high-temperature metamorphism and the role of magmatic heat

High-Temperature Metamorphism and theRole of Magmatic Heat Sources at theRogaland Anorthosite Complex inSouthwestern Norway

MATHIAS WESTPHAL1,*, JOHN C. SCHUMACHER1,y ANDSTEFAN BOSCHERT2

1INSTITUT FUÈ R MINERALOGIE, PETROLOGIE UND GEOCHEMIE DER UNIVERSITAÈ T FREIBURG IM BREISGAU,

ALBERTSTRASSE 23B, D-79104 FREIBURG, GERMANY

2KRISTALLOGRAPHISCHES INSTITUT, DER UNIVERSITAÈ T FREIBURG IM BREISGAU, D-79104 FREIBURG,

GERMANY

RECEIVED JULY 10, 1998; ACCEPTED JANUARY 2, 2003

The Rogaland complex covers �1000 km2 in southwesternNorway and consists mainly of anorthosite massifs and thelayered Bjerkreim±Sokndal lopolith (BSL). These rocksintrude charnockitic migmatites containing intercalated marblesand garnetiferous migmatites. High-temperature mineralisograds (pigeonite, osumilite and orthopyroxene) in the meta-morphic basement are subparallel to and increase in gradetowards the intrusive complex. P±T estimates from the countryrocks show a roughly linear increase in temperature towardsthe BSL consistent with the distribution of isograds. The peakP±T conditions at 20 and 2�5 km from the contact at �5 kbarrange from 700 to 41000�C. Field relations and age determi-nations link the high-T metamorphism and the magmatism.The two-dimensional thermal modelling indicates that heatfrom a single magmatic cooling unit is not sufficient to producethe array of isograds and the peak metamorphic temperatures.Two magmatic episodes separated by �3Myr, however, canaccount for the high-temperature metamorphism. In this model,the emplacement and crystallization of the anorthosite produces aregional thermal gradient (from 750 to 600�C). After a briefhiatus a second, smaller body (BSL) provides an additionalthermal input that results in an array of high-temperatureisograds and country-rock temperatures 41000�C.

KEY WORDS: Rogaland; UHT; thermal model; osumilite

INTRODUCTION

Proterozoic terrains (e.g. Antarctica and NorthAmerica) are the most common hosts of large anortho-site complexes (Ashwal, 1993; and references therein).Metamorphic contact aureoles of various dimensionscommonly accompany these intrusions. Anorthositecomplexes are typically emplaced into high-grademetamorphic rocks and caused high-temperaturecontact metamorphism in the proximal country rocks.The intrusion temperatures of the anorthosites andrelated rocks are �1000±1200�C, and the associatedmetamorphic aureoles are commonly composed ofgranulite-facies mineral assemblages characterized byhigh-temperature mineral-in isograds, such as andalu-site, orthopyroxene, pigeonite or osumilite. As thegranulite-facies country rocks appear to be essentiallyanhydrous, heat transfer from the intrusions to thecountry rocks is likely to have been conductive ratherthan advective.In southwestern Norway a mid-crustal (�5 kbar)

contact metamorphic aureole is associated with theemplacement of the Rogaland anorthosite complex(Hermans et al., 1975; Jansen et al., 1985; Maijer,1987). The metamorphic aureole extends to �20 kmfrom the intrusive contact, corresponding to the loca-tion of the orthopyroxene-in isograd in quartz-bearing

JOURNAL OF PETROLOGY VOLUME 44 NUMBER 6 PAGES 1145±1162 2003

*Present address: Institut fuÈ r Mineralogie, Petrologie und Geo-chemie der Universitt TuÈ bingen, Wilhelmstrasse 56, D-72074TuÈ bingen, Germany.yCorresponding author. Present address: Department of EarthSciences, University of Bristol, Bristol BS8 1RJ, UK. E-mail:[email protected]

Journal of Petrology 44(6) # Oxford University Press 2003; all rightsreserved.

metapelites and plagioclase±clinopyroxene-bearingmetabasites. An osumilite-in isograd lies �10±13 kmfrom the intrusive contact, and a pigeonite-in isogradoccurs �5 km from the contact (Tobi et al., 1985).Age determinations of Sch�arer et al. (1996) have

shown that massif-type anorthosites and the layeredseries of the BSL are nearly coeval, rather than beingseparated by a significant time interval (150±250Myr)as was previously thought (Maijer, 1987). The massif-type anorthosites that form the major part of theintrusive complex were emplaced into high-grademetamorphic rocks, which had equilibrated at 600±700�C at 6±8 kbar ( Jansen et al., 1985) before theanorthosite event.Ultrahigh-temperature (UHT) metamorphism

around the Rogaland complex appears to be moreextensive than that found at other anorthosite com-plexes. For example, at the Nain complex (Speer,1975; Berg & Wheeler, 1976; Berg, 1977) and theLaramie anorthosite complex (Snyder et al., 1988) thetotal widths of the aureoles are only 3±4 km comparedwith 20 km at Rogaland, and the peak temperaturesare lower than in Rogaland.The aim of this study is to understand the contact

metamorphism induced by the interaction of theanorthosite and the mafic intrusive bodies based on awell-characterized, high-temperature terrain, by two-dimensional thermal modelling of the intrusive com-plex. This experiment focuses on understanding thehigh-temperature conditions that led to pigeonite andosumilite stability in the country rocks, resulted in theobserved distribution of isograds and formed the widegranulite-facies aureole of the Rogaland anorthositecomplex. Heat conduction modelling was carried outusing the commercial software package FIDAP (1993),which uses the finite-element method. We comparedthe calculated thermal effects of single- and two-intrusive event scenarios with the distribution oftemperatures in the country rocks proximal to theanorthosite complex, estimated using a range ofgeothermometers. Temperatures within the aureolerange from 700�C at the orthopyroxene-in isograd to�1000�C at a distance of 2�5 km from the contact withthe intrusions.Our modelling results demonstrate that the emplace-

ment of the igneous complex as two separate eventscan account for many observations not explainedby the single intrusion scenario. Initial anorthositeemplacement heats the country rocks and produces athermal gradient. A discrete second phase of magmaemplacement �3Myr after the anorthosite providesthe additional heat input that causes the high-temperature metamorphism and the distribution ofthe observed mineral-in isograds in the preheatedcountry rocks.

GEOLOGICAL SETTING

In Rogaland, southwestern Norway (Fig. 1), largeanorthosite bodies have intruded polymetamorphicgneisses of the Proterozoic of the Baltic shield, whichare granitic to charnockitic migmatites with inter-calated bodies of mafic rocks and metamorphosed sedi-mentary rocks (Tobi et al., 1985). The charnockiticmigmatites (Hermans et al., 1975) contain orthopyrox-ene, quartzo-feldspathic lenses, schlieren and layeredunits containing both leucosomes and melanosomes.The metasedimentary series consist mainly of eithercalcareous rock types, the Faurefjell Formation(Sauter, 1983; Jansen & Tobi, 1987), or more abun-dant pelitic rock types, `garnetiferous migmatites'(Huijsmans et al., 1981). As in other areas of southernNorway, the basement rocks intruded by the Rogalandcomplex give deposition ages of �1�5Ga (Verschure,1985) and underwent regional metamorphism togranulite-facies conditions at �1�2Ga (Versteeve,1975; Wielens et al., 1981).The igneous complex (Fig. 2) consists of several

massif-type anorthosite bodies and the layered series(anorthosite to quartz mangerite) of the Bjerkreim±Sokndal lopolith (BSL). Late-stage intrusive activityconsists mainly of a jotunitic dyke system that includesthe Tellnes ore body inside the AÊ na±Sira massif(Sch�arer et al., 1996). Large iron-rich, syenitic to grani-tic sheet intrusions are present in the country rocks andhave compositions analogous to parts of the anorthosi-tic igneous complex. However, these intrusions havecrystallization ages of 1�2Ga (Rietmeijer, 1979) thatpre-date the final anorogenetic (post-Sveconorwegian)emplacement of the intrusive complex from about 930to 920Ma (Sch�arer et al., 1996).

