A structural, geochronological & geochemical investigation of the Abt Schist; Ad Dawadimi Terrane; central Saudi Arabia

Abstract

The Ad Dawadimi Terrane lies on the eastern margin of the Arabian-Nubian Shield between the ~800-600 Ma Ar Rayn Terrane to the east and the Palaeo-Neoproterozoic (1800-630 Ma) Afif Terrane to the west. The Ad Dawadimi Terrane is considered to be one of the most homogeneous terranes of the Arabian Shield and is dominated by the Abt Schist, a greenschist facies metamorphic unit of fine-grained sandstones and siltstones. Structural data shows variations in the foliations between the Ad Dawadimi Terrane’s eastern and western margins. The east is dominated by westsouth-west verging sub-horizontal, moderately inclined, open, asymmetrical folds with a subhorizontal to gentle south-south-east plunge. The western margin, although less well constrained, is dominated by folds with a west-north-west vergence, sub-horizontal, moderately – gently inclined, open, asymmetrical geometry; with a north-east south-west extensional component. 206Pb/238U data for Abt Schist samples within the Ad Dawadimi Terrane give a maximum depositional age of 594 ±14 Ma (2σ error at 95-105% concordance). Two 40Ar/39Ar metamorphic muscovite step-heating analyses yield peak metamorphic of ages ~616 ± 3 Ma and ~613 ± 6 Ma thus being within error of the youngest 206Pb/238U detrital zircon age. A number of zircon grains yield 206Pb/238U ages >1 Ga suggest a Mesoproterozoic, or older, influence on the source for the Abt sediments. This is supported by TIMS TDM model ages ~1154 ± 19 Ma suggesting Abt Schist protolith sediments were influenced by late Mesoproterozoic sources. Sedimentary geochemical analyses of immobile elements suggest sediments derived from the Afif Terrane were deposited into a fore-arc setting above a west dipping subducting slab. The very short time period between deposition and basin closure also suggests a fore-arc setting. Post deformation mafic dykes cross-cut the dominant foliation of the Ad Dawadimi Terrane, suggesting a period of extension after basin closure.