high-grade contact metamorphism of calcareous rocks from...

American Mineralogist, Volume 82, pages 1241-1254, 1997

High-grade contact metamorphism of calcareous rocks from the Oslo Rift,Southern Norway

BrOnN Jnvrrvnrrr' Svnn Dlnr,cnnNr2 AND HalxoN Ausrnnnrvr3

'Department of Geology, University of Oslo, PO. Box 1047 Blindern, N-0316 Oslo, Norway'?The Norwegian Petroleum Directorate, PO Box 600, 4001 Stavanger, Norway

rMineralogical-Geological Museum, Sars 9t.1, N-0562 Oslo, Norway

Ansrnact

Shallow-level plutons caused extensive contact metamorphism of Lower Paleozoic shaleand carbonate sequences in the Permian Oslo Rift. A >500 m long and 100 m wide shale-limestone xenolith embedded within monzonites belonging to the Skrim plutonic complexexperienced high-grade contact metamorphism and generation of minerals and mineralassemblages rarely reported from metamorphic rocks. The peak metamorphic (Stage I)assemblages in calcite-saturated rocks include wollastonite, melilites, fassaitic pyroxenes,phlogopite, titanian grossular, kalsilite, nepheline, perovskite, cuspidine, baghdadite, pyr-rhotite, and occasional graphite. Mineral reactions involving detrital apatite produced aseries of silicate apatites, including the new mineral species Ca..(Th,U),.Si.O,r(OH). Thisassemblage equilibrated at T : 820-870'C with a C-rich, internally buffered pore-fluid(20-40 mol%o CO, + CI{"). During cooling the shale-limestone xenolith experienced infil-tration of C-poor (< 0.7 mol%o CO,) fluids, triggering the formation of retrograde (StageII) mineral assemblages comprising monticellite, tilleyite, vesuvianite, grandite garnets,diopside, and occasional hillebrandite. Rare potassium iron sulfides (rasvumite and djer-fisherite) formed at the expense of primary pyrrhotite. These assemblages probably formednear 700 'C. Formation of diffuse sodalite-bearing veinlets was associated with breakdownof nepheline and the replacement of kalsilite and wollastonite by potassium feldspar. Thesodalite-bearing Stage III assemblage formed by the infiltration of saline brines at a max-imum temperature of 550 "C. Low-temperature (Stage IV) retrogression of the Stage I-I[assemblage produced scawtite, giuseppettite, hydrogrossulars, phillipsite, thomsonite, andthree hitherto undescribed mineral species.

INrnooucrroN

Mineral assemblages and mineral reactions occurringduring contact metamorphism of calcareous rocks havebeen extensively studied and described since the pio-neering work by Goldschmidt (1911) in rhe Oslo rifr. Ob-served mineral assemblages reflect a wide range of tem-perature and fluid-composition space (Tiacy and Frostl99l). Equilibrium temperatures approaching and evenexceeding 1000 "C are reported for limestone assem-blages in aureoles around mafic inffusions (e.g., Joesten1983). Because most mafic intrusions crystallize withoutreleasing large quantities of volatiles, most high-gradecontact metamorphic carbonate rocks have buffered thelocal pore-fluid composition during heating and devola-tilization. High-grade assemblages refl ecting equilibrationwith HrO-dominated fluids are rarely reported from shal-low level (P < 1000 bars) environments (see however,Williams-Jones 1981 and Inderst 1987).

In the Oslo area, contact metamorphism is related tovarious shallow level intrusions. Frequently devolatiliza-tion reactions in calcareous rocks are driven by infiltra-tion of externally derived HrO-dominated fluids (Jamtveit

et al. 1992a,1992b; Jamtveit and Andersen 1993; Sven-sen and Jamtveit 1998). Here, we present the first de-scription of high-grade mineral assemblages in contactmetamorphic carbonates from the Oslo rift. A large xe-nolith of Lower Paleozoic sedimentary rock within thePermian Skrim monzonite complex in the Southern Oslorift experienced metamorphic temperatures exceeding thesolidus of the crystallizing monzonite. Mineral reactionsinvolving carbonate and silicate minerals led to the pro-duction of various of Si-poor silicates, including numer-ous phases rarely or never previously reported from meta-morphic rocks. Reactions between silicates and pyrrhotiteproduced rare potassium iron sulfides, and reactions in-volving detritial apatite produced LREE and Th-rich sil-icate apatites including a new mineral species[Ca..Th, rSi.O,, (OH)].

Gnor,ocrcal, sETTING

The Permo-Carboniferous Oslo paleorift is situated inthe southwestern part of the Baltic shield. The exposedparls of the rift are composed of two half grabens filledwith Carboniferous sedimentarv rocks. alkaline volcanic

0003-004x/97 | 1 | 1 2-124 l$0 5.00 t241

1 1 / 1 JAMTVEIT ET AL.: CONTACT METAMORPHISM OF CALCAREOUS ROCKS

Frcunn 1. Simplified geological maps of the Flekkeren area (left; Dahlgren 1998) and the Southern margin of the Oslo rift(right; Dahlgren 1998)

and intrusive rocks, and Lower Paleozoic sedimentaryrocks (Fig. 1). The latter are dominated by shales andlimestones (Bjgrlykke 1974). Extensive intrusive activityin the time interval 300-250 Ma (Sundvoll et al. 1990)included emplacement of various monzonitic to graniticrocks and caused widespread contact metamorphism andhydrothermal activity within the originally unmetamor-phosed sedimentary rocks.

The Lower Paleozoic shales are composed of illite,chlorite, quafiz, diagenetic feldspar, and occasional do-lomite, whereas the limestones are dominated by calcite(Bj0rlytct<e 1974). Both shales and limestones contain sig-nificant but generally low contents of organic carbon (<I wtVo). During contact metamorphism, the most notablemetamorphic reactions took place at the carbonate-shalecontacts where metasomatic calc-silicate zones formed atthe expense of the carbonate and shale layers. Maximumtemperature conditions near the intrusive contacts wereestimated to be in the range 400-550 "C (Jamtveit et al.

1992b; Svensen 1996), consistent with dominantly con-ductive heating of the aureoles.

Recent stable isotope results have demonstrated wide-spread pervasive infiltration of aqueous fluids with amagmatic O isotope signature around the various coolingintrusions (Jamtveit et al. 1992a, 1992b, 1997). Decar-bonation reactions leading to formation of the contactmetamorphic mineral assemblages are to a large extentdriven by this infiltration. Reaction-enhanced porosityand permeability at the shale-carbonate contacts was asignificant factor in controlling fluid release from theseshallow intrusions (Jamtveit et al. 1997). The currentmodel for fluid flow during peak contact metamorphicconditions around granitoid intrusions in the Oslo rift in-cludes early pervasive and slow infiltration of magmaticfluids of low salinity in an overpressurized system (rela-tive to hydrostatic), intemrpted by rapid and focused re-lease of saline solutions with associated pressure drops inhigh permeabil ity zones.

