Polymetamorphic Complex Research Articles

Unraveling the polymetamorphism of calc-silicate rocks from 639 to 561 Ma in the Western Gondwana (Passo Feio Complex, Dom Feliciano Belt, South Brazil) based on U-Pb dating in titanite, apatite and zircon

Titanite is a valuable tool for studying polymetamorphism and understanding the evolution of orogenic belts as it potentially records different stages of metamorphic evolution. The combination of geochronology and Zr-in-titanite geothermometry has been widely used in petrochronology studies. However, there is ongoing debate regarding the significance of ages and temperatures obtained from titanite. Here, we conducted the first integrated titanite, zircon, and apatite UPb study, along with Zr-in-titanite geothermometry, on calc-silicate rocks from a polymetamorphic complex of the central Dom Feliciano Belt (Western Gondwana). Our aim was to investigate the impact of various metamorphic events on titanite grains. We carried out a geochronological, geochemical, and petrographic analysis of a calc-silicate sequence of the Passo Feio Complex, which was intruded by the Caçapava do Sul Granitic Complex (CSGC). The metamorphic complex experienced regional, contact, and hydrothermal metamorphism during the Neoproterozoic, but the ages and conditions of these metamorphic events remain widely debated. UPb analyses on over one hundred titanite grains from a K-feldspar-diopside schist revealed three distinct titanite populations. The older group, dated at 639 ± 3/7 Ma (2s; n = 19), was associated with the regional metamorphism event (M1), likely initiated by the primary collision of the Dom Feliciano Belt. The intermediate group exhibited an age of 596 ± 1/6 Ma (2s; n = 91). Given the presence of high-K magmatism and a carbonatite intrusion in the study area, producing zircon UPb ages around 600 Ma, this group was associated with contact metamorphism (M2), belike influenced by these intrusive igneous activities. The youngest titanite population showed an age of 566 ± 3/6 Ma (2s; n = 6), which closely aligns, within analytical error, with the age of 561 ± 1/6 Ma (2s; n = 46) obtained from UPb dating on apatite sourced from a diopside-phlogopite schist. The younger ages observed in both titanite and apatite are attributed to alterations resulting from interactions with hydrothermal fluids (M3) during the cooling phase of the CSGC. Chemical analyses conducted with an electron microprobe assessed the Zr concentrations in fifty titanite grains, examining both bright and dark zones observed in back-scattered electron images. The lowest temperature recorded among the 50 grains was 629 °C, which corresponds to dark BSE zones. In the light zones, the minimum temperature was 639 °C. While temperatures estimated using the Zr content in titanite may not correspond precisely with UPb ages, combining titanite ages with those from other accessory minerals like zircon and apatite, along with microstructural analysis, can provide a more comprehensive understanding of orogenic belt evolution.

Quartzites of the Khobeinskaya suite of the Subpolar Urals: Material composition, age limitations and possible sources of terrigenous material

Research subject. Quartzites that make up the ridge part of the ridge Rosomaha in the northern part of the Lyapinsky meganticlinorium in the Subpolar Urals. Material and methods. Detrital zircons were isolated from quartzites and their optical and isotope-geochronological U-Pb (LA-SF-ICP-MS) studies were performed. On the basis of chemical analyzes of rocks using indicator ratios and coefficients, the conditions for the formation of deposits were established. Results. The stratigraphic position of the quartzite ridge Rosomaha was determined. The time and conditions for the formation of the Khobeinskaya formation were specified. The role of polymetamorphic complexes of the Subpolar Urals as possible sources of terrigenous material removal during the formation of the Upper Precambrian section of the Subpolar Urals was estimated. Conclusions. It was established that the quartzites belong to the ridge Rosomaha to the Khobeinskaya formation, whose age is limited to the interval of 850–800 Ma (Inzersky level). It is shown that the crystalline complexes of Fennoscandia and the Central Russian orogen, as well as polymetamorphic complexes of the crystalline basement of the Timan-Severoural margin of the East European Platform, could be sources of terrigenous material.

