Geological-structural and petrological data acquired by the authors on the most widely spread metamorphic associations of the Belomorian Mobile Belt in northern Karelia, Russia, indicate that the rocks belong to five age groups (0–IV), which were produced by metamorphism of protoliths of different composition and genesis. The Paleoproterozoic metamorphic associations were formed by three successive metamorphic episodes, which corresponded to discrete evolutionary episodes of the Lapland–Karelian Orogen. The first episode of Paleoproterozoic metamorphism was high-pressure transformations that produced steep zones of ductile flow of submeridional and northeastern trend. The second episode was high-temperature metamorphism at intermediate and high pressure, which was triggered by the development of gently sloped ductile flow zones and tectonic mélange of roughly latitudinal trend. The third metamorphic episode involved amphibolization of the rocks during high-temperature decompression cooling and was likely related to the development of randomly oriented fractures when the rocks of the Belomorian Mobile Belt were brought to middle and upper crustal levels. Each of the episodes was characterized by different P–T parameters of metamorphism, which are recorded in the compositions of the rock-forming minerals. The former two episodes corresponded to the prograde metamorphic evolution, and the third one was a retrograde evolution. The beginning of the first episode was responsible for the origin of amphibolites at Т = 560–640°C and Р = 6.7–9.5 kbar, with the metamorphic peak producing eclogites at Т = 600–690°C and Р = 11.3–12.9 kbar and eclogite-like rocks at higher temperatures of Т = 650–780°C and Р = 10.3–13.0 kbar. The peak of the second metamorphic episode corresponded to the origin of abundant plagiomigmatite leucosomes, which were formed within a broad temperature range (640–840°C) and a narrow pressure range (9.0–11.9 kbar). The widespread amphibolization of the metabasites of all types occurred only during the third metamorphic episode, whose P–T parameters broadly varied: 590–740°C and Р = 7.5–10.8 kbar. All dike varieties of the complex of eclogitized garnet metagabbro were formed at the metamorphic peak at the same P–T parameters as the host metamorphic rocks in the corresponding structures: metagabbro I was formed in steep ductile flow zones, and metagabbro II was produced in the gently sloped zones.

Проведена оценка изотопного Sm-Nd-возраста апоамфиболитовых эклогитов, эклогитоподобных гранат-клинопироксеновых кристаллических сланцев, отобранных в Чупинском и Энгозерском сегментах Беломорского подвижного пояса (БПП). Эклогиты и эклогитоподобные породы показали по паре минералов гранат–клинопироксен практически один и тот же изотопный возраст: эклогиты – 2119 ± 170, эклогитоподобные гранат-клинопироксеновые породы – 2191 ± 39 млн лет. Полученный возраст -2,1 млрд лет – наиболее древний указатель начала палеопротерозойского метаморфизма в БПП.

The Belomorian Mobile Belt (BMB) in northern Karelia mostly consists of gently sloping shear zones, whose gneisses and migmatized amphibolites and blastomylonites are typically thinly banded, with their banding consistently dipping north- and northeastward. These gently sloping shear zones were not affected by folding after they were produced and are not cut by Paleoproterozoic metabasite dikes. Intrusive metabasites in the gently sloping shear zones make up relatively small (usually <5 m) equant or elongate bodies and occur as fragments of larger bodies. These fragments are often concentrated in stripes. Metabasites in the gently sloping shear zone are sometimes also found as lenses and tabular bodies of relatively small thickness, which are conformable with the foliation of the host rocks. The gently sloping shear zones cut across older domains of more complicated structure, which suggests that these zones are gently sloping ductile shear zones. Along these zones, the nappes were thrust south- and southwestward, and this process was the last in the origin of major structural features of BMB when the Paleoproterozoic Lapland–Kola orogen was formed. Practically identical age values were obtained for the gently sloping shear zone in the two widely separated Engonozero and Chupa segments of BMB: 1879 ± 21 Ma ( 40 Ar/ 39 Ar amphibole age of amphibolite whose protolith was mafic rock) and 1857 ± 13 Ma (Sm–Nd mineral isochron age of garnet amphibolites after gabbronorite). The P – T metamorphic parameters in these gently sloping shear zones are remarkably different from the metamorphic parameters outside these zones: the pressure is 3–4 kbar lower and the temperature is 60–100°C lower. Thrusting-related decompression triggered the transition from the older high-pressure episode of Paleoproterozoic metamorphism to a younger syn-thrusting higher temperature metamorphic episode. The peak metamorphic parameters corresponding to the boundary between the amphibolite and granulite facies were reached only in the central portions of the shear zones: T = 680–760°C, P = 8.0–11.9 kbar. In areas of the most intense migmatization, temperature estimates in the central portions of the shear are as high as 810–830°C. The marginal portions of the shear zones were formed at lower temperatures of 610–630°C. The temperature heterogeneous and rock heating in the gently sloping shear zones may have resulted from flows of high-temperature metamorphic fluid that were focused to the central portions of the zones.

