Earth Science Research; Vol. 13, No. 1; 2024 ISSN 1927-0542 E-ISSN 1927-0550 Published by Canadian Center of Science and Education 13 Plagioclase-hosted Crystallized Melt Inclusions within Hypabyssal Volcanic Rocks of the Torud-Ahmad Abad Magmatic Belt, Iran: Analysis of Origin and Fractionation Processes Fazilat Yousefi 1 , David R. Lentz 1 & Lambrini Papadopoulou 2 1 Department of Earth Science, University of New Brunswick, Fredericton, NB E3B5A3, Canada 2 Department of Mineralogy-Petrology-Economic Geology, Aristotle University of Thessaloniki, Thessaloniki 998560, Greece Correspondence: Fazilat Yousefi, Department of Earth Science, University of New Brunswick, Fredericton, NB E3B5A3, Canada. E-mail: Fazilat.yousefi@unb.ca Received: October 4, 2024 Accepted: November 13, 2024 Online Published: November 14, 2024 doi:10.5539/esr.v13n1p13 URL: https://doi.org/10.5539/esr.v13n1p13 Abstract 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. Keywords: melt inclusions, post-entrapment crystallization, subvolcanic rocks, Torud-Ahmad Abad Magmatic belt, Iran 1. Introduction The composition of these Eocene adakitic subvolcanic igneous rocks in the Torud-Ahmad Abad Magmatic Belt (Iran) are predominantly andesite, trachy-andesite, with minor dacite and basaltic andesite (Yousefi et al., 2021, 2020, 2019, 2017a, b). Melt inclusion trapping depth varies as it happens during magma ascent with increased plagioclase crystallization associated with depressurization and cooling. During ascent, plagioclase phenocrysts grow more rapidly, which enhances melt entrapment as droplets within these growing crystals. In the Torud-Ahmad Abad magmatic rocks, cognate xenoliths, enclaves, micro-enclaves, and mafic clots-clusters (glomerocrysts) are common (Yousefi et al., 2017b). The ferromagnesian mineral clusters are fragments of crystallized magma that formed during earlier stages of crystallization and (or) cogenetic mafic magmas, and which were subsequently incorporated and transported within the younger magmatic pulses. Magma mixing as noted above, with assimilation, and fractional crystallization processes (AFC) partially controlled the evolution of the magmas in the Torud-Ahmad Abad Magmatic Belt (Yousefi et al., 2017a, b); the primary end-member mantle- derived magma is mafic in comparison to these mafic-intermediate to felsic hypabyssal intrusions. The existence