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    3.65–2.10 Ga history of crust formation from zircon geochronology and isotope geochemistry of the Quijingue and Eucides plutons, Serrinha nucleus, Brazil [查看] D.C.Riosa、D.W.Davis、H.Conceic¸ão、M.L.S.Rosaa、W.J.Davis、A.P.Dickin、M.M.Marinho、R.Stern
    The Serrinha nucleus is one of the Archaean granite-gneiss terrains of São Francisco craton, Brazil. In this area, many granitic rocks formed during the Trans-Amazonian orogeny, and intruded during and following a greenschist metamorphic event. Zircon geochronology and Nd isotopes record a history of extensive crustal reworking that began at 3.6 Ga and culminated in the emplacement and deformation of a continental arc during the Trans-Amazonian accretionary orogeny at 2.1 Ga.The basement to the arc is represented by migmatitic gneiss of theUauáComplex, which records a period of zircon growth at 3195±11Ma but contains cores as old as 3314±20 Ma, aswell as metamorphic zircon as young as 3090 Ma. The Meso to Paleoarchean ages, as well as the absence of evidence for widespread Neoarchean magmatism are similar to what is observed in the Kaapvaal craton in South Africa. The Quijingue trondhjemite was emplaced into Uauá Complex migmatitic gneiss at 2155±3Ma probably during a period of TTG magmatism associated with arc construction on the edge of a Mesoarchean continent. Although older cores appear to be absent from the Quijingue trondhjemite, its depleted mantle Nd age of 3.3 Ga suggests that it was derived from partial melting of the basement. Xenocrystic zircon grains with ages as old as 3.62 Ga are present in this pluton and indicate that Eoarchean crustwas present and may still be preserved in the area. Ocean convergence was probably followed by a Trans-amazonian collisional event that resulted in widespread syn to post-tectonic alkaline magmatism such as the shoshonitic Euclides pluton. Primary zircon from this pluton records an emplacement age of 2097±8Ma but the depleted mantle Nd age of 2.87 Ga indicates derivation of these magmas from Archean sources. The inherited zircon population from Euclides preserves a complex record of growth. One possible xenocryst gives a discordant and relatively imprecise age of 3654±125 Ma. SHRIMP dating of other zircon grains gives concordant ages that range from 2350Ma to 2500Ma as well as a 2.14-Ga Trans-Amazonian component. The provenance of the very early Proterozoic ages is unknown but may indicate zircon growth or resetting events at deep crustal levels.These data combined with other results suggest that mid to early Archean continental crust could underlie much of the São Francisco craton.
    Palaeozoic arc magmatism in the Central Asian Orogenic Belt of Kazakhstan SHRIMP zircon ages and whole-rock Nd [查看] A.Kroner、E.Hegner、B.Lehmann、J.Heinhorst、M.T.D.Wingate、D.Y.Liu、P.Ermelov
    Early Palaeozoic tonalite to granodiorite intrusions in northern Kazakhstan are associated with lode gold mineralization and have SHRIMP zircon ages of 457.3 ± 6.6 Ma (Aksu), 452.9 ± 5.6 Ma and 447.4 ± 5.4 Ma (both Zholymbet). The Stepnyak intrusion contains large xenoliths with an age of 480.6 ± 5.0 Ma. One early Palaeozoic zircon from a porphyritic diorite at Stepnyak has a core with a nearconcordant 207Pb/206Pb age of 3888 ± 1.5 Ma, whereas other xenocrystic grains are between 983 and 2698 Ma old. The early Archaean age is probably inherited from unexposed basement of the Kokchetav Massif and represents the oldest crustal material so far known from the Asian continent. It appears that the Aksu, Zholimbet and Stepnyak granitoids were emplaced in the late Ordovician in an Andean- or Japan-type continental arc environment on the margin of the Kokchetav Massif.Late Palaeozoic granitoids in central Kazakhstan have Devonian zircon ages of 407.1 ± 3.9, 381.1 ± 3.1 and 369.2 ± 4.9 Ma, whereas the youngest sample from the Topar Massif has a late Carboniferous emplacement age of 314.1 ± 5.1 Ma. Initial eNd values range from +5 to -1 corresponding to Nd-model ages of 1.1–0.6 Ga. The isotopic data are similar to those of other Phanerozoic granitoids of the Central Asian Orogenic Belt and corroborate melting of predominantly juvenile crustal protoliths. Our data also confirm that Devonian and Carboniferous arc magmatism in central Kazakhstan assimilated older and already accreted island arc systems, yet is derived from primitive sources, probably in a subduction setting.The range of eNd values with corresponding Nd mean crustal residence ages in the range of 0.6 1.1 Ga for most samples indicates variable recycling of late Proterozoic largely juvenile continental crust in the formation of the Palaeozoic magmatic arcs of central Kazakhstan. The orogenic evolution terminated with Permian anorogenic, rift-related alkaline granite magmatism (εNd = +2 to +8) which reflects melting of juvenile, mantle-derived lower crust.
