Title

A. M. Belyaev

Petrology of volcanic rocks from the rapakivi formation (Hogland Island)

Geological, geochemical and isotope-geochemical researches of volcanites from the Island Gogland were conducted. It is established that subhorizontal strata of porphyritic rhyodacites and porphyritic basalts form covers with typical pillow structures, which characteristic for underwater sea outpourings. Geochemical and isotope-geochemical data confirm that porphyritic rhyodacites (1640 Ma) were formed contemporaneously with the rapakivi granites of Vyborg massif, and porphyritic basalts are similar to gabbro-anorthosites. Abnormal proportions of alkalis in the porphyritic rhyodacites and porphyritic basalts are interpreted here as a consequence of the interaction between magmas and hot K-rich sea water. It is supposed, that potassium-saturated hydroterms were formed due to release of fluids derived from high potassium magmas of the basalts and rapakivi granite into sea waters.
Key words: rapakivi formation, Wiborg massif, Hogland, petrology, mineralogy, geochemistry, isotope geochemistry, trace element’s, rhyodacites, basalts, pillow lavas, quartzites, conglomerates, chertes, granite gneiss, rapakivi granites, waste mantle, metasomatism, sericitization.