Journal of Petrology Volume 42 Number 11 Pages 1971-1993 2001
© Oxford University Press 2001
Petrogenesis of the Post-kinematic Magmatism of the Central Finland Granitoid Complex I; Radiogenic Isotope Constraints and Implications for Crustal Evolution
1DEPARTMENT OF GEOLOGY, PO BOX 64, FIN-00014 UNIVERSITY OF HELSINKI, HELSINKI, FINLAND
2GEOLOGICAL SURVEY OF FINLAND, PO BOX 96, FIN-02151 ESPOO, FINLAND
3DEPARTMENT OF PHYSICS AND EARTH SCIENCES, UNIVERSITY OF NORTH ALABAMA, POBOX 5172, FLORENCE, AL 35632-0001, USA
Received October 11, 2000; Revised typescript accepted April 15, 2001
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONGEOLOGICAL SETTING AND SAMPLINGANALYTICAL METHODSU-Pb GEOCHRONOLOGYNd-Pb-Sr ISOTOPE GEOCHEMISTRYDISCUSSIONCONCLUDING REMARKSREFERENCES |
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Post-kinematic granitoids of the Central Finland Granitoid Complex (CFGC) were emplaced into the Palaeoproterozoic crust of central Finland shortly after the
1·89 Ga peak of the Svecofennian orogeny. They are found as discordant, generally non-foliated plutons with A- and C-type granite characteristics and a bimodal (mafic–felsic) magmatic association. Conventional and ion microprobe U–Pb data show that the post-kinematic magmatism commenced in the northeastern CFGC at 1·885 Ga and gradually moved to the west, where the plutons are dated at 1·870 Ga. Radiogenic isotope (Nd, Sr, Pb) composition of the post-kinematic plutons indicates homogeneous initial ratios: 
Nd (at 1·875 Ga) 0, 87Sr/86Sr 0·703, and Stacey & Kramers-type Pb with low long-term Th/U (2). These help to constrain the protolith history of the post-kinematic magmatism. Nd and Pb isotopic data for Palaeoproterozoic granitoids, Pb isotopic data for associated sulphides, and Nd and Pb isotopic composition of the classic 1·6 Ga Finnish rapakivi granites reveal several distinct crustal domains in the Finnish Svecofennian orogen, and indicate that the central part of the Svecofennian orogen consists of several terranes (microcontinents) that were amalgamated in Palaeoproterozoic times. The CFGC region is the oldest (2·0 Ga) among these terranes. KEY WORDS: crustal evolution; granite; Proterozoic; radiogenic isotopes; U–Pb geochronology
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTING AND SAMPLINGANALYTICAL METHODSU-Pb GEOCHRONOLOGYNd-Pb-Sr ISOTOPE GEOCHEMISTRYDISCUSSIONCONCLUDING REMARKSREFERENCES |
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In many Precambrian shield areas, extensive Palaeoproterozoic granitoid magmatism marks the mature stage of crustal accretion that led to rapid growth of juvenile terranes around pre-existing Archaean cratons (Patchett et al., 1981
; Van Schmus & Bickford, 1981; Patchett & Bridgwater, 1984; Huhma, 1986; Sadowski & Bettencourt, 1996). On a global scale, these processes led to the establishment of the Palaeoproterozoic supercontinent NENA (Northern Europe–North America; Gower et al., 1990; Rogers, 1996) the scattered remnants of which currently constitute a substantial fraction of the continental crust of the northern hemisphere.
In the Finnish part of the Fennoscandian (or Baltic) Shield (Fig. 1), the evolution of the 1·9 Ga Svecofennian orogen involved several magmatic events (e.g. Korsman et al., 1999). Emplacement of early orogenic (1·93–1·91 Ga) tonalites was followed by predominant synorogenic (1·90–1·87 Ga) magmatism characterized by I-type tonalite, granodiorite, and granite and arc-type volcanism. At 1·84–1·81 Ga, late-orogenic (mainly S-type) granites were emplaced as a broad belt across southernmost Finland. These and associated mafic rocks have been ascribed to crustal shortening (Väisänen & Hölttä, 1999). Small post-orogenic (or post-collisional) bimodal stocks, also in southernmost Finland, are shoshonitic and were emplaced at 1·81–1·77 Ga during crustal extension (Patchett & Kouvo, 1986; Eklund et al., 1998; Väisänen et al., 2000). The classic 1·65–1·54 Ga rapakivi granites (e.g. Haapala & Rämö, 1990) record the latest major magmatic event in southern Finland.
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The Central Finland Granitoid Complex (CFGC) forms an important, yet petrologically fairly poorly known section of the Palaeoproterozoic crust that was accreted onto the Archaean craton of the Fennoscandian Shield at 2·0–1·9 Ga (e.g. Huhma, 1986; Nurmi & Haapala, 1986; Patchett & Kouvo, 1986; Lahtinen, 1994; Nironen, 1997; Korsman et al., 1999). The complex covers 40 000 km2 and mainly consists of synorogenic granitoid rocks with U–Pb ages of 1·89–1·87 Ga (Huhma, 1986; Vaasjoki, 1996; Lahtinen & Huhma, 1997). These may be divided into synkinematic (1·89–1·88 Ga) and post-kinematic (1·88–1·87 Ga) groups, which differ in terms of field relations, geochemistry, and petrography (Nironen et al., 2000a). The synkinematic plutons are typically intensively foliated and are related to the second main collisional phase of the Svecofennian orogeny at 1·89 Ga (Lahtinen, 1994; Nironen, 1997). The post-kinematic granitoids barely post-date the synkinematic plutons, yet they clearly intrude the latter and are only slightly, if at all, foliated. They also show a bimodal (mafic–felsic) magmatic association typical of silicic magmatism in an extensional tectonic setting and presumably record an extensional or transtensional magmatic event in central Finland that was contemporaneous with crustal shortening in southern Finland (Nironen et al., 2000a). In southern Finland, comparable extensional magmatism took place later, during the post-orogenic (1·8 Ga) and rapakivi (1·6 Ga) events. Recent mineral chemical (Elliott et al., 1998) and whole-rock geochemical (Nironen et al., 2000a) data have shown that the post-kinematic, typically quartz monzonitic and granitic plutons of the CFGC were crystallized from relatively high-temperature, low-aH2O magmas that had both A-type and C-type (charnockitic; Kilpatric & Ellis, 1992) granite characteristics. In many respects, they resemble the rapakivi granites of southern Finland (see Rämö & Haapala, 1995).
The purpose of this paper is to present new U–Pb (conventional and ion probe), Nd, Pb, and Sr isotopic data for the post-kinematic plutons of the CFGC (silicic rocks and associated mafic intrusions) and some of their synkinematic country rocks. Combined with previously published data, these will be used to (1) constrain the emplacement sequence of the post-kinematic plutons, (2) scrutinize the isotopic character of the CFGC region, and (3) complement a petrologic model of the post-kinematic magmatism presented in a companion paper (Elliott, submitted). The radiogenic isotopic data for the silicic and mafic rocks of the CFGC will be also used to assess the composition of deep crust and subcontinental mantle. In addition, we will compare the isotopic composition of the post-kinematic plutons with that of the anorogenic rapakivi granites further south. This allows us to identify several Palaeoproterozoic crustal terranes (microcontinents) that were amalgamated in the Palaeoproterozoic, provided protolith material for the post-kinematic and rapakivi granitoids, and now form an essential part of the Fennoscandian Shield.
