Petrogenesis of Early-Orogenic Diorites, Tonalites and Post-Orogenic Trondhjemites in the Nubian Shield     

ABDEL-RAHMAN  ABDEL-FATTAH M. 《Journal of Petrology》1990,31(6):1285-1312

The gabbro-diorite-quartz diorite-tonalite (GDT) suite representsthe oldest crustal component in northeastern Egypt; it was emplacedat 881?58 Ma during the early stage of the Pan-African orogeny.Renewed tectono-magmatic processes at the post-orogenic stageproduced the 516?7 Ma trondhjemite (TR) suite that was emplacedinto GDT host rocks. The GDT suite is compositionally broad,with a wide range of SiO₂ contents (50–64 wt.%), and isdepleted in K, Rb, Y, Nb, Hf, and REE. The suite shows a smoothvariation in both major and trace elements from gabbro to tonaliteand has a very low Rb/Sr ratio (0?05) and a low initial ⁸⁷Sr/⁸⁶Srratio (0?7042). It is calc-alkaline and shows the petrologicalcharacteristics of M-type granites and the trace-element characteristicsof arc-lavas. The GDT suite exhibits REE profiles typical ofisland-arc basalt (IAB), with an (La/Yb)_N ratio of 5?5. The trondhjemite is a relatively homogeneous felsic unit (71–75wt. % SiO₂) and belongs to the high- Al₂O₃ continental trondhjemitetype. The TR has a low Rb/Sr ratio (0?16), a low Sr-initialratio (0?7047) and trace-element characteristics of volcanic-arcgranites. It is relatively enriched in Al, Sr, and Ba, depletedin 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 geochemicallyprimitive source. The GDT suite resulted first by the emplacement of a gabbroicmagma, derived possibly from a mantle wedge above an early Pan-Africansubduction zone, and fractionated (48?8% plagioclase, 12?4%augite, 22% amphibole, and 2?9% magnetite, as documented byfractionation modelling) to give the more felsic varieties.Petrogenetic modelling, combined with petrographic and geochemicalfeatures, suggests that the TR magma was produced by non-modalpartial melting of GDT rocks at depth; fractional crystallizationof plagioclase and amphibole controlled the evolution of thisTR magma. This and other studies suggest that post-Archean trondhjemitesare similar to Archean trondhjemites in that they were mostlyproduced via multi-stage development and crustal recycling whichinvolved partial melting of a mafic crustal source.  相似文献