Andersson1992.pdf

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Charnockites, pyroxene granulites, and garnet-cordierite gneisses at a boundary between Early Svecofennian rocks and Småland-Värmland granitoids, Karlskoga, southern Sweden Ulf B. Andersson , Lena Larsson & Anders Wikström Published online: 06 Jan 2010.

To cite this article: Ulf B. Andersson , Lena Larsson & Anders Wikström (1992) Charnockites, pyroxene granulites, and garnet-cordierite gneisses at a boundary between Early Svecofennian rocks and Småland-Värmland granitoids, Karlskoga, southern Sweden, Geologiska Föreningen i Stockholm Förhandlingar, 114:1, 1-15, DOI: 10.1080/11035899209453457 To link to this article: http://dx.doi.org/10.1080/11035899209453457

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Charnockites, pyroxene granulites, and garnet-cordierite gneisses at a boundary between Early Svecofennian rocks and Smiland-Varmland granitoids, Karlskoga, southern Sweden

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ULF B. ANDERSSON, LENA LARSSON and ANDERS WlKSTR~hl

Andersson, U.B.. Larsson, L. & Wikstrom, A., 199203 10: Charnockites, pyroxenegranulites, and garnet-cordieritegneisses at a boundary between Early Svecofennianrocks and SmAlandVarmland granitoids, Karlskoga, southern Sweden. Geologiska Foreningens iSfockholtnForhandlingar. Vol. 114, Pt. 1. pp. 1-15. Stockholm. ISSN 0016-786X. A lobe of Early Svecofennianhigh-grade.metamorphicrocks surrounded and intruded by rocks of the Smiland-Vilrmland batholith east of Karlskoga, central southern Sweden, has been studied. Application of geothcrmobarometry reveals that these rocks have suffered granulite facies metamorphism at conditions constrained to 670-770°C, 4.0-4.5 kbar and aHp 0.1-0.3. The metamorphism has transformed biotite granite into charnockite. intermediate volcanite into pyroxene granulite, and lower grade presumably semipelitic gneiss into garnetcordierite gneiss. Extensive partial melting accompanied the metamorphism in the garnetcordierite gneisses and granulites. but not in the charnockites. The metamorphism is attributed to a local contact metamorphic peak, associated with the emplacement of the Smiland-VLrmland granitoids and related mafic plutonics, in the peneconternporaneous,amphibolite facies, regional “serorogenic Svecofennian” episode. 0 Charnockife.high-grade (garnef-cordierife) gneks, granulifefacies mefamorphism,rnigmafizafion.geofhermobaromefry.Smdland- Vdrmland granifoid, Svecofennian. UljB. AnderssonandLena Larsson. InsfifufeofGeology, Depl. ofhfineralogyandPefrology. Uppsala Universify,Box555, S-75122 Uppsala. Sweden. Anders Wikrfr&n.Geological Survey of Sweden (SGU), Box 670, S-75128 Uppsala. Sweden. Manuscrip1 received 22 December 1958. revised manuscripf received 12 September 1991. Revised and accepfed28 November 1991.

Granulite facies metamorphism and the formation of charnockites are intimately connected, as shown by the worldwide association of pyroxene granulites and other high-grade rocks together with charnockites and anatectic high alkali granites. Such a n association is also present in the occurrence here described. A granulite facies dehydration metamorphism of granitic to granodioritic plutonics is the presently most accepted hypothesis for charnockite formation (e.g. Ashworth 1985, Gopalakrishna et al. 1986), although there is still a controversy regarding theextent and importance of associated melting (e.g. Ashworth 1985, Clemens & Vielzeuf 1987, Frost &Frost 1987, Wickham 1988). Some workers emphasize a formation totally in the solid state by flushing of the rocks with C02-rich vapours, causing dehydration, as evidenced mainly by COZ-rich fluid inclusions (e.g. Newton et al. 1980, Hansen et al. 1984, Newton 1989). This process has been advocated especially for the south Indian granulite terrains (e.g. Gopalakrishnaet al. 1986. Jianget al. 1988), whereas other terrains like the Adirondacks of New York lack evidence for pervasive C 0 2 IGFF 111992

flooding (e.g. Jiang et al. 1988, Edwards & Essene 1988, Lamb&Valley 1988).TheoriginoftheCO*. mantle or crustal, is however not yet confirmed, although a mantle origin has recently gained increasing support for the south Indian terrains (Stahle et al. 1987, Jiang et al. 1988). Others point t o the possible generation of COzrich fluid inclusions by the preferential partitioning of H 2 0 into a melt phase during migmatization, thus depleting the fluid in HzO (c.g. Touret & Dietworst 1983, Powell 1983, Lamb & Valley 1984, Ashworth 1985, Touret &Olsen 1985). This discussion applies to all granulites, not only charnockites. Still others maintain the existence of charnockitic magmas as the explanation for the large massive charnockites, such as those present in south India (e.g. Wickham 1988). It has also been suggested that they represent residues after partial melting and removal of alkali granitic melts t o higher crustal levels (e.g. Fyfe 1973, Clernens & Vielzeuf 1987, Hubbard 1988,1989). Other models integrate COrstreaming with melts moving through the crust (Frost & Frost 1987). Most granulite terrains have yielded P-T condi-

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2 UlJ0. Anderssori el at.

tions in the order of 8 kbar and 750°C (e.g. Gopalakrishna et al. 1986, Raase et al. 1986, Edwards & Esscne 1988. Sandiford et al. 1988, Lamb & Valley 1958, Perrault & Martignole 1988). There are, however, also areas exhibiting more shallow, lowP high-T, dehydration metamorphism of 2-6 kbar and more than 700°C (e.g. Berg & Docka 1983, Korsman et al. 1984, Frost & Frost 1987, Young et al. 1989, Harrison et al. 1989). As far as the present authors know, the occurrences of charnockites and granulite facies rocks in Sweden have until now only been recorded in the SW Swedish Gneiss Region (Quensel1951, Larsson 1966, 1968, Hubbard 1975, Wikman & Bergstroni 1987. Johansson et al. 1991). Recently, P-T conditions of mafic granulites in this region have been calculated to around 700-770°C and 8-10 kbar, and dated by Sm/Nd mineral isochrons to late Sveconorwegian (c. 900 Ma, Lindh et al. 1990, Johansson et al. 1991). Rocks in this area have been attributed to regional metarrnorphism with local granulitic peak dehydrations (Quensel 195 I , Larsson 1966, 1968) or such a process followed by melting, due to heat influx supplied by mafic intrusions, accompanied by the accumulation of charnockitic restite after removal of granitic melt (Hubbard 1988, 1989). In the Svecofennian terrain of southern Finland several distinct granulite areas have been described (e.g. Korsman et al. 1984, HolttS 1986, Schreurs & Westra 1985, 1986). The P-T calculations from these areas have suggested low-P high-T conditions of 3-6 kbar and 700--825°C. possibly resulting from thermal domes (Schreurs & Westra 1986). The rocks investigated here seem to be related to the batholithic intrusion of the SmBIand-V&mland granitoids and also yield low-P high-T conditions, indicating a formation by contact metamorphism. In the Ketilidian of southern Greenland a situation very similar in age and setting has also produced granulite facies rocks (Bridgwater et al. 1974) with P-T conditions of 7OO-8OO"C and 3-4 kbar (Harrison et al. 1989). This paper provides a first description of the rocks present, attempts a tentative outline of the PT conditions, and proposes a possible geological history. The work largely constitutes parts of a B.Sc.-thesis at Uppsala University (Larsson 1989). The area mapped in that work (Fig. 2) contains the pyroxene granulites and garnet-cordierite gneisses on which geothermometry and barometry have been applied. The charnockites have been found just south of that area (Fig. 1) by one of us (AW) during the SGU-mapping on the Karlskoga SE map-sheet.

