The Cenozoic alkaline volcanism of the Senegalo-Mauritania sedimentary basin presents an episodic long-lasting volcanic activity from the Eocene-Oligocene boundary up to the Quaternary. Two volcanic episodes are usually distinguished on stratigraphical grounds: a Miocene one and a Quaternary one separated by a period of quiescence of several million years corresponding to the main phase of lateritic weathering. The Tertiary lavas are highly silica-undersaturated alkaline rocks ranging from nephelinites to basanites. They contain nepheline and fassaite-type clinopyroxene in their mineralogy. The Quaternary lavas are more evolved with Hy-normative in comparison to the Tertiary ones; they are composed of basanites and medium to coarser-grained dolerites. The distribution of the REE and other incompatible elements are typical of alkaline lavas with generally strong LREE enrichment without significant Eu anomaly. The Cenozoic Cap-Vert lavas have OIB (oceanic islands basalts)-affinities as shown by their relatively radiogenic Nd and unradiogenic Sr characters closed to the HIMU-OIB. This suggests a HIMU-type end member in the magma sources which is fairly comparable to those erupted in the Canaries and Cape Verde archipelagoes during the same period.
The West African Atlantic margin is spotted by three main alkaline volcanic provinces: the Canary Archipelago, the Cape Verde Archipelago and the Cap-Vert peninsula (Dakar, Senegal). The Cap-Vert peninsula is located at the westernmost part of the Senegalo-Mauritania basin about 1000 km at the east of the Cape Verde Archipelago. The volcanic province (
Reference [
grounds during the Miocene age for the first occurence and an early Quaternary age for the second. Since the 50’s, many descriptive studies of volcanic occurrences have been carried out [
References [
Reference [
On the continental shelf, the activity of the Cenozoic volcanism is marked by intrusive rock sand hydrothermal vents in Paleozoic and Cretaceous sediments coinciding with the development of the Cayar Seamount and the volcanic eruptions on the Cape Verde archipelago [
Several hypotheses have been advanced to explain the geodynamic significance of this volcanism. [
This paper deals the interpretation from new geochemical data of the evolution of the Cenozoic volcanism of the Cap-Vert peninsula in relation with its geodynamic environment. The results are discussed and compared with the two other provinces of the Canary and Cape Verde Archipelagoes.
The Senegalo-Mauritania sedimentary basin is geologically pretty well known [
The magmatism acted along ancient zones of crustal weakness, which are related with inland extensions of transform faults. It is associated with two major tectonic events that occurred between the late Lutetian and late Eocene [
The Tertiary volcanism consists about thirty small occurrences of hectometric to kilometric extension scattered over an area about 100 km long from Dakar to the east of Thiès (
1) Ankaratrite, nephelinite, basanitelavas flows or sills with columnar structure well spread in the Cap-Vert peninsula (Gorée, Cap Manuel, Iles des Madeleines, Pointe de Fann). The columns are generally vertical and regular shaped with polygonal section and some are bowed toward the edge of the flow;
2) basanite and nephelinitedykes with pegmatitoid veins and tuffs of limited extension in the Rufisque area (Cap des Biches, Diokoul); The dykes oriented N20˚, intrude the paleogene limestone sand the contact is marked by a weak thermal metamorphism. The rocks contain numerous vesicles filled of calcite and zeolite elongate NNE to NE trends.
3) Intrusions of basanites and tuffs located at the intersection of WNW-ESE and sub-meridian main fractures in the Ndiass horst area (Khazabe, Thiéo, Bandia);
4) Andin the Thiès region where the main outcrop would correspond to the remains of a lava lake located in a maar [
Petrographical features:
1) Nephelinites with aphyrictexture are usually associated with porphyritic (or pegmatitoids) nephelinites with coarser doleritic texture. In the microliticfacies, phenocrystalor microcrystalolivine (75% to 90% Fo) are usually corroded by anhyalinemesostasis. The clinopyroxene with fassaite to diopside compositions (Wo 47 - 55 En 30 - 38, Fs 9 - 17) represent the main mineral phase of the rock. An hedralnepheline is associated with needles of apatite and fine grains of olivine and clinopyroxene in the mesostasis. In the porphyritic facies, the clinopyroxenes of fassaitetype (Wo 50 - 55 En 29 - 38, Fs 11 - 16) exhibit occasionally hourglass twinning oroscillatoryzonation features. A few tabular minerals of plagioclase (An 50% - 66%) are associated with olivine (66% to 84% Fo). Fine grains of apatite, zeolites, biotite and oxides are abundant in the rock.
2) The basanites are the most representative facies of this volcanism and are frequently associated with nephelinites. The common facies have aphyric texture and may have vacuolar structure with vesicles filled of calcite. The euhedral to subhedral crystals of olivine (67% to 85% Fo) are sometimes surrounded by a thinrim ofidding site. The clinopyroxene with augite compositions (Wo 47 - 52, En 31 - 41, Fs 10 - 15) are corroded by
the mesostasis. The microlitic plagioclases (An 52% - 80%) are relatively abundant. Oxides are grouped into fine granulesorin inclusion in olivine or clinopyroxen eminerals. The hyaline ground mass contains microcrystals of olivine, clinopyroxene, plagioclase andoxides.
