Dynamic Metasomatism: Stable Isotopes, Fluid Evolution, and Deformation of Albitite and Scapolite Metagabbro (Bamble Lithotectonic Domain, South Norway)

New stable isotopic data from mineral separates of albite, scapolite, amphibole, quartz, and calcite of metasomatic rocks (Bamble lithotectonic domain) give increased knowledge on fluid type, source, and evolution duringmetamorphism. Albite from a variety of albitites givesδ18OSMOW values of 5.1–11.1‰,while quartz fromclinopyroxene-bearing albitite gives 11.5–11.6‰. δ 18OSMOW values for calcite samples varies between 3.4 and 12.4‰ and shows more consistent δC values of −4.6 to −6.0‰. Amphibole from scapolite metagabbro yields a δ18OSMOW value of 4.3 to 6.7‰and δDSMOW value of−84 to−50‰,while the scapolite gives δ 18OSMOW values in the range of 7.4 to 10.6‰.These results support the interpretation that the originalmagmatic rocks weremetasomatised by seawater solutions with a possible involvement from magmatic fluids. Scapolitisation and albitisation led to contrasting chemical evolution with respect to elements like P, Ti, V, Fe, and halogens. The halogens deposited as Cl-scapolite were dissolved by albitisation fluid and reused as a ligand for metal transport. Many of the metal deposits in the Bamble lithotectonic domain, including Fe-ores, rutile, and apatite deposits formed during metasomatism. Brittle to ductile deformation concurrent with metasomatic infiltration illustrates the dynamics and importance of metasomatic processes during crustal evolution.


Introduction
Metasomatism is the pervasive alteration of rocks with respect to both mineralogical and chemical composition.It results from interaction with fluids, sometimes causing albitisation by replacement of rock units by Na-rich feldspar, and scapolitisation forming scapolite-bearing rocks.These fluids can infiltrate under highly variable geological settings and PT conditions and originate from a meteoric, magmatic, or metamorphic environment.Albitisation is reported in deep weathering profiles [1,2], in epiclastic sediments during diagenesis and low-grade metamorphism [3], in granitoids during late magmatic alteration [4], and in association with fluid migration in mobile belts [5].Regional-scale metasomatism is a widely recognized phenomenon in a number of rock types and tectonic settings [6,7].Metasomatism is an important guide to hydrothermal ore deposits and represents a characteristic feature of many orogenic gold deposits, iron oxide-apatite (IOA), iron oxide-Cu-Au (IOCG), and U deposits [6,[8][9][10][11].
The metasomatic processes affecting the Bamble lithotectonic domain have locally transformed the rocks so strongly that we cannot trace the precursor, and therefore a full understanding of the processes is still lacking.However, a number of papers have solved various aspects of the metasomatic processes including widespread formation of scapolite metagabbro [19][20][21]27] through Mg-Cl metasomatism, replacement textures in apatite [28][29][30], rutile formation [31], carbonate deposition [32], tourmaline formation [33], and sapphirine-corundum crystallization [34].While the scapolitisation process with respect to mineral reactions is relatively well understood in the Kragerø region, albitisation is a more complex process and less constrained.Extensive albitisation is seen along veins, as brecciation, as formation of foliated albititic felsites and chlorite schists, as carbonate-rich albitite, and as large-scale albitite bodies [23,35].
In this paper, we present stable O-, H-, and C-isotope data on mineral separates from albitites and scapolite metagabbro with the purpose of constraining the fluid type and source.Different models for fluid evolution are then discussed.Whole rock geochemical data is presented in order to illustrate the chemical changes and discussed relative to mineralogical replacement and mineral deposition.Brittle and ductile structural elements associated with the metasomatism are used to discuss the dynamics of fluid processes.
The Kragerø area (Figure 1(b)) consists of a layered complex of mafic rocks and variable gneisses and quartzites.The mafic rocks are amphibolites and metagabbros including bodies of gabbro [56].Orthogneisses are of granitic, granodioritic, quartzdioritic, and tonalitic composition.Quartzites containing sillimanite are interlayered with garnet amphibolite, felsic gneiss, and garnet-and cordierite-bearing mica gneiss.
Na-metasomatism in the form of albitisation is regionally extensive in the Precambrian crust of southern Scandinavia and is particularly widespread in the Bamble and Kongsberg-Modum lithotectonic domains and the Norwegian part of the Mylonite Zone (Figure 1(a)) [23].In the Bamble lithotectonic domain, albitisation is present from the northeastern boundary to the Oslo Rift and southwestwards through the domain.Large bodies of albitite are found in the vicinity of Kragerø and towards Arendal [23,[57][58][59].Mg-Cl-metasomatised rocks in the form of scapolite metagabbros occur widespread as part of the mapped amphibolites and metagabbro, commonly in conjunction with the albitites [19,20,50].

