Study of physical conditions and magmatic evolution of Volcanic rocks in East of Qazvin

Volcanic rocks including olivine basalt، basalt، andesite basalt، dacite، trachyandesite and their associated tuffs with the age of Eocene to Plio-Quaternary are exposed in the east of Qazvin. In order to investigate the kinetic of crystallization of magma، plagioclase crystals، in selected olivine basalt، basalt، andesitic volcanic rocks of Eocene to Plio-Quaternary have been selected. The rocks mainly present porphyritic، glomoroporphyritic and occasionally microlitic textures. In This research، volumetric phase proportion، residence time and nucleation rate for plagioclase crystals in 8 samples estimated by crystal size distribution (CSD) method and results for different samples compared with each other. In general، in spite of compositional and age similarity between samples of different unites، plagioclases present sharp different physical properties. Volumetric phase proportion of this mineral varies from 24. 5% in basalt of Plio-Quaternary to 61. 25% in basalt of Eocene. Based on the slope of lines in crystal size distribution diagrams، plagioclases growth in different residence times. Eocene unites present 21 to 55 years and this time for Pli Quaternary unites varies from 22. 8 to 30. 7 years. Fractional crystallization was the main magmatic process in samples، however، variation of nucleation rate in different rocks with different ages، suggesting the different physical conditions during the crystallization. An extraordinary value for andesitic basalt of Eocene is due to presence of magmatic mingling.