Eclogites and polyphase P–T cycling in the Caledonian Uppermost Allochthon in Troms, northern Norway

1. metamorphic Geol., 1990, 8, 289-309

Eclogites and polyphase P-T cycling in the Caledonian Uppermost Allochthon in Troms, northern Norway .E. J . KROGH', A . ANDRESEN', 1 . BRYHNI', T . M . BROKS' a n d S . E . KRISTENSEN' 'Institutt for Biologi og Ceologi, Universitetet i Tromsa, PO Box 3085, Guleng, N-9001 Tromss, Norway

Mineralogisk-Geologisk Museum, Sarsgt. 1, N-0528 Oslo 5, Norway

ABSTRACT Edogites in the Tromsai area, northern Norway, are intimately associated with meta-supracrustals within the Uppermost Allochthon of the Scandinavian Caledonides (the Tromsa Nappe Complex). The whole sequence, which includes pelitic to semipelitic schists and gneisses, marbles and calc-silicate rocks, quartzofeldspatbic gneisses, metabasites and ultramafites, has undergone three main deformational/ metamorphic events (DJM,, D2/M2 and DJM,). Detailed structural, microtextural and mineral chemical studies have made it possible to construct separate P-T paths for these three events.

Chemically zoned lute syn- to post-Dl garnets with inclusions of Bt, PI and Qtz in Ky-bearing metapelites indicate a prograde evolution from 636°C. 12.48 kbar to c. 720"C, 14-15 kbar. This latter result is in agreement with Grt-Cpx geothermometry and Grt-Cpx-PI-& geobarometry on eclogites and trondhjemitic to dioritic gneisses. Maximum pressures at c. 675°C probably reached 17-18 kbar based on Cpx-PI-Qtz inclusions in eclogitic garnets, and Grt-Ky-PI-Qtz and Jd- Ab-Qtz in trondhjemitic gneisses. Post-D,/pre-D2 decompressional breakdown of the high-P assemblages indicates a substantial drop in pressure at this stage. Inclusions and chemical zoning in syn- to posr-D2 garnets from metapelites record a second episode of prograde metamorphism, from 552' C, 7.95 kbar, passing through a maximum pressure of 10.64 kbar at 644"C, with final equilibration at c. 665"C, 9-10kbar. The corresponding apparently -facial paragenesis Grt + Cpx + PI + Qtz in metabasites yields c. 635" C, 8-10 kbar. In the metapelites post-D, Grt in apparent equilibrium with Bt, Phe and PI yield c . 630" C, 9 kbar. The DJM, and DJM2 episodes are exclusively recorded in the T r o w Nappe Complex and must thus predate the emplacement of this allochthonous unit on top of the underlying Lyngen Nappe, while the D3/M, episode is common for the two units.

A previously published Sm-Nd mineral isochron (Grt-Cpx-Am) on a partly retrograded and recrystallized ecologite of 598 f 107 Ma represents either the timing of formation of the eclogites or the post-eclogite/pre-D, decompression stage, while a Rb-Sr whole rock isochron of an apparently post-DJpre-D, granite of 433 f 11 Ma is consistent with a K-Ar age of post-D,/pre-D2 amphiboles from a retrograded eclogite of 437 f 16 Ma which most likely record cooling below the 475-500" C isotherm after the M3 metamorphism.

Key words: Caledonides; eclogite; geothennobarometry; high-pressure granulite; K-Ar; metapelite; plymetamorphism; P-T-f paths; Rb-Sr; Sm-Nd; Uppermost Allochthon.

Abbrcvtbbns: Abbreviations tor mineral names are adopted from Kretz (1983).

INTRODUCTION

Eclogites and eclogitic rocks occur within a wide belt in the basal gneiss region of western Norway from Sognefjord to Trondheim (Griftin, Austrheim, Brastad, Bryhni, Krill, Krogh, Merk, Wale & T~rudbakken, 1985). In recent ycars scattered Occurrences of similar rocks have been described from the allochthonous units of the Scandinavian Caledonides: in the Bergen anorthosites of the Middle Allocbthon (Austrbeim 8i Griffin, 1985), in the Seve Nappe of tbe Upper Allochthon (van Roennund, 1982, 1985; Andreasson, Gee & Suktjo, 1985) and in the Tromsai

Nappe Complex of the Uppermost Allochthon. The 'basal gneiss eclogites' have been thoroughly studied by numerous geologists for more than a century. and Griffin et ul. (1985) and Griffin (1987) have recently presented a review of their Occurrence and genesis. Until recently, however, little attention has been paid to the eclogites within the Caledonian allochthons. In the light of the unusual P-T conditions required for eclogite formation, the allochthonous eclogites certainly represent an important key to the understanding of the tectonometamorphic evolution of the Scandinavian Caledonides. Whereas the Seve eclogites (van Roermund, 1982, 1985; Andreasson et

m9

290 E . J . KROCH Er A L .

al., 1985) occur in a Nappe of Baltoscandian affinity, the Tromw eclogites are located in a nappe clearly exotic to Baltica. The latter occur as pods, lenscs and larger bodies within a sequence of continental platform type metasedi- ments (Tromxlalstind sequence) in the Uppermost Allochthon (Tromw Nappc Complex; Andresen, Fareth, Bergh, Kristensen & Krogh, 1985). This paper presents field, petrographical, mineralogical

and isotope data on the Troms0 eclogites and associated rocks. The data are interpreted in terms of successive deformational and metamorphic events, and an attempt is made to construct the P-T paths for these events using geothermobarometric methods. Geochronological studies, including an internal Sm-Nd isochron on a partly retrograded and recrystallized eclogite, a Rb-Sr whole rock isochron on a structurally dated microcline gneiss, and K-Ar mineral ages on post-eclogite/pre-D, amphiboles, broadly constrain the timing of the events between about 600 and 430Ma. The significance of these data is discussed in the context of the tectonic evolution of the Scandinavian Caledonides in the Troms region of northern Norway.

REG I 0 N AL C LO LO C I CAL S ETTl N G

The Troms segment of the Scandinavian Caledonides (Fig. 1) is characterized by a series of rather flat lying nappcs and nappe complexes. In the east the nappes overlie both the autochthonous Precambrian gneisses and granites of the Baltic Shield and an unconformably overlying veneer of autochthonous Late Precambrian (Vendian) to Cam- brian sediments (Binns, 1978). The Precambrian basement rocks can be traced westwards through several windows to the coastal parts of Troms where they make up most of the outer islands. Here, the autochthonous cover sequence is missing and the nappes sit directly on the gneissts and granites of the western part of the basement (Western Basement Region/WBR; Andresen ef al., 1985). It is generally accepted that the eastern and western parts of the basement are continuous without any suture separating them, and that the Caledonian nappes were emplaced from west-north-west across the WBR (Binns, 1978; Andresen ef uf., 1985). The nappe terminology, tectonostratigraphy and the location of the main nappe boundaries in the area are as yet not well established

Q. 1. Simplified geologicpl map of the Scandinavian CPlcdoNdes in Finamark and northemmost Troms, showing the main tectonic units and terrana (from Aodrescn. 1988).

(Binns, 1978; Zwaan & Roberts, 1978; Andresen ct d., 1985; Andresen, 1988). Binns (1978) subdivided the allochthonous units in Troms and Finnmark into seven nappes or nappe complexes. Based on recent mapping in western Troms the tectonostratigraphy presented by Binns (1978) has been revised somewhat (Andresen et al., 1985). This revised tectonostrahgraphy with minor modifications based on new data from Bergh & Andresen (1987) and Andresen (1988) will be used throughout this paper (Fig. 1).

The T r o w Nappe Complex (TNC), which contains the cclogites, probably represents the tectonically highest unit within the entire Scandinavian Caledonides (Binns, 1978; Andresen et al., 1985; Andresen, 1988). The Lyngen

0 L m. 2. Simplified geological map of the

T r o w area to show the distribution of eclggites (E), and the localities referred in the text.

to

ECLOCITES FROM THE TROMS AREA, NORWAY 291

Nappe or Lyngen Composite Terrane, which underlies the TNC, is dominated by an upper OrdoviaadSilurian dominantly sedimentary sequence - the Balsfjord group (Andresen & Bergh, 1985). In certain areas the Balsfjord group has a depositional contact against the Lyngen gabbro, now interpreted as an ophiolitc fragment (Minsaas & Sturt, 1985). The metamorphic grade of the Balsfjord group is generally in the middle to upper greenschist facie, but locally shows an inverted metamorphic gradient up to kyanite grade (amphibolite faaes) towards the overlying TNC (Humphreys, 1981; Kristensen. 1983).

Reconnaissance mapping has shown that the TNC can be subdivided into three major lithotectonic units (Fig. 2). The lower part is not well investigated, but appears to be

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

. . . . . . TROMSDALSTIND SEOUENCE

TROMSB NAPPE COMPLEX

wlECLOGITE AND SERPENTINITE BODIES

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . SKATTBRA GNEISS

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . + ::: A

292 E. 1. KROCH ET AL.

dominated by folded quartzo-feldspathic gneisses, locally migmatized, with minor bodies of amphibolite and granitoid intrusions. This lower part is overlain by a middle sequence composed mainly of banded and foliated, and partly migmatized amphibolites and amphibole gneisses (Skattara gneiss) locally containing layers of anorthositic composition and pods and lenses of pyroxenite and serpentinite. Characteristic of the Skattara gneiss are numerous cross-cutting dykes ranging in composition from diabase via diorite to oligoclasite (Landmark, 1953). The dykes show little or no sign of ductile deformation, except towards the overlying Tromsdalstind sequence (Broks, 1985), but are locally cut by late brittle faults. The plagioclase making up 90-100 per cent of the otigoclasitic dykes has the composition An,,=. The Skattera gneiss is particularly well developed and exposed in the area around the city of Tromse, but apparently pinches out southwards and westwards (Kristensen, 1983).

The dominantly supracrustal Tromsdalstind sequence (Andresen et al., 1985), which forms the upper part of the TNC, includes marbles, calc-silicate rocks, garnet amphibolites, garnet-mica schists and gneisses (locally kyanite-bearing), quartzo-feldspathic gneisses, hornblende-biotite gneisses, biotite-minocline gneisses, game- tiferous clinopyroxene gneisses (high-pressure granulites), ultrabasites and eclogites. The boundary between the Skattara gneiss and the overlying Tromsdalstind sequence is strongly mylonitized, and in places clearly truncates the layering of the latter (Broks, 1985).

