magnetic fabric and emplacement of the post-collisional pomovaara granite complex in northern...

Ž .Lithos 45 1998 131–145

Magnetic fabric and emplacement of the post-collisionalPomovaara Granite Complex in northern Fennoscandia

Marit Wennerstrom ), Meri-Liisa Airo¨Geological SurÕey of Finland, P.O. BOX 96, FIN-02150 Espoo, Finland

Received 8 December 1997; accepted 10 June 1998

Abstract

The Proterozoic Pomovaara Granite Complex in northern Finland comprises three separate highly magnetic granitestocks. They are discordant, apparently unfoliated and according to isotope data, a significant Archaean componentcharacterizes the source of these granites. The three stocks are aligned in an array parallel to major trans-crustal faults asinterpreted from both aeromagnetic and gravity data. Their younger age of 1.8 Ga, compared to the main tectonic events at1.9 Ga in northern Fennoscandia, indicates their post-collisional nature with respect to these events. The anisotropy of

Ž .magnetic susceptibility AMS was studied together with magnetic, gravity and geological data in order to assess theemplacement mechanisms of the Pomovaara Granite Complex, and the possible tectonic control of fault systems on theascent and emplacement of granitic magma. The orientation of magnetic fabrics within the granite stocks indicates that thedirection of the original magma upwelling was from the SW, parallel to the major fault zones that controlled, at the crustalscale, the ascent of granite magmas. The predominant NW–SE orientations of the minimum magnetic axes of the magneticellipsoid and the elongate shapes of the stocks indicate compression normal to the deep fault trend during the crystallizationof the granite magma. q 1998 Elsevier Science B.V. All rights reserved.

Keywords: Granites; Magnetic fabric; Emplacement; Aeromagnetic maps; Finland

1. Introduction

Post-collisional granite intrusions are present invarious parts of the Fennoscandian Shield. For exam-ple, more than 10 granite batholiths and plutonsoccur within an E–W trending belt, nearly 500 kmlong and 100 km wide near the southern coastline of

Ž .Finland reported by Eklund et al., 1998 . In north-ern Finland, post-collisional magmatism is repre-

) Corresponding author.

sented by the so-called Nattanen-type granite intru-Ž .sions Fig. 1 , which represents a relatively young

Ž .geological event ca. 1.8 Ga in northern Fennoscan-Ž .dia Front et al., 1989 . The Nattanen-type granites

are characterized by high SiO , and are typically2ŽI-type and highly magnetic Haapala et al., 1987;

.Front et al., 1989 . Their petrology and geochemistryhave been studied systematically, but their structureand emplacement mechanisms have not been investi-gated in detail.

The three distinct stocks of the Pomovaara Gran-ite Complex have been assigned to the Nattanen-type

0024-4937r98r$ - see front matter q 1998 Elsevier Science B.V. All rights reserved.Ž .PII: S0024-4937 98 00029-2

( )M. Wennerstrom, M.-L. AirorLithos 45 1998 131–145¨132

Ž . Ž .Fig. 1. Simplified geological map of northern Fennoscandia, modified from Gorbunov and Papunen 1985 and Korsman et al. 1997 . TheŽ . Ž . Ž . Ž .frame of Fig. 2 is shown. 1 Rocks of the Caledonian Orogeny; 2 Nattanen-type granites; 3 Palaeoproterozoic granitoids; 4 Lapland

Ž . Ž .Granulite belt; 5 Palaeoproterozoic supracrustal rocks; 6 Archaean granitoids and gneisses.

granites on the basis of their petrological and geo-chemical properties, and form a discrete group ofdiscordant and unfoliated intrusions. The anisotropy

Ž .of magnetic susceptibility technique AMS has beensuccessfully used in the interpretation of structureand emplacement history of a number of granite

Žbodies for example, Chlupacova et al., 1975; Guillot´ ´et al., 1983; Hrouda and Lanza, 1989; Bouchez etal., 1990; Bouillin et al., 1993; Cruden and Launeau,

.1994; Leblanc et al., 1994; Archanjo et al., 1995 . InŽ .Finland, Puranen 1991 has studied the magnetic

fabrics within and around the Wiborg rapakivi plu-ton. The initial measurements of AMS in the Po-

Žmovaara granites were so promising Wennerstrom,¨.1993 that oriented samples from every possible

outcrop were taken and measured. These AMS re-sults are used here together with magnetic, gravityand geological data to discuss the emplacement

mechanisms of the Pomovaara Granite Complex, andthe relation of their intrusion to the main regionaltectonic events.

