Associations and Formation Conditions of a Body of Melilite Leucite Clinopyroxenite (Purtovino, Vologda Oblast, Russia): an Alkaline–Ultrabasic Paralava

Abstract

A novel petrogenetic scheme is discussed for the formation of a melilite leucite clinopyroxenite body from an alkaline–ultrabasic paralava in the Purtovino area. Its protolith was likely a mixture of Upper Permian sedimentary rocks (aleurolite, marl, among others). Degassing, evaporation, and thermal (contact) metamorphism have significantly influenced the petrogenesis to produce a wide diversity of species present in mineral associations. The crystallization of paralava in a shallow setting was accompanied by an intense degassing and vesiculation of the melt, causing locally high porosity in the rock. An elevated degree of oxidation of the initial melt and progressive rise of fO₂ were likely related to the H₂ loss during the vesiculation and dissociation of H₂O. Consequently, ferrian magnesiochromite (Mchr) and chromian spinel (Fe³⁺-enriched) were the early phases to crystallize; they were followed by members of the magnesioferrite–magnetite series. In situ melting of quartz-bearing and carbonate–clay rocks led to the development of domains of peralkaline felsic glass that surround partially resorbed quartz grains. Numerous grains of wollastonite and rare larnite formed during contact pyrometamorphism. The alkalis increased progressively during crystallization, with a notable enrichment in Na (up to 0.30 apfu) in the åkermanite–gehlenite series. The formation of leucite following melilite is indicated. Euhedral grains of Cpx display concentric cryptic zonation, with a zone of extreme Mg enrichment due to a local deficit in Fe²⁺. As consequences of the continuing rise in fO₂, esseneite crystallized in the rim of zoned clinopyroxene. Two schemes of coupled substitution account for the composition of Cpx grains analyzed in various textural relationships: Mg²⁺ + Si⁴⁺ → (Fe³⁺ + Al³⁺) and (Ti⁴⁺ + Al³⁺) + (Na + K)⁺ → 2Mg²⁺ + Si⁴⁺. The pre-existing grains of olivine (associated with Mchr) were likely replaced completely by sepiolite–palygorskite associated with brownmillerite and its probable Fe³⁺-dominant counterpart, srebrodolskite. The investigated layer of alkaline microclinopyroxenite is unique in the Russian Plate, and a search is thus required to recognize other pyrogenic products. Also, further research is required to evaluate the contents and volumes of coal (or other sources of hydrocarbons) that could cause spontaneous and long-lasting combustion to form the considerable volume of paralava recognized in the Purtovino area.

INTRODUCTION

In this work, mineral associations of an unusual body of alkaline microclinopyroxenite in the Purtovino area (~20 km southeast of Velikii Ustyug, Vologda oblast) are studied in detail to draw petrogenetic conclusions. The small sheeted body is situated among Upper Permian sedimentary rocks exposed in the lower reaches of the Sukhona River. This body was previously considered as a manifestation of alkaline–ultrabasic magmatism (Trufanov and Masaitis, 2007) likely related to the “stage of Mesozoic tectonomagmatic activation” in the northern Moscow Syneclise (Buslovich, 2000). Alkaline ultrabasic rocks attract attention as a peculiar marker of tectonic setting and sources of diamonds and other mineral resources (e.g., Kogarko, 2004; Woolley et al., 1995; Mitchell, 2020). In addition to the concept of melanoleucite intrusion (Trufanov and Masaitis, 2007), we put forth and discuss a novel hypothesis for the generation and crystallization of ultrabasic paralava. Such rocks are relatively scarce but known worldwide, for instance, in the Kuznetsk coal basin (e.g., Cosca et al., 1989; Pyrogenic Metamorphism, 2005; Peretyazhko et al., 2018, 2021; Sharygin, 2019; Savina et al., 2020; Zhang et al., 2020; Savina and Peretyazhko, 2023).

The main tendencies of paralava crystallization are similar to those of the volcanic and subvolcanic complexes. Therefore, our data and observations could provide insight into conditions of subsurface crystallization of ultrabasic melts. The pyrogenic petrogenesis of the alkaline microclinopyroxenite body in the Purtovino area of the northwestern Russian Plate, here established for the first time, highlights the prospects for the recognition of other pyrogenic products.

GEOLOGICAL BACKGROUND AND SAMPLES

A heterogeneous sheeted body (8 × 3 m) of alkaline microclinopyroxenites and associated thermally altered rocks are exposed in the Sukhona River bed, near the settlements of Purtovino and Isada (Figs. 1a, 1b). The walls of the river valley are made up of Upper Permian horizontally stratified sedimentary rocks. Light marls and limestones are mainly developed near the water line. Up section, they are overlain by the carbonate–clayey rocks, including argillites, aleurolites, marls, dolomitic limestones, as well as weakly cemented varieties of quartz sandstones. Similar rocks in the region contain coaly segregations and fragmented tetrapod fossils, which indicate their affiliation to the Permian system (Tatarian stage) (Verzilin et al., 1993).

Fig. 1.

Position and geological scheme of the Purtovino area (a, b) compiled using pre-Quaternary geological maps of the Vologda oblast (Buslovich, 2000, and others). (c) Photo image of eliminate the exposed body. (d) Geological scheme of the body (compiled using materials provided by A.I. Trufanov): (1) greenish gray fractured marl, (2) brick red mudstone, with precipitation of calcium carbonate singular along planar fractures; (3) fractured, brownish to brick-red thermally altered aleurolite with carbonate films along planar fractures; (4) thermally altered brick-red marl with whitish carbonate film; (5) thermally altered, fractured, light yellow to greenish gray aleurolite (brownish carbonate–ferruginous formations along fractures); (6) red-brownish marl with light gray-green spots. Carbonatization and development of brown to gray mudstones with greenish gray tint and conchoidal fracture; (7) brownish aleurolite with thin intercalations of mudstone and marl developed beyond the thermally altered halo; (8) clastic rocks; (9) alkaline microclinopyroxenite body. Whitish carbonate–siliceous film locally up to 0.5 cm thick in the upper contact; (10) outcrop fragments, clastic rocks or sediments.

