New U-Pb ages from the Wiborg rapakivi area: constraints on the temporal evolution of the rapakivi granite-anorthosite-diabase dyke association of southeastern Finland

Precambrian Research, 51 ( 1991 ) 227-243

227

Elsevier Science Publishers B.V., Amsterdam

New U-Pb ages from the Wiborg rapakivi area: constraints on the temporal evolution of the rapakivi granite-anorthositediabase dyke association of southeastern Finland Matti Vaasjoki

a

O. Tapani R~im6 b and Matti Sakko a

a Unit for Isotope Geology, Geological Survey, SF-02150 Espoo, Finland b Department of Geology, University of Helsinki, P.O. Box 115, SF-O0171 Helsinki, Finland (Received November 10, 1989, revised and accepted July 15, 1990 )

ABSTRACT Vaasjoki, M., R~im6, O.T. and Sakko, M., 1991. New U - P b ages from the Wiborg rapakivi area: constraints on the temporal evolution of the rapakivi granite-anorthosite-diabase dyke association of southeastern Finland. In: I. Haapala and K.C. Condie (Editors), Precambrian Granitoids--Petrogenesis, Geochemistry and Metallogeny. Precambrian Res., 51: 227-243. New U - P b data on zircons, monazites, and baddeleyile suggest that the Wiborg rapakivi batholith and associated mafic rocks in southeastern Finland were emplaced mainly between 1650 and 1625 Ma. The earliest anorogenic magmatism related to the intrusion of the rapakivi granites was the emplacement of some diabase dykes about 1665 Ma ago, while the youngest porphyries intruded the rapakivi granites at 1615 Ma. The process involved three peaks of diabase activity at 1665, 1645, and 1635 Ma and two major granite events at 1640 +_5 and 1630 +_5 Ma, the former of which comprises also the intrusion of minor gabbroic-anorthosite bodies. On the whole, the result was the emplacement of at least 105 km 3 of rock material over a period of 50 Ma. Within southern Finland, the rapakivi magmatism continued until 1540 Ma by emplacement of the West Finnish intrusions, which combined are as extensive as the Wiborg area. Globally, the Proterozoic rapakivi event probably represents the largest pulse of intracratonic magmatism which occurred during geological history and may be a consequence of rapid growth of continental masses in the Early Proterozoic.

Introduction The first isotopic ages on the Finnish rapakivi granites were published in 1958 by Olavi Kouvo, who found that several rocks from the Wiborg rapakivi area registered U - P b zircon ages at around 1700 Ma. These data and further samples were later used by Vaasjoki (1977) who was ale to show that there was a definite difference in age between the east and west Finnish rapakivi batholiths, the western ones being ~ 1570 Ma old while the ages within the Wiborg area varied from 1670 to 1640 Ma. As much of the early data was based on single fractions from any one sample, the interpretations of Vaasjoki ( 1977 ) relied heavily on 0301-9268/91/$03.50

the use of the radiation damage diffusion model of Wasserburg (1963) and composite discordia lines derived from analyses involving several samples. It was thus apparent that in order to get the most out of new S m - N d data on the rapakivi granite-anorthosite-diabase dyke association of Finland (R~im6, in prep. ) new analyses on the old samples had to be carried out. Moreover, advances in analytical techniques, especially the development of the air abrasion method (Krogh, 1982 ), had made it possible to obtain more concordant zircon data than before, and thus ages determined by the new methods would be more precise than those previously measured. The material used for this study were partly

© 1991 - - Elsevier Science Publishers B.V.

228

M. VAASJOKI E'F AI

new zircon and monazite fractions separated from old samples. In order to support the S m Nd study, new samples were collected especially from the northern part of the Wiborg area (Suomenniemi complex ), and thus a more detailed picture of the various rock types and their age relationships could be obtained. The purpose of this paper is to report and to discuss the new results and to point out some misinterpretations of the earlier data.

is wiborgite, the rapakivi granite proper, which comprises over 80% of the Finnish part of the batholith (Vorma, 1976); other major rock types in the Finnish part are pyterlite (6%), tirilite (3%), porphyritic rapakivi granite ( 1% ), and various biotite granites ( 8% ). In the northern part of the Wiborg area there are two satellite rapakivi complexes. Ahvenisto and Suemenniemi (Fig. 1). The Ahvenisto satellite consists of horseshoe-shaped gabbro-anorthosite rim and a central rapakivi granite batholith in which the major rock type is biotite granite (Savolahti, 1956). Associated with the gabbros, anorthosites, and granites are intermediate monzodioritic rocks (Johanson, 1989). The Suomenniemi complex is composed of various granites and mafic and silicic dyke rocks. Moreover, a small anorthositic body is also present. Recently, the Suo-

Sample material The Wiborg batholith, the classical Finnish rapakivi granite area, is an epizonal multiple intrusion with an approximate diameter of 150 km and a surface area of more than 18.000 km ~ (Fig. 1 ). About one third of it is located in the U.S.S.R. The main rock type of the batholith

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NEW U - P b AGES FROM THE WIBORG RAPAKIVI AREA

menniemi complex has been the subject of detailed geochemical and isotopic work (R~im6, in prep.; see also Haapala and R~im6, 1990). Associated with the rapakivi granites and transecting the Svecofennian juvenile crust of southern Finland there are mafic dyke rocks (Subjotnian diabases) that occur as mainly northwest or west-northwest trending dyke sets (Fig. 1). The most extensive of them is the roughly 300 km long H~ime diabase dyke swarm, first described in detail by Laitakari (1969). In the Suomenniemi complex these mafic melts have occasionally intruded along the same fractures as the rapakivi-related quartz porphyry magmas, forming composite dykes (R~im~5, 1989). The sample material from the Wiborg batholith consists of ten old samples collected for the works of Kouvo (1958) and Vaasjoki (1977). From these, the remaining zircon fractions were reseparated and air abrasion fractions were analyzed for most of these. In addition, eleven samples from the Suomenniemi complex, the Sinkko granite within the Wiborg batholith, a diabase belonging to the H~ime dyke swarm (Laitakri, 1969, 1987), and a porphyritic granite from a roof pendant within the Wiborg batholith were analyzed in order to complete the set of samples and to provide further support for the S m - N d data set. Altogether, the present material consists of 97 mineral fractions representing 23 samples. They are summarized in Table 1, and their geographic locations are shown in Fig. 1. Sample A I 8 from V~irt~5, Lemi is an evengrained dark, fayalite-bearing hornblende granite named tirilite (Hackman, 1934). It is the oldest rapakivi variety in the Lappeenranta area (northern part of the Wiborg batholith), and is cut at Myllylampi by dykes of typical wiborgite (Al148), which is the "proper" rapakivi displaying ovoidal potassium feldspar phenocrysts mantled by oligoclase. Close to Myllylampi, within the Ruoholampi roof pendant, there occurs a hypabyssal rapakivi variety known as the Hiidenniemi

229

porphyry (Vorma, 1975), of which sample A422 was taken. According to Hackman (1934), the tirilite grades into the Lappee granite which in its turn grades into the Sinkko granite, which is an even-grained biotite granite. The single chemical analysis of the Sinkko granite (cf. Vorma, 1976) indicates a major element distribution similar to proper rapakivis, but the rock lacks most of the textural features typical of rapakivi, including the conspicuous "drop" quartz (originally crystallized as fl-quartz). Sample A1190-Sinkkola was collected from the type locality of this granite. The rapakivi granites are cut in places by quartz porphyry dykes, some of which form composite dykes together with the rapakivi-age diabases. These quartz porphyries were sampled at four different localities: Hamina (A323), Kiesil~i (A99), Nikkari (A1100), and Viitalampi (Al163). Also the granite porphyry dyke of Mentula, (A21 ) cross-cutting the Svecofennian rocks northwest of the Suomenniemi batholith, was reanalyzed. Close to the Suomenniemi complex, at the northern contact of the Wiborg batholith, additional analyses were carried out on the contact varieties of the Wiborg batholith at Parola and at Pes~int~ij~irvi. Sample A629 comes from the very contact and represents a fine-grained biotite granite with some fayalite and scattered potassium feldspar phenocrysts. There is distinct layering, interpreted as laminar flow structure (Vaasjoki, 1977), and the innermost of the layers grades into a pyterlite from which sample A630 was taken. The pyterlite grades within 200 m into a wiborgite, which is represented by sample A631 taken about 2 km away from the contact. Within the Suomenniemi complex, new samples from the main granite types of the batholith were analyzed. These are A1043Pohjalampi representing the hornblende granite occurring on the southeastern part of the intrusion, and A 1042-Uiruvuori representing the biotite granite which forms the bulk of the

