Island arc character and late Precambrian age of volcanics at Wadi Shwas, Hijaz, Saudi Arabia: geochemical and Sr and Nd isotopic evidence

A summary description of two volcanic sequences named the Surgah and the Shwas Formation, situated in the Wadi Shwas area at long. 42°0′, lat. 20°0′ in the Arabian Shield, is given. Major and trace element concentrations including REE abundances are presented, as well as Sr and some Nd isotope results. One sequence (the Surgah Formation) shows a tholeiitic differentiation trend from basaltic to rhyolitic rocks, whereas the Shwas Formation has calc-alkaline affinities. Both sequences have low concentrations of incompatible elements, notably Rb, Nb, Zr and Ti. The coexistence of a tholeiitic and a calc-alkaline suite, along with these low concentrations, is strongly reminiscent of some present-day island arc environments. An Rb-Sr errorchron age of 721 ± 55 Ma was obtained on the Surgah Formation, with high initial ⁸⁷Sr/⁸⁶Sr ratios around 0.705. The rare earth abundances are overall very low and the patterns show a relative depletion of the light REE in the least evolved rocks, indicating a relatively light-REE-depleted mantle source. A time-integrated light-REE-depleted source is also indicated by the fairly radiogenic ¹⁴³Nd/¹⁴⁴Nd ratios (ε{lunate}_Nd values around 8). The Nd "model age" of source depletion (T_CHUR^Nd) is around 2.2 Ga.