NGU -BULL 44 2 , 2004 - PAGE 23
TOMANDERSEN, ROBERT FREI, HENNING S0RENSEN & NIELS LANGAGER WESTPHAL
Porphyritic syenite at Lake Mykle, the Oslo Rift
a possible derivative of larvikite
TOM ANDERSEN, ROBERT FRE I, HENN ING S0RENSEN & NIELS LANGAG ER WESTPHA L
Andersen, T., Frei, R., Sore nse n, H. & West phai, N.L. 2004: The occurrence of porphyritic syenite at Lake Mykle, th e
Oslo Rift - a possible derivative of larvikit e. Norgesgeologiske undersokelse Bulletin 442, 23- 28.
New Rb-Sr and Sm-Nd isot op e data on porphyrit ic syenite from lake Mykle in th e so ut hern part of t he Oslo Rift confirm a close compo sit ional and genet ic relat io nship bet ween thi s rock and th e associated larvikite.The porph yriti c
syent it e has initi al 87Sr/86Sr at 280 Ma betw een 0.7044 and 0.7048 , and EN d between + 1.56 and + 2.52. This patte rn
of variat ion is compati ble wit h contam inat ion of a mantl e-derived parent mag ma wit h small amo unt s of
Precambrian calc-alkaline gneisses in th e deep crust.
Tom Andersen, Institutt for geofag, Universitetet i Oslo, Postboks 1047 Blindern, N-0316 Oslo, Norge; Robert Frei and
Henning Serensen, Geologisk lnstitut, Kobenhavns Universitet, 0 ster Voldgade 10, DK-1350 Kobenhavn K, Danmark; Niels
L.Westphal, Mcersk Olie og GasAlS, Esplanaden 50, DK-1263 Kebenhavn K, Danmark.
Introduction
zonitic pluto nic rock .The matrix of th e syenit e is very close
in co mposition to nordmarkitic syenite which in trud es t he
porphyritic syenite. Based on petrography and majo r and
t race element data, Pet ersen & Sorensen (1997) proposed
th at t he porphyri tic syenite was derived from a larvi kit ic
m elt by fr actionation processes invo lving plagioclase,
c1inopyroxene, Fe-Ti oxides and pos sibly apat ite .
An occurre nce of porphyriti c syenit e located at the so ut h
end of lake Mykle, on the 1:50 000 geological map-sheet
1713 I Siljan in the southwe stern part of the Oslo Rift, wa s
described in some detai l by Petersen & Sorensen (1997).This
syenite contains phenocrysts of plag iocl ase almost identica l
t o t he fe ldspa r of t he surround ing larv ikite, whic h is a mon-
59° 3D' 00"
Oslo
reg ion ...
"'. j?
1:==========1Larv ikit e
l: fi~ :~o: 1
Por phyriti c sye nite
liHW
}1 Syen ite
o
Microsyenite
~ Net-veined com plex
----=-=0=-=-=-=-=-=-=--_-0 _-_-_-_-_-_-_-
o
1
2
3 4
5 km
Fig. 1.Geolog ical map of th e Lake Mykle area. Sample locati on s: Stars:Porphyritic syenite. Diamon ds: Larvikite.
~ Volcanic rocks
NG U -BUL L 4 4 2 , 2 0 0 4 - PA G E 24
TOM ANDERSEN, ROBERTFREI, HENNING S0 RENSEN s NIELSLANGAGERWESTPHAL
syenite, as demo nstrated by the presence of larvi kite roo f
pendants in the po rphyritic syenite. Towards t he north and
east it is in cont act wit h g ranites (includin g ekerite) and
mino r bod ies of nord markit ic syenite. Cont acts between
larvi kite and porph yriti c syenite are generally sharp and
th ere are no chill zon es in the porphyriti c syenite in contact
wi t h larvikite. Locally, however, th ere is a t ransition from
larvikit e into porphyritic syenite in t he form of an increasing
density of plagioc lase phenocryst s tow ard th e larviki te over
a distance of a few cm. The porphyrit ic syenite con tains
xenolit hs of larvikit e which app ear to disinte grate int o clusters of feldspar (Fig. 2a). It also intrudes th e larvikite in the
fo rm of dykes (Fig. 2c). The nordmarkitic syenite and the
granites are yo unge r th an t he porphyrit ic syenite (Fig. 2b);
t hese you nger intru sions have ob lit erat ed t he ori ginal
nor th ern contact of th e larvikit e massif.