The main phase of the Caledonian deformation and metamorphism in Finnmark took place around 550- MOMa (Finnmarkian phase; Sturt, Pringle & Roberts, 1975; Sturt, Pringle & Ramsay, 1978; Dallmeyer, 1988), with a later thermal overprint at 4 1 w Ma (Sturt et d., 1975, 1978; Dallmeyer, 1988). The presence of upper

Ordovician/Silurian fossils in the Balsfjord group (Olaus- sen, 1977; Bjarlykke & Olaussen, 1981; Binns & Matthews, 1981) and in the Goulas marble of the Vaddas terrain (Binns & Gayer, 1980) testify that the main deformation in Central Troms is of Silurian or younger age. The nature as well as the location of the boundary between rock units involved in these two orogenic phases, the Finnmarkian phase and the Scandian phase, is obscure. However, it certainly has to be below the Vaddas terrain (Fig. 1). The preserved depositional basement (Lyngen gabbro) to the Balsfjord group suggests that rocks of ‘Finnmarkian age’ may be involved in the zone of Scandian deformation, shuffled in between rocks of upper Ordovician/Silurian depositional age. It is not clear whether the rocks of the TNC have been through only the Scandian phase of deformation, the Scandian and the Finnmarkian phases, or the Scandian and Finnmarkian phases plus one or more Precambrian deformational events. However, an internal Sm-Nd isochron based on Cpx, Am and Grt from a partly retrograded and recrystallized eclogite sample (TE-4) yields an age of 598 f 107Ma (Griffin & Brueckner, 1985). which puts a lower age limit on the eclogite formation.

FIELD OCCURRENCE OF THE TROMS0 ECLOCITES

The TromsB eclogites were first described by Petersen (1878). and later by Endell (1913). These earlier workers considered the eclogites and their retrograde alteration products to be metamorphosed gabbros which had intruded supracrustal rocks. In a later description Landmark (1973) suggested an origin by metamorphism and metasomatism of Mg-rich calcareous sediments.

Q. 3. Elliptical lens (boudin) of eclogite in impure marble, Lanesodden.

E C L O G I T E S FROM T H E T R O M S A R E A , N O R W A Y 293

Three main Occurrences of eclogite and related rocks are known within the Tromsdalstind sequence (Landmark, 1973). These are Lanesodden, Btintuva and Tromsdalstind (Fig. 2). The eclogites appear to be confined to the marble dominated lower part of the Tromsdalstind sequence. The size of the individual bodies vanes from a few centimetres up to several hundred metres.

lanesodden

On the southern tip of TromsB Island, at Lanesodden (Fig. 2) numerous metabasite lenses occur in intensely folded schistose calc-silicate rocks and marbles (Fig. 3). Most of the mafic lenses are slightly ellipsoidal in shape and seldom exceed 5m in length. Their mineralogical composition is variable and includes dark garnet amphibolites (with kelyphitized garnets up to lm large in places), black hornblendites, grey-green diopside-tremolite rocks, green garnet-clinopyroxene gneisses and dark green retrograded eclogites. Some of the eclogitic bodies are distinctly zoned. In some cases they have a blackish green hornblendized marginal zone, and in other cases an inner marginal zone enriched in garnet, with an outer margin consisting mainly of fine-grained dull green clinopyroxeneftremolite- plagioclase symplectites. In this outer zone the relative amount of tremolite increases towards the surrounding calc-silicate marble. Amphibolitization along fractures in the eclogites is also common. Quartz has segreBated along the margin of, as well as inside, some eclogite bodies. In the latter case amphibole is commonly associated with the quartz segregations. One spectacular eclogite-like calc- silicate rock (represented by sample 1A) occurs as a c. 5 an thick boudinaged and isoclinally folded layer within the foliated silicate marble.

Pegmatitic schlieren of plagioclase, quartz and black equidimensional hornblende (up to 3 cm across) are found commonly within the eclogites. A zone of garnetite locally separates the pegmatite from the eclogite host rock. In other places the garnet is evenly distributed throughout the pegmatitic schlieren. The Occurrence of similar pegmatites outside the eclogite bodies may suggest that they represent transposed dykes which originally cut both basic inclusions and enclosing marbles and calc-silicates. However, some of them may as well result from an in s#u partial fusion of the host eclogite.

Basic rocks of variable composition are found as lenses in both mica schist and marble at and around Bantuva (Fig. 2), a small mountain peak on the mainland just east of T r o w . The mountain top itself is made up of a c. 200 x 100 mz body of magnetite-rich garnet-clinopyroxene rock overlying a coarse-grained marble. Several smaller basic lenses, including both garnet amphibolites and garnet-clinopyroxene rocks are found in the underlying marble as well as in the interbedded mica schist. The garnet-clinopyroxene rocks comprise both eclogites and calc-silicate rocks. The relative proportions of garnet and

clinopyroxene vary considerably in the calc-silicate rocks, from almost pure garnetites to a clinopyroxene to garnet ratio of c. 70:30. Plagioclase-amphibole pegmatites commonly cut the garnet-clinopyroxene rocks. Horn- blende, commonly as large equidimensional poikiloblasts, is observed in many of the garnet-clinopyroxene rocks. Garnet is generally flattened in the plane of foliation and surrounded by black hornblende (Kelyphite). The original omphacite in the eclogitic rocks is commonly altered to pale green fine-grained symplectites. Other garnet- clinopyroxene-bearing rocks include garnetiferous q u m - feldspathic gneisses and green garnet-clinopyroxene gneisses. Layers of garnet quartzites with up to 50% garnet are also present.

Tromsdalstind

The largest of the Tromscb eclogites forms a banded body at the summit of Tromsdalstind (Fig. 2), a l238m high peak east of the city of Tromscb. This large (approximately 0.4km’) eclogite body commonly has a distinct foliation, with aggregates of reddish brown garnet in a pale green matrix of symplectitized omphacite, flattened in the plane of foliation. Less common are weakly foliated to unfoliated eclogites with essentially red-brown garnet and grass green omphacite. In the latter varieties, the grain-size varies considerably from layer to layer. Garnets may vary from C0.5 mm up to 3 mm, and omphacite from 0.5 mm to 10 mm. There are also variations in the relative proportions of garnet, and omphacite, the latter being dominant in the coarser varieties. In some of these coarse-grained layers lath-like aggregates of garnet occur, giving the rock of gabbro-like texture. White zoisite is commonly seen as randomly oriented and evenly distributed poikiloblasts in the garnet-clinopyroxene matrix, and it also occurs with kyanite in irregular lenses and pods in some parts of the eclogite. Some layers of eclogite contain pale blue kyanite as a matrix mineral. Intimately interlayered with the eclogite are layers and bands of garnetiferous hornblende schist and meta- trondhjemite/tonalite (internal gneisses, Fig. 4). The latter rocks contain sodic plagioclase + quartz + clinopyroxene + garnet as major phases. Locally minor kyanite and phengite are observed in the more felsic varieties. A gradational change in mineralogical composition in terms of relative amounts of mafic (garnet and clinopyroxene) and felsic (plagioclase and quartz) minerals, from a plagioclase-free eclogite, via intermediate compositions to a garnet- and clinopyroxene-poor meta-trondhjemite has been observed locally. Rutile porphyroblasts, up to 4 an in length, are locally formed along cracks and fault surfaces. The eclogite is cut by granitic and plagioclase-amphibole- rich pegmatites.

STRUCTURAL EVOLUTION OF THE ECLOGITE HOST ROCKS

Four major deformationdl events are recorded in the Tromsdalstind sequence (Kristensen, 1983; Broks, 1985).

2~ E. J . KROCH Er A L .

The D, episode is only locally recognized by an internal foliation (S,) in the earliest (late- to post-D,) formed garnets, now occurring in the metapelites together with plagioclase, kyanite, biotite and phengite as pre-kinematic porphyroclasts relative to the dominant plane foliation (S,) (Figs 6a and 7). The D2 episode is characterized by tight to isoclinal folds ranging in scale from microscopic to macroscopic, with formation of a transposition foliation, locally defined as a penetrative axial plane cleavage to the F, folds (Fig. 5) . S, can be recognized as banding in gneisses and marbles, and as schistosity in mica-rich gneisses. Lenses of mafic and ultramafic rocks within gneisses and calc-silicate rocks are always oriented with their shortest axis perpendicular to the S2 plane. Pegmatitic dykes, lenses and xhlieren are also oriented parallel to &.

The D, episode is characterized by asymmetric microscopic to mesoscopic close to tight folds, which in places are refolding F, folds. In mica-rich rocks S , is defined by a crenulation cleavage, microscopically by a non-penetrative zonal cleavage defined by bent and polygonized D2 mica along the cleavage plane. In more competent rocks S3 is locally developed as a fracture cleavage parallel to the axial plane of mesoscopic F3 folds. S3 is always oriented at an angle to S2. The thrust zone

between TNC and the underlying Lyngen Composite Terrane (LCT), as well as the upper part of Lm itself has been folded by F, me- and macroscopic folds.

The D, episode is characterized by open asymmetric mesoscopic to macroscopic parallel folds. A secondary crenulation cleavage S, is locally developed in mica-rich rocks, but is absent in more competent rocks. F4 microfolds are seldom developed, but are characterized by bending of S, and/or S, micas with formation of a weakly developed conjugating S, zonal cleavage, when observed.

PETROGRAPHY

Metapelites

Microtextural studies of mineral growth in metapelites related to the defonnational episodes D,-D, in the Tromscialstind sequence reveal a complex deformational evolution. The dominant foliation in the area is the S, foliation. All the other defonnational/metamorphic stages are here related to S,.

D, (pre-D,) mineralgrowth

Minerals formed during the earliest recognizable deformation/metamorphism are found as planar inclusion trails (S,) at an angle to the dominant matrix foliation &), in late syn- to post-D, garnets (Fig. 6a). Common inclusion minerals are Qtz, PI, Bt, Phe, Ep. n m and Rt. h r g e grains of Bt, Phe, Ky and PI, all showing undulatory extinction, and with the S, foliation wrapping around, are interpreted to be isofacial with the earliest garnets (Fig. 7).

+ 1 cm , Fq. 5. Quartz string. probably representing S, , isoclinally folded by F, folds, with development of S, axial plane cleavage. Garnet mica gneiss. Bentuva.

D, mineral growth

Early syn-D, and late syn- to post-D, garnets are locally common. Within the early syn-D, garnets weakly

ECLOCITES FROM THE TROMS AREA, NORWAY 2 S

a b

- \ , - - ‘ C d

Fig. 6. Textural relationships of garnets in mctapclitts: (a) late syn/pt-D,-pre-D,, Malangshalveya; (b) w l y sp-D, garnet. Malanghalv@ya; (c) late syn- to post-D, garnet, Mdanghalveya; (d) p 6 t - D ~ g a t . Malangahatveya.

crenula td S, is preserved, while the external S, foliation wraps around these garnets, with the formation of quartzcnriched pressure shadows (Fig. 6b). In the late syn- to post-D, garnets the internal foliation in the marginal parts of the garnets is continuous with the external S, foliation. This foliation is also bent around the core of the garnet (Fig. 6c). Minerals included in D2 garnets are mainly Qtz and PI, but locally Pg, Mrg. Cld and St occur. The S, foliation is defined by parallelism of Bt and Phe and locally Pg, with porphyroblasts of PI and Qtz (locally Ky and St). Fibrolite has locally grown on interfaces between Grt and PI, white mica forms after Ky, and chloritization of Grt and Bio is also observed.