Reconstruction of petrophysical zoning of the Blagodatnoye gold deposit in the Yenisei Ridge: Geodynamic and physical-chemical aspect

Research subject. Petrophysical zoning of the Blagodatnoye gold-sulfide deposit in the Yenisei Ridge. Aim. To determine indicative petrophysical characteristics of the products of the main occurrence stages and to develop an evolutionary petrophysical model of the investigated deposit.Materials and methods. Physical fields were studied by the methods of magnetic and electrical exploration and gamma-spectrometry. The petromagnetic heterogeneity and mineralogical-geo chemical features of formation of polymetamorphic complexes, metasomatites and ores were studied by a neutron activation analysis of the content of rare earth and radioactive elements, petrochemical x-ray fluorescence analysis, as well as by an electron-probe microanalysis of pyrite.Results. The syncollisional fold-overthrust fault (785 Ma) of the preparatory stage provided structural control over the ore-bearing mineral-forming system. The signs of zone dislocation metamorphism include geophysical anomalies: magnetic and natural electrical anomalies due to pyrrhotite and graphite mineralization of cleavage zones on fold limbs, and specific electrical resistance from silicification zones in fold hinges. The metasomatism of the pre-ore (753 Ma) and ore (698 Ma) stages took place under rifting conditions. Pre-ore quartz-muscovite and chlorite metasomatites with carbon mineralization and supra-background Au concentrations were formed under the action of reducing reaction solutions; they remained unaltered in the non-productive part of the deposit. These formations are characterized by elevated concentrations of radioactive elements and natural electrochemical polarizability. During the ore stage, Au was concentrated by fluids with hydro-carbonate-sulfide composition under the violation of the strike-slip kinematics, which caused significant petrophysical transformations of the productive part of the deposit. Early carbon metasomatites in the sub-ore and root sections of the ore bodies were depleted in terms of U, at the same time as retaining their electrochemical activity. Uranium accumulated in the upper horizons of the productive part, whose rocks lost their polarizability due to scattered carbonate mineralization. Magnetic pyrrhotite crystallized as part of sulfides with a regular increase in its proportion in the root sections of the ore bodies. At the final stage (368 Ma), the mineralized zone was broken into a series of blocks with unequal vertical displacements and levels of erosional truncation by upcasts. This led to the exposure of various-depth sections with contrast petrophysical characteristics.Conclusions. The Blagodatnoye deposit was formed in four stages: preparatory, two ore-generating and final. The petrophysical features of the products of each stage formed the basis for the developed evolutionary petrophysical model, which will be tested on the materials of geophysical studies of the Yenisei Ridge territories.

On the relationships between the Al2SiO5 polymorphs during formation of blastomylonites (North Yenisei Ridge)

Relevance. When mapping the vast areas of the Precambrian polymetamorphic complexes in the North Yenisei Ridge, there is a problem of metamorphism interpretation and phasing of geological development of a particular area along with thrust tectonics. The solution of these issues is also of great importance for the purposes of areas delineation of metamorphic rocks that are favorable for the detection of high-alumina (andalusite, kyanite, sillimanite) schists. Purpose of the work: to substantiate and itemize some geological prospecting, mineralogical and petrological indicator criteria for the development of high-alumina garnet-kyanite-staurolite blastomylonites of dislocation metamorphism formed by andalusite-bearing rocks of regional metamorphism. Research methodology: detailed mapping of structural-metamorphic zoning of dislocation (collisional) metamorphism in the Mayakon key area with sampling of polymetamorphic rocks for petrographic studies of mineral parageneses. Investigation of polished thin sections of polymetamorphic rocks by microprobe analysis with elucidation of minerals zoning, their chemical composition, calculation of the Р–Т paths of metamorphism and determination of the absolute age of blastomylonite formation based on the 40Ar/39Ar dating of biotite. Analysis and generalization of the results obtained for the Mayakon area and their comparison with other key areas of the North Yenisei Ridge. Results. At the Mayakon potential area, a progressive metamorphic zoning of kyanite-bearing blastomylonites has been identified, and the transitional I, outer II, middle III, and inner zones are determined as the dislocation metamorphism intensifies towards the Panimba thrust fault. Based on the compositions of garnets, biotite, and plagioclase, the P–T paths of the early regional metamorphism of andalusite-sillimanite type and late local kyanitesillimanite type were calculated. A list of geological prospecting, petrological and isotope-geochronological criteria for recognizing blastomylonites among rocks of regional metamorphism in thrust zones has been substantiated and itemized. Conclusions. Method of polymetamorphism reconstruction in the North Yenisei Ridge shows that tectonic inversion conditions took place in the Neoproterozoic, in the late Tonian era (~850 Ma ago In terms of occurrences, they are related to the final stage of the Grenville orogeny (1.1–0.85 Ga). The formation of blastomylonites of dislocation (collisional) metamorphism by metapelites of regional metamorphism in thrust zones is accompanied by an increase in the number of mineral phases and leads to a reduction in usable space of high-alumina andalusite schists.

