XIV semana de Geoquímica /VIII Congresso de Geoquímica dos Países de Língua Portuguesa
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K/Ar AGES OF CARBONATITES FROM THE ISLAND OF FOGO (CAPE VERDE)
J. Madeira1a, J. Munhá1b, C.C.G. Tassinari2, J. Mata1b, A. Brum da Silveira1a S. Martins1b
1-
Departamento Geologia/(a- LATTEX; b-Centro Geologia;), FCUL, Ed.C6, 2ºpiso, Campo Grande, 1700 Lisboa
2- Centro de Pequisas Geocronológicas, Instituto Geociências, Universidade de São Paulo, São Paulo, Brasil
RESUMO: Datações K/Ar obtidas em biotites nos carbonatitos da Ilha do Fogo indicam idade Pliocénica
Inferior a Médio, o que é significativamente mais antigo que a admitida para o processo de edificação do vulcão
do Fogo. Estas datações suportam a hipótese de que os carbonatitos fazem parte de substracto mais antigo que
inclui a plataforma submarina entre o Fogo e Brava, da qual se elevam também os Ilhéus Secos/Rombos.
ABSTRACT: The K/Ar ages obtained during this study indicate an Early to Middle Pliocene age for carbonatite
rocks, relatively older than the age admitted for the Fogo volcanic building. This supports the hypothesis that the
carbonatites belong to an older basement. This basement must be responsible for the submarine platform
between Fogo and Brava, which includes the Secos/Rombos islets.
Introduction
The island of Fogo, in the archipelago of Cape Verde, is a relatively simple island-volcano, in
the sense that it is formed by a single central volcano. Its morphology corresponds to a conic volcanic
edifice, 27 km in diameter at sea-level, truncated by a 9 km wide, 1000 m deep summit caldera, inside
which a secondary cone (Pico do Fogo) grew up to the present maximum altitude of the island, 2.829
m. A flank collapse affected the oriental side of the volcano after the caldera formation and prior to the
growth of Pico do Fogo (Brum da Silveira et al., 1997). This collapse involved a 65º wide sector of the
volcano flank, opening the caldera depression to the east. Presently the collapse scar is marked by the
8,5 km wide opening of the caldera and by its lateral scarps. The northern scarps are almost
completely fossilized by recent lava flows, whereas the southern scarp still presents a 400 m high
steep slope.
The volcanic rocks of Fogo are divided into three main stratigraphic units: a carbonatite unit,
assumed by most authors to be the oldest, a major volcanic sequence related to the sub-aerial shieldbuilding of the island volcano, and a post caldera sequence that includes several historic eruptions.
This paper presents new K/Ar ages for the carbonatite rocks and discusses their relations with the
other units and the volcano structure.
Volcanic stratigraphy of Fogo
Crystalline carbonate rocks of Fogo are known to the scientific community since the
publication, in 1913, of the work of Friedlander and Bergt (Beiträge zur Kenntnis der kap-verdischen
Inseln), but were only recognized as carbonatites by Assunção and co-workers in 1966. However,
these rocks were already known to the population that used carbonatite to draw white motifs in black
stone pavings. Carbonatite rocks outcrop in three locations on the west slope of Fogo, close to the city
of S. Filipe (two in the valley of Ribeira do Pico near Monte Almada, and one in Ribeira da Trindade
north of Monte Barro). Only two of the three outcrops mentioned in earlier reports were found. The
dense network of nefelinite, melilite and phonolite dikes (Machado and Assunção, 1965) that cut the
carbonatite bodies, together with the limited dimension of the outcrops, makes it unfeasible to
understand the original geometry of the carbonate rocks (however, sub-horizontal flow structures
suggests that they might correspond to lava flows). The carbonatites are exposed on the bed and valley
walls of two rivers as a result of fluvial incision on recent (post-caldera) lava flows and torrential
conglomeratic and sandy sediments that unconformably overlie the older sequence (Fig.1). The
carbonatite rocks must be either a part of the main shield building volcanic unit or older than that
sequence. However, this geometric relation is not visible on outcrop.