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GEOLOGICAL SETTING AND SAMPLING |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTING AND SAMPLINGANALYTICAL METHODSU-Pb GEOCHRONOLOGYNd-Pb-Sr ISOTOPE GEOCHEMISTRYDISCUSSIONCONCLUDING REMARKSREFERENCES |
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The CFGC area in central Finland is primarily composed of calc-alkaline granitoids (mainly granodiorite and granite) but also includes minor mafic intrusions, subvolcanic rocks, and remnants of supracrustal belts (e.g. Korsman et al., 1997
). The calc-alkaline plutons are typically foliated and considered synkinematic in regard to orogenic deformation that peaked at 1·91–1·90 and 1·89–1·88 Ga in the Finnish part of the Svecofennian orogen (Lahtinen, 1994). The post-kinematic plutons of the CFGC are found as relatively small bodies that sharply cut the slightly older synkinematic granitoids throughout the complex (Fig. 1). In general, they consist of massive to slightly foliated quartz monzonite and monzogranite with alkaline affinity and thus clearly differ from the calc-alkaline synkinematic granitoids. Available U–Pb data (e.g. Vaasjoki, 1996) show that the post-kinematic plutons are only slightly (10–20 Ma) younger than the synkinematic granitoids, and they are considered post-kinematic relative to orogenic movements within the CFGC (Nironen et al., 2000a). Overall, they have been related to extensional or transtensional events that modified the Svecofennian crust that had been tectonically thickened by convergent processes at 1·91–1·88 Ga (Lahtinen, 1994; Nironen, 1997; Nironen et al., 2000b).
The post-kinematic plutons of the CFGC may be divided into three types according to their petrographical, mineral chemical, and geochemical characteristics (Table 1; Elliott et al., 1998; Nironen et al., 2000a). Type 1 plutons on the southern flank of the CFGC (Fig. 1) are composed of peraluminous, coarse-porphyritic biotite granodiorite and granite. Type 2 plutons consist of metaluminous to peraluminous monzogranites that vary from equigranular to coarse-porphyritic and contain biotite or biotite and hornblende as the main mafic silicates; several of the Type 2 plutons are multiple intrusions. On average, the Type 2 plutons are more enriched in Fe relative to Mg and in Rb relative to Sr than the other types (Table 1). Fluorite is a characteristic accessory mineral of Type 2 plutons in the western part of the CFGC. Some Type 2 plutons are associated with coeval mafic rocks (Fe-enriched gabbros and diorites) that are found as small bodies on the margins of the plutons (Fig. 1). Type 3 plutons are composed of metaluminous to marginally peraluminous biotite–hornblende quartz monzonite and monzogranite (occasionally also granodiorite and quartz monzodiorite) and either have a two-pyroxene (± fayalite) bearing margin or contain pyroxenes (mainly clinopyroxene) throughout. Most of the Type 3 plutons are found in the eastern part of the CFGC (Fig. 1).
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Conventional U–Pb (mainly zircon) studies were performed on six Type 2 plutons (Honkajoki, Parkano, Puula, Kaipola, Saarijärvi, Juupajoki) and an associated mafic intrusion (Perämaa), four Type 3 plutons (Kurikka, Jämsä, Petäjävesi, Muurame), and a synkinematic granodiorite in the western part of the CFGC. In total, 77 zircon, three monazite, and two titanite fractions from 19 samples were analysed. Zircons from two Type 2 plutons with heterogeneous conventional fractions (Puula, Honkajoki) were also analysed by ion microprobe in search of inheritance patterns and more precise igneous ages. In total, 32 samples from 15 post-kinematic plutons and five synkinematic granitoids were chosen for Nd–Pb–Sr isotopic studies. Sample locations are shown in Fig. 1 and, in more detail for the U–Pb samples, in Electronic Appendix 1.
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ANALYTICAL METHODS |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTING AND SAMPLINGANALYTICAL METHODSU-Pb GEOCHRONOLOGYNd-Pb-Sr ISOTOPE GEOCHEMISTRYDISCUSSIONCONCLUDING REMARKSREFERENCES |
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U–Pb geochronology
Conventional U–Pb zircon, monazite, and titanite analyses were made at the Unit for Isotope Geology, Geological Survey of Finland. Heavy minerals were extracted from 10–20 kg samples and separated using mainly Clerici’s solution. The samples were sieved using 160 and 70 µm sieves and only the non-magnetic (1·4 A, 1° tilt) fractions from a Frantz isodynamic separator were used. Part of the heaviest (least metamict) zircon fractions from the granitoids was air-abraded. Dissolution of the samples and chemical purification of U and Pb were carried out using the method of Krogh (1973)
. The total Pb blank was 500 pg for 4–10 mg zircon samples (old analyses) and 30 pg for samples smaller than 1 mg. Isotopic ratios of Pb and U were measured on a VG SECTOR 54 mass spectrometer and on a non-commercial Nier-type mass spectrometer built at the Geological Survey of Finland. The fitting of the discordias to the datasets was carried out using the Isoplot/Ex program (Ludwig, 1998). As the analyses were carried out over a rather long period of time between 1973 and 2000 (see Electronic Appendix 2), error estimates for individual analyses vary a great deal. Generally, calculated errors have been used. However, for some old (hand scan) analyses, estimated long-term average errors on the 2
level (0·65% for the Pb–U ratios, 0·15% for 207Pb/206Pb, error correlation 0·97) were used.
Zircons from two plutons (Puula and Honkajoki) were also measured using the Cameca IMS 1270 ion microprobe (NORDSIM) at the Swedish Museum of Natural History, Stockholm. The zircons were mounted in epoxy, polished, and coated with gold. The spot diameter for the 4 nA primary O2- ion beam was 30 µm and oxygen flooding in the sample chamber was used to increase the transmission of Pb. The mass resolution (M/
M) was 5600 (10%). Four counting blocks, each including three cycles of the Zr, Pb, Th, and U species, were measured for each spot. The raw data were calibrated against a zircon standard (91500) and corrected for background (204·2) and the average isotopic composition of feldspar Pb determined for the post-kinematic plutons (this study). For a more detailed description of the analytical process, readers are referred to Whitehouse et al. (1999).
