sign in sign up
<<
Home , Granulite, Karigasniemi

BulletinBulletin of theof the Geological Geological Society Society of ,of Finland, Vol. Vol. 78, 77, 2006, 2005, pp. pp. 71–105 00–00

Evolution of Migmatitic Complexes: Implications from Granulite Belt, Part I: Metamorphic geology

Pekka Tuisku1)* , Perttu Mikkola2) and Hannu Huhma3) 1) Department of Geosciences, University of Oulu, P.O. Box 3000, FI-90014 University of Oulu, Finland 2) Geological Survey of Finland, P.O. Box 1237, FI-70211 KUOPIO 3) Geological Survey of Finland, P.O. Box 96, FI-02151 Espoo, Finland

Abstract The Palaeoproterozoic Lapland granulite belt was juxtaposed between Archaean and Prot- erozoic terrains in the NE part of the Fennoscandian concurrently with the accretion of Svecofennian arc complexes at ~1.9 Ga. The belt consists mainly of aluminous migmatitic metagreywackes. Abundant noritic to enderbitic magmas were intruded concordantly into the metasediments and were probably an important heat source for , which took place during the crystallization of the magmas. This is supported by structural and con- tact relations of metasediments and igneous rocks, and by the lack progressive metamor- phic reaction textures in the igneous series. The peak of metamorphism took place above the dehydration melting temperature of the biotite--- as- semblage at 750−850°C and 5−8.5 kbar which lead to formation of a restitic palaeosome and peraluminous granitic melt in metapelites. Subsequently, the rocks were decompressed and cooled below the wet melting temperature of pelitic rocks (650°C) under the stability fi eld of andalusite coexisting with potassium (2−3 kbar). Cooling was accompanied by the crystallization of the neosomes, often carrying aluminium-rich phases. Postmetamor- phic duplexing of the LGB is clearly seen in the distribution of calculated PT conditions.

Key words: , Lapland Granulite Belt, , metasedimentary rocks, metapelite, enderbite, mineralogy, metamorphism, P-T conditions, Paleoproterozoic, Lap- land Province, Finland

*Corresponding author e-mail: pekka.tuisku@oulu.fi 1. Introduction

The Palaeoproterozoic Lapland granulite belt (LGB) 1937; Meriläinen, 1965, 1976; Simonen, 1980). Re- forms an extensive, 400 km long and 90 km wide, cent bedrock mapping and use of geophysical data middle to lower crustal sequence in the NE Fenno- (Korja et al., 1996; this study) have shown that, in ad- scandian (Baltic) Shield (Eskola, 1952; Meriläinen, dition to migmatitic metapelites, intermediate to ba- 1976; Korja et al., 1996). It was earlier thought to sic rocks form an important constituent of the belt. consist mainly of acid granulite facies with These were earlier called the Lapland charnockitic only minor basic or intermediate rocks (Mikkola, complex (Barbey et al., 1986), but later loosely called 72 P. Tuisku, P. Mikkola and H. Huhma enderbites or norite-enderbite series according to ma- imentary granulites, ranging in composition from jor types (Korja et al., 1996; Fig. 1). psammitic to pelitic, form the main rock type and are Also, the LGB was earlier thought to form part of the also called khondalites or khondalite series (e.g. Bar- Archaean basement complex of the shield (i.e. Simo- bey et al., 1986; Korja et al., 1996). The series is com- nen, 1980) but later structural (Marker, 1988; Korja posed mostly of acid migmatitic gneisses or kinzigites, et al., 1996) and geochronological data showed that it which often have a strong S-tectonitic or blastomy- is in most part a Palaeoproterozoic high-grade mobile lonitic structure usually dipping gently to NE, except belt surrounded by Archaean complexes (Meriläinen, at the northeastern margin, where the dips are steep- 1976; Bernard-Griffi ths et al., 1984; Huhma & Mer- ly south-westwards. Some quartz-poor or quartz-defi - iläinen, 1991; Sorjonen-Ward et al., 1994; Tuisku & cient rocks, called laanilites by Hackman (1905) and Huhma, 1998, 1999; Daly et al., 2001). Eskola (1952) are found sporadically among metased- The aim of our study was to reveal the metamor- imentary granulites. phic evolution of the migmatitic metapelites and Two major metasedimentary units contain almost the igneous series of the LGB. Unraveling the his- 50 percent of concordant, enderbitic sill-like intru- tory of the LGB could clarify the nature of the pro- sions, which also are dominantly tectonized (Korja posed models invoking divergent to convergent, ca. et al., 1996; this study, Fig.1). These have also been 2.0−1.8 Ga plate tectonic processes in the Fennos- called the Lapland charnockitic complex (Barbey et candian Shield (Gaál & Gorbatschev, 1987). Gran- al., 1986). Because norites, orthopyroxene quartz di- ulites commonly occur in signifi cant tectonic zones, orites and enderbites form the majority of the pluton- for example in the internal parts of orogenic belts ic rock series in the LGB, we prefer the term norite- (Harley, 1989) and plate tectonic and crustal thicken- enderbite series. Even opdalites or charno-enderbites ing processes were important in the development of are rare and true (orthopyroxene gran- high-grade granulite complexes also during the Palae- ites) are not found. Anorthosites are found as one ma- oproterozoic era (Newton, 1987). The present study jor unit called the Angeli anorthosite massif, cover- could also clarify the possible connection between the ing an area of about 30 km2 (Meriläinen, 1976), and tectonic evolution in the NE part of the shield and some smaller concordant sheets discovered during this the evolution in the Svecofennian arc domain, now study. Some later, post-orogenic intrusions, di- exposed in the central and SW part of the shield. The abase dikes (Meriläinen, 1976) and one syenite intru- geological history of the LGB also has a key role in sion have also been found in the area of the LGB. the interplate correlation of ca. 1.8−2.0 Ga orogen- Considering to the size of the LGB, published high ic events between the Canadian and Fennoscandian quality geochronological data are sparse. According Shields (Gaál & Gorbatschev, 1987; Hoffman, 1988, to the available evidence, most of the anorthositic, 1989; Bridgwater et al., 1990; Hall et al., 1995). enderbitic and metapelitic rocks have a Palaeopro- Because the LGB is lithologically bimodal, un- terozoic age range from about 2.0 to 1.9 Ga (Mer- raveling the evolution of the igneous rocks of the LGB iläinen, 1976; Bernard-Griffi ths et al., 1984; Huh- and their relation to the metasedimentary migmatites ma & Meriläinen, 1991; Sorjonen-Ward et al., 1994; could help to clarify the role of underplating or intra- Daly et al., 2001). plating processes in the evolution of high-grade com- The NE and SW sides of the LGB are bordered by plexes in the lower crust. thrusts and other deformation zones (Marker, 1988; Korja et al., 1996, Braathen & Davidsen, 2000). On 2. Geological setting the SW-side, there is a marginal series containing a tectonic mixture of highly strained, banded rocks, The Lapland granulite belt is one of the classical ex- sometimes called the Tanaelv belt (Marker, 1988). It amples of large granulite terrains (Fig. 1). Metased- contains some relatively high-pressure rocks, which Evolution of Migmatitic Granulite Complexes: Implications . . . 73

5 0 0 K M 1 8 E LAPLANDGRANULITEBELT3 0 A N D F I G . 1 5 0 k m F I G U R E 2

TS 9 1E 5 3 5 I 8 1 0 4 7 8 A 1 6 89 8 3 A T S 7 7 6 0 3 4 3 0 U t s j o k i 6 7 T S rt a ) p i o c z rto re o ( P e n r a e T r i a I n _ 1 0 4 8 5 6 5 N 6 5 N 1 0 0 C T S E 5 5 1 91 9 1 0 0 F T S

8 4 B T S NORWAY i wo d n vo e K 8 6 B T S )n(r a e a A ch 1 0 4 8 9 60 6 0 N 8 8 B T S RAPAKIVIGRANITES 1.8-1.5Ga ARCHAEAN >2.5GaSVECONORW EGIAN etc1.7-1Ga KARELIAN 2.5-1.9GaMESO-NEOPROTEROZOIC E 5 0 6 GRANULITE BELTSCALEDONIANMOUNTAINBELT 1.9Ga SVECOFENNIAN1 8 2 4PHANEROZOIC 1.9-1.8Ga3 6 L E G E N D O F M A I N M A P : R u o h t i r 5 7 T S

A i l i5 g a s4 T S P elitic m igm atites Karigasniem4 9 T S i D 8 8 5 4 8 T S 5 1 T S 4 6 T S N orite-enderbite series 1.9 G a D 8 7 8 I rae n iT r en a ra p t )n (ra e a A ch A northosites D 8 9A 6 3 1 7 M afic intrusive rocks 3 8 T S 7 C 9 1 G ranites 1.8 -1.77 G a M arginalThrust

8 T SK a a m a n e n F a u l t E 3 5 4 Strikes ofS-surfaces in SW E 5 2 4E 3 5E 5 3 6 3 L a k e E 4 9 6 Inarinjärvi E 4 4E 6 4 5 2 m arginalzone D 9 0 4 1 2 9 1 Sam le used in therm o b a r o m e t r y A n g e l i 2 4 T S Isotopic dating sam ple (PartII) 1 5 T S 2 1 T S N e l l i m ö D 9 1 3 A 4 3 1 D 9 1 4

1 3 9A 1 3 1 8 A 2 7 6 A 1 1 1 IA S S U R A 2 6T 8 ö r m ä s P a l a e o p r o t e r o z o i ct r a n s p o s e d L e n t o C h i g sh t r a i zn o n (e f o r e l a n sd l a b ) i v e r I v a l o j o k i R

5 9 5 K u t t u r a A 1 6 8 1A 1 6 8 2 S a a r3 i C s 9 e 5 l k ä 2 0 9 3 6 9 3 L a a n i o j a I 1 I 2 A 2 1 4 2 2 9 3 I 5 3 5 9 3 Kaarle Kustaa shaft

