![. RESI]LTS OF ELECIRON.MCROPROBE ATiIALYSES OF REPRESENTATTT/E CATAPLEIITE Cc-IIYDROCATAPLEIITB?) A}ID EUDIALYTEFROM THE LOS ARCIIIPELAGO](https://www.researchgate.net/profile/Daniel-Demaiffe/publication/235939315/figure/tbl3/AS:643568356036609@1530450061381/RESILTS-OF-ELECIRONMCROPROBE-ATiIALYSES-OF-REPRESENTATTT-E-CATAPLEIITE_Q320.jpg)
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28L
Thz C anndian M ine ralo
gi
st
Vol. 34, pp. 281-299
(1996)
THE LOS ARCHIPELAGO NEPHELINE
SYENITE
RING.STRUGTURE:
A MAGMATIC MARKER OF
THE EVOLUTION
OF THE GENTRAL AND EOUATORIAL
ATLANTIC
CHRISTIANMOREAU
Ddpanement dzs Sciences de l,a Tete et CNRS
- UM10, Pble Sciences et Technalogie,
Universitd de Ia Rochelle, Avmue Marillnc, 17M2 In Rochelle
Cedex I, France
DANIEL OHNENSTETTER
Centre
de Recherchz Pdtographiqucs et G4ochimiques
- Cmtre Natiorul de La
Recherchz Scientifiqw,
B,P. 20. 5450 1 Vandoewre-l?s-Nancv
Cedex,
Fraru:e
DANIELDEMAIFI1E
Ddpartemmt
des Sciences d.e IaTerre, C.P.
160/02, Universit€
Libre d.e
Brwelles,
50, avenue F.D. Roosevelt, 1050
Bru,xelles, Belgique
BERNARD ROBINEAU
Ddpartement des
Sciences
de la Terre, Universit€
de La R€unioa
15,
avenue Reni Cassin 97489 Saint-Denis
Cedex,
Ia. Rdunion
ABSTRACI
The Los Archipelago ring structure
(Guineq West Africa) is composed
of peralkaline
nepheline
syenite, which crops out
over two long crescent-shaped islaads
Qamara Kassa), five islets and a central islald (Roume).
A detailed
geological
study,
which led to a new geological
map,
recognizes
several series
within two main petographic suites, on the basis of textural and
structural
field criteria- The fust suite is miaskitic and is composed ofhastingsite-augite
nepheline
syenite, whereas
the second
suite is agpaitic and is mainly composed of arfuedsonite-aegirine
nepheline syenite. These formations are intruded by
microsyenite of the same
composition
and then by ring-shaped
monchiquitic and radial phonolitic dykes. Late pegmatites
cross-cut
al1 the rocks. A Rb-Sr whole-rock isochron gives an age of 104.3 x. 1.7 (2,o) Ma (MSWD = 1.28). The initial
Sr isotopic composition
(87SrF6Sr0
= 0.7040) is compatible with a maatle origin with minor or no crustal influence. The
geological
environment and the emplacement age of the Los Archipelago
point to formation during the continental
breakup
between the West Africa and South America and early rifting of the Equatorial Atlantic Ocean.
Keywords: miaskitic nepheline syenite, agpaitic nepheline
syenite, Rb-Sr geochronology,
continental
breakup,
Equatorial
Atlantic Ocean,
los Archipelago, Guinea.
Sotrcuenn
L'archipel de Los (Guin6e,
Afrique de I'Ouest), est un complexe
annulaire compos6
de sy6nite
n6ph6linique hyperalcaline
affleurant sur deux lles en forme de croissant
(Iamara et Kassa), cinq llots et ltle centrale
de Roume.
Une 6tude
gdologique
d6taill6e a permis d'6tablir une nouvelle carte g6ologique
of I'on a distingu6 au sein de deux grandes
suites de sy6nite
ndphdlinique
plusieurs
sdries
selon des critbres
texturaD(
et structuraux
6tablis sur
le terrain. la premibre
suite, miaskitique,
est
compos6e essentiellement de sy6nite n6phdlinique i hastingsite et augite.
La seconde
suite, agpartique,
est constitu& de sy6nite
n6ph6linique i aegyrine
et arfvedsonite. L'ensemble de ces sy6nites
est inject6 par des venues de microsy6nite de m6me
composition,
puis travers6
par des filons annulaires de monchiquite et radiaires de phonolite.
Des filons de pegmatite
tardive
recoupent l'ensemble des formations de la stnrcture. Une isocbrone Rb-Sr sur roche totale a donn6 un dge
de lM.3 t 1.7 (26)
Ma (MSWD = 1.28) pour I'ensemble des formations sy6nitiques. k rapport isotopique
initial (87SrF6Sr)e,
6gal i 0.7040,
est compatible avec une source
mantellique
l6g0rement
appauwie,
sans
(ou avec
peu) d'influence
crustale. L'environnement
g6ologique
et I'dge de mise en place de l'archipel de Los permettent
de le considdrer
comme un bon marqueur magmatique
intervenu lors du changement de r6gime tectonique, correspondant h la sdparation de I'Afrique de I'Ouest
et de I'Am6rique
du
Sud
et donnant naissance d l'Oc6an Atlantique Equatorial.
Mots-cl6s: sy6nite n6ph6linique miaskitique, sy6nite n6ph6linique agpartique,
datation Rb-Sr, s6paration
des continents, oc6an
Atlantique 6quatorial, archipel de Los, Guin6e.
282 TIIE CANADIAN MINERALOGIST
INTRoDUcnoN
The Los Archipelago
(9'30'N, 15o30'E),
located
5 km offshore Conakry (Guinea"
western
Africa), is
composed
oftwo long crescent-shaped
islands
(Kassa
and Tamara)
and five islets (Corail, Blanche, Cabri,
Fousset
and Poulet) that encircle
an inlernal
lagoon and
its cenfral island, Roume
(Figs. 1, 2). This nepheline
syenite complex was studied by Giirich (1887),
Lacroix
(1911),
Delaire
& Renaud
(1955),
Lazarenkov
(1975)
and
Moreau et al. (1986).
The circular
shape of
the archipelago
was first commented
upon
by Millot &
Dars (1959),
who interpreted
the islands as the expres-
sion of a ring structure comparable
to those observed in
Cameroon,
Nigeria" Madagascar,
South Africa and
western
Africa. Moreau et al. (L986) emphasized
the
importance
of the magmatic
structures in the syenites
and described
the xenotths, dykes,
and veins. In terms
of size
(12 km in diameter, 115 km2),
petrology
and
high level of emplacement, the Los complex is similar
to the Tlfmaussaq
massif (100 km2) in Greenland
(Jptan L974, Larsen & Sorensen 1987), but smaller
than the Khibina-Ixivozero pluton (2000 km2) in the
Kola peninsula
@udkin & Mitrofanov 1994,
Semenov
1994).
Most of the anorogenic ring-structures intrude either
a continental
infraplale environment, such
as
in the Arr,
Nigeria and Cameroon
provinces, or an oceanic
innaplate environmento
such as at Kerguelen Island
@lack et al. 1,985). In conffast, the Los complex
innudes a passive continental margin; it thus bears
some resemblance to the Canary and Cape Verde
complexes
(Cr€vola
et al. 1994), which also
intrude the
passive
margin of west Africa, and the Trinidade and
Fernando do Noronha intrusions, located on the
Brazilian passive
margin (Ulrich & Gomes 1981).
Fic. l. Geological
sketch
map
showing the location of the Los Islands
(Millot & Dam 1959) in the prolongation
of the Conakry
Peninsula-
Insert: location
of the studied area in West
Africa. l: Paleozoic white sandstone of the Bove basin;
2: hoterozoic
calc-alkaline
porphyritic biotite granite; 3: Kakoulima gabbro
and peridotite;
4: Los Islands nepheline syenite; 5: marine
alluvium.
r1 ffiffiffi
1234
EffiffiM
567E
br
Ta 35
a
Ta 16
{}'
{'
lEd
F
I
Ro 13e
I"S
_l
=l
Ta7
CORAIL
cosag,fo6a o 1 2km
\.7
TI{E LOS ARCHIPEI.AGO RING-STRUCTTJRB
Frc. 2. Geological
map of Los Archipelago. 1 : bauxitic lareite. Agpaitic suite (aegiine - arfuedsonite
- l6venite
- kupletskite
nepheline syenite): 2: fine-grained and microcrystalline tlpe; 3: medium- to coarse-grained
type; 4: brecciated
type.
Miaskitic suite
$astingsite
- augite nepheline syenite): 5: medium-
to coarse-grained
type; 6: "nebulous"
type; 7: mesocratic
type; 8: leucocratic type; br: occutence of endogenous
breccia.
283
A new 1:50 000 map of the complex
(Fig. 2),
completed as a result of a detailed structural and
petro-
logical study
(Moreau
et al. L986), shows that the main
rock-types
pass gadationally from one to the other;
sharp
contacts are completely lacking, which points to
a single phase of intrusion. Detailed mineralogical,
petrological
and geochemical
data have been
obtained
on the main rock-types
of the Los complex. An Rb-Sr
whole-rock isochron allows us to place ttre Los
archipelago magmatic event temporally in relation to
the evolution of the Central and Equatorial Atlantic
Ocean. A new interpretation of the emplacement of this
complex in its geodynamic
environment
is presented.
Gpor,ocrcar, SstnNc
From the geodynamic point of view, the Los
complex
intrudes a passive
continental margin made of
early Mesozoic continental plaform sediments that
overlie a Proterozoic basement
(Villeneuve 1984).
The Los intrusive complex was emplaced
during the
Cretaceous,
at the western edge of the Conakry
Peninsula,
consisting essentially of the elongate
(50 km) Kaloum ultramafic body. The eastem
part of
this complex,
known as
the Kakoulima
massif,
is made
of a thick sequence
of layered
peridotite
and
gabbro,
of
tholeiitic affinity, forming a huge
laccolith @elaire &
Renaud
1955,
Barrdre
1959, Diallo et al. 1992)
that
intruded the West African Proterozoic
basement
and
the Paleozoic
sediments
of the Bove basin
(Fig. 1).
The
Freetown layered
complex
(Sierra
t eone)
, dated at 193
t 3 Ma (Rb-Sr
age:
Beckinsale
et al. L977),
is similar
1o
ghs
lkkqulima massif.