GFF II4(1992)

Fig. 1. hlap of the investigated area. Preliminary results of SGU-mapping on the topographic map-sheet 10 E Karlskoga SO and the special investigation of Fig. 2. hlajor map symbols as in Fig. 2, Additional symbols: Elon-

gated crosses= biotite granite, Ch=charnockite, Kv= quartz segregations. Legend of lower right insert figure: hatched, area= SmAland-Varmland granites, SF = Svecofennian province, SWSGR = southwest Swedish gneiss region, PZ= Protogine Zone.

Regional setting The eastern contact of the Smiland-Virmland granites (mostly the coarse porphyritic Filipstadtype) has a strongly variable character which has been described elsewhere from the F i n s p h g area (Wikstrom 1987, 1991). Although sharply discordant in areas in low to intermediate metamorphic grade (e.g. south of Grythyttan, Lundstrom 1991), other areas of high t o intermediate amphibolite facies, as those of the Finspjng map-sheets, show a rangeof relationships from discordant to conformable, marginally folded layers. These layers, which are structurally interpreted as gravitationally overturned and folded marginal facies of diapir intrusions, imply that the viscosity contrast between the intruding body and the country rocks cannot have been large (Ramberg 1981). For structural reasons it is thus probable that the intrusion of the SmAland-Varmland granite magmas in that area took

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GFF I14 (1992)

Ctiarnockifes,pyroxene granitlifes, arid garnet-cardierite gneisscs 3

place in connection with regional heating, migmatite formation and some melting in high amphibolite facies. The sharp, cross-cutting contacts, e.g. in the Grythyttan area with much lower grade, also make it probable that this regional heat generation is not directly linked to thegranite intrusion. (Very local contact metamorphic facies rocks have been described from western Bergslagen by Hellingwerf 1985.) The common "Postorogenic-Svecofennian" label often attached to these granites has therefore been questioned in the above mentioned papers from the Finspang area. In the investigated area, part of a northeastwards protruding lobe from the major batholith of Filipstad-type granite is found (Fig. 1, right). This granite is here, in general, structurally discordant to regional fold structures and shows intrusive contacts. Some of the contacts are also coincident with shear zones of Sveconorwegian age where the main component is dip-slip (Wahlgren & Ronnlund

1988). Such a shear zone coincides with the contact to the investigated high-grade metamorphic rocks

(Fig. 2). However, small satellite bodies of Filipstad-type granite, in part hybridized with comagmatic norite, seem to have intrusive contacts to the high-grade gneisses. The intrusive contact of the major lobe is therefore believed t o be not far away from these rocks. This type of relationship is also present further south in the charnockite area (Fig. 1). The general coincidence in time (1.84-1.76 Ga) of the granites of the Smdland-Varmland belt and the late orogenic "Svecofennian" granites (Johansson 1988), indicates a major heat input and crustal reworking at that time. This is also consistent with conformable structures in plastically deformed amphibolite facies Svecofennian country rocks in zones adjacent to larger heat conducting plutonic bodies. The SmAland-Varmland granitoids and the late orogenic granites should therefore

RED.FELSIC,HEltROGEKEOUS GR4NITE PARTLY W l l H GARNET ~ ~ X ~ O R P H Y R IFILIPSIAO-IYPE IIC

a %

YCRllE I l E F l l HYPERSIHEKE I C N A L I I E I R I G H T I

S I R O l G L Y M081LIZED HETCROGENEOUS MlGF!AlIlE.~l:NtY CF SUFTIACRVSTALCR!GI!

W R I c R U S l A L R N K S IN G E H E R A L l L f F l l G A R h E l - L A R I N G (RIGHT1 PYROXLEE G R A W L I I E S T R C S L Y MCEILIZED GARILT-CORO!ERITE GtiElSS DARK (NIRT.Z-~OPXRITE MICA-SUPHIOE ROCK

a

EXTENSIVE GRAMTIC LEOSCME VElHlhO

I POSl VISIIGSO S A U O S T O N L FAULT

--OF SVECOHORWLGIANAGE .-- SHLARZCKE IMAlKLY DIP SLIP1

-6-

OOLLPUE DIKE DIKES OF FELSIL G A R N E T - B A R I N G G R A NII E

@ SAMPLf YLWBLR

---- SMALL ROAD

Fig. 2. Specially investigated area, containing the garnet-cordierite gneisses and pyroxene granulites. For location, see Fig. I.

4

U u B. Andersson el ol.

be considered a s formed by the same crustal event, although presumably in different tectonic regimes (Johansson 1988). They also display different proportions of added mantle material (e.g. Patchett et at. 1987, Andersson & \VVikstr&jm 1989, Andersson 1991).

