A phlogopite-amphibole-plagioclase bearing websterite from Guinadji volcano (Adamawa Plateau, Northern Cameroon): evidence of a cumulate origin and crustal event

Kagarabi, Paterne Mulimbi; Ntaflos, Theodoros; Tamen, Jules; Nkouathio, David G.; Tumba, Kaniki

doi:10.2478/mipo-2025-0011

Figures & Tables

(a) Location of Adamawa plateau (red star) and other principal volcanic centers along the CVL (modified according to Nkouathio et al., 2008). Dashed lines are limits between the CVL segments: ocean, COB and continent. (b) Location map of the CVL. The principal geologic features of Africa (Congo Craton, CVL, East African Rift, Hoggar, Kalahari Craton, Tibesti, West African Craton) are indicated. COB, continental ocean boundary; CVL, Cameroon volcanic line.

Photomicrographs of the studied websterite. (a) Cumulative texture. (b) Interstitial phlogopite. (c) Edenite as rim around cpx. (d) Three aspects of opx. Dashed lines represent their triple junctions. hbi, host basalt infiltration, op, opaque minerals; Phl, phlogopite.

BSE images of the studied websterite. (a) Association phlogopite-amphibole-rutile. (b) Amphibolitization of clinopyoxene. (c) Amphibole elongated exsolution lamellae following clinopyroxene cleavage. (d) Plagioclase as intercumulus phase. (e) Image showing that amphibole and phlogopite are of metasomatic origin. (f) Clinopyroxene remants as inclusion in plagioclase. sul, sulfides.

Chondrite normalized REE (a) and spider diagram (b) of the Guinadji volcano websterite; normalization values are from Sun and McDonough (1989). REE, rare earth elements.

Pyroxene composition from the studied websterite plotted on the En-Wo-Fs ternary diagram after Morimoto et al. (1988).

Clino- and orthopyroxene mineral chemistry plots. (a) CaO (wt%) versus Mg# content of clinopyroxene, (b) Al2O3 versus Mg# for clinopyroxene, (c) CaO (wt%) versus Mg# content of clinopyroxene, (d) Al2O3 versus Mg# for clinopyroxene. Plagioclase-bearing olivine websterite from Serri et al. (1988), Layered websterite from Wilson and Chaumba (1997), Abyssal crustal cumulate pyroxenite after Dantas et al. (2007); Mantle websterites from Rogkala et al., 2017; Garnet websterites from Lu et al., 2018, Befang websterite from Tedonkenfack et al. (2021).

(a) Chemical composition of phlogopite on the Mg/(Fe + Mg) versus Altot classification diagram of Rieder et al. (1998). Red circles represent chemical compositions of phlogopite. (b) Amphibole classification diagram after Leake et al. (1997). The blue field represents amphiboles described by Pintér et al. (2015) and Njombie Wagsong et al. (2018). TFA, Tetra-Ferri-Annite; TFP, Tetra-Ferri-Phlogopite.

Whole-rock CaO–Al2O3–MgO plot of the Guinadji volcano websterite (references of the fields after Selim et al., 2025).

