A. M. Fedoseenko

Composition, sources, and mechanism of continental crust growth in the Lake zone of the Central Asian Caledonides: I. Geological and geochronological data

Data on the composition, inner structure, and age of volcanic and siliceous-terrigenous complexes and granitoids occurring in association with them in the Caledonian Lake zone in Central Asia are discussed in the context of major relations and trends in the growth of the Caledonian continental crust in the Central Asian Foldbelt (CAFB). The folded structures of the Lake zone host basalt, basalt-andesite, and andesite complexes of volcanic rocks that were formed in distinct geodynamic environments. The volcanic rocks of the basalt complex are noted for high concentrations of TiO2 and alkalis, occur in association with fine-grained siliceous siltstone and siliceous-carbonate rocks, are thus close to oceanic-island complexes, and were likely formed in relation to a mantle hotspot activity far away from erosion regions supplying terrigenous material. The rocks of the basalt-andesite and andesite complexes have lower TiO2 concentrations and moderate concentrations of alkalis and contain rock-forming amphibole. These rocks are accompanied by rudaceous terrigenous sediments, which suggests their origin in island-arc environments, including arcs with a significantly dissected topography. These complexes are accompanied by siliceous-terrigenous sedimentary sequences whose inner structure is close to those of sediments in accretionary wedges. The folded Caledonides of the Lake zone passed through the following evolutionary phases. The island arcs started to develop at 570 Ma, their evolution was associated with the emplacement of layered gabbroids and tonalitetrondhjemite massifs, and continued until the onset of accretion at 515–480 Ma. The accretion was accompanied by the emplacement of large massifs of the tonalite-granodiorite-plagiogranite series. The postaccretionary evolutionary phase at 470–440 Ma of the Caledonides was marked by intrusive subalkaline and alkaline magmatism. The Caledonides are characterized by within-plate magmatic activity throughout their whole evolutionary history, a fact explained by the accretion of Vendian-Cambrian oceanic structures (island arcs, oceanic islands, and back-arc basins) above a mantle hotspot. Indicators of within-plate magmatic activity are subalkaline high-Ti basalts, alkaline-ultrabasic complexes with carbonatites and massifs of subalkaline and alkaline gabbroids, nepheline syenites, alkaline granites, subalkaline granites, and granosyenites. The mantle hotspot likely continued to affect the character of the lithospheric magmatism even after the Caledonian folded terrane was formed.

Early paleozoic granitoids in the Lesser Khingan terrane, Central Asian Foldbelt: Age, geochemistry, and geodynamic interpretations

The U-Pb zircon dates obtained for the Sutara (480 ± 4 Ma), Kabalinskii (471 ± 10 Ma), and Durilovskii (461 ± 5 Ma) massifs reliably confirm an Early Proterozoic orogenic event, which took place after granulite metamorphism at approximately 500 Ma (Wilde et al., 2003) in the Lesser Khingan (Jiamusi) terrane. The rocks emplaced most shortly after the main metamorphic event are the granites of the Sutara Massif and leucogranites of the Kabalinskii Massif, whose geochemistry is close to that of collision granites. The quartz diorites and subalkaline granites of the Durilovskii Massif, whose geochemistry suggests their origin in a postcollision environment with the participation of an enriched mantle source, were emplaced longer after metamorphic event and after the aforementioned massifs.

Vendian stage in formation of the Early Caledonian superterrane in Central Asia

Granitoids and metamorphic rocks of the Baidarik basement block of the Dzabkhan microcontinent are studied in terms of geology, geochronology (U-Pb dating of zircon microfractions and individual grains) and Nd isotopic-geochemical systematics. As is established, the formation history of metamorphic belt (disthene-sillimanite facies) in junction zone of the Baidarik block and Bayankhongor zone of the Late Riphean (∼665 Ma) ophiolite association characterizes development of the Vendian (∼560–570 Ma) active continental margin. The high-P metamorphic rocks of that time span evidence formation of structures with the Earth’s crust of considerable thickness. In Central Asia, events of the Vendian low-gradient metamorphism are established also in the Tuva-Mongolian massif, Kan block of the East Sayan Mountains, and South Chuya inlier of the Caledonides in the Altai Mountains. Based on these data, it is possible to distinguish the Late Baikalian stage in development of the Early Caledonian superterrane of Central Asia, which antedated the subsequent evolution of this structure during the Late Cambrian-Ordovician. The high-gradient metamorphism that affected most intensively the southeastern part of the Baidarik block can be correlated with the Early Paleozoic (525–540 Ma) evolution of active continental margin and associated development of the Vendian oceanic basins and island arcs of the Ozernaya zone.

