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Bulletin of the Geological Society of Finland, Vol. 75 (1–2) pp. 51–68

The 3.5 Ga Siurua trondhjemite gneiss in the Archaean Pudas- järvi Belt, northern Finland

Tapani Mutanen1)* and Hannu Huhma2) 1)Geological Survey of Finland, P.O. Box 77, FIN-96101 Rovaniemi, Finland 2)Geological Survey of Finland, P.O. Box 96, FIN-02151 Espoo, Finland

Abstract In the Archaean Pudasjärvi Complex the pyroxene-bearing rocks are considered to form a belt, the Pudasjärvi Granulite Belt (PGB). The major types of the PGB are meta- igneous mafi c and , and trondhjemite gneisses. Red alaskites, white leu- cogranites and trondhjemitic pegmatoids are locally abundant. Ion microprobe U-Pb anal- yses on zircons suggest a magmatic age of ca. 3.5 Ga for the trondhjemite gneiss in Siu- rua, considered the oldest rock so far identifi ed in the Fennoscandian Shield. The old age is supported by the Sm-Nd depleted mantle model age of 3.5 Ga, and by convention- al U-Pb zircon data, which have provided a minimum age of 3.32 Ga. The U-Pb sims-da- ta on the Siurua gneiss are, however, heterogeneous and suggest several stages of zircon growth, mostly at 3.5–3.4 Ga. An inherited core in one crystal provided an age of 3.73 Ga, whereas the youngest two analyses yield ages of 3.1 and 3.3 Ga. Metamorphic mon- azite formed in the Siurua gneiss ca. 2.66 Ga ago, roughly contemporaneously with the high-grade recorded by zircon in a mafi c granulite. Magmatic zircons from a felsic high-grade rock provide ages of ca. 2.96 Ga, but no zircons coeval with the 2.65 Ga metamorphism were detected by ion-microprobe. As a whole the PGB seems to be a tectonic block-mosaic containing rocks with Sm-Nd crustal formation ages ranging from 3.5 to 2.8 Ga.

Key words: gneisses, granulites, , alaskite, chemical composition, absolute age, U/Pb, zircon, Sm/Nd, , Siurua, Pudasjärvi, Ranua, Finland

1. Introduction

The exposed Archaean crust in the Fennoscan- facies rocks were encountered in what proved to dian Shield consists of several natural or formal be an extensive belt. Four samples representing subdivisions, variously called “blocks”, “areas”, various rock types were chosen for isotopic dat- “complexes” or “belts”. Here, the names pro- ing, and the results of these studies are report- posed by Nironen et al. (2002) are used. The ed here. They reveal that a trondhjemite gneiss Pudasjärvi Complex is bounded to the north from Siurua, Pudasjärvi, seems to be the old- and south-southwest by Palaeoproterozoic est primary crustal rock in the Fennoscandi- belts and to the east by Palaeoproterozoic an Shield, with a minimum conventional U-Pb zones, rocks of the Central Granitoid zircon age of 3.3 Ga. The age is noticeably old- Complex and the Archaean Eastern Finland er than the 3.1–3.2 Ga ages of the orthogneiss- Complex. Until recently the geology of the area es from Tojottamanselkä, Sodankylä (Kröner et has been relatively poorly known, partly due to al., 1981; Jahn et al., 1984; Kröner & Comp- limited outcrop. ston, 1990), Lapinlahti (Paavola, 1986; Hölttä During exploration in 1998 in are- et al., 2000) and other parts of the Fennoscan- as west and northwest of Pudasjärvi, granulite dian Shield (Shurkin et al., 1979; Mints et al., 1982; Hyppönen, 1983; Lobach-Zhuchenko et * Corresponding author al., 1986; Sergeev et al., 1990; Chekulaev et al., e-mail: [email protected] 1997). 52 Mutanen, T. and Huhma, H.

Fig. 1. General geology of the Archaean Pudasjärvi Complex. The map is simplifi ed after Korsman et al. (1997). Area of Fig. 3 outlined.

Rocks with Sm-Nd model ages (TDM) sug- rock or are inherited from an older source re- gesting formation of sialic crust at 3.4–3.6 Ga gion (Kamber et al, 2001). are known from the Fennoscandian Shield (Jahn et al., 1984; Huhma et al., 1995; Hanski et al., 2. General geology 2001), and direct evidence of the ancient crust in that age range has been provided by the ion The north-south trending Pudasjärvi Granulite probe analyses from few detrital zircons (Claes- Belt (PGB) is located in the middle of the Ar- son et al., 1993; Huhma et al., 2000). In Eu- chaean Pudasjärvi gneiss complex (Figs. 1 and rope, zircons older than 3.5 Ga have been found 2), ca. 70 km northeast of Oulu. On the gener- in the Novopavlovsk Complex in the Ukrainian al geological map (1:400 000; Enkovaara et al., Shield (Bibikova & Williams, 1990). The old- 1952; 1953) some of the rocks in the belt have est reliably dated rocks on the Earth, e.g. the It- been portrayed as containing either saq Gneiss Complex, Greenland, give ages in the pyroxene or . The boundaries of the range 3.7–3.8 Ga (Nutman et al., 1996; Kam- belt have not yet been delineated by systemat- ber et al., 2001). It has been questioned wheth- ic mapping. Based on the old map (Enkovaara er the 4 Ga old zircons in the Acasta gneisses et al., 1952) and new mapping data, the length in Canada (Stern & Bleeker, 1998; Bowring & of the PGB is at least 55 km; the width seems to Williams, 1999) are cogenetic with their host vary between 15 and 40 km. The PGB is fl anked The 3.5 Ga Siurua trondhjemite gneiss in the Archaean Pudasjärvi Granulite Belt, northern Finland 53

Fig. 2. Regional Bouguer gravity map of the Pudasjärvi Complex. The northern part of the block structure contain- ing the Pudasjärvi Granulite Belt (PGB) is outlined. The PGB is located along the eastern slope of a marked (30 mGal) regional positive Bouguer anomaly. by wide areas of Archaean gneisses, still known 9–11 kbar, and 800–900oC (Hölttä & Paavola, in general features only. West of the PGB lies 2000) has taken place at 2.6–2.7 Ga (Hölttä et also the Oijärvi , with associated al. 2000; Mänttäri & Hölttä, 2002). gold deposits (Tolppi, 1999). Reports of meta- The Pudasjärvi Granulite Belt runs along the morphic pyroxene amongst Archaean gneiss- eastern fl ank of a regional positive gravity anom- es east of the PGB (for example, Enkovaara et aly in the western part of the Pudasjärvi gneiss al., 1953, p. 26) suggest that outlier occurrences complex (Fig. 2). On the aeromagnetic map the of granulites are more widespread in the eastern PGB appears as a subdued feature, bounded part of the Pudasjärvi Complex and the north- and intersected by faults and ductile shears (Fig. ern part of the Archaean Eastern Finland Com- 3). As a whole the PGB seems to be a tecton- plex. Archaean granulites are well exposed in ic block-mosaic. Two larger internal blocks with the Iisalmi Complex, Central Finland (Paavo- regular magnetic banding and fold structures la, 1986), where high-grade metamorphism at can be discerned (Fig. 4), but otherwise patterns 54 Mutanen, T. and Huhma, H.

