Magnetic and compositional variations inProterozoic mafic dykes in Finland, northernFennoscandian shield

Meri-Liisa Airo

Abstract: Proterozoic mafic dykes of both tholeiitic basaltic and more magnesian compositions from four areas inFinland have been studied for their magnetic and compositional variations to explain the variation in their aeromagneticpatterns. The highly magnetic more magnesian dykes are characterized with distinctive aeromagnetic anomalies,whereas the tholeiitic dykes are weakly magnetic and associated with faint magnetic signatures. To evaluate the factorsthat control the primary magnetization and the effects of later geological processes on their ferrimagnetic mineralogy,petrophysical properties of the dykes were correlated with their bulk mineral compositions. The role of ferric iron wasof particular interest, as those iron-bearing silicates which participate in the reactions where titanomagnetites areformed or destroyed contain ferric iron in their crystal structures. The early crystallization of hydrous mafic silicates,biotite and amphibole, versus the anhydrous mafic silicate, clinopyroxene (augite), seems to be of key importance toformation of titanomagnetites. As clinopyroxenes can accommodate only minor amounts of ferric iron compared tohydrous mafic silicates, the excess ferric iron can be consumed to form primary titanomagnetites. Therefore it isconcluded that the water vapour pressure at the emplacement depth of the dykes is most relevant in terms of theprimary ferrimagnetic mineralogy. The originally high total magnetization of rocks that already contain abundantferrimagnetic minerals will not be greatly changed in secondary processes, as the volume percentage of the secondaryfine-grained magnetite is generally small. In addition, the total magnetization of the rocks that are originally deficientin titanomagnetites does not increase significantly.

Résumé : Les dykes mafiques protérozoïques de quatre régions en Finlande, avec des compositions basaltiquestholéiitiques ou plus magnésiennes, ont été étudiés pour déterminer leurs variations des propriétés magnétiques et descompositions chimiques, afin d’expliquer les changements observés de leurs anomalies aéromagnétiques. Les dykesfortement magnétiques plus riches en magnésium sont caractérisés par des anomalies aéromagnétiques distinctes, tandisque les dykes tholéiitiques ne sont que faiblement magnétiques et associés à de faibles signatures magnétiques. Afin depouvoir évaluer les facteurs qui contrôlaient l’acquisition de l’aimantation primaire, ainsi que les effets des processusgéologiques plus tardifs sur leur minéralogie ferrimagnétique, on a mis en corrélation les propriétés pétrophysiques desdykes avec leurs compositions minéralogiques globales. Le rôle du fer ferrique présente un intérêt particulier, car lessilicates ferrifères, qui ont participé dans les réactions produisant ou détruisant les titanomagnétites, contiennent du ferferrique dans leurs structures cristallines. La cristallisation précoce des silicates mafiques hydroxylés, biotite etamphibole, versus le silicate mafique anhydre,clinopyroxène (augite), apparaît comme un facteur déterminant de laformation des titanomagnétites. Vu que les clinopyroxènes ne peuvent incorporer que de petites quantités de ferferrique comparativement aux silicates mafiques hydroxylés, le surplus de fer ferrique pouvait alors cristalliser sousforme de titanomagnétite primaire. Par conséquent, nous concluons que la pression de vapeur d’eau à la profondeur demise en place des dykes était en rapport direct avec la minéralogie ferrimagnétique primaire. La forte aimantationtotale initiale des roches contenant déjà une proportion abondante de minéraux ferrimagnétiques, ne sera pasgrandement affectée par les processus secondaires, en raison généralement du faible pourcentage par volume de lamagnétite secondaire à grain fin. À cet égard, l’aimantation totale des roches initialement déficientes entitanomagnétites n’a pas été intensifiée significativement.

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Introduction

High-resolution aeromagnetic data have been available inFinland since the early 1970s. It has been produced as part

of the aerogeophysical mapping programme conducted bythe Geological Survey of Finland (GTK). This programmewas at first directed to exploration for base-metal deposits(Peltoniemi 1982) but was later expanded to cover the wholecountry. At present these data, measured with a terrain clear-ance of 30–40 m and with a 200 m line spacing, cover morethan 75% of the area of Finland. The magnetic surveys ini-tially included the total field intensity, and, from 1975, itshorizontal gradient (Korhonen 1984). The two wing-tipmagnetometer sensors and the observation density of four

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Received April 1, 1998. Accepted January 21, 1999.

M-L. Airo. Geological Survey of Finland (GTK),P.O. Box 96 (Betonimiehenkuja 4), FIN-02151 Espoo,Finland (e-mail: meri-liisa.airo@gsf.fi).

times per second (sampling interval of ~12 m) make thesedata suitable when interpreting even small-scale geologicalfeatures. Due to the wide coverage of high-quality surveys,large-scale geological structures can also be mapped. Thedata are used routinely in systematic bedrock mapping.

