Rb−Sr isotopic studies of the plutonic rocks of the Oslo region

Rb-Sr ISOTOPIC ROCKS OF THE K.S. H E I E R

STUDIES, OF THE OSLO REGION

PLUTONIC

A N D W. C O M P S T O N

HExErS, K.S. & CO.~WSTON, W. 1969: Rb-Sr isotopic studies of the plutonic rocks of the Oslo Region. Lithos 2, 133-145. 4 granites, 7 ekerites, 1 ekerite-nordmarkite, 4 nordmarkites, 4 larvikites, 6 lardalites, and 2 kjelsfisites of the Oslo Series rocks were analysed by the Rb-Sr method. The data were treated by the regression method of McIntyre et al. (1966). A Permian age is confirmed and the time of intrusion and crystallization defined to 276 m.y. (k ~ 1.39 • 10-11yr-~). T h e internal precision is 4- 7 m.y. when the ekerites are excluded from the regression. This is improved to 4- 1.7 m.y. when the ekerites are considered with the other rocks. It is an interestingly short time for the intrusion and crystallization of rocks ranging from 50 to 77% SiO,. The common initial SW/Sr s6 ratio of the rock series is 0.7041 4- 0.0002. It restricts the magma source to the upper mantle or a deep crust of very different composition for the Precambrian gneisses and Palaeozoic sediments of the surface crust in the region.

Introduction The Oslo Series suite of (comagmatic ?) subvolcanic, mildly alkaline rocks of Permian age is one of the most intensely studied igneous rock suites in the world. Since 1944 the majority of papers discussing the rocks have been published in the series 'Studies on the igneous rock complex of the Oslo region', Skrifter Det Norske Videnskaps Akademi; Oslo, I. Mat.-Naturv. Klasse. More than 20 papers have appeared in this series to date. The major element compositions of a large number of samples of these rocks were published by Br6gger (1933). Barth (1945) published the systematic petrography of the plutonic rocks based on thin-section studies of the rocks analysed by Br6gger. The petrographic relations of the principal Oslo rocks are illustrated by their 'family tree' which is reproduced here in Fig. 1. The chief types of the Oslo magmatic province are in the kjelsfisite-ekerite series. They may be derived by fractional crystallization of a syenitic magma. The mineral reactions leading from one petrographic type into the other are illustrated in Fig. 2. The distinctions are mainly on the basis of the feldspar reaction series. The transition larvikite - lardalite branches out from the major series and is caused by a sharp decline in silica with formation of nepheline. Lardalite is quantitatively least important in the Oslo series, where it covers only 1.3 per cent of the total area. The genetic position of the granite is uncertain. Barth (1945) considered

134

K.S. tIEIER & W. COMPSTON

Soda o.rlhoclase~Soda orthoclase,,,, Labfadorite Andesine Oligoclase - ~ Anorthoclase.

Or, Ab

~

3O

0

Eker.

..t-- 0

,

. , , 0 ._l_

-0 ,) ('Nel~h.~ L,,,_\Pegm./\

70 An

50

30

10

Content of An in feldspar decreasing

0

7-0

>

20r ~

-10 >

Fig. 1. 'Family-tree' of the plutonie rocks of the Oslo region (after Barth 1945).

Orthoclase Andesine

t.

:- Soda orthoclase >- O l i g o c l a s e - - ~ A n ~ 1 7 6

I Kjelsfisite I ~ I Augite

,t Larvikite

I

----Or, Ab

t

I--~l

~ [ Nordmarkite

I

~, Augite ~ H o r n b l e n d e

'

l Ekerite

I

1'

> Soda amphibole

Fig. 2. T h e mineral reactions leading from one plutonic rock type to the other (after Barth 1945).

it not related to the other rocks within the series through a magmatic differentiation process. The generation of the Oslo magma has been a matter of some controversy and is discussed by Barth (1954) and Oftedahl (1959, 1960, 1967). Both authors considered the only magma derived from the mantle to be the one which formed the subvolcanic Oslo-essexites and their derivatives (the Oslo essexite series, Barth 1945) which are very insignificant in volume (and not studied here). From the volume relationships, it seemsunlikely that the monzonitic magma was derived from the gabbroic (Oslo-essexite) magma by fractional crystallization. The graben-area now occupied by the Permian rocks was previously composed of Precambrian gneisses overlain by Palaeozoic sediments. Barth (1954) advocated that the geochemistry of the Oslo rocks is essentially dominated by the pre-existing rocks which were completely melted by percolating 'emanations', and the Oslo province formed through a