D3 mineral growth

Post-D3 Grt is locally seen overgrowing F3 microfolds (Fig. 6d) with a helicitic inclusion pattern. The rims observed on

D2- Phe

1 mm W

4 . 7 . Pre-D, (D,?) porphyroclasts of plagioclasc and phengite in garnet mica gneiss, Malangshalveya.

some late syn- to post-D, garnets may have grown. st-D,. Unstrained polygonized and recrystallized Bt and Phe OCCUT in the S, cleavage plane, and are thus related to the D, episode. Randomly oriented chlorite overgrowing F3 folds has grown po~t-D3.

D. No mineral growth related to the D, episode has been observed.

Edogites (I) Primary phases observed in the Troms0 eclogites are

Grt (Grt I) + Omp (Cpx I) f Am (Am I), Qtz, Ky, Zo, Phe, high-Al (>6wt % A1203) Ttn, Rt and Cal. Parallel oriented Omp (and occasionally Am) define the earliest recognizable foliation in the eclogites. In a few relatively coarse-grained samples from the Tromsdalstbd edogite, larger (up to lonun), strained early Omp (Cpx 1’) porphyroclasts OQW in a finer grained mosaic textured matrix of Cpx I. In some of these coarser-grained samples from the Tromsdalstind cclogite lath-shaped aggregates of garnet are common, as are corona-like textures of garnet around omphacite aggregates. Garnets may contain minute inclusions of Rt. Qtz, Bt, Phe, brown Am (AmIi) and l d y omphaate (CpxIi). Growth of Zo seems to post-date dl the other primary minerals, but it nevertheless appears to be part of the stable eclogite facie assemblage. In one sample Ky and Cal together with Grt and Omp occuf in mutual textural equilibrium with each other (Fig. 8).

2 S E . J . KROGH E T A L .

1 mm t I

Fii. 8. Sketch from micrograph of cclogite from Tromsdalstind, showing textural relationship berween garnet, omphacitc (with incipient sympldtization), kyanite and calcite.

Secondary (post-eclogite) alterations are common. The following breakdown reactions have been observed.

(IIa) Breakdown of Omp I to fine symplectitic aggregates (Fig. 8 ) of PI (PI IIa)+high-Al (>4.5wt % A1,03) Na-Aug (Cpx IIa) and/or Am (Am na'), gradually recrystallizing to granular Cpx + PI or a coarser grained assemblage of PI (PI Ha") + Am (Am IIa") along the rims of the symplectitic aggregates, or to large equidimensional poikiloblasts (up to I0 mm) of brown Am (Am IIa"') in a PI-rich matrix.

(IIb) Secondary growth of Grt (Grt IIb) on primary garnet, or as atoll Grt (Grt IIb') around Cpx I.

(Ilc) Reaction between Grt and Qtz to produce coronas of low-A1 (<2.0wt% A1203) Na-Aug (Cpx IIc)+PI (PI IIc) or Opx (Opx IIc) + PI.

(Ild) Kelyphitic rims of dark green Am (Am IId) + PI (PI IId) + Bt + Mag around Grt.

(He) Rims of low-Al (c2.5 wt % AIZ03) Ttn (Ttn IIe) or Ilm on Rt.

(IIf) Lath-shaped symplectitic aggregates of Bt + PI + Ms replacing primary Phe.

At an apparently later stage the further reactions listed below occur.

(ng) Reaction between Cal and Qtz to yield Am. (IIh) Myrmekitic intergrowth of Ep + Qtz surrounding

GI-t. (IIi) Reaction between secondary PI (PI 11) and

Fe-oxide to form intergrowths of Ep and Qtz. (IIj) Growth of secondary Grt as atolls around Cal, Bt

and secondary PI (PI n). (IIk) Growth of late large poikiloblasts of Am

containing inclusions of atoll Grt, Cal, PI, Bt and Ttn. This may be coeval with the poikiloblasts described in IIa"'.

(111) Total recrystallization to a mosaic textured assemblage of Grt + Cpx + Am + PI f Ep + Qtz + Ttn.

The alteration phenomena described above are more pronounced in the smaller Lanesodden and B~ntuva

eclogite bodies than in the larger Tromsdalstind eclogite. In a few localities an almost total sequence of reactions and recrystallization has been recognized.

Internal gneisses

Primary phases in gneisses intercalcated within the large Tromsdalstind eclogite body (internal gneisses) are Grt (Grt I) (*Omp/Jd (Cpx I)} + PI (PI I) + Qtz f Ky (* Phe)+ Rt + Mag (phases in { - ) are only indirectly observed from breakdown textures). The following alteration phenomena are observed in the internal gneisses.

(Ia) Breakdown of primary Omp/JdI to a coarse- grained aggregate of secondary Omp (Cpx Ia; NaMz= 0.30) + PI (PI Ia).

(IIa) Breakdown of secondary Omp Ia to symplectitic aggregates of coarser high-A1 (>5 wt % A1203; Cpx IIa) or finer low-Al (<4wt % Al,03) Na-Aug (Cpx IIa') + PI (PI IIa') (Fig. 9a). and breakdown of primary Omp/Jd I to fine-grained granular aggregates of low-Al (<4 wt % Al2O3) Na-Aug (Cpx IIa) + PI (PI Ira) (Fig. 9b).

(IIb) Secondary growth of Grt (GrtIIb) on primary garnets, or as atoll Grt (Grt IIb') around Cpx Ia.

(IIc) Reaction between Grt and Qtz to produce coronas of I O W A (<4wt% A1203) Na-Aug (CpxIIc) and PI

(IId) Breakdown of Grt to aggregates of dark green Am

(ne) Aggregates of intergrown St +Bt +P1 in Ky-

(IIf) Lath-shaped aggregates of symplectitic inter-

(IIg) Dark green Am replacing Cpx IIa.

(PI nc) . (Am IId) + Mag + PI (PI IId).

bearing gneissts seem to replace Grt + Ky.

growths of Ms + Bt + PI replacing primary Phe.

External gneisses

Garnet clinopyroxene-bearing trondhjemitic to tonalitic gneisses from other parts of the Tromsdalstind Sequence (external gneisses) commonly have the granular textured apparently stable mineral assemblage Grt + low-Al (< 3 wt % Al,03) Na-Aug (Cpx 1Ia) + P1 (PI IIa) + Qtz + Ttn (Fig. 9e). Additional phases may be brownish green Am (Am 11). Bt, Ep and Mag. Parallel oriented minerals and mineral aggregates commonly define a foliation parallel to the dominant S, in the metapelites. In a few samples larger strained grains of high-A1 (>5 wt % A1203) Na-Aug have a massive inner zone (Cpx Ib) partly transformed into symplectitic low-Al Na-Aug (Cpx IIa') + PI (PI IIa') along cracks and the rim (4. Fig. 9c). In other samples aggregates of granular low-Al Na-Aug (Cpx IIa)+PI (PlIIa) occur, probably pseudomorphing an earlier high-Al Na-Aug (Fig. 9d). Myrmekitic intergrowths of green Am (Am 111) + Qtz commonly replace Cpx 11.

Amphibdites

Gamet-bearing amphibolites within the Tromsdalstind Sequence commonly show a mosaic textured Grt+


m. 9. Textural relationships for breakdown of clinopyroxenes. (a) Breakdown of omphaate (Cpx la) to Na-augite (Cpx Ua’) + plagiodasc (An,,) sympleaite (see Table 2). Internal tonalitic gneiss, sample 82010, TromsdPlstind. (b) Aggregate of recrystallized Na-augite (Cpx Ira) + albite (As). Internal troodhjemitic gneiss, sampk 82002, Tromsdalstind. (c) Breakdown of high-Al Na-augite (Cpx Ib) to low-Al Na-augite (Cpx IIa’) + plagiodase (An,,,) sympkctite (see Table 2). Amphibolite. sample 2753. Ramfjord. (d) Aggegate of recrystallized low-Al Na-augite (Cpx IIa) + plagioclasc (An,) probably pseudornorphing an earlier lugh-AI Na-augite. External tonalitic gneiss, sample 1K. Lanesodden. (e) Totally recryslallized D2 assemblage of garact + low-Al Na-augite + plagioclasc + quartz. External tonalitic gneiss, sample TD-2, Beotuva.

so E . J . KROCH Er AL.

brownish Am f low-Al Na-Aug + PI + Qtz + Ep, defining a foliation parallel to S, in the metapelites. Additional phases are Bt, Mag, Ttn and Cal. In some cases the amount of Na-Aug exceeds that of Am. As in the gneisses, two generations of Na-Aug can be found, where larger strained early high-Al Na-Aug (Cpx Ib) is partly replaced by symplectitic low-Al Na-Aug (Cpx IIa’) + PI (PI IIa’) intergrowths (Fig. SC). Myrmekitic intergrowths of green Am + Qtz after Cpx, and Ep + Qtz (after Grt?) are commonly observed. In a few cases coronas of Am + Ep+ PI + Qtz in intimate intergrowths are seen replacing Grt.

green

Garnet-clinopyroxene calc-silicate rocks

The garnetiferous calc-silicate rocks essentially contain Grt+Cpx. Additional phases are Zo, Ep, Qtz, Ttn, Cal and Dol. Secondary Am locally replaces Cpx, and may also appear as large (up to 10mm) equidimensional poikiloblasts in a similar manner as in the eclogites. Other secondary features are intergrowths of Am+Qtz, and reaction between ZO and Cal to yield Scp. The content of Mag is locally very high (up to 30%). The Grt and Cpx in the Mag-rich calc-silicate layers are darker brownish red and darker green respectively, than in the ordinary calc-silicate assemblages.

Phgioclase-rich pegmatites

Plagioclase-rich pegmatites occur commonly in the eclogites, amphibolites and calc-silicate rocks and they invariably contain large (up to 20mm) short-prismatic brownish green Am and large (up to 8 nun) euhedral Ttn. In some pegmatites within eclogites relict reddish-brown Grt is common and evenly distributed in the plagioclase-

amphibole matrix. Minute paler brownish Grt grains may occur as sparse inclusions in the poikilitic amphiboles (e.g. sample Ml). Na-Aug is locally present in the pegmatites, both as minute inclusions in the poikilitic amphiboles and as larger grains in the matrix. Ep is present as a minor phase.

M I N E R A L CHEMISTRY

Mineral aaalyses were done on an ARL-EMX electron microprobe fitted with a LINK energy dispersive system at h4incralogisk-Geologisk Musturn, Oslo. Matrix corrections were done by the m-4 programme of Statham (1976). Various natural and synthetic standards were used for calibration.

Garnet

Tabk 1 prescnts selected analyxa of garnet from the different lighologies in the Tromsdalstind Sequence. The compositions of the analysed garnets with respect to the mole proportions of Mg, Fe, Mn and Ca are also presented in Fig. 10. Generally the eclogite garnets arc the most Prp-rich while garnets h m the calc-silicate rocks and recrystallized gneisses are rather Gn-rich. Garnets from metapelites generally have the lowest content of Gn. High Sps contents (up to 19mol. %) are found in garnets from some of the external gneisses. Then is a slight overlap in composition between garnets from the cclogites and garnets from the internal gncisses. In the metapclites D, garnets am distinctly more Ca-rich than rhe D, and D, garnets, which overlap in composition. Zoning is generally abscnt in Grt from cclogites, internal and external gneisses, but a slight outward incrcasc in the Mg/Fe ratio has been found in some samples. Garnets from metapelites show more distinct zoning. Tbc late-syn- to post-D, garnets commonly show either a constant or a slightly outward decrease in the Mg/Fc ratio, while there is a relatively strong incrcase in Ca towards the rim (Fig. lla). Sp-D, garnets from the metapclites show more pronounced zoning mth an outward incrcasc in the Mg/Fe ratio, than do the D, garnets (Fig. llb). In contrast, post-D, garnets show an o w a r d decrease in the Mg/Fc ratio (Fig. llc).

