Plutonic and metamorphic rocks from the Keraf Suture (NE Sudan): a glimpse of Neoproterozoic tectonic evolution on the NE margin of W. Gondwana

Abstract

The Keraf Shear Zone (KSZ) of NE Sudan formed in Neoproterozoic time when juvenile crust of the Arabian–Nubian Shield collided with reworked older crust of the Saharan Metacraton. Orthogneisses and deformed granitoids which intrude carbonate sediments are calc-alkaline, medium-K, metaluminous and I-type diorites and granodiorites, with subduction-related trace element signatures. 207 Pb / 206 Pb zircon single grain ages are ∼730 Ma for the high-grade gneiss and ∼710 Ma for granitoids. K/Ar ages are 660 Ma for hornblende from the granitoids and 560 Ma for biotite from the high-grade gneiss and the plutons. The geochronologic data indicate that the crust formed ∼730–710 Ma and that orogenic activity ended by ∼565 Ma. Cooling occurred over much of NE Africa at about the same time, possibly caused by orogenic collapse or unroofing caused by extensive glaciation. Pb isotopic ratios indicate an oceanic source for both the magmatic and metamorphic rocks in the KSZ. Sm/Nd model ages are 620–770 Ma for the post-tectonic plutons and 830–900 Ma for the gneiss, indicating negligible involvement of a pre-Neoproterozoic material in the evolution of Keraf igneous rocks. The ( 87 Sr / 86 Sr ) i values range from 0.7026 to 0.7046, which are indistinguishable from initial ratios for juvenile crust of the Arabian Nubian Shield and are lower than initial Sr isotopic compositions for the Saharan Metacraton. The isotopic data indicate that Keraf igneous rocks are juvenile additions to the crust and that the boundary between the Saharan Metacraton and the Arabian–Nubian Shield lies to the west.