Origin of carbonate-silicate rocks of the Porya Guba (the Lapland-Kolvitsa Granulite Belt) revealed by stable isotope analysis (δ18O, δ13C)

Introduction

The carbonate-silicate geochemical cycle (the inorganic carbon cycle [1]) includes the processes of weathering, sedimentation (with silicate to carbonate transformations), metamorphism and volcanic activity (with conversion of carbonates to silicates). Over an interval of several million years or more, this cycle controls the balance of CO₂ during interactions of the atmosphere, hydrosphere, crust, and mantle of the Earth and largely determines global climate changes [2-4]. Carbonate-silicate rocks in metamorphic complexes (hereafter, for brevity, the obsolete term calciphyres is used) represent a sequence of phase transformations in the cycle stages associated with formation of the carbonate substrate, metamorphism, degassing, and the release of significant amounts of CO₂ into the atmosphere [5-7]. Phase reactions in the carbonate-silicate cycle are accompanied by isotopic reactions, and the ratios of stable isotopes (¹⁸O/¹⁶O,¹³C/¹²C), as well as “unconventional” isotopes of magnesium, calcium silicon and titanium can serve as the most important geochemical tracers [8-10] and provide an opportunity to estimate the influence of different reservoirs on the composition of gases, temperatures of processes, as well as CO₂/H₂O balance in the atmosphere, hydrosphere and gases of deep origins. Nevertheless, stable isotope data have been mostly limited to sedimentary carbonate rocks of low metamorphic degrees; the sources of matter and conditions of calciphyre transformations within highly metamorphosed (granulite) terranes have been less studied [11, 12].

Ancient granulite terranes which are widely developed within the shields and in the crystalline basement of platforms provide important evidence on the composition and degree of transformation of the continental crust formed during early stages of the Earth history [13-15]. The Lapland-Kolvitsa Granulite Belt is a part of the Fennoscandinavian Shield, which in recent years provides a scientific testing ground for various geological models and concepts [16-18]. The Paleoproterozoic events that led to the formation of rock associations of the Lapland-Kolvitsa Belt have also been widely manifested in the adjacent tectonic structures, for example in the rocks of the Belomorian Mobile Belt [19-21].

Within the Lapland-Kolvitsa Granulite Belt among other formations, rare outcrops of calciphyres have been observed, so far with unclear origin. A general description, sporadic measurements of the chemical composition [22-24] and carbon isotope ratios [25] suggest a sedimentary precursor and possible biogenic component of some rocks of the belt, but the alternation of calciphyres with presumably metaintrusive rocks makes their assignment to metasediments doubtful. The aim of the present work is to study oxygen and carbon isotopic compositions accomplished by the phase equilibria modeling of the Porya Guba calciphyres, and to reconstruct possible protolith origins and metamorphic conditions. The results of the isotopic measurements are supplemented by new data on chemical and phase compositions of the rocks.

Methods

The isotope composition measurements

Standard procedures [26, 27] were used to extract CO₂ from carbonates (100 % H₃PO₄) and O₂ from silicates (the fluorination followed by the conversion to CO₂). The extraction yield (at least 98 %) was controlled by measuring the gas volume, the completeness of the conversion was checked by measuring the residual oxygen pressure. CO₂ isotope ratios were determined on the SIRA-9 mass spectrometer (VG-Isogas). The entire extraction and measurement cycle for each sample was repeated at least twice. In addition, the oxygen isotopic composition of the in-lab CO₂ standard was determined every 4-5 experiments. Calcite samples calibrated relative to the NBS-18 standard were used to control the carbonate determinations. The accepted acid fractionation factor of calcite (T = 25 °C) is 1.01049 [28]. Samples of quartz and potassium feldspar calibrated relative to NBS-28 were used to control the results of the silicate measurements. Isotopic data are given according to the conventional δ-notation relative to V-SMOW (for oxygen) and PDB (for carbon) standards: δ = [R_sam/R_std – 1]1000, where R_sam/R_std is the ratio of heavy and light isotopes in the sample and the standard. The average error of the values δ¹⁸O is 0.07 ‰, δ¹³C 0.05 ‰ (1σ). Sample preparation and measurement of oxygen isotope composition were performed at the Institute of Petrology and Mineralogy (Bonn).