H 5 9 1 3 4 3 0 7 5 6 0 B 7 4 3

7 5 5 0 3 5 7 0

Fig. 1. Geological map of the Lapland Granulite Belt in Finnish territory. The map shows major lithological subdi- vision of the LGB, major tectonic elements of the LGB and adjacent areas, location of samples used for geologi- cal thermo-barometry in this paper and, in uppermost part, location of Fig. 2. The sites of samples used for isoto- pic dating in Part II of this contribution are also shown. The inset shows the location of the study area in the Fen- noscandian shield. 74 P. Tuisku, P. Mikkola and H. Huhma

LEGEND GRANITE VEINS 7 7 6 6 NORITESCHISTOSITY

LINEATION 3 4 9 0 ENDERBITE SAMPLE POINT ENDERBITE GARNET TONALITE / GRANODIORITE N M ETAPELITIC GRANULITE BLASTOM YLONITICTeno M ETA / - river N o r w a y PELITIC GRANULITE

5 2 F i n l a n d T a n a r i v e r T e n o / 1 k m

5 5 5 0

3 4

7 0

4 5

P ã d d a 6 0 5 0 5 0 6 0 K istuskaidi 3 4 8 4

7 7 6 2

Fig. 2. Map from area, where Tenojoki (Tana elv) canyon crosscuts the LGB and forms the Finnish-Norwegian fron- tier, showing typical belt parallel layering (banding) and mutual relationships of the rocks.

reached equilibrium at 12 kbar. This, together with Ga granitic plutons intrude discordantly both the the intensive transposition during the thrusting of LGB and the complexes to the southwest and north- the LGB, suggests that the areas to the SW were in- east of it (Meriläinen, 1976; Huhma, 1986; Kors- cluded in the subducting slab and formed a fore- man et al. 1997). land area (Tuisku & Makkonen, 1999). Yet the ar- The petrography and petrology of the Lapland eas to the SW contain Archaean complexes granulite belt has been described earlier in several pa- (3.1−2.6 Ga) and Palaeoproterozoic, greenstone- pers, including Eskola (1952), Meriläinen (1976), dominated supracrustal sequences (ca. 2.3–2.0 Ga), Hörman et al. (1980, Raith & Raase (1986), Barbey which were folded before thrusting of the LGB to & Raith (1990), Leibinger (1996) and Perchuck et the SW (Mikkola, 1937; Meriläinen, 1965, 1976; al. (2000). However, the only attempts to construct a Kröner et al., 1981; Lehtonen et al., 1992; Korja et metamorphic evolution (PT) path were made by Leib- al., 1996; Korsman et al., 1997; Braathen & David- inger (1996) and Perchuck et al. (2000) for the cool- sen, 2000; Vaasjoki, 2001). In the NE, the LGB is ing stage of the LGB. bordered by a gneiss complex, called the Inari Ter- rane, which is dominantly composed of Neoarchae- 3. Petrography an basement but also contains notable Palaeopro- terozoic granodioritic to gabbroic plutons having Profi le mapping was carried on throughout the ex- an age of 1.95−1.9 Ga (Meriläinen, 1976; Kesola, posed section of the LGB, usually in major or mi- 1995; Korsman et al. 1997). Younger, ca. 1.77−1.88 nor river valleys and hilly highlands. Several samples Evolution of Migmatitic Granulite Complexes: Implications . . . 75

Fig. 3. Enderbite from Vui- jeminhaara, representing the most typical, meso- or leuco- cratic , lineated member of the norite-enderbite series of the LGB.

were selected for thermo barometric work after petro- irregularly shaped or labradorite and quartz graphic studies and/or preliminary probing. grains are surrounded by biotite and orthopyrox- ene and the texture may be mortar. Garnet occurs 3.1. Enderbites, norites and related ortho- as poikiloblasts or skeletal grains with inclusions and gneisses embayments of quartz, plagioclase, biotite, and, in many cases, rims of plagioclase and orthopyroxene. Enderbites, garnet enderbites and norites are, in that Accessory phases include , ilmenite, mag- order, the major types of orthogneisses. They occur netite, , apatite and monazite. exclusively as conformable veins or sills in metapelites Norites usually form conformable layers in en- (Fig. 2). The thickness of the bodies varies from a few derbite intrusions (Fig 2). They are more melano- centimeters to more than one kilometer. The intru- cratic than enderbites and the plagioclase is labra- sions are often composed of several rock types, which dorite. The texture is granoblastic and the rocks are usually change to each other gradually. generally less lineated and more massive and coarse- Enderbites occur in almost every mapped intru- grained than the enderbites. Orthopyroxene is usu- sion. Andesine, sometimes labradorite, is prominent ally the dominant dark mineral but also substan- feldspar and forms a relatively fi ne-grained granoblas- tial amounts of hornblende may occur. Only small tic matrix of the rock. Rodded quartz grains together amounts of clinopyroxene are encountered and only with polycrystalline aggregates of orthopyroxene give few rocks are classifi ed as clinopyroxene norites. Bi- a lineated texture to the rock (Fig. 3). Minor biotite otite may be prominent and accessory minerals in- and/or hornblende and clinopyroxene may occur, clude ilmenite, magnetite, sulphides, apatite, and usually with orthopyroxene. Other accessory min- orthoclase. erals include ilmenite, magnetite, apatite, interstitial Garnet tonalites are less abundant than the pre- orthoclase, sulphides, zircon, and monazite. viously described intrusive rocks. They occur con- Garnet enderbites occur generally in a zone be- formably with enderbite intrusions and in contrast to tween enderbites and metapelites, without clear con- garnet enderbites lack orthopyroxene. They are also tacts to either rock types (Fig. 2). Medium-grained, more quartz-rich, coarse-grained and less foliated that 76 P. Tuisku, P. Mikkola and H. Huhma

Fig. 4. A schlieren type diatexite from Kaamanen area. The leucosomes consist of garnet and cordierite bearing gran- ite while the mesosomes consist of sillimanite rich garnet-cordierite gneiss.

Fig. 5. A Stromatic metatexite from Kaamanen area. The leucosomes consist of granite containing sporadically cor- dierite and garnet. The mesosome and melanosome bands consist of biotite and sillimanite rich garnet ± cordier- ite gneiss. Evolution of Migmatitic Granulite Complexes: Implications . . . 77

Fig. 6. The most common rock of the Lapland granulite belt, a blastomylonitic intensively banded diatexite, half way between Kuttura and Kaarle Kustaa sampling sites. Leucosomes, having a composition of garnet granite (ligth band in the upper middle part of the photograph), are tectonically mixed with sillimanite rich garnet gneiss (palaeosome) and garnet sillimanite bands (melanosome).

