the los archipelago nepheline syenite ring.strugture

19 Pages·2006·1.49 MB·English

28L
Thz Ca nndianM iner alog i st
Vol. 34, pp. 281-299( 1996)
THEL OSA RCHIPELAGNOE PHELINSEY ENITER ING.STRUGTURE:
A MAGMATICM ARKERO F THE EVOLUTION
OF THE GENTRALA ND EOUATORIALA TLANTIC
CHRISTIANMOREAU
Ddpanementd zs Sciencesd e l,aT ete et CNRS- UM10, Pble Sciencese t Technalogie,
Universitdd e Ia Rochelle,A vmue Marillnc, 17M2 In RochelleC edexI , France
DANIEL OHNENSTETTER
Centred e RecherchzP dtographiqucs et G4ochimiques- Cmtre Natiorul de LaR echerchzS cientifiqw,
B,P. 20. 54501 Vandoewre-l?s-NancvC edex,F raru:e
DANIELDEMAIFI1E
Ddpartemmtd esS ciencesd .eI aTerre, C.P. 160/02, Universit€ Libre d.eB rwelles,
50, avenueF .D. Roosevelt,1 050B ru,xellesB, elgique
BERNARD ROBINEAU
Ddpartementd es Sciencesd e la Terre, Universit€ de LaR €unioa
15,a venueR eni Cassin 97489 Saint-DenisC edex,I a. Rdunion
ABSTRACI
The Los Archipelago ring structure( Guineq West Africa) is composedo f peralkalinen ephelines yenite,w hich crops out
over two long crescent-shapeidsl aads Qamara Kassa),f ive islets and a central islald (Roume).A detailedg eologicals tudy,
which led to a new geologicalm ap, recognizess everals eriesw ithin two main petographic suites,o n the basiso f textural and
structuralf ield criteria-T he fust suite is miaskitic and is composedo fhastingsite-augiten ephelines yenite,w hereast he second
suite is agpaitic and is mainly composedo f arfuedsonite-aegirinen epheline syenite. These formations are intruded by
microsyeniteo f the samec omposition and then by ring-shapedm onchiquitic and radial phonolitic dykes. Late pegmatites
cross-cuta l1 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 (87SrF6Sr=0 0.7040) is compatible with a maatle origin with minor or no crustal influence. The
geological environmenta nd the emplacemenat ge of the Los Archipelago point to formation during the continentalb reakup
betweent he West Africa and SouthA merica and early rifting of the EquatorialA tlantic Ocean.
Keywords: miaskitic nepheline syenite, agpaitic nepheline syenite, Rb-Sr geochronology,c ontinental breakup, Equatorial
Atlantic Ocean,l os Archipelago,G uinea.
Sotrcuenn
L'archipel de Los (Guin6e,A frique de I'Ouest),e st un complexea nnulairec ompos6d e sy6niten 6ph6liniqueh yperalcaline
affleurant sur deux lles en forme de croissant( Iamara et Kassa),c inq llots et ltle centraled e Roume.U ne 6tudeg dologique
d6taill6e a permis d'6tablir une nouvelle carte g6ologique of I'on a distingu6 au sein de deux grandess uites de sy6nite
ndphdliniquep lusieurss driess elond esc ritbrest exturaD(e t structuraux6 tabliss ur le terrain. la premibres uite,m iaskitique,e st
compos6ee ssentiellemendte sy6niten 6phdliniquei hastingsitee t augite.L a secondes uite,a gpartiquee, st constitu& de sy6nite
n6ph6liniquei aegyrine et arfvedsonite.L 'ensembled e ces sy6nitese st inject6 par des venuesd e microsy6nited e m6me
composition,p uis travers6p ar des filons annulairesd e monchiquitee t radiairesd e phonolite. Des filons de pegmatitet ardive
recoupentl 'ensembled esf ormationsd e la stnrcture.U ne isocbroneR b-Sr sur rochet otale a donn6u n dged e lM.3 t 1.7 (26)
Ma (MSWD = 1.28) pour I'ensembled es formations sy6nitiques.k rapport isotopique initial (87SrF6Sr)e6,g al i 0.7040,
est compatiblea vec une sourcem antelliquel 6g0rementa ppauwie,s ans( ou avec peu) d'influencec rustale.L 'environnement
g6ologiquee t I'dge de mise en place de l'archipel de Los permettentd e le considdrerc ommeu n bon marqueurm agmatique
intervenul ors du changemendt e r6gime tectonique,c orrespondanht la sdparationd e I'Afrique de I'Oueste t de I'Am6riqued u
Sud et donnantn aissanced l'Oc6anA tlantique Equatorial.
Mots-cl6s:s y6niten 6ph6liniquem iaskitique,s y6niten 6ph6liniquea gpartiqued, atationR b-Sr, s6parationd es continents,o c6an
Atlantique 6quatorial,a rchipeld e Los, Guin6e.
