phase relations in transitional and alkali basaltic glasses from iceland

53 Pages·2015·3.86 MB·English

NORR NA ELDf JALLASTQStN
NORDIC VOLCANOLOGICAL INSTITUTE 8202
UNIVERSITY OF ICELAND
PHASE RELATIONS IN TRANSITIONAL
AND ALKALI BASALTIC GLASSES
FROM ICELAND
by
Peter Thy
Nordic Volcanological Institute
Reykjavik, Iceland
1 9 8 2
NORDIVCO LCANOLOGIICNASLT ITUT8E2 02
UNIVERSOIFT Y !CELANO
PHASREE LATIOINNTS R ANSITIONAL
ANDA LKALBIA SALTGILCA SSES
FROMI CELAND
by
PetTehry
NordViocl canolIongsitciatlu te
ReykjaIvciekl,a nd
1 9 8 2
PHASREE LATIIONNT SR ANSITAINOONA ALLK ABLAIS ALTIC
GLASSFERSOI MC ELAND
Abstract
Basaltgilca ssefsr omt he threael kalairce aosf I celand:
SnaefellsVnoelsc anZiocn e,S udurleVnodl canZiocn ea nd
VestmannaeVyojlacra nAirce ac ontapilna giocloalsiev,i ne,
clinopyroxcehnreo,m iasnp ineta nd titanomagnetite as
phenocrypshta sesT.h eg lassaerse n ephelitnohe y persthene
normatailvek albia saltwiict hF eO/MgrOa tiobse twee1n. 4-
4.7. Olivinrea ngei n compositfiroonm F ogto Fo ,
0 55
plag1oclafsreo m An toA n and clinopyroxefnreo m
90 50
En Fs wo to En Fs7wo• Clinopyroxerneev eals
45 10 45 40 1 43
systemaTtii:cA mLe tastacbrlyes talliztarteinodrnse latteod
thec ompositoifo nt he enclosignlga ss.T wo typeso f
phenocrayrsetp reseinntm osgtl asssehso winag bimodality
in sizaen dc ompositiMoinc.r ophenocirsyt shteps h asmeo st
Likeltyoh ave crystaflrloimtz heeed n closignlga ssw,h ile
macrophenocmrayys htasv ec rystallifzreodma liquiodf a
slightlleys se volved compoTshietg iloans.s sehso wc omplex
phenocryst-grlealsast ionwsh ichc anb e relatetdo a
polybareifcf ect.T hen ormatigvlea ssc ompostioanrse
relatetdo divariasnutr facienst heb asaltte trahedarnodn
definteh ep ositioofnt hef ourp hasec otectiLci ne.I n
generawli thi ncreasFienOg/ MignOt heg lastsh ep henocryst
assemblavgaersfy r omc linopyrooxleinvei,an ned p lagioclase
alonga clinopyroxene-osluirvfiancteeo olivinaen d
plagioclaasleo nga olivine-plagiosculrafsaec e.A
deflectiiosn s eeni nt hen ormatiglvaes cso mpotsiionfsr om
clinopyroxceonnet ainignlga sses ctloi nopyroxferneee
glasses.T he appearanocfe p lagiocltaosgee thewrit h
clinopyroaxnedon lei vicnaen b ee xplainiend t heL ighto f
experimenitnavle stigatoifo nst heef fecotf p ressuorne
phasree lationTsh.em ajoerl emevnatr iatiooft nh eg lasses
is interpreatser de presenmtainntgld ee rivmeadg mbaa tches
B-1
ofp rimarLyi quidmso difietdo varioudse grebey high,
intermediaantde L owp ressucrrey stfarla ctionattoiwoanr ds
equilibrpihuamsr ee latiodnusr inags cenatn dr esidenicne
magmac hambersT.h e bimodaliints yi zea ndc ompositoifo n
plagioclaansdoe l ivipnhee nocrycsatnbs e r elatteodh ight o
intermedpiraetses ucrrey stfarla ctionaitnit ohnem elt.T he
relativeevloyl vegdl assc ompositifornosm V estmannaeyjar
(mugeariatned) F e-Tib asalgtl assefsr omS udurlaanred
belivetdo be quenchehidg hp ressur(ew itho r without
volatilecso)m positioonnlsy w ithm inorL owerp ressure
crystfarla ctionation.
