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    Geochronologic Constraints on the Magmatic Underplating of the Gangdise Belt in the India-Eurasia CollisionEvidence of SHRIMP II Zircon U-Pb Dating [查看] DONGGuochenMOXuanxueZHAOZhidanGUOTieyingWANGLiangliangCHENTao
    Abstract Abundant small mafic intrusions occur associated with granitoids along the Gangdisê magmatic belt. In addition to many discrete gabbro bodies within the granitoid plutons, a gabbro-pyroxenite zone occurs along the southern margin of the Gangdisê belt to the north of the Yarlung Zangbo suture. The mafic intrusion zone spatially corresponds to a strong aeromagnetic anomaly, which extends ~1400 km. The mafic intrusions consist of intermittently distributed small bodies and dikes of gabbro and dolerite with accumulates of pyroxenite, olivine pyroxenite, pegmatitic pyroxenite and amphibolite. Much evidence indicates that the Gangdisê gabbro-pyroxenite assemblage is most likely a result of underplating of mantle-derived magma. Detailed field investigation and systematic sampling of the mafic rocks was conducted at six locations along the Lhasa-Xigazê segment of the mafic intrusive zone, and was followed by zircon SHRIMP II U-Pb dating. In addition to the ages of two samples previously published (47.0±1 Ma and 48.9±1.1 Ma), the isotopic ages of the remaining four gabbro samples are 51.6±1.3Ma, 52.5±3.0 Ma, 50.2±4.2Ma and 49.9±1.1Ma. The range of these ages (47−52.5 Ma) provide geochronologic constraints on the Eocene timing of magma underplating beneath the Gangdisê belt at ca. 50 Ma. This underplating event post-dated the initiation of the India-Eurasia continental collision by 15 million years and was contemporaneous with a process of magma mixing. The SHRIMP II U-Pb isotopic analysis also found several old ages from a few zircon grains, mostly in a range of 479−526 Ma (weighted average age 503±10 Ma), thus yielding information about the pre-existing lower crust when underplating of mafic magma took place. It is believed that magma underplating was one of the major mechanisms for crustal growth during the Indian-Eurasia collision,possibly corresponding in time to the formation of the 14−16 km-thick “crust-mantle transitional zone” characterized by Vp = 6.85−6.9 km/s. Key words: SHRIMP U-Pb dating, mafic intrusion, gabbro, underplating, the Gangdisê, Tibet
    Discovery of the Early Paleozoic post-collisional granites in northern margin of the Erguna massif and its geological significance [查看] WUGuangSUNFengyueZHAOCaishengLIZhitongZHAOAilinPANGQingbangLIGuangyuan
    Abstract The Luoguhe intrusion, located in Mohe County, Heilongjiang Province, is mainly composed of monzogranite, quartz diorite and granodiorite, with minor diorite, tonalite, quartz monzodiorite, quartz monzonite, syenogranite and alkali-feldspar granite. The intrusion can be divided into two lithological units, i.e. quartz diorite and monzogranite units, with affinities to high-K calc-alkaline series. The quartz diorite unit (SiO2: 54.79% ― 58.30%,Na2O/CaO: 0.79―1.53 and Shand index: 0.77―0.82) belongs to metaluminous rocks. And the monzogranite unit (SiO2:65.29%―66.45%, Na2O/CaO: 1.73―3.43 and Shand index <1.05) can be considered as weakly peraluminous rocks. The intrusion is characterized by high REE abundance (∑REE =180.2―344.3 µg/g), medium-strong negative Eu anomalies (δ Eu = 0.33―0.82), weak REE fractionation [(La/Yb)N=4.12―10.45], enrichments in Rb, Th, U, K, La, Ce, Nd, Hf,Zr and Sm, but strong depletions of Ba, Nb, Ta, Sr, P and Ti.These characteristics of major, REE and trace elements indicate that the intrusion was formed in a transitional tectonic setting from compressional to extensional regime, which can be classified as post-collisional granitoids. SHRIMP U-Pb zircon analyses yield ages of 517±9 and 504±8 Ma for the quartz diorite and monzogranite units, respectively. The discovery of Early Paleozoic post-collisional granites in the northern margin of the Erguna massif indicates that the northern branch of Paleo-Asian Ocean between Siberian plate and Erguna massif was closed in the Early Paleozoic and the Salair orogeny ended ca. 500 Ma ago. Keywords: post-collisional granite, SHRIMP U-Pb dating, Early Paleozoic, geochemistry, Erguna massif.
