LATE PROTEROZOIC ISLAND-ARC VOLCANICS FROM GEBEIT, RED-SEA HILLS, NORTHEAST SUDAN

The area of Gebeit Mine in the northern Red Sea Hills, Sudan, is built up of voluminous volcanic rocks and minor volcaniclastic and clastic sequences. According to their chemical and modal compositions the Gebe it volcanics can be divided into four groups: (a) cpx-phyric basalts w ith clinopyroxene and plagioclase as the dominant phenocrysts and mino r opaques; (b) hbl-phyric basalts with hornblende, clinopyroxene, plag ioclase and subordinate magnetite including one rare dacite; (c) plphy ric andesites with plagioclase phenocrysts in a matrix that is rich in magnetite; and (d) aphyric basalts. The compositional variation withi n the distinct volcanic groups can only partly be explained by fractio nal crystallization, and more than one magma source reservoir is requi red. Mineral and whole rock Sm/Nd data for the cpx-phyric and hbl-phyr ic basalts yield an isochron age of 832 +/- 26 Ma (epsilonNd(t) = 6.74 +/- 0.19, MSWD = 0.12) which is interpreted as the age of eruption. T he epsilonNd(t) values for the aphyric basalts and pl-phyric andesites range from 6.7 to 8.3, indicating the involvement of different deplet ed magma sources. The Nd and Sr isotopic data rule out any significant influence of older continental crust in the formation of the Gebeit v olcanics and indicate an intra-oceanic origin. This implies that the G ebeit terrane is a segment of juvenile crust that originated in a subd uction-related environment and supports the arc accretion model for th e Arabian-Nubian Shield.