Alkaline basalts of Bafang and its environs are consisted of feldspars, olivines, pyroxenes and oxides which appear as phenocrysts, microphenocrysts and microcrysts. Feldspars are plagioclases (An<sub>67.97-15.8...Alkaline basalts of Bafang and its environs are consisted of feldspars, olivines, pyroxenes and oxides which appear as phenocrysts, microphenocrysts and microcrysts. Feldspars are plagioclases (An<sub>67.97-15.84</sub>Ab<sub>69.19-30.43</sub>Or<sub>20.59-1.51</sub>) and anorthoclases (Ab<sub>68.11-61.20</sub>Or<sub>33.87-20.91</sub>An<sub>10.98-4.93</sub>). Plagioclases are the most abundant amount these feldspars. Anorthoclases appear only in mugearite (BAF 3 and BAF 37) the most differentiated of the studied alkaline-basalts. In High Magnesian basalt, (HMg-B) plagioclases are labradorites (An<sub>67.97-59.3</sub><sub>0</sub>Ab<sub>38.74-30.43</sub>Or<sub>2.75-1.60</sub>) and sanidine (An<sub>45.44-31.82</sub>Ab<sub>62.66-51.79</sub>Or<sub>5.52-2.77</sub>), whereas in Low Magnesian basalt (LMg-B) there are labrador (An<sub>67.4.75-51.96</sub>Ab<sub>44.98-33.72</sub>Or<sub>3.06-1.51</sub>), andesine (An<sub>45.44-31.82</sub>Ab<sub>62.66-51.79</sub>Or<sub>5.52-2.77</sub>), oligoclase (An<sub>26.65-15.84</sub>Ab<sub>69.19-63.57</sub>Or<sub>20.59-8.55</sub>) and anarthoclase (Ab<sub>68.11-61.20</sub>Or<sub>33.87-20.91</sub>An<sub>10.98-4.93</sub>). Olivines are magnesian (Fo<sub>86.7-50.1</sub>) and ferriferous (Fo<sub>48.8-37.8</sub>). In HMg-B, olivine are only magnesian. These olivines are chrysolites and hyalositerite. In LMg-B, olivines are magnesian and ferriferous with the predominance of ferriferous. They are chrysolites, hyalositerite and hortonolite. Pyroxenes are Ca, Mg and Fe clinopyroxenes. There are diopsides (Wo<sub>51.94-45.02</sub>En<sub>44.41-33.16</sub>Fs<sub>16.42-10.70</sub>) and augites (Wo<sub>44.88-43.64</sub>En<sub>41.03-37.04</sub>Fs<sub>18.25-14.43</sub>). Oxides are magnetites represented by ulvospinel (Usp<sub>90-75</sub>Sp<sub>2-7</sub>Mt<sub>5-23</sub>). Fractionation of ferromagnesian minerals (opaque oxide, olivine and pyroxene) is the main differentiation process. Two stages of fractional crystallization can be distinguished: the first stage comes with basanites and the second with hawaiites to mugearites. Chemical compositions of phenocrystals in studied basaltics lavas record signatures of magma recharge by pulsatory intrusions of new magma into the existing magma reservoir before the eruptions.展开更多
The Barombi Mbo Maar (BMM), which is the largest maar in Cameroon, possesses about 126 m-thick well-preserved pyroclastic deposits sequence in which two successive paleosoil beds have been identified. The maar was tho...The Barombi Mbo Maar (BMM), which is the largest maar in Cameroon, possesses about 126 m-thick well-preserved pyroclastic deposits sequence in which two successive paleosoil beds have been identified. The maar was thought to have been active a million years ago. However, layers stratigraphically separated by the identified paleosoils have been dated to shed lights on its age and to reconstruct the chronology of its past activity. The results showed that the BMM formed through three eruptive cycles: the first ~0.51 Ma ago, the second at ~0.2 Ma and the third ~0.08 Ma B.P. The ages indicate that the BMM maar-forming eruptions were younger than a million years. The findings also suggested that the maar is polygenetic. At a regional scale, the eruptive events would have occurred during some volcanic manifestations at Mt Manengouba and Mt Cameroon. Therefore, with the decrease in the recurrence time of eruptions from ~0.3 Ma to 0.1 Ma, and given the possible relation between its eruptive events and those of its neighboring polygenetic volcanoes, the BMM is expected to erupt within the next 20 ka.展开更多