282
TIIE CANADIAN MINERALOGIST
INTRoDUcnoN of size (12 km in diameter,1 15 km2),p etrologya nd
high level of emplacementt,h e Los complexi s similar
The Los Archipelago( 9'30'N, 15o30'E),l ocated to the Tlfmaussaqm assif (100 km2) in Greenland
5 km offshore Conakry (Guinea"w estern Africa), is (Jptan L974, Larsen & Sorensen1 987), but smaller
composedo ftwo long crescent-shapeids lands (Kassa than the Khibina-Ixivozero pluton (2000 km2) in the
and Tamara) and five islets (Corail, Blanche, Cabri, Kola peninsula@ udkin & Mitrofanov 1994,S emenov
Fousseta ndP oulet)t hat encirclea n inlernal lagoona nd 1994).
its cenfral island, Roume (Figs. 1, 2). This nepheline Most of the anorogenicr ing-structuresin trude either
syenite complex was studied by Giirich (1887), a continentali nfraplalee nvironment,s ucha si n the Arr,
Lacroix (1911),D elaire& Renaud( 1955),L azarenkov Nigeria and Cameroon provinces, or an oceanic
(1975)a ndM oreaue t al. (1986).T he circulars hapeo f innaplate environmentos uch as at Kerguelen Island
the archipelagow as first commentedu pon by Millot & @lack et al. 1,985).I n conffast, the Los complex
Dars (1959),w ho interpretedt he islandsa s the expres- innudes a passive continental margin; it thus bears
sion of a ring structurec omparableto thoseo bservedin some resemblancet o the Canary and Cape Verde
Cameroon, Nigeria" Madagascar,S outh Africa and complexes( Cr€vola et al. 1994),w hich alsoi ntrude the
westernA frica. Moreau et al. (L986) emphasizedth e passivem argin of west Africa, and the Trinidade and
importanceo f the magmatics tructuresi n the syenites Fernando do Noronha intrusions, located on the
and describedt he xenotths, dykes,a nd veins. In terms Brazilian passivem argin (Ulrich & Gomes1 981).
Fic. l. Geologicals ketchm ap showingt he location of the Los Islands( Millot & Dam 1959)i n the prolongationo f the Conakry
PeninsulaI-n sert: location of the studieda reai n West Africa. l: Paleozoicw hite sandstoneo f the Bove basin; 2: hoterozoic
calc-alkalinep orphyritic biotite granite; 3: Kakoulima gabbro and peridotite; 4: Los Islands nephelines yenite;5 : marine
alluvium.
283
TI{E LOS ARCHIPEI.AGO RING-STRUCTTJRB
r1 ffiffiffi
1234 Ta 35
EffiffiM
567E
br
lEd
Ta 16
F
I
{}'
{'
Ro 13e
I"S
a
Ta7
_l
=l
CORAIL
cosag,fo6a o 1 2km
\.7
Frc. 2. Geologicalm ap of Los Archipelago. 1: bauxitic lareite. Agpaitic suite (aegiine - arfuedsonite- l6venite- kupletskite
nepheline syenite): 2: fine-grained and microcrystalline tlpe; 3: medium- to coarse-grainedty pe; 4: brecciatedt ype.
Miaskitic suite $astingsite- augiten ephelines yenite):5 : medium-t o coarse-grainetdy pe; 6: "nebulous"t ype; 7: mesocratic
type; 8: leucocratict ype; br: occutence of endogenousb reccia.
A new 1:50 000 map of the complex (Fig. 2), The Los intrusive complex was emplacedd uring the
completeda s a result of a detaileds tructurala nd petro- Cretaceous, at the western edge of the Conakry
logical study (Moreaue t al. L986),s howst hat the main Peninsula, consisting essentially of the elongate
rock-types pass gadationally from one to the other; (50 km) Kaloum ultramafic body. The eastemp art of
sharpc ontactsa re completelyl acking, which points to this complex,k nown as the Kakoulimam assif,i s made
a single phase of intrusion. Detailed mineralogical, of a thick sequenceo f layeredp eridotitea nd gabbro,o f
petrologicala nd geochemicadl ata have been obtained tholeiitic affinity, forming a huge laccolith @elaire &
on the main rock-typeso f the Los complex.A n Rb-Sr Renaud1 955,B arrdre1 959,D iallo et al. 1992)t hat
whole-rock isochron allows us to place ttre Los intruded the West African Proterozoicb asementa nd
archipelagom agmatic event temporally in relation to the Paleozoics edimentso f the Bove basin (Fig. 1). The
the evolution of the Central and Equatorial Atlantic Freetownl ayeredc omplex( Sierrat eone), dateda t 193
Ocean.A new interpretationo f the emplacemenot f this t 3 Ma (Rb-Sr age:B eckinsalee t al. L977),i s similar
complexi n its geodynamice nvironmenti s presented. 1o ghsl kkqulima massif. Thesei ntrusions have been
relatedt o the formation of the CentralA tlantic passive
Gpor,ocrcarS, stnNc margin @iallo el aI. 1992). There is, however, no
direct field relation betweent he Kaloum-Kakoulima
From the geodynamic point of view, the Los tholeiitic intrusive complex and the Los silica-under-
complexi ntrudesa passivec ontinentalm arginm adeo f saturated alkaline suite. In particular, there is no
early Mesozoic continental plaform sedimentst hat evidencet hat the youngerL os complex cross-cutst he
overlie a Proterozoic basement (Villeneuve 1984). tholeiitic stratiform complexa t depth.
284 TIIE CANADIAN MINERAI'GIST
Ftg-o RBlenoNsHrpsA NDP g:nocnepny The nephelinem icrosyenitei s locally porphyritic, with
alkali feldspar phenocrys*. This rock type is found
Two main petrographics uitesh ave been described mostly on the externalp art of the complex, along the
in the Los Islands, a miaskitic oneo and the other westernc oast of Tamara and eastgrnc oast of Kassa.
agpaitic (Fig. 2) (Lacroix 1911, Delaire & Renaud 3) A brecciatedn ephelines yeniteh asb eeno bservedo n
1955, Moreau et al. 1986). The distinction between Roume Island, locally on the southeastemp art of
agpaitic and miaskitic suites is classically done on Tamara, and on the external parts of Kassa and
the basis of minsral6gical composition (Sgrensen Temala.
