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    SHRIMP U-Pb zircon geochronological evidence for Neoarchean basement in western Arnhem Land, northern Australia [查看] JulieA.Hollis、ChrisJ.Carsonb、LindaM.Glass
    The Pine Creek Orogen, located on the exposed northern periphery of the North Australian Craton,comprises a thick succession of variably metamorphosed Palaeoproterozoic siliciclastic and carbonate sedimentary and volcanic rocks, which were extensively intruded by mafic and granitic rocks. Exposed Neoarchean basement is rare in the Pine Creek Orogen and the North Australian Craton in general.However, recent field mapping, in conjunction with new SHRIMP U–Pb zircon data for six granitic gneiss samples, have identified previously unrecognised Neoarchean crystalline crust in the Nimbuwah Domain, the eastern-most region of the Pine Creek Orogen. Four samples from the Myra Falls and Caramal Inliers, the Cobourg Peninsula, and the Kakadu region have magmatic crystallisation ages in the range 2527–2510 Ma. An additional sample, from northeast Myra Falls Inlier, yielded a magmatic crystallisation age of 2671±3 Ma, the oldest exposed Archean basement yet recognised in the North Australian Craton. These results are consistent with previously determined magmatic ages for known outcropping and subcropping crystalline basement some 200km to the west. A sixth sample yielded a magmatic crystallisation age of 2640±4 Ma. The ca. 2670Ma and ca. 2640Ma samples have ca. 2500Ma metamorphic zircon rims, consistent with metamorphism broadly coeval with emplacement of the volumetrically dominant ca. 2530–2510Ma granites and granitic gneisses. Neoarchean zircon detritus, particularly in the ca. 2530–2510Ma and ca. 2670–2640Ma age span, are an almost ubiquitous feature of detrital zircon spectra of unconformably overlying metamorphosed Palaeoproterozoic strata of the Pine Creek Orogen,and of local post-tectonic Proterozoic sequences, consistent with this local provenance. Neoarchean zircon is also a common detrital component in Palaeoproterozoic sedimentary units across much of the North Australian Craton suggesting the existence of an extensive, if not contiguous, Neoarchean crystalline basement underlying not only a large part of the Pine Creek Orogen, but also much of the North Australian Craton.
    SHRIMP U-Pb age constraints on magmatism and high-grade metamorphism in the Salem Block, southern India [查看] ChrisClark、AlanS.Collins、NicholasE.Timms、PeterD.Kinny、T.R.K.Chetty、M.Santosh
    In this paper, we present Sensitive High Resolution Ion Microprobe (SHRIMP) U and Pb isotope analyses of zircon from a charnockite and a charnockite-hosted leucosome in order to determine the age of magmatism and anatexis related to high-grade metamorphism immediately to the north of the Palghat Cauvery Shear System (PCSS), a proposed Neoproterozoic terrane boundary within Southern India. Weakly luminescent, oscillatory-zoned cores in I06–128 and analyses with high Th/U ratios in I06–129 yield weighted mean 207Pb–206Pb ages of 2538±6 Ma and 2529±7 Ma. These two ages are statistically indistinguishable and are interpreted to reflect the crystallisation age of the original magmatic protolith to the charnockite. Low Th, Th/U ratio and brightly luminescent overgrowths and complete zircon grains in I06–128 and I06–129 give weighted mean 207Pb–206Pb ages of 2473±8 Ma and 2482±15 Ma respectively. The two ages are statistically distinct and are interpreted to constrain the timing of post-crystallisation high-grade metamorphism and partial melting of the magmatic rocks that crystallised at ~2530 Ma. The new ages of the charnockites are consistent with the suggestion that this activity is related to accretionary processes on the margin of the Dharwar craton and may be related to a second stage of convergent tectonics and collision on the margin of Kenorland. There is no evidence of a pervasive Neoproterozoic high-grade metamorphic event associated with the amalgamation of Gondwana recorded in these rocks. However, the possibility that deformation was localised along discrete shear zones at this time cannot be discounted. The data from this study add weight to the hypothesis that the PCSS represents a Neoproterozoic suture zone along which the Dharwar Craton and the microcontinent Azania, with its constituent Madurai and Trivandrum Blocks in the Southern Granulite terrane, collided at ca. 530 Ma Ga during the closure of the Mozambique ocean.
