Petrogenesis of a rare Ediacaran tonalite-trondhjemite-granodiorite suite, Egypt, and implications for Neoproterozoic Gondwana assembly

Abstract

Most tonalite-trondhjemite-granodiorite (TTG) suites are Archean-Palaeoproterozoic in age, but those of Neoproterozoic-Phanerozoic age are scarce. A rare Ediacaran high-Al TTG suite has been identified at the Fannani Igneous Complex (FIC) in the northern Arabian-Nubian Shield, which is essentially composed of amalgamated Neoproterozoic island-arc Pan-African composite terranes that contain several ophiolitic sutures. The FIC exhibits a wide range of SiO2, Al2O3, Sr and Zr, shows moderate rare earth element (REE) enrichment, and K, Ti, Nb, Y and heavy REE depletion. It is a subsolvus suite with clear orogenic affinities and strong arc-geochemical signatures. The precise U-Pb zircon thermal ionization mass spectrometry age obtained (607.4 ± 1.95 Ma) indicates oceanic subduction extended to late stages of the East African Orogeny. The FIC exhibits 87Sr/86Sr compositions of 0.70346-0.71091 (Sr(i) ratio, 0.70284), and 143Nd/144Nd of 0.51254-0.51270 ( Nd(t) = +5.12 to +7.16), typical of modern oceanic-arc rocks (as Japan-arc basalts), and suggestive of mantle sources and island-arc settings. The FIC possesses low values of Yb (1.55 ppm), Nb (14 ppm) and Y (24 ppm), and high ratios of Sr/Y (27), Zr/Sm (46) and Nb/Ta (11.8), typical of magmas produced by anatexis of a basaltic slab. Partial melting models show that the FIC magma was generated by melting (F = 0.25-0.50) of a subducted oceanic crust transformed into eclogite, leaving 10-25% garnet in the residue. The FIC and similar complexes produced via slab melting during the closure of the Mozambique Ocean formed large juvenile belts along the East African Orogen that sutured East and West Gondwana together into a united supercontinent. © The Author(s), 2020