DEFINITION OF CLASSIFICATION OF ROCK UNITS

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 million years ago) and includes rocks with

continental affinity. The volcanic rocks that belong to this

unit are characterized by basaltic tholeiite compositions and

by the domination of basaltic rocks that are older than 800

million years. 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

that were formed in an island arc environment and that are

characterized by basic tholeiitic volcanic rocks (Baish and

Bahah Groups) and calc-alkaline rocks (Jeddah Group).

These rocks overlie in some places highly metamorphosed

rocks of continental origin (Sabia Formation and Hali

schists) that are considered to have been brought into the system

either from a nearby craton such as the African craton,

or from microplates that were 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 in the period between 800 and 700 million years

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 million years 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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