These intrusions
have been
related to the formation of the Central
Atlantic passive
margin @iallo el aI. 1992). There is, however, no
direct field relation between
the Kaloum-Kakoulima
tholeiitic intrusive complex and the Los silica-under-
saturated alkaline suite. In particular, there is no
evidence that the younger
Los complex
cross-cuts
the
tholeiitic stratiform
complex
at depth.
284 TIIE CANADIAN MINERAI'GIST
Ftg-o RBlenoNsHrps
AND
Pg:nocnepny
Two main petrographic
suites
have been described
in the Los Islands, a miaskitic oneo
and the other
agpaitic (Fig. 2) (Lacroix 1911, Delaire & Renaud
1955, Moreau et al. 1986).
The distinction between
agpaitic and miaskitic suites is classically done on
the basis of minsral6gical
composition
(Sgrensen
1974). Xenolithsn
veins and dyke rocks cutting the
complex are grouped into families (phonolite,
microshonkinite,
and
monchiquite)
on the basis of their
petrography.
Detailed field and sfructural studies
of the whole
Los intrusive complex (Moreau et al. 1986) suggest
four stages of emplacement 1) the agpaitic and
miaskitic nepheline
syenite - microsyenile,
forming
the largest
part of the
complex,
were emplaced
together
at an early staget
2) the nepheline
microsyenite
and
aplite were emplaced later, followed by 3) ring dykes
(and
endogenous
breccias)
and, finally, 4) radial dykes
and late pegmatites.
The general
shape
of the whole
complex
and its internal structures
correspond
to those
of a lopolith. The center
of igneous
activity seems
to be
located on Roume Island, where a widespread
endogenous
breccia
is found. There is no evidence
for
the possible
existence
of two centers.
Agpaitic syenite
Agpaitic syenite cover only one quarter of the
surface
at los. Such rocks contain
l6venite-
and astro-
phyllite-group minerals (Gtrich 1887) and rare
minerals
such
as villiaumite (Lacroix 1908),
serandite
(.acroix 1931)
and steacyite
(Parodi
& Della Ventura
1987).
The
texture is foyaitic to locally tachytoid, as a
result of flow of the syenitic magma during its
emplacement.
A transition toward a microcrystalline
texture
has been
noted,
as well as toward a pegmatitic
facies.
Mineralized
druses occur,
especially
on Roume
Island
and in a quarry at the north ofKassa.
A detailed survey _permits to distinguish three
different types. 1) A coarse- to medium-grained
aegirine nepheline syenite shows an intergranular
foyaitic texture,
as in the extemal
rim (Ftg. 2) of some
islands
(northeast
of Kassa:
Ka2Tb, Ka ld; northwest
of Tamara:
Ta 35) and in the center
of the structure ar
Roume
Island @o 1, Ro 9). This syenile
is composed
of euhedral laths of perthitic feldspar, commonly
mantled by albite, and interstitial, anhedral
feldspathoids (nepheline,
sodalite, and analcime),
associated
with poikilitic Mn-rich arfvedsonite.
Euhedral
and zoned crystals of l8venite, kupletskite,
and
aegirine
are
associated
with small crystals
of pyro-
phanite,
rare
pyrochlore,
with small hexagonal
crystals
of catapleiite and fluorite in the interstices. 2) An
aegirine
- lAvenite
- nepheline
microsyenite
with rare
kupletskite shows a nfqesrystalline foyaitic texture,
locally with well-developed
frachytic fluidal features.
The nepheline
microsyenite is locally porphyritic, with
alkali feldspar phenocrys*. This rock type is found
mostly on the external
part of the complex,
along the
western coast of Tamara and eastgrn coast
of Kassa.
3) A brecciated nepheline
syenite has been
observed on
Roume Island, locally on the southeastem
part of
Tamara, and on the external parts of Kassa and
Temala.
Note that the transition between agpaitic and
miaskitic syeniles is progressive;
it has
been
observed
in Tamara
(Ta 35), Corail (Co 6a), Cabri and Blanche
(Bl7).
Miaskitic syenite
Miaskitic syenite occupies the outer rim of the
structure
(Fig. 2), the main part of the islands
of Kassa
(Ka 3) and
Tamara
CIa 16, Ta 7). Such rocls display
a
coarse-grained intersertal texture, with euhedral to
subhedral
alkali feldspar,
and nepheline
and analcime
within the interstices.
Augite commonly is included
within the poikilitic hastingsite
and is associated
with
euhedral crystals of titanite, apatite and titaniferous
magnetite.
At the scale of the map (Fig. 2), the following pet-
rographic types have been distinguished:
1) a meso-
cratic medium- to coarse-grained
nepheline syenite
represents the main lyps; it displays
an intergranular
to
intersertal
texture with local feldspathic
intergrowths
and mosaic concentrations of has 'ngsite,
titanite, and
auglte; 2) a 'onebulousoo
type of nepheline syenite,
characterized
by dark grey spotted polycrystalline
aggxegates
of nepheline; 3) a mesocratic nepheline
microsyenite,
characterized by a slightly porphyritic
heteromicrogranular texture; such rocks outcrop
mainly as narrow bands (a few decimelers) in the
northeastern
and northwestern
parts of Tamara and
along the western
coast ofKassa, and 4) a leucocratic
type occurs as a light grey, feldspar- and nepheline-
rich, medium to coarse-grained
rock, locally with a
pegmatitic (soutl of Kassa) or a microcrystalline
texture
(eastem
and
northeastem coast of Tamara).
The
dykzs, xenoliths and
pegmatites
Two types of dykes have been distinguished.
l)
Ring dykes
(decimeter
to meter thick) with low inward
dip. They consist mainly of phonolite. They are
centered on Roume Island, but also crop out on
Tamara, Kassa
and Corail islands.
The
phonolite
dykes
present either a porphyritic texfure or a fluidal
pilotaxitic lexture. The porphyritic texture (Ta 9) is
defined by sanidine phenocrysts
in a groundmass
composed
of spherulites
with radial intergrowths of
alkali feldspar and aegirine needles,
associated
with
nosean, and biotite. The fluidal piloaxitic
lexture (Ca 4) is composed
of needles of aegirine,
sanidine
microphenocrysts
that outline a flow-induced
T}IE LOS ARCHIPET,AGO
RING.STRUCTURE 285
lamination, and microlites in a nepheline- and
analcime-rich groundmass
with rare crystals of
fluorite. 2) Radial dykes (centimeter to decimeter
thick) are homogeneously
distributed throughout the
intrusive
complex.
They consistmainly
of monchiquite
and shonkinite. The monchiquite dyke from Roume
Island (Ro 13e) shows an intersertal textwe: zoned
titaniferous augite and hastingsite
phenocrysts
locally
occur as glomeroporphyritic
clusters
in a groundmass
composed
of intersertal alkali feldspar and microlites
of titaniferous augite. Hastingsite, apatite, and
titaniferous
magnetite,
analcime, and
nepheline fill the
interstices; calcite occurs as a secondary mineral. The
microshonkinite
dyke (Ta 18d) is a medium-grained
porphyritic rock with an intersertal texture. The
phenocrysts are titaniferous augite and olivine
(replaced by a smectite-goup minsl6l). The alkali
feldspar laths define
an intersertal
texture; microlites
of
titaniferous augite, hastingsite, biotite, and apatite
occur within an analcime-
and nepheline-rich
ground-
mass.
Magmatic
xenoliths are
coutmon, but represent
less
that L voL%o;
they are mainly found in miaskitic
syenite.
They vary in size from a few centimeters to
several
decimeters, and are usually ovoid, round or
ellipsoidal. Two main types have been identified:
l) leucocratic xenoliths of nepheline microsyenite
composition, and
2) mafic xenoliths of basanitic
com-
position, located only in the southem
paft of Tamara
Island (Ta 5c). On Corail Island, a large screen
(50 x 2 m) of micro-essexite
(Co 3a) occurs
within the
syenite.
It contains
syenitic xenoliths and is cross-cut
by pegmatitic
syenitic veins.
A phonolitic dyke cuts
all
the rocks.
A basanite
enclave from Tamara Island (Ta 5c)
shows an intersertal lexture with olivine (Fo6aa)
phenocrysts,
and plagioclase,
augite, and hastingsite
microlites, euhedral titaniferous magnetite,
analcime
and apatite.
The micro-essexite
screen
(Co 3a) from
Corail Island shows a microheterogranular
texture,
with zoned and resorbed
phenocrysts
of augite,
nnhedral alkali feldspar associated
witl augite
and hastingsite
microlites, titaniferous
magnetite,
and
apatite,
with interstitial nepheline and analcime
in the
groundmass.
Dykes of pegmatite are ubiquitous;
they are
related
to the main agTaitic
and miaskitic nepheline
syenites,
but they are
also associated
with various
dykes and
cut
some
of them (Moreau
et al. 1986).
Aller,vncar MsrHoDs
Concentrations
of major and
tace elements
in whole
rocks were determined
by inductively coupled
plasma
(CP) - atomic
emission
spectrometry
at the
University
of Clermont-Fenand
tr (Cantagrel
& Pin 1994).
The
FeO content
was established
by wet chemistry
at the
University of Nancy I. Analyses
of the minerals
were
obtained
with a Camebax
Microbeam
and
SX 50 elec-
fton microprobes with wavelength-dispersion
spec-
fometers attle joint CNRS-BRGM facility. Operating
conditions
were:
accelerating
voltage 15 kV and
beam
current
10
nA; counting
times
were 10 s for major ele-
ments and
20 s for minor elements.
Oxides and
natural
silicates
were
used as standards.
Data reduction
for the
microbeam analyses
was performed using the ZAF
method
of Henoc & Tong (1978) and,
for the SX 50,
6ing the PAP method
of Pouchou
& Pichoir (1984,
r99L).
