M. M. Fugzan

Crystalline complexes of the Tarbagatai block of the Early Caledonian superterrane of Central Asia

The oldest crystalline complexes of the Early Caledonian superterrane of Central Asia were formed in the Early Precambrian. They are exposed in the basement of microcontinents, which represent old cratonic fragments. Among the latters are the crystalline complexes of the Tarbagatai block previously ascribed to the Dzabkhan microcontinent. It was shown that the crystalline complexes of the Tarbagatai block have a heterogeneous structure, consisting of the Early Precambrian and later Riphean lithotectonic complexes. Structurally, the Early Precambrian complexes are made up of tectonic sheets of gneisses, migmatites, and gneiss granites of the Ider Complex that are cut by gabbroanorthosite massif. The Riphean Jargalant Complex comprises alternating hornblende crystalline schists and biotite (sometimes sillimanite-bearing) gneisses with marble horizons. The upper age boundary of the Riphean Complex is determined by the subautochthonous granitoids with age about 810 Ma. The presence of the Riphean high-grade rocks indicates that structures with newly formed crust were formed in the paleooceanic framing of the Early Precambrian blocks of the Rodinia supercontinent by the Mid-Late Riphean. Divergence that began at that time within old Rodinian cratons and caused rifting and subsequent break-up of the supercontinent was presumably changed by convergence in the paleooceanic area.

The Early Baikalian Crystalline Complex in the Basement of the Dzabkhan Microcontinent of the Early Caledonian Orogenic Area, Central Asia

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.

Sarmatia-Volgo-Uralia junction zone: Isotopic-geochronologic characteristic of supracrustal rocks and granitoids

The geochronologic (U-Pb isotopic system of zircons) and isotopic-geochemical (Sm-Nd isotopic system of the bulk rock) studies were performed along the profile extending from the eastern Sarmatia (in the west) to the Middle Volga megablock of Volgo-Uralia (in the east), i.e., across the entire junction zone for dating the integration of Sarmatia and Volgo-Uralia, representing two segments of the East European Craton. It is established that the examined rocks are characterized by the Paleoproterozoic Nd isotopic model age, which varies from 2.1 and 2.4 Ga, except for some samples indicating a similar age of the crust through the entire Sarmatia-Volgo-Uralia junction zone. The highly metamorphosed complexes of the granulite and amphibolite facies constituting the southwestern margin of Volgo-Uralia are Paleoproterozoic, not Archean, in age, contrary to previous views. Two Early Paleoproterozoic lithotectonic complexes are defined in Volgo-Uralia: South Volga metasedimentary and Tersa metasedimentary-volcanogenic. The obtained data confirm the asynchronous integration of individual segments into the East European Craton: the integration of Sarmatia and Volgo-Uralia approximately 2100–2000 Ma ago was followed by the conjunction of this newly-formed continent with Fennoscandia ca. 1800 Ma ago.

Late Riphean episode in the formation of crystalline rock complexes in the Dzabkhan microcontinent: Geological, geochronologic, and Nd isotopic-geochemical data

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.

Isotope-geochronological (U-Th-Pb, Lu-Hf) study of the zircons from the archean magmatic and metasedimentary rocks of the Podolia domain, Ukrainian shield

Zircons from the oldest magmatic and metasedimentary rocks of the Podolia domain of the Ukrainian shield were studied and dated by U-Pb method on a NORDSIM secondary-ion mass spectrometer. The age of zircon cores in the enderbite gneisses taken in the Kazachii Yar and Odessa quarries on the opposite banks of the Yuzhnyi Bug River reaches 3790 Ma. Cores of the terrigenous zircons in the quartzites from the Odessa quarry as well as in the garnet gneisses from the Zaval’e graphite quarry have an age within 3650–3750 Ma. Zircon rims record two metamorphic events at 2750–2850 Ma and around 1900–2000 Ma. Extremely low U content in the zircons of the second age group indicates conditions of the granulite-facies metamorphism in the Paleoproterozoic within the Podolia domain. Obtained data on the orthorocks (enderbite-gneiss) and metasedimentary rocks unambiguously suggest the existence of ancient Paleoarchean crust in the Podolia (Dniester-Bug) domain of the Ukrainian shield. They contribute in our knowledge of scales of the formation and geochemical features of the primordial crust.

The Uspensk intrusion in south Primorye as a reference petrotype for granitoids of the transform continental margins

Identification of the granitoid complexes as petrotypes of definite tectonic settings remains one of the most important geological problems taking into account the large volume of fundamental tectonic and applied regional works. To the present, the granitoids of within-plate environments, island arcs, subductionrelated continental margins, and collisional settings have been substantiated and studied in detail [1‐3 and others]. At the same time, the granitoid associations of transform plate boundaries have been studied significantly less well. Transform continental-margin settings (Californiantype complex settings) related to subsidence of the oceanic ridge beneath a continent with the formation of slab windows and wide development of strike-slip faulting were distinguished for the first time at the western margin of North America [4 and others]. A similar regime was later substantiated for the Early Cretaceous and Paleogene stages of the Sikhote Alin evolution [5, 6]. Unlike the Californian coast, the transform margin setting in the Russian Far East was not complicated by mantle plumes that were unrelated to subduction. Therefore, this region is a unique object for distinguishing magmatic complexes typical of the transform‐continental setting.

