The early evolution of the Southwest Swedish Gneiss Province: geochronological and isotopic evidence from southernmost Sweden

Precambrian Research, 64 ( 1993 ) 361-388

361

Elsevier Science Publishers B.V., Amsterdam

The early evolution of the Southwest Swedish Gneiss Province: geochronological and isotopic evidence from southernmost Sweden Ake Johansson a,*, Martin Meier b, Felix Oberli b and Hugo Wikman c aLaboratoryfor Isotope Geology, Swedish Museum of Natural History, Box 50007, S- 104 05 Stockholm, Sweden blnstitute for Crystallography and Petrography, ETH-Zentrum, CH-8092 Zfirich, Switzerland CGeologicalSurvey of Sweden, Kiliansgatan 10, S-223 50 Lund, Sweden Received June 24, 1991; revised version accepted October 26, 1992

ABSTRACT The continental crust in southwestern Sweden has a complicated history of formation, apparently commencing ~ 1.75 Ga ago and continuing until the end of the Sveconorwegian orogeny ~ 0.9 Ga ago. Most of the rocks are polymetamorphic gneisses, but massifs of intrusive rocks can commonly be recognized. This study reports new U - P b zircon ages from the province of Sk~ne in the southernmost part of the region. Two gneiss samples and four samples of granitoid rocks were dated by the U - P b zircon method. In the two gneiss samples, selected single zircon crystals were analyzed in addition to the standard multi-grain size fractions. The multigrain zircon analyses yielded an upper intercept age of 1613 + 6 Ma for a grey gneiss from the central part of the province of Sk~ne, and 1557_+,372Ma for a red aplitic gneiss from southern Sk~ne. Individual zircon crystals from these samples gave 2°7pb/2°6pb-ages between 1335 and 1669 Ma. For the grey gneiss, the individual zircons define a separate discordia with an upper-intercept age of 1640 + 16 Ma; in the red gneiss sample they scatter around a line with an upper intercept at 1675+25 Ma. Core-bearing zircons from a red gneissic granite in central Sk~me yielded an age of 1575_+77 Ma, while similar zircons from a strongly foliated red granite from the Kullaberg horst in northwestern Sk~tne gave an age of 1497_+447Ma. Core-free zircons from the Beden granodiorite in southern Sk~ne have an age of 1449_+ 23 l mMa, and core-bearing zircons from a gneissic charnockite near ~rkelljunga in northern Sk~ne yielded an age of 1452_5o +35o Ma. Although the latter age is poorly defined, it allows for similar intrusion ages of the Orkelljunga charnockite and the Beden granodiorite. Sm-Nd isotopic analyses yield TDM ages of 1.84 to 1.99 Ga, and TCHURages of 1.48 to 1.60 Ga for these rocks, while Rb-Sr analyses give TUR ages between 1.46 and 1.71 Ga. The combined evidence suggests that the main crust-forming episode in this part of southwestern Sweden occurred 1.6 to 1.7 Ga ago, with presumably subduction-related magmatism forming the protoliths of the red and grey gneisses. Around 1.5 to 1.6 Ga ago red anatectic granites were generated. The Beden granodiorite and the ~rkelljunga charnockite, as well as the Varberg charnockite farther north, were all formed ~ 1.45 Ga ago, possibly by mantle-generated anorogenic magmatism. Some 0.9 Ga ago, after the Sveconorwegian crustal thickening and high-pressure metamorphism, uplift within the Southwest Swedish Gneiss Province exposed lower- to midcrustal granulitic and charnockitic rocks in its southern part.

1. Introduction

The Precambrian of southern and central Sweden can be divided into three parts (inset, Fig. 1): the ~ 1900-1750 Ma old Svecofennian Province in the east, the ~ 1800-1650 Ma *Corresponding author.

0301-9268/93/$06.00

old Transscandinavian (Granite-Porphyry) Belt in the centre, and the ~ 1750-900 Ma old Southwest Swedish Gneiss Province in the west. The latter forms part of the Southwest Scandinavian Domain (Gahl and Gorbatschev, 1987), which also includes the Precambrian crust of southern Norway. Because this domain has been affected by Sveconorwe-

© 1993 Elsevier Science Publishers B.V. All rights reserved.

SSD10301-9268 (93)EO070-S

362

gian-GrenviUian reworking, it is also known as the Sveconorwegian Province (e.g. Park et al., 1991 ), and is regarded as the Scandinavian counterpart of the Grenville Province of eastern Canada (e.g. Gower and Owen, 1984; Gower, 1985). The boundary between the Southwest Swedish Gneiss Province and the Transscandinavian Belt is a major intraplate tectonic zone, known as the Protogine Zone (Gorbatschev, 1980; Ga~il and Gorbatschev, 1987). Several other N-S trending tectonic zones (the Mylonite Zone, the G6ta Alv Zone, the Dalsland Boundary Thrust; cf. Berthelsen, 1980; Gorbatschev, 1980; Gafil and Gorbatschev, 1987; Park et al., 1991) divide the Southwest Swedish Gneiss Province into different crustal segments. Blekinge (Fig. 1 ) can also be considered part of the Southwest Swedish Gneiss Province, despite its location east of the Protogine Zone (Johansson and Larsen, 1989 ). However, the older Blekinge granitoids

A. JOHANSSON ET AL,

may also be a southerly, more deformed continuation of the Transscandinavian Belt granitoids. Southwestern Sweden has a long and complex geological evolution, apparently commencing ~ 1.75 Ga ago during the Gothian orogeny (Ga~ll and Gorbatschev, 1987) and continuing until the end of the Sveconorwegian orogeny ~0.9 Ga ago. Considerable efforts have been made to date and decipher the older history of the northern part of southwestern Sweden (e.g. Welin and K~ihr, 1980; Welin et al., 1982; Persson et al., 1983, 1987; Welin and Samuelsson, 1987; Hansen et al., 1989; Ah~ill and Daly, 1989;/~h~ill, 1990;/i,h~ill et al., 1990). Part of this work has attempted to identify an older, possibly Svecofennian, bedrock into which the westernmost part of the 1.65-1.8 Ga old Transscandinavian Belt granitoids could have intruded. However, no ages older than 1.8 Ga have so far been found. The

E <06Go 09GG

I 2Go 12GQ 12Oa 1 35-I ~ Oa

145

16Ga ~1 6Ge 1 7Ga

Fig. 1. Geological map of SkAne, southernmost Sweden, simplified after Wikman and Bergstr6m (1987) and Kornf'~ilt and BergstriSm (1990), showing sample locations. Inset map shows the main geological divisions of southern and central Sweden. PZ= Protogine Zone; M Z = Mylonite Zone; S L M = Stora Le-Marstrand Belt; TGPB= Transscandinavian Granite-Porphyry Belt; V=Varberg.

THE EARLY EVOLUTION OF THE SOUTHWEST SWEDISH GNEISS PROVINCE

only ages older than 1.7 Ga are from a granitic gneiss believed to be a deformed Transscandinavian Belt granite ~t,1777_+lgl l I,,~,~AQU-Pb; Welin and K ~ r , 1980), and from a mafic metavolcanite ( 1758 ___78 Ma, Sm-Nd; ]~la~illand Daly, 1989). It may be, however, that the intense and repeated metamorphism and deformation in southwestern Sweden have both reset the U Pb systems of the zircons and the Rb-Sr wholerock ages (cf. Welin et al., 1982; Hansen et al., 1989; Ah~ill, 1990). In addition, movements along the Protogine Zone have largely obscured the original relationships between the commonly massive granitoids of the Transscandinavian Belt and the gneisses of the Southwest Swedish Province. In the southern part of southwestern Sweden only limited geochronological work has been carded out, mainly centred on the Varberg charnockite, where a Rb-Sr age of 1420___52 Ma was obtained by Welin and Gorbatschev (1978; recalculated to the new decay constant). The significance of the so-called "Hallandian" event at 1.4-1.5 Ga (Hubbard, 1975; cf. also G a ~ and Gorbatschev, 1987 ) has been a matter of debate. New Sm-Nd ages on mafic granulites from the granulite area south of Varberg suggest that this high-grade metamorphism is Sveconorwegian ( ~ 0.9 Ga; Johansson et al., 1991 ) rather than Hallandian. The purpose of the present study was to obtain additional U - P b zircon ages from the previously little known southernmost part of the Southwest Swedish Gneiss Province (Fig. 1 ) and to discuss their implications for the geotectonic evolution of southwestern Sweden. The area investigated belongs to the eastern segment of the Southwest Swedish Gneiss Province, i.e. the area between the Protogine and the Mylonite Zones (cf. Lindh, 1987; Park et al., 1991 ). It is characterized by high-grade, partly granulitic metamorphism (Johansson et al., 1991 ). In its southernmost part, in the province of Skhne, the Southwest Swedish Gneiss Province is transected by NW-trending Phanero-

363

zoic faults of the Tornquist Zone (the Fennoscandian Border Zone) and is overlain by Phanerozoic sedimentary rocks. The Precambrian rocks reappear in upfaulted blocks, such as the Kullaberg, S/Sderhsen, and Romele~tsen horsts (Fig. 1 ). The bedrock geology of Kullaberg has been described by Forsell ( 1962 ) and by Norling and Wikman (1990), and that of Romelehsen by Hjelmqvist (1934). An overview of the geology of the whole area is given by Wikman and Bergstr/Sm (1987a). One of the samples studied, a red gneissic granite (84093), is from the Kullaberg horst, and two samples, a red aplitic gneiss (85015 ) and a granodiorite (85016), come from the Romelehsen horst. A grey gneiss sample of intermediate composition (85017) and a sample of coarse-grained red gneissic granite (85018) are both from central Skhne, while one sample, a charnockitic syenite (85019), is from northern Sk/ine (Fig. 1 ). Sample coordinates and descriptions are given in Appendix I, and chemical compositions in Appendix II. A description of the analytical procedures used is given in Appendix III. 2. Results of multi-grain U-Pb zircon analyses The results of the U-Pb analyses on multigrain size fractions of zircons carded out at the Museum of Natural History in Stockholm are reported in Table 1, and the ages obtained are summarized in Table 2. Photomicrographs of zircons are shown in Fig. 2, and U-Pb data are plotted in conventional concordia diagrams in Fig. 3. In the following, the results are discussed in order from older to younger rocks.

85017." Grey gneiss, Vdgasked, central Skdne Sample 85017 is derived from the oldest rock unit in this part of southwestern Sweden, which forms an extensive complex of highly deformed and partly banded, relatively finegrained grey and red gneisses. These gneisses comprise rocks of supracrustal as well as plu-

364

A. JOHANSSONET AL

TABLE 1 U - P b data for zircons from southwest Sweden No.

