Pb Evaporation Research Articles

Kinetics study of Pb evaporation from pure Pb and Pb–Ag alloy in vacuum evaporation process

Traditional method for treatment of Pb–Ag alloy has disadvantages of a long metal production cycle, long process, and high energy consumption. Vacuum distillation has obvious advantages, such as a short flow, short production cycle, high direct yields of metal, and low cost. Kinetics is the key factor that determines production efficiency. Therefore, evaporation kinetics of Pb evaporation from pure Pb and Pb–Ag alloy has been investigated. The evaporation rate of pure Pb was experimentally determined as 0.044, 0.13 and 0.32 g cm−2·min−1 at 1173 K, 1273 K and 1373 K, respectively. Pb evaporation from Pb–Ag alloy followed first order kinetics and the rate controlling step were mainly mass transfer in liquid which indicated that increasing the stirring of the melt during the production process can improve the removal efficiency of lead. In addition, some kinetic parameters were obtained such as the apparent activation energy of Pb evaporation from Pb–Ag alloy was 148.8 kJ mol−1, the mass transfer coefficient was 6.48 × 10−8, 1.90 × 10−7 and 6.00 × 10−7 m s−1 at 1173 K, 1273 K and 1373 K, respectively. The research enriches the Pb evaporation kinetic database and can provide theoretical guidance for the vacuum distillation of noble lead alloys. In addition, the research designs a set of alloy configuration and vacuum evaporation device with simple assembly and convenient operation which can be used for the research on vacuum evaporation kinetics of other non-ferrous metal alloy systems.

Kinetics of Pb evaporation from Pb-Sn liquid alloy in vacuum distillation

In this work, the kinetics of Pb evaporating from Pb-Sn alloy has been investigated. The experiments were performed in the temperature range 1200 K–1400 K and the pressure of 10 Pa. The results indicated that the evaporation of Pb follow first order kinetics. The overall mass transfer coefficients of lead evaporation from Pb-Sn alloy are 2.68 × 10−6 m s−1, 5.01 × 10−6 m s−1 and 1.92 × 10−5 m s−1 at 1200 K, 1300 K and 1400 K, respectively. Additionally, the apparent activation energy of lead evaporation from molten Pb-Sn alloy was calculated. It revealed that the mass transfer of lead in the vapor phase is not rate limiting in the vacuum distillation. The Pb evaporation from Pb-Sn alloy was mix-controlled by vaporization and liquid mass transfer. The mass transfer in the liquid phase is a major rate-limiting step under the present experimental conditions.

First U-Pb data on age of granitoids in the basement of the Pechora basin

The Pechora basin, a large regional tectonic struc� ture in the Pechora region, is located between the Timan and Ural mountains (Fig. 1). Its section is com� posed of Upper Cambrian sedimentary-metamorphic and igneous rocks of the basement (Timanides) uni� versally overlain by a platformal cover represented by Ordovician-Cenozoic sedimentary and volcanic rocks. The basement comprises the Izhma, Pechora, and Bol'shezemel'skaya zones. The last two constitute the Bol'shezemel'skaya megablock, and the Izhma zone together with the Timan and Kanin Peninsula form the Timan megablock. According to geophysical and drilling data, the megablocks distinctly differ from each other in the composition and type of magmatism (7). The boundary between them corresponds to the zone of the NWextending Pripechora and Ilych- Chikshino deepseated faults. The thickness of the platformal cover varies from 4-7 km in central areas of the Pechora basin to 1-4 km in arches and uplifts. In this connection, information on the structure and geo� dynamic development of the basement beneath the basin is derived from the core of over 200 boreholes drilled with different purposes up to a depth of 5 km (7). Granitoids have been recovered by 26 boreholes throughout the entire region being more abundant in the Izhma and Pripechora-Ilych-Chikshino zones. Until the end of the 20th century, the age of granitoids was known from the limited K-Ar data (1, 13), most of which correspond to the period of 600-500 Ma. They provided grounds for assuming that granitoids are Vendian-Cambrian in age. Subsequently, geo� chronological dates were obtained by Pb-Pb (Pb evaporation) measurements in single zircon crystals from granites and diorites recovered from several drill� holes (13). These measurements yielded uniform ages in the interval of 567-551 Ma corresponding in the recent stratigraphic scale to the boundary of the Lower and Upper Vendian (12). The dating of zircons from igneous rocks using the radiogenic Pb isotopes is based on the assumption that the 207 Pb/ 206 Pb ratios (ages) remain unchanged after the formation of zircons and host rocks, although this is frequently not the case. The subsequent Rb-Sr investigations of granitoids from the Izhma and Pripechora-IlychChikshino zones (4) recovered by the same boreholes, where zir� con was analyzed, revealed that the Rb-Sr ages are confined to the period of 608-595 Ma, i.e., to the Riphean-Vendian boundary interval (12). These data are quite consistent with the ages obtained for grani� toid massifs in the North Timan structure of the Timan megablock. Their Rb-Sr ages fall into the interval of 597-587 Ma (2), while the Pb-Pb (Pb evaporation) method yielded zircon ages of 621 ± 3.5 Ma (3). Here�

