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 2).
Located near Batoke village (
The Cameroon Hot Line is interpreted as a megashear zone, developed on both oceanic and continental domains [
Mineral compositions were studied in detail and the compositional ranges given below are based on electron microprobe (CAMEBAX SX 50 at the University of Marie Curie, Paris) measurement of mafic phases in only a few representative sample of nephelinite. The measurement were made according to standard analyzed data, under the condition expressed in kv (acceleration) nA (beam current) and (counting times at the peak). Olivine (15 kv, 40 nA, 20s and Si, clinopyroxene (15 kv, 40 nA, 20 s for Si, Al, Fe, Mg, Ca, Na Mn and 30 s for Ti and Zr), nepheline (15 kv, 40 nA, 15 s for Si, Al, and 20 s for Ca, Na, K ), garnet (15 kv, 40 nA, 20 s for Si, Al, Fe, Mg, Ca, Na, Mn and 15 s for Ti), melilite ((15 kv, 40 nA, 20 s for al elements), perovskite and sphene (15 kv, 40 nA, 5 s for all
elements), (oxides (15 kv, 40 nA, 20 s for Ti, Fe, Mn, Mg, 20 s for Si, 20 s for Cr and 30 s for Al) natural silicates and oxides as standards, PAP corrections were made using CAMECA software [
Thermodynamic condition of crystallization during the eruption of Etinde nephelinite was determined by the QUILF95 software [
Our description of the nephelinite is based on samples collected from the flank of Mount Etinde and the Batoke beach, possibly representing the volcanic event. The textures are generally porphyritic (15% - 30% phenocrysts), with groundmass exhibiting various textural types including intergranular, intersertal. Photomicrograph, taken under crossed polarizers, of nephelinte rocks shows olivine, clinopyroxene, nepheline and garnet phenocrysts. The photograph, taken plane polarized light, shows the typical shape of olivine and cliopyroxene: the irregular cracks and slight alteration along the craks (
haüynophyre. Spinel, described here from the nephelinitic lavas, occurs as tin (up to 2 mm) opaque or brownish euhedral inclusions in olivine and euhedral inclusions in aluminous clinopyroxene. Spinels found are xenomorphic, probably because it crystallized late. Small magnetites are irregularly distributed between the silicates, the locally form lenticular zones of interstitial Ti-magnetite. The groundmass consists of prismatic clinopyroxene and nepheline up to 0・2 mm in length microphenocrysts set in a matrix with smaller crystals, perovskite, magnetite, sphene and carbonate. The prismatic Zeolite is automorphe in LPA and replaced by analcime product (
