Associations and formation conditions of a body of melilite leucite clinopyroxenite (Purtovino, Vologda oblast, Russia): an alkaline-ultrabasic paralava

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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 growth 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 akermanite–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.

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About the authors

A. Y. Barkov

Cherepovets State University

Email: anderez@mail.ru

Research Laboratory of Industrial and Ore Mineralogy

Russian Federation, Cherepovets

A. A. Nikiforov

Cherepovets State University

Email: anderez@mail.ru

Research Laboratory of Industrial and Ore Mineralogy

Russian Federation, Cherepovets

R. F. Martin

McGill University

Email: anderez@mail.ru

Department of Earth and Planetary Sciences

Canada, Montreal

V. N. Korolyuk

V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Science

Email: anderez@mail.ru
Russian Federation, Novosibirsk

S. A. Silyanov

Siberian Federal University

Email: anderez@mail.ru

Institute of Non-Ferrous Metals

Russian Federation, Krasnoyarsk

B. M. Lobastov

Siberian Federal University

Author for correspondence.
Email: anderez@mail.ru

Institute of Non-Ferrous Metals

Russian Federation, Krasnoyarsk

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2. Fig. 1. Location and diagram of the geological structure of the Purtovino area (a, b), compiled on the basis of maps of pre-Quaternary geology of the Vologda region (Buslovich, 2000, and others). (c) Photograph of clearing of the studied body. (d) Diagram of the geological structure of the body (compiled using materials of A.I. Trufanov): 1 - greenish-gray marl, fractured; 2 - brick-red argillite, with calcium carbonate deposits along fracturing planes; 3 - brown to brick-red siltstone, fractured, thermally altered, with carbonate films along fracturing planes; 4 - brick-red marl, thermally altered, with a whitish coating of carbonates; 5 – light yellow to greenish-gray siltstone, thermally altered, fractured (brown carbonate-ferruginous formations along the fractures); 6 – reddish-brown marl with light gray-green mottling. Carbonatization and development of brown to gray argillite with a greenish tint and conchoidal fracture are noted along the fractures; 7 – brown siltstone with thin interlayers of argillite and marl, developed beyond the aureole of thermally altered rocks; 8 – sodded scree; 9 – body of alkaline microclinopyroxent. In the upper contact there is a mealy white coating of carbonate-siliceous material in places up to 0.5 cm thick; 10 – scree material on the slope (sodded in places) and fragments of rocks from clearing the outcrop.

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3. Fig. 2. Samples of alkaline microclinopyroxenite from a sheet-like body in the Purtovino area, representing two characteristic types. (a) Rocks of type 1 and type 2, having a discrete contact line, represent distinguished endo- and exocontact facies, respectively. Exocontact rocks are mostly glassy. They are characterized by vertically oriented cracks of contraction origin. (b) Fragments of highly porous textures. (c) Top view.

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4. Fig. 3. Backscattered electron images showing characteristic structures and associations of the Purtovino body. (a, b) Varieties of microgranular clinopyroxenite (Cpx). (c) Cpx grains that contain leucite (Lct) inclusions and are associated with large veinlet grains of calcite (Cal). The tabular grain of melilite (Mll) surrounded by leucite (Lct) in (d) is associated with small zoned grains of Cpx. Symplectitic segregations of Lct are contained in the host melilite, Mll (lower part of Fig. 3d). (e) Aggregates of minerals of the magnesioferrite–magnetite (Mfr–Mag) series associated with leucite (Lct) and silicate glass: Glass (K–Na–(Al)-bearing). Note the development of Cpx crystallites in Fig. 3e. Subhedral magnesiochromite grain (Mchr in Fig. 3e) is associated with microgranular Cpx (zoned) and interstitial leucite (Lct) segregations.

