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 Gebeit volcanics can be divided into four groups: (a) cpx-phyric basalts with clinopyroxene and plagioclase as the dominant phenocrysts and minor opaques; (b) hbl-phyric basalts with hornblende, clinopyroxene, plagioclase and subordinate magnetite including one rare dacite; (c) plphyric andesites with plagioclase phenocrysts in a matrix that is rich in magnetite; and (d) aphyric basalts. The compositional variation within the distinct volcanic groups can only partly be explained by fractional crystallization, and more than one magma source reservoir is required. Mineral and whole rock Sm/Nd data for the cpx-phyric and hbl-phyric 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. The epsilonNd(t) values for the aphyric basalts and pl-phyric andesites range from 6.7 to 8.3, indicating the involvement of different depleted magma sources. The Nd and Sr isotopic data rule out any significant influence of older continental crust in the formation of the Gebeit volcanics and indicate an intra-oceanic origin. This implies that the Gebeit terrane is a segment of juvenile crust that originated in a subduction-related environment and supports the arc accretion model for the Arabian-Nubian Shield.