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Hassan M.,University of Graz | Hassan M.,Suez Canal University | Hassan M.,Egyptian Institute of Geodynamic | Stuwe K.,University of Graz | And 5 more authors.
Gondwana Research | Year: 2015

In active tectonic regions, shear zones play an important role in re-configuring the structure of the lithosphere. One of the largest shear zones on Earth is the Najd Fault System of the Arabian-Nubian Shield. The main active phase of this shear zone was during the last stages of the Pan-African Orogeny (ca. 630-540. Ma). Six samples of intrusive rocks that were emplaced into the shear zone at different stages during its active phase are used to illustrate the progressive evolution of the Ajjaj shear zone. A sample of coarse-grained diorite, with an intercept U-Pb zircon age of 696. ±. 6. Ma, shows very weak deformation. Two samples from deformed granodiorite-tonalite intrusions at the border of the Ajjaj shear zone show conspicuous degrees of deformation, and define two U-Pb clusters of concordia ages at 747. ±. 12. Ma-668. ±. 8. Ma and 742. ±. 5. Ma-702. ±. 12. Ma. Two samples of granites show mylonitic foliation with flattened quartz and biotite parallel to the trend of the shear zone. These samples yield U-Pb ages of 601. ±. 3. Ma-584. ±. 3. Ma. Another granite sample is undeformed and shows cross-cutting relations with the shear foliation of the Ajjaj shear zone. It yields a concordia age of 581. ±. 4. Ma. The metamorphic rocks of the Hamadat complex host the Ajjaj shear zone, and have been useful in determining the metamorphic P-T conditions attending the activity of the shear zone. The peak metamorphism of the Hamadat Complex is 505-700. °C at two ranges of pressure 8-11 and 14.5. ±. 2. kbar. New data confine the activation of the Ajjaj shear zone in a limited period of time between 604. Ma and 581. Ma and the operation at different crustal levels with a maximum depth of 58. km. © 2015 International Association for Gondwana Research. Source

Hassan M.,University of Graz | Hassan M.,Egyptian Institute of Geodynamic | Hassan M.,Suez Canal University | Abu-Alam T.S.,University of Graz | And 4 more authors.
Precambrian Research | Year: 2014

Recently published age data indicate that the Sa'al-Zaghra metamorphic complex in Sinai, Egypt contains the oldest rocks found in the northernmost Arabian-Nubian Shield, preserving evidence for a 1110-1030. Ma rift-related volcanic system formed during Rodinia break-up (Be'eri-Shlevin et al., 2012). As such, its metamorphic evolution provides evidence for an important part of the geological history of the shield. Here we use petrographic, mineral chemistry and thermodynamic modeling, in combination with structural data from the field, to derive a P T D t path for the complex. It is shown that the metamorphic rock of the complex equilibrated during an early deformation event that involves a flat lying fabric and is interpreted as an extensional event. P T conditions attained during this event are between 370-420. °C and around 3. kbar. These conditions correspond to a geothermal gradient of 38-41. °C/km which is much higher than that documented elsewhere in the metamorphic complexes of Sinai (i.e. 25-27. °C/km). We suggest that this is because metamorphism in the Sa'al-Zaghra complex records an earlier stage of metamorphism and deformation during breakup of Rodinia, whereas the lower gradients documented elsewhere is related to the Gondwana collision. During the subsequent East-West-Gondwana collision, the Sa'al-Zaghra complex remained at shallow crustal levels (<9. km), and therefore it escaped the deep crustal metamorphism of the Pan-African event. © 2013 Elsevier B.V. Source

Abu-Alam T.S.,Tanta University | Abu-Alam T.S.,Norwegian Polar Institute | Abu-Alam T.S.,Egyptian Institute of Geodynamic | Hamdy M.M.,Tanta University
Journal of African Earth Sciences | Year: 2014

The Arabian-Nubian Shield is the largest tract of juvenile continental crust of the Neoproterozoic. This juvenile crust is composed of intra-oceanic island arc/back arc basin complexes and micro-continents welded together along sutures as the Mozambique Ocean was closed. Some of these sutures are marked by ophiolite decorated linear belts. The Sol Hamed ophiolite (808 ± 14 Ma) in southeastern Egypt at the Allaqi-Heiani-Onib-Sol Hamed-Yanbu arc-arc suture represents an uncommon example of rocks that might be less deformed than other ophiolites in the Arabian-Nubian Shield. In order to understand fluid-rock interactions before and during arc-arc collision, petrological, mineral chemistry, whole-rock chemistry and thermodynamic studies were applied to the Sol Hamed serpentinized ophiolitic mantle fragment. These studies reveal that the protolith had a harzburgite composition that probably originated as forearc mantle in the subducted oceanic slab. We propose that these rocks interacted with Ti-rich melts (boninite) in suprasubduction zone, which latter formed the Sol Hamed cumulates. Spinel's Cr# associated with the whole rock V-MgO composition suggest that the harzburgites are highly refractory residues after partial melting up to 29%. The melt extraction mostly occurred under reducing conditions, similar to peridotites recovered from the subducted lithosphere. Protolith alteration resulted from two stages of fluid-rock interaction. The first stage occurred as a result of infiltration of concentrated CO2-rich fluid released from carbonate-bearing sediments and altered basalt at the subduction zone. The alteration occurred during isobaric cooling at a pressure of 1 kbar. The fluid composition during the isobaric cooling was buffered by the metamorphic reactions. The second stage of fluid-rock interactions took place through prograde metamorphism. The increase in pressure during this stage occurred as a result of thrusting within the oceanic crust. In this process the forearc crust was loaded by roughly 20-30 km of overthrust rocks. © 2014 Elsevier Ltd. Source

