Magmatic–Hydrothermal Transition of Granitic System: Evidence from Mineral Reaction Overprints in the Homrit Waggat Granite Intrusion (Central Eastern Desert, Egypt)

Abstract

The Precambrian Homrit–Waggat granite is a post-orogenic batholithic intrusion located in the northern region of the Nubian Shield, characterized by a typical annular morphology and significant secondary alteration. This study aims to elucidate the processes that have shaped the intrusion in both macroscopic and microscopic perspectives, employing a combination of field observation and petrographic analysis alongside major and trace element compositions of minerals. Within the central region of the pluton, biotite and amphibole are observed sporadically, while the predominant crystallization of anhydrous oligoclase in the outer regions has led to a progressive increase in volatile components within the residual melt, ultimately resulting in a volatile-saturated aluminosilicate melt. The exsolved fluids subsequently interacted with the previously crystallized mineral assemblage, producing metasomatic overprinting. As the cooling and crystallization continued, the water pressure within the magma chamber gradually escalated until it equaled or surpassed the confining pressure, leading to the formation of fractures and veins filled with minerals that crystallized from the residual aqueous fluids. The ongoing degassing and expulsion of aqueous fluids from the magma chamber's interior ultimately contributed to the collapse of the chamber's roof, resulting in the annular ring-dike morphology observed in the Homrit Waggat pluton.