1974). Xenolithsnv eins and dyke rocks cutting the Note that the transition between agpaitic and
complex are grouped into families (phonolite, miaskitic syenilesi s progressive;it has beeno bserved
microshonkinite,a ndm onchiquite)o n the basiso f their in Tamara( Ta 35), Corail (Co 6a), Cabri and Blanche
petrography. (Bl7).
Detailed field and sfructural studies of the whole
Los intrusive complex (Moreau et al. 1986) suggest Miaskitic syenite
four stages of emplacement 1) the agpaitic and
miaskitic nepheline syenite - microsyenile, forming Miaskitic syenite occupies the outer rim of the
the largestp art of the complex,w ere emplacedto gether structure( Fig. 2), the main part of the islandso f Kassa
at an early staget2 ) the nephelinem icrosyenite and (Ka 3) and TamaraC Ia 16,T a 7). Suchr ocls display a
aplite were emplacedl ater, followed by 3) ring dykes coarse-grainedi ntersertal texture, with euhedral to
(and endogenoubs reccias)a nd,f inally, 4) radial dykes subhedrala lkali feldspar,a nd nephelinea nd analcime
and late pegmatites.T he general shapeo f the whole within the interstices.A ugite commonly is included
complex and its internal structuresc orrespondto those within the poikilitic hastingsitea nd is associatedw ith
of a lopolith. The centero f igneousa ctivity seemsto be euhedral crystals of titanite, apatite and titaniferous
located on Roume Island, where a widespread magnetite.
endogenousb recciai s found. There is no evidencef or At the scaleo f the map (Fig. 2), the following pet-
the possiblee xistenceo f two centers. rographic types have been distinguished:1 ) a meso-
cratic medium- to coarse-grainedn epheline syenite
Agpaitic syenite representsth e main lyps; it displaysa n intergranulart o
intersertal texture with local feldspathic intergrowths
'ngsite,
Agpaitic syenite cover only one quarter of the and mosaic concentrationso f has titanite, and
surfacea t los. Suchr ocks containl 6venite- and astro- auglte; 2) a 'onebulousotyop e of nepheline syenite,
phyllite-group minerals (Gtrich 1887) and rare characterizedb y dark grey spotted polycrystalline
minerals such as villiaumite (Lacroix 1908),s erandite aggxegateso f nepheline; 3) a mesocratic nepheline
(.acroix 1931) and steacyite( Parodi & Della Ventura microsyenite, characterizedb y a slightly porphyritic
1987).T he texturei s foyaitic to locally tachytoid, as a heteromicrogranular texture; such rocks outcrop
result of flow of the syenitic magma during its mainly as narrow bands (a few decimelers) in the
emplacementA. transition toward a microcrystalline northeasterna nd northwesternp arts of Tamara and
texture has beenn oted, as well as toward a pegmatitic along the westernc oasto fKassa, and 4) a leucocratic
facies.M ineralized druseso ccur, especiallyo n Roume type occurs as a light grey, feldspar- and nepheline-
Island and in a quarry at the north ofKassa. rich, medium to coarse-grainedro ck, locally with a
A detailed survey _permits to distinguish three pegmatitic (soutl of Kassa) or a microcrystalline
different types. 1) A coarse- to medium-grained texture (eastema nd northeastemc oasto f Tamara).
aegirine nepheline syenite shows an intergranular
foyaitic texture,a s in the extemalr im (Ftg. 2) of some The dykzs,x enolithsa nd pegmatites
islands (northeasto f Kassa:K a2Tb, Ka ld; northwest
of Tamara:T a 35) and in the centero f the structurea r Two types of dykes have been distinguished. l)
RoumeI sland @o 1, Ro 9). This syenilei s composed Ring dykes( decimetert o metert hick) with low inward
of euhedral laths of perthitic feldspar, commonly dip. They consist mainly of phonolite. They are
mantled by albite, and interstitial, anhedral centered on Roume Island, but also crop out on
feldspathoids( nepheline, sodalite, and analcime), Tamara,K assaa nd Corail islands.T he phonolited ykes
associated with poikilitic Mn-rich arfvedsonite. present either a porphyritic texfure or a fluidal
Euhedral and zoned crystals of l8venite, kupletskite, pilotaxitic lexture. The porphyritic texture (Ta 9) is
and aegirinea re associatedw ith small crystalso f pyro- defined by sanidine phenocrysts in a groundmass
phanite,r are pyrochlore,w ith small hexagonacl rystals composedo f spherulitesw ith radial intergrowths of
of catapleiite and fluorite in the interstices. 2) An alkali feldspar and aegirine needles,a ssociatedw ith
aegirine- lAvenite- nephelinem icrosyenitew ith rare nosean, and biotite. The fluidal piloaxitic
kupletskite shows a nfqesrystalline foyaitic texture, lexture (Ca 4) is composedo f needles of aegirine,
locally with well-developedf rachytic fluidal features. sanidinem icrophenocryststh at outline a flow-induced
285
T}IE LOS ARCHIPET,AGOR ING.STRUCTURE
lamination, and microlites in a nepheline- and and apatite. The micro-essexites creen( Co 3a) from
analcime-rich groundmassw ith rare crystals of Corail Island shows a microheterogranulart exture,
fluorite. 2) Radial dykes (centimeter to decimeter with zoned and resorbed phenocrystso f augite,
thick) are homogeneouslyd istributed throughout the nnhedral alkali feldspar associated witl augite
intrusive complex.T hey consistmainlyo f monchiquite and hastingsitem icrolites, titaniferous magnetite,a nd
and shonkinite. The monchiquite dyke from Roume apatite,w ith interstitial nephelinea nd analcimei n the
Island (Ro 13e) shows an intersertal textwe: zoned groundmass.
