Geophysical interpretation of the crustal and upper mantle structure in the Wiborg rapakivi granite area, southeastern Finland

Precambrian Research, 64 (1993) 273-288 Elsevier Science Publishers B.V., Amsterdam

273

Geophysical interpretation of the crustal and upper mantle structure in the Wiborg rapakivi granite area, southeastern Finland S. Elo *,a and A. Korja b a Geological Survey of Finland, Betonimiehenkuja 4, SF-02150 Espoo 15, Finland b University of Oulu, SF-90570 Oulu, Finland Received February 15, 1991; revised version accepted December 20, 1992

ABSTRACT The rapakivis in southeastern Finland and Russian Karelia cover a total area of more than 20,000 kin 2 and comprise the Proterozoic (1.6-1.3 Ga) anorogenic Wiborg, Ahvenisto, Suomenniemi, and Onas batholiths, and associated mafic rocks. A model of the crustal and upper mantle structure in this area based on gravity, aeromagnetic, deep seismic sounding, and petrophysical data is presented. The average density (2625 kg/m 3) of the batholiths is lower than that of the surrounding Svecofennian rocks by about 80 kg/m 3. The density histogram of the rapakivi granites is divided into two normally distributed components with mean densities of 2625 kg/m 3 (major) and 2690 kg/m 3 (minor), respectively. There are related local gravity anomalies. The susceptibility histogram of the rapakivi granites is decomposed into five log-normally distributed components. The two major components have mean susceptibilities of ( 165 and 1305) X 10 - 6 SI units. The former component (40%) is virtually paramagnetic, and the latter (60%) is weakly ferromagnetic. The two components with mean susceptibilities of (6000 and 13,5000) X 10-6 SI units constitute only 3% of all the samples. Associated gabbro- anorthosites also have high susceptibilities (1450-30,370) × 10-6 SI units. These ferrimagnetic rocks cause noticeable local aeromagnetic anomalies. An extensive Bouguer anomaly low is associated with the batholiths. A pronounced regional high surrounds this low. The range of values is nearly 60 mGal. The anomaly pattern is primarily the result of superposition of the gravity low, which is due to the relatively low-density upper and middle crust, onto the broader gravity high, that is due to the uplifted Moho. According to the published deep seismic sounding results, the crust is 41 km thick in the rapakivi region. Thus it is 620 km thinner than to the west, north and south of the rapakivi batholiths. The refraction model also implies a 6 km thick high-velocity body at a depth of 10 km. This body has a high vp/vs-ratio and is interpreted as gabbro-anorthosite. The magnetic anomaly values in the rapakivi area are slightly higher than those in the surrounding schist areas but lower than the values in the metavolcanic belt to the west. Within the Wiborg batholith there is a zone of positive regional magnetic anomalies with a separate, well defined magnetic high of about 400 nT in the western part. This anomaly is interpreted as a pipe-like magnetic body with an abnormally strong magnetization. The upper surface of the body lies at depths between 12 and 15 km. The body extends from the upper to the lower crust. The model derived from geophysical data supports a Proterozoic mantle upwelling, which provided excess heat to cause uplift and partial melting of both the upper mantle and the lower crust. Partial melting of the upper mantle produced intrusions of gabbro-anorthosites and diabases, while extensive partial melting of the lower crust produced rapakivi magmas and caused their emplacement into the upper crust.

1. Introduction

tered in the southern parts of the Baltic Shield

Extensive batholiths of anorogenic rapakivi granite and associated rock units are encoun-

( F i g s . 1 a n d 2 ). T h e b a t h o l i t h s w e r e e m p l a c e d into Palaeoproterozoic crust. The southernmost batholiths are covered by younger Phan e r o z o i c p l a t f o r m s e d i m e n t s ( B o y d et al., 1985 ). T h e W i b o r g r a p a k i v i g r a n i t e a r e a c o r n -

*Corresponding author.

0301-9268/93/$06.00 © 1993 Elsevier Science Publishers B.V. All fights reserved. SSD10301-9268 (93) EO084-P

274

S. ELO AND A. KORJA

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Fig. 2. The rapakivi batholiths and associated gabbro-anorthosites and diabases of SE Finland modified after H~irme (1980) and R~im6 (1990). The southeastern rapakivi area includes the Wiborg and its satellite batholiths Suomenniemi, Ahvenisto and Onas. The Lapinj~irvi area (Fig. 11 ) is outlined by a small rectangle on the right.

prises more than 20,000 km 2 in southeastern Finland and Russian Karelia. The area consists of the Wiborg batholith and its satellites: Suomenniemi, Ahvenisto, and Onas massifs. These include several separate intrusions dis-

tinguished by lithological variation, age differences and sometimes by local magnetic and gravity anomalies. Most rapakivi granites and coeval diabases and gabbro-anorthosite complexes in Finland