O Tholeiitic
--=5 0k m § Alkalic
jarIIlllllJ Transitional
Figur1e. Samplleo calit(ireesf etroT abl1e) a nd
generoault lionfet hea ctivveo lcanziocn eosf I celand
simplififerdo m Palmaso&n Saemundss(o1n9 74).
Petrochemidciavli siboent wetehno leiitailck,a laincd
transitiaolnkaall biacs albtass eodn J akobss(o1n9 72).
Thet ransitizoonnaselh owas c omplienxt errelationship
betwetehno leiitaelsk,a lain dF e-Tbia salt(sJ akobsson
19721,9 79I;m sla1n9d7 8)T.h eF e-Tbia salhtasv eb een
referrteoda sa trantsiionaall kalsiecr ie(sJ akobsson
1979)S.n .V.-Z.S naefellVsonlecsa nZiocn eR;. V.Z-.
ReykjaVnoelsc aniZco ne;S .V.Z-.S udurlaVnodl canic
Zone.
B-2
INTRODUCTION
Alkalbia salptlsa ya ni mportarnotl e aibny sspaelt rogenesis
ins pitoef t heilro wa bundan(cGea s1t9 68)A.l kalbia salts,
togethewri tht holeiibtaisca lwtist hv ariabelner ichmienn t
incompateilbelmee ntasp,p eatro b ec onfinetdo topographic
highsa,b normcarlu sttahli cknessefsr aacntduz roen e(se .g.
Shibateat al. 1979a).T hec loses patiraell ationship
betwetehno leiietnidca lkalbia salitns interplaotcee anic
islanhdass b een interipnrt eetremdos f d ifferecnotn ditions
andg reatdeerp thosf a lkalmia gmgae nerat(ieo.ng .M cBirney
& Gass1 967;G ast 19F6r8e;ey t a l. 1978)c,o mpositional
variatiiontn h eimra ntlseo urc(eS u&n Hanso1n9 75o)r high
pressufrrea ctionatfiroonmt holeii(tee. gM.a cdona1l9d6 8;
MaalD&e P eters1e9n7 6).
Thes ystemarteigci onaell ementvaalr iatiaonnd the,
relativteo primitiMvied -OceRaind gbea saltusn,d epleted
naturoef m ostt holeiitbiacs altisn thea ctivveo lcanic
zoneso f Icelanadn d associaHtiedd- Ocaeraen ahsa veb een
explainiend t he lighto f a mantlpel umet heory( e.g.
Sigvalda1s9o7n4 )A. binarmya ntlseo urchea sb eena dvocated
(Schilli1n9g7 3). Varioudse greeosf heterogeneties,
previoduesp letieovne ntasn dm ixinign t hes ourcree gioann d
int heo ceancircu sste emh,o wevetro,b ep referrbeads edo n
tracee lemenatn d isotopset udie(se .g.O 'Nioentsa l.
1976W;o ode ta l. 1979Z;i ndleetra l. 1979).T he alkali
basalthsa veb eena ttributteod a lowd egreoef p artial
meltionfga relativuenldye pletmeadn tle( O'Nioents al.
1973;O 'Nion&s Granvo1l9d7 3a)n dm ayb er elatteodd eep­
seatefdr actuzroen ecsu ttitnhgi cckr ustpaill e[sS igurdsson
1970a)O.s karssetoa nl . (1982)h,o wever, proposed that
alkalbia salitnsI celacnodu lhda veor iginaftreodmp artial
meltionfga hydratoecde ancircu sats a resulotf a thermal
impacftr oman a ctivrei ftz one.
