The crystalline complexes of the Hercynian South Altai Metamorphic Belt (SAMB), which is a part of the Central Asian Foldbelt more than 1500 km long. They compose different-scale tectonic plates, the level of metamorphism in which at the early stages reached the conditions of high-temperature amphibolite subfacies and, in places, granulite facies. In terms of tectonics, the band of their outcrops is confined to the margin of the North Asian Caledonian continent, stretching from southeast to northwest along the southern slope of the Gobi, Mongolian, and Chinese Altai to Eastern Kazakhstan, where they are represented in the Irtysh shear zone. The SAMB includes poly- and monometamorphic complexes. The age of granitoids formed at the late episode of metamorphism was determined for the Tsel tectonic plate of the Gobi Altai in the southeastern part of the SAMB: from 374 ± 2 to 360 ± 5 Ma. These and previously obtained results show that the early stage of low-pressure metamorphism and the late stage of high-pressure metamorphism occurred in the age intervals of 390–385 and 375–360 Ma, respectively, almost throughout this belt. A short-term stage of stabilization was between these stages. These processes occurred during the closure of the basin with the Tethys-type oceanic crust of the South Mongolian Ocean (paleo-Tethys I). The spatial position of the SAMB is determined by the asymmetric structure of the basin, in which the active continental margin is represented along its northern part, and the passive one is represented along the southern one (in modern coordinates).

Tectonic sheets of various size along the southern slope of the Mongolian and Chinese Altai ranges and in eastern Kazakhstan include high-grade metamorphic rocks, which are collectively referred to as the Southern Altai Metamorphic Belt. Rocks of the sheets show traces of amphibolite-facies elevated-pressure metamorphism of the kyanite–sillimanite type M2. Some of the tectonic sheets display evidence of polymetamorphism: the rocks preserve textures and mineral assemblages of an earlier metamorphic episode (of elevated temperature and relatively low pressure) of the andalusite–sillimanite facies series M1. The earlier metamorphic episode occurred at 390–385 Ma, and the later one, at ~370–356 Ma. The protoliths of the high-grade metamorphic rocks were mostly Early Paleozoic terrigenous rocks and subordinate amounts of volcanic rocks analogous to the weakly metamorphosed or unmetamorphosed rocks in their northern surroundings. Typical rocks of the tectonic sheets are mafic dikes and massifs of the Gashun Nuur Complex, which were emplaced between metamorphic episodes M1 and M2. According to their geochemistry and Nd isotopic parameters, most of the metabasites are similar to enriched basalts of mid-oceanic ridges and oceans plateaus. The quantitatively subordinate group of the layered mafic bodies displays geochemical characteristics of subduction-related rocks. Correlations between the metamorphic events and magmatism in the continental (Mongolian and Chinese Altai) and paleoceanic (Trans-Altai Gobi and eastern Junggar) regions led us to suggest a geodynamic model for the development of the Southern Altai Metamorphic Belt. The volcano-terrigenous rocks, which were later metamorphosed, were accumulated mostly in the Early Paleozoic as an accretion wedge on an active continental margin. The earlier episode of high-temperature metamorphism M1 and coeval large-scale calc–alkaline magmatism occurred at the same active continental margin after the magmatic front shifted southward (in modern coordinates). The emplacement of the swarms of mafic bodies of the Gashun Nuur Complex and simultaneous rifting in the southern Chinese Altai were triggered by the subduction of an spreading ridge of an oceanic or backarc basin beneath the active margin. The second metamorphic episode (elevated-pressure metamorphism) M2 and overthrusting in the structures of the Altai are correlated with deformations at low angles and the transition from oceanic to continental volcanism in the Trans-Altai Gobi and Junggar. These tectonic processes were induced by the accretion of a system of mid-Paleozoic ensimatic island arcs of the Trans-Altai Gobi and Junggar to the Altai margin of the Siberian paleocontinent.