Isotopic
analyses
were
carried
out at the Laboratoire
de P6trologie - G6odynamique
chimique of the
Universit6 Libre de Bruxelles. The samples were
processed
following standard chemical procedures
(1e., IIF-HCIOa dissolution and separation using an
anion-exchange
column). The isotopic composition
of Sr was measured
on double Re filaments with a
TABLE I. MINEMIIXIICAL COMPOSITION
OF
SS.ECTED SAI\{PI.ES
FROIVI
TIIE LOS
ARCIIPEI.AGO
lb2-rrb rgpEltboyonlto I N,A,s 1 + | t t + a +
no08gp8ltbE6nfterN,A,3+t++++
not agpe[lo8ysnns r il,A,S + + + I + +
'(ald agpcltlosyenlto r t{A,S + t a + + + +
Tqgtegpahbsyonlt€+N,Atttt+t+
Bl7 td$kltlosymi€ r NA + + 4 +
Tr td nlaldilr 8tqlo r N, A, E r r
Kr3 nhrktbsydlo I N,A + + I + +
Tr? nlaskillasysno + N,A,a e r + +
Co 0r mleskilb ryenne + N,A + + ? +
E
7
8
10
11
12
't3
lrl
Tr0
Cr4
Ro l39
Ta l8d
phonon + N,A"A r
phomlto. + N,
AS +
nonchhllto+N,Arr
nbrc€honKnlls { }{.A i + r +t
++
tt
t 5 Co 3a nbeaqqllo r N, A + r
tO TaSr ba@nlto r A + r +
Ield.: fttdspg0: Foid: N: nophdlng
.{. analoine,
S: so{tat{s;
OL olivinq Ae$ agirine,Ha$.: haciryfite; Arf: arFdsoaitq' I,tv.: lavenite
ltpl: tupl€0sLil€;
Im': ilneurhs;
TiEgl: eadftrous-roagde nrA: orfiatyto;
Crt.: c*apl€iit6,
fbo.: ftrorilo; Tii: titoito, Pyro.:
pyrocblorc; Ap.: apde'
286 THE CANADIAN MINERALOGIST
Fimigan MAT 260 mass spectrometer.
Between-run
precision
is better than
5 x 10-5. The
NBS 987 standard
gave
a value for 875t7t65r
of 0.71023
t I (2o on
the mean for 30 measurements. normalized to
865.7885t
= 0.1198). Rb and Sr concentrations
have
been measured
by X-ray fluorescence
or by isotope
dilution (8?Rb
and eSr'spike) where concentrations
were less than 30 ppm. The error on the Rb/Sr ratio is
32Vo.
T\e Rb/Sr age has been calculated
following
Williamson
(1968),
with an assignedenor
of 2Vo
onthe
87Rb/d6sr
ratios and of 4 x 10-5
(average
between-run
error) on the 8751165r
ratios,
except
where the isotopic
measurement
for a sample
was less
precise.
MnrsRAl,ocy
The petrographic
features and mineral associations
of the analyzed samples
are summarized
in Table l.
The two main suites
are distinguished
by mineralogy
and crystal chemisty. The agpaitic syenites
invariably
contain aegirine, arfredsonite commonly very much
enriched
in lvln (up to 9.2 wtVo
MnO), Mn-rich mica
(up to 6.8 wtVo
MnO), pyrophanile,
and Mn-bearing
minerals
such
as kupletskite
and l8venite. Villiaumite,
serandite, steacyite, and phases
rich in rare-earth-
elements
(RE4 and Zr, such
as eudialyte, catapleiite,
rosenbuschite
and pyrochlore, also characterize
the
agpaitic rocks. The miaskitic syenites are mineral-
ogically characlerized
by hastingsitic
amphibole,
with
lower Mn content
(up to 3.9 wtVo
MnO), Mn-bearing
mica, and an abundance
of titanite. The transition
between the two main suites is marked in some
samples
(Ta 35 and Bl 7) by intermediate features,
such
as the presence
of ltvenite instead of titanile. The
evolufion from the miaskitic to the agpaitic facies is
marked by the disappearance
of titanite, augite,
fuast'ngsite, and titaniferous magnetite,
whose roles are
played by the kupletskite-livenite association,
aegirine, arfuedsonite,
and
pyropharfte,
respectively.
The composition of the
pyroxenes,
calculated
on the
basis offour cations and six atoms of oxygen
(Iable 2),
evolves from augite in the miaskitic suite to aegirine-
augite and aegirine
in tle agpaitic suiie. In some rocls,
aegirine-augite occurs
as a relict core within aegirine.
The prismatic crystals of aegirine commonly show
oscillatory Tening. The concentration
of zirconium
is invariably high in the aegirine (ZrO2 may
reach L.73 wt%o) and comparable
to that in the
Illmaussaq suite
(Shearer
& Larsen 1994).
The concen-
tration ofmanganese
is high i1 both quadrilateral
and
Na-pyroxenes
(nomenclature
according to the IMA
recommendations: Morimoto 1988), with MnO content
up to 4.70 wtTo; thus in most cases
the pyroxene
could
be called manganoan
aegirine, manganoan
aegirine-
augite and manganoan
augite. Only a few alkaline
intrusive complexes are known to contain pyroxene
as Mn-rich as the Los manganoan aegjine (i.e.,
Kangerdlugssuaq: Kempe & Deer 1970, Layne et al.
1982). Compared
to the crystallization
trends of
sodium-rich
pyroxenes
from the Uterature
@g. 3), the
compositional
trend at Los is close
to the Tlfmaussaq
trend (Larsen 1976) for aegirine-rich
pyroxene, and
close to the Uganda
(Tyler & King 1967) and South
Q6roq (Stephenson
1972)
trends for the quadrilateral
TABLE 2 RESIJLTS OF
ELBCTRON.MICROPROBE
ANALYSES
OFREPRESENTATT!tsPYRO'GNESFROM THEI.oS ARCHIPELACO
@!le Kanb Kat?b RS R09t2 R0915 Rol Kald Ta35 Rol Tal6 Td6 Co6a Co6a Co6a
slo2 32-16 51.A 5L79 5139 5121 51J8 5rJ4 50.46 5{It5 49.q} 4.@ 4F.67 49.45 4&46
\a !.14 t8 0.49 039 111 096 0.35 0.09 lJt) 0.4 0.4r 12 0t6 lO1
ADDI l.m 090 0.91 0.71 ljo 088 l.O 039 0_57 2fi 1.65 2l1 /4r 2.70
F4OlelA 9,61 4"2 n,g 2&45 tl.6 nn 3L61 29,4t 2t.m 4 19 6J6 6.47 t.tl
FO€lq 0.83 4:70 238 I.4l 1.6 ?A 0.4 068 53? rrJT rLOg 6n 6.48 7.49
rrno 0.t/ 191 1.43 X6 1"51 u3 065 /6 t.A *2 4:10 LU L46 2
MsO 0 032 021 034 Ln 01? 0A 0J0 0.r8 6.86 132 833 &8., 1&
c@ 0.41, 2& 2.c9 395 0.11 Zt4 0.?3 Zu 5.35 l8J7 17.65 19.41 19.45 1892
l.la2o l3r3 ll3l 11.89 ll34 r27 11.89 UA7 1136 1025 t.trt Z.@ 2Or 2A 2s2
K20 0.o 0,m 0.m 0.01 o0l 0.@ 00r 0,t2 0.0r oo0 o.o2 0.00 o.m ooo
7tg2 0.46 1.09 O55 035 Lm 0J6 02 1.?3 0.69 O O.tl 0.06 O-A O.m
Nb205 0.m o10 0,m 0.00 0@ o3t 0.m 0.D 052 o.m o.m o.@ 0.00 o.0o
Tobl 1m.64 9897 lfr.@ ,9J4 1O3 1@.65 9.J0 9430 l@.46 99.64 9&7E 98J8 98.trr 99.16
0.031 0037 0.035 0@ 0.030 0.016 0.032 0.m 0.m1 o.oj7 0.016 0.00 0.018 o.ml
0.m3 ().68 0.014 00u 0.061 ooa 0.010 0.@ 0.044 0.013 0.0!2 0.036 0.0a5 o.(D
oajr, 0-o8 0.&B 0.7n 0.798 0.789 0il7 0.665 0"640 0.125 0120 o8 0.t88 0137
0,026 0.1t4 0!96 0JX6 0.047 0.106 0013 0.@ 0.1?4 03?0 0369 0.219 0,8 o2A1
oll8 0.063 0.047 0.U0 0.049 0056 0.12t 0.068 0.t@ 0.t2, 0.158 o.m 0080 0.0?4
0.0!4 0019 0.0lj 0@ 0.018 0.m9 0.011 0.@ 0011 0399 0168 0J15 oJ10 0456
0016 0110 0.086 0164 0.04 0.088 0.030 0.69 08 01n 0.753 0.804 0.80t 0.?85
0"vr6 0358 0.888 0"S1 0.949 0.686 0960 oJ99 07n O.Mt Ot01 0.1@ 0.165 0.175
0.00 0.m qm 0.m oml 0.@1 0.(m 0.m1 0.m 0.00 o.ml o.mo o.mo o.mo
0.009 0.@1 0.010 0.0, o0r9 0,010 0.o4 0.033 0,013 o.(ro 0.m o.mr o.m 0.60
o,m 0.m4 0.0m om 0.m 0.010 o.qD 0.m 0.014 0.00 0.m 0.@ om omo
rJ66 1.86 L9D4 1.994 1.9@ tS76 1.9$ r9A ,"975 1943 1.938 1.@ 1.909 Lyr8
0I)14 0.m4 0.m6 0.06 0.038 0@4 0.017 0.01E 0125 0.0J7 0.062 0.114 o.Dl 0.1it2
st
Al lv
Al el
Tt
Fo3+
F€Ar
IUn
MS
Co
l.h
K
k
Nb
Co!@s I to & Na Wmq 6lm 9: C&l\trB
pyrm; olm l0 to 14:
Quad
!y@ CoEpositi@
qes€d itr wgo.
Na
THE LOS ARCHIPELAGO
RING-STRUCTURB 287
Frc. 3. Na - Mg - Fe2+ + Mn
triangular plot showing
the Los suite ofpyroxene
compositions. Dots:
miaskitic suite; open cir-
cles: agpaitic suite. The
Ilfrnaussaq
(Larsen
I 976),
South Q6roq suite
(Stephenson 1972), and
Ugantla (Tyler & King
1967) trends also are
reported.
