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    U-Pb SHRIMP and Sm-Nd geochronology of granite-gneiss complexes and implications for the evolution of the Central Brazil Archean Terrain [查看] Cla´udiaLimaQueiroz、HardyJost、LuizCarlosdaSilva、NealJ.McNaughton
    The paper is a first attempt to unravel the Archean multi-stage metaplutonic assemblage of the Meso/Neoarchean terrane of the State of Goia´s, Central Brazil, by means of the U–Pb SHRIMP zircon and Sm–Nd techniques. Two stages of granitic plutonism,spanning ca. 140 m.y., were precisely established for the accretion of the gneiss protoliths. The earliest stage embraces tonalitic to granodioritic and minor granitic orthogneisses with Nd juvenile signature, emplaced from ca. 2845 to ca. 2785 Ma, interpreted as the roots of an early arc. Inherited zircon xenocrysts and Nd isotopic data indicate that the juvenile magmas underwent contamination from a sialic crust as old as 3.3 Ga, from which there are, so far, no recognizable exposures. The second stage comprises granodioritic to granitic gneisses and lasted from ca. 2711 to 2707 Ma. Based on their Nd isotopic signatures and on inherited zircon crystals, their protoliths are interpreted as dominantly crustal-derived. The SHRIMP data from zircon crystals did not depict a Paleoproterozoic overprinting on the Archean gneisses, which is due to geological processes with prevailing temperatures below the isotopic stability of the U/Pb/Th system in the mineral. These processes comprise crustal extension and intrusion of a mafic dike swarm at ca. 2.3 Ga, followed by low grade events mostly related to shear zones between ca. 2.15 and 2.0 Ga. The study also revealed the extent of the Pan- African tectono-thermal overprinting on the Archean orthogneisses. Most of the zircon populations show morphological evidence of metamorphic peripheral recrystallization dated between ca. 750 and 550 Ma. One of the banded gneisses with a crystallization age of ca. 2700 Ma (2r) has a more complex zircon population including magmatic new grains, which yielded a precise 206Pb/238U crystallization age of 590 ± 10 Ma (2r). These new grains are interpreted to have grown in anatectic veins injected within strongly sheared gneiss.The data characterize a widespread Pan-African-aged metamorphic overprinting, culminating with localized anatexis of the Archean orthogneisses.
    The Palaeoproterozoic evolution of the Litchfield Province, western Pine Creek Orogen, northern Australia Insight from SHRIMP U–Pb zircon and in situ monazite geochronology [查看] ChrisJ.Carson、KurtE.Wordena、IanR.Scrimgeour、RichardA.Stern
    The Litchfield Province, located on thewestern margin of the Palaeoproterozoic Pine Creek Orogen, northern Australia, comprises variablymetamorphosed pelitic and psammitic sediments, extensively intruded by voluminous granitoids and mafic rocks. SHRIMP U–Pb in situ monazite geochronology of undeformed granulite-facies migmatitic Hermit Creek Metamorphics (ca. 730 ◦C, ca. 5 kb) and mid-amphibolite-facies Fog Bay Metamorphics (ca. 600 ◦C, ca. 4 kb) indicates mid- to high-grade metamorphism affected these rocks at ca. 1855 Ma. SHRIMP U–Pb analysis on low Th/U metamorphic zircon rims from the Fog Bay Metamorphics also indicate metamorphism at ca. 1855 Ma. The undeformed nature of the Hermit Creek and Fog Bay Metamorphics suggest that metamorphism was not accompanied by significant penetrative strain. These observations suggest that the elevated geotherm at this time may have been due to conductive and advective thermal input within a thinned lithosphere during intra-crustal extension and rifting.Metamorphism at ca. 1855Ma in the Litchfield Province is similar in age to that reported fromthe Hooper Orogen and the Western and Central Zones of the Halls Creek Orogen, but not with the Eastern Zone.Furthermore, there appears no evidence for the ca. 1845–1835Ma Halls Creek Orogeny in the Litchfield Province, suggesting that the inferred suture in the Halls Creek Orogen between the Kimberley Craton and the proto North Australian Craton, may not extend into the Litchfield Province and that the Litchfield Province may have been contiguous with, or forms part of, the Kimberley Craton. The upper-amphibolitefaciesWelltree Metamorphics (ca. 700 ◦C, ca. 4 kb), in contrast, preserve monazite grains that are aligned with a penetrative biotite fabric and are dated at 1813±3 Ma, thus recording a previously unrecognised tectono-thermal event in the Pine Creek Orogen.
