BERNARD BONIN & HENNING S!2JRENSEN NGU-BULL 44 1,2003 - PAGE 17
The granites of the Mykle region in the southern part of the Oslo igneous province, Norway
BERNARD BONIN &HENNINGS0RENSEN
Bonin,B.&Serensen,H.2003:The granitesof the Mykle regionin thesout hern part ofthe Osloigneousprovinc e, Norway.NorgesqeoloqiskeundersekelseBulletin441,17-24.
The granitesexposedaroundlakeMykle consti t ute twodiscretemassifs.The weste rngranitemassif,the subjectof thepresent paper, isanintrusive complex,withlarvikiteas countryrocksmakingup the topographic highsand occurring as sto ped blocks within thegranite.Twotypesof granit e were ident ified: a marginal,fine-to medium- grained,reddishgraniteand a central,younger,coarse-grained, whitetogreygranite akin to ekerite.Thereddish granitedisplayschilled margi nsatthecontactswit h larvikit e and locally containsmafic rocks asnet-veined com- plexes.It ismetaluminous toweakly peralumi no usand yieldsthe highest contentsof incomp at ible elements.
Thoughlaterthansyenite and the redd ishgranite,theekeritedisplays sharp contacts only,wit h nochilled margin s, indi cati ng thatnosignificanttemperaturegradient s were created during itsemplacement.Miarolit ic cavities,abun- dant inall rocktypes, are evidenceof fluidexsolution from volati le-rich magmas.Thepresentsurface levelcorre- spo nds tothe poorl y exposedroof nearpart of alargerintrusion,emplacedwit hin larvikit e and rhom b porphy ry lavasbymagmati c stop ingandperhapscauldronsubsidence.
BernardBonin,UPS-CNRSFRE2566«Orsayterre»,Universite de Paris-Sud,F-9140s OrsayCedex 05, France.
HenningSetensen,Geologisklnstitut,Kobenhavns Universitet,0ster Voldgade 10,DK- 13s0 KebenbavnK,Danma rk.
Introduction
Igneous rocks cover about75%of the areaof the Oslo Riftof southern Norway(Oftedahl 1978).Granites makeup about 20%of this area andconstit uteabo ut33%ofthe areaof plu- toni crocks (calculatedfrom Table 1,in Oftedahl 1978).The related extrusive rocks (rhyoliteandtrachyte)are compara- tivelyrare,only 0.4%of the area,and are especial lyassoci- atedwit hcauldronst ruct ures(Oftedahl 1978).Theplut oni c rocks,including granites,are concentrated in three main complexes,geographically separated by outcrops of sedi- ment ary rocks and lavas,namelythe nort hern Nordmarka- Hurdalen area, the cent ral Dramm en-Finnemarka area,and the southern Larvi k-Skrim area(Fig.1).In the southe rnarea, theLarvik complexis apparently devoidof granitesand, to our know ledge, the single sheet of granit e intersect ing larvikit e that can be observed at Eidanger, about 30 km south of Mykle (unp ublished observation),representsthe southernmost granite occurrence withi n the entire province.
The Skrim area includestheMykle granites,exposed in the hills dominating lake Mykle,thusformi ngthesouthern- most large granitemassif of the provi nce.On oldergeologi- cal maps (e.g., Dietr ich etal. 1965), the Mykle granitesare shown as a sout hwesternextensio nofthe peralkaline vari- ety of granite at lake Eikeren which was named ekerit e (Brogger 1906).The detailed geologica l mapping of the Siljan map-sheet carriedoutby staffand stud ent sfromthe Geological Institute of the University of Copenhagen has
Fig.1.Generalisedgeological map ofthe OsloIgneousProvi nce.
I {.{~~~~:I
Plut onic igneousrocks1 -- -1
Volcan ic rocks/:::: >1
Cambro-Silurian sediments1 -=- = -=-=-1
Precambrian basementz".
~Oslo .'
~~ff region4 \
~ - s.
. ~ ~ )
F Finn ema rka E
Ei
Eidange r Eikeren
G M
Glitrevann Mykl e
NGU-BULL 441,20 0 3 -PAGE 18 BERNARD BONIN
s
HENNINGS0RENSEN--_.--
Fig.2.Simplified geolog ical map of the Myklearea.Thecontact betwe enthefine- to medium-gra inedgraniteand theekerite is not shown due to poor exposures in large parts of thearea.Theekeriteoccupies the central part,andthe fine- to medium - grained granit e the marginalpartsof the westerngranitemassif.
h oslo
'f1d
region~ ~ :
' ri-
MYklep \,V I
~Volcanicrocks D L arvikite
§~~§ D
Granite~11111111 0
MicrosyeniteII IIIII II ~
Net-veinedcomplexshown that the exposuresof the Mykle and Eikeren granites are separatedby a zone of larvikite,syenite and lavas(Fig. 2), thoughit is notruled out that the two granite masses may be connectedat depth.As amatt er of fact,alarge part ofthe Mykle granites is indistinguishable in the field from the Eikeren ekerite.
