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Journal of Petrology | Volume 44 | Number 6 | Pages 1145-1162 | 2003
© Oxford University Press 2003
High-Temperature Metamorphism and the Role of Magmatic Heat Sources at the Rogaland Anorthosite Complex in Southwestern Norway
1 INSTITUT FÜR MINERALOGIE, PETROLOGIE UND GEOCHEMIE DER UNIVERSITÄT FREIBURG IM BREISGAU, ALBERTSTRASSE 23B, D-79104 FREIBURG, GERMANY
2 KRISTALLOGRAPHISCHES INSTITUT, DER UNIVERSITÄT FREIBURG IM BREISGAU, D-79104 FREIBURG, GERMANY
Present address: Department of Earth Sciences, University of Bristol, Bristol BS8 1RJ, UK. E-mail: j.c.schumacher{at}bris.ac.uk
RECEIVED JULY 10, 1998; ACCEPTED JANUARY 2, 2003
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONGEOLOGICAL SETTINGTHERMAL MODELLINGCOMPUTATIONSRESULTS AND DISCUSSIONCONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
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The Rogaland complex covers
1000 km2 in southwestern Norway and consists mainly of anorthosite massifs and the layered Bjerkreim–Sokndal lopolith (BSL). These rocks intrude charnockitic migmatites containing intercalated marbles and garnetiferous migmatites. High-temperature mineral isograds (pigeonite, osumilite and orthopyroxene) in the metamorphic basement are subparallel to and increase in grade towards the intrusive complex. P–T estimates from the country rocks show a roughly linear increase in temperature towards the BSL consistent with the distribution of isograds. The peak P–T conditions at 20 and 2·5 km from the contact at 5 kbar range from 700 to >1000°C. Field relations and age determinations link the high-T metamorphism and the magmatism. The two-dimensional thermal modelling indicates that heat from a single magmatic cooling unit is not sufficient to produce the array of isograds and the peak metamorphic temperatures. Two magmatic episodes separated by 3 Myr, however, can account for the high-temperature metamorphism. In this model, the emplacement and crystallization of the anorthosite produces a regional thermal gradient (from 750 to 600°C). After a brief hiatus a second, smaller body (BSL) provides an additional thermal input that results in an array of high-temperature isograds and country-rock temperatures >1000°C. KEY WORDS: Rogaland; UHT; thermal model; osumilite
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGTHERMAL MODELLINGCOMPUTATIONSRESULTS AND DISCUSSIONCONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
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Proterozoic terrains (e.g. Antarctica and North America) are the most common hosts of large anorthosite complexes (Ashwal, 1993; and references therein). Metamorphic contact aureoles of various dimensions commonly accompany these intrusions. Anorthosite complexes are typically emplaced into high-grade metamorphic rocks and caused high-temperature contact metamorphism in the proximal country rocks. The intrusion temperatures of the anorthosites and related rocks are
1000–1200°C, and the associated metamorphic aureoles are commonly composed of granulite-facies mineral assemblages characterized by high-temperature mineral-in isograds, such as andalusite, orthopyroxene, pigeonite or osumilite. As the granulite-facies country rocks appear to be essentially anhydrous, heat transfer from the intrusions to the country rocks is likely to have been conductive rather than advective.
In southwestern Norway a mid-crustal (5 kbar) contact metamorphic aureole is associated with the emplacement of the Rogaland anorthosite complex (Hermans et al., 1975; Jansen et al., 1985; Maijer, 1987). The metamorphic aureole extends to 20 km from the intrusive contact, corresponding to the location of the orthopyroxene-in isograd in quartz-bearing metapelites and plagioclase–clinopyroxene-bearing metabasites. An osumilite-in isograd lies 10–13 km from the intrusive contact, and a pigeonite-in isograd occurs 5 km from the contact (Tobi et al., 1985).
Age determinations of Schärer et al. (1996) have shown that massif-type anorthosites and the layered series of the BSL are nearly coeval, rather than being separated by a significant time interval (150–250 Myr) as was previously thought (Maijer, 1987). The massif-type anorthosites that form the major part of the intrusive complex were emplaced into high-grade metamorphic rocks, which had equilibrated at 600–700°C at 6–8 kbar (Jansen et al., 1985) before the anorthosite event.
Ultrahigh-temperature (UHT) metamorphism around the Rogaland complex appears to be more extensive than that found at other anorthosite complexes. For example, at the Nain complex (Speer, 1975; Berg & Wheeler, 1976; Berg, 1977) and the Laramie anorthosite complex (Snyder et al., 1988) the total widths of the aureoles are only 3–4 km compared with 20 km at Rogaland, and the peak temperatures are lower than in Rogaland.
The aim of this study is to understand the contact metamorphism induced by the interaction of the anorthosite and the mafic intrusive bodies based on a well-characterized, high-temperature terrain, by two-dimensional thermal modelling of the intrusive complex. This experiment focuses on understanding the high-temperature conditions that led to pigeonite and osumilite stability in the country rocks, resulted in the observed distribution of isograds and formed the wide granulite-facies aureole of the Rogaland anorthosite complex. Heat conduction modelling was carried out using the commercial software package FIDAP (1993), which uses the finite-element method. We compared the calculated thermal effects of single- and two-intrusive event scenarios with the distribution of temperatures in the country rocks proximal to the anorthosite complex, estimated using a range of geothermometers. Temperatures within the aureole range from 700°C at the orthopyroxene-in isograd to 1000°C at a distance of 2·5 km from the contact with the intrusions.
