TOM ANDERSEN& HENNINGS0 REN SEN
- - - -
NGU-BU L L441, 200 3 -PA G E25Microsyenite from Lake Mykle, Oslo Rift: Subvolcanic rocks transitional between larvikite and nordmarkite
TOM ANDERS EN&HENNING S0RENS EN
Andersen, T.& Sorensen,H.2003:Mikrosyenite from Lake Mykle,OsloRift : Subvolcan icrocks transitionalbetween larvikite and nordmarkite.Norgesgeol ogiskeundersek elseBulletin441,25-31.
A small bod y of trachyt e or microsyenite croppingouteastoflake Mykleinthe Vestfo ld Graben of the Oslo Rift probablyrepresentsa subvolcanicint rusion emplaced atafairlyearly stageof theintrusivehistory of the Silj an- Myklearea. The microsyeniteshows chemical, pet rographical and mineralog ical feat ures int ermediat ebetw een larvikite and nordmarkite, suggesting thatitrepresentsatransit ionalmagm atyp e.The microsyeniteshowspyrex- ene zo nationpatt ernsindicatingadiscontinuous,two-stage crystallizat io n history, whoseiniti al st ageresem bles that observedinnormal larvi kit e.At thefinal stage ofpyroxenegrowt h,the melthad developedto a morealkaline com posit ion,resemblin gsome oftheless alkaline(alkali)syenites in therift.
Tom Andersen, tnstituttfor geofag,UniversitetetiOslo,Postboks1047 Blindern,N-0316Oslo,Norge.Henning Serensen, Geologisklnstitu t,KobenhavnsUniversitet,0sterVoldg ade 10,DK-1350 Kobenhav nK,Danmark
Introduction
The Oslo Riftigneou sprovince in SoutheastNorw ayconsists of large plateaus of volcanic rocks,major plutonic com- plexes,cauld ronstr uct ures and a multitud eof dykes.At the present level of exposure, the predominant volcani c rocks are the well- known rhomb porphyri es which are charac- terised by rhomb-shaped pheno cry sts of anorthoclase feldspar.They range in.composition from tephri-phonolite to trachyandesite/lat it e and correspond chemically and mineralog icallyto the plutonicrocklarvikit e.Basaltsarerela- tively less abundant.Inspi te of thewidesp read occurrence ofnord markit e and other typesofsyenit es in the OsloRift, trachyticlavais a rarerock type,onlyfound associ ated wit h cauldrons (e.g., Oftedahl & Petersen 1978).In the present paper,we report the discovery of a small bodyofsubvol- canic trachyte or microsyenite on the east coast of lake Myklein thesouthwestern part of theOsloRift, in a region underla inby plutonic larvikite,syenitesand granites;itsdis- tance to exposed volcanicrocksisabout 10km(Fig. 1).This occurrence of microsyeniteis briefly described, and its pet- rogeneticalrelat ions hip to the major pluto nic rocktypes of the province is evaluated from whole-rockand mineralcom- positions.
Field occurrence
Thearea around lake Mykle(Fig.1)forms part ofthe 1:50 000 geological map-sheet Siljan,and is characterised by the occurrence of a considerable variety of plutonic igneous rocks, which is an indication that this part of the Oslo igneous province has been the sit e of intensive igneo us act ivit y during long period s.It is dominatedby largeplutons of larvikit e whic hareexposed to the northand southof the lake. In the western partof thearea,thehighest partsof the land scape are madeupof larvikiteformingtheroof of gran-
ite and syenite intrusions.Thisindicatesthat larvi kite may orig inally haveform ed onecont in uo us bod y,whichisnow separated by large intrusive massesof granite and minor masses of syenite, thatform the landscapearound thelake.
In the centralpart of themap-sheetthehighestpeaksare composedofgranite.
Geologicalmapping ofthe Siljan map-sheet,carriedout by staff and st udent s from the Geological Institute, University of Cop enhagen, has established the follo win g successionof magmaticeventsin the Mykle area:
1. Larvikite,making up a large pluton which was most probablyformed byseveral intrusive events.
2. Intrusion of sma ll bodies of gabbro/diorite (Pedersen 1994,Pedersen &Seren sen 2003)and neph eline syenite (Anderse n&Sorensen 1993).Thegabb ro conta insxeno- liths of larvi kite,and thelarvikiteisinte rsecte dbydykes of nepheline syenite, showin g that the gabb ro s and nepheline syenites areyou ngerthan the larvikit e.The gabbro andtheneph elin esyenite areintrudedby syen- ite and granite. Their mut ual age relationship is unknow n.
