Metamorphic petrology of the Froland corundum bearing rocks: the cooling and uplift history of the Bamble Sector, South Norway
TIMO G. NIJLAND, FRANK L1 AUW, DIEDERIK VISSER, CORNEllS MAIJ ER & ANTONY SENIOR.
Nijland, T. G., Liauw , F., Visser , D., Maijer, C. & Senior, A. 1993: Metamorphic petro logy of the Froland corundum-bearing rocks : the cooling and uplift history of the Bamble Sector , South Nor way. Nor. geol. unaers . Bull. 424, 51-63.
Corundum-bearing rocks from Kleggasen , Froland, preserve metamorphic textures which reflect the cooling and uplift path of the Bamble Sector. The poT path may be correlated with a T-t path constructed from published ages. The P-T path starts at the thermal climax of prograde metamorphism, M I (c. 750·C, 7 kb), characte rized by Sil, PI, Bt, Rt and Crn. SUbsequent near isobaric cooli ng results in M 11 , characte rized by Ky + Ms :t Chi veinlets (600-700·C, 7 kb). This marks the onset of rehydration. M III assemblages of Mrg :t Crn formed around 500-570·C and 3-7 Kb, and are followed by M IV (c. 400·C, 2-4 kb) characterized by Ms, Bt and Ep. The trajectory from M 11 via M III to M IV is believed to reflect the uplift of the area in response to Sveconor wegian upthrusting. The uplift was completed before c. 940 Ma, which is the age of post-te cto nic granites and K-Ar biotite cooling ages. The latest episode recorded by the Froland rock s, M V (175-280·C, 2-3 kb), is characterized by Prh, Pmp, Scp and Tur , indicating local influx of B and Cl-bearing hydrous fluids. MV is interpreted as adapt ion to upper crustal cond itions between c. 920 and 760 Ma.
T. G. Nijland, F. Liauw, D. Visser , C. Maijer & A. Senior, Departm ent 01 Geoctiemistry, Utrecht University, P.O. Box 8002 1, 3508 TA Utrecht, The Netherlands.
Introduction are composed mainly of plagioclase, corun The Bamble Sector (Fig, 1) is part of the South dum, biotite and sillimanite, and cut by vein west Scandinavian Domain of the Fennoscand lets of green mica and kyanite. They occur in a ian Shield (Gaal & Gorbatschev 1987), It is 2 m-wide , concordant lens, which is separated composed of a Proterozoic supracrustal suite from steeply dipping banded and granitic gneis that was metamorphosed and intruded by ses by a 20 cm-thick biotitite. A small, intens acidic and basic magmas during the Gothian ively altered part of the outcrop features large (1750-1500 Ma) and Sveconorwegian brown tourma line and a rare chromian rnont (1250-950 Ma) oroqen ies. For a detailed des morillonite, volkonskoite (Nilssen & Raade cription of the litholog ies and structural geolo 1973), The rocks provide information about the gy of the area, the reader ls referred to Star successive stages of post-peak metamorphism mer (1985, 1991), and have been studied to unravel the cooling In the Nelaug-Fro land-Arendal area (Fig. 1), and uplift history of the Bamble Secto r. the grade of metamorphism ranges from up per-amphibolite facies in the north up to granu lite facies in the area around Arendal in the Petrography south (Bugge 1940, Touret 1971, Lamb et al. 1986), poT estimates range from c. 836°C, 7.7 Oldest mineral assemblage (M I) kb in the core of the granulite facies area to The oldest assemblage, M la, cons ists of plagi c. 752°, 7.1 kb in the amphibolite facies area oclase , biotite , fibrol itic and euhedral sillimani (Nijland & Maijer 1992), te, and rutile, with accessory zircon , allanite Corundum-bearing rocks occur at Klegga and apatite (Table 1). Microfolds of fibrolite sen, Froland, 11 km northwest of Arendal, and are enclosed by large plagioclase crysta ls. were first descr ibed by Oftedahl (1963). The Plagioclase is in some cases antiperth itic. rocks have been interpreted as metamorpho Large subhedra l brown biotites, which com sed kaolinite-bauxite weathering crusts (Serd monly contain small sagenite needles , const itu yuchenko 1968), They are leucocratic, and te the foliat ion together with fibrolite. Younqer gneissic in their biotite-rich parts, The rocks euhedral sillimanite generally also follows this 52 Timo G. Nijland et al. NGU - BULl. 424. 1993
100 km
~ Caledonian area ITIIIlll Oslo Graben D Precambrian
Granites. granitic ~fn f l gneisses
Quartzites. cale 1, ::Ili l -silicate rocks. marble
Tonalitic to granodioritic gneisses with metape A litic intercalations N Amphibolites - Metagabbros 1.5 km II Lakes - "not mapped" I I 64850