Our modelling results demonstrate that the emplacement of the igneous complex as two separate events can account for many observations not explained by the single intrusion scenario. Initial anorthosite emplacement heats the country rocks and produces a thermal gradient. A discrete second phase of magma emplacement 3 Myr after the anorthosite provides the additional heat input that causes the high-temperature metamorphism and the distribution of the observed mineral-in isograds in the preheated country rocks.
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GEOLOGICAL SETTING |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGTHERMAL MODELLINGCOMPUTATIONSRESULTS AND DISCUSSIONCONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
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In Rogaland, southwestern Norway (Fig. 1), large anorthosite bodies have intruded polymetamorphic gneisses of the Proterozoic of the Baltic shield, which are granitic to charnockitic migmatites with intercalated bodies of mafic rocks and metamorphosed sedimentary rocks (Tobi et al., 1985). The charnockitic migmatites (Hermans et al., 1975) contain orthopyroxene, quartzo-feldspathic lenses, schlieren and layered units containing both leucosomes and melanosomes. The metasedimentary series consist mainly of either calcareous rock types, the Faurefjell Formation (Sauter, 1983; Jansen & Tobi, 1987), or more abundant pelitic rock types, ‘garnetiferous migmatites’ (Huijsmans et al., 1981). As in other areas of southern Norway, the basement rocks intruded by the Rogaland complex give deposition ages of
1·5 Ga (Verschure, 1985) and underwent regional metamorphism to granulite-facies conditions at 1·2 Ga (Versteeve, 1975; Wielens et al., 1981).
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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 activity consists mainly of a jotunitic dyke system that includes the Tellnes ore body inside the Åna–Sira massif (Schärer et al., 1996). Large iron-rich, syenitic to granitic sheet intrusions are present in the country rocks and have compositions analogous to parts of the anorthositic igneous complex. However, these intrusions have crystallization ages of 1·2 Ga (Rietmeijer, 1979) that pre-date the final anorogenetic (post-Sveconorwegian) emplacement of the intrusive complex from about 930 to 920 Ma (Schärer et al., 1996).
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The igneous complex
Structure of the intrusive complex
The igneous complex consists of three massif-type anorthosite bodies, the Egersund–Ogna, Helleren and Åna–Sira, and the layered mafic series of the Bjerkreim–Sokndal lopolith (BSL, Fig. 2). The Egersund–Ogna massif is a 20 km diameter anorthosite dome, composed primarily of unzoned plagioclase with compositions ranging from An40 to An50 with a grain size of 1–3 cm. The central part of the anorthosite body contains giant Al-rich orthopyroxene (<1 m) and 5–50 cm plagioclase (An55) megacrysts (Duchesne & Maquil, 1987). The foliation of plagioclase grains in the margin of the intrusive body is consistent with deformation accompanying diapiric emplacement of low melt-fraction crystal mushes (Duchesne et al., 1985).
The Helleren and the Åna–Sira massifs (Fig. 2) are similar to the coarse-grained Egersund–Ogna anorthosite massif (Duchesne et al., 1985). The Åna–Sira massif contains the well-known Tellnes Fe–Ti deposit (Krause et al., 1985), which is a part of a leuconoritic dyke system (Michot, 1960) representing the 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 southeastern lobes (roughly upper part) cover an area of 40 km x 9 km. Recent mapping (Paludan et al., 1994) shows that the northern lobe is a trough-like, discordant intrusion that has the shape of a major isoclinal syncline, whose axis plunges roughly up to 45° to the SE. Both the northeastern and southwestern contacts dip at 80–90° towards the centre of the intrusion, which is thickest in the axial region of the syncline. The roof of the BSL is not preserved within the area of present outcrop (Wilson et al., 1996).
The igneous stratigraphy of the BSL is well documented (e.g. Nielsen & Wilson, 1991; Paludan et al., 1994; Wilson et al., 1996) characterized by anorthosite to troctolite, leuconorite, jotunite (= hypersthene monzodiorite) to mangerite (=hypersthene monzonite), quartz mangerite and igneous charnockite (Duchesne & Wilmart, 1997). Rhythmic layering (Michot, 1960; Paludan et al., 1994) characterizes the structure of the BSL magma chamber. Michot (1960) divided the BSL into an upper part that contains mangerite and quartz mangerite and a lower part containing anorthosite, leuconorite, norite and gabbronorite. Wilson et al. (1996) explained the BSL as a sequence of six (intrusive) megacycle units (MCU I–IV), which repeat characteristic sequences of cumulates.
Geophysical data indicate that the Åna–Sira massif is 4 km thick (Smithson & Ramberg, 1979) and that the layered series of the BSL may be 9 km in thickness (Paludan et al., 1994; Wilson et al., 1996). Assuming an average vertical thickness of 5 km for the whole intrusive complex and a length and average width of 50 km x 20 km, the volume of magmatic material is 5000 km3 (Schärer et al., 1996). This is a minimum estimate, as palaeomagnetic data from the North Sea indicate a much larger area of igneous activity. The Rogaland anorthosite complex might have had a total volume of the order of 20 000 km3 (Schärer et al., 1996).
