Geologicheskii Vestnik

The Northern Kraka massif represents a large outcrop on the Earth's surface of rocks from the so-called "mantle section" of the ophiolite association, covering an area of more than 200 sq km. Lherzolites play the primary role in the massif's structure, with harzburgites and dunites being subordinate. Despite its long history of study, geological maps of the massif constructed by various authors differ significantly from each other, due to the low contrast of the constituent rocks and their frequent alternation in the section. Macroscopic or petrographic study-based mapping of the massif fails to adequately characterize its heterogeneity. This work undertakes mineralogical-geochemical mapping of the ultramafics based on the composition of the main rock-forming mineral-olivine-and the accessory chromian spinel. Using SEM/EDS analysis of these minerals' compositions at approximately 350 points distributed across the entire massif area, the distribution of key mineralogical-geochemical depletion indicators of the mantle substrate was analyzed, such as the forsterite content and NiO in olivine, and the relative chromian index #Cr = Cr/(Cr+Al+Fe³⁺) in chromian spinels. Maps of the density of Fo, NiO, and #Cr values were made. Since changes in these characteristics reflect the intensity of partial melting processes in mantle peridotites, the heterogeneity of the mantle substrate's depletion was established, as reflected on the composite map. The main result of the study is a map of the relative depletion of ultramafics, showing zones of lowest depletion in the northeast and east of the massif, and highest depletion in its central and western parts.