    Tectonics of the southern Asian Plate margin along the Karakoram Shear Zone Constraints from field observations and U-Pb SHRIMP ages [查看] ArvindK.Jain、SandeepSingh
    New geological observations, recent published data andU–Pb SHRIMP zircon dating from the Karakoram Mountains along the Nubra and Shyok Rivers reveal that the initial subduction of the Tethyan oceanic lithosphere took place ~110 Ma beneath the Paleozoic–Mesozoic platform of the southern edge of the Asian Plate. This has produced the I-type plutons within the Karakoram Batholith Complex, well before the juxtaposition of the Asian Plate along the Karakoram Shear Zone. Within this shear zone, U–Pb zircon crystallisation ages of ~75 Ma from mylonitised granitoids and 68 Ma from undeformed Tirit granodiorite constrain the timing of suturing of the Karakoram terrain with the Trans-Himalaya between 75 and 68 Ma.Post-shearing leucogranite was episodically generated within frontal migmatised Karakoram Metamorphic Belt and emplaced between 20 and 13 Ma within the shear zone. Presence of a low resistivity zone as a possible indication of mid-crustal partial molten crust underneath the Higher Himalaya–Ladakh–Karakoram terrains manifests the impingement of the Indian Plate along the Main Himalayan Thrust at depth.Physical continuity of the Baltoro granite belt into the Karakoram Batholith is established as well as the continuity of the Shyok suture as the Shiquanhe Suture Zone in western Tibet through the Chushul–Dungti sector. The Karakoram Shear Zone, therefore, displays a complex geological history ofmovements since ~75 Ma and plays a very significant role in the overall India–Asia convergence, rather thanmerely being a strike-slip fault for eastward extrusion of a segment of Asia in Tibet.
    SHRIMP U-Pb zircon age constraints on the Late Archaean tectonostratigraphic architecture of the Eastern Goldfields Superterrane,Yilgarn Craton, Western Australia [查看] NatalieKositcin、StuartJ.A.Brown、MarkE.Barley、BryanKrapez、KevinF.Cassidy、DavidC.Champion
    Reassessment of Late Archaean tectonostratigraphic architecture of the Eastern Goldfields Superterrane is based on 35 new SHRIMP U-Pb zircon ages of supracrustal sequences. The superterrane comprises at least five terranes with ∼2810 to ≤2657 Ma volcanic and sedimentary rocks amalgamated by∼2660 Ma. Each terrane has distinctive stratigraphy, volcanic facies, geochemistry and timing of volcanism. The KalgoorlieTerrane comprises a >2715 to ∼2692 Ma plume-influenced, but deep-marine, mafic-ultramafic succession (Kambalda Sequence) overlain unconformably by a 2686±5 to ≤2666±6Ma arc-related, deep-marine felsic volcaniclastic-shale succession of tonalite-trondjemite-diorite (TTD) affinity (Kalgoorlie Sequence). Felsic volcaniclastic rocks of the Agnew Domain and northern Yandal Belt are included in the Kalgoorlie Terrane, based on their similar age and geochemistry. The terrane also contains enclaves of >2740 Ma volcanic sequences that are remnants of autochthonous basement to the Kambalda Sequence.Arc-related bimodal and intermediate volcanic rocks of the Gindalbie Terrane, ranging in age from 2693±4 to 2676±5 Ma, are coeval with but geochemically distinct from the Kalgoorlie Sequence. They overlie sequences similar in age and composition to volcanic rocks of the Kurnalpi Terrane. The Kurnalpi Terrane comprises deep-marine, calc-alkalic mafic and intermediate volcanic rocks (Kurnalpi Sequence) ranging in age from 2715±5 to 2704±4 Ma, overlain conformably by mafic to ultramafic lavas (Minerie Sequence). Although of overlapping age, arc-related rocks in the Kambalda and Kurnalpi-Minerie Sequences are geochemically different. SHRIMP dating has identified older volcanic sequences in the Laverton (≤2808±8 Ma) and Duketon-Burtville (2805±5 Ma) Terranes, with komatiite-hosted