Age and Chemical Characteristics of some pre-Pan-African Rocks in the Egyptian Shield

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U-Pb ages on zircons from several types of sialic rocks, together with a variety of geochemical data on one sample of early plutonic activity within the Egyptian Shield are presented. The earliest autochthonous granitic rocks in the Egyptian Shield are probably quartz diorites (tonalites) emplaced just prior to the main peak of Pan-African igneous activity (550–650 Ma). The studied example has an age of 711 Ma ± 7 Ma, and is characterized by very low initial Sr ratio (87Sr/86Sr = 0.7026) and very low abundances of K2O and related large-ion lithophile trace elements. REE contents are low (less than 20 × chondritic abundances). Abundance patterns show only moderate light rare-earth enrichment (La/Yb = 4.5). These characteristics are inconsistent with any models requiring fractional fusion of pre-existing continental crustal material. Granitic and arkosic cobbles occur in rare conglomerate beds within volcanoclastic and greywacke sedimentary sequences. The cobbles show a wide range in ages (1.1–2.3 Ga) and have no obvious source within the Egyptian Shield. It is suggested that they are not indicative of any sialic plutons in the Egyptian Shield which might predate deposition of the enclosing immature sediments. More probably, they were derived from adjacent continental areas such as the Uweinat region to the west, and were deposited in an evolving arc-ocean basin complex.

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Precambrian Research, v. 14, issue 2, p. 119-133