Abstract

The Late Cretaceous rocks of the Abu Khuruq ring complex (AKRC), exposed in the southern Eastern Desert, Egypt, comprise phonolite, trachyte, syenogabbro, essexite, nepheline syenite, and quartz syenite, as well as quartz- and nepheline-bearing pegmatites. The rocks of the complex are dominantly metaluminous and enriched in both large-ion lithophile (LIL) and high-field-strength (HFS) elements with a lack of a negative Nb anomaly. All rocks have high LREE content relative to HREE, show weak to steep fractionated REE patterns [5.6 < (La/Yb)n < 18.6], and possess the geochemical characteristics of anorogenic suites. Syenogabbros display REE patterns [11.8 < (La/Yb)n < 14.2] not significantly different from those of the essexite unit and the essexitic xenoliths found in the nepheline syenite [(La/Yb)n in the ranges 12.0–13.7 and 15.6–18.6, respectively]. They have relatively weakly fractionated HREE patterns [3.0 < (Gd/Yb)n < 3.7], low Ybn and Lun values (< 10), and no Eu anomaly. The syenogabbros (<3 wt.% MgO, <20 ppm Ni, and negligible Cr) presumably formed by fractional crystallization of an alkaline basaltic magma. The syenogabbroic melt yielded the essexite by removal of plagioclase and clinopyroxene. The silica-undersaturated evolved rocks have conformable trace-element and REE patterns [10.8 < (La/Yb)n < 14.2], without significant variations in LILE and HFSE, and negative Ba, Sr, Ti, and Eu anomaly. The normalized trace-element and REE patterns of these two units are quite similar. We infer a comagmatic relationship for the phonolites and nepheline syenites, both formed by simple fractional crystallization of an essexitic melt. The trachytes have lower MgO and CaO contents than the phonolites. The quartz syenite unit exhibits moderately fractionated REE patterns [9.3 < (La/Yb)n < 14.4], with (Eu/Eu*)n in the range 0.5–1.1]. This marginal unit may well have been derived from the SiO2-undersaturated syenitic magma, but it was contaminated by crustal material.

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