Vorticity Analysis and Deformation History of the Mizil Gneiss Dome, Eastern Arabian Shield, Saudi Arabia

Abstract

The Mizil gneiss dome is an elliptical structure consisting of an amphibolite-facies volcanosedimentary mantle and a gneissic granite core. This dome is located at the northern tip of the Ar Rayn terrane only a few kilometers from the eastern edge of the Arabian shield. Previous investigations have shown the intrusive core to be an adakitic diapir with a U–Pb zircon age of 689 ± 10 Ma; this age is 50–80 Ma years older than other granites in this terrane. Vorticity analysis was carried out on samples from the intrusive core and volcanosedimentary cover; the Passchier and Rigid Grain Net (RGN) methods were used to obtain the kinematic vorticity number (Wk) and the mean kinematic vorticity number (Wm). The Wk and Wm values show a marked increase towards the south; such a pattern indicates a N-S movement of the core pluton thus creating an inclined diapir tilted to the south. Analogue experiments simulating the flow of magma diapirs rising form a subducted slab through the mantle wedge have shown that supra-subduction zone oblique diapirs are produced close to the trench and are elongated normal to the convergence direction as is the case in the Mizil pluton. This effect was found to increase with increasing slab dip due to enhanced drag along the upper surface of the subducted lithospheric plate. Spontaneous subduction which is often associated with rollback resulting in back-arc extension and steep dipping slabs is thought to have occurred in the Mozambique Ocean by 700 Ma. The Mizil pluton is coeval with the back-arc Urd ophiolite from the adjacent Dawadimi terrane, and could therefore have been produced by incipient subduction of a relatively cold slab as observed in many Pacific margin adakites. The tectonic evolution of the eastern shield, as deduced from the Mizil dome and other data from Ar Rayn and neighboring terranes, begins with the subduction of >100 My-old lithosphere beneath the Afif terrane resulting in back-arc spreading and the splitting of the Ar Rayn arc from the Afif microplate, with the concomitant production of a small volume of adakite melt. Other arc terrane(s) docked east of Ar Rayn with the westward-directed subduction still going but a lower angles and greater depth due to trench jump; this phase produced the more prevalent non-adakitic group-1 granites. A major collisional orogeny affected the entire eastern shield between 620–600 Ma and sutured the eastern shield terranes with northern Gondwana.