Late Pan‐African Magmatism and Crustal Development In Northeastern Egypt: Reply

Abstract

Some of our interpretations on the nature of Pan-African magmatism and crustal development in northeastern Egypt (Abdel-Rahman and Martin 1987) have been challenged by Stern and Gottfried (1989), and we welcome this opportunity to respond to the three points raised.Point 1: a wide variety of <600 Ma igneous and sedimentary rocks occur in northeastern Egypt. Instead of reviewing the literature in an encyclopedic way, we deliberately (according to Stern and Gottfried) ignored evidence to make our point. Our interpretations concerning the tectonic setting of the area were based largely, but not exclusively, on a study of the Ras Gharib composite batholith (700 km2, 552 Ma). Information on this pluton was supplemented by data on other granitic complexes exposed in the same area and emplaced between 596 and 565 Ma (e.g. Fawakhir, Mons Claudianus, Abu-Kharif, Qena-Safaga, and Wadi Endiya,Figures 9 and 11 of our paper). Thus we do not consider our data base to be restricted, as claimed. On the contrary, the study area is considered quite representative of the crust of northeastern Egypt, where granitic rocks represent the major component (more than 70 per cent: Schurmann 1966).Points 2 and 3: geochemical data for <600 Ma igneous suites indicate the presecce of 'A-type' melts that formed in an extensional setting; active subduction younger than 600 Ma is inconsistent with accepted tectonic reconstructions. In fact, ours is not an iconoclastic interpretation, as almost all investigators of Late Proterozoic magmatism in the Arabian-Nubian Shield have concluded that Andean-type calc-alkaline magmas were involved in the period 600-550 Ma (cJ references quoted in Abdel-Rahman and Martin 1987, and those mentioned below). Hussein er al. (1982) proposed that most of the 'Younger' or 'Gattarian'granites of Egypt are subduction-related and of I type.As the example that was the focus of our investigation does not conform to the expected profile of an 'A-type complex', Stern and Gottfried suggested that it must therefore be older than 600 Ma, with its Rb-Sr system reset as a result of an indeterminate thermal event. The wide range of Rb/ Sr values and the relative absence of scatter along the %point isochron (Abdel-Rahman and Doig 1987), the agreement between our age and the K-Ar age of 550 Ma reported by Schurmann (1966), and the lack of evidence of thermal resetting, for example in the structural state of the K-feldspar in these rocks (Abdel-Rahman 1986), suggest to us that 552 Ma does represent the approximate age of crystallization of the pluton.The parallelism between <600 Ma magmatism in the Gharib block and in the Sinai Peninsula and Saudi Arabia is considered striking. Bielski (198 1) documented intensive calc-alkaline plutonism in the Sinai Peninsula between 600 and 580 Ma. Halpern and Tristan (1981) established ages between 588 and 552 Ma for several monzonite and calc-alkaline granitic complexes from the Sinai Peninsula. Segev (1987) showed that rhyodacitic rocks there (633-548 Ma) belong to the orogenic stage, whereas ...