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Biostratigraphy versus isotope geochronology: Testing the Urals island arc model

Geoscience Frontiers 2019年第1期10卷 119-125页

作者： Svetlana Tessalina Cristina Talavera Michael E.Pritchin victor puchkov John de Laeter Centre for Isotope Research Curtin University The Institute for Geoscience Research (TIGeR) Curtin University School of GeoSciences Institute of Geology and Geochemistry Urals Branch of Russian Academy of Sciences Institute of Geology Ufimian Scientific Investigation Centre

Formation of the Urals volcanic-hosted massive sulphide(VHMS) deposits is considered to be related with the intra-oceanic stage of island arc(s) development in the Upper Ordoviciane Middle Devonian based on the biostratigraphic record of ore-hosting sedimentary rocks. However, the direct Re-Os dating of four known VHMS systems in the Urals gives significantly younger Re-Os isochron ages ranging from355 ± 15 Ma up to 366 ± 2 Ma. To address this discrepancy, we performed SHRIMP U-Pb dating on zircons extracted from rhyodacites(Eifelian biostratigraphic age of 393 -388 Ma) from the footwall of the Alexandrinka VHMS deposit which has a Re-Os isochron age of sulphides of 355 ± 15 *** ^(206) Pb/^(238) U mean age of 374 ± 3 Ma(MSWD ? 1.4 and probability ? 0.11) is considered to be the crystallisation age of the host volcanic rock. This age is ca. 15 Ma younger than the Eifelian(393 -388 Ma)biostratigraphic age and overlaps the Frasniane Famennian boundary(372 ± 2 Ma), characterised by the final stages of Magnitogorsk Arc e East European continent collision. Such an inconsistency with geochronological age may be due to a reburial of conodonts during resedimentation as a result of erosion of older rocks in younger sedimentary sequences.

关键词： Urals Volcanic-hosted massive sulphide deposits U-Pb dating Geochronology

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Timanide(Ediacaran–Early Cambrian) Metamorphism at the Transition from Eclogite to Amphibolite Facies in the Beloretsk Complex,SW-Urals,Russia

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Journal of Earth Science 2019年第6期30卷 1144-1165页

作者： Arne P.Willner Michael Gopon Johannes Glodny victor N.puchkov Hans-Peter Schertl Institute of Geology Mineralogy and GeophysicsRuhr-University BochumD-44780 BochumGermany College of Earth Science and Engineering Shandong University of Science and TechnologyQingdao 266590China Deutsches Geoforschungszentrum GFZ、Telegrafenberg 14473 Potsdam、Germany Institute of Geology and Geochemistry Uralian Branch of Russian Academy of Science、620016 EkaterinburgRussia

The Beloretsk Metamorphic Complex in the SW Urals formed at a convergent eastern margin of Baltica during the Neoproterozoic–Early Cambrian Timanide *** comprises three major units with lenses of facies-critical metabasites within metasedimentary rocks:A lowermost eclogite unit,an intermediate garnet amphibolite unit and an upper amphibolite-greenschist ***(P)-temperature(T)-paths of four rocks from the two lowermost units were determined mainly by PT pseudosection techniques showing similar clockwise loops at different peak metamorphic,watersaturated conditions:A phengite-bearing eclogite shows peak PT conditions of 16.5–18.5 kbar/525–550℃(stage I)followed by stage II at 11.5–13.0 kbar/585–615℃.A garnet amphibolite from the intermediate unit yields lower peak conditions of 11.7–14.5 kbar/480–510℃(stage I)followed by stage II at 9.5–11.0 kbar/535–560℃.However,a granite gneiss in the eclogite unit shows similar maximum pressures as the eclogite,but higher temperatures at 15.6–16.2 kbar/660–675℃,whereas a garnet micaschist contains comparable high pressure relicts,but underwent an advanced midcrustal reequilibration at 7.5–9.0 kbar/555–610℃.We dated the eclogite by a 7-point Rb/Sr mineral isochron(phengite,omphacite,apatite)at 532.2±9.1 Ma interpreted as age of crystallisation of the eclogitic peak PT *** age is the youngest compared to the known Timanide metamorphic and magmatic ages.

关键词： Urals eclogite high pressure metamorphism Timanide orogeny Rb-Sr dating PT pseudosections PT-path

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