In the activveo lcanziocn eosf I celaanldk ali basalts
arer estricttoet dh reaer ea(sF ig1.) , whichi n perta re
separatfreodm t hem aint holeiirtiifctz one[ Jakobs1s9o7n2 ;
Imslan1d9 78).A lkalbia saltasr e founidn t hew estward
extendSinnage fellVsonlecsa niZco ne( Fig1.) . Them agma
productihoans beenr elatteoda flanpko sitiroenl atitvoe
thet holeiitriicf tz one( Jakobss1o9n7 2),d igressive
spreadionfg p latet rappevdo lcanciecn tefrrso ma former
rifste gmen(tO skarsestoa nl . 1982)a, transcurrfeanutl t
zone (Sigurdss1o9n7 0a)o,r to en extincrti ftz one
(Johanne1s9s8o0n) T.h eS udurleVnodl canZiocn e iisn, the
southerpna rt, geinne rtarla nsititoona allk alibcu,ts hows
a
complreexl ationbsehtiwpe en tholaelikiatliaicnc,d Fe-Ti
basaltty pes.T heF e-Ti bashaalvtebs e enr eferrteoda sa
transitiaolnkaall sie rie(sJ akobs1s9o7n9 ).T heS udurland
VolcanZiocn ec anb ev ieweads a southwaorfdf shforto mt he
mainr iftz one( Jakobs1s9o7n2 S;a emunds1s9o7n8 )o,r m ay be
a
transgresvsoilvcea nziocn weh erveo lcaniissrm e generated
ina coolipnlga tdeu et ot hermiamlp acftr oma na ctivrei ft
zone[ Oskersesotan l . 1982)T.h eV estmannaeVyojlacra nic
Areias d ominatbeyad l kalbia saltasn,dh asb eenv ieweads a
southweaxrdt ensiooftn h eS udurlaVnodl canic (ZJoankeo bsson
1972a)n dt hed eepersetg eneraotfim oang mian transgressive
volcani(sOms karsestoa nl . 1982).
B-3
Petrogenemtoidce lfso ra lkalbia salhtasv em ainlbye en
baseodn w holreo ckl avag eochemiswthreyr,ae c cumulatoifo n
phenocrystcsa n restrict detcaoimlpeadr iswoint h
experimepnhtaaslee q uilibrium stundaiteusr abloa ns altic
systemsD.e tailaenda lysoefsr apidqluye nchmeadg mawsi th
relativehliyg hp ropoorntsi of glassw illa llowl iquid
variatiaonnd crystal-Lirqeuliadt iotnobs e s tudieTdh.e
presecnotn triburteipoonr tthse resultosf majore lement
analyseosfc oexistgilnags asn dp henocrpyhsats eisn a lkali
basaltgilca ssferso mI celanTdh.e t ermg lasiss , following
Bryan( 1979)u,s edt od enoteei thear r ocko ra phaseT.h e
actuamle anisnhgo ulhdo,w evebre,c learf romt he context.
The aim of the studyi s to examinteo w hate xtentth e
phenocrayssste mbliasgi en e quilibrium thwei etnhc losing
glassa,n d toc ompatrhee g lass compoasnidte iqouni librium
phenocrywsittshe xperimepnhtaaslre e latioinns t he basalt
system.T he ultimataei m is tos eet ow hate xtent is it
possibtloee xplopreet rologciocn straionnt sp etrogenesis
ande volutioofan l kalbia saltmiacg maisn I celand.
PETROGRAPHY
A totaolf 3 5s amploefsb asaltgilca ssferso m1 7l ocalities
were stu(dFiiegd1. ) . Localitaineds p etrograpdheitca ils
of thea nalysseadm plaerse g iveinn T abl1e. Them ajority
oft hes ample(s3 1a)r eP leistocgelnaec iaolr subaquatic
quenchegdl assesF.o urs amplefsr omV estmannaeayrjea r
tephrfar omt her ecenStu rtseayn d Heimaeye ruptions
(Thorarinsson 1916986;5T ,h orarinsesto anl . 1973;
Jakobsestoa nl . 1973).