The acquisition, processing, and publication of high-quality aeromagnetic data are relatively routine in manyparts of the world, but techniques for the geological interpre-tation of these data are still being developed to benefit fromthis material as much as possible. Fast and reliable geologi-cal interpretation of airborne geophysical surveys requiresknowledge of the real rock petrophysical parameters for thesource rocks. Typically, a wide variation in magnetic suscep-tibilities can be found within an individual rock type. Ac-cording to field experience, it is common for a singlelithological type to be represented by both weakly andstrongly magnetic variants even in the same outcrop. Onaeromagnetic maps, these local variations are smoothed outand hence estimations of the average properties and theirgeneral statistical behaviour must be based on a large num-ber of samples.

The Geophysics Department at GTK has conducted a re-gional petrophysical mapping programme (Korhonen et al.1997) covering all of Finland. The extensive collection ofpetrophysical data will provide a broad constraint on localvariations and can be used as a standard in different kinds ofclassifications as well as in lithological and areal compari-sons (e.g., M. Puranen et al. 1968; R. Puranen et al. 1978).Measurements have been incorporated into a regionalpetrophysical database, currently comprising measurementsof more than 130 000 samples. The petrophysical parametersmeasured are bulk density, magnetic volume susceptibility,and intensity of remanent magnetization. Coordinates ofsampling sites and the lithological name of every sample arealso included. Other Scandinavian countries also have expe-rience in using systematic petrophysical data in geologicalinterpretation. These studies have resulted in summaries ofthe overall statistical behaviour of petrophysical parametersin the Fennoscandian shield (Henkel 1991). Similar resultshave been presented in Australia by Clark and Emerson(1991) and Clark (1997).

The aim of this article is to illustrate some practical expe-rience gained at the GTK in the geological interpretation ofthe aeromagnetic data combined with regional petrophysicsand studies of magnetic petrology. The Proterozoic maficdyke swarms represent good reference material for petro-physical analysis because their mineral compositions arecommonly well-known and the petrophysical properties androck magnetic data on dykes are available, as are com-positional and chemical information and age data (e.g.,Vuollo et al. 1992; Nykänen et al. 1994). The dykes repre-sent different age groups, and they are commonly used inpaleomagnetic studies (Mertanen 1995; Neuvonen et al.1997). As in other continents, Proterozoic mafic dyke mag-matism was intense in the Fennoscandian shield. Becauseseveral factors may affect the magnetic mineralogy duringthe geological history of rock, analysis of magnetic proper-ties is easier and less ambiguous for rocks of standard com-positions and simple deformation history. The dykes includeboth unmetamorphosed variants and those which have meta-

morphosed together with their country rocks. Often thedykes are related to the last tectonic event and cut all theother lithological units, and their relationship to other mag-netic patterns that they cut can be used to infer the tectonichistory of their environment.

General petrophysical properties for maficdyke rocks

The regional petrophysical database maintained by theGTK contains more than 4000 mafic dyke rock samples,classified into quite broad lithological groups: diabases–dolerites (about 3000 samples), metadiabases (about 1000samples) which cover different metamorphosed dyke compo-sitions but are mainly tholeiitic metabasalts, and moremagnesian dyke rocks (about 300 samples) under the namesolivine diabases, pyroxene diabases, or albite diabases, thelast ones often corresponding to differentiated sills. The uni-modal density histograms for mafic dyke rocks (Fig. 1) re-flect their overall (olivine–) clinopyroxene–plagioclasecompositions, with mean densities of about 3000 kg/m3. Forcomparison the density histogram for gabbros is very simi-lar, reflecting the same relative content of the high-densitymafic silicates. Bimodal distributions of magnetic suscepti-bilities are typical of almost all Precambrian rock types ofmafic composition (Puranen 1989; Henkel 1991), in contrastto recent mafic igneous rocks, for example those in Iceland(Airo 1990). The magnetic dualism of mafic dyke swarmswas discussed by Korhonen (1987). The lower susceptibilitymode at about 0.001 (SI) is determined by the paramagnet-ism of mafic silicates in the rock, and through densities theparamagnetic susceptibilities are related to the mineralogicaland chemical compositions of rocks (Puranen 1989). Fig-ure 1 shows that about half of the dolerite and gabbro sam-ples have susceptibilities less than 0.002 SI. The highersusceptibilities arise from the presence of ferrimagnetic min-erals, and mainly range between 0.01 and 0.1 SI. TheKönigsberger ratios (Q values) reflect the relative impor-tance of remanent magnetization and are also indicative ofthe amount of magnetic minerals, their composition, andgrain sizes. The dolerites include more fine-grained variantsthan do the gabbros, which can presumably be related totheir faster cooling rates. The high Q values for gabbros of-ten indicate lamellar magnetite–ilmenite intergrowths, whichtypically form during deuteric cooling. Despite the magneticdualism in general, experimental studies of the petrophysicsof plutonic rocks have shown that specific intrusions can berecognized on the basis of their distinctive and relativelyuniform densities and magnetizations. Based on mineralogi-cal and magnetic comparisons, this applies particularly tomafic and ultramafic dykes and intrusions with simple defor-mation histories, and to granitoids from different tectonicsettings.