Paleoproterozoic Metamorphism of the Archean Tuntsa Suite, Northern Fennoscandian Shield

The Tuntsa Suite is a polymetamorphic Archean complex mainly consisting of metasedimentary gneisses. At least two strong metamorphic events can be distinguished in the area. The first took place at high temperatures in the Neoarchean at around 2.70–2.64 Ga, indicated by migmatisation and U-Pb ages of metamorphic zircon. During the Paleoproterozoic, metasedimentary gneisses were penetratively deformed and recrystallized under medium pressures producing staurolite, kyanite and garnet-bearing mineral assemblages. The suggested Paleoproterozoic PT path was clockwise where the temperature and pressure first increased to 540–550 °C and 6 kbar, crystallizing high Ca/low Mg garnet cores. The mineral compositions show that commonly garnet core was not in chemical equilibrium with staurolite but crystallized earlier, although garnet-staurolite-kyanite assemblages are common. The temperature and pressure increased to c. 650 °C and 8 kbars where staurolite and kyanite coexist. This was followed by decompression down to c. 550–600 °C and 3–4 kbars, shown by andalusite crystallization and cordierite formed in the breakdown of staurolite and biotite + kyanite. The observed garnet zoning where Mg increases and Ca decreases from the core to the rim was developed with both increasing and decreasing pressure, depending on the effective bulk composition. The U-Pb and Sm-Nd age determinations for monazite and garnet show that the Paleoproterozoic metamorphic cycle took place at 1.84–1.79 Ga, related with thrusting of the Lapland granulites onto the adjacent terranes and subsequent exhumation.

Geological study in the Iilmeny state reserve during 2010–2020

The paper presents the main results of isotopic-geochronological and mineralogical-geochemical studies at the territory of the Ilmeny State Reserve over 2010–2020. A summarized age scheme is given for various rock types of the Ilmenogorsk polymetamorphic complex. The textural-structural features of milonites and main directions of metasomatic processes in the rocks of the complex are shown.

The Lower Precambrian in the structure of paleozoic in the Subpolar Urals

In many polymetamorphic complexes of the Urals, rocks with Early Precambrian age marks have been established. But only with respect to the two polymetamorphic complexes located on the western slope of the Southern Urals: the Taratash and Aleksandrov, their indisputable belonging to the Archean-Paleoproterozoic section is recognized. They are framed by weakly metamorphosed Lower Riphean sediments and reliable geochronological data are obtained from them, which unambiguously indicate the Early Precambrian age of rock metamorphism. The available Early Precambrian age datings for other Ural polymetamorphic complexes (with the prevailing number of Late Precambrian and Paleozoic age values) are interpreted differently. Therefore, their attitude to the Lower Precambrian section has been disputed by many researchers. In the article, for the first time, based on the results of mass U-Pb dating of metamorphic zircons from the gneiss of the Nyrtin polymetamorphic complex of the Subpolar Urals, taking into account the available data, the Paleoproterozoic age of the earliest stage of rocks metamorphism (2127 ± 31 Ma) is substantiated. This gives grounds to assert that the complex under consideration, as well as the Taratash and Alexandrov complexes of the Southern Urals, belongs to the Lower Precambrian formations involved in the structure of Uralides.