All sampalreesh ighlvye siculaarn d fragmental,
containindgi fferenatm ountso f phenocrystbsr,o wn
translucseindte romegllaans so,p aquet achylitgel ass,
palagonitigzleads s,h olocrystalblaisnael tiacn d rare
silicgilca sfsr agments. stTuhdiiyss concernweidt ht he
non-oxidisziedde romegllaans saensd t heiprh anocrysts. The
relatiavmeo unotfsp henocryvsatrsi efsr omt racaem ounttso
moret han2 5v ol.c%r ystalasnd,n oc orrelatbieotnw etehne
amountosf phenocryasntds c hemistorfy theg lassi s
observedA. morede tailsetdu doyf t hem odavla riatieonnd
wholer ockm ajora nd tracee lemengte ochemisitsr iyn
progress.
All analyserse portewde rep erformed aw iftuhl ly
automatAiRcL -SEmMiQc roprowbiet ho n-lindea tar eduction
usinga standaarldp ha-faccotrorre ctpiroonc edurAe t.o tal
ofo ver2 00p0o inatn alysweesr ceo mpilaenddu sed.A lli ron
isc alculaatseF de Oe xcepwth ereo therwisstea ted.Th e
precisiaonnda ccuraocfyt hem icropraonbael ysiessg iveinn
Tabl2e. One standardde viatioonrn e plicaatnea lysaerse
generalbleyl o0w. 50f orm ajoerl ementasn d0 .10f or minor
elements.
B-4
TABLE 1 Petrologidceatla ilosf thes amplesan d samplel ocalitiesT.h et ablien dicattehse m ineral
assamblagperse senwti thitnh eg lasisn individusaalm ples
Samplneo . macroehenoCElSCS micro�henoc�sts Locality COllllllents
pl cpx ol pl cpx ol chr mt
Snaefellsnes
ICE x x x x KerlingarfJall hysloclasti1ee
l(E x x x x KerlingarfJall hyaloclast1te
I l
KE )A x x x x Kerlingarfjall hysloclastite
1
ICE: 38 x x x x Kerlingarfjall hyalloacs!:it)e
KE 4A x x x x Kelri ngfaj r l 11l hy11locl11s2t1 1ce
2
KE 48 x x x x Kerlingarfjall hyaloclast>i te
GR x x x x Grettisbaeli hyllloclastite
CR tr x x x x Grettisbaeli hyaloclast:ite
RI x x xx x 1Uifsg1lll,i tarvatn hyalocLastite
HI2 f x x xx x KlifagiBli,t arvatn hyaloc-laa3t) ite
SI2 c x x xx x K.lifsgill!,i tarvatn hyalcclas3t) ite
"
EN x x xx xx x x EnniO,l afsvik hyaloclastite
EN s x " xx x x x x EnniO,l afsvik hyaloclastite
"
S1'I x x xx x x StapafelSlu,p i hysloclastite
ST 2 x x x xx x x x StapafelSlt,ap i hyaloclas4t1 ite
Vestmannae�jar
4
VE 20 x x xx x Ellidaey hyaloclastli te
VE 46 xx x l( x Brandur hyaloclastite
"
VE 49 x x x Hallisey hyaloclastite
" 4
V£ 16 xx xx xx x SaefelBle,i maey hyaloclast)i te
VE 112 xx x x EldfelRle,i .maey teph5r1a
VE 156 xx x x Eldfel!lle,i .maey teph6r1a
SU1 9 x ,ex xx xx x NE Surtsey teph7r1a
SY x xx x x x Syrtlingur teph8r1a
Sudurlar.d
LA x x SW ofL aufafell hyaloclas9t1 ite
LA x x SW cfL aufafel.l hyaloclastite
RE x x x x tr Reidskard hyaloclastite
RE 2 x x x eungurfiRte,l dskard hyaloclastite
RE 4 x x x x x tr KerlingafjRaelild.s lcardh yaloclas1t01i te
BA 1 x x x Bagafelslc ,f L aufafelhly aloclastit.e
HA 2 )( x )( li&gafelsl o,f La ufafellh yaloclastite
BA 3 tr x x x Sagafelslo, f L au.ffae ll hyaloclas4t1 ite
HA 4A tr !iagafells,o fL aufafelhly aloclaset4i11t11
SA 4B tr x x BagafelSl o,f L aufafelhly aloclas1t11i te
IIA s tr x " x Bagafelslo, ! L &ufafelhly aloclatset i
"
BA6 x x x Hagafelsl o,f L aufa!elhly aloclllstite
B-5
pl- plagiocclpax-s ec,l inopyrool-x oelniev,i cnher-, c hromsipainn amlt-, t it.anomagnetite
x -pre.se#n txx- abundantt,r -t.race..