Selected electron microprobe analyses of pyroxene and garnet phenocrysts are represented in the figure. These data are plotted on conventional Diopside-Hedenbergite-Enstatite- Ferrosilite and Almandine-Pyrope-Spertartine quadrilateral and ternary diagrams, respectively, comparing the current results against previously published data from the Etinde Mountain [
Rage homogeneous olivine grains in the néphélinite are Mg-rich (Fo86) and moderately high levels of CaO content solid-solution, and are similar composition to olivine grain from xenolith
The nomenclature of [
Sample | 36 | 35 | 31 | 38 | 33 | 34 | 32 | 37 | 39 |
---|---|---|---|---|---|---|---|---|---|
c | c | c | c | c | r | r | r | r | |
SiO2 | 45.34 | 48.71 | 47.22 | 44.12 | 42.74 | 46.33 | 45.19 | 45.74 | 45.41 |
TiO2 | 3.23 | 1.29 | 1.81 | 3.61 | 4.17 | 2.67 | 3.25 | 3.22 | 3.33 |
Al2O3 | 6.96 | 3.59 | 4.77 | 7.76 | 8.79 | 5.61 | 6.68 | 6.97 | 7.08 |
Cr2O3 | 0.01 | 0.01 | 0.01 | 0.00 | 0.00 | 0.00 | 0.01 | 0.01 | 0.01 |
FeO | 10.33 | 10.50 | 10.40 | 9.11 | 9.01 | 9.64 | 8.72 | 8.27 | 7.92 |
MnO | 0.41 | 0.46 | 0.46 | 0.34 | 0.24 | 0.38 | 0.26 | 0.25 | 0.25 |
MgO | 9.94 | 10.56 | 10.24 | 10.09 | 9.97 | 10.58 | 10.74 | 11.40 | 11.14 |
CaO | 22.82 | 23.54 | 22.53 | 23.38 | 23.07 | 22.75 | 22.99 | 23.50 | 23.34 |
Na2O | 0.93 | 0.96 | 1.01 | 0.74 | 0.72 | 0.85 | 0.78 | 0.64 | 0.76 |
ZrO2 | 0.14 | 0.06 | 0.09 | 0.14 | 0.14 | 0.12 | 0.12 | 0.10 | 0.12 |
TOTAL | 100.11 | 99.68 | 98.54 | 99.29 | 98.85 | 98.93 | 98.74 | 100.10 | 99.36 |
Si | 1.7093 | 1.8391 | 1.8040 | 1.6731 | 1.6279 | 1.7619 | 1.7176 | 1.7104 | 1.7096 |
Ti | 0.0916 | 0.0366 | 0.0520 | 0.1030 | 0.1195 | 0.0764 | 0.0929 | 0.0906 | 0.0943 |
Al | 0.3092 | 0.1597 | 0.2148 | 0.3468 | 0.3946 | 0.2514 | 0.2992 | 0.3072 | 0.3141 |
Cr | 0.0003 | 0.0003 | 0.0003 | 0.0000 | 0.0000 | 0.0000 | 0.0003 | 0.0003 | 0.0003 |
Fe3+ | 0.1541 | 0.1576 | 0.1461 | 0.1528 | 0.1612 | 0.1326 | 0.1347 | 0.1353 | 0.1310 |
Fe2+ | 0.1715 | 0.1739 | 0.1862 | 0.1361 | 0.1258 | 0.1740 | 0.1425 | 0.1233 | 0.1184 |
Mn | 0.0131 | 0.0147 | 0.0149 | 0.0109 | 0.0077 | 0.0122 | 0.0084 | 0.0079 | 0.0080 |
Mg | 0.5587 | 0.5944 | 0.5832 | 0.5704 | 0.5661 | 0.5998 | 0.6086 | 0.6355 | 0.6253 |
Ca | 0.9217 | 0.9522 | 0.9221 | 0.9499 | 0.9414 | 0.9269 | 0.9362 | 0.9414 | 0.9414 |
Na | 0.0680 | 0.0703 | 0.0748 | 0.0544 | 0.0532 | 0.0627 | 0.0575 | 0.0464 | 0.0555 |
Total cations | 4.0000 | 4.0000 | 4.0000 | 4.0000 | 4.0000 | 4.0000 | 4.0000 | 4.0000 | 4.0000 |
Wo | 50.08 | 52.16 | 50.42 | 51.23 | 50.65 | 49.92 | 50.27 | 49.96 | 50.47 |
En | 39.36 | 38.09 | 38.48 | 40.80 | 42.02 | 39.72 | 41.36 | 43.17 | 42.73 |
Fs | 10.57 | 9.75 | 11.11 | 7.97 | 7.33 | 10.36 | 8.37 | 6.87 | 6.80 |
AlVI | 0.0185 | 0.0000 | 0.0187 | 0.0199 | 0.0225 | 0.0133 | 0.0168 | 0.0175 | 0.0238 |