    Precambrian crystalline basement in southern Mongolia asrevealed by SHRIMP zircon dating [查看] AntoineDemoux、AlfredKroner、DunyiLiu、GombosurenBadarch
    Single zircon ages determined by ion microprobe (SHRIMP II) for granitoid gneisses from the southern slope of the Baga Bogd massif (Gobi-Altai,southern Mongolia) reveal several episodes of zircon growth, ranging from late Palaeoproterozoic to late Cambrian.The oldest events are documented by a zircon crystallization age for a gneiss protolith at 1519 ± 11 Ma and by a xenocrystic zircon from a dark grey augen-gneiss yielding an age of c. 1701 Ma. Discrete igneous events are recorded in granite-gneisses with protolith emplacement ages of 983 ± 6, 956 ± 3 and 954 ± 8 Ma. These ages provide the first record of early Neoproterozoic magmatic activity in this region. A much younger and discrete magmatic event is recorded by several dioritic to granitic orthogneisses which are tectonically interlayered with the older gneisses and have protolith emplacement ages between 502 and 498 Ma. These late Cambrian granitoids of calc-alkaline affinity are likely to have been emplaced along an active continental margin and suggest that the Baga Bogd Precambrian crustal fragment was either docked against the southward (present-day coordinates) growing margin of the CAOB or was a large enough crustal entity to develop an arc along its margin. We speculate that the Precambrian gneisses of this massif may be part of a crustal fragment rifted off the Tarim Craton.
    Paleoarchaean charnockite in the Ntem Complex,Conga Craton,Camerooninsights from SHRIMP zircon U-Pb ages [查看] TallaTAKAM\MakotoARIMA、JosephKOKONYANGI、DanielJ.DUNKLEY、EmmanuelN.NSIFA
    SHRIMP U-Pb analysis of zircon from charnockites and granite of the Archaean Ntem Complex,southern Cameroon was conducted to clarify the magmatic evolution of the northern part of Congo craton.Zircon morphologies and bulk chemistry are consistent with a magmatic origin for the charnockites,Zircons collected from two charnockites in the southern part of the complex yield differing magmatic ages of 3266±5Ma and 2883±11Ma.A xenocrystic zircon in a granite collected from the northern part of the complex gives a single grain age of 3477±16Ma,the oldest age ever reported from the Congo craton,while majority of zircons in the same rock yield magmatic age of 2853±12Ma.In addition,an inherited magmatic age of 3399±6Ma was obtained from the ~2883Ma charnockite.The present data provide evidence of early Paleoarchaean magmatic activity including charnockitic plutonism in the Congo craton.