Fig. ·1. Geological sketch map 01 the Froland area west of the Nidelva river with relevant localities (modilied after Visser & Senior 1990). Coord inates along the margins of the map are accord ing to the grid of Norge s geogr aliske oppmAling. PKF = Porsgrunn-Kristiansa nd Fault, which is the boundary between the Barnble and Telernark Sectors . Inset: Division 01 the Southwest Scandinavian Domain (modilied after Verschure 1985). Numbers denote : 1 - Western Gneiss Region. 2 - Rogaland I Vest-Agder Sector . 3 - Telemark Sector , 4 - Bamble Sector, 5 - Kongsberg Sector. 6 - 0stfold Sector , 7 . Stora Le Marst rand Belt. foliation . but its crysta l faces truncate the fibro prisms at the place where they are overgrown lite (Fig. 2). Large bronze rutile occurs inter by corundum. The M Ib corundum occurs in grown with euhedral sillimanite. Rutile shows two differe nt microstructures: either as large lamellar twinning and is common ly conc entra subhedral to euhedra l cryst als (up to several ted within the hinges of microfolds. cm) grown in the centre of sigmoidal thick Large colourless to pink. pleochroic. corun enings of the fibro lite foliation. or as large dum crystals enclose both fibrolite and euhed archipelago -like crys tals (Fig. 3). Corundum ral sillimanite. biotite, rutile and plagioclase. shows lamellar twin ning. and displays slight and is therefore denot ed as M lb. Boundaries colour zoning with a bluish tinge. espec ially between corundum and the other minerals are along the margins and accord ing to cry stallog usually sharp. and show no signs of reaction; raphic planes; the blue colour become s mor e only sillimanite bottle-neck textures are spora intense along altered margins. dically observed. i.e. the thinning of sillimanite NGU · BULL. 424. 1993 Metamorphic pe trology of the Froland 53
M I M 11 M III M IV M V
S F P B C R Z A ATK M C M CP B M CC E P P S T i i I t r t r I p u y s h r r I t S h a p r m c u I b n n I r I g n I I h PP r
TN75 x x x x x x x x x x x x x TN 76 x x x x x x x x x x x x x TN77 x x x x x x x x x x x x x x x x x x x x x TN 78 x x x x x x x x x x x x x x x x x TN79 x x x x x x x x x TN81 x x x x x x x x x x x x TN82 x x x x x x x x x x x x TN8 3 x x x x x x x x x x TN84 x x x x x x x x x MA993 x x x x x x x x x x x x x FL1 43 x x x x x x x x x x x FL144 x x x x x x x x x x x x x x FL145 x x x x x x x x x FL14 6 x x x x x x x x x x x x x FL1 47 x x x x x x x x x FL14 8 x x x x x x x x x x x x FL 149 x x x x x x x x x x x x x x x FL 150 x x x x x x x x x x x x x x x x FL1 51 x x x x x x x x x x
Table 1. Distribution 01 the successive metamorphic assemblages in the different samples. Mineral abbreviations according to Kretz (1983). except Fib - librolite.
subhedral grains. Rare relics of biotite and fibrolite occur in the veinlets. Although M 11
III minerals are mainly rest ricted to veinlets, gre ;;: .- en muscovite and chlorite also occur disper '. sed throughout the rocks. Here too , they for m ' I .". RT relatively large. subhedral grains, in contrast 'i 0 to M IV muscovite and chlorite which usually occur as rims and fine-grained aggregates I S ll around older phases . M 11 Muscovite outside the veinlets has been found to enclose micro folds of fibrolite. Pl
Margaritization reactions (M Ill) In the host rock (M I assemblage). aggregates of sheaf-like bundles of margarite locally repla ced M la plagioclase. These aggregates are usually present in the vicinity of M Ib corun Fig . 2. M la fibr olite truncated by M la eu hedral silliman ite . dum crysta ls, but the grain boundaries of cor Photomicrogr aph 01 sample TN 84. undum are virtually unaffected. Sporadically. small coru ndum grains occur in the margarite aggregates; these may represent relics. Rutile Kyanite-bearing veinlets (M 11) and biotite occur unaffected in these aggrega Veinlets of kyanite and muscovite with minor tes. chlorite cut through the M I assemblages (Fig. In the M 11 veinlets, kyanite is partially repla 4). Kyanite occurs in the centre of the ve in ced by fine-grained aggregates and rosettes lets, and is weakly bluish pleochroic and tw in of margarite. This replacement has preferent i ned . Muscovite and chlorite occur along the ally taken place along cracks , cleavage planes margins of the veinlets . Both display a pale and grain bounda ries. The margarite is usual green pleochroism. They form relatively large ly accompanied by small cloudy grains of eo- 54 Tima G. Nijland et al. NGU · BULL. 424. 1993
Fig. 3. Archipelago crystal of M Ib corund um enclosed by plagioc lase. Photomicrograph of sample TN8 1.
rundurn (Fig. 5), which are situated in the cent re of the cracks. Traces of muscovite occur in the margarite aggrega tes.
