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Solid-State Redistribution of Mineral Particles in the Upwelling Mantle

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Solid-State Redistribution of Mineral Particles in the Upwelling Mantle GEORESURSY = GEORESOURCES 2019. V. 21. Is 1. Pp. 31-46 ORIGINAL RESEARCH ARTICLE DOI: https://doi.org/10.18599/grs.2019.1.31-46 Solid-state redistribution of mineral particles in the upwelling mantle flow as a mechanism of chromite concentration in the ophiolite ultramafic rocks (by the example of Kraka ophiolite, the Southern Urals) D.E. Saveliev1*, V.B. Fedoseev2 1Institute of Geology of the Ufa Federal Research Centre of the Russian Academy of Sciences, Ufa, Russian Federation 2Razuvaev Institute of Organometallic Chemistry of the Russian Academy of Sciences, Nizhny Novgorod, Russian Federation Abstract. The main regularities of the structure of chromitite-bearing zones of ultramafic rock of the ophiolitic association are considered on the example of Kraka massifs. In all studied chromitite-bearing zones, olivine demonstrates a strong preferably crystallographic orientation, indicating that plastic flow was one of the main factors of petrogenesis and ore formation. A critical review of existing ideas about the origin of ophiolitic chromitites has been carried out. It is shown that for models involving the reaction and magmatic formation of dunites and chromitites, there are a number of difficulties. In particular, the application of the magma mixing model to the mantle ultramafiс rocks for the formation of chrome ores is faced with the problem of “free space”. Free space is necessary for the deposition of large volumes of ores, which is absent in a very low-porous crystalline upper mantle. In the “melt-mantle” interaction model, it is difficult to explain the often observed abrupt contacts of dunites and harzburgites, as well as an increase in the content of orthopyroxene in the near-contact parts of harzburgites, which is very often observed in ophiolite massifs. In addition, there is no mechanism for the formation of chromitites as geological bodies in this model. We have shown that the main trend in the composition and structure of the mantle section of ophiolites is stratification, accompanied by the separation of the rheologically most “weak” aggregates of polycrystalline olivine (dunites), which are host rocks for chrome ores. The stratification of the mantle material occurred during the solid-phase redistribution of minerals in the rocks, which are a dispersion system. In this work, a thermodynamic model is substantiated, which demonstrates the possibility of the emergence of solid-state flows in the conditions of the upper mantle and which makes it possible to eliminate some of the difficulties and contradictions characteristic of the magmatic and reaction-magmatic hypotheses. Keywords: ultramafic rocks, olivine, chromitite, plastic flow, stratification, rheomorphic segregation Recommended citation: Saveliev D.E., Fedoseev V.B. (2019). Solid-state redistribution of mineral particles in the upwelling mantle flow as a mechanism of chromite concentration in the ophiolite ultramafic rocks (by the example of Kraka ophiolite, the Southern Urals). Georesursy = Georesources, 21(1), pp. 31-46. DOI: https://doi.org/10.18599/grs.2019.1.31-46 Introduction metasomatic transformations of the peridotite (enstatite) The origin of ultrabasic rocks of ophiolitic complexes substrate (Bakirov, 1963; Moskaleva, 1974; Saveliev, and related chrome deposits has been controversial 1977, etc.) was caused mainly by the inability to explain for many years. In the first half of the 20th century, magmatic differentiation of “epigenetic” properties of chromitites were considered as differentiates of dunite and chromitite in relation to the surrounding ultrabasic magma (dunite or peridotite one) (Loginov et harzburgite. In the works of A.G. Betekhtin, G.G. al., 1940; Sokolov, 1948; Kravchenko, 1969; Pavlov et Kravchenko, A.S. Varlakov, great importance of fluids al., 1979; Marakushev, 1988, etc.). The development of in the genesis of chromitites was suggested. Nowadays, the metasomatic hypothesis regarding dunite-harzburgite due to numerous findings of submicron inclusions of complexes with chrome ore mineralization as a product of “fluid-containing” phases in chromitites, these ideas have more and more supporters (Chashchukhin et al., 2007; Pushkarev et al., 2007; 2015). * Corresponding author: Dmitry E. Saveliev In the 1960s and 1970s the ophiolitic assemblages, E-mail: [email protected] which include ultramafic rocks, were compared with © 2019 The Authors. Published by Georesursy LLC This is an open access article under the CC BY 4.0 license relics of the paleooceanic crust and upper mantle (Peive, (https://creativecommons.org/licenses/by/4.0/) 1969). The ubiquitous distribution of deformational www.geors.ru GEORESURSY 31 Solid-state