The dominant ferrimagnetic mineral in mafic dyke rocksand gabbros is magnetite, as evidenced by their high suscep-tibilities but low Q values, mainly below 5. In the scatter di-agrams of susceptibility versus Q value, the magnetite-bearing dolerites and rocks containing monoclinic pyrrhotiteplot in almost mutually exclusive fields (Fig. 2). Thepyrrhotite-bearing black schists in Finland (Paleoproterozoic

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metamorphosed black shales) have Q values > 10 and sus-ceptibilities an order of magnitude less than those of themagnetite-bearing dolerites (e.g., Airo 1997). The Precam-brian banded iron ore samples in the regional petrophysicaldatabase have Q values comparable to those of black schists.Similar magnetic properties for magnetite- and pyrrhotite-bearing rocks have been presented by Henkel (1994) andClark (1997), and it seems clear that the predominating mag-netic mineral can be estimated on the basis of distributionsof magnetic susceptibilities and Q values.

The effect of monoclinic pyrrhotite on the magnetic prop-erties of mafic dyke rocks is negligible compared to that ofmagnetite, since pyrrhotite is typically only present in smallamounts. The effect of larger amounts of pyrrhotite on themagnetic anomalies of different rock types is local and re-sults in distinctive magnetic anomaly patterns. Such anoma-lies in the Precambrian are mainly related to black schists orhighly metamorphic intermediate gneisses (Henkel 1991,1994; Airo and Loukola-Ruskeeniemi 1991; Airo 1995,1997). In the same way as pyrrhotite, hematite has a minor,predominantly local role as a source of magnetic anomalies.This is because the exposed rocks we now see were formedat mid-crustal or still deeper levels, where conditions weremore favourable for the formation of magnetite than hema-tite. It has been observed that the replacement of magnetiteby hematite (martitization) in some granitoids has led to adecrease in the overall magnetic susceptibility for the graniteintrusion.

Investigations of the petrophysical distributions of variouslithologies in the Finnish petrophysical database have clearly

shown that the magnetic properties of Precambrian rocks aremainly controlled by the abundance, composition, and tex-ture of magnetite. These are generally studied by applyingmethods of magnetic petrology to petrophysical and mag-netic studies. Based on experimental studies at GTK, mag-netic petrology can in many instances be estimated by usingstatistical analysis of large numbers of petrophysical sam-ples, at least in regional-scale investigations (e.g., Airo1999).

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Fig. 1. Frequency and cumulative histograms of petrophysical properties for Paleoproterozoic mafic dyke rocks (diabases–dolerites) andgabbroic rocks (data from petrophysical database, GTK). Q value (Königsberger’s ratio) is the ratio of the remanent magnetization tothe induced magnetization. N, number of samples.

Fig. 2. Magnetic properties of typical magnetite-bearing (circles)and pyrrhotite-bearing (triangles) Precambrian rocks in thepetrophysical database, GTK (based on Airo 1997). Black schistsdenote Early Proterozoic metamorphosed black shales.

Optically determined textural features of magnetite grainswere correlated with magnetic properties to interpret the re-lation of magnetic grain size with Q values. In summary, Qvalues give a good estimate of the predominant magneticgrain size but are affected by the shapes of magnetic grains.The studies at GTK included samples from Paleoproterozoictholeiitic metalavas, metamorphosed to greenschist–amphibolite facies, and from mafic to ultramafic igneousrocks in northern Finland (Airo 1993, 1995). Titano-magnetite compositions, as determined by microprobe andCurie-point measurements, were of pure titanium-free mag-netite in all the samples. The effective magnetic grain sizeswere determined by hysteresis measurements. Remanentmagnetization and Q values > 4 predominated in sampleswhere magnetization was carried by magnetite grains withthe pseudo-single-domain (PSD) behaviour. Unexpectedly,quite high remanences and Q values of 3–4 also were associ-ated with larger magnetite grains exhibiting the multi-domain (MD) behaviour. Such grains had broken edges andirregular shapes due to deformation. Those samples contain-ing euhedral magnetite grains with sharp edges were domi-nated by induced magnetization and Q values < 2. Single-domain (SD) magnetite was not found.

It can be generalized that the magnetization is carried bycoarse MD magnetite in the Finnish Precambrian rocks withQ values less than 2 (also Korhonen et al. 1997). Higher Qvalues between 2 and 10 are associated with either a finergrain size (PSD) or irregularly shaped magnetite grains. Thehighest Q values (10–100) are indicative of ore mineral-

izations or a relationship with rocks containing monoclinicpyrrhotite.

