IN NORTHERN FINLAND. Marit Wennerström Risto Puranen Seppo Elo Q 19/22/3723/94/ 1 Koskee 3724 MAGNETIC FABRIC OF THE POMOVAARA GRANITE COMPLEX

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1 Marit Wennerström Risto Puranen Seppo Elo Q 19/22/3723/94/ 1 Koskee 3724 MAGNETIC FABRIC OF THE POMOVAARA GRANITE COMPLEX IN NORTHERN FINLAND (Modified from the paper given by Marit Wennerström in the Symposium on 'The Svecofennian Domain", , Turku, Finland)

2 CONTENTS 1. GEOLOGICAL AND GEOPHYSICAL BACKGROUND 2. PETROPHYSICAL PROPERTIES 3. MAGNETIC FABRIC 4. GEOPHYSICAL MODELS BASED ON AEROMAGNETIC AND BOUGUER ANOMALY PROFILES 5. CONCLUSIONS

19 Fig. 13. DIRECTIONS OF MAXIMUM (i), INTERMEDIATE (+) AND MINIMUM (0) SUSCEPTIBILITIES IN THE NORTHERN STOCK OF THE POMOVAARA COMPLEX

20 In the Central stock (Fig.14.) similar exchange of principal susceptibility directions occurs in the porphyritic granite. In two sites (NW, E) quite close to the margins, the magnetic foliation planes follow the direction of the outer contact. Foliation plane is vertical in E and slopes somewhat outwards in NW. The dominant direction of magnetic lineation points to southwest in this stock, as well. We suggest that this is the direction of the original magma flow. The AMS data in the two even-grained granites are tightly grouped. In the biotite granite the fabric is fairly strongly developed and planar-parallel. The foliation plane is nearly vertical sloping somewhat to the southeast. In the biotite-muscovite granite, close to the centre of the central stock, the magnetic foliation plane slopes gently (about 30 degrees) to the south-southwest. Here the fabric is strongly developed and planar-parallel. It could be a roof fabric indicating that the uppermost part of this granite phase is now exposed. The three granite phases in the central stock can be distinguished from each other on the basis of their magnetic fabric. A greater number of samples is needed to verify the dif ferences. In the Southern stock (Fig.15.) all the sites have well aligned fabrics. They are strongly developed and linear-parallel. The magnetic foliation planes are subhorizontal. The granite materia1 was compressed against the roof of the stock. This is observed in the microscopic scale as moderate deformation of the rock, like fine-grained recrystallization of quartz and bending of K-feldspar phenocrysts.

23 4. Geophysical models based on aeromagnetic and Bouguer anomaly prof iles In figure 16 there are three profiles across the northern, central and southern stocks of the Pomovaara complex. These combined aeromagnetic low altitude and Bouguer anomaly profiles go in the NW-SE direction through the central points of the intrusions. Maximum amplitude of the Bouguer anomaly due to the Pomovaara complex is about -25 mgal and the average magnetic anomaly high about 500 nt. The density contrast between the Pomovaara granites and the country rock in the models is -140 kg/m3. Figure 17 shows the interpreted models of the Pomovaara stocks. The different blocks in the central and southern stocks indicate the different suscept-ibility values used in the models. The effective susceptibility value used in the northern stock was SI. In the central stock from west to east values were SI, SI and SI. In the southern stock effective susceptibilities were from west to east SI, SI, SI and SI. The minimum thickness of the intrusions varies from about 3 km, in the southern stock, to 6 km, in the central stock. The contacts are vertical in the northern stock, they dip somewhat outwards in the central stock, and in the southern stock contacts slope more gently outwards, which is in agreement with the interpretation of magnetic fabric data. The combined profile across the whole complex in the NE-SW direction indicates that the intrusions are very closely packed to each other (Fig.18.). The northern and central stocks are separated at the surface by a km thick body of host rocke caueing the local gravity and magnetic highs (marked by A in figure 18).

27 5. Conclusions Magnetic fabric can be used in the structural interpretation of granite intrusions in defining: - direction of contacts - erosion level of an intrusion (for example roof fabrics) - existence of separate granite phases - direction of magma movement Together with geophysical modelling anisotropy of magnetic susceptibility is an efective tool in granite Studies. REFERENCES Damm, V Gesteinsmagnetische Anisotropien in Magmatitten und deren strukturgeologische Bedeutung. 2. Geol. Wiss. 16, Front, K., Vaarma, M., Rantala, E. & Luukkonen, A Keski- Lapin varhaisproterotsooiset Nattas-tyypin graniittikompleksit, niiden kivilajit, geokemia ja mineralisaatiot. Geological Survey of Finland, Report of Investigation 85, 77 p. Henkel, H Petrophysical properties (density and magnetization) of rocks from the northern part of the Baltic Shield. Tectonophysics 192, Hrouda, F Magnetic anisotropy of rocks and its application in geology and geophysics. Geophysical Surveys 5, Jelinek, V Precision A.C. bridge set for measuring magnetic suscept-ibility of rocks and its anisotropy. Studia geophysica et geodaetica 17, Rochette, P., Jackson, M. and Aubourg, C Rock magnetism and the inter-pretation of anisotropy of magnetic susceptibility. Reviews of Geophysics 30 (3),

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