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Field relations and petrography The rocks of the area are very complex, consisting of highly migmatitic polymetamorphic supracrustals associated with different granites, some of them anatectic, and intrusive basites. Later Sveconorwegian overprinting, related to the Protogine Zone, is also present, in the form of brittle-ductile shear zones and faulting. The bedrock of the area is composed off wo main unifs, (a) the coarse porphyritic Filipstad-type granites and associated mafic rocks, and (b) metamorphic rocks including some plutonic rocks of uncertain age. The rocks of unit (a) are unaffected by the high-grade metamorphism that all the other rocks have suffered and thus have their igneous textures preserved. In the centre of Fig. 1 the norites and Filipstad granites occur together and display definate signs of hybridization through magma mixing, e.g. diffuse contacts with feldspar xenocrysts mixed into a d a r k noritic matrix. Therefore the noritic rocks are considered part o f the coeval suite of mafic rocks associated with the Smiland-Varmland batholith. Some scattered unmetamorphosed dolerite dikes of unknown age are also present. These rocks are not described any further since the main theme of the paper concerns the metamorphic rocks. The metamorphic rocks (b) o f the area form a part of the supracrustals of the oldest Svecofennian. In the central parts of Fig. 1, they consist of strongly mobilized, fairly homogeneous garnetcordierite gneisses (Gt-Cd gneisses) of presumably semipeliticorigin. In thecentreofthearea, dikesof coarse garnet-bearing felsic granite sharply cut the mafic and hybrid rocks of the Smiland-Varmland generation. Thedike material is very similar to the leucosome of the adjacent Gt-Cd gneisses and is thus regarded as apophyses injected from the partially melted gneiss. The areal extension of the GtCd gneisses (Fig. 2) is limited in the west by a NNEstriking fault zone, and in the northeast by migmatized supracrustals of lower grade and varying composition. Southeastwards the Gt-Cd gneisses grade into less mobilized, orthopyroxene-bearing rocks (see below). In thesouthwest thereis adistinct massofgarnetiferous, red, felsie granite, separated from the

GFF 114 (1992)

gneisses but probably part of the same anatectic event. The mineralogy is dominated by potassium feldspar and quartz with only small amounts of garnet. The texture and grain-size change somewhat from place t o place and sometimes there is a resemblance with recrystallized acid voleanics. South of this granite the Gt-Cd gneisses progressively grade into lower grade gneisses. In the easfern part of the gneiss area, the migmatitic gneisses grade into a local, dark, sulphide-bearing variety. This is also affected by migmatization and carries orthopyroxene and cordierite. The garnet-eordierite gneisses contain the assemblage: Gt-Cd-Bt-Pl-Kfs-Qz(-Sill). The garnets are present in both the mobilized parts and in fragments of psammitic character (Fig, 3). In the latter they display large (c. 1 cm) white regular haloes depleted in biotite, whereas in the mobilized parts haloes are very weakly developed and irregular. This points to a synmigmatitic mineral growth. Such textures have been described by Stuwe & Powell (1989), who relate them to melt reactions of the type Q z + B t + C d = G t + K f s + L , where the haloe is part of the reaction products, occurring at around 4.5 kbar and 750°C. The texture of the mobilized parts is anatectic-blastic with a coarse intergrowth of large poikiloblastic garnets, cordierites, biotites, feldspars, and quartz with small amounts of sillimanite. Some retrogression is observed where muscovite developes in the cracks of garnet and cordierite marginally is transformed to biotite-sillimanite intergrowths. The Sveconorwegian tectonic overprinting is more intense in the western parts closer to the large fault zone. It is marked by shearing and breaking u p of the metamorphic minerals and retrogression, mainly of cordierites. to micas and chlorites. The mineral com-

Fig. 3. Strongly mobilized garnet-cordierite gneiss with a psammitic fragment in the lower right corner.

Cirarnockites,pyroxene granulites. and garnel-cordieritegneisses 5

GFF 114 (1992)

Table I . Average mineral chemical analyses.

+ /-

denotes measured compositional ranges. Smplc 47 gamctcordicritc gnciss (n=3)

Sample 48 gmct-curdierite gneiss (n=3)

a

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0.01 0.01 0.96 3.57 0.74 36.37 0.03 20.97 37.20

Tot

& 0.03 0.04 0.06 0.9

0.06 1

0.05 3 0.2

99.85

k

&

rd

0.6 0.2

0 0.09

0.04

0.02 6.69 0.06 10.75 0.01 32.23 41.25

9.21 0.05 0.06 9.15 0.01 19.72 3.45 17.16 34.66

4 0.05 4 0.2 0.5 0.5

94.08

5k

0 0.01 1.14 4.12 1.14 34.80 0.02 21.14 37.47

Tot

99.83

0.04 0.02 0.5 0.3 0.4 0.08 0.5

0.08

rp

9.68 0.2

0.01 0.04 0.02 7.63 0.11 9.22 0.02 32.25 48.32

0.03 8.95 0.01 20.51 4.10 17.41 31.60

0.04 0.9 0.01 2 0.2 I 0.8

95.40

& 0.02 0.09 0.01 0.4 0.08 0.6 0.009 0.6 0.8

97.61

100.07

94.89

positions of the analyzed high-grade metamorphic assemblages are very s,imilar in different parts of the area (Table 1). indicating homogeneous metamorphic conditions. A more detailed account has been presented elsewhere (Larsson 1989). Southeastwards the Gt-Cd gneisses are gradually succeeded by orfiiopyroxene-bearirlg rocks. These are more typical migmatites separated into leucosome and mesosome material. In the parts closest to the Gt-Cd gneisses the mesosome material (which contains the orthopyroxene assemblage)

0.6 0.03 0.4 0.2

21.27 35.68 98.14

h

&

9.33 0.10 0.02 9.48 0.02 19.45 3.92 17.21 35.35

0. I 0.007 0.03 0.2 0.08 0.3

0.3 0.3 0.3

94.87

rd 0 0.08 0.02 7.40 0.04 9.72 0 32.46 47.92

& 0.01

0.04 0.3 0.2 0.2 1

1

97.64

Synplc 43 pymxcnc granulite (ngt,bt=3;nopx=8;npl'l) L f f i !ax& g l d 0 9.40 0.2 0.01 0.007 0.28 0.1 0.02 0.006 0.03 0.003 0.02 0.02 9.12 0.6 0.01 0.01 0.08 0.02 3.83 0.2 0.82 0.09 0.58 0.3 4.20 2 11.40 0.3 13.88 0.2 2.25 0.3 0.09 0.06 0.84 0.06 0.01 0.1 34.40 3 18.96 0.6 32.97 0.4 0.04 0.4 0.01 0.04 3.99 0.4 0.12 0.02 0.01 0.01 21.37 0.3 15.23 0.5 2.97 0.08 22.41 0.4 1 36.43 0.9 48.88 0.4 63.58 0.8 37.30

5 5 5

100.38

99.82

0.06

0.01

Smplc 55 pyroxcnc p u l i t c (ngsbt=3: nopx=6;n -5) PI5 5 k k & Q E & p 0.01 0.02 9.46 0.08 0 0.02 0.04 0.04 0.04 0.2 0.02 0.02 2.01 0.2 0.01 0.21 0.03 4.45 1 12.14 1 0.3 14.33 2.22 . 0.2 0.05 0.02 0.07 0.77 32.25 1 11.32 2 0.4 32.48 0.1 0.10 0.06 0.05 3.86 0.02 15.88 4 2.15 0.2 21.50 0.2 1.0 36.14 0.9 0.3 37.62 49.76