    First U-Pb SHRIMP age of the Hauterivian stage,Neuque´n Basin, Argentina [查看] M.BeatrizAguirre-Urreta、PabloJ.Pazos、Darı´oG.Lazo、C.MarkFanning、VanesaD.Litvak
    A high-resolution ion-microprobe (SHRIMP) U–Pb zircon age from a tuff layer intercalated in the ammonoid bearing sedimentary succession of the Neuque´n Basin in Argentina provides a robust geochronologic date to add to the absolute ages and to improve the relative chronology of the Early Cretaceous Hauterivian stage. The tuff layer appears interbedded between shales of the upper member (Agua de la Mula) of the Agrio Formation within the Spitidiscus riccardii ammonoid zone (base of the Late Hauterivian) yielding a date of 132.5 ± 1.3 Ma. This date confirms and supports an accurate correlation between the ammonoid biostratigraphy of the Neuque´n Basin with the Western Mediterranean Province of the Tethys during the Early Cretaceous and matches with the most recently published time scale. It also casts doubts on the validity of K–Ar ages on glauconite-grains recently reported from the Lower Cretaceous of the Vocontian Basin of France.
    Evolution of Pre-1.8 Ga basement rocks in the western Mt Isa Inlier,northeastern Australia—Insights from SHRIMP U-Pb dating and in-situ Lu-Hf analysis of zircons [查看] FrankP.Bierlein、LanceP.Black、JanetHergt、GeordieMark
    We present new SHRIMP zircon U–Pb data from pre 1.8 Ga basement rocks (Yaringa Metamorphics, Kurbayia Migmatite) on both sides of the Mt Isa Fault in the western Mt Isa Inlier. These data confirm that felsic intrusions were emplaced into the western Kalkadoon-Leichhardt Belt at 1849±4Ma (2σ), and constrain the Barramundi Orogeny in the western Fold Belt to ca. 1.87 Ga, at the bottom part of the previously reported range of ca. 1.90–1.87 Ga. Integration of in situ Hf isotope analysis and SHRIMP zircon age data support the notion that there is no lithospheric break across the Mount Isa Fault. Furthermore, the 176Hf/177Hf isotope data confirm that Archaean–Palaeoproterozoic magmatic zircons on both sides of the Mount Isa Fault were sourced from the same parental lithospheric reservoir which evolved over time from more primitive mantle to supracrustal compositions, without significant contributions from juvenile sources in the Palaeoproterozoic. The oldest inherited zircons in samples from the Yaringa Metamorphics reflect the participation of Archaean (ca. 3300–3600 Ma) crustal components in the western Mt Isa Inlier. These zircons, together with isotopic data from other studies, may allow for a tectonic reconstruction involving Archaean crust underlying much of the Proterozoic succession at least in the western Mt Isa Inlier. Alternatively, the Archaean zircons could represent detrital components incorporated from Palaeoproterozoic metasedimentary rocks. As detrital zircons can be transported laterally for hundreds of kilometres, these grains cannot be diagnostic of the nature and age of the basement beneath the western Mt Isa Inlier. Consequently, neither model can be conclusively excluded given the current data set.