Vacuum Separation Behavior of Pb from Copper-Rich Particles of Crushed E-Wastes

Due to lack of proper treatment methods, Pb pollution caused by improper treatment of electronic wastes (e-waste) has been attracting increasing attention. This paper investigates the separation behavior of Pb in the presence of Cu and other metallic particles (Cd, Bi, and Zn), in order to separate and recover various metals effectively from the copper-rich particles of crushed e-wastes. Vacuum metallurgy separation method is adopted to separate and recover Pb from the copper-rich particles of crushed e-wastes. Due to the variety and complexity of metals in copper-rich particles, this paper studies the effects of Cu, Cd, Bi, and Zn on vacuum separating and recycling of Pb. It is found that Cu particles have both a positive dispersing and a negative blocking effect on Pb evaporation. Cd can be evaporated preferentially, and then Pb can be subsequently separated for their huge vapor pressure gap. The formation of Pb-Bi alloy with a low vapor pressure makes the separation of Pb more difficult. As a result of different condensation characteristics, Pb and Zn can be evaporated together from Cu particles, and then be respectively condensed on different positions as pure Pb and Zn.

Fabrication of reproducible, integration-compatible hybrid molecular/si electronics.

Reproducible molecular junctions can be integrated within standard CMOS technology. Metal-molecule-semiconductor junctions are fabricated by direct Si-C binding of hexadecane or methyl-styrene onto oxide-free H-Si(111) surfaces, with the lateral size of the junctions defined by an etched SiO2 well and with evaporated Pb as the top contact. The current density, J, is highly reproducible with a standard deviation in log(J) of 0.2 over a junction diameter change from 3 to 100 μm. Reproducibility over such a large range indicates that transport is truly across the molecules and does not result from artifacts like edge effects or defects in the molecular monolayer. Device fabrication is tested for two n-Si doping levels. With highly doped Si, transport is dominated by tunneling and reveals sharp conductance onsets at room temperature. Using the temperature dependence of current across medium-doped n-Si, the molecular tunneling barrier can be separated from the Si-Schottky one, which is a 0.47 eV, in agreement with the molecular-modified surface dipole and quite different from the bare Si-H junction. This indicates that Pb evaporation does not cause significant chemical changes to the molecules. The ability to manufacture reliable devices constitutes important progress toward possible future hybrid Si-based molecular electronics.

A new approach to Pb evaporation and condensation from zircon enabling sub-Ma age determination on Precambrian rocks

Sub-million year age precision on low to moderately metamict Precambrian zircon is often reproducible using the isotope dilution thermal ionization technique (ID-TIMS) exploiting U–Pb double spiking for precise mass fractionation correction. However, this requires total dissolution of sample and laborious chemical processing. On the other hand whole grain evaporation techniques of Pb extraction are simple and less time consuming but provide relatively inefficient ionization and are not amenable to double Pb isotope spiking for precise mass fractionation correction, which limits the precision of ages that they can provide (often restricted to ±few million years). A modified evaporation–condensation method is described that overcomes these limitations with the ability to use 202Pb–205Pb double spiking for precise mass fractionation correction in order to achieve sub-million year age precision on low to highly metamict Precambrian samples. Examples are shown of age determinations on previously dated zircon populations from the Marmion tonalite and Nevison Lake rhyolite, western Superior province (U<200ppm), which give 3001.8±0.5Ma and 2998.2±0.3Ma, respectively, and from the black norite and the felsic norite in the Sudbury igneous complex (U>1000ppm), which give 1848.9±0.2Ma and 1851.3±0.5Ma, respectively (all errors are 95% confidence).