Fig. 3. Grey-tone low-altitude aeromagnetic map of the Pudasjärvi Granulite Belt and surroundings, with sample lo- calities. Outlined area is shown enlarged in Fig. 4. The 3.5 Ga Siurua trondhjemite gneiss in the Archaean Pudasjärvi Granulite Belt, northern Finland 55 of primary structures cannot be distinguished in the grey-tone low-altitude aeromagnetic maps. It appears, too, that inside the PGB slices of dif- ferent metamorphic grades ( and granulite facies) and crustal ages are juxtaposed, which attests to considerable differential vertical movements. Inside the blocks, however, signs of penetrative deformation (such as ) seem to be generally lacking. Under microscope the rocks have a polygo- nal mosaic granoblastic and, apparently, non- 7280 foliate textures, with straight or slightly serrated grain boundaries. Triple boundary junctions at 120° are common in mafi c granulites. Preferred orientation, when found in microscope (Lalli, 2002) might indicate proximity of syntectonic, healed shear zones. The PGB forms the southern part of a ma- jor long-lived, battered zone that registers in- termittent post-PGB faulting and magmatic events. Southwards from the down-bulge at the northern contact of the Pudasjärvi gneiss com- plex with Palaeoproterozoic supracrustal rocks (Fig. 1), the zone includes blocks of 2.44 Ga (Ala pieti, 1982; Perttunen & Vaasjoki, 2001) layered intrusions, diorites and other intrusive rocks in the Ranua area, magnetic mafi c dykes of westerly, northerly and north-northwesterly trends. Non-magnetic gabbroic dykes, assumed feeders of the layered intrusions, occur near the 3480 northwestern (upper) border of the gneiss com- Fig. 4. Detail of aeromagnetic map of Fig. 3. The length plex (Mutanen, 1989). of the magnetic structure from north to south is ca. The most prominent faults run in norther- 16 km. ly, northeasterly and north-northwesterly direc- tions. The maximum (apparent) horizontal shift magmatism; the felsic members, however, could along a right-lateral fault is 8 km. The young- also be seen to represent new sialic crust charac- est, anastomosing system of faults has a general terized by an incomplete, or atypical TTG (to- trend between northwest and west-northwest. nalite-trondhjemite-granodiorite) association. The Ranua diorites, just north of the PGB, are 3. Petrography and geochemistry massive igneous rocks. The major and trace el- ement compositions of some type samples are Most rocks of the Pudasjärvi Granulite Belt given in Table 1 and the chondrite-normalized encountered in outcrops are presumed or- REE patterns in Fig. 5. thogneisses of mafi c and felsic composition The mafi c granulites, with pyroxenes and (trondhjemites, tonalites, alkali , high-T metamorphic are called py- alaskites). As a whole the assemblage of igneous ribolites (pyroxene ). These mas- rocks may represent bimodal (basaltic/felsic) sive, granoblastic, medium-grained and non- 56 Mutanen, T. and Huhma, H.

Table 1. Major and trace element analyses for rocks of the Pudasjärvi Granulite Belt and Ranua . Sample A1601 A1602 A1603 A1611 TM-00-13

SiO2 49.4 70.3 73.3 56.0 75.3 TiO2 0.66 0.45 0.14 0.77 0.02 Al2O3 15.1 15.6 14.8 18.1 13.3 Fe2O3(t) 10.9 2.33 1.28 7.73 0.91 MnO 0.19 0.02 0.02 0.13 0.04 MgO 7.89 0.92 0.44 2.98 <0.03 CaO 11.7 3.17 2.61 6.92 0.58

Na2O 2.34 4.48 4.17 4.73 3.81 K2O 0.19 1.83 2.79 0.90 5.32 P2O5 0.04 0.03 0.04 0.34 <0.01 Sum 98.41 99.13 99.59 98.60 99.30 Cr 306 <30 <30 30 <30 Ni 121 8 5 6 <4 Co 42 5.6 2.4 15 <0.5 V 241 32 14 126 <0.5 Sc 46 3.6 1.8 16 2.5 Sr 127 311 449 1047 18 Ba 33 503 970 665 170 Rb 0.4 57 59 14 91 Zr 32 294 66 142 55 Hf 1.0 6.9 1.7 3.2 2.8 Ta 0.2 <0.2 <0.2 <0.2 1.8 Nb 1.8 5.2 0.7 4.3 2.3 Pb <30 32 <30 <30 34 U <0.2 2.0 <0.2 <0.2 0.5 Th <0.5 46 <0.5 0.6 4.7 Y 19 7.5 1.5 20 21 La 4.03 123 11.4 27.3 8.59 Ce 10.6 213 17.2 62.8 16.2 Pr 1.72 22.2 1.70 8.58 1.65 Nd 8.08 75.8 5.83 36.2 5.61 Sm 2.17 10.0 0.69 6.65 1.01 Eu 0.83 1.35 0.35 1.75 0.12 Gd 2.70 6.77 0.66 5.98 1.24 Tb 0.49 0.59 <0.1 0.72 0.29 Dy 3.20 1.90 0.29 3.76 2.65 Ho 0.70 0.27 <0.1 0.67 0.72 Er 2.01 0.60 <0.15 1.87 2.31 Tm 0.27 <0.1 <0.1 0.26 0.38 Yb 0.96 0.55 <0.15 1.63 2.64 Lu 0.26 <0.1 <0.1 0.23 0.38 S 340 <100 <100 <100 <100 Cl <60 170 <60 210 <60 Cu 49 <5 14 11 <5 Ga 14 22 15 23 16 Zn 85 52 22 105 <10 F 600 600 300 600 <100 Major oxides (wt. %) and S, Cl, Cr, Sr, Ba, and Pb (ppm) by XRF; REE and other trace elements (ppm) by ICP-MS; F (ppm) by potentiometric NaOH-fusion; all analyses are made at the laboratory of the Geological Survey of Finland,