3. Intrusion of smallbodies ofporphyriticsyenite(Petersen
& Serensen 1997) which conta in xenolit hs of larvik it e and gabbro but areintersectedby several generation sof dykesof syeniteandgranit e.
4. Bodiesofsyenite(no rdmarkite)andgranitewhichint er- sect the porphyritic syenite .They contain net-vein ed complexesintheir contact zones wit h larvikite(Morogan
&Serensen 1994).
5. Alarge body of coarse-gr ain edalkaligraniteofekeritic type(Brogger 1906).
The microsyenite described in the present paper is locat ed on the east coastoflakeMykle whereitforms the southerncontactof theekerit icgranite.Itistheonly exam-
NGU- BULL 441,2003 - PAGE26
~: : :::~
:] Larvikite~ G Diorite-gabbro
Porphyrit ic syenite
TO M ANDERS EN &HENNfNG S0RENSEN
Os lo reqio n,
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SyeniteN
Nephelinesyenite-
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TO M AN D ERSEN&HENNINGS0RENSEN NGU-BUL L441,2003- PAGE 27
Fig.2.The outcrop of microsyenit ealong theforestroadon theeast coastof Mykle.Notethe pronounced horizont alsheet joint ingin the microsyenite.
Fig.3. Coarse-grained ekeriteintrud ing microsyenitealong horizontal sheetjoint ing .
pieofits kind recorded inthe area.It isolderthan thisgran- ite but its age relation sto the other intrusive rocksare unknown. The microsyenite is expo sed along the forestroad on the west coast of Mykle (UTM reference539465893)fora distance of abou t 200 m to the south and nort h of the bridge over Smalvann elva(Fig.1).At thenorthernendofthe exposure,the microsyeniteisin contactwith ekerit ic granite and isintruded by sheetsof ekerite and pegmatite (Fig. 2) along horizontal sheet jointing of themicrosyenite (Fig.3).
Theboundariesof the microsyenite towardsthe east, south and westare concealed under a cover of veget ation and loosedeposits.The shapeand size of themicrosyenite body are therefore unknown,and it cannotbe decidedfromfield evidence if it represents remnants of a dyke,plug or lava flow. Over adistanceof about 100 m to the northof thecon- tact, the granite contains numerous angular to rounded xenolit hs of microsyenite up to about 30 cm across.The veinsof graniteand pegmatiteintersectingthemicrosyen- iteare highlymiaroli ti c.
Fig. 1. Simplifi ed geological map ofthe Myklearea,from the north- western partof the1:50,000map-sheetSiljan.
Petrography
The microsyeniteis white and fine-grained,wit han average grainsize<1 mm, but wit hscatte red largergrainsof alkali feldspar attaining 2-3mm insize.Thepredomin ant mineral of the microsyenit e is alkali feldspar (:2:95 %)occurring as rectangular grains which only except ionally show weakly developed cross-hatchedmicrocline twinning.Some of the largergrain s ofalkalifeldspar havecores of plagioclasesimi- lar to the feldspars of larvikite and rhomb porph yry.The platy alkali feld spar grains have rimsof albite and vesicles are surroundedbyplates of albite.Thetextureis interlocking wit houtany preferredorient ation of the feldspars.There are scatte red grains of green clinopy roxene and more rare blui sh amphibo le,the latterin places wit hcores ofc1inopy- roxene. Accessory minerals are titanite, somet imes wit h cores of ilmenite,Fe-Ti oxidesand apatite.Brownish grains are most prob ably allanite. The feldspar, pyroxene and amphibo le grains arepract ically unalte red in somesamples, but the feldspar grainsbecome turbid and the pyroxenes and amphi bole are altered into biotite and rust-coloured mate rial in some thin-sect ions.Some samples also have plates of biotit e.ln one thin-section,thereis alittle inte rst i- tial quartzassociatedwithbioti te,andsecondarymuscovite, chloriteandcalcit e.
