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    Depositional History of the Chhattisgarh Basin, Central India Constraints from New SHRIMP Zircon Ages A Discussion [查看] P.P.Chakraborty、K.Das、Y.Tsutsumi、K.Horie
    In the past 4 decades, extensive field-based studies in unmetamorphosed and mildly deformed Indian Proterozoic basins have produced significant findings in physical sedimentology, in general, and the uniqueness of vegetationless yet biomat-influenced sedimentation patterns, in particular. In comparison,geochronological and geochemical appraisal of the basin fills was limited, rendering the time framework for these basins highly tentative. Use of more and more high-precision instrumental facilities in recent times has provided significant information on time constraints and sediment provenances of these Proterozoic Indian basins (Chakraborti et al. 2007; Malone et al. 2008; Absar et al. 2009). The contribution by Bickford et al.(2011) is a welcome addition to this list. While these contributions have undoubtedly enriched Indian Precambrian geology by adding a new database,the conclusions drawn from this new database have swept the field observations into oblivion in many instances. The contribution of Bickford et al.(2011) is one such example. From U-Pb SHRIMP age determinations of zircon grains from interbedded tuffaceous layers and coarse arenaceous rocks,Bickford et al. (2011) constrained the time domain of Chhattisgarh depositional history (i.e., between ~1400 and ~1000 Ma) and concluded that there had been a change in provenance through Chhattisgarh depositional history.
    A Paleozoic subduction complex in Korea SHRIMP zircon U-Pb ages and tectonic implications [查看] SungWonKim、SanghoonKwon、M.Santosh、IanS.Williams、KeewookYi
    The Wolhyeonri complex in the southwestern margin of the Korean Peninsula is divided into three lithotectonic units: Late Paleozoic Zone I to the west, Middle Paleozoic Zone II in the middle and Early Paleozoic Zone III to the east. Zones II and III display characteristics of continental arc magmatic sequence.Zone II is dominated by mafic metavolcanics, whereas zone III is characterized by the presence of dismembered serpentinite bodies including chaotic mélange. These zones are proposed to have been formed in a convergent margin setting associated with subduction. Here we present zircon SHRIMP U–Pb ages from the various units within the Wolhyeonri complex which reveal the Paleozoic tectonic history of the region.The Late Carboniferous ages obtained from the main shear zone between the Wolhyeonri complex and the Paleoproterozoic Gyeonggi massif are thought to mark the timing of continental arc magmatism associated with the subduction process. In contrast, Zone I with Neoproterozoic arc magmatic remnants might indicate deposition in a forearc basin. The Wolhyeonri complex also preserves strong imprints of the Triassic collisional event, including the presence of Middle Triassic high-pressure metabasites and eclogites near the eastern boundary of the Zone III. These range of radiogenic ages derived from the Wolhyeonri complex correlate well with subduction and accretion history between the North and South China cratons. Similar geochronological features have also been indentified from the Qinling, Tongbai–Xinxian, and northern Dabie areas in east-central China. The existence of Paleozoic coeval subduction in East Asia prior to the Triassic collision is broadly consistent with a regional tectonic linkage to Gondwana
    30 million years of Permian volcanism recorded in the Choiyoi igneous province(W Argentina) and their source for younger ash fall deposits in the Paraná Basin SHRIMP U-Pb zircon geochronology evidence [查看] A.C.Rocha-Campos、M.A.Basei、A.P.Nutman、LauraE.Kleiman、R.Varela、E.Llambias、F.M.Canile、O.deC.R.daRosa
    We present four SHRIMP U–Pb zircon ages for the Choiyoi igneous province from the San Rafael Block, central–western Argentina. Dated samples come from the Yacimiento Los Reyunos Formation (281.4±2.5 Ma) of the Cochicó Group (Lower Choiyoi section: andesitic breccias, dacitic to rhyolitic ignimbrites and continental conglomerates), Agua de los Burros Formation (264.8±2.3 Ma and 264.5±3.0 Ma) and Cerro Carrizalito Formation (251.9±2.7Ma Upper Choiyoi section: rhyolitic ignimbrites and pyroclastic flows) spanning the entire Permian succession of the Choiyoi igneous province. A single zircon from the El Imperial Formation, that is overlain unconformably by the Choiyoi succession, yielded anearly Permian age (297.2±5.3 Ma),while themain detrital zircon population indicated an Ordovician age (453.7±8.1 Ma). The new data establishes a more precise Permian age (Artinskian–Lopingian) for the section studied spanning 30Ma of volcanic activity. Volcanological observations for the Choiyoi succession support the occurrence of explosive eruptions of plinian to ultraplinian magnitudes, capable of injecting enormous volumes of tephra in the troposphere–stratosphere. The new SHRIMP ages indicate contemporaneity between the Choyoi succession and the upper part of the Paraná Basin late Paleozoic section, fromthe Irati up to the Rio do Rasto formations, encompassing about 24 Ma. Geochemical data show a general congruence in compositional and tectonic settings between the volcanics and Paraná Basin Permian ash fall derived layers of bentonites. Thickness and granulometry of ash fall layers broadly fit into the depletion curve versus distance fromthe remote source vent of ultraplinian eruptions. Thus, we consider that the Choiyoi igneous provincewas the source of ash fall deposits inthe upper Permian section of the Paraná Basin. Data presented here allow a more consistent correlation between tectono-volcanic Permian events along the paleo-Pacificmargin of southwestern Gondwana and the geological evolution of neighboring Paleozoic foreland basins in South America and Africa.