RB-SR ISOTOPIC STUDIES

135

burst of degassing of the Earth's interior. Oftedahl (1959) assumed that the monzonitic magma was generated by melting of the lower portion of the crust, and Oftedahl (1967) discussed the following possibilties for the magma formation. (1) Anatectic magma formation in the deeper crust, moving upwards and changing gradually to more acidic products. (2) Anatectic magma formation in the deeper crust, accompanied by gravitative differentiation in situ. (3) Magma formation in the deeper crust, with comprehensive differentiation near the surface. (4) Basaltic magmas from the upper mantle, undergoing a comprehensive crustal differentiation. (5) Possible differentiation in the upper mantle. He found the possibilities (1) and (2) to be most probable for the plutonic series but suggested that modern trace element studies, and especially isotope studies, could give better information than the classical methods. Recent gravity measurements over the Oslo graben carried out by the Norwegian Geodetic Survey (Norges Geografiske Oppm{iling) have shown large positive anomalies to be associated with the rocks in the area. This may suggest that dense material of the mantle has risen to a high level in the Oslo graben. The magmatic rocks may therefore be associated with large amounts of basic rocks at depth. The derivation of the magmatic rocks from basic magma and the origin of the graben through an expansion of the crust should therefore be reconsidered. The surface area occupied by the different rock types was estimated by Barth (1945) and given here in Table 1. Table 1. Surface area occupied by the different subvolcanic rock types (after Barth 1945) Oslo-essexite ' Akerite Kjels~site Lardalite Larvikite Nrordmarkite-Pulaskite Ekerite Biotite granite

Km z 15.3 52.2 201.0 65.0 1670.0 1425.0 821.0 840.0

% 0.3 1.0 4.0 1.3 32.8 28.0 16.1 16.5

5089.5

100.0

T h e age of t h e Oslo province A thin series of Permian sedimentary rocks are conformably overlain by a thick series of volcanic rocks which mark the beginning of the igneous activity in the area. The sediments contain fossils and fossiliferous strata occur also between the lowermost lava flows. Some fossils indicate a lower Permian age, probably the middle part of lower Permian (Oftedahl 1960), corresponding to about 270 to 280 m. years ago on Kulp's time scale (Kulp 1961). Radiometric age determinations carried out by different methods were summarized by Neumann (1960). Isotopic lead age determinations on Oslo Series rocks have been reported

136

K.S. tIE1ER & Vr COMPSTON

by Nier (1939) and by Faul et al. (1959), who give UZ38/pb z~ ages of 243 m.y. and 259 m.y. respectively for thorite from a locality near Brevik at the southernmost extension of the province, and for zircon from nordmarkite near Oslo. Both authors also report the Pb 207 abundance, which in the case of Faul et al. appears sufficiently accurate to warrant the calculation of the U235/pb2~ age, at 296 m.y. Faul et al. also give the Pb2~ 2~ age at 400 m.y., with wide uncertainty limits, which they rate as of little use owing to the presence of non-radiogenic Pb of unknown origin and isotopic composition. Nevertheless, it adds to the impression that the U]Pb age system in the zircon is discordant and that the U238[pb 2~ result is a minimum age, as indeed Faul et al. emphasize. The coneordia-diagram, Fig. 3, shows the data plotted as a point surrounded by a box which represents our estimates of the maximum limits for independent y and x errors. The coordinates were calculated as Radiogenic Pb 2~ y-x=-

U23.