This paper reports new geological, geochronological (U-Pb method) and Sm-Nd isotope-geochemical data on the metamorphic and magmatic complexes of the South Hangay metamorphic belt located in the southern part of the Hangay group of the Precambrian blocks, along the junction zone between the Baidarik block and the Vendian-Late Riphean ophiolite complex of the Bayanhongor zone. A lower age limit of the early metamorphic episode of the disthene-sillimanite facies series (M2) is provided by the age of 603 ± 3 Ma obtained for gabbroids of the island arc association distinguished in the Tatsaingol complex. An upper age limit is determined by granites with ages of 562 ± 2, 564 ± 5, and 571 ± 9 Ma. Later superimposed metamorphism of the andalusite-sillimanite facies series (M2) was established in the southeastern part of the belt. The lower boundary of this metamorphism is constrained by the emplacement of quartz diorites at 552 ± 2 Ma, while the upper boundary is marked by postkinematic granites of the Tatsaingol pluton with an age of 545 ± 5 Ma. The protoliths of high grade metamorphic rocks contained Early and Late Precambrian rocks. The former could belong to the basement of the Baidarik block and/or its shelf, while the latter, to the rocks of the oceanic and island-arc complexes of the Bayanhongor paleobasin. Virtually, the continental crust, fragments of which are represented in the Tatsaingol complex, was formed within an interval around 610-560 Ma. The final stages in the formation of the South Hangay metamorphic belt coincided with the initiation of the Vendian-Early Paleozoic paleoocenic basins, volcanic islands, and island arcs in the Early Caledonian framing of the Precambrian Hangay block structure in the Late Vendian. By the time of amalgamation of structures of the Early Caledonian superterrane of Central Asia in the Late Cambrian-Early Ordovician (500-470 Ma), an “agglomerate” of the Precambrian Hangay blocks has been already formed.

The Dzabkhan microcontinent was earlier considered as a fragment of an ancient craton in the structure of the Central Asian Orogenic Belt. Deposits of the Tsagaan Oloom Formation were included in the shelf zone, under the assumption that they were related to the regional unconformity between the Early-Late Precambrian crystal formations. The carbonate sequence of the Tsagaan Oloom Formation overlaps crystalline rocks only in the eastern part of the Dzabkhan microcontinent, where dolomites lie unconformably on high-grade metamorphic rocks intruded by granitoids of the Bogdyngol massif. The latter were included in the composition of both the Early Precambrian basement and the Middle Riphean intrusive complex. We have determined the U-Pb zircon age of these granitoids at 717 ± 5 Ma and the Nd model ages of granitoids and gneisses of the basement of the Tsagaan Oloom Formation at 2.0–1.9 Ga at εNd = −10.0...−6.6. Recent geochronological and Nd and Pb-Pb isotopic and geochemical data indicate that intrusive and high-grade metamorphic complexes are absent in the crystalline basement of the Dzabkhan microcontinent, similar to those in ancient cratons. One can assume that the Late Riphean carbonate cover (Tsagaan Oloom Formation) deposited on the Late Precambrian continental block.

Дзабханский микроконтинент рассматривался ранее как фрагмент древнего кратона в структуре Центрально-Азиатского складчатого пояса. Отложения цаганоломской свиты включались в состав его шельфового чехла. Предполагалось, что они фиксируют региональное несогласие между кристаллическими образованиями раннего и позднего докембрия. Непосредственное залегание толщи карбонатных пород цаганоломской свиты на кристаллических породах установлено только в восточной части Дзабханского микроконтинента, где доломиты с несогласием перекрывают высокоградные метаморфические породы и прорывающие их гранитоиды Богдынголского массива. Последние включались в состав то раннедокембрийского фундамента, то среднерифейского интрузивного комплекса. В настоящей работе определен возраст этих гранитоидов 717 ± 5 млн лет (U-Pb метод по циркону) и Nd-модельные возрасты этих гранитоидов и гнейсов фундамента цаганоломской свиты 2.0–1.9 млрд лет при Nd = –10.0... –6.6. Полученные к настоящему времени геохронологические, Nd и Pb-Pb изотопно-геохимические данные свидетельствуют об отсутствии в фундаменте Дзабханского микроконтинента пород интрузивных и высокоградных метаморфических комплексов, сопоставимых с образованиями древних кратонов. Можно полагать, что формирование позднерифейского карбонатного чехла (цаганоломской свиты) происходило на блоке позднедокембрийской континентальной коры.