Pressure estimates for the igneous rocks
P–T estimates at Rogaland based on geothermobarometry studies of the massif-type anorthosites (Fig. 3) indicate that they reached final equilibrium at a minimum pressure of 4–7·5 kbar (Duchesne & Maquil, 1987; Wilmart & Duchesne, 1987). An experimental study of a jotunite from the lower part of the BSL indicates that the currently exposed levels represent an intrusion depth equivalent to a pressure of 5 kbar (Vander Auwera & Longhi, 1994).
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Relative ages and age determinations
The field relationships provide evidence for the sequence of intrusive events (Fig. 2). The Helleren massif cuts the foliation of and is therefore younger than the Egersund–Ogna anorthosite. The layered BSL is younger than the Åna–Sira and Egersund massifs as it contains xenoliths of both anorthosite bodies (Wilson et al., 1996). The final stage of anorthosite evolution was the emplacement of a system of jotunite dykes (the monzonoritic Lomland dyke system), which cuts all the massifs (Duchesne et al., 1989).
Radiometric ages of anorthosites are difficult to obtain (Ashwal, 1993), as the rocks show generally insufficient variability in parent/daughter ratios (e.g. K/Ar, Rb/Sr or Sm/Nd) for reliable whole-rock isochrons. Dating using the U/Pb method on zircons is generally difficult because zircons in anorthosite are scarce. Hence, the ages of many anorthosites are known only indirectly, as a coeval origin for the associated zircon-bearing mangerites, charnockites and granites is only an assumption.
Intrusive activity associated with the Rogaland anorthosite complex was thought to have spanned some 100–250 Myr (Demaiffe & Michot, 1985; Duchesne et al., 1985; Maijer, 1987). In a new attempt to date the anorthosite, Schärer et al. (1996) used zircon and baddeleyite from large aggregates of orthopyroxene megacrysts. The new age data suggest that magma emplacement actually occurred over a much shorter interval. The emplacement ages (Fig. 2) of the major anorthosite units (Schärer et al., 1996) are closely spaced at 929 ± 2, 932 ± 3 and 932 ± 3 Ma for the Egersund, Helleren and Åna–Sira massifs. The jotunite dykes (931 ± 5 Ma) and the Tellnes dykes (920 ± 3 Ma) cut the other units and are later.
Unfortunately, there are no age data available for the BSL because these rocks lack zircon-bearing megacrysts of plagioclase or orthopyroxene. Additionally, as the BSL consists of a number of intrusive units (MCU I–IV), there can be no single age of intrusion. From field relationships the BSL intruded after the Egersund, Helleren and Åna–Sira anorthosites, but before the jotunite dyke system. Based on the available age determinations and their errors, the maximum time span over which the BSL could have been emplaced is 5 Myr (931–926 Ma).
The metamorphic envelope to the anorthosite complex
Stages of metamorphism
Hermans et al. (1975), Jansen et al. (1985) and Maijer (1987) inferred four separate phases (M1–M4) of metamorphism in the region. The first high-grade metamorphic event (M1) probably occurred before 1·2 Ga (Versteeve, 1975; Wielens et al., 1981). Duchesne & Michot (1987) related M1 to the emplacement of the massif-type anorthosites. However, dating (1050–870 Ma) 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 that the high-temperature and low-pressure metamorphic event (M2) is correlated with the intrusion of the BSL (Maijer et al., 1981; Maijer, 1987). Recent work by Möller et al. (2002) indicates metamorphic ages of 925 Ma for high-temperature M2 phase. A subsequent low-temperature and low-pressure, retrograde event (M3) overprints the M2 assemblages (Maijer, 1987). The youngest metamorphic event, the Caledonian greenschist-facies M4 stage, locally overprints the higher-grade assemblages.
The recent age determinations of Schärer et al. (1996) indicate that the massif-type anorthosites and associated rocks were emplaced into high-grade metamorphic regional rocks, which equilibrated at 600–700°C at 6–8 kbar pressure during an earlier M1 event that preceded the intrusions of the anorthosite complex (Jansen et al., 1985). Furthermore, the interval that separates the intrusion of the massif-type anorthosites and the BSL decreases from 150–250 Myr (Maijer, 1987) to 5 Myr (Schärer et al., 1996).
Metamorphic mineral assemblages and isograds
More than 5 km from the anorthosite complex, monzonorite and pyroxene syenite in the country rocks contain orthopyroxene, clinopyroxene, Ca-amphibole and biotite (Dekker, 1978). Within 5 km of the intrusive contact, inverted pigeonite may be present (Hermans et al., 1975; Tobi et al., 1985), indicating that the thermal maximum exceeded 825°C (Lindsley, 1983; Ranson, 1986; Sandiford & Powell, 1986) near the contact. The mineral assemblages and compositions of phases that were used for the P–T estimates are given in Table 1 (see Fig. 1 for the localities). The mafic country rocks that recrystallized during M2 metamorphism show equant mineral grains with triple junctions suggesting textural equilibrium among orthopyroxene, clinopyroxene, F-bearing hornblende, F-bearing biotite, plagioclase and quartz, which occur in variable proportions.