titaniferous augite and hastingsitep henocrystslo cally Dykes of pegmatitea re ubiquitous;t hey are related
occur as glomeroporphyriticc lustersi n a groundmass to the main agTaitica nd miaskitic nephelines yenites,
composedo f intersertal alkali feldspar and microlites but they are also associatedw ith various dykesa nd cut
of titaniferous augite. Hastingsite, apatite, and someo f them (Moreaue t al. 1986).
titaniferousm agnetite,a nalcime,a nd nephelinef ill the
interstices;c alcite occursa s a secondarym ineral. The Aller,vncar MsrHoDs
microshonkinite dyke (Ta 18d) is a medium-grained
porphyritic rock with an intersertal texture. The Concentrationso f major andt ace elementsin whole
phenocrysts are titaniferous augite and olivine rocks were determinedb y inductively coupledp lasma
(replaced by a smectite-goup minsl6l). The alkali (CP) - atomice missions pectrometrya t the University
feldsparl aths definea n intersertalt exture;m icrolites of of Clermont-Fenandt r (Cantagrel& Pin 1994). The
titaniferous augite, hastingsite, biotite, and apatite FeO content was establishedb y wet chemistry at the
occur within an analcime-a nd nepheline-richg round- University of Nancy I. Analyseso f the minerals were
mass. obtainedw ith a CamebaxM icrobeama nd SX 50 elec-
Magmatic xenoliths are coutmon,b ut representle ss fton microprobes with wavelength-dispersions pec-
that L voL%o;t hey are mainly found in miaskitic fometers attle joint CNRS-BRGM facility. Operating
syenite.T hey vary in size from a few centimeterst o conditionsw ere: acceleratingv oltage 15 kV and beam
several decimeters,a nd are usually ovoid, round or current 10 nA; countingt imes were 10 s for major ele-
ellipsoidal. Two main types have been identified: mentsa nd 20 s for minor elements.O xidesa ndn atural
l) leucocratic xenoliths of nepheline microsyenite silicatesw ere useda s standardsD. ata reductionf or the
composition,a nd 2) mafic xenoliths of basaniticc om- microbeam analysesw as performed using the ZAF
position, located only in the southemp aft of Tamara method of Henoc & Tong (1978) and, for the SX 50,
Island (Ta 5c). On Corail Island, a large screen 6ing the PAP method of Pouchou& Pichoir (1984,
(50 x 2 m) of micro-essexite(C o 3a) occursw ithin the r99L).
syenite.I t containss yenitic xenoliths and is cross-cut Isotopic analysesw ere carriedo ut at the Laboratoire
by pegmatitics yeniticv eins.A phonolitic dyke cuts all de P6trologie - G6odynamiquec himique of the
the rocks. Universit6 Libre de Bruxelles. The samples were
A basanite enclave from Tamara Island (Ta 5c) processed following standard chemical procedures
shows an intersertal lexture with olivine (Fo6aa) (1e., IIF-HCIOa dissolution and separationu sing an
phenocrysts,a nd plagioclase,a ugite, and hastingsite anion-exchangec olumn). The isotopic composition
microlites, euhedral titaniferous magnetite, analcime of Sr was measuredo n double Re filaments with a
TABLE I. MINEMIIXIICAL COMPOSITIONO F SS.ECTEDS AI\{PI.ESF ROIVTI IIE 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+
E Bl7 td$kltlosymi€ r NA + + 4 +
7 Tr td nlaldilr 8tqlo r N, A, E r r
8 Kr3 nhrktbsydlo I N,A + + I + +
Tr? nlaskillasysno + N,A,a e r + +
10 Co 0r mleskilb ryenne + N,A + + ? +
11 Tr0 phonon + N,A"A r
12 Cr4 phomlto. + N,A S +
't3 Ro l39 nonchhllto+N,Arr +t
lrl Ta l8d nbrc€honKnlls { }{.A i + r
t 5 Co 3a nbeaqqllo r N, A + r ++
tO TaSr ba@nlto r A + r + tt
Ield.: fttdspg0F: oid:N : nophdlng. {. analoine,S : so{tat{sO; L olivinq Ae$ agirine,Ha$.: haciryfite; Arf: arFdsoaitq' I,tv.: lavenitel tpl: tupl€0sLil€;
Im': ilneurhsT; iEgl: eadftrous-roagde nrA: orfiatyto;C rt.: c*apl€iit6, fbo.: ftrorilo; Tii: titoito, Pyro.:p yrocblorcA; p.: apde'
286
THE CANADIAN MINERALOGIST
Fimigan MAT 260 mass spectrometerB. etween-run samples( Ta 35 andB l 7) by intermediatefe atures,s uch
precisioni s bettert han 5 x 10-5.T he NBS 987 standard as the presenceo f ltvenite instead of titanile. The
gave a value for 875t7t65or f 0.71023t I (2o on evolufion from the miaskitic to the agpaitic facies is
the mean for 30 measurements. normalized to marked by the disappearanceo f titanite, augite,
865.7885=t 0.1198).R b and Sr concentrationhs ave fuast'ngsitea, ndt itaniferousm agnetite,w hoser oles are
been measuredb y X-ray fluorescenceo r by isotope played by the kupletskite-livenite association,
dilution (8?Rba nd eSr'spike) where concentrations aegirine,a rfuedsonitea, nd pyropharfte,r espectively.