AlIV | 0.2907 | 0.1597 | 0.1960 | 0.3269 | 0.3721 | 0.2381 | 0.2824 | 0.2896 | 0.2904 |
AlVI/AlIV | 0.0637 | 0.0000 | 0.0956 | 0.0610 | 0.0605 | 0.0558 | 0.0597 | 0.0604 | 0.0819 |
Fe(+3) Y | 0.1541 | 0.1565 | 0.1461 | 0.1528 | 0.1612 | 0.1326 | 0.1347 | 0.1353 | 0.1310 |
Ti/Al | 0.2962 | 0.2293 | 0.2422 | 0.2969 | 0.3028 | 0.3037 | 0.3105 | 0.2948 | 0.3002 |
Mg/(Mg + Fe2+) | 0.63 | 0.64 | 0.64 | 0.66 | 0.66 | 0.66 | 0.69 | 0.71 | 0.71 |
Garnet composition falls within the compositional range of garnets reported in other lavas and ijolites from Oldoinyo Lengai [
Representative chemical analyses and structural formulae (O2− = 21) of haüyne is presented in
Perosvkite and Sphene are trace- and rare element-rich. The Trace elements content
Samples | 22 | 23 | 24 | 25 | 26 | 27 | 28 | 29 | 30 | 31 | 32 | 33 |
---|---|---|---|---|---|---|---|---|---|---|---|---|
c | c | c | c | c | r | r | r | r | r | r | r | |
SiO2 | 35.19 | 36.15 | 34.70 | 33.65 | 34.45 | 33.64 | 34.18 | 34.43 | 34.52 | 35.18 | 36.11 | 35.49 |
Al2O3 | 30.93 | 31.52 | 30.69 | 29.94 | 29.90 | 30.15 | 30.34 | 30.80 | 30.68 | 30.90 | 30.50 | 31.35 |
FeO | 0.55 | 0.54 | 0.41 | 0.58 | 0.75 | 0.20 | 0.67 | 1.85 | 0.48 | 0.48 | 0.29 | 0.29 |
CaO | 1.62 | 1.66 | 3.09 | 2.98 | 3.13 | 3.24 | 3.18 | 1.55 | 2.16 | 1.60 | 1.80 | 1.06 |
Na2O | 18.45 | 20.44 | 18.51 | 17.68 | 15.87 | 17.19 | 17.98 | 17.94 | 20.14 | 18.73 | 17.41 | 18.74 |
K2O | 3.43 | 3.30 | 2.86 | 3.28 | 3.70 | 3.49 | 3.15 | 3.19 | 2.85 | 3.50 | 3.54 | 3.32 |
SO3 | 8.10 | 8.21 | 11.67 | 11.50 | 10.86 | 11.84 | 11.55 | 10.29 | 10.13 | 8.16 | 6.67 | 8.19 |
Total | 98.28 | 101.82 | 101.94 | 99.61 | 98.66 | 99.75 | 101.05 | 100.05 | 100.95 | 98.53 | 96.32 | 98.44 |
O | 21.00 | 21.00 | 21.00 | 21.00 | 21.00 | 21.00 | 21.00 | 21.00 | 21.00 | 21.00 | 21.00 | 21.00 |
Si | 4.588 | 4.570 | 4.367 | 4.344 | 4.469 | 4.329 | 4.350 | 4.425 | 4.407 | 4.580 | 4.777 | 4.602 |
Al | 4.753 | 4.698 | 4.552 | 4.556 | 4.572 | 4.574 | 4.552 | 4.666 | 4.617 | 4.742 | 4.755 | 4.791 |
Fe | 0.060 | 0.057 | 0.043 | 0.063 | 0.081 | 0.021 | 0.072 | 0.199 | 0.051 | 0.052 | 0.032 | 0.032 |
Ca | 0.226 | 0.224 | 0.417 | 0.412 | 0.435 | 0.447 | 0.433 | 0.214 | 0.295 | 0.222 | 0.255 | 0.148 |
Na | 4.665 | 5.011 | 4.517 | 4.427 | 3.991 | 4.289 | 4.436 | 4.470 | 4.984 | 4.728 | 4.466 | 4.712 |
K | 0.571 | 0.532 | 0.460 | 0.541 | 0.612 | 0.573 | 0.512 | 0.522 | 0.464 | 0.581 | 0.597 | 0.549 |
S | 1.790 | 1.760 | 2.490 | 2.518 | 2.388 | 2.582 | 2.492 | 2.243 | 2.192 | 1.800 | 1.496 | 1.801 |
Total | 16.653 | 16.852 | 16.845 | 16.862 | 16.547 | 16.815 | 16.847 | 16.738 | 17.009 | 16.704 | 16.378 | 16.633 |
