Mineralogy of Peralkaline Silicic Volcanics: Information from Kone Volcano, Ethiopian Rift Valley

Abstract

The presented paper described in detail the mineralogy of silicic peralkaline eruptives from Kone volcano within the Ethiopian rift system, which is formed predominantly by rhyolite with some small occurrence of trachyte. The majority of eruptive rocks in the Kone volcanic area are phenocryst-poor. The studied rocks contain alkali feldspars (anorthoclase and sanidine), quartz, clinopyroxene (hedenbergite), aenigmatite and olivine (fayalite), accompanied by rare Fe-To oxides (ilmenite) and apatite. All these minerals are described in detail. These data are very interesting for all researchers, who study similar eruptive rocks.

1. Introduction

The Ethiopian rift valley, forming a part of the East African rift system, is the largest active continental rift on Earth (Figure 1). This rift shows transitional character between continental rift and seafloor spreading [1] as it is underlain by thinned crust intruded by mafic dykes. The Ethiopian rift is characterized by numerous volcanic centres most of which are still active as evidenced by strong fumarolic activity and episodes of deformation, such as rapid uplift [2]. Kone is one of these silicic volcanic edifices.

Figure 1.

Location of the active quaternary magmatic segments - zones of dense faulting and aligned eruptive centres which are the current locus of strain within the Ethiopian rift (southern Red Sea and main Ethiopian rift). Arrows show plate motions relative to stable Nubia. Danakil block is a microplate between the Nubian and Arabian plates. TGD is Tendaho-Goba’ad discontinuity marking the active and ancient boundary between the east African and the Red Sea rifts.

Kone volcano (also previously known as Gariboldi volcano [3, 4]) is composed of a series of silicic cones with a summit caldera. It is located (8.8 N, 39.69 E) at the end of the Ethiopian rift valley, near the junction with Afar depression (Figure 1). Kone covers an area of 250 km². The caldera is elliptical in shape (5 × 7.5 km wide structure) trending E-W [5, 6]. The rim of the caldera rises about 100 m above the caldera floor. The summit of the volcano has an elevation of 1619 m above mean sea level. Roughly N-S-trending regional normal faults and fissures, forming a part of the Wonji fault belt, cut across the caldera and its flanks, especially the eastern side of the volcano.

Despite their petrological and volcanological interest, there is very little published data on the mineralogy of peralkaline silicic volcanics. Here, we present electron microprobe analyses for comendites of Kone volcano within the Ethiopian rift valley (Figure 2). The aim of this paper is to document in detail the phenocryst compositions of cemendites.

Figure 2.

Simplified geological map of Kone volcano and its surroundings.

2. Geologic background

Volcanic activity commenced around 45 Ma in southern Ethiopia [7], resulting in volumetrically significant basaltic flows and associated rhyolites. Nevertheless, the peak of magmatism had occurred c. ~30 Ma ago, resulting from the impinging Afar mantle plume at the base of the Ethiopian lithosphere and leading to flooding basalt eruptions in Ethiopia and Yemen [8]. The Yemen plateau basalts were united to their Ethiopian counterparts prior to the opening of the Red Sea basin. At ~25 Ma continental rifting commenced in the southern Red Sea [9]. In Southern Ethiopia, extension began ~18 Ma ago [10] and was accompanied by basaltic magmatism, active for about seven to eight million years [7]. The southern Ethiopian rift propagated northward, reaching the present central MER ~14 Ma ago and ultimately joining the southern Red Sea rift ~11 Ma ago [11]. Contemporaneously to the connection between the main Ethiopian and Red Sea rifts, a flood basalt event occurred in this area. Beginning in the late Miocene and continuing throughout the Pliocene, silicic volcanic centres emerged from the rift floor [6]. Progressive weakening of the lithosphere in Afar, associated with heating and the thermomechanical erosion of the lower crust generated by the Afar mantle plume, resulted in the onset of oceanic rifting at 5.3 Ma [9]. Oceanic rifting is still active in Afar, whereas it has not commenced in the MER yet [11].

The Pliocene-Pleistocene boundary was marked by a change in the stress field, giving rise to oblique rifting [12]. Since then, an extension has been localized in narrow (50 km long, maximum 20 km wide) en-echelon arranged segments on the rift floor [6], with a system of bounding faults that are referred to as the Wonji Fault Belt [12]. Moreover, these segments have been the locus of volcanic activity throughout the Quaternary and are thus referred to as magmatic segments by [6]. Volcanic activity associated with the magmatic segment was initially characterized by large volumes of felsic lavas. When these faults reached the upper mantle in recent times, basaltic volcanism commenced [12]. At present, volcanic activity within the magmatic segment is dominated by fissural basalt eruptions [1, 6].