Tabk 1. Selected analysts of garnet. ECL = cclogitc; IG = internal gneiss; EG = external gneiss and amphibolite; CS = calf-silicate rock; MP = metapelite. The different stages are d d b c d in the text.

s.mpLm. 1338 1338 82017 82017 I A gzmeB gzmeB gaOl0 82010 2753 IH T-3 TD-14 316 332 H4 448 45% 462 R&L ECL ECL ECL ECL ECL IG IG IG IG EG EG CS CS MP MP MP MP MP W hlc Ii I I I l h I I 1. IN II. 11. U. n. M, M, y M, M, M,

SiOa y1.u 3.a 3.34 r).w 39.n ~ . z a 38.65 B.IS B.OS 37.65 38.50 n . 6 1 38.46 38.17 n.70 n.m 3a.m 38.m n.15 TO1 0.07 o m 0.09 0.04 0.08 0.04 0.10 0.04 0.04 0.05 0.15 0.00 0.26 0.00 0.00 0.15 0.00 0.00 0.00

21.w 22.15 n . n 2275 n . ~ 21.75 21.w n.2~ n.01 21.37 n.05 21.43 21.65 21.75 ~ 1 . 5 5 21.70 21.91 21.4s 21.59 u.m n.u n . 4 0 22.55 15.65 24.63 23.n 22.35 24.m 23.61 n.34 1 8 . s ~ 10.29 26.32 m.01 m.70 30.49 3 1 , s 32.92

4 0 3 r+o Mno 1.75 1.37 0.49 0.61 0.39 0.59 O.M 0.52 0.80 1.01 0.69 4.40 0.20 1.76 0.85 ~ 4 2 1.00 2.23 0.35 w 5.25 6.55 9.45 9.07 6.47 3.40 6.18 7.19 5.38 2.50 2.67 1.33 0.60 4.11 4.26 6.14 5.55 4.56 4.84 I30 8.00 9.64 5.62 7.14 16.36 11.54 9.14 8.34 8.31 14.00 13.24 15.88 29.05 7.85 7.47 465 4.49 4.11 3.U

Twl 59.28 1 0 i . t ~ 59.66 1rn.00 i o i . ~ i m . ~ 1m.36 99.84 99.79 10u.u 100.64 99.0 im.51 99.96 59.85 1m.m 101.u 102.60 100.29

12 orycmr

Si 2.992 2.995 2.998 2.929 2.980 2.991 2.980 3.002 3 . m 2.962 2.591 2.978 2.951 2.997 2.976 2.931 2.958 2.999 2.948 Al 2.026 1.980 2.001 2.005 1.- 2.003 1.996 2.011 2.010 1.962 2.019 2.000 1.9% 2.013 2.005 1.983 2.010 1.962 ZQD Ti 0.004 0.m 0.m 0.m 0.005 0.002 0.m 0.m 0.m 0,003 0.m 0.m 0.015 0.m 0.m 0.009 0 . m 0.m 0.m Fe 1.m 1.424 1.428 1.410 0.981 1.609 1.533 1.433 1.568 1 3 % 1.516 I.= 0.660 1.m 1.850 1.958 1 . W 2044 2.185 Mn 0.116 0.m 0.032 0.039 0.025 0.039 0.W 0.034 0.052 0.068 0.045 0.29S 0.013 0.117 0.057 0.M 0.066 0.146 0.U2.4

MI 0.612 0.741 1.074 1.011 0.723 0.3% 0.710 0 . a 0.621 0.293 0.309 0.157 0.069 0.461 0.501 0.710 0.644 0.527 O.S%3 CI 0.671 0.784 0.459 0.572 1.314 0.966 0.755 0.685 0.690 1.180 1.101 1.347 2.388 0.660 0.632 0.306 0.374 0.341 0.293

TOUl 7.991 8.013 7.996 8.017 8.019 8 . m 8.016 7.990 7 . W 8.044 7.591 8.023 8.055 7.997 8.021 8.069 8.W 8.020 8.042

x, 0.226 0.258 0.153 0,189 0.432 0.321 0.245’ 0.230 0.235 0.381 0.371 0.442 0.763 0.Pl 0.m 0.123 0.122 0.112 0.095 0.m 0.029 0.011 0.013 0.008 0.013 0.012 0.011 0.018 0 . a 0.015 0.097 0.W 0.039 0.019 0.D.W 0.021 0.018 0.m

0.206 0.244 0.359 0.333 0.236 0.132 0.234 0.276 0.212 0,095 0.104 0.052 0.022 0.161 0.165 0.225 0.210 0.172 0.186

XMa x, 0 . m 0.469 0.477 0.465 0.322 0.535 0.505 0.- 0.535 0.m 0.510 0.409 0.211 o m 0.609 o m 0.647 0.668 0.711

xw

ECLOCITES F R O M THE TROMS AREA, NORWAY B9

pb. 10. Mole proportions of Ca-Mn-Fe- Mg for g a t s in various Lithologies within the T r o d s t i n d Sequence. 0 = eclogite; 0 = internal gneiss; 0 = external gneiss;

metapelite; 2 = D, garnets. metapelite; A = dc-silicate mh; 1 = D, g-ts,

3 = D, garnets, metapelite. - h + Un

Pyroxene

Full analyses of m e pyroxenes are presented in Table 2. and condcnscd compositions of all analysed pyroxenes are given in Table 6 and Fig. 12. Stage I d i n m o x c n e s in cdogitts are mostly ornpbacitic (Na, = 0.3Hl.44). although Na-augites (Na,, = 0.114.23) do occut (Fig. 12). Inclusions of Omp (together with Ab) in Grt in one sample (1338) have a higher Na content (Na,=0.54). Stage la dinopyroxenes in internal gncisteS are omphacitic to chloromelanitic in composition (Jd,7-U, A%-). A 'reconstructed' pyroxene based on modal proportiom of Na-augite (25%) and albite, A b , (75%) aggregates in internal gneiss sample 8uKn (Fig. %) indicates a jadcitic com-

0.5 mm a -4

Rim Core

b

potation ( J h ) .

0.045 CaAISiOs + 0.705 NaAlSi,O, (0.75 Ab, in PI)

+ 0.25 N+JFe, ~g)o.poAb.16Si1.5?606 (0.25 Na-Aug)

= 1 .a N%.7&+~4Fe, Mg)o,&b.&1.&6 + 0.74 SiO,. 1 .oO Jdm

High-AI Na-augltes (A1203 > 5 wt %) are found as (a) sympled- ites in cdogites, (b) coam symplcctites in internal gncisus and (c) larger grains in external goeisscs and amphibolitts. Low-Al

1.0 mm c_

0

0.5 mm C

A o

A A A A

16

10

Rim Core Rim

plr. 11. Zoning profiles of (a) D,. (b) D2 and (c) D, gatnets from metapetites. 0 = almanac; 0 = pyropt; O=spessamoc;A=grossullU.

300 E. J . KROGH E T A L .

T.bk 2. Selected analyses of pyroxene. Abbreviations as in Table 1. Formula calculation after Ncumann (1976). The different stages are described in tbe text.

