The Cameroon hot line is dominated by magmatic rocks. The variations of magma and chemistry are generally due to the difference of physical conditions and chemistry in the magma source region during the ascent of magm...The Cameroon hot line is dominated by magmatic rocks. The variations of magma and chemistry are generally due to the difference of physical conditions and chemistry in the magma source region during the ascent of magma. The Mt Etinde and the Mt Cameroon, both edifices belong to the Cameroon Hot line, have a particularity some rare rocks such as camptonite and nephelinite. The relationship between the silica undersaturated rocks in the both edifices is characterized by the lateral variation appear through the petrography of the different rocks. The concerned geochemical data allow to compare the Mount Etinde nephelinite and Mount Cameroon camptonite where the differentiation process reflects geochemical affinities from a basaltic magma source on the Cameroon hot line. The compatible elements between the Mount Etinde nephelinite and the Mount Cameroon camptonite and basalt correlate with the difference in modal compositions of mineral phases. The lateral variation of major and trace element contents in the Mount Cameroon camptonite and Mount Etinde nephelinite seem to be related to the difference in the fractional crystallization processes of mineral phases, the difference in the partial melting processes and the metasomatism source rich in volatile. The silica-undersaturated character of the camptonite and nephelinite could be attributed to assimilation of carbonate rocks within depth-level magma chambers. Trace element AFC modelling revealed that the parental magmas of both edifice volcanic rocks were mostly affected by fractional crystallisation coupled with metasomatism process in Ca rich source.展开更多
Mount Etinde is a Recent (<1 Ma) strombolian-type volcano located on the southern flank of Mount Cameroon. Mount Etinde lavas are distinguished on the basis of the mineralogical compositions of their phenocrysts: o...Mount Etinde is a Recent (<1 Ma) strombolian-type volcano located on the southern flank of Mount Cameroon. Mount Etinde lavas are distinguished on the basis of the mineralogical compositions of their phenocrysts: olivine-pyroxene, olivine-melilite and clinopyroxene-nepheline. Some magnetite and ilmenite occur as inclusions in these early phases. Mafic mineral composition indicates that fractionation involved only limited Fe-enrichment. Oscillatory, normal and sectorial zoning in clinopyroxene relates to the differentiation and fractional crystallization of the magma. Based on the primitive mantle-normalized trace and rare earth element patterns, all nephelinites have high abundances of incompatible elements (103 < La < 281;131 < Ce < 503), with negative anomalies for high field strength element Ti and low Nb/Y (0.1 - 0.2) and Rb/Y (<0.03) ratios, suggesting derivation from a similar source. Textural characteristics and mineral chemical data, as well as whole-rock compositions, suggest that the nephelinitic lavas may have been derived from basaltic magma from a heterogeneous lithospheric mantle. Geochemical modeling of major and trace element variations indicates that the Etinde lavas could not have been produced by only fractional crystallization. Pneumatolytic reaction probably affected the pyromagmas (basaltic magma composition) and was responsible for the nephelinite rocks at lower oxygen fugacity (fO<sub>2</sub>).展开更多
文摘The Cameroon hot line is dominated by magmatic rocks. The variations of magma and chemistry are generally due to the difference of physical conditions and chemistry in the magma source region during the ascent of magma. The Mt Etinde and the Mt Cameroon, both edifices belong to the Cameroon Hot line, have a particularity some rare rocks such as camptonite and nephelinite. The relationship between the silica undersaturated rocks in the both edifices is characterized by the lateral variation appear through the petrography of the different rocks. The concerned geochemical data allow to compare the Mount Etinde nephelinite and Mount Cameroon camptonite where the differentiation process reflects geochemical affinities from a basaltic magma source on the Cameroon hot line. The compatible elements between the Mount Etinde nephelinite and the Mount Cameroon camptonite and basalt correlate with the difference in modal compositions of mineral phases. The lateral variation of major and trace element contents in the Mount Cameroon camptonite and Mount Etinde nephelinite seem to be related to the difference in the fractional crystallization processes of mineral phases, the difference in the partial melting processes and the metasomatism source rich in volatile. The silica-undersaturated character of the camptonite and nephelinite could be attributed to assimilation of carbonate rocks within depth-level magma chambers. Trace element AFC modelling revealed that the parental magmas of both edifice volcanic rocks were mostly affected by fractional crystallisation coupled with metasomatism process in Ca rich source.
文摘Mount Etinde is a Recent (<1 Ma) strombolian-type volcano located on the southern flank of Mount Cameroon. Mount Etinde lavas are distinguished on the basis of the mineralogical compositions of their phenocrysts: olivine-pyroxene, olivine-melilite and clinopyroxene-nepheline. Some magnetite and ilmenite occur as inclusions in these early phases. Mafic mineral composition indicates that fractionation involved only limited Fe-enrichment. Oscillatory, normal and sectorial zoning in clinopyroxene relates to the differentiation and fractional crystallization of the magma. Based on the primitive mantle-normalized trace and rare earth element patterns, all nephelinites have high abundances of incompatible elements (103 < La < 281;131 < Ce < 503), with negative anomalies for high field strength element Ti and low Nb/Y (0.1 - 0.2) and Rb/Y (<0.03) ratios, suggesting derivation from a similar source. Textural characteristics and mineral chemical data, as well as whole-rock compositions, suggest that the nephelinitic lavas may have been derived from basaltic magma from a heterogeneous lithospheric mantle. Geochemical modeling of major and trace element variations indicates that the Etinde lavas could not have been produced by only fractional crystallization. Pneumatolytic reaction probably affected the pyromagmas (basaltic magma composition) and was responsible for the nephelinite rocks at lower oxygen fugacity (fO<sub>2</sub>).