The fractionation of isotopes between phases A and B is quantified by the value α_AB = R_A/R_B = (δ_A +1000)/(δ_B +1000). The value α_AB can be obtained using so-called “β-factors”, so that provided isotopic equilibrium 10³lnα_AB(T) = 1000lnβ_A(T) – 1000lnβ_B(T). Temperatures of the isotopic equilibrium are determined by the measured isotopic ratios of phases given the temperature dependences of the β-factors. For the purposes of the present work, β(T) dependences in the form of the cubic polynomials determined on the basis of isotopic frequency shifts [29] have been adopted.

Calculation of phase equilibria

Phase equilibria were modeled for the system SiO₂-TiO₂- -Al₂O₃-FeO-MgO-CaO-Na₂O-K₂O-H₂O-CO₂ (STAFMCNK-H₂O-CO₂) using the Perple_X (version 6.9, www.perplex.ethz.ch) [30, 31] and GeoPS [32] software. For the fluid (H₂O-CO₂), the equation of state and thermodynamic database ([33], modified in 2004) have been adopted. The cal-culations account for the phases present in the rocks, their possible precursors and transformation products. The results of  X-ray fluorescence spectrometry (Philips PW-1480 device) with an analytical error less than 3 % were taken as the input data to calculate the phase equilibria (pseudosections). Compositions of individual phases were determined on the JEOL JXA-8230 de-vice (IGGD RAS) at 15 kV accelerating voltage, 20 nA electron probe current, and 10 μm beam diameter. Calcite CO₂ content was quantified after drying the sample (for 1 h at T = 105 ^оC) using value of the gas extracted during reaction with 10 % hydrochloric acid solution. Termination of the carbonate decomposition was recorded by the cessation of the gas release.

Geological background and rock composition

The metacarbonate rocks had been distinguished as part of the lower sequence of the Kolvitsa granulite zone (Ploskotundrovskaya series [22]), and are also noted at the southeastern end of the Lapland block (Salny and Tuadash tundras).
The rocks are mostly observed in the Porya Guba area of the Kandalaksha Bay of the White Sea on the Medvezhy Island and the western coast of the Porya Guba, where outcrops of calciphyres are traced for 10-15 km along the strike (NW-SE). This paper presents data on the rocks from the Medvezhy Island and the Tamarkina Luda Island (West of the Porya Guba, Fig.1). Within the outcrop on the Medvezhy  Island, calciphyres form interlayers (extending up to several tens of meters) with a thickness from few centimeters to 50-60 cm, alternating with mafic schists (two-pyroxene, garnet-two-pyroxene, amphibole-diopside, actinolite, scapolite-bearing, etc.), acidic granulites, and variably migmatized gneisses. In places, the schists and gneisses contain sulfide mineralization. The rocks of the interbedded calciphyres and mafic schists are characterized by boudinage and delamination structures due to the different resistance of carbonates and silicates.

Metamorphic age of the rocks from the granulite complex are estimated from 1925 Ma (the beginning of metamorphism) to 1870 Ma (its termination) [34]. The protolith formation refers to the Early Proterozoic, although there is evidence of the pre-Proterozoic (the Late Archean?) age [35].

The calciphyres are presented by medium to large-grained, gray, banded rocks, usually of granoblastic (heterogranoblastic) texture. According to the mineral composition, two main groups can be distinguished within the calciphyre layers:

