Elsevier

Geochemistry

Volume 68, Issue 4, 25 September 2008, Pages 431-450
Geochemistry

Post-orogenic and anorogenic A-type fluorite-bearing granitoids, Eastern Desert, Egypt: Petrogenetic and geotectonic implications

https://doi.org/10.1016/j.chemer.2007.01.001Get rights and content

Abstract

The present study focuses on four A-type fluorite-bearing granitic plutons in the Eastern Desert of Egypt which are classified into post-orogenic subsolvus (Homrit Waggat, 535 Ma; Homer Akarem, 541 Ma and Ineigi, 571 Ma) and anorogenic hypersolvus (Gabal Gharib, 476 Ma) granites. All the granites are Si- and alkali-rich and Mgsingle bondCasingle bondTi poor. Whereas both granite types appear relatively homogeneous in terms of most of their major and trace elements, they differ in that the subsolvus granites are depleted in TiO2, FeO*, Ba, Sr and Zr and enriched in Rb and Y with respect to the hypersolvus granites. The two granite types, however, can be distinguished more easily by their rare-earth element (REE) patterns. Chondrite-normalized REE patterns of the hypersolvus granite display a gull-wing shape, characterized by a large negative Eu anomaly and moderate-to-high LREE contents. Relative to the hypersolvus granite, subsolvus granite is depleted in LREE and more enriched in HREE contents. The increase of HREE in the subsolvus granite is presumably caused by F complexing during the late stage of its evolution. This is supported by the abundance of fluorite veins cross-cutting the subsolvus granite. The negative Eu anomalies in the subsolvus granite point to the role of feldspars as residual phase in the source, and as a crystallizing phase during magmatic differentiation.

Field relations, textural, mineralogical and geochemical data of the post-orogenic subsolvus granite are consistent with its derivation from a parental basic magma through crystal–liquid fractionation of alkali feldspar, plagioclase, amphibole, Fesingle bondTi oxides, titanite, zircon, monazite and allanite. Crystallization occurred in a water-enriched and rather oxidizing environment, as a result of which the entire suite has a transitional character between that of a post-orogenic and an anorogenic setting. On the other hand, the most credible mechanism for the origin of the anorogenic hypersolvus granite is partial melting of I-type granodiorite–monzogranite source rocks in the study area.

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 Mgsingle bondMn-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)

  • M.A. Hassanen

    Post-collision, A-type granites of Homrit Waggat complex, Egypt: petrological and geochemical constraints on its origin

    Precamb. Res.

    (1997)
  • M.A. Hassanen et al.

    Geochemistry, Sr- and Nd isotopic study on the rare-metals bearing granitic rocks, Central Eastern Desert, Egypt

    Precamb. Res.

    (1996)
  • G. Mahood et al.

    Large partition coefficients for elements in high-silica rhyolites

    Geochim. Cosmochim. Acta

    (1983)
  • C.F. Miller et al.

    Extreme fractionation in felsic magma chambers: a product of liquid state diffusion or fractional crystallization?

    Earth Planet. Sci. Lett.

    (1984)
  • M.M. Pimentel et al.

    Intracrustal REE fractionation and implications for Smsingle bondNd model age calculations in late-stage granitic rocks: an example from central Brazil

    Chem. Geol.

    (1991)
  • C.W. Ponader et al.

    Rare earth elements in silicate glass/melt systems, II. Interactions of La, Gd and Yb with halogens

    Geochim. Cosmochim. Acta

    (1989)
  • J.J.W. Rogers et al.

    Plutonism in Pan-African belts and the geologic evolution of northeastern Africa

    Earth Planet. Sci. Lett.

    (1978)
  • A.E. Shimron

    The Red Sea line – a late Proterozoic transcurrent fault

    J. Afr. Earth Sci.

    (1990)
  • S.P. Turner et al.

    Derivation of some A-type magmas by fractionation of basaltic magma: an example from the Padthaway Ridge, South Australia

    Lithos

    (1992)
  • W. Unzog et al.

