formation; the upper stratigraphic boundary of their formation was specified as the Middle–Upper Ordovician.

Newly discovered uranium mineralization at ~2.0 Ma in the Menggongjie granite-hosted uranium deposit, South China

The southeastern part of the Nanling metallogenic province, South China contains numerous economically important granite-hosted, hydrothermal vein-type uranium deposits. The Miao’ershan (MES) uranium ore field is one of the most important uranium sources in China, hosts the largest Chanziping carbonaceous-siliceous-pelitic rock-type uranium deposit and several representative granite-hosted uranium deposits. The geology and geochemistry of these deposits have been extensively studied. However, accurate and precise ages for the uranium mineralization are scarce because uranium minerals in these deposits are usually fine-grained, and may have formed in several stages, thus hindering the understanding of the uranium metallogenesis of this province.The Menggongjie (MGJ) uranium deposit is one of the largest granite-hosted uranium deposits in the MES ore field. Uranium mineralization in this deposit occurs at the central part of the MES granitic complex, accompanied with silicification, fluorination, K-metasomatism and hematitization. The ore minerals are dominated by uraninite, occurring in quartz or fluorite veinlets along fractures in altered granite. In-situ SIMS U-Pb dating on the uraninite yields the U–Pb isotopic age of 1.9±0.7Ma, which is comparable to the chemical U-Th-Pbtol uraninite age of 2.3±0.1Ma. Such ages agree well with the eruption ages of the extension-related Quaternary volcanics (2.1–1.2Ma) in South China, suggesting that the uranium mineralization have formed at an extensional setting, possibly related to the Quaternary volcanic activities. Therefore, our robust, new dating results of the MGJ uranium deposit make it the youngest granite-hosted uranium deposit reported so far in South China and the mineralization event represents a newly identified mineralization epoch.

Neoarchean arc magmatism and crustal growth in the north-eastern North China Craton: Evidence from granitoid gneisses in the Southern Jilin Province

Late Neoarchean granitoid gneisses are widespread in the Jiapigou–Jingyu area of the Southern Jilin Province (SJP) of the North China Craton (NCC). These rocks are composed of tonalitic–trondhjemitic–granodioritic–dioritic (TTGD) gneisses and preserve important information on the geodynamic evolution and Archean crustal growth along the northeastern margin of the NCC. Here we present LA–ICP–MS zircon U–Pb isotopic ages which reveal that the magmatic precursors of the tonalitic–trondhjemitic gneisses were emplaced at ∼2578–2574Ma, and the granodioritic–quartz dioritic magmas crystallized at ∼2546–2524Ma. The rocks were subjected to regional amphibolite facies metamorphism at ∼2510–2490Ma.Geochemically, the tonalitic–trondhjemitic gneisses can be subdivided into two subgroups. The magmas of high-Yb subgroup were derived by partial melting of dominantly ancient basaltic materials at the crustal level in the stable field of amphibole, clinopyroxene and significant amounts of garnet, and subsequently subjected to varying degrees of fractionation. Magmas of the low-Yb subgroup were generated by partial melting of dominantly juvenile basaltic materials with minor greywackes at the lower crustal level in the stability field of garnet and rutile, with a possible eclogitic residue. The magmatic precursors of granodioritic gneisses were produced by partial melting of subducted oceanic slabs and minor sediments, with the melts contaminated by the mantle wedge materials during their ascent. The parental magmas for the quartz dioritic gneisses were possibly generated by partial melting of a depleted mantle source that was significantly metasomatized by slab-derived fluids or melts.Combined with their petrogenesis, ∼2578–2574Ma tonalitic–trondhjemitic gneisses display positive zircon εHf(t2) values (+0.4 to +6.2) with TDM(Hf) ages of 2856–2643Ma, suggesting a crustal reworking process. On the other hand, ∼2546–2524Ma granodioritic to quartz dioritic gneisses have highly depleted zircon εHf(t2) values (+4.5 to +7.7) and TDM(Hf) ages of 2682–2550Ma that are close to their magmatic crystallization ages, indicating a significant crustal growth. Integrated with our recent studies on ∼2683–2654Ma metavolcanic rocks in Helong granite-greenstone belt and voluminous ∼2588–2536Ma metavolcanics in Jiapigou granite-greenstone belt, we infer that the Southern Jilin Province experienced Neoarchean subduction-related evolution history, with an Andean-type active continental margin along the northern margin of the NCC, and witnessed two major episodes of subduction-related crustal growth at ∼2.7Ga and ∼2.6–2.5Ga, with the latter period coupled with reworking of old crustal materials.