230

M. V-~,ASJOKIt l A I

TABLE 1 The samples for isotopic age determinations from the rapakivi granites and associated rocks of southeastern Finland Sample

Map

Northing

Easting

Location

Rock type

A 1135 40323 A0096 A0069 A0118 A0629 A0630 A0631 A0098 A 1048b A 1043 A0099 A I 100 A I I 11 1130

214311 304201 311306 311407 311409 313202 313202 313202 314107 313206 313208 313209 313209 313209 313209

6814.08 6719.10 6770.00 6772.50 6797.40 6781.20 6781.12 6780.04 6801.65 6799.46 6788.77 6797.72 6791.30 6796.46 6799.24

2570.52 3509.80 3470.30 3480.50 3480.60 3505.88 3505.68 3505.40 3529.30 3510.45 3528.02 3521.56 3527.98 3527.84 3528.34

V irmaila Itamina Toyryla Veda Nurmaa Parola Parola Pesantajarvi Luotolahti Vaaralampi Pohjalampi Kiesila Nikkari Heinladenniemi Sikolampi

A I 156 A I 190 A0018 A0422 A I 148 A 1163 A0021 A1042

313305 313309 313404 313404 313404 314101 314104 314104

6759.80 6768.50 6775.60 6770.48 6771.02 6800.09 6804.21 6803.95

3558.60 3568.95 3551.40 3557.70 3555.66 3507,52 3511.96 3519.31

Sateenkankaankalliot Sinkkola Varto Ruoholampi Myllylampi Viitalampi Mentula t Jiruvuori

Diabase Quartz orphyry Hornblende granite Porphyritic biotite granite Leucogabbronorite Contact variety Pyterlitc Wiborgite Biotite granite Gabbronorite Hornblende granile Quartz porphyry Quartz porphyry Aegirine-augite syenite Hornblende-augite-fayalite granite Porphyritic biotite granite Biotite granite Tirilite Quartz-feldspar porphyry Wiborgite Quartz porphyry Granite porphyry Biotite granite

Suomenniemi batholith. In addition, more exotic rock types, such as an aegirine-augite syenite vein (A1111-Heinlahdenniemi) and a hornblende-augite-fayalite granite (A 1130Sikolampi), were analyzed from the eastern part of the batholith. Gabbronorite (A 1048bV~i~ir~ilampi ) from the anorthosite body at the northwestern flank of the batholith was also investigated. In the northwestern part of the Wiborg area, the Jaala-Iitti dyke (A96), which is considered to be the youngest rapakivi phase in that area, and its immediate country rock, the porphyritic granite of Veda (A69) were reanalyzed using old material. In the Toivarila roof pendant some 20 km south of Lappeenranta (Hackman, 1934; Simonen, 1987), there occurs a porphyritic biotite granite (AI 156) which macroscopically is curiously like the surrounding rapakivi granites. However, this rock does not exhibit the "drop" quartz typical of the rapakivi granites,

and it also contains cordierite and garnet, which are certainly not minerals characteristic of rapakivi granites.

Analytical methods The zircons were separated using mainly Clerici's solution, which when heated in a water bath, can theoretically attain a density of 4.6 g / c m 3. In practice, standard solutions of 3.8, 4.0, 4.2, 4.3, and 4.5 g / c m 3 were used. In order to assist the handpicking, the samples were usually sieved by 160 and 70/tm sieves. Only the non-magnetic (1.4 A, 1 ° tilt) fractions from a Frantz isodynamic separator were used. The usual procedure was to take part of the heaviest (and the least metamict) fraction for air abrasion treatment to create a most concordant data point and to analyze 2-3 lighter fractions for obtaining a useful downward extension of the discordia. In some cases, the samples were treated with hydrofluoric acid

N E W / ~ - P b AGES F R O M T H E W I B O R G RAPAKIV1 AREA

(HF) in order to remove surface pigments from uncrushed or inclusions from crushed samples. The dissolution of the samples and the chemical purification of U and Pb were carried out using the m e t h o d of Krogh (1973). The fitting of the discordias to the data sets was done using the method of York (1969), assuming 20.8% errors on the Pb/U-ratios and a 90% error correlation. All error estimates are given on a 2a-level. Results

The analytical results are summarized in Tables 2, 3 and 4 and in Fig. 2. The new results show that many of the samples do not conform to the continuous diffusion model of Wasserburg (1963), as the lower intercept ages are often close to 0 Ma. Thus the main intrusive phases of the Wiborg batholith are somewhat younger than believed so far, ranging from 1646 + 4 Ma for the even-grained darkish rapakivi variety (tirilite) to 1615 _+8 Ma for the quartz porphyry dyke at Hamina. The reputedly youngest rapakivitic rock in the northwestern corner of the Wiborg area, the JaalaIitti dyke, is at 1629 + 7 Ma slightly younger than its country rock, the porphyritic and highly siliceous Verla granite dated at 1638 _+4 Ma. The association of rapakivi granites and anorthosites and gabbros has been discussed on several occasions (e.g., Kranck, 1969; Bridgwater and Windley, 1973; Barker et al., 1975; Vorma, 1976; Vaasjoki, 1977; Emslie, 1978; Anderson, 1983; Haapala, 1988). The gabbroic samples analyzed during this study (A118-Nurmaa from the Ahvenisto complex and A1048b-V~i~ir~ilampi from the Suomenniemi complex) demonstrate that although the emplacement of these rocks preceded the main rapakivi activity, no analytically significant age differences are encountered. The isotopic ages from the Subjotnian diabase dyke (Laitakari, 1987; Siivola, 1987 ) demonstrate that many of these rocks are coeval with the gabbroic rocks,

231

but the concordant result from the Virmaila dyke (A1135) shows that diabase activity occurred also before the gabbro and anorthosite emplacement associated with the intrusion of the rapakivi granites. The quartz porphyry dykes in the Suomenniemi complex register ages at ~ 1635 Ma, and are thus slightly younger than the main granites of the Suomenniemi batholith (A1042Uiruvuori, A1043-Pohjalampi). The quartz porphyry of Viitalampi (Al163) is from a composite dyke that consists of quartz porphyry magmas and a central diabase. In between the silicic margins and the mafic interior of the dyke there are zones of mingled rock composed of diabase globules in quartz porphyry matrix. According to R~im6 ( 1989 ), the silicic magma intruded before the mafic magma, but the mafic magma was emplaced before the silicic melt in the fracture was completely solidified. The implication is that as the diabase of the composite dyke demonstrably is not older than the quartz porphyry margins, the time span of the Subjotnian diabase activity is extended from the previously demonstrated 20 Ma (Vaasjoki and Sakko, 1989) to at least 30 Ma, having commenced before 1665 Ma and having ended after 1635 Ma. The present data suggest that the diabases were emplaced at least in three different pulses dated at 1665, 1645, and 1635 Ma. The quartz porphyry dyke at Hamina, with a previous apparent age of 1700 Ma, is, in the light of the new analyses, definitely the youngest rock encountered within the Wiborg area. The data from this rock demonstrate particularly well the benefits of the new separation and sample preparation techniques, as the previous three almost identical analyses (see fig. 2 in Vaasjoki, 1977 ) are now supplemented by a number of more concordant results, facilitating the calculation of a reliable upper intersection age estimate. In the Lappeenranta area, the tirilite is the oldest rapakivi phase at 1646 + 4 Ma while the cross-cutting wiborgite at Myllylampi is mar-