    The complex age of orthogneiss protoliths exemplified by the Eoarchaean Itsaq Gneiss Complex (Greenland) SHRIMP and old rocks [查看] KenjiHorie、AllenP.Nutmanc、ClarkR.L.Friend、HiroshiHidaka
    Field studies integrated with cathodoluminescence petrography and SHRIMP U–Pb dating of zircons from >150 orthogneisses and metatonalites from the Eoarchaean Itsaq Gneiss Complex (southern West Greenland) shows that only a minority contain ≥3840Ma zircons, whereas the majority carry only younger ones. Rocks containing ≥3840Ma zircons vary from very rare single-phase metatonalites to morecommoncomplexly banded tonalitic migmatites. The former metatonalites have simple oscillatoryzoned ≥3840Ma zircon with limited recrystallisation and overgrowth, whereas the more common migmatites have much more complicated zircon populations with both ≥3840Ma and 3650–3600Ma oscillatory-zoned zircon, more extensive recrystallisation and widespread complex core-rim multiple growth relationships.With only 100–160ppm Zr in the tonalites and likely melt generation temperatures of >1000 ◦C, the experimentally determined zircon solubility–melt composition relationships established by other workers shows that the precursor melts to the Itsaq Complex tonalites were strongly undersaturated in zircon, thus any entrained xenocrystic zircon would have been rapidly dissolved. Therefore, the≥3840Ma oscillatory-zoned zircons crystallised out of tonalitic melt and gives magmatic age of the rock in which they occur.With an established igneous age of ≥3840Ma established from such relationships, we interpret the correlated variation between the field nature of these rocks and their zircon petrography/age structure as due to superimposition onto ≥3840Ma tonalite protoliths of variable amounts of heterogeneous strain, heterogeneous distribution of melt patches formed during in situ anatexis at up to ∼800 ◦C, plus granitic veining. This explains why geologically simple metatonalites have simple zircon populations,whereas complex orthogneisses have complex zircons. The large amount of integrated field, geochemical and zircon data rule out an alternative interpretation, that the ≥3840Ma zircons represent an igneous xenocrystic component, present in younger rocks to varying degrees. If this were true, then the structurally simple (less reworked) rocks should still display complex zircon populations.Gneisses with ≥3840Ma zircon are commonest on Akilia and neighbouring islands, in Itilleq fjord (∼65km east Akilia) and on the north of Ivisaartoq (∼150km northeast of Akilia). These include from Itilleq a 3891±6Ma gneissic tonalite (with minor neosome)—which is currently the oldest rock recognised in the Itsaq Gneiss Complex. Overall, the ≥3840Ma tonalites are a widespread and unevenly distributed in the Itsaq Gneiss Complex, and they are a volumetrically minor component compared with ~3800, 3750 and 3700Ma tonalite generations.Using the subset of our data covering Itilleq and the neighbouring fjords, migmatite samples with ≥3800Ma igneous zircon are mutually exclusive from migmatite samples with ~3700Ma igneous zircon.This suggests that prior to an amalgamation event followed by 3660–3600Mahigh-grade metamorphism,≥3840Ma tonalites might have resided in a terrane discrete from ∼3700Ma tonalites. This is in accord with interpretation of the non-migmatised part of the Complex in the Isua area, where a terrane of ~3800Matonalites with a minor associated≥3840Macomponent and a terrane with ~3700Matonalites were tectonically juxtaposed at ~3660 Ma.
