Neoproterozoic metamorphic evolution in the Transangarian Yenisei Ridge: Evidence from monazite and xenotime geochronology

Chemical mapping and in situ dating of U-Th-rich minerals in zoned garnets from gneisses of the Garevka metamorphic complex were used to constrain multiple metamorphic events in the Transangarian Yenisei Ridge. The data provide supporting evidence for three distinct metamorphic stages. The first episode occurred as a result of the Grenville orogeny during the Late Mesozoic and Early Neoproterozoic (1050–850 Ma) and was marked by low-pressure zoned metamorphism and a metamorphic field gradient with dT/dH = 20?30°C/km typical of orogenic belts. At the second stage, the rocks experienced Late Riphean (801–793 Ma) syn-collisional medium-pressure metamorphism with a low metamorphic field gradient (dT/dH ≤ 10°C/km). The final stage evolved as a synexhumation dynamic metamorphism (785–776 Ma) with dT/dH ≤ 12°C/km and reflected rapid exhumation of rocks in shear zones. The sequence of collisional events within the western margin of the Siberian craton affected by the Valhalla orogen suggests that Siberia and cratons of the North Atlantic region were in close proximity to one another at about 800 Ma, which is supported by recent paleomagnetic reconstructions.     

P-T-t constraints on the metamorphic evolution of the Transangarian Yenisei Ridge: Geodynamic and petrological implications

Two metamorphic complexes of the Yenisei Ridge with contrasting composition are analyzed to unravel their tectonothermal evolution and geodynamic processes during the Riphean geologic history of the area. The structural, mineralogical, petrological, geochemical and geochronological data are used to distinguish two stages of the evolution with different ages, thermodynamic regimes, and metamorphic field gradients. Reaction textures, chemical zoning in minerals, shapes of the P-T paths, and isotope dates provide convincing evidence for a poly metamorphic history of the region. The first stage is marked by the formation of the ~ 970 Ma low-pressure zoned And-Sil rocks (P = 3.9-5.1 kbar, T = 510–640 °C) of the Teya aureole and a high metamorphic field gradient with dT/dH = 25–35 °C/km typical of many orogenic belts. At the second stage, these rocks experienced Late Riphean (853–849 Ma) collisional medium-pressure metamorphism of the kyanite-sillimanite type (P = 5.7-7.2 kbar, T = 660–700 °C) and a low metamorphic field gradient with dT/dH < 12 °C/km. This metamorphic event was almost coeval with the Late Riphean (862 Ma) contact metamorphism in the vicinity of the granitic plutons, which was accompanied by a high metamorphic field gradient with dT/dH > 100 °C/km. At the first stage, the deepest blocks of the Garevka complex in the vicinity of the Yenisei regional shear zone underwent high-pressure amphibolite-facies metamorphism within a narrow range of P = 7.1-8.7 kbar and T = 580–630 °C, suggesting the burial of rocks to mid-crustal depths at a metamorphic field gradient with dT/dH ~ 20–25 °C/km. At the second stage, these rocks experienced the Late Riphean (900–850 Ma) syn-exhumation dynamometamorphism under epidote-amphibolte facies conditions (P = 3.9-4.9 kbar, T = 460–550 °C) and a low gradient with dT/dH < 10 °C/km accompanied by the formation of blastomylonitic complexes in shear zones. All these deformation and metamorphic events identified on the western margin of the Siberian craton are correlated with the final episodes of the Late Grenville orogeny and provide supporting evidence for a close spatial connection between Siberia and Laurentia during early Neoproterozoic time, which is in good agreement with recent paleomagnetic reconstuctions.     

Exhumation rate of rocks from Neoproterozoic collisional metamorphic complexes of the Yenisei Ridge

Doklady Earth Sciences -     

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.     

Geochemistry,protolith origin,and age of Lower Proterozoic Fe- and Al-rich metapelites in the Transangarian Yenisei Ridge

Doklady Earth Sciences -     

Zoning in garnets—Diffusion models

When reference frames and diffusing components are properly selected, the flux equations for diffusion of the major components in natural garnets may often be approximated by: J DC. In such cases it is shown that, for reasonable diffusion coefficients and boundary conditions, observed zoning profiles in natural garnets may be explained with pure diffusion models. These models allow for original inhomogeneities in the host rock and may be used to explain why single hand specimens may show such a variety of zoning profiles within a single mineral species.     