232

M. Va, ASJOKI ET 41..

TABLE 2

U-Pb analyses of zircons, monazites, and baddeleyite from the rapakivi granites and associated rocks of southeastern Finland Sample

Fraction

Concentrations e~sU

Virmaila diabase A 1135 baddeleyite

A I 135A

2°6pb/2°4pb

2°6pb ( rad )

Lead ratios, 2°6pb= 100 2O4pb

>7pb

2ospb

373.2

95.22

266

0.3707

15.33

32.0~

370.7 542.3 410.8 196.0 186.7 485.0 264.3 229.7

6 t. 77 89.35 68.27 45.24 44.00 117.85 46. l 2 47.60

145 219 146 391 960 669 165 118

0.6896 0.4192 0.671 I 0.2545 0.1025 0.1487 0.6060 0.8431

18.75 15.64 19.24 13.47 !1.37 t2.0i l 8.26 21.55

37, i 8 25, i 3 ]7~83 .... i0.85 !(~.24 3t~.34 46.97

488.5 535. t 496.8

106.95 111.82 I 15.95

495 5697 5551

0.1609 nd 0.0129

12.31 10.06 10.18

17.82 12. l i; 12.5'4

635.0 2161.0 296.9 460.0 829.0 252.8 255.9 284.8

129.14 402.23 68.67 92.06 205.35 61.41 59.72 62.44

185 241 711 304 686 2555 397 1063

0.4965 0.4030 0.1392 0.3282 0.1430 0.0374 0.2507 0.0921

16.9t 15.39 11.79 14.44 t2.13 10.58 3.47 1.24

53.05 33.8~ 24.23 39.71 13890 20.9O 28.8:' 23.74

281.9 768.5

69.59 190.58

3155 4349

0.0316 0.0229

0.25 0.30

18.99 2194

Parolacontactvariety A629 A0629A 4 . 0 - 4 . 2 / + 160 B 3.8-4.0 C 3.6-3.8 D 4.0-4.2

1365.3 2611.3 1990.7 1697.9

326.23 581.49 384.05 406.02

12264 9610 2067 20409

0.0035 0.0074 0.0380 0.0001

0.10

i 2.72

10.06 10.36 10.07

14.31 17.46 1£77

Parola pyterlite A630 A0630A + 4 . 0 / - 70 B - 4 . 0 / + 160 C 4.3-4.5 D 4.2-4.3 E 4.3-4.5/abr

1045.1 1505.2 443.0 735.6 422.1

210.54 317.88 92.45 159.45 93.54

3812 2694 4950 4890 2729

0.0204 0.0346 0.0167 0.0180 0.0350

10.30 10.59 10.23 10.26 10.53

i£55 14.93 110; !1.4(i t 5.20

Pesant~grvi wiborgite A631 A0631A + 4 . 2 / - 160 B 3 . 8 - 4 . 2 / - 70 C 4.3-4.5/abr D 4.3-4.5

452.0 912.5 396.5 427.6

79.46 155.29 84.56 84.43

1697 753 3672 2982

0.045l 0.1183 0.0226 0.0297

0.62 I t .65 10.31 10.41

13.52 16.96 12.59 12.49

Luotolahti biotite granite A98 A0098A + 200

305.4

67.41

5700

0.0175

I 0.147

14.25¢~

Hamina quartz porphyry A323 A0323A

B C D E F G H

+ 4.2 3.8-4.1 total +4.6/HF + 4 . 6 / + 150 4.2-4.6 + 4.6 +4.6/HF

Jaala-liti dyke, hornblende granite A96 A0096A B C

total total total/abr

Verla porphyritic biotite granite A69 A0069A +4.1/70-160 B 3 . 6 - 3 , 8 / + 160 C +4.3/70-100/HF D +4.3/70-160 E monazite F +4.3/70-160/HF

G

+4.3/+160/HF

H

+4.3/-70/HF

Nurmaaleucogabbronorite A l l 8 A0118A B

+ 4.1 3.8-4.1

233

NEW U-Pb AGES FROM THE WIBORG RAPAKIVIAREA TABLE 2 (continued) Sample

Fraction

Concentrations

2O6pb/zo4pb

Lead ratios, 2°6pb= 100 2o4pb

2oTpb

2093 1030 622 1144 773 466

0.0443 0.0953 0.1596 0.0854 0.1276 0.2133

10.66 11.67 12.11 11,17 11.69 12.78

24.38 43.83 47.82 46.47 43.86 47.50

57,63 55.97 115.68 133.33 45.77

3099 2929 1954 1397 2294

0.0305 0.0307 0.0492 0.0681 0.0382

10.45 10.45 10.54 10.73 10.60

10.35 10.49 9.55 10.32 11.50

229.8 306.0 648.5 156.2

36.68 40.87 110.03 33.63

759 605 1495 2025

0.1283 0.1629 0.0659 0.0469

11.62 11.86 10.68 10.64

32.14 38.33 29.02 23.81

100.6 112.3 181.3

24.66 26.25 41.49

2193 1710 1047

0.0294 0.0379 0.0846

10.46 10.57 11.22

17.49 18.29 20.27

Heinlahdenniemiaegirine-augitesyenite A l l l l AIlIIA 4,3-4.5/+70 242.4

48.11

371

0.2657

13.69

28.31

Sikolampihornblende-augite-fayalitegranite A l l 3 0 AII30A +4.5 259.9 B 4 . 3 - 4 . 5 / + 160 1166.6 C 4 . 2 - 4 . 3 / + 160 526.5 1) 4 . 0 - 4 , 2 / + 160 617.8 E + 4 . 3 / + 160/abr 254.8 F + 4 . 5 / - 160 424.9

46.21 103.51 164.37 190.30 74.01 56.46

198 219 4683 2429 16377 36

0.5044 0.4570 0.0214 0.0413 0.0061 0.2750

16.50 15,53 10.35 10.64 10.18 13,33

49.82 31.41 17.06 17.05 15.43 75.64

Sateenkankaankalliotporphyritic biotite granite Al156 AII56A +4.3/abr 591.5 B +4.3 612.6 C 4.2-4.3 769.0 D monazite 2646.6

169.04 173.90 217.21 645.14

4132 9124 11224 60850

0.0228 0.0091 0.0078 0.0016

11.78 11.56 11.47 10.27

5.47 7.26 7.31 611.28

Sinkko biotite granite A l l 9 0 AII90A +4,55/abr B +4.55 C 4.3-4.55/+70 D 4.2-4.3/+70

291.9 315.1 400.7 597.5

86.64 86.51 11.80 168.63

2324 2165 1524 1490

0.0430 0.0462 0.0656 0.0671

10.64 10.64 10.93 10.92

12.74 12.59 14.21 17.78

135.2 268.9 1106.6 227.7 125.2

42.53 82.32 241.07 70.04 40.04

92100 7952 11900 5848 16820

0.0011 0.0126 0.0084 0.0171 0.0059

10.15 10.22 10.03 10.32 10.21

16.72 16.25 21.87 16.54 17.36

238U

2°6pb ( rad )

232.9 536.3 885.5 570.0 511.8 1132.0

54.95 122.17 190.77 137.78 116.31 234.92

P o ~ a l a m p i h o r n b l e n d e g r a n i t e A1043 AI043A +4.3/abr3h B +4.3/70-160 C 4.2-4.3/+70 D 4 . 0 - 4 . 2 / + 70 E + 4 . 3 / a b r 6h