Tot

0.02 0.05 0.5

0.73 4.39 0.64 35.41

97.10

! x & 0.11 0.1

0 0.01

0.02 0.04 0.3 0.02 0.5 0.02 0.6 0.3

Smplc 24 gamctcordicrilc gnciss (n=3)

5 5 5

g l 5 k

95.54

99.77

99.86

chmrrkitc (nbt: =3:nopx=l) l 0.16 5.77 10.04

i& 0.02 0.2 0.2

0.06 0.02 0.03 0.01 27.68 55.85

0.04 0.03 0.05 0.02 0.5 0.4

99.62

h 8.97 0.06 0.02 6.81 0.25 25.81 4.12 13.68 35.58 95.31

G

QE

0.3 0.1 0.05

0 0.03 0.50 7.88 1.30 4 I .32

1

0.2 2 0.6 0.8 2.0

0.14 I .09

47.32 99.58

dominates over the fine to medium-grained granitic Ieucosome material. Here the term pyroxene granulite has been applied, and this rock is best developed in the southern parts. To the east (around sample site 55, Fig. 2) the rocks become dominated by the autochtonous migmatite granite ( >SO%) and have been termed granitoid with pyroxene granulite fragments, but there is no sharp boundary. The migmatitic nature of the pyroxene granulites is shown in Fig. 4. The more fine-grained mesosomes contain the

6 U!f B. Andersson el al.

GFF 114 (1992)

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Fig. 4. Pyroxene granulite. Hypersthene is also present in the small veins.

assemblage: Gt-Opx-Bt-PI-Kfs(-Qz), where the garnets and orthopyroxenes have grown blastically at the expense of biotite. K-feldspars are probably orthoclase since no twinning is present. Antiperthitic exsolution is not uncommon in the plagioclases. The amount of quartz is usually very low and the protolith is tentatively interpreted to be a dacitic-latitic volcanite. Thin mobilized veins with a coarser texture contain mesoperthite together with larger orthopyroxenes and some oxides. Biotite and quartz are sparse in the veins and garnet is absent. Larger and coarser quartzofeldspathic veins also occur, containing K-feldspar with microcline twinning, plagioclase, quartz with sutured grain boundaries, some biotite and traces of garnet and cordierite. The quartz in these has presumably been released from the mesosome, now deficient in quartz. Mineral analyses of the assemblage of the mesosome from two localities (43 and 5 5 ) are listed in Table 1. A more thorough description of the rocks in Fig. 2 is given by Larsson (1989). Charnockitic rocks are present about 5 km south of the high-grade gneisses and granulites (Fig. 1). (Charnockites are also found both further to the southwest and to the north (Wahlgren and Stephens, pers. comm. 1991).) The protolith of the charnockites consists of greyish red, mediumgrained, slightly foliated biotite granite. The age of this granite is unknown but is presently regarded as the oldest granite variety of the SmAland-Varmland batholith. The greyish-green charnockites are found as irregular patches with diameters ranging from some decimetres to several hundred metres. The contact zone between the two rocks is often transitional but can also be sharp within a few

centimetres. This is easily recognized in fresh outcrops but disguised in weathered ones. It is by n o means a magmatic contact, but merely a sudden change in colour, representing a metamorphic boundary mainly marked by the incoming of orthopyroxene and the typical charnockitic dark colouring of the feldspars. So far, the charnockites have been followed along the contact to the Filipstad-type granite for about 5 km and about 2 km away from the contact (Fig. 1). The fexfrrreof the charnockite is heterogranular xenoblastic with annealed quartz grains (Bard 1986). Quartz is developed as small rounded xenomorphic grains included in the fringes of large Kfeldspars or as coarser grains with faint polygonal boundaries in the matrix associated with K-feldspar and plagioclase (Fig. 5). This texture indicates annealing during prograde metamorphism of myrmekites of quartz and plagioclaseexsolved from Kfeldspar, since plagioclase is always present together with the rounded quartz grains and also as more regular myrniekites. Large. probably primary, quartz grains have also been recrystallized showing lobate boundaries. K-feldspars have not been as strongly recrystallized as quartz and consist of both large grains and matrix grains. They have, in addition to extensive myrmekite exsolution also suffered microperthitic exsolution. The typical microcline twinning is absent or only very faintly developed, pointing to a structural state of'intermediate microcline-orthoclase. The structural state of the K-feldspars have been further explored by X-ray diffraction. Grains were handpicked and ground for analysis. The spacing

GFF I14 (1992)

Charnockifes.pyroxene granulifes,and garnet-cordieritegneisses 7

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Fig. 5. Typical quartzfeldspar texture of charnockite.

ofthepeaksofthereflectionsfromthe 131 and 1-31 lattice planes of the K-feldspar can be used to determine its crystallographic nature (e.g. Goldsmith 6: Laves 1954). I f these reflections are distinct and well separated the feldspar is triclinic (low microcline). Often, however, the peaks become broader and less well separated (intermediate microcline) and also merge into one peak, indicative of monoclinic feldspar, at least with regard t o the resolution of X-ray powder diffraction. Recent transmission electron microscopy (TEM) investigations have shown this t o be related to the development of submicroscopic triclinic twin domains (e.g. Smith & Brown 1988, Bambauer et al. 1989, Brown 6: Parsons 1989). which also can be correlated with the optical properties as observed under the microscope. The coarseness and distribution of the twin domains, determine the optical appearance. Hence, the well developed cross-hatched twinning and distinct triclinic X-ray pattern of low (regular) microcline result from relatively large domains. This is in contrast to orthoclase which appears monoclinic under the microscope and in the X-ray analysis as a result of an “averaging” over very small twin domains (“t\veed”-orthocIase, Smith 6: Brown 1988, Bambauer et al. 1989. Brown & Parsons 1989). Transitional varieties of these two, i.e. intermediate o r irregular microcline, are characterized by intermediate-sized domains and usually display transitions between areas of differentsized domains. This is usually found in optically inhomogeneous K-feldspars, showing areas of faint o r blurred cross-hatching. Theamount of different size domains also determines the X-ray pow-

der result. Fig. 6A shows a part of the diffractogram of a K-feldspar from the charnockite, with one distinct 13 I-peak, with a d-value of 2.9916, and no 131 reflection. This indicates a monoclinic symmetry suggesting the K-feldspar t o be orthoclasewith very fine twin domains. Several different

1 Fig. 6. X-ray diffractograms of K-feldspars from the charnockite (A) with a distinct peak, d-value 2.9916, for lattice plane 131, supportingamonoclinicsymmetry,and (B) from the biotitegranite with a broad diffuse reflection indicating the feldspar to be intermediate microcline. The peaks at d = 2.9 are K-feldspar 011.