    Progressive development of lattice preferred orientations (LPOs) naturally deformed quartz within a transpressional collision zone (Panafrican Orogen in the Eastern Desert of Egypt)

    J. Struct. Geol.

    (2000)
  • H. Yurimoto et al.

    Are discontinuous chondrite-normalized REE patterns in pegmatitic granite systems the results of monazite fractionations?

    Geochim. Cosmochim. Acta

    (1990)
  • A.M. Abdel-Rahman et al.

    The Rbsingle bondSr geochronological evolution of the Ras Gharib segment of the northern Nubian Shield

    J. Geol. Soc. Lond.

    (1987)
  • A.M. Abdel-Rahman et al.

    Late Pan-African magmatism and crustal development in northeastern Egypt

    Geol. J.

    (1987)
  • A.M. Abdel-Rahman et al.

    The Mount Gharib A-type granite, Nubian Shield: petrogenesis and role of metasomatism at the source

    Contrib. Mineral. Petrol.

    (1990)
  • J. Bennett et al.

    Tiered-tectonics and evolution, Eastern Desert and Sinai, Egypt

  • B. Bonin

    From orogenic to anorogenic settings: evolution of granitoid suites after a major orogenesis

    Geol. J. W. S. Pitcher Spec. Issue

    (1990)
  • B. Bonin et al.

    Plutonic alkaline series: daly gap and intermediate compositions for liquids filling up crustal magma chambers

    Schweiz. Mineral. Petrogr. Mitt.

    (1990)
  • B. Charoy et al.

    Zr-, Th-, and REE-rich biotite differentiates in the A-type granite pluton of Suzhou (Eastern China): the key role of fluorine

    J. Petrol.

    (1994)
  • W.J. Collins et al.

    Nature and origin of A-type granites with particular reference to south eastern Australia

    Contrib. Mineral. Petrol.

    (1982)
  • R.A. Creaser et al.

    A-type granites revisited: assessment of a residual-source model

    Geology

    (1991)
  • R.L. Cullers et al.

    Origin and chemical evolution of the 1360 Ma San Isabel batholith, Wet Mountains, Colorado: a mid-crustal granite of anorogenic affinities

    Geol. Soc. Am. Bull.

    (1992)
  • A. Ebadi et al.

    Beginning of melting and composition of first melts in the system Qzsingle bondAbsingle bondOrsingle bondH2Osingle bondCO2

    Contrib. Mineral. Petrol.

    (1991)
  • G.N. Eby

    Chemical subdivision of the A-type granitoids: petrogenetic and tectonic implications

    Geology

    (1992)
  • A.T. Egeberg et al.

    The Bonifatio peralkaline granites (NW Corsica): a possible case of evolution through volatile transfer

    Bull. Soc. Géol. Fr.

    (1993)
  • El-Manharawy, M.S., 1977. Geochronological investigation of some basement rocks in central Eastern Desert, Egypt,...
  • M.F. El-Ramly et al.

    On the tectonic evolution of the Wadi Hafafit area and environs, Eastern Desert of Egypt

    Bull. Fac. Earth Sci., King Abdulaziz Univ.

    (1984)
  • M.M. El-Sayed

    Tectonic setting and petrogenesis of the Kadabora pluton: a late Proterozoic anorogenic A-type younger granitoid in the Egyptian Shield

    Chem. Erde

    (1998)
  • M.M. El-Sayed et al.

    Geochemistry and petrogenesis of late Precambrian tonalite–granodiorite–syenogranite series at Umm Shaddad district, Egypt

    Neues Jahrb. Mineral., Abhandl.

    (1999)
  • El-Sayed, M.M., Hassanen, M.A., Moghazi, A.M., Mohamed, F.H., 1999b. Geochemistry and tectonogenetic evolution of late-...
  • M.M. El-Sayed et al.

    The Mueilha intrusion, Eastern Desert, Egypt: a post-orogenic, peraluminous, rare-metal bearing granite

    Chem. Erde

    (2001)
  • H. Furnes et al.

    Pan-African magmatism in the Wadi El Imra district, Central Eastern Desert, Egypt: geochemistry and tectonic environment

    J. Geol. Soc. Lond.

    (1996)
  • Cited by (0)

    View full text