Abstract
This article documents the evolution of pressure and temperature conditions and the successive influence of hydrous melts and aqueous fluids on the operation of extensional shear zones, which produced exhumation of mantle slivers from deep lithospheric or asthenospheric depths, nearly reaching the surface, in a rift-to-drift setting. These results are based on a re-analysis of 40 samples from the three peridotite massifs of Zabargad island in the northern Red Sea. By integrating high-resolution mapping of the microstructure by electron backscattered diffraction with recent developments in barometry for plagioclase- bearing peridotites and in thermodynamic modelling of peridotitic compositions, this study: (1) constrains the temporal and spatial evolution of petrological and tectonic processes in the shallow mantle during rifting and (2) documents the presence of melts or aqueous fluids throughout the functioning of the shear zones, unravelling substantial feedback between petrological and tectonic processes. Thermobarometry and thermodynamic modelling constrained by the microstructural observations document progressive strain localization during shearing, which occurred under decreasing pressure and temperature from near solidus conditions at >1 GPa (north and central peridotite massifs) or ~0.7 GPa (southern massif) to < 600°C and <0.3 GPa in all three massifs. The data substantiate local aqueous fluid saturation in the shear zones with increasing hydration during their operation, indicating fluid focusing, with seawater ingress extending to 10 km depth. The presence of melts or fluids enabled concurrent dislocation and dissolution-precipitation creep, which resulted in weakening in the shear zones. However, fluid supply was spatially heterogeneous and likely intermittent, with equilibrium achieved only locally in the ultramylonites. The present study documents therefore how progressive strain localization and fluid-focusing in extensional shear zones may contribute to the thinning and exhumation of the mantle during continental rifting and the rift-to-drift transition.