Pb 2~ [ ( Pb 2~ ~ -

-

-

U_,3s L\ pb~o, ]

[Pb2~

obso.ed-/v

) i. t al

]

Radiogenic Pb 2~ Pb2~ [ ( Pb2~ / /'Pb2~ ] -- - UZ3S UZ3S [ \ p b 2~ ] observed- ~p---~i) initial 1

In constructing the box we have assumed maximum uncertainties of 2% for Pb2~176 1% for Pb2~ 2c~ and 0.5% for Pb2~ 2~ in Faul et al.'s primary data. Covariant y, x error due to the terms PbZ~ and to the calculation of Pb2~ 2~ as (Pb2~ 2~ (Pb2~176 are not shown in Fig. 3 as their locus is effectively parallel to concordia, which is nearly a straight

,.,

3,o

[ .05 CO

eq

.0t,

o

.03

I

.Z

.3 Radiogenic 207pb 235 u

I

.4

Fig. 3. Uranium-lead isotopic data on zircon from nordmarkite (Faul 1959) plotted on the concordia diagram. Box represents our estimate of the maximum limits for independent y and x errors in the original data.

RB-SR ISOTOPIC STUDIES

137

line on tile scale employed. We have assumed an error of -b 10%, (about 18.6) for the initial Pb2~ TM which is strongly demagnified in calculating the yerror, and 1% (about 15.6) for initial Pb2~ T M which is slightly multiplied. Total independent y- and x-errors amount to -4- 3.5% and -_k 2.5% respectively, using the above assumptions. Although the error-box just overlaps concordia at 265 m.y., the predominant impression from Fig. 3 is that the zircon data lies significantly below concordia. Extrapolation from the zircon point along a diffusive Pb loss line would intersect concordia at about 400 m.y., which clearly predates the emplacement of the rock. However, the intersection is inaccurate owing to the uncertainty in radiogenic Pb2~ 235, and we conclude that there need be no conflict of the concordia 'age' with the stratigraphic control between 270 and 280 m.y. A number of zircon lead-~, ages of Oslo rocks cited by Neumann (1960) range from about 135 m.y. to 285 m.y.; like the U23s/pb 2~ results, these should be regarded as minimum ages. The available K-At ages are also discordant, from 259 m.y. for biotite in granite near Drammen (Faul et al. 1959) 284 m.y. for biotite in basalt at Sande in Vestfold (Goldich, unpubl.), to 315 m.y. for lepidomelane in net~heline syenite pegmatite from Langesundsfjord (Polkanov & Gerling, unpubl.). P u r p o s e of t h e p r e s e n t s t u d y It was considered that some of the problems and controversies related to the genesis of the monzonitic magma, for example the cosanguinity of the different rock types, particularly the relation between the granite and the other rocks, were amenable to solution from a study of their Rb-Sr isotope geochemistry. A clear definition of the initial SrST/Srs6 ratios of the different rock types within the series was considered to be particularly useful (Oftedahl 1967). This is the first report on whole rock Rb/Sr age determinations within the Oslo province and it is of interest to see how these compare with age estimates from other methods. S a m p l e location a n d description A total of 28 rocks were examined comprising 4 granites, 7 ekerites, 1 ekeritenordmarkite, 4 nordmarkites, 4 larvikites, 6 lardalites, and 2 kjels{,sites. Samples with no letter prescript were from the type-specimens described by Br6gger (1933) and Barth (1945) in the collections of the Geologisk Museum in Oslo. Samples with prescript E were described by Dietrich et al. (1965) and Dietrich & Heier (1967). Samples with prescript S were collecetd and described by Professor H. Wright, Pennsylvania State University and we acknowledge the use of these samples collected by him. His petrographic descriptions and sample localities which are not previously published are included in the appendix, p. 139.

138

K.S. tlEIER & ~,V. COMPSTON

Analytical p r o c e d u r e The rocks were crushed in a steel jaw-crusher and finely ground in a tungsten-carbide Sieb mill. Rb and Sr were determined by X-ray fluorescence spectrography on all samples following the method described by Norrish & Chappell (1967). Analyses were performed with a Philips PW 1540 X-ray spectrograph, operating under the following conditions: X-ray tube: Molybdenum anode (55 kV, 36 mA) Primary collimator: 160 ~tm Analyzing crystal: LiF (200) Detector: Scintillation counter Analytical lines: Rb K~, Sr K~. The precision obtained under the above conditions are given in Table 2. The Rb and Sr concentrations were recalculated into the atomic ratios Rb87/ Sr 86 as described by White et al. (1967) following the measurement of Sr isotopic compositions. Table 2. Precision of the X-ray spectrographic Rb and Sr determinations

