The Neoproterozoic evolution of the central-eastern Bayuda Desert (Sudan)

Abstract

In the central-eastern Bayuda Desert (Sudan), a border area between the Saharan Metacraton and juvenile Pan-African crust of the Nubian Shield, the Rahaba-Absol and the Abu Harik-Kurmut Terranes were amalgamated along suture and shear zones. U–Pb zircon ages by laser-ablation dating of metamorphosed granitoid rocks reveals ages interpreted as magmatic and metamorphic activities throughout the Neoproterozoic. The meta-granitoids are calc-alkaline porphyroblastic varieties of biotite metagranite (969±5Ma), quartzfeldspathic metagranite (914±6Ma), biotite–muscovite metagranite (912±4Ma) and coarse-grained meta-monzodiorite (909±9Ma, 818±19Ma, 669±13Ma) associated with the Rahaba-Absol Terrane, and medium-grained metagranites (813±4Ma, 808±5Ma, 799±16Ma), a meta-quartz–monzonite (810±10Ma), biotite metagranites (794±15Ma, 783±13Ma, 700±7Ma), alkali metagranite (645±5Ma), and porphyritic meta-quartz-monzonite (630±4Ma) associated with the Abu Harik-Kurmut Terranes. Th/U ratios of the zircon were used to distinguish between intrusion and metamorphic ages.The Neoproterozoic ages are interpreted as geodynamic pulses: a pre-Pan-African event 1000–900Ma characterized by metamorphism and magmatism ending with the known Bayudian Event (920–900Ma) in the Rahaba-Absol Terrane; a cycle between 850 and 775Ma of subduction-related magmatism and metamorphism with a peak around 825–800Ma; the final collision of the Rahaba-Absol and Abu Harik-Kurmut Terranes began before 714Ma and ended probably before 645–630Ma, documented by discordant E-W trending alkali metagranites and NE trending porphyritic meta-quartz–monzonite respectively. Theses ages of discordant intrusions constrain the post-collisional horizontal movement along the southern Keraf Shear Zone between 630 and ∼590Ma. The existence of inherited zircon grains in some samples of Pan-African age indicated sedimentary activity contemporaneous with the island-arc magmatism and metamorphism. This was probably a period of extensive erosion of the earlier island arc material and later stacked crustal material after terminal collisional between the Abu Harik-Kurmut arc and the Rahaba-Absol continental terrane. Pb isotope data (feldspar, whole rock) and Sr isotopes corroborate the division into two terranes, the Rahaba-Absol Terrane as an old cratonic part of the Saharan Metacraton, and the Abu Harik-Kurmut Terrane as juvenile island-arc additions during the Pan-African.