Petrography
Fig. 2. (a)Two larvikite xenolit hs in a st ate of disaggregat ion in porp hyrit ic syenite. Sout heast coast of Lake Mykle. (b) Porphyrit ic syenit e
(right) int ruded by syenite dyke. Sout hwest coast of Lake Mykle. (c)
Dyke of po rphyrit ic syenite wi th sharp cont act s against larvikit e. South
end of Lake Mykle .
Rb-Srand Sm-Nd isoto pe data for the por phyriti c syenite
are present ed and discussed in t he present paper wi t h th e
aim of constraining th e mo de of origin of the porphyri tic
syenite.
Field relationships
The porph yritic syenite form s a dom e-shaped body measuring 4.5 x 2 km (Fig. 1), and th e exposed vertical thickness is
about 150 m. Towards th e south and west, t he po rphyritic
syenite is in contact w it h larvi kit e, w hich also overlies th e
The porphyritic syenit e is charact erised by evenly distributed ph enocrysts of plagioclase (andesine-oligoclase)
w hich measure up to abou t 2 cm and make up 30-70 % of
th e rock. They are rimm ed by crypt oper thi t ic t ernary
feldspar. The fine-grained mat rix consists of cryp to- or
microp ert hitic An-poor alkali feldspar, t he g rains of which
may have cores of plagioclase, and of edenit ic amphibole,
augi t ic pyroxene, Fe-Ti-oxides, bio tite and abou t 5 % interstitial quartz. Accessory minerals are zircon, allanite,chevkinite,
apatite, fluor ite and calcite (Pet ersen & Sorensen 1997).
The larvikite of t he area is porphyritic and is dom inated
by large grains of plagioclase (50 t o 20 % An), w hich make
up about 75 % of t he rock. The plagio clase is rimm ed by
microp erth itic ternary feldspar. Aug itic c1i nopyroxene and
som e orthopyroxene make up about 10 %, edenitic amphibole 5 %, Fe-Ti oxides 5 % and accessory minerals 5 % ( zircon, apat ite, biot ite, quar tz, allanit e, chevkinit e and f1uorite).
The mat rix is composed of alkali feldspar and inte rgrowths
of oligocla se and pot ash feldspar. Petersen (1992) disti nguished two minor t ypes of larvi kite:(1 ) a coarse-grained
type with a larger con te nt of interst it ial alkali feld spar, and
(2) a fin e-grained typ e containing subcalcic and pigeonitic
clinopyroxenes and a high-t emperatu re plagioc lase. The
fine-grained variet y of larvikite occurs as scattere d, minor
bod ies, and its orig in remains uncert ain.
The plagioclase phe nocrysts of the po rphyri ti c syenit e
have rims of ternary feldspar, and are almost iden tical to t he
feldspar of t he host larvikit e.
Major and tra ce element data
Majo r and t race elem ent composit ions of larvikite and porphyriti c syenit e are presented in Table 1,based on data from
Petersen & Sorensen (1997), supplemented by isoto pe dilution tr ace elemen t analysesfrom t he present st udy (Table 2).
The larvikite analyses are of th e most abu ndant variety, wi th
the exception of 81543 w hich represent s t he fine-grai ned
type. In terms of maj or element s, t here is a gradual tran sit ion
NGU-BULL 44 2 , 2 0 0 4 - PAGE 2 5
TOM ANOERSEN, ROBERT FREI, HENNING S0RENSEN & NIELSLANGAGER WESTPHAL
Tabl e 1. Whole-rock major (wt%) and trace element (p p m) data of larv iki te and po rp hyritic syentie from the Myk le area.