3. Eruptions at Kone volcano

Volcanological investigation reveals that three silicic eruptive episodes have occurred at Kone volcano [3, 4, 13, 14], followed by a basaltic phase (Figure 2). The first eruptive phase produced lavas, forming the main topographic expression of the volcano. The second phase of activity formed ignimbrite outflow sheets. The third silicic eruptive phase built widespread pumice deposits, varying in thickness from 50 cm to over 20 m. A separate, recent basalt eruption, erupted during the first half of the 19th century, formed cinder cones and associated lava flows inside and outside the caldera [15].

4. Analytical methods

Phenocryst and glass matrix compositions were determined by electron microprobe at Université de Loraine (Nancy, France). Prior to analyses, samples were coated with a thin film of carbon. Analytical conditions were an accelerating voltage of 15 kV, a probe current of 20 nA, and a beam spot diameter of 5 μm.

5. Mineralogy of Kone silicic eruptives

Kone volcano is predominantly rhyolite in composition with minor trachyte [13]. The majority of kone eruptive rocks are phenocryst-poor with total modal contents of <6%. Given the textural diversity of Kone silicic volcanics (lava, ignimbrite and pumice) we have only selected samples from the first phase (lava) and second phase (ignimbrite) eruptives to illustrate the mineralogy of the silicic melt. The dominant mineral assemblage, with decreasing order of abundance, is alkali feldspar, quartz, clinopyroxene, aenigmatite and olivine, accompanied by rare Fe-Ti oxides and apatite.

Alkali feldspar is overwhelmingly the most abundant phenocryst and forms tabular crystals. Most of the crystals are fragmented. Quartz occurs as rounded microphenocrysts. Clinopyroxene forms euhedral to subhedral pale green crystals. The Na-Fe-Ti silicate aenigmatite is a distinctive phenocryst constituent, recognized by its blood-red to black pleochroism. Olivine occurs as colorless, partly resorbed grains. Fe-Ti oxides form discrete equant crystals, but more commonly occur as inclusions within clinopyroxene. Small euhedral apatite prisms are a common accessory mineral. Quartz and alkali feldspar are common groundmass phases.

Phenocryst compositions of Kone silicic volcanics are reported in Tables 1–4. Compositions of alkali feldspar span the anorthoclase-sanidine boundary (Figure 3a). There is no notable compositional variation between individual samples (Table 1). Zoning is uncommon. Where present is usually normal, whereby the rim is richer in K₂O than the core (Table 1). Microlites of alkali feldspar have slightly more potassic composition than the alkali feldspar phenocrysts. It is noteworthy that analyses of alkali feldspar matrix are broadly comparable to those of alkali feldspar phenocryst rim. Clinopyroxene is generally hedenbergite (Figure 3b). Hedenbergite is essentially unzoned (Table 2). Aenigmatite shows very restricted variation in TiO₂, FeO and Na₂O (Table 3). Olivine appears almost pure fayalite and has a high content of MnO (4.33 wt.%, Table 4), which is a characteristic feature of peralkaline rhyolites [16]. Fe-Ti oxides are essentially ilmenite (Table 4). It is noteworthy that similar phenocryst and groundmass mineralogies occur.

Sample	7Cc	7Cr	11B	5D	4A	4A	4Am	3Am	3A	3Am
SiO2	67.69	66.64	67.00	67.28	67.78	67.57	67.93	67.32	67.65	67.90
Al2O3	18.16	17.85	17.86	19.07	18.69	18.30	18.43	18.30	18.65	17.96
FeO	0.63	1.30	0.97	0.41	0.61	0.42	0.64	0.86	0.47	0.69
CaO	0.00	0.00	0.01	0.04	0.01	0.02	0.04	0.00	0.06	0.00
Na2O	7.99	6.93	7.10	8.21	7.59	7.85	7.65	7.29	8.29	6.93
K2O	5.39	7.31	7.03	4.99	5.80	6.19	6.34	6.55	4.80	6.90
Total	99.87	100.03	99.97	99.99	100.49	100.34	101.03	100.31	99.93	100.38
Structural formula (8 oxygen)
Si	3.02	3.01	3.01	2.99	3.01	3.01	3.01	3.01	3.01	3.03
Al	0.96	0.95	0.95	1.00	0.98	0.96	0.96	0.96	0.98	0.94
Fe	0.02	0.05	0.04	0.02	0.02	0.02	0.02	0.03	0.02	0.03
Ca	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
Na	0.69	0.61	0.62	0.71	0.65	0.68	0.66	0.63	0.71	0.60
K	0.31	0.42	0.40	0.28	0.33	0.35	0.36	0.37	0.27	0.39
Total	5.00	5.03	5.02	5.00	4.99	5.02	5.01	5.01	4.99	4.99
%An	0.00	0.00	0.05	0.17	0.07	0.09	0.19	0.02	0.30	0.00
%Ab	69.27	59.02	60.51	71.32	66.50	65.76	64.61	62.84	72.18	60.40
%Or	30.72	40.98	39.44	28.50	33.43	34.15	35.20	37.14	27.52	39.60