were less than 30 ppm. The error on the Rb/Sr ratio is The compositiono f the pyroxenes,c alculatedo n the
32Vo. T\e Rb/Sr age has been calculated following basiso ffour cationsa nds ix atomso f oxygen( Iable 2),
Williamson( 1968),w ith ana ssignedenoor f 2Voo nthe evolvesf rom augite in the miaskitic suite to aegirine-
87Rb/d6sr
ratios and of 4 x 10-5( averageb etween-run augitea nd aegirinei n tle agpaitics uiie. In somer ocls,
error) on the 8751165rra tios, exceptw heret he isotopic aegirine-augiteo ccursa s a relict core within aegirine.
measuremenfto r a samplew as lessp recise. The prismatic crystals of aegirine commonly show
oscillatory Tening. The concentration of zirconium
MnrsRAl,ocy is invariably high in the aegirine (ZrO2 may
reach L.73 wt%o) and comparablet o that in the
The petrographicf eaturesa nd mineral associations Illmaussaqs uite( Shearer& Larsen1 994).T he concen-
of the analyzeds amplesa re summarizedi n Table l. tration ofmanganesei s high i1 both quadrilaterala nd
The two main suites are distinguishedb y mineralogy Na-pyroxenes( nomenclaturea ccording to the IMA
and crystal chemisty. The agpaitic syenitesi nvariably recommendationsM: orimoto 1988),w ith MnO content
contain aegirine, arfredsonite commonly very much up to 4.70 wtTo;t hus in most casest he pyroxenec ould
enrichedi n lvln (up to 9.2 wtVoM nO), Mn-rich mica be called manganoana egirine, manganoana egirine-
(up to 6.8 wtVo MnO), pyrophanile, and Mn-bearing augite and manganoana ugite. Only a few alkaline
minerals such as kupletskitea nd l8venite.V illiaumite, intrusive complexesa re known to contain pyroxene
serandite, steacyite, and phases rich in rare-earth- as Mn-rich as the Los manganoan aegjine (i.e.,
elements( RE4 and Zr, such as eudialyte,c atapleiite, KangerdlugssuaqK: empe & Deer 1970, Layne et al.
rosenbuschitea nd pyrochlore, also characterizet he 1982). Comparedt o the crystallization trends of
agpaitic rocks. The miaskitic syenites are mineral- sodium-richp yroxenesf rom the Uterature@ g. 3), the
ogically characlerizedb y hastingsitica mphibole,w ith compositionalt rend at Los is close to the Tlfmaussaq
lower Mn content (up to 3.9 wtVoM nO), Mn-bearing trend (Larsen 1976) for aegirine-rich pyroxene, and
mica, and an abundanceo f titanite. The transition close to the Uganda (Tyler & King 1967) and South
between the two main suites is marked in some Q6roq (Stephenson1 972) trends for the quadrilateral
TABLE 2 RESIJLTSO F ELBCTRON.MICROPROBAEN ALYSES
OFREPRESENTATT!tsPYRO'GNESFROTMH EI.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 [email protected] 9.J0 9430 [email protected] 99.64 9&7E 98J8 98.trr 99.16
st rJ66 1.86 L9D4 1.994 1.9@ tS76 1.9$ r9A ,"975 1943 1.938 1.@ 1.909 Lyr8
Al lv 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
Al el 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
Tt 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
Fo3+ oajr, 0-o8 0.&B 0.7n 0.798 0.789 0il7 0.665 0"640 0.125 0120 o8 0.t88 0137
F€Ar 0,026 0.1t4 0!96 0JX6 0.047 0.106 0013 0.@ 0.1?4 03?0 0369 0.219 0,8 o2A1
IUn 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
MS 0.0!4 0019 0.0lj 0@ 0.018 0.m9 0.011 0.@ 0011 0399 0168 0J15 oJ10 0456
Co 0016 0110 0.086 0164 0.04 0.088 0.030 0.69 08 01n 0.753 0.804 0.80t 0.?85
l.h 0"vr6 0358 0.888 0"S1 0.949 0.686 0960 oJ99 07n O.Mt Ot01 0.1@ 0.165 0.175
K 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
k 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
Nb o,m 0.m4 0.0m om 0.m 0.010 o.qD 0.m 0.014 0.00 0.m 0.@ om omo
Co!@s I to & Na Wmq 6lm 9: C&l\trBp yrm; olm l0 to 14:Q uad! y@ CoEpositi@ qes€d itr wgo.
287
THE LOS ARCHIPELAGOR ING-STRUCTURB
Na
Frc. 3. Na - Mg - Fe2++ Mn
triangular plot showing
the Los suite ofpyroxene
compositions. Dots:
miaskitic suite; open cir-
cles: agpaitic suite. The
Ilfrnaussaq( LarsenI 976),
South Q6roq suite
(Stephenson 1972), and
Ugantla (Tyler & King
1967) trends also are
reported.