    LA-MC-ICPMS and SHRIMP U–Pb dating of complex zircons from Quaternary tephras from the French Massif Central Magma residence time and geochemical implications [查看] AlainCocherie、C.MarkFanning、PierreJezequel、Miche`leRobert
    Analyses of zircon grains from the Queureuilh Quaternary tephras (pumice) provide new information about their pre-eruptive history. U–Pb dating was performed in situ using two methods: SHRIMP and LA-MC-ICPMS equipped with a multi-ion counting system. Both methods provided reliable 207Pb/206Pb and 206Pb/238U ratios as well as U and Th abundances required for U–Pb Concordia intercept age determination, after initial 230Th disequilibrium correction. The new LA-MC-ICPMS method was validated by dating a reference zircon (61.308B) and zircons from a phonolitic lava dated independently with the two techniques. A time resolution of about 20 kyr for 1 Ma zircon crystals was achieved for both methods.The clear euhedral zircon population from Queureuilh tephras is quite complex from several points of view: (1) some grains are reddish or yellowish while others are colorless; (2) the U and Th composition changes by more than an order of magnitude and Th/U is generally high (1–2); (3) there are three discrete ages recorded at 2.35 ± 0.04, 1.017 ± 0.008 and 0.640 ± 0.010 Ma.From the previously determined 40Ar/39Ar age at 0.571 ± 0.060 Ma [Duffell H. (1999) Contribution ge´ochronologique a` la stratigraphie volcanique du Massif des Monts Dore par la me´thode 40Ar/39Ar. D.E.A. Univ. Clermont-Ferrand, 56 p.], the discontinuous zircon age populations, the color of the grains and their composition, we favor the following model as explanation:The oldest, less numerous group of reddish zircons represents xenocrystic grains resulting from assimilation of the local material during magma ascent. A primitive magma chamber, perhaps deep in crustal level, was formed at 1.0 Ma.The related magma, previously characterized by high Th/U ratio (2.2 ± 1.1), underwent rejuvenation during ascent to a new chamber at shallow depth and/or during injection of more mafic magmas. During this stage, at 0.64 Ma, the colorless zircon grains of lower Th/U ratio (1.3 ± 0.5) crystallized. This last stage defined the magma residence time of 70 kyr prior to eruption dated by the 40Ar/39Ar method. However, if the primitive magma is considered, the magma residence time as a whole from this first stage reached 446 kyr.In the light of the complex history of such magmas, which commonly involves recycling of zircon grains that precipitated tens to hundreds of kyr earlier than eruptions, the use of Zr concentration in geochemical modeling of whole rock compositional data can be problematic.
    Granitoid evolution in Sinai, Egypt, based on precise SHRIMP U–Pb zircon geochronology [查看] B.H.Ali、S.A.Wilde、M.M.A.Gabr
    Late-stage Pan-African granitoids, including monzogranite, syenogranite and alkali granite, were collected from four separate localities in Sinai. They were selected to represent both the calc-alkaline and alkaline suites that have been viewed as forming separate magmatic episodes in the Eastern Desert of Egypt, with the transition to alkali granite at ~610 Ma taken to mark the onset of crustal extension. Although intrusive relations were observed in the field, the emplacement ages of the granitoids cannot be distinguished within analytical uncertainty and they all formed within a restricted time span from 579 to 594 Ma. This indicates that the two suites are coeval and that some calc-alkaline rocks were also likely generated during the late extensional phase. These ages are identical to those recently obtained from similar rocks in the North-Eastern Desert, confirming that Sinai is the northern extension of the Eastern Desert Pan-African terrane of Egypt.Rare inherited zircons with ages of ~1790 and ~740 Ma are present in syenogranite from northeastern Sinai and indicate that older material is present within the basement. A few zircons record younger ages and,although some may reflect later disturbance of the main zircon population, those with ages of ~570 and 535 Ma probably reflect thermal events associated with the extensive emplacement of mafic and felsic dykes in both northeastern and southern Sinai.