文摘Alkaline basalts of Bafang and its environs are consisted of feldspars, olivines, pyroxenes and oxides which appear as phenocrysts, microphenocrysts and microcrysts. Feldspars are plagioclases (An<sub>67.97-15.84</sub>Ab<sub>69.19-30.43</sub>Or<sub>20.59-1.51</sub>) and anorthoclases (Ab<sub>68.11-61.20</sub>Or<sub>33.87-20.91</sub>An<sub>10.98-4.93</sub>). Plagioclases are the most abundant amount these feldspars. Anorthoclases appear only in mugearite (BAF 3 and BAF 37) the most differentiated of the studied alkaline-basalts. In High Magnesian basalt, (HMg-B) plagioclases are labradorites (An<sub>67.97-59.3</sub><sub>0</sub>Ab<sub>38.74-30.43</sub>Or<sub>2.75-1.60</sub>) and sanidine (An<sub>45.44-31.82</sub>Ab<sub>62.66-51.79</sub>Or<sub>5.52-2.77</sub>), whereas in Low Magnesian basalt (LMg-B) there are labrador (An<sub>67.4.75-51.96</sub>Ab<sub>44.98-33.72</sub>Or<sub>3.06-1.51</sub>), andesine (An<sub>45.44-31.82</sub>Ab<sub>62.66-51.79</sub>Or<sub>5.52-2.77</sub>), oligoclase (An<sub>26.65-15.84</sub>Ab<sub>69.19-63.57</sub>Or<sub>20.59-8.55</sub>) and anarthoclase (Ab<sub>68.11-61.20</sub>Or<sub>33.87-20.91</sub>An<sub>10.98-4.93</sub>). Olivines are magnesian (Fo<sub>86.7-50.1</sub>) and ferriferous (Fo<sub>48.8-37.8</sub>). In HMg-B, olivine are only magnesian. These olivines are chrysolites and hyalositerite. In LMg-B, olivines are magnesian and ferriferous with the predominance of ferriferous. They are chrysolites, hyalositerite and hortonolite. Pyroxenes are Ca, Mg and Fe clinopyroxenes. There are diopsides (Wo<sub>51.94-45.02</sub>En<sub>44.41-33.16</sub>Fs<sub>16.42-10.70</sub>) and augites (Wo<sub>44.88-43.64</sub>En<sub>41.03-37.04</sub>Fs<sub>18.25-14.43</sub>). Oxides are magnetites represented by ulvospinel (Usp<sub>90-75</sub>Sp<sub>2-7</sub>Mt<sub>5-23</sub>). Fractionation of ferromagnesian minerals (opaque oxide, olivine and pyroxene) is the main differentiation process. Two stages of fractional crystallization can be distinguished: the first stage comes with basanites and the second with hawaiites to mugearites. Chemical compositions of phenocrystals in studied basaltics lavas record signatures of magma recharge by pulsatory intrusions of new magma into the existing magma reservoir before the eruptions.
文摘The Barombi Mbo Maar (BMM), which is the largest maar in Cameroon, possesses about 126 m-thick well-preserved pyroclastic deposits sequence in which two successive paleosoil beds have been identified. The maar was thought to have been active a million years ago. However, layers stratigraphically separated by the identified paleosoils have been dated to shed lights on its age and to reconstruct the chronology of its past activity. The results showed that the BMM formed through three eruptive cycles: the first ~0.51 Ma ago, the second at ~0.2 Ma and the third ~0.08 Ma B.P. The ages indicate that the BMM maar-forming eruptions were younger than a million years. The findings also suggested that the maar is polygenetic. At a regional scale, the eruptive events would have occurred during some volcanic manifestations at Mt Manengouba and Mt Cameroon. Therefore, with the decrease in the recurrence time of eruptions from ~0.3 Ma to 0.1 Ma, and given the possible relation between its eruptive events and those of its neighboring polygenetic volcanoes, the BMM is expected to erupt within the next 20 ka.
