The early Paleozoic tectonic evolution of the Russian Altai: Implications from geochemical and detrital zircon U-Pb and Hf isotopic studies of meta-sedimentary complexes in the Charysh-Terekta-Ulagan-Sayan suture zone

The Charysh-Terekta-Ulagan-Sayan suture zone was regarded as a tectonic boundary separating two distinct subduction-accretion systems in the Central Asian Orogenic Belt (CAOB). In the north, magmatic arcs, such as the Gorny Altai terrane, formed in the southwestern periphery of the Siberian continent, whereas in the south, arc-prism systems, such as the Altai-Mongolian terrane, formed around the so-called Kazakhstan-Baikal composite continent with Gondwana affinity. When did these two systems amalgamate and whether the metamorphic complexes in the suture zone represent Precambrian micro-continental slivers are critical for our understanding of the accretionary orogenesis and crustal growth rate in the CADS. A combined geochemical and detrital zircon U-Pb-Hf isotopic study was conducted on the meta-sedimentary rocks from the Ulagan (also referred to Bashkaus) and Teletsk Complexes in the suture zone. The results indicate that the protoliths of these rocks were dominated by immature sediments deposited in a time period between 500 and 420 Ma. Thus, Precambrian micro-continental slivers may not exist in the suture zone and even in the whole Altai Orogen. The meta-sedimentary rocks from the Ulagan Complex yield geochemical compositions between those of common intermediate and felsic igneous rocks, implying that these kinds of rocks possibly served as dominant sources. Detrital zircons from this complex consist of a major population of ca. 620-500 Ma, a subordinate one of ca. 931671 Ma and rare grains of ca. 2899-1428 Ma. This age spectrum is compatible with the magmatic records of the western Mongolia. We propose that the Ulagan Complex possibly represents part of a subduction-accretion complex built upon an active continental margin of the western Mongolia in the early Paleozoic. The remarkable similarities in source nature, provenance, and depositional setting to the early Paleozoic meta-sedimentary rocks from the northern Altai-Mongolian terrane imply that the Ulagan Complex was possibly fragmented from this terrane. The meta-sedimentary rocks from the Teletsk Complex show similar detrital zircon populations but contain higher proportions of mafic sediments and have more depleted whole-rock Nd isotopic compositions. Our data suggest that the detritus mostly came from the same source as that for the Ulagan Complex but those from the Gorny Altai terrane also contributed. This implies that the Gorny Altai and Altai-Mongolian terranes possibly amalgamated prior to the early Devonian rather than in the middle Devonian to early Carboniferous as previously thought. Thus, the widespread Devonian to early Carboniferous magmatism within these two terranes was possibly generated in a similar tectonic setting. Moreover, the dominant Neoproterozoic to early Paleozoic detrital zircons from the Teletsk Complex yield largely varied Gamma(Hf)(t) values of -23.8 to 12.4, indicating that crustal growth and reworking are both important in the accretionary orogenesis. (C) 2016 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.