According to aeromagnetic and gravity data, theNattanen-type granite intrusions are situated at atransition zone of the main lineament directions innorthern Finland, striking NE–SW and NW–SE.These lineaments were formed and reactivated dur-ing several tectonic events, but the framework of thefault pattern was mainly caused by two early events:

Žcontinental rifting during 2.4–2.1 Ga Sorjonen-.Ward, 1997 and collision of the Kola continental

block from NE towards the Karelian craton, produc-ing the high-grade metamorphic rocks of the LaplandGranulite belt ca. 1.9 Ga ago and resulting to crustal

Ž .thickening Berthelsen and Marker, 1986 . The NE–SW trending major faults are supposed to representtransform faults associated with the NW–SE-di-

( )M. Wennerstrom, M.-L. AirorLithos 45 1998 131–145¨ 133

Fig. 2. Location of the Pomovaara Granite Complex on theŽ .high-altitude 150 m aeromagnetic DGRF-65 anomaly map of

Ž .Finland Korhonen, 1980 . One kilometer spacing between datapoints. Three main lineament trends discussed in text are shownby arrows and the southwestern margin of Lapland Granulite beltis marked by the dashed line. The area of Fig. 3 is outlined.

Ž .rected rifting. In the aeromagnetic image Fig. 2 ,they appear as interruptions in the anomaly patternsof the Granulite belt and as linear changes in themagnetic field level. The southwestern front of theGranulite belt parallels the main rifting direction. Asthe major faults can be followed for at least 100 km,they must extend deep into the crust. The appearanceof the Nattanen-type granite intrusions in two arraysparallel to the faults suggests a tectonic control to theascent and emplacement of these plutons.

2. Geological and tectonic setting

The Pomovaara Granite Complex consists of threeseparate, rounded, multiple intrusions, called herethe Northern, the Central and the Southern stocksŽ .Fig. 3a . One radiometric age of 1772 Ma has been

Ždetermined from the Pomovaara Complex Lehtonen. Ž .et al., 1985 site marked by star in Fig. 3a . The

Complex has intruded Archaean, granodioritic totonalitic granite–gneiss and streaky, reddish arkosic

Ž .gneisses Manninen and Pihlaja, in preparation . Inspite of the poor exposure, the structural relationshipbetween the stocks and their environment can beconfidently established using geophysical data. Dueto their high magnetization, the Pomovaara stocksare readily discernible in high-resolution aeromag-netic data and the magnetic structure of the discrete

Ž .stocks is well-defined Fig. 3b . The Pomovaarastocks are also clearly visible in gravity data, due totheir lower densities compared to the surroundinggranodioritic and metamorphosed supracrustal coun-try rocks. The stock-like character and the steepdipping of the contacts have been interpreted bymodel calculations from several aeromagnetic and

ŽBouguer anomaly profiles Wennerstrom et al.,¨.1993 . Fig. 4 illustrates the cross-section of the

ŽCentral stock the aeromagnetic anomaly profile.shown in Fig. 3b , based on aeromagnetic and grav-

ity modeling. The interpreted vertical extent of theCentral stock is about 8 km, and 4 km of theNorthern stock. Interpretation of the vertical thick-ness of the Southern stock was more difficult be-cause of the low density contrast with its surround-ings, but it extends at least to the depth of 2.5 km.

To the west of the Pomovaara Complex, there isthe thick pile of weakly magnetic metavolcanic rocksbelonging to the Kittila Greenstone belt, produced¨during the continental rifting. The thickness of thebelt has been interpreted by gravity modeling to be

Ž .about 6 km Elo et al., 1989 . To the east, thePalaeoproterozoic mafic and ultramafic volcanicrocks, quartzites and micaschists are strongly foldedŽ .Fig. 3a . These folded structures appear in the aero-magnetic image as alternating magnetic and nonmag-netic banded patterns. To the west of the Complex,the folding is more open and the layering dips aremore gentle. The three stocks are discordant withrespect to the regional folding and have sharp con-tacts with their country rocks. Near the contacts, thefolds in the wall-rock locally follow the direction ofthe contacts. The Pomovaara stocks have been af-fected by younger tectonic events after their em-placement, partly coinciding with the old weaknesszones and enhancing the earlier structural patterns.The faults and fractures, illustrated as sharp nonmag-netic lineaments like those fragmenting the Northern


Ž . Ž . Ž .Fig. 3. Pomovaara Granite Complex and its surroundings. a Geological map. Simplified from Manninen and Pihlaja 1993 . bŽ .Aeromagnetic image and location of profile C1. Low-altitude airborne measurements terrain clearances35 m, line spacings200 m by

the Geological Survey of Finland.