81561
81566
81571
81577
81541
81543
81456
8159 5
81594
81559
LK
LK
LK
LK
LK
PS
PS
PS
PS
PS
57.30
1.25
18.77
1.43
FeP 3
FeO
3.63
M nO
0.12
MgO
1.36
5.16
CaO
5.54
Na20
K20
3.27
P20 S
0.60
Volatiles
0.69
Tota l
99.12
(Na+ K)/ AI 0.67
60 .04
1.44
15.82
1.72
4.29
0.16
1.38
3.20
4.90
4.82
0.54
0.69
99 .00
0.84
58.62
1.36
17.39
1.81
3.52
0.13
1.44
4.22
5.48
4.42
0.55
0.63
99.57
0.79
57.53
1.33
18.09
1.46
4.02
0.11
1.50
5.17
4.89
3.43
0.60
1.00
99.13
0.65
60.74
1.33
16.17
1.80
3.67
0.15
1.28
3.38
5.08
4.61
0.47
0.67
99.3 5
0.83
59.80
1.43
16.16
2.02
3.90
0.15
1.47
3.87
5.01
4.28
0.56
0.71
99.36
0.80
58.81
1.42
16.36
2.26
3.73
0.15
1.49
3.80
5.37
4.68
0.54
0.68
99 .29
0.85
60.52
1.30
16.17
1.67
3.75
0.14
1.29
3.35
5.13
4.71
0.45
0.68
99.16
0.84
59.19
1.52
16.25
2.15
3.87
0.15
1.57
4.08
4.96
4.25
0.62
0.71
99.32
0.79
109
3.1
573
728
939
24.2
119
7.9
104
204
85
14.3
4.2
1.9
5.7
0.8
57
17
104
45
8
157
3.8
144
112
211
4.5
n.d.
215
2.7
218
n.d.
155
3.7
20 1
n.d.
n.d.
n.d.
296
488
5 15
435
329
394
379
334
439
1000
1180
31. 1
139
9.4
118
239
9 15
907
780
750
695
1040
n.d.
n.d.
n.d.
99
n.d.
n.d.
111
223
95
188
124
264
604
1300
31.1
204
13.2
147
288
582
1300
137
627
1190
33.6
165
11.1
138
273
640
10 10
n.d.
948
819
19.8
117
7.8
106
202
107
98
85
85
127
109
20
4.4
2.9
8.4
1.1
80
22
124
23
11.1
n.d.
n.d.
n.d.
n.d.
n.d.
16
66
21
101
40
8
59
16
84
52
8
19.2
3.7
2.9
10
1.2
87
38
106
38
9.7
20
n.d.
n.d.
n.d.
n.d.
14
4.4
2
5.7
0.8
59
20
94
32
8
Weight percent oxides
Si0 2
Ti0 2
AI20 3
59.9 1
1.11
17.64
1.20
3.55
0.12
1.12
3.56
5.87
4.14
0.45
0.75
99 .42
0.80
Parts per million
Rb
Cs
Sr
Ba
Zr
Hf
Nb
Ta
La
Ce
Nd
Sm
Eu
Tb
Yb
Lu
Y
Th
Zn
V
Sc
n.d.
151
n.d.
n.d.
n.d.
n.d.
n.d.
83
39
105
55
12
191
133
136
n.d.
n.d.
139
278
117
253
119
115
106
22
3.7
3.1
9.9
1.3
95
41
93
54
10.3
21
20
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
93
38
94
42
11
81
103
55
13
LK: Larvikit e, PS: Porp hy rit ic syeni te. n.d. no t de te rm ined
Major elements by XRF on fu sed discs pre pa red w it h sod ium tet rabora te flux at th e Geo lo gical Survey of Denm ark and Green land; tr ace elements by
XRF on pressed powde r pe llets at Geo logica l Inst it ute, Unive rsity of Cop en hagen and by Instrum ental neutr on act ivat ion analysis By Tracechem AlS,
Cop en hagen . Rb, Sr, Sm and Nd da ta in it alics: Isoto pe d ilu tio n analyses (Table 2).