Ni-sulphide deposits atWindarra, in the Laverton Sequence, linked to mafic-ultramafic magmatism ∼100 million years older than in the Kalgoorlie Terrane. Older sequences in the Laverton Terrane are autochthonous basement to the Kurnalpi Sequence.Synorogenic siliciclastic sequences occur as erosional remnants either in synclinal structures or adjacent to terrane-scale faults, and overlie a regional-scale unconformity. Detrital zircon ages indicate deposition after 2657±4 Ma, placing important age constraints on the principal shortening event (D2) and peak metamorphism. Detrital zircon ages also indicate older sources at ∼2730, ∼2800, ∼2950, ∼3000 Ma and, less commonly, >3400 Ma. Many zircon age-populations correspond to known supracrustal and crustal rocks in the Yilgarn Craton, but some have no known source in the Eastern Goldfields Superterrane. Superterrane-wide distribution and evidence for erosion of all crustal and supracrustal rocks ≥2660 Ma in age requires deposition of these sequences after terrane amalgamation, most likely synorogenic to collision between the Eastern Goldfields Superterrane and the Yilgarn Craton.
    SHRIMP Ages of Zircon in the Uppermost Tuff in Chattisgarh Basin in Central India Require ∼500-Ma Adjustment in Indian Proterozoic Stratigraphy A Reply [查看] S.Patranabis-Deb、M.E.Bickford、B.Hill、A.K.Chaudhuri、A.Basu
    SHRIMP Ages of Zircon in the Uppermost Tuff in Chattisgarh Basin in Central India Require ∼500-Ma Adjustment in Indian Proterozoic Stratigraphy A Discussion [查看] A.Mukherjee、R.K.Ray
    Sedimentology, structure and SHRIMP zircon provenance of the Woodline Formation, Western Australia Implications for the tectonic setting of the West Australian Craton during the Paleoproterozoic [查看] C.E.Hall、S.A.Jones、S.Bodorkos
    The Paleoproterozoic Woodline Formation forms a 50 km long, northeast-trending belt of metasedimentary rocks that unconformably overlies Archean granites and greenstones of the Eastern Goldfields Superterrane near the southeastern margin of the Yilgarn Craton. The Woodline Formation is dominated by mature, quartz-rich sandstone units interbedded with siltstones, and represents the remnants of a broader siliciclastic sequence deposited along a passive margin after c. 1737 Ma. The formation is weakly deformed and metamorphosed to lower greenschist facies,with upright northeast-trending folds resulting from northwest–southeast shortening during the Albany–Fraser Orogeny between 1345 Ma and 1140 Ma. An extensional event at c. 1210 Ma is recorded by the emplacement of the northeast-trending Fraser dyke swarm and formation of the grabens that preserved the Woodline Formation. Three hundred and seventy-one U–Pb SHRIMP detrital zircon analyses from six samples from coarse-grained sandstone units of the Woodline Formation indicate a maximum depositional age of 1737±28 Ma (95% confidence). The age data from all six samples show heterogeneity of age spectra from the Eoarchean to the Paleoproterozoic, with an up-sequence diversification of source terranes. Significant probability maxima at 2687 Ma, 2655 Ma and 2629 Ma are consistent with erosion of the underlying Archean granite–greenstone rocks of the Eastern Goldfields Superterrane and the adjacent terranes of the Yilgarn Craton. In comparison, the Paleoproterozoic zircons with maxima at 2245 Ma, 2034 Ma and 1886 Ma do not match known tectonomagmatic events within theWest Australian Craton. Correlation of detrital zircon results from theWoodline Formation with other Paleoproterozoic siliciclastic units around the margin of the Yilgarn reveal a remarkable similarity with the Earaheedy Group, especially with the Paleoproterozoic age peaks indicating a common protosource(s) that is either buried, no longer exists, or is part of an exotic terrane beyond the West Australian Craton.
