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ISSN 0145 8752, Moscow University Geology Bulletin, 2015, Vol. 70, No. 2, pp. 84­96. © Allerton Press, Inc., 2015. Original Russian Text © N.V. Lubnina, V.S. Zakharov, M.A. Novikova, V.P. Vorontsova, 2015, published in Vestnik Moskovskogo Universiteta. Geologiya, 2015, No. 2, pp. 10­21.

Paleoproterozoic Remagnetization in the White Sea Mobile Belt, Karelia: Petro Paleomagnetic Evidence and Supercomputer Modeling
N. V. Lubnina, V. S. Zakharov, M. A. Novikova, and V. P. Vorontsova
Department of Geology, Moscow State University, Moscow, Russia e mails: natalia.lubnina@gmail.com, vszakharov@yandex.ru, mari_1989@mail.ru, vall_nett@mail.ru
Received November 12, 2014

Abstract--The detailed petro paleomagnetic study of the Paleoproterozoic eclogite complexes of the White Sea mobile belt in the area of Gridino dike field revealed two stages of rock remagnetization. The Early Pale oproterozoic gabbronorite dikes contain a secondary component of NRM that is pointed NNW with a strong positive slope that is correlated with the post orogenic collapse of 1.95­1.88 Ga B.P. Formation of the second alternating magnetization phase of rocks is estimated at 1.8­1.75 Ga B.P., which corresponds to the influence of post orogenic hydrothermal fluids. Keywords: paleomagnetism, remagnetization, Paleoproterozoic, White Sea mobile belt, supercomputer modeling DOI: 10.3103/S0145875215020052

INTRODUCTION Remagnetization of rocks is traditionally thought to be a process that indicates that the ferromagnetic fraction of a rock is magnetized under the effect of an external magnetic field in a direction that corresponds to the time of this field application. Remagnetization of rocks is often a factor that partially smoothes or completely destroys the primary magnetization com ponent (that formed at the moment of rock forma tion), and therefore complicates paleomagnetic stu dies. In this respect, many attempts have been made both to investigate the mechanism of the remagnetiza tion of rocks and to elaborate the methods for the detection of the component of the secondary magne tization. The main factors that determine remagneti zation are P­T regimes and the presence of fluids; these lead to either partial or complete destruction of host minerals and/or formation of a new mineral frac tion. Depending on the combination of these two fac tors, remagnetization can be thermoviscous or chemi cal (see the review in (Lubnina, 2009; Zwing, 2003, and others)). Thermoviscous remagnetization occurs during rapid cooling of rock at low temperatures (Nagata, 1961; Dunlop et al., 1997) with vertical motions (for example, rapid exhumation of rocks to the surface) being the controlling factor. The characteristic feature of thermoviscous remagnetization is a homogeneous distribution of the secondary component over the volume, with different host minerals of the magnetiza tion demonstrating the same mean direction.
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Chemical remagnetization is attributed to the for mation of secondary minerals, magnetization carriers, under the influence of hydrothermal fluids, which leads to complete replacement of the initial host min eral of magnetization. The characteristic features of chemical remagnetization are, first, selective remag netization of rocks within the united section with the same properties, and, second, nonuniform remagneti zation within the same stratum. Resulting from the chemical remagnetization, a bipolar secondary mag netization component, which clearly depends on the host minerals­carriers of magnetization, is formed. Subdivision of the full magnetization vector in metamorphic rocks into components (magnetization components that occurred in different time intervals) by using component analysis is obligatory in modern paleomagnetic studies. However, due to the close spectrum of blocking temperatures of host minerals of magnetization, such a subdivision is complicated or impossible. Nevertheless, the adequate subdivision of secondary (metachronous) magnetization compo nents allows self consistent reconstructions to be made and provides better understanding of the geody namics of the studied area as a whole. A secondary magnetization component that occurs as a result of metamorphism can either completely destroy the pri mary one or be presented in the form of low and mid temperature magnetization components in rocks. The time of the settling of the secondary magnetization component is traditionally estimated by comparison with the most reliable paleomagnetic poles that were