Theintrud ing alkali granite is coarse-grained and composed of perthitic alkali feldspar,quartz andsome interst it ialalbite.
Aggregates of biotit e, chlorite and Fe-Ti oxides are most prob ablyform edby alterationof theoriginalpyroxene and amphi bo le.Accessories are zirconas crystals and irregularly shapedgrains,titanite andpyrochlore.
Mineral chemistry
Pyroxenes and amphi bo les in samples 86062 and 23354 were analysed by elect ron microprobe,using a five-spec- trometer CAMECA SX100, wavelengt h-dispersive instru- ment at the Department of Geosciences,University ofOslo.
Grains were imaged by backscatt ered electrons prior to analysis.
Pyroxene
The pyroxenes in 86062 and 23354 have pale green and weakly pleoch roic cores surrounded by grass-green rims.
The rim sare bright in backscatt ered electrons,andthe core- rim interface isin general,quitesharp butirregular,which suggests that the coreshavegrow n as skeletal grains,or that theyhavebeen resorbedprior to crysta llization of therims (Fig.4).Select edanalysesare givenin Table1.The fullset of data can be obtainedfromthe aut horsonrequest.
The pyroxenecores aresodicaugites/salites comparable in composition to pyro xenes in larvikit efrom the Oslo Rift (Neumann 1976), whereas the rim s plot along a trend towardsAc(Fig. 5).The maximumAc-enr ichment observed in theouter rims is 55%,but most analysed pointsgiveAc wellbelow50%.Thelevelof Hdenrichmentalong thistrend is similar to what has been observedin peralkalinesyenit es
NGU-BU LL 441,2003 - PAGE28 TOM ANDERSEN & HENNIN G S0 RENSE N
Table 1. Selected analysesof clinopyroxeneand amphibole.
Sample 86062 23354 23354
Point HS1-1-4 HS1-1-5 HS1-1-6 HS1-1-] HS1-5-1 HS1-5-3 HS1-5-4 HS2-3-6 HS2-3-8 HS2-4-3 HS2-4-4 HS2-4-5 HS2-4-6 HS2-1-1b HS2-1-2 HS2-1-2b HS2-1-2c
Mineral Clinopyroxene Clinopyroxen e Amphibole
core elrinterface rim rim green rimpalezone tip Green 81ue Blue Blue Si02 52.23 52.33 51.55 51.37 52.79 51.08 51.14 52.34 50.86 51.13 51.67 51041 51.53 52.68 50.06 50.35 50.80
AI203 1.24 1.19 0.34 0.28 0.56 0043 0.29 0.98 0.21 1.96 0.32 1.59 0049 0.72 0.86 0.72 0.76
Ti02 0045 0040 0.17 0.18 0.34 0.16 0.21 0.36 0.00 0.74 0.17 0.58 0.25 0.52 0.28 0.21 0.22
FeO 10.79 10.31 22.14 22.25 12.12 18.81 22.83 11.32 22.66 10.32 22049 10.87 17041 24.04 33045 33.38 33.54
MgO 11.63 12.12 4.27 4.13 11.07 6.14 3.73 11.09 2.70 11.72 3.59 11046 6.55 6.93 1.15 1.19 1.53
MnO 0.93 0.89 2.15 2.18 1.93 2.37 2.18 1.03 2.31 0.60 2.58 0.67 1.76 2048 1.62 1.83 1.84
CaO 22.13 22048 14043 13.77 20.13 18.37 13.37 22.17 17.63 22.69 13.17 22.84 18.97 7.96 2.19 2.02 2.69
Na20 1.05 1.05 4.95 5043 1045 2.92 5.63 1.20 3.16 1.03 5.51 1.04 2.79 2.82 5043 5.73 5040
K20 0.00 0.00 0.01 0.00 0_00 0.00 0.01 0.04 0.01 0.00 0.01 0.00 0.02 0044 0.77 0.64 0.75