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.

Исследованы поведение редкоземельных элементов и изотопно-геохимические (Sm–Nd) особенности в метаморфизованных дайках низко-титановых толеитов в Красногубском дайковом поле в восточной части Беломорского подвижного пояса. Проведено сравнение дайек низко-титановых толеитов Красногубского и Гридинского дайковых полей.

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.

Geochemical and Nd isotope data are reported for Late Riphean metamorphic complexes and granitoids of the Bayannur zone of the Songino block in the Early Caledonian superterrane of Central Asia. Geological, geochronolgical, geochemical, and isotope data were integrated to discuss rock sources and main mechanisms responsible for the formation and evolution of the Late Riphean continental crust. It was established that lithotectonic complexes of the Bayannur zone were formed on a convergent plate during Late Proterozoic tectonogenesis (around 1.3–0.78 Ga). This period primarily produced a juvenile crust represented by paleooceanic (N- and E-MORB types) and island arc basalts. An interval of 800–880 Ma was marked by the formation of rocks of the Bayannur complex and metaterrigenous sequence (accretionary wedge) of the Kholbonur complex, and the emplacement of quartz diorites and granodiorites of the Gashunnur pluton due to erosion and melting of both Late Riphean juvenile sources and ancient possibly Early Precambrian crustal material in a setting of ensialic island arc. At the final stage of the Late Riphean evolution of the Bayannur zone, postkinematic granitoids of the Bayannur pluton, and gabbrodiorites and anorthosites of the Ontsula pluton were derived from mantle juvenile and crustal sources in a within-plate setting. In terms of isotope characteristics, the crystalline complexes of the Bayannur zone are comparable with the Japan-type modern island arc systems. A synthesis of geological, geochronological, and isotope-geochemical data indicates a much wider distribution of the Late Riphean juvenile crust-forming processes than considered previously and remobilization of continental crust in the eastern segment of the Central Asian Fold Belt. The Vendian-Paleozoic stage in the evolution of this segment was characterized by an intense growth of juvenile crust, while magmatism during Late Riphean stage was determined by mixing of Late Riphean juvenile and ancient Early Precambrian sources.

Part II of this paper reports geochemical and Nd isotope characteristics of the volcanogenic and siliceous-terrigenous complexes of the Lake zone of the Central Asian Caledonides and associating granitoids of various ages. Geological, geochronological, geochemical, and isotopic data were synthesized with application to the problems of the sources and main mechanisms of continental crust formation and evolution for the Caledonides of the Central Asian orogenic belt. It was found that the juvenile sialic crust of the Lake zone was formed during the Vendian-Cambrian (approximately 570–490 Ma) in an environment of intraoceanic island arcs and oceanic islands from depleted mantle sources with the entrainment of sedimentary crustal materials into subduction zones and owing to the accretion processes of the amalgamation of paleoceanic and island arc complexes and Precambrian microcontinents, which terminated by ∼490 Ma. The source of primary melts for the low-Ti basalts, andesites, and dacites of the Lake zone ophiolites and island arc complexes was mainly the depleted mantle wedge above a subduction zone. In addition, an enriched plume source contributed to the genesis of the high-Ti basalts and gabbroids of oceanic plateaus. The source of terrigenous rocks associating with the volcanics was composed of materials similar in composition to the country rocks at a minor and varying role of ancient crustal materials introduced into the ocean basin owing to the erosion of Precambrian microcontinents. The sedimentary rocks of the accretionary prism were derived by the erosion of mainly juvenile island arc sources with a minor contribution of rocks of the mature continental crust. The island arc and accretion stages of the development of the Lake zone (∼540–590 Ma) were accompanied by the development of high- and low-alumina sodic granitoids through the melting at various depths of depleted mantle reservoirs (metabasites of a subducted oceanic slab and a mantle wedge) and at the base of the island arc at the subordinate role of ancient crustal rocks. The melts of the postaccretion granitoids of the Central Asian Caledonides were derived mainly from the rocks of the juvenile Caledonian crust at an increasing input of an ancient crustal component owing to the tectonic mixing of the rocks of ophiolitic and island arc complexes and microcontinents. The obtained results indicate that the Vendian-Early Paleozoic stage of the evolution of the Central Asian orogenic belt was characterized by the extensive growth of juvenile continental crust and allow us to distinguish a corresponding stage of juvenile crust formation.