-m. 1338 133s 8017 8017 8017 m i 7 IA mom garnee g~oga ~ 2 m s mi0 8010 smio 82010 2753 2753 2753 IH T-3 m i 4 Raci ECL u1. E a Ea JXL ECL ECL IG IG IG IG IG 10 IG IG EG EG EG EG CS CS S W li I I'c I'r I IIa I IJ Ua 11.' Uc la U. UJ' Ilc Ib 11.' Ila 1Ia IIa IIa

~~~~~

55.m Y.U 55.m 55.62 55.83 54.3 n.m n.25

4.78 6.08 4.81 4.m 5.08 5.78 3.68 8 . 3

0.11 0.11 0.15 0.16 0.14 0.11 0.04 0.25 12.99 9.31 9.78 10.28 10.27 6.51 4.28 8.12

0.08 0.14 0.03 0.02 0.05 0.05 0.M 0.13 7.08 9.44 8.98 8.74 8.95 I l .% 14.23 8.74

11.0.5 14.85 14.m 13.68 13.47 18.20 P.57 17.47 7.75 5.3l 6.38 6.43 6.32 3.61 1.67 4.15

99.72 99.n 99.95 99.81 100.11 im.% 100.n 99.44

n .M 0.18 4.51 7.78 0.15

11.18 19.57 2.84

99.25

si Mu'" dun

Fc'* Fc" L(n

MI c. N. Tow

Md.%Jd M.%k M . % A A . l

TI

1.583 1.960 0.017 0 . M 0.526 0.355 0.m 0.m 0.019 0.053 0.123 0.130 0.002 0.004 0.374 0.507

0.533 0.375

4.00 4.000

51.40 32.15 I.@ 5.32

46.72 62.53

0.420 o m

1.988 1.- 0.012 0.015 0.399 0.417 0.004 0.m 0.w 0.m 0.098 0.111 0.001 0.001

0.535 0.523 0.441 0.445

4.000 4.000

0 . m 0.465

m.51 41.41 457 3.48 u.93 5.552

1.9811 0.012 0.419 0.aY 0.023 0.129 0.m 0.475 0.514 0.436

4.00

41.36

56.1 zm -

1.953 1.953 0.M7 0.017 0.229 0.134 0.003 0.001 0.064 0.027 0.110 0.064 0.002 0.m 0.641 0.764

0.252 0.117

4.000 4.000

18.76 8.93 6.39 2.73

74.84 88.33

0 . m o m

1.920 0.080 0.271 0.m 0.091 0. 165 0.004 0.4W 0.688 0.296

4.000

20.46

m.44 9.10

-

1 .wo 0.040 0.156 0.005 0.m 0.163 O.CQ.5 0.616 0.ns 0.M)

4.000

n S 7

79.66 7 . n

52.54 52.6.l 52.81 9.15 0.m 0.03 0.25 0.30 2.51 2.14 8.39 5.49

10.73 11.19 6.26 7.21 0.18 0.16 0.07 0.M

11.06 11.35 9.89 11.37 19.66 19.17 17.08 19.n 2.42 2.10 4.22 2.45

99.17 99.57 98.98 99.79

4atar r ;6oxygms

1.966 1 . w 1.931 1.954

0.076 0.W 0.293 0.1s 0.002 0.001 0.001 0.006

0.206 0.178 0.130 0.210 0.- 0.005 0.m 0.m 0.617 0.629 0.539 0.623 0.788 0.164 0.669 0.m 0.176 0.1% 0.299 0.175

4.000 4.000 4 . m 4.000

o m o.ms 0.w 0.016

0.m 0.m 0.062 0.01~

4 s 2.18 23.73 16.26 12.97 16.99 6.19 1.m a.45 m.53 70.09 &?.Y

51. 53 0.26 3.97 7.91 0.06

11.56 19.71 2.13

98.13

n.iz 51.24 0.12 0.25 3.44 5.60 7.35 8.65 0.05 0.08

13.36 10.55

1.79 1.78 m.71 m.65

1 0 0 . ~ 9a.m

52.21 51.53 52.44 52.76

3.09 2.96 2.97 0.89 7.93 8.09 9.50 11.22 0.08 0.07 0.10 0.52

11.97 12.12 11.56 10.67 21.90 21.61 22.57 U.30

1.17 1.23 1.10 0.56

0.25 0.26 0.05 0.00

98.60 9u.m im.29 99.92

50.92

1.65 12.95 0.34 R.%

22.48

0.m

0.87

98.17

1.972 0.028 0.148 0.m 0.021 0.228 0.m 0.647 0.793 0.155

4.000

13.44 2.06

84.50

1.968 1.m 0.m 0.m 0.115 o.1m 0.003 0.m 0.m 0.023 0.lM 0.248 0.m 0.00) 0.724 0.590 0.m 0.830 0.126 0.129

4 . m 4 . m

8.90 10.60 3.7'2 2.35 81.38 m.05

1.964 1.910 1.950 1.999 0.036 0.030 0.050 0.001 0.101 0.101 0.080 0.038 0.m 0.0117 0.001 0.m 0.005 0.m 0.046 0.m 0.244 0.m 0.250 0.352 0.003 0.002 0.003 0.017 0.671 0.678 0.641 0.602 0.883 0.868 0.899 0.946 0.085 0.069 0 . 0 0.011

4.000 4.000 4.000 4 .00

7.99 8.62 3.36 3.R

91.46 91.M 92.07 95.89 0 .n 0.33 4.58 0.39

1.975 0025 0.051 0000 0.039 0.381 0.01 I 0.518 0.594 0.065

4.000

2.62 3.93

93.46

Chloromlarrte

0

Omohactto

Fig. 12. Compositional variations of clinopyroxenes from various lithologies and stages, Tromsdalstind Sequence. 0 =

0 = internal gneiss. stage I; x = internal 0 eclogite, stage I; 0 = cclogitc. stage n;

0 0 .

AUQ 10 20 30 40 50 60 II; A = calc-silicate rocks. A mr. .z* gneiss, stage II; 0 = external gneiss, stage

ECLOCITES FROM THE TROMS AREA, N O R W A Y 341

Tabk 3. Selected analyses of amphibole. Formula calculation after Neumann (1976). Abbreviations as in Table 1. The different stages are described in the text.

s.mp*m. 82015 ZP ZP ZP zp 2753 M-1 St-1 ROCL ECI EKL Ea ECL ECL EG PEOM OUG SurC I li nd nd I I r Il 11

42.93 0.98 15.74 8.55 0.00

14.38

3.69 0.00

97.63

11.56

6.146 1.854 0.802 0.106 0.m

0.000 3.069

1.024 0.000

u.790

o . 7 ~

I .m

41.90 50.23 43.71 43.60 42.95 42.m 0.M 0.29 0.45 1.23 0.92 0.98

18.74 6.111 14.19 13.57 U . R 14.10 13.68 10.37 12.89 11.91 16.05 13.55 0.00 0.15 0.13 0.12 0.02 0.12

10.54 11.64 1140 11.55 11.74 11.62

0.42 0.16 0.37 0.56 0.16 1.68

10.33 15.43 11.n 11.110 1o.m 10.89

2.94 1.26 2.13 2.m 2.37 1.80

w.41 96.33 w.59 w.n 97.13 97.51

23 0-M; a h - (C.+ Na+ K) = 13.ooO

5.955 7.234 6.418 6.429 6 397 6.330 2045 0.766 1.m 1.571 1.a 1.670 1.094 0.Y)P 0.874 0.787 0.631 0.792

0.671 0.M 0.346 0.128 0 . W 0.136 0.955 0.909 1.237 1.340 1.695 1.543 0.000 0.018 0.016 0.OU 0.003 0.015

1.605 1.796 1.793 1.825 l.gl4 1.W 0.810 0.352 0.606 0.629 0 . W 0.517 0.076 0 . 0 0.069 0.105 0.030 0.318

15.491 15.177 L3.W 15.559 15.588 15.679

0.m o.mi 0.050 0.136 o m 0.109

2 . m 3.312 2 . 4 ~ 1 2.593 2.m 2 . a

42.71 1.14 13.09 IS.23

10.63 I 1 92 2.84 0.65

98.49

0.m

6.314 1.686 0.594 0.127 0.126 17% 0 010 2.M 1888 0.814 0.123

u.824

bomblende, while poikilitic (Am na*) and kelyphitic (AmIIc) amphiboles from the cclogites, poikilitic amphibole from the PI-rich pegmrtita, and amphibole porphyroblasts from amphibolites are feman pargasitic horoblendes of very similar COmpoeitiOnS.

Mia

Selected malyscs of micas arc presented in Table 4. Phcngites WM ocw as inclusions in Omp in eclogite have relatively high

Si'+ contents (6.68-6.74 based on 22 oxygcns). Larger strained prc-D, grains in the metapelites have an average Si4+ content of 6.45. while that of D, phengites lies around 6.20. D, phengites have values in the range 6.12-6.16.

Tiinite

Selected d y x s of titanite are presented in Table 5. Primary Al-rich titanite from sample 1A is zoned. Maximum substitution of Al for li in the core is 48.4%. decreasing to c. 25% at the rim. A similar degree of substitution (25%) is found in titanite from other eclogites in the area. Slightly lower substitution (15-24%) is found in titanite from external gncisses and calc-silicate rocks. !kmndary titanitc. which replaces rutile in eclogites, and recrystallized titanite in the amphibolitcs and PI-Am-rich pegmatitcs has significantly lower degree of substitution (5-1096). Semiquantitative analyses show up to 2.5 wt 96 F in the core of titanite in ample 1A. The maximum substitution of Ti by Al coupled witb the high F content of the titanite is similar to that reported by Franz & Spear (1985) from thc Tauern Window. A Substitution of c. 25 % is also reported from some of the eclogites from the Western Gneiss Region of Norway (Smith 1 9 n . 1980; Smith & Lappin 1982).

CEOTHERMOBAROMETRY

The physical conditions at various stages of apparent equilibrium in the rocks described from the Tromsdalstind sequence are evaluated using the methods described below. For geothermometry on the eclogites, internal and external gneisses and amphibolites the Grt-Cpx (GC) calibration of Krogh (1988). combined with calculation of femc/ferrous ratio in Cpx by the method of Neumann (1976), has been used. The Grt-Bt (GB) geothermometer preferred here (Perchuk & Lavrent'eva, 1983) for the metapelites has been evaluated elsewhere (Chipera & Perkins, 1988) to give the most precise and accurate results of all the available GB geothennometers. This method does not, however, take into consideration the effect of

T.Lk 4. Selected analyses of mica. Abbreviatioas Bs in Table 1. The merent Sampkm.

11.84 1H 316 322 322 H4 H-4 H4 448 448 45% 45% 462 462 Rock E C L E G W MP MP MP MP MP UP MP MP MP MP MP

stages are described in the text. Miuml h B 1 B1 B c P b c B t h P # B ~ P b c B t P b e B ~ p b c DI, 4 'I DI DI % D2 6 4 4 Dl D3 D3 D3

50.48 37.00 37.35 37.31 49.07 3700 45.10 46.30 37.28 45.06 37.07 46.05 36.65 45.67 1.06 2.52 1.44 1.94 0.78 1.62 0.79 0.00 1.14 0.42 1.44 0.24 1.60 0.33

0.82 16.17 15.18 16.09 1.62 14.10 3.09 1.53 14.111 3.41 15.21 l.W 16.20 2.12

3.71 13.45 13.15 12.42 2.57 14.30 1.06 0.00 13.76 1.09 13.36 1.65 12.81 1.43 0.00 0.16 0.04 0.00 0.04 0.09 0.02 0.35 0.04 0.00 0.10 0.05 0.04 0.02 0.39 0.14 0.50 0.39 0.70 0.67 1.37 6.23 0.62 1.53 0.47 2.02 0.30 1.94 11.07 9.13 8.78 9.28 10.87 9.25 9.57 1.86 9.11 9.29 9.21 8.45 9.00 8.58

27.41 15.80 18.67 18.30 30.94 18.011 =.a 38.m 18.40 n.30 17.57 3.n i8.m 3.9

0.08 0.18 0.16 0.18 0.03 0.07 0.00 0.07 om 0.011 0.13 0.00 0.09 0.00

9s.m 94.~5 95.n 9s.91 %.a s.18 n u ) 94.62 9s.i~ 93.18 94.s 94.69 s.08 94.68

22 0-

6.741 5.592 5.536 5.535 6.- 5.492 6.lW 6.022 5.536 6.203 5.565 6.19 5.m 6.116 1.259 2 . 4 6 2.464 2.465 1.531 2.508 1.810 1.978 2.464 1.797 2.435 1.643 2.515 1.884 3.056 0.- 0.W 0.734 3.276 0.655 3.431 3.878 0.757 3.444 0.674 3.557 0.728 3.576 0.1Of1 2.286 0.161 0.216 0 . a 0.181 0.W 0.tCO 0.127 0.043 0.163 0.024 0.180 0.033 0.092 2 . W 1.8112 1.9% 0.179 1.750 0.355 0.166 1.838 0.393 1.910 0.219 2.W 0.237 0 . W 0 . a 0.m 0.U2 0.003 0.009 0.000 0.006 0.000 0.009 0.017 0.000 0.011 0.000 0.736 3.W 2.905 2.746 0.93 3.164 0.217 0.ooO 3.016 0.224 2.909 0.329 2.m 0.2115 O.Oo0 0.W 0.006 0.000 0.006 0.014 0.003 0.049 0.006 0.mO 0.016 0.W 0.006 0.003 0.101 0.041 0.144 0112 0.179 0.193 0.365 1.586 0.179 0.408 0.137 0.524 0.087 0304 1.886 1.760 1.660 1.7% 1.828 1.752 1.676 0.312 1.7215 1.632 1.764 1.441 1.718 1.466

T d 13.m 15.616 15.575 15.563 14.053 15.718 14.128 13.94) 15.678 14.153 15.666 14.101 15.616 14.1M

