Geochemical, U–Pb zircon, and Nd isotope investigations of the Neoproterozoic Ghawjah Metavolcanic rocks, Northwestern Saudi Arabia

New geochemical, Nd-isotope and U–Pb zircon data from Neoproterozoic volcanic rocks from Wadi Sawawin in northwestern Saudi Arabia provide important constraints on the evolution of the crust in this part of the Arabian–Nubian Shield (ANS). The Ghawjah volcanic rocks range from tholeiitic to calc-alkaline and are metamorphosed to greenschist facies. U–Pb zircon analyses for Ghawjah andesite yield a weighted mean 206Pb/238U age of 763 ± 25 Ma, indicating that these are some of the oldest rocks of the Midyan terrane. Ghawjah volcanic rocks are mostly moderately fractionated, as indicated by Mg-numbers between 28 and 67, Cr between 5 to 537 ppm and Ni from 4 to 175 ppm, REE patterns are slightly fractionated [(La/Yb)N = 1.2 to 4.0], and multi-element diagrams show Ba, Sr, Rb and K enrichments and Nb and Ta depletions, typical of modern convergent-margin igneous rocks. Ghawjah volcanic rocks have positive εNd (+ 5.4 to + 8.2) and a mean model age of 0.71 Ga. Ghawjah volcanic rocks are similar to the “Younger Volcanic” rocks from the Central Eastern Desert (CED) of Egypt, in terms of stratigraphic relations, chemical compositions, Nd-isotopic compositions, and U–Pb zircon ages (~ 750 Ma), indicating that both were generated by partial melting of subduction-modified depleted mantle. The Ghawjah volcanic rocks are interpreted to have formed at ~ 750 Ma in an arc setting during an important episode of crust formation.

Research Highlights
► Geochemical data for Ghawjah volcanic rocks indicate these are tholeiitic and calc-alkaline and range from primitive basalt to dacite.
► U-Pb zircon geochronology of the Ghawjah andesite gives a weighted mean 206Pb/238U age of 763 ± 25 Ma.
► Geochemical data show that Ghawjah tholeiitic and calc-alkaline lavas have strong affinities to those of modern arcs.
► Nd isotopic data for the Ghawjah volcanic rocks show strongly positive initial εNd (+ 6.1 to + 8.2) and a mean Nd model age (0.71 Ga) that is similar to the U-Pb zircon crystallization age (763 Ma).
► Ghawjah volcanic rocks represent juvenile additions from asthenospheric mantle to the crust.