Magma fractionation and emplacement mechanism in a subvolcanic plumbing system in a continental region: constraints from the late Neoproterozoic Wadi Dib ring complex in the Eastern Desert, Egypt

抄録

We examined an alkaline ring complex in the Eastern Desert of Egypt, Wadi Dib ring complex (WDRC), to understand formation mechanisms of the intimately related ring structure and chemical diversity. The WDRC consists of multiple circular rings of the oldest volcanic units, the middle-stage plutonic unit, and the youngest dike unit, and these units show overlapping whole-rock major element compositions. The compositional variation of the volcanic and plutonic units can be accounted for by a stepwise fractional crystallization starting with a trachytic magma without significant magma replenishment or crustal contamination. From the margin to the center and oldest to youngest, the plutonic unit consists of an outer ring (syenite), inner rings 1 (quartz-bearing syenite) and 2 (quartz syenite), and a granitic core (syenogranite). The whole-rock chemical composition of the plutonic unit is progressively more fractionated inwards from the outer ring to the granitic core through the inner rings. Syenites from the innermost outer ring show high degree of deformation, which gradually decreases outwards in the outer ring and abruptly decreases inwards in the outermost inner ring 1. The deformed syenites show microstructures suggesting reactive melt transportation. The country rocks neighboring the ring complex and equivalent blocks present in the periphery of the outer ring, the overlying volcanic unit and equivalent blocks present in the inner rings all show microstructures indicative of pyrometamorphism. Spatial variations in the microstructures in the plutonic unit indicate an increase in cooling rate from the outer ring to the granitic core and thus with time. These geological, chemical, and microstructural features of the WDRC suggest that the ring complex represents an evolving roof zone of a subvolcanic magma body located at a depth of a few km. Intimate coupling of growing roof and sidewall mushy boundary layers and later collapse of the roof boundary layer with occasional involvement of the overlying volcanic piles induced segregation of interstitial fractionated melt from the wall boundary layer to the collapse space driven by a pressure gradient induced by the collapse. The ascended fractionated melt started to crystallize to form a roof boundary layer of the next generation by increasing cooling efficiency, which thickened until the next collapse on a smaller horizontal scale inducing melt segregation from the wall boundary layer towards the roof zone. Repetition of the sequence with shallowing and decreasing the horizontal scale produced the ring structure and chemical diversity of the WDRC.