Espoo. Total as Fe2O3(t).

foliate rocks consist of (An36-72, Lal- tite (with exsolved fl ake pentlandite) and chalco- li, 2002), clinopyroxene, orthopyroxene and pyrite. Exsolved lamellar ilmenite in magnetite brown or greenish brown hornblende; sparse is common. Secondary are pyrite and is sometimes present. Accessory miner- carbonate. Rare zircon occurs as euhedral stub- als are fl uorapatite, magnetite, ilmenite, pyrrho- by crystals. The 3.5 Ga Siurua trondhjemite gneiss in the Archaean Pudasjärvi Granulite Belt, northern Finland 57

Retrograde alteration, where present, has 600 produced dark green hornblende and minor A1602 e colourless amphibole (cummingtonite?) from it A1611 100 A1603 pyroxene. The alteration advanced from grain n d r A1601 boundaries and sometimes consumed the orig- h o inal pyroxene entirely, but even then brown 1 C

/C 10 hornblende usually has remained intact. Other le

retrograde minerals are (from ilmenite), m p , biotite and pyrite. S a A These mafi c granulites are interpreted to be 1 .6 basaltic rocks (see Table 1, A1601) with a fair- La Ce Pr Nd SmEu Gd Tb Dy Ho Er Tm Yb Lu ly fl at REE pattern (Fig. 5), probably metalavas not materially affected by crystal fractionation 600 or crustal contamination. e

The Siurua trondhjemite gneiss is the domi- it 100

nant rock in a large outcrop area. It is the only n d r TM-00-13.1 rock type among the samples of PGB which h o is not, at least in its present state, of granu- 1 C

