THE ARABIAN-NUBIAN SHIELD FROM ISLAND ARC TO CRATON: TIMESCALES OF JUVENILE CRUST FORMATION AND THE EXTENT OF PRE-NEOPROTEROZOIC CRUST

The Arabian-Nubian Shield (ANS) is ideal for investigating juvenile crust formation by plate tectonics in Neoproterozoic time. We studied Neoproterozoic arc terranes that flank the Bi'r Umq suture zone (BUSZ) in Saudi Arabia, a well-preserved suture spanning >600 km in NE Africa and Arabia. The terranes consist of nested arc plutons and coeval volcano-sedimentary rocks, new U-Pb zircon ages of which reveal a complex and protracted (~820-520 Ma) history. They define 4 magmatic episodes (M1-4) recording the transition of the ANS from discrete arcs to composite terranes to craton. M1 (825-800 Ma) and M2 (785-745 Ma) represent the main subduction-related crust-forming events and were pre- and syntectonic regarding terrane suturing along the BUSZ. Syntectonic plutons record suture-related deformation at 780 and 750 Ma. M1-M2 units are juvenile, as shown by high initial εNd values (+5.8 to +8.5) and similar igneous and Nd model ages. M3 (700-633 Ma) and M4 (600-565 Ma) reflect minor post-tectonic magmatism, possibly due to anatexis during collision along the East African Orogen. M3-M4 units show lower initial εNd values (+7 to +4) and more disparate igneous and Nd model ages.

Although the SE Arabian Shield was influenced, or is underlain, by Mesoproterozoic-Archean crust, the core of the shield where our samples originate is considered juvenile, with no crust older than 870 Ma. However, SHRIMP-RG analyses reveal abundant (56 of 403) Early Neoproterozoic to Archean Pb-Pb zircon ages, one of which (2840 Ma) is the oldest yet reported from the Arabian Shield. Our results greatly increase the known extent of the “contaminated” shield, but the source of the inherited zircons is unclear. Most occur within M2 volcanic rocks and M4 post-tectonic granites, the lower initial εNd values of which support some involvement of pre-existing crust in their formation. Some zircon morphologies suggest a detrital origin, whereas others are more like juvenile grains extracted in situ from parental igneous rocks. Inheritance also affects BUSZ ophiolitic rocks, which may best be explained by rifting of older continental crust. The above evidence suggests that previously unrecognized pre-Neoproterozoic crust played a cryptic role in the evolution of the juvenile core of the ANS.