Fe2+
+ Mn
TABLE 3. RESULTS OF ELECIRON-MICROPROBB AI.IALYSBS
OF REPRESENTATTVE AI\4PHIBOLES
FROM
TIIE LOS ARCHIPELAGO
Sqpb RoO R!'9 Ro9 Ro1 K8ld T!35 Ta16 Ts16 Kr3 T87 Ts? Tc1
!o12345678910U12
so2 5lt1 ,1950 49.88 45.(X /M.18 48.55 4o.30 48 3737 39.9 9.06 39,41
Tiql 033 0.69 0.46 Ltg 131 099 3.1r 2-61 1.11 3.\7 lJ6 1.63
Ano3 l3E l,dt 1.43 392 3.81 30 10.95 10.14 l4ffl 1058 11.04 10.86
FoD3 oli 254 48 3.8 ?.18 8.09 657 5.12 5.S 6:t3 l.7l 4.t2 6.61
Feoelq 16.66 18.95 n.9 m37 $21 13.75 12E8 13,p 13.49 13.94 1650 13,73
MlO 9.17 1.69 a.v2 567 856 5.12 2-70 3.13 3.86 236 3.13 324
MgO 3I2 L@ 2-g 1.63 235 7.01 E.87 E43 6.08 9.91 6J0 7g
cao 0J9 0J5 0.6? 1.41 0.62 153 10.1)8 10.06 10',t8 1032 rcA' 9,92
Ns2O tl9 &A &16 82A EJ4 ES2 3.16 3.00 L74 3.19 3.08 3.10
K20 28 Lt3 2r9 l.61 1.62 157 1.60 1.68 l.9l 1.68 1.86 1.?l
7.192 0.10 0.16 0.30 0n 0.4 027 0,06 0.08 0.00 02t 0r0 0.@
F rJg 1.26 1.6 0J9 1J5 r.43 1.99 L6l 1.42 235 2M 1.83
q 0.m 0,v, 0.m 0.01 0.00 0.00 0.@ 0.m 0,00 0.04 0.08 0.02
IDO qle 1.05 128 L,A 1"r9 1.14 126 l.ts 031 L26 0.&| 0.92 1.(b
.O.F 4:,5 4J3 453 46 {.65 4.61 -0.84 -1.10 4.60 {.99 -0.87 4:n
.-o4l 0,00 0.0 0.00 0.00 0.00 0.m {.01 0.00 0.00 {.01 4.01 0.00
Totrl
st
Al lv
Al vi
Ti
k
Fo3{
Be,+
Ma
Mg
C€
Na
K
9831 98A7 98.,18 99,47 :fl 9.37 loI.Q, 9.95 10012 99.E8 9930 99.65
8.09 1.w2 ?958 1U, 1372 7,494 6.114 6il1 5.8,t6 6.137 6.194 6.115
0.000 0.0s 0$42 0.718 0,628 0J06 1.886 l:19 Ll54 1.863 1.846 1.865
0237 0,M O,28 0.0m 0.089 0.0t1 o.tn 0.043 0.419 0.050 0.2(E o,ln
0041 0.082 0.055 0 3 0.157 0.u4 0354 03@ 0.130 0366 0.185 0.191
0.00t 0.012 0,@ 0.015 0.64 0rm 0.00s 0.006 0.000 0.02,, 0.015 0.0m
0392 0J10 0.460 0J67 o,CB 0X62 0.584 0J89 0.786 0100 0.4n 0,774
Lr99 X{ \&t 2.732 LVlt ftn 1.635 tJl4 1.751 1.789 LL74 l,1tl
Lni 1.69 r8 0n 1.157 0SCt 0346 0,4{p 05gt 0307 0.411 0.4n
0A?5 0.618 O.6X 0388 0J6 1.610 2005 1537 1.,106 L%7 $n 1.694
0009 0.095' 0.115 0241 0.106 0 9 1.638 1.661 l:143 tng ljr2j l.6g
25V1 2l,67 252 2l,62 ZU 2573 0.98 0.896 0.8a 0.950 0941 0.995
0.4J4 O.4y 0.,145 0345 0329 0309 0309 0330 o5l9 0329 0373 03|o
F 0s9l 0.636 0.632 0.W 0.781 0.706 0.955 tnL 0.698 1,141 1,O2t 0.89
cl 0,0@ 0.@ oom 0.m4 0.000 0.001 0.007 0.m1 0.m0 0.009 0.@ 0.004
oH 1.109 1360 1368 1.698 L2l9 1,g3 1.6E 0i?8 1302 0.85 0963 1.(B6
Total t8rl6 l&096 18iE3 18,184 18.080 18.150 17.875 17J88 17"94 18.059 18.69 17.!n
Colunos 1 to 6: arftdsolite; colums 7 to 12: hasingsite. Conpositions expressed
in wt9zo.
Ca> 1.34
Ti< 0.50
288
and Na-Ca pyroxenes,
although
the manganese
conlent
is higher at Los. The niobium conlent
can reach
up to
0.52 vttVo NbrO, at Los: similar values have been
obtained
in Zr-rich pyroxene
from peralkaline
trachyte
and comendite from the Wamrmbungle volcano
(Duggan
1988).
The composition
of the amphiboles,
also classffied
according
to IMA recommendations
(Leake 1978), is
calculated
on the
basis of 13 cations
in the tetrahedrally
coordinated
alid C (IrIl, M2 and M3) sites.
Two groups
Sl+Nr+K
raa
hastingsite a
ll
arfvedsodte
tr o
Ftc.4. Composition
of the Los amFhiboles. A. Amphibole
compositions
plotted
in the Ca
+ ryAl yersas
Si + Na + K diagram
(Gret et aL 1980). The field of arfvedsonite from
the Oslo
Rift (Neumann
1976)
and the hornblende hastingsite
fron the Iskou complex
(Gret et al. 1980) also are shown. B. Cau versus Si diagram O{itchell 1990).
Dots: miaskitic suite; open
circles: agpaitic suite.
75
have been found: the agpaitic syenite suite is charac-
teized by manganoan
arfvedsonite, whereas
the
amphiboles from the miaskitic suite are mostly
manganoan-magnesian hastingsite or F-rich
manganoan-magnesian
hastingsite
(Table
3). In a plot
of rvAl + Ca versw Si + Na + K (Giret et al. L980)
(Fig. 4A), the amphibole compositions
from the
agpaitic suite evolve from richterite toward arfved-
sonite,
and
plot within the same field as those
from the
Oslo rift (Neumann 1970. The Los arfvedsonite
TIIE I,OS ARCHIPEI..AGO
RING-STRUCTT]RE 289
S@pli Ta35 Ta3t TA Ta7 Ts? Bf,
TABI.E 4. RESI,JLTS OF ELECTRON-MICROPROBE
AI{ALYSES
OF
REPRESENTATTVE MICA FROM TT{E LOS ARCHIPELAGO et aI. L982).
The astrophyllite-group minerals occur only in
the agpaitic
suite.
Their composition
(calculated
on the
basis
of Si + Al = 8 cations)
evolves
from asfrophyllite
@e end-member)
to kupletskile (Mn end-member)
Clable 5). In a single crystal, Mn emichment
can be
documented from core to rim (anal. I and 2).
Compared to astrophyllite from Kangerdlugssuaq
Q-,ayne
et al. 1982), that at Los is richer in Nb (up to
234 wtvo
MzOs).
The l&venite
(calculated
on the basis of Si + Al =
2 cations)
also
is a characteristic
mineral
of the agpaitic
suile. At Los, the lAvenite
(Iable 6) is enriched
in Ti
and
Mn compared
to that at Kangerdlugssuaq
(Kempe
& Deer 1970). As at Lovozero
(Vlasov et al. 1966),
it
evolves
toward titanium-rich
l&venite.
In the miaskitic suite,
the dominant
oxide phase
is
titaniferous magnetite (calculated on the basis of
3 cations and 4 atoms
of oxygen)
locally enriched
in
ulvdspinel component
(Table 7). In contrast, in
the agpaitic suite, the dominant
oxide belongs
to the
ilmenite group, invariably enriched
in Mn, with up to
74mol.Vo
pyrophanite
(Iable 8).
The zirconosilicates also are restricted to the
agpaitic suite. Representative
compositions
of cata-
pleiite and
partial analyses
of eudialyte
(without REE
detenninations)
are given in Table 9. A mineral close
to catapleiite,
or more probably
to cr-hydrocatapleiite,
NaHZSi3Oe'2H2O.aq
@ortnov
et al. 1972),
calculated
on the basis
of 3 (Si + Ti + Al) atomso
overgrows
the
l8venite and the eudialyte. Eudialyte (calculated
on
the basis of 6 Si atoms) is enriched in Mn (up to
9 wtTo
MnO), as has been noted
by Kunitz (1936) in
eudialyte
associated
with villiaumite and astrophyllite
from Roume Island, compared to eudialyte from
Lovozero
(Khomyakov
1995).
The feldspathoids
occur in both suites;
representa-
tives compositions
are grven
in Table 10.
The sodalite
(calculated
on the basis of Si + Al + Fe3+
= 12
cations)
is nearly
pure.
As in the case of sodalite,
analcime
(cal-
culated on the basis of 96 atoms of oxygen) also is
neady pure. On the conrary, nepheline
(calculated
on
the
basis of 32 atoms
of oxygen)
shows
a wide range of
K content, from 5.80 to 7.64 wtVo K2O. In some
samples,
the nepheline
is transformed
into cancrinite.
In conclusion,
the evolution from miaskitic toward
agpaitic suites is marked by drastic changes
in the
mineral
associations
and
by a general
increase
in levels
of Mn, Zr, and Ti in both the silicates
and the oxides
that crystallize from the residual liquid. Detailed
mineralogical
data and
discussion
of crystallochemical
substitution within the different minerals will be
deferred
to a later paper.
