Plagioclase-hosted Crystallized Melt Inclusions within Hypabyssal Volcanic Rocks of the Torud-Ahmad Abad Magmatic Belt, Iran: Analysis of Origin and Fractionation Processes

Investigating crystal-rich clots hosted in phenocrysts and phenoclasts within Eocene subvolcanic rocks in Torud-Ahmad Abad, south-southeast of Shahrood (northern part of central Iran zone). These crystal-rich clots and clusters (alias stone inclusions, nanogranitoids, microcrystal clots) are interpreted as crystallized melt inclusions (cMIs) within phenocrysts and phenoclasts, highlighting plagioclase-hosted inclusions. Least-altered hypabyssal igneous rocks are trachy-andesitic, basaltic andesitic, and dacitic porphyries. These porphyries have porphyritic, glomeroporphyritic, granular, and trachytic textures with variable-sized phenocrysts of plagioclase (albite-labradorite), green hornblende (magnesio-hastingsite), and clinopyroxene (diopside-augite), with minor biotite, and Fe-Ti oxides; large plagioclase phenocrysts, exhibiting clear normal oscillatory zoning, were consistently utilized as plagioclase-hosted inclusions due to their abundance in rocks. MIs exhibit complete post-entrapment crystallization (PEC), generally with a slightly finer grain size than the igneous groundmass, i.e. no preserved glassy MI were observed in these phenocrysts, only cMIs. These variably sized, cryptocrystalline to microcrystalline clots in various phenocrysts seem to also represent primary igneous assemblages, manifested as clusters of microphenocrysts; these are referred to as crystal clots. SEM-EDS analyses determined the composition of crystal-rich clots in various plagioclase phenocrysts forming inclusions. The major-element composition of these crystal-rich clots in plagioclase are basalt, basaltic andesite, andesite, trachy-andesite, and trachyte that seem to be melt trapped during plagioclase phenocryst growth; these trapped interface melts then form microphenocryst assemblages that are preserved in phenocrysts, which are trapped when some process interferes with the growth of a phenocryst. These cMIs exhibit compositional variations from their bulk host rock, resulting from entrapment during magma mixing during plagioclase growth.