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ABDEL-FATTAH M. ABDEL-RAHMAN, Petrogenesis of Early-Orogenic Diorites, Tonalites and Post-Orogenic Trondhjemites in the Nubian Shield, Journal of Petrology, Volume 31, Issue 6, December 1990, Pages 1285–1312, https://doi.org/10.1093/petrology/31.6.1285
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Abstract
The gabbro-diorite-quartz diorite-tonalite (GDT) suite represents the oldest crustal component in northeastern Egypt; it was emplaced at 881±58 Ma during the early stage of the Pan-African orogeny. Renewed tectono-magmatic processes at the post-orogenic stage produced the 516±7 Ma trondhjemite (TR) suite that was emplaced into GDT host rocks. The GDT suite is compositionally broad, with a wide range of SiO2 contents (50–64 wt.%), and is depleted in K, Rb, Y, Nb, Hf, and REE. The suite shows a smooth variation in both major and trace elements from gabbro to tonalite and has a very low Rb/Sr ratio (0·05) and a low initial 87Sr/86Sr ratio (0·7042). It is calc-alkaline and shows the petrological characteristics of M-type granites and the trace-element characteristics of arc-lavas. The GDT suite exhibits REE profiles typical of island-arc basalt (IAB), with an (La/Yb)N ratio of 5·5.
The trondhjemite is a relatively homogeneous felsic unit (71–75 wt. % SiO2) and belongs to the high- Al2O3 continental trondhjemite type. The TR has a low Rb/Sr ratio (0·16), a low Sr-initial ratio (0·7047) and trace-element characteristics of volcanic-arc granites. It is relatively enriched in Al, Sr, and Ba, depleted in K and Rb, and highly depleted in Y, Nb, Zr, Hf, Ta, and REE, with LREE-depleted, less fractionated patterns [(La/Yb)N=5·3]. These features suggest that the TR was derived from a geochemically primitive source.
The GDT suite resulted first by the emplacement of a gabbroic magma, derived possibly from a mantle wedge above an early Pan-African subduction zone, and fractionated (48·8% plagioclase, 12·4% augite, 22% amphibole, and 2·9% magnetite, as documented by fractionation modelling) to give the more felsic varieties. Petrogenetic modelling, combined with petrographic and geochemical features, suggests that the TR magma was produced by non-modal partial melting of GDT rocks at depth; fractional crystallization of plagioclase and amphibole controlled the evolution of this TR magma. This and other studies suggest that post-Archean trondhjemites are similar to Archean trondhjemites in that they were mostly produced via multi-stage development and crustal recycling which involved partial melting of a mafic crustal source.