GeocgEtltsrRv
eNo Acs oF
EMPLAcEMENT
Concenhations
of major and some trace elements
(Zr, Nb, Y, Rb, Sr) in representative
samples of
sio2 vtf/o
\ql
ADo.3
FO
lrnO
Mgo
cd
Na2o
r20
q
E2O@lc
.-o'F
.O=cl
Tobl
st
Al iv
n33 An v.6 \tn $9
L6 r:r9 1.'2 Ln 31r
9.70 9.C' 1493 l4l8 lJ31
19.41 1E77 24.45 2231 A6
5.63 sJO 436 452 3.(B
957 10.04 5.,n 6J5 78
0.o 0.6 orn o.lt 0.115
ili'1 03,f ori 032 0.61
9J4 9J9 9.01 9.11 t.tl
l,a 1.,16 0.16 oln 014
0.r5 0.06 0.04 0.01 o.lxt
335 3:IE 3.70 3.80 3fr
0.o 0.66 0.o, 0.ll) 0.06
0.lr, o.oj 0.o2 0.m 0.{D
38Jr
2"t3
1055
13.f,
5A
t293
0.m
0.64
9,2
233
034
3,42
1.S
0.l.tt
l@26
!m.,|6 t0036
Alvt
Fo2r
Ivln
Ms
06
C4
N6
K
F
OH
T@l
3.m6 3.r'36 2:r* L1U 26tl
0913 0.CZ3 LzA 12t6 1319
0.0m 0.q[ 0.180 0.166 0.108
0.138 0.r9, o10J 0,161 oal
r3tl1 1 1.6t4 1.471 r.43
0381 0396 03411 03D 02Jr
r.140 1.187 0.661 O'n 0923
LW 2.915 2.940 L{ro 2EE8
0.m o.qD 0.m 0.o9 0.m5
oo4t 0.62 0.039 0.049 0,091
09t3 09$ 0934 0916 0.t89
0J.t4 0366 0.041 0.m 0.05
0.oa) 0l)08 o.ffi 0.ml 0.0ft
1.606 L.A6 1954 Lg 1.95t
3o}l
0gt5
0.(m
0.120
0.89'
034
1J12
2ffi
0.m
o.wl
094
0578
0.0itt
LM
9.37B
Clable 3) is richer in Mn (up to 9.17 fi7o MnO) than
the arfuedsonite from Kangerdlugssuaq
(Kempe &
Deer l97l,Layne et al. 1982). The amphibole compo-
sitions from the miaskitic rocks evolve from hastingsite
to hasringsitic homblende and edenite. The magnesian
6astingsite contains up to 3.86 wtVo MgO. It is also
characterized by a high fluorine content
(up to 2 wITo),
In the Ca6 versus Si diagram
(Fig. 4B), fts amphibole
compositions evolve from magnesian
hastingsite
toward arfuedsonite. as in other undersaturated
complexes
(Mitchell 1990).
The compositions of biotite, calculated on the basis
of I I atoms of oxygen,
are
given
in Table 4. The biotite
from the rock transitional toward the agpaitic suite
(Bl7 and Ta35) shows a lower Al content (<70 tutVo
Alror) and
hon content, and higher Mn content
(up to
5.9 wtVo MnO) than the biotite from the miaskitic suite
(34 vrr%o MnO). The composition of the low-Al -
high-Mn biotite from the agpaitic syenite at Los is very
similar to that described
at Kangerdlugssuaq
(Layne
290 THE CANADIAN MINERALOGIST
TABIJ 5. REST]LTS
OF ELECTRON.MCROPROBE
ANALYSES
OF
RFRESENTAffi/E ASTROPFIYLLITE
AND KUPLETSKIIE FROM
TIIE rrOS
ARCIITPAAGO
Kald9
stg2
1I@
Zrg2
iJ,ot
&o
Md
Mlo
@
N!2o
K20
BaO
tt205
n2Oelc
crP
Tet tml9
r3J9
ot14
0.lD
r.611
32.96 n55 t+.@ tLn ,Laz n.62 y.9 '454 33.9 33.40
9.12 9.60 9.ll 1.92 8Jt IO83 &98 10?6 gA a;23
da Lx 3.6 4.n 335 052 !.!6 oot r9 40J
l5 lJ7 l.t4 t.&, t52 {}@ L6 2g 1.36 Lm
Dls 16.2 v.ta t&ol lTJt rL36 tTAt ZI98 m.6 ril
13.83 l&99 2tg2 t7.t3 l8J8 4& l&91 15.63 16'18 19.88
062 0.60 0.82 0J2 040 0, 0.94 t.4l oo 0.68
t23 t.@ l.4t l.!€ lJ6 09 1.03 IJI lJ' 1.33
2.ll 1.8t ZVz t.1t 1.68 Z6 28 l.6t 2.t1 Lgt
t9, 612 5.89 &14 6.01 JJ6 5.& 6A, 6.A 3.n
O24 ol2 0A, O{D dd. ral O,t4 O03 O.g, O.gt
1:ll l.$ !n /y 2B O70 tJ3 Ld. &! tJ9
a4 0.49 C96 088 o.r2 0'm 076 o,n O.6J O.m
455 4J8 4.49 LU 4.45 493 4& 486 4t2 4,9
4A 4l,, {.43 "0.40 {.37 qO 4.9 -O4l 1}.30 -0,4t
6&3 7,4 ?.0J1
l.rt o:t6 0t49
&m &@ s.fltc
t.603 L4n 1309
omt 0.301 0.415
0.151
l.@4 r:tB 1.8?5
t476 t.61 LIM
262J 29fl 3J55
o4l5 0rt8 0.214
6515 68@ 6.472
o38o 0310 03@
4f43 o.sn o.g2
l-t'l t6l, !lJ3
om3 0.06 0.o6
2,@ 2-An 2@
o57 o.44 0.61I
6423 6-50 63
9t30 99,U
6'90
0,m
E 000
1.tza 8.m
oa77 0.m
8.m g@
1.388 r:rX2
0355 00t4
0r0t 0.6,
lr49 tar3
rfig 5.790
3.415 0J36
ot30 0.063
67l' 6.690
0,360 0
0.?@ t3gl
t.663 1J08
ZTg M
0J5t 0.m
&441 1M
7.@S 7.359 7.Ur1
0.905 064t 09a
&m &0@ &m
t574 t"49 1r8J
oil2 0317 0@
0.t36 - 0.u
!953 tn$ rgn
2-956 2543 3259
sJ05 1938 t.t16
o.tq, 0258 0.1@
6555 eW 6.638
0380 0320 0J40
u766 0829 0tt3
I.'U' TJ93 L69I
0.010 o0l9 0.r,
zw z7@ LIn
ol}7 o64J 0.60l
6.663 6.3!5 6399
6.9m
t.lq!
8.m
02at
6436
o,'2
mgr
1.,188
0l)I1
2n0l
0483
6Jr
st
z
Zr
Nb
Y
Fit+
M!
Mg
x
&
Na
K
Ea
v
OH
r359
o.lt4
ol39
l.6n
3&,
0r3l
6!5
0.m
0.891
l.,st
0.0!l
0,0@
0183
6!7
T&l W+X+Y+Z 16.660 19.4m DAO 19360 t9.,t(o t93ro t&6 t&n l93r
Colums l, 5, 7, 8: astropMite; colums a 3, 4 6, 9: kupletskito. Compodtions
eqprsssed
in wPlo.
a$)aitic and
miaskitic syenites
and in some
dykes and
enclaves
are given in Table LL, together with their
CIPW norrns.
Al1 fhe analyzed
samples
are strongly
silica-undersaturated, with normative nepheline
content as high as 3l7o for some agpaitic rocks. In
agreement
with their modal minslalogy (presence
of
sodic pyroxene and sodic amphibole), the agpaitic
rocks contain up to 7.9Vo aegirine and sodium meta-
silicate
(ns)
in their norms,
whereas
the miaskitic
rocks
have higher levels of nonnative magnetite and
ilmenite.
The two series
of syenite
display a restricted range
of composition,
with SiO, befweea
54 a$d
60 wtqo, and
KrO + Na2O
in excess of 1.3
wtVo. They typically plot
in the alkaline field of the alkali-silica diagram
(Fig. 5). The samples of agpairic syenire
have signin-
cantly higher levels of Na and Mn, lower Ti and Ca"
higher Na/K, and lower Fez+/Ire3+
than the samples of
miaskitic syenite. The agpaicity
index [the molar ratio
(Na + K)/A11,
defined by Sprensen
(1960), is grearer
than 1 for the agpaitic rocks and less than I for the
miaskitic rocls.
Rb, Z, Nb and Y contents
are high for the both
types of syenite, as in alkaline rocks in general
(Bowden & T\rner L974). T\e samples
of agpaitic
syenite usually show higher concentrations of these
elements
(with more than 2000 ppm 7r) than those
of miaskitic syenite. Agpaitic samples are also
characterized by high levels of Rb (usually
>300
ppm)
and low contents of Sr (<30 ppm), resulting in much
higher Rb/Sr values
(>20) than in the miaskitic rocks
(Rb/Sr
< 10).
Samples of phonolite from the ring dykes have
major- and trace-element
contents
roughly similar to
those of agpaitic syenite. The two analyzed samples
taken from the radial dykes (monchiquite and
shonkinite) and the two enclaves
(basanite
and
micro-
essexite) have an alkali-rich mafic composition
(SiO2<
45Vo).
Rather imprecise K-Ar ages on whole rocls led
Lazarenkov
(1975) andl-azarenkov & Sherif (1975)
to
suggest that the Los pluton was emplaced in three
phases
during the Cretaceous, between 105 to 80 Ma.
Ten representative
samples of the two syenite series
have been analyzed for Rb-.Sr
isotopes,
together
with
four samples of dykes (two of phonolite from early
ring-dykes and two of mafic alkaline character from
late radial dykes), a micro-essexite from a large screen
and a basanite xenolith. Analytical data are reported in
Table L2 and
plotted on Figure 6.
TI{E LOS ARCHIPET,AGO
RING-STRUCTT.IRE 291
TABLE 6. RESI]LTS OF EI.ECTRON-MCROPROBE
ANALYSES
OF
REPRESENTATIVE
LAVEMTE
FROM TIIE IOS ARCHIPELAGO
mple Ro9
1Rol
2Rol Ka27b
5
6.036 s.w 6.065
29.56
8.37
r6.66
4.95
0.94
0.04
4:Tl
10.76
0.11
9.77
10.05
0.00
0.00
3Jl
0.01
0J6
-1.2l8
0.00
98.58
\ryz
0.@8
0.4v1
0.548
0.151
o269
0.614
0.0u
0.6E7
1.313
0.000
0.0m
0.749
0.000
0251
6.gN
Conceffidion of m{or elemcs
(reported
as oxides),
fluorine
and chlorine reported ia w9o.
The computed regression-line calculated for the
16 samples
gives a very high MSWD (mean
square of
the weighted deviates) value of 22.9, which means
either that all the samples
are not strictly cogenetic
(i.e., they have different initial Sr isotopic composi-
tion) or that they are not of the same age. Secondary
alteration of the isotope system can be ruled out, as
petrographically fresh samples only have been
measured. If one considers
the samples of syenite only
(which constitutes ttre main rock-type of the Los
complex), it seems that sample Ka 27b (agpaitic
syenite), which has an extremely high 87RbF6Sr
value
(528) owing to its very low Sr content (2.66 ppn,
isotope dilution), plots distinctly below the regression
line. This behavior has been observed before for
alkaline
rocks with low Sr content, for example in the
Noqui peralkaline granite
inZaire (Cahen
et al. 1976)
TABI.B 7 RESULTS OF ELEffEON.MICTOPROBE ANALYSES
OF REPRFSEMTATIVB TITANIFBROUS MAGNBITTE
FROM TI|E LOS AlCHIPEIr'q.q)
Tar6
1
sio2
Tto2
hgz
Nb205
Ta20s
4t2o.3
FeO
IvlnO
Mgo
CaO
Na2O
K20
BaO
F
cl
II2O
calc.