^"'f\ l )

Itkisilianiii i i

$i13l't

iv

I '

r - l t f t i r tf i f + f t t t t- + f + i i t i +t f f t t + + +f t t t f f t tt r t + f t t t rf t t f r i t f

: : : : : j :: :: :: :: : :: :: : : : : : : : : i-N:i!\

\.. @l LakeFlekkerenlttletamorPhicfaull -

Ordovician Limestones

PermianSkrimBatholith Mesoproterozoic

liit+*lrufatisyenrte N\AmphibolitesandMetaleucogaooroEiJJI urititr llllilQuartzofetdspathicgneisses

Carboniferous.Permian LateProterozoic

iiil various bathotiths p rencarbonatite complex

4l1niorladt lvlesoproterozoicFtl:]I! .. : nl:iii,;]] Volcanics FFf,l Pegmatites

Cambrian.Silurian \-' 66561s and Amphibolite\\\ 56s1s5, limestones ffi Variousgneisses

JAMTVEIT ET AL.: CONTACT METAMORPHISM OF CALCAREOUS ROCKS

TaeLe 1. Abbreviations and formulas for observed minerals in limestones and calc-silicate rocks from Flekkeren

1243

Name Petrografic status

abaoakalanapauoabgbr

albiteandraditeakermanitealabanditeanorthiteapatiteaugite (fassaite)baddeleyitebaghdaditebritholitecalsitechromitecuspidinediopsidedierfisheritegalenagraphitegrossurargiuseppettitehillebranditehibschitekatoileK{eldsparkimzeyitekalsilitel0llingitemarialitemarcasitemagnetitemonticellitenephelineopalperovskitephlogopitephillipsitepentlanditepyritepyrrhotitequartzrasvumilesaponitescawtitesodalitescheeliteschorlomitesphaleritethaumasite?tobermoritethoritetitanitethomsonitethorianitetilleyiteuraniniteuvarovitevesuvianitewollastonitezrconnew specresnew zeolite?new species?new species?unid speciesunid species

NaAlSi.OuCa.FerSi"O,,Ca,MgSi2O?MnSCaAl,Si,OuCa.(PO.)"(OH)Ca(Mg, Fe,Al,TiXSi,Al),O6ZrO"Ca"ZrSi"On(LREE,Ca).(SiO., PO4)3(OH)CaCO.FeCrrOoCa4Si,O?(F,OH),CaMgSi,OuK"(Fe,Ni),.S,uClPbS

Ca3Al,Si3O,,(Na,K,Ca),_"(Si ,Al)1,O,4(S04,Cl) , ,Ca,SiO"(OH),Ca3A|,S|3-,O1,(OHL, (x : 0 2-1 5)Ca"Al.Si"

"O,r(OH)", (x - 1 5-3)

KAlSi3OsCa"(Zr,Ti),(Si,Al)"O,,KAtSiO4FeAs,Na4AlsSisOriClFeS,Fe"OoCaMgSiO4NaAlSiO4sio,CaTiO"KMg"AlSi"O,o(OH).(K,Na,Ca),- , (Si ,Al)sO16.6 H,O(Fe,Ni)"SuFeS,Fe, ,Ssio,KFe,S"(Ca/2,Na)"

"(Mg,Fe)3(Si,Al)oO,o(OH),.4 H.O

Ca?Si6(CO3)O,s 2 H,ONa"AluSiuO.oCl,CaWOoCa.Ti.(Fe,Si).O,,ZnSCa6Si, (CO3),(SO ) 2(OHl p 24 H,OCanSi,,O"o(OH)u.4 H,OThsi04CaTiSiOuNaca,AlsSiso,o.6 H,OTho"CauSi,O,(CO")"UO,Ca.Cr.Si.O,,Ca,oMg,Alo(SiOo).(Si,O,), (OH)"CaSiO3ZrSiOoCa" ufih,U), u(SiOo)"(OH)Na,CaAl4Si4O16 X H,ONa,Ca.Si"O,.(OH).Ca-ouAl:o,SiO.(OH)- X H"OLREE,Ca-Silicate(stillwellite ?)Ca, LREE,Th, U,Ti, Nb-oxide

Stage lVStage lVStage IStage l?Stage l-detritalStage Iinclusions in zirconStage IStage Iall assemblagesdetritalStage IStage llStage lldetritalStage l?Stage llStage lVStage ll?Stage lVStage lVStage l l or l l lStage IStage Idetrital?Stage l.Stage lVdetritalStage llStage IStage lVStage IStage IStage lVStage IStage lVStage IStage l-Stage llStage lVStage lVStage llldetritalStage IdetritalStage lVStage lVdetritalStage l.Stage lVStage lVStage lldetritalStage IStage llStage l-lllStage l.Stage IStage lVStage lVStage lVStage l??detrital

cccrcudiolgagpgrguhbh ikakfskikslomamcmgmoneoppepnplnt

pvpynqzrs

soshsosph

tbthtiIM

toryurUVVSWOztnX 1x2X3x4Y1Y2

- Only in peak assemblage of calcite-{ree rocks.

Tlte Flekkeren limestone raft is located within the south-western pafi of the Permian Skrim batholith (Fig. 1; Datrl-gren 1998). The Skrim batholith consists largely of a seriesof alkaline plutons of larvikite (morzonite with ternary feld-spar), syenites and granites, which transects north-ftending

Sveconorwegian (1200-1000 Ma) gneisses, quartzites, andamphibolites, easterly dipping Cambro-Silurian sedimentaryrocks, and Upper Paleozoic basaltic volcanic rocks. Thelimestone raft (Fig. 1) is totally embedded in larvikite thatwas emplaced at 281 -+ 1 Ma (Pedersen et al. 1995). Boftt

1244

TaeLe 2. Observed mineral assemblages in contactmetamorohic limestones

Stage I Stage ll

Sample FLl5 79D 79A 79A 798 79D FL17

JAMTVEIT ET AL: CONTACT METAMORPHISM OF CALCAREOUS ROCKS

xX

x

X

X

xX

xxxxxx

xxxxxx

xxX

x

XXX

Notes: Not including phases outside the KNaCaFeMgAITiSiCOH systemand late (post Stage lll) reaction products Abbreviations used for mineralgroups, rather than end-members, start with an upper case letter as fol-lows: Grt : garnet; Mel : melilite, Cpx : clinopyroxene; Sca : scapolite

TeeLe 3. Representative mineral analyses

the limestone raft and the larvikite are ffansected by veinsfrom a younger alkali syenite pluton that occurs irnrnediate-ly east of Flekkeren. The limestone raft is separated by alarvikite screen less than l0 m thick from Sveconorwegianmetagabbros in the west. The stratigraphic position of themetasediments in the Flekkeren raft is not evident becausethe protolith was modified by the sffong metamorphism.Most likely it consists of the Upper Ordovician (Caradoc-Ashgiil) nodular limestones (the Steinvika formation) in thewest, shales of the Venstpp formation in the middle, andnodular limestones (the Herpya formation) in the east. Southof Lake Flekkeren the upper part of the Steinvika formationis underlain by about 350 m of Cambrian and Ordoviciansediments (Dahlgren 1998). These underlying sediments aremissing at Lake Flekkeren, and, because the distance be-tween the basement and the limestone raft is less than 10m, evidently the raft is displaced from its original position.The Flekkeren body is poorly exposed on the northern shoreof the small lake Flekkeren, and as scattered small outcropsin the boggy area north of the lake. It constitutes a lens atleast 500 m long and less than 100 m wide, and it is locallyftansected by small veins injected from the host plutons. Theoriginal composition of the initial sedimentary layers (mil-limeter to centimeter thick) varied from nearly pure lime-stone (calcite) layers to marly shales. The described samplematerial was collected from loose blocks from a recent roadblasting and is limited to samples of marly limestones.