    Rheic Ocean ophiolitic remnants in southern Iberiaquestioned by SHRIMP U-Pb zircon ages on the Beja-Acebuche samphibolites [查看] A.Azor、D.Rubatto、J.F.Simancas、F.Gonza´lezLodeiro、D.Martı´nezPoyatos、L.M.Martı´nParra、J.Matas
    The Rheic Ocean was a major oceanic domain betweenAvalonia andGondwana in Ordovician-Silurian times. Most of the Paleozoic plate reconstructions assume that the Rheic Ocean suture lies within southern Iberia, coinciding with the contact between the South Portuguese Zone and the Ossa-Morena Zone.This paper reports four Sensitive High Resolution Ion Micro-Probe (SHRIMP) U-Pb zircon ages from midocean ridge basalt (MORB)-featured rocks of the Beja-Acebuches Amphibolite unit, which crops out along the boundary between the Ossa-Morena and the South Portuguese Zone, and is considered its most conspicuous suture unit. The obtained ages range from 332 ± 3 to 340 ± 4 Ma, corresponding to the crystallization of the mafic protoliths. These Early Carboniferous ages for the Beja-Acebuches amphibolites imply that this unit can no longer be viewed as an ophiolite belonging to the Rheic Ocean suture, since this oceanic domain was presumably closed in Devonian times. Tectonic reconstructions joining in a single suture line the Beja-Acebuches Amphibolite unit in southern Iberia to either the Devonian Lizard ophiolite in southern England or the root zone of the Devonian/Ordovician ophiolitic units in northwest Iberia must be therefore reconsidered because of the age difference.We interpret the Beja-Acebuches Amphibolite unit to represent a narrow and very ephemeral realm of oceanic-like crust that opened in Early Carboniferous times, after total consumption of the Rheic Ocean. We suggest that a mantle plume underneath southern Iberia in Early Carboniferous times is the most plausible large-scale geodynamic scenario for the formation of theseMORB-featured rocks.