Fraction a

Weight (mg)

U (ppm)

Pb-rad (ppm)

Pb-com (ppm)

2°6pb/2°4pbb

84093:M611egranite, Kullaberg 1 NM > 150 2 NM 106-150 3 NM 74-106 4 NM 45- 74 5 N M + M <45 6 M > 150 7 M 106-150 8 M 74-106 9 M 45- 74

18.93 17.92 22.60 18.98 14.81 16.47 19.43 17.53 21.83

806 818 790 786 902 911 919 919 916

171 174 171 168 183 179 185 183 180

0.7 0.6 0.4 0.7 1.2 1.1 1.1 1.2 1.2

13850 16270 20400 14280 8600 9870 10200 9080 8750

85015:Stenbergetredgneiss, Romelehsen 1 M <74 2 M > 74 3 NM <45 4 NM 45- 74 5 NM 74-106 6 NM > 106

16.34 15.40 16.50 15.86 20.36 12.75

996 852 912 842 779 754

172 151 180 168 150 144

6.6 5.6 5.5 3.9 3.2 4.5

1590 1640 1980 2560 2790 1900

6.79 6.58 7.65 5.90 4.36

346 333 326 344 344

82 79 79 83 85

0.4 0.1 0.1 0.1 0.1

7090 10280 12040 9710 9720

640 432 366 316 284

125 93 82 74 69

4.6 1.6 1.8 1.8 4.2

1620 3260 2570 2360 970

5.68 3.28 3.38 2.71 3.79 4.52

1315 1487 1558 1710 1973 1624

239 264 266 290 333 254

4.9 7.6 8.3 7.8 10.4 7.9

2900 2030 1890 2200 1900 1900

85019: OrkelOungachamockite, n o ~ h e m S k h n e 1 NM > 150 2.26 2 NM 106-150 2.56 3 NM 74-106 2.06 4 NM 45- 74 5.17 5 NM <45 4.13 6 M > 106 3.59

370 436 467 440 453 413

84 94 106 101 I01 91

85016: Bedengranodiofite, Romele~sen 1 NM > 150 2 NM 106-150 3 NM 74-106 4 NM 45- 74 5 NM <45 85017: V~gaskedgreygneiss, centralSkhne 1 NM <45 2 NM 45- 74 3 NM 74-106 4 NM 106-150 5 NM > 150 -85018: Sk~iralid granite, central Skhne 1 NM > 150 2 NM 106-150 3 NM 74-106 4 NM 45- 74 5 NM <45 6 M > 150

25.3 19.5 24.2 24.2 23.0

0.1 0.03 0.2 0.1 0.3 1.0

14600 24900 13710 33870 16250 5200

THE EARLYEVOLUTIONOF THE SOUTHWESTSWEDISHGNEISSPROVINCE

365

Error ¢ corr.

2°Tpb/235Ud _+2tr

2°6pb/23SUd _+2a

2°Tpb/2°6pbd _+2a

2°Spb/206pbd

207pb/206pb age

0.714 0.792 0.748 0.722 0.560 0.648 0.663 0.580 0.739

2.678+ 9 2.688 -+ 8 2.729 -+ 7 2.692 + 8 2.509 + 13 2.468 + 9 2.526 + 11 2.490_+ 11 2.441+ 6

0.2146+4 0.2153 -+4 0.2178 -+4 0.2149 + 4 0.2045 + 4 0.1999-+ 4 0.2042 + 5 0.2016+4 0.1978-+3

0.0905+2 0.0906 + 2 0.0908 + 2 0.0908 + 2 0.0890 + 4 0.0895 + 2 0.0897 _+3 0.0896+ 3 0.0895-+2

0.0571 0.0578 0.0629 0.0666 0.0659 0.0567 0.0565 0.0604 0.0640

1436 1437 1444 1443 1404 1416 1420 1417 1415

0.891 0.889 0.796 0.918 0.933 0.875

2.160+ 2.236_+ 2.503+ 2.536-+ 2.454_+ 2.424_+

8 6 8 9 9 7

0.1675-+5 0.1727+4 0.1924+5 0.1944-+6 0.1881 +6 0.1859+5

0.0935+2 0.0939+ 1 0.0943+2 0.0946-+ 1 0.0947+ 1 0.0946+ I

0.1086 0.1055 0.0976 0.1012 0.1023 0.1032

1499 Ma 1506 Ma 1515 Ma 1520Ma 1521 Ma 1519 Ma

0.567 0.585 0.654 0.619 0.662

2.985 + 14 2.992 -+ 14 3.009+ 9 3.001 + 11 3.050-+ 9

0.2379 + 4 0.2382 -+ 5 0.2396+4 0.2384+4 0.2427+4

0.0910 + 4 0.0911 + 4 0.0911 +2 0.0913+ 3 0.0911 +2

0.0736 0.0742 0.0810 0.0852 0.0960

1447 1448 1448 1453 1450

Ma Ma Ma Ma Ma

0.892 0.816 0.921 0.832 0.855

2.307+ 2.588+ 2.729+ 2.846+ 2.959+

6 6 6 8 9

0.1863+4 0.2034+4 0.2124+4 0.2194+ 5 0.2266+6

0.0898+ 1 0.0923+ 1 0.0932+ 1 0.0941 + 1 0.0947_+2

0.1308 0.1354 0.1368 0.1426 0.1570

1421 1473 1492 1510 1523

Ma Ma Ma Ma Ma

0.550 0.774 0.518 0.536 0.665 0.894

2.297+ 2.200 + 2.117+ 2.100+ 2.095 + 1.915+

10 8 10 11 7 9

0.1794+2 0.1740 + 5 0.1676+2 0.1669+3 0.1668 + 3 0.1537+6

0.0928+4 0.0917 + 2 0.0916+4 0.0913+4 0.0911 + 2 0.0904+2

0.0846 0.0964 0.0933 0.0890 0.0866 0.0944

1484 1462 1460 1452 1449 1433

Ma Ma Ma Ma Ma Ma

0.616 0.590 0.555 0.701 0.589 0.582

2.845 + 2.720+ 2.874+ 2.946 + 2.857+ 2.760 +

11 11 19 10 17 13

0.2305 + 5 0.2192+4 0.2320+ 5 0.2349 + 5 0.2273+ 5 0.2232 + 4

0.0895 + 3 0.0900+3 0.0898 + 5 0.0910 + 2 0.0912+4 0.0897 + 3

0.0502 0.0522 0.0513 0.0508 0.0534 0.0622

1415 1426 1422 1446 1450 1419

Ma Ma Ma Ma Ma Ma

Ma Ma Ma Ma Ma Ma Ma Ma Ma

a Fraction: NM=non-magnetic at 1.6 A and 3-5 ° side slope in the Frantz Isodynamic Separator; M = magnetic at the same conditions; size fractions in lzm. b Uncorrected ratio. c Error correlation 2°7pb/235U- 2O~pb/23sU. dlsotope ratios corrected for fractionation (Pb 0.12% per AMU; U 0.01-0.033% per AMU ), blank (Pb 0.13-1.6 ng; U 0.10 ng), and common lead (2°6pb/2°4pb = 15.95-16.25; 2°7pb/2°4pb = 15.35-15.40; 2°spb/2°4pb = 35.60-35.90).

366

A. JOHANSSON ET A L

TABLE 2 S u m m a r y o f n e w U - P b zircon ages f r o m s o u t h w e s t Sweden Sample

Fraction

U p p e r intercept age ( M a )

Lower intercept age ( M a )

MSWD

Model a

84093

1-9

1497 _+47

421-157+ 15o

4.9

84093

5 excl.

1488__+9

370 + 37

1.8

1

85015

1-6

l. q. .g. 7 + 327 2

7.0

2

85016

1-5

i,t4o . . . . +23 11

+88 150_90 +5Ol 0-493

0.8

1

85017 85018 85019

1-5 1-6 1-6

1613+6 1 . .q7 . . ~ +77 61 +347 1452--47

540__ + 13 119 291 _+ 128 +813 317_966

1.5 4.9 20.7

1 2 2

2

a Model 1: internal errors b a s e d on weighted error propagation. Model 2: external errors based on equal weights.

tonic derivation, the latter seemingly dominating (Wikman and Bergstrtim, 1987a, b; Wikman and Sivhed, 1993 ). The sampled rock is a grey, fine- to medium-grained, relatively homogeneous gneiss with a marked lineation, outlined by strongly attenuated feldspar megacrysts. Superficially, it bears resemblance to the "coastal gneisses" of Blekinge, although it is located west of the Protogine Zone. In the total alkali versus silica diagram (TAS; Le Maitre, 1984), the sample plots at the boundary between syenite and granodiorite. In the alkalinity classification diagram (log CaO/ (Na20 + K20 ) v e r s u s S i O 2 ; Peacock, 1931 ), it is calc-alkaline, and in the Rb versus Y + Nb diagram (Pearce et al., 1984), it plots within the field of volcanic arc granites. The zircons (Fig. 2D) are buff to brown, ranging from clear and almost euhedral crystals to rounded, turbid and fractured ones. They are often rich in inclusions, but seem to lack cores. Five non-magnetic size fractions were analyzed; they have low to intermediate uranium contents of 280-640 ppm, 1.6-4.6 ppm common lead, and 69-125 ppm radiogenic lead. In the concordia diagram (Fig. 3D ), they plot rather discordantly, yielding an upper-intercept age of 1613 + 6 Ma. The nominal age of 1613 Ma suggests that the grey gneiss is substantially younger than the ~ 1700 Ma old V~istanh metavolcanics and the "coastal gneiss" of Blekinge (Johansson and

Larsen, 1989), and the 1750-1810 Ma old Smhland granitoids (Patchett et al., 1987; Jarl and Johansson, 1988; Mansfeld, 1991 ). However, the possibility that the age has been metamorphically reset cannot be excluded, and it is best regarded a minimum estimate. The single-zircon U-Pb results reported below demonstrate that the U-Pb systematics of the zircon population are more complex than indicated by the multi-grain analyses.

85015: Red gneiss, Stenberget, Romelettsen, southern Skdne Sample 85015 is a red, weakly foliated, fineto medium-grained, leucocratic and aplitic rock of uncertain origin. Similar red aplitic gneisses dominate the Romelehsen horst, and have been interpreted by Hjelmqvist (1934) as hypabyssal intrusives. However, in the Stenberget area, the bedrock is dominated by a younger, medium-grained granite, referred to as the "Romele granite" by Hjelmqvist (1934), who compared it to the Spinkam~a-type granites of Blekinge (1.35-1.4 Ga old; Aberg, 1988, and references therein). Such a correlation was supported by Welin and Blomquist (1966), who obtained an upper-intercept age of 1395 Ma (recalculated to new decay constants) for a U-Pb discordia combining pegmatite minerals from Stenberget and Blekinge. Our sample comes from the southwestern part of the

367

THE EARLY EVOLUTION OF THE SOUTHWESTSWEDISHGNEISS PROVINCE

. 84093

D.

