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    The tectonic evolution of the Neoproterozoic Brasília Belt, central Brazil, based on SHRIMP and LA-ICPMS UePb sedimentary provenance data A review [查看] MárcioM.Pimentel、JoseneusaB.Rodrigues、MariaEmiliaS.DellaGiustina、SergioJunges、MassimoMatteini、RichardArmstrong
    The Brasília Belt is a Neoproterozoic orogenic belt in central Brazil, developed between the Amazon, São Francisco-Congo and Paranapanema cratons. It consists of a thick sedimentary pile, made up of several stratigraphic units, which have been deformed and metamorphosed along the western margin of the São Francisco Craton during the Brasiliano orogenic cycle. In the western part of the belt, a large, juvenile magmatic arc is exposed (the Goiás Magmatic Arc), consisting of calc-alkaline plutonic suites as well as volcano-sedimentary sequences, ranging in age between ca. 860 and 650 Ma. Regional-scale, westdipping thrusts and reverse faults normally mark the limits between the main stratigraphic units, and clearly indicate tectonic transport towards the east. The age of deposition and tectonic significance of the sedimentary units comprising the Brasília Belt have been a matter of continuous debate over the last three decades. In the present paper, recent provenance data based on LA-ICPMS UePb ages of detrital zircon grains from several of these units, are reviewed and their significance for the age of deposition of the original sediments and tectonic evolution of the Brasília Belt are discussed.The Paranoá, Canastra and the Vazante groups, in the central part of the Belt, have detrital zircon grains with ages older than ca. 900 Ma and are interpreted as representative of the passive margin sequence deposited on the western margin of the São Francisco Craton. On the other hand, samples from the Araxá and Ibiá groups have a much younger population of Neoproterozoic zircon grains, as young as 650 Ma, and have been interpreted as syn-orogenic (fore-arc?) deposits. The Bambuí Group, exposed in the easternmost part of the belt and covering large areas of the São Francisco Craton also has young zircon grains and is interpreted, at least in part, as the foreland basin of the Brasília Belt.
    U-Pb SHRIMP Dating of Detrital Zircons from the Nzilo Group(Kibaran Belt)Implications for the Source of Sediments and Mesoproterozoic Evolution of Central Africa [查看] JosephW.Kokonyangi、AliB.Kampunzu、RichardArmstrong、MakotoArima、MasaruYoshida、TakamotoOkudaira
    The Kibaran Supergroup, a 13000-km-long belt of metasedimentary and igneous rocks in the southeastern Congo, is in a critical location between the Congo Craton (sensu stricto) and the Tanzania-Bangweulu Block. Understanding its tectonic evolution will shed much-needed light on the amalgamation history of sub-Saharan Africa. This study presents U-Pb SHRIMP age data for 150 detrital zircons from four metasedimentary formations of the Nzilo Group,the middle lithostratigraphic unit within the Kibaran Supergroup in Katanga Province. These samples yielded dates between 3214±7 and 1329±32 Ma. Prismatic Mesoproterozoic detrital zircons (dated at 1499±49 to 1329±32 Ma, with a peak at 1380 Ma) occur in all samples and are inferred to be derived from the Mitwaba orthogneisses,which intrude the Kiaora Group, the oldest lithostratigraphic unit of the Kibaran Supergroup. More than three-quarters of the zircon population is composed of recycled grains, yielding dates between 2434±5 and 1696±18 Ma, with peaks at 2050 and 1850 Ma. These peaks overlap with the timing of geological events in the adjacent Paleoproterozoic Bangweulu Block, including the Ubendian-Usagaran belts. Archean zircons were not found in the oldest Nzilo units and are restricted to the youngest Nzilo rocks, where they formonly aminor component (＜6%) of the zircons recovered.The Archean zircons are inferred to be derived from the unroofing of successively older crust in the East African lithosphere. U-Pb data support field observations and indicate that the Nzilo Group sediments postdate the ~1.38-Ga Kibaran syn-D1 igneous rocks and received some detritus from them. The lithostratigraphic and geochronological data, coupled with the regional geology, indicate that a substantial portion of the Nzilo Group detritus came from interbasinal reworking of the underlying Kiaora Group and its associated 1.38-Ga orthogneisses, with a significant contribution from the Tanzania-Bangweulu Block. The data support a previously proposed subductional model for the Kibaran belt and constrain the paleotectonic environment during the deposition of the Nzilo Group.