    Geochemistry, Nd isotopes and U–Pb SHRIMP zircon dating of Neoproterozoic volcanic rocks from the Central Eastern Desert of Egypt New insights into the ∼750Ma crust-forming event [查看] KamalA.Ali、RobertJ.Stern、WilliamI.Manton、Jun-IchiKimura、HossamA.Khamees
    Submarine tholeiitic and calc-alkaline basalt and andesite with subordinate dacite, metamorphosed to greenschist facies, are exposed at Wadi Kareim and Wadi El Dabbah, Central Eastern Desert of Egypt.These metavolcanic sequences are representative of early stages of Neoproterozoic crust formation in the northern part of the Arabian-Nubian Shield. The magmas were primitive to moderately fractionated, as indicated by average Mg# (54), Cr (191 ppm) and Ni (69 ppm). Their REE patterns are slightly fractionated [(La/Yb)N = 1.3–3.9], and multi-element diagrams show Ba, Sr, K and Rb enrichments and Nb and Ta depletions, typical of modern convergent margin igneous rocks. Tectonic discrimination diagrams also show affinities with modern convergent margin magmas, suggesting that Wadi El Dabbah lavas formed in a volcanic arc that experienced cryptic crustal contamination, whereas Wadi Kareim lavas formed in a back-arc basin. Positive initial Nd (+5.1 to +8.9) and Nd model ages (0.64–0.79 Ga) indicate that the crust is juvenile andwas extracted from a depleted mantle source.Wadis Kareim and El Dabbah metavolcanic rocks are dated at∼750Ma(U–Pb SHRIMP) but contain abundant pre-Neoproterozoic zircons which might be derived from underlying sediments or perhaps older continental crust. These results indicate that a major ∼750Ma crust-forming event involving ophiolite generation and eruption of juvenile melts was important for the formation of the crust.
    Evidence for subduction at 3.8 Ga Geochemistry of arc-like metabasalts from the southern edge of the Isua Supracrustal Belt [查看] F.E.Jenner、V.C.Bennett、A.P.Nutman、C.R.L.Friend、M.D.Norman、G.Yaxley
    We present new major and trace element data for rare examples of preserved pillow basalts from a locality of little studied ＞3800 Ma rocks at the southwestern edge of the Isua Supracrustal Belt (ISB), West Greenland.The 20 samples have 47.3–59.0 wt.% SiO2, 4.9–12.2 wt.% MgO, 6.5–11.2 wt.% CaO, 60–340 ppm Ni and 63–1094 ppm Cr, consistent with pre-metamorphic basaltic compositions. Nb/La(n) and Ti/Gd(n) are lower than primitive mantle compositions (0.1–0.6 and 0.3–0.8, respectively), with these ratios interpreted to reflect the composition of their source. Correlations of Nb/La with La/Sm, Ti/Gd and Zr/Sm and between Zr/Nb and Nb/Th are comparable to those of modern subduction related magmas, whereby slab melts enriched the overlying mantle wedge and HFSE were retained by Ti-rich phases. Trends in Ba/La vs. Ba/Nb are comparable to modern day arc related basalts, which reflect a sediment influence and may indicate that, at least early in the evolution of Eoarchean subduction zones, the subducted Eoarchean oceanic crust retained a pelagic sediment cover, with this cover influencing the chemistry of the slab-derived fluid. Three of the ISB pillow lavas show extreme enrichments in LREE (La/Sm(n)=1.9–2.6), large depletions in Nb, Ta, Zr and Hf relative to REE that are similar to rare high-K arc basalts from the Clark volcano of the presently active Tongan–Kermadec–New Zealand arc (Gamble, J.A., Christie, R.H.K., Wright, I.C., Wysoczanski, R.J., 1997. Primitive Krich magmas from Clark volcano, southern Kermadec arc: a paradox in the K–depth relationship. The Canadian Mineralogist, 35, 275–290). The compositions of these highly enriched basalts indicate that the mantle source region of the Isua basalts was occasionally overprinted by small-volume sediment dominated melts rather than hydrous fluids. The compositional affinities of these 3.8 Ga pillow basalts with modern island-arc basalts provide strong evidence for the role of slab fluids and melts in basalt genesis in the Eoarchean, further demonstrating arc-like signatures in the oldest rock sequences.The compositions of Archean mafic rocks including samples from the ISB that are interpreted to have a petrogenesis similar to that of modern island-arc basalts, are compared with Archean ‘non-arc’ rocks (i.e.plume-related komatiites, komatiitic basalts and associated tholeiitic basalts). The two groups are distinguished by differences in La/Sm, Gd/Yb, Nb/La, Ti/Gd, Ba/La, Ba/Nb and Al2O3/TiO2. Thus clear compositional distinctions between different basaltic types, attributed to arc and non-arc origins, are a feature of the rock record for at least the last 3.8 Ga.