GEOPHYSICAL INTERPRETATION OF CRUSTAL AND UPPER MANTLE STRUCTURE IN THE WIBORG AREA

have been dated to 1.6-1.5 Ga, but the main phases of the Wiborg batholith range from 1646 Ma to 1630 Ma in age (Vaasjoki et al., 1991 ). The bimodal magmatism is well displayed in composite diabase-quartz porphyry dykes in the Suomenniemi batholith (R~im6, 1990). The roof pendants of the Wiborg batholith consist of Subjotnian ( 1.8-1.5 Ga) metasedimentary rocks, mafic to felsic volcanic rocks, diabases, anorthosites and gabbros, and Svecofennian (1.9-1.8 Ga) granitoids and gneisses (Vorma, 1975; Simonen, 1987). Jotnian ( < 1.5 Ga) sandstone is found east of the Wiborg batholith in Russian Karelia, and again in southwestern Finland, where it is associated with mafic dyke swarms and underlying and surrounding rapakivi granites. The rapakivi granites, being A-type granites, crystallized from silicic magmas with high Fe/ Mg and K / N a ratios (Nurmi and Haapala, 1986 ). The geochemistry of the rapakivi granites has been studied by, among others, Sahama (1945), Savolahti (1962) and Vorma (1976). The petrology, mineralogy and petrography have been described by e.g. Vorma (1976) and Simonen (1987). On the basis of Sm-Nd-isotope results, Haapala and R~im6 (1990) and R~im6 (1990) concluded that the rapakivi magma was a product of partial melting of the Svecofennian crust. The coeval diabase dykes and gabbro-anorthosites are of mantle origin with a minor crustal contamination component. There are also a few papers on the geophysical features of the rapakivi batholiths in Finland. Laur6n (1970), Shustova et al. (1978) and Elo ( 1981 ) have discussed gravity models; Puranen et al. ( 1978 ), LS.hde ( 1985 ) and Puranen (1989) petrophysical properties of the rapakivi granites; and Tuomi (1988) and Luosto et al. (1990) the seismic structure of the crust underneath the Wiborg and Suomenniemi batholiths. The structure and dimensions of the southeastern rapakivi area have been described as batholith, laccolith or sheet. After Laur6n

275

(1970) published his gravity interpretation (cf. Fig. 6), the Wiborg batholith has been taken to be a 10 km thick laccolith with a deep root under the Ahvenisto intrusion. In this paper, we present an alternative interpretation, which takes into account the entire crust and the three-dimensional character of the gravity anomaly. Elo ( 1981 ) showed that the gravity anomaly pattern would arise if the mass deficit due to the rapakivi batholith were compensated by a mass surplus located below the batholith. In this case, the anomaly is understood as a sum of two components, a relatively sharp negative anomaly on top of a relatively smooth positive one. In 1982, the Deep Seismic Sounding (DSS) profile BALTIC was shot across the Wiborg and Suomenniemi batholiths. According to Luosto et al. (1990), the crust is 41 km thick under the rapakivi batholiths. Thus it is 6 to 20 km thinner than in the area to the west, north, and south of the rapakivi granites (Luosto, 1991 ). This fact must be taken into account in any valid interpretation of the gravity anomaly. Since 1971, quite a number of petrophysical measurements have been performed on both rapakivi granite and Svecofennian rock samples (Puranen et al., 1978; L~ihde, 1985; Korhonen et al., 1989; Puranen, 1989). These measurements give a good basis for evaluating the density and magnetic susceptibility contrasts in geophysical models. Puranen (1989) summarized the petrophysical data into mass susceptibility-density (XD) diagrams, which can be used to identify the para- and ferrimagnetic parts of the geologic units. According to him, the cumulative susceptibility distributions and XD-diagrams ofWiborg rapakivi varieties suggest that rapakivi granites contain exceptionally iron-rich mafic silicates, which have partly been decomposed into small amounts of fine-grained iron oxides. In this paper, we try to combine and reinterpret earlier separate results concerning the regional and local geophysical features of the

276 Wiborg batholith, introduce new interpretation profiles and petrophysical data, and present some constraints on the genesis of the batholith. The paper is mainly based on the following data: aeromagnetic (altitude 150 m), regional gravity and petrophysical measurements provided by the Geological Survey of Finland, the gravity data measured by the Finnish Geodetic Institute, and the DSS interpretations made at the Institute of Seismology of the University of Helsinki. Preliminary results were discussed in Elo and Tuomi (1989) and Korja and Elo (1990).

