[SVC43-05] Heterogeneities in Crystal from YTT (74 ka) and the Implications for Toba Magma Chamber Systems
Keywords:Toba Caldera, the Youngest Toba Tuff (YTT), Heterogeneities in Crystal, Conduit Process, Chamber Process, Multiple eruptions
Characterizations of Toba Tuffs have been made but not been enough to discuss YTT in detail. In this study, we focus on spatial heterogeneities in YTT deposits. Samples from four different locations were employed for the analyses. Component analysis was carried out on components larger than 2 mm. Whole-rock geochemical data were obtained by XRF. Petrography analysis for 37 thin sections was conducted using optical microscope. Textural analysis was carried out for 170 free-crystals and 25 selected thin sections using microphotographs taken by SEM and further analyzed using image processing software. Chemical analysis for free-crystals was carried out by SEM-EDS, while for pumices grain of 22 thin sections was conducted using EPMA.
Geochemical data showed that YTT magma is rhyodacitic to rhyolitic in whole-rock compositions with wide range of SiO2 (69.15–76.83 wt.%). There are differences in abundance and type of pumices, free-crystals, and lithic in each location. Major minerals are plagioclase, biotite, sanidine, and quartz. Common characteristics of northern and southern part deposit is that most of crystals are fractured, some forming aggregates, has anhedral shape and wide variation in size (0.003 mm2-13.113 mm2). However, there are differences between northern and southern deposits: presence of amphibole with larger size, orange quartz, sieve texture, patchy zoning, oscillatory zoning, crystal clots, domination of quartz and sanidine as free-crystals, and wider range of anorthite (An25– An87) is mostly found in northern deposits.
Plagioclase composition from northern part shows bimodal distribution suggesting that crystallization does not occur simultaneously by single process. Furthermore, plots of anorthite number versus size and of average anorthite number versus crystal content show random distribution, suggesting the complex crystallization of plagioclase: other processes than fractional crystallization in magma chamber. Moreover, presence of antecryst and disequilibrium textures in northern deposit indicates intervention from older rocks or even other systems. Heterogeneities in crystal from northern and southern deposits suggest that YTT deposits are generated by multiple eruptions from independent, at least two magma chambers.