The igneous complex

Structure of the intrusive complexThe igneous complex consists of three massif-typeanorthosite bodies, the Egersund±Ogna, Helleren andAÊ na±Sira, and the layeredmafic series of the Bjerkreim±Sokndal lopolith (BSL, Fig. 2). The Egersund±Ognamassif is a 20 km diameter anorthosite dome, composedprimarily of unzoned plagioclase with compositionsranging from An40 to An50 with a grain size of 1±3 cm.The central part of the anorthosite body contains giantAl-rich orthopyroxene (51m) and 5±50 cm plagio-clase (An55) megacrysts (Duchesne & Maquil, 1987).The foliation of plagioclase grains in the margin of theintrusive body is consistent with deformation accom-panying diapiric emplacement of low melt-fractioncrystal mushes (Duchesne et al., 1985).The Helleren and the AÊ na±Sira massifs (Fig. 2)

are similar to the coarse-grained Egersund±Ogna

JOURNAL OF PETROLOGY VOLUME 44 NUMBER 6 JUNE 2003

1146

Fig. 1. Generalized geological map of Rogaland, SW Norway (after Hermans, 1975).

Fig. 2. A summary of age determinations, relative ages from field relations and locations of the anorthosites and some related intrusive rocks(Sh�arer et al., 1996). High-temperature mineral-in isograds after Hermans (1975).

WESTPHAL et al. ROGALAND ANORTHOSITE COMPLEX, NORWAY

1147

anorthosite massif (Duchesne et al., 1985). The AÊ na±Sira massif contains the well-known Tellnes Fe±Tideposit (Krause et al., 1985), which is a part of aleuconoritic dyke system (Michot, 1960) representingthe late stages of intrusive activity.The layered series of the BSL (Fig. 2) consists of three

lobes (Duchesne, 1987). The large northwestern lobe(lower part) and the smaller southern and southeasternlobes (roughly upper part) cover an area of �40 km �9 km. Recent mapping (Paludan et al., 1994) showsthat the northern lobe is a trough-like, discordantintrusion that has the shape of a major isoclinal syn-cline, whose axis plunges roughly up to 45� to the SE.Both the northeastern and southwestern contacts dipat 80±90� towards the centre of the intrusion, which isthickest in the axial region of the syncline. The roof ofthe BSL is not preserved within the area of presentoutcrop (Wilson et al., 1996).The igneous stratigraphy of the BSL is well docu-

mented (e.g. Nielsen & Wilson, 1991; Paludan et al.,1994; Wilson et al., 1996) characterized by anorthositeto troctolite, leuconorite, jotunite (� hypersthene mon-zodiorite) to mangerite (�hypersthene monzonite),quartz mangerite and igneous charnockite (Duchesne& Wilmart, 1997). Rhythmic layering (Michot, 1960;Paludan et al., 1994) characterizes the structure of theBSL magma chamber. Michot (1960) divided the BSLinto an upper part that contains mangerite and quartzmangerite and a lower part containing anorthosite,leuconorite, norite and gabbronorite. Wilson et al.(1996) explained the BSL as a sequence of six (intru-sive) megacycle units (MCU I±IV), which repeatcharacteristic sequences of cumulates.Geophysical data indicate that the AÊ na±Sira massif

is �4 km thick (Smithson & Ramberg, 1979) and thatthe layered series of the BSL may be�9 km in thickness(Paludan et al., 1994; Wilson et al., 1996). Assumingan average vertical thickness of 5 km for the wholeintrusive complex and a length and average width of50 km � 20 km, the volume of magmatic material is�5000 km3 (Sch�arer et al., 1996). This is a minimumestimate, as palaeomagnetic data from the North Seaindicate a much larger area of igneous activity. TheRogaland anorthosite complex might have had a totalvolume of the order of �20 000 km3 (Sch�arer et al.,1996).

Pressure estimates for the igneous rocksP±T estimates at Rogaland based on geothermobaro-metry studies of the massif-type anorthosites (Fig. 3)indicate that they reached final equilibrium at a mini-mum pressure of 4±7�5 kbar (Duchesne & Maquil,1987; Wilmart & Duchesne, 1987). An experimentalstudy of a jotunite from the lower part of the BSL

indicates that the currently exposed levels representan intrusion depth equivalent to a pressure of �5 kbar(Vander Auwera & Longhi, 1994).

Relative ages and age determinationsThe field relationships provide evidence for thesequence of intrusive events (Fig. 2). The Hellerenmassif cuts the foliation of and is therefore youngerthan the Egersund±Ogna anorthosite. The layeredBSL is younger than the AÊ na±Sira and Egersund mas-sifs as it contains xenoliths of both anorthosite bodies(Wilson et al., 1996). The final stage of anorthositeevolution was the emplacement of a system of jotunitedykes (the monzonoritic Lomland dyke system), whichcuts all the massifs (Duchesne et al., 1989).Radiometric ages of anorthosites are difficult to

obtain (Ashwal, 1993), as the rocks show generallyinsufficient variability in parent/daughter ratios (e.g.K/Ar, Rb/Sr or Sm/Nd) for reliable whole-rock iso-chrons. Dating using the U/Pb method on zircons isgenerally difficult because zircons in anorthosite arescarce. Hence, the ages of many anorthosites areknown only indirectly, as a coeval origin for the asso-ciated zircon-bearing mangerites, charnockites andgranites is only an assumption.Intrusive activity associated with the Rogaland

anorthosite complex was thought to have spannedsome 100±250Myr (Demaiffe & Michot, 1985;Duchesne et al., 1985; Maijer, 1987). In a new attemptto date the anorthosite, Sch�arer et al. (1996) usedzircon and baddeleyite from large aggregates of

Fig. 3. Compilation of pressure and temperature estimates usingvarious conventional geothermobarometers. N&P, Newton &Perkins (1982); P&H, Powell & Holland (1988); H, Harley (1984);L&G, Lee & Ganguly (1988). The data are from Westphal (1998),and some of the localities are shown in Fig. 1. Pressure of crystal-lization estimates for associated intrusive rocks also indicate �5 kbar(Vander Auwera & Longhi, 1994).


1148

orthopyroxene megacrysts. The new age data suggestthat magma emplacement actually occurred over amuch shorter interval. The emplacement ages (Fig. 2)of the major anorthosite units (Sch�arer et al., 1996) areclosely spaced at 929 � 2, 932 � 3 and 932 � 3Mafor the Egersund, Helleren and AÊ na±Sira massifs. Thejotunite dykes (931 � 5Ma) and the Tellnes dykes(920 � 3Ma) cut the other units and are later.Unfortunately, there are no age data available for

the BSL because these rocks lack zircon-bearing mega-crysts of plagioclase or orthopyroxene. Additionally, asthe BSL consists of a number of intrusive units (MCUI±IV), there can be no single age of intrusion. Fromfield relationships the BSL intruded after the Egersund,Helleren and AÊ na±Sira anorthosites, but before thejotunite dyke system. Based on the available age deter-minations and their errors, the maximum time spanover which the BSL could have been emplaced is�5Myr (931±926Ma).