2. Petrophysics The main rock types in the batholith are wiborgite, pyterlite, porphyritic rapakivi, tirilite or dark wiborgite, and even-grained rapakivi. Wiborgite is a hornblende-bearing rapakivi granite in which ovoidal potassium-feldspar phenocrysts are mantled with plagioclase. Dark wiborgite is similar in texture to wiborgite but contains fayalite and/or pyroxene and more plagioclase. Pyterlite is a porphyritic biotite granite in which the ovoidal potassium-feldspars are without plagioclase mantles. The porphyritic and even-grained rapakivi granites are biotite granites. A regional petrophysical sampling was carried out along the DSS profile BALTIC and along its crossline in the Finnish part of the Wiborg batholith (Korhonen et al., 1989 ). Altogether 835 unweathered rapakivi granite samples from this survey were classified into three groups on the basis of the visually determined contents of dark minerals. Samples containing only biotite were included in the pyterlite group, biotite- and hornblende-bearing samples in the wiborgite group, and hornblende- andpossibly pyroxene- or olivinebearing samples in the dark wiborgite group. The histograms for density, magnetic susceptibility, and remanent magnetization values of each group are presented in Fig. 3. Table 1

S. ELO AND A. KORJA

shows the mean values of the petrophysical parameters for each group. The density of the rapakivi granite samples varies between 2525 and 2760 kg/m 3. The mean density of the pyterlite group is 2600, of the wiborgite group 2620 and of the dark wiborgite group 2665 kg/m 3. The increase of mean density reflects the increase of dark mineral content. The range of magnetic susceptibility values is (20 to 46,000 ) × 10-6 SI units. The pyterlite samples are para/ferrimagnetic (mean k=450×10-6), the wiborgite samples are para/ferrimagnetic (mean k= 1200× 10 -6) and the dark wiborgite samples are ferrimagnetic (mean k=2280× 10-6). The remanent magnetization of individual samples varies between 0 and 16,000× 10 -3 A/m but is below 200)< l0 -3 A/m in 95% of the samples. Susceptibility increases with increasing mafic mineral content or, at a much higher rate, with increasing amounts of ferrimagnetic minerals. According to Puranen (1989), the rapakivi granite sample material is dominated by weakly ferrimagnetic samples, whose mean Q-ratio is 6.7. High Q-ratios and relatively low susceptibilities indicate fine-grained (single domain) magnetite in small amounts. The mafic silicates of rapakivi granites are very iron-rich (Simonen and Vorma, 1969), and thus the low magnetite content is tentatively explained by Puranen (1989) to have been formed by the low-temperature re-equilibration of biotite and amphiboles. This would explain the small amount and fine grain size of magnetite. The mean Q-ratios of our rapakivi groups vary between 1.2 and 2.7, but they are still higher than the mean value (0.69) for all granites in Finland (Puranen, 1989) Complex distributions resulting from the mixing of different populations are often analysed by means of quantitative methods. We applied standard non-linear least-squares optimization. The components were assumed to be normally distributed for density and lognormally distributed in regard to susceptibil-

GEOPHYSICALINTERPRETATIONOF CRUSTALAND UPPER MANTLESTRUCTUREIN THE WlBORGAREA

SUSCEPTIBILITY

DENSITY (kg/m 3)

( .10-' Sl)

277

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Fig 3. Density, magnetic susceptibility and remanent magnetization histograms of rapakivi granites.

ity. We think that these methods can provide useful parameters also for geophysical interpretations. In addition to our 835 new samples (Fig. 3 ), 259 old samples from the petrophysical data bank of the Geological Survey of Finland were included in the analysis. The density histogram was decomposed into two normally distributed components. The

major component (96% of samples) has a mean density of 2625 kg/m 3 (standard deviation s = 35 kg/m 3) and the minor component (4%) a mean density of 2690 kg/m 3 ( s = 4 0 kg/m 3). The major component corresponds to wiborgite, pyterlite, and porphyritic and evengrained rapakivi samples, and the minor component to dark wiborgite samples.

278

S. ELO AND A. KORJA

TABLE 1 Measured density, magnetic susceptibility, and remanent magnetization and calculated Q-ratio (Koenigsberger ratio ) of rapakivi granites and surrounding Svecofennian rocks Rock type

Rapakivigranite N=835, 100% Pytedite N= 150,18% Wiborgite N=590,71% Dark wibor~te N=95,11%

Density (kg/m 3)

Susceptibility ( X 10 - 6 SI)

Remanence ( X 10 -3 A/m)

Q-ratio mean

mean

st.dev,

mean

st.dev,

mean

st.dev.