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obtained earlier. Recent developments in isotopic methods, including U­Pb dating of sphenes and rutiles (whose isotopic systems close near the Curie points of the main host minerals­carriers of magneti zation), has allowed the time of the settling of second ary magnetization component to be dated by using independent methods. STUDY OBJECTS The White Sea mobile belt is composed mainly of Meso and Neoarchean granite gneisses, metavolcano genic, and paragneiss complexes. Its distinctive fea tures are repeated manifestation of intensive deforma tions and metamorphism at higher and moderate pres sure both in the Neoarchean and Paleoproterozoic ((Slabunov, 2008) and references therein). The boun dary between the White Sea mobile belt and the adja cent structures was completely formed in the Paleopro terozoic ((Slabunov, 2008) and references therein). In the eastern White Sea mobile belt, there is a Neoarchean eclogite containing complex whose forms are tectonic slabs, the so called Gridino zone, which is approximately 50 km long and 5­6 km wide. In terms of texture features, this complex is compara ble to mixtites (Granulitovye i eklogitovye..., 2011). The clastic part of the mixtite complex is presented by numerous lens shaped, rarely irregularly shaped bod ies that are nonuniformly distributed in the matrix. Mafic rocks are sharply predominant in the composi tion, viz., eclogites of different degrees of alteration, amphibolites, and metamorphosed gabbroids. The large variety of rocks in fragments of different compo sitions, degrees of deformation, and metamorphism is an argument that the eclogite containing mixtite is a melange formed in a subduction zone (Granulitovye i eklogitovye..., 2011). Based on the geological and geo chronological data, the upper age limit of the eclogite containing mixtite formation is no younger than 2701.3 ± 8.1 Ma ((Volodichev et al., 2009) and refer ences therein). Another age and genetic type of eclogites that occurs in the area of the Gridino village is Paleopro terozoic apo gabbroid eclogites (Morgunova and Per chuk, 2011; Slabunov et al., 2011). There are up to three generations of dikes of eclogitized gabbroids, often with hardening zones; dikes cut the intensively altered Neoarchean eclogite containing complex, which is metamorphosed under the amphibolite facies condition. The predominant part of this territory and the mafic dike magmatism, along with the consider able density of the clustering of the intrusions, suggest that this area can be considered as the Gridino dike field (Fig. 1) (Stepanov and Stepanova, 2005). Within the limits of the Gridino dike field, among the first generation dikes, gabbroids of high Fe, tholeiite, and subalkali compositions can be distin guished in terms of petrochemical characteristics. Eclogite alteration involved gabbroids of all men
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tioned groups, but parageneses of eclogite facies have been found only in tholeiite and subalkali ones. Igne ous minerals in the first generation dikes are preserved only in a subalkali dike at Cape Peschanyi. The second generation dikes are commonly repre sented by a complex of magnesian ultramafic to mafic rocks, which is well known within the limits of the White Sea mobile belt, or by the lherzolite­gab bronorite complex, which is 2393 Ma in age on U­Pb (Volodichev et al., 2009). In this area, two intrusion phases of gabbronorites of this complex are distin guished, with the rocks possessing similar petrochem ical characteristics. Eclogite alterations in the second generation dikes are nonuniform, analogous to se cond generation dikes, both in the areal distribution of all dikes and within the limits of particular bodies. The levels of P­T metamorphism are different, e.g., on Eklogitovyi Is., the dike of eclogite altered gab bronorites of the second intrusion phase cuts the body of gabbronorites of the first phase, where the degree of metamorphism chiefly corresponds to the high pres sure amphibolite facies. The third generation dikes, which cut second gen eration ones, are represented by two petrochemical groups: high iron (Fe­Ti) and tholeiite (Fe) gabbroids (Stepanov and Stepanova, 2005). The former are com parable to the coronite gabbros of the White sea mobile belt and dolerites of the Karelian Craton, being 2.12 Ga in age (Stepanova et al., 2005). The degree of metamorphism of these rocks does not exceed the high pressure amphibolite facies. Dikes composed by tholeiite gabbroids (from 10­20 cm to 4 m thick) have been found only on Vorotnaya Luda Is. and at Cape Gridin; they have a remarkable geochemical peculiar ity in their distribution of REEs: they do not contain traces of crustal contamination, i.e., they are compa rable to basalts (Stepanov and Stepanova, 2005). To carry out the paleomagnetic studies, we made a detailed sampling of the dikes of first two generations and the host eclogites on Vorotnaya Luda Is. as well (Fig. 1). METHODS OF SAMPLING AND LABORATORY PALEOMAGNETIC STUDIES The oriented specimens for paleomagnetic studies were sampled with a hand drill. We also sampled oriented hand specimens, from which 2 cm cubes were cut later. Magnetic and solar compasses were used for the orien tation of the core and hand specimen samples. Sampling of paleomagnetic specimens was made by the site based method. During the selection the preferable specimens were fine grained units from contact zones of mafic intrusive bodies. A number of specimens were collected in thin dikes (up to 2 m thick) was 10­15; for thick (more than 3­5 m) dikes, the number of collected specimens was 15­20; all specimens were collected across the strike of each dike. If possible, exocontact and endocontact zones
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34°6