The Early Caledonian Central Asian Orogenic Belt hosts fragments of continental blocks with Early and Late Precambrian crystalline basement. One of the structures with an Early Precambrian basement was thought to be the Dzabkhan microcontinent, which was viewed as an Early Precambrian “cratonal terrane”. The first geochronologic data suggest that the basement of the Dzabkhan microcontinent includes a zone of crystalline rocks related to Late Riphean tectonism. Geological, geochronological (U-Pb zircon dates), and Nd isotopic-geochemical data were later obtained on the northwestern part of the Dzabkhan microcontinent. The territory hosts the most diverse metamorphic complexes thought to be typical of the Early Precambrian basement. The complexes were determined to comprise the Dzabkhan-Mandal and Urgamal zones of high-grade metamorphic rocks. Gabbrodiorites related to the early metamorphic episode and dated at 860 ± 3 Ma were found in the Dzabkhan-Mandal zone, and the gneiss-granites marking the termination of this episode were dated at 856 ± 2 Ma. The granitoids of the Dzabkhan batholith, whose emplacement was coeval with the termination of the late high-grade metamorphic episode in rocks of both zones, have an age of 786 ± 6 Ma. Similar age values were determined for the granitoids cutting across the Late Precambrian rocks of the Songino and Tarbagatai blocks, which mark the stage when the mature Late Riphean continental crust was formed. The Late Riphean magmatic and metamorphic rocks of the Dzabkhan microcontinent were found out to have Nd model ages mostly within the range of 1.1–1.4 Ga at ɛNd(T) from +1.9 to +5.5. The Nd model age of the metaterrigenous rocks is 2.2−1.3 Ga at ɛNd(T) from −7.2 to +3.1. The results of our studies provide evidence of convergence processes, which resulted in the Late Riphean (880−780 Ma) continental crust in Central Asia. Simultaneously with these processes, divergence processes that were responsible for the breakup of Rodinia occurred in the structures of the ancient cratons. It is reasonable to suggest that divergence processes within ancient continental blocks and Rodinia shelf were counterbalanced by the development of the Late Riphean continental crust in the convergence zones of its surrounding within established interval.

The Early Caledonian folded area in Central Asia (Early Caledonian superterrane) hosts micro-continent fragments with an Early and Late Precambrian crystalline basement, the largest of them being the Dzabkhan and Tuva-Mongolian fragments. Their junction zone hosts exposures of crystalline rocks that were previously thought to be part of the Early Precambrian Dzabkhan microcontinent. The Bayannur zone in the southern part of the Songino block hosts the Baynnur gneiss-migmatite and Kholbonur metavolcanic-terrigenous metamorphic complexes. The former is believed to be the Early Proterozoic crystalline basement, and the latter is thought to unconformably overly the Late Riphean cover complex of the Songino block. Various rocks of the tectono-stratigraphic complexes in the Bayannur zone were studied geologically and geochronologically (by the U-Pb technique of zircon). Regional metamorphism and folding in the Bayannur Complex were dated at 802 ± 6 Ma. The Nd model ages lie within the range of 1.5–2.0 Ga and thus preclude the correlation of these rocks with those in the Archean and Early Proterozoic basement of the Dzabkhan microcontinent. The upper age limit for folding and metamorphism in the Bayannur zone is marked by postkinematic granites dated at 790 ± 3 Ma, and the lower limit of the volcano-sedimentary complex is determined by the Nd model age of the sandstone (1.3 Ga). The upper age limit of the volcano-plutonic rocks in this zone is set by the gabbroids and anorthosites: 783 ± 2 and 784 ± 3 Ma, respectively. The complex of island-arc granitoids in the Bayannur zone is dated at 859 ± 3 Ma. The age constraints make it possible to correlate crystalline rocks in the Bayannur Complex of the Sangino block and the Dzhargalant Complex in the Tarbagatai block. Currently available data testify that the Precambrian Khangai group of blocks in the Early Caledonian Central Asian superterrane includes continental crustal blocks related to the processes of Early Precambrian, Late Riphean, and Vendian tectonism.