Ž .stock in NW–SE direction Fig. 3b , were probablyintroduced during such younger events, as the Cale-

Ždonian orogeny 0.6–0.4 Ga ago Gaal and Gor-´.batschev, 1987 .

The southwestern margin of the Lapland Gran-ulite belt is about 30 km to the NE from the Po-

Ž .movaara Complex Fig. 2 . During the overthrust ofthis belt, low-angle lateral movement towards SW


Fig. 4. Bouguer and aeromagnetic anomaly profiles C1 with 2.5dimensional models, the Central stock of the Pomovaara granite

Ž .Complex. Observed values thick solid lines , calculated anoma-Ž .lies thin solid lines and regional backgrounds for the calculated

Ž .anomalies dashed lines . The Bouguer anomaly has been interpo-lated from the results of the regional gravity measurements by theGeological Survey of Finland. The average interval between origi-nal observations in this case is approximately 1 km. The aeromag-

Ž .netic anomaly has been interpolated from the low-altitude 35 mairborne measurements by the Geological Survey of Finland.Location of profile C1 is shown in Fig. 3b.

caused shearing along the delaminated granulite–gneiss layers and parallel to the margin. Differentialstepwise movement of the granulite–gneiss blockstook place in the direction of the NE–SW trendingfault zones and produced vertical movements alongthe faults. Vertical displacement is indicated by recti-linear change in both magnetic and gravity fieldstrengths across the faults. For example, in the crustalblock outlined by the major faults in Fig. 2, themagnetic field level is 70–80 nT lower compared tothe adjacent structural units and it coincides with a

Bouguer-anomaly low. According to an analysis ofthe systematic density and magnetic property datafrom the petrophysical data base provided by theGeological Survey of Finland, this change cannot beentirely caused by compositional variations of thesurface rocks, but also has a deeper source. As aconsequence, the pile of the dense granulite gneissesis thinner in the unit between the major faults thanoutside it, and the underlying weakly magnetic andlower density granodioritic basement rocks are closerto the present surface. Therefore, based on the re-gional potential field data and model calculations, itis postulated that the structural crustal unit whichcontains the granite stocks, represents an upliftedcrustal block.

3. Petrography and opaque minerals

The most abundant granite type in the PomovaaraComplex is a coarse to medium-grained, generallyporphyritic monzogranite with a high SiO content2Ž . Ž .)70 wt.% Front et al., 1989 . This granite is

Žmostly unfoliated, with sporadic K-feldspar -3. Ž .cm phenocrysts in varying orientations Fig. 5a . In

the Northern stock, the phenocrysts are abundant andthe granite is mostly coarse-grained. In the Southernstock, the subparallel orientation of feldspar crystalsand fine-grained crystallization of quartz indicate

Ž .slight deformation in this granite Fig. 5b . Near theNW–SE trending fault zones, foliation increases, itsorientation is parallel to the faults and the rocks arecataclastic in texture. The content of dark mineralsŽ .mostly biotite is less than 10%, generally 4–6%.Some hornblende occurs infrequently in these gran-ites, too.

In the Central stock, two even-grained granitephases, namely light biotite granite and reddish bi-otite–muscovite granite, have the same principal

Žminerals as the surrounding porphyric granite Fig..3a . The content of dark minerals varies between

2–6%. Biotite is the main dark mineral in the biotitegranite, and is invariably accompanied by muscovitein the biotite–muscovite granite. Sporadic porphyryand aplite dykes are also associated with the Po-movaara granite stocks. The available isotope dataindicate a significant Archaean component in the


Ž . ŽFig. 5. Microphotographs showing microstructures in the Pomovaara Complex. a Porphyritic granite from the Northern stock Sample No..141-HGW-90 . Field of view is 9.6 mm wide. Various orientations of K-feldspar phenocrysts, typical of porphyritic granites, and euhedral

Ž . Ž .magnetite grains are shown. b Porphyritic granite from the Southern stock Sample No. 145-HGW-90 . Field of view is 5 mm wide in bothŽ .figures. K-feldspar phenocrysts are smaller than in a and bent, and fine-grained recrystallization of quartz is obvious.


Žsource of the Pomovaara Granite Complex Front et.al., 1989 .