The aimof this paperis to provide a summary of the geo- logical features and compositionsof the rock types and to discuss briefly the magmatic evolution and mode of emplacement of the Mykle granite massif.
Geological setting
Two discrete granite massifs were distinguished in the Mykle area (Fig. 2).The western massif,centred on Lake Mykle, will be described in furtherdetail. The eastern massif constitutes a ring structure (Segalstad 1975)and is sepa- rated from the western massif by a'screen'of syenit eand larvikite up to 1 km in width.
The eastern granite massif
The eastern granite massif cross-cutsrhomb porphyry lavas in the east and larvikitein the north and south.An inner core of different granite varieties isrimmed by an outer ring of larvikite and syenite (Fig. 2).The origin of the ring structure is st ill a matter of debate.Its present annular shape can result from eitheraring intrusionoflarvikite and syenite post-dat- ing granites,which is cont rary to the ubiq uitou s granites post-datinglarvikite elsewherein the Oslo rift, or a compos- ite granite intrusion which has partly up-do med its roof of larvikite and syenite.
In the second altern ative,erosion would have remo ved most of theflat-lying roof and left the steeper outerwalls.
This interpretation is supported by the occurrenceof disin- tegrated roof penda nts,rafts andfragme ntsoflarvi kite and syenit e in thegranite and by granite veins intersectingthe rocks of theouter ring. This latter interpretation is,therefore,
favoure d.Theeast erngranitemassif will notbefurt her dis- cussed in the present paper, whichisrestricted to thewest- ern granite massif.
The western granite massif
Granite s aroun dlakeMykleare interpre tedas formingasin- gle,6km x5 km-large,compo siteint rusive body(Fig.2).A smallisolated granite int rusion occursto the south of the main mass of graniteonthesout hwesternshore of the lake.
It consistsof afine- to medium-gra ined,greyto red granite which cont ainsxenolit hs of porp hyritic syenite along its southe rncontactand dior it e-gabb roxenolit hs inthe nor th (Pedersen&Serensen2003).
Country rocksinclude larvikite, locally with porphyrit ic syenite(Petersen&Serensen 1996),in the north,west and south,and a small occurrence of microsyeniteinthesouth- east(Andersen and Serensen2003).Thetops of the hills in the western part of the granitemassif are mostlymade up of larvikite thatisexposedat nearlythe samealt it ude.They are likely to representremnants of theroofof thegranitemassif.
The granite containsnumerousxenolit hs oflarvikite in all stages ofdisintegration.Granite occurring in topograph ical depressions wit hin larvikite farther south of the massif affo rdevide ncethatgranites underlie larvi kiteinthispart of the region(see also Segalstad1975).
Two main variet ies of granite were distingu ished: (i) a fine- to medium -grained,commonly reddi sh granit e, (ii) a coarse-grain ed whit e to grey granite which is sost rikingly sim ilarto ekerite oftheEikeren type locality that itwill be refer redto as ekeriteinthispaper.Both varieti es arefullof miarolit iccavit ies,especiallyin the contact zones,and have crusts ofmanganeseoxidesfillingjointsand fractures.
The fine- to medium-grainedgranite
Themarginal zoneof the granitemassif isoccupied bythe fine-to medium-grained granite.Chilled margins,afew mm
BERNARD BONIN& HENNINGS0REN SEN
Fig.3.Contactbetween larvikite(toppart ofthephoto)and the under- lyinggranitewhich, intheupperpart,isdevoid of mafic globulesbut,in thelowerpart,containssuch globules.Southwestcoastof Mykle.
Fig. 4.Dyke of ekeriteintersectingthe mafic andgraniticpartsofanet- veined complex.Northof Mykle.
NGU-BUL L 441,2003 - PA GE 19
wid e,are developed at contacts with larvikite. Theyarerich in quartz and display granophyric interg row t hs of quart z and alkalifeldspar. Larvikit ecan,in turn, containfluor it e and be fenitized in the immediat e contact zo ne (unpublished observ at io ns).