/C 10

lite facies. Salic segregations in trondhjemite in le

the Siurua outcrop and, locally, in mafi c granu- m p

lites, as well as narrow (usually < 20 cm), gneis- S a B sosity-parallel veins of in Siu- 1 .6 rua trondhjemite suggest anatexis. The rock is La Ce Pr Nd SmEu Gd Tb Dy Ho Er Tm Yb Lu granoblastic, medium-grained and consists of Fig. 5. REE (chondrite normalized) patterns of, a) Siu- plagioclase (An27), , potassium feldspar rua gneiss (A1602), Ranua diorite (A1611), and felsic (slightly perthitic microcline), dark biotite and (A1603) and mafi c (A1601) granulite samples, b) ”tim- accessory fl uorapatite, zircon, monazite (inclu- berman alaskite”. C1 chondrite values from Sun and McDonough (1989). sions in biotite), ilmenite, magnetite and meta- mict allanite. According to electron microprobe analyses the monazite contains 0.26–0.66 wt% pletion of phosphorus and strong enrichment U and 0.80–1.01 wt% Pb. In some samples zir- of Th and LREE. con has peculiar goethite mantles. Dark green The felsic granulites a arere medium-grained,medium-grained, hornblende and occur in one sample. Al- granoblastic and apparently unfoliated rocks teration products are carbonate, chlorite (from consisting mainly of plagioclase (An27-35), biotite), epidote and (from plagio- quartz, orthopyroxene and clinopyroxene. Po- clase), titanite and (from ilmenite) and tassium feldspar occurs in interstices and as an- hematite (”martite”, from magnetite). Textur- tiperthite in plagioclase. Biotite is a regular mi- al relations suggest that part of the biotite was nor constituent in the most felsic types, but in formed from garnet. Some of the microcline more mafi c variants brown hornblende takes its may also arise from material emanated from stead. Accessory phases identifi able under mi- granite veins. Gneissic banding is croscope are fl uorapatite, zircon (sometimes seen on outcrop but penetrative deformation is with discordant growth zones), magnetite and not noticeable under the microscope. ilmenite (sometimes ilmenite alone). As in ma- The chemical composition of the Siurua fi c granulites there is also lamellar exsolved il- trondhjemite gneiss (Table 1, A1602) is close menite in magnetite. Chalcopyrite, bornite and to the average Archaean TTG gneisses (Mar- chalcocite are occasionally encountered. Pyr- tin, 1994). The major differences are the de- rhotite occurs in very small amounts either as 58 Mutanen, T. and Huhma, H. small free grains or as tiny inclusions in mag- Compared to average crustal composition netite. Rare pyrite may be a secondary phase. the alaskites are strongly depleted of Mg, Ca, Typical retrograde minerals are titanite, hema- Fe, P, Ti and Sr (Table 1, TM-00-13). Their tite (“martite”), carbonate, clinozoisite-epidote, REE pattern (Fig. 5b) is peculiar, having the chlorite, marcasite and goethite. The complete general form of timberman´s (AcanthocinusAcanthocinus ae- lack of exsolved hematite in ilmenite attests to dilis) tentacles (hence the appellation ”timber- reducing crystallization (equilibration) condi- man alaskite”), so that from LREE and HREE tions (e.g., Frost 1991), while absence of marti- ends the curve drops toward Eu. The REE pat- tization of magnetite implies that no retrograde tern of another granite from the Isokumpu area (and supergene) oxidation took place. The rocks is similar but HREE are still more enriched (Lal- are completely recrystallized in prograde meta- li, 2002, App. 1). The alaskites are also high in morphism to the extent that no tokens of pri- Y (20–60 ppm). The modal and major element mary igneous textures are left. composition of the timberman alaskite magma On account of their modal and major ele- represents a dry granite minimum (> 900°C, ment compositions (Table 1, A1603) the fel- e.g., Sood 1981; Carmichael et al. 1974), or sic granulites (alkali charnockites) pass as trond- metamorphic recrystallization within the same hjemites (O´Connor, 1965). Typically they are temperature range. The strong to extreme deple- leucocratic; in the type sample A1603 the sum tion of the elements listed above indicates that of normative quartz and feldspar is 95 %. How- the alaskite magma resulted from considerable ever, larger sample set suggests spreading over crystal fractionation. to the tonalite fi eld (Lalli, 2002). The REE The araree geochemically rrelatedelated to patterns of the felsic granulites and the Siurua alaskites. They are white subsolvus with trondhjemite have a similar, LREE-enriched independent oligoclasic plagioclase. The micro- shape; however, the REE concentrations of the cline contains little or no perthite. The amounts Siurua trondhjemite are about an order of mag- on dark minerals (biotite, garnet) and musco- nitude higher (Fig. 5). Both show a negative Eu vite are small. Zircon occurs as euhedral short (CN) anomaly. Distinct from these is the fl at- prismatic crystals. Other accessory minerals en- ter LREE pattern of the intrusive diorites of the countered are fl uorapatite, ilmenite, magnet- Ranua area, just north of the PGB. ite, monazite, pyrrhotite, pyrite, chalcopyrite, Alaskites a arere formallyformally high-silicahigh-silica hypersol-hypersol- mackinawite and molybdenite. Secondary min- vus granites. These medium-grained, granular erals are green hydrobiotite (from garnet), chlo- (granoblastic), texturally massive rocks are com- rite (from biotite), rutile (from ilmenite), hem- posed of perthitic microcline, quartz and very atite (“martite”, from magnetite), marcasite small amounts of independent plagioclase. Spes- (from pyrrhotite) and carbonate. sartite-rich garnet is a scarce but regular constit- With the decrease of microcline the leu- uent and has partly reacted to biotite. Second- cogranites grade to granodiorites and trond- ary and accessory minerals are muscovite, chlo- hjemites. rite, magnetite, hematite (martite after magnet- Trondhjemite pegmatoids formform narrownarrow (usu-(usu- ite), rutile, zircon and a few tiny grains of cassi- ally less than one metre) bands in the proxim- terite and gold (these latter found only by EMP ity of other granitic rocks. These coarse-grained scanning). The proportion of perthite in mi- reddish rocks consist of plagioclase, variable crocline is so high as to contain almost all of amounts of quartz and biotite. The amount of the plagioclase in the rock. The marginal parts microcline (often as antiperthite in plagioclase) of microcline contain a smaller proportion of is typically small. In places quartz is practical- perthite. The interiors of the independent pla- ly lacking. Zircon and monazite occur as large gioclase grains are characteristically smudged by crystals, often visible with the naked eye. Other iron hydroxides. accessory and secondary minerals are magnetite, The 3.5 Ga Siurua trondhjemite gneiss in the Archaean Pudasjärvi Granulite Belt, northern Finland 59 ilmenite (partly altered to titanite and rutile), along with relatively feeble manifestations of chalcopyrite, pyrite, pyrrhotite (as inclusions in retrograde processes, suggests exceedingly rapid pyrite), galena (?), chlorite, carbonate, musco- ascent and exhumation of the PGB fault block vite and epidote. These rocks are very rich in Th terrain. (up to 481 ppm), LREE (Ce and La up to 4720 ppm and 2500 ppm, respectively). 4. Methods for isotopic analyses The Ranua diorites, just north of the Pudas- järvi Granulite Belt, are massive igneous rocks. Zircons from four rock samples were separat- Diorite occurs in a relatively large area charac- ed and analysed following methods by Krogh terized by a positive regional gravity anomaly, (1973). Procedures for conventional analyses and forms large outcrops in the northern part involved aliquoting the HCl solution and ad- of the supposed intrusion area. These medi- dition of 208Pb/ 235U isotopic tracer (206Pb/ 235U um-grained rocks consist of plagioclase, green tracer for monazite). Measurements were made or bluish green hornblende and dark brown using a VG Sector 54 mass spectrometer. The biotite, and always contain a fair amount of performance of the ion counter was checked by quartz in interstices. The original lath-shaped repeated measurements of a NBS 983 standard. plagioclase is altered to coarse epidote and to al- U-Th-Pb spot analyses of zircons were per- bite-. Accessories and alteration prod- formed using Cameca IMS1270 ion-micro- ucts are magnetite (partially or wholly oxidized probe at the Swedish Museum of Natural His- to hematite), ilmenite (with exsolved hematite), tory, Stockholm (NORDSIM facility). The an- fl uorapatite, titanite, rutile, pyrite and zircon. alytical procedure is described in Whitehouse Vestiges of igneous origin are seen in the shape et al. (1997, 1999). The spot-diameter for the of plagioclase grains and rare trellis texture of il- 4nA primary O - ion was ca. 30 µm, and oxy- 2 menomagnetite, but otherwise the granoblast- gen fl ooding was used to improve the ionization ic textures and paragenesis represent later meta- of Pb. Calibration of the U/Pb ratio was based morphic overprint(s) and retrograde reactions. on analyses of the Geostandards zircon 91500, Apart from local sheared variants the rocks are which has an age of 1065 Ma (Wiedenbeck massive. et al., 1995). Corrections for common Pb are The granulite mineral assemblages of the Pu- based upon the measured 204Pb signal and the dasjärvi Granulite Belt represent water-defi cient present day terrestrial average Pb-isotopic com- low-P high-T metamorphic conditions. For fel- position is used for this correction (Stacey & sic granulites, however, the P-T conditions es- Kramers, 1975). Data reduction was performed timated from mineral reactions are surprisingly using Nordsim software written by M.J. White- mild (2.5–3.5 kbar, 500–600°C); only one sam- house, and age calculations were made using ple gave 5.5 kbar and 650°C (Lalli, 2002). For Isoplot/Ex 2.49 (Ludwig, 2001). Using BSE- mafi c granulites the metamorphic temperatures images 30 zircon analysis points were selected are 700–800 °C and maximum possible pres- and measured from two rock samples. sures ca 6.5–7 kbar (op.cit.). According to Lal- For Sm-Nd analyses the sample dissolution li (2002) after the metamorphic peak conditions was performed in sealed tefl on bombs, and the rocks underwent a rapid, practically isother- a mixed 150Nd-149Sm spike solution was add- mal decompression, and the minerals equilibrat- ed to the sample prior the dissolution. Chem- ed at tempered conditions. In the PGB a ubiqui- ical separation of Sm and Nd followed meth- tous cooling “speedometer” is available: lamellar ods by Richard et al. (1976). Measurements ilmenite exsolution bodies in magnetite (in con- were made on a VG Sector 54 mass spectrome- trast to granule exsolution) indicate rapid cool- ter using Ta-Re triple fi laments and a dynamic ing through the Fe-Ti oxide solvus (at or below mode of measurements. Nd ratios are normal- 600°C, e.g., Price 1981; Lindsley, 1991). This, ized to 146Nd/144Nd=0.7219. Measurements of 60 Mutanen, T. and Huhma, H.