3) In the Diack pluton, the basanites, dolerites, and gabbrosare arranged into aconcentric structure from the periphery to the inner parts of the pluton. In the dolerites, the clinopyroxenes with augitic composition (Wo 44 - 45, En 40 - 44, Fs 10 - 14) and the olivine (60% to 61% Fo) are grouped into the spaces between the plagioclase laths (An 44% - 63%). Oxides are abundant and exhibit a graphitic structure. The gabbros are composed of euhedral olivine (67% Fo) locally surrounded by a fringe of alteration of idding site. The plagioclases (An 44% - 52%) are abundant and partially saussuritized. Abundant oxides are associated with clinopyroxene.
The Quaternary volcanism is confined at the Head of the Cap-Vert peninsula commonly named Mamelles volcano (
From the bottom to the top, the Quaternary volcanismis divided into:
・ A lower unit consisting of tuffs and thin flows of olivinebasalts known only by drilling;
・ An intermediary unit is composed of dolerite sand basalts with aferruginous fringe of alteration;
・ An upper unit composed of volcanic flows corresponding to the Mamelles volcano with several phases of volcanic activities: a) maar deposits established during an initial phase of phreatomagmatic eruption; b) strombolian activity building acinder cone with emission of lavas at the base of the cone; c) edification of alava lakein a large crater by a ring collapsein the cone; d) eruption of doleritic cone-sheet feeding the terminal effusive phase.
These products are structured into four columnar or lamellar flows separated byscories and ferruginized sandy-claymaterial [
Petrographical features:
The basanites with fluidal texture are composed of olivine (64% to 91% Fo) sometimes surrounded by a rim of iddingsite. Zoning clinopyroxene of augitic composition (Wo 45 - 50 En 36 - 44 Fs 10 - 15) are associated with abundant plagioclase (An 46% - 76%) in the upper flows. Oxides are usually included in plagioclase or in clinopyroxene crystals or dispersed in the mesostasis.
The dolerites with sub-ophitic to fluidal texture are composed of abundant laths of plagioclase with rare clinopyroxene and olivine crystals. In the upper dolerites, the plagioclase (An 27% - 56%) may represent the only mineral phase where it is associated with augitic clinopyroxene (Wo 31 - 47 En 38 - 49 Fs 11 - 24), olivine (58% - 83% Fo) partially idding sitized and Fe-Ti oxides.
Twenty three samples (present study) associated with eighteen samples ([
The Cenozoic volcanic rocks exhibit distinctive geochemical compositions. The Tertiary lavas aresilica-under- saturated (40% - 48%) with Na2O/K2O ratios values varying between 2.12 and 4.94 except in the porphyritic (or pegmatitoids) nephelinites where these values are more important (7.91 and 40.75). They are characterized by