In the contact zones so ut hwest and northw est of the granite massif,thefine- to medium-grained granite forms net -vein ed com plexes wit h num ero us enclaves of mafic rocks (Fig. 3).Thisoccurrenceof maficrocks isinterpret ed as resulting from the inj ection of trachybasalt ic to trachy an- desiticmagm as alo ngthewallsofthegrani te magm a cham- ber in a zone of weakn essbet w een the marginal, almost com pletely consolid ated grani teand thest illliq uidgranitic mag ma(Mo roga n&Se ren sen 1995).Pillo w sof mafic mag- mas were subseq uent ly disrupted wit hin the granit ic mag matofor m the net- veinedcomplexes.Inseveralplaces, thenet-vein ed com plexesare cross-cutbyekerite(Fig.4).
Last,inone locality,thefin etomed ium-g rain edgran it e contai ns xeno lithsof nep heli ne syenite whichwereintrude d by syenite prior to int rusio n of the grani te (Andersen &
Sorensen1995).
The ekerite
Thecoarse-grainedgran it e occupie sthe cent ralpartof the granite massif.It showswelldeveloped sheetjoi nti ng,which is generally alm ost horizontal,suggesting that the roofof thegranite massif is situ atedata short distance above.
The ekerite contains numerous xenolit hs of larv ikit e, measuringfrom tensof metrestoafew centi met res.Acloser examinat io n of the xenolit hs reveals that larvikite was intruded by the fine- to med ium -grain edtype and, insome cases, by syenite prior to ekerite. This assem blage was int rude d and partlydig ested byekerite. Larvikit exenolit hs show all stages of recrystallisati on and disint egration int o ekerite .
PreciseU-Pb zirco n dating gave a Mid Permian age of 279.8±0.7Ma for the Mykle ekerit e,consid ered asthe age of emplacem ent (Pedersen et al. 1995).The Eikeren ekerit e yielded a slightly younger ageof 271 ± 2Ma bytheRb-Sr method(Sundvo ll etal. 1990).
Relationships between the two granite types Sharp contacts between ekerite and thefine-to med iu m- grained granite were observed in som e places along the west coast of Mykle(Fig. 5).No chilled margins are displayed at the contact of ekeritewith the fine-to medium-gr ained granitebut, insome localit ies,crystals of quart zandamphi- bolehave grown withinekeriteperpendicularto the co ntact against screens of thefine-to mediu m -g rained granit e(Fig.
6), suggestingthattheeke rit ewasemplaced into aconsoli- dated,but still hot,fine- to medium-grained granite.
Along somecontact zo neson the west coast of Mykle, alternating sheets of fine-to medium-gr ained granite and ekeritewere observedagainstoverly inglarvikit e (Fig.7).This seq ue nce could be causedbyfluctu at ingcond it io ns of crys-
NGU-BULL 441,20 0 3 - PAGE 20 BERNARD BONIN& HENNING S0RENSEN
Fig.5.Cont actbetween ekerite(lowerpart of the photo below the feetof theperson)andthe overly- ing fine-grainedgranite which,in thebackgrou nd, is overlain by a net-veined complex of trachy- andesitic glob ules ingranite.Southwes tcoast of Mykle.
Fig.6.Contactbet weenthe enclosedscreenof fine-grainedgranit e wit h blackdot s(left side of the photo)and the enclosing ekeritic granite (lower rightside).The contactis marked by crystalsofamphibolegrow- inginto ekerit e.Nort hwestcoastof Mykle.(Photo:LonePedersen).
tallisation in theroof of the granit emagmachamber,suchas variations of PH,O,but itis morelikelytheresultofinjection of ekerite into the sheet joints of the already crystalli sed fine-to medium-grained granite.This interpretati onis sup- ported by the occurrenc e of (i) ekerite dykesintersecti ng both larvikit e andfine- to medium-grainedgranite,and(ii) screens of the fine- to medium-grained granit e wit hin ekerite(Fig.6).
Associated dykes and veins
Several generat ionsof granit ic dykes and veinsintersectthe country rocksandshowbeaut iful examplesof dilationphe- nomena.On thesoutheasterncoastof lakeMykle, sheets of ekerite int rude a small occurr ence of microsyenit e along horizont al sheet joint s (Andersen & Sorensen 2003).The sheets display pegmatitic patches and abundant miaroli tic cavit ies, whichcan beup to10cm indiamet er.
Fig.7.Contactbetween larvikite{darkrock inthe upper part of thephoto}andtheund erlying gran- ite whichismade up of alterna tingsheets of fine- and coarse-graine d granite.Note the horizontal sheet jointingin larvikite as wellas in granite.West coastof Mykle.
