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A. A. Rashwan
H. Schleicher

 


Granitic melts can be produced in different geotectonic settings. The resulting rocks differ in terms of their mineralogical, geochemical and isotopic composition, and thus it is possible to trace the compositional and structural evolution of a given segment of continental crust in time by the petrological study of its granitoid rocks.
I. Petrology of Panafrican Leucogranite Series ("Younger Granites")

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Distribution of Panafrican "Younger Granites" of the Eastern Desert (red) and Granite Complexes investigated by our research group (yellow)

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Granite Mountains in the Hamrat el Feg and Agab el Nogum Area, Southern Eastern Desert
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The formation of the Arabic-Nubian Shield (ANS) during the Pan-African orogeny represents one of the most important events of formation of juvenile crust in the Earth's history. This Pan-African basement is considered to have evolved by the accretion of a series of magmatic arcs and terranes. The evolution from initial island arc situations via different stages to final cratonization and subsequent rifting was accompanied by a large number of granite intrusions of quite different type and origin (plagio-granites, 'syn-collision' granites ['older' granites and gabbro-diorite-tonalite complexes], and 'younger' or 'pink' granites).

The so-called 'younger' granites ('late-collision' or 'late-tectonic' granites; ca. 600 - 550 Ma) are frequently and widespread distributed (compare the map). They are characterized by a leucogranitic, very uniform mineralogy and major element geochemistry with compositions close to the eutectic, they are epizonal and usually undeformed. Their origin is interpreted in very different ways: 1) as late differentiation products of calc-alkaline magma series from active continental margins, or 2) as 'late/post collision' granites, created by largescale anatectic melting at the climax of the Panafrican orogenesis, and thus of crustal provenance.

Up to now, 18 granite intrusions and two alkaline ring complexes were investigated by our research group, namely in the southern and central segment of the ED (marked in yellow colours in the map). Beside major and trace element geochemistry and electron microprobe, we used zircon typology and synchrotron XRF at HASYLAB/DESY to measure in-situ trace element distributions in minerals.

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Microscopic image (polarized light) of typical 'Fishbone' structure of perthitic alklaline feldspar from leucocratic hypersolvus granite


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Chemical variation of bulk rock samples from leucogranites from different parts of the Eastern Desert revealing different magmatic series (namely alkaline A-type granites - high Zr+Ce+Y contents - and calcalkaline volcanic arc granites - lower Zr+Ce+Y)
Zircon crystal (size 1 mm) with perthitic alkaline feldspar, biotite, ilmenite, apatite and quartz
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Discrimination diagram Zr vs. Ga/Al for leucogranites of the Eastern Desert

The investigated granites are all leucogranites with a narrow range of SiO2 between 72 and 78% (exception: Um Lasifa) and thus consists mainly of quartz and feldspars with only minor amounts of mafic minerals (biotite, hornblende) leading to nearly eutectic compositions. However, in spite of this uniform feature most of the geochemical parameters clearly show the existence of two distinct types of leucogranitic, late- to post-tectonic rocks (compare the geochemical diagrams). One series stands for a typical calc-alkaline volcanic arc rock suite, represented by G. Filat, Um Lasifa, G. Nasiya, parts of Abu Khareef, and Kadabora granites. Their differentiation patterns mainly results from the fractionation of hornblende, biotite, and plagioclase. In the Pearce discrimination plots all of these rocks plot within the field of 'volcanic arc granites' (VAG).

The second group of leucogranites (represented by Abu Khareef, Abu Hamer, and Hamrat el Feg) are lowered in Al, but enriched in Fe, alkalies, REE, Zr and other trace elements like Y, Nb etc. (see diagram above). According to the Pearce discrimination they are 'within plate granites'; according to the discrimination of Whalen et al.(1984) we are dealing with A-type granites (see diagram left side).

II. A-Type Granites of the Eastern Desert

Late- to postorogenic A-type granites (ca. 600-540 Ma) are widespread and common in the Arabic-Nubian Shield (ANS). Their origin is discussed in different ways: (a) as anhydrous fractionation products of basaltic melts, or (b) as anatectic partial melts derived from a basic to intermediate lower crust, probably with a metasomatic overprinting (c). Beside alkaline feldspar and quartz, these very leucocratic "hypersolvus" granites contain alkaline amphiboles, Fe-rich biotite and rather high amounts of accessory zircon, allanite, monazite, apatite, fluorite, and ilmenite. Geochemically they are characterized by high enrichments of LIL and HFS elements, especially Zr, Nb, Y, and REE.

All petrographic and geochemical features give evidence for rather anhydrous high-temperature melts, e.g. formation of feldspars, high Zr temperatures, very high intrusion level. Zircon typology studies (see Fig. below) underline these findings.

Thus, together with the very common and widespread occurrence in all parts of the Eastern Desert, the A-type granites are a hint for an unusual high temperature regime during the late- to post-tectonic stage of the Panafrican orogeny within the ANS.

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Landsat 7 Image of Kadabora Granite, Central Eastern Desert
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Quite often the A-type granites exhibit clear evidence for a rather young, late-magmatic to (auto-) metasomatic formation of their minerals. Beside newly formed albite and microcline, late allanite, fluorite and secondary zircon occur. Geochemically we do not find fractionation trends but rather diffuse elemental patterns. In some cases (e.g. Kadabora, Um Lasifa), a close geologic coexistence with VAG leucogranites is striking, forming multiple intrusion granite complexes (compare Landsat image of Kadabora granite above, with A-type granites in the inner part).


   Geochemical "Spidergram" of
   Kadabora Granite Rocks in
   proof of the existence of a
   multiple granite intrusion
   with two distinct magmatic
   series
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