Petrological and microstructural evolution of syn-tectonic granitoids rocks, Migif-Hafafit area, South Eastern Desert, Egypt
Authors: Thabet Ismail 
Published: July 2014

The studied granitoids cored huge open regional fold trending NW-SE. The granitoids cored domes of the Wadi Hafafit Culmination (WHC) which is bordered by latitudes 24° 35´ 00´´ N and 24° 51´ 00´´ N and longitudes 34° 30´ 00´´ E and 34° 45´ 00´´ E. The basement rocks of the domes consist of banded amphibolites, overthrusted by ultramafic rocks, alternating bands of biotite- and hornblendegneisses and psammitic gneiss at the rim of the domal structure. In some parts, the amphibolites are associated with metagabbro. Granitoids (tonalites granodiorites and quartz diorites) with gneissic texture intrude the cores of the domes The distribution of zircon among the minerals on different types of Hafafit granitoid domes was determined in order to calculate the relative abundance ratio of zircon in the rock, forming minerals and at boundaries of minerals. The following features of zircon crystals were taken into account: colour, inclusions, crystal length, crystal breadth, elongation ratio, crystal habit and forms, and degree of roundness and corrosion. The geochemistry of the granitoids cored in Hafafit domes was studied in order to identify their geochemical behavior, petrochemical characteristics, magma type and tectonic setting. The cored granitoids of Hafafit are classified by the geochemical analyses and their modal composition as tonalites to granodiorites in all domes (A, B, C, D and E) and quartz diorite in dome D. The magma type of the granitoids cored Hafafit domes show a calc-alkaline affinity. The tectonic setting of granitoids (cored Hafafit domes) and their geochemical characteristics are indicative of island arc regime. The Mineral chemistry concluded the granitoids cored Hafafit domes is characterized by more calcic composition in the former rock, and show positive correlation between CaO in the rock and An% in the plagioclase. In order to understand the conditions under which the studied granitoids were crystallized, the corresponding pressure and temperature was determined for each dome. The results showed that the crystallization pressure and temperature of the granitoids range respectively: (i) from 4.7 to 5.3 kbar and from 716oC to 758oC for tonalities and granodiorites of dome A, (ii) from 5 to 6.3 kbar and from 729oC to 766oC for tonalities and granodiorites of domes B&C, (iii) from 4.4 to 4.6 kbar and from 764oC to 770oC for tonalities and granodiorites of dome D, and 3.8 to 5.5 kbar and from 664oC to 688oC for quartz diorite of dome D and (iv) from 4.45 to 6.3 kbar and from 677oC to 689oC for tonalities and granodiorites of dome E. From these it concluded that the tonalities and granodiorites of dome D are characterized by high T and low P conditions, oppositevely to the granitoids of the rest domes(A,B,C and E) that show relatively lower T and higher P. Optical microstructures, which have been formed as a result of deformation, are observed in plagioclase, quartz, biotite and hornblende. The deformation of the biotite and hornblende was intensive in gneissic tonalite which forms the core of the northern dome A and the southern dome E, and in gneissic tonalite and gneissic granodiorite which occupy the cores of the center domes B, C and D of Hafafit. The studied microstructures or substructures include dislocation gliding (synthetic microfractures), undulose extinction, deformation band, deformation twin and subgrain development. The most common substructures observed in plagioclase and quartz includes undulose extinction and subgrain development. The strain analysis showed that the final strain ellipsoids in domes A, B, C and E have oblate and prolate strain symmetry, but the strain ellipsoids in dome D have only oblate symmetry. The four domes A, B, C and E are affected by more than one deformation events related to extension and compression but dome D is affected by one extension deformation event. The domes are not uniform deformed throughout the WHC, according to the radial pattern of stretching lineation and were subjected and affected by late-orogenic extension, controlled by the Najd transform faults and resulted in the exhumation of the WHC domes are oriented NW-SE parallel to the trend of the left-lateral shear zones of Najd fault system. Finaly, we concluded that the tectonic evolution of Wadi Hafafit complex was controlled by five successive stages: -Frist stage: (>700 Ma) this early stage started with rifting and ocean floor spreading and contemporaneously with deposition in an oceanic basin along passive continental margins. - Second stage: (680-700 Ma) subduction took place (during this stage), which resulted in the generation of calc alkaline magmatism and the formation of Hafafit island arc. -Third stage: (670-680 Ma) advanced subduction caused the syentectonic metamorphic phases (M1) and (M2). Early collision between Hafafit island arc and another arc to its northwest lead to phases of shearing and thrusting. This shearing and thrusting have different directions SE or NW and NE or SW as shown by structural analysis. -Fourth stage: (590-600 Ma) late collision took place between Hafafit arc and the other volcanic arc, advancing further to the southwest. During this stage the ophiolitic melange and the Wadi Ghadir ophiolite were obducted onto the eastern part of WHC by regional thrusting. Regional nappe transport occured in NW direction, separating the Nugrus unit metamorphosed under green schist facies conditions (M1) from Hafafit unit metamorphosed under amphibolite facies conditions (M2). -The late fifth stage: (580-590 Ma) orogenic extension and crustal thinning took place, controlled by the Najd transform faults and resulted in the exhumation of the Hafafit domes through a combination of transpression and lateral extrusion. The orientation of C-axis of the two generations zircon crystals is SE-NW, being parallel to the main dominated lineation of hosting minerals (plagioclase, hornblende and biotite). All granitoids domes were subjected to this deformation event, and therefore the orientation of long axis (X) of strain ellipsoid is the same (i.e. SE-NW) in all domes.