Gravity Interpretation of the Egersund Anorthosite Complex, Norway: Its Penological and Geothermal Significance

Gravity interpretation of the Egersund anorthosite complex, Norway: Its penological and geothermal significance

SCOTT B. SMITHSON Department of Geology, University of Wyoming, Laramie, Wyoming 82071 IVAR B. RAMBERG Institute for Geology, University of Oslo, Oslo, Norway

ABSTRACT any given complex and whether associated anorthosite and norite syncline. Michot granitic rocks (mangerites) are comagmatic. (1968) and Duchesne (1972) suggested that The Egersund anorthosite complex con- If such large amounts of monomineralic the igneous complex evolved from a pla- sists of several bodies of anorthosite and a anorthosite were derived from gabbroic gioclase magma. From rare-earth element large syncline of layered norite and anor- magma, a dense mafic residuum rich in dark studies, Duchesne and others (1974) sug- thosite with granitic rocks in the core. This minerals would be left behind. We would gested that monzonoritic magma could be igneous complex is emplaced in granulite- generally expect any such residuum to be the source for anorthosite and could have facies gneisses. These gneisses have a mean buried; therefore, geophysical methods been derived from melting of kaersutite in density of 2.70 g/cm3, indicative of granitic would have to be used to detect such the mantle. A recent study of plagioclase in rocks. A Bouguer gravity anomaly map bodies. In addition, various hypotheses of the anorthosite shows that it is recrystal- shows no distinctive gravity anomaly over petrogenesis imply different amounts of lized (Zuno-Mahmalat and Krause, 1976). anorthosite, but a sharp 25-mgal positive rock types associated in a given anorthosite In an early study, Barth (1936) proposed anomaly is present over the norite syncline. complex. that the complex formed from a Gravity models indicate that the norite The Egersund anorthosite is one of the granodioritic magma. syncline is about 4 km thick and that rela- well-known localities (Kolderup, 1896; tively minor amounts of granitic gneiss are Barth, 1936). Gravity measurements should GRAVITY MEASUREMENTS present in the core of the syncline. Anor- be able to detect such a buried mass (re- thosite masses cannot be modeled directly siduum) as well as reveal geometry of sur- Approximately 300 gravity stations were but can be inferred to have a thickness of at face rocks in an anorthosite complex; we measured to supplement 300 stations that least 4 km. Relative amounts of the rock have thus conducted a gravity study of the had previously been measured by the Geo- types in the anorthosite complex are anor- Egersund anorthosite complex, one of the graphical Survey of Norway (Anonymous, thosite, 70%; norite, 25%; and granitic classic anorthosite occurrences located on 1961). Gravity measurements and reduc- rocks, 5%. No evidence is found for a dense the southeastern coast of Norway. tions were carried out according to methods mafic residuum that would be formed if the outlined by Dobrin (1960) and Smithson anorthosite differentiated from a basaltic GEOLOGY (1963). Gravity stations were largely mea- magma in place. If the rocks of the complex sured on known spot elevations with an ac- are cogenetic, the parent magma would be Country rocks in the Egersund area con- curacy of ±1 m; the relatively few stations noritic anorthosite, and the volume of sist of migmatitic charnockites, banded whose elevations were measured with granitic rocks is so small that it would not gneisses, and metasedimentary rocks that barometers have an accuracy of ±3 m. Ter- change the presumed composition of the have been recrystallized in the granulite rain corrections have been applied out parent magma appreciably. Heat flow facies of regional metamorphism and be- through zone M. The maximum error in the through the anorthosite is so low (0.45 long to the deep catazonal level of the crust Bouguer anomalies is ±1 mgal; the Bouguer HFU) that mantle heat flow in this area can (Michot and Michot, 1968). The several gravity anomaly map has been contoured hardly be greater than 0.2 to 0.3 HFU. bodies of the Egersund anorthosite complex on a 2.5-mgal interval. Granulitic gneisses or other rocks produced are emplaced in these catazonal gneisses. at low heat must compose the entire crust Michot and Michot (1968) recognized three BOUGUER GRAVITY beneath the anorthosite complex. possible origins for anorthosite. The dome- ANOMALY MAP shaped Egersund (westernmost) body was a INTRODUCTION magmatic synkinematic intrusion, and the The Bouguer gravity anomaly map of norite syncline developed during the same southern Norway (in Smithson and others, The petrogenesis of anorthosites has been magmatic event (Michot, 1968). Small 1974, Fig. 1) shows a regional increase in debated for many years (Buddington, 1939; amounts of anorthosite were formed gravity of 0.2 to 0.3 mgal/km to the south- Isachsen, 1968). The major problem in metasomatically during migmatization of west toward the coast. This regional in- anorthosite petrogenesis concerns the the regional gneisses. Finally, the Haaland crease in gravity is probably caused by a amount of anorthosite relative to the anorthosite massif (southernmost) rose as coastward rise in the Moho (Sellevoll, amount of gabbro and other mafic rocks in an anatectic diapir into the slightly earlier 1973). Superimposed on this northeast-

Geological Society of America Bulletin, Part I, v. 90, p. 199-204, 3 figs., February 1979, Doc. no. 90214.