fro m larvikit e ove r porphyritic syen ite to nord markit ic syenite, as shown by Harker var iatio n diag rams (Figs. 6 an d 7 in
Peterse n & Serensen 1997 ). When compared w ith t he
larvikite, nordma rkite and porphyritic syen ite are enri ched
in Si, K, Rb,Th, Nb,Ta, RE E (wi t h the except io n of Eu), Y, Zr and
Hf and relatively depleted in AI, Ca, Na, Eu, Sr and Ba, that is
in the compon ents of pl agioclase . The t wo syenit es have
prac tically ide nt ical REE patterns; larv ik it e has a w eak posi tive Eu anoma ly, w hereas t he syen ites have very weak negative anoma lies (Fig. 8 in Pet ersen & Sorensen 1997) .
Nordmarkite and porp hyr it ic syenite have almost ind ist ing uishab le Iit hop hile element patt erns, w hic h d iffe r o nly in Sr
(higher in t he porphyritic syenite) and Zr,Th and Hf (lowe r in
t he porp hy ritic syenit e).These differences reflect differences
in modal mi ne ralogy; plag io clase is present in the per-
p hy rit ic syenite, w hereas zir con is m o re abunda nt in t he
no rdm ar kit e (Fig . 9 in Petersen & Sorense n 1997).
Sr and Nd Isot ope data
Analyt ical methods
Rb, Sr, Sm and Nd concentrations of four samples of porphyritic syenite and th ree samples of associat ed larvikit e
(Pete rsen & Ser en sen 1997) were analysed by X-ray fl uores cence and by isotope d ilution t herm al io nization mass spectr ometry at th e Geo log ical In stitute, Cope nhage n Un iversit y.
The sam ple powders (200 mg) were at ta cke d fo llo w ing a
sequen tia l di ssolution first w it h 8N HBr, foll o w ed by 14N
HN0 3 - 32% HF mixtures, in Tefl on b eakers on a hot pla te (T=
150 °C) for three days. A 149Sm - 1So Nd spike was added before-
N GU- BUL L 442 , 200 4 - PAG E 26
TOM ANDERSEN, ROBERT FREI, HENNINGS0RENSEN& NIELS LANGAGER WESTPHAL
Table 2.Rb-Sr and Sm-Nd data for intrus ive rocks from the Mykle Area. Oslo Rift.
Sample
Rb
pp m
Porphyric syenite
191
81577
81566
215
81559**
221
81567**
237
81582**
206
81571
218
81561
201
Larvikite, S Lake Mykle
81543
157
81594
155
81595
112
144
81456**
81541 **
109
Larvi kite, E Lake Mykle
74
76461 *
76465*
90
76468*
54
77805*
80
77814*
66
81236*
77
79389*
102
Granit e
86044*
82
Sr
ppm
87Rb/ 86Sr
439
379
329
328
426
334
394
1.301
1.682
1.945
2.093
1.4
1.935
1.492
0.709559
0.711482
0.712205
0.712697
0.71001
0.712187
0.710534
0.000020
0.000020
20
22
106
119
0.1082
0.1039
0.512556
0.512591
0.000021
0.000019
0.70438
0.70478
1.56
2.40
0.000020
0.000020
21
20
115
109
0.1054
0.1071
0.512577
0.512603
0.000012
0.000013
0.70448
0.70459
2.07
2.52
296
435
515
488
573
1.556
1.043
0.625
0.854
0.551
0.711099
0.707204
0.707015
0.707331
0.70601 7
0.000020
0.000020
0.000020
20
15
16
107
85
85
0.1068
0.1002
0.1048
0.512590
0.512637
0.512609
0.000012
0.000011
0.000011
0.70490
0.70305
0.70453
2.28
3.43
2.72
709
650
870
664
725
697
558
0.301
0.399
0.180
0.350
0.263
0.320
0.529
0.705987
0.706144
0.705332
0.706097
0.705598
0.705833
0.706519
0.00042
0.00003
0.00003
0.00003
0.00003
0.00003
0.00003
19
16
19
20
20
19
22
105
87
101
108
108
105
117
0.1119
0.1096
0.1136
0.11 13
0.1145
0.1119
0.1133
0.512621
0.512615
0.512609
0.512628
0.512611
0.512602
0.512628
0.000010
0.000010
0.000010
0.000010
0.000012
0.000010
0.000010
0.704786
0.704554
0.70461 2
0.704702
0.704551
0.704558
0.704411
2.70
2.66
2.41
2.86
2.41
2.33
2.78
9.820.742261548
0.000009
7.8
44.4
0.1068
0.512622
0.000010
0.70315
2.90
24
87Sr/ 86Sr
2a
Sm
ppm
Nd 147Sm/ ' 44 Nd
ppm
143Nd/ ' 44 Nd
Sr;
280Ma
2a
E Nd
280Ma
" : Analysed at th e Laboratory of Isot ope Geology, Mineralogical- Geological Museum, University of Oslo. ** : Analyst: P.M. Holm, Copenhagen University.