Ghoneim M.F.,Tanta University | Heikal M.Th.S.,Tanta University | El Dosuky B.T.,Tanta University | Abu-Alam T.S.,Tanta University | And 3 more authors.
Arabian Journal of Geosciences | Year: 2013

Calc-alkaline and alkaline intrusions of the late Neoproterozic form essential part of the Arabian-Nubian Shield. They were formed during the collision between East- and West-Gondwana. Sharm El-Sheikh area, Sinai, includes wide compositional array of these intrusions that can be considered as a case study. Variations in both tectonic and thermobarometric condition for granitic intrusions are studied. Four mappable granitic types are recognized namely monzogranite, syenogranite, alkali feldspar granites, and riebeckite-bearing granites. The monzogranite and the syenogranite of the study area are mostly I-type, whereas the alkali feldspar granite and the riebeckite-bearing granite belong to A-type granitoid. The calc-alkaline intrusions were formed in compressional setting due to decompressional melting of mafic lower crust. Partial melting and anatexing of crustal rocks are suggested to explain the protolith of the alkaline intrusions. The transition from the calc-alkaline magma to the alkaline one occurred as a result of the tectonic transition from compression regime to tectonic relaxation (extension setting) during the last stage of the Pan-African Orogeny. The amphiboles of the studied granites are classified as calcic- and alkali-amphiboles. The calcic-amphiboles are ferro-edenite while the alkali-amphiboles are typically riebeckite. Both amphibole types are of magmatic nature. Coexisting amphiboles and plagioclases are used to estimate the physicochemical parameters of magma crystallization. The syenogranite underwent temperature and pressure of formation range of 520-730 °C, <3 kbar. The alkali feldspar granite records 450-830 °C, <4 kbar, while the riebeckite-bearing granite records the lowest temperature condition among all varieties and estimate formation at 350-650 °C, <4 kbar. © 2013 Saudi Society for Geosciences. Source

Ghoneim M.F.,Tanta University | Noweir M.A.,Tanta University | Abu-Alam T.S.,Tanta University | Abu-Alam T.S.,Norwegian Polar Institute | Abu-Alam T.S.,Egyptian Institute of Geodynamic
Arabian Journal of Geosciences | Year: 2015

A group of intrusive and extrusive igneous rocks is located around Wadi Kariem, Central Eastern Desert. These rocks have diversed petrographic compositions ranging from gabbros to granites with their volcanic equivalents. They belong to four distinct Neoproterozoic units of the Eastern Desert, namely “metagabbros (MG), older granites (OG), metavolcanics (MV) and younger granites (YG)”. Both major and trace elements are compiled to deduce their genetic relationships. 1/Sr versus Rb/Sr and Rb/Ba versus Rb plots suggest that these rock units exhibit comparative magmatic relationships. The trace element data and the numerical modelling are treated according to the general equation of partial melting (Shaw 1970) and Rayleigh equation of fractional crystallization. These rock types favour complex petrogenetic processes during their generation. The magmatic model is based on “in-sequence” genesis between partial melting and fractional crystallization as well as assimilation and/or magma mixing processes in the late stage. It is evident that these rocks resulted from five essential stages during magmatic evolution of the area: (1) Meta-andesite (MV) was generated throughout partial melting (5 %) of the oceanic crust followed by fractional crystallization (25–50 %); (2) Gabbroic rocks (MG) were derived by partial melting (46 %) of the oceanic crust followed by fractional crystallization (20–40 %); (3) Granodiorite (OG) were derived throughout partial melting (6–6.5 %) of gabbroic parent followed by (5–20 %) fractional crystallization; (4) Alkali-feldspar granite (YG) was derived throughout (45–60 %) fractional crystallization of granodiorite (OG); And finally, (5) the recorded hybrid granodiorite rocks (HG) were generated by partial melting of granodiorite (OG) (24.5 %), assimilation and/or partial melting of metagabbros (MG) (11.7 %), followed by magma mixing and (35–55 %) fractional crystallization. © 2015 Saudi Society for Geosciences Source

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