Tholeiitic and more magnesian dykeswams

Despite the sometimes poor exposure, the extent of Pro-terozoic dyke swarms in Finland is well known due to thehigh-resolution aeromagnetic surveys. On high-resolutionaeromagnetic maps, the Proterozoic mafic dykes are associ-ated with often distinctive, characteristic magnetic anomalyamplitudes and patterns. Differences in the proportions andmorphology of magnetite affect these anomalies. The major-ity of Paleoproterozoic dyke swarms in Finland are doleritic,and they are commonly associated with weak magneticanomalies. The more magnesian (noritic) and picritic dykesare typically highly magnetic, and their anomaly patterns of-ten reflect their intrusive mechanism. Because the essentialcompositional differences in terms of rock magnetization inthe first place seem to be the Mg content and the presence ofclinopyroxene (augite), the dykes are subdivided here intotholeiitic and more magnesian. Four groups of mafic dykes,representing various ages and metamorphic grades, were se-lected to study the factors that most strongly influence theirmagnetizations and anomaly patterns. The locations of theaeromagnetic images chosen to illustrate various types ofdykes and their typical magnetic signatures are shown inFig. 3. Their petrophysical properties are compared inFig. 4, where the dykes are divided into three main magneticgroups in terms of content and grain size of magnetic miner-als.

Kuhmo tholeiitic dyke swarmsThe Paleoproterozoic (2.2–1.97 Ga) mafic dykes in the

Kuhmo region, eastern Finland, are tholeiites and Fe-tholeiites in composition, intruded into the rifted Kareliancraton (Nykänen et al. 1994; Vuollo 1994). They were sub-sequently metamorphosed under greenschist- to amphibolite-facies conditions together with their Archean granitoid–gneiss country rocks most likely during the 1.9–1.8 GaSvecofennian orogeny. With image-processing techniquesthe enhanced dyke anomalies (Fig. 5) can be seen to followat least three different fault directions, around a northwest–southeast modal trend. What we now are looking at is a“tilted projection” of a fragmented crustal segment. At par-ticular depths where fault orientations were favourable, theintrusion of dyke magma may have followed the joint struc-tures both laterally and vertically depending on the densityof magma compared with the mean density of the surround-ing crustal segments. Therefore, the three dyke anomaly setswith different trends do not necessarily represent differentages or variations in degree of differentiation or differentmagma pulses.

The weak magnetic anomalies correlate well with thepetrophysical properties of Kuhmo tholeiitic dykes. InFig. 4. most of the Kuhmo samples plot within the low mag-netic susceptibility field, having values <0.002 (SI), indicat-ing a deficiency of ferrimagnetic minerals.

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Fig. 3. Generalized geological map of the Fennoscandian shield(based on Gaál and Gorbatschev 1987). Locations of theaeromagnetic maps representing various groups of mafic dykeswarms: 1, Kuhmo; 2, Häme; 3, Laanila; 4, Hyrynsalmi.

Häme olivine dolerite dyke swarmThe Subjotnian (mid-Proterozoic) Häme mafic dyke

swarm in southern Finland is composed of unmeta-morphosed olivine diabase dykes (Laitakari 1987). Thetholeiitic magma of Häme dyke swarm has a plagioclase–olivine–clinopyroxene–orthopyroxene composition (with mi-nor biotite and K-feldspar). Amygdaloidal structures in someof the Häme dykes indicate crystallization near the surface.The known length of the swarm is about 300 km, the widthof individual dykes ranging from a couple of centimetres toapproximately 1 km. The dykes contain primarily coarse-grained magnetite and produce two crosscutting sets ofstrong magnetic anomalies that transect all other magneticpatterns (Fig. 6). The MD behaviour of the coarse-grainedmagnetite is demonstrated in Fig. 4b by the low Q values ofabout unity. This means that induced magnetization predom-inates and the remanence directions are mainly parallel to

the present Earth’s field (PEF). The relative proportion ofplagioclase differs slightly between the differently strikingHäme dyke sets. This indicates a difference in their degreeof differentiation or in the water content of magma and is re-flected in somewhat lower densities for the more evolvedset.

Laanila serpentinite diabaseIn northern Finland the Laanila diabase dyke, 998 Ma in

age (Mertanen et al. 1996), cuts discordantly across thegranite and granulite gneiss complexes. It can be traced forabout 100 km and has a maximum width of 150–200 m. AtLaanila the outcropping dyke is 20 m wide. According toPihlaja (1987) the mineral composition is plagioclase–clinopyroxene–serpentine (olivine having been completelyserpentinized), and it is unmetamorphosed. On the AFM dia-gram it plots within the tholeiitic field and represents a con-

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Fig. 4. Petrophysical properties of Proterozoic mafic dykes in Finland: (a) density vs. susceptibility, and (b) Q value versussusceptibility. abdb, albite diabases (differentiated mafic dykes).

tinental, within-plate basalt. The existence of primaryplagioclase refers to conditions of low water pressures, forexample crystallization either at a high crustal level or underextensional tectonics. These conditions have also favouredthe formation of primary magnetite.

