Kryvdik S.G.¹, Sharygin V.V.², Gatsenko V.O.¹, Lunev E.S.¹
1 – M.P. Semenenko Institute of Geochemistry, Mineralogy and Ore Formation of the National Academy of Sciences of Ukraine
2 – V.S. Sobolev Institute of Geology and Mineralogy of the Siberian Branch of the RAS, Novosibirsk, Russia
Language: Ukrainian
Geochemistry and ore formation 2016, 37: 3-14
https://doi.org/10.15407/gof.2016.37.003
Micas of the Pokrovo-Kyriyivo massif (Azov Sea region, Ukraine).
The article represents new microprobe data for micas from malignites, their xenoliths and dike subalkaline gabbroids of the Pokrovo-Kyriyivo massif. Micas from gabbroids are phlogopites with moderate Al2O3 (14-16 wt. %) and TiO2 (4,3-6,3 wt.%). Malignite xenoliths contain tetraferriphlogopite, which is high in magnesian number (0,8 Mg #), low in alumina (7,9-11,2 wt.% Al2O3) and TiO2 (0,7-2,5 wt.%). Two generations of micas are documented in malignites. The first early generation includes varieties, which form single large rhomboid, rectangular or irregular grains with numerous inclusions of nepheline, clinopyroxene, titanite, magnetite and Sr-rich fluorapatite. These micas in malignites are low in Al2O3 (6.4-8.8 wt.%), high in TiO2 (up to 4.9 wt. %) and F (to 3.3 wt. %). The amount of the K cations is more than Al (K > Al), and the sum (Si + Al) is < 4. In chemical composition these micas are intermediate between tetraferriannite and tetraferriphlogopite. However, the reserve absorption scheme is rarely observed in these micas (optically tetraferriphlogopites are present only in mica-rich xenoliths from malignite). The second late generation is represented by small crystals from miarole-like minute inclusions (commonly in gцtzenite), which also contain zeolites, fluorite, strontianite and later (II generation) nepheline. Micas of second generation are characterized by low TiO2 (0.8-2.5 wt.%), MgO (1.4-10 wt.%), F (0-0.2 wt.%), and high content of Al2O3 (12.6-15.9 wt.%) which is 1.3-1.5 Al cations in the formula. They are represented by annite to siderophуllite. Annite is formed in a medium and low temperature conditions. It is assumed that complement deficiency of cations in tetrahedra (Si + Al <4) preferably Ti4+, but not Fe3+, is characteristic for micas of alkaline rocks and some carbonatites. Low Al2O3 content in early micas of malignites (K > Al) is due to agpaitic conditions of malignite formation and high K content. Alumina-understurated micas are crystallized together with such characteristic agpaitic minerals as alkaline pyroxene and amphibole, nepheline and sodalite, gцtzenite, catapleiite, Sr-rich fluorapatite. Fluorine is typical for high-magnesian micas. It decreases up to a total absence with increasing of Fe2O3. This is probably due to the decreasing temperature because fluorine is not incorporated in Fe2O3-rich annite, which crystallize in microinclusions even if fluorite is present (i.e., when the system was saturated with fluorine). It is also possible that during the formation of malignite the Fe increase in system is a division between the fluorine-rich mica and fluid – melt as determined by experiment for the system of iron-rich biotite – granite melt.
Keywords:Pokrovo-Kyriyivo massif, subalkaline gabbroid, malignite, phlogopite, tetraferriphlogopite, tetraferriannite, annite.
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