Geological Studies on the Abu Fannani Mylonitic Amphibolites and Related Amphibolite Xenoliths in the Juvenile Neoproterozoic Meatiq Dome Area, Central Eastern Desert, Egypt.

Abstract

The Neoproterozoic crustal rocks exposed in the Meatiq dome area, Central Eastern Desert of Egypt, comprise medium -to high -grade mylonites. They form strongly foliated and folded mappable units, including (in the core): Um Baanib granite-gneiss, followed structurally upward by Fiadiya augen schists, Gabal Meatiq phyllonitic mylonites, Abu Zohleiqa augen gneisses, Um Esh El-hamra quartzofeldspathic mylonites, and Abu Fannani mylonitic amphibolites. These appear to have been largely derived from granitic, pelitic to semipelitic, and basaltic to basaltic andesite rocks. The present work is focused only on the Abu Fannani mylonitic amphibolites which occur mainly in the shear zone constituting the frame of the Meatiq Dome. The amphibolite xenoliths hosted by the Um Baanib granite-gneiss are supposed to be originally belonging to the Abu Fannani unit. Field and petrographic characteristics of both amphibolitic types indicate that they were undergone a complex evolution of repeated deformations and metamorphisms. These were genetically connected with three major events, including subduction, obduction and updoming processes. Due to subduction, the original basaltic to basaltic andesites, together with the pelitic to semipelitic sediments formed a strongly folded and highly metamorphosed (M 1 ) orogenic belt. The lower part of this belt was migmatized and intruded by a synkinematic granite (i.e., Um Baanib granite-gneiss). During obduction, thick nappes overrode the highly metamorphosed belt leading to its transformation into high-to mediumgrade mylonites having the amphibolite facies grade metamorphism (M 2 ). Thereafter, updoming processes originally linked with the Najd Fault System took place where brittle-ductile shear zones accompanied with greenschist facies metamorphism (M 3 ) were achieved. Regarding geochemistry of the amphibolites, major elements refer to their derivation from tholeiitic basalt to basaltic andesites. In Harker variation diagrams, immobile and incompatible elements (Zr,Ti,Y, and P) show good linear correlation relative to the mobile elements. Tectonic discrimination diagrams suggest the presence of two tectonic settings for both amphibolites, i.e., within-plate and MORB environments.