RELATIONSHIP BETWEEN PETROGRAPHY AND UNIAXIAL COMPRESSIVE STRENGTH OF SOME CRYSTALLINE BASEMENT COMPLEX ROCKS OF SOME AREAS IN SOUTHWESTERN NIGERIA

The petrography and uniaxial compressive strength of six crystalline basement complex rocks of Akure and Igarra areas were studied, analyzed and correlated. The outcrops of porphyritic biotite granite, biotite granite and lamprophyre (in Igarra) as well as gneiss, granite gneiss and charnockitic rocks (in Akure) were examined and sampled.

Thin sections were prepared from the samples and investigated for the petrographic characteristics. Photomicrographs were taken and analyzed with the aid of “ImageJ”. The UCS of the rock types was determined with aid of Instron Universal tester “3369”. The Schmidt rebound hammer was also used to estimate the UCS.

The average modal compositions of the major minerals range from 16 to 29% (quartz), 19 to 36% (plagioclase), 5 to 41% (biotite) and 4 to 37% (microcline). The petrography further showed that some of the rocks types particularly charnockitic rocks possessed micro-structures (micro-cracks, bent lamellae, distorted/deformed twinning and undulose extinction). The UCS of the rock types are 171MPa (lamprophyre), 149MPa (granite gneiss), 117MPa (biotite granite), 101MPa (gneiss), 82MPa (charnockitic rocks) and 56MPa (porphyritic biotite granite). The correlation coefficient (r) of the relationship between the mineral content and the UCS evaluated the highest value of 0.6229 for quartz.

Textural and micro-structural characteristics of the rock types were discovered to have more influence on the UCS of the rocks than the mineral content.