2625

35

1190

1940

110

650

2.3

2600

25

450

890

30

110

1.9

2620

30

1200

1140

130

760

2.7

2665

35

2280

4690

110

320

1.2

The susceptibility histogram was decomposed into five log-normally distributed components. The first component (32% of the samples) is virtually paramagnetic and has a mean susceptibility of 165)< 10 -6 (standard deviation of logarithmic distribution in 10 -6 units s~=0.23). The second and third components (8% and 5 7 0 respectively) are weakly ferrimagnetic and have susceptibilities of 4 4 5 ) < 10 - 6 (Sl=0.16) and 1 3 0 5 ) < 10 - 6 (s~=0.26). The fourth and fifth components (2% and 1%) with mean susceptibilities of about 6000)< 10 -6 and 13,500)< 10 -6 cause the most notable aeromagnetic anomalies within the batholith.

(1.86) in anorthosite rock samples from the POLAR profile region in Finnish Lapland. The overall high Vp/Vs-ratio of the Wiborg batholith area was suggested by Luosto et al. (1990) to be the result of low rock density and relatively great plasticity. According to a more detailed seismic model for the upper crust (Tuomi, 1988), the Suomenniemi batholith is only 3 km thick. A local gravity minimum caused by the batholith is seen on profiles 3 and 4 of Fig. 5. The following gravity interpretations indicate that there is a larger rapakivi body beneath the Suomenniemi batholith. In conclusion, the Suomenniemi batholith is a separate intrusion above the underlying main rapakivi batholith.

3. Seismic models 4. Gravity models According to Luosto et al. (1990), the upper and middle crust in the rapakivi area extend to a depth of 30 km, the Moho is at a depth of 41 km, and another refraction discontinuity exists at a depth of 50 km. Between 8 and 14 km, a high velocity body (Vp = 6.4-6.5 k m / s ) is detected. This body has also a high P- to S-wave velocity ratio of 1.76-1.82. The high Vp/Vs-ratio together with high Vp-values w a s interpreted by Tuomi ( 1988 ) to represent gabbroanorthositic rock types. Recently, Kern et al. (1990) have observed a very high Va/Vs-ratio

An areally large Bouguer anomaly low is associated with the Wiborg batholith and its satellites (Fig. 4). A pronounced regional high surrounds the Bouguer anomaly low. Profiles 1-4 in Fig. 5 show a general symmetry. The anomaly pattern suggests that the gravity low is superimposed on a broader gravity high. Here it must be remembered that the map covers only the western, northwestern, and northern parts of the anomaly pattern. The steepest gradient outlines the area of the rapakivi bath-

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Fig. 4. Bouguer anomaly map of southern Finland (Elo, 1992). The size of the rectangular frame is 750 by 400 km. The data are those of the Finnish Geodetic Institute (see Kiviniemi, 1980). Class intervals are 2.5 mGal. The values increase from deep green towards dark red-brown. l'he range of values is 90 reGal. A large Bouguer anomaly low (green) is associated with the Wiborg batholith and its satellites. A pronounced regional high (red-brown) surrounds the Bouguer anomaly low. The anomaly minimum forms a ring around the centre of the rapakivi batholth. Note that our data do not cover the eastern, southeastern and southern sectors of the anomaly pattern. The pattern suggests that the gravity o w is superimposed on a broader gravity high. See also the profiles in Fig. 5.

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S. ELO AND A. KORJA BOUGUER ANOIdALYPROFILE 1 regal 0

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Laur6n's (1970) profileA-A' (Fig. 6) traverses over the Wiborg and Ahvenisto batholiths starting some 80 km northwest of the contact, where the Bouguer anomaly ( - 10 mGal) is affected by the regional Bouguer anomaly high. Using two-dimensional modelling tech-

oliths. Profile 2 coincides with the profile interpreted by Laur6n (1970), but it has been continued towards the northwest in order to better define the zero level of the anomaly pattern. In this profile the zero level is around - 2 5 mGal.

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lie 15 2O

G E O P H Y S I C A L I N T E R P R E T A T I O N O F C R U S T A L A N D U P P E R M A N T L E S T R U C T U R E IN T H E W I B O R G AREA

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Fig. 7. Gravity anomaly interpretation of the Wiborgbatholith accordingto Shustovaet al. ( 1978). MGAL

PROFILE

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Fig. 8. Gravity model of the crust and upper mantle of the Wiborg batholith area along profile 2. The profile runs from the northwest to the southeast, The model is modified from Elo (1981) taking into account information given by Luosto et al. (1990) and Korja et al. (1992).

