Geochronological and Petrological Constraints on the Evolution of the Pan African Ajjaj Shear Zone, Saudi Arabia

Abstract:

In the active tectonic regions, shear zones play an important role to re-configure the structure of the lithosphere. One of the largest shear zones on the Earth is the Najd Fault System of the Arabian-Nubian Shield. Literature data record the main active phase of this shear zone during the last stages of the Pan-African Orogeny (ca. 630 - 540 Ma). The Najd Fault System is composed of several shear zone segments, one of them is the Ajjaj shear zone. Determination of the age of variably deformed intrusions is expected to give approximated age of deformation in Ajjaj shear zone. Six samples of intrusive rocks showing variable composition were used to illustrate the time progress and evolution of the Ajjaj shear zone. One sample is from a very coarse grained diorite lying within the Ajjaj shear zone. It has very weak deformation and produces an intercept U-Pb zircon age of 696 ± 6 Ma. Two samples are from granodiorite-tonalite intrusions to the tenant of the Ajjaj shear zones. They show conspicuous degree of deformation and define two U-Pb clusters of concordia ages at 747 ± 12 Ma - 668 ± 8 Ma and 742 ± 5 Ma - 702 ± 12 Ma. Three samples are granites from variable plutons along the Ajjaj shear zone. Two of them show mylonitic foliation of flattened quartz and platy minerals such as biotite parallel to the main deformation trend of the shear zone. They yield U-Pb ages of 601 ± 6 Ma - 584 ± 3 Ma. The third sample is undeformed and has a cross-cut contact relationship with the foliation of the Ajjaj shear zone. It yield concordia ages of 581 ± 4 Ma. These data confine the activity of the Ajjaj shear zone to a limited period between 605 Ma and 577 Ma. As the activity of the Ajjaj shear zone was responsible for the exhumation of the Hamadat metamorphic complex, we also constrained the vertical motions that occurred during the shear zone activity using mmetamorphic rocks. It is shown that peak metamorphism occurred around 505 - 700 ºC at two ranges of pressure 8 - 11 and 14.5 ± 2 kbar with highest pressure rocks being central to the shear zone and lower pressure occurring in more distal parts. This suggests exhumation from about 44 - 58 depth with the largest exhumation depths occurring in the most central part of the shear zone.