(b)

34°8
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66 68 82 70 72 80 55 70 75 55 70­80 70 80 60 75 80 80 75

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Quaternary deposits
a b

Granodiorite dikes: a aplites, b granites Metagabbro Olivine gabbronorite complex Mg tholeiites

70 60 80 80

70

WHITE SEA
80 52

100 m
70 70

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Archean eclogites with low a and high b content of inclusions Occurrence elements

Fig. 1. The geological scheme of Vorotnaya Luda Island (a), after (Granulitovye i eklogitovye..., 2011), with sampling points drawn. In inset (b), the study area location within the White Sea mobile belt is shown.

were also sampled and these objects were considered as one site. The first generation dike is folded. To per form the paleomagnetic validity test, we sampled ori ented specimens from different limbs of the fold. To estimate the age of magnetization acquired by the rocks (contact zone test), we sampled the host meta morphic complexes (eclogites) from both the contact zone and at a distance of up to 200 m from it. In total, 146 oriented samples were collected for paleomagnetic studies. The laboratory studies were carried out at the Petromagnetic Laboratory of Moscow State Univer sity (Moscow, Russia) and at the Paleomagnetic Lab oratory of Lund University (Lund, Sweden). The specimens underwent a complete cycle of treatment in accord with the modern methods of paleomagnetic and petromagnetic studies (Paleomagnitologiya..., 1982). Petromagnetic studies were carried out on a KLY 4S kappabridge equipped with a CS4 oven add on unit (manufactured by AGICO company, Czech Republic). All specimens were treated by step wise temperature purification at up to 590­600°C, some were purified by an alternating magnetic field of up to 100 mTl. The number of purification stages was at least

15­20. For demagnetization, a TD 48 nonmagnetic oven (ASC company, United States) was used. Residual magnetization was measured using a JR 6 spin magne tometer (AGICO company) and a SQUID magne tometer (2G Enterprises, United States). Possible sec ondary alterations during temperature purification were controlled by magnetic susceptibility measure ment on the KLY 4S kappabridge after each step of demagnetization. The components of residual natural magnetization were distinguished by using Zijderveld orthogonal plots; the directions of these components were calculated by the least squares method. Com puter processing of the measurement results was made using special purpose software. In making Precambrian reconstructions the study of the magnetic texture of the rocks plays a large role: it allows the absolute spatial orientation of strain and stress directions during settling of the secondary changes of host minerals of magnetization that are to be estimated (Tarling and Hrouda, 1993; McElhinny and McFadden, 2000). We studied the magnetic texture of the rocks on the basis of initial anisotropy of magnetic susceptibility (AMS) measured on a KLY 4S kappabridge with the
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PALEOPROTEROZOIC REMAGNETIZATION IN THE WHITE SEA MOBILE BELT Table 1. The paleomagnetic directions for Paleoproterozoic complexes in the White Sea mobile belt, Gridino dike field Direction Ord. nos. Component N Dec° 354.5 28.6 336.8 13.4 37.9 23.7 332.8 40.3 334.6 28.6 Inc° 37.6 60.5 47.2 ­21.8 58.0 18.9 48.9 57.7 47.9 58.5 K ° 95 6.3 7.4 3.6 13.4 4.8 9.3 3.8 4.5 Supposed age of magnetization components, Ma

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Host eclogites 1 2 3 4 5 6 7 8 Average direction of GR component Average direction of PR component GR1 PR1 GR2 GR2S PR2 PR2S GR3 PR3 14 14 23 23 28 28 25 25 3 3 44.3 30.0 70.9 6.1 33.7 9.6 58.4 42.7 1980 1980 1800 1800 1980 1980 1800 1980 1800 1980

First generation dike

Second generation dike

N is the number of specimens; Dec° and Inc° are the declination and inclination, respectively, of the average directions of components in the geographic coordinate system; K is the clustering of vectors; ° is the radius of the confidence circle at a 95% probability for the 95 average direction; boldface denotes average paleomagnetic directions for GR and PR components, used in interpretation.