85017

E.

85018

d

i B.

85015

C.

85016

F.

85019

Fig. 2. P h o t o m i c r o g r a p h s o f zircons from samples 84093-85019. Horizontal w i d t h A: 0.54 m m ; B-F: 0.7 m m .

large quarry at Stenberget, where a light-red, fine-grained aplitic rock is present. Hjelmqvist (1934) interpreted this lithology as related to the Romele granite, but a connection with the older aplitic gneisses appears equally plausible. The sampled rock is dominantly red and very leucocratic, but diffuse, 1-5 cm large grey patches are sometimes present. According to the TAS classification, the sample has a granitic composition, and in the alkalinity diagram

it plots close to the boundary between the calcalkaline and alkali-calcic fields. In the Rb versus Y + Nb diagram, it plots within the field of volcanic arc granites, but close to that of syncollisional granites. According to Pearce et al. (1984), this boundary is not very well defined for Precambrian granites; Rb mobility would also impair the discrimination between volcanic arc and collision-related granitoids. The zircons (Fig. 2B) are brown to buff,

368 A. J O H A N S S O N E T AL.

A ' 206pb/238u

.

.

.

.

.

.

.

.

.28

.26

84093 .24

B

2O6pb/2~% .

15 0 ~ . J "/" . ~ I

MSWD - 4.9

f

~

I ~ / --~4- MO 1: NM >150 2: NU 106-150 74-106 45-- 74 <45 6: M >15o ~: ~ 1o6-!~o

//

1300J

~

~,v

.22

~

4 j,~

1200

~

-~y./

/,

s,,,/7

3: NM 4: NM 5: NM+M

-" ~_ 3

2

'

85015 Stenberget red g n e i s s

/

.20 1000~

.20

....

/"~UU /

~ 7

i

....

/

__

+32 ..

.24

/ ,"~.3 4 //~5 ° / o

J

2: 3: 4: 5:

M >74 NM <45 NM 4 5 - 74 NM 74-106

.16

/ 2.0

2.4

2.8

3.2

2

3

C

.26

D

2OB'pb/238u 8501 6 .25

4

145y ./~l

B e d e n granodiorite

MSWD = 0.8

1400 ~ " J / /

'

1 6 o o ~

85017 Vagasked grey gneiss

.28

~ ; j /

1613 ± 6 Ma

1449 +_23 Mcl .24

.24

S

,~

_75 2 ~

~

/

1:NId>150

5

4

./

2: NM 106-150

3: NM 74-106

/

.'4

120 ~ 7 :Uy/~" y~",/j"

.20

1: NM <4-5

,./_ 47"" ~ 2

NM 74-106 4: NM 106--150 5: NM >150 3:

.23 2.8

3.0

3.2

2

3

4

E 2o

.,238o

'

,

.26

'

.24

85018 Skt~ralid granite MSWD= 4.9

j

1200J J J

,

,

,

1450

85019 1400 J f ' ~ , ~ , ' + 7 7 . , f f ~ 1575 '-61 Mo

/

J

1: NM>150 NId 106--150 ~: NU 74-1o6 4: NM 45-- 74 5: NId <45 6: M >150

~

Orke,ju~g~

..~ / 1000 / _ " 1 -~/" 4 ~ " , 2

-7,~

Mswo = 20.7

j I ~

"~nn ~

l ~ j ~"

.22

/ .0

~0

/

/

135y~,/

2:

J

"

14oo ~ J

charnockite .24

.20

.

206pb/238U

,28

/

I ....

-t-347 ,,

I'~OZ --47 --, MQ

/

~/ 4

1: NM >150

3

NM 106--150 3: NM 74-106

5

~

2:

4: NM 5: NM

4s- 74 <45

.16

/ 2

3

4

, 2.4

~ 1 7

+8,13/--966 M,a 2.8

207pb/235 U 3.2

Fig. 3. Concordia diagrams for conventional multi-grain analyses of standard size fractions of zircon. The data points are represented by their error ellipses of the 2a level; numbering refers to Table 1 (NM= non-magnetic, M= magnetic fraction, grain size in micrometres). Note the different scales employed in the diagrams. MSWD=mean square of weighted deviates. (A) 84093; (B) 85015; (C) 85016; (D) 85017; (E) 85018; (F) 85019. rounded and mostly turbid, fractured and rich in inclusions and diffuse cores, signifying a complex multi-stage history. The cores domi-

nate over the overgrowths in volume. Two magnetic and four non-magnetic size fractions have been analyzed. They contain 7 5 0 - 1 0 0 0

THE EARLYEVOLUTIONOF THE SOUTHWESTSWEDISHGNEISS PROVINCE

ppm uranium, 3.2-6.6 ppm common lead, and 144-180 ppm radiogenic lead. In the concordia diagram (Fig. 3B), the rather discordant data define a line yielding an upper-intercept g~7 -27 +32 Ma. age of 1.,., Considering the uncertainty regarding the origin of the sampled rock and the complex character of the zircons, this age result is difficult to interpret. The rock could belong to the older complex of red and grey gneisses with a true age around or above 1600 Ma (cf. sample 85017) that has been lowered somewhat due to metamorphic overgrowths (cf. Hansen et al., 1989). However, the rock could also belong to the ~ 1.4 Ga old suite of "anorogenic" granites, with the apparent U-Pb age having been increased substantially due to the presence of zircon cores. Core-bearing zircons are common in the "anorogenic" granites of southeastern Sweden, particularly in the Vhnga granite, where they substantially affect the U - P b age (Aberg et al., 1985b; Aberg, 1988 ). The singlecrystal results presented below clearly support the former hypothesis, revealing the complex nature of the zircon population.

85018: Gneissic granite, Skiiralid, central S1Cme The gneissic granite at Sk~iralid is part of a large granitic massif west of the Protogine Zone. The sampling locality is just north of the northern fault escarpment of the S/Sder~tsen horst. The rock is a reddish, coarse-grained granite with feldspar megacrysts and abundant garnet. It displays a diffuse gneissic texture but lacks strong foliation. According to the TAS classification, the sample is a true granite, and in the alkalinity classification it is transitional between calc-alkaline and alkali-calcic rocks. In the Rb versus Y + N b diagram, it plots in the field of within-plate granites, but is close to the triple junction with volcanic arc and syn-collisional granites. If it is post-collisional, it could plot in any of these fields depending on the source material; post-collisional granites

369

therefore cannot be discriminated on the basis of this diagram (Pearce et al., 1984). The zircons (Fig. 2E) are rusty brown and surprisingly euhedral, forming prismatic crystals with pyramidal terminations, although rounded grains are also common. They are very turbid and rich in inclusions. A large proportion of the grains has conspicuous cores, occasionally even several cores within one single crystal, testifying to the complex history of the zircons. The cores greatly dominate over the overgrowths in volume. One magnetic and five non-magnetic size fractions were analyzed. The zircons are fairly uranium-rich (1310-1970 ppm), and also rich in common lead (4.8-10.4 ppm) and radiogenic lead (240-330 ppm). They are very discordant, yielding a poorly defined upper-intercept age of 1575-61+77xvzu]~A(Fig. 3E). Due to the complex nature of the zircons, their strong discordancy and the limited spread of the data points, the geological significance of the upper-intercept age is unclear. Inherited cores could have increased the U-Pb age, whereas metamorphic overprinting could have lowered it. Nevertheless, the intercept age agrees with the ages of red microcline granites farther north ( ~ 1500-1580 Ma; Welin et al., 1982; Persson et al., 1983, 1987; Lindh, 1987; /~da~ill et al., 1990), and also with the T U R Sr and TCHUR Nd model ages obtained for this sample (cf. below). We thus prefer the interpretation that the Sk~iralid granite is a late Gothian anatectic granite originating from material of short crustal residence and with only minor metamorphic resetting of its primary age. Alternatively, the rock could be a younger ( ~ 1.4 Ga old?) anatectic granite with abundant old restitic zircon material (cf. the Vhnga granite east of the Protogine Zone; Aberg et al., 1985b).

84093: Gneissic granite, Mrlle, Kullaberg, northwestern Sk~ne Sample 84093 is derived from a small massif of red gneissic granite on the Kullaberg pen-

370

insula (Fig. 1 ). It has a strong, steep, N-S trending foliation, parallel to the trend in the surrounding gneiss-amphibolite complex. According to Noding and Wikman (1990), the massif has a dominantly granodioritic composition. The sampling site is at the seashore at M611e. The rock is a red medium-grained gneissic granite with a strong foliation outlined by highly attenuated streaks of mafic minerals. According to the TAS classification, the sample is a granite, and in the alkalinity classification it is transitional between calc-alkaline and alkali-calcic rocks (similar to samples 85015 and 85018). In the Rb versus Y + Nb diagram, it plots in the field of volcanic arc granites; this field, however, can also be occupied by post-collisional granites (Pearce et al., 1984). The zircons (Fig. 2A) are brown, rounded, and often irregular in shape. They are turbid and fractured, displaying diffuse (metamorphosed?), core-like structures, suggesting a complex, multi-stage history. Four magnetic and four non-magnetic size fractions plus one mixed fraction were analyzed. The zircons are intermediate in uranium (780-920 ppm), relatively low in common lead (0.4-1.2 ppm), and contain 168-185 ppm radiogenic lead. They plot discordantly, yielding an upper-intercept age of 1497_+~7 Ma with a MSWD-value of 4.9 (Fig. 3A). If the deviating fraction 5 is excluded, the upper intercept becomes 1488_+9 Ma, with MSWD= 1.8. This, again, is a result that is difficult to interpret, considering the complex history of the rock and the complex nature of the zircons. The M/511egranite much resembles the late Gothian anatectic microcline granites, and our preferred interpretation is that it belongs to this group and was formed some 1500-1580 Ma ago. The zircon age may have been lowered due to recrystallization induced by the strong deformation that affected the area. Alternatively, the sample could be a younger ( ~ 1.4 Ga old?) anatectic granite containing abundant older restitic material.