    An Early Aged Ophiolite in the Western Kunlun Mts.,NW Tibetan Plateau and Its Tectonic Implications [查看] XIAOXuchangWANGJunSULiJIWenhuaSONGShuguang
    A combined study of SHRIMP U-Pb dating ,trace elemnt and mineral inclusions on high-pressure metamorphic overgrowth zircon in eclogite from Qinglongshan in the Sulu terrane [查看] LIQiuliLIShuguangHOUZhenhuiHONGJi'anYANGWei
    Zircon U-Pb dating of jadeitite from the Syum-Keu ultramafic complex, Polar Urals, Russia Constraints for subduction initiation [查看] FancongMengAlexanderB.MakeyevJingsuiYang
    U-Pb SHRIMP data constraints on calc-alkaline granitoids with 1.3e1.6 Ga Nd TDM model ages from the central domain of the Borborema province, NE Brazil [查看] I.P.GuimarãesA.F.SilvaFilhoC.N.AlmeidaM.B.MacambiraR.Armstrong
    The Tabira, Itapetim and Timbaúba granitoids are intruded into metasedimentary sequences and Cariris Velhos (Tonian) orthogneisses from the Central Domain of the Borborema Province, NE Brazil. They have U-Pb SHRIMP ages of 593±7 Ma; 615±9 Ma and 616±5 Ma respectively. The studied granitoids have zircon cores inherited from the protholith, with a large number of analyses showing 206Pb/238U ages ranging from 950 to 1200 Ma. Oscillatory zoning typical of magmatic zircon is common, although it is faint in some inherited cores.The studied granitoids are calc-alkaline and show Nd TDM model ages ranging from 1.30 to 1.56 Ga and 3Nd (600 Ma) ranging from -2.40 to -5.34. These values are similar to those recorded in the country rocks. The lowest values of 3Nd (600 Ma) were recorded in enclaves of dioritic composition. Nd and U-Pb SHRIMP data suggest a significant participation of the metasedimentary rocks in the protholith of these granitoids. The Mesoproterozoic Nd TDM model ages recorded in the studied granitoids are interpreted as the result of a hybrid source involving melting of metagraywackes, metamafic rocks of Tonian ages and/or biotite e bearing orthogneisses (Cariris Velhos Orthogneisses). The resulted melting was modified by mingling with juvenile Brasiliano melts, diorite in composition.The Timbaúba granitoids intrusions are coeval with high-T metamorphism and flat-lying foliation forming event in an intracontinental setting, during the Brasiliano convergence and contractional deformation. The Itapetim Pluton was emplaced in the convergence - lateral escape setting and the Tabira granitoids were intruded after the flat-lying foliation event, representing sin transcurrent intrusions.Our data show that within the Central Domain of the Borborema Province, granitoids with similar petrographic and geochemical compositions can have distinct ages and be intruded in distinct tectonic regimes.
    Thermochronology of the PoSen complex, northern Vietnam Implications for tectonic evolution in SE Asia [查看] Pei-LingWangChing-HuaLoChing-YingLanSun-LinChungTung-YiLeeTranNgocNamYujiSano
    The PoSen complex, located closely adjacent to the southwestern margin of the Red River shear zone represents the uplifted basement of north Vietnam and may record the motion of the shear zone. However,its thermochronological history has not been fully examined yet. Here we applied U–Pb and 40Ar/39Ar dating methods to reveal its thermochronological history. U–Pb analysis of composite zircon grains by TIMS yielded an average age of 760 ± 25 Ma, clustering on the concordia line. Twelve SHRIMP U–Pb analyses also yielded a consistent result of 751 ± 7 Ma. Along with the geochemical features, the U–Pb dating results suggest the PoSen complex was a late Proterozoic magmatic complex, which could correspond to the Chengjiang orogeny, a widespread thermal event in southwest China. Results of 40Ar/39Ar dating of micas and K-feldspars were in the range of 36–30 Ma, revealing a rapid cooling and exhumation history of the PoSen complex during the late Paleogene. The time span of cooling and exhumation of the PoSen complex is slightly older than the main cooling phases of the Ailao Shan–Red River (ASRR) metamorphic massifs (28–17 Ma), but is synchronous with the early igneous activity stage in the eastern Indo-Asian collision zone of southeast China and north Vietnam. Owing to the ongoing debate about the initiation and offset of the ASRR shear zone, the tectonic force for the late Paleogene cooling of the PoSen complex is still inconclusive. The rapid exhumation of the PoSen complex could be in response to either the detachment of the Neo-Tethyan slab or a transpressional phase of continental subduction along the ASRR shear system in the eastern Indo-Asian collision zone.
    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.PimentelJoseneusaB.RodriguesMariaEmiliaS.DellaGiustinaSergioJungesMassimoMatteiniRichardArmstrong
    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.
    The North Patagonian batholith at Paso Puyehue (Argentina-Chile). SHRIMP ages and compositional features [查看] EugenioAragónAntonioCastroJuanDíaz-AlvaradoD.-Y.Liu
    SHRIMP dating of titanite from metasyenites in the Central Zone of the Limpopo Belt, South Africa [查看] M.J.RigbyR.A.Armstrong
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