Analytical Methods
Different types of albitites and scapolite metagabbro were mapped and sampled in the Kragerø area of Bamble lithotectonic domain (Table 1; Figure 1).Polished thin sections were studied via optical and scanning electron microscopy (SEM), using a LEO 1450 VP instrument at the Geological Survey of Norway (NGU).
Whole rock major and trace element analyses (Table 2) were carried out at the NGU.Major elements were measured on fused glass beads prepared by 1 : 7 dilution with lithium tetraborate.Trace elements were measured from pressed tablets.The samples were analysed on a PANalytical Axios XRF spectrometre equipped with a 4 kW Rh X-ray endwindow tube, using synthetic and international standards for calibration as described by Govindaraju [60].Rock samples used for whole rock geochemistry were selected as being representative and homogenous, with good control on mineralogy and petrography.
Stable isotopic data are presented in Table 3.The oxygen isotope composition ( 16 O, 17 O, and 18 O) of handpicked mineral separates of albite, scapolite, amphibole, and quartz was measured at the University of Tübingen using a method similar to that described by Sharp [61] and Rumble III and Hoering [62], which is described in more detail in Kasemann et al. [63].Between 2 to 4 mg of sample was loaded onto a small Pt sample holder, which was pumped to a vacuum of about 10 −6 mbar.After prefluorination of the sample chamber overnight, the samples were heated with a CO 2 -laser in 50 mbars of pure F 2 .Excess F 2 was separated from the O 2 using KCl at 150 ∘ C by producing KF and releasing Cl 2 .The extracted O 2 was collected quantitatively by adsorption on a molecular sieve (13X) at liquid nitrogen temperature in a sample vial.Subsequently the vial was removed from the For the D/H analysis of the minerals, an extraction line as described in [65] was used.Depending on the water content, a sufficient amount of hydrous minerals was loaded into 12 cm long quartz tubes in order to obtain >1 mg H 2 O. Water was released by heating the minerals in the tubes using a torch.H 2 O was then converted to H 2 using Zn (see also Vennemann and O'Neil [65] for further details).H 2 was then subsequently measured by a Finnigan MAT 252 Mass Spectrometer, using the dual inlet device.External precision is typically ±2‰, and all values are reported relative to SMOW.
Stable isotope analysis (C, O) of carbonate samples was performed using a Finnigan MAT 252 gas source mass spectrometer combined with a Thermo Finnigan GasBench II/CTC Combi-Pal autosampler.Both devices are connected using the continuous flow technique with a He stream as carrier gas.About 0.1 mg dried sample powder is loaded into a 10 ml glass exetainer, sealed with rubber septum.The exetainers are placed in an aluminium tray and set to 72 ∘ C.After purging with pure He gas, 4-6 drops of 100% phosphoric acid are added.After a reaction time of about 90 minutes, the released CO 2 is transferred (using a GC gas column to separate other components) to the mass spectrometer using a He carrier gas.The sample CO 2 is measured relative to an internal laboratory tank gas standard, which is calibrated against internal and international carbonate standards (e.g., Laser marble, NBS-19).All values are given in ‰ relative to PDB (Vienna Pee Dee Belemnite) for C and SMOW/PDB for O.The external precision calculated over 10-15 standards is typically in the range of 0.05-0.06‰for  13 C and 0.06-0.08‰for  18 O.For further details see Spötl and Vennemann [66].
Albitisation affects both mafic and granitoid lithologies in the Kragerø area, usually associated with the scapolitebearing rocks, and normally postdating the scapolitisation.Albitisation takes place along veins and in breccias.Albite is the dominant mineral in foliated felsites, in chlorite schist, in carbonate-rich albitite, and in large-scale albitite bodies [23,50].Albitisation has been studied in detail in the Ringsjø-Ødegården Verk area [20,35].Both mafic (gabbro, scapolite metagabbro, and amphibolite) and granitoid protolith are transformed to albitite along veining, where the central vein consists of nearly pure albite (Figures 3(a At the Langøy locality, albitite extends over a 3 × 2 km area and follows a mapped vein-pattern through gabbro, metagabbro, and scapolite metagabbro rocks (Figure 1(b)).It includes massive carbonate-rich albitite, brecciated and altered host rock with albite-carbonate groundmass, and foliated albitic felsites.The massive carbonate-rich albitite usually occurs as several-meter thick deposits (Figure 3(d)) with the largest albitite body being more than 150 m wide and 1500 m long (Figure 1(b)).They are brecciated along their margins (Figure 3(e)) to the scapolite metagabbro with a gradational contact.The initial transformation and disintegration of the metagabbro protolith are observed along and adjacent to the individual albititic veins (Figure 3(f)).Progressive deformation and infiltration caused brecciation, with an albititic groundmass infiltrating angular clasts of greenishgrey, retrograded mafic rock, and progressively developing a foliation fabric.These foliated albite-rich felsites are rocks with layers of light carbonate-albite dominated bands layered with green-grey chlorite schist, after veined, brecciated, and flattened metagabbro (Figure 3(g)).
