Geochemical constraints from the Hafafit Metamorphic Complex (HMC): Evidence of Neoproterozoic back-arc basin development in the central Eastern Desert of Egypt

Abstract

The Hafafit Metamorphic Complex (HMC) is a part of the Precambrian belt in the central Eastern Desert of Egypt. Two distinct metamorphic units were identified: gneisses and amphibolites. The gneisses are subdivided on mineralogical grounds into granitic gneiss, biotite-gneiss, hornblende-gneiss and psammitic gneiss. Using major elements discrimination criteria to discriminate between orthogneiss and paragneiss, the granitic gneiss shows igneous origin, whereas biotite-gneiss, hornblende-gneiss and psammitic gneiss show sedimentary origin. The mineralogical and chemical compositions of the granitic gneisses indicate that they are tonalitic to trondhjemitic and have compositions consistent with hydrous partial melting of a mafic source, suggesting subduction-related magmatism. Based on Si, Al and alkali contents of paragneisses, the psammitic gneiss could be classified as metamorphosed lithic arenite, whereas biotite- and hornblende-gneisses are classified as metamorphosed greywacke. Sedimentation may have occurred in a back-arc basin setting with transitional deposition from shallow-marine to terrestrial environment. This sedimentation was probably occurred on a tholeiitic basaltic oceanic crust. The amphibolites are subdivided according to mineralogical basis into clinopyroxene-amphibolite, garnet-amphibolite and garnet-free massive amphibolite. Chemical data of amphibolites shows tholeiitic affinity, which suggests a back-arc geotectonic setting. A generation of the leucogranite along thrust zones is related to the late phase of metamorphism of Hafafit rocks. This interpretation is supported by the similarity between metamorphic age and granite emplacement age.