1)KE 3Ai sa coarse-rgerlaaitnuienvdce o nsolliadyaeatrboe dv ea fine-grmai.nienreadl l-aryiec(rhKE 3 8).
2)KE 4Ai sa coarse-grroacifknr eodtm h es amleo calaistKE y 4 8w hicihsa fine-grained rock.
3)B I2 ci sa coarse-gparratif nreodtm h es ames amplaes B I2 fw hicihsf roam f ine-grained layer.
4)S ampplaer tallyt erbeypd a lagonitization.
5)T epbfraal Jla n2.3 ,1 973.
61T ephfraal Mla y2 -31,9 73.
7)T ephfraal Nlo v1.9 6-3J an1.9 64.
6)T ephfraal Jlu n6e, 1 965.
9)Xenocryosfts si licgilca sasr eo ccasiopnraelsleyn t.
10Tw)o typeosfg lasasn dr arxee nocroyfsa tlsb ite.
11l H A4 Ai sf r0a1D l ighatn dB A4 8f roam d arcko louvraerdi etTyh.ed iffereinsdc uee t oh igcho ncentroaft ion
cu.bicz eol!(taensa lcimient? hlel atter.
Alls amplefsr oVme st:mannaareeyf jraotrmh ec ollectaitMo unsse oumf N aturHails toRrye,y kjavik.
T A 8 2L. E Precisainodan c curoafcm yi croparnoabley ses
WRAB-4 VG-A99
2 3 4 l 2 3 4
Sio 47.944 7.950 .5) 0.0 50.905 0.90 0.47 0.0
2
Tio 0.69 0.64 0.07 7.2 4.06 3.79 0.15 6.7
2
Alo 17.751 7.910 .24 0.9 12.971 2.260 .15 5.5
23
FeO 8.58 8.46 0.19 1.4 131.8 13.280 .25 0,6
MnO 0.1 2 0.17 0.034 17. 0.19 0.20 0.03 5.3
MgO 9.95 9.83 0.08 1.2 5.18 4.82 0.05 6.9
CaO 11.521 1.590 .11 0.6 9.38 9.41 o·.100.3
Nao 2.442.46 0.04 0.8 2.73 2.59 0.1 0 5.1
2
Ko 0.1 1 0.09 0.031 8.2 0.80 0.76 0.04 5.0
2
P0 0.06 0.06 0.03 0.0 0.41 0.46 0.051 2.2
25
Total 99.169 9.16 99.809 8.47
1. Accepatneadl yosfiW sR AB(-B4.E vanpse,r sc.o mm.a)n dV G-A(9J9a rosewich
eta l.1 979b)a salgtliacs ses.
2. Averaogf5e 0 m icropanraolbyes es.
3.Ones tanddaervdi ation.
4.Percendteavgiea tfiroonam c cepantaeldy sis.
B-6
Typeosf phenocrysts
The termp henocriyssu ts edt o denaoltlec rystaflosu nidn
theg lasssa mplreesg ardloefst sh eiorr iginT.h eo rigionf
the varioutsy peso fp henocrywsitlslo nt heo thehra ndb e
cruciianlo rdetro i nterprpehta sree lationTso.f acilitate
thed iscussai doins tinctbieotnw eveanr iopuhse nocrtyyspte s
isu sedt hroughtohuetp aper.
