Further Evidences of Cataclasis in the Ife-Ilesa Schist Belt, Southwestern Nigeria.

Abstract

The studied rock types form part of the lithologic unit of the Ife-Ilesa schist belt. The exposures of the rocks were studied in the field for their field relations, structure, texture and mineralogy and in the laboratory for their mineralogy, texture and micro-structure. Further analysis of the photomicrographs was carried out with “ImageJ” software. The results of the study complement each other. Field observation shows porphyroclasts of quartz and feldspar in fine-grained groundmass, mineral segregation and bands that are asymmetrically folded for study site 1 while it shows whitish rock with dark spots indicative of ground and congealed rock for study site 2. The rock exposed in study site 1 exhibited joints in various directions while that in study site 2 exhibited joints in two directions. The petrographic study reveals porphyroclastic texture, mineral segregation, rotated large grains, broken/jagged margins of the large grains and the growth of post deformational minerals in site 1 and in study site 2; it revealed textural characteristic that is similar to that of the rock in site 1, broken/jagged boundaries of the large quartz grains as in site 1, micro-fault, preferred orientation of grains and presence of new mineral grains. “ImageJ” revealed the same observations as the petrography as well as widest range (statistical) for minerals that fall into the minimum category, shortest range for minerals that fall into the maximum category and similarity between very coarse and fine grains. The very wide range of minimum and low range of maximum when compared with other rock types indicate uncoordinated production of the fine-grained materials and crushing effect of high pressure on the large grains, respectively. These observations point to shearing, crushing and grinding due to cataclasis. The similarity between large and fine crystals indicates that they are products of the similar parentage. The fabric revealed by this work points to the mylonitization of the rocks. The results of the study point to relic cataclastic fabric in a completely reconstituted rock. It is inferred that at least two episodes of high pressure metamorphism might have affected the rock. It is also inferred that the rocks experienced an episode of the chemical reconstitution with the attendant formation of new minerals. The rocks and by extension parts of the Ife-Ilesa schist belt must have experienced cataclastic metamorphism. The rocks can therefore be classified as mylonites.

Keywords: Cataclasis; Mylonite; Porphyroclasts; ImageJ; Ife-Ilesa.