BERN ARDBONIN&HENNINGS0REN SEN NGU-BUL L 441,2003 - PAGE21
1000
PI
1.15
::
1.10 1.05
1.00 0.95
LOS1---:~---I----;::::r:::::::;::==r===;1
• Fine-gr ained granite
• Ekerit e 1.0 0- j - - -t-±+---.---''<f'-<;::---i - Glit revanngranites
- Eikereneke rite
0.8S+--- -- - + -- - - + - - - - 1 - - -- -1-- - - - 1 0.90
10
86044 86035 860 22 86069
~~8~~
86060
100
:g2
c;o .co
•
PI -ASI Mykle
ASI 0.95I--I"-=~;;::;;::=t==t=-I"'-I--I
Bulk-rock chemical compositions
Major elements were analysed by X-ray fluorescence on fused glass discs at the Laboratoire de Petroqraphie et Volcano loq ie,Universite deParis-Sud. Most traceelements wereanalysed byX-ray fluorescenceonpowd eredsamp les, andLi and Be byatomi cabsorption spectro photomet ryat theGeolog icalInst itute,University ofCop enhagen. Cs, REE, Hf,Ta,Sc and Uwereanalysed byinst rumentalneutronacti- vationanalysisatTracechem A/S,Copenhagen.
Theanalyt ical result s are listedinTable1a,b.One sample of the southwesternsatellitegranite(81084),foursamplesof thefine-to medium -grained granite (86018, 86035,86044 and 86069), and four samples of ekerite (86022, 86045, 86057and86060)werechosenfor bulk-rockchemicalanaly- ses. As for the fine- to medium-grained granite,sample 86018was taken atthe contactwith larvikite,sample86035 atthecont act withtheunderlyin g ekerite,sample86069 at the contactofanet- veinedcomplex,andsample86044 isa granite enclosing globules of trachyandesite of the net-
Fig.8.PI-ASIdiagr am.TheAlumin a Sat uration Ind exisdefinedasthe AbO,/(CaO +Na,O +1(,0) molar ratio.The Peralkalinity Index PI is definedasthe (Na,O+K,O)/AbO,molar ratio.Thepolygon alareasare thecomposit ionfieldsoftheGlit revannCauldron (adapte d fro mJensen 1985)and theEikeren ekerite(adapted fromNeumann et al.1990).
The fine- to medium-grained reddish granite
The grain sizeof the granite rangesfrom1to 5 mm,locally up to 1cm.Ithas a granular text ure and consistsofturbid grainsof mesopert hiticalkali feldsparwhichmayberim med by lucidalbite,andofquartz,associated with clusters of Fe- Ti oxides,silicicbiotite andchlorite.Theseclustersform ed at the expenseof primary pyroxene and amphibole,which occur only spo radically. Clinop yroxene, when present, is almost pureaegirine and amphibolevaries fromferro-eden- ite(Petersen&Sorensen 1997)toferrori chteriteto arfved- sonite (M'Rabet- Maamar 1994).Accessory mineralsarezir- con,Fe-Tioxides (Ti-magnet ite converted into maghemite and ilmenite coated by ilmeno-ruti le and rutile),allanite, apat iteandfluorite.
Thefine-to medium-grained,grey to redgranite ofthe smallsatellite intrusion on the sout hwestern shoreof lake Mykle consists of mesopert hitic alkali feldspar, quartz, kato phorite to ferror icht erit e (Pedersen 1994), bioti te, ilmenite,magnetite,zircon, apatite,titanit eand allanite.
Petrography
Thetwo varieties of granit e are hypersolvus rocksdomi- nated by mesoperthiticalkalifeldspar.
The ekerite
The grain size can reach up to one centimetr e and even morein pegmatitic patc hes.Thetexture andmineralogyare identi cal toekerite of theEikerentyp elocality.Euhedra lto subhedralgrainsofwhite mesoperthitic alkalifeldsp ar and roun ded grainsofquartzmaybesegregatedinto elongated patchesdueto flowageduring emplacementof thegranitic melt.Therearesmallinterstiti algrain s ofalbite. Aegirine and green to blue arfvedsonite are present asminor compo- nents,mainly asinterstit ial smallgrains,but aegirine may occur asprismatic grainsup to onecm in length.Insome samples,aegir ine formsrims on arfvedsonite,whereas the oppositerelationshipisseen in others,andthe twominerals also con stituteparallel inte rgrowths.Pyroxene and amphi- bole grains arecommonly convertedint o aggregatesofsili- cic biot ite,stilpnomelane and Fe-Ti oxide s(ilmenite,ruti le) (M' Rabet-Maamar 1994).Crystalsofastr ophyllite up toone centimet re in sizeare seen in some localitieson the east coast ofMykle.Zirconis alwayspresent as small,clear crys- tals andasmore irregular metamict grains. Pyrochlore and fluorit e may also be present.Zon ed dykes and veins of ekerite have feldspar-rich rims and a quartz-rich cent ral core.
