The Eastern Boundary of the Baikal Collisional Belt: Geological, Geochronological, and Nd Isotopic Evidence

摘要

New geological, geochronological, and Nd isotopic data are reported for the rocks occurring at the interlluve of the Barguzin, Nomama, and Katera rivers, where the main structural elements of the Early Paleozoic collisional system have been established. The strike-slip and thrust Tompuda-Nomama and Barguzin boundary sutures separate the Svetlaya and the Katera zones of the Baikal-Muya Belt from the Barguzin terrigenous-carbonate terrane. The age estimates of syntectonic (prebatholithic) gneissic granite and gabbrodior-ite intrusive bodies (469 ± 4 and 468 ± 8 Ma, respectively) coincide with the age of collisional events in the Ol'khon, Southwest Baikal, and Sayan regions (480-470 Ma). A linear zone with zonal metamorphism and granite-gneiss domes dated at 470 Ma is revealed in the allochthonous fold-nappe packet of the Upper Riphean Barguzin Formation. This zone of Caledonian remobilization marks the collisional front between the Riphean structural units of the Barguzin Terrane consolidated 0.60-0.55 Ga ago and the Baikal-Muya Belt. New data allow us to recognize this zone as the northeastern flank of the Baikal Collisional Belt. The Nd isotopic data for the reference igneous complexes of the collisional zone indicate that the Late Riphean juvenile crust was involved in the Ordovician remobilization in the zone of conjugation of the consolidated Baikalian structural elements at the northeastern flank of the Baikal Belt and likely was a basement of the entire Barguzin Terrane or, at least, its frontal portion. The lateral displacements of the terranes to the northeast during the Early Ordovician collision were constrained by the rigid structural framework of the Baikalides in the Muya segment of the Baikal-Muya Belt, where the Riphean blocks were involved in strike-slip faulting and the Vendian-Cambrian superimposed basin underwent deformation. Finally, it may be concluded that the Early Ordovician was an epoch of collision, complex in kinematics, between heterogeneous blocks of the continental crust: the Baikalides of the Baikal-Muya Belt and polycyclic Barguzin-Vitim Superterrane.