In the Storkollen-Åtangen area, west of the town of Kragerø, large-scale albititic bodies covering >1 km 2 are enveloped by amphibolites, metagabbro, and scapolite metagabbro.The albitite is clinopyroxene-and titanite-bearing.It characteristically takes the form of a medium-grained, granoblastic, light grey, or pink leucocratic rock (Figure 3(h)).It is either massive or has a gneissic banding formed by alternating leucocratic and amphibole-bearing melanocratic layers.The clinopyroxene-bearing albitite and its melanocratic layers show replacement to rutile-bearing, light pink, fine-grained albitite.The contact to the enveloping amphibolite unit is associated with a greenish-grey transformation of mafic phases.Analysed amphiboles show edenitic, pargasitic, and actinolitic compositions.In addition, Dahlgren et al. [32] report dolomite-dominated deposits in this area with veining and brecciation of the metagabbro and amphibolites.

Carbonate-Rich Albitite and Albitite
Felsite.The carbonate-rich albitite consists of fine to medium grains of near end-member albite (Ab 97-100 ), calcite, and dolomite (Figure 4(d)).Minor quartz and chlorite are present with rutile and Fe-oxides as accessories.The albitite host clasts consist of mafic, greenish-grey, fine-grained, and retrograded metagabbro, partly replaced by albitite and characterised by a higher content of chlorite and Fe-oxide.In the related, banded, albite-rich felsitic schist, the light bands are composed of fine-grained albite, calcite, chlorite (Mg# = 0.85-0.89),and amphibole.The darker bands also contain clinopyroxene and some phlogopite (Mg# = 0.82), with rutile, apatite, zircon, and magnetite as accessory phases.In addition to the banding, reflected by modal variation, a parallel fabric is defined by planar-oriented phlogopite and chlorite (Figure 4(e)).

Whole Rock Geochemistry.
Whole rock geochemical data from the gabbro/metagabbro and tonalite protolith, together with the metasomatic scapolite-bearing metagabbro and albitites, are presented in Table 2 and Figure 5.While scapolite metagabbro has a gabbro protolith, the albitites are derived from a variety of rocks including a gabbro or scapolite metagabbro protolith for the samples with SiO 2 < 70, whereas for albitites with SiO 2 > 70 a granitoid or unknown protolith is inferred (Table 2).For the major elements, systematic geochemical changes are seen for the elements Na, Ca, Fe, and Mg in the metasomatic rocks compared to the protoliths.For scapolite metagabbro and albitite derived from gabbro, Na 2 O increases and CaO decreases with increasing SiO 2 (Figures 5(a) and 5(b)).For a specific content of SiO 2 , the Na 2 O is higher for the scapolite metagabbro than for the albite, which reflects that the Na : Si ratio in marialite is 2 : 1, while the same ratio for albite is 1 : 1.We regard the two trends, defined by increasing Na 2 O with increasing SiO 2 for scapolite metagabbro and albitites with SiO 2 < 70, to represent increasing degree of scapolitisation and albitisation.Fe 2 O 3 (Figure 5(c)) is generally lower in the scapolite metagabbro compared to the gabbro/metagabbro and shows especially low values in the albitites.MgO (Figure 5(d)) decreases with increasing SiO 2 for both albitite and scapolite metagabbros.An increase of P 2 O 5 with increasing degree of scapolitisation (increasing SiO 2 ) is apparent for the scapolite metagabbros, while for albitites the P 2 O 5 decreases with albitisation (Figure 5(e)).The concentration of the trace elements Zn and Cu (Figures 5(f) and 5(g)) decreases with increasing degree of scapolitisation and both elements are below 15 ppm for all albitites, while one of the gabbro samples contains around 90 ppm for both Zn and Cu (Table 2).Bromine which is absent in the protolith rocks increases with increasing degree of scapolitisation up to a level of 80 ppm, while this element is below the detection limit in the albitites (Figure 5(h)).No analyses of Cl are available, but we assume, based on the mineralogical evolution and mineral chemistry, that Cl must parallel the evolution of Br at a much higher level.Like P 2 O 5 , TiO 2 increases with increasing degree of scapolitisation (Figure 5(i)).Albitites with low SiO 2 values contain the highest TiO 2 content (ca 4 wt%), while increasing the degree of albitisation apparently results in decreasing the TiO 2 content.Vanadium, an element that typically follows Ti, displays a clear increase with degree of scapolitisation and a reduction during progressive albitisation (Figure 5(j)).For most of the metasomatised samples analysed, there is a negative correlation between TiO 2 and Fe 2 O 3 , while for the gabbro an overall positive correlation between these two oxides exists (Figure 5(k)).TiO 2 values up to 4.31 wt% are found in some of the scapolite metagabbros and albitites.