Paleoproterozoic granitoids from the northern limit of the Archean Amapá block (Brazil), southeastern Guyana Shield: Pb–Pb evaporation in zircons and Sm–Nd geochronology

Central Amapá, northern Brazil is located at the boundary between: (a) a northern Paleoproterozoic domain, consisting mainly of granite-greenstones terrains and (b) a southern Archean continental block (Amapá block), including an Archean basement reworked during the Transamazonian orogeny (2.26–1.95 Ga). Field investigations, Pb–Pb zircon and Sm–Nd whole rock geochronology supported by geochemical data on granitoids brought further constraints on Paleoproterozoic crustal growth in the southeastern Guyana Shield. A first magmatic episode, dated at 2.26 Ga, is marked by the crystallization of metaluminous low-K tholeiitic tonalites and quartz-diorites, which geochemical affinity with volcanic arc and association with T-MORB amphibolites suggest that they formed in a back-arc basin – island arc system. This event is coeval to the oceanic stage registered in French Guyana during the Eorhyacian (2.26–2.02 Ga). A second magmatic episode is represented by peraluminous, medium- to high-K calc-alkaline tonalite and granodiorite, which revealed some similarities with Mesorhyacian TTG rocks of French Guyana. For granitoids of both episodes, TDM and εNd values indicate the contribution of some Archean crustal component, probably by assimilation or contamination. This second magmatic episode occurred at 2.10 Ga, indicating that the period of successive calc-alkaline magmatic arcs formation may have extended until the Neorhyacian. Meanwhile, during this time, tectonic accretion by collision of the newly formed continental landmass was the prevailing process in French Guyana. The latter magmatic episode, even though poorly constrained, was registered around 2.08–2.02 Ga in central Amapá. It corresponds to the emplacement and solidification of high-K collisional granitoids, produced by partial melting of the Archean continental crust, as testified by the Archean TDM, inherited Pb–Pb zircon ages and strongly negative εNd values. Our results point toward the existence of a protracted episode of crustal growth during the Neorhyacian in the southeastern Guyana Shield. This episode has been predominantly driven by magmatic arc accretion during, at least, 160 My, along the period of 2.26–2.10 Ga. This cycle ended with diachronic closure of the oceanic basins and arc–continent collision.

Ultra-high temperature granulite-facies metamorphic rocks from the Mozambique belt of SW Tanzania

The metamorphic rocks in the Neoproterozoic (Pan-African) Mozambique belt of southwestern Tanzania, around the town of Songea, can be subdivided into one- and two pyroxene bearing charnockitic gneisses, migmatitic granitoid gneisses and amphibolite-facies metapelites. Lower-grade amphibolite-facies rocks are rare and can be classified as sillimanite- and/or garnet-bearing metapelites. Most of the studied charnockitic gneisses show excellent corona textures with large orthopyroxene grains rimmed by clinopyroxene, followed by quartz and well developed garnet rims due to the reaction Opx+Pl=Grt+Cpx+Qtz that formed during isobaric cooling. These and other charnockitic gneisses show symplectites of orthopyroxene and An-rich plagioclase that resulted from the breakdown of garnet during isothermal decompression due to the reaction Grt+Cpx+Qtz=Opx+Pl. Geothermobarometric calculations yield up to ~1050°C and up to ~12kbar for peak metamorphic conditions. These are higher temperature and slightly lower pressure conditions than reported for other granulite-facies terrains in the Mozambique belt of Tanzania. Single zircon Pb–Pb evaporation and U-Pb SHRIMP ages for magmatic zircons extracted from two charnockitic and two granitic gneisses cluster in two groups, one at ~750Ma and one at ~1150Ma with the older reflecting the time of emplacement of the igneous precursors, and the younger approximating the time of charnockitization. These protolith ages are similar to those farther east in the Masasi area of southern Tanzania, as well as in northern Mozambique and in southern Malawi, and suggest that the Mozambique belt consists of chronologically heterogeneous assemblages whose pre-metamorphic tectonic setting remains obscure.

