Early Neoproterozoic crustal growth and microcontinent formation of the north-central Central Asian Orogenic Belt: New geological, geochronological, and Nd-Hf isotopic data on the Melange Zone within the Zavkhan terrane, western Mongolia

In this study, new geological, geochronological, geochemical, and Nd-Hf isotopic data are presented for the Melange Zone within the Zavkhan terrane, Mongolia, and the terrane structure, early Neoproterozoic continental crust growth, and microcontinent formation in the north-central part of the Central Asian Orogenic Belt (CAOB) are discussed. The Melange Zone separates high-grade complexes of the northwestern part of the Zavkhan terrane and unmetamorphosed Neoproterozoic Zavkhan Formation covered by Ciyogenian-Cambrian shelf deposits of the southwestern part. Zone consist of a lower-grade association of basalts, basaltic andesites, rarely felsic volcanic rocks, trondhjemites of the Kharuul Massif and variably metamorphosed from greenschist- to upper amphibolite-facies, and high-grade metamorphic rocks including quartzite-gneisses, hornblende schists, and amphibolites with relics of eclogite and blueschist-facies metamorphism assemblages. Emplacement of trondhjemites and gabbro dykes of the Kharuul Massif occurred at about 960-930 Ma. Geochemical, Nd whole-rock, and Hf-in-zircon isotopic data indicate an oceanic island arc setting for the lower-grade association. LA-ICP-MS dating and Hf-in-zircon data for detrital zircons from the quartzite-gneisses of the Melange Zone indicate that the sources comprise Palaeoproterozoic (ca. 2.02 and 2.48 Ga), Neoarchean (ca. 2.59 and 2.67 Ga), and Mesoarchean (ca. 2.8 and 3.0 Ga) magmatic and metamorphic crustal rocks as well as 2.6-2.5 Ga juvenile complexes. High-grade amphibolites show geochemical similarity to continental arc and within-plate basalts, and were formed from enriched mantle sources. Obtained combined with published data demonstrate that the Zavkhan terrane represents composite terrane composed of ca. 960-930 Ma island arc complex, and ca. 860-800 Ma active continental margin complexes in the north and reworked at ca. 800-720 Ma early Precambrian continental crust in the south, which are separated by a Melange Zone. We speculate that formation of the latest Mesoproterozoic-early Neoproterozoic island arc and active continental margin complexes in the north-central CAOB could be related to the assembly of the Rodinia supercontinent at ca. 1.1-1.0 Ga, which initiated subduction in Mirovian Ocean and led to the development of accretionary orogens around supercontinent margins. (C) 2020 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

Automated summary

This study presents new geological, geochronological, geochemical, and Nd-Hf isotopic data for the Melange Zone within the Zavkhan terrane in Mongolia and discusses the terrane structure, early Neoproterozoic continental crust growth, and microcontinent formation in the north-central part of the Central Asian Orogenic Belt. The findings imply that the Zavkhan terrane is a composite terrane consisting of different complexes separated by a Melange Zone.