Miarolitic cavities contain crystals of quart z, albite, f1uo- rit eand calcite.Larvikit eandgabbro overlying thehorizon- talsheetsofgranit e and quart z syenitemay display str ings of miarolit ic cavit iesthatarearrangedat astee pangle tothe contactwith theint rusive sheets.
La Ce Nd SrnEu Tb Yb Lu
Fig.9. Cho nd rite-normalisedREEdiag ram.
NGU-BU L L44 1,20 03 -PAG E22
Table1a.Chemic alanalyses ofgranit esfro mtheMykle area:major elements(wt.%) andCIPWweightnor ms
BERNA RD BONI N & HENNIN G S0RE NSE N
Table1b.Chem ical analysesofgranites fromthe Mykle area:trace elemen ts(ppm)
Fine-and medium-grained granites Ekerites Fine-and medium -grainedgranites Ekerites
86057 86060 Sample
SiO, TiO, AI,O, Fe,O, FeO MnO MgO CaO Na,O K,O P,O, vol atiles total A.5.I.***
P.I.* * **
q or ab an c ac en fs wo mt hm 11 rt
81084* 860 18 86035 86044 86069 86022 72.84 77.14 72.80 74.26 76.47 75.13 0.29 0.09 0.33 0.30 0.19 0.39 13.00 11.38 13.18 13.49 12.13 11.61 1.12 1.42 1.95 1.55 2.23 1.87 1.10 0.41 0.92 0.13 0.14 0.61 0.09 0.06 0.05 0.03 0.03 0.13 0.16 n.d.** n.d. 0.02 n.d. n.d.
0.19 0.14 0.16 0.44 0.17 0.25 4.40 3.58 3.96 4.04 3.75 4.01 5.31 4.83 5.67 5.69 5.03 4.96 0.02 n.d. n.d. n.d. n.d. n.d.
0.42 0.47 0.48 0.47 0.37 0.42 8.94 99.52 99.50 100.42 100.51 99.38 0.98 1.00 1.02 0.99 1.02 0.93 1.00 0.98 0.96 0.95 0.96 1.03 29.17 37.97 27.95 28.21 34.68 33.35 30.81 28.79 33.80 33.38 29.77 29.72 36.47 30.47 33.72 34.23 31.76 32.28
o
0.67 0.79 1.82 0.84 0o
0 0.23 0 0.20 0o
0 0 0 0 1.600.40 0 0 0.05 0 0
0.77 0.11 0.09 0.05 0 0
0.38 0 0 0.15 0 0.47
1.46 1.07 2.02 0.42 0.08 1.39
o
0.69 0.57 1.27 2.17 0.34 0.52 0.16 0.62 0.27 0.30 0.62o
0 0 0.16 0.04 0 8604576.77 0.23 11.17 1.98 0.37 0.12 n.d.
0.17 4.31 4.82 n.d.
0.38 100.32 0.89 1.10 34.29 28.60 30.55
o o
5.27
o
0.40 0.3 0.25
o
0.43
o
76.08 0.27 11.42 2.52 0.09 0.14 n.d.
0.28 4.20 4.99 n.d.
0.46 100.45 0.89 1.08 33.60 29.54 30.97
o o
3.92
o o
0.57 0.45 0.88 0.13 0.17
76.95 0.24 10.94 1.65 0.52 0.12 n.d.
0.18 3.99 4.78 n.d.
0.35 99.72 0.91 1.07 36.34 28.44 29.45
o o
3.93
o
0.31 0.35 0.63
o
0.46
o
Sampl e 81084* 86018
Li n.a.** n.a.
Rb 282 230
Cs n.a. 1.0
Be n.a. n.a.
Sr 10 6.3
Ba 43 26
Pb 11 77
Zr 616 176
Hf n.a. 6.7
Nb 214 73
Ta n.a. 5.1
La 102 15
Ce 203 38
Nd 81 19
Sm n.a. 3.5
Eu n.a. 0.4
Tb n~ . Q8
Yb n.a. 3.2
Lu n.a 0.5
Y 78 29
Sc 6 1.3
Zn 38 73
Th 4.5 11
U n~. 5.4
Ga 27 25
* FromPedersen(1994)
** Not analysed
86035 14 270 2.7 11 16 84 11 618 21 218 12.
109 260 98 16.1 1.2 2.6 8.3 1.3 87 4.4 41 40 12.5 32
86044 86069 n.a. n.a.
239 241 1.7 2.9 n.a. n.a.