258.0 251.1 637.5 801,6 185.9

Kiesil~i quartz porphyry A99 A0099A + 4.3/HF B +4.3 C 4.2-4.3/HF D + 4.3/HF/crush Nikkariquartz AII00A B ("

V ~ r ~ l a m p i g a b b r o n o r i t e A1048b A1048bA +4.5 B 4.3-4.5 C 4.2-4.3 D 4.3-4.5/abr E 4.3-4.5 F 4.0-4.2

porphyry A l l 6 0 + 5 . 4 / + 160/abr + 4 . 5 / + 160 4 . 3 - 4 . 5 / + 160

Vfirt6tirilite AI8 A0018B + 4 . 5 / - 100 C + 4 . 2 / - 100 D -4.0 E + 4 . 2 / - 100 F +4.6/abr

2O~pb

1

.34

•

M Va,.~.S.I()KI E l '~

TABLE 2 ( continued )

Sample

Fraction

2O6pb/el>4pb

( 'oncentrations -'~'L

Lead ratios, 2m'Pb--- 100

>~Pb( rad )

Ruoholampi quartz-feldspar porphyry A422 A0422A B (7 D E F (i H

- 4.0 + 4 . 5 / a b r 2h + 4 . 5 / + 70 4 . 3 - 4 . 5 / + 70 4 . 2 - 4 . 3 / + 70 4 . 0 - 4 . 2 / + 70 + 4 . 5 / a b r 4h monazitc

Myllylampi wiborgite A 1148 -\1148A + 4 . 5 / + 160/abr B +4.5 (" 4 . 3 - 4 . 5 / + 160

36!).4 84,6 97,9 192, 7 375.8 378,6 74.2 ~52.b

86.81 20.86 23.77 29.63 91.13 92,01 18.55 16 l.66

724 627 381 456 658 726 835 1806

0.1327 0+ 1444 0.2590 0.2169 0.14t~3 0.1355 0.1103 0.0412

I 1.92 12.05 13.62 [ 3.I)4 12.12 11.91 11.6{? 11).6 :'

34.0i 23.c,2 ~ 1,(~7 29.63 2'-L';() ~,t.54 24~02 926.3a,

194.t~ 244.2 492.7

45.38 46.85 80.39

4386 3062 1804

0.017b 0.0256 0.0520

I (i.28 10.3~ 10.5 t

i 3.1 ,~ t2 5 [.3.58

43A; 56.2 63.!)

10.78 12.09 14.05

606 498 213

0.1623 0.2008 0.4690

12.31 t 2.9<) 16.53

2¢5.(~5 28.87 38.(~2

I 176+0 110.3 142.5 331.8 1115

209.91 26.87 27.43 25,93

585 4375 705 815 740

I). 1683 0.0192 0.1393 (11.1215 0.1318

12.22 10.32 I I.C72 11.61 11.90

28.30 22.8(~ 27.7t 27+34 ?6.98

38[.2 718.2 3174 375.b 976.6

73.66 139.84 73.43 75.48 182.44

938 790 1506 675 1129

0. 1042 0.1250 (/.0618 0.1432 0.087t)

11.4,4 l 1.75 10.92 11.99 11.14

26.04 26.[C 21.21 2727 21.82

Viitalampi quartz porphyry A 1163 •\ I 163A B ('

+ 4.5/abr +4.5 4 . 3 - 4 . 5 / + 160

Mentula granite porphyry A21 ~002 IA 3,8-4, I B + 4.6/HF/crush (+ +4.6

D

4.4-4.6/+ 70

E

+4.6/HF

60.12

Uiruvuori granite A 1042 klO42A B ( D E

+ 4 . 3 / - 160 4 . 2 - 4 . 3 / - 160 + 4 . 3 / + 160/abr + 4 . 3 / + 160 4 . 0 - 4 . 2 / - 160

Concentrations in/lg/g. Corrected for blank.

ginally younger. The result for the Sinkko granite, 1636 + 8 Ma, clearly establishes that this rock belongs to the rapakivi suite, but the age difference suggests that this rock may not be a direct derivative of the tirilite magma. Results from the three samples from the northern contact of the Wiborg batholith at Parola (A629, A630, and A631 ) demonstrate that these rocks were emplaced at about 1630 Ma, and thus the northern fringe of the Wiborg rapakivi batholith is in this area definitely younger than the Suomenniemi batholith, a relationship already suggested by Vorma ( 1972 )

on the basis of crystallographic data on K-feldspars. Previously, when the data were interpreted according to a diffusion model, the contact variety seemed to be significantly younger than the wiborgite and the pyterlite in its vicinity (cf. Vaasjoki, 1977). The new data, however, have removed this discrepancy, and these three rocks may be regarded coeval within experimental error. The result from the hornblende-augite-fayalite granite A 1 130-Sikolampi, 1636 + 23 Ma, shows that this rock is part of the rapakivi suite. It has, however, an abnormal discordancy pat-

NEW U-Pb AGES FROM THE W1BORG RAPAKIV1AREA

235

TABLE 3 U / P b ratios and apparent isotopic ages for zircons, monazites, and baddeleyite from the rapakivi granites and associated rocks of southeastern Finland Sample

Atomic ratios

Apparent ages ( Ma )

206pb/238

2°vpb/235U

l°Tpb/2°6pb

T~6/8~

T(7/5)

T~ 7/61

AlI35A

0.2949

4.160

0.1020

1665

1666

1667

A0323A B C D E F G H

0.1926 0.1904 0.1921 0.2667 0.2724 0.2808 0.2017 0.2395

2.639 2.587 2.645 3.661 3.740 3.855 2.751 3.270

0.0993 0.0985 0.0998 0.0996 0.0996 0.0996 0.0989 0.0990

1126 1124 1133 1526 1553 1596 1185 1384

1312 1297 1313 1564 1580 1605 1343 1474

612 596 621 616 616 616 604 1606

A0096A B (7

0.2529 0.2408 0.2697

3.513 3.339 3.721

0.1008 0.1006 0.1000

1449 1391 1539

1524 1490 1575

1630 1634 1625

A0069A B C D E F G H

0.2351 0.2151 0.2673 0.2313 0.2863 0.2808 0.2698 0.2534

3.264 2.914 3.639 3.160 4.010 3.897 3.725 3.486

0.1007 0.0983 0.0987 0.0991 0.1016 0.1007 0.1002 0.0998

1361 1256 1526 1341 1622 1595 1539 1455

1472 1385 1557 1447 1636 1612 1576 1524

1637 1591 1600 1607 1653 1636 1627 1620

A0118A B

0.2853 0.2866

3.978 3.994

0.1011 0. lOll

1617 1624

1629 1632

1645 1644

A0629A B C D

0.2762 0.2574 0.2230 0.2764

3.809 3.518 3.007 3.814

0.1000 0.0991 0.0978 0.1001

1571 1476 1297 1573

1594 1531 1409 1595

1625 1608 1583 1625

A0630A B C D E

0.2328 0.2441 0.2412 0.2505 0.2561

3.218 3.386 3.327 3.459 3.546

0.1002 0.1006 0.1000 0.1001 0.1004

1349 1407 1393 1441 1469

1461 1501 1487 1517 1537

1628 1635 1624 1626 1632

A0631A B C D

0.2032 0.1967 0.2464 0.2282

2.786 2.697 3.397 3.147

0.0994 0.0995 0.0999 0.1000

1192 1157 1420 1324

1351 1327 1503 1444

1613 1614 1623 1624

A0098A

0.2551

3.529

0.1003

1464

1533

1630

A1048bA B C D E F

0.2727 0.2633 0.2490 0.2793 0.2626 0.2398

3.778 3.763 3.403 3.849 3.596 3.252

0.1005 0.1037 0.0991 0.0999 0.0993 0.0983

1554 1506 1433 1588 1503 1385

1588 1584 1504 1603 1548 1469

1633 1690 1607 1623 1611 1593

236

M, VAASI()KI E l AI

FABLE 3 (continued) Sample

Atomic ratios

Apparent ages 1 Ma )