Representative EMPA of phlogopite and amphibole (wt_ %)_

	Phlogopite								Amphibole
SiO₂	38.93	39.62	39.32	39.48	39.16	38.91	39.53	38.8	44.75	45.58	44.20
TiO₂	3.56	3.61	3.58	3.61	3.67	3.69	3.56	3.56	1.81	1.64	1.65
Al₂O₃	14.03	13.93	13.7	14.01	14.04	14.07	13.73	14.11	11.49	10.52	11.25
FeO	6.05	5.96	6.12	6.02	6.23	5.92	6.11	6.08	6.43	6.19	6.39
MnO	0.02	0.04	b.d.l	0.04	0.02	0.03	0.02	0.06	0.13	0.09	0.1
MgO	20.59	21.27	20.75	20.97	20.6	20.47	20.83	20.46	16.53	17.35	16.96
CaO	b.d.l	b.d.l	0.01	0.01	b.d.l	0.01	0.02	b.d.l	11.78	11.7	11.85
Na₂O	0.57	0.56	0.55	0.57	0.58	0.59	0.57	0.57	1.81	1.75	1.79
K₂O	9.61	9.57	9.67	9.55	9.51	9.42	9.56	9.49	1.71	1.57	1.64
F	2.95	3.04	2.95	3.14	2.79	3.00	2.65	2.87	n.a	1.50	n.a
Cl	n.a	n.a	n.a	n.a	n.a	n.a	n.a	n.a	0.02	0.02	0.02
BaO	0.37	0.39	0.36	0.39	0.38	0.43	0.33	0.36	n.a	n.a	n.a
Total	96.67	98	97.03	97.8	96.99	96.54	96.92	96.36	96.45	97.91	95.85
Si(a.p.f.u)	5.69	5.71	5.73	5.71	5.70	5.69	5.74	5.69	6.51	6.61	6.48
Ti	0.39	0.39	0.39	0.39	0.40	0.41	0.39	0.39	0.20	0.18	0.18
Al	2.42	2.37	2.35	2.39	2.41	2.43	2.35	2.44	1.98	1.80	1.94
Al^VI									0.48	0.41	0.42
Al^IV									1.49	1.39	1.52
Fe²⁺	0.74	0.72	0.75	0.73	0.76	0.72	0.74	0.74	0.78	0.75	0.78
Mn									0.02	0.01	0.01
Mg	4.49	4.57	4.51	4.52	4.47	4.47	4.51	4.47	3.59	3.75	3.71
Ca									1.84	0.82	1.86
Na	0.16	0.16	0.16	0.16	0.16	0.17	0.16	0.16	0.51	0.49	0.51
Na (M4)									0.11	0.08	0.04
Na (A)									0.41	0.41	0.47
K	1.79	1.76	1.80	1.76	1.77	1.76	1.77	1.77	0.32	0.29	0.31
F	1.36	1.39	1.36	1.44	1.29	1.39	1.21	1.33
Mg#	0.86	0.86	0.86	0.86	0.86	0.86	0.86	0.86	0.82	0.83	0.83
Na + K	1.95	1.92	1.96	1.92	1.93	1.93	1.93	1.93	0.83	0.78	0.83

Representative EMPA of rutile and iron oxide (wt_%)

Rutile			Iron oxide
SiO₂	0.09	0.07	0.04
TiO₂	96.73	98.97	b.d.l
Al₂O₃	0.18	0.05	b.d.l
Cr₂O₃	0.94	b.d,l	0.02
Nb₂O₅	0.15	0.28	b.d.l
FeO	0.74	0.65	90.50
MnO	b.d.l.	b.d.l	0.03
CaO	0.58	0.02	0.01
NiO	0.03	b.d.l	0.14
ZnO	0.01	0.02	0.03
V₂O₃	0.21	0.02	b.d.l
Total	99.65	100.09	90.78
Si (a.p.f.u)	0.001	0.001
Ti	0.980	0.993
Al	0.003	0.001
Cr	0.010	b.d.l
Nb	0.001	0.002
Fe²⁺	0.008	0.007
Ca	0.008	b.d.l.