The geological structure of the body and mapping data are shown in Figs. 1c, 1d. The alkaline microclinopyroxenites compose a thin (0.2–0.3 m) narrow sheeted body lying conformably in the bottom part of the thermally metamorphosed aleurolites and marls (Fig. 1d). The alkaline clinopyroxenite has a green or pistachio tint and a microgranular texture. The upper near-contact parts of the sheeted body are composed of glassy varieties locally having a fine vesicular structure. The elements of columnar jointing (up to 5 cm across) are accompanied by contraction fractures. Grains of partially resorbed quartz ~2 mm across are locally discernible on rock chips.

During the study, we have examined in detail mineral associations in ten representative samples from a sheet of alkaline microclinopyroxenites, which were previously described as “ultrabasic foidites or melanoleucites” (Trufanov and Masaitis, 2007). Collected samples include different clinopyroxenite varieties, which locally have a darker green color, crystalline appearance and porous structure. The rocks have a sharp contact with the rocks of the exocontact facies (Fig. 2a). The latter have a pistachio color, distinct glassy appearance with elements of finely vesicular structure, and are covered by a film of finely dispersed carbonate–siliceous material (Figs. 2a–2c). It is noteworthy that the observed vertical fractures do not intersect the contact line (Fig. 2a).

Fig. 2.

Specimens of alkaline microclinopyroxenite from sheeted body in the Purtovino area, representing two characteristic types: (a) Rocks of types 1 and 2, separated by discrete contact line, represent the endo- and exocontact facies, respectively. The exocontact rocks are usually glassy. They are characterized by the vertically oriented fractures of contraction origin. (b) Fragments of highly porous textures. (c) Image: the view from above.

METHODS

Clinopyroxene was analyzed at the Analytical Center of Multielement and Isotope Studies, Institute of Geology and Mineralogy of the Siberian Branch, Russian Academy of Sciences, Novosibirsk, on a JEOL JXA-8100 microprobe equipped with wavelength dispersive spectrometers (WDS). The general methodology and approaches are published in (Korolyuk et al., 2009; Lavrentiev et al., 2015). The measurements were carried out at an accelerating voltage of 20 kV, a beam current of 50 nA, and a beam diameter of 1–2 μm. Lines Kα are used as analytical for all elements, except for chromium. Cr was measured using less intense line Kβ to avoid V superposition. To compensate loss of intensity, the peak was measured using a high-aperture channel of the spectrometer. The superposition of TiKβ₁ on VKα was corrected using the overlap correction software. Calibration standards were diopside (Ca, Mg, and Fe), pyrope (Mg, Fe, Si, and Al), Cr-bearing garnet (Cr), Mn-bearing garnet (Mn), Ti-bearing diopside glass (Ti), albite (Na), and orthoclase (K). The minimum detection limits for oxides were as follows (in wt %): ≤0.01 (Mg, Fe, Ca, K, Mn, Cr, Ti), ≤0.02 (Al, Na), ≤0.03 (Si). The ZAF correction method was applied. The accuracy and reproducibility of analytical procedures were estimated according to (Korolyuk et al., 2009).

Most of the analyses were carried out using quantitative scanning electron microscopy and energy dispersive spectrometry (SEM-EDS) at the R&D Center of NorNickel, Siberian Federal University, Krasnoyarsk. Over two thousand point determinations were made on a Tescan Vega III SBH (Tescan Orsay Holding) electron microscope equipped with Oxford X-Act (Oxford Instruments Nanoanalysis) EDS at an accelerating voltage of 20 kV, a beam current of 1.2 nA, and certified standard samples MAC (Micro-Analysis Consultants Ltd, Great Britain; reg. no. 11192). The beam current was measured on a MAC cobalt standard (reg. no. 9941) every 60 minutes.

MINERAL ASSOCIATIONS, STRUCTURES, AND COMPOSITIONS

Trufanov and Masaitis (2007) noted that the clinopyroxenite body contains clinopyroxene, leucite, and wollastonite. Table 1 presents a complete list of established minerals. The rock can be classified as a leucite–melilite clinopyroxenite, since the melilite content in it locally reaches and even slightly exceeds 10 vol % (Mitchell, 1996). Representative WDS and SEM-EDS analyses of clinopyroxene and spinels are presented in Tables 2, 3. More complete data are given in Supplementary¹ 1, ESM_1 (clinopyroxene), ESM_2 (melilite), ESM_3 (leucite), ESM_4 (wollastonite), ESM_5 (calcite), ESM_6 (larnite), ESM_7 (sepiolite–palygorskite), ESM_8 (brownmillerite), ESM_9 (spinel-group minerals), and ESM_10 (likely, qeltite). The characteristic structures of rocks and examples of mineral associations are shown in Figs. 3а–3f, 4a–4g, and 5a–5d.

Table 1. List and relative abundance of minerals in a body of the leucite–melilite microclinopyroxenite in the Purtovino area

Table 2. Representative compositions of clinopyroxene grains in melilite microclinopyroxenite in the Purtovino area

Table 3. Representative compositions of spinel-group minerals from leucite–melilite microclinopyroxenite body in the Purtovino area

Fig. 3.