    Petrological and in situ SHRIMP geochronological constraints on the tectonometamorphic evolution of the Committee Bay belt, Rae Province, Nunavut [查看] R.G.Berman、M.Sanborn-Barriea、N.Rayner、C.Carson、H.A.Sandeman、T.Skulski
    Petrology and in situ SHRIMP monazite geochronology provide new constraints on the polymetamorphic and polydeformational history of the Committee Bay belt, Rae Province, Nunavut, Canada. Rare ca. 2.58 Ga monazite inclusions in garnet porphyroblasts in the eastern part of the belt are interpreted to reflect regional contact metamorphism (M1) associated with voluminous 2.61–2.58 Ga plutonism. Monazite also occurs as ca. 2.35 Ga inclusions in andalusite in the central supracrustal belt, but is absent from the east. These data complement previous studies to the west, and suggest that the Committee Bay belt represents a ca. 2.35 Ga tilted crustal section that was tectonically thickened±imbricated during the Arrowsmith orogeny (D1–M2). Penetrative reworking of the belt occurred during a diachronous D2–M3 tectonometamorphic event which involved northwest-vergent thickening to depths of ∼5 kbar across most of the belt. Although D2 folds and fabrics across the belt are geometrically correlative, monazite is ~20 Myr older in the western migmatite domain (ca. 1.86 Ga) than in the western supracrustal belt (ca.1.84 Ga). This temporal difference is attributed to burial of more radiogenic lithologies in the migmatite domain. Tectonic thickening is attributed to ca. 1.87 Ga collision of Meta Incognita microcontinent with the southeastern flank of the Rae Province. The eastern supracrustal belt exposes similar amphibolitefacies (M3) northeast-striking, northwest-vergent D2 structures that are dated at 1.815 Ga. This younger reworkingmayreflect the influence of a major northward projecting promontory of the Superior Province which continued to affect the softened hinterland of the Rae Province during terminal collision at ca.1.82 Ga. The central Committee Bay belt records a younger, ca. 1.79 Ga structural overprinting (D3–M4) which involved dextral shearing and differential burial to temperatures above monazite growth in a gentle synformal structure formed during the final stages of amalgamation of Laurentia.
    Neoproterozoic SHRIMP U–Pb zircon ages of silica-rich Dokhan Volcanics in the North Eastern Desert, Egypt [查看] ChristophBreitkreuz、HassanEliwa、IbrahimKhalaf、KhaledElGameel、BenjaminBühler、SergeiSergeev、AlexanderLarionov、MamoruMurata
    Chronology of Neoproterozoic volcanosedimentary successions remains controversial for many regions of the Arabian–Nubian Shield, including the Dokhan Volcanics of NE Egypt. New U–Pb zircon SHRIMP ages have been obtained for 10 silica-rich ignimbrites and two subvolcanic dacitic bodies, mapped as Dokhan Volcanics, from the North Eastern Desert of Egypt. Crystallization ages range between 592±5 and 630±6Ma (Early Ediacaran). Apparently, the late consolidation of the Arabian–Nubian Shield was accompanied by the evolution of isolated volcanic centres and basin systems which developed during a period of approx. 40 Ma, independently in space and time and probably under changing tectonic regimes.The obtained age data together with other previously published reliable ages for Dokhan Volcanics suggest two main pulses of volcanic activity: 630–623Ma and 618–592 Ma. Five samples contain inherited zircons, with ages of 669, 715–746, 847 and 1530 Ma, supporting models that North Eastern Desert crust is mainly juvenile Neoproterozoic crust.
    Detrital zircon SHRIMP U–Pb age study of the Cordillera Darwin Metamorphic Complex of Tierra del Fuegosedimentary sources and implications for the evolution of the Pacific margin of Gondwana [查看] F.HERVE´、C.M.FANNING、R.J.PANKHURST、C.MPODOZIS、K.KLEPEIS、M.CALDERO´N、S.N.THOMSON
    Abstract: The Cordillera Darwin Metamorphic Complex in the southernmost Andes includes a basement of probable Palaeozoic age, a mid-Jurassic and younger volcano-sedimentary cover, and a suite of Jurassic granites, all of which were jointly metamorphosed during the Cretaceous. Detrital zircon ages presented here show that some of the amphibolite-facies metamorphic rocks previously mapped as basement have a Jurassic protolith. Overall the detrital zircon age patterns for samples of the Cordillera Darwin basement differ from those of the Madre de Dios Terrane of the western Patagonian Andes with which they had been correlated;instead, they are more comparable with those from the Eastern Andes Metamorphic Complex, which apparently developed in a passive margin setting. The paucity of Cambrian detrital zircons indicates that the meta-igneous basement of the Magallanes foreland basin of central and northern Tierra del Fuego was not the main source of detritus for the protolith of the Cordillera Darwin Metamorphic Complex. The possibility is envisaged that the Magallanes Fagnano transform fault boundary between the Scotia and South America plates resulted from reactivation of an older, pre-Jurassic suture zone between the basement terranes of north–central Tierra del Fuego and Cordillera Darwin.
