The Neoproterozoic layered mafic-ultramafic intrusion of Gabal Imleih, south Sinai, Egypt: Implications of post-collisional magmatism in the north Arabian-Nubian Shield

摘要

The Imleih layered mafic-ultramafic body is a Neoproterozoic intrusion covering about 45 km~2 in south ern Sinai. The field relations as well as the fine grained chilled margins indicate that the layered intrusion is younger than the surrounding calc-alkaline syn-tectonic granodiorite and Iqna Shara volcanics and older than the post-tectonic Iqna granite. There is a general consensus that there are no ophiolitic rocks younger than the syn-tectonic granitoids in the Arabian-Nubian Shield (ANS), which supports the non-ophiolitic origin of the Imleih layered intrusion. The studied layered intrusion is tilted to the north, with lower layers (peridotite and pyroxenite) exposed to the south and upper layers (cumulus gabbro and anorthosite) exposed to the north. Imleih rocks are cumulates, dominated by cumulus gabbro with sub sequent amounts of peridotite, pyroxenite and anorthosite. Pegmatitic gabbro is locally observed. These rocks are unmetamorphosed. In the peridotite, the Fo content of olivine ranges from 0.81 to 0.83. Clino-pyroxenes of the studied rocks range in composition from diopside to augite and have subalkaline affin ities. Plagioclase in the cumulus gabbro is mostly bytownite (An_(73-82)). Two types of spinel are observed, chromite and pleonaste. Chromite has Cr# (Cr/(Cr +Al) of 0.60-0.81 whereas pleonaste have Cr# lower than 0.02. The present study revealed that the mafic-ultramafic complex of Gabal Imleih fractionated from a single batch of high Al-basaltic magma and evolved by fractional crystallization at the base of a Neoproterozoic magma chamber. Olivine and chromite were the first crystallized phases followed by pyroxene and plagioclase. Amphibole appears at nearly the same time as plagioclase. The Imleih mafic-ultramafic body was emplaced post-tectonically in a transitional phase between the calc-alkaline and alkaline magmatism.