Mass-selected photodissociation studies of AlPbn+clusters (n =7–16): Evidence for the extraordinary stability of AlPb10+and AlPb12+

We report fragmentation pathways and dissociation energies of AlPb${}_{n}^{+}$ ($n=7$--16) clusters. The clusters are produced with pulsed laser vaporization and studied in a supersonic molecular beam setup. They are mass selected and photodissociated with 532 and 355 nm laser light. Photofragments are thereafter mass separated in a tandem reflectron time-of-flight mass spectrometer. Bare Pb${}_{n}^{+}$ ($n$ $=$ 8--16) clusters preferentially evaporate Pb atoms, with the exception of Pb${}_{15}^{+}$ that fragments by loss of a Pb${}_{2}$ dimer to form the stable Pb${}_{13}^{+}$ cluster. The smallest AlPb${}_{n}^{+}$ ($n$ $=$ 7--11) clusters also show mainly atomic Pb evaporation, whereas the favored fragmentation pathway of the larger clusters ($n$ $=$ 12--16) involves Pb${}_{2}$ and Pb${}_{3}$ fragments. AlPb${}_{10}^{+}$ and AlPb${}_{12}^{+}$ are the most intense fragments of several larger cluster sizes, demonstrating the high stability of these two sizes. Dissociation energies corresponding to the most facile fragmentation channel of AlPb${}_{n}^{+}$ ($n$ $=$ 11--15) are bracketed from the measured laser fluence dependencies of the fragment intensities using constraints imposed by unimolecular reaction rates.

Impedance spectroscopy of PZT ceramics- measuring diffusion coefficients, mixed conduction, and Pb loss

Sintering of lead zirconate titanate (PZT) at high temperatures results in loss of Pb unless an ambient Pb activity is maintained. The tell-tale sign of Pb loss is an increased conductivity, usually manifested in unacceptably high values of tanδ. The conductivity is caused by oxygen vacancies and/or electron holes which are a byproduct of Pb evaporation. In the first part of this paper, it is shown how impedance spectroscopy can be used to separate ionic and electronic conductivity in a properly designed sample by selection of appropriate boundary conditions. Subsequently, impedance is used to probe defect concentrations in PZT during prolonged annealing at 700°C. It is found that oxygen vacancies are generated during annealing in air but the rate of generation actually decreases upon lowering the ambient pO(2). These results are explained by a model of Pb evaporation which, in this case, leads predominantly to oxygen vacancy generation. In principle, this effect could be used to generate a specific vacancy concentration in similar Pb-based oxides.

Carbon isotope geochemistry and geochronological constraints of the Neoproterozoic Sirohi Group from northwest India

The early Neoproterozoic carbonates of the Sirohi Group, northwestern India, were studied for geochronology as well as for oxygen and carbon isotopes across three different sections to understand their relationship with contemporaneous global events. New single zircon Pb–Pb evaporation data constrain the age of the Sirohi Group between ca. 920 and ca. 820Ma, though actual span for deposition of the carbonates may be little shorter taking into allowance the time for peneplanation. Narrow range in variation of δ18O values is generally observed in the carbonate which is consistent with generally well-preserved original compositions. Two sections (SA and SS) out of the three that were studied had similarly small variations in δ18O (about 2‰), whereas the third section (SN) had a slightly wider range of δ18O (around 5‰). The δ18O values of the SN section varied between −11.9 and −17.6‰, which can be attributed to marginal modifications in primary composition caused by contact metamorphism which is evident by the occurrence of wollastonite in the carbonates.The δ13Ccarb values of these carbonates varied between −4.1 and +4.7‰ in all sections. Positive and near-zero δ13Ccarb values are from SA and SS sections which are least affected by the metamorphism. The δ13Ccarb values of these two sections are interpreted as primary and can be correlated with contemporary sea-water compositions. The SN section showed negative δ13Ccarb values which might be marginally affected by contact metamorphism, though we avoided sampling from the contact aureole. However, the metamorphic equilibration with 13C-depleted fluids leading to insignificant alteration in isotope compositions cannot be totally ruled out for the analyzed samples.Positive and near-zero carbon isotope values of Sirohi carbonates (SA and SS sections) closely match the global trends of carbonate dated between 900 and 850Ma but the negative carbon isotope anomaly (SN section) shown by the Sirohi Group carbonates is unmatched. Since the SN section is stratigraphically lower to SA and SS sections we suggest that the negative carbon isotope values of the Sirohi Group as one of the oldest Neoproterozoic negative carbon anomaly possibly close to 900Ma and not associated with glaciation. The carbon isotopic compositions of carbonates from the Sirohi Group suggest a variable degree of preservation of original compositions that represent yet another locality with moderately heavy values between 850 and 900Ma and an unmatchable example of the associated negative values. This reflects a unique biogeochemical signature of changing carbon cycles prior to the onset of Snowball Earth Events in the Neoproterozoic.