• Calciphyres, dominated by calcite, monoclinic pyroxene (diopside), garnet (up to 13 vol.%), quartz (0.1-0.5 mm, 5-15 %), sometimes plagioclase (0.01-1.5 mm). Calcite occurs as large polysynthetic grains up to 2-3 mm. Large colorless segregations of diopside (0.3-2 mm, reaching 30 % of the rock volume) have been replaced by tremolite and phlogopite. Garnet
  poikiloblasts (up to 1 cm) are dominated by the grossular-andradite series, with a small content of pyrope and almandine. However, in rocks with the significant plagioclase content (reaching 10-15 vol.%) garnet (sometimes relic, up to 5-7 vol.%) is dominated by almandine with pyrope. Magnesite, dolomite, sometimes olivine (forsterite), titanite or ilmenite, and apatite are present as accessories (< 5 %). For the sake of brevity, this variety will be referred to as the diopside calciphyres (di-calciphyres).
• Calciphyres dominated by forsterite, calcite with a small amount of monoclinic pyroxene. Within some rocks idioblastic (up to a few mm) calcite segregations are found. Olivine grains
  (0.3-0.5 mm) are replaced by serpentine (0.2-0.3 mm). Other secondary hydroxyl-bearing minerals are also present: mainly mica, chlorite, epidote, and amphibole (actinolite-tremolite, up to 0.5 mm). It should be noted that, despite the significant MgO content in the olivine-bearing varieties of
  the studied calciphyres, dolomite is usually found as relics. Apatite, titanite, ilmenite, and spinel (hercinite) are observed as accessories. This variety of the carbonate-silicate rocks is further referred to as the olivine calciphyres (ol-calciphyres).

In addition to the mineral composition, the two calciphyre varieties differ also by the content of the major oxides (Table 1). The olivine calciphyres have significantly higher MgO/(MgO + CaO) ratio (about 0.4) if compared with the calciphyres of the first group (less than 0.1). FeO and MnO contents of the ol-calciphyres are slightly lower than in the di-calciphyres, higher alkaline contents are noted in the olivine-bearing varieties. Differences in the concentrations of other components (including CO₂, SiO₂, Al₂O₃) are insignificant.

Table 1

The whole-rock composition of the calciphyres and spatially related Porya Guba mafic schists

Notes: sample 13 – the Tamarkina Luda Island, others – the Medvezhy  Island; LOI – loss on ignition (105 °С).

Oxygen and carbon isotopes

Measured isotopic ratios of calcite (δ¹⁸O and δ¹³C relative to SMOW and PDB, respectively) and silicates (δ¹⁸O) are given in Table 2. The δ¹⁸O value of calcite in the calciphyres vary from 15.0 до 17.8 ‰ (the mean value at 16.44±0.92 ‰) with δ¹³C varying from –3.6 to –6.2 ‰ (the mean at –4.90±0.89 ‰), suggesting the sedimentary nature of the rocks. At the same time, average δ¹⁸O of calcite from the diopside calciphyres is 17.05 (±0.61) ‰, whereas average δ¹⁸O of calcite in the olivine-bearing varieties is lower, 15.68 (±0.59) ‰. Average δ¹³C values of calcite are –4.35 (±0.65) and –5.58 (±0.67) ‰, respectively. Calcite δ¹⁸O and δ¹³C increase following the increase of the calcite content in the rock (which is expressed for example, as a linear dependence of δ¹⁸O_cal ≈ 0.15CO₂ + 13.02 wt.%, R² = 0.7). In the limit of the pure calcite rock (CO₂ = 44 wt.%), δ¹⁸O_cal, according to this relationship, is estimated at 19.6 ‰, which corresponds to the isotopic composition of the Precambrian diaphtorised carbonate sediments [36, 37]. Another noted pattern – the relationship δ¹⁸O and δ¹³C of calcite – may reflect metamorphic degassing of the rocks. Representative isotopic analyses of the calciphyres taken from the Tamarkina Luda Island for comparison follow the same pattern.

Temperature dependences of the stable isotope fractionations are determined based on the β-factors (the reduced partition function ratios):

where ν_q, i are the vibrational frequencies with the wave vector q and phonon branch index i from 1 to 3N_at; N_at is the number of atoms in a unit cell; T temperature, K; h and k – the Planck and Boltzmann constants; N is the number of atoms undergoing isotopic substitution; N_q is the number of vectors q considered in the summation; the superscript * refers to the heavier isotope.