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The granite migmatites of the country rocks are generally orthopyroxene- and quartz-bearing assemblages (Hermans et al., 1975). Most of these mineral assemblages developed during the M1 event, but were overprinted by the subsequent, higher-temperature M2 event. The migmatites are probably relicts of the M1 event. The melanosomes of the metapelitic, ‘garnetiferous migmatites’ (Huijsmans et al., 1981) commonly contain either the low-Al mineral assemblage garnet–quartz–orthopyroxene–plagioclase–spinel–biotite or the high-Al mineral assemblage garnet–cordierite–sillimanite–plagioclase–spinel–graphite–sulphides (Hermans et al., 1975). The grain size of the matrix assemblage of the melanosome varies between 0·1 and 0·5 cm. The matrix contains rare garnet porphyroblasts up to 3 cm in diameter. We infer that the relict assemblage garnet + quartz + plagioclase + orthopyroxene ± spinel formed during M1. The reaction garnet = orthopyroxene + spinel records the prograde overprint of rocks proximal to the igneous complex during the M2 contact metamorphic event. The garnet + quartz overgrowths around orthopyroxene grains and zoning in orthopyroxene and garnet (Westphal & Schumacher, 1997; Schumacher & Westphal, 1999) are indicators of partial retrograde re-equilibration.
In places, osumilite is present (Maijer et al., 1977) in gneisses bearing K-feldspar, quartz, garnet, orthopyroxene and cordierite. As with pigeonite, osumilite occurs only in rocks proximal to the igneous complex. The presence of osumilite indicates final equilibrium of the rocks within the contact aureole at granulite-facies conditions, 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 isograds (pigeonite, osumilite and orthopyroxene) are conformable to the northern lobe of the BSL, suggesting a genetic relationship (Figs 1 and 2). Generally, the spatial distribution of the mineral isograds indicates that the grade of M2 metamorphism increases towards the igneous complex. At distances farther than 15–20 km from the intrusive complex, the effect of the high-temperature metamorphism (M2) is no longer detectable and all the country rocks in the study area show mineral assemblages typical of the granulite-facies conditions (M1) before the emplacement of the igneous complex (Jansen et al., 1985). A Caledonide greenschist-facies M4 stage has not affected the higher-grade assemblages in the study area (see also Tobi et al., 1985).
Pressure and temperature estimates for the country rocks
P–T estimates for the thermal maximum of the M2 episode inferred from mineral equilibria on rocks within the contact metamorphic aureole (see localities, Fig. 1) are consistent with the high-grade conditions inferred from the mineral assemblages (Westphal & Schumacher, 1996). Based on the net-transfer reaction orthopyroxene + plagioclase = garnet + quartz and Fe–Mg exchange between orthopyroxene and garnet, Westphal & Schumacher (1996) inferred temperatures of 900°C and 700°C at 5 kbar at points 5 km and 15·5 km from the igneous complex (Fig. 3). The result of multiple phase equilibrium (TWEEQU) estimates (Berman, 1991) on a sample 13 km (Fig. 1, and see Fig. 5 below) from the contact indicates temperatures of 765°C at 5 kbar. These estimates are consistent with the experimental results of Vander Auwera & Longhi (1994), who estimated BSL emplacement at an original pressure exceeding 5 kbar. We consider that uncertainties related to the analysed composition of minerals together with the uncertainties derived from the thermodynamic data yield an overall uncertainty of ±1 kbar and ±50°C, which, based on suggestions made by Essene (1989) for single thermobarometers, represent a conservative and therefore, maximum uncertainty.
Zoning profiles in coexisting orthopyroxene and garnet from the country rocks between the pigeonite-in and the orthopyroxene-in isograds indicate that cooling associated with M2 occurred over 20 Myr (Westphal & Schumacher, 1997; Schumacher & Westphal, 1999). Coexisting orthopyroxene and garnet from localities above the pigeonite-in isograd do not preserve high-temperature equilibrium compositions because of retrograde Fe–Mg exchange. Orthopyroxene and spinel completely replace primary garnet in the country rocks close to the igneous contact; garnet is a retrograde phase, found as rims around spinel and orthopyroxene in these rocks. Petrographic interpretations are better evidence of the peak temperature conditions within the pigeonite-in zone. These T–distance relations are shown below in Fig. 9.
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The presence of inverted pigeonite, which defines the pigeonite-in isograd (Maijer, 1987), suggests high temperatures within the narrow zone of the contact aureole. In addition, some samples at
2–3 km from magmatic contact show thin-section-scale textures that suggest high-temperature, fluid-absent melting of biotite (M2 stage) (Westphal & Schumacher, 1997). Symplectites of biotite + quartz between biotite and osumilite are associated with ilmenite, spinel and quartz. Quartz occurs as small grains, 50 µm in size, inside the osumilite. Biotite, ilmenite and spinel grains are 200–300 µm across. Osumilite occurs as large crystals (up to 1 cm) and the biotite + quartz symplectite zones range from 100 to 200 µm and enclose single grains of biotite. This texture suggests a very small-scale and general melting reaction such as biotite + quartz = melt with subsequent recrystallization to biotite + quartz symplectite upon cooling (see also Barboza & 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 of both F and Ti in phlogopite and biotite increases their thermal stability (e.g. Munoz & Ludington, 1977; Trønnes et al., 1985; Patiño Douce, 1993; Tareen et al., 1995, 1998). The stability field of F–Ti-biotite may be shifted to higher temperatures by as much as 450°C relative to KMASH (Dooley & Patiño Douce, 1996). Consequently, if the biotite + quartz symplectites indicate fluid-absent melting of high Ti/F-biotite in rocks containing osumilite + biotite + ilmenite + spinel + quartz, this would be consistent with temperatures higher than 1000°C near the intrusive contact. Thus, the thermal aureole may have reached temperatures over 1000°C near the contact with the anorthosite decreasing to 700°C or lower at distances >15–16 km from the complex.
