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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] DONGGuochen、MOXuanxue、ZHAOZhidan、GUOTieying、WANGLiangliang、CHENTao

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] WUGuang、SUNFengyue、ZHAOCaisheng、LIZhitong、ZHAOAilin、PANGQingbang、LIGuangyuan

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] XIAOXuchang、WANGJun、SULi、JIWenhua、SONGShuguang

Abstract The early aged ophiolites have attracted attention of many geologists in recent decades, because the early aged ophiolites can provide the information about the ancient oceanic processes relevant to the evolution of plate tectonics in the early period of the earth, and also concern such problems as whether there existed a “Proto-Tethys” and the break-up and convergence of the Rodinian Supercontinent. This paper reveals a definite complete ophiolite of Neoproterozoic-Early Paleozoic, named Kuda ophiolite in the western Kunlun Mts., NW Tibetan Plateau, and reports the recent reasonable SHRIMP zircon U-Pb ages of 510±4 Ma, and 502±13 Ma for the cumulates of the Kuda ophiolite, using the most powerful dating tool, the SHRIMP-II. The geochemical and geochronology data integrating with the geological setting suggest that the Kuda ophiolite might have formed in an archipelago oceanic basin, not in a vast ocean, the so-called “Proto-Tethys”, and was tectonically emplaced during the Early Paleozoic.

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] LIQiuli、LIShuguang、HOUZhenhui、HONGJi'an、YANGWei

Abstract Methods recently advanced for discrimination on the genesis of metamorphic zircon,such as annalysis of mineral inclusions and trace elements,provide us powerful means to distinguish zircon overgrowth during high-pressure metamorphism.Zircons in ultrahigh-pressure eclogite from Qinglongshan in the Sulu terrane were studied by the SHRIMP U-Pb method in combining with trace element and mineral inclusion analyses. No inherited core was identified in the annalyzed zircons by means of cathodoluminescence image.The occurrence of high-pressure metamorphic mineral inclusions in zircon,such as garnet,omphacite,rutile,and the flat HREE pattern in zircon indicate that the zircon formed at high-pressure metamorphic conditions.Therefore,a weighted average U-Pb age of 227.4 ±3.5Ma obtained from such a kind of zircon is interpreted to represent the timing of peak metamorphism for the Qinglongshan eclogite

Zircon U-Pb dating of jadeitite from the Syum-Keu ultramafic complex, Polar Urals, Russia Constraints for subduction initiation [查看] FancongMeng、AlexanderB.Makeyev、JingsuiYang

High-pressure, low-temperature metamorphic rocks record subduction of oceanic crust and sediments into the mantle, but the timing of subduction initiation is commonly difficult to determine. However,the appearance of fluid activity in the mantle wedge above a subduction zone can be correlated with incipient subduction. Here we describe jadeitite veins up to 5 m wide, which are hosted in serpentinites of the Syum-Keu ultramafic complex, Polar Urals, Russia. The jadeitites consist of both jadeite (86–88 mol% jd) and omphacite (54–75 mol% jd). Multi-stage growth of the jadeite is indicated by variations in color and structure, such as growth zoning and the distribution of fluid inclusions. We consider that the jadeitite veins formed from Na–Al–Si-rich fluids under HP/LT conditions during active subduction, and that the source material was subducted ocean crust. SHRIMP U–Pb dating of zircons from the jadeitite yielded a concordant weighted mean 206Pb/238U age of 404 ± 7 Ma, which we interpret as the age of peridotite serpentinization and jadeitite formation in the suprasubduction zone mantle. This age indicates that subduction was initiated about 40 Ma earlier than peak metamorphism (360–355 Ma) of eclogite in the Polar Urals.

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.Guimarães、A.F.SilvaFilho、C.N.Almeida、M.B.Macambira、R.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-LingWang、Ching-HuaLo、Ching-YingLan、Sun-LinChung、Tung-YiLee、TranNgocNam、YujiSano

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.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.

The North Patagonian batholith at Paso Puyehue (Argentina-Chile). SHRIMP ages and compositional features [查看] EugenioAragón、AntonioCastro、JuanDíaz-Alvarado、D.-Y.Liu

New U-Pb SHRIMP zircon ages together with a geochemical and isotopic (Sr and Nd) study have been carried out on a hornblende gabbro, a biotite granodiorite, and host andesitic rocks of the North Patagonian batholith at Paso Puyehue, close to the Argentinean-Chilean border. The results yield ages of 18±1 Ma for the gabbros and 12.4±0.3 Ma for the granodiorites. An age of 125±2 Ma was obtained for the host volcanic rock, and an age of 22±2 Ma for the basaltic trachy-andesite. The intrusive suite is composed of dominant hornblende-biotite granodiorites and subordinate hornblende gabbros and diorites. These form a typical metaluminous calc-alkaline series. The time-isotopic magmatic evolution of plutonic and volcanic rocks shows an early Eocene beginning of disparate isotopic ratios as a consequence of coeval primitive and more crustal compositions with no spatial constraints. This disparate isotopic assemblage is also coeval with the presence of the Aluk-Farallon-SAM triple junction at this latitude and the shift of the calc-alkaline magmatism from the batholith axis to the back-arc. Considerations are made for the possibility of a Paleogene slab window by slab detachment, and the development of a new subduction front since the beginning of Nazca Plate development at the 23 Ma major ocean plate rearrangement.

SHRIMP dating of titanite from metasyenites in the Central Zone of the Limpopo Belt, South Africa [查看] M.J.Rigby、R.A.Armstrong

SHRIMP dating of titanite from metasyenites in the Central Zone of the Limpopo Belt yields a mean 207Pb/206Pb age of 2010.3 ±4.5 Ma calculated from 23 analyses. This age, combined with petrographic and field observations, suggests the metamorphism in the syenites occurred during Palaeoproterozoic event.

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