the 143Nd/144Nd in the La Jolla standard during zircon are discordant. In spite of some scatter in 1995–2000 have yielded a ratio of 0.511850 ± the analytical results it is suggested that small 0.000007 (standard deviation, n=60). The Sm/ amounts of metamorphic zircon had formed Nd ratio of the spike was calibrated against the ca. 2.65 Ga ago in this mafi c rock. This age is Caltech mixed Sm/Nd standard (Wasserburg et within the range obtained for the metamorphic al, 1981). Based on duplicated analyses the error monazite and zircon in the Varpaisjärvi gran- in 147Sm/144Nd is estimated to be 0.4%. Initial ulites in eastern Finland (Huhma et al., 1995; 143Nd/144Nd and e were calculated with the fol- Hölttä et al., 2000; Mänttäri & Hölttä, 2002). lowing parameters: Ê147Sm=6.54x10-12a-1, 147Sm/ 144Nd=0.1966 and 143Nd/144Nd=0.51264 for Sample A1602 Siurua, Pudasjärvi, trond- present CHUR. T was calculated according , map sheet 3514 08, coordinates DM hjemite gneiss to DePaolo (1981). Programs by Ludwig (1991, x=7267.09, y=3480.52. 2001) have been used for age calculations. The sample was collected from a relatively large outcrop area consisting mainly of slight- ly banded trondhjemites, with lesser amounts of 5. Results and Discussion darker granitoids containing garnet and horn- 5.1. Zircon ages blende. In the southern part of the outcrop area boudins of dark green mafi c rocks, from 10 cm Sample A1601, Rankkila, Pudasjärvi, mafi c gran- to >2 m wide, appear to be metamorphosed ulite, map sheet 3513 09, Finnish national grid dykes. Zircons separated from the sample are coordinates x=7241.20 (northing), y=3485.00 generally small euhedral prisms, short to mod- (easting). erately elongate and rounded on edges due to re- The outcrop by the Oulu–Pudasjärvi road sorption. The colour ranges from pale brown to consists of grey-black granoblastic pyroxene am- colourless, the grains are generally translucent. phibolite (pyribolite), with only slight retrogres- In the HF-etched section oscillatory zoning is sive alteration of pyroxenes to secondary amphi- very distinct. The sample also yielded yellow boles. The sample yielded a small amount of zir- grains of monazite in the magnetic fraction. con, which consists mainly of very transparent, The monazite analysis shows very high U and colourless, equant, and subhedral grains with- Pb and provides a concordant age of 2660±2 Ma out distinct internal structure. Many grains have (Table 2 and Fig. 6). This is similar to the age of a rounded shape, and the appearance of zircon is the metamorphic zircon in the mafi c granulite likely metamorphic. (A1601) discussed above. The regression of the four discordant U-Pb The fi ve conventional U-Pb analyses on zir- analyses (Table 2) provides intercepts with the con are discordant and do not give any decent concordia curve at 2650±20 Ma and 156±250 chord on the concordia diagram (Fig. 6). Partic- Ma (MSWD=5.7, Fig. 6). The data reveal that ularly the analysis A1602B is off and has a radio- the U concentration in the zircon is very low (10 genic 208Pb/ 206Pb markedly higher than the oth- ppm), and thus accidental procedural blank may er analyses (Table 2). This is likely due to a mi- have some infl uence on the analyses. Rejecting nor amount of monazite in the aberrant anal- the relatively poor analysis B, the other data are ysis. As the Pb concentration in monazite is perfectly on a chord, which gives intercepts at very high, only 0.0005 mg (less than one grain) 2652±4 Ma and 157±48 Ma (MSWD=0.6). In would be needed to change the Pb/Pb ratios. any case, the 207Pb/ 206Pb age of 2648±3 Ma for However, even rejecting this analysis the four the least discordant analysis (D) with a relatively points do not plot exactly on a line as the cal- high 206Pb/ 204Pb-ratio should be considered the culation gives intercepts at 3361±78 Ma and minimum age for zircon. It remains somewhat 1223±840 Ma, and a large MSWD=16. Never- obscure why the analyses on such low-uranium theless, it is obvious that the 207Pb/ 206Pb ages of The 3.5 Ga Siurua trondhjemite gneiss in the Archaean Pudasjärvi Granulite Belt, northern Finland 61 Pb 206 2649 2630 2624 2648 3299 3270 2660 3289 3317 3273 2922 2913 2851 2939 2699 2705 2703 2702 Pb/ 207 U 235 2588 2543 2298 2613 3217 3180 2666 3175 3247 3157 2854 2803 2774 2866 2698 2708 2705 2697 Pb/ 207 U 238 APPARENT AGES / Ma AGES APPARENT 2511 2436 1950 2568 3088 3040 2672 2997 3134 2976 2758 2653 2670 2764 2697 2711 2708 2691 Pb/ 206 0.99 0.99 0.98 0.99 0.98 0.98 0.97 0.98 0.98 0.98 0.98 0.98 0.96 0.98 0.92 0.97 0.98 0.99 corr. Error %  0.3 0.5 0.4 0.2 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.3 0.1 0.12 0.15 0.25 0.12 2 Pb 206 Pb/ 0.17960 0.17751 0.17693 0.17948 0.26877 0.26389 0.18082 0.26710 0.27196 0.26444 0.21210 0.21092 0.20302 0.21432 0.18503 0.18575 0.18556 0.18539 207 % 2 3 2 2 1  0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.6 0.6 0.75 2 U 235 Pb/ 8.615 11.794 11.239 12.112 22.776 21.918 12.806 21.800 23.476 21.392 15.614 14.810 14.360 15.818 13.251 13.390 13.356 13.242 207 ISOTOPIC RATIOS* ISOTOPIC % 2 3 2 2 1  0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.7 0.6 0.6 2 U 238 Pb/ 0.47628 0.45920 0.35315 0.48944 0.61460 0.60239 0.51363 0.59188 0.62607 0.58670 0.53391 0.50925 0.51301 0.53529 0.51938 0.52282 0.52205 0.51805 206 Pb 206 7.1 0.1 0.1 0.08 0.37 0.076 0.098 0.077 0.066 0.063 0.065 0.061 0.083 0.087 0.215 0.205 0.185 0.195 Pb/ radiogenic 208 Pb 204 3068 3150 2504 3389 4350 2597 2723 1342 1705 1280 2074 8182 913.5 685.7 38880 2452.7 1008.1 1635.9 Pb/ measured 206 76 55 Pb 372 557 424 358 441 479 304 163 451 5.51 5.34 5.33 4.78 62.1 54.3 ppm 1.21 (%) U 89 520 647 618 492 648 780 512 273 732 114 8.97 95.8 86.7 ppm 3280 10.62 10.59 13.73 0.51 0.34 0.45 1.35 0.43 0.25 0.14 0.53 0.20 0.26 0.52 0.43 0.28 0.35 0.13 0.40 0.40 0.69 Sample weight / mg weight x hours U-Pb data for zircons and monazite from the Pudasjärvi Granulite Belt and Ranua diorite density/mesh/abraded Sample information Sample A1601 Rankkila, Pudasjärvi, mafi c granulite mafi A1601 Rankkila, Pudasjärvi, A1601A +4.0 +200 a5h clear rounded A1601B +4.0+200 clear rounded A1601C +4.0 -200 a3h clear A1601D +4.0 +200 a6h gneiss trondhjemitic Pudasjärvi, A1602 Siurua, A1602A +4.3 a2h pale clear A1602B +4.3 pale clear +monazite? +4.2 A1602C Monazite magn A1602D 4.2-4.3 a5h A1602E +4.3 a16h translucent A1602F 4.2-4.3 trans- lucent felsic granulite Pudasjärvi, A1603 Isokumpu, A1603A +4.0 +200 a4h pale turbid A1603B +4.0 +200 pale turbid A1603C +4.0 -200 clear A1603D +4.0 -200 total a5h Ranua, diorite A1611 Korkia-aho, A1611A +4.2 a16h A1611B +4.2 a6h A1611C +4.2 A1611D +4.2 a16h & Kramers 1975). common lead (Stacey for fractionation, blank (Pb 40 pg, U 10 pg)) and age related ratios corrected *) Isotopic Table 2. Table 62 Mutanen, T. and Huhma, H.