Fig. 7. A cut hand sample of a blastomylonitic diatexite, showing prominent S-C structure (in the lower part), stretch- ing lineation and thin, few millimeters wide banding of mesosomes and granitic leucosomes (one, for example, in the center of the sample). Half way between Kaamanen and Karigasniemi. 78 P. Tuisku, P. Mikkola and H. Huhma the garnet enderbites. Poikiloblastic garnet has only derbites via sillimanite-free garnet gneisses and gar- inclusions and embayments of quartz. Oligoclase oc- net enderbites. In some places, crosscutting intrusive curs with biotite as polycrystalline aggregates between rocks are also found in metapelite bedrock, but these quartz grains. Accessory minerals include orthocla- are garnet±sillimanite-bearing or granodior- se, zircon, graphite, and some opaque oxides and sul- ites, olivine-diabase veins, and biotite-granites. phides. The metapelites are always migmatitic. The struc- Garnet granodiorites are relatively rare and are ture varies from massive to extremely foliated and found mostly in those parts of the enderbite associ- blastomylonitic. Massive types often have a high ation, where crosscutting granitic veins are met with. neosome/palaeosome ratio and display nebulitic or They are poorly to moderately foliated. Poikiloblas- schlieren structures, while foliated types have lower tic garnet contains inclusions and embayments of neosome/palaeosome ratio and are usually more or quartz, oxides and biotite. Relatively coarse-grained less stromatic (Figs. 4 and 5). This is especially the quartz and orthoclase grains are embedded in a fi ne- case at the NE margin of the belt in the vicinity of Ina- grained matrix of oligoclase, biotite, quartz and or- rijärvi. However, in the other parts of the belt, me- thoclase with myrmekitic embayments. Accessory tapelites are usually foliated and banded S-tectonites minerals include monazite, zircon, and some or blastomylonites and have a high proportion of ne- graphite, opaque oxides, and sulphides. osome (Fig. 6). Due to intensive tectonization, the migmatites are generally transformed into banded 3.2. Anorthosites gneiss (Figs. 6 and 7). In the NE part of the LGB, the migmatites may contain abundant biotite, being thus The Angeli massif is partly conformable to the layer- darker than in the other parts of the belt. ing of the LGB in the eastern side of the massif and The palaeosomes (mesosomes) generally have a partly fault bounded in the south while in the west it fi ne or medium-grained, foliated matrix of quartz, is bordered by the marginal zone of the LGB (Tan- plagioclase (oligoclase or andesine), biotite, silliman- aelv belt). The rock is moderately foliated; medium- ite prisms ± rutile, ilmenite, and orthoclase. Shear grained (∼2 mm) and has a granoblastic texture. Lab- band foliation or schistosity is bent around round- radorite or bytownite is a major mineral and acces- ish or fl attened garnet , which range sories include hornblende, apatite, ilmenite, rutile ± in diameter from few millimeters to several centime- garnet ± scapolite. The smaller bodies are conforma- ters and contain oriented inclusions of sillimanite, bi- ble with the metapelite-enderbite sequence although otite, plagioclase, quartz, ilmenite, and rutile but nev- they have sheared margins. er orthoclase. Often margins of garnet are slightly re- placed by biotite and sillimanite. In other places, es- 3.3. Migmatitic metapelites pecially in the area around Inarijärvi, cordierite may be present. It occurs as matrix grains or small por- The majority of the paragneisses are composed of alu- phyroblasts and has inclusions of biotite, sillimanite, minous, pelitic metatexites and diatexites. In some and accessory phases. Margins of garnet porphyrob- parts of the LGB, the bedrock consists exclusively of lasts are usually replaced by cordierite in these rocks. these rocks (for example the hilly highlands east of Graphite, sulphides, apatite, zircon, and monazite Saariselkä), while in other areas, metapelites form the are common accessory phases in palaeosomes. Rare- country rock for the conformable enderbite-norite se- ly, hercynite-rich spinel, högbomite, , or- ries intrusions and their amount may be only half of thopyroxene, kornerupine, or andalusite is found. the total rock volume (Figs. 1 and 2). The contacts to- The neosomes are divided to melano- and leuco- wards the enderbites may be sharp but in some cas- somes. The amount of melanosome is small and they es, the sillimanite-bearing metapelites change to en- are best visible at the margins of leucosomes in stro- Evolution of Migmatitic Granulite Complexes: Implications . . . 79 matic migmatites in the vicinity of Inarijärvi (Fig. 5). times surrounded by micropegmatitic intergrowths Melanosomes are less abundant in schlieren migma- of feldspar and quartz. Also sillimanite is found as tites and they may be totally lacking especially in the large prismatic grains up to 5 cm long. Monazite may blastomylonitic migmatites. The most probable rea- be concentrated at the margins of leucosomes as rela- son for this is that primary biotite has mostly disap- tively large, 0.5 mm grains. peared and/or the structure is obscured though tec- tonization; biotite mostly occurs as a back-reaction 3.4. Silica defi cient granulites product around and in shear bands togeth- er with sillimanite (Figs. 4 and 6). The melanosomes Hackman (1905) proposed the name laanilite for are usually less than one centimetre wide, well foli- quartz-poor granulite found at Laanila near Saariselkä. ated and consist primarily of biotite, prismatic silli- Eskola (1952) reconsidered the mineralogy and chem- manite, garnet ± ilmenite, rutile, graphite, and apa- istry of these rocks, which are low in silica and alka- tite and sometimes cordierite. The grain size is fi ne or lis and extremely rich in aluminium, magnesium and medium with the exception of garnet and cordierite iron. Laanilites occur sporadically within metapelites, which, when present, occur as porphyroblasts. The are composed of mesosome and leucosome por- inclusions of porphyroblasts are the same as in pal- tions and may be considered as a variant of metape- aeosomes. Accessory phases may include apatite, zir- litic . The mesosomes consist of cordierite, con, monazite and rarely spinel. Compared to palae- garnet, plagioclase, biotite, quartz, ilmenite, and pyr- osomes the amount of zircon may be notably high. rhotite. Their texture is crystalloblastic and relatively In some schlieren migmatites, larger, several centime- coarse. Garnet porphyroblasts contain inclusions of sil- ters wide and irregularly shaped melanosomes are en- limanite, biotite, quartz, pyrrhotite and, rarely, plagi- countered. They occur either associated with palaeo- oclase and spinel. Sillimanite is commonly found as somes or as individual schlieren in leucosomes. relict inclusion bands in cordierite together with spi- Mineralogically the leucosomes resemble peralu- nel and ilmenite. Other inclusions in cordierite are bi- minous granites: The major phases include quartz, otite, garnet, and opaques. Orthopyroxene is found in plagioclase (oligoclase or less frequently andesine), some samples associated with quartz, garnet, biotite, orthoclase, and garnet. Accessories include zircon, sillimanite, and cordierite. Zircon, monazite and chal- monazite, and occasionally rutile, ilmenite, silliman- copyrite are common accessory phases. Andalusite, ite, and biotite. Cordierite and/or andalusite may oc- chlorite and quartz occur in some samples as alteration cur as accessory or major phases, especially in the re- products of cordierite. The leucosome consist of or- gion near Inarijärvi. The leucosomes are generally thoclase, oligoclase, and cordierite, which may be en- medium or coarse-grained, but may also be fi ner due closed or intergrown in poikilitic quartz grains up to 2 to mortar or blastomylonitic texture. The rocks often cm large. Garnet grains, up to 1 cm in diameter, con- look relatively massive in outcrop, but microscopical- tain inclusions of quartz and less abundant biotite, sil- ly they are usually strongly foliated and have elongat- limanite, orthoclase and accessory zircon and mona- ed rodded quartz grains in a feldspar matrix and/or zite. Garnet may be partly replaced by biotite and sil- mono- or polycrystalline σ-shaped feldspar porphy- limanite at the margins while orthoclase is replaced by roclasts in a quartz matrix. Garnet porphyroblasts are myrmekite. Accessory phases include ilmenite, rutile, usually round in shape and have an even grain size pyrrhotite, chalcopyrite, and pyrite. compared to the lens shaped garnets in palaeosomes. They also have orthoclase inclusions in addition to 3.5. Granite veins quartz, plagioclase and biotite. Cordierite is often of a deep-blue, jewel-grade variety. Andalusite is rarer and Granite veins occur locally in the metapelitic rocks. occurs as large, reddish, transparent grains. It is some- Sometimes they are associated with garnet bearing 80 P. Tuisku, P. Mikkola and H. Huhma gneissic granodiorites (Fig. 2), but often occur inde- increasing mole fraction of almandine, which varies pendently in the migmatitic rocks. They crosscut both from 0.50 to 0.74. The highest almandine contents palaeosome and neosome parts of the rock and seem were found in metasedimentary biotite-silliman- to be massive, coarse-grained and undeformed in out- ite gneisses and cordierite-biotite sillimanite gneiss- crops. However, under the microscope, they are de- es in the NE marginal zone of the LGB as well as formed; i.e. quartz shows strong chessboard extinction in some gneisses in the Archaean Inari terrain and pattern and orthoclase is granulated in the margins. -biotite gneiss in the S-marginal zone of the Potassium feldspar is perthitic orthoclase and up to 1 LGB. All these samples represent a lower metamor- cm in size. In some parts of the granulated portions, phic grade than the proper metasedimentary granu- cross-hatching of microcline is developed. Orthocla- lite, which usually has garnets with almandine con- se is generally replaced by myrmekite in the margins. tents from 0.50 to 0.64. The grossular and spessartine Other major phases are biotite and garnet, which is re- contents are low in the metasedimentary granulites, placed by biotite-oligoclase symplectite and sillimanite usually < 0.05 and 0.03, respectively. In the ender- prisms in the margins. Accessory phases include graph- bite series, there is more variation in the garnet com- ite, rutile, zircon, monazite, ilmenite, pyrrhotite, and position in the granulite facies proper: the almand- chalcopyrite. Rutile is replaced by ilmenite and, in one ine content varies between 0.5 and 0.7 and the gros- sample, rimmed with ilmenite and sillimanite. sular fraction may reach 0.16. The garnets are usu- ally unzoned. Sometimes a very narrow rim was ob- 4. Methods served, having a higher Fe/Mg-ratio than the inner parts of the grain. Microprobe analyses were performed using the JEOL JCXA733 WDS equipped with LINK AN10 auto- 5.2. Plagioclase mation at the Institute of Electron Optics, Universi- ty of Oulu. The analytical procedure is described in Plagioclase analyses are arranged according to the host Muhongo and Tuisku (1996). A few hundred min- rock type and increasing content in Table eral analysis were performed with wide area coverage 2. In the metasedimentary rocks, the lowest anorth- of the LGB. The cations are calculated according to ite (<30%) and orthoclase (< the detection limit of number of anions in mineral formula. K2O) content is encountered in the biotite-silliman- ite gneisses of the NE-marginal zone and in the afore- mentioned kyanite gneiss. Plagioclase with

3). The range in XFe is from 0.34 to 0.56. The most component. iron-rich orthopyroxene was found in a vein in a of the Inari Terrane. The 6. Key metamorphic equilibria and Mg/Fe-ratio of orthopyroxene correlates with that in features garnet in the garnet-bearing enderbites and is always higher than in the associated garnet. The orthopy- Excluding the innermost cores of some garnet grains, roxenes are generally unzoned. Sometimes a very nar- biotite and sillimanite are abundant inclusions in gar- row rim was observed, having a lower Fe/Mg-ratio net in metapelites (Fig. 8a, c and d). This is true both than the inner parts of the grain. in the marginal zone of the LGB in the NE where bi- otite and sillimanite occur as a stable matrix assem- 5.4. Ilmenite blage, and in granulites proper, where either silliman- ite or biotite is absent in the matrix. These inclusions Representative microprobe analyses of ilmenite are may be used to demonstrate the prograde reaction given in Table 4. The calculated ferric iron content is history of the LGB. It is most probable that silliman- highest in some palaeosomes of the metapelite series ite was the fi rst aluminium silicate generated because rocks and in some garnet-free enderbites. The mole not a single grain of other polymorphs was found fraction of ilmenite component was usually almost within the garnet. The fi rst possible reaction to pro- one in most samples and even the lowest values were duce an aluminium silicate in the sillimanite fi eld is higher than 0.8. In one porphyritic orthoclase gran- ite ilmenite contains 15 % of the pyrophanite (Mn) (1) Staurolite + chlorite + + quartz = bi- component. otite + aluminium silicate (sillimanite) + H2O

5.5. Sillimanite and rutile This reaction is further supported by numerous sillimanite and biotite inclusion in garnet, suggest- Sillimanite was found to be almost pure aluminium ing that these phases were abundant before and dur- silicate. In one sample only the FeO content was ca. ing the growth of majority of the garnet. The major wt. 0.9 %. Rutile was also pure. Only a few samples part of garnet was then most probably produced by contained less than 0.5 wt% FeO or Cr2O3. reaction

5.6. Cordierite (2) Staurolite + muscovite + quartz = garnet + silli- manite + biotite +H2O Cordierites were analyzed only from some spinel- bearing metapelites (laanilites) in order to calculate This reaction produced the common marginal the reaction relationships of spinel-forming reactions. zone assemblage, which, in addition to garnet por-

