Structural geology of felsic plutonic rocks in the Arabian Shield; styles, modes and levels of emplacement

Abstract

Previous attempts to relate Arabian granite types to tectonic setting, depth and time of emplacement relative to orogeny are simplistic. They give little importance to the role of lineaments in magma genesis and ascent, do not consider the style of emplacement, either forceful or passive, and over-generalize the relationship between pluton emplacement and regional deformation. Whereas a great deal is known of the geochemistry and geochronology of these rocks, very little has been reported of their structural geology. Depth, style and time of emplacement provide an additional frame of reference through which to view these felsic plutonic rocks. Although plutons with features of forceful catazonal emplacement are reported in the southern Arabian Shield, they were predominantly emplaced transitionally between the catazone and mesozone. True catazonal plutons are rare, but parts of the Shield, notably the northern and eastern areas, contain country rocks of sufficiently high metamorphic grade to indicate that detailed mapping may eventually identify more. Diapiric plutons in the central and eastern Shield are examples of forceful, post-tectonic emplacement in the mesozone and, transitionally, between the mesozone and epizone. Passive, mesozonal emplacement involves the stoping and assimilation of the envelope. Epizonal plutons occur throughout the Shield and include many ring complexes, bell-jar shaped and nested plutons typical of passive emplacement, as well as some examples of forceful emplacement by piercement, diapiric deformation of the envelope and gas coring. Many of the plutons described are post-tectonic but one is syn-tectonic relative to a shear zone, and some are pre-tectonic. Others show evidence of a combination of forceful and passive emplacement. The localization of magmas due to control by major structural lineaments is common within the Shield, but only one instance, a shear zone in the northwestern Hijaz, has been studied in detail.