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    Zircon ‘microvein’ in peralkaline granitic gneiss, western Ethiopia Origin, SHRIMP U-Pb geochr onology and trace element investigations [查看] TesfayeKebede、KenjiHorie、HiroshiHidaka、KentaroTerada
    Azircon‘microvein’ composed of several hundred crystals occurs in peralkaline granitic gneiss of western Ethiopian Precambrians.U–Pb ages and trace element (U, Th, Hf, Y, REE, P, Ca, Al, Fe, andMn) abundances of the ‘microvein’ and host granitic gneiss zircon were determined using a sensitive high mass resolution ion microprobe (SHRIMP) and electron probe microanalyzer (EPMA). Back-scattered electron (BSE) imaging of the ‘microvein’ zircon and host granite zircon, hereafter referred to as Type-I and Type-II zircon, respectively,reveal prevalent low and high mean atomic number contrast domains within individual crystals. Ubiquitous fluorite microinclusions in bright BSE domains of Type-I and less commonly, Type-II zircon suggest an early formation of fluorite that buffers F activity, causing zircon supersaturation and precipitation from a late-magmatic melt/fluid-enriched in high field strength elements (HFSEs) including Zr.The textural make up of the host peralkaline granitic gneiss and internal structural features of Type-I and Type-II zircon indicate that darkgrey BSE domains were formed by dissolution–reprecipitation owing to fluid infiltration and interaction with the primary zircon crystals.The bright and dark-grey BSE domains in Type-I zircon yield U–Pb ages of 779±69Ma and 780±35Ma, and similar domains in Type-II zircon dated at 778±49 Ma and 780±31 Ma, respectively. The primary and recrystallized domains in both zircon types have indistinguishable ages, suggesting initial crystallization shortly followed by fluid-driven alteration. The ages are identical,within analytical uncertainties, to the 776±12Ma zircon U–Pb emplacement age of a protolith of a leucocratic granitic gneiss determined from a different sample. Hence, zircon crystals forming ‘microvein’ and aggregate structures, the relatively high Th/U ratios (reaching up to 1.5) in the primary domains, high LREE/HREE, and the formation of Type-I and Type-II zircon during emplacement support a late-magmatic–hydrothermal origin. Extensive alteration of the host rock, recrystallization of young and non-metamict zircon corroborate the infiltration of or thomagmatic or hydrothermal fluids containing fluorides as a major constituent,which expelled a considerable amount of trace elements,namely,Hf, U, Th,Y, and theREEs, from the recrystallized domains of Type-I and Type-II zircon. The trace element depleted recrystallized domains characteristically contain microfractures apparently caused by differential volume expansion of the U and Th enriched primary domains or volume change during cation exchange reactions, and anomalously high Th/U ratios (∼0.5 to 1.0). Furthermore, the ca. 780–776Ma emplacement age of the protolith of the peralkaline granitic gneiss and late-stage orthomagmatic or hydrothermal activity shed light on the occurrence of older anorogenic granitoid magmatism and associated structures in western Ethiopian Precambrian terranes.
    U–Pb SHRIMP monazite ages of the giant Morro Velho and Cuiabá gold deposits, Rio das Velhas greenstone belt, Quadrilátero Ferrífero, Minas Gerais, Brazil [查看] L.M.Lobato、J.O.S.Santos、N.J.McNaughton、I.R.Fletcher、C.M.Noce
    U–Pb SHRIMP results of 2672±14 Ma obtained on hydrothermal monazite crystals, from ore samples of the giant Morro Velho and Cuiabá Archean orogenic deposits, represent the first reliable and precise age of gold mineralization associated with the Rio das Velhas greenstone belt evolution, in the Quadrilátero Ferrífero, Brazil. In the basal Nova Lima Group, of the Rio das Velhas greenstone belt, felsic volcanic and volcaniclastic rocks have been dated between 2792±11 and 2751±9 Ma, coeval with the intrusion of syn-tectonic tonalite and granodiorite plutons, and also with the metamorphic overprint of older tonalite–trondhjemite–granodiorite crust. Since cratonization and stable-shelf sedimentation followed intrusion of Neoarchean granites at 2612+3/−2 Ma,it is clear that like other granite–greenstone terranes in the world, gold mineralization is constrained to the latest stages of greenstone evolution.
    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.