Stable Isotopic Compositions
Mineral separates from the scapolite metagabbro and albitites have been analysed for the stable isotopes of O ( 18 O), H (D), and C ( 13 C; Table 3).Albite separates from different types of albitites, quartz separates from clinopyroxene-bearing albitites, calcite separates from carbonate-rich albitite, and scapolite separates from scapolite metagabbros have been analysed with respect to  18 O.The albite, calcite, and scapolite are presumed to have formed during metasomatism, while the quartz equilibrated with these minerals during the same event.The stable isotopic composition of these minerals should give constraints on the infiltrating fluid chemistry, but O in the silicate crystal structure should also retain information regarding the origin of the rock.(g) Cu-SiO 2 ; (h) Br-SiO 2 ; (i) TiO 2 -SiO 2 ; (j) V-SiO 2 ; (k) TiO 2 -Fe 2 O 3 ; and (l) V-TiO 2 .
In addition, the D composition of amphibole separates from the scapolite metagabbro is presented.The amphibole crystallized during the metasomatic alteration of the dry gabbro by the infiltration of an external fluid [20].Consequently, the D-values give direct information on the chemistry of the metasomatising fluid.Carbon, in the form of CO 2 , was also supplied externally during the metasomatic event resulting in the formation of calcite, which was analysed for  13 C.
Albite mineral separates from the Ringsjø-Ødegården Verk area give  18 O SMOW values of 5.1 to 8.4‰ for samples of albitite originating from a mafic/gabbro protolith and 8.5 to 10.8‰ for samples originating from a granitoid/tonalite.Albite, from carbonate-bearing albitite samples from Langøy, gives a  18 O SMOW of 5.5 to 7.0‰.Albite from a clinopyroxene-bearing albitite in the Åtangen-Storkollen area yields a  18 O SMOW of 10.8 to 11.1‰, while quartz from the same samples gives a  18 O SMOW of 11.5 to 11.6‰.Scapolite separates from a scapolite metagabbro sampled at the Ødegården Verk and Ringsjø localities give  18 O SMOW values in the range of 7.4 to 10.6‰.Calcite from different albitites shows a wide range in  18 O SMOW between 3.4 and 12.4‰, but with a quite consistent  13 C of −4.6 to −6.0‰ (Figure 6).Amphibole separates from the same scapolite metagabbro samples yield  18 O SMOW from 4.3 to 6.7‰ and D SMOW of −84 to −50‰.[18,23].Earlier work in the Bamble lithotectonic domain has shown that scapolitisation transforms mafic rocks into scapolite metagabbros by infiltration of Cl-Mg-rich solutions and that albitites form from both mafic and granitoid protoliths by Na-rich solutions [20,35].As expected, Na 2 O increases and CaO decreases during albitisation.Addition of albite to a gabbroic protolith will dilute the nonadded elements in equal proportion.Although the overall trend displayed by Figure 5 can be explained by addition of albite and scapolite to a gabbroic protolith, the TiO 2 -Fe 2 O 3 relationship shown in Figure 5(k) strongly indicates that addition of albite and scapolite alone cannot explain the chemical evolution displayed and that other elements must have been mobile.The strong reduction in Fe 2 O 3 suggests that this oxide is removed during albititisation and to some extent during scapolitisation.The measured variation in Br and Cl which is assumed to parallel Br suggests that these elements are added during the scapolitisation but were removed from the rock during albitisation.The mineralogical evolution, where Cl-scapolite formed during scapolitisation and later broke down during albitisation, suggests that halogens will be present in the fluid also during albitisation and are available for complexing with metals (e.g., Fe, Cu, and Zn).We suggest that such a complexing can explain the many ore deposits in the area and in particular the Langøy Fe-mines.