The time length of formation of the Angara-Vitim batholite: Results of U-Pb geochronological studies

This paper describes the results of geochronological studies (U-Pb method over micro lots and single grains of zircon) of autochtonous and allochtonous granitoids of the Barguzinskii complex of the Angara-Vitim batolite of the petrotypical area in the basin of the Dzhirga and Kovyli rivers (tributaries of the Barguzin River). The age of crystallization of gneissose granitoids is 297 ± 5 Ma, and that of intrusive leucocratic biotite granites is 291 ± 1 Ma. The estimates of the age finalize the discussion on the age of granitoids of the Barguzin complex and cannot be considered as “rejuvenated.” The analyses of the geochronological data that have been obtained up to the present for granitoids of the Angara-Vitim batolite with the SHRIMP and U-Pb methods for large samples of zircons show that in the majority of cases they cannot be used for precise estimation of the age of their crystallization. The geochronological data obtained with use of the U-Pb method over micro samples and single grains of zircon allow one to make a conclusion on the formation of granitoids of the described complexes of the Angara-Vitim batholite that occurred within 303 ± 7–281 ± 1 Ma. Thus, the time length of formation of the largest in the eastern segment of the Central Asian belt of the Angara-Vitim batholite is not more than 22 Ma (minimum 6 Ma), which allows us to consider it as a large granitic province and is a boundary condition for development of the geodynamic models of its formation.

Crustal growth stages in the Songino block of the Early Caledonian superterrane in Central Asia: I. Geological and geochronological data

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.

Convergent boundaries and related igneous and metamorphic complexes in caledonides of Central Asia

Fragments of the crystalline complexes where Vendian metamorphism of moderate and elevated pressure predated Early Paleozoic metamorphism have been established in the accretionary-collisional domain of the eastern segment of the Central Asian Foldbelt (Early Caledonian superterrane of Central Asia). The geodynamic setting of the Vendian (∼560–570 Ma) South Hangay metamorphic belt located in the junction zone of the Baydrag Block and the Late Riphean (∼665 Ma) ophiolite complex of the Bayanhongor Zone is considered. The origination of this belt was related to the formation of the convergent boundary in the framework of the Zabhan microcontinent about 570 Ma ago. At the same time, an island-arc complex was formed in the paleo-oceanic domain. Metamorphism of elevated pressure indicates that Vendian structures with sufficiently thick continental crust were formed in the framework of the continental blocks. Vendian metamorphism is also established in the Tuva-Mongolia Massif and the Kan Block of the Eastern Sayan. These data show that the Late Baikalian stage predated the evolution of the Early Caledonian superterrane of Central Asia. The development of its accretionary-collisional structure was accompanied by Late Cambrian-Early Ordovician low-pressure regional metamorphism. Granulite-facies conditions were reached only at the deep levels of the accretionary-collisional edifice. The outcrops of crystalline complexes in the southern framework of the Caledonian paleocontinent are regarded as fragments of the Early Paleozoic Central Mongolian metamorphic belt.

