Mn-Ti-RICH MAGNETITE FROM MALIGNITES OF POKROVO-KYRYIVSKYI MASSIF (AZOV SEA REGION, UKRAINIAN SHIELD)

Abstract

Malignites are the one of alkaline components of Pokrovo-Kyryivskyi massif, lo-cated at the junction of Pryazovskyi megablock of the Ukrainian shield and the folded structure of the Donbas. Rocks contain several minerals of specific composition: Zr- and Nb-rich götzenite, low-aluminiferous micas with a high content of Ti, Fe-containing nepheline, Ca-Na-amphiboles with high content of K, Sr-fluorapatite, britholite, rare earth niobate-titanates, REE-Sr-carbonates, catapleite etc. Magnetite from malignites has quite unusual chemistry. It is a rare mineral, and usually presents in the form of microscopic inclusions in rock-forming minerals. How-ever, the case is relatively large (> 200 μm) idiomorphic or subidiomorphic magnetite crystals, which contain small or microscopic inclusions of other minerals – nepheline, Sr-fluorapatite, titanite. The presence of such inclusions indicates the emergence of magnetite on the main (magmatic) stage of malignites formation. Much more magnetite is in the xenolith from malignites, where it is in the form of twisting irregularly shaped grains among the scales of phlogopite and tabular grains of Mg-richterite. The studied magnetite is characterized by a high content of MnO and TiO2, higher – V2O3 and very low – MgO, Al2O3 and Cr2O3. Clear positive correlation between TiO2 and MnO has been determined, and negative – between these oxides and FeO* with no correlation with V2O3. Magnetite contains inclusions of nepheline and Sr-fluorapatite, associates with minerals of the series MnTiO3–FeTiO3. Magnetite of such chemical composition has been investigated in Ukraine for the first time. Magnetite from xenolith in malignites is characterized by high contents of Cr2O3. We also have analyzed very rare in malignites minerals of series ilmenite FeTiO3–pyrophanite MnTiO3 that occur as inclusions in götzenite. They have high content of Nb2O5 (up to 2.3 wt. %) and low – ZnO (0.5 wt. %). It is concluded that the magnetite in malignites is syngenetic and high-temperature mineral that has been formed at the magmatic stage of rock crystallization. Discovered peculiarities of magnetite chemistry and the paragenesis of minerals (K-feldspar, nepheline, aegirine-salite pyroxene, alkali amphibole) are characteristic for alkaline ig-neous rocks. We can also assume that malignites have been crystallized quickly in hypabyssal conditions, in which oxidation of ulvospinel component and formation of separate la-mellae of FeTiO3–MnTiO3 in magnetite matrix did not happen. It is known that there are crypto- and fine-grained rocks (the so-called phonolitoid) in Pokrovo-Kyryivskyi massif, which chemical composition is similar to malignites or is the same. Perhaps these rocks are the effusive analogues of malignites. As you know, the melts of alkaline rocks are crystallized very well even on the Earth's surface in lava flows.