Low-grade asse mblages (M IV) Several low-grade minerals developed after the margaritization reactions. A rim of anort hite rich plagioclase is locally present around both M Ib and M III corundum. Small M III corun dum is in places entirely replaced by aggrega tes of anorthite-rich plagioclase and micas. Anorthite-rich plagioclase and M la matrix pla gioclase have never been observed in con tact. Former grain boundaries and characteris tic cleavage planes of M Ib corund um can be retraced in the anorthite-rich plagioclase. Plagi oclase, biotite and aluminium silicates are local ly altered to sericite I muscovite ± calcite. I xrl B. _ Rt Some pseudomorphs of fine-gr ained muscov i te after kyanite occur. Margarite is in some _ Ms /Chi case s altered to chlori te and calcite. Fine g KY grained brown biotite occurs in small veinlets . Epidote occurs as a few isolated grains. R crn Both anort hite-rich plagioclase and the other phases are grouped in M IV because they 10 mm clearly postda te M III and predate even lower grade phases . Their mutual relationships are unclear, but anorthite-rich plagioclase is likely Fig. 4. M 11 veinlet of muscovite and cnlonte, with kyanite in the centre. cutting the M I assemblage and foliation. to be slightly older , as it has locally been alte Drawn from a thin-section of sample MA993. red to sericite. NGU - BULL. 424, 1993 Metamorphic petrology of the Froland 55
Fig. 5. Replacement of M 11 kyan i le by M III marg arite and cor un dum. Photomicrograph of sample TN77.
Lowest grade alteration (M V) grams, respect ively. Selected analyses are Prehnite and potash feldspar occur together presented in Table 2. A more comprehensive with chlorite as small lenses along the cleav list of analyses may be obtained upon requ age planes of biotite. Seapolite with a birefrin est from the first author. gence of 0.020, has replaced plagioclase in a patchy way, and encloses M 11 kyanite and M $ 7Table 2. Selected analyses. Fib, Sil, J Ta ble 2. Con tinu ed. Biotite Phase ChI xs ~ r q Crn Ms PI Sep Only M I biotites have been analysed . X , Il Il III III I V IV V Mg 5i02 2 6 . 3 8 .. 5.55 32 .60 H. 8 2 5~ . 70 51.82 defined as Mg/(Fe+Mn +Mg ), ranges from 0.69 Ti 0 2 . 0 5 . 9 8 .15 .85 to 0.73. TiO, is present in the range 1.80-2.55 wt.%. Ti shows negative correl ations with AIIV 10. 1 2°) 2 1.90 35 .20 .. 7.9.. 97 .93 H .82 28 .86 2 s . 9 .. Cr 20 J nd . 0 .I. ... . s9 nd nd and XMg. The biotites conta in 0.64 to 0.99 wt.% Cr,O,. The Cr content of biotites from reo 16 . 6 1 ." . '7 . 2 6 1. 10 MnO . 0 6 MA993 is much lower (0.30-0.42 wt%). The Mg O 2 0 . 78 .4. . 7 1 . 6 9 biotites conta in trace amounts of Cl ( ;;'i 0.04 Ca O 8 .94 10 .47 1 1. 22 at 0=22), whereas F ranges from 0 to 0.15 Na 20 1.1 2 1.93 1. 25 5 .89 6 .81 K,O 9 .71 .69 9 .55 . 2R atoms per 22 oxygens ( ;;'i 0.32 wt.%). The P20S nd F-content of the biotites increases towards SO, nd nd nd nd . .. 2 their core. Cl . 0 3 .01 .01 nd :2.07 . 0 7 nd To t al 85 .88 9 3.96 9) .52 99.07 9 ,L 17 100.82 97 .56 si 5.41 6 .12 4.38 6.05 2 . .. 8 7 .66 Margarite Ti . 0 1 . 10 . 0 2 .09 M III margarites contain substa ntial quantities Al 5 .29 5 .58 7 .60 1.98 5 .54 1. 52 e . 34 of Na,O (Table 2). In some margarites, Si is Cr . 0 5 .02 .01 . 1 0 higher than the ideal stoichiometric 4 atoms , f e :2. 8 5 . 0 5 . 04 .00 . 13 Mn . 01 suggesting that the presence of Na is due to Mg 6.34 . 10 . 15 . 1. the plagioclase subst itution NaSiCa.,A1. , (Frey Ca 1. 28 . 50 1. 78 et al. 1982), giving paragonite solid solution. Na . 2 9 . 5 0 . 33 . 5 1 1.95 In other margar ites, the number of Ca atoms 1. 66 . 11 1. 6 4 . 0 5 is low , but Si normal , as has been observed . 0 5 at other localities where marg arite has grown Cl . oi . 0 0 . 0 0 .52 out of aluminium silicates (e.g. Guidotti & . 04 Cheney 1976, Gibson 1979, Baltatzis & Kata gas 1981). Froland margarites have Ca/(Ca+ Aluminium silicates Na) ratios betwee n 0.72 and 0.79. Ca/(Ca+Na) All aluminium silicates have a near ideal stoi ratios in margarite are cons iderably higher chiometrical composition. Fe is present in than those of coex isting plagioclase (0.38-0. minor amounts only, but Cr contents are high. 43), as is commonly observed (Ackermand & M la fibrolite conta ins 0.72 ± 0.05 wt.% Cr,O, Morteani 1973, Gibson 1979, Frey et al. 1982). (mean ± 1a ), M la euhedra l sillimanite 0.69 The amount of K,O in the margarites shows ± 0.07 wt.%, and M 11 kyanite 0.70 ± 0.09 a weak positive corr elation with the amount wt.% in samples TN77, 78, 84 and FL144. The of Na,O. Cr content of the aluminium silicates is much Cr,O, contents ( ;;'i 0.59 wt.%) are higher lower in sample MA993: Kyanites contain only than usual, and even higher than in the Cr 0.24 ± 0.07 wt% Cr,O, and sillimanites 0.50 rich margarites reported by Morand (1990) from ± 0.04 wt%. No systema tic difference in Cr,O, New South Wales, Australia, which contain conte nt is shown by aluminium silicates from up to 0.37 wt.