2015 GSA Annual Meeting in Baltimore, Maryland, USA (1-4 November 2015)

Paper No. 287-12
Presentation Time: 11:15 AM


EL-SHAZLY, Aley K., Geology Department, Marshall University, Huntington, WV 25755 and KHALIL, Khalil I., Geology Department, Faculty of Science, Alexandria University, 21511, Moharam Bek, Alexandria, +203, Egypt, elshazly@marshall.edu

The Central Eastern Desert (CED) of Egypt, part of the Arabian Nubian shield, has the following unique features: (i) thirteen bodies of Banded iron formations (BIFs) intercalated with arc – related volcanic and volcaniclastic rocks, all regionally metamorphosed under greenschist to amphibolite facies conditions and intruded by syn- to post- tectonic magmas; (ii) lack of syn- or post-orogenic calcalkalic Dokhan volcanics; and (iii) high abundance of ophiolitic rocks compared to the Northern (NED) and Southern Eastern Desert. Areas to the south of Wadi El-Dabbah in CED are characterized by N to NE – verging overturned to recumbent folds, whereas areas to the north are predominated by SW – directed folding and thrusting. Thermobarometry on metavolcanic and metavolcaniclastic rocks indicates a northward decrease in metamorphic grade from 520 ± 30°C, 5 ± 2 kbar in the south, to 400 ± 50°C, 3.5 ± 2 kbar in northern CED. Magmatic zircons from metavolcaniclastics yield three age populations of 828 ± 5; 772 ± 5; and 728 ± 4 Ma; a few crystals have bright hydrothermal overgrowths that yield ages of 650–720 Ma. Magmatic crystals from felsic intrusions give ages of 680–550 Ma.

We conclude that Wadi El Dabbah represents remnants of an island arc formed on top of a south dipping subduction zone. Oceanic crust in its fore- and back- arc basins formed 850–700 Ma, contemporaneously with arc activity. Three pulses of arc eruptions delivered volcaniclastic debris to these basins, imposing anoxic conditions that led to increased Fe2+ in seawater. During periods of arc quiescence, Fe2+ was oxidized and precipitated as BIF precursor minerals. During the Pan-African orogeny, back arc basin lithosphere and overlying BIFs and volcaniclastics were emplaced northward onto the arc. Stacking of thrust sheets tapering to the north caused regional metamorphism with highest grades in the southern CED. Transpression along the northeastern margin of the arc closed the fore-arc basin, emplacing ophiolites north of Wadi El Dabbah with SW verging folds. Subduction polarity reversal led to intrusion of syntectonic granitoids into the ophiolitic sequence, and eruption of Dokhan volcanics in NED. Ages of c. 680 Ma on syntectonic granitic intrusions suggest that the collisional stage of the Pan-African Orogeny began c.10–40 Ma earlier than previously thought.