Geochemistry, age and origin of the Mons Claudianus TTG batholith (Egypt): insight into the role of Pan-African magmatism in uniting plates of Gondwana

摘要

The tonalite-trondhjemite-granodiorite (TTG) Mons Claudianus Batholith (MCB) of Egypt is subsolvus, metaluminous to mildly peraluminous, exhibits wide ranges of SiO2, Al2O3, Sr, Rb, Zr, shows large Ba enrichment, is moderately enriched in rare earth elements (REE) and is depleted in K, Ti, Nb, Y, Hf and heavy REE (HREE), reflecting strong arc geochemical signatures. Moderate fractionation of REE and lack of Eu anomaly characterize the MCB. It is typical of high-Al TTGs of volcanic-arc affinities. U-Pb-zircon dating produced a Pan-African age of 664.12 +/- 0.38 Ma. The MCB exhibits Sr-87/Sr-86 isotopic compositions of 0.70352-0.70626 (initial Sr ratio of 0.70259) and Nd-143/Nd-144 ratios of 0.51261-0.51276 (epsilon(Nd); -0.5 to +2.4), suggestive of a mantle source. Anatexis of a basaltic slab under eclogitic conditions leaving garnet in the residue produces high-Al TTG rocks characterized by low Yb values (<1.8 ppm). Values (in ppm) of Yb (0.65-1.8), Y (2.2-19), Nb (1.2-6.4), and ratios of Nb/Ta (7-17), (La/Yb)(N) of 11.7, Sr/Y and Zr/Sm (58 and 45, respectively) are all consistent with anatexis of a basaltic slab under eclogitic conditions, leaving garnet in the residue to produce this high-Al TTG suite. The data conform to magma generation via partial melting (F = 0.25-0.50) leaving 15-25 % garnet in the residue. Voluminous synorogenic magmatic pulses, resulting from slab melting during the closure of the Mozambique Ocean via convergence of east and west Gondwana, produced the MCB and similar large batholiths forming the core of Pan-African belts. These belts welded together vestiges of fragmented Rodinia, assembling them into a united Gondwana.