%. The highest Cr cont ents ( ;;'i the success ive metamorphic phases . The Cr 1.93 wt.%) recorded in retrograde margarites contents of Froland sillimanite and kyanite are are from the New Zealand Alps (Cooper high for metape lites , which both typically conta 1980). The Cr,O, contents in the Froland marga ins less than 0.20 wt.% Cr,O, (Kerrick 1991, rites reflect the who le rock composition, and and refer ences there in). are lower for MA993 margarites than those fro m other samples. The FeD conten t (;;; 0.45 Corundum wt.%) is norma l for margarites. TiO, in the Both M Ib and M III corund um conta in small Froland margarites is less than 0.35 wt.%. amounts of Fe and Cr. In spite of its colou r Only traces of Mg, Cl and F have been detected. zoning, M la corundum does not show any detectable chemical zoning. M III corundum Muscovite s./. has a lower Cr,O, content than the M Ib corun Chemically, no distinction could be made bet dum, respectively 0.78 ± 0.28 wt.% and 1.07 ween M 11 and M IV muscovites. The muscov ± 0.10 wt.%. ites show limited solid solution with paragon- NGU · BULL. 424. 1993 Metamorp hic petrology of the Froland 57 ite, and minor Ca subst itution. All muscov ites Neither quartz nor other silica-bearing reac conta in FeO (0.37-1.29 wt.%), c-.o, ( ~ 0.90 tions products are present. Therefore, the wt.%) and Ti02 (0.30-1.42 wt.%). The muscov growth of corundum must have involved remo ites contain no Cl and only traces of F ( ~ val of silica by a fluid phase: 0.13 wt.%). (1)AI,SiO, (Sil) + 2 H,O'" AI,O, (Crn) + Si(OH), Plagioclase and scapolite Both M la and M IV plagioclase are almost This reaction is not restricted to a narrow P-T orthoclase free. M la plagioclase has an anor space, but mainly depends on the activity of thite content of 38-43 %. M IV anorthite-rich SiO, in the coex isting fluid, and may have plagioclase has 48-51 %An. The scapol ites occurred at temperatures higher than c. 450 are Cl-rich (1.78-2.10 wt.%) and SOl-poor ( ~ 500°C (cf. Hemley et al. 1980) for the pressu 0.42 wt.%), with trace amounts of fluor ine ( ~ res recorded in the Bamble Sector (7-8 kb). 0.14 wt.%). The meionite percentage ranges With exception of the replace ment of fibro from 45 to 47%. lite by euhedral sillimanite, no trace of the prograde P-T history has been found in the Rutile Froland rocks. Neither early mineral inclusions nor chemical zonat ions have been observed. Cr is the only element substituting significant Nevertheless, M I is interpreted to represent ly in rutile. Cr.O, is present in moderately high the thermal climax of metamorphism, because amounts (0.70-1.05 wt.%). sillimanite is the ubiquitous aluminium silicate present in the Bamble Sector (Starmer 1976, Chlorite Touret & Falkum 1987). From the MI assem XMg of the chlorites ranges from 0.64 to 0.70. blage, it is impossible to estimate peak meta AIIVvaries between 1.85 and 2.72. TiO, occurs morphic P-T cond itions. However, as M I pha in considerable amounts , up to 1.02 wt.%, and ses sillimanite, biotite and plagioclase are traces of Mn, Ca, Na, K, and S may be present. peak metamorphic minerals in this part of the Bamble Sector, we will adopt the peak meta Reaction history and P-T path morph ic P-T estimates for the Froland area. Nijland & Maijer (1992) estimated P-T cond i Thermal climax (M I) tions at 752 ± 34°C and 7.1 ± 0.4 kb based The oldest mineral assemblage, M la, is repre on mineral pairs in amphibolites. Visser & sented by plagioclase, sillimanite, biotite and Senior (1990) obtained nearly identical results rutile. The observed textures do not direct ly of 740 ± 60°C, 7 kb, from aluminous encla reveal the relationship between fibrolite and ves in cordierite-orthoamphibole rocks. the euhedral sillimanite. However , the euhed raj sillimanite truncates the fibrolite foliation Initial cooling (M 11) (Fig. 2). This is interpreted as evidence that The M I assemblage is replaced by M 11 kyan euhedral sillimanite replaces the fibrolite (et. ite, muscovite and minor chlorite along vein Vernon 1987). lets. As relics of biotite and sillimanite are M Ib corundum encloses both types of silli still present in these veinlets, the replacemen t manite, but mainly grew at expense of the fibro probab ly took place by the generalized reac lite, as indicated by the occur rence of large tion: corundum crysta ls in the centres of fibrolite aggregates . This may be explained by the lar (2) K,.,Na..IMgllFe,.,Tio., ger effect ive reaction surface of the fibrolite Cr.,Alu Si,.,O,.