Во второй части статьи рассмотрены данные о геохимических и Nd-изотопных особенностях вулканогенных и кремнисто-терригенных комплексов Озерной зоны каледонид Центральной Азии, ассоциирующих с ними разновозрастных гранитоидов, и на основе синтеза геологических, геохронологических, геохимических и изотопных данных обсуждаются вопросы об источниках пород и ведущих механизмах формирования и эволюции континентальной коры каледонид Центрально-Азиатского складчатого пояса. Установлено, что ювенильная сиалическая кора Озерной зоны была образована в венд-кембрийское время (около 570–490 млн. лет назад) в обстановке внутриокеанических островных дуг и океанических островов из деплетированных мантийных источников с вовлечением в зоны субдукции корового материала в форме осадков, а также во время аккреционных процессов амальгамации палеоокеанических и островодужных комплексов и докембрийских микроконтинентов, завершившихся к рубежу около 490 млн. лет назад. Источником исходных расплавов низко-Ti базальтов, андезитов и дацитов офиолитовых и островодужных комплексов Озерной зоны являлась, главным образом, деплетированная мантия надсубдукционного клина. В петрогенезисе высоко-Ti базальтов и габброидов океанических плато участвовал также обогащенный плюмовый источник. Источниками сноса терригенных осадочных пород, ассоциирующих с вулканитами, являлись породы, близкие по составу к вмещающим, при подчиненной варьирующей роли древнего корового материала, поступавшего в океанический бассейн вследствие размыва докембрийских микроконтинентов. Осадочные породы аккреционной призмы формировались при размыве преимущественно ювенильных островодужных источников с незначительным участием пород зрелой континентальной коры. На островодужной и аккреционной стадиях развития Озерной зоны (около 540–490 млн. лет назад) происходило становление гранитоидов натровой серии высоко- и низкоглиноземистого типов посредством разноглубинного плавления деплетированных мантийных источников – метабазитов субдуцирующей океанической плиты и мантийного клина, а также в основании островной дуги при подчиненном участии древнекорового материала. Источниками расплавов постаккреционных гранитоидов каледонид Центральной Азии выступали преимущественно породы ювенильной каледонской коры при прогрессивной добавке древнего корового компонента, вследствие тектонического смешения пород офиолитовых и островодужных комплексов и микроконтинентов. Таким образом, полученные данные свидетельствуют о том, что венд – раннепалеозойский период эволюции Центрально-Азиатского складчатого пояса характеризуется обширным ростом ювенильной континентальной коры и позволяют выделить соответствующий этап ювенильного корообразования.

The paper presents data on the structure, composition, and age of granitoid associations (Tokhtogeshil’skii Complex) composing the Kharanur and Sharatologoi polychronous plutons in the northern part of the Ozernala zone in western Mongolia. The Tokhtogeshil’skii Complex was determined to consist of a number of independent magmatic associations, which were formed at 540–450 Ma, within three age intervals (540–520, 510–485, and 475–450 Ma), have different composition, were derived from different sources, and were emplaced in different geodynamic environments. During the first, island-arc stage (540–520 Ma), high-Al plagiogranites were produced, which belong to tonalite-plagiogranite (531 ± 10 Ma) and diorite (529 ±6 Ma) associations in the Kharanur pluton, low-Al plagiogranites of the tonalite-plagiogranite association (519 ± 8 Ma) in the Sharatologoi pluton, and rocks of the Khirgisnur peridotite-pyroxenite-gabbronorite complex (Kharachulu and Dzabkhan massifs). The rocks of the diorite and plagiogranite associations of the Kharanur pluton have ɛNd(T) from +7.9 to +7.4, TNd(DM) = 0.65 Ga, and (87Sr/86Sr)0 = 0.7038–0.7039. The plagiogranites of the Sharatologoi pluton (tonalite-plagiogranite association) are characterized by ɛNd(T) from +6.5 to +6.6, TNd(DM) = 0.73–0.70 Ga, and (87Sr/86Sr)0 = 0.7038–0.7039, which suggest predominantly juvenile subduction sources of the parental melts at a subordinate role of ancient crustal material. During the second, accretionary stage (510–485 Ma), low-Al plagiogranites of the diorite-tonalite-plagiogranite association of the Sharatologoi pluton (494 ± 10 Ma, M type) were formed. The Sr-Nd isotopic characteristics of these rocks ɛNd(T) = +6.6, (87Sr/86Sr)0 = 0.7039 are analogous to those of the plagiogranitoids of the early type. This suggests that the melted sources were similar in composition. During the third, postcollisional stage (475–450 Ma), rocks of the diorite-granodiorite-granite association were formed (459 ± 10 Ma, type I) in the Kharanur pluton. These rocks have ɛNd(T) = +5.1, TNd(DM) = 0.74 Ga, and (87Sr/86Sr)0 = 0.7096. The parental melts were supposedly derived by means of partial melting of “the Caledonian” juvenile crust with the addition of more ancient crustal material.