В аккреционно-коллизионной области восточного сегмента Центрально-Азиатского складчатого пояса (раннекаледонском супертеррейне Центральной Азии) установлены фрагменты кристаллических комплексов, в которых раннепалеозойскому предшествовал вендский метаморфизм умеренного–повышенного давления. В работе рассмотрена геодинамическая обстановка формирования вендского ( 560 570 млн. лет) Южно-Хангайского метаморфического пояса, расположенного в зоне сочленения Байдарикского блока и позднерифейского ( 665 млн. лет) офиолитового комплекса Баянхонгорской зоны. Становление пояса отражает образование на рубеже около 570 млн. лет конвергентной границы в обрамлении Дзабханского микроконтинента. В это же время в палеоокеанической области происходило формирование островодужного комплекса. Проявление метаморфизма повышенного давления свидетельствует о том, что в венде в обрамлении континентальных блоков были сформированы структуры с достаточно мощной континентальной корой. Вендский метаморфизм установлен также в Тувино-Монгольском массиве и в Канском блоке Восточного Саяна, что позволяет выделять позднебайкальский этап в развитии раннекаледонского супертеррейна Центральной Азии, предшествовавший становлению его структуры в позднем кембрии – раннем ордовике. Становление аккреционно-коллизионной структуры раннекаледонского супертеррейна Центральной Азии сопровождалось в позднем кембрии – ордовике региональным метаморфизмом пониженного давления, который достигал в глубинных сечениях аккреционно-коллизионного сооружения условий гранулитовой фации. Выходы кристаллических комплексов, локализованных в структурах южного обрамления каледонского палеоконтинента, рассматриваются как фрагменты раннепалеозойского Центрально-Монгольского метаморфического пояса.

В раннекаледонской складчатой области Центральной Азии (в раннекаледонском супертеррейне) представлены фрагменты континентальных блоков (микроконтинентов), наиболее крупные из которых – Дзабханский и Тувино-Монгольский с ранне- и позднедокембрийским кристаллическим фундаментом соответственно. В зоне сочленения этих микроконтинентов известны также кристаллические породы Тарбагатайского и Сонгинского блоков, рассматриваемых как образования раннедокембрийского энсиалического фундамента супертеррейна. В их составе установлены глубокометаморфизованные породы, сформированные в ходе раннебайкальского тектогенеза ( 790 820 млн. лет). Проведенные геохронологические (U-Pb метод по цирконам) и Nd-изотопные исследования позволили выделить в составе северо-западной части Дзабханского микроконтинента Дзабхан-Мандалскую зону развития кристаллических пород, связанных с рифейским корообразующим процессом. Возраст формирования субстрата гнейсов этой зоны определяется в интервале 1.3–0.86 млрд. лет. Возраст раннего эпизода метаморфизма оценивается около 856 ± 2 млн. лет. Полученные к настоящему времени данные свидетельствуют о гетерогенном строении фундамента Дзабханского микроконтинента, в пределах которого представлены раннедокембрийские и ранне- и позднебайкальские кристаллические образования.

Fragments of continental blocks or microcontinents are represented in the Early Caledonian orogenic area of Central Asia (or Early Caledonian superterrane); the largest of these are the Dzabkhan and Tuva-Mongolian microcontinents, with Early and Late Precambrian crystalline basements, respectively. In the linkage zone of these microcontinents, crystalline rocks of the Tarbagatai and Songino blocks that are considered as units of the Early Precambrian ensialic basement of the superterrane are also known. They are composed of strongly metamorphosed rocks formed during the Early Baikalian orogeny about 790 to 820 Ma. U-Pb zircon dating and Nd isotope studies revealed, within the northwestern Dzabkhan microcontinent, the Dzabkhan-Mandal zone of crystalline rocks associated with the Riphean crust-forming process. The age of the gneiss substrate of this zone is estimated as 1.3 to 0.86 Ga. An early episode of metamorphism is dated at about 856 ± 2 Ma. The data available so far indicate a heterogeneous structure of the Dzabkhan microcontinent basement represented by Early Precambrian and Early and Late Baikalian crystalline formations.