The β-factors (calcite, quartz, monoclinic pyroxene, olivine, and garnet) between 0 and 1500 °C are interpolated by the cubic polynomials against x = 10⁶/T²:1000lnβ¹⁸O = ax + bx² + cx³ [29]. The vibration frequencies of the isotopologues of the studied phases were determined by the frozen phonon method (CRYSTAL software: www.crystal.unito.it) within the density functional theory (DFT).

Table 2

Isotopic composition of pure fraction separates

Notes: F – fraction of CO₂, remaining in calcite after degassing; CO₂(i) – CO₂ content in a rock prior to degassing (see Table 1). For samples 21, 34 the measured CO₂ concentrations are 27.22 and 30.50, respectively.

The value of δ¹⁸O within the silicate fraction of the calciphyres (quartz, pyroxene, olivine, garnet, Table 2) depends both on the isotopic composition of the carbonate fraction (the higher δ¹⁸O_cal, generally the higher δ¹⁸O of each particular phase), and the ¹⁸O/¹⁶O fractionation between different phases depending on temperature. δ¹⁸O variation of quartz in the di-calciphyres is 17.1-19.9 ‰, pyroxene 13.1-16.5 ‰. δ¹⁸O values of olivine from the ol-calciphyres vary between 12.0 and 14.8 ‰. Generally, the decrease of δ¹⁸O values occurs in the sequence quartz-calcite-(garnet-pyroxene). The lack of equilibrium between minerals is indicated by the large scatter of temperatures calculated from the distribution of isotopes between different phases of each particular sample (Table 2). For example, temperatures calculated using the quartz-calcite isotope geothermometer vary from 430 °C (sample 13) to 990 °C (sample 7) and 1110 °C (sample 34), calcite-clinopyroxene from 620 to 810 °C (on two samples).

Within rocks without carbonates, both at contacts with calciphyres and at some distance (5-10 m) the evolved δ¹⁸O (reaching 11.6-13.5 ‰ for pyroxene and plagioclase) is also observed.

Revealing primary composition of metamorphic rocks

The oxygen and carbon isotopic ratios imply primary sedimentary nature of the Porya Guba calciphyres and possibly, of the entire rock sequence studied (NW coast of the Porya Guba). Hence, the phase compositions of primary sediments were reconstructed based on the rock chemistry. Using the Perple_X program (see the methods), the restored (“model”) phase composition of rocks in the conditions of the sedimentation (25 °C, 1 bar) was calculated (Table 3).

Sediments chemically corresponding to the di-calciphyrescontained 70-85 wt.% of the carbonate fraction dominated by calcite (reaching 78 %), with subordinate amounts of dolomite (up to 10 %) and ankerite (up to 15 %). The clastic component includes quartz (reaching 22 wt.%), while kaolin dominates in the clay fraction (up to 9 wt.%). The detrital component (rutile and titanite) content is estimated at 0.1-0.2 %. The primary composition of sediments transformed into the ol-calciphyres (mostly forsterite) was characterized by significant amount of dolomite within carbonate fraction (up to 62 wt.%) together with ankerite (up to 8 %), siderite (up to 4 %), and magnesite (less than 1 %). Calcite content did not exceed 35 wt.%.

Table 3

Reconstructed composition of rocks in the conditions of sedimentation, wt.%

Notes. Composition calculated from data in Table 1 at ambient conditions T = 298.15 °C, P = 1 bar (Werami module of the Perple_X software [30]). CO₂ content calculated from the measured composition accounting the degree of degassing F and given at Table 2; in the absence of the respective data, calculations were performed under conditions of the volatile saturation (H₂O > 5 %).

Sedimentary rocks currently represented by mafic schists, contained less than 30 wt.% carbonates (as reconstructed with CO₂ excess), and probably were dominated either by the clastic components (up to 43 % quartz, sample 9) or the clay components (sample 10). Oxygen isotopic variations of the sediments in interaction with seawater at 25 °C are estimated at 31.8±2.3 ‰ (probable oxygen isotopic composition prior to the sediments lithification).