Nd–Pb–Sr isotopes
Silicic whole-rock powders (150–200 mg) were dissolved for a minimum of 2 days in a Teflon bomb at 180°C in HF–HNO3. Mafic whole-rock powders (200–300 mg) and feldspar fractions (200 mg) were dissolved in Savillex screw-cap Teflon beakers. After evaporation the samples were dissolved in HCl to obtain clear solutions. In analyses performed on the Type 1 plutons, most of the Type 2 plutons, mafic rocks, and the synkinematic Nokia pluton, the clear HCl solutions were aliquoted and spiked with 149Sm–145Nd, 87Rb–84Sr, and 206Pb (mafic rocks) tracers. For the Type 3 plutons and three synkinematic plutons that were analysed after the Type 1 and 2 plutons, the HCl solutions were totally spiked with the 87Rb–84Sr and a new 149Sm–150Nd tracer. The two methods yielded compatible results—this was checked by measuring Sm/Nd and Rb/Sr for one of the mafic samples with both methods; the ratios obtained (see Tables 4 and 5, below) were within the long-term maximum error of 0·5% based on duplicate samples. Pb, Rb, Sr, Nd, and Sm were separated from the same dissolved sample fraction. Pb was purified according to the anion exchange–anodic electrodeposition procedure of Gulson & Mizon (1979). Rb, Sr, and light rare earth elements (LREE) were separated using standard cation exchange chromatography, and Sm and Nd were purified using a modified version of the Teflon–HDEHP method of Richard et al. (1976). The total procedural blanks were <3 ng for Pb, <2 ng for Sr, and <300 pg for Nd.
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Isotopic ratios of Sm, Nd, Sr, and Pb were measured on a VG SECTOR 54 mass spectrometer (those of Nd and Sr in dynamic mode). Isotope dilution for Rb was performed on a non-commercial Nier-type mass spectrometer built at the Geological Survey of Finland. Nd isotopic ratios were normalized to 146Nd/144Nd = 0·7219. Repeated analyses of the La Jolla Nd standard gave 143Nd/144Nd of 0·511852 ± 0·000013 (mean and external 2 error of 63 measurements). The external 2 error on 143Nd/144Nd was thus 0·0025% and the Sm–Nd ratios are estimated to be accurate within 0·5%. The maximum error in the Nd values is ±0·35 -units. Sr isotopic ratios were normalized to 86Sr/88Sr = 0·1194. Repeated analyses of the NBS 987 Sr standard yielded 87Sr/86Sr of 0·710253 ± 0·000024 (mean and external 2 error of 46 measurements). The external 2 error on 87Sr/86Sr is 0·0035% and the error in Rb/Sr ratio is 0·5%. A mass fractionation correction of +0·1% per a.m.u. was made to normalize the measured Pb isotopic ratios to the accepted values (Gulson et al., 1984) of the NBS 981 Pb standard (206Pb/204Pb = 16·937, 207Pb/204Pb = 15·491, 208Pb/204Pb = 36·69). External 2 errors are 0·15% on 206Pb/204Pb and 207Pb/204Pb, and 0·2 % on 208Pb/204Pb. Each reported Pb isotopic ratio is the average of two separate mass spectrometer runs.
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U–Pb GEOCHRONOLOGY |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTING AND SAMPLINGANALYTICAL METHODSU-Pb GEOCHRONOLOGYNd-Pb-Sr ISOTOPE GEOCHEMISTRYDISCUSSIONCONCLUDING REMARKSREFERENCES |
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Conventional U–Pb analyses were performed for 11 post-kinematic plutons and a synkinematic granodiorite, and two post-kinematic plutons with heterogeneous zircon populations (Puula, Honkajoki) were also studied by ion microprobe. Sample locations for U–Pb analyses are shown in Fig. 1 and Electronic Appendix 1. Results for conventional work are summarized in Table 2 and given in detail in Electronic Appendix 2. Electronic Appendix 3 shows the conventional U–Pb data in concordia diagrams. The ion microprobe data are given in Table 3. Back-scattered electron images of some of the zircons analysed by ion microprobe are shown in Fig. 2 and the ion microprobe data are shown in concordia diagrams in Fig. 3.
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Conventional U–Pb analyses
The conventional U–Pb geochronological results on the 77 zircon, three monazite, and two titanite fractions are described in detail in Electronic Appendix 4. In general, the Type 2 plutons (Honkajoki, Parkano, Puula, Kaipola, Saarijärvi, Juupajoki) have more heterogeneous zircon populations than the Type 3 plutons (Kurikka, Jämsä, Petäjävesi, Muurame), which all show concordant or nearly concordant fractions. The upper intercept and/or 207Pb/206Pb ages of the zircons (Table 2) are in most cases interpreted as the crystallization ages of the plutons. These range from 1867 ± 3 Ma (Perämaa mafic intrusion associated with the Type 2 Honkajoki pluton in the western CFGC) to 1885 ± 3 Ma (Type 3 Muurame pluton in the eastern CFGC).
Ion microprobe analyses
Post-kinematic Puula pluton (sample A924)
Zircons chosen for ion microprobe work were mainly prismatic with length to width ratio (L/B) between two and four, had at least one developed bipyramidal edge, and showed typical magmatic zoning (Fig. 2a–d). Some grains had a small zircon nucleus of probable inherited origin (Fig. 2b); these were, however, too small for ion microprobe dating. A total of 30 spots were analysed for sample A924 (Table 3). The U concentrations vary usually from 200 to 300 ppm for zoned zircon domains and from 700 to 4600 for structurally more homogeneous inner domains. The 206Pb/204Pb vary from 400 to 11 000 and demonstrate high common Pb contents in many of the zircons.
Twenty-seven of the 30 spots gave concordant results within the 2 error limits (Table 3, Fig. 3a). These may be further split into two apparent age groups according to their 207Pb/206Pb ages. Twenty-two of the analysed spots form a tight cluster with an upper intercept age of 1870·9 ± 9·2 Ma [mean square of weighted deviates (MSWD) 0·69] and a weighted average 207Pb/206Pb age of 1867 ± 3 Ma. These are, within the analytical error, indistinguishable from the 207Pb/206Pb ages of the three monazite fractions analysed from the pluton (1875 ± 4, 1874 ± 5, 1874 ± 4 Ma; Table 2). We consider the average of the ion microprobe upper intercept age and monazite ages, 1874 ± 3 Ma, the emplacement age of the pluton.
Five of the concordant spots show younger 207Pb/206Pb ages (Fig. 3a). The apparent upper intercept age and the weighted average are 1812 ± 18 Ma and 1819 ± 13 Ma, respectively (Fig. 3a). The five analyses are from varying domains of the zircons and no common factor for the younger ages could be discerned. They might represent zircon growth or resetting of older zircon domains during a subsequent event that seems to be registered by the almost concordant titanite with a 207Pb/206Pb age of 1783 ± 4 Ma (Table 2).
Only three of the data points from Puula have discordant ages (dashed ellipses in Fig. 3a). The most discordant point (23a) was analysed on a centre domain of a zoned zircon and the low 206Pb/204Pb (400) indicates that the analysis may have hit on cracks surrounding the inner domain. The other two discordant analyses show 207Pb/206Pb ages of 1882 ± 12 and 1944 ± 5 Ma (35a and 32a in Fig. 3a, respectively; see also Fig. 2d); the latter of these indicates presence of zircon from older crustal sources.
Post-kinematic Honkajoki pluton (sample A588)
Zircons analysed by ion microprobe were mainly prismatic with L/B of 2–4 and with growth zoning typical of magmatic zircons (Fig. 2e–i). Several crystals had inner domains that appeared structurally homogeneous and, on back-scattered electron images, rather light, and thus were composed of heavier (less metamict) material (e.g. Fig. 2h). Some of these inner zones had been altered to darker (more metamict) material (Fig. 2g and i). Possible inherited cores were visible in a few zircons (Fig. 2e); these were, again, too small to be dated.