A. JOHANSSON ET AL.

85016: Granodiorite, Beden, Romelettsen, southern Sk&ne The Beden granodiorite (the "Beden granite" of Hjelmqvist, 1934), forms a relatively undeformed massif of intermediate composition in the eastern part of the Romelehsen horst. Hjelmqvist (1934) considered it to be younger than the surrounding aplitic gneisses and tentatively related it to the "Romele granite". Sample 85016 is from a quarry 1 km northwest of Beden. It is a dark grey, mediumgrained, massive to weakly foliated rock of intermediate composition. Its texture is mostly even-grained, apart from a few scattered rectangular feldspar megacrysts, up to 1 cm in size and with a faint greenish tint. In the TAS classification, the sample is at the boundary between syenite and granodiorite; in the alkalinity diagram it is calc-alkaline. In the Rb versus Y + Nb and Nb versus Y diagrams, it plots in the fields of within-plate granites. The zircons (Fig. 2C) are pale buff to almost colourless and mainly consist of irregular fragments (fragmented larger crystals?). In high-refractive liquid, they are transparent and lack visible cores, growth zoning, or other internal structures, but are fairly rich in both opaque and transparent inclusions. Thus they appear to have had a less complex history than most other zircons from southwestern Sweden. Five non-magnetic size fractions were analyzed. The zircons are low in uranium (326-346 ppm), radiogenic lead (78-85 ppm) and common lead (0.1-0.4 ppm). They are only weakly discordant, the most fine-grained fraction curiously enough being the most concordant one (Fig. 3C). The very limited range of U/Pb-ratios adversely affects the precision of the concordia intercepts, despite a MSWDvalue of only 0.8. The upper-intercept age is 1449_+23 11 Ma, while the lower-intercept age is close to zero. The upper-intercept age is thus the same as the average of the 2°7pb/2°6pb ages, which range from 1447 to 1453 Ma. The absence of cores in the Beden zircons

THE EARLY EVOLUTION OF THE SOUTHWEST SWEDISH GNEISS PROVINCE

and their limited discordancy indicate that the ~ 1450 Ma date approximates the intrusion age of the Beden granodiorite. Such a relatively young age also agrees with the more or less undeformed nature of the Beden granodiorite.

85019: Charnockite, N W of Orkelljunga, northern Sk~ne Sample 85019 is from a small massifofcharnockitic gneissic granite on the eastern extension of the Hallands~sen horst (Wikman and Bergstr6m, 1987a). The sample is a greenishgrey, foliated, medium-grained rock containing pyroxene, amphibole, garnet and opaques as the mafic minerals. In the TAS classification, the rock is a syenite; according to the alkalinity classification it is alkali-calcic, and in the Rb versus Y + Nb and Nb versus Y diagrams it plots in the fields of within-plate granites. The zircons (Fig. 2F) are pale buffand prismatic but have rounded edges; some irregular fragments are also present. There are distinct cores surrounded by transparent mantles. In some grains, the core volume exceeds 50%, whereas other grains apparently lack cores. There are few inclusions. One magnetic and five non-magnetic size fractions were analyzed. The zircons are low in uranium (370470 ppm), in common lead (0.03-1.0 ppm) and in radiogenic lead (83-106 ppm). They plot somewhat discordantly, showing considerable scatter around the discordia line, possibly due to variable amounts of cores in the crystals. The 2°7pb/2°6pb-ages of the individual fractions are between 1415 and 1450 Ma, while the discordia yields a poorly defined upper-intercept age of ln~+35o • -,.,---5o Ma, with a MSWD-value of 20.7 (Fig. 3F). Thus, the nominal upper-intercept age is almost identical to that of the Beden granodiorite, albeit with a large associated error. Since both the compositions and the zircons of these two rocks are similar, a common intrusive age of ~ 1450 Ma appears probable.

371

3. Single-zircon and titanite U-Pb analyses The results of the single-zircon and titanite U - P b analyses carded out at the ETH in Ziirich on samples 85015 and 85017 are listed in Table 3 and illustrated in Figs. 5 and 6. Photomicrographs of some of the crystals analyzed are shown in Figs. 4 A-F.

85017: Grey gneiss, Vgigasked, central Sla~ne Seven zircon crystals were selected from sample 85017. Zircons Nos. 7 and 15 were clear, transparent, and free from fractures. They were reasonably euhedral with 7 being more elongated than 15. Zircons Nos. 4, 11 and 38 were similarly transparent grains which were abraded in order to remove the rims and increase the concordancy. Zircon 22 was fractured, but still reasonably euhedral, while 31 was a dark and turbid grain. The sample weights ranged from 2.3 to 20 gin. Except for No. 3 l, the zircons were low in uranium ( 115-326 ppm), common lead (01.0 ppm), and radiogenic lead (135-830 p g = 3 6 - 8 5 ppm). As expected, No. 22 plots more discordantly than Nos. 7 and 15 (Fig. 5 ), while the abraded zircons 4, 11 and 38 are least discordant. Zircon 31 is very rich in U, radiogenic Pb and common Pb and plots very discordantly. The analyzed single crystals plot along a separate discordia subparallel to the multi-grain line, yielding an upper-intercept age of 1640_ 16 Ma (MSWD 7.9). The 2°7pb/ 2°Spb-ages of the individual zircons range from 1335 to 1650 Ma. The reason for the difference between the discordias obtained from the multi-grain and single-grain analyses is not entirely clear. An analytical artefact, e.g. due to an intedaboratory bias in spike calibration, is ruled out, since such an error would shift the data points either towards or away from the origo of the plot, along trends subparallel rather than perpendicular to the discordia lines. A large amount of translation, and thus an exceedingly large

A. JOHANSSONET AL.

372 TABLE 3

Single zircon (zr) and titanite (ti) U - P b results Sample

Weight (/~m)

U (ppm)

Pb-rad (ppm)

Pb.com a (ppm)

Pb-com b (pg)

2O6pb/2°4pb¢ _+95% c.1.

I

85015: Stenberget red gneiss, Romele~tsen 3 zr 8.0 8 zr 7.3 10 zr 6.3 12 zr 10.3 17 zr 16.4 20 zr 10.9 26 zr abraded 3.8 30 zr abraded 4.2

454 1169 1339 642 935 422 309 255

104 171 203 136 187 104 93 79

2.1 33 2.7 3.9 4.2 0.9 ~0 ~0

26.8 252 27.3 50.1 78.4 20.1 5.2 5.8

85017: Viigasked grey gneiss, central Sldme 4 zr abraded 3.0 7 zr 11.3 11 zr abraded 3.8 15 zr 20.2 22 zr 9.3 31 zr 8.5 38 zr abraded 2.3 A ti abraded 39.4 B ti abraded 48.3

183 166 115 149 326 1337 216 120 193

64 45 36 41 85 197 68 18 30

0.8 ~0 0.7 0.3 1.0 154 ~0 9.3 64

12.8 9.9 12.8 17.2 19.4 1314 5.2 379 3124

bias, would be necessary for the discordias to coincide. The effect is more likely caused by a small Sveconorwegian component that is more abundant in the multi-grain samples than in the crystals selected for single-grain analysis. Such a Sveconorwegian influence is seen in the titanite analyses. Sample A consists of five strongly abraded, light-coloured inclusion-free titanite grains yielding semi-concordant U - P b model ages of 910-930 Ma, and sample B contains eighteen strongly abraded, slightly darker grains yielding model ages of 920-960 Ma. In the context of single-stage (closed-system) evolution of the radiogenic Pb component of the titanites, the U - P b data corrected only for analytical lead contribution yield a lower intercept age of 910 ___3 Ma in a Tera and Wasserburg (1972) diagram. Although the former model ages are highly dependent on the choice of values for common Pb correction, and the relatively precise 910 Ma result is only accurate for closed-system evolution, the data clearly indicate a Sveconorwegian influence

1770+32 304+ 1 2870+ 31 1571+8 2158+ 12 3225 + 66 3884 + 92 3215+29

725+31 2781 + 52 586+ 14 2618+60 2155+51 83.1+0.1 1639 + 72 133+ 1 45.2+0.1

and are in good agreement with other U - P b data on titanites from southwestern Sweden (Johansson, 1990) as well as S m - N d mineral ages of maflc granulites from that area (Johansson et al., 1991 ). In conclusion, the single-zircon results from the grey gneiss suggest that it may be as old as ~ 1650 Ma. Some metamorphic resetting may have affected the analyzed multi-grain fractions, lowering their upper-intercept age to ~ 1610 Ma. It is noteworthy that the selected clear crystals do not represent newly crystallized zircon material, but are as old as, or even slightly older than, the bulk of the population. There are no signs of any component older than ~ 1650 Ma.

85015: Red gneiss, Stenberget, Romelettsen, southern Sktme Totally clear, transparent and fracture-free crystals were impossible to find in this sample. Due to the presence of fractures, the crystals

THE EARLYEVOLUTION OF THE SOUTHWEST SWEDISHGNEISS PROVINCE

373

207pb/235Ua -+ 95% c.l.

2°6pb/23sUa _+95% c.1.

Error.

2OSpb/2O6pbd _+95% c.l.

2OTpb/2O6pbd _+95% c.l.

207pb/2O6pb

corr.

2.949+ 11 1.8029+76 1.9216_+55 2.6732_+67 2.4448 -+ 55 3.1923+77 3.928-+ 13 3.958_+ 12

0.21661 + 5 3 0.14399-+39 0.15518+32 0.19904_+31 O. 18536 -+ 26 0.23551 + 2 8 0.27954+62 0.28024+44

0.783 0.818 0.870 0.853 0.885 0.816 0.831 0.723

0.13038+58 0.09068-+62 0.04860-+29 0.13715+39 O. 15581 + 33 0.11591 -+42 0.15185+52 0.17849+63

0.09874+24 0.09081 -+24 0.08981 -+ 13 0.09741 + 14 0.09566 + 12 0.09831 + 16 0.10192+ 19 0.10243+23

1600+5 1443-+5 1421 + 3 1575+3 1541 + 2 1592-+3 1659_+3 1669-+4

3.801 + 48 3.311 _+ 16 3.824_+44 3.403 _+ 12 3.069 -+ 12 1.565-+ 13 3.796_+ 29 1.468 _+ 17 1.504 _+34

