Abstract
Idi-ayunre and Akure areas are part of the basement complex of southwestern Nigeria and are predominantly consisted of gneisses, granite and migmatite with some minor quartz veins and pegmatite. These rocks have been greatly weathered to form clay, laterite and soils.Chemical analysis were carried out on basement rocks and exposed profiles. The weathering profile was subjected to X ray diffraction (XRD) analysis to determine mineralogical compositions whereas Chemical Index of Alteration (CIA) was calculated from the elemental concentrated data.Weathering of basement rocks in Idi-Ayunre and Akure districts resulted in the formation of soil layer which ranged 0-0.4m, laterite layer 1.2-2.2m, and clayey zone 3.8-6.6m. Quartz, plagioclase, microcline, and biotite were the main minerals in parent rocks. Some of the primary minerals such as biotite and K-feldspar have been weathered to form kaolinite. Quartz, kaolinite and goethite formed the dominant minerals revealed by X-ray diffraction on decomposed granite sequences. The results from chemical analysis showed that Al and Fe have been enriched in weathering profiles of banded gneiss, migmatite gneiss and porphyritic granite whilst on the other hand Ca, Mg, Mn, Na, K, Ti were reported to be depleted.. Silica was relatively stable from basement to the topsoil in the profile. The CIA generally ranged between 80 - 99The lateritic profiles over banded gneiss, granite and porphyritic granite of Idi-Ayunre and Akure areas varied with the composition of the parent rocks. The thick clayey layers could be of great economic importance for the production of ceramics wares and for constructional purposes.
Highlights
The majority of the soils present in the tropics are reddish soils called lateritic soils or laterite soils according to Anupam S. and Rajamani V. (2000)
Clay, laterite and soil were obtained from fresh road cuts at specific intervals from top soil down to fresh basement rocks
Lateritization is the dominant process of chemical weathering in tropical and subtropical regions of the world
Summary
The majority of the soils present in the tropics are reddish soils called lateritic soils or laterite soils according to Anupam S. and Rajamani V. (2000). Lateritic soils are essentially product of tropical or subtropical weathering of various crystalline igneous rocks, sediments, detrital deposits and volcanic ash. Some of the basement rocks have been greatly weathered to form residual soils. (1995) examined the secondary geochemical and mineralogical dispersion patterns associated with lateritization process in Ile-Ife, southwestern Nigeria. Kehinde-Phillips et al (1995) described the mineralogy and geochemistry of the weathering profiles over amphibolites, anthophylite and talc schists in the Ilesha schist belt, southwestern Nigeria. According to Bolarinwa (2006) examined the mineralogy and geochemistry of the weathering profiles above basement rocks in Ibadan and concluded that the weathering profiles are composed of quartz, kaolinite, goethite and limonite. Ige et al (2005) studied the mineralogy and geochemistry of lateritic weathering profiles on ultramafic rock www.ccsenet.org/jgg According to Bolarinwa (2006) examined the mineralogy and geochemistry of the weathering profiles above basement rocks in Ibadan and concluded that the weathering profiles are composed of quartz, kaolinite, goethite and limonite. Ige et al (2005) studied the mineralogy and geochemistry of lateritic weathering profiles on ultramafic rock www.ccsenet.org/jgg
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