NEOPROTEROZOIC OPHIOLITES AS DEVELOPED IN SAUDI ARABIA AND THEIR OCEANIC AND PERICONTINENTAL DOMAINS

The initial stages of formation of the Arabian shield are depicted in its ophiolites, which belong to two domains: an older (~ 800 Ma) oceanic domain of the western shield, and a younger (~ 700 Ma) pericontinental domain of the eastern and central shield. The oceanic domain ophiolites occur along the Yanbu and Bir Umq sutures, which juxtapose the ensimatic island arc terranes. The pericontinental domain ophiolites lie along the Ad-Dafinah, Halaban, and Al-Amar sutures. Ophiolites of the Ad-Dafinah suture were formed from volcanic islands on the western periphery of the Afif microcontinental terrane. Ophiolites of the Halaban and Al-Amar sutures developed above an Andean type subduction zone. Apart from these suture-associated ophiolites, certain ultramafic–mafic bodies (~ 627 Ma) occur along a later fault zone within the Asir terrane. Although displaced by major faults, ophiolites are the best available criteria for suture demarcation. The Arabian shield ophiolites are prominent among other contemporary ophiolites, which are less- abundant and lesser-known ophiolites. Certain primary features and chemical parameters are maintained even after their multistage histories of alteration, strain and metamorphism. The petrological attributes of these Neoproterozoic oceanic lithosphere sections compare well with Mesozoic ophiolites of the same (Arabian) plate. However, certain aspects are unique to the former, such as crust / mantle thickness ratios, the abundance of isotropic gabbros, and scarcity of layered gabbros. The scarce occurrences of segregated chromite are relatively more abundant in the oceanic domain ophiolites than in the pericontinental ophiolites. Pericontinental chromites with higher Cr / (Cr+Al) ratios, reflect higher degrees of partial melting of the mantle source, corroborating their island arc-related environments as postulated earlier, despite no reports on blueschists in the shield. However, a fore-arc regime for most ophiolites as stated previously, is not supported. The ophiolitic rocks show extensive alteration, including hydration of the ultramafic rocks, and rodingitization and chloritization of the basic rocks. Like arc-related rocks, the Ti-contents of oceanic ophiolites are low in most samples, but slightly higher in a few. The trace elements in volcanic rocks, as well as rare earth element patterns in different lithological components of some ophiolites indicate composite tectonic settings. Mantle heterogeneities may have caused the components with diverse geochemical signatures that are indicative of calc-alkaline rocks, mid-ocean ridge basalts and the enriched rocks of the oceanic islands or seamounts. A seawater effect is noticeable as a negative Ce anomaly in many bulk-rock REE patterns. Adakite in the Al’Ays ophiolite supports its formation in a small ocean basin.