Fe2++ Mn
TABLE 3. RESULTSO FE LECIRON-MICROPROBABI. IALYSBS
OFR EPRESENTATTVAEI\4 PHIBOLEFSR OMT IIE 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 9831 98A7 98.,18 99,47 :fl 9.37 loI.Q, 9.95 10012 99.E8 9930 99.65
st 8.09 1.w2 ?958 1U, 1372 7,494 6.114 6il1 5.8,t6 6.137 6.194 6.115
Al lv 0.000 0.0s 0$42 0.718 0,628 0J06 1.886 l:19 Ll54 1.863 1.846 1.865
Al vi 0237 0,M O,28 0.0m 0.089 0.0t1 o.tn 0.043 0.419 0.050 0.2(E o,ln
Ti 0041 0.082 0.055 0 3 0.157 0.u4 0354 03@ 0.130 0366 0.185 0.191
k 0.00t 0.012 0,@ 0.015 0.64 0rm 0.00s 0.006 0.000 0.02,, 0.015 0.0m
Fo3{ 0392 0J10 0.460 0J67 o,CB 0X62 0.584 0J89 0.786 0100 0.4n 0,774
Be,+ Lr99 X{ \&t 2.732 LVlt ftn 1.635 tJl4 1.751 1.789 LL74 l,1tl
Ma Lni 1.69 r8 0n 1.157 0SCt 0346 0,4{p 05gt 0307 0.411 0.4n
Mg 0A?5 0.618 O.6X 0388 0J6 1.610 2005 1537 1.,106 L%7 $n 1.694
C€ 0009 0.095' 0.115 0241 0.106 0 9 1.638 1.661 l:143 tng ljr2j l.6g
Na 25V1 2l,67 252 2l,62 ZU 2573 0.98 0.896 0.8a 0.950 0941 0.995
K 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 expressedin wt9zo.
288
Ca> 1.34
Ti< 0.50
Sl+Nr+K
raa
hastingsite a
ll
arfvedsodte
tr
o
75
Ftc.4. Compositiono f the Los amFhibolesA. . Amphibole compositionsp lotted in the Ca
+ ryAl yersasS i + Na + K diagram( Gret et aL 1980).T he field of arfvedsonitefr om
the Oslo Rift (Neumann1 976)a nd the hornblendeh astingsitefr on the Iskou complex
(Gret et al. 1980) also are shown. B. Cau versus Si diagram O{itchell 1990).
Dots: miaskitic suite; openc ircles: agpaitics uite.
andN a-Ca pyroxenes,a lthought he manganesceo nlent have been found: the agpaitic syenite suite is charac-
is higher at Los. The niobium conlent can reachu p to teized by manganoana rfvedsonite,w hereas the
0.52 vttVo NbrO, at Los: similar values have been amphiboles from the miaskitic suite are mostly
obtainedi n Zr-rich pyroxenef rom peralkalinet rachyte manganoan-magnesian hastingsite or F-rich
and comendite from the Wamrmbungle volcano manganoan-magnesiahna stingsite( Table 3). In a plot
(Duggan1 988). of rvAl + Ca versw Si + Na + K (Giret et al. L980)
The compositiono f the amphiboles,a lso classffied (Fig. 4A), the amphibole compositionsf rom the
accordingt o IMA recommendations(L eake 1978), is agpaitic suite evolve from richterite toward arfved-
calculatedo n the basiso f 13 cationsi n the tetrahedrally sonite,a ndp lot within the samef ield as thosef rom the
coordinateda lid C (IrIl, M2 andM 3) sites.T wo groups Oslo rift (Neumann 1970. The Los arfvedsonite
289
TIIE I,OS ARCHIPEI..AGOR ING-STRUCTT]RE
TABI.E 4. RESI,JLTSO F ELECTRON-MICROPROBAEI{ ALYSESO F et aI. L982).
REPRESENTATTVMEI CA FROMT T{EL OS ARCHIPELAGO The astrophyllite-group minerals occur only in
S@pli Ta35 Ta3t TA Ta7 Ts? Bf, the agpaitics uite.T heir composition( calculatedo n the
basiso f Si + Al = 8 cations)e volvesf rom asfrophyllite
sio2 vtf/o n33 An v.6 \tn $9 38Jr @e end-member)t o kupletskile (Mn end-member)
\ql L6 r:r9 1.'2 Ln 31r 2"t3 Clable 5). In a single crystal, Mn emichmentc an be
ADo.3 9.70 9.C' 1493 l4l8 lJ31 1055 documented from core to rim (anal. I and 2).
FO 19.41 1E77 24.45 2231 A6 13.f, Compared to astrophyllite from Kangerdlugssuaq
lrnO 5.63 sJO 436 452 3.(B 5A
Mgo 957 10.04 5.,n 6J5 78 t293 Q-,aynee t al. 1982),t hat at Los is richer in Nb (up to
cd 0.o 0.6 orn o.lt 0.115 0.m 234 wtvoM zOs).
Na2o ili'1 03,f ori 032 0.61 0.64 The l&venite (calculatedo n the basis of Si + Al =
r20 9J4 9J9 9.01 9.11 t.tl 9,2
l,a 1.,16 0.16 oln 014 233 2 cations)a lsoi s a characteristicm ineral of the agpaitic
q 0.r5 0.06 0.04 0.01 o.lxt 034 suile. At Los, the lAvenite( Iable 6) is enrichedi n Ti
E2O@lc 335 3:IE 3.70 3.80 3fr 3,42 and Mn comparedt o that at Kangerdlugssuaq(K empe
..-Oo='Fcl 00..olr, 0o..6o6j 00..oo,2 00..lml) 00..0{D6 01..lS.tt & Deer 1970).A s at Lovozero (Vlasov et al. 1966),i t
evolvest oward titanium-rich l&venite.
