Tectonic Position of the Neoproterozoic Gabbro-Ultrabasite and Gabbroid Complexes of the Bayan Nuur Block of the Songino Ledge, Central Asian Orogenic Belt

The Early Caledonian folded region of the southern framing of the Siberian platform, between the Zavkhan and Tuva-Mongolian terranes, contains blocks of the Songino ledge crystalline rocks. In the Bayan Nuur block (southern part of the Songino ledge), the Neoproterozoic (890–780 Ma) Bayan Nuur gneiss-migmatitic and Kholbo Nuur metaterrigenous-volcanogenic complexes are distinguished. The zone of their junction is traced by thrust structures, which are reflected in all structural-material complexes of the Bayan Nuur block. There are tectonic slabs in the junction zone. Their peculiarity is the presence of rocks formed before the formation of thrust gabbro-ultrabasite complex and post-thrust gabbroids and gabbro-diorites. The upper limit of the thrust formation interval is defined by Bayan Nuur pluton granitoids having the age of 790 ± 3 Ma and by gabbroids and anorthosites of Onzula tectonic plate giving the ages of 785 ± 3 and 784 ± 3 Ma. The lower limit of the thrust formation interval is determined by the age of the Bayan Nuur ultrametamorphic granitoid complex (802 ± 6 Ma). Bodies of massive (post-thrust) pegmatoid gabbros and rocks of gabbro-ultrabasite complex possessing clearly manifested thrust-associated structures are recognized in the tectonic slab of the ridge reaching the area of Mount Sant Uul. These rocks are dated at 782 ± 2 and 806 ± 10 Ma, respectively (ID-TIMS). The data indicate that the post-thrust and pre-thrust units cannot be parts of a united layered complex. In turn, pre-thrust units can be considered as paleooceanic fragments in the accretionary structure of the Bayan Nuur block, Songino ledge.