The metamorphic envelope to theanorthosite complex

Stages of metamorphismHermans et al. (1975), Jansen et al. (1985) and Maijer(1987) inferred four separate phases (M1±M4) ofmetamorphism in the region. The first high-grademetamorphic event (M1) probably occurred before1�2Ga (Versteeve, 1975;Wielens et al., 1981). Duchesne&Michot (1987) relatedM1 to the emplacement of themassif-type anorthosites. However, dating (1050±870Ma) of the country rocks close to the BSL (e.g.Dekker, 1978; Pasteels et al., 1979; Maijer et al., 1981;Wielens et al., 1981; Rietmeijer, 1984) suggests thatthe high-temperature and low-pressure metamorphicevent (M2) is correlated with the intrusion of the BSL(Maijer et al., 1981; Maijer, 1987). Recent work byM�oller et al. (2002) indicates metamorphic ages of�925Ma for high-temperature M2 phase. A sub-sequent low-temperature and low-pressure, retrogradeevent (M3) overprints the M2 assemblages (Maijer,1987). The youngest metamorphic event, the Caledo-nian greenschist-facies M4 stage, locally overprints thehigher-grade assemblages.The recent age determinations of Sch�arer et al.

(1996) indicate that the massif-type anorthosites andassociated rocks were emplaced into high-grade meta-morphic regional rocks, which equilibrated at 600±700�C at 6±8 kbar pressure during an earlier M1event that preceded the intrusions of the anorthositecomplex ( Jansen et al., 1985). Furthermore, the intervalthat separates the intrusion of the massif-type anortho-sites and the BSL decreases from 150±250Myr (Maijer,1987) to �5Myr (Sch�arer et al., 1996).

Metamorphic mineral assemblages and isogradsMore than 5 km from the anorthosite complex, mon-zonorite and pyroxene syenite in the country rockscontain orthopyroxene, clinopyroxene, Ca-amphiboleand biotite (Dekker, 1978).Within 5 km of the intrusivecontact, inverted pigeonite may be present (Hermanset al., 1975; Tobi et al., 1985), indicating that thethermal maximum exceeded 825�C (Lindsley, 1983;Ranson, 1986; Sandiford & Powell, 1986) near thecontact. The mineral assemblages and compositions ofphases that were used for the P±T estimates are given inTable 1 (see Fig. 1 for the localities). The mafic countryrocks that recrystallized during M2 metamorphismshow equant mineral grains with triple junctions sug-gesting textural equilibrium among orthopyroxene,clinopyroxene, F-bearing hornblende, F-bearingbiotite, plagioclase and quartz, which occur in variableproportions.The granite migmatites of the country rocks are

generally orthopyroxene- and quartz-bearing assem-blages (Hermans et al., 1975). Most of these mineralassemblages developed during the M1 event, but wereoverprinted by the subsequent, higher-temperatureM2 event. The migmatites are probably relicts of theM1 event. The melanosomes of the metapelitic, `gar-netiferous migmatites' (Huijsmans et al., 1981) com-monly contain either the low-Al mineral assemblagegarnet±quartz±orthopyroxene±plagioclase±spinel±biotiteor the high-Al mineral assemblage garnet±cordierite±sillimanite±plagioclase±spinel±graphite±sulphides(Hermans et al., 1975). The grain size of the matrixassemblage of the melanosome varies between 0�1 and0�5 cm. The matrix contains rare garnet porphyro-blasts up to 3 cm in diameter. We infer that the relictassemblage garnet � quartz � plagioclase � orthopyr-oxene� spinel formedduringM1.The reactiongarnet�orthopyroxene � spinel records the prograde overprintof rocks proximal to the igneous complex during theM2 contact metamorphic event. The garnet � quartzovergrowths around orthopyroxene grains and zoning inorthopyroxene and garnet (Westphal & Schumacher,1997; Schumacher &Westphal, 1999) are indicators ofpartial retrograde re-equilibration.In places, osumilite is present (Maijer et al., 1977) in

gneisses bearing K-feldspar, quartz, garnet, orthopyr-oxene and cordierite. As with pigeonite, osumiliteoccurs only in rocks proximal to the igneous complex.The presence of osumilite indicates final equilibrium ofthe rocks within the contact aureole at granulite-faciesconditions, with temperatures exceeding 700�C(Olesch & Seifert, 1981) or 875�C (Carrington &Harley, 1995).Regional mapping (e.g. Hermans et al., 1975; Tobi

et al., 1985) has shown that high-temperature mineral


1149

isograds (pigeonite, osumilite and orthopyroxene) areconformable to the northern lobe of the BSL, suggest-ing a genetic relationship (Figs 1 and 2). Generally, thespatial distribution of the mineral isograds indicatesthat the grade of M2 metamorphism increases towardsthe igneous complex. At distances farther than �15±20 km from the intrusive complex, the effect of thehigh-temperature metamorphism (M2) is no longerdetectable and all the country rocks in the study areashow mineral assemblages typical of the granulite-facies conditions (M1) before the emplacement of theigneous complex (Jansen et al., 1985). A Caledonidegreenschist-facies M4 stage has not affected thehigher-grade assemblages in the study area (see alsoTobi et al., 1985).

Pressure and temperature estimates for thecountry rocksP±T estimates for the thermal maximum of the M2episode inferred from mineral equilibria on rockswithin the contact metamorphic aureole (see localities,Fig. 1) are consistent with the high-grade conditionsinferred from the mineral assemblages (Westphal &Schumacher, 1996). Based on the net-transfer reactionorthopyroxene � plagioclase � garnet � quartz andFe±Mg exchange between orthopyroxene and garnet,Westphal & Schumacher (1996) inferred temperaturesof 900�C and 700�C at 5 kbar at points 5 km and15�5 km from the igneous complex (Fig. 3). The resultof multiple phase equilibrium (TWEEQU) estimates(Berman, 1991) on a sample 13 km (Fig. 1, and see

Table 1a: Mineral compositions of phases used for P±T estimates (see Figs 3 and 5)

Sample 28 Sample 35 Sample 40

Area: 28a2 28a2 28a2 35b1 35b1 35b1 35b4 35b4 35b4 40b1 40b1 40b1

Analysis: 1045

garnet

1040

opx

1046

plag

1220

garnet

1216

opx

1215

plag

1239

garnet

1244

opx

1241

plag

1257

garnet

1260

opx

1261

plag

SiO2 38.31 48.97 60.02 37.66 48.75 56.26 38.01 48.62 55.12 38.04 49.39 59.26

TiO2 0.05 0.14 0.00 0.04 0.16 0.00 0.05 0.14 0.00 0.06 0.21 0.02

Al2O3 21.61 3.54 25.55 22.17 3.82 28.87 22.12 3.83 28.45 22.46 3.93 26.68

FeO 34.83 33.28 0.24 33.05 31.67 0.17 32.53 32.17 0.24 33.34 30.09 0.15

MnO 0.74 0.21 0.06 0.77 0.22 0.00 0.60 0.27 0.00 1.22 0.31 0.04

MgO 5.21 14.55 0.01 6.09 15.66 0.01 6.43 15.43 0.01 6.07 16.68 0.00

CaO 2.02 0.21 7.60 2.06 0.28 9.59 1.88 0.26 9.54 1.60 0.31 7.60

Na2O 0.01 0.03 7.40 0.01 0.01 6.32 0.00 0.02 5.92 0.02 0.02 7.55

K2O 0.00 0.01 0.18 0.01 0.01 0.13 0.00 0.00 0.18 0.00 0.00 0.38

F

Cl

Total 102.78 100.92 101.05 101.86 100.57 101.34 101.62 100.73 99.45 102.80 100.94 101.68