The Laanila dyke was emplaced into a major fracturezone. It is strongly magnetic, and the negative inclination ofthe magnetization vector results in a negative magneticanomaly (Fig. 7). The aeromagnetic maps show other paral-lel deep fractures at 10–15 km distance on both sides ofLaanila dyke. The fracture to the east of Laanila dyke wasintruded by similar magma and has very similar magneticproperties, but the magnetic anomaly is weakly positive(Eastern dyke in Fig. 7). Although the primary magnetiza-tion in both dykes was thermoremanent with a negative in-clination (Mertanen et al. 1995), the large contribution of theviscous component in the direction of the PEF due to acoarser grain size results in a positive magnetic anomaly forthe Eastern dyke. The major difference between the dykes isin the high Q values of the Laanila dyke (Table 1), indicat-ing a finer magnetic grain size, which makes it a more effec-tive carrier of the primary thermoremanent magnetization(TRM). The PSD grains of Ti-poor magnetite were also veri-

fied by hysteresis measurements (Mertanen et al. 1996). Thegrain-size difference has been interpreted to indicate that theoutcropping dykes represent cooling at different crustal lev-els, followed by differential vertical block movements anduplift. Possibly the more severely serpentinized upper part ofLaanila dyke was eroded, although it still remains in theeastern dyke. The vertical movement of crustal segments hasbeen evidenced by geophysical modelling of potential fields.

MgO-bearing gabbro–wehrlites in HyrynsalmiMetamorphosed 2.2–2.1 Ga mafic dykes and sills are very

common in the Paleoproterozoic Kainuu schist belt and theadjacent Karelian craton in eastern Finland. The Archeanbasement areas east of the Kainuu schist belt are cut by nu-merous tholeiitic metadiabase dykes (see Fig. 8) withplagioclase and uralitic amphibole, metamorphosed underamphibolite-facies conditions. Primary compositions of thestrongly differentiated dykes and sills within the schist belt(gbw in Fig. 8) range from wehrlite to gabbroic and quartz-plagioclase-rich rocks (Kontinen 1987). Similar distinctiveirregular aeromagnetic patterns as associated with gabbrowehrlite sills in Fig. 8 can be followed along the easternmargin of the schist belt for several hundred kilometres.

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Fig. 5. Gray-scale aeromagnetic map (GTK) illustrating the Kuhmo mafic dykes in eastern Finland (all the narrow, linear northwest–southeast-trending features cutting other formations; the weakly magnetic basement rocks are Archean granitoid gneisses). Darkanomalies are positive.

There is a striking uniformity of very high magnetic suscep-tibilities (>0.1 SI) and Q values between 0.1 and 1 (Fig. 4b),indicating the presence of coarse-grained magnetite in thesesills. The high MgO content of magma results in liquidswith higher temperature and lower viscosity than dolerites(Cadman et al. 1990), making the intruding of magma be-tween the metasedimentary layers easy. In Labrador dykesuites these factors also resulted in a coarse magnetite grainsize. The mean density of such a magma largely correspondsto the mean densities of sedimentary rocks, thereby resultingin lateral magma movement between the sedimentary layers.The magnetic signature is influenced by late intrusion of thehigh-MgO magma, when the enclosing metasediments wereunconsolidated.

Relation of ferric iron to the magneticproperties of dykes

Ferric iron in mafic silicatesMagnetic properties of rocks are largely defined by the re-

actions between ferric iron in mafic silicates and iron oxides(Grant 1985; Frost 1991a, 1991b). The role of ferric iron ingeological processes is poorly understood because ferric ironcannot be routinely analyzed by the electron microprobe.The oxide–silicate reequilibration process involves the ex-change of ferrous iron, and to a lesser extent ferric iron andtitanium, between the oxides and coexisting silicates (Frost

1991b). The incorporation of ferric iron in either spinel ornon-spinel phases in the rock primarily controls the forma-tion of titanomagnetites. The cation (the iron) will be prefer-entially incorporated into the spinel structure (O’Reilly1984), but any excess of ferric iron over that required toform magnetite will crystallize in non-spinel phases. Particu-larly those iron-bearing silicates, which contain ferric iron intheir crystal lattice, participate in the reactions where mag-netite is either produced or destroyed. The active role of ironions is based on their transition element cation nature and at-tributed here to the monoclinic crystal structure, where Fe3+

occupies a specific location at the outer edge of crystal lat-tice, in the same way as the tetrahedral sites in spinel struc-tures.

According to the above comparisons of magnetic anomalypatterns and intensities, and correlations of petrography withthe magnetic properties of dykes, intense magnetizationseems mainly related to those rocks which contain clino-pyroxene (augite). Both the unmetamorphosed and metamor-phosed highly magnetic dykes are characterized by thepresence of clinopyroxene, either primarily or as a result ofmetamorphic reactions. The early formation of clino-pyroxenes relates to MgO content of mafic magma and de-pends on the ambient redox conditions. As clinopyroxenescan accommodate only a minor amount of ferric iron, the ex-cess ferric iron can be consumed in formation oftitanomagnetites. Primary biotite and amphibole, formed un-

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Fig. 6. Gray-scale aeromagnetic map (GTK) showing the Häme dyke swarm. Dark anomalies are positive. The Early Proterozoicmigmatitic pyrrhotite-bearing gneisses are traversed by the northwest–southeast-striking linear dyke anomalies.

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Fig. 7. Gray-scale aeromagnetic map (GTK). Dark anomalies are positive. Laanila dyke and the related deep fracture are shown by thenegative anomaly trending southwest from the top of the map and cutting the highly magnetic beds containing mafic granulite gneisses.The rounded positive anomalies (10–15 km wide) in the southwestern corner are related to Early Proterozoic porphyritic granitoidintrusions.