70 17
218 70
14 21
671 638 21.6 25.0 151 263 10.3 15.6
142 66
260 158
116 70
17.6 14.2 1.9 1.0 2.8 2.8 8.8 9.2 1.4 1.4
88 88
4.3 2.9 151 151
44 45
8 13.6
23 25
86022 36 251 1.6 11 3.1 45 38 704 24.1 142 13.4 83 186 90 17.4 2.0 3.2 9.7 1.3 98 3.3 115 18 5 25
86045 51 264 3.9 8 2.4 16 22 538 16.6 228 15.6 60 142 69 16.0 1.8 3.9 14.3 1.9 161 2.7 152 21 8.7 27
86057 86060
41 45
260 220
2.8 4.3
36 8
30 22
23 36
17 12
859 669
28.3 25.7
208 203
13.7 14.1
48 43
105 101
48 49
9.4 11.8 1.1 1.5 2.4 2.9 11.2 8.4 1.6 1.3
98 101
3.4 2.3
173 118
15 16
9 9.6
29 34
fromPedersen(1994) n.d.=not detected
A.5.1.
=
aluminasat urationindex:AI203/(CaO+Na20+K20,molar rat io P.I.=peralkalinityindex:(Na20+K20)/A1203,molarrat ioAnalyst: Mrs.R.Coqu et,Laboratoire de Petr oqraph ie-Volcano loqie,Universite de Paris-Sud,Orsay (France)
veined complex. Samples 86022, 86045 and 86057 are ekerite from various localitie s on the west ern and eastern sid es ofthe lake, whereas sample 86060 was takenat the contac twit h microsye niteonthe sout heaste rnshoreoflake Mykle.
Table 1aillust rat es the difference sofchem icalco mposi- tio nsof the fine- to medium-gra in ed graniteand ekeri te.
Apartfrom the two quartz-rich contact samples,86018and 86069, the fine- to medi um -gr ain ed granite yield s lower contents of SiO,and normative q and higher contents ofor thanekerit e.The ekerite sampleshave a peralkalin it y index P.I.,calculated as(Na20 +KP)/AIP3(m olarratio).markedly hig herthan one,whilethe metaluminousto weaklyperalu- ruino us,fine- tomedi um-g rained granite samplesyieldval- ues lowerthanor close to one(Fig.8).LowCaO abundances explainexceedingly low contents of normativean and cpx.
Accordingly,allekeritesam p les have normative ac, two fine- tomed ium-gr ain edgran it e samples haveno rm at ive c,and fourfine- to medium-grainedgranitesamples have norma- tivecpx.
When compared with ekerite, the fine - to medium-
Analysts:J.c.Bailey andB. Damgaard,GeologicalInstitute, Universityof Copenhage R.Gw ozdz,TracechemA/S,Copenhagen. Methods:INAA:Cs,REE,Ta,Hf,Sc, U;atom absorptionspectrometr y:Li,Be; XRF:otherelemen ts.
grainedgranit e islowerinLi,Cs,Y andZn,andalittl e higher inAI,K,Rb,Ba,the component s of alkalifeldspar in ag ree- mentwit h hig her normative or.Addit io nally,the low-silica, fine- to medium-grained granite sampl es are enriche d in LREE(Fig.9),Sc,Th and perhaps U relativeto ekerit e.
Among the fine-to medium-gr ained granit e sample s, 86018 and 86069 differ from the two other sam ples by higher contents ofSi02and lower co nt ent s of most other elements.These samples were taken nearthecont act s and thehighcontentsofquartz mayhave'dil uted' thecontent s of the other elements.The two other samp les,86035and 86044, have lowe r cont ents of Si02 than ekerite .Sample 81084ischem ically(and mineralogically)so sim ilarto sam- ples 86035and86044 that the smallsatelliteintrusio ntothe so ut h of the main massof granite islikely to repre sent a sout hern extension ofthe fin e-tomedi um-grained granite of the easte rn granitemassif.
The ekeritecomposit ions are high lysimilar,wit h the only exception of higher Sr contents in sam ples 86957 and 86060.Thismay becausedbythe occurre nceof astrophyllite in the case of sam p le 86057.Sam ple 86060 comes from ekerit eintr uding andassimilating a microsyeni telocatedat thesoutheasterncontact of thegranite massif.This rockcon- tains about 750ppm Sr(Andersen&So rensen2003).
BERNARDBONIN&HENNING S0RENSEN
----_.-
Discussion
Two topi cs will be addressed:(i) the relationshipsof the Mykle granites wit h the ot herekerite occurrences in the Oslo Igneous Province, (ii) the magmatic evolution and mode ofemplacement of the Mykle weste rn granitemassif.