The modal proportion of opaque minerals in thePomovaara granites, mostly Fe–Ti-oxide minerals,varies between ca. 0.1% in the aplite dykes andsomewhat more than 1.0% in the porphyritic granite.According to microscope investigations, magnetite isthe most abundant Fe–Ti-oxide mineral. Electronmicroprobe analyses confirm that it is nearly pure

Ž .iron oxide Ti-content-0.5 at.% .Most of the magnetite grains are euhedral, though

Ž .some are rounded and some are subhedral Fig. 5a .Ž .Grain size 0.2–0.3 mm in average varies from 0.01

mm to 0.8 mm. Ilmenite, the second most abundantFe–Ti-oxide mineral, is present in all lithologies inthe Central stock, and in some porphyritic andmedium-grained granites and aplite granite dykes ofthe Northern stock. Ilmenite is always anhedral andhas mostly crystallized between magnetite grains.Grain sizes up to 1 mm have been observed. Mag-netite has altered to hematite through martitization toa variable extent. The martite is an almost pure ironoxide with Ti and Mn contents less than 0.2 wt.%.Hematite also occurs as isolated anhedral grainsŽ .diameters0.02–0.05 mm and as micro-inclusionsin ilmenite. Euhedral grains of sphene, in contactwith magnetite, are very common in the porphyriticgranite and appear to be a relatively late phase.Sometimes magnetite coexists with biotite, ilmeniteor sphene as elongate polycrystalline aggregates.

4. Petrophysical properties

Ž .Densities, magnetic volume susceptibilities MSand intensities of remanent magnetization were ex-amined for 141 oriented samples. They are comparedin Fig. 6 to the overall properties of the Pomovaara

Ž .granites, reported earlier by Front et al. 1989 . Themean density is 2593 kgrm3, which is low com-pared to granitic rocks in general. The density ofgranitic crust in Finland ranges from 2610 to 2660

3 Ž .kgrm Puranen et al., 1978; Elo, 1997 and themean density of rapakivi granites in southern Finland

3 Ž .is 2630 kgrm Elo, 1997 . The susceptibilities ofthe Pomovaara granites are typically more than 0.01SI due to their magnetite content. On average, theintensity of remanent magnetization is low, around0.2 Arm.

The petrophysical data in Fig. 6 are illustrated asŽ .X–Y plots of susceptibility vs. density Fig. 6a and

susceptibility vs. Q-ratio, the ratio of remanent toŽ .induced magnetization Fig. 6b . These plots are

indicative of main mineral compositions with respectŽ .to their relative content of Fe, Mg silicates and

various aspects of magnetic mineralogy, that is, theconcentration, composition and grain size of ferri-

Ž .magnetic minerals Puranen, 1989; Henkel, 1991 .MS value of 0.002–0.003 SI divides the magneticdata into two groups. This limit corresponds to theapproximate magnetite content of 0.1–0.2 vol.%.Below this limit, the magnetization is predominantly

Ž . Ž .Fig. 6. Petrophysical properties of the 141 oriented samples black diamonds , with the data of all Pomovaara granite samples circles as aŽ . Ž . Ž .reference. a Density vs. magnetic susceptibility. b Magnetic susceptibility vs. Q-value Konigsberger-ratio .¨


due to the paramagnetic mafic silicates, which aremainly biotites in the Pomovaara granites. Most ofthe samples plot above this limit, where magnetiza-tion is predominantly carried by magnetite. Bimodalsusceptibility distributions are typical of Precambrianrocks, partly as a result of magnetite consumptionand production in metamorphic and alteration pro-cesses. Uniform magnetic properties of the Po-movaara granites indicate that after crystallization,no essential changes affecting the magnetization haveoccurred. This can be connected to reflect the post-metamorphic nature of the Pomovaara granites. Thecommon coarse grain size of magnetite is verified by

Ž .Q-ratios focusing around 0.5 Fig. 6b . Q-valuesmainly below 1.0, together with the high MS valuesgenerally indicate that the magnetization is carriedby coarse-grained magnetite. Q-values up to 10 inFig. 6b refer to finer-grained magnetite in a fewsamples. However, Q-ratios of 3.0–7.0 may also beassociated with irregular magnetite grain shapes, as a

Žconsequence of their high remanences Airo, 1993;.Airo, 1995 . Euhedral magnetite grains, typical of

Pomovaara granites, are generally associated withlow remanence intensities due to their multi-domainstructure and the predominance of the viscous rema-

Ž .nence magnetization component VRM . For aero-magnetic interpretation, this means that the directionof remanence in a broad sense follows the directionof the present Earth’s field, and that the magneticanomalies reflect the structural trends of their sourcerocks.