Metamorphic phase equilibria of the calciphyres

Phase equilibrium diagrams with the stability fields of phase associations in P-T coordinates (pseudosections) were constructed accounting for chemical composition of representative samples of the diopside calciphyres from the Medvezhy and the Tamarkina Luda islands, the olivine calciphyres, and garnet-pyroxene plagioclase schists (see Table 1). The pseudosections constrain P-T conditions of phase transformations and in particular, probable temperatures of fluid component releases, which is essential when evaluating isotopic composition of rocks. Close spatial relations of the studied rocks imply similar P-T conditions of metamorphism, which should correspond to the intersection of the stability fields observed in different mineral associations of the samples. For clarity, the results of pseudosection modeling are presented without considering components presented in insignificant amounts (Fig.2).

Decarbonatization reactions that accompany phase transformations of all the calciphyres studied deserve special attention to explain the change in the isotopic composition of rocks, because these reactions cause isotopic fractionation of both oxygen and carbon; the isotopic fractionation factors between CO₂ and solid phases are significant even at elevated temperatures; large volume increase associated with the formation of fluid components causes complete or partial removal of CO₂.

Considering the di-calciphyres, CO₂ release is caused by the decomposition reactions of dolomite or ankerite, which becomes unstable as temperature increases:

If carbonates with iron (ankerite, sample 13) prevail in the source rock at low temperatures, carbon dioxide is released for example, by the reaction

The source of silica can be quartz as well as other silicates including:

In the di-calciphyres of the Medvezhy Island (sample 7) at pressures below 6 kbar, the decarbonatization reactions start within the range of 580-630 °C (Fig.2, a, red lines ). The simplified anhydrous projection of the system is shown for clarity. At higher pressures, the onset temperature of CO₂ release also rises (e.g., if P = 8 kbar, theonset of CO₂ release shifts to T ≈ 700 °C). By the sequence of phase reactions, primary sediments (see Table 1) transform into the observed grs-ttn-di-hed-qtz-cal association which at 8 kbar is stable from 790 to 970 °C (at another pressure the stability field shifts by about 30 °C/kbar). At lower temperatures, hedenbergite is stable rather than almandine and at higher temperatures, wollastonite appears instead of quartz.

Association of the di-calciphyres from the Tamarkina Luda Island (sample 13) is cal-cpx(di + hed)--grt(grs)-qtz-ttn. The onset of decarbonatization at a reduced pressure is possible from 560 °C (P = 3 kbar). At higher pressures, the temperature of CO₂ release increases to 705 °C (P = 8 kbar), with an estimated ∂T/∂P gradient of approximately 29 °C/kbar (Fig.2, b). Similar to the Medvezhy Island diopside calciphyres (sample 7), the stability field of the observed grs-ttn-cpx-qtz-cal association with increasing temperature is limited by the association with wollastonite instead of quartz. It is noteworthy that for these rocks, carbonates which are stable up to temperatures of approximately 570-650 °C consist of calcite and ankerite rather than dolomite (as in sample 7) and accordingly, decarbonatization is caused by the decomposition of ankerite by a reaction similar to (3).

The ol-calciphyres (sample 8, Fig.2, c). The observed fo-fa-di-ilm-hс-cal association remains in equilibrium with the fluid components (H₂O + CO₂) starting from 690 °C at 3 kbar or 860 °C at 8 kbar; the upper stability boundary is 750 to 905 °C, respectively. The lower-temperature association includes dolomite, relics of which are observed in thin sections; in the higher-temperature association fo-fa-di-usp-hc-cal-(H₂O-CO₂) ulvospinel is formed at the expense of ilmenite. It should be noted that, in contrast to diopside calciphyres, the beginning of decarbonatization (marked by red line in Fig.2, c) does not lead to the total breakdown of dolomite, and calcite becomes stable only from 680 °C (3 kbar) to 860 °C (8 kbar), which coincides with the lower temperature limit at which the observed association remains stable (Fig.2, c, blue line).