Thermochronological models for the evolution of A-type leucogranites in the Neoproterozoic collisional orogen of the Yenisei Ridge

Results of complex geological, petrological, geochemical, and isotope-geochronological studies of Neoproterozoic postcollisional A-type granites of the Glushikha complex, Yenisei Ridge, are presented, as well as results of mathematical modeling. The localization, sizes, and depths of formation of magmatic bodies and the physicochemical conditions of the formation of granite intrusions and their magmatic sources are considered. The research is focused on the modeling of the thermal history of the formation and subsequent cooling of granites in the collision orogen. The modeling of heat transfer during the acid-magma intrusion was performed by the example of the Lendakha and Glushikha leucogranite plutons. With the combined methods of the modeling of heat transfer and the behavior of K/Ar isotopic system, the models for leucogranite pluton cooling have been verified for the first time. The time of formation and cooling of granite bodies is estimated, and a generalized geodynamic model for the formation of the Glushikha postcollisional A-type leucogranites is proposed.     

Metamorphic evolution of ultrahigh-temperature Fe- and Al-rich granulites in the south Yenisei Ridge and tectonic implications

This study provides the first evidence for the occurrence of ultrahigh-temperature (UHT) granulite-facies metamorphism in the Yenisei Ridge (Angara–Kan block). UHT metamorphism is documented in Fe-Al-rich metapelites on the basis of the garnet–hypersthene–sillimanite–cordierite–plagioclase–biotite–spinel–quartz–K-feldspar assemblage. Microtextural relationships and compositional data for paragneisses of the Kan complex attest to three distinct metamorphic episodes: (M1) pre-peak prograde (820?900°C/5.5–7 kbar), (M2) peak UHT (920–1000°C/7–9 kbar), and (M3) post-peak retrograde (770?900°C/5.5–7.5 kbar). The observed counterclockwise P–T evolution at a high geothermal gradient (dT/dP = 100–200°C/kbar) suggests that UHT metamorphic assemblages were formed in an overall extensional tectonic setting accompanied by underplating of mantle-derived mafic magmas, which may be sourced from ~1750 Ma giant radiating dike swarms linked to the Vilyuy mantle plume as part of the Trans-Siberian LIP. The broad synchroneity of UHT metamorphism (1744 ± 26 Ma; monazite–zircon isochron age) and rift-related endogenic activity in the region can provide an additional line of evidence for the two-stage evolution of granulite-facies metamorphism in the Angara–Kan block. The Aldan–Stanovoy, Anabar, and Baikal basement inliers of high-grade metamorphic rocks within the Siberian craton record two Paleoproterozoic peaks (1.9 and 1.75 Ga) of granulite-facies metamorphism. The synchronous sequence of tectonothermal events at the periphery of the large Precambrian Laurentian, Baltica, and Siberian cratons provide convincing evidence for their spatial proximity over a wide time interval, which is consistent with the most recent paleomagnetic reconstructions of the Proterozoic supercontinent Nuna.     

Comparison of diagenetic and low-grade metamorphic evolution of chlorite in associated metapelites and metabasites: an integrated TEM and XRD study

Chlorite is a common sheet silicate that occurs in various lithologies over a wide grade range involving diagenesis and low‐grade metamorphism. Thus, the reaction progress of chlorite offers a unique opportunity for direct correlation of zonal classification of metasedimentary rocks based on illite crystallinity with metabasite mineral facies. To provide such correlation, chlorite crystallinity indices, apparent mean crystallite sizes and lattice strains, crystallite size distributions and compositions of chlorite from coexisting metapelites and metabasites were determined by X‐ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), analytical electron microscopy (AEM) and electron microprobe (EMP) methods. Samples were from Palaeozoic and Mesozoic formations of the Bükkium (innermost Western Carpathians, Hungary) that underwent Alpine (Cretaceous) orogenic metamorphism. Metapelites range in grade from late diagenesis to epizone, whereas metabasites vary from prehnite–pumpellyite through pumpellyite–actinolite to greenschist facies. Despite significant differences in composition, mineral assemblages and textures, reaction progress, as measured in part by chlorite crystallinity, in metapelites paralleled that in metabasites. Chlorite crystallinity and mean crystallite size increase and the proportion of mixed layers in chlorite decreases, whereas the calculated lattice strain does not change significantly with increasing metamorphic grade. Similar trends, but (especially at higher grades) significant differences, were found in mean crystallite size values using various methods for XRD line profile analyses. The increase in crystallite size with increasing grade was demonstrated also by direct TEM measurements on ion‐milled whole‐rock samples, but with a larger scatter of data at higher grades. In spite of the different kinds of mixed layering in chlorite (Mg‐rich smectitic, mostly random, local corrensite‐like units in metabasites, and Fe‐rich berthierine and dioctahedral smectite in metapelites), XRD‐calculated and TEM‐measured parameters were found to be reliable tools for measuring reaction progress and metamorphic grade of the same degree in both lithotypes.     