Tertiary volcanism | |||||||||||||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Samples. | 1 | 30 | 31 | 32 | 33 | 34 | 35 | 36 | 37 | 38 | 39 | DK18 | DK13 | DK15 | DK14 | DK1 | DK17 | DK16 | DK3 | DK2 | DK6 | DK7 | DK8 |
SiO2 | 41.72 | 41.58 | 39.26 | 41.14 | 42.46 | 39.39 | 39.87 | 38.76 | 42.33 | 47.85 | 47.22 | 40.46 | 41.50 | 39.80 | 43.34 | 39.96 | 45.31 | 46.91 | 40.81 | 40.62 | 40.60 | 39.56 | 47.21 |
Al2O3 | 11.51 | 11.71 | 13.12 | 11.55 | 13.49 | 12.14 | 12.55 | 15.91 | 12.23 | 13.62 | 15.95 | 11.67 | 12.00 | 12.80 | 14.02 | 12.63 | 13.20 | 13.20 | 12.03 | 12.96 | 17.27 | 11.26 | 13.55 |
Fe2O3 | 13.41 | 12.56 | 10.42 | 12.39 | 11.41 | 11.57 | 11.91 | 12.09 | 12.37 | 11.57 | 11.93 | 11.67 | 13.10 | 11.80 | 12.03 | 11.85 | 11.43 | 11.14 | 10.94 | 11.49 | 10.39 | 11.96 | 11.85 |
MnO | 0.19 | 0.19 | 0.17 | 0.18 | 0.18 | 0.19 | 0.20 | 0.22 | 0.20 | 0.16 | 0.19 | 0.19 | 0.20 | 2.00 | 0.19 | 0.17 | 0.17 | 0.16 | 0.17 | 0.17 | 0.17 | 0.20 | 0.16 |
MgO | 12.72 | 12.73 | 7.69 | 12.25 | 9.15 | 11.71 | 10.62 | 5.80 | 10.77 | 9.80 | 3.52 | 12.07 | 11.30 | 11.10 | 9.26 | 11.05 | 11.94 | 11.23 | 12.82 | 11.14 | 4.90 | 13.53 | 9.69 |
CaO | 12.84 | 12.33 | 15.95 | 12.56 | 12.05 | 14.97 | 14.55 | 13.77 | 12.32 | 10.06 | 8.74 | 15.38 | 12.40 | 14.40 | 12.72 | 14.83 | 11.64 | 11.99 | 15.53 | 14.86 | 11.08 | 14.33 | 10.48 |
Na2O | 2.64 | 2.73 | 3.30 | 2.72 | 3.46 | 1.97 | 2.94 | 4.69 | 4.10 | 2.86 | 4.86 | 2.84 | 3.54 | 2.83 | 3.39 | 3.41 | 2.83 | 2.85 | 3.12 | 3.29 | 5.58 | 3.04 | 3.16 |
K2O | 0.96 | 0.85 | 1.56 | 0.69 | 1.00 | 0.25 | 0.26 | 0.12 | 0.88 | 0.76 | 2.02 | 1.08 | 0.92 | 0.27 | 1.17 | 7.00 | 1.14 | 0.90 | 0.76 | 0.68 | 2.50 | 0.73 | 0.64 |
TiO2 | 2.50 | 1.82 | 2.07 | 1.80 | 1.96 | 1.96 | 2.05 | 1.99 | 2.22 | 1.55 | 2.49 | 2.13 | 2.50 | 2.04 | 2.04 | 2.37 | 2.14 | 1.80 | 1.97 | 3.08 | 1.68 | 1.93 | 1.43 |
P2O5 | 0.83 | 0.90 | 2.77 | 0.85 | 0.84 | 0.89 | 1.00 | 1.59 | 0.58 | 0.28 | 1.02 | 1.05 | 0.90 | 1.05 | 0.91 | 0.94 | 0.58 | 0.57 | 0.93 | 0.76 | 1.62 | 1.04 | 0.51 |
PF | 0.88 | 1.77 | 2.61 | 2.80 | 2.92 | 4.31 | 4.10 | 5.17 | 2.37 | 1.26 | 1.87 | 1.84 | 1.66 | 4.42 | 2.66 | 1.59 | 0.66 | 0.76 | 1.32 | 1.42 | 3.61 | 2.18 | 1.03 |
Total | 100 | 99.16 | 98.92 | 98.93 | 98.91 | 99.37 | 100 | 100 | 100 | 99.76 | 99.80 | 100 | 100 | 100.71 | 101.73 | 99.48 | 101.04 | 101.51 | 100 | 100 | 99.40 | 99.76 | 99.71 |
NaO/K2O | 2.75 | 3.19 | 2.12 | 3.96 | 3.46 | 7.91 | 11.12 | 40.75 | 4.65 | 3.78 | 2.41 | 2.63 | 3.85 | 10.48 | 2.90 | 5.01 | 2.48 | 3.17 | 4.11 | 4.84 | 2.23 | 4.16 | 4.94 |
Ne' | 11.7 | 10.5 | 13.7 | 9.75 | 10.1 | 8.63 | 13.4 | 21.2 | 19.2 | 0 | 10.6 | 13 | 15.3 | 13 | 12.5 | 15.6 | 7.11 | 4.43 | 14.3 | 15.1 | 25.58 | 1.35 | 13.9 |
Co | 83.3 | 77.9 | 82.6 | 76.6 | 73.8 | 69.8 | 68.2 | 55.9 | 78.4 | 123 | 49.9 | 72 | 76 | 73 | 60 | 82 | 69 | 67 | 74 | 65 | 42 | 64 | 59 |
