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    Geodynamic problems of the junction between the Agin andArgun zones of Transbaikalia Evidence from U-Pb SHRIMP dating of rocks ofthe Tsugol gabbro-plagiogranite massif [查看] D.A.Lykhin、S.L.Presnyakov、G.E.Nekrasov、S.V.Ruzhentsev、B.G.Golionko、Yu.S.Balashova
    Fluid flow during exhumation of deeply subducted continental crust zircon U-Pb age and O-isotope studies of a quartz vein within ultrahigh-pressure eclogite [查看] Y.-F.ZHENG、T.-S.GAO、Y.-B.WU、B.GONG、X.M.LIU
    Quartz veins in high-pressure to ultrahigh-pressure metamorphic rocks witness channelized fluid flow that transports both mass and heat during collisional orogenesis. This flow can occur in the direction of changing temperature/pressure during subduction or exhumation. SHRIMP U-Pb dating of zircon from a kyanite-quartz vein within ultrahigh-pressure eclogite in the Dabie continental collision orogen yields two age groups at 212 ± 7 and 181 ± 13 Ma, which are similar to two groups of LA-ICPMS age at 210 ± 4 and 180 ± 5 Ma for the same sample. These ages are significantly younger than zircon U-Pb ages of 224 ± 2 Ma from the host eclogite. Thus the two age groups from the vein date two episodes of fluid flow involving zircon growth: the first due to decompression dehydration during exhumation, and the second due to heating dehydration in response to a cryptic thermal event after continental collision. Laser fluorination O-isotope analyses gave similar d18O values for minerals from both vein and eclogite, indicating that the vein-forming fluid was internally derived. Synchronous cooling between the vein and eclogite is suggested by almost the same quartz–mineral fractionation values, with regularly decreasing temperatures that are in concordance with rates of O diffusion in the minerals. While the quartz veining was caused by decompression dehydration at 700–650 C in a transition from ultrahigh-pressure to high-pressure eclogite-facies retrogression, the postcollisional fluid flow was retriggered by heating dehydration at 500 C without corresponding metamorphism. In either case, the kyanite–quartz vein formed later than the peak ultrahigh-pressure metamorphic event at the Middle Triassic, pointing to focused fluid flow during exhumation rather than subduction. The growth of metamorphic zircon in the eclogite appears to have depended on fluid availability, so that their occurrence is a type of geohygrometer besides geochronological applicability to dating of metamorphic events in orogenic cycles.
    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.
    Early Archean to Middle Jurassic Evolution of the Korean Peninsula and Its Correlation with Chinese CratonsSHRIMP U-Pb Zircon Age Constraints [查看] HeejinJeon、MoonsupCho、HyeoncheolKim、KenjiHorie、HiroshiHidaka
    U-Pb zircon ages of tuffs and sandstones of the Daedong Supergroup (Bansong and Nampo groups) in the Korean Peninsula were determined using a sensitive high-resolution ion microprobe (SHRIMP) in order to constrain their age of sedimentation and to unravel discrete geologic events as recorded in detrital zircons. The ages of four tuffaceous samples from the Bansong Group imply that the Daedong Supergroup formed at ca. 187–172 Ma in association with the Early-Middle Jurassic orogeny. These data are in marked contrast with paleomagnetic arguments suggesting that the Bansong and Nampo groups are precollisional Early-Middle Triassic deposits that are correlative with the North and South China blocks, respectively. Detrital zircons of the Daedong Supergroup define seven age components: (1)Early-Middle Archean (3.64–2.97 Ga), (2) Late Archean–middle Early Proterozoic (2.63–2.33 Ga), (3) late Early Proterozoic(1.98–1.75 Ga), (4) Middle-Late Proterozoic (1.2–0.6 Ga), (5) Devonian (400–355 Ma), (6) Early Permian (280–255 Ma), and (7) Middle Triassic–Early Jurassic (240–180 Ma). These age distributions, together with available geochronological data, suggest that crustal growth of the Korean Peninsula has continued since ca. 3.6 Ga and culminated at ca. 2.5 and 1.9–1.8 Ga. Major age populations of detrital zircons of the Bansong and Nampo groups are similar, except for the presence of Middle-Late Proterozoic ages in the latter. Inasmuch as these ages are characteristic for the South China Block, the Gyeonggi massif, or at least the local source of the Nampo Group, is most likely a correlative of the South China Block.