During data acquisition, several analyses were discarded because 206Pb/204Pb were extremely low, and meaningful results were gained for only 11 spots (Table 3). Characteristic for most of the dated zircons are low 206Pb/204Pb and, in some cases, fairly high U values (>1000 ppm). Only four spots yielded concordant age data with an upper intercept age of 1867·1 ± 8·6 Ma and a weighted average of 1864·8 ± 4·4 Ma (2). A discordia line with an upper intercept age of 1866·5 ± 5·6 Ma (MSWD 0·80) can be constructed using the concordant analyses and four discordant points (Fig. 3b). Three discordant data spots (dashed ellipses in Fig. 3b) with very low 206Pb/204Pb (140–320) fall on the younger side of the discordia and may indicate subsequent Pb loss. The 1867 ± 6 Ma age derived from the eight spots is considered the emplacement age of the pluton.
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Nd–Pb–Sr ISOTOPE GEOCHEMISTRY |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTING AND SAMPLINGANALYTICAL METHODSU-Pb GEOCHRONOLOGYNd-Pb-Sr ISOTOPE GEOCHEMISTRYDISCUSSIONCONCLUDING REMARKSREFERENCES |
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Nd isotopes
Nd isotopic data for the post-kinematic plutons of the CFGC and their country rocks are presented in Table 4 and Fig. 4. The post-kinematic plutons show substantial variation in Sm and Nd concentrations and wide ranges in 147Sm/144Nd (0·068–0·159) and 143Nd/144Nd (0·51099–0·51213). Their initial
Nd (at 1875 Ma) values are, however, remarkably similar, -1·1 to +0·5 (Table 3). No clear difference exists between the initial Nd isotopic compositions of the three pluton types either (Fig. 4). The most juvenile composition [highest Nd (at 1875 Ma), +0·5] was measured for the Type 2 Honkajoki pluton (sample A588) in the western CFGC and the lowest Nd (at 1875 Ma) value (-1·1) for the Type 3 Muurame pluton (A1426) in the east–central CFGC (Fig. 1). One of the Type 2 plutons, Juupajoki (or Koppelojärvi), was analysed previously by Patchett & Kouvo (1986). Their data indicate an Nd (at 1875 Ma) of -0·5, which is in good agreement with our data (Nd -0·3; Table 4). Depleted mantle model ages, TDM (DePaolo, 1981), of the post-kinematic plutons vary from 2·11 to 2·27 Ga and average at 2·19 ± 0·06 Ga (mean and 1 of 17 samples), if samples with anomalously high (7-BAE-96) and low (36-BAE-98, A1426) Sm/Nd are not taken into account (Table 4).
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The mafic samples analysed from the Perämaa intrusion in the western CFGC (Fig. 1) show a variation in rock type (Table 4) and also slight, but probably significant, variation in the initial Nd isotopic composition (Fig. 4). A mafic cumulate (R5-53.4) has Nd (at 1875 Ma) of +0·5 whereas the gabbro (OTR-83-16), diorite (OTR-83-14.8), and quartz diorite (OTR-83-11) are more unradiogenic with Nd (at 1875 Ma) of -0·4, -0·3, and -0·7, respectively. The Kälä gabbro (126-BAE-96) in the southeastern CFGC (Fig. 1) is similar, with an Nd (at 1875 Ma) of -0·5. The TDM of the mafic rocks is between 2·20 and 2·28 Ga.
The five synkinematic silicic and intermediate rocks (13-MN-93, 131-MN-94, A567, A1091, OTR-99-8) all have low 147Sm/144Nd (0·110–0·117) but show some variation in initial composition. The synkinematic granodiorite at Honkajoki (A567) and the quartz diorite at Rautalampi (OTR-99-8) both have Nd (at 1875 Ma) of -0·2 and are more radiogenic than the Luopajärvi tonalite (A1091), Nokia granodiorite (13-MN-93), and Jämsä granodiorite (131-MN-94), which have Nd (at 1875 Ma) of -0·9, -1·3, and -1·6, respectively (Fig. 4). The TDM of the synkinematic samples ranges from 2·18 to 2·29 Ga.
Pb isotopes
Pb isotopic data for the post-kinematic granitoids and mafic rocks and one synkinematic granitoid (Nokia) are shown in Table 5 and Fig. 5. The Pb isotopic ratios of the feldspar fractions are considered reasonable estimates of the initial Pb isotopic composition of the granitoids and they were used for the common Pb correction in the U–Pb analyses. Whole-rock values were determined to examine the time-integrated evolution of uranogenic and thorogenic Pb. In search of initial compositions, time-corrected uranogenic Pb isotopic ratios were determined for the mafic rocks.
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The feldspar Pb isotopic ratios of the post-kinematic plutons are fairly homogeneous; variation in 207Pb/204Pb and 208Pb/204Pb barely exceeds the external error, 206Pb/204Pb shows a somewhat larger variation (Table 5, Fig. 5). The average ratios of the 17 feldspars from the post-kinematic plutons are 206Pb/204Pb = 15·704 ± 0·125 (1
), 207Pb/204Pb = 15·287 ± 0·020, and 208Pb/204Pb = 35·177 ± 0·071. They plot rather close to the Stacey & Kramers (1975) growth curve for average crustal Pb (Fig. 5).
In the 207Pb/204Pb vs 206Pb/204Pb diagram (Fig. 5a), the post-kinematic samples yield a trend (not shown) that corresponds to an age of 1812 ± 82 Ma. This is compatible with the overall emplacement age of the plutons. The samples from the Perämaa mafic intrusion fall on a 1891±230 Ma isochron; this age is compatible with the U–Pb zircon age of the Perämaa gabbro (Table 2) but the small spread in the isotopic ratios results in a large calculated error. The Kälä gabbro has a more radiogenic character than the Perämaa samples. Using the U and Pb values of the five mafic rocks (Table 6), time-corrected uranogenic Pb isotopic ratios were calculated. The Perämaa gabbro (OTR-83-16) and diorites (OTR-83-14.8, OTR-83-11) are best preserved of the five samples and yielded similar time-corrected ratios that average at 206Pb/204Pb = 15·64 and 207Pb/204Pb = 15·28. This is close to the composition of the feldspar fractions of the post-kinematic granitoids (Table 5). The synkinematic granodiorite (13-MN-93) shows somewhat higher relative 207Pb/204Pb than the post-kinematic plutons on average (Fig. 5).
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In the 208Pb/204Pb vs 206Pb/204Pb diagram (Fig. 5b) the Type 1 post-kinematic plutons are heterogeneous with Th/U between 2 (Karkku) and 6 (Siitama). The Type 2 and Type 3 plutons are more homogeneous with Th/U of 2·26 ± 0·65 and 1·98 ± 0·33, respectively. The samples from the mafic intrusions scatter widely and, very roughly, conform to a Th/U of 4.