(OH), (Bt) aggregates with respect to the euhedral sillima + 3 AI,Mgo.o,Si,.oO, (Fib) + 0.5 Ca. ,Nao.,AI,.,Si,.,O, (PI) nite, whereas removal of silica will have been + 4 H,O + 0.25 FeH much easier along the grain boundary net .. K,.,Na. ,Mgo.,Tic,, works in fibrolite aggregates than from massi Cro,,AI,.eSi. ,020(OH), ve euhedral sillimanite. Contacts between co (Ms) + 0.5 Mg. 1Fe,.,AIl,lSi,.,020(OH )" (Chi) + AI,.Si,.O, (Ky) + 0.1 rr- rundum and the euhedral sillimanite are usual H ly sharp crystal faces . However, bottle-neck + 0.08 MgH + 0.2 Ca + 0.1 Na' textures involving this sillimanite indicate that As in many terrains, the occurrence of the M it was also partially consumed by corundu m. 11 assemblage in veinlets show s that kyanite 58 Timo G. Nijland et al. GU - BULL. 424 , t993 developed not directly from sillimanite, but by Uplift and furthe r cooling (M Ill) a 'transport reaction', i.e. by means of an inter Rehydration progressed by the margar itization mediary fluid phase. The development of the of plagioclase and kyanite during M Ill. As M 11 assemblage is the first response of the outlined above, margarite form ed both in the rock s to re-intro duction of hydrou s fluids after host rock and in the M 11 kyanite-bearing vein the metamorph ic climax. A small drop in tempe lets. Textura l relations of margarite with re rature at nearly constant pressure might have spect to corundum are different in both environ been sufficient to effectuate the growth of ments. In the host rock (MI assemblage), kyanite and hydrous phyllosllica'es.M 11 is corundum remains unaffected or occur s as constrained by the kyanite-sillimanite phase reaction relics within margarite aggregates. In boundary and reaction (3) which marks the the M 11 veinlets, corundu m occurs, and was dawn of M Ill. This implies that M 11 took place most likely formed together with margar ite in at temperatures of about 600-700°C (Fig. 6a), cracks in kyanite. We will first discuss the or slightly lower (See below). reactions in the host rock (reactions 3-4), and subsequently consider the M 11 veinlets (reacti 10-,------,------:------:,------, ons 5-8). In the host rock, margarite may have been 8 produc ed by the following reaction: (3) CaAl,Si,O (An) + AI,O, (Crn) + H,O .. CaAI. Si,OIO(OH), (Mrg) lS cL 4 (Gibson 1979, Dymek 1983). This reaction has been calibrated experimentally, and provides 2 an upper temperature limit of c. 650°C at 7 a kb (Chatterjee et al. 1984). However, in case 0 200 400 600 800 T(OC) of PH20 < P otat- reaction (3) will be shifted to 200 lower temperatures. The addition of Na to the CASH system will stabilize plagioclase over 400 margarite (as indicated by Ca/(Ca+Na) ratios) and consequently also shift reaction (3) to _ 600 lower temperatures. Cr is likely to favour coru n ~'" dum. Concluding, reaction (3) will in reality be 800 situated at somewhat lower temperatures than its ideal pos ition in the CASH system (Fig. 6a). 1000 Another possible reaction resulting in the s m-Nd growth of margarite in the Froland rocks , 1200 which involves the consumpt ion of plagioclase b with fibro lite inclusions (which were already metastable as the rocks had passed into the Fig. 6. The cooling and uplift history of the Bamble Secto r. kyanite field), may be reaction (4): (a) The P-T path as deduced from the Frotano rocks . Mine ral abbreviations are after Kretz (1983). Indicated are: the aluminum silicates after Holdaway (1971), the peak meta (4) CaAl,Si,O (An) + AI,SiO, (Fib) + 3 H,O morphic poT box after Nijland & Maijer (1992), the fields of metamorphic episodes M I-V, relevant reaction lines 3, CaAI.Si,O,o(OH), (Mrg) + Si(OH). 6, 9, and 10 as discussed in the text , the prehnite-p umpel Iyite tacies field of Frey et al. (1991) with 2 and 3 kb. iso In this reaction, silica again has to be remo bars (See text). ved by a fluid phase. In a silica-saturated (b) The r -r path as constructed from separate ly published mineral ages. The T-t path was constructed with the mean environment, the reaction would occur betw e ~ one sigma of the follow ing published mineral ages (All en 3 and 7 kb ( PTotal = P H20 ) and in the tempe data recalculated to current decay const ants): Sm-Nd Mineral ages of Kullerud & Dahlgren (1992). K-Ar (Hbl) rature range 500-570°C (Chatterjee et al. O 'Nio rt s et a l . (1 9 6 9). K -Ar ( 8 1) - Kulp & Neumann (1 9 6 1), 1984). Fibrolit e is a lr e ady metastable, which O'Nions et al. (1969), de Haas et al. (1992c). Fission tracks enhances the growth of margarite. The under (Grt) and (Ves) -Haack (1975). Fission tracks (Ap) - Van Haren & R6hrman (1988). The following closure temperatu saturation of silica will also favour margarite, res were used:Sm-Nd (Grt) - Mezger et al. (1992). K-Ar (Hbl) and shifts the reaction (4) to higher tempera tu - Harrison (1981). K-Ar (Bt) - verscnure et al. (1980). Fissi on tracks (Grt) and (v es) - Haack (1977). Fission tracks (Ap) res. This will be opposed, however, by the - Sharma et al. (1980). effect of Na (see above). NGU- BULL. 424, 1993 Metamorphic petrology of the