Lithostratigraphy and geochemistry of Upper Vendian‒Lower Cambrian deposits in the northeastern Baltic monocline

The results of investigations of Upper Vendian‒Lower Cambrian deposits in the northeastern part of the Baltic monocline specify views on the evolution of depositional environments of sedimentary successions constituting the basal part of the sedimentary cover in inner areas of the northwestern East European Platform. It is shown that the Late Vendian and initial Cambrian were characterized by the consecutive influx of relatively mature terrigenous detrital material that originated from both the weathering crust of the Baltic Shield and new sources. Its deposition was interrupted by notable, although likely asynchronous, hiatuses, which are registered at the base of the Upper Vendian Vasileostrovskaya and Voronkovo formations and Lower Cambrian Lomonosov Formation. In the Late Vendian, sedimentary material was transported from the Baltic Shield, while beginning from the initial Early Cambrian the additional contribution to the formation of the sedimentary cover of the Baltic monocline was provided by coarse-grained sedimentary material from the Timan margin of the Baltica as follows from U‒Pb isotopic ages obtained for detrital zircons. At the same time, lithogeochemical parameters of fine-grained rocks experienced no substantial changes.

U–Pb and Re–Os Geochronology of the Hucunnan Copper–Molybdenum Deposit in Anhui Province, Southeast China, and its Geological Implications

AbstractThe Middle–Lower Yangtze Region (MLYR) is one of the most important metallogenic belts in China that hosts numerous Cu–Fe–Au–S deposits. The Hucunnan deposit in the central part of MLYR is a newly discovered porphyry–skarn‐type copper–molybdenum deposit during recent drilling exploration. Laser ablation ICP–MS analysis carried out in this study yields U–Pb isotopic ages of 137.5 ± 1.2 Ma for the Cu–Mo bearing granodiorite rock and 125.0 ± 1.5 Ma for the Cu‐bearing quartz diorites. The Re–Os isotopic dating of seven molybdenite samples gave an isochron age of 139.5 ± 1.1 Ma, suggesting a syn‐magma mineralization of molybdenite in the Hucunnan deposit. Since porphyry‐type molybdenum deposits are rare in central MLYR, the discovery of the Hucunnan deposit suggests possible molybdenite mineralizations in the deep places of the Cu–Mo bearing granitoids. In addition, the U–Pb isotopic age of 125 Ma for the Cu‐bearing quartz diorites implies a new Cu mineralization period for the MLYR that was rarely reported by previous studies.

Detrital zircon ages from the Ross Supergroup, north Victoria Land, Antarctica: Implications for the tectonostratigraphic evolution of the Pacific-Gondwana margin

We present new U–Pb isotopic age data for detrital zircons from 16 deformed sandstones of the Ross Supergroup in north Victoria Land, Antarctica. Zircon U/Th ratios primarily point to dominantly igneous parent rocks with subordinate contributions from metamorphic sources. Comparative analysis of detrital zircon age populations indicates that inboard stratigraphic successions (Wilson Terrane) and those located outboard of the East Antarctic craton (the Bowers and Robertson Bay terranes) have similar ~1200–950Ma (Mesoproterozoic–Neoproterozoic) and ~700–490Ma (late Neoproterozoic–Cambrian, Furongian) age populations. The affinity of the age populations of the sandstones to each other, as well as Gondwana sources and Pacific-Gondwana marginal stratigraphic belts, challenges the notion that the outboard successions form exotic terranes that docked with Gondwana during the Ross orogeny and instead places the terranes in proximity to each other and within the peri-Gondwana realm during the late Neoproterozoic to Cambrian. The cumulative zircon age suite from north Victoria Land yields a polymodal age spectra with a younger, primary 700–480Ma age population that peaks at ~580Ma. Cumulative analysis of zircons with elevated U/Th ratios (>20) indicating metamorphic heritage yield ~657–532Ma age probability peaks, which overlap with the younger dominantly igneous zircon population. The data are interpreted to give important new evidence that is consistent with ongoing convergent arc magmatism by ~626Ma, which provided the dominant zircon-rich igneous rocks and subordinate metamorphic rocks. Maximum depositional ages as young as ~493–481Ma yielded by deformed sequences in the outboard Bowers and Robertson Bay terrane samples provide new support for late Cambrian to Ordovician deformation in outboard sectors of the orogen, consistent with tectonic models that call for cyclic phases of contraction along the north Victoria Land sector of the Ross–Delamerian orogen.