0.27332 + 76 0.24318 _+89 0.27641 _+82 0.25030 + 54 0.22907 + 50 0.13217+34 0.27146_+ 83 O. 15206 _+38 O. 15343 _+38

0.656 0.845 0.672 0.755 0.738 0.682 0.552 0.604 0.796

0.3750+ 27 O. 19072 + 73 0.1963_+28 O. 18978 + 61 0.22298 + 70 0.2105-+20 0.2528 _+16 0.0595 + 19 O. 1009 _+46

0.1009__+ 11 0.09874 + 26 0.10034_+99 0.09859 -+ 24 0.09715 -+ 26 0.08586_+63 O. 10143 + 67 0.07003 _+74 0.0711 _+ 16

1640_+ 20 1600 -+ 5 1630_+ 18 1598 -+ 5 1570-+ 5 1335+ 14 1650_+ 12 929_+ 22 960 _+45

age (Ma)

aCorrected for analytical blank. Wl'otal amount of common Pb present in analysis. c Measured ratio corrected for mass fractionation only (correction factor 1.0009 _+0.0008 A M U - l ). Pb blank data (all uncertainties given at the 95% confidence level): 85015/3-20 and 85017/4-22: 85015/26 and 30, and 85017/31 85017 titanites A and B: and 38: 1.25 + 0.43 × 10-14 moles 2°6Pb 0.71 _ 0.35 × 10-14 moles 2°6pb 1.29 _+_0.64 × 10-14 moles ( = 10.3 _+3.6 pg Pb-tot) ( = 5.8 _+2.9 pg Pb-tot ) ( = 10.8 _ 5.4 pg Pb-tot) 2°Spb/2°6pb = 2.076 _ 0.009 2°7pb/2°6pb = 0.857 _ 0.008 2°4pb/2°6pb = 0.0550 -+ 0.0024

2°Spb/2°6pb = 2.060 _+0.032 2°7pb/2°6pb-- 0.850_ 0.023 2°4Pb/2°6Pb = 0.0533 -+0.0012

2°6pb

2°Spb/2°6pb = 2.111 + 0.009 2°7pb/2°6pb = 0.879 _+0.004 2°4pb/2°6pb= 0.0567 + 0.0011

dAll ratios corrected for mass fractionation, blank lead and common lead. Common lead compositions used (Stacey and Kramers, 1975): 85015 zircon: 85017 zircon: 85017 titanite:

2°Spb/2°4pb= 35.70, 2°Tpb/2°4pb= 15.40, 2°6pb/2°4pb= 16.05 ( 1550 Ma) 2°spb/2°4pb= 35.60, 2°Tpb/2°4pb= 15.35, 2°6pb/E°4pb= 15.95 ( 1600 Ma) 2°Spb/2°4Pb= 36.93, 2°7pb/2°4pb= 15.53, 2°6pb/2°4pb= 17.23 (910 Ma)

selected for abrasion disintegrated into small fragments rather than becoming polished at the surface. However, amongst the eight zircons analyzed, Nos. 3, 20, 26 and 30 were reasonably clear and euhedral, and fragments of the latter two grains were abraded. Zircon 20 was a more fractured crystal, and No. 17 was a short and stubby turbid crystal. Zircons 8 and 10 were round, turbid, brown grains, initially be-

lieved to represent the oldest component within the zircon population (corroded remnants lacking later overgrowths). The sample weights ranged from 4 to 16/zm. The grains analyzed had variable uranium contents (255 to 1340 ppm). Radiogenic lead contents ranged from 330 to 3060 pg (79 to 203 ppm). The abraded crystals Nos. 26 and 30 were lowest in uranium and plot least dis-

374

A. JOHANSSON ET AL.

. 85017:4

&.

85015:3

B.

85015:10

E.

85017:7

C.

85015:12

F.

8S017:]I

Fig. 4. Photomicrographsof some of the singlezircon crystalsanalyzed. Lengthof zircons 180-360#m. (A) 85015 zircon 3; (B) 85015 zircon 10; (C) 85015 zircon 12; (D) 85017 zircon 4 (prior to abrasion); (E) 85017 zircon 7; (F) 85017 zircon 11 (prior to abrasion). cordantly, followed by Nos. 20 and 3. No. 12 is somewhat more discordant (Fig. 6). These five single crystals all plot to the right of the discordia defined by the multi-grain analyses. Zircon 17 plots close to the multi-grain fractions. The round, turbid grains 8 and 10 were highest in uranium and plot most discordantly and to the left of the multi-grain discordia. There is therefore no evidence that these two grains are older than the bulk of the zircon population.

The 2°7pb/2°6pb-ages of the single zircons range from 1421 to 1669 Ma. A regression line fitting the single crystal results produces an upper intercept of 1675 _+25 Ma. However, the extremely high MSWD-value of 200 testifies to the scatter of the data points and thus to the complex composition of the zircon population. The fact that this complexity has been camouflaged by the integrating effect inherent to analyses of composite fractions consisting of

THE EARLY E V O L U T I O N O F THE S O U T H W E S T SWEDISH GNEISS PROVINCE

206pb/258U

Vagosked grey gneiss

Titanites~

0.0

1

4. Sm-Nd and Rb-Sr isotopic results

1640-+ 16 Me ~ j / ' / 7

'

, / ~ . ,,/~d~x,~ 5 ~ ob~

in\ 2

3

4

Fig. 5. Concordia diagram for sample 85017 (grey gneiss, V~gasked) showing the results of single zircon and titanite analyses (labelled error ellipses at the 95% confidence level) in relation to results obtained by conventional multigrain analyses of standard size fractions (crosses; cf. Fig. 3D). The single zircons define a separate subparallel discordia with an upper intercept at 1640+ 16 Ma and a MSWD of 7.9. 206pb/258U

1675 ± 25 Mo.,~ /

85015

1 5 ~ ~

Stenberget red gneiss

/.~/~/

° ab~

26 obr,

lOOO / L ) 1

/.~ 0.0

0

Si MgsiweDz i rCOo2nS

\ 577 ± 65 Ma 1

207pb/235U 2

3

4

37 5

5

Fig. 6. Concordia diagram for sample 85015 (red gneiss, Stenberget) showing the results of single zircon analyses (labelled error ellipses) in relation to results obtained by conventional multi-grain analyses of standard size fractions (crosses; cf. Fig. 3B). The single zircons scatter around a line with an upper intercept at 1675 + 25 Ma.

hundreds or thousands of crystals, clearly reveals the limitations of conventional zircon dating. The results from the abraded zircons suggest an age of 1.65 to 1.7 Ga for the precursor of the red gneiss, whereas the upper-intercept age derived from the multi-grain fraction analysis is considerably younger due to the higher susceptibility of these lower-quality samples to younger metamorphic overprinting.

S m - N d and Rb-Sr isotopic analyses were carded out on whole-rock samples of the six dated rocks; the results are listed in Tables 4 and 5 and illustrated in Figs. 7 and 8. The TDM Nd model ages of the individual samples are between 1840 and 1990 Ma, the TcnuR Nd model ages are between 1480 and 1600 Ma, and the TVR Sr model ages are between 1460 and 1710 Ma. Although the TDM ages could be taken to indicate Svecofennian ( ~ 1750-1900 Ma old) precursors to these rocks, it is well known that most Svecofennian rocks do not have the initial Nd-isotopic signatures of a fully depleted mantle (e.g. Patchett et al., 1987). The similar TCHUR and TUR ages rather suggest an origin with a near-chondritic mantle or Bulk Earth composition at a time close to the ages indicated by the zircons (cf. Table 2 ). If the zircon ages of Table 2 are used to calculate initial isotope ratios, initial eNd-Values between - 1 and + 1 (Table 4 and Fig. 7 ), and initial esr-values between - 8 and + 53 (Sr(i)=0.7021-0.7065, Table 5 and Fig. 8) are obtained. The three Esr-values around + 50 (samples 84093, 85015, 85019) may indicate the involvement of some older crust, although some scatter of the apparent initial Sr-values may also be expected due to later mobility in the Rb-Sr system. If 1675 Ma is used instead as the age of sample 85015, initial eNd becomes + 1.7 and initial Esr - 110. For at least the three most siliceous rocks (84093, 85015, 85018) a crustal origin appears likely, but the Sr isotope data show that the crustal residence time of the precursors must have been short. The Nd and Sr isotope data thus confirm the general time range indicated by the U - P b zircon data, but cannot resolve the finer particulars of crustal evolution. They suggest that the main crustforming event west of the Protogine Zone in southernmost Sweden did not occur long before 1650 Ma. However, the presence of some earlier proto-crust in the area, or of some unrecognized older units within the red and grey

A. JOHANSSON ET AL.

376 TABLE 4 S m - N d data for gneisses and granitoids from southwest Sweden No. Locality

Sample

Sm a Nd a (ppm) (ppm)

147Sm/t,4Nd a

143Nd/t44Ndb +2trm

ENd(0) ¢ ~Nd(T)e TCHUR d (present) (initial) (Ma)

TDM e (Ma)

1

M611e

84093

6.79

38.4

0.1068

0.511710± 5

-18.1

1570

1900

2

Stenberget

85015

6.66

39.6

0.1015

0.511676± 8

-18.8

1540

1860

3

Beden

85016

12.00

62.4

0.1163

0.511855± 13

- 15.3

1480

1860

4

V~gasked

85017

6.60

37.9

0.1052

0.511673± 7

- 18.8

1600

1930

5

Sk~ralid

85018

13.73

78.5

0.1058

0.511737± 8

-17.6

1510

1840

6

OrkelOunga

85019

9.88

45.3

0.1318

0.511976± 7

-12.9

1550

1990

-0.8 (1497) +0.2 (1557) -0.3 (1449) +0.1 (1613) +0.8 (1575) -0.8 (1452)

a Sm and Nd contents and t47Sm/144Ndratio from isotope dilution analysis. Estimated analytical uncertainty of 1475m/144Nd ratio 1.0°6. b ~43Nd/~,~Nd ratios corrected for Sm interference and normalized to ~46Nd/144Nd=0.7219. Normalized ratio increased by 0.000040 in order to correct for the low result obtained for the La Jolla Nd-standard (0.511802 ± 20). Analytical errors shown are 2 standard deviations of the mean. ¢ ~Nd-values according to the model of Jacobsen and Wasserburg ( 1984): present-day chondritic 1475m/~44Nd= 0.1967; presentday chondritic 14aNd/t'~Nd = 0.512638. d Model age calculated relative to the chondritic uniform reservoir ( C H U R ) of Jacobsen and Wasserburg ( 1984 ). e Model age calculated relative to the depleted mantle curve ( D M ) of DePaolo ( 1981 ).

TABLE 5 R b - S r data for gneisses and granitoids from southwest Sweden No.