2o6pb/238 U

~'O7pb/23511

l°Vpb/2°6pb

T~ ,'s)

I/; :)

]'i 6 ~

AI043A B C D E

0.2582 0.2577 0.2097 0.1922 0.2845

3.571! 3,562 2.852 2.596 3.954

0,1003 0,1003 0.0986 0.0979 0.1008

1480 1477 1227 1133 1614

1542 1541 i369 t29 () 1624

62~ 629 598 585 638

A0099A B ( D

0.1844 0.1544 0.1961 0.2488

2.503 2,043 2,642 3,428

0.0984 0.0960 0.0977 0.0999

1091 926 1154 1432

i271 tl30 !312 151J

594 548 581 622

>XlI()0A B C

0.2833 0.2701 0.2645

3,929 3.743 3.668

0.1006 0.1005 (/.1006

1608 1541 1512

t619 1580 1564

~35 633 634

Al 11 IA

0.2294

3.172

0.1003

1331

1450

62 ~)

AII30A B (" D E F

0.1129 0,0644 0.2818 0.2764 0.2666 0.0773

1.482 0.814 3.907 3.838 3.710 1.014

0.0952 0.0917 0.10(/6 0, t007 0.10119 0.0952

689 402 1600 1573 1523 479

~22 604 i615 1600 1573 71()

53i 461 !634 1637 164i i531

AI156A B (" D

0.3264 0.3281 0.3303 0.2817

5.003 5.173 5.204 3.982

0.1136 0.1143 0.1143 0.1025

1821 1829 1839 1600

1838 1848 1853 1630

1857 1869 i868 i670

AI190A B C D

0.2757 0.2551 0.2545 0.2502

3.819 3,520 3.519 3.449

0.1005 0.1001 0.1003 0,100(I

1569 1464 1461 1439

1596 !53l !531 I515

~632 !~25 i¢,29 [(~24

A0018 B ( D E g

0.2861 0.2787 0.2518 0.2817 0.289l

3.998 3.860 3.441 3.919 4.036

0. I1114 O. 1005 0.0991 0.1009 0.1013

1621 1584 1447 1599 1636

11133 ] ¢~05 1513 1617 1641

i 649 I(~33 J6()" i641 i647

,XO422A B (" D g F G H 1

0.2751 0.2851 0.2806 0.2768 0.2803 0.2809 0.2888 0.2784 0.2863

3.836 3,955 3.893 3.836 3.888 3.892 4.014 3.875 3.987

0.101! 0.1006 0.1/106 0.1005 0.1006 0.1005 0.1008 0.1010 0.1010

1566 1617 1594 1575 1592 1595 1635 t583 1622

1600 1625 1612 1600 1611 1611 1637 1608 16131

i645 !()35 1635 !633 1635 i633 1639 1641 1642

AII48A B (77

0.2695 0.2217 0.1886

3.729 3.049 2.546

0.1004 0.0998 0.0979

1538 1290 1113

1577 1420 1285

!630 1019 1585

237

NEW U-Pb AGES FROM THE WIBORG RAPAKIVIAREA TABLE 3 (continued) Sample

Apparent ages (Ma)

Atomic ratios 2°6pb/238U

2°7pb/z35U

l°Tpb/2°6pb

T~6/s)

T~ v/5 ~

TI v/6

AlI63A B C

0.2836 0.2484 0.2543

3.939 3.471 3.531

0.1007 0.1013 0.1007

1609 1430 [460

1621 1520 1534

1637 1649 1637

A0021A B C D E

0.2064 0.2815 0.2225 0.2094 0.2640

2.817 3.901 3.065 2.867 3.669

0.0990 0.1004 0.1000 0.0993 0.0999

1210 1599 1296 1226 1510

1360 1614 1424 1373 1565

1605 1633 1623 1611 1622

A1042A B C D E

0.2233 0.2250 0.2674 0.2322 0.2152

3.080 3.112 3.710 3.206 2.951

0.1001 0.1003 0.1006 0.1001 0.0994

1299 1308 1527 1346 1256

1427 1435 1573 1458 1394

1625 1629 t636 1626 1613

Atomic ratios corrected for common lead. 6/4:15.91 ; 7/4:15.37; 8/4:35.38 (rocks of rapakvi association). 6/4:15.71:7/4:15.28; 8/4:35.21 (A1156). TABLE 4 Upper and lower intercept data for the age determinations from the rapakivi granites and associated rocks in southeastern Finland (when the calculation of the intercepts is not feasible, the 2°Tpb/2°6pb age of the most concordant sample is shown ) Sample and locality

Rock type

Concordia intercepts Upper

AI 135 Virmaila A0323 Hamina A0096 T6yryl/i A0069 Verla A0118 Nurmaa A0629 Parola A0630 Parola A0631 Pes~int~ij~irvi A0098 kuotolahti A 1048b V~i~ir~ilampi A 1043 Pohjalampi A0099 Kiesil~i A1100 Nikkari A1111 Heinlahdenniemi A I 130 Sikolampi A1156 Sateenkankaankalliot A 1190 A0018 A0422 A 1148 A1163 A0021 A 1042

Sinkkola Vart/5 Ruoholampi Myllylampi Viitalampi Mentula Uiruvuori

Diabase Quartz porphyry Hornblende granite Porphyritic biotite granite Leucogabbronorite Contact variety Pyterlite Wiborgite Biotite granite Gabbronorite Hornblende granite Quartz porphyry Quartz porphyry Aegirini-augite syenite Horblende-augite-fayalite granite Porphyritic biotite granite, zircon Porphyritic biotite granite, monazite Sinkko biotite granite Tirilite Quartz-feldspar porphyry Wiborgite Quartz porphyry Granite porphyry Biotite granite

Lower

7 / 6 : 1 6 6 7 +_9 1615 -+ 6 42_+ 38 1630 +_ 5 10+_101 1639 _+ 2 230-+ 18 7/6:1645+_5 1631 -+ 4 284_+ 41 1634 + 24 49_+201 1631 _+ 9 70 _+ 43 7/6:1641 _+ 1 1636 _+ 14 339_+ 137 1641 + 2 191-+ 9 175+_ 71 1639_+ 9 10+ 67 1635 _+ 2 7/6:1629_+6 1636 + 23 83+ 16 7/6: 1868+_ 1 7/6:1670+3 128+ 107 1636 + 8 1646 -+ 4 414_+ 86 296 ± 178 1641 +_ 4 171+ 91 1642 +_22 - 103_+253 1636 _+ 16 140+- 56 1638 +_32 81+ 43 1639 -+ 6

M. VAASJ()KI ET At,

238

206Pb/238U 06/238UFHE SUOMENNIEMI BATHOLITH

ANORTHOSITE AND DIABASE

30

1700 / f _/~

30

i"

I

r

A 1135 V,rmaila A01~8 Nurrnaa [ ' A1045b V&ar~lampl

.[3-

.

21

/

~;" A0099-Kiesila ! A l t 0 0 NIkk;~r~

/ 207Pb/235U

32

f"

i

"4~J"

- 2

~'J

.....

~ f f

1500 / ~ 1 5 0 0 . ~

I fob

Quartz porphyry dykes

-~"" ~

42

207Pb/235U

26

4

206Pb/238U -206Pb/238U THE SUOMENNIEMI BATHOLITH THE LAPPEENRANTA AREA

30

! ;'00 . ., I~"

1700 . /

25 500 ~

~ A2/

:'~F 7l¢ ~

,ooo~~ ~ 21

/

/

~

"El'/

/

'

:::12:

iorphyry

~::i::e

/-

,

Ay

:: ::::; :::°:: ...........