    Dating Prograde Amphibolite and Granulite Facies Metamorphism Using In Situ Monazite U-Pb SHRIMP Analysis [查看] C.J.Forbes、D.Giles、P.G.Betts、R.Weinberg、P.D.Kinny
    In situ SHRIMP U-Pb analysis of monazite grains from pelites within an early-formed high-temperature shear zone in the southern Broken Hill Block, Australia, has been used to constrain the timing of prograde amphibolite facies and peak granulite facies metamorphism within the terrain. Geochronological analyses from grains inhabiting texturally distinct localities revealed two distinct age populations, ca. 1619 and ~1600 Ma. The older age was obtained from grains that occur as inclusions completely enclosed within coarse-grained K-feldspar and garnet grains, which evidently armored the monazite inclusions against resetting during younger deformation and metamorphic events. The ca.1619-Ma monazite population occurs as part of an amphibolite facies inclusion assemblage hosted within the peak granulite facies mineral assemblage and constrains the timing of prograde amphibolite facies metamorphism within the Broken Hill Block. The younger ∼1600-Ma monazite age population was from grains within the pervasive shear fabric of the pelites or adjacent to fractures or grain boundaries in the matrix. The ∼1600-Ma age population represents either a later stage of monazite growth or isotopic resetting during deformation along the high-temperature shear zone at peak granulite facies conditions.
    Age and sedimentary provenance of the Southern Granulites, South India U-Th-Pb SHRIMP secondary ion mass spectrometry [查看] AlanS.Collins、M.Santosh、I.Braun、C.Clark
    Southern India lies at a junction in the Gondwana-forming orogenic belts, between the East African Orogen and the Kuunga Orogen. It contains voluminous high-grade metasedimentary gneisses that make up an important component of the record of collision and amalgamation of Gondwana. Here we present U-Pb Secondary Ion Mass Spectrometry (SIMS) isotopic data from detrital zircon cores from throughout southern India that demonstrate dominant Neoarchaean to Palaeoproterozoic age components that are incompatible with the known ages of potential southern and central Indian source regions. The original sediments to the Trivandrum Block gneisses were deposited between∼1900 and∼515 Ma, whereas a sample from the Achancovil Unit, and possible also a sample from the Madurai Block, were deposited in Neoproterozoic times. We speculate that these rocks broadly correlate with southern and western Malagasy metasedimentary rocks (including the Itremo and Molo Groups) and formed an extensive basin (or basins) that lay on the west side (present orientation) of the Neoproterozoic continent Azania. In addition, metamorphic zircon from four samples yielded an age of 513±6Ma that is interpreted as dating high-grade metamorphism throughout much of the Southern Granulite Terrane.
    Adjacent terranes with ca. 2715 and 2650 Ma high-pressure metamorphic assemblages in the Nuuk region of the North Atlantic Craton, southern West Greenland Complexities of Neoarchaean collisional orogeny [查看] AllenP.Nutman、ClarkR.L.Friend
    In the gneiss complex of the Nuuk region of the North Atlantic Craton in southern West Greenland, terrane juxtaposition was followed by Neoarchaean folding under amphibolite facies conditions, with widespread low-pressure recrystallisation (5 kbar and 550–700 ◦C). The complex metamorphic overprinting requires that the P–T history related to actual terrane assembly has to be extracted from very small relicts of older metamorphic assemblages combined with the U/Pb dating, petrology and geochemistry of metamorphic zircons.In the south of the region, the Færingehavn terrane (Eoarchaean orthogneisses) is tectonically overlain by a supracrustal package of amphibolites and paragneisses (ca. 2840 Ma felsic volcano-sedimentary protoliths). This package is juxtaposed against a higher tectonic level represented by the Tre Brødre terrane (2825 Ma orthogneisses) and the Tasiusarsuaq terrane(2920–2810 Ma orthogneisses). The terranes were assembled by 2710–2720 Ma, as shown by dating of granitic sheets intruded along the terrane boundary mylonites. In the Færingehavn terrane and in the overlying 2840 Ma supracrustal package, relict early high-pressure assemblages (12–8 kbar, 700–750 ◦C) are clinopyroxene + garnet + plagioclase + quartz±hornblende in mafic rocks and garnet + kyanite + rutile bearing assemblages in paragneisses. These are commonly replaced by lower pressure assemblages (7–5 kbar) such as cordierite±sillimanite±garnet in paragneisses and hornblende + plagioclase + quartz±garnet in mafic rocks.In situ partial melting took place during both low- and high-pressure regimes. Metamorphic zircon in the high- and low-pressure assemblages yields dates of ca. 2715 Ma, mostly with errors of <±5 Ma, thereby demonstrating rapid decompression at high temperatures.Zircons in the overlying Tre Brødre and Tasiusarsuaq terranes show little response to the ca. 2715 Ma event supporting structural interpretations that they were at a higher structural level at ca. 2715 Ma. High temperature recrystallisation continued after ca. 2715 Ma, as demonstrated by intergrowth of sillimanite with 2680 Ma metamorphic zircon.The Kapisilik terrane (3050–2960 Ma orthogneisses) and another supracrustal assemblage of amphibolites and ca. 2800 Ma quartzo-feldspathic metasedimentary rocks are exposed north of the Færingehavn terrane and within fold cores along its western margin, are bounded by folded Neoarchaean mylonites. The Kapisilik terrane and this supracrustal assemblage include high-pressure metamorphic remnants in amphibolites and metasediments (metamorphic segregations with garnet + clinopyroxene and kyanite,respectively) that formed at ca. 2650 Ma. These remnants are overprinted by high temperature, but lower pressure metamorphic events at ca. 2630, 2610 and 2580 Ma. Thus remnants of early metamorphism