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THERMAL MODELLING |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGTHERMAL MODELLINGCOMPUTATIONSRESULTS AND DISCUSSIONCONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
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Field studies, petrological studies and the geochronology suggest that the high-temperature (M2) metamorphic event was caused by the emplacement of the Rogaland anorthosite complex into granulite- to near granulite-facies host rocks. The pigeonite-in (at 5 km distance), osumilite-in (at 10 km distance) and locally the orthopyroxene-in (at
15 km distance) isograds represent the extent of prograde metamorphic reaction during contact metamorphism at 5 kbar pressures. A thermal modelling study was undertaken to clarify the relationship between intrusion history (timing of major intrusions) and the UHT metamorphism indicated by mineral-in isograd arrays observed in the country rocks.
Model assumptions
Factors that can influence the width of the contact metamorphic aureole around an igneous intrusion include the intrusion temperature, the thermal conductivities of the wall rocks, latent heat of crystallization, and the ambient country rock temperature at the time of intrusion. The physical properties of the country rocks and the intrusive units place limits on the model parameters, and are described briefly below.
Contact metamorphism occurred as the thermal gradient between the higher temperature of the intrusions and the lower temperature of the country rocks began to equilibrate thermally. Modelling the interplay of heat conduction, heat advection, and heat sources allows for a greater understanding of the metamorphism. Analytical solutions exist that approximate the thermal evolution of country rock adjacent to intrusions with a simple geometry (e.g. Lovering, 1935; Jaeger, 1964). Dykes and tabular bodies can be represented in one-dimensional (1-D) thermal models that treat the heat source as a tabular body with two infinite planar dimensions. Heat transport is linear in one direction that is perpendicular to the finite dimension. For roughly cylindrical intrusive bodies two-dimensional (2-D) thermal modelling can be used. The cylindrical intrusions are infinite in one and finite in two dimensions. The heat flow is planar in the two dimensions that are perpendicular to the third infinite dimension. Magma chambers or intrusive bodies that are 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 the only realistic solution to geological situations. However, depending on geometry and scale, portions of igneous 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 present level of exposure and are roughly cylindrical (see Paludan et al., 1994). This indicates that realistic thermal modelling can be done two dimensionally. In the worst case, the 2-D modelling will give the maximum extent of isotherms around the intrusions and would still place limits on the thermal evolution around the intrusive complex.
An additional consideration is the heat transport mechanism. If a fluid phase is absent or minor, then advective heat transport can be ignored. Several observations suggest that a fluid phase was not volumetrically important in the metamorphic rocks around the intrusive complex. Osumilite is not stable under conditions of high water activity (Schreyer & Seifert, 1967; Olesch & Seifert, 1981), and the overall preservation of the orthopyroxene + quartz assemblage in the country rock gneisses indicates a lack of significant hydration. Swanenberg (1980) examined the country rocks from the Rogaland anorthosite complex and suggested that the compositions of the fluid inclusions, which range from H2O rich to CO2 rich, are related to rock type rather than metamorphic grade. Further, Bol et al. (1995) inferred from oxygen and carbon isotope studies on samples from both the Faurefjell Formation and graphite-bearing ‘garnetiferous migmatites’ that no pervasive fluid was present during the high-T metamorphism. As a result, it is possible to treat all heat transport at Rogaland as conductive, which simplifies the modelling.
Two scenarios of intrusion were modelled. The simplest scenario is to treat all the intrusive bodies of the whole complex as a single intrusion. This treatment is justified by the nearly identical emplacement ages (Fig. 2) of the major anorthosite units (Schärer et al., 1996), which are closely spaced at 929 ± 2, 932 ± 3 and 932 ± 3 Ma for the Egersund, Helleren and Åna–Sira, respectively. The jotunite dykes give ages of 931 ± 5 Ma, and the observation that the BSL intruded after the anorthosite but before the dyke system would support this single event scenario. The Tellnes dykes (920 ± 3 Ma) are volumetrically small and represent only a minor contribution to the total heat budget.
The second intrusion scenario we tested was to separate the anorthosite intrusions and the later BSL intrusion into two major intrusive phases. The anorthosites were treated as a single elliptical unit, whereas the BSL is more complex (Nielsen & Wilson, 1991; Paludan et al., 1994). The BSL consists of three major and three minor cycles that were emplaced sometime within 5 Myr after the anorthosite. For the modelling, we simplified then BSL to three magmatic pulses at 3, 3·7 and 4·5 Myr after the anorthosite intrusion.
