DEFINITION OF INTRUSIVE ROCKS

The intrusive rocks that cut the Arabian shield are divided into

three main groups. These groups are called (from the older to

the younger) pre-orogenic, syn-orogenic, and post-orogenic.

The pre-orogenic intrusions are those intrusions that cut

through the lower layered rocks unit only and not the other

layered rock units. It is considered older than the middle layered

rock unit but younger than the lower layered rock unit.

These intrusions are characterized by their calcic to calcalkaline

composition. They are dominated by gabbro, diorite,

quartz-diorite, trondhjemite, and tonalite. These intrusions

are found in the southern, southeastern, and western parts of

the shield and coincide with the areas of the lower layered

rocks unit. These intrusions are assigned ages between 1,000

and 700 million years old. Geochemical signatures including

strontium isotope ratios show that these intrusions were

derived from magma that came from the upper mantle.

The syn-orogenic intrusions are those intrusions that cut

the lower and the layered rock units as well as the pre-orogenic

intrusions, but that do not cut or intrude the upper layered

rocks unit. These intrusions are considered older than

the upper layered rocks unit and younger than the pre-orogenic

intrusions, and they are assigned ages between 700 and

620 million years old. Their chemical composition is closer to

the granitic calc-alkaline to alkaline field than the preorogenic

intrusions. These intrusions include granodiorite,

adamalite, monzonite, granite, and alkali granite with lesser

amount of gabbro and diorite in comparison with the preorogenic

intrusions. The general form of these intrusions is

batholithic bodies that cover wide areas. They are found

mostly in the eastern, northern, and northeastern parts of the

Arabian shield. The initial strontium ratio of these intrusions

is higher than that of the pre-orogenic intrusions and indicates

that these intrusions were derived from a magma that

was generated in the lower crust.

Post-orogenic intrusions are intrusions that cut through

the three upper Proterozoic layered rocks units as well as the

pre- and syn-orogenic intrusions. These are assigned ages

between 620 and 550 million years old. They form circular,

elliptical, and ring-like bodies that range in chemical composition

from alkaline to peralkaline. These intrusions are made

mainly of alkaline and peralkaline granites such as riebeckite

granite, alkaline syenite, pink granite, biotite granite, monzogranite,

and perthite-biotite granite.

Ringlike bodies and masses of gabbro are also common,

and the post-orogenic magmatic suite is bimodal in silica content.

These intrusions are found scattered in the Arabian

shield, but they are more concentrated in the eastern, northern,

and central parts of the shield.

The initial strontium ratio of the post-orogenic intrusions

ranges between 0.704 and 0.7211, indicating that these

intrusions were derived from a magma that was generated in

the lower crust.

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