Sample

Number of Analyses

ppm Rb

Coefficient of variation for population

ppm Sr

Coefficient of variation for population

G-2 AGV-1 BCR-1 W-1

22 13 12 17

169.2 67.4 47.0 22.2

0.50 0.65 0.69 1.6

471.9 649.7 322.1 187.2

0.34 0.36 0.10 0.33

Sr was also determined by isotope dilution using a Sr 84 spike in rocks of low Sr concentration ( < 3 0 ppm.). The X-ray values are systematically lower by approximately 1 ppm., probably due to bias in making correction for the non-linear background under the Ks Sr peak, and the isotope-dilution values are preferred. Comparative data for higher Sr concentrations (to be reported elsewhere) show that this difference becomes a progressively smaller fraction until, at about 100 ppm, the results by the two methods cannot be distinguished. Unspiked measurements of Sr87/Sr s6 and SrSS/Srs6 were made for all other samples, and for these and the spiked analyses, variable mass-discrimination in Sr87/Sr 86 was corrected by normalising SrsS/Sr 86 to 8.3752. The unspiked Sr87/SrSS were also corrected for 0.05 ~tgms of common St, having Sr87/Sr 8~of approximately 0.705, which is introduced by reagents during chemical processing. The mass-spectrometer used is a 6 inch 90 degree sector-field machine having triple filament source, single Faraday-cup collector and vibrating-reed electrometer with digitized output; isotopic peaks were selected by switching the magnet current. For this machine and using the same techniques, the standard error for a single normalised Sr87/Sr s6 measurement,

RI~-SR ISOTOPIC STUDIES

139

including error introduced during chemical processing, is 3 • 10 -4 for samples having Sr87/Sr86 up to about 0.75 and for runs having high internal precision (A.W. Webb 1968). For a standard K-feldspar with SrST/Sr86 of 1.200, the standard error is significantly greater at 10 • 10 -4.

Presentation of data The analytical data are listed in Table 3, and details of regression analysis for a number of different groupings of the samples are given in Table 4. The regression method employed is described by Mclntyre et al. (1966). The coefficient of variation for RbST/Srs6 due to experimental error was taken as 0.5% for all samples, but in contrast to the example given by Mclntyre et al., the variance for SrST/Srs6 was not taken as uniform. For the samples E 1, E 2 Table 3. Analytical Data. Rb

Sr

ppm

ppm

RbST/Srs6

SrST/Srs6

63 Granite 64 a G A 3198 ~ G A 3199 a

272.4 208.8 248.7 249.9

94.3 130.7 90.2 99.5

8.36 4.62 7.98 7.27

0.7360 0.7215 0.7350 0.7329

160 212 -

61 E-1 E-2 E-3 E-5 E-9 E-10

Ekerite * ~ * * * *

226.5 253.1 275.4 184.7 155.1 91.5 113.9

19.41 164.0 207.6 62.13 15.24 17.40 20.82

0.7778 1.2918 1.4971 0.9447 0.7591 0.7734 0.7852

187 149 125 213 305 427 399

60

Nordmarkite-Ekcrite

E-6 53 47 44

No.

Rock

33.9* 4.7* 4.1" 8.8* 29.5* 15.3" 15.9"

K/Rb

26.1

169.7

0.4440

0.7064

354

Nordmarkite ~ ~ ~

121.3 57.0 136.5 177.5

124.6 21.6" 352.6 468.1

2.81 7.64 1.118 1.095

0.7151 0.7335 0.7084 0.7084

416 951 353 217

42 38 41 40

Larvikite * ~ ~

115.2 95.5 178.4 68.7

782.2 953.9 759.8 983.8

0.425 0.2890 0.678 0.201

0.7059 0.7051 0.7066 0.7044

281 337 190 435

$20 S14 45 $3 $8 $5 34 32

Lardalite ~ ~ ~ ~ ~ Kjelsaasite ~

250.5 131.8 127.9 155.1 82.9 74.1 23.5 88.0

492.2 757.0 777.7 313.3 340.8 44.6 650.9 973.7

1.470 0.503 0.475 1.429 0.702 4.80 0.104 0.261

0.7105 0.7065 0.7059 0.7089 0.7064 0.7221 0.7042 0.7053

174 299 306 306 561 673 400 293

* determined by isotope dilution.

140

K.S. ttEIER & W. COMPSTON

Table 4.