subsequent calculation of the full ellipsoid of magnetic susceptibility. In order to analyze the character of the distribution for directions of the main ellipsoid axes, viz., the maximal (K1), intermediate (K2), and mini mal (K3), they were drawn on stereogram plots in pro jection to the lower hemisphere. Additionally, we calculated such AMS parameters as the average value of magnetic susceptibility (Kav = (K1 + K2 + K3)/3), degree of elongation (L = K1/K2), and degree of flattening (F = K2/K3). The degree of anisotropy was measured by using the parameter P (P = K1/K3): if P = 1, the magnetic susceptibility ellip soid is of a spherical shape and the degree of anisot ropy is 0%; at P = 1.15, the degree of anisotropy is 15%, and so on. The shape of the magnetic susceptibility ellipsoid was determined by calculation of the T param eter (T = [2 ln(K2/K3)/ln(K1/K3)] ­ 1), after (Jelinek, 1981): this parameter changes from +1 in the case of a flat shape to ­1 in the case of an elongated one. RESULTS OF THE PALEOMAGNETIC STUDIES AND DISCUSSION Analysis of magnetic purification shows that con siderable part of specimens from both host eclogites and dikes of two generations contain three magnetiza tion components (Fig. 2). The least stable of these three is the low temperature component (PDF) which usually is destroyed at up to 250°C. The direction of
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this component is close to that of the modern mag netic field in the studied area, and this was excluded from further consideration. A metachronous NNW pointed component of moderately positive inclination is distinguished in the eclogite specimens at temperatures of 250­500°C (Figs. 2a­2c, GR1 component). This component is monopolar. The distribution of the distinguished GR1 components on the sphere is given in Table 1. The high temperature component (PR1) is distinguished as a characteristic one (the most stable, reaching the origin of coordinates of Zijderveld orthogonal plots). The component is destroyed at temperatures of 510­ 555°C (Figs. 2a­2c). For most of the specimens, the high temperature component points NE and has a moderate to steep positive inclination (Figs. 2a­2c). The distribution of the distinguished PR 1 compo nents on a sphere is shown in Fig. 3; the average pale omagnetic direction of this component is given in Table 1. In specimens from the first generation dike, step wise temperature purification revealed two high tem perature natural residual magnetization components. The first component has a blocking temperature of 450­510°C (Figs. 2d­2f). The average direction of this component in the present day coordinate system is N = 23, Dec = 336.8°, Inc = 47.2°, K = 70.9, 95 = 3.6° (Fig. 3b, Tables 1 and 2). Since the first genera tion dike is folded and the oriented specimens were
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LUBNINA et al. I. Host rocks (Archean eclogites) (a) 510°C N Top 250°C GR1 M/Mmax (b) 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0 100 200 300 400 500 600 °C (c) N 555°C 530°C GR1 3 NRM W

PR1 99.9 mA/m W

555°C E

S Down (d) N Top GR2 450°C NRM (e)

II. First generation dikes (f) N

PR2 W

153 mA/m E 555°C

M/Mmax 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0 100 200 300 400 500 600 °C

GR2 9 555°C 350°C NRM W

S Down III. Second generation dikes (g) M/Mmax 1.0 0.9 0.8 GR3 0.7 0.6 E 0.5 629 mA/m 0.4 0.3 0.2 0.1 0 100 200 300 400 500 600 700 °C W N Top (h) (i) N GR3 21 510°C 590°C NRM

PR3 W

PDF S Down

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Fig. 2. Examples of step wise temperature purification of specimens from the dikes of two generations and the host eclogites on Vorotnaya Luda Island (Gridino dike field): I host Archean eclogites (a­c); II, first generation dike (d­f); III, second generation dike (g­i). Each specimen is characterized by, left to right: a Zijderweld plot in the geographic coordinate system (a, d, g); a curve of the change in value of natural residual magnetization during temperature cleaning (b, e, h); and stereoprojection of directions in the geographic coordinate system of the host eclogites (c), the first generation dike (f) and second generation one (i). The empty circles are the projections of vectors to the upper hemisphere (to the vertical plane for Zijderweld plots); filled ones, pro jections of vectors to the lower hemisphere (to the horizontal plane for Zijderweld plots). The digits near the circles indicate the temperature (°C) of magnetic cleaning. The encircled letters denote the distinguished components: PDF, contemporary (present day) magnetization component; GR, low temperature; PR, high temperature (see explanation in text).