redistribution… D.E. Saveliev, V.B. Fedoseev structures in ultramafic rocks of ophiolites and the solid-phase segregation of chromite in ascending plastic ocean floor has been established, which made it possible flows of ultramafic rocks. to consider the rocks of both assemblages as “mantle tectonites” (Coleman, 1979) and to state their significant Factual material similarity to each other. In the future, more and more Kraka massifs located within the Zilair mega-zone researchers began to consider chromium mineralization of the western slope of the Southern Urals (Fig. 1) were in ophiolites as a result of the depletion of initially chosen as the object of study. Ultramafic rocks occupy homogeneous, undepleted mantle substrate, similar in an area of about 900 km2 and form four large bodies of composition to lherzolite (Savelieva, 1987; Savelieva, oval or isometric shape. The structure of the massifs is Saveliev, 1991; Perevozchikov, 1995). dominated by variously serpentinized spinel peridotites Detailed studies of the structural features of ultramafic with a subordinate value of spinel-plagioclase varieties rocks of chromite-bearing zones were carried out on and rare dunite bodies. the Uralian massifs (Goncharenko, 1989; Denisova, The main feature of peridotites is the almost 1989; 1990, Savelieva, Saveliev, 1991; Shcherbakov, ubiquitous distribution of primary banding, which is 1990). On the basis of large-scale geological mapping, expressed in frequent alternation of bands with different petrostructural analysis, generalization of data obtained quantitative ratios of rock-forming minerals – olivine, as a result of a large amount of drilling and mining, by orthopyroxene, clinopyroxene and Cr-spinel. The the early 1990s, the most publications made a conclusion thickness of bands varies from tenths of a centimeter about the leading role of plastic flow in the genesis of to the first tens of meters. Almost always banding is ultramafic rocks and chromitites, but a model linking the accompanied by foliation and lineation (Fig. 2a, b). plastic flow and the processes of differentiation of matter Macroscopically, mineral slaty cleavage is expressed in the upper mantle was never formulated. in the regular arrangement of tabular pyroxene Since the early 1990s, the main hypothesis crystals; when studying rocks under a microscope, a claiming to explain the genesis of mantle dunites and similar orientation is observed even more for olivine. chromitite is the reactive porous melt flow through Lineation – the elongation of individual crystals in one the mantle peridotites, first formulated in the works direction – is also characteristic of olivine grains. At the by P. Kelemen (Kelemen et al., 1992; 1995) and same time, Cr-spinels and pyroxenes most often show supported by various researchers (Gonzalez-Jimenez et al., 2014; Miura et al., 2012; Zhou, 1994; Zhou et al., 1996, and others). It should be noted that despite the different terminology, this idea is very close to the above-mentioned metasomatic hypothesis, widely used in the works by a number of Soviet geologists (Moskaleva, 1974; Varlakov, 1978, etc.). However, in the reaction-magmatic model there is no mechanism for the formation of chromitites as geological bodies. Even if we assume that chromite crystallizes in dunite at the dissolution of pyroxenes, the factors that cause it to form separate bodies with differing concentrations, from rarely disseminated to massive ores, remain unclear. In addition, within the framework of the reaction hypothesis, it is difficult to explain the often observed abrupt contacts of dunites and harzburgites, as well as an increase in the content of orthopyroxene in the near-contact parts of harzburgites, which is very often observed in ophiolites. In a number of publications, we have substantiated Fig. 1. Overview map of Kraka massifs. 1 – enclosing the rheomorphic model for the formation of ore Paleozoic sedimentary rocks of the Zilair mega-zone, 2 – concentrations of Cr-spinels in mantle ultramafic rocks mafic and ultramafic rocks of the transitional mantle-crust complex (gabbro, verlite, clinopyroxenite), 3 – predominantly (Saveliev, 2013; Saveliev et al., 2008; etc.) and started spinel peridotites with subordinate dunites, 4 – melange the development of a physical model of rheomorphic serpentinites, 5 – chrome deposits and ore occurrences, differentiation (Saveliev, Fedoseev, 2011, 2014) as and the most significant of them are: K – Klyuchevsky, S – a logical continuation of studies that established the Sakseysky, Sh – Shatransky, Ap – objects of Apshaksky Area, tectonic nature of mantle section of ophiolites. This BB – Bolshoy Bashart, M – named after Menzhinskiy, MB – paper presents the results of thermodynamic modeling of Maly Bashart. 32 GEORESOURCES www.geors.ru GEORESURSY = GEORESOURCES 2019. V. 21. Is 1. Pp. 31-46 The microstructural
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