312 , E . J. KROCH € T A L .

Tabhe 5. Selected analyses of titanite. Abbreviations as in Table 1. The different stages arc described in tbe text.

SMlple110. T-24 Rock ECL stage Ib

SiO, 31.81 TiOz 31.04 A 2 0 3 6.71 FCO 0.36 MnO 0.06

0.12 29.70

MsO G O Total 99.86 Si 1.001 Al 0.249 Ti 0.733 Fe 0.009

1A ECL

I

31.64 22.01 13.23 0.25 0.06 0.12

30.05 97.36 0.984 0.485 0.515 0.007

1A ECL Ib

31.85 30.83 6.83 0.21 0.00 0.14 29.66 99.52 1 .an 0.254 0.730 0.006

2P ECL IId

30.57 38.43

1.44 0.19 0.03 0.02

28.75 99.43 0.995 0.055 0.940 0.005

1H TD-14 EG CS I b I b

30.94 30.34 34.40 32.10 4.32 5.62 0.64 0.55 0.00 0.00 0.06 0.05

28.75 28.85 99.11 97.51 0.995 0.983 0.164 0.215 0.872 0.782 0.017 0.015

compositional range [Xca+,,,Jo" = 0.10-0.38 has been proposed (see Appendix).

For pressure estimation, the methods based on the paragencses Grt-P1-Ky-Qtz (GPKQ) (Ghent, 1976), Grt-PI-Bt-h4s (GMBP) (Ghent & Stout, 1981), Grt- Cpx-PI-Qtz (GCW) (Newton & Perkins, 1982) and Cpx-PI-Qtz (CPQ) (Holland, 1980) have been used. The expressions used here for the first three methods are taken from Powell & Holland (1988). Composition-activity relationships used here are taken from Wood & Banno (1973; diopside), Holland (1980; jadeite). Newton (1983; albite), and Hodges & Royden (1984); garnet, muscovite, phlogopite and anorthite). For the Cpx-PI-Qtz method, the relationship XJd=XN- is used as an approximation (cf. Newton, 1983).

Tables 8-12, and Fins 14 and 15 summarize the Ma 0.002 0.002 0.000 0.001 0.000 0.000 calculated P-T relations at the different stages of Mg &@M 0.006 0.007 0.001 0.003 0.m metamorphic evolution of the Tromsdalstind Sequence. ca '.Oo0 '*OO2 "Ool "OO2 o'990 "002 Generally, the various methods used for temperature and Total 3.000 3.000 3.000 3.000 3.000 3.000 pressure ddat ions give very consistent results. - Al/(Al+Ti) 0.253 0.485 0.258 0.055 0.164 0.215

MI metamorphism

Xc. and XMm in the garnet on the distribution of Fe2' and Successive inclusions of Bt and PI from core to rim in one Mg between garnet and biotite. Based on data for a suite of these garnets from a Ky-bearing sample (592) describe of D, Grt-Bt pairs in this work (Fig. 13). a new empirical an apparent prograde P-T evolution from 636"C, correction of the experimentally calibrated method for the 12.48 kbar to a maximum of c. 720" C. 14-15 kbar (Tables

T J k 6 . Coodcnscd aaalysesof miaerals from cdogites, internal and external

calculations.

R Bc - Gn

x, xLll x, x, x, xy x, x, x, Ti Al- Fe Ma gneises used in the gtothermobarometric

cacr w MI? 133ElN 0.529 0.236 0.2% 0.039 0.123 0374 0.420 0333 0.09 - - - - - Mrrirvlm M, I336 0.169 0.244 0.258 0 . W 0.130 0.W 0.m 0.375 - - - - - - 2933 0.- 0.151 0.230 0.011 0.26;) 0.416 0.605 0.353 - - - - - - KBIS 0.389 0.m om 0.m 0.017 0.sm 0 . a 0.384 - - - - - - -16 0.m 0.389 0.m 0.009 0.W 0 . M 0.612 0.365 - - - - - - 82017 0.477 0.359 0.19 0.011 0.129 0.47s 0.514 0.436 - - - - - - 17% 0.3740.4340.1740.0180.1260.7590.8!J20.l11 - - - - - - m 0.347 0.382 0.m 0.011 0.w 0.m 0.778 0.203 - - - - - - 11.81 0.361 0.437 0.195 0.m 0.066 0.616 0.671 0.3UZ - - - - - - I A 0.m 0.238 0.437. 0.m 0.064 0.764 0.1171 0.117 - - - - - - T-8 0.W 0.411 0.216 0 . W 0.061 0528 0.m 0.399 - - - - - - T-ll 0.W 0.180 0.267 0.010 0.17s 0.431 0.544 0.423 - - - - - - 7-13 0.363 0.367 0 . X 0.009 0 . a 0.691 0.716 0.237 - - - - - - T-'& 0.439 0.148 0.399 0.014 0.m 0.4Y 0.573 0.396 - - - - - - r-u 0 . W 0.156 0.288 0.014 0.189 0.454 0.m 0.365 - - - - - - T-26 0.- 0.192 0.314 0.013 0.130 0521 0.647 0.358 - - - - - - -plllr Modnv MI T-17 0.614 0.114 0.2% 0.01s 0.236 0.322 0.m 0.442 0.06 - - - - - 82003 0.648 0.m 0.144 0.001 - - - - 0 . 0 6 - - - - -

0.240 0.760 0.m - - - - - samz 83006 0.m O . l R 0.253 0.016 0.194 0.109 0.635 0331 0.14 - - - - - 81ooBA 0.5290.1920.1680.0110.1500.4330.6220.3Y0.13 - - - - - gm(B 0.535 0.137. 0.321 0.OW - - - - 0.14 - - - - - 82CaeB 0.505 0.234 0.249 0.012 0.165 0.479 0.688 0.296 0.16 - - - - - 82010 0.W 0.276 0.230 0.011 0.11) 0.539 0.669 0.299 0.20 - - - - -

_ - - - - -

-- 0.SaZ 0.055 0.381 0 . a 0 . W 0.671 0.683 0.OgJ 0.31 - - - - -

L.lr 4 21s) IH 0510 0.104 0.371 0.015 0.m 0.641 0.899 0.019 0.25 0.286 0.406 2.044 3.030 0.m m 2 0.453 0.015 0.461 0.062 0.355 0.557 0.910 0.080 0.21 - - - - - MI 0,429 0.101 0 . M 0.030 0.203 0.708 0.931 0.0% 0.31 - - - - -

ECLOGITES FROM THE TROMS AREA, NORWAY 303

1.8

1.6

T.#C 7. Condensed analyses of minerals Gn B1 PtK PI from metapelites used in the - gcothcrmobarometric calculations. X, X- X, Xlb li Al" Fc Mg Lb laKd X , X, X z X, -

-

-

D, 592. I S92.t 592.3 592.4 m.5 a n.83 ma 316 322 350 m w - 2

4 To4. I " 0 8 . 2 m . 3 m . 4 9 22.15 m a07 212 254 su 448 H-4

4 1 45 45%

462 urn

0.667 0.066 0.243 0.637 0.m 0.247 0.681 0.m 0.200 0.653 0.139 0.190 0.649 0.145 0.aaZ 0.611 0.136 0.231 0.627 o.os1 0.271 0.616 0.139 0.219 0.99 0.161 0.221 0.609 0.165 0.208 0.614 0.154 0.200 0.m 0.099 0.261 0 . a 0.IYI 0.210

0.686 0.718 0.681 0.687 0.634 0.681 0.m 0 . 6 6 0.644 0.581 0.721 0.647 0.622

0.217 0.090 0.184 0.093 0.174 0.138 0.214 0.091 0.181 0.138 0.191 0.101 0.173 0.137 0.155 0.1u 0.233 0.m 0.lW 0.131 0.142 0.110 0.210 0 . m 0.226 0.123

0.653 0.lCn 0.126 0.m 0.148 0.716 1.m 3202 0.014 1.fSZO 0.751 0.27. 0.- 0.28

0.668 0.172 0.112 0.048 0.163 0.674 1.910 2.989 0.017 1.- 0.733 OX7 0.857 0.22 0.646 0.164 0.133 0.057 0.171 0.681 1.893 2.970 0.013 1.8213 0.751 0 2 7 0.SI 0.27

0.m 0.094 0.144 0.m o m 0.779 2 . a 2.148 0.004 I.- o . 8 ~ 0.13s 0 . 8 ~ 1 0.16

~ ... ~

0.711 0.186 0.095 0.008 0.lSD 0.728 2.021 2.857 0.011 1.W1 0.743 0.255 0.865 0.30

0.W 0.117 0.753 0.oU 0.257 0.619 0.014 0.226 0.601 0.018 0.226 0 . n 0.003 0.226 0.728 0.022 0.2.49 0.714 0.M2 0.341 0.556 0.m 0.2u 0.730 0.039 0.161 0.797 0.019 0.216 0.734 0.m 0.2117 0.607 0.018 0.220 0.659 0.028 o.m 0.609

2 . m 2 . o ~ 0.m 2 . m - 2.356 2 . a 0.W 1.9551 - 2.426 2 . M 0.ml 1.9160 - 2.304 2363 0.m 1 . m - 2.- 236) 0.005 1.5m - 2.395 2.283 0.013 1.4545 - 3.132 1.5% 0.009 1.8349 - 2.145 2.528 0.000 1.6531 - 1.882 2 .W 0.010 1.7140 - 1.996 2.746 0 . a 1.- 0.908 2.214 2.587 0.015 1.5387 - 2 . m 2.309 0.013 1.7501 - 2.216 2.73~3 0.m 1.~542 0.807

0.26 0.23 0.18 0.18 0.22 0.22 0.25 0.24 0.25 0.P

0.23 0.21

0.20

0.m 0.179 0.781 1.307 3.472 0.006 2.1280 0.796 0.191 0.w 0.25

0.006 0.150 0.781 1.960 2.881 0.000 1.7512 0.827 0.164 0.862 0.20 0.009 0.150 0.781 l.W 2881 0.000 1.5516 0.m 0.164 0.862 0.20 0.W 0.2U 0.660 1.851 2.988 0.018 1.7324 0.627 0.168 0.848 0.25 0.018 0.248 0.78s 1.66% 2.732 0.001 1.62% 0.890 0.110 0.m 0.29 0.011 0.210 0.741 1.959 2.798 0.m l.67W 0.840 0.160 0.850 0.40 0.054 0.183 0.846 2.012 2.679 0.000 1.7137 0.881 0.118 0.891 0.28

0.093 0.150 0.730 1.675 3.183 0.006 1.7544 0.817 0.181 0.858 0.34 0.W 0.176 0.813 2.293 2.397 0.009 1.- 0.862 0.136 0.693 0.22 0.UZ 0.127 0.757 1.838 3 .W 0.000 1.6304 0.796 0.201 0.837 0.25 0.030 0.181 0.655 1.750 3.164 0.009 1.- 0.820 0.179 0.840 0.29

0.m 0.169 0.809 1.568 3.125 0.011 Z.MIZ o . 8 ~ 0.187 0.m 0.20

0.027 0.151 0.m 1.133 3.171 0.018 1 . 6 ~ 7 0.819 0.120 0.a~ 0.27

8b and 9b). Inclusions of Omp (Na, = 0.54) and PI (An,) in garnet from one of the eclogite samples (1338) yield T and P of 669" C, 15.94 kbar (CPQ) and 675" C, 18.20 kbar

1.4 I 1 I I I 0 0.10 0.20 0.30 0.40

Fig. U. In Grt-Bt pairs from thc Tromsdalstind Sequence.

vcfsus (X, + X,,,,,)O" for D,, D, and D,

(GCPQ), respectively (Table 8a). The latter pressure value m a y be overestimated due to the low Ancontent of the plagioclase (cf. Ashworth & Evirgen, 1985). A few samples from the internal gneisses also indicate an earlier somewhat higher pressure, recorded by the assemblages Grt + PI + Ky + Qtz and jadeitic pyroxene (Jd,& + Ab +

18 m M,IG

8 .x 1 6 - MIMP .. x . x

X

a I

600 700 800 6- ' 500

r (OC)

Flg. 14, Calculated maximum P- T values for different litbologies and stages in the Tromsdalsthd Sequcna. Methods described in the text. IG = internal gneiss; EG = external gneiss; MP = metapelite.

a4 E. I . KROCH Er AL.

18 -

16 -

14 - - I n : 1 2 -

EM, ECLllG A

0 \ \

\ 598Ma? I I I

I I I

1 I I 500 600 700 800

7 (Oc)

m. 15. Average P- T values and inferred P- T-I paths for dietrent lithologics and stages in the TromsdPlstind Sequence. EM, = w l y M,; MM, = maximum MI; EM2 = early M2;

IG = internal gneiss; EG = external gm5.s; MP = metapelite. Stability curves: Jd + Qtz = Ab (H~llpod. 1980); Ky = Si (Holdaway, 1971); Fe-St + Qtz = A h + Ky + H 2 0 (Pigage & Greenwood. 1982).

MM2 = mpXim~m MZ; M M 3 = M3; ECL = edogite;

Qtz (max. P MI, Table 8a). No exact temperature estimate can be made for this stage. However, pressures around 17 kbar at 675°C are indicated.

Late- to post-D, garnets combined with apparently associated large pre-D2 porphyroclasts of Bt + Phe + PI + Ky+Qtz in metapelites yield 721 f23"C, 14.75f 0.65 kbar (GPKQ) and 712" C, 12.45 kbar (GMBP) (Table 9b). h n d a r y Omp+PI+Qtz in internal gneisses combined with Grt+Cpx gives 713f29"C, 15.97f 0.41 kbar (CPQ) and 713 f 31"C, 15.13 f 1.13 kbar (GCPQ), respectively (Table 9a). One of the samples (T-17) gives deviating pressures for the t3t-Cpx-PI-Q~ geobarometer. This sample contains albite (An& which

