Mylonites in OBP are of quartzitic to quartzo-feldspathic or granitic composition and may be garnet or hypersthene bearing. Structurally, they vary from coarsely banded or lenticular to very thinly banded and reach a stage whereby they can be classified as ultramylonite. The mylonites show various degrees of granulation and recrystallization whereby quartz responded to flattening much more than the feldspars which have apparently nucleated at a greater rate than quartz and are consequently of smaller grain size. Thin quartz ribbons or blades are developed in the ultramylonites whereas more stoutly plates or individual ovum have developed in all other mylonite types. Moreover, elongated quartz porphyroblasts may occur in uniformly sutured form. Below is a brief mesoscopic description of OBP mylonites. Only the most conspicuous features are listed.
1) Quartzite mylonite: finely banded, dense, and has a sugar-like fabric, virtually 100% quartz; 2) Muscovite-biotite, finely- to coarsely-banded granitic mylonite: globular to oblate porphyroclasts deflecting foliation, porphyroclasts may show various degrees of deformation and are locally cored by minute inclusions of mafic minerals; matrix constituting the bulk is feldspathic. Mylonite found near migmatite zones; 3) Biotite lenticular mylonite: of quartzo-feldspathic composition, coarsely banded and is marked by dark grey to black, thin slivers of biotite undulating around white to locally grey feldspar porphyroblasts 4-8 mm across that are either round or oval in shape and in part enveloped by translucent to sugary, grey quartz; 4) Garnetiferous lenticular mylonite: also of dominantly quartzo-feldspathic composition and is coarsely foliated, wavy foliation around white, tapered augens of alkali feldspar measuring about 2-4 mm in length, quartz is translucent to light grey and is warped around the feldspar porphyroblasts; equant porphyroblasts of garnet 1-2 mm across are sparsely interspersed within the feldspar-quartz lithology and are dark greenish to black in color; 5) Garnetiferous, finely banded granulitic ultramylonite: of quartzo-feldspathic composition and is very finely to locally coarsely foliated and may be layered; garnet porphyroblasts 1-2 mm across are unevenly distributed and may not exceed 1-2% of the rock per volume, has a cherty appearance and its foliation may become difficult to detect where the rock is fresh; 6) Charnockitic ultramylonite: also of generally quartzo-feldspathic composition and is finely to very finely foliated and tightly folded, prominent mineral lineation grading into rodding on foliation surfaces, desert varnish lends the rock a flinty appearance, streaks of translucent, strongly flattened quartz lenses.
Mylonites in the subsurface of Sirt Basin occur in a variety of rocks ranging from phyllite or schist to marbles and medium-grade amphibolites. Locally, where porphyroblasts of pyrite or pyroxene are components, they are subjected to rotation, granulation, and recrystallization. The Mylonites were commonly folded tightly. In contrast to the Mylonites of Al’Oweinat Basement Province, faulting and fracture filling may accompany mylonitization. Because of coring, it was not possible to decipher whether the mylonites occur as concordant or discordant belts. However, it was found in certain cases that mylonite was the only rock extracted from the well, albeit showing varying degrees of mylonitization.