Samples 76461 to 79389 from th e larvikite ring structure east of lake Mykle (Fig. 1) were provided by Uffe Larsen.
ha nd. 5r a nd REE f racti on s we re sep a rat ed over 15 ml g la ss
an ag e o f 280 Ma and a n i n it ial 875r/865r of 0.7045 (Fig. 3).
stem
One, off-ly ing lar v ikite sam ple (8 159 4) plots sig nifican tly
co lu m ns
c ha rged
wi th
AG
SOW
cation
res i n.
Purificat ion o f t he 5r fr act ion w as achieved b y a pa ss over
belo w th is line , w ith 875r/ 86 5r2s0Ma= 0 .7031 .
micro -col u m n s co nta in ing 5r5pec™ res in . REEs we re f u rther
Lar v ikit e an d porp h yr iti c syeni te show small an d over-
se pa ra ted over HDEHP-co at ed bio b ead s (Bio Rad ™) loaded
lap p i n g ran ges of 1475m/J44N d a nd 143Nd/' 44N d . At 280 Ma , all
i n 6 ml g lass st e m col u m ns. 5r and N d isotopes we re
sa m p les ha ve po siti v e epsilo n Nd values, rangi ng from 1.56
a n alysed in dyna m ic m u lti-co l lect in g routine s, 5m in a static
mode, on a VG 54 5e ctor IT mass spect ra mete r. Th e mean
va lue fo r ou r i nternal JM N d standa rd (refe re n ced agai nst La
0.720
Joll a) d u ring t he period of measurement w as 0.5111 15 fo r
'43Nd /' 44N d, wit h a 2a externa l rep roducibil ity o f
o 0
± 0 .000013
(fiv e m easure m ents).The mean 875r/865r va lue of the NB5 98 7
0.716
.. •
•
Porphyritic syenite
Larvikite
5r standard was 0.710 24 8, wi th a 2a extern al reprodu ci bility
o f 0.00001 1 (f o u r m easu rem en t s).
Ad ditio n al
a na ly sed
at
sa m p les
the
of
larvikite
Lab o rat o ry
of
and
gra nite
Isotope
we re
Geology,
M ine ra log ica l-Geological M u seum, Uni ve rsity of O slo , b y
...
en
0.712
<D
ec
-.::
....en 0.708
ec
m eth o d s desc ribed b y Andersen et al. (20 0 1).
0.704
Results
Rb -5r a n d
Sm-Nd iso t ope data for selected sample s o f por-
ph yriti c sye ni te, larv ik it e an d gran ite f ro m th e Myk le area are
g iven in Tab le 2 . Larvik ite and porp hyrit ic sy e n it e show m ar g i na lly ove rla pp ing ra nges of 87Rb / 865 r and 875r/865r. Ne it her
of th e t w o
rock t y p es defi ne
sta tist ica lly v alid
Rb -5r
isoc h ro ns; t h e sam p les of po rp hyri t ic sye n it e a nd a majority
of th e la rvi kite sam p les sca tter aro u nd a refe re nc e line w ith
0.700
+----+---+__--_ - - -I-----+----l
0.0
0.4
1.6
2.0
Fig. 3. Rb-S r correlati on diagram showing larvikite and porphyritic syenite, compared to a 280 Ma reference line w it h an initia l 87Sr/ 86 Sr ratio of
0.7045.Circles and squares: Samples from th e sout h end of Lake Mykle.