niques, Laur6n has interpreted the anomaly pattern as being caused by outcropping crustal units (Fig. 6). To create the anomaly pattern, he defines two blocks of irregular shape. The block with a density contrast of - 100 k g / m 3 ( - 0.1 g / c m 3) represents rapakivi granites. The other block with a density contrast of + 100 k g / m 3 ( + 0.1 g / c m 3) is located on the northwestern side of the batholith. This block could correspond to a large body of relatively dense rocks, which, however, is not exposed at

the surface. In order to produce the deep gravity low, Laur6n places a deep root under the Ahvenisto area. Because of the circular symmetry of the anomaly, this model implies a continuous root on all sides of the batholith. The structure resembles an inverted dish. The effect of the crust-mantle boundary is neglected. Shustova et al. ( 1978 ) published a generalized gravity model for the Wiborg batholith based on a profile over the entire batholith (Fig. 7). The Bouguer anomaly is similar on both sides as predicted from the general symmetry of the anomaly pattern. The profile covers Laur6n's (1970) profile and our profile No. 2. In order to obtain a good fit between the measured and calculated gravity values, Shustova et al. used in their model an abnormally dense (2900 k g / m 3) upper crust around the Wiborg batholith. Such density would be typical for gabbros, but is an unlikely mean density for the upper parts of the crust. Elo (1981) has presented an alternative model explaining the Bouguer anomaly pattern across the Wiborg batholith. In his model, a low-density upper and middle crust is more or less compensated by uplifted upper mantle. This idea is demonstrated by the three-dimensional model shown in Fig. 8, in which the

19 °

60°+ ;Stl

Fig. 9. Aeromagnetic total intensity map of southern Finland from Korhonen ( 1980). Contour intervals are 200 nT, class intervals of the colour scale are 50 nT. The values increase from deep blue to dark purple. The size of the rectangle frame is 750 by 400 kin.

300nT

200nT

I00 nT

O~T

I00 nT

!0~ nT

]00 nT

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283

GEOPHYSICAL INTERPRETATION OF CRUSTAL AND UPPER MANTLE STRUCTURE IN THE WIBORGAREA

block boundaries were taken from the seismic cross-section (Luosto et al., 1990) and from the map of the Moho depth (Luosto, 1991 ). The density contrasts and the zero level were solved by optimization methods. The model is 200 km long, 140 km wide and 60 km deep. It includes a lower block (between 41 and 60 km of depth) with a density contrast of + 350 kg/ m 3, and an upper block (between 0 and 30 km of depth) with a density contrast of - 145 kg/ m 3. The density contrast of the lower block represents the density difference between lower crust and mantle. The density of the upper block characterizes the upper and middle crust including rapakivi granites and other low-density rock types. The average density contrast between the rapakivi granites and the surrounding rocks is - 8 0 kg/m 3 at the surface, but the contrast between the rapakivi granite and the metavolcanic belt immediately to the west of the batholith is - 105 kg/m 3 (see Table 2 ). The gravity interpretation implies that there are abnormal amounts of low-density material in the upper and middle crust, and that the average density contrast of the lowdensity block is greater than that observed at the surface.

5. Magnetic models The magnetic anomaly values in the rapakivi area are slightly higher than those in the surrounding schist areas but distinctly lower than the values in the metavolcanic belt to the west (Fig. 9). Positive regional magnetic anomalies form a well-defined magnetic high of about 400 nT in the western part of the batholith (Fig. 10). This anomaly is not associated with any obvious gravity signature. According to the quantitative interpretation of this positive anomaly, the depth to the upper surface of the magnetic body is between 12 and 15 km, and the body thus extends from the upper crust to the lower crust. The anomaly cannot be explained by the observed induced and remanent magnetization values of rapakivi

nT

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4000

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3.9 A/M OBSERVED CALCULATED ZERO LEVEL

50 Fig. 10. A two-dimensional magnetic interpretation model o f the regional magnetic a n o m a l y along the profile M - M ' (for location see Fig. 2). T h e m a g n e t i z a t i o n contrast is given in A / m .

TABLE 2 Generalized mean densities and mean susceptibilities of rocks in SE Finland Rock unit

Density ( k g / m 3)

Susceptibility ( X 10 -6 SI)

Svecofennian bedrock Svecofennian metavolcanic belt Ahvenisto gabbro-anorthosites Rapakivi granites

2705 2730 2870 2625

8450 16400 l 100

granites, because an abnormally strong magnetization (3.9 A / m ) is needed for the interpretation. However, the anomaly can be explained either by the Svecofennian metavolcanic rocks and gabbros, or by the gabbro-anorthosites associated with the rapakivi granites if their remanent magnetization is strong enough (see Tables 1 and 2 ). Large volumes of these rocks should also produce a gravity signature. Van Schmus et al. (1989) describe an anorogenic granite (1433 Ma) which is located in northwestern Iowa, U.S.A., and has its upper surface at a depth of about 500 m. The granite contains up to 2.8% mag-

