Ultrapotassic rocks from bottom zones of basaltic bodies from Southern Timan Ridge (Russia)

 Udoratina O.V. *, Varlamov D.A. **, Kulikova K.V. *, Savelyev V.P. ***

* IG Komi SC UB RAS, Syktyvkar, Russia, udoratina@geo.komisc.ru

** IEM RAS, Chernogolovka, Russia,

*** JSC Ukhtageoservice, Ukhta, Russia,

 

The Upper volcanic-sedimentary Dzherskoy (D₃dz) suite of the Southern Timan contain bodies of a basaltoids (sills, injection veins etc.). In the lower parts of these bodies the bleached metasomatic  zones are observed. The thickness of zones varies from centimeters to the first tens centimeters. These rocks are greenish-gray with transition to grayish-beige, they have microamygdaloid texture and microporphyric, sometimes microglomeroporphyric fabric with predominantly trachytic-like groundmass. The rock contains potassium feldspar (Kfs), carbonate, chlorite, kaolinite, anatase, pyrite. The roundish amygdales take about 5 vol.% of rocks and have complex zoned structure (carbonate→opal→chlorite→kaolinite).  The size of amygdales varies from 0.15 to 0.6 mm. Phenocrysts are represented by elongate-tabular Kfs crystals, frequently in  glomeroporphyric accretions with relic grains of former mafic mineral which usually are  pseudomorphically replaced during metasomatism process by fine-grained aggregate of ferriferous carbonate. The size of phenocrysts doesn't exceed 0.15–0.3 mm. The groundmass is composed of randomly oriented small Kfs microlites, carbonate grains, chlorite flakes, cryptogranular aggregate of kaolinite. The reduction of crystallization degree and increase of kaolinization (toward bottom) are observed.

Kfs in phenocrysts and in groundmass represents practically pure potassium feldspar, and data on the results of X-ray diffraction it corresponds to orthoclase. Carbonates always represents ferriferous (ankerite series). Ore minerals are submitted by pyrite, chalcopyrite, anatase and different iron oxides. Petrochemically rocks are classified as the high-potassic alkali basalt: SiO₂ – just about 46.0, K₂O – 7-8.5, TiO₂ – 1.2-2.0 wt.%. The increase of aluminum and the decrease of iron, magnesium and calcium contents are observed toward the edge of the sole.