Magnetic susceptibilities are lowest in the CentralŽ .stock mean 0.012 SI and slightly higher in the

Ž . Ž .Southern 0.014 SI and Northern 0.016 SI stocks.These differences are reflected into the aeromagnetic

Ž .image Fig. 3b , where the Northern stock is associ-ated with the highest magnetic anomalies and theCentral stock with the lowest. The distribution of MScorrelates with the lithological variations within thestocks. The highest MS values are associated withporphyritic granite, which is the predominating gran-ite type in the Northern and the Southern stocks andwhich is responsible for their distinctive magneticanomalies. In the Central stock, the porphyritic typeis mainly confined at the outer rim of the intrusion.The inner type of the Central stock consists ofso-called biotite granite and biotite–muscovite gran-ite, which have broadly similar mineral composi-

tions, except for the relative proportions of biotiteand muscovite. Aplite granite is the most weakly

Žmagnetic type generally MS values less than 0.005.SI . It mainly occurs as small discrete veins at the

boundaries of the stocks and does not contribute totheir overall magnetization. The moderate magnetitecontent of Pomovaara granites related to their lowcontent of dark silicates, compared to Svecofenniangranitoids in general, can be regarded to result fromthe partitioning of iron between the oxide and sili-cate phases during cooling of systems bearing K-

Žfeldsparqmagnetiteqbiotite Frost and Lindsley,.1991 . Production of magnetite and K-feldspar at the

expense of biotites in the porphyritic granites ofPomovaara can be explained by highly oxidizingconditions, which could mean cooling near surfaceŽ .in the upper crust . Oxidizing conditions may alsobe provided by the ascent of magma along theextensional deep fault.

The effect of surface weathering on densities andmagnetizations has often been discussed, particularlybecause the country-wide systematic petrophysicalsampling in Finland is carried out mainly by collec-

Žtion of hand samples from outcrops Korhonen et al.,.1997 . Weathering has affected the densities of some

samples from the Pomovaara granites, notably thoseshowing the lowest density values in Fig. 6. On themagnetic properties, weathering has had no substan-tial effect. This is obvious because there is no markeddifference in the MS values of the weathered sam-ples compared to the overall MS level of the Po-movaara granites. In contrast to silicate minerals,magnetite is highly resistant to weathering, due to itsspinel structure. Furthermore, the alteration of mag-netite to less magnetic oxides such as hematite re-quires a more drastic change of conditions, espe-cially under ambient temperatures, than is normallyassociated with surface weathering. Such alterationwould rather require a deuteric or a metasomaticre-equilibration process in most highly oxidizing

Ž .conditions Frost, 1991 during the slow cooling ofgranitic magmas.

5. Anisotropy of magnetic susceptibility

The magnetic susceptibility of an anisotropic sub-stance can be represented by a symmetric tensor of


second rank, which is uniquely specified by sixŽindependent components K11, K 22, K 33, K12,

.K 23, K 31 . The tensor can be estimated from themeasured susceptibilities by means of the least

Ž .squares method Nye, 1957 . The susceptibility ten-sor can also be specified using three principal sus-ceptibilities and their directions. The principal sus-

Žceptibilities k1smaximum, k2s intermediate, k3.sminimum can be used to define the magnitude

and susceptibility ellipsoids, which both describegeometrically the anisotropy of magnetic susceptibil-ity.

The shape and orientation of the susceptibilityellipsoid are controlled by the fabric of the magnetic

Žminerals in the rock, the magnetic fabric Hrouda,.1982 . If ferromagnetic minerals, such as accessory

iron oxides, magnetite or hematite, are present inquantities greater than 0.1 vol.% of the total rock,they will dominate the observed magnetic properties

Žbecause of their high magnetic susceptibilities Tar-.ling and Hrouda, 1993 . In the Pomovaara granites,

the amount of magnetite is generally greater than 0.1vol.%, and therefore dominates the magnetic suscep-tibility and in practice, the magnetic anisotropyŽ .AMS of these rocks.

Ž .A high value of AMS degree Psk1rk3 indi-cates an intense shape preferred orientation of ferro-

Ž .magnetic minerals Hrouda, 1982 . The linear anisot-Ž Ž . .ropy, Lsk1rk2 or ls k1yk2 rk , may be used

to describe the intensity of the linear–parallel orien-Žtation, while the planar anisotropy, Fsk2rk3 or

Ž .fs k2yk3rk , characterizes the intensity of theplanar–parallel orientation of ferromagnetic miner-

Ž .als. The direction D1, I1 of maximum susceptibil-Ž .ity k1 determines the linear anisotropy, and the

Ž . Ž .direction D3, I3 of minimum susceptibility k3 isŽ .normal to the magnetic foliation plane Table 1 .