Metasomatism and Mineralisation. Metasomatism is extensive in south Norway
The Bamble lithotectonic domain is characterised not only by widespread metasomatic alteration, but also by a high density of mineral deposits (Geological Survey of Norway Ore Database [24][25][26]).The high density of apatite and rutile deposits follows the regional distribution of metasomatic alteration in the Bamble lithotectonic domain [23].While ilmenite is the main Ti-bearing mineral in the gabbro protolith, Ti occurs as rutile (Figures 6(a)-6(b)) and in amphibole (<0.34 a.p.f.u.) and biotite (<0.69 a.p.f.u.) within the scapolite metagabbro [20].Replacement of ilmenite by rutile is illustrated in Figures 6(a)-6(b).During albititisation, biotite and amphibole break down and Ti is released as titanite [20,31].The whole rock geochemical data (Figures 5(i) and 5(j)) illustrates that TiO 2 and V increased during scapolitisation and decreased during albitisation.The high values of TiO 2 in some of the albitites are probably inherited from the scapolite enrichment.Fe 2 O 3 decreases during scapolitisation and albitisation and the TiO 2 -Fe 2 O 3 relationships (Figure 5(k)) cannot be the result of pure dilution by adding albite and scapolite but suggest that Fe is removed.The whole rock geochemistry in Figure 5(e) illustrates the P, which is increased during scapolitisation and that the P resources at Ødegården Verk owe their existence to this event rather than the albitisation event which leads to a reduction of P.This is in accordance with earlier works which show that scapolite metagabbros commonly have both a high apatite content in the Bamble lithotectonic domain and host vein-related apatite deposits (Figure 6(c)) [19,28,29].Scapolitisation and albitisation are documented as having formed chlorapatite and hydroxyfluorapatite at Ødegården Verk in Bamble (Figure 6(c)) [19,28,29].
As discussed above, metasomatism of the gabbro causes extensive Fe-depletion (Figure 5(c)) [20].In addition, the whole rock geochemistry shows that the concentration of Cu and Zn is lowered during the scapolitisation of the gabbro/metagabbro (Figures 5(f)-5(g)) and is nearly completely depleted during albitisation of the same protolith.The fluid mobilization of these elements could have caused the widespread occurrences of metal deposits in the Bamble lithotectonic domain [23].Fe-oxide ores are present as hematite-carbonate veins in the rutile-rich albitites in the Kragerø area and are widespread in the Bamble lithotectonic domain [23,58], as hematite-rich albitites, orthoamphibolehematite veins, and albite-magnetite veins.Cu-Zn-bearing base metal deposits are frequent in the Kragerø-Bamble area (Geological Survey of Norway Ore Database).The association of Fe-ores with albitites and altered granites has been reported worldwide, for example, as in magnetite-apatite deposits from the Lyon Mountain area, Adirondacks, New York, USA [67].

Stable Isotopic Results:
Fluid and Rock Origin.The stable isotopic composition of silicate mineral separates can reflect the origin of both the rocks and the infiltrating fluid [68,69].It will retain information from the protolith phases, but, depending on the degree of alteration and replacement, the isotopic composition will undergo a shift during fluid infiltration.Oxygen is already present in significant concentrations in the silicate minerals.To shift the  18 O composition in a silicate mineral will require large amounts of infiltrating fluids.This must be the case for the albitite rocks in the Bamble lithotectonic domain, which have undergone complete alteration to a new mineralogy, involving large chemical changes [20,23,50].
A  18 O SMOW composition of 5.1 to 8.4‰ is seen for the albite separates from albitite formed from a gabbroic protolith (Table 3).Results for albite from a carbonate-rich albitite deposited in metagabbro at Langøy fall in the same range.A  18 O SMOW composition of 8.5 to 10.8‰ is obtained for albite, which originated from a granitoid protolith in the Ringsjø-Ødegården Verk area.From a clinopyroxenebearing albitite in the Åtangen-Storkollen area, the albite gives a  18 O SMOW composition of 10.8 to 11.1‰ and the quartz 11.5 to 11.6‰.The results from measured albitites from both mafic and tonalitic magmatic precursors are in accordance with the original values from such protoliths [70] coupled with the influence of a fluid with both a magmatic and seawater origin [69].Depending on the temperature, the reported O-isotopic signature could originate from a magmatic fluid, although a magmatic fluid would normally give a higher value.Seawater could explain the reported values since it could lower the isotopic ratio relative to the magmatic protolith values.As metasomatic fluid infiltration is often spatially inhomogeneous, this could possibly also explain variations in the resulting values.A meteoric water source can clearly be ruled out, as meteoric water would have led to a significantly lower  18 O SMOW composition of about +2 to −10‰.Mark and Foster [71] document a similar  18 O SMOW composition associated with albitisation in the Cloncurry district, Australia, and concluded that it is due to magmatic processes.