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8 UrJB. Anderson et al.

GFF 114 (1992)

ites. Oxides of apparent primary origin are accessory, together with some zircon and stubby apatite. The biofitegruniteshows the same development of exsolved and annealed myrmekites with rounded quartz grains. T h e K-feldspars, on the other hand, have more well developed microcline twinning. however with diffuse patches. This suggests a more ordered structural state with coarser twin domains than in the charnockites, but not pure low microcline. The X-ray diffractogram in Fig. 6B shows a typical result of a separated K-feldspar megacryst from the biotite granite. There is one broad reflection with no distinct d-spacing. This supports the optical interpretation of intergrowths of triclinic twin domains that are coarser compared to those in the charnockite. Small, nonmegacrystic K-feldspars bitotite + 3 quartz = 3 orthopyroxene + K-feldhave X-ray characteristics intermediate between spar + H 2 0 the megacrysts and those observed in charnockitic K-feldspars. These data suggest that the biotite (e.g. Loomis 1966). The composition of the biotite granite has suffered less metamorphic heating than and orthopyroxene is presented in Table 1. The the charnockite. biotite is Fe-rich and contains appreciable amounts The large plagioclases are primary with some seof Ti. The corresponding formula is (normalized to ricitic alteration. Biotite has been observed t o 7 cations (IV + VI)): break down to oxides, K-feldspar and probably (Nao.olKo.~2)(Mgo.~~Mno.02Fe2+1.,JTio.~,Ab.lt)(Al~.~~ Mg-rich biotite, forming pseudomorphic aggreSi2.87)010.47(OH)I.53 gates of “pre”eulite. Amphiboles are not present. Accessories are as in the charnockites. The granite (Fe/Fe+ Mg = 0.68). The pyroxene is therefore also very Fe-rich and corresponds to eulite of the hence seems to be in a state of incipienfcharnockifization and indicate somewhat lower temperatures, following formula (based on 4 cations): as suggested by the structural state of K-feldspar. ( C a o . o z ~ ~ . , g M n o . o ~I.40Fe3 F ~ +o.oA.od(Ab.cusiI .%I The granite-charnockite transformation is iso0 6 chemical with respect to the analyzed major and trace elements (Table 2). Although only two ana( = Fs,*). Amphibole has not been observed in the charnocklyses have been performed so far, the results show

grains have been analyzed with the same result, supporting the optical interpretation. This is consistent with metamorphic heating, which increased the tetrahedral disorder and erased the coarse, low microcline twin domains of the biotite granite, followed by cooling, which was not slow enough for these coarse domains to be developed again. Plagioclase occurs, except in myrmekitic exsolutions, also as large primary albite twinned grains. Large patches of antiperthitically exsolved K-feldspar are also observed. The mafic minerals are represented by biotite and orthopyroxene. Biotite and quartz are consumed by the prograde growth of orthopyroxene (Fig. 7). whereas K-feldspar is released, according to:

+

Fig. 7. Orthopyroxene groath at the expense of biotite. In charnockite.

Charnockites, pyroxene granulites, and garnet-cordierite gneisses 9

GFF I14 (1992)

Table 2. Whole-rock chemical analyses (wt To oxide) and trace elements (ppm). Samples from 1 km NE of 61sdalen (topographic map-sheet 10 E Karlskoga SO, coordinates 657 1801142825). Granite SiO, Ti02

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AIZO,

Fez03 FeO hlnO MgO CaO Na,O K,O

Rb Sr

Y Zr

Nb

68.6 0.50 13.9 1.11 2.76 0.065 0.62 1.74 2.94 5.60 210 110 50 390 10

Charnockite 68.4 0.51 14.0 1.24 2.79 0.074 0.70 1.60 2.93 5.58

200 110 50 390 10

very close correlation. This is also valid for elements that are known to be depleted during charnockitizationelscwherc, e.g. K and Rb(e.g. Tarney & Windley 1977, Condie & Allen 1984). The elements released during the transformation have thus been taken u p by newly formed minerals, e.g. Rb and K by K-feldspar. Fluid removal of elements (metasomatism) during metamorphism thus seems t o be of very limited or no significance in this transformation.

them. Therefore temperatures are also given at Xf,'pmax,which causes a considerable increase in T (8-129O). depending on the difference between Xf,'*+"and Xf,'g.,,,ax.The largest differences are recorded in sample 43. It is suggested that these values represent conditions closer t o the metamorphic peak and that the average values are minimum values. The compositions of the cordierites. orthopyroxenes, plagioclases and biotites are more uniform. The average values have therefore been employed. An exception is sample 48, where one biotite is considerably richer in Fe. This value is combined with Xfig.,,,= to arrive at a maximum garnet-biotite temperature (Table 3). The applied thermometers display some scatter of temperatures. T h e values derived from the model by Indares & Martignole (1985) are consistently lower than the others. This is probably due to overcompensating for the minor constituents, Ca and Mn in garnet and Ti and A1 in biotite (e.g. Schreurs & Westra 1986, Chipera & Perkins 1988). The esti-

Table 3. Geothermobarometry. Calcula~ionsarc made according to: F&S: Ferry& Spear 1978, P&L: Perchuk & Lavrenteva 1983, ]&hi: Indares & Martignole 1985, BhlS: Bhattacharya et at. 1988, H: Harley 1984, S&B: Sen & Bhattacharya 1984, P: Perchuk et at. 1985 (combined thermobarometer), P&C: Perkins & Chipera 1985. Temperatures are given at 4 kbar and pressures at 700°C. The second values for each sample show temperatures at Xf& The third value of gt-bt of sample 48 represents XRI,.,. and Xf: Temperature ("C) Pressure (kbar) F&S P&L I&hI DAIS H S&D P P k$'&CrI

Geothermobarometry Geothermometry has been performed using garnet-orthopyroxene models (Harley 1984, Sen & Bhattacharya 1984, Perchuk et al. 1985) for thepyroxene granulites, garnet-cordierite models (Perchuk & Lavrenteva 1983, Uhattacharyaet al. 1988) for the garnet-cordierite gneisses and garnet-biotite models (Ferry & Spear 1978, Perchuk &L Lavrenteva 1983, Indares&Martignole 1985) for both. Barometry has been applied according to Perkins & Chipera (1985) on the pyroxene granulites (GtOpx-PI-Qz) and according to Perchuk et al. (1985) on the garnet-cordierite gneisses (Gt-Cd-Sill-Qz). The average mineral chemical data of Table 1 have been used for the thermobarometric calculations presented in Table 3. Since these high-grade gneisses arc believed to have equilibrated during peak o f metamorphism, the cores of the grains have been analyzed in order t o avoid retrogressive effects. There is, however, a significant spread in Fe/Mg of the garnets, indicating that retrogression has affected some of