The AMS of magnetite is controlled by grainshape, indicated by the maximum susceptibility par-allel to the long axes of the multidomain grainsŽ .Thompson and Oldfield, 1986 . Magnetic grains in

Ža rock may interact magnetically Tarling and.Hrouda, 1993 , thus, the AMS reflects the arrange-

ment and orientation either of individual magneticgrains or of grain clusters. Interaction between mag-netic grains tends to increase the susceptibility mag-

Žnitude parallel to grain alignment Gregoire et al.,´.1995 . In the Pomovaara granites, clusters of mag-

Ž .netite and magnetiteqbiotite are observed Fig. 5a .

6. Magnetic fabrics in the Pomovaara granites

The AMS measurements of the Pomovaara gran-ites were performed using the Kappabridge KLY-2

Table 1Anisotropy of magnetic susceptibility: means and parameters

Ž .Sample No. N K Sl " K " D1 11 K " D3 13 P " F " L "max min

139-HGW-90 5 0.010 0.004 0.010 0.005 184 37 0.009 0.004 203 7 1.23 0.05 1.13 0.07 1.09 0.04140-HGW-90 5 0.029 0.010 0.031 0.010 245 4 0.026 0.009 300 56 1.19 0.03 1.09 0.02 1.09 0.03141-HGW-90 8 0.026 0.004 0.028 0.005 199 20 0.024 0.004 107 28 1.14 0.03 1.07 0.02 1.06 0.05142-HGW-90 6 0.038 0.005 0.039 0.006 88 40 0.036 0.005 225 18 1.08 0.01 1.04 0.02 1.05 0.01143-HGW-90 10 0.016 0.003 0.018 0.004 12 12 0.014 0.003 152 72 1.25 0.03 1.11 0.01 1.12 0.02144-HGW-90 10 0.015 0.001 0.016 0.002 230 4 0.013 0.001 149 70 1.24 0.02 1.08 0.02 1.15 0.01145-HGW-90 11 0.018 0.002 0.020 0.002 103 3 0.016 0.002 168 69 1.26 0.04 1.07 0.03 1.17 0.02146-HGW-90 5 0.043 0.005 0.049 0.006 55 9 0.038 0.005 298 72 1.28 0.03 1.08 0.01 1.18 0.02154-HGW-90 9 0.011 0.002 0.013 0.002 219 24 0.010 0.001 15 63 1.30 0.03 1.18 0.01 1.04 0.01155-HGW-90 6 0.018 0.004 0.018 0.004 205 25 0.017 0.004 141 10 1.09 0.01 1.05 0.01 1.04 0.01156-HGW-90 5 0.014 0.003 0.015 0.003 202 35 0.013 0.003 154 26 1.12 0.04 1.04 0.02 1.08 0.03157-HGW-90 11 0.015 0.001 0.017 0.001 243 35 0.014 0.001 290 11 1.21 0.02 1.14 0.03 1.07 0.02158-HGW-90 8 0.018 0.003 0.019 0.003 213 21 0.017 0.002 240 18 1.13 0.02 1.06 0.03 1.07 0.03159-HGW-90 7 0.020 0.005 0.021 0.005 183 60 0.018 0.004 211 3 1.18 0.06 1.13 0.03 1.05 0.03160-HGW-90 5 0.017 0.004 0.017 0.004 183 59 0.016 0.004 154 11 1.10 0.03 1.08 0.02 1.03 0.01161-HGW-90 6 0.010 0.002 0.011 0.002 200 76 0.010 0.002 148 9 1.07 0.02 1.03 0.02 1.03 0.01162-HGW-90 11 0.006 0.004 0.007 0.004 240 48 0.006 0.003 82 25 1.20 0.10 1.08 0.07 1.11 0.06163-HGW-90 13 0.009 0.003 0.010 0.003 219 53 0.008 0.002 87 13 1.20 0.16 1.11 0.04 1.09 1.09


Fig. 7. Dependence between magnetic lineation L and foliation Fbased on mean values of sample sites.

Ž .apparatus of AGICO Tcheck Republic working at300 Arm magnetic field and 920 Hz. The programANISO 16 developed by AGICO computes the com-ponents of the susceptibility tensor, principal suscep-tibilities and their directions, and several anisotropyparameters. The results of measurements on 141specimens from 18 sites with two to five cores persite and one to five specimens per core are shown inTable 1.

