Neoproterozoic paleogeography and displacement of the Mongolian blocks constrained by detrital zircon ages and Hf isotopic systematics (Zavkhan Block, W Mongolia)

Abstract

This study presents results of provenance analysis based on U–Pb ages and Hf isotopic composition of detrital zircons from (meta-)sedimentary rocks of two sections of the Zavkhan Block in W Mongolia. Detrital zircon age populations for most of the studied samples show a dominant Tonian–Cryogenian peak (c. 750 Ma), considered as detritus derived from the local rift-related magmatic sources. Some samples also have significant Neoarchean–Paleoproterozoic (c. 2.7–1.7 Ga) zircons sourced from cratonic basement. The large range of ε_Hf(t) values (–30 to +15) of the Tonian–Cryogenian zircons indicates significant mixing of mantle-derived magmas with mature crustal material, interpreted as a result of crustal melting during rifting. The Hf isotopic composition of the older zircons suggests contributions from both juvenile magmas and crustal recycling. Maximum depositional ages indicate that although dominantly Tonian–Cryogenian, the studied sequences also include subordinate older (pre-rift) stratigraphic members. The zircon age and Hf isotopic systematics of the younger part of studied rocks are interpreted to reflect sedimentation in Tonian–Cryogenian post-rift sequences evolving towards continental passive margin during the break-up of Rodinia. On the other hand, the pre-Tonian detritus from post-rift strata were dominantly recycled directly from the older pre-rift strata or indirectly from the Tonian anatectic magmatic rocks. These data are correlated with corresponding datasets from potential cratonic sources, which indicate close similarity and possible paleogeographic connection of the Zavkhan Block to the western Siberian Craton. Thus, it is proposed that the Zavkhan Block together with other Mongolian continental fragments were rifted from western Siberia in the Tonian–Cryogenian. The whole archipelago was subsequently dextrally translated along the Siberian margin into its Ediacaran–early Paleozoic position south of the Siberia Craton.