Representative EMPA of pyroxene (wt_%)_

cpx

SiO₂	52.53	51.34	51.01	51.73	52.42	51.58	51.44	51.41	51.5	52.16
TiO₂	0.35	0.48	0.50	0.40	0.37	0.47	0.4	0.49	0.56	0.46
Al₂O₃	3.07	4.05	4.21	3.54	3.22	3.95	3.66	4.16	4.5	3.98
Cr₂O₃	0.31	0.38	0.4	0.38	0.32	0.40	0.38	0.40	0.39	0.40
FeO	5.63	5.38	5.37	5.46	5.47	5.53	5.56	5.49	5.53	5.53
MnO	0.16	0.11	0.17	0.16	0.17	0.13	0.16	0.15	0.14	0.12
MgO	15.47	14.97	14.85	15.28	15.42	14.95	15.04	14.85	14.68	14.95
CaO	22.00	22.08	21.96	21.95	21.82	21.91	21.99	21.97	21.8	21.94
Na₂O	0.45	0.49	0.48	0.48	0.45	0.5	0.44	0.51	0.53	0.48
NiO	0.01	0.01	0.02	b.d.l	0.04	0.02	0.01	0.01	0.02	0.01
Total	99.98	99.28	98.98	99.39	99.71	99.44	99.08	99.44	99.64	100.04
Si (a.p.f.u)	1.93	1.9	1.89	1.91	1.93	1.9	1.91	1.9	1.9	1.91
Ti	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.02	0.01
Al	0.13	0.18	0.18	0.15	0.14	0.17	0.16	0.18	0.20	0.17
Cr	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.01
Fe³⁺	0.01	0.03	0.03	0.03		0.02	0.03	0.02
Fe²⁺	0.16	0.14	0.14	0.14	0.16	0.15	0.14	0.15	0.17	0.17
Mn			0.01		0.01		0.01
Mg	0.85	0.82	0.82	0.84	0.85	0.82	0.83	0.82	0.81	0.82
Ca	0.87	0.87	0.87	0.87	0.86	0.87	0.87	0.87	0.86	0.86
Na	0.03	0.04	0.03	0.03	0.03	0.04	0.03	0.04	0.04	0.03
Mg#	0.84	0.86	0.86	0.86	0.84	0.84	0.85	0.85	0.83	0.83
Al^VI	0.06	0.07	0.08	0.06	0.07	0.08	0.07	0.08	0.09	0.09
Al^IV	0.07	0.10	0.11	0.09	0.07	0.10	0.09	0.10	0.10	0.09
Al^VI/Al^IV	0.83	0.7	0.69	0.67	0.97	0.79	0.69	0.77	0.91	1.02
Wo	45.91	46.88	46.91	46.24	45.9	46.59	46.53	46.84	46.84	46.63
En	44.92	44.2	44.14	44.79	45.13	44.23	44.28	44.03	43.88	44.2
Fs	9.17	8.92	8.95	8.98	8.98	9.18	9.19	9.14	9.28	9.17

opx

SiO₂	53.89	53.3	54.02	54.26	54.27	54.55	54.16	53.77	53.84	53.72	54.07	54.23	54.12	54.1
TiO₂	0.12	0.12	0.10	0.12	0.11	0.12	0.12	0.11	0.10	0.11	0.12	0.12	0.14	0.11
Al₂O₃	2.23	2.7	2.32	2.24	2.29	2.05	2.32	2.28	2.28	2.54	2.21	2.48	2.78	2.23
Cr₂O₃	0.17	0.23	0.17	0.14	0.17	0.13	0.20	0.20	0.21	0.23	0.20	0.23	0.25	0.15
FeO	14.07	14.23	14.2	14.08	14.23	14.17	14.17	14.19	14.08	14.24	14.14	14.25	14.29	14.43
MnO	0.3	0.28	0.28	0.29	0.28	0.27	0.29	0.29	0.31	0.28	0.29	0.27	0.31	0.29
MgO	27.79	27.64	27.65	27.43	27.39	27.79	27.54	27.36	27.55	27.45	27.61	27.38	27.3	27.4
CaO	0.89	0.61	0.78	0.75	0.75	0.83	0.80	0.85	0.86	0.70	0.81	0.80	0.73	0.78
Na₂O	0.03	0.01	0.03	0.02	0.02	0.01	0.02	0.03	0.02	0.09	0.02	b.d.l	0.03	0.02
NiO	0.02	0.02	0.02	0.01	0.05	b.d.l	0.02	b.d.l	0.04	b.d.l	0.02	0.01	0.04	0.01
Total	99.51	99.14	99.59	99.36	99.6	99.91	99.66	99.07	99.3	99.38	99.48	99.77	100	99.53
Si (a.p.f.u)	1.94	1.92	1.94	1.96	1.96	1.96	1.95	1.95	1.94	1.94	1.95	1.95	1.94	1.95
Al	0.09	0.11	0.10	0.10	0.10	0.09	0.10	0.10	0.10	0.11	0.09	0.10	0.12	0.09
Fe³⁺	0.02	0.02	0.01						0.01	0.01
Fe²⁺	0.40	0.41	0.42	0.42	0.43	0.42	0.43	0.43	0.42	0.42	0.43	0.43	0.43	0.44
Mn	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.01
Mg	1.49	1.49	1.48	1.48	1.47	1.49	1.48	1.48	1.48	1.48	1.48	1.47	1.46	1.47
Ca	0.03	0.02	0.03	0.03	0.03	0.03	0.03	0.03	0.03	0.03	0.03	0.03	0.03	0.03
Mg#	0.79	0.79	0.78	0.78	0.77	0.78	0.78	0.78	0.78	0.78	0.78	0.77	0.77	0.77
Al^VI	0.03	0.04	0.04	0.05	0.05	0.04	0.05	0.04	0.04	0.04	0.04	0.06	0.06	0.05
Al^IV	0.06	0.08	0.06	0.04	0.04	0.04	0.05	0.06	0.06	0.06	0.05	0.05	0.06	0.05
Al^VI/Al^IV	0.54	0.52	0.74	1.26	1.2	0.95	0.9	0.76	0.7	0.69	0.79	1.10	1.03	0.9
Wo	1.76	1.21	1.55	1.51	1.51	1.65	1.58	1.69	1.71	1.4	1.62	1.6	1.46	1.56
En	76.51	76.65	76.42	76.47	76.27	76.47	76.37	76.15	76.39	76.38	76.42	76.16	76.17	75.98
Fs	21.74	22.13	22.03	22.03	22.22	21.88	22.05	22.15	21.9	22.22	21.96	22.24	22.37	22.45