(BaO, SrO, ZrO2 and Nb2O3) represented slightest 4.0 wt%. Sphene’s rare elements content is low ≈ 2 wt%, and for perovskite they vary between 3 - 8 wt%. The ternary classification CaO-SiO2-TiO2 diagrams shows that the crystallization temperature condition of sphene is low compared to perovskite [
Phenocryst cores contain around 19 - 22 mol % kalsilite (Ks) and generally contain excess Si. Although some cores are homogeneous, others are complex, with zones containing differing K and Fe concentrations. Enrichment in K and Fe is a feature of the rims seems of nepheline described by [
Spinels chemical compositions were calculated on the basis of three cations and the proportions of Fe3+ and Fe2+ were determined on the basis of charge balance [
The minerals of the melilite group consist mainly of the solid-solution series between åkermanite (Ca2MgSi2O7), gehlenite (Ca2Al2SiO7) and ferro-akermanite (Ca2Fe2+Si2O7). The most melilite however contain appreciable amounts of Na replacing Ca as well as Fe2+ replacing Mg [
The variations observed in the major element composition (
concentration (Cl, Na, Fe, Mn, P, and Ti). The low MgO (<2 wt%), Ni (<15 ppm) and Cr (<12 ppm) concentrations indicate that these are evolved lavas. It also has Al2O3 concentrations similar to the other late nephelinite and despite the presence of Fe-rich rims on nepheline, sodalite and pyroxene phenocrysts.
On the Harker diagram (
Trace element analyses of the nephelinite from Etinde massif are reported in
Pyroxene nephelinite | Melilite nephelinite | Olivine nephelinite | Garnet nephelinite | Perovskite nephelinite | haüynorphyre | ||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
ME35 | ME24 | ME133 | ME135 | ME12 | ME134 | ME34 | ME158 | ME152 | ME153 | ME36 | ME154 | ME155 | ME132 | ME32 | |
SiO2 | 40.66 | 39.92 | 41.40 | 40.00 | 36.69 | 39.20 | 40.01 | 41.20 | 40.16 | 43.59 | 43.54 | 43.34 | 44.19 | 39.89 | 40.23 |
TiO2 | 4.15 | 4.14 | 3.60 | 4.21 | 6.77 | 4.60 | 3.62 | 3.31 | 2.93 | 2.00 | 2.09 | 2.11 | 2.75 | 2.98 | 4.23 |
Al2O3 | 15.01 | 14.58 | 13.50 | 14.50 | 11.10 | 13.01 | 14.03 | 17.01 | 17.98 | 19.88 | 19.67 | 19.65 | 19.44 | 17.92 | 15.20 |
Fe2O3 | 12.00 | 12.28 | 12.20 | 13.00 | 13.02 | 13.69 | 12.06 | 10.85 | 10.00 | 7.80 | 7.97 | 7.73 | 7.20 | 9.40 | 12.12 |
MnO | 0.20 | 0.33 | 0.20 | 0.22 | 0.30 | 0.24 | 0.43 | 0.40 | 0.31 | 0.65 | 0.37 | 0.27 | 0.96 | 0.22 | 0.24 |
MgO | 5.12 | 5.25 | 8.90 | 6.23 | 6.40 | 8.40 | 5.22 | 3.98 | 3.55 | 0.78 | 1.20 | 1.23 | 1.38 | 3.50 | 4.44 |
CaO | 14.20 | 14.34 | 12.60 | 14.56 | 18.20 | 15.77 | 14.78 | 12.33 | 10.46 | 6.20 | 6.12 | 6.22 | 5.79 | 11.01 | 12.98 |
Na2O | 3.65 | 3.14 | 3.70 | 3.01 | 2.72 | 2.41 | 3.02 | 5.90 | 7.02 | 8.46 | 8.09 | 7.30 | 6.14 | 7.09 | 4.10 |