No. of O: 12 6 8 12 6 8 12 6 8 12 6 8

Si 2.974 1.904 2.654 2.932 1.889 2.498 2.952 1.887 2.494 2.935 1.891 2.611

Al 1.977 0.162 1.332 2.034 0.175 1.511 2.025 0.175 1.517 2.042 0.177 1.385

Ti 0.003 0.004 0.000 0.002 0.005 0.000 0.003 0.004 0.000 0.003 0.006 0.000

Mg 0.603 0.843 0.001 0.707 0.905 0.000 0.745 0.893 0.001 0.697 0.952 0.000

Fe2� 2.261 1.082 0.009 2.152 1.027 0.006 2.113 1.044 0.009 2.151 0.964 0.006

Mn 0.049 0.007 0.002 0.051 0.007 0.000 0.040 0.009 0.000 0.079 0.010 0.001

Ca 0.168 0.009 0.360 0.172 0.012 0.456 0.156 0.011 0.462 0.132 0.013 0.359

Na 0.001 0.002 0.634 0.001 0.001 0.544 0.000 0.001 0.519 0.003 0.001 0.645

K 0.000 0.000 0.010 0.001 0.000 0.007 0.000 0.000 0.010 0.000 0.000 0.022

Clÿ

Fÿ

Sum 8.035 4.013 5.002 8.051 4.019 5.022 8.033 4.023 5.012 8.043 4.015 5.029


1150

Fig. 5 below) from the contact indicates temperaturesof 765�C at 5 kbar. These estimates are consistent withthe experimental results of Vander Auwera & Longhi(1994), who estimated BSL emplacement at an originalpressure exceeding 5 kbar. We consider that uncertain-ties related to the analysed composition of mineralstogether with the uncertainties derived from the thermo-dynamic data yield an overall uncertainty of �1 kbarand�50�C,which, based on suggestionsmade byEssene(1989) for single thermobarometers, represent a conser-vative and therefore, maximum uncertainty.Zoning profiles in coexisting orthopyroxene and gar-

net from the country rocks between the pigeonite-inand the orthopyroxene-in isograds indicate thatcooling associated with M2 occurred over �20Myr(Westphal & Schumacher, 1997; Schumacher &Westphal, 1999). Coexisting orthopyroxene and garnetfrom localities above the pigeonite-in isograd do notpreserve high-temperature equilibrium compositions

because of retrograde Fe±Mg exchange. Orthopyroxeneand spinel completely replace primary garnet in thecountry rocks close to the igneous contact; garnet is aretrograde phase, found as rims around spinel andorthopyroxene in these rocks. Petrographic interpreta-tions are better evidence of the peak temperature con-ditions within the pigeonite-in zone. These T±distancerelations are shown below in Fig. 9.The presence of inverted pigeonite, which defines the

pigeonite-in isograd (Maijer, 1987), suggests high tem-peratures within the narrow zone of the contact aur-eole. In addition, some samples at �2±3 km frommagmatic contact show thin-section-scale texturesthat suggest high-temperature, fluid-absent melting ofbiotite (M2 stage) (Westphal & Schumacher, 1997).Symplectites of biotite � quartz between biotite andosumilite are associated with ilmenite, spinel andquartz. Quartz occurs as small grains, �50 mm in size,inside the osumilite. Biotite, ilmenite and spinel grains

Sample 42 Sample 43 Sample 44

Area: 42a3/4 42a3/4 42a3/4 42a6 42a6 42a6 43a1 43a1 43a1 44a 44a 44a

Analysis: 1420

garnet

1426

opx

1418

plag

1405

garnet

1401

opx

1403

plag

1104

garnet

1107

opx

1113

plag

1542

garnet

1565

opx

1557

plag

SiO2 38.81 47.84 55.95 38.74 48.31 56.36 38.80 46.94 60.01 38.65 48.19 56.84

TiO2 0.01 0.15 0.03 0.06 0.16 0.00 0.01 0.05 0.01 0.11 0.25 0.00

Al2O3 22.84 7.99 28.39 22.88 8.36 28.57 22.57 9.68 25.64 22.89 7.35 27.74

FeO 25.42 24.26 0.17 25.95 23.04 0.21 28.32 25.05 0.33 28.96 25.02 0.25

MnO 3.41 1.07 0.00 3.45 1.05 0.01 1.08 0.26 0.04 0.59 0.12 0.05

MgO 9.22 19.54 0.01 9.45 20.13 0.00 9.04 18.05 0.00 9.83 18.83 0.00

CaO 1.32 0.09 9.74 1.21 0.08 9.46 0.77 0.05 6.36 1.29 0.10 9.39

Na2O 0.00 0.03 4.86 0.00 0.00 5.92 0.05 0.00 8.06 0.01 0.01 6.24

K2O 0.00 0.00 0.19 0.00 0.00 0.19 0.01 0.00 0.23 0.02 0.00 0.24

F

Cl

Total 101.02 100.97 99.34 101.74 101.13 100.72 100.65 100.07 100.68 102.34 99.87 100.74

No. of O: 12 6 8 12 6 8 12 6 8 12 6 8

Si 2.960 1.789 2.521 2.942 1.790 2.513 2.976 1.769 2.660 2.922 1.819 2.537

Al 2.054 0.352 1.507 2.048 0.365 1.502 2.040 0.430 1.340 2.040 0.327 1.459

Ti 0.001 0.004 0.001 0.003 0.004 0.000 0.001 0.001 0.000 0.006 0.007 0.000

Mg 1.048 1.089 0.001 1.070 1.112 0.000 1.033 1.014 0.000 1.108 1.060 0.000

Fe2� 1.622 0.758 0.007 1.648 0.714 0.008 1.816 0.790 0.012 1.832 0.790 0.009

Mn 0.220 0.034 0.000 0.222 0.033 0.001 0.070 0.008 0.002 0.037 0.004 0.002

Ca 0.108 0.004 0.470 0.098 0.003 0.452 0.064 0.002 0.302 0.104 0.004 0.449

Na 0.000 0.002 0.424 0.000 0.000 0.512 0.007 0.000 0.692 0.001 0.001 0.540

K 0.000 0.000 0.011 0.000 0.000 0.011 0.001 0.000 0.013 0.002 0.000 0.013

Clÿ

Fÿ

Sum 8.012 4.032 4.942 8.031 4.023 4.997 8.008 4.014 5.022 8.053 4.011 5.010


1151

are �200±300 mm across. Osumilite occurs as largecrystals (up to 1 cm) and the biotite� quartz symplectitezones range from 100 to 200 mm and enclose singlegrains of biotite. This texture suggests a very small-scale and general melting reaction such as biotite �quartz � melt with subsequent recrystallization tobiotite � quartz symplectite upon cooling (see alsoBarboza & Bergantz, 2000).The electron microprobe analyses of biotite gave

compositions with 2±3�5% TiO2, F/(F � OH) �0�4±0�6 and XMg �0�83 (Westphal, 1998). Substitution ofboth F and Ti in phlogopite and biotite increases theirthermal stability (e.g. Munoz & Ludington, 1977;Trùnnes et al., 1985; Pati~no Douce, 1993; Tareen et al.,1995, 1998). The stability field of F±Ti-biotite may beshifted to higher temperatures by as much as 450�C

relative to KMASH (Dooley & Pati~no Douce, 1996).Consequently, if the biotite � quartz symplectites indi-cate fluid-absent melting of high Ti/F-biotite in rockscontaining osumilite � biotite � ilmenite � spinel �quartz, this would be consistent with temperatureshigher than 1000�C near the intrusive contact. Thus,the thermal aureole may have reached temperaturesover 1000�C near the contact with the anorthositedecreasing to 700�C or lower at distances 415±16 kmfrom the complex.