Fig.Fig. 6.6. U-PbU-Pb concordiaconcordia plotplot ofof con-con- ventionalventional zirconzircon andand monazitemonazite datadata forfor rocksrocks ofof thethe PudasjärviPudasjärvi Granu-Granu- litelite BeltBelt andand RanuaRanua diorite.diorite.

the individual analyses are very old, and give a allow indisputable determination of exact mag- minimum age of 3317 Ma for the zircon. matic age for the Siurua gneiss. This is not un- Ion-microprobe U-Th-Pb analytical data for common for ancient gneisses, which often tend A1602 zircons are presented in Table 3 and in to provide complicated age patterns and prob- Fig. 7. Most of the data points appear to be re- lems with interpretation (e.g. Whitehouse et al verse discordant, although there appeared to be 1999). Several zircons in sample A1602 are ca. no problems with the Pb/U calibration per- 3.5 Ga old, which is considered as the best es- formed on 91500 during this session; the rea- timate for igneous crystallization of the Siu- son for this problem remains unclear but may rua gneiss. Abundant 3.4 Ga ages may be ex- for example be related to localized charging is- plained by sub-solidus age-resetting of igneous sues around the zircons, or signifi cant compo- zircons, metamorphic recrystallisation of igne- sitional differences between these zircons and ous zircons or precipitation of new zircon from the 91500 standard. However, repeated analyses metamorphic fl uids aided by breakdown of Zr- to the same spots yield consistent Pb/Pb ratios bearing minerals such as hornblende (Mezger (n1176-01a, -01d and n1176-10a, -10b) and and Krogstad, 1997; Fraser et al. 1997). There suggest that the 207Pb/206Pb ages are reliable. Fif- are four analyses on the crystal n1176-01, from teen grains of the total 18 analysed yield 207Pb/ which three give ages of 3.5 Ga (01a and 01d 206Pb ages of 3.4–3.5 Ga. One of these crystals from the same spot, Fig. 8b), whereas 01c gives has a distinct core, which gives an age of 3.73 Ga an age of 3.4 Ga. The low Th/U in analysis 01c (Fig. 8a). The three youngest 207Pb/206Pb ages are (0.12) suggests metamorphic recrystallisation. 3.35 Ga, 3.27 Ga and 3.12 Ga. The cathodolu- However, in CL image the domain is not clear- minescence (CL) images reveal that zircons in ly distinct. As the gneiss is slightly migmatitic, sample A1602 are CL-dark and internally rela- the formation of felsic segregations may be asso- tively homogeneous, many grains showing faint ciated with some zircon growth (e.g. 3.12 Ga). prismatic zoning (Fig. 8b). Most crystals have a None of the sims ages are close to the 2.66 Ga very thin (2μm) CL-light rim close to the sur- monazite. However, the conventional data with face. There is no distinct structural control be- 207Pb/206Pb ages of ca. 3.3 Ga suggest that the tween the different age groups, and the zircon proportion of younger zircon might be larger population appears to be relatively homogene- than expected from the sims analyses. It is possi- ous also in optical microscope. The data do not ble that the outer rims with thin CL-light bands The 3.5 Ga Siurua trondhjemite gneiss in the Archaean Pudasjärvi Granulite Belt, northern Finland 63  ± 43 37 36 37 43 41 37 42 42 40 41 42 44 45 42 41 40 50 34 35 44 37 38 32 32 32 33 32 34 46 U 238 3810 3571 3348 3620 3740 3591 3123 3735 3696 3436 3630 3737 3994 3833 3670 3585 3478 3868 3228 3351 3696 3537 3497 2974 3042 2959 3092 2991 3118 3212 Pb/ age (Ma) 206  ± 16 14 14 14 15 15 15 15 15 16 15 15 15 17 16 15 15 21 13 14 20 15 15 13 13 15 14 13 14 21 U 235 Pb/ 3528 3379 3541 3590 3500 3124 3604 3570 3329 3485 3576 3818 3768 3532 3464 3428 3660 3304 3436 3539 3446 3458 2944 3022 2961 3019 2976 3017 3054 3619 age (Ma) 207  5 5 5 5 5 5 4 5 7 5 5 5 9 9 5 7 19 5 8 21 10 9 6 3 13 7 3 6 18 7 ± Pb 206 Pb/ 3504 3397 3496 3507 3448 3124 3531 3499 3265 3403 3487 3727 3733 3454 3394 3399 3547 3351 3486 3451 3394 3435 2923 3009 2962 2971 2966 2950 2951 3515 age (Ma) 207 limit 1.6 5.3 2.1 4.3 4 3.1 5.3 5.9 6.1 4.2 4 5.9 -1.9 -1.7 3.1 1.7 1.8 3.9 5.3 7.3  Disc. % Disc. 2 0.97 0.97 0.97 0.98 0.98 0.98 0.98 0.98 0.96 0.97 0.97 0.97 0.94 0.93 0.98 0.95 0.81 0.98 0.93 0.76 0.90 0.93 0.96 0.99 0.85 0.95 0.99 0.96 0.85 0.95 rho  % 1.3 1.4 1.3 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.6 1.5 1.5 1.5 1.7 1.3 1.4 1.6 1.3 1.4 1.3 1.3 1.3 1.3 1.3 1.4 1.8 1.5 ± Pb. U Pb 204 238 206 0.7401 0.6809 0.7534 0.7864 0.7455 0.6233 0.7850 0.7741 0.7040 0.7561 0.7854 0.8583 0.8124 0.7670 0.7439 0.7153 0.8221 0.6499 0.6817 0.7741 0.7308 0.7204 0.5861 0.6031 0.5825 0.6154 0.5903 0.6219 0.6460 0.8058  % 1.4 1.4 1.4 1.5 1.5 1.5 1.5 1.5 1.6 1.5 1.5 1.5 1.7 1.6 1.5 1.5 2.1 1.4 1.5 2.1 1.5 1.5 1.4 1.4 1.6 1.4 1.4 1.4 2.1 1.6 ± U Pb 235 207 31.293 26.877 31.685 33.308 30.401 20.677 33.778 32.631 25.532 29.954 32.846 41.971 39.884 31.404 29.300 28.267 35.748 24.905 28.490 31.633 28.777 29.137 17.157 18.622 17.470 18.554 17.737 18.509 19.240 34.318 error.  % 0.2 206 f 0.14 0.01 0.01 0.01 0.01 0.02 0.01 0.09 0.04 0.05 0.22 0.62 0.71 0.01 0.02 0.04 0.03 0.02 0.01 0.01 0.63 0.17 0.11 0.07 0.08 0.09 0.45 0.17 0.01 meas 0.34 0.12 0.40 0.39 0.33 0.36 0.32 0.39 0.21 0.32 0.40 0.56 0.51 0.31 0.32 0.25 0.33 0.33 0.15 0.37 0.26 0.36 0.48 0.49 0.65 0.44 0.32 0.81 0.42 0.45 Th/U .  95 92 51 84 Pb contributed by common Pb, estimated from estimated from common Pb, Pb contributed by [Pb] 360 503 894 768 341 253 444 870 622 266 696 363 511 311 531 362 255 649 819 325 139 781 249 548 592 ppm 206 1030 73 76 63 [U] 349 558 835 690 330 304 404 797 669 256 629 344 498 320 459 407 279 598 827 323 177 954 303 711 669 100 893 ppm Ion-microprobe U-Th-Pb analytical data for zircons from Pudasjärvi gneisses % is the percentage of % is the percentage 206 n1176-01b n1176-01c n1176-01d n1176-02a n1176-03a n1176-04a n1176-05a n1176-06a n1176-07a n1176-08a n1176-09a n1176-10a n1176-10b n1176-10c n1176-11a n1176-12a n1176-13a n1176-14a n1176-15a n1176-16a n1176-17a n1176-18a granulite: A1603 (n1178) Isokumpu n1178-01a n1178-02a n1178-03a n1178-04a n1178-04b n1178-05a n1178-06a 1 on ratios and ages are Errors f “ discordant. reverse numbers are positive in percent; of discordance %: degree “Disc. within 2 indicate that analysis is concordant Blanks Sample/ Sample/ spot # A1602 (n1176) Siurua gneiss: A1602 (n1176) Siurua n1176-01a Table 3. Table 64 Mutanen, T. and Huhma, H.