Paleoproterozoic Rock Metamorphism of the Nyartin Complex, the Subpolar Urals

In the Urals, Archean and Paleoproterozoic formations are revealed as a part of some polymetamorphic complexes which crop out onto the day surface in relatively small tectonic blocks (up to the first thousand square kilometres in area). Belonging of rocks composing polymetamorphic complexes within the Western tectonic zone of the Urals, which located west of the Main Ural Fault, to the Archean-Paleoproterozoic section is most reliable. These complexes are interpreted as fragments of the heterogeneous crystalline basement of the Ural part of the East European craton. Nevertheless, belonging of only two South Ural complexes: the Taratash and Aleksandrov to the Archean-Paleoproterozoic section causes no special disputes due to the relatively weak geochronological study of the polymetamorphic formations. They are framed by the weakly metamorphosed Lower Riphean deposits; reliable geochronological data are obtained on them, which unambiguously indicate the early Precambrian age of rock metamorphism. The existing Early Precambrian dates (with the prevalence of Late Precambrian and Paleozoic ages) for other Ural polymetamorphic complexes are interpreted in different ways. Therefore, their attachment to the Lower Precambrian section is disputed by many researchers. Taking into account the already available data and the first results of mass U-Pb dating of metamorphogenic zircons from gneisses of the Nyartin polymetamorphic complex in the Subpolar Urals, the Paleoproterozoic age of the earliest stage of rock metamorphism is substantiated in the paper. This gives grounds for the conclusion that this geological object belongs, as well as the Taratash and Alexandrov complexes in the Southern Urals, to the Lower Precambrian formations involved into the Uralides’ structure.

Metamorphism, P-T-t Conditions of Formation, and Prospects for the Practical Use of Al2SiO5 Polymorphs, Chloritoid, and Staurolite (Yenisei Ridge)

The Yenisei Ridge is an accretion–collisional orogen located in the southwestern frame of the Siberian Craton in the interfluve between Podkamennaya Tunguska, Angara, Kan, and Yenisei rivers. The Precambrian mono- and polymetamorphic complexes composed predominantly of the Mesoarchean–Neoproterozoic metapelitic rocks have been studied. Based on the typification of metamorphic complexes by pressure, temperature, metamorphic gradient, as well as age of metamorphism, the location scheme of the fields of the Precambrian sedimentary–metamorphic rock which are prospective for searching deposits of high-alumina metamorphic minerals (andalusite, kyanite, and sillimanite, chloritoid, and staurolite) in the Trans-Angara segment of the Yenisei Region, was compiled. The Teya sillimanite and Panimbinsk andalusite deposits, which are confined to the fields of regional metamorphic complexes of iron-alumina metapelites of the And–Sill facies series, are recommended as a priority for the organization of prospecting works and the subsequent involvement to the metallurgical industry. These metapelites are classified as monomineral. Owing to widespread occurrence and abundance of andalusite and sillimanite, the above deposits have significant inferred resources. Stratiform deposits of garnet–staurolite and chloritoid high-alumina rocks are still insufficiently studied and should be investigated further.The prospects for the possible use of high-alumina andalusite and sillimanite together with Middle Tatarka and Kiya nepheline syenite massifs and the bauxites of the Chadobets uplift, already being explored in the region, for production of aluminum oxide, silumin, and aluminum, as well as, the prospects for the expansion of the raw material base of the Boguchansk Electrometallurgical Complex, brought into operation in 2016 in the Lower Angara region, are considered.

Structure and Metamorphism of Rocks as a Criterion of Dissection and Correlation of Polymetamorphic Complexes (On Example of the Subpolar Urals)

According to the officially accepted stratigraphic schemes, pre-Upper Riphean stratigraphic units of the Subpolar Urals are divided (bottom-up) into the Nyartin metamorphic complex (PR1), Manhobeyu (RF1), Schekurya (RF1), and Puiva (RF2) suites. Studying the structure and metamorphism of the above-mentioned structural-material units, it was found that intensively and repeatedly deformed and metamorphosed formations of the Nyartin complex, Manhobeyu and Schekurya suites can be assigned to the same structural stage. Considering the available isotopic data on the early stages of metamorphic manifestations in rocks of the Nyartin complex, it can be stated that all of these stratigraphic units are attributed to the Lower Proterozoic, with a high degree of probability. The Upper Proterozoic section in the Subpolar Urals begins with sediments of the Middle Riphean Puiva suite. In contrast to the southern regions of the Urals, there are no Lower Riphean formations there.

