Investigation of physical processes of crystallization in igneous rocks from SE Birjand using 3-D shape modeling of plagioclase crystals

Shape or habit of crystal is controlled by physicochemical conditions of crystallization, thus quantitative measurements of crystal shape can reveal aspects of crystallization environment. In this paper, based on statistical calculations, numerical modeling software and microscopic observations. various methods of quantitative measurement of theshape of crystal have used More than 3200 plagioclase crystals in 9 thin sections of the two groups of rocks including basaltic andesite and quartz diorite were processed. Study area is located about 120 Km southeast of Birjand. The results of calculated crystals aspect ratio (Short: Intermedite: Long or S: I: L) have showed that both groups have tabular habit (I≈L), but tabulation amount in the first group is to be more (I, L>2). Analysis demonstrates that microlites in the first group were formed under conditions with high undercooling, fast growth rate and environment with high advection (relative movement between the crystal and growing medium) and the high mechanical movement of magma and high chemical potential gradient had significant role on ctystal shapes. In the second group, contrastingly, low growth rate and high diffusion rate were established. Also, textural coarsening process was affected on shift of crystal shapes from tabular to more equant forms and reduction of aspect ratio with increment ofmaximum length of crystals.