U-Pb isotopic evidence for accretion of a continental microplate in the Zalm region of the Saudi Arabian Shield

Abstract

The region covered by this work includes three of the main tectonic units of the Arabian Shield: the Afif continental terrane, the Nabitah suture with its associated mobile belt, and the Asir ensimatic arc terrane. The geology of the area is well understood, and this geochronologic and isotopic study confirms that the Afif terrane was a continental microplate in the late Proterozoic. The study also provides a time frame for the crustal evolution of this part of the Shield. U-Pb zircon age data from a pelitic garnet-sillimanite gneiss of the Kabid formation shows that this part of the continental basement in the Afif terrane may be as old as 1830 Ma. Isotope data indicate that lead from the Kabid gneiss resided in the upper continental crust for a long period before 1830 Ma, and require that Archean rocks exist at depth beneath the Afif terrane. Lead isotope data reveal a change in the nature of the underlying crust, from continental basement in the northeast, to less radiogenic, marginal arc rocks in the southwest. This change is coincident with both aeromagnetic data, and a fades change within a pre-collision marginal basin. Miogeosynclinal continental shelf fades of the Siham group lie unconcormably over the Kabid formation, and are in the area of continental lead signatures. Eugeosynclinal deep water sediments and volcanics, in association with ultramafic rocks, occur in the area of marginal arc signatures. U-Pb zircon age determinations show that this "Andean" continental margin developed before about 720 Ma, and emplacement of calc-alkaline plutonic rocks continued until about 690 Ma. During the period 690-640 Ma, the continental Afif microplate collided with the Asir terrane as part of the Nabitah orogeny. At approximately 640 Ma ago, the Najd strike-slip orogen commenced with a dextral phase that controlled the emplacement of granitic plutons as well as the development of a series of large pull-apart grabens. Some of these grabens were floored by new oceanic crust and were filled with volcano-sedimentary rocks of the Bani Ghayy group. Subsequently, the Najd fault system changed to sinistral strike slip motion at about 620 Ma ago.