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southwest gravity gradient is the gravity ef- and the surrounding regional gneisses ex- anorthosite, 2.71 g/cm3; norite, 3.00 g/cm3; fect of the Egersund anorthosite. The most cept on the northwest flank of the anortho- granitic rocks, 2.75 g/cm3; surrounding distinct feature of the local gravity field is site, where a positive anomaly in the gneis- gneisses, 2.70 g/cm3. easily the strong 30-mgal gravity high cen- ses indicates some sort of subsurface mass tered over the northern part of the norite excess. A gravity high in anorthosite just GRAVITY MODELS syncline (Fig. 1). The entire syncline involv- south of Egersund indicates a mass excess, ing norite is marked by a positive gravity and small gravity lows are found over mon- Gravity models have been calculated anomaly ranging from 10 to 30 mgal. Posi- zonite and quartz monzonite in the core of along four profiles through the anorthosite tive anomalies follow both of the southern the norite syncline. complex (Figs. 2, 3). Because the density of arms in the bifurcation of the norite anorthosite is so close to that of surround- syncline (Fig. 1). The anorthosite bodies ROCK DENSITY ing gneisses, the anorthosite is not modeled; represent gravity lows in relation to the no- gravity models involve the norite syncline. rite syncline. Almost no change in the grav- Densities have been measured for 92 rock Gravity anomaly variations within the ity field is apparent between the anorthosite samples. Mean densities are as follows: anorthosite are most likely caused by unex-

58*40'

Figure 1. Bouguer gravity anomaly map of Egersund anorthosite complex. Contour interval is 2.5 mgal.

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posed dense rocks like norite or possibly by positive gravity anomaly in the area is syncline. The models (Figs. 2, 3) show that noritic anorthosite; evidence does not caused by downfolded dense rocks in the the norite trough has a thickness of about 4 suggest that density variations of anortho- syncline. This trough of noritic rocks is the km in most of the profiles. Lower density site itself are significant. major feature that is modeled in the gravity granitic rocks must be included in the The measured high density of norite and interpretation. The gravity high over the models in order to match the observed the manner in which positive gravity norite trough is characterized by a smaller gravity profiles. The maximum thickness of anomalies and isoanomaly contours follow gravity low over the granitic rocks (mang- granitic rocks in the trough is 2 km, and the the outcrop pattern show that the major erites) of lower density in the center of the amount of norite greatly exceeds the amount of granitic rock. The two southern extensions or tails of norite gneiss also have thicknesses of 3 to 4 km. Most of the gravity profiles cannot be matched with models that have vertical contacts; therefore, most models display contacts with moderate dips, generally to the east. A body of noritic gneiss may cause a positive gravity anomaly within anorthosite at the west end of profile B-B'. Although anorthosite has not been modeled directly, its thickness must be at least 4 km because of the structural rela- tionship between anorthosite and the adjacent norite trough — that is, if the norite syncline is formed by diapiric rise of anorthosite.

PETROLOGIC IMPLICATIONS

Two major problems concerning the genesis of anorthosites exist. These are (1) to determine the nature of anorthosite parent magma, such as whether anorthosite is derived from basaltic magma (Bowen, b) 1917) or from a more felsic magma (Barth, 1936; Green, 1969), and (2) to determine whether granitic (commonly syenite and monzonite) rocks associated spatially with anorthosite are comagmatic or are totally unrelated (Buddington, 1939, 1972). Grav- ity interpretation can shed some light on both of these problems. If anorthosite is directly derived from basaltic magma, it will be associated with a highly positive gravity anomaly caused by a 10 20 30 40 km cumulate of mafic minerals. In areas where anorthosite is associated with fractional crystallization from basalt, such as the Stillwater Complex (Bonini, 1969) and the Duluth Gabbro (Ikola, 1968), positive gravity anomalies occur over the intrusions. Such may be the case for the Bankura anorthosite in India (Verma and others, 1975). On the other hand, Simmons (1964) found a negative gravity anomaly over the Adirondack anorthosite and calculated that if it were underlain by gabbro and/or peridotite, it would have a positive anomaly Figure 2. Gravity profiles and models through norite syncline. Position of profiles is of 30 to 50 mgal. Hodge and others (1973) shown in Figure 1. also found a negative gravity anomaly over