Tobl !m.,|6 t0036 l@26 In the miaskitic suite, the dominant oxide phasei s
st 3.m6 3.r'36 2:r* L1U 26tl 3o}l titaniferous magnetite (calculated on the basis of
Al iv 0913 0.CZ3 LzA 12t6 1319 0gt5 3 cations and 4 atoms of oxygen) locally enrichedi n
ulvdspinel component (Table 7). In contrast, in
Alvt 0.0m 0.q[ 0.180 0.166 0.108 0.(m the agpaitic suite, the dominant oxide belongst o the
0.138 0.r9, o10J 0,161 oal 0.120
Fo2r r3tl1 1 1.6t4 1.471 r.43 0.89' ilmenite group, invariably enrichedi n Mn, with up to
Ivln 0381 0396 03411 03D 02Jr 034 74mol.Vop yrophanite(I able 8).
Ms r.140 1.187 0.661 O'n 0923 1J12 The zirconosilicates also are restricted to the
06 LW 2.915 2.940 L{ro 2EE8 2ffi agpaitic suite. Representativec ompositions of cata-
C4 0.m o.qD 0.m 0.o9 0.m5 0.m pleiite and partial analyseso f eudialyte( without REE
N6 oo4t 0.62 0.039 0.049 0,091 o.wl detenninations)a re given in Table 9. A mineral close
K 09t3 09$ 0934 0916 0.t89 094 to catapleiite,o r more probably to cr-hydrocatapleiite,
F 0J.t4 0366 0.041 0.m 0.05 0578 NaHZSi3Oe'2H2O.aq@ ortnove t al. 1972),c alculated
0.oa) 0l)08 o.ffi 0.ml 0.0ft 0.0itt on the basis of 3 (Si + Ti + Al) atomsoo vergrowst he
OH 1.606 L.A6 1954 Lg 1.95t LM l8venite and the eudialyte. Eudialyte (calculated on
T@l 9.37B the basis of 6 Si atoms) is enriched in Mn (up to
9 wtToM nO), as has been noted by Kunitz (1936) in
eudialytea ssociatedw ith villiaumite and astrophyllite
from Roume Island, compared to eudialyte from
Lovozero( Khomyakov1 995).
The feldspathoidso ccur in both suites;r epresenta-
tives compositionsa re grveni n Table 10. The sodalite
Clable 3) is richer in Mn (up to 9.17 fi7o MnO) than (calculatedo n the basiso f Si + Al + Fe3+= 12 cations)
the arfuedsonitef rom Kangerdlugssuaq( Kempe & is nearlyp ure.A s in the caseo f sodalite,a nalcime( cal-
Deer l97l,Layne et al. 1982).T he amphibolec ompo- culated on the basis of 96 atoms of oxygen) also is
sitionsf rom the miaskitic rocks evolvef rom hastingsite neady pure. On the conrary, nepheline( calculatedo n
to hasringsitich omblendea nd edenite.T he magnesian the basiso f 32 atomso f oxygen)s howsa wide rangeo f
6astingsitec ontains up to 3.86 wtVo MgO. It is also K content, from 5.80 to 7.64 wtVo K2O. In some
characterizedb y a high fluorine content( up to 2 wITo), samples,t he nephelinei s transformedin to cancrinite.
In the Ca6v ersusS i diagram( Fig. 4B), fts amphibole In conclusion,t he evolution from miaskitic toward
compositions evolve from magnesianh astingsite agpaitic suites is marked by drastic changesi n the
toward arfuedsonite. as in other undersaturated mineral associationsa ndb y a generali ncreasein levels
complexes(M itchell 1990). of Mn, Zr, and Ti in both the silicatesa nd the oxides
The compositionso f biotite, calculatedo n the basis that crystallize from the residual liquid. Detailed
of I I atomso f oxygen,a reg iven in Table4 . The biotite mineralogicald ataa nd discussiono f crystallochemical
from the rock transitional toward the agpaitic suite substitution within the different minerals will be
(Bl7 and Ta35) shows a lower Al content (<70 tutVo deferredt o a later paper.
