Late Neoproterozoic volcanics and associated granitoids at Wadi Ranga, south Eastern Desert, Egypt: A transition from subduction related to intra-arc magmatism

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• 作者：Moustafa Esmail Gharib ; ^{gharibme@yahoo.com} ; Ahmed Hassan Ahmed¹
• 关键词：Wadi Ranga volcanics ; Intra-arc basin ; Pitchstone porphyry ; Granite porphyry ; Egypt
• 刊名：Lithos
• 出版年：2012
• 出版时间：15 December, 2012
• 年：2012
• 卷：155
• 期：Complete
• 页码：236-255
• 全文大小：3607 K

文摘

The late Neoproterozoic of Wadi Ranga area, south Eastern Desert was characterized by extensive magmatism consisting predominantly of calc-alkaline older granitoids, Ranga volcanics and granite porphyry intrusions forming a belt of volcanic-intrusive complexes. Geological studies of the Ranga volcanics revealed two sequences of volcanic suites: the older mafic sequence composed essentially of basalt/basaltic andesite lava flows and the associated pyroclastics, while the younger felsic one represented by pitchstone porphyry. Geochemically, the older granitoids are exhibit low K-calc-alkaline characters and most likely formed in a convergent margin (arc) tectonic environment. Basalt/basaltic andesites are compositionally uniform with tholeiitic affinity. They have trace element characteristics of arc related volcanics with low Nb (0.2-0.9 ppm), Zr/Nb ratios of 40-81 with an average 54 compared to 32 in average N-MORB, signify a convergent margin setting with a depleted mantle wedge. Low Mg# (45 to 60 with an average 51), Co and Ni contents may be indicative of a non-plume-influenced mantle wedge. Compositionally, these mafic volcanics are commensurate with modern intra-arc basalts. REE modeling indicates that the magma from which these mafic volcanics were formed was generated by about 17-30 % partial melting of a depleted mantle that composed mainly of spinel lherzolite in an extensional marginal basin. Geochemical characters of major, trace and REE patterns of the granite and pitchstone porphyries proved their formation from the same magma source which is originally different from the magma producing the mafic volcanics. The relative enrichment in K₂O and LREE of the pitchstone porphyry along with the depletion in CaO and MgO relative to granite porphyry probably resulted from crustal contamination processes.