Amphiboles in scapolite metagabbros were produced during fluid infiltration into the dry protolith gabbro [20,29].This implies that the H-isotopic content of the amphiboles, in contrast to the O-isotopes, will give more accurate information regarding the metasomatic fluid.The D-composition of the amphibole from the scapolite metagabbro generally varies between −50 and −59‰ and is in accordance with an igneous precursor [70] infiltrated by magmatic or metamorphic H 2 O [69,72].A hydrothermal saline solution would not affect the D-composition, as it will give similar D-values compared to magmatic and metamorphic fluids.Again, a meteoric water origin can be excluded as it would give significantly lower values for the D composition down to −90 to −140‰.The stable isotopic composition of scapolite-bearing rocks is known from Mary Kathleen, Queensland, Australia [73], where the scapolitisation is interpreted to have been caused by magmatic fluids, and the Greenville Province, Ontario, Canada [74], where scapolitisation was caused by metamorphic fluids originating from a carbonate source.
For the carbonate-rich albitite, the  18 O SMOW values show values similar to silicate rocks, indicating a magmatic source for C (Figure 7).This is supported by the  13 C PDB values, which fall between −6.0 and −4.6‰ and give signatures similar to those for carbonatitic magma.Our petrographic investigations show in addition that breakdown of scapolite during albitisation produces carbonate [50].Dahlgren et al. [32] described vein deposited dolomite marbles giving  18 O = 9.6 to 10.7‰,  13 C = −8.5 to −6.2‰, and high 87 Sr/ 86 Sr ratios of 0.706 to 0.709, which overlaps the values reported from these studies (Figure 6) and values from the Bamble hyperites [37] and vein carbonates [40].Dahlgren et al. [32] suggested that the dolomite marbles were formed from hydrothermal solutions that were channeled into a large degassing zone, which now takes the form of a deformed, regional zone with hydrothermal dolomite deposits, albitites, apatite-veins, and widespread scapolitisation.These authors speculated that the fluids were derived from charnockite intrusions in the region.
As mentioned above, while metasomatism is able to significantly alter the chemical composition of the precursor rock, this alteration may vary spatially.This also applies to the isotope composition of the rocks.Probably this is due to varying temperatures as well as the different water/rock (w/r) ratios that caused the alteration.The variable oxygen isotope  3) and diamonds indicate data from dolomite marble deposits/veins and a calcite + albite + quartz dike in the Kragerø area by Dahlgren et al. [32].BG = Bamble hyperites [37]; CBT = world carbonatites [38]; PC = nonmetamorphic proterozoic carbonates [39]; VC = vein carbonate [40].
composition in all altered rock types from this study, in combination with relatively homogeneous H-and C-isotope ratios, corroborates this assumption.Varying degrees of alteration, variable , and variable w/r ratios can produce isotopic signatures that reflect the values that we have measured and are shown in Table 3.In addition, fluid compositions can also have been varied, even on a local scale, and scales of equilibrium might also have been local, regardless of the widespread regional occurrence of the metasomatic rocks.
For the sampled localities, Engvik et al. [20] reported a Cl-and B-rich environment, Sr-signatures in the scapolite with an initial 87 Sr/ 86 Sr ranging from 0.704 to 0.709, and a regional distribution of lithologies, indicating that the fluid originated from evaporites that were mobilized during regional metamorphism.Our new data on the stable isotopic composition of the albitites and scapolite metagabbro support the interpretation that the original magmatic mafic and granitoid rocks were metasomatised by fluids reflecting a seawater origin or with a possible magmatic component.Depending on , w/r, and the degree of alteration, both fluid types (seawater and magmatic) may lead to the same approximate pattern.What can be ruled out from the H and O stable isotope data is meteoric water as it would have led to significantly lower  18 O SMOW and D SMOW values and also to different  13 C PDB values.
Other stable isotopic constraints in the Kragerø area of the Bamble lithotectonic domain support a mixture of magmatic and metamorphic fluid signatures coupled with seawater as being responsible for the metasomatism.Bast et al. [33] analysed B isotope compositions in tourmaline in order to constrain the possible sources of and the evolution of hydrothermal fluids. 11 B values were found to range from −5 to +27‰ (relative to SRM-951), which suggests marine evaporites interlayered with continental detritus and pelagic clay as a possible B source reservoir.Negative  11 B values were explained by the influence of pneumatolytic fluids associated with granitic pegmatites.Variations in  11 B on a regional km-scale, with small local variations, were explained by fluid infiltration during several generations of pulses.