BSE images showing characteristic textures and associations in the body of the Purtovino area. (a, b) Varieties of microgranular clinopyroxene (Cpx). (c) Cpx grains with leucite inclusions (Lct) in association with large veinlet-like calcite grains (Cal). Prismatic melilite grain (Mll) surrounded by leucite (Lct) in figure (d) is associated with small zoned Cpx grains. Symplectic Lct in host melilite, Mll (lower part of Fig. 3d). (e) Aggregates of minerals of the magnesioferrite–magnetite series (Mfr–Mag), associated with leucite (Lct) and silicate glass: Glass (K–Na–(Al)-bearing). Cpx crystallites are shown in Fig. 3e. Subhedral grain of magnesiochromite (Mchr in Fig. 3f) is associated with microgranular Cpx (zoned) and interstitial grains of leucite (Lct).

Fig. 4.

BSE images (a–c) show drop-like grain of sepiolite (Sep) in association with host melilite (Mll) and rims of porous brownmillerite (Bmlr). There are inclusions of Cal (calcite), Lct, Wo (wollastonite), and grains of Cpx. Aggregate of larnite grains (Lrn) and rims of brownmillerite (Bmlr) in association with melilite Mll (d). Xenogenic grains of quartz (Qz), one of which is fractured and partially resorbed, are rimmed by silicate glass (Glass), enriched in K, Na and Al, with the finest inclusions of wollastonite (Wo), calcite (Cal), and clinopyroxene (Cpx) (Figs. 4e, 4f). (g) Images of radial textures made up of two-layer intergrowths of acicular melilite (Mll) and plagioclase (Pl) in association with dendrites of hedenbergite (Hd); Cal – calcite.

Fig. 5.

BSE images show characteristic examples of zoning in the clinopyroxene grains (Cpx) associated with melilite (Mll), wollastonite (Wo), and leucite (Lct). (a–d) shows the position of initial and final analysis points in the detailed microprobe profiles (ab and cd), results of which are discussed in the text and shown in Figs. 7a–7i.

Clinopyroxene close in composition to diopside is present in different structural–textural forms and is the major rock-forming mineral of the alkaline-ultrabasic body. Diopside usually occurs as micrograins no more than 0.1 mm wide. Comparatively large subhedral crystals up to 2 mm long are locally present. The microgranular Cpx can fill cavities of pre-existing fluid inclusions (Figs. 3a, 3b). Crystals and skeletal grains of Cpx are frequently associated with domains of visible contamination in the contact zone with grains of quartz or its polymorphic modification (cristobalite, tridymite). Relict partially resorbed quartz grains could be tentatively termed “xenogenic” as they were entrapped by melt as a solid phase. The grains of xenogenic quartz are usually rimmed by K–Na–(Al)-bearing silicate glass (Figs. 4e–4f). Some subhedral Cpx crystals are concentrically zoned with the development of oscillatory and cryptic zoning (Figs. 5a–5d). Marginal parts, as cores, show clear evidence for the resorption of early zones by melt.

The rock also contains unusual scarce dendritic Ti-bearing hedenbergite (3.70 wt % TiO₂ and 0.25 apfu Fe³⁺: no. 13 in Table 2) associated with radial textures, which are made up of acicular grains of melilite and plagioclase (Fig. 4g). The high calculated Fe³⁺ content in hedenbergite in the endocontact facies of clinopyroxenites indicates an oxidized state of the melt. The composition of clinopyroxene in the main volume of the body (Supplementary 1, ESM_1) corresponds to the diopside–esseneite series (Morimoto et al., 1988). The Mg and Fe concentrations in the Cpx grains widely vary. The MgO and FeO_tot contents account for from 2 to 20 wt % (Fig. 6a). The recalculation of clino-pyroxene compositions indicates the moderate to high Fe³⁺ concentrations. This fact is consistent with the elevated content of ferric iron in the representative whole-rock analysis (Trufanov and Masaitis, 2007), in wt %: SiO₂ 42.70, TiO₂ 0.54, Al₂O₃ 10.30, Fe₂O₃ 5.10, FeO 2.44, MnO 0.14, MgO 8.87, CaO 24.60, Na₂O 0.30, K₂O 3.43, P₂O₅ 0.22, L.O.I. 0.67, total 99.31.

Fig. 6.

Variations of FeO_tot–MgO in wt % (a), as well as Fe³⁺–Mg (b), Al–Mg (c), Fe³⁺–Al (d), Al–Si (e), and Ti–Al (f), expressed in atoms per formula units (apfu), based on results of 394 analyses (n = 394) of clinopyroxene grains in different structural–textural forms in the body of the Purtovino area. Values of correlation coefficient (R) are shown in the plots.

Data on the diopside–esseneite series reveal significant and strongly negative correlations in the pairs Fe³⁺–Mg (Fig. 6b), Al–Mg (Fig. 6c), and Si–Al (Fig. 6e). The Fe³⁺–Al and Al–Ti correlations, in contrast, are positive (Figs. 6d, 6f). The highest Ti clinopyroxene in this series corresponds to esseneite (5.69 wt % TiO₂: no. 14 in Table 2). According to WDS analysis, the compositions of rims of zoned grains (Figs. 5a, 5c) also correspond to esseneite (no. 6 in Table 2): (Ca_0.97Na_0.03)_Σ1.00(${\text{Fe}}_{{0.48}}^{{3 + }}$Mg_0.27Ca_0.08Ti_0.08Al_0.06Mn_0.02${\text{Fe}}_{{0.01}}^{{2 + }}$)_Σ1.00(Si_1.35Al_0.65)_Σ2.00O₆ (Supplementary 1, ESM_1). The mineral contains only trace Fe²⁺ contents (0.37 wt % FeO), whereas the calculated Fe₂O₃ is as high as 16.05 wt %. The maximum Fe³⁺ concentration in esseneite is 0.54 apfu.

