The layered rocks in the Arabian shield are classified into three major rock units, each of them belonging to one of the three tectonic cycles mentioned above. These major layered rock units are the lower, middle, and upper layered rock units.
The lower layered rock unit covers those rock groups that formed in the early upper Proterozoic tectonic cycle (older than 800 Ma) and includes rocks with continental affinity. The volcanic rocks that belong to this unit are characterized by tholei-itic basalt compositions and by the domination of basaltic rocks older than 800 Ma. The rock groups of this unit are located mostly in the southwestern and eastern parts of the shield.
The rock groups of the lower layered unit include rocks formed in an island arc environment and characterized by basic tholeiitic volcanic rocks (Baish and Bahah Groups) and calc-alkaline rocks (Jeddah Group). In some places these rocks overlie highly metamorphosed rocks of continental origin (Sabia Formation and Hali schists) considered to have been brought into the system either from a nearby craton such as the African craton, or from microplates rifted from the African plate such as the Afif microplate.
The middle layered rock unit includes the layered rock groups that formed during the middle upper Proterozoic tectonic cycle between 700 and 800 Ma ago. The volcanic rocks are predominately intermediate igneous rocks characterized by a calc-alkaline nature. These rocks are found in many parts of the shield with a greater concentration in the north and northwest, and scattered outcrops in the southern and central parts of the shield.
The upper-layered rock unit includes layered rock groups that formed in the late upper Proterozoic tectonic cycle in the period between 700 and 560 Ma ago and are predominately calc-alkaline, alkaline intermediate, and acidic rocks. These rock groups are found in the northeastern, central, and eastern parts of the shield.
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