Diffusion modelling is applied to layered garnet-pyroxene-quartz coronas, formed by a pressure-induced reaction between plagioclase and primary pyroxene in a metabasic granulite. The reconstructed reaction involves some change in composition of reactant minerals. The distribution of minerals between layers is satisfactorily explained by diffusion-controlled reaction with local equilibrium, in which the diffusion coefficient for Al was smaller than those for Fe, Mg and Ca by a factor of approximately four. Diffusion of Mg towards plagioclase implies a chemical-potential gradient for MgO component in a direction opposite to the changing Mg content of garnet; this is explained by the influence of Al 2 O 3 on the chemical potential of the pyrope end-member. Grain-boundary diffusion is suggested to have operated, possibly with composition gradients different from those in the bulk minerals. Chemical-potential differences across the corona are estimated from the variation in garnet composition, enabling affinity (the free energy change driving the reaction) to be estimated as 6.9±1.8 kJ per 24-oxygen mole of garnet produced. This implies that the pressure for equilibrium among the minerals was overstepped by 1.4±0.4 kbar. The probable P-T conditions of reaction were in the range 650-790°C, 8-10 kbar. Assuming a timescale of reaction between 106 and 108 years, estimated diffusion coefficients for Fe, Mg and Ca are in the range 9×10−23 to 5×10−20 m2 s−1. These are consistent with experimental values in the literature for solidstate diffusion, including grain-boundary diffusion.
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 com pleted 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 Pre cambrian Laurentia and Baltica cratons. This is evidenced by paleocontinental reconstructions, which con firm close spatiotemporal links of Siberia with cratons in the northern Atlantic 1400-600 Ma ago and indi cate incorporation of the Siberian Craton into the ancient Nuna and Rodinia supercontinents.
Basic and ultrabasic rocks in high- and ultrahigh-pressure collision belts can provide important petrological information. Mantle-derived and “crustal” peridotites and pyroxenites are recognized among these rocks in Phanerozoic orogenic zones. The former were emplaced as mantle magma intrusions or tectonically transferred solid fragments of mantle material in the deeply subsided lithosphere, while the latter are shallow complexes of dikes and sills, which were altered and metamorphosed during subduction process. Both rock types were later exhumed at the Earth’s surface. For geochemical comparison of these types, four groups of rocks were chosen: two sample sets of mantle-derived rocks and two sets of crustal rocks. The mantle-derived rocks include a set of spinel and garnet peridotites of alpine-type bodies from the Eastern and Western Alps and the Ronda massif in Spain as well as a set of pyroxenites from the Eastern Alps. The “crustal” rocks include a set of garnet and spinel peridotites from the Kokchetav massif, northern Kazakhstan, and garnet peridotites from the Western Gneiss Region, western Norway, together with a set of pyroxenites from the Kokchetav massif. Geochemical investigation has revealed that mantle-derived peridotites are characterized, as a rule, by high contents of MgO (35–46 wt.%), Cr (1750–12,770 ppm), and Ni (900–2500 ppm), low contents of FeO (5–10 wt.%), TiO2 (0.01–0.3 wt.%), Zr (0.002–1.2 ppm), Nb (0.001–0.3 ppm), Sm (0.003–0.5 ppm), La (0.005–1 ppm), and Yb (0.006–0.54 ppm), and the total content of REE equal to 0.06–5.2 ppm. Mantle-derived pyroxenites contain 27–35 wt.% MgO, 2300–3300 ppm Cr, 5.5–9 wt.% FeO, 0.02–0.08 wt.% TiO2, 0.2–1.4 ppm Zr, 0.007–0.06 ppm Nb, 0–0.13 ppm Sm, 0.007–0.23 ppm La, 0.02–0.2 ppm Yb, and 0.05–1.6 ppm total REE. “Crustal” peridotites are characterized by high contents of FeO (12–25 wt.%), TiO2 (0.64–2.6 wt.%), Zr (33–179 ppm), Nb (3.4–13.8 ppm), Sm (0.7–4 ppm), La (1–8 ppm), Yb (0.8–3.3 ppm), and total REE (11.5–48 ppm) as well as by comparatively low contents of MgO (15–26 wt.%), Cr (79–244 ppm), and Ni (450–730 ppm). “Crustal” pyroxenites contain 6–21.5 wt.% MgO, 90–230 ppm Cr, 11–21 wt.% FeO, 0.7–1.3 wt.% TiO2, 45–493 ppm Zr, 1–8 Nb, 1.6–4.3 ppm Sm, 4.7–14 ppm La, 1.3–7.4 ppm Yb, and 27–80 ppm total REE. These data permit us to develop D. Carswell’s idea of distinctions between the mantle-derived and “crustal” peridotites and suggest some promising geochemical criteria. The criteria are based on distinctions between the contents of MgO, FeO, TiO2, Cr, Ni, Zr, Nb, REE, etc. in peridotites and pyroxenites. Binary MgO–Cr, FeO–TiO2, La–Yb, Lu–Nd, Eu–Gd, and Sm–ΣREE diagrams give an opportunity to discriminate the compositions in detail and are the most appropriate for practical use. The obtained information may be helpful in understanding the nature of protoliths when studying mafic/ultramafic granulites in high-grade metamorphic rocks.
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.
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 polymetamorphic 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.
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