Stageill

X

X

X

X

X

X

xxx

woMelGrtcpxphcupeKSne

morykfssdt i

X X XX X XX X X

X Xx

Clinopyroxene

Stage I Stage I Stage I Stage ll Stage tVSample Melanite Uvarovite Kimzeyite Grossular Katoiteno. FL17 FL-1 FL-B FL-C FL-C

Stage I Stage llAugite DiopsideFL-A FL-E

Stage I Stage IFL-A FL-A

Stage I Stage IFL-A FL-B

Phlogopite Kalsilite Nepheline

Stage I Stage IFL-A FL21

31.68 35 409.44 1327.09 0.60

11.02 2.39o .12 0 .121.23 0.22

34 66 33 16

415 27390.57 0 00

N B O . C a + M n + M g : 3 0 0 0

2 433 2.954o 547 0 0830 643 0 0600.639 0 1500 008 0 0080 141 00272 859 2973

0.253 1.812o.o21 0.0001 940 0.184

19 68 39 29 21.9510.01 0.1 1 0.048.1 3 17 .98 23 606 48 5.20 0 290 06 0.05 0 221 5 2 0 0 0 0 1 5

31 73 37 27 42.18

1.87 0.00 0 041 8 4 1

100 60 97.89 99.91 88 47

sio,Tio,Al,o3FerO"MnOMgoCaONaroKrOc%o"ZrO,FrO

Total

SiTiAIFeMnMg

NaKCrZroH'F

1 625 2949 1.4440 623 0 006 0 0020 .792 1592 1 .8370.403 0 294 0.0150.005 0.003 0 0120.188 0.000 0 0142812 3.000 2 986

o 122 0 000 0 0020.743

0.204 6 224

4640 52421 63 0.568.14 1 .704.59 3.650 05 0 18

12.55 14.9025.19 25300 1 1 0 2 8

067 0 64

99 34 99 66

cations:4000

1 729 1.9440.046 0 0160.358 00740 1 4 3 0 . 1 1 30 002 0 0060.697 0 8241 .006 1.0050.008 0.020

0.020 0.019

42.940 0 0

2.080 0 4924

C O T T

2390 1 60 0 0

99.1 5

cations:5000

1 .9670 000o 2970 0800 0020 6301 8040 2 1 00.009

34.89 39 57 38 30 38 89 41 67000 047 058 0 .00 000

17.91 17 19 17 39 30 96 34.480.24 3.97 3 89 0.14 0 19o 12 0.o4 0.00 0.01 0.006 23 24.06 23 00 0 02 0.00

38 87 0.05 0.08 2 13 0.061 78 0.08 0 08 0.16 16 230 .12 9 .90 10 .16 26 81 8120.00 0.24 0.24 0.00 0 00

0 8 0 0 8 8100.17 96.36 94 64 99 12 100.75

cations - Si + Al -N a - K - 2 0 0 0

1 40001 574 5.523 5 483 1 0320 000 0.049 0.063 0.0000.952 2.827 2 934 0.9680.009 0 463 0.465 0 0030 004 0 004 0.000 0 0000 .419 5004 4908 00011 879 0 008 0.012 0.0610 160 0.020 0.023 0.0080 007 1 762 1 856 0.908

0027 0027

1.O120.0000.9880 0040 0000.0000 001o 764o 252

0.360 0 400

Nofe.'N B O. : Normalization based on" oH : 4-[3000 - si - Ti(4+)]

PBrnocn.lpHy AND MINERAL cHEMrsrRy

The analyzed samples from Flekkeren contain morethan 60 different mineral species formed at various stagesduring the contact metamorphic history. Peffographic andmicroprobe studies were carried out by the JEOL JSM-840 electron microscope at the Department of Geologyand by the CAMECA CAMEBAX electron microprobeat the Mineralogical-Geological Museum, both at theUniversity of Oslo, using natural and synthetic standards.All analyses were run at an accelerating voltage of 15 kV.Beam currents varied from l0 to 20 nA depending on thephase to be analyzed. Peak and background intensitieswere counted for 10 s for the most abundant elements(e.g., Si) and for 20 s for elements with lower concentra-tions. Mineral abbreviations, end-member compositions,and mode of occurrence are given in Table I and ob-served mineral assemblages relevant for the phase pet-rological analysis are listed in Table 2. Representativemineral analyses are given in Tables 3 and 4.

In the following, the observed minerals and mineralassemblages are related to four different stages (I-IV) inthe contact metamorphic evolution. This does not implythat we assume that the metamorphic process has takenplace as a series of events, but is an attempt to separateminerals and assemblages that seem to be related in spaceand time.

Teete 3-Extended

t245

Stage I

Stage I represents the peak metamorphic conditions. Atthis stage, calcite in carbonate-rich rocks is typicallycoarse grained (grain size of several millimeters) and dis-plays polygonal foam textures. Carbonate-poor rocks aremore fine grained (typical grain diameter 0.01-l mm),and the silicate minerals commonly display textures in-dicative of very short transport distances during growth(such as complex graphic intergrowths). Carbonate-bear-ing lithologies contain wollastonite, kalsilite, nepheline,melanite garnet, perovskite, cuspidine, baghdadite, pyr-rhotite (Fe/S : 0.84-0.93), occasional alabandite, one ormore apatite-group minerals, and occasional graphite asscattered, apparently corroded grains.

The bulk composition of the analyzed layered samplesvaries significantly on millimeter scale. Some domains con-tain the assemblage Mel+Cpx+Ph+Wo+Cc+Ks (Fig. 2a),which represents an isobaric invariant assemblage in theKAIO,-CaO-MgO-SiO,-H,O-CO, system.

Clinopyroxene commonly occurs as poikiloblasts (occa-sionally many millimeters across) with inclusions of wol-lastonite, phlogopite, and kalsilite. A continuous ftmge oc-curs in clinopyroxene compositions from the Stage I augites(fassaites) containing up to 10.9 wtvo A7,O. (0.5 Al apfu),2.6 wt%o TiO,, and 0.15 wtTo Cr,O. to the nearly pure di-opsides formed during retrogression (Stage tr; see Thble 3).

