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

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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. 199 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/90/2/199/3429742/i0016-7606-90-2-199.pdf by guest on 28 September 2021 200 SMITHSON AND RAMBERG 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. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/90/2/199/3429742/i0016-7606-90-2-199.pdf by guest on 28 September 2021 EGERSUND ANORTHOSITE COMPLEX, NORWAY 201 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 grav- ity 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 adja- cent norite trough — that is, if the norite syncline is formed by diapiric rise of anor- thosite. PETROLOGIC IMPLICATIONS Two major problems concerning the genesis of anorthosites exist. These are (1) to determine the nature of anorthosite par- ent 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.
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