Post-orogenic and anorogenic A-type fluorite-bearing granitoids, Eastern Desert, Egypt: Petrogenetic and geotectonic implications
Introduction
The Arabian–Nubian shield (ANS) is a Pan-African accretionary belt (Stoeser and Camp, 1985). Kröner (1983) suggests that the ANS represents an assemblage of accreted terranes consisting of juvenile arcs, oceanic plateaus and microcontinents. According to Stern (1994), the evolution of the ANS encompasses four major tectono-magmatic episodes that occurred between 900 and 550 Ma. These are: (1) rifting of the supercontinent Rodinia (∼900–850 Ma), (2) sea-floor spreading, arc and back-arc basin formation, and accretion of the juvenile crust (870–690 Ma), (3) continental collision (750–650 Ma), and (4) east–west crustal shortening (640–550 Ma) as East and West Gondwana collided. In a structural synthesis of the ANS in Egypt, Greiling et al. (1994) stressed that the collision ended at 615–600 Ma. The subsequent extensional collapse occurred between 595 and 575 Ma and was followed by transpressional tectonics along major shear zones until 530 Ma.
The Neoproterozoic evolution of the Egyptian Eastern Desert is characterized by felsic-dominated magmatism that marks the culmination of the Pan-African igneous activity (650–550 Ma; Rogers et al., 1978). A part of these rocks can be classified as post-orogenic to anorogenic A-type granitoids (Hassanen and Harraz, 1996; El-Sayed, 1998; El-Sayed et al., 1999a, El-Sayed et al., 2001; Moghazi, 1999; Mohamed, 1999; Mohamed et al., 1999), post-dating the emplacement of I-type calc-alkaline granitoid batholiths. These A-type granites mainly occur as small circular massifs (∼0.5–10 km2) emplaced as discordant bodies at shallow crustal levels. El-Sayed (1998) classifies the Egyptian granitoids into: (1) I-type orogenic, arc-related granitoids, which were further subdivided into calc-alkaline and highly fractionated granitoids and (2) A-type anorogenic granitoids. Egyptian granitoid magmatism at 610–550 Ma (Beyth et al., 1994; Furnes et al., 1996; El-Sayed, 1998; Moghazi, 1999) is supposed to be related to extensional tectonics (Abdel-Rahman and Martin, 1990; Beyth et al., 1994; El-Sayed, 1998) and/or to post-collisional deformation (El-Sayed et al., 1999b, El-Sayed et al., 2001; Mohamed, 1999; Mohamed et al., 1999) with granite emplacement being related to strike-slip movements that followed the collisional event by about 25–75 Ma (Moghazi et al., 1999). The low initial strontium isotope ratio of 0.7025 and low Th/U (∼2.5) of these granites persuaded Rogers et al. (1978) of their mantle derivation. The origin and evolution of the alkaline A-type granitoids – whether anatexis of lower crust (Klemenic and Poole, 1988) or fractional crystallization of mantle-originating mafic magma (Beyth et al., 1994; El-Sayed, 1998) – remains unresolved, although a mantle source followed by fractional crystallization seems to be the dominant petrogenetic process (Nédélec et al., 1995). The location of the alkaline granitoid province at the junction between ENE transform faults and NNW deep-seated tectonic zones (Garson and Krs, 1976; Bowden, 1985; Fig. 1) indicates the emplacement of the granites in pull-apart opening zones related to the post-orogenic evolution (Moghazi et al., 1999; El-Sayed et al., 1999a, El-Sayed et al., 2001).
The present study focuses on four A-type granitic plutons (Homrit Waggat, Homer Akarem, Ineigi and Gabal Gharib) exposed in the Egyptian Eastern Desert and the associated fluorite mineralization (Fig. 1). The purpose of this paper is to investigate the evolution of the A-type granitic plutons and the role of the associated fluorite deposits in their genesis. The paper also discusses the geotectonic implications of the A-type granitoid magmatism in Egypt. The older I-type granite of the Gharib area is used in the present study for comparison with A-type granites and for petrogenetic considerations.