XMg was relatively high, ca. 0.8. phyroblasts or poikiloblasts, exclusively contains abundant and relatively coarse-grained biotite and 5.7. Spinel sillimanite in the matrix. The fate of muscovite is un- clear but if any remained in reaction (2), it probably Spinel was analyzed, together with cordierite and gar- then disappeared during the succeeding migmatiza- net, to reveal its reaction relationship in metapelites. tion by the reaction The composition of spinel is degenerate with that of garnet and sillimanite. The hercynite content is ca. 66 (3) Muscovite + plagioclase + quartz = liquid + K- %. In addition to hercynite and Mg-spinel, the spi- feldspar + sillimanite Evolution of Migmatitic Granulite Complexes: Implications . . . 91 a) b)

c) d)

e) f)

Fig. 8. Photomicrographs of granulite samples and their reaction textures. All fi gures are 3 mm wide. a) Relic, abun- dant sillimanite and biotite inclusions in a garnet which lies in biotite free, sillimanite rich rock ma- trix. This is cut by orthoclase rich leucosome, indicating different stages of biotite dehydration reaction (4). b) Re- action textures related to biotite dehydration melting reaction (5): Relic, abundant sillimanite and biotite inclusion in a cordierite porphyroblast which lies in biotite free rock matrix with abundant orthoclase, sillimanite and gar- net porphyroblasts (not in Figure). Arrows indicate places where interstitial orthoclase replaces plagioclase. c) Tex- tures indicating both heating i.e. dehydration melting of biotite (reactions 4 and 5) and decompression reaction (7): Biotite and sillimanite inclusions in a garnet porphyroblast which is in turn replaced by a reaction rim composed of cordierite. Plagioclase of the matrix is replaced by a large, interstitial orthoclase grain in the upper right corner. d) Textures indicating both dehydration melting of biotite (4 and 5) and reaction (13): Biotite and sillimanite inclu- sions in a garnet porphyroblast which is in turn replaced by a reaction rim composed of cordierite. Prismatic silli- manite of the matrix is replaced by spinel (shown by arrows). e) and f) cooling stage, retrogressive reactions. e) re- verse of reaction (4) producing sillimanite-biotite-plagioclase-quartz sympectite in the expence of orthoclase and garnet and f) granitic leucosome where large andalusite grains have crystallized with simultaneous formation of pla- gioclase-quartz symplectite (myrmeckite). Some biotite is formed together with andalusite. 92 P. Tuisku, P. Mikkola and H. Huhma Evolution of Migmatitic Granulite Complexes: Implications . . . 93

Excluding some outcrops, which have at least two Which of the reactions (4) to (6) takes place de- leucosome phases, there is little evidence for reac- pends both on the pressure of metamorphism and tion (3) and the most extensive migmatization was rock composition. Reaction (4) predominates at produced by the following reactions (Fig. 8a, b and higher pressures and in rocks with higher Fe/Mg ra- c): tios (see the review in Thomson, 2001). In some out- crops of the NE marginal zone, the rock composition (4) Biotite + sillimanite + plagioclase + quartz = liquid varies so much, probably due to original bedding of + K-feldspar + garnet the metasediment, that all reactions have taken place in different parts of the rock producing either garnet, (5) Biotite + sillimanite + plagioclase + quartz = liquid garnet and cordierite or cordierite-bearing assemblag- + K-feldspar + garnet + cordierite es. Cordierite, however, is usually found only in the NE part and NE marginal zone of the LGB, which (6) Biotite + sillimanite + plagioclase + quartz = liquid means that a relatively high pressure prevailed during + K-feldspar + cordierite the dehydration melting. In cordierite-bearing migmatites, the garnet-bear- All metapelites in the LGB show evidence for ing assemblage is regularly replaced by cordierite and some of these reactions. In the NE marginal zone, additional orthoclase has grown. This is revealed the reactions did not progress to the end (Fig. 5) and by cordierite and orthoclase-bearing reaction rims both biotite and sillimanite are abundant in palaeo- around the garnet. Apparently this occurred due to somes. In the granulite belt proper, the palaeosomes decompression during the melting by a continuous always lack either biotite or sillimanite, indicating reaction (Fig. 8c): the completion of the reactions (Fig. 6) and relative- ly higher temperature during the peak of metamor- (7) Garnet + liquid1 = cordierite + orthoclase + liquid2 phism. The temperature difference and continuous nature of the melting reactions is also refl ected in The reactions discussed above are represented in the mineral compositions as the marginal zone me- Fig. 9, except that reaction (7) is omitted for clarity. tapelites have the highest almandine content in gar- Also divariant fi elds of reactions (4) to (6) are omit- net and the highest and lowest orthoclase con- ted, while only the onset of the reactions is shown. tent in plagioclase. The minimum temperature achieved in the NE mar-

Fig. 9. Results of thermobarometry and petrogenetic (grid) criteria to defi ne conditions of metamorphism of me- tagreywackes throughout the LGB. See text for detailed discussion on thermobarometry and each profi le presented in Figs. 9a-d. Sources of reaction curves (circled symbols) are following: (1) St+Chl = Bt+Als after Spear (1993); (2) St = Bt+Als+Grt after Pattison & Tracy (1991); (3) dehydration melting of muscovite after Pëto (1976); (4) beginning of dehydration melting of biotite to form L+Kfs+Grt after Le Breton and Thompson (1988); (5) dehydration melt- ing of biotite to form L+Kfs+Grt+Crd as wel as (10) Ms+Qtz = Kfs+Als+V after Pattison & Tracy (1991); (20) wet melting of pelitic rocks after Thompson (1982); (21) wet melting of Ms+Pl+Qtz after Tuttle & Bowen (1958); (P) al- uminium silicate stability after Pattison & Tracy (1991); dotted curves numbered by squared 4 to 8 indicate liquidus curves for granitic magma with equivalent H2O content after Holtz et al. (2001). Grey arrows indicate average PT- paths for the profi les and it should be noted, that e.g. in Nellimö-Kuttura profi le the NE part underwent relatively lower pressure path as the average was. Dots on the arrows indicate reactions, which probably took place during the evolution of the LGB, and the numbers besides them correspond to the reactions in the text. Grey quadrilat- erals give average maximum PT-conditions with error of about 1 kbar in P and grey circles give retrograde condi- tions of the profi les. Note that the pressure constraints for reaction points 1–4 in the fi gures are poor and pres- sure is only limited by the presence of sillimanite during reactions 2 and 4. 94 P. Tuisku, P. Mikkola and H. Huhma ginal zone, necessary for the beginning of dehydra- of granites and leucosomes took place below the wet tion melting of biotite, is ca. 750°C. All the reac- melting temperature of granitic rocks and beneath tions from (4) to (7) may be onset and will be pro- the pressure of the reaction curve: moted by decompression in a temperature range of 720–760 °C. (10) Muscovite + quartz = K-feldspar + aluminium The fi rst signs of cooling are given by granitic silicate + vapor migmatite leucosomes and some larger peraluminous crosscutting granites, which most probably were gen- Corresponding reactions are presented in Fig. 9. erated by melt segregation from pelitic migmatites. The pressure was below 3 or 2 kbar and temperature Garnets in these rocks are regularly more or less re- ca. 650°C, depending on which aluminium silicate placed by a mixture of biotite and sillimanite or bi- phase diagram is accepted. otite-sillimanite-quartz symplectite (Fig. 8e), which Some assemblages and reactions involving spi- are produced by the reverse of reaction (4). Textures nel in laanilites and less commonly migmatitic me- indicating reverse reactions (5) and (6) also occur tapelites need a special attention, although these do but are less common. This might suggest that cool- not have regional extent. The metapelite sample I5 ing took place with considerable decompression and from Kaarle Kustaa shaft consists of quartz, plagiocla- the PT path had a similar slope to that of the reaction se, garnet, biotite, rutile, and interstitial orthoclase curves of (5) and (6). Another indication of decom- probably representing melt extraction. Garnet con- pression of garnetiferous granite bodies is the replace- tains inclusions of biotite and it seems clear that melt- ment of rutile and garnet by ilmenite and sillimanite ing reaction (4) took place in the rock. Besides, there by the reaction: are narrow bands of garnet and sillimanite, not in contact with quartz. Both sillimanite and garnet have (8) Garnet + rutile = ilmenite + sillimanite + quartz nucleus of spinel and sporadic inclusions of quartz. Evidently, a reverse of the reaction The most important evidence of lowered pres- sure (during cooling) is given by generation of anda- (11) Garnet + sillimanite = spinel + quartz lusite, which is encountered in two contexts. First, it is found within pseudomorphs after cordierite by the took place in the bands. Because the equilibrium reaction: conditions were on the right side, the maximum tem- perature exceeded ca. 900–950°C at a pressure of at (9) Cordierite = andalusite + chlorite + quartz least 6.5 kbar, according to petrogenetic grids of Das- However, this is common only in the NE parts of gupta et al. (1995) and Sengupta et al. (1991, 1999; the LGB, where cordierite is present. Regionally more see also other references therein). The intermediate important is crystallization of andalusite in granitic pressure is also supported by the presence of rutile leucosomes and in some larger granitic pods. Anda- and lack of ilmenite and cordierite. lusite may occur as large up to several centim- Migmatitic laanilites near the NE marginal zone, eters long and most probably crystallized from a melt, near the sampling site E496 (Fig. 1) contain assem- because no replacement textures were found. This blages of 1) garnet-cordierite-quartz-orthopyroxene- kind of andalusite is most common in the NE part of plagioclase, with relics of sillimanite and spinel and the LGB, but is also found in the most SW part, for 2) garnet-cordierite-spinel-plagioclase, with relics of example in sample I1.1 from Kuttura, used for dating sillimanite and quartz. In both types, abundant silli- in Part II of this contribution (Fig. 8f). Because anda- manite and biotite inclusions in garnet suggest that lusite apparently crystallized together with K-feldspar an early melting reaction (4) or more probably (5) and quartz, the fi nal crystallization of these portions took place. The latter assemblage (Fig. 8d) may then Evolution of Migmatitic Granulite Complexes: Implications . . . 95

