Deciphering the multistage magmatic processes in the Nuqara caldera, the northern Egyptian Nubian-Shield: Insights from zircon morphology and geochemistry

Trace elements in igneous zircon crystals exhibit variability within single crystals or among populations of crystals, demonstrating heightened sensitivity to changes in melt composition. The three distinct types of zircons in the Nuqara caldera complex (659 ± 16 Ma andesites, 602.3 ± 4.4 Ma rhyolites, and 589.4 ± 6.1 Ma rhyolite porphyry; A, B, and C, respectively) signify a collective geological history influencing the multistage magmatic evolution. Significantly, the studied zircons demonstrate growth rate and variable length-to-width ratios that progressively increase from A to C. Ti-in-zircon geothermometer (T_Ti-in-zrc = 924°C) along with the internal structure and geochemistry of type A zircons, such as very weak cathodoluminescence (CL) brightness, zoning, and higher concentrations of some trace elements content, suggest their formation during the early, hotter, and less-evolved melt stage of volcanic activity. Type B zircons exhibit T_Ti-in-zrc (833°C) and commonly display resorption with an absence of singular dark CL, indicating substantial reheating of the magma reservoir. The interaction between the incoming evolved magma and the resident magma results in the formation of zircon rims during the magma cooling, featuring significant overlaps in zircon trace elements. This final phase in the Nuqara caldera complex marks the complete hybridization of the initially distinct magmas, culminating in a gradual cooling process. The newly formed zircons (type C) are characterized by light CL features with weak zoning occurring either as the rim of the oscillatory zoned zircon or as an individual zircon grain. Their less evolved chemical signature and T_Ti-in-zrc (708°C) highlight the significance of this final stage in shaping the overall geochemical and thermal evolution. The obtained zircon data, spanning from the initial crystallization to the subsequent recharge, mixing, and hybridization stages, delineate the discernible phases in the formation of the Nuqara caldera, providing insights into the transitions from subduction to collision-related geological processes.