Cr | 447 | 399 | 99.6 | 386 | 217 | 400 | 362 | 12.2 | 393 | 467 | 43 | 386 | 358 | 335 | 203 | 258 | 577 | 548 | 577 | 531 | 18 | 356 | 49 |
Cs | 0.37 | 0.37 | 0.26 | 0.36 | 0.93 | 0.39 | 0.27 | 0.48 | 1.26 | 0.88 | 1.53 | nd | nd | nd | nd | nd | nd | nd | nd | nd | nd | nd | nd |
Hf | 5.28 | 3.30 | 3.75 | 3.62 | 4.55 | 6.69 | 4.53 | 3.05 | 4.69 | 2.22 | 4.67 | nd | nd | nd | nd | nd | nd | nd | nd | nd | nd | nd | nd |
Nb | 80.9 | 61.1 | 108 | 61.6 | 99.8 | 118 | 156 | 236 | 122 | 39.1 | 156 | 91 | 119 | 118 | 104 | 96 | 88 | 54 | 103 | 98 | 134 | 46 | 84 |
Ni | 371 | 301 | 105 | 289 | 142 | 217 | 199 | 54.4 | 260 | 305 | 50.5 | 298 | 293 | 172 | 120 | 210 | 307 | 306 | 269 | 201 | 59 | 236 | 308 |
Rb | 27.9 | 23.1 | 55.6 | 17.6 | 26.7 | 14.2 | 12.1 | 8.82 | 44.4 | 24.9 | 63.9 | 31 | 30 | 11 | 30 | 23 | 34 | 18 | 23 | 27 | 89 | 24 | 29 |
Sr | 802 | 828 | 1540 | 872 | 1997 | 2732 | 2799 | 3571 | 802 | 552 | 843 | 1583 | 709 | 2812 | 1490 | 1302 | 784 | 694 | 1073 | 1011 | 1774 | 579 | 1088 |
Ta | 7.26 | 4.14 | 7.45 | 4.61 | 6.11 | 6.81 | 7.74 | 11.01 | 9.88 | 3.39 | 11.38 | nd | nd | nd | nd | nd | nd | nd | nd | nd | nd | nd | nd |
Th | 5.27 | 5.11 | 9.67 | 5.44 | 11.85 | 10.77 | 12.17 | 18.23 | 5.74 | 2.20 | 8.80 | nd | nd | nd | nd | nd | nd | nd | nd | nd | nd | nd | nd |
U | 1.25 | 1.24 | 2.09 | 1.28 | 3.64 | 2.59 | 3.18 | 4.79 | 1.38 | 0.53 | 2.10 | nd | nd | nd | nd | nd | nd | nd | nd | nd | nd | nd | nd |
V | 228 | 191 | 225 | 180 | 196 | 227 | 268 | 314 | 249 | 218 | 232 | 233 | 220 | 242 | 206 | 219 | 242 | 220 | 220 | 284 | 202 | 179 | 197 |
Y | 25 | 25 | 38.6 | 25.9 | 28.8 | 29.7 | 33.6 | 40.4 | 24.9 | 20.6 | 36.4 | 29 | 27 | 31 | 31 | 27 | 24 | 23 | 25 | 22 | 33 | 29 | 21 |
Zr | 212 | 152 | 201 | 155 | 222 | 329 | 257 | 257 | 227 | 99.2 | 250 | 182 | 228 | 226 | 212 | 165 | 231 | 156 | 200 | 212 | 156 | 89 | 178 |
La | 45.62 | 55.40 | 112.90 | 57.99 | 103 | 87.63 | 97.77 | 140 | 42.32 | 17.59 | 65.04 | 80.90 | 49.50 | 88.30 | 101.00 | 83.52 | 47.20 | 38.40 | 82.41 | 84.64 | 107 | 44.34 | 74.00 |
Ce | 88.57 | 107.70 | 216.10 | 113.40 | 174 | 164 | 181.60 | 252 | 78.71 | 34.32 | 118.30 | 158 | 95.2 | 167 | 176 | 155 | 92.6 | 77.5 | 152 | 101 | 184.8 | 74.65 | 141 |
Pr | 10.43 | 12.40 | 23.85 | 13.05 | 18.60 | 18.27 | 19.71 | 26.42 | 8.90 | 4.08 | 12.90 | nd | nd | nd | nd | nd | nd | nd | nd | nd | nd | nd | nd |
Nd | 41.61 | 48.29 | 89.79 | 50.68 | 67.47 | 67.26 | 72.25 | 92.71 | 34.46 | 16.79 | 48.19 | 66.10 | 41.60 | 67.90 | 67.30 | 63.27 | 40.17 | 34.30 | 63.12 | 46.02 | 72.14 | 28.67 | 61.60 |
Sm | 8.44 | 9.02 | 15.65 | 9.54 | 11.42 | 11.45 | 11.92 | 14.58 | 6.82 | 3.78 | 9.08 | 9.23 | 7.75 | 9.80 | 10.70 | 12.25 | 5.17 | 5.98 | 11.75 | 9.43 | 13.02 | 6.20 | 11.93 |