Sr isotopes
Sr isotopic compositions were measured for the Type 2 Honkajoki pluton, the Type 3 Jämsä pluton, and the Perämaa and Kälä mafic intrusions (Table 5). Samples from Honkajoki are clearly more radiogenic than those from Jämsä and the mafic intrusions, and fail to register a reliable initial ratio for the pluton. The two quartz monzonites (82a-MN-94, 102b-BAE-96) and granites (106-BAE-96, 231-MN-94) of the Jämsä pluton define an age of 1851±80 Ma and initial 87Sr/86Sr [(87Sr/86Sr)i] of 0·7030±0·0009, and the four samples from the Perämaa intrusion (R5-53.4, OTR-83-16, OTR-83-14.8, OTR-83-11) fall on an isochron of 1823±15 Ma, (87Sr/86Sr)i of 0·70287±0·00003, and MSWD of 0·76. These ages are fairly compatible with the U–Pb ages of Jämsä and Perämaa, and the calculated initial ratios are probably close to those of the magmas from which the plutons crystallized.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTING AND SAMPLINGANALYTICAL METHODSU-Pb GEOCHRONOLOGYNd-Pb-Sr ISOTOPE GEOCHEMISTRYDISCUSSIONCONCLUDING REMARKSREFERENCES |
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Temporal evolution of the post-kinematic magmatism
The geographical positions of the post-kinematic plutons dated by the U–Pb method are shown in Fig. 6. In general, a trend with decreasing ages from the NE to the west can be observed: the plutons in the NE have ages of
1885 Ma, those in the west of 1870 Ma. This shift in age is accompanied by a change in the character of the granitoid magmatism: the older plutons in the NE are primarily C-type and the younger plutons in the west primarily A-type (see Nironen et al., 2000a). The thickness of the continental crust across the CFGC region also shows a drastic change from east to west. The high-velocity lower crust is 5–8 km thicker in the east than in the west (Korsman et al., 1999). It is possible that the thicker crust in the east promoted anatexis, as opposed to the thinner (and thus more rigid) lower crust in the west, allowing the post-kinematic magmas in the eastern part of the CFGC to be formed (and emplaced) earlier. The post-kinematic plutons probably include a major component from mafic lower crust that was formed in response to mafic underplating shortly before the onset of the post-kinematic extension or transtension (Elliott, submitted). Considering the large area of the mafic underplate (entire CFGC) and the age distribution of the post-kinematic magmatism, it is also probable that the locus of mafic underplating moved westward (see Nironen et al., 2000a). This may have been accompanied by a consanguineous shift in crustal extension.
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The search for inherited zircon cores by ion microprobe was focused on the Type 2 plutons, which crystallized at lower temperatures than the Type 3 plutons (Elliott et al., 1998) and thus could have retained inherited U–Pb systematics better. The studied zircons included cores but yielded only one spot (Fig. 2d) with an age clearly older than the 1870 Ma emplacement age of the plutons. This 1944 ± 5 Ma zircon (Table 3, Fig. 3a) and the scattered conventional U–Pb data (Fig. d in Electronic Appendix 3) show that older material was involved at least in the Type 2 post-kinematic magmatism. The 1950 Ma age is younger than the proposed >2·0 Ga age (e.g. Claesson et al., 1993; Lahtinen & Huhma, 1997) of the primitive Svecofennian crust.
In addition to the old age, titanite and zircon from Puula also register a younger event (or events) at 1·8 Ga (Tables 2 and 3, Fig. 3). The 1820 Ma (zircon) event could be related to the peak of regional migmatization, high-T metamorphism, and the emplacement of microcline granites in southern Finland; this tectonothermal event has been recorded by zircon, monazite, and titanite in several lithological units (e.g. Korsman et al., 1984; Nironen, 1989). The 1783 ± 4 Ma titanite age of Puula is similar to that of the nearby 1·8 Ga Luonteri intrusion (Korsman et al., 1984) and probably reflects cooling through a 500°C isotherm. The nature of this cooling is uncertain as it could have occurred either on a regional scale, in the aftermath of the migmatization, or locally, as a result of block uplift. Nevertheless, the complexity of the isotopic ages from the Puula pluton may be related to its location at the ‘hinge’ of three major crustal terranes (see Nironen et al., 2000b).
Isotopic characteristics of the CFGC
Combined, the initial Nd isotopic composition of the synkinematic and post-kinematic granitoids of the CFGC shows remarkably little variation. The Nd (at 1875 Ma) values of the post-kinematic granitoids range from -1·1 to +0·5 (Table 4) with an average at -0·2 ± 0·4 (1, n = 20). The Type 1 and Type 2 plutons in the southeastern, southern, and western parts of the CFGC have Nd (at 1875 Ma) values of -0·6 to +0·2 (the mean value for the Honkajoki pluton). The Type 3 plutons show Nd (at 1875 Ma) values of -1·1 to +0·4. Nd isotopic data are available for 10 synkinematic granitoids of the CFGC [this study and data from Huhma (1986) and Patchett & Kouvo (1986)]. These have Nd (at 1875 Ma) between -1·6 and +1·1 and average Nd (at 1875 Ma) of -0·4 ± 0·8 (1, n = 10).
The Bulk-Earth-type initial Nd isotopic composition of the CFGC granitoids (Nd 0) indicates that no Archaean crustal material was directly involved in their genesis. It also shows that these granitoids were not directly derived from a depleted mantle source or from material recently separated from depleted mantle. The homogeneity of the initial Nd isotopic composition over the entire CFGC region also requires a homogeneous source and implies that the crust-forming process was probably fairly rapid. Assuming that the CFGC lithosphere ultimately originated from an LREE-depleted mantle, the Nd isotopic composition of the granitoids implies a substantial time gap between the crust–mantle differentiation and the 1·89–1·87 Ga magmatism. This would indicate that the CFGC region represents a relatively old Palaeoproterozoic lithospheric domain (see also Lahtinen, 1994; Lahtinen & Huhma, 1997; Rämö et al., 1998).
Nd isotopic data for mafic rocks of the CFGC are relatively few. The four samples from the Perämaa intrusion in the western CFGC have Nd (at 1875 Ma) values that cluster around zero (+0·5 to -0·7) and are thus similar to those of the post-kinematic granitoids (Fig. 4). The same is true for the Kälä gabbro in the eastern CFGC with Nd (at 1875 Ma) of -0·5. A gabbro from the southern flank of the CFGC has a slightly positive Nd (at 1875 Ma) of +1·2 (Patchett & Kouvo, 1986), and metabasalts associated with the Haveri Formation, which is considered to represent the lowermost supracrustal unit on the southern flank of the CFGC (see Fig. 1), have Nd (at 1875 Ma) ranging from +1·4 to -0·8 and averaging +0·4 (Vaasjoki & Huhma, 1999). The range of initial Nd values of the Perämaa mafic intrusion is roughly twice the experimental error (Fig. 4) and indicates that the magmatic evolution of the pluton involved open-system processes. The mafic cumulate with a slightly positive initial Nd (+0·5) could approach the Nd isotopic composition of a melt extracted from a Bulk-Earth-type mantle and the gabbro (Nd -0·4) and diorites (Nd -0·3, -0·7) could reflect crustal fractionation and incorporation of relatively unradiogenic Nd into the evolving mafic magma.