Locality

Sample

Rb a (ppm)

Sr a (ppm)

S7Rb/S6Sr a

243

1.770

S7Sr/S6Sr b ±2tr~

Esr(0) c (present)

~sr(T) c (initial)

TUR ¢

0.743953± 16

+560

1630

6.527

0.851850±23

+2092

+46 (1497) +47 (1557) +5 (1449) + 12 (1613) -8 (1575) +53 (1452)

1

M611e

84093

148

2

Stenberget

85015

214

3

Beden

85016

144

190

2.202

0.748940± 18

+631

4

V~gasked

85017

104

434

0.6966

0.719597± 13

+214

5

Skaralid

85018

263

9.434

0.915458± 12

+2994

6

Orkel~unga

85019

138

2.750

0.763805± 16

+842

96.2

82.3 146

(Ma)

1590 1460 1710 1570 1550

a Rb and Sr contents and STRb/S6Sr ratio from XRF. Estimated analytical uncertainty of the S7Rb/S6Sr ratio is 1.0%. b S7Sr/S6Sr ratios corrected for Rb interference and normalized to S6Sr/SSSr = 0.1194.2 standard deviations of the mean analytical error. ¢ ~sroValues and TUR Sr model age according to McCulloch and Chappell ( 1982): present-day SVRb/S6Sr mantle ratio = 0.0827; present-day S7Sr/S6Sr mantle ratio = 0.7045.

THE EARLY EVOLUTION OF THE SOUTHWEST SWEDISH GNEISS PROVINCE

CNd .I0

377

gneiss complex, cannot be excluded at the present stage of investigation.

.5

" : -.'t-"~: - ~2~..

CHUR

o

: :5o!9

~ge IOa)

J 20

~ !5

s

1'0

20

015

Fig. 7. end vs. age for gneiss and granitoid samples from southwest Sweden. Dots indicate initial eNd-Values using the zircon ages listed in Table 2, dotted lines show extrapolations of the evolution trends for each sample back to the Depleted Mantle curve of DePaolo ( 1981 ). ~Sr q000

.900

.@00

.700

.500

.500

,/*00 I ~.300

',,'

//

.200

~y.. ~ / / /

',

/ ../"

/

~

-~

//--~,5'~1

. Age [Go)

+I00

--

UNIFORM RESERVOIR (BULK EARTH) i 15

i 10

i 05

O

100

Fig. 8. esr vs. age for gneiss and granitoid samples from southwest Sweden. Dots indicate initial esr-values using the zircon ages listed in Table 2, dotted lines show extrapolations of the evolution trends for each sample back to the Uniform Reservoir/Bulk Earth line of McCulloch and ChappeU ( 1982 ).

5. Discussion: a geotectonic model for the Southwest Swedish Gneiss Province The results of earlier U - P b zircon and R b Sr whole-rock age determinations of Precambrian rocks on both sides of the Protogine Zone in southern Sweden are listed in Table 6. Resuits believed to reflect the primary crystallization ages of the rocks are shown in Fig. 9. Apparently reset Rb-Sr ages have been omitted, as well as some U - P b ages derived from zircons with abundant cores which are believed to reflect the ages of inherited materials (the V&nga, G6temar and Jungfrun granites; Aberg, 1988, and references therein ). Also excluded are Rb-Sr and S m - N d ages of mafic dykes. The U - P b zircon ages of the Sm&land granitoids from the Transscandinavian GranitePorphyry Belt are between 1760 and 1810 Ma, while one porphyry has an age as high as 1u., Qa 7 -29 + 46 Ma (Aberg and Persso n, 1984 ). Some rocks along the western edge of the Transscandinavian Belt (the future site of the Protogine Zone) and in western Blekinge give U - P b ages around 1700 Ma. Continuing farther west, U Pb ages decrease to 1600 Ma or less in the southern part of southwestern Sweden, whereas in the north (outside the area of Fig. 9) a few ages at, or above, 1700 Ma have been recorded. Although the zircon ages obtained in the present investigation are, at least partially, disturbed, neither the grey nor the red gneiss sample investigated are likely to be older than ~ 1650-1700 Ma. The presence of older rock units within the Southwest Swedish gneiss complex cannot be excluded, but 1600-1700 Ma appears to be the general age of crust formation in southwestern Sweden, a conclusion supported by the present Nd and Sr isotope data. It remains enigmatic that even the oldest,

A. JOHANSSONET AL.

378 TABLE 6 Review of U - P b and Rb-Sr datings of Precambrian rocks from southern Sweden No.

Rock/Locality

Method

Age (Ma)

SVECOFENNIAN ENCLAVES IN THE TRANSSCANDINAVIAN BELT ~ 1845 1 Loftahammar granite U - P b zr 1620±40) (Rb-Sr WR 1660± 35) (Rb-Sr WR ~1845 2 U - P b zr Or6-Hamn6 granite 1630±59) (Rb-Sr WR 1825+ 177/ 3 U - P b zr Virserum arkose -59 1754±10) 4 Eksj6 tonalite ( U - P b zr 1800-+ 57) 5 Vetlanda gr.diorite (Rb-Sr WR SMALAND GRANITES AND PORPHYRIES (TRANSSCANDINAVIAN BELT) (Rb-Sr WR 1690 _+64 ) 6 Granite, Str~lsn~is +46 7 Porphyry, Storebro U - P b zr ,t Qa o.I 7 --29 (Rb-Sr WR 1645 + 20) U - P b zr 1768 + 9 8 Granite, Vimmerby 177~+40 U-Pbzr • 0_47 9 V~ixj6 granite + porph. U - P b zr 1769 + 9 10 Viixj6 granite (Rb-Sr WR 1686 + 50) 11 Sm~dand granites in NE Blekinge U - P b zr 1802 +4 12 V~ixj6-type granite, Ramnebo U-Pbzr 1786~ 13 Filipstad-type granite, S~ivsj6 U - P b zr 1664 ± 9 14 Hagshult granite BLEKINGE PROVINCE 15 V~istan~i metavolc. 16 Coastal gneiss 17

Tving granitoid

U - P b zr U - P b zr (Rb-Sr WR U - P b zr Rb-Sr WR

ANOROGENIC GRANITES IN SOUTHEASTERN SWEDEN 18 Karlshamn granite U - P b zr Rb-Sr WR K-hamn leucogranite Rb-Sr WR (U-Pb zr 19 V~nga granite (Rb-Sr WR Rb-Sr WR 20 Eringsboda and Rb-Sr WR Klagstorp granites 21 G6temar w. granite ( U - P b zr U - P b mz G6temar east + west Rb-Sr WR 22

G6temar e. granite

23

Uthammar granite Uthammar-V~nevikVirbo-Br~tnhult gr. Jungfrun granite

24

Reference

Aberg, 1978 Aberg, 1978 Priem and Bakker, 1973 Aberg, 1978 Aberg, 1978 Aberg and Persson, 1984 Aberg and Persson, 1986 R6shoff, 1973

Aberg, 1978 Aberg and Persson, 1984 Aberg, 1978 Patchett et al., 1987 Patchett et al., 1987 Jad and Johansson, 1988 Johansson and Larsen, 1989 Mans~ld, 1991 Mans~ld, 1991 Jarl, 1992

1705 + 8 1690_+ 39 1642 + 43 ) 1771 _+4 1762 _+88

Johansson Johansson Johansson Johansson Johansson

and and and and and

Larsen, Larsen, Larsen, Larsen, Larsen,

1989 1989 1989 1989 1989

+121 1403_86 1422 + 31 1358 + 24 1i J uce4+54x --451 1452_+24) 1347 _ 41 l 1358 _+114

Aberg et al., 1985a Springer, 1980 Springer, 1980 Aberg et al., 1985b Aberg et al., 1985b Aberg et al., 1985b Aberg and Kornflilt, 1986

1468_+453) 1383+ 14 1377 _+27

U - P b zr U - P b mz ( U - P b WR U - P b zr Rb-Sr WR

1382._+7~ 1397 + 14 1357 + 140) ~ 1400 1350 + 11

Aberg et al., 1984 Aberg et al., 1984 Aberg, 1978; Aberg et al., 1984 Aberg et al., 1984 Aberg et al., 1984 Smellie and Stuckless, 1985 Aberg, 1986 Aberg, 1986

( U - P b zr Rb-Sr WR

148034°) 1386 _ 21

Aberg et al., 1983 Aberg et al., 1983

THE EARLYEVOLUTION OF THE SOUTHWESTSWEDISHGNEISSPROVINCE

379

TABLE 6 (cont.) No.

Rock/Locality

Method

SOUTHWEST SWEDISH GNEISS PROVINCE 25 Gn. granite, Alvesta U - P b zr 26 Gneissic granite, U - P b zr W. Glim6kra 27 Grey gneiss, V~igaU - P b zr sked 28 Red gneiss, StenU - P b zr berget 29 Gneissic granite, U - P b zr Sk~iralid 30 Gn. granite, M611e U - P b zr 31 Beden granodiorite U - P b zr 32 t3rkelljunga charU - P b zr nockite 33 Varberg charnockite R b - S r WR 34 Lerum granite U - P b zr R b - S r WR 35 Askim granite U - P b zr ( R b - S r WR 36 Bunketorp granite U - P b zr ( R b - S r WR 37 Horred metavolcanite U - P b zr 38 Horred granodiorite U - P b zr PROTOGINE ZONE SYENITES AND GRANITES 39 Vaggeryd syenite U - P b zr ( R b - S r WR 40 Skhne syenites R b - S r WR 41 Onnestad syenite U - P b zr 42 G6rbj6rnarp syenite U - P b zr 43 Guml6sa-Glim~kra U - P b zr granite U - P b zr

Age (Ma)

Reference

1713 + 3 1531 + 8

Johansson, 1990 Johansson, 1990

1613 + 6

This study

1557 __+372

This study

1~7 ~ +77 J J-6~

This study

14o7 ~ , / _+47 34 1449__+2~ 1 4J-~~ +47 347

This study This study This study

1420 + 52 1603 + 40 1611 + 28 1362+9 1293+14) 1279+62 1192+ 14) +36 1643_23 1593_+~°

Welin and Gorbatschev, 1978 Welin and Samuelsson, 1987 Welin and Samuelsson, 1987 Welin and Samuelsson, 1987 Welin and Samuelsson, 1987 Welin and Samuelsson, 1987 Welin and Samuelsson, 1987 Ah~ill et al., 1993 Ah~ill et al., 1993

1203 + 8 1127 + 67 ) 1184 + 38 + 140 1224_14 1204_+~4 1204__+~56 + 8o 1232_46

Jarl, 1992 Patchett, 1978 Klingspor, 1976 Johansson, 1990 Hansen and Lindh, 1991 Johansson, 1990 Johansson, 1990

Method: zr = zircon upper-intercept age, mz = monazite 2°Tpb/2°6pbage, WR = whole-rock isochron age. The ages in brackets are presumably not primary crystallization ages due to reset Rb-Sr, inherited zircon components, etc., and have not been included in Fig. 9. The compilation covers the map area in Fig. 9, mafic rocks excluded.

multiply deformed rock units west of the Protogine Zone appear to be younger than the essentially undeformed Smfland granitoids. Hitherto no rocks sufficiently old to serve as host rocks for the Smhland intrusions, i.e. rocks at least Svecofennian in age, have been found along the western margin of the Transscandinavian Belt. Their absence can be explained either by large-scale lateral displacement along the Protogine Zone, or by a combination of crustal shortening and uplift in the west. The latter alternative is outlined in Fig. 10, which depicts a tentative plate-tectonic reconstruction of the evolution of southern Sweden from

~ 1700 to 900 Ma. 1700 Ma ago (Fig. 10A ), Svecofennian continental crust, including the Transscandinavian Granite-Porphyry Belt, already existed in the present east. A possible continuation of Svecofennian crust is also shown west of the Transscandinavian Belt. Magmatism occurred along the western edge of this belt (Johansson, 1990; Jarl, 1992) and in Blekinge (cf. Fig 9A; Johansson and Larsen, 1989), possibly in response to subduction farther west. In the western part of southwest Sweden (Stora Le-Marstrand Belt), greywackes are intercalated with mafic volcanics, with a S m - N d age of ~ 1760

380

A. JOHANSSON ET AL.

,°!

