Tectono-magmatic evolution of the Russian Altai, northwestern Central Asian orogenic belt

Abstract

The Russian Altai is a key area in the northwestern Central Asian Orogenic Belt (CAOB), comprising the northern segment of the Altai-Mongolian terrane (AM) in the south, the Gorny Altai terrane (GA) in the north and the Charysh-Terekta-Ulagan-Sayan (CTUS) suture zone between these two terranes. The widely exposed Paleozoic (meta-)sedimentary sequences and granitoid intrusions recorded multiple tectono-magmatic events in this area.

The Cambrian-Ordovician meta-sedimentary rocks from the northern AM were dominated by immature sediments possibly sourced from intermediate-felsic igneous rocks. Geochemical data show that the sediments were possibly deposited in a continental arc-related setting. Zircons separated from these rocks are mainly 566-475 Ma and 1015-600 Ma old. The similar source nature, provenance and depositional setting of these rocks to the counterparts from the Chinese Altai (i.e., the southern AM) imply that the whole AM possibly represents a coherent accretionary prism in the early Paleozoic. Meta-sedimentary rocks from the Ulagan Complex along the CTUS suture zone show indistinguishable geochemical compositions and detrital zircon populations to the counterparts in the AM. Accordingly, this complex possibly represents a fragmented tectonic unit of the AM.

The Cambrian to early Devonian (meta-)sedimentary rocks from the GA contain significant amount of 640-470 Ma detrital zircons which were potentially sourced from the Kuznetsk-Altai intra-oceanic island arc in the east of this terrane. The low abundance of 640-540 Ma zircons (5%) may attest that this arc was under a primitive stage in the late Neoproterozoic, when mafic igneous rocks dominated. However, the voluminous 530-470 Ma zircons (95%) suggest that this arc possibly evolved toward a mature one in the Cambrian to early Ordovician with increasing amount of intermediate-felsic igneous rocks, highlighting both crustal growth and recycling.

Geochemical modeling shows that the meta-sedimentary rocks from the Teletsk Complex along the CTUS suture zone possibly consisted of ca. 30-40 % sediments from the GA and ca. 60-70% from the source of the Ulagan Complex (i.e., part of the northern AM). The minimum depositional age of ca. 420 Ma indicates that the AM and GA possibly had amalgamated prior to the early Devonian. This conclusion is also supported by the occurrence of AM-derived ca. 2431-772 Ma detrital zircons in the early Devonian sedimentary sequences from the GA. Zircon U-Pb-Hf and whole-rock Sr-Nd isotopic data constrain that the granitoids from the Yaloman intrusive complex of the GA were emplaced in ca. 389-387 Ma and they were mainly derived from partial melting of Neoproterozoic juvenile crustal components. The occurrence of coeval igneous mafic enclaves implies that mantle-derived mafic melts played a crucial role in the formation of this complex. This study suggests that the Yaloman intrusive complex were formed by subduction of the Ob-Zasain Ocean.

Collectively, the AM and GA represented two separated subduction-accretion systems in the early Paleozoic, which were amalgamated prior to the early Devonian. Subsequently, both terranes were under the same tectonic setting, i.e., active continental margin, with emplacement of voluminous granitoids in the Devonian. Therefore, the Russian Altai documented complicated accretionary orogenesis in the northwestern CAOB.