Geology of the Fontane talc mineralization (Germanasca valley, Italian Western Alps)

ABSTRACTThe 1:5000 scale Geological Map of the Fontane talc mineralization (FTM) aims to give new information about the origin and geological structure of an important talc mineralization occurring in the axial sector of the Italian Western Alps. The FTM is hosted within a pre-Carboniferous polymetamorphic complex which was deformed and metamorphosed during both Variscan and Alpine orogenesis, and is part of the Dora-Maira continental crust. Field mapping and underground investigations highlight that the talc bodies (i) never crop out but occur at depth along a well-defined lithostratigraphic association between micaschist, marble and gneiss and (ii) were deformed during different Alpine-related deformation phases (i.e. D1, D2 and D3 syn-metamorphic phases and post-metamorphic extensional faulting). The here defined lithostratigraphic and structural characterization of talc bodies, is an input for further research into the geodynamic context of where talc forms and for new mineral exploration outside the mapped area.

ARCHEAN AND PALEOPROTEROZOIC HISTORY OF ROCK METAMORPHISM IN THE URALS CRUSTAL SEGMENT

In the Urals, Archean and Paleoproterozoic formations were found in several polymetamorphic complexes which crop out as relatively small tectonic blocks (within the first thousands sq. km in area). It is most likely that the rocks forming the polymetamorphic complexes located within the paleocontinental Ural area extending to the west of the Main Uralian Fault belong to the Archean-Paleoproterozoic section. These complexes are interpreted as fragments of the crystalline basement of the Ural part of the East European Craton, which are included in the structure of the Uralides. Two stages of the granulite facies metamorphism - earlier Neoarchean and later Paleoproterozoic - were established in the rocks of the polymetamorphic complexes in the paleocontinental (and partially paleooceanic) Ural region. High-pressure mineral parageneses are the products of the earliest endogenous rock transformational processes in eclogite-bearing complexes, where high-pressure metamorphism is apparently complementary to the Paleoproterozoic granulite metamorphism.

Late Neoproterozoic granulite facies metamorphism in the Menderes Massif, Western Anatolia/Turkey: implication for the assembly of Gondwana

The Menderes Massif is a major polymetamorphic complex in Western Turkey. The late Neoproterozoic basement consists of partially migmatized paragneisses and metapelites in association with orthogneiss intrusions. Pelitic granulite, paragneiss and orthopyroxene-bearing orthogneiss (charnockite) of the basement series form the main granulite-facies lithologies. Charnockitic metagranodiorite and metatonalite are magnesian in composition and show calc-alkalic to alkali-calcic affinities. Nd and Sr isotope systematics indicate homogeneous crustal contamination. The zircons in charnockites contain featureless overgrowth and rim textures representing metamorphic growth on magmatic cores and inherited grains. Charnockites yield crytallization age of ~590 Ma for protoliths and they record granulite-facies overprint at ~ 580 Ma. These data indicate that the Menderes Massif records late Neoproterozoic magmatic and granulite-facies metamorphic events. Furthermore, the basement rocks have been overprinted by Eocene Barrovian-type Alpine metamorphism at ~42 Ma. The geochronological data and inferred latest Neoproterozoic–early Cambrian palaeogeographic setting for the Menderes Massif to the north of present-day Arabia indicate that the granulite-facies metamorphism in the Menderes Massif can be attributed to the Kuunga Orogen (600–500 Ma) causing the final amalgamation processes for northern part of the Gondwana.

Fluid regime of initial granite-forming processes in metamorphic complexes of various pressure: A case study of the western Baikal metamorphic belt

Syncollisional granite generated in zonal metamorphic complexes at 500–700°C and various pressure and in the presence of fluid of various composition are proved to differ in proportions of granite material generated by metamorphic, metasomatic, and magmatic processes. Monometamorphic low-pressure complexes devoid of the influx of foreign solutions are characterized by the most intense metamorphic differentiation: the dissolution and redeposition of quartz and plagioclase into lower pressure zones and the development of plagiomigmatites. The process starts with deformations of the schists at 500–550°C. The main factors (at a constant total composition of the rock) is an uneven pressure distribution and metamorphic solutions. This stage is followed by metasomatic granitization, which is initiated if shear motions and decompression are associated with influx of silicic alkaline solutions into the rocks and their transformation into migmatite (up to granite-gneiss). Granitization during this stage is controlled by an increase in the permeability of the rocks and the introduction of perfectly mobile Si, Na, and K and removal of Mg, Ca, and Fe, which modifies the composition of the rocks and makes them similar to granite. Metasomatic granitization may take place in low-pressure polymetamorphic complexes. Under a high lithostatic pressure and in the presence of water-carbon dioxide fluids (kyanite-sillimanite facies series of metamorphism), the magmatic process begins at higher temperatures. The early stages are dominated by metamorphic differentiation and metasomatic sodic migmatization. The rocks modified by these processes (their salic and mafic minerals are segregated) are affected by anatexis at 650–700°C (partial melting of the granite eutectic) and, at higher temperatures, by diatexis, which generates granite melts able to migrate. These processes are variably pronounced in all metamorphic complexes.

