Journal of Economic Geology (Sep 2023)
Physical Conditions of the Formation of Plagioclase Crystals Using Crystal Size Dispersion (CSD) in the Mafic Migmatites of Northeastern Takab
Abstract
The Zanjan-Takab complex is a metamorphic belt with NW-SE trend that includes gneiss, amphibolite and gneissic amphibolite, old metagranites, pelitic schists with migmatites and metaophiolites. Takab mafic migmatites, based on field evidence and partial melting degree, are divided into two main groups of metatexites with patchy, ophthalmitic, diktyonitic, agmatic, stromatic structures and diatexites with schollen, ptygmatic, folded, stictolithic, phlebetic, schliren, nebulitic structures. These migmatites consist of mesosome part with textures of porphyroblastic, xinoblastic, granoblastic and nematoblastic and main minerals plagioclase, hornblende, biotite, melanosome part with nematoblastic, xinoblastic, granoblastic and oriented textures and main minerals hornblende and plagioclase. The leucosome part is composed of granular, sympletic and myrmekitic textures and the main minerals are plagioclase, quartz, k-feldspare, titanite, hornblende and biotite. Investigating the shape of the crystal size scatter diagrams (CSD) shows the physical conditions and petrological processes are effective in the studied rocks. In order to investigate these processes, plagioclase crystals in 4 leucosome samples were quantitatively analyzed with the help of Jmicro vision and CSD Corrections software, and then the results obtained from the analysis of different leucosome samples were compared. The crystal size scatter diagrams of plagioclase crystals show two stages of growth with different speeds in the studied samples, so that the larger crystals (the end part of the curved diagram on the right) belong to a melt that is at greater depths or it cooled in a calmer environment. Nevertheless, the initial part of the diagram on the left side crystallized in more superficial areas and at a higher speed. Introduction The crystallization history of a rock is recorded in the size and distribution of its minerals (Muller et al., 2009). The most common method of quantitative measurement of textures is investigation of crystal size dispersion (CSD) (Higgins, 1998). Crystal size dispersion (CSD) deals with the quantitative measurement of crystals of a specific mineral with a unit volume in size intervals (Cashman, 1993; O'Driscoll et al., 2007). Many researchers, including Cashman and Marsh (1988), Cashman (1993), Zieg and Marsh (2002), Higgins and Roberge (2003), Higgins and Roberge (2007), Kaneko et al., (2005), Cashman and Ferry (1988), Higgins (1996) and O'Driscoll et al., (2007) in their studies of CSD diagrams to determine kinetic indices of crystallization of magmatic systems, cooling history, temperature, magmatic mixing, texture maturity, dominant size of crystals, density Crystals and partial volume of crystals and thermodynamic and kinetic models were used. In addition, researchers such as Kaneko et al., (2005), Cashman and Ferry (1988), Moazzen and Modjarrad (2005) and Muller et al., (2009) studied the crystal size distribution in metamorphic rocks. In this research, an attempt has been made to investigate the plagioclase crystals in the leucosome section in the migmatites of North-Eastern Takab using the crystal size dispersion (CSD) technique, and by examining the resulting diagrams, the petrological processes and the nucleation rate of the crystals plagioclase should be expressed during melting and finally the formation of leucosomal parts. Materials and methods In the study of Ghareh naz migmatites, after taking digital photos of the leucosomal part, they were put together with Adobe Illustrator software for better coverage, and then all the plagioclase crystals were drawn and measured separately. Then the images were transferred to JMicroVision v1.2.7 software and the necessary measurements for all 4 locosome samples (7n, 6p, 6m, mh) including length, width, area, angle, location of the center of crystals (coordinates of X and Y points) in the environment of this software was done. It should be mentioned that considering that the studies are done on thin section images, so to eliminate the errors caused by this problem, all the software settings were entered based on the Higgins (1998) method. Then the obtained data were transferred to CSD Corrections 1.40 software and according to the information obtained from the frequency and size of plagioclase crystals, a natural semi-logarithmic diagram based on the method provided by Higgins (1998) for 4 leucosome samples of migmatite Ghareh naz was drawn separately. In these diagrams, the population density axis, Ln(n)(mm-4) is plotted against the size axis of the largest crystal dimension, (mm) equal to L. The unit of measurement for bulk density is mm-4 (Marsh, 1988; Higgins, 2006; Bindeman, 2003; Higgins ana Roberge, 2003, Higgins ana Roberge, 2007; Gulda, 2006; Higgins and Chandrasekharam, 2007; Brugger and Hammer, 2010) and for crystal size is mm. Result Studies show that downward concavity in the direction of small crystal sizes indicates coarsening (Higgins ana Roberge, 2007; Vanderzwan et al., 2013) or, in other words, indicates the cessation of nucleation along with successive crystal growth (Lentz and Mcsween, 2000; Higgins ana Roberge, 2007; Vanderzwan et al., 2013). The amount of low concavity in some samples can be considered as an almost linear trend. The almost linear trend on the right side of some graphs indicates the successive growth and nucleation of crystals, the process of subtraction (crystal separation during subtractive crystallization) and crystal accumulation (Higgins, 2009). As can be seen in the diagrams, the very small break created in the crystal size scatter diagram indicates the accumulation or increase of large grains, or in other words, the sedimentation and separation of crystals (Lentz and Mcsween, 2000). In other words, the coarsening process in the studied migmatite locosome occurs when small grains in multiphase mixtures have a higher surface free energy per unit volume than large grains and for this reason, they are less stable (Higgins, 1998, Higgins, 1999). Discussion The scatter diagrams of the crystal size in the studied leucosomal samples show that the larger crystals (the end part of the curved diagram on the right) belong to a melt that cooled at greater depths or in a calmer environment. Nevertheless, the initial part of the diagram in the left side crystallized in more superficial areas and at a higher speed. Based on the studies conducted on the plagioclase crystals in the migmatite locosome, it was observed that the plagioclase crystals have a non-linear CSD trend, which indicates two stages of growth with different speeds for these crystals. In the initial stages of crystallization, plagioclase is in the form of coarse crystals with low number of spores, high gradient and high growth rate, and in the next stage, smaller plagioclase with high nucleation rate, low gradient and low growth rate are developed. Plagioclase crystals with their large number and relatively large size indicate their slow cooling and low nucleation.
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