The southern parts of both the BSL and the anorthosites are not included in the thermal model, as the high-temperature mineral assemblages in the country rock are not spatially related (Fig. 2) (in the following discussion BSL refers to the lower part).
Intrusion temperatures and the latent heat of crystallization
Both the orthopyroxene megacrysts and the large plagioclase crystals of the Egersund massif crystallized at 1100–1200°C (Duchesne & Maquil, 1987). The BSL intrusion consists of several igneous rock types, which range from anorthosites to leuconorites. We assume liquidus and solidus temperatures of 1200°C and 1100°C, respectively, for both the massif-type anorthosite 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 compared with the Ballachulish Igneous Complex, which shows a similar temperature interval of crystallization (Weiss & Troll, 1991). At Ballachulish the foliation of mineral grains at the margins of the intrusion suggests that the crystal mush contained 50 vol. % crystals (Troll & Weiss, 1991). Peacock (1989) suggested that the latent heat of a magma that was 50% crystallized at the time of intrusion would be about half that of a magma on the liquidus. Consequently, we estimate the latent heat of crystallization for the anorthosites to have a value of 2·5 x 105 J/kg. On the other hand, textural evidence indicates that the parental magmas for the BSL intruded almost devoid of crystals (Nielsen & Wilson, 1991; Paludan et al., 1994). A crystal-free melt would have a value for the latent heat of crystallization of 5 x 105 J/kg, which is similar to that of basalt (e.g. Murase & McBirney, 1973; Thompson, 1992). We did not account for the heat absorbed during partial melting of wall rocks and heat released during subsequent crystallization as the field observations suggest that partial melting was rare in the osumilite–biotite-bearing sequences that make up 1% of the supracrustal section of the contact aureole.
We assume that thermal conductivity, and, therefore thermal diffusivity, is independent of temperature and have adopted a value of 
= 0·72 x 10-6 m2/s (Peacock, 1989). Additionally, we assume that textural anisotropy, for example in schists (Buntebarth, 1991), does not affect the thermal properties.
Initial and boundary conditions
It is impossible to make an independent estimate of the country rock temperature at the time of intrusion, but it is possible to constrain the probable range. We assume initial P–T conditions of the supracrustal section before the emplacement of the anorthosite magma were 500–750°C and 5 kbar. These temperatures would be consistent with the emplacement of Rogaland anorthosite complex into crustal rocks already at amphibolite- to granulite-facies conditions (Jansen et al., 1985) and with mantle heat flow (55–85 mW/m2) inferred for anorthosite emplacement (e.g. Emslie, 1985; Wiebe, 1986, 1992; Corrigan & Hanmer, 1997).
Numerical treatment of the heat conduction equation
We have adopted a continuum formulation of the heat conduction equation to model heat transfer in the contact aureole. A comprehensive discussion of the basic heat conduction equation has been given by, for example, Carslaw & Jaeger (1959) and Norton & Knight (1977) (see also Voller, 1987). The partial differential equation that describes 2-D, time-dependent heat conduction is
 | (1) |
The above equation is solved numerically by the finite-element method (FEM). The 120 km x 240 km computational domain consists of a grid of 1945 nodes that are refined proximal to the igneous complex (Fig. 4). For the modelling, we assumed the solution to be symmetric around the lower margin of the computational domain and conducted a grid refinement study to ensure that the numerical solution was independent of the spatial and temporal grid discretization. Heat flux varied by <1% after increasing the density of nodes by a factor of two.
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We account for the latent heat of fusion in the definition of an effective specific heat (cp*). The latent heat L corresponds to the change in the enthalpy H at the transitional melt temperature (Tm), and the following relationship is introduced:
 | (2) |
 |
 | (3) |
is the Dirac delta function. For the application of (2) to a system that solidifies over the range of temperatures, the Dirac delta function in (2) is replaced with the -form function *(T – Tm, 
T), which has a large but finite value over the interval T centred about Tm and is zero outside the interval. The interval T corresponds to the difference between the liquidus and the solidus temperatures for the material. The cp value is for each node using input enthalpy–temperature curves. The effective specific heat is calculated using equation (3). |
COMPUTATIONS |
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Two-dimensional modelling of a large, single intrusion
The simplest assumption was to model the anorthosite bodies and BSL as a single cooling unit. We conducted three series of simulations using uniform initial temperatures in the supracrustal section of 500, 600 and 750°C. Figure 5 compares calculated temperatures and P–T estimates from various locations around the intrusive contact. This shows that model country rock temperatures that are lower than 600°C are inconsistent with P–T determinations. The initial country rock temperature of 600°C results in maximum temperatures that are lower than the P–T determination near the intrusive contact, but fit well with the P–T determinations at
15–20 km from the intrusion (Fig. 5). Using an initial country rock temperature of 750°C in the model gives temperatures near the contact that fit well with the observations, but temperatures that are too high at greater distances (Fig. 5). Further, a uniform country rock temperature of 750°C suggests a mantle heat flux of 100 mW/m2, which is the kind of value associated with oceanic intraplate hot spots (Sleep, 1990), and is unrealistically high for the setting of the anorthosites. The thermal modelling of a single, elliptical cooling unit shows that no uniform country rock temperature can satisfactorily explain the observed temperature–distance profile at Rogaland. However, these results suggest that a thermal gradient present in the country rocks could account for the observed temperature array around the magmatic complex.