Sample 24

gt-bt

737 154

gtcd

651 659 635 643

608 620 717 736

693 5.1

701 5.1

Sample 47 gt-bt

708 . 733

gtcd

661 619 673

565 581

6S9

735 755

135 5.5 154 5.7

71 I 751

718 5.0 149 5.3

Sample 48 gt-bt

626 659 815

gtcd

614 648 717 662 686

525 558 665

613 625

519

Sample 43

gt-bt

616 700

601

609 642 683

gt-opx

5.2

4.5

110 771 164

Sample 55 gt-bl 609 637

gt-opx

605 623

544 574 655 725 163 701 763 785

6.0 4.2

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10

GFF 114 (1992)

Ulf B. Anderssorr et al.

mationsaccording to Perchuk& Lavrenteva (1983) are also a bit low when compared to Ferry 6:Spear (1978) (Gt-Bt) or Bhattacharya et al. (1988) and Perchuk et al. (1985) (Gt-Cd). The garnet-cordierite temperatures of Bhattacharya et al. (1988) and Perchuk et al. (1985) for the garnet-cordierite gneisses and the garnet-orthopyroxene temperatures for the pyroxene granulites show generally good agreement, whereas the garnet-biotite estimations are lower than the garnet-orthopyroxene ones for the pyroxene granulites. The temperature calculations according t o garnet-biotite models are generally lower than those obtained from garnetcordierite and garnet-orthopyroxene pairs. This is an indication of the more ready readjustment of biotite during cooling. The model of Ferry & Spear (1978) seems t o be most in agreement with the garnet-cordierite and garnet-orthopyroxene thermometers. The one unretrogressed (?) biotite of sample 48 records temperatures in the order of the other thermometers. The Mn-component of the garnets in the pyroxene granulites is relatively high (Sp,). In the calculations according to Harley (1984). Mn is combined with Ca, thereby raising the temperatures by approx. 15". The other two models d o not consider the Mn-component. Temperatures have been given a t 4 kbar. The effect of pressure on the temperatures is low. in the order of 5--lO"/kbar, for all thermometers. If the real pressure should depart from the one used, this would not significantly change the overall temperature estimates. In spite of the differences in the calculated temperatures, the bulk of the data lie in the range 650--77O"C, typical of the granulite facies. Lower temperatures are recorded from analyses of grains in contact with each other, from the GtCd gneisses. Margins of biotites, garnets and cordierites have been analyzed close t o their mutual interfaces and the calculated garnet-biotite and Tuble 4. Geothermobarometry; contacts. Calculations are made according to: F&S: Ferry & Spear 1978. PBL: Perchuk & Lavrenteva 1983, I&hl: Indares RC hlartignole 19S5. BhTS: Bhattacharya et al. 1988, P: Perchuk et al. 1985 (combined thermobarometer). Temperatures are given at 4 kbar. Pressure (kbar) Temperature ("C) F&S PBI, 1BhI BhiS P P Sample 1:gt-bt 543 579 438 23 2:gt-bt 553 586 445 Sample 47 Sample 48

I:gt-cd 2:gt-cd

616 544

1:gt-bt 516 517 4M 2:gt-bt 412 541 383

700 671 4.8 655 593 4.0

garnet-cordierite temperatures are 70-200" and 35-140" respectively, lower. than peak temperatures (Table 4). These readjusted temperatures lie in the range 470--700°C. with an average of c. 570°C (excluding Indares & hlartignole 1985), i.e. about 100-150° lower than peak metamorphic temperatures. The pressure calculations according t o Perchuk et al. (1985) are derived by combining two equations of P and T a n d simultaneously solving for the assemblage Gt-Cd-Sill-Qz, present in the garnetcordierite gneisses. This gives pressures o f 5 - 5 5 kbars, corresponding to at depth of 20 km. Pressures have also been obtained from the models presented by Perkins & Chipera (1985). for the assemblage Gt-Opx-PI-Qz (pyroxene granulites). The latter lies in the range4.2-6.0 kbar (c. 15-22 km) at 700°C. and are thus in good agreement with the result from the Gt-Cd gneiss above. The GtOpx pressuresare calculated according t o two independent calibrations for the Fe and Mg end member reactions. These should yield nearly identical results. Discrepancies may be attributed to disequilibrium of the phases or improperly calibrated models. At X r > O . 5 , larger differences were observed between the Fe and Mg end member models (Perkins 6: Chipera 1985). Xf{' values in our samples are approx. 0.55, possibly explaining the difference of 0.7-1.8 kbar. The pressure of formation for the peak metamorphic assemblages in the area is thus suggested to be in the range 4.2-6.0 kbar (15-22 km). Pressures (Perchuk et al. 1985) have also been calculated for the contact grains mentioned above with the result of 4.0-4.8 kbar, which is about 1 kbar lower than the peak metamorphic pressures.

Discussion In Fig. 8, the solid frame outlines the high-T and low- to intermediate-P conditions derived in this study. It is superimposed on the stability data of high-grade pelitic rocks presented by Holdaway & Lee (1977). The garnet-cordierite gneisses contain the appropriate assemblage for applying this diagram. This rock has obviously reached the line of the reaction Bt + Sill + Qz = Cd + Alm-Py + Kfs + V, since large amounts of garnet have formed. It is observed togrowat theexpenseofbiotite, andsillimanite has been almost totally consumed. The analysed cordierites of the gneiss have compositions of Fe/Fe + Mg = 0.4-0.5, which further constrain theassemblagein P-Tspace (shaded area). The P-T conditions delineated in Fig. 8 are thus 670-770°C and 4.0-4.5 kbar.

GFF 114 (1992)

Charnockites, pyroxene granulites, and garnet-cordierite gneisses

I

I

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500

600

700

800 T"C

--

Fig. 8. Stabilityrelations i high-gradepeliticroclis(from Holdaway 6: Lee 1977, fig. 7B) at P ~ 1 , ~ = 0P,,,. . 4 Lines labelled 20-80, denotes Fe/Fe+ hlg in cordierite. The solid frame outlines the P-T conditions inferred in this study. Further constraints are imposed by the composition of cordierite (shaded area). Frame of broken lines outlines P-l'areaofgrainsinmutualcontact. Arrowsuggest a tentative retrograde P-T path (near isobaric cooling), consistent with metamorphism associated with magma intrusions in the middle crust.

