Temporal evolution of magmatic processes in a subvolcanic plumbing system in a continental region: constraints from geochemistry of the late Neoproterozoic Wadi Dib alkaline ring complex in the Eastern Desert, Egypt

We examined trace element compositions and Sr, Nd, Pb isotope ratios of the Wadi Dib ring complex (WDRC), one of the alkaline ring complexes in the Eastern Desert of Egypt, to better understand temporal evolution of an alkaline magma in a small crustal magma body. The ring complex consists of plutonic, volcanic, and dike units and represents a continental subvolcanic plumbing system developed above a magma body. The whole-rock Rb-Sr isochron age was newly determined as 586.8 ± 10.1 Ma with initial 87Sr/86Sr = 0.70333 ± 0.00011 by critically evaluating isotopic heterogeneity at the time of solidification and excluding samples unsuitable for the age estimation, which have information on open magmatic processes involving isotopically distinct exotic materials. The consistent Rb-Sr isochron age and the systematic major and trace element variations show that trachybasalts of the dike unit represent a parental magma of the WDRC. We developed a new approach to quantify open magmatic processes governing temporal evolution of the magma body by combining geochemical data of multiple samples to extract model parameters. We adopted an assimilation and fractional crystallization (AFC) model to rocks of the plutonic units with SiO2 > 63 wt%, whose radiogenic isotopic ratios requires involvement of exotic materials and successfully estimated the AFC parameters including major and trace element compositions and Nd isotope ratios of the exotic melts, which are geochemically similar to the country rocks. We adopted a boundary layer fractionation (BLF) model to rocks of the volcanic and plutonic units with SiO2 < ∼ 63 wt% without evidence for assimilation and successfully estimated the BLF parameters including major and trace element compositions of a ‘precursor magma’ initially emplaced into the magma body. Diverse rock types of the WDRC were derived from the precursor magma by open-system fractional crystallization with or without assimilation. The temporal evolution of the WDRC constrained by the modeling results features: (1) BLF operated in the early stage of the complex evolution was driven by formation of a fractionated melt in the sidewall boundary layer, which was transported to the roof zone to form magmas erupted as the volcanic unit by mixing with the precursor magma and (2) AFC became extensive in the later stage, during which the exotic melt was derived from the bottom boundary layer by melting of the country rocks sunk from the carapace of the magma body by stoping in the earlier stage. The ratio of extent of assimilation and crystallization (Ma/Mc) increases from ~ 0 to 1-2 with time and the overall Ma/Mc integrated over the entire evolution of the WDRC is estimated to be ∼ 0.2, which indicates a critical role of heat generated in the earlier BLF stage to induce the later extensive assimilation.