The Taif area occupies 170 km2 at the western margin of the Saudi Arabian Hijaz plateau, where granitic rocks, which form about 55% of the total outcrop, have been intruded into amphibolites and quartzo-feldspathic gneisses. The host rocks have been metamorphosed to the almandine-amphibolite facies and have petrographic and geochemical features that suggest that they were originally igneous rocks of calc-alkalic affinity. The granitic intrusions have been divided into four groups (G1, G2, G3, and G4) on the basis of field relations, chemistry, and textures; G1 through G3 are synkinematic, and G4 is postkinematic. The granitic rocks are calc-alkalic. Isotopic data suggest that they were emplaced between 595 and 525 m.y. ago; initial 87Sr/86Sr ratios (0.703 to 0.710) indicate an origin by partial fusion of the mantle or anatexis of the lower crust. Field, geochemical, and petrographic studies suggest emplacement in an island-arc environment. This is compatible with other investigations that have led to the conclusion that the crystalline basement of Arabia and northeast Africa is the product of cratonization of island arcs over a period of 600 m.y. (from 1,100 to 500 m.y. ago). The Taif granitic rocks represent final phases in the episodic cratonization process.
- Geological Society of America