Laanila dyke Eastern dyke

Density D (kg/m3) 2954 2920Magnetic susceptibility K (SI) 0.03 0.04Natural remanent magnetization

NRM (A/m)3–11 3–5

Declination D (°) 350 1Inclination I (°) –30 +30Königsberger ratio Q 9 2.5Grain size Single to pseudo-single domain Multi-domain

Notes: Data from Mertanen et al. (1996).

Table 1. The magnetic properties of Laanila dyke and the parallel trending dyke to the east.

der high water vapour pressures, incorporate greateramounts of ferric iron than clinopyroxene, with the resultthat magnetite crystallization is prevented. In conclusion, ahigh primary magnetite content is expected from the highMgO content, as is the case for the gabbro–wehrlite dykes inHyrynsalmi and for ultramafic compositions in general.

In basaltic rocks, the formation of magnetite is dependenton the differentiation path and stage. The more primitiveMgO-rich basalts tend to become more magnetic than themore evolved, SiO2-rich basalts, as their composition fa-vours the early formation of magnetic spinel phases. A smalldifference in the differentiation path may be critical for theformation of magnetic phases. In the more Fe-rich basalts,the oxidizing conditions at the level of emplacement influ-ence the incorporation of ferric iron either in oxide or in sili-cate phases. The oxidizing conditions also control thedistribution of magnetite. The oxidation state does not nec-essarily change in these reactions. At lower crustal levelswhere the oxidation state is constant, the partial oxygenpressure decreases during crystallization. Under high watervapour pressures ferric iron is incorporated to biotite andhornblende at the expense of magnetite formation. At highercrustal levels under more oxidizing conditions and at lower

water pressures plagioclase is formed along with clino-pyroxene, allowing more abundant formation of magnetite.Primitive tholeiitic magmas (e.g., Kuhmo dyke swarm) areinitially in a reduced state, as they derive from partial melt-ing of reduced mantle material. In a reduced magma undernon-oxidizing mid-crustal conditions, Fe2+ is probably notoxidized to Fe3+ in the amounts needed to form abundantprimary titanomagnetites. This means that the ferrous ironand the available ferric iron are consumed for hydrous ironbearing silicates which form under conditions of moderatelyhigh water pressures (Cadman et al. 1990). The Kuhmotholeiites contain primary biotite and amphiboles, and it isproposed that the dyke magma was primarily weakly mag-netic. The titanomagnetites in the Häme dyke swarm or theLaanila dykes must inherit from the primary cooling of theolivine dolerite magma, as metamorphism has not affectedthem. The extensional tectonic setting presumably offeredoxidizing conditions, and the Mg content resulted in forma-tion of primary clinopyroxene instead of hydrous iron bear-ing silicates. Near-surface crystallization of the Häme dykeswarm would suggest fast cooling, which is at variance withthe coarse overall grain size. In comparison, the coarse grainsize in the quartz-tholeiitic dykes of Labrador is attributed to

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Fig. 8. Gray-scale aeromagnetic map (GTK), Hyrynsalmi. Dark anomalies are positive. The Archean basement is cut by bandedanomalies due to weakly magnetic metadiabase dykes. The gabbro–wehrlite sills (gbw) appear as strong anomalies at the easternmargin and within the Early Proterozoic schist belt.

the high water content of dyke magma (Cadman et al. 1990).The emplacement at higher crustal levels under conditionsof low water activity resulted in plagioclase crystallizationalong with titanomagnetite and clinopyroxene, with no pri-mary hornblende or biotite. The Labrador dyke suites con-tain abundant plagioclase phenocrysts. At lower crustallevels plagioclase was not formed.

Ferric iron in magnetiteThe determination of the ratio of ferric to ferrous iron is

commonly based on estimates of their abundances calculatedfrom the results of whole rock silicate analyses. Althoughgenerally regarded as a controlling factor of the oxidationstate of rock, the ratio of ferric to ferrous iron rather is a re-sult of oxidation conditions and therefore can be regarded asa measure of this ratio. Furthermore, this ratio does not rep-resent the ferric–ferrous iron ratio in magnetite, which af-fects its internal magnetic properties. The subsolidusreactions that affect the oxides during cooling of plutonicrocks also concern the factors that control magnetite compo-sition (Frost 1991b). The ferric–ferrous iron ratio in magne-tite grains in metabasalts from Finnish Lapland was studiedby Mössbauer spectroscopy using facilities at the HelsinkiUniversity of Technology, the aim being to explain thehigher remanences observed in association with the brokenmagnetite grains having irregular shapes. Pure magnetitetypically has a weak natural remanent magnetization (NRM)when Fe3+:Fe2+ = 2:1. An increase in the relative proportionof Fe2+ means that there are more extra electrons and theNRM will grow, as ferrimagnetism is associated with theferrous iron (O’Reilly 1984; Butler 1992). In euhedral mag-netite grains the ferric–ferrous iron ratio was 2:1 as ex-pected. In those samples containing broken magnetite grainsand exhibiting a higher remanence in addition to the high in-duced magnetization, this ratio deviated from 2:1 and therelative proportion of ferrous iron was higher.

