Neoproterozoic contaminated MORB of Wadi Ghadir ophiolite, NE Africa: Geochemical and Nd and Sr isotopic constraints

• Published in: Journal of African earth sciences (1994) 2011-02, Vol.59 (2), p.227-242
• Main Authors: Basta, Fawzy F., Maurice, Ayman E., Bakhit, Bottros R., Ali, Kamal A., Manton, William I.
• Format: Article
• Language: English
• Subjects: Contaminated MORB; Eastern Desert; Geochemistry; Marine; Neoproterozoic; Sr and Nd isotopes
• ISSN: 1464-343X; 1879-1956
• DOI: 10.1016/j.jafrearsci.2010.10.008

► Similar to MORB ophiolites, the metabasalts of Wadi Ghadir are characterized by high TiO 2 values. ► Enrichment in LREE, Th and U indicate that Wadi Ghadir metabasalts are similar to contaminated MORB. ► The variable LILE-enrichment is due to source & crustal contamination & post-magmatic alteration. ► The low ε Nd(t) values of some samples reflect their contamination by slightly older arc crust. ► The degree of contamination of the Eastern Desert ophiolites increases from south to north. The ophiolitic metabasalts (pillowed and sheeted dikes) of Wadi Ghadir area, Eastern Desert, Egypt, were analyzed for their major, trace and rare earth elements, Nd and Sr isotopes and the chemistry of their plagioclase, amphibole and chlorite was also reported. Geochemically these rocks range from tholeiitic basalt to basaltic andesite. The generally low MgO, Cr and Ni and high Zr contents are consistent with derivation of these rocks from an evolved magma. The high TiO 2 contents (mostly between 1.76% and 2.23%) classify Wadi Ghadir ophiolitic metabasalts as MORB ophiolite. The chondrite-normalized REE patterns of most samples display small LREE-enrichment with (La/Yb) n ranging from 1.44 to 2.56. The MORB-normalized spider diagram shows variable LILE abundances, which are either similar to or enriched relative to MORB, and most samples display small Nb depletion. The abundances of some LILE (Ba, Rb and K) as well as Na and Si were modified by post-magmatic seafloor hydrothermal alteration. Enrichment of the least mobile LILE (Th & U) indicates that Wadi Ghadir ophiolitic metabasalts are akin to C (contaminated)-MORB. These geochemical characteristics are similar to BABB modified by contamination. Wadi Ghadir metabasalts have low initial Sr ratios (0.7010–0.7034) which are similar to those of MORB, while their ε Nd( t ) values (+7.7–+4.5) are either more or less positive than the value of depleted mantle (DM). The more positive ε Nd( t ) values indicate DM source for these basalts, while the less positive ε Nd( t ) values reflect the involvement of slightly older component in Wadi Ghadir ophiolite. We suggest that the parent magma of metabasalts was contaminated by slightly older material, most probably oceanic-arc crustal rocks, which caused enrichment in LREE, and by analogy LILE, but did not significantly affect Nd isotopic systematics or modify Sr isotopes. Such contaminated MORB character also revealed by other ophiolitic metavolcanics in the Central Eastern Dese