Eu | 2.73 | 2.79 | 4.68 | 2.90 | 3.52 | 3.42 | 3.65 | 4.26 | 2.28 | 1.37 | 2.98 | 2.47 | 2.15 | 2.52 | 2.73 | 3.51 | 1.40 | 1.78 | 3.26 | 2.77 | 3.72 | 1.88 | 3.52 |
Gd | 7.42 | 7.46 | 12.55 | 7.57 | 9.14 | 9.00 | 9.60 | 11.60 | 6.20 | 4.02 | 8.47 | 7.90 | 6.81 | 8.08 | 8.48 | 9.77 | 4.90 | 5.68 | 9.18 | 8.01 | 10.07 | 5.73 | 9.59 |
Tb | 1.02 | 1.02 | 1.62 | 1.08 | 1.18 | 1.22 | 1.28 | 1.52 | 0.87 | 0.62 | 1.21 | nd | nd | nd | nd | nd | nd | nd | nd | nd | nd | nd | nd |
Dy | 5.39 | 5.34 | 8.38 | 5.64 | 6.13 | 6.21 | 6.55 | 7.77 | 4.73 | 3.58 | 6.78 | 5.68 | 4.95 | 6.07 | 5.87 | 5.68 | 4.46 | 4.21 | 5.38 | 4.92 | 6.71 | 4.23 | 6.10 |
Ho | 0.92 | 0.91 | 1.37 | 0.95 | 1.02 | 1.05 | 1.11 | 1.33 | 0.82 | 0.67 | 1.20 | nd | nd | nd | nd | nd | nd | nd | nd | nd | nd | nd | nd |
Er | 2.32 | 2.21 | 3.36 | 2.37 | 2.62 | 2.67 | 2.83 | 3.49 | 2.05 | 1.76 | 3.11 | 2.32 | 2.24 | 2.70 | 2.67 | 2.36 | 1.85 | 1.88 | 2.24 | 2.00 | 2.94 | 1.94 | 2.49 |
Tm | 0.30 | 0.29 | 0.43 | 0.31 | 0.35 | 0.37 | 0.38 | 0.47 | 0.28 | 0.24 | 0.42 | nd | nd | nd | nd | nd | nd | nd | nd | nd | nd | nd | nd |
Yb | 1.83 | 1.79 | 2.51 | 1.87 | 2.10 | 2.26 | 2.36 | 2.92 | 1.70 | 1.46 | 2.66 | 1.34 | 1.71 | 1.89 | 1.99 | 1.72 | 1.14 | 1.35 | 1.64 | 1.44 | 2.23 | 1.54 | 1.81 |
Lu | 0.28 | 0.26 | 0.35 | 0.28 | 0.31 | 0.34 | 0.34 | 0.42 | 0.25 | 0.22 | 0.38 | 0.16 | 0.24 | 0.24 | 0.27 | 0.31 | 0.14 | 0.19 | 0.30 | 0.25 | 0.37 | 0.26 | 0.29 |
normative nepheline (9% to 25%) with the exception indoleriticfacies showing normative hypersthene and weakly differentiated with mg numbers ranging from 0.38 to 0.70. On the (Na2O + K2O) versus SiO2 diagram [
The Al2O3 exhibits a positive correlation with the degree of silica saturation, whereas MgO,
The concentrations of trace elements exhibit substantial differences between Tertiary and Quaternary rocks. The compatible elements (Cr, Ni, Co, Sc and V) are generally more abundant in Tertiary lavas, indicating various extents of fractionation of ferro-magnesian minerals whereas in the Quaternary lavas, the concentrations of these elements are less abundant.
Quaternary volcanism | ||||||||||||||||||
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Sample | 6 | 17 | 25 | 26 | 27 | 28 | 14 | A | 16 | 19 | 29 | 18 | DK12 | DK11 | DK5 | DK10 | DK4 | DK9 |
SiO2 | 48.23 | 47.54 | 46.91 | 47.90 | 48.24 | 47.97 | 50.27 | 49.35 | 51.30 | 50.65 | 50.51 | 47.13 | 48.42 | 51.72 | 46.90 | 49.84 | 48.20 | 51.75 |
Al2O3 | 14.80 | 15.04 | 14.60 | 14.87 | 14.69 | 14.66 | 17.27 | 14.90 | 14.72 | 14.69 | 15.44 | 14.82 | 15.18 | 15.21 | 14.92 | 15.60 | 15.08 | 14.66 |
Fe2O3 | 10.41 | 10.61 | 9.81 | 10.32 | 10.96 | 10.82 | 12.08 | 11.30 | 10.44 | 9.71 | 10.63 | 10.26 | 10.91 | 10.30 | 10.46 | 10.71 | 10.89 | 10.91 |