The Mykle western granite massif and other ekerite occurrences in the Oslo Igneous Province Intheir monographonekeriteof theOslo regi on, Diet rich et al. (1965) describe associations of the granit e varieties described in the present paper and varioussyenit ic rocks (see also Dietrich &Heier 1967).The late-stage granites,com- pared withthe early stag e quartz-poor rocks,arehig her in Si, Be,
u.
Rb,Cs,Nb,Zr, Pband Taand low er in Ti,AI,Fe,Mg,Ca, Na,K,Ba,Sr,Laand Nd.Ekerit e isconsideredtohaveformed fromvolatile-rich resid ual liquidsundergoingchemica lfrac- tionationas aresultof mineral settlin g andvolati le transfer, part ly resulting in loss ofvolatile s.Inagreement with this model, the massive rocksare hig her in incomp at ibl e ele- mentsthanthe miarolitic variet ies.Rasmussen et al. (1988) and Neumann et al. (1990) emphasise the roleplayedbya comb inationof crystal frac- tionation and volatile transfer in the formation of the Eikeren ekerite.It wasnot possibleto subdivide the Eikeren granite intoseparateint rusiveunit s on thebasis of field,pet- rographicor compositional crite ria, and cross-cutting rela- tionsin the Eikeren ekerite are believed to reflect move- ment swithin thecrystallising magmabodyrat her than sep- arate intrusions (Neumann et al. 1990).
The Mykle western granite massifis also composite but, by contrast with the Eikeren ekerite, separate rock units could be defined. Chilledmargins are displayed atthe con- tactswith the earlier larvikit e and enclavesof nephelin e syenite,implying thatthesealready conso lidat ed rocks were cool enough to create a sharp gradient of temperatur e when thegranitic magmas wereemplaced. On the contrary, though later than syenite, granites wereemplaced when syenitewere sti ll hot,implyi nga sho rt timelag bet weenthe two intrusive events.Accordingly,no chilled marginsoccur in thegranitesat the contactwithsyenite.The two granit e varieties,also,were emplacedinashort periodof tim e;sharp contacts were observed,but wit h no chilled margins.The sequence:fine- to medium-grained granit e -> ekerite is show n byxenolit hsof larvikit e andmafic rocksofthenet- veined complexesinvaded byandenclosedwit hin thefin e- to medium-grained granite, prior to the emplacement of ekerite.
Anoth er example of a compo site granite st ockin the Oslo rift hasbeendescribedfrom theGlitrevann cauldron (Jensen 1985).Threedistinct granit etypesaredistingu ished, a medi um -grai ned granite, a porphyrit ic granite and an aplitic granit e.The porphyriticgranite mayrepresenta con- tact faciesof themedium-grainedtype,but theaplit ic gran- ite is youn gest and intrudesthe two ot hers.Thechem ical compositio ns(Fig.8)of thetwofirst-namedtyp es are very
NGU-BUL L441, 2003 -PAGE23
closeto thatof the fine-to medium-grained Mykle granite, e.g.,relativelylow Si02,relativelyhigh AlP3and normativec.
Thechem icalcompositionof the aplit ic type,though similar, is less peralkalin ethan the Mykle ekerite,e.g.,high Si02,low A1203,but low normative ac. Accordingly, nosodic pyro xene or amphiboleare ment ioned in theaplitictype which con- tains biotit e.ThemagmatictrendoftheGlitrevann granites matchesthatof the Myklefine-tomedium -grainedgranit e and differsmarkedly from themoreperaluminousbiot it e granit es of the Drammen and Finnemarka bat holiths (Trennes &Brandon 1992).
Implications for magmatic evolution and modes of emplacement
Granit ecomplexes are frequ ent ly composite,whatever their sites of emplacement.In the Oslo IgneousProvince,they wereemplacedatshallow depth s wit hin theupp er crust ofa LateCarboniferou sto Permi ancontinent alrift.
The granites of theOslo Rift can be classifiedat a first ste p into two major variet ies:biotitegranit e andekerite,cov- ering areasof similar size.This classific ation should also includ etherelatedsyenit es.Thereispresent ly a consensus that syenites and ekerit e are genetically related (e.g., Rasmussen etal. 1988),but thisislessclearforbiot ite gran- ite.Follow ingSeether(1962),Gaut(198 1) introduceda two- fold classification of biot it egranit e basedonfield relat ion- ships, with BG I corresponding to granites forming large massifsand show ing no transit ionsto other rock groups, and BG 11 generally formi ng smaller bodies in association wit hsyenite and ekerite.Trennes &Brandon (1990)showed that BG Iismild lyperalum inou s,while BG 11,basedon the exampleof theGlitrevannCauldron (Jensen1985),is meta- luminous,weakly peraluminou s and even weakly peralka- line.The shift from peraluminousto peralkaline composi- tion sfor small chemical variatio nsismostly related to the exceeding ly low abundances of CaO, resulting in trends passingthrough or nearthe (1.00/1.00) coordin atesinan PI- ASI plot(Fig.8).