Stages and regularities in the development of Late Paleozoic anorogenic volcanism in the southern Mongolia Hercynides

811 An important role in the formation of the Central Asia Hercynides belonged to postaccretionary pro� cesses, both orogenic and anaorogenic. The accre� tionary structures proper were formed in the Middle Paleozoic in response to the convergence between the Siberian and Tarim paleocontinents. This conver� gence was responsible for the gradual closure of the intervenient PaleoAsian ocean branch (1-3) and was accompanied by the convergence at its boundaries, which resulted locally in the formation of accretionary terranes, island arcs, and active margins along conti� nental blocks. The most significant consequence of these processes was accretion of juvenile crustal blocks to the margin of the Siberian continent. This led to the formation of the Hercynides folded domain, which is traceable in the presentday structure from eastern Kazakhstan through northwestern China to southern and southeastern Mongolia (2). After the formation of this folded domain, its struc� tures were involved in a long period of their transfor� mation by postaccretionary magmatism that lasted from the Early Carboniferous until the Early Permian. Two stages are distinguishable in its history. The early stage was related to the formation of the active margin

Age and tectonic position of the Chiney Layered Massif, Aldan shield

The Chiney Complex of basic rocks includes several layered gabbroid massifs. The largest and the best stud- ied massif is the Chiney pluton, which is located among the terrigenous sequence of the Udokan Group, in the southeastern part of the Chara-Olekma geoblock of the Aldan shield. The Udokan sequences are also intruded by the rapakivilike granites of the Kodar Complex. The Chiney Massif is widely known owing to its associated economic mineralization represented by the unusual association of Ti-magnetite V-bearing and PGE-bearing sulfide copper ores. The age of this massif is presently estimated on the basis of 20-year old data: Thus, the age of the considered massif is bracketed between 2180 ± 50 and 1830 ± 50 Ma. This range is too wide to determine the position of the massif in the mod- ern integrated geodynamic models of the evolution of the Precambrian complexes of the Aldan shield (3, 4) and to estimate the promise of this region for Fe-Ti-V and Cu-PGE mineralization. In addition, more data need to be obtained to constrain the upper and lower age limits of the different units of the Udokan Group, which is the Lower Proterozoic stratotype of the Siberia and Far East and serves as an age marker in the regional stratigraphic scale. In order to solve this problem, we conducted U-Pb geochronological studies of the Chiney Massif, which are presented in this paper. The Chiney Massif is situated in the southeastern part of the Kodar-Udokan trough, which is filled mainly with metasedimentary rocks of the Udokan Group. It penetrates terrigenous-carbonate rocks of the upper Chiney (Aleksandrovskaya and Butunskaya For- mations) and lower Kemen (Sakukan Formation) sub- groups of the Udokan group. The massif is exposed as a W-E-trending elongated body with the maximal size of 9 × 16 km and an area of 150 km 2 . The study results are reported in detail in the generalized monographs (5-8).

Protoliths of the metamorphic rocks of the Fedorov Complex, Aldan shield: Character, age, and geodynamic environments of origin

Geochemical data indicate that the protoliths of the overwhelming majority of the metamorphic rocks composing the Fedorov Complex in the Aldan granulite megacomplex were volcanic rocks of three groups, which occur in different proportions in the complex: (i) volumetrically predominant (no less than 90%) continuous differentiated island-arc basalt-andesite-dacite-rhyolite series, (ii) within-plate basalts, whose composition was similar to that of low-Ti traps, and (iii) basalts of composition similar to that of continentalrift basalts. The U-Pb zircon crystallization age of the metamorphosed basaltic andesites of the Fedorov Complex was estimated at 2006 ± 3 Ma, which testifies, when considered together with preexisting geochronological data, that the complex was produced during a time span of no longer than 25 m.y. A model is proposed according to which the complex was produced within the geodynamic system of the active continental margin of the Olekma-Aldan continental microplate and the Fedorov island arc.

Age, sources, and geological position of anorthosites of Precambrian terranes of Central Asia: Example from the Khunzhilingol Massif, Mongolia

Large anorthosite massifs (or autonomous anorthosites) are typical magmatic complexes of Early Precambrian structures. They are usually formed at the postorogenic stages of their evolution and serve as indicators of their intraplate activation. These massifs remain enigmatic from the petrological point of view, but provide important information on the geology and evolution of host structures. Anorthosite massifs were found in Mongolia in the second half of the 20th century. These are the small Mustulin and Khodzhulingol massifs in the Tarbagatai block, and Olonkhuduk Massif in the Dzabkhan microcontinent (Fig. 1). One more large anorthosite massif was recently found by us in the northern part of the Tarbagatai block, in the lower reaches of the