GF
4{t
Total
si
AI
Ti
Zt
Nb
Fez+
Mn
Mg
Ca
Na
K
Ba
29,13 B.9l
5,77 6.83
m.74 1851
4.82 4.67
0.@ 0.00
0.@ 0.01
5J8 4.45
70.47 10.91
0.05 0.11
&05 9.93
1049 10.14
a.vz 0.00
0,o3 0,4
428 3.68
0.00 0.00
0.16 050
-1.80 -155
0.@ 0.00
97.19 9854
2.000 t.999
0.000 o.ml
0.267 0.743
0.695 0.603
0.150 0.141
0.32t 0.249
0.609 0.6t7
0.005 0.0u
05y2 0.74,
1.396 r.3t4
0.002 0.000
0.001 0.012
0.y29 0.779
0.000 0.0@
0.071 4.221
8,6r A.77
7,3r 6.y2
19,{t 18.4
3.83 4.47
o.n 0.69
0.03 0.@
4.18 4.44
11J1 10.83
0.14 0.11
10.33 10.80
9.95 9.83
0.01 0.00
0.14 0.18
3.70 3.79
0.05 0,06
0.48 0.43
-1J6 -1.60
-0.01 {.01
0.11 0,12 0.6 0.14 019 0'04
8J3 6.78 1!' 1Xr 7&. 6.m
0,9 0.49 0J1 0r8 0.x 0.6€
J0.46 55.19 6.9 v51 53.86 55J8
N 308t 30.93 31.96 3139 3r.90
1136 6.69 4J6 AE 7n 5.5
o6 0m 0.o o,@ 0m oll
Ro1
o1n4 0.04 0.03 0m5 0l!', 0.(m
0250 0.194 0.r5 02ra 0.u, 0.191
0,0n 0.o2 iaz ().0l2 0,010 0030
1.4?8 lj8l l.T2l 7.92 1"531 lS
0.rt6 0981 n976 r.@4 r.@5 l.0ll
0375 0216 o.lt 0rl8 0 01?5
0,m 0.@ um o@1 0,m o,ffi
0250 0.194 0,125 0218 0& 0191
0.m4 0,m4 0m3 0.6 0,97 (L@
on1 0116 0.r9 0rl8 023 [l?t
0.m3 0.m 0.m o.ml 0.000 0m6
0.tr1 0.09 0.0il o.mt o.ru om9
0.& 0.t5 0.709 0J53 0J4l 0617
scr2 0.05
To2 1633
Al20' 4€0
F20d d 30.69
H6L 4319
MnO 1.10
MsO l.l0
Td n.fr
st 0.(r2
'n o4,t)
a 02t2
R3+ 0365
&2+ 136J
Mr 0035
M8 0.61
m1r 10.11 10134 Dr.48 r@.90 16-4
UdaFtul 0.160
&?sror 0.m
@l 0.m
SDhrl 0.tr1
kye 0.09
!rag* 0.&
9J0 99.05
conporiE@ qp@ed in vPlo.
TABLE 8. RESULTS
OF
ELECTRON-
MCROPROBE
AI.{ALYSES
OF
REPRESENTATTI'E
PYROPHANITE
FROM TI{E LOS ARCHIPELAGO
Sample Krld Katd Kald Rol
n'1234
sio2 0.04 0.03 0.09 027
T1o2 51.67 5r.00 s0.91 4927
7tg2 O.l2 0.O7 0.G] 0.m
A12o/3 0.r7 0.00 0.00 0.00
Fe2O3
cals 2.6 2,72 L& 6.80
Feo ets 1039 14.08 1354 f3.08
MnO t5.73 31.44 31.94 31.06
MsO 0.01 0.m 0.01 0.01
CaO 0.03 0.00 0.0r 0.00
F
cl
OH
Total
1.996 2.m0
0.m4 0,000
0.370 0.350
0.638 0.605
0.116 0.136
0235 0.247
0.656 0.617
0.014 0.0u
03v 0.77r
l.ryt L.A!3
0.mr 0.0@
0.004 0.005
0.786 0.807
0.000 0.0m
0.214 0.193
Total
st
Ti
7t
AI
Fe3+
Fe2+
Mn
Mg
Ca
1rsJ4 98.13 99,m r0p'.42
0.001 0.001 0.004 0.005
0.971 0.978 0.972 0.931
0.001 0.001 0.m 0.@
0.m5 0,000 0.000 0.m0
0.049 0.041 0.051 0.128
0.277 0.300 0237 0.n5
0.756 0.619 0.687 0.66r
0.000 0.000 0.0m 0.m0
0.000 0.000 0.m 0.000
Toal Zm 2.m 2.0t10 2.m
Endm€mbss
Geudeure0000
ryrophantte 74 67 67 62
Ilm€niB 21 29 n 26
Hemarfts54512
Conoentration
ofmajor eletnents
(reported
as
oxides),
reponed
in wtrlo.
292 TIIE CANADIAN MINERALOGIST
TABLE 9. RESI]LTS OF ELECIRON.MCROPROBE ATiIALYSES
OF
REPRESENTATTT/E
CATAPLEIITE Cc-IIYDROCATAPLEIITB?)
A}ID EUDIALYTEFROM THE LOS ARCIIIPELAGO
ffipls KatlbIK&lb
4
Kald KatTb
23 @pts KAZIb Ksld KaXIb E&lb
sio2
Tic/2
7.\Oz
Nb205
Ta2O5
Ano3
FeO
DItrO
Mgo
CaO
Na2O
po
BaO
F
ct
IDO cclc.
a;3
o{l
Toel
47.9i 46
0.s 0.00
34.63 33.4r
0.m 0.m
0.m o,4
0.04 0.12
0.0 0.00
02r 0,00
0.m 0.00
orE 08
&M 8JB
0.g2 0.04
51.17
0.n
14,8
021
0.0
03r
3Itr
9,%
0.09
634
7,v2
o.n
0.00
0J8
0.64
0.93
0.12
0.19
95J9
0.m 0.{D
0.m 0,q,
7"$ t.@
0.00 0.00
0.00 0.00
5L45
0.45
11.78
utt
0.17
0a
2-Q
E.15
0.05
5.68
r233
0r8
0.m
0.rz
0.71
l.o
0.0t
0.16
n.6
si
AI
7t
Nb
Ta
&2+
Ilfn
lrlg
Ca
Na
K
Ba
F
o
w
Tolrl
6.m
0.060
0.q39
0s57
0.089
0253
0.7E9
0.@
0.700
\7X
o.eu
0.m
0,u2
0.r18
0.820
2993 Lytg 6.m
0.{r/ 0JJ2r 0.096
0.m um 0.€6
1.054 r,67 0.817
0.m 0.m 0.011
0.m 0.m
0.m 0,m o@
0015 0.0m 0,930
orm 0.0m 0.005
0.019 0.c2r 0.8m
09?8 l.016 l.E{tl
0.@ 0.m 0.(2!t
0.m
0.m0 0.m 0.104
0.m 0,0m 0.167
am em 0J29
98J9
1-OKl 7.1n tO-*70 11-17n
Cmc€nbathn0f rtr@ol€o@s (r€pofied
asdd€s)
frnrrne md chlodm npdtrd in wt7o. @mlel and
2 Cst4lellq ohon 3 8rd4: Eudialy'
TABLE IO. RESULTS
OF
ELECTRON-MICROPROBE
A}{ALYSES
OF REPRESENTATTVE
SODALTTE,
NEPHELINE.
AND A}IALCIME
FROM
TIIE LOS ARCHIPELACO
Sanpb Rol Krrfrb Ro9 Tat6 Kaztb
tro!2145 Roo
6Rol
7Ta35 Tal6 K&?