Table 5. Result of thermobarometry GAES + GAFS + Grt-Opx Profi le T °C used in P kbar P kbar Sample s GASP and s P kbar s T °C s (GASP) (GRISP) GRISP Nellimö - Kuttura D914 6.1 0.9 825 1391 6.6 (max)* 1.0 825 595 5.4 1.0 825 Törmäs 6.4 0.9 825 3C95 825 6.9 1.0 LentoC 7.3 1.0 825 Laanioja 6.3 1.0 825 H591 7.8 1.0 825 3593 7.3 1.0 825 693 8.2 1.1 856 77 2093 6.8 1.0 825 2293 8.2 1.0 825 2RJK 7.1 1.0 825 I2 825 7.0 1.0 B743 9.2 1.0 700 Kaamanen - Angeli E355 6.2 0.9 825 E524 5.9 0.9 825 E496 6.0 0.9 825 D904 5.3 0.9 825 A431 4.2 1.0 825 1291 4.9 0.9 825 15TS 6.1 0.9 825 21TS 5.7 0.9 825 24TS 6.6 1.0 825 Kaamanen - Karigasniemi 7C91 6.5 1.0 750 F E452 4.7 1.1 648 72 t E363 6.6 1.1 752 75 n E354 8.3 1.0 825 s E446 5.5 1.0 750 8TS 7.4 1.0 800 38TS 6.0 1.0 825 D896 7.0 1.0 825 46TS 6.5 0.9 825 48TS 7.2 1.0 825 49TS 7.3 1.0 825 51TS 7.6 1.0 825 54TS 825 6.8 0.9 57TS 7.6 1.0 825 D878 7.8 1.0 825 D885 8.0 1.0 825 - Karigasniemi 10478core 7.1 1.1 740 66 E591rim 4.6 1.1 584 69 E519core 5.9 1.1 692 73 E506 7.4 1.1 679 65 I8 6.8 1.0 825 E535 7.3 0.8 825 91TS 7.4 0.8 825 98ATS 6.7 0.9 825 67TS 7.4 1.1 976 93 10485rim 6.7 1.0 804 80 10485core 8.0 1.1 931 83 100CTS 5.2 1.1 734 85 100FTS 6.6 0.9 84BTS 8.1 1.1 942 87 86BTS 7.1 1.0 760 83 10489 8.6 1.1 788 72 88DTS 8.7 1.1 798 77 *Rutile absent, max P assuming rutile present 96 P. Tuisku, P. Mikkola and H. Huhma

10000 reaction scheme is supported, in addition to textur- 10485 84BTS 10489 al features, by the degenerate composition of garnet, 9000 88DTS 693

8000 E506 10478 spinel, and sillimanite.

7000 The enderbite-norite series lacks prominent pro- E363 67TS 6000 gressive reaction textures. Neither does orthopy- E519 5000 86BTS roxene or garnet have inclusions of preceding low-

4000 E452 100CTS er-grade minerals. This means, that no progressive Pressure bars Pressure 3000 metamorphic evolution may be constructed for these 2000 rocks. Contrary, for example orthopyroxene may 1000 contain abundant exolutions of clinopyroxene, which 0 0 200 400 600800 1000 1200 suggests cooling from a very high temperature. Ret- Temperature C rogressive textures are common as is partial replace- ment of orthopyroxene by biotite or biotite-quartz Fig. 10. Results of thermobarometry to defi ne condi- tions of metamorphism of garnet-bearing rocks of the symplectite. This indicates cooling by the reverse of norite-enderbite series of the LGB. Calculated error is the reaction shown by the eclipses. See text for detailed discussion. (16) Biotite + quartz = orthopyroxene + orthocla- se + fl uid have developed by the continuous reactions As orthoclase is rare in the enderbite series, this re- (12) Garnet + sillimanite + quartz = cordierite and, action may imply some metasomatic infl ow of potas- after consumption of quartz, sium (together with fl uid). Alternatively, potassium was derived from the orthoclase component of the (13) Garnet + sillimanite = cordierite + spinel ternary feldspar solid solution. In the garnet-bearing enderbites, garnet is often more or less replaced by or, if the temperature of reaction (11) was exceed- plagioclase and orthopyroxene by the reaction ed and sillimanite consumed, by the continuous re- action (17) Garnet + quartz = orthopyroxene + plagioclase

(14) Garnet + spinel +quartz = cordierite most probably having taken place during decompres- sion of the LGB. The former assemblage (garnet-cordierite-or- Some assemblages and reactions in the rocks from thopyroxene-quartz-plagioclase) may have developed the marginal zone, outside the granulite belt prop- by univariant reaction (11) followed by a reaction er, have important petrogenetic and tectonic conse- (14) and then, after consumption of spinel, by con- quences. A relatively high pressure-temperature-gra- tinuous reaction dient during the metamorphism of the SW margin- al zone is indicated by the assemblage kyanite-gar- (15) Garnet + quartz = cordierite + orthopyroxene net-biotite-muscovite-plagioclase-quartz. It indi- cates that the breakdown temperature of staurolite The development of both assemblages suggests a was exceeded but the dehydration melting temper- relatively high temperature of ca. 900°C and a rel- ature of muscovite was not reached in the kyanite atively low pressure below 6 kbar. All the continu- fi eld, i.e. pressure was at least 7 kbar and the tem- ous reactions (12) to (15) considered above are high- perature 650−700 °C. From the same zone, Tuisku ly pressure-dependent and favor decompression. The and Makkonen (1999) found metamorphosed ma- Evolution of Migmatitic Granulite Complexes: Implications . . . 97