Xenocryissa ts u seda true non-comagmatic crystal (i.e.
likeltyo h avec rystallfirzoemad significadnitflfye reonrt
moree volvleidq uicdo mposittihoentn h ato f thee nclosing
glass).M icroa-nd macrophenocrysts were preliminarily
classifaicecdo rditnosg i ze( 0.3m0m ).M ecrophenocrysts ere
likeltyo b ec omagmapthiacs ebsu,t m ighth avec rystallized
froma liquiodf a slightllye sse volvecdo mposition.
Microphenocarryets htesp hasetsh atm ostl ikelcyr ystallized
fromt hee nclosignlga ssM.i croliatreeos n lyp resenitn a
few sampleasn d haveg eneralbleye na voideddu et ot he
posisbiliotfym etastagbrloew tohr m oree volvesdt ageosf
liquicdo mpositioMnasc.r ophenocarnydxs etnso cryasrtesi n
genereaulh edrtaols ubhedreenld, msahyo wr esorptiwoint h
embayegdr ainm arginesn d slighzto natioorno vergrowth.
Microphenoctreynsdtt so be compositionhaolmloyg eneous,
euhedreolr sometimsekse letalA.l l typeso ccasionally
contaiinnc lusioofng sl ass.
Ordeorf n ucleation
Determinaotfio ornd ero f nucleatiaosn r ecordeidn the
mutuailn tergroowft ht he minerpahla seiss i mportafnotr
understanpdhiansge equialnidbp reitar ogeneBsoitsh.m i cro­
andm acrophenocorcycsuitrns g eneraals discretcrey stals
inidcatingh omogeneounsu cleatiofnr om the melt.
Monominercalluisct erinpga,r ticularolfy olivineo r
clinyorpoxenea,n d morer arelpyo lymineraclliucs tering
betweemna ficp haseasn d plagiclasies, occasionally
observedA. few exampleosf 'bow-tiaen'd radiating
intergrobwetthw ecelni nopyrooxreo nlei vine and plagioclase
are seeni n the Sudurlegnlda ssebsu t rareliyn the
Snaefellsanneds V estmannaegyljaasrs es.T hesel etetr
texturecasn be interpreetser de sultifnrogm s imultaneous
nucleatainodnc rystgarlo wtfhr oma commonnu cleu(sB ryan
1979),w hereasg lomerocrymsatys indicatsee condary
nucleatuipoonne arlipehre nocrpyhsats e(sD owt1y9 80).I n
somes amplepsl agiocloacsceu rass r arem inutlea thosn ly
CKET,a bl1e) . Thism ay indicatae Laten ucleatioofn
plagioclesree lativet o olivine{ Jekobss1o9n7 9).
Corresponcdoinnsgi deratciaonn bse appltioe dt he rare
occurrenocfe o livinien theF e-Tgil assferso mS udurland
(HA).
A clossep atiraell ationship cbhertowmeiseepnni nealn d
olivine ptooia n dtesp endennuctl eatoifo nc hromiasnp inal
on olivinger owth.V eroyf tecnh romisapni neils fouinnd
clusteirnsc ludiend o r closelayt tachetdo the olivine
phenocrystTsi.t anomagneotcictuer ass primardyi screte
phenocryisntt sh eg lasbsu ta lsoa s inclusioinns o livine
and occasionailnlp yl agioclaTshees. p ineplh asies h ence
ane arlnyu cleatpihnags beu tw itha dependent nucleation of
chromisapni naoln o livignreo wt(hT h1y9 82).