Froland 59 In the M 11 veinlets, margarite formed within (aCa2+laH.') vs. temperature and log(asi02(aq)) cracks in kyanite. Retrograde reactions (5) vs. temperature diagrams show that margarite (Guidotti et al. 1979, Cooper 1980, Feenstra and corundum are stable with respect to anort 1985) and (6) (Yardley and Baltatzis 1985) hite and corundum below a maximum tempera have been proposed to explain the growth ture of 583°C at PTotal = PH20 = 5 kb, provided of margarite at the expense of kyanite in oth that the silica activity in the coex isting fluid is er rocks : lower than 0.1. This temperature is in fair agreement with cond itions for reaction (6) in (5) 2 AI,SiO, (Ky) + Ca> + H,O + 'I, 0 , ... the silica saturated system (Chatterjee et al. CaAI,Si,O,o(OH), (Mrg) 1984). The stability of margarite plus corundum with respect to aluminium silicates depends (6) AI,SiO, (Ky) + CaAI,Si,O, (An) + 3 H,O ... entirely on the silica and lime saturation of the CaAI,Si,O,o(OH), (Mrg) + Si(OH), coex isting fluid. Moderately low log(asi02(aq)) and relatively high log (aca2+la H.') stabilize the Quartz is not present in any of the Froland assemblage margarite plus corundum , where samples , which excludes the possibility of as lower silica and higher Ca'+ activities fa reaction (6), unless all silica has been removed vour the form ing of clinozoisite I epidote plus by a fluid phase . Reaction (5) may have procee corundum. Addition of Na and Fe to the sys ded in the Froland rocks , but does not ac tems tends to narrow the stability field of count for the occurrence of small grains of margarite plus corundum in favour of epidote corundum in the centre of the margarite-filled plus corundum. cracks in kyanite. The latter also holds for Margarite and an epidote group mineral (6). Therefore, other reactions have to be cons i have not been observed together, indicating dered for the margaritization of kyanite. A that reaction (9) did not take place. poss ible reaction, involving the consumption of anorth ite, is: (9) 5 CaAI,Si,O, (An) + 2 H,O ... CaAI,Si, OIO(OH), (Mrg) + 2 Ca,AI,Si,O,,(OH) (Zo) + 2 SiO, (7) 2 AI,SiO, (Ky) + CaAI,Si,O (An) + 5 H,O ... CaAI,Si,O,o(OH), (Mrg) + AI,O, (Crn) (Chatterjee et al. 1984). This constrains the + 2 Si(OH), lower temperature side of M Ill. The reaction line may slightly deviate from its ideal positi However, there is no textural evidence in the on. Addition of Na will stabilize plagioclase ; Froland rocks for the involvement of plagiocla regard ing the anorth ite-content of M I plagioc se in the breakdown of kyanite. Kyanite and lase (38-48%), this will inevitably have occur plagioclase have not been found in contact red. PH20 < PTotal will have the same effect. with each other, but late scapolite (proba bly Addition of Fe to the CASH system will favour replacing plagioclase) has been found enclo zoisite + margarite, and, under silica-undersa sing kyanite . This indicates that plagioclase turated conditions zoisite (epidote) over marga and kyanite may have coex isted in the M 11 rite. veins. The reaction may also have taken place by a fluid phase : Late-stage cooling (M IV & MV) The growth of M IV minerals , Le. muscovite, (8) 3 AI,SiO, (Ky) + ea- + 3 H,O + 'I, 0 , ... new biotite , epidote , etc., indicates adapt ion CaAI,Si,O,o(OH), (Mrg) + AI,O, (Crn) + Si(OH), to greenschist facies conditions. No mineral reactions could be deduced , so M IV is not To our knowledge, reactions produc ing marga well constrained. However, its PoT cond itions rite plus corundum from aluminium silicates are indicated by the following: (1) The new have not been reported until now. The only growth of biotite, indicating that the rocks reaction published so far resulting in the growth were still in the biotite stability field; (2) The of both minerals is: 8 Dsp + Prl + 2 Cal = 2 absence of diaspo re; and (3) The nearby occur Mrg + Crn + 2 CO, + 3 H,O, occur ring progra rence of late andalusite. de in marbles (Okrusch et al. 1976). The absence of diaspore indicates that reac Rosing et al. (1987) calculated equilibria tion (10) did not occur: betwee n aqueous solutions and minerals in the CASH and NCFASH syste ms. Their log (10) AI,O, (Crn) + H,O ... 