Table 1.

Composition of alkali feldspar from Kone silicic eruptives.

The symbol C, R and M represent core, rim and matrix, respectively.

Sample	7C	11B	5D	4A	3A
SiO2	50.08	49.09	47.64	48.73	49.26
Al2O3	0.18	0.11	0.13	0.10	0.13
TiO2	0.41	0.34	0.44	0.21	0.23
FeO	28.46	28.95	29.13	28.26	28.87
MnO	1.29	1.13	1.69	1.48	1.33
MgO	0.40	0.69	1.06	1.77	0.70
CaO	15.44	15.87	18.39	17.79	16.19
Na2O	3.65	3.30	1.51	1.44	2.86
K2O	0.00	0.00	0.00	0.00	0.00
TOTAL	100.18	99.47	100.02	99.86	99.59
Structural formula (6 oxygen)
Si	2.01	1.98	1.94	1.98	2.00
Al^IV	0.00	0.01	0.01	0.00	0.00
T	2.01	1.99	1.95	1.99	2.00
Al^VI	0.01	0.00	0.00	0.00	0.00
Fe^3+	0.24	0.27	0.20	0.13	0.21
Ti	0.01	0.01	0.01	0.01	0.01
Mg	0.02	0.04	0.06	0.11	0.04
Fe^2+	0.71	0.71	0.79	0.82	0.76
Mn	0.04	0.04	0.06	0.05	0.05
M1	1.05	1.07	1.13	1.13	1.07
Ca	0.66	0.69	0.80	0.77	0.70
Na	0.28	0.26	0.12	0.11	0.22
K	0.00	0.00	0.00	0.00	0.00
T + M1 + M2	4.00	4.00	4.00	4.00	4.00
%En	1.43	2.37	3.36	5.67	2.39
%Fs	59.15	58.21	54.74	53.38	57.84
%Wo	39.42	39.42	41.90	40.95	39.77

Table 2.

Composition of clinopyroxene in Kone silicic volcanics.

Sample	7C	11B	3AC	3AR	3AM
SiO2	41.20	41.29	40.56	51.406	50.777
Al2O3	0.36	0.19	0.4673	0.1813	0.0887
TiO2	8.56	8.73	8.8818	2.2242	1.3667
FeO	41.18	41.02	41.664	31.074	29.278
MnO	1.42	1.07	1.1592	1.231	1.9151
MgO	0.23	0.30	0.238	0.5307	1.4578
CaO	0.29	0.20	0.3953	0.3743	0.6536
Cr2O3	0.00	0.01	0.0002	0.0002	0.0078
NiO	0.00	0.09	0.0745	0.0062	0.0002
Na2O	7.19	7.01	7.0118	5.7041	6.6961
K2O	0.00	0.00	0.0001	1.526	1.4608
Total	100.44	99.894	100.45	94.258	93.702
Structural formula (6 oxygen)
Si	1.6794	1.695	1.6576	2.1756	2.123
Al^IV	0.0175	0.009	0.0225	0	0
T	1.6969	1.7039	1.6801	2.1756	2.123
Al^VI	0	0	0	0.009	0.0044
Fe^3+	0.6677	0.6195	0.672	0.0486	0.2842
Ti	0.2623	0.2695	0.273	0.0708	0.043
Cr	6E-06	0.0002	6E-06	7E-06	0.0003
Ni	3E-06	0.0028	0.0024	0.0002	7E-06
Mg	0.0139	0.0183	0.0145	0.0335	0.0908
Fe^2+	0.7292	0.7821	0.745	1.0507	0.7366
Mn	0.0492	0.0373	0.0401	0.0441	0.0678
M1	1.7222	1.7298	1.747	1.257	1.227
Ca	0.0125	0.0086	0.0173	0.017	0.0293
Na	0.5685	0.5576	0.5556	0.4681	0.5428
K	5E-06	5E-06	5E-06	0.0824	0.0779
T + M1 + M2	4.00	4.00	4.00	4.00	4.00

Table 3.