Results of detailed microprobe profiles, ab and cd, performed using WDS with a step of 2 μm over the length and width of the euhedral zoned clinopyroxene grain (Figs. 5a, 5c) are presented in Figs. 7a–7h and in Supplementary 1, ESM_1. A significant outward increase of Fe³⁺ (caused by the presence of the esseneite component) is observed in a zoned grain. This is accompanied by the increase of K, Na, Mn, Ti, and Al. According to Mg variations, the observed trends indicate the existence of four crystallization stages schematically shown in Fig. 7i.

Fig. 7.

Compositional variations of the zoned clinopyroxene grain from the body of the Purtovino area, which are based on electron-microprobe profiles ab and cd. The analyzed grain of Cpx and position of the profiles are shown in Figs. 5a–5d. The contents of Na₂O (a), K₂O (b), MnO (c), TiO₂ (d) are presented in wt %, whereas Mg (e), Fe³⁺ (f), Fe²⁺ (g), and Al (h) are given in atoms per formula unit (apfu). Distance is shown in micrometers. Figure 7i schematically shows four stages of crystallization, which are discussed in the text.

Melilite is the second in abundance (Table 1). Its grains have a prismatic habit and frequently reach 0.5 mm (Fig. 3d), being usually in a close association with clinopyroxene, leucite, wollastonite or sepiolite, and calcite (Figs. 4a, 4c, 5b–5d). A series of solid solution extends from åkermanite, Ca₂MgSi₂O₇, to its boundary with the gehlenite field, Ca₂Al(Al,Si)O₇ (Fig. 8; Supplementary 1, ESM_2). Significant amount of Na (up to 0.30 apfu) isomorphically substitutes for Ca in this series (Fig. 9). Acicular melilite crystals are observed in the intergrowths with calcic plagioclase Or_6.1–8.8Ab_17.5–19.3An_71.9–76.3 (Fig. 4g). The representative results of SEM–EDS analyses are as follows (in wt %): SiO₂ 50.10, Al₂O₃ 28.74, Fe₂O₃ 3.34 (all Fe as Fe₂O₃), CaO 14.96, Na₂O 2.22, K₂O 1.54, Total 100.90, which corresponds to the formula (Ca_0.74Na_0.20K_0.09)_Σ1.03(Si_2.30Al_1.56${\text{Fe}}_{{0.12}}^{{3 + }}$)_Σ1.98O₈.

Fig. 8.

Compositional variations of melilite, i.e., members of the åkermanite–gehlenite series (Åk–Gh) in the diagram Mg–^VIAl–Fe²⁺ based on results of 83 analyses (n = 83).

Fig. 9.

Compositional variations of minerals of the åkermanite–gehlenite series in the Ca–Na diagram expressed in atoms per formula unit (apfu) (n = 83).

Leucite is developed as unevenly disseminated small grains or forms veinlets no more than 0.1 mm across, microcrystals and their aggregates or symplectites in melilite, as well as rims around it (Figs. 3d–3f, 5a–5d). Its grains contain a significant admixture of Na and Fe: up to 1–1.5 wt % Na₂O and FeO, respectively (Supplementary 1, ESM_3).

Wollastonite forms small grains (up to 0.1 mm) in association with clinopyroxene and melilite (Figs. 5a–5d) or inclusions in glass (Figs. 4e, 4f). It usually contains up to 2 and 1 wt % MgO and FeO, respectively (Supplementary 1, ESM_4). Inclusions and veinlets of calcite reach 0.3–0.4 mm long (Figs. 3c, 4b, 4c). Its grains could be significantly enriched in Mg (6 wt % MgO; Supplementary 1, ESM_5). Calcite ascribed to the later generation is associated with sepiolite (Figs. 4a, 4c). Micrograined clusters of larnite are associated with melilite (Fig. 4d). The composition of larnite (Supplementary 1, ESM_6) closely corresponds to Ca₂SiO₄.

The sepiolite–palygorskite grains up to 0.5 mm across contain 17–25 wt % MgO (Supplementary 1, ESM_7). The average composition of sepiolite (based on analyses of 19 grains) can be satisfactorily recalculated (on a water-free basis) per 32 oxygen atoms: (Mg_6.84Ca_0.63Fe_0.17K_0.16Mn_0.03)_Σ7.83(Si_11.29Al_1.09)_Σ12.38O₃₂. It is pertinent to mention that the inner rims of the brownmillerite developed in host sepiolite (Fig. 4a) have a low-Al composition: Ca_1.97(${\text{Fe}}_{{0.96}}^{{3 + }}$Al_0.64Si_0.15Mg_0.12Ti_0.07Mn_0.07)_Σ2.01O₅, which corresponds to srebrodolskite (Fe³⁺-dominant analogue of brownmillerite: Chesnokov and Bazhenova, 1985) (Supplementary 1, ESM_8).