I

%

9

~rberg

Age| 11J 12 I 3 1L 15 ~6

B

WEST SOUTH-WEST SWEDEN Ir PZ I r I 4 3 e 0 ~ ' 2 39 41 • "~3

Age 11

I

12 13

,36 e35 033

o19 32,

34• e38 •37

29e

21 22 23~ °24

180o2O 818

,31

,30 • 28

• 26

16

e27

17

• 14 e16 15eo25

ee

17

t~ 9 10

18 19 O0

EAST

SM~,LAND, BLEKINGE AND SVECOFENNIAN ENCLAVES

• U - P b zircon age

o Rb-Sr

~17 o8

o3 e7

w h o l e rock age

e12 2~1

18 19 Ga 300km

Fig. 9. (A) Geological map of southern Sweden, simplified and modified after Magnusson et al. (1957), showing published U-Pb zircon ages (dots) and Rb-Sr whole-rock ages (circles) of Precambrian granitoid rocks. Rb-Sr ages showing a significant degree of metamorphic resetting, and U-Pb zircon ages believed to be influenced by inherited zircon components have been excluded from the compilation. (B) Age distribution versus east-west geographic distribution (in relation to the Protogine Zone) of the same samples. Numbering refers to Table 6, where references can also be found. Ma (Ah~ill and Daly, 1989), suggesting an oceanic environment. About 1 6 0 0 - 1 6 5 0 Ma ago (Fig. 10B) most of the continental crust in southwestern Sweden was formed, perhaps as a result o f continued subduction that had m o v e d farther west. Fig. 10C depicts the final phase o f the Goth-

ian orogeny ~ 1550 Ma ago, with continued crustal accretion in the west leading to compression and deformation, crustal shortening and thickening, and the generation o f anatectic red microcline granites (e.g. M611e and Sk~iralid, and "Group C 1" o f A_h/ill et al., 1990) in the already formed part of the Goth-

THE EARLYEVOLUTIONOF THE SOUTHWESTSWEDISHGNEISSPROVINCE

A

381

1700 Mo: Early Gothion subduction SLM

BLEKINGE + + ~_

SF,

SMZL+AL(?P?~..~

~ B

>

>

-

H

i

.~r_. +

>

)

q

_

SF _

_

_

1600-1650 Me: Mid-Gothion subduction .~IhJ SW SWEDEN

C

SF"~

BLEKINGE SM~LAND SF + + ++ ++++, + ++++ + ++~++++~++

1550 Ma: Late Gothion co[[ision and compression S.W SWEDEN

SF?

BLEKINGE

SM~LAND <~SZS]

; ~

+ + ~

+

SF

+ + + + + + + + + +

D SW SWEDEN

E

SF~

SF?

~

PZ E:~ BLEKINGE

SMJ~LAND

SF

1000 Mo: Mid-Sveconorwegion collision and compression

O ~ ~ < ~ SF

-

G

SM~LAND

1200 Mo: Early Sveconorwegion tensional mogmotism S.W. SWEDEN

F

BLEKINGE

+ + + +++~+

++++

900 Mo: Late Sveconorwegion thrusting, uplift and erosion

i;o

2;0

300 km

Fig. 10. Seven schematic and hypothetical E-W profiles across southern Sweden depicting the geotectonic evolution from ~ 1700 to 900 Ma. For details see text. SF= Svecofennian; SF?= hypothetical Svecofennian extension or remnant west of the Transscandinavian Belt/Protogine Zone; TGPB= Transscandinavian Granite-Porphyry Belt; S L M = Stora Le-Marstrand Belt; PZ= Protogine Zone; M Z = Mylonite Zone. Vertical scale is approximately half of horizontal scale.

ian orogen. Southeastern Sweden apparently acted as a stable block in the east. The Gothian orogeny in southwestern Sweden is comparable in age to the Labradorian orogeny in eastern Canada (Gower and Owen, 1984; Gower, 1985 ) and the Mazatzal orogeny in the southwestern U.S.A. (Karlstrom et al., 1987; Karlstrom and Bowring, 1988). Thus, the crust created between 1.5 and 1.8 Ga ago forms a semi-continuous belt along the southern margin of a Mesoproterozoic superconti-

nent (Gower et al., 1990). In southwestern Sweden, the end of the Gothian orogeny was apparently followed ~ 1500 Ma ago by an extensional episode, marked by the intrusion of mafic hypabyssals in V~irmland (Welin et al., 1980; Johansson and Johansson, 1990), some of the "C2" intrusives of Ah~ill et al. (1990) and other mafic rocks. Between 1350 and 1450 Ma ago (Fig. 10D), the magmatism of the "Hallandian event" followed (Hubbard, 1975), with the ~ 1450 Ma

382

old Beden granodiorite and 0rkelljunga charnockite as well as the Varberg charnockite (Welin and Gorbatschev, 1978) representing lower-crustal manifestations of this activity and the "anorogenic" granites east of the Protogine Zone (/l,berg, 1988) possibly being somewhat younger upper-crustal equivalents. Related to this activity is also a regional 1400 Ma old resetting of the K-Ar ages in southeastern Sweden (Aberg, 1978), and a metamorphic disturbance affecting the U-Pb zircon ages of some granitoids in the northern part of southwestern Sweden ~ 1400 Ma ago (Hansen et al., 1989). The "Hallandian" magmatism in southern Sweden appears to be part of a major, possibly world-wide event of anorogenic magmatism, particularly evident in the areas of newly formed Mesoproterozoic crust in North America, where it is subdivided into two discrete magmatic pulses, at 1.48-1.45 Ga and 1.401.34 Ga (Anderson, 1983; Bickford et al., 1986). It has been suggested that the mechanism responsible for this magmatism was a mantle upwelling due to overheating beneath a stationary supercontinent, in turn causing partial melting within the lower crust (Hoffman, 1989). Around 1200 Ma (Fig. 10E), mantle-generated tensional magmatism occurred concentrated along the Protogine Zone (Klingspor, 1976; Johansson, 1990; Johansson and Johansson, 1990; Hansen and Lindh, 1991; Jarl, 1992). Farther north, 1200-1250 Ma old granites occur to the west of the Protogine Zone ("Group C3" ofAh~ill et al., 1990). This magmatism appears related to the break-up of the Mesoproterozoic supercontinent in the North Atlantic area (Patchett and Bylund, 1977; Gower et al., 1990). A collision with North America took place at some time between ~ 1100 and 900 Ma (Fig. 10F), during the Grenvillian-Sveconorwegian orogeny. Sveconorwegian Sm-Nd mineral ages in marie granulites from southwestern Sweden (Johansson et al., 1991 ) suggest that signifi-

A. JOHANSSON ET AL.

cant Sveconorwegian crustal thickening occurred. Around 900 Ma ago (Fig. 10G), these tectonic movements were followed by isostatic uplift and erosion of the thickened crust west of the Protogine Zone, accompanied by the intrusion of dolerites along and east of the Protogine Zone (Patchett and Bylund, 1977; Patchett, 1978; Johansson and Johansson, 1990). This uplift of southwestern Sweden was discovered already by Magnusson (1960) and Welin and Blomquist ( 1966 ), based on K-Ar closure ages of 900-1000 Ma west of the Protogine Zone. It appears likely that the uplift was largest in the south, where granulitic and charnockitic rocks yielding 880-930 Ma Sm-Nd mineral ages are exposed (Johansson et al., 1991 ), and where the Protogine Zone is characterized by steep subvertical shears. Therefore, in the southern part of southwestern Sweden we find exposed a Mesoproterozoic deep crustal section with granulitic and charnockitic rocks, while in the north, especially to the west of the Mylonite Zone, a more shallow crustal level is seen. Also Blekinge features a mid-crustal level. The Transscandinavian Granite-Porphyry Belt may represent an even shallower level, and has acted as a stable block undergoing little compression or post-tectonic uplift. The hypothetical Svecofennian crust west of the Transscandinavian Belt may have been uplifted and eroded away in the area west of the Protogine Zone.

6. Conclusions

( 1 ) The formation of continental crust in the southern part of the Southwest Swedish Gneiss Province took place between ~ 1.6 and 1.7 Ga ago, possibly by subduction-related magmatism that formed the precursors of the red and grey gneisses. (2) Compression and crustal thickening during the final phase of the Gothian orogeny

THE EARLYEVOLUTIONOF THE SOUTHWESTSWEDISHGNEISSPROVINCE

~ 1.5 to 1.6 Ga ago led to the formation of red anatectic granites. (3) During the "Hallandian event", mantlegenerated anorogenic magmatism led to the intrusion of granodioritic and charnockitic rocks in southwestern Sweden at ~ 1.45 Ga. (4) Following an episode of tensional magmatism along the Protogine Zone ~ 1.2 Ga ago, and renewed compression and crustal thickening during the main phase of the Sveconorwegian orogeny, the crustal block west of the Protogine Zone was uplifted and eroded ~ 0.9 Ga ago. (5) In broad terms, the crustal evolution within the southern part of the Southwest Swedish Gneiss Province (the eastern gneiss segment) appears similar to the evolution of the northern part and within the western gneiss segment.

383

Acknowledgements We acknowledge the technical assistance given by the staff of our laboratories, particularly Paula Allart (Stockholm) for assistance with mineral separation, Knut Christiansson (Stockholm) for maintenance of the chemical laboratory, and Magnus Hedberg (Stockholm) for mass spectrometer maintenance and computer programming. Part of the figures were drawn by Solveig Jevall (Stockholm). Stefan Claesson's (Stockholm) and Roland Gorbatschev's (Lurid) critical comments led to substantial improvements of the paper. The work of A.J. was funded by the Swedish Natural Science Research Council (NFR), which also contributed a special grant to A.J. enabling a two-month stay in Ziirich.