fi c rocks with the assemblage olivine-orthopyroxene- throughout the LGB and thus allows the GASP (gar- clinopyroxene-pargasite-spinel giving metamorphic net-aluminium silicate-silica-plagioclase) barometer conditions of ca. 11−12 kbar and 670 °C. A relative- to be applied over a regional extent. Another useful ly high P/T ratio is also indicated by the common oc- barometer is GRIPS (garnet-rutile-ilmenite-plagi- currence of garnet-bearing diopside amphibolites in oclase-silica) because either rutile or ilmenite or both the SW marginal zone, while decompression is dem- usually occur in the palaeosomes of garnet-bearing onstrated by a development of plagioclase rim around metapelites. Even if ilmenite is absent, this barometer garnet by the reaction may be used to calculate the minimum possible pres- sure and in the absence of rutile, a maximum possible (18) Garnet + quartz = clinopyroxene + anorthite pressure is obtained, because both phases are almost pure when they occur in the LGB. The latter may also The plagioclase rims may be strongly elongated par- be applied to the garnet-bearing enderbites, which allel to the NNE dipping stretching lineation of the usually contain ilmenite, thus giving a maximum amphibolites which indicates that the decompression possible pressure. Also, it may be used to receive ref- took place prior to or during SSW-directed thrusting erence data from adjacent tonalites gneisses outside of the LGB. Garnet amphibolites are also found in the the LGB proper. The third useful set of thermo-ba- NE marginal zone, where orthopyroxene-plagioclase- rometers is found in the enderbite series where the symplectite was developed between clinopyroxene and garnet-orthopyroxene-plagioclase-quartz assemblage garnet during decompression by the reaction is relatively common and has a similarly wide regional extent to the metapelite assemblages. Both the GAFS (19) Garnet + clinopyroxene +quartz = orthopyrox- and GAES barometers (garnet-anorthite-ferrosilite/ ene + plagioclase -silica) and garnet-orthopyroxene Fe/Mg ex- change thermometers were used in this study. In summary, according to the mineral assemblages Calculations were made using numerous pub- and reaction textures described above, metapelites of lished calibrations of the aforementioned thermoba- the granulite belt proper show evidence for progres- rometers, but to maintain consistency in the calcula- sive heating in intermediate pressure conditions at tion and to allow readers to receive a picture of the least up to 750°C and prominent dehydration melt- regional variation in the metamorphic conditions in ing and migmatization. Locally, the temperature may the LGB, we concentrate here on reporting results have exceeded 900°C, while the marginal areas of the obtained by the Berman thermodynamic data set LGB underwent only partial dehydration melting. and TWEEQU software (Berman, 1988; 1991; and The progressive stage was followed by decompression June/92 update). The error calculation was made by to ca. 2−3 kbar and cooling. In contrast, enderbites INVQ method and software (Gordon, 1992; Gordon do not show evidence of progressive heating but de- et al., 1994; Ghent & Gordon, 2000). Both core and compression and cooling is obvious. Marginal zones rim composition of the minerals was used in calcula- of the LGB were metamorphosed at lower tempera- tions. Rims always gave a little lower T (~20–50°C) tures of ca. 650−700°C with a higher P/T ratio than and P (~0.2–0.5 kbar) than the cores. Summary of the granulite belt proper. Post-peak decompression results is given in Table 5 where samples are grouped and cooling is obvious also in the marginal zone. according to mapped cross-section of the LGB and arranged from E to W or from NE to SW within 7. Thermobarometry each of the profi les (Fig. 1) to illustrate across-the- belt variation in the metamorphic conditions. Re- The mineral assemblage garnet-plagioclase-silliman- sults from cores in metagreywackes are given in Fig- ite-quartz occurs in the palaeosomes of the metapelites ures 9a-d. Each of the Figures represents results of 98 P. Tuisku, P. Mikkola and H. Huhma one cross-section of the LGB to illustrate the along- this profi le. Comparison with the metamorphic con- strike variation in the metamorphic conditions. They ditions of enderbites is thus not possible here. The also show some important reference equilibria of me- lowermost pressure was calculated for sample A431, tapelitic rocks discussed above. The continuous con- which is a sample from the Lake Möyssäjärvi area, be- tours mark the results from GASP barometry and tween this and the previous profi le, which may repre- long dashed the GRISP barometry. The sample num- sent an even lower pressure part of the LGB. The cal- bers referring to the localities shown in Fig. 1 are giv- culated low pressures are supported by the fact that all en with each contour. Results calculated by combina- samples, except E355 which is from the higher-pres- tion of the GAES and GAFS barometers and garnet- sure part of the array, contain abundant cordierite. orthopyroxene thermometer are given in Figure 10. The third profi le begins in the area of the marginal Most of these results are from the fourth, Utsjoki–Ka- zone, east of Kaamanen, with more abundant biotite- rigasniemi profi le. sillimanite gneisses and diopside amphibolites mixed The easternmost profi le extends from Nellimö at with migmatitic metapelites. It extends 80 km NW the Russian border to ca. 110 km SW at Kuttura. across the LGB to Karigasniemi on the western side of The results are shown in Fig. 9a, including also some the belt. The results are shown in Fig. 9c. The GASP- scattered samples east of the profi le. The rocks are isopleths of samples E355 and E446 representing the migmatitic metapelites and therefore the isopleths are migmatitic biotite-sillimanite gneiss area east of Kaa- shown above the dehydration melting temperature of manen are drawn to cross-cut the dehydration-melt- biotite and sillimanite. The calculated pressure var- ing curve while the GASP-isopleths of the rest of the ies from ca. 5−6 kbar at the NE to ca. 8 kbar at the samples are drawn on the high-T side of it. The garnet- SW. Tectonic slicing of the LGB during thrusting and orthopyroxene thermobarometry of enderbite samples uplift after the peak of metamorphism most proba- E452 and E363 from the biotite-sillimanite gneiss area bly cause some exceptions from the systematic pres- support the lower average temperature east of Kaa- sure increase. Sample B743 from the SW marginal manen. The calculated pressures increase towards the zone is included for reference. The rock is garnet–ky- NW part of the profi le and near Karigasniemi they are anite gneiss with abundant biotite and muscovite and as high as in the Kuttura area of the fi st profi le. hence the isopleth is drawn between the reaction con- The fourth section represents the northernmost tours bordering the stability fi eld of the muscovite- part of the LGB. It begins at Utsjoki in the eastern plagioclase-biotite-kyanite-garnet-quartz assemblage margin of the belt and ends 30 km north of Karigas- either in hydrous or dry conditions. Sample 693 in niemi at the western margin of the belt. The GASP Figure 10 is a garnet-bearing enderbite from the SW- assemblages show completion of the dehydration re- part of the profi le and the result of calculation is in action (4) and calculated pressures are similar to those a good agreement with those obtained from the me- in the fi rst and third profi le (Fig. 9d). Garnet-bear- tapelites in this area. ing enderbites are abundant in this section, which The next section is from the central part of the suggests a relatively high pressure/temperature ratio. LGB. It extends from Kaamanen, situated to the The results of garnet-orthopyroxene thermobarom- north of Inarijärvi, 70 km west to Angeli at the Nor- etry confi rm this but also show considerable scatter wegian border. The results are presented in Fig. 9b. in the PT-space and P/T-ratio (Fig. 10). This can be Again, the dehydration melting temperature of bi- explained by the palaeotectonic position. For exam- otite constrains the low temperature limit of isop- ple, samples E506, 10489 and 88TS, which have the leths. The calculated pressures are considerably low- highest P/T ratio, similar to that of sample B743 from er than in the Nellimö-Kuttura profi le. The lower the fi rst section, are situated next to the basement and pressure (about 4−6 kbar) is also supported by the probably represent the lowermost part of the belt dur- fact that no garnet-bearing enderbites were found in ing granulite metamorphism. The same is seen in the Evolution of Migmatitic Granulite Complexes: Implications . . . 99 series of samples 100CTS, 100FTS, 84BTS, 86BTS, completion of reaction (4). Also results of thermo 88TS and 10489, which show quasi-systematic in- barometry suggest that this temperature was exceed- crease in the P/T ratio and absolute pressure (Table5). ed in a regional scale. The series most probably represents a single tectonic Mineral assemblages, as well as thermobarometry unit from roof to fl oor. The pressure difference of 3.5 indicate, that there was considerable regional as well kbar corresponds to a difference of more than 10 km as local variation in pressure from about 5 to 9 kbar in the crustal load during metamorphism, which is during the peak of metamorphism, which is shown also the thickness of the unit measured in the fi eld. in Figures 9a-d. This may be explained by variation The difference of the present data from that of the in depth. Considering the present 35 km maximum earlier works (Hörmann et al., 1980; Raith & Raase, thickness of the LGB the lithostatic pressure differ- 1986; Barbey & Raith, 1990; Perchuck et al., 2000) ence of 12 kbar could have been possible. This is, is that there is greater aerial coverage, four profi les are however, not observed and it may be concluded that studied instead of one and the profi les are more con- the present day structure of the LGB is a result of tec- tinuous. According to our data it seems that there is tonic duplexing after the peak of metamorphism, as more variation in the PT regime and the across-belt proposed already by Korja et al. (1996). Observed change of metamorphic conditions is not as system- pressure variation indicates that maximum difference atic as earlier interpreted. It also appears that along- of lithostatic load between the lower and upper parts stike variation in metamorphic conditions is consid- of the LGB was about 12 km (compare with Fig. 4 in erable. Our PT data support the suggestion of Korja Korja & al., 1996). In the present day cross-sections, et al. (1996) that the LGB consists of several structur- sudden upward jumps in calculated pressures occur, al units, which were juxtaposed after the peak of the as for example between samples 100CTS and 10489 regional metamorphism. in Utsjoki-Karigasniemi profi le. This feature togeth- er with a shear zone observed between these sampling 8. Discussion sites is in accordance with the post-peak duplexing and thus strongly supports the model presented in Korja et al. (1996; Fig. 5). 8.1. Evolution of the metasedimentary sequence The retrograde part of the PT evolution of the of the LGB LGB metapelite is governed by decompression and The progressive heating path of the metasedimenta- cooling, which is shown by the numerous reaction ry rocks of the Lapland granulite belt can be deter- textures and low-pressure assemblages in the leuco- mined from the petrological data (Fig. 9.). Abundant somes, especially those where orthoclase occurs in biotite and sillimanite inclusions in garnet through- equilibrium with andalusite. PT paths are given in out the belt indicate that biotite-sillimanite-gar- Figures 9a-d for each sub-area of the LGB. Cooling net gneiss was a prevailing rock type during heating from ca. 850°C to less than 650°C and decompres- from reaction (2) to (4). Bt-Sil-Grt gneiss- sion from 5–9 to as low as 2 kbar is recorded. It is ap- es were generated by reaction (2) at 620−650°C and parent, that leucosomes were not completely - 3.6−6.5 kbar from staurolite-bearing lized until the end of decompression. On the oth- or gneisses. Bt-Sil-Grt gneisses still exist in some er hand, pressure differences (i.e. load) between var- lower grade areas in the marginal zones, especially ious parts of the LGB was considerably reduced at in NE part of the LGB, often mixed with migmatit- the end of decompression. This suggests that the de- ic granulite, indicating that maximum tempera- compression and duplexing took place simultaneous- ture was here near the reaction isograd (4) at about ly by crustal scale thrusting as proposed by Korja et 750°C. However these conditions are exceeded in al. (1996). the major part of the LGB, as shown by regional 100 P. Tuisku, P. Mikkola and H. Huhma

8.2. Evolution of the norite-enderbite series decompression textures. Because the thermobaro- metry gives similar PT-conditions both for meta- Already Hörman et al. (1980), and later Barbey and sediments and metaigneous rocks, and the miner- Raith (1990) proposed that the LGB igneous series als are homogeneous in the metaigneous rocks, the evolved in a convergent volcanic arc setting. The norite-enderbite series evidently crystallized within primitive mantle-normalized trace element spec- the same conditions where metapelites suffered the trum of the norite-enderbite series has similarities maximum temperature of metamorphism. This is to recent subduction-related mafi c and intermedi- also supported by fi eld data, as igneous rocks form ate arc volcanics (e.g. Shinjo et al., 2000; Churik- fl at, sill-like and deformed bodies sandwiched be- ova et al., 2001; Jolly et al., 2001; Hochstaedter et tween metapelite layers and often show assimilated al, 2001). In particular the peaks of Rb, Ba, K, and contacts and degree of mixing with the metapelites. Sr, observed in the LGB, are considered to indicate Chilled margins were not observed. Evidently, the a subduction-related extra component in the spec- igneous series subsequently shared the evolution trum (Pearce, 1983) while the relative even deple- with the metapelites, cooling and decompressing to- tion of Nb, Ta and Ti might indicate removal of gether with them. these components to the mantle due to a refracto- ry eclogitic slab (MacDonough, 1991). The latter is 8.3. SW Marginal zone considered one of the strongest pieces of evidence for subduction-related magmatism even in palaeoig- Metamorphic pressure and temperature may also neous rocks (Leybourne et al., 1999). Also, the neg- be determined for the marginal zones, especially the ative anomalies at Zr and Th observed in the LGB southwestern footwall zone. Tuisku and Makkonen are common in subduction environment magma- (1998) calculated 11–12 kbar and 670°C for the tism (Pearce, 1983; Jolly et al., 2001). It is thus ev- Vaulo mafi c-ultramafi c complex and 8.2–9.8 kbar ident that a volcanic arc existed between the Kare- and 650–720°C is calculated for Korvatunturi area lian province, now in the south and southwest, and in this work (sample B743 in Fig. 9a). These data is the Kola Province including the Inari Terrane, now in accordance with the southwest-directed thrusting in the northeast of the LGB. model of the LGB presented in many papers (Mark- If we take into an account the great volume of the er, 1988; Barbey and Raith, 1990; Korja et al., 1996 igneous rocks in the LGB, one of the major reasons and Tuisku and Makkonen, 1998). of high-grade metamorphism of the metapelites in the LGB could have been heat liberated from cooling 8.4. Evolution of migmatitic granulite and crystallization of arc magmas. complexes Thermobarometry, mineral chemistry, textural and fi eld data suggest that the pressure was high dur- Lapland granulite belt is signifi cant migmatitic gran- ing the intrusion of norites and enderbites into the ulite belt which has common features with many oth- metasedimentary rocks. On the other hand, it seems er granulite terrains, such as a high temperature/pres- that the metapelites were remarkably heated during sure ratio and a clockwise PT path. Our study has the burial because sillimanite, not kyanite, is the pre- shown that the LGB has much greater amount of in- vailing prograde aluminium silicate (Fig. 9). trusive rocks than previously thought. Arc related As described in the section of the key reactions, magmas apparently were an important heat source all visible reaction textures in enderbites and norites for metamorphism of the LGB. It is possible, that col- were generated due to decompression and cool- lision related magmatism could be predominant heat ing. In contrast, metapelites show abundant pro- source also in other granulite belts rich in metasedi- gressive reaction textures in addition to cooling and mentary rock, regardless of the exposure of the igne- Evolution of Migmatitic Granulite Complexes: Implications . . . 101