The main shield building eruptive sequence includes the volcanic rocks and intercalated
sediments that constitute the main volcano of Fogo (outer cone of Ribeiro, 1954, or pre-caldera lavas
of Machado and Assunção, 1965). This sequence is exposed on the caldera walls (Bordeira), around
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the depression of Chã das Caldeiras. It is composed a pile of aa lava flows (alternating clincker and
massive flows), basaltic, basanitic, phonolitic and trachitic in nature, exposed in the stratigraphically
lowest part of the sequence at Monte Amarelo (Portela) spur, passing upwards to alternating aa and
pahoehoe lava flows (basanites, basalts, and ankaramites) and thick (4-24 m) conglomeratic sediments
(debris flows or lahars) in the southern caldera wall, or primarily basaltic lava flows in the northern
caldera wall. Locally, some consolidated deposits present piroclastic flow facies. The sequence is cut
by abundant dikes, which constitute a dense dike swarm in the central area of the Bordeira, including
tefrites, nephelinites, basanites, augitites, limburgites, nepheline or hauyne bearing leucitites,
ankaratrites with nepheline, and phonolites.
W
E
clincker
fluviatile sandstone
10 m
basaltic
lava flows
valley floor
conglomerate
thermal
metamorphism
Figure 1. Geometric relations
between post-caldera lava flows and
sediments unconformably overlying
the carbonatites, as observed in
Ribeira do Pico valley. The relation
with the pre-caldera formations is not
observed.
dikes
carbonatite
The post-caldera sequence includes most of the surface lavas inside and outside the caldera,
which almost completely resurfaced the island. Inside the caldera the exposed materials include the
piroclastic cone of Pico do Fogo, its parasitic cinder cones and associated lava flows, and the torrential
sediments that cover part of the east slope, inside the flank collapse scar. Most of the eruptions that
formed these cones and lava flows occurred after the settlement of Fogo in mid 15th century (which led
to the abandon of the original name of the island, S. Filipe). Based on coeval documents, Ribeiro
(1954), Machado (1962) and Machado and Assunção (1965) refer that the Pico do Fogo cone may
have been in more or less permanent strombolian summit activity since discovery of the island, with
sporadic effusion of lavas in 1500, 1564, 1604 e 1664, until mid 18th century when eruptions acquired
a individual character and became located around the base of the central cone. There are written
accounts of eruptions in 1769, 1785, 1799, 1847, 1852, 1857, 1951 and 1995 (Ribeiro, 1954; Machado
and Assunção, 1965; Torres et al., 1998). Outside the caldera no historic events occurred, but several
cones erupted in very recent times. These are located in the south-southeast flank, in the north close to
Mosteiros, and around S. Jorge to the northwest; in all three areas lavas flowed over the sea cliffs
creating littoral platforms. One of them produced a surtseyan cone, presently surrounded by sub-aerial
lavas of later eruptions. Besides these very young eruptions, whose lavas can still be cartographically
individualized, the surface volcanic rocks of the outer slopes of the Fogo volcano became older from
south to north. Post-caldera lavas are predominantly basanites, olivine or pyroxene basalts,
ankaramites, limburgites and augitites (Silva et al., 1997).
Fogo Carbonatites
The petrography of Fogo carbonatites has been described by Assunção et al. (1966);
geochemical (elemental/isotopic) data on those rocks has been recently reported by Hoernle et al.