Two generations of spinel-group minerals are distinguished: magnesiochromite and magnesioferrite–magnetite series. The latter solid-solution series includes spinels most enriched in Fe³⁺ (Figs. 3e, 3f). The representative compositions of spinel-group minerals are shown in Table 3, while Supplementary 1, ESM_9 displays the wider spectrum of observed compositions. It is seen that the body contains spinel varieties, chromian spinel and magnesioferrite with high Fe³⁺ concentration, aluminous magnesiochromite (also enriched in Fe³⁺), aluminous magnesioferrite, and magnesian magnetite. Significant enrichment in Fe³⁺ and Al is expressed in the high contents of magnesioferrite, magnetite, as well as spinel components. The observed crystallization trend likely extends from the earliest magnesiochromite to the high-Mg magnesioferrite and further to the magnetite series, in which Mag content subsequently increases during crystallization (Figs. 10, 11a, 11b).

Fig. 10.

Compositional variations of magnesiochromite (Mchr; n=10) and members of the magnesioferrite–magnetite series (Mfr–Mag; n = 30) in the triangular diagram Cr–Fe³⁺–Al. Several compositions of spinel grains enriched in Fe³⁺ (Mfr component) are provisionally included in the observed Mfr–Mag series.

Fig. 11.

Compositional variations of magnesiochromite (Mchr) and members of the magnesioferrite–magnetite series (Mfr–Mag) in the diagrams Fe²⁺–Mg (a) and Fe³⁺–Al (b), expressed in atoms per formula units (apfu). The values of correlation coefficient (R), shown in Fig. 11b, were calculated based on the compositions of members of the Mfr–Mag series (n = 30).

Of interest are veinlets (≤10–15 mm) and inclusions of irregular shape in melilite, which correspond to an unidentified Zr-bearing Ca–Fe titanosilicate (Supplementary 1, ESM_10). The compositions of several inclusions (n = 5) have close compositions, wt %: SiO₂ 25.50–27.30, TiO₂ 12.71–14.06, ZrO₂ 0–0.82, Al₂O₃ 3.17–4.04, Fe₂O₃ (tot.) 20.94–22.66, Cr₂O₃ 0–0.88, MgO 0.66–1.14, CaO 32.53–33.26, total 97.53–100.23, and correspond to the formula Ca_3.00(${\text{Fe}}_{{1.55 - 1.69}}^{{3 + }}$Ca_0.50–0.55Mg_0.10–0.17)_{Σ2.23–2.35}(Ti_0.94–1.05 Zr_0.01–0.04)_{Σ0.95–1.09}(Si_2.54–2.69Al_0.38–0.47)_{Σ2.92–3.16}O₁₄. It is possible that this mineral is an analogue of qeltite, Ca₃TiSi₂(${\text{Fe}}_{2}^{{3 + }}$Si)O₁₄, a new species recently discovered in the pyrometamorphic rocks of Palestine (Galushkina et al., 2021). Xenogenic quartz grains correspond to SiO₂ in composition. Domains of silicate glass in contact with quartz (Figs. 4e–4f) usually display elevated concentrations of K, Na, and Al, which suggests their formation through assimilation of fragments of quartzofeldspathic material entrapped by a melt from the host metasedimentary sequence. All analyzed domains of glass in the different parts of the body have close compositions, i.e., are felsic, essentially potassic and peralkaline (n = 9), wt %: SiO₂ 71.99 (65.16–77.19), TiO₂ 0.40 (0–0.62), Al₂O₃ 8.35 (5.11–13.75), FeO_tot 3.14 (1.56–5.80), MgO 1.07 (0–1.89), CaO 2.59 (1.50–4.70), Na₂O 3.82 (2.43–4.62), K₂O 8.70 (7.60–10.29), total 100.05.

The finest grains of fluorapatite (≤5 μm) are confined to the clinopyroxene boundaries. Their typical composition is as follows, wt %: P₂O₅ 36.25, SiO₂ 5.80, CaO 53.65, Na₂O 0.49, F 3.21, O≡F 1.35, total 98.05, which corresponds to the formula (Ca_9.49Na_0.16)_Σ9.65(P_5.07Si_0.96)_Σ6.03O₂₄(F_1.68OH_0.32), calculated per 25 oxygen atoms. A submicron-sized inclusion of copper oxide was also found. Nickel minerals were not encountered in the studied samples.

DISCUSSION

Formation of Paralava and Pyrogenic Genesis of Clinopyroxenite from the Purtovino Area

The peculiar mineral associations and geological features (Fig. 1d) indicate the high probability of crystallization of alkaline ultrabasic rocks from paralava. The evidence for such an origin is the uniqueness of this body for the region and the entire Russian Plate, its geological structure, morphology, size, and the presence of the external halo of thermally altered and fused rocks, glassy structures, as well as widespread pores and cavities. The exotic mineral association characterized by the predominance of microcrystalline Cpx, development of esseneite and other species with the extremely high Fe³⁺ content (diopside, spinels, brownmillerite, and srebrodolskite), calcic plagioclase, melilite, leucite, larnite, and wollastonite, are consistent with this petrogenetic scheme. A characteristic feature also is the significant amount of ferric iron (3.34 wt % Fe₂O₃) in plagioclase.