Fig. 7. U-Pb concordia plot of zircon data from A1602 and A1603.

in zircons represent this younger metamorphic Four crystals give 207Pb/206Pb ages of ca. 2.96 effect. This is also supported by the relatively Ga, the other two are 2.92 Ga and 3.01 Ga. In low Th/U ratio in conventional bulk analyses CL-images the 2.96 Ga crystals show marked (ca. 0.25 calculated from the radiogenic 208Pb/ regular oscillatory zoning (Fig. 8c). The 3.01 Ga 206Pb in table 2) compared to the Th/U in sims 207Pb/206Pb-age was measured from a CL-dark analyses (table 3). and homogeneous core. The age of the mag- As the zircon population has primarily a matic-looking zircons is ca. 2.96 Ga, which is magmatic and relatively homogeneous appear- considered the best estimate for the igneous age ance and the rock itself is considered magmat- of the sample A1603. No distinct metamorphic ic, it can be concluded that the Siurua trond- zircon growth was detected in isotopic studies, hjemite gneiss is the oldest rock so far identifi ed but thin rims with CL-light band on zircons in the Fennoscandian Shield. Signs of an even may well represent younger overgrowths. older crustal contribution are obtained from a 3.73 Ga old zircon core. Sample A1611, Korkia-aho, Ranua, diorite, map sheet 3524 05, x=7322.230, y=3476.746. Sample A1603, Isokumpu, Pudasjärvi, fel- The dated diorite sample contains less mafi c sic granulite, map sheet 3514 06, coordinates minerals and is fi ner in grain size than outcrop- x=7279.40, y=3476.55. ping diorites in general. Abundant zircon crys- This rock is relatively homogeneous, grano- tals separated from the sample A1611 (TM-99- blastic, medium-grained and massive. The zir- 33.1) are nearly colourless, clear, subhedral to con crystals are mostly light-coloured, moder- anhedral and moderately elongate. Many grains ately elongate euhedral prisms that are round- show regular oscillatory zoning, which may be ed on edges, probably due to resorption. Many interpreted as magmatic. grains, especially in the coarse fraction, are rel- The four U-Pb analyses on zircon are prac- atively turbid. tically concordant and provide an age of The conventional U-Pb data are discordant 2.703±0.003 Ga, which should represent the and heterogeneous. The 207Pb/ 206Pb ages range crystallization age of the diorite (Fig 6). Conse- from 2.85 Ga to 2.94 Ga; the latter is consid- quently, the Ranua diorite is not related to the ered a minimum age for the average zircon. 2.44 Ga magmatism, as has been occasionally Six zircon crystals from sample A1603 were speculated. analysed by ion-microprobe (Table 3, Fig. 7). The 3.5 Ga Siurua trondhjemite gneiss in the Archaean Pudasjärvi Granulite Belt, northern Finland 65

in this study are shown in Table 4, which also re- A n1176-10 ports ε values calculated using an age of 2.7 Ga. If Sm/Nd fractionation took place during late 3.45 Ga c Archaean, this epsilon value should be more re- liable than model ages when comparing differ- ent materials. The composition of the mafi c granulite 3.73 Ga a b A1601 is typical for many basalts, with only slight LREE enrichment. The calculation of a crustal residence age for such a composition is not warranted, but the initial εNd (T) of +1.0 (2.7 Ga ago) is common for late Archaean juve- B n1176-01 nile rocks. The other rocks in this study exhibit Sm/ d a Nd ratios quite normal for felsic crust (Ta- c ble 4). The initial ε value (+0.8 at 2.7 Ga) for b the Ranua diorite A1611 is similar to the ma- fi c granulite above, and the model age of 2.8 Ga