(2002). The studied carbonatites display a coarse-grained calcite matrix, including (relatively large)
biotite crystals, together with variable amounts of apatite, pyrochlore, iron oxides, fluorite and
aegirine. Analysed carbonatites are sovites that have high CaO (48.88 – 54.08 wt%) and very low
MgO (0.52 – 1.70 wt%) contents; their trace element characteristics (Nb/Ta = 47 – 96; Zr/Hf = 70 153; Nb/U = 9 – 12; Ce/Pb = 75 – 117) are in accord to those reported by Hoernle et al. (2002) for
Cape Verde oceanic carbonatites. Biotites separated for geochronological analysis have homogeneous
major element compositions (Mg# = 0.65 – 0.67; Al2O3 = 12.2 – 12.5 wt%; TiO2 = 2.69 – 2.74 wt%)
and preserve the characteristically high Nb/Ta, Zr/Hf and Nb/U elemental ratios of their host
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carbonatite rocks; representative biotite geochemical patterns, as normalized to their respective
carbonatite host rocks, are shown in figure 2. K/Ar geochronological results for (four) biotite separates
are summarized in Table I. Biotites yielded ages ranging from 3.2 Ma up to 5.1 Ma, which are in
general agreement with the ages of 3.7 Ma, 4.1 and 4.8 Ma (see Hoernle et al, 2002) previously
reported for Fogo carbonatites. All the available carbonatite ages are significantly older than the
relatively recent ages admitted for the Fogo volcanic building.
Figure 2. Biotite geochemical patterns, normalised relative to their respective host carbonatite
Table I: K/Ar geochronology of biotites from Fogo carbonatites
% Atm Ar
Age  1 (Ma)
0.92
86.91
4.1  0.5
6.0965
0.89
91.47
3.8  0.7
CV-2B B43
6.2876
1.25
82.41
5.1  0.9
CV-IND2 B43
4.6885
0.59
91.00
3.2  0.7
Biotite Sample
K wt%
CV-1A B43
5.8413
CV-1B B43
40
Ar Rad (*10-6)
CCSTP/g
Conclusions
The eccentric and very shallow position of the carbonatite outcrops in the volcanic construction
of Fogo is compatible with the hypothesis that they could constitute some of the youngest lavas in the
pre-caldera sequence, instead of an older substrate. However, the degree of intrusion by dikes of the
carbonatite bodies, and the fact that these rocks do not outcrop in the sea cliffs near by, suggest that
they should constitute some irregular residual relief of older rocks, which is in agreement with the
hypothesis of Assunção et al. (1966) that they might belong to a common basement with the island of
Brava and the Secos or Rombo islets, where carbonatite rocks were also described (Fig. 3. 2). The
K/Ar ages obtained during this study indicate an Early to Middle Pliocene age for carbonatite rocks,
relatively older than the age admitted for the Fogo volcanic building. This supports the hypothesis that
the carbonatites belong to an older basement (Fig. 3. 3). In this case, this basement must be
responsible for the submarine platform between Fogo and Brava, which includes the islets and
presents a circular shape. It is unlikely that this basement is part of the Fogo volcanic edifice, in which
case it wouldn’t outcrop because the degree of erosion in the island is clearly insufficient to expose its
own basement.
Acknowledgements
This study is a contribution to research projects INTERGEOFOGOS (POCTI/CTA/35614/2000),
POCA-PETROLOG (Centro Geologia U. Lisboa, UI: 263; POCTI/FEDER) and GEODYN
(LATTEX, POCTI-ISFL-5-32).
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25º W
3. 2
23º W
24º W
3. 1
Santo Antão
0
300
17º N
17º N
Sal
Santa Luzia
S. Vicente
S. Nicolau
Boa Vista
16º N
100
0
10
0
16º N
0
200
0
300
Maio
Fogo
15º N
15º N
Santiago
Brava
4000
0
50
100 km
23º W
25º W
WSW
ENE
3. 3
Post-caldera volcanism
Post-caldera volcanism
+ +
+ + + +
+ + + + +
+ + + + + +
+ + + + + + + + + +
+ + + + + + + + + + +
Fogo-Brava basement
Post-caldera flank collapse
Pre-caldera volcanism
A
B
Figure 3. Geographic relation between Fogo, Brava and the Rombos or Secos Islets in the Cape
Verde archipelago (3. 1). A common basement, to which the carbonatites belong, may be represented
by the submarine platform from which Fogo, Brava and the islets rise. Crosses indicate the locations
where carbonatite rocks were observed in Fogo and Brava (from Assunção et al., 1966) (3. 2). This
basement may be present just underneath the west flank of the Fogo volcano, as represented in the
geologic structure section below (3. 3).
References
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Verde Islands. Bol. Soc. Geol. Portugal 16(1/2): 179-188.
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