The early Precambrian history of rock metamorphism in the Urals segment of crust

Early Precambrian rock units in the Urals are present in several polymetamorphic complexes, which are exposed in the Urals in the form of small (<1500 km2) tectonic blocks. Their ages are Archaean (as old as 3.5 Ga) and Palaeoproterozoic. During the formation of these complexes in the early Precambrian, two stages of ultra-high-temperature (granulite) metamorphism occurred. The maximum age of the early Neoarchaean stage of metamorphism is 2.79 Ga. Evidence of this metamorphic event includes the dating of the Taratash gneiss-granulite complex of the South Urals. Gneiss-migmatite complexes, which dominate the lower Precambrian section of the Urals, were formed in the Palaeoproterozoic during the sequential appearance of granulite facies metamorphism followed by amphibolite facies metamorphism and accompanying granitization. The maximum age of the Palaeoproterozoic stage of granulite metamorphism in the Alexandrov gneiss-migmatite complex, the most well-studied complex in the South Urals, is 2.08 Ga.

The Bossons glacier protects Europe's summit from erosion

The contrasting efficiency of erosion beneath cold glacier ice, beneath temperate glacier ice, and on ice-free mountain slopes is one of the key parameters in the development of relief during glacial periods. Detrital geochronology has been applied to the subglacial streams of the north face of the Mont-Blanc massif in order to estimate the efficiency of erosional processes there. Lithologically this area is composed of granite intruded at ~303Ma within an older polymetamorphic complex. We use macroscopic features (on ~10,000 clasts) and U–Pb dating of zircon (~500 grains) to establish the provenance of the sediment transported by the glacier and its subglacial streams. The lithology of sediment collected from the surface and the base of the glacier is compared with the distribution of bedrock sources. The analysis of this distribution takes into account the glacier's surface flow lines, the surface areas beneath temperate and cold ice above and below the Equilibrium Line Altitude (ELA), and the extent of the watersheds of the three subglacial meltwater stream outlets located at altitudes of 2300m, 1760m and 1450m.Comparison of the proportions of granite and metamorphics in these samples indicates that (1) glacial transport does not mix the clasts derived from subglacial erosion with the clasts derived from supraglacial deposition, except in the lower part of the ice tongue where supraglacial streams and moulins transfer the supraglacial load to the base of the glacier; (2) the glacial erosion rate beneath the tongue is lower than the erosion rate in adjacent non-glaciated areas; and (3) glacial erosion beneath cold ice is at least 16 times less efficient than erosion beneath temperate ice. The low rates of subglacial erosion on the north face of the Mont-Blanc massif mean that its glaciers are protecting “the roof of Europe” from erosion. A long-term effect of this might be a rise in the maximum altitude of the Alps.

Tectonometamorphic evolution of the Garevka polymetamorphic complex (Yenisei Ridge)

The Garevka metamorphic complex (GMC), located at the junction of the Central Angara and Isakovka terranes (western part of the Transangarian Yenisei Ridge), was studied in terms of its tectonometamorphic evolution and geodynamic processes in the Neoproterozoic history of the region. Geological, structural, geochronological, and petrological data permitted the recognition of two stages in the GMC evolution, which differ in thermodynamic regimes and metamorphic field gradients. These stages were related to crustal contraction and extension within the Yenisei regional shear zone, a large lineament structure in the region. Stage 1 was marked by the formation of metamorphic complexes in the middle to upper amphibolite facies moderate-pressure regional metamorphic settings at ∼960 Ma, P = 7.7–8.6 kbar, and T = 582–631 °C. This suggests subsidence of the area to the middle continental crust with dT/dH = 20–25 °C/km. During stage 2, the rocks experienced Late Riphean (∼880 Ma, SHRIMP II U–Pb and 40Ar–39Ar dating) dynamic metamorphism under epidote-amphibolite facies conditions (P = 3.9–4.9 kbar; T = 461–547 °C), indicating a metamorphic field gradient of dT/dH no greater than 10 °C/km, with the formation of blastomylonites in narrow zones of ductile and brittle deformations. In these zones, high-grade GMC blocks were exhumed to the upper continental crust and underwent low-temperature metamorphism. Comparison of the structural, geologic, and other evolutionary features (nearly identical age constraints in view of exhumation rate, similar PT-paths, and different types of metamorphism associated with different geodynamic settings, etc.) of the Garevka and Teya complexes suggests that they constitute a single polymetamorphic complex.

