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.

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The Chandman massif, a typical structure of the Mongolian Altai, consists of a migmatitemagmatite core rimmed by a lower-grade metamorphic envelope of andalusite and cordieritebearing schists. The oldest structure in the migmatite-magmatite core is a sub-horizontal migmatitic foliation S1 parallel to rare granitoid sills. This fabric is folded by upright folds F2 and transposed into a vertical migmatitic foliation S2 that is syn-tectonic, with up to several tens of metres thick granitoid sills. Sillimanite-ilmenite-magnetite S1 inclusion trails in garnet constrain the depth of equilibration during the S1 fabric to 6-7 kbar at 710-780°C. Reorientation of sillimanite into the S2 fabric indicates that the S1-S2 fabric transition Accepted ArticleThis article is protected by copyright. All rights reserved. occurred in the sillimanite stability field. The presence of cordierite, and garnet rim chemistry point to decompression to 3-4 kbar and 680-750°C during development of the S2 steep fabric, and postectonic andalusite indicates further decompression to 2-3 kbar and 600-650°C. Widespread crystallization of post-tectonic muscovite is explained by the release of H 2 O from crystallizing partial melt. In the metamorphic envelope the subhorizontal metamorphic schistosity S1 is heterogeneously affected by upright F2 folds and axial planar subvertical cleavage S2. In the north, the inclusion trails in garnet are parallel to the S1 foliation, and the garnet zoning indicates nearly isobaric heating from 2.5-3 kbar and 500-530°C. Cordierite contains crenulated S1 inclusion trails and has pressure shadows related to the formation of the S2 fabric. The switch from the S1 to the S2 foliation occurred near 2.5-3 kbar and 530-570°C; replacement of cordierite by fine-grained muscovite and chlorite indicates further retrogression and cooling. In the south, andalusite containing crenulated inclusion trails of ilmenite and magnetite indicates heating during the D2 deformation at 3-4 kbar and 540-620°C. Monazite from a migmatite analyzed by LASS yielded elevated HREE concentrations. The grain with the best-developed oscillatory zoning is 356±1. Pb), considered to date the crystallization from melt in the cordierite stability around 680°C and 3.5 kbar, whereas the patchy BSE-dark domains give a date of 347±4.2 [±7] Ma interpreted as recrystallization at subsolidus conditions. The earliest subhorizontal fabric is associated with the onset of magmatism and peak of P-T conditions in the deep crust, indicating important heat input associated with lower crustal horizontal flow. The paroxysmal metamorphic conditions are connected with collapse of the metamorphic structure, an extrusion of the hot lower crustal rocks associated with vertical magma transfer and a juxtaposition of the hot magmatite-migmatite core with supracrustal rocks. This study provides information about tectono-thermal history and time scales of horizontal flow and vertical mass and heat transfer in the Altai orogen. It is shown that, similar to collisional orogens, ...

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Orthogneiss and meta-rhyolite bodies from different crustal levels of the Tseel Terrane in the Mongolian Altai were examined using multidisciplinary approach involving structural geology, whole-rock geochemistry and U-Pb zircon geochronology. The orthogneisses form sheet-like bodies parallel with dominant sub-horizontal metamorphic fabric which was heterogeneously verticalized along localized zones of deformation at boundaries of lower and middle crustal domains. Three samples of orthogneisses yielded Late Devonian LA-ICP-MS U-Pb zircon ages of 373 ± 3, 377 ± 5 and 379 ± 2 Ma (2σ), which are interpreted as crystallization ages of felsic magmas. The meta-rhyolite displays poorly constrained, older U-Pb zircon ages of 380 ± 4 and 403 ± 5 Ma, which are also considered as intrusive. Whole-rock geochemistry, including relatively little fractionated REE patterns, as well as radiogenic whole-rock Nd and zircon Hf isotopic signatures point to a rather primitive source of the granitic protoliths. The high-K calc-alkaline chemistry and LILE over HFSE enrichments in the NMORB-normalized spider plots indicate an arc-related origin. Juvenile character of the studied rocks was confirmed by Nd and Hf crustal residence ages that are mostly 0.8-0.9 Ga. The origin of the metaigneous rocks is interpreted in terms of partial melting of Neoproterozoic to Cambrian magmatic arc-derived material, probably dominated by immature psammitic sediments (graywackes). This study brings important arguments that the orthogneisses do not represent an old crystalline basement previously assumed in the Mongolian Altai. A model is proposed suggesting formation of mature and layered continental crust by syn-orogenic melting of youthful volcanosedimentary wedge and emplacement of sub-horizontal syn-orogenic magmatic sheets at all crustal levels during crustal-scale vertical shortening. The vertical shortening was probably connected to lithospheric-scale extensional event associated with massive heat influx and emplacement of juvenile magmas at the bottom of the crust. It is suggested that this mechanism represents potentially a viable model for cratonization of accretionary systems worldwide.

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Three crystalline complexes were newly described in the area of the junction between the Mongolian Altay and Gobi Altay in the surroundings of Chandman Sum, SW Mongolia. Khan Khayrkhan, Chandman Khayrkhan and Unegt Uul crystalline complexes were distinguished on the basis of their geological position, distinct petrography, metamorphic style and contrasting geochronological data. These units are situated along the northern margin of the Gobi-Altay Terrane and at first sight seem to have a very simple and uniform history. However, detailed studies reveal a more complex and varied evolution. The Unegt Uul Crystalline Complex is a tectonic mélange of leucogranites, amphibolites and mica schists exposed between the branch of the Bogd fault in the north and Lower Palaeozoic sediments in the south. Metamorphic rocks suffered a prograde metamorphic event reaching temperatures of c. 650 °C and pressures of 6-7.5 kbar estimated from amphibolites. This Complex represents the oldest of the three units; the age of leucogranite formation corresponds to Cambrian (518 ± 5 Ma). Chandman Khayrkhan Crystalline Complex is also restricted by Bogd fault from the north but southern boundary is limited by an intrusive contact with the Chandman Massif. It is composed of orthogneisses and migmatites with amphibolite and calc-silicate lenses. The Complex was affected by a HT, likely periplutonic, metamorphic event and subsequent retrogression. While the rocks of the Chandman Khayrkhan Crystalline Complex remain undated, the age of the granitic rocks in the Chandman Massif itself are Early Carboniferous (345 ± 2 Ma). The Khan Khayrkhan Crystalline Complex has tectonic contacts with the surrounding Palaeozoic volcanosedimentary units. It is built by orthogneisses, amphibolites, paragneisses and mica schists. Metamorphic style corresponds to a prograde event with peak at c. 670 °C and 7-10 kbar. The crystallization age for the granitic protolith to the orthogneisses was the latest Devonian (363 ± 3 Ma).

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