THERMAL MODELLING

Field studies, petrological studies and the geochrono-logy suggest that the high-temperature (M2) meta-morphic event was caused by the emplacement of the

Table 1a: continued

Sample 48 Sample B96

Area: 48b 48b 48b 48b3 48b3 48b3 231-1 231-1 231-1 231-8

Analysis: 1153

garnet

1157

opx

1154

plag

1178

garnet

1174

opx

1176

plag

1807

bi

1808

bi

1809

bi

1819

bi

SiO2 37.19 47.57 57.85 36.83 46.21 57.33 39.53 39.64 39.25 39.12

TiO2 0.06 0.09 0.05 0.05 0.20 0.05 3.51 3.34 3.23 3.31

Al2O3 21.87 3.97 26.76 21.90 4.82 26.85 13.71 13.70 13.56 13.93

FeO 32.54 32.82 0.35 32.64 32.72 0.32 7.62 7.42 7.29 7.80

MnO 1.68 0.46 0.02 1.68 0.44 0.00 0.07 0.01 0.08 0.00

MgO 5.60 14.98 0.01 5.96 14.68 0.00 20.81 21.11 21.10 21.08

CaO 1.92 0.20 7.50 1.50 0.18 7.82 0.00 0.00 0.00 0.01

Na2O 0.02 0.03 7.23 0.03 0.03 7.04 0.26 0.26 0.26 0.26

K2O 0.00 0.02 0.55 0.00 0.01 0.46 9.17 8.24 8.91 8.42

F 4.53 4.47 4.48 4.44

Cl 0.02 0.02 0.00 0.01

Total 100.88 100.14 100.32 100.57 99.28 99.87 99.22 98.20 98.17 98.37

Corrected for F & Cl: 95.07 94.07 94.06 94.24

No.: 12 6 8 12 6 8 11 11 11 11

Si 2.932 1.869 2.588 2.914 1.835 2.577 2.858 2.874 2.861 2.842

Al 2.033 0.184 1.411 2.042 0.225 1.423 1.169 1.170 1.165 1.193

Ti 0.004 0.003 0.002 0.003 0.006 0.002 0.191 0.182 0.177 0.181

Mg 0.659 0.878 0.001 0.703 0.869 0.000 2.244 2.282 2.293 2.283

Fe2� 2.145 1.078 0.013 2.160 1.086 0.012 0.460 0.450 0.444 0.474

Mn 0.112 0.015 0.001 0.113 0.015 0.000 0.004 0.001 0.005 0.000

Ca 0.162 0.008 0.360 0.127 0.008 0.376 0.000 0.000 0.000 0.001

Na 0.002 0.002 0.627 0.004 0.002 0.614 0.036 0.036 0.037 0.036

K 0.000 0.001 0.031 0.000 0.000 0.027 0.846 0.762 0.829 0.780

Clÿ 0.002 0.003 0.000 0.001

Fÿ 1.037 1.025 1.034 1.021

Scations 8.049 4.038 5.033 8.065 4.047 5.030 7.807 7.758 7.812 7.789

Cations � Fÿ & Clÿ: 8.846 8.786 8.846 8.811


1152

Rogaland anorthosite complex into granulite- to neargranulite-facies host rocks. The pigeonite-in (at 5 kmdistance), osumilite-in (at 10 km distance) and locallythe orthopyroxene-in (at �15 km distance) isogradsrepresent the extent of prograde metamorphic reactionduring contact metamorphism at �5 kbar pressures.A thermal modelling study was undertaken to clarifythe relationship between intrusion history (timing ofmajor intrusions) and the UHT metamorphismindicated by mineral-in isograd arrays observed inthe country rocks.

Model assumptions

Factors that can influence the width of the contactmetamorphic aureole around an igneous intrusioninclude the intrusion temperature, the thermal con-ductivities of the wall rocks, latent heat of crystalliza-tion, and the ambient country rock temperature at thetime of intrusion. The physical properties of the coun-try rocks and the intrusive units place limits on themodel parameters, and are described briefly below.Contact metamorphism occurred as the thermal gra-

dient between the higher temperature of the intrusionsand the lower temperature of the country rocks beganto equilibrate thermally. Modelling the interplay ofheat conduction, heat advection, and heat sourcesallows for a greater understanding of the metamorph-ism. Analytical solutions exist that approximate thethermal evolution of country rock adjacent to intru-sions with a simple geometry (e.g. Lovering, 1935;Jaeger, 1964). Dykes and tabular bodies can berepresented in one-dimensional (1-D) thermal modelsthat treat the heat source as a tabular body with twoinfinite planar dimensions. Heat transport is linearin one direction that is perpendicular to the finite

dimension. For roughly cylindrical intrusive bodiestwo-dimensional (2-D) thermal modelling can be used.The cylindrical intrusions are infinite in one and finitein two dimensions. The heat flow is planar in the twodimensions that are perpendicular to the third infinitedimension. Magma chambers or intrusive bodies thatare finite in three dimensions require three-dimensional(3-D) thermal modelling (e.g. Alcock et al., 1999).Clearly, no intrusive body is infinite in any direction;

consequently, 3-D modelling would appear to be theonly realistic solution to geological situations. How-ever, depending on geometry and scale, portions ofigneous complexes can be modelled two (e.g. Norton& Knight, 1977) and even one dimensionally (e.g.Dipple, 1992).As stated above, the intrusions at Rogaland appear

to have extended both above and below the presentlevel of exposure and are roughly cylindrical (see

Table 1b: Mineral assemblages in thesamples that were used for P±T estimates

Sample: 28 35 40 42 43 44 48 B96

garnet x x x x x x x

orthopyroxene x x x x x x x x

plagioclase x x x x x x x x

K-feldspar x x x x x x x x

quartz x x x x x x x x

osumilite x

cordierite x x

sillimanite x

spinel x x x x x x

biotite x x x x x x x

ilmenite x

Table 1c: P±T estimates based on orthopyroxene±garnet±plagioclase±quartz assemblages

H NP H PH H LG

T (�C) P (kbar) T (�C) P (kbar) T (�C)@5 kbar T (�C)@5 kbar

28a 700 5�1 695 4�6 700 730

35b1 750 5�5 745 4�8 750 805

35b4 780 5�6 775 4�9 775 840

40b1 790 6 785 5�7 790 850

40b2 750 5�4 745 4�8 745 790

42a34 795 5�3 785 4�7 790 850

42a5 800 855

42a6 790 5�3 785 4�6 790 845

43a1 800 5�6 795 4�9 795 855

44ab 800 5�6 795 4�9 795 850

LG NP LG PH

T (�C) P (kbar) T (�C) P (kbar)

28a 730 5�2 725 4�635b1 805 5�7 800 4�935b4 840 5�8 840 5

40b1 850 6�3 860 5�940b2 790 5�6 790 4�942a34 850 5�4 850 4�842a5

42a6 855 5�4 840 4�743a1 845 5�8 855 5�144ab 855 5�7 850 4�9

Temperature estimates: H, Harley (1984); LG, Lee&Ganguly(1988). Pressure estimates: NP, Newton & Perkins (1982);PH, Powell & Holland (1988).