First results of U–Pb dating of detrital zircons from the Upper Ordovician sandstones of the Bashkir uplift (Southern Urals)

The first results of U–Pb dating of detrital zircons from Upper Ordovician sandstones of the Bashkir uplift in the Southern Urals and U–Pb isotopic ages available for detrital zircons from six stratigraphic levels of the Riphean–Paleozoic section of this region are discussed. It is established that the long (approximately 1.5 Ga) depositional history of sedimentary sequences of the Bashkir uplift includes a peculiar period lasting from the Late Vendian to the Emsian Age of the Early Devonian (0.55–0.41 Ga). This period is characterized by the following features: (1) prevalence of material from eroded Mesoproterozoic and Early Neoproterozoic crystalline complexes among clastics with ages atypical of the Volga–Urals segment of the East European Platform basement; (2) similarity of age spectra obtained for detrital zircons from different rocks of the period: Upper Vendian–Lower Cambrian lithic sandstones and Middle Ordovician substantially quartzose sandstones.

Equigranular eclogites from the V. Grib kimberlite pipe: Evidence for Paleoproterozoic subduction on the territory of the Arkhangelsk diamondiferous province

nological Institute, Russian Academy of Sciences. The U–Pb isotope age was determined for 19 zircon grains. The chemical composition of eclogites was reconstructed on the basis of mineral densities, their chemical composition, and the modal concentration in the rock. The modal proportion of 40% garnet + 60% clinopyroxene was used for reconstruction of the REE concentrations. The compositions of rutile (0.3– 0.4%) and ilmenite (1%) were taken into consider�

Geochronology, petrology and Hf–S isotope geochemistry of the newly-discovered Xiarihamu magmatic Ni–Cu sulfide deposit in the Qinghai–Tibet plateau, western China

This paper reports the first set of data for the newly-discovered Xiarihamu magmatic Ni–Cu sulfide deposit in the Eastern Kunlun Paleozoic arc terrane which is located in the northern part of the Qinghai–Tibet plateau. An on-going drilling campaign reveals ~100million tons of sulfide mineralization with the average grade of 0.8wt.% Ni and 0.1wt.% Cu for the deposit. This makes the Xiarihamu deposit one of the 20 largest magmatic Ni–Cu sulfide deposits in the world and the largest ever found in arc settings. The deposit is hosted in a small ultramafic body intruding older gabbroic and metamorphic rocks. New zircon U–Pb isotope age data reveal that the ultramafic body (411.6±2.4Ma) is ~20Ma younger than the host gabbroic intrusion (431.3±2.1Ma). The ultramafic body is composed predominantly of lherzolite and olivine websterite, with minor dunite, websterite and orthopyroxenite. Mineralization mainly occurs as sub-horizontal to gently dipping (<30°) disseminated sulfide zones that are generally concordant with the lithological structure of the ultramafic intrusion. The lateral extension and thickness of individual mineralized zones are up to ~200m and ~100m, respectively. Sulfide mineral assemblages are composed of pyrrhotite, pentlandite and chalcopyrite. The Xiarihamu ultramafic rocks show light REE enrichments and pronounced negative Nb anomalies, plus significant Ca-depletion in olivine (<700ppm Ca), which are characteristic of many arc basalts in the world. Olivine crystals in the Xiarihamu ultramafic rocks have relatively primitive compositions, with Fo contents up to 90mol%, close to the mantle value. The contrasting Ni contents of olivine crystals with similar Fo contents from different sulfide-mineralized zones in a single drill core indicate that at least two pulses of sulfide-laden magma with different Ni compositions were involved in the development of the deposit. Estimated parental magma for the Xiarihamu lherzolites contains 52.4wt.% SiO2 and 9.8wt.% MgO, which are within the ranges of boninites worldwide, supporting the interpretation that the Xiarihamu sulfide ore-bearing ultramafic intrusion is the product of subduction-related boninitic magmatism. The εHf (t) values of zircon crystals from the Xiarihamu ultramafic rocks vary from 1 to 5, indicating minor crustal contamination of the parent magma. The involvement of external sulfur in the genesis of the Xiarihamu magmatic sulfide deposit is evident from higher δ34S values of sulfide ores (3.5 to 6.8‰) than typical mantle value (0±2‰), although the actual contaminants are yet to be identified.