MnO | 0.13 | 0.14 | 0.13 | 0.13 | 0.14 | 0.14 | 0.11 | 0.15 | 0.14 | 0.10 | 0.12 | 0.14 | 0.12 | 0.12 | 0.12 | 0.13 | 0.13 | 0.13 |
MgO | 7.88 | 7.99 | 7.83 | 7.40 | 8.41 | 8.20 | 3.52 | 7.04 | 7.33 | 7.39 | 5.56 | 7.98 | 7.18 | 7.24 | 8.21 | 7.26 | 7.76 | 7.01 |
CaO | 7.33 | 8.00 | 9.42 | 8.75 | 8.59 | 7.91 | 6.83 | 8.88 | 9.01 | 8.50 | 9.07 | 9.07 | 7.84 | 8.96 | 8.32 | 9.36 | 8.36 | 8.73 |
Na2O | 4.06 | 4.79 | 4.30 | 3.89 | 3.66 | 4.51 | 3.78 | 3.72 | 3.94 | 3.78 | 4.05 | 4.16 | 5.05 | 4.01 | 4.20 | 3.84 | 4.74 | 3.95 |
K2O | 1.97 | 1.46 | 1.52 | 1.43 | 1.24 | 1.68 | 0.59 | 1.04 | 0.69 | 0.76 | 0.47 | 1.45 | 1.62 | 0.61 | 1.43 | 0.70 | 1.51 | 0.63 |
TiO2 | 2.18 | 2.13 | 1.89 | 1.90 | 1.81 | 2.13 | 1.61 | 1.70 | 1.52 | 1.49 | 1.55 | 1.88 | 2.12 | 1.35 | 2.00 | 1.43 | 1.86 | 1.54 |
P2O5 | 0.69 | 0.60 | 0.56 | 0.59 | 0.50 | 0.55 | 0.30 | 0.47 | 0.33 | 0.34 | 0.33 | 0.58 | 0.53 | 0.36 | 0.60 | 0.34 | 0.61 | 0.40 |
PF | 2.66 | 1.07 | 1.84 | 1.97 | 1.19 | 0.88 | 3.63 | 1.16 | -0.04 | 1.86 | 1.85 | 1.69 | 0.58 | nd | 2.48 | 0.57 | 0.51 | 0.03 |
Total | 100 | 99.36 | 98.80 | 99.13 | 99.41 | 99.43 | 99.99 | 99.70 | 99.39 | 99.28 | 99.56 | 99.15 | 99.55 | 99.88 | 99.64 | 99.78 | 99.65 | 99.74 |
NaO/K2O | 2.06 | 3.29 | 2.83 | 2.72 | 2.95 | 2.69 | 6.37 | 3.58 | 5.68 | 4.97 | 8.70 | 2.87 | 3.12 | 6.57 | 2.94 | 5.49 | 3.14 | 6.27 |
Ne' | 2.93 | 8.01 | 8.33 | 2.62 | 1.29 | 8.82 | 0 | 0 | 0 | 0 | 0 | 6.52 | 8.52 | 0 | 5.72 | 0 | 7.76 | 0 |
Co | 54.9 | 56.9 | 68.3 | 84.9 | 68.8 | 67.5 | 54.7 | 57.1 | 70.5 | 47.2 | 60.6 | 50.8 | 55 | 63 | 51 | 65 | 60 | 60 |
Cr | 238 | 253 | 225 | 239 | 262 | 307 | 274 | 257 | 237 | 215 | 242 | 215 | 246 | 261 | 236 | 244 | 257 | 35 |
Cs | 0.66 | 1.66 | 0.36 | 0.43 | 0.20 | 0.58 | 0.23 | 0.41 | nd | nd | nd | nd | nd | nd | ||||
Hf | 4.34 | 4.08 | 3.45 | 3.59 | 3.14 | 3.81 | 2.55 | 2.88 | 2.15 | 2.30 | 2.29 | 3.73 | nd | nd | nd | nd | nd | nd |
Nb | 66.5 | 58.2 | 52.8 | 53.3 | 46.1 | 55.8 | 30.1 | 41.2 | 30.6 | 31.1 | 32.1 | 51.5 | 56 | 31 | 59 | 27 | 57 | 33 |
Ni | 226 | 231 | 215 | 212 | 237 | 268 | 151 | 218 | 136 | 125 | 116 | 190 | 229 | 146 | 199 | 197 | 227 | 140 |
Rb | 57.4 | 17.2 | 40.9 | 38 | 28.2 | 45 | 12.3 | 25.4 | 15.3 | 16.9 | 4.47 | 37.4 | 39 | 17 | 35 | 19 | 44 | 17 |
Sr | 858 | 749 | 772 | 741 | 674 | 714 | 442 | 595 | 466 | 433 | 487 | 742 | 710 | 496 | 768 | 491 | 758 | 451 |
Ta | 5.01 | 4.54 | 3.84 | 3.92 | 3.20 | 4.19 | 1.76 | 2.71 | 1.89 | 1.80 | 1.86 | 3.80 | nd | nd | nd | nd | nd | nd |
Th | 4.56 | 5.23 | 4.07 | 4.17 | 3.51 | 3.79 | 2.88 | 2.87 | 2.77 | 3.18 | 3.16 | 4.60 | nd | nd | nd | nd | nd | nd |
U | 0.63 | 1.19 | 0.79 | 0.94 | 0.72 | 0.86 | 0.67 | 0.73 | 0.48 | 0.82 | 0.67 | 1.09 | nd | nd | nd | nd | nd | nd |
V | 114 | 141 | 126 | 131 | 136 | 137 | 97.2 | 138 | 133 | 122 | 131 | 132 | 145 | 134 | 146 | 47 | 151 | 143 |