Regression

Number of specimens M S W D

F at 5~

Age (m.y.) ) , = 1.39 x 1 0 - " y r -t

IsoInitial chron SrST/Sr~6 model

28

8.74

1.87

273.0 4-1.4

.7043 4.0002

1

26 25

2.76 1.86

1.89 1.90

276.1 :J: 1.7

1

7

28.48

2.53

269.4-1.7

16 5. Oslo Series specimens except ekerites 4 6. Ekerites except El, E5, E9 4 7. Granites only

1.37

2.02

275.74-6.6

5.11

3.32

2574-7

3.65

3.32

2884-59

.7041 • .0002 .7054.006 .7041 4.0002 .7054.006 .7034.006

1. All specimens including granites 2. All except ekerites E1 and E5 3. All except El, E5, E9 4. Ekerites only

4 1 3 2

and E 3 which are relatively enriched in radiogenic Sr, this variance was taken as 1.0 X 10 -6, as found empirically for NBS standard K-feldspar 70A, and 0.1 X 10 -6 w a s u s e d f o r t h e o t h e r s .

The first regression (Table 4) tests the hypothesis that all the samples analyzed had the same initial Sr87/Sr 86 and that all became chemically closed to Rb and Sr diffusion over a period which is short compared to the experimental precision of age-measurement. An affirmative answer to this hypothesis would be indicated by a value for the Mean Square of Weighted Deviates (MSWD) for the regression which does not statistically exceed unity, i.e. the McIntyre Model 1 fit, ~n which all scatter about the isochron can be assigned to experimental error. In geological terms, the affirmative answer implies that the following situations were simultaneously achieved in the different rock-types: (a) A common magma source for both the principal Oslo rock series and the associated granites. (b) No contamination of any of the samples during emplacement by Sr from the country rock. (c) A time for differentiation of the principal series which is less than about 2 m.y., with the granites also emplaced within this period. (The 95% confidence limits of precision for the age-determination at Model 1 is 1.4 m.y., Table 4). (d) No subsequent disturbance of Rb or Sr in any of the rocks due to later metamorphism or weathering. The negative answer implies that one or more of the above situations is not true. The result obtained, 8.74 for MSWD, is clearly negative. Regressions of smaller groups of the samples are now called for, and the first objective is to check whether one or several samples in particular might be responsible for

RB-SR ISOTOPIC STUDIES

141

most of the excess scatter. If the ekerites E 1 and E 5 are deleted, M S W D is significantly reduced to 2.76 (Table 4) which however still exceeds Model 1 ; if in addition the ekerite E 9 is also deleted, M S W D becomes 1.86 which is a Model 1 fit at the 95% level. The deletion of none of the above 3 samples was anticipated for prior geological or technical reasons. We present, however, the following points a postiori which deserve to be mentioned: (1) All the ekerites were difficult to mass-analyze, possibly owing to their high Rb/Sr and their high rare-earth and zirconium contents. Samples E 1 and E 5 in particular gave rather unstable ion-beam intensities, although the internal variance for their mean SrST/Sr86 did not exceed 1.0• 10 -6. (2) Sample E 9 plots above the isochron indicating (i) higher age, (ii) higher initial SrST/Sr86 ratio, (iii) loss of Rb or gain of Sr sT. A higher age can probably be ignored but a higher initial SrST/Srs6 ratio caused by contamination from the country rocks is a distinct possibility. The plutonic rocks often contain xenoliths of country rocks and on the whole it is rather surprising that contamination with radiogenic Sr is not more evident, particularly in the ekerites with their general low absolute Sr contents. The petrography and geochemistry of these ekerites were described in detail by Dietrich et al. (1965) and Dietrich & Heier (1967). E 9 is closely related to the typical ekerites E 1, E 2 and E 3 in mineralogy and major element chemistry but fits better into the low silica nordmarkite category when the more mobile trace elements are considered. Rb is in this context a 'mobile' element and the K/Rb ratio of E 9 is high (Table 3). Morphologically E 9 is markedly miarolitic and the low content of volatile trace elements could be ascribed to loss of the magmatic vapor phase upon crystallization (Dietrich & Heier 1967). However, if loss of Rb is the cause of the abnormal position of E 9 it must have happened at some time significantly after the crystallization. The minerals occurring in the miaroles are perthitic microcline, quartz, riebeckiticarfvedsonite, aegirin-augite, sphene, zircon, fluorite, pyrite, and the more exotic minerals catapleite, gearksutite, narsarsukite, and zeophyllite. The minerals in the cavities (except perthite) are Ca, Na-minerals where Sr will substitute for Ca and they are expected to contain only small amounts of Rb. Miarolitic rocks will weather more easily than the massive types and a continuous loss of Rb may take place through weathering. Sr liberated together with Rb may be incorporated in the minerals of the cavities. It may be relevant that the ekerite here is full of pegmatite nests (Oftedahl 1953), but E 3 and E 10 which fit the Model 1 isochron are from the same ekerite massif. E 9 was collected on the west side of lake Eikeren, E 3 and E 10 were from the east side. The ekerites as a group definitely contribute most of the excess scatter to the first regression. Regression 4 in Table 4, the ekerites alone, shows M S W D of 28.48 whereas regression 5, the remaining members of the Oslo Series, gives a Model 1 fit. In general terms this was expected on the grounds that the ekerites would be most susceptible to Sr contamination and that, being the I0--