    Determining high precision, in situ, oxygen isotope ratios with a SHRIMP II Analyses of MPI-DING silicate-glass reference materials and zircon from contrasting granites [查看] R.B.Ickert、J.Hiess、I.S.Williams、P.Holden、T.R.Irel、P.Lanc、N.Schram、J.J.Foster、S.W.Clement
    The development of new techniques and instrumentation on the ANU SHRIMP II ion microprobe has made it possible to measure the oxygen isotope ratios of insulating and conducting phases (e.g. silicates, carbonates, phosphates and oxides) on a 25 μm scale with better than 0.4‰ precision and accuracy at 95% confidence.Instrumentation changes include the installation of a multiple collector, charge neutralization using an oblique-incidence low-energy electron gun, and the addition of Helmholtz coils to counter mass dispersion by the Earth's magnetic field. A redesign of sample mounts and mount holders has effectively eliminated differences in variable isotope fractionation across the mount surface during analysis. Techniques have been developed to minimize the effect of electron-induced secondary ionization of oxygen. During a 6-minute analysis involving 100–140 s of data collection, δ18O values can be measured on one 25 μm spot with an internal precision of better than 0.2‰ (2 standard errors). Analyses of MPI-DING silicate-glass reference material demonstrate that the external reproducibility of single spots can be better than 0.4‰ at 95% confidence, and that for matrix-matched samples and reference material, accuracy is commensurate with precision. MPI-DING glasses are acceptable ion microprobe reference materials for oxygen isotope measurements of glasses, although KL2-G is possibly heterogeneous. Zircon reference materials TEMORA 2 and FC1 appear to be acceptable as preliminary oxygen isotope reference materials. SHRIMP II analyses of FC1 indicate that it has a δ18O value of 5.4‰ (VSMOW). Analyses of zircon oxygen isotopic compositions from a gabbro, a tonalite and a granodiorite from southeastern Australia are presented. Zircon from the gabbro has a δ18O value of 5.6‰, the tonalite has an I-type affinity and slightly heterogeneous δ18O values around 6.6‰, and the granodiorite has an S-type affinity and a range of igneous, melt precipitated zircon δ18O values between 8.2 and 10.2‰. These results suggest that the gabbro is mantle-derived and slightly contaminated with crustal material, and that the I-type granodiorite has evolved in a similar manner from a mantle-derived source. The δ18O values of the zircon from the S-type granodiorite are not only higher than from the I-type, but also more heterogeneous, consistent with partial melting of a poorly-mixed, metasedimentary source.
    Dating the Oldest Greenstone in India A 3.51-Ga Precise U-Pb SHRIMP Zircon Age for Dacitic Lava of the Southern Iron Ore Group,Singhbhum Craton [查看] JoydipMukhopadhyay、N.J.Beukes、R.A.Armstrong、UdoZimmermann、GautamGhosh、R.A.Medda
    This article reports a precise 3506:8 ± 2:3-Ma U-Pb SHRIMP zircon age for dacitic lava in a well-preserved low-grade metamorphic and low-strained greenstone belt succession of the southern Iron Ore Group, Singhbhum craton, India.This age makes the succession the oldest-known greenstone belt succession in India and one of the oldest low-strain greenstone successions in the world after the 3.51-Ga Coonterunah Group of the Pilbara craton,Western Australia, and the moderately deformed 3.54-Ga Theespruit Formation of the Barberton Greenstone Belt, Kaapvaal craton, South Africa. The geochemical composition of the dacitic lava and related volcanic rocks suggests that they formed in a volcanic arc setting. The succession also contains a major ∼120-m-thick oxide facies banded iron formation that distinguishes it from the slightly older successions of the Pilbara and Kaapvaal cratons. This banded iron formation may well be one of the oldest and most well preserved, and together with associated volcanics, it may have immediate implications for understanding >3:5-Ga surface and tectonic processes on Earth.