The initial (feldspar) Pb isotopic composition of the post-kinematic granitoids of the CFGC is homogeneous and corroborates the picture emerging from the Nd data. The feldspars plot, on average, slightly below the Stacey & Kramers (1975) growth curve (Fig. 5)—the average µ2 value (present-day second-stage 238U/204Pb) of the feldspars is 9·59 ± 0·13 (1, n = 17) whereas that of the Stacey & Kramers (1975) growth curve is 9·74. Thus the Pb that was incorporated into the post-kinematic granitoid magmas had an overall crustal character.
Three initial Pb isotopic compositions calculated for the Perämaa mafic intrusion are shown in Fig. 5a. They are indistinguishable from those of the feldspars of the post-kinematic plutons and are markedly different from the initial Pb isotopic composition of the Haveri Formation (see also Vaasjoki & Huhma, 1999). Vaasjoki & Huhma (1999) proposed that the Haveri metabasalts and associated sulphides record the Pb (as well as Nd) isotopic composition of an enriched lithospheric mantle source. The Perämaa mafic intrusion has a similar initial Nd isotopic composition to the Haveri metabasalts but clearly more radiogenic initial Pb isotopic composition. This may indicate contamination of the depleted mafic magma with crustal radiogenic Pb or that the subcontinental mantle north of the Haveri Formation had a more radiogenic Pb isotopic composition. In view of the remarkable homogeneity of the Nd and Pb composition of the post-kinematic plutons of the CFGC, the latter hypothesis is favoured (see also Lahtinen, 1994).
An overall mantle Th/U of 4 at 2 Ga (Zindler & Hart, 1986; Zartman & Haines, 1988; O’Nions & McKenzie, 1993; Rudnick & Fountain, 1995; Kramers & Tolstikhin, 1997) is in agreement with the Th/U of the Haveri metabasalts and is compatible with their direct mantle origin. However, the Th/U of the post-kinematic plutons of the CFGC (2·3 for Type 2, 2·0 for Type 3) cannot be ascribed to such a mantle source. A petrochemical model based on the Honkajoki and Jämsä plutons (Elliott, submitted) indicates that the Type 2 and Type 3 post-kinematic plutons represent low to moderate degree partial melts of evolved yet mafic (ferrodioritic) lower crust with minor incorporation of restitic material from the deep crust. In this process, the mafic lower-crustal source retained Th more efficiently than U and thus resulted in a lower Th/U in the resultant anatectic melts, largely by way of plagioclase in the source retaining more Th than U. The Perämaa mafic intrusion is not an anatectic melt of mafic lower crust and, according to the model above, should originally have had a Th/U comparable with that of the Haveri mantle source. In Fig. 5b, the mafic samples show such a pronounced scatter that the Th/U of 4 is most uncertain. At face value, however, it is compatible with the presumed mantle Th/U at that time.
The low initial 87Sr/86Sr of the Type 3 Jämsä pluton (0·7030 ± 0·0009) and the Perämaa mafic intrusion (0·70287 ± 0·00003) show that no old high-Rb/Sr material was incorporated into these magmas. The low mantle-like initial ratio of the Jämsä pluton indicates that it was either derived from a mantle source or generated by remelting of crustal material with a low overall Rb/Sr. The latter hypothesis is consistent with a mafic juvenile ferrodiorite or gabbro as the principal protolith of the Honkajoki and Jämsä plutons (Elliott, submitted). For the Perämaa intrusion, the low initial 87Sr/86Sr probably reflects (1) the low ratio of the mantle source and (2) the low ratio in the possible crustal contaminant.
Crustal terranes in the Finnish Svecofennian orogen
The uniform isotopic composition of the post-kinematic granitoids shows that the CFGC region was differentiated from the upper mantle in a relatively short period of time. The age of this mantle–crust differentiation is yet to be established—present estimates include >1·91 Ga (Nironen, 1997) and 2·1–1·9 Ga (Claesson et al., 1993; Lahtinen & Huhma, 1997). Previous studies have shown regional differences within the Finnish Svecofennian domain (e.g. Vaasjoki, 1981; Lahtinen & Huhma, 1997). We will explore these differences in detail to show the exotic nature of the CFGC region.
Recently, the Finnish Svecofennian orogen has been divided into three major crustal units: (1) the (accretionary) arc complex of southern Finland; (2) the arc complex of central and western Finland; (3) the primitive arc complex of central Finland (Korsman et al., 1997, 1999). These are depicted in Fig. 7 and will be referred to in this paper as Terrane I, Terrane II, and Terrane III, respectively. Terrane I covers a roughly east–west-oriented unit that consists of sedimentary rocks, cross-cutting synorogenic (1·89–1·88 Ga) granitoids and volcanic rocks, late-orogenic (1·84–1·81 Ga) granites, minor post-orogenic (1·81–1·77 Ga) intrusions, and the 1·65–1·54 Ga rapakivi granites. It extends from the Russian border in the east to the archipelago of southwestern Finland and probably further west to south–central Sweden (Bergslagen area; Nironen, 1997). Terrane II includes the CFGC and the surrounding schist belts. It probably continues to northern Sweden and contains blocks of slightly older (>1·90 Ga) crustal material (Nironen, 1997). Terrane III is a relatively narrow unit between the Archaean craton in the NE and the other Proterozoic terranes in the SW. It is composed of early orogenic (1·93–1·91 Ga) granitoids and 1·89 Ga synorogenic plutonic and volcanic rocks.
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Nd isotopic evidence
Geographical variation of Nd isotopic composition of the silicic and mafic rocks in the Proterozoic terranes is shown in Fig. 7a. In general, clear differences exist between the three terranes, and to a certain extent, also within the terranes themselves. Terrane I shows a bimodal distribution of Nd (at 1875 Ma) values. Except for the southernmost coast of Finland, the silicic rocks have Nd (at 1875 Ma) of +1·5 to +3·0 with a mean at +2·2 ± 0·6 (1, n = 5), whereas the mafic rocks show a range from +0·4 to +3·3 and a mean value at +2·0 ± 0·9 (n = 18). The southernmost part of Terrane I shows lower Nd (at 1875 Ma) values, -0·9 to +0·7 for silicic and 0 for mafic rocks. Similar bimodality is also present in Terrane II. The bulk of this terrane has very uniform Nd (at 1875 Ma) values for the silicic rocks, -1·3 to +1·1 (average -0·3 ± 0·6; n = 30). The associated mafic rocks have Nd (at 1875 Ma) of -0·8 to +1·2 with a mean at +0·2 ± 0·7 (n = 13). The northwesternmost part of Terrane II is more juvenile with Nd (at 1875 Ma) of +2·9 and +3·3 (silicic rocks) and +2·5 and +4·6 (mafic rocks). Terrane III shows a markedly juvenile character with Nd (at 1875 Ma) values for silicic rocks of +0·9 to +4·0 (average +2·5 ± 0·7; n = 18). Three mafic rocks from Terrane III have Nd (at 1875 Ma) of -0·3 to +2·7 (Fig. 7a).