The curve of granite melting included (Kerrick 1972). clearly allows partial melting in these rocks. This is also concistent with Thompson (1982), who

11

pyroxene in the pyroxene granulites indicates, applying o u r P-T constraints, a value of aHZO in the range 0.1-0.3 (Fig. 9). The conditions thus determined arc characteristic of granulite facies although shallower than granulite areas in south India or the Adirondacks (c. 8 kbar, e.g. Raaseet al. 1986, Edwards6:Essene 1988). Recent investigations (Johansson et al. 1991) in thesouthwest Swedish Gneiss Region have demonstrated the existence of granulite facies rocks over wider areas than previously known, with 1'-T-conditions calculated at 700-77OoC and 8-10 kbar. This medium to high pressure metamorphism is, however, much younger (c. 910 Ma) than in the presently studied area and related to a late Sveconorwegian collisional and crustal thickening event (Johansson et al. 1991). The P-T conditions in the present area correspond, o n the other hand, with thelow-Pgranuliteareas investigated in the Svecofennian of Finland, which consists of rocks of the same age and a similar geological setting (e.g. Korsman et al. 1984, 3461tta 1986, Schreurs & Westra 1985, 1986). In those studies are, however, the heat source for the metamorphism unclear, and thermal domes have been invoked (Schreurs & Westra 1986). The high-grade rocks in the present area aresuggested to be the result of thermal metamorphism associated with the intrusion of the Smdland-VPrmland granitoids and related mafic rocks, presumably superimposed on a n early Svecofennian amphibolite facies rnetamorphism. This is supported by the relatively lowP estimate ("shallow") and the spatial distribution of the high-grade rocks in a lobe surrounded and intruded by Smlland-Vrrmland plutonics.

calculated dehydration melting in pelitic rocks t o start at about 700OC at 4 kbar. Garnet and cordierite occur abundantly in the mobilized gneiss, indicating that these minerals formed together with melt. h k l t was produced during prograde metamorphism by several reactions, but the main reaction in this case was probably the dehydrationmelting reaction B t + S i l l + Q z = G t + C d + K f s + L , occurring at around 75OOC (Thompson 1982, Waters 1988), where almost all sillimanite was consumed. It is also possible to put some constraints on wa6 ter pressure. The assemblage garnet-cordierite-Kfeldpar-quartz requires PHZo< P,,, to form (Hol5 daway & Lee 1977). hloreover, the stability relations at Pllz0 = p,,, are not consistent with the 4 P-T conditions derived for the rocks in this study (Holdaway 6: Lee 1977, fig. 7A). Fig. 8, which 23 -Y shows stability relations at Pllz0 = O.4Pt,, is, how- a 2 ever, consistent with our P-T data. To further constrain water activity the results of Lamb & Val1 ley (1988) have been applied (Fig. 9). Our determined P-T area has been superimposed on the re550 650 750 T"C action curves of the reaction Phl+ Qz= En + Kfs + V Fig. 9. Equilibrium lines of the assemblage phlogopiteat different aHp. This is based o n experiments in (Lamb & Valley the pure K20-h~g0-A1203-Si02-H20-system. Ex- quartz-enstatite-K-feldspar-vapour 1988). Curve of granite melting at Xl120 = 0.5, after Kerperiments in the Fe-rich system suggest somewhat rick (1972). Stability fields of A12SiO,-silicates from higher ak,20,but this is not yet accurately modelled Holdaway (1971). Constrained P-T area of our study, from Pig. 8. (Lamb 6: Valley 1988). The formation of orthoL

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12

U!fB Anderson el al.

The P-T recorded in contact grains suggest a lowering of pressure with c. 1 kbar and temperature with 100-ISO”C. The tentativeretrograde P-Tpath indicated in Fig. 8 would not be inconsistent with a model of heat supplied by intruding magmas (near isobaric cooling, cf. Waters 1986, Bohlen 1991). The mobilized and homogenized structure of the garnet-cordierite gneisses is very similar t o the rocks mentioned by Ahlin & Lundqvist (1988) and Lundqvist (1990), which have been characterized as hornfelses in the contact zone to the Revsund granite in north-central Sweden. They are also very similar to homogenized garnet-cordierite gneisses in the contact zone of the NygArden norite pluton, which have been found to be mobilized and to back-vein the norite (Larsson 1935). Other garnetcordierite gneisses in the neighbourhood, further east (Landergren 1934, Wikstrom 1991). could be explained by localised thermal domes connected to the intrusion of ”late orogenic Svecofennian” granites, which are generated penecontemporaneously with the SmAland-Varmland granites. Further east, all the way to the Baltic coast, the paragenesis garnet-cordierite is ubiquitous in Svecofennian amphibolite facies, less mobilized, supracrustal gneisses. This regionally metamorphosed assemblage cannot be related t o any granite intrusions, but the boundary towards the more clearly contact metamorphic assemblages has not been established. The migmatization of the here investigated gneisses appears to be contemporaneous with the growth of the metamorphic minerals. This is evidenced by the large, white, biotite depleted haloes around garnets growing in psammitic fragments, whereas garnets growing in the mobilized material show weakly developed and irregular haloes. The latter are hence considered to have grown in a mobilized, partly melted environment. Furthermore. dikes of garnet-bearing felsic granites cut basites and hybrids belonging to the SmAland-Varmland intrusions (Fig. 2). These are regarded as apophyses of leucosome material generated during thepartial melting of the neighbouring Gt-Cd-gneiss, back-veining into rocks that supplied the heat for the same melting. This is because the composition of the latter made them crystallize at higher temperatures, while the gneiss leucosome was still molten. This is evidence that the granulite-melting event is connected to the intrusion of the S m i land-Varmland plutonics (contemporaneous and younger). The pyroxene granulite contain different types of veins. Coarse-grained quartzofeldspathic, and finer grained small ones. The latter contain mesoperthitic feldspars and larger orthopyroxenes than