    Coolwater culmination Sensitive high-resolution ionmicroprobe (SHRIMP) U-Pb and isotopic evidence for continental delaminationin the Syringa Embayment, Salmon River suture, Idaho [查看] KarenLund、J.N.Aleinikoff、E.Y.Yacob、D.M.Unruh、C.M.Fanning
    During dextral oblique translation along Laurentia in western Idaho, the Blue Mountains superterrane underwent clockwise rotation and impinged into the Syringa embayment at the northern end of the Salmon River suture. Along the suture, the superterrane is juxtaposed directly against western Laurentia, making this central Cordilleran accretionary-margin segment unusually attenuated. In the embayment, limited orthogonal contraction produced a crustal wedge of oceanic rocks that delaminated Laurentian crust. The wedge is exposed through Laurentian crust in the Coolwater culmination as documented by mapping and by sensitive high-resolution ion microprobe U-Pb, Sri, and eNd data for gneisses that lie inboard of the suture. The predominant country rock is Mesoproterozoic paragneiss overlying Laurentian basement. An overlying Neoproterozoic (or younger) paragneiss belt in the Syringa embayment establishes the form of the Cordilleran miogeocline and that the embayment is a relict of Rodinia rifting. An underlying Cretaceous paragneiss was derived from arc terranes and suture-zone orogenic welt but also from Laurentia.The Cretaceous paragneiss and an 86-Ma orthogneiss that intruded it formed the wedge of oceanic rocks that were inserted into the Laurentian margin between 98 and 73 Ma, splitting supracrustal Laurentian rocks from their basement. Crustal thickening, melting and intrusion within the wedge, and folding to form the Coolwater culmination continued until 61 Ma. The embayment formed a restraining bend at the end of the dextral transpressional suture. Clockwise rotation of the impinging superterrane and overthrusting of Laurentia that produced the crustal wedge in the Coolwater culmination are predicted by oblique collision into the Syringa embayment.
    Constraining the age of the Iporanga Formation with SHRIMP U-Pb zirconImplications for possible Ediacaran glaciation in the Ribeira Belt, SE Brazil [查看] G.A.C.Campanha、M.S.Basei、C.C.G.Tassinari、A.P.Nutman、F.M.Faleiros
    The Ribeira belt in SE Brazil is a Neoproterozoic to Early Palaeozoic orogen, whose architecture and history is not yet fully understood. The depositional age of many of the sedimentary sequences in the Ribeira Belt remains unconstrained, and with debate concerning their depositional environment and tectonic setting. In this paper we present SHRIMP zircon U/Pb age constraints for one such problematic unit in the Ribeira Belt–the Iporanga Formation – and discuss the significance of this age with regards to the timing of Neoproterozoic glacial events in southeast Brazil.Using a felsic volcanic unit immediately under the Iporanga Formation and granite cobbles from breccias in its basal parts a reconnaissance SHRIMP U/Pb zircon maximum depositional age of 580 Ma is assigned for the base of this unit. This age is marginally younger than the 625–605 Ma ages for intrusions into the Lajeado and Ribeira subgroups, with which the Iporanga Formation is in tectonic contact. This indicates that the Lajeado and Ribeira subgroups are not stratigraphically equivalent to the Iporanga Formation, as thought previously by some workers. The maximum depositional age of 580 Ma also places a maximum time constraint on the tectonic juxtaposition of the Iporanga Formation with other supracrustal units, and on the greenschist facies metamorphism and isoclinal folding that affected it. The potential glacial origin for the Iporanga Formation, if correct, would place it in the late Ediacaran — provisionally equivalent to the Gaskiers glaciation.