89r0 Ta7 Co6a Co6. Knb
tl lt t1 ta Ki3
t( R09
l6
sto2 n52 3t.8E 37,3 n.to 42ist 44.18 4r.51 15.14 Q96 45.0S U.m &A5 43.n 50.6, 5336 5469
Tlol 0.03 000 0.00 0.0t o.fir o03 0.m o,m 0.00 om 0.00 ots o.(D o00 0.03 o.v2
Anoo 97l'1 t3,12 31.31 31,70 ?/.5t 3243 tt,fis 32EE 32:t9 325t lg,a 3L9 3?:t6 XM 30.Ot U.t6
P€ao3 0.49 0.36 o,8 o15 0.m o@ 0.00 o.m 0.00 ooo o.(x) om 0.@ o.ot 0,00 o,u
D'ls 0.(n 0.m 0.04 00 o.00 o.oo 0.045 o,0o o.o3 0,01 o.0r 006 0.03 0.00 ol4 (l.{B
Mp 0,00 oq, o.ot o.ot o.(xt om oo0 oot 0.00 o0o 0.00 o(r0 0.m o.0o o.m o.(It
cao 0.01 0.o 0.6 o04 0.(t) ooo 0.00 0,00 0.33 0.30 0.16 atz 0.08i oq, 0.01 0.@0
BaO &r0 0,@ 0.30 0.06 0.m 0.m o.t2 0r4 o.(x
Na2o u35 u.sg 24t3 23,or 15.94 r6:m r.90 t6.31 ts.yz 15,62 rs.89 1629 Ls,y, tssl 9.47 rzm
K20 0.05 0.(n o.t2 0.m 7,5t 3:n 7.9 5.62 537 5,t1 5.66 5s2 5.80 0ll o.ql 0.13
so3 0.3E 0.47 0.m o(n
F 0.m 0.00 0.00 000
(r 7,m 674 8.t2 &oo
.o.4 -r.62 .1.52 -1.83 -1.r0
O.S 4.05 -0.05 0.@ O{Xr
Ir2o calE &47 g.io a,zt
Torsl 1(n.83 101.63 t@.04 l0o4r t@.16 99.31 9.00 roo.B D,4 9&96 gg,u 99.11 9g.65 r(X).22 tor,5E n,s4
st t9l0 5.966 6.015 5.986 t.11, s.555 8.1?6 s.tt3 8J5t t.6?3 8.444 8.qr2 S.{R n.nA n.nz 3r.yt9
Tl 0.m4 0'm 0'm0 o.0t 0.000 0.q)4 o.oo0 0,000 o.mo 0.m0 o.mo o.m5 o,(Xlo o.mo 0.014 o.0r
Al 6,(B9 5.91 5.951 5.96 7.sJ3 ?.a5 7.601 7.159 7,t54 73,7 7A97 7,459 ?.439 18.@t 20.0S9 1659
h3+ 0.051 0.041 o,vv 0.018 0,032 0.160 0.023 0.068 0.$r o.@r 0.062 0.69 0.06E 0.005 0,m0 0.151
Itr 0'm oo0 o.mi o.otxt o.o0o 0,o0 0.m8 o,oo0 0.006 o.m2 0.002 0.010 0.004 o.@o 0.06t 0.043
Mg 0.000 0'm4 (L06 o,ot 0.0oo 0.m o.@ o,w2 o.o0o o.mo 0.000 o.mo 0.0(n o.{no 0.002 0.a2
c8 0.001 0.m0 00(b 0.m6 0.000 0.m0 0.000 o.mo o.odt o.6t 0.032 o.m5 o.olg o.or9 0.006 o.(m
Ba o.(xt 0,oo o.v8 0,m5 o.mo ooo o.oo2 0.010 o.(B
Ng 7.559 7'169 7X3l 7:l8z t95S J.9E9 6.O2 6,Wt 5.8n fiy 5,8E4 6.(r,6 5.965 17.860 tO.4lE 135t
K 0.m9 0,m0 (Lm' o.0o l.so r.M r.vA t,r62 l.3o! l.3lr 1.3s4 l3:4 rAn 0.G3 o,v2 0.w7
F 0.0fit 0.m0 (L000 0.m
(l Lyn n$ 2,m 2.n5
s 0.o4t 0.64 o.mo 0.ooo
Il2o $lr3 r6.w r6.6s
Toraf 21540 m.w a,ns 2t,w a,uz a,9y7 .V24 ir.376 8,24 A.6t z'srn A.80 A,47 65.yt6 60.8E8 6246E
Columns
I to 4: sodalite;
columns 5 to 13:
n€p@
o.presdinwyo. Compositions
TIIE LOS ARCHIPEIAGO RING-STRUCTTJRE 293
TABLE 1I. CHEMICAL
COMPOSIuON
OF REPRESENTATII/E
SAMPLES
FROM TIIE LOS
ARCIIIPELAGO
Ka27b Ro9 RoI Kald Ta35 L:OOa BUt 'rA16 Krt 'ltil 'l a!, 1}n4 KOIJe 'rardo Co3a Ta5c
it02
n02
{r203
7e2O3
.?eo
11t80
vlgO
30
\la2o
'c20
2o5
-or
fotal
hppm
Vb
ppm
rppm
,r
D
ul
te
tc
ta
li(wo)
li (eo)
li (fs)
ro
tl (fo)
)l (fa)
Dt
I
rp
Na+KVAI
55.59 54.00 55.28 58.26 60.63
02.6 023 0.n 0.2r 0J?
20.89 2132 2rc7 2n.$ nJ6
2J2 l.@ 1.93 t.54 zJr
0.70 0.95 0.50 0.8? 0.41
0.48 0.46 0.36 0.45 0.30
0.13 0.16 024 0.17 0.13
0.59 0.98 0.86 0.n 0J9
11.69 l0.zl8 9.U 10.62 7.91
4.70 6.Vt 5.49 4.58 6.74
0.01 0.u2 0.o2 0.0r 0.03
1.25 2.n 3.4 1.18 2.m
99.01 992/t 99.60 98.81 99.70
n72 n+r LM 2M7 74
439 449 331 39E 255
37 30 36 26 49
nn 35.{t 32.U n.m 3629
n.p n35 n.50 4a.29 41.t8
0.m 0.m 0.m 0.@ 0.m
29.3t StiO 28.09 21.85 9:78
1.gt 4.61 3.6 4.45 724
2.03 t:16 0.00 0.98 0.00
rn 1.9E t.32 0.53 0.64
020 033 0.60 0.r0 0.32
r.@ 1.tr2 0.7r 0.48 031
0.00 0.m 0.41 0.00 0.49
0.09 0.05 0.m 0.23 0.00
0.51 031 0.00 r24 0.00
0.00 0.00 0.96 0.00 0.30
0.49 0.44 0.3E 0.,m 1.08
0.o2 0.05 0.05 0.v2 0.o7
t.l6 1.09
1.04t.t2 .(p
54.98 59.36 56.12 56.40 59.4,
r.g2 0.43 0.52 0.36 0.7(
18.93 19.53 2t.K ?2;05 9.A
128 2.20 0.76 1.01 t.4t
\61 1.55 1.15 1.59 1.1(
0.37 0.48 0.2r 0.36 oil
o.77 021 0.34 0.39 0.34
L82 0.?5 l.l8 1.56 7.61
7,74 8.75 8.!t8 8.49 6.4
673 4.98 1.Vt 6.26 7.4t
0.14 0.03 0.05 0.m 0.0i
1.77 1.38 Ln 0.90 1.6t
99.16 99.65 99.71 99.4A 9.8:
39:n 29.43 41.78 36.99 4.09
16.88 216.66
22.4 26,98 Y.8e
0.00 0.00 0.70 3.57 2.39
23.31 t4.72 26.70 2t1.30 9,q
3.77 1.16 0.m 0.m o.fr
o.32 0.00 0.m 0.00 0.0(
5.46 t.47 l.r 1.66 l.&
1.88 0.39 0.85 0.53 0.81
333 1.16 LAz 1.18 0.Ol
0.0E 0.00 0.14 0.@ t.23
0.02 0.09 0.m 031 o.fr
0.05 0.30 0.m 0.75 0.d
0.@ 2.6r Llo 1.46 LrC
t.94 0.82 0.99 0.68 t.44
o.32 0.(n 0.11 0.0? 0.1(
1.06 l.ot 0.99 0.94 0.91
5m '32r 368 tnl 345
208 47 tQ 280 19r
494324373/,
99.65
56.34 56.47
o.32 024
20.19 20.63
t.2t t23
1.04 l.M
0.31 029
o.20 0.19
1.08 r.ul
9.66 954
5.81 5.68
0.6 0.08
3.6 2.n
99.36
1243 1135
270 205
29 25
34.34 3357
?3.t8 2923
0.00 0.00
23.{t 2452
3.50 356
1.46 0.51
2.15 2.00
0.47 0.40
1.83 t:74
0.00 0.00
0.92 0.05
0.10 024
0.m 0.@
0.61 o.M
0.m 0.1E
l.l0 1.06
99.84
4d.J8 43.{l
r.93 2.53
15.6 14.21
3.90 4.86
5.2r 5.4
025 0.18
5.09 ?.3r
7.72 t0.4
5.49 3.37
3n 2.14
0.69 0.63
5.9t 5.n
99.65
289 215
135 106
33 28
tg.vt t2.65
tt33 11.90
8.V3 11.33
18.81 9.00
0.00 0.@
0.m 0.00
to.76 tzJl
7.63 10.08
Lr8 120
0.00 0.00
3.54 5.70
t.ll 0.75
5.65 7.O5
3.6t 4.80
159 1.45
0.55
0.El
4.M 43.62
2.30 2.52
t2.t7 t3.47
3.54 5.21
1.t2 7.53
2r o.zi
8.n 8.98
12.19 tLn
4.17 L18
223 1.05
059 0.49
224 t.t4
9937 99.83
13.18 6.21
0.00 9.26
7.90 27.07
19.12 7.73
0.@ 0.00
0.(p 0.00
20.y 16.61
14.30 lz0l
431 3.08
0.00 0.00
s.il 7.26
r.88 2"05
5.13 7.56
4.37 4.79
1.36 1.13
0:76 0.43
273 2t3
68 62
29 29
Conceffiation of major elemefs rerported
as oxide$ in wgq md of trace elemens reported in ppm"
and the Kidal ring-complex of the Iforas, Mali
(Li6geois & Black 1984). Disregarding
this sample,
the nine samples
of syenite
plot on a good isochron
(MSWD = 1.28), which
gives
an Albian age, 104.3 t
1..7 Ma (2c), with an initial ratio of 0.70401 t 0.00008
(2a).
The micro-essexite
screen
(sample
Co 3a) and the
basanite
enclave
(sample
Ta 5c), which have virtually
the same measured
Sr isotopic composition
and very
low 87RbF6Sr
ratios
(0.19
and 0.16, respectively),
plot
very close to the isochron. The computed
line for the
11 samples
nM.5 r 1.6
Ma; (87SrF6Sr)0
=O.70399
x.
0.000M; MSWD = 1.041 is, within erel limits,
indistinguishable
from the "syenite"
isochron. The two
analyzed samples of phonolite dykes, which are
geochemically
similar to the syenites,
also
plot, within
grsls limitg, on the "syenite" isochron, suggesting
a
comagmatic
origin.
The late radial dykes (monchiquite and micro-
shonkinite) on the contrary, plot slighfly but signifi-
cantly below the isochron: they have initial ratios
(calculated
at 105 Ma) close
to 0,7033. This value is
compatible with a mantle-derived mafic magma
without crustal influence; the slightly higher isotopic
ratio of the syenitic and phonolitic rocks (0.7040)
points either to a different mantle source-region
or
to some degree of crustal contamination
during
emplacement
and differentiation of the Los complex.
The lack or low level of crustal contamination
in
nepheline syenite
compared to quartz
syenite has been
reported in other syenitic complexes, such as
Marangudzi, 7-lmbabwe and Mont Brome, Quebec
(Foland
et al. L993) and from the Abu Khurq complex,
Egypt, in which the contamination is related to the
silica saturation
of the magma
Q,andoll et al, L994).
The lack of quartz syenite in the Los archipelago is
atributed to the emplacement
of the archipelago
at the
edge
of a passive
margin.
294 TIIE CANADIAN MINERALOGIST
62
Ftc. 5. Total alkali veru,rs silica (TAS) diagram. Volcanic fields after Le B
u et al. (1,986)
and Ir Maitre (1989).