Zircon evaporation ages and geochemistry of metamorphosed volcanic rocks from the Vinjamuru domain, Krishna Province: evidence for 1.78 Ga convergent tectonics along the southeastern margin of the Eastern Dharwar Craton

Palaeoproterozoic intermediate to potassic felsic volcanism in volcano‐sedimentary sequences could either have occurred in continental rift or at convergent magmatic arc tectonic settings. The Vinjamuru domain of the Krishna Province in Andhra Pradesh, SE India, contains such felsic and intermediate metavolcanic rocks, whose geochemistry constrains their probable tectonic setting and which were dated by the zircon Pb evaporation method in order to constrain their time of formation. These rocks consist of interlayered quartz–garnet–biotite schist, quartz–hematite–baryte–sericite schist as well as cherty quartzite, and represent a calc‐alkaline volcanic sequence of andesitic to rhyolitic rocks that underwent amphibolite‐facies metamorphism at ~1.61 Ga.Zircons from four felsic metavolcanic rock samples yielded youngest mean 207Pb/206Pb ages between 1771 and 1791 Ma, whereas the youngest zircon age for a meta‐andesite is 1868 Ma. A ~2.43 Ga zircon xenocryst reflects incorporation of Neoarchaean basement gneisses. Their calc‐alkaline trends, higher LILE, enriched chondrite‐normalized LREE pattern and negative Nb and Ti anomalies on primitive mantle‐normalized diagrams, suggest formation in a continental magmatic arc tectonic setting. Whereas the intermediate rocks may have been derived from mantle‐source parental arc magmas by fractionation and crustal contamination, the rhyolitic rocks had crustal parental magmas. The Vinjamuru Palaeoproterozoic volcanic eruption implies an event of convergent tectonism at the southeastern margin of the Eastern Dharwar Craton at ~1.78 Ga forming one of the major crustal domains of the Krishna Province. Copyright © 2012 John Wiley &amp; Sons, Ltd.

Pb loss in Pb(Zr,Ti)O3 ceramics observed by in situ ionic conductivity measurements

Analysis of the impedance spectra of Nb-doped Pb(Zr,Ti)O3 (PZT) embedded capacitors revealed that the ionic conductivity increased monotonically during annealing at 700 °C. Furthermore, the rate of increase was lowered by a reduction in the ambient pO2. The results could be explained by a model in which oxygen vacancies are generated as a consequence of Pb evaporation from the PZT. At 700 °C, this process is most likely limited by surface kinetics rather than Pb bulk diffusion. It was shown that the Pb loss could be completely recovered by annealing in a high activity Pb source with a commensurate reduction in oxygen vacancy concentration. The electronic conductivity was predominantly p-type and was relatively unaffected by the Pb loss throughout the course of the experiment.

Zircon geochronology, geochemistry and origin of the TTG suites of the Rio Maria granite-greenstone terrane: Implications for the growth of the Archean crust of the Carajás province, Brazil

