Neoproterozoic nascent island arc volcanism from the Nubian Shield of Egypt: Magma genesis and generation of continental crust in intra-oceanic arcs

Abstract

The Neoproterozoic Wadi Ranga metavolcanic rocks, South Eastern Desert of Egypt, constitute a slightly metamorphosed bimodal sequence of low-K submarine tholeiitic mafic and felsic volcanic rocks. The mafic volcanic rocks are represented by massive and pillow flows and agglomerates, composed of porphyritic and aphyric basalts and basaltic andesites that are mostly amygdaloidal. The felsic volcanic rocks embrace porphyritic dacites and rhyolites and tuffs, which overlie the mafic volcanic rocks. The geochemical characteristics of Wadi Ranga volcanic rocks, especially a strong Nb depletion, indicate that they were formed from subduction-related melts. The clinopyroxene phenocrysts of basalts are more akin to those crystallizing from island-arc tholeiitic magmas. The tholeiitic nature of the Wadi Ranga volcanics as well as their LREE-depleted or nearly flat REE patterns and their low K 2O contents suggest that they were developed in an immature island arc setting. The subchondritic Nb/Ta ratios (with the lowest ratio reported for any arc rocks) and low Nb/Yb ratios indicate that the mantle source of the Wadi Ranga mafic volcanic rocks was more depleted than N-MORB-source mantle. Subduction signature was dominated by aqueous fluids derived from slab dehydration, whereas the role of subducted sediments in mantle-wedge metasomatization was subordinate, implying that the subduction system was sediment-starved and far from continental clastic input. The amount of slab-derived fluids was enough to produce hydrous magmas that follow the tholeiitic but not the calc-alkaline differentiation trend. With Mg# > 64, few samples of Wadi Ranga mafic volcanic rocks are similar to primitive arc magmas, whereas the other samples have clearly experienced considerable fractional crystallization. The low abundances of trace elements, together with low K 2O contents of the felsic metavolcanic rocks indicate that they were erupted in a primitive island arc setting. The felsic volcanic rocks are characterized by lower K/Rb ratios compared to the mafic volcanic rocks, higher trace element abundances (~ 2 to ~ 9 times basalt) on primitive arc basalt-normalized pattern and nearly flat chondrite-normalized REE patterns, which display a negative Eu anomaly. These features are largely consistent with fractional crystallization model for the origin of the felsic volcanic rocks. Moreover, SiO 2-REE variations for the Wadi Ranga volcanic rocks display steadily increasing LREE over the entire mafic to felsic range and enriched La abundances in the felsic lavas relative to the most mafic lavas, features which are consistent with production of the felsic volcanic rocks through fractional crystallization of basaltic melts. 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. We emphasize that the geochemical characteristics of these rocks such as their low LILE and nearly flat REE patterns can successfully discriminate them from other Egyptian Neoproterozoic felsic volcanic rocks, which have higher LILE, Zr and Nb and fractionated REE patterns.