Presentation Information
[T2-P-14]First zircon U-Pb ages of basement granites from Lake Manoun, west-central Cameroon
*Miharu NAKAYASHIKI1, Takeshi HASEGAWA1, Hisatoshi ITO2, Festus Tongwa AKA1 (1. Ibaraki university, 2. Central Research Institute of Electric Power Industry)
Keywords:
Pan African granite,zircon,U-Pb age,Cameroon,Gondwana
Introduction
Lake Monoun is an infamous maar volcano located on the Cameroon Volcanic Line (CVL) in West-Central Africa. In 1984, the lake discharged a huge amount of CO2 gas that killed 37 people. Lake Monoun was formed in the Holocene by at least three successive explosive eruptions (Nche et al., 2025). Although juvenile material in the lake-forming eruption products is alkali basalt, the deposits also contain lots of accidental fragments from basement rocks. No detailed studies have been done on the geochemistry and age of the basement rocks in this area of the CVL situated in the northern domain of the Central African Fold Belt. Such studies are essential to further understand the formation processes and dynamics of the Gondwanan supercontinent. Here, we report the first zircon U-Pb age of xenolithic basement rocks in Lake Monoun maar-forming eruption deposits.
Methods
We carried out modal analysis, cathodoluminescence (CL) imaging, and U-Pb dating of zircon grains in accidental lithic fragments of Monoun eruptive products. U-Pb dating was performed from 48 analysis points on 35 zircon grains in sample M21A, mainly following the procedure of Ito (2014). We compared the 238U−206Pb age and 235U−207Pb age obtained from a single analytical spot, calculated their ratio, and accepted as concordant those analyses where the age values were within ± 10 % of each other. Dating results are shown with 2σ errors.
Results
Rock forming minerals of sample M21A mainly consist of K-feldspar, quartz, and plagioclase. Most of the quartz is re-crystallized, fine-grained, and mylonitized. Apatite and chlorite occur as secondary minerals, with chlorite distributed in veins. Based on the modal composition and the QAPF diagram (Le Bas and Streckeisen, 1991), sample M21A classifies as monzogranite. Considering its metamorphic features, the rock can be described as a granitic mylonite. CL images show that most of the zircons have sector and fir-tree zoning textures. Zircon U-Pb dates show a strong peak at 500-800 Ma. Two analysis points yielded ages between 1,800 and 2,000 Ma (Fig. 1). The weighted average age is 559 ± 22 Ma (n = 27, MSWD = 29).
Discussion
The two zircon grains dated between 1,800 and 2,000 Ma might be derived from remobilization of Paleoproterozoic crust. Our U-Pb zircon age of 559 ± 22 Ma for the Lake Monoun monzogranite basement can be compared to the weighted mean of 4 biotite 40Ar/36Ar age of 528 ± 3 Ma for quartz monzonite basement in Lake Nyos area (Dalrymple and Lockwood, 1990). In the Dschang area, 30 km southwest of Lake Monoun, granitoid rocks with zircon U-Pb aged at 578 ± 11 Ma and 563 ± 8 Ma are distributed (Kwékam et al., 2020), that might be related to right-step lateral faulting (Central Cameroon shear zone) during the late Pan-African orogenic activity. The occurrence of mylonitized quartz grains in our rock samples suggests that the monzogranite basement was metamorphized and deformed by the regional fault movement. Crustal evolution in the Monoun and Dschang areas may have been similar.
References
Le Bas and Streckeisen. (1991) J. Geol. Soc. London. 148, 825-833.
Dalrymple and Lockwood. (1990) Natural Hazards 3: 373-378.
Ito, H. (2014) JVGR. 289, 210-223.
Kwékam et al., (2020) Geol. Soc. London Spec. Publ. 502.
Ludwig, K.R. (2012) Berkeley Geochronol. Center Spec. Publ. 5, 75.
Nche et al., (2025) JVGR. 460, 108265.
Lake Monoun is an infamous maar volcano located on the Cameroon Volcanic Line (CVL) in West-Central Africa. In 1984, the lake discharged a huge amount of CO2 gas that killed 37 people. Lake Monoun was formed in the Holocene by at least three successive explosive eruptions (Nche et al., 2025). Although juvenile material in the lake-forming eruption products is alkali basalt, the deposits also contain lots of accidental fragments from basement rocks. No detailed studies have been done on the geochemistry and age of the basement rocks in this area of the CVL situated in the northern domain of the Central African Fold Belt. Such studies are essential to further understand the formation processes and dynamics of the Gondwanan supercontinent. Here, we report the first zircon U-Pb age of xenolithic basement rocks in Lake Monoun maar-forming eruption deposits.
Methods
We carried out modal analysis, cathodoluminescence (CL) imaging, and U-Pb dating of zircon grains in accidental lithic fragments of Monoun eruptive products. U-Pb dating was performed from 48 analysis points on 35 zircon grains in sample M21A, mainly following the procedure of Ito (2014). We compared the 238U−206Pb age and 235U−207Pb age obtained from a single analytical spot, calculated their ratio, and accepted as concordant those analyses where the age values were within ± 10 % of each other. Dating results are shown with 2σ errors.
Results
Rock forming minerals of sample M21A mainly consist of K-feldspar, quartz, and plagioclase. Most of the quartz is re-crystallized, fine-grained, and mylonitized. Apatite and chlorite occur as secondary minerals, with chlorite distributed in veins. Based on the modal composition and the QAPF diagram (Le Bas and Streckeisen, 1991), sample M21A classifies as monzogranite. Considering its metamorphic features, the rock can be described as a granitic mylonite. CL images show that most of the zircons have sector and fir-tree zoning textures. Zircon U-Pb dates show a strong peak at 500-800 Ma. Two analysis points yielded ages between 1,800 and 2,000 Ma (Fig. 1). The weighted average age is 559 ± 22 Ma (n = 27, MSWD = 29).
Discussion
The two zircon grains dated between 1,800 and 2,000 Ma might be derived from remobilization of Paleoproterozoic crust. Our U-Pb zircon age of 559 ± 22 Ma for the Lake Monoun monzogranite basement can be compared to the weighted mean of 4 biotite 40Ar/36Ar age of 528 ± 3 Ma for quartz monzonite basement in Lake Nyos area (Dalrymple and Lockwood, 1990). In the Dschang area, 30 km southwest of Lake Monoun, granitoid rocks with zircon U-Pb aged at 578 ± 11 Ma and 563 ± 8 Ma are distributed (Kwékam et al., 2020), that might be related to right-step lateral faulting (Central Cameroon shear zone) during the late Pan-African orogenic activity. The occurrence of mylonitized quartz grains in our rock samples suggests that the monzogranite basement was metamorphized and deformed by the regional fault movement. Crustal evolution in the Monoun and Dschang areas may have been similar.
References
Le Bas and Streckeisen. (1991) J. Geol. Soc. London. 148, 825-833.
Dalrymple and Lockwood. (1990) Natural Hazards 3: 373-378.
Ito, H. (2014) JVGR. 289, 210-223.
Kwékam et al., (2020) Geol. Soc. London Spec. Publ. 502.
Ludwig, K.R. (2012) Berkeley Geochronol. Center Spec. Publ. 5, 75.
Nche et al., (2025) JVGR. 460, 108265.