Ferric iron in alteration and metamorphic processesDuring prograde metamorphic processes secondary mag-

netite is commonly formed in mafic rocks during decompo-sition of mafic silicates, with an increasing rate until upperamphibolite and granulite facies (Grant 1985). Secondarymagnetite production requires the presence of those maficsilicates which contain ferric iron: biotite, amphiboles, py-roxenes (especially clinopyroxene). Both in prograde andretrograde metamorphism, the reactions between Fe oxidesand Fe3+ in the silicates may be the most important oxide–silicate equilibria in mafic rocks (Frost 1991b). There areseveral indications that the Fe3+ bound in silicates is releasedas magnetite during the transition from amphibolite togranulite facies (e.g., Olesen et al. 1991).

Regional prograde metamorphism at mid-crustal levels isattributed to a moderately constant oxidation state. Duringthe silicate dehydration reactions, magnetite grains growalong ferric iron bearing mafic silicate grain boundaries andminor magnetite inclusions are precipitated within the sili-cate crystal lattices. Under more oxidizing conditions athigher crustal levels ferric and ferrous iron may be availablein suitable proportions, and the amount of magnetite maygrow through an increase in magnetite grain size, since mag-

netite has a strong tendency to form euhedral crystals. Suchcrystals are dominated by induced magnetization and exhibitthe MD behaviour. However, originally weakly magneticrocks are never able to produce magnetite in concentrationsneeded to substantially change the early magnetic properties.Greenschist–amphibolite-facies metamorphism of maficrocks originally poor in magnetite never produces secondarymagnetite in sufficient amounts as to significantly changerock magnetization, as indicated by the weak magnetizationof Kuhmo dykes. On the other hand, if the primary magneti-zation is strong, the effect of prograde metamorphism is tofurther strengthen the magnetic properties, especially theremanence, by the formation of fine-grained secondary mag-netite. It could be expected that secondary fine-grained mag-netite would have been produced during progrademetamorphism of ultramafic or gabbro–wehrlitic rocks.However, the effect of this on magnetic properties would bemasked by the overwhelming predominance of abundant pri-mary coarse-grained magnetite, as is shown by the Q valuesbelow unity for the differentiated sills of Hyrynsalmi.

Retrograde metamorphic and hydrothermal reactions tendto destroy magnetite through the oxidizing role of water. Asignificant change of conditions such as intense deformationand introduction of hydrothermal oxidizing fluids is neededto cause radical changes in magnetization. Low-temperaturehydrothermal alteration (or zeolite-facies metamorphism) ofbasaltic rocks results in weakly magnetic rocks and can beregarded as a primary process, since it affects the magneticproperties directly after or already during cooling. All theweakly magnetic samples from recent Icelandic basalts, asstudied for their petrophysical properties by Airo (1990),were from high-temperature areas. The destruction of coarsemagnetite grains begins with fracturing at grain margins,leading to progressive fragmentation and grain replacement.During the early stages of this process there is only a smallchange in induced magnetization, but it may increase theremanence profoundly. The reduction in magnetization isalso reflected in the magnetic pattern by relatively weakanomaly intensities.

Secondary magnetite is commonly produced in the pro-cess of serpentinization, filling the cracks of the fracturedolivine or orthopyroxene grains. This coarse-grained magne-tite is favoured by the highly oxidizing conditions and lowwater pressures, and leads to generally high inducedmagnetizations associated with serpentinized ultramafics.However, serpentinization is a retrograde process, since wa-ter is introduced into the rock material, and has a tendencyto destroy the primary titanomagnetites. In Laanila dykes theolivines are fully serpentinized and the magnetic carriers areskeletal magnetite and titanomagnetite grains, possibly as aresult of retrograde reactions. In addition, secondary magne-tite was produced in some Finnish ultramafic formationsduring the regional metamorphism. Their high remanence isthen due to PSD magnetite inclusions in pyroxenes, basedon optical and hysteresis measurement evidence.

The present erosional level of shear zones in Precambrianshield areas represents compressional deformation undermid-crustal conditions. As stated above, ferric iron is incor-porated into hydrous iron silicates at the expense of magne-tite under a constant oxidation state and a high water

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pressure. That is one of the reasons why shear zones onmagnetic maps are related to magnetic lows. Other reasonsare the intrusion of K-granites and migmatization associatedwith shearing. A similar situation is applied to contact zonesof felsic plutons. In highly deformed shear zones magne-tite can be destroyed at the expense of biotite (magnetite +K-feldspar → biotite) at high oxygen fugacities (Grant1985; Clark 1997). The same reaction is regarded as anoxide–silicate reequilibration process by Frost (1991b), inwhich case it is a cooling reaction rather than a later alter-ation process.