Pyroxene megacrysts in Proterozoic anorthosites: Implications for tectonic setting, magma source and magmatic processes at the Moho

Proterozoic anorthosites from the 1630–1650 Ma Mealy Mountains Intrusive Suite (Grenville Province, Canada), the 1289–1363 Ma Nain Plutonic Suite (Nain–Churchill Provinces, Canada) and the 920–949 Ma Rogaland Anorthosite Province (Sveconorwegian Province, Norway), all entrain comagmatic, cumulate, high-alumina orthopyroxene megacrysts (HAOMs). The orthopyroxene megacrysts range in size from 0.2 to 1 m and all contain exsolution lamellae of plagioclase that indicate the incorporation of an excess Ca–Al component inherited from the host magma at pressures in excess of 10 kbar at or near Moho depths (>30–40 km). Suites of HAOMs from each intrusion display a large range in 147Sm/144Nd (0.10 to 0.34) making them amenable for precise age dating with the Sm–Nd system. Sm–Nd isochrons for HAOMs give ages of 1765±12 Ma (Mealy Mountains), 1041±17 Ma (Rogaland) and 1444±100 Ma (Nain), all of them older by about 80 to 120 m.y. than the respective 1630–1650, 920–949 and 1289–1363 Ma crystallization ages of their host anorthosites. Internal mineral Sm–Nd isochrons between plagioclase exsolution lamellae and the orthopyroxene host for HAOMs from the Rogaland and Nain complexes yield ages of 968±43 and 1347±6 Ma, respectively – identical within error to the ages of the anorthosites themselves. This age concordance establishes that decompression exsolution in the HAOM was coincident with magmatic emplacement of the anorthosites, ∼100 m.y. after HAOMs crystallization at the Moho. Correspondence of Pb isotope ages (206Pb/204Pb vs. 207Pb/204Pb) with Sm–Nd ages and other strong lines of evidence indicate that the older megacryst ages represent true crystallization ages and not the effects of time-integrated mixing processes in the magmas. Nd isotopic evolution curves, AFC/mixing calculations and the age relations between the HOAMs and their anorthosite hosts show that the HAOMs are much less contaminated with crustal components and are an older part of the same magmatic system from which the anorthosites are derived. Modeling of these anorthositic magmas with MELTS indicates that their ultramafic cumulates would have sunk in the magma and been sequestered at the Moho, where they may have sunk deeper into the mantle resulting in large-scale compositional differentiation. The HAOMs thus represent a rare example of part of a cumulate assemblage that was carried to the upper crust during anorthosite emplacement and, together with the anorthosites, illustrate the dramatic influence that magma ponding and differentiation at the Moho has on residual magmas traveling towards the surface. The new geochronologic and isotopic data indicate that the magmas were derived by melting of the mantle, forming magmatic systems that could have been long-lived (e.g. 80–100 m.y.). A geologic setting that would fit these temporal constraints is a long-lived Andean-type margin.