JAMTVEIT ET AL,: CONTACT METAMORPHISM OF CALCAREOUS ROCKS

Cuspidine Baghdadite Vesuvianite Monticellite

Sample noStage IFL-B

Stage IFL-B

Stage IFL-B

Stage llFL17

Stage llFL-B

Stage I Stage llFL.B FL-D

Stage llFL-D

sio,Tio,Al,o3FerO"MnOMgoCaONaroKrOCr,O"ZrO,FrO

Total

N B O

32 240.02

0 0 00.03

60.05

1 3 210 4699.34"

cat ions :6000

1.9890.001

0.0000 001

3.969

32 540 0 2

0 0 00 0 0

59 93

1 7 010 3499 80-

2 0000 001

0 0000.000

3 947

0.051

2 .010

29.092.70

41 42

25 52

98 73

1.9840.1 39

3 028

0 849

29 363 0 2

36.94o 7 7

16 .931 .840 0 43.32

JO CJ

0 0 3002

96 48

10 8060 132c t c o

o 2710 0061 1217 4890 0 1 00 0 1 0

36.840.001 4 79 5 10 4 6

16.24J C . t Z

0.01

99 65

1 0 1 60 0000.0480.2190 0 1 10 6671 038

0 000

37 500 0 0o.o21 7 5070

22.95JO JU

0 0 0

99 22

0 9980 0000 0010 0390 0 1 60 9 1 11 035

0 000

37 181 0 3

16 671 .850 0 03 3 8

36 810 0 3o.o2

23.72

98 25 96 98

cations - 25000

SiTiAIFeMnMg

NaKCrZloH.F

1.9920.1 54

10.8290 .1775 0540.2710.0001 . 1397 5150 0 1 00 .010

J .UOO

0 7850 040

2 041

. T o t a l - 0 = F

r246 JAMTVEIT ET AL.: CONTACT METAMORPHISM OF CALCAREOUS ROCKS

Tee|-e 4. Representative silicate-phosphate analyses

Sampleno. FL-7 FL-7 FL-7 FL-7 FL.7 FL-7 f L - I IL - I fL - I

sio, 19.52P,O. 1 65CaO 2327Ce,O" 3 01LarO3 2 36Nd,o3 o 44Tho, 43 58UO. 3 .17so" 0.10cl o.22

Total 97.31

si 2579P 0 1 8 6Ca 3294L a 0 1 1 0Ce 0 150Nd 0.020Th 1 .310u 0.090s 0 .010cl 0.050

1878 18 811 95 421

23 87 24.833.71 4 262.80 3.210.91 0.80

39 41 38 58230 2630 0 0 0 3 30 1 6 0 1 8

2032 20.66 17.461 1 1 0 . 9 8 6 1 3

20 05 19.45 26 361 2 0 8 1 1 8 5 1 1 2 08 01 7 .85 7 102 36 2 68 2.21

25.16 26.57 20 966 .15 6 09 4610 .12 0 .15 0 570 1 6 0 . 1 8 0 . 2 1

95.52 96 46 96 82

2 700 2.763 2.0610 . 1 2 6 0 1 1 2 0 6 1 92854 2787 3.3350 390 0 390 0 3100 590 0 580 0 4800 .110 0 130 00900.760 0.810 0.5600 . 1 8 0 0 1 8 0 0 1 2 00.010 0.020 0.0500 040 0.040 0.040

17 63 18.055 6 1 6 1 8

25 89 25.3011 02 13767 1 7 8 7 52 39 2.59

20 56 1637454 4 950 6 5 0 5 50 1 3 0 1 9

YJ.3d vb bv

2 .108 2 .1470 573 0 6283 317 3.2230 320 0 3800 480 0 6000 1 0 0 0 1 1 00 560 0 4400 120 0 1300.060 0 0500.030 0.040

17.39 18.68 0.456.54 4 88 40.63

25.61 21 51 56.231 1 7 7 2 0 6 1 0 4 87 68 12 95 0322.25 4.64 0 00

1 7 4 3 8 5 3 0 1 86 4 1 3 3 8 0 0 70 45 0.35 0 020 16 0.19 0.00

95 69 95.72 98 38

2 091 2257 0.0370672 0 504 28643.299 2.785 4 9710 340 0 580 0 010o 520 0 910 0 0100 100 0200 0.0000 480 0 230 0 0000 .170 0090 00000.040 0.030 0.0000.030 0.040 0.000

19.063.45

24.744 2 53 1 20 8 3

40 572840.200 . 1 9

99.24 93 90 97.85

Normalization based on Ca + REE + Th + U = 50002.402 2465 23940.372 0219 04583.341 3.357 3 3860 150 0 .140 0 .1500 200 0.180 0.2000 040 0 040 0.040' l 1 6 0 1 1 8 0 1 . 1 2 00 080 0 070 0.0700 020 0 000 0 0300 040 0 040 0.040

Melilite compositions vary continuously from almostpure akermanite to akermanite..gehleniteo.. The Mg/(Mg*Fe'?* ) ratio is generally high (0.96-0.98), but rhemost akermanite-rich compositions show more extensiveFeMg ,exchange [Mg/(Mg + Fe2+) : 0.88-0.90].

Phlogopite from the Flekkeren locality is characterizedby Mg/(Mg + Fe) > 0.9 and considerable substitutiontoward the eastonite end-member (2.8-3.1 Al apfu whennormalized to total cations - K : 14.00). F typicallysubstitntes for about lj%o of the OH.

Kalsilite commonly occurs intergrown with wollaston-ite. Spectacular graphic intergrowths of kalsilite and wol-lastonite with minor nepheline resemble textures formedby spinoidal decomposition (Fig. 2b). As these phaseshardly can form by decomposition of a single pre-existingsolid phase, we speculate that this texture results fromrapid (metastable) reaction between the original potassi-um feldspar and calcite:

K f s + C c : K s + W o * C O , ( l )

Nepheline commonly occurs as irregular lamellae with-in kalsilite grains, but may also form separate grains thatfrequently are fringed by complex intergrowths of can-crinite and zeolite-group minerals (see below). Kalsilitecontains very little Na whereas nepheline contains about25 mol%o kalsilite component.