Fault and fracture zones emerge as rectilinear magneticminima on aeromagnetic maps. The oxidation of magnetiteto hematite has been proposed as the reason (Henkel andGuzman 1977) for the reduced magnetization associatedwith fracture zones. The Precambrian upper crustal rockshave been almost totally eroded away from the Finnish partof the Fennoscandian shield. At mid-crustal levels under theamphibolite-facies conditions, instead of oxidizing to hema-tite, magnetite may be destroyed and hydrous mafic silicatesproduced, in the same way as described above in associationwith shear zones. In fractured rocks water is introduced tothe crystal lattices of silicates, weakening the crystal struc-tures. Recent faults and fractures are often interpreted on thebasis of their sharp and rectilinear features in the aeromag-netic data. However, magnetite is quite resistant to surfaceweathering, and does not oxidize readily, particularly in lowtemperatures. Moreover, silicates weather before magnetite,so limited amounts of weathering cannot significantly alterthe magnetic susceptibilities of surficial rocks. Therefore ithas been proposed that the decrease in magnetization relatedto many fault and fracture zones in the Fennoscandian shieldhas been produced during earlier and more severe, mid-crustal processes than surface weathering (Airo 1999). Thepresent aeromagnetic image of these fault and fracture zoneshas been enhanced by the repeated kinematic reactivation ofthe same weakness zones at different evolutional stages.

Summary

The high iron content is no universal guide to estimaterock magnetic properties, which are dependent on many fac-tors during the geological history of rock. Combined geo-physical and geological studies on mafic dyke rocks showedthat their magnetization was predominantly determined dur-ing emplacement, where the bulk composition (largely con-trolled by the Fe, Mg, and Ca contents) and the ambientredox state of the parental dyke magma play an importantrole. As the oxidation conditions in the Earth’s crust are gen-erally laterally continuous, abrupt linear changes in magneticfield may be indicative of vertical displacement between ad-jacent crustal segments. Differences in magnetic field reflectcompositional and geometrical variations, and can be uti-lized in tectonic and structural interpretations. It is proposedthat the metamorphic history after emplacement had onlyminor effects on the magnetization of the studied mafic dykerocks. Based on correlations of mineral compositions andmagnetic properties, the highly magnetic Proterozoic maficdyke rocks and their metamorphosed derivatives in Finlandare characterized by clinopyroxene (augite) coexisting in thesame compositions with often a large amount of magnetite.

Oxidation state and water vapour pressure at the emplace-ment environment have a major influence on the partitioningof ferric iron between hydrous and anhydrous mafic sili-cates, at the same time resulting in formation and destructionof magnetite. Mafic rocks that were primarily nonmagneticwill never be very highly magnetic.

Major compositional changes do not happen unless thereis a radical change in environmental conditions. In the Finn-ish Precambrian such a large-scale production of secondarymagnetite was possible during the Svecofennian orogeny (at1.9–1.8 Ga). A strong Svecofennian overprint characterizesthe remanent magnetizations within the Precambrian rocks(Mertanen 1995). The secondary fine-grained (PSD) magne-tite occurring as inclusions within (clino)pyroxene (and (or)plagioclase) crystals carries a strong, stable thermoremanentcomponent, reflecting the direction of the geomagnetic fieldvector at the time of cooling through the Curie point (Butler1992). This produces distinctive anomaly patterns when as-sociated with high magnetite contents (magnetic susceptibil-ities > 0.01 SI), and on high-resolution aeromagnetic mapsthe predominance of remanence can be recognized from thestyle of the magnetic signature. A remanence vector that de-viates substantially from the present Earth’s field directionwill only affect the anomaly pattern when it is at a very highangle and when the intensity of remanent magnetization ex-ceeds a certain level (Q >> 1), for example, the effect of thenegative inclination in Laanila with Q values greater than 8.This has been demonstrated by model calculations of variousdyke-like anomalies by varying the magnetic parameters.The collection of oriented samples for detailed studies canbe focused on such formations. With lower magnetite con-tents (magnetic susceptibilities < 0.01 SI) even the highremanences (= high Q values) are not enough to change themagnetic patterns.

The induced magnetization dominates in many of the Pro-terozoic mafic dyke swarms. In rocks containing large mag-netite grains (MD), characterized by viscous magnetization,the rock may possess a collection of remanence componentswith different directions formed at different times. Thesetend to cancel each other out, and the direction of the resul-tant remanence vector approaches that of the present Earth’sfield. At the present high latitudes of the Fennoscandianshield, this makes the geometrical interpretation of the well-defined dyke anomalies unambiguous, as the anomalies re-flect the structural trends of the dykes.

Acknowledgments

The earlier draft of this paper benefited from the criticismof Dr. D.A. Clark CSIRO, Sydney, Australia. I would like toacknowledge the critical comments of ProfessorM. Peltoniemi from the Technical University of Helsinki. Ialso thank S. Mertanen from the Geological Survey of Fin-land for giving some helpful advice about the manuscriptand M. Kurimo, who supervised me in preparing the aero-magnetic images. The reviews of the manuscript by Profes-sor W.A. Morris and Professor R.W. Kerrich were muchappreciated. The primary geophysical data were provided bythe Geological Survey of Finland.

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