Major (wt_%) and trace elements (ppm) bulk rock compositions of the phlogopite-amphibole-plagioclase-bearing websterite xenolith of Guinadji volcano_

SiO₂ (wt%)	51.54
TiO₂	0.43
Al₂O₃	4.88
Cr₂O₃	0.25
FeO^*	9.5
MnO	0.19
MgO	19.55
CaO	12.98
Na₂O	0.58
K₂O	0.12
P₂O₅	0.07
BaO	0.01
LOI	0.32
Mg#	0.79
Total	100.42
Ba (ppm)	100
Ce	11.1
Cr	1690
Cs	0.05
Dy	1.89
Er	1.15
Eu	0.61
Ga	7.8
Gd	2.07
Hf	0.85
Ho	0.41
La	3.8
Lu	0.12
Nb	1.3
Nd	8.4
Pr	1.68
Rb	3.9
Sc	46.3
Sm	2.36
Sr	76.5
Tb	0.35
Th	0.2
Ti	0.28
Tm	0.12
U	0.08
V	248
Y	9.3
Yb	0.87
Zr	30

Representative EMPA of plagioclase (wt_%)_

SiO₂	52.92	52.15
TiO₂	b.d.l	0.01
Al₂O₃	29.06	29.77
FeO	0.14	0.15
MnO	0.02	0.01
MgO	0.05	0.06
CaO	12.04	12.60
Na₂O	4.42	4.04
K₂O	0.51	0.49
Total	99.17	99.28
Si (a.p.f.u)	2.42	2.38
Al	1.56	1.60
Fe²⁺	0.01	0.01
Ca	0.59	0.62
Na	0.39	0.36
K	0.03	0.03
An	58.30	61.47
Ab	38.75	35.71
Or	2.94	2.82

A phlogopite-amphibole-plagioclase bearing websterite from Guinadji volcano (Adamawa Plateau, Northern Cameroon): evidence of a cumulate origin and crustal event

Figures & Tables

Figure 1.

Figure 2.

Figure 3.

Figure 4.

Figure 5.

Figure 6.

Figure 7.

Figure 8.

Representative EMPA of phlogopite and amphibole (wt_ %)_

Representative EMPA of rutile and iron oxide (wt_%)

Representative EMPA of pyroxene (wt_%)_

Major (wt_%) and trace elements (ppm) bulk rock compositions of the phlogopite-amphibole-plagioclase-bearing websterite xenolith of Guinadji volcano_

Representative EMPA of plagioclase (wt_%)_

Paradigm

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