1153

Paludan et al., 1994). This indicates that realisticthermal modelling can be done two dimensionally.In the worst case, the 2-D modelling will give themaximum extent of isotherms around the intrusionsand would still place limits on the thermal evolutionaround the intrusive complex.An additional consideration is the heat transport

mechanism. If a fluid phase is absent or minor, thenadvective heat transport can be ignored. Several obser-vations suggest that a fluid phasewas not volumetricallyimportant in the metamorphic rocks around the intru-sive complex.Osumilite is not stable under conditions ofhigh water activity (Schreyer & Seifert, 1967; Olesch &Seifert, 1981), and the overall preservation of the ortho-pyroxene�quartzassemblage in thecountry rockgneissesindicates a lack of significant hydration. Swanenberg(1980) examined the country rocks from the Rogalandanorthosite complex and suggested that the composi-tions of the fluid inclusions, which range fromH2O richto CO2 rich, are related to rock type rather than meta-morphic grade. Further, Bol et al. (1995) inferred fromoxygen and carbon isotope studies on samples fromboththe Faurefjell Formation and graphite-bearing `gar-netiferous migmatites' that no pervasive fluid waspresent during the high-T metamorphism. As a result,it is possible to treat all heat transport at Rogaland asconductive, which simplifies the modelling.Two scenarios of intrusion were modelled. The sim-

plest scenario is to treat all the intrusive bodies of thewhole complex as a single intrusion. This treatment isjustified by the nearly identical emplacement ages(Fig. 2) of the major anorthosite units (Sch�arer et al.,1996), which are closely spaced at 929� 2, 932� 3 and932 � 3Ma for the Egersund, Helleren and AÊ na±Sira,respectively.The jotunite dykes give ages of 931� 5Ma,and the observation that the BSL intruded after theanorthosite but before the dyke system would supportthis single event scenario. The Tellnes dykes(920 � 3Ma) are volumetrically small and representonly a minor contribution to the total heat budget.The second intrusion scenario we tested was to sepa-

rate the anorthosite intrusions and the later BSL intru-sion into two major intrusive phases. The anorthositeswere treated as a single elliptical unit, whereas the BSLis more complex (Nielsen & Wilson, 1991; Paludanet al., 1994). The BSL consists of three major andthree minor cycles that were emplaced sometimewithin 5Myr after the anorthosite. For the modelling,we simplified then BSL to three magmatic pulses at 3,3�7 and 4�5Myr after the anorthosite intrusion.The southern parts of both the BSL and the anortho-

sites are not included in the thermal model, as the high-temperature mineral assemblages in the country rockare not spatially related (Fig. 2) (in the following dis-cussion BSL refers to the lower part).

Intrusion temperatures and the latent heat ofcrystallizationBoth the orthopyroxene megacrysts and the largeplagioclase crystals of the Egersund massif crystallizedat 1100±1200�C (Duchesne &Maquil, 1987). The BSLintrusion consists of several igneous rock types, whichrange from anorthosites to leuconorites. We assumeliquidus and solidus temperatures of 1200�C and1100�C, respectively, for both the massif-type anortho-site and the average of the BSL.The massif-type anorthosites appear to have

intruded as a crystal mush (Duchesne & Maquil,1987). The anorthosites can be roughly comparedwith the Ballachulish Igneous Complex, which showsa similar temperature interval of crystallization (Weiss& Troll, 1991). At Ballachulish the foliation of mineralgrains at the margins of the intrusion suggests that thecrystal mush contained �50 vol. % crystals (Troll &Weiss, 1991). Peacock (1989) suggested that the latentheat of a magma that was 50% crystallized at the timeof intrusion would be about half that of a magma onthe liquidus. Consequently, we estimate the latent heatof crystallization for the anorthosites to have a value of�2�5 � 105 J/kg. On the other hand, textural evidenceindicates that the parental magmas for the BSLintruded almost devoid of crystals (Nielsen & Wilson,1991; Paludan et al., 1994). A crystal-free melt wouldhave a value for the latent heat of crystallization of5 � 105 J/kg, which is similar to that of basalt (e.g.Murase & McBirney, 1973; Thompson, 1992). We didnot account for the heat absorbed during partial melt-ing of wall rocks and heat released during subsequentcrystallization as the field observations suggest thatpartial melting was rare in the osumilite±biotite-bearing sequences thatmakeup�1%of the supracrustalsection of the contact aureole.We assume that thermal conductivity, and, therefore

thermal diffusivity, is independent of temperatureand have adopted a value of k � 0�72 � 10ÿ6 m2/s(Peacock, 1989). Additionally, we assume that texturalanisotropy, for example in schists (Buntebarth, 1991),does not affect the thermal properties.

Initial and boundary conditions

It is impossible to make an independent estimate ofthe country rock temperature at the time of intrusion,but it is possible to constrain the probable range. Weassume initial P±T conditions of the supracrustal sec-tion before the emplacement of the anorthosite magmawere 500±750�Cand 5 kbar. These temperatures wouldbe consistent with the emplacement of Rogalandanorthosite complex into crustal rocks already atamphibolite- to granulite-facies conditions ( Jansen et al.,


1154

1985) and with mantle heat flow (55±85mW/m2)inferred for anorthosite emplacement (e.g. Emslie,1985; Wiebe, 1986, 1992; Corrigan & Hanmer, 1997).

Numerical treatment of the heatconduction equation

We have adopted a continuum formulation of the heatconduction equation to model heat transfer in thecontact aureole. A comprehensive discussion of thebasic heat conduction equation has been given by, forexample, Carslaw & Jaeger (1959) and Norton &Knight (1977) (see also Voller, 1987). The partial dif-ferential equation that describes 2-D, time-dependentheat conduction is

@T@t� k

�@2T@x2� @

2T@y2

�: �1�

The above equation is solved numerically by thefinite-element method (FEM). The 120 km � 240 kmcomputational domain consists of a grid of 1945 nodesthat are refined proximal to the igneous complex(Fig. 4). For the modelling, we assumed the solutionto be symmetric around the lower margin of the com-putational domain and conducted a grid refinementstudy to ensure that the numerical solution was inde-pendent of the spatial and temporal grid discretization.Heat flux varied by 51% after increasing the densityof nodes by a factor of two.We account for the latent heat of fusion in the defini-

tion of an effective specific heat (cp�). The latent heat L

corresponds to the change in the enthalpy H at thetransitional melt temperature (Tm), and the followingrelationship is introduced:

H�T��TTref

�cp�T� � LY�T ÿ Tm�� 2�

Y�z� � 1 z � 0

0 z 5 0:

�The cp

� is

c�p �dH

dT� cp�T� � Ld�T ÿ Tm,DT� �3�

where d is the Dirac delta function. For the applica-tion of (2) to a system that solidifies over the range oftemperatures, the Dirac delta function in (2) isreplaced with the d-form function d�(T ± Tm,DT),which has a large but finite value over the interval DTcentred about Tm and is zero outside the interval. Theinterval DT corresponds to the difference between theliquidus and the solidus temperatures for the material.The cp value is for each node using input enthalpy±temperature curves. The effective specific heat iscalculated using equation (3).