T.bk 8. P-Tconditions calculated for early MI in (a) cclogita (ECL) and internal gaeisses (IG) and (b) metapelites (W). Gcothcrmometers used are Grt-Cpx (Krogh, 1988) for ECL and IG and Grt-Bt (this paper) for MP. Barometers used are Grt-Cpx-PI-Qtz (Gcw; Powell &I Hollaod. 1988). Qx-PI-Qz (CPQ; Holland, 1980) and Grt-PI-Ky-Qtz (GPKQ; Powell & Holland. 1988).

(a) Cc logk and intend gnciva

GPCQ cw GPKQ

sample 00. T(%) P(khr) T K ) P (kbar) T(T) P(khu)

1338 Ii (ECL) 675 18.20 669 15.94 - - 32001 (IG) - - - 8zom (W m B OG)

- 673 17.34

671 16.81

M a 675 18.20 670 16.39 672 17.07

(b) M - h

5% 5% 593,

- 671 16.85 - - - - - - -

- 636 12.48 - 653 13.46 - 643 13.39

- - - - - -

- - -

T.WC 9. P-Tcooditions calculated for maximum MI (MM,) in (a) internal gncisses and (b) mctapclites. Metbods used are those given in Table 8. except Grt-Ms-Bt-PI (Gh4BP; Powell & Holland, 1988). Valua in parentheses are deleted from the calculation of mean values. ~ ~~ ~

(a ) I ~ I c ~ ~ ~ GCPQ cw

Sample no. T("C) P(kbar) T("C) P(kbar)

no06 683 14.36 686 15.45 ((UX#(A 798 16.34 707 15.93 ((UX#(B 759 15.82 760 16.42 82010 689 13.99 694 15.69 T-17 727 (18.81) 7'20 16.33 Mean 713 15.13 713 15.97 *(la) 31 1.13 29 0.41 ( b ) Metapelira

GPKQ GMBP

sample no. T("C) P (kbar) T("C) P (kbar)

27.83 695 14.34 - - 33 767 16.14 - - 858 71 1 14.29 - - 316 702 14.03 - - 322 713 14.57 702 12.24 350 738 15.32 - -

722 15.14 - - 73 1 14.81 - - 592. 701 14.38 - - 5925

5m TO-2 732 14.87 722 12.66 MSUl ?2l 14.75 712 12.45 *:(la) 23 0.65

may result in too high an estimated pressure (cf. Ashworth & Evirgen, 1985). Using the average values for the Cpx-PI-Qtz and Grt-Cpx-PI-% geobarometers from the internal gneisses, Grt-Cpx geothemometry on the eclogites and garnet pyroxenites yield 744*32"C, 16.9 f 0.55 kbar and 742 f 31" C, 1579 f 0.41 kbar, resp ectively. Coexisting Grt ( X G m = 0.28) + Ky + Cal + Qtz in the Tromsdalstind eclogite is stable between Pa=P,,= 11 kbar (X- = 1.00) and Pa = P,, = 20 kbar (Xc% = 0.15) at 700°C (K. Bucher-Nurminen, personal communica- tion). The Occurrence of omphacite- and rutile-bearing quartz veins, and zoisite-kyanite-quartz segregations strongly suggest the presence of a fluid phase during the maximum M, metamorphism. The stability of zoisite in the Tromsdalstind eclogite further indicates that X, was low. Equilibrium pressures should thus be closer to 20 kbar.

The frequent dtcompressional and hydration reactions observed in the eclogites suggest that a period of uplift and influx of H,O-rich fluids followed the high-P D, event. At this post-eclogite/pre-D2 stage partial melting seems to have taken place both in the metapelites and the mafic rocks

The last recorded mineral growth related to this event is secondary Sill, which indicates that pressures below c. 8 kbar at 700" C were reached (Holdaway, 1971).


4 metamorphism

The earliest recorded metamorphic assemblage from the D2 event is found as inclusions of diagnostic minerals as Cld, Pg, Mrg, St and Qtz in some of the syn- to post-D, garnets from metapelites. These are succeeded by the stable matrix assemblage Grt + PI + Bt + Phe + Pg + Ky + St+Qtz. This observation suggests that the equilibrium reactions

Cld + Pg+ Qtz= St + Ab + H20

Mrg+ Qtz= An + Ky + H20 (1)

(2) and

were overstepped, while equilibrium for the matrix assemblage should be close to the

and

stability curves. Reaction (1) is located by Baltatzis & Wood (1977) to be

c. 575" C at 5 kbar and c. 610" C at 8.5 kbar. With Mrg as an additional phase, upper stability would be lowered somewhat. Reaction (2) is located at c. 6 kbar at 570" C (Storre & Nitxh, 1974). Reactions (3) and (4) are both located at t. 675" C, 10 kbar (Holland, 1979; Pigage & Greenwood, 1982). Calculated P-T values based on successive inclusions from core to rim of Phe, Bt and PI in Grt (sample T W ) yield an apparent prograde P-T evolution from 552°C. 7.95kbar through a P,= 10.65 kbar at 644" C, while the outermost rim of garnet and matrix minerals give 671" C, 10.23 kbar (Tables 10 and llb). Average temperature and pressure for 10 samples containing the assemblage Grt + Bt + Phe + PI are 662 f 7" C, 9.55 f 0.58 kbar, while three samples containing Ky as an additional phase average 669f8"C. 10.34f 0.61 kbar (Table llb). These calculated values are thus in excellent agreement with the observed phase relations. The r e q s t a h d * ma6c assemblage Grt-Cpx-PI-Qtz (four samples) yields average P-T values of 634 f 11" C, 8.18 f 1.17 kbar (ma) and 638 f 14" C, 10.14 f 0.59 kbar (Gcw) (Table lla).

Pg + Qtz= Ab + Ky + H,O

St + Qtz= A h + Ky + H20 (3)

(4)

& metamorphism Post-D, Grt in apparent equilibrium with Bt. Phe and PI in metapelites yields 631 f 7" C, 9.23 f 0.62 kbar for five

T1uc 18. P-Tconditions calculated for early M2 in rnetapclita. Metbods used arc tbose given in Tables 8 and 9.

Meiqpclirr GMBP

Sample no. T P (kbar)

T0-8, 552 7.95 T88, Ta8,

M7 8.56 644 10.65

Tabk ll. P-Tconditions calculated for maximum M2 in (a) external peisses (EG), amphibolitcs (A) and pegmatitcs (PEGM), and (b) metapclitcs. Methods wed are those given in Tables 8 and 9.

(a ) External gneisses (EG) amphibolites ( A ) and pegmatifa ( P E C M )

GCPQ C W

Sample no. T('C) P (kbar) T("C) P (kbar)

2753 (A) 1H (EG) TD-2 (EG) M1 (PEGM) M a

( b ) Mempelim *(la)

Sample no.

62 1 9.31 652 10.57 635 10.15 645 10.55

638 10.14 14 0.59

GPKQ

T("C) P(kbar)

62 1 9.32 647 8.56 63 1 8.30 635 6.55

634 8.18 11 1.17

GMBP

T("C) P (kbar)

9 205 207 212 254 424 448 22.15 H 4 m,

658 659 664 659 659 654 667 656 673 67 1

10.04 8.48

9.39 8.88 9.50

10.19 9.12 9.90

10.23

9.77

M W 669 10.34 662 9.55 *( la) 8 0.61 7 0.58

samples, while one of the samples containing Ky as an additional phase yields 639" C. 9.08 kbar (Table 12).

GEOCHRONOLOGY

Landmark (1953,1973) considered the Skattera Gneiss to be of Precambrian age based on lithological similarities with Precambrian gneisses and anorthosites on Ringvass~y north-west of TromsB. A pre-Caledonian equilibration age was also favoured by Bryhni, Krogh & Griffin (1977) for the Tromsdalstind Sequence eclogite fades mineral assemblage. Griffin & Brueckner (1985) reported Sm-Nd isotope data on Cpx, Am and Grt from an eclogite sample

T.bk l2. P -T conditions calculated for M3 in metapelites Methods used are t h e given in Tables 8 and 9.

Metapelim GMBP GPKQ

Sample no. T('C) P (kbar) T r C ) P (kbar)

1 620 9.16 45 627 10.06 455a 633 9.41 45% 637 9.19 462 636 8.34 639 9.08 Mean 63 1 9.23 639 9.08 f l l d 7 0.62

- - - - - - - -

306 E . j . KROGH E T A L .

Tab& W. Rb-Sr data for biotite-K-feldspar gneiss (Hclla gneiss).

Sample Rb (ppm) Sr (ppm) nRb/%r "Sr/%r (20)

He-1 He-2 He-3 H e 4

He41 He-7

He-5

ne-8 He-9

89.3 97.6

124.8 129.3 90.3

105.1 111.0 86.7 97.3

130.1

85.5 74.1 41.1

106.4 96.4

122.5 109.6

120.7 1.988 2.341 4.266 5.064 6.391 2.847 3.335 2.052 2.570

0.72431 f 6

0.73864 12 0.74312 f 8 0.74977 f 7 0.72990 f 15 0.73284 f 7 0.72482 f 7 0.72498 f 8

o.mn * 8

(431 f 7 Ma. & = 0.71108) is obtained on a late/post- D2/pre-D, granitic intrusion in the upper part of the Lyngen Composite Terrane (Lindstrsm, 1988).

K-Ar dating of secondary amphibole from a retrograded eclogite yields an age of 437 f 16 Ma. Similar ages (448 f 20 and 436 f 20 Ma) are obtained on amphiboles from the Skattsra gneiss (Table 14). It is now generally accepted that K-Ar ages date a point on a cooling trend, in this casc when the T r o w Nappc Complex was uplifted above the crustal 475-500°C isotherm (Armstrong, 1966; Dodson, 1973). -

1 = 1.42 X lo-" y-' (Steiger & JPger, 1977)

DISCUSSION

from the TNC, and calculated an isochron age of 598 f 107 Ma. This sample is. however. partly retrograded and recrystallized (exsolution lamellae of PI in Cpx. domains of granular Cpx and PI, secondary growth of Grt and Am), and the meaning of this apparent age is uncertain.

A seven-point Rb-Sr whole rock isochron of a fine-grained biotite-microcline gneiss in the Tromsdalstind Sequence yields 433 f 11 Ma (2a) with initial "Sr/"bsr = 0.71217f0.00023. One point is omitted from this regression (Table 13, Fig. 16). A similar Rb-Sr age

I He1 La Gneiss, I

TROMSB NAPPE COMPLEX / 0.75

Fig. 16. Isochron plot of the analysed sampla of microcline gneiss from HeUa. Unanaintia given at the 2a level.

T.blt 14. K-Ar data on amphiboles from retrograded eclogite (T0-1). Skattera gneiss (SK-1) and oligoclasite dyke (SK-2).

Structural, textural and P-T data indicate that the Tromsdalstind Sequence rocks underwent a complex tectonometamorphic evolution. Different P-T regimes at different stages can be identified. The coherence of P-T data calculated by the different methods is striking, and indicates both a high degree of confidence in the methods, and a high degree of equilibrium in the assemblages. The constructed P -T paths during the D1, D2 and D3 episodes recorded in the Tromsdalstind Sequence (Figs 12 and 13) reveal the complex polymetamorphic character of this eclogite-bearing terrain.