Lithos 2 : 2

142

K.S. tlEIER & W. COMPSTON

youngest members of the Oslo Series, they might be detectably younger than the other samples. If the specimens E 1, E 5 and E 9 are excluded from the ekerite group, regression 6 for the remainder still exceeds the Model 1 fit and the reduction in MSWD from 28.48 to 5.11 is not statistically significant. The granites alone, regression 7, constitute a valid separate group as their relationship to the main Oslo Series is a point in question and they were not included in regression 5. Their fit to the regression line is close to Model 1 but their range in Rb87/Sr sGis too small to allow the independent measurement of an age and initial Sra7/Sr 86with much precision. Discussion The most conservative estimate for the Rb-Sr whole-rock age of the Oslo Series is given by regression 5, Table 4, in which the ekerites as a group are deleted. The result is 276-4-7 m.y. ( ~ = 1.39 • 10-11 yr-1), in good agreement with the stratigraphic age of lower Permian. However it should be noted that the uncertainty of 4- 7 m.y. refers only to the internal precision of the data, and whatever systematic error exists in the Rb 87 half-life employed, 5.0 • 101~years, and in the analytical determination of RbS7/Srs6 by X-ray fluorescence spectrography, will be additional. An alternative approach is to group together all specimens, except for 3 ekerites on the qualitative grounds already described, and including the 4 granite specimens. The age is unchanged at 276 m.y. (regression 3, Table 4) but the internal precision is considerably improved to ~ 1.7 m.y. No selective contamination or difference in the source or the age of the granite is detected. The ekerites are either slightly contaminated by Sr from the countryrock, both radiogenic and non-radiogenic Sr (Model 3 regression) or their chemistry has been slightly disturbed after emplacement but they are not detectably different in age to the main Oslo Series (Table 4, regressions 4 and 6). An interestingly short time interval is suggested for the crystallization of the plutonic rocks which range from 50-77#/0 SiOz. This point was also emphasized by Oftedahl (1967) for the volcanic series. This short active period is of general interest for considerations about the development of rift valleys. In the Oslo area this similarity in age of the plutonic rocks is supported by their field relations. Though the more basic rocks can be shown to be the oldest there is a considerable extent of overlap as indicated by 'simultaneous' contacts, BriSgger (1898), Oftedahl (1960). Two rocks show a common boundary surface without any contact relations such as chilled borders, apophyses, or contact metamorphism. Such contacts are observed between larvikite/lardalite, larvikite/basic nordmarkite, and akerite/nordmarkite (Oftedahl 1960). The initial S r 8 7 / S r 86 ratio of 0.7041 4- 0.0002 is well defined and puts restrictions on hypotheses for the origin of the magma. It is within the range previously reported by Czamanske (1965) on two rocks from the Finnmarka granite pluton. The most likely source of magma with this SrSW/Sr86 ratio is

RB-SR ISOTOPIC STUDIES

143

the upper mantle, but a deep crustal origin cannot be excluded because of lack of knowledge of the Rb/Sr ratio in this material (see also Oftedahl 1967). Contamination from the Precambrian granites and gneisses, and the Palaeozoic sediments into which the plutonic rocks were extruded may safely be ruled out. The short period of magmatic activity and the sequence with the basic rocks being the oldest indicate that the rocks are related through magmatic differentiation rather than through differential melting in the source region. This statement need only be modified if a large segment of the deep crust and upper mantle with different chemical composition is found to have identical, isotopic compositions. The position of the granite relative to the Oslo series rocks has not been ascertained by this study.

