Geochemical Characterisation of Magnesian Intrusives within High Grade Migmatite Gneiss Terrain: Insight from Plutons around Iwo Area, Southwest Nigeria

• 발행기관: 대한자원환경지질학회
• 저자명: Ogungbesan, Gbenga O Afolabi, Adegoke O Mustapha, Adedamola H Jimoh, Razak O Ajibade, Olumuyiwa M Okunola, Olufemi W
• 간행물 정보: 『자원환경지질』57권 5호, 609~632쪽, 전체 24쪽
• 주제분류: 자연과학 > 지질학
• 파일형태: PDF
• 발행일자: 2024.10.31

국문 초록

Magnesian granitoids, ranging from quartz-syenite to granodiorites of varied mineralogical composition, are poorly studied in metamorphosed terrains of Proterozoic eon, unlike their ferroan variety. Geochemical traits of magnesian granitoids in southwest Nigeria's Precambrian basement are investigated to understand their chemistry and evolutionary origins, such as continental collision events and tectonic settings. Four intrusive units based on their mineralogical compositions were identified as quartz syenite, porphyritic granodiorite, tonalite-trondhemite-graniodiorite (TTG) component of the high-grade migmatite gneiss, and charnockite (with granodioritic compositions). These rocks contain alkali feldspar, plagioclase, quartz, and biotite, the main mineral phases that are common to them. Pyroxene and garnet were observed in the quartz-syenite and charnockite, while hornblende crystals were found in quartz syenite, porphyritic granodiorite, and TTG. Geochemical analysis showed average silica and alumina concentrations accordingly: quartz syenite (59.28% SiO2, 13.28% Al2O3), porphyritic granodiorite (58.80% SiO2, 16.59% Al2O3), TTG (59.07% SiO2, 15.56% Al2O3), and charnockite (53.43% SiO2, 18.06% Al2O3). The average Fe/Mg ratios were 1.14 (quartz syenite), 1.78 (porphyritic granodiorite), 1.66 (TTG), and 1.80 (charnockite), and total alkali values were 9.98% (quartz syenite), 7.79% (porphyritic granodiorite), 9.11% (TTG), and 6.56% (charnockite). Based on their Fe/Mg ratio, alumina saturation index (ASI) (0.63-0.88), and Modified Alkali Lime Index (MALI) these rocks were characterised as metaluminous magnesian with alkali-calcic to alkalic nature. Variable LREE enrichment and europium anomalies were observed, with the quartz-syenite having the highest LREE enrichment and lowest Eu/Eu* (av.0.67). The plot of Rb vs Y+Nb showed that these intrusives are post-collision plutons, with the quartz syenite samples plotting in the syn-collision granite (syn-COLG) field while the porphyritic granodiorite and the charnockite plotted in the volcanic arc granite (VAG) field. These rocks must have been derived from partially melting the upper continental crust and deeper crust of possible mantle materials and emplaced as Pan-African post-orogenic plutons. The tectonic discrimination diagram for the granitoids implied late orogenic to post-collision uplift, collision arc events, and granite magmatism as the dominant events which characterised the Pan-African orogeny.

목차

1. Introduction

2. Geological Setting

3. Methodology

4. Results and Discussion

5. Discussion

6. Conclusion

Acknowledgement

References

키워드

granitoids magnesian metaluminous magma sources post-collision