Total 100045 100.76 100.01 99.59 100.39 100.27 99.38 100.53 99.53 100.19 99.50 100045 99.77 98.58 95.80 96.07 97.52
Si 1.954 1.946 1.985 1.981 1.985 1.966 1.979 1.96 1 2.013 1.91 3 2.000 1.923 1.985 7.922 7.904 7.925 7.885 AI 0.055 0.052 0.016 0.013 0.025 0.019 0.013 0.043 0.010 0.086 0.014 0.070 0.022 0.127 0.160 0.133 0.138 Ti 0.013 0.01 1 0.005 0.005 0.010 0.005 0.006 0.010 0.000 0.021 0.005 0.01 6 0.007 0.058 0.03 3 0.025 0.026 Fe3+ 0.089 0.110 0.373 00421 0.090 0.257 00441 0.104 0.207 0.121 0.389 0.127 0.203 0.441 10407 10412 1.375 Fe2+ 0.249 0.210 0.340 0.297 0.291 0.348 0.298 0.25 1 0.54 3 0.202 0.339 0.213 0.358 2.583 3.010 2.98 1 2.979 Mg 0.648 0.672 0.245 0.238 0.621 0.352 0.215 0.6 19 0.159 0.654 0.207 0.639 0.376 1.553 0.271 0.280 0.355 Mn 0.029 0.028 0.0 70 0.071 0.062 0.077 0.072 0.033 0.077 0.019 0.085 0.021 0.057 0.315 0.216 0.244 0.242 Ca 0.887 0.895 0.595 0.569 0.811 0.758 0.554 0.890 0.748 0.910 0.546 0.915 0.783 1.282 0.371 0.340 0_447 Na 0.076 0.076 0.369 00406 0.105 0.218 00423 0.087 0.243 0.075 00414 0.075 0.208 0.822 1.663 1.748 1.625 K 0.000 0.000 0.000 0.000 0.000 0.000 0.001 0.002 0.001 0.000 0.000 0.000 0.001 0.085 0.156 0.128 0.148
NaB 0.718 1.629 1.660 1.553
(Na+K)A 0.189 0.189 0.216 0.219
Weigh t percent oxides by electron microprobe analysis.Structural formulae have been calcu late d on the basis of 4 cations for pyroxene, and Si+AI +Ti+Fe+Mg +Mn=13fo ram p hibol e.
Fe3+has been estim ated from stoichiometric charge balance to 4 oxyg enforpyroxen eand 46negat ive cha rgeuni tsforamphibole.
and granitesfromtheOslo Rift (Neumann 1976,Andersen 1984),butdoesnotreachthe strongly Ac-enrich edcomposi- tions(Ac~90%)seenin peralkalinesyenit es andgranit es in the rift(Neumann 1976).Itshouldbenoted that the pyrex- enetrend is notcontinuou s,but showsa distinct composi-
tionalbreakbet weencoreand rimcomposition s.
The substi tuti on mechani sms of the pyroxenes are illus- tratedinFig. 6.The compositionof the cores is controlledby theFe-Tssubstitut ion(Fel-Ml+Air),and the rims bythe Ac substi t uti on(NaM2+Fel-Ml).
Fig.5.Pyro xeneco mpo sit ion splottedin the Di-Hd-Acplane.
- 0.25
• 23354
o 86062
0.75 0.50
Hd
- .
~ .
;., :c-
. -(~AOv
o . p
"
o
0.50
..
0.75
1.00f -- - / - - . , - -- - - - r - -- ---r- - - -+0.00
0.00 0.25 1.00
Cores
Fig.4.Elect ro n backscatterimage ofpyroxene (grey) with magneti te (white)in perthitic feldsparmatrix(d ark)insample8602.Thedarkgrey, skeletalorresorbedcoreinthepyr oxene(analy sisHS1-5- 1)consistsof Ac-poo r pyroxene,and the backscatter brig ht rim or overgrowth of aegirine-augiteiszon edtow ard s increasi ngAc(analysis HS1-5-4)
TOM ANDERS EN&HEN NINGS0REN SEN NGU-BULL441,2003 -PAG E 29
Table 3.Trace eleme ntconce ntrat ions (p pm),whole rocks.