In Fig. 14 all the calculated maximum P -T values from the three main metamorphic events are plotted. This shows a distinct difference between the M, episode on one hand and the M2 and M, episodes on the other. The two latter do overlap, and are not easily distinguished in t e r n of P and T. However, in some cases the textural relationships are distinctly different. In Fig. 15 the average of calculated P-T values for different stages are plotted, and inferred P-T paths are indicated.

The metamorphic evolution during the D, episode of the Tromsdalstind Sequence is compatible with rapid crustal thickening, possibly by crustal subduction or imbrication in a continental collision zone, succeeded by a period of heating with slight decompression and later uplift at constant or decreasing temperature, as shown in theoretical models describing P-T paths of regional metamorphism in areas of thickened continental crust (England & Richardson, 1977; England & Thompson, 1984; Oxburgh & Turcotte, 1974; Richardson & England, 1979; Thompson, 1981; Thompson & England, 1984; Thompson & Ridley, 1987). The curved P-T path for this episode is similar to that recorded for eclogites in the Western Gneiss Region of Norway (e.g. Jamtveit, 1987). The Sm-Nd mineral age of 598 f 107Ma on a partly retrograded and recrystallized eclogite may either record the eclogite stage or this latter uplift stage. In this context it may be of interest to compare with the Sm-Nd mineral age (605 f 37 Ma) on similar postcclogite high-pressure granulites from the northeast Ox inlier, north-west Ireland (Sanders, Daley & Davies, 1987). This age has been interpreted as a 'cooling age'.

The D2 P-T path indicates a second episode of rapid crustal thickening, followed by a slight pressure decrease at increasing temperature. Then followed a new event of

ECLOCITES FROM THE TROMS AREA, NORWAY 3Q7

deformation (D3), possibly at decreasing T, with a subsequent growth of Grt, Bt, Ms, PI and Qtz. This could be related to the emplacement of the sequence on top of colder rocks (Lyngen Composite Terrane). As maximum temperatures for this episode exceeded the apparent blocking temperature for the K-Ar system in amphiboles (475-500" C), the age of 436 f 16 Ma probably represents the post-D, cooling. The D2 and D3 episodes were probably closely related in time.

TECTONIC IMPLICATIONS

Iocorporation of the Tromsdalstind Sequence into a Caledonian tectonic madel must take into consideration the following important observations.

1. The depositional environment/geotectonic setting indicated by the country rocks in the Tromsdalstind

2. Tbe tectonostratigraphic position of the Tromsa Nappe Complex, particularly the high-pressure metamorphic Tromsdalstind Sequence. 3. The calculated P-T paths for the eclogites and their

country rocks. 4. Tbe ages obtained for the eclogites and the country

rocks as indicated by the present Sm-Nd, Rb-Sr and K-Ar ages.

The structural positioo of the Tromsdalstind Wuence on top of the tectonic fragments of oceanic spreading ridges, oceanic islands and island arcs of the Upper AJlochthoo (Stephens & Gee, 1985) indicates that the tectonometamorphic evolution of the T r o w eclogites and their country rocks should be viewed independently of Caledonian tectonic models based on eclogites elsewhere in the Scandinavian Caledonidcs (West Norwegian Basal Gneiss Complex: Cuthbert, Harvey & Carswell, 1983; Medaris & Wang, 1986; Jamtveit, 1987; Scve N a p : Dallmeyer, Gee & Beckholmen, 1985).

The complete obliteration of primary sedimentary features makes identification of the depositional environment for the Tromsdalstind Sequence somewhat uncertain. However, the presence of a relatively thick marble sequence together with substantial thicknesses of metapelites suggest a shallow marine environment. The high proportion of basic rocks with magmatic geochemical characteristics indicates an extensive influx of volcanogenic and/or plutonic material into the sedimentary pile. The Tromsdalstind eclogite is regarded as a metagabbro, based on relict textural evidence, while the boudins and lenses of eclogite within marble and metapelite probably represent dismpted basaltic flows and/or minor intrusions in this shallow marine sequence.

Tbe P-T path recorded by the various quilibrium mineral assemblages and mineral-chemical variations indicates a complex tectonometamorphic evolution after the deposition of the Tromsdalstind Sequence. The first loop on the P-T path, starting at c. 12.5 kbar and 638°C. indicates compression from a lower continental crustal position to depths corresponding to about 60km (17-19 kbar) by overloading of additional crust, before

Sequence.

decompression to a level corresponding to pressures less than c. 8 kbar. This is interpreted in terms of subduction of continental crust with subsequent erosion and uplift, and probably large-scale imbrication within the underthrusted plate. The pre- or early Caledonian Sm-Nd mineral age of 598 f 107 Ma on garnet-clinopyroxene-amphibole from a partly retrograded and recrystallized cclogite sample could represent either the peak (eclogite faaes) metamorphic conditions. or, more likely, the later pre-D, decompressional stage. Humphries & Cliff (1982) calculated a garnet Sm-Nd blocking temperature of 600°C. This value may, however, involve large uncertainties as the calculations are based on a few high-temperature diffusion data (Harrison & Wood, 1980). It is therefore suggested that neither the M2 nor the M, metamorphism, which both excceded rn- . af€ectcdtheSm-Nsystunatics ix-the -

eclogites. As a consequence of the uplift, the relatively hot Tromsdalstind Sequence was tectonically brought into contact with the oow underlying Skattera gneiss. Partial melting with formation of pegmatites in various lithologies most probably occurred at this stage. After a period of cooling and probably further decompression, the Troms- dalstind !jcquence and the now associated Skattera gneiss were deformed (DJ and brought as one unit to depths equivalent to 10-11 kbar, reaching maximum temperatures of c. 665" C. The calculated P-T path for this event is best explained by underthrusting of the two units along a deep crustal shear zone (A-type subduction?). The consequent thickening of the crust resulted in uplift and erosion of the overriding plate, and decompression and cooling of the amalgamated Tromsdalstind Sequence and Skattera gneiss (the TrornM N a p Complex) of the lower plate. This uplift (isostatic and/or tectonic) probably resulted in the emplacement of the T r o w Nappc Complex on top of the upper Ordovician/Silurian Balsfjord Group of the Lyngen Nappe. resulting in deformation and metamorphism (locally to Ky-grade) of the latter rocks, and a consequent deformation (D3) during cooling of the overlying T r o w Nappe Complex. This ended the semnd loop of the P-T path for the Tromw Nappe Complex. The late Ordovician/Silunan age of the Balsfjord Group puts a lower age limit on the D, event. The 433 f 11 Ma Rb-Sr age on a microcline-biotite gneiss, and the 437f 16MaK-Ar age on secondary hornblende in a retrograded eclogite, together with two K-Ar ages on hornblendes from the Skattera gneiss on 436 f ul Ma and 448 f 20 Ma probably dates the post-D, cooling below c. 500°C. The greenschist facies metamorphism recorded in the authochthonous/parauthochthonous western basement rocks documents that the D,-, events represent a series of transported tectonometamorphic events.

ACKNOWLEDGEMENTS

We would like to thank Drs Tony Carswell, Bill Griffin, Christine Miller and Ian Sanders for valuable and constructive criticism on earlier versions of the manuxript. Figures were kindly drafted by Hilkka Falkseth and Lis Olsen. The research was funded by a grant from the

E . 1. KROCH E l A l .

Norwegian Research Council for Science and the Humanities (NAVF, grant D.47.31-035). This is publica- tion no. 49 in the Nomegian ILP project.

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Received 3 February 1988; revision accepted 19 September 1989,

APPENDIX GARNET-BIOTITE CEOTHERMOMETRY

Equilibrium temperatures for metapelites are calculated using an em irical corrected version of the experimental calibration of the Fe -Mg distribution between garnet and biotite (Perchuk & Lamnt’eva, 1983). This correction is based on 1 1 D, Grt-Bt pairs from the Tromsdalstind Sequeace, showing a range in (X , + XMm) from 0.100 to 0.386. The relationship between In K, and this compositional parameter is shown in Fig. 13, and linear regression yields the expression

In KD = 1.425(X, + XMn) + 1.436 (r2 = 0.97%).

At (X,+X,)=O, a mean temperature for the D, episode is calculated to 664’C. By using a realistic pressure estimate (10 kbar for D2), substitution of this expression into thc Perchuk & Lavrent’eva (1983) Grt-Bt equation. using an intermediate value for pressure correction given by Pcrchuk & Lavrcnt’cva,

p+

yields

T (“C) = {I3824 + 1341(X, + XMn) + 20.7P (kbar)]/ (In K, + 2.868))-273.

Documents

Eclogites and polyphase P–T cycling in the Caledonian Uppermost Allochthon in Troms, northern Norway