Measurements of  37 Cl, together with F, Cl, Br, and I concentrations, were used to trace the metasomatic evolution of gabbroic bodies and to understand the interplay between localized and pervasive fluid flow [27,30].The reported Br/Cl and I/Cl ratios (3 × 10 −3 and 25 × 10 −6 ) overlap with the range of ratios measured for marine pore fluids.The unaltered gabbro has  37 Cl values near 0% and a similar value is inferred for the infiltrating fluid.Minimally altered samples have negative  37 Cl values (average = -0.6 ± 0.1‰). 37 Cl values increase (up to +1‰) with increasing evidence of fluid-rock interaction.Measured Cl-stable isotope values of individual apatite grains are heterogeneous and range from −1.2 to +3.7‰.High  37 Cl values are generally correlated with OH-rich zones formed during fluid-aided metasomatic alteration of the chlorapatite, whereas low  37 Cl values, measured in the host chlorapatite, are interpreted to have been of magmatic origin.

Fluid Evolution.
Changes in fluid conditions will affect the geochemical and mineralogical evolution during metasomatism.Fluids with a high Mg-and Cl-content cause scapolitisation and phlogopite formation [20,29], while Na-rich solutions cause albitisation [12,20,21,23].The replacement of scapolite by albite during the albitisation also releases Cl into the albitisation fluid.Metasomatism is enhanced by Cl, which has been shown to be an effective ligand for transporting Fe [75,76].A high CO 2 concentration in the fluid enhances carbonitisation [32].The complexity and evolution of metasomatic fluids penetrating the Kragerø area can be explained by a series of different possible models, which include (1) phase separation of volatiles; (2) internal recycling; and (3) external infiltration, which are further expanded as follows.
(1) Phase Separation of Volatiles.Fluid composition evolves as a function of changes in physical conditions.A decrease in temperature will affect separation of volatiles into different phases [77,78].Separation of hydrous and CO 2 -dominated fluid phases and brines could possibly explain the complex pattern in the spatial distribution of metasomatic rocks containing scapolite metagabbros, different varieties of albitites, and carbonate deposits in the Bamble lithotectonic domain.
(2) Internal Recycling.Albitisation can be controlled by internal recycling of fluids.The observed fluid composition and mineralogical reactions can be an effect of local replacement reactions.Mineral reactions can both release and consume fluid components and solutes, and dissolved elements in one reaction can be used in another reaction.The scapolite gabbro in the Kragerø area is composed mostly of major Cl-CO 2 -dominated scapolite and Ti-, Fe-, and Cl-bearing amphibole.During albitisation, both minerals break down and disappear as the rock is transformed into albitite.During these reactions, all CO 2 , H 2 O, and Cl are released as fluids [50].
Breakdown of scapolite during albitisation results in albite, CO 2 , and Cl via the following reactions: Here and CO 2 react to form calcite (Engvik et al. [50], Figures 4(a)-4(d)) and Cl can be reused as a ligand for metal complexing and transport.Also, replacement of rutile and scapolite by titanite releases CO 2 and Cl, whereas replacement of ilmenite and scapolite will also release Fe: or Breakdown of amphibole during albitisation occurs in two stages: Breakdown of the Cl-bearing amphibole and, subsequently, chlorite releases H 2 O and Cl.Titanium from amphibole crystallizes as rutile [35], while Fe is either deposited as nanoinclusions of magnetite or hematite in the albite [79] or transported and deposited as ores associated with the albitite [23].Excess Na, Al, and Si are used to produce albite or Al-Sirich phases [34], and the Ca is incorporated into the calcite.
(3) External Infiltration.Metasomatism can be controlled by an influx of external fluids.As discussed above, this work, combined with earlier isotopic studies in the Kragerø area [20,27,30,32,33], indicates a mixture of magmatic and evaporitic/seawater signatures.This is in agreement with the regional lithological distribution, which consists of a mixed gneiss region with magmatic rocks and metasedimentary sequences [43,80] metamorphosed during the late Sveconorwegian tectonometamorphic event [20,50].Remobilized volatiles in sediments, possibly together with fluids derived during magmatic activity, were behind the regional metasomatism.Engvik et al. [50] discuss the variety in lithology, mineral assemblages, and replacement related to albitisation, indicating changing physical conditions during albitisation, which possibly occurred in several stages over a longer time interval.Similar metasomatic processes have been also reported in other regions such as Australia [5,81,82], which have been affected by various tectonic processes and crustal movements.The scapolitisation of dry gabbro requires the infiltration of an external fluid.The breakdown of a scapoliteamphibole-dominated metagabbro to albitite could possibly contribute a substantial amount of the necessary fluids.The question as to whether metasomatism occurred in an open or closed system therefore depends on the scale, that is, if we regard the metagabbro rim zone capable of producing reactive solutions, which cause local albitisation in a closed system, or if the large-scale Bamble lithotectonic domain can be considered as a closed system, including both magmatic and metasedimentary sequences.
