PLAGIOCLASE ZONATION OF CLINOPYROXENITE–GABBRONORITE–DIORITE KAALAMO MASSIF (NORTHERN LADOGA AREA, RUSSIA)

Abstract

The gabbroids of Kaalamo massif of the Northern Ladoga area contain plagioclases with a mixed type of zoning, which makes it possible to establish some features of crystallization and postmagmatic evolution of intrusive rocks. The study of minerals by the method of X-ray spectral microanalysis and numeric modeling of mineral formation during crystallization of rocks show ranges of compositions changes in plagioclases. The studied grains of plagioclase from the real samples were compared with theoretical (modelled) ones. The conclusions about the magmatic and postmagmatic evolution of gabbroids based on the data obtained were made. As the temperature dropped below the liquid’s of plagioclase, a solid solution precipitated from magma of ultrabasic – basic composition in the form of basic plagioclase with An 80 . Further, the Na-Si enrichment of plagioclase occurs due to fractional crystallization, in which the composition of the melt shifts to the side rich in silicic acid and sodium. Crystallization of plagioclase, reaching An 50 , is associated with mixing of the compositions of two magmas (fractionated early and new portions of the melt). In plagioclases An 50–40 , oscillatory zoning was formed due to diffusion processes at the crystal – melt interface during slow cooling of the intrusion and increased magma viscosity due to the shift of its composition to the silicic acid part. The latest plagioclases (to An 30 and lower) were formed during the recrystallization of magmatic plagioclases and reflect the stages of late-, postmagmatic alteration and superimposed regional metamorphism.