The Flekkeren samples contain an extreme variety ofgarnets (see Table 3). Because of the presence of OH andZr in many of these gamets, the structural formula hasbeen calculated by assuming that Ca + Mg + Mn : 3.0.For the melanites, structural formulas have also been cal-culated by normalization to a total charge of 24, but theresults are only marginally different. Three garnet typesare classified as Stage I phases. Uvarovite (X"" = 0.9) ttratpredates or dates the peak assemblage nucleated on de-

tritial chromite grains and is responsible for the stronggreen color of some hand specimens. Rare zirconian gar-net (kimzeyite) was found in veinlets through the pre-Stage I uvarovite along with the Stage I Zr phase bagh-dadite (see below) and probably grew in alocally Zr-en-riched system. Kimzeyite contains up to 18.4 wt%o ZrO,(0.7 Zr pfu) and are low in Si because of the extensiveZf+ Al4 + Sil + -, Alu*-, substitution. Ti- and Cr-contents arehigh: 10-l I wtvo TiO, and 1.7-2.4 wt%o Cr,O. (0.6-0.7Ti and 0.1-0.15 Cr pfu). The dominating Stage I garnetgrains are subhedral to anhedral melanite, occasionallywith a core of uvarovite. The melanite compositions showgood positive correlation between Mg, Ti, Cr, and Zr,reaching a maximum of approximately 9.4 wt%o TiOr,5.7wt%o Cr.O,, and 2.0 wt%o ZrOr. A good negative corre-lation exists between the Mg and Si content of these gar-nets (Fig. 3), presumably reflecting increasing Tio*-con-tent with increasing Ti-content. The uvarovitecompositions plot on the Mg vs. Ti and Mg vs. Si trendsdefined by the melanite and Stage II grandite. Ignoringthe uvarovites, there seems to be a gap in garnet com-position in the range Mg : 0.06-0.1 apfu, separatingStage I melanites from Stage II grandite. Furthermore thisgap reflects abrupt changes in the composition of zonedgarnet with melanitic cores and granditic rims.

Cuspidine [Ca"Si,Or(EOH),] and baghdadite (Ca.ZrSi,On)are additional Ca phases that occur as small (- 100 pm)grains in equilibrium with Cc+Wo+Mel (Fig. 4a). Bagh-dadite was originally described from a melilite skarn byAl-Hermezi et al. (1986). This occurrence of baghdadite isto our knowledge only the second reported. Baghdadite pre-sumably formed by the breakdown of original detrital zirconthrough the reaction

Zm + 2Cc + Wo : Bg + 2CO.. (2)

Frcunn 2, (a) Peak metamorphic assemblage composed ofcalcite, wollastonite, clinopyroxene (fassaite), phlogopite, meli-lite, and titanian garnet (melanite). Field of view 1 mm long (tr)BSE-image showing graphic intergrowth of wollastonite and kal-silite Note variations in the orientation of the wollastonite la-mellas along boundaries interpreted as original grain boundaries

1247

(c) Formation of Stage II phases tilleyite, monticellite, and gran-dite garnet along original calcite-wollastonite grain boundaries.Also note the late growth of scawtite along fractures through thetilleyite and along the tilleyite-wollastonite boundary. (d) Ras-vumite (core) and djerfisherite (rim) replacing an original pyr-rhotite srain.


f , . . 5w€ , r

Baghdadite analyses show nearly perfect stoichiomet-ric compatibility with the formula given by Al-Hermeziet al. (1986) (Table 3). In the Flekkeren samples aboutl5%o of the Zr is substituted by Ti, giving rhe approximateformula Ca,Zro",Tio,.Siroe. The cuspidine is close to theF end-member and contains variable Zr contents (0-1.7wt%o ZrOr). The most Zr-rich samples coexist with bagh-dadite and may in fact reflect limited solid solution to-ward baghdadite, which also is a member of the wcihler-ite-cuspidine group.

Calcite-free, calc-silicate layers have a Stage I assem-blage composed of wollastonite, clinopyroxene, melaniticgarnet or uvarovite, and potassium feldspar. An additionalphase is scapolite, which has an ambiguous petrographicstatus in these rocks. It sometimes occurs as millimeter-sized poikiloblasts with inclusions of the peak assemblageminerals and in many cases clearly replaces potassiumfeldspar. Scapolite growth is associated with alkali-ex-change and introduction of Cl to the metasedimentary

rocks. Accessory minerals include zitcon, pentlandite,pyrrhotite, ldllingite, chromite, scheelite, galena, sphal-erite, uraninite, a LREE-silicate (possibly stillwellite), anunidentified oxide of Ca, LREE, Th, U, Ti, and Nb andSi-rich apatite group minerals. Zircon is recrystallizedand remobilized during the high-grade metamorphismand occurs as euhedral porphyroblasts or as irregularmasses along the silicate grain-boundaries.

Various Th and LREE-enriched silicate apatites formedat the expense of original apatite in layers enriched inheavy accessory minerals during contact metamorphism.These can occasionally be observed as coronas aroundthe original apatite grains (Fig. 4b). Brownish, Th-rich,varieties (X1 in Table 1) are commonly associated withthorite. Thorianite is observed as a reaction product whenthe original silicate apatites break down during hydrationof metamict grains to an amorphous substance with ap-atite-like composition. Silicate phosphate analyses arelisted in Table 4. The composition space of the silicate

tr grandites+ zoned melanite' melanites. kimzeyitesa uvarovitesr hydrogarnets

. a+ l' a

a a a

3

a 1..

a

r248

t . 6

l r 1 )

0.8

0.4

0.6

0.5

._ 0.4F

0 0.04 0.08 0.12 0.16 0.2

Mg atoms p.f.u.

Ftcunn 3 Cr, Ti, and Si apfu vs. Mg atoms for the varioustypes of garnet observed in the Flekkeren samples. Crosses(zoned melanite) refer to data from a single-zoned garnet grain.

apatites can be largely described by the end-members ap-atite [Ca.P.O,,(OH)], britholite [LREE.Ca,Si.O,,(OH)],and the new species Xl [Ca..(Th,U)r sSi.O,,(OH)]. Minorsubstitutions occur along the vectors C(OH) , (0.035 t0.005 Cl pfu) and SiSP , (0.035 t 0.015 S pfu). Silicarephosphate analyses have been normalized assuming thatCa+Th+U+La+Ce+2Nd : 5.000. Doubling of rhe Ndcontent was done to account for the presence of REEheavier than Nd and is based on the relative concentra-tions of the three analyzed LREE assuming a log-linearREE-cation size vs. concentration pattern. Errors intro-


0.3

0.2

0.1

(n Frcunn 4. BSE-images showing (a) Coexistence of bagh-dadite and the related mineral cuspidine in a matrix of meliliteand wollastonite. This system is calcite saturated. Scale bar is100 pm. (b) Th-rich silicate apatite (X1) forming as a coronaaround an original apatite grain. Scale bar 10 p"m.

duced by doing this are small in these exffemelyLREE-enriched apatites. Normalization including the tet-rahedral coordinated cations is not possible as these sili-cate apatites may contain significant amounts of CO. andB, which are not measured by the EMP The aralyzedsamples range in composition from ap,rXlr,bru, toapuXlr.brn, but cluster around aproXlooo.broo.. @ig. 5).The U content is constant (4.5 + 1.5 wt%o UOr) and doesnot vary systematically with Th content but is low (2.3-3.2wt%o UO,) in the most Th-rich samples (>1.0 Th pfu).The limited variations in the apatite content of the ana-lyzed samples indicate that the main substitution mech-anism is ThCaLREE-r. However, a positive correlationexists between the Th + U content and the Si deficiency(defined as Th + 2LREE - Si) in the tetrahedral position.which may be explained by moderate coupled Th + Bsubstitution for Ca and P or LREE and P [e.g., along thevector ThB(CaP)_,1 (Burt 1989). The total sum of theEMP analyses is low for Th-poor samples (Table 3). This

X1ca, rr\ r(sioo \ (oH)

Car(POr)r(OH)

apante

LREE, Car(SiOo ), (OH)

britholite

Frcunn 5. Compositional variations among apatites and sili-cate apatites showing that most analyses plot within the field ofthe new end-member species X1. Open symbols show the com-position of grains that give low totals in the electron microprobeanalyses. This is probably partly due to hydration of metamictgrains and partly due to a significant concentration of B and or C.

is probably due to a significant HrO content, possiblyresulting from hydration of metamict grains.