L'S

.,po,235U

-{J" ,

-206Pb/238U

•

30

~ 1043 HOrnblende gi,ri:,

A 1 1 9 0 Sinkk . . . . . . . . .

...............

'Z_ ....

206Pb/238U

THE PAROLA CONTACT

!700 ~ / ..~

THE NORTHWEST CORNER

3O

17 .0 I

"

...li:W r; .......

21

/

A630-Pyterlde AE31-Wiborgib~

~2"3

207Pb/235U

A96 ;he Jaala--lilti dyku

/S / "~'/2i3

41

30

£ AOg-Porphyriticgranll~ i

207Pb/235U

PORPHYRY DYKES

"

ROOF PENDANT PORPHYRITIC GRANITE

1700 ~ 1 / "

30

1500

~

'

1

~ '

-2, / / " / .

~/J,

411

206Pb/238U

" 206Pb/238U

•

~/;::S:~

~o/~

j+/~/"

/ ~ 22,66

,

207Pb/235U 20" ,

: A0021-Menlula + A 0 3 2 3 Hamina

,

,

4 1

p'J;/÷// i _

190~,,~1

700 Monazde

~

4-J AAlt148-Wiborgite156-Porphyritgic....... .0,P°,..°

L//

214

,

i

~'-

i

i

i

.__~

L

56 --a-- .....

Fig. 2. The analytical data presented on concordia diagrams. Note that the lengths o f the axes vary between the individual diagrams.

NEW U-PI3 AGES FROM THE WIBORG RAPAKIVI AREA

tern, as the heavier fractions are turbid, very high in uranium and extremely discordant. The three lighter fractions consist of clear, zoned crystals typical of the rapakivi granites and would alone determine an upper intercept age of 1632 + 1 Ma, but he lower intercept is then negative ( - 216 + 12 Ma). A totally new rock type in the Finnish rapakivi suite, peralkaline syenite, is represented by sample A1111-Heinlahdenniemi, which is from a dyke that cross-cuts the even-grained hornblende granite of the Suomenniemi batholith. Although the zircon content of the rock is so small that no proper discordia line could be constructed, the one zircon fraction analyzed proves unequivocally that the rock is of rapakivi age. As it cross-cuts the batholith, circumstantial evidence and its own 2°7pb/2°6pb age dictate that it should be between 1640 and 1630 Ma old. The three zircon analyses from the porphyritic granite in the Toivarila roof pendant at Sateenkankaankalliot (A1156) demonstrate clearly that this rock is considerably older than the rapakivis, probably of Svecofennian ( ~ 1900 Ma) age. The discordancy pattern of the analyzed fractions probably reflects continuous diffusion of the lead both before and after the rapakivi event, as well as an episodic lead loss during the immersion of the roof pendant in the rapakivi magma. This interpretation is supported by the nearly concordant monazite result (2°Vpb/2°6pb age of 1670+3 Ma), which demonstrates that the roof pendant was heated close to 600 °C during the intrusion of the rapakivi magma. It is worth noting in this context that the monazites from the rapakivi granites register similar ages as their zircons (Fig. 2) and consequently the intrusion of the Wiborg batholith is the latest hightemperature regional event in southeastern Finland. Discussion

According to the new data, it seems that the intrusion of the rapakivi granites in southeast-

239

ern Finland commenced about 1645 to 1650 Ma ago in a number of areas. Anorthositic rocks, which probably are cogenetic but evidently not comagmatic with the rapakivi granites (e.g., R~im6 and Haapala, 1990), were the first rocks to be emplaced both in the Wiborg, Suomenniemi, and Ahvenisto areas. Evengrained rapakivi granites, excluding the Sinkko granite, intruded in the Suomenniemi and Lappeenranta areas not much later, as all rocks in these areas are within experimental error in the 1645 to 1640 Ma age bracket. The Sinkko granite is slightly younger, and the present result casts some doubt on the earlier ideas that it would belong to the tirilite-Lappee granite sequence and thus would be older than the wiborgites in the Lappeenranta area. As is shown by the porphyry dyke evidence, the magmatic activity in the Suomenniemi area terminated about 1635 Ma ago. The youngest major intrusive phases within the Wiborgite rapakivi batholith are the rocks of the northern contact at Parola and a wiborgite in the southeastern part of the batholith (A29-Muurikkala, now dated at 1633 +_8 Ma; V. Suominen, pers. commun., 1989). The intrusion of these rocks did not commence until about 1635 Ma ago, i.e. after the emplacement of the final members of the Suomenniemi complex. This evidence from the isotopic data is also corroborated by field observations: e.g., the Lovasj~irvi mafic intrusion, dated at 1645 _+3 Ma (Siivola, 1987 ), is brecciated by rocks belonging to the Wiborg batholith. In southeastern Finland, the youngest rock of rapakivi affinity is the quartz porphyry at Hamina (A323), which cross-cuts the surrounding wiborgite. As it has been dated at 1615 + 5 Ma, it is evident that in the Wiborgite batholith, the intrusion of the rapakivi granites lasted for the considerable time period of 30 Ma. An interesting feature of the zircon data is that many of the Finnish rapakivi granites exhibit elevated 2°spb/2°6pb ratios (up to 0.75, Table 2) when compared to zircons from many

240

other granitic rocks. This means in effect that the T h / U ratio in the rapakivi zircons and consequently in the magmas from which they crystallized must have been abnormally high. The present material suggests that the enrichment in 2°Spb is particularly high in the early intrusive rocks and the late quartz porphyry dykes, while most of the rocks of the main intrusive phases seem to be more normal. One interpretation of the data is that the early and the late magmas were rather small in volume and thus did not absorb much of the country rock material during their ascent from a Thenriched and U-depleted lower crust. The major intrusive phases, in contrast, incorporated more of the U-enriched pre-existing upper crust into them and thus exhibit more normal 2°Spb/ 2°6pb ratios in their zircons. An opposite character is shown by the Svecofennian granite A 1156 from the country rock roof pendant in the northwestern part of the Wiborg batholith. The three zircon fractions of this sample show 2°8Pb/2°6pb ratios ranging from 0.05 to 0.07 (Table 2 ). These values are clearly lower than the typical e°Spb/2°rPb ratios of zircons in granitoid rocks (usually the radiogenic 2°8pb/2°6pb ratios are around 0.1; Huhma, 1986 ), and they probably register the low time-integrated T h / U ratio of the sedimentary protolith of this S-type granite. It should be noted in this context that as the fractionation of Sm and Nd between upper and lower crust is rather small (e.g., Ben Othman et al., 1984) and as the Finnish rapakivi granites most probably derive from crust which formed during Svecofennian time, i.e. only 200 to 300 Ma earlier (R~im6 et al., 1989; Haapala and R~imr, 1990), no substantial differences in the S m / N d data should be expected. The intrusion of the Subjotnian diabase dykes which began 1665 Ma ago, possibly peaked at ~ 1645 Ma (Vaasjoki and Sakko, 1989; Laitakari and Leino, 1989), and as is suggested by the Viitalampi data continued until 1635 Ma. This evidence shows that the extensional rupturing of the Fennoscandian