TABLE 12. Rb-Sr IIiOTOPIC DATA FOR SELBCTED
SAMPLES
FROM THE
LOS ARCHIPELAC'O
66
60f,6
56
5250
484644
40
Na2O +K2O
phonolite
Roe. .T#
notl $ ?"'o
o*to
cooa n Yal
Co6aO'143 ar^)
\ Bl7 vta7-/
foidite
trachyte
(
I
I
I
a t LBoli"
basanite
ICo3u
Tatu L
TaSc
I
picrobasalt
andesite dacite
basalt basltic
andesite
SiO2
tephritic phonolite
phonolitic tephrite
lf SamDlss Rock-tyDe Rb fDDml Sr {DDml aTRb/86Sr 87Sr/ABSr 2 sldm
1 Ka 27b
2 Ro9
3 Rol
4 Kald
5 Ta35
6 Bt?
7 Tat6
8 Ka3
g Taz
10 Co6a
't1 Ta9
12 ca4
l3 Fo 13s
14 Ta l8d
15 Coga
16 Tasc
agpalllc syenlto
agpaltlo
€yenlts
agpaltic syenlte
agpaltic syenlte
agpalllc sy€nlta
mlasldllc syenlte
mlaskltlc syenltB
mlasldtlc slonlle
mlasklllc syenlto
mlasldllc
syenlte
phonollte.
phonoltte
monchlqulle
mlcro6honldntte
mlcro-gssexlto (scrEen)
basanlte
(xenollth)
528 1.63375 0.00040
1
59 0.93749 0.00035
120 0.88729 0.00030
66.3 0.80547 0.00027
98,5 0.83237 0.00007
23,1 0,73748 0.00006
a,44 0.71331 0.00003
3.29 0.70891 0.00004
2.11 0.70715 0.00003
0.43 0.70466 0.00005
112 0.87417 0.00010
14.9 0.72443 0.00007
0.65 0.70443 0.00002
0.1 8 0.70360 0.00004
0.19 0.70427 0.00002
0.16 0.70422 0.00003
445
412
371
404
296
254
192
253
179
198
Jlo
271
192
58
7S
43
2,AA'
7.67'
9.10.
17.80'
9.80'
32.0
.
6l
223
246
1 335
857
950
1 177
778
' lsotops dllutlon mothod
Drscussron:
Tlu Los Ancrmsl-Aco nr RELATToN
WInl THE EVOTUNON OT CbNTRAL.EQUATORIAL
Arlamc OcraN
Post-Paleozoic magmatic activity in West Africa is
commonly subdivided into three
periods
(Sykes
1978,
Guiraud
et al. L987):1) a short and widespread Liassic
magmatic event, well dated from 203.7 x.2.7 Ma to
197.1 x. 1.8 Ma (Sebai
er al. l99l), has affected a
2000-krn-long area along the edge of the West
African
craton. It has produced
abundant continental tholeiite
occurring as diabase dykes @ars 1960) and sills
@ertrand
1991). Two laccolithic intrusive bodies
are
related to this event the Kakoulima body in Guinea
(Lazarenkov 1975, Diallo et al. L992) and the
Freetown body in Sierra [,eone @riden et al. I97L).
2) Numerous pipes and dykes of kimberlite are
scattered over Liberia"
Sierra
[,eone, Guinea
and Mali;
their early Cretaceous age
(140 to 92Ma; Odin 1994)
is based
on paleomagnetic
evidence
(Haggerfy
1982).
3) Cenozoic
alkaline
volcanism occurs
along the West
African margrn
(Cape
Verde and Canary archipelagos,
Dakm Peninsula). A huge volcanic seamount has been
discovered
on the southern
margin of the Guinean
continental shelf @erfand et al. 1988, 1989).
The Guinean continental shelf is bounded to the
West by the southemmost
segment of the Central
Atlantic passive margin of Jurassic age, and to
the South by a transform-type margin related to the
Equatorial Atlantic opening
(Berrand et al. 1989). The
295
rtr1
Guinean margin has been subjected to several
magmatic
events related
to tectonic
changes
in the drift
of the southern
continents
(Africa" South America).
The tholeiitic magmatism
at -200 Ma has
been linked
(Bertrand 1991)
to extensional
tectonism
related to a
rifting mechanism that eventually led to continent
disruption. In early Cretaceous
time, the margin
became a transform margin with the development
of
E-W fracture shear-zone
0ike the actual
St. Paul and
Romanche faults
in the Equatorial
Atlantic). This shear
or transform
zone allowed a connection
between rifting
in the Northern and in the Southern
Atlantic (Mascle
et al. 1995).
This Barremian
to Albian (117 to 96 Ma)
transtensional
tectonic
phase
resulted in the formation
of small
pull-apart
basins
characterized
by the thinning
of the continental crust
(Mascle
et al. 1986, 1988),
and
the progressive
opening of the Equatorial Atlantic
Ocean between
West Africa and South
America.
This
phase was accompagnied
by the emplacement
of
alkaline igneous
rocks along fault zones, like the
Liberian and Sierra
Leone kimberlites,
which intruded
along reactivated
crustal fractures, as suggested
by
Haggerty (1982). The southem
Guinean
margin was
formed by the reactivation of one of the
Guineo-Nubian lineaments
(Guiraud et al. L987), of
Jurassic age.
The final disruption
between the West African and
South American continents
resulted
in a pronounced
tectonic unconformity
of upper
Albian age
(-100 Ma)
(Mascle
et al. 1986, 1988,
1995, Wilson & Guiraud
L992).
k resulted
in the complete relaxation
of shear
TTIE I,OS ARCIIIPELAGO RING-STRUCTTJRE
FIc. 6. Rb-Sr whole-rock isochron for the Los suite of syenites.
a: agpaitic suite;
b: miaskitic suite; c: micro-essexite; d: basanite;
e: dykes
(phonolite,
shonkinite
and
monchiquite). For more details, see Table
2 a:rd
text.
fta
120
LOS alkaline comolex
9WR(tr,I)
104.3 t L.7 Ma
0.70401 r 8
MSWD = I
296 TIIE CANADIAN MINERALOGIST
Ftc. 7. Paleoenvironmental
maps
(modified after Dercourt et aL 1993). A. Early Aptian
(113-108 Ma) plate tectonic reconstruction, with the future position of the Los
Archipelago.
B. Late Cenomanin (96-92 Ma) plate teclonic reconstruction, with the
location
of the Los Archipelago
after its emplacement.
sftesses,
During Cenozoic
times,
minsl changes
in the
pattem
of drifting in the cental Atlantic have
produced
local tensional stresses
(along
old fractures
of the West
Aftican margin),
which are
possibly
responsible
for the
alkaline volcanism.
The Los syenite ring-structure,
dated at lA43 x,
1.7 Ma (Albian),
thus appears
to have been emplaced
at
the end of this transtensional
lectonic phase,
during
the early opening of the Equatorial Atlantic Ocean.
On the paleoenvironmental
map for Aptian times
(l 13-108
Ma) @g. 7A, from Dercotxr
et al. 1993), the
geological
setting ofthe Los area is the
Jurassic
passive
margin of the cental Atlantic. The magmatic
pulse
thus occurred in an attenuated continental crust by
reactivation of a prominent east-west fracture. This
happened as a result of relaxation of shear sfressn
implying a tensional gash oblique to the Guineo-
Nubian lineament. The paleoenvironmental
map for
late Cenomanian (96-92 Ma) times (Fig. 7B,
Dercourt
et al.1993) shows clearly the opening
ofthe
Equatorial Atlantic Ocean and the progressive
west-
ward offset of the spreading ridge along prominent
transform faults in the prolongation of the
Guineo-Nubian
lineaments.
TI{E LOS ARCHIPELAGO RJNG-STRUCTT]RE 297
CoNcr-usrous
l. The Los ring-complex intruded the Guinean
passive
continental margin, which consists
of a Proterozoic
basement covered by early Mesozoic sediments.
2. The Los complex consists mainly of nepheline
syenites
cut by phonolite
dykes and
by alkaline mafic
dykes
(monchiquile,
shonkinite).
3. Two suites of rocks have been recognized,
one
miaskitic, the other agpaitic.
4. These suites
differ by their mafic minerals.
Samples
of miaskitic syenite contain augite, hastingsitic
amphi-
bole, Mn-bearing mica and titanite. Samples of agpaitic
syenite are characterized by aegirine, Mn-rich
arfvedsonite,
Mn-rich mica, pyrophanile,
kupletskile
and l&venite, and by 7-r- and REE-rich accessory
minerals
(eudialyte,
catapleiite,
pyrochlore).
5. All the syenites
are enriched in elements
(Rb, Zr,
Nb, Y) typically found in alkaline rocks, the agpaitic
syenite being more strongly enriched
(>300 ppm Rb,
up to 2000
ppm Zr) than the miaskitic syenite.
6. The phonolite
found in dykes is similar gssshsmi-
cally to the agpaitic syenite.
7. An Albian age
(104.3
t 1.7 Ma; Rb-Sr whole-rock
isochron)
has been
obtained for the emplacement of the
Los complex.
8. The low inid.al87SrF6Sr value
(0.7M01
r 0.00008)
points to a mantle origin without significant crustal
influence.
9. The intrusion of the Los complex is related
0o
the
end of a tanstensional tectonic regime during
the early
opening of tle Equatorial Atlantic ocean, along
reactivated Guinee-Nubian lineaments.
AcIo{owI;EDcHvENTs
Thanks me extended to Dr. K. Bell (Carleton
University, Ottawa) and Dr. L. Kogarko (Vernadsky
Institute of Geochemistry, Moscow) for I.G.C.P.
(project 314 "Alkaline and carbonatitic magmatism")
funding to D.O. to participate
at the special symposium
on oThe petrology, minsmlsgy and geochemistry
of
alkaline
rocks" at the Waterloo's GAC-MAC meeting
in May L994. C.M. acknowledges with gratifude
the
C.N.R.S research
grant'Action Sp6ciale Programm6e
Afrique", coordinator Dr. M. Deynoux. D.D. thanls
the FNRS, which gives financial support for isotopic
measurements
at Bruxelles. Dr. K.A. Foland (Ohio
State University, Colombus) and an anonymous
referee, as well as R.F. Maftin, are thanked for their
carefrrl review and improvement of an earlier draft of
fte manuscript.
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Received September 13, 1994, revised manuscrtpt accepted
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