SOURCE AND TECTONIC SETTING OF SHARM EL-SHIEKH ALKALINE A-TYPE GRANITOIDS , SINAI , EGYPT

The Sharm El-Shiekh alkaline granite is a typical Late Pan-African A -type granitoid pluton in Sinai segment of the Arabian-Nubian Shield. Such granitoids represent in general the last major · magmatic activity, and in specific the termlnal granitic plutonism in Sinai massive. They are hypersolvous textured, ranging in composition from alkali feldspar perthitic granite to quartz alkali · feldspar syenite. The most striking compositional feature of the studied granites is their extreme enrichment in Kz(J and total alkalis that led to formation of normative acmite and sodium metasilicate, and consequently a strong alkaline and peralkaline nature. The present granitoids exhibit typical features of A -type granitoids, characterized by low CaO content (averaging 0. 71 wt.% ), high FeO.jMgO ratios (9.28-115.5) and NKJA ratios (1.01-1.36), with exceptionaUy high concentrations of the trace elements Y, Nb, Zr, , Zn and Ga and low Sr and Ba contents. K20 The use of different petrogenetic discrimination criJeria and diagrams reveal that they are anorogenic A-type granites, evolved in within-plate (rift-related) setting. However, the Yb/Nb ratio has given rise to a contrasting dual source material derivation and tectonic setting of these A-type granitoids. The studied samples are discriminated between granitoids derived from sources like those of oceanic-island basalts but emplaced in continental rifts or during intraplate magmatism (Anorogenic), and granitoids derived from continental crust or underplated crust through a cycle of · continent-continent collision or island-arc magmatism (Post-orogenic). INTRODUCTION Alkaline granite plutons occur without any recognizable alignment, all over the Arabian Shield with estimated combined area of about 5000 Km (Harris & Marriner, 1980). In Sinai, these granites seem to be more widespread than in any other part of the Arabian-Nubian . Shield, accounting for some 20% of all granites and covering more than 1000 Km (Bentor 1985). The worldwide distribution of A-type granites has been recognized more than 25 years ! ago (Loiselle and Wones 1979; White and Chappell 1983). The composition of A-type ~granites is diverse; they comprise syenogranite, peralkaline and alkali-feldspar granite and f syenite, rapakivi granite, monzogranite and F-rich topaz granite (e.g.; Whalen et al. 1987; Eby . 1992; Whalen et al. 1996). In the vast crustal provinces of Central Asia and the Arabian-Nubian Shield three rock types are dominant among the A-type granites (Bentor 1985; Litvinovsky et al. 2002): metaluminous syenogranite (e.g. G. Firani; El-Masry 1998), . peralkaline granite, (e.g. G. Gharib; Abdel-Rahman & Martin 1990) ) and alkali-feldspar · (mostly perthitic) granite (e.g. the investigated A-type granite). The intrusion of anorogenic (A-type) granitic magma into the upper crust commonly, follows a cycle of compressive tectonic activity and orogenic magmatism (Boullier et al. El-Bialy, M.Z. & El Omla, M.M. 1986; Liegeois and Black 1987). Anorogenic alkaline magmas are believed to be emplaced into the Arabian-Nubian Shield crust between the waning stages of the Pan-African orogeny and the opening of the Red Sea 23 Ma ago (l-Iashad 1980; Harris 1982; Vail 1985; El-Ramly and Hussein 1985). They were identified in Egypt by Hussein et al. (1982) as ; 03-granites ; intraplate, anorogenic granites, related to hot spots and incipient rifting, El Gaby et al. (1990) as Phanerozoic alkaline A-type granites and Ragab & El Alfy (1996) as late collision granites. Further, In Sinai, Agron & Bentor (1981) and (Bentor 1985) grouped these alkaline granites as intrusive partner of late alkaline batholithic phase, the so called "Katharina Superprovince ". The A-type alkaline granites under consideration were reported in previous works and interpreted in various connotatjons (e.g. Ghoniem et al. 1991; Higazy et al. 1992 a& b; Khalaf ct al. 1994; Abdel Maksoud et al. 1994; Abu El-Leil et al. 1995 ; El-Nashar 2000). The published absolute ages of Sinai alkaline granites range from 600±24 to 560±10 Ma (Baubron et al. 1976; Stoeser & Elliott 1980; Halpern 1980; Bielski 1982). On the other side, the Mt. Gharib A-type granite (nmth Eastern Desert of Egypt) yielded a much younger ages of 476±2 Ma(Abdel-Rahman & Martin 1990) and 483 Ma (Hashad 1980). Recently, Katzir et al. (2006) gave a Rb-Sr isochron age of Kathelina pluton A-type granite, S. Sinai, of 593±16 Ma. METHODOLOGY & ANALYTICAL TECHNIQUES Mapping The construction of the geological map (Fig. 1) was based on a TM satellite image at scale of 1:100,000 (JPEG format) and a base topographic map (1:250,000), aided by the available former geologic maps (e.g. Abdel Maksoud et al. 1994 Khalaf et al. 1994 and Azzaz et al. 2000). Tracing, drawing, hashering and other processmgs were produced using CORELDRA W version 5.0 software. Petrography A total of 27 representative thin sections were prepared and studied using a polarizing microscope. Modal analysis was carried out for some selected 18 thin sections, using a Swift automatic point counter and a mechanical stage. Reliable modal analysis was accomplished at 1300-1500 counts for each sample. Such reliable modal counts are achieved either by ·following the method recommended by Nisbett (1964) or by counting multiple thin sections of the same rock sample to reduce the modal variation (error) within each sample by about 50% and 15.38% respectively (Emerson 1964). Ternary plots on the QAP diagram were produced using Triplot version 4.01 software. Anorthite content (An%) of the plagioclase feldspar was determined optically by measuring the extinction angle of the twinned plagioclase using the appropriate charts and methods described in Shelly (1975) and Kerr (1977). The grain size of different grains or crystals was measured microscopically using an eyepiece micrometer.