Influence of crude oil on the genesis of the Lanjiagou porphyry molybdenum deposit, western Liaoning Province, China

The Lanjiagou porphyry molybdenum deposit in western Liaoning Province, China, is hosted in fine-grained Jurassic granites. LA-ICP-MS zircon U–Pb analyses indicate that the crystallization of the ore-hosting granites took place 185.0±1.8Ma (MSWD=1.4). Molybdenum mineralization in the deposit can be divided into three stages: the stockwork quartz vein stage, the planar quartz vein stage, and the fissure-filling quartz vein stage. Re–Os isotopic ages for the molybdenite from the stockwork quartz vein-type ores yielded an isochron age of 188.8±9.9Ma (MSWD=3.0), while six samples from the planar quartz vein-type ores yielded a similar isochron age of 185.6±1.2Ma (MSWD=0.5). Re–Os isotopic ages for the molybdenite identical, within error, to zircon U–Pb isotopic ages indicate that the molybdenum mineralization is related to the host intrusions. Apart from primary inorganic fluid inclusions (IFIs), a large number of primary organic fluid inclusions (OFIs) are found in the latter two stages of vein quartz, and minors found in the first stage. The components and characteristics of OFIs in the three stages of vein quartz differ from each other, which is also true for the IFIs. OFIs in stockwork vein quartz are characterized by halite-bearing inclusions, and organic liquids in the inclusions are brown and do not fluoresce under ultraviolet (UV) light. Homogenization temperatures (Th) for the primary IFIs coeval with OFIs of this stage ranges from 300°C to >450°C, while the salinity varies from 10 to 53wt.% NaCl equiv.. In planar vein quartz, OFIs are predominately two-phased (liquid and gas), and salt daughter minerals (halite) are absent. Organic liquids are light brown to colorless and show blue fluorescence under UV light; The Th range for the IFIs of this stage is 250–360°C, and the salinity range is 3–17wt.% NaCl equiv. Finally, OFIs in fissure-filling vein quartz are marked by liquid–gas inclusions. Organic liquids are generally colorless and show yellow fluorescence under UV light. The Th range for the primary IFIs is 180–240°C and the salinity range is 4–11wt.% NaCl equiv. Organic geochemical analyses indicate that organic matter in the Lanjiagou deposit was derived from mature crude oil. We suggest that large volumes of crude-oil-bearing non-magmatic fluids were flushed into the Lanjiagou porphyry hydrothermal system during all phases of ore formation and likely played important roles in mineralization.

Sequence of Late Jurassic–Early Cretaceous magmatic–hydrothermal events in the Xiong’ershan region, Central China: An overview with new zircon U–Pb geochronology data on quartz porphyries

Recent investigations have revealed several large Au and Mo deposits in the Xiong’ershan region, Central China. Most quartz porphyries associated with the mineralization occur as dikes and apophyses, or as rubbles cemented in mineralized breccia pipes. Three types of quartz porphyries were sampled from the Leimengou Mo deposit, the Qiyugou Au deposit, and the Niutougou Au deposit. LA–ICP–MS zircon U–Pb analysis was performed in zircons from two quartz porphyries; the results yielded ages of 125.4±0.77Ma for Leimengou Mo deposit and 150.1±1.1Ma for Qiyugou Au deposit. The magma source of Leimengou quartz porphyry is similar to that of the mineralized cementing material in breccia pipes of the Qiyugou Au deposit, whereas the magma source of Qiyugou quartz porphyry is the same as that of quartz porphyries in Niutougou Au deposit. Based on the new U–Pb isotopic ages of granitic plutons reported in this study, together with the age data in the literature, we identify distinct magmatic pulses in the Xiong’ershan region at ca.160, 150, 143, 133, 125, and 115Ma during the Late Jurassic–Early Cretaceous. The ages of Au and Mo mineralization coincide with the thermal events at about 115, 125, 133, and 143Ma are considered to be co-eval with granitic magmatism. No mineralization ages of 150 and 160Ma thermal events have been previously reported. Our study demonstrates Late Jurassic–Early Cretaceous multiple magmatic pulses and mineralization in the Xiong’ershan region.

Mineralogical criteria for the diamond potential of Upper Triassic placers on the northeastern margin of the Siberian Platform