M. VAASJOKI EI Al..

Shield had already commenced some 20 Ma prior to the emplacement of the first rapakivi granite magmas. Whether the emplacement of the rapakivi granite batholiths was a consequence of the extensional rupturing, or whether the ascent of the rapakivi magmas from deeper levels actually caused deeply penetrating fractures to tap the mantle, as was suggested by Laitakari and Leino (1989), is a question which cannot be answered on the basis of the present data set alone. The association of rapakivi granites with more basic rocks (anorthosites, mangerites, etc.) was first discussed by Kranck (1969), who considered that the magmas were formed deep in the continental crust as a result of interaction of mantle/lower crust-derived basalt magma and crust-derived anatectic granitic magma. Bridgwater and Windley ( 1973 ) proposed that the emplacement of these rocks was associated with crustal rifting and coined the expression "anorthosite event", which occurred on a global scale in middle Proterozoic times. In Greenland, the anorthosite activity commenced 1750 Ma ago (Gulson and Krogh, 1975). In Labrador, three distinct episodes of concurrent intrusion of anorthosites, mangerites, igneous charnockites, and granites have been recorded, dated about 1640, 1360, and 1150 Ma (Emslie, 1978; Emslie and Hunt, 1990 ). Most of the anorogenic granites of midcontinental and southwestern U.S. register ages around 1350 to 1500 Ma (Anderson, 1983; Van Schmus et al., 1987 ), and later anorogenic activity in west-central U.S. is represented by the 1015 Ma old Pikes Peak batholith (Barker et al., 1975 ). According to Emslie ( 1978 ), the mid-continental belt of anorthosite-adamellite suites in North America is a manifestation of melting and magma reduction in the mantle that ultimately matures into full-scale continental rifting. Anderson (1983) emphasized the role of the mantle-derived anorthositic and mangeritic magmas in providing the heat to fuse the lower crust to produce granitic magmas, and considered the tectonic framework an

241

NEW U-Pb AGES FROM THE WIBORG RAPAKIVI AREA

incipient rift that failed to integrate into a world-wide plate system. A different view was held by Nelson and DePaolo (1985) who suggested that the 1400 Ma old granites of midcontinental U.S. may be related to subduction processes related to formation of new crust in the southern U.S. A relation to orogeny for these granites was also proposed by Van Schmus and Bickford ( 1981 ) (see also Vorma, 1976). Certainly, the dimensions of some of the rapakivi batholiths are staggering. The outcropping area of the Wiborg batholith exceeds 18,000 km 2 and its volume is probably of the order of 105 km 3. As the rapakivi intrusions of southwestern Finland (Alan& Vehmaa, and Laitila) are probably connected underground (Vaasjoki et al., 1988 ), they together would be of a similar size. Moreover, smaller granite bodies and quartz porphyry dykes of rapakivi age occur also elsewhere in southern Finland (Vaasjoki, 1977; T6rnroos, 1984; Idman, 1989). A fact worth contemplation is that few other intracratonic plutons resembling the size of the rapakivi granite batholiths are known to have formed during geological history. Although each individual rapakivi batholith formed over a considerable period of time and the rocks of the anorthosite-granite association formed within Laurasia during the time interval of about 700 Ma, the "anorthosite-rapakivi event" nevertheless occurred in a geologically relatively restricted time. Ample evidence suggests that the continental plates had attained 60-80% of their present size in Early Proterozoic times, with about one third of them formed after the Archaean (Moorbath, 1985; Ashwal, 1989 ). The Proterozoic continental crust masses, formed by differentiation of the mantle and admixture of detritus from pre-existing Archaean crust (Patchett and Bridgwater, 1984; Huhma, 1986; Patchett and Kouvo, 1986) formed the framework for all subsequent geological history. As far as the anorthosite-rapakivi association is

concerned, the process could have been simple. When a continental plate reached a suitable thickness and was struck by a mantle heat source, its lowest parts started to melt and the formation of anatectic melts commenced. The anatectic melts rose upward, were emplaced as epizonal batholiths in the upper crust, and as a consequence, the continental plates lost from their lower parts most of the material necessary for creating granitic bodies. Recently, a probable mechanism for the process was elegantly introduced by Hoffman ( 1989 ).

Conclusions Combined, the new data suggest that the Wiborg rapakivi batholith is slightly younger than previously thought, with the main intrusive activity having occurred between 1650 and 1625 Ma. The earliest magmatism related to the intrusion of the rapakivi granites was the emplacement of some diabase dykes ~ 1665 Ma ago, while the last quartz porphyries intruded the rapakivi granites at 1615 Ma. Thus the whole process lasted for about 50 Ma, with the main magmatism occurring at least in two different pulses dated at 1640+ 5 and 1630 + 5 Ma. Moreover, it should be noted that the Wiborg batholith is still markedly older than the rapakivi granites of southwestern Finland, which have been dated at around 1580 to 1540 Ma. Although the emplacement of the rapakivi batholiths involved considerable amounts of time, and the formation of the Laurasian anorthosite-granite complexes span a time interval of 700 Ma, the formation of the Proterozoic rapakivi granites can be considered a more or less singular event, which probably is related to the formation of large permanent continental masses in the Early Proterozoic.

Acknowledgements Our particular thanks go to Dr. Olavi Kouvo who performed the analyses for samples A69, A99, A18, A323, and A21 "aus Liebe zur

242

Kunst", as the German saying goes. In the treatment of those and the other samples as well, Mrs. Tuula Hokkanen and Mrs. Marita Niemel~i (chemical preparation) together with the staff of the Unit for Mineralogy of the Geological Survey of Finland (mineral separation) were of invaluable assistance. The field work in the Suomenniemi area was supported by the Academy of Finland (SA 01/349 ). This paper is IGCP Project 257 (Precambrian dyke swarms) Publication No. 29. As with many geological theories, the principles of the last few paragraphs were trashed out in evening sessions between the present Prof. Atso Vorma (then a research geologist) and the present senior author (then a field hand) during field work some 20 years ago. Since then, night-time theories have become more plausible and must have occurred to several other gangs as well. References Anderson, J.O., 1983. Proterozoic anorogenic granite plutonism of North America. In: L.G. Madaris, Jr., C.W. Myers, D.M. Mickelson and W.C. Shanks (Editors), Proterozoic Geology. Geol. Soc. Am. Mem., 161 : 13-154. Ashwal, L.D., 1989. Introduction. In: L.D. Ashwal (Editor), Growth of the Continental Crust. Tectonophysics, 161: 143-352. Barker, F., Wones, D.R., Sharp, W.N. and Desborough, G.A., 1975. The Pikes Peak Batholith, Colorado Front Range, and a model for the origin of the gabbro-anorthosite-syenite-potassic granite suite. Precambrian Res., 2: 97-160. Ben Othman, D., Polv6, M. and All~gre, C.J., 1984. N d Sr isotopic composition of granulites and constraints on the evolution of the lower continental crust. Nature, 307: 510-515. Bridgwater, D. and Windley, B.F., 1973. Anorthosites, post-orogenic granites, acid volcanic rocks and crustal development in the North-Atlantic shield during the mid-Proterozoic. Geol. Soc. S. Aft. Spec. Publ., 3: 307316. Emslie, R.F., 1978. Anorthosite massifs, rapakivi granites, and late Proterozoic rifting of North America. Precambrian Res., 7:61-98. Emslie, R.F. and Hunt, P.A., 1990. Ages and petrogenetic significance of igneous mangerite-charnockite suites