Geochemistry,petrogenesis and age of metamorphic rocks of the Angara complex at the junction of South and North Yenisei Ridge

The mineralogical, petrological, geochemical and geochronological data were used to evaluate the age and petrogenesis of compositionally contrasting metamorphic rocks at the junction between Meso-Neoproterozoic Transangarian structures and Archean-Paleoproterozoic complexes of the Angara–Kan inlier of the Yenisei Ridge. The studied metabasites and metapelites provide clues for understanding the evolution of the region. The magmatic protoliths of low-Ti metabasites were derived by melting of depleted (N-MORB) upper mantle, and their high-Ti counterparts are interpreted to have originated from an enriched mantle source (E-MORB). The petrogeochemical characteristics of protoliths of the metabasite dikes resemble those of within-plate basalts and ocean island tholeiites. The Fe- and Al-rich metapelites are redeposited and metamorphosed products of Precambrian weathering crusts of kaolinite and montmorillonite-chlorite-hydromica compositions. The Р–Т conditions of metamorphism (4.9–5.5 kbar/570–650°С for metabasites; 4.1–7.1 kbar/500–630°С for metapelites) correspond to epidote–amphibolite to amphibolite facies transition. The evolution of the Angara complex occurred in two stages. The early stage (1.18–0.85 Ga) is associated with Grenville tectonics and the late stage is correlated with accretion/collision episodes of the Valhalla orogeny, with the peaks at 810–790 and 730–720 Ma, and the final stage of the Neoproterozoic evolution of the orogen on the southwestern margin of the Siberian craton. The correlation of regional crustal processes with globalscale geological events in the Precambrian evolution of the Earth supports recent paleomagnetic reconstructions that allow a direct, long-lived (1400–600 Ma) spatial and temporal connection between Siberia, Laurentia, and Baltica, which have been parts of ancient supercontinents.     

Late Vendian postcollisional leucogranites of Yenisei Ridge

The Late Vendian (540–550 Ma) U–Pb zircon age of postcollisional granitoids in the Osinovka Massif was obtained for the first time. The Osinovka Massif is located in rocks of the island-arc complex of the Isakovka Terrane, in the northwestern part of the Sayany–Yenisei accretion belt. These events stand for the final stage of the Neoproterozoic history of the Yenisei Ridge, related to the completing accretion of the oceanic crust fragments and the beginning of the Caledonian orogenesis. The petrogeochemical composition and the Sm–Nd isotopic characteristics support the fact that the granitoid melt originated from a highly differentiated continental crust of the southwestern margin of the Siberian Craton. Hence, the granite-bearing Late Riphean island-arc complexes were thrust over the craton margin at a distance considerably exceeding the dimensions of the Osinovka Massif.     

Grenville tectonic events and evolution of the Yenisei Ridge at the western margin of the Siberian Craton

Geological, petrologic, geochemical, and isotopic geochronological evidence for Grenville events at the western margin of the Siberian Craton are considered. These events were related to assembly of the Rodinia supercontinent. Multiple manifestations of riftogenic and within-plate magmatism at the final stage of orogenic evolution gave rise to breakdown of Rodinia and the formation of the Paleoasian ocean. The results allowed us to develop a new concept on the Precambrian geological evolution of the Yenisei Ridge and the processes that created its tectonic structure. The chronological sequence of events in the history of the Transangarian Yenisei Ridge is based on geological evidence and isotopic dating of Precambrian complexes variable in geodynamic nature. Four tectonic stages dated at 1.4?1.1, 1.1?0.9, 0.90?0.85, and 0.8?0.6 Ga were controlled by collision and extension recognized from large regional linear crustal structural elements. The evolution of the Transangarian Yenisei Ridge, which lasted for ～650 Ma, corresponds in duration to supercontinental cycles that begin from rifting and breakdown of the predated supercontinent and was completed by orogeny and the formation of a new supercontinent. The regional geodynamic history correlates with the synchronous sequence and similar style of tectonothermal events at the periphery of the large Precambrian Laurentia and Baltica cratons. This is evidenced by paleocontinental reconstructions, which confirm close spatiotemporal links of Siberia with cratons in the northern Atlantic 1400?600 Ma ago and indicate incorporation of the Siberian Craton into the ancient Nuna and Rodinia supercontinents.     

Chemistry and genesis of garnets in metamorphic rocks

250 analyses of garnets in metamorphic pelitic, arkosic, and pyroclastic rocks from the Stavanger area have been made by means of a microprobe (ARL-EMX). The relations between chemistry and metamorphic grade are discussed. The dependence of the Mn content upon temperature, pressure, chemistry of the parent rocks and oxygen fugacity is a very sophisticated one.It is shown that the Mn content of the garnets depends on the oxygen fugacity of the system and that the oxygen fugacity is strongly linked to the behaviour of graphite during increasing metamorphism.Examples described by Miyashiro (1953) and Engel and Engel (1960) are in agreement with the model presented here.Some observations show an influence of primary sedimentary enrichments of manganese upon the first occurrence of garnets in the course of metamorphism.