TTG suites are the most voluminous granitoid rocks exposed in the Mesoarchean Rio Maria granite-greenstone terrane (RMGGT), southeastern Amazonian craton. Extensive field work in key areas of the RMGGT, integrated with petrographic, geochemical, and geochronological studies, the latter employing the Pb–Pb evaporation and U–Pb LA-MC-ICPMS on zircon techniques, indicates that the TTG magmatism in the RMGGT can be divided into three episodes: (1) at 2.96 ± 0.02 Ga (the older rocks of the Arco Verde tonalite and the Mogno trondhjemite); (2) at 2.93 ± 0.02 Ga (Caracol tonalitic complex, Mariazinha tonalite, and the younger rocks of the Arco Verde tonalite); and (3) at 2.86 ± 0.01 Ga (Água Fria trondhjemite). The new data demonstrate that the Mogno trondhjemite is significantly older than thus far assumed, reveal the existence of a new TTG suite (Mariazinha tonalite) and indicate that the volume of TTG suites formed during the 2.86 event was relatively small. The Arco Verde tonalite yielded significant age variations (2.98–2.93 Ga), but domains with different ages could not be discerned. The tonalitic-trondhjemitic suites of the RMGGT derived from sources geochemically similar to the metabasalts of the Andorinhas supergroup that were extracted from the mantle during the Mesoarchean (3.0–2.9 Ga). Three groups of TTG granitoids were distinguished in Rio Maria: (1) high-La/Yb group, with high Sr/Y and Nb/Ta ratios, derived from magmas generated at relatively high pressure (>1.5 GPa) from sources with residual garnet and amphibole; (2) medium-La/Yb group with magmas formed at intermediate pressure (∼1.0–1.5 GPa), but still in the garnet stability field; and (3) low-La/Yb group with low Sr/Y and Nb/Ta ratios, crystallized from magmas generated at lower pressures (≤1.0 GPa) from an amphibolitic source with residual plagioclase. These three geochemical groups do not have a direct correspondence with the three episodes of TTG generation and a single TTG unit can be composed of rocks of different groups. A model involving hot subduction underneath a thick oceanic plateau was envisaged to explain the tectonic evolution of the RMGGT. In this context, the low-La/Yb group was formed from magmas originated by the melting of the base of a thickened basaltic oceanic crust at comparatively lower pressures (ca. 1.0 GPa), whereas the medium- and high-La/Yb groups were derived from slab melting at differing pressures (1.0–1.5 and >1.5 GPa, respectively). Part of the TTG magmas formed between 2.98 and 2.92 Ga reacted during their ascent with the mantle wedge and were consumed in the process, leaving behind a metasomatized mantle. 50 m.y. later, at ca. 2870 Ma, thermal events possibly related to slab-break-off and resultant asthenosphere mantle upwelling or to a mantle plume, induced melting of the metassomatized mantle to form sanukitoid magmas. These magmas may have heated the base of the Archean continental crust and could have led to local melting of basaltic crust, forming the Água Fria trondhjemite magma.

Chapter 29 The Tsagaan Oloom Formation, southwestern Mongolia

Abstract The Tsagaan Oloom Formation (Fm.) in southwestern Mongolia contains two Neoproterozoic glacial deposits, with diamictite in the Maikhan Ul Member (Mb.) and in the Khongoryn Mb., which are separated by over 500 m of limestone. The Maikhan Ul Mb. ranges in thickness between 5 m and greater than 300 m, expanding in deeper-water sections towards the SW, where it is composed of two massive diamictites separated by over 100 m of sandstone, siltstone and shale. The basal 10 m of the overlying Tayshir Mb. of the Tsagaan Oloom Fm. consists of a fine-laminated, dark grey limestone. The Khongoryn Mb. is composed primarily of limestone clasts in a shale matrix, and is between 0 and 23 m thick. The overlying Ol Mb. contains sedimentary structures characteristic of basal Ediacaran cap carbonates including micropeloids, tubestone stromatolites, giant wave ripples and former aragonite crystal fans. U–Pb evaporation ages from zircons in the underlying Dzabkhan Volcanics constrain the Tsagaan Oloom Fm. to &lt;773 Ma, and tuffs within the Maikhan Ul and Tayshir members testify to the potential for additional geochronology. The Cryogenian organic-rich limestone of the Tayshir Mb., which lies between the two glacial deposits, is ideally suited for geochemical studies and has been the subject of several carbon, strontium and rare earth element investigations. Limited palaeomagnetic studies suggest a mid- to low-latitude position of the Dzabkhan platform during deposition of the glaciogenic strata, and additional studies are in progress.

SHRIMP zircon dating and Nd isotopic systematics of Palaeoproterozoic migmatitic orthogneisses in the Epupa Metamorphic Complex of northwestern Namibia

The Epupa Metamorphic Complex constitutes the southwestern margin of the Congo Craton and is exposed in a hilly to mountainous terrain of northwestern Namibia, bordering the Kunene River and extending into southern Angola. It consists predominantly of granitoid gneisses which are migmatized over large areas. This migmatization locally led to anatexis and produced crustal-melt granites such as the Otjitanda Granite. We have undertaken reconnaissance geochemical studies and single zircon U–Pb SHRIMP and Pb–Pb evaporation dating of rocks of the Epupa Complex. The granitoid gneisses, migmatites and anatectic melts are similar in composition and constitute a suite of metaluminous to peraluminous, calc-alkaline granitoids, predominantly with volcanic arc geochemical signatures. The zircon protolith ages for the orthogneisses range from 1861 ± 3 to 1758 ± 3 Ma. Anatexis in the migmatitic Epupa gneisses was dated from a melt patch at 1762 ± 4 Ma, and the anatectic Otjitanda Granite has a zircon age of 1757 ± 4 Ma. Migmatization and anatexis therefore occurred almost immediately after granitoid emplacement and date a widespread high-temperature Palaeoproterozoic event at ∼1760 Ma which has not been recorded elswhere in northern Namibia. The Nd isotopic systematics of all dated samples are surprisingly similar and suggest formation of the protolith from a source region that probably separated from the depleted mantle about 2.4–2.0 Ga ago. A major Archaean component in the source area is unlikely. Structural reworking of the Epupa gneisses during the transpressional Neoproterozoic to early Palaeozoic Kaoko orogeny led to partial or complete obliteration of the older structures and resulted in spectacular low- to high-grade shear and mylonite zones. This reworking did not affect the U–Pb isotopic system in the zircons but documents partial destruction of the Congo cratonic margin. The Epupa granitoid rocks formed during an event generally referred to in Africa as the Eburnian orogeny, but the nature and tectonic setting of the Congo craton of southwestern Africa during this time remain largely unknown.