Whole-rock chemistry
Two samplesofmicrosyenite(86062,86064)and one sample of the intrudi ng granite (86060) have been analysed for major andtrace elements(Tables 2 and 3).Majorelement analyses were made by XRF at the Laboratoire de Pet roqraphie et Volcanolog ie,UniversitedeParisSud,Orsay, France(Mme.R.Coq uet);trace elements at theGeological Institute,University ofCopenhagen byXRF(J.c.Bailey)and at TracechemA/Sby INAA(R.Gwozdz).Add it ional data for a T1 trachyte flow fromtheVestfold plateauc.30 kmENEof Mykle (Oftedahl & Petersen 1978), and a syenite (81462) from the west coast of Lake Mykle (Petersen & Sorensen 1997)are given forcomp arison.
Samp le 86064is weakly miarolit ic, which may explain its elevated Na,Oand Cl. Otherwi se,there ispractically no dif- ferenceinchemistry between thetwo samples.Thechon - drite-normalisedREEdiagram show sa rathersteepslopein theLREE partandanalmo st horizont alHREE part of thedia- gram (Fig.7a).Cont rarytothe graniteof thearea,which has
Ekerite Microsyenite Syenite
86060 86062 86064 81462**
Li 45 10 n.d. n.a.
Cs 4.2 0.8 3.7 4.8
Rb 220 135 195 250
Be 8 7 n.d. n.a.
Ba 36 1440 n.a. 700
Pb 12 9 n.a. 18
Sr 22 748 779 216
Zr 669 581 488 1630
Hf 25.7 14 10.9 39.6
Nb 203 182 136 205
Ta 14.7 10.8 10.2 13.6
V 3 22 n.a. 38
Ni 7 7 n.a. 5
Se 2.3 2.3 2.5 9.5
Ga 34 28 n.a. 28
Zn 118 158 135 103
Cr 3 2.6 3.1 5
La 42.6 104 119 148
Ce 101 188 200 300
Nd 49 76 78 127
Sm 11.8 10.5 10.8 19.4
Eu 1.5 2.4 2.7 3.5
Tb 2.9 1.6 1.6 3
Yb 8.4 5.6 4.3 9.5
Lu 1.3 0.9 0.6 1.4
Y 101 59 n.a. 90
Th 16 25 26 56
U 9.6 7.6 7.5 12
Table 1.The amphibo les range in composition from fer- rorichteri tetoriebeckite with(Na+K)A<O.5.lncontrast, most plutonic sodi cand sodic-calcic amphiboles from theOslo Rift have higher A-sit e occupancies, ranging from sodic edenite to magnesioarfvedsonite/arfvedsonite (Neumann 1976,Andersen1984).
**. West coastof lakeMykle
n.a:Not analysed n.d.:Belowdet ecti onlimit
o.so
000062 . 23354
0.15 0.10
AI 0.05
Ams O.j)
0.00- \ - - - -...- - -...- - - - . . . - - - -...
000 0.40
0.10
nzo
om ,..-;;;---,
;I~0.3)
Fig. 6.Plot oftot al calculatedFe
,.
and AI in c1inop yroxen e,witharrows representing the acmite (MgM,CaM,=Fe"M,NaM') and ferri-tschermak(MgM,Sir=Fe" M,AIr) su bstit ut ions.
Amphibole
Amphibole isabsentfrom86062,and issparsely present in 23354, where it occurs asblue to green pleochroic grains intergrown wit h pyroxene.Selected analyses are given in
Table 2. Major eleme nt compositions of microsye niteand comparabl e ro cks.
Ekerite Microsyenite Trachyte Syenite
86060 86062 86064
rrn -
81462**Wei9htpercent oxides
SiO, 76.95 64.33 62.58 64.30 60.80
TiO, 0.24 0.62 0.69 1.10 1.27
AbO, 10.94 17.05 18.13 14.17 15.83
Fe,O, 1.65 2.60 3.38 3.79 2.40
FeO 0.52 1.21 0.66 1.00 3.45
MnO 0.12 0.13 0.15 0.12 0.15
MgO n.d. 0.39 0.48 1.30 1.31
CaO 0.18 1.57 1.28 1.40 2.29
Na,O 3.99 5.90 6.68 5.68 5.05
K,O 4.78 5.18 4.88 3.60 5.33
P,O, n.d. 0.13 0.26 n.d. 0.42
H,O+ 0.23 0.33 0.49 0.35 0.86
H,O- 0.12 0.18 0.25 0.09 n.d.