B-7
Phenocrayssste mblages
Systematriecg ionaclh angeasr er ecordiendt hep henocryst
assemblawgietsh iann db etwetehne v arioussu barea(sT able
1). Thes malsli zeo fa thin section qmuaeksetsi ointa ble
whethtehre m acrophenoacsrsyesmtb laigne sT abl1e canb e
takena s representaftoirv teh es amplaen,d t hef ollowing
discussiwoinl lf or thisr easonb e limitedt o the
microphenocarsyssetm blageAsl.s ot he smalln umbeorf
samplesr estricttsh e representatiavnidt yf urther
investigatmiaoynv se ryw ellc hangteh e generpaalt tern
outlinbeedl ow.
SneefellVsonlecsa nZiocn eO.l ivisnter ongldyo minatoevse r
plagioclaisne m osts amplefsr omS naefellsnwehse,r eas
clinopyroixseo nnel yf ounidn a fews ample(sT abl1e) . Four
samplefsr omt he westerpna rt( STE,N ; Fig.1 ) contain
microphenocorfy sotlsi vinpel,a gioclaansdce l inopyroxene.
Chromisapni naalr e presenats euhedrailn clusioinns
olivineb,u t are alsof ounads d iscrectrey stailnsg lass
adjacetnoto livineF.u rtheears twarCdKsE c)l inopyroixse ne
notf ounads a microphenoactra ylslt anpdl agiocloacsceu r
onlya sa subordinmaitcer ophenocryst.d omOilniavtaiennsde
chromiasnp inails stillp resentF.u rtheears tC HIG,A )
plagiocliassa en a bundapnhte nocrtyosgte ther owliitvhi ne,
andc hromisapni nails o nlyp reseinntt heG Rs amples.
Previoupse trograpahnidcm ineralogiwcoarlko nt he
alkali basraolctkioscf t heS naefellVsonelsc aniZco neh as
mainlbye enc oncernweidt ht hee volutioofvn o lcanciecn ters
froml ateT ertiary, during tQou Raetceernntat riym es
(Sigurds1s9o7n0 bJ;o hannes1s9e7n5 }.S igurds(s1o9n7 0b)
describaeldk albia saltfsr omt heS etberagr eas howing
evolutitoon benmoritaensd possailbklaylr ih yoliteIsn.
theb asaltosf thisse riepsh enocryasrtes o livinaen d
plagioclbauste c linopyrooxcecnuer s ionna l fy ews amples.
Apatei toccurass phenocryisnt rockso f mugearitic
composition,r araenC da -poor pyroaxreefn oeusn idn the
groundmaJsosh.a nnes(s1e9n7 5d1e scribterda nsitioannadl
alkalib asaltsf romt he extremeea sterpna rto f the
SnaefenlelsVs olcanZiocn ec ontainpihnegn ocryosft osl ivine
and plagioclaCslei.n opyropxheenneo crywsetrsen otetdo b e
raree xcepitn h awaiitliacv asA.d ditiognraolu ndmpahsass es
areF e-Toix idecsl,i nopyroaxnedn rea reC a-poor pyroxene.
The findingisn thisst udayr ei na ccordawnicteht hosoef
Sigurds(s1o9n7 0abn)d J ohannes(s1e9n7 5).
SudurlaVnodl caniZco ne.T he Sudurlagnlda ssessh owt wo
difefrentt ypeso fp henocrayssste mbla(gTeasb l1e)r elated
tot hec ompositoifot nh e glassesA. Fe-Tib asalgtl ass
containpsl agioclacslei,n opyroxaennde r areloyl ivine.
Titanomagniesto intleyf ounads a phenocriynst tw oo ft hese
samplesA.n alkalbia salgtl assc ontainpsl agioclase,
olivinaen d sometimcelsi nopynreo.x Oenltyr acaem ounotfs
chromisapni naalr ef oun(dT abl1e} .
TheS udurland VoZlocnaewn aisic n vestigaitne dde tail
by Jakobsso(n1 979)w,h od istinguissheevde raRle cent
volcaniscy stemwsi the ithetrh oleiitoirc t ransitional
alkalcio mpositioTnhse. l attesre rieiss r eferrteodi n
thipsa pears F e-Tbia saltsT.h em icrophenoacsrsyesmtb lages