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
Bioko Island（3008 m a.s.l） is located in the presently more active volcanic zone of the Cameroon Line and composed essentially of alkaline basalts and hawaiites, and lesser mugearites. The rocks show microlitic porp...Bioko Island（3008 m a.s.l） is located in the presently more active volcanic zone of the Cameroon Line and composed essentially of alkaline basalts and hawaiites, and lesser mugearites. The rocks show microlitic porphyritic texture with phenocrysts of olivine（83% 〈 Fo 〈 87%） and clinopyroxene in a matrix of plagioclase, clinopyroxene and oxides. Hawaiites and mugearites also include phenocrysts of plagioclase（An62-67Ab35-32Or3-1）. Major element variation diagrams show an increase in Si O2, Al2O3, Na2 O and K2 O with increasing Mg O for the studied rock groups. The rocks are characterized by low（^86Sr/^87Sr）i ratios（0.70320e0.70406）, high 3Nd（t） values（2.56e4.33） and high（^206Pb/^204Pb）i ratios（20.032e20.035） values.Basalts are enriched in LILE and LREE, and have（Hf/Sm）N= 0.57e1.16. These geochemical signatures are similar to those of the Mount Cameroon rocks, and might be attributed to low degrees of partial melting from a garnet-amphibole-bearing mantle source. The trace elements and isotopic compositions suggest that the parental magma source might have involved HIMU- and EM1-components.展开更多
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>).
基金financially supported by the Ministère des Affaires Economiques (Project SGB/ NAT 91-98)
文摘Bioko Island（3008 m a.s.l） is located in the presently more active volcanic zone of the Cameroon Line and composed essentially of alkaline basalts and hawaiites, and lesser mugearites. The rocks show microlitic porphyritic texture with phenocrysts of olivine（83% 〈 Fo 〈 87%） and clinopyroxene in a matrix of plagioclase, clinopyroxene and oxides. Hawaiites and mugearites also include phenocrysts of plagioclase（An62-67Ab35-32Or3-1）. Major element variation diagrams show an increase in Si O2, Al2O3, Na2 O and K2 O with increasing Mg O for the studied rock groups. The rocks are characterized by low（^86Sr/^87Sr）i ratios（0.70320e0.70406）, high 3Nd（t） values（2.56e4.33） and high（^206Pb/^204Pb）i ratios（20.032e20.035） values.Basalts are enriched in LILE and LREE, and have（Hf/Sm）N= 0.57e1.16. These geochemical signatures are similar to those of the Mount Cameroon rocks, and might be attributed to low degrees of partial melting from a garnet-amphibole-bearing mantle source. The trace elements and isotopic compositions suggest that the parental magma source might have involved HIMU- and EM1-components.
