干旱区生态环境知识资源中心

Granitic rocks are commonly used as means to study chemical evolution of continental crust, particularly, their isotopic compositions, which reflect the relative contributions of mantle and crustal components in their genesis. New SIMS and K-Ar geochronology, isotope, geochemical, and mineral chemistry data are presented for the granitoid rocks located in and around Gabal Data in the Northern Eastern Desert of Egypt. The granitoid suite comprises quartz diorites, Muscovite (Mus) trondhjemites, and granodiorites intruded by biotite-hornblende (BH) granites and alkali feldspar (AF) granites. Mus trondhjemite, granodiorite and BH granite exhibit I-type calc alkaline affinities. Mus trondhjemite and granodiorite show medium-K calc-alkaline and metaluminous/mildy peraluminous affinities, whereas BH granites have high-K calc-alkaline and metaluminous character. Concordant Pb-206/U-238 weighted mean ages together with geochemical peculiarities suggest that Mus trondhjemites (741 Ma) followed by granodiorites (720 Ma) are genetically unrelated, and formed in subduction-related regime by partial melting of lower oceanic crust together with a significant proportion of mantle melt. The genesis of Mus trondhjemites is correlated with the main event in the evolution of the Eastern Desert, called "similar to 750 Ma crust forming event".

The field and geochemical criteria together with age data assign the high-K calc-alkaline BH granites (608-590 Ma) and alkaline AF granites (600-592 Ma) as post-collisional granites. The differences in geochemical traits, e.g. high-K calc-alkaline versus alkaline/peralkaline affinities respectively, suggest that BH granites and AF granites are genetically unrelated. The age overlap indicating coeval generation of calc-alkaline and alkaline melts, which in turn suggests that magma genesis was controlled by local composition of the source. The high-K calc-alkaline BH granites are most likely generated from lithospheric mantle melt which have been hybridized by crustal melts produced by underplating process. AF granites exhibit enrichment in K2O, Rb, Nb, V. and Th, and depletion in Al2O3, TiO2, MgO, CaO, FeO, P2O5, Sr, and Ba as well as alkaline/peralkaline affinity. These geochemical criteria combined with the moderately fractionated rare earth elements pattern (La-N/Yb-N = 9-14) suggest that AF granite magma might have been generated by partial melting of Arabian-Nubian Shield (ANS) arc crust in response of upwelling of hot asthenospheric mantle melts, which became in direct contact with lower ANS continental crust material due to delamination. Furthermore, a minor role of crystal fractionation of plagioclase, amphibole, biotite, zircon, and titanomagnetite in the evolution of AF granites is also suggested. The low initial Sr-87/Sr-86 ratios (0.7033-0.7037) and positive epsilon(Nd)(T) values (+ 2.32 to +4.71) clearly reflect a significant involvement of depleted mantle source in the generation of the post-collision granites and a juvenile nature for the ANS. (C) 2013 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.