Sum 99.72 99.62 99.91 96.9 99.16
CIPWnorm.weightpercent
qz 36.01 7.11 2.36 13.52 5.05
co 0.00 0.00 0.16 0.00 0.00
or 28.24 30.60 28.83 21.27 31.49
ab 29.66 49.90 56.49 48.04 42.71
an 0.00 4.75 4.65 2.55 4.80
ac 3.60 0.00 0.00 0.00 0.00
di 0.80 1.86 0.00 3.58 3.24
hs 0.02 0.22 1.38 1.82 4.54
ilm 0.46 1.18 1.31 2.09 2.41
mt 0.59 2.53 0.62 0.43 3.48
ap 0.00 0.31 0.62 0.00 0.99
P.1. 1.07 0.90 0.90 0.93 0.89
D.1. 93.91 87.6 1 87.68 82.83 79.25
NGU-BULL 441,2003 - PAGE 30
]
-~-<>-8606086062~ -co-.86064
~c:
0 .J::.
-
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.><
u0 Q:
a La Ce Nd SrnEu Tb Yb Lu
Rb U K Ce Sr d Zr Eu Tb Yb V Mg Th Nb La Pb P Na Srn Ti Y AI Ca Ni
1000
100
~~~
-.>--86060CD ~
..\ , r.
<o-.-86062Q: 10 \~ 1\ --<>--86064
0~ I ~ I , -0- 81462
\ \l ;-~ ~\ ~
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0 b
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1
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~
b
Fig.7.(a).Chondrite -nor malisedREEdiagram of microsyenite(86062 and 86064)and granite (86060).Chondrit edatafromBoynton(1984)_
(b)Spider diagram. normalised to N-MORB.MORB data from Wood (1979).
marked,negativ e Eu anom alies,the microsyeniteshows very weak or non-exis tin g Eu anomalies, as is also the case for larvikiteand syenit esof the Mykle area(Petersen &Serensen 1997). Contrarytoekerite, the Mykle microsyenit e isnotper- alkaline; it s peralkalin it y ind exis0.90,simil ar to the indi ces of thesyenite from lake Mykleand the trachyte from the Vestfold plateau (Table 2).The syenite at lakeMykle hasdis- tinctly higher contents of LREE, Th and Zr than the microsyenite (Tab le 3,Fig.Zb), whichis also slightly low erin Si02,Ab03and Nap and high er in CaOand Ti02•The T1tra- chyte is sligh tly richer in femic com po nents and slightly poorer in the alkali feldspar components than the Mykle microsyenite.
Discussion
The lack of exposures preventsdetermination of the shape and sizeof the microsyenite body at lake Mykle. The fine grain size combined with thesize of the outcrop makes a plu tonic orig in highly unlikely. The shape of the outcrop does not indi cate a dyke- or sheet-like form for the microsyenit e body.Theeven -g rained, holocry stalli nenature of the rock,and theabsence oftrachytictexture makes an origin as a lava flow highlyunlikely. Thebest interpreta tion of the microsyeniteistherefo re thatit is a remnant ofa sub- volcanicbody of trachyt e,whichmost probablybelongs to
TOMANDERSEN&HENN ING SI2JRENSE N
stage 2 of the int rusive historyof theMykle area.Apart from pigmentation of the feldspar,rust-coloration of pyroxene and amphibole,and the rare occurrence of quartz,biotite and muscovite,the microsyen ite does not appear to have been contact-metasomatised by the ekerit ic granite emplacedin stage 5.
The few trachytic lavas known from the Oslo ign eou s province are associated wit h cauldronstructures,which are deep sectionsthrough evolvedcentral volcanoes(Oh edahl
&Petersen 1978,Ramberg & Larsen1978). Taking into con- sideration that the primary contact relati onshi ps of the diverse rock types formed during stage 2 havebeen obliter - ated by younger intrusions,we cannot exclude the former presence of a cent ralvolcanoatthis stage.In any case,our observat ion saddone moretyp eofigneous rock to the vari- ety of igneou s rocks in the comp licated contact zone betw een the northern granite and the sou the rn larvikite exposedon the east coast of Mykle.Mostof thisarea is cov- ered by forest,and the relat ionship between the various rock types can only be unravelled where the bedrock is exposed along lakes,roads and rivers. Itis thereforepossib le that trachyt icrocks aremorewidesp readin the regi on,but concealedbeneathvegetati on and loose deposits.
