PHYSICOCHEMICAL CONDITIONS OF CRYSTALLIZATION OF DUNITES OF THE NIZHNII TAGIL PLATINUM-BEARING MASSIF (Middle Urals)
V.A. Simonov1,2, V.N. Puchkov3, V.S. Prikhod’ko4, S.I. Stupakov1, A.V. Kotlyarov1, N.S. Karmanov1, A.S. Stepanov5
1V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia
2Novosibirsk State University, ul. Pirogova 2, Novosibirsk, 630090, Russia
3Institute of Geology, Ufa Science Center, Russian Academy of Sciences, ul. Karla Marksa 16/2, Ufa, 450000, Russia
4Yu.A. Kosygin Institute of Tectonics and Geophysics, Far Eastern Branch of the Russian Academy of Sciences, ul. Kim-Yu-Chena 65, Khabarovsk, 680000, Russia
5ARC Centre of Excellence in Ore Deposits (CODES COE), University of Tasmania, Private Bag 79, Hobart, Tasmania 7001, Australia
Keywords: Physicochemical conditions of crystallization, silicate melt inclusions, Cr-spinels, dunites, Pt-bearing ultramafic massifs
Abstract
Studies of primary multiphase silicate inclusions in accessory Cr-spinels from the fine-grained dunites of the Nizhnii Tagil Pt-bearing massif reveal their similarity to melt inclusions trapped by chromite during its growth. The analyzed Cr-spinels with multiphase silicate inclusions differ in composition from ore chromites of the same massif and from chromites (with melt inclusions) from ultramafic oceanic complexes but are similar to Cr-spinels in dunites from Pt-bearing alkaline ultramafic massifs (Konder and Inagli). According to petro- and geochemical data on heated multiphase silicate inclusions, the studied Cr-spinels crystallized with the participation of subalkalic picrobasaltic melts similar to the magmas of the Konder Pt-bearing massif and having almost the same chemical composition as tylaites. The differences between the compositions of olivines formed within the multiphase silicate inclusions and of the rock-forming minerals show that the studied Cr-spinels formed from an intercumulus liquid melt in the olivine crystal interstices during the cumulate crystallization of most of the Nizhnii Tagil massif dunites in the intrusive chamber. Numerical modeling based on the compositions of heated multiphase silicate inclusions in accessory Cr-spinels demonstrates that olivines and Cr-spinels from the studied dunites crystallized at 1430 to 1310 ºC and then olivine formation continued to 1280 ºC during the evolution of melts.
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