ACI,~NOWLEDGE*IENT. -- We thank Professor T.F.W. Barth, Professor Chr. Oftedahl, and Mr. J.A. Dons for critically reading the manuscript, and Mr. M.J. Vernon for the chemical preparations of the samples.

APPENDIX

Petrographic data of analysed samples (Table 3) For samples with no letter prescript see descriptions by BrSgger (1933) and Barth (1945). For samples with prescript E see Dietrich et al. 1965; Dietrich & Heier 1967. T h e samples with prescript S were collected and described by Professor H. VCright, Pennsylvania State University. T h e samples marked G A are from.collections of Mineralogical Geological Museum, Oslo. G A 3198, GA 3199. Granite; Heggedal, R6yken. Quartz ; alkalifeldspar patch perthite, serisitized, albite ; biotite and sphene. S 3. Lardalite, 500 m north of Uroa. 88% Alkalifeldspar in irregular crystals, perthitic, generally untwinned but with a few patches of microcline twinning and occasional plagioclase lamellae showing albite twinning. Indices range downward from a little below 1.537. (-) 2 V N 8 0 ~ A few crystals show plagioclase predominating over potassic feldspar. < 1 % Albite in interstitial crystals. < 1 % Olivhte. 5% Augite colorless, ( + ) 2 V = 6 0 ~ 4 % Biotite. 1% Apatite. 2% Opaque. Feldspar crystals up to 1 cm. long, augite and biotite crystals several mm. across. No nepheline is found in thin section, but the rock is considered to be a lardalite. S 5. Lardalite, south of road to Nau at Aaserumsvatn, no thin section available. S 8. Lardalite at contact with larvikite, Aaserumsvatn. 88% Alkali feldspar, perthitic, plagioclase bands well twinned (albite law). Crystals pronouncedly tabular, some Carlsbad-twinned. Refractive indices of the potassic m e m b e r are below 1.537 and those of plagioclase member range downward from about 1.537; maximum extinction angle on albite twins is 18 ~ indicating plagioclase m e m b e r is albite. In some crystals albite appears to predominate over potassic feldspar. Continuous gradation observed in ratio of the two. 1% Albite in very clear, interstitial crystals. 5% Biotite medium brown, 2 V ~ 6 0 ~ 2% Augite largely altered to calcite, biotite. < 1 % Sphene. 1% Apatite. 2% O baque. < 1 % Calcite, alteration.

144

K.S. IIEIER & W. COMPSTON No nepheline or quartz found. Feldspar tablets with flat, very regular outlines, crystal length few m m to 2 cm Miarolitic texture.

S14. Lardalite, Lunde. 83% Alkali feldspar is patch-perthitic, indices all below 1.537. Plagioclase lamellae show albite twinning in places. Perthitic texture relatively coarse. 5% Nepheline. 3~ Biotite, deep red brown, 2 V = 1 2 ~ 3 % Olivhte(-)2V~80 ~ somewhat serpentinized. 30/0 Augite. 1% .4patite. 2% Opaque. Feldspar tablets are 1-2 cm long, biotite crystals up to 89cm across. Augite and olivine are in clusters of smaller crystals. $20. Lardalite just west of Heum. 72% Alkali feldspar, in irregular crystals, perthitic, potassic lamellae very finely twinned in places; some of plagioclase lamellae show albite twinning. Indices of potassic portion all below 1.537; those of plagioclase member range downward from very slightly above 1.537. A few crystals show plagioclase as predominant. 10~'o Nepheline. 1% Pyroxene. 2% Hornblende. 12% Biotite very dark brown, 2 V = 1-2 ~

1% SpheRe. 1% Apatite. 10/'o Opaque. Feldspar ct3"stals up to 189 cm long, biotite crystals 89cm across. Dark minerals somewhat segregated. Small rounded nepheline crystals scattered through feldspar in addition to large nepheline crystals.

