Kaersutite lamprophyres of the Pokrovo-Kyriivo massif (the Azov area, Ukraine): petrographic features, mineral composition, geochemistry

Abstract

The paper represents results of the petrographical, geochemical and mineralogical studies of a dyke of porphyry-like mafic rock from the Pokrovo-Kyriivo massif of the Azov area, the Ukrainian Shield. This dyke of fine-grained rock has unique mineral composition as the main mineral in the groundmass is kaersutite. According to mineralogical and petrographic features, it can be identified as kaersutite lamprophyre. The Devonian Pokrovo-Kyriivo massif is located at a junction of the Azov Domain of the Ukrainian Shield and the folded structure of Donbas. This massif is composed of subalkaline (pyroxenite, gabbro) and alkaline (malignites, nepheline syenite) rocks which are cut by numerous dykes varying in composition from ultramafic to intermediate. The dyke of finegrained kaersutite lamprophyre was discovered in the bottom hole of the drillhole 36, where it is intruded into mediumgrained gabbroids and pyroxenites. The groundmass of the rock is composed of kaersutite, titanomagnetite, albite, chlorite, titanite, pyrite, chalcopyrite, apatite, monazite. The phenocrysts are represented by chlorite and talc pseudomorphoses after a diamond-shaped mineral with shortened obtuse angles. According to electron microprobe analysis, kaersutite has low TiO2 content and intermediate mg# values of 0.54—0.67. According to the chemical composition, studied kaersutites differ from the same mineral in the microgabro dyke in the Pokrovo-Kyriivo massif, and from kaersutites in camptonites of the Khlibodarivka open pit in the Azov area. At the same time, it is similar to kaersutites from camptonites of the Novoukrainka dyke field of the Inhul terrane. Kaersutite lamprophyre also differ from dyke rocks of the Pokrovo-Kyriivo massif by the absence of clinopyroxene, mica, ilmenite and low content of TiO2 in titanomagnetite, which does not exceed 6 %. This is due to a low TiO2 content in the rocks (up to 3.5 %), relative to other dyke gabbro of the area. According to mineralogical-petrographic and chemical characteristics, kaersutite lamprophyres are unique rocks that have no direct counterparts in the Azov area or on the Ukrainian Shield in the whole.