Origin and Evoluti
Origin and Evolution History of Magmatic Garnet-Bearing Pegmatites and Associated Granitoids, Abu Had Area, South Eastern Desert, Egypt: Inference From Petrology and Geochemistry.
Moustafa Esmail Gharib
Geology Department, Faculty of Science, Helwan University, Egypt. email@example.com
Abstract: Garnet is an uncommon accessory mineral in igneous rocks but is petrologically significant. Pegmatite hosted garnet in Abu Had area is a zoned type and concentration of garnet increase toward the core zone. They intruded the gneissose tonalite-granodiorite and metagabbro country rocks parallel tothe Wadi El Gemal major thrust. Isolated large elongated masses or dyke-like bodies of leucogranite intrude the older gneissose tonalite-granodiorite and the metagabbro country rocks along the major thrust fault of Wadi El Gemal in NE-SW direction.
The garnets of Abu Had pegmatite have frequently euhedral shape. They are spessartine-almandine rich type displaying reversal zoning with almandine-rich cores and spessartine-rich rims and plotted in the field of magmatic garnet derived from peraluminous S-type granites. Chondrite-normalized REE patterns display a strong enrichment of HREE relative to LREE, and pronounced negative Eu anomaliescharacteristic features of spessartine-rich garnets crystallized from silicic melts and of garnet in peraluminous granites. The gneissose tonalite-granodiorite suite has a calc-alkaline metaluminous affinity. It displays the characteristics of I-type granites formed in an island-arc environment and were derived through partial melting of amphibolite source rocks. The leucogranites posses high SiO2, Al2O3and alkali content, with low TiO2, MgO, CaO and Fe2O3 contents and strongly peraluminous (A=CNK>1.1) characters. They have the geochemical characters of S-type granites and most likely formed in syn-collision regime. Crustal thickening during the collision stage of the Arabian Nubian shiled led to partial- melting of metasedimentary source and other associated crustal rocks along the NW- SE Wadi El Gemal major thrust. Textural features of the studied garnet-bearing pegmatites such as micrographic intergrowth and absence of myrmekite, beside the data of mineral chemistry for the fresh rock-forming minerals support magmatic origin for these pegmatites. Compositional data of primary minerals from the Abu Had pegmatite proved peraluminous source rock magmas. The close spatial distribution of the garnet-bearing pegmatites to the leucogranite country rocks, textural and mineralogical similarity between the leucogranite and the neighboring pegmatites, beside the data of whole rock composition of the leucogranite probably indicate that these rocks are direct precursors to the garnet- bearing pegmatites of Abu Had area.Field and petrographical observations indicate that the garnet-bearing pegmatite affected by post magmatic deformation and metasomatism which often restricted to the microshear zones and fracture sets associated with them. Finally, pinch and swell of the pegmatite dyke, besides cracking and stretching of some garnet crystals in direction perpendicular to the direction of microshear zones, prove that the study pegmatite and the associated country rocks exposed to extension tectonic event post-date the formation of garnet and the associated primary minerals.
[Moustafa Esmail Gharib. Origin and Evolution History of Magmatic Garnet-Bearing Pegmatites and Associated Granitoids, Abu Had Area, South Eastern Desert, Egypt: Inference From Petrology and Geochemistry. J Am Sci 2012;8(10):536-554]. (ISSN: 1545-1003). http://www.jofamericanscience.org. 77
Keywords: Magmatic garnet, pegmatite, Abu Had, Wadi El Gemal thrust, Arabian Nubian Shield Full Text 77