Five new Rb-Sr whole-rock isochrons for volcanic successions assigned to 'sequence' A in the Central Arabian Shield indicate that very late Precambrian c.690–570 Ma volcanicity was an important event in the evolution of the shield. The following ages are considered to define or approximate to the age of lava extrusion: Fatimah group— 688 ± 30 Ma; Jahhad unit—616 ± 13 Ma; Juqjuq formation— 612 ± 22 Ma; Arfan formation— 608 ± 9 Ma; Hummah unit— 572 ± 23 Ma. Initial 87Sr/86Sr ratios range from 0.7027 to 0.7036, and Rb/Sr ratios are uniformly low (<0.4), except for a few metarhyolites. All the lavas appear to have been derived from a primitive mantle-type source, although a slight increase of initial 87Srl86Sr ratio for the Jahhad and Hummah samples reflects the involvement of less-depleted mantle or possibly slight crustal contamination.

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