Triangles: Samples from NEof Lake Mykle.
TOMANDERSEN, ROBERT FREI, HENNING S0RENSEN & NIELSLANGAGER WESTPHAL
(87S r/86Sr)
0.700
0.710
10
5
I
r
I
1
0.720
0.730
0.740
0.750
v
~ Larvikite
1 l_~~_~J
~
r---,
~tl
~ '~i~
Syenites and granites
Precambrian crusta I
~~~~:~~n::s
0
N
1:1. 1
o
~
-z"O -5
l lj
I
~ -- -
-15
(- - - - - - - - - -,
r
: GRA
i
iI
_
rL-S-R-
f
I l
11 c.9
!it
-10
1.0
0.760
:
0.703
4.0 +-........
( - ---------- -)
0.705
-----4
-------i;::.------+--.:-------'r-i.!..r:!~) .
\, - NOG
1
- ----- --- - -'
3.0
I
I
-20 "'----
-
- - --
-
-
-
-
-
• • Larvikite
-
2.0
o Porph. syenite
!:l Granite
1.0 ~----'"
Fig.4. Sr and Nd isotopic composition of porphyritic syenite and associated rocks from t he Mykle area. Black tria ng les rep resent larvikites fro m
NE of Lake Myk le. Ruled/cross-hat ch ed areas repr esent range s of regiona l variation of larvikite and syenite and granite in t he Oslo Rift (data
from Neumann et al. 1988). Ranges of Precambrian crus ta l com ponents
are taken fro m And ersen & Knudsen (2000): M M U: 1.15-1.50 Ga Mafi c
underpl ate, GRA:c. 0.93 Ga gr anites, NDC:'Normal d eep crust; i.e.,moderately L1 LE-enrich ed rocks in the d eep crust , UCE: Up pe r crustal rocks
east of th e rift, LSR: Upper crustal rocks of t he Telemark area, showing
elevated Rb/ Sr ratios at nor ma l Rb co ncentra t io ns. TIG: Precambrian
(1.6-1.2 Ga) cale-alka line m et a-ign eous rocks, in cludin g ton alite, tr on d hj em ite, gr an od iorite and t heir ext rusive equivalents (Kn ud sen &
And ersen 1999, Andersen & Knud sen 2000 and references therein ).
The inset is an expa nsio n of th e ro cks fr om the Mykle area, showing
data points w it h 2s erro r bars.
to 2.54 for t he porphyritic syenite, and from 2.28 t o 3.43 for
the larvikite (Table 2). Regardless of composi tion, the maj ority of samples for m a cluster in a E Nd vs.87Sr/86Sr280Ma diagram
(Fig. 4).Variations in init ial Sr and Nd composition within this
cluster are small, but exceed analytical error, and are uncorrelated (Fig. 4, inset). One sample of larviki te (81594), and
one granite show significantly lowe r time-corrected 87Sr/ 86 Sr
than the other samples.This is probably due to loss of radiogeni c stron tium d uring late (postglacial ?) weat hering. The
gran ite sample has a high Rb/Sr rati o compared to the other
samples, and only moderate loss of radiogen ic Sr from this
sample could increase th e Rb/Sr ratio enough to cause significant overcorrection for radioge nic growth w hen recalculated to 280 Ma.
The total ranges of Srand Nd isotopic variation at 280 Ma
reported in Table 2 fall within the overall range of variation
of larvikitic rocks from th e Oslo Rift (Fig. 4, data from
Neumann et al. 1988). However, in most othe r felsic rocks in
the rift, a reductio n in initial 143Nd/ 144Nd is commonly cou-
NGU- B ULL 44 2 , 2 0 0 4 - PAGE 2 7
pled t o a pronounced increase in initial 87Sr/ 86Sr; see crosshatched area in Fig.3 (Andersen & Knudsen 2000).This is not
observed in the porphyritic syenite from the Mykle area.