Early Miocene granite formation by detachment tectonics or not? A case study from the northern Menderes Massif (Western Turkey)

The northern border of the Menderes Massif is dominated by gneiss–schist–marble association and metagranites. The high-grade units of the massif are tectonically overlain by a meta-platform sequence of the Afyon Zone and the Cretaceous-early Tertiary HP/LT Tavşanlı Zone. The Simav Magmatic Complex, comprising the shallow-seated, I-type, calc-alkaline, post-collisional Eğrigöz and Koyunoba granites, and associated subvolcanic-volcanic sequences crosscuts the Menderes Massif, Afyon Zone and tectonic contact between them. In this paper, the age relation between gneisses, metagranites of the Menderes Massif and the Eğrigöz and Koyunoba granites was evaluated by isotope dating. 207 Pb/ 206 Pb evaporation ages between 607 and 501 Ma and U–Pb zircon ages of 543 ± 10 and 30.04 ± 0.56 Ma were obtained for two gneiss samples and a metagranite in the Menderes Massif, respectively. U–Pb zircon analyses yield crystallization ages of 21.7 ± 1.0 Ma for the Koyunoba granite and 19.4 ± 4.4 Ma for the Eğrigöz granite. A cooling age of 18.77 ± 0.19 Ma for the Eğrigöz granite was obtained by Rb–Sr (whole rock, biotite) analyses. Field occurrences, geochemical characteristics and geochronological data obtained from the Eğrigöz and Koyunoba granites show close similarities with other Oligo-Miocene granitoids in northwestern Anatolia indicating that they emplaced along an E-W trending regional magmatic belt and are not individual bodies related to local, north-dipping, low-angle detachment faults as suggested in previous papers.

Zircon and titanite age evidence for coeval granitization and migmatization of the early Middle and early Late Proterozoic Saharan Metacraton; example from the central North Sudan basement

Central North Sudan, west of the Keraf suture, is part of the Saharan Metacraton whose crystalline basement encompasses migmatite gneisses and granites. Granites intrude migmatites in form of small plutons, veins, lenses and pods, indicating a complex chronology. This study, based on whole rock element concentrations, isotope geochemistry and single mineral geochronology, is aimed to unravel the petrogenesis of these basement rocks. Whole rock geochemistry indicates an I-type potassic calc-alkaline meta- to peraluminous composition. Granite zircon U–Pb and Pb–Pb evaporation analyses yield an identical age range (597 ± 25–602 ± 3.5 Ma). Similar ages (597 ± 8.6–603.8 ± 2 Ma) are obtained for the migmatite gneisses. Titanite U–Pb ages are also similar in both rock types, but are younger or closely conform with zircon ages. Biotite Rb/Sr ages are younger and identical (566 ± 11–570 ± 17 Ma). These age data suggest coeval granitization and migmatization during the Pan-African period and somewhat later cooling of the central North Sudan basement. Older zircon U–Pb ages, ranging from 613 to 1322 Ma, are thought to be signatures of inheritance, while younger ones (336–594 Ma) suggest radiogenic Pb loss. Sr initial ratios (0.70257–0.72102) and ε Nd values (−2.3 to −8.8), calculated for the zircon crystallization age of ∼600 Ma indicate a crustal signature. Coupled with Nd model ages of 1460–1990 Ma, isotope data indicate that the central North Sudan basement is recycled Middle to Late Proterozoic continental crust.