All the observed mineral associations are stable within PT-conditions determined by the intersection of the equilibrium fields for different rocks (Fig.3). At the minimum possible pressures (5.5-6 kbar), the temperature limiting the stability of all associations is 740-870 °C. If the metamorphic pressure is higher, temperature estimates increase(dT/dP ≈ 30-40 °C/kbar), and at P = 8kbar the temperature interval at which all the observed associations can coexist is 860-910 °C.

Discussion

Evolution of the isotope composition during metamorphism

A significant correlation between δ¹³C and δ¹⁸O values of calcite (R² = 0.86; Fig.3) is generally observed (within samples composing all varieties of the studied calciphyres). The reactions of CO₂ release characterizing metamorphic transformations of the calciphyre precursors are accompanied by fractionation of oxygen and carbon isotopes with the restite (solid phase residue after gas extraction) depletion in the heavy isotope (1000lnα between CO₂ and carbonates and (or) silicates is always greater than zero for the equilibrium oxygen isotope fractionation, and for temperature above ca. 150-200 °C for the carbon isotope fractionation also [29]). Provided constant volume, continuous removal of the released CO₂ from the system is likely, so that the change in the isotopic composition of the solid phases can be evaluated with equation of the Rayleigh's distillation, the latter expressed by the relation [38] for isotope fractionation:

where F stands for the mole fraction of an element remaining in the restite after gas extraction; δ expresses the isotopic ratio of an element (oxygen, carbon) in a solid; α is the isotopic fractionation factor between fluid and solid phases: (¹⁸O/¹⁶O)fluid/(¹⁸O/¹⁶O)restite; f and i denote values aftergas release and beforegas release, respectively.

The linear array expressing simultaneous changes of oxygen and carbon isotopic ratios agrees with the analytical dependence of δ¹³C_cal and δ¹⁸O_cal (Fig.4), provided that the isotopic equilibrium of CO₂ and the produced carbonate (calcite and (or) dolomite) was maintained during degassing, isotopic reactions of CO₂ with carbonates were hardly accompanied by isotopic reactions with silicates. The latter assumption can be justified by the disagreement of the calculated array considering isotopic equilibrium involving silicates with the analytical data (dashed line in Fig.4).
In addition, the measured distribution of oxygen isotopes between calcite and silicates (see Table 2) significantly deviates from the equilibrium distribution. In the absence of the isotope equilibrium with silicates, values of F for oxygen and carbon coincide (F_C = F_O [39]). The cal-CO₂ isotope fractionation in the latter case corresponds to 700 °C, which is wholly endorsed by the estimates of decarbonatization temperatures obtained from phase relations modeling (with the construction of P-T pseudosections).

The averaged dependence combines analytical data over all samples, including the diopside and olivine calciphyres. Some deviations of the analytical points can be caused by fluctuation of the fractionation values (variations of the decarbonatization temperature, changes of the composition of carbonates in equilibrium with CO₂ of the olivine calciphyres). In particular, however, the measured variations of δ¹⁸O and δ¹³C reflect the degree of distillation F of equation (6) provided the same isotopic composition of carbonate at the initial stage of distillation (F = 1, the beginning of CO₂ release) and similar degassing temperature (at T = 700 °C the values of δ¹⁸O_cal = 17.96 and δ¹³C_cal = –3.4 ‰ closely follow the analytical data).

Depleted δ¹⁸O values of calcite from the ol-calciphyres relative to δ¹⁸O of calcite from the di-calciphyres with a general trend δ¹⁸O vs δ¹³C can be interpreted as a result of larger degassing degree of the former. Thus, maximum values of δ¹⁸O_cal = 17.78 and δ¹³C_cal = –3.57 ‰ (sample 13) reflect initial stages of the distillation (F = 0.96), whereas minimum values of δ¹⁸O_cal = 15.02 and δ¹³C_cal = –6.06 ‰ (sample 8) correspond to more advanced distillation (F = 0.5). The results are consistent with the process of decarbonatization with decomposition of dolomite (or Fe-carbonate, sample 13) producing calcite (see Fig.2, b) due to a series of sequential reactions with removal of the released CO₂. Temperature of the degassing obtained from the isotopic analysis is supported by the phase modeling (T = 700 °C at P ≈ 8 kbar).