K2O | 2.00 | 2.63 | 2.30 | 2.22 | 1.60 | 0.40 | 0.59 | 1.17 | 3.94 | 7.78 | 7.23 | 7.13 | 4.33 | 3.28 | 2.20 |
P2O5 | 1.22 | 1.08 | 0.60 | 1.21 | 2.20 | 0.63 | 1.01 | 0.65 | 0.76 | 0.16 | 0.20 | 0.17 | 0.83 | 1.00 | 1.12 |
SO3 | 0.04 | 0.22 | 0.12 | 2.2 | 0.12 | 0.43 | 0.21 | 2.02 | 0.02 | ||||||
Cl | 0.01 | 0.11 | 0.05 | 0.04 | 0.01 | 0.3 | 0.23 | 0.39 | 0.57 | 6.29 | 0.4 | 0.03 | |||
PF | 1.08 | 1.42 | 2.19 | 1.21 | 1.33 | 5.2 | 2.45 | 0.56 | 1.52 | 2.2 | 3.11 | 0.78 | 2.44 | ||
Total | 99.34 | 99.44 | 99.00 | 101.35 | 100.21 | 99.73 | 100.13 | 99.26 | 100.17 | 99.17 | 99.50 | 99.04 | 99.30 | 99.49 | 99.35 |
Ba(ppm) | 1003 | 1078 | 1858 | 826 | 781 | 766 | 795 | 1168 | 1011 | 3272 | 2903 | 3500 | 2999 | 889 | 1700 |
Rb | 47 | 93 | 100 | 96 | 55 | 77 | 100 | 94 | 115 | 248 | 221 | 199 | 232 | 108 | 73 |
Th | 14 | 17 | 17 | 19 | 30 | 13 | 13 | 13 | 17 | 11 | 10 | 10 | 4 | 16 | 13 |
Sr | 2693 | 2498 | 2897 | 2368 | 2625 | 1598 | 2919 | 3610 | 2400 | 5423 | 7102 | 6567 | 8169 | 2149 | 3283 |
Zr | 594 | 632 | 540 | 724 | 605 | 444 | 666 | 742 | 618 | 1001 | 932 | 932 | 1094 | 612 | 747 |
Hf | 4.5 | 6 | 5.1 | 4.3 | 3.4 | 5.2 | 7 | 4.1 | 5.2 | 9 | 6.8 | 6.1 | 8.9 | 6.1 | 5.6 |
Nb | 213 | 254 | 273 | 150 | 141 | 93 | 228 | 267 | 298 | 295 | 316 | 288 | 275 | 276 | 247 |
Ni | 12 | 13 | 14 | 23 | 25 | 45 | 22 | 7 | 9 | 3 | 4 | 3 | Na | 10 | 14 |
Co | 43 | 38 | 23 | 66 | 87 | 37 | 33 | 38 | 17 | 19.2 | 41 | 55 | 122 | 24 | 51 |
Zn | 121 | 141 | 138 | 85 | 97 | 102 | 147 | 129 | 154 | 222 | 222 | 211 | Na | 142 | 149 |
Cr | 8 | 9 | 13 | Na | Na | 113 | 27 | 7 | 4 | 3 | 6 | 4 | Na | 4 | 12 |
La | 214 | 256 | 233 | 219 | 205 | 103 | 193 | 281 | 237 | 114 | 143 | 136 | 233 | 232 | 266 |
Ce | 467 | 501 | 456 | 392 | 431 | 211 | 366 | 503 | 434 | 131 | 192 | 157 | 417 | 444 | 487 |
Nd | 172 | 187 | 163 | 111 | 141 | 89 | 149 | 185 | 154 | 26 | 47 | 39 | 122 | 155 | 160 |
Sm | 19 | 10 | 15 | 26 | 30 | 19 | 19 | 10 | 20 | 7 | 12 | 10 | 11 | 18 | 16 |
Eu | 6.1 | 5.1 | 3.2 | 9.1 | 11.8 | 4 | 6.9 | 5 | 5.1 | 6.3 | 5.7 | 3.8 | Na | 4.8 | 5.02 |
V | 243 | 225 | 297 | 245 | 287 | 222 | 133 | 267 | 289 | 304 | 288 | 233 | 376 | 244 | 289 |
Yb | 4 | 4.3 | 4.5 | 9.1 | 11.8 | 4 | 3.89 | 4.6 | 3.6 | 4.6 | 3.5 | 3.36 | Na | 5.1 | 5.4 |
Y | 36 | 42 | 39 | 48 | 36 | 29 | 37 | 36 | 41 | 42.7 | 36 | 32 | 33 | 56 | 51 |
negative Nb anomaly is more significant for Etinde. They also show small Zr and Y positive anomalies where as small negative anomalies for these two elements are observed in the olivine nephelinite.