2 AIO(OH) (Dsp) 60 Timo G. Nijland et al. This constrains the lower temperature limit to started at slightly higher temperatures than 400°C. At PH20 < PTotal' reaction (10) will be those of the prehnite-pumpellyite facies. shifted to lower temperatures. Summar izing, the retrograde poT path (Fig. Metapelites at Grimassn (Fig. 1), in the di 6a) starts with near-isobaric cooling (M 1-11 ), rect vicinity of the Froland locality (c. 4.5 km) followed by near-isothermal decompression (M contain late blast s of andalusite and stauro Ill), and is concluded by slight decompression lite, which overgrow the old biotite foliation. during cooling from greenschist to prehnite Cordierite-orthoamphibole rocks from the near pumpellyite facies conditions (M IV-V). From by Boyletoss locality (Fig. 1) contain both late the discuss ion above, however, it appears stage andalusite and kyanite (Visser & Senior that especially the latest stages indicate sliding 1990). This indicates equilibration at or below changes rather than discrete episodes . the andalusite-kyanite cotectic line. The first part of the deduced retrograde P-T MV is cons trained by the coexistence of path (Fig. 6a) essentially confirms the paths prehnite and pumpellyite (Fig. 6a), which limits proposed by Touret & Olsen (1 985) and Vis the temperature to 175-280°C and pressure ser & Senior (1990). However, the stability fi from 0.5 to 4.5 kb (Frey et al. 1991). Pressure elds of the margarite-bearing assemblages is more narrowly constrained to 2-3 kb by the require a PH20 considerably higher than the fluid inclusion data of Touret & Olsen (1985), " ,PTotal suggested by Visser & Senior (1990) who observed that H20-rich inclusions were for the same metamorphic stage (their M4). spatially associated with prehnite and pumpel The retrograde P-T path of the Froland rock s Iyite. Growth of M V hydrous minerals like also agrees with the cooling path of the Mo prehnite, pumpe llyite, Cl-rich scapo lite and dum Complex of the Kongsberg Sector (Munz tourmaline record the local influx of CI- and 1990), which at that time may have constituted B-bear ing hydrous fluids. one continuous terrain with the Bamble Sector (Bugge 1936, Starmer 1985). A small increase in press ure between M I and M 11 , as sugge Retrog rade P-T pa th sted in the Modum Complex, is not found in The cooling and uplift path of the Froland the Bamble Sector. rocks starts at M I, for which we adopt P-T estimates from the same area (Fig. 6a). To form the M 11 veinlets, while anorth ite + corun Time constraints dum + vapour remain stable, limited cooling M I in the Froland rocks is most likely equiva must have occurred. As discussed above, lent to the M 3b of Visser & Senior (1990), addition of Na and Cr to the CASH system which is the thermal climax of their prograde will shift reaction (3) to lower temperatures P-T path, and the sole sillimanite-producing than indicated in Fig. 6a. The field of M 11 stage in their rocks . These authors considered will have been larger, and the rocks may have this stage to be related to Gothian continental experienced a slight uplift. collision. This interpretation may be supported The formation of margarite from M 11 kyan by the fact that sillimanite occur s in the folia ite implicates that the rock s passed reaction tion cut by the basic intrusions of the Jarnrnas line (6), which constrains the upper limit of M knutene Gabbro (Fig. 1), which have been Ill. The lower temperature limit of M III is less dated at 1766 ± 190 Ma (Sm-Nd whole rock , well constrained. Reaction (9) has not been de Haas et al. 1992a). observed in the Froland rocks , but may be However, Kullerud & Dahlgren (1992) recent situated at slightly lower temperatures than ly obtained Sm-Nd mineral ages betwee n 1068 indicated in Fig. 6a. and 1107 Ma for granulite facies assemblages The M IV stability field must have been along near Arendal. Sm-Nd mineral isotopic systems the andalusite-kyanite reaction line (Fig. 6a). are still poor ly understood, and blocking tempe Its upper temperature limit is constra ined by ratures not defined for most minerals. Recent the lower limit of M Ill. The M V stability fields ly, Mezge r et al. (1992) concluded that the are well constrained by the occurrence of preh block ing temperature of granulite facies gar nite + pumpellyite (Fig. 6a). However , rocks nets is ca. 600 ± 30°C for the Sm-Nd sys from the alteration zone contain an over tem. This is ca. 150°C below our peak meta whelm ing amount of prehnite with respect to morph ic conditions, and would reflect M 11 pumpe llyite. Therefore, it is likely that M V rather than M 1 temperatures. Consequently, NGU • BULL. 424. 1993 Metamorphic petrology of the Froland 61 M I will have occurred during or before the Tectonic history Early Sveconorweg ian. The combined poT and T-t path s show that Early Sveconorwegian gabb roic intrusions after 1150 Ma the Sveconorwegian was domi in the Froland area (1.2 ± 0.1 Ga, Sm-Nd nantly a coo ling and uplift event. The slow whole rock + minerals; de Haas et al. 1992c) uplift points to prolonged residence at lower still enjoyed amph ibolite facies metamorphism crustal levels after M I, and indicates a crust (de Haas et al. 1992b). Garnet-hornblende ther thickened by overthr usting, probably with mometry yields temperatures of ea. 720°C for magmatic addit ion and slow erosion. Evidence this metamorphism (G.J.L.M . de Haas pers . for magmatic addition is provided by the evolu comm . 