Guli massif is a world-largest alkaline-ultramafic complex. It located in transition zone between Siberian platform and Khatanga syncline. Dunites are the first intrusive phase of Guli massif. They cover approximately 60% of present-day surface of the massif (Egorov, 1991).
Chromite-spinel-magnetite solid solutions are supposed to be the most informative minerals of olivine- ultramafic and mafic rocks. They are widely used for determination of formation type of ultramafic and mafic rocks and estimation of rock-forming conditions.
Composition vatiations of spinel-group minerals in Guli massif was previously studied by Egorov (1991), who attributed the olivine rocks of Guli massif as dunites instead of olivinites, and Malich with co-authors (Malich, 1999), who found that spinel-minerals composition depend on type of rocks, serpentinization ratio and ontogeny character of spinel minerals.
We have studied the spatial variations of spinel minerals composition. The mineral composition was estimated by electron-microprobe analyse in GEOKHI RAS (Cameca-100). The samples collection was kindly given by L.N. Kogarko and A.M. Asavin.
According with the data of previous investigators, compositions of spinel minerals vary between ferrichromite and titanomagnetite with minor but variable amount of aluminium and titanium components. Spinel-minerals composition can be shown in Al3+-Cr3+-Fe3+ diagram (fig 1).
Fig. 1. Variations of spinel-group minerals from Guli massif. Numbered arrows show: 1- core-rim zoning in chromite from maimechite(Kogarko, Ryabchikov, 1999); 2-evolution of chromspinel composition during fraction crystallisation of maimechite melt; 3 – evolution of chromspinel composition during equilibrium crystallisation of maimechite melt; 4- chromspinel composition variation in Khaluda hill, from bottom to top.
We hasn’t found the noticeable core-rim zonation and chemical difference between compositions of spinel-minerals crystals included in olivine, subhedral crystals between olivine and euhedral crystals between serpentine in the same sample. At the same time some samples contain compositionally contrast spinel minerals. Some grains are ferrichromite or chrom-magnetite and some grains are titanomagnetite. It result in bimodal distribution of compositions in Cr3+/Fe3+ ratio.
We can see the accordance between aluminium content in ferrochromite and in magnetite from the same sample or series of samples from one place of massif.
The richest in aluminum are a ferrichromite and magnetite from Ingarynga valley, the next are samples from district of r. Vostochny and mt. Khaluda (they are similar to the external part of chromite grains in maimechite by Al content), the next - from Epishkin Hill and from the South part of massif. The lowest aluminum content we have found in samples from Iolite hill.
Samples of dunites from mt. Khaluda show chrome-spinel composition variations depending on vertical position: on the bottom part of mountain present only the ferrichromite and Cr-free magnetite, in the medium part of slope – ferrichromite with lower average Cr/Fe3+ ratio and chrom-magnetite and in the top of mountain – only chrome-magnetite. This composition trend conform to the crystallization differentiation of ultramafic magma.
We cannot explain the difference in aluminum content in spinel-group minerals from the different parts of Guli massif by the fractionation in single magma portion, because differentiation trends are sub-parallel to the Cr-Fe side of the diagram. On the other hand, we cannot explain this difference by the postmagmatic reequilibration, because the diffusion coefficients of Al and Cr in chrome-spinels are very low, therefore their concentrations practically not change in post-crystallisation processes unlike Fe and Mg concentrations (Roeder, Cambell, 1985).
The only explanation we can propose that different parts of dunite-core of Guli massif were formed from different portions of primary melt. It means that we have to consider the dunite core of Guli complex as combination of dunite massifs.
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Kogarko L.N., Ryabchikov I.D. Geochemical evidence for meimechite magma generation in the subcontinental lithosphere of Polar Siberia // Journal of Asian Earth Sciences. 1999. P. 1-9.