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the Laramie anorthosite. Any reasonable in- insignificant amount of granitic rocks are thosites were formed from a quartz diorite terpretation of the Egersund anorthosite aspects of our interpretation that could parent magma. Anorthosite is the high- suggests that the amount of anorthosite far hardly be drastically changed. temperature residuum and granitic rocks exceeds the amount of norite and/or Our best volume estimates indicate that are the low-melting fraction from this initial peridotite. We therefore conclude that the the Egersund anorthosite complex was composition. Green pointed out the dif- Egersund anorthosite is not associated with formed from a noritic anorthosite magma; ficulty of deriving an aluminous noritic a large mafic residuum and was not derived a similar conclusion was reached by Hodge anorthosite magma from basalt. However, directly from a basaltic magma; however, and others (1973) for the Laramie anor- the volumetric amount of granitic rocks in we cannot exclude the possibility that anorthosite. On the basis of laboratory experi- this anorthosite complex is far too small to thosite was derived from basalt at a deep ments, Green (1969) suggested that anor- be derived from a quartz diorite magma un- position and then intruded into the crust. 40 Gravity interpretation can definitely rule out the presence of extensive mafic differ- entiates near the anorthosite; however, our 30 PROFILE C-C' main goal is estimation of relative amounts of rock types and bulk composition of ¡20 magma that formed this anorthosite. There -OBSERVED g are two complicating factors. First, on the -CALCULATED g basis of aeromagnetic anomalies, Sellevoll 10 and Aalstad (1971) have extended the Eger- sund anorthosite complex well out into the ASSUMED REGIONAL I " i Skagerrak, and the aeromagnetic signature +* * . * +|vvloo|vv |oo o o o ooo| >Si| + * ++I indicates that the seaward extension is

mostly anorthosite. Second, the thickness of 10 20 30 40 km anorthosite has not been modeled directly. We make the plausible assumption based 3.00 on structural relationships of anorthosite ^^ 2.75 forming diapiric domes that anorthosite is 3.00 2.70 at least as thick as the norite trough. Grav- ity interpretation clearly shows the extent of rocks more dense than anorthosite and, importantly, also shows that the amount of granitic rock (mangerite) is small even though the outcrop is fairly large. On the basis of these interpretations, amounts of major rock types are about as follows:

anorthosite, 70%; norite, 25%; and grani- OBSERVED g tic rocks, 5%; and the proportion of anorthosite to other rocks is this great or CALCULATED g greater. The amount of granitic rocks is so small that whether they are comagmatic or not will hardly change the composition of the original magma, although granitic rocks may have risen and been preferentially re- 10 20 30 40 km moved by erosion. The fact that granitic rocks make up only a small proportion of the total complex and the fact that similar associations are found all over the world 2.70 suggest that they are comagmatic, as do the concordant granitic rocks in the core of the syncline. Michot and Michot (1968) have suggested a more widespread anorthositic layer and that anorthosite in the complex may have several different origins. Such features would change our estimates of relative amounts of rocks; however, strong prepon- Figure 3. Gravity profiles and models through norite syncline. Position of profiles is derance of anorthosite over norite and an shown in Figure 1.

