Oziegbe, and two co-authors, reported in early 2020 that primary, secondary and accessory minerals in tonalitic rocks from Iwo region of the Precambrian Basement Complex of Southwestern Nigeria were identified and analyzed with the aim of determining the various processes involved during the crystallization of magma.
Thin sections of tonalite were prepared and studied with the aid of a petrographic microscope. The mineral assemblages observed are biotite, plagioclase, alkali-feldspar, amphiboles, pyroxene, quartz, muscovite and chlorite. Allanite, titanite, apatite and zircon occur as accessory minerals. Muscovite and chlorite are found to be secondary minerals. The mineral allanite has a characteristic form of zoning and shows evidence of metamictization, and is surrounded by dark-colored biotite having radioactive haloes. Titanite is anhedral to subhedral crystals and forms reaction rim round opaque minerals.
Plagioclase shows evidence of compositional zoning as well as plastic deformation of the twin lamellae. The allanite observed is primary in nature and has undergone radioactive disintegration; chlorite and muscovite are formed by secondary processes of chloritization and sericitisation, respectively.
The tonalite is formed as a result of rapid cooling of magma close to the Earth’s surface.
Keywords: zoning, titanite, radioactive haloes, chloritization, sericitization
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