Figure 8 shows a histogram of the Nd (at 1875 Ma) values in the three terranes. Data for Terranes I and II, for which a wealth of both silicic and mafic rocks have been analysed, show that the initial Nd isotopic compositions of silicic and mafic rocks are similar to each other within each terrane. Terrane I and III are both more juvenile than Terrane II, and Terrane III is somewhat more juvenile than Terrane I. The two Bulk-Earth-type (Nd 0) compositions of the mafic rocks of Terrane III probably reflect the influence of the Archaean craton, and the average of the positive Nd values, +2·5 ± 0·7 (1, n = 18), is considered characteristic of the crustal history of Terrane III. The southwesternmost part of Terrane I is distinct from the rest of Terrane I, having initial Nd isotopic composition not much different from that of Terrane II. Terrane II also includes a juvenile block in the northwesternmost part, comparable with Terrane III.
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Pb isotopic evidence
Pb isotopic compositions of sulphides analysed from the Svecofennian domain of Finland since the 1970s are shown as Stacey & Kramers (1975) second-stage µ values (µ2) in Figs 7b and 8. Most of the data come from Terranes II and III. For Terrane II, the µ2 values average at 9·68, whereas Terrane III shows a clearly more unradiogenic character with mean µ2 at 9·39. Sulphide Pb data on Terrane I are less extensive (Vaasjoki, 1981). It appears, however, that the southern part of this terrane is more radiogenic in character with mean µ2 of 10·03 than the northern part; for the latter, two determinations have been published (µ2 = 9·40 and 9·89; Fig. 7b).
Figure 8 shows that, in terms of sulphide Pb, Terrane I is the most radiogenic, Terrane III the least radiogenic, and Terrane II is intermediate between the two. These data probably delineate crustal blocks of different origin and thus corroborate the Nd isotopic data (Figs 7a and 8) with similar implications. The Haveri metabasalts and sulphides (Vaasjoki & Huhma, 1999) are indicative of the least radiogenic (µ2 = 9·06; Fig. 8) Pb isotopic composition of the Finnish Svecofennian orogen. This is comparable with that of the ophiolite-associated Outokumpu mantle located east of the suture between the Archaean craton and the Svecofennian orogen (Fig. 7; Vaasjoki, 1981; Peltonen et al., 1996).
Concept of terranes
As the Nd and common Pb isotopic systems were first applied to the study of the origin of the Palaeoproterozoic Fennoscandian crust (e.g. Vaasjoki, 1981; Wilson et al., 1985; Huhma, 1986; Patchett & Kouvo, 1986; Patchett et al., 1987; Billström, 1989), the juvenile character and relatively small spread of initial Nd values was explained as the result of mixing of small (yet variable) amount of detritus from pre-existing Archaean crust with mantle-derived (juvenile) material. The distinct Nd and Pb compositions of the terranes in the Finnish Svecofennian orogen do, however, call for an allochthonous relationship for the terranes. This was suggested by Lahtinen & Huhma (1997), who considered the CFGC region to represent an older Proterozoic (2·0–2·1 Ga) crustal block surrounded by more juvenile island-arc accretions. The Nd and Pb data (Figs 7 and 8) also lend support to the idea that at least two of the juvenile accretions (Terrane I and Terrane III) have different origins.
An important feature of Terrane II is that the detailed Nd isotopic sampling has been unable to identify substantial differences in initial isotopic composition. In the following, we will focus on the origin of the CFGC granitoids and implications regarding the origin of Terrane II. As part of this, we will make a comparison with the 1·6 Ga rapakivi granites of Terrane I. These resemble the post-kinematic plutons of the CFGC in many respects and help to resolve the relative age of the protoliths of the two granitoid suites.
Timing of mantle separation
The initial Nd isotopic compositions of granitoid rocks of the three crustal terranes of the Finnish Svecofennian orogen are shown in Nd vs age diagrams in Fig. 9. The samples that were plotted in Fig. 9 are those with Sm/Nd in the range typical of continental crustal material (corresponding to 147Sm/144Nd between 0·09 and 0·14; see Rudnick & Fountain, 1995; Rämö & Calzia, 1998) and thus probably representative of the evolution of continental crust in general. In Fig. 9, two depleted mantle models are shown—the original model by DePaolo (1981) and the more recent model by Nägler & Kramers (1998). At the time of interest (2 Ga), the two models are almost consanguineous.
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The initial compositions of the Terrane III granitoids (Fig. 9a) plot close to depleted mantle models. This shows that the protoliths of the granitoids differentiated from a depleted mantle source very shortly before the formation of the granitoids. The same is true for the samples from the northwesternmost part of Terrane II (Fig. 9b) and one from Terrane I (Fig. 9c). Terrane III constitutes a relatively narrow crustal segment (Fig. 7) and probably represents a zone of mantle upwelling that was localized between two major (Archaean and Proterozoic) crustal domains. Lahtinen & Huhma (1997) related this to rifting that eventually tapped a depleted asthenospheric mantle reservoir. The juvenile composition of Terrane III granitoids shows also that input from the Archaean craton was minimal. Considering the proximity of the present exposed Archaean (Fig. 7) this is surprising and indicates that the granitoid magmatism in Terrane III took place before the Proterozoic and Archaean domains were juxtaposed. This is compatible with the tectonic model of Nironen (1997) in which the CFGC area and the Archaean domain collided at 1·91 Ga.
The Nd isotopic composition of both syn- and post-kinematic granitoids of the CFGC region of Terrane II is very homogeneous with Nd values varying from slightly positive to slightly negative and with TDM model ages on the order of 2·2 Ga (Fig. 9b). The Palaeoproterozoic granitoids of Terrane I (Fig. 9c) show a range of variation in initial Nd isotopic composition, with samples in the southernmost part having a clearly less radiogenic character than the others; this is compatible with the suggested older (>2·0 Ga) crust in the southern part of Terrane I (Lahtinen & Huhma, 1997). Figure 9c also shows the initial Nd isotopic composition of the classic rapakivi granites of southern Finland. These represent primarily crustal melts from the deep crust of Terrane I (e.g. Haapala & Rämö, 1990) and provide a reference point for further assessment of the age difference of Terrane I and Terrane II.
Post-kinematic granitoids vs rapakivi granites
The post-kinematic granitoids of the CFGC (Terrane II) and the rapakivi granites (Terrane I) probably represent average isotopic compositions of the deep parts of the crustal domains they occupy. Both granitoid suites represent high-temperature, water-deficient systems and are also geochemically comparable with each other (see Rämö & Haapala, 1995; Elliott et al., 1998; Nironen et al., 2000a). Modelling of partial melting of the protolith of these two groups of granitoids suggests similar degrees of fusion and fractionation of Sm/Nd (Rämö, 1991; Elliott, submitted). Rämö (1991) (see also Rämö & Haapala, 1995) concluded that the initial magma of rapakivi granites in the northern part of the Wiborg batholith (Fig. 1) was generated by relatively small degrees of melting of deep crust with the anatectic melt having a 15% lower Sm/Nd value than the protolith. In a companion paper (Elliott, submitted), a similar overall decrease in the Sm/Nd for the generation of the Type 2 and Type 3 post-kinematic plutons of the CFGC is implied. Thus the two suites show similar parent–daughter Sm–Nd systematics and yield constraints on the age of their protoliths.