Appendix I--Sample locations and descriptions Sample no.--Rock type--Locality Map sheet---Coord. Sw. nat. grid--Latitude/longitude (Swedish nat. grid = "Rikets n~it) Sample description and mineralogy~ 84093---Gneissic granite--Mtille, Kullaberg, northwest Sk/ine 3B NO 8i--62 44 90 N/12 94 60 E--56 ° 17' 15"N/12°29'20"E Reddish, strongly foliated, medium-grained gneissic granite with qz, kfsp, plag, amph, bi, opq (rot, py), ap, zr (marie minerals in bands) 85015--Red gneiss---Stenberget, Romeletsen, southern Sk~ne 2D SV 2b--61 61 45 N/13 55 40 E--55°33'40"N/13°31'25"E Reddish, weakly foliated, fine- to medium-grained leucocratie aplitic gneiss of uncertain origin, with qz, kfsp, plag, minor bi and opq (mt, py) plus zr 85016---Granodiorite--Beden, Romeletsen, southern Sk~ne 2D SV le--61 58 55 N/13 60 95 E--55°32 ' 15"N/13°36'20"E Grey, relatively unreformed, even- and medium-grained granodiorite ("Beden granite" ), with qz, kfsp, plag, amph, bi, opq (mr, py), ti, ap, zr (marie minerals in aggregates) 85017--Grey gneiss--V~gasked, central Skane 3D SV 2 a l 6 2 12 80 N/13 54 65 E--56°01 '20"N/13°28 ' 35"E Grey, strongly lineated, fine- to medium-grained gneiss of intermediate composition and uncertain origin, with qz, kfsp, plag, amph, hi, opq (mr, py), ti, ap, zr (strongly attenuated feldspar megacrysts) 85018---Gneissic granite--Sk~ralid, central Sk/me 3C SO 3i--62 15 40 N/13 41 20 E---56°02'25"N/13 ° 15'40"E Reddish, medium- to coarse-grained gneissic granite, with qz, kfsp, plag, amph, hi, gt, opq (rot, py), ap, zr (maric minerals in aggregates, feldspars partly as diffuse megacrysts) 85019---CharnockiteqNW of (~rkelljunga, northern Sk~ne 3C NO 9i--62 46 75 N/13 40 45 E--56 ° 19' 15"N/13 ° 13'45"E Greenish grey, foliated, medium-grained charnockitie rock of syenitic composition, with qz, kfsp, plag, amph, px, gt, opq (mr, py), ap, zr (marie minerals in aggregates) =Mineral abbreviations: qz = quartz, kfsp = K-feldspar, plag = plagloclase, amph = amphibole, bi = biotite, px = pyroxene, gt = garnet, opq=opaques (mt =magnetite, py=pyrite), t i = titanite, ap=apatite, zr=zireon

A.JOHANSSONETAL.

384

Appendix II--Chemical composition of gneisses and granitoids from southwest Sweden 84093 SiO 2 (wt%)

A1203 Fe203 MnO TiO2 MgO CaO K20 Na20 P205 Sum LOI Ba (ppm) Be Co Cr Cu La Mo Ni Pb Sc Sn Sr V Y Zn Zr W Nb Yb

70.9 14.0 3.07 0.07 0.50 0.76 1.79 4.69 3.72 0.13 99.6 0.6 1264 1.6 <5.7 101 20 45 7.7 11 <11 6.9 <5.7 231 35 36 40 201 <11 8.5 3.8

85015

85016

76.4 12.5 1.53 0.04 0.24 0.17 0.69 5.08 3.45 0.03 100.1 0.3 352 1.6 <5.6 121 16 48 8.0 6.9
85017

64.3 13.8 7.91 0.14 1.30 1.42 3.61 4.27 3.32 0.39 100.5 0.3 1087 2.2 14 61 26 55 <5.6 13 <11 14 6.7 226 72 61 94 355 <11 14 6.7

67.5 15.6 4.46 0.12 0.58 1.37 3.27 3.61 4.19 0.18 100.8 0.3 1378 1.4 8.5 70 75 40 <5.1 13 <10 11 <5.1 444 66 27 63 196 <10 7.7 3.0

85018 73.1 13.0 3.06 0.08 0.36 0.31 1.30 5.59 3.01 0.07 99.8 0.2 547 1.8 <5.4 103 13 99 <5.4 6.4
85019 66.5 15.5 4.50 0.19 0.70 0.58 1.99 5.55 4.23 0.16 99.8 0.2 2583 1.6 <5.6 65 13 39 <5.6 <5.6 <11 18 <5.6 153 16 62 102 828
Analyzed by the Swedish Geological Company (SGAB), LuleA, using ICP on LiBO2 melt. All Fe given as Fe203

Appendix llI--analytical procedures

Multi-grain U-Pb zircon analyses (in Stockholm) Multi-grain size fractions of zircon were dissolved in H F + H N O 3 following Krogh (1973) and spiked with separate 235U and 2°SPb tracers. For samples 85018 and 85019, insets with 0.4 ml sized microcapsules were used for dissolution (Parrish, 1987). U and Pb were extracted by standard anion exchange techniques in HC1, and Pb was further purified by electroplating. Lead was analyzed on a Finnigan MAT 261 multicollector mass spectrometer. For samples 85016, 85018 and 85019, uranium was also analyzed on the MAT 261 mass spectrometer. For the remaining samples, uranium was analyzed on an AVCO 901A single-collector mass spectrometer. Calculations were made following Ludwig (1980). During the course of the work, Pb blank levels decreased from 1.6 ng to 0.13 ng. For uranium, a blank level of 0.10 ng was assigned to each analysis. For Pb blank composi-

tion, the following ratios were used: 2°6pb/2°4pb = 18.5 + 2.0, 2°7pb/2°4pb = 15.6 + 0.2, 2°spb/ 2°4pb= 38.5 _+1.5. Depending on the ages of the samples, the following values were adopted for common lead correction: 2°6pb/2°4pb= 15.95-16.25, 2°Tpb/2°4pb= 15.3515.40, 2°Spb/2°4pb = 35.60-35.90 ( 1600-1450 Ma on the Stacey and Kramers (1975 ) curve). Regression calculations of discordia lines were made according to York (1969), and ages calculated using the decay constants in Steiger and J~iger (1977). All errors given are at the 2a confidence level.

Single zircon and titanite U-Pb analyses (in Ziirich) The single zircon crystals analyzed from samples 85015 (red gneiss ) and 85017 ( grey gneiss ) included both clear, core-free and euhedral crystals, some of which were abraded with pyrite in order to reduce discordancy (cf. Krogh, 1982), and fractured, turbid and rounded grains for comparison.

THE EARLYEVOLUTIONOF THE SOUTHWESTSWEDISHGNEISSPROVINCE The grains were weighed on a Mettler UM3 ultramicrobalance, and washed in acetone, alcohol and 1% HNO3. They then were leached with hot 1 M HNO3 for 15 min, and hot 4.6 MHC1 for 30 rain. The leach solutions of zircons Nos. 3-20 from sample 85015 and of zircons Nos. 4, 11, 22 and 31 from sample 85017 were spiked with a mixed U - T h - P b tracer and analyzed for lead and uranium. The amounts of U and radiogenic Pb leached were relatively small (0.04-0.7% and < 0.6% of the total contents of U and radiogenic Pb, respectively), the uranium-rich and strongly discordant zircons being most susceptible to leaching. Because of the partially compensating effect of both U and Pb loss, no correction for leached U and Pb was applied to the zircon data. The zircons were transferred to 0.7 ml Krogh-type teflon bombs, and spiked with an amount of mixed U-Th-Pb-spike corresponding to 3080 pg 2°Spb and 3-8 ng each of 233U and 235U, prior to dissolution in 0.3 m150% H F + 8/21 conc. HNO3 at 215 ° C for 7 days. U and Pb were extracted by standard HCI anion column procedures (0.05 ml resin volume). The titanite samples consisted of 5 and 18 strongly abraded grains, respectively. After dissolution, Pb was separated from U by anion exchange in 0.05 ml resin in H F medium, and purified by standard HBr anion exchange procedures. U was further purified by anion resin exchange in HNO3 and HC1. Mass spectrometry and data reduction procedures used at the ETH have been described by Oberli et al. ( 1981 ) and Barth et al. (1989). Parameters used for Pb data reduction are listed in the footnote to Table 3. Uranium mass fractionation was corrected by 233U-235U double-spike techniques. Uranium blanks were negligible.

Sm-Nd and Rb-Sr whole-rock analyses (in Stockholm) The method used for S m - N d and Rb-Sr whole-rock analysis in Stockholm follows that described by Claesson (1987), with some modifications added. About 150 mg of rock powder were dissolved with H F and HNO3 in teflon bombs at 200°C for a week. Prior to dissolution, a combined 147Sm-15°Nd tracer was added. After dissolution the samples were put through a standard HC1-HNO3 ion exchange procedure for separation of Sr and REE as a group. Sm and Nd were then separated using the a-hydroxyisobutyric acid method. Spiked Sm and Nd were run as metal ions on Re double filaments on a Finnigan MAT 261 mass spectrometer in static multicollector mode. Nd isotope compositions were calculated from the spiked runs, the Nd isotope ratio being corrected for Sm interference and normalized to ~46Nd/ 144Nd = 0.7219. Eleven runs of the La Jolla Nd-standard during the relevant time period gave a consistently low 143Nd/144Nd-value of 0.511802+20 (2 st.dev.). To adjust all values to our normal La Jolla value of 0.511842, the ~43Nd/~44Nd ratios of the samples were increased by +0.000040. For the calculation of model ages, a 147Sin decay constant of 6.54.10-t2 yr-~ and model parameters given by Jacobsen and Wasserburg (1984) and DePaolo

385

( 1981 ) were used (see footnotes to Table 4). Strontium was also analyzed on the MAT 261 mass spectrometer, with the ratio being corrected for Rb interference and normalized to S6Sr/SSSr=0.1194. Five runs of NBS SRM 987 Sr standard during the relevant time period gave a STSr/S6Sr-value of 0.710227 + 64 (2 st.dev. ). Rb and Sr contents were determined by duplicate XRF analysis, and are considered accurate to + 1%. A STRb decay constant of 1.42.10- ~~y r - ~and model parameters of McCulloch and Chappell ( 1982; see footnotes to Table 5 ) were used in the calculations of model ages.

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