The Bangaani area is located in the northwestern part of the Karthala Massif on the island of Grande Comore, about 10 km from the capital Moroni. It is essentially constituted of basalt plateaus outcropping in the for...The Bangaani area is located in the northwestern part of the Karthala Massif on the island of Grande Comore, about 10 km from the capital Moroni. It is essentially constituted of basalt plateaus outcropping in the form of pahoehoe and aa lava flows or in the form of massive vesicular or non vesicular basalt flows. Petrographic analysis of the massive basalts studied shows a porphyritic microlitic texture marked by the successive crystallization of olivines, opaque minerals, clinopyroxenes and plagioclases in a relatively abundant mesostasis. This crystallization sequence is typical of a high pressure environment corresponding to primary alkaline magmas. The geochemical study of major and trace elements shows that the basalts studied are under saturated in silica (47.24%) but rich in alkali (2.26%) and titanium (2.66%). They correspond to intraplate alkaline basalts of type oceanic island basalts (OIB) and present a primary character marked by relatively high contents of magnesium (6.69%), chromium (151.23 ppm), nickel (107.53 ppm) and scandium (27.15 ppm). The REE and multi-element diagrams confirm that the Karthala basalts are alkaline basalts of type OIB by their enrichment in LILE and their depleted character in HREE and HFSE. In terms of isotopic ratios, they are comparable to HIMU and EM I.展开更多
Hydrogeological modeling is an interesting and widely-used approach to improving our understanding of groundwater,both to test existing hypotheses on the behavior of hydrosystems and to predict their responses to vari...Hydrogeological modeling is an interesting and widely-used approach to improving our understanding of groundwater,both to test existing hypotheses on the behavior of hydrosystems and to predict their responses to various natural or man-made problems.Today,software such as Leapfrog Geo offers the possibility of building a 3D geological model with a more accurate representation of the subsurface.Statistical tools such as ordinary kriging can be used to simulate the spatial distribution of groundwater.These modeling approaches were combined to improve knowledge of the groundwater flow context within three massifs on the island of Grande Comore.The delineation of the 3D geometry of litho-stratigraphic units has enabled a more detailed conceptualization of groundwater flows in a complex volcanic environment.Piezometric interpolations were used to validate aquifer geometry.It has been demonstrated that an implicit geological model coupled with piezometry can provide very interesting information on the hydrogeological configuration of a volcanic massif.In the Karthala and Badjini massifs,the respective confined and semi-confined configurations of the aquifers are observed,with thicknesses that progressively decrease with distance from the coast.In the Grille massif,on the other hand,the aquifer configuration is unconfined,with thickness increasing with distance from the coast.In all three massifs,the flow of water in the underground hydrosystems is from the central part towards the coast,naturally following the geological configuration of the ground.It should be noted that the absence of data in the central parts of the massifs still leaves uncertainties about the geometry in these parts of the aquifers.However,the models that have been established provide valid hypotheses for characterizing the hydrogeological configuration at the scale of each massif.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The Bangaani area is located in the northwestern part of the Karthala Massif on the island of Grande Comore, about 10 km from the capital Moroni. It is essentially constituted of basalt plateaus outcropping in the form of pahoehoe and aa lava flows or in the form of massive vesicular or non vesicular basalt flows. Petrographic analysis of the massive basalts studied shows a porphyritic microlitic texture marked by the successive crystallization of olivines, opaque minerals, clinopyroxenes and plagioclases in a relatively abundant mesostasis. This crystallization sequence is typical of a high pressure environment corresponding to primary alkaline magmas. The geochemical study of major and trace elements shows that the basalts studied are under saturated in silica (47.24%) but rich in alkali (2.26%) and titanium (2.66%). They correspond to intraplate alkaline basalts of type oceanic island basalts (OIB) and present a primary character marked by relatively high contents of magnesium (6.69%), chromium (151.23 ppm), nickel (107.53 ppm) and scandium (27.15 ppm). The REE and multi-element diagrams confirm that the Karthala basalts are alkaline basalts of type OIB by their enrichment in LILE and their depleted character in HREE and HFSE. In terms of isotopic ratios, they are comparable to HIMU and EM I.
文摘Hydrogeological modeling is an interesting and widely-used approach to improving our understanding of groundwater,both to test existing hypotheses on the behavior of hydrosystems and to predict their responses to various natural or man-made problems.Today,software such as Leapfrog Geo offers the possibility of building a 3D geological model with a more accurate representation of the subsurface.Statistical tools such as ordinary kriging can be used to simulate the spatial distribution of groundwater.These modeling approaches were combined to improve knowledge of the groundwater flow context within three massifs on the island of Grande Comore.The delineation of the 3D geometry of litho-stratigraphic units has enabled a more detailed conceptualization of groundwater flows in a complex volcanic environment.Piezometric interpolations were used to validate aquifer geometry.It has been demonstrated that an implicit geological model coupled with piezometry can provide very interesting information on the hydrogeological configuration of a volcanic massif.In the Karthala and Badjini massifs,the respective confined and semi-confined configurations of the aquifers are observed,with thicknesses that progressively decrease with distance from the coast.In the Grille massif,on the other hand,the aquifer configuration is unconfined,with thickness increasing with distance from the coast.In all three massifs,the flow of water in the underground hydrosystems is from the central part towards the coast,naturally following the geological configuration of the ground.It should be noted that the absence of data in the central parts of the massifs still leaves uncertainties about the geometry in these parts of the aquifers.However,the models that have been established provide valid hypotheses for characterizing the hydrogeological configuration at the scale of each massif.
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.