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less, as Green (1969) has suggested, the than normal (Polyak and Smirnov, 1968). experiments on melting of noritic anortho- granitic fraction moved upward away from Whether a mantle heat flow of about 0.3 site (Green, 1969) show that alkali-rich no- the anorthosite. His experiments with melt- HFU is typical of old stable areas or rite is the low-melting fraction. This agrees ing of noritic anorthosite show that the low whether it is unusually low is uncertain at with field relations in the anorthosite com- melting fraction is composed of alkali-rich present. plex. Derivation of noritic anorthosite norite, a fact that agrees with this anortho- magma still remains a problem. site occurrence. Presence of minor granitic CONCLUSIONS The anorthosite complex represents a rocks and norite bordering anorthosite deep, catazonal exposure of the crust. The domes seems to agree with his experiments Thickness of the norite syncline in the crust, which is 28 km thick here, must have on melting of noritic anorthosite. Egersund anorthosite complex is about 4 a very low heat contribution to heat flow of km, and the amount of granitic rock in the 0.1 to 0.2 HFU. This value can be regarded GEOTHERMAL IMPLICATIONS core of the syncline is small compared with as the heat contribution of the lower crust. surface exposure of the granitic rock. Struc- For this area the mantle heat flow can Heat production in anorthosite is so low tural considerations suggest that anortho- hardly be greater than 0.3 HFU, and it that we may regard an anorthosite body as site is at least as thick as the norite syncline. might even be as low as 0.2 HFU. Quartzo- a window through which we can view the The anorthosite could be thicker. The feldspathic granulitic gneisses are exposed thermal contribution of the lower crust and probable tremendous seaward extension of on the surface in this area, and granulites or upper mantle. Heat flow through the anor- anorthosite means that 70% is a minimum rocks of equally low heat production must thosite ranges from 0.4 to 0.5 HFU (Swan- estimate for the proportion of anorthosite. make up the entire crust, or mantle heat berg and others, 1974). This means that The source area for the anorthosite com- flow would be negative. heat flow from the mantle in this Precamb- plex must also have been very large and is rian shield can hardly be greater than 0.3 to most likely still buried. No gravity evidence 0.4 HFU. The Egersund anorthosite has a for a dense mafic residuum is found near the REFERENCES CITED heat production of 0.2 HGU and a geo- anorthosite complex, so it could not have formed by fractional crystallization of a Anonymous, 1961, Egersund area, Bouguer thermally estimated thickness of 3.5 km anomalies: Geographical Survey of Nor- (Swanberg and others, 1974). It is emplaced basaltic magma in place or at moderate way. in a granulite-facies terrain and represents a depth beneath the complex. Even though Barth, T.F.W., 1936, The large Precambrian in- deeply eroded section through the crust. gravity interpretation is ambiguous, density trusive bodies in the southern part of Nor- Lambert and Heier (1968) showed that contrast between granitic rocks and norite way: International Geological Congress, in the syncline is so great that if much 16th, Washington, D.C. 1933, Report, granulite-facies terrains have heat-pro- p. 297-309. duction values of about 1.4 HGU. The crus- granitic rock were present it would cause a Bonini, W. E., 1969, Gravity studies in Montana, tal thickness near Egersund is about 28 km large gravity effect. Best estimates of the Wyoming, and Washington: EOS (Ameri- (Sellevoll, 1973). We may assume that the overall composition of the anorthosite can Geophysical Union Transactions), v. anorthosite complex is underlain by rocks complex is that it is a noritic anorthosite; 50, p. 531-533. Bowen, N. L., 1917, The problem of anortho- of low heat production; otherwise the therefore, this is the most reasonable com- sites: Journal of Geology, v. 25, p. 205— mantle heat contribution would be zero or position for the parent magma. The amount 243. even negative. We use a maximum heat of granitic rocks is so small that inclusion of Buddington, A. F., 1939, Adirondack igneous production of 1.5 HGU for the crust be- them hardly changes estimates of overall rocks and their metamorphism: Geological Society of America Memoir 8, 354 p. neath 4 km of anorthosite, and the geo- composition; granodiorite or quartz diorite 1972, Differentiation trends and parental thermal flux from the upper mantle is 0.1 seems to be a composition that is too silica magmas for anorthositic and quartz mang- HFU. For an absolute minimal heat pro- rich for the primary magma unless almost erite series, Adirondacks, New York, in duction of 0.5 HGU (corresponding to the entire granitic fraction moved upward Shagam, R., and others, eds., Studies in pyroxene granulite), the upper-mantle away from the anorthosite complex. The Earth and space sciences: Geological Soci- mean density of 2.70 g/cnr' for the granuli- ety of America Memoir 132, p. 477-488. geothermal flux is 0.3 HFU. Even if we in- Dobrin, M. B., 1960, Introduction to geophysical clude uncertainties such as effect of refrac- tic gneisses surrounding the anorthosite prospecting: New York, McGraw-Hill tion in anorthosite (Pollack and Roy, 1968) complex indicates that these rocks are es- Book Co., 446 p. or unrecognized climatic effects and arbi- sentially granitic, even though metamorphic Duchesne, J., 1972, Iron-titanium oxide minerals trarily increase these values by 0.1 HFU, facies suggests a deep level of crustal expo- in the Bjerkrem-Sogndal massif, southwestern Norway: Journal of Petrology, v. 13, sure. The low density of the country rocks heat flux from the upper mantle must lie be- p. 57-82. tween 0.2 and 0.4 HFU at the very most. A also means that gravity-driven diapirism of Duchesne, J., Roelandts, I., Demaiffe, D., and value of 0.4 HFU requires that heat pro- the granitic fraction from the anorthosite others, 1974, Rare-earth data on mon- duction through 20 to 24 km of crust is as complex will not be very effective, because zonoritic rocks related to anorthosites and low as 0.5 HGU. A plausible value of man- the present density of the granitic fraction their bearing on nature of parental magma of the anorthositic series: Earth and Plane- (2.75 g/cm') is greater than that of the coun- tle heat flow is 0.3 HFU, a value lower than tary Science Letters, v. 24, p. 325—335. other estimates. try rocks. The huge volume of anorthosite Green, T., 1969, High-pressure experimental would require formation of a great granitic studies on the origin of anorthosite: Cana- Low heat flow seems to be typical for the batholith if the primary magma were quartz dian Journal of Earth Sciences, v. 6, Baltic Shield (Lubimova and others, 1972). dioritic or granodioritic. On the other hand, p. 427-440. Old shields commonly have lower heat flow Hodge, D. S., Owen, L. B., and Smithson, S. B.,

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