It is unclear which high-temperature source caused melting of such a significant volume of aleurolites and marls and widespread thermal contact metamorphism in the Upper Permian sedimentary rocks. As known, the main thermal source for the formation of paralavas and pyrometamorphism is a natural combustion of coal beds (e.g., Cosca et al., 1989). At the same time, the following conditions are required to form the pyrogenic lavas by combustion of concealed coal seams: (1) spontaneous combustion is hardly probable in a completely closed system and requires partial influx of air as an agent of oxidation and burning; (2) stable and long-lasting combustion to generate high-temperature conditions (up to 1450°C) with formation of lavas (or large fragments of fused volcanosedimentary rocks) requires circulation with a systematic air influx and removal of volatile products; (3) a sufficiently great volume of coal seam or bed burning for a long time. In relation to these factors, the formation of paralavas is a scarce phenomenon, even in the coal-bearing fields such as the Kuznetsk Basin. It is known that pyrometamorphism is widespread in the naturally burned spoil heaps (Sokol et al., 1998, 2002). The formation of paralavas of broad chemical composition with the development of diverse and anomalous mineral associations is controlled by several factors. The most significant are the compositions of initial remelted sedimentary rocks, temperature conditions of melting and cooling, immiscible splitting, convective processes, saturation of mineral-formation medium in volatiles, and the level of fluctuation of fO₂ in the system (Cosca et al., 1989; Peretyazhko et al., 2018, 2021; Sharygin, 2019; Guy et al., 2020; Zhang et al., 2020).

Concepts of the pyrogenic genesis of a body in the Purtovino area require additional studies. It is necessary to estimate the contents and volumes of coal in the coal-bearing deposits, spontaneous and long-lasting combustion of which could provide melting of the Upper Permian host rocks. The heating power and the probability of natural and long-lasting burning of coal-bearing rocks, observed in the Upper Permian sequences of the Sukhona River area, are unknown. However, finely dispersed coal is locally present in some of Triassic sediments (Avdoshenko and Trufanov, 1989).

Magnesiochromite found in the body of the Purtovino area is unknown in pyrometamorphic rocks. This is largely related to the high-temperature crystallization of magnesiochromite, which is typical of ultramafic rocks: dunites, chromitites, serpentinites, kimberlites, lamproites, komatiites, and other high-Mg volcanic rocks as well as xenocrysts in lamproites and basalts of mid-ocean ridges. At the same time, maximum temperatures, reached in pyrogenic processes (1400–1450°C; Cosca et al., 1989; Peretyazhko et al., 2018, 2021), do not exclude magnesiochromite crystallization. But this is hardly likely in view of the Cr-depleted composition of protolith, i.e., carbonate–clayey and quartz-bearing rocks subjected to pyrometamorphism. For example, the equilibrium crystallization of magnesiochromite at 1400°C in a haplobasaltic melt requires significant Cr contents, up to 3500–6800 ppm (Borisova et al., 2020). However, such high-Cr source is absent in the Purtovino stratigraphic sequence. The Cr content in the rocks of the alkaline microclinopyroxenite body is as low as 82 ppm (Trufanov and Masaitis, 2007), which is not higher than the average Cr content in marls. For instance, marl samples associated with paralava of the Central Apennines contain 106–321 ppm Cr (Melluso et al., 2003).

Thus, the formation of crystallites of magnesiochromite (Fig. 3f) and chromian spinel likely reflects the disequilibrium and metastable conditions of crystallization of the alkaline ultrabasic body. A xenogenic origin of magnesiochromite grains (Mchr) seems to be unlikely. We suggest that they crystallized from melt. 1. Compositions of several Mchr grains (nos. 7–12 in Table 3) significantly vary and are enriched in Fe³⁺, which is a typomorphic characteristic of mineral associations of the body. Such a high oxidation state is not typical of Mchr. 2. In addition to Mchr, the high Cr content was found in associated spinel grains (no 3 in Table 3). 3. The alternative origin for these grains in the Upper Permian sequences cannot be substantiated. No ultrabasic rocks or high-Mg intrusions are known in the Vologda region, except for several bodies located at great depths in the crystalline basement. However, they cannot be a source of Mchr, because the paralava layer was formed in situ in the subsurface environment of the sedimentary cover. An inferred presence of Mchr grains in sedimentary rocks of the area (aleurolites, marls, limestones, and quartz sandstones) is not plausible. A cosmogenic origin is inconsistent with the oxidized compositions. In addition, occasional and selective incorporation of grains of only cosmogenic Mchr in a small volume of paralava is considered unlikely.

The following observations should be also mentioned. Similar mineral associations were described in calc-silicate (skarnoid) xenoliths in alkaline basalts (Reato et al., 2022), where evidence for hydrocarbon combustion is absent. Previously, esseneite was found in coarse-grained pyroxene–anorthite xenoliths (Yakubovich et al., 2017), which obviously have no pyrogenic origin. The petrogenesis of rocks of such type remains rather controversial. For instance, wollastonite- and melilite-bearing rocks similar to our associations were ascribed to pyrometamorphogenic paralavas (Melluso et al., 2004), whereas other researchers regard them as intrusive and associated effusive facies of melilite-bearing and carbonatite occurrences of the Central Apennines igneous province (Stoppa et al., 2005). Nevertheless, the data described above and our observations suggest a pyrogenic genesis of body in the Purtovino area.

Conditions of Crystallization of Paralava Body in the Purtovino Area

We suggest that the natural combustion of unestablished or previously existing coal seams produced melting of Upper Permian laminated sedimentary sequence, with accumulation of a layer of alkaline–ultrabasic paralava at the bottom of thermally altered aleurolites (Fig. 1d). The volume of paralava subsequently increased with melting and seepage of droplets of forming melt. A relatively rapid crystallization of the accumulated volume of alkaline ultrabasic paralava occurred in a shallow setting, which resulted in the micrograined, aphanitic, and glassy textures with development of crystallites (and dendritic grains) of clinopyroxene and wollastonite, as well as abundant fracturing of rocks in the marginal facies due to contraction. Degassing of paralava and abundant release of volatiles led to the vesiculation (separation of bubbles of water-bearing gas phase) (Fig. 2b). Dehydration, decarbonization, and assimilation of fragments of host rocks facilitated the release of volatiles and more rapid cooling of alkaline–ultrabasic melt. Contamination of the accumulated lava portion in situ is evident from the presence of numerous quartz crystals (partially resorbed: Figs. 4e, 4f) with domains of silicate glass of subpantellerite composition, which was formed in contact with these grains through the assimilation of xenogenic material by melt. Resorbed quartz xenocrysts likely represent relics of the relatively refractory phase in the former heterogeneous material (quartzofeldspathic). The formation of wollastonite and larnite points to skarnoid processes, which caused thermal metamorphism and anatexis of calcareous sedimentary rocks.

