Timing of formation and tectonic setting of Paleozoic granitoids in the eastern Mongol–Okhotsk Belt: Constraints from geochemical, U[sbnd]Pb, and Hf isotope data

Abstract

The Mongol–Okhotsk Belt is one of the major structural elements in East Asia. However, many aspects of the evolution of this structure from the oceanic domain to the suture zone remain unresolved. The most controversial of these aspects are the timing and polarity of subduction processes as well as the timing of the final closure of the Mongol–Okhotsk Ocean. Here, we present whole-rock geochemical and UPb geochronological data for granitoids in the eastern Mongol–Okhotsk Belt to constrain their timing of formation and tectonic setting. Our new data suggest three stages of granitic magmatism in the eastern part of the Mongol–Okhotsk Belt: Cambrian (ca. 511 Ma), Middle Devonian (ca. 393 Ma), and Late Permian–Early Triassic (257–250 Ma). Cambrian (511 ± 4 Ma) plagiogranites in the Galam Terrane have adakitic geochemical signatures, and εHf(t) values of zircons from these granites vary mostly from +8.8 to +14.8 with Hf model ages (tHf(DM)) ranging from 0.77 to 0.53 Ga. These plagiogranites most likely represent the roots of an island arc that formed above a NW-dipping (present-day coordinates) subduction zone beneath the edge of the Siberian Craton. Middle Devonian (393 ± 7 Ma) granites from the Selemdzha Terrane, as well as Late Permian–Early Triassic (257 ± 4, 253 ± 3, 253 ± 2, 252 ± 3, and 250 ± 4 Ma) granodiorites and granites from the Selemdzha and Tokur terranes, have supra-subduction whole-rock geochemical signatures. Slight negative and positive zircon εHf(t) values of −6.1 to +8.4, tHf(DM) ages of 1.1–0.6 Ga, and tHf(C) ages of 1.4–0.6 Ga suggest that the studied rocks contain a continental crustal component. The Middle Devonian and late Permian–Early Triassic granodiorites and granites formed either in a continental island arc or on an active continental margin. During the Middle Devonian and at the late Permian–Early Triassic boundary, the Mongol–Okhotsk oceanic crust was subducting beneath the edge of the Siberian Craton. Thus, middle Cambrian, Middle Devonian, and Permian–Triassic granitoids in the eastern Mongol–Okhotsk Belt reflect three stages of subduction, meaning that the accretionary wedge terrane assemblages started to form during the Cambrian then continued during the Devonian and at the Permian–Triassic boundary, but was not started at Jurassic as previously proposed.