Abstract

Absence of specific name and classification of the Isiagu intrusive rock and definition of its petrographic province became a challenge. To this effect, we collected rock samples at various depths from a deep quarry pit at Ishiagu and subjected the samples to both physical and optical analysis. This analysis was further tied to the basic principles of magma fractionation and crystallization. Physical analysis of hand specimens obtained at 0, 15, 30, 45, 60, 75 feet depths found the rock as hollo-crystalline, consisting of phenero-crystals. The texture is finer top-down, with mafic minerals increasing bottom-up, and felsic minerals greater top-down.  This describes the intrusive as intermediate between felsic and mafic, rather than the use of basic and acidic in existing literature. Average specific gravity (SG) of the rock samples is 2.8, but the 45 ft sample has the highest SG of 3.0, and consists mainly of mafic minerals. Others have light to intermediate colours. A calcite vein of average width (30 cm) runs top-down of the intrusive body. This calcite (secondary mineral) is vitreous, colourless, has perfect cleavage, and hardness less than 4. From both physical and optical analysis, the major minerals of the Isiagu intrusive are in the order of abundance: Augite, Plagioclase, Quartz, Olivine, Hornblende, Biotite and Magnetite (main accessory). Plagioclase however is more abundant at depths ≥ 60 ft. The percentage concentrations of these minerals vary with depth and described the Ishiagu intrusive as Olivine-diabase of Lacolith structure.

Key words: Olivine-diabase, intermediate intrusive, petrographic province, Isiagu.