干旱区生态环境知识资源中心

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 nonplume-influenced mantle wedge. Compositionally, these mafic volcanics are commensurate with modem 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 Iherzolite 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. Published by Elsevier B.V.