PETROLOGY AND GEOCHEMISTRY OF WADI ATALLA EL-MURR METABASALTS, EASTERN DESERT, EGYPT: VOLCANISM IN A MATURE NEOPROTEROZOIC BACK-ARC BASIN

Abstract

The mafic volcanic rocks of Wadi Atalla El-Murr, Eastern Desert, Egypt, comprise aphyric and porphyritic metabasalts composed of variably altered pyroxene and plagioclase. In the porphyritic metabasalts, the phenocrysts are dominated by augite either as individual crystals or as aggregates forming glomeroporphyritic texture. The plagioclases encompass a wide compositional range from labradorite (An = ~73%) to albite (An = 0.61%), reflecting seafloor hydrothermal alteration and metamorphism. The composition of the augite indicates crystallization from a subalkaline magma, overlapping the chemical characteristics of pyroxenes from island arc and ocean-floor basalts. The whole-rock compositions of representative samples reveal that these basalts are characterized by low K2O contents (<1 wt%) and have tholeiitic affinity. The chondrite-normalized REE patterns of the metabasalts display slight enrichment in LREE relative to HREE (La/Ybn = 1.08-1.70) and their MORB-normalized spider diagrams show variable LILE enrichment, variable depletion in most HFSE and a small negative Nb anomaly, suggesting derivation from a mantle source modified by a subduction component. However, these tholeiitic basalts differ in their geochemical features from Neoproterozoic nascent intra-oceanic island arc tholeiites (IAT), and mostly occupy the MORB or BABB field on conventional tectonomagmatic discrimination diagrams. The mixed MORB/IAT geochemical characteristics of the Wadi Atalla El-Murr basalts most closely resemble those of basalts generated in modern back-arc environments affected by minor subduction input, implying eruption during the earliest mature stage of a Neoproterozoic back-arc basin. The present and published data of the metavolcanic rocks of Egyptian ophiolites revealed that they comprise BABB and MORB compositions. The majority of ophiolite metavolcanics of Egypt formed in subduction-related settings, from mantle sources slightly to significantly modified by input of a subduction component, implying generation at different stages of development of Neoproterozoic supra-subduction zone systems.