Stage II

The ffansition from Stage I to Stage II assemblages incalcite-saturated layers is marked by the occurrence ofmonticellite, tilleyite, vesuvianite, grandite garnet (poorin Ti and Cr, see Table 3), Al-poor diopside, and occa-sional hillebrandite. Retrogression defining the onset ofStage II occurs pervasively through the Stage I assem-blages. Tilleyite forms as a corona between calcite andwollastonite (Fig. 2c) and is in textural equilibrium withmonticellite probably forming at the expense of melilite.Tilleyite and monticellite may form through the reactions:

2Wo + 3Cc : Ty + CO, (3)

3Cc + 2Ak: Ty + 2Mo + CO, (4)

Monticellite also coexists with garnet and vesuvianiteand is observed in reaction rims around melilite with wol-lastonite and calcite. In this case it mav form bv the fluid-absent reaction:

Ak: Wo + Mo (5)

Monticellite falls into two compositional groups, onewith Mg/(Mg + Fe) : 0.67 - 0.77 and one coexistingwith tilleyite and with Mg/(Mg + Fe) > 0.95.

Vesuvianite occasionally forms euhedral porphyro-blasts with inclusions of wollastonite and melilite or oc-curs as millimeter-thick layers alternating with granditicgarnet layers. The Mg/(Mg + Fe) ratios for four samplesare within the range 0.7-0.8 (all Fe assumed divalent).

t249

Grossular-rich Stage II garnet (Xe, > 0.6), typically isobserved as rims on Stage I melanites or as alterationrims between other Stage I minerals but sometimes aseuhedral poikiloblasts with numerous calcite and wollas-tonite inclusions. On the basis of the textural observationsthe most likely candidates for reactions responsible forvesuvianite and garnet formation are:

3Wo + 2Ak + 2Ge + CO, + zH,O : vs * Cc (6)

2 W o + G e + C O , : G r + C c ( 7 )

Both these reactions include breakdown of the gehlen-ite (ge : Ca,AlrSiO,) component of the Stage I melilites.Al-poor diopside formed as a rim or neoblast aroundpeak-metamorphic augitic pyroxene (fassaite) and is com-monly associated with grandite garnet. The formation ofthese phases may be described by the breakdown of thecalcium-tschermaks (cats) component of the pyroxenethrough the continuous equilibrium:

Cats + Wo : Gr (8)

The Stage I pyrrhotite is frequently sunounded by a co-rona of rasvumite, which again may be replaced by djer-fisherite (Fig. 2d). The formation of these sulfides is spa-tially associated with formation of other Stage IIminerals. Djerfisherite may also replace pynhotite with-out any intervening stage of rasvumite growth. Rasvumiteand djerfisherite are rare potassium iron sulfides previ-ously reported from alkaline intrusions (e.g., Czamanskeet al. 1979) and from enstatite chondrites (Heide andWlotzka 1995). These phases are interpreted to haveformed by reactions such as (stoichiometry dependent onthe pyrrhotite composition):

Wo + 2Cc + 2Ks + 6Pyh : 2Rs + Gr + 2CO, (9)

3Wo + 6Cc + 6Ks + 25Pyh: Dj + 3Gr + 6CO' (10)

or by the addition of K from externally derived fluids. Ras-vumite is nearly stoichiomeffic KFerS.. Djerfisherite hasbeen described as KCu(Fe,Ni),rS,o, as KuNa(Fe,Ni)r"SroCl,and as I((Fe,Ni),.S,uCl (cf. Czamanske et al. 1979). Theanalyzed. samples fall into two populations, one Cl-bearingand one essentially Cl-free. Ni was only analyzed in theCl-poor samples, which contain 4-5 Ni apfu. Furthermorethe X-ray spectrum indicated significant amounts of Co andCu.

Stage III

Examining the samples from Flekkeren using UV lighthas revealed diffuse veinlets that contain highly fluores-cent sodalite in a symplectite (grain size 10-50 pm) con-taining kalsilite, potassium felspar, and calcite (Fig. 6a).Wollastonite, clinopyroxene, and titanite occur in thissymplectitic matrix. These assemblages are referred to asStage III assemblages below. Clinopyroxene grains inthese veinlets are extensively zoned and show sharp com-positional changes from a green acmite rich core (0.17-0.18 Na pfu), to an intermediate zore (0.071-0.076 Na


r250

FrcunB 6. BSE-images showing (a) Symplectite composedof the Stage III phases kalsilite, K-feldspar, sodalite, nepheline,and calcite between larger wollastonite and calcite grains. Scalebar is 100 pm. (b) Complex intergrowths of guiseppettite, phil-lipsite, thomsonite, calcite, and an unknown zeolite-like mineral(X2) replacing an original nepheline grain. Nepheline can stillbe found as inclusions in the fassaitic clinopyroxene porphyro-blast in the upper right corner of the BSE-image. Field of viewabout 0.7 mm long.

pfu) to a colorless rim (0.048-0.056 Na pfu), which is inoptical discontinuity with the core.

Stage IV

Stage IV includes minerals occuring as late productsof retrogressive reactions affecting Stage I-I[ mineralsand in most cases their occurrence is confined to localsubdomains in the samples. Many of these minerals arevery fine grained and care was taken to avoid inter-growths and poorly defined grains during electron micro-probe analysis. All phases described below show uniformextinction in cross-polarized light and are homogenous atthe 1 pm scale. Microprobe analyses of different grainshave given consistent results.

Alteration products after mafix nepheline grains are com-plexly intergrown (Fig. 6b) and include several phases in-cluding the sulfate-bearing cancrinite-group mineral giusep-


I pettite [representative formula: Kr.Na..Ca,,rAluSi6Or"(SO4)r.-CL.l and the zeolites thomsonite (nearly ideal end-membercomposition) and phillipsite (representative formula: KNao,-Ca, ,A1..Si.rO,o.6HrO). An additional, and to our knowledgenot yet described, zeolite-like mineral with the approximateformula Na,.C4.AloSi,O,u . X H,O (X2 in Thble 1) is alsopresent. Low-grade tectosilicates formed along with calciteduring wollastonite consumption. Scawtite occurs as fringesalong grain boundaries and fractures through tilleyite (Fig.2c).

