Geochemical and K–Ar age constraints on the Late Neoproterozoic (?) gneisses at Um Tenassib area, north Eastern Desert, Egypt

Abstract

Wadi Um Tenassib metamorphic rocks consist mainly of biotite gneiss and biotite-hornblende gneiss with subordinate intercalations of amphibolite, migmatitic gneiss, and aplitic granite. Biotite-hornblende gneiss, biotite gneiss, and aplitic granite are geochemically characterized and their cooling ages are determined by using the K–Ar method on biotite. The Um Tenassib gneisses (UTG) range in composition from quartz diorite/monzodiorite to granodiorite. They were derived from igneous rocks that pertain to calc alkaline and metaluminous to weakly peraluminous affinities and were generated in continental volcanic arc setting. REE patterns of the UTG are moderately fractionated (La N/Lu N = 5.9–7.5) relative to those of the aplitic granite (La N/Lu N = 33). The similarity in the geochemical characteristics and REE patterns of both gneiss types indicate their magmatic consanguinity. Amphiboles of the UTG biotite-hornblende gneisses are mainly hornblende, together with few paragasitic hornblende and edenite. Plagioclase composition is oligoclase to andesine (An 21–46) in the biotite-hornblende gneiss, and oligoclase (An 11–26) in the biotite gneiss. Mineral chemistry of amphibole and plagioclase indicate that the gneisses were metamorphosed under low- to medium-pressure of 2.6–6.4 kbar and at medium to high temperatures of 660–755 °C. The K–Ar biotite cooling ages (seven samples) range from 585 ± 12 Ma to 598 ± 12 Ma for the UTG, except one biotite-hornblende gneiss sample gives age of 577 ± 11 Ma. These ages suggest a latest metamorphic cooling event at ca. 585–600 Ma time span, which is consistent with the proposed cooling ages of ∼600 Ma for the Elat metamorphic rocks [Cosca, M.A., Shimron, A., Caby, R., 1999. Late Precambrian metamorphism and cooling in the Arabian–Nubian Shield: petrology and 40Ar/ 39Ar geochronology of metamorphic rocks of the Elat area (southern Israel). Precamb. Res. 98, 107–127]. It may indicate that the metamorphism of the UTG might have been contemporaneous with the suggested regional metamorphism at 620 ± 10 Ma for Sinai metamorphic rocks (Cosca et al., 1999) and/or the emplacement age at 614 Ma for the granodiorite in the study area [Stern, R.J., Hedge, C.E., 1985. Geochronological and isotopic constraints on the Late Precambrian crustal evolution in the Eastern Desert of Egypt. Am. J. Sci. 285, 97–127]. These ages also lie within the range of magmatic activity of the Younger Granites in the North Eastern Desert (575–600 Ma).