Early Paleozoic Granitoid Magmatism and Metamorphism on the Derba Microcontinent, Eastern Sayan Region: New Isotope-Geochronological Data

摘要

The Derba microcontinent with a thick metaterrige-nous-carbonate complex is among the largest structural elements of the Neoproterozoic accretion-collision belt at the southwestern and southern framings of the Siberian Craton. In the model proposed by N.A. Berzin and N.L. Dobretsov, the Derba and Sangilen terranes are considered microcontinents that was separated from the Siberian Craton or Rodinia in the Riphean. Based on indirect evidence, the hidden crystalline basement of this microcontinent is believed to be different from the Archean-Early Proterozoic basement of the Siberian Craton. Terrigenous-carbonate sediments could be deposited on a thin continental or suboceanic crust that included Proterozoic metaterrigenous-volca-nic complexes. The geodynamic history of the eastern Sayan region has been variously interpreted because of the lack of modern high-precision isotope-geochronological data on the age of metaterrigenous-carbonate sequences and the timing of metamorphism and collisional granitoid magmatism in the Derba microcontinent. Previously, the metaterrigenous-car-bonate sequences were considered Lower-Middle Proterozoic rocks; at present, they are considered Riphean formations. The age of specific (mainly, vein-type) granitoids of the Derba Complex was estimated with a significant error at ～650 Ma (U-Th-Pb allanite dating) or 360-930 Ma (maximum 360-460 Ma) (Pb-Pb zircon dating). The major part of K-Ar isotope dates (370-540 Ma) obtained for metamorphic rocks and granitoids in the late 1950s was attributed to the rejuvenation of Precambrian rocks owing to the Caledonian tectogenesis and magmatism.