LATE PRECAMBRIAN CRUSTAL EVOLUTION IN NE SUDAN - ISOTOPIC AND GEOCHRONOLOGICAL CONSTRAINTS

The continental crust of NE Africa and Arabia was formed or modified during the late Precambrian (ca. 900-550 Ma) Pan-African orogenic cycle. The relationship between juvenile crust of the Arabian-Nubian shield (formed by accretion of ensimatic terranes) and older rocks of the Nile craton to the west (thermally reworked at the same time), is poorly understood. A regional geochronologic and isotopic study, including U-Pb zircon and Rb-Sr ages and Sm-Nd, Rb-Sr, and feldspar Pb isotopic studies indicates that the crust of NE Sudan formed between about 810 and 580 Ma, with an apparent peak in the volume of crust generated at about 700 Ma. Isotopic data indicate that the sources for these melts were juvenile, with initial Sr-87/Sr-86 = 0.7019 to 0.7030, epsilon(Nd(T)) = 5.1-7.7, and with Pb isotopic characteristics indicating unresolvable contributions from older crust. In most cases, neodymium model ages (T(DM)) are very similar to the crystallization ages, and both initial Sr-87/Sr-86 and epsilon(Nd(T)) are indistinguishable from that inferred for the late Precambrian lithospheric mantle, These ages and isotopic characteristics are indistinguishable from those of adjacent juvenile terranes in Arabia and Egypt but contrast markedly with samples along and west of the Nile near the eastern margin of the Nile craton. The ages and isotopic signatures of rocks north of the Nakasib suture seem distinctly younger and derived from a more depleted mantle source than those from south of the suture, suggesting that this is one of the most important terrane boundaries in the Arabian-Nubian shield. Age data also provide an independent constraint on the time of suturing between East and West Gondwanaland to 600-700 Ma.