The anisotropy degree in the Pomovaara granitesvaries between 1.03–1.37. The data show a peakaround the value 1.1, and a great number of sampleshave values between 1.2 and 1.3. The magneticsusceptibility ellipsoids in the Pomovaara granites

Ž . Ž .show both prolate E-1.0 and oblate E)1.0Ž Ž .shapes Esk2=k2r k1=k3 . The shape of the

susceptibility ellipsoids are illustrated by the mag-Ž .netic anisotropy plot Fig. 7 . Seven sites have triax-

ial ellipsoids, which means that the three principalŽaxes have different values. Both linear fabrics pro-

. Ž .late and planar fabrics oblate occur.The orientations of principal susceptibilities are

represented on the lower hemisphere of an equal-areaprojection in Fig. 8 and a detailed examination of theAMS data of the different stocks is illustrated in Fig.9. In the Pomovaara granite stocks, the most com-mon direction of maximum anisotropy axes points to

Ž .the SW Fig. 8a . The minimum susceptibility axesŽ .point either to NW or SE Fig. 8b , which imply

directions normal to the magnetic foliation planes.Ž .The fairly high degree of anisotropy P implies

deformation in the stocks during the emplacementand crystallization of the magmas. In the stereoplots

Ž .of the Northern stock Fig. 9a , there is a dispersionin the orientations of the magnetic axes. However, atevery site, at least one susceptibility axis, maximum

Žor minimum is quite well-defined. At three sites I,.II, III , the minimum directions are quite well-

grouped, and magnetic foliation planes are parallel to

Ž . Ž .Fig. 8. Directions of a maximum and b minimum anisotropy axes presented on the lower hemisphere of an equal-area projection.


Ž . Ž . Ž .Fig. 9. The AMS data presented on the lower hemisphere of an equal-area projection. Maximum B and minimum e susceptibilities. aŽ . Ž .Northern stock. b Central stock. c Southern stock.

the outer contact of the stock. These planes arenearly vertical, indicating a vertical attitude of thecontacts. Close to the margins of the Northern stock,magnetic foliation planes trend also subparallel tothe contacts. In the southern part of the stock, wheredirections of planes are subnormal to the main trendof the Complex, the anisotropy degree is one of the

Ž . Žlowest Ps1.08"0.01 in the whole Complex Ta-.ble 1 .

Ž .In the Central stock Fig. 9b , there is a change inthe principal susceptibility directions in the por-

Ž .phyritic granite. At two sites I, II quite close to themargins, the magnetic foliation planes follow thedirection of the outer contact. The foliation plane is


Ž .vertical in the east II and dips somewhat outwardsŽ .in northwest I . The dominant direction of magnetic

lineation points to the southwest in this stock.In the two even-grained granites of the Central

stock, the AMS directions are tightly grouped. In thebiotite granite, the fabrics are fairly strong and pla-nar–parallel. The foliation plane is nearly vertical,dipping somewhat to the southeast. In the biotite–muscovite granite, close to the center of the Central

Žstock, the magnetic foliation plane dips gently about.308 to the southwest. Here the fabric is strongly

developed and planar–parallel, too, and might repre-sent a roof-parallel fabric. This suggests that thepresent level of exposure is close to the uppermostpart of this granite. The orientation of magneticfoliation planes in two measured sites of the granitephases in the center of the Central stock differ fromthe orientations in the porphyritic granite. At least,directions of magnetic lineations point to the south-west in these granites, too.

Ž .The four sites of the Southern stock Fig. 9c havewell-aligned fabrics that are strongly developed andlinear–parallel. The magnetic foliation planes arenearly horizontal. In addition to this, moderate defor-mation of the rock, with fine-grained recrystalliza-tion of quartz and bending of K-feldspar phenocrystsŽ .Fig. 5b , suggests that the granite material wascompressed or sheared against the roof above thestock. This indicates that the original top part of thisstock is now exposed. Almost horizontal foliationplanes near the margins imply gently outward slop-ing outer contacts of the stock.

7. Discussion

Granites usually ascend as partial melts and thedegree of magma viscosity is important in control-ling internal processes, such as flow, heat transferand settling of crystals. The preferred mineral orien-tations are likely to be attributable to magmatic flowas well and the strong preferred orientation resultsfrom deformation during magma crystallizationŽ .Pitcher, 1979 . The orientation of magnetic anisot-ropy in granites indicates the direction of magmaticflow or an alignment of magnetic minerals havingdeveloped because of deformation during crystalliza-tion. The long cooling time of a granite massif

allows substantial re-equilibration between titano-Žmagnetite and the groundmass Frost and Lindsley,

.1991 . In the Pomovaara granites, the presence ofnearly pure magnetite and a late sphene phase sup-port extensive re-equilibration and reorganization ofFe-ions between Fe-silicate and Fe-oxide phases dur-ing crystallization. Because the kinematic memory ofa crystalline fabric appears to be quite shortŽ .Bouchez, 1997 , it is suggested that the magneticfabric in the Pomovaara granites mostly records strainrelated to the local stress field pertaining during thefinal stage of magma emplacement and during thecrystallization of the granite magma.