Stage IV hydrogrossular is invariably Al-rich (>1.8 Alpfu) and occurs along grain boundaries or fracturesthrough Stage II grandite garnet. It contains 1.45-1.7 Sipfu, (5.2-6.2 OH-groups pfu) and are classified as hib-schites and katoites (cf. Passaglia and Rinaldi 1984).

Fractures through large wollastonite grains (100 pm to1 mm long) in the Stage III veinlets invariably contain acolorless unidentified phase (X3 in Table 1) that may rep-resent a new mineral species. The formula inferred fromelecffon microprobe analyses is [Na"CaoSiuO,.(OH)"]. An-other unknown Ca,Si-rich mineral species (X4 in Tablel) was found as fracture fillings through clinopyroxenegrains along with mineral X3. Tobermorite and smectite(saponite) represent the latest and very fine-grained alter-ation products from calc-silicates observed in these sam-ples, and opal was identified as a coating on late fractures.

PBrnor,ocv

In the following, we attempt to determine the varyingtemperature and fluid composition conditions during themetamorphic history of these rocks through an analysisof heterogeneous phase equilibria. As the activity-com-position relations for Ti-rich garnets and vesuvianite sol-id-solutions are essentially unknown, the Flekkeren as-semblages were analyzed within the composition spaceKAIO,-CaO-MgO-SiO,-H,O-CO,. Pressure estimates onthe basis of stratigraphic reconstructions (Vogt 1907;Dahlgren, unpublished results) and fluid inclusion work(Olsen and Griffin 1984; Andersen 1990) are in the range700-1000 bar. Figure 7 and 8 show isobaric T-X.,., andf-X(O) diagrams for this end-member system calculatedat 1000 bar, applying the VERTEX program of Connolly(1990) and a revised version of the thermodynamic da-tabase of Holland and Powell (1990). For binary H,O-CO, fluids, the CORK equation of state (Holland andPowell 1991) was used. For graphite saturated assem-blages, we applied the GCOH and GCOHS equations ofstate of Connolly and Cesare (1993).

As the petrographic status of the observed graphite inthese rocks is uncertain, phase diagrams were calculatedby assuming binary H,O-CO, fluids or assuming graphitesaturation. The observed Stage I mineral assemblageCc-Wo-Ph-Cpx-Ak-Ks-fluid (Table 2) corresponds toequilibration near the isobaric Invariant Point 8 in Figure7a, located at T : 870 "C, X..r, : 0.38. The diopside-absent reaction is stable on the low temperature side ofInvariant Point 8, and thus there is a net production ofpyroxene in this point (cf. Carmichael 1991). This is con-

0.008 0.016 0.024 0.032x(c02)

Frcunn 7. Simplified T-)i.o, diagrams calculated at P - 1000bar for the pure KAIO,-CaO-MgO-SiO,-H,O-CO. system ex-cluding equilibria that are irrelevant with the present variabilityin bulk composition (i e., equilibria involving forsterite, Ca-poorpyroxene, and amphibole). Reactions are always written with thehigh-temperature assemblage to the right of the equality sign. (a)T-X.o, diagram for the region relevant to the Stage I assemblage.(b) f-X.", diagram showing in more detail equilibria relevant forthe retrograde Stage II and III assemblage. The regions of pos-sible stability of these assemblages are limited by the reactionsshown by bold lines. Abbreviations not included in Table 1: Sa: sanidine; Lc : leucite; Q : quartz, Bq : beta quartz; Spu :spurrite.

l25l

0.100 0.200 0 300 0.400x(o)

FrcunB 8. T-X(O) projections calculated at P : 1000 bar for

:l;uro"l,:"r.*to, -C ao-M gO - S iO' -C - o-H sys tem with graphite

sistent with the observed poikiloblastic texture of the py-roxene in these rocks. Uncertainties in the position ofInvariant Point 8 mainly stems from possible errors in theactivity corrections of the phase components Di, Ak, andPh. The univariant reactions intersect at acute angles andthus minor errors in the relevant activities give large ef-fects on the calculated X"o.-value. Test experiments withseveral activity models and with varying compositions ofthe phases involved suggest a maximum range for Z of820-870 "C and X.o, of 0.2-0.4. Lowering the pressureto 700 bar gives a reduction in Z by about 25 'C and anincrease in X.., by 0.03 units for the end-member system.The inferred peak temperature is slightly higher than theinferred solidus (approximately 800 "C) for a composi-tionally similar monzonite intrusion in the Sande Caul-dron further north in the Oslo Rift (Andersen 1984).

The stability of the retrograde assemblage Ty-Mo-Wo-Cc, can be inferred from Figure 7b. Wollastonite + mon-ticellite puts an upper limit on the metamorphic temper-ature of about 710'C, and the presence of tilleyite at suchtemperatures requires that X"o, = 0.016. Coexistence ofmonticellite and kalsilite restricts the stability of the StageII assemblage to a region near Invariant Point l0 at T -

710 "C, X"",- 0.005. Hence, these diagrams suggest achange from 20-4OVo CO, in the metamorphic fluid dur-ing peak metamorphic conditions to almost C-free fluidsduring formation of the Stage II assemblage.

The Stage III assemblage Ks-Kfs-Wo-Cc constrain themaximum temperature to about 550'C and the compo-sition of a binary HrO-CO,fluid to &.,< 0.016 (Fig. 7b).

At graphite saturation, the assumption of a binary HrO-CO, fluid is incorrect. Figure 8 shows a T-X(O) diagramconstructed for the end-member svstem at 1000 bars. A

JAMTVEIT ET AL,: CONTACT METAMORPHISM OF CALCAREOUS ROCKS

UF

U-

0.100 0.200 0.300x(co2)

0.400

oF

c

r252

projection of COH-fluid compositions through graphiteleads to a binary representation of the fluid in the H-Osystem where X(O) t: O(O + H)l varies from 0 to I asthe fluid composition changes from the C-H join to theC-O join and is 7s for pure H,O (Connolly 1995). X(O)is proportional to the oxygen fugacity in GCOH fluids.The estimates of the peak metamorphic temperature (nearInvariant Point 8) is not sensitive to the absence or pres-ence of graphite. In the T-X(O) diagram Invariant Point8 is located at T - 840 "C, X(O) - 0.4. However, thefluid composition corresponding to this X(O) value de-viates significantly from a binary H'O-CO, fluid and con-tains about 14 molTo CH". Adding S as a fluid componentand buffering the fluid composition by pynhotite with aFe/S ratio of 0.93 (the most Fe-rich pynhotite measuredin the Flekkeren samples), the resulting fluid compositionis 407o H,O,307o CO,, lU%o H,S, lUVo CHo, 5Vo CO, and5Vo H,.

Phase relations, calculated including AlrO. as an ad-ditional thermodynamic component, are consistent withthe formation of garnet and vesuvianite by Reactions 6and, 1 at temperatures below peak temperature. Howeveqthe exact positions of these equilibria are associated withconsiderable uncertainty because of uncertainties in ther-modynamic data and solid solution models for vesuvi-anite and melanitic garnets.

