Geochemistry and petrogenesis of late Neoproterozoic peraluminous anorogenic leucogranite, Zabara area, Central Eastern Desert, Egypt

摘要

The Neoproterozoic basement in the Zabara area, central Eastern Desert of Egypt, comprises metasedimentary rocks, an ophiolitic sequence (metagabbro, metabasalt and serpentinite), "Shadli" younger metavolcanic rocks and leucogranite. The leucogranite samples have high Al_2O_3 contents (> 14 wt percent), an alumina saturation index (ASI) mostly > 1.1, and normative corundum (-4 percent), indicating a peraluminous affinity. They exhibit geochemical features of highly fractionated calc-alkaline magmatic rocks and A-type granitoids as can be inferred from their high concentrations of SiO_2, Na_2O + K_2O, Y, Rb, and Nb and low contents of CaO, TiO_2, MgO, P_2O_5, Sr, and Ba. Chondrite-normalized rare earth element patterns show, on average, light rare earth element enrichment (La/Sm_N = 3.7), mostly unfractionated heavy rare earth element patterns (Gd/Lu_N = 0.4), and pronounced negative Eu anomalies. The latter suggest a significant plagioclase fractionation. The overall uniform composition of the leucogranite samples, peraluminous nature and enrichment of incompatible elements (K, Rb, Ba and Th) strongly suggest a homogeneous source. Trace element evidence suggests that fractional crystallization of plagioclase, K-feldspar, biotite, Fe-Ti oxide with minor hornblende from a mafic magma may have produced the leucogranite in a shallow-level crastal magma chamber.