COMPUTATIONS

Two-dimensional modelling of a large,single intrusion

The simplest assumption was to model the anorthositebodies and BSL as a single cooling unit. We conductedthree series of simulations using uniform initial tem-peratures in the supracrustal section of 500, 600 and750�C. Figure 5 compares calculated temperatures andP±T estimates from various locations around the intru-sive contact. This shows that model country rock tem-peratures that are lower than 600�C are inconsistentwith P±T determinations. The initial country rocktemperature of 600�C results in maximum tempera-tures that are lower than the P±T determination nearthe intrusive contact, but fit well with the P±T deter-minations at �15±20 km from the intrusion (Fig. 5).Using an initial country rock temperature of 750�C inthe model gives temperatures near the contact that fitwell with the observations, but temperatures that aretoo high at greater distances (Fig. 5). Further, auniform country rock temperature of 750�C suggestsa mantle heat flux of �100mW/m2, which is the kindof value associated with oceanic intraplate hot spots(Sleep, 1990), and is unrealistically high for the settingof the anorthosites.

Fig. 4. Locations of nodes used in the finite-element modelling. Theextent of the area ensures that the boundary conditions have anegligible influence on the temperature field near the intrusions.The lower part of grid is the area of interest.


1155

The thermal modelling of a single, elliptical coolingunit shows that no uniform country rock temperaturecan satisfactorily explain the observed temperature±distance profile at Rogaland. However, these resultssuggest that a thermal gradient present in the countryrocks could account for the observed temperaturearray around the magmatic complex.

Two-dimensional modelling of multipleintrusions (two stages)

For the second set of simulations the geometries of theintrusion phases were modelled as two ellipses (Fig. 6).In this model, a massif-type anorthosite complex(30 km � 40 km) initially heats the country rocks,and, after a brief hiatus, a second, smaller complex(9 km � 12 km), which represents the northern lobeof the BSL, intrudes.Figure 7a shows time vs temperature profiles for

points at various distances from the contact. Figure 7aalso shows that, if the second phase (BSL) wereemplaced almost immediately after the first phase,the results would be very similar to those of a singlecooling unit (above), as almost no thermal gradientfrom the initial intrusion would be present in the coun-try rocks. An interval of 10±15Myr allows enough timeto pass that the initial intrusion and the country rocksapproach thermal equilibrium, and the country rocktemperatures are effectively uniform. Large to moder-ate thermal gradients in the country rocks are transi-tory and present only from �1�5 to 7�5Myr followingthe first stage of intrusion (Fig. 7a). The most

pronounced thermal effects of the second phase ofintrusion would occur over this short span of time. Inour model calculations, after�3Myr a significant ther-mal gradient extends to 20 km in the adjacent countryrocks (Fig. 7b).For the second phase of the thermal model (BSL) we

simplified the complicated structure of the nested(onion-shaped) megacycle units (Wilson et al., 1996).Disregarding their characteristic geochemical features,there are approximately three equal-sized units. Wemodelled these units as three pulses of fresh, hotmagma into the chamber over a total time interval ofassumed 1�5Myr.The calculations were performed in four consecutive

steps. The effects of the first phase (anorthosite)assumed an initial intrusion temperature of 1200�Cand a uniform 600�C for the country rock. After3Myr the second phase (BSL) started. The initialmagma temperature was 1200�C, but this intrusionwas emplaced into country rock with a thermal gradi-ent that was the result of anorthosite emplacement.This procedure was repeated at 3�7 and 4�5Myr tosimulate the second and third cycles (simplified) ofthe second phase (BSL). The timing between pulsesallows for the maximum thermal effect on the adjacentrocks. The calculations were ended 20Myr after theemplacement of the first phase.

Fig. 5. Temperature vs distance profiles in the country rocks calcu-lated by assuming a large, single intrusion and various homogeneouscountry rock temperatures. These are compared with the geothermo-metry and other estimates of temperature. Shaded area defines theapproximate limits on the maximum metamorphic temperature atincreasing distances from the magmatic contact. Labels (e.g. B96,é48) are sample localities (see Fig. 1). Data from Westphal &Schumacher (1997) and Westphal (1998).

Fig. 6. Comparison of the real extent and modelled intrusions. Thelarge ellipse represents the massive anorthosite intrusion phase andcovers an area of 30 km � 40 km � 942�5 km2. The small ellipserepresents the second phase (multiple intrusions) of the northernlobe (lower part) of the BSL, which covers an area of 9 km �12 km � 84�8 km2. Together, the total area of the ellipses(1027�3 km2) approximates the actual area of exposure. It shouldbe noted that the anorthosite complex extends into the North Sea[Bol (1990) and references therein; Sch�arer et al. (1996) andreferences therein].


1156

RESULTS AND DISCUSSION

At the end of the finite-element calculation, we haddata for 1945 nodes (geographical locations) spreadover an area of 30 km � 70 km (see Fig. 4). Each nodecontains 111 data points that represent temperature±time information spanning 20Myr.Figure 8 shows temperature vs time profiles for seven

locations (see also Fig. 1) at increasing distances fromthe contact with the intrusive complex. On all theprofiles the effects of the initial heating (anorthosite)and of each of the three phases of the BSL cycle areevident. Extremely high temperatures are reached onlynearest the intrusive complex, where the preheating ofthe anorthosite was highest (0�5±5�0 km, Fig. 8). Atincreasing distances (10�5±15�5 km, Fig. 8) from thecontact the effects are less pronounced. Figure 8 alsoshows this temporal delay in the thermal maximum,which is attained at about �4�5Myr at 0�5 km but at�7�5Myr at 15 km. This is in excellent agreement withrecent determinations of metamorphic ages of�925Ma (M�oller et al., 2002; A. M�oller, personal com-munication, 2002).Figure 9 compares the calculated maximum tem-

peratures (Tmax) with observed peak temperatures asa function of distance from the intrusive contact. Themodel and observed values show excellent agreement.The model also gives temperatures for the pigeonite-inand osumilite-in isograds of �860 and 750�C. Thesevalues are reasonable for the established stabilities ofthese minerals (see Lindsley, 1980, 1983; Carrington &Harley, 1995).

The modelling also shows the 2-D spatial distribu-tion of maximum temperatures (Fig. 10). The lower-temperature isotherms form distinct narrow zones thatparallel the anorthosite contact. The pattern of thehighest-temperature isotherms in the country rocks(T higher than �900�C) broadens in the area betweenthe modelled BSL intrusion and anorthosite body. This

Fig. 7. (a) Sets of time vs temperature curves for the country rocks that are generated by simulated intrusion of the anorthosite alone. (b) Thedistance vs temperature diagram shows the thermal gradient in the country rocks that is generated by the anorthosite at �3�0Myr. Thistemperature distribution pattern is used for the BSL intrusions in the 2-D thermal model.

Fig. 8. Time vs temperature profiles for the double intrusion modelat various distances from the BSL contact. Inset shows the modelledsample locations.