Section snippets
Regional geological setting
The structural evolution of eastern Egypt during the Neoproterozoic is dominated by either compressional, extensional or strike-slip linked faulting (Kamal El Din et al., 1992; Wallbrecher et al., 1993; Greiling et al., 1994; Fritz et al., 1996). Several contrasting models have been proposed to explain the structural evolution of the ANS, including: (1) high-angle block faulting and “block tectonics” (Sultan et al., 1988), (2) low-angle normal faulting (Kamal El Din et al., 1992), (3) reverse
Petrography
This study is based on 60 samples collected from the four granitic plutons and the associated fluorite mineralization. Two groups of granites are distinguished petrographically as well as geochemically, namely subsolvus and hypersolvus granites. In the following, a brief petrographical description of the two granite types will be given.
Analytical methods
Twenty-eight granite samples (21 samples for the subsolvus granites, 5 samples for the hypersolvus granites and 2 samples for the older I-type granites forming the country rocks of the Gabal Gharib intrusion) and six fluorite samples from the four studied plutons were analysed for major and trace elements. Major oxides and trace elements for the granitic samples, using fused and pressed pellets, respectively, were determined by X-ray fluorescence spectroscopy (XRF), with a Rigaku 3080EZ
Granite-type and tectonic setting
A-type granites are relatively alkaline in composition with the magmas having an anhydrous character (Loiselle and Wones, 1979). A-type classification (unlike the I- and S-type) does not imply a specific source or mode of origin. Bonin (1990) proposes that alkaline granites post-dating a major orogenic episode can be subdivided into two groups: (1) post-orogenic granites, characterized by MgMn-rich mafic minerals, high Ba and Sr abundances, crustal Sr isotopic signatures and (2) early
Conclusions
The principal geochemical characteristics of the studied granites, including high contents of SiO2, K2O and HFSE (Zr, Nb, Y, REE), the high Fe/(Fe+Mg) ratio and the very low MgO and low CaO contents, as well as their REE patterns are coincident with those found in A-type granites elsewhere. The studied granitic rocks are subdivided into two different magmatic pulses, which are mineralogically and chemically distinct, namely: A2-subtype subsolvus and A1-subtype hypersolvus granites. Field
Acknowledgments
The analytical work was done at Tohoku University, Japan during a research fellowship granted to F.H. Mohamed by the Matsumae International Foundation, Japan. The authors would like to express their sincere thanks to Prof H. Fujimaki and Dr. Ishikawa, Tohoku, Japan, for permission to use XRF instrument and technical advice. REE analyses for selected samples were made available through the help of Dr. T. Oberthür, Hannover, Germany. The authors thank B. Bonin and R. Sallet for their critical
References (82)
- et al.
The late Precambrian Timna igneous complex, southern Israel: evidence for comagmatic-type sanukitoid monzodiorite and alkali granite magma
Lithos
(1994) The geochemistry and mineralization of alkaline ring complexes in Africa (a review)
J. Afr. Earth Sci.
(1985)Aluminium saturation in I- and S-type granites and the characterization of fractionated haplogranites
Lithos
(1999)- et al.
Petrogenesis of Mesoproterozoic Oak Creek and West McCoy Gulch plutons, Colorado: an example of cumulate unmixing of a mid-crustal, two-mica granite of anorogenic affinity
Precamb. Res.
(1993) - et al.
Effects of water and fluorine on the viscosity of albite melt at high pressure: a preliminary investigation
Earth Planet. Sci. Lett.
(1985) The A-type granitoids: a review of their occurrence and chemical characteristics and speculations on their petrogenesis
Lithos
(1990)- et al.
Formation of Neoproterozoic metamorphic core complexes during oblique convergence (Eastern Desert, Egypt)
J. Afr. Earth Sci.
(1996) The application of trace elements to the petrogenesis of igneous rocks of granitic composition
Earth Planet. Sci. Lett.
(1978)- et al.
Geochemistry and petrogenesis of a peralkaline granite complex from the Midian Mountains, Saudi Arabia
Lithos
(1980) - et al.
Relative and absolute terrestrial abundances of the rare earths