Representative sampling of a diamond-bearing basal horizon in the Carnian Stage (Upper Triassic) on the northeastern margin of the Siberian Platform revealed a wide spectrum of indicator minerals, first of all, garnets, whose compositions are the same as in the inclusions in the regional diamonds. Of special interest are garnets of potential eclogite paragenesis with an abnormally high impurity of MnO (0.5–3.2 wt.%), which was earlier detected in more than 20% of garnets present as inclusions in diamonds of northern Quaternary placers and recommended as a new mineralogical criterion for diamond presence. Subcalcic Cr-pyropes of dunite–harzburgite paragenesis were also found in variable amounts, from 0.7 to 3.9 rel.%, in the sample of 973 grains of pyropes of lherzolite and websterite parageneses. Three grains contain 11.9, 12.6, and 16 wt.% Cr2O3, which corresponds to the presence of 30–34% of Mg–Cr-knorringite component. Such pyropes have been revealed for the first time in the study region. Cr-spinels are a mixture of compositions typical of kimberlites and the regional alkali-ultrabasic rocks. All studied samples contain picroilmenites with a variable content of Cr2O3 impurity. Since Mg–Fe–Ca-garnets with Mg# < 35 can be partly hosted in metamorphic rocks of the Anabar Shield, the elevated content of Na2O impurity (>0.09 wt.%) was also taken into account. The different contents of indicator minerals in the samples might be due to the variable composition of the diamond orebodies. The Carnian placers call for new systematic sampling. Special attention should be given to estimation of the composition of garnets of presumably eclogite paragenesis with elevated contents of TiO2, MnO, CaO, and Na2O and to search for perovskite and Nb-containing rutile. These minerals, together with zircons, are of interest for determining the U–Pb isotopic age of probable diamond orebodies—kimberlites.

Electron-microprobe Th–U–Pb monazite dating in Early-Palaeozoic high-grade gneisses as a completion of U–Pb isotopic ages (Wilson Terrane, Antarctica)

The electron microprobe (EMP) Th–U–Pb monazite bulk chemical dating method was applied to granulite-facies rocks of the Wilson Terrane in Antarctica. A combination of this method to isotopic U–Pb-SHRIMP ages for the evaluation of metamorphic processes required the analysis of reference monazites. These can be subdivided into three groups: a) Monazite with variable total Pb at constant Th (e.g. VK-1) is unsuitable for EMP data evaluation; b) Monazite with highly variable total Pb and Th, but with at least some Th/Pb approximating an apparent isochrone (e.g. MPN) is partly useful; and c) Monazite with constant Th/Pb at high Th (e.g. Madmon monazite) is best suitable for the combined approach and can be additionally used to improve the Th calibration for EMP. Study of monazite in grain mounts and in thin sections led to partly different but complementary results: Older monazites with EMP ages up to 680Ma occur mainly in a grain mount from diatexite and metatexite and are interpreted as detrital relics. Some of these monazites show structures and mineral–chemical zonation trends resembling metasomatism by alkali-bearing fluids. A marked mobility of Th, P, Ce, Si and U is observed. The age of the metasomatic event can be bracketed between 510 and 450Ma. Furthermore, in the grain mount and in numerous petrographic thin sections of migmatites and gneisses, the EMP Th–U–Pb and SHRIMP U–Pb monazite data uniformly signal a major metamorphic event with a medium-pressure granulite facies peak between 512 and 496Ma. Subsequent isothermal uplift and then amphibolite-facies conditions between 488 and 466Ma led to crystallisation of pristine monazite. The high-grade metamorphic event, related to the Ross Orogeny, can be uniformly traced more than 600km along strike in the Wilson Terrane.

Central/Eastern Indian Bundelkhand and Bastar cratons in the Palaeoproterozoic supercontinental reconstructions: A palaeomagnetic perspective