    The 880-864 Ma granites of the Yenisey Ridge, western Siberian margin Geochemistry, SHRIMP geochronology, and tectonic implications [查看] V.A.Vernikovsky、A.E.Vernikovskaya、M.T.D.Wingate、N.V.Popov、V.P.Kovach
    The results of geological, petrological, geochemical, and geochronological investigations of the Teya, Kalami, andYeruda granites are discussed in the context of the formation the western margin of the Siberian craton and the Central Asian Orogenic Belt. These granites occur within Meso- to Neoproterozoic greenschist and amphibolite facies metamorphic rocks of the Central Angara terrane in the Yenisey Ridge region. Based on previous geochronology, these rocks have been implicated in alleged late Mesoproterozoic (Grenville-age) collision events along the western Siberian craton margin during the assembly of Rodinia. In all three plutons, the granites have mostly subaluminous—slightly peraluminous chemical compositions, and are characterized by variable normative corundum (1.1–3.3%) and Na2O/K2O (0.4–1.8). Sm–Nd and Rb–Sr isotopic data show that they could be formed from continental crust magmatic sources with Paleoproterozoic model ages (TNd(DM-2st) = 1.8–2.1 Ga). SHRIMP U–Pb zircon ages of 868–864 Ma were obtained for three samples from the Teya granite and 875 Ma for a sample from the Kalami granite. Together with the previously dated 878 Ma Yeruda granite, these intrusive rocks are the most ancient granites in the Transangara region of the Yenisey Ridge.It is likely that the Teya, Kalami, and Yeruda granites were part of the Central Angara terrane prior to its collision with Siberia at 760–720 Ma.
    SHRIMP U-Pb zircon and Sm-Nd garnet ages from the granulite-facies basement of SE Kenya evidence for Neoproterozoic polycyclic assembly of the Mozambique Belt [查看] CHRISTOPHA.HAUZENBERGER、HOLGERSOMMER、HARALDFRITZ、ANDREASBAUERNHOFER、ALFREDKRO¨NER、GEORGHOINKES、ECKARTWALLBRECHER、MARTINTHONI
    The Taita Hills–Galana River region is a key area to demonstrate the polycyclic nature of the Mozambique Belt in SE Kenya. On the basis of petrological and tectonic data, this area is composed of two different granulite-facies terranes, which are separated by the 20–30 km wide Galana Shear Zone. The Taita Hills and adjoining Sagala Hills exhibit a metamorphic overprint at 630–645 Ma, similar to areas in Tanzania.An emplacement age for the magmatic precursor rocks of 850–960 Ma was derived from zircon cores. Sm–Nd garnet–whole-rock analyses give an age of 585 Ma, interpreted as the cooling age after 630–645 Ma metamorphism. Nd crustal residence ages are between 1000 and 1500 Ma. The Galana Shear Zone east of the Taita Hills contains strongly deformed tonalitic migmatites with interlayered pegmatites that date a younger tectonometamorphic event at 560–580 Ma. East of the shear zone only a young metamorphic age of 550 Ma was found. The Nd model ages are c. 1500 Ma to c. 2900 Ma. In a continental configuration prior to Gondwana break-up our study area was located close to Madagascar, where several large shear zones are observed. One of these shear zones (Ranotsara Shear Zone) may be a continuation of the Galana Shear Zone.
    SHRIMP ion probe zircon geochronology and Sr and Nd isotope geochemistry for southern Longwood Range and Bluff Peninsula intrusive rocks of Southland, New Zealand [查看] RICHARDC.PRICE、TREVORR.IRELAND、ROLANDMAAS、RICHARDJ.ARCULUS
    Permian–Jurassic ultramafic to felsic intrusive complexes at Bluff Peninsula and in the southern Longwood Range along the Southland coast represent a series of intraoceanic magmatic arcs with ages spanning a time interval of 110 m.y. New SHRIMP U-Pb zircon data for a quartz diorite from the Flat Hill complex, Bluff Peninsula, yield an age of 259 ± 4 Ma, consistent with other geochronological and paleontological evidence confirming a Late Permian age.The new data are consistent with an age of c. 260 Ma for the intrusive rocks of the Brook Street Terrane. SHRIMP U-Pb zircon ages for the southern Longwood Range confirm that intrusions become progressively younger from east to west across the complex. A gabbro at Oraka Point (eastern end of coastal section) has an age of 245 ± 4 Ma and shows virtually no evidence of zircon inheritance. The age is significantly different from that of the Brook Street Terrane intrusives. Zircon ages from the western parts of the section are younger and more varied (203–227 Ma), indicating more complex magmatic histories. A leucogabbro dike from Pahia Point gives the youngest emplacement age of 142 Ma, which is similar to published U-Pb zircon ages for the Anglem Complex and Paterson Group on Stewart Island.