284

S. ELO A N D A. KORJA

netite and causes a magnetic anomaly of 1200 nT but no significant gravity anomaly. It is possible that parts of the Wiborg batholith at deeper levels also become more magnetic than at the surface. "

60°45,

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G E O P H Y S I C A L PROFILE

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6. Lapinjiirvi Besides regional scale modelling, local structures can also be studied by gravity and magnetic methods. For example at Lapinj~irvi (Fig. 11 ) different rapakivi types are distinguished on the basis of their gravity and magnetic signatures. At Lapinj~irvi, a southwest-northeast directed body of dark wiborgite (2665 k g / m 3 ) is cut by even-grained biotite rapakivi granite ( 2 6 0 0 k g / m 3 ) . In the southeast, the dark wiborgite is in contact with wiborgite (2620 kg/ m3). The dark wiborgite body is associated with gravity and magnetic anomalies that are

Fig. 11. Lithological map of the Lapinj~irvi area after Simonen and Laitakari (1962). The location of the area is shown in Fig. 2.

A G R A V I M E T R I C INTERPRETATION MGAL

- -34

-

+65KG /m a

~

OBSERVED

+25 K G / m a

[~

CALCULATED

-36

ZERO LEVEL

MANTLE UPWELLING

MANTLE

-38 -40 -42

0

2.5

t

5.0

i

7.5

/

I

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f

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INTERPRETATION

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GRANITE

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Fig. 12. Gravity and magnetic interpretation of the Lapinj~irvi profile L - L ' . Density contrasts are given in kg/m 3 and magnetization contrasts in A/m.

Fig. 13. Schematic geophysical and geological models of the crustal structure in the Wiborg batholith area. (a) A geophysical model based on seismic, magnetic and gravity data. (b) The origin of rapakivi granites based on a geological interpretation of the geophysical model. The figure is modified after Huppert and Sparks (1988) and R~im6 (1990).

GEOPHYSICAL INTERPRETATION OF CRUSTAL AND UPPER MANTLE STRUCTURE IN THE WIBORG AREA

elongated in the strike direction of the body. The Bouguer anomaly profile (Fig. 12) across the dark wiborgite shows a gravity high of 3.5 mGal caused by a 3.6 km wide body consisting of two parts. However, the corresponding magnetic anomaly of 130 nT is caused by an only 0.7 km wide body. In the interpretation model (Fig. 13 ), the background density represents biotite rapakivi or pyterlite. The two blocks, differing from the background by density contrasts of +65 and +25 kg/m 3' correspond to dark wiborgite and wiborgite, respectively. Highly magnetic dark wiborgite is restricted to the northwestern side of the intrusion. Here the most magnetic part is also the most dense one. The local magnetic and gravity anomalies within the Wiborg batholith suggest that the batholith consists of separate intrusions and intrusion phases, which occasionally have rather sharp contacts. 7. Discussion

Model interpretations of seismic and gravity data suggest an upwelling mantle structure beneath the Wiborg rapakivi batholith. The lowdensity upper crust is a 30 km thick ovoidal sheet consisting of several separate rapakivi intrusions, which are connected to the lower crust by way of a magnetic body. The high velocity body within the low-density rapakivitic material represents the remnants of coeval gabbro-anorthositic magmatism (Fig. 13 ). Because the magnetic body (Fig. 13 ) crosscuts the generally layered structure, it is assumed to be an intrusion that could be interpreted as a feeder channel of the rapakivi and gabbroic magmas. A mixture of rapakivi and gabbroic rocks would explain the high magnetization level and the absence of a gravity signature. Although the overall magnetite content of the exposed rapakivi batholith in southeastern Finland is quite low, there are elsewhere examples of anorogenic granites With magnetite

285

contents from 1 to 3% as reported e.g. by Van Schmus et al. (1989) from northwestern Iowa, U.S.A. Moreover, available petrophysical data imply that the proportion of deafly ferrimagnetic rapakivi varieties is larger in SW Finland than in SE Finland. The interpreted gabbro-anorthosite sheet could explain the zone of positive regional magnetic anomalies within the western parts of the Wiborg batholith close to its contact. Pesonen et al. ( 1989 ) have proposed that the extremely rapid movement of plates during the Subjotnian rifting stage may be due to movement over a thermal dome or a hotspot. The one-way movement is not, however, supported by the latest geochronological data. Actually the Wiborg batholith (1646-1630 Ma) is older than the Salmi batholith ( 1539 Ma) to the east of it and also older than the Laitila, Vehmaa and Aland batholiths ( 1570-1580 Ma) in the west (Suominen, 1991; Vaasjoki et al., 1991 ). The map of Moho depth in Fennoscandia (Luosto, 1991 ) shows thinning of the crust around the Gulf of Finland where the rapakivi granites are situated. The E-W oriented structures of the thinner crust (41-46 kin) that is associated with rapakivi batholiths cut and disturb the structures of the thick (56-65 km) Svecofennian crust. The intrusions could therefore be the result of crustal thinning or an incomplete rifting process. As demonstrated by the Moho uplift beneath the Wiborg batholith, the crust is not uniformly stretched in southern Finland. Klein and Hsui ( 1987 ) postulated that anorogenic magmatism is caused by an excess of mantle-derived heat, which caused partial melting of both the lower crust and the upper mantle. Preceding collisional events can produce a thick continental crust, which, in turn, acts as a cap on the mantle and thus prevents its venting. The focusing of heat is followed by the emplacement of anorogenic granitoids. On a global scale, partial melting and intrusions of anorogenic granitoids took place during or be-