    Records of Precambrian–Early Palaeozoic volcanic and sedimentary processes in the Central European Variscides A review of SHRIMP zircon data from the Kaczawa succession (Sudetes, SW Poland) [查看] RyszardKryza、JanZalasiewicz
    The early, pre-orogenic stages of evolution in the Variscan belt, i.e. rifting processes, opening of sedimentary basins and associated igneous activities, are often obscure because many successions have yielded little or no biostratigraphic data, have a strong metamorphic overprint and are tectonically deformed and dislocated.The increasing application of SHRIMP zircon dating has provided useful constraints on magmatic and metamorphic processes, helped locate probable source areas for detritus within sedimentary successions and facilitated large-scale palaeogeographic correlations. This methodology has recently thrown considerable light on the age and relationships of the previously poorly constrained rock units of the Kaczawa Complex in the Polish West Sudetes. Thus, recent SHRIMP studies in the Kaczawa Mountains have yielded Early Ordovician ages of the initial rift type bimodal volcanic suites at the bottom part of the Kaczawa Succession: c. 503 Ma for metarhyodacites of crustal derivation, and c. 485 Ma for alkaline metatrachytes of mantle signature. These dates provide a firm temporal constraint on the initial rift magmatism interpreted as related to the continental break-up of the northern peripheries of Gondwana. New SHRIMP data from metavolcaniclastic and metasedimentary rocks of the Kaczawa Complex have yielded results that have provided significantly changed interpretations on their age and relationships. For instance, a siliciclastic sequence interpreted as belonging to the lower part of the Kaczawa Complex (the Gackowa Sandstones) and seemingly sourced (using an array of geochemical and mineralogical evidence) from nearby early Ordovician volcanic rocks has, surprisingly, yielded zircon ages not younger than Precambrian and thus this unit has tentatively been reinterpreted as a possible correlative of the Neoproterozoic Lusatian Graywackes. Felsic metavolcaniclastic rocks embedded in the carbonate succession of the Wojcieszów Limestone have yielded SHRIMP zircon ages of c. 497 Ma, thus supporting a late Cambrian/early Ordovician age for this shallowwater limestone sequence (which had been proposed to be late Ordovician/Silurian or younger in age, based on controversial foraminifera evidence, and more recently assigned to the Mid Cambrian, based on newly found archaeocyathid fragments). And, age spectra of zircon populations in the metamudstone sequence of the Radzimowice Slates of thus far enigmatic stratigraphic position indicate that these deposits may have accumulated during early Palaeozoic rifting, with source areas similar to those known from parts of Gondwana (e.g. NW Africa). Overall, the new SHRIMP zircon studies, both in metavolcanic and metasedimentary sequences, have considerably illuminated their affinities, and should provide encouragement for further application of this technique in this region. In general the results provide support and detail for interpretation of this rock complex as reflecting the early stages of development of the volcanic and sedimentary processes operating along North Gondwana during latest Precambrian and early Palaeozoic times.
    Provenance and tectonic setting of Paleoproterozoic metasedimentary rocks along the eastern margin of Hearne craton Constraints from SHRIMP geochronology,Wollaston Group, Saskatchewan, Canada [查看] HaiThanhTran、KevinM.Ansdell、KathrynM.Bethune、KenAshton、MikeA.Hamilton
    Single detrital zircon grains from various parts of theWollaston Group, a Paleoproterozoic metasedimentary succession deposited along the southeastern margin of the Hearne Province, northern Saskatchewan,Canada,were analyzed by SHRIMP U–Pb geochronological techniques. Zircon analyses are mostly concordant and yield ages ranging from ca. 2800 to 1780 Ma, although distinct age populations were detected in all samples. The stratigraphically oldest sample (Geoch 4) is dominated by a bimodal distribution of zircon ages (ca. 1.90 and 2.4–2.6 Ga), which is similar to that preserved in the sample (Geoch 2) from the middle portion of theWollaston Group. The stratigraphically youngest sample (Geoch 9) contains ca.2.1Ga zircons, aswell as zircons with the same ages as observed in Geoch 4 and Geoch 2. Zircon ages older than 2450Ma appear to be consistent with the age of the Hearne Province basement, suggesting that part of the sedimentary detrituswas locally derived. Zircons with ages in the 2430–2350Ma range, found in all samples, may have been derived froma more distant source, such as Rae Province rocks thatwere affected by the recently identified Arrowsmith orogeny. Significant amounts of 1920–1880Ma zircon grains are found in all samples; these are interpreted to represent sedimentary detritus derived from juvenile volcanic terranes. Zircons younger than 1860Ma are interpreted to be the product of post-Wollaston Group thermal overprinting. Our data, together with field relationships and geochemical data, suggest that most of the preservedWollaston Group was deposited in a back-arc to foreland basin environment. It received detritus from both Archean continental crust to the west and a juvenile continental magmatic arc, likely located to the east, as the youngest zircon ages are not consistent with the age of Taltson Orogen rocks to the west.
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