    SHRIMP Ages of Zircon in the Uppermost Tuff in Chattisgarh Basin in Central India Require ~500-Ma Adjustment in Indian Proterozoic Stratigraphy [查看] SarbaniPatranabis-Deb、M.E.Bickford、BarbaraHill、AsruK.Chaudhuri、AbhijitBasu
    The Chattisgarh Basin of east central India and many unmetamorphosed Proterozoic sedimentary basins of Peninsular India have been considered mostly Neoproterozoic (1000–545 Ma) in age. A newly recognized succession of rhyolitic ignimbrite, ash beds, and volcaniclastic sandstones near the top of the ∼2.2-km-thick sedimentary fill of the Chattisgarh Basin is a chronostratigraphic marker. Euhedral igneous zircons from these units give U-Pb SHRIMP ages of 990–1020 Ma, indicating that the basin fill beneath this marker horizon is pre-Neoproterozoic. On the basis of newly reported zircon ages of 16315 Ma from the basal part of the Vindhyan Basin and accepting the consensus that all virtually undeformed and unmetamorphosed craton-interior Proterozoic sedimentary basins in peninsular India are approximately coeval, we conclude that these basins are approximately Mesoproterozoic (1600–1000 Ma) in age. The reassigned age for these rocks (1650 to 900 or possibly ∼1000 Ma), up to 500 Ma in variance with the current notion (∼1100 to ∼518 Ma; Naqvi 2005), calls for a thorough rethinking of contemporary models concerning tectonics,sedimentation, and other geological activity that affected the Indian shield in the Proterozoic Era.
    Precambrian crustal contribution to the Variscan accretionary prism of the Kaczawa Mountains (Sudetes, SW Poland) evidence from SHRIMP dating of detrital zircons [查看] RyszardKryza、JanZalasiewicz、StanisławMazur、PawełAleksandrowski、SergeySergeev、AlexanderLarionov
    SHRIMP dating of detrital zircons from sandstones of the Gackowa Formation (Kaczawa Complex, Sudetes, SW Poland) indicates input from late (550–750 Ma) and early Proterozoic to Archaean sources (~2.0–3.4 Ga, the latter being the oldest recorded age from the Sudetic region). These dates preclude within-terrane derivation from seemingly correlatory acid volcanic rocks of early Palaeozoic age.Rather, they indicate provenance from Cadomian and older rocks that currently form part of other, geographically distant terranes; the most likely source identified to date is the Lusatian Block in the Saxothuringian Zone. Hence, the Gackowa Formation may be late Proterozoic rather than early Palaeozoic in depositional age, possibly coeval with the late Proterozoic (pre-Cadomian) greywackes of Lusatia, being subsequently tectonically interleaved with early Palaeozoic volcanic rocks into the Kaczawa accretionary prism during the Variscan orogeny. However, correlation with the lithologically similar early Ordovician Dubrau Quartzite of Saxothuringia, and so assignation to the early Paleozoic (post-Cadomian) rift succession deposited at the northern margin of Gondwana, cannot yet be precluded.
    Permian to Cretaceous polymetamorphic evolution of the Stewart River region, Yukon-Tanana terrane, Yukon, CanadaP-T evolution linked with in situ SHRIMP monazite geochronology [查看] R.G.BERMAN、J.J.RYAN、S.P.GORDEY、M.VILLENEUVE
    Textural relationships and the trace element chemistry of accessory minerals and garnet can provide the linkage between in situ SHRIMP ages and quantitative pressure–temperature data that is required to decipher complex polymetamorphic and polydeformational histories. Application of these methods to lower amphibolite facies rocks of the Stewart River area, Yukon (Canada) yields robust new constraints on the tectonic evolution of central Yukon Tanana Terrane (YTT). A TIMS U/Pb titanite age of 365–350 Ma is interpreted to date low-P metamorphism (M1) and D1 deformation associated with arc plutonism above an east-dipping subduction zone. Monazite inclusions in garnet porphyroblasts record a transition from low to high pressure (9 kbar and 600 C) at c. 239 Ma. These data help to establish a c.260–240 Ma tectonometamorphic event (M2–D2) reflecting intra-arc thickening during west-dipping subduction of Slide Mountain Ocean. Another transition from low- to high-P (M3–D3; 7.8 kbar and 595 C), dated by c. 195–187 Ma monazite, is interpreted to reflect the change from regional contact metamorphism during arc plutonism to internal duplication of YTT during initial collision of YTT with the North American craton. The Mt Burnham (north-eastern) region records a different history because of its proximity to later plutons and its late exhumation via extensional faulting. Monazite growth at 146 Ma dates 9 kbar metamorphism (M4), interpreted to reflect a previously unrecognized period of plutonism associated with auriferous quartz veins in the Klondike region. Monazite growth at 114–107 Ma reflects low-P (<4.6 kbar) contact metamorphism (M5) accompanying regional plutonism and extension.
    Oldest (3.5 Ga) zircons of the Urals U-Pb (SHRIMP-II) andT DM constraints [查看] Yu.L.Ronkin、S.Sindern、CorrespondingMemberoftheRASA.V.Maslov、D.I.Matukov、U.Kramm、O.P.Lepikhina
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