Geochemical and Isotopic Constraints on the Origin of Rare-Metal Enriched Alkaline A-Type Granites from the Nubian Shield: Role of Mantle Metasomatism

Abstract

Abstract The U-Pb zircon TIMS age of 637.47 ± 0.23 Ma identifies the oldest anorogenic complex in the northern Nubian Shield as that at Mount Hamr, emplaced as elongated plutons along deep-seated faults, intruding the Pan-African shield rocks. The hypersolvus peralkaline granites of Mount Hamr are dominated by perthite, quartz, and arfvedsonite, along with accessory zircon, chevkinite-(Ce), monazite, aenigmatite, apatite, fluorite, and opaque ilmenite. The rocks are depleted in Al, Ca, and Sr and enriched in Rb, high field strength elements (Zr, 528–1115 ppm), and rare earth elements (ΣREE, 416–1648 ppm), showing fractionated, light rare earth element-enriched patterns [(La/Yb)N = 17]. The rocks are classified as ferroan, reduced A-type granites (A1-subtype) and exhibit age-corrected (143Nd/144Nd)(i) ratios ranging from 0.5115 to 0.5117, with [ϵNd(t) = +5.0 to +5.9] similar to HIMU-OIB, and have lower age-corrected (87Sr/86Sr)(i) ratios (avg. Sr(i) = 0.702). The data yield Nd-TDM2 model ages of 860–929 Ma. High Zr/Hf, (Ce/Pb)N and low Y/Nb, (Th/Nb)N in these rocks reflect OIB-geochemical-signatures. The rocks crystallized at high temperature (TZr = 900–1185 °C) from H2O–depleted melt via extensive low-pressure fractionation of OIB-type parent magma, initially developed from a Na-F-rich metasomatized mantle source. The latter may have led to the formation of similar rare-metal-enriched alkaline intrusions within the vast Arabian-Nubian Shield and possibly within some magmatic provinces occurring in other shields.