Discussion
Larvikite magma in the Oslo igneous province is consid ered
to have been derived by fractiona l crystallizat ion of basaltic
melts formed by part ial melting of a mildly deple ted somew hat heterogeneous mantle source (Neumann 1980,
Neumann et al. 1988, Rasmussen et al. 1988). The same
auth ors proposed t hat crustal contaminati on was involved
in the formation of the syenit ic and granitic rocks of the
region, w hich is also suppo rted by publis hed radiogenic isotope data (Andersen & Knudsen 2000).
Based on major and trace-element data, Petersen &
Sorensen (1997) pointed out tha t larvik ite, porp hyritic syenite and nordmarkitic syenite may have been form ed from
successive pulses of magma from a common source, and
that feldspar fractiona tion played an im por tant role in this
process. The Harker variation diagrams of Pet ersen &
Serensen (1997) indicat e that fractionation of c1inopyroxene, Fe-Ti oxides,and perha ps,apatite, was also involved.The
nordmarkitic syenite and the microsyenite at Lake Mykl e
described by Andersen & Sorensen (2003) mark a mor e
evolved stage t han the porphyritic syenite.
The overall out crop pattern (Fig. 2), and the presence of
roof pendants of larvikite in the porphyritic syenite, indicate
t hat th e porphyritic syenite is located inside the larvikite
massif, but close to its contacts to (younger) granites and
nordmarkitic syenite.
The contact relation ships and th e presence of disaggregated xenoliths of larvik ite in the porphyritic syenite may be
taken as evidence th at the plagioclase phenocrysts of the
porphyritic syenite originated by disintegration of larvikit e.
Such an origin for th e por phyritic syenite is, however, hard to
reconcile wi th th e even distribution of phenocr ysts over t he
entire volume of the po rphyritic syenite, including dykes
w hich intersect t he larvikite with sharp intrusive contacts
(Fig. 2c).
The similarity in struct ure and compositi on of the plagioc1ase of larvikite and porphyritic syenit e, includi ng the ir rim s
of ternary feldspar, led Petersen & Serensen (1997) to pro pose that th e porphyrit ic syenite may have been formed by
extended crystallization of larvikit ic melts caused by an
increase in HP , ot her volat iles and inco mpatible element s
in response to the fract ionat ion of anhydrous minerals such
as feldspar, c1inopyroxene, Fe-Ti oxides and possibly apatite .
Such evolved melts may have colle cted in cupola in the
upper part of th e still hot, solidifying larvikite. This can
explain the generally sharp, non -chilled contacts and local
tr ansitional phenomena. The even distribution of plagioc1ase phenocr ysts in th e porphyrit ic syenite may then be
explained by assumi ng t hat t he magma, whi ch formed the
porphyritc syenite,w as at its liquid us at th e time of emplacement ,and that early nucl eation of plagioclase to ok place in a
supercoo led melt which solidified rapid ly to for m th e fine -
s
N G U -B ULL 442 , 2 0 04 - PAGE 2 8
gra ined , qu art z-b earing ,
Soren sen 1997).
syen it ic
TOM ANDERSEN, ROBERT FREI, HENNING S0RENSEN NIELSLANGAGER WESTPHAL
m at rix
(Petersen
&
Simi larit ies w it h both larvikit ic roc ks of th e Oslo Rift in
gene ral, and wi t h larviki tes and g ranit ic rocks of t he Mykle
area in particular, are also hig hlig hted by th e Sr and Nd isotop e data (Fig.4).