    SHRIMP U-Pb dating of detrital zircons from the Sanbagawa Belt,Kanto Mountains,Japanneed to revise the framework of the belt [查看] YukiyasuTSUTSUMI、AtsushiMIYASHITA、KentaroTERADA、HiroshiHIDAKA
    Radiometric ages of detrital zircons in three samples of psammitic schists from the Sanbagawa Belt,Kanto Mountains,were obtained from the 238U/206Pb ratio and isotopic compositions of Pb using a Sensitive High Resolution Ion MicroProbe(SHRIMP II）.Most of the zircon ages cluster around Cretaceous,with a few ages corresponding to older zircons.The origins of the detrital zircons are mainly Cretaceous igneous rocks.The ages of the youngest zircons in samples AM48p,SnbE,and AM29p indicate Late Cretaceous time,and they are 78.8±1.3Ma,91.4±1.4Ma,and 95.3±1.5Ma,respectively.The age difference between the youngest detrital zircon age and white mica K-Ar age is 13 Myr.The Sambagawa Belt is believed to be a metamorphosed phase of the Chichibu Belt,which is a Middle Jurassic to earliest Cretaceous accretionary complex;however,the results of this study suggest that the protolith of the Sanbagawa belt was accreted in Late Cretaceous,similar to the Shimanto belt that runs parallel to the Sambagawa and Chichibu belts.
    U-Pb SHRIMP ages and tectonic setting of the Munster Suite of the Margate Terrane of the Natal Metamorphic Belt [查看] P.Mendonidis、R.A.Armstrong、G.H.Grantham
    New U–Pb Zircon SHRIMP ages of 1091±7.1 Ma and 1093.1±5.8 Ma have been determined for two discrete phases of the Munster Suite. The Munster Suite is a calc-alkaline mafic to intermediate suite of intrusive igneous rocks that form part of the southern-most tectonic Terrane of the Mesoproterozoic-aged Natal Metamorphic Belt. Previously published geochemical data indicate that the discrete phases of the Suite are consanguineous and that these rocks originated within an oceanic island arc environment. The new age determinations now show that the different phases are also coeval. Moreover, the ages also indicate that the intrusions were, within statistical error, coeval with S-type granites within the Terrane. This is interpreted to indicate that magmatic underplating provided both the magma by way of a number of progressively more evolved pulses to produce the Munster Suite, as well as the heat necessary for crustal melting to produce the S-type granites within an island arc environment. Therefore, these new age determinations indicate a period of crustal growth at circa 1090 Ma. This moreover, is a maximum constraint on the age of the northwardverging structures within the Margate Terrane.
    SHRIMP U-Pb zircon geochronology of high-grade rocks and charnockites from the eastern Amery Ice Shelf and southwestern Prydz Bay, East Antarctica Constraints on Late Mesoproterozoic to Cambrian tectonot [查看] XiaochunLiu、YueZhao、BiaoSong、JianLiu、JianjunCui
    The eastern Amery Ice Shelf (EAIS) and southwestern Prydz Bay are situated near the junction between the Late Neoproterozoic/Cambrian high-grade complex of the Prydz Belt and the Early Neoproterozoic Rayner Complex. The area contains an important geological section for understanding the tectonic evolution of East Antarctica. SHRIMP U–Pb analyses on zircons of felsic orthogneisses and mafic granulites from the area indicate that their protoliths were emplaced during four episodes of ca. 1380 Ma, ca. 1210–1170 Ma, ca. 1130–1120 Ma and ca. 1060–1020 Ma. Subsequently, these rocks experienced two episodes of high-grade metamorphism at N970 Ma and ca. 930–900 Ma, and furthermore, most of them (except for some from the Munro Kerr Mountains and Reinbolt Hills) were subjected to high-grade metamorphic recrystallization at ca.535 Ma. Two suites of charnockite, i.e. the Reinbolt and Jennings charnockites, intrude the Late Mesoproterozoic/Early Neoproterozoic and Late Neoproterozoic/Cambrian high-grade complexes at N955 Ma and 500 Ma, respectively. These, together with associated granites of similar ages, reflect late- to post-orogenic magmatism occurring during the two major orogenic events. The similarity in age patterns suggests that the EAIS–Prydz Bay region may have suffered from the same high-grade tectonothermal evolution with the Rayner Complex and the Eastern Ghats of India. Three segments might constitute a previously unified Late Mesoproterozoic/Early Neoproterozoic orogen that resulted from the long-term magmatic accretion from ca. 1380 to 1020 Ma and eventual collision before ca. 900 Ma between India and the western portion of East Antarctica. The Prydz Belt may have developed on the eastern margin of the Indo-Antarctica continental block, and the Late Neoproterozoic/Cambrian suture assembling Indo-Antarctica and Australo-Antarctica continental blocks should be located southeastwards of the EAIS–Prydz Bay region.
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