    A granitic inclusion suite within igneous zircons from a 3.81 Ga tonalite(W. Greenland) Restrictions for Hadean crustal evolution studies using detrital zircons [查看] AllenP.Nutman、JoeHiess
    This paper presents an electron microprobe study of inclusions within igneous zircons from ca. 3800 Ma metatonalite sample G97/18. Crystalline inclusions are K-feldspar, plagioclase, quartz, hornblende, biotite,ilmenite and apatite. Additionally, one globular polymineralic inclusion interpreted as crystallised melt occurs at the surface of a polished grain mount. This consists of plagioclase + quartz + K-feldspar around a biotite lath. Other similar, but rare globular inclusions were seen buried within other zircons below the polished surface. These inclusions, particularly the presence of K-feldspar and plagioclase in broadly equal amounts, entail crystallisation of the zircons from a (residual) granitic (sensu-stricto) melt rather than a tonalitic melt. SHRIMP (Sensitive High Resolution Ion MicroProbe) U/Pb dating of zircons with inclusions indicates that they grew at ca. 3800 Ma (the accepted igneous age of the rock).This inclusion suite is compatible with the low Ti-in-zircon temperatures for G97/18 igneous zircon ( Hiess et al., 2008). Therefore, low Ti-in-zircon temperatures and “granitic” inclusions found in Hadean detrital zircons do not necessitate crystallisation from low temperature granites, because such features would also be found if Hadean zircons had crystallised (late) from higher temperature zircon-undersaturated melts of tonalitic composition.
    U-Pb zircon ages (SHRIMP) for Cadomian and Early Ordovician magmatism in the Eastern Pyrenees New insights into the pre-Variscan evolution of the northern Gondwana margin [查看] PedroCastiñeiras、MarinaNavidad、MontserratLiesa、JordiCarreras、JosepM.Casas
    New geochronological data from low- to medium-grade metamorphic areas of the Eastern Pyrenees (Canigó,Roc de Frausa and Cap de Creus massifs) confirm the presence of two significant pre-Variscan igneous events:Ediacaran–Early Cambrian and Early Ordovician. The Ediacaran–Early Cambrian (580–540 Ma) magmatism is characterized by metavolcanic plagioclasic gneisses (metatuffs) coeval with sedimentation and by sheets of granitic orthogneisses emplaced in the lower part of the metasedimentary series. In the Canigó and Roc de Frausa massifs, the metatuffs are spatially associated with metabasites. Both lithologies occur as massive layers of lava flows, discontinuous lense-shaped, subvolcanic, gabbroic bodies or volcanoclastic tuffs interbedded in the lower and middle part of the pre-Upper Ordovician metasedimentary succession. This magmatism is bimodal and has a tholeiitic and calc-alkaline affinity. The granitic orthogneisses represent thick laminar intrusions of subaluminous and aluminous composition. Early Ordovician (475–460 Ma) magmatism is represented by laccoliths of aluminous granitic orthogneisses emplaced in the middle part of the pre-Upper Ordovician succession.These geochronological data reveal the existence of an Ediacaran metasedimentary sequence and Cadomian magmatism in the Pyrenees and allow their correlation along the Eastern Pyrenean massifs. The data also show ages ranging from Neoproterozoic to Early Ordovician of the large bodies of granitic orthogneisses that intruded into the series at different levels. Both events represent the final stages of the Cadomian orogeny and its transition to the Variscan cycle in the Eastern Pyrenees. A Cambrian rifting event linking both cycles has not been identified in the Pyrenees to date.Our findings provide a better fit for the pre-Variscan sequences of the Pyrenees with those of the Iberian Massif and allow their comparison with other pre-Variscan massifs in Europe.
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