    Evolution of polycyclic basement complexes in the Arac¸ua´ı Orogen based on U-Pb SHRIMP data Implications for Brazil-Africalinks in Paleoproterozoic time, [查看] CarlosM.Noce、AntˆonioCarlosPedrosa-Soares、LuizCarlosdaSilva、RichardArmstrong、DaniellePiuzana
    Paleoproterozoic basement of the Arac¸ua´ı Orogen is composed of amphibolite and granulite facies orthogneiss units known as Mantiqueira and Juiz de Fora complexes. Six U–Pb SHRIMP crystallization ages for the Mantiqueira Complex range from 2137±19 to 2041±7 Ma. Studied samples are characterized by the abundance of inherited Archean zircon grains, TDM model ages ranging from 2.9 to 3.2 Ga, and strongly negative εNd(t) (−9 to −13). Gneiss protholits were mainly generated by partial melting of older continental material, and the Mantiqueira Complex is related to active margin and syn-collisional magmatism. High-grade gneiss samples of the Juiz de Fora Complex yields crystallization ages of 2119±16 and 2084±13, and inherited zircon grains are absent.This plutonic unit much probably evolved within an oceanic magmatic arc setting, or on a very stretched continental crust.The Mantiqueira and Juiz de Fora complexes, together with the Kimezian basement of the West Congo Belt, were parts of a Paleoproterozoic orogenic system disrupted and deeply reworked during the evolution of the Arac¸ua´ı–West Congo Orogen.Neoproterozoic metamorphic overprint is dated at ca. 590–574 Ma.
    Zircon sensitive high mass-resolution ion microprobe UPb and fission-track ages for gabbros and sheeted dykes of the Taitao ophiolite, Southern Chile, and their tectonic implications [查看] RYOANMA、RICHARDARMSTRONG、TORUDANHARA、YUJIORIHASHI、HIDEKIIWANO
    The Late Miocene–Pliocene Taitao ophiolite is composed of a complete sequence of classic oceanic lithosphere and is exposed approximately 50 km southeast of the Chile triple junction, where the Chile Ridge subducts beneath the South American Plate. Gabbros and ultramafic rocks are folded into a complex pattern, but only evidence for block rotation has been reported in the overriding sheeted dyke complex. In the present study,sensitive high mass-resolution ion microprobe U–Pb and fission-track dating methods were applied to zircon crystals separated from gabbros and sheeted dykes. Two sets of radiometric ages of gabbros range between 5.9±0.4 and 5.6±0.1 Ma. These ages coincide within their error ranges and imply rapid intrusion and cooling of gabbros. The U–Pb age of a dacite dyke intruded into the sheeted dyke complex was determined to be 5.2±0.2 Ma.These data indicate that the magmas of the Taitao ophiolite were formed during the 6 Ma Chile Ridge collision event and emplaced in a shorter period than previously thought. A short segment of the Chile Mid-oceanic Ridge must have been emplaced during the 6 Ma event.
    SHRIMP U–Pb zircon ages of pyroclastic rocks in the Bansong Group,Taebaeksan Basin, South Korea and their implication for the Mesozoic tectonics [查看] RaeheeHan、Jin-HanRee、Deung-LyongCho、Sung-TackKwon、RichardArmstrong
    The Bansong Group (Daedong Supergroup) in the Korean peninsula has long been considered to be an important time marker for two wellknown orogenies, in that it was deposited after the Songnim orogeny (Permian–Triassic collision of the North and South China blocks) but was deformed during the Early to Middle Jurassic Daebo tectonic event. Here we present a new interpretation on the origin of the Bansong Group and associated faults on the basis of structural and geochronological data. SHRIMP (Sensitive High-Resolution Ion MicroProbe) U–Pb zircon age determination of two felsic pyroclastic rocks from the Bansong Group formed in the foreland basin of the Gongsuweon thrust in the Taebaeksan Basin yielded ages of 186.3T1.5 and 187.2T1.5 Ma, respectively, indicating the deposition of the Bansong Group during the late Early Jurassic.Inherited zircon component indicates ca. 1.9 Ga source material for the volcanic rocks, agreeing with known basement ages. The Bansong Group represents syntectonic sedimentation during the late Early Jurassic in a compressional regime. During the Daebo tectonic event, the northeast-trending regional folds and thrusts including the Deokpori (Gakdong) and Gongsuweon thrusts with a southeast vergencedeveloped in the Taebaeksan Basin. This is ascribed to deformation in a continental-arc setting due to the northwesterly orthogonal convergence of the Izanagi plate on the Asiatic margin, which occurred immediately after the juxtaposition of the Taebaeksan Basin against the Okcheon Basin in the late stage of the Songnim orogeny. Thus, the Deokpori thrust is not a continental transform fault between the North and South China blocks,but an “intracontinental” thrust that developed after their juxtaposition.
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