THE MESOPROTEROZOIC MUCAJAI ANORTHOSITE - MANGERITE - RAPAKIVI GRANITE COMPLEX, AMAZONIAN CRATON, BRAZIL

The Mesoproterozoic Mucajai anorthosite – mangerite – rapakivi granite (AMG) complex, in northern Brazil, comprises rapakivi granites, fayalite–pyroxene quartz mangerite and syenite, and a massif-type anorthosite forming an asymmetrically zoned igneous complex without counterpart in the Amazonian craton. The rapakivi granites consist of biotite–hornblende granite, including pyterlites and subordinate wiborgites, and porphyritic biotite granite, and exhibit strong similarities with the classic rapakivi granites of Finland. The presence of a fayalite + ilmenite + quartz assemblage in the mangerites indicates low f (O 2 ) conditions during crystallization. The mangerites and rapakivi granites exhibit geochemical characteristics of A-type granites, including high contents of HFS elements (especially Zr), high Ga/Al, Rb/Ba and Rb/Sr values, high K 2 O contents and high FeO t /(FeO t + MgO), typical of reduced rapakivi granites. The biotite granite and the biotite–hornblende granite are related to each other by fractional crystallization, but the mangeritic rocks probably have an independent origin. A 1538 ± 5 Ma Pb–Pb evaporation age has been obtained for a fayalite–pyroxene mangerite. This age overlaps within analytical errors with the 1527 ± 7 Ma (U–Pb SHRIMP) and 1544 ± 42 Ma (U–Pb conventional method) ages previously reported for the anorthosite and rapakivi granites, respectively. The Nd data, including T DM model ages, in the range of 2.07–2.01 Ga, and e Nd (t) values between −2.4 and −1.3 suggest a juvenile crustal source for the granitic rocks of the complex. The e Nd (t) value calculated for the anorthosite (−2.9) probably reflects crustal contamination. Prominent bodies of rapakivi granite coeval with the Mucajai AMG complex are distributed along a NW–SE belt in the western part of the Guyana shield. The Mucajai AMG complex intruded 1.94 Ga country rocks at least ca . 400 my after the stabilization of this part of the Guyana shield, indicating an intraplate setting, in a geological scenario similar to those of the classical rapakivi complexes of the Fennoscandian shield.

Rhyacian (2.23–2.20Ga) juvenile accretion in the southern São Francisco craton, Brazil: Geochemical and isotopic evidence from the Serrinha magmatic suite, Mineiro belt

Abstract The Serrinha magmatic suite (Mineiro belt) crops out in the southern edge of the Sao Francisco craton, comprising the Brito quartz-diorite, Brumado de Cima and Brumado de Baixo granodiorites, granophyres and felsic sub-volcanic and volcanic rocks, part of which intruded into the Nazareno greenstone belt. The suite rocks have petrographic features that are consistent with magma supercooling due to the low water content combined with volatile loss, leading to crystallization of quartz and alkaline feldspar at the rims of plagioclase phenocrysts (granophyric intergrowth). The investigated rocks are sub-alkaline, calc-alkaline and show low content in rare earth elements. The U–Pb zircon crystallization ages for the Brumado de Cima granodiorite [2227 ± 22 (23) Ma] and a coeval granophyre [2211 ± 22 (23) Ma], coupled with available single-zircon Pb evaporation ages for the Brito and Brumado de Baixo plutons, are significantly older than the “Minas orogeny” (ca. 2100–2050 Ga) of Quadrilatero Ferrifero area, eastward from the Serrinha suite. Our data establish an early Rhyacian event tectonically linked with the evolution of the Mineiro belt. The bulk Nd isotopic signature [low negative to positive eNd(t) values] of the Serrinha samples are consistent with the important role of Paleoproterozoic mantle components in the magma genesis. The integrated geologic, geochemical and isotopic information suggests that Paleoproterozoic evolution of the Mineiro belt initiated in a passive continental margin basin with deposition of the Minas Supergroup at ca. 2500 Ma. This stage was succeeded by outboard rupture of the oceanic lithosphere with development and coalescence of progressively younger magmatic arcs during Rhyacian time. One of the earliest arcs formed the Serrinha suite. The tectonic collage of the Serrinha and Ritapolis (2190–2120 Ma) arcs produced the NE–SW Lenheiro shear zone, resulting in mylonitization and recrystallization of both the granitoid intrusions and host rocks. As a matter of fact juxtaposition of distinct magmatic units in age and origin took place along the Lenheiros structure in this sector of the Mineiro belt.