6.4.On the Dynamics of Fluid Infiltration.As described above, scapolitisation and albitisation have occurred not only as static replacement of rocks, but also throughout those parts, which consist of dynamic deformed veining, breccias, and foliated schists.Fluid infiltration and metasomatic replacement both occur as a pervasive replacement of larger rock volumes as observed in the scapolitisation of gabbros.In addition, a high fluid pressure will cause fracturing, which channelizes the fluids, resulting in metasomatism that is widespread in a brittle deformed structure as single veins, networks of veins, and breccias (Figures 2 and 3 In addition, the role of deformation, as well as existing lithological contacts and lineaments, will affect the spatial distribution of the metasomatic rocks.Concurrent metasomatic infiltration and deformation caused a progression of the resulting foliation into the major regional structure, a process which was synchronous with a regional tectonometamorphic event.
Formation of metasomatic scapolite metagabbros in the Kragerø area is constrained at 600 to 700 ∘ C at mid-crustal levels [19].Formation of the clinopyroxene-bearing albitite in the Kragerø area is calculated to 410-420 ∘ C by Engvik et al. [50], while Mark and Foster [71] constrain similar albitites to 450-550 ∘ C from the Cloncurry district in Australia [71].The local presence of prehnite, pumpellyite, and analcime indicates a low-grade albitisation event at temperatures < 350 ∘ C. The tectonometamorphic setting indicates that the albitisation processes occurred over a time span at middle to upper crustal levels.Although the scapolitisation conditions refer to a ductile crustal regime, fracturing and formation of breccias caused by high fluid pressure [83][84][85] have been described as a precursor stage for ductile deformation in the lower crust [86,87].A variation in fluid pressure can possibly explain the change between brittle and ductile deformation during metasomatism.The ductile formation of foliation in both the scapolite metagabbro and scapolite-bearing amphibolites at Åtangen, following scapolite-cemented brecciation, and similar formation of foliated albitic and chlorite schists at Langøy, illustrate that the deformation changed from brittle to ductile during metasomatism.Both breccias and ductile rock fabrics are well known elsewhere in albitised and scapolitised crust [5,20].
In the Kragerø area, single, metasomatised, large (>1 km 2 ) albitites and scapolite metagabbro bodies have been mapped.These replacement zones, resulting from metasomatic infiltration, are widespread on the regional scale similar to features mapped in the Modum area [21].Age dating of the metasomatism indicates that these processes were part of the regional Sveconorwegian amphibolite-facies, tectonometamorphic phase.These ages are constrained by U-Pb ages from metasomatically generated rutile, titanite, and monazite between 1100 and 1070 Ma in the Bamble area [50] and by a U-Pb titanite age of 1080 Ma in the Modum area [21].A later event connected to Permian Oslo Rift activity is evidenced by Ar-Ar age dating of metasomatically produced K-feldspar [88] and can possibly reflect the low-grade albitisation stage.Fluid infiltration during the Permian is indicated by alteration in the Bohus granite east of the Oslo Rift [89].
The progressive development of albitised schists and scapolite-bearing amphibolites described above illustrates the importance of metasomatic processes during crustal evolution.These mineral phases and lithologies have a widespread occurrence, extending outside the mapped albitites in Figure 1(b).
Although not yet quantified, our results indicate the importance and extent of metasomatic influences on rock formation and structure on a regional scale.
)-3(b)).The replacement zone to the mafic host rock shows a widespread replacement of the mafic phases to chlorite (Figure 3(c)).Intensive albitisation affects part of the area resulting in a 0.5 × 2 km albitite body (Figure 1(b)).

Table 1 :
Key samples with mineral assemblage.

Table 2 :
Whole rock geochemical data, major and trace elements.

Table 3 :
[64]le isotopic data.andheated to room temperature; thus, O 2 is released as a gas and eventually analysed isotopically using a Finnigan MAT 252 isotope ratio mass spectrometer.Oxygen isotope compositions are given in the standard -notation and expressed relative to SMOW (Vienna Standard Mean Ocean Water) in permil (‰).Replicate oxygen isotope analyses of the standards, using NBS-28 quartz and UWG-2 garnet[64], generally have an average precision of ±0.1‰ for 18O.The accuracy of 18O values is commonly better than 0.2‰ compared to the accepted 18O values for NBS-28 of 9.64‰ and UWG-2 of 5.8‰.