Geological Society of America

266 Precambrian Geology

Wednesday, 21 October 2009
1:30 PM - 5:30 PM
B117/118/119

U-PB ZIRCON AGES AND GEOCHEMISTRY OF THE NEOPROTEROZOIC OPHIOLITE ASSEMBLAGES FROM WADI ALLAQI, SOUTH EASTERN DESERT OF EGYPT

Paper number: 266-7
Program: Technical Sessions
Time: 3:00 PM – 3:15 PM

Authors

K.A. Ali Presenting Author
University of Texas at Dallas, Richardson, TX
M.K. Azer
National Research Centre, Cairo
H.A. Gahlan
Assiut University, Assiut, N/A
S.a. Wilde
Curtin University of Technology, Perth, N/A
Robert Stern
University of Texas at Dallas, Richardson, TX
M.D. Samuel
National Research Centre, Cairo, N/A

Abstract

Neoproterozoic ophiolites cover an extensive area in the Eastern Desert of Egypt. These assemblages are part of nappe complexes that were thrust over continental margin type sediments and arc terranes.

We report new U-Pb zircon SHRIMP ages from the southernmost ophiolites in Egypt, along Wadi Allaqi near the border with Sudan. Layered gabbro associated with the ophiolite in Wadi Abu-Fas yielded a concordia age of 731 ± 6 Ma, while a gabbro sample associated with ophiolite nappe from Jabal Moqsim ~ 12 km to the east yielded a concordia age of 700 ± 7 Ma. Zircon separated from a quartz diorite associated with Wadi Umm Ashirah ophiolite nappe at the entrance of Wadi Allaqi near Lake Nasser yielded a concordia age of 709 ± 4 Ma. An intermediate metavolcanic sample from an inferred arc calc-alkaline and subvolcanic succession from Wadi Shilman ~ 50 km north of Wadi Allaqi, yielded a weighted mean ²⁰⁶Pb/²³⁸U age of 733 ± 7 Ma; a post-tectonic granite intruded into the ophiolite nappe from the same region yielded a concordia age of 629 ± 5 Ma.

Published ages for two ophiolite occurrences in the Eastern Desert of Egypt are established by zircon dating, 746 ± 19 Ma for a plagiogranite sample from Wadi Ghadir ophiolite (Pb-Pb zircon evaporation age) ~ 300 km north of Wadi Allaqi, and 736.5 ± 1.2 Ma for a gabbro sample from the Fawakhir ophiolite (U-Pb zircon TIMS age) ~ 500 km north of Wadi Allaqi similar to our new zircon data from the Wadi Allaqi ophiolite. Published εNd value from Jabal Gerf gabbro samples in Wadi Allaqi vary between +6.5 and +8.9, indicate that the rocks were derived from depleted mantle.

Our new data combined with published zircon ages and Sm/Nd isotope data further indicate that a major event between ~ 750 and 700 Ma ago involving ophiolite generation and eruption of juvenile melts was responsible for the formation of the crust of this part of NE Africa.