Discussion on the age of the Feiran basement rocks, Sinai: implications for late Precambrian crustal evolution in the northern Arabian-Nubian Shield

Abdel-Fattah M. Abdel–Rahman & R. Doig write: Stern & Manton (1987) produced a tectonic model for the evolution of the Sinai basement dependent on their claim that there is no reliable evidence for ages older than about 670 Ma; a claim challenged by Shimron (1988). One of the arguments in this discussion centred on a 4-point Rb–Sr isochron age of 881 ± 58 Ma for a gabbro–diorite–tonalite (D–T) suite, published by us in 1987 and criticized by Stem & Manton (1988). We first comment on this and then discuss their 1987 tectonic model. Specific comments with which we wish to take issue were: (1) that our sample localities were widely scattered and represent different intrusive bodies; (2) that the D–T suite is closely associated with trondhjemites of 516 ± 7 Ma age; (3) that the age of the D–T suite is suspect because it is based on 4 points with a limited range of Rb/Sr ratios. With regard to point (l), the suite covers a large area (>100 km 2 ) and extends beyond the map area of our 1987 paper. It has been intruded by younger magmatic complexes (Dokhan volcanic rocks, calc-alkaline granites and trond-jhemites) giving discontinuous exposures of the older rocks. These exposures do not form two different intrusions, as incorrectly assumed by Stern & Manton, but all exhibit the same mineralogical and geochemical features as well as textural and field characteristics. The widely spaced sampling was designed to give the best representation of this extensive unit. Our reply