Two-dimensional modelling of multiple intrusions (two stages)
For the second set of simulations the geometries of the intrusion phases were modelled as two ellipses (Fig. 6). In this model, a massif-type anorthosite complex (30 km x 40 km) initially heats the country rocks, and, after a brief hiatus, a second, smaller complex (9 km x 12 km), which represents the northern lobe of the BSL, intrudes.
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Figure 7a shows time vs temperature profiles for points at various distances from the contact. Figure 7a also shows that, if the second phase (BSL) were emplaced almost immediately after the first phase, the results would be very similar to those of a single cooling unit (above), as almost no thermal gradient from the initial intrusion would be present in the country rocks. An interval of 10–15 Myr allows enough time to pass that the initial intrusion and the country rocks approach thermal equilibrium, and the country rock temperatures are effectively uniform. Large to moderate thermal gradients in the country rocks are transitory and present only from
1·5 to 7·5 Myr following the first stage of intrusion (Fig. 7a). The most pronounced thermal effects of the second phase of intrusion would occur over this short span of time. In our model calculations, after 3 Myr a significant thermal gradient extends to 20 km in the adjacent country rocks (Fig. 7b).
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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. We modelled these units as three pulses of fresh, hot magma into the chamber over a total time interval of assumed 1·5 Myr.
The calculations were performed in four consecutive steps. The effects of the first phase (anorthosite) assumed an initial intrusion temperature of 1200°C and a uniform 600°C for the country rock. After 3 Myr the second phase (BSL) started. The initial magma temperature was 1200°C, but this intrusion was emplaced into country rock with a thermal gradient that was the result of anorthosite emplacement. This procedure was repeated at 3·7 and 4·5 Myr to simulate the second and third cycles (simplified) of the second phase (BSL). The timing between pulses allows for the maximum thermal effect on the adjacent rocks. The calculations were ended 20 Myr after the emplacement of the first phase.
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RESULTS AND DISCUSSION |
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At the end of the finite-element calculation, we had data for 1945 nodes (geographical locations) spread over an area of 30 km x 70 km (see Fig. 4). Each node contains 111 data points that represent temperature–time information spanning 20 Myr.
Figure 8 shows temperature vs time profiles for seven locations (see also Fig. 1) at increasing distances from the contact with the intrusive complex. On all the profiles the effects of the initial heating (anorthosite) and of each of the three phases of the BSL cycle are evident. Extremely high temperatures are reached only nearest the intrusive complex, where the preheating of the anorthosite was highest (0·5–5·0 km, Fig. 8). At increasing distances (10·5–15·5 km, Fig. 8) from the contact the effects are less pronounced. Figure 8 also shows this temporal delay in the thermal maximum, which is attained at about +4·5 Myr at 0·5 km but at +7·5 Myr at 15 km. This is in excellent agreement with recent determinations of metamorphic ages of 925 Ma (Möller et al., 2002; A. Möller, personal communication, 2002).
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Figure 9 compares the calculated maximum temperatures (Tmax) with observed peak temperatures as a function of distance from the intrusive contact. The model and observed values show excellent agreement. The model also gives temperatures for the pigeonite-in and osumilite-in isograds of
860 and 750°C. These values are reasonable for the established stabilities of these minerals (see Lindsley, 1980, 1983; Carrington & Harley, 1995).
The modelling also shows the 2-D spatial distribution of maximum temperatures (Fig. 10). The lower-temperature isotherms form distinct narrow zones that parallel the anorthosite contact. The pattern of the highest-temperature isotherms in the country rocks (T higher than 900°C) broadens in the area between the modelled BSL intrusion and anorthosite body. This feature is very similar to the pattern of the pigeonite and osumilite isograds, which can be treated as the rough approximates of isotherms. The zones defined by these isograds are broadest around the BSL and narrow drastically around the anorthosite.
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Another interesting feature seen in Fig. 10 is that the high-temperature isotherms cross from country rocks into the intrusive bodies. As a consequence, isotherms obtained from detailed geothermobarometry or high-temperature isograds in the country rocks may appear to be truncated by the intrusion, which could lead to misinterpretation 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) multiple intrusions (Fig. 11b). The large single intrusion generates isotherms that are roughly parallel to the contacts. A narrow zone within 2·8 km of the contact reaches temperatures of 850–950°C. Higher-temperature isotherms (1000–1150°C) lie within the intrusion. The multiple intrusion model (Fig. 11b) produces a very different pattern of isotherms. The isotherms above 900°C effectively abut the anorthosite and a broad zone of up to 5 km across reached temperatures higher than 950°C. Additionally, the 850–950°C isotherms extend farther into the country rocks (10 km).
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The distribution of maximum temperatures and duration isochrons, which represent the length of time the rocks spent above 750°C during heating and cooling, are compared in Fig. 12a. The duration isochrons roughly parallel the contact of the initial and larger intrusion, whereas the maximum temperature isotherms are strongly deflected by the smaller and later intrusion. This indicates that, whereas the second intrusion has a major impact on the local distribution of peak temperatures, it does not significantly prolong the heating and cooling cycle. The 750°C isotherm and the duration isochron for 0–5 Myr are roughly parallel over the entire area. The duration isochrons of 10–14 Myr crosscut the isotherms at steep angles near the smaller and later intrusion (near point C in Fig. 12a), whereas near point A, at greater distances from the smaller intrusion, the isotherms and duration isochrons are nearly parallel.
