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<p>The granite of Abeokuta is part of the Older Granite suite of the basement complex of southwestern Nigeria. Field observation has identified the granite as porphyritic in texture, with a preferred alignment of megacrysts of K-feldspar >5 cm that are mostly euhedral to subhedral in shape. K-feldspar megacrysts are predominantly yellowish, but whitish and pinkish colouration has also been observed. The dark grey porphyritic, mafic rock composition occurs as enclaves within the granite, with phenocrysts having a preferred alignment as the enclosing granite. Also, K-feldspar megacrysts show zoning defined by concentric arrangement of inclusions of biotite relative to the crystal faces. Petrographic study shows the following minerals in order of abundance: K-feldspar, biotite, quartz, and plagioclase, with K-feldspar having a microperthitic intergrowth and biotite crystals aligned in a preferred direction. Evidence such as the crystal shape of K-feldspars, preferred alignment of K-feldspar phenocrysts, and concentric crystallographic arrangements of inclusions of biotite in K-feldspar support a magmatic/phenocrystic origin of K-feldspar megacrysts of Abeokuta granite rather than originating by growing in a solid state as porphyroblasts.</p>
</abstract>

The granite of Abeokuta is part of the Older Granite suite of the basement complex of southwestern Nigeria. Field observation has identified the granite as porphyritic in texture, with a preferred alignment of megacrysts of K-feldspar >5 cm that are mostly euhedral to subhedral in shape. K-feldspar megacrysts are predominantly yellowish, but whitish and pinkish colouration has also been observed. The dark grey porphyritic, mafic rock composition occurs as enclaves within the granite, with phenocrysts having a preferred alignment as the enclosing granite. Also, K-feldspar megacrysts show zoning defined by concentric arrangement of inclusions of biotite relative to the crystal faces. Petrographic study shows the following minerals in order of abundance: K-feldspar, biotite, quartz, and plagioclase, with K-feldspar having a microperthitic intergrowth and biotite crystals aligned in a preferred direction. Evidence such as the crystal shape of K-feldspars, preferred alignment of K-feldspar phenocrysts, and concentric crystallographic arrangements of inclusions of biotite in K-feldspar support a magmatic/phenocrystic origin of K-feldspar megacrysts of Abeokuta granite rather than originating by growing in a solid state as porphyroblasts.

Field Observations, Petrography, and Microstructures of Granite from Abeokuta Southwestern Nigeria

Materials and Geoenvironment

This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 3.0 License.

Terenska opažanja, petrografija in mikrostrukture granita iz Abeokuta v jugozahodni Nigeriji

The granite of Abeokuta is part of the Older Granite suite of the basement complex of southwestern Nigeria. Field observation has identified the granite as...

Field Observations, Petrography, and Microstructures of Granite from Abeokuta Southwestern Nigeria

Article Category: Original Scientific Article

Publié en ligne: 04 sept. 2023

Pages: 69 - 77

Reçu: 03 août 2021

Accepté: 24 nov. 2021

DOI: https://doi.org/10.2478/rmzmag-2021-0009

Mots clés
megacrysts, microperthitic, K-feldspar, zoning, porphyroblasts

© 2021 Ehitua Julius Oziegbe et al., published by Sciendo

This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 3.0 License.

Field Observations, Petrography, and Microstructures of Granite from Abeokuta Southwestern Nigeria

Article Category: Original Scientific Article

Publié en ligne: 04 sept. 2023

Pages: 69 - 77

Reçu: 03 août 2021

Accepté: 24 nov. 2021

DOI: https://doi.org/10.2478/rmzmag-2021-0009

Mots clésmegacrysts, microperthitic, K-feldspar, zoning, porphyroblasts

© 2021 Ehitua Julius Oziegbe et al., published by Sciendo

This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 3.0 License.

Mots clés
megacrysts, microperthitic, K-feldspar, zoning, porphyroblasts