ophiolites, ultramafics, olivine, chrome spinelides

1. Betekhtin A.G. (1933) On the study of chromite iron ore deposits. Zapiski Leningradskogo gornogo institute. V. VIII, 31–66 (In Russian).
2. Bryanchaninova N.I. Rock-forming silicates of ultrabasites as indicators of formation conditions and ore content. Syktyvkar: Institute of Geology, Komi science center UB RAS, 1990, 22 p. (In Russian).
3. Knyazev Yu.G., Knyazeva O.Yu. (2006) State Geological Map of the Russian Federation. Scale 1:200,000. Second edition. South Ural Series. Sheet N-40-XXIII (Beloretsk). Explanatory notes. Ufa: Bashkirgeologiya, 194 p. (In Russian).
4. Knyazev Yu.G., Knyazeva O.Yu., Karimov T.R. (2015) State Geological Map of the Russian Federation. Scale 1:200,000. Second edition. South Ural Series. Sheet N-40-XXVIII (Burzyan). Explanatory notes. Moscow: VSEGEI Publishing House. 237 p. (In Russian).
5. Larionov N.N., Bergazov I.R. (2015) State Geological Map of the Russian Federation. Scale 1:200,000. Second edition. South Ural Series. Sheet N-40-XXP (Tukan). Explanatory notes. Moscow: VSEGEI Publishing House. 247 p. (In Russian).
6. Mavrinskaya T.M., Yakupov R.R. (2016) Ordovician deposits of the western slope of the Southern Urals and their correlation based on conodonts and chitinozoans. Geologiya i geofizika. 57(2), 333–352. (In Russian). DOI: 10.15372/GiG20160204
7. Makeev A.B., Bryanchaninova N.I. (1999) Topomineralogy of ultramafic rocks of the Polar Urals. Saint Petersburg: Nauka. 252 p. (In Russian).
8. Moskaleva S.V. (1974) Hyperbasites and their chromite mineralization. Leningrad: Nedra. 279 p. (In Russian).
9. Ringwood A.E. (1981) Composition and petrology of the Earth’s mantle. Moscow: Nedra. 585 p. (In Russian).
10. Saveliev D.E. (2018) Ultramafic massifs of Kraka (Southern Urals): structural and compositional features of peridotite-dunite-chromitite associations. Ufa: Bashkir Encyclopedia. 204 p. (In Russian).
11. Saveliev D.E., Artem’ev D.A. (2021) Geochemical features of plastically deformed olivine from ophiolitic peridotites and dunites of the Kraka massifs (Southern Urals). Zapiski Rossiyskogo mineralogicheskogo obshchestva. 150 (1), 101–126. (In Russian).
12. Saveliev D.E., Shilovskikh V.V., Sergeev S.N. (2020) Microstructural features of ophiolitic chromitites of the Kraka massifs (Southern Urals). II. Podiform massive ores. Zapiski Rossiyskogo mineralogicheskogo obshchestva. 149 (5), 59–81. (In Russian).
13. Savelieva G.N. (1987) Gabbro-ultramafic complexes of Ural ophiolites and their analogues in the modern oceanic crust. Moscow: Nauka. 246 p. (In Russian).
14. Yakupov R.R., Mavrinskaya T.M., Abramova A.N. (2002) Paleontological justification of the Paleozoic stratigraphic scheme of the northern part of the Zilair megasynclinorium. Ufa: Institute of Geology, Ufa Scientific Center, RAS. 160 p. (In Russian).
15. Ahmed A.H., Arai S., Abdel-Aziz Y.M., Rahimi A. (2005) Spinel composition as a petrogenetic indicator of the mantle section in the Neoproterozoic Bou Azzer ophiolite, Anti-Atlas, Morocco. Precambrian Research. 138 (3–4), 225–234. https://doi.org/10.1016/j.precamres.2005.05.004
16. Arai S. (1994) Characterization of spinel peridotites by olivine-spinel compositional relationships: review and interpretation. Chemical Geology. 113 (3–4), 191–204. https://doi.org/10.1016/0009-2541(94)90066-3
17. Bhat I.M., Ahmad T., Subba Rao D.V. (2019) Geodynamic significance of Cr-spinels from ophiolite mantle peridotites of the Northwestern Himalaya. Journal of the Geological Society of India. 93, 657–662.
18. Bodinier J.-L., Dupuy C., Dostal J. (1984) Geochemistry of Precambrian ophiolites from Bou Azzer, Morocco. Contributions to Mineralogy and Petrology. 87, 43–50. https://doi.org/10.1007/BF00371401
19. Coleman R.G. (1971) Plate tectonic emplacement of upper mantle peridotites along continental edges. Journal of Geophysical Research. 76 (5), 1212–1222. https://doi.org/10.1029/jb076i005p01212
20. Dilek Y., Furnes H. (2011) Ophiolite genesis and global tectonics: geochemical and tectonic fingerprinting of ancient oceanic lithosphere. GSA Bulletin. 123 (3–4), 387–411. https://doi.org/10.1130/b30446.1
21. Ghosh B., Morishita T., Bhatta K. (2013) Significance of chromian spinels from the mantle sequence of the Andaman Ophiolite, India: paleogeodynamic implications. Lithos. 164–167, 86–96. https://doi.org/10.1016/j.lithos.2012.08.004
22. Ishii T., Kojitani H., Akaogi M. (2018). Phase relations and mineral chemistry in pyrolitic mantle at 1600–2200 °C under pressures up to the uppermost lower mantle: Phase transitions around the 660-km discontinuity and dynamics of upwelling hot plumes. Physics of the Earth and Planetary Interiors. 274, 127–137. https://doi.org/10.1016/j.pepi.2017.10.005
23. Kelemen P.B. (1990) Reaction between ultramafic rock and fractionating basaltic magma, 1, Phase-relations, the origin of calc-alkaline magma series, and the formation of discordant dunite. Journal of Petrology. 31, 51–98. https://doi.org/10.1093/petrology/31.1.51
24. Kelemen P.B., Dick H.J.B. (1995) Focused melt flow and localized deformation in the upper mantle: Juxtaposition of replacive dunite and ductile shear zones in the Josephine peridotite, SW Oregon. Journal of Geophysical Research: Solid Earth. 100 (B1), 423–438. https://doi.org/10.1029/94JB02063
25. Li H.-Y., Chen R.-X., Zheng Y.-F., Hu Z., Xu L. (2018) Crustal metasomatism at the slab–mantle interface in a continental subduction channel: geochemical evidence from orogenic peridotite in the Sulu Orogen. Journal of Geophysical Research: Solid Earth. 123, 2174–2198. https://doi.org/10.1002/2017JB014015
26. Lin K.-Y., Warren J.M., Davis F.A. (2023) Trace elements in abyssal peridotite olivine record melting, thermal evolution, and melt refertilization in the oceanic upper mantle. Contributions to Mineralogy and Petrology. 178 (10). https://doi.org/10.1007/s00410-023-02044-6
27. Moores E.M. (1982) Origin and emplacement of ophiolites. Reviews of Geophysics and Space Physics. 20(4), 735–760. https://doi.org/10.1029/rg020i004p00735
28. Olfindo V.S.V., Payot B.D., Valera G.T.V., Arai S. (2020) Petrogenesis of heterogeneous mantle peridotites with Ni-rich olivine from the Pujada Ophiolite, Philippines. Journal of Asian Earth Sciences: X. 4, 100039. https://doi.org/10.1016/j.jaesx.2020.100039
29. Pomonis P., Magganas A. (2017) Petrogenetic implications for ophiolite ultramafic bodies from Lokris and Beotia (Central Greece) based on chemistry of their Cr-spinels. Geosciences. 7 (1), 10. https://doi.org/10.3390/geosciences7010010
30. Ricolleau A., Fei Y., Cottrell E., Watson H., Deng L., Zhang L., Fiquet G., Auzende A.-L., Roskosz M., Morard G., Prakapenka V. (2009). Density profile of pyrolite under the lower mantle conditions. Geophysical Research Letters. 36 (6). https://doi.org/10.1029/2008GL036759
31. Ringwood A.E. (1975) Composition and Petrology of the Earth’s Mantle: New York, McGraw Hill. 604 p.
32. Rui H.-C., Namur O., Lian D.-Y., Cai P.-J., Li J., Valdes-Mariño Y., Yang J.-S., He H.-P. (2025) Modification of oceanic lithospheric mantle by percolated melts sourced from recycled ancient crust: Evidence from Ca-Os isotopes of refractory harzburgites. Chemical Geology. 695, 123077, https://doi.org/10.1016/j.chemgeo.2025.123077
33. Shabutdinov T.D., Abdrakhmanov R.F., Saveliev D.E., Poleva A.O., Mashkova E.A., Snachev A.V., Gataullin R.A., Durnaeva V.N., Samigullin A.A. (2025) Geochemical Features of Ultramafic Rocks and Formation of Magnesium–Bicarbonate Groundwaters in the Kraka Massif Area (Southern Urals). Geosciences. 16 (1), 8. https://doi.org/10.3390/geosciences16010008
34. Snachev A.V., Rassomakhin M.A. (2024) Gold and Platinum Group Element Occurrence Related to Black Shale Formations in the Southern Urals (Russia): A Review. Minerals. 14 (12), 1283. https://doi.org/10.3390/min14121283
35. Yao J., Cawood P.A., Zhao G., Han Y., Xia X., Liu Q., Wang P. (2021) Mariana-type ophiolites constrain the establishment of the modern plate tectonic regime during Gondwana assembly. Nature Communications. 12 (1). https://doi.org/10.1038/s41467-021-24422-z
36. Zhou M.F., Robinson P., Malpas J., Li Z. (1996). Podiform Chromitites in the Luobusa Ophiolite (Southern Tibet): Implications for Melt-Rock Interaction and Chromite Segregation in the Upper Mantle. Journal of Petrology. 37 (1), 3–21. DOI: 10.1093/petrology/37.1.3