Characteristics and Origin of Egyptian Younger Granites

The post-tectonic Younger Granite plutons of Egypt are products of a single magmatic episode 603 to 575 m.y. ago which marked the end of a “cratonizing process” and the beginning of truly anorogenic activity in the Egyptian Red Sea Hills exposure of the Nubian-Arabian Shield. Prior to Younger Granite magmatism, the region is interpreted as having been the locus of island-arc, small ocean-basin development with later compressive coalescence of arcs and basins. This early period was characterized by the formation of deformed eugeosynclinal sediantary and volcanic rocks, ophiolites, and orogenic batholiths. A tectonic environment including the deposition of terrigenous sediments in isolated, fault-bounded basins and intermediate to felsic volcanism followed the culmination of orogenic activity. The Younger Granites intrude all of these earlier units. After the time of Younger Granite intrusion and prior to Red Sea rifting, magmatism was limited to minor alkaline volcanism and intrusions ranging from early Paleozoic to Tertiary in age. The Egyptian Younger Granites occur as three petrologic-geochemical groups, but all three are post-tectonic and epizonal. Other characteristics common to all groups are homogeneity and an absence of tectonic foliation and pegmatites. Younger Granite quartz veins occur in place of pegmatites, which are common in older, orogenic plutons. The modal abundances of quartz, plagioclase, and alkali-feldspar in the Younger Granites classify them as true granites with a few granodiorite samples among the least differentiated group III plutons. Group I granites have been extensively albitized and silicified, resulting in the modification of original hypersolvus quartz-feldspar fabrics. Group III rocks are strictly subsolvus and contain larger proportions of plagioclase (other than hydrothermal-metasomatic albite) and mafic mineral phases than do groups I or II. Group II granites are similar to group I but are less albitized and have higher mafic contents. The eighteen plutons studied range from peraluminous to slightly peralkaline granitic rocks. Group III plutons are the most mafic, with high MgO, total Pe, CaO, TiO 2 , A1 2 O 3 , and Sr contents. Group I granites are very siliceous (to 80% SiO 2 ) and also contain the most Rb, Y, Nb, and U, all reflecting their hydrothermal alteration, Group I and, to a lesser extent, group II are very depleted in MgO, TiO 2 , and Sr relative to “calc-alkaline granites.” Taken as e whole, the analyzed Younger Granite groups are not as rich in large ion lithophylic (LIL) elements as similar post-tectonic plutons from other areas. Egyptian Younger Granites typically have low 87 Sr/ 86 (0.701 − 0.7025) and are not associated with mafic differentiates nor with other diverse rock types often found in many post-tectonic alkali granite provinces. From the available data, the Younger Granites can be considered as low-temperature products of a eutectic partial-melting process that took place in depleted upper mantle or lower crustal material. The postulated geologic history of the region prior to Younger Granite intrusion indicates that the source of the granite melts was probably “cratonized” oceanic, continental margin, or intra-oceanic are and basin material. The chemistry and tectonic setting of the Younger Granites strongly demonstrates their anorogenic character and tends to argue against their derivation from subduction-zone activity.