3.5 Ga suggests negligible involvement of much older a crustal material in its genesis. The Siurua trond- n1176-02 hjemite A1602, in contrast, has a TDM age of 3.48 Ga. It should be noted that the LREE con- centration is high in this rock. Due to the low C Sm/Nd ratio the initial epsilon at 2.7 Ga is very b n1178-04 low (-10.8), and thus this analysis can be regard- ed as a reliable indication of ancient crustal ma- terial, strongly supporting the old zircon age for

a this rock. The REE concentrations are rather low in 2.96 Ga the felsic granulite A1603 and alaskite (granite) TM-00-13, yet the LREE patterns (Sm/Nd) of n1178-05 a these rocks are similar. The Sm-Nd analysis on A1603 gives an ε of -3.0 and a model age of 3 Ga. This suggests that the origin of the rock is different from the ancient crust recorded by the Fig. 8. BSE and CL images of selected zircon grains from, Siurua gneiss above. The result from the gran- a) Siurua gneiss A1602, grain n1176-10, b) grains n1176- ite TM-00-13 (with unknown age) indicates a 01 and n1176-02, and c) Isokumpu granulite A1603, grains n1178-04 and n1178-05. still younger average protolith, as the TDM is ca. 2.76 Ga. This statement relies on the general as- 5.2. Sm-Nd whole rocks analyses sumption that the Nd-isotopic composition in the newly formed granite represents the aver- The Sm-Nd analyses on whole rocks give con- age protolith. There are indications of isotop- straints on the crustal residence age, especial- ic disequilibrium associated with anatexis (Dav- ly when the rocks analysed have typical crus- ies & Tommasini, 2000), and it remains ob- tal REE patterns. If metamorphic Sm/Nd frac- scure whether granites with low REE concentra- tionation has occurred, the results are obvious- tion are more suspect. It is also conceivable, that ly misleading. The data on fi ve samples analysed some Sm/Nd fractionation has occurred during 66 Mutanen, T. and Huhma, H.

Table 4. Sm-Nd data on whole rocks of the Pudasjärvi Granulite Belt and Ranua diorite. Sample Rock type Sm Nd 147Sm/144Nd 143Nd/144Nd Nd-epsilon T DM (ppm) (ppm) (±0.4%) (± 0.002%) (at 2.7 Ga) (Ma) A1601 Rankkila mafi c granulite 2.27 8.27 0.1660 0.512145 1.0 A1602 Siurua trondhjemite 78.18 0.0807 0.510025 -10.8 3481 gneiss A1603 Isokumpu felsic granulite 0.81 6.10 0.0799 0.510408 -3.0 3009 A1611 Korkia-aho, Ranua diorite 6.67 35.79 0.1126 0.511186 0.8 2808 TM-00-13 alaskite 1.04 6.41 0.0984 0.510956 1.3 2763 143Nd/144Nd is normalized to146Nd/144Nd = 0.7219. Errors are 2 standard errors of mean. T is calculated according to DePaolo (1981). DM metamorphism, and the model ages should be The ion-microprobe zircon U-Pb age of 3.5 taken with some reservation. Ga reveals that the trondhjemite gneiss from Si- urua, Pudasjärvi, is the oldest rock so far iden- 6. Conclusions tifi ed in the Fennoscandian Shield. Signs of an even older crustal contribution are obtained The Archaean Pudasjärvi gneiss complex con- from a 3.73 Ga old zircon core. tains a wide range of rocks with igneous ages from 3.5 Ga in the Siurua gneiss (A1602) to Acknowledgements 2.7 Ga in the Ranua diorite (A1611). The Sm- Nd depleted mantle model ages range from 3.5 We thank Heikki Juopperi, Pekka Tuisku, Katja Lalli, Bo Johanson, Lassi Pakkanen, Eero Hanski, Viena Ar- to 2.8 Ga, respectively, and suggest that the Ar- vola, Eija Hyvönen, Seppo Aaltonen, Pauli Vuojärvi, chaean complex consists of rocks with large Matti Karhunen, Leena Järvinen, Tuula Hokkanen, range of primary crustal formation ages. Lasse Heikkinen, Marita Niemelä, Arto Pulkkinen and The orthopyroxene-bearing high-grade rocks Irmeli Mänttäri for help in fi eld, offi ce and laborato- ry. Chemical analyses are made at the laboratory of the within the complex are considered to form a Geological Survey of Finland (GTK). Matti Vaasjoki belt, the Pudasjärvi Granulite Belt, which is, is acknowledged for comments on early version of the however, poorly delineated at the moment. manuscript. Reviews by Stephen Daly and an anony- Metamorphic zircon formed in a mafi c granu- mous referee are acknowledged. On our wish the technical department of the Pu- lite (A1601-Rankkila) ca. 2.65 Ga ago, roughly dasjärvi municipality kindly exposed and cleansed contemporaneously with the 2.66 Ga monazite large areas of the Siurua gneiss outcrop, for which we in the Siurua gneiss. The age is close to the high- thank the municipality director Paavo Pikkuaho, ad- grade metamorphism encountered in the Var- ministrative manager Esko Malinen, technical manag- er Ilpo Koponen and all the other persons who were paisjärvi granulite area, eastern Finland (Hölt- engaged in the cleansing work. tä et al., 2000). The rocks at Varpaisjärvi were The NORDSIM facility is operated and fund- metamorphosed at 9-11 kbar (Hölttä & Paa- ed under an agreement between the research councils vola, 2000), whereas at Pudasjärvi preliminary of Sweden, Finland, Denmark and Norway and the Swedish Museum of Natural History. Martin White- pressure estimates are much lower (Lalli, 2002). house, Kerstin Lindén and Lev Ilyinsky are greatly ac- Magmatic zircons in a felsic granulite (A1603- knowledged. This is a NORDSIM contribution #87. Isokumpu) in the PGB provide U-Pb ages of ca. 2.96 Ga, but no zircons coeval with the 2.65 Ga metamorphism were detected. It is conceivable that thin rims on zircons both in felsic granulite (1603) and Siurua gneiss (A1602) were formed Editorial handling: Yrjö Kähkönen and Petri Peltonen during regional metamorphism. The 3.5 Ga Siurua trondhjemite gneiss in the Archaean Pudasjärvi Granulite Belt, northern Finland 67

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