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.     

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.     

Quantitative analysis of mass transfer during polymetamorphism in pelites of the Transangarian Yenisei Ridge

The study provides geological, structural, mineralogical, petrological, and geochronological evidence for polymetamorphic evolution of gneisses from the Garevka complex of the Yenisei Ridge. The results of the study provide significant insight into the geochemical behavior of major and trace elements in zoned garnet crystals and mineral inclusions formed during prograde and retrograde metamorphism of pelitic rocks. It was shown that the concentrations of Y and HREE in garnet decrease with increasing P and T and increase with decreasing pressure and temperature. The combined study of multicomponent chemical zoning patterns of coexisting minerals and metamorphic mineral reactions in metapelites was conducted. The results show that the main reason for a drastic increase in CaO content in garnets during collisional metamorphism is a mass exchange between garnet and plagioclase. The deviation from this trend, as indicated by the concurrent increase inthe grossular content of garnet and anorthite content of plagioclase, arises from the breakdown of epidote. The calculated metamorphic reactions, mass balance analysis, and changes in mineral chemistry during metamorphism reinforce the evidence for the isochemical character of processes with respect to most components of the system. The minimum volume of the system in which chemical exchange between reacting phases is balanced for all major and trace elements did not exceed ~ 1 mm³. The total HREE balance requires a greater reaction volume (up to ~ 8 mm ) involved in the redistribution of these elements, which provide evidence for their relatively higher mobility during metamorphism relative to other rare earth elements. The specific distribution and quite substantial mass transport of HREE are controlled by heterovalent isomorphic substitution between these elements and CaO in garnet.     

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

Doklady Earth Sciences -     

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.     

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

Doklady Earth Sciences -     

Age and Source Areas of Detrital Zircons from the Rocks of the Yenisei Tectonic Zone: to the Problem of Identification of Archean Metamorphic Complexes in the Transangarian Yenisei Ridge

The petrogeochemical and geochronological correlations were carried out between boudined fragments of tonalitic rocks previously dated at Neoarchean, quartzite sandstones, and host amphibolites in the Yenisei Regional Shear Zone of the Yenisei Ridge in order to solve the problem of age of the Transangarian Yenisei Ridge basement. Detrital zircons in metasandstones can be derived from the Neoarchean–Paleoproterozoic crystalline rocks of the Angara–Kan block. Interpretation of available data does not confirm the inferred presence of the Early Precambrian basement of the Siberian Craton beneath the Transangarian Yenisei Ridge.     

Nature and Age of Detrital Zircons from Rocks of the Shear Zone: The Problem of Occurrence of the Archean Basement in the Transangarian Yenisei Ridge

Petrogeochemical and geochronological study of boudined quartzite sandstones and tonalites, as well as host amphibolites, in the shear zone showed that potential sources of detrital zircons were Neoarchean–Paleoproterozoic rocks of the Angara–Kan block and products of their metamorphism. Interpretation of the available data does not confirm the inferred presence of the Early Precambrian basement of the Siberian Craton in the Transangara region.     

Zoning of garnets as an indicator of metamorphic evolution in metapelites of the Yenisei Ridge

The geochemical patterns of major and trace elements in zonal garnets and the mineral inclusions in them formed by progressive and regressive metamorphism of pelites are established. It is shown that an increase in temperature and pressure led to a decrease in the Y and HREE contents in garnets, and the increase in their contents is related to a decrease in the PT-parameters of their formation. A negative correlation between the CaO and REE contents in garnet indicates their isomorphism. The main reason for the sharp increase in the CaO content in garnets during collision metamorphism is mass transfer between the garnet and the plagioclase. The deviations from this situaiton, which are expressed in simultaneous increase in the grossular component in garnet and the anorthite component in plagioclase, are caused by metamorphic reactions related to the epidote decomposition. The mass transfer of major and trace elements between the reacting phases in metamorphic reactions mostly occurred with preservation of the balance of matter. The mirror shape and the character of the REE patterns of the rock-forming minerals relative to the composition of the rock indicate the equilibration of the HREE and Y contents between garnet, the major concentrator of these elements in the rock, and other phases. The balance between the LREEs and HREEs in the rock is achieved by the presence of variable amounts of monazite.     