GFF 114 (1992)

in the mesosome. This is interpreted in terms of prograde metamorphism. The coarse veins represent the first stages of melting at higher alcp, as shown by their eutectic composition and content of biotite. These veins are interconnected and collects to larger amounts of migmatite granite, especially in theeastern parts. This may be related t o an earlier stage in the migmatitic evolution, where considerable amounts of water-rich phases reacted t o produce melts, leaving solid water-poor residues (mesosomes). As the metamorphism proceeds the water becomes progressively partitioned into the melt phase (e.g. Powell 1983). The earlier stages of melting thus removes much of the water vapour, lowering aHZO.T h e later stages of melting occur undtr almost anhydrous conditions. The small veins with increased growth of orthopyroxene and non-eutectic, quartz deficient composition, represent this later stage. The relevant reactions involved in producing pyroxene granulites and melts have been thoroughly discussed by Waters (1988). who concluded that the dehydration - melting reaction Bt + PI Qz= Opx + Kfs + L is of major importance. In the present case this reaction is suggested to be responsible for the formation of the small veins with larger orthopyroxenes compared to the mesosome. This reaction occurs over a n interval between 700 and 80O0C, depending on the Fe-Mg-partitioning between the phases (Waters 1988), consistent with the determined temperatures above. The fine-grained mesosome contains the assemblage garnet-orthopyroxene-biotite-plagioclase-K-feldspar(-quartz). where the growth of garnet diminished and was succeeded by the growth of orthopyroxene as the temperature increased and allyo decreased, since garnet is not present in the small. high-temperature veins. A later subsolidus metamorphic overprint o n an already formed migmatite would not be expected to develop such inhomogeneous mineral growth in closely associated mesosome and veins. The described granulite nietamorphism is regarded as the culmination of an event of large heat input in the crust manifested by theextensive regional metamorphism and migmatization of the early Svecofennian rocks and the penecontemporaneous generation and intrusion of the “late orogenie Svecofennian” granites and the Smiland-Varmland batholith. Thus, the occurrence described here is regarded as a local granulitic contact metamorphic peak in the overall amphibolite facies metamorphism. The charnockites represent another form of granulite facies metamorphism spatially related to the intrusion of the Smiland-Varmland granitoids. They are also associated with high grade garnet-

+

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GFF 114 (1992)

Charnockites, pyroxene granulites, and garner-cordierite gneisses 13

cordierite gneisses (Fig. I). The patchy development of the charnockitic assemblage records ternperaturemaximain the biotitegranite, asshown by the growth of eulite at the expense of biotite, and the structural state of K-feldspar. The lack of appropriate P-T calibrations for the assemblage biotite-orthopyroxene has made such calculations impossible. Melting appears not t o be connected with this transformation in contrast to the pyroxene granulites. This poses the problem of a somewhat different mechanism of metamorphism. The patchy inhomogcneous development resembles the amphibolite-granulite transition zone of south India which have been attributed to COz-streaming along fracture zones in the protolith (e.g. Janardhanet al. 1982, Condieet al. 1982, Gopalakrishna et al. 1986). Thiswould beconsistent with themodels of Waters (1988), who shows that with a high content of.COz in a vapour phase, and accordingly a low aHlO, the reaction Bt + Qz = Opx + Kfs + V will proceed with rising temperature, until the melting reaction Bt + Q z + PI = Opx + Kfs + L is reached. The latter has obviously not been achieved in the charnockite indicating somewhat lower temperatures compared t o the pyroxenegranulite, but still around 700°C. Regarding the origin of the presumed C 0 2 , a mantle source is unlikely due to the shallower level (c. 15 km) recorded in the adjacent high-grade gneisses. A generation from the intruding granitoids as envisaged by Frost &Frost (1987) could be suggested. The transition has, however. not caused any chemicaI depletion in the charnockites. Further work e.g. o n fluid inclusions, is needed to constrain the importance of C02. Sm/Nd-dating of the metamorphic mineral assemblages of the area would b e helpful in constraining the timing of metamorphism.

(3) The metamorphic conditions have been constrained by geothermobarometry to approx. 670770°C. 4.0-4.5 kbar (=15 km) and = 0.1-0.3. (4) Extensive partial melting is associated with the metamorphism in the Gt-Cd gneisses. This is indicated by the a) homogenized mobilized structure with remnants of unmelted, mainly psammitic fragments, b) the diffuse development of haloes around garnets in the mobilizate compared t o the regular ones in the fragments, c) the size and amount of garnet which is greater in the mobilizate, and d) the related generation of garnetiferous felsic granite. Melting is corroborated expcrimcntally for pelitic systems of the determined P-T conditions (e.g. Thompson 1982, 1988, Waters 1988), and the reaction Bt + Sill + Qz= Gt + C d + Kfs + L is suggested to have played a major role in the development of the gneisses. Melting associated with the development of the metamorphic assemblage also applies to the opxbearing parts of the area (pyroxene granulites). Here the formation of a migmatite granite and different generations of veins attest to melting, where garnet and cordierite occur sparsely in the granite and larger veins. Orthopyroxene is mainly restricted to the mesosome and thesmall veins, where they have grown larger in the latter. This suggests the reaction B t + P I + Q z = O p x + K f s + L (cf. Waters 1988) to be important in the later stages of melting. The charnockites, on the other hand, are not associated with partial meIting, but result from a pure metamorphic, isochemical, prograde transformation. This suggests the reaction Bt + Q z = Opx Kfs + H20to be of major importance, and the solidus is not reached (cf. the pyroxene granulites), suggesting somewhat lower temperatures and probably initally drier conditions in the biotite granite compared to the gneisses. ( 5 ) The granulite metamorphism and related meltConclusions ing is interpreted as resulting from the heat suppThe main conclusions reached in this study encomlied by the intruding Smiland-Varmland plutonics passes: a t about 1.8 G a (Jar1 & Johansson 1988). This is (I) Granulite facies metamorphism has been atconsistent with: a) the spatial arrangement of the tained in a lobe of early Svecofennian supracrustals, rocks, and b) dikes of garnet-bearing granitic leusurrounded and intruded by SmAland-Varmland cosome material, emanating from the Gt-Cdgranitoids and associa!cd mafic plutonics, east of gneisses, cutting basitcs and hybrids of the Karlskoga, southern Sweden. SmAland-Varmland generation, and c) the relative(2) Themineral assemblages characterizing the mely low pressure (shallow level) type metamorphism. tamorphism are garnet-cordierite-biotite-plagio- The later Svcconorwegian event is of low-grade, reclase-K-feldspar-quartz(-sillimanite)in the semi sulting in mineral retrogression and brittle-ductile pelitic gneisses, garnet-orthopyroxene-biotite-pla- tectonism but no melting. gioclase-K-feldspar(-quartz) in the pyroxene granulites, and orthopyroxene-biotite-plagioc1ase-Kfeldspar-quartz in the charnockitized biotite granite.

+

14

Acknowledgements. - We ackno\rledge the help from CarlHenric Wahlgrcn. SGU. who in thc first place suggcstcd a n in\cstigation of the relationship between t h e garnet-cordierite assemblage and ductile shear zones in the area, a u o r k Hhich later developed into the present paper. Anna Sch)?t. Stockholm, \\ho working fortheSGUmadethefieldwork resultinginmost o f t h e map in Fig. I. Claes Alinder. SGU. for performing the microprobe analysts. Christina Wcrnstrdm. Geol. Inst., for drairing thc figurcs and Olle Wallner. Geol. inst.. for making the thin scclions. The review of Fred Hubbard irnprovcd the manuscript significantly.

References

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GFF 114 (1992)

U!f B. Andersson et 01.

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