Lead Isotope Systematics of some Igneous Rocks from the Egyptian Shield

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Lead isotope data on late Precambrian igneous rocks from the eastern desert of Egypt are presented. Previous work has indicated that this igneous suite is characterized by uniformly low initial ratios of 37Sr86Sr (< 0.7035). The Pb data define three groups, loosely corresponding to age. An older tonalite to granodiorite (OTG) suite, with ages in the range 610–710 Ma, has Pb isotope characteristics similar to modern, mantle-derived oceanic mafic rocks. The age-corrected initial Pb isotope ratios of the OTG group lie near the “ocean regression line” in correlation diagrams of 206Pb204Pb vs. 207Pb204Pb and 203Pb204Pb. The isotope data imply an origin for the OTG group analogous to calc-alkalic igneous rocks in modern intra-oceanic island arcs. A younger suite of K-rich plutonic rocks (570–595 Ma) has similar 206Pb204Pb and 208Pb204Pb initial ratios relative to the OTG suite, but have higher 207Pb204Pb initial ratios when calculated from measured whole-rock U and Pb contents. However, K-feldspar and galena, associated with this suite, indicate initial Pb isotope ratios close to the OTG suite. We interpret this to indicate that the younger granites could have originated by anatexis of OTG-type material, but they probably experienced an episode of metasomatism after emplacement. A suite of volcanic rocks (the Dokhan Volcanics) from the central eastern desert are intermediate between the younger granites and OTG in both age (∼ 610 Ma) and Pb isotope characteristics. Limited data on initial 208Pb204Pb ratios suggest that all the Egyptian samples originated from a source with anomalously low ThPb ratios. Although the relative abundance of granitic rocks in the Egyptian Shield decreases to the south, no evidence of north-south heterogeneity in the Pb isotope composition of the respective source regions is apparent in the data. However, the westernmost sample studied, from the Aswan area, contains distinctly more radiogenic Pb relative to the mean. This location may mark the boundary between the Late Precambrian ensimatic orogen now outcropping in the Egyptian and Saudi Arabian Shields, and an older sialic craton to the west.

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Precambrian Research, v. 20, issue 1, p. 63-77