Finite Strain Analysis of Xenoliths and Phenocrysts in Akure Granitoids, Ondo State, Southwestern Nigeria

Abstract

Aim: This study was carried out to measure the strain conditions in the Akure granitiods with a goal to understand the impact of the pan-African orogenic cycle on this granitic body that extends from Ore through Akure to Ado-Ekiti axis. Methods: Lengths, strike, and dips of the principal stress directions, σ1,σ2 and σ3 in 101 xenoliths and 887 phenocrysts were measured. The length of the principal axes were measured using a centimeter rule while the strike and dip of xenoliths and trends of phenocrysts were measured using a compass clinometers. The extent of elongation and shortening, stretch and quadratic elongations were also determined. The Flinn’s, logarithmic Flinn’s and the Hsu diagrams were employed to understand the impact of the tectonism on rocks in the area. A total of 101 xenoliths and 887 phenocrysts were measured on the field. The lengths and trends of the maximum, intermediate and minimum directions were measured.  Results: The data obtained from the field analysis revealed that the maximum strain direction trends from NE-SW which corresponds to the direction of the stress field of the Pan-African orogeny. The original shape of these geological bodies ranges between 14.38 and 36.83 cm. Furthermore, the results showed that there is a correlation between the length and width of the xenoliths which confirms that they must have been acted upon by tectonic activity. The study further revealed that the xenoliths and phenocrysts trend in the NE-SW and NW-SE directions which correspond to the principal directions of the Pan-African Orogeny. The elongation of xenoliths in the maximum, intermediate and minimum strain directions ranges between 1.05 and 37.38, -1.94 and -1.14 and -3.15 and -0.20 respectively. The quadratic elongation is between 1.90 and 2.66 for the maximum strain direction while it ranges between 0.39 and 0.60 in the intermediate strain direction. In minimum strain direction, quadratic elongation ranges between 0.51 and 0.61. Flinn’s plot revealed that 100% of the xenoliths were strained by tectonism while logarithmic Flinn’s and Hsu plots revealed that 97% and 3% of the xenoliths were deformed by constrictional and flattened strain respectively. Conclusion: This study revealed that the pan-African orogeny played a significant role in deforming the xenoliths in the porphyritic granite. The investigation further revealed that the stress that acted on the Akure granitoids produced a constrictional change in most of the xenoliths while only a few of them were flattened.