The crystallization of paralava bodies at the early stages followed mainly the known tendencies in the evolution of effusive and subvolcanic complexes. The high-Mg grains of spinel-group minerals, magnesiochromite and chromian spinel (Figs. 3e, 3f), were likely first to crystallize. The earliest magnesiochromite grains have a high Mg index (Mg#_max = 77.0) in combination with high Fe³⁺# = 68.4. Such significant enrichment in Mg, for instance, is observed in chromian spinels from the Lyavaraka complex, Kola Peninsula, which crystallized from a komatiitic melt (Barkov et al., 2021a, 2022). One may suggest that the parental ultrabasic paralava bearing alkali metals initially was in a highly oxidized state. At the same time, fO₂ continued to increase, which led to the subsequent crystallization of members of the magnesioferrite–magnetite series (Fig. 10). Aggregates of these spinels accompanied by leucite are closely associated with domains of felsic glass (Fig. 3e). Hence, the contamination of melt by SiO₂ (Qz) component could cause a shift of its composition to the crystallization field of spinel-group minerals. Such an assumption is consistent with a model suggesting contamination by felsic material as a trigger for the formation of chromite zones in the layered intrusions (Irvine, 1975; Alapieti et al., 1989; Kinnaird et al., 2002).

The occurrence of magnesiochromite indicates the probable crystallization of olivine in this paragenesis. Grains or relics of olivine in the studied samples are not observed, but droplike grains of sepiolite–palygorskite are developed (Fig. 4a), which could be formed through the complete replacement of initial Ol grains, probably with the intermediate formation of serpentine as a dissolution and substitution product (e.g., Mulders and Oelkers, 2021). For example, fibrous sepiolite was formed in the cavities of an ophiolite mélange in relation with the transformation and serpentinization of primary Ol (Yalçin and Bozkaya, 2004). Sepiolite veinlets closely associated with brownmillerite and calcite were formed at the autometasomatic stage owing to the accumulation of water vapor singular in the fluid. The development of sepiolite indicates the H₂O presence in a melt, which is an important characteristic of body in the Purtovino area, unlike other pyrogenic rocks.

In the Mg–Fe diagram (Fig. 6a), clinopyroxene defines an extended compositional series in spite of the comparatively small size of alkaline ultrabasic body. This feature can be related to the high volatile content, which significantly widened crystallization intervals and caused extended Cpx trends (Figs. 6a–6f). The largest Cpx grains (~2 mm long) crystallized in the areas with elevated contents of volatile components. Four crystallization stages are distinguished for zoned Cpx grains (Figs. 5d–5d, 7a–7h). Stage 1 in Fig. 7i marks the earliest period of normal crystallization within a relatively oxidized melt, which led to the formation of a Fe³⁺-bearing core. By the end of this stage, physicochemical conditions in the medium of mineral formation changed sharply, likely owing to the rapid cooling of the body. The core of the crystal was slightly resorbed (Figs. 5a, 5c) prior to the next stage. Stage 2 (Fig. 7a) involves anomalous or metastable crystallization, which is recorded by a sudden increase of Mg content, which is correlated with an increase in fO₂, thus enabling a conversion Fe²⁺ → Fe³⁺ and an increase of Mg# in a differentiated melt. During stage 3 or the stage of “adaptation”, the zoned crystal tends to crystallize under modified conditions. Stage 4 or final stage reflects the main stage of decreasing temperature in combination with continuing increase of fO₂, which caused the development of an esseneite rim enriched in Al, Ti, Mn, and alkali metals (Figs. 7a–7h). Two schemes of isomorphic substitutions are identified in clinopyroxene in different textural positions (Figs. 6a–6f): Mg²⁺ + Si⁴⁺ → (Fe³⁺ + Al³⁺); (Ti⁴⁺ + Al³⁺) + (Na + K)⁺ → 2Mg²⁺ + Si⁴⁺.

Thus, newly formed esseneite in the Purtovino area likely reflects the following petrogenetic circumstances: (1) rapid crystallization of alkaline-ultrabasic melt under shallow conditions and (2) initially elevated fO₂ which, however, continued to increase in the melt up to the final stage of crystallization of zoned clinopyroxene. The composition of esseneite reaches the maximum content of Fe³⁺ = 0.48–0.54 apfu in the rim of zoned grains.

