Origin of low-18O metamorphic rocks from a Late Proterozoic shear zone in the Eastern Desert of Egypt

Abstract

Anomalously low and variable 18O/16O ratios (whole rock δ18OSMOW=−1.9 to +6.0) have been measured in several Late Proterozoic metamorphic units associated with a large scale low-angle ductile shear zone at Meatiq Dome in the central Eastern Desert of Egypt. The low-18O units consist of granite gneiss, quartzofeldspathic mylonite, and quartz phyllonite. These are intruded by syntectonic diorite-tonalite-granodiorite and post-tectonic granite, both having normal δ18O. No evidence for other low-18O country rock units has been found. Oxygen isotope fractionation between quartz and magnetite in the granite gneiss (ΔQZ-MT=8.0 to 9.3) is consistent with equilibration at amphibolite grade temperatures, suggesting that 18O depletion occurred prior to metamorphism. Whole rock geochemical analyses indicate that the low-18O units were derived from two granitic protoliths, one of which apparently underwent local sericitic alteration prior to metamorphism; it is proposed that 18O depletion and sericitic alteration occurred simultaneously in a high-temperature meteorichydrothermal system associated with emplacement of these granitic protoliths. The relatively high ductility of the sericitically altered rock may have been an important factor in the subsequent localization of the shear zone.