1157

feature is very similar to the pattern of the pigeoniteand osumilite isograds, which can be treated as therough approximates of isotherms. The zones definedby these isograds are broadest around the BSL andnarrow drastically around the anorthosite.Another interesting feature seen in Fig. 10 is that the

high-temperature isotherms cross from country rocksinto the intrusive bodies. As a consequence, isothermsobtained from detailed geothermobarometry or high-temperature isograds in the country rocks may appearto be truncated by the intrusion, which could lead tomisinterpretation of the field relations.Figure 11 shows the distribution of isotherms based

on the thermal effects of considering the intrusions as(1) a large single event (Fig. 11a) or (2) multipleintrusions (Fig. 11b). The large single intrusion gener-ates isotherms that are roughly parallel to the contacts.A narrow zone within �2�8 km of the contact reachestemperatures of 850±950�C. Higher-temperature

isotherms (1000±1150�C) lie within the intrusion. Themultiple intrusion model (Fig. 11b) produces a verydifferent pattern of isotherms. The isotherms above�900�C effectively abut the anorthosite and a broadzone of up to �5 km across reached temperatureshigher than 950�C. Additionally, the 850±950�C iso-therms extend farther into the country rocks (�10 km).The distribution of maximum temperatures and

duration isochrons, which represent the length of timethe rocks spent above 750�C during heating and cool-ing, are compared in Fig. 12a. The duration isochronsroughly parallel the contact of the initial and largerintrusion, whereas the maximum temperature iso-therms are strongly deflected by the smaller and laterintrusion. This indicates that, whereas the secondintrusion has a major impact on the local distributionof peak temperatures, it does not significantly prolongthe heating and cooling cycle. The 750�C isotherm andthe duration isochron for 0±5Myr are roughly parallelover the entire area. The duration isochrons of 10±14Myr crosscut the isotherms at steep angles near thesmaller and later intrusion (near point C in Fig. 12a),whereas near point A, at greater distances from thesmaller intrusion, the isotherms and duration isochronsare nearly parallel.These relative orientations of the isotherms and the

duration isochrons show the combined thermal effectsof the two intrusions. Not surprisingly, the effects arestrongest in the area between the intrusions (near point

Fig. 9. A distance vs temperature diagram comparing the simulatedtemperatures with the thermal maximum inferred from geothermo-barometry. Also shown are the distances of the mapped isograds forpigeonite, osumilite and orthopyroxene.

Fig. 10. The spatial distribution of maximum temperatures from thefinite-element modelling. The highest-temperature isotherms in thecountry rocks are related to the small, second intrusion. Dashed whitelines are schematic pigeonite (P) and osumilite (O) isograds.

Fig. 11. Comparison of the distribution of the isotherms assuming alarge single event (a) or a multiple intrusion (b). Dashed lines in eachdiagram show key features from the alternative model.


1158

C in Fig. 12a), and the effect of the smaller intrusiondissipates noticeably at distances 410 km (near pointB in Fig. 12a). Additionally, the area around point C(Fig. 12a), where the isotherms and duration isochronsshow the least parallelism, gives an impression of theextensive variation in heating and cooling rates thatcan occur over relatively small distances. This varia-tion of heating can be shown in detail for four locations(points A, B, C and D in Fig. 12a) in temperature±timediagrams (Fig. 12b).Point A is located 6 km from the intrusive contact

along the extension of the long axis of the large, ellipticalintrusion. The maximum temperature is �750�C, so

this location never rises above 750�C and the durationof the temperature interval greater than 750�C is zero.Point B is located 5 km from the large and 10 km

from the small intrusion and lies on 10Myr durationisochron. The maximum temperature that is attainedis�820�C, and the thermal effect of the small intrusionis considerable at point B. The initial rapid heating andlong period of cooling for these localities (A and B) arecontrolled by the anorthosite (Fig. 12b). The differencein the temperatures at points A and B after 3Myrreflects the geometrical effect (elliptical shape) of themodelled intrusion (see also point C).Point C is located 5 km from the large intrusion and

4 km from the small intrusion. The locality lies on the12Myr duration isochrons, and it reached a maximumtemperature of nearly 950�C. Point C has experiencedan extremely different heating and cooling history frompoints A and B (Fig. 12b). The variations in heatingand cooling rates and the higher maximum tempera-ture are due to the major influence of the second intru-sion (modelled BSL).Point D (Fig. 12b) is located 5 km from the intrusive

contact along the extension of the short axis of thesmaller intrusion. The maximum temperature at thislocality is �900�C, and the total time with tempera-tures higher than 750�C is 7Myr. The variations inheating and cooling rates mimic those at point C. Thethermal effects of the second intrusion on the countryrocks (heating and cooling rates) are more drastic thanat point C, but the maximum temperature is lowerbecause of the lower initial temperatures at the timeof the second intrusion. The only difference betweenthe thermal development at points C and D (Fig. 12b)is the country rock temperature at the time of thesecond intrusion (i.e. the magnitude of the thermalgradient caused by the anorthosite).All four points are located at about the same distance

(�5 km) from an intrusive contact, but show differenttemperature±time histories, which are related to therelative distance to both plutons. The heating and theextremely high temperatures are related to proximityto the small intrusion, whereas the duration of heatingand cooling cycle is controlled by the much largerintrusion. The temperature±time development ofpoint C (Fig. 12b) is the result of extensive influenceof both the large and the small intrusions; this localityreached the highest temperature and remained at hightemperatures (4750�C for about 12Myr).

CONCLUSIONS

Thermal modelling suggests that the Rogaland intru-sive complex cannot be treated as a single intrusiveevent. Modelling two intrusive cycles with a 3Myrhiatus between the cycles can account for the

Fig. 12. (a) The distribution of maximum temperatures (continuouslines) and duration isochrons (dashed black lines), which indicate thetotal amount of time that the rock spent above 750�C. Dashed whitelines are schematic pigeonite (P) and osumilite (O) isograds. (b) TheT±t development of locations A, B, C and D [see (a)], which showsvariations in both maximum temperatures and heating and coolingrates at various localities. (See text for discussion.)


1159

high-temperature metamorphism and the observeddistribution of isotherms (isograds). In this model, theinitial emplacement and crystallization of the anortho-site (first phase) produces a regional thermal gradientranging from �640�C to �880�C. The smaller BSLintrusion (second phase) intrudes adjacent to theanorthosites and provides a second thermal pulse.This combination of events can explain the observedarray of high-temperature isograds and the maximumtemperatures of 41000�C at �2 km distance from themagmatic contact.Until recently, intrusive activity associated with the

Rogaland anorthosite complex was thought to span100Myr, which would effectively preclude the two-phase intrusive model suggested above. However,new age data from Sch�arer et al. (1996) suggest thatthe entire magmatic emplacement of the main intru-sive complex occurred over a much shorter time inter-val of �5Myr between �931 and 926Ma, which isentirely consistent with the timing of the intrusionsthat the thermal modelling suggests is necessary toexplain the metamorphism of the country rocks.

ACKNOWLEDGEMENTS

We are grateful to the staff of the InstitutfuÈ r Mineralogie±Petrologie±Geochemie, Universit�atFreiburg for their help during sample preparation(especially K. Fesenmeier) and microprobe measure-ments (especially H. MuÈ ller-Sigmund). We would alsolike to acknowledge the staff and students of theUniversity of Utrecht for the excellent fieldwork andmapping they carried out many years earlier in thisregion. We thank A. Scheld, J. Alcock, S. Peacock andC. Maijer for their opinions, and K. Bucher for review-ing an earlier version. M. Sandiford's review andS. Barboza's very detailed comments helped toimprove this paper. Funding for this work by grantSchu 919/4-1 of the Deutsche Forschungsgemeinschaftis gratefully acknowledged. This work is part of thedoctoral thesis of M.W., which was completed at theUniversit�at Freiburg. The geological framework of thiscontribution developed over time as the result ofmutually beneficial discussions between M.W. andJ.C.S. The numerical solutions to test these ideas arethe result of co-operation betweenM.W. and S.B., whowas supported by the Kristallographisches Institut(Freiburg).

SUPPLEMENTARY DATA

For supplementary data, please refer to Journal ofPetrology Online.

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high-temperature metamorphism and the role of magmatic heat

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