The Katherina Ring Complex (Sinai Peninsula, Egypt): Sequence of emplacement and petrogenesis

Abstract

The Katherina Ring Complex (KRC) in the central Sinai Peninsula, Egypt, was formed in three consecutive stages: volcanic, subvolcanic, and plutonic. The outer Katherina ring dike, about 30 km in diameter, marks the contour of a paleocaldera. Volcanic ignimbrite extrusions representing the earliest stage of the KRC were followed by emplacement of subvolcanic peralkaline microgranite bodies. The ring dikes are composed mainly of porphyritic quartz monzonite and plagioclase-rich quartz syenite, with less abundant alkali feldspar quartz syenite and peralkaline granite. The central alkaline granite pluton (ca. 210 km²) was emplaced at ∼595 Ma. The quartz monzonite–syenite group is characterized by positive Eu anomalies (Eu/Eu* = 1.1-1.6), which is consistent with its enrichment in accumulated plagioclase crystals (xenocrysts). These features, along with the positive ε_Nd(T) values in quartz monzonite (up to +5.6) suggest that the initial silicic magma was hybridized by plagiclase-rich mafic magma. Mineral geothermometry and melt inclusion studies point to the formation of the silicic magmas at high temperatures, up to 900 to 1000 °C. Oxygen and Sr-Nd isotope data suggest that the source of the magmas was moderately depleted mantle or young juvenile crust. The trend of compositional change from quartz alkali feldspar syenite to alkali feldspar and peralkaline granite is consistent with a fractional crystallization model. A specific feature of the magma differentiation process is that the residual melt separation could occur when the magma was ∼55 percent crystallized (“rigid percolation threshold”) and clusters of crystals formed a rigid skeleton in the magma. At this stage, residual melt flowed pervasively through the pore space. The suggested model of melt separation alleviates the problem of a differentiation process since it does not require crystal settling that seems unrealistic in highly viscous silicic magmas at shallow depth. Chemical and Nd isotopic distinctions between the leucocratic volcanic–subvolcanic rocks (ε_Nd(T) = 4.2-4.6) and the Katherina pluton granite (2.6-3.9) suggest that the silicic magmas of the volcanic-subvolcanic and the plutonic stages were probably produced from different mantle-derived sources.