    Cambrian ensialic rift-related magmatism in the Ossa-Morena Zone(Évora–Aracena metamorphic belt, SW Iberian Massif) Sm-Nd isotopes and SHRIMP zircon U-Th-Pb geochronology [查看] M.Chichorro、M.F.Pereira、M.Díaz-Azpiroz、I.S.Williams、C.Fernández、C.Pin、J.B.Silva
    The Late Ediacaran (c. 560–550 Ma) Série Negra sediments of the Évora–Aracena metamorphic belt, Ossa-Morena Zone, SW Iberian Massif, preserve a record of the erosion of an Avalonian–Cadomian magmatic arc and subsequent related turbiditic sedimentation. Detrital zircon from the Série Negra is characterized by predominantly Ediacaran and Cryogenian ages, with few Paleoproterozoic and Archean cores, and a marked lack of Grenvillian ages. These features, when combined with the metasediments' enrichment in LREE (La/Yb=14), negative Eu-anomalies, low 147Sm/144Nd values (0.121) and negative εNd550=−5.5, indicate that the protolith Série Negra sediments were derived from a continental magmatic arc. A period of Late Cadomian (ca. 560–540 Ma) tectonism was followed by an extended episode of widespread bimodal magmatism related to Cambrian (ca. 540–500 Ma) rifting. This tectonic inversion is expressed in the geological record by a regional Early Cambrian unconformity.SHRIMP zircon U–Th–Pb ages from four felsic orthogneisses from the Évora Massif record Cambrian (527±10 Ma, 522±5 Ma, 517±6 Ma and 505±5 Ma) crystallization ages for their igneous protoliths. This confirms the existence of widespread Lower Paleozoic igneous activity in the Ossa-Morena Zone: (i) a Lower Cambrian (ca. 535–515 Ma) igneous–felsic dominated–sedimentary complex (with calc-alkaline signature and associated carbonate and siliciclastic deposition), and (ii) a Middle Cambrian–?Ordovician (ca. 515–490 Ma) igneous–bimodal–sedimentary complex (with calc-alkaline and tholeiitic signatures and associated dominant siliciclastic deposition, but also carbonate sediments).The Cambrian felsic magmatism was characterized by negative Eu-anomalies, (La/Lu)N=0.8–11, 147Sm/144Nd=0.1289–0.1447 and εNd500 ranging from −1.5 to −0.8. A tendency towards peraluminous compositions suggests late fractionation, low degrees of partial melting, or the mixing of crustal and mantle-derived material in the magma source region. Some felsic rocks possibly represent the last residual melts of hightemperature,zircon-undersaturated mafic magmas later affected by crustal contamination, while others indicate partial melting of crustal metasediments variably contaminated by basaltic liquids.The transition from early felsic dominated to later more mafic magmatism suggests the gradual opening of the system to tholeiitic N–E-MORB products (ThN/TaNb1.0). The as yet undated (Cambrian–?Ordovician) E-MORB amphibolites have 147Sm/144Nd=0.1478–0.1797 and εNd500 values ranging from +6.4 to +7.3, while the N-MORB amphibolites have 147Sm/144Nd=0.1818–0.1979 and εNd500 values of +5.8 and +7.0,reaching a maximum of +9.1. In contrast, other amphibolites have a negative Ta-anomaly (1.35bThN/TaNb2.41) reminiscent of lavas from “orogenic” settings or alternatively, typical of crustally-contaminated within-plate magmas. These “VAB-like” amphibolites have 147Sm/144Nd values ranging from 0.1639 to 0.1946 and εNd500 values of +3.5 to +5.2, suggesting derivation by crustal assimilation processes. The subalkaline igneous precursors of the amphibolites were most likely generated in a rift setting by asthenospheric upwelling.
    A revised Palaeoproterozoic chronostratigraphy for the Pine Creek Orogen,northern Australia Evidence from SHRIMP U-Pb zircon geochronology [查看] KurtWorden、ChrisCarson、IanScrimgeour、JamesLally、NigelDoyle
    New SHRIMP U–Pb zircon geochronology for volcanic and volcaniclastic sedimentary rocks from the central Pine Creek Orogen provides constraints on the Palaeoproterozoic evolution of northern Australia. An intermediate agglomerate from the Stag Creek Volcanics (Namoona Group) and two samples of volcaniclastic sedimentary rock from the Wildman Siltstone (Mount Partridge Group) were dated to constrain the age of the lower parts of the Palaeoproterozoic succession. These samples all have a youngest zircon population with an age in the range ca. 2030–2020 Ma, which is interpreted to approximate the age of volcanism and volcaniclastic deposition. Two widely separated samples of Gerowie Tuff from the overlying South Alligator Group have depositional ages of 1864±3Ma and 1862±4 Ma. Zircons of the Berinka Volcanics andWarrs Volcanic Member within the overlying Finniss River Group have crystallisation ages of 1861±4Ma and 1862±3 Ma, respectively. These newdata suggest that the basal units of the Pine Creek Orogen were being deposited in part during a previously unrecognised episode of ca. 2030–2020Ma felsic volcanism and sedimentation in northern Australia, which was followed by a major hiatus. The ca.1865–1860Ma ages for volcanic rocks within the South Alligator and Finniss River Groups provide a maximum age on the timing of deformation and metamorphism in the central Pine Creek Orogen, and allow correlation with felsic volcanic events across large areas of the Pine Creek Orogen and theNorth Australian Craton.
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