Close agreement of the analytical data in the δ¹³C vs δ¹⁸O diagram with the Rayleigh distillation model suggests that the isotopic composition of the removed CO₂ and the remaining carbonate was controlled by the equilibrium exchange between the remaining carbonate (calcite) after carbon dioxide extraction and the continuously removed gas. The process occurred without isotopic exchange with silicates. Isotopic composition of the produced CO₂ in equilibrium with calcite vary depending on the degree of degassing, from δ¹⁸O = 21.9 and δ¹³C = –1.0 ‰ (F ≈ 1) up to 18.9 and –3.7 ‰ (F ≈ 0.5), respectively. The integrated (batch) isotopic composition of the resulting CO₂ falls between these values and is determined by the material balance relations. In case of insignificant change of the fractionation during decarbonatization, the isotopic composition of all formed CO₂ is estimated at δ¹⁸O =19.9 and δ¹³C = –2.2 ‰ (F = 0.5).

Equilibrium of the released gas with calcite without silicates [40] does not restrict F by the so-called calc-silicate limit (F = 0.6 [38]), and the minimum values of the measured isotope ratios correspond to degassing degree at about 0.5 (the ol-calciphyres).

Conclusion

The results of oxygen and carbon isotope analysis indicate that the Porya Guba calciphyres and the intercalated mafic schists with the evolved δ¹⁸O and δ¹³C resulted from metamorphism of sedimentary rocks (limestones, marls, and graywackes). Lithochemical reconstructions suggest marly sediments as the precursors for the carbonate-silicate rocks, while chemical composition of the former varied depending on the ratio of clay, carbonate, and clastic components. Carbonates comprise 58-84 wt.% (in the sediment precursors of the calciphyres) and 10-30 wt.% (in the possible protoliths of the mafic schists). The isotopic composition of the sediments (δ¹⁸O 17.9 ‰ SMOW and δ¹³C –3.4 ‰ PDB) lies within the values typical for the Precambrian diagenetically transformed sedimentary carbonates [36, 37, 41] without any influence of both organic carbon [25] and carbon of the magmatic origin [42].

The most significant shifts in the isotopic composition during metamorphism of the rocks are due to the sequence of reactions producing CO₂ following decomposition of dolomite or other carbonates (siderite, ankerite), which become unstable when temperature rises to 700 °C (P ≈ 8 kbar) and are replaced by magnesium/iron silicates (e.g., monoclinic pyroxene, garnet). The degassing was accompanied by the simultaneous reduction of the calcite δ¹⁸O and δ¹³C as a result of the isotopic fractionation (accompanied by the Rayleigh distillation) yielding values of 15.0 and –6.2 ‰, respectively (half of the initial CO₂ removed) and producing the linear relationship δ¹⁸Ovs δ¹³C. The isotopic composition of the carbon dioxide released (and possibly added to the atmosphere) is estimated at 20 ‰ (SMOW) and –2 ‰ (PDB).

The shift of the calcite isotopic composition thus reflects different degrees of degassing, which in turn, depends on the composition of the parent rocks. For example, the more dolomite prior to metamorphism contained in a sedimentary rock, the higher the degree of degassing. In the olivine calciphyres (MgO up to 20 wt.%), δ¹⁸O and δ¹³C are generally lower than in the diopside calciphyres. At the same time, variations in the isotopic composition of calcite of the interbedded rocks are consistent with the internal (local) control of the emitting fluid [40] and contradict the large-scale interaction with any external (deep or surface) reservoir. Among other things, our results are not consistent with the possibility of mixing (assimilation) of the studied carbonate rocks with magmatic rocks and/or metasomatic transformations during the skarn formation.

It can be noted that at metamorphic temperatures below 650-700 °C dolomite remains stable and the observed dolomite calciphyres of the Kolvitsa granulite zone [24], chemically similar to the olivine calciphyres considered in the present work, might be the “precursors” of olivine calciphyres at lower peak metamorphic temperatures.

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