Late Cryogenian–Ediacaran history of the Arabian–Nubian Shield: A review of depositional, plutonic, structural, and tectonic events in the closing stages of the northern East African Orogen

Abstract

During the late Cryogenian–Ediacaran (650–542 Ma), the Arabian–Nubian Shield (ANS) underwent final assembly and accretion to the Saharan Metacraton concurrent with the assembly of eastern and western Gondwana. At the end of the Precambrian it lay at one end of the East African Orogen, with its northern margin (present coordinates) forming a low-relief stable shelf facing an open ocean; to the south the ANS transitioned into the Mozambique Belt. The geologic history of the ANS during this period provides insight into the closing developmental stages of one of the world’s largest accretionary orogens. Following a 680–640 Ma orogenic event reflecting amalgamation of a core grouping of island-arc terranes (the proto-Arabian–Nubian Shield; pANS), the region underwent extensive exhumation, erosion, and subsidence. Depositional basins formed in the northern and eastern pANS, with those in the east below sea level and connected to an ocean. Periodic basin closure and formation of new basins in other parts of the ANS followed. Many basins were filled by terrestrial, molasse-type sediments interfingering with subordinate to predominant amounts of volcanic rocks. Magmatism was extensive throughout the period, initially characterized by tonalite–trondhjemite–granodiorite (TTG) and granite (monzogranite, syenogranite), but also characterized, from ∼610 Ma on, by increasing amounts of alkali-feldspar granite and alkali granite. The plutons are largely undeformed, except where cut by brittle–ductile shear zones. The magma sources of the late Cryogenian–Ediacaran granitoids were dominated by juvenile crust and(or) depleted mantle and magmas mostly originated in anorogenic, post-collisional, commonly extensional, settings. They were derived by melting and fractionation of anhydrous high-grade metamorphosed lower crust, mafic- to intermediate calc-alkaline crust, and(or) subduction-modified mantle wedges associated with slab break-off or delamination. By ∼630 Ma, the region was affected by oblique (transpressional) convergence of continental blocks that formed eastern and western Gondwana—the pANS was approaching the Saharan Metacraton; north-trending shear and shortening zones developed in the southern ANS; and northwest-trending strike-slip shear zones of the Najd fault system dominated farther north. In the northwestern ANS, convergence and Najd transpression buckled the crust causing structural highs with domes of gneissic infracrust overlain by supracrust composed of ophiolitic and volcanosedimentary assemblages dating from the Tonian–middle Cryogenian period of island-arc activity. The supracrust was extensively translated to the northwest above a high-strain zone. Extension and tectonic escape augmented exhumation of the gneissic infracrust particularly between ∼620–580 Ma. In the northeastern ANS, linear belts of gneiss formed from reworked older intrusive bodies or syntectonic intrusions that were emplaced along Najd faults. By ∼620 Ma a marine basin on the eastern margin of the pANS (present coordinates) was beginning to close. A thick sedimentary assemblage (Abt formation) in this basin underwent metamorphism and folding, and subduction-related magmatism and volcanism farther into this basin (Al Amar arc; >690–615 Ma) was coming to an end. Amalgamation of the Abt formation, Al Amar arc, and the pANS occurred between ∼620 and ∼605 Ma, and terminal collision between the pANS and the Saharan Metacraton was complete by ∼580 Ma. At this time, the ANS was fully assembled. Granite magmatism continued until ∼565–560 Ma and orogeny ceased by ∼550 Ma. During these terminal events, the region underwent strong chemical weathering and became a vast low-relief surface on which Lower Paleozoic sandstone was eventually deposited.