New U-Pb titanite age data on the rocks from the Karelian craton and the Belomorian mobile belt, Fennoscandian Shield

We test the possibility of applying titanite as indicator of the boundaries between the Belomorian mobile belt and the Karelian craton of the Fennoscandian Shield. U-Pb isotope dating established wide variations of titanite ages in the Belomorian mobile belt and the Karelian craton. The titanites from the Karelian craton are mainly Archean in age (2.52–2.86 Ga), whereas the Belomorian mobile belt contains, with few exceptions, Paleoproterozoic titanites (1.74–1.95 Ga). In the Karelian craton, the age of titanite, in general, records the cratonization of the Earth’s crust (2.8 Ga and 2.6–2.7 Ga, respectively). In the Belomorian mobile belt, it presumably reflects the timing of the exhumation of tectonic nappes from the mid-crustal depths during the collisional stage of the evolution of the Lapland-Kola orogen.

Geothermochronology based on noble gases: I. Stability of the U-Xe isotopic system in nonmetamict zircons

The paper presents data on persistent tendencies and relations in the migration of noble gases in U-bearing minerals of various composition: uraninite, pitchblende, metamict zircon, khlopinite, samarskite, betafite, and ampangabeite from various regions worldwide. The escape curves of all noble gases (starting with radiogenic He, Kr, and Xe and ending with nucleogenic 38Ar) during annealing in the laboratory are demonstrated to include three or four extrema, which suggest a change (increase or decrease) in the escape of rare gases at certain temperatures. This, in turn, implies that noble gas atoms are contained in the structures of minerals in various energy states and that all calculations of migration parameters of noble gases according to the classic diffusion model based on Fick’s laws are inaccurate. The paper presents a brief description of an alternative mechanism underlain by an exponential dependence of the migration kinetics on temperature and time. A principle is proposed for the isotopic dating of minerals based on the application of the neutron-induction U-Xe techniques. The paper presents a technique for studying the escape of noble gases from minerals. Experimental data are obtained on the escape kinetics of radiogenic and neutron-induced (in a nuclear reactor) Xe components from four nonmetamict zircon samples. One of them (zircon from Sri Lanka) is characterized by a particularly homogeneous crystal structure. Its optical examination indicates that this transparent gem-quality zircon contains no inclusions or structural flaws or defects. Its cathodoluminescence images show only thin, weakly contrasting zoning of the magmatic type. Mass spectrometric SHRIMP II data reveal a very homogeneous distribution of U and Th in the zircon grain. Nevertheless, the curve of Xe escape kinetics for this sample shows well pronounced extrema, which suggest that Xe atoms are characterized by different settings in the zircon structure: they can be accommodated at defects and structural zones of various types (crystalline, amorphous, and metamict). According to literature data, such zones are nanometer-sized. For each of the detected extrema, which correspond to the discrete energy settings of Xe atoms in the structure, migration parameters are calculated: the activation energy and frequency factor, which are proved to be strongly correlated. The stability of radiogenic Xe in the structure of crystalline (nonmetamict) zircons calculated from these parameters was demonstrated to be extremely high.

The early crust of the Volgo-Uralian segment of the East European Craton: Isotope-geochronological zirconology of metasedimentary rocks of the Bolshecheremshanskaya Formation and their Sm-Nd model ages

We present the results of isotope-geochronological study of metasedimentary rocks of the Bolshecheremshanskaya Formation of the Volgo-Uralian segment of the East European Craton carried out to identify their protoliths. 16 samples of high-alumina gneisses from well cores were studied using the Sm-Nd isotope method and TNd(DM) model ages. Accessory zircons were selected from rocks with the most ancient model ages (more 3.2 Ga) in three wells: Minnibaevskaya 20000, Novo-Elkhovskaya 20009, and Zai-Karatayskaya 12930 in South Tatarstan. The isotope U-Pb dating of 200 zircon grains was performed on a Cameca 1280 NORDSIM secondary ion mass spectrometer at the Natural History Museum (Stockholm, Sweden). The most applicable sites for analysis of zircon crystals were pre-selected based on cathodoluminescence images. The analytical results demonstrate the diversity of zircon groups in age from 3.8 to 2.6 Ga and together with geochemical features of metasedimentary rocks of the Bolshecheremshanskaya Formation suggest the heterogeneous composition and age of provenance areas under denudation. Occurrence of Eoarchean and Paleoarchean zircons in the clastic material of the protolith of the Bolshecheremshanskaya gneisses indicates the existence of Early Archean crustal terrains in Volgo-Uralia.

Late Riphean age of the crystalline basement of the carbonate cover of the Dzabkhan microcontinent

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.