To elaborate on this in more detail, we considered the effect of Sm/Nd fractionation on the time-integrated evolution of the two granitoid suites and their protoliths. For this, we used the available Nd isotopic data for the granitoids with 147Sm/144Nd between 0·09 and 0·14 (the range for typical continental crust; see above). The initial compositions and evolution lines of these samples are shown in Fig. 10a. The average Nd (at 1875 Ma) value of the post-kinematic granitoids is -0·12 ± 0·74 (2) and the mean 147Sm/144Nd is 0·1141. A 15% decrease in Sm/Nd as these granitoids were formed indicates a 147Sm/144Nd of 0·1312 for the protolith. With an Nd of -0·12 and a 147Sm/144Nd of 0·1312, a TDM of 2246 Ma can be calculated for the protolith. Considering the 2 variation in initial Nd values, a range of 2175–2318 Ma is obtained for the protolith model ages. For the rapakivi granites, the mean Nd (at 1635 Ma) value is -1·39 ± 0·76 and the mean 147Sm/144Nd is 0·1093. A 15% decrease in Sm/Nd during partial melting indicates a 147Sm/144Nd of 0·1257 for the source, which, with an Nd (at 1635 Ma) of -1·39, yields a TDM of 2138 Ma. The 2 variation in initial Nd results in a model age range of 2070–2207 Ma.
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The results of these calculations are depicted in Fig. 10b. The evolution paths for the two protoliths show some overlap but the
Nd (at 1875 Ma) values of the rapakivi protolith (+0·1 to +1·6) are, on average, slightly, yet probably significantly, higher than those of the post-kinematic granitoids (-0·9 to +0·6). The rapakivi granites may thus have derived from a slightly more juvenile protolith than the post-kinematic granitoids. Our model suggests an 100 m.y. age difference for the protoliths and, for that matter, for the two crustal domains (Terrane I and Terrane II).
There is also a subtle difference in the Pb isotopic composition of the post-kinematic granitoids and the rapakivi granites. In Fig. 11, feldspar fractions from the rapakivi granites plot on the Stacey & Kramers (1975) growth curve whereas feldspars from the post-kinematic plutons are somewhat less radiogenic, falling, on average, slightly below the growth curve. The mean µ2 value for the rapakivi feldspars is 9·77 ± 0·13 (n = 15) and that for the post-kinematic plutons 9·59 ± 0·13 (n = 17). The lower overall µ value of the post-kinematic plutons points to a lower overall U/Pb and can be interpreted as a feature of a slightly older protolith (see Haapala & Rämö, 1992). Combined, the Pb and Nd isotopic data demonstrate differences for the two granitoid suites and their protoliths and are compatible with the idea that the CFGC region represents an older Palaeoproterozoic crustal domain (see Lahtinen, 1994; Lahtinen & Huhma, 1997; Nironen, 1997). Our Nd model suggests that Terrane II could be 100 m.y. older than Terrane I.
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Previously published U–Pb ion microprobe data (Huhma et al., 1991; Claesson et al., 1993) show that 2·1–1·9 Ga zircons are fairly commonly encountered in the detrital zircon populations of southern and central Finland and elsewhere in the Svecofennian domain. In line with this, the conventional and ion microprobe U–Pb data presented in this study for the CFGC region are not indicative of crustal domains older than 2·0 Ga. The Nd isotopic composition of the mafic rocks of Terrane II is similar to that of the granitoid rocks associated with them. The mafic rocks may reflect (1) the composition of subcontinental lithospheric mantle attached to Terrane II and/or (2) contamination by Terrane-II-type crust. The small volume and enriched character of the mafic rocks of Terrane II show that this mantle lithosphere was not subject to large-scale invasion by depleted asthenospheric melts before the formation of the mafic rocks. Evidence for a clearly depleted mantle exists only for the areas surrounding the CFGC region and thus, again, stresses the exotic nature of Terrane II.
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CONCLUDING REMARKS |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTING AND SAMPLINGANALYTICAL METHODSU-Pb GEOCHRONOLOGYNd-Pb-Sr ISOTOPE GEOCHEMISTRYDISCUSSIONCONCLUDING REMARKSREFERENCES |
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The isotopic compositions of the Palaeoproterozoic post-kinematic plutons of the CFGC, associated synkinematic granitoids, and granitoids located in adjacent crustal domains allow the following observations and conclusions:
- the CFGC region in the central part of the Fennoscandian Shield is rather uniform in terms of its Nd and Pb isotopic composition and probably represents the oldest (2·0 Ga) Proterozoic terrane of the shield.
- The post-kinematic plutons of the CFGC were emplaced within a remarkably short period of time (1885–1870 Ma) and very shortly after the second main collisional phase of the Svecofennian orogeny. The post-kinematic magmatism migrated across the CFGC from the NE to the west, possibly as a result of thicker crust in the eastern part of the CFGC.
- The protolith of the post-kinematic granitoids was relatively mafic deep crust characterized by Bulk-Earth-type Nd isotopic composition and low overall Rb/Sr.
- Mafic rocks associated with the post-kinematic granitoids are small in volume and show very similar Nd, Pb, and Sr isotopic characteristics to the granitoids. They are thus not indicative of direct involvement of depleted mantle in the formation of the CFGC crust.
- The Finnish part of the Svecofennian domain of the Fennoscandian Shield comprises at least three (possibly four) Proterozoic crustal terranes with distinct Nd and Pb isotopic compositions. In the southernmost terrane, Nd isotopes reveal an older nucleus.
- In each terrane, silicic (primarily crust-derived) and mafic (mantle-derived) rocks have grossly similar Nd isotopic compositions. This is compatible with the different terranes representing crust–mantle entities with distinct histories and origin.
- Combined Nd and Pb isotopic mapping of post-kinematic granitoid rocks and sulphides may be used as a tool for Proterozoic lithospheric reconstructions.
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ACKNOWLEDGEMENTS |
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We are much obliged to O. Kouvo, who performed the conventional U–Pb analyses on samples A16, A240, A922, A923, A924, A588, A589, and A952 ‘aus Liebe zur Kunst’, as the German saying goes. The separation and chemical preparation of the zircon fractions were in the firm hands of T. Hokkanen, M. Karhunen and M. Niemelä. Our sincere thanks go to H. Huhma, who provided us with unpublished Nd data on the Luopajärvi tonalite and with a very useful review of an earlier version of the manuscript. Journal reviews by T. Andersen, J. Patchett and an anonymous reviewer, and editorial comments by M. Wilson improved the paper. This work was funded by the Academy of Finland (project 36002), and is a contribution to IGCP Project 426 (Granite Systems and Proterozoic Lithospheric Processes).
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FOOTNOTES |
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Electronic Appendices 1–4 can be found at http://www.petrology.oupjournals.org
*Corresponding author. Telephone: +358-9-191-50810. Fax: +358-9-191-50826. E-mail: tapani.ramo{at}helsinki.fi
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REFERENCES |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTING AND SAMPLINGANALYTICAL METHODSU-Pb GEOCHRONOLOGYNd-Pb-Sr ISOTOPE GEOCHEMISTRYDISCUSSIONCONCLUDING REMARKSREFERENCES |
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