Several solid-solid continuous equilibria can be writtenamong the possible phase components to constrain meta-morphic temperatures from fluid-independent reactions,but none have proved useful because of small entropychanges and correspondingly great sensitivity to activity-concentration models and uncertainties in available ther-modynamic data. However, several useful observationscan be made from variations in the mineral composition.The extensive Al substitution in the pyroxene reflects thehigh temperature of metamorphism and is buffered byReaction 8, which is characterized by a positive changein both entropy and volume. This equilibrium is probablyone of the most useful continuous reactions in recordingrelative temperature variations within and among contactaureoles in calcareous rocks where the fluid compositionmay be variable. Likewise, the Mg content of the garnetis likely to correlate positively with the metamorphic tem-perature because almost every Fe-Mg exchange equilib-rium between garnet and other ferromagnesian mineralsproduces relative Fe enrichment in the garnet. As the Alcontent in pyroxene and Mg content in garnet correlatepositively with the temperature of equilibration, the Ticontents of both pyroxene and garnet also correlate pos-itively with temperature. Cr shows the same behavior asTi, but the Cr content of the garnet is strongly affectedby local Cr sources in its environment and thus showsmuch more scatter than does the Ti-content.

DrscussroN

The mineral assemblages from the Flekkeren localityare unusual for metamorphic rocks, mainly because rocksof such bulk composition rarely experience such high


I oooF

300

time

Frcunn 9. Schematic temperature-time evolution for theFlekkeren megaxenolith superimposed on a T-t curve believedto have a shape typical for regions near the top of a coolingintrusive. Heating during the prograde stage lead to devolatili-zation reactions causing a relatively high C content in the porefluids at Stage I. This was followed by an infiltration controlledincrease in the HrO content through influx of magmatic (upwarddirected arrows) and later of meteoric fluids (downward directedarrows) during Stage IV.

temperatures in combination with low pressures. Many ofthe observed phases are indeed more common in someSi-poor alkaline intrusions such as ijolite-series rocks andultramafic lamprophyres (e.g., Sprensen 1979; Dahlgren1981).

The large number of minerals present in these rocks isclearly related to metamorphic reactions taking placethroughout the metamorphic history, and these reactionsare partly driven by the interactions with externally de-rived magmatic and meteoric fluids. The metamorphichistory inferred from the petrology is schematically illus-trated in Figure 9 and is superimposed on a T-t cwve thatis believed to be typical for regions near the top of acooling intrusion (cf. Hanson 1995; Fig. 8). We believethat the petrologic analysis gives a reasonably reliableindication of the conditions of formation for the observedStage I mineral assemblage, indicating a relatively CO,-rich (or at least C-rich) pore fluid during peak metamor-phism. However, in the present geological setting, wherea megaxenolith of impure limestone fell into a partly mol-ten intrusion, we cannot expect the rapidly heating meta-sedimentary rock to follow a progressive reaction pathpredicted from an equilibrium phase diagram. The ob-served graphic intergrowths between kalsilite and wollas-tonite (Fig. 2b) reflect shon transport distances during thereaction and are more readily explained by rapid non-progressive metamorphic breakdown of original potassi-um feldspar and calcite than by progressive reactionsalong a buffered T-X.n, path that would have lead to theproduction of leucite as an intermediate but non-observedspecles.

During rapid heating of a calcareous rock embedded ina partly crystallized intrusion, the fluid pressure withinthe xenolith rapidly rose through devolatilization reac-

HrO, NaCl -({a_%

""

III IV

tions to reach or even exceed the fluid pressure in thesurrounding intrusive. This hindered major fluid infiltra-tion into the xenolith in agreement with available O iso-tope results obtained from the carbonates. At this stage(Stage I) the fluid composition in the xenolith was eitherbuffered by decarbonation reactions or, in the non-equi-librium case, controlled by the stochiometry of the on-going reactions. This caused a rise in the CO, content(and CH. content at reducing conditions) of the meta-morphic fluids toward peak temperature conditions. Whencooling started, the total solid volume of the xenolith wasconsiderably reduced because of volatile loss (occasion-ally exceeding l07o of the initial volume, depending onthe initial carbonate-silicate ratio). Unless this volumeloss was accommodated by compaction, which may havebeen a relatively slow process at the time-scales of theheating of these contact metamorphic rocks, the reductionin total solid volume made the xenolith more permeableand susceptible to pervasive influx of external fluids.Thus, the progressive stage of metamorphism leading tothe Stage I assemblage is strikingly similar to the indus-trial production of highly porous cement clinker by rapidheating at low P of a clay-limestone mixture, with a bulkcomposition similar to some of the Flekkeren samples.

This model explains why the post-peak (Srage II-IV)assemblage reflects C-poor, HrO-dominated magmatic ormeteoric fluids. The pervasive formation of the Stage IIassemblage is in many respects in contrast to the morecommon situation during retrogressive metamorphismwhere volatilization reactions usually depend on fluid ac-cess around fractures, faults, or other highly permeablezones generated after the formation of the peakassemblage.

Howeveq Stage III retrogression is associated withveins or veinlets. This is consistent with retrosression atlower temperature where thermal contractionhay haveled to fracturing and more localized access of magmaticand possibly meteoric fluids to generate veins in the meta-sedimentary rocks. The growth of sodalite during StageIII implies an addition of Cl to the system. Influx ofbrines through brittle fractures is a common feature incontact aureoles in the Oslo rift during cooling from peakmetamorphic conditions and is ascribed to the late releaseof saline magmatic liquids that were relatively immobileduring peak metamorphism because of the low perrne-ability of the country rocks at this stage (Jamtveit andAndersen 1993).

Stage IV marks the onset of the growth of very finegrained low-temperature phases such as zeolites. At thisstage the magmatic system was completely crystallizedand the most likely source for externally derived fluid ismeteoric water.

AcxNowr-nncMENTS

Discussions and assistance from Odd Nilsen and Henrik Svensen at theDepartment of Geology, University of Oslo, and the supporting engage-ment of Alf Olav Larsen at Norsk Hydro, Research Centre, Porsgrunn, aregreatly acknowledged Very thorough reviews by an anonymous referee

1253

and by Tom Hoisch significantly improved this paper. This work was sup-poted by grant no 110578/410 from the Norwegian Research Council.

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Jamtveit, B., Grorud, H.F, and Bucher-Nurrninen, K (1992b) Contactmetamorphism of layered shale-carbonate sequences in the Oslo rift: IIMigration of isotopic and reaction fronts around cooling plutons Earthand Planetary Science Letters, I 14, 131-148

Jamtveit, B, Grorud, H.F, and Ragnarsdottir, KV (1997) Flow and trans-port during contact metamorphism and hydrothermal activity: Examples


high-grade contact metamorphism of calcareous rocks from...

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