A Comparison of Nd Isotopes of Granitoids from the Central Asian Orogenic Belt and Qinling‐Dabie Orogen, and Implications for Understanding of Crustal Growth from Accretion to Collision

Abstract

A number of ancient continental crustal fragments has been described in Mongolia, Kyrgyzstan, and most of Kazakhstan (Badarch et al., 2002; Byamba, 2009; Degtyarev et al., 2017; Tomurtogoo, 2005). However, contextual interpretation and integration of the available geologic data are still need to be completed. The Gargan block makes up the basement of the Tuva-Mongolia terranes (TMT; e.g. Kuzmichev, 2015), which occupies a large region in southern Russia and northernmost Mongolia sharing a tectonic boundary with the Siberian Craton to the northeast (Fig. 1A). Geologic descriptions of the Gargan block has been restricted to southern Russia only and based primarily on lithostratigraphic comparisons, their possible southern extensions were inferred in Mongolia. Here we present field observations and new zircon U-Pbgeochronologic data from previously undated basement gneisses and the overlying Neoproterozoic meta-sedimentary rocks of southern part exposed in Mongolia (Fig. 1A and 1B). By constraining/refining the age of its basement, extent, and provenance in regional tectonic framework, we integrated the geologic data into a global geotectonic framework of the CAOB with a proposed Proterozoic to early Paleozoic tectonic evolution model of the Gargan block (Fig. 1C). Obtained data confirm the existence of NeoarcheanNeoproterozoic basement in the TMT and for the first time reveal multiple stages of Neoproterozoic metamorphic events at ~ 1 Ga and between ~ 814 and ~ 782 Ma. Older event may relate to an assembly of several of the ancient continental crustal fragments in Mongolia. The basement age comparison and detrital zircon provenance do not favor any known cratons in and around the CAOB suggesting that the Gargan block may have travelled alone in most of the Proterozoic. 2019 Second workshop of Project IGCP-662 04-10 July, 2019, Ulaanbaatar and Govi-Altai, Mongolia Abstract volume 29 Figure 1. Tectonic evolution model of the Gargan block(Bold et al., accepted). A) Archean-Proterozoic continental crustal fragments described in north-central Mongolia (after Badarch et al., 2002). Regional outline of the Tuva-Mongolia terranes: Basement ages are shown with respective references. B) Time-space diagram of the Gargan block. C) Tectonic evolution model of the Gargan block at ~ 2500, ~ 1000, ~ 780, and ~ 450 Ma.