Author: W.R. Church
Company: None

Granitic and metamorphic rocks of the Taif area, western Saudi Arabia: Discussion and reply
 Discussion W. R. CHURCH Department of Geology, University of Western Ontario, London, Ontario N6A 5B7, Canada

In their comments on the regional setting and petrogenesis of the Arabian Shield and northeastern Africa, Nasseef and Gass (1977) supported the view that this region is the product of cratonization of oceanic island arcs over the period 1,100 to 500 m.y. B.P.  In doing so, they once more raised the possibility (Bakor and others, 1976; Neary and others, 1976; Gass, 1977) that the ophiolites of Egypt and Saudi Arabia "mark the positions of former basins between individual arc systems" (Bakor and others, 1976, p. 7), the ultramafic zones of Saudi Arabia and the Eastern Desert of Egypt being located adjacent to collision sutures separating as many as seven island arcs that were swept together and cratonized over a period of 600 m.y. In view of the general importance of this model in theories concerning the origin of continental crust, I offer the following remarks.

First, there is some doubt that the distribution of ultramafic rocks of the Eastern Desert of Egypt delineates "zones" as shown by Nasseef and Gass (1977, Fig. 4). In the region of the Eastern Desert of Egypt between Safaga and Marsa Alam, the distribution of ultramafic rocks as shown on the geological map of the basement rocks in the Eastern Desert of Egypt (El Ramly, 1972; Fig. 1 here) does not seem to justify the assertion that two independent ultramafic zones exist in this region. More important, the ultramafic rocks of the Eastern Desert, which occur both as isolated serpentinite masses of varying dimensions (tens of kilometres to kilometres; for example, Bir Barramiya — Shukri and Lotfi, 1955) and as parts of relatively coherent ophiolite suites (for example, the Atalla Serpentinite Range so well exposed along the Qift-Quseir road — Akaad and Noweir, 1972) are enclosed within mixtite, graywacke, and shale units of what can be termed the "geosynclinal sequence" (El Ramly, 1972, p. 8). The primary control on the occurrence of the ultramafic rocks may therefore be stratigraphic in which case the lack of surface exposures of ultramafic in the area between Marsa Alam and the region of Gebel Zarget Naam (Ghanem, 1972) and the latter area and the region of Gebel Muosim could be due to the presence in these areas of first-order northwest-trending anriformal structures that have been eroded sufficiently deeply to expose stratigraphic levels beneath the ultramafic-bearing units of the geosynclinal sequence. 

Second, according to Akaad and Shazly (1972), the geosynclinal sequence meta- sedimentary rocks pass down gradationally into a highly deformed sequence of mature quartzites and arkoses (Meatiq Group); therefore, it seems unlikely that the geosynclinal sequence was deposited in situ in an oceanic trench environment. Some support for this view is provided by the finding of clasts of mature quartzite containing microcline and rounded grains of zircon in pebbly mudstones of the geosynclinal sequence southwest of Marsa Alam. The possibility that the geosynclinal sequence of the Eastern Desert is underlain by a middle Prorerozoic cratonic platform sequence overlying Eburnean basement cannot, therefore, be ruled out. Elsewhere, rocks similar to the Meatiq Group occur in the Sudan, where they are known as the Kashebib Group, and perhaps also in Saudi Arabia in the form of the Hali Group. An alternative to the hypothesis that the geosynclinal sequence rocks are trench deposits is that they were laid down on a sub-siding sialic continental margin as exogeosynclinal deposits analogous to those of the western margins of the Alpine and Appalachian systems, a view presaged by Akaad and Shazly (1972, p. 235). In this event, and as is also possible in the case of the upper Proterozoic ophiolites of Bou Azzer, Morocco (Church, 1976), and Rhyd YBont, Wales, emplacement of the ultramafic and associated mafic rocks of the Eastern Desert of Egypt by obduction may have taken place relatively early during the Pan-African event rather than during arc-arc collision at the end of the cratonization cycle. It may perhaps be equally reasonable to regard the evolution of the Pan-African system in terms of intracontinental crustal basification and attenuation leading to the development of rift ocean basins and subsequent secondary subduction-related phenomena. Bertrand and others (1975), Ducrot and Lancelot (1975), and Ba and others (1976) have shown that in the Hoggar region of Algeria pre-Pharusian cratonic rocks are present both to the west of the(West African craton) and east (East Saharian craton) of the 650- to 560-m.y.-old Hoggar Pan-African system. Also sedimentary and volcanic sequences of the upper Proterozoic (about 800 to 650 m.y. old) Pharusian system are invariably underlain by older Pre-cambrian tectonites and/or platform sequences, rather than by penecontemperaneous, cratonized volcanic-arc rocks. It seems doubtful, therefore, that the Pan-African system of northern Africa could have been produced entirely by intra-oceanic island-arc evolution and progressive cratonization of a large area of upper Proterozoic oceanic crust east and north of the West African and Congo cratons, re-spectively (Gass, 1977). If analogy is required, a comparison with the Pacific region between eastern Australia and New Zealand is perhaps as credible as the intra-oceanic island-arc model. Alternatively, there may be no simple present-day plate tectonic analogue for the Pan-African system, and attempts to explain the late Proterozoic evolution of northern Africa in terms of Pacific-type plate tectonic models may be only partly valid.

 ACKNOWLEDGMENTS
I express my gratitude to Dr. Ahmed Hashad and Dr. Mohamed El-Sharkawy for the opportunity they provided to examine the ophiolitic rocks and metasedimentary units of the "geosynclinal sequence" of the Eastern Desert of Egypt, as well as to Dr. E. M. El Shazly for many amiable courtesies during my visit.

According to Pegram and others (1976) platform sediments are also present along the western margin of the Tibesti Massif batholith located halfway between the East Saharan craton and the Eastern Desert of Egypt.

The article discussed was published in the Bulletin, v. 88,p. 1721-1730.

Figure 1. Distribution of ultramafic rocks in Eastern Desert of Egypt. Taken from Geological map of Eastern Desert of Egypt (El Ramly, 1972).
Geological Society of America Bulletin, Part 1, v. 90, p. 893-896, 2 figs., September 1979, Doc. no. 90913.
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