The Blagodatnoe gold-sulfide deposit (Yenisei Ridge,Russia): the nature of geophysical anomalies and the succession and causes of formation of petrophysical zoning

Data on geophysical fields and petrophysical heterogeneity, parageneses and the thermodynamic conditions and age of their formation, and fluid inclusions were used for the genetic reconstruction of petrophysical zoning at the Blagodatnoe gold-sulfide deposit. Petrophysical associations of the preore and ore stages of the deposit formation are clearly reflected in anomalies of the magnetic and natural electric fields and the aureoles of radioactive elements. At the early preore stage (752 Ma), reduced solutions with high activity of K, enriched in U, Th, and, probably, Au, were supplied to intensely foliated tectonic zones. Their interaction with initial metasedimentary rocks gave rise to contiguous zones of quartz–muscovite and chlorite metasomatic rocks. Accompanying graphitization led to a high electrochemical activity of the metasomatic rocks, which generated anomalies of up to –300 mV in the natural electric field; the most intensely carbonized zones became enriched with U (up to 6.5 × 10^–4%) and, probably, Au. The quartz–muscovite metasomatic rocks accumulated Th and K (up to 29 × 10^–4% and 4%, respectively), whereas the chlorite metasomatic rocks accumulated rock-forming elements (particularly Fe), which led to the compaction of these rocks and the acceleration due to gravity in local positive anomalies. The nonmagnetic character of the fresh pre-ore metasomatic rocks suggests the predominantly pyritic composition of early sulfides. At the ore stage (698 Ma), the minerals were deposited from H₂O–CO₂–As–S solutions at 560 to 315 °C. The activity of these solutions caused a redistribution of radioactive elements and a high petrophysical differentiation of the ore-bearing structure. The amplitudes of the anomalies above this structure vary from 500 to 80 nT in the magnetic field and from –130 to + 10 mV in the natural electric field. It has been found that the hydrothermal fluid hardly affected the polarization properties of graphitized rocks at the maximum temperatures but caused an intense removal of U and the development of magnetic pyrrhotite after pyrite. The temperature decrease in the mineral-forming system was favorable for the formation of siderite. Carbonaceous schists which experienced carbonatization lost their electrochemical activity. The binding of carbon dioxide in the solid phase influenced the migration capability of trace elements and their zonal distribution. With this evolution of the solution, Th accumulated at the lower levels of the mineralized zone, whereas the upper levels of the deposit became enriched with U. Productive gold–arsenopyrite–pyrite–pyrrhotite paragenesis with anomalous magnetic susceptibility evolved at the ore stage. The late galena–sphalerite–chalcopyrite paragenesis (365 Ma) was of strictly local occurrence and reduced the magnetic susceptibility of ores.     

P-T-t reconstructions of South Yenisei Ridge metamorphic history (Siberian craton): Petrological consequences and application to the supercontinental cycles