Alror) and hon content,a nd higher Mn content( up to
5.9 wtVoM nO) than the biotite from the miaskitic suite GeocgEtltsrRve No Acs oF EMPLAcEMENT
(34 vrr%oM nO). The composition of the low-Al -
high-Mn biotite from the agpaitics yenitea t Los is very Concenhationso f major and some trace elements
similar to that describeda t Kangerdlugssuaq(L ayne (Zr, Nb, Y, Rb, Sr) in representatives amples of
290 THE CANADIAN MINERALOGIST
TABIJ 5. REST]LTSO FE LECTRON.MCROPROBAEN ALYSES
OFR FRESENTAffi/E ASTROPFIYLLITEA ND KUPLETSKIIE FROMT IIE rrOSA RCIITPAAGO
Kald
9
stg2 32.96 n55 t+.@ tLn ,Laz n.62 y.9 '454 33.9 33.40
1I@ 9.12 9.60 9.ll 1.92 8Jt IO83 &98 10?6 gA a;23
Zrg2 da Lx 3.6 4.n 335 052 !.!6 oot r9 40J
&iJ,oot Dll5s 1l6J.72 vl..tta4 t&t.o&l, lTt5J2t rL{}3@6 tTLA6 t Z2I9g8 m1.6.3 6 riLlm
Md 13.83 l&99 2tg2 t7.t3 l8J8 4& l&91 15.63 16'18 19.88
Mlo 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
N!2o 2.ll 1.8t ZVz t.1t 1.68 Z6 28 l.6t 2.t1 Lgt
K20 t9, 612 5.89 &14 6.01 JJ6 5.& 6A, 6.A 3.n
BaO O24 ol2 0A, O{D dd. ral O,t4 O03 O.g, O.gt
tt205 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
n2Oelc 455 4J8 4.49 LU 4.45 493 4& 486 4t2 4,9
crP 4A 4l,, {.43 "0.40 {.37 qO 4.9 -O4l 1}.30 -0,4t
Tet tml9 9t30 99,U
st 6.9m 7.@S 7.359 7.Ur1 1.tza 8.m 6'90 6&3 7,4 ?.0J1
z 8t..lmq! 0&.m90 5 0&604@t 0&9ma o8a.m7 7 0g.@m 0E, 0m00 &l.rmt o&:@t6 0s.tf4lt9c
r3J9 t574 t"49 1r8J 1.388 r:rX2 r359 t.603 L4n 1309
Zr ot14 oil2 0317 0@ 0355 00t4 o.lt4 omt 0.301 0.415
Nb 0.lD 0.t36 - 0.u 0r0t 0.6, ol39 0.151
Y r.611 !953 tn$ rgn lr49 tar3 l.6n l.@4 r:tB 1.8?5
Fit+ 2-956 2543 3259 rfig 5.790 t476 t.61 LIM
M! sJ05 1938 t.t16 3.415 0J36 3&, 262J 29fl 3J55
Mg 02at o.tq, 0258 0.1@ ot30 0.063 0r3l o4l5 0rt8 0.214
x 6436 6555 eW 6.638 67l' 6.690 6!5 6515 68@ 6.472
& o,'2 0380 0320 0J40 0,360 0 0.m o38o 0310 03@
Na mgr u766 0829 0tt3 0.?@ t3gl 0.891 4f43 o.sn o.g2
K 1.,188 I.'U' TJ93 L69I t.663 1J08 l.,st l-t'l t6l, !lJ3
Ea 0l)I1 0.010 o0l9 0.r, 0.0!l om3 0.06 0.o6
v 2n0l zw z7@ LIn ZTg M 0,0@ 2,@ 2-An 2@
0483 ol}7 o64J 0.60l 0J5t 0.m 0183 o57 o.44 0.61I
OH 6Jr 6.663 6.3!5 6399 &441 1M 6!7 6423 6-50 63
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: kupletskitoC. ompodtiones qprssseidn wPlo.
a$)aitic and miaskitic syenitesa nd in somed ykes and syenite usually show higher concentrationso f these
enclaves are given in Table LL, together with their elements( with more than 2000 ppm 7r) than those
CIPW norrns. Al1 fhe analyzeds amplesa re strongly of miaskitic syenite. Agpaitic samples are also
silica-undersaturated, with normative nepheline characterizedb y high levels of Rb (usually >300 ppm)
content as high as 3l7o for some agpaitic rocks. In and low contentso f Sr (<30 ppm), resulting in much
agreementw ith their modal minslalogy (presenceo f higher Rb/Sr values (>20) than in the miaskitic rocks
sodic pyroxene and sodic amphibole), the agpaitic (Rb/Sr< 10).
rocks contain up to 7.9Voa egirine and sodium meta- Samples of phonolite from the ring dykes have
silicate (ns) in their norms,w hereasth e miaskitic rocks major- and trace-elementc ontentsr oughly similar to
have higher levels of nonnative magnetite and those of agpaitic syenite. The two analyzeds amples
ilmenite. taken from the radial dykes (monchiquite and
The two serieso f syenited isplay a restrictedr ange shonkinite)a nd the two enclaves( basanitea nd micro-
of composition,w ith SiO, befweea5 4 a$d6 0 wtqo, and essexite) have an alkali-rich mafic composition
KrO + Na2Oi n excesso f 1.3w tVo.T hey typically plot (SiO2<4 5Vo).
in the alkaline field of the alkali-silica diagram Rather imprecise K-Ar ages on whole rocls led
(Fig. 5). The sampleso f agpairic syenireh ave signin- Lazarenkov( 1975) andl-azarenkov& Sherif (1975)t o
cantly higher levels of Na and Mn, lower Ti and Ca" suggestt hat the Los pluton was emplacedi n three
higher Na/K, and lower Fez+/Ire3t+h an the sampleso f phasesd uring the Cretaceousb, etween 105 to 80 Ma.
miaskitic syenite.T he agpaicity index [the molar ratio Ten representatives ampleso f the two syenite series
(Na + K)/A11,d efined by Sprensen( 1960), is grearer have been analyzedf or Rb-.Sri sotopes,t ogetherw ith
than 1 for the agpaitic rocks and less than I for the four sampleso f dykes (two of phonolite from early
miaskitic rocls. ring-dykes and two of mafic alkaline characterf rom
Rb, Z, Nb and Y contents are high for the both late radial dykes),a micro-essexitefr om a large screen
types of syenite, as in alkaline rocks in general and a basanitex enolith. Analytical data arer eportedi n
(Bowden & T\rner L974). T\e sampleso f agpaitic Table L2 andp lotted on Figure 6.