Late Precambrian evolution of Afro-Arabian crust from ocean arc to craton

摘要

Large parts of northeast Africa and Saudi Arabia consist of a telescoped, Proterozoic island-arc ocean-basin complex less than 1 b.y. old. In the central Eastern Desert of Egypt, the oldest units in the complex are a mafic and ultramafic sequence representing an oceanic substrate. Concentrations of elements in constituent rocks least altered by metamorphism, including Cr, Ni, Ti, and REE in the ultramafic rocks and pillow basalts, are essentially those found in similar rocks of contemporary oceanic crust. Thick sequences of calc-alkaline volcanic rocks and related volcanogenic metasediments, including wackes, breccias, and banded iron formations, overlie the oceanic substrate. The andesitic volcanic rocks are similar to those in modern circum-Pacific island arcs, although amounts of Cr and Ni tend to be higher. Rare cobble beds in the metasediments in the Eastern Desert of Eygpt contain granitic and quartzitic clasts derived from old Proterozoic and Archean forelands, presumably those exposed west of the Nile River. Stratiform ultramafic sills as much as 1 km thick, as well as thinner gabbroic sheets, are intruded as magmas within the metasediments. The composition of the ultramafic sills approximates that of basaltic komatiites. In eastern Egypt, granitic plutons, ranging from syntectonic quartz diorites to a post-tectonic flood of LIL-enriched granite emplaced 550 to 570 m.y. B.P. partly engulf and surround much of the ocean-arc complex. Age studies suggest that the Egyptian segment of the ocean-arc complex could have evolved, been telescoped, and intruded by progressively more voluminous and fractionated granitic rocks between about 550 and 850 m.y. B.P. In southwest Saudi Arabia, limited radiometric data obtained by the U.S. Geological Survey suggest that these processes may have begun slightly earlier. Both the volcanic rocks within the complex and the granitic rocks intrusive into it have low initial ratios of Sr87/Sr86 (< 0.704), suggestive of a mantle origin. There is no evidence of older sialic roots beneath the complex.