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collected from the limbs, the time of settling for this component in rocks was estimated by a step wise unbending test. The maximum clustering of the distri bution of single vectors on the sphere was reached at 0% unbending. The clustering of the GR2 component in the present day coordinate system is higher than
N (a) GR1 (b) GR2 N

that in the stratigraphic coordinate system (Kg/Ks = 11.7), indicating that this magnetization component settled in the rock after the dike was folded. For further analysis, we used the direction of the GR2 component in the present day coordinate system. The average direction of the GR2 component is close to that of the
N (c) GR3

GR2S

W W E

E

N

N

(d)

PR1

N

(f) PR3

PR2S W (e) PR2 E

W 1 2 3 4

E

Fig. 3. The distribution of the directions for mid temperature (GR, gray circles) and high temperature (PR, gray squares) residual magnetization components on a sphere in the geographic coordinate system, as revealed for the host Archean eclogites and the dikes of two generations on Vorotnaya Luda Is., Gridino dike field. The empty triangles are the directions of the mid temperature (GR2S) and high temperature (PR2S) magnetization components that were revealed for specimens of the first generation dike in the stratigraphic coordinate system. In the stereograms, the empty figures are projections of the vectors to the upper hemi sphere; the filled circles are the projections of the vectors to the lower hemisphere. For the letter notations of the magnetization components, see Table 1. MOSCOW UNIVERSITY GEOLOGY BULLETIN Vol. 70 No. 2 2015


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Table 2. Paleomagnetic poles for Paleoproterozoic complexes in the White Sea mobile belt that were used in reconstructions Sampling area Gridino dike field, Vorotnaya Luda Is. Gridino dike field, Vorotnaya Luda Is., remagnetization Central Karelian terrane, remagnetization Vodlozero terrane, remagnetization Vodlozero terrane, Ropruchei sill Index PR GR RC VD RS , °N 47.2 68.2 45.2 40.8 34.8 , °E 218.3 245.6 192.4 205.4 209.6 A95 5.7 8.4 8.0 6.3 13.3 m, °N 21.1 41.9 19.8 12.4 6.2 Age, Ga 1.98 1.98 1.80 1.80 1.75 Source Present work Present work (Lubnina, 2009) (Lubnina, 2009) (Bogdanova et al., 2013)

° and ° are the latitude and longitude of the paleomagnetic pole, respectively; A95 is the radius of the confidence circle, in degrees; m, °N is the paleolatitude (in degrees north latitude).

GR1 component in host eclogites (Fig. 3). The nega tive contact test indicates partial secondary remagne tization of rock during the formation of the GR1 and GR2 low temperature components. The second natural residual magnetization compo nent (PR2) was distinguished in specimens from the first generation dike in the temperature interval of 510­580°C (Figs. 2e and 2f). Since the high temper ature magnetization component is completely destroyed at 560­580°C, its main carrier is most likely magnetite. The average direction of the PR2 high temperature component in the present day coordinate system was N = 28, Dec = 37.9°, Inc = 58.0°, K = 33.7, 95 = 4.8° (Fig. 3e, Table 1). After unfolding, the average direction of this component in the strati graphic coordinate system was N = 28, Dec = 23.7°, Inc = 18.9°, K = 9.6, 95 = 9.3° (Fig. 3e, Table 1). Clustering in the present day coordinate system is considerably higher than that in the stratigraphic coordinates (Kg/Ks = 3.5), indicating the secondary nature of this high temperature magnetization com ponent. The average direction of the PR2 high tem perature component from the first generation dike falls within the confidence interval of the average direction of the PR1 high temperature component from the host eclogites (Fig. 4), indicating remagneti zation of rocks after penetration of the first generation dike. Negative folding and contact tests indicate the secondary nature of this high temperature magnetiza tion component. For the second generation dikes, two behavioral types are characteristic for vectors of natural residual magnetization during temperature purification. In one case, only the NNE pointing high temperature magnetization component of a moderate positive inclination is distinguished (Figs. 2h­2j). This com ponent is destroyed at up to 555°C. In another case, rounded portions can be seen in the Zijderveld plots in the temperature interval of 350­520°C; this indicates

overlap of the spectra of the blocking temperatures of two different host minerals of magnetization and the impossibility of complete separation of these two high temperature natural residual magnetization compo nents. However, in two narrow intervals (250­350°C and 520­580°C), GR3 and PR3 magnetization com ponents are distinguished The GR3 component points NW and has a moderate positive inclination (Fig. 3c). The average direction of this component in the present day coordinate system is N = 25, Dec = 332.8°, Inc = 48.9°, K = 58.4, 95 = 3.8° (Fig. 3c, Table 1). The average direction of the GR3 component from the second generation dike fits, within the limits of the confidence intervals, the directions of the GR1 and GR2 mid temperature magnetization compo nents from the host eclogites, and the first generation dike, respectively. The negative contact test suggests a secondary nature of these magnetization components. Remagnetization of the rocks took place after penetra tion of the second generation dikes, as is shown by the negative contact and folding tests. The Magnetic Texture of the Rocks The secondary nature of the distinguished magne tization components is also verified by the results of AMS studies. These studies of specimens from the host eclogites and the dikes of two generations allowed us to determine the magnetic texture of the rocks, i.e., the peculiarities and regularities of the distribution of the magnetic fraction in the studied specimens (Fig. 4). In the host eclogites, the value of the magnetic suscep tibility falls within the interval from 80 to 234 â 10­6 SI units (Fig. 4a). A negative correlation is observed between the value of the magnetic susceptibility and the composition of the magnetic fraction (Fig. 4d) and a high degree of anisotropy is reported (up to 51%, with the average AMS value varying from 10 to 22%). The high degree of anisotropy and the dependence of
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(a)

