Tsymbal S.M., Kryvdik S.G., Tsymbal Yu.S.
M.P. Semenenko Institute of Geochemistry, Mineralogy and Ore Formation of the National Academy of Sciences of Ukraine
Language: Ukrainian
Geochemistry and ore formation 2016, 36: 89-102
https://doi.org/10.15407/gof.2016.36.089
The results of petrographic and mineralogical studies of dunites in Malyi Ostrozhok intrusion located in the northern part of the Podil megablock of the Ukrainian shield are presented. At the present erosional level its size is about 150´350 m, falling to the south at an angle of 45–50. This intrusion has penetrated in the Berdichev granites (2,05–2,02 Ga) rocks and contains typical for the last almandine and biotite xenokrists. For the first time the chemical composition of rockforming minerals – olivine, chromite, phlogopite, amphibole and pyroxenes is studied in detail. Olivine is presented by forsterite (Fo89–91) with high NiO (average 0.45 %) and low CaO content. Ortopyroxene composition corresponds to enriched in Al enstatite or bronzite (Al2O3 – 1,0–3,8 %, f = 10–14 %). Clinopyroxene is rare and belongs to diopside (f = 11–13 %) or low-chromian diopside (f = 4,3 %). In most cases phlogopite with high magnesium content (MgO – 23–26%, f = 4–8 %) and amphibole (hastingsyte, edenite, actynolite and hornblende) are interstitial. Chromspinelides are idiomorphic and form crystals of octahedral habit. They are presented as inclusions in olivines and phlogopites or individual crystals or interstitial segregation. Their content in hosted dunites is about 1–3 % and their composition varies (%): Cr2O3 – 58–46; Al2O3 – 12–23; FeO – 20–25; Fe2O3 – 0–5; MgO – 6–9. They are represented by two groups: chromites and alumochromites with medium magnesity and low titanium contents. The inverse relationship between Cr2O3and Al2O3+ Fe2O3content and less significant between Cr2O3 and MgO are observed. It is proposed that all chromites (f= 55–70 %) have the crust origin. Among them absen are high Mgvarieties that are typical for mantle dunite and peridotite xenoliths from kimberlites. Investigated dunite intrusion has signs of crystalline differentiation and olivine fractionation. It is supposed that a parental ultramafic melt was formed by melting of depleted mantle by low oxygen fugacity and insignificant influence of deep metasomatism. Taking into account the sequence mineral separation, evolution trends of their chemical composition and relatively low temperature of olivine and chromite equilibriumu is concluded that the dunite is the end member melt of crystallization with increased of Fe, Cr, Al, Ca, K and Na content. By characteristic of chromite compositions they are similar to dunites of Kapitanka intrusion in Golovanivsk suture zone of the Ukrainian shield and are considered as chromitebearing hyperbasites of Proterozoic age.
Key words: dunite, olivine, chromite, phlogopite, amphibole, Malyi Ostrozhok intrusion.
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