1991), indicating that high-temperature tion of gabbroic intrusions perforating this crust cond itions prevailed at this time. Therefore, (Frost et al. 1989, de Haas et al. 1992ab). the onset of coo ling is likely to be Early to The drop in press ure after initial coo ling, Mid Svecono rweg ian (Fig. 6b). The end of reflected by the M III and M IV, represents uplift (Fig. 6b) is constrained by K-Ar biotite upthrusting of the Bamble Secto r, probably coo ling ages that average 940 Ma (Kulp & accompanied by eros ion or tecton ic unroo fing. Neumann 1961, O'Nions et al. 1969, de Haas The intrusion of undeformed granites betwee n et al. 1992c). The timing of the latest metamorphic phase, 990-950 Ma throughout the Southwest Scandi MV, is less evident. Prehnite, pumpellyite and navian Domain (Priem et al. 1973, Pedersen & Maalee 1990, Kullerud & Machado 1991' scapo lite are also present in several post b iot i ~ tectonic microdolerite dykes in the area, and among others) and slightly younger K-Ar have been reported from other rocks as well te cooling ages of 940 Ma (Kulp & Neumann 1961, O'Nions et al. 1969, de Haas et al. (e.g. Field & Rodwell 1968). The dykes are generally cons idered to be of Permian age, 1992c) indicate that the process of upthrusting but may partly be contemporaneous with ba was completed before this time. Sveconor sic dyke swarms in the Rogaland Sector, that weg ian upward movement is likely to have were dated by Sundvoll (1990) as Late Prote ro occurred along deep ly penetrating, low-angle zoic. Preliminary results from radiometric stu shear zones. The imbricated structure of the ~ve co no rweg ia n dies in the Arendal area seem to confirm this terr ain in southern Norway (M.M. Moree pers . comm . 1992). Corre lation IS not as clearly demonstrated as in SW Swe of M V with Caledon ian activity (Verschure den (Eugeno-S Work ing Group 1988, Park et 1982) is unlikely because of the large distance al. 1991), but the deep, gent ly dipping, seis from the Caledon ian front. Sauter et al. (1983) mic reflector s cutt ing the Moho below south suggest that the growth of prehn ite was due ern Norway and the Skagerrak (Husebye et to Permian thermal disturbance. Available fissi al. 1988, Lie et al. 1990, Pedersen et al. on-track ages from the Bamble Sector are illus 19990, Kinck et al. 1991) may be remnants trated in Fig. 6b. Apatite fission -track ages of of the zones along which upthrusting occur c. 240 Ma (van Haren & R6hrma n 1988) indica red. After upthrusting, the formerly lower crus te that temperatures were too low to prod uce tal rocks adapted to their new upper crustal prehnite and pumpe llyite during the Permian. environment. Therefore, MV was probably Haack (1975) obta ined fission-track ages on not related to a separate thermal event, but garnet (924 Ma) and vesuvian ite (761 Ma) from just reflects the adapt ion of lower crusta I rocks the Arenda l skarns . Andriesse n (pers. comm. to their new upper crustaI environment. 1991) obtained slightly younger ages for gar net (c. 900 Ma) and slightly older ages for Acknowledgements R.P.E. Poorter and T. Boute n assisted with the microprobe vesuvianite. Both minera l ages suffer from not analyses .. J.B.H. .Jansen and G.J.L.M . de Haas critically well establ ished annealing temperatures (P. read. earlier versions of this paper. Blakstad Yrkessk ote provided lodging during fieldwork. This study was made A.M. Andriessen pers . comm . 1991). Haack possible by financial suppo rt of AWON (NWO grant 751· (1977) estimated annealing temperatures of 353·023 . TGN). The paper benefitted from the constructive 135-155°C and 280-300°C for vesuvianite and reviews by I.A. Munz and an anonymous reviewer. garnet, respectively, although the latter may actually be a little too low . This indicates that MV, which equilibrated betwee n 175 and 280°C, should have equilibrated between c. 760 and 920 Ma. 62 Timo G. Nijland et al. GU - BULL. 424. 1993 References Isotopic constraints on the timing of the crustal accreti Acke rman c , D. & Morteani, G. 1973: Occurrence and break on of the Barnbte Secto r, orw ay. as evidenced by down 01 paragon ite and margan te in the Greiner Scrue coronitic gabbros . Precamb. Res.. in press . fer Series (ZlIIerthal Alps . TyrOl). Contrib. Mineral. Pet Haas . G.J.L.M . de . Versch ure . A.H.. Ma ijer, C. & Val ro l. 40. 293-304. brac ht, P.J . 1992c: Isotopic age deter minat ion s in South Baltatzls. E. & Katag as, C. 1981: Margante pseucomorphs orwa y. Ill. Ab·Sr isotope syste matics of the cc ro runc after kyanit e in Glen Esk, Scotland. Am. Minera l. 66. Vestre Dale Gabb ro and its country rock s. Bamb/e. 213-216. orw ay. 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Proterozoic as revealed by major and mega-tectonics Manuscript received September 1992; revised types cript November 1992; accep ted February 1993.