Mesoarchean bimodal volcanic rocks of the Onot greenstone belts, southwestern Siberian craton: Implications for magmatism in an extension/rift setting

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Abstract

This paper shows that Mesoarchean bimodal felsic and mafic metavolcanic rocks of the Onot greenstone belt (Sharyzhalgay uplift, the southwestern margin of the Siberian craton) formed as a result of rifting of Paleoarchean continental crust. Evidence for this comes from U-Pb zircon ages, whole-rock geochemical and Nd isotope data and Hf-in-zircon isotope data from a metarhyolite-basaltic unit. The over 80 km long Onot greenstone belt consists of tectonic sheets composed of metasedimentary-volcanogenic and tonalite-trondhjemite-granodiorite (TTG) rocks thrusted to the southwest onto high-grade rocks of the Kitoy terrane. The greenstone sequence includes a bimodal metavolcanic unit overlain by metavolcanogenic-sedimentary units. The felsic volcanic rocks formed at ~2.88 Ga and were metamorphosed at 660–690 °C and ~6 kbar during the Late Paleoproterozoic. The high-Fe metarhyolites are enriched in the REE and HFSE and are compositionally similar to felsic volcanic rocks of Archean greenstone belts with lower La/Yb and Zr/Y (FIIIa and FIIIb types after Hart et al., 2004), and A-type granitoids. The high- and low-Ti metabasalts (amphibolites) possess geochemical affinities to both subduction-related basalts and continental flood basalts contaminated by crustal material. The metarhyolites yielded negative εNd(t) values (−3.8 to −0.8). Their zircons yielded negative to positive εHf(t) values (−8.5 to +1.1). The isotopic data suggest melting of heterogeneous crust, consisting of Paleoarchean plagiogneisses of the TTG basement complex and a juvenile source. The contribution of a mafic source resulted in enrichment of metarhyolites in FeO, MgO and TiO2. The crustal melting probably occurred at shallow depths at 2–4 kbar and at high temperatures higher than 900 °C. The coeval formation of Onot basalts and low-P - – high-T felsic melts could be triggered by decompressional mantle upwelling and subsequent extension and thinning of the subcontinental lithosphere. Worldwide, evidence for extension/rifting comes from Meso-Neoarchean mafic dikes and coeval intra-plate bimodal basalt-rhyolite volcanism. The events of extension/rifting and the formation of subcontinental lithospheric mantle suggest formation of stable and rheologically rigid continental plates.

Original languageEnglish
Article number105731
Number of pages16
JournalPrecambrian Research
Volume343
DOIs
Publication statusPublished - Jul 2020

Keywords

  • Geochemistry
  • Nd and Hf isotopes
  • Onot greenstone belt
  • Stabilization of subcontinental lithospheric mantle
  • U-Pb zircon age
  • A-TYPE GRANITES
  • TRACE-ELEMENT GEOCHEMISTRY
  • HF ISOTOPE COMPOSITION
  • U-PB
  • SHARYZHALGAI-UPLIFT
  • CONTINENTAL-CRUST
  • LU-HF
  • PILBARA CRATON
  • SM-ND
  • RE-OS

OECD FOS+WOS

  • 1.05.KY GEOLOGY

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