SW m arginal zone, Vaulo 2 0 S u b d u c t e d 3 4 s l a b ? s 1 k c o r 0 c i t i l e p f o g n i t l e m t e W 1 0 Low er, high-P parts of the LG B SW m arginal zone, Korva- t u n t u r i I n t r u s i o n , 8 co o l i n g a n d crystallization P om f a g m a s k b a r

6 6

M igm atization

heatingo f t hof e Lm G etasedim B ents a n d U pper, low -P B u r i a l parts of the LG B

4 NE m a r g i n a l z o n e Trusting, erosion, cooling and decom pression of the com plete LG B sequences 2 (m etasedim entary and igneous series) F i ncrystallization a l o f 1 0 leucosomexhum es, ation to upper crustal lew el com pleted

8 0 0 1 0 0 0 5 0 0 6 0 0 7 0 0To C 9 0 0

Fig. 11. Summary of the PT evolution of the LGB, including evolution of different parts of the LGB proper as well as the marginal zones. The evolution of the supracrustal rocks of the LGB as well as the evolution of marginal zone assemblages are given in continuos grey arrows while the evolution of norite-enderbite series is given by dashed arrows. Most important petrogenetic and tectonic stages are indicated by text. 102 P. Tuisku, P. Mikkola and H. Huhma ous rocks. The homogeneous composition of miner- lization of the leucosomes of migmatitic metapelites als in rock samples of the LGB evidently is a result of took fi nally place at ~ 650°C and 2−3 kbar, at the end relatively long residence in high temperature condi- of thrusting and exhumation stage of the LGB. Our tions. Cooling rate of the migmatites was probably data have also important constraints on the evolution slow because of the heat of crystallization and cool- of other decompressing granulite belts. Crystalliza- ing of the intrusions was transported to the metased- tion of melts and cooling of the rocks unquestionably imentary units in the beginning of the uplift. Even takes time; especially if mantle derived melts are im- during the uplift the cooling was somewhat delayed portant heat sources of the metamorphism but also which is shown by well developed decompression tex- the heat of crystallization of the leucosomes greatly tures, then followed by cooling related retrograde tex- affects the late evolution. tures, and, a very high T/P-ratio in the end of leu- cosome crystallization. The temperature still was ca. Acknowledgements 650°C in this stage but the pressure only ca. 2 kbar, We are grateful to Eero Hanski for suggestions that corresponding to a lithostatic load of only 6–7 km. greatly improved the fi rst version of the manuscript. Evidently; the heat of crystallization of abundant leu- Financial support by Finnish Cultural Foundation (A. cosomes greatly affected to the late stage evolution of E. Nordenskiöld fund) is greatly acknowledged. The the LGB, and this surely is the case in other granulite journal reviewers Dr. Stefan Bergman and Dr. Pentti belts with high amount of migmatitic rocks (Brown, Hölttä are greatly acknowledged especially for propos- als that made the illustrations and metamorphic reac- 2002). tion sequence more understandable.

9. Conclusion References The fi rst published, complete PT paths of the La- Barbey, P. & Raith, M., 1990. The granulite belt of Lap- pland granulite belt are presented in Fig 9. Moreo- land. In: Vielzeuf, D. & Vidal, Ph. (eds.) Granulites and crustal evolution. Kluwer Academic Publishers, Dor- ver, this is done in regional scale covering most of the trecht, p. 111−132. LGB and also some marginal areas. The results are Barbey, P., Bernard-Griffi ths, J. & Convert, J., 1986. The schematically summarized in Fig. 11 together with Lapland charnockitic complex: REE geochemistry and petrogenesis. Lithos 19, 95−111. some important petrogenetic data and tectonic in- Berman, R.G. 1988. Internally-consistent thermodynamic terpretation. data for stoichiometric minerals in the system Na2O - The metapelites were heated in sillimanite fi eld at K2O - CaO - MgO - FeO - Fe2O3 - Al2O3 - SiO2 - TiO2 - H O - CO . Journal Petrology 29, 445−522. about 5±1 kbar pressure. Mineral assemblages and 2 2 Berman, R.G. 1991. Thermobarometry using multiequilib- thermo barometry determine the peak metamorphic rium calculations: a new technique with petrologic appli- conditions, viz. ~ 850°C and 5-9 kbar in this migm- cations. Canadian Mineralogist 29, 833−855. atite forming stage Norite-enderbite series, arc type Bernard-Griffi ths, J., Peucat, J.J., Postaire, B., Vidal, P., Con- vert, J. & Moreau, B., 1984. Isotopic data (U-Pb,Rb-Sr, magmas were intruded in the buried metapelite se- Pb-Pb and Sm-Nd) of the mafi c granulites from Finnish quence and their crystallization and cooling proba- Lapland. Precambrian Research 23, 325−348. bly was an important heat source for the metamor- Braathen, A. & Davidsen, B., 2000. Structure and stratigra- phy of the Palaeoproterozoic Greenstone Belt, phism of the metasediments. After the crystalliza- North Norway – regional implications. Norsk Geologisk tion of magmas, the metamorphic maximum and Tidsskrift 80, 33−50. partial melting stage of metasediments, both rock se- Bridgwater, D., Autrheim, H., Hansen, B.T., Mengel, ries apparently shared a common cooling and decom- F., Pedersen, S. & Winter, J., 1990. The Proterozoic Nagssugtoqidian mobile belt of southeast Greenland: A pression path. This was caused by southwest direct- link between the eastern Canadian and Baltic shields. ed thrusting of the complete rock sequence and rap- Geoscience Canada 17, 305−310. id erosion of the uplifted thrust sheets. The crystal- Brown, M., 2002. Retrograde processes in migmatites and Evolution of Migmatitic Granulite Complexes: Implications . . . 103

granulites revisited. Jounal of Metamorphic Geology 20, A.R. (eds.) The Geology of North America-An Over- 25–40. view. Geological Society of America, Boulder, Colora- Churikova, T., Dorendorf, F. & Wörner, G., 2001. Source do, p. 447−512. of fl uids in the mantle wedge below Kamchatka, evidence Hochstaedter, A.J., Gill, J., Peters, P., Broughton, P., Hold- from across-arc geochemical variation. Journal of Petrol- en, P. & Taylor, B., 2001. Across-arc geochemical trends ogy 42, 1567−1593. in the Izu-Bonin arc: Contributions from the subduct- Daly, J.S. Balagansky, V.V., Timmerman, M.J. Whitehouse, ing slab. Geochemistry, Geophysics, Geosystems 2, Pa- M.J., de Jong, K., Guise, P, Bogdanova, S., Gorbatschev, per number 2000GC000105 R & Bridgwater, D., 2001. Ion microprobe U-Pb zircon Holtz, F., Johannes, W. Tamic, N. & Behrens, H., 2001. geochronology and isotopic evidence for a trans-crustal Maximum and minimum water contents of granitic suture in the Lapland–Kola Orogen, northern Fennoscan- melts generated in the crust: a reevaluation and impli- dian Shield. Precambrian Research 105, 289−314 cations. Lithos 56, 1−14. Dasgupta, S., Sengupta, P., Ehl, J., Raith, M. & Bardhan, Hörmann, P.K., Raith, M., Raase, P., Ackermand, D. & S., 1995. Reaction textures in a suite of spinel granu- Seifert, F., 1980. The granulite complex of Finnish Lap- lites from the Eastern Ghats Belt, India: evidence for land: petrology and metamorphic condidions in the Iva- polymetamorphism, a partial in the lojoki-Inarijärvi area. Geological Survey of Finland, Bul-