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5. Fig. 4. Backscattered electron images (a–c) show a teardrop-shaped sepiolite grain (Sep) in association with host melilite (Mll) and a rim-like segregation of porous brownmillerite (Bmlr). Inclusions of Cal (calcite), Lct, Wo (wollastonite) and Cpx grains are present. An aggregate of larnite grains (Lrn) and a rim-like segregation of brownmillerite (Bmlr) are in association with melilite, Mll (d). Xenogenic quartz grains (Qz), one of which is fractured and partially resorbed, are surrounded by silicate glass (Glass) enriched in K, Na and Al, with minute inclusions of wollastonite (Wo), calcite (Cal) and clinopyroxene (Cpx) (Figs. 4e, 4f). (g) Images of fan-shaped textures composed of two-layer intergrowths of acicular crystals of melilite (Mll) and plagioclase (Pl) in association with hedenbergite (Hd) dendrites; Cal – calcite.

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6. Fig. 5. Backscattered electron images show characteristic examples of zoning in clinopyroxene (Cpx) grains associated with melilite (Mll), wollastonite (Wo) and leucite (Lct). Letters a–d indicate the positions of the start and end points of the detailed microprobe profiles (ab and cd), the results of which are discussed in the text and presented in Figs. 7a–7i.

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7. Fig. 6. Variations in the contents of FeOtotal–MgO in wt.% (a), as well as Fe3+–Mg (b), Al–Mg (c), Fe3+–Al (d), Al–Si (d) and Ti–Al (e), expressed in atoms per form. units (a. f. u.), observed from the results of 394 analyses (n=394) of clinopyroxene grains in its various textural and structural forms in the Purtovino region body. The graphs show the values ​​of the correlation coefficient (R).

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8. Fig. 7. Variations in the composition of zoned clinopyroxene grains from the Purtovino area body established from ab and cd electron microprobe profiles. The analyzed Cpx grains and the location of the profiles are shown in Figs. 5a–5d. The contents of Na2O (a), K2O (b), MnO (c), and TiO2 (d) are presented in wt. %, while Mg (d), Fe3+ (f), Fe2+ (g), and Al (h) are expressed as atoms per form unit (a. f. u.). The distance along the abscissa axis is given in micrometers. Fig. 7i schematically shows the four distinguished crystallization stages, which are discussed in the text.

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9. Fig. 8. Variations in the compositions of melilite, i.e. members of the akermanite–gehlenite series (Åk–Gh) on the Mg–AlVI–Fe2+ diagram based on the results of 83 analyses (n = 83).

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10. Fig. 9. Variations in the composition of minerals of the akermanite–gehlenite series on the Ca–Na diagram, expressed in atoms per form. units (a. f. u.) (n = 83).

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11. Fig. 10. Variations in the compositions of magnesiochromite (Mchr; n = 10) and members of the magnesioferrite–magnetite series (Mfr–Mag; n = 30) on the Cr–Fe3+–Al triangular diagram. Several compositions of spinel enriched in Fe3+ (a component of Mfr) are conditionally included in the observed Mfr–Mag series.

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12. Fig. 11. Variations in the compositions of magnesiochromite (Mchr) and members of the magnesioferrite–magnetite (Mfr–Mag) series in the Fe2+–Mg (a) and Fe3+–Al (b) diagrams, expressed in atoms per form units (a. f. u.). The correlation coefficient (R) value shown in Fig. 11b was calculated based on the compositions of the members of the Mfr–Mag series (n = 30).

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13. ESM_1.xlsx – Composition clinopyroxene; ESM_2.xlsx – Structure melilita; ESM_3.xlsx – Composition leucite; ESM_4.xlsx – Composition of wollastonite; ESM_5.xlsx – Composition calcite; ESM_6.xlsx – Composition of larnite; ESM_7 hlsx – The composition of the grain is sepiolite-palygorskite; ESM_8.xlsx – Composition of brownmillerite; ESM_9.xlsx – Composition of minerals of the spinel group; ESM_10.xlsx – Composition Ca-Fe titanosilicate.
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