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    30 million years of Permian volcanism recorded in the Choiyoi igneous province(W Argentina) and their source for younger ash fall deposits in the Paraná Basin SHRIMP U-Pb zircon geochronology evidence [查看] A.C.Rocha-Campos、M.A.Basei、A.P.Nutman、LauraE.Kleiman、R.Varela、E.Llambias、F.M.Canile、O.deC.R.daRosa
    We present four SHRIMP U–Pb zircon ages for the Choiyoi igneous province from the San Rafael Block, central–western Argentina. Dated samples come from the Yacimiento Los Reyunos Formation (281.4±2.5 Ma) of the Cochicó Group (Lower Choiyoi section: andesitic breccias, dacitic to rhyolitic ignimbrites and continental conglomerates), Agua de los Burros Formation (264.8±2.3 Ma and 264.5±3.0 Ma) and Cerro Carrizalito Formation (251.9±2.7Ma Upper Choiyoi section: rhyolitic ignimbrites and pyroclastic flows) spanning the entire Permian succession of the Choiyoi igneous province. A single zircon from the El Imperial Formation, that is overlain unconformably by the Choiyoi succession, yielded anearly Permian age (297.2±5.3 Ma),while themain detrital zircon population indicated an Ordovician age (453.7±8.1 Ma). The new data establishes a more precise Permian age (Artinskian–Lopingian) for the section studied spanning 30Ma of volcanic activity. Volcanological observations for the Choiyoi succession support the occurrence of explosive eruptions of plinian to ultraplinian magnitudes, capable of injecting enormous volumes of tephra in the troposphere–stratosphere. The new SHRIMP ages indicate contemporaneity between the Choyoi succession and the upper part of the Paraná Basin late Paleozoic section, fromthe Irati up to the Rio do Rasto formations, encompassing about 24 Ma. Geochemical data show a general congruence in compositional and tectonic settings between the volcanics and Paraná Basin Permian ash fall derived layers of bentonites. Thickness and granulometry of ash fall layers broadly fit into the depletion curve versus distance fromthe remote source vent of ultraplinian eruptions. Thus, we consider that the Choiyoi igneous provincewas the source of ash fall deposits inthe upper Permian section of the Paraná Basin. Data presented here allow a more consistent correlation between tectono-volcanic Permian events along the paleo-Pacificmargin of southwestern Gondwana and the geological evolution of neighboring Paleozoic foreland basins in South America and Africa.
    Evidence for subduction at 3.8 Ga Geochemistry of arc-like metabasalts from the southern edge of the Isua Supracrustal Belt [查看] F.E.Jenner、V.C.Bennett、A.P.Nutman、C.R.L.Friend、M.D.Norman、G.Yaxley
    We present new major and trace element data for rare examples of preserved pillow basalts from a locality of little studied ＞3800 Ma rocks at the southwestern edge of the Isua Supracrustal Belt (ISB), West Greenland.The 20 samples have 47.3–59.0 wt.% SiO2, 4.9–12.2 wt.% MgO, 6.5–11.2 wt.% CaO, 60–340 ppm Ni and 63–1094 ppm Cr, consistent with pre-metamorphic basaltic compositions. Nb/La(n) and Ti/Gd(n) are lower than primitive mantle compositions (0.1–0.6 and 0.3–0.8, respectively), with these ratios interpreted to reflect the composition of their source. Correlations of Nb/La with La/Sm, Ti/Gd and Zr/Sm and between Zr/Nb and Nb/Th are comparable to those of modern subduction related magmas, whereby slab melts enriched the overlying mantle wedge and HFSE were retained by Ti-rich phases. Trends in Ba/La vs. Ba/Nb are comparable to modern day arc related basalts, which reflect a sediment influence and may indicate that, at least early in the evolution of Eoarchean subduction zones, the subducted Eoarchean oceanic crust retained a pelagic sediment cover, with this cover influencing the chemistry of the slab-derived fluid. Three of the ISB pillow lavas show extreme enrichments in LREE (La/Sm(n)=1.9–2.6), large depletions in Nb, Ta, Zr and Hf relative to REE that are similar to rare high-K arc basalts from the Clark volcano of the presently active Tongan–Kermadec–New Zealand arc (Gamble, J.A., Christie, R.H.K., Wright, I.C., Wysoczanski, R.J., 1997. Primitive Krich magmas from Clark volcano, southern Kermadec arc: a paradox in the K–depth relationship. The Canadian Mineralogist, 35, 275–290). The compositions of these highly enriched basalts indicate that the mantle source region of the Isua basalts was occasionally overprinted by small-volume sediment dominated melts rather than hydrous fluids. The compositional affinities of these 3.8 Ga pillow basalts with modern island-arc basalts provide strong evidence for the role of slab fluids and melts in basalt genesis in the Eoarchean, further demonstrating arc-like signatures in the oldest rock sequences.The compositions of Archean mafic rocks including samples from the ISB that are interpreted to have a petrogenesis similar to that of modern island-arc basalts, are compared with Archean ‘non-arc’ rocks (i.e.plume-related komatiites, komatiitic basalts and associated tholeiitic basalts). The two groups are distinguished by differences in La/Sm, Gd/Yb, Nb/La, Ti/Gd, Ba/La, Ba/Nb and Al2O3/TiO2. Thus clear compositional distinctions between different basaltic types, attributed to arc and non-arc origins, are a feature of the rock record for at least the last 3.8 Ga.
    Constraining the age of the Iporanga Formation with SHRIMP U-Pb zirconImplications for possible Ediacaran glaciation in the Ribeira Belt, SE Brazil [查看] G.A.C.Campanha、M.S.Basei、C.C.G.Tassinari、A.P.Nutman、F.M.Faleiros
    The Ribeira belt in SE Brazil is a Neoproterozoic to Early Palaeozoic orogen, whose architecture and history is not yet fully understood. The depositional age of many of the sedimentary sequences in the Ribeira Belt remains unconstrained, and with debate concerning their depositional environment and tectonic setting. In this paper we present SHRIMP zircon U/Pb age constraints for one such problematic unit in the Ribeira Belt–the Iporanga Formation – and discuss the significance of this age with regards to the timing of Neoproterozoic glacial events in southeast Brazil.Using a felsic volcanic unit immediately under the Iporanga Formation and granite cobbles from breccias in its basal parts a reconnaissance SHRIMP U/Pb zircon maximum depositional age of 580 Ma is assigned for the base of this unit. This age is marginally younger than the 625–605 Ma ages for intrusions into the Lajeado and Ribeira subgroups, with which the Iporanga Formation is in tectonic contact. This indicates that the Lajeado and Ribeira subgroups are not stratigraphically equivalent to the Iporanga Formation, as thought previously by some workers. The maximum depositional age of 580 Ma also places a maximum time constraint on the tectonic juxtaposition of the Iporanga Formation with other supracrustal units, and on the greenschist facies metamorphism and isoclinal folding that affected it. The potential glacial origin for the Iporanga Formation, if correct, would place it in the late Ediacaran — provisionally equivalent to the Gaskiers glaciation.
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