systemKFMASH and the role of ZnO and Fe2O3. Jour- letin 308, 100 p. nal of Petrology 36, 435−461. Huhma, H., 1986. Sm-Nd, U-Pb and Pb-Pb isotopic evi- Eskola, P., 1952. On the granulites of Lapland. American dence for the origin of the Early Proterozoic Svecokare- Journal of Science, Bowen volume 1, 133−171. lian crust in Finland. Geological Survey of Finland, Bul- Essene, E.J., 1989. The current status of thermobarometry letin 337, 48 p. in metamorphic rocks. In Daly, J.S., Cliff, R.A. & Yard- Huhma, H. & Meriläinen, K.,1991. Provenance of para- ley, B.W.D. (eds.) Evolution of metamorphic belts. Geo- gneisses from the Lapland granulite belt. In Tuisku, P. logical Society Special Publication 43, 1−44. & Laajoki, K. (eds.) Metamorphism, deformation and Gaál, G. & Gorbatschev, R., 1987. An outline of the Pre- structure of the crust, abstracts (IGCP 275). Res Terrae, cambrian evolution of the Baltic Shield. Precambrian Re- Series A, No.5, p. 26. search 35, 15−52. Jolly, W.T., Lidiak, E.G., Dickin, A.P. & Wu, T.-W., 2001. Ghent, E.D. & Gordon, T.M., 2000. Application of INVQ Secular geochemistry of Central Puerto Rican island arc to the geothermobarometry of metamorphic rocks near a lavas: Constraints on Mesozoic tectonism in the Eastern kyanite-sillimanite isograd, Creek, British Colum- Greater Antilles. Journal of Petrology 42, 2197−2214. bia. American Mineralogist, 85, 9−13. Kesola, R., 1995. Näätämö. Explanation to the Geological Gordon, T.M., 1992. Generalized thermobarometry; Solu- map of Finland 1:100 000, pre-Quarternary rocks, sheets tion of the inverse chemical equilibrium problem using 3934+4912+4914. Geological Survey of Finland, 88 p. data for individual species. Geochimica et Cosmochim- (in Finnish with English summary) ica Acta, 56, 1793−1800. Korja, T., Tuisku, P., Pernu, T. & Karhu, J., 1996. Field, Gordon, T.M., Aranovich, L.Ya. & Fed'kin, V.V., 1994. Ex- petrophysical and carbon isotope studies on the Lapland ploratory data analysis in thermobarometry: an example Granulite Belt: implications for deep . from the Kisseynew Sedimentary Gneiss Belt, Manitoba, Terra Nova 8, 48−58. Canada. American Mineralogist, 79, 973−982. Korsman, K., Koistinen, T., Kohonen, J., Wennerström, Hackman, V., 1905. Die chemische Beschaffenheit von M., Ekdahl, E., Honkamo, M. Idman, H. & Pekkala, Eruptivgesteinen und der Halbinsel Kola. Bul- Y. (eds.), 1997. Suomen kallioperäkartta/Bedrock map letin de la Commission Géologique de Finlande 15, 143 of Finland, 1:1 000 000. Geological Survey of Finland, p. Espoo. Hall, J., Wardle, R.J., Gower, C.F., Kerr, A., Cofl in, K., Kröner, A., Puustinen, K. & Hickman, M., 1981. Goechro- Keen, C.E. & Carrol, P., 1995. Proterozoic orogens of the nology of an Archaean tonalitic gneiss dome in northern northeastern Canadian Shield: new information from the Finland andits relation with an unusual overlying volca- Lithoprobe ESCOOT crustal refl ection seismic survey. nic conglomerate and komatiitic greenstone. Contribu- Canadian Journal of Earth Sciences 32, 1119−1131. tions to Mineralogy and Petrology 76, 33−41. Harley, S.L., 1989. The origin of granulite: a metamorphic Le Breton, N. & Thompson, A.B., 1988. Fluid-absent (de- perspective. Geological Magazine 126, 215−147. hydration) melting of biotite in metapelites in the early Hoffman, P.F., 1988. United plates of America, the birth of stages of crustal anatexis. Contributions to Mineralogy a craton: Early Proterozoic assembly and growth of Lau- and Petrology 99, 226−237. rentia. Annual Rewievs of Earth and Planetary Scienc- Leibinger, S., 1996. The North-Eastern Marginal Zone es 16, 543−603. of the Lapland Granulite Belt at Kaamanen, Northern Hoffman, P.F., 1989. Precambrian geology and tectonic Finland: Lithology and Tectono-Metamorphic Develop- history of North America. In: Bally, A.W. & Palmer, ment. Unpublished Graduate Thesis, Johann-Wlfgang- 104 P. Tuisku, P. Mikkola and H. Huhma

Goethe-Universität, Frankfurt, 95 p. H2O to 15 kbar total pressure. Programs in Experimen- Lehtonen, M.. Manninen, T., Rastas, P. & Räsänen, J., tal Petrology, Natural Environment Research Council 3rd 1992. On the Early Proterozoic metavolcanic rocks in report, 41−45. Finnish central Lapland. In: Mitrofanov, F. (ed.) Cor- Raith, M. & Raase, P., 1986. High grade metamorphism relation of Precambrian Formations of the Kola-Karelia in the granulite belt of Finnish Lapland. In: Dawson, Region and Finland. Russian Academy of Science, Apa- J.B. et al. (eds.) The Nature of the Lower Continental tity, p. 65−85. Crust. Geological Society of London, Special Publica- Leybourne, M.I., Van Wagoner, N. &Ayres, L.D., 1999. tion 24, 283−295. Partial melting of refractory subducted slab in Palaeo- Sengupta, P., Karmakar, S., Dasgupta, S. & Fukuoka, M., proterozoic island arc: Implications for global chemical 1991. Petrology of spinel granulites from Araku, East- cycles. Geology 27, 731−734. ern Ghats, India, and a petrogenetic grid for sapphirine- Marker, M., 1988. Early Proterozoic thrusting of the Lap- free rocks in the system FMASH. Journal of Metamor- land Granulite Belt and its geotectonic evolution, north- phic Geology 9, 451−459. ern Baltic Shield. Geologiska Föreninge i Stockholm Sengupta, P., Sen, J., Dasgupta, S., Raith, M., Bhui, U.K. Förhandlingarna 110, 405−410. & Ehl, J., 1999. Ultra-high temperature metamorphism McDonough, W.F., 1991. Partial melting of subducted oce- of metapelitic granulites from Kondapalle, Eastern Ghats anic crust and isolation of its residual eclogitic litholo- Belt: implications for the Indo-Antarctic correlation. gy. Royal Society of London Philosophical Transactions, Journal of Petrology 40, 1065−1087. Ser. A 335, 407−418. Shinjo, R., Woodhead, J.D. & Hergt, J.M., 2000. Geo- Meriläinen, K., 1965. Geological Map of Finland 1:400 chemical variation within the northern Ryukyu Arc: 000, Sheet C8-9. Geological Survey of Finland. magma source composition and geodynamic implica- Meriläinen, K., 1976. The granulite complex and adjacent tions. Contributions to Mineralogy and Petrology 137, rocks in Lapland, northern Finland. Geological Survey 323−341. of Finland, Bulletin 281, 129 p. Simonen, A., 1980. Pre-Quarternary Rocks of Finland 1: Mikkola, E., 1937. Geological Map of Finland 1:400 000, 1000 000. Geological Survey of Finland. Sheet C7. Geological Survey of Finland. Sorjonen-Ward, P., Claoué-Long J. & Huhma H., 1994. Mikkola, P., 1998. The pressure-temperature conditions of SHRIMP isotope studies of granulite and their metamorphism of the Lapland granulite belt and their relevance to early Proterozoic tectonics in northern Fen- tectonic consequences (in Finnish). M.Sc Thesis, Uni- noscandia. U.S.G.S. Circular 1107, p. 299. versity of Oulu, Finland, 79 p. Spear, F.S., 1993. Metamorphic Phase Equilibria and Pres- Muhongo, S. & Tuisku, P., 1996. Pan-African high pressure sure-Temperature-Time Paths. Washington D.C.: Min- isobaric cooling : evidence from mineralogy and thermo- eralogical Society of America Monograph Series. barometry of the granulite-facies rocks from the Ulug- Thompson, A.B., 1982. Dehydration melting of pelitic

uru Mountain Ranges, eastern Tanzania. Journal of Af- rocks and the generation of H2O-undrsaturated granitic rican Earth Sciences 23, 443−463. liquids. American Journal of Science 276, 1567−1595. Newton, R.C., 1987. Petrologic aspects of Precambrian gran- Thomson, J.A., 2001. A counterclockwise P-T path for ana- ulite facies terrains bearing on their origins. In: Kröner, tectic , south-central Massachusetts. Contributions A. (ed.) Proterozoic Lithospheric Evolution, Geodynamic to Mineralogy and Petrology 141, 623−641. Series Vol. 17, AGU, Washington, p. 11−26. Tuisku, P. & Huhma, H., 1998. from the margin- Pattison, D.R.M. & Tracy, R.J., 1991. Phase equilibria and al zone of the Lapland granulite belt: evidence from the thermobarometry of metapelites. In: Kerrick, D.M. (ed.) 1.90−1.88 Ga subduction zone. Geological Survey of Contact Metamorphism. Mineralogical Society of Amer- Finland, Special Paper 26, p. 61. ica, Reviews in Mineralogy 26, 105−206. Tuisku, P. & Huhma, H., 1999. SIMS U-Pb dating of zir- Pearce, J. A., 1983. Role of the sub-continental lithosphere cons from migmatite khondalites and enderbite from in magma genesis at active continental margins. In: Lapland granulite belt, Finland. Terra Abstracts 11, Hawkesworth, C.J. (ed.) Continental basalts and man- 710−711. tle xenoliths. Shiva, United Kingdom, 230−249. Tuisku, P. & Makkonen, H.V., 1999. Spinel-bearing sym- Perchuk, L.L., Gerya, T.V., van Reenen, D.D., Krotov, plectites in Palaeoproterozoic ultramafi c rocks from two A.V., Safonov, O.G., Smit, C.A. & Shur, M.Yu., 2000. different geological settings in Finland: thermobaro- Comparative petrology and metamorphic evolution of metric and tectonic implications. Geologiska Förening i the Limpopo (South Africa) and Lapland (Fennoscan- Stockholm Förhandlingarna 121, 293−300. dia) high grade terrains. Mineralogy and Petrology, 69, Tuttle, O.F. & Bowen, N.L., 1958. Origin of granite in 69−107. the light of experimental studies in the system NaAl-

Pëto, P., 1976. An experimental investication of melting re- Si3O8-KalSi3O8-SiO2-H2O. Geological Society of Amer- actions involving muscovite and paragonite in the silica- ica Memoir 74, 54−63.

saturated portion of the system K2O-Na2O-Al2O3-SiO2- Vaasjoki, M., (ed.), 2001. Radiometric age determinations Evolution of Migmatitic Granulite Complexes: Implications . . . 105 from Finnish Lapland and their bearing on the timing of Precambrian volcano-sedimentary sequences. Geological Survey of Finland, Special Paper 33, 279 p.