Detailed work on Quaternary volcanism has been carried out in the Mermoz and Fann sectors of western Senegal. In the Mermoz sector, the main emission zone is a collapsed crater located at the intersection of three maj...Detailed work on Quaternary volcanism has been carried out in the Mermoz and Fann sectors of western Senegal. In the Mermoz sector, the main emission zone is a collapsed crater located at the intersection of three major fractures: NE-SW, NW-SE and N-S. The lithological succession in this Mermoz sector comprises, from bottom to top: 1) a substratum with at its base Eocene limestones on which lie Quaternary sands surmounted by stratified tuffs;2) a vesicular ball dolerite which deforms the stratified tuffs;3) a dark early breccia;4) two generations of basanites: the first is vesicular, the second non-vesicular;5) a clear intermediate breccia and finally 6) a late breccia. The Fann sector contains several emission zones, most of which are currently located in the ocean. The lavas may have reached the coast through E-W and NE-SW faults. The lithological succession includes from bottom to top: 1) scoria-rich early volcanic breccias;2) a first generation of non-vesicular mesocrate dolerite (D1);3) a second generation of melanocrate vesicular dolerite (D2);4) basanites and finally 5) a late breccia. The geochemical characteristics of the lavas studied are compatible with a very enriched and very deep magmatic source of the garnet lherzolite type located in the lower mantle. The magma from this source would have risen in the form of mantle plumes through major NE-SW and E-W faults in a continental intraplate context.展开更多
The Karthala massif,which covers the entire central and southern part of the island of Grande Comore,is made up of massive basalt flows,both vesicular and non-vesicular,pahoehoe and aa-type lavas and oceanite boulders...The Karthala massif,which covers the entire central and southern part of the island of Grande Comore,is made up of massive basalt flows,both vesicular and non-vesicular,pahoehoe and aa-type lavas and oceanite boulders.Petro-graphic observations of massive basalt flows show porphyritic and vacuolar microlitic textures marked by successive crystallization of opaque minerals,olivines,clinopyroxenes,orthopyroxenes and plagioclases in little or no mesostasis.This crystallization sequence is characteristic of a high-pressure geodynamic environment.The chemical variation of major and trace elements shows that the basalts studied are characterized by an under-saturation in sil-ica(47.44%)and an enrichment in alkalis(1.95%)and titanium(2.33%).The rare-earth spectra of these basalts show a subparallel pattern,suggesting the same source.In the rare earth and multi-element diagram,the Karthala basalts show an alkaline nature,characterized by enrichment in light rare earths(LREE)and large ion lithophile elements(LILE)and depletion in heavy rare earths(HREE)and high field strength elements(HFSE).They also show pos-itive Nb and Ta anomalies,typical of oceanic island basalts(OIBs)emplaced by mantle plume dynamics.These geochemical characteristics are compatible with an enriched,deep-seated garnet lherzolite-type source.展开更多
文摘Detailed work on Quaternary volcanism has been carried out in the Mermoz and Fann sectors of western Senegal. In the Mermoz sector, the main emission zone is a collapsed crater located at the intersection of three major fractures: NE-SW, NW-SE and N-S. The lithological succession in this Mermoz sector comprises, from bottom to top: 1) a substratum with at its base Eocene limestones on which lie Quaternary sands surmounted by stratified tuffs;2) a vesicular ball dolerite which deforms the stratified tuffs;3) a dark early breccia;4) two generations of basanites: the first is vesicular, the second non-vesicular;5) a clear intermediate breccia and finally 6) a late breccia. The Fann sector contains several emission zones, most of which are currently located in the ocean. The lavas may have reached the coast through E-W and NE-SW faults. The lithological succession includes from bottom to top: 1) scoria-rich early volcanic breccias;2) a first generation of non-vesicular mesocrate dolerite (D1);3) a second generation of melanocrate vesicular dolerite (D2);4) basanites and finally 5) a late breccia. The geochemical characteristics of the lavas studied are compatible with a very enriched and very deep magmatic source of the garnet lherzolite type located in the lower mantle. The magma from this source would have risen in the form of mantle plumes through major NE-SW and E-W faults in a continental intraplate context.
文摘The Karthala massif,which covers the entire central and southern part of the island of Grande Comore,is made up of massive basalt flows,both vesicular and non-vesicular,pahoehoe and aa-type lavas and oceanite boulders.Petro-graphic observations of massive basalt flows show porphyritic and vacuolar microlitic textures marked by successive crystallization of opaque minerals,olivines,clinopyroxenes,orthopyroxenes and plagioclases in little or no mesostasis.This crystallization sequence is characteristic of a high-pressure geodynamic environment.The chemical variation of major and trace elements shows that the basalts studied are characterized by an under-saturation in sil-ica(47.44%)and an enrichment in alkalis(1.95%)and titanium(2.33%).The rare-earth spectra of these basalts show a subparallel pattern,suggesting the same source.In the rare earth and multi-element diagram,the Karthala basalts show an alkaline nature,characterized by enrichment in light rare earths(LREE)and large ion lithophile elements(LILE)and depletion in heavy rare earths(HREE)and high field strength elements(HFSE).They also show pos-itive Nb and Ta anomalies,typical of oceanic island basalts(OIBs)emplaced by mantle plume dynamics.These geochemical characteristics are compatible with an enriched,deep-seated garnet lherzolite-type source.