The tectonic control is proposed to the ascent ofthe Pomovaara Granite Complex. Geophysical evi-dence supporting the uplift of the structural crustalblock which contains five of the Nattanen-type gran-ite intrusions, are well in harmony with geologicalinformation. The brittle character of the upliftedcrustal block is indicated by weakness zones parallelto the major NE–SW trending faults, and intrusionof the Laanila dyke swarm reactivating the same

Ž .fault system at 1.0 Ga Mertanen et al., 1995 .Shallow level of emplacement of the Pomovaaragranites is strengthened by the abundant miaroliticcavities and relatively high whole rock ferricrfer-

Ž .rous ratio Wennerstrom, 1993 . The direct contact¨of the Pomovaara stocks with the Archaean granodi-oritic basement dome, which is now at the exposurelevel, suggests that the uplift happened before thegranite magmatism took place. Decrease in the litho-spheric pressure after the uplift of crustal blockcaused melting in the Archaean basement and forma-tion of granitic magma. The deep faults parallel tothe trend of the Complex have served as channels forthe upwelling. The dominant SW-trend of magneticlineation suggests that this was the direction of theoriginal magma flow. During the crystallization ofthe Pomovaara intrusions, compression normal to theorientation of the deep faults affected the magneticfabric by enhancing the trend of the dominant NE–SW lineation orientation. No evidence of shearingalong these faults has been observed. The minimummagnetic axes, which point to southeast or northwestand the slightly elongated shapes of the stocks alonga southwest–northeast trend indicate compressionnormal to this trend. It is suggested that this com-pression is related to the closure of the faults that


controlled the ascent of granite magmas. Gravity andaeromagnetic modeling and AMS results concerningthe structures of the stocks correspond to each other.

The magnetic fabric in the Central Stock impliesthe existence of three distinct magma pulses duringthe emplacement. The three granite phases differfrom each other in terms of rock type, magnetitecontent and the orientation of magnetic foliationplanes. The present outcrop level of the biotite–muscovite granite in the center of this stock corre-sponds to the original top of the magma pulse. In theSouthern stock, all the magnetic fabric elements arewell-defined, showing subhorizontal magnetic folia-tion planes, which also indicate the original top ofthe stock.

8. Conclusions

The emplacement of the Pomovaara Granite Com-plex was studied by using aeromagnetic, gravity,anisotropy of magnetic susceptibility and geologicaldata. According to geophysical evidence, the Po-movaara Granite Complex is situated at a transitionzone of the main fault systems in northern Finland.NW–SE directed continental rifting between 2.4–2.1

ŽGa fragmented the Archaean crust Sorjonen-Ward,.1997 , and produced deep NE–SW trending trans-

form faults. After that, the continental convergenceŽ .from NE towards SW Berthelsen and Marker, 1986

caused overthrusting of the Lapland Granulite Belttowards the SW. This resulted in crustal thickeningand lateral block movement along the NW–SE trend-ing weakness zones related to the earlier rifting. Alinear change in both the magnetic and gravity fieldintensities across the deep NE–SW trending faults isinterpreted to be indicative of a vertical block move-

Ž .ment uplift of the crustal segment which containsthe Pomovaara Complex and which is outlined bythese faults.

The direct contact of the Pomovaara stocks withthe Archaean dome at the present exposure level, andthe epizonal character of the granites, support thatthe uplift took place before the granite magmatism.The appearance of the Pomovaara granite intrusionsin an array along the NE–SW trending deep faultsuggests a tectonic control of this fault to the ascent

and emplacement of the Complex. The compressionduring the emplacement of the intrusions was per-pendicular to the fault, and the direction of magmaupwelling parallels the fault trend according to theresults of AMS studies. The three Pomovaara graniteintrusions are clearly discordant to the regional de-formational features, which were formed during the1.9 Ga collisional events in northern Fennoscandia.Thus the emplacement of these granites is regardedas post-collisional, however, originally linked to thefault movements and the uplift.

Acknowledgements

The Geological Survey of Finland provided uswith the opportunity for field work in the Finnishtundra and for analysing the data. We thank ourcolleagues in the survey: Risto Puranen, for hisvaluable opinions concerning the manuscript, BoJohansson who made the microprobe analyses, AinoLehti and Anneli Lindh who participated in compil-ing the figures, and Peter Sorjonen-Ward, for check-ing the English. Especially we want to thank ourreviewers, Dr. J.L. Bouchez and Dr. K. McKaffrey,for their valuable advice for correcting themanuscript, and their comments, which will be use-ful also in our future literary tasks.

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