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These relative orientations of the isotherms and the duration isochrons show the combined thermal effects of the two intrusions. Not surprisingly, the effects are strongest in the area between the intrusions (near point C in Fig. 12a), and the effect of the smaller intrusion dissipates noticeably at distances >10 km (near point B in Fig. 12a). Additionally, the area around point C (Fig. 12a), where the isotherms and duration isochrons show the least parallelism, gives an impression of the extensive variation in heating and cooling rates that can occur over relatively small distances. This variation of heating can be shown in detail for four locations (points A, B, C and D in Fig. 12a) in temperature–time diagrams (Fig. 12b).
Point A is located 6 km from the intrusive contact along the extension of the long axis of the large, elliptical intrusion. The maximum temperature is 750°C, so this location never rises above 750°C and the duration of 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 10 Myr duration isochron. The maximum temperature that is attained is 820°C, and the thermal effect of the small intrusion is considerable at point B. The initial rapid heating and long period of cooling for these localities (A and B) are controlled by the anorthosite (Fig. 12b). The difference in the temperatures at points A and B after 3 Myr reflects the geometrical effect (elliptical shape) of the modelled 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 the 12 Myr duration isochrons, and it reached a maximum temperature of nearly 950°C. Point C has experienced an extremely different heating and cooling history from points A and B (Fig. 12b). The variations in heating and cooling rates and the higher maximum temperature are due to the major influence of the second intrusion (modelled BSL).
Point D (Fig. 12b) is located 5 km from the intrusive contact along the extension of the short axis of the smaller intrusion. The maximum temperature at this locality is 900°C, and the total time with temperatures higher than 750°C is 7 Myr. The variations in heating and cooling rates mimic those at point C. The thermal effects of the second intrusion on the country rocks (heating and cooling rates) are more drastic than at point C, but the maximum temperature is lower because of the lower initial temperatures at the time of the second intrusion. The only difference between the thermal development at points C and D (Fig. 12b) is the country rock temperature at the time of the second intrusion (i.e. the magnitude of the thermal gradient caused by the anorthosite).
All four points are located at about the same distance (5 km) from an intrusive contact, but show different temperature–time histories, which are related to the relative distance to both plutons. The heating and the extremely high temperatures are related to proximity to the small intrusion, whereas the duration of heating and cooling cycle is controlled by the much larger intrusion. The temperature–time development of point C (Fig. 12b) is the result of extensive influence of both the large and the small intrusions; this locality reached the highest temperature and remained at high temperatures (>750°C for about 12 Myr).
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CONCLUSIONS |
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Thermal modelling suggests that the Rogaland intrusive complex cannot be treated as a single intrusive event. Modelling two intrusive cycles with a 3 Myr hiatus between the cycles can account for the high-temperature metamorphism and the observed distribution of isotherms (isograds). In this model, the initial emplacement and crystallization of the anorthosite (first phase) produces a regional thermal gradient ranging from
640°C to 880°C. The smaller BSL intrusion (second phase) intrudes adjacent to the anorthosites and provides a second thermal pulse. This combination of events can explain the observed array of high-temperature isograds and the maximum temperatures of >1000°C at 2 km distance from the magmatic contact.
Until recently, intrusive activity associated with the Rogaland anorthosite complex was thought to span 100 Myr, which would effectively preclude the two-phase intrusive model suggested above. However, new age data from Schärer et al. (1996) suggest that the entire magmatic emplacement of the main intrusive complex occurred over a much shorter time interval of 5 Myr between 931 and 926 Ma, which is entirely consistent with the timing of the intrusions that the thermal modelling suggests is necessary to explain the metamorphism of the country rocks.
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SUPPLEMENTARY DATA |
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For supplementary data, please refer to Journal of Petrology Online.
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ACKNOWLEDGEMENTS |
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We are grateful to the staff of the Institut für Mineralogie–Petrologie–Geochemie, Universität Freiburg for their help during sample preparation (especially K. Fesenmeier) and microprobe measurements (especially H. Müller-Sigmund). We would also like to acknowledge the staff and students of the University of Utrecht for the excellent fieldwork and mapping they carried out many years earlier in this region. We thank A. Scheld, J. Alcock, S. Peacock and C. Maijer for their opinions, and K. Bucher for reviewing an earlier version. M. Sandiford's review and S. Barboza's very detailed comments helped to improve this paper. Funding for this work by grant Schu 919/4-1 of the Deutsche Forschungsgemeinschaft is gratefully acknowledged. This work is part of the doctoral thesis of M.W., which was completed at the Universität Freiburg. The geological framework of this contribution developed over time as the result of mutually beneficial discussions between M.W. and J.C.S. The numerical solutions to test these ideas are the result of co-operation between M.W. and S.B., who was supported by the Kristallographisches Institut (Freiburg).
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FOOTNOTES |
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*Present address: Institut für Mineralogie, Petrologie und Geochemie der Universität Tübingen, Wilhelmstrasse 56, D-72074 Tübingen, Germany.
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