    2635 Ma amphibolite facies gold mineralisation near a terrane boundary (suture) on Storø, Nuuk region,southern West Greenland [查看] AllenP.Nutmana、OleChristiansen、ClarkR.L.Friend
    Gold on Storø (in the Nuuk region of southern West Greenland) occurs in a slice of strongly deformed, amphibolite facies,Neoarchaean quartzo-feldspathic metasedimentary rocks and amphibolites in tectonic contact with the Eoarchaean Færingehavn terrane and the Meso- to Neoarchaean Akia terrane. The gold is associated with either l¨ollingite (FeAs) + arsenopyrite or pyrrhotite, which belong with the amphibolite facies silicate mineral assemblages. Zircons were U/Pb dated by SHRIMP to constrain the timing of mineralisation. The 2700–2830 Ma oscillatory-zoned, high Th/U volcano-sedimentary zircon as whole grains and cores are interpreted to give the maximum depositional age of the rocks hosting the gold. A granite sheet barren of gold that cuts gold mineralised rocks gives an age of ca. 2550 Ma, which is the absolute minimum age of mineralisation. Metamorphic zircons associated with auriferous arsenopyrite (particularly close to secondary l¨ollingite), pyrrhotite and in the coexisting amphibolite facies silicate minerals were dated in situ, in polished thin sections. These have lower average Th/U and yielded dates of ca. 2635 Ma, which is interpreted as the age of the gold-bearing mineral assemblages and hence probably the mineralisation.Supracrustal packages containing abundant quartzo-feldspathic sedimentary rocks with depositional ages of ca. 2800 Ma and metamorphosed at 2650–2600 Ma also occur as folded tectonic intercalations along the western edge of the Færingehavn terrane south of Storø. Moreover, in the 3080–2960 Ma Kapisilik terrane to the north and east of Storø, high-grade metamorphic events also occurred between 2650 and 2600 Ma. However, the Eoarchaean Færingehavn terrane to the south and east of Storø shows widespread zircon growth and recrystallisation during 2720–2700 Ma regional metamorphism, but not at 2650–2600 Ma. The Storø gold prospect is thus located near an important tectonic terrane boundary (cryptic suture?) between domains with different Neoarchaean metamorphic history. Storø mineralisation at ca. 2635 Ma probably occurred during deformation of this boundary shortly after it was created, and at Storø maybe was focussed in an area of low strain in a major antiform. The general tectonic frameworks for ca. 2635 Ma gold mineralisation on Storø and in the Yilgarn Craton are similar. These gold provinces are compared and contrasted, and the likely reason why so much less gold occurs in the Nuuk region is discussed.
    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.
    Two distinct Precambrian terranes in the Southern Prince Charles Mountains,East Antarctica SHRIMP dating and geochemical constraints [查看] E.V.Mikhalsky、B.V.Beliatsky、J.W.Sheraton、N.W.Roland
    The Southern Prince Charles Mountains (SPCM) are mostly occupied by the Archaean Ruker Terrane. The Lambert Terrane crops out in the northeastern part of the SPCM. New geochemical and zircon U–Pb SHRIMP ages for felsic orthogneisses and granitoids from both terranes are presented. Orthogneisses from the Ruker and Lambert terranes differ significantly in their major and trace-element compositions. Those from the Ruker Terrane comprise two distinct groups: rare Y-depleted and abundant Y-undepleted. U–Pb isotopic data provide evidence for tonalitetrondhjemite emplacement at 3392T9 and 3377T9 Ma, pre-tectonic granite emplacement at 3182T9 Ma, metamorphism(?) at c.3145 Ma, and thermal events at c. 1300(?) and 626T51 Ma. The Lambert Terrane orthogneisses probably originated in a continental magmatic arc. Zircon dating shows a very different geological history: pre-tectonic granitoid emplacement at 2423T18 Ma, metamorphism at 2065T23 Ma, and syn-tectonic granitoid emplacement at 528T6 Ma, syn-tectonic pegmatite emplacement at 495T18 Ma. The Lambert Terrane can be correlated with neither the Meso- to Neoproterozoic Beaver Terrane in the Northern PCM, which differs in isotopic composition, nor with the Archaean Ruker Terrane, which differs in both granitoid chemical composition and the timing of major geological events. It represents a Palaeoproterozoic orogen which experienced strong tectonic re-activation in Pan-African times. The Lambert Terrane has some geochronological features in common with the Mawson Block, which comprises south Australia and some areas in East Antarctica.
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