(b)

(c)

1 80 Km â 10­6 SI units 234 L 1.259 (d) Linear Flat 1 1 (g)
N

1 477 Km â 10­6 SI units 1190 L 1.416 (e) Linear Flat

1 459 Km â 10­6 SI units 976 L 1.192 (f) Linear

Flat 1 1 1 (h)
N

1.259 F

1.416 F (i)

1

N

1.192 F

W

EW

EW

E

W

W

W

Fig. 4. The main characteristics of the magnetic textures of specimens from the host eclogites and the dikes of two generations on Vorotnaya Luda Is., Gridino dike field. Dependence of magnetic anisotropy, P, on the initial magnetic susceptibility Km for spec imens from eclogites (a), the first generation dike (b) and second generation one (c). Magnetic texture types (Flinn diagrams) for specimens from eclogites (d) and the first generation dike (e) and second generation dike (f). Stereograms showing distribu tion of directions of main AMS ellipsoid axes in projection to the lower hemisphere for specimens from eclogites (g), the first generation dike (h) and second generation one (i). In the stereograms, the projections of the major, intermediate, and minor AMS ellipsoid axes are indicated with squares, triangles, and circles, respectively.

the value of the magnetic susceptibility on the size of the magnetic fraction indicate the formation of sec ondary host minerals of magnetization. In this group of specimens the smooth flat type of magnetic anisot ropy is predominant (the mode value of the parameter T falls within the interval of 8­15%), indicating the formation (secondary transformation) of a magnetic texture of the main volume of the host eclogites in the absence of stress deformations (Fig. 4g). The average direction of the major axis of the magnetic susceptibility ellipsoid lies in the vertical plane, while the minor one lies in the plane of secondary mineralization (Fig. 4a). The formation of such a type of magnetic susceptibility anisotropy is most probably related to secondary changes in the magnetic fraction. The specimens from the first generation dikes are characterized by magnetic susceptibility values from
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477 to 1190 â 10­6 SI units (Fig. 4e). Analogous to specimens of the host eclogites, they demonstrate a negative correlation between the value of the magnetic susceptibility and the degree of anisotropy, P. In spec imens from the first generation dike, a high degree of anisotropy of the magnetic susceptibility is observed, from 24 to 57% (Fig. 4h). The shape of the magnetic anisotropy is characterized by a flat type caused by uniaxial compression, with the parameter P varying from 0.530 to 0.810 (Fig. 4h). Analysis of the direc tions of the magnetic susceptibility ellipsoids of the main axes shows the orientations of the major axes in most specimens along the strike of the first generation dike (Fig. 4b). The directions of the minor axes are per pendicular to the contact plane of the dike (Fig. 4b). Under uniaxial compression, the value of the mag netic susceptibility decreases due to the occurrence of
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microflaws in magnetic crystals and the shift of the domain boundaries towards these microflaws. Under these stresses, the easy magnetization axis shifts towards the lowest pressure source (Sholpo, 1977), or, in the present case, in the plane that is oriented in parallel to the contact plane between the dike and the host eclogites. In specimens of olivine gabbronorites (second generation dikes), the value of magnetic susceptibility changes from 459 to 976 â 10­6 SI units (Fig. 4c). Analysis of the magnetic anisotropy shows that the degree of anisotropy falls within the range of 13­24% (Fig. 4f). A negative correlation is observed between the value of the magnetic susceptibility and the degree of anisotropy (Fig. 4c). The shape of the magnetic anisotropy is linear in most of the specimens (Fig. 4f) due to secondary fine magnetite crystal growth. The T parameter, which characterizes the shape of the AMS ellipsoid, varies from 0.070 to 0.910. The orientations of the main axes of the magnetic susceptibility ellip soids in olivine gabbronorites is consistent along the entire section: the major axis lies nearly in the plane of the strike of the second generation dikes, while the minor and intermediate ones lie in a plane that is ori ented perpendicularly to the contact plane of the dike (Fig. 4j). The formation of such anisotropy can be explained by the following mechanism: the flat parts of magnetic minerals that formed under higher tempera tures are oriented in parallel to the contact plane, while the elongated parts of these crystals are oriented along the direction of the flow of secondary fluids. Paleomagnetic Poles The GR and PR were recalculated from the average directions of the GR and PR high temperature mag netization components to the coordinates of the sam pling points = 65.9° N, = 34.7° E (Table 2). Since both magnetization components are secondary and formed after the penetration of the dikes of two gener ations, the time when the rocks of these components acquired magnetization components can be estimated only from indirect evidence. As a result of the inde pendent U­Pb dating of sphenes, the regular decrease in their absolute age was revealed within the limits of the White Sea mobile belt: from 1.92 Ga in the north eastern part to 1.8 Ga in the southwestern part, at the boundary with the Karelian Craton (Bibikova et al., 1999). Since the closing temperature of the sphene isotope system is approximately 80°C, we can suggest that all the rocks in the White Sea mobile belt were remagnetized within the limits of this time interval. The paleomagnetic pole that was recalculated from the average direction of the PR high temperature magne tization component lies in the Paleoproterozoic part of the apparent polar wandering path (APWP) for the Karelian Craton, in the time interval of 1.95­1.88 Ga (Fig. 5, Table 2). In addition, the paleomagnetic pole we obtained is close to that of 1.98 Ga B.P. for the Vod

