A zircon study has been made on 11 samples of igneous rocks from the Saudi Arabian craton. Ages of sized and magnetic fractions of zircon concentrates show variable degrees of discordance, which seem to result from a very young disturbance that produces linear arrays in the Concordia plot. Model age calculations based on a statistically and geologically reasonable lower intercept produce very consistent internal relationships. The Pan African Orogeny, considered to be responsible for loss of radiogenic argon and strontium from minerals of many rocks, does not appear to have affected the zircon data, even though uplift had exposed the rocks of the Arabian Shield at that time.
Tonalite, granodiorite, and crosscutting leuco-adamellite bodies in the southern part of the An Nimas batholith yield ages in the time range 820-760 m.y. A narrow time range of 660 to 665 million years was indicated for ages of widely separated and compositionally different intrusive bodies all to the east of the An Nimas batholith. This work suggests that the younger end of the age spectrum established from regional K-Ar and Rb-Sr measurements may be underestimated, and that magmatic activity could have been more episodic than previously assumed.