One of the first finds of esseneite pyroxene (0.33 Fe³⁺ and 0.28 Mg apfu) was made in a xenolith in association with garnet and anorthite in the Udachnaya kimberlite pipe (Shatsky, 1983). Esseneite from the Purtovino area has the following composition: (Ca_0.97Na_0.03)_Σ1.00(${\text{Fe}}_{{0.48}}^{{3 + }}$Mg_0.27Ca_0.08Ti_0.08Al_0.06Mn_0.02 ${\text{Fe}}_{{0.01}}^{{2 + }}$)_Σ1.00(Si_1.35Al_0.65)_Σ2.00O₆, which closely corresponds to Ca_0.99${\text{Fe}}_{{0.52}}^{{3 + }}$Mg_0.32${\text{Fe}}_{{0.06}}^{{2 + }}$Ti_0.05Mn_0.01Si_1.34Al_0.71O₆ from ultramafic xenoliths in the dacite lavas of the Ten’-01 paleovolcano (Yakubovich et al., 2017). At the type locality of esseneite (Wyoming, USA), the relatively low-Mg esseneite, (Ca_1.01Na_0.01)_Σ1.02(${\text{Fe}}_{{0.72}}^{{3 + }}$Mg_0.16Al_0.04Ti_0.03${\text{Fe}}_{{0.02}}^{{2 + }}$)_Σ0.97 (Si_1.19Al_0.81)_Σ2.00O₆, was formed in a fused sedimentary rock owing to coal combustion. This paragenetic association, including melilite, anorthite, and minerals of the magnetite–hercynite series, was formed by pyrometamorphic reactions at high fO₂ close to the hematite–magnetite buffer (Cosca and Peacor, 1987). Esseneite enriched in the kushiroite component, CaAl[AlSiO₆], was successfully synthesized: Ca(${\text{Fe}}_{{0.82}}^{{3 + }}$Al_0.18)(Si_1.00Al_0.82${\text{Fe}}_{{0.18}}^{{3 + }}$)O₆ (Ghose et al., 1986). A significant amount of this component is found in esseneite (0.48 Fe³⁺ apfu) from calc-silicate xenoliths in the alkaline basalts from Carpathians (Reato et al., 2022). An unusual variety of clinopyroxene (0.51 Fe³⁺ apfu) was found in the Chelyabinsk coal basin (Kabalov et al., 1997). Sectorially zoned diopside macrocrysts containing up to 0.18 Fe³⁺ apfu were documented in the marginal part of the Mont Royal gabbroic complex, Canada. These zoned crystals, like clinopyroxene from the Purtovino area, contain anomalous zones enriched in Mg–(Cr) (Barkov and Martin, 2015). It should also be noted that Fe³⁺ content in Cpx could serve as a sensitive indicator for the typification of volcanogenic associations (Bindi et al., 1999).

The åkermanite–gehlenite series extends toward an end member containing 30 at % Fe²⁺ and 70 at % ^VIAl (Fig. 8). The main type of isomorphic substitution, Mg + Si⁴⁺ → 2Al, is combined in this series with significant Na–Ca substitution (Fig. 9), following a scheme of coupled substitution to ensure charge balance. Thus, the gehlenite structure in this series likely contains iron atoms in two valence states.

The mineralogical data obtained clearly indicate that the body in the Purtovino area crystallized at progressively increasing fO₂. Such trend is similar to the evolution of komatiitic melts of the Paleoproterozoic Serpentinite belt in the Kola Peninsula, where an increase of fO₂ is thought to be related to the removal of hydrogen at vesiculation and upon the dissociation of water during crystallization under subvolcanic conditions (Barkov et al., 2019, 2021, 2022). This mechanism proposed for the first time in (Czamanske and Wones, 1973) can be universal and applied to a wide spectrum of ultrabasic melts crystallizing under shallow conditions.

CONCLUSIONS

A wide diversity of mineral associations was established and their chemical variations were characterized. It is proposed that the body from the Purtovino area was formed from paralava. The protolith of the body was likely a mixture of Upper Permian sedimentary rocks (aleurolites, marls, quartz sandstones, among others). Degassing, evaporation, and contact thermal metamorphism significantly affected its genesis. The new model requires further studies to reveal the content and volume of coal (or other hydrocarbon source) in the carbon-bearing deposits, the long-lasting combustion of which provided the formation of significant volume of paralava. In this regard, borehole drilling is desirable to examine the lower levels of the stratigraphic sequence that host the alkaline microclinopyroxenite body.

A relatively rapid crystallization of paralava under shallow conditions was accompanied by the intense degassing and vesiculation of melt, which caused locally significant porosity of the rocks. In situ melting of carbonate–clayey and quartz-bearing rocks produced segregations of peralkaline felsic glass in contact with partially resorbed xenogenic quartz grains. Wollastonite and larnite crystals were formed during contact thermal metamorphism.

Inclusions of sepiolite–palygorskite-group minerals indicate the presence of H₂O in the initial melt, which is a distinctive feature of the pyrogenic body. High oxidation of the melt and prograde growth of fO₂ likely due to H₂ removal during vesiculation and water dissociation resulted in the early crystallization of Fe³⁺-rich magnesiochromite associated with spinel (also Fe³⁺-bearing), which was followed by the crystallization of the magnesioferrite–magnetite series.

Clinopyroxene crystals show a cryptic concentric zoning. A zone of extreme Mg enrichment revealed in zoned Cpx crystals was formed due to the local deficit in Fe²⁺. Owing to a continuing increase of fO₂, esseneite сompositions were reached in the rim of zoned clinopyroxene grains. Two schemes of coupled isomorphic substitutions are traced in Cpx in different textural relations: Mg²⁺ + Si⁴⁺ → (Fe³⁺ + Al³⁺); (Ti⁴⁺ + Al³⁺) + (Na + K)⁺ → 2Mg²⁺ + Si⁴⁺. The pre-existing olivine grains were likely completely replaced by sepiolite–palygorskite in association with brownmillerite. A prograde accumulation of alkalis in the melt led to the significant increase of Na content in the minerals of the crystallizing åkermanite–gehlenite series. Leucite was formed after melilite.

A pyrogenic sheet of alkaline clinopyroxenite in the Purtovino area is unique for the Russian Plate. A search is required to recognize other pyrogenic products.