M. V*k,~,S,It)KI EJ . k l

associated with massif anorthosltes, Grcnxille t'rovince. J. Geol., 98: 213-231. Gulson, B.L. and Krogh, T.E., 1975. Evidence of multiple intrusion, possible resetting of U - P b ages, and ne~ crystallization of zircons in the post-tectonic intrusions ("Rapakivi granites" ) and gneisses from South Greenland. Geochim. Cosmochim. Acta, 39: 65-82. Haapala, I., 1988. Metallogeny of the Proterozoic rapakivi granites of Finland. In: R.P Taylor and D. F. Strong (Editors), Recent Advances in the Geology of Granite-Related Mineral Deposits. (?an. Inst. Min. Metall. Spec. Vol., 39:124-132. Haapala, 1. and R~im6, O.T., 1990. Petrogenesis of the Proterozoic rapakivi granites of Finland. In: H.J. Stei n and J.L. Hannah (Editors), Ore-Bearing Granite Systems: Petrogenesis and Mineralizing Processes. Geol. Soc. Am., Spec. Pap., 246: 275-286. Hackman, V., 1934. Das Rapakiwirandgebiet der Gegend von Lappeenranta (Willmanstrand). Bull. (2omnL G6ol. Finlande, 106, 82 pp. Hoffman, P.F., 1989. Speculations on Laurentia~s first gigayear (2.0 to 1.0 Ga). Geology, 17:135-138. Huhma, H., 1986. Sm-Nd, U - P b and Pb-Pb isotopic evidence for the origin of the Early Proterozoic Svccokarelian crust in Finland. Geol. Surv. Finland B u l l 337, 48 pp. Idman, H.,, 1989. The Siipyy granite--a new rapakivi occurrence in Finland. Geol. Soc. Finland B u l l ¢~l: 123127. Johanson, B.S., 1989. Monzodioritic rocks of the gabbro-anorthosite complex associated with the Ahvenisto rapakivi batholith, southern Finland. In: I. Haapala and Y. K~ihk6nen (Editors), Symposium Precambrian Granitoids Abstracts. Geol. Surv. Finland Spec. Pap,, 8: 74. Kouvo, O., 1958. Radioactive age of some Finnish preCambrian minerals. Bull. Comm. G6ol. Finlande, 182~ 70 pp. Kranck, E.H., 1969. Anorthosites and rapakivi magmas from the lower crust. In: Y.W. Isachsen (Editor), Origin of Anorthosites and Related Rocks, NY. State Mus. Sci. Serv. Mem., 18: 93-97. Krogh, T.E., 1973. A low-contamination method for hydrothermal decomposition of zircon and extraction U and Pb for isotopic age determinations. Geochim. Cosmochim. Acta, 37: 485-494. Krogh, T.E., 1982. Improved accuracy of U-Pb zircon ages by the creation of more concordant systems using air abrasion technique. Geochim. Cosmochim. Acta, 46: 637-469. Laitakari, 1., 1969. On the set of olivine diabasc dikes in H~ime, Finland. Bull. Comm. G6ol. Finlande, 241, 65 PP. Laitakari, l., 1987. The Subjotnian diabase dyke swarm of H~ime. In: K. Aro and I. Laitakari (Editors), Diabases and Other Mafic Dyke Rocks in Finland. Geol.

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Surv. Finland Rep. Invest., 76:99-116 (in Finnish with English abstract and figure and table captions). Laitakari, I. and Leino, H., 1989. A new model for the emplacement of the H/~me diabase dyke swarm, central Finland. In: S. Autio (Editor), Geological Survey of Finland Current Research 1988. Geol. Surv. Finland Spec. Pap., 10: 7-8. Moorbath, S., 1985. Crustal evolution in the early Precambrian. Origins Life, 15:251-261. Nelson, B.K. and DePaolo, D.J., 1985. Rapid production of continental crust 1.7 to 1.9 b.y. ago: Nd isotopic evidence from the basement of the North American midcontinent. Geol. Soc. Am. Bull., 96: 746-754. Patchett, P.J. and Bridgwater, D., 1984. Origin of continental crust of 1.9-1.7 Ga age defined by Nd isotopes in the Ketilidian terrain of South Greenland. Contrib. Mineral. Petrol., 87:311-318. Patchett, J. and Kouvo, O., 1986. Origin of continental crust 1.9-1.7 Ga age: Nd isotopes and U - P b zircon ages in the Svecokarelian terrain of South Finland. Contrib. Mineral. Petrol., 92: 1-12. R~im6, O.T., 1989. Bimodal silicic-basic magmatism associated with rapakivi granites: petrography and petrology of composite diabase-quartz porphyry dykes and K-feldspar diabases in the Suomenniemi complex, southeastern Finland. In: I. Haapala and Y. K~ihk6nen (Editors), Symposium Precambrian Granitoids Abstracts. Geol. Surv. Finland Spec. Pap., 8: 105-106. R/iraqi, O.T. and Haapala, I., 1990. The rapakivi granites of eastern Fennoscandia: a review with insights into their origin in the light of new S m - N d isotopic data. In: C.F. Gower, T. Rivers and B. Ryan (Editors), MidProterozoic Laurentia-Baltica. Geol. Assoc. Can. Spec. Pap., 38. In press. R~im/5, O.T., Vaasjoki, M. and Huhma, H., 1989. S m - N d and Pb-Pb isotopic constraints on the origin of the rapakivi granites and associated tholeiitic dyke rocks in southern Finland. In: I Haapala and Y. K~ihk6nen (Editors), Symposium Precambrian Granitoids Abstracts. Geol. Surv. Finland Spec. Pap., 8: 107-108. Savolahti, A., 1956. The Ahvenisto massif in Finland. Bull. Comm. G6ol. Finlande, 174:96 pp. Siivola, J., 1987. The mafic intrusion of Lovasj~irvi. In: K. Aro and I. Laitakari (Editors), Diabase and Other Mafic Dyke Rocks in Finland. Geol. Surv. Finland Rep. Invest., 876:121-128 (in Finnish with English abstract and table and figure captions).

243 Simonen, A., 1987. Kaakois-Suomen rapakivimassiivin kartta- alueiden kallioper~i. Summary: Pre-Quaternary rocks of the map-sheet areas of the rapakivi batholith in SE-Finland. Explanation to the maps of Pre-Quaternary rocks, Sheets 3023+3014, 3024, 3041, 3042, 3044, 3113, 3131,3133. Geological Survey of Finland, 49 pp. T6rnroos, R., 1984. Petrography, mineral chemistry and petrochemistry of granite porphyry dykes from Sibbo, southern Finland. Geol. Surv, Finland Bull., 326, 43 Pp. Vaasjoki, M., 1977. Rapakivi granites and other postorogenic rocks in southern Finland: their age and the lead isotopic composition of certain associated galena mineralizations. Geol. Surv. Finland Bull., 294, 64 pp. Vaasjoki, M. and Sakko, M., 1989. The radiometric age of the Virmaila diabase dyke: evidence for 20 Ma of continental rifting in Padasjoki, southern Finland. In: S. Autio (Editor), Geological Survey of Finland Current Research 1988. Geol. Surv. Finland Spec. Pap,, 10: 43-44. Vaasjoki, M., Pihlaja, P. and Sakko, M,, 1988. The radiometric age of the Reposaari granite and its bearing on the extent on the Laitaila rapakivi batholith in western Finland. Bull. Geol. Soc. Finland, 60:129-134. Van Schmus, W.R. and Bickford, M.E., 1981. Proterozoic chronology and evolution of the midcontinent region, North America. In: A. Kr6ner (Editor), Proterozoic Lithospheric Evolution. Am. Geophys. Union and Geol. Soc. Am. Geodyn. Set., 17: 43-68. Vorma, A., 1972. On the contact aureole of the Wiborg rapakivi granite batholith in southeastern Finland. Geol. Surv. Finland Bull., 255, 28 pp. Vorma, A., 1975. On two roof pendants in the Wiborg rapakivi massif, southeastern Finland. Geol. Surv. Finland Bull., 272, 86 pp. Vorma, A., 1976. On the petrochemistry of rapakivi granites with special reference to the Laitila batholith, southwestern Finland. Geol. Surv. Finland Bull., 285, 98 pp. Wasserburg, G., 1963. Diffusion processes in lead-uranium systems. J. Geophys. Res., 68: 4823-4846. York, D., 1969. Least squares fitting of a straight line with correlated errors. Earth Planet. Sci. Eett., 5: 320-324.