In terms of majo r and trace element composition,the Mykle microsyen ite is less evo lved than the nordmarkite intrusionsin the OsloRift.It s REEdistr ibutionpattern resem- bles larvikite more than any other rock types from the region (cf.Neumann 1980).The composit iona levo lution of the pyroxenes is discontinuous, wit h cores resembling pyroxenes in larvikite, overgrown by pyroxene trending tow ards the sodic composit ions seen in nor dm arkite and ekerite, but wit hout reaching the extreme compos it io ns observedin the most evolved alkalineplutons(et.Neumann 1976, Andersen 1984).The core-rim relat ionsh ips of the pyroxene suggesttwo distinctstagesof crystal growth,the firstofwhich involved growthof augitic pyroxene,and the laterof alkaline pyroxene.The irregularcore-rim interfaces suggestthat these processes were separated by a stage of resorption.
The larvikitesin the Oslo Rift formed by polybaricdiffe r- entiat ionof a mafic parent magma, wit h anini ti al stage of evolutionin magma chambersin the deep to middl ecrust
(Neumann1980),but pyroxenesand amp hibo lesformed at
a late stage of evo lut ion,more or lessin situ ( euma nn 1976). In nordmarkite and ekerite,mafic silicatesare late- stage,interstit ialphaseswhich grewinsitu (Neumann 1976, Andersen 1984).The first stage ofpyroxene growt h in the Mykle microsyenite may be related to the fractio nat ion ofa larvikite-like magma during transport to the shallo w cru st.
An initial,larvikite-related stage of evolution is also sug- gestedby the presence of cored feldspars.Whilepondedin a shallow magma chamber,this melt must have developed towards more alkaline composit ionsthan those observed in larvikite and related rocks.Thismay be due to continued fractionalcrystallizat ion,or to mixing wit h a more evo lved
TO M AND ERSEN & HEN N I N G S0RENSEN
magma. If magma mixing isthereason,homogeni zation of the mixed magma must have been essentia lly comp lete, andmusthavetakenplacebefo re finalemplacementof the magm a.
Conclusions
The microsyeniteon the eastern shore of Lake Mykle may havebeen formed during stage 2 of the evolut ion of the Mykle magmatic complex,whichalsocom prises emplace- ment of gabbro and nepheline syenite.Theprim ary str uc- tural relationships ofthe rocksformed at this stage were obli terated by late r, voluminou s syenite and granit eintr u- sio ns.Themost likely originof themicrosyenit e is as a shal- low,subvolcanicint rusion, possibly relate d tothe eruption of nolonger preservedtrachyticlavas.lntermsof majorand trace elementcomposition,the Mykle microsyeniteisinter- mediatein compositionbetweenlarvi kit eand nordmarkit e.
Pyroxene and feld spar zonat ion patt ernssuggest that the trachyti c magma developed throughtwodistinctstages of evoluti on.The Mykle microsyenite is therefore a transitional rock type in the Oslo Rift,representi ng either acogenet ic 'missinglink'between larvikit icand nordmarkit ic trendsof evo lution,ora well-homogenized hybridbetween the two.
Acknowledgemen ts
Thefield and laboratoryinvestig ation s of HS weresu p por te d bythe Geological Survey of No rw ay,the Danish Natural Science Research Counciland theCarlsberg Foundati on.We aregratefu lfor analytical work provid edby Mme.R.Coq uet,Univer sit edeParis-Sud,Dr.J.e.Bailey, University of Cop enha g en and Dr. R. Gwoz d z, Tracech em A/S, Cop enhag en;andtoDr.MurielEram bert,Univ ersityof Oslo forassis- tance in the electron microp ro be lab. BrittaMun ck and Ole Bang Bert helsen,University of Copenhag en, assisted withtheprep arationof illustrations.Thanks are dueto Prof. E.-R.Neu mann and Dr.B.Sundvoll, Universit y ofOslo,forcomments and helpful sug ge st io ns,and toOdd NilsenandTorePrestvikforhelpf ul review s.
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