lozero terrane of the Karelian Craton (Pasenko and Lubnina, 2014). Proceeding from these data, we esti mate the formation age of the PR high temperature magnetization component at 1.95­1.88 Ga B.P. The formation age of the high temperature magne tization component is probably related to post oro genic collapse. The collapse of collisional systems occurred due to the gravity instability of an anoma lously thick continental crust that formed during the collision. As a result, collapse took place under a ten sional setting and at an abrupt decrease in pressure and temperature during a short time interval. Magnetiza tion formed under such a regime is thermoviscous in nature (an abrupt drop of temperature below the Curie points of host minerals of magnetization for a short term time interval) and is completely destroyed when the primary magnetization settles during the forma tion of rocks. The newly formed secondary compo nent is monopolar and most often is the only one that is analyzed in the component analysis, excluding the viscous (contemporary) magnetization component. Estimation of the time when the GR mid temper ature metachronous magnetization component formed is somewhat complicated. The paleomagnetic pole recalculated from the direction of this component to coordinates of sampling points (Fig. 5) demon strates a 60°­90° scatter in latitude relative to the coordinates that were obtained earlier for the Pale oproterozoic paleomagnetic poles of the Karelian Craton (Fedotova et al., 1999; Bogdanova et al., 2013). Nevertheless, the direction of metachronous magneti zation component that we determined in the present work is close to that of Svecofennian magnetization (~1.8 Ga) for the Karelian Craton. The high blocking temperatures (up to 510°C) of the host minerals of this magnetization suggest that this component formed under the effect of fluids. Since collapse is accompa nied by the release of a large amount of fluids, whose effect can last for tens of millions years after the main collision stage, the formation time of the secondary metachronous magnetization component may be "delayed" by this time period relative to the main phase of post orogenic collapse. The effects of fluids of different compositions lead to the formation of new host minerals­carriers of magnetization that result from oxidation of primary minerals and also to settling of the secondary (metachronous) magnetization com ponent in rocks. The newly formed component can be either monopolar or bipolar. The directions of natural residual magnetization vectors that correspond to dif ferent carrier minerals nearly coincide with each other, in this case. The secondary component can form before, during, and after folding. It also should be noted that the U­Pb age of rutiles in rocks of the Karelian Craton and the White Sea mobile belt is 1760 ± 20 Ma (Slabunov, 2008), indicating a secon dary thermal effect (400­500°C) in this time interval (Bibikova et al., 1999a, 1999b). Thus, formation of the
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PR

60° N 1840 1790 1880 2300­2100 2058 1384 1100 0° 2450 30° S 900 940 980 60° S GR 1950­1850 30° N

1475 1461 1452 1265

1770

1200

750

90°

120°

150°

180°

210°

240°

270°

300°

90° S 330°

Fig. 5. The apparent polar wandering path for the Karelian (dark gray arrow) and Kola (light gray dashed line) cratons in the time interval of 2.45­0.75 Ga B.P., after (Lubnina, 2009). The star denotes the paleomagnetic pole that was recalculated from the direction of the PR high temperature magnetization component to the coordinates of sampling points; the rhombus indicates the paleomagnetic pole that was recalculated from the direction of GR mid temperature magnetization component to the coordi nates of sampling points.