Tectonic interpretation of the thermochronological data and PT-conditions of rock metamorphism in the Bodonchin zonal complex ( Mongolian Altay)

Based on the new petrological and thermochronological data, analysis of the metamorphism conditions and tectonic evolution of the Bodonchin zonal complex in the Mongolian Altay was performed. Using mineral geothermometers and geobarometers, the parameters of the thermal state of the Mongolian Altay crust site during the collision of terranes were estimated, and the paleogeotherm at the peak of syncollisional metamorphism was reconstructed. The thermal state of the crust was determined by either a high concentration of radioactive heat sources or a high mantle heat flow. The estimated metamorphism temperatures and pressures of rocks in two zones (staurolite-kyanite schists and migmatites) of the Bodonchin complex correspond to the paleogeotherms with average temperature gradients ∂ T/∂ z = 25.5 and 27.2 ºC/km. The results of isotope dating of zircons and metamorphic minerals were used to construct a thermochronological model for the regressive stage of evolution of the polymetamorphic complex. The rates of the ascent of metamorphic rocks to the surface as a result of thrusts in the Bulgan Fault zone were estimated at 0.3–1 mm/year.

The Teya polymetamorphic complex in the Transangarian Yenisei Ridge: An example of metamorphic superimposed zoning of low- and medium-pressure facies series

Two successive phases of metamorphism can be recognized based on mineralogical and petrological observations coupled with geothermobarometric estimates for chemical zoning in Fe- and Al-rich metapelites from the Teya crystalline rocks of the Transangarian Yenisei Ridge. The first phase is marked by the formation of low-pressure regional metamorphic complexes of the andalusite-sillimanite type (P = 3.9–5.1 kbar; T = 510–640°C), which were most likely related to the Middle Riphean Grenville events. In the second phase, metapelitic rocks underwent Late Riphean medium-pressure collisional metamorphism of the kyanite-sillimanite type (P = 5.7–7.2 kbar, T = 660–700°C), which resulted locally in an increase in pressure in the vicinity of thrusts. These results suggest that medium-pressure kyanite-bearing metapelitic rocks were formed as a result of collision-related metamorphism caused by thrusting of the Siberian cratonal blocks onto the Yenisei Ridge in the vicinity of the Tatarka deep fault.

Isotope-geochronological timing of metamorphic events in the boundary zone between the Aldan Shield and the Dzhugdzhuro-Stanovoi Folded Area

Previous studies demonstrated [1–3] that age esti� mation of leucosomes of anatectic and diatectic mig� matites and products of granitization (granite gneiss) is an effective way to understand the sequence of the formation of polymetamorphic complexes, because their relative sequence is easily established on the base of simple, uniquely inter preted geological observa� tions. Migmatites of the Stanovoi complex in the zone of direct junction between the Dzhugdzhuro–Stano� voi Folded Area (DSFA) and the Aldan Shield (AS), which were investigated in the section along the Malyi BAM in the area of Nagornyi Village and Kholodni� kan Pass, are the object of this study. We already stud� ied the relations between the Aldan and Stanovoi com� plexes in the early 1960s [4]. It was revealed that over� thrust–sheet structure was developed in this zone and the tectonic plate moderately dips to the south. They are deformed in linear open or isoclinal folds oriented in the latitudal direction parallel to the whole junction zone. Biotite granite gneiss (Lc 1 ) replacing biotite– amphibole gneiss and amphibole–pyroxene crystalline schist is the earliest in the whole sequence of ultrameta� morphogenetic granitoids. At least three generations of anatectic leucosomes (Lc2–4 ) are observed in granite gneiss; they form isoclinal folds in plates of tectonic lay