Abstract

Two northeast-trending late Precambrian ensimatic island-arc systems in western Saudi Arabia are described. Each displays lithotectonic provinces that conform to those found in modern examples of plate convergence. The younger Hijaz arc system (800–700 Ma) contains a thick fore-arc sequence of Al Ays group turbidites overlying an accretionary prism of metamorphosed and fragmented volcanosedimentary rocks and associated ophiolites of the Farri group and the Al Hinu formation. The older Asir arc system (> 900–700 Ma) is exposed at a much deeper erosional level, in which only remnant arc and fore-arc deposits are present. It is identified in part by a frontal-arc assemblage of abundant dioritic intrusions and high-grade metamorphic rocks that are interpreted as a high-T-low-P belt. A mafic volcanic assemblage in the back-arc area appears to delineate an interarc basin situated between the At Ta'if frontal arc and a lithologically similar remnant arc west of Bishah.

About 700 Ma ago, the Hijaz island arc collided with the Asir arc system along a southeast-dipping subduction zone now recognized as the Bi'r Umq–Port Sudan suture. Epeirogeny during this period led to the creation of an intramassif basin in the Hijaz arc system and an accretionary basin in the Asir arc system. These were filled by volcanic and shallow-water to continental molasse deposits of the Hadiyah and Fatimah groups, respectively. Continued convergence between 700 and 675 Ma ago resulted in development of the northeast-trending Samran fold belt. The collision event marks the cessation of an episode of southeast convergence that is interpreted as the initial phase in the progressive eastward evolution of the Arabian-Nubian Shield.

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