Tectonic position of the neoproterozoic gabbro-hyperbasite and gabbroid complexes of the Bayannur block of the Songino ledge, Central Asian orogenic belt

The Early Caledonian folded region of the southern framing of the Siberian platform between Dzavkhan and Tuva-Mongolian terranes contains blocks of Songino ledge crystalline rocks. In the Bayannur block of the southern part of the Songino ledge, Neoproterozoic (890–780 Ma) gneiss-migmatitic Bayannur and metaterrigenous-volcanogenic Kholbonure complexes are selected. The zone of their junction is traced by thrust structures, which are reflected in all structural and real complexes of the Bayannur block. At the junction zone, tectonic plates are presented. Their peculiarity is the presence of rocks formed before the formation of the gabbro-hyperbasite complex and post-thrust gabbroids and gabbro-diorites. The upper age limit of the interval of the formation of thrust faults is defined by Bayannur pluton granitoids with age of 790 ± 3 Ma and gabbroids and anorthosites of Onzula tectonic plate with ages of 785 ± 3 and 784 ± 3 Ma. The lower boundary of the formation of thrust faults is determined by the age of Bayannur complex ultrametamorphic granitoids (802 ± 6 Ma). Bodies of massive (post-thrust) pegmatoid gabbro and rocks of gabbro-hyperbasite complex with clearly manifested structures associated with the thrusts are recognized in the tectonic plate of the ridge overlooking the area of Mount San Node. These rocks are dated at 782 ± 2 and 806 ± 10 Ma, respectively (ID TIMS). The obtained data indicate that the post-thrust and before-thrust formations can not be parts of a single stratified complex. The latter can be considered as fragments of paleooceanic formations in the accretion structure of the Bayannur block of the Songino ledge.