The Mykle weste rn granit e massif is composed of a sequ enceof three intrusions:(i) syenite,(ii)fine-tomedium- grained granite, akinto the above -ment ioned BG11,and(iii) ekerite.Thetimelag betwee n the intr usiveeventswas short enough to prevent any significant gradientof temperatur e betweenthe alreadysoli di fiedwall rocks and the intr uding magma.It is suggested that the int rusive magm as were tapped successively from the quickly evolving top of a magma chamber and,then,emplaced atshallowe r dept hs wit hin thethick cap of larvikit eand relatedrhomb porphyry lavas.Theabundant miarol iticcaviti es occurringwit hin the intrusiverocksand their wall rocksnearthe contacts are evi- denceofa subvolcanic level of emplacement,ind ucingfluid exsolut ion from volatile-richmagmas (Raade 1972)and con- tinuing transport of fluids at subsolidus temp eratu res (Neumannetal.1990).
Themaximumshapeof theMykle weste rngraniternas-
NGU-BULL 441,2003 - PAGE24
sifisnotdisplayed at thepresent surfacelevel,asshownby theflatcontactsbetweentheintrusiverocksandtheirover- lying countryrocks.Onthebasisoftopographic low s,aero- magneti canomaliesand the observation of a 4-8 m-wide ring dyke of syenite nearFjellvann, 10 kmsout hwestoflake Mykle,5egalstad(1975)suggested that theMykle granites correspond tothe exposedroof of a larger ring structure,up to 20 km indiameter, intowhich larvikiteand related rocks subsided and were locally disrupted by stoping.This sug- geststhat theemplacement of thegranites tookplaceby cauldron subsidence in combination with magmatic stop- ing, processes frequently described in plutonic comp lexes emplaced in anorogenic geodynamic sett ings(e.g.,Bonin 1986).
From an examination of the aeromagnetic map (5egalst ad1975,fig. 2),itis suggestedthatthe Mykle ekerite islikelytobedisconnected at depth fromtheEikeren ekerite and the eastern granitemassifcould constitutea satellite of thewestern granite massif.
Summary and conclusions
The granites exposedaround lake Mykle constitute awest- ern and an easte rnmassif.Onlythewesternmassifis treated in thepresentpaper.Itformsanintrusive complex withthe followingcharacterist ics:
* Country rocks are larvikite occurring in the topographic highs andas stopedblockswithin the granites.
* Twotypesofgranite wereident ified:anearly, marginal, fine-to medium-grained,reddish granite;and a younger, central,coarse-grained,whiteto grey ekerite.
* The fine- to medium-grained reddish granite displays chilled margins at the contact swithlarvikite and locally contains mafic rocks as net-veined complexes.The gran- ite,akin totype BG11 of the biotite granite classification ofGaut(1981),ismetalumi noustoweaklyperaluminou s and yields the highest conte nts of incompatible ele- ments.
* Though later than syenite and the fine- to medium- grainedgranite,the ekerite displays sharpcontactsonly, wit h no chilled margins,indicating that no significant temperature gradients were created during its emplace- ment.
* Miarolitic cavities,abun dant in all rock types,are evi- dence of fluid exsolutionfrom volatile-richmagmas and continuingtransfer of fluids at subsolidus temperatures.
* The present surface level corresponds to the poorly exposed roof of a larger intrusion, emplaced within larvikiteand rhomb porphyry lavas by magmaticstoping and perhapsby cauldron subsidence.
Acknowledgements
The field work of H.S. was supported by grants from the Geo lo g ical Sur vey of Norwa y (NGUl and fro mtheDani sh Natu ralScienceResearch Council.Major element roc kanalyses were providedby Mrs.R.Coquet, Labor at oi re de Petroqraph ieetVolcan o logie,Universit ede Paris-Sud.
Orsay (Fran ce), trace elem ent analyses byDr.J.c. Baile y,Dr.R.Gw ozdz
BERNARD BONIN&HENNING 50REN5EN
andMrs.B.Dam g aard,Geo log ical Institute,University of Copenhagen.
Cand.scient. Lon e Pedersen,Britta Mu nch and Ole Bang-Bert helsen offered valuab le assistance with the preparat ion of illustrations.
Con struct ive criticism ofthe manuscript from Drs.Jean-PaulLieqeois and0ysteinNordgu lenisgrat efully acknowledged.
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