Studies of gneisses from the Yenisei regional shear zone (YRSZ) provide the first evidence for Mesoproterozoic tectonic events in the geologic history of the South Yenisei Ridge and allowed the recognition of several stages of deformation and metamorphism spanning from Late Paleoproterozoic to Vendian. The first stage (~ 1.73 Ga), corresponding to the period of granulite-amphibolite metamorphism at P = 5.9 kbar and T = 635 °C, marks the final amalgamation of the Siberian craton to the Paleo-Mesoproterozoic Nuna supercontinent. During the second stage, corresponding to a hypothesized breakup of Nuna as a result of crustal extension, these rocks underwent Mesoproterozoic dynamic metamorphism (P = 7.4 kbar and T = 660 °C) with three peaks at 1.54, 1.38, and 1.25 Ga and the formation of high-pressure blastomylonite rocks in shear zones. Late-stage deformations during the Mesoproterozoic tectonic activity in the region, related to the Grenville-age collision processes and assembly of Rodinia, took place at 1.17-1.03 Ga. The latest pulse of dynamic metamorphism (615–600 Ma) marks the final stage of the Neoproterozoic evolution of the Yenisei Ridge, which is associated with the accretion of island-arc terranes to the western margin of the Siberian craton. The overall duration of identified tectonothermal processes within the South Yenisei Ridge during the Riphean (~ 650 Ma) is correlated with the duration of geodynamic cycles in the supercontinent evolution. A similar succession and style of tectonothermal events in the history of both the southern and the northern parts of the Yenisei Ridge suggest that they evolved synchronously within a single structure over a prolonged time span (1385–600 Ma). New data on coeavl events identified on the western margin of the Siberian craton contradict the hypothesis of a mantle activity lull (from 1.75 to 0.7 Ga) on the southwestern margins of the Siberian craton during the Precambrian. The synchronous sequence and similar style of tectonic events on the periphery of the large Precambrian Laurentia, Baltica, and Siberia cratons suggest their spatial proximity over a prolonged time span (1550–600 Ma). The above conclusion is consistent with the results of modern paleomagnetic reconstructions suggesting that these cratons represented the cores of Nuna and Rodinia within the above time interval.     

The application of ionic equilibria to metamorphic differentiation: An example

Mineral segregations formed by metamorphic differentiation are an important source of information on diffusion processes in metamorphism. Segregations consisting of andalusite-biotite-quartz cores surrounded by a quartz-feldspar mantle in sillimanite-biotitefeldspar-quartz gneiss near Västervik, Sweden (Loberg, 1963) formed by core-to-mantle migration of K, and mantle-to-core migration of Fe, Mg and Ca. These migrations can be represented by a set of interconnected ionic equilibria involving reaction of microcline and Fe(OH)⁺ in the core to form andalusite plus biotite, and reaction of K⁺, sillimanite and biotite in the mantle to form microcline. Equilibrium constants for these reactions, calculated for conditions inferred from the mineral assemblage and biotite composition, indicate gradients of K⁺ activity (higher in core) and Fe(OH)⁺ activity (higher in mantle). These gradients result simply from the free energy difference between andalusite and sillimanite, without invoking pre-existing megascopic inhomogeneities in the rock or surface energy effects. Although small, these gradients appear to be capable of driving the segregation process.     

Calculated phase equilibria for low- and medium-pressure metapelites in the KFMASH and KMnFMASH systems

Petrogenetic grids in the KFMASH and KMnFMASH model systems calculated with the software thermocalc 3.1 are presented for the P–T range 0.5–12 kbar and 450–900 °C, for assemblages involving garnet, muscovite, chloritoid, biotite, chlorite, staurolite, cordierite, spinel, orthopyroxene, K‐feldspar, Al₂SiO₅ phases, quartz, water and melt. Based on calculated compatibility diagrams and P–T and T–M_Mn [Mn/(Mg + Fe + Mn)] pseudosections for different metapelitic bulk compositions, the principal conclusions are that the addition of Mn to the KFMASH system: (i) enhances the stability of garnet, and, to a lesser extent, aluminosilicates; (ii) reduces the stability of staurolite, cordierite and, to a lesser extent, chlorite; and (iii) extends the medium pressure stability of muscovite and the low‐P stability field of K‐feldspar. The influence of Mn on individual mineral stabilities is strongly related to rock composition, in particular, to the relative contents of Al₂O₃ and K₂O. For metapelites of a range of compositions and M_Mn values, P–T pseudosections in the KFMASH system, in most cases, do not adequately predict the mineral assemblages observed in natural assemblages under medium and low‐pressure conditions. In contrast, the P–T pseudosections in the KMnFMASH system generally provide more satisfactory results, suggesting that MnO is one of the non‐KFMASH components that should not be neglected in documenting the phase equilibria of medium‐ and low‐P metapelites.