Mbzeralogisk-Geologisk l~hzseum Sarsgt. 1, Oslo 5. Norway (K.S.H.) Dept. of Geophysics and Geochemistry Australian National University Canberra, (IV.C.)

REFERENCES BARTH, T.F.'~V. 1945: Studies on the igneous rock complex of the Oslo region. If. Systematic petrography of the plutonic rocks: Skrifter Det Norske Vid.-Akad. i Oslo. I. l]lat.Naturv. kl. 1944, No. 9, 103 pp. BARTH, T.F.W. 1954: Studies on the igneous rock complex of the Oslo region. XIV. Provenance of the Oslo magmas. Skrifter Det Norske Vid.-,4kad. i Oslo. I. i~lat.-Naturv. kl. 1954, No. 4, 20 pp. BRiSOGER, W.C. 1898: Die Eruptivgesteine des Kristianiagebietes. III. Die Ganggefolge des Laurdalits. Vid. Selsk. Skr. 1897. BR/SCCER, W.C. 1933: Same series. VII. Die chemische Zusammensetzung der Eruptivgesteine des Oslogebietes. Skrifter Det Norske Vid.-Akademi i Oslo I. l]lat.-Naturv, kl.

1933, No. 1. CZAMANSKE, G.K. 1965: Petrologic aspects of the Finnmarka igneous complex, Oslo area, Norway. flour. Geol. 73, 293-322. DIr'TntCH, R.V., HmER, K.S. & TAYLOR, S.R. 1965: Studies on the igneous rock complex of the Oslo region. XX. Petrology and geochemistry of ekerite. Skrifter Det 1Vorske Vid.Akad. i Oslo. I. i~lat.-Naturv, kl. 1965, No. 19, 31 pp. DIzrRIcH, R.V. & HEIER, K.S. 1967: Differentiation of quartz-bearing syenite(nordmarkite) and riebeckitic arfvedsonite granite (ekerite) of the Oslo series. Geochhn. et Cosmochbn. Acta 31, 275-80. FAUL, H., ELMOI~, P.L.D. & Bro,N,~OCK, W.W. 1959: Age of the Fen carbonatite (Norway) and its relation to the intrusives of the Osto region. Geochbn. et Cosmochim. Aeta 17, 153-5. KuLr', J.L. 1961: Geologic time scale, Science 133, 1105-1114. ~ICINTYRE, G.A., BROOKS, C., COMPSTON,W. ~ TUREK, A. 1966: T h e statistical assessment of R b - S r isochrons, your. Geophys. Res. 71, 5459-68. NEOMAI~N, H. 1960: Apparent ages of Norwegian minerals and rocks. Norsk Geol. Tidsskr. 40, 173-91.

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NIER, A.O. 1939: T h e isotopic constitution of radiogenic leads and tile measurement of geological time. II. Phys. Rev. 2rid set. 55, 153-63. NORRISII,K. & CtIAPPELL,I].W. 1967: X-ray fluorescence spectrography. In J. Zussman (editor). Physical methods in determhlative mhzeralo(y. Academic Press, London. OFTEDAtlL, C. 1953: Studies on the igneous rock complex of the Oslo region. X I I I . T h e cauldrons. Nkrifter Det Norske Vid.-,'tkad. i Oslo. I i)fat.-Naturv, kl. 1953, no. 3, 108 pp. OFTEDAIIL,C. 1959: Volcanic sequence and magma formation in the Oslo region. Geol. Rundsehau 48, 18-26. OFTEDAttL, C. 1960: Permian rocks and structures of the Oslo region. In O. Holtedahl(editor) Geology of Norway. Norges Geol. Unders. 208, 298-343. OFTEDAHL,C. 1967: M a g m e n - E n t s t e h u n g nach Lava-Stratigraphie im siidlichen OsloGebiete. Geol. Rundscl, au, 57, 203-18. WEBB, A.W. 1968: Unpublished Ph.D. dissertation. T h e Australian National University, Canberra, A.C.T. WroTE, A.J.R., CO.XtPSTO.'r W. & KLEE.MAN, A.W. 1967: T h e Palmer granite - a study of a granite within a regional metamorphic environment. Jour. Petr. 8, 29-50.

Accepted for publication September 1968

Printed April 1969