Neoproterozoic‐Early Paleozoic Peri‐Pacific Accretionary Evolution of the Mongolian Collage System: Insights From Geochemical and U‐Pb Zircon Data From the Ordovician Sedimentary Wedge in the Mongolian Altai

Abstract

Neoproterozoic to early Paleozoic accretionary processes of the Central Asian Orogenic Belt have been evaluated so far mainly using the geology of ophiolites and/or magmatic arcs. Thus, the knowledge of the nature and evolution of associated sedimentary prisms remains fragmentary. We carried out an integrated geological, geochemical, and zircon U‐Pb geochronological study on a giant Ordovician metasedimentary succession of the Mongolian Altai Mountains. This succession is characterized by dominant terrigenous components mixed with volcanogenic material. It is chemically immature, compositionally analogous to graywacke, and marked by significant input of felsic to intermediate arc components, pointing to an active continental margin depositional setting. Detrital zircon U‐Pb ages suggest a source dominated by products of early Paleozoic magmatism prevailing during the Cambrian‐Ordovician and culminating at circa 500 Ma. We propose that the Ordovician succession forms an “Altai sedimentary wedge,” the evolution of which can be linked to the geodynamics of the margins of the Mongolian Precambrian Zavhan‐Baydrag blocks. This involved subduction reversal from southward subduction of a passive continental margin (Early Cambrian) to the development of the “Ikh‐Mongol Magmatic Arc System” and the giant Altai sedimentary wedge above a north dipping subduction zone (Late Cambrian‐Ordovician). Such a dynamic process resembles the tectonic evolution of the peri‐Pacific accretionary Terra Australis Orogen. A new model reconciling the Baikalian metamorphic belt along the southern Siberian Craton with peri‐Pacific Altai accretionary systems fringing the Mongolian microcontinents is proposed to explain the Cambro‐Ordovician geodynamic evolution of the Mongolian collage system.

Structural and Geochronological Constraints on Devonian Suprasubduction Tectonic Switching and Permian Collisional Dynamics in the Chinese Altai, Central Asia

“…The 2,000‐km‐long Char‐Erqis fault zone is considered by some authors as the southwestern limit of the Mongolian Collage (e.g., Xiao et al, ). The Mongolian Collage itself consists of the accreted Mongolian Precambrian blocks overthrust by late Proterozoic ophiolites and accretionary wedge during a main Early Cambrian accretionary event (Buriánek et al, ; Jiang et al, ). The Proterozoic wedge was intruded by an ~1,800‐km‐long Cambro‐Ordovician Ikh‐Mongol arc system (e.g., Janoušek et al, ).…”

“…Together with the Ikh‐Mongol arc system (Janoušek et al, ), it represents a huge early Paleozoic Altai accretionary system, rimming the active margin of the Precambrian Zavhan and Baydrag ribbon continents to the northeast (Figure ; Jiang et al, ; Soejono et al, ). This accretionary system developed above a long‐lasting and retreating Pacific‐type oceanic plate that produced Devonian to Carboniferous oceanic assemblages further south (Jiang et al, ; Nguyen et al, ; Xiao et al, ). In the Chinese Altai, the structurally deepest Ordovician sedimentary sequence of the wedge is called the Habahe Group that represents the oldest lithological unit in the region.…”

“…Here we focus on the Altai accretionary wedge, which represents a key component of the Mongolian Collage forming an eastern tract of the CAOB (Figure ; Şengör et al, ; Windley et al, ). This wedge represents an ~1,500 km long early Paleozoic volcano‐sedimentary accretionary prism rimming the active margin of the Mongolian Precambrian blocks (Jiang et al, ). The wedge was transformed into a mature orogenic system during the Devonian‐Carboniferous (420–360 Ma) Altai orogeny associated with extensive magmatism and synchronous metamorphism (e.g., Cai, Sun, Yuan, Long, et al, ; Jiang et al, ; Wang et al, ).…”

“…Simplified geological map of the Mongolian Collage System (modified from Jiang et al, ). Extent of the Altai accretionary wedge and the location of the Chinese Altai are outlined by dashed lines.…”

Revision of the Chinese Altai‐East Junggar Terrane Accretion Model Based on Geophysical and Geological Constraints

“…490 Ma and a broad minor peak in the interval of 1,200–700 Ma. Such a detrital pattern was interpreted as clastic material derived mainly from the northerly Proterozoic Lake Zone and margins of Precambrian Mongolian microcontinents (e.g., Janoušek et al, 2018; Jiang et al, 2017; Soejono et al, 2018). The difference in amplitude of the ca.…”