286

fore the breakup of a major Precambrian supercontinent. Klein and Hsui also postulate that the crust immediately above a partially melted zone would be thinned. The intrusion of granite would be expressed in domal structures beneath areas of mechanical and thermal subsidence. Subsidence basins should thus be floored by anorogenic granitoids. A good example of these relationships is the Satakunta graben which is floored by Laitila rapakivi granites. This model predicts correctly (Luosto, 1991 ) that the crust is thinned also under Satakunta and ]kland. Huppert and Sparks (1988) described how silicic magmas are generated by emplacement of basaltic magmas into continental crust. If the crust is cold, the basaltic magmas reach the surface. When a large region of the crust is close to melting, large silicic magma bodies can be generated which ascend to the surface while basaltic magmas may still reach the surface in peripheral regions. Haapala and Riim/5 (1990) suggested that the partial melting of the lower crust was caused by mafic mantle-derived magmas in extensional tectonic settings. The upwelling mantle in a rift environment causes thermal anomaly and results in melting of the lower crust and formation of rapakivi granites (Riim/5, 1990). The proposed model fits well with the seismic data (Luosto et al., 1991 ) that indicate a thinner crust, a secondary reflective boundary in the upper mantle, and several reflectors within the Wiborg area. The second mantle reflector could represent the underplated maficultramaflc rocks. Coeval bimodal magmatism should be an appropriate situation to produce horizontal reflectors with high reflection coefficients between flat lying granites and gabbroic sills. The gravity model also implies an uplifted Moho and large amounts of low-density rocks in the upper and middle crust.

S. ELO AND A. KORJA

8. Conclusions

The gravity, magnetic and seismic interpretations together with petrophysical and geological data provide quantitative constraints for the present upper mantle and crustal structure and the geological evolution of the rapakivi areas. The gravity anomaly pattern is the result of a superposition of a gravity low due to the relatively low-density upper and middle crust onto a gravity high due to an uplifted Moho. In the western part of the Wiborg batholith, there is a strongly magnetized pipe-like body extending from the middle to the lower crust. It is tentatively interpreted as a magnetic feeder pipe of the rapakivi and gabbro-anorthositic magmas. No obvious gravity anomaly is associated with this magnetic feature. The rapakivi granites were formed in an extensional tectonic setting above an upwelling mantle (Fig. 13). The rising mantle magmas intruded into the crust and formed diabases and gabbro-anorthosites. Additional mantlederived material may be preserved at depths between 8 and 14 km where there is a high velocity body with a high Vp/Vsratio. The uppermost mantle (41-50 km) in the seismic refraction model may represent the mafic and ultramafic underplating of the crust which led to the melting of the overlying Svecofennian crust and resulted in the formation of A-type granite rapakivi batholiths. The crust was not stretched uniformly as is demonstrated by the Moho uplift beneath the Wiborg batholith. Small, local magnetic and gravity anomalies within the Wiborg batholith are caused by lithological variation. The local anomalies can be well explained by the measured values of density, magnetic susceptibility and remanent magnetization. In favourable places, gravity and magnetic mapping can be used to delineate boundaries of separate intrusions within the batholith.