Mafic dykes in the central Indian Bundelkhand and eastern Indian Bastar cratons are potential sources for tracing the location of the Indian shield within Palaeoproterozoic supercontinent reconstructions. A total of 610 oriented core samples were collected from 27 mafic dykes (342 samples) in Bundelkhand craton and 20 dykes (268 samples) in Bastar craton. The characteristic magnetisations identify distinct groups of directions. The derived palaeopoles are integrated with recently reported U–Pb isotopic ages from the Bundelkhand and Bastar cratons and correlated with palaeopoles from the mafic dykes in the Dharwar craton, South India. Characteristic remanence (ChRM) identified in these dykes are classified into (i) steep upward/downward components further sub-grouped as ca. 2.37Ga steep 1 (λ=10.2°S; Ф=75.0°E; A95=18.1° in Bundelkhand and λ=22.3°N; Ф=71.4°E; A95=21.6° in Bastar), component comparable with ca. 2.4–2.45Ga steep 2 group from one dyke in Bundelkhand (λ=14°S; Ф=101°E; A95=26.1°) and one dyke in Bastar (λ=6°N; Ф=113°E; A95=26.5°). A steep component (λ=60.4°N; Ф=45.3°E; A95=9.7° in Bundelkhand and a comparable component λ=49°N; Ф=129°E; A95=15.1° from one dyke in Bastar) is not assigned an age at present. (ii) ca. 2.18Ga shallow easterly and antipodal shallow westerly components (λ=0.4°S; Ф=347°E; A95=21.6° in Bundelkhand and λ=18.0°N; Ф=344.0°E; A95=8.1° in Bastar) and (iii) 1.99Ga shallow northwest and antipodal shallow southeast (λ=57.5°N; Ф=309.0°E; A95=4.7° in Bundelkhand and λ=39°N; Ф=321°E; A95=28° in Bastar). A group (iv) of ∼2.2Ga northeast shallow components (λ=36.0°S; Ф=357.0°E; A95=9.4°) is found only in the Bundelkhand craton. The distinct groups of palaeomagnetic pole determinations from dykes of the Bundelkhand and Bastar craton exhibit a remarkable match with palaeomagnetic poles determined from Precambrian mafic dykes in the Dharwar craton. The close comparison of mafic dyke magnetisations between the cratons suggests close proximity since 2.45–2.5Ga. Models suggesting amalgamation of crustal blocks along the Central Indian Tectonic Zone at 1.8Ga or a 1.0Ga collision along this zone to form Rodinia are untenable. Testing of proposed NeoArchaean-Palaeoproterozoic supercontinent reconstructions showing a north China–India linkage or India's close proximity to Slave craton to form a supercraton ‘Sclavia’ are not supported. Instead, the data are compatible placing India in close proximity to the Yilgarn block of Western Australia.

Provenance and Pb isotopic ages of lunar volcanic and impact glasses from the Apollo 17 landing site

Glassy spherules found in the lunar regolith can be used to investigate the volcanic and impact history of the Moon. Here we report the first study combining petrography with microbeam geochemical data for major-element, trace-element and Pb isotopic compositions of individual spherules that were separated from a sample of regolith collected at the Taurus-Littrow Valley on the Moon. Petrographic characteristics of 216 spherules were established by optical and electron microscopy. About half of these spherules show obvious evidence of formation by meteorite impacts into the lunar regolith, such as relict clasts, schlieren, vesicles, FeNi metal blebs, irregular shapes and heterogeneous nucleation textures. The remaining spherules have textures indicating rapid quenching of melt droplets, including clast-free glass and microcrystalline to vitrophyric textures. These were produced by either impact melting or volcanic fire fountaining. Chemical compositions of clast-poor spherules were determined by electron microprobe and laser ablation ICPMS, and the provenance of each spherule was classified as either feldspathic highlands, mare or mixed highlands-mare. Spherules with feldspathic highlands compositions are droplets of impact melt that provide a regional-scale sampling of regolith and rock compositions in the vicinity of the landing site. A group of picritic, high-TiO2 mare spherules has a limited range of major and trace-element compositions similar to the famous Apollo 17 orange glass; these are likely to be volcanic in origin. A small number of mare spherules with very low Ti (VLT) and unusually aluminous compositions were also found. We tentatively conclude that the VLT spherules reflect igneous compositions, whereas the aluminous mare spherules are likely to be impact mixtures. We also conducted a reconnaissance LA-ICPMS study of Pb isotopic compositions and U–Th–Pb concentrations to investigate the feasibility of dating individual spherules. 207Pb/206Pb model ages and U–Th–Pb chemical ages of the picritic mare spherules are similar to those of the 3.8–3.5 Ga Apollo 17 mare basalts, consistent with an igneous origin for the glasses. In contrast, impact spherules have systematically younger chemical ages (≤2 Ga). About half of the impact spherules appear to be ≤500 Ma, consistent with the recent discovery of a young spike in the spherule age populations at other landing sites. The abundance of relatively young ages with a predominantly local provenance suggests that most of the impact spherules were produced by small impact events, perhaps related to craters ≤1 km in diameter.