Y | 18.9 | 20.2 | 19.8 | 20.6 | 20.4 | 19.7 | 23.6 | 19.9 | 16.6 | 16.9 | 19.1 | 20.3 | 19 | 17 | 20 | 42 | 21 | 19 |
Zr | 181 | 161 | 148 | 149 | 132 | 154 | 86.1 | 113 | 79.4 | 84.4 | 87.3 | 148 | 143 | 71 | 146 | 74 | 156 | 79 |
La | 37.71 | 34.88 | 32.84 | 33.44 | 30.07 | 30.80 | 36.95 | 27.79 | 24.36 | 26.26 | 26.19 | 33.88 | 28.88 | 24.46 | 33.80 | 36.91 | 33.68 | 25.65 |
Ce | 68.64 | 64.48 | 61.09 | 61.81 | 55.47 | 56.23 | 52.74 | 51.44 | 43.70 | 47.24 | 46.39 | 63.01 | 51.03 | 49.13 | 63.23 | 38.26 | 67.13 | 48.37 |
Pr | 7.92 | 7.45 | 7.02 | 7.15 | 6.55 | 6.53 | 7.84 | 6.00 | 4.79 | 5.18 | 5.25 | 7.30 | nd | nd | nd | nd | nd | nd |
Nd | 32.13 | 30.07 | 28.05 | 28.85 | 26.13 | 26.89 | 31.49 | 24.51 | 18.55 | 20.02 | 20.67 | 29.40 | 26.79 | 18.56 | 28.39 | 26.53 | 28.39 | 19.49 |
Sm | 7.14 | 6.65 | 6.26 | 6.38 | 5.82 | 6.37 | 7.17 | 5.37 | 4.09 | 4.38 | 4.66 | 6.41 | 6.98 | 4.69 | 6.64 | 6.62 | 6.89 | 4.67 |
Eu | 2.48 | 2.33 | 2.18 | 2.22 | 2.06 | 2.21 | 2.53 | 1.89 | 1.56 | 1.62 | 1.67 | 2.28 | 2.25 | 1.64 | 2.26 | 2.22 | 2.27 | 1.65 |
Gd | 6.52 | 6.21 | 5.64 | 5.78 | 5.36 | 5.69 | 7.08 | 5.27 | 4.26 | 4.49 | 4.60 | 6.08 | 6.58 | 4.78 | 6.20 | 7.93 | 6.41 | 4.75 |
Tb | 0.87 | 0.87 | 0.79 | 0.82 | 0.78 | 0.82 | 1.01 | 0.76 | 0.62 | 0.65 | 0.69 | 0.85 | nd | nd | nd | nd | nd | nd |
Dy | 4.31 | 4.57 | 4.20 | 4.37 | 4.18 | 4.30 | 5.37 | 4.03 | 3.36 | 3.58 | 3.83 | 4.49 | 4.35 | 3.43 | 4.39 | 5.54 | 4.43 | 3.75 |
Ho | 0.68 | 0.75 | 0.71 | 0.74 | 0.70 | 0.71 | 0.90 | 0.70 | 0.59 | 0.62 | 0.68 | 0.75 | nd | nd | nd | nd | nd | nd |
Er | 1.51 | 1.81 | 1.71 | 1.75 | 1.67 | 1.68 | 2.17 | 1.76 | 1.45 | 1.57 | 1.67 | 1.82 | 1.72 | 1.53 | 1.70 | 2.54 | 1.77 | 1.61 |
Tm | 0.18 | 0.23 | 0.22 | 0.23 | 0.22 | 0.22 | 0.27 | 0.22 | 0.19 | 0.21 | 0.23 | 0.23 | nd | nd | nd | nd | nd | nd |
Yb | 1.01 | 1.36 | 1.27 | 1.34 | 1.29 | 1.20 | 1.65 | 1.34 | 1.18 | 1.25 | 1.33 | 1.42 | 1.11 | 1.12 | 1.25 | 1.61 | 1.30 | 1.28 |
Lu | 0.13 | 0.18 | 0.17 | 0.19 | 0.18 | 0.17 | 0.24 | 0.19 | 0.16 | 0.17 | 0.19 | 0.20 | 0.17 | 0.22 | 0.21 | 0.29 | 0.23 | 0.23 |
The thorium considered as a strongly hygromagmaphile element shows a positive correlation with the incompatible elements and a negative correlation with the compatible elements (
The distribution of the rare-earth elements (REE) is typical of alkaline lavas, with generally strong light-REE
(LREE) enrichments defining regularly sloping patterns without significant Eu-anomaly (
The good correlations formerly observed between thorium and trace elements and the parallelism of the representative REE patterns suggest in first approximation a common magmatic source. The Y/N bratios (0.17 - 0.78) and (La/Yb)N values (8.63 - 34.98) are closed to those alkali basalts where Y/Nb < 1 and(La/Yb)N > 12.