secondary metachronous magnetization component can be attributed to the effect of fluids 1.8­1.75 Ga ago. SUPERCOMPUTER MODELING RESULTS To reconstruct the possible conditions of crustal heating to high temperatures, we used the original self consistent petro­thermo­mechanical 2D models by T.V. Gerya (Gerya and Yuen, 2003; Sizova et al., 2014). Modeling was performed using the supercom puter complex of the Moscow State University (Voevodin et al., 2012). A description of the model that reproduces the plate convergence at a rate of 5 cm/yr and the conti nental collision, as well as some results, were given in (Zakharov et al., 2015). The thickness of the continen tal lithosphere is 150 km; the set mantle temperature
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exceeded the contemporary one by 150°C; the radio genic heat generation of the crust was higher than the contemporary value by a factor of 1.5. These parame ters correspond to the Archean­Paleoproterozoic boundary, according to some authors (for example (Abbott et al., 1994)). The modeling results show that during subduction preceding a continental collision considerable heat generation takes place in the zone where the oceanic lithosphere sinks. The crust rapidly (~100 ka) heats to 500°C in the zone of the ocean­mantle transition at a depth of about 5­10 km from the surface (Fig. 6). In some versions, repeated heating of the crust is possi ble, but these results require a thorough analysis and adequate interpretation.
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94 0 20 40 60 80 Z, km 100 120 140 160 180 200 2450 1 2 3 4 5

LUBNINA et al. Time is 10.33 mln year

2500 6 7 8 9 10 11 12 13 14 15

2550 X, km 16 17 18 19 20

2600 21 22 23 24 25

2650 26 27 28 29

Fig. 6. The results of supercomputer modeling of the plate convergence in the Precambrian at a 150 km thickness of the conti nental crust, isothermals are in °C: (1) air; (2) water; (3) sediments 1; (4) sediments 2; (5) upper continental crust; (6) lower con tinental crust; (7) upper oceanic crust (basalts); (8) lower oceanic crust (gabbros); (9) dehydrated "dry" lithospheric plate; (10) dehydrated "dry" asthenosphere; (11) hydrated lithospheric mantle; (12) hydrated mantle; (13) serpentinized lithosphere; (14) depleted peridotites; (15) crystallized melts extracted from partially melted metasedimentary rocks; (16) crystallized tonalite trondhjemite granodiorites (TTG) extracted from partially melted basalt; (17) crystallized TTG extracted from partially melted gabbro; (18) crystallized basalts extracted from partially melted peridotite; (19) partially melted sediments 1; (20) par tially melted sediments 2; (21) partially melted upper continental crust; (22) partially melted lower continental crust; (23) par tially melted basalts; (24) partially melted gabbros; (25) partially melted lithospheric mantle; (26) partially melted asthenospheric mantle; (27) melts (basalt, gabbro) produced from peridotite; (28) felsic melts (TTG) produced from basalt; (29) felsic melts (TTG) produced from gabbro.

CONCLUSIONS (1) As a result of paleomagnetic studies of Pale oproterozoic complexes in the Gridino dike field, it has been found that rocks of this complex suffered mineral transformation and complete or partial remagnetization under the effect of superimposed process. Two magnetization components have been

distinguished, whose secondary natures were proven on the basis of negative contact and folding tests. (2) It has been shown that the secondary magneti zation component that is distinguished in the rocks of the White Sea mobile belt can be subdivided into two age groups: 1.95­1.88 and 1.8­1.75 Ga. (3) The first cluster includes dates that refer to the formation of the secondary component during col
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lapse. This component is local in distribution. The characteristic magnetization component is high tem perature, in this case. (4) The second remagnetization group is formed by the dates that were obtained from rocks that under went Paleoproterozoic endogenous activation. Petro graphic and microprobe studies suggest a hydrother mal­metasomatic nature of the secondary changes in these rocks as a result of the interaction with hydro thermal fluid (chemical remagnetization). In this case, the rocks are completely or partially remagne tized and the age when they acquired a secondary metachronous component can be estimated at 1.8­ 1.75 Ga. (5) The secondary component that formed as a result of the effect of hydrothermal fluids in Late Pale oproterozoic is widespread within the limits of the Karelian block and the White Sea mobile belt, with both being related to the East European Craton. ACKNOWLEDGMENTS The work was supported by the Russian Founda tion for Basic Research (projects nos. 13 05 01 33 and 14 05 00731). The instruments that were used in the present work were bought in the framework of the Development Program of the Moscow State University. Modeling was performed on the supercomputer complex of Moscow State University. REFERENCES
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Translated by N. Astafiev

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