GEOPHYSICAL INTERPRETATION OF CRUSTAL AND UPPER MANTLE STRUCTURE IN THE WIBORG AREA

Acknowledgements A. Korja was supported by the Finnish Cultural Foundation and the Academy of Finland. T. Koistinen and R. Puranen, two reviewers and the editor made useful comments on the original manuscript. Ms. Salme N~issling and Ms. Paula P/Snti/5 helped in preparing the figures. We wish to thank them and all the others for useful discussions and encouragement. References :Aro, K.,1986. Mafic dyke rocks in Finland 1:2,000,000. In: K. Aro and I. Laitakari (Editors), Diabases and Other Mafic Rocks in Finland. Geol. Surv. Finl., Rep. Invest., 76, 254 pp. Boyd, R., Nilsson, G., Papunen, H., Vorma, A., Zagorodny, V. and Robonen, V. (Compilers), 1985. General geological map of the Baltic Shield 1:2,500,000. In: G.I. Gorbunov and H. Papunen (Editors), NickelCopper Deposits of the Baltic Shield and Scandinavian Caledonides. Geol. Surv. Finl., Bull., 333, Map 1. Elo, S., 1981. A gravity study of the Wiborg rapakivi granite massif. Paper presented at the 43rd EAEG Meeting, May 26-29, 1981, Venice, 20 pp. (Prints available from the author.) Elo, S., 1992. Gravity anomaly maps. The Geochemical Atlas of Finland, Part 2. Geological Survey of Finland, pp. 70-75. Elo, S. and Tuomi, A., 1989. Gravimetric, magnetic and seismic interpretations of rapakivi massifs in South° eastern Finland. In: I. Haapala and Y. K~hktnen (Editors), Symposium Precambrian Granitoids, Abstracts. Geol. Surv. Finl., Spec. Pap., 8: 45. Gorbatschev, R., Lindh, A., Solyom, Z., Laitakari, I., Markov, M.S., Ivliev, A.I. and Bryhni, I., 1987. Mafic dyke swarms of the Baltic Shield. In: H.C. Halls and W.F. Fahrig (Editors), Mafic Dyke Swarms. Geol. Assoc. Can., Spec. Pap., 34:361-372. Haapala, I.J. and R~imt, O.T., 1990. Petrogenesis of the Proterozoic rapakivi granites of Finland. In: H.J. Stein and J.L. Hannah (Editors), Ore-Bearing Granite Systems; Petrogenesis and Mineralizing Processes. Geol. Soc. Am. Spec. Pap., 246: 275-286. H~rme, M., 1980. The general geological map of Finland 1:400,000, Sheet C1-D1, Helsinki. Geological Survey of Finland. Huppert, H.E. and Sparks, S.J., 1988. The generation of granitic magmas by intrusion of basalt into continental crust. J. Petrol., 29: 599-624. Hutton, D.H.W., Dempster, T.J., Brown, P.E. and Becker, S.D., 1990. A new mechanism of granite emplace-

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288 magnetic constraints. Tectonophysics, 162 ( 1/2): 2749. Puranen, R., 1989. Susceptibilities, iron and magnetite content of Precambrian rocks in Finland. Geol. Surv. Finl., Rep. Invest., 90, 45 pp. Puranen, R., Elo, E. and Airo, M-L., 1978. Geological and areal variation of rock densities, and their relation to some gravity anomalies in Finland. Geoskrifter, 10: 123-164. R~im6, O.T., 1990. Diabase dyke swarms and silicic magmatism----evidence from the Proterozoic of Finland. In: A.J. Parker, P.C. Rickwood and D.H. Tucker (Editors), Mafic Dykes and Emplacement Mechanisms. Balkema, Rotterdam, pp. 185-199. RiimS, O.T., Vaasjoki, M. and Huhma, H., 1989. Sm-Nd and Pb-Pb isotopic constraints on the origin of the rapakivi granites and associated tholeiitic dyke rocks in southern Finland. In: I. Haapala and Y. K~ihk6nen (Editors), Symposium Precambrian Granitoids, Abstracts. Geol. Surv. Finl., Spec. Pap., 8: 107. Sahama, Th.G., 1945. On the geochemistry of the East Fennoscandian rapakivi granites. Bull. Comm. G6ol. Finl., 136: 15-67. Savolahti, A., 1962. The rapakivi problem and the rules of idiomorphism in minerals. Bull. Com. G6ol. Finl., 204:33-112. Shustova, L.E., Puura, V.A. and Velikoslavinskiy, D.A., 1978. Glubinnoye stroyenie Viborskogo plutona po geofisichiskim dannym. In: D.A. Velikoslavinskiy, A.P. Birkis, O.A. Bogatikov, V.P. Buharev, S.D. Velikoslavinskiy, L.I. Gordienko, O.V. Einchenko, Ya.Ya. Kivisilla, Yu.E. Kirs, Yu.I. Kononov, Yu.F. Levitskiy, M.I. Niyn, V.A. Puura, M.I. Hcorov and L.E. Shustova, Anortosit-rapakivigranitnaya formatsiya. Akad. Nauk SSSR, Leningrad, pp. 37-43. Simonen, A., 1987. The Pre-Quaternary rocks of the mapsheet area of the rapakivi massif in SE Finland. Geological map of Finland 1:100,000. Explanation to the

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