Felsic intru sive rocks in the Oslo Reg ion show an overall
tend ency to ward s initi al 87Sr/ 86Sr rat io s of 0.710 or hig her,
and near-zero or w eakly negat ive epsilo n-Nd values (crosshat ch ed field in Fig. 4). Based o n a co mpilation of geo chemi cal data on Precambrian rock s fro m So ut h No rw ay, Andersen
& Knud sen (2000) defin ed six composit io nally and geo graphically con strained crusta I end m em ber s, which may
have act ed as cru stal contaminants in t he magmatic system
of t he Oslo Rift. The regi onal variation in the felsic int rusive
roc ks can be accounted fo r by con tam ination wi t h moderately L1LE-enr iched, Precam br ian rocks in t he dee p crust ,
repre sented by t he 'No rm al Deep Cru st' (NOC) and
'Precam brian granite' (GRA) co mponents in Fig. 4 (Andersen
& Knudsen 2000). Som e g ran itic rocks sho w evide nce of con tamination by st ro ng ly evo lved crusta l rocks w it h low Sr
concentrations and co rrespondi ngly hi g h Rb/ Sr rat io s (LSR
co m pon ent in Fig. 4). In th e Myk le larvikit e and porphy riti c
syeni te, a m inor shift tow ards lowe r epsilo n-Nd is no t cou p led to an increa se in initi al 87Sr/ 86Sr, w hich sugg ests t hat
neit her of th ese crusta l co m pon ents can have had any significant influ enc e o n th e interm ediat e and felsic magmas of
th e Myk le area. How ever, th e samp les analysed in t his st udy
ov erlap in Sr and Nd co m position w it h th e hig h eps ilon-N d
rang e of 1.6-1.2 Ga cale-alkaline g neisses at 280 Ma. Such
rocks are w ide sp read in t he 0stfo ld -Ak ershus, Kong sberg
and Bamble secto rs in t he Precam b rian of South No rway
(And ersen & Griffin 2002 ), i.e.,adja cent to t he sout hern part
of t he Oslo Rift . These po tential contam in ant s are repr esented by th e TTG co mponent in Fig. 4, w hi ch is co nstrai ned
by data fro m a range of subd uction -relat ed, m eta -ign eo us
Precam b rian rocks (And ersen & Knudsen 2000 and refe rence s th erein ). From th eir geog rap hical d istribut ion, TIG
co nt aminants are most likely t o have influe nced magmas in
th e sout he rn part of th e rift , and th e pr esent data are com pat ible w it h moderate amounts of such con taminati on in
t he ma gmas of t he Myk le area. However, th e cale-alkaline
me ta-ig neou s Precam br ian rocks are in ge ne ral m et alu m inous t o peraluminou s, and major contam inatio n w it h such
mater ial is not like ly in roc ks tr end ing to w ard s increasing
(Na+ K)/ AI (Tab le 1).
Conclusions
Sr and Nd isoto pe data o n po rphy rit ic syen ite and associate d larv ikite in the Mykle area of th e Oslo Rift co nfirm th e
close genet ic relat ionsh ip bet ween the t wo rock types in di cat ed by pet rog rap hy and m ajor and tra ce elem ent geochem istr y (Pet ersen & Sorensen 1997). A lim ited var iation in
ini t ial Nd isoto pi c composit ion (fro m epsilon -Nd = + 3.4 to +
1.6 at 280 Ma). wi t hou t co rrespo nd ing variat ion in int it ial
87Sr/ 86Sr sugg ests th at th e larvikite and po rp hyrit ic syen ite
mag mas w ere contam inat ed by m inor amo unts of
Precam brian cale-al kaline rocks, sim ilar to sub du ct io nrelat ed meta-igneous roc ks fou nd at both side s of t he southern on shor e segment of t he Oslo Rift.
Acknowledgements
Field w ork was sup ported by Norges geologiske undersokel se, the
Dani sh Nat ural Scienc e Research Cou ncil and t he Carl sberg Foundation .
Uffe Larsen kind ly p rovided samples of larvikite from t he area east of
Lake My kle. Toril Eng er assisted in t he isoto pe labora tory in Oslo. Britt a
M unck and Ole Bang Berth elsen, Geol ogical Insti tu te, Univers ity of
Copen hag en provid ed valuable assistance wi th the illus tra t io ns.Thanks
to Poul Ma rtin Holm for Sr isotope analyses, and to Tore Prestvik and
0 yst ein No rdgu len for criti cal com me nts on the manuscript.
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