الفهرس 
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Abstract
The Precambrian rocks of the Wadi Ranga area, South Eastern Desert, Egypt, are essentially a Pan-African assemblage comprising a metavolcanic group, intrusive rocks and molasse clastic rocks (Hammamat sediments). The Wadi Ranga metavolcanic rocks are mafic and felsic lavas and pyroclastic rocks. The mafic volcanic rocks cover about 200 km2 and crop out as two large belts, in the central and southern parts of the study area, and comprise massive and pillow lavas and agglomerates. The mafic volcanic rocks include porphyritic and non-porphyritic varities, which are mostly amygdaloidal. The felsic volcanic rocks crop out in the eastern part of the study area, covering about 50 km2 and consist essentially of porphyritic dacite–rhyolite lava flows, lapilli and crystal tuffs and agglomerates. The mafic and felsic lavas are low-K tholeiitic rocks, which have LREE-depleted or almost flat chondrite-normalized REE patterns. The geological and geochemical characteristics of Wadi Ranga volcanic rocks suggest subsea extrusion during primitive island arc stage. The geochemical features of the volcanic rocks indicate ultra-depleted mantle source for the mafic volcanic rocks and origin of the felsic volcanic rocks through fractional crystallization of the mafic magmas. The low-K tholeiitic nature and flat REE patterns of the felsic volcanic rocks of Wadi Ranga indicate that they do not belong to the Dokhan felsic volcanic rocks of Egypt, as previously thought; however they are more akin to the Shadli felsic volcanic rocks.
The intrusive rocks comprise gabbros and granitoids emplaced during oceanic island arc and post-collision stages. The plutonic rocks of the island arc stage include hornblende gabbros (Dabbah pluton), trondhjemite (Abu Ghalaga pluton) and tonalites with subordinate quartz gabbro and quartz diorite (Reidi and Abu Ghusun plutons), whereas the post-collision intrusives include granodiorite and monzogranite (Helifi-Hamata pluton). The gabbros and granitoids of the island arc stage are largely calcic, low-K rocks which have either tholeiitic (gabbro and trondhjemite) or transitional tholeiitic to calcalkaline nature (tonalites). On the other hand, the granitoids of the post-collision stage are medium to high-K calcalkaline rocks. All the investigated granitoids are metaluminous. The spider diagrams and the REE patterns of the gabbro and trondhjemite are similar to those of the Wadi Ranga low-K tholeiitic basalts and silicic volcanics, respectively, suggesting that the gabbro and trondhjemite are the plutonic equivalents of the Wadi Ranga immature island arc extrusives, and they were derived from mantle source at the early immature island arc stage. On the other hand, the tonalites have geochemical characteristics consistent with derivation through high degree partial melting of juvenile basaltic arc crust during a late immature island arc stage. The post-collision granitoids show geochemical characteristics consistent with origin through lower degrees of partial melting of old basaltic arc crust and subsequent fractional crystallization. The trondhjemite and tonalites, and the granodiorite–monzogranite belong to M-type and I-type granitoids, respectively.
The relatively large volume of Wadi Ranga silicic volcanic rocks implies that significant volume of silicic magmas can be generated in immature island arcs by fractional crystallization and indicates the significant role of intra-oceanic arcs in the production of Neoproterozoic continental crust. Silicic magma production through partial melting of the early arc volcanic rocks during the evolution of the arc and the post-collision stage, drives the middle and upper oceanic arc crust towards a composition closer to that of the continental crust. The present study indicates that the intra-oceanic island arcs continued to play a role in the generation of the continental crust after the Archean.
Keywords:
Wadi Ranga, Eastern Desert, Neoproterozoic, Continental Crust, Oceanic Island Arcs, Metavolcanic Rocks, M- and I-types granitoids.