The spider-diagrams (
The enriched character of the Tertiaryrocks compared to the Quaternary rocks could be explained by a subcontinental enriched mantle source or an interaction of metasomatic fluids with the magmatic source just before the melting. The prophyritic (or pegmatitoids) nephelinites which are significantly more enriched than other Tertiary and Quaternarylavas, contained apatite, amphibole, biotite and calcite interpreted as an early interaction of magma with fluid phases rich in H2O and CO2 ([
The Nd and Sr isotopes [
All the Tertiarylavas are characterized by low radiogenic Srvalues (0.703064 - 0.703254) and positive ԐNd (+5.07 to +6.5) excepted porphyritic (or pegmatitoids) nephelinites which are more radiogenic Sr (0.704105 - 0.704652) for comparable ԐNd values (+4.92 to +5.56). Moreover, the Quaternary lavas are slightly less Sr radiogenic (0.70299 to 0.70302) with positive ԐNd (+5.48 to +5.79). The quaternary rock DK 10 differs from others, with more radiogenic Sr(0.70320) and less radiogenic Nd (ԐNd = +3.26). This high 87Sr/86Sr value could be interpreted as reflecting the effects of alteration which modified the magmatic Sr isotopic signatures.
The weak radiogenic strontium and more radiogenic neodymium with positive ԐNd (+3.26 to +6.5) are intermediary between DMM and HIMU OIB-type suggesting an origin from a mantle less depleted than those of ocean ridges basalts (
Several lines of evidence indicate that the studied volcanic rocks did not suffer from significant crustal contamination. The existence of mantle xenoliths and the lack of differentiated lavas suggest that the magmas were not retained long enough within the crust to be contaminated. A possible contamination by lithospheric crust may be considered to explain the increase of silica contents in Quaternarylavas. According to [
The Cenozoic volcanism of the Cap-Vert peninsula is marked by a magmatic activity spread over 35 Ma, from Late Eocene up to Middle Pleistocene. It belongs to the last magmatic episode known in the western and the northwestern parts of the West Africa during the Permian up to the Quaternary. It is characterized by different periods of activities where no age progression is related to the geographic localization of outcrops as it is shown on the map (cf.
The Cap-Vert peninsula volcanism is previously interpreted as anintra-plate volcanism associated with the migration of hot spot activities [
In fact, the evolution of the Cap-Vert peninsula volcanism is characterized by different periods of magmatic activities (
Ref. Samples | (87Sr/86Sr)i | 143Nd/144Nd | ԐNd | |
---|---|---|---|---|
Quaternary volcanism | DK11 | 0.702994 | 0.512935 | 5.79 |
DK10 | 0.703196 | 0.512805 | 3.26 | |
DK4 | 0.703016 | 0.512919 | 5.48 | |
Tertiary volcanism | DK18 | 0.704076 | 0.51289 | 4.92 |
DK13 | 0.703254 | 0.512971 | 6.5 | |
DK14 | 0.704636 | 0.512923 | 5.56 | |
DK1 | 0.703191 | 0.512932 | 5.74 | |
DK3 | 0.703064 | 0.512898 | 5.07 | |
DK8 | 0.702908 | 0.51293 | 5.7 |
between 48 and 45 Ma by [
The ages recorded in the Cape Verde and Canary Archipelagoes volcanism are relatively more recent in comparison to those observed in the Cap-Vert peninsula. In the Cape Verde Archipelago, the first events of the Cenozoic volcanism are dated around 25.6 Ma in the Old Eruptive Complex of Sal Island [
In the Senegalo-Mauritanian basin, an early plutonicalkaline phase took place in the early Maastrichtian. Then the Cenozoic volcanism began in the Cap-Vert peninsula at the limit Eocene-Oligocene and continued up to 0.6 Ma, while it was still active in Cape Verde (Fogo Island) and Canary Islands (Tenerife and La Palma).
This Cenozoic volcanism is connected to the uplift of lithospheric blocks in response to the Alpine tectonic events occurring in the northwestern Africa. In the Cape Verde and most of the Canary Archipelagoes, the volcanism was at first submarine and becoming subaerial later while in the Cap-Vert peninsula and possibly the easternmost Canary Islands, the volcanism sets on a transitional crust [
In fact, in all these three provinces, the Cenozoic magmatism is strongly alkaline and mainly undersatured, with olivine nephelinites and olivine-melilite nephelinites being fairly common. However, the volcanic series of
the Cap-Vertpeninsula remain less differentiated in comparison to the Cape Verde and Canary archipelagoes ones, where mildly alkaline to transitional series are reported [
The heterogeneity of the Cenozoic volcanism source in the Cape Verde and the Canary Archipelagoes is widely discussed [
The sources of this volcanism are related to the sub-lithospheric plume channeling processes which are beginning at the Triassic-Jurassic stage in the realm of the future central Atlantic Ocean. This process has continued throughout the Cenozoic stage leading to alkaline magmatism along a north-northeast trend with decreasing ages from the Cape Verde archipelago up to Europe [
Thank you to Professors Gilbert Crévola and Christian Moreau for supplementary analytical data for the production of this document and to Umberto Cordani and Pierre Barbey for help to improve the presentation of the document.