Nephelinite rocks has lower overall REE abundances compared with the haüynophyre, exhibiting different concave up REE pattern with steeper LREE part (La/Sm) = 18 compared with 4 - 5 in nephelinite. The trace element profile resembles that of nepheline from the Tanzania alkaline complex [
Primitive mantle-normalized trace-element patterns of Etinde samples (
Geochemical characteristics observed on the nephelinite lavas are similar or identical of the Mount Cameroon lavas [
The nephelinite rocks described here are insignificant volume when compared to the basaltic rocks of Mont Cameroon, but probably abundant ultramafic rocks of the Cameroon Hot Line. The interstitial mineral reveals ambiguous textural position. Although forming the analcime, there is ample textural evidence that it has replaced zeolite. This suggests that it is late magmatic origin. The oxygen fugacity lower and silica activity evolution at the late-magmatic and metasomatic process of evolution in the nephelinite could be constrained from mineral equilibria of the Equations (1), (2) and (3). The nephelinites have high Ba and Zr, and to a lesser degree high K. The Ba/Zr (1.4) and Ba/Nb ratios are relatively high compared to some nephelinite (Ba/Zr < l, Ba/Nb > 3) which suggests either amphibole was low in the mantle source [
Mineralogical, geochemical and trace elements are reported for the main nephelinite rocks on the Mount Etinde. The major and trace element composition of the host alkaline nephelinite suggest that fractional crystallization is not the only process involved in the petrogenesis of the nephelinitic lavas. The variation observed in their major and trace element position could be partly explained by fractional crystallization processes. The negative P anomaly may indicate that apatite may be a residual phase. These variations are consistent with crystallization processes dominated by removal of clinopyroxene ± olivine and nepheline by low aSiO2 and low fO2. The residual minerals in their source are mainly amphibole and garnet, possibly subordinate orthopyroxene (Equation (1)). The large variations documented by the isotopic data suggest the presence of heterogeneous mantle involving HIMU and EMI component [
The high Zr/Hf (113 - 163) observed in the nephelinite lavas indicated the assimilation of carbonatic liquid near the magma chamber after the pneumatolytic processes of the basaltic magmas origin in agreement by [
We are grateful to the following individuals who have greatly aided this study.
Ntoumbé, M., Déruelle, B., Mbowou, I.B.G. and Ngounouno, I. (2016) New Petrological and Geochemical Data of the Nephelinitic Lavas and Geodynamic Implications of Mount Etinde (Cameroon). International Journal of Geosciences, 7, 1452-1470. http://dx.doi.org/10.4236/ijg.2016.712101