Compositions of aenigmatite from Kone silicic eruptives.

The symbol C, R and M represent core, rim and matrix, respectively.

Sample	5D	Sample	5D	4A/5–2	4A/3–2	5D
	Fayalite		Ilmenite	Ilmenite	Apatite	Glass
No	34	SiO2	0.00	0.03	3.26	69.28
SiO2	29.07	Al2O3	0.00	0.00	0.00	9.62
FeO	64.98	TiO2	56.20	51.85	0.00	0.57
MnO	4.33	FeO	39.70	44.04	0.91	6.84
MgO	0.74	MnO	2.70	2.08	0.06	0.27
CaO	0.35	MgO	0.11	0.00	0.00	0.02
Total	99.47	CaO	0.07	0.05	46.84	0.03
		Na2O	0.02	0.00	0.40	2.59
		K2O	0.01	0.00	0.00	5.70
		P2O5	0.00	0.01	29.71	0.00
		Total	98.79	98.06	81.18	94.90

Table 4.

Compositions of olivine, ilmenite, apatite and glass matric in Kone silicic eruptives.

Figure 3.

Compositional ranges for alkali feldspar (a) and Ca-rich clinopyroxene (b) in silicic volcanics of Kone volcano.

Matrix glass has a rhyolite composition with SiO₂ content of 69.3 wt.% (Table 4). It has high K₂O content (5.7 wt.%), but extremely low Na₂O content (2.6 wt.%). The low Na₂O content may be due to deutric mobilization.

6. Peralkaline affinity of Kone silicic volcanics

Kone silicic eruptives are characterized by the phenocryst assemblage of alkali feldspar, quartz, hedenbergite, aenigmatite and fayalite. This assemblage is accompanied by rare ilmenite and apatite. The appearance of aenigmatite in silicic volcanics has been attributed to reflect a peralkaline affinity [17]. Peralkaline magmas likely form when fractional crystallization removes a high proportion of plagioclase relative to mafic minerals [18]. The coexistence of aenigmatite and hednbergite is thought to be a typical feature of the mildly peralkaline silicic rocks such as comendite and trachycomendite [19].

The presence of quartz as a major phenocryst phase in Kone silicic volcanics indicates an advanced stage of fractionation (i.e., the more SiO₂-rich magma). Furthermore, the SiO₂ content of the glass matrix (69.3 wt.%, Table 4) shows a rhyolite composition. Thus, the phenocryst mineralogies reflect the SiO₂ content of the magma from which they crystallized. On this basis, the alkali feldspar + hedenbergite + aenigmatite + fayalite assemblage only crystallizes in the most evolved magmas (rhyolites) with the appearance of quartz as a phenocryst phase.

On the basis of the mineral assemblage, the Kone silicic volcanics can be regarded as more mildly peralkaline rhyolite (comendite) as evidenced by the appearance of hedenbergite, which appears to cease crystallizing in strongly peralkaline rhyolite (pantellerite), generally coinciding with the crystallization of aegirine or aegirine-augite [19]. This is probably related to the low fO₂, at or close to FMQ, at which the magmas evolve [20]. We conclude that the alkali feldspar + quartz + hedenbergite + aenigmatite + fayalite assemblage is essentially restricted to the mildly peralkaline rhyolite (comendite), but close to the comendite-pantellerite boundary as crystallization of aenigmatite is restricted to pantellerite [16].

7. Conclusions

Kone silicic eruptives are characterized by the phenocryst assemblage of alkali feldspar (anorthoclase and sanidine), quartz, hedenbergite, aenigmatite and fayalite, accompanied by rare ilmenite and apatite. The existence of aenigmatite is a typical feature of the peralkaline silicic rocks. We conclude that the alkali feldspar + quartz + hedenbergite + aenigmatite + fayalite assemblage is essentially restricted to the mildly peralkaline rhyolite (comendite) as hedenbergite appears to cease crystallizing in strongly peralkaline rhyolite (pantellerite